Combinatorial Synthesis and High-Throughput Characterization of Multinary Vanadate Thin-Film Materials Libraries for Solar Water Splitting’ Supervisors: Prof

Combinatorial Synthesis and High- Throughput Characterization of Multinary Vanadate Thin-Film Materials Libraries for Solar Water Splitting

Dissertation zur Erlangung des Grades Doktor-Ingenieurin

der Fakultät für Maschinenbau der Ruhr-Universität Bochum

von

Swati Kumari

aus Begusarai, India

Bochum 2020

The presented work was carried out from August 2016 to April 2020 at the chair of Materials

Discovery and Interfaces (MDI) under the supervision of Prof. Dr.-Ing. Alfred Ludwig in the

Institute for Materials, Department of Mechanical Engineering, Ruhr-Universität Bochum,

Germany.

Dissertation eingereicht am: 21.04.2020

Tag der mündlichen Prüfung: 27.05.2020

Vorsitz: Prof. Dr.-Ing. Andreas Kilzer

Erstgutachter: Prof. Dr.-Ing. Alfred Ludwig Zweitgutachter: Prof. Dr. rer. nat. Wolfgang Schuhmann

“Child-like curiosity – seeking, questioning, and understanding – is the basis of knowledge that opens up new avenues that we call progress.”

– Klaus von Klitzing –

Abstract

Solar water splitting using a photoelectrochemical cell (PEC) is an attractive route for the generation of clean and renewable fuels. PEC cell converts solar energy into an intermediate chemical energy carrier such as molecular hydrogen and oxygen. The solar-driven oxygen evolution at photoanode is limited due to the lack of materials with suitable optical, electronic properties, and operational stability. In this thesis, combinatorial synthesis and high- throughput characterization methods were used to investigate the suitability of multinary vanadium-based material systems for PEC application. Multinary ternary (M-V-O) and quaternary (M1-M2-V-O) metal vanadium oxide photoanodes: Fe-V-O, Cu-V-O, Ag-V-O, W-V-O, Cr-V-O, Co-V-O, Cu-Fe-V-O, and Ti-Fe-V-O materials libraries were synthesized using combinatorial reactive magnetron sputtering with a continuous composition and thickness gradients. All as-deposited materials libraries were subsequently annealed in air at different temperatures. Several high-throughput characterization techniques such as energy- dispersive X-ray spectroscopy, X-ray diffraction, stylus profilometer, ultraviolet-visible spectroscopy, optical scanning droplet cell, and scanning electron microscopy were used to establish correlations between composition, structure, and functional properties. High- throughput characterization of the M-V-O (M is Fe, Cu, Ag) systems revealed to have composition-dependent crystal structures such as for Fe-V-O: from 10 to 80 at.% Fe,

Fe2V4O13 → FeVO4 → Fe2O3, for Cu-V-O: from 18 to 84 at.% Cu, CuV2O6 → -Cu2V2O7 →

-Cu2V2O7 → Cu11V6O26 and Cu3V2O8 → Cu5V2O10, and for Ag-V-O: from 22 to 77 at.%

Ag, AgV6O15 → Ag2V4O11 → AgVO3 → Ag4V2O7 influencing the PEC behavior of the systems. The Ag-V-O photoanode with porous nanowire like-structure exhibited an enhanced PEC activity when compared to the nanocrystalline structures. Two new M1-M2-V-O photoanodes: Cu-Fe-V-O and Ti-Fe-V-O were investigated based on the results obtained from the M-V-O systems screening. Seven mixed vanadate phase regions were identified in the

Cu-Fe-V-O materials system exhibiting the highest photocurrent density at (Cu45Fe21V34)Ox consisting of the dominant -Cu3Fe4V6O24 phase. For the Ti-Fe-V-O system, (Ti29Fe34V37)Ox composition consists of the major FeVO4 phase and minor Fe-titanate phases FeTi3O7 and

Fe2TiO5 in the photoactive region. Correlations between the structural and functional properties for both the quaternary systems are discussed. Besides structure-property screening of the vanadium-based systems, PEC stability tests were carried out on selected ternary M-V-O photoanodes (M is W, Cr, Co) in varying electrolytic environments such as alkaline, neutral, and acidic media.

Table of contents

Chapter 1: Introduction ……..……………………………………………...... 1

Chapter 2: State of art …………………………………………………………………. 4

2.1 Photoelectrochemical (PEC) water splitting …………………………………………. 4 2.1.1 The PEC cells ………………………………………………………………... 4 2.1.2 Semiconductor photoelectrode materials …………………………………….. 6 2.1.2.1 Bandgap and band edge positions ………………………………………... 6 2.1.2.2 Semiconductor - electrolyte interface …………………………………..... 9 2.1.2.3 Stability in aqueous electrolyte …………………………………………. 11 2.2 PEC cell efficiency …………………………………………………………………. 11 2.3 Literature survey on multinary vanadate photoanodes for PEC water splitting ……. 12 2.4 Combinatorial material science ……………………………………………………... 20 2.5 Thin Film synthesis …………………………………………………………………. 21 2.5.1 Magnetron sputtering ………………………………………………………... 21 2.5.2 Reactive magnetron sputtering ……………………………………………… 23 2.5.3 Thin film growth modes …………………………………………………….. 24

Chapter 3: Research Methodology ………………………………………………… 26

3.1 Instrumentation ……………………………………………………………………... 26 3.1.1 Combinatorial reactive magnetron sputtering ………………………………. 26

3.2. Materials characterization ……………………………………………………………… 29 3.2.1 Film thickness measurements ………………………………………………. 29 3.2.2 Energy-dispersive X-ray spectroscopy (EDX) ……………………………… 30 3.2.3 Scanning electron microscopy (SEM) ……………………………………… 31 3.2.4 Transmission electron microscopy (TEM) …………………………………. 32 3.2.5 X-ray diffraction (XRD) ……………………………………………………. 33 3.2.6 Ultraviolet-visible (UV-Vis) spectroscopy …………………………………. 35 3.2.7 Optical scanning droplet cell (OSDC) ……………………………………… 36

Chapter 4: Results and Discussion …………………………………………………. 40

4.1 Screening of ternary M-V-O photoanode systems ………………………………….. 40 4.1.1 Fe-V-O ……………………………………………………………………… 40

4.1.1.1 Synthesis of Fe-V-O materials libraries ………………………………... 40 4.1.1.2 Composition and thickness analysis ……………………………………. 41 4.1.1.3 Composition and thickness dependent structural analysis ……………… 41 4.1.1.4 PEC analysis ……………………………………………………………. 44 4.1.1.5 Correlations between composition, thickness, structure, and photocurrent density …………………………………………………………………... 46 4.1.1.6 Morphology of the Fe-V-O thin films …………………………………... 48 4.1.1.7 Composition, phase constituency, and color of Fe-V-O thin films …….. 49 4.1.1.8 Optical properties and analysis of hit Fe-V-O sample ………………….. 50 4.1.2 Cu-V-O ……………………………………………………………………… 53 4.1.2.1 Fabrication of Cu-V-O materials libraries ……………………………… 53 4.1.2.2 Elemental composition and thickness analysis of Cu-V-O materials libraries ………………………………………………………………….. 53 4.1.2.3 Composition-dependent crystal structure analysis ……………………… 56 4.1.2.4 PEC analysis ……………………………………………………………. 58 4.1.2.5 Surface morphology of Cu-V-O thin films ……………………………... 63 4.1.2.6 Analysis of optical properties …………………………………………... 64 4.1.3 Ag-V-O ……………………………………………………………………... 67 4.1.3.1 Synthesis of Ag-V-O materials libraries ………………………………... 67 4.1.3.2 Composition and thickness analysis ……………………………………. 68 4.1.3.3 PEC, structure, and morphology of Ag-V-O materials libraries ……….. 68 4.1.3.4 Analysis of optical properties …………………………………………... 76 4.1.3.5 Detailed analysis of highest photocurrent (Ag-V-O) sample …………… 77 4.2 Screening of quaternary M1-M2-V-O photoanode systems ………………………… 80 4.2.1 Cu-Fe-V-O ………………………………………………………………….. 80 4.2.1.1 Synthesis of Cu-Fe-V-O materials libraries …………………………….. 80 4.2.1.2 Compositional and PEC analysis ……………………………………….. 80 4.2.1.3 Phase identification of Cu-Fe-V-O materials libraries ………………….. 82 4.2.1.4 Incident photon-to-current efficiency (IPCE) …………………………... 86 4.2.1.5 Optical properties analysis of Cu-Fe-V-O materials libraries ………….. 87 4.2.1.6 Surface morphology of Cu-Fe-V-O materials libraries ………………… 88 4.2.2 Ti-Fe-V-O …………………………………………………………………... 91 4.2.2.1 Synthesis of Ti-Fe-V-O materials library ………………………………. 91 4.2.2.2 Compositional analysis …………………………………………………. 91

4.2.2.3 Composition-dependent PEC performance of Ti-Fe-V-O materials library……………………………………………………………………. 93 4.2.2.4 Crystal structural analysis of Ti-Fe-V-O materials library ……………... 93 4.2.2.5 Surface morphology of Ti-Fe-V-O materials library ………………….. 100 4.3 Screening and PEC stability test of M-V-O photoanode systems ………………… 102 4.3.1 Synthesis of W-V-O, Cr-V-O, and Co-V-O materials library …………….. 102 4.3.2 Photographs of M-V-O systems in different states ………………………... 103 4.3.3 Compositional analysis of M-V-O materials libraries …………………….. 105 4.3.4 Composition, PEC stability test (alkaline electrolyte), and crystal structural analysis …………………………………………………………………….. 106 4.3.5 PEC stability test in acidic electrolyte …………………………………….. 110 4.4 Summary …………………………………………………………………………... 113

Chapter 5: Conclusions ……………………………………………………………… 118

References ………………………………………………………………………………. 121

Appendix ………………………………………………………………………………... 135

Chapter 1 Introduction For the development of a long-term sustainable energy economy, harvesting of energy directly from sunlight serves as a desirable approach towards the need for clean fuel.[1] The continuous increase in the population growth globally leads to raise the emission of greenhouse gases like carbon dioxide (CO2) and methane (CH4) resulting in global warming which causes harmful effects on the ecosystem, biodiversity, and human health.[2] In 2018, global energy consumption increased by 2.9% as reported by BP’s 2019 “Statistical Review of World Energy”.[3] Renewable powers like solar and hydrogen rose by 14.5%. Solar alone provided more than 40% of renewables growth and the production grew by 30 megatonnes of oil equivalent (mtoe). Despite an increase in renewable power consumption, the demand for coal consumption raised by 1.4% leading to an increase in carbon emission by 2.0%.[3] Therefore, energy roadmaps must be developed in order to eliminate carbon emission in air and reduce climate change by developing fossil fuel replacement technologies that could provide clean energy and are cost-effective before it gets too late.[4]

In order to have carbon-free fuels, hydrogen is considered as an excellent energy carrier that can be produced via electrolysis of water and/or steam reforming.[5] However, in the steam reforming process, hydrogen is produced on a large industrial scale mainly from natural gas, oil, and coal which releases carbon in the atmosphere.[5] Thus, steam reforming cannot be considered renewable. The production of hydrogen via electrolysis allows the splitting of water molecules into molecular hydrogen and oxygen by applying electricity which costs approx. 80% of hydrogen operational production, considering this method to be an expensive path.[5] Another approach for the generation of power (hydrogen) is through photovoltaic cell. However, in the photovoltaic cell, the challenge is to develop a suitable chemical storage method.

Therefore, a potential approach towards the generation of renewable hydrogen involves combination of sunlight absorption in an electrolyzer and a photovoltaic cell into a single device. This approach is known as photoelectrochemical (PEC) water splitting. A PEC cell converts solar energy into hydrogen from the most abundant natural resources ‘water and sunlight’.[6–9] The future energy triangle is shown in Fig. 1 which appears to be an attractive route towards carbon-free clean fuel.[6]

Figure 1. Possible future energy triangle. Adapted from Ref.[6]

A possible set-up of a PEC water splitting is schematically shown in Fig. 2, where solar energy is directly converted into chemical energy to form molecular hydrogen and oxygen without the production of any by-products using a photoabsorber.[6,10] In order to perform PEC water splitting efficiently, the photoabsorber should have bandgap energy ranging from 1.2 to 2.6 eV to absorb a sufficient portion of the solar spectrum, exhibit long-term stability, high efficiency, cost-efficient.[11] To-date, a maximum of 12.4% solar-to-hydrogen efficiency was achieved using a multijunction III-V PEC semiconductor but lacks long-term stability.[12] Therefore, the key challenge is to find a suitable photoabsorber material that fulfills the criteria aforementioned.

Figure 2. Schematic of a PEC cell where water is reduced to H2 at photocathode and oxidized to O2 at photoanode. Adapted from Ref.[13,14]

Combinatorial material science (CMS) is an efficient approach to search for new suitable PEC materials for solar water splitting.[15,16] In this thesis, a contribution to this field of research for the development of ternary and quaternary photoanode systems focusing on multinary metal vanadium oxides (M-V-O) is presented. Eight multinary vanadate photoanode materials systems: Fe-V-O, Cu-V-O, Ag-V-O, W-V-O, Cr-V-O, Co-V-O, Cu-Fe-V-O, and Ti-Fe-V-O were screened using CMS approach and were investigated via high-throughput characterization techniques in order to establish correlations between composition, structure, and functional properties.

Outline of the thesis

This thesis consists of an investigation of ternary (M-V-O) and quaternary (M1-M2-V-O) material systems, and PEC stability screening of ternary M-V-O photoanodes. The research work is presented in two sections: firstly, the thin film photoanodes were synthesized using combinatorial reactive magnetron sputtering and screened via high-throughput characterization methods. Secondly, the photoelectrochemically active regions, “hit samples”, within each materials system were studied in detail by identifying the influencing factors such as composition, phase constituency, film thickness, and microstructure exhibiting the enhanced PEC performance. Chapter 2 introduces PEC water splitting, working principles of PEC cells, and lists the essential requirements for the selections of suitable photoelectrodes for efficient PEC performance. The literature survey for selected multinary vanadate photoanodes is briefly discussed. This chapter also describes the CMS approach and thin film synthesis and growth modes. In chapter 3, research methodologies are discussed. This chapter includes working principles and functionalities of the thin film deposition methods for the materials library synthesis and materials characterization techniques. Chapter 4 comprises the results and discussion part of the thesis. This chapter has 4 subsections - the investigation of the ternary M-V-O photoanode systems, quaternary M1-M2-V-O photoanode systems, PEC stability tests of ternary M-V-O photoanode systems, and the last subsection summarizes this chapter. The concluding remarks of the research work are presented in Chapter 5.

Chapter 2 State of Art

2.1 PEC water splitting

A green and sustainable approach helps to deal with the ever-rising global energy demand. The need for clean carbon-free fuel can be fulfilled by harvesting energy directly from the most abundant renewable resource, ‘sunlight’. The harvested solar energy needs to be converted into usable chemical fuel which can be stored and used as per the requirement. The PEC cell is one of the promising technologies which convert solar energy directly into an intermediate chemical energy carrier in the form of molecular hydrogen and oxygen.[10] The advantage of using a PEC cell for the splitting of water is that it combines the harvesting of solar energy and electrolysis of water to a single device which is clean, efficient and cost- effective.[6] In 1972, Fujishima and Honda demonstrated the concept of PEC water splitting [8] by using TiO2 as the photoabsorber as well as photocatalyst. The concepts introduced in this chapter loosely follow the PEC water splitting textbooks by Chen, Dinh, and Miller,[11] Krol and Grätzel[6] and an electrochemistry textbook by Bard.[17]

2.1.1 The PEC cell

The main key components of a PEC cell are a light-absorbing semiconductor photoelectrode (working electrode), an aqueous electrolyte and, a metal counter-electrode. The mechanism of a PEC water splitting is schematically depicted in Fig. 3. The basic concept is that a semiconductor working electrode is immersed in an aqueous electrolyte and is illuminated with sunlight. “The incident photon (hv) generates electron (e-) - hole (h+) pairs with an efficiency labeled 휂푒−/ℎ+ that are separated in the opposite direction due to the presence of an electric field inside the semiconductor.”[11] The efficiency of the charge transport is labeled as

ηtransport. The photogenerated electrons are transported to the metal counter-electrode through an ohmic contact and an external electrical connection reduces water to form hydrogen gas, resulting in hydrogen evolution reaction (HER). The photogenerated holes oxidize water at the semiconductor/electrolyte interface which drives oxygen evolution reaction (OER) forming oxygen gas. “The charge transfer efficiency at the solid-liquid interface (for both electrons and holes) is labeled ηinterface. The thermodynamic potential (ΔE°) to perform solar water splitting is 1.23 V”.[11] The overall PEC water splitting takes place in two half-cell

reactions: (I) OER from n-type semiconductor (photoanode) and (II) HER from p-type semiconductor (photocathode).

In acidic electrolyte, pH < 7, the half-cell reactions, that is water reduction and oxidation reactions are:[6]

+ - Cathode(HER) : 4H + 4e ⇆ 2H2 ………………………………...……(1)

+ + Anode(OER): 2H2O + 4h ⇆ 4H + O2…………………………...……(2)

In alkaline electrolyte, pH > 7, the half-cell reactions for water are:[6]

- - Cathode(HER): 4H2O + 4e ⇆ 2H2 + 4OH ……………………………(3)

- + Anode(OER): 4OH + 4h ⇆ 2H2O + O2…………………...…..………(4)

Therefore, the overall reaction for both the half-cell reaction results in:

- 4e + 2H2O ⇆ 2H2 + O2 ……………………...………………………(5)

Figure 3. Schematic illustration of a PEC cell operation with key components, adapted from Ref.[11]

2.1.2 Semiconductor photoelectrode materials

Identifying potential photoelectrode materials for the generation of chemical fuels from sunlight has remained a major challenge. PEC materials should fulfill key requirements such as optimized bandgap, band edge positions, charge transport, long-term stability in aqueous electrolyte, low-cost fabrication, and abundance of the constituent materials in order to perform the water splitting reaction.

2.1.2.1 Bandgap and band edge positions

The electronic structure of the semiconductor influences the PEC performance of the system. Fig. 4 adapted from Ref.[18], explains the electron band model of a semiconductor. In a semiconductor, the valence band is the highest energy band consisting of occupied atomic orbitals whereas, the lowest energy band with unoccupied atomic orbital is the conduction band. The highest energy level is known as valence band edge, EV and lowest energy level is called conduction band edge, EC. The energy difference between the top of the valence band, [18] EV and bottom of the conduction band, EC is called bandgap energy (Eg). The Fermi level

(EF) in an intrinsic semiconductor is equidistant from both the EV and the EC representing the probability of an energy level to be occupied by an electron in the semiconductor is 50%.[18] In this case, the semiconductor is capable of carrying charges. The electrons are excited from the valence band to the conduction band leaving behind mobile positively charged vacancies in the valence band which is referred as holes, h+. Therefore, with the migration of the electrons and holes, a current is generated in the semiconductor. [18]

On addition of the elements in the structure of the semiconductor, the EF position of the semiconductor is changed that is if EF moves near the EC, then n-type semiconductor (electron donor) is formed and if EF moves closer to the EV, then p-type semiconductor (electron acceptor) is formed.[17,18]

Figure 4. Schematic representation of the electronic band structure of a semiconductor a) where the

Fermi level (EF) lies in the middle of the bandgap energy (Eg) with respect to the vacuum as the zero- energy reference. A – electron affinity, ɸ - work function (electronegativity), and I – ionization energy,

EC – conduction band edge and EV – valance band edge. On addition of elements, b) the position of EF shifts towards EC, forming n-type semiconductor, whereas c) for p-type semiconductor, the EF position [18] lies closer to the EV.

Thermodynamically splitting of water (H2O), photoelectrolysis into H2 and O2 under standard Gibbs free energy, 훥퐺° is expressed as:[6]

푘퐽 훥퐺° = −푛퐹훥퐸° = 237.2 ……………………………..(6) 푚표푙 퐻2

Where, F is Faraday’s constant, F = 96485 C/mol, n is the number of moles of transferred electrons, n = 2, and ΔE° is the standard potential of the electrochemical cell, ΔE° = 1.229 V.

According to the Nernst equation, 훥퐺° corresponds to 1.229 V (훥퐸°) per electron transfer. Therefore, to derive this reaction, the semiconductor must absorb a significant portion of solar spectrum with photon energy greater than the Eg of the semiconductor (1.23 eV). Thus, converting the photon energy into H2 and O2. “During this conversion process, either two electron-hole pairs per H2 molecule (2 x 1.23 eV = 2.46 eV) or four electron-hole pairs per O2 molecule (4 x 1.23 eV = 4.92 eV) are generated.”[10] Fig. 5 illustrates a semiconductor under illumination with Eg large enough to absorb sufficient solar spectrum, and drive the HER and

+ OER at electrochemical potentials E°(H /H2) or E°(O2/H2O) using photogenerated electrons or holes.[10]

Figure 5. Energy band edge position of ideal semiconductor material for PEC water splitting under illumination with EV as the valence band edge, EC as the conduction band edge and E° is the + [10,19] electrochemical potential for (H /H2) and (O2/H2O) redox couples.

In order to carry out OER or HER without charge carrier (electrons and holes) recombination that is electrons and holes are separated in opposite directions. These charge carrier needs to reach the semiconductor-liquid electrolyte interface where they react only with the solution species. At the semiconductor-electrolyte interface, the electron transfer takes place which leads to loss and hence, a kinetic overpotential is required to perform the OER and HER.[10,20]

Therefore, to effectively perform the PEC water splitting, Eg should range between 1.2 to 2.6 eV.[10,20–22]

Fig. 6 shows the energy band edge positions of various PEC water splitting semiconductor materials. Among these materials, TiO2 is most extensively studied due to its ability to drive the overall water splitting as the band edge positions straddle the redox potential of water.

However, due to a large Eg of TiO2, ⁓ 3.2 eV, only a small portion of the solar spectrum (UV light) can be absorbed resulting in poor solar efficiency.[20] For an ideal single electrode material, the positions of the EV should be more positive than the water oxidation potential and the EC position should be more negative than the water reduction potential. Moreover, apart from fulfilling the characteristic requirement for an ideal single electrode material, the 8

semiconductor should be stable against corrosion when immersed in an aqueous electrolyte. For example, CdS seems to fulfill the band edge positions criterion in order to drive the water splitting reaction but is unstable in aqueous electrolytes.[20,23]

Figure 6. The bandgap and band edge positions of ideal and existing PEC semiconductors..[20]

2.1.2.2 Semiconductor - electrolyte interface

According to the Nernst equation, the electrochemical potential of electrons in redox electrolyte is:[24]

푅푇 퐶표푥 Eredox = E°redox + ln [ ] ………………………………....(7) 푛퐹 퐶푟푒푑

Where Cox and Cred are the concentration of the oxidized and reduced species in the redox couple. The redox potential (Eredox) can be determined with the Fermi level (EF.redox) in the redox electrolyte. In semiconductors, the electron energy in a vacuum is considered as the reference and in electrochemistry, the standard hydrogen electrode (SHE) represents the reference which is found to be - 4.5 eV to the vacuum level. Therefore, the expression for the [24] relation between Eredox and EF.redox versus vacuum reference is:

EF.redox = - 4.5 eV - Eredox ……………………………………(8)

To attain a thermodynamic equilibrium, the flow of electrons takes place between the semiconductor and an aqueous electrolyte. After equilibrium in the dark, the electrochemical 9

potential (Fermi level) equilibrates in the semiconductor with the electrolyte’s redox potential resulting in band bending.[10] Fig. 7 shows the schematic of band energy for n-type semiconductors after thermodynamic equilibrium in dark and under illumination with the electrolyte redox potential. Here, all descriptions for n-type semiconductors are analogous to the p-type semiconductors. For n-type semiconductors (O2/H2O redox species), excess positive charge arises in the semiconductor due to the ionized dopants and the negative charge in the electrolyte. Therefore, band bending in n-type semiconductors directs the photogenerated free minority charge carrier (holes) to flow into the electrolyte. The length of band bending can be described as the depletion region or space charge layer, and is occupied by positive charge carriers. Negative charge carriers spread over a narrow region known as the Helmholtz layer.[10] Upon illumination, electron-hole pairs are generated, resulting in an increase in the Fermi level. The maximum Fermi level is called the flat band potential. This shift in the Fermi level is known as the photovoltage.[6] Therefore, free energy generated by the semiconductor is the difference between the quasi-Fermi level hole (EF,p) and quasi-Fermi level electron (EF,n), under illumination, in other words, the energy difference between the majority carriers and the photoexcited minority carriers.[10,25] The potential at which no excess charge exists as well as no space charge layer/depletion region was created is defined as a potential of zero current. Photovoltage is determined in open circuit potential (OCP) against the reference electrode. Under illumination, the OCP shifts to negative potentials for n-type semiconductors whereas OCP shifts to positive potentials for p-type semiconductors.[10]

Figure 7. The band energies of n-type semiconductors immersed in an electrolyte and connected to a metal counter electrode in equilibrium in a) dark and b) under illumination. Illumination raises the Fermi level and decreases the band bending. The photovoltage is the difference between the quasi- [6,25] Fermi level of electrons (EF,n) and the quasi-Fermi level of holes (EF,p), adapted from Ref. 10

2.1.2.3 Stability in aqueous electrolyte

To drive the overall water splitting reaction, the semiconductor photoelectrodes must exhibit long-term stability in aqueous electrolyte against photocorrosion. During photocorrosion, a thin insulating oxide layer is formed on the semiconductors limiting the charge transfer across the semiconductor-electrolyte interface.[6] In general, oxide semiconductors are more stable but are likely to have anodic or cathodic decompositions. Thermodynamic stability of the semiconductor predicts that the oxidation decomposition potential of the semiconductor should be more positive than the valence band edge positions for water oxidation. For water reduction, the reduction decomposition potentials should be more negative than the [23] conduction band edge position of the semiconductors. CdS has a suitable Eg (2.4 eV) and band edge positions to perform solar water splitting reaction (see Fig. 6) but is unstable. RuO2 is stable in an acidic electrolyte and is known to be the most active material for water oxidation. However, at high overpotentials, RuO2 is unstable and dissolves as ruthenate in alkaline electrolyte. For alkaline electrolytes, spinel- and perovskite-like structures are [26] recommended. Moreover, the addition of inert metal oxides such as TiO2, SnO2, etc. can [10] stabilize RuO2 and reduce anodic photocorrosion. “It is the quasi-Femi level that should straddle the water redox potentials. Although the interpretation of the quasi-Fermi level as a thermodynamic driving force may not be entirely appropriate under all conditions.”[6]

2.2 PEC cell efficiency

The solar to hydrogen (STH) conversion efficiency is a major factor to determine the overall splitting of water in a PEC cell at an intensity of Air Mass 1.5 Global (AM 1.5 G). It is expected that the photoelectrode fulfills all the necessary criteria discussed above and all photogenerated electrons and holes are used to drive the water splitting reaction efficiently. STH efficiency is measured under zero bias conditions such that there is no applied voltage between the working and the counter electrodes, and (sun) light being the only source for energy. For STH efficiency measurements, both the working electrode and the counter electrodes should be immersed in the same pH aqueous electrolyte without any sacrificial [11] donor or acceptor reagent in the electrolyte. For direct conversion, STH efficiency (ηSTH) is defined as (equation, e.q. 9) the free energy stored in the hydrogen. The chemical energy produced in terms of hydrogen (mmol H2/s) is multiplied by the Gibbs free energy per mole [11] of H2 (ΔG° = 237 kJ/mol at 25°C) and divided by incident power density (Ptotal).

