Xiao-Ju Wen, Cheng-Gang Niu*, Lei Zhang, Guang-Ming Zeng   College of Environmental Science Engineering, Key Laboratory of Environmental  Biology Pollution Control, Ministry of Education, Hunan University, Changsha  410082, China  

ABSTRACT

In this work, an n–p heterostructure SnO2/BiOI photocatalyst was successfully  fabricated through a facile chemical bath method. The photocatalysts was applied to  minimize antibiotic oxytetracycline hydrochloride(OTTCH) and methyl orange(MO)  under visible light irradiation. SnO2/BiOI composite exhibited excellent  photocatalytic performance for the refractory pollutant OTTCH and MO  decomposition. Especially, the sample of 30wt%SnO2/BiOI possessed the best photocatalytic performance in all the obtained catalysts. Several reaction parameters  affecting OTTCH degradation such as initial concentration, ion species and  concentration were investigated systematically. The optical and electrical properties of  materials demonstrate that the transfer rate of electron-hole pairs dramatically  improve though forming an n–p junction in SnO2/BiOI hybrid. Moreover, the energy  band alignments of SnO2/BiOI junction were confirmed via combining DRS and XPS  analysis, which provided strong support for the proposed mechanism. This work  could provide a new approach to construct new p–n junction photocatalysts and a  reference for the study of other heterojunction catalysts.

Keywords: SnO2; BiOI; p-n junction; Photocatalysis; Oxytetracycline  hydrochloride.

INTRODUCTION

Oxytetracycline hydrochloride(OTTCH), one of the broad-spectrum antibiotic,  especially employed in veterinary medicine.1  Because of its high stability, a large  amount of them cannot be biologically degraded or eliminated in treatment plants,  ending up in effluents of lakes and rivers.2  It can seriously pose a threat to a large  number of living organisms. Besides, traditional treatment processes is refractory to  disposal of it.3  Thereby, it is strongly desired to search for an efficient and  cost-effective method to destruct its complex structure and remove its biotoxicity.  Especially, photocatalytic degradation has become hot spot because it offers an  economic and environmentally friendly solution.4-5 Unfortunately, traditional photocatalysts TiO2 only utilize ultraviolet light which just accounts for 3-5% of the  solar light.6  As a result, visible light-driven(VLD) photocatalysts have aroused  widespread concern.  

    BiOI, a p-type semiconductor, has been demonstrated to be a promising VLD  photocatalyst owing to its strong visible-light response, non-toxicity.7-8 Unfortunately,  the photocatalytic performance of bare BiOI is still unsatisfied owing to its fast  recombination rate of electron-hole pairs.9  Generally, constructing semiconductor  heterostructure is effective in accelerating photoinduced electron-hole pair’s  separation efficiency. Particularly, fabrication of BiOI-based p–n junction structure  has been validated a very efficient approach to overcome the detrimental effects  inherent to BiOI and further enhanced its photocatalytic activity. For instance, Ao and  his coworkers reported novel p-n heterojunction BiOI/La2Ti2O7 composite and its  highly photocatalytic performance for multiple organic pollutants degradation under  visible light irradiation.10 Lately, Wen at al successfully designed BiOI/CeO2 p-n  junction photocatalyst via a simple method and the catalyst displayed highly  photocatalytic activity for pollutants degradation.11 Therefore, fabrication of p-n  heterostructure through combining BiOI to another semiconductor with suitable  valence band and conduction band positions is a promising strategy for acquiring a  good visible light-driven(VLD) photocatalyst. Among various candidate, Tin oxide  (SnO2), a traditional intrinsic n-type semiconductor material, is a possible candidate,  which has been widely used in various fields such as energy storage, gas sensors,  electronics, solar cells, and photocatalysis.12-16 Normally, pure SnO2 cannot be easily  used as VLD photocatalysis due to its wide band gap. 17 Fortunately, SnO2 has high  electrical conductivity, which is beneficial to improve charge carrier transportation.18 By means of coupled it with visible light-driven(VLD) photocatalysts, it can facilitate  the separation of photo-induced electron–hole pairs in system, consequently  enhancing the photocatalytic efficiency, such as SnO2/g-C3N4, 19-20 Ag3PO4/SnO2, 21 SnO2/Cu2O,22 and SnO2/Co3O4. 23 All in all, SnO2 may be a good candidate for  fabricating p-n junction towards BiOI.

    Based on the above narration, we successfully fabricated SnO2/BiOI n-p  heterojunction by means of loading SnO2 nanoparticles on the surface of BiOI  nanosheets. The SnO2/BiOI composites display enhanced photocatalytic performance  towards MO and OTTCH degradation under visible light irradiation. Subsequently,  micro-structure, surface chemical states, Specific surface area, optical and  photoelectrocatalytic properties of the heterojunction system were systematically  studied. Meanwhile, several factors that affect the OTTCH degradation have also been  investigated in detail. The synergistic interaction and an alternative  visible-light-induced photocatalytic mechanism were also discussed.  

