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Investigation of the charge dynamics and roles of cocatalysts is crucial for understanding the reaction of photocatalytic water splitting on semiconductor photocatalysts. In this work, the dynamics of photogenerated electrons in Ga2O3loaded with Cr2O3-Rh cocatalysts was studied using time-resolved mid-infrared spectroscopy. The structure of these Cr2O3-Rh cocatalysts was identified with high-resolution transmission electron microscopy and CO adsorption Fourier-transform infrared spectroscopy, as Rh particles partly covered with Cr2O3. The decay dynamics of photogenerated electrons reveals that only the electrons trapped by the Rh particles efficiently participate in the H2evolution reaction. The loaded Cr2O3promotes electron transfer from Ga2O3to Rh, which accelerates the electron-consuming reaction for H2evolution. Based on these observations, a photocatalytic water-splitting mechanism for Cr2O3-Rh/Ga2O3photocatalysts has been proposed. The elucidation of the roles of the Cr2O3-Rh cocatalysts aids in further understanding the reaction mechanisms of photocatalytic water splitting and guiding the development of improved photocatalysts.

Junctions are an important structure that allows charge separation in solar cells and photocatalysts. Here, we studied the charge transfer at an anatase/rutile TiO2phase junction using time-resolved photoluminescence spectroscopy. Visible (∼500 nm) and near-infrared (NIR, ∼830 nm) emissions were monitored to give insight into the photoinduced charges of anatase and rutile in the junction, respectively. New fast photoluminescence decay components appeared in the visible emission of rutile-phase dominated TiO2and in the NIR emission of many mixed phase TiO2samples. The fast decays confirmed that the charge separation occurred at the phase junction. The visible emission intensity from the mixed phase TiO2increased, revealing that charge transfer from rutile to anatase was the main pathway. The charge separation slowed the microsecond time scale photoluminescence decay rate for charge carriers in both anatase and rutile. However, the millisecond decay of the charge carriers in anatase TiO2was accelerated, while there was almost no change in the charge carrier dynamics of rutile TiO2. Thus, charge separation at the anatase/rutile phase junction caused an increase in the charge carrier concentration on a microsecond time scale, because of slower electron-hole recombination. The enhanced photocatalytic activity previously observed at anatase/rutile phase junctions is likely caused by the improved charge carrier dynamics we report here. These findings may contribute to the development of improved photocatalytic materials.

A series of WO3samples with different crystalline phases were prepared by the thermal decomposition method from ammonium tungstate hydrate. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy, and N2adsorption-desorption were used to characterize the crystalline phase, morphology, particle size, chemical composition, and surface area of the WO3samples. The formation of hexagonal (h-WO3) and monoclinic (m-WO3) crystal structures of WO3at different temperatures or different times was confirmed by XRD. m-WO3is formed at 600 °C, while m-WO3starts to transform into h-WO3at 800 °C. However, h-WO3, which forms at 800 °C, may transform into m-WO3by increasing the calcination temperature to 1000 °C. SEM results indicate that m-WO3particles exhibit a bulky shape with heavy aggregates, while h-WO3particles exhibit a rod-like shape. Moreover, m-WO3crystals are sporadically patched on the surface of the h-WO3rod-like particles, resulting in the exposure of both m-WO3and h-WO3on the surface. It is observed that the monoclinic phase (m-WO3)/hexagonal phase (h-WO3) junction was fabricated by tuning the calcination temperature and calcination time. The relative ratios between m-WO3and h-WO3in the phase junction can readily be tailored by control of the calcination time. The photocatalytic activities of WO3with different crystalline phases were evaluated by the photocatalytic degradation of rhodamine B as a model pollutant. A higher photocatalytic activity was observed in the WO3sample with the m-WO3/h-WO3junction as compared with the sample with only m-WO3. The improvement of photocatalytic activity can be attributed to the reduction of the electron-hole recombination rate owing to the formation of the phase junction, whose presence has been confirmed by HRTEM and photoluminescence spectra.

In situ transmission infrared spectroscopy was used to study the role of Pt cocatalyst in Pt/TiO2using CO as a probe molecule. An 11 cm−1redshift of CO adsorbed on Pt/TiO2was observed under irradiation in the absence of changes in the CO coverage or sample temperature. In contrast, no CO shift was detected on Pt/Al2O3. This indicates that the redshift of the CO adsorption peak is due to the photogenerated electron transfer from TiO2to Pt, and this accounts for the increased photocatalytic activity by the loaded Pt cocatalyst.