Co3O4 Thin Films: Sol-Gel Synthesis, Electrocatalytic Properties &PhotoelectrochemistryTHESISPresented in Partial Fulfillment of the Requirements for the Degree Master of Science inthe Graduate School of The Ohio State UniversityByTushar S KabreGraduate Program in ChemistryThe Ohio State University2011Master's Examination Committee:Associate Professor Yiying Wu, AdviserAssistant Professor Joshua GoldbergerCopyright byTushar S Kabre2011iiAbstractWorld energy consumption is bound to increase with the increasing population. Fossilfuels widely used today, are limited in their supply. Moreover, high CO2

emission fromtheir widespread usage is believed to have caused the global warming. Clearly, in thelong run an economy based entirely on the fossil fuels is not a sustainable economy. It istherefore very important than ever before, to innovate new and sustainable ways toharvest the solar energy. With this view, work presented here investigates the electrocatalyticand photo-electrochemical properties of Co3O4 thin films prepared by the sol-gelmethod, for the purpose of water splitting and solar cells. The thin films synthesized werecharacterized by using x-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS).The electrocatalysis studies were done for the oxygen evolution reaction (OER). OERreaction is the half reaction occurring at anode in water splitting cell. It has largeoverpotential, which brings the overall efficiency of the electrochemical water splittingdown. Therefore, better OER electrocatalysts are required.In the OER experiments, we used the electrochemical milling (ECM) on Co3O4

thin

films, to generate the nanoporous Co3O4. Electrochemical measurements were done iniii1M NaOH solution with the thin films as working electrode in a typical three-electrodeassembly. OER performance for these ECM treated sample increased compared tosamples without ECM treatment. Scanning electron microscopy (SEM) images ofsamples were taken at different stages of experiments to detect any change in surfacemorphology. The planar surface of the thin films was found to have changed tohexagonal plates after ECM treatment, and they were only formed after exposing ECMtreated samples to 1M NaOH. XPS investigation revealed the formation of CoO fromCo3O4 because of ECM.These hexagonal plates were identified to be cobalt oxyhydroxide, using Ramanspectroscopy and X-ray diffraction (XRD). CoO(OH) is known to have higherconductivity. Hence increased OER performance of CoO(OH) thin films is maybe due tothe overall increase in effective active sites.Wide band gap semiconductors such as TiO2 or ZnO are limited to UV region of solarspectrum. Therefore, small band gap semiconductors such as Co3O4 with absorption invisible solar spectrum can potentially harvest higher amount of the solar energy.Reported studies of photo-electrochemical (PEC) properties of Co3O4 are very scarce.Hence, this work is partly focused on developing working procedure of stableivphotocurrent measurement for Co3O4 thin films. High absorption in the visible region ofthe solar spectrum was the initial motivating factor for pursuing the PEC study of Co3O4.Photocurrent measurements were done using a Xe lamp as the light source. In allelectrochemical measurements, a typical three-electrode assembly was used with Co3O4

used as working electrode. Many different electrolytes - aqueous as well as non-aqueous -were used. The photocurrent obtained was found to be very low, ~ 10-20 μA at the

maximum. Transient photocurrent was observed, maybe due to the surface trappedminority carriers. Large background dark current was present in almost all systems. Itcould be due to porous nature of the thin films, exposing FTO to the electrolyte.vDedicationThis thesis is dedicated to all my teachers.viAcknowledgmentsI would like to thank my advisor, Dr. Yiying Wu for giving me all essential guidancethrough different stages of this work. I am also thankful to Dr. Yanguang Li for thethorough discussions and practical help in getting the initial set up going forelectrochemistry experiments.I would like to specially mention Ms. Gayatri Natu, Dr. Zhiqiang Ji and Mr. PanitatHasin for all their help, encouragement and support. Moreover, thanks are due to all thenew graduate and undergraduate students of our group, for keeping a sincere and friendlyworking atmosphere.Sincerely,TkTushar S KabreviiVita2005................................................................B.S. Chemistry, University of Mumbai2007................................................................M.S. Chemistry, University of Mumbai2008 to present ..............................................Graduate Teaching Associate, Departmentof Chemistry, The Ohio State UniversityField of StudyMajor Field: ChemistryviiiTable of ContentsAbstract ............................................................................................................................... iiDedication ........................................................................................................................... vAcknowledgments.............................................................................................................. vi

Vita ................................................................................................................................... viiList of Tables ...................................................................................................................... xList of Figures ................................................................................................................... xiiList of Abbreviations ........................................................................................................ xvChapter 1: Introduction ...................................................................................................... 1Chapter 2: Co3O4 thin films – synthesis and characterization ........................................... 52.1 Introduction ........................................................................................................... 52.2 Sol-gel synthesis .................................................................................................... 52.3 Co3O4 thin film synthesis by sol-gel method......................................................... 82.4 Characterization of the Co3O4 thin films ............................................................. 10Chapter 3: Electro-catalytic study of Co3O4

.................................................................... 183.1 Introduction ......................................................................................................... 18ix3.2 Introduction to electrochemical milling .............................................................. 243.3 Experiments ......................................................................................................... 263.4 Results ................................................................................................................. 283.4 Discussion ............................................................................................................ 47Chapter 4: Photoelectrochemical study of Co3O4

............................................................ 54

4.1 Introduction ......................................................................................................... 544.2.1 Fundamentals of Photoelectrochemistry .......................................................... 554.2.2 Introduction to Photoelectrochemistry of Co3O4

.............................................. 584.3 Experiments ......................................................................................................... 604.4 Results ................................................................................................................. 67Chapter 5: Conclusions and future work .......................................................................... 815.1 Electrocatalysis .................................................................................................... 81References ......................................................................................................................... 84xList of TablesTable 1: Assignment of Co3O4 optical absorption peaks13

............................................... 16Table 2: Literature survey of Co3O4 OER experiments .................................................... 23Table 3: Electrochemical charge and discharge time for Co3O4 thin film samples. Refer toFigure 9 ............................................................................................................................. 28Table 4: Tafel slope and Exchange current density for Co3O4 thin film samples beforeand after ECM as calculated from Tafel plot in Fig. 10. (Sample 2 plot is not shown inFig. 10) .............................................................................................................................. 32Table 5: Summary of Co2p and O1s peak positions for different samples of Co3O4 thinfilms – from experiments and literature ............................................................................ 42Table 6: Literature survey of different methods for synthesis of CoO(OH) ..................... 49Table 7: Comparison of Co3O4 with CoO(OH) ................................................................ 50

Table 8: Different band gap transitions in Co3O4, ............................................................ 59Table 9: Different reference electrode used ...................................................................... 62Table 10:Preparation of different electrolytes used in the PEC experiments ................... 65Table 11: Summary of the PEC data obtained under different experimental conditions.Working electrode is thin film of Co3O4 on FTO, unless specifically mentioned. ........... 72xiTable 12: Summary of the PEC data obtained under different experimental conditions.Working electrode is thin film of Co3O4 on FTO, unless specifically mentioned. ........... 73xiiList of FiguresFigure 1: World marketed energy consumption from year 1990 to 2035 ........................... 2Figure 2: Image (a) blank FTO slide (b) Co3O4 thin film on FTO substrate .................... 10Figure 3: XRD spectrum of Co3O4 thin films on FTO substrate ..................................... 11Figure 4: SEM micrograph of Co3O4 thin films on FTO substrate .................................. 13Figure 5: UV-Vis spectrum (K-M transformed DRS spectrum) of Co3O4 thin film onglass substrate. .................................................................................................................. 15Figure 6: The overpotential for oxygen evolution as a function of the enthalpy changefrom the lower to higher oxide transition in acidic and basic medium for different metaloxides ............................................................................................................................... 20Figure 7: Schematic illustration of structural Changes occurring in electrode duringdischarging and charging process by conversion mechanism .......................................... 24Figure 8: SEM micrographs of starting CuO (a) and discharged product (metallic Cu)obtained at the current density of 0.32mA/cm2 (b), 0.065mA/cm2 (c) and 0.032mA/cm2

(d). .................................................................................................................................... 25

Figure 9: Electrochemical charge-discharge curves for Co3O4 thin film samples cycledbetween 0-3V at constant current of 30 μA. ..................................................................... 29xiiiFigure 10: (a) Cyclic voltammograms of sample before and after ECM in 1M NaOH at5mV/s scan rate. (b) Tafel plots of sample before and after ECM. .................................. 31Figure 11: SEM micrographs of Co3O4 thin film samples after various treatments ......... 33Figure 12: SEM micrograph of exposed (to NaOH) and unexposed (outside of circle) areaof the sample. .................................................................................................................... 36Figure 13: SEM micorgraphs of ECM trated samples after NaOH exposure for differentamount of time. a) 15 min b) 30 min c) after CV d) after OER measurements ................ 37Figure 14: Co2p region of XPS spectrum of Co3O4 thin film samples without ECM andwith ECM ......................................................................................................................... 39Figure 15: O1s region of XPS spectrum of Co3O4 thin films without ECM .................... 40Figure 16: O1s region of XPS spectrum of Co3O4 thin film samples after ECM (CoO) . 41Figure 17: XRD spectrum of ECM treated sample after OER. ........................................ 45Figure 18: Raman spectrum of ECM treated samples after OER measurements ............. 46Figure 19: Crystal structure of CoO(OH) ......................................................................... 53Figure 20: Energetic position of fermi level under different conditions .......................... 57Figure 21: Schematic diagram and photograph of PEC cell, front and lateral view ........ 64Figure 22: LSV curve for TiO2 sample under chopped light at pH of 6.884 ................... 68Figure 23: Photocurrent square against potential vs N.H.E For TiO2 PEC experiment inacetate buffer ..................................................................................................................... 69

xivFigure 24: Change in flat band potential with pH of the acetate buffer used in TiO2

aqueous PEC experiment .................................................................................................. 70Figure 25: Chopped light LSV curve for Co3O4 thin films in ferrocene based nonaqueouselectrolyte............................................................................................................ 71xvList of AbbreviationsBTU British Thermal Unit (1BTU = 1055 joules)CV Cyclic voltammetryCVD Chemical vapor depositionECM Electrochemical millingFTO Fluorine doped Tin oxideGAA Glacial acetic acidIPCE Incident photon to electron conversion efficiencyK-M Kubelka-MunkLaser Light amplification by stimulated emission of radiationLSV Linear sweep voltammetryOER Oxygen evolution reactionPDF Powder diffraction filePEC PhotoelectrochemistryPLD Pulse Laser DepositionSEI Solid electrolyte interfaceSEM Scanning electron microscopyTCO Transparent conducting oxideTEM Transmission electron microscopyTEOS Tetraethyl orthosillicateTGA Thermo-gravimetric analysisUV-Vis Ultra violet and visibleXPS X-ray photoelectron spectroscopyXRD X-ray diffraction1Chapter 1: IntroductionThe demand for the energy supply required to maintain the steady growth in economyincreases with the increasing population. Today’s industrial economy is based on therapid consumption of fossil fuels, which are non-renewable and finite in their amount.Hence, there is an urgent need to find new renewable and cheap sources of energy. Yearwise energy demand of the world, starting from the year 1990 and projected into year2035 is shown in Figure 1. The projection in future is based on the current data of the

total marketed energy consumption of the world and the percentage GDP growth. It willhold true, only if the current trend continues in a business as usual fashion.The current research on the alternative energy focuses on different areas includingvarious types of solar cells for capturing the solar energy, fuel cells for cleaner andefficient consumption of existing fuels, hydrogen generation and so on.1

