Investigation of the charges separation and transfer behavior of BiOCl/BiF3 heterojunction

Abstract

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-...

Synthesis of “walnut‐like” BiOCl/Br solid solution photocatalyst by electrostatic self‐assembly method

“…Selective electron diffraction (SAED) shows a circular pattern (illustrated in Figure B), showing its polycrystalline properties. The diffraction point set can be indexed as the [001] band axis of BiOX, and the adjacent angle points labeled as (200) and (110) planes was 45°, which confirmed that the high‐exposure plane of the prepared “walnut‐like” BiOCl/Br solid solution was {001} plane, which was consistent with the XRD results.…”

“…As shown in the figure, the TOC value of RhB decreased significantly, and the mineralization rate of BiOCl/Br solid solution under visible light and ultraviolet was 74.2% and 78.7%, respectively, which was significantly higher than that of 001‐BiOCl nanosheet 1.9% and 63.6%, respectively. Hence, the RhB mineralization on the “walnut‐like” BiOCl/Br solid solution is dominant, indicating that most RhB can be completely decomposed into CO 2 and H 2 O, and only a small amount of small molecular organic matter exists in the residual solution, which is consistent with the results of relevant literature …”

“…Second, in 001‐BiOCl nano and “walnut‐like” BiOCl/Br solid solution in the visible light and ultraviolet light catalytic system, respectively, to be joined the 1,4‐benzoquinone (BQ), ammonium oxalate (AO) and isopropyl alcohol (IPA) three capture agent to ·O 2 − , ·OH and h + capture experiment was carried out to examine the possible active species in the photocatalytic system . Results as shown in Figure C, three kinds of trapping agent were joined, reducing the “walnut‐like” the activity of the 001‐BiOCl and the BiOCl/Br solid solution, which shows ·O 2 − , ·OH and h + are the reaction of the main species.…”

“…In order to verify that the “walnut‐like” BiOCl/Br solid solution has higher photocatalytic activity than other samples, the charge generation and transfer of photocarriers in the samples were studied by using electrochemical workstation to measure the transient photocurrent densities and electrochemical AC impedance of the photocatalysts . The result is shown in Figure A,B.…”

“…Hence, the RhB mineralization on the "walnut-like" BiOCl/Br solid solution is dominant, indicating that most RhB can be completely decomposed into CO 2 and H 2 O, and only a small amount of small molecular organic matter exists in the residual solution, which is consistent with the results of relevant literature. 10,51 The degradation experiment was conducted with phenol without photosensitive property to verify that the photosensitive effect of dye did not play a dominant role in the photocatalytic degradation experiment of RhB. Figure 13E, F showed that the "walnut-like" BiOCl/Br solid solution had higher photocatalytic degradation activity than that of 001-BiOCl.…”