Photoelectrochemical (PEC) water splitting, using sunlight and appropriate semiconductors to produce hydrogen (H2) fuel, is a promising route to solve both the production of clean H2 fuel and storage for solar energy. Owing to its various advantages, hematite (α-Fe2O3) has emerged as a promising photoanode material for PEC water splitting. However, its poor electrical conductivity, low carrier mobility, short-hole diffusion length, and fast recombination rates of the electron-hole pairs have greatly limited its full potential for PEC performance. One way to improve the PEC activity of α-Fe2O3 is by doping with other elements. In particular, surface doping is proved to be more beneficial than bulk doping because it reduces the distance moved by the charge carriers from inside the bulk to the surface where they are required for interfacial transfer. In this study first-principles calculations based on density functional theory (DFT) were carried out to investigate the influence of Cu, Zn, Ti and Zr on the {0001} and {01 2} hematite surfaces for enhanced PEC water splitting. Various surfaces of hematite were constructed and their thermodynamic stabilities were determined by calculating surface and formation energies. The {0001} and {01 2} surfaces were found to be the most stable. Besides, all the doped systems were found thermodynamically stable. Furthermore, it was found that Cu doped surface systems does not only decrease the bandgap but also leads to the correct conduction band alignment for spontaneous water splitting. In all calculations, the charge density difference plots and the Bader charge analysis showed accumulation of charge at the top outmost surface, implying the photogenerated charge carriers can efficiently diffuse to the surface for enhanced interfacial charge transfer to the adsorbates. Morever, it was found that even with mono doping of Zn on the topmost layer of the {0001} α-Fe2O3 surface, the bandgap can be decreased without impurity states in the band structure which normally acts as recombination centres. Furthermore, the energetic stability and electronic properties of bimetallic doped {0001} α-Fe2O3 surface with (Zn, Ti) and (Zn, Zr) pairs for enhanced PEC water splitting was also studied. Bimetallic doping is viewed as an important and executable way of not only increasing the conductivity of a semiconductor material but also reducing the quick recombination of the electron-hole pairs. The doped systems showed negative formation energies under both O-rich and Fe-rich conditions implying that they are thermodynamically stable and could be prepared experimentally. Additionally, bimetallic doping of (Zn, Ti) and (Zn, Zr) on the {0001} surface is expected to enhance the PEC performance of α-Fe2O3 because Ti or Zr is capable of increasing the conductivity of α-Fe2O3 due to the substitution of Fe3+ with Ti4+ or Zr4+, while Zn can foster the surface reaction and reduce quick recombination of the electron-hole pairs. We hope that our results provided here will be of great interest to both experimental and theoretical researchers. / Thesis (PhD (Physics))--Univesity of Pretoria, 2020. / Ministry of Higher Education, Copperbelt University, Zambia / The University of Pretoria, Department of Physics / Centre for High-Performance Computer (CHPC), Cape Town / Physics / PhD (Physics) / Restricted

