UPTEC Q 16006 Examensarbete 30 hp Juni 2016

Synthesis and characterisation of delafossite CuFeO2 for solar energy applications

Axel Forslund Abstract Synthesis and characterisation of delafossite CuFeO2 for solar energy applications Axel Forslund

Teknisk- naturvetenskaplig fakultet UTH-enheten Delafossite CuFeO2 is an intrinsic p-type semiconductor with a band gap around 1.5 eV. Further, it is composed of relatively abundant, nontoxic Besöksadress: elements, and therefor have potential to be an attractive material for solar Ångströmlaboratoriet Lägerhyddsvägen 1 energy harvesting.This work examines three routes to synthesise this material. Hus 4, Plan 0 The first includes a sol-gel deposition and then relies on solid state reaction above 650 degrees Celsius in inert gas atmosphere. In this work, no delafossite Postadress: is obtained with this method.The second method is a hydrothermal route to Box 536 751 21 Uppsala make particles under hydrostatic pressure in an autoclave. Delafossite is obtained mixed with other phases.The third route includes aqueous Telefon: precipitation similar to the second route, but a temperature of 70 degrees 018 – 471 30 03 Celsius and ambient pressure is sufficient to produce a pure delafossite particle Telefax: phase. It provides a robust and simple way to make delafossite CuFeO2 018 – 471 30 00 particles.The resulting particles are deposited and compressed on glass into thin films.The films have a band gap slightly below 1.5 eV and show some Hemsida: photoactivity in electrochemical measurements. http://www.teknat.uu.se/student

Handledare: Gerrit Boschloo Ämnesgranskare: Tomas Edvinsson Examinator: Åsa Kassman Rudolphi ISSN: 1401-5773, UPTEC Q 16006 Syntes och karakterisering av CuFeO2-delafossit för solenergitillämpningar

Axel Forslund

Som en del i att minska den mänskliga påverkan på klimatet måste användningen av fossila bränslen reduceras. Samtidigt som det globala energibehovet ökar, är också flera nya förnybara energislag på väg upp. Produktionen av solceller har de senaste åren ökat och priserna på moduler har fallit. Effektiv energiproduktion från solinstrålningen med tillgängliga och billiga material kan vara viktigt för att uppnå de klimatmål vi har satt upp.

Kommersiella solceller domineras fortfarande av kiselsolceller. Tunnfilmsceller finns dock på marknaden och även färgämnessensiterade solceller, med den engelska förkortningen DSSC, används idag. Många alternativ använder dock ovanliga ämnen som inte alltid bryts för sitt eget värde och därmed blir dyra eller prisberoende av marknadsfluktuationer. Andra ämnen är mycket giftiga och medför risker vid utvinning och tillverkning av solceller.

Nya solcellsmaterial finns på forskningsstadiet, så som CZTS-solceller (koppar- zink- tenn- sulfid/selenid) och hybridorganiska perovskitsolceller. En del av dessa använder vanliga och lättillgängliga ämnen och skulle möjligen kunna tillverkas med lösningsbaserade tekniker, vilket kan sänka tillverkningskostnader.

Utöver att generera elektricitet direkt från solstrålningen kan man bland annat också tänka sig produktion av vätgas med hjälp av solens strålning.

CuFeO2-delafossit är ett material av koppar- och järnoxid som kan absorbera solljus och skulle kunna användas i solenergitillämpningar. Det utgörs dessutom av relativt tillgängliga och ofarliga material. Potential finns för att tillverka filmer med enkla och energisnåla tekniker.

I det här arbetet undersöks tre metoder för att tillverka CuFeO2-delafossit.

1. I den första metoden beläggs ett substrat med en gel av koppar- och järnsalter. Gelen bränns sedan bort tillsammans med saltresterna och kvar blir en amorf film, utan kristallstruktur. Tanken är sedan att anlöpning vid över 650 ˚C i ett argonflöde ska skapa en syrefattig atmosfär där den deponerade filmen av termodynamiska skäl omvandlas till delafossit. I det här arbetet erhålls dock ingen delafossit, utan enbart andra faser av koppar- och järnoxid bildas.

Det är alltså en svår metod, som dessutom kräver ett mycket värmetåligt substrat av specialglas. Vidare medför den stor energiåtgång med flera anlöpningar vid höga temperaturer och ett högt flöde av inert gas.

I de två andra metoderna tillverkas partiklar av CuFeO2-delafossit som sedan kan beläggas på att substrat.

i 2. Med en “hydrotermisk” metod tillverkas partiklar vid förhöjt tryck och temperatur i vatten i en tryckkokare, eller autoklav. I det här arbetet används temperaturer mellan 100 ˚C till 200 ˚C. För att bilda delafossit krävs en basisk lösning och NaOH används för att höja pH- värdet innan reaktion i autoklaven sker. Så fort NaOH tillsätts fälls ett förstadium till partiklar ut, som sedan åldras i autoklaven. Som utgångsmaterial används en lösning av koppar- och järnsalter, i det här fallet Cu(NO3)2 och FeCl2. I det här arbetet erhålls CuFeO2-delafossit- partiklar tillsammans med en blandning av andra partiklar av koppar- och järnfaser.

3. Med en betydligt enklare metod tillverkas liknande CuFeO2-delafossitpartiklar vid enbart 70 ˚C i vatten i en laboratorieglasflaska i en ugn. NaOH används även här för att fälla ut ett förstadium till partiklar, men dessa behöver inte lika hög temperatur för att bilda enbart delafossit. En del av orsaken till den lägre temperaturen är att sulfater, Cu(SO4) och Fe(SO4), används här – istället för Cu(NO3)2 och FeCl2. Dessa partiklar blir flakformade och ca 200 nm breda, men betydligt tunnare.

I det här arbetet har förhållandet i det initiala skedet av reaktionen, vid tillsättanden av NaOH och utfällningen, visat sig vara mycket viktigt i styrandet av bildade faser. Det är kon- centrationen av NaOH, snarare än mängden NaOH under åldringen, som påverkar vilka faser som bildas. I det här arbetet har pellets av ren NaOH tillsatts, vilket gett en mycket robust syntes av delafossit. Delafossit har bildats som enda kristallina fas synlig i röntgen- kristallografi, även då den tillsatta mängden NaOH varit för liten för att alla koppar- och järnjoner ska kunna reagera.

Det är viktigt att ha en tillräckligt basisk miljö för att kunna reducera Cu2+ till Cu1+. Vid tillräckligt basiska förhållanden kan då delafossit bildas, med oxidationstillstånden 1+ 3+ Cu Fe O2. Men när inte enbart delafossit bildas på grund av ändrade initiala förhållanden, 1+ 1+ bildas ofta Cu 2O som bifas. Även där är oxidationstalet Cu och kopparjonen är reducerad från Cu2+. För att utreda detta vidare krävs analys av de initialt bildade faserna vid olika förhållanden.

Det pulver som tillverkats har deponerats på glas med en ledande beläggning på, flourdopad tennoxid – även kallat FTO-glas. ”Spin coating” och ”doctor blading” har använts som deponeringsmetoder. De deponerade filmerna har mycket dåliga mekaniska egenskaper, och pulvret lossnar lätt. För att förbättra filmerna har de tillverkade proverna pressats under 15 kN/cm2 till 20 kN/cm2. De pressade filmerna har bättre mekanisk stabilitet och stannar på substratet även vid nedsänkning i vatten. De visar även fotoaktivitet i elektrokemiska mätningar. Vidare förbättring av filmerna behövs dock innan en fungerande fotovoltaisk eller fotoelektrokemisk anordning är aktuell.

Metod 1 kan jämföras med metod 3 utan hänsyn till om filmer erhållits eller ej, och utan att se till eventuella filmers prestanda. Metod 1 kräver mer energi och arbete med att belägga utgångsmaterial, men ger en film direkt på ett substrat. Metod 3 utgör en enkel och robust metod att tillverka delafossit, men kräver efterarbete för att tillverka en tunnfilm. Å andra sidan kan partiklarna beläggas på en mängd olika substrat, vilker ger större flexibilitet och kanske även en prismässig fördel.

Examensarbete 30 hp på civilingenjörsprogrammet Teknisk fysik med materialvetenskap Uppsala universitet, juni 2016

ii Acknowledgements

This work have been conducted with professor Gerrit Boschloo as supervisor and I wish to thank him for this opportunity to investigate an interesting material. As co-supervisors, Malin Johansson and Xiaoliang Zhang have also provided valuable input and helped me with mea- surements and synthesis. Thank you, Malin for all the constructive suggestions and comments. During this work, I have also had help from many people in the group at Physical Chemistry, which I am very thankful for. Leif Häggman helped me in the lab, Wenxing helped me with electrochemical measurements and you all helped explaining areas which were new to me. I have also had a great time with you in the group. I will address a special thank to Pedro Berastegui at Inorganic Chemistry, who helped me with the tube furnaces. I also wish to thank Tomas Edvinsson for his useful comments and valuable input.

iii Acronyms

CIGS copper indium gallium selenide

CZTS copper zinc tin sulfide/selenide

DSSC dye sensitised solar cell

FTO fluorine doped tin oxide

GIXRD gracing incidence X-ray diffraction

LSV linear sweep voltammerty

PV photovoltaic

RHE reversible hydrogen electrode

SE secondary electron

SEM scanning electron microscopy

SHE standard hydrogen electrode

UV-Vis ultraviolet-visible spectroscopy

XRD X-ray diffraction

iv Contents

Populärvetenskaplig sammanfattning i

Acknowledgements iii

Acronyms iv

List of Figures vii

List of Tables viii

1Introduction 1

2Background 3 2.1 Semiconductors ...... 3 2.1.1 Direct and indirect band gap ...... 3 2.1.2 Doping ...... 4 2.1.3 The pn-junction ...... 4 2.1.4 Photovoltaic effect ...... 4 2.2 Solar energy ...... 5 2.2.1 Solar water-splitting ...... 5 2.3 Methods for characterisation and analysis ...... 7 2.3.1 XRD ...... 7 2.3.2 UV-Vis ...... 7 2.3.3 SEM ...... 8 2.3.4 Electrochemical measurements ...... 9 2.4 Delafossite CuFeO2 ...... 9 2.4.1 Properties ...... 9 2.4.2 Applications ...... 10 2.4.3 Synthesis ...... 10

3Experimental 13 3.1 Synthesis ...... 13 3.1.1 Sol-gel route with subsequent annealing ...... 13 3.1.2 Hydrothermal direct growth on substrate ...... 14 3.1.3 Powder synthesis ...... 15 3.1.4 Compression film making ...... 17 3.2 Characterisation methods/equipment ...... 17 3.3 Chemicals ...... 18

v 4Resultsanddiscussion 19 4.1 Sol-gel route ...... 19 4.2 Powder synthesis ...... 22 4.2.1 Hydrothermal process ...... 22 4.2.2 Low temperature aqueous precipitation process ...... 22 4.3 Compression of film ...... 26 4.4 Choosing an ‘easy route’ ...... 32

