UC Davis

Solar water splitting offers a clean and renewable alternative to fossil fuel for energy production. This dissertation investigates a novel concept of utilizing ferroelectric polarization in photocatalysts to enhance overall solar water splitting activity. Our approach encompasses material synthesis, thin film fabrication, material doping, ferroelectric polarization, and surface photovoltage spectroscopy. Characterization mainly consists of electron microscopy, optical spectroscopy, powdered X-ray diffraction, and X-ray photoelectron spectroscopy. Photoelectrochemical activity is measured under simulated conditions using gas chromatography and electrochemistry. Our goal is to gain a better understanding of the origin of ferroelectrics in perovskite metal oxides and how it can alter photocatalytic activity, thereby improving the overall efficiency for solar water splitting.In chapter 2, chromium-doped strontium titanate (SrTiO3:Cr) nanocrystals of perovskite structure type and 45 nm (±15 nm) edge lengths were obtained by hydrothermal synthesis in water from titanium oxide, strontium hydroxide, and chromium(III) nitrate. According to XPS, the majority of the surface chromium (68.3%) is present in the 3+ state and the remainder (32.2%) in the 6+ state. Optical spectroscopy confirms a broad absorption at 2.3-2.9 eV from Cr3+ dopant states, in addition to the 3.2 eV band edge of the SrTiO3 host. After modification with Pt nanoparticles, Cr-doped SrTiO3 nanocrystals catalyze photochemical H2 evolution from aqueous methanol under visible light illumination (>400 nm) and with an apparent quantum yield of 0.66 % at 435 nm. According to surface photovoltage spectroscopy (SPS), Cr-doped SrTiO3 nanocrystals deposited onto gold substrates are n-type and have an effective band gap of 1.75 eV. SPS and transient illumination experiments at 2.50 eV reveal an anomalous surface photovoltage that increases with prior light exposure to values of up to -6.3 V. This photovoltage is assigned to ferroelectric polarization of the material in the space charge layer at the Au/SrTiO3:Cr interface. The polarization is stable for 24 h in vacuum but disappears after 12 h when samples are stored in air. The electric polarizability of SrTiO3:Cr is confirmed when films are exposed to static electric fields (1.20 MV m-1) in a fixed capacitor configuration. The discovery of a ferroelectric effect in Cr-doped SrTiO3 could be significant for the development of improved photocatalysts for the conversion of solar energy into fuel. In chapter 3, we demonstrate an unexpected ferroelectric effect in oxygen deficient SrTiO3-x and its application to improve photoelectrochemical water oxidation, a pathway to solar hydrogen fuel. After hydrogen-annealed SrTiO3-x is polarized under an electric field of 8.65 kV cm-1 under Ar flow the anodic water oxidation photocurrent is found to increase from 0.99 to 2.22 mA cm-2 at 1.23 V RHE (60 mWcm-2, UV illumination), or to decrease to 0.50 mA cm-2, for the opposite field orientation. The polarization also increases or decreases the surface photovoltage signal of the material and it modifies it flat band potential, based on Mott-Schottky measurements. These findings are attributed to the presence of an electric dipole near the SrTiO3-x surface, which modifies the potential drop across the depletion layer and affects the photochemical charge separation. Based on XPS, the electric polarization does not change the composition near the SrTiO3-x surface. Instead, the polarization effect is attributed to the electric field-induced migration of oxygen vacancies in SrTiO3-x surface region. This is supported by DFT calculations, which show that VO transfer from the surface to the sub-surface layer strongly invert the electric dipole near the surface. Such a surface effect also explains the limited lifetime of the polarization effect and its dependence on the environment (24 h in air and 48 h under argon). The ability to induce an electrical polarization in a non-ferroelectric material after introduction of oxygen vacancies is of interest for solar energy conversion and information technology applications. In chapter 4, the results of a systematic study on the facet-dependence of photoelectrochemical water oxidation with hydrogen annealed SrTiO¬3 single crystals are demonstrated. After H2 annealing, [100, 110, and 111] oriented SrTiO3-x single crystals exhibit variable water oxidation photocurrents (0.34, 0.82, 1.36 mA cm-2, respectively) at 1.23 V versus RHE under UV illumination (60 mW cm-2). The average photocurrent onsets were observed at -0.172, -0.286, and -0.294 V RHE for [100], [110], [111] SrTiO3-x crystals, respectively. Surface photovoltage spectra of the crystals exposed to 0.50 M aqueous Na2SO4 show surface photovoltage signals of -0.31, -0.57, and -0.67 V for the [100], [110], [111] crystals respectively. Mott Schottky measurements in aqueous K3/4[Fe(CN)6] show facet-dependent flatband positions of -0.60, -0.69, and -0.75 V RHE for the [100], [110], and [111] crystals respectively. These collective results confirm that the photoelectrochemical water oxidation performance of SrTiO3-x crystals is facet-dependent and controlled by the barrier height of the junctions at each facet. These results provide an explanation for the facet-dependent charge separation in SrTiO3 photocatalysts. In chapter 5, we demonstrate ferroelectric enhancement (FE) in photocatalytic hydrogen evolution and photoelectrochemical water oxidation with barium titanate nanocrystals. Nanocrystals of the ferroelectric tetragonal structure type were obtained by hydrothermal synthesis from TiO2 and barium hydroxide in 63% yield. BaTiO3 nanocrystal films on tantalum substrates exhibit water oxidation photocurrents of 0.141 mA cm-2 at 1.23 V RHE under UV light (60 mW cm-2) illumination. Electric polarization at 80 kV cm-1 increases the photocurrent by a factor of 2 or decreases it by a factor of 3.5, depending on the field orientation. It also shifts the onset potential by 0.15 V or +0.09 V, depending on the polarity of the applied field, and it modifies the surface photovoltage signal. Lastly, exposure to an electric field increases the H2 evolution rate of Pt/BaTiO3 by a factor of ~1.5 and it raises the selectivity of photodeposition of silver onto the (001) facets of the nanocrystal. All FE enhancements can be removed by heating samples above the Curie temperature of BaTiO3. These findings can be explained with FE dipole-induced changes to the potential drop across the space charge layer of the material. The ability to use the ferroelectric effect to enhance hydrogen evolution and water oxidation is of potential interest for the development of improved solar energy-to-fuel conversion systems.