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Since the discovery of titanium dioxide's (TiO2) capability for water-splitting and photocatalytic degradation of organic compounds, semiconductor photocatalysis has received great attention and promises an environmentally clean and sustainable solution by solar hydrogen production and waste water treatment.1,2 In this thesis, the photocatalytic performance of two different photocatalyst systems based on Pd nanoparticle decorated n-type TiO2 nanorods and Ag incorporated ptype BiFeO3 thin films were investigated for solar hydrogen and oxygen production and photodecolourisation of a common textile dye, Rhodamine B. High surface area TiO2 nanorods were grown on glass fibre substrates by a hydrothermal method to produce a mechanically robust photocatalytic filter. Metallic Pd nanoparticles were deposited onto TiO2 nanorods via a photochemical method. It was found that the hybrid Pd/TiO2 catalyst system showed higher photoactivity with a doubled kinetic rate for the photodecolourisation of RhB. Full decolourisation has been achieved in 180 minutes with as-grown TiO2 nanorods whereas this time was reduced to only 90 minutes for the Pd/TiO2 hybrid catalyst. This enhancement was associated with the localised surface plasmon resonance (LSPR) effect due to the interaction of Pd with visible light and the electron scavenging role of Pd for efficient charge separation. The same hybrid Pd/TiO2 photocatalyst system was then developed on FTO coated glass substrates so that the photoelectrochemical experiments can be carried out using a potentiostat. Mott-Schottky curves demonstrated a positive shift in flat band potential and an increased charge carrier density after Pd deposition. The facilitated charge transfer at the interface of Pd and TiO2 was shown by EIS data with a smaller arc size for Pd/TiO2 in Nyquist plots. The photoelectrochemical performance of the bare TiO2 and hybrid Pd/TiO2 samples were compared through the photoelectrocatalysis of Rhodamine B (RhB) and solar hydrogen production in different electrolyte solutions at various applied voltage values. A higher amount of hydrogen by Pd/TiO2 was photogenerated in methanol solution whereas bare TiO2 produced a higher amount of hydrogen in 0.01M Na2SO4 and pure deionised water under the same conditions. The results were discussed by proposing possible reaction mechanisms with an emphasis on the charge trapping role of Pd nanoparticles. P-type BiFeO3 (BFO) thin films were deposited on large scale FTO coated glass substrates by a sol-gel method. A photocurrent density of -0.004mA/cm2 was achieved at 0V vs NHE under AM1.5 G illumination and 1.2μmol of O2 was produced in 2h at an external bias of -0.5V vs Ag/AgCl. These values were significantly increased upon the incorporation of Ag into the BFO matrix. Ag was incorporated into the BiFeO3 matrix at different concentrations as metallic Ag structures and Ag nanowires. The enhancement by Ag modification was attributed to enhanced light absorption due to light scattering effect and efficient charge separation by Ag as they act as electron sinks. These explanations were supported by shifts in flat band and onset potentials after Ag modification in detailed measurements of Mott-Schottky plots and j-v curves.

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.

Research into dye sensitised solar cells has increased in recent years as the search for a viable low cost, renewable energy source continues. The synthesis and characterisation of an array of symmetrical and asymmetrical zinc and ruthenium centred phthalocyanines and naphthalocyanines are presented in this work. Certain compounds were designed so that they would possess a carboxylic acid group which could be utilised to chemisorb the compound to a titanium dioxide surface. The dye sensitised titania electrodes were studied as potential photoanodes in dye sensitised solar cells. The use of symmetrical and asymmetrical compounds in the solar cells enabled conclusions to be drawn about the effects on electron injection of the HOMO energy level and the number and position of binding groups. The highest incident photon-to-current conversion efficiency (IPCE) of 4 % and overall conversion efficiency (η) of 0.09 % were obtained when 2,3:9,10-(22,92-carboxyl)benzo(b,k)-15,18,22,25-tetrakis(octyl)phthalocyaninatozinc(II) (63) was utilised as a sensitiser. This response was concluded to be due to the molecule possessing two binding groups and phthalocyanine like energy levels. When the ruthenium centred and zinc centred compounds were compared as sensitisers in DSCs, an increase in photovoltage and photocurrent was observed with the use of the ruthenium centred compounds. This is due to the binding group being attached to the axial ligand and therefore being situated closer to the LUMO electron density which is found at the centre of the molecule. As the binding group is closer there is less hindrance to electron injection into the TiO2 conduction band. Aggregation studies were also conducted on the acid and ester substituted zinc naphthalocyanine with and without the use of additives. It was found that the ester existed primarily as a dimer whose formation is concentration dependent. The acid also existed as a dimer but produced a "fake" monomer peak due to the formation of J aggregates. It was found that upon dilution the angle of the J aggregates shifted so that they formed face-to-face aggregates. It was found that the peripherally binding additive cetyltrimethylammonium bromide (CTAB) prevented aggregation at a concentration 20 times that of the compound but upon dilution rearranged itself so that aggregation was no longer inhibited.