5Summaryandconclusion 33

6References 34

Appendix A SEM 39

Appendix B UV-Vis 42

Appendix C XRD 43

Appendix D LSV 47

Appendix E Synthesis conditions 48

Appendix F Peak broadening of powder XRD with Scherrer equation 51

vi List of Figures

2.1 Two variations of the delafossite structure ...... 11

4.1 Photos of annealed films on glass ...... 19 4.2 SEM picture of amorphous film ...... 20 4.3 SEM cross section of amorphous film ...... 20 4.4 Diffractograms from samples annealed in 575 ◦C argon flow ...... 20 4.5 Diffractograms from samples annealed in 700 ◦C argon flow ...... 21 4.6 Comparison of diffractograms from annealed sample at 700 ◦C and 800 ◦C ... 22 4.7 SEM pictures of LT12d...... 23 4.8 Diffractogram from LT12 at different stages in synthesis ...... 24 4.9 SEM pictures of compressed film ...... 28 4.10 SEM cross section of compressed film ...... 29 4.11 UV-Vis from compressed film ...... 30 4.12 Absorption coefficient for compressed film ...... 30 4.13 Tauc band gap estimation of compressed film ...... 31 4.14 Urbach tail band gap estimation of compressed film ...... 31

vii List of Tables

2.1 Classification system for solar water photolysis ...... 6

3.1 Chemicals used in sol-gel route ...... 13 3.2 Annealing conditions for sol-gel route samples on sola lime glass ...... 14 3.3 Annealing conditions for sol-gel route samples on aluminoborosilicate glass ... 15 3.4 Precursor solution composition for hydrothermal syntheses ...... 16 3.5 Conditions for different hydrothermal syntheses ...... 16 3.6 LT12 precursor solution composition ...... 16 3.7 Conditions for low temperature synthesis LT12 ...... 17

4.1 Parameters and results for low temperature synthesis LT12 ...... 25 Benefits and drawbacks of sol-gel preparation ...... 32 Benefits and drawbacks of low temperature process ...... 32

viii 1Introduction

Non-fossil energy is becoming more and more important in our energy system and in our strive to reduce the emissions of carbon dioxide and reduce our effect on the global climate. The global energy demand is still increasing and fossil fuels provide the majority of the energy supply today [1]. Renewable energy is emerging and in recent years we have seen decreasing prices in solar energy and increasing production of photovoltaic devices [2]. Solar energy harvesting with high efficiency and without using materials with high environmental impact might play a key role in reducing our environmental footprint. Silicon solar cells remains a commercially well-established technique, but the manufacturing is energy intensive and requires extremely pure silicon. The silicon layer also has to be relatively thick due to the indirect band gap, requiring more material. There are some alternatives commercially available such as thin film GaAs, copper indium gallium selenide (CIGS) and CdTe. However, some are made up of exotic materials with low abundance in the earths crust, making them expensive with possibly unstable and market dependent prices and some are quite toxic and require special care, both during manufacturing and when in use. Further, many alternatives have simply not been around long enough for long term stability guaranties. Electrochemical solar sells such as dye sensitised solar cells (DSSCs) use an approach dif- ferent from the ordinary pn-junction and exist on the market today, but they traditionally use metal-organic complexes with precious metals such as ruthenium as dye and suffer from stability issues. To achieve the best efficiencies they also need to use a liquid electrolyte with the risk of leakage [3], making them less practical if not used mainly for their tunable colour. New solar cell materials such as copper zinc tin sulfide/selenide (CZTS) and hybrid organic perovskites could provide a solution with relatively abundant materials and possibly also made with solution based techniques cutting the manufacturing costs. Other materials could also be considered for photovoltaic applications. Another route utilising the solar energy is solar water-splitting. Hydrogen is an attractive energy carrier as it combusts only into water and is functional in a fuel cell. Production of this fuel directly, from only water and sunlight seems like an ideal system. It also reduces the intermittency problem associated with renewable power generation from sun and wind, as the fuel produced can be stored for an arbitrary time (and transported), until combusted or used in a fuel cell. In theory, solar water-splitting can reach high efficiencies, close to the Shockley-Queisser limit above 30 % for 1 Sun illumination, more practical limits have been estimated to between 10 % and 20 % depending on the configuration [4, 5]. The later estimation requires a tandem system. Delafossite CuFeO2 is made up of comparatively abundant materials and could possibly be used as an absorber material in a solar cell or to reduce water in a solar water-splitting device. It has been reported to be an intrinsic p-type semiconductor and has a band gap of about 1.5eV.Howeverithasalsobeenreportedtobechallengingtosynthesise[6, 7]. In this study three earlier reported routes synthesising delafossite CuFeO2 are examined and thin films are made either directly in the synthesis or from the product resulting from the

1 delafossite synthesis. In only one of the synthesis routes, delafossite CuFeO2 is produced as the only phase after slight altering of the reported method. In the other routes, no delafossite or a mix of phases are produced. Out of these three routes, two are compared from a synthesis point of view: A sol-gel route requiring annealing and a low temperature particle route requiring deposition and compression after the synthesis.

2 2Background

In the following section an introduction about semiconductors, solar energy and the measuring techniques used in this work is given. Next, a short background follows about the material system examined in the work. Some earlier reports for the synthesis of and measurements on the material is included.

2.1 Semiconductors

A semiconductor is a solid with a conductivity that is intermediate that of a metal and an insulator [8]. Alternatively, it can be more appropriate to define a metal as having it’s Fermi level where the density of states is not vanishing [9, 10]. A semiconductor or insulator on the other hand, have the Fermi level in a region of vanishing density of states. Bands of electron energy states and band gaps arise when introducing quantum mechanical models together with periodic potentials in solid crystals. The crystal structure of a crystalline chemical compound determines the band structure with energy states and the electron proper- ties of the compound determines the Fermi level and in which way the electrons occupy these states. Metals can have bands that are not completely filled, and can easily — with a small amount of energy — excite electrons into free electronic states within the band to be able to act nearly as free electrons. Metals are therefor often good conductors. But a material can also be a metal if bands overlap with each other. This gives a similar effect, as there are no band gaps hindering the electrons to get excited, but a continuum of states still allow them to easily get excited. Semiconductors and insulators on the other hand, have band gaps with the valence band fully occupied. The electrons therefor need larger amounts of energy to reach a free state, which reduces their ability to conduct current. The difference between semiconductors and insulators is not always clearly defined, but generally the band gap of a semiconductor is small enough to allow electrons from the valence band to get thermally excited into the conduction band at reasonable temperatures.

2.1.1 Direct and indirect band gap There are two types of band gaps: Direct and indirect. A direct band gap have its valence band maximum at the same position in the Brillouin zone as the conduction band minimum. An electron will thus need extra energy to cross the gap. An indirect semiconductor on the other hand, does not have its valence band maximum in the same place in reciprocal space as the conduction band minimum. To cross the gap, an electron will therefor need not only photon energy, but also energy from a crystal momentum. As photons carry little momentum, the excitation by a photon needs to be combined with some momentum from the crystal lattice, which drastically decreases the probability for light

3 absorption for indirect band gap semiconductors. Two examples are GaAs as a direct band gap semiconductor and Si as an indirect band gap semiconductor. In practical terms this forces a Si solar cell to be much thicker than a GaAs solar cell.

2.1.2 Doping To increase the carrier density and thus the conductivity of a semiconductor, small amounts of impurities can be implanted into the crystal lattice, so called doping. There are two types of doping, n-type and p-type. For n-type doping electron donors are inserted, elements with one valence electron more than the semiconductor. They bind by forming hybrid orbitals with the surrounding semiconductor atoms, but the extra electron is then left over, not included in any binding hybridisation. The electron is then just loosely bound to the positively charged dopant atom, otherwise acting like a free electron. By using the similarity to the hydrogen atom — substituting the electron mass to an ef- fective electron mass and the dielectric constant of vacuum to the dielectric constant of the semiconductor — the binding energy can be calculated relative to the conduction band mini- mum. The energy is rather small and the donor electrons are easier to excite than the intrinsic electrons. In some intermediate temperature region when the donors are thermally ionised but the intrinsic are not the electron density is practically the same as the donor concentration in the semiconductor. The same reasoning is used for p-type doping where an element with one electron less than the semiconductor is inserted. This creates an extra state to where an electron can easily be excited and thereby create a hole in the valence band.

2.1.3 The pn-junction Apn-junctionisinprinciplemadeupofann-typesemiconductorandap-typesemiconductor joined together. In the boundary between the two types of semiconductors the electrons and holes diffuse to the other side. As they recombine a depletion layer will appear where only the charged, fixed dopant atoms are left. As only the ionized atoms remains the layer is polarised and hinders the charges to diffuse further over the boundary, and equilibrium can be reached. If a minority electron from the p-side enters the depletion region it will drift to the n-side driven by the depletion region electric field. This giver rise to a drift current that is compensated by the diffusion current of electrons still diffusing from the n-side to the p-side at equilibrium. If external bias is applied, the chemical potential on either side can be changed. This in turn changes the diffusion current to increase for forward bias, or decrease for reverse bias (eventually approaching zero, leaving almost only the drift current at a practically constant value).

2.1.4 Photovoltaic effect There are several effects that can give rise to the buildup of a voltage in a semiconductor. One is the direct excitation of electrons by photons. If a semiconductor is exposed to light, electrons in the valence band can be excited to the conduction band by absorbing the energy of a photon. The energy of the photon must then be at least as high as the energy gap between the valence band and the conduction band of the semiconductor. For smaller energy, there is no density of states for the electron to go to. For absorption of higher energy photons, the electron gets excited up above the conduction band edge, but will soon loose excess energy and relax to close to the conduction band edge. If the electron in the conduction band and the resulting hole in the valence band can be separated, then a voltage is built up and work can be extracted from the material. If an

4 electron-hole pair is generated in the depletion region in a pn-junction, they are effectively separated by the built in voltage. This is the way for conventional solar cells to utilise light to build up a voltage that can be used to perform work in a circuit [9, 10], and these solar cells are called photovoltaic (PV) devices, or cells, in this work. The maximum efficiency of a PV device is predicted by Shockley & Queisser [11]andreaches just above 30 % for a single junction device.

2.2 Solar energy

The basic functioning of a conventional PV device is described in section 2.1.4. The most common material for PV devices is Si, and Si-cells have in recent years decreased in price, and the worldwide installed capacity increases [12]. However, there are also other ways of generating a voltage from sunlight, for example in a photoelectrochemical cell, where a semiconductor is immersed in an electrolyte. DSSCs is a subtype of these and use dyes attached to a large band gap semiconductor. The dye acts as an absorber, absorbing photons to excite electrons into higher energy states. These excited electrons ideally injects into the high band gap semiconductor. Simultaneously the empty state in the dye attracts an electron from — or injects a hole into — the electrolyte, oxidising a species in the electrolyte. This species is transported to the counter electrode where it can get into a reduced state again [13]. - - The electrolyte can be made of a solution with I /I3 redox species, or of some hybrid metal organic complex with e.g. Co [14]. When the excited electrons injects into the large band gap semiconductor the risk of recom- bination decreases and the electrons can be transferred to a contact more efficiently.