퐻 (푚푚표푙⁄푠) 푋 ΔG°(푘퐽⁄푚표푙) 휂 = [ 2 ] ………………………………(9) 푆푇퐻 ( 2) 2 푃푡표푡푎푙 푚푊⁄푐푚 푋 퐴푟푒푎 (푐푚 ) 퐴푀 1.5 퐺 11

Alternatively, e.q. 10 is used to calculate the STH efficiency as a product of current and voltage: [6]

푗푝ℎ표푡표(푉푟푒푑표푥 − 푉푏𝑖푎푠) 휂푆푇퐻 = [ ] …………………………....…(10) 푃푡표푡푎푙 퐴푀 1.5 퐺

Where, ηSTH is solar to hydrogen conversion efficiency, jphoto is the photocurrent 2 density (mA/cm ), Vredox is the redox potential for water splitting (thermodynamically 1.23 V at room temperature), Vbias is the external bias, and Ptotal is the incident power density (mW/cm2).

Another important parameter to diagnose the efficiency of a PEC cell is the external quantum efficiency (EQE) or incident-photon-to-current efficiency (IPCE). It represents the photocurrent density (jphoto) produced per incident photon flux (ɸ) as a function of the wavelength (λ) of the incident light:[17]

푗(휆) 퐼푃퐶퐸 (휆) = 퐸푄퐸 (휆) = ……………………...... …(11) 푒ɸ(휆)

2.3 Literature survey on multinary vanadate photoanodes for PEC water splitting

Multinary transition metal oxides can be explored with the aim to find new material(s) for [8] [27] [27–29] PEC water splitting. Binary metal oxides (M-O) such as TiO2 , WO3, and Fe2O3 have been extensively investigated. These M-O semiconductors satisfy the major requirements such as high abundance in nature, non-hazardous, and stable in aqueous media. However, their PEC performance is limited by poor electronic conductivity resulting in the recombination of photogenerated charge carriers (electron-hole pairs). Another limitation for [30–32] suitable PEC material is it’s wide Eg. To improve PEC performance, the search is focused on multinary metal oxides, ternary or quaternary semiconductors. Ternary semiconductors can be a) combination of two binary M-O i.e. M1-O + M2-O→ M1-M2-O or b) doping or mixing of single metal into binary M-O such as M1-M2-O semiconductors, for [33–35] [36–38] [39] [40,41] [42] example, BiVO4, FeVO4, Fe-W-O, Ti-W-O, and V-doped TiO2. Ternary metal oxides offer the opportunity to tune the compositions as well as the electronic structure of the semiconductors which enables tailoring PEC properties. Among various ternary metal oxide systems, transition metal orthovanadate (M-VO4) is considered as a promising class of photoanode materials for PEC.[22] Yan et al. reported a list of potential ternary metal vanadate photoanode materials with VO4-based compounds such as FeVO4, α-Cu2V2O7, α-Ag3VO4, 12

[22] α-CoV2O6, etc. exhibiting Eg within 1.2 to 2.8 eV energy range shown in Table 1. They are being explored as a catalyst for fuel cells, electrodes for batteries, gas sensors, solar cells, photoluminescence, microwave applications, and humidity sensors. Therefore, the incorporation of transition metals into a vanadate based photoanode is considered a promising search space for PEC.

Table 1 – List of promising ternary vanadate phases with the corresponding direct and indirect Eg energies, and photocurrent density at OER potential, adapted from Ref.[22]

Phases Direct Indirect Photocurrent bandgap bandgap density at OER (eV) (eV) potential (mA/cm2)

Cr2V4O13 2.52 2.30 0.139

orth-CrVO4 2.59 2.38 0.20

mon-CrVO4 2.27 <2.27 0.036

tri-FeVO4 2.51 <2.3 1.3

α-CoV2O6 2.25 <2.25 0.015

Co3V2O8 2.34 2.22 0.006

Ni2V2O7 2.73 <2.5 0.003

Ni3V2O8 2.66 <2.5 0.003

α-Cu2V2O7 2.43 2.06 1.6

β-Cu2V2O7 2.42 2.03 2.0

γ-Cu3V2O8 2.40 1.80 1.8

Cu11V6O26 2.49 1.87 0.95

α-Ag3VO4 2.38 2.14 0.062

β-Ag3VO4 2.51 <2.51 0.41

mon-BiVO4 2.46 2.35 0.396

Fe-V-O photoanodes

Iron-vanadate (Fe-V-O) photoanodes consist of the transition metals Fe and V which are widespread in nature. Fe-V-O is considered to be promising for energy-related technological applications. Fig. 8 shows the phase diagram of the V2O5-Fe2O3 system and 400-950°C temperature range adapted from the ASM alloy phase diagram database.[43] Iron (III) vanadate, FeVO4 has a triclinic crystal structure in equilibrium state and is stable under 3+ ambient conditions when compared to other iron vanadates. “In FeVO4, Fe ions have three crystallographic sites such that two sites are distorted octahedral FeO6 and one site is distorted

[44] trigonal bipyramidal FeO5.” The structure consists of double-bent chains of six edges surrounding six Fe atoms formed by the Fe-O polyhedra and the chains are interconnected by [44] tetrahedra VO4 to form three-dimensional frameworks.

Figure 8. Partial phase diagram of the iron vanadate system (Fe-V-O).[43]

Arai et al. discovered FeVO4 (n-type semiconductor) as a potential candidate for PEC water [45] splitting using high-throughput screening methods. The Eg of several iron vanadate phases is reported to be in the range between 2.0 eV and 2.6 eV.[22,36,37,46,47] Fan et al. reported a hierarchical hollow FeV composite material, exhibiting high electrocatalytic activity in dark, 2 [48] reaching 10 mA/cm at 390 mV overpotential. Yan et al. found FeVO4 to exhibit a photocurrent density of 1.3 mA/cm2 at OER potentials.[22] Iron vanadate photoanodes, such as [38,49] [46,47] [47] [50] [51] FeVO4, Fe2VO4, FeV2O4, Fe2V4O13, and FeV3O8, have been explored using various synthesis routes, such as sonochemical method,[38] precipitation,[52] solid-state,[46] and [50] ion layer deposition. FeV3O8 nanoplatelets synthesized by a pH-controlled hydrothermal method were reported to exhibit good stability and high photocatalytic activity in visible light [51] illumination. Mandal et al. reported the semiconductor FeV2O4 to generate a maximum photocurrent density of 0.18 mA/cm2 with 22% IPCE value.[47] 14

Cu-V-O photoanodes

Copper vanadate (Cu-V-O) materials have gained attention in early 1958 due to their interesting structural aspects such as crystal chemistry.[53] This system is considered as a potential candidate for applications such as thermal batteries,[54] flame retardants,[55] lithium- intercalating battery cathodes/anodes,[54,56] and catalysts for solar thermochemical water splitting.[57,58] Clark and Garlick in 1977 studied the polymorphic transformation of copper divanadate system in a temperature range from 500 to 730°C. Three polymorphs α, β, and γ of

Cu2V2O7 were reported at different temperatures. For temperatures < 550°C, the formation of

β-Cu2V2O7 takes place and at temperature range form 560 to 680°C, α-Cu2V2O7 forms. For temperatures > 680°C, a new polymorph is γ-Cu2V2O7 which transforms to β-Cu2V2O7 on [59] cooling to 600°C. At 605°C, β-Cu2V2O7 is reported to transform to α-Cu2V2O7. Similarly,

Rao and Palanna reported the polymorph Cu3V2O8 in a temperature range between 460 and 560°C.[60] High-throughput methods revealed a variety of polymorphs of Cu vanadate phases [61,62] such as α-CuV2O6, α-Cu2V2O7, β-Cu2V2O7, γ-Cu3V2O8. The phase diagram of the

CuV2O5-V2O5 system in a temperature range from 620 to 760°C adapted from the ASM alloy phase diagram database is shown in Fig. 9.[63]

[63] Figure 9. Phase diagram of the CuV2O5-V2O5 system. 15

Cu-V-O systems have gained attention for PEC application due to their Eg of ⁓ 2 eV and [61,62,64–67] stability against photocorrosion. Newhouse et al. synthesized Cu1-xVxOz composition space using inkjet printing and reported four Cu vanadate phases: α-CuV2O6, α-Cu2V2O7,

β-Cu2V2O7, and Cu5V2O10. The phases α-CuV2O6 and α-Cu2V2O7 are photoelectrocatalytically active for the OER in borate-buffer (pH 9.2) and are highly stable as 2 the dark currents were consistently near 0 mA/cm . However, the β-Cu2V2O7 exhibits the highest PEC activity in the presence of a ferri/ferrocyanide redox species with enhanced stability in pH 13 electrolyte.[62] Similarly, Zhou et al. established the structure-property relations of CuO-V2O5 composition space for OER photoelectrocatalysts using high- throughput methods (Cu1-xVxOz). Among the four Cu-V-O photoelectrocatalysts: Cu11V6O26

(Fingerite), γ-Cu3V2O8 (McBirneyite), β-Cu2V2O7 (Ziesite), α-Cu2V2O7 (Blossite), Fingerite was stable in strong alkaline electrolyte (pH 13).[61] Correspondingly, Lumely et al. enhanced the PEC performance of Fingerite by the addition of Mo and W.[68] However, McBirneyite exhibited a maximum photocurrent of 1.5 mA/cm2 and showed stability against [61,69] photocorrosion in pH 9. The PEC behavior of the McBirneyite (γ-C3V2O8) was improved by doping of 0.75 at.% Cr which resulted in five-fold enhancement of the charge separation efficiency at 1.23 V vs. reversible hydrogen electrode (RHE).[70] PEC behavior of the Cu-V-O system dependence on the presence copper vanadate phases. PEC stability of Cu-V-O was investigated thermodynamically using the Pourbaix diagram of Cu-V-H2O systems based on the Materials Project database (Fig. 10).[64,71]

Figure 10. Pourbaix diagram calculated for Cu-V-H2O systems. The water redox potentials are labeled by green dashed lines and stable regions are colored according to the label towards the right of the diagram.[64] 16

Ag-V-O photoanodes

Silver vanadate (Ag-V-O) systems have potential applications in rechargeable lithium batteries,[72] sensors,[73] photocatalysts,[74,75] and medical implant devices such as cardioverter defibrillators, and drug infusion devices.[76] Silver vanadates consist of multiple phases depending on reaction conditions and material stoichiometry. The phase diagram for the

Ag2O-V2O5-VO2 system through the reaction of Ag with V2O5 and AgVO3 with V2O5 was [77] reported. Four silver vanadate phases were identified from the reaction of AgVO3 with

V2O5 in air. The reaction temperatures ranged from 350 to 640°C for V:Ag ratio 1:19. The

β-phase of AgVO3 was phase I and the Ag2V4O11 was phase II formed at 500°C. With an increase in oxygen pressure, phase III, Ag1+xV3O8 coexists with phase IV (β-phase of

Ag0.33V2O5). For a V:Ag ratio > 2.5, phase IV was recorded and its pure form was formed for [77] V:Ag = 6. Another equilibrium diagram of V2O5-AgVO3 system was discussed in the [77] literature survey by Takeuchi et al. The reactions of V2O5 with AgVO3 takes place at annealing temperatures ranging from 400 to 450°C for 200 h in air. Three silver vanadate phases were identified from chemical and X-ray diffraction analysis: β-AgxV12O30 for 1.7 < x

< 2.0, γ-Ag1.12V3O7.8, and ε-Ag2V4O10.85. The equilibrium diagram of the V2O5-AgVO3 system in the temperature range from 400 to 750°C, adapted from the ASM alloy phase diagram database is shown in Fig. 11.[78]

[78] Figure 11. Equilibrium diagram of V2O5-AgVO3 system. 17

Ag-V-O has been explored as a water oxidation photocatalyst. Silver vanadate phases such as [79] [80–82] [80,83,84] [83] [85,86] α-AgVO3, β-AgVO3, α-Ag3VO4, β-Ag3VO4, and Ag2V4O11 have shown propitious PEC properties. In the Ag-V-O system, the Eg range between 2.0 eV and [22,84,85] 2.5 eV, lying within the solar spectrum. Gao et al. fabricated Ag3VO4/β-AgVO3 nanocomposites with a molar ratio of 30% of Ag3VO4 (nanoparticles) to β-AgVO3 nanowires using chemical precipitation method and reported 2.4 times enhanced photocatalytic activity for RhB degradation when compared to that of pure Ag3VO4 and 9 times higher than pure [80] β-AgVO3 nanowires. Chemelewski et al. reported α-Ag3VO4 thin films with a Eg of 2.2 eV which were synthesized using successive ionic layer adsorption and reaction technique and performed PEC measurements, but reported unstable and decreasing photocurrent density.[84]

Ag2V4O11 nanotubes as well as nanowire structures were fabricated by a hydrothermal process and reported to have an indirect Eg of ⁓ 2.0 eV. Both the Ag2V4O11 nanotubes and nanowires revealed to have high photocatalytic activity due to low Eg, nanostructured morphology and highly mobile charge carrier.[85,86]

W-V-O, Cr-V-O, and Co-V-O photoanodes

The transition metal tungsten (W) is a hard metal and can be used as tungsten steels for automobiles and high-speed cutting tools. Moreover, W is also used in lamps, fluorescent light tubes, as well as flame retardants in textile industries. Very few works have been reported on tungsten vanadate (W-V-O) systems for PEC water splitting. Karuppasamy et al. fabricated a V-WO3 thin film photoanode by electron beam evaporation as well as DC magnetron sputtering at room temperature for investigation of electrochromic and photocatalytic properties: (I) Thin films prepared by electron beam evaporation revealed an increase in photocatalytic activity with V doping and the Eg reduced to 3.15 eV from 3.28 eV. (II) For the films synthesized by DC magnetron sputtering with varying V concentration from 9 to 11%, reported to have enhanced photocatalytic response for maximum V concentration [87,88] (11%), high work function (4.83 eV) and reduced Eg (3.01 eV). Similar to the W-V-O system, chromium vanadate (Cr-V-O) is least investigated for PEC application. Yan et al. fabricated Cr-V-O photoanodes using RF magnetron sputtering with subsequent annealing in air and reported chromium vanadate phases such as Cr2V4O13, orth-CrVO4, and mon-CrVO4 as potential photoanodes for water with Eg between 2.27 and [22] [89] 2.38 eV. Kalal et al. investigated Cr2V4O13 synthesized via a wet chemical method. Cobalt vanadate (Co-V-O) materials have a crystalline structure in a Kagomé staircase [90] geometry. Co3V2O8 materials act as anode and positive electrode in Li batteries and

supercapacitors respectively.[91,92] Co-V-O materials are found to show interesting OER electrocatalytic activity: Xing et al. synthesized Co3V2O8 nanoparticles using a chemical coprecipitation method and resulted in electrocatalytic current up to 429.7 mA/cm2 at 2.05 V [93] vs. RHE and promising OER stability. Similarly, Hyun et al. fabricated Co3V2O8 one- and zero-dimensional (1-D, 0-D) nanostructures using electrospinning. Here, morphology influences the electrocatalytic OER behavior. 1D-Co3V2O8 showed enhanced electrocatalytic activity and long-term stability in 0.1 M KOH at 0.35 V vs. RHE.[94]

Cu-Fe-V-O photoanodes

From the literature survey, Fe vanadates and Cu vanadates showed sufficient proof of being PEC active, so the mixture of these two vanadate systems, Cu-Fe-V-O, could be a reasonable choice in the search of potential PEC materials. In the CuO-V2O5-Fe2O3 system, Cu3Fe4V6O24 is formed with two polymorphs α and β; α-Cu3Fe4V6O24 is known with its mineral name [95–97] Lyonsite and β-Cu3Fe4V6O24 has Howardevansite like structure. Blonska-Tabero et al. reported the phase diagram of the subsolidus areas of the system CuO-V2O5-Fe2O3 presented in Fig. 12. The H- and L-type in the diagram represent the Lynosite- and Howardevansite- type phase.[98]

[98] Figure 12. Phase diagram of the CuO-V2O5-Fe2O3 system in subsolidus area. Wieczorek-Ciurowa et al. studied physicochemical and catalytic properties of the Cu-Fe-V-O system using a mechanochemical synthesis route and reported both the α- and β-polymorphs of Cu3Fe4V6O24 to be unstable during steam reforming (450°C) of methanol and form binary metal oxides.[99] 19

2.4 Combinatorial material science

Combinatorial materials science (CMS) aims to discover new materials by synthesizing and characterizing (“screening”) a large number of samples (typically hundreds to thousand samples) in materials libraries (MLs).[100–102] Since the 1990s, CMS has gained worldwide attention when Xiang and co-authors reported combinatorial methods for the investigation of high-temperature superconductors.[100] Reddington et al. in 1998 used the combinatorial approach in electrochemistry and developed optical screening methods.[103] Since then, numerous efforts have been made for the development of automated high-throughput screening technology which allows rapid synthesis, processing, interpretation of large datasets, and reducing cost as well as time consumption for the characterizations of new materials.[20,100,103–109] CMS screening enables to efficiently measure and obtain structural- functional properties such as composition, crystal structure, phase constitution, and photocurrent densities using high-throughput characterization techniques to identify promising materials systems.[15,110–112] A large number of datasets obtained from high- throughput characterization methods can be explored with the help of data science and machine learning tools.[113,114]

The chair of ‘Materials Discovery and Interfaces (MDI)’ lead by Prof. Dr.-Ing. Alfred Ludwig at Ruhr-Universität Bochum focuses on the discovery of new materials using CMS approach for different applications such as energy conversion,[36,39,40,115,116] shape-memory alloys,[117,118] and magnetic materials.[119] Fig. 13 represents a visualization of the CMS cycle followed in the chair of MDI, starting with the synthesis of thin-film MLs within a composition space identified from data mining such as from the database of the chair of MDI, literature survey or predictions from theoretical/computational studies such as density functional theory, etc. The obtained as-deposited, as well as annealed MLs, are examined using high-throughput characterization methods by measuring the measurement areas (MAs) within the ML. High- throughput characterizations bring a large number of multidimensional datasets that are explored by data visualization and analysis enabling to obtain correlations between compositional, structural, and functional properties. These correlations help in identification of interesting regions on the MLs also referred as “hit regions” which exhibit promising functional properties such as high photocurrents, low Eg values, etc. After identifying a hit region, a sample with interesting hit region is synthesized and characterized for in-depth analysis. The MLs are fabricated using physical vapor deposition (PVD) typically magnetron

sputtering due to controlled deposition rate in order to have continuous composition spread on a single substrate.[120]

Figure 13. A visualization of the combinatorial materials science cycle which includes combinatorial synthesis, high-throughput characterization methodology, and data visualization & analysis at the chair of Materials Discovery and Interfaces (MDI), Ruhr-Universität Bochum (Prof. Dr.-Ing Alfred Ludwig).

2.5 Thin film synthesis

Thin film depositions are performed by preparing a thin layer onto a surface also known as a substrate. The thickness of the thin film deposition is typically in the range of a few nanometers to micrometers.

2.5.1. Magnetron sputtering

Sputter deposition is a PVD technique widely used for the deposition of thin films. In sputtering, a metallic or alloy target material is bombarded with energetic ions of inert gas, typically Ar. The collision of energetic ions with the target surface ejects the target atoms also known as “sputtered atoms” into the space due to energy transfer of momentum.[121,122] These sputtered metal atoms are then condensed and get deposited on the substrate material forming a thin film. This process is performed in a vacuum chamber and is schematically presented in 21

Fig. 14 consisting of magnetron sources, a substrate table (with heater), gas inlets, water cooling source, and vacuum pumps. For sputtering, inert gases with high atomic mass are preferred in order to achieve high sputter yield. “Sputter yield is defined as the ratio between the number of ejected sputtered atoms and the number of incident projectiles.”[122] Ar gas is typically used as a process gas for sputtering as it is chemically inert.[122] As shown in Fig. 14, in the sputter chamber, the cathode and an anode are positioned opposite to each other. Using the vacuum pumps, the chamber is pumped to a base pressure of the order to 1 x 10-4 Pa or lower. Low pressures are required during sputtering process in order to maintain high ion energies and to prevent too many atom-gas collisions after ejection from the target.[123] Ar gas is supplied into the chamber through a gas inlet valve controlled by the mass flow controllers, reaching a pressure of approx. 4 Pa. Within this pressure range, the plasma is ignited (glow discharge) when a high voltage (using direct current - DC or radiofrequency - RF power) is applied between the cathode and an anode (deposition pressure is lower than the ignition pressure ranging between 0.13 to 0.66 Pa). This results in the acceleration of the Ar+ ions generated in the glow discharge towards the cathode and dislodge the (cathode) target atoms forming a thin film layer on the substrate.[124–127] The plasma is maintained by the ionization caused by the secondary electrons generated by the ions bombardment of the target surface.[122] In magnetron sputtering, the secondary electrons are constrained by the magnetic field to move in the direction perpendicular to both the electric (E) and magnetic (B) field.[128] This is called E x B drift where the electrons show a cycloidal motion in the glow discharge. The magnetic field is oriented such that the E x B drift of the emitted secondary electrons forms a closed loop.[124] This, in turn, increases the probability of collision between the ionizing electrons and atoms which results in increased ionization leading to increased plasma density by trapping the electrons near the target region.[129] The high plasma density increases the target erosion rate due to increased ion flux because of which magnets are placed along a ring directly behind the target as shown Fig. 14 to maximize the use of the target.[122]

Figure 14. Schematic representation of a magnetron sputter deposition chamber (not to scale), chair of Materials Discovery and Interfaces (MDI), Ruhr-Universität Bochum (Prof. Dr.-Ing Alfred Ludwig).[130]

2.5.2 Reactive magnetron sputtering

Reactive magnetron sputtering is generally used to deposit oxide and nitride thin films.

During sputtering, a reactive gases such as O2 or N2 is supplied into the sputter chamber, allowing the gas to react with the target surface as well as with the sputtered deposited atoms forming an oxide or nitride thin film on the substrate.[128] With an increase in the reactive gas flow (for example O2), an oxide layer is formed on the (metallic) target surface converting from metallic to compound target. During reactive sputtering, the rate of deposition is reduced as compared to the metallic target deposition due to the effect of the poisoning of the target. Fig. 15 shows the hysteresis behavior of the a) deposition rate and b) chamber pressure as a function of the reactive gas flow rate respectively.[131] Initially, with the low reactive gas flow (Fig. 15a), the target surface changes from metallic to compound (target poisoning), however, in this stage, no noticeable change in the rate of deposition is observed as compared to metallic target deposition. With further increase in the reactive gas flow crossing the critical flow point, the target is completely poisoned leading to a sharp drop in the deposition rate and remains low for high reactive gas flows. In order to reach back the metallic deposition rate, the reactive gas flow is reduced but the target remains in the oxidized state. After crossing a certain reduced gas flow point, the deposition rate drastically rises, reaching back the metallic 23

deposition rate. Similar behavior can be observed in chamber pressure (Fig. 15b) with the reactive gas flow.[128,131]

Figure 15. Hysteresis curves in reactive sputtering for (a) the deposition rate and (b) the chamber pressure.[131]

2.5.3. Thin film growth modes

During the sputter deposition process, the sputtered target atoms condense on the substrate resulting in the formation of a thin film. Film formation includes nucleation and growth processes. Nucleation stage occurs when several sputtered atoms adsorb on the substrate resulting in the formation of the film. In this stage, small and mobile clusters or islands formation takes place. Prior to nuclei formation, large clusters are formed while island density saturates. For the next stage, the merging of islands takes place which decreases the island density where further nucleation occurs. Merging of islands or clusters continues until the gap between them is filled. The continuous depositions lead to fill the voids developed while clusters merging resulting in forming a uniform thin film.[121]

Three basic growth modes are observed during the thin film formation, which is shown schematically in Fig. 16. (1) Volmer-Weber or island growth, (2) Frank-van der Merwe or layer growth, and (3) Stranski-Krastanov or island + layer growth.

Figure 16. Three thin-film growth modes: 1) Island growth mode, 2) Layer growth mode, and 3) Stranski-Krastanov growth mode.[121]

For the first growth mode, island growth/Volmer-Weber, small clusters nucleate and grows to form islands on the substrate. In this growth mode, the deposited atoms-atoms/atoms- molecules bonding are stronger than the atoms/molecules-substrate bonding.[121] Layer growth/Frank-van der Merwe is the second growth mode where small clusters nucleate and grow in the form of a layer. Here, the atoms-substrate bonding is stronger than the atom-atom bonding. Single-crystal epitaxial growth of semiconductor film is a typical example of layer growth mode. The third growth mode is Stranski-Krastanov which involves the combination of both island and layer growth. In this growth mode, after the formation of one or more monolayers, the layer growth becomes unfavorable which further results in the formation of islands. This growth mode is often observed in metal-metal and metal-semiconductor systems. The lattice mismatch between the film and the substrate causes accumulation of the strain energy in the growing film which when released causes high energy at the deposit- intermediate-layer interface which may trigger the island formation.[121,124]

Chapter 3 Research methodology

3.1 Instrumentation

3.1.1 Combinatorial reactive magnetron sputtering

The combinatorial reactive magnetron sputtering unit (AJA International ATC-2200V) illustrated in Fig. 17a was used for the fabrication of multinary vanadate thin-film materials libraries (MLs). This sputter down cylindrical chamber is equipped with four magnetron cathodes placed on top of the chamber facing down to the rotatable substrate table which is integrated with the heater. The substrates are transferred to and from the sputter chamber through the load-lock using a mechanical manipulator. The cathodes are positioned in a special design configuration i.e. an azimuthal angle of 90° (when cathodes are aside) and/or 180° (when cathodes are opposing side) and an inclination angle of 45° with respect to the substrate. This cathode configuration enables the fabrication of well-defined continuous composition and thickness gradients within a single ML. Fig. 17b and c depict the top view and side view of the film deposited in this design configuration with composition and thickness gradients almost perpendicular to each other. The substrate region facing closer to the target source will have the highest concentration of that target material. Each cathode can be connected to direct current (DC), pulsed-direct current (p-DC) or radiofrequency (RF) power supplies. Prior to the deposition of the multinary vanadate thin-film MLs, the chamber was brought down to a base pressure of ≤ 2.8 x 10-5 Pa. All films were deposited at a deposition pressure of 0.66 Pa in the reactive environment of Ar and O2 gases without intentional heating (except Fe-Ti-V-O system). The deposition was started with the cleaning of the substrate by using a substrate bias (20 W RF power) for 15 min. Metallic targets with 99.99% purity (4-inch diameter) were used for the fabrication of the thin-film MLs.

For the presented work, the multinary vanadate ternary and quaternary systems were deposited in a configuration illustrated by the schematic drawing of Fig. 18a for M-V-O (ternary) and Fig. 18b for M1-M2-V-O (quaternary) systems. Table 2 lists the deposition parameters such as deposition pressure, process gas flow, reactive gas flow, deposition temperature, and deposition duration for all explored multinary vanadate systems presented in this thesis. All MLs were annealed in air at a ramp rate of 5°C per min and allowed to cool down naturally. Table 3 describes the post-deposition annealing parameters such as annealing temperature and duration. The sputter power details of the MLs are mentioned in the results 26

and discussion section for the individual system. The thin-film MLs were prepared on three different substrates at identical deposition parameters for different high-throughput characterizations. For PEC measurements, a homogeneous 40 nm Pt back electrode layer [DC 100 W] was deposited onto the 10 nm Ti adhesion layer [p-DC 100W] prior to the multinary vanadate thin films on thermally oxidized 100 mm (4-inch) diameter Si substrate (1.5 µm

SiO2 as a diffusion and reaction barrier). On the deposited film, energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and scanning electron microscopy (SEM) characterizations were performed. For optical properties, the thin films were deposited on a fused silica substrate and films were prepared on SiO2/Si substrate with photoresist patterns for film thickness measurements.

Figure 17. a) AJA International ATC-2200V (reactive) magnetron sputtering unit at the chair of MDI, Ruhr-Universität Bochum. Schematic view (not to scale) of continuous composition and thickness gradient within a single materials library b) top view and c) side view.[36] 27

Figure 18. a) Schematic representation of combinatorial (not to scale) a) two magnetrons and b) three magnetrons sputter setup (illustration from Dr. H. Stein, MDI, RUB).[132]

Table 2: List of process parameters for the deposition of M-V-O and M1-M2-V-O systems.