The recent momentum in energy research has simplified converting solar to electrical energy through photoelectrochemical (PEC) cells. There are numerous benefits to these PEC cells, such as the inexpensive fabrication of thin film, reduction in absorption loss (due to transparent electrolyte), and a substantial increase in the energy conversion efficiency. Alpha-hematite ([U+F061]-Fe2O3) has received considerable attention as a photoanode for water-splitting applications in photoelectrochemical (PEC) devices. The alpha-hematite ([U+F061]-Fe2O3) nanomaterial is attractive due to its bandgap of 2.1eV allowing it to absorb visible light. Other benefits of [U+F061]-Fe2O3 include low cost, chemical stability and availability in nature, and excellent photoelectrochemical (PEC) properties to split water into hydrogen and oxygen. However, [U+F061]-Fe2O3 suffers from low conductivity, slow surface kinetics, and low carrier diffusion that causes degradation of PEC device performance. The low carrier diffusion of [U+F061]-hematite is related to higher resistivity, slow surface kinetics, low electron mobility, and higher electro-hole combinations. All the drawbacks of [U+F061]-Fe2O3, such as low carrier mobility and electronic diffusion properties, can be enhanced by doping, which forms the nanocomposite and nanostructure films. In this study, all nanomaterials were synthesized utilizing the sol-gel technique and investigated using Scanning Electron Microscopy (SEM), X-ray Diffractometer (XRD), UV-Visible Spectrophotometer (UV-Vis), Fourier Transform Infrared Spectroscopy (FTIR), Raman techniques, Particle Analyzer, Cyclic Voltammetry (CV), and Chronoamperometry, respectively. The surface morphology is studied by SEM. X-Ray diffractometer (XRD) is used to identify the crystalline phase and to estimate the crystalline size. FTIR is used to identify the chemical bonds as well as functional groups in the compound. A UV-Vis absorption spectral study may assist in understanding electronic structure of the optical band gap of the material. Cyclic voltammetry and chronoamperometry were used to estimate the diffusion coefficient and study electrochemical activities at the electrode/electrolyte interface. In this investigation, the [U+F061]-Fe2O3 was doped with various materials such as metal oxide (aluminum, Al), dichalcogenide (molybdenum disulfide, MoS2), and co-catalyst (titanium dioxide, TiO2). By doping or composite formation with different percentage ratios (0.5, 10, 20, 30) of aluminum (Al) containing [U+F061]-Fe2O3, the mobility and carrier diffusion properties of [U+F061]-hematite ([U+F061]-Fe2O3) can be enhanced. The new composite, Al-[U+F061]-Fe2O3, improved charge transport properties through strain introduction in the lattice structure, thus increasing light absorption. The increase of Al contents in [U+F061]-Fe2O3 shows clustering due to the denser formation of the Al-[U+F061]-Fe2O3 particle. The presence of aluminum causes the change in structural and optical and morphological properties of Al-[U+F061]-Fe2O3 more than the properties of the [U+F061]-Fe2O3 photocatalyst. There is a marked variation in the bandgap from 2.1 to 2.4 eV. The structure of the composite formation Al-[U+F061]-Fe2O3, due to a high percentage of Al, shows a rhombohedra structure. The photocurrent (35 A/cm2) clearly distinguishes the enhanced hydrogen production of the Al-[U+F061]-Fe2O3 based photocatalyst. This work has been conducted with several percentages (0.1, 0.2, 0.5, 1, 2, 5) of molybdenum disulfide (MoS2) that has shown enhanced photocatalytic activity due to its bonding, chemical composition, and nanoparticle growth on the graphene films. The MoS2 material has a bandgap of 1.8 eV that works in visible light, responding as a photocatalyst. The photocurrent and electrode/electrolyte interface of MoS2-[U+F061]-Fe2O3 nanocomposite films were investigated using electrochemical techniques. The MoS2 material could help to play a central role in charge transfer with its slow recombination of electron-hole pairs created due to photo-energy with the charge transfer rate between surface and electrons. The bandgap of the MoS2 doped [U+F061]-Fe2O3 nanocomposite has been estimated to be vary from 1.94 to 2.17 eV. The nanocomposite MoS2-[U+F061]-Fe2O3 films confirmed to be rhombohedral structure with a lower band gap than Al-[U+F061]-Fe2O3 nanomaterial. The nanocomposite MoS2-[U+F061]-Fe2O3 films revealed a more enhanced photocurrent (180 μA/cm2) than pristine [U+F061]-Fe2O3 and other transition metal doped Al-[U+F061]-Fe2O3 nanostructured films. The p-n configuration has been used because MoS2 can remove the holes from the n-type semiconductor by making a p-n configuration. The photoelectrochemical properties of the p-n configuration of MoS2-α-Fe2O3 as the n-type and ND-RRPHTh as the p-type deposited on both n-type silicon and FTO-coated glass plates. The p-n photoelectrochemical cell is stable and allows for eliminating the photo-corrosion process. Nanomaterial-based electrodes [U+F061]-Fe2O3-MoS2 and ND-RRPHTh have shown an improved hydrogen release compared to [U+F061]-Fe2O3, Al-[U+F061]-Fe2O3 and MoS2-[U+F061]-Fe2O3 nanostructured films in PEC cells. By using p-n configuration, the chronoamperometry results showed that 1% MoS2 in MoS2-[U+F061]-Fe2O3 nanocomposite can be a suitable structure to obtain a higher photocurrent density. The photoelectrochemical properties of the p-n configuration of MoS2-α-Fe2O3 as n-type and ND-RRPHTh as p-type showed 3-4 times higher (450 A/cm2) in current density and energy conversion efficiencies than parent electrode materials in an electrolyte of 1M of NaOH in PEC cells. Titanium dioxide (TiO2) is known as one of the most explored electrode materials due to its physical and chemical stability in aqueous materials and its non-toxicity. TiO2 has been investigated because of the low cost for the fabrication of photoelectrochemical stability and inexpensive material. Incorporation of various percentages (2.5, 5, 16, 25, 50) of TiO2 in Fe2O3 could achieve better efficiencies as the photoanode by enhancing the electron concentration and low combination rate, and both materials can have a wide range of wavelength which could absorb light in both UV and visible spectrum ranges. TiO2 doped with [U+F061]-Fe2O3 film was shown as increasing contacting area with the electrolyte, reducing e-h recombination and shift light absorption along with visible region. The [U+F061]-Fe2O3-TiO2 nanomaterial has shown a more enhanced photocurrent (800 μA/cm2) than metal doped [U+F061]-Fe2O3 photoelectrochemical devices.