2.2.1 Solar water-splitting Previously, methods of generating electricity from sunlight have been mentioned. Another way of extracting energy from the solar radiation is to use it directly to split water into hydrogen and oxygen. This can be done in a way similar to the electrochemical solar cell, but the reversible redox reaction for the electrolyte is substituted to oxidation of water into oxygen at the anode and reduction of the thereby released protons into hydrogen at the cathode. Either of, or both, the anode and the cathode can be a photodevice, absorbing photons and driving the cell half-reactions [5]. The formal electrochemical potential required to drive the water-splittingis around 1.23 V. and can be divide into the following half reactions. The potential is given relative to the reversible hydrogen electrode (RHE): Reduction reaction:

+ red 2H +2e− H2 (E =0.0V v.s. RHE) → 0 Oxidation reaction:

+ 1 + ox H O +2h O2 +2H (E = 1.23 V v.s. RHE) 2 → 2 0 − Complete reaction:

1 H O H2 + O2 (∆E = 1.23 V v.s. RHE) 2 → 2 − The overall reaction has a negative standard potential and energy must be added for the reaction to occur. Even though 1.23 V theoretically is enough for some reaction, higher voltages

5 Table 2.1 – Classification system for solar water photolysis schemes. S: singel photosystem; D: dual photosystem operating at different threshold wavelength λ1 and λ2. After Bolton et al. [15].

Scheme No. of Minimum no. Reaction classification photosystems of absorbed photons per H2 1hv S1 1 1 H O H + 1 O 2 −−→ 2 2 2 2hv 1 S2 1 2 H2O H2+ 2 O2 −−→4hv S4 1 4 H O H + 1 O 2 −−→ 2 2 2 hv1+hv2 1 D2 2 2 H2O H2+ 2 O2 2−−hv −+2 − −→hv D4 2 4 H O 1 2 H + 1 O 2 −− − − − −→ 2 2 2 are required in practice. The excess voltage required is called overvoltage and is caused by the difference in chemical potential required to drive the reaction and by losses in the system [15]. The extra chemical potential required is inevitable and can be calculated [16], and some losses will also aways be in the system but have to be estimated to a reasonable value. The maximum efficiencies for a photolytic system are also different depending on which principal features are utilised. Bolton et al. [15] sort some different photoelectrochemical (PEC) systems into five different classes, denoted S1, S2, S4, D2 and D4. These notations indicates the number of photosystems used (S for singel photosystem and D for dual photosystem) and the number of photons (1,2 or 4) used to produce one H2 molecule. These classes are summarised in table 2.1. In the same paper, Bolton et al. suggests the ideal efficiency limit for the S2 system to be 30.7%,atλg =775nm,andfortheD4systemtobe41.0%, with λ1 =655nmand λ2 =930nm. However, Bolton et al. also estimates some more realistic efficiencies of around 17 % and 27 % for the same systems respectively, still assuming 100 % quantum yeld, absorption of all incoming sunlight and neglecting losses from reflections at interfaces. Further, they predict practical efficiencies of 10 % and 16 % for the S2 and D4 systems respectively, by counting in likely losses [15]. The optimal band gap energy for the maximum efficiency varies with the assumed losses, but in the analysis of Bolton et al. [15] and the similar analysis of Prévot & Sivula [5]the optimal band gap energy is around 1.3eV and 1.9eV for each photoelectrode respectively. Hu et al. [17]getoptimumbandgaplevelsof1.0eV to 1.2eV and 1.6eV to 1.8eV respectively, for different resistive losses, losses due to a fill factor <1, etcetera. So in theory, solar water-splitting can reach high efficiencies, close to the Shockley-Queisser limit, but it has been suggested that reasonable practical limits could lie at 10 % and 16 % for single photosystems and dual photosystems respectively. Prévot & Sivula [5]useasimilar analysis but with a tandem device with two different semiconductors with different band gaps and predicts a solar to hydrogen efficiency of around 22 %. In their discussion Prévot & Sivula compare this with a tested solar to hydrogen efficiency of 9.3% in a system with a commercial PV silicon cell and a high pressure electrolyser. This comparison could be questioned as their calculated efficiency is for a tandem device while the tested system is for a single junction, commercial silicon cell with about 15 % solar to current efficiency (not a state-of-the-art cell). Further, the produced hydrogen in the tested system comes out pressurised — an inevitable step for practical hydrogen storage which is not counted into the 22 % efficiency claimed by Prévot & Sivula. Further, as stated by Hu et al. [17], PV cells coupled with electrolysers can be tuned

6 and wired to match their I-V characteristics so that the total efficiency of the system will simply be the product of the efficiency of the solar cell and the electrolyser. In the case of a state-of-the-art system with a multijunction PV cell under concentrated solar illumination, 43.5% 73 % = 31.8%efficiency, or recently even higher, should be possible to reach now [17]. × By contrast, Jacobsson et al. [18] argue that PV/electrolyser systems and PEC cells do not differ considering the physical processes involved. This would mean that they essentially are affected by the same losses and therefor have the same theoretical efficiencies. Factors that would then matter are more coupled to properties of materials required for e.g. stability in PECs, inevitable light absorption losses for solution immersed systems or grid losses in grid connected PV/electrolyser systems. From this point of view a PV/electrolyser system is the most developed and commercially mature, due to the fact that each technique can — and have been — developed independently. An obstacle for commercialisation have been the low price for natural gas derived hydrogen, rather than technology development [18].

2.3 Methods for characterisation and analysis

2.3.1 XRD To characterise the crystal structure of the samples made in solid state reaction, hydrothermal and low temperature synthesis, X-ray diffraction (XRD) was used. An X-ray beam with a specific wavelength is sent towards the sample. The wavelength is small — around the size of the lattice spacing of the sample, e.g. Cu-Kα was used in this work, which is around 1.54 Å— and thus the x-ray interacts with the sample through constructive and destructive interference. Constructive interference is achieved for certain angles determined by the Bragg-condition:

nλ =2d sin(θ) where n is an integer number, λ is the wavelength, d is the distance between two crystal planes and θ is the incident angle of the beam. For thin films a constant grazing incidence angle can be used to enhance the relative intensity of the thin film at the surface compared to the substrate. The method is called gracing incidence X-ray diffraction (GIXRD).

2.3.2 UV-Vis In a typical ultraviolet-visible spectroscopy (UV-Vis) measurement the absorbance and/or the reflectance of a sample is recorded over a span of wavelengths. Light from a light source is lead through a monochromator (for example a prism and a slit) to obtain a small span of wavelenghts. The light is then allowed to pass through a sample or reflect against a sample before collected in a detector. This gives an absorbance or reflectance measurement respectively. After interaction with the sample the light can be collected with an integrating sphere as to include scattered light for detection. The integrating sphere ideally consists of a sphere whose walls scatters all light. Estimation of the band gap energy can be done following the Tauc method [19, 20], using the following equation for measurement data in the absorption edge in the absorption spectrum:

αhv = B(hv E )r − g Here α is the absorption coefficient, h is Boltzmann’s constant, v is the photon frequency, B is a constant, Eg is the band gap energy and r is an index associated to indirect and direct

7 band gap, among other things [21]. r =2corresponds to an indirect band gap and r =1/2 to 1/r adirectbandgap.If(αhv) is plotted against the photon energy (hv), then Eg can easily be determined by extrapolating a line in the absorption edge — for some suitable r —toαhv =0, where then the corresponding hv = Eg. The absorption coefficient in case of uniform absorbance through the film is defined as [19]: 1 α = ln(T ) −d where d is the thickness and T the transmittance of the layer of interest. During measure- ments, some of the incident light is reflected before entering the material and the reflectance contribution must be extracted from the measured transmittance:

1 Rmeasured T − . ≈ Tmeasured Here R is the measured reflectance. This is an approximation and does not take multiple reflexes from the several interfaces into account. The resulting formula is 1 1 R α = ln − measured −d T ! measured " There could be several layers other than that of interest in the sample and these have to be compensated for as well. For example fluorine doped tin oxide (FTO) coated glass is used in this work, and even though the glass is assumed to be completely transparent the FTO needs to be taken into account. A first order correction αd = αtotdtot αFTOdFTO can be used as an − approximation to remove the FTO absorbance.

2.3.3 SEM A scanning electron microscopy (SEM) accelerates electrons from an electron emitting source (e.g. aLaB6 crystal or a hot W-filament can be used.) through a system of lenses and detects scattered electrons resulting from interactions with the accelerated electrons and the sample. The beam of accelerated electrons is scanned stepwise across the sample, and a detector counts electrons resulting from interactions for each point at which the beam is focused. Then, a computer creates an image of the gathered information from these scanned points. The SEM thus gives a black and white picture of the sample with contrast due to geometry and compositional differences at the sample ‘surface’. The information given in the picture results from interactions in the interaction volume of the electron beam and the sample at a specific point. The interaction volume is determined by the spot size and the penetration depth of the electrons. The electron penetration depth depends on the acceleration voltage and the material, but is typically around 0.5 µm to 5 µm, though the depth from which different mechanisms give information always differs. The spot size is the cross section of the cone formed by the electron beam, at the sample surface. It is changed by focusing the beam, but limited by not only the lens system but also the sample roughness and height differences in the sample as in that case not a single object plane can be obtained — some depth of field is needed to see the whole picture clearly. A tilting sample is also to be avoided, as the beam then has to refocus during the scanning to get a clear picture over the whole scanning area. There are several types of detectors used in SEM.Oneisthesecondary electron (SE) detector which is situated not right above the incident electrons, but at an angle from the normal of the surface. This detector detects the SEs that have escaped from the material after energy transfer from the incident electrons. As they have to escape the material, they loose energy and

8 therefor only the electrons close to the surface can escape outside the sample. This detector is therefor more surface sensitive and suitable for topological measurements. Two other types of detectors used in SEM are back scattered electron (BDE) detectors and energy dispersive X-ray spectrometry (EDS or EDX) detector. However, they are not used in this work.

2.3.4 Electrochemical measurements To measure the behaviour of a species at different potentials, electrochemical methods can be used. The measurements depend not only on the electrical field in the cell, but also on chemical events during the measurement. Voltages are often given relative to some standardised reference. The standard hydrogen electrode (SHE) is defined to be the reference electrode for which, 0 + under standard conditions and at all temperatures, E (H / H2)=0, i.e. the reduction potential of a hydrogen ion to hydrogen gas is zero. In an aqueous solution at 25 ◦C this is calculated to be between 4.4V to 4.5V v.s. the potential of an electron at rest in vacuum. In electrochemical standards the voltage is given with the positive direction reversed com- pared to the physical convention of decreasing potentials into the negative region from the electron at rest in vacuum. In practical measurements a reference electrode is used. One such is the Ag/AgCl electrode, where Ag and AgCl is in equilibrium with a KCl-solution. The potential of Ag/AgCl depends on the concentration of KCl in the solution, but is usually around 0.2Vv.s. SHE.Ifthesolution in the reference electrode is saturated the potential is 0.197 V v.s. SHE. If the pH in the electrolyte is not 0, as in the SHE,thereductionpotentialofhydrogen will be different. Therefor RHE is often defined to have the potential of hydrogen reduction at any pH. It therefor coincide with SHE at pH 0, but can generally be estimated from the relationship: E =0.0000 V 0.0591 V pH (v.s. SHE) − × E.g. if measured voltages are given with respect to a saturated Ag/AgCl reference electrode in 1 m NaOH solution, then Ag/AgCl is

0.197 V (v.s. SHE), and SHE is 0.0591 V 14 = 0.8274 V (v.s. RHE), × which gives a total difference of about 0.197 + 0.827 = 1.024 1.0V for Ag/AgCl v.s. RHE. ≈ So the reduction potential for hydrogen at pH 14 is then around 1.0V v.s. Ag/AgCl. − In electrochemistry, a cathodic current goes into the electrode, i.e. electrons goes into the solution and reduce the electrolyte. Correspondingly, an anodic current goes from the electrode into the solution, i.e. the electrons move from the solution to the electrode, oxidising the electrolyte. Ameasurementthatscansthevoltageinaconstantratefromonevoltagetoanother,and measures the current during the scanning is called linear sweep voltammerty (LSV).

