Hydrogen has been identified as a potential energy carrier due to its high energy capacity and environmental harmlessness. Compared with hydrogen production from hydrocarbons such as methane and naphtha in a conventional hydrogen energy system, photocatalytic hydrogen evolution from water splitting offers a more economic approach since it utilizes the abundant solar irradiation as energy source and water as initial reactant. Powder photocatalyst, which generates electrons and holes under illumination, is the origin where the overall reaction happens. High solar energy conversion efficiency especially from visible range is commonly the target. Besides, cocatalyst for hydrogen and oxygen evolution is also playing an essential role in facilitating the charge separation and enhancing the kinetics. In this thesis, the objective is to achieve high energy conversion efficiency towards water splitting from diverse aspects. The third chapter focuses on a controllable method to fabricate metal pattern, which is candidate for hydrogen evolution cocatalyst while chapter 4 is on the combination of strontium titanium oxide (SrTiO3) with graphene oxide (GO) for a better photocatalytic performance. In the last chapter, photoelectrochemical water splitting by Ta3N5 photoanode and FeOOH as a novel oxygen evolution cocatalyst has been investigated.

The role of high doping levels and the interfacial structure on the junction behavior between n-type silicon microwires and the conducting polymer, PEDOT:PSS, was investigated using tungsten probes, an established technique for Ohmic contact to individual microwires. The resistance and the doping density of carriers as a function of length along each microwire as well as the junction resistance between individual microwires and the conducting polymer were characterized by making Ohmic contact to microwires. The junction between highly-doped n-Si microwires and the conducting polymer had relatively symmetric current-voltage characteristics and a significantly lower junction resistance as compared to low-doped microwires. The current-voltage response of junctions formed between the polymer and low-doped microwires, which still incorporated the metal catalyst used in the growth process, was also studied. Junctions incorporating copper at the interface had similar current-voltage characteristics to those observed for the highly-doped microwire, while junctions incorporating gold exhibited significantly lower resistances

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

Doctor of Philosophy / Department of Chemistry / Kenneth J. Klabunde / New photocatalystic materials Ti-In oxy(nitride) and nanosized Ru-loaded strontium titanate doped with Rh (Ru/SrTiO3:Rh) have been synthesized. The textural and surface characteristic properties were studied by nitrogen BET analysis, diffuse reflectance UV-vis spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy and powder XRD. The photocatalytic properties were enhanced by the binary metal oxides of titanium dioxide and indium oxide. The XRD patterns confirmed the oxygen exchange between two metal oxides during the synthesis. Moreover, the presence of titanium dioxide can help the stabilization of InN during hot NH3(g) treatment. On the other hand, the particle sizes of aerogel prepared Ru/SrTiO3:Rh varied from 12 to 25 nm depended on different Rh doping. A mixture of ethanol and toluene was found to be the best binary solvent for supercritical drying, which yielded a SrTiO3 sample with a surface area of 130 m2 Enhanced photocatalytic hydrogen production under UV-vis light irradiation was achieved by ammonolysis of intimately mixed titanium dioxide and indium oxide at high temperatures. Gas chromatography monitored steadily the formation of hydrogen when sacrificial (methanol or ethanol) were present. XRD patterns confirmed that the photocatalysts maintain crystalline integrity before and after water splitting experiments. Moreover, the presence of InN may be crucial for the increase of hydrogen production activities. /g and an average crystallite size of 6 nm. These Ru/SrTiO3:Rh photocatalysts have been studied for photocatalytic hydrogen production under visible light. The band gap of the bulk SrTiO3 (3.2 eV) does not allow response to visible light. However, after doping with rhodium and loaded with ruthenium, the modified strontium titanates can utilize light above 400 nm due to the formation of valence band or electron donor levels inside of the band gap. Moreover, the surface areas of these photocatalysts are much larger than conventional solid-state synthesized samples (1-2 m 2/g), which yielded more Ru loading and reaction sites. The areogel and hydrothermal synthesized samples required basic (alkaline) conditions for hydrogen generation facilitation compared with acidic conditions for conventional solid-state samples.

Thesis advisor: Dunwei Wang / Due to the increasing energy demand from human activities, efficient utilization of renewable energy, such as wind, solar and geothermal energies, becomes necessary and urgent. Photoelectrochemical water splitting offers a great example to convert solar energy and storage it in the term of chemical bond. Seeking suitable photoanode materials becomes the research focus of my study, because the development of photoanode materials significantly lags that of robust photocathode (such as Si). The main challenge is to fabricate an efficient and stable photoanode material which can deliver high photocurrent and sufficient photovoltage which can match well with those of photocathode when made into tandem cell configuration. Hematite (α-Fe2O3) represents a promising metal oxide photoanode material, with a suitable band gap (2.1 eV), low cost and toxicity. Applying nanostructures and appropriate surface modification layers help address existing research challenges. As a result, a much lower turn on potential and greater photocurrent density is achieved. Another photoanode material attracts our attention is tantalum nitride (Ta3N5), with a similar band gap to hematite but much better light absorption properties, shows a poor stability in aqueous electrolyte. For both photoanode materials, thermodynamic and kinetic aspects are studied in details when tested in water splitting devices. These works provide new ideas and insights on the future studies. / Thesis (PhD) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

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.

