TiO 2 -ZnO Composite Sphere Decorated with ZnO Clusters for Effective Charge Isolation in Photocatalysis Lun Pan, Guo-Qiang Shen, Jing-Wen Zhang, Xiao-Chu Wei, Li Wang, Ji-Jun Zou,* and Xiangwen Zhang* Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Collaborative Innovative Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ABSTRACT: TiO 2 and ZnO are extensively used photocatalysts, but their activity needs improvement due to rapid charge recombination. Herein, we designed and synthesized a novel structure of a TiO 2 -ZnO composite sphere decorated with ZnO clusters by a one-pot solvethermal method. TEM and EDX characterizations show this structure contains a TiO 2 core, TiO 2 - ZnO composite (type II heterojunction) surface layer, and surface c-axis ZnO clusters. The in situ Au and PbO 2 photodeposition shows that the photoinduced electrons and holes are driven to ZnO clusters and TiO 2 , respectively, attributed to the synergy of the type II heterojunction of TiO 2 -ZnO and high electron mobility of ZnO. The PL spectra confirm that such a structure is much more efficient in retarding the charge recombination than the sole TiO 2 -ZnO sphere. Importantly, this structure shows higher photoactivity in degradation of rhodamine B and isomerization of norbornadiene than pure TiO 2 , ZnO, and TiO 2 -ZnO composite spheres. 1. INTRODUCTION TiO 2 and ZnO are technologically important semiconductors and have been widely used in a variety of applications like solar cells, photocatalysis, and environmental remediation. 1-11 However, there are still some problems with these materials, among which the major one is fast charge recombination. The utilization of surface electron trappers and/or construction of heterojunctions are effective to solve this problem. 1,6,12-16 The composite consisting of TiO 2 and ZnO displays a profoundly improved charge isolating capability compared with pure ZnO and TiO 2 because their band levels match with each other to form a type II heterojunction that makes photoinduced electrons enriched in ZnO and holes confined in TiO 2 , 17,18 retarding the charge-pairs recombination rate and increasing their lifetime. This increases the availability of charge pairs on the surface of the photocatalyst and consequently an improvement in redox process can be expected. Generally, noble metals like Pt and Ag are used as surface electron trappers, 13,16,19 but they are cost ineffective. Recently, we reported that TiO 2 quantum dots (QDs) can work as electron trappers on TiO 2 nanosheets to make electrons isolated on QDs and holes on the nanosheet and significantly improve the photoactivity. 20 Actually, ZnO has higher electron mobility than TiO 2 (ZnO has mobility of 115- 155 cm 2 V -1 s -1 , while TiO 2 has mobility of 10 -5 cm 2 V -1 s -1 ), 21 suggesting that ZnO particles or clusters may also serve as the good electron trappers. It is expected that the combination of a type II heterojunction and surface electron trapper could produce more efficient charge transfer and isolation. 1,22 Herein, we synthesized a novel TiO 2 -ZnO composite sphere decorated with surface ZnO clusters. This structure not only possesses a type II heterojunc- tion (TiO 2 -ZnO composite) that can separate the electron and hole in ZnO and TiO 2 , but also has additional electron trappers (surface ZnO clusters) that can further isolate the photoinduced charges. As a result, the charge recombination is suppressed, and the photoactivity is improved greatly. 2. EXPERIMENTAL SECTION 2.1. Materials. Rhodamine B (RhB, 96%) and norbornadiene (NBD, 97%) were purchased from J&K Scientific, Ltd. Ethanol (chromatographic grade), tetrabutyl titanate (TBT, 99%), zinc acetate (reagent grade), HAuCl 4 (reagent grade), and Pb(NO 3 ) 2 (reagent grade) were purchased from Tianjin Guangfu Fine Chemical Research Institute. Deionized water (18.0 MΩ·cm) was used in all experiments. 2.2. Sample preparation. For the preparation of TiO 2 - ZnO composite, 0.5 mL TBT (1.5 mmol), 0.327 g (1.8 mmol) or 0.164 g (0.9 mmol) zinc acetate and 79 mL ethanol were mixed and solvothermally treated in a 100 mL Teflon-lined autoclave at 150 °C for 24 h, and the produced white powders were calcined at 500 °C in air for 1 h. Pure ZnO or TiO 2 was synthesized with the same procedure respectively using TBT and zinc acetate as precursors. 2.3. Characterization. XRD characterization was conducted using a D/MAX-2500 X-ray diffractometer equipped with Cu Kα radiation. SEM images were observed using a field-emission scanning electron microscope (Hitachi S-4800). High-resolution TEM observations were carried out with a Tecnai G 2 F-20 trans- mission electron microscope. Energy dispersive spectrum (EDS) characterization was performed with an EDX system attached to TEM. Steady-state photoluminescence (PL) spectra were measured by a Horiba JobinYvon Fluorolog3-21 with the excitation light at 325 nm. Surface composition and chemical states were analyzed with a PHI-5000 X-ray photoelectron spectroscope (XPS) equipped with Al Kα radiation, and the binding energy was calibrated by the C 1s peak (284.6 eV) of the contamination carbon. UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded with a Hitachi U-3010 spectrometer Received: April 18, 2015 Revised: June 25, 2015 Accepted: July 3, 2015 Published: July 3, 2015 Article pubs.acs.org/IECR © 2015 American Chemical Society 7226 DOI: 10.1021/acs.iecr.5b01471 Ind. Eng. Chem. Res. 2015, 54, 7226-7232