Enhanced Photocatalytic Properties of Mesoporous SnO2 Induced by Low Concentration ZnO Doping Zhenhai
Wen,†,‡
Geng
Wang,†
Wu
Lu,†
Qiang
Wang,†
Qian
Zhang,†
and Jinghong
Li*,†
Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Tsinghua UniVersity, Beijing 100084, China, and College of Chemistry and Chemical Engineering, Graduate UniVersity of Chinese Academy of Sciences, Beijing 100039, China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1722-1725
ReceiVed NoVember 11, 2006; ReVised Manuscript ReceiVed May 19, 2007
ABSTRACT: Mesoporous SnO2 doped with a low concentration of ZnO (m-SnO2/ZnO), prepared through a hydrothermal method by using cetyltrimethylammonium bromide (CTAB) as a structure-directing agent, has been characterized by transmission electron microscopy, powder X-ray diffraction, and nitrogen adsorption-desorption technologies. It was found that doped m-SnO2 showed different properties compared with undoped mesoporous SnO2 (m-SnO2). The former not only had a larger Brunauer-EmmettTeller (BET) surface area but also showed a more stable mesoporous structure against high-temperature sintering than did the latter. Furthermore, m-SnO2/ZnO exhibited a significant enhancement of photocurrent and photocatalytic capability toward degrading rhodamine B (RhB) compared to undoped mesoporous SnO2. The improvement of photocatalytic activity for m-SnO2/ZnO might be attributed to its higher specific surface area and enhanced charge separation that derive from the coupling of ZnO with SnO2. Introduction
Experimental Procedures
Tin dioxide (SnO2), a wide band gap semiconductor (3.6 eV) with rutile-type crystal structure, has been widely investigated for various potential applications such as gas sensors,1 solar cells,2 lithium batteries,3 catalysis,4 and transparent conductive electrodes5 due to its unique optical, catalytic, and electrical properties. Two ways are usually utilized to improve the electrical, optical, and catalytic performance of SnO2. One is doping some extraneous elements into SnO2, which would induce a significant effect on its physical and chemical properties.6 The other is to synthesize nanostructure SnO2, which would contribute to a high specific surface area and quantumsize effects.7 Recently, the discovery of mesostructured materials provides a novel route to obtain powerful functional materials since the large surface area and nanosize active pore wall likely give them novel or improved functional properties superior to traditional solids. Mesoporous SnO2 (m-SnO2) has been previously prepared through various methods including sol-gel and sonochemical methods utilizing supramolecular templates.8 However, m-SnO2 synthesized through these methods usually had a pore wall with low crystallinity and poor thermal stability, which would result in the collapse of the mesostructures and intensively restrict their extensive application.9 Therefore, it is highly desirable to synthesize mesoporous SnO2 with a highly crystalline wall and considerable thermal stability. In this paper, we demonstrate a convenient and facile method to synthesize m-SnO2 and low concentration ZnO doped m-SnO2 (m-SnO2/ZnO) with a crystalline pore wall by utilizing cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. It was found that introducing zinc ion into the reaction system facilitated the production of a larger surface area and more stable structure against high-temperature sintering in comparison with pure phase m-SnO2. Moreover, the doped mesoporous composites produced evident enhancement of photocurrent and photocatalytic efficiency for degrading rhodamine B (RhB).
Materials and Synthesis. In a typical experiment to synthesize m-SnO2/ZnO, 0.55 g of CTAB and 0.08 g (2 mmol) of sodium hydroxide were added in 30 mL of a mixed solvent of alcohol and water to form a homogeneous, clear solution. Afterward, 1.5 mmol of tin chloride pentahydrate (SnCl4‚5H2O) and 0.5 mmol of zinc chloride (ZnCl2) were slowly added to the above solution. After the sample was agitated vigorously for 10 min, the mixture was then transferred to a 40 mL Teflon-lined stainless steel autoclave and heated at 160 °C for 15 h. The pure phase m-SnO2 was obtained as mentioned above except that ZnCl2 was added. The final precipitates were filtered and washed repeatedly with distilled water and absolute alcohol and dried at 60 °C. Characterization. The morphologies of the samples were observed utilizing a Hitachi model H-800 transmission electron microscope (TEM). X-ray fluorescence (XRF) spectrometric measurements were obtained on a Shimadzu XRF-1700 spectrometer. Powder X-ray diffraction (XRD) was performed on a Bruker D8-Advance X-ray powder diffraction. Specific surface areas were measured at 77 K by Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption (Shimadzu, Micromeritics ASAP 2010 Instrument). Measurement of Photocatalytic Properties. Photocurrent was measured at zero bias by using a two-electrode configuration and homebuilt experimental system, where the sintered m-SnO2 or m-SnO2/ZnO film served as the working electrode and a platinum wire was used as the counter electrode. A 500-W Xe lamp with a monochromator acted as light source, and the illumination area of the electrode was about 0.12 cm2. The photocurrent signal was collected utilizing a lock-in amplifier (Stanford instrument SR830 DSP). The monochromatic illuminating light intensity was about 15 µW/cm2 estimated with a radiometer (Photoelectronic Instrument Co. IPAS). All measurements were performed after bubbling electrolyte with N2 for 20 min and were automatically controlled by computer. The photocatalytic degradation of RhB on m-SnO2/ZnO and m-SnO2 was carried out in a home-built reactor. Typically, 0.01 g of the catalyst was added to 10 mL of the RhB solution (1 × 10-5 mol L-1). A 200-W Hg mercury discharged lamp with a water filter was used as a light source to initiate the reaction. The concentration of the RhB solution was monitored using a UVvis spectrophotometer.