Deposition Process Reactive Deposition Deposition

Systems pressure gas; Ar gas; O2 duration temperature (Pa) (sccm) (sccm) (min) (°C)

ML1: 240 Fe-V-O 0.66 40 10 ML2: 240 RT ML3: 270 ML1: 180 Cu-V-O 0.66 40 10 ML2: 180 RT ML3: 390 ML1: 150 Ag-V-O 0.66 40 10 ML2: 150 RT ML1: 270 W-V-O 0.66 40 10 ML2: 240 RT ML1: 240 Cr-V-O 0.66 40 10 ML2: 240 RT ML1: 240 Co-V-O 0.66 40 10 ML2: 270 RT ML1: 180 Fe-Cu-V-O 0.66 40 10 ML2: 150 RT

Fe-Ti-V-O 0.66 40 10 ML1: 210 400

Table 3: Post-deposition annealing parameters of M-V-O and M1-M2-V-O MLs.

Anneal Anneal Systems temperature duration

(°C) (h)

Fe-V-O 600 5

Cu-V-O 600 6

Ag-V-O 300 10

W-V-O 600 6

Cr-V-O 500 5

Co-V-O 600 5

Fe-Cu-V-O 600 5

Fe-Ti-V-O 600 5

3.2 Materials characterization

All M-V-O and M1-M2-V-O MLs were investigated using high-throughput characterization techniques. Each ML was segmented in a measurement grid of 4.5 mm x 4.5 mm resulting in a total of 342 MAs. Therefore, in order to investigate these well-defined and comparable samples within the MLs, high-throughput characterization methods such as surface profilometer for film thickness, EDX for composition, SEM for morphology, XRD for crystal structure, ultraviolet-visible (UV-Vis) spectroscopy for Eg, and PEC for photocurrent density was performed to establish correlations between them.

3.2.1 Film thickness measurements

Surface profilometry is a (surface) topographical measurement technique that determines the thickness (on steps) and roughness of the film. The stylus profilometer is connected to a small-tip probe which scans the surface of the sample and senses the variation in the movement of the tip to determine the surface height profile. To run the measurement, the

sample surface moves under the stylus tip. A linear variable differential transformer (LVDT) detects the vertical motion of the stylus and converts the detected signal into height data.[133] Styli are typically made of hard materials like diamond. In order to avoid damage to the surface during the measurement, the load of the stylus tip on the surface can be varied from 0.1 mg to 50 mg.[133]

High-throughput film thickness measurement of the MLs was performed using surface profilometry (Ambios XP2). Photoresist patterns with a measurement grid of 4.5 mm x

4.5 mm were prepared on to the Si/SiO2 substrate surface before film deposition. After the lift-off process by sequential ultrasonication of the MLs in organic solvents (acetone and isopropanol), the film thickness map was determined.

3.2.2 Energy-dispersive X-ray spectroscopy (EDX)

EDX allows determining the elemental composition of a sample. In Bohr’s atomic model (Fig. 19),[134,135] focused beam of high energy electrons bombard the sample’s surface, resulting in the excitation of the electrons from innermost shell of an atom. The ejection of excited electrons creates a hole that is filled by electrons from the higher energetic levels resulting in an emission of characteristic X-ray radiation which corresponds to the difference of the energetic levels. The shells from low to high energy levels are named K, L, M, etc. A Si drift detector is commonly used to collect and count the number of emitted X-rays at each energy level. The elements are characterized by the energy level which emits the X-rays within the same energy level and determines the amount of the element present in the sample.[135,136]

Figure 19. Schematic of energy-dispersive X-ray spectroscopy, adapted from Ref.[134,135] 30

The compositional analysis for heavy elements in the thin-film MLs was performed using EDX (INCA X-act OXFORD instruments). Prior to the EDX measurement, a Co standard was used for the calibration of the instrument with a compositional error ± 1 at.%. Complete ML was investigated using a measurement grid of 4.5 mm x 4.5 mm at an acceleration voltage of 20 kV. However, an exact quantification of light elements like oxygen is not possible via EDX because of their low fluorescence quantum yield and absorption of their X- rays. The obtained data allows plotting the color-coded composition maps for each element in the MLs.

3.2.3 Scanning electron microscopy (SEM)

SEM is an electron microscopy technique with a high energy focused beam of electrons that scans the sample surface to obtain a high-resolution image e.g. surface morphology, grains shape, and sizes in nanometer to micrometer scale. In SEM, an electron beam is generated by means of thermionic emission from an electron source. Electrons are accelerated by a high voltage energy ranging from 0.1 to 30 keV. When an electron beam is incident on the specimen surface, the electrons penetrate to a depth of few microns and interact with the atoms in a droplet-shaped volume producing a variety of signals such as secondary electrons, backscattered electrons, and X-rays. A schematic illustration of the interaction volume between the electron beam and the specimen is presented in Fig. 20. The penetration depth of the electrons and the interaction volume depends on the density of the materials, energy of the electron beam as well as the atomic number of the specimen. Secondary electrons originate by inelastic scattering from a penetration depth of few nm (near the sample surface) and have a low energy of 50 eV. The secondary electron beam is sensitive to the angle of incidence which makes it suitable for the investigation of the sample topography. Backscattered electrons are emitted elastically from a deeper interaction level (depth dependent on the interaction volume). The backscattered electrons are dependent on the atomic number of the specimen such that heavy atoms scatter electrons much stronger than lighter atoms and thus much higher signal is produced which enables to differentiate between different phases.[137] The morphology of the multinary vanadate thin films were characterized using SEM, Leo 1530VP at an acceleration voltage of 4 kV. SEM images were taken at selected MAs on the MLs. ImageJ software was used to perform the analysis of the SEM images.[138]

Figure 20. Scheme (not to scale) of electron beam interaction volume ‘droplet-shaped’.[139]

3.2.4 Transmission electron microscope (TEM)

TEM is a characterization technique used to study elemental, structural, and morphological properties.1 Cross-sectional TEM lamella were prepared from the most promising photoelectrochemically active regions on the MLs using a focused ion beam (FIB, FEI Helios G4) operated at 30 kV. TEM provides information such as bright field (BF) and dark field (DF) images for visual interpretation of structures. Crystallographic information is obtained from the selected area diffraction (SAD) patterns. BF TEM images and SAD patterns were obtained in an FEI Tecnai Superwin F20 operated at 200 kV and were analyzed using JEMS software involving crystallographic data from the ICSD database.[140,141] The scanning transmission electron microscope (STEM) and EDX images were acquired on the Titan Themis TEM microscope operated at 300 kV in Prof. Dr. Christina Scheu’s group at Max- Planck-Institut für Eisenforschung GmbH, Düsseldorf.2 The aberration-corrected electron beam has a convergence semi-angle of 24 mrad, and a probe current of ~100 pA. The high angle annular dark-field (HAADF) detector was positioned to collect diffracted electrons of semi-angles between 73 and 200 mrad. Multivariate statistical analysis was applied to reduce the noise and identify significant components.[142]

1Dr. Aleksander Kostka from Center for Interface-Dominated High Performance Materials (ZGH), RUB is acknowledged for performing the TEM measurements and diffraction pattern analysis. 2Prof. Dr. C. Scheu, Dr. Katharina Hengge, and Dr. Siyuan Zhang are acknowledged for performing the STEM and EDX measurements at Max-Planck-Institut für Eisenforschung (MPIE) GmbH, Düsseldorf. 32

3.2.5 X-ray diffraction (XRD)

XRD is a fast and non-destructive analytical technique which gives information about the material’s structural properties such as phase of a crystalline material, lattice parameters, crystallite size, preferred orientation, etc. In XRD, a collimated beam of X-rays generated by a cathode ray tube filtered to produce monochromatic radiation is incident on the sample. Constructive interference occurs as the incident rays interact with the specimen at a specific angle from a particular set of lattice planes.[143] The X-ray beams commonly produced by a

Cu source with the wavelength, e.g. Cu Kα radiation equal to ⁓ 0.15406 nm. Bragg’s Law states a relation between the wavelength of the X-rays, angle at which the beam of X-rays incident on the parallel planes of atoms in a crystal and the interplanar distance:[144,145]

n λ = 2dhkl. sin 휃 ……………………………………………(12) where, n is an integer (order of diffraction), λ is the wavelength of the X-rays, dhkl is the interplanar spacing within the same family of the plane which causes constructive interference, and 휃 is the angle between the incident X-ray and the diffracted atomic planes. The diffraction of X-rays by a periodic arrangement of atoms (Bragg’s equation) is illustrated in Fig. 21.[146] The crystallographic orientation of the materials can be identified by scanning over a large angular range and different angles, the intensity of the diffracted X-rays are measured.[145]

Figure 21. Constructive interference of X-rays adapted from Ref.[146]

High-throughput XRD analysis was performed in Bragg-Brentano geometry (Bruker D8

Discover, equipped with a VANTEC-500 area detector and Cu Kα radiation). MLs mapping were scanned in 휃/2휃 geometry at 50 kV X-ray generator and measured the library at 342 MAs with 4.5 mm distance between each MA. The obtained raw data is imported to the DIFFRAC.EVA V4.2.1 software to extract a 1D view of the diffraction patterns. For the XRD analysis of quaternary vanadate systems, the analysis was performed by using machine learning. An in-house built software known as ‘htAx’ (high-throughput analysis of X-ray and functional properties data)[147] developed by Dr. Helge Sӧren Stein (MDI, RUB) and updated by MSc. Lars Banko3 (MDI, RUB) was used to develop the clustering (component) of the XRD diffraction patterns. The weight map for each component was calculated using the non- negative matrix factorization (NMF) of the components.[148] To identify the phases, another software named ‘Phase Identification’ developed by Dr. Bin Xiao4 (MDI, RUB) was used. MLs with promising PEC performance were further investigated using high-throughput synchrotron-XRD measured at 15.5 KeV energy beamline 1-5 at Stanford Synchrotron Radiation Lightsource (SSRL)5. A MARCCD detector was used for data collection. The scan was performed with a grazing incidence angle at 1° in order to minimize diffraction from the substrate. Standard LaB6 powder was used to extract detector distance, tilting, rotation, and direct beam position parameters to the 2D detector for data analysis. Those obtained geometric parameters were used to calibrate, integrate, and normalize the raw images to 1D patterns. The diffraction patterns and the interplanar spacings were used to identify the crystal structure of the material. Grazing incidence synchrotron-XRD helps in identification of surface and near-surface structures which are difficult to identify from other techniques.[149] High-intensity grazing incidence X-rays help in fast in-situ processes, high signal-to-noise ratio produces qualitative data and “the angle of incidence controls the penetration depth of high energy X-rays which leads to different sensitivity of experiment with respect to bulk and surface signals”.[149,150] Thus, surface roughness, morphology, particle shape, size, lattice strain, and surface tension from peak’s shape are obtained.[149] Moreover, crystallization defects and film thickness can also be determined. The obtained X-ray diffraction patterns are matched with the databases such as Pearson’s Crystal database and inorganic crystal structure database (ICSD) for the identification of the crystal structures.[141,151]

3 MSc. Lars Banko from MDI, RUB is acknowledged for modifying the htAx software. 4 Dr. Bin Xiao from MDI, RUB is acknowledged for developing phase identification software. 5Dr. Apurva Mehta and Dr. Suchismita Sarker from SSRL, Stanford University, USA are acknowledged for performing high-throughput synchrotron-XRD measurements and providing the data. 34

3.2.6 Ultraviolet-visible (UV-Vis) spectroscopy

UV-Vis spectroscopy allows determining the optical properties of materials such as light absorption as a function of wavelength as well as the optical Eg. The high-throughput UV-Vis (optical) transmission (HOTT) an in-house built setup (MDI, RUB) covers a wavelength range from 300 to 1000 nm which includes all UV and visible light spectrum. High- throughput measurements were performed by scanning the MLs in a measurement grid of 3 mm x 3 mm (exposed area) with 4.5 mm separation between each MA. The measurement starts with the switching on the Xe lamp and lamp warm-up for at least 40 min. Then, a fused silica reference sample was placed in the path of the light and the reference transmission spectra were recorded at several (7) randomly-selected MAs. After the reference measurement, the vanadate thin films prepared on fused silica substrate was placed in the path of the light and the transmission spectra were collected from the complete ML (342 MAs) by moving the stage in x- and y-direction. ‘The light impinges upon the sample and is partially absorbed in a characteristic wavelength corresponding to electronic transition in the sample’.[11] According to the Beer-Lambert law, the absorption coefficient (α) is calculated using:[152,153]

퐼 훼(휆) = −log ( ) ………………………………………..…(13) 퐼표 where λ is the wavelength of the light, I is the intensity of the transmitted light, and Io is the intensity of incident light source (reference sample). Using the absorption coefficient α, the Eg of the thin-film semiconductors can be calculated using Tauc plot analysis.[154] ‘This is done by plotting y(E) which is the product of the wavelength-dependent absorption coefficient (α) and the photon energy (E) to the power n’:[132]

푦(퐸) = (훼(휆) 푥 퐸)푛………………………….……………..(14)

ℎ푐 1239.8 (푒푉 푥 푛푚) 퐸 (푒푉) = = ………………………………(15) 휆 휆 (푛푚)

For direct transition Eg, n = 2 and for indirect transition Eg, n = ½. The Eg was determined by plotting the (푎(휆)퐸)푛 (ordinate) vs. E (abscissa) and fitting the straight part of the curve. The [154] fit intercept intersecting the abscissa is the Eg value. Eg values for all MAs were manually determined by extrapolation of the Tauc plots using OriginLab software.

3.2.7 Optical scanning droplet cell (OSDC)

The PEC performance of the MLs was measured using an automated high-throughput OSCD developed by Dr. Kirill Sliozberg in the group of Prof. Dr. Wolfgang Schuhmann at the Center for Electrochemical Sciences (CES) at Ruhr-Universität Bochum.[40,155] The OSCD is a three-electrode configuration setup with counter electrode, reference electrode, and working electrode which can be used to perform a variety of (photo)electrochemical experiments on MAs of the MLs. The OSDC allows measuring PEC properties of hundreds to thousands of MAs with high reproducibility.

Fig. 22 is a schematic representation of the OSDC setup. The setup consists of an electrolyte inlet source (1), Pt-wire counter electrode (2), optical fiber (3), electrolyte outlet source (4), Ag/AgCl 3 M KCl reference electrode (5), and a polytetrafluoroethylene (PTFE) tip (6). The photograph of the OSDC setup is presented in Fig. 23. Here, the OSDC is mounted on a movable stage (1) (three directions) controlled via step-motor-driven micrometer screws (2). The pressure of the PTFE tip of the cell on the sample surface (working electrode) is controlled by means of a force sensor (3). The applied force can be varied from 50 to 400 mN depending on the tip size and geometry which can range from 0.2 mm to several mm diameter. Typically, the working electrode area exposed during the measurement is ⁓ 0.78 mm2. A thin PMMA optical fiber connected with a xenon (Xe) lamp (Hamamatsu), 150 W (wavelength range from 370 to 700 nm) coupled to a monochromator with a shutter is used as a light source (4). A syringe (5) and a peristaltic pump (6) are used to fill the tip of the cell with the electrolyte and a valve system (7) controls the electrolyte section. The cell is rinsed with a new electrolyte solution after every measurement.

Typical measurements using OSDC are open-circuit potential (OCP), linear potential sweep (LSV) or potentiodynamic photocurrent measurement under chopped light illumination6. On illuminating the sample, the shift of open circuit potential (ΔOCP) allows determining the type of semiconductor: n- or p-type. For n-type semiconductors, the ΔOCP shift is expected to be negative and positive for p-type semiconductors. A potentiodynamic photocurrent measurement determines the photocurrent density within an applied potential range with the chopped illumination of the light i.e. opening and closing the shutter. For n-type semiconductors, the potential is swept in anodic direction and cathodic direction for p-type semiconductors.

6João R. C. Junqueira from Center for Electrochemical Sciences (CES), RUB is acknowledged for the help with PEC measurements. 36

Figure 22. Schematic (not to scale) of the OSDC with (1) electrolyte inlet source, (2) Pt-wire counter electrode, (3) optical fiber, (4) electrolyte outlet source, (5) reference electrode (typically Ag/AgCl 3 M KCl), and (6) PTFE capillary tip, drawn by Dr. Kirill Sliozberg.[155]

Figure 23. Photograph of the complete OSDC setup: (1) three direction movable OSDC stage, (2) stage motor, (3) force sensor, (4) light source, (5) syringe, (6) peristaltic pump, and (7) electrolyte in- and outlet valve system. Photograph by Dr. K. Sliozberg.[40] 37

The photocurrent densities were calculated by subtracting the dark current from the current measured upon illumination and divided by the geometric area. The applied potential was converted to the RHE using the equation:[17]

퐸푅퐻퐸 = 퐸퐴𝑔⁄퐴𝑔퐶푙 + 0.210 + (0.059 푥 푝퐻)……………….………(16)

For PEC characterization in this thesis, all MLs were investigated using the three-electrode OSDC setup: Pt-wire as a counter electrode, Ag/AgCl (3 M KCl) as a reference electrode and sample surface (MAs) as a working electrode. The aqueous electrolytes of different pH used for the PEC measurements for the multinary vanadate systems are listed in Table 4. Weak alkaline electrolytes (pH 8 and 9.3) were used for PEC measurements as pH near neutral (weak alkaline electrolyte) improves the movement of protons on the electrolyte surface and contribute to increase the mass-transport flux. Thus, weak alkaline electrolytic conditions are considered to be suitable for OER in PEC water splitting.[156] MAs were illuminated with a light intensity of 100 mW/cm2. The chronoamperometry measurements were performed by applying a constant bias potential of 1.63 V vs. RHE for all multinary vanadate systems. This bias potential was deliberately chosen for high-throughput screening of the systems to identify the potentially active regions within the MLs.

IPCE measurements were performed using a monochromator integrated with a shutter (Instytut Fotonowy). A 150 W Xe lamp (Ushio) was used as the light source. Selected MAs (⁓ 0.5 cm2) were exposed through a quartz window as a working electrode. Prior to the IPCE measurements, the light intensity for each wavelength was calibrated (Pmono) with a thermopile connected to a power meter (Thorlabs). An autolab PGSTAT12 (Metrohm) was used to apply the bias potential and record the current. The IPCE values were calculated using: [11]

2 푗푝ℎ표푡표(푚퐴⁄푐푚 ) 푥 1239.8 퐼푃퐶퐸 = 2 …………………………………(17) 푃푚표푛표(푚푊⁄푐푚 ) 푥 휆 (푛푚)

Where jphoto is the measured photocurrent density and λ is the wavelength of the incident light.

Table 4. List of systems, electrolytes, pH and bias potentials used for the PEC measurements.

Systems pH Electrolyte Potential (V vs. RHE) Fe-V-O 8 0.1 M sodium phosphate 1.63 Cu-V-O 9.3 0.1 M sodium tetraborate 1.63 Ag-V-O 9.3 0.1 M sodium tetraborate 1.63 W-V-O 9.3 0.1 M sodium tetraborate 1.63 7 0.1 M sodium sulfate Cr-V-O 4.5 0.1 M sodium perchlorate 1.63 7 0.1 M sodium sulfate 8 0.1 M sodium phosphate

Co-V-O 4.5 0.1 M sodium perchlorate 1.63 7 0.1 M sodium sulfate 8 0.1 M sodium phosphate Cu-Fe-V-O 9.3 0.1 M sodium tetraborate 1.63 Ti-Fe-V-O 9.3 0.1 M sodium tetraborate 1.63

Chapter 4 Results and Discussion

4.1 Screening of ternary M-V-O photoanode systems

4.1.1 Fe-V-O7

4.1.1.1 Synthesis of Fe-V-O MLs

As discussed in the literature survey of chapter 2, the Fe-V-O system is promising for PEC.[22,47,51] To explore a wide range of Fe-V-O compositions, three MLs were synthesized using process parameters described in Table 2 by a combinatorial reactive magnetron sputtering see section 3.1.1. For the preparation of Fe-V-O thin-film MLs, Fe and V targets (99.99% purity) were sputtered using RF and p-DC power supply respectively. Table 5 lists the details of sputter power in watt (W) supplied for the preparation of the thin-film MLs. All as-deposited Fe-V-O MLs were annealed in air at 600°C for 5 h with a ramp rate of 5°C/min and allowed to naturally cool down (see Table 3). Each ML was fabricated using identical parameters on three different sets of substrates (i) Pt back electrode with Ti adhesion layer on

Si/SiO2 substrate for PEC, XRD, and EDX measurements; (ii) Si/SiO2 substrate with photoresist patterns for thickness measurements; and (iii) fused silica substrate for optical properties investigation.

Table 5. Sputter power, Fe content, and thickness for the three Fe-V-O MLs.

Materials Deposition power (W) Fe content Thickness library RF (Fe) p-DC (V) (at.%) (nm) ML1 110 160 10-50 ML2 180 200 19-61 140-425 ML3 210 180 34-80

7 This work is reprinted from Kumari et al.[36]. The contributions for this publication are: I synthesized the MLs and performed EDX, XRD, SEM, and UV-Vis characterizations. The PEC measurements were performed by Dr. R. Gutkowski and J. R. C. Junqueira. TEM and STEM measurements were obtained by Dr. A. Kostka and Dr. K. Hengge respectively. I analysed all the data obtained and wrote majority of the paper. The experiments, analysis, and writing of the paper was supervised by Prof. Dr. C. Scheu, Prof. Dr. W. Schuhmann, and Prof. Dr.-Ing. A. Ludwig.

4.1.1.2 Composition and thickness analysis

Three Fe-V-O MLs (ML1, ML2, and ML3) were fabricated to investigate a large compositional space (Fe10-79V21-90)Ox. ML1 covers (Fe10-50V50-90)Ox, ML2 covers

(Fe19-61V39-81)Ox, and ML3 covers (Fe34-80V20-66)Ox composition spread (see Table 5). All MLs have a similar thickness gradient ranging from 140 to 425 nm (Table 5). The structural and functional properties of the Fe-V-O thin films are discussed in dependence of the Fe content in atomic percentage (at.%). Fig. 24 represents the color maps of (a) thickness and (b, c, d) composition gradients of the three MLs. The black dots in the color maps indicate the MAs where high-throughput characterizations were performed. The composition maps of the three MLs are presented in the color scale of the total Fe composition spread i.e. from 10 to 80 at.% Fe.

Figure 24. Color-coded maps of a) film thickness, b) Fe composition maps of ML1 c) ML2, and d) ML3, adapted from Kumari et al.[36]

4.1.1.3 Composition and thickness-dependent structural analysis

To study the crystallinity and the occurring phases in the system, high-throughput XRD analysis was performed on the annealed Fe-V-O MLs. Fig. 25 shows diffraction patterns of 10 41

selected MAs along the center of each ML representing the complete composition spread of the system. Three different crystalline phases were identified: the monoclinic phase Fe2V4O13, the triclinic FeVO4 and Fe2O3 phases matching to the Pearson’s Crystal Database entries:

1002729, 1414851, and 1925934 & 1603007 respectively. The Fe2V4O13 phase with strong peaks matching to the (002), (022), (023) planes and smaller peaks corresponding to (113),

(020), (210), (220) planes were recorded along with the (111) plane of the FeVO4 phase for the films with low Fe content (ML1 and 2) that is < 36 at.% (Fig. 25a, b). The formation of the monoclinic Fe2V4O13 phase, a Fe:V composition ratio of 1:2, as reported by Tang et al., is [50] in good agreement with the presented data. Traces of hematite (Fe2O3) structure were observed in the MLs with Fe content > 23 at.%. Corresponding to the literature, the Fe2O3 phase is reported in several Fe-V-O thin films.[47,50,157] The diffraction peak intensity of the

(111) plane of the FeVO4 phase starts to decrease from 26 at.% Fe and completely disappears at around 30 at.% Fe. Similarly, the diffraction peak intensities of the Fe2V4O13 phase discussed above decrease from 36 at.% Fe and vanish beyond 42 at.% Fe (Fig. 25b). With an increase in Fe content > 42 at.%, the crystallinity of the FeVO4 phase with (012) and (112) planes were observed to increase. This behavior indicates a change in the texture of the film, that is, the preferred orientation of the grains relative to the substrate (Fig. 25b, c). For Fe

> 36 at.%, the Fe2O3 phase peak position is observed to shift towards a lower 2Ɵ angle which is likely due to substitution of V atoms by Fe atoms in the crystal lattice (VFe, Krӧger-Vink [158] notation). For Fe content from 42 to 66 at.%, the intensity of the Fe2O3 phase was very low, suggesting change in the texture of the film i.e. increasing crystallinity of the FeVO4 phase. However, the presence of the Fe2O3 phase is confirmed in the region by means of TEM analysis (discussed in the analysis of the hit region section). The Fe-V-O materials system is [157] reported to form the FeVO4 phase when annealed above 600°C. Therefore, for Fe contents up to 55 at.%, the peak intensities of the two main reflections of the FeVO4 phase, from (012) and (112) planes were recorded to increase but for Fe > 60 at.%, the intensity of (112) plane starts to decrease. However, the intensity of the (012) plane of the FeVO4 phase remains unchanged until 66 at.% Fe that can be seen in ML3 (Fig. 25c). As expected, MLs with high

Fe content (68 to 80 at.%) showed increased crystallinity of the Fe2O3 phase whereas the intensity of the FeVO4 phase was decreased. To conclude the this section, three phases were recorded throughout the composition spread: Fe2V4O13 (11 to 36 at.% Fe), FeVO4 (42 to

66 at.% Fe), and Fe2O3 (23 to 80 at.% Fe).

Figure 25. XRD patterns of 10 selected MAs with respective composition information from a) ML1, b)

ML2, and c) ML3. Three phases are observed along the composition spread: Fe2V4O13 (11 to 42 at.%

[36] Fe), FeVO4 (32 to 80 at.% Fe), and Fe2O3 (23 to 79 at.% Fe). 43

The crystallinity of the Fe-V-O thin films influenced by the film thickness can also be explained by the color-coded diffraction intensity maps of the main reflections of the

Fe2V4O13 and FeVO4 phases shown in Fig. A1 (Appendix). The intensity maps of (002) plane of the Fe2V4O13 phase, (012) and (112) planes of the FeVO4 phase for all MLs revealed to exhibit the highest peak intensity towards higher film thickness that is 280 to 395 nm and 355 to 400 nm respectively. With increase in thickness of the film, the volume increases which results in high XRD peak intensity. Therefore, the phases with low peak intensity can be identified in films with higher thickness.

4.1.1.4 PEC analysis

For PEC water splitting, photocurrent density is one of the adequate screening parameters for the characterization of the photoelectrodes (section 3.2.7). Photocurrent measurements were performed using the OSDC setup. Fig. 26 shows the color maps of steady-state photocurrent density with the same scale bar of a) ML1, b) ML2, c) ML3, and d) chronoamperometry chopped-illumination (j-t curve) data of the selected MAs marked within colored circles in Fig. 26c. As observed from Fig. 26a, ML1 exhibited a local maximum photocurrent density of ⁓ 40 µA/cm2 at 41 to 45 at.% Fe, whereas an increase in PEC activity (⁓ 80 µA/cm2, local maximum) was recorded in ML2 (Fig. 26b) with Fe content ranging from 53 to 58 at.%. The PEC active regions (local maximum) within ML1 and 2 have film thicknesses ranging from 230 to 250 nm. These results suggest the PEC activity to be composition-dependent. A similar trend was observed for ML3 (Fig. 26c) with Fe up to a maximum of 80 at.%. Photocurrents increases with an increase in Fe content. A global maximum photocurrent density of ⁓ 190 µA/cm2 was recorded within a composition spread from 54 to 66 at.% Fe. The enhanced photocurrent region (160 to 190 µA/cm2) within ML3 was observed throughout the thickness gradient which suggests that in this case film thickness does not influence the PEC performance. However, beyond 66 at.% Fe, the photocurrent values decrease. The reason behind this PEC behavior of the Fe-V-O photoanode system is explained in detail in the next section (correlation of composition, structure and functional properties). The obtained results show the importance of the CMS approach which enables the identification of the compositions (hit region) exhibiting the highest photoactivity. Fig. 26d shows the j-t curve from selected MAs highlighted in Fig. 26c exhibiting stable photocurrents for about 120 s. These results suggest that the photoactive region could be stable against photocorrosion without any contribution from photooxidation during OER.

