Graphene Coating of TiO2 Nanoparticles Loaded on Mesoporous

Aug 19, 2010 - X. F. Qian , T. Kamegawa , K. Mori , H. X. Li , and H. Yamashita. The Journal of ... A. Maamor. International Journal of Environmental ...
0 downloads 0 Views 260KB Size
Download PDF
J. Phys. Chem. C 2010, 114, 15049–15053

15049

Graphene Coating of TiO2 Nanoparticles Loaded on Mesoporous Silica for Enhancement of Photocatalytic Activity Takashi Kamegawa, Daiki Yamahana, and Hiromi Yamashita* DiVision of Materials and Manufacturing Science, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ReceiVed: June 16, 2010; ReVised Manuscript ReceiVed: August 2, 2010

TiO2 nanoparticles supported on a mesoporous silica surface (TiO2/MCM-41) were selectively coated with graphene through the formation of surface complexes between TiO2 nanoparticles and 2,3-dihydroxynaphthalene and following carbonization under N2 flow. The pore structure as well as the high surface area of MCM-41 was retained even after carbonization at 1073 K. The selective graphene coating led to the enhancement of photocatalytic activities of TiO2/MCM-41 for the decomposition of 2-propanol in water compared with unmodified samples due to the appropriate adsorption properties of organics to transfer to the surface of TiO2 nanoparticles. 1. Introduction TiO2-based photocatalytic systems have attracted much attention because of their fascinating properties for dealing with the environmental problems, that is, purification of air as well as the wastewater polluted with dilute organic compounds.1-6 The design of reduction technology of volatile organic compounds (VOCs), such as aldehyde and aromatic compounds emitted from interiors of new buildings, which were related to the sick house syndrome, was also intensively performed using TiO2 photocatalysts due to their exceptional high activity, nontoxicity, chemical stability, and low cost. In this regard, for enhancement of catalytic activities of TiO2-based photocatalysts, modification of their bulk or surface properties was performed by doping of transition metals and nitrogen, ion implantation, surface coating with adsorbents, and anchoring of hydrophobic functional groups.7-11 The designs of composite materials were also frequently performed by anchoring of TiO2 nanoparticles on various adsorbents, such as zeolites, mesoporous silicas, and carbon materials.12-21 In these systems, adsorbents provide higher concentration environments of dilute organic compounds around supported TiO2 nanoparticles, and thereby, dilute organic compounds in air or water were efficiently decomposed into CO2 and H2O compared with those on naked TiO2 photocatalysts. It was also reported that the surface modification of adsorbents by using silylation reagents, which changes their surface properties from hydrophilic to hydrophobic through the decreasing amounts of surface hydroxyl groups, was effective for improving their adsorption properties of organic compounds.19-21 However, in some cases, functional moieties anchoring by using silylation reagents prevent the transfer of organic compounds to the catalytically active site within their pore or frameworks. On the other hand, recently, nanostructured carbon materials, such as carbon nanotubes and graphene sheets, offer exciting research fields.17,22-29 These materials were definitely applied as an efficient support in the fields of catalysts and photocatalysts owing to their structurally interesting as well as unusual properties. In the carbon materials and TiO2 composite systems, * To whom correspondence should be addressed. Fax/Tel: +81-6-68797457. E-mail: [email protected].

carbon exhibited the multifunctional properties, for example, the important roles for efficient adsorption of substances as well as preventing the recombination of photoformed electron-hole pairs.28,29 In the present study, we dealt with the coating method of TiO2 surfaces selectively by graphene using 2,3-dihydroxynaphthalene (DN) as a precursor and their spectroscopic investigations. Selective graphene coating of TiO2 nanoparticles supported on a mesoporous silica surface (TiO2/MCM-41) was performed to design more efficient composite systems. The effect of the graphene coating on their photocatalytic activities was evaluated through the comparative