In this dissertation, α-Fe2O3 thin film deposition techniques were first evaluated to understand their effects on the structural, optical and photoelectrochemical (PEC) properties of the films. α-Fe2O3 films were deposited by dip, spin and combined dip/spin coating techniques on fluorine-doped tin oxide (FTO) substrates at an annealing temperature of 500°C. Structural properties suggest better crystallinity for films prepared by dip and combined dip/spin coating techniques as compared to spin coated films. Field emission scanning electron microscopy showed spherical nanoparticles with some agglomeration into small larvae-shape nanostructures for all the films. All films absorb in the visible region due to their bandgap of 1.98 ± 0.03 eV. Maximum photocurrent densities of 34.6, 7.8, and 13.5 µA/cm2 were obtained at 1.23 V vs reversible hydrogen electrode (RHE) for dip, spin and combined dip/spin coated films with the thickness of 740-800 ± 30 nm respectively. Improved crystallization, low charge transfer resistance at the solid/electrolyte junction, high surface states capacitance, and a more negative flat band potential values obtained for dip coated films using electrochemical techniques, have been associated to their improved photocurrent response. Furthermore, the annealing approach for preparing multi-layered α-Fe2O3 films using the dip coating technique was modified to enhanced their PEC performance. The first three layers of the films were annealed at 500°C and the fourth layer at 500, 600, 700, 750 and 800°C respectively. Films annealed at 750°C recorded the best performance, producing 0.19 mA/cm2 photocurrent at 1.23 V vs RHE; 5.3 times more than what was recorded for films sintered at 500°C, and the onset potential yielded a cathodic shift of 300 mV. The enhanced performance was linked to improved crystallization and absorption coefficient, lowered flat band potential, increased charge carrier density, decreased charge transfer resistance at the solid/liquid interface and increased surface states capacitance for films annealed at 750°C. Also, nanostructured heterojunction of α-Fe2O3 and porous copper (II) oxide (CuO) composites represented as α-Fe2O3/CuO was prepared for the enhancement of PEC water splitting. Structural studies confirmed the high purity of α-Fe2O3/CuO heterostructures produced. Enhanced photocurrent density of 0.53 mA/cm2 at 1.0 V vs RHE was achieved for α-Fe2O3/CuO photoanodes, representing a 19-fold increase compared to the value recorded for α-Fe2O3. The formation of a heterojunction coupled with the porous surface morphology of α-Fe2O3/CuO facilitated charge separation of photogenerated electron-hole pairs, reduced the bandgap and increased the charge carrier density of the heterostructure, enhancing PEC water splitting. / Thesis (PhD (Physics))--University of Pretoria, 2021. / National Research Foundation - The World Academy of Sciences (NRF) grant #110814 and South African Research Chairs Initiative (SARCHI) grant #115463. / Physics / PhD (Physics) / Restricted

Metal oxide materials could offer earth-abundant, non-toxic alternatives to existing lightabsorber materials in thin-film photovoltaic and photoelectrochemical cells. However, efficiency of these devices based on existing metal oxides is typically low due to poor material properties. In this research, novel Sb:SnO2 nanorod and nanotube electron collectors have been synthesized, investigated and were used to improve the photo-conversion efficiency of top-performing BiVO4 photoelectrochemical cell. The performance of Sb:SnO2/BiVO4 photoanode achieved a new record for the product of light absorption and charge separation efficiencies (ηabs × ηsep) of ~ 57.3% and 58.5% under front- and back-side illumination at 0.6 VRHE and Sb:SnO2/BiVO4 PV cell achieved 1.22% solar power conversion efficiency. In addition, a new promising metal oxide material (CuBiW2O8) has been synthesized and its optoelectronic properties have been investigated to make photovoltaic cell which has potential to achieve over 30% solar power conversion efficiency.