Invariably,almost all these technologies involve the use of semiconductors as the energy transducer.Current “grid” infrastructure may or may not be usable with these renewable energysources, depending on the amount and rate of energy generation. However, the currentscenario clearly indicates that there will be a huge volume of “off grid” usage ofelectrical energy in the form of batteries, supercapcitors or fuel cells.2Most of these forms of electrochemical storage or generation involve the use ofsemiconductor in one way or the other. This underlines the importance of the study of thesemiconductor electrochemistry, especially metal oxide semiconductors, as they haveshown potential for application in almost all the research directions mentioned above.Co3O4 has been extensively studied for various applications such as electro-catalyst forthe oxygen evolution reaction (OER), as a surface coating of solar selective absorber, as aphoto-electrode in photo electrochemical (PEC) studies, as a catalyst for hydro- crackingof crude oil and as an anode in a lithium ion battery. A brief survey of literature publishedon the application of Co3O4 as the OER catalyst and for the PEC will be covered in therespective introductions of the following chapters.02004006008001990 1995 2000 2007 2015 2020 2025 2030 2035Quadrillion BTUHistory ProjectionsFigure 1: World marketed energy consumption from year 1990 to 2035, history and projections based on

current data 23Physical and chemical properties of bulk Co3O4 have been extensively investigated. It isknown to have a spinel type crystal structure. It can be described as a regular cubic closedpacked oxygen framework with Co3+ filling half of the octahedral interstices and Co2+

occupying one eighth of tetrahedral interstices. Such site occupancy has been widelystudied using magnetic susceptibility and Mössbauer spectroscopy. An antiferromagnetictransition observed in Co3O4 at around 40K has been explained based onthe site occupancy of Co2+ and Co3+ metal ions oxygen framework of Co3O4.3For both PEC and OER experiments, mechanically and chemically stable Co3O4 filmswere required. Sol-gel type of method was used to prepare thin films on differentsubstrates. Experimental details and characterization of these films are covered in chapter2.Co3O4 is also known to be a good electro-catalyst for variety of reactions. Reactionstudied in this work is oxygen evolution reaction (OER). Co3O4 is one of the goodcandidates as an anode for this reaction. Work reported here shows a simple and effectivemethod of synthesizing nanoporous Co3O4 with higher surface area. Nanoporous Co3O4

was then used as electro-catalyst for the OER reaction. An increase in the OER activitywas observed along with a change in chemical composition. In fact Co3O4

was found tohave changed into CoO(OH), and it was responsible for the increased the OER activity. Itwill be described in chapter 3.4Co3O4 is a p-type semiconductor. Various studies involving Mott-Schottky4

measurementand Seebeck coefficient5 measurement have been done to confirm that.This current work uses the photo-electrochemical approach to investigate Co3O4

semiconductor nature, in order to explore its use in liquid junction solar cells and watersplitting. The work on PEC of Co3O4 is discussed in detail in chapter 4. Moreover, all the

work is summarized in chapter 5 along with the future work plans.5Chapter 2: Co3O4 thin films – synthesis and characterization2.1 IntroductionIn the current work, thin films of Co3O4 were chosen as the form of the samples to besynthesized because it serves as a good starting point for basic experiments. Thin filmscan be synthesized by various methods such as spray pyrolysis, chemical vapordeposition (CVD), chemical vapor transport reactions, pulse laser deposition (PLD),sputtering, electrophoresis, and electrochemical deposition. 6-9 For the purpose of ourOER and PEC experiments, we synthesized the thin films using sol-gel synthesis.General introduction to sol-gel synthesis, experimental details of the sol-gel synthesismethod used, and characterization results of the thin film sample will be covered in thefollowing sections of this chapter.2.2 Sol-gel synthesisSol-gel synthesis has a long history. Early work in this field was focused on the synthesisof ceramic materials. Basic principle of the sol-gel synthesis involves formation ofcolloidal sol, which disperses the precursors of the final intended product in uniformmanner. Then this sol was coated in layer-by-layer fashion on a suitable substrate.6Choice of the substrate depends on the requirement constraint set by the experiment.Each layer is air dried to form a gel. Hence, the name is sol-gel. The gel holds residualamount of solvents used as trapped droplets. However, the overall volume of the layers isreduced; such reductions in volume can potentially build up stress. It may result in agranular, brittle structure. Hence, a drastic change in volume is not desired. The gelformed can be annealed at required temperature to get desired product.Most of the initial studies were focused on silica based sols such as tetraethylorthosilicate(TEOS), Si(OC2H5)4. Various other systems are possible as long as some

precursor is available. The stability of aqueous precursor is greatly affected by the pH. Incase of non-aqueous sols, various ligands can be used to increase the stability of themetal-based system.As evident from its basic principles, the sol-gel method gained wide popularity becauseof its robustness, procedural control over composition of precursor and thus thestoichiometry of the final product. Ease of doping at various concentrations is anadditional advantage in some cases. Most of the sols are based on organic solvents, whichreduce the time required for gel formation.7Following is the summary of basic processes involved in sol-gel synthesis.1. Formation of sol: This involves the mixing of precursor solutions to formsuspension of colloidal particles in the solvent of choice. As mentioned earlier,water based systems are affected by the choice of the pH. Colloid can bestabilized through charge stabilization. Hence, pH can be chosen to have someresidual amount of charge on colloidal particles. Thorough mixing improves thehomogeneity and phase pureness of the final product.2. Casting: This is a step to mold the sol in particular shape, this can be replaced bydip coating or spin coating procedures. For the dip coating and the spin coating,higher viscosity sols with small amount solvents are required.3. Gelation: In this step, colloidal particles slowly form 3-D network and trap smalldroplets of solvent remaining in those networks. Time required for the gelationdepends on the viscosity of the sol. In case of polymer-based sols, fibers can bespun from such sols.4. Aging: This is a process of syneresis of colloids. During aging, condensationcontinues along with localized solution and reformation of the gel network, whichincreases the thickness of the inter particle necks, decreases the porosity. Hence,with aging gel becomes strengthened. There should be a resistance to the stressdeveloped in gel after aging to avoid cracking later in the drying process.

5. Drying: During drying, the liquid is removed from the interconnected porenetwork. Large capillary stresses can develop during drying when the pores aresmall (< 20 nm). These stresses will cause the gels to crack catastrophically, if the8drying process is not controlled by decreasing the liquid surface tension. Theliquid surface tension can be lowered by addition of surfactants or elimination ofvery small pores, by hypercritical evaporation - which avoids the solid-liquidinterface, or by obtaining monodisperse pore sizes by controlling the rates ofhydrolysis and condensation.6. Dehydration: Dehydration removes any residual hydrated material remaining atthe surface of the sample.7. Densification and annealing: This is the final step of heating to make thematerial compact, remove any residual organic groups remaining. Hightemperatures are preferred for this step. Thermo gravimetric (TGA) curve data forthe final intended product can be used to choose appropriate range of temperaturefor annealing. Atmosphere of heating can be changed from neutral air to reducingor to oxidizing based on the specific requirement.2.3 Co3O4 thin film synthesis by sol-gel methodCobalt oxide thin films were prepared using a previously described method10

with somemodifications. Co3O4 thin films were dip coated on pre-cleaned Ti foils using cobaltoxide precursor sol. The sol preparation was started with a 12.5mmol Co(II) nitrateprecursor (Co(NO3)2.6H2O, Sigma Aldrich). Cobalt was precipitated as Co(OH)2 usingequivalent amount of NaOH. Precipitate was washed three times with deionized water,and was re-dispersed using 25mL of glacial acetic acid (GAA). Sol thus obtained was9heated at 140 ºC to remove excess water and was replenished again with glacial aceticacid to 25 mL. Sol stored in closed container was observed to be stable for a day.For the oxygen evolution reaction (OER) experiments thin films were coated on one sideof Ti foil cut into appropriate dimensions, whereas for photoelectrochemical

measurements thin films were coated on Fluorine doped Tin oxide (FTO) coated glassslides. FTO is a transparent conducting oxide (TCO) which provides good electricalcontact to the cobalt oxide thin film and acts as transparent window, for back illuminationon the sample in PEC cell. Sol was then dip coated at a withdrawal speed of 90 mm/minon pre-cleaned Ti foils. The withdrawal speed was optimized to get thick and uniformlycoated thin films. The thickness of the gel is related to the withdrawal rate.Cleaning of the substrate is important for sol-gel film to adhere firmly. Ti foils werecleaned with alcoholic KOH and rinsed first with water and then with acetone, and driedin oven for 15 min at 60 ºC. Titanium does not react with alcoholic KOH at temperaturesbelow 80 ºC. After each dipping, samples were annealed at 350 ºC for 5 min and then leftto cool in air. After four such dip-anneal cycles, samples were heated at 400 ºC for 1 hrfor final annealing treatment. Number of dip-anneal cycles can be changed to change thethickness of the thin film. Four cycles were used in our experiment.A similar procedure was used to coat Co3O4 thin films on FTO for PEC experiments.Washing procedure used was same as in case of Ti foils. After 3 dip-anneal cyclessamples were heated to 500 ºC for 4 hours. The higher temperature and longer heating10time leads to better crystallinity11, 12. It is a favorable condition for higher photocurrents.This will be explained in details in discussion section of PEC experiments in chapter 4.2.4 Characterization of the Co3O4 thin filmsCo3O4 is known for the non-stoichiometry in its composition. It is a stable oxide of cobaltin the temperature range of 350 ºC to 750 ºC. Beyond 850 ºC, Co3O4 is decomposed intocobalt monoxide. Characterization was done using x-ray diffraction (XRD), Scanningelectron microscopy (SEM) and UV-Visible spectroscopy.Figure 2: Image (a) blank FTO slide (b) Co3O4 thin film on FTO substrate after 3 dip-annealing cycles(a) (b)