Figure 26. Color-coded photocurrent density maps of a) ML1, b) ML2, c) ML3 of Fe-V-O photoanode system with the same photocurrent scale bar and d) chopped illumination data of selected MAs highlighted in c), adapted from Ref.[36]

IPCE measurements were performed to investigate the relation between composition-dependent photocurrent density and wavelength of the light ranging from 370 to

600 nm. IPCE curves from the dominant Fe2V4O13 (38 at.% Fe), FeVO4 (61 at.% Fe), and

Fe2O3 (72 at.% Fe) phase regions of ML3 are presented in Fig. 27. The experiments were performed in 0.1 M sodium phosphate solution at 1.63 V vs. RHE with a calibrated monochromatic light source. As shown in Fig. 27, the dominant FeVO4 phase (61 at.% Fe) with a global maximum photocurrent density, exhibits a maximum efficiency of 7% at

370 nm wavelength. The IPCE decreased to 6.4% at 340 nm for the Fe2O3 dominant region

(72 at.%). The Fe2V4O13 dominant phase region (38 at.% Fe) exhibits the least PEC active

region on ML3 with an efficiency of only 0.5% at 370 nm wavelength. On comparing IPCE values of the three dominant phase regions, the FeVO4 phase with 61 at.% Fe displays maximum efficiency which shows the importance of the FeVO4 phase in the Fe-V-O photoanode system. For higher wavelength range (> 420 nm), higher IPCE values are only detected from the region consisting of the FeVO4 phase (green and blue line) whereas, low

IPCE values (< 1%) are recorded for the region without the FeVO4 phase (red line).

Figure 27. IPCE curve of the three MAs consisting of Fe2V4O13 (38 at.% Fe), FeVO4 (61 at.%), and [36] Fe2O3 (72 at.%) dominant phase regions measured at 1.63 V vs. RHE.

4.1.1.5 Correlations between composition, thickness, structure, and photocurrent density

Fig. 28 shows the correlations between film thickness, composition, crystal structure, and photocurrent density. The color-coded scatter plot of film thickness and Fe content in dependence of photocurrent density is presented for a total of 1026 MAs obtained from three MLs. The plot is divided into five phase regions (labeled with roman numbers ‘I-V’) with dashed lines of different colors, indicating crystal structure (with respective planes) information. As discussed in section 4.1.1.2. (composition and thickness analysis section), the thickness gradients for all three MLs are similar ranging from 140 to 425 nm. As discernible from Fig. 28, the region I with Fe content < 26 at.% (dominating phase: FeVO4, with (111) plane, left of red dashed line) shows a minimal photocurrent of ⁓ 15 µA/cm2 from ML1 and

ML2. Region II (left of green dashed line) consisting of the prominent Fe2V4O13 phase with preferred (002), (022), and (023) orientation overlaps the region I, covering a large compositional range (up to 42 at.% Fe). The Fe2O3 phase in region III spans from around 23

to 42 at.% Fe. A local maximum photocurrent density of ⁓ 28 µA/cm2 is recorded within the region II and III (ML1 and 2). For Fe content > 36 at.% (yellow dashed line) labeled with region IV, consists of the FeVO4 phase with preferred (012) and (112) orientations along with the Fe2O3 phase. For compositions > 45 at.% Fe, the phase present in region II completely disappears. Region IV with a global maximum photocurrent density of ⁓ 190 µA/cm2 recorded from 54 to 66 at.% Fe consists of the mixture of the dominant FeVO4 phase with the

Fe2O3 phase (ML3). Within this region (between yellow and blue dashed lines) increasing photocurrent densities from 40 to 190 µA/cm2 were recorded with an increased Fe content up to 66 at.% throughout the thickness gradients. The trend of increasing photocurrents with increasing Fe content was consistent until 66 at.% Fe. For films with Fe content ≥ 68 at.% (region V), the photocurrents were observed to decrease to ⁓ 100 µA/cm2 due to the presence of the dominant Fe2O3 phase.

Figure 28. Color-coded scatter plot for the assessment of correlations between film thickness, Fe content and photocurrent density at 1.63 V vs. RHE for 1026 MAs from the three MLs. The different phase regions (I-V) are separated by dashed lines of different colors.[36]

The Fe2O3 phase plays an important role in the PEC activity of the Fe-V-O thin film system.

Though the Fe2O3 phase was identified in the regions II, III, IV, and V from 23 to 80 at.%, the photocurrent densities were not identical in all regions of the MLs. The global maximum 47

photocurrent density was recorded only in region IV. On comparing region I (FeVO4 phase without Fe2O3 phase) and region IV (FeVO4 phase with Fe2O3 phase), much higher photocurrents were observed from region IV which suggests that the combination of the two phases FeVO4 and Fe2O3 has a synergistic effect resulting in an enhanced PEC activity.

Region V with the prominent Fe2O3 phase showed decreased photocurrent values which is [30] possibly due to surface recombination. As reported by Singh et al., Fe2O3 is reported to [159] form a passivation layer over the FeVO4 thin film. Films with the dominant Fe2V4O13 phase covering the regions I, II, and III (up to 42 at.% Fe) exhibit a low photocurrent density 2 of 28 µA/cm . Correspondingly, Tang et al. reported the Fe2V4O13 surface to suffer from poor charge carrier transport as a low photocurrent density (28 µA/cm2) was recorded even in the [50] presence of the hole scavenger. Region IV (54 to 66 at.% Fe) with the dominant FeVO4 phase was observed to exhibit the global maximum photocurrent density up to ⁓ 190 µA/cm2, which is twice the value reported by Morton et al.[157]

4.1.1.6 Morphology of the Fe-V-O thin films

PEC performance is also known to be dependent on the surface morphology of the photoelectrodes. SEM images of selected MAs from three ML with maximum photocurrent density are presented in Fig. 29. Surface topographical images from the MLs along the composition and thickness gradients are shown in Fig. A2. With an increase in Fe content, the grain size decreases, but the grains show similar morphology across the thickness gradient. Prismatic-shaped grains are observed towards the PEC active regions. The average size of the grains in the ML1 was ⁓ 265 nm at 42 at.% Fe (Fig. 29a), whereas the grain size decreases to ⁓ 200 nm at around 54 at.% Fe in ML2 (Fig. 29b). For the highest photocurrent region (ML3), an average grain size of ⁓ 125 nm at 61 at.% Fe (Fig. 29c) was calculated. Thin films with small grains exhibit high PEC activity due to the presence of large surface areas. Despite the presence of large surface area towards Fe-rich region with the prominent Fe2O3 phase, the photocurrent densities were observed to decrease (see Fig. A2, ML3 top row), which could be related to its extremely short hole diffusion length (2 to 4 nm).[160] Moreover, accumulation of positive charge on the Fe2O3 photoanode surface results in band banding causing rapid electron-hole recombination.[30] These results underline the importance of identifying the right crystal structure and chemical composition for enhanced PEC activity of the films which can be efficiently achieved using CMS. Here, a large composition spread (10 to 80 at.% Fe) of the Fe-V-O materials system was investigated. An interesting region 54 to 66 at.% Fe exhibits promising PEC performance.

Figure 29. SEM images after PEC measurements from a) ML1, b) ML2, and c) ML3 at the compositions (Fe42V58)Ox, (Fe54V46)Ox, and (Fe61V39)Ox respectively exhibiting maximum photocurrent density. The Fe content is labeled top left, photocurrent density top right, and film thickness is labeled bottom left for all MLs.[36]

4.1.1.7 Composition, phase constitution, and color of the Fe-V-O thin films

In Fig. 30, (a, d) color-coded Fe composition maps, (b, e) photographs of Fe-V-O MLs, and (c, f) color maps of XRD peak intensities from ML2 and 3 are presented beside each other in order to study the color of the Fe-V-O films in terms of composition and phase constitution. Fe-V-O thin films were prepared on transparent fused silica wafers with subsequent annealing in air at 600°C for 5 h. The complete composition spread recorded from ML2 and ML3 is 19 to 80 at.% Fe. With the gradual increase in Fe content, the color of the film gradually changes from light yellow to deep orange (see Fig. 30a, b, d, e). Moreover, from XRD analysis (see section 4.1.1.3), three crystal structures i.e. Fe2V4O13, FeVO4, and Fe2O3 were identified along the Fe composition gradient (low to high Fe). Fig. 30c, f shows the XRD peak intensity maps of the dominant phases found in ML2 and ML3 throughout the composition spread. As mentioned earlier, with increased film thickness (left side of the substrate) the volume increases due to which highest XRD peak intensity is recorded towards thicker film. Different colors of the Fe-V-O film correspond to the presence of different crystal structures in the film.

The yellow color belongs to the Fe-V oxide phases Fe2V4O13 and FeVO4. Moreover, due to the presence of the dominant Fe2O3 phase towards the Fe-rich region, the deep orange color film is formed. Correspondingly, Mandal et al. reported a change in color of the film/solution precursor from deep red to light yellow color due to the addition of V into the Fe2O3 matrix.[157]

Figure 30. (a, d) Color-coded Fe-composition map, (b, e) optical images, and (c, f) XRD-peak intensity color maps of the Fe2V4O13, FeVO4, and Fe2O3 phases from (a-c) ML2 and (d-e) ML3 respectively. Optical images were obtained from MLs prepared on fused silica wafers after annealing in air at 600°C for 5 h.

4.1.1.8 Optical properties and analysis of hit Fe-V-O sample

The optical properties of the Fe-V-O MLs were investigated using UV-Vis spectroscopy. The color map of the Eg values of ML3 with promising PEC performance is shown in Fig. 31. Eg values were calculated using the Tauc plot analysis. The direct and indirect Eg ranged from 2.55 to 2.83 eV (Fig. 31a) and 1.92 to 2.49 eV (Fig. 31b) respectively. The dominant

Fe2V4O13 phase showed a large direct Eg (2.75 to 2.83 eV) and an indirect Eg ranging from

2.32 to 2.49 eV. The indirect Eg narrowed with the change in crystal structure to the prominent FeVO4 and Fe2O3 phases i.e. Eg reduced to 1.92 eV. The phase region with the dominant FeVO4 phase showed an enhanced PEC performance. For FeVO4 and Fe2O3, the conduction band position is observed to be at -0.15 V and -0.10 V respectively, which is near their flat band positions.[47] Moreover, due to charge transfer from O 2p-orbital to V 3d- orbital, FeVO4 corresponds to have direct transition and the indirect transition is determined from O 2p-orbital and Fe 3d-orbitals.[161] Fig. 31c shows the Tauc plots of the global 2 maximum PEC active region (190 µA/cm ) on ML3 with FeVO4 as the dominant phase (61 at.% Fe). The photoactive region corresponds to have 2.04 eV indirect and 2.80 eV 50

direct Eg. The indirect Eg value is in agreement with the reported literature but direct Eg is larger than the reported one i.e. 2.50 eV.[22]

Figure 31. Color-coded maps of a) direct and b) indirect Eg values of ML3. c) Tauc plots for the determination of the direct and indirect Eg of the dominant FeVO4 phase where the maximum photocurrent density was recorded. Figure adapted from Kumari et al.[36]

The hit sample ‘(Fe61V39)Ox’ where the global maximum photocurrent density was recorded in ML3 was analyzed in greater detail. A HAADF cross-sectional image from the hit sample is shown in Fig. 32a. The zoomed image of the region highlighted in Fig. 32a is shown in Fig. 32b which confirmed the film thickness of the hit sample to be ⁓160 nm. Two different structures in the hit sample cross-section image are indicated with A and B in Fig. 32b, and suggest the film to be inhomogeneous. A qualitative elemental distribution map was acquired to identify these structures (Fig. 32c-e). An overlay map of Fe, V, and O is presented in Fig. 32f. Region A is Fe-rich (98 at.%) whereas region B consists of both Fe (57 at.%) and V (43 at.%), however, an exact quantification of light elements like O is not possible via EDX because of their low fluorescence quantum yield and absorption of their X-rays.

Region A and B are indicated in an overlay map of Fe and V in Fig. 33a. The BF TEM images at higher magnification are shown in Fig. 33b, c, where regions A and B are tilted in zone axes due to which the respective areas appear dark. SAD patterns were performed to identify the crystal structures in these indicated regions. The diffraction patterns at different zone axes were acquired for each highlighted region (dashed circle) in Fig. 33a. The presence of the Fe2O3 phase (Fig. 33d) and the FeVO4 phase (Fig. 33e) was confirmed from a set of SAD patterns with corresponding JEMS simulation for regions A and B respectively. From

XRD analysis (Fig. 25), a region with the dominant FeVO4 phase showed the maximum PEC activity. This phase is confirmed by SAD patterns (Fig. 33d, e). Region B with the FeVO4 phase covers a major area of the hit sample which is decisive for the enhancement of the PEC performance of the film.

Figure 32. a) HAADF cross-section TEM image of hit sample ‘(Fe61V39)Ox’ exhibiting highest photocurrent density in ML3 and b) zoomed HAADF image of the region highlighted in panel a. c-f) Corresponds to EDX compositional maps with an overlay map of Fe, V, and O.[36]

Figure 33. a) Cross-sectional overlay map of Fe and V. BF TEM images from b) region A and c) region B in Bragg contrast. SAD patterns from the highlighted regions on panel (a) belonging to d) [36] Fe2O3 phase at [-1,1,2] zone axis and e) FeVO4 phase at [6,1,1] zone axis respectively. 52

4.1.2 Cu-V-O

4.1.2.1 Fabrication of Cu-V-O MLs

Cu-V-O is considered as an interesting system for PEC water splitting.[61,62,64–67] Three Cu-V-O MLs were fabricated using combinatorial reactive magnetron sputtering at a deposition pressure of 0.66 Pa with 40 sccm Ar and 10 sccm O2 gas. High purity (99.99 %) 4-inch diameter Cu and V targets were sputtered using RF and DC sputter power respectively. The sputter power supplied to the respective Cu and V cathodes are shown in Table 6. All MLs were fabricated at RT with subsequent annealing in air at 600°C for 6 h with a ramp rate of 5°C /min and cooled down naturally. Cu-V-O thin films were synthesized at identical deposition parameters on three different substrates such as Pt back electrode with Ti adhesion layer on Si/SiO2 substrate, Si/SiO2 substrate with photoresist patterns, and fused silica wafers in order to perform several high-throughput characterizations.

Table 6. List of sputter parameters for Cu-V-O MLs. Materials Deposition power (W) Cu content Thickness Anneal library RF (Cu) p-DC (V) (at.%) (nm) (°C / h) ML1 105 235 18 - 61 141 - 336 ML2 160 210 46 - 84 600 / 6 ML3 145 210 40 - 78 220 - 656

After oxidation of the MLs, different colors were observed indicating the presence of several phases along the composition gradient of the Cu-V-O MLs as shown in Fig. 34a, b, c. In the presented photographs, the bottom of the MLs is V-rich and the top consists of Cu-rich region.

4.1.2.2 Elemental composition and thickness analysis of Cu-V-O MLs

To investigate qualitatively the compositional space within the Cu-V-O MLs, EDX analysis was performed on a measurement grid of 4.5 mm x 4.5 mm resulting in a total of 342 MAs on each library. The color-coded composition and thickness maps of the three MLs on the XY coordinate axes are shown in Fig. 35a, b, c. The composition spread of the Cu-V-O MLs comprises of 19 to 84 at.% Cu and 16 to 81 at.% V content: ML1 – (Cu19-61V39-81)Ox, ML2 –

(Cu46-84V16-54)Ox, and ML3 – (Cu40-78V22-60)Ox also listed in Table 6. The composition maps for all MLs are plotted with the same color scale from blue (low Cu at.%) to red (high Cu at.%) in order to visualize the complete composition spread within the MLs.

Figure 34. Photographs of oxidized Cu-V-O a) ML1, b) ML2, and c) ML3 prepared on 100 mm diameter Pt/Ti/Si/SiO2 substrate after annealing at 600°C for 6 h in air.

Cu content increases from the bottom (blue color) to the top of the substrate (red color) and the upper region of the ML comprises the highest Cu content (Fig. 35b). The thickness measurements were performed on the same MAs as that of EDX MAs revealing to have thickness gradient ranging from 141 to 656 nm as shown in Fig. 35a, b, c and are listed in Table 6. ML 1 and 2 have similar film thickness spread i.e. from 141 to 336 nm whereas ML3 covered higher film thickness from 220 to 656 nm. The thickness gradient is recorded from right to left of the substrate such that the right side of the wafer has lower film thickness (141 nm) i.e. blue in color and higher film thickness (656 nm) with red color is recorded towards the left side of the Cu-V-O MLs. Similar to the composition maps, the thickness maps for all the MLs are presented with the same color scale for direct comparison.

Figure 35. Color-coded composition maps of Cu and thickness maps from a) ML1 b) ML2 and c) ML3 comprising a total composition spread of (Cu18-84V16-82)Ox and 141 to 656 nm film thickness for Cu-V-O MLs. 55

4.1.2.3 Composition-dependent crystal structure analysis

High-throughput XRD measurements in Bragg-Brentano geometry were performed to investigate the composition-dependent structural state of the Cu-V-O thin films. In Fig. 34, different colors on the annealed MLs suggests the formation of different crystal structures in the films. Fig. 36 shows the XRD patterns of 10 selected MAs through the center of each ML representative of complete composition spread i.e. from top to bottom of ML1, ML2, and ML3 (measurement direction shown in Fig. 35c Cu at.% with black arrow). The complete composition gradient is divided into five mixed phase regions, listed in Table 7. Region I: dominant CuV2O6 and minor β-Cu2V2O7, region II: dominant β-Cu2V2O7 and minor

α-Cu2V2O7, region III: dominant α-Cu2V2O7 and minor β-Cu2V2O7, Cu11V6O26, γ-Cu3V2O8, region IV: dominant Cu11V6O26 and minor α-Cu2V2O7, γ-Cu3V2O8, and region V: dominant

Cu5V2O10 and minor Cu11V6O26, γ-Cu3V2O8. These Cu vanadate phases are also identified by their mineral names such as Ziesite (β-Cu2V2O7), Blossite (α-Cu2V2O7), Fingerite

(Cu11V6O26), McBirneyite (γ-Cu3V2O8), and Stoiberite (Cu5V2O10).

Table 7. Phase regions, Cu content, major and minor phases.

Phase Cu content Major phase Minor phase regions (at.%)

I 18 – 40 CuV2O6 β-Cu2V2O7 (Ziesite)

II 33 – 50 β-Cu2V2O7 α-Cu2V2O7 (Ziesite) (Blossite)

III 47 - 63 α-Cu2V2O7 Ziesite, (Blossite) Fingerite (Cu11V6O26)

IV 54 - 76 Cu11V6O26 Blossite, (Fingerite) McBirneyite (γ-Cu3V2O8)

V 62 – 84 Cu5V2O8 Fingerite, (Stoiberite) McBirneyite

As shown in Fig. 36a, for ML1 with low Cu concentration ranging from 18 to 40 at.% (region

I), the majority of the peaks matched to the triclinic CuV2O6 phase (1100685 - Pearson’s crystal database), however, in the same region, a small signal of the monoclinic β-Cu2V2O7

(Ziesite) phase matching to the 308436 - Pearson’s crystal database was recorded. The highest peak intensities of the CuV2O6 phase were observed at 2휃 angles of 20.20° and 29.30°. Cao et al. fabricated a CuV2O6 cathode using a sol-gel method at synthesis temperatures ranging from 500 to 620°C and reported the as-prepared cathode to form CuV2O6 at 550°C, 600°C, and 620°C.[56] With an increase in Cu content from 33 to 50 at.% (region II), the crystallinity of the CuV2O6 decreased and disappeared. The peak intensities matching to the β-Cu2V2O7 phase gradually increased. The highest peak intensity of the β-Cu2V2O7 phase was observed at 2휃 angle 24.58°. This change in the crystal structure of the film was probably due to oxidation as well as an increase in the Cu content. In region II, weak signals from orthorhombic

α-Cu2V2O7 (Blossite: 1900935 – Pearson’s crystal database) phase was observed at 27.31° angle from 47 at.% Cu. For region III (47 to 63 at.%), the majority of the peaks matched to the α-Cu2V2O7 phase with a maximum signal intensity at 27.31° angle whereas, in the same phase region III, few peaks corresponds to the β-Cu2V2O7 phase. From 50 at.% Cu, Ziesite

(β-Cu2V2O7) phase became invisible in phase region III which suggests that the film has transformed into the Blossite (α-Cu2V2O7) phase. Zhou et al. reported the phase transition [61] from Ziesite to Blossite structure, that is from β-Cu2V2O7 to α-Cu2V2O7, at 605°C. Similarly, Slobodin et al. observed the transition in the polymorphs of the copper pyrovanadate from low (500°C) to high (780°C) temperatures.[162,163] Correspondingly, Kim et al. annealed Cu-V-O films from 200 to 600°C and found the film to be amorphous until 400°C. Films were observed to be crystalline after annealing at and above 500°C [66] corresponding to the CuV2O6 and Cu2V2O7 phases. These observations are in agreement with the presented results (MLs annealed at 600°C). From 54 to 76 at.% Cu, region IV consists of a mixture of Cu vanadate phases. In this region, the Cu11V6O26 phase (mineral name Fingerite, 2070357 – Pearson’s crystal database) was identified as the dominant phase whereas few peaks matched to the γ-Cu3V2O8 phase (McBirneyite, 1901865 – Pearson’s crystal database) and α-Cu2V2O7 phase (up to 63 at.% Cu). The presence of the minor McBirneyite phase is more pronounced in ML3 with higher film thickness (220 to 656 nm). In region IV, only three peak signals matched to the McBirneyite phase at 14.73°, 20.88°, and 22.28° 2휃 angle, while the other peaks belong to the dominant Fingerite phase. Seabold et al. synthesized a semiconductor using nanoparticle inks of a hydroxide precursor and confirmed [69] the formation of the γ-Cu3V2O8 phase using XRD analysis. Similarly, Jiang et al. studied electronic, optoelectronic and PEC properties of γ-Cu3V2O8 thin films prepared by magnetron sputtering.[164] However, for ML3, the presence of the dominant Fingerite phase was observed within a composition range between 50 and 66 at.% Cu with the highest peak intensity at 57

angle 24.60°. For Cu contents from 66 to a maximum of 84 at.%, another Cu vanadate phase:

Cu5V2O10 (Stoiberite - 1100190 Pearson crystal database) was recorded (region V). The

Cu5V2O10 phase showed the maximum peak intensity at 34.55° 2휃 angle towards maximum Cu content. In this region, weak signals from the Fingerite and McBirneyite phases were observed which gradually decreased towards Cu content 68 to 76 at.%. The Cu-rich (> 78 at.%) region was observed to have a pure Stoiberite phase for ML2. But for ML3, the Fingerite phase disappeared beyond 66 at.% Cu resulting the diffraction patterns to have the major Stoiberite phase and the only minor Mcbirneyite phase up to 78 at.% Cu (see Fig 36c). Similar to the Cu vanadate phase transition results observed here, Zhou et al. and Jiang et al. reported Cu vanadate phases such as β-Cu2V2O7, α-Cu2V2O7, γ-Cu3V2O8, Cu11V6O26, and [61,64,165] Cu5V2O10. The library annealed at 550°C showed a major β-Cu2V2O7 (Ziesite) phase and the ML annealed at 610°C was reported to have dominant α-Cu2V2O7 (Blossite) phase.

The γ-Cu3V2O8 (McBirneyite) phase was found towards high Cu content. The authors reported a change in crystal structures from γ-Cu3V2O8 → Cu11V6O26 → Cu5V2O10 with an increase in Cu content.[61] These structural analyses confirmed the formation of several Cu vanadate phases with increasing Cu content (as suggested by the photograph of annealed MLs, Fig 34).

4.1.2.4 PEC analysis

High-throughput PEC measurements for Cu-V-O were performed in an OSDC cell (see section 3.2.7) using Pt wire as a counter electrode, Ag/AgCl (3 M KCl) as a reference electrode and MAs with 0.785 mm2 surface area as working electrode. An aqueous electrolyte of pH 9.3 (0.1 M sodium tetraborate) was used (see Table 4, section 3.2.7). PEC measurements for all the three MLs were performed at a constant bias potential of 1.63 V vs. RHE. For calculation details of the photocurrents see equation 16. The correlations of the measured photocurrents with identified phases and composition are discussed in this section. Fig. 37 represents the color maps of the photocurrent density (with the same color scale) and the XRD peak intensity maps of the main Cu vanadate phases from ML1, 2, and 3. The photocurrent maps are plotted beside the color maps of the respective XRD peak intensities identified for each ML for direct comparison between PEC activity and crystal structure. ML1 and 2 (Fig. 37a, b) with 18 to 84 at.% Cu content showed a local maximum photocurrent density of 157 µA/cm2 at 53 at.% Cu and 284 to 320 nm film thickness. As discussed in the previous section, the complete composition spread (18 to 84 at.% Cu) is divided into five Cu vanadate phase regions.

Figure 36. XRD patterns of 10 selected MAs representative for the complete composition spread from low Cu to Cu-rich region of a) ML1, b) ML2 and c) ML3 along with respective compositions shown on the right side of the diffraction patterns. The diffraction patterns show six Cu vanadate phases along the Cu composition gradient: CuV2O6, β-Cu2V2O7 (Ziesite), α-Cu2V2O7 (Blossite), Cu11V6O26

(Fingerite), γ-Cu3V2O8 (McBirneyite), and Cu5V2O10 (Stoiberite).

Region I (18 to 40 at.% Cu) is present only in ML1 (Fig. 37a) with the dominant CuV2O6 phase exhibiting local minimum photocurrents ranging from 0 to 70 µA/cm2. For region II (33 2 to 50 at.% Cu), the photocurrent density ranged from 40 to 80 µA/cm where the β-Cu2V2O7 (Ziesite) was observed to dominate the region (Fig. 37a, b). The increase in photocurrents 2 from 0 to 80 µA/cm is due to phase transition from hexaoxodivanadate (CuV2O6) to 2 pyrovanadate (Cu2V2O7). The local maximum photocurrent density of 157 µA/cm was observed from phase region III (47 to 63 at.% Cu) which consists of the major α-Cu2V2O7

(Blossite) phase. As observed in Fig. 37a, b, the intensity maps of the α-Cu2V2O7 (Blossite) phase overlap the region with the local maximum photocurrent (157 µA/cm2) making the Blossite phase interesting to investigate. It is important to note that the XRD peak intensity maps exhibit strong intensity towards the thicker region of the films because with increased film thickness the measured volume increases resulting in stronger XRD signal intensity (Fig. 37c). ML3 (40 to 78 at.% Cu) with maximum film thickness ranging from 220 to 656 nm exhibited a global maximum photocurrent density of 170 µA/cm2 in the composition range between 50 and 55 at.% Cu throughout the thickness gradient. This global maximum photocurrent was again recorded in the region with the dominant α-Cu2V2O7 (Blossite) phase (region III) similar to the one observed for the thinner film (ML2). The XRD intensity maps of the major α-Cu2V2O7 phase and the minor phase β-Cu2V2O7 almost overlap the photoactive region of ML3 as shown in Fig. 37c. Region III (47 to 63 at.% Cu) with the major Blossite structure was observed to produce enhanced photocurrent density ranging from 80 to 2 170 µA/cm . Guo et al. reported the phases CuV2O6 and Cu2V2O7 as promising photoanodes exhibiting 25 to 35 µA/cm2 photocurrents at 1.23 V vs. RHE.[166] Moreover, similar to the presented results, Kim et al. identified Cu2V2O7 to exhibit higher photocurrents 2 [66] (0.65 mA/cm ) than CuV2O6. Thus, Cu2V2O7 is more suitable for PEC water splitting. The intensity maps of the dominant Cu11V6O26 (Fingerite) phase and the minor γ-Cu3V2O8 (McBirnetiye) phase from region IV (54 to 76 at.% Cu) as shown in Fig. 37a, b, c, are observed in the region closer to the photoactive region which suggests the Fingerite and the McBirneyite phases can be interesting Cu vanadate phases to investigate. Moreover, the highest XRD intensity for both the phases (Fingerite and the McBirneyite) are observed towards left of the library due to the presence of the thicker film. The photocurrents in region IV were observed to be unstable as it exhibited a maximum of ⁓ 100 µA/cm2 photocurrents in ML2 (Fig. 37b) towards 260 to 280 nm film thickness. However, the photocurrent reduced to ⁓ 0.03 µA/cm2 when measured in thick ML3 (220 to 656 nm thickness, Fig. 37c) which is probably due to the strong recombination of the electron-hole pairs leading to loss of holes

reaching the surface of the thick film. Moreover, these results suggest the importance of the film thickness influencing the PEC activity. However, γ-Cu3V2O8 (McBirneyite) has been identified as an interesting photoanode to exhibit promising PEC performance.[69,164,165] In the present case, McBirneyite structure does not exhibit enhanced photocurrents which might be due to its ‘weak’ crystallinity. Lumley et al. studied pristine and (Mo, W)-doped Cu11V6O26 electrodes and reported improved and stable photocurrents (in pH 9.2) of 60 and 95 µA/cm2 at [68] 1.4 V vs. RHE for Mo:Cu11V6O26 and W:Cu11V6O26 electrodes respectively for up to 2 h.