Solar hydrogen has the potential to replace fossil fuels with a sustainable energy carrier that can be produced from sunlight and water via &quotewater splitting&quote. This study investigates the use of hematite (Fe&sub2O&sub3) as a photoelectrode for photoelectrochemical water splitting. Fe&sub2O&sub3 has a narrow indirect band-gap, which allows the utilization of a substantial fraction of the solar spectrum. However, the water splitting efficiencies for Fe&sub2O&sub3 are still low due to poor absorption characteristics, and large losses due to recombination in the bulk and at the surface. The thesis investigates the use of nanostructured composite electrodes, where thin films of Fe&sub2O&sub3 are deposited onto a nanostructured metal oxide substrate, in order to overcome some of the factors that limit the water splitting efficiency of Fe&sub2O&sub3. Doped (Si, Ti) and undoped Fe&sub2O&sub3 thin films were prepared using vacuum deposition techniques, and their photoelectrochemical, electrical, optical and structural properties were characterised. The doped Fe&sub2O&sub3 exhibited much higher photoelectrochemical activity than the undoped material, due to an improvement of the surface transfer coefficient and some grain boundary passivation. Schottky barrier modeling of Fe&sub2O&sub3 thin films showed that either the width of the depletion region or the diffusion length is the dominant parameter with a value around 30 nm, and confirmed that the surface charge transfer coefficient is small. An extensive review of the conduction mechanisms of Fe&sub2O&sub3 is presented. ZnO and SnO&sub2 nanostructures were investigated as substrates for the Fe&sub2O&sub3 thin films. Arrays of well-aligned high aspect ratio ZnO nanowires were optimised via the use of nucleation seeds and by restricting the lateral growth of the nanostructures. The geometry of the nanostructured composite electrodes was designed to maximise absorption and charge transfer processes. Composite nanostructured electrodes showed lower quantum efficiencies than equivalent thin films of Fe&sub2O&sub3, though a relative enhancement ofcollection of long wavelength charge carriers was observed, indicating that the nanostructured composite electrode concept is worthy of further investigation. The rate-limiting step for water splitting with Fe&sub2O&sub3 is not yet well understood and further investigations of the surface and bulk charge transfer properties are required in order to design electrodes to overcome specific shortcomings.

Solar hydrogen has the potential to replace fossil fuels with a sustainable energy carrier that can be produced from sunlight and water via &quotewater splitting&quote. This study investigates the use of hematite (Fe&sub2O&sub3) as a photoelectrode for photoelectrochemical water splitting. Fe&sub2O&sub3 has a narrow indirect band-gap, which allows the utilization of a substantial fraction of the solar spectrum. However, the water splitting efficiencies for Fe&sub2O&sub3 are still low due to poor absorption characteristics, and large losses due to recombination in the bulk and at the surface. The thesis investigates the use of nanostructured composite electrodes, where thin films of Fe&sub2O&sub3 are deposited onto a nanostructured metal oxide substrate, in order to overcome some of the factors that limit the water splitting efficiency of Fe&sub2O&sub3. Doped (Si, Ti) and undoped Fe&sub2O&sub3 thin films were prepared using vacuum deposition techniques, and their photoelectrochemical, electrical, optical and structural properties were characterised. The doped Fe&sub2O&sub3 exhibited much higher photoelectrochemical activity than the undoped material, due to an improvement of the surface transfer coefficient and some grain boundary passivation. Schottky barrier modeling of Fe&sub2O&sub3 thin films showed that either the width of the depletion region or the diffusion length is the dominant parameter with a value around 30 nm, and confirmed that the surface charge transfer coefficient is small. An extensive review of the conduction mechanisms of Fe&sub2O&sub3 is presented. ZnO and SnO&sub2 nanostructures were investigated as substrates for the Fe&sub2O&sub3 thin films. Arrays of well-aligned high aspect ratio ZnO nanowires were optimised via the use of nucleation seeds and by restricting the lateral growth of the nanostructures. The geometry of the nanostructured composite electrodes was designed to maximise absorption and charge transfer processes. Composite nanostructured electrodes showed lower quantum efficiencies than equivalent thin films of Fe&sub2O&sub3, though a relative enhancement ofcollection of long wavelength charge carriers was observed, indicating that the nanostructured composite electrode concept is worthy of further investigation. The rate-limiting step for water splitting with Fe&sub2O&sub3 is not yet well understood and further investigations of the surface and bulk charge transfer properties are required in order to design electrodes to overcome specific shortcomings.