112.4.1 X-ray diffractionThin films prepared on FTO substrate were characterized using XRD. (XRD, Cu K_,Geirgerflex, Rigaku Co, Japan). Scanning angle range was limited to 30 to 60 degrees asmajor peaks for Co3O4 occur in that region. Small peaks for Co3O4 were observed withrelatively intense peaks of Tin oxide (SnO2). Peaks for Co3O4were indexed according tothe JADE database. (PDF # 71-0816). From previous SEM experiments on cross-sectionof the Co3O4 coated FTO, thickness of the thin films is known to be of the order of 50-100 nm. Because of such a low thickness very small amount of sample is interacting withthe x-rays, hence the diffracted peaks are low in intensity.Figure 3: XRD spectrum of Co3O4 thin films on FTO substrateCo3O4 PDF# 71-0816

122.4.2 Scanning electron microscopySEM measurements were done using Sirion (SEM, Sirion, FEI Co., Hillsboro, OR).Sample showed uniformly thick porous surface. The porous nature is maybe due to thegel 3D network formed during the Sol-gel process, which is annealed afterwards. Porousfilms with grains attached to each other may help for better contact with electrolyte.Films were firmly attached to the substrate, good mechanical stability is important forrobustness of the experiments.SEM images at two different magnifications are shown in Figure 4 from various regionsof the sample to show the film quality in different regions. Some of the SEM imagesshow cracks on the surface of about 20 nm width. Such cracks may have been developedduring dip annealing part of the procedure. Films were cooled to room temperatureduring each dip-annealing cycle to avoid building up of stress in the films.132.4.3 UV-Vis SpectroscopyUV-Vis spectrum was obtained with a Lambda 950 spectrophotometer using the diffuse

reflectance spectroscopy mode (DRS). (Perkin Elmer, Waltham, MA, USA). Filmsdeposited on the glass substrate were used for the measurement. A blank glass slide wasFigure 4: SEM micrograph of Co3O4 thin films on FTO substrate (a) showing uniform coating of thin filmand (b) its porous nature.(a)(b)14used as a reference. Kubelka-Munk transformation was done on the DRS data to get UVVisiblespectrum of the Co3O4 thin films.Kubelka-Munk (K-M) is a widely used theoretical model of reflectance. It assumes ahomogeneous and continuous sample, and that the angular distribution of intensities isequal. The K-M equation used for the conversion can be written as follows.____ ___ _ ___

____Where, k = molar absorption coefficientR = Absolute reflectances = scattering coefficientFigure 5 shows the UV-Vis spectra obtained after the K-M conversion of the DRS spectrafor dip-annealed sample. Co3O4 has been reported to show all the four types oftransitions.15Co3O4 is known to show strong optical absorption in visible and infrared region, it iseasily evident from its dark black color. Absorption bands are assigned to the transition in3d level electrons from Co2+ and Co3+ ions.13 The optical absorption for semiconductorsis generally further analyzed using the classical relation, __ _ ___ _ ____

Where _ = absorption coefficienth = Planck’s constantv = frequency of light in mEg = optical band gap energy in eVn = 1,2,3 or ½ depending on the types of transitionFigure 5: UV-Vis spectrum (K-M transformed DRS spectrum) of Co3O4 thin film on glass substrate.

16Such an analysis of the absorption spectrum has been reported in the earlier works9. Itwill be discussed in chapter 4.Observed Energy (eV) Assigned transition0.8 Co2+ (_*e) _ Co2+ (_*t2)1.0 Co2+ (_2t2) _ Co3+(_*e)1.3 Co3+ (_2t2) _ Co2+(_*t2)2.1 O2- (_*T) _ Co2+(_*t2)In the Table 1 above, Co2+ and Co3+ along with their oxygen ligands were consideredtogether rather than non-interacting individual sites. Valence orbitals for oxygen are thethree 2p orbitals and for Co2+ and Co3+ 3d, 4s and 4p. Initial valence state ionizationenergies for Co3+, Co2+ and O2- ions were calculated using Pauling’s electro-neutralityprinciple. According to Pauling’s electro-neutrality principle, stable ionic structurepreserves local electronic neutrality. Moreover, orbital energies of isolated gas phaseions are obtained from empirical ionization potentials. The final assignment of thetransitions is understandably approximate as no orbital can be described as a pure metalor a pure oxygen orbital. Table 1 summarizes the different charge transfer transitionsallowed in unit cell of Co3O4. The first transition shown in Table 1 is the most localizedtransition having ligand field character of Co2+ ion. However, oxygen ligandsparticipation as ligands for both Co2+ and Co3+ results in broader band with higherintensity. The second transition shows internal oxidation-reduction process within latticeTable 1: Assignment of Co3O4 optical absorption peaks13

17for Co2+ - Co3+ charge transfer process. This is peculiar to most the mixed valence statesoxides. The third transition is exactly opposite of second one and occurs at higher energy.The fourth transition is ligand to metal charge transfer transition. In simple binary oxides,such transitions are simple in nature. However, in mixed valence oxides they are morecomplicated. Unequal inter-electrons repulsion splits the ligand orbitals, which results in

many more such transitions with broader bands. O2- to Co3+ ligand to metal chargetransfer transition occurs at even more higher energy however, it is not resolvable fromthe broad bands of O2- - Co2+ ligand to metal charge transfer transitions. Anyirregularities in the symmetric environment of the Co3O4 lattice, such as defects or latticedistortions brought by doping will destroy the local symmetries and hence reduce thedegeneracy of orbitals. This may introduce new transitions in case of doping withdifferent intensities and bandwidth.18Chapter 3: Electro-catalytic study of Co3O4

3.1 IntroductionOverpotential is an inherent problem with any electrode reaction. It is a kinetic constraint,which results in the need of application of higher magnitude bias than what is expectedbased on thermodynamic standard reduction potentials. Since voltage is just the amountof energy needed to pass a unit charge across the circuit, this results in low efficiencyoutput of the system. An electro-catalyst effectively brings down this overpotentialactivation barrier.The oxygen evolution reaction, which is oxidation of water, is one of the importantreactions that occur at the anode. Since it is coupled with other important reactions suchas hydrogen evolution in water splitting, it is important to investigate and develop newstrategies to get better electro-catalysts. Many factors such as mechanism of electrodereaction, kinetics, thermodynamics, effective surface area, conductivity and chemical andmechanical stability decide the overall performance of an electro-catalyst.19Metal oxides have proven to be good electro-catalysts. Early work in this field includesresearch efforts of S Trassiti and Rasiyah14, 15. Efforts were made to correlate metaloxygenbond strength with the enthalpy change associated with increasing the oxidationstate of metal ion with overpotential. Such correlation mostly show a volcano shaped

curve, which suggest that lowest overpotential is observed at an optimum value of metaloxygenbond strength. Any deviation from this optimum value results in a higheroverpotential. This is illustrated in Figure 6. Data is presented for both acidic and basicmedium. Metal oxides at the top of this volcano curve, such as RuO2, have really lowoverpotential and have proven to be really good electro-catalysts. Unfortunately, they areeconomically not feasible for wide scale application. Hence, the main thrust in this fieldhas been to improve the efficiency of metal oxides like MnO2, Co3O4. A good review ofthe history of the research in this area can be found here16.The most commonly proposed mechanism in literature for oxygen evolution on Co3O4

can be written as follows17

________

________ _____________ ____

_____________ ___________________ ____

________ __

__ ____ ______ _ !_ ______20Considering the low current densities obtained in practical water splitting cell, increasingthe surface area will increase the exchange current density. Therefore, we have attemptedto increase the surface area of the thin film Co3O4 electrode, and the results are reportedhere. In contrast to the solution-based synthesis of Co3O4 nanostructures, a top downapproach was used in this study with thin films as the starting point.Figure 6: The overpotential for oxygen evolution as a function of the enthalpy change from the lower tohigher oxide transition in acidic and basic medium for different metal oxides18

21In the previous work reported in literature, various approaches have been taken to

correlate different properties of metal oxide surfaces to the electrochemical overpotentialobserved. The step of desorption rather that adsorption was found to be the ratedetermining step in the OER. Many studies involving homo nuclear isotopic exchange onoxide catalyst has been reported, which in a way measure looseness of the surface oxygenbonds. Work done in this direction is reported by Boreskov et al19 and Winter et al20-22.The volcano curve in Figure 6 is based on data obtained from these references. Tseng andJasem23 found that during the different intermediate reaction steps, the metal oxidesurface changes from lower oxidation state to higher oxidation state. For example, cobaltoxide changes from Co3O4 to Co2O3, which is today understood as an oxidation of Co(II)to Co(III). The enthalpy change associated with such a transition was found to becorrelated with the free energy change in adsorption. Hence, the plot of free energychanges during adsorption/desorption against overpotential of different metal oxidesshows volcano behavior. Their work led to the prediction that RuO2 and the spinelNiCo2O4 are two of the best available electro catalysts. Since then many studies havebeen done on RuO2 and NiCo2O4.In general, doping cobalt oxides with Li, Ni or Cu have been shown to enhance theirelectro-catalytic property. Nikolov et al in their work have showed that different extent ofdoping affects the geometric and electronic factors which change the electro-catalyticbehavior.24 A decrease in the Tafel slope with increase in the dopant content wasobserved, which was thought to be due to the increase in the number of active sites. This22results in larger charge transfer coefficients. Mechanistic studies on NiCo2O4

in KOHusing various electro-analytical methods show formation of higher valence oxides of Niand Co on NiCo2O4 surface before the oxygen evolution maxima 25-28. Since such

formation of higher-valence-oxide was found to be limited to the surface, higher surfacearea of electrode facilitates the formation of more active centers. It has been shown thatCo3O4 nanowires (NW) doped with Ni gives higher performance over pure cobalt oxideNW29. The increased performance can be understood based on the higher effectivesurface area. Ni doping also increases conductivity of the NWs and makes more activesites accessible to electrons. This demonstrates that increasing the surface area andchanging the electronic factors by doping are two viable research approaches to get betterperforming electro-catalysts.S. Trassiti et. al have shown that OER performance of Co3O4 is affected by pH, presenceof support layer and calcination temperatures. Support layer reduces the current drop atthe electrode-substrate interface. 30 Recently it has also been shown by Frei H. et al that amesoporous silica as support layer works well for Co3O4 and MnO2

