Dynamics of charge carriers in bismuth vanadate photoanodes for water splitting using solar energy
Creators: name: Ma, Yimeng
Publisher: name: Imperial College London
Year: 2015

description: This thesis described an investigation of charge carrier dynamics in dense, flat bismuth vanadate (BiVO4) photoanodes using transient absorption spectroscopy and photoelectrochemical measurements including transient photocurrents. Transient absorption spectroscopy was employed to probe directly the photogenerated charge carrier population change as a function of time from microsecond (μs) to second (s) timescales. Transient photocurrent measurements were used to monitor charge extraction under chopped light conditions. Photo-induced absorption spectroscopy was employed to investigate charge carriers under working photo-electrochemical (PEC) conditions. The transient absorption signals due to photogenerated holes in BiVO4 were determined through using electron/hole scavengers and applied electrical bias in a complete photoelectrochemical cell. In ‘un-doped’ BiVO4, photogenerated holes were found to absorb from 500 nm to 900 nm. The dynamics of photogenerated holes were studied as a function of applied potential and excitation intensity. The population of long-lived (milliseconds–seconds) holes increased with increasing the width of space charge layer as a function of applied potential. A recombination process in kinetic competition with water oxidation on these long timescales was found to limit the photocurrent amplitude and onset potential in un-doped BiVO4 photoanodes. Using transient photocurrent measurements, this recombination process was identified as recombination of surface-accumulated holes with electrons from the bulk of the semiconductor (back electron/hole recombination). Doping molybdenum (MoVI) in un-doped BiVO4 has been reported to be an effective method to increase photocurrent amplitude. Impedance measurements were carried out to determine the donor density increased by the presence of MoVI doping. The increased donor density limited efficient generation of the space charge layer to retard fast recombination on microseconds to milliseconds timescales, thus limiting the long-lived hole yield under modest applied potentials. MoVI dopants were shown to improve the electron transport determined by front/back side illumination in PEC and transient absorption spectroscopy (TAS) measurements. Cobalt phosphate (CoPi) surface-modified un-doped BiVO4 photoanodes were also studied using transient absorption spectroscopy and transient photocurrent measurements. Transient absorption spectra of CoPi-modified BiVO4 were similar to those of unmodified BiVO4, and the kinetics on milliseconds to seconds did not change in the presence of CoPi surface modification. Both results indicated that photogenerated holes in BiVO4 rather than CoPi species were monitored by transient absorption spectroscopy. However, the negative shift of photocurrent onset and increased photocurrent could be explained by efficient suppression of back electron/hole recombination in BiVO4 photoanodes. In terms of the function of CoPi water oxidation catalysts on BiVO4, photo-induced absorption (PIA) was employed to further study the CoPi/BiVO4 system. CoPi species oxidised by BiVO4 holes were observed in PIA measurements due to the high extinction coefficient of oxidised CoPi and significant hole accumulation generated by continuous illumination. However, these oxidised CoPi species did not appear to drive catalytic water oxidation, as evidenced by results from spectroelectrochemical measurements of CoPi/FTO electrodes; water oxidation still occurred via BiVO4, consistent with the transient absorption results. Therefore, I concluded that in the CoPi/BiVO4 system, CoPi did not act as a catalyst, although hole transfer to CoPi can take place.