Abstract: 

The BiOCl with {010} facets could be promising materials for  photodegradation and energy conversion devices such as dye self-photosensitization  photocatalytic fuel cells (DSPFCs). However, the {010} facets usually diminish  rapidly during the growth process as the result of its high surface energies. In this  work, we reported a simple and efficient method to prepare BiOCl with tunable  exposed {010} facets. It was found that the solvent used in synthesis process acted an  important roles on the formation of ultrathin construction and the growth of {010}  facets by controlling the [H+ ]. For decreasing the surface energy and promoting the  growth of high-active {010} facets, the thickness of BiOCl and the areas of {001}  were reduced in its forming process. We had demonstrated that the enhancement of  visible light-harvesting and photosensitization activity of BiOCl was primarily  attributed to the decrease of thickness and the growth of {010} facets which could  provide large surface areas and more active sites for dye absorption and  photoelectrons transfer. The BiOCl with tunable exposed {010} areas were evaluated  as photoanode materials in DSPFCs. As expected, owing to its strong dye absorption  capability and high transfer efficiency of charge carriers, the DSPFC with optimal  performance was obtained by employing RhB as fuel when BiOCl possessed the  larger areas of {010} facets and became a thinner nanosheet structure. And the Jsc and  Voc of DSPFC were measured to be 0.0058mA/cm2  and 0.689V respectively. Meanwhile approximately 67% color removal was achieved on BiOCl{010}-Pt cell  by treating 40mL of 5mg/L RhB under visible light for 240min, which was much  higher than P25-Pt (4%).

INTRODUCTION  

The environmental pollution and excessive energy consumption have been caused  great attention worldwide. Since the discovery of photocatalytically splitting of water  into hydrogen over TiO2 semiconductor materials under UV light,1  the photo fuel cells  (PFCs) have been demonstrated as a promising and potential technique for addressing  and relieving the intense pressure between environment and energy.2-5 In PFCs system,  the photoanode utilize solar energy as impetus and contaminant as fuel to  simultaneously achieve the energy conversion and pollutants degradation.6  Currently,  the research of PFCs devices assembled with TiO2 semiconductor materials achieved  impressive achievement under UV, 3,7-8 but the limited visible light-harvesting and the  presence of surface state seriously impede their wide application.9,10 Although the  quantum dot, such as CdS, PbS, can be employed to enhance the visible light  absorption capacity and photoelectrons transfer efficiency,11,12 the toxicity of quantum  dot13 and the instability of CdS14 are of application concern. 

 The dye self-photosensitization photocatalytic fuel cells (DSPFCs) are  developed to fulfill the high visible light-harvesting and effective photosensitization  degradation.15,16 Compared to PFCs, the generation of photoelectrons came from  excited dye molecules usually is much easier owing to the wide light absorption  region of dye molecules.17 Moreover, the loss of photoelectrons is few and the 

recombination rate of charge carriers is low in DSPFCs, because the valence band of  the semiconductor is not involved in the process of photosensitization reaction.18,19 BiOCl can be used as novel promising material for DSPFCs application owing to its  unique electronic structure and surface properties. Although the BiOCl cannot be  photo-excited directly under visible light because of its wide band gap, it was  demonstrated that the BiOCl with {010} facets exhibited excellent dye  self-photosensitization performance. On the one hand, the BiOCl layered structure  consisting of [Bi2O2] 2+ layers sandwiched between two slabs of halogen ions can  induce the formation of internal static electric fields and endow it with open channel  feature, which are beneficial for the photoelectrons transfer.20 Moreover, the  conduction band of BiOCl (-1.1 eV)21,22 is more negative than TiO2 (-0.29 eV),23 therefore the formation of O2· - active species (E (O2/O2·-) (-0.046 eV))24 on BiOCl is  much easier. The O2· - active species possess high oxidation activity for further  pollutant decomposition. On the other hand, the BiOCl with dominant {010} facets  has excellent dye absorption capacity and photosensitization performance due to its  large surface areas and more surface active sites, which are favorable for enhancing  the light-harvesting.18 The dye absorption capability and mobility of charge carriers  are the two major impact factors on DSPFCs performance. Therefore, the controlled  synthesis of BiOCl with high percentage of {010} facets has important meaning for  obtaining high-performance DSPFCs. Unfortunately, the high active facets usually  diminish rapidly during the growth process as the result of their high surface  energies.25 26

In this study, a series of BiOCl with tunable exposed {010} facets were  synthesized by a facile strategy. The impacts of solvent used in synthesis process on  nanostructure of BiOCl and the growth of {010} facets were discussed in detail. The  experiments of photodegradation Rhodamine B contaminant were carried out to  assess the photosensitization performance of as-prepared BiOCl and investigate the  relationship between {010} facets and photocatalytic activity. Moreover, the  photovoltaic performances of DSPFCs assembled with BiOCl photoanodes were  researched by employing RhB as fuel. Owing to the strong dye absorption capability  and high photoelectrons transfer efficiency of {010} facets, the BiOCl with largest  {010} facets exhibited superior photovoltaic and RhB decomposition performance  than P25 and BiOCl{001}. Our work expects to open a new avenue for the wide  application of BiOCl in environment protection and energy, and provide new ideas on  exploitation and synthesis of photoanodes materials for high-performance DSPFCs.

