It is still a big challenge for Ag3PO4 to be applied in practice mainly because of its low stability resistant to photo corrosion, although it is an efficient photocatalyst. Herein, we have mainly investigated its activity and stability under indoor weak light for the degradation of dye pollutants. It is amazing that under indoor weak light irradiation, rhodamine B (RhB) can be completely degraded by Ag3PO4 polypods after 36 h, but only 18% of RhB by N-doped TiO2 after 120 h. It is found that under indoor weak light irradiation, the degradation rate (0.08099 h(-1)) of RhB over Ag3PO4 polypods are 46 times higher than that (0.00173 h(-1)) of N-doped TiO2. The high activity of Ag3PO4 polypods are mainly attributed to the three-dimensional branched nanostructure and high-energy {110} facets exposed. After three cycles, surprisingly, Ag3PO4 polypods show a high stability under indoor weak light irradiation, whereas Ag3PO4 have been decomposed into Ag under visible light irradiation with an artificial Xe light source. This natural weak light irradiation strategy could be a promising method for the other unstable photocatalysts in the degradation of environmental pollutants.

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-...

In this work, Zn(OH)F is prepared by an initiative, simple post-synthesis method, in which the molar ratio of F/Zn (R) was varied to investigate the effect of the NHF amounts added on the samples. Further, we have mainly investigated their energy bands and photochemical properties. Under UV light irradiation (λ£420nm), the samples (R=0,1,2) show the high degradation activities of methylene blue (MB) dye, namely, 80% of MB can be degraded after 8min. It is found that the hydroxyl and fluorine have greatly down shifted the conduction band (CB, 0.99eV) and valence band (VB, 4.17eV) of Zn(OH)F, compared with ZnO (CB=-0.31eV, VB=2.89eV), but with the nearly same band gap. For the degradation of MB dye, the main oxidative species are holes and hydroxyl radicals for ZnO and Zn(OH)F, respectively. This study suggests that this simple post-synthesis fluorination approach could be extended to develop the other photocatalysts; moreover, we can facilely tune the band structure and photocatalytic activity by introducing or removing hydroxyl and fluorine, which could benefit to develop new photocatalysts.

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