The field of solar energy conversion has experienced resurgence in recent years due to mounting concerns related to fossil fuel consumption. The sheer quantity of available solar energy and corresponding opportunity for technological improvement has motivated extensive study of novel light-absorbing semiconductors for solar energy conversion. Often, these studies have focused on new ways of synthesizing and altering thin film semiconductor materials with unique compositions and morphologies in order to optimize them for higher conversion efficiencies. In this dissertation, we discuss the synthesis and electrochemical characterization of a variety of candidate semiconductor materials exhibiting promising characteristics for photoelectrochemical solar energy conversion. Three specific methods of thin film deposition are detailed. The first is a physical vapor deposition technique used to independently tune the morphology and composition of hematite (α-Fe2O3) based materials. Because of hematite’s poor electronic properties, these modifications were able to significantly improve its performance as a photoanode for water oxidation. The second technique is electrodeposition, which was employed to deposit the novel ternary metal oxide, CuBi2O4. The study of these films, along with those prepared by physical vapor deposition, provided insight into the factors limiting the ability of this photo-active material to function as a photocathode for hydrogen evolution from water. The third technique is chemical spray pyrolysis, which was employed to deposit and optimize films of the bismuth chalco-halides BiOI and BiSI. These studies were used to obtain previously unknown properties of these materials relevant to their utilization in photoelectrochemical cells. The manipulation of deposition temperature had significant effects on these properties and dictated the films’ overall photoconversion performance. / text

Photoelectrochemical studies were performed on bismuth vanadate (BiVO₄) to understand chemical and physical properties of the photocatalysts, and to improve the photoactivity for water oxidation. Scanning electrochemical microscopy (SECM) was used to screen various dopants for BiVO₄, to calculate the photoconversion efficiencies to chemical energy at BiVO₄ electrodes, and to study the water oxidation intermediate radicals at the surface of BiVO₄. Tungsten and molybdenum doped BiVO₄ (W/Mo-BiVO₄) shows a photocurrent for water oxidation that is more than 10 times higher than undoped BiVO₄. Photoelectrochemical measurements and material analysis were done to discuss the factors that affect performance of BiVO₄. Finite elements analysis was also performed to explain the electron-hole transport and electrochemical reactions at W/Mo-BiVO₄ electrodes in solutions. Addition of conductive or electron accepting materials, e.g. reduced graphene oxide, into BiVO₄ was tried to study the electron-hole transport phenomena in the metal oxide electrodes. Surface adsorbed radicals produced during the water oxidation at W/Mo-BiVO₄ were interrogated by using SECM that the surface coverage and decay kinetics of adsorbed hydroxyl radicals at W/Mo-BiVO₄ were measured. The quantum efficiencies of the injected photon conversion to chemical energy were obtained from the photoelectrochemical measurements by using SECM. SECM techniques and finite elements analysis were also used to measure the faradaic efficiency of water oxidation at W/Mo-BiVO₄ under irradiation. Finally, unbiased water splitting to generate hydrogen and oxygen from water splitting only using photon energy at W/Mo-BiVO₄ electrodes was demonstrated in a dual n-type semiconductor or Z-scheme device. / text

While prices of solar energy are becoming cost competitive with traditional fossil fuel resources, large scale deployment of solar energy has been limited by the inability to store excess electrical energy efficiently. One promising route towards both the capture and storage of solar energy is through photoelectrochemical water splitting, a process by which a semiconducting material can collect energy from the sun and use it to directly split water (H2O) into hydrogen fuel and oxygen. Unfortunately, photoelectrochemical water splitting devices are limited by the low efficiencies and high overpotentials of the oxygen evolution reaction (OER). To improve kinetics of OER, different electrocatalyst are often coated on the semiconductor. However, the role of the catalyst and the mechanism of charge transfer at the semiconductor|catalyst interface is not clear. It is important to understand this interface if we are to rationally design high performance water splitting cells. The research presented in this dissertation takes on two aims: 1) obtaining a fundamental knowledge of the charge transfer processes that take place at the semiconductor catalyst interface of photoanodes and 2) developing new experimental approaches that can be applied towards achieving the first aim. Specifically, this dissertation begins with a prospectus that outlines the state of the field, and the what was known about the semiconductor|electrocatalyst interface at the outset of the presented work (Chapter II). Next, the testing and application of new nanoelectrode AFM probes to study an array of electrochemical phenomena will be discussed (Chapter III). These probes will then be applied towards the study of hematite (Fe2O3) semiconductors coated with cobalt phosphate (oxy)hydroxide (CoPi) electrocatalyst (Chapter IV) and bismuth vanadate (BiVO4) semiconductors coated with CoPi electrocatalyst (Chapter 5). This dissertation includes previously published and unpublished co-authored material. / 2020-01-11