La séparation photoélectrochimique de l'eau (PEC) est un moyen direct de produire un combustible solaire tel que l'hydrogène à partir de l'eau. Le goulot d'étranglement de ce processus se situe dans la photoanode, qui est responsable du côté oxydation de la réaction1,2. Dans ce travail, l'utilisation de BiVO4 en tant que photoanode a été largement étudiée afin d'améliorer sa photoactivité. L’optimisation de la synthèse de photoanodes BiVO4 par électrodéposition en couche mince sur du FTO a été réalisée. Les facteurs influant sur l'activité photoélectrochimique, tels que le temps d'électrodéposition, le rapport Bi-KI/benzoquinone-EtOH dans le bain de dépôt et la température de calcination, ont été étudiés à l'aide de la conception composite centrale d'expériences. Les états de surface sur la surface de BiVO4 donnent lieu à des niveaux de défaut pouvant induire une recombinaison électron-trou via le mécanisme de Shockley-Read-Hall5. Afin de minimiser les inefficacités dues à la recombinaison électron-trou et passiver les états de surface, des couches de recouvrement ultra-fines d'Al2O3 et de TiO2 ont été déposées sur les électrodes en film mince BiVO4 d'une manière analogue à l'ALD. Cela a également été réalisé afin de protéger la surface de BiVO4 de la photocorrosion et d’augmenter sa stabilité. Une densité de photocourant de 0,54 mA/cm2 à 1,23 V vs RHE a été obtenue pour les 2 cycles de BiVO4 modifié par Al2O3, comme le montre la Figure 2, soit une amélioration de 54% par rapport à la BiVO4 nue qui démontrait une densité de photocourant de 0,35 mA/cm2. à 1,23 V vs RHE. Une augmentation de 15% de la stabilité de l'électrode de BiVO4 modifiée par Al2O3 a également été observée au cours de 7,5 heures d'irradiation continue. De plus, grâce aux mesures de capacité de surface présentées à la Figure 3, il a été montré que la surcouche de Al2O3 passivait effectivement à passiver les états de surface des électrodes de BiVO4. La nature de la surface de BiVO4 a été étudiée en étudiant la réactivité de la poudre de BiVO4 avec un titrant chimique. L’existence de groupes hydroxyle de surface sur BiVO4 a été confirmée et quantifiée (max. 1,5 OH / nm2) par titrage chimique. La réaction de la poudre de BiVO4 avec une impulsion de AlMe3 et une impulsion de H2O a montré qu'il existait 1,2 molécules de CH4 dégagées par Bi-OH. Dans ce travail, nous avons pu mettre en évidence les facteurs importants dans la synthèse de BiVO4 et leur incidence sur la photoactivité résultante. Nous avons également réussi à passiver les états de surface de BiVO4 en utilisant Al2O3, ce qui n’est pas bien exploré dans la littérature. De plus, nous avons pu sonder et discuter de la nature de la surface de BiVO4. Ceci est une connaissance très fondamentale et le premier rapport à ce sujet, à notre connaissance. Une bonne compréhension de cette surface semi-conductrice importante et de ses interactions facilitera la conception d'un photoanode BiVO4 plus efficace / Photoelectrochemical (PEC) water splitting is a direct way of producing a solar fuel like hydrogen from water. The bottleneck of this process is in the photoanode, which is responsible for the water oxidation side of the reaction1,2. In this work, the use of BiVO4 as a photoanode was extensively studied in order to improve its photoactivity. The optimization of BiVO4 photoanode synthesis via thin film electrodeposition on FTO was performed. The factors affecting the photoelectrochemical activity such as the electrodeposition time, ratio of the Bi-KI to benzoquinone-EtOH in the deposition bath, and the calcination temperature, have been investigated by using the Central Composite Design of Experiments.Surface states on the BiVO4 surface give rise to defect levels, which can mediate electron-hole recombination via the Shockley-Read-Hall mechanism5. In order to protect the BiVO4 surface and minimize the inefficiencies due to electron-hole recombination and passivate the surface states, ultrathin