Region V (62 to 84 at.% Cu) with the Cu5V2O10 (Stoiberite) structure exhibited the least photocurrent density values ranging from 0.01 to 0.05 µA/cm2. In region V, the least photoactivity was recorded due to the presence of the Stoiberite structure which limits the charge transport facilitating electron-hole recombination and causes a decrease in the photocurrent density. Jiang et al. reported Cu5V2O10 to undergo surface recombination at potential range (0.8 to 1.6 V).[165] Similar to the presented results, Zhou and Jiang et al. investigated Cu vanadate phases such as CuV2O6, β-Cu2V2O7, α-Cu2V2O7, γ-Cu3V2O8,

Cu11V6O26, and Cu5V2O10 using combinatorial material science approach, and identified the

γ-Cu3V2O8 phase to exhibit the highest PEC performance when measured in pH 9 borate buffer.[61,62,165] To conclude this section, Table 8 summarized the lists of phase regions, Cu contents, identified major and minor phases, and photocurrent densities.

Table 8. List of phase region identity, Cu content, major and minor phases, and photocurrents of the Cu-V-O thin-film MLs.

Phase region Cu content Major phase Minor phase Photocurrent ID (at.%) density (µA/cm2)

I 18 – 40 CuV2O6 Ziesite 0 – 70

β-Cu2V2O7 II 33 – 50 Blossite 40 - 80 (Ziesite)

α-Cu2V2O7 Ziesite, III 47 - 63 80 - 170 (Blossite) Fingerite

Cu11V6O26 Blossite, IV 54 - 76 McBirneyite 0.03 - 100 (Fingerite) (γ-Cu3V2O8)

Cu5V2O10 Fingerite, V 62 – 84 0.001 - 0.05 (Stoiberite) McBirneyite

Figure 37. Color-coded photocurrent density maps of a) ML1, b) ML2, and c) ML3 along with color maps of the XRD intensity of the Cu vanadate phase identified in the MLs: region I: dominant CuV2O6 and minor β-Cu2V2O7, region II: dominant β-Cu2V2O7 and minor α-Cu2V2O7, region III: dominant

α-Cu2V2O7 and minor β-Cu2V2O7 & Cu11V6O26, region IV: dominant Cu11V6O26 and minor α-Cu2V2O7

& γ-Cu3V2O8, and region V: dominant Cu5V2O10 and minor Cu11V6O26 & γ-Cu3V2O8.

4.1.2.5 Surface morphology of Cu-V-O thin films

The surface morphology of thin-film semiconductors plays a major role in the enhancement of the PEC performance. SEM images of the morphologies of the Cu-V-O MLs (ML1, ML2, and ML3) are shown in Fig. 38. Each image is labeled with Cu content top left, photocurrent density bottom left, film thickness top right, and phase region bottom right. The images show that the films of MLs are discontinuous and the morphology is composition-dependent. ML1 with the dominant CuV2O6 phase (region I) consists of low Cu content ranging from 18 to 40 at.% (Fig. 38a). Discontinuous column-like crystals with ⁓ 2 µm length and ⁓ 800 nm width were observed. Similar morphology was reported for a CuV2O6 cathode synthesized by the sol-gel route and annealed at 600°C.[56] For Cu contents from 33 to 50 at.%, region II with

β-Cu2V2O7 as the dominant phase exhibited large interconnected irregular shaped grains of

⁓ 1 µm size. Morphology for the region with the dominant α-Cu2V2O7 phase (region III, 47 to 63 at.% Cu) showed similar surface structure as that of the region II. For ML2 (Fig. 38b), towards low Cu content between 46 to 55 at.% (region II, III), the SEM images show interconnected irregular shaped grains of ⁓ 750 nm size. However, for Cu content > 55 at.% consists of regions III (α-Cu2V2O7), IV (Cu11V6O26, γ-Cu3V2O8), and V (Cu5V2O10) showed compactly packed irregular shaped grains with an average grain size ranging between 120 and 180 nm. Corresponding to ML1 and 2, up to 55 at.% of Cu content in ML3 (Fig. 38c), the surface morphology was observed to have a mixture of column-like structure and globular shaped grains with ⁓ 2 µm length and ⁓ 800 nm width along the regions II (β-Cu2V2O7), III

(α-Cu2V2O7), and IV (Cu11V6O26, γ-Cu3V2O8). For Cu contents between 55 to 75 at.%, covers phase regions III and IV with grains ranging from 500 to 800 nm size. Similarly, Guo et al. [166] observed an interconnected morphology for Cu2V2O7, synthesized via drop-casting.

Meanwhile, the Cu-rich region with the dominant Cu5V2O10 (region V) and the only minor

γ-Cu3V2O8, McBirneyite (region IV) phase showed much smaller homogeneous grains with an average size ranging between 45 nm and 80 nm. Jiang et al. synthesized homogeneously distributed small grains by reactive sputtering reported the formation of the γ-Cu3V2O8 phase.[164,165] The morphology for the global maximum photocurrent region shows a mixture of globular and column-like structure towards the dominant phase region III (α-Cu2V2O7) along with minor phases II and IV. These observations confirmed the morphology of the Cu-V-O thin films to be composition-dependent. Thus, the discontinuity of the film might be due to Cu content such that films with low Cu content (18 to 58 at.%), large gaps were observed between the microstructures whereas the regions with high Cu content (60 to

81 at.%), the morphology appeared to be densely packed agglomeration of particles. Similar [167] observations were reported by Newhouse et al. for a Cu1-xVxOz library.

Figure 38. SEM images of selected MAs from a) ML1, b) ML2, and c) ML3 representing the morphology of the complete library. Cu content in at.% is labeled top left, film thickness top right, photocurrent density in µA/cm2 is labeled bottom left, and identified phase region bottom right.

4.1.2.6 Analysis of optical properties

The optical Eg values of the identified Cu vanadate phases regions were investigated using the Tauc plot analysis. For the optical measurements, another set of the Cu-V-O MLs was prepared on a transparent substrate (fused silica). The Cu vanadate phases have high absorption coefficients with increasing Cu:V ratio which leads to increase the partial density states of Cu at the conduction band minimum showing higher optical transition possibility.[165] The conduction band edge is derived from V 3d-orbitals and the valence band edge is formed from hybridization of Cu 3d9 with O 2p-orbitals.[67,165] Fig. 39a, b, c represents the direct and indirect Eg values for all three MLs. The Eg values were calculated from the identified five phase regions (see XRD results, section 4.1.2.3). Each Tauc plot is labeled with the dominant

phase found in the region. Phase region I (Fig. 39a) with the dominant CuV2O6 phase shows

2.38 eV and 2.33 eV direct and indirect Eg. However, the Eg values for the CuV2O6 dominant phase are reported to be approximately 1.95 to 2.0 eV which is smaller than the presented [65,166] result. This small increase in the Eg might be due to the presence of a mixed phase in the region I. For phase region II (dominant β-Cu2V2O7), the Eg values ranged from 1.90 to

2.28 eV for direct and 1.65 to 2.23 eV indirect Eg. Similarly, the Eg for the major α-Cu2V2O7 phase in region III corresponds to have 1.77 to 1.98 eV direct and 1.72 to 1.94 eV indirect Eg for all three Cu vanadate MLs. Zhou et al. reported similar results for several Cu vanadate [61] phases exhibiting indirect Eg between 1.8 to 2.0 eV. Region IV with the dominant

Cu11V6O26 phase corresponds to have 2.03 to 2.65 eV direct Eg and 1.84 eV indirect Eg. The major Cu5V2O10 phase (region V), belonging to the Cu-rich region, was determined to have

2.68 to 2.78 eV and 1.92 to 1.96 eV direct and indirect Eg respectively. Similarly, Zhou and Jiang et al. reported the formation of various Cu vanadate phases with the change in the Cu:V ratio. They observed Eg values between 1.83 to 2.03 eV for β-Cu2V2O7, α-Cu2V2O7,

γ-Cu3V2O8, Cu11V6O26, and Cu5V2O10 phases. These Cu vanadate phases are reported to be [61,165] indirect Eg semiconductors. The Eg values of all identified phase regions along the Cu contents are listed in Table 9.

Table 9. List of phase region identity, Cu contents, major and minor phases, and Eg values of the Cu-V-O thin-film MLs.

Phase Cu content Bandgap Eg region (at.%) Major phase Minor (eV) ID phase Direct Indirect

I 18 – 40 CuV2O6 Ziesite 2.38 2.33

β-Cu2V2O7 II 33 – 50 Blossite 1.90 – 2.28 1.65 – 2.23 (Ziesite)

α-Cu2V2O7 Ziesite, III 47 - 63 1.77 – 1.98 1.72 – 1.94 (Blossite) Fingerite

Cu11V6O26 Blossite, IV 54 - 76 2.03 – 2.65 1.84 (Fingerite) McBirneyite (γ-Cu3V2O8)

Cu5V2O8 Fingerite, V 62 – 84 2.68 – 2.78 1.92 – 1.96 (Stoiberite) McBirneyite

Figure 39. Direct and indirect Eg of the Cu-V-O a) ML1, b) ML2, and ML3 determined from the Tauc plot analysis using a high-throughput UV-Vis spectroscopy (optical) transmission test stand. Tauc plots are obtained from the identified five phase regions labeled with the dominant phase: Region I:

CuV2O6 with 2.38 eV direct and 2.33 eV indirect Eg, Region II: β-Cu2V2O7 with 1.90 to 2.28 eV direct and 1.65 to 2.23 eV indirect Eg, Region III: α-Cu2V2O7 with 1.77 to 1.98 eV direct and 1.72 to 1.94 eV indirect Eg, Region IV: Cu11V6O26 with 2.03 to 2.65 eV direct and 1.84 eV indirect Eg, and Region V:

Cu5V2O10 with 2.68 to 2.78 eV direct and 1.92 to 1.96 eV indirect Eg.

4.1.3 Ag-V-O8

4.1.3.1 Synthesis of Ag-V-O MLs

The nanostructured silver vanadate (Ag-V-O) system has been explored for PEC water oxidation in recent years.[79–86] The structure-property details and functionalities of the Ag-V-O system are summarized in section 2.5. To explore the Ag-V-O materials system for PEC water splitting, two Ag-V-O thin-film MLs (ML1 and ML2) were fabricated using combinatorial reactive magnetron sputtering (AJA International ATC 2200V) on a stationary 4-inch diameter substrate. The metallic targets with 4-inch diameter were positioned at an angle of 90° between each other and 45° with respect to the substrate as discussed in section 3.1.1, Fig. 20. V (99.99%) purity was sputtered using a p-DC power supply with a frequency f = 10 kHz and a reverse time t = 5 µs. Ag (99.99% purity) was deposited using RF magnetron sputtering. All depositions were performed at room temperature with a deposition pressure of

0.66 Pa in 40 sccm Ar / 10 sccm O2 reactive environment: see Table 2 for the details of the deposition parameters. The sputter power, distribution of Ag content, and film thickness for both the MLs are listed in Table 10. All as-deposited MLs were annealed at 300°C for 10 h in air with a ramp rate of 5°C per min and allowed to cool down naturally (see Table 3). Corresponding to the Fe-V-O and Cu-V-O materials systems, Ag-V-O MLs were fabricated on three different substrates (Pt/Ti/SiO2/Si wafer, SiO2 wafer with photoresist patterns, and fused silica wafer) at identical parameters for various high-throughput characterizations using 4.5 mm x 4.5 mm measurement grid.

8 This work is reprinted from Kumari et al.[168]. The contributions for this publication are: I and Lukas Helt synthesized the MLs and performed EDX, XRD, SEM, and UV-Vis characterizations. The PEC measurements were performed by J. R. C. Junqueira. TEM and STEM measurements were obtained by Dr. A. Kostka and Dr. S. Zhang respectively. High-throughput synchrotron XRD measurements were performed by Dr. S. Sarker and Dr. A. Mehta. Lukas and I analyzed all the data obtained and I wrote majority of the paper. The experiments, analysis, and writing of the paper was supervised by Prof. Dr. C. Scheu, Prof. Dr. W. Schuhmann, and Prof.-Dr.-Ing. A. Ludwig. 67

Table 10. Overview of the deposition power, Ag and V contents, and film thickness of the Ag-V-O MLs.

Materials Deposition power (W) Ag content V content Thickness library RF (Ag) p-DC (V) (at.%) (at.%) (nm) ML1 35 350 22 - 62 38 - 78 123 - 395 ML2 46 330 37 – 77 23 - 63 198 - 714

4.1.3.2 Composition and thickness analysis

Results of the elemental composition of the Ag-V-O MLs were investigated using high- throughput EDX analysis. The color-coded composition and thickness maps of both libraries are plotted on the XY coordinate axes are shown in Fig. 40. The total composition spread of the Ag-V-O MLs comprise of 22 to 77 at.% Ag and 23 to 78 at. % V. The Ag and V composition gradient for each ML is listed in Table 10. ML1 covers (Ag22-62V38-78)Ox and

ML2 encloses (Ag37-77V23-63)Ox. The composition maps for all MLs are plotted with the same color scale i.e. from blue (low Ag at.%) to red (high Ag at.%) in order to anticipate the complete composition spread throughout the MLs (Fig. 40a, b). The Ag composition is increased from the bottom (blue color) to the top of the substrate (red color) such that the bottom region of the library covers low Ag content (Fig. 40a) and the upper region of the library comprises the highest Ag content (Fig. 40b). The thickness measurements were performed at identical MAs to that of EDX and observed to have thickness gradient ranging from 123 to 714 nm for both the MLs as shown in Fig 40c, d. Thickness maps are plotted with the same color scale and range from 123 to 395 nm and 198 to 714 nm for ML1 and ML2 respectively (Table 10). Film thickness increases from right to left side of the MLs (right side covers films with a low thickness (blue color) and higher film thickness (red color) is recorded towards the left side of the MLs).

4.1.3.3 PEC, structure, and morphology of Ag-V-O MLs

PEC properties of the Ag-V-O thin films were investigated using the three-electrode OSDC setup (see section 3.2.7). High-throughput PEC measurements (see section 3.2.7) were performed at 1.63 V vs. RHE using an aqueous electrolyte of pH 9.3 (see Table 4). PEC measurements performed to identify the active regions within the Ag-V-O MLs are shown in Fig. 41. Change in PEC performance of the Ag-V-O photoanode was observed with an increase in Ag concentration. MLs with low Ag content (< 30 at.%, Fig. 41a), exhibited photocurrents < 50 µA/cm2, however, for Ag content ranging between 30 to 45 at.%,

enhanced photocurrents from ⁓ 300 to 554 µA/cm2 were recorded. The global maximum photocurrent density of ⁓ 554 µA/cm2 was recorded at 32 at.% Ag and 352 nm film thickness, marked with a star in Fig. 41a. For ML2 (Fig. 41b), similar PEC activity to that of ML1 was recorded towards 37 to 45 at.% Ag, where a local maximum photocurrent of ⁓ 355 µA/cm2 was obtained. MLs with Ag concentration > 46 at.%, decrease in photocurrent density to ⁓ 0 µA/cm2. These results showed the PEC activity of the Ag-V-O system to be composition- dependent, however, no significant change in photocurrents was recorded along the thickness gradient.

Figure 40. Color-coded Ag composition maps (a and b) and thickness maps (c and d) for Ag-V-O ML1 and ML2 respectively.[168]

Figure 41. Color-coded photocurrent density maps of a) ML1 and b) ML2 measured at 1.63 V vs. RHE. The star in (a) indicates the MA with a global maximum photocurrent density.[168]

The PEC characteristics of the Ag-V-O thin films can be correlated to the composition- dependent (i) crystal structure as well as the (ii) morphology of the films.

(i) The phase constitution of the annealed Ag-V-O MLs was identified using high- throughput XRD analysis prior to PEC measurements. From a large number of diffraction patterns (684 MAs), those of 10 selected MAs describing the composition spread from each ML are displayed in Fig. 42a, b for ML1 and ML2 respectively. Four Ag vanadate phases were identified throughout the composition spread:

monoclinic AgV6O15 (Pearson's crystal database-1707760), monoclinic Ag2V4O11

(Pearson’s crystal database-1123000, 1403921), monoclinic AgVO3 (Pearson’s crystal

database-1801127), and orthorhombic Ag4V2O7 (Pearson’s crystal database-2090137). The composition gradient on each ML is separated by four different phase regions which are labeled with roman numerals (I-IV). Phase region I with 22 to 30 at.% Ag

consists of the phases AgV6O15 and Ag2V4O11. The majority of the peaks match the

AgV6O15 phase in region I. Phase region II with Ag concentration up to 51 at.%

corresponds to have Ag2V4O11 as the dominant phase. The crystallinity of the

Ag2V4O11 phase increases with an increase in Ag content up to 47 at.%. However, above 47 at.%, the peak intensity decreases due to the formation of another Ag

vanadate phase: AgVO3. Region III with the dominant AgVO3 phase was identified

between 51 to 61 at.% Ag, however, weak signals of the AgVO3 phase were confirmed towards lower Ag content (26 at.%) from high-throughput synchrotron- XRD analysis (discussed later in this section). With further increase in Ag content from 62 to maximum 77 at.%, the Ag4V2O7 phase was observed along with the preceding AgVO3 phase, region IV (mixed-phase region). Several research articles [72,169–171] reported the presence of the Ag2V4O11 phase in the Ag-V-O system. PEC activity of the Ag-V-O materials system revealed to have crystal structure- dependency. Region II with the dominant Ag2V4O11 phase from 30 to 45 at.% Ag exhibits enhanced photocurrents ranging from ⁓ 300 to 554 µA/cm2. However, the photocurrents reduced to ⁓ 0 µA/cm2 in regions I, III, and IV where the phases

AgV6O15, AgVO3, and Ag4V2O7 were recorded as the dominant phase regions. High-throughput screening of compositional, structural, and functional (PEC) properties of the Ag-V-O photoanode reveals ML1 to be propitious for PEC water splitting. Therefore, for a qualitative investigation of the PEC active regions, ML1 was studied in-depth for crystal structure and phase constituency details. High-throughput synchrotron-XRD mapping was performed on ML1 after PEC measurements. Synchrotron-XRD was performed on identical MAs as that of the standard-XRD measurements (discussed above, Fig. 42a). 10 selected diffraction patterns of the standard-XRD measurements (θ/2θ geometry) and synchrotron-XRD (grazing incidence angle) measurements along the composition gradient are presented in Fig. 43a, b. In order to compare the synchrotron-XRD results with that of the standard- XRD, both the diffraction patterns are presented in d-spacing (nm). The synchrotron-

XRD measurements confirmed the presence of three phases AgV6O15, Ag2V4O11, and

AgVO3 previously detected by means of standard-XRD. As synchrotron-XRD helps in identifying the phases with low crystallinity, the presence of the AgVO3 phase was confirmed along with the dominant Ag2V4O11 and minor AgV6O15 phases in the PEC active region.

Figure 42. XRD patterns of 10 selected MAs of a) ML1 and b) ML2 with corresponding composition information. The peak marked with a star is from the Pt back electrode layer matching to Pt (111) orientation.[168] 72

Figure 43. XRD patterns of selected MAs from ML1 measured in a) 휃/2휃 geometry (standard- XRD) and b) grazing incidence angle (synchrotron-XRD).[168] 73

(i) Another important parameter influencing the PEC performance of the systems is the surface morphology of the films. The morphology of the Ag-V-O thin film was obtained before PEC measurements and is illustrated in Fig. 44a, b for ML1 and ML2 respectively along the composition gradient (bottom to top) and the thickness gradient (left to right). Each SEM image is labeled with the Ag composition (at.%), thickness (nm), phase ID (phase regions I-IV discussed in XRD section), and 2 photocurrent density (µA/cm ). Region I (AgV6O15 dominant phase) with < 30 at.% Ag shows tightly packed agglomerated nanoscaled grains of ⁓ 45 nm size exhibiting a very low photocurrent density of < 50 µA/cm2. Morphology of the Ag-V-O thin films changed from nanoscaled grains to porous nanowire-like structure with an increase in Ag content. Region II with the porous nanowire-like structure exhibited enhanced photocurrents ranging between ⁓ 300 to 554 µA/cm2 within 30 to 45 at.% Ag. Porous nanostructured films exhibit a large surface area that ameliorates the PEC performance of the films.[41] Moreover, holes generated in the porous nanowire-like structures can be efficiently transferred to the electrolyte resulting to improve the lifetime of the photogenerated electron-hole pairs and reducing recombination. Within the

photoactive region II, the dominant Ag2V4O11 phase consists of nanowire-like structure with a width of 52 to 55 nm (Fig. 44a). With increase in Ag content (> 51 at.%), length and width of the nanowires increases and form clustered nanorod-

like structures. Region III with 51 to 61 at.% Ag (dominant AgVO3 phase) shows a dense clustered nanorod-like structure having a width of ⁓ 200 nm. Similarly, region IV (62 to 77 at.% Ag) showed clustered nanorod-like structure with increased number of nanorods leading to increase recombination of the charge carrier and thus suppresses photocurrent densities. Gao et al. reported similar microstructure as that of regions III and IV (Fig. 44b) and observed reduced photocatalytic response when 40%

concentration of Ag3VO4 nanoparticles were deposited on the surface of β-AgVO3 nanowires.[80] No specific variation in the PEC activity was observed along the thickness gradient, meanwhile, a large variation in photocurrent values was recorded along the composition gradient.

Figure 44. SEM images of selected MAs (before PEC measurement) of a) ML1 and b) ML2 along with the composition (bottom to top) and the thickness (left to right) gradients. The Ag content in at.% is labeled top left, the film thickness in nm top right, phase ID (I: AgV6O15/Ag2V4O11, II: Ag2V4O11, III: 2 [168] AgVO3, and IV: AgVO3/Ag4V2O7) bottom right and photocurrent density in µA/cm bottom left.

The topographical images of PEC active ML1 were obtained from a high-throughput atom force microscope (AFM, Bruker Dimension Fastscan) using Scanasyst mode.9 The images were measured after PEC measurements along the Ag composition gradient (Fig. 45). Similar to SEM images (Fig. 44a), AFM images show the change in surface morphology from a dense nanoscaled structure to porous nanowires and then to a densely clustered nanorod like- structure. According to the roughness analysis, with an increase in Ag content, the roughness of the Ag-V-O film increased. From low Ag content (23 at.%) to maximum Ag content (61 at.%), the roughness increased from 5.9 nm to 19.6 nm. The roughest surface morphology is recorded for maximum Ag content which might be due to clustered nanorod-like structure. However, the PEC active region (30 to 45 at.% Ag) showed a roughness of ⁓ 9.7 nm.

9 MSc. Xiao Wang from MDI, RUB is acknowledged for the help with AFM measurements at ZGH. 75

Figure 45. AFM images of selected MAs of ML1 (after PEC measurement) along the Ag composition gradient (left to right). The Ag content (at.%) is labeled top left, the photocurrent density (µA/cm2) bottom left, and the color bar indicates the surface height.[168]

4.1.3.4 Analysis of optical properties

Transmittance spectra of the Ag-V-O materials system were measured using high-throughput

UV-Vis spectroscopy and the Eg values were calculated by Tauc plot analysis using OriginLab software (see section 3.2.6). The photographs of the annealed Ag-V-O MLs are shown in Fig. A3. The variation in the colors on the photograph of the MLs correlates with the presence of different Ag vanadate phases in the film. The conduction band of the identified Ag vanadate phases Ag2V4O11, AgVO3, and Ag4V2O7 are formed from V 3d- and Ag 5s-orbitals whereas the valence bands are formed from hybridized Ag 4d- and O 2p- orbitals resulting to have narrow Eg and allows visible light absorption. The hybridized orbitals of the valence band are positioned towards more positive energy level than O 2p- [85,172] orbitals. The color maps of direct and indirect Eg are illustrated in Fig. 46a, b. For both

MLs, the direct and indirect Eg values are plotted with the same color scale (Fig. 46a, b) for direct comparison. Calculated direct and indirect Eg values are in the range of 2.3 to 3.1 eV and 1.4 to 2.7 eV respectively. MLs with the dominant AgV6O15 phase, region I ( < 30 at.%

Ag) show a direct Eg ⁓ 2.5 eV and indirect Eg between 2.0 to 2.2 eV. Region II with the major

Ag2V4O11 phase (between 30 to 51 at.% Ag), the Eg is recorded to range between 2.3 to

3.1 eV for direct and 1.4 to 2.0 eV indirect transition. Correspondingly, the Ag2V4O11 phase corresponds to have an indirect Eg of ⁓ 2.0 eV and is reported to be an indirect Eg semiconductor.[85,86] MLs with Ag concentration between 51 to 61 at.% Ag consisting of the

AgVO3 as the major phase (region III) showed the energy range between 2.8 to 3.0 eV and

1.8 to 2.5 eV for direct and indirect Eg respectively. However, the AgVO3 phase is reported to [80] show a low direct Eg of 2.2 eV. Region IV with 62 to 77 at.% Ag consisting of a mixture of

AgVO3 and Ag4V2O7 phases corresponds to have direct Eg ranging between ⁓ 2.7 to 3.0 eV and 2.3 to 2.7 eV for indirect Eg. 76

Figure 46. Color-coded maps of direct and indirect Eg values for a) ML1 and b) ML2 of Ag-V-O.

4.1.3.5 Detailed analysis of highest photocurrent sample (Ag32V68)Ox

A cross-sectional TEM lamella was prepared using FIB (see section 3.2.4) from ML1 exhibiting the global maximum photocurrents with a composition (Ag32V68)Ox. A HAADF cross-sectional image of the hit sample (Ag32V68)Ox is presented in Fig. 47a. The cross- sectional image shows the Ag-V-O hit film to have a nanowire-like structure and thus complementing the SEM and AFM results shown in Fig. 44 and 45 respectively. Moreover, the film thickness of 348 nm is similar to the one obtained from the profilometer mapping (Fig. 40 c). Fig. 47b corresponds to the EDX qualitative elemental composition maps of V, Ag, O, Pt (back electrode), Ti (adhesion layer between Pt and the substrate), and Si

(substrate). The composition map suggests that oxygen is uniformly distributed throughout the film, however, due to low fluorescence quantum yield and absorption of X-rays, the exact quantification of oxygen is not possible.

