Author: Yazhi Wang Shuping Zhu Xiangrong Chen Yougen Tang Yifan Jiang Zhiguang Peng Haiyan Wang

Abstract

Mesoporous ZnO/TiO2 hollow microspheres have been successfully  synthesized by a facile one-step hydrothermal route without any template. The effect of  reactants, temperature and hydrothermal time on the morphology and photocatalytic  degradation activity has been investigated in details. The hollow microspheres with  average diameter of 0.5~2 μm showed time-dependent growth process and consisted of  nanoparticles with a diameter of 10-20 nm. The photocatalytic activity of ZnO/TiO2 hollow microspheres was evaluated via the photocatalytic degradation of methyl  orange (MO). Compared with the pristine TiO2 hollow microspheres or ZnO  nanosheets, the ZnO/TiO2 hollow microspheres produced significantly positive effect  on the band-gap energy shift (from 3.16 eV (TiO2) to 2.80 eV (ZnO/TiO2)) and  photocatalytic degradation (with a high apparent reaction rate of 10.19×10-2 min-1)  probably owing to the hybrid hollow microsphere morphology and their synergistic  catalytic effect.

Introduction

Among various semiconductor oxides, TiO2 has been considered as one of great  important materials because of their high photocatalytic activity, absence of toxicity,  chemical stability and relatively low cost [1-4]. Since Fujishima and Honda [5] found a  TiO2 photocatalytic anode could split water into hydrogen and oxygen as a clean source under ultraviolet illumination, it has received extensive attention for its destruction of  pollutants [6, 7] and production of hydrogen [8-10]. The photocatalytic ability of TiO2 can be significantly improved by modifications on its surface and coupling with other  sulfides or oxides [11-15]. Besides the wide application as electronic devices, ZnO is  also an important photocatalytic material with a versatile wide band gap (3.37 eV) [16]  compared with pristine TiO2 (3.2 eV), and the photodegradation mechanisms of ZnO  and TiO2 have been proven to be similar [2]. The ZnO/TiO2 composite not only  achieves efficient electron/hole pairs under illumination and higher reaction rates than  other semiconductor couples, but also introduces impurity and defect levels to the  forbidden band of TiO2 [15, 17-19], which may decrease the band gap of TiO2 and  reduce the excitation energy of the composite. On the other hand, the coupling can  prolong the duration period of the separation of electrons and holes, thereby reducing  their recombination chance.

Spherical hollow structures are receiving a great deal of attention because of the  lowest surface/volume ratio for aggregated products. With this regard, nanometer or  micrometer scale TiO2 [20, 21] and ZnO [22, 23] hollow spheres have been well  studied, which is beneficial to photocatalytic properties probably due to the large  specific surface area, low density and highly efficient light-harvesting abilities. Kunio  et al. concluded that the high photoreactivity of TiO2 hollow spheres could be attributed  to the multiple diffraction and reflection of the irradiation on the surface of the  materials [24].

Experimental

Photocatalytic measurements

The photocatalytic activities of ZnO/TiO2 composite hollow spheres for the  degradation of MO solution were carried out under UV light source (a 250 W  high-pressure mercury lamp with a mainly emitted wavelength of 365 nm) and visible  light source (CEL-HXF300; Beijing Aulight Co. Ltd, Beijing, China) with a 400 nm  cut off filter (λ>400 nm) emitted about 10 cm above the reaction break. The reaction  suspension was prepared by adding the sample (20 mg) into 200 mL of an MO (20  mg/L) solution under stirring. Prior to irradiation, the suspension was stirred  continuously in the dark for 30 min to ensure the establishment of an adsorption/  desorption equilibrium. At each intervals (5 min under UV light and 30 min under  visible light) of given irradiation time, 2 mL of the suspension was collected and  centrifuged to remove the particles. The dye concentration was analyzed with a  UV-vis spectrophotometer (Unico (Shanghai) Instrument Co., Ltd, UV-3802) by  recording variation of the absorption band maximum (465 nm) in the UV-vis  spectrum of MO.

