pubs.acs.org/Langmuir © 2009 American Chemical Society
Uniform Carbon-Coated ZnO Nanorods: Microwave-Assisted Preparation, Cytotoxicity, and Photocatalytic Activity Yang Guo,† Hongsheng Wang,‡ Chuanglong He,‡ Lijun Qiu,‡ and Xuebo Cao*,† †
Key Laboratory of Organic Synthesis of Jiangsu Province and Department of Chemistry, Soochow University, Suzhou, Jiangsu 215123, China and ‡Institute of Biological Sciences and Biotechnology, Donghua University, Shanghai 201620, China Received October 23, 2008. Revised Manuscript Received January 10, 2009 This manuscript describes the accurate deposition of carbon on the surface of ZnO nanorods by a simple, microwave-assisted method and the studies on the cytotoxicity and photocatalytic activity of the C/ZnO hybrids. For the coating of carbon, the surface of the preformed ZnO nanorods were first modified by amino groups and then grafted by glucose, and finally they were irradiated in a microwave field to induce the transformation of glucose into carbon. With this method, the as-prepared carbon-coated product preserved the good dispersity and uniformity of the initial ZnO nanorods. Studies on the effects of carbon-coated ZnO nanorods and pure ZnO nanorods on cultured mouse fibroblast cells revealed that the coating of biocompatible carbon remarkably reduced the cytotoxicity of ZnO nanorods. In addition, benefiting from the synergy effect of carbon and ZnO, carbon-coated ZnO NRs also exhibited excellent photocatalytic activity toward the decomposition of methylene blue in a short time (∼14 min).
1. Introduction Zinc oxide (ZnO), a semiconductor with a direct bandgap of 3.37 eV and a large exciton binding energy of 60 meV, possesses unique electrical, optoelectronic, and luminescent properties. Because of these attractive properties, nanostructured ZnO, including particles, rods, wires, ribbons, and tubes have many important practical applications in catalysis, photoluminescence, and functional devices (e.g., solar cells, resonators, field-effect transistors, and gas sensors).1 Although pure ZnO has rich physical properties, there are also intense interests in the modification of nanostructured ZnO with another nanophase materials in order to impart new properties to it and to extend its applications in more fields. Currently, these functional nanophase materials include nanoparticles of noble metal, magnetic clusters, quantum dots, and so on. For instance, the combination of ZnO with γ-Fe2O3 generated bifunctional materials that possess magnetic and photoluminescent properties simultaneously;2 the attachment of CdTe and CdSe quantum dots onto ZnO improved the optical absorption ability toward visible lights of the latter as well as the photovoltaic performance.3 The epitaxial growth of Ag and Au on ZnO nanoparticles accelerated the separation process of photogenerated electrons and holes in ZnO and increased the photocatalytic activity of ZnO.4 *Corresponding author. E-mail:
[email protected]. (1) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, R829. (2) Cao, X. B.; Lan, X. M.; Guo, Y.; Zhao, C.; Han, S. M.; Wang, J.; Zhao, Q. R. J. Phys. Chem. C 2007, 111, 18958. (3) (a) Levy-Clement, C.; Tena-Zaera, R.; Ryan, M. A.; Katty, A.; Hodes, G. Adv. Mater. 2005, 17, 1512. (b) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Bary Carter, C.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 7, 1793. (c) Cao, X. B.; Chen, P.; Guo, Y. J. Phys. Chem. C 2008, 112, 20560. (4) (a) Wang, X.; Kong, X. G.; Zhang, H. J. Phys. Chem. C 2007, 111, 3836. (b) Zheng, Y. H.; Zheng, L. R.; Zhang, Y. Y.; Lin, X. Y.; Zheng, Q.; Wei, K. M. Inorg. Chem. 2007, 46, 6980. (c) Zheng, Y. H.; Chen, C. Q.; Zhang, Y. Y.; Lin, X. Y.; Zheng, Q; Wei, K. M.; Zhu, J. F. J. Phys. Chem. C 2008, 112, 10773.