EXPERIMENTAL SECTION

Photocatalytic Measurements. The photocatalytic activity experiments of samples were evaluated by photodegrading the Rhodamine B (RhB) under visible light  irradiation at ambient temperature using a 300 W Xe arc lamp (CEL-HXF300, Beijing)  with a cutoff filter (λ> 420 nm). Briefly, a 0.02 g photocatalyst was added into 100mL  aqueous solution containing 40 mg/L RhB solution. Prior to irradiation, the  suspension was magnetically stirred in dark for 1 h to ensure desorption-adsorption  equilibrium. During the procedure of degradation, the analytical samples (about 4 mL)  were taken every 5min and centrifugated at a rate of 8000 rpm to remove the  remaining particles, And the RhB concentration was monitored by recording the  absorbance of supernatants at maximum absorption wavelength using an UV-vis  spectrophotometer (Shimadzu 2550, Japan).  

RESULTS AND DISCUSSION

The characterization of XRD patterns was carried out to describe the crystal structure  of samples. From the Figure 1.a, all of the diffraction patterns were corresponded to  the BiOCl tetragonal crystals (JCPDS no.06-0249), and no other diffraction peaks  were observed. However, the samples prepared in different solvent exhibited distinct  differences of diffraction peaks intensity on {001} and {010} facets. Compared to the  other solvent, the sample obtained in HNO3/ethanol possessed highest (001) peaks  which was the characteristic diffraction peaks of {001} facets. Because the hydrogen  ions in solution preferred to absorb on oxygen terminated {001} facets, therefore the  BiOCl with {001} facets was easily formed under high [H+ ] condition.18 Once the [H+ ]  in solution was controlled by using different solvent, the intensity of (001) peaks  became weak gradually, indicating the growth rate of BiOCl was decayed along the  {001} facets direction.30 Furthermore, the (110) peaks, that as characteristic  diffraction peaks of {010} facets, grew rapidly. For reducing the surface energy and  promoting the growth of high active {010} facets, the thickness of BiOCl became thin  and the areas of {001} also decreased. As clearly showed in Figure 1.b, it could be  seen that the specific values of the I(110)/I(001) gradually increased with the decrease  of {001}. These results suggested that the crystal growth speed was inhibited along  the {001} facets and increased along the {010} facets as the result of preferred  orientation under the low [H+ ]. Different surface properties endow semiconductors  with distinctive optical and electronic properties.31,32 The BiOCl with exposing large  percentage area of {010} facets exhibited superior dye photosensitization activity  under visible light.18,19

The typical scanning electron microscopy (SEM) images of BiOCl samples were  shown in Figure 2. With the growth of {010} faces, the size of BiOCl samples  gradually decreased, and the nanostructures changed from the 2D nanosheet to the 3D  nanoflower (Figure 2.a~e). This change was ascribed to the change of [H+ ] in ethanol  solution. All detailed parameters are displayed in Table 1. During the growth of  BiOCl, the hydrogen ions in solution preferred to absorb on oxygen terminated {001}  facets and increased the crystal growth speed along the ab plane. Therefore, the  uniform square-shaped BiOCl nanosheets with exposing {001} were obtained in  HNO3/ethanol solution. Once H2O was used as solvent, the hydrogen ions mainly  came from the hydrolysis of Bi(NO3)3 and therefore the [H+ ] was much lower than  HNO3 solution which resulted in the slow crystal growth speed along the ab plane. It  was clearly observed that the morphology of BiOCl changed from uniform nanosheet  to nanodiscs. And the BiOCl nanodiscs were much thinner and smaller than nanosheet  (Figure 2.b). Compared to the H2O/ethanol, the size and thickness of BiOCl prepared in CH3COOH/ethanol became slightly smaller owing to the low degree of ionization  of CH3COOH. In EG/ethanol solution, Bi3+ preferred to absorb on oxygen terminated  of EG molecular to form a relatively stable complex Bi2(OCH2CH2O)3. Relatively,  the stronger dissolving capacity between EG and ethanol solution is beneficial for the  formation and growth of BiOCl tiny nuclei. Moreover, the tiny nuclei are not easy to  reunite together in ethanol solution during the growth, leading to the final products  with small crystal size (Figure 2.d). These results corresponded to the literature  reports.33,34 Finally, the 3D porous nanoflower structure of BiOCl was formed through  the Ostwald ripening and self-assembly process.35,36 With a further decrease of [H+ ],  the BiOCl ultrathin nanosheet units were formed in NH4OH/ethanol solution first  owing to the growth of {010} facets, and then the ultrathin nanosheets aggregated  freely into 3D porous nanoflower to reduce the surface energy (Figure 2.e). The  nanoflower structure with interlaced ultrathin nanosheets can not only greatly increase  the surface areas of photocatalyst, but also enhance the multi-reflection of light.37 The  decrease of size and the change of structure can influence significantly on the physical  and chemical of BiOCl.  

