Surface recombination velocities for the (100) and (001) crystal faces of bismuth vanadate single crystals

Abstract

Bismuth vanadate (BiVO4) is widely used as a photocatalyst for water splitting, and its carrier lifetime is the most essential parameter for photocatalysts. In this study, we characterized the carrier lifetime in BiVO4 single crystals on the (100) and (001) crystal faces using the microwave photoconductivity decay (μ-PCD) method. For the (001) face, the observed μ-PCD curves with excitation by a 266 nm laser had no injected photon density dependence. In contrast, those excited using a 355 nm laser had an injected photon density dependence. The decay at a high injected photon density was faster than that at a low injected photon density. For the (100) face, the decay curves at both excitations of 266 and 355 nm were not significantly different, and they depended on the injected photon density. These results indicate that the carrier lifetime is dominated by surface recombi-nation only under the 266 nm excitation condition for the (001) face, whereas under other conditions, this is dominated by the Schokley–Read–Hall recombination. The temperature independence of the μ-PCD curves indicates that the recombination center is sufficiently deep in the bandgap. We estimated the surface recombination velocities and bulk lifetimes of the samples by fitting the experimental results to the calculations. We believe that the estimated surface recombination velocity and bulk lifetime will aid in the design of BiVO4 photocatalysts.

Effect of dislocations on carrier recombination and photoelectrochemical activity in polished and unpolished TiO2 and SrTiO3 crystals

“…Moreover, the μ-PCD curves of the unpolished face show slight injected photon density dependence, while those of the polished face show no injected photon density dependence. This indicates that the carrier recombination of the unpolished face is dominated by the Shockley-Read-Hall (SRH) recombination, 63 while that of the polished face is dominated by the sufficiently fast recombination at the bulk recombination center which is similar to SrTiO 3 . 60,62 In addition, the Auger and bimolecular recombination which strongly depend on charge carrier concentration are negligible.…”

“…Yttrium aluminum garnet (YAG) lasers with wavelengths of 266 and 355 nm and pulse width of 1 ns were used as excitation light sources (FTSS 266-200 and FTSS 355-50, CryLas). 58,60,61,63,65 The spot size 63 of the two lasers was 0.13 cm -2 . The laser intensity was 0.06-0.6 mW for the 266 nm laser and 0.2-4.0 mW for the 355 nm laser.…”

“…For μ-PCD measurements, microwaves generated by a Gunn diode at a frequency of 10 GHz were irradiated through the waveguide to the unexcited side of the sample and used as a measurement probe. 61,63 The reflected microwaves were detected using a Schottky barrier diode, and the intensity of the reflected microwaves was converted to a voltage signal for μ-PCD measurements. 60 To reduce the influence of the sample conductivity, we normalized the original signal from the peak in the carrier recombination observation.…”

“…Microwave photoconductivity decay (μ-PCD) measurement is a sensitive method for detecting carrier recombination even at low charge carrier densities. 52,[58][59][60][61][62][63] Hence, we used undoped TiO 2 and SrTiO 3 single crystals to observe carrier recombination. In PEC water-splitting experiments, the generated photocurrent is a reliable indicator of the photoelectrode performance.…”

Analysis of defects dominating carrier recombination in CeO2 single crystal for photocatalytic applications

“…Therefore, it can also be concluded that defects directly dominate carrier recombination, which affects the material performance. Several measurements have been performed to observe the carrier recombination process [22,23]. Among these techniques, microwave photoconductivity decay (µ-PCD) and time-resolved photoluminescence (TR-PL) are the highly sensitive measurements used to observe the minority carrier recombination process when the charge carrier density is low [24].…”