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Amorphous carbon is known to widely serve as an intercomponent to be deposited on the surface of polymers, ceramics, metals, and oxides, for the purpose of modifying the corrosion resistance, thermal stability, adsorbability, or electronic properties of these materials.5 Especially, sputtering a layer of carbon to the surface of biomedical materials (e.g., implants, cell scaffolds) is a conventional method to improve the biocompatibility of the materials. If the surface of nanostructured ZnO is modified by carbon, the good properties of ZnO and carbon will be integrated into the hybrids, which is advantageous to overcoming some intrinsic defects of ZnO. In the first, carbon is a more inert material than ZnO and is less affected in physiological conditions. Thus, the coating of carbon on ZnO will make the latter more environmentally safe. Second, ZnO is an amphoteric compound and easily suffers from the corrosion of acid and base; the coating of carbon should help to increase the resistance. Finally, amorphous carbon and other related materials are low work function materials, and hence the combination of them with ZnO will modify the optical and electronic properties of ZnO.6 To date, the reports about the coating of ZnO nanostructures with carbon are rather limited; the common methods are related to the direct sputtering of carbon or annealing of the blends of ZnO and polymers. For instance, Liao et al. prepared ZnO nanowires coated with carbon by magnetron sputtering,6 and they found that the coated :: nanowires exhibited improved field emission properties; Ozkal et al. reported the coating of ZnO nanoparticles with carbon through the pyrolysis of polymers (e.g., poly(vinyl alcohol)).7 The sputtering method has the advantage of growing a homogeneous carbon layer on the surface of ZnO nanostructures, but special (5) Silva, R., Ed. Properties of Amorphous Carbon; Institution of Engineering and Technology: Hertfordshire, U.K., 2003. (6) Liao, L.; Li, J. C.; Wang, D. F.; Liu, C.; Liu, C. S.; Fu, Q.; Fa, L. X. Nanotechnology 2005, 16, 985. :: (7) Ozkal, B.; Jiang, W.; Yamamoto, O.; Fuda, K.; Nakagawa, Z. J. Mater. Sci. 2007, 42, 983.
Published on Web 2/27/2009
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Scheme 1. Diagrammatical Description of the Surface Modification of ZnO NRs by Amino Groups, the Graft of Glucose, and MicrowaveAssisted Deposition of Carbon on the Surface of ZnO NRs
equipment is required. On the other hand, the direct annealing of blends of ZnO and polymer can not ensure that the carbonization occurs accurately on the surface of ZnO, and hence the final product will be the mixture of carbon particles and ZnO, rather than the expected well-defined ZnO core/ carbon shell. Herein, we described a microwave-assisted carbonization strategy for the accurate deposition of carbon on ZnO. The principle of this method is described in Scheme 1. In the first step, the surface of the preformed ZnO nanorods (NRs) was functionalized with amino groups by interacting with (3-aminopropyl)triethoxysilane (APTES). Then, glucose molecules were grafted onto NRs through the reaction between surface amino groups and aldehyde groups of glucose. Finally, ZnO NRs grafted by glucose were irradiated in a microwave field to induce the transformation of glucose into carbon. Relative to conventional carbonization methods, this microwave irradiation route not only provides low-temperature, environmentally sustainable carbonization methodology but also does not bring bad influences on the preformed rodshaped pattern of ZnO. Studies on the influences of carboncoated ZnO NRs on the growth of mouse fibroblasts cells revealed that the cytotoxicity of ZnO NRs was remarkably reduced after the coating of carbon. In addition, the asprepared carbon-coated ZnO NRs also exhibited excellent photocatalytic activity toward the decomposition of methylene blue (MB), an organic dye.
2. Materials and Methods 2.1. Preparation of ZnO NRs Coated with Carbon. a. Synthesis of ZnO NRs. ZnO NRs were synthesized according to a previously published procedure by us.8 Briefly, 14.75 g of zinc acetate and 7.4 g of potassium hydroxide were dissolved with 60 and 32 mL of methanol, respectively. After that, the two solutions were mixed and stirred for 72 h at 60 °C. Then, solid ZnO was separated from the solution by centrifugation, washed repetitively with distilled water and alcohol, and dried at 40 °C under vacuum. b. Introduction of Amino Groups into ZnO NRs9. ZnO NRs (0.1 g) were first dispersed in 10 mL of fresh distilled dimethyl sulfoxide (DMSO) to form homogeneous colloidal solution by ultrasound. Then, 1.35 g of APTES was added to the solution. The solution was stirred for 3 h at 120 °C before finishing the reaction. The amino-functionalized ZnO NRs (8) (a) Guo, Y.; Cao, X. B.; Lan, X. M.; Zhao, C.; Xue, X. D.; Song, Y. Y. J. Phys. Chem. C 2008, 112, 8832. (b) Song, Y. Y.; Cao, X. B.; Guo, Y.; Chen, P.; Zhao, Q. R.; Shen, G. Z. Chem. Mater. 2009, 21, 68. (9) Kim, J. Y.; Osterloh, F. E. J. Am. Chem. Soc. 2005, 127, 10152.