2.4 Delafossite CuFeO2 2.4.1 Properties +1 +3 Delafossite crystals typically have the chemical formula A B O2 [22]. It is constructed by + 3+ alternating layers of close packed standing O–C –O dumb-bells and Fe O6 octahedrons. There

9 are two common crystallised states of delafossite: One with rhombohedral 3R-(R3m) symmetry (3R delafossite) and one with hexagonal 2H- (P6c/mmc) symmetry (2H delafossite), with the stacking sequences ‘AaBbCcAa. . . ’ and ‘AaBbAa. . . ’ respectively. An illustration of the structures can be seen in figure 2.1. (The illustrations are made with ‘VESTA’, a crystal structure illustration program [23].) CuFeO2 can be an intrinsic p-type conductor and conducts well compared to some other intrinsic p-type materials [24]. The good conductivity have suggestively been attributed to Cu vacancies and interstitial O in the delafossite crystal structure [24], but also to the strong covalent nature of the Cu-O bond [7]. The conductivity of single crystal delafossite CuFeO2 is higher perpendicular to the c-axis compared to parallel to the c-axis, and in the order of 1 1 1Ω− cm− [24–27].

2.4.2 Applications 1 Delafossite CuFeO2 has reportedly an optical band gap at around 1.1eV to 1.6eV [6, 7, 24, 27–30] with the conduction band positioned at around 0.4eV relative to RHE at pH 13.6, − according to Prévot et al. [6]. This is about 3.2V below vacuum level and is in the same region as measured earlier [27, 31]. This is a suitable position for the reduction of water into hydrogen as a photocathode in a tandem water-splitting cell. One of the greatest advantages of delafossite CuFeO2 is its long term chemical stability under neutral and alkaline conditions [32]. It has been tested to show small declining in photoactivity in electrochemical measurements after hours and even months of operation [6, 32]. It could also possibly be used as a hole conductor in PV devices. However, the conductivity features some anisotropy [25] with good conductivity perpendicular to the c-axis, but worse conductivity parallel to the c-axis.

2.4.3 Synthesis Solid state reaction

The conventional way of preparing delafossite CuFeO2 uses a high temperature reaction in oxygen deficient atmosphere, where delafossite CuFeO2 seems to be thermodynamically stable [20]. Delafossite CuFeO2 may form in temperatures above 650 ◦C,butinmanyearlierworks temperatures of up to more than 1000 ◦C have been used [6, 7, 25, 27, 29, 31, 33].

Sol-gel preparation Asol-gelprocessshouldincludethetransitionofasol into a gel [34]. A sol is often defined as asuspensionofsmallparticlesinaliquidthatremainsdispersedduringtheprocess,andagel as a three dimensional network extending uniformly over the liquid phase. There are more and less rigorous definitions of the expression, but according to a wider definition the gel can be formed from e.g. metal-organics and produces a uniform solid or highly viscous liquid matter with no precipitation of the compounds. The ‘Pechini route’ [35]includesusinganalpha-hydroxycarboxylicacidlikecitricacidin combination with metal oxides or salts. After adding a polyhydroxy alcohol such as ethy- lene glycol, a polyesterification creates a polymer gel from metal chelate complexes with the carboxylic acid and the alcohol [34, 36]. Prévot et al. [6] have earlier prepared films with a sol-gel2 route, or modified Pechini route, to get a uniform thin-film with the desired Cu-Fe stoichiometry. They spin coat an equimolar

1Whether the band gap is direct or indirect differs in the reports. 2According to a broader definition [34].

10 (a) (b)

(c) (d)

Figure 2.1 – There are two structures of delafossite CuFeO2: (a) and (c) with rhombohedral 3R-(R3m) symmetry (3R delafossite) viewed from the a and c axis, respectively; (b) and (d) with hexagonal 2H-(P6c/mmc) symmetry (2H delafossite), also viewed from the a and c axis, respectively. The stacking sequences of the Cu–Fe layers are ‘AaBbCcAa. . . ’ for 3R delafossite and ‘AaBbAa. . . ’ for 2H delafossite.

11 solution with Cu and Fe nitrates in ethanol with citric acid and ethylene glycol. In the solution, the alpha-hydroxycarboxylic acid (the citric acid) forms a chelate with the metal ions and during spin coating this reacts with the polyhydroxy alcohol (the ethylene glycol) into a polymer gel when ethanol is evaporating, evenly distributing the different metal ions through the film [34, 36]. The organic polymer is then burned away at elevated temperature and the first non-organic film obtained is amorphous and must undergo a solid state high temperature reaction in oxygen deficient atmosphere to form delafossite CuFeO2,asdescribedunder‘Solidstatereaction’.

Hydrothermal synthesis

Instead of heating the reactants up to > 650 ◦C, high pressures can be used in combination with relatively low temperatures in a liquid to prepare delafossite structured powders [22, 37–43]. In autoclave, delafossite powders have been obtained at temperatures as low as 100 ◦C [37]. In these syntheses, large amounts of NaOH is used [37, 38]andthepHduringthereactioncan be quite high. However, some of the NaOH seems to be consumed in the reaction and it is not clear whether the high pH catalysts the reaction or just acts as a ‘consumable’. John et al. [44] used something similar to a hydrothermal synthesis using Cu(SO ) 5H O 4 • 2 and Fe(SO ) 7H Oinwaterat70 ◦C adding differently concentrated NaOH-solutions. However, 4 • 2 the vessel was not sealed to build up a pressure and was thereby not strictly a hydrothermal synthesis. In this report, it will be named ‘low temperature (synthesis/route/method)’. This low temperature method is reported to result in precipitated delafossite CuFeO2 powder in a NaOH solution after annealing in an oven at 70 ◦C.

Compression There have been earlier reports of making metal oxide films on glass with a compression method [45]. Lindström et al. use a suspension of 20 %wt to 25 %wt TiO2 powder in ethanol, doctor bladed on a glass substrate. When the ethanol has evaporated the sample is compressed using a hydraulic press with a pressure in the order of 10 kN/cm2. By this method µm-thick films are made in a simple way. The films are conducting and the porosity can be controlled by adjusting the pressure. Powder of delafossite CuFeO2 could possibly be prepared in a similar way.

12 3Experimental

In this part of the report the experimental details are given. All the analytical equipment used are listed in section 3.2 and the chemicals used are specified in section 3.3.

3.1 Synthesis

3.1.1 Sol-gel route with subsequent annealing The ‘sol-gel route’1 used in this thesis is based upon the synthesis method used by Prévot et al. [6]. The method includes the following initial steps:

1. Mixing of equimolar Cu(NO ) 3H OandFe(NO) 9H Oinethanoluntildissolved 3 2• 2 3 3• 2 2. Adding citric acid

3. Stirring for 2h

4. Adding ethylene glycol

5. Stirring overnight

6. Spin coating on FTO at 3000 rpm for 1min

7. Drying on hot plate at 100 ◦C for 10 min

8. Annealing in a furnace at 450 ◦C for 30 min to burn away organic material. (2h is used by Prévot et al.)

The amounts of precursor chemicals used are listed in table 3.1.

1The route could be called ‘sol-gel route’ by a broader definition. Modified ‘Pechini route’ could possibly be considered more correct, see section 2.4.3 under ‘Sol-gel preparation’.

Table 3.1 – Chemicals used in sol-gel route with subsequent solid state reaction

Chemical Amount (mmol) Amount Cu(NO ) 3H O 2.00 482 mg 3 2• 2 Fe(NO ) 9H O 2.00 808 mg 3 3• 2 Ethanol — 10 ml Citric acid 4.00 768 mg Ethylene glycol 4.48 0.25 ml

13 Table 3.2 – Annealing conditions for sol-gel prepared samples on FTO-coated soda lime glass. The ramping rate while heating was 10 ◦C/min, while the samples where cooled naturally in the furnace. The gas flow could not be measured with the furnace used in this case.

Sample Temperature (◦C) Type of gas A2 500 Air C3 575 Air C5 650a Air C2 500 Ar C4 575 Ar

a At this temperature the substrate softened, so no annealing in argon was done at 650 ◦C

This results in a brownish, transparent film. The described process can be repeated several times to increase the thickness of the thin film. After each cycle the colour changes into more dark brown and the film appears less transparent upon ocular investigation. It should be noted that repetition of the process was tried without the last annealing step. However, the resulting films did not change colour, which seems reasonably to be due to dissolution of the previous layer – so that it washes away during spin coating of the new layer. Further, according to earlier investigations with termogravimetric analysis [6, 20, 46, 47] crystalline phases of CuFe2O4 and CuO start forming at 450 ◦C and above. Therefore, the temperature of the first annealing should not exceed this temperature to obtain a uniform, non-crystalline film. After this first deposition of Cu and Fe nitrates, the film is annealed for a second time in higher temperature:

Annealing in tube furnace at >500 ◦C in controlled atmosphere • Prévot et al. use a temperature of 700 ◦C in argon. In this work both lower and higher temperatures are also investigated for the second annealing. The conditions used for samples on FTO-coated soda lime glass are presented in table 3.2. Sample C5 was annealed in air at 650 ◦C but did not withstand the temperature, so no sample was annealed in argon for this temperature. When annealing at 700 ◦C aspecialglassmustbeusedastheglasstransitiontemperature and softening point of usual soda lime glass lies slightly above 500 ◦C [48]andcannotstand higher temperatures, as noticed for sample C5. Therefore FTO-coated aluminoborosilicate glass from Solaronix SA was used above these temperatures. The annealing conditions for these samples are presented in table 3.3. Before heating the samples in table 3.3,argongaswassettoflowthroughthetubefurnace for 3h to 6h at room temperature to decrees the amount of oxygen present. Prévot et al. [6] use an argon flow of 300 ml/ min,andthisflowratewasusedalsointhiswork.Significantly lower argon flow rates were investigated but resulted in e.g. metallic tin phases from the FTO.

3.1.2 Hydrothermal direct growth on substrate In section 3.1.3 under ‘Hydrothermal process’ a route to make particles is described. In an attempt to grow a film by precipitating particles directly on FTO-coated soda lime glass in hy- drothermal conditions, substrates were immersed into an autoclave together with the precursor solution. The full description of the hydrothermal process can be seen in the named section.