overlayers of Al2O3 and TiO2 were deposited to the BiVO4 thin film electrodes in an ALD-like manner. A photocurrent density of 0.54 mA/cm2 at 1.23 V vs RHE was obtained for the 2 cycles Al2O3-modified BiVO4, which was a 54% improvement from the bare BiVO4 that demonstrated a photocurrent density of 0.35 mA/cm2 at 1.23 V vs RHE. A 15% increase in stability of the Al2O3- modified BiVO4 electrode was also observed over 7.5 hours of continuous irradiation. Moreover, through surface capacitance measurements, it was shown that the Al2O3 overlayer was indeed passivating the surface states of the BiVO4 electrodes. The nature of the BiVO4 surface was studied by investigating the reactivity of powder BiVO4 with a chemical titrant. The existence of surface hydroxyl groups on BiVO4 was confirmed and quantified (max 1.5 OH/nm2) via chemical titration. The reaction of the BiVO4 powder with one pulse of AlMe3 and 1 pulse of H2O showed that there were 1.2 molecules of CH4 evolved per Bi-OH. In this work, we were able to highlight which factors are important in the synthesis of BiVO4, and how they affect the resulting photoactivity. We have also achieved the passivation of the BiVO4 surface states using Al2O3, which is not well-explored in literature. Moreover, we were able to probe and discuss the nature of the BiVO4 surface. This is a very fundamental knowledge and the first report of such, to the best of our knowledge. A good understanding of this important semiconductor surface and its interactions will aid in the design of a more efficient BiVO4 photoanode

Tin disulfide (SnS2) is a two-dimensional (2D) material with excellent properties and high prospects for low-cost solutions to catalytic and optoelectronic applications. In this work, vertical nanoflakes of SnS2 have been synthesized using custom-designed close space sublimation (CSS) system and investigated for applications in photoelectrochemical (PEC) water oxidation and metal-semiconductor-metal (MSM) photodetector. For the PEC application, vertical SnS2 nanoflakes grown directly on transparent conductive substrates have been used as photoanodes, which produce record photocurrents of 4.5 mA cm−2 for oxidation of a sulfite hole scavenger and 2.6 mA cm−2 for water oxidation without any hole scavenger, both at 1.23 VRHE in neutral electrolyte under simulated AM1.5G sunlight, and stable photocurrents for iodide oxidation in acidic electrolyte. This remarkable performance has been attributed to three main reasons: (1) high intrinsic carrier mobility of 330 cm2 V−1 s−1 and long photoexcited carrier lifetime of 1.3 ns in the nanoflakes, (2) the nanoflake height that balances the competing requirements of light absorption and charge transport, and (3) the unique stepped morphology of these nanoflakes that improves photocurrent by exposing multiple edge sites in every nanoflake. In another application, these SnS2 nanoflakes have been used to enhance the performance of lead sulfide quantum dot (PbS QDs) photodetectors by providing a high-mobility channel for photoexcited charges from PbS QDs, which results in 2 orders of magnitude enhancement in responsivity. The physical models and experimental findings presented in this dissertation can help engineer more cost-effective solutions for PEC water splitting and optoelectronics based on 2D metal dichalcogenides.

In 1972 Fujishima and Honda conceptualised a photoelectrochemical cell for hydrogen generation via PEC water splitting. Hydrogen as a clean energy carrier provides environmentally friendly energy storage solutions or can fuel certain applications. This idea has since then been further built upon with new materials and combinations with the aim of improving efficiency. In this project n-type cubic silicon carbide thick layers were grown by a sublimation method and characterised for water splitting performance. A generated photo-current density of 0.45 mA/cm2 was measured with no bias between the working and counter electrodes.

