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-...