14 Table 3.3 – Annealing conditions for sol-gel prepared samples on FTO-coated alu- minoborosilicate glass.

Sample Temperature (◦C)Ar-gasflow(ml/ min) Comment F1 700 300 old film E1 700 300 Spin coated on glass E2 700 300 Spin coated on FTO

However the glass did not stand the highly alkaline environment used for making the particles during heating in the autoclave and the glass substrates were destroyed. Further, precipitation from the precursor solution seems to occur directly when adding NaOH. Thus, direct growth from this system seems hard for this reason, not only for the hydrothermal process, but also the low temperature process described under ‘Low temperature aqueous precipitation process’.

3.1.3 Powder synthesis To synthesise powder a hydrothermal method and a low temperature method has been tested. The hydrothermal method [22, 37, 39–43]involvesheatinganaqueoussolutioninanautoclave container maintaining a constant volume and thus raising the pressure within the container as the temperature increases. In the synthesis a high pH have been used before, as noted in section 2.4.3 under ‘Hydrothermal synthesis’, and NaOH seems to be consumed in the process. The low temperature method [38, 44]alsousesanaqueoussolutionbutinatemperature below 100 ◦C.Johnet al. [38]use50 ◦C to 90 ◦C to obtain delafossite CuFeO2. After taking the samples named in the following sections out of the oven, they have been washed with deionised water twice and in ethanol once. To extract the particles from the washing liquid the suspension was centrifuged for 3min at 4000 rpm.

Hydrothermal process The hydrothermal synthesis method was taken mainly from Xiong et al. [37]althoughmany other similar routs have been presented before [22, 39–43]. The route included:

1. Dissolving Cu(NO ) 3H O and FeCl 4H Oindeionisedwaterbystirringandsome 3 2• 2 2• 2 heating

2. Adding NaOH

3. Stirring for 10 min

4. Pouring the solution into autoclave and sealing

5. Putting the autoclave into an oven at a temperature in the range of 100 ◦C to 160 ◦C for 17 h to 40 h

6. Natural cooling to room temperature

The composition of the precursor solutions and the amount of added NaOH is presented in appendix E, but a typical example is shown in table 3.4. The hydrothermal synthesis was altered in search for a method giving the most delafossite CuFeO2 phase. It was altered as presented in table 3.5.

15 Table 3.4 – Example of precursor solution composition for hydrothermal syntheses.

Chemical Amount (mmol) Amount Water 1100 20 ml Cu(NO ) 3H O 4.2857 1035.4mg 3 2• 2 FeCl 4H O 4.2857 852.0mg 2• 2 NaOH 31.4 1260 mg

Table 3.5 – Conditions for different hydrothermal syntheses.

Sample Temperature (◦C)AddedNaOH(g) Time in oven (h) HT1 100 1.257 21 HT4 120 1.25 17 HT5 140 2.5 66 HT6 160 1.25 22 HT7 160 2.5 22 HT8 160 2.5 40

Low temperature aqueous precipitation process For the low temperature powder synthesis with aqueous precipitation the route followed was mainly that of John et al., with some differences worth of notice for sample LT12. LT10 and LT11 were prepared as close to the route used in John et al. [38]aspossible.However,the adding of NaOH solution showed to be a critical moment, and monitoring the pH as the NaOH was added proved to be very hard. Therefor the NaOH was weighted and added in solid form for sample LT12, and the pH was measured with indicator paper for the fresh samples just as the precipitate had settled at the bottom of the vessel, and after 24 h ageing in the same vessel. Hereafter, the term ageing will be used for the period during which the samples are stored at elevated temperature (70 ◦C), after the NaOH have been added and the initial reaction have occurred. Table 3.6 shows the composition of the precursor solution for the low temperature particle synthesis of LT12 samples. A summary of the added amount of NaOH into samples LT12a, LT12b, LT12c, LT12d and LT12e is shown in table 3.7. The temperature is held constant at 70 ◦C for all samples. The molar amount of Cu and Fe approximately corresponds to a concentration of 10 g/l.

Table 3.6 – LT12 precursor solution composition.

Chemical Amount (mmol) Amount Water — 10 ml Cu(SO ) 5H O 1.574a 393.0mg 4 • 2 Fe(SO ) 7H O 1.574a 437.6mg 4 • 2 a Corresponds to an approximate Cu2+/Fe2+-concentration of 10 g/l.

16 Table 3.7 – Conditions for low temperature synthesis LT12.

Sample Temperature (◦C) Added NaOH (g) LT12a 70 0.20 LT12b 70 0.25 LT12c 70 0.30 LT12d 70 0.40 LT12e 70 0.50

In table 3.7 the conditions for synthesis LT12 are listed. The reaction temperature is 70 ◦C for all samples and NaOH is added to the sample solutions with the amount increasing for the samples in alphabetical order from LT12a to LT12e. After the synthesis, the samples were cooled naturally from the reaction temperature and washed with deionised water twice and in ethanol once. The samples were stored in ethanol until characterisation could be undertaken.

3.1.4 Compression film making

In this work the principle when making CuFeO2 is the same as for the method used by Lindström et al. [45]. CuFeO2 powder is mixed with ethanol to a suspension. The suspension is then doctor bladed or spin coated onto a FTO glass substrate, or in some cases to a SnO2 coated FTO glass. The resulting film is then compressed in a hydraulic press under a pressure of around 15 kN/cm2 to 20 kN/cm2.Forthecompression,thesampleisplacedinbetweentwopolishedstainlesssteel cylinder plates. Aluminum foil is separating the sample film and one of the steel plates to avoid contamination of the plate.

3.2 Characterisation methods/equipment

SEM: Zeiss LEO 1550 FEG • XRD: • – Powder XRD Siemens D-5000 Th-Th ∗ Bragg-Brentano setup ∗ Motorised slits ∗ Instrumental broadening around 0.1◦ ∗ – GIXRD: Siemens D-5000 Th-2Th ∗ Parallel beam setup ∗ X-ray mirror ∗ Instrumental broadening around 0.3◦ ∗ – Both use a 1.54 Å Cu-Kα X-ray-source UV-Vis: Perkin Elmer Lamba 900 spectrometer • – Double-monochromator – 50-mm in diameter BaSO4 coated integrating sphere

17 3.3 Chemicals

Cu(NO ) 3H O, 99.5%, analysis grade, Merck KGaA • 3 2• 2 Fe(NO ) 9H O, 98 %–101 %,FlukachemieAG • 3 3• 2 Citric acid, 99.5%, reagent grade, Sigma-Aldrich Chemie GmbH • CuSO 5H O, 99 %, analysis grade, Merck KGaA • 4• 2 FeSO 7H O, 99 %, reagent grade, Sigma-Aldrich Chemie GmbH • 4• 2 FeCl 4H O, 99 % analysis grade, Riedel-de Haën AG • 2• 2 SnO ,colloidal,15 %wt SnO ,K+ pH 10, NYASOL SN15, The PQ Corporation • 2 2 Polyethylene glycol (PEG), 20 000(16 000–24 000), Fluka chemie AG • NaOH, 98 %, Sigma-Aldrich Chemie GmbH • Ethylene glycol, Sigma-Aldrich Chemie GmbH • Terpineol, Sigma-Aldrich Chemie GmbH • Salicylic acid, 99 %, Sigma-Aldrich Chemie GmbH • Ethanol, VWR •

18 4Resultsanddiscussion

Three methods for synthesising delafossite CuFeO2 have been investigated and the results are described and discussed below.

4.1 Sol-gel route

(a) (b) (c)

Figure 4.1 – Photos of annealed films on glass. (a) Film of 1 layer spin coating sol-gel prepared film on FTO coated aluminoborosilicate glas annealed at 450 ◦C. (b) ‘6 layer’-film on FTO coated aluminoborosilicate glas annealed at 450 ◦C. (c) ‘6 layer’-film on FTO coated aluminoborosilicate glas annealed at 700 ◦C.

In section 3.1.1 the method used to make oxide films on glass have been described. Some films in different stages of this process can be seen in figure 4.1.Figure4.1a and 4.1b is a picture of a single layer precursor film and a ‘6-layer’ precursor film, respectively, on high temperature resistant aluminoborosilicate glass and annealed at 450 ◦C.XRDmeasurementsonsuchfilms showed no crystalline features. Only FTO can be seen in a diffractogram with an incident angle of 0.2◦, which should have enhanced any peaks for crystalline phases of the topmost film relative to the FTO (the XRD diffractograms are found in appendix C,figureC.1). Such films also look very uniform in SEM, as can be seen (on ordinary soda lime glass with FTO)in figure 4.2. The thickness of a 6-layer film is around 400 nm as determined from SEM pictures showed in figure 4.3. Figure 4.1c show a ‘6-layer’ film prepared as the films just described, and then annealed a second time at higher temperature in an atmosphere with low oxygen content. The tem- peratures tried here range from 500 ◦C to 800 ◦C.Noneofthesamplesshowedanydelafossite phase peaks in XRD measurements. Only CuO and CuFe2O4 could be identified in XRD measurements seen in figure 4.4. The samples annealed at 700 ◦C in 300 ml/ min argon flow — which is exactly the condi- tions used by Prévot et al. [6]—didnotresultinanydelafossitephaseinthisstudy.XRD measurement results from an as prepared film on glass are shown in figure 4.5. Only Cu2O and maghemite Fe2O3 peaks are present in the diffractogram. The diffractogram peaks from

19 Figure 4.2 – SEM pictures of an amorphous film on FTO coated soda lime glass after 6 cycles of sequential spin coating and annealing at 450 ◦C.

Figure 4.3 – SEM cross section of an amorphous film deposited on FTO-glass by sequential spin coating and annealing at 450 ◦C.

2,000 C4_575deg_Ar FTO_ref CuFe2O4_34-0425 1,500 CuO_45-0937 SnO2tinoxide_41-1445

1,000 A.u.

500

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦)

Figure 4.4 – Diffractogram from annealed sample in 575 ◦C in an argon flow and a reference diffractogram from only FTO-glass. The furnace used could not measure the argon flow. Reference peaks are shown at the bottom with colours corresponding to each reference in the legend.

20 2,000 E1_on_glass_700deg_Ar Cu2O_05-0667 1,500 Fe2O3maghemite_39-1346

1,000 A.u.

500

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦)

Figure 4.5 – Diffractograms from annealed samples in 700 ◦C in an argon flow of 300 ml/min. Reference peaks are shown at the bottom with colours corresponding to each reference in the legend.

maghemite Fe2O3 closely resembles them from magnetite Fe3O4 separated only by a small shift. However, almost every peak in question are shifted in favour for maghemite, and the picture of the film in figure 4.1c is red — whereas magnetite is black — which should indicate that the phase is maghemite Fe2O3. Annealing at even higher temperature, 800 ◦C,resultedinmaghemiteFe2O3 as the major phase and Cu2O peaks were no longer present in the diffractogram, which can be seen in figure 4.6. The ovens used here and by Prévot et al. [6] are both tube furnaces with adjustable gas flow and the same diameter. The amorphous precursor films seen herein and in [6]alsoseem similar. Possibly, the oven used in this work could differ from the one used by Prévot et al., not holding exactly the same temperature as measured by the oven thermometer or leaking gas. However, the oven is used frequently by others and errors with the oven does not seem likely. Further, Chen & Wu [29] vary the annealing parameters in a similar process with thin films prepared by spin coating and obtained varying relative amount of delafossite phase for temperatures under 700 ◦C. The non delafossite phases found for lower temperatures, CuFe2O4 and CuO, are the same as found earlier for similar temperatures [29]. Cu2OandFe2O3,foundfor700 ◦C,havealso been reported before [33, 49], though as a product of decomposition of CuFeO2 at 1180 ◦C.As seen when comparing figure 4.5 and 4.6 for the sample annealed at 700 ◦C on glass and on FTO coated glass respectively, the appearance of Cu2OcorrelatingpeakscomparedtothoseofSnO2 and Fe2O3 seem to be prohibited by the FTO coating. This could indicate that the formation of crystalline Cu2O is less favourable at FTO glass than on bare glass, at least in the presence of Fe2O3. Two samples similar to sample E2 were annealed at 700 ◦C in nitrogen flow and at signifi- cantly lower flow rate, around 20 ml/min. These films were gray with white tints and showed traces of metallic tin in XRD measurements, seen in appendix C,figureC.3. Whether this was caused by too high oxygen partial pressure or by the nitrogen is not inves- tigated in this work. Though Chen & Wu [29]useanitrogenatmospheretoobtaindelafossite. However, this was on quartz substrates, and not on FTO coated glass. Thus the combination of FTO, Cu and Fe precursors and nitrogen cannot be excluded as a reason for the metallic tin formation.