31-33. Nano-rodbundled structures in these mesoporous catalyst were reported to be the main factorincreasing the surface area and turnover frequency. The turnover frequency is the numberof oxygen molecules evolved per second per active site. Table 2 summarizes up to dateinformation on the different literature published on Co3O4 OER experiments. It has partlybeen adopted from supporting information of reference 29.23ReferenceNumberPreparationMethodExperimentalConditionsTafel plot(mV/dec)ExchangeCurrent Density(A/cm2)29ChemicalGrowth

1M NaOH @ RT606.6 x 10-12

34 Electrodeposition 1M NaOH @ 25 ºC 40 NR24 Teflon bondedfilm fromthermaldecomposition@ 350 C@400 C3.5 KOH5042NR35 Electrodeposition 1M NaOH 43 3.2 x 10-6

36 Spray Pyrolysis@ 400 C1M KOH, 25 ºC 60 NR10 Dip coated solgelfim annealed@300 C@500 C1M KOH53-5557-59NR37 Spray Pyrolysis@ 500C1M KOH 50-68 NRTable 2: Literature survey of Co3O4 OER experiments243.2 Introduction to electrochemical millingThe method used to increase the surface area is called electrochemical milling (ECM). Inthis process, the sample is used as a cathode, and electrode is repeatedly lithiated and delithiatedin a Li ion battery. Because of structural changes occurring during the dischargeand charging processes, electrode materials tend to form smaller grains of 30-50 Å size.Figure 7 illustrates such changes in a typical metal oxide electrode38. Metal oxide ondischarging forms metal domains of various sizes in the matrix of Li2O, depending on thedischarge current density. During the charging, the reaction reverses and these metaldomains react with Li2O to form nano-sized metal oxide. This is also known as the

process of pulverization.Figure 7: Schematic illustration of structural Changes occurring in electrode during discharging andcharging process by conversion mechanism 25

25The first work published on this topic 39 includes the use of a CuO cathode, which turnedinto nanostructures of Cu after discharging. Depending on the discharging current,morphology of the electrode material was changed. In a way it gives better control overthe size of grains as we change discharging/charging currents.Figure 8: SEM micrographs of starting CuO (a) and discharged product (metallic Cu) obtained at thecurrent density of 0.32mA/cm2 (b), 0.065mA/cm2 (c) and 0.032mA/cm2 (d). 26

26Similar work on Pt and RuO2 has shown the applicability of such a phenomenon inapplications such as super capacitors and catalysis. 40 Another similar attempt on Si, ZnO,and Ag NWs was done to show that with different current densities applied for ECM,nanopores of 1-10 nm size could be developed in 1D nanostructure. Improved supercapacitor performance of such porous Si NW was then demonstrated. 41

3.3 ExperimentsPreparation of the Co3O4: Thin films of the Co3O4 were prepared on Ti foil substrate asdescribed in section 2.3.Electrochemical Milling:Battery cycling was done in Swagelok cells using Maccor battery testing unit. (Model4300, 4-channel unit)Samples of cobalt oxide dip coated on one side of Ti foil were cut to the size of ~1cm x 1cm. Swagelok assembly was used in which cobalt oxide samples were placed as cathodeagainst Li metal anode. Samples were cycled for one discharge-charge cycle at a constantcurrent of 30μA. Entire procedure was carried out inside a glove box under argonatmosphere.OER activity measurement:OER activity was measured for the same sample, both before and after theelectrochemical lithiation, to avoid any apparent change in the current due to the variationin the mass of Co3O4 per unit area of the Ti foil.

27Samples obtained after battery cycling were washed three times with Isopropyl alcohol(IPA), and were transferred to the indigenously made OER cell. The electrochemicalexperiments were performed with a Gamry Reference 600 model of computer-controlledpotentiostat-galvanostat. A typical three-electrode system was used, in which Co3O4

coated on Ti foil acted as the working electrode. Hg | HgO (1M NaOH) electrode wasused as a reference (Eref= 0.135V vs. NHE) and Graphite as the counter electrode. Onlyan area of 0.246 cm2 of the working electrode was exposed to the electrolyte through acircular window at the base of the cell. Approximately 10ml of 1M NaOH was used asthe electrolyte.Scanning electron microscopy:Samples at various stages of experiment were examined under SEM microscope (SirionFEG SEM) to record the changes in surface morphology.X-ray photoelectron spectroscopy (XPS):XPS was performed with a Kratos axis instrument for examining the surface compositionof the samples. Monochromatic Al source was used with charge compensation module toneutralize the residual charge building up on surface of poorly conducting samples. Forquantitative comparison, the non-linear curve fitting was done using CasaXPS package.283.4 ResultsLithiation cycling:A typical battery-cycling curve is reported in Figure 9. Charge-discharge behavior isquite similar to what was previously reported for Co3O4. Reaction taking place indischarge step is38, 42

Co3O4 + 8Li _ 4Li2O + 3 CoThis reaction is not surprising given that the Li is a strong reducing agent. The reactionforms small domains of about 3nm diameter of Co metal embedded in Li2O. There arenumerous literature reports providing same observation. 42, 43 Since the current used for

the battery-cycling is same in both the samples, the variation in the time to complete thecycle can only be due to the variation in the amount of Co3O4 per unit area of the Ti foil.Sample DischargeTime(hour)ChargeTime(hour)Sample 1 5.4975 3.4855Sample 2 7.0961 4.6427Table 3: Electrochemical charge and discharge time for Co3O4 thin film samples. Refer to Figure 929During the charging step, delithiation of this matrix occurs in which Li2O reactsbackwards to Co metal under the applied potential to give CoO. 43-45 The chargingreaction is therefore can be written as follows.Co + Li2O _ CoO + 2LiSuch formation of CoO on charging can be explained based on similarity in crystalstructure of CoO and Li2O. Li2O has antifluorite crystal structure and CoO has rock saltstructure, hence both Li2O and CoO have same Oxygen lattice of cubic close packing,therefore the conversion is inherently more favored. 43

Figure 9: Electrochemical charge-discharge curves for Co3O4 thin film samples cycled between 0-3V atconstant current of 30 μA. Time taken for discharging and charging is shown in Table 1.ChargeDischarge11 2230It should be noted that all the experiments in reports mentioned above were eithermesoporous or thin films or nano size structures. Other reports involving bulk Co3O4 aselectrode show formation of Co3O4 after charging step. 42, 46 Our Co3O4

samples werefound to be converted into CoO after ECM of Co3O4 thin films which was confirmedwith XPS analysis of those samples. Results pertaining that is discussed in corresponding

section.OER measurement (before and after Lithiation cycling):Cyclic voltammetry scans were done in the potential range of 0.0-0.85 V (Vs. Hg | HgOreference). Four fold increases in the current at 0.8V (vs. ref. electrode) was observed forthe same geometrical area exposed to the electrolyte. Multiple samples were used tocheck the reproducibility of this observation. The increase in magnitude of the currentwas found to be reproducible in all the samples with the same ~4 fold increase.A representative comparison of the current before and after the lithiation of the samesample is shown in Figure 10 a. This increase in current was initially thought to be onlydue to increase in the surface area because of the nanopores evolution as reportedpreviously for ZnO41.31Figure 10: (a) Cyclic voltammograms of sample before (black) and after (red) ECM in 1M NaOH at 5mV/sscan rate. (b) Tafel plots of sample before (black) and after (red) ECM. Only sample 1 data is shown here.(a)(b)Before ECMAfter ECMAfter ECMBefore ECM32This increase in the current at the same potential is reflected on the Tafel plots asdecrease in the slope. If the surface area is increasing, then it can be expected that theexchange current density would also increase. Instead, an anomalous decrease in theexchange current density was observed. This is explained in the discussion in section 3.4.SEM analysis:Surfaces of the samples were observed under SEM to notice the change in the surfacetexture with lithiation and OER processing. Figure 11shows all the SEMs obtained for thethin film samples after various treatments. Figure 11 a, b and c are samples without ECM

and Figure 11 d and e are samples after ECM. Samples without lithiation showed noformation of nanoplates on the surface after exposure to NaOH or after OERSample Tafel Slope j0 in mAcm-2 R2

Sample 11011.8E-050.995Sample 1 after ECM804.4E-060.999Sample 2911.5E-050.990Sample 2 after ECM 77 6.3E-06 0.986Table 4: Tafel slope and Exchange current density for Co3O4 thin film samples before and after ECM ascalculated from Tafel plot in Fig. 10. (Sample 2 plot is not shown in Fig. 10)33measurements. On the other hand, sample after ECM clearly shows nanoplates growth onthe surface after OER measurement in Figure 11(e).continuedFigure 11: SEM micrographs of Co3O4 thin film samples after various treatments(a)(a) Pristine Co3O4 thin film on Ti foil (b) dipped in NaOH for 1day (c) after OERmeasurement (d) after ECM (e) OER measurement after ECM34Figure 11 continued(c)(b)35Figure 11 continued(d)(e)36Further investigation showed (Figure 12) that only the region exposed to the sodiumhydroxide during the OER measurements is converted to nanoplates.To check whether nanoplates are formed during, the initial exposure to 1M NaOH ofECM treated Co3O4 or at some later stage, the ECM treated samples were exposed to the1 M NaOH for different amount of time and at different stages of the OER experiment.

