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

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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
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