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were separated from the solution by centrifugation and washed repetitively with absolute ethanol. c. Grafting Glucose onto ZnO NRs. The amino-functionalized ZnO NRs were dispersed in 6 mL of ethanol dissolved by 0.2 g of glucose. Then the suspension was stirred for about 3 h at 60 °C to promote the reaction of glucose with the surface amino groups of ZnO NRs. After that, solids were collected and washed three times with ethanol. Thus, ZnO NRs grafted by glucose were obtained. d. Microwave-Assisted Deposition of Carbon onto ZnO NRs. The above ZnO NRs grafted by glucose were transferred to a 100 mL quartz flask filled with 50 mL of glycerol. Glycerol was chosen as the solvent and thermal medium because its high viscosity allows ZnO NRs to be well dispersed in the solvent and prevents the aggregation of the carbonization product. Then, the flask was placed into a SinoMSZ-1 microwave reactor equipped with a temperatureprogrammed system. After purging by nitrogen for 10 min, the system was irradiated at 100 °C for 30 min. It was found that the solid product had changed its color from the initial white to brown, suggesting the existence of carbon component in the product. The brown solids were separated from the solution by centrifugation, washed several times with distilled water and ethanol, and dried in vacuum at 40 °C for 4 h. 2.2. Characterization. The phase compositions of the products were measured by an X’Pert PRO SUPER rA rotation anode X-ray diffractometer with Ni-filtered Cu KR radiation (λ = 1.54178 A˚). The morphologies of the products were observed by a FEI Tecnai G20 transmission electron microscope (TEM) and a Hitachi S-4700 fieldemitting scanning electron microscope (FESEM). High-resolution TEM (HRTEM) images were taken at 200 kV with a JEOL-2010 TEM. Fourier transform infrared (FT-IR) spectra were recorded on a Bio-Rad FTS 575C instrument. Thermogravimetric analysis (TGA) of the products was performed on a STA-409 PC thermal analyzer. Optical absorption spectra were measured by a Shimadzu 3150 UV-vis-near-infrared spectrophotometer. 2.3. Evaluations of Cytotoxicity of ZnO NRs and Carbon-Coated ZnO NRs10. ZnO NRs and carbon-coated ZnO NRs were sterilized at 121 °C for 20 min, and then dissolved in 50 mg/mL phosphate-buffered saline solution (PBS). For each NR, five solutions at concentrations of 20, 10, 2, 0.2, and 0.1 mg/mL were prepared. 3-(4,5-Dimethylthiazol-2-yl)2,5-diphenyltetrazoliumbromide (MTT) (Sigma) (5 mg/mL) was dissolved in PBS and filtrated with 0.22 μm film. All solutions were stored at 4 °C in dark. A mouse fibroblast cell line (3T3), obtained from Shanghai Institutes of Biological Sciences, Chinese Academy of Sciences, was cultured in Dulbecco’s modified Eagle’s medium (DMEM, GIBCO) with 10% fetal bovine serum (FBS, GIBCO) and grown at 37 °C in a 5% CO2 humidified incubator. The cells were plated in 96-well culture plates (Falcon) at a concentration of 4 103 cells/well, and allowed to grow to be 80% confluent. Then, in each well, the medium was replaced with 95 μL of fresh culture medium plus 5 μL solutions of the NRs mentioned above. After that, the final concentrations of the NRs were 1000, 500, 100, 10, and 5 μg/mL, respectively. The cells without adding any particles were used as the control, and the wells without cultured cells but with different density of particles were also tested as the blank contrast. The cells were treated by the NRs for 36 h, and each concentration was set (10) Lewinski, N.; Colvin, V.; Drezek, R. Small 2008, 4, 26.