The following list shows the different time intervals used.1. 15 min exposure without any CV.2. 30 min exposure without any CV3. After CV measurementsFigure 12: SEM micrograph of exposed (to NaOH) and unexposed (outside of circle) area of the sample.374. After OER measurementsImages obtained are presented below. It clearly indicates that nanoplates were formedduring the first 15 minutes of exposure to the NaOH solution.continuedFigure 13: SEM micorgraphs of ECM trated samples after NaOH exposure for different amount of time. a)15 min b) 30 min c) after CV d) after OER measurements(a)(b)38Figure 13 continued(c)(d)39XPS analysis:XPS spectrums were collected for Co3O4 thin film samples before and after ECM tostudy the possible change in chemical composition of Co3O4 because of ECM.810 800 790 780 770500010000150002000025000Co2p3/2 Co2p1/2

CPSBinding Energy (eV)ss Co2p1/2

ss Co2p3/2

with ECMwithout ECMFigure 14: Co2p region of XPS spectrum of Co3O4 thin film samples without ECM (bottom) and with ECM(top)40536 534 532 530 528 526020004000600080001000012000

CPSBinding Energy (eV)Without ECM530.9529.3Figure 15: O1s region of XPS spectrum of Co3O4 thin films without ECM41536 534 532 530 528 526020004000600080001000012000CPSBinding Energy (eV)with ECM531529.3Figure 16: O1s region of XPS spectrum of Co3O4 thin film samples after ECM (CoO)42SampleCo2p3/2

(eV)Satellite(eV)Co2p1/2

(eV)Satellite(eV)O1s(eV)IntensityRatioCo2p1/2

Sat./Main.Co3O4 solgelthinfilm779.0780.0789.6794.1795.5803.7529.3531.00.20Co3O4 thinfilm afterECMtreatment780.5

785.3796.3802.3529.3531.00.46Co3O447

779.6780.7789.5794.5796.0804.5529.5530.80.32CoO 48 780.5 786.4 796.3 803.0 529.6531.20.90Figure 14 shows the Co2p region of thin film samples Co3O4 both with and without ECMtreatment. It clearly shows that satellite peaks for the Co3O4 sample without the ECMtreatment are smaller that the peaks for Co3O4 samples with ECM treatment. In addition,the binding energy difference in main peak and satellite peak is higher in sample withoutthe ECM treatment. Based on literature values of binding energies and height of satellitepeaks, the Co3O4 sample with ECM treatment was found to be matching with Co2pregion of CoO. 48

Table 5: Summary of Co2p and O1s peak positions for different samples of Co3O4 thin films – fromexperiments and literature43Moreover, for Co3O4 sample without ECM the peak fitting of the Co2p region, mainpeaks were de-convoluted in two constituent peaks due to Co3+ and Co2+ in which Co3+

peaks appear at lower binding energy that that of Co2+. This can be explained based onfinal state relaxation effects in addition to Koopman’s theorem. 49 Koopman’s theoremcalculates the binding energy of the electron assuming that all the other electrons are

frozen in their respective energy levels and don’t have any effect on the binding energyof the electron “knocked out” by X-Ray photon. However, in real samples there areatomic as well as extra-atomic factors which cause the binding energy of the electron tobe less than the one expected based on Koopman’s theorem. 49 For CoO (Co3O4 withECM treatment) Co2p region is very different with only one peak each at 2p1/2 and 2p3/2

positions with binding energies matching to Co2+

in CoO. 48

In theory, O1s region of Cobalt Oxides should have only peak corresponding to thelattice oxygen appearing at 529.3 eV. However, in reality surface of these oxide sample iscovered with OH- and other common Oxygen containing species, hence additionally onesmall peak is present in the O1s region of the Co3O4 sample in Fig 15.Similarly, Figure 16 shows the O1s region for CoO thin film sample, main peak around529.3 eV is due to lattice Oxygen. Other peak around 531 eV is due to chemi-adsorbedOH- species. 48

44Table 5 summarizes all the binding energy values form all the samples. Ratio of heightsof Co2p1/2 satellite peak to that of the corresponding main peak was used to compare withliterature results mentioned in bottom two rows. From the absence of overlapping peaksand increased height of the satellite peaks, it is confirmed that thin films of Co3O4 areconverted in to CoO after ECM treatment. This agrees with the literature reportsmentioned in section 3.1 about the lithiation cycling of Co3O4.In addition to the XPS experiments, XRD and GAXRD measurements were done onECM treated Co3O4 thin film sample. However no peaks from the sample were observedeven in the GAXRD experiments. This could likely be due to the sample beingamorphous in nature.XRD analysis:XRD spectrum of the samples was taken after ECM treatment and OER measurement to

identify the product formed. XRD of thin films on the Ti foil was taken. It is shownbelow in Figure 17. Peaks obtained for the sample were very small compared to the peaksobtained for Ti foil substrate. Peaks for the sample were identified to be due to CoO(OH)phase and none of the peaks of Ti interfered with peak positions of CoO(OH).45Peak matching was done according to PDF card # 73-1213 for CoO(OH) and card # 44-1294 for Ti. CoO(OH) crystal structure is brucite type layered structure. Crystal structureand related aspects are treated in the discussion section.Figure 17: XRD spectrum of ECM treated sample after OER.46Raman analysis:Raman spectrum was obtained for these samples with a He-Ne laser (633 nm edge).Spectrum obtained was compared with the reference spectrum of CoO(OH). Maincharacteristic peak was observed at 503 cm-1. This peak is unique to CoO(OH). Otherpossible products with Co and O are Co3O4 and Co(OH)2 have characteristic peaks at481cm-1 and 523 cm-1 respectively which were not observed in the spectrum.Figure 18: Raman spectrum of ECM treated samples after OER measurements473.4 DiscussionThe complete conversion of Co3O4 thin films to CoO(OH) can be schematically writtenas follows.Co3O4 thin films form CoO after ECM treatment. This ECM treated solid layer was mostlikely amorphous hence it was not observed in GAXRD spectra. This CoO layer reactswith NaOH to form CoO(OH). Each aspect of this multistep process is discussed below.3.4.1 Formation of CoO(OH) from CoO.As evident from the XRD spectrum in Figure 17 and Raman spectrum in Figure 18, it isclear that the hexagonal shaped nanoplates are CoO(OH) (heterogenite) phase and notoxides of Cobalt. To understand the formation of CoO(OH) it was important to identifythe phase formed after ECM treatment of Co3O4 thin films.

The XPS studies on after ECM Co3O4 thin film samples clearly suggest formation ofCoO from Co3O4 thin films after ECM treatment. And CoO must be reacting with NaOHto form CoO(OH). Co3O4 has two different oxidation states of Cobalt namely 3+ and 2+ inCo3O4ECM CoONaOHCoO_OH_482:1 ratio. Moreover, it is known that the Co2+ is more stable in water compared to Co3+.50

CoO on the other hand is composed entirely of Co2+ which is more easy to react with OHtoform CoO(OH). A plausible mechanism for such formation of CoO(OH) from CoOcan be written as follows,a) Dissolution of solid CoO in basic medium to form Co_OH_,__-._

In very basic solution such as 1M NaOH used in our experiment, Co(II) forms chargedspecies such as Co_OH_,

__CoO_/_ _ OH_

_-._ _ H_O_0_ 1Co_OH_,__-._

b) Oxidation of Co(II) to Co(III) with dissolved oxygen4Co_OH_,_

_-._ _O__-._ 1 4CoO_OH__-._ _ H_O_-._ _ 4OH__-._

c) Precipitation of CoO(OH) to form hexagonal shaped crystalsAnd this formed CoO(OH) then precipitates back as hexagonal platelets.CoO_OH__-._ 1 CoO_OH__/_

Net reaction is therefore,4CoO_/_ _O__-._ _H_O_0_ 1 4CoO_OH__/_

This conclusion also agrees with the Pourbaix diagrams for Cobalt which predictsCobaltic acid HCoO_

_as the stable phase at the pH =14 and E=0. 51

49

The following table summarizes the different methods of synthesizing CoO(OH) underdifferent conditions.Reference Method DescriptionProduct morphologyApplication testedfor and othercomments52 Co-electro-depositionfrom Co containingNi(NO3)2 solutionGrainy in appearance OER, high current dueto synergistic effect ofCo and Ni hydroxides53 Conversion of Co toCo(OH)2 and then toCoOOH under appliedbiasNRNiMH batteryelectrode54 Chemicalprecipitation:_-CoOOH formationfrom CoSO4 precursormixed with H2O2

_-CoOOH formationfrom Co(OH)2 andKOHNR(NO SEM done)Positive electrode forNi batteries55 Air oxidation ofcommercial and labmade sample ofCo(OH)2 at 100CBroken hexagonalshaped platesCharacterization usingTEM was done.SAED patternsobtained to identify56 Potentiostaticelectrolysis of Cocontaining nitratesolution at pH = 7.4Gathered crystallitewell adhered to thesubstrate57 Synthesis of

CoO(OH) fromCo(OH)2 precursor byozonationRough surfaceplatelets of varyingsizeChemical stabilityunder variouselectrochemicalconditions wasstudiedMost methods reported above have used electrochemical method or chemical method toconvert Co precursor into CoO(OH). Co(OH)2 is the most commonly used precursor, andTable 6: Literature survey of different methods for synthesis of CoO(OH)50mostly alkaline solutions are used to convert the precursor to the final desired productunder ambient conditions. In the current work reported here, formation of CoO(OH) takesplace spontaneously without any assistance of heat, oxidizing agents or electric potential.Moreover, CoO(OH) formed is growing on the substrate itself which is therefore likely tohave better contact with substrate and hence smaller current drop across the activematerial – substrate interface.3.4.2 Higher OER performance of CoO(OH)The following table summarizes the difference in conductivity and Co3+

contentpercentage of Co3O4 and CoO(OH). Most of the values are obtained from one or moreliterature reports.Physical or ChemicalProperty /natureCo3O4

CoO(OH)ReferencesMolecular weight240.797 g/mol91.940 g/molCalculatedCrystal Type Spinel Brucite [58, 59, 60 ]Electrical resistivity 14.2 Ohm.cm 0.2 Ohm.cm [9]Co3+ % of total cobaltcontent bystoichiometry

66% 100% CalculatedTable 7: Comparison of Co3O4 with CoO(OH)51Co3O4 has spinel structure, which can be described as corner sharing octahedra andtetrahedra of oxide ions with Co3+ in the center of the octahedra and Co2+ in the center ofthe tetrahedra. It is known that normal spinels have hopping kind of mechanism forcarrier conductivity, which tends to put spinel structured metal oxide in less conductingcategory. 61

On the other hand CoO(OH) structure can be described as edge sharing Co(III)oxohydroxo octahedra. Conductivity of as high as 5 Scm-1 has been reported in theliterature.62 This higher conductivity has already been utilized in the Ni-MH type ofbatteries, in which use of CoO(OH) coating on NiO(OH) electrode has shown to increasethe electrode performance. 63

Co4+ sites formed through oxidation of Co3+ on surface are known to act as the activesites. 17 The reaction of desorption of adsorbed OH- species is known to be the ratedetermining step in most cases. Higher conductivity might be reducing the Ohmicoverpotential due to current transport in the electrode.Most of the Co3+ sites for CoO(OH) are exposed through edge planes and not throughbasal planes. (See Figure 19) Hydrogen atoms occupy the planes in between two layers,they have not been show in Figure 19 for more clarity. There is an interesting study ondissolution of CoO(OH). It shows that dissolution only occurs through the edge sites.64 Intheir experiment, they dissolved CoO(OH) with chelating ligand. If dissolution occurred52through basal planes, then the height of the plates is expected to decrease. However, nosuch decrease in height was observed.From the SEM images, it can be seen that most of the surface is made up of basal planesand not edge type surface. It suggests a decrease in OER activity rather than increase.