Le mécanisme élémentaire de l'électrodépôt de films de MnO2 fût étudié sur des électrodes de Pt massif dans des électrolytes aqueux. Il se révèle être une réaction multi-étapes sensible au pH et à la force ionique. La chronoampérométrie couplée à des électrolytes neutres peu concentrés favorise l'électrodépôt de films stables de MnO2. Le FTO est un meilleur substrat que l'ITO parce qu'il présente une activité électrochimique plus élevée et favorise la stabilité mécanique de films électrodéposés de MnO2. De plus, le potentiel d'électrodépôt influence à la fois la structure et la morphologie des films de MnO2. Les films amorphes de MnO2 obtenus à potentiel élevé possèdent une activité électrocatalytique et une stabilité plus élevées que la birnessite. Un traitement thermique peut améliorer amplement leur activité électrocatalytique et leur stabilité mécanique. Une transition de phase des films de MnO2 apparaît à 500 °C. Leur morphologie change de façon dramatique après chauffage au-delà de cette température. Les échantillons chauffés à 500 °C ont la meilleure activité électrocatalytique pour l'OER. Les cations Na+, K+, Ca2+ and Mg2+ sont insérés en petites quantités dans la structure des films de MnO2 au cours de la démarche d'électrodépôt, mais ils influencent néanmoins la structure et la morphologie des films. Finalement, les films de birnessite ou amorphes apparaissent comme des candidats prometteurs en tant que catalyseurs pour la dissociation photoélectrochimique de la dissociation de l'eau, puisqu'ils génèrent des photocourants considérables sous lumière solaire. Pour cela, des films de MnO2 épais, amorphes et recuits à 500 °C produisent les meilleures performances. / The basic electrodeposition mechanism of MnO2 films was studied first on bulk Pt electrodes in various aqueous electrolytes. It was revealed that MnO2 electrodeposition is a multi-step reaction that is sensitive to pH and ionic strength. Chronoamperometry coupled to low concentration neutral aqueous solutions favors the electrodeposition of stable MnO2 films. FTO was found to be a better substrate than ITO, because it has a higher electrochemical activity and could enhance the mechanical stability of electrodeposited MnO2 films. Moreover, the potential used for electrodeposition has great influence on both the structure and the morphology of MnO2 films. Amorphous MnO2 films obtained at high potential possess higher electrocatalytic activity and stability than the birnessite-type MnO2 variety. The heat treatment can greatly enhance the electrocatalytic activity and mechanical stability. A phase transition of MnO2 films appears at 500 °C. The morphology changes dramatically after heating above this temperature. Samples heated at 500 °C are found to have the best electrocatalytic activity towards OER. Na+, K+, Ca2+ and Mg2+ cations were found to be inserted in small amounts into the structure of MnO2 films during the electrodeposition procedure but they influence the structure and morphology of the films. Finally, birnessite type and amorphous MnO2 films appear to be promising candidates as catalysts for photoelectrochemical water splitting, as they are able to generate considerable photocurrents under solar light illumination. In this purpose, thick and amorphous films with 500 °C heat treatment are supposed to produce the best performances.

Synopsis of thesis entitled “Electrochemical and Photoelectrochemical Investigations of Co, Mn and Ir-based Catalysts for Water Splitting” by Ahamed Irshad M (SR No: 02-01-02-10-11-11-1-08823) under the supervision of Prof. N. Munichandraiah, Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore (India), for the Ph.D. degree of the Institute under the Faculty of Science. Hydrogen is considered as the fuel for future owing to its high gravimetric energy density and eco-friendly use. In addition, H2 is an important feedstock in Haber process for ammonia synthesis and petroleum refining. Although, it is the most abundant element in the universe, elemental hydrogen is not available in large quantities on the planet. Consequently, H2 must be produced from its various chemical compounds