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Figure 1. TEM (a) and FESEM (b) images of ZnO NRs. Inset in Figure 1a was the diameter distribution curve of the NRs, which was extracted from 200 rods. for four parallel samples during the process. The cell viability after adding NRs was determined by the MTT method. All samples in the 96-well plates were incubated with 100 μL of serum-free medium with 5 mg/mL MTT for 4 h. After that, the culture media were drawn off, 100 μL of DMSO was added, and the solutions were oscillated slightly for about 15 min. Then the 96-well plates were placed into an enzymelabeled instrument (MK3, Thermo, USA), and the absorbance at 492 nm for each well was measured. 2.4. Photocatalytic activity of Carbon-coated ZnO NRs. Carbon-coated ZnO NRs (0.04 g) or pure ZnO NRs were dispersed into 100 mL of aqueous MB solution (concentration: 2 mg/L) by ultrasound. The dispersion was stirred for 5 min. Then, the solution was exposed to a 200 W Xe lamp for a certain time. In the process of irradiation, circulating water was used to ensure that the temperature of the solution was constant. The supernatant was collected every 2 min by centrifugation. The concentration of MB in the separated supernatant was determined by a UV-vis spectrometer (Shimadzu 3150).
3.
Results and Discussions
Figure 1 shows the morphology characterization of the initial ZnO NRs for the coating of carbon. As seen from the TEM image (Figure 1a) and the FESEM image (Figure 1b), the product is composed of uniform NRs on a large scale. Almost no irregular particles were found in them. According to our previous study,8a these NRs were highly crystalline and preferentially grown along the common [001] direction. The inset in Figure 1a is the diameter distribution curve extracted from 200 NRs; the average diameter of the NRs was calculated to be 12.07 nm. These NRs were then functionalized with amino groups (-NH2) by interacting them with APTES, a silane-coupling agent. Active sites were generated on the surfaces of ZnO NRs after such treatment (Scheme 1), which could further react with the aldehyde group of glucose. To provide evidence of the modification of amino groups of the surface of ZnO NRs and the grafting of glucose, FT-IR spectra of the products were recorded (Figure 2). In Figure 2a, the IR absorption characteristics associated with APTES can be distinguished 4680
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Figure 2. IR spectra of ZnO NRs modified by APTES (a) and glucose (b).
easily:11 the bands at 2936 and 2880 cm-1 due to the stretching vibration of C-H bonds; the band at 1659 cm-1 due to the contribution of -NH2; the strong bands at 1130 and 1020 cm-1 due to the stretching vibration of C-N and Si-O bonds, respectively. As for the band at 1587 cm-1, it is caused by CdO bonds, and such a band is familiar in the IR spectrum of ZnO synthesized from a precursor of zinc acetate.8a,12 After reacting with glucose, the product shows a quite different IR spectrum (Figure 2b) relative to that modified by APTES. The major differences were marked by circles. The broadband around 3400 cm-1 was contributed by OH groups in glucose. The weak absorption band around 1616 cm-1 revealed that the reaction between glucose and APTES occurred, and a CdN bond was formed. In addition, the band at 1130 cm-1 was obviously weakened, which further confirmed the reaction between glucose and APTES and the formation of CdN bonds. The band at 1200 cm-1 was caused by the stretching vibration of C-O bonds in glucose. Consequently, according (11) Sainsbury, T.; Fitzmaurice, D. Chem. Mater. 2004, 16, 3780. (12) Cozzoli, P. D.; Curri, M. L.; Agostiano, A.; Leo, G.; Lomascolo, M. J. Phys. Chem. B 2003, 107, 4756.
