Nanophotothermolysis of Poly-(vinyl) Alcohol Capped Silver Particles

Laser-induced thermal fusion and fragmentation of poly-(vinyl) alcohol (PVA)-capped silver nanoparticles in aqueous medium have been reported. PVA-capped silver nanoparticles with an average size of 15 nm were prepared by chemical reduction technique. The laser-induced photo-fragmentation of these particles has been monitored by UV-visible spectroscopy and transmission electron microscopy. The morphological changes induced by thermal and photochemical effects were found to influence the optical properties of these nanoparticles

Wavelength-Dependent Ultrafast Charge Carrier Separation in the WO3/BiVO4 Coupled System

Due to its 3c2.4 eV band gap, BiVO4 is a very promising photoanode material for harvesting the blue portion of the solar light for photoelectrochemical (PEC) water splitting applications. In WO3/BiVO4 heterojunction films, the electrons photoexcited in BiVO4 are injected into WO3, overcoming the lower charge carriers\u2019 diffusion properties limiting the PEC performance of BiVO4 photoanodes. Here, we investigate by ultrafast transient absorption spectroscopy the charge carrier interactions occurring at the interface between the two oxides in heterojunction systems to directly unveil their wavelength dependence. Under selective BiVO4 excitation, a favorable electron transfer from photoexcited BiVO4 to WO3 occurs immediately after excitation and leads to an increase of the trapped holes\u2019 lifetime in BiVO4. However, a recombination channel opens when both oxides are simultaneously excited, evidenced by a shorter lifetime of trapped holes in BiVO4. PEC measurements reveal the implication of these wavelength-dependent ultrafast interactions on the performances of the WO3/BiVO4 heterojunction

Dynamics of photogenerated charge carriers in WO3/BiVO4 heterojunction photoanodes

Photoelectrochemical water splitting is a promising way to convert and store solar energy into hydrogen as fuel. Among semiconductor materials, metal oxides are commonly used as photoanodes due to their bandgap positions and high stability under oxidative conditions. BiVO4 and WO3 are widely studied n-type metal oxides with small optical bandgaps (ca. 2.5 and 2.7 eV respectively). While BiVO4 alone exhibits poor electron transport, by combining it with WO3 highly enhanced photoelectrochemical performances have been obtained due to increased charge carrier separation in the photoanode. Individual WO3 and BiVO4 have been synthesized by solution processed techniques and the photoanodes, including the WO3/BiVO4 heterojunction system, have been prepared by spin coating. The so obtained electrodes were tested in the reaction of photoelectrochemical water splitting. The photocurrent of the WO3/BiVO4 electrode was highly increased as compared to those of the two pure WO3 and BiVO4 electrodes. Femtosecond transient absorption spectro-scopy was used to investigate the charge carrier dynamics of the studied photoanodes. The spectra of BiVO4 could be assigned to trapped electrons (460-600 nm) and trapped holes (600-850 nm). Electron injection from BiVO4 to WO3 was observed in the heterojunction system, as shown by the faster decay of the 470 nm signal assigned to trapped electrons. Thionine photoreduction experiments carried out under continuous irradiation at 400 nm confirmed the electron injection process between excited BiVO4 and WO3

Dynamics of Photogenerted Charge carriers in WO3/BiVO4 Heterojunction Photoanodes

Bismuth vanadate (BiVO4) with a band gap of similar to 2.4 eV has emerged as one of the visible photocatalysts that can absorb light below 520 nm. The electron/hole pairs that are generated following BiVO4 band gap excitation are effective for water splitting, especially when BiVO4 is combined with other metal oxides such as WO3. We report a solution processed method for designing transparent WO3/BiVO4 heterojunction electrodes and observe a synergistic effect on the photoelectrochemical activity of WO3/BiVO4, with the combined system performing dramatically better than either individual component. Using ultrafast transient absorption spectroscopy, we elucidated the electronic interaction between WO, and excited BiVO4. Moreover, the photocatalytic reduction of thionine by WO3/BiVO4 as well as by each individual oxide component is used to track electron injection processes and determine the energetics of the studied systems. In the composite WO3/BiVO4 film a shifted quasi-Fermi level results, due to electronic equilibration between the two materials. The better performance of WO3/BiVO4 heterojunction electrodes is thus a consequence of the electron injection from BiVO4 into WO3, followed by back electron transfer from WO3 to the holes in BiVO4