* Corresponding author. Tel/Fax: +86-10-62795290. E-mail: jhli@ mail.tsinghua.edu.cn. † Tsinghua University. ‡ Graduate University of Chinese Academy of Sciences.
Results and Discussion TEM was utilized to observe the different morphologies of the as-prepared m-SnO2 and m-SnO2/ZnO. Figure 1A shows a representative TEM image of the as-prepared m-SnO2, from which one can observe disordered wormhole-like pores. After
10.1021/cg060801z CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007
Photocatalytic Properties of Mesoporous SnO2
Crystal Growth & Design, Vol. 7, No. 9, 2007 1723
Figure 3. X-ray diffraction patterns of m-SnO2/ZnO (A), m-SnO2 (B), m-SnO2/ZnO calcined at 450 °C for 5 h (C), and m-SnO2 calcined at 450 °C for 5 h (D).
Figure 1. TEM images of m-SnO2 (A), m-SnO2 calcined at 450 °C for 5 h (B), m-SnO2/ZnO (C), and m-SnO2/ZnO (D) calcined at 450 °C for 5 h; (E) SAED and (F) EDS of calcined m-SnO2/ZnO.
composites, which are dominated by Sn signals and Cu peaks (derived from the copper foil substrate) accompanying several small Zn peaks. The analysis results from EDS suggested that the as-synthesized composites, when controlling the raw material with a Sn/Zn mole ratio of 3:1, comprised 98.78 mol % SnO2 and 1.22 mol % ZnO. Additionally, X-ray fluorescence (XRF) spectrometric measurements further confirmed that the Zn doped in the SnO2 sample was about 1.31 mol %. No carbon element was found according to XRF spectrometry, indicating that CTAB was completely removed after washing and heat treatment. The analysis above demonstrated that only a small quantity of ZnO was doped in the composites under the experimental conditions even though a high concentration of Zn precursor was introduced into the reactive system. First, there were 1.5 mmol of SnCl4‚5H2O, 0.5 mmol of ZnCl2, and 2 mmol of NaOH in the raw materials; the following reaction previously would occur:
Sn4+ + 4OH- f Sn(OH)4 V
(1)
Zn2+ + 2 OH- f Zn(OH)2 V
(2)
Reaction (1) meant that 1 mol of Sn4+ would consume 4 mol of OH-. Obviously, most of the Sn4+ did not take part in the above reaction as there was a lack of OH-. The remaining Sn4+ would continue to hydrolyze according to reactions (3) and (4) under hydrothermal conditions. Figure 2. Schematic mechanism for the formation of m-SnO2 and m-SnO2/ZnO.
calcinations at 450 °C for 5 h, m-SnO2 exhibited less welldefined porous structures (Figure 1B), suggesting that most of the porous structures were destroyed due to the collapse of the mesostructure wall and aggregation of nanoparticles induced by heat treatment. A TEM image of m-SnO2/ZnO is presented in Figure 1C, which clearly shows that m-SnO2/ZnO had a pore structure similar to m-SnO2. In contrast to m-SnO2, the pore structure showed little change after calcinating at 450 °C for 5 h (Figure 1D). It is thus reasonable to conclude that the ZnOdoped composites had more stable mesostructures. Selected area electron diffraction (SAED) patterns for m-SnO2/ZnO exhibit several bright concentric rings, as shown in Figure 1E, suggesting that the composite had a polycrystalline structure attributable to various diffraction planes of tetragonal polycrystalline SnO2. Figure 1F shows typical energy-dispersive X-ray spectroscopy (EDS) results from the calcined m-SnO2/ZnO
Sn4+ + 4 H2O f Sn(OH)4 V + 4 H+
(3)
Sn(OH)4 f SnO2 + H2O
(4)
Actually, the pH of the mixed solution after the reaction was found to be acidic, further demonstrating that the above reaction occurred under hydrothermal conditions. The acidity of the mixed solution hindered the formation of a large amount of ZnO because the H+ released from reaction (3) could dissolve Zn(OH)2 formed by reaction (2). On the other hand, a spot of ZnO always existed in the final product although the as-repared samples had been washed repeatedly with highly pure water and alcohol. Therefore, the small quantity of ZnO could be formed during the nucleation and growth processes of the m-SnO2 rather than adsorbed on the surface of SnO2. A possible mechanism was proposed to explain the formation of SnO2 with unordered mesoporous structures (Figure 2). Because of hydrogen bonding and static interactions, the micelles of CTA+ were initially surrounded by OH- when
1724 Crystal Growth & Design, Vol. 7, No. 9, 2007
Figure 4. Nitrogen adsorption/desorption isotherms of m-SnO2/ZnO (A), m-SnO2 (B), m-SnO2/ZnO calcined at 450 °C for 5 h (C), and m-SnO2 calcined at 450 °C for 5 h (D).
sodium hydroxide was added. The Sn4+ ions added afterward reacted with OH- at the interface of the micelle and yielded a mesoporous SnO2 crystalline structure during hydrothermal treatment. The m-SnO2 with a disordered porous structure was formed after CTAB was removed through heat treatment. It was noted that when CTAB was controlled at a relative low concentration (