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to IR spectroscopy results, it is believed that the glucose molecules were grafted onto ZnO NRs via the proposed route. Besides IR spectroscopy, the results of TGA (Figure 3) also provided evidence that APTES and glucose were linked to the ZnO NRs. Figure 3a-c corresponds to the TGA profiles (weight W vs temperature T plots) of pure ZnO NRs, ZnO NRs modified by APTES, and ZnO NRs grafted by glucose, respectively. For pure ZnO NRs, a total weight loss of 6.7% was observed in the temperature range of 25-700 °C (Figure 3a), in which the weight loss of 3.9% that occurred at temperatures lower than 125 °C was caused by the evaporation of free water, and the weight loss of 2.7% in the range of 250-400 °C may be attributed to the deadsorption of bound water. For ZnO NRs linked by APTES, the total weight loss was 11.9% (Figure 3b), and it mainly occurred between 200 and 600 °C, which is related to the thermolysis of APTES at high temperatures. When glucose molecules were grafted, a sharp weight descent at temperatures higher than 100 °C was observed (Figure 3c), and obviously it was caused by the decomposition of thermally instable glucose molecules. In this
Figure 3. TGA curves of ZnO NRs (a), and NRs modified by APTES (b) and glucose (c).
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case, the total weight loss of ZnO NRs had reached 24.1%. By comparison of curves b and c, it is learned that the weight percentage of glucose in the product was 12.2%. So, we can infer that, after the carbonization of this glucose, about 4.4% (wt %) carbon will be left in the product. Figure 4 exhibits TEM and FESEM images of the products carbonized in microwave field. The TEM image (Figure 4a) revealed that the product was still composed of well-dispersed NRs, like pure ZnO. But because of the deposition of carbon, their surface was rough relative to that of initial ZnO NRs. In addition, no byproducts such as carbon spheres or particles were seen during TEM observations, which suggested that the carbonization occurred accurately on the surface of the NRs. Figure 4b displayed the FESEM image of the carbonization product, which confirmed the TEM result that the products were uniform NRs on a large scale. Figure 4c was a representative HRTEM image taken on the edge of an individual carbon-coated NR. For the purpose of comparison, an HRTEM image taken on the edge of uncoated ZnO NRs is shown in Figure 4d. Obviously, uncoated ZnO NRs have a highly crystalline and smooth surface. However, after the coating of carbon, an amorphous carbon layer with a thickness of 2 nm was observed, as indicated by the arrow in Figure 4c. Consequently, HRTEM characterizations provide direct evidence that glucose was carbonized in situ on the surface of ZnO NRs via the present route. The structure of ZnO NRs coated with carbon was further characterized by X-ray diffraction (XRD, Figure 5), and, as a comparison, the XRD pattern of pure ZnO NRs was also included. As seen, all of the diffraction peaks in Figure 5 can be indexed to ZnO in a hexagonal structure; their strong and sharp features suggest that NRs coated with carbon were still well crystalline, like the pure ones. The difference between the XRD patterns of carbon-coated ZnO NRs and pure ZnO NRs is that the noise level of the former is high, which is
Figure 4. (a,b) TEM and FESEM images of carbon-coated ZnO NRs. (c,d) Representative HRTEM image of carbon-coated ZnO NRs and pure ZnO NRs, where the arrow indicates the carbon layer on the surface of ZnO NRs, and the insets are the selected-area electron patterns (SAED). Langmuir 2009, 25(8), 4678–4684
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Figure 5. Typical XRD patterns of pure ZnO NRs (top) and carbon-coated ZnO NRs (down). reasonable because the amorphous carbon layer can cause the scattering of X-rays. Figure 6 displays the relative activity levels of mouse fibroblast cells after incubating with different concentrations of ZnO NRs and carbon-coated ZnO NRs. For pure ZnO, despite the concentrations, they show an obvious toxicity, as also demonstrated by other researchers.13-16 They pointed out that ZnO could dissolve in culture medium and endosomes that would disrupt cellular homeostasis and lead to lysosomal and mitochondria damage and ultimately cell death.16,17 For