Synthesis of Bismuth Vanadate Thin Films for Solar Water Splitting Photoelectrode and Their Photoelectrochemical Characterization

Abstract

Energy harvesting technology with a clean and renewable source of energy has drawn significant attention because of its possibility for solving the global energy challenge. Among the renewable energy sources including solar, wind, geothermal, and hydropower energy, solar seems the most desirable since it can support future societies in a sustainable way with the use of hydrogen gas as an energy carrier. Along with that consideration, the solar-derived hydrogen production techniques such as photoelectrochemical (PEC) water splitting has made significant progress. In general, the PEC water splitting system consists of photoelectrodes (photoanode or photocathode), electrolyte, and a light source. Mostly, light-absorbing semiconductors have been used as the photoelectrode, and the electrode materials have been being of great interest in the field of PEC water splitting. Among various materials, this dissertation focuses on bismuth vanadate (BiVO4) which is small band gap oxide semiconductors. There are some advantages of using BiVO4 as photoanode. Briefly, it is composed of inexpensive elements and, because of its low band gap energy of 2.4 eV, it can utilize visible light. The latter becomes even more important when used in terrestrial environments, because about half of the solar radiation on earth corresponds to the visible light. However, they suffer from excessive charge carrier recombination, poor charge transport, and slow water oxidation kinetics. In order to solve the problems, various strategies are investigated and relevant experimental results are presented and discussed further in each part of the dissertation. In the first part, deposition of high quality stoichiometric BiVO4 thin films by pulsed laser deposition (PLD) at process temperature of as low as 230°C is investigated. And also, capabilities of facile morphology control of the film by simply controlling the process temperature during deposition are successfully demonstrated. This topic was motivated by an idea that a porous structure of BiVO4 thin films can be formed during the PLD process as a result of suppressed diffusion of deposited BiVO4 nuclei at low temperature followed by grain growth at the nuclei surface. It is reasonable considering the thermodynamics and kinetics of film growth, where conditions are expected to satisfy the three-dimensional island growth mode at low temperature (the Volmer-Weber growth). As expected, the results show that enhancement of PEC performance is exhibited from the film deposited at low temperature and the enhancement is attributed to increased surface area. In the second part, development of fully solution-deposited silver nanoparticle (Ag NP) impregnated nanocomposite BiVO4 (Ag-BiVO4) photoanode for maximum utilization of localized surface plasmon resonance (LSPR)-induced enhancement effects is reported. The LSPR represents the response of oscillatory electrons in metal with respect to the incident light wave. Since a peak resonant wavelength of the given metal NP can be tailored to the visible region by controlling its size and shape, the improvement of photocatalytic activity of nanoparticle modified photocatalysts can be obtained under the illumination of visible light. This strategy is particularly referred to as ‘plasmonic photocatalysis’. In this topic, exceptionally simple process for synthesis of Ag-BiVO4 is proposed, and structural characteristics and photocatalytic performance of the synthesized Ag-BiVO4 film are fully examined. The results show that the nanocomposite Ag-BiVO4 film exhibits the drastic increase in the current density at low applied potential, and shows the improved kinetics of the carrier generation and separation as a result of LSPR-mediated effect. In the last part, a photochemical activation-assisted synthesis of BiVO4 photoanode under irradiation of monochromatic deep ultra-violet light from excimer lamp (ELUV) which enables low temperature crystallization of visible-light-active monoclinic BiVO4 is demonstrated. It is well known that monoclinic phase of BiVO4 should be achieved in order to realize the visible light hydrogen production. Thus, in order to fulfill phase transformation condition from amorphous to monoclinic phase, sufficient heat should be applied via annealing at high temperature. However, high temperature annealing processes inevitably causes side effects such as undesirable crystal phase formation and thermal shock on device, which frequently lead to fast charge recombination and poor charge transfer. In this topic, the PEC device employing the BiVO4 thin film grown at a low temperature of 350°C exhibits 3.14 mA/cm2 at 1.23 V versus the potential of the reversible hydrogen electrode under AM 1.5G illumination, which is decently comparable with the best performance reported for single layer of BiVO4 film via solution-based synthesis. The outstanding PEC performance is attributed to 3-dimensionally porous structure and fully covered feature of synthesized BiVO4, which are enabled by low temperature annealing process. Moreover, interestingly, unreported phase transformation of BiVO4 by photochemical activation is observed using in-situ TEM analysis. Based on microscopic and spectroscopic observations, effect of ELUV irradiation on observed phenomena are fully discussed.