21 E2_on_FTO_700deg_Ar 6,000 E2_800deg E2_800deg_th2th2 Cu2O_05-0667 Fe2O3maghemite_39-1346 4,000 SnO2tinoxide_41-1445 A.u.

2,000

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦)

Figure 4.6 – Diffractograms from annealed sample with film on FTO glass in 700 ◦C (red) com- pared with the same sample after annealing in 800 ◦C (blue) in an argon flow of 300 ml/min.A spectrum for the film after 800 ◦C recorded with a 2Th-2Th (not GIXRD) setup is also shown (green). Reference peaks are shown at the bottom with colours corresponding to each reference in the legend.

4.2 Powder synthesis

4.2.1 Hydrothermal process To obtain delafossite with a hydrothermal process, mainly the method of Xiong et al. [37]was 2+ 2+ followed. In the salts used, Cu(NO3)2 and FeCl2, the oxidation states are Cu and Fe .In 1+ 3+ CuFeO2 however, the oxidation states are Cu and Fe . Thus, a change of oxidation states is required in the synthesis. The synthesised powders show varying amounts of delafossite phase — it is mainly HT7 and HT8 where delafossite is detectable in XRD to any significant extent, as seen in the diffractograms for powders from the hydrothermal synthesis in appendix C,figureC.4.Other than delafossite, iron oxides and copper oxides are detected. SEM pictures of HT7 show particles of irregular size and shape (see appendix A,figureA.1). Some particles are cubic, others are flakes and yet others are ‘star shaped’. The particles in the dispersion could be a mix of delafossite CuFeO2 particles and particles of other phases. These synthesis routes did not provide a robust way to make delafossite CuFeO2,forthe conditions in this work. (However, e.g. the synthesis pressure could be varied by varying the grade of filling of the autoclave[39], which could possibly improve the results.) Instead, a low temperature route seemed more promising, also removing the need of an autoclave and lowering the required temperature.

4.2.2 Low temperature aqueous precipitation process

In the low temperature synthesis Cu(SO4)andFe(SO4)areusedasprecursorsalts.Herethe oxidation states are Cu2+ and Fe2+,justasinthehydrothermalmethod.However,thechange of salts seem have a large effect on which phases form. The delafossite particles formed are flake shaped and around 200 nm wide, but significantly thinner, as seen in figure 4.7. John et al. [38] use different concentrations of the NaOH solution added to precipitate from the precursor solution. Also in this work, NaOH solutions of different concentrations were used. The first, LT10, from the low temperature synthesis contained mainly Cu2Oas the main crystalline phase after 20 h and showed only small traces of delafossite CuFeO2 in XRD measurements. LT11 contained more delafossite phase but still significant peaks from

22 Figure 4.7 – SEM pictures of LT12d.

Cu2Oweredetectedinsamplesagedfor20 h. The diffractogram can be seen in appendix C. The pH in these samples was held around 9 or around 12, to the best of the authors ability. But for practical reasons it was hard to measure the pH when precipitation in the solutions was occurring. Anyhow, the final pH was measured to around 12.5 to 13 and 13 to 14 for sample LT11a and b respectively. Larger amounts of NaOH solution was added to LT11bthan a, but XRD measurements are almost identical. Further the concentration of the added NaOH solution was higher for LT11 than for LT10, which could indicate that the concentration of the added NaOH solution is more important than the amount of NaOH. To investigate this, NaOH was added as solid pellets in LT12. Indeed, this resulted in samples with delafossite CuFeO2 as the only identifiable crystalline phase in the precipitated suspension. This was the case even if the amounts of NaOH where to small to sustain the reaction of all the Cu and Fe. A summary of the low temperature synthesis LT12 with added amount of NaOH and measured pH just after the precipitation and after 24 h is presented in table 4.1. For samples LT12a, LT12b and LT12c the pH had even decreased (to pH 3 for LT12a) after 24 h compared to the fresh samples. For the other two samples, LT12d and LT12e, the pH was significantly higher, despite the added amount of NaOH being in the same order as for LT12a, LT12b and LT12c. This should indicate that the relative amount of NaOH consumed in the process is inbetween that added to LT12c and LT12d. The amount is 0.30 g and 0.40 g respectively. This corresponds to 30 g/l and 40 g/l in the precursor solution solvent. With the molecular mass of NaOH, 3 3 4 40 g/mol,thisgives 4 mol/l and 1mol/l, which in turn corresponds to a ratio of 0.1574 =4.76 1 mol/lNaOH NaOH or 0.1574 =6.35 mol/lCu = Cu . Molar ratios for all LT12samples are presented in table 4.1.Johnet al. [#38] suggest a reaction$ scheme where 4 OH- is required for the reaction which is only slightly less than the amount required in this work. John et al. [38]alsodescribethatthe‘reactionpH’playsaroleintheformationofdelafossite. In this work, the NaOH was consumed very fast when adding it to the solution. It would therefor be hard to speak of any ‘reaction pH’. Only after a certain amount of NaOH added, the pH started to rise. This sudden rise in pH can probably be accounted to that at this time, all of the Cu and Fe present in the solution have been consumed in the process. When no Cu or Fe remains, the reaction into delafossite — or precursor phases for delafossite — comes to an end. This allows the excess NaOH to raise the pH in the solution. As stated, if the NaOH is added as pellets delafossite forms even in conditions opposite to those just described: When the amount of NaOH is not enough for all the Cu and Fe precursors to react. In this case, the excess Cu and Fe must either be present integrated

23 2,000 LT12a_3h LT12b_3h LT12c_3h 1,500 LT12d_3h LT12e_3h CuFeO2v3R_39-0246 CuFeO2v2H_79-1546 A.u. 1,000

500

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦) (a) 3,000 LT12a_6h LT12b_6h LT12c_6h LT12d_6h 2,000 LT12e_6h CuFeO2v3R_39-0246 CuFeO2v2H_79-1546 A.u.

1,000

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦) (b) 4,000 LT12a_24h LT12b_24h 3,000 LT12c_24h LT12d_24h LT12e_24h CuFeO2v3R_39-0246 2,000 CuFeO2v2H_79-1546 A.u.

1,000

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦) (c)

Figure 4.8 – Diffractogram from LT12 taken from reaction temperature and washed after: (a) 3h, (b) 6hand (c) 24 h. Reference peaks for 3R and 2H delafossite CuFeO2 respectively shown as bars and indicated with stars in the label.

24 Table 4.1 – Added NaOH, resulting pH and XRD implications for low temperature synthesis LT12.

a Sample Added NaOH/Cu pHfresh pH24 h Major phase detected

LT12a 3.2 5 to 6 3 CuFeO2 —3R

LT12b 4.0 <6 4 CuFeO2 —3R

LT12c 4.8 6 5 CuFeO2 —3R

LT12d 6.4 12.5 to 13 13 to 14 CuFeO2 —3R

LT12e 7.9 13 to 14 14 CuFeO2 —3R a Molar ratio of added NaOH to Cu in the solution. with the delafossite, or as minor phases, most not crystalline enough to be visible in the XRD measurements. In sample LT12, SEM pictures (figures A.2, A.3 and A.4 in appendix A)indeed show not only the beginning of small flakes, but also other features more resembling blobs clutched together. XRD measurements in figure 4.8 indicate the structural composition of the LT12 samples from 3 different occasions in the reaction process. Even after just 3h,infigure4.8a,theonly clearly visible crystalline phase is delafossite CuFeO2. This is also the case for the first three samples, LT12a, LT12b and LT12c, where the added amount of NaOH was small — even if the peaks are very broadened and noisy. The major phase present in this synthesis is 3R delafossite. Samples LT12d and LT12e also showed some traces of 2H delafossite in the diffractograms in figure 4.8. This is in accord with the observations by John et al. that higher NaOH concentration encourages growth of 2H delafossite phase. But after 6h the 2H phase was no longer detectable in XRD measurements. Further analysis could perhaps confirm that higher ageing pH will give more 2H delafossite. However, the ageing pH does not seem to have any effect on the formation of other crystalline phases than delafossite. Even when the pH was as low as pH 3 after 24 h the only identifiable phase in XRD was delafossite — though the peaks are very broad and noisy, and several other phases could possibly be present. Further, when the concentration of the added NaOH is very high, i.e. added as pellets of pure NaOH, 3R delafossite seem to form within 3h if a large enough amount of NaOH is added. However, an increased concentration of NaOH in the solution added clearly decreased the relative intensity of the Cu2O compared to CuFeO2, comparing LT10 and LT11a and LT11b. LT11a and LT11b differed only in the amount of solution added, which seems to have negligible effects on the crystalline structure of the resulting particles in the suspension. This somewhat confirms the effects seen for LT12, where the crystalline phase is the same for the different amounts of NaOH — they only differ in particle size or crystallinity. In summary, the particle formation is very sensitive to the initial conditions during the precipitation. This is indicated not only be the concentration dependency on added NaOH, but also on the dependency on the precursor salts used. The need to reduce Cu2+ to Cu1+ to form delafossite could be a reason to the dependency on high concentrations of NaOH. However, when not only delafossite is formed, Cu2O is present as a biphase — which is also composed of Cu1+ — as for sample LT10 and LT11. So even there, the Cu is reduced. Of course, also the oxidation of Fe2+ into Fe3+ is important, but to investigate this further, analysis of the initially formed phases at different conditions should be performed. The temperature sensibility in air seen for the particles makes any annealing significantly harder and prevents isolating organic ligands to be used. The sensibility is not inconsistent with performance losses at similar temperatures of a thin film with 3R delafossite in [6]. The same

25 paper also reports thermogravimetric indications of oxidation of a powder sample (prepared in the same way) at above 400 ◦C. They attribute this difference to tensions in the film, whereas tensions would be absent in the particles. However the particles prepared in this work have the same structure but seem to be significantly more sensible to temperature. Possibly, this could be due to smaller particle size, though this should be further investigated. Changes at similar temperatures have been reported earlier, at 400◦Cfor3Rdelafossite with decomposition into CuFe2O4 and CuO [50].