carbon-coated ZnO NRs, they also exhibit strong toxicity at high particle concentrations (1000 and 500 μg/mL). However, their toxicity was significantly reduced as the concentration of the particles decreased. Instead, at particle concentrations of 100, 10, and 5 μg/mL, they were found to promote the growth of the cells. The reduced cytotoxicity in carbon-coated ZnO NRs was understandable because the carbon layer on the surface of ZnO NRs not only increases the biocompatibility of ZnO NRs but also slows the dissolution of ZnO in the culture medium.18,19 However, we are not clear as to why low concentrations of carbon-coated ZnO NRs can promote the growth of the cells. Further investigations will be required to clarify it. The optical property of carbon-coated ZnO NRs was studied by measuring their UV-vis absorption spectra. Carbon-coated ZnO NRs exhibited a narrow UV absorption and a wide visible absorption (Figure 7). The strong, narrow UV absorption band centered at 365 nm is the intrinsic exciton absorption of ZnO, whereas the wide visible absorption (13) Bechtel, D. B.; Bulla, L. A.Jr J. Bacteriol. 1976, 127, 1472. (14) Feldmann, C. Adv. Funct. Mater. 2003, 13, 101. (15) Lin, W. S.; Xu, Y.; Huang, C. C.; Ma, Y. F.; Shannon, K. B.; Chen, D. R.; Huang, Y. W. J. Nanopart. Res. 2009, 11, 25. (16) Huang, Z. B.; Zheng, X.; Yan, D. H.; Yin, G. F.; Liao, X. M.; Kang, Y. Q.; Yao, Y. D.; Huang, D.; Hao, B. Q. Langmuir 2008, 24, 4140. :: (17) Xia, T.; Kovochich, M.; Liong, M.; Madler, L.; Gilbert, B.; Shi, H. B.; Yeh, J. I.; Zink, J. I.; Nel, A. E. ACS Nano 2008, 2, 2121. (18) (a) Saito, Y. Carbon 1995, 33, 979. (b) Zalich, M. A.; Baranauskas, V. V.; Riffle, J. S.; Saunders, M.; St. Pierre, T. G. Chem. Mater. 2006, 18, 2648. (19) (a) Pauser, S.; Reszka, R.; Wagner, S.; Wolf, K. J. Anti-Cancer Drug Des. 1997, 12, 125. (b) Fernandez-Pacheco, R.; Ibarra, M. R.; Valdivia, J. G.; Marquina, C.; Serrate, D.; Romero, M. S.; Gutierrez, M.; Arbiol, J. NanoBiotechnology 2005, 1, 300.
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Figure 6. The relative activities level of a 3T3 cell treated by different concentrations of ZnO NRs and carbon-coated ZnO NRs. a, b, c, d, and e stand for concentrations of 1000, 500, 100, 10, and 5 μg/mL, respectively.
Figure 7. UV-vis absorption spectra of ZnO NRs and carboncoated ZnO NRs. should originate from carbon in the product.20 That is, the coating of a carbon layer leads to the extension of the optical absorption of ZnO NRs to the visible region. It means that more photons can be absorbed and utilized for the photocatalytic reaction if they were used as photocatalysts. (20) Sun, X. M.; Li, Y. D. Langmuir 2005, 21, 6019.
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Figure 8. (a,b) Temporal spectral changes of MB catalyzed by carbon-coated ZnO NRs and pure ZnO NRs. (c) Relationships of the concentration of MB with the irradiation time. Figure 8a,b displays the photodegradation behaviors of MB catalyzed by carbon-coated ZnO NRs and ZnO NRs, respectively. Figure 8c plots the dependence of the concentration of MB on the irradiation time. The optical absorption spectrum of MB features three bands centered at 664, 292, and 246 nm, of which the 664 nm absorption band arises from the chromophore in MB, and two bands in the ultraviolet region originate from benzene rings. Just as in previous reports,21 the band at 664 nm was selected to monitor the temporal concentration changes of MB. When carbon-coated ZnO NRs were used as the photocatalysts, the absorption band at 664 nm decreased sharply (by 72%) after irradiation for 2 min, while the bands at 292 and 246 nm disappeared, and a new band centered at 262 nm was generated (Figure 8a). These results demonstrate that the conjugated structure of MB was broken up and transformed into small aromatic intermediates.21 As the irradiation time increased, the absorption intensity of the bands at 262 nm also decreased smoothly, suggesting the further decomposition of the aromatic intermediates. Additionally, the complete photodegradation of MB in the presence of carbon-coated ZnO NRs only required 14 min. Under the same experimental conditions, only 62% of MB was degraded if pure ZnO NRs were used as photocatalysts (Figure 8b). Furthermore, it is worth noting that the absorption bands of MB at 292 and 246