Figure 47. a) STEM-HAADF cross-sectional image of the maximum photocurrent sample (Ag32V68)Ox and b) EDX composition maps of V, Ag, O, Pt, Ti, and Si.[168]

A BF-TEM image of (Ag32V68)Ox is displayed in Fig. 48a. The selected area diffraction

(SAD) pattern confirmed the presence of the monoclinic phases Ag2V4O11 and AgVO3 (Fig. 48b, c). The two silver vanadate crystal structures identified with the SAD patterns support the high-throughput synchrotron-XRD analysis. Due to the low symmetry of the

AgV6O15 phase, the AgV6O15 phase was not detected in SAD pattern, however, its presence was confirmed from XRD analysis (see Fig. 42, 43).

The optical properties of the maximum photocurrent sample (Ag32V68)Ox were evaluated in the wavelength range between 350 and 900 nm. Their absorption edge and Eg are discernible from Fig. 49 The absorption edge (Fig. 49a) of the film with the dominant Ag2V4O11 phase is up to 600 nm wavelength which covers the region from ultra-violet to visible wavelength range. Fig. 49b shows that Eg obtained from Tauc plot analysis corresponds to have ⁓ 2.30 eV direct and ⁓ 1.87 eV indirect Eg and is identified to be smaller than the one for pure Ag2V4O11 nanostructured material (2.0 eV).[85,86]

Figure 48. a) Cross-sectional BF-TEM image of (Ag32V68)Ox (hit) sample, b) inset SAD pattern from the region presented in a), and c) matching to the simulated powder diffraction patterns of the [168] monoclinic phases Ag2V4O11 and AgVO3.

Figure 49. a) Absorbance spectrum from the PEC active sample (Ag32V68)Ox and b) Tauc plots of the

[168] dominant Ag2V4O11 phase with 2.30 eV direct and 1.87 eV indirect Eg.

4.2 Screening of quaternary M1-M2-V-O photoanode systems

4.2.1 Cu-Fe-V-O

4.2.1.1 Synthesis of Cu-Fe-V-O MLs

The mixture of potentials ternary vanadate systems such as Fe-V-O and Cu-V-O could lead to the identification of promising PEC materials. With the knowledge gained from literature and the results obtained from screening ternary vanadates (sections 2.5, 4.1.1, 4.1.2, 4.1.3), a new quaternary photoanode system Cu-Fe-V-O was investigated using CMS. Two Cu-Fe-V-O MLs were deposited using a combinatorial reactive magnetron sputter system (AJA International ATC 2200V, see section 3.1.1) from metallic targets of Fe, Cu, and V in a reactive environment on 100 mm diameter Si/SiO2 substrate coated with 40 nm Pt layer serving as the back electrode and 10 nm Ti adhesion layer underneath the Pt layer. The elements were sputtered from high purity 4-inch diameter Fe, Cu, and V metal targets (99.99% purity) using RF, DC, and p-DC power supply respectively as listed in Table 11. The targets were positioned with an inclination angle of 45° with respect to the substrate and 90° between V – Fe and V – Cu respectively, and 180° between Fe - Cu targets. Thin-film MLs were synthesized at a deposition pressure of 0.66 Pa in reactive atmospheres of Ar (40 sccm) and O2 (10 sccm). The process parameters of the MLs are listed in Table 2. The as-deposited Fe-Cu-V-O MLs were subsequently annealed in air at 600°C for 5 h with a ramp rate of 5°C per min in a conventional furnace (Nabertherm) (see Table 3). The MLs have ⁓ 250 nm film thickness at the center.

Table 11: List of the deposition power used for the fabrication of Fe-Cu-V-O MLs.

Materials Deposition power (W) Cu content Fe content V content Library DC (Cu) RF (Fe) p-DC (V) (at.%) (at.%) (at.%) ML1 36 215 280 29-72 4-27 22-57 ML2 36 330 280 11-55 27-73 12-34

4.2.1.2 Compositional and PEC analysis

High-throughput EDX analysis revealed the annealed Cu-Fe-V-O to cover elemental compositions from 11 to 72 Cu at.%, 4 to 73 Fe at.%, and 12 to 57 at.% V. The color-coded ternary composition maps of ML1 and ML2 are shown in Fig. 50a. ML1 and ML2 covers the composition space (Cu29-72Fe4-27V22-57)Ox (blue color) and (Cu11-55Fe27-73V12-34)Ox (red color)

respectively. With the information obtained from the screening of the ternary systems Fe-V-O and Cu-V-O (sections 4.1.1 and 4.1.2), two Cu-Fe-V-O MLs were fabricated in order to cover both the photoactive regions identified from the Fe-V-O and Cu-V-O systems. Fig. 50b represents the color-coded photocurrent density maps in the ternary composition space obtained from ML1 and ML2. The high-throughput PEC measurements were performed using an OSDC setup, (section 3.2.7). The measurements were performed in a three-electrode setup consisting of Ag/AgCl/3 M KCl reference electrode and an aqueous solution of 0.1 M

Na2B4O7 (sodium tetraborate), pH 9.3 was used as the electrolyte (Table 4). The photocurrents were recorded under chopped illumination of light at a constant bias potential of 1.63 V vs. RHE using equation 16 (section 3.2.7) and in a measurement grid of 4.5 mm x 4.5 mm identical to that of the EDX measurements. PEC screening revealed ML1 to exhibit two photoactive regions with enhanced photocurrents (Cu53-58Fe6-8V36-40)Ox and 2 (Cu38-48Fe16-27V33-39)Ox with local maximum photocurrent of ⁓ 100 µA/cm and global maximum photocurrent density of ⁓ 108 µA/cm2 respectively. Both photoactive regions can be observed in the color-coded ternary diagram of the photocurrent density map (Fig. 50b). However, the screening of ML2 displayed maximum photocurrents of only 40 µA/cm2 in the composition range (Cu14-25Fe53-65V19-25)Ox.

Figure 50. Color-coded ternary plot of a) composition maps of Cu, Fe, and V. ML1 (blue color)

(Cu29-72Fe4-27V22-57)Ox and ML2 (red color) consist of (Cu11-55Fe27-73V12-34)Ox composition. b) The photocurrent density maps of ML1 and ML2 for the Cu-Fe-V-O system in the composition coordinate 2 axes showed the highest photocurrent density of 108 µA/cm at (Cu45Fe21V34)Ox.

4.2.1.3 Phase identification of Cu-Fe-V-O MLs

To identify the occurring phases within the Cu-Fe-V-O system, high-throughput XRD measurements were performed in Bragg-Brentano geometry (section 3.2.5) resulting to have a large dataset analyzed by using machine learning. ML1 with enhanced PEC activity resulted to have a total of 7 XRD clusters (components/phase regions). The diffraction patterns obtained from these 7 XRD clusters (phase regions) are shown in Fig. 51. The diffraction patterns are matching with the mixed ternary and quaternary metal vanadate phases revealing the crystal structure information of complete ML1. Fig. 52a shows the color-coded ternary plot of 7 XRD clusters identified in ML1 and the photocurrent density map in the ternary diagram is presented in Fig. 52b for direct comparison with the identified crystal structures.

The phase regions identified within the library are phase region I: -Cu3FeV6O26 / FeVO4,

II: Cu5V2O10 / FeVO4 /  Cu3Fe4V6O26, III: Cu5V2O10, IV: Cu5V2O10 / FeVO4, V: FeVO4 /

γ-Cu2V2O7 / -Cu3Fe4V6O26, VI: -Cu2V2O7 / -Cu3Fe4V6O26 / FeVO4, VII: -Cu3Fe4V6O26 /

FeVO4 matching to the ICSD database 67999 (β-Cu3Fe4V6O24), 202361 (α-Cu3Fe4V6O24),

10329 (FeVO4), 158375 (β-Cu2V2O7), 171028 (γ-Cu2V2O7), and 2557 (Cu5V2O10) as illustrated with the XRD (components) weight maps in Fig. 52c. The color-code of the diffraction patterns (Fig. 51) and the XRD clusters (Fig. 52a) corresponds to the presence of different phase regions labeled in the right of Fig. 52a. Diffraction patterns obtained from phase regions I, V, and VI are matched to the major phases α-Cu3Fe4V6O24 (Lynosite),

FeVO4, β-Cu2V2O7 (Ziesite) and minor γ-Cu2V2O7 phase and the weight maps of phase regions I, V, and VI are shown in Fig. 52 c.i, c.v, c.vi respectively. Within phase regions, I and V, two photoactive regions (Cu53-58Fe6-8V36-40)Ox and (Cu38-48Fe16-27V33-39)Ox were 2 recorded to exhibit enhanced photocurrents of ⁓ 100 and ⁓ 108 µA/cm at (Cu53Fe7V40)Ox and

(Cu45Fe21V34)Ox compositions respectively. Both photoactive phase regions consist of the major phases -Cu3Fe4V6O24 (Lynosite) and FeVO4. The phase regions II, III, IV, and VII consists of the major phases β-Cu3Fe4V6O24 (Howardevansi structure) and Cu5V2O10

(Stoiberite) along with the minor FeVO4 phase as depicted in Fig. 51 (XRD diffraction patterns). The compenent weight maps of these II, III, IV, VII phase regions are shown in Fig. 52 c.ii, c.iii, c.iv, c.vii respectively. These phase regions exhibit the least photoactivity with photocurrent density < 5 µA/cm2. A sharp decrease in photocurrent density was observed from region I to region II due to the influence of the dominant crystal structure. In Region II, majority of the peaks match the Cu5V2O10 (Stoiberite) phase showed decreased photocurrent density due to increased surface recombination of the Cu-rich region with the Cu5V2O10

phase.[165] The crystal structure analysis revealed two  and  polymorphs of the quaternary

Cu3Fe4V6O24 phase such that -Cu3Fe4V6O24 (Lynosite) showed global maximum photocurrent density whereas -Cu3Fe4V6O24 (Howardevansi) exhibited the least photocurrent values. However, Wieczorek-Ciurowa et al. reported both polymorphs of the Cu3Fe4V6O24 exhibit similar catalytic behavior.[99] Fig. 52d shows the photograph of the ML1 overlaid with the XRD clusters in the XY coordinate axes with 342 MAs. The colors in the overlaid XRD clusters belong to the presence of different phase regions discussed above.

Figure 51. Seven XRD patterns obtained from XRD clustering using htAx program[147] matching to the phases: -Cu3FeV6O26, -Cu3Fe4V6O26, Cu5V2O10, γ-Cu2V2O7, -Cu2V2O7, and FeVO4.

Figure 52. Color-coded ternary diagrams with composition coordinate axes of a) XRD clusters, b) photocurrent density map, and c) weight maps of identified seven different phase regions

(components): c.i) -Cu3FeV6O26 / FeVO4, c.ii) Cu5V2O10 / FeVO4 /  Cu3Fe4V6O26, c.iii) Cu5V2O10, c.iv) Cu5V2O10 / FeVO4, c.v) FeVO4 / γ-Cu2V2O7 / -Cu3Fe4V6O26, c.vi) -Cu2V2O7 / -Cu3Fe4V6O26 /

FeVO4, and c.vii) -Cu3Fe4V6O26 / FeVO4 with respective intensity color scale in the Fe-Cu-V-O system (ML1). d) Photographic image of the Cu-Fe-V-O ML1 with overlaid XRD clusters in the XY coordinate axes. 84

As discussed above, a sharp decrease in photocurrent density was recorded from the dominant

-Cu3Fe4V6O26 phase region I to phase region II with the major Cu5V2O10. Therefore, high- throughput synchrotron-XRD measurements were performed in this interesting region (fine grid, 1.5 mm x 4.5 mm). Fig. 53 shows the diffraction patterns from the global maximum photocurrent region (I) to the least PEC active region (II), direction marked with a white arrow in Fig. 52d. Each diffraction pattern is labeled with Cu content (at.%) and photocurrent density (µA/cm2). The peaks marked with a star (*) correspond to match with the Pt (back contact) in Fig. 53. Towards low Cu contents from 33 to 47 at.%, majority of the peaks at 2휃 angle 26.51°, 27.20°, 32.54°, 35.38°, 35.92° belong to the major -Cu3Fe4V6O26 (Lynosite) phase along with minor FeVO4 phase (phase region I) resulting in an increased crystallinity of the Lynosite structure. Unlike the XRD analysis discussed above, minor signals from the

Cu5V2O10 phase was recorded here. However, towards further increase in Cu content up to

59 at.%, the crystallinity of the Cu5V2O10 (Stoiberite) phase is recorded to increase at peak positions 2휃 angles 29.37°, 30.10°, 31.81°, 32.32°, 35.10°, 42.84°. In this region (II), a mixture of phases along with the major Stoiberite phase, minor FeVO4 and Lynosite structure was observed. The dominant Lynosite structure in the photoactive region I and Stoiberite structure in region II (inactive region) was confirmed by both standard and synchrotron XRD analysis.

. Figure 53. Synchrotron-XRD patterns from the line scan (marked with a white arrow in Figure 52d) along the photoactive to inactive region labeled with Cu content in at.% and photocurrent density in µA/cm2. 85

4.2.1.4 Incident photon-to-current efficiency (IPCE)

IPCE measurements were performed at different MAs on ML1 to investigate the photocurrents in dependence of the wavelength ranging from 250 to 650 nm. The IPCE spectra of five different MAs from ML1 with compositions such as (1) (Cu51Fe18V31)Ox, (2)

(Cu45Fe21V34)Ox, (3) (Cu40Fe15V45)Ox, (4) (Cu50Fe8V42)Ox, and (5) (Cu66Fe8V26)Ox are shown in Fig. 54 with an inset photocurrent density map at 1.63 V vs. RHE. The inset is labeled with the numbers (1-5) where the IPCE measurements took place. The measurements were recorded in 0.1 M sodium tetraborate solution (Na2B4O7) with a calibrated monochromatic light source. The IPCE result obtained from (2) (Cu45Fe21V34)Ox where the global maximum photocurrent density (108 µA/cm2) was recorded, exhibited an efficiency > 30% at 310 nm wavelength. For the region with local maximum photocurrents (⁓ 90 to 103 µA/cm2), marked with numbers 3 and 4 showed an efficiency > 12% at 300 nm wavelength. The efficiency of the PEC inactive regions (marked with 1 and 5) is almost zero. These results indicate the influence of the crystal structure present in the investigated regions such that region with the highest efficiency consists of the major α-Cu3Fe4V6O24 and minor FeVO4 phases whereas the region (1) and (5) with least IPCE values consists of the dominant Cu5V2O10 along with minor signals of FeVO4.

Figure 54. IPCE spectra showing PEC activity in dependence of the wavelength for Cu-Fe-V-O films

(ML1) at 1.63 V vs. RHE in 0.1 M Na2B4O7 (pH 9.3) with an inset photocurrent density map (at 1.63 V vs. RHE) plotted in XY coordinate axes highlight with numbers (1-5) where IPCE measurements were performed. 86

4.2.1.5 Optical properties of Cu-Fe-V-O MLs

The Cu-Fe-V-O MLs (ML1 and ML2) were screened using a high-throughput UV-Vis spectroscopy (optical) transmission test stand, (HOTT) by measuring 342 MAs of 3 mm x 3 mm with 4.5 mm distance between them (section 3.2.6). The MLs were prepared on fused silica wafers using the same fabrication parameters as described previously for the conducting substrate (Table 2 and 11). Eg values for each MA were manually obtained using OriginLab software by the extrapolation of the Tauc plots to determine the direct and indirect and indirect Eg. The Eg maps obtained from Tauc plot analysis are shown in Fig. 55a, b for the direct and indirect transition. The color-coded ternary plot reveals both the MLs to exhibit direct Eg ranging between 2.07 to 2.77 eV (Fig. 55a) and indirect Eg from 1.71 to 2.16 eV

(Fig. 55b) which is in the range of interest for PEC water splitting. The direct Eg range from 2.07 to 2.77 eV for ML1 and 2.0 to 2.48 eV for ML2 whereas for both MLs (ML1 and ML2), the indirect Eg corresponds to range between 1.71 to 2.16 eV and 1.78 to 2.07 eV respectively.

For ML1, the color-coded direct Eg map showed large Eg values (2.59 to 2.77 eV) at around

22 to 40 at.% V towards the dominant Stoiberite (Cu5V2O10) structure. However, the direct Eg value is observed to decrease down to 2.07 eV with an increase in V content up to 57 at.%

(ML1) consisting of the dominant β-Cu3Fe4V6O24 (Howardevansi). The direct Eg analysis for

ML2 reveals to be dependent on the Cu concentration. The direct Eg increased from 2.0 to

2.48 eV with variation in Cu content from 11 to 55 at.% (Fig. 55a). For the indirect Eg

(Fig. 55b), in ML1, a small Eg of 1.79 to1.94 eV was observed towards the highest photoactive region. However, the indirect Eg does not show much variation along the composition gradient or the identified crystal structures phase regions as observed for direct

Eg transition (see Fig. 55b). The enhanced PEC activity was recorded from ML1 showing two photoactive regions of interest such as (Cu45Fe21V34)Ox and (Cu50Fe8V42)Ox which corresponds to have 2.58 eV and 2.47 eV respectively for a direct Eg and both the hit samples showed ⁓ 1.87 eV indirect Eg. As shown in Fig. 55c, the Tauc plots from the highest 2 photoactive region of ML1 (Cu45Fe21V34)Ox exhibiting 108 µA/cm photocurrent density which corresponds to have direct Eg of 2.58 eV and 1.87 eV of indirect Eg with the major

α-Cu3Fe4V6O24 phase.

Figure 55. a) Direct (2.1 to 2.8 eV) and b) indirect (1.7 to 2.2 eV) Eg of Cu-Fe-V-O MLs determined from Tauc plot analysis using high-throughput UV-Vis spectroscopy (optical) transmission test stand. Each MA illuminated with an area of 3 mm x 3 mm with a 4.5 mm gap between each MA. c) Tauc plots of the direct and indirect transition of (Cu45Fe21V34)Ox highest photoactive sample results in Eg of 2.58 eV and 1.87 eV respectively.

4.2.1.6 Surface morphology of Cu-Fe-V-O MLs

SEM images were obtained from selected MAs highlighted in Fig. 56 with the white arrows in the ternary color-coded photocurrent plot for ML1 and ML2 of the Cu-Fe-V-O system. Fig. 56a shows SEM images obtained from ML1 starting with the highest photocurrent density (108 µA/cm2) region to the least photoactive region (< 10 µA/cm2). For the composition spread (Cu33-48Fe14-27V33-45)Ox, the morphology appears identical, exhibiting a

PEC activity from 60 to 108 µA/cm2. The morphology showed irregular shaped grains with an average size of ⁓ 180 to 220 nm. With a further increase in Cu content from > 50 to 72 at.%, smaller grains of with average size of ⁓ 100 to 120 nm are observed to be embedded into the matrix of the ⁓ 180 to 220 nm larger grains. Additionally, the number of smaller grains relatively increases towards the Cu-rich region showing the least PEC performance. In ML2, 100 to 103 nm size uniform circular-shaped grains were observed for Cu contents < 20 at.%. However, films with Cu contents from 20 to 55 at.% showed irregular-shaped grains similar to that of ML1 producing maximum photocurrents of only 40 µA/cm2. The photocurrent density can be influenced by the change in morphology of the film due to the presence of large surface area (ML1), although, the difference in the morphology of the film is not as strong as the photocurrents recorded in different regions. The influence of the morphology in the enhanced photoactive region seems to be minor when compared to the crystallinity of the dominant α-Cu3Fe4V6O24 phase see Fig. 51. However, it should be noted that the morphology and texture of sputtered thin films are not independent of each other, although, the deposition parameters such as deposition pressure, or rate or overall film thickness might influence the morphology and texture without changing the crystal structure.[39]

Figure 56. Selected SEM images from the Cu-Fe-V-O a) ML1 and b) ML2 at different compositions marked with white arrows in the ternary color-coded (composition-photocurrent) plot. The V, Fe, and Cu contents in at.% are labeled in the bottom left, top left, and bottom right respectively. The photocurrent density in µA/cm2 is labeled top right for each image.

4.2.2 Ti-Fe-V-O

4.2.2.1 Synthesis of Ti-Fe-V-O ML

The thin film system Ti-Fe-V-O system was investigated using CMS. The ternary systems Fe-V-O,[22,36,47,51] Fe-Ti-O,[173–175] and Ti-V-O[42,176] are well studied for PEC water splitting. [22,36,37,46,47] These systems have Eg values in the range between 1.9 – 2.1 eV for Fe-V-O, [174,177] [178] ⁓ 2.1 eV for Fe-Ti-O, and ⁓ 1.6 – 2.7 eV for V-Ti-O. The Eg for each ternary system lies within the desirable range of the solar spectrum for PEC water splitting. No reports are available on the investigation of the Ti-Fe-V-O system for PEC water splitting using CMS. Using combinatorial reactive magnetron sputtering, a thin-film Ti-Fe-V-O ML was fabricated at a deposition pressure of 0.66 Pa (see Table 2). The thin films were deposited in a reactive environment of 10 sccm O2 and 40 sccm Ar gas from 4-inch metallic targets of Ti, Fe, and V with 99.99% purity. Ti, Fe, and V targets were sputtered using DC, RF, and p-DC power supply respectively, and their sputter powers are listed in Table 12. In order to achieve a ML with a large composition gradient, the targets were placed confocal in the cylindrical deposition chamber with an inclination angle of 45° with respect to a static substrate and Fe and Ti were placed at opposite sides and V with an azimuthal angle of 90°. The films were fabricated at a deposition temperature of 400°C on a 100 mm diameter Pt (40 nm) / Ti (10 nm) / Si/SiO2 substrate with ⁓ 250 nm thickness. The as-deposited ML was annealed in air at 600°C for 5 h with a ramp rate of 5°C per min in a conventional furnace (Nabertherm) (see Table 3).

Table 12: List of deposition power and Ti, Fe, V composition spread.

Materials Deposition power (W) Ti content Fe content V content library DC (Ti) RF (Fe) p-DC (V) (at.%) (at.%) (at.%)

Ti-Fe-V-O 285 155 100 25-61 17-44 21-39

4.2.2.2 Compositional analysis

High-throughput EDX analysis revealed the composition gradient of the as-deposited Ti-Fe-V-O ML to be 25 to 61 at.% Ti, 17 to 44 at.% Fe, and 21 to 39 at.% V (Table 12). No significant change in the composition space was recorded for the annealed Ti-Fe-V-O library. Fig. 57 represents the (a) photograph of the as-deposited ML and (b, c, d) composition gradient on a spatial XY coordinate. The photograph of the ML is overlaid with the schematic

of the measurement grid that is 4.5 mm x 4.5 mm (black circles) where high-throughput EDX measurements were performed (Fig. 57a). The color-code is kept the same for all the elemental composition maps in order to see the elemental composition gradient from blue (low at.%) to red (high at.%) colors. The position of the metallic targets in the sputter chamber can also be determined from the color-coded composition maps such that the Ti-rich region (Fig. 57b) is recorded opposite to that of the Fe-rich region (Fig. 57c) and V-rich region (Fig. 57d) observed in between them. Thus, the fabrication of the

(Ti25-61Fe17-44V21-39)Ox ML was confirmed which was further investigated for structural and functional properties.

Figure 57. a) Photograph of the as-deposited Ti-Fe-V-O ML (100 mm diameter) with an overlay scheme of the MAs (black circles) and b) color-coded EDX composition maps of Ti, Fe, and V in at.%.

4.2.2.3 Composition-dependent PEC performance of Ti-Fe-V-O ML

PEC measurements were performed using a high-throughput OSDC setup[40] (section 3.2.7) by João R.C. Junqueira (CES, RUB) in an aqueous electrolyte of 0.1 M sodium tetraborate (pH 9.3) (see Table 4) at 1.63 V vs. RHE. Fig. 58 shows the steady-state photocurrent density map of the complete a) as-deposited and b) air annealed Ti-Fe-V-O ML. For as-deposited ML, the color-coded ternary diagram represents the local maximum photocurrent density of 2 ⁓ 64 µA/cm at (Ti33Fe42V25)Ox. This local maximum photocurrent region was observed from middle to top region on the ML as depicted from Fig. 58a. i) which is Fe-rich region (Fig. 57c). The upper half of the as-deposited ML is photoactive with photocurrents ranging between 35 to 64 µA/cm2 (Fig. 58a. i). After oxidizing the ML in air at 600°C for 5 h, the photocurrent density increased from 64 to 82 µA/cm2 as shown in Fig. 58b. However, for the annealed ML, the photoactive region revealed to shift towards the V-rich region (Fig. 57d) as observed from Fig. 58b. i. The global maximum photocurrent density of 82 µA/cm2 was obtained at (Ti29Fe34V37)Ox. Interestingly, the as-deposited (Fig. 58a. i) and annealed (Fig. 58b. i) ML showed similar local maximum photocurrent density (upper left region of the Ti-Fe-V-O film). Therefore, these results indicate that the post-deposition oxidation of the film might influence the structure and morphology which improves the PEC performance of the system.

4.2.2.4 Crystal structural analysis of Ti-Fe-V-O ML

As-deposited Ti-Fe-V-O ML

High-throughput XRD analysis of as-deposited Ti-Fe-V-O ML showed the presence of four different phase regions. Fig. 59a represents the XRD diffraction patterns from each phase region labeled with the Roman numerals I-IV (top right corner of Fig. 59a representing the structural information of the complete ML). Fig. 59b shows the color-coded ternary diagram of the crystal structure cluster in the composition coordinate axes. Each phase region also shows a Pt peak at 39.9° 2휃 marked with a star (*) (Fig. 59a). The diffraction pattern from phase region I is recorded to have several peaks with low intensity between 2휃 angles 16.5° to

40.0°. All the peaks present in the phase region I match to the triclinic FeVO4 phase (ICSD number: 10329). Phase region I is observed towards low Ti content that is from 25 to 46 at.% (Fig. 59b). Within phase region I, the local maximum photocurrent density of 62 µA/cm2 was recorded (see Fig. 58a). The color-coded weight maps from each phase region (components) are illustrated in Fig. 60.

Figure 58. Color-coded ternary plot of the photocurrent density maps of a) as-deposited and b) annealed Ti-Fe-V-O ML in the composition coordinate axes with the same color scale. Color-coded photocurrent density maps in XY coordinate axes for (a.i) as-deposited and (b.i) annealed ML shows a shift in the photoactive region after oxidizing the ML in the air at 600°C for 5 h. MAs with blue color in b) were damaged during annealing.

Fig. 60a shows the weight map from phase region I supporting the presence of the FeVO4 phase in the region with low Ti content (Fe-rich). For phase region II, along with the dominant FeVO4 phase, minor signals from the orthorhombic FeTi3O7 phase (ICSD number

190042) was observed. Peak intensity and the number of peaks matching to the FeVO4 phase are recorded to increase in phase region II as compared to phase region I which might be due to change in the composition of the film. The color-coded weight map for phase region II (Fig. 60b) shows the maximum weight towards low Ti content. Similar to region II, the 94

diffraction pattern of phase region III showed an increase in the peak intensity at 2휃 27.53° angle matching to the FeVO4 phase along with an increase in the intensity of the FeTi3O7 phase at 2휃 34.55° and 36.07° angles. Moreover, the composition spread covered by phase region III is similar to phase region II from 28 to 60 at.% Ti (Fig. 59b). Weight map obtained from region III (Fig. 60c) shows the maximum weight towards mid to high Ti content resulting in increased intensity of the FeVO4 and the FeTi3O7 phase. For region IV, a major peak matching to the FeTi3O7 phase was observed at 2휃 40.50° and other peaks matching to the FeVO4 phase were recorded with very low peak intensity. From the XRD clusters plot (Fig. 59b), phase region IV is recorded to overlap the composition spread from phase region II and III. Corresponding to the identified phases in the phase region IV, weight map (Fig.60d) confirms the presence of the maximum weight of the FeTi3O7 phase towards the Ti-rich region.

Figure 59. a) Four XRD patterns (휃/2휃 geometry) representing the phase information of the complete as-deposited Ti-Fe-V-O ML. b) Color-coded ternary XRD clusters of the as-deposited Ti-Fe-V-O ML showing different phase regions.