More details about the crystal structure and morphology of BiOCl were obtained  from the transmission electron microscopy (TEM) images. The square-shaped BiOCl  nanosheet structure was acquired in HNO3/ethanol, whose diameter and thickness  were 3µm and 0.25µm (Figure 3.a). A clear lattice spacing of 0.275nm and the  interfacial angle of 90。were observed (Figure 3.b), which were corresponded to the  (110) lattice planes. When the [H+ ] was controlled, the size of BiOCl became small  and the morphology changed to nanodiscs. The thickness of samples were estimated  to be 61.5nm, 50.6nm, and 27.5nm by using H2O, CH3COOH, (CH2OH)2 as solvent,  respectively (Figure 3.c~e). And the nanoflower structure of BiOCl was presented in  Figure 3.f. It was clearly observed that the BiOCl nanoflower was composed of a  large amount of interlaced ultrathin nanosheets with 15nm in thickness. As  aforementioned, the solvent have a significant effect on the morphology and thickness  of BiOCl samples. The surface areas of BiOCl can be enlarged greatly owing to the reduction of nano-sized and the formation of nanoflower structure, which can provide  larger space for dye molecule absorption and chemical reaction. Moreover, the  predominant defect on the surface of BiOCl would be changed from VBi ‴ to  VBi ‴VO ••VBi ‴ with the decrease of thickness. The negatively charged VBi ‴VO ••VBi ‴  preferred to absorb cationic dye molecular.38 A clear and continuous lattice fringes  with the lattice spacing of 0.37nm was determined by high-resolution TEM (HRTEM)  image (Figure 3.g), that was corresponding to the {002} atomic planes. The physical  and chemical performance of semiconductor materials is closely related with its  surface properties and geometric structure.

The XPS characterization was carried out to record the surface chemical  properties and composition of as-prepared BiOCl. As shown in Figure 4.a, bismuth,  oxygen and chloride elements were included in the sample. The elements molar ratio  was estimated to be 1:1.42:0.93 corresponding with the stoichiometric molar ratio in  BiOCl. The peaks of binding energies for Bi(4f) core level were located in 164.78 and  159.48eV, which were belonged to Bi (4f5/2) and Bi (4f7/2) respectively (Figure 4.b).  And the position of characteristic peaks was the same in BiOCl-HNO3 and  BiOCl-NH4OH. These results suggested that the bismuth ions in BiOCl samples were  in tri-valence chemical state. The binding energies of O1s in different BiOCl samples  were presented in Figure 4.c. It could be clearly seen that the O1s peak position in  BiOCl-NH4OH had a slightly shifts (0.31 eV) along low binding energy orientation.  This suggested that the BiOCl-NH4OH surface had more negatively charge than  BiOCl-HNO3, which could be attributed to the decrease of the BiOCl thickness. With  the growth of {010}, the thickness of BiOCl decreased gradually, leading to the  change of predominant defects on the surface of BiOCl from VBi ‴ to VBi ‴VO ••VBi ‴ . The  negatively charge of VBi ‴VO ••VBi ‴ defects is favorable for cationic dye absorption.38

The Nitrogen adsorption-desorption isotherm analysis were conducted to survey  the special surface area of BiOCl samples. With the growth of {010} facets, the  surface areas of BiOCl reduced first and then increased, which was attributed to the  change of its nanostructure (Figure 5.a). The large special surface area can offer more  active sites and open space for dye molecular absorption. The BiOCl prepared in  NH4OH/ethanol solution possessed the largest surface area than others due to its 3D  interlaced nanoflower structure. All these detailed parameters are displayed in Table 1.  The RhB adsorption curves over the as-prepared samples under darkness for 60 min  were showed in Figure 5.b. As light receptors and electron donor, the adsorption  efficiency of photosensitizer is an important influencing factor on the generation of  photoelectrons and light-harvesting. In comparison, the RhB adsorption capability of  BiOCl-NH4OH is largest than others, which is primarily ascribed to its large surface  area and thin nanosheet structure. Although the BiOCl prepared in HNO3 had  relatively large surface area (17.302 m2g -1), the RhB adsorption capability was lowest  than others, revealing that the physical performance of BiOCl was closely related with  {010} facets. The {010} facets could provide more active sites for cationic RhB  molecules absorption. For DSPFCs, the large special surface areas are plays a vital  role on enhancing the harvest of visible light and dye absorption capability.39 The DRS was employed to analyze the optical absorption properties of the  BiOCl samples as shown in Figure 6.a. Generally, the energy gap of photocatalys is  directly determines its optical absorption performance.