However, a four times increase in OER activity was observed, which is currently thoughtto be due to the improved kinetics of the OER reaction because of the increasedconductivity of CoO(OH).Surface area argument is yet to be proven decisively. BET surface area measurement wasnot allowed because of the microscopic quantities of sample produced. Electrochemicalmethods were proved to be unsuccessful because of very low current in potentials regionsaway from the redox peaks.It is difficult to correlate the Tafel plot data of two chemically different electro-catalysts.Generally, less overpotential at the same current density or high current density at thesame overpotential is needed to prove any improvement. Increase in the current density atsame overpotential can be seen in Figure 10 b. However, at the same time, there issubstantial decrease in exchange current density. More investigation is needed tounderstand the causes of decrease in the exchange current density.53Figure 19: Crystal structure of CoO(OH)54Chapter 4: Photoelectrochemical study of Co3O4

4.1 IntroductionIn the photoelectrochemistry (PEC) of semiconductors, the electrochemical response ofthe system is studied under the illumination of light. A typical PEC device is a threeelectrodesystem with either p or n type semiconductor as the working electrode, acounter electrode – typically platinum, a suitable reference electrode in an electrolyte.Incident photons generate electron-hole pairs in the semiconductor. Energized electronsget excited to the conduction band and corresponding holes are created in the valenceband. Further movement and recombination lifetime of the minority and the majoritycarriers depend on the type of semiconductor and its physical properties. Many factorsaffect the overall performance of the PEC cell. They are discussed at length in section4.5.

The first reported experiment in PEC was carried out in the year 1839. In that experimentphotocurrent was obtained after the light illumination of the two platinum electrodesimmersed in the electrolyte of metal halide salts65. Later, after the oil embargo of 1970’s,research on liquid junction solar cells again picked up.55An important paper in this direction was published by Fujishima and Honda in Nature in1972.66 It involved the use of TiO2 as the semiconductor electrode for water photolysis.Later in 1991 M. Grätzel and co-workers demonstrated the possibility of using the TiO2

nano particles along with the dye for efficient solar energy harvesting.67 TiO2

soonbecame the material vastly studied for its properties and used as the photo-anode indifferent electrochemical systems. TiO2 has a wide band gap of about 3.1 eV, whichrestricts its use to the UV region of the sunlight. Visible radiations lack the energy to getthe necessary excitation required. Hence, it is important to investigate othersemiconductors with optical absorption in the visible solar spectrum.4.2.1 Fundamentals of PhotoelectrochemistryIn principle, PEC is the electrochemistry of a semiconductor electrode surface in asuitable electrolyte, under light illumination.Semiconductors can be considered as a giant connected molecule, which forms broadbands of electronic energy levels. The valence band is analogous to the highest occupiedmolecular orbital (HOMO) and conduction band is analogous to the lowest unoccupiedmolecular orbital (LUMO). The energy separation between the valence and conductionband in such a picture is called the band gap. The band gap of a semiconductor governsthe optical properties of it.56The Fermi level of a semiconductor specifies the electrochemical potential of an e- in thesemiconductor. The probability of the Fermi level being occupied with the electron isexactly ½. In the case of the p-type semiconductors, it is closer to the valence band and in

the case of the n-type semiconductor; it is closer to conduction band. Doping thesemiconductor changes the carrier concentrations and correspondingly changes Fermilevel. At a fixed doping level, the Fermi energy level can be manipulated using appliedbias across the semiconductor.Generally, the Fermi level of the semiconductor and the Fermi level of electrolyte (thestandard reduction potential for the electrode reaction of the electrolyte can be consideredas its Fermi level) are different from each other. Thus, when a semiconductor electrode iskept in contact with an electrolyte, charge transfer happens in order to equilibrate theFermi level, which forms a depletion region inside the semiconductor and a Helmholtzlayer at the interface of semiconductor and electrolyte. The thickness of the depletionregion depends on various factors such as doping level, carrier conductivity of thesemiconductor and surface trap states. This process is similar in its effect as a p-njunction. Charge balancing builds up a potential difference called as the space-chargelayer, and it helps in the separation of the electron-hole pair generated with the incidentlight. Figure 20 schematically shows the band levels from bulk to surface both n and ptype semiconductor at accumulation, depletion, and inversion.57Effectively, all the radiation of required wavelength absorbed in this region contributestowards the overall photocurrent registered in the PEC cell. The minimum energyrequired for the electron hole pair generation depends on the band gap of thesemiconductor material.Figure 20: Energetic position of fermi level under different conditions68

58In the PEC method, the flat band potential of the semiconductor can be measured fromthe onset of the photocurrent. Photocurrent is the difference in the current magnitude atthe same potential under dark and under light conditions. In practice, dark and light I-V

curve are measured together as a chopped light I-V curve, where the working electrode isscanned across the voltage range at a constant sweep rate and the current is measured.The light falling on the working electrode is chopped at a constant periodic rate.Chopping of the light is done in part to minimize the effect of the temperature increasecaused by the local heating of the sample surface because of the light exposure. Theeffect of local heating causing the temperature variation and the other environmentalfactors are difficult to reproduce or eliminate by blank correction method. Hencechopping serves as an effective practical approach to avoid such experimental variationsaltogether.4.2.2 Introduction to Photoelectrochemistry of Co3O4

Co3O4 is a p-type semiconductor. It is black in color and has a high absorption in thevisible region of the solar spectrum. K-M transformed diffuse reflectance spectrum ofCo3O4 is presented in Figure 5 of section 2.4.3. It demonstrates the ability of Co3O4 toabsorb a high percentage of the incident solar radiation. However, this high opticalabsorption efficiency has not been observed to translate into high photocurrent. PreviousPEC experiments done with Co3O4 thin films in 1M NaOH as electrolyte have shownreally small photocurrents of the order of 10 μA.69 Clearly, more investigation is requiredon the PEC of Co3O4.The band gap values of Co3O4 reported in the literature vary from591.50 to 2.07eV.69, 70 In reality, Co3O4 shows the different types of the band gaps asmentioned inTable 8. Assignment of the different transitions based on theoretical studies has beendiscussed in section 2.4.3. Further discussion in terms of direct/indirect allowed andforbidden band gap transitions has been done extensively in literature. 9, 71

P.S. Lokhandeet. al have discussed different types of transitions in Co3O4 thin films prepared by spray

pyrolysis. They recorded optical density (_t) in the wavelength range of 350-550 nm at300 K, and used classical relation 2_ _ _2_ _ ____ to analyze the data. They found thepresence of all the four types of the transitions. The following table summarizes the datapresented in their work. 9Type of band gap Band Gap in eVDirect Allowed 2.06 and 1.44Direct Forbidden 1.38 and 1.26Indirect Allowed 1.10 (Ep = 0.02)Indirect Forbidden 0.75 (Ep = 0.27)In the PEC work presented here for Co3O4, thin films were investigated for theirmagnitude of photocurrent response. Pilot experiments were done on TiO2 as a referencesystem to test the accurate reproduction of experiments from literature. Because of therobust nature of the TiO2 - PEC experiment, it is a good method to check the correctnessof the working protocol.Table 8: Different band gap transitions in Co3O4,Ep = Energy of phonon assisting indirect transition60Initial experiments with Co3O4 thin films showed small photocurrents of the order of 10-20 μA. Higher photocurrent of the order of 100μA was required to further theinvestigation of variation of photocurrent with intensity and wavelength of the incidentlight. Results of these experiments have been discussed in the next few sections andfuture studies will be proposed in chapter 5.4.3 ExperimentsPreparation of Co3O4 thin films: Thin films of Co3O4 were prepared on FTO-coated glasssubstrate as described in section 2.3. The transparent conductive oxide (TCO) is requiredin this case to allow back illumination of the PEC cell set up.Preparation of TiO2 thin films on FTO:Thin films of TiO2 were obtained by making P-25 TiO2 slurry in ethylene glycol andacetic acid 1:1 mixture, and applying it by doctor blade method on the conductive side ofthe pre cleaned FTO slides, of 2.5cm * 2.5 cm dimension by doctor blade method.Typically, thin films were only deposited on 1 cm2 area at the center of the FTO, this area