Figure 60. Color-coded weight maps of the four components (phase regions) shown in Figure 59 consisting of a) phase region I: FeVO4, b) phase region II: dominant FeVO4 and minor FeTi3O7, c) phase region III: mixed FeVO4 and FeTiO3 and d) phase region IV: dominant FeTiO3 and minor

FeVO4 with respective weight scale bar.

Air-annealed Ti-Fe-V-O ML

The as-deposited Ti-Fe-V-O ML was annealed in air at 600°C for 5 h with a ramp rate of 5°C/min. The annealed ML was measured again using high-throughput XRD mapping in θ/2θ geometry. As discernible from Fig. 61a, two diffraction patterns defining the structural character of the complete annealed ML with more pronounced crystallinity when compared to the as-deposited ML. Pt peak (conducting electrode layer) marked with (*) observed at 39.9° 2휃 angle for the annealed ML. Both the diffraction patterns are identified as (two) phase regions within the annealed ML labeled as regions I and II. Phase region I consist of the dominant FeVO4 phase at peak position 27.50° 2휃 angle (highest peak intensity) along with the several minor peaks matched to the aforementioned phase. Within phase region I, peaks

with low intensities are recorded from the FeTi3O7 phase at 35.24° and 36.12° 2휃 angles. The XRD clusters in Fig. 61b shows the presence of phase region I with red color. White circles in the XRD cluster ternary diagram are the MAs which were damaged after annealing in air due to the presence of Ag glue used for as-deposited PEC measurements. The global maximum photocurrent density ⁓ 82 µA/cm2 (Fig. 58b) was recorded in phase region I which covers low Ti content from 25 to 45 at.%. Phase region II (blue color) is recorded in the composition spread from medium to maximum Ti content (35 to 61 at.%). The crystallinity of the FeVO4 and FeTi3O7 phases was observed to decrease when compared to phase region I. The color- coded weight maps from both phase regions are shown in Fig. 62a, b. The maximum weight map for the region I was observed towards the V-Fe rich region (Fig. 62a) resulting to have the FeVO4 as the dominant phase in the region. In Fig. 62b, color-coded weight map from phase region II showed the maximum weight towards Ti-rich concentration. Due to reduced peak intensity and increased noise in the diffraction patterns, the annealed Ti-Fe-V-O ML was investigated using high-throughput synchrotron-XRD in order to identify all possible phases formed in the film.

Figure 61. a) Two XRD patterns representing the phase information of the complete air-annealed Ti-Fe-V-O ML labeled with the Roman numerals (I & II) phase regions. b) Color-coded ternary XRD clusters of the annealed Ti-Fe-V-O ML showing different phase regions in the MLs (MAs with white circles were damaged during annealing).

Figure 62. Color-coded weight maps of the two components (phase regions) shown in Figure 61 consisting of a) Region I: dominant FeVO4 and minor FeTi3O7 phase and b) Region II: mixed FeVO4 and FeTi3O7 with a respective weight scale bar.

Air-annealed Ti-Fe-V-O ML (Synchrotron-XRD)

The air-annealed Ti-Fe-V-O ML was screened using high-throughput synchrotron-XRD for the investigation of crystal structures. Synchrotron-XRD mapping revealed the presence of three crystal structures in the annealed Ti-Fe-V-O ML as shown in Fig. 63a. Three XRD patterns in Fig. 63a shows the presence of mixed crystal structures throughout the ML. The peaks marked with (*) belong to the Pt (conducting back electrode layer) for each diffraction pattern. Phase region I showed the presence of three different crystal structures: FeVO4,

FeTi3O7, and Fe2TiO5. The FeVO4 and the FeTi3O7 phases were already identified by standard-XRD measurement (see Fig. 61a) but identification of the orthorhombic Fe2TiO5 phase (ICSD number: 88379) was possible through synchrotron measurements. In phase region I, FeTi3O7 is recorded as the dominant phase with highest peak intensities at 33.73°, 36.07°, and 40.60° 2휃 angles. However, within this phase region (I), several peaks with low intensity are confirmed to match with the phases FeVO4 and Fe2TiO5. The color-coded ternary diagram of XRD clusters in Fig. 63b shows the presence of phase region I with yellow color circles covering the complete Ti composition gradient (25 to 61 at.%). Fig. 64a shows the color-coded weight map of phase region I, exhibiting the maximum weight intensity towards Ti-rich compositions, suggesting the FeTi3O7 as the dominant phase in the region.

For phase region II, the peak intensity of the phase FeVO4 was observed to increase at 27.74°

2휃 when compared to region I. Meanwhile, the peak intensities of the phases FeTi3O7 and

Fe2TiO5 were recorded to decrease. Within the XRD cluster ternary diagram (Fig. 63b), phase region II with a mixture of all identified phases are presented with red color circles having 30 98

to 58 at.% Ti content. Fig. 64b shows the color-coded weight map of phase region II covering the maximum weights for the top and bottom of the annealed ML. In phase region III, the diffraction pattern is recorded to be shifted towards a low 2휃 angle (see Fig. 63a). With the help of the Pt peak, it is possible to correct the peak positions of the film (this small shift might be an instrumental error caused during synchrotron measurement). The diffraction pattern in phase region III confirmed to have the FeVO4 as a major phase having the highest peak intensity at 27.74° 2휃. However, the peak intensities of Fe2TiO5 and FeTi3O7 phases decrease. The XRD cluster ternary diagram (Fig. 63b) confirmed that the global maximum photocurrent density (82 µA/cm2, Fig. 58b) was generated from phase region III (blue color circles). Moreover, the color-coded weight map (Fig. 64c) shows the highest weight intensity from phase region III towards V and Fe-rich regions.

Therefore, the high-throughput XRD patterns of the as-deposited and air-annealed Ti-Fe-V-O ML obtained from the standard- and the synchrotron-XRD revealed the presence of the major

FeVO4 phase along with the minor FeTi3O7 and Fe2TiO5 phase in the enhanced PEC active region.

Figure 63. a) Three synchrotron-XRD patterns representing the crystal structure information of the complete air-annealed Ti-Fe-V-O ML labeled with the Roman numeral (I - III) phase regions. b) Color-coded ternary XRD clusters of the annealed Ti-Fe-V-O ML with different phase regions (MAs with white circles were damaged during annealing).

Figure 64. Color-coded weight maps of three components (phase regions) shown in Figure 63 consists of a) Region I: dominant FeTi3O7 and minor FeVO4, Fe2TiO5 phases, b) Region II: mixed FeVO4,

FeTi3O7, Fe2TiO5 phases, c) Region III: dominant FeVO4 and minor Fe3TiO7, Fe2TiO5 phases with a respective weight scale bar.

4.2.2.5 Surface morphology of Ti-Fe-V-O ML

SEM images were taken from selected MAs of the as-deposited and annealed ML labeled with numbers (1-15) on the photograph of the Ti-Fe-V-O ML as shown in Fig. 65. The SEM images obtained from both the as-deposited and annealed ML are placed side to each other for direct comparison. Each SEM image is labeled with the MAs (top left) and the photocurrent density (top right). For as-deposited ML (Fig. 65a), MAs labeled as 1, 2, and 3 with Fe content ranging between 17 to 28 at.% consist of irregular shaped separated grains with an average size ranging from 500 to 800 nm. For 4 to 9 MAs, the irregular shaped grains size reduced to 315 to 450 nm with increased Fe content from 19 to 34 at.%. Moreover, the number of grains are observed to increase with increasing Fe content forming compact structure when compared to low Fe content region (1, 2 and 3 MAs). The photocurrent density in these regions (1 to 9 MAs) were recorded to range between 18 to 40 µA/cm2. Reduced

100

photoactivity could relate to the poor charge carrier transport due to the presence of large separated irregular shaped grains. For films with high Fe content (30 to 44 at.%), the surface morphology revealed to have dense grains of 100 to 200 nm size, as shown in MAs 10 to 15, Fig. 65a. The enhanced PEC activity (local maximum photocurrents, 62 µA/cm2) was measured in these regions. It is known that smaller grains result to have a large surface area which corresponds to have a large solid/electrolyte interface beneficial for charge carrier transport resulting to improve PEC performance of the film. However, for the annealed Ti-Fe-V-O ML (Fig. 65b), no distinct change in the shape and size was observed when compared to the as-deposited ML. The global maximum photocurrent density of 82 µA/cm2 was recorded from the annealed ML. It showed similar surface morphology as that of local maximum photocurrents (Fig. 65a) due to the presence major the FeVO4 phase along with minor signals from the iron titanate phases (discussed in the previous section).

Figure 65. SEM images of selected MAs of a) as-deposited and b) annealed ML highlighted in the photograph of the Ti-Fe-V-O ML. MA is labeled in the top left and the photocurrent density in µA/cm2 is labeled top right for each SEM image. 101

4.3 Screening and PEC stability test of M-V-O photoanode systems

4.3.1 Synthesis of W-V-O, Cr-V-O, and Co-V-O MLs

Potential M-V-O photoanode systems such as W-V-O, Cr-V-O, and Co-V-O were synthesized as thin-film MLs using combinatorial reactive magnetron sputtering (section 3.1.1) from 99.99% purity metal targets of W, Cr, Co, and V. During deposition, the sputter configuration such as 90° angle between the targets and 45° inclination angle between the targets and the stationary substrate was used. This configuration design was similar to the M-V-O (Fe-V-O or Cu-V-O, etc.) photoanode systems which enable to have a continuous spread of composition and thickness gradient (see Fig. 18). All M-V-O thin films were prepared on 100 mm diameter Si/SiO2 substrate at a deposition pressure of 0.66 Pa and a reactive gas pressure of

1.33 Pa (1:4 ratio of Ar/O2) listed in Table 2 of section 3.1.1. The process parameters of the M-V-O MLs are summarized in Table 13. All MLs were prepared at room temperature with consecutive annealing in air (see Table 3). 100 nm Pt back conducting electrode was deposited on Si/SiO2 substrate (10 nm Ti adhesion layer between Si/SiO2 surface and Pt layer at 100 W DC for Pt and 100 W DC for Ti sputter powers) for PEC measurements prior to each M-V-O thin film deposition.

Table 13. Deposition power supplied for the fabrication of W-V-O, Cr-V-O, and Co-V-O MLs.

M-V-O Deposition powers (W) Annealing system MLs temperature/duration

RF p-DC °C / h

ML W1 W: 163 V: 230 W-V-O 600 / 6 ML W2 W: 220 V: 170

ML Cr1 V: 330 Cr: 60 Cr-V-O 500 / 5 ML Cr2 V: 300 Cr: 110

ML Co1 V: 340 Co: 65 Co-V-O 600 / 5 ML Co2 V: 460 Co: 53

Two sets of MLs from each ternary M-V-O (W-V-O, Cr-V-O, and Co-V-O) system were fabricated and investigated in different electrolytic environments such as acidic, neutral, and alkaline. Firstly, PEC mapping was performed at a constant bias potential of 1.63 V vs. RHE in weak alkaline electrolyte: 0.1 M sodium phosphate (pH 8) for the Cr-V-O and Co-V-O 102

systems and 0.1 M sodium tetraborate (pH 9.3) for the W-V-O system. Post-PEC (in the weak alkaline electrolyte) treated MLs were further investigated in neutral electrolyte. The MLs were initially soaked in neutral media, 0.1 mol/L Na2SO4, pH 7 (12 h for W-V-O and 1 h for Cr-V-O and Co-V-O systems). PEC measurements were again performed on the stable regions of the MLs at 1.63 V vs. RHE in acidic media using pH 4.5, 0.1 M sodium perchlorate electrolyte. Table 4 lists the electrolyte, pH, and bias potential used for the PEC measurements for the W-V-O, Cr-V-O, and Co-V-O MLs. Prior to the PEC stability test measurements, each ML was screened using high-throughput characterization methods for compositional and structural analysis.

4.3.2 Photographs of M-V-O systems in different states

To understand the PEC stability of the systems W-V-O, Cr-V-O, and Co-V-O, the M-V-O MLs were tested in different electrolytic environments such as (i) PEC measurements in weakly alkaline electrolytes (pH 8 and 9.2), (ii) MLs were soaked in neutral electrolyte (pH 7) and (iii) PEC measurements performed in acidic electrolyte (pH 4.5). The photographs of the M-V-O MLs in different states such as as-deposited, air-annealed, post-PEC (pH 9.3: W-V-O MLs and pH 8: Cr-V-O and Co-V-O MLs), and MLs soaked in the neutral electrolyte (pH 7) are shown in Fig. 66. The as-deposited W-V-O and Co-V-O MLs were annealed in air at 600°C for 6 h (W-V-O MLs), 5 h (Co-V-O MLs) and 500°C for Cr-V-O MLs (5 h) at a ramp rate of 5°C/min. All the MLs were allowed to cool down naturally. The photographs of each as-deposited and annealed M-V-O system are presented in Fig. 66a, b, c for ML1_W, ML1_Cr, and ML1_Co respectively. Post-PEC images shown in Fig. 66 revealed that the M-V-O thin films were dissolved with the electrolyte drops from the PEC measurements in weakly alkaline (pH 8 and 9.3) electrolytes. For the W-V-O system, the films were completely dissolved after soaking the ML in pH 7 for 12 h (see Fig. 66a, post-soaked in the electrolyte). EDX analysis (Table 14) confirmed the presence of W and V in the white area of the ML1_W whereas the remaining surface showed the presence of Si from the substrate

(Si/SiO2). However, no change in the visual image of the ML1_Cr and ML2_Co was observed after soaking the ML in Ph 7 for 1 h (Fig. 66b, c post-soaked in the electrolyte). Therefore, the stable M-V-O MLs (Cr-V-O and Co-V-O) were investigated in acidic environment.

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Figure 66. Photographs of the M-V-O MLs: a) ML1_W, b) ML2_Cr and c) ML1_Co in different states: as-deposited (room temp), air annealed (T = 500°C for Cr-V-O; 600°C for W-V-O, Co-V-O), post-PEC treated (pH 8: Cr-V-O, Co-V-O; pH 9.3: W-V-O), and after soaking in neutral electrolyte (pH 7).

Table 14. Elemental composition of W-V-O ML1 at two different regions marked with 1 and 2 on the substrate after soaking in pH 7 (Figure 66a).

MAs Si O W V

(at.%) (at.%) (at.%) (at.%)

1 47 53 - -

2 65 25 0.48 0.22

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4.3.3 Compositional analysis of M-V-O MLs

As mentioned earlier, compositional and crystal structure mappings were performed on the annealed MLs before the PEC stability test measurements. High-throughput EDX measurements were performed to screen the compositional gradient within the MLs. To visualize the compositional space of the W-V-O system, the color-coded composition maps of ML1_W and ML2_W are presented in Fig. 67. The color scale is the same for W and V for both MLs in order to have a clear visualization of the composition spread within the system. The compositional map showed that ML1_W consists of 4 to 54 at.% W and 46 to 93 at.% V (Fig. 67a) whereas ML2_W showed a composition gradient of 23 to 76 at.% W and 24 to 77 at.% V (Fig. 67b). Similar composition gradients were obtained for the Cr-V-O and Co-V-O MLs presented in Fig. A4. The concentration of W, Cr, and Co ranges from 7 to 93 at.%, 24 to 83 at.%, and 23 to 80 at.%, respectively. Table 15 summarizes the overall compositional spread for all M-V-O systems.

Figure 67. Color-coded composition maps of a) ML1_W and b) ML2_W with W concentration (ai and bi) and V concentration (aii and bii) for W-V-O system.

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Table 15. List of compositional gradients for M-V-O MLs

M - V - O ML 1 (at.%) ML 2 (at.%)

W - V - O (W7-54 V46-93)Ox (W23-76 V24-77)Ox

Cr - V - O (Cr24-67 V33-76)Ox (Cr42-83 V17-58)Ox

Co - V - O (Co23-62 V38-77)Ox (Co40-79 V21-60)Ox

4.3.4 Composition, PEC stability test (alkaline electrolyte), and crystal structure analysis

The crystal structure is an intrinsic property determining the PEC activity of the photoanode system. The XRD patterns of the systems W-V-O, Cr-V-O, and Co-V-O are shown in Fig. A5. In this section, correlations between composition, crystal structure, and photocurrent density (measured in weakly alkaline electrolyte, pH 8 and 9.3) are discussed. A total composition spread for the W-V-O system ranging from 7 to 76 at.% (Fig. 68a) exhibited a maximum photocurrent density of ~ 7.1 µA/cm2 at 42 at.% W (Fig. 68b, ML1_W). However, towards maximum W content (76 at.%), the photocurrent density decreased to 0 µA/cm2 (see Fig. 68a, b, for ML2_W). The XRD patterns reveal the presence of two dominant phases (i) orthorhombic V2O5 (towards V-rich region) and tetragonal W5O14 (towards W-rich region) matching to the Pearson's crystal database: 1910043 and 1251922 respectively (see Fig. A5 a). The diffraction peak intensity color-maps of the identified dominant phases from both the

MLs are illustrated in Fig. 68c, d. The XRD intensity map of the V2O5 phase with (110) plane showed maximum intensity for low W content region (V-rich). However, the dominant W5O14 phase showed highest intensity for W-rich region, see Fig. 68d. As observed from Fig. 68 b, c, d, the W-V-O system exhibited the PEC active region only towards the maximum intensity of the W5O14 phase whereas the region with the highest intensity of the V2O5 phase showed almost zero photocurrents. Meyer et al. reported the W5O14 phase to be promising for [39] enhanced PEC performance of the Fe-W-O system. Moreover, V2O5 is considered to be the most stable vanadate phase, a good choice for enhancing the PEC behavior, and acts as a [179,180] catalyst. However, in the present case, the combination of the V2O5 and W5O14 phases does not seem to influence the PEC performance of the W-V-O systems.

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Figure 68. Color-coded maps of (a) W content, (b) photocurrent density measured in a weak alkaline electrolyte (pH 9.3) at 1.63 V vs. RHE, and (c and d) XRD peak intensities of the V2O5 and W5O14 phases for ML1_W and ML2_W of W-V-O system.

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The Cr-V-O MLs with a total composition spread of 24 to 83 at.% Cr produced low photocurrents of 0.80 µA/cm2 at 47 at.% Cr as shown in Fig. 69a, b. ML1_Cr exhibited < 1 µA/cm2 photocurrent density for < 50 at.% Cr concentration whereas the upper half region of the ML with Cr content > 51 at.%, the film was unstable due to which no photocurrents were detected. The structural analysis revealed the presence of two monoclinic crystal structures: (i) CrVO4 (1811684 Pearson’s Crystal database) and (ii) Cr2V4O13 (1224521 Pearson’s Crystal database) throughout the composition gradient (Fig. A5.b). The color-coded highest peak intensity maps from the respective phases are shown in Fig. 69c, d for both the MLs. Fig. 69c represents the color-map of the highest peak intensity of the

CrVO4 phase with (220) plane and Fig. 69d for the Cr2V4O13 phase with (002) plane orientation. Therefore, on comparing the intensity maps of the identified phases with the PEC activity (in alkaline media) of ML1_Cr, the region with the dominant Cr2V4O13 phase showed

PEC activity whereas the region with the dominant CrVO4 phase, no photocurrents were recorded. Similar behavior was observed for ML2_Cr with a maximum photocurrent density 2 of ⁓ 0.80 µA/cm towards the Cr2V4O13 phase. The peak intensity of the Cr2V4O13 phase decreases resulting in an increase in the intensity of the CrVO4 phase (Fig. A5.b) up to maximum Cr content of 83 at.% making more than half of the ML2 inactive (Fig. 69b, d).

Yan et al. reported a list of vanadate phases including Cr2V4O13 and CrVO4 exhibiting 0.139 mA/cm2 and 0.036 mA/cm2 photocurrents respectively for OER photoanodes.[22]

Moreover, the phase Cr2V4O13 is reported to be an efficient catalyst with photodegradation efficiency of 79%.[89] For Co-V-O, ML1_Co with 23 to 62 at.% Co (Fig. 70a) exhibited a photocurrent density of ⁓ 1 µA/cm2 (Fig. 70b). The highest photoactive region on ML1_Co consists of Co content ranging from 24 to 30 at.% whereas, with an increase in Co content up to 62 at.%, more than half of the ML was observed to be inactive. Similarly, ML2_Co, the ML was recorded to be completely inactive for the composition gradient from 30 to 80 at.% Co (see Fig. 70a, b). XRD analysis revealed the presence of three Co vanadate phases such as the monoclinic

CoV2O6 (1705986 Pearson's crystal database) towards the low-Co rich region, the monoclinic

Co2V2O7 (543605 Pearson’s crystal database) for Co mid to rich region, and the orthorhombic

Co3V2O8 (1900786 Pearson’s crystal database) towards Co-rich region (Fig. A5 c). The color- coded intensity maps of the three cobalt vanadate phases exhibited the highest peak intensity from ML1_Co as shown in Fig. 70c, d, e for CoV2O6, Co2V2O7, and Co3V2O8 phases. The

CoV2O6 phase showed a maximum peak intensity towards the PEC active region of ML1_Co (Fig. 70b, c) influencing the PEC behavior, though the photocurrents recorded are not enough

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to consider it to be a promising system. The regions with the dominant Co2V2O7 and Co3V2O8 phases exhibited photocurrents < 1 µA/cm2. However, the literature survey revealed the [93,94] Co3V2O8 phases to be highly active and stable for OER.

Figure 69. Colo-coded maps of (a) Cr content, (b) photocurrent density measured in a weak alkaline electrolyte (pH 8) at 1.63 V vs. RHE, and (c and d) XRD peak intensities of Cr2V4O13 and CrVO4 phases for ML1_Cr and ML2_Cr for Cr-V-O system. 109

Figure 70. Color-coded maps of ML1_Co and ML2_Co having (a) Co content, (b) photocurrent density measured in a weak alkaline electrolyte (pH 8) at 1.63 V vs. RHE respectively. XRD peak intensities of (c) CoV2O6, (d) Co2V2O7 and (e) Co3V2O8 phases from ML1_Cr for the Co-V-O system.

4.3.5 PEC stability test in acidic electrolyte

To assess the stability of the M-V-O systems, PEC measurements were firstly performed using weakly alkaline electrolytes (section 4.3.1.4) and then the MLs were soaked in neutral electrolyte (section 4.3.1.2). Thereafter, the second PEC measurements were performed on the post-electrolyte soaked MLs operated in an acidic electrolyte (pH 4.5) on the stable regions of the Cr-V-O and Co-V-O MLs. For Cr-V-O MLs (Fig. 71a, b), the maximum photocurrent 110

density of ⁓ 5 µA/cm2 at 1.63 V vs. RHE was recorded which was found to be higher than the one measured in an alkaline electrolyte (see Fig. 69b). Interestingly, the photoactive regions remain the same for both the alkaline and the acidic electrolytes, just the photocurrents were enhanced. For the Cr-V-O system, an acidic electrolyte was found to be suitable for PEC measurement. For the Co-V-O system (Fig. 71c, d), ML2_Co exhibited a larger PEC active region when compared to the photoactive region operated in an alkaline electrolyte (Fig. 71b). No change in photocurrents values was observed on comparing the PEC results from alkaline and the acidic environment. Under these conditions, sodium perchlorate (pH 4.5) an acidic electrolyte is considered suitable for PEC measurements of Cr-V-O and Co-V-O systems.

Figure 71. Color-coded photocurrent density maps of (a and b) Cr-V-O MLs and (c and d) Co-V-O MLs operated in acidic electrolyte, pH 4.5 at 1.63 V vs. RHE.

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Apart from the presented M-V-O systems, one Fe-V-O ML showed an interesting behavior after performing PEC measurements at 1.63 V vs. RHE in pH 8. The ML was annealed at 600°C for 6 h in air (ramp 5°C/min). Fig. 72 shows the photographs of a) annealed, b) post- PEC, c) color-coded map of Fe content and d) photocurrent density at 1.63 V vs. RHE for Fe-V-O ML. The MAs where PEC measurements were performed are clearly visible due to the electrolyte drops towards the lower half of the ML with Fe content ranging from 17 to 28 at.% (low Fe content), whereas the upper half of the ML (Fe-rich region) seems to be stable. This transition from unstable to a stable region on the library indicates to be composition-dependent (Fe content). The stable Fe-rich region exhibited a comparatively enhanced photocurrent density of ⁓ 21 µA/cm2 when compared to the unstable low Fe composition region (> 10 µA/cm2) which is in agreement with the work discussed in section 4.1.1 for the Fe-V-O screening section.[36]

Figure 72. Photographs of a) annealed, b) post-PEC treatment, c) color-coded maps of Fe content and d) photocurrent density for Fe-V-O ML. 112

4.4 Summary

In this thesis, multinary vanadate systems: Fe-V-O, Cu-V-O, Ag-V-O, W-V-O, Cr-V-O, Co-V-O, Cu-Fe-V-O, and Ti-Fe-V-O were investigated via combinatorial synthesis and high- throughput methods. For the first part of the thesis, each ternary M-V-O system (M: Fe, Cu, Ag) demonstrated a correlation between compositional, structural, and functional properties. All three M-V-O systems revealed to have composition-dependent structure and PEC performance. The Fe-V-O materials system exhibited composition regions of interest ranging from (Fe54V46)Ox to (Fe66V34)Ox. Within this region, the presence of the major FeVO4 and minor Fe2O3 phases were confirmed via structural analysis (XRD). The indirect Eg of

Fe2V4O13, FeVO4, and Fe2O3 dominant phase regions ranged from 1.92 to 2.49 eV. The optical properties and PEC activity are reported[161] to be influenced by the electronic structure of the film. The conduction band of FeVO4 and Fe2O3 is observed to be near their [47] flat band positions that is -0.15 V and -0.10 V respectively. The direct transition for FeVO4 (⁓ 2.80 eV) is probably due to charge transfer from O 2p-orbital to V 3d-orbitals and indirect transition (2.04 eV) is determined from O 2p-orbitals and Fe 3d-orbitals.[161] The highest photoactive region (Fe61V39)Ox consists of the dominant FeVO4 phase and ⁓ 2.04 eV indirect

Eg which should allow the visible light absorption up to 600 nm wavelength. Additionally, wavelength-dependent photocurrents in this photoactive region showed 7% IPCE values with an onset wavelength at 470 nm. PEC limitations like low efficiency and less light absorption can be ameliorated by thicker films that can sufficiently absorb a large portion of visible light from the solar spectrum.[161] Another factor that plays an important role to improve PEC performance is crystal structure and crystallinity of the material. The Fe-V-O system showed mixed-phase regions throughout the MLs and exhibited maximum photocurrents of 2 190 µA/cm at FeVO4 (major) and Fe2O3 (minor) phases which might be improved by preparing pure FeVO4 phase photoanodes with increased crystallinity resulting to increase the absorption coefficient. In Fe2O3 bulk recombination occurs due to small space-charge width and poor intrinsic hole-transport property. This limitation could be overcome by nanostructured surface morphology (see Fig. 31, A2) which can efficiently reduce the migration length for photogenerated holes (minority carriers) and increases surface area resulting to suppress the charge recombination.[161] To understand the functional properties of the Cu-V-O system, MLs with Cu and V composition gradients were deposited. Five Cu vanadate mixed phases were identified across the composition gradient: (I) CuV2O6 (major) and β-Cu2V2O7 (minor), (II) β-Cu2V2O7 (major) and α-Cu2V2O7 (minor), (III) α-Cu2V2O7 (major) and β-Cu2V2O7, Cu11V6O26, γ-Cu3V2O8 113

(minor), (IV) Cu11V6O26 (major) and α-Cu2V2O7, γ-Cu3V2O8 (minor), (V) Cu5V2O10 (major) and Cu11V6O26, γ-Cu3V2O8 (minor). All five phase regions correspond to have an indirect Eg ranging between 1.65 to 2.33 eV. The Eg values of Cu vanadate phases correspond to have high absorption coefficients with increasing Cu:V ratio which results in an increased partial density of Cu states at the conduction band minimum demonstrating higher optical transition possibility.[165] The valence band edge of the Cu vanadate phases are derived from hybridization of Cu 3d9 with O 2p-orbitals and conduction band edges are formed from V 3d- [67,165] orbitals. Among all identified Cu vanadate phases, the dominant α-Cu2V2O7 phase (III) 2 exhibited the highest photocurrent density (170 µA/cm ) at (Cu50-55V45-50)Ox. Region IV with 2 the dominant Cu11V6O26 phase (Cu-rich) exhibited an enhanced photocurrent of 100 µA/cm for lower film thickness 260 to 280 nm but the films with (dominant Cu11V6O26 phase) higher thickness (220 to 656 nm) exhibited zero PEC activity due to loss of holes reaching the surface of thick films causing strong recombination of photogenerated charge carriers. The surface morphology with respect to the Cu:V ratio showed column-like structures to large interconnected irregular shaped grains to small compactly packed uniform shaped grains.

