Jingxiu Yang

Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4

Charge separation is crucial for increasing the activity of semiconductor-based photocatalysts, especially in water splitting reactions. Here we show, using monoclinic bismuth vanadate crystal as a model photocatalyst, that efficient charge separation can be achieved on different crystal facets, as evidenced by the reduction reaction with photogenerated electrons and oxidation reaction with photogenerated holes, which take place separately on the {010} and {110} facets under photo-irradiation. Based on this finding, the reduction and oxidation cocatalysts are selectively deposited on the {010} and {110} facets respectively, resulting in much higher activity in both photocatalytic and photoelectrocatalytic water oxidation reactions, compared with the photocatalyst with randomly distributed cocatalysts. These results show that the photogenrated electrons and holes can be separated between the different facets of semiconductor crystals. This finding may be useful in semiconductor physics and chemistry to construct highly efficient solar energy conversion systems.

A Theoretical Study on the Mechanism of Photocatalytic Oxygen Evolution on BiVO4 in Aqueous Solution

The oxygen evolution reaction (OER) is regarded as one of the key issues in achieving efficient photocatalytic water splitting. Monoclinic scheelite BiVO(4) is a visible-light-responsive semiconductor which has proved to be effective for oxygen evolution. Recently, the synthesis of a series of monoclinic BiVO(4) single crystals was reported, and it was found that the (010), (110), and (011) facets are highly exposed and that the photocatalytic O(2) evolution activity depends on the degree of exposure of the (010) facets. To explore the properties of and photocatalytic water oxidation reaction on different facets, DFT calculations were performed to investigate the geometric structure, optical properties, electronic structure, water adsorption, and the whole OER free-energy profiles on BiVO(4) (010) and (011) facets. The calculated results suggest both favorable and unfavorable factors for OER on the (010) and the (011) facets. Due to the combined effects of the above-mentioned factors, different facets exhibit quite different photocatalytic activities.

Enhancing charge separation on high symmetry SrTiO3 exposed with anisotropic facets for photocatalytic water splitting

One of the challenging issues in photocatalytic overall water splitting is to efficiently separate the photogenerated charges and the reduction and oxidation catalytic sites on semiconductor-based photocatalysts. It has been reported that the photogenerated charge can be separated between different facets of a semiconductor crystal with low symmetry. However, many semiconductor crystals possess high symmetry (such as the cubic phase) and expose isotropic facets, which are not suitable for charge separation between the facets. Herein, using a nanocrystal morphology tailoring strategy, we synthesized the exposed facets of high symmetry SrTiO3 nanocrystals from isotropic facets (6-facet SrTiO3) to anisotropic facets (18-facet SrTiO3), which leads to the exposure of different crystal facets. We found that the reduction and oxidation catalytic sites can be separately distributed only on the anisotropic facets of 18-facet SrTiO3 nanocrystals, but randomly distributed on every facet of 6-facet SrTiO3 nanocrystals. Based on these findings, the selective distribution of dual-cocatalysts on the anisotropic facets of 18-facet SrTiO3 nanocrystals leads to a fivefold enhancement of apparent quantum efficiency. The superior performance can be attributed to the charge separation between anisotropic facets and the separation of the reduction and oxidation catalytic sites to reduce the charge recombination. These findings will be instructive for the rational design of a high efficiency photocatalytic system for solar energy conversion.

Effects of Zn2+ and Pb2+ dopants on the activity of Ga2O3-based photocatalysts for water splitting

Zn-doped and Pb-doped β-Ga2O3-based photocatalysts were prepared by an impregnation method. The photocatalyst based on the Zn-doped β-Ga2O3 shows a greatly enhanced activity in water splitting while the Pb-doped β-Ga2O3 one shows a dramatic decrease in activity. The effects of Zn(2+) and Pb(2+) dopants on the activity of Ga2O3-based photocatalysts for water splitting were investigated by HRTEM, XPS and time-resolved IR spectroscopy. A ZnGa2O4-β-Ga2O3 heterojunction is formed in the surface region of the Zn-doped β-Ga2O3 and a slower decay of photogenerated electrons is observed. The ZnGa2O4-β-Ga2O3 heterojunction exhibits type-II band alignment and facilitates charge separation, thus leading to an enhanced photocatalytic activity for water splitting. Unlike Zn(2+) ions, Pb(2+) ions are coordinated by oxygen atoms to form polyhedra as dopants, resulting in distorted surface structure and fast decay of photogenerated electrons of β-Ga2O3. These results suggest that the Pb dopants act as charge recombination centers expediting the recombination of photogenerated electrons and holes, thus decreasing the photocatalytic activity.

