To date, it is still a big challenge to investigate the charge transfer behavior from bulk to surface for the solar energy conversion and utilization. Herein, the BiF 3 /BiOCl heterojunction has been prepared through a mild post-synthesis method. Surface photovoltage spectra (SPV) results show that only negative SPV signal can be observed for BiOCl, suggesting that the photogenerated electrons mainly move to the surfaces and accumulate on the surface; both negative and positive signals can be observed for 38% BiF 3 /BiOCl, indicating that photogenerated electrons and holes can both move to the surfaces and accumulate on the surface; but nearly no SPV signal can be observed for BiF 3 , demonstrating that nearly no electrons or holes can accumulate on the surface. Furthermore, under ultraviolet light irradiation (λ ≤ 420 nm), the degradation rate is 5.3 and 5.8 times higher than that of BiOCl and BiF 3 for the degradation of 2-nitrophenol, respectively. We hold that the charges transfer and separation efficiency of BiF 3 /BiOCl have been significantly improved by the synergetic effect of the surface electric field, bulk internal electric field and interface electric field. This work could help us to intensively understand the charge transfer behavior of a heterojunction photocatalyst. Keywords: charge transfer; surface electric field; bulk internal electric field; interface electric field IntroductionThe conversion and utilization of solar energy, e.g., photocatalysis, has caused great attention because of its potential application in energy conversion, purifying wastewater and noxious gas [1][2][3][4][5]. To date, it is still a big challenge to investigate the charge transfer behavior from bulk to surface for the solar energy conversion and utilization. Many semiconductors have been developed, such as TiO 2 [1], ZnO [6.7], Ag 3 PO 4 [8-10], CdS [11] and so on. However, the low charge separation efficiency limits the application in practices. Currently, bismuth-based semiconductors, including BiVO 4 [12,13], BiWO 4 [14], Bi 3 PO 4 [15], Bi 2 O 2 CO 3 [16,17], BiOX (X=Cl, Br, I) [18-21], etc. have shown efficient photocatalytic activities in wastewater and noxious gas purification. Among them, layer structured BiOCl (composed of [Bi 2 O 2 ] 2+ layers interleaved with Cl layers) have attracted great interests due to its outstanding optical and electrical properties [22,23]. However, its photocatalytic activity is obviously limited by the wide band gap and high recombination rate of photogenerated carries [24,25]. Moreover, it is still a big challenge to investigate the charge transfer behavior of photocatalysts. Up to now, many efforts have been made to enhance the photocatalytic performance of BiOCl, such as metal doping (Fe, Zn, Cu, Mn, etc.) [23,26-28], nonmetal doping (F, C, etc.) [19,29], co-catalyst modification (Ag, Au, Bi etc.) [30-35] and semiconductor heterojunctions (BiOI/BiOCl [36], Bi 2 S 3 /BiOCl [37], Ag/AgX/BiOX [21,38], BiOCl/Ag 3 PO 4 [39], g-C 3 N 4 /BiOCl [25,40], Bi 2 O 2 CO 3 /BiOCl [41], TiO 2-...

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In this work, we mainly investigate the influence of structure water on crystal structure, electronic structure, band structure, and charge separation of WO3·2H2O. It is found, for the first time, that although water is a weak electron donor, a ligand-to-metal charge transfer (LMCT) from H2O to W occurs. The structure water contributes to the conduction band (CB) of WO3·2H2O, and the band gap of WO3·2H2O is obviously narrowed, thus increasing the light absorption obviously. Moreover, the results of EIS, photocurrent spectra, and PL show that structure water in WO3·2H2O also improves the charge separation and transfer efficiency of the catalyst. This is the first investigation on the LMCT transfer from structure water (a weak electron donor) to tungsten, which obviously improves light absorption and charge separation. Under visible light irradiation (λ ≥ 420 nm), WO3·2H2O nanosheets have a photocatalytic activity 2.3 times higher than that of WO3 for the degradation of methylene blue (MB). The kind number of photochemical materials can be increased greatly, because structure water-contained compounds widely exist in nature.

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The big Jahn–Teller distortion of α-MoO3 results in a high CWAO activity.

Nitrogen photoreduction is a promising ammonia production technology since it can be carried out at the atmospheric pressure by merely utilizing the abundant solar energy. However, the low conversion efficiency is still a big challenge for practical applications. In this work, Bi-decorated g-C 3 N 4 nanosheets are successfully prepared by an in situ solvothermal method. The ultraviolet−visible diffuse reflection spectrum shows that Bi decoration conspicuously enhances the visible light absorption because of the surface plasmon resonance effect of the Bi semimetal. Moreover, the photoluminescence spectrum demonstrates that Bi decoration greatly improves the charge separation efficiency because the high work function of Bi (4.5 eV) favors electron transfer from the conduction band of g-C 3 N 4 to the Bi surface. Meanwhile, nitrogen sorption isotherms show that Bi decoration obviously increases the Brunauer−Emmett−Teller surface area (4.3 m 2 g −1 for g-C 3 N 4 vs 7.7−8.4 m 2 g −1 for Bi/g-C 3 N 4 ), which is beneficial for N 2 adsorption. Additionally, the density functional theory calculation shows that the adsorption energy (E ad ) of N 2 on Bi/g-C 3 N 4 (−2.3 eV) is 6.2 times higher than that on g-C 3 N 4 (−0.3 eV), which indicates that N 2 is easier to adsorb on Bi/g-C 3 N 4 thermodynamically. It is assumed that the decorated Bi can also provide new active sites that significantly improve the adsorption and activation of N 2 . Therefore, Bi/g-C 3 N 4 exhibits a high nitrogen photoreduction activity under visible light irradiation (λ > 420 nm): the ammonia yield over 30-Bi/g-C 3 N 4 (3075 μmol L −1 g −1 ) is 1.8 times higher than that over g-C 3 N 4 (1076 μmol L −1 g −1 ) and the ammonia production rate over 30-Bi/g-C 3 N 4 (1025 μmol L −1 g −1 h −1 ) is 1.8 times higher than that over g-C 3 N 4 (359 μmol L −1 g −1 h −1 ). The improved activity is mainly attributed to the Bi decoration on the g-C 3 N 4 surface, which improves the adsorption and activation of N 2 , the electron−hole separation efficiency, and the light-harvesting ability. This work demonstrates that similar to precious metals, Bi semimetal can also greatly improve the nitrogen photoreduction activity. Most importantly, Bi semimetal is more abundant and cheaper than precious metals, and it could be more promising for practical applications.