Cu-rich phases, γ-Cu3V2O8 and Cu11V6O26 are considered interesting photoanodes but do not exhibit the highest photocurrent unlike reported in[61,64,165] suggesting the importance of crystallinity of the Cu-vanadate phases as these phases showed very low signal intensities in XRD diffraction patterns. The Pourbaix diagram of the Cu-V-O generated by [71] materialsproject.org showed Cu(s) and VO4(s) to be stable above pH 7. The complete findings from the Cu-V-O MLs are summarized in Table 16 confirming this photoanode system suitable for solar water splitting. Table 16. List of major and minor phases in phases, Cu composition, phase region identity, Bandgap energies (Eg), and photocurrents of five Cu vanadate phases identified in the Cu-V-O systems.

Cu Bandgap energy Photocurrent Major phase content Minor phase (eV) density Direct Indirect (at.%) (µA/cm2)

I - CuV2O6 18 – 40 Ziesite 2.38 2.33 0 – 70

II - β-Cu2V2O7 33 – 50 Blossite 1.90 – 2.28 1.65 – 2.23 40 - 80 (Ziesite) III - α-Cu2V2O7 47 - 63 Ziesite, 1.77 – 1.98 1.72 – 1.94 80 - 170 (Blossite) Fingerite IV – Cu11V6O26 54 - 76 Blossite, 2.03 – 2.65 1.84 0.03 - 100 (Fingerite) McBirneyite (γ-Cu3V2O8) V - Cu5V2O8 62 – 84 Fingerite, 2.68 – 2.78 1.92 – 1.96 0.001–0.05 (Stoiberite) McBirneyite 114

Further, MLs containing Ag-V-O were deposited. Corresponding to Fe-and Cu-V-O materials systems, four Ag-vanadate phases (AgV6O15, Ag2V4O11, AgVO3, and Ag4V2O7) were identified along the composition gradient. The Ag-V-O materials system with

(Ag30-45V55-70)Ox composition range exhibited the enhanced photocurrent density ranging from ⁓ 300 to 554 µA/cm2 along the thickness gradient. This photoactive region consists of the dominant Ag2V4O11 and minor phases AgV6O15 and AgVO3. The Ag-vanadate phases correspond to have an indirect Eg ranging from 1.4 to 2.7 eV. The optical properties can be influenced by the electronic structure of the system. The conduction band of the identified Ag-vanadate phases is derived from V 3d- and Ag 5s-orbitals whereas the valence band is formed from the hybridized Ag 4d- and O 2p-orbitals resulting to have narrow Eg and allows visible light absorption as the hybridized orbitals of the valence band are positioned towards more positive energy level than O 2p-orbitals.[85,172] Apart from the electronic structure of the Ag-V-O system, surface morphology is considered to influence the PEC performance of the system. The morphology changed from nanoscale structure to porous nanowires to the clustered nanorod-like structure across Ag:V composition ratio. The film with the dominant

Ag2V4O11 phase corresponds to have a nanowire-like structure which is considered to improve the PEC behavior of the system due to the presence of large surface area leading to increase in the number of active sites and efficiently acquire charge separation at the photoelectrode surface. Detailed analysis of photoactive region via high-throughput synchrotron XRD and TEM analysis confirmed the presence of the major phase Ag2V4O11 as well as the phases with low crystallinity AgV6O15 and AgVO3. Two quaternary metal vanadate systems (M2-M1-V-O) Cu-Fe-V-O and Ti-Fe-V-O and their PEC behavior in terms of composition, structural and functional property were investigated. Cu-Fe-V-O MLs were deposited to discover new potential PEC materials by combining two photoactive Fe-V-O and Cu-V-O systems. The Cu-Fe-V-O materials system exhibited two compositional regions of interest (Cu53Fe7V40)Ox and (Cu45Fe21V34)Ox. By utilizing machine learning algorithms in htAx based analysis,[147] 7 mixed ternary and quaternary vanadate phase regions: (I) -Cu3FeV6O26 / FeVO4, (II) Cu5V2O10 / FeVO4 / -Cu3Fe4V6O26, (III)

Cu5V2O10, (IV) Cu5V2O10 / FeVO4, (V) FeVO4 / γ-Cu2V2O7 / -Cu3Fe4V6O26, (VI) -Cu2V2O7

/ -Cu3Fe4V6O26 / FeVO4, and (VII) -Cu3Fe4V6O26 / FeVO4 were identified. The indirect Eg of Cu-Fe-V-O is found to range from 1.71 to 2.16 eV which lies within the desirable range for [22] the absorption of the solar spectrum. The indirect Eg for both the regions of interest correspond to have 1.87 eV. Materials composition (Cu45Fe21V34)Ox with the dominant

-Cu3FeV6O26 (Lynosite) phase (I) exhibits the highest photocurrent density up to 115

108 µA/cm2. The wavelength-dependent photocurrent density shows an efficiency (IPCE) of 30% at 310 nm which is much higher than the efficiency from the ternary Fe-V-O. Within the Cu-Fe-V-O ML, regions exhibiting photocurrents < 5 µA/cm2 consists of dominant phases

Cu5V2O10 (Stoiberite) and β-Cu3Fe4V6O24 (Howardevansi structure). The Cu5V2O10 phase is considered to suppress the PEC performance due to “surface recombination whose probability is high for Cu-rich phases that is electronically active surface states are associated with Cu cations at the photoanode surface”.[165] The Ti-Fe-V-O materials system was deposited to study the structural and functional behavior with respect to deposition (400°C) and annealing (600°C) temperatures. Using machine learning algorithms in htAx based analysis,[147] the as-deposited ML was identified to have four phase regions consisting of FeVO4 and FeTi3O7 phases. A maximum photocurrent 2 density of 64 µA/cm was recorded for as-deposited ML from (Ti33Fe42V25)Ox composition corresponding to have FeVO4 phase. For annealed ML, the photocurrent density increased up 2 to 82 µA/cm at (Ti29Fe34V37)Ox due to an increased crystallinity of the identified phases. Feng et al. reported to increase the absorption coefficient by engineering the crystallinity which results to improve the PEC performance.[161] Structural analysis of annealed ML performed via high-throughput synchrotron XRD revealed the presence of three phase regions consisting of the major FeVO4 and minor FeTi3O7 and Fe2TiO5 phases within the photoactive region. No significant change in the surface morphology of the as-deposited and annealed ML was observed. The PEC active regions in both states (as-deposited and annealed) showed dense grains of 100 to 200 nm size resulting to have a large surface area which is an important factor to enhance the PEC performance of Ti-Fe-V-O materials system. To this end, metal vanadate systems M-V-O (M: W, Cr, Co) were deposited to investigate the PEC stability tests in different electrolytic environments such as acidic, neutral, and alkaline. PEC stability test was started with the photocurrent measurements using weakly alkaline electrolytes, pH 8 (Cr-V-O and Co-V-O MLs) and pH 9.3 (W-V-O MLs). The W-V-O materials system exhibited a maximum photocurrent of ⁓ 7 µA/cm2 at 42 at.% W whereas for the Cr-V-O and Co-V-O materials systems < 1 µA/cm2 photocurrents were observed. For the second PEC stability test, each M-V-O ML (post-PEC treated in weakly alkaline electrolyte) was soaked in the neutral electrolyte (pH 7) for 12 h for W-V-O and 1 h for Cr-V-O and Co-V-O respectively. Due to poor adhesion of the W-V-O thin films with the substrate, the films were removed when soaked in the neutral electrolyte, however, the Cr-V-O and Co-V-O thin films remain stable. The MLs with stable thin films (Cr-V-O and Co-V-O) were measured using an acidic electrolyte, pH 4.5. The photocurrents were recorded to reach up to

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5 µA/cm2 for the Cr-V-O MLs whereas, for the Co-V-O MLs, no change in the photocurrent density was observed, however, a larger photoactive region was covered when compared to the photoactive region observed in alkaline medium. The structural analysis confirmed the presence of the tetragonal W5O14, monoclinic Cr2V4O13, and monoclinic CoV2O6 phases towards the PEC active regions within the W-V-O, Cr-V-O, and Co-V-O MLs respectively. The PEC stability test results suggest that the investigated M-V-O systems are more stable in the acidic electrolyte of pH 4.5 than the alkaline electrolyte (pH 8 and 9.3). Moreover, the enlarged photoactive region was recorded when measured in acidic electrolyte. However, under these investigated conditions, the obtained photocurrents are not high enough to consider M-V-O, M: W, Cr, Co photoanodes as promising for water splitting. In conclusion, this work presents the screening of vanadium-based metal oxides for PEC water splitting using combinatorial synthesis and high-throughput methods for the rapid analysis of large datasets. Out of several multinary vanadate systems Fe-V-O, Cu-V-O, Ag-V-O, W-V-O, Cr-V-O, Co-V-O, Cu-Fe-V-O, and Ti-Fe-V-O characterized via high- throughput EDX, XRD, UV-Vis, SEM, and OSDC methods, Fe-V-O, Cu-V-O, Ag-V-O, Cu-Fe-V-O, and Ti-Fe-V-O are identified to be leading materials with promising functional properties.

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Chapter 5 Conclusions

In this thesis, the combinatorial materials science approach was used to investigate the ternary and quaternary vanadium-based materials systems for PEC water splitting. Starting with the screening of ternary metal vanadium oxide systems, the Fe-V-O materials system with (Fe10-79V21-90)Ox composition spread revealed to have a composition-dependent crystal structure and PEC behavior. The crystal structure changed with an increase in Fe content from Fe2V4O13 → FeVO4 → Fe2O3. The highest photocurrent region at 61 at.% Fe corresponds to have the major FeVO4 and minor Fe2O3 phases with an indirect Eg of 2.04 eV.

This narrow Eg as well as increased crystallinity of the FeVO4 phase helps in a large range of visible light absorption. An efficiency of 7% (IPCE) was recorded for the FeVO4 dominant phase whereas < 2% efficiency was observed by the dominant Fe2V4O13 phase.

For Cu-V-O materials system, MLs with (Cu19 –84V16-81)Ox composition gradient corresponds to have five phase regions from low Cu to Cu-rich region: CuV2O6 → β-Cu2V2O7 →

α-Cu2V2O7 → Cu11V6O26, γ-Cu3V2O8 → Cu5V2O10. The highest photoactive region was recorded close to (Cu50-55V45-50)Ox composition along the thickness gradient (220 to 656 nm).

Crystal structure analysis revealed the presence of a major α-Cu2V2O7 and minor mixture of phases β-Cu2V2O7, Cu11V6O26, γ-Cu3V2O8 in the highest photocurrent region. This region of interest corresponds to have an indirect Eg of 1.94 eV which can significantly absorb ⁓ 600 nm wavelength of visible light. Another ternary metal vanadate system, Ag-V-O was investigated. Corresponding to the

Fe-V-O and Cu-V-O systems, four crystal structures AgV6O15 → Ag2V4O11 → AgVO3 →

Ag2V4O were identified across the composition (Ag22-77V23-78)Ox spread. The analysis of SEM images revealed the morphology to change from dense nanoscale structure → porous nanowire-like structure → clustered nanorod-like structure with an increase in Ag content.

The highest photocurrent region consists of the dominant Ag2V4O11 and minor AgV6O15,

AgVO3 phases exhibiting an indirect Eg of 1.87 eV. From theoretical calculations, Ag2V4O11 exhibited to have highly mobile charge carriers which promote charge transportation in the system.[85] Additionally, the porous nanowire-like structure showed improved PEC performance due to enlarged surface area which increases the number of active sites resulting in sufficient charge separation at the surface. The results obtained from ternary metal vanadates motivated to investigate the quaternary metal vanadium oxide systems: Cu-Fe-V-O and Ti-Fe-V-O. The structural analysis of both

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the systems were investigated using machine learning algorithm based htAx and phase identification software. Cu-Fe-V-O MLs showed two photoactive regions, (Cu53Fe7V40)Ox 2 2 and (Cu45Fe21V34)Ox exhibiting 103 µA/cm and 108 µA/cm photocurrent density respectively. Structural analysis using htAx[147] identified mixed vanadate regions phases:

-Cu3FeV6O26 / FeVO4 (I), Cu5V2O10 / FeVO4 / -Cu3Fe4V6O26 (II), Cu5V2O10 (III), Cu5V2O10

/ FeVO4 (IV), FeVO4 / γ-Cu2V2O7 / -Cu3Fe4V6O26 (V), -Cu2V2O7 / -Cu3Fe4V6O26 / FeVO4

(VI), and -Cu3Fe4V6O26 / FeVO4 (VII). Materials composition with the dominant

α-Cu3Fe4V6O24 phase and minor FeVO4 phase showed the highest photoactivity and exhibited an IPCE value > 30%. Using UV-Vis spectroscopy, both photoactive regions showed an indirect Eg of 1.87 eV. However, due to the presence of the Cu-rich phase Cu5V2O10, the PEC performance is reduced because of increased surface recombination at the Cu-rich region. Therefore, another materials system without Cu was fabricated. Ti-Fe-V-O was investigated using high-throughput methods both prior and post annealed of the ML. As-deposited ML (400°C) exhibited a photoactive region close to (Ti33Fe42V25)Ox composition whereas the annealed ML (600°C) showed increased photocurrent density close to (Ti29Fe34V37)Ox compositions. Both the photoactive regions consist of the major FeVO4 phase and minor Fe-titanate phases FeTi3O7 and Fe2TiO5, analyzed via high-throughput synchrotron XRD analysis. No significant change in the surface morphology of the as-deposited and annealed ML was recorded. This finding confirmed the importance of the crystal structure and crystallinity of the FeVO4 phase in the film which results in the enhancement of the PEC performance. Screening of vanadium-based systems motivated to investigate the M-V-O (M: W, Cr, Co) systems in different electrolytic environments like acidic, neutral, and alkaline in order to perform the PEC stability test. The PEC stability test was performed in three steps. Firstly, the PEC measurements were carried out using a weakly alkaline electrolyte and thereafter, secondly, the materials systems were soaked in the neutral electrolyte. Lastly (third PEC stability step), the stable MLs (Cr-V-O and Co-V-O) were measured using an acidic electrolyte for performing PEC measurements. PEC measurements performed in different aqueous media exhibited significantly low photocurrents at an overpotential of 1.63 V vs. RHE. These results suggested the W-V-O, Cr-V-O, and Co-V-O MLs are not suitable for water oxygen under investigation conditions.

Summary of the thesis is listed in Table 17 consisting of all investigated materials systems, identified phases, highest photocurrent density at 1.63 V vs. RHE, and indirect Eg.

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Table 17. List of materials systems, identified phases, photocurrent density, and indirect bandgap energies (Eg) of all investigated systems in this thesis.

Photocurrent Bandgap S.No Materials systems Identified phases density energy (µA/cm2) (eV) 1 (Fe10-79V21-90)Ox Fe2V4O13, FeVO4, Fe2O3 190 1.92 - 2.49

CuV2O6, -Cu2V2O7,

2 (Cu19-84V16-81)Ox -Cu2V2O7, Cu11V6O26, 170 1.65 - 2.33

Cu3V2O8, Cu5V2O10

AgV6O15, Ag2V4O11, AgVO3, 3 (Ag22-77V23-78)Ox 500 1.4 - 2.7 Ag4V2O7 3.15 - 3.28 4 (W7-76V24-93)Ox W5O14, V2O5 7 [88] 2.27 - 2.30 5 (Cr24-83V17-76)Ox CrVO4, Cr2V4O13 5 [22] 2.22 - 2.25 6 (Co23-79V21-77)Ox CoV2O6, Co2V2O7, Co3V2O8 1 [22]

-Cu3Fe4V6O26,

7 (Cu11-72Fe4-73V12-57)Ox -Cu3Fe4V6O26, Cu5V2O10, 108 1.71 - 2.16

FeVO4, γ-Cu2V2O7, -Cu2V2O7 1.9 - 2.1 8 (Ti25-61Fe17-44V21-39)Ox FeVO4, FeTi3O7, Fe2TiO5 82 [174]

The present work shows the advantage of using combinatorial materials science for the identification of materials for PEC application which not only minimizes the fabrication cost but also reduces the time and energy consumption. Among eight multinary vanadium-based materials systems, Fe-V-O, Cu-V-O, Ag-V-O, Cu-Fe-V-O, and Ti-Fe-V-O with promising

PEC performance confirmed FeVO4, -Cu2V2O7, Ag2V4O11 and -Cu3Fe4V6O26 photoanodes as potential candidates to efficiently perform PEC water splitting reaction. Therefore, for the extension of this work, these identified candidates can be used as the base material for other multi-element systems (e.g.: M-FeVO4, M is metal) using combinatorial and high-throughput methods towards the search for novel PEC materials.

120

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Appendix

1 List of abbreviations and symbols

CMS - Combinatorial material science hυ - Photon energy PEC - Photoelectrochemical e- - electrons M-V-O - Metal vanadium oxides h+ - holes ML - Materials library 휂푒−/ℎ+ - Efficiency (electron – hole pair) PVD - Physical vapour deposition ηtransport – Efficiency (charge transport) CVD - Chemical vapour deposition ηinterface – Efficiency (solid/liquid interface) NHE - Normal hydrogen electrode ηSTH – Standard hydrogen electrode efficiency SHE - Standard hydrogen electrode ΔE° - Thermodynamic potential = 1.229 V RHE – Reversible hydrogen electrode at.% - Atomic percentage OER - Oxygen evolution reaction Ec – Conduction band edge HER - Hydrogen evolution reaction Ev – Valance band edge EDX – Energy dispersive X-ray spectroscopy Eg – Bandgap energy XRD – X-ray diffraction EF – Fermi level SEM – Scanning electron microscope H2O - Water AFM – Atom force microscope α – Absorption coefficient UV-Vis – Ultraviolet - visible spectroscopy Å - Armstrong HOTT – High-throughput optical test stand °C – Degree Celsius OSDC – Optical scanning droplet cell λ - Wavelength IPCE - Incident photon-to-current efficiency jphoto – Photocurrent density OCP – Open circuit potential 훥퐺° - Standard Gibbs free energy LSV – Linear sweep voltammogram F - Faraday’s constant = 96485 c/mol TEM – Transmission electron microscope FIB – Focused ion beam STEM – Scanning transmission electron miscroscope SAD – Selected area diffraction patterns HAADF – High angle annular dark field RT – Room temperature SCCM – Standard cubic centimetres per minute Mtoe - Megatonnes of oil equivalent Pa – Pascal E – Electric field B – Magnetic field DC – Direct current p-DC – Pulsed direct current RF – Radio frequency

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2 Additional Information

Color-coded XRD peak intensity maps of the major Fe-V-O phases

Figure A1. Color-coded maps of main XRD peak intensities of the FeVO4 and the Fe2V4O13 phases of all three materials libraries.

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SEM images of the Fe-V-O materials libraries along the composition and thickness gradients

Figure A2. SEM images of all Fe-V-O MLs along composition and thickness gradients. The Fe content is labeled top left, photocurrent density top right, thickness bottom left, and bottom right is the image position on the sample.

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Photographs of annealed Ag-V-O materials libraries

Figure A3. Photograph of annealed Ag-V-O a) ML1 and b) ML2 with color variation along the composition gradient due to presence of different Ag vanadate phases.

Color-coded composition maps of Cr-V-O and Co-V-O for ML1 and ML2

Figure A4: Color-coded composition maps of a) ML 1 and b) ML 2 for M-V-O (M: Cr & Co) systems with Cr and Co concentration (ai & bi) and V concentration (aii & bii) in at.% .

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XRD mapping of W-V-O, Cr-V-O, and Co-V-O materials libraries

Figure A5. XRD patterns of 10 selected MAs through the center of each ML representing the overall compositions spread for the M-V-O MLs: (a) W-V-O (b) Cr-V-O, (c) Co-V-O.

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3 Publications and presentations

Publication list 1. S. Kumari, R. Gutkowski, J. R.C. Junqueira, A. Kostka, K. Hengge, C. Scheu, W. Schuhmann, A. Ludwig ‘Combinatorial synthesis and high-throughput characterization of Fe-V-O thin-film materials libraries for solar water splitting’ ACS Combinatorial Science, 2018, 20, 544-553.

2. S. Kumari, C. Khare, F. Xi, M. Nowak, K. Sliozberg, R. Gutkowski, P. S. Bassi, S. Fiechter, W. Schuhmann, A. Ludwig ‘Combinatorial search for new solar water splitting photoanode materials in the thin-film system Fe-Ti-W-O’ Zeitschrift für Physikalische Chemie, 2019, DOI: 10.1515/zpch-2019-1462.

3. S. Kumari, L. Helt, J. R.C. Junqueira, A. Kostka, S. Zhang, S. Sarker, A. Mehta, C. Scheu, W. Schuhmann ‘High-throughput characterization of Ag-V-O nanostructured thin-film materials libraries for photoelectrochemical solar water splitting’ International Journal of Hydrogen Energy, 2020, 45, 12037-12047.

4. S. Kumari, J. R. C. Junqueira, S. Sarker, A. Mehta, W. Schuhmann, A. Ludwig ‘Assessment of Structural and Photoelectrochemical Properties of Cu-Fe-V-O Thin Films’, Submitted in Journal of Chemical Physics, 2020. Conference and seminar presentations 1. Exploration of Multinary Vanadate Thin-Film Systems for Solar Water Splitting using Combinatorial and High-throughput Methods, International Bunsen-Discussion- Meeting, 2019, Taormina, Italy 2. Investigation of V-Ag-O Thin-Film Materials Libraries for Solar Water Splitting by Combinatorial Synthesis and High-Throughput Characterizations methods, IMPRS- SurMat Seminar 2019, Schloss Ringberg, Germany 3. Thin-film combinatorial materials science for the design of materials, Institute day, 2019, Ruhr-Universität Bochum, Germany 4. Investigation of Vanadium-Metal-Oxide Thin-Film Materials Libraries for Solar Water Splitting, nanoGe Fall Meeting 2018, Torremolinos, Spain 5. Combinatorial Synthesis and High-Throughput Characterization of Fe-V-O Thin-Film Materials Libraries for Solar Water Splitting, IMPRS-SurMat Seminar, 2018, Meschede, Germany 6. High-Throughput Screening of V-Fe-O Thin-Film Semiconductor Materials Libraries, 1st IMPRS - RECHARGE Scientific Symposium 2017, NETZ- NanoEnergieTechnikZentrum, Universität Duisburg-Essen, Dusiburg, Germany

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7. Combinatorial screening of Fe-V-O Thin-Film Materials Libraries for solar water splitting, SPP Project Meeting, 2017, Hofgeismar, Germany 8. High-Throughput Screening of Fe V-O Thin-Film Materials Libraries, RUB Materials Day, Ruhr Universität Bochum, 2017, Bochum, Germany 9. DFG SPP 1613 Summer School, Solar water splitting, 2016, Berlin, Gemany

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4 Acknowledgements

First and foremost, I would like to express my deepest gratitude to my supervisor Prof. Dr.- Ing. Alfred Ludwig for offering me the opportunity to join his research group. His continuous scientific support and suggestions have always encouraged me to be motivated from an early stage of this research work. I am thankful to him for showing confidence in me and giving me the freedom to carry my research independently. I would like to sincerely thank my second supervisor, Prof. Dr. Wolfgang Schuhmann, for his scientific advices during our meetings. I am really thankful to him for his lessons of life which helped me to see things differently. I am pleased to thank both of my supervisors for all the scientific discussions and quality time spent during our conference visits which helped me to stay on the right track of my research work.

I express many thanks to all my group members for having a comfortable atmosphere and sharing their knowledge and experience with me. I thank Dr.-Ing Helge Stein for picking me at the airport, helping in all administrative stuff, and introduced me to everyone in the group during my first week. Special thanks to Alan Savan for always being ready to fix any technical issues that occurred in the lab especially the sputter chambers. I would like to thank Xiao Wang for sharing the sputter chamber (K3) (as we both worked with vanadium) and fruitful discussions we had while target changing and chamber cleaning. I thank Steffen Salomon for helping with any kind of issues that occurred while XRD and EDX measurements. I would like to thank Lars Banko for developing machine learning databases like Compact, HDF5, which made our life easier to find specific data and especially for modifying the htAx software and Dr. Bin Xiao for developing phase identification software which made XRD analysis way easier. I also thank Dr. Aleksander Kostka for TEM measurements. I would like to thank Dr. Ramona Gutkowski and especially João RC Junqueira for performing all PEC measurements, introducing me to the OSDC setup, and discussion about the electrochemistry. I would also like to thank my Ph.D. colleagues Nadine Ziegler and Valerie Strotkӧtter for having a nice atmosphere in the office.

I thank Dr. Sigurd Thienhaus for helping with all the administrative stuff, writing scripts for optical test stand and for sharing his cool (piano) keyboard which I learned and enjoyed playing.

From MPIE Dusseldorf, I would like to acknowledge Prof. Christina Scheu, Katharina Hengge, and Siyuan Zhang for extending their support in performing high-resolution TEM

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measurements. Dr. Apurva Mehta and Dr. Suchismita Sarker from SLAC National Accelerator Laboratory, Stanford University, U.S.A. is acknowledged for performing the high-throughput synchrotron-XRD measurements.

International Max-Planck Research School for Interface Controlled Materials for Energy Conversion (IMPRS-SurMat) is greatly acknowledged for the financial support. I am thankful to Dr. Christoph Somsen and Elke Gattermann for helping out with all administrative paperwork and organizing the SurMat curriculum.

Last but not least, my family member and friends. I thank my parents for their support and understanding they showed during past three and a half years. Special thanks to my husband Anurag for believing in me and always standing by my side. You were always there when I was stressed and motivated me to start with positive energy. Pragati, Aparna, Sandeepan, Akshay, Ujas, Dipali, Vaibhav, and Sagar are thanked for meeting time to time and organizing get-togethers which made my stay in Germany easy. Thank you all for being there!

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Curriculum Vitae Personal Information Name: Swati Kumari Date of Birth: 02.05.1992 Place of Birth: Begusarai, India Nationality: Indian Educational Qualification 08/2016 – Present Doctoral Student

Institute for Materials, Faculty of Mechanical Engineering Ruhr-Universität Bochum, Germany Thesis entitled: ‘Combinatorial Synthesis and High-Throughput Characterization of Multinary Vanadate Thin-Film Materials Libraries for Solar Water Splitting’ Supervisors: Prof. Dr.-Ing. A. Ludwig, Prof. Dr. W. Schuhmann

07/2010 – 06/2015 Integrated Master of Technology

Center for Nanotechnology, Central University of Jharkhand, India Thesis: ‘Development of hard and optically transparent AlSiN nanocomposite coatings’ Scholarships

08/2016 – 07/2019 International Max-Planck Research School for Interface Controlled Materials for Energy Conversion (IMPRS-SurMat)

08/2019 – 07/2020 Post Graduate Scholarship from Faculty of Mechanical Engineering, Ruhr-Universität Bochum, Germany Work experience

08/2016 - Present Scientific Staff at Chair of Materials Discovery and Interfaces, Faculty of Mechanical Engineering, Ruhr- Unveristät Bochum, Germany.

11/2015 – 05/2016 Research Assistant at Corrosion and Surface Engineering department, National Metallurgical Laboratory, C.S.I.R Lab, India

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