4.3 Compression of film

The LT12 particles were not easy to spin coat or doctor blade on FTO glass, further described in appendix E in E. The particles easily fell of the substrate, and they seemed to be sensible to higher temperatures than 300 ◦C. The temperature sensibility and bad adhesion after simple doctor blading or spin coating leaves the need for an easy coating technique not requiring any higher temperatures. For this reason compression of doctor bladed or spin coated films was investigated. It has earlier been used for TiO2 films [45]resultinginmechanicallystableandelectricallyconductingporous films. When compressing doctor bladed or spin coated films with pressures of 15 kN/cm2 to 2 20 kN/cm the adhesion on FTO or SnO2 coated substrates increased and the films remained mechanically stable even for gentle scratching. Pressures lower than 15 kN/cm2 significantly decreased the mechanical stability. The films were smooth and appeared homogeneous. One example of a compressed film on SnO2 coated FTO glass can be seen in a SEM picture in figure 4.9.ASEM cross section is shown in figure 4.10. These films gave p-type photoresponse in LSV with chopped light, showed in appendix D. The photoresponse of a known n-type semiconductor, SnO2,wasintheotherdirectioninthe same measurement serie. In figure 4.11 UV-Vis measurements for pressed films on FTO glass are shown. At short wavelengths some interference fringes can be seen in the reflectance spectrum. These can also be seen in the UV-Vis spectrum for FTO glass in figure B.1 in appendix B,sothesecould possibly be accounted to the FTO glass beneath the pressed particle film. Between around 800 nm to 1000 nm the film starts absorbing, and then again, after 400 nm the transmittance goes down to close to zero. Above 1500 nm the reflectance goes up slightly, but this feature is also seen for only the FTO coated glass and cannot be said to be a property of the pressed delafossite film. In figure 4.12 the absorption coefficient for a compressed film can be seen. The contribution from the reflectance and the FTO glass are compensated for to obtain the line in the graph. The thickness of the film have been estimated from the SEM pictures in figure 4.10 and assumed to be 4 µm. In figure 4.13 the band gap have been estimated through the Tauc method described in section 2.3.2. Two band gap energy values have been estimated with r =2and r =1/2,to 1.07 eV (indirect band gap) and 1.42 eV (direct band gap) respectively. With 95 % confidence interval from the fitted lines they span 0.0877 eV and 0.1380 eV respectively. There are previous reports on an indirect band gap at similar values, [27]butoverall,around1.3eVto 1.5eVseem like a more common value. However, r =2gives a good fit in this work and a direct band gap at 1.30 eV have been predicted by theoretical calculations [28]. Interpolating lines around the Urbach tail can be another method of estimating the band gap and this can be seen in figure 4.14.Fromthefigurethebandgapisestimatedto1.3eV to 1.4eV. The line fitted to the data in the absorption edge was hard to fit to a certain value

26 but depending on which data points are chosen the band gap position varies from 1.29 eV to 1.37 eV. This would strengthen the indications of a (possibly direct) band gap at around 1.4eV.

27 (a) (b)

Figure 4.9 – SEM pictures of compressed CuFeO2 particle film on SnO2, FTO and soda lime glass. (a) is an overview and (b) is a magnified picture on the same area. The surface looks quite flat and homogeneous.

28 (a) (b)

(c)

Figure 4.10 – SEM cross section pictures of compressed CuFeO2 particle films on SnO2, FTO and soda lime glass. (a) and (b) is a series with different focus to compare the layer thickness and characteristics. The films have split at different places during cross section sample preparation. In (a) a layer of SnO2 can be seen on top of FTO coated soda lime glass at the edge of the cross section prepared sample. In (b), with the edge in the foreground, the compressed CuFeO2 is in focus with small hexagonal particles pressed together. (c) shows a similar picture (SnO2 seems to be lacking here — it’s thickness is suggested to vary over the substrate for this sample by SEM pictures in appendix A) with a CuFeO2 particle film around 4 µm to 5 µm thick. The left red bar is only as a vertical 1 µm reference. The film seems to have a similar thickness in (b).

29 100 T R 80 A

60

A,R,T (%) 40

20

0 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 Wavelength (nm)

Figure 4.11 – Transmittance (T), reflectance (R) and absorptance (A) from a sample of LT12d particles pressed on FTO glass. The sharp decrease in transmittance (and increase in absorptance) at around 360 nm is probably due to FTO absorption at these wavelengths, and should not be accounted to the delafossite film.

0.7

0.6

0.5

) 0.4 -1 m µ (

α 0.3

0.2

0.1

0 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 Wavelength (nm)

Figure 4.12 – The absorption coefficient for pressed CuFeO2 particles on FTO glass, estimated from the assumption that the film thickness is 4 µm. The film is not completely homogeneous and the thickness is based on SEM cross section pictures of a similar film pressed on FTO and SnO2. The absorption of the film have been corrected for the FTO absorption.

30 3 30 P2 P2 2.5 Eg =1.13 25 Eg =1.43

2 20 2 1/2

1.5 15 energy) energy) × × d

d 1 10 × × α ( α ( 0.5 5

0 0

0.5 5 − 1 1.5 2 − 1 1.2 1.4 1.6 1.8 2 2.2 Photon energy (eV) Photon energy (eV) (a) (b)

Figure 4.13 – The absorption coefficient multiplied by the thickness and photon energy, squared or to the square root, plotted against the photon energy to estimate the band gap energy from reflection and transmission data for a sample of delafossite particles from LT12 on FTO coated glass. The band gap estimated from the fit with (αdhv)1/2 (for indirect band gap) is between 1.02 eV to 1.11 eV (a) and the band gap estimated from the fit with (αdhv)2 (for direct band gap) is between 1.35 eV to 1.49 eV (b).

2 P2 Eg =1.3 1.5

1

0.5 d)

× 0 α ln( 0.5 −

1 −

1.5 −

2 − 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 Photon energy (eV)

Figure 4.14 – Logarithm of absorption coefficient times thickness plotted against photon energy. The flat region after the Urbach tail is fitted to a line that intersects with a fit in the absorption edge. The intersection is the estimated band gap energy, here around 1.3eV.

31 4.4 Choosing an ‘easy route’

Assuming that the sol-gel route would have resulted in 2H delafossite, this synthesis method can be compared to the low temperature route, not taking into account the performance of the ready films in electrochemical measurements (which could not be investigated or compared without a delafossite film from both routes). Here, some benefits and drawbacks of each synthesis method are listed. Some benefits and drawbacks of the sol-gel preparation with subsequent annealing are:

Sol-gel Benefits + Afilmissynthesiseddirectlyonthesubstrate + Easily controllable film thickness Drawbacks Requires high temperature and inert gas flow − Necessary to use a substrate withstanding high temperature − Precursor deposition is time consuming − Seemingly hard to obtain a film of delafossite CuFeO − 2 For the low temperature particles prepared with coating and compression the following can be concluded:

Low temp. Benefits + Easy and robust particle synthesis + May be possible to control phase composition and particle size + No high temperature or controlled atmosphere is needed + Can be coated and compressed onto many different substrates Drawbacks Requires deposition and compression of particles after synthesis − Conduction between particles may be a problem − Optimisation involves both particle properties and film properties −

32 5Summaryandconclusion

Delafossite CuFeO2 have potential to be a good solar absorber material with its band gap at around 1.5eV. Further, Cu and Fe are relatively cheap and abundant elements, but the synthesis of delafossite remains a challenge. In this work three delafossite CuFeO2 synthesis routes have been studied: A sol-gel route with subsequent annealing to make a film directly on a substrate • Ahydrothermalroutetomakeparticles • Alowtemperatureaqueousprecipitationroutetomakeparticles • The synthesised particles have been spin coated or doctor bladed onto substrates. The resulting thin films are characterised with XRD, SEM and spectrometry. The sol-gel films were hard to crystallise into delafossite CuFeO2.Otherphasesformed instead and despite following a known method, no delafossite was formed. However, delafossite CuFeO2 was synthesised with a hydrothermal route. No optimisation of pressure and temperature was attempted — nevertheless the method requires an autoclave and a temperature of above 160 ◦C in this work. Hydrothermal syntheses of delafossite CuFeO2 at 100 ◦C have been reported earlier. For this method, a highly alkaline solution is required. Finally, a low temperature synthesis of delafossite CuFeO2 was used. The method provides arobustroutetomakeparticlesataround70 ◦C without the need for alkaline conditions in the solution. In this synthesis, NaOH is added to precipitate precursor states to delafossite particles, before ageing in a laboratory bottle at the named temperature. The concentration of the added NaOH have a large impact on the outcome, and if added as pure NaOH delafossite appears to form in mild alkaline, neutral and possibly even slightly acid environments. The synthesised particles can be spin coated or doctor bladed onto several different sub- strates and compressed to break the agglomerates formed by the particles. Photoelectrochemi- cal measurements have shown some photoactivity for the compressed films. However, compres- sion condition optimisation and particle/substrate matching could be further investigated. The particles have shown some temperature induced phase change, although it have not been fully investigated. Slight annealing of the compressed films is not completely excluded at temperatures well below 450 ◦C,andpossiblythiscouldenhanceconductivityandother particle interconnection related properties.

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44. John, M. et al. Low-temperature synthesis of CuFeO2 (delafossite) at 70 ◦C: A new process solely by precipitation and ageing. Journal of Solid State Chemistry 233, 390–396. issn: 0022-4596 (Jan. 2016). 45. Lindström, H. et al. Anewmethodformanufacturingnanostructuredelectrodesonglass substrates. Solar Energy Materials and Solar Cells 73, 91–101. issn: 0927-0248 (May 2002). 46. Jain, G. C. & Das, B. K. A thermogravimetric study of the oxidation of CuFeO2. Journal of Materials Science 12, 1903–1908. issn:0022-2461(1977). 47. Derbal, A., Omeiri, S., Bouguelia, A. & Trari, M. Characterization of new heterosys- tem CuFeO2/SnO2 application to visible-light induced hydrogen evolution. International Journal of Hydrogen Energy 33, 4274–4282. issn:0360-3199(Aug.2008). 48. ODonnell, M. D., Watts, S. J., Law, R. V. & Hill, R. G. Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties Part II: Physical properties. Journal of Non-Crystalline Solids 354, 3561–3566. issn:0022-3093(July15, 2008).

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37 Appendices

38 ASEM

Figure A.1 – SEM pictures of HT7.

39 (a) (b)

Figure A.2 – SEM pictures of LT12a.

(a) (b)

Figure A.3 – SEM pictures of LT12b.

(a) (b)

Figure A.4 – SEM pictures of LT12c.

40 (a) (b)

Figure A.5 – SEM pictures of LT12d.

(a) (b)

(c)

Figure A.6 – SEM pictures of LT12e.

41 BUV-Vis

100 T R 80 A

60

A,R,T (%) 40

20

0 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 Wavelength (nm)

Figure B.1 – UV-Vis measurement results for FTO glass used to remove the FTO contribution in the band gap estimation for a film of pressed delafossite CuFeO2 on FTO glass.

42 CXRD

2,000 A2_02incident FTO_ref CuFeO2_2H_79-1546 1,500 SnO2tinoxide_41-1445

1,000 A.u.