available on earth. Currently, H2 is produced in large scale from methane by a process called steam-methane reforming (SMR). This process releases huge amount of CO2 into atmosphere as the by-product causing serious environmental issues. The development of alternate clean methods to generate H2 is a key challenge for the realization of hydrogen economy. Production of H2 gas by water splitting using electricity or sunlight is known. Low cost, high natural abundance and carbon neutrality make water as the best source of hydrogen. Thermodynamically, splitting of H2O needs 237 kJ mol-1 of energy, which corresponds to 1.23 V according to the equation, ΔG = -nFE. However, commercial electrolyzers usually operate between 1.8 to 2.1 V, due to the need of large overvoltage. The high overvoltage and subsequent energy losses are mainly associated with the sluggish kinetics of oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode. The overvoltage can be considerably reduced using suitable catalysts. Hence, the design and development of stable, robust and highly active catalysts for OER and HER are essential to make water splitting efficient and economical. Attempts in the direction of preparing several novel OER and HER catalysts, physicochemical characterizations and their electrochemical or photoelectrochemical activity are described in the thesis. A comprehensive review of the literature on various types of catalysts, thermodynamics, kinetics and mechanisms of catalysis are provided in the Chapter 1 of the thesis. Chapter 2 furnishes a brief description on various experimental techniques and procedures adopted at different stages of the present studies. Chapter 3 explains the results of the studies on kinetics of deposition and stability of Nocera’s Co-phosphate (Co-Pi) catalyst using electrochemical quartz crystal microbalance (EQCM). The in-situ mass measurements during CV experiments on Au electrode confirm the deposition of Co-Pi at potential above 0.87 V vs. Ag/AgCl, 3 M KCl (Fig.1a and b). The catalyst is found to deposit via a nucleus mediated process at a rate of 1.8 ng s-1 from 0.5 mM Co2+ in 0.1 M neural phosphate solution at 1.0 V. Further studies on the potential and electrolyte dependent stability of the Co-Pi suggest that the catalyst undergoes severe corrosion at high overpotential and in non-buffer electrolytes. Current/ Fig.1 (a) Cyclic voltammograms and (b) mass variations vs. potential of Au-coated quartz crystal in 0.1 M potassium phosphate buffer solution (pH 7.0) containing 0.5 mM Co(NO3)2 Chapter 4 deals with the electrochemical deposition of a novel OER catalyst, namely, Co-acetate (Co-Ac) from a neutral acetate electrolyte containing Co2+ ions. Use of acetate solution instead of phosphate avoids the solubility limitations and helps to get thick layer of the catalyst in a short time from concentrated Co2+ solutions. In addition, the Co-Ac is found to be catalytically superior to Co-Pi (Fig. 2a). It is also observed that the Co-Ac catalyst undergoes ion exchange with electrolyte species during electrolysis in phosphate buffer solution, which results in the formation of a hybrid Co-Ac-Pi catalyst (Fig. 2b). The presence of both acetate and phosphate ions in the catalyst and their synergistic catalytic effect enhance the OER activity. Fig.2. (a) Linear sweep voltammograms of Co-Ac in (i) phosphate and (ii) acetate electrolytes, and that of Co-Pi in (iii) acetate and (iv) phosphate electrolytes. (b) SEM image showing the formation of two layers of the catalysts after electrolysis in phosphate solution. In Chapter 5, high OER activity of an electrodeposited amorphous Ir-phosphate (Ir-Pi) is investigated. The catalyst is prepared by the anodic polarization of a carbon paper electrode in neutral phosphate solution containing Ir3+ ions (Fig. 3). The Ir-Pi film deposited on the electrode has Ir and P in an approximate ratio of 1:2 with Ir in an oxidation state higher than +4. Phosphate ions play a major role for both the electrochemical deposition process and its catalytic activity towards OER. The Ir-Pi catalyst is superior to similarly deposited IrO2 and Co-Pi catalysts