Investigations on the roles of surface states on BiVO4 photoanodes and CuBi2O4 photocathodes for photoelectrochemical water splitting

Climate change is an alarming issue, which has a devastating impact on our planet. Especially CO2 gas emissions due to the wide-spread utilization of fossil fuels play a major role in global warming. To remove fossil fuels from our lives, alternative energy sources are needed to satisfy the ever-increasing energy demand. Hydrogen is a one of the most promising green energy carriers due to its zero-carbon emission upon utilization, and its extremely high gravimetric energy density. Photoelectrochemical water splitting is a promising route to obtain green hydrogen directly from water and sunlight. Oxide semiconductors utilized as photoelectrodes offer significant potential in terms of light absorption and stability in aqueous electrolytes however their poor surface activities and ineffective charge carrier utilization properties hamper their widespread use. Understanding the origin of these poor activities is crucial to de-bottleneck the performance of photoelectrodes. BiVO4 photoanodes, one of the most promising materials for water oxidation reaction, suffer from overwhelming surface recombination of charge carriers which limit their activity. However, the recombination dynamics are complex and the detailed mechanisms of the recombination taking place in the bulk and the surface regions are typically not addressed. In the first part of this study, BiVO4 photoanodes with high surface area were synthesized using electrodeposition method. We show that the water oxidation activity of BiVO4 photoanode is significantly boosted by the TiO2 overlayer prepared by atomic layer deposition (ALD). With a TiO2 overlayer of an optimized thickness, the photocurrent at 1.23 VRHE increased from 0.64 to 1.1 mA.cm-2 under front illumination, corresponding to 72% enhancement. We attribute this substantial improvement to enhanced charge separation and suppression of surface recombination due to surface state passivation. We provide direct evidence via transient photocurrent (TPC) measurements that TiO2 overlayer significantly decreases the photogenerated electron-trapping process at the BiVO4 surface. Electron-trapping passivation leads to enhanced electron photoconductivity, which results in higher photocurrent enhancement under front illumination, rather than back illumination. Even though the electron trapping process is eliminated completely at higher TiO2 overlayer thicknesses, the charge transfer resistance at the surface also increases significantly, resulting in a diminished photocurrent. We demonstrate that ultrathin TiO2 overlayer can be used to fine-tune the surface properties of BiVO4. CuBi2O4 is one of the most promising photocathodes for HER due to its low band gap and high flat-band potential. However, compared to BiVO4, it is a lot less studied, so the roles of the charge carrier dynamics are not yet fully understood. In the second part of this study, we synthesized CuBi2O4 photocathodes via electrodeposition method. The synthesized photocathode films were characterized by XPS, XRD, FESEM, UV-vis-NIR absorption, and Raman spectroscopy. Using electrochemical impedance spectroscopy (EIS) and TPC measurements, we have identified the presence of surface-states, and photogenerated electron trapping process at these states. The results indicate that surface-states near the flat-band potential act as photogenerated electron traps, which results in a delay in the photocurrent onset to 0.9 VRHE -1.0 VRHE even though photocurrent is observed as early as 1.4 VRHE. The TiO2 overlayer was grown on CuBi2O4 surface via ALD with different thicknesses to tune the charge carrier dynamics at the surface and inhibit the electron trapping process. However, even with low coverage of TiO2 overlayer, CuBi2O4 activity significantly decreased. Furthermore, we have identified significant photocorrosion, which is detrimental for the stability of the photocathode. These results suggest that interfacial dynamics are important for the CuBi2O4 photocathode activity and stability. Thus, the surface charge carrier dynamics need to be tuned by other materials than TiO2 to passivate the electron trapping process to improve the performance of CuBi2O4.