nm were only weakened step by step, and no new bands appeared, unlike the case catalyzed by carbon-coated ZnO NRs. It suggests that the degradation of MB undergoes quite different routes under the catalysis of pure ZnO NRs and carbon-coated ZnO NRs. The studies of the structures of the intermediates and the photodegradation mechanisms of MB catalyzed by ZnO NRs and carbon-coated ZnO NRs are now in progress. The results will be reported in future work. In previous reports, the complete decomposition of MB usually requires 1 h or longer in the presence of TiO2 or ZnObased nanoparticles.21,22 Obviously, herein carbon-coated ZnO NRs show much higher photocatalytic activity than they do, which is due to the synergy effect of carbon and ZnO. First, amorphous carbon is known to have good absorbability, and hence MB can be enriched on the surface of photoactive ZnO, resulting in the acceleration of the rate of photocatalytic reaction. In the studies of the photocatalytic degradation of direct blue 53 (DB53), Sobana and Swaminathan found that the addition of commercial activated carbon into ZnO enhanced the adsorption of DB53 and lead to the increase of the photocatalytic efficiency of ZnO by a factor of 4.21.23 Second, the coating of amorphous carbon improve the absorption of ZnO toward visible light and ultraviolet light (Figure 7), which means that more photons
(21) (a) Zhang, T. Y.; Oyama, T.; Aoshima, A.; Hidaka, H.; Zhao, J. C.; Serpone, N. J. Photochem. Photobiol. A: Chem. 2001, 140, 163. (b) Houas, A.; Lachheb, H.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J. Appl. Catal. B: Environ. 2001, 31, 145.
(22) (a) Wang, X.; Hu, P.; Yuan, F. L.; Yu, L. J. J. Phys. Chem. C 2007, 111, 6706. (b) Ullah, R.; Dutta, J. J. Hazardous Mater. 2008, 156, 194. (23) Sobana, N.; Swaminathan, M. Sol. Ener. Mater. Sol. Cells 2007, 91, 727.
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can be absorbed and be utilized for the photocatalytic reaction. To confirm it, we studied the degradation behaviors of MB irradiated with only UV light or visible light. Since ZnO is a wide band gap semiconductor (Eg = 3.37 eV, corresponding to a wavelength of 368 nm), it only absorbs the light in the UV region. Under the irradiation of visible light or UV light, we found that carbon-coated ZnO NRs both exhibited a higher activity to induce the decomposition of MB than pure ZnO NRs, which means carbon-coated ZnO NRs can utilize the light more efficiently. Finally, amorphous carbon is a low work-function material (3.5-4.0 eV). The coating of carbon onto ZnO generates C/ZnO heterostructures that can direct photogenerated electrons and holes in ZnO to flow toward an opposite direction (the band structure diagrams of C/ZnO heterostructures have been described in detail in ref 6). For photocatalysts, the blocking of the recombination of photogenerated charges in them plays a key role in the enhancement of their photocatalytic activity.
4. Conclusion In summary, we have proposed a simple, microwaveassisted method for the accurate coating of carbon on the surface of ZnO NRs at low temperature. This method
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undergoes the process in a sequence consisting of the introduction of surface amino groups into ZnO NRs, the grafting of glucose, and microwave-assisted carbonization. With this method, the as-prepared carbon-coated product preserved the good dispersity and uniformity of initial ZnO NRs, which will facilitate their applications in technical fields. Studies on the effects of carbon-coated ZnO NRs and pure ZnO NRs on the cultured mouse fibroblast cells revealed that the coating of biocompatible carbon remarkably reduced the cytotoxicity of ZnO NRs. In addition, benefiting from the synergy effect of carbon and ZnO, carbon-coated ZnO NRs also exhibited excellent photocatalytic activity toward the decomposition of MB in a short time. Consequently, these as-prepared, low-toxicity, carbon-coated ZnO NRs should be ideal photocatalysts in the removal of dyes and other organic pollutants. Acknowledgment. This work was financially supported by the Key Laboratory of Organic Synthesis of Jiangsu Province (P. R.China), the Education Department of Jiangsu Province, Program of Innovative Research Team of Soochow University, and the National Natural Science Foundation of China (20601020).
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