Nitrogen-doped layered oxide Sr5Ta4O15−xNx for water reduction and oxidation under visible light irradiation

Development of a photocatalyst with wide visible light absorption is of vital importance in solar-chemical energy conversion. In this work, we introduce a new nitrogen-doped layered oxide, Sr5Ta4O15−xNx, which exhibits a significantly extended absorption edge compared with the undoped oxide Sr5Ta4O15. The extension of the visible light absorption has been ascribed to the substitution of nitrogen for oxygen atoms as well as the formation of Ta–N bonds, which was confirmed by X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Judged by the first principle calculation, the N 2p states mixed with pre-existing O 2p states shift the valence band maximum upward and result in wide visible light absorption. Band structure analysis combined with UV-Vis diffuse reflectance spectrum (DRS) and Mott–Schottky (M–S) measurement shows that the conduction and valence bands of Sr5Ta4O15−xNx are sufficient for water reduction and oxidation, respectively. The photocatalytic water splitting performances of Sr5Ta4O15−xNx are strongly related to the deposited cocatalyst. With an optimized cocatalyst, the Sr5Ta4O15−xNx shows both H2 and O2 evolution activities under visible light irradiation using CH3OH and AgNO3 as scavengers respectively. Following the optimized cocatalyst deposition of the Sr5Ta4O15−xNx, the cocatalyst-modified nitrogen-doped tantalum-based layered oxides Sr2Ta2O7−xNx and Ba5Ta4O15−xNx also exhibit activities for both the water splitting half reactions. This work demonstrates that the nitrogen-doped tantalum-based layered oxides may be a new type of potential photocatalyst with wide visible light absorption for solar water splitting.

Selective photocatalytic conversion of glycerol to hydroxyacetaldehyde in aqueous solution on facet tuned TiO2-based catalysts

Glycerol is selectively converted to hydroxyacetaldehyde (HAA) and H2 in aqueous solution on TiO2-based photocatalysts. The product selectivity was verified to be strongly dependent on the facets of TiO2. Rutile with high percentage of {110} facets results in over 90% superior selectivity of HAA, while anatase with {001} or {101} facets gives only 16% and 49% selectivity for HAA, respectively.

Theoretical Study on the Role of Ca2+ at the S2 State in Photosystem II

In photosynthesis, calcium is crucial for oxygen evolution. In the absence of Ca(2+), the Kok cycle has been proven to stop at S2 with Yz•. To explore the reason, photosystem II (PSII) S2 models (in total 32452 atoms) with different metal ions (Ca(2+), Sr(2+), and K(+)) or without Ca(2+) involved in the oxygen evolution complex (OEC) have been theoretically studied based on the previous dynamic study of PSII. It is found that the portion of the Mn1 d-orbital decreases in the highest occupied molecular orbitals for Ca(2+)-depleted PSII. This feature is unfavorable for the electron transfer from the OEC to the Yz•. Furthermore, the proton donor-acceptor distance was found elongated by the alternation of the binding water in the absence of Ca(2+). The isolated vibrational modes of the key water molecules along the path and their high frequency of the OH stretching modes also suggested the difficulty of the proton transfer from the OEC toward the proton exit channel. This work provides one mechanistic explanation for the inactivity of Ca(2+)-depleted PSII.

Theoretical insight into the roles of cocatalysts in the Ni–NiO/β-Ga2O3 photocatalyst for overall water splitting

The formation and stability of Nin and (NiO)n (n = 1–4) clusters on the β-Ga2O3 surface have been studied by means of first-principles density functional theory calculations. It is found that the optimum interaction of the Nin and (NiO)n clusters with the surface requires different surface sites. This optimizes the formation of interfacial bonds between the atoms from clusters and the coordinatively unsaturated atoms from the surface. The stability of the adsorbed Ni clusters increases with the number of Ni atoms. In a Nin/Ga2O3 system, as the Ga unoccupied states overlap with the unoccupied Ni state, the excited electrons transferred from Ga to Ni participate in the proton reduction reaction. Our calculations show that (NiO)n clusters strongly adsorb on the Ga2O3 surface due to the negative adsorption energies within −1.9 eV to −3.7 eV. For (NiO)n/Ga2O3, occupied states from the (NiO)n cluster may accept the holes from O atoms in the Ga2O3 surface to take part in the photocatalytic water oxidation reaction.

A Novel Sr2CuInO3S p-type semiconductor photocatalyst for hydrogen production under visible light irradiation

Abstract A novel Sr2CuInO3S oxysulfide p-type semiconductor photocatalyst has been prepared by solid state reaction method and it exhibits intriguing visible light absorption properties with a bandgap of 2.3 eV. The p-type semiconductor character of the synthesized Sr2CuInO3S was confirmed by Hall efficient measurement and Mott-Schottky plot analysis. First-principles density functional theory calculations (DFT) and electrochemical measurements were performed to elucidate the electronic structure and the energy band locations. It was found that the as-synthesized Sr2CuInO3S photocatalyst has appreciate conduction and valence band positions for hydrogen and oxygen evolution, respectively. Photocatalytic hydrogen production experiments under a visible light irradiation (λ>420 nm) were carried out by loading different metal and metal-like cocatalysts on Sr2CuInO3S and Rh was found to be the best one among the tested ones.