is enough to cover the circular window of PEC cell. Excess TiO2 is wiped off gently andthe film is air dried and then annealed at 550 ºC for 1.5 hours. Before using for the PECexperiment, the mechanical strength of the film was checked by gently tapping the FTOedges. This is important in order to avoid dissolution or falling off the particles during theexperimental workflow.61PEC cell set up:A typical PEC cell set up is shown in the Figure 21 below. A Co3O4 thin film on the FTOsubstrate is used as the working electrode. A limited area of the working electrode isexposed to the electrolyte, through a hole in the wall of the teflon cell, with an O-ring inbetween to ensure a good seal that would prevent the leakage. The counter electrode is agraphite rod in the case of aqueous electrolytes and a platinum wire in the case of nonaqueouselectrolytes. A stable reference electrode can be chosen depending on theelectrolyte. Details of the different reference electrodes used are summarized in Table 9.Figure 21 shows the photograph of a working PEC cell used for non-aqueous PECexperiments. One edge of the conductive side of the working electrode is taped withdouble-sided Cu tape to provide a convenient connection to the electrode. The conductiveside of the working electrode is then pressed against the circular window of the PEC cell,through which it is exposed to electrolyte. The O-ring seals the edges of the window andprevents the leakage of the electrolyte. A metal support tightened with screws is thenplaced to secure the working electrode in place. The opposite window holds only clearquartz glass secured in similar fashion. In case of PEC cell used for non-aqueouselectrolytes, reference, and counter electrode are connected to the top, which is threadedto fit into the main body of PEC cell and seal the cell completely. In case of the PEC cell62

for an aqueous electrolyte, no such sealing is required; the reference and counterelectrodes are dipped in the cell directlyReference ElectrodeUsed forelectrolytes (Error!Reference sourcenot found. formore details)Method ofpreparationCommentsR1Non aqueousReference electrode S6, S7 and S8Pt wire dipped in 0.5M TBPNew referenceelectrode is preparedevery time in a smallcapillary with porousglass bottomR2Aqueous ReferenceelectrodeS1-S5Ag wire dipped insaturated KCl solution,This is standardreference electrodefrom Accumet for 1MKCl, 1M KCl waschanged to sat. KClReference electrodewas standardizedagainst commerciallyobtained Ag | AgCl |KCl (1M) fromAccumetR3Non aqueousreference electrode S9, S10Pt wire dipped inTriiodide solution with0.04LiI + 0.02M I2New referenceelectrode is preparedevery time in a smallcapillary with porousglass bottom

The non-aqueous cell setup was assembled in a glove box maintained at low humidityunder argon atmosphere. It is important to avoid traces of water in the electrolyte,Table 9: Different reference electrode used63because even a small amount of water quenches the overall photocurrent observed,because of strong adsorption at photoelectrode and affects the reproducibility ofphotocurrent magnitude. In case of an aqueous PEC, cell was set up under ambientconditions, and argon gas was flowed at a moderate rate for 10-15 minutes before theactual PEC experiment in order to flush out dissolved oxygen, otherwise oxygen redoxchemistry might affect the I-V curves recorded.64Other experimental details for various experiments carried out are mentioned as requiredin the following sections. Preparation of different electrolytes is summarized as in Error!Reference source not found..Figure 21: Schematic diagram (Top) and photograph of PEC cell, front (bottom left) and lateral (bottomright) view65Electrolyte Preparation CommentsS1 acetate Buffer 0.1 M Acetic acid + 0.1 MDipotassium phosphoricacid + 1M KClpH = 7 maintained withacetate buffer andmeasured with calibratedpH meter. pH was then beadjusted in the same waywithin pH range of 4-10S2 sodium hydroxide 0.1M NaOH --S3 FeCl3 1M KCl + 0.01 M FeCl3 +0.1M Disodiumphosphoric acidpH = 7 maintained withPhosphoric acid andNaOH, Small amount ofFe precipitates as Fe(OH)3

so remaining solution wasused as electrolyte afterfiltration

S4 Aqueous triiodide system1M KCl + 0.01 M I2 +0.02 M LiI + 0.1 MDisodium phosphoric acidpH was maintained at 7with Phosphoric acid andNaOHS5 K3Fe(CN)6

1M KCl + 0.01MK3Fe(CN)6 + 0.1 MDisodium phosphoric acidpH was maintained at 7with Phosphoric acid andNaOHS6 Triiodide0.7M LiI + 0.05 M I2 inpure ACN in side gloveboxPrecaution must be takento avoid any introductionof moisturecontinuedTable 10:Preparation of different electrolytes used in the PEC experiments66Table 10:Preparation of different electrolytes used in the PEC experiments continuedS7 DSSC electrolyte0.02 M I2 + 0.04 M LiI +0.5 M 1,2-DMPII (1,2-Dimethyl-3-propylimidazolium iodide)in pure ACN in side glovebox--S8 DSSC + 0.5 TBP S7 electrolyte + 0.5 TBP(4-t-butylpyridine) in pureACN in side glove box--S9 Triiodide 0.2 M LiI + 0.1M I2 inpure ACN in glove boxDifferent compositionotherwise same as S6S10 Ferrocene 0.05M Ferrocene + 0.1MLiClO2 in pure ACN inglove box--PEC measurement with Gamry:Gamry reference 600 was used for electrochemical measurements with the PEC cell in

the standard three-electrode configuration as described above. CV was measured initiallyfor each set of experiment to get the understanding of the general behavior of the systemand to determine the range of voltage, over which photocurrent measurement can bedone. The first CV is not stable because the surface of working electrode is notequilibrated with the electrolyte. After 5-10 cycles of scanning the selected voltage range,CV stabilizes. Then the linear sweep voltammograms (LSV) were recorded in dark as67well as light condition, at a rate of 100 mV/s. This helps to identify dark and light parts ofthe chopped light I-V curve. In case of n-type semiconductor, photocurrent is anodic innature and in case of p-type semiconductor, it is cathodic in nature. 100W Xe arc lamp(Oriel Apex) was used as the light source.4.4 ResultsPilot experiment with TiO2

TiO2 photo anode was used in PEC cell with electrolyte S1 acetate buffer, seeTable 9 for detailed composition. A Ag | AgCl | Sat KCl (R2) electrode was used as thereference electrode.pH of the acetate buffer electrolyte was changed by varying the ratio of acid and salt. pHvalues were measured before the PEC measurement using a pH meter. A three-pointcalibration of the pH meter was done on each day of the pH measurement, using standardbuffers of 4.0, 7.0 and 10.0 pH.Figure 22 shows a typical LSV curve obtained under the chopped light source. Onset ofphotocurrent in this graph indicates the flat band potential. Photocurrent graduallyincreases and reaches a stable saturated value and forms the plateau on the right side ofthe graph. Photocurrent is anodic in nature, which is consistent with the fact that TiO2 isn-type. Photocurrent is the difference in the light and dark current in the chopped LSVcurve. Maximum Photocurrent obtained is about 175 μA. Figure 23 shows the plot of68

square of the photocurrent vs. voltage. From the extrapolation of the straight line asshown in figure 23, one can obtained the flat band potential.Figure 22: LSV curve for TiO2 sample under chopped light at pH of 6.88469In aqueous electrolyte such as the acetate buffer, Helmholtz layer on the surface of theworking electrode is made up of OH- ions hence it is affected by the pH. Effect of pH onflat band potential of working electrode can be studied by repeating same procedure asmentioned above at different pH of the electrolyte. Figure 24 shows plot of flat bandpotential of TiO2 from graphs similar to Figure 23, at different pH of the acetate buffer.Figure 23: Photocurrent square against potential vs N.H.E For TiO2 PEC experiment in acetate bufferpH = 6.88470Typically it shows slope of 59mV/ pH. Value of 54.6mV/pH was obtained in case of thispilot experimental. It proves the correctness of the working protocol.PEC results for Co3O4 thin filmsThe procedure of the PEC experiments for Co3O4 was same as TiO2. The thickness of thefilms was typically ~ 70 nm -100 nm as roughly estimated from SEM measurements.Photocurrent obtained with different electrolytes is summarized in Table 12.Figure 24: Change in flat band potential with pH of the acetate buffer used in TiO2 aqueous PECexperiment71Figure 25 shows a typical chopped light LSV curve obtained for the Co3O4

thin films.Photocurrent is cathodic in nature because Co3O4 being p-type semiconductor. Darkcurrent is non-zero and increasing in magnitude as scanned from 0.4V to -1.0V. Inaddition, the maximum photocurrent obtained is ~3μA, in contrast to ~200μAphotocurrent in TiO2 in an acetate buffer solution and near zero dark current.Table 12 summarizes the data obtained for different PEC experiments with the set ofconditions used. Since this initial part of the work is focused on optimizing theFigure 25: Chopped light LSV curve for Co3O4 thin films in ferrocene based non-aqueous electrolyte,

electrolyte S10 in table 7.72experimental conditions to achieve higher photocurrent, only the maximum photocurrentsfor those graphs are reported here.PEC cellconfigurationMax.PhotocurrentμAOther comments on the observationsAqueous: Counter electrode is graphite.R.E. = R2Electrolyte = S10 μANo chopping in current-voltage curve except smallcontinuous changes which could be due to localtemperature variationR.E. = R2Electrolyte = S2~ 1.5μA Noisy Ohmic behavior in overall IV curveR.E. = R2Electrolyte = S30 μA Near zero dark in -0.25V to 0.5 V region.R.E. = R2Electrolyte = S50 μA Well defined peaks around 0.2234 V (vs R2) inCVR.E. = R2Electrolyte = S43 μA Very small photocurrentNon-aqueous: Counter electrode is Pt wireR.E. = R1Electrolyte = S66 μA Scanned only in -0.3 to 0.0V regioncontinuedTable 11: Summary of the PEC data obtained under different experimental conditions. Working electrode isthin film of Co3O4 on FTO, unless specifically mentioned.73Error! Reference source not found. continuedR.E. = R1Electrolyte = S7~ 0.5 μA Noisy data with transient spikes at the point wherelight is choppedR.E. = R1Electrolyte = S84μA Almost flat dark current from -0.4 to 0 VR.E. = R3Electrolyte = S9

