Jun Yanga,∗, Longjun Xua,∗ ∗ ∗, Chenglun Liua,b,∗∗, Taiping Xiea a State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, PR China b College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China

a b s t r a c t

Porous Bi5O7I nanosheets were successfully prepared by a facile thermal decomposition of BiOI nanosheets in air at 500 ◦C for 2 h. The crystal structure, surface morphology, specific surface area, and optical properties of resulted materials were characterized by XRD, SEM, TEM, nitrogen adsorption–desorption isotherms and UV–vis diffuse reflectance spectroscopy, respectively. Moreover, the photodegradation ability of porous Bi5O7I nanosheets on target pollutant Rhodamine B (RhB) under visible light irradiation ( ≥ 400 nm) and simulated sunlight irradiation was investigated. The results indicated that porous Bi5O7I nanosheets were mainly composed of irregular nanosheets. With the layer thickness of 30–50 nm and pore diameter distribution around 25 nm, it belonged to mesoporous material. Compared with BiOI, the photocatalytic activity of porous Bi5O7I nanosheets under visible light was appropriately 2 times than that of BiOI, and 2.3 times under simulated sunlight. The major reasons for the improvement in catalytic performance were the existence of multiple pores and the special band structure.

1. Introduction

Since Fujishima and Honda [1] discovered the TiO2 electrode was featured with photocatalytic water splitting in 1972, the photocatalytic materials have received increasing attentions. In recent years, the photocatalytic materials have been extensively applied in the purifification of water pollution and air pollution [2–8]. Bi oxyhalide (BiOX, X= Cl, Br and I) [9–12] is a new type of photocatalyticmaterial.Itsunique layer structuremay generate electrostatic fifield that is perpendicular to each layer, which is benefificial to the effective segregation of photocarrier. Hence, it shows high photocatalytic activity [11]. Besides some ordinary BiOX (X:O = 1:1) in the BiOX family, there are still some poor BiOX (X:O < 1), such as Bi3O4Cl [13], Bi4O5Br2 [14], Bi4O5I2 [15], Bi7O9I3 [16] and Bi5O7I, which are obtaining increasing attention as well. According to the investigations of Sun and Wang [17], Bi5O7I exhibited excellent photocatalytic activity under visible light radiation. They prepared Bi5O7I with hydrothermal method and used it in the photocatalytic degradation of RhB under visible light irradiation for the first time, which demonstrated much superior catalytic performance than that of Bi2O3. Xiao [18] utilized Bi2O3 as the precursor to react with KI in the presence of HNO3 so as to prepare Bi5O7I which played a certain desorption role in bisphenol-A under visible light irradiation. To improve the photocatalytic activity of Bi5O7I, Zhang [19] applied Er3+ and Yb3+ for co-doping Bi5O7I, which significantly enhanced its ability in photocatalytic degradation of pollutants. Though these methods can be successfully used to prepare Bi5O7I, the photocatalytic performance of ultimate obtained samples varies from each other and some even are equipped with no photocatalytic activity. For instance, the Bi5O7I prepared by Li [20] through hydrothermal method presented no photocatalytic degradation competence in the degradation test of methyl orange. Besides, the Bi5O7I prepared by Yu [21] via roasting method almost showed no photocatalytic performance as well. This might be caused by the surface morphology and low specifific surface area of their prepared samples, which indicated that the surface morphology of Bi5O7I had significant influences on its photocatalytic performance.

In view of this, a facile approach was adopted to conduct thermal decomposition for the prepared BiOI in advance to get porous Bi5O7I and the photodegradation ability of porous Bi5O7I on organic dye RhB under visible light irradiation ( ≥ 400 nm) and simulated sunlight irradiation were investigated. Compared with BiOI, it showed excellent photodegradation ability for organic pollutants. In addition, the pore formation mechanism of Bi5O7I and the reasons for improving photocatalytic performance were explored preliminarily. As far as we know, there is no literature report on porous Bi5O7I up to now.

2. Experimental

2.3. Measurement of photocatalytic performance

The photocatalytic performance of porous Bi5O7I nanosheets was characterized by degrading RhB aqueous solution. With xenon lamp (CEL-HXF300, AULTT) of 300W as the light source and equipped with UV cut-off filter ( ≥ 400 nm), the self-made circulating water system maintained the temperature of reaction system at 25 ± 5 ◦C. Dispersed 0.1 g catalysts into 100 mL of RhB (10 mg/L) aqueous solution and stirred for 1 h in the dark so as to reach adsorption and desorption balance between catalysts and dye. Besides, during the photodegradation process, keep the light source 20 cm away from the liquid level, and the absorbance of supernatant centrifuged from the extracted 4 mL liquid was measured every 0.5 h of irradiation.

3. Results and discussion

3.1. Crystal phase and morphology

Fig. 1 presents the XRD patterns of BiOI nanosheets and porous Bi5O7I nanosheets as well as their corresponding standard patterns. From Fig. 1a, it can be seen that the prepared BiOI is the pure phase (cell parameters: a = b = 3.99 ˚A and c = 9.14 ˚A) with excellent degree of crystallinity and all peaks could accurately correspond to the below standard cards (JCPDS no. 10-0445) without the emergence of other impurity phase. Fig. 1b is the XRD pattern of Bi5O7I prepared by thermal decomposition of BiOI nanosheets in air at 500 ◦C for 2 h. It is found that the cell parameters of Bi5O7I were a = 16.26 ˚A, b = 5.35 ˚A and c = 11.50 ˚A respectively. This is consistent with the theoretical value in the corresponding below standard cards (JCPDSno.40-0548), andthepresence of other impurityphase is not observed as well. In other words, BiOI was successfully transformed into Bi5O7I through thermal decomposition at 500 ◦C for 2 h. According to the calculation of Scherrer equation, the crystal diameters of BiOI and Bi5O7I are 24.2 nm and 40.1 nm respectively. 

The SEM images of BiOI and Bi5O7I samples are shown in Fig. 2. From images (a, b), it is obvious that BiOI is mainly piled up by the smooth irregular nanosheets. While the SEM images (c, d) of Bi5O7I clearly show that Bi5O7I is also composed by irregular nanosheets with smooth surface. The layer thickness of nanosheets was measured to be around 30–50 nm. However, it was worth noticing that many pores on the sample surface still could be seen. To further observe the microtopography of pores, Bi5O7I was characterized by TEM and HRTEM, and the results are shown as in Fig. 3. 

From Fig. 3a, the presence of pores on Bi5O7I nanosheets can be more precisely viewed. The pore shapes are not regular circles but circles or ellipse of all sizes, which is consistent with the results observed in SEM pictures. Moreover, the Nano measure software is utilized to conduct statistical analysis on the pore size. Two hundred pores were selected randomly and the statistical results are shown in Fig. 3d. Moreover, the pore diameter is mainly distributed between 15 and 30 nm, around 25 nm. Fig. 3b displays theHRTEMof circular regioninFig. 3a, whichmeasuredthe fringe spacing between neighboring crystal lattices are 0.316 nm, 0.287 nm, 0.274 nm and 0.229 nm respectively. The crystal faces corresponding to Bi5O7I are (3 1 2),(0 0 4),(2 0 4) and (0 0 5) respectively. Taking into account another circular region in Fig. 3a as selected area electron diffraction (SAED), the results show that the porous Bi5O7I nanosheet is a polycrystal, and its diffraction point can perfectly comply with the results of HRTEM and XRD.