both in terms of onset potential and current density at any potential in the OER region. Tafel measurements and pH dependence studies identify the formation of a high energy intermediate during oxygen evolution. Fig.3. (a) Cyclic voltammograms during the Ir-Pi deposition and (b) SEM image of Ir-Pi on C. Chapter 6 is on the preparation of a composite of Mn-phosphate (MnOx-Pi) and reduced graphene oxide (rGO) and its utilization as an OER catalyst. The composite is prepared by the simultaneous electrochemical reduction of KMnO4 and graphene oxide (GO) in a phosphate solution (pH 7.0). Various analytical techniques such as TEM, XPS, Raman spectroscopy, etc. confirm the formation of a composite (Fig. 4) and electrochemical studies indicate the favourable role of rGO towards OER. Under identical conditions, MnOx-Pi-rGO gives 6.2 mA cm-2 at 2.05 V vs. RHE whereas it is only 2.9 mA cm-2 for MnOx-Pi alone. However, the catalyst is not very stable during OER which is ascribed to slow oxidation of Mn3+ in the catalyst. Fig.4. (a) Raman spectrum and (b) TEM image of MnOx-Pi-rGO. In Chapter 7, an amorphous Ni-Co-S film is prepared by a potentiodynamic deposition method using thiourea as the sulphur source. The electrodeposit is used as a catalyst for the HER in neutral phosphate solution. The composition of the catalyst and the HER activity are tuned by varying the ratio of concentrations of Ni2+ and Co2+. The bimetallic Ni-Co-S catalyst exhibits better HER activity than both Ni-S and Co-S (Fig. 5a). Under optimized deposition conditions, Ni-Co-S requires just 150 mV for the onset of HER and 10 mA cm-2 is obtained for 280 mV overpotential. The Ni-Co-S shows two different Tafel slopes, indicating two different potential dependent HER mechanisms (Fig. 5b). Presence of two different catalytic sites which contribute selectively in different potential regions is proposed. Fig.5. (a) Linear sweep voltammograms of HER at 1 mV s-1 in 1 M phosphate solutions (pH 7.4) using (i) Ni-S, (ii) Co-S and (c) Ni-Co-S. (b) Tafel plot of Ni-Co-S showing two Tafel slopes. Photoelectrochemical OER using ZnO photoanode and Co-acetate (Co-Ac) cocatalyst is studied in Chapter 8 of the thesis. Randomly oriented crystalline ZnO nanorods are prepared by the electrochemical deposition of Zn(OH)2 followed by heat treatment at 350 ºC in air. Co-Ac is then photochemically deposited onto ZnO nanorods by UV illumination in the presence of neutral acetate buffer solution containing Co2+ ions. The hybrid Co-Ac-ZnO shows higher photoactivity in comparison with bare ZnO towards PEC water oxidation (Fig. 6). Co-Ac acts as a cocatalyst and reduces the charge carrier recombination at the electrode/electrolyte interface. Fig.6. (a) Linear sweep voltammograms of ZnO under (i) dark and (ii) light conditions, and that of Co-Ac-ZnO in (iii) dark and (iv) light in 0.1 M phosphate (pH 7.0) electrolyte. Chapter 9 deals with PEC water oxidation using α-Fe2O3 photoanode and Ir-phosphate (Ir-Pi) cocatalyst. α-Fe2O3 is prepared by direct heating of Fe film in air which in turn is deposited by the electrochemical reduction of Fe2+. Thickness of the film as well as calcination temperature is carefully optimized. In order to further enhance the OER kinetics, Ir-Pi is electrochemically deposited onto α-Fe2O3. Under optimized conditions, Ir-Pi deposited α-Fe2O3 shows around 3 times higher photocurrent than that of bare α-Fe2O3 at 1.23 V vs. RHE (Fig. 7). Ir-Pi acts as a cocatalyst for OER and reduces the photogenerated charge carrier recombination. Fig.7. Photocurrent variation of α-Fe2O3 electrode at 1.23 V vs. RHE for (i) front and (ii) back side illuminations, against Ir-Pi deposition time. The thesis ends with a short summary and future prospectus of studies described in the thesis. The research work presented in the thesis is carried out by the candidate as the part of Ph.D. program. Some of the results have already been published in the literature and some manuscripts are under preparation. A list of publications is included at the end of the thesis. It is anticipated that the studies reported in the thesis will constitute a worthwhile contribution.