EXPERIMENTAL

Materials

Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), Tin chloride pentahydrate  (SnCl4·5H2O), Sodium hydroxide (NaOH), polyvinyl alcohol (PVA), ethylene glycol  (EG), ethanol, sodium sulfate (Na2SO4) potassium iodide (KI), isopropyl alcohol  (IPA), sodium oxalate (Na2C2O4), p-benzoquinone (BQ), methyl orange (MO),  oxytetracycline hydrochloride(OTTCH) were all purchased from Shanghai chemical  Reagents Co., Ltd. All ultrapure water used in all the experiments was obtained from  a Milli-Q ultrapure (18.2 MΩ cm) system.

Synthesis of pure SnO2

SnO2 was synthesized via a hydrothermal method according to following  procedure. Firstly, 10mmol SnCl4·5H2O and 50mmol NaOH dissolved in 50mL  deionized water, respectively. After the two solutions were mixed, 50 mL ethanol  solution was added to the mixture solution. Then, the resulting mixture was sealed in  a 200 mL Teflon-lined stainless-steel autoclave and maintained at 160 °C for 12 h.  Finally, the resulting precipitates were obtained through vacuum filtration. After  washing with deionized water and absolute ethanol several times, the obtained  precipitate was dried at 70 °C for 12 h.  

Fabrication of BiOI/SnO2 p-n junction  

SnO2 nanoparticles were loaded in situ on the BiOI nanosheets by a facile  chemical bath method. In brief, 1mmol SnO2 was ultrasonically dispersed in 40mL of  distilled water to form a uniform solution A. Meanwhile, 1mmol Bi(NO3)3·5H2O and  KI were dissolved in 20mL ethylene glycol and get a clear yellow solution. Then the  solution was added dropwise to the solution A under vigorous stirring. The mixture  was strong stirred for 30 minutes at room temperature. Afterwards, the mixture was stirred in an water bath at 80℃ for 2 h. After cooling down to room temperature, the  product was collected by vacuum filtration and washed several times with ultrapure  water and alcohol. Eventually the 30wt%SnO2/BiOI samples were obtained after  dried at 70℃ for 12h. The schematic diagram for preparation of the SnO2/BiOI  junction is illustrated in Scheme 1. By changing the weight of SnO2, different weight  percentage ratio of SnO2/BiOI composites (15wt%, 45wt% and 60wt %) were  obtained, respectively. The pure BiOI was prepared by the same method described  above without the addition of SnO2 precursor. 

 Scheme 1. Schematic illustration for the preparation procedure of the SnO2/BiOI p-n  junction 

 For comparison, a mechanically physically mixed SnO2 and BiOI sample  (SnO2/BiOI weight percentage ratio = 30wt%, denoted as Mixture) was prepared.  .

Photocatalytic activity tests  

MO and a typical antibiotic OTTCH, were used to evaluate the photocatalytic  activities. The visible light was provided through a 300 W Xe lamp(Zhong jiao jin  yuan, CEL-HXF300) with a UV Cut-off filter(UVCUT420). 

Briefly, 50 mg catalyst  was added into MO (50 mL, 10 mg/L, pH=6.15) and OTTCH (50 mL, 10 mg/L,  pH=5.49) solutions. Before turn on the light, the suspensions were place on a  magnetic stirrer in the dark for 30min to reach adsorption-desorption equilibrium.  During the irradiation, an approximately 3.0mL suspension was sampled and  separated at given time intervals to measure the changes of the pollutants  concentration. The concentration of the MO and OTTCH were measured with a  UV-vis spectrophotometer at their maximum absorption wavelength (464nm for MO,356 nm for OTTCH). The amount of adsorption is excluded when the degradation  efficiency is calculated. The degradation efficiency could be calculated according to  the following equation:  

RESULTS AND DISCUSSION

The crystal structure of the SnO2 as well as BiOI and 30wt%SnO2/BiOI  composites were analyzed using XRD, as displayed in Fig.1. All the XRD peaks of  pure SnO2 could well correspond to the tetragonal phase (JCPDS 01-0625).24 In pure  BiOI, the XRD peaks could be indexed to the tetragonal BiOI (JCPDS 73-2062).25 For the patterns of composites, some new typical diffraction peak can be observed at  2θ=26.2°, 33.6° and 51.4°, which were ascribed to the (110), (101) and (211) crystal  facet of SnO2 phase, respectively. It is noteworthy that with the SnO2 weight  increasing, the diffraction peak of SnO2 became more remarkable. However, the  diffraction peaks intensity of SnO2 is weak, which is due to lower crystallinity. The  average crystallite size of the pure SnO2 was calculated by Scherrer’s formula.