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-...

It is still a big challenge to facilely tune the energy bands of a semiconductor. Herein, we have mainly investigated energy bands and photochemical properties of Bi6S2O15 and Bi2O(OH)2SO4, which have very similar layered structures. It is found that the hydroxyls have down shifted the conduction band (CB, 0.21 eV) and valence band (VB, 4.39 eV) of Bi2O(OH)2SO4, compared with those (CB = 0 eV; VB = 3.36 eV) of Bi6S2O15. Moreover, the main oxidative species of Bi6S2O5 and Bi2O(OH)2SO4 are holes (h(+)) and hydroxyl radicals (˙OH) for the degradation of rhodamine B (RhB) dye, respectively. This obvious difference has been mainly attributed to the hydroxyls, which have changed the energy band structure and the band gap. In addition, we have also investigated the morphology-dependent properties of Bi2O(OH)2SO4. Under ultraviolet light irradiation (λ ≤ 420 nm), Bi2O(OH)2SO4 microspheres show an activity 1.3 times and 2.2 times higher than the long flakes and straw sheaves for the degradation of (RhB), respectively. This study provides us a new idea that we can facilely tune the energy band of a semiconductor by introducing or removing hydroxyl or other anions.

In this work, we report the facile conversion from BiOCl nanosheets to BiOF, Bi 7 F 11 O 5 and BiF 3 by a novel ion exchange approach, in which the effects of fluorine source, F/Bi molar ratio and reaction medium (ethanol/water) on the products are mainly investigated. A plausible conversion mechanism is proposed to illustrate the formation of BiOF, Bi 7 F 11 O 5 and BiF 3 . Furthermore, the photocatalytic activities of the samples are also investigated. It is amazing that under ultraviolet light irradiation (λ ≤ 420 nm), the activity of the as-formed Bi 7 F 11 O 5 /BiOCl sample is 3.28 times higher than that of BiOCl for the degradation of methyl orange (MO). It is demonstrated that the improved activity is mainly attributed to the formation of Bi 7 F 11 O 5 /BiOCl heterojunction, which has significantly improved the separation and Inspired by the results above, a simple ion exchange approach is developed to convert BiOCl to BiOF, Bi 7 F 11 O 5 and BiF 3 , in which fluoride is mainly employed as the fluorination chemical. A plausible conversion mechanism is proposed and discussed; meanwhile, we have investigated their photocatalytic activities for the degradation of MO. The results show that Bi 7 F 11 O 5 /BiOCl heterojunction shows the The simulations of energy band structures, total and a part of densities of states (T-and P-DOS) were calculated by density functional theory (DFT) as implemented in the CASTEP. The calculations were carried out using the generalized gradient approximation (GGA) level and Perdew-Burke-Ernzerh (PBE) formalism for combination of exchange and correlation function. The cut-off energy is chosen as 18 orbit contributes to both CB and VB of both BiOCl and BiOF. Hu et al. have reportedthat F 2p and O 2p oribits mainly contribute to the VB of Bi 7 F 11 O 5 , and Bi 6p oribit mainly contributes to the CB. 20 Meanwhile, the VB top of BiF 3 is mainly attributed by the F 2p orbital, and the CB bottom is mainly attributed by the Bi 6p, as well as a little of F 2p orbits (Fig. 10f). Comparing BiF 3 with BiOF, it is interesting that F 2p orbit contributes to both CB and VB of BiF 3 , which is different from BiOF. This further confirms that with the increase of fluorine, the band gaps of BiOCl, BiOF, Bi 7 F 11 O 5 and BiF 3 increase, similar to the experimental result above. Conversion mechanismIt is well known that BiOCl, BiOF have tetragonal structures and Bi 7 F 11 O 5 , BiF 3 have monoclinic, cubic structures; respectively. Due to the layer structure of BiOCl, Cl, instead of O, is easily substituted by F. Thus, BiOCl can be converted to BiOF, Bi 7 F 11 O 5 and BiF 3 . In the presence of NH 4 F at V Eth /V w =18/2, BiOCl can be facilely converted to BiOF, Bi 7 F 11 O 5 and BiF 3 . At V Eth /V w =18/2, however, we cannot achieve the complete conversion from BiOCl to phase-pure BiOF while only changing the amount of NH 4 F added. Neverheless, BiOCl can completely convert to phase-pure BiOF at low V Eth /V w ratios (0/20, 2/18, 10/10) at R F =2. According to the experiment results, the schematic ...

For the first time, anatase TiO2 twin crystals have been synthesized by a simple hydrothermal method. The samples are characterized using X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction, ultraviolet–visible diffused reflectance spectra, and nitrogen sorption isotherms. It is found that a prolonged reaction time significantly contributes to the structure evolution and that twin crystals form via the oriented attachment (OA)- and Ostwald ripening (OR)-controlled secondary growth mechanism. Remarkably, the photoactivity of TiO2 twin crystals is 2.57 and 8.74 times higher than that of single-crystalline anatase TiO2 nanosheets and rutile TiO2, respectively. This has been mainly ascribed to the unique twin crystal structure. This work is a big step toward further improvement of the photocatalytic properties of of TiO2.

In this work, V2O5with an interesting “L”-shape is successfully prepared by a simple hydrothermal method without using any surfactant or template.