3μA Stable photocurrent with ohmic behavior of IVcurveR.E. = R3Electrolyte = S1015 μA Quite stable photocurrent with high backgrounddark current , it shows ohmic behavior in –veregion from – 0.5 onwardsR.E = R3Electrolyte = S9W.E. = Co3O4 NW0μA Completely ohmic nature in IV curve nophotocurrent except small current, possibly due tolocal temperature variation4.5 Discussion:As summarized above, IV curves obtained for Co3O4 thin films in different electrolytemedium have three main issues to be addressed to get higher photocurrent.A. Small photocurrentB. Ohmic Behavior of the dark currentC. Transient currentTable 12: Summary of the PEC data obtained under different experimental conditions. Working electrode isthin film of Co3O4 on FTO, unless specifically mentioned.744.5.1 Small photocurrentThere are several ways in which small photocurrent for Co3O4 thin films can beexplained. Photocurrent generation broadly involves three parts, light absorption, chargecarrier separation, and charge carrier collection.Light absorption depends on the band gap and extinction coefficient of the semiconductorand also the thickness of the film. Wide band gap semiconductors absorb in UV region.Most of the solar energy reaching the earth is in the form of visible and NIR radiations;therefore, semiconductor with absorption in this region of solar spectrum is desired for apractical device.Light penetration depth is decided by absorption coefficient (_) of the semiconductormaterial. Higher thickness beyond this light penetration depth does not absorb light andhence will not contribute to the photocurrent. In fact for the charge separation to occur,local electric field of depletion region is necessary. Hence, most of the electron hole pair

generated outside the depletion region, beyond minority diffusion length will most likelyrecombine to give away heat. Therefore, optimization of the thickness of the films isnecessary.75Photocurrent generated by a semiconductor is modeled by Gartner model. Following is atypical equation:_34__ _ 5_ _ 6 7 _ 34___689:__5_qua4tumYie3d_Jphoto/Io6_absorptio4coefficie4tw_spacechargeregio4widthLmi4_diffusio43e4gthGartner model is a theoretical model to find theoretical quantum yield of thesemiconductor photo-electrode, using space charge region width, diffusion length, andextinction coefficient. However, this model does not take into account the effects such asmulti-electron processes causing photocurrent doubling, effect of surface states, andrecombination effects. Hence, effectively Gartner equation predicts the upper limit for thephotocurrent, except in case of multi-electron processes, where photocurrent obtainedends up being higher than the upper limit set by Gartner equation. In the actual PECexperiments, the photocurrent density ends up being lower due recombination effects.Thickness of the depletion region depends on doping density and crystal purity. In case ofCo3O4, it is known to have varying stoichiometry depending on the method ofpreparation. P-type nature of the Co3O4 is due to O2- deficiency in Co3O4

lattice. In a sol76gel method, extent of deficiency depends on variety of conditions such as, pH of the sol,temperature of annealing, atmosphere of annealing and precursor used. It is difficult tocontrol it very precisely.As evident from its UV-Vis absorption spectrum, Co3O4 absorbs very strongly in thevisible region. However, it does not translate into high photocurrent. This could be due tofast recombination of the electron and hole generated after the light absorption.

Recombination of a minority carrier and a majority carrier can be seen as a pseudo firstorder reaction.72 Because majority carrier is present in excess compared to minoritycarrier, majority carrier concentration practically stays constant during recombinationprocess. Lifetime of the minority carrier can be used to characterize the extent ofrecombination. This lifetime of minority carrier is a measure of time for which it existsbefore recombining. Following expression relates minority carrier lifetime to minoritycarrierdiffusion coefficient. It is another way of comparing the different materials;judged for potential application in PEC cell as photo-electrode. Minority-carrier diffusionlength is a measure of distance, that minority carrier travels before recombining withmajority carrier. With diffusion length, it is convenient to compare that to thickness of thedepletion region and overall thickness of the film.77Following equation underlines the relation of minority-carrier diffusion length tominority carrier lifetime.89:_ _ _ P9:__Q_

Lmin = Minority Diffusion lengthD = Diffusion coefficient_min = Minority Carrier lifetimeIn a strict sense, this equation is only applied in a field free diffusion of minority carriers.However, the general relation of lifetime to diffusion length even holds true in thedepletion region. It shows that short-lived minority carrier diffuses smaller distance andmakes it more likely to recombine. In fact, most spinel structured metal oxides havehopping mechanism for carrier mobility. In case of cobalt oxide valence and conductionband is made up of weak overlap of d-orbitals of cobalt ions. 70 Hence, it puts an intrinsiclimit on minority-carrier diffusion length. This may explain low photocurrent observed inthe PEC experiments for Co3O4 thin films.

Higher doping in a semiconductor facilitates recombination. In the case of p-type Co3O4,higher doping means more O2- deficiency or lower O/Co ratio. Theoretically, in absenceof O2- deficiency, O/Co ratio of 1.33 is expected. However in practice O/Co ratio of as78low as 1.15 has been reported in literature.70 This could be one possible cause of highrecombination.4.5.2 Ohmic behavior of dark currentDark current occurs due to reactions occurring at working electrode in absence of light.Ideally very low or no current is observed under dark, within a certain potential biasrange. This is known as the diode behavior of the semiconductor. Figure 22 showschopped photocurrent spectrum for the TiO2 thin films. TiO2 is n-type semiconductor,hence it acts as photo-anode, and anodic photocurrent is observed under illumination oflight. Moreover, a nearly zero dark current observed suggests that TiO2

shows diodebehavior. In this flat region, no faradic current is observed.If applied bias is too positive or negative, it results in breakdown of diode behavior. Thislimit is called Zener limit. Outside of this potential window, semiconductor electrodeeffectively acts as metal electrode. Zener limit is dependent on surface state concentrationand electric field applied. This may explain the large increase in the dark current at -1.0V.In case of Co3O4, all the IV curves obtained under chopped illumination had non zerodark current, which went on increasing as scanned beyond -0.5 V. Figure 25 shows onesuch curve as a representative example. Curve in Figure 25 shows almost ohmicbehavior. This is maybe due to the tunneling of electrons across the barrier region, which79is likely to be thin if doping or O2- is high. Such tunneling would increase as scan intomore negative region.Moreover, such dark current depends on the types of reactions that are possible for a

given electrolyte at the semiconductor surface. Co3O4 surface has many Co3+

sites, whichcan act as catalytic sites. In case of aqueous PEC experiments, partial dissolution wasobserved on prolonged illumination. This indicates that Co3O4 surface reacts withelectrolyte.The non-zero dark current could also be due to the exposure of FTO to the electrolyte. Incase of K3Fe(CN)6 as electrolyte, well formed redox peaks of Fe(CN)6

3+ were obtained incyclic voltammetry. Such peaks are likely to appear on metal like electrode. It suggeststhat either Co3O4 is behaving like metal at too negative bias, or electrolyte is in directcontact with FTO.4.5.3 Photocurrent TransientPhotocurrent transient response is the sudden spikes observed in current density, as thelight is chopped in the PEC experiment. The transient overshoot is dependent on thecapacitance of the space charge layer, and concentration of surface states. When light isswitched on, photocurrent shoots to give a high transient value, which subsequentlydecays to a stable steady-state photocurrent value. Magnitude of this stable photocurrentdepends on magnitude of rate of recombination and rate of charge transfer. 72 When80going from light to dark in chopped light experiment, electrons on surface continue torecombine with the holes, in case of p-type semiconductor. Since holes are effectivelymoving in opposite direction, current has negative sign. This recombination continues todecay and shows gradual decrease to steady state photocurrent. To check whether this isbecause of surface recombination, surface passivation studies can be carried out.81Chapter 5: Conclusions and future workClearly more work is required to get the complete understanding of thephotoelectrochemistry and the OER electrocatalysis on Co3O4. All the work presented inchapters 2, 3 and 4 is summarized here, and future work is outlined.

5.1 ElectrocatalysisConclusionECM treatment was used to generate nanopores in thin films of Co3O4. Such an ECMtreatment showed a significant increase in the OER activity of the thin film samplecompared to the samples without the ECM treatment. The SEM images of the thin filmsafter the OER measurement showed different surface morphology for samples with ECMand without ECM treatment. Hexagonal plates observed after OER measurement on thesurface of ECM treated thin film samples, were identified as CoO(OH) using XRD. Onfurther characterization done using Raman spectroscopy, it was confirmed to beCoO(OH) with possible minute contamination of Co3O4 and Co(OH)2.82CoO(OH) is known to have higher conductivity compared to Co3O4. Hence, it may bedecreasing overall Ohmic overpotential in electrode, and increasing the OER activity. Insubsequent experiments it was confirmed that CoO(OH) plates are formed as they areexposed to 1M NaOH. Formation of CoO(OH) from Co3O4 happened only in case ofECM treated sample, and not in case of untreated samples. This method of synthesis ofCoO(OH) is unique compared to the solution-based synthesis methods, and produces wellformed hexagonal plates.XPS analysis Co3O4 of thin film samples after ECM treatment showed formation of CoO.Future workThe reasons for the increase in OER performance of CoO(OH) electro-catalyst are stillnot completely clear. Many aspects of the experiment such as variation of pH, change inproduct morphology with thickness of original thin film, are yet to be investigated. Forexample, TEM diffraction patterns could be obtained to check for disorder in crystallattice.73

5.2 PhotoelectrochemistryConclusion

In PEC studies, we first studied TiO2 as a model system in acetate buffer solution aselectrolyte. The chopped I-V curve showed photocurrent of the order of ~150 μA. Theflat band potential was calculated from the plot of square of photocurrent vs. potential.The variation in flat band potential with pH was Nearnstian in behavior.83In case of the Co3O4 PEC, we used different aqueous as well as non-aqueous electrolyteswith Co3O4 as photocathode in a three-electrode type cell assembly. However, a smallphotocurrent of the order of ~3-5 μA was obtained in almost all experiments. Most of thechopped I-V curves showed a high dark current value with no diode behavior. It wasdifficult to find the flat band potential because of such a small photocurrent. The smallphotocurrent obtained is thought to be due to the small diffusion length of the carriers inCo3O4.Future workIn the PEC studies of Co3O4, a higher photocurrent is required, to further theinvestigation of the band positions and the diffusion length measurement. The flat bandposition can be obtained using the Butler-Volmer method. Also the IPCE measurementcan be done to demonstrate the wavelength dependence of the photocurrent. Betterpractical methods are needed to control the recombination of photo-generated carriersand to get the higher and reproducible photocurrent of at least ~50-70 μA. The surface ofthe semiconductor plays an important role in change collection. Interfacial kinetics ofdifferent possible reactions also needs to be considered while using different electrolytes.Annealing at temperatures higher than 550 ºC may improve crystallinity in the Co3O4 thinfilms, which may improve the carrier diffusion length. However, glass supported FTOsubstrate used for these thin films, tends to deform at temperatures higher than 600 ºC.This puts inherent upper limit for annealing temperature.84References

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