In the synthesis method of a BiVO4 photoanode via BiOI flakes, a BiOI film is formed by electrochemical deposition in Step 1, and a vanadium (V) source solution is placed by drop-casting on the BiOI film in Step 2. Following this, BiVO4 particles are converted from the BiOI–(V species) precursors by annealing. However, it is challenging to evenly distribute vanadium species among the BiOI flakes. As a result, the conversion reaction to form BiVO4 does not proceed simultaneously and uniformly. To address this limitation, in Step 2, we developed a new electrochemical deposition method that allowed the even distribution of V2O5 among Bi–O–I flakes to enhance the conversion reaction uniformly. Furthermore, when lactic acid was added to the electrodeposition bath solution, BiVO4 crystals with an increased (040) peak intensity of the X-ray diffractometer (XRD) pattern were obtained. The photocurrent of the BiVO4 photoanode was 2.2 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) under solar simulated light of 100 mW/cm2 illumination. The Faradaic efficiency of oxygen evolution was close to 100%. In addition, overall water splitting was performed using a Ru/SrTiO3:Rh–BiVO4 photocatalyst sheet prepared by the BiVO4 synthesis method. The corresponding hydrogen and oxygen were produced in a 2:1 stoichiometric ratio under visible light irradiation.
To investigate the Rh-doping effect on the photooxidative degradation activity of titanate nanosheets prepared by exfoliation of an H 2 Ti 3 O 7 crystal in aqueous media, a colloidal aqueous suspension of titanate nanosheets doped with Rh atoms at the Ti sites (TiNS:Rhz, z ¼ amount of Rh, [Ti 3Àz Rh z O 7 ] 2À ) was prepared. Oxidative degradation of methylene blue (MB) by TiNS:Rhz in aqueous media under UV light irradiation was studied. Using X-ray diffraction, diffuse reflection spectroscopy, and photoelectrochemical measurements, the electronic band structure of TiNS:Rhz was elucidated and was shown not to change with the amount of Rh doping at the Ti sites. However, the degradation of MB increased with an increase in Rh doping. From these results, it is suggested that the redox reaction between Rh 3+ and Rh 4+ strongly contributes to the oxidative degradation of MB in the presence of TiNS:Rhz under UV light irradiation. † Electronic supplementary information (ESI) available: The schematic of H 2 Ti 3 O 7 crystal structure and their lattice parameters, the XRD patterns of Rh-doped sodium titanate, Na 2 Ti 3Àx Rh x O 7 , H 2 Ti 3Àx Rh x O 7 , and non-exfoliated TiNS:Rhz, the AFM of TiNS:Rhz, and XPS data for Ti and Rh compared between TiNS:Rh 0 and TiNS:Rh 10 . See
Various amounts of Rh-doped titanate nanosheets (Ti3NS:Rh(x), where x is doped amount) were prepared to develop a new nanostructured photocatalyst based on metal oxide compounds that can split water to produce H2 under sunlight. Ti3NS:Rh(x) was obtained by acid exchange, intercalation, and exfoliation of Rh-doped layered sodium titanate compound (Na2Ti3–x Rh x O7). A new energy gap was found in the diffuse reflection spectrum of the Ti3NS:Rh(x) colloidal suspension solution; this new energy gap corresponds to electrons in the 4d level of Rh3+ or Rh4+, which are doped in the Ti4+ site. A photocatalyst activity of Ti3NS:Rh(x) for H2 evolution in water with triethylamine (TEA) as an electron donor was investigated. The appropriate amount of Rh doping can improve the photocatalytic activity of Ti3NS for H2 evolution from water using triethylamine (TEA) as a sacrifice agent. The reason was related to the rich state of Rh3+ or Rh4+ doped in the Ti4+ site of Ti3NS. Doping Rh 1 mol % of Ti, Ti3NS:Rh(0.03) shows the H2 evolution rates up to 1040 nmol/h, which is about 25 times larger than that of nondoped Ti3NS under UV irradiation (>220 nm) and 302 nmol/h under near-UV irradiation (>340 nm). These results show that the development of new nanostructured photocatalyst based on Rh-doped titanate compounds that can produce H2 under near-UV irradiation present in sunlight was a success.
We investigated the adsorption and photocatalytic decomposition of methylene blue (MB) by colloidal suspensions of Rh-doped titanate nanosheets ([Ti4−xRhxO9]2−; Ti4NS:Rhx) prepared by exfoliating H2Ti4−xRhxO9 in aqueous tetramethylammonium hydroxide, revealing that doped nanosheets showed a slightly enhanced adsorption capacity compared to their non-doped counterparts. This behavior was ascribed to the elevated ionic charge of doped Ti4NS and the thus increased volume charge density. Accordingly, Ti4NS:Rhx exhibited a higher MB adsorption capacity than previously reported [Ti3−xRhxO7]2−. Moreover, we found that Ti4NS:Rhx catalyzed the photodegradation of MB under UV light irradiation, additionally showing that the activity of this catalyst can be improved by Rh doping, probably due to the large contribution of Rh3+/Rh4+ shuttling to effective MB degradation.
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