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.

Potential electrolytes for dye-sensitized photoelectrochemical solar cells have been synthesized and their applicability has been investigated. Different experimental techniques were used in order to characterize the synthesized electrolytes, such as elemental analysis, electrospray ionisation/mass spectrometry, cyclic voltammetry, dynamic viscosity measurements, as well as impedance, Raman and NMR spectroscopy. Some crystal structures were characterized by using single crystal X-ray diffraction. In order to verify the eligibility of the ionic compounds as electrolytes for photoelectrochemical solar cells, photocurrent density/photovoltage and incident photon-to-current conversion efficiency measurements were performed, using different kinds of light sources as solar simulators. In electron kinetic studies, the electron transport times in the solar cells were investigated by using intensitymodulated photocurrent and photovoltage spectroscopy. The accumulated charge present in the semiconductor was studied in photocurrent transient measurements. The ionic liquids were successfully used as solar cell electrolytes, especially those originating from the diethyl and dibutyl-alkylsulphonium iodides. The highest overall conversion efficiency of almost 4 % was achieved by a dye-sensitized, nanocrystalline solar cell using (Bu2MeS)I:I2 (100:1) as electrolyte (Air Mass 1.5 spectrum at 100 W m-2), quite compatible with the standard efficiencies provided by organic solvent-containing cells. Several solar cells with iodine-doped metal-iodidebased electrolytes reached stable efficiencies over 2 %. The (Bu2MeS)I:I2-containing cells showed better long-term stabilities than the organic solvent-based cells, and provided the fastest electron transports as well as the highest charge accumulation. Several polypyridyl-ruthenium complexes were tested as solar cell sensitizers. No general improvements could be observed according to the addition of amphiphilic co-adsorbents to the dyes or nanopartices of titanium dioxide to the electrolytes. For ionic liquid-containing solar cells, a saturation phenomena in the short-circuit current densities emerged at increased light intensities, probably due to inherent material transport limitation within the systems. Some iodoargentates and -cuprates were structurally characterized, consisting of monomeric or polymeric entities with anionic networks or layers. A system of metal iodide crownether complexes were employed and tested as electrolytes in photoelectrochemical solar cells, though with poorer results. Also, the crystal structure of a copper-iodide-(12-crown-4) complex has been characterized / QC 20101013