Results and discussion

Fig. 1 shows the XRD patterns of as-synthesized TiO2, ZnO and ZnO/TiO2 composite. As can be seen, the sole TiO2 can be well indexed to anatase structure  (JCPDS No.65-5714) without any impurity diffraction peaks. Good crystallization is  also demonstrated from sharp diffraction peaks. Meanwhile, the XRD pattern of ZnO is  in good agreement with wurtzite hexagonal phase (JCPDS No.36-1451). As for  ZnO/TiO2 composite, it is interesting to note that the diffraction peaks of both anatase  TiO2 and wurtzite ZnO could be observed in the as-prepared composites, implying that  the composites consist of TiO2 and ZnO. And there is no other impurity phase from the  XRD diffraction peaks. This is similar to the reported results [11, 33]. The average  crystalline sizes of above three samples calculated using the diffraction peaks (101) of  anatase phase for TiO2 or wurzite phase for ZnO from Scherer’s formula [34] are in the  range of 10-20 nm which is consistent with the TEM result in Fig. 2c.

The SEM image in Fig. 2a shows the overview of the uniform spherical  morphology of ZnO/TiO2, and it can be clearly observed that the ZnO/TiO2 microspheres have a diameter of 0.5~2 μm. The hollow and nanoparticles aggregated  structure can be judged by a cracked ZnO/TiO2 microsphere in high magnification  image in Fig. 2b. TEM image in Fig. 2c can also indicate that the shell of the products is made up of the aggregated nanoparticles with a diameter of 10-20 nm (can also be seen  in inner image of Fig. 2c). The corresponding SAED pattern (Fig. 2d) reveals the  polycrystalline nature of the as-prepared microspheres and the diffraction rings from  inside to outside can be indexed to anatase (101), wurtzite (100), wurtzite (102),  anatase (105) and wurtzite (103). The HRTEM (Fig. 2e) from the edge of the observed  ZnO/TiO2 hollow sphere displays three types of clear lattice fringes, which are  embedded in amorphous matrix. Two sets of the fringes spacing are 0.25 nm and 0.28  nm, corresponding to the (101) and (100) planes of wurtzite ZnO. Another set of the  fringes spacing (0.35 nm), corresponds to the (101) lattice spacing of the anatase TiO2.  EDS analysis result can give the chemical composition of the ZnO/TiO2 hollow sphere  catalyst on the surface. As shown in Fig. 2f, the peaks of O, Zn and Ti can be clearly  seen while no other elements are detected besides Au (arising from the conductive gold  coating prior to imaging) and C (originating from the underlying conductive carbon  tapes), further supporting the existence of ZnO/TiO2 composite.

The optical band gaps of as-prepared TiO2 and ZnO/TiO2 hollow spheres were  studied by means of UV-vis optical absorbance spectra (Fig. 3). The absorption onsets  are determined by linear extrapolation from the inflection point of the curve to the  baseline. From the absorption spectra between 230 and 800 nm, the absorption onsets  of pure TiO2 and ZnO/TiO2 hollow spheres are around 392 nm and 442 nm,  corresponding to band gap energy of 3.16 eV and 2.80 eV. The reduction in band gap  energy of ZnO/TiO2 composite can be due to the synergistic effect between the  conduction band of ZnO and TiO2 [35].

Conclusion

In summary, mesoporous ZnO/TiO2 hollow spheres with an average diameter of  0.5-2 μm were prepared via a facile one-step template-free method, which is based on  the similarities in chemical properties between TiO2 and ZnO. In comparison with  TiO2, the band gap energy of ZnO/TiO2 hollow spheres was decreased from 3.16 eV to  2.80 eV because of the coupling of ZnO. Based on various morphologies and the K of  photodegradation observed in different conditions, we found that the addition of  fluorine and ammoniate, hydrothermal temperature and hydrothermal time are essential  to the structure and photocatalytic activity. Mesoporous ZnO/TiO2 hollow  microspheres exhibited much better photocatalytic degradation activity of MO in  comparison to bare ZnO nanosheets and TiO2 hollow spheres, because the coupling of  two sole simiconductor and red shift from near UV to the visible region could  significantly enhance the separation and transport of photogenerated electron-hole  pairs. A plausible time-dependant formation mechanism of ZnO/TiO2 hollow spheres  was proposed based on the detailed morphologies variations of as-prepared products in  different conditions.