500

0 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦)

Figure C.1 – Annealed samples in 700 ◦C in an argon flow of 300 ml/min. Reference peaks can be seen at the bottom with a label with a box of the corresponding colour in the legend box.

1,400 C2_500deg_Ar C3_575deg_air 1,200 C4_575deg_Ar FTO_ref CuFe2O4_34-0425 1,000 CuO_45-0937 SnO2tinoxide_41-1445 800 A.u. 600

400

200

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦)

Figure C.2 – Annealed samples in 500 ◦C to 575 ◦C in air or an argon flow of 300 ml/min. Refer- ence peaks can be seen at the bottom with a label with a box of the corresponding colour in the legend box.

43 3,000 cufeo2_700deg_in_n2 Cu2O_05-0667 2,500 Fe2O3maghemite_39-1346 Fe3O4magnetite_89-0691 CuFe2O4_34-0425 2,000 SnO2tinoxide_41-1445 Sn_86-2264 1,500 A.u.

1,000

500

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦)

Figure C.3 – Annealed samples in 700 ◦C in nitrogen flow of around20 ml/min. Reference peaks can be seen at the bottom with a label with a box of the corresponding colour in the legend box.

44 104 otmwt ae ihabxo h orsodn oori h eedbox. legend the in colour corresponding the of box a with label a with bottom Cu are features C.4 Figure · 2.2

HT1 HT2 2 HT3

R pcr rmtefis w o eprtr ytee.Ietfidcrystalline Identified syntheses. temperature low two first the from spectra XRD – HT4

2 HT5 n hmoerldlfsieCuFeO delafossite rhombohedral and O HT6 1.8 HT7 HT8 Cu2O_05-0667 CuFeO2v3R_39-0246 1.6 CuFeO2v2H_79-1546 Fe2O3hematite_33-0664 CuO_45-0937

1.4

1.2 45

1 2 eeec ek a ese tthe at seen be can peaks Reference .

0.8

0.6

0.4

0.2

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80 1,200 LT10 LT11a 1,000 LT11b Cu2O_05-0667 CuFeO2v3R_39-0246 800

600 A.u.

400

200

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦)

Figure C.5 – XRD spectra from the first two low temperature syntheses. Identified crystalline features are Cu2O and rhombohedral delafossite CuFeO2. Reference peaks can be seen at the bottom with a label with a box of the corresponding colour in the legend box.

46 DLSV

10 4 · − 1

0.5

0

Light off

0.5 −

Light on 1 − Current (A)

1.5 −

2 −

2.5 −

Compressed particle film

3 − 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 Voltage (V)

Figure D.1 – LSV on a pressed sample. The photoresponse indicated in the figure is opposite to that of SnO2, a n-type semiconductor.

47 ESynthesisconditions

Conditions for LT process

Table E.1 – LT10 precursor solution composition and added NaOH solution

Chemical Amount (mmol) Amount Water — 20 ml Cu(SO ) 5H O 3.148a 786.0mg 4 • 2 Fe(SO ) 7H O 3.148 875.2mg 4 • 2 32 wt% NaOH — 3mlb a Corresponds to an approximate Cu2+-concentration of 10 g/l. b This amount should correspond to around 1.41 g of pure NaOH.

Table E.2 – LT11 a & b precursor solution composition

Chemical Amount (mmol) Amount Water — 20 ml Cu(SO ) 5H O 3.148a 786.0mg 4 • 2 Fe(SO ) 7H O 3.148 875.2mg 4 • 2 32 mol% NaOH in a — 0.75 mlb 32 mol% NaOH in b — 1.65 mlc a Corresponds to an approximate Cu2+-concentration of 10 g/l. b This amount should correspond to around 0.75 g of pure NaOH. c This amount should correspond to around 1.65 g of pure NaOH.

Table E.3 – LT12 precursor solution composition

Chemical Amount (mmol) Amount Water — 10 ml Cu(SO ) 5H O 1.574a 393.0mg 4 • 2 Fe(SO ) 7H O 1.574 437.6mg 4 • 2 a Corresponds to an approximate Cu2+-concentration of 10 g/l.

48 Table E.4 – Summary of suspension of LT12-particles for spin coating.

Solvent Additive Comment Ethanol — Suspension separates quickly, bad adhesion after spin coating PET Not dissolving Citric acid Colloid dispersion, possibly forming chelate gel and needs annealing Citric acid and ethylene glycol Colloid dispersion, forming gel and needs annealing Terpinol Suspension separates quickly, bad ad- hesion after spin coating Water — Suspends better than in ethano, not wetting FTO PET Not wetting FTO

Observations when spin coating/doctor blading particle sus- pension

The particles synthesised in this work does not seem to differ much from each other in the ability to disperse in liquid. Even though no specific measurements investigating this have been made, a summary of simple observations will be presented here. All particles synthesised as in 3.1.3 seem to stay somewhat longer in suspension in water compared to ethanol and terpineol. Some LT12d particle suspensions with ethanol where mixed with citric acid and some with citric acid and ethylene glycol. They formed a colloidal dispersion, which did not separate at all. These dispersions could be spin coated on FTO glass. The citric acid and ethylene glycol forms an insulating gel which must be annealed to decom- pose. Possibly, the citric acid also forms some sort of chelate gel with the dispersed particles. None of these films were conducting and therefor would have to be burned away. Further, they dissolved very easily in water and it would not be possible to do any electrochemical mea- surements with water electrolyte for these films before annealing. A summary is presented in table E.4. However, an evident colour change was seen even when heated at a hot plate below 100 ◦C, and the delafossite peaks in XRD where significantly decreased. As seen in figure E.1,after annealing at 350 ◦C the peaks are almost completely gone. As the particles thus where sensible to even slight temperature increments, the organic material could not be burned away. The particles also seem to be agglomerated without the citric acid additive. They cluster together in ethanol, if not in high enough concentration. This makes spin coating result in dotted films with small clusters of particles spread out over the substrate. However, this effect could be somewhat decreased if the substrates were heated just before spin coating, most likely making the ethanol evaporate faster leading to visibly more even films. This also made more of the dropped dispersion deposit on the substrate.

49 3,000 FTO_corr HT12d_eth_onFTO_scAT1000rpm_annealed350deg HT12d_eth_onFTO_scAT3000rpm_annealed350deg 2,500 HT12d_eth_onFTO_scAT3000rpm_hpAT100deg Cu2O_JPC2-2CA-00-005-0667 CuFeO2v3R_39-0246 2,000 Fe2O3hematite_JPC2-2CA-00-033-0664 SnO2tinoxide_41-1445

1,500 Intensity

1,000

500

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (◦)

Figure E.1 – LT12d with citric acid spin coated on FTO and annealed. The green line shows the XRD result for a film spin coated at 3000 rpm and dried at a hotplate at 100 ◦C. The blue line and red line both show XRD measurements for samples annealed in an oven at 350 ◦C,theblue spin coated at 1000 rpm and the red at 3000 rpm.

50 FPeakbroadeningofpowderXRD with Scherrer equation

In the following section the particle size is estimated through the Sherrer equation based on peak broadening in XRD measurements. The Sherrer equation is written Kλ Dhkl(2θ)= Bhkl cos(θ) where Dhkl is the particle size, K is a shape depending constant (typically K 0.9), λ is the ∼ wavelength of the diffracted radiation, Bhkl is the peak broadening and θ is the angle for the peak. Note that it is cos θ and not cos 2θ in the denominator. The tables present the calculations of the size for different powder samples from LT12. βinstr is the instrumental broadening, here about 0.1◦. The λ-value used is 0.154 nm. K is set to 0.9.

Table F.1 – Sample LT12b. The crystalline size is calculated through the Scherrer equation. βinstr is the instrumental broadening.

Peak (h k l) 2θ FWHM FWHM βinstr (rad/π) Crystallite size (nm) − (0 0 3) 15.63◦ 1.5704◦ 0.008 169 5

(0 0 6) 31.20◦ 1.5710◦ 0.008 172 6

(0 1 2) 35.45◦ 1.4233◦ 0.007 352 6

(1 0 4) 40.32◦ 1.7419◦ 0.009 122 5

(1 1 0) 61.01◦ 0.4307◦ 0.001 837 27

(0 1 11) 70.25◦ 0.6552◦ 0.003 084 17

51 Table F.2 – Sample LT12c. The crystalline size is calculated through the Scherrer equation. βinstr is the instrumental broadening.

Peak (h k l) 2θ FWHM FWHM βinstr (rad/π) Crystallite size (nm) − (0 0 3) 15.50◦ 0.4494◦ 0.001 941 23

(0 0 6) 31.30◦ 0.5275◦ 0.002 375 19

(0 1 2) 35.83◦ 0.2682◦ 0.000 934 50

(1 0 4) 40.28◦ 0.3957◦ 0.001 643 29

(0 1 8) 55.31◦ 0.4903◦ 0.002 168 23

(1 1 0) 61.10◦ 0.2860◦ 0.001 033 50

(0 0 12) 65.02◦ 0.7243◦ 0.003 468 15

(0 1 11) 70.26◦ 0.4336◦ 0.001 853 29

(2 0 2) 72.80◦ 0.3292◦ 0.001 273 43

Table F.3 – Sample LT12d. The crystalline size is calculated through the Scherrer equation. βinstr is the instrumental broadening.

Peak (h k l) 2θ FWHM FWHM βinstr (rad/π) Crystallite size (nm) − (0 0 3) 15.55◦ 0.3600◦ 0.001 444 31

(0 0 6) 31.28◦ 0.3600◦ 0.001 444 32

(1 0 1) 34.61◦ 0.1800◦ 0.000 444 104

(0 1 2) 35.77◦ 0.2214◦ 0.000 68 68

(1 0 4) 40.28◦ 0.2700◦ 0.000 944 50

(0 1 8) 55.31◦ 0.4500◦ 0.001 944 26

(1 1 0) 61.12◦ 0.2700◦ 0.000 944 54

(1 0 10) 64.85◦ 0.2500◦ 0.000 833 63

(0 0 12) 65.14◦ 0.5500◦ 0.0025 21

(0 1 11) 70.16◦ 0.4500◦ 0.001 944 28

(2 0 2) 72.77◦ 0.6300◦ 0.002 944 19

52 Table F.4 – Sample LT12e. The crystalline size is calculated through the Scherrer equation. βinstr is the instrumental broadening.

Peak (h k l) 2θ FWHM FWHM βinstr (rad/π) Crystallite size (nm) − (0 0 3) 15.55◦ 0.27◦ 0.000 944 47

(0 0 6) 31.28◦ 0.27◦ 0.000 944 49

(1 0 1) 34.61◦ 0.27◦ 0.000 944 49

(0 1 2) 35.78◦ 0.27◦ 0.000 944 49

(1 0 4) 40.28◦ 0.27◦ 0.000 944 50

(0 1 8) 55.31◦ 0.36◦ 0.001 444 34

(1 1 0) 61.12◦ 0.27◦ 0.000 944 54

(1 0 10) 64.88◦ 0.316◦ 0.0012 44

(0 0 12) 65.15◦ 0.272◦ 0.000 956 55

(0 1 11) 70.26◦ 0.36◦ 0.001 444 37

(2 0 2) 72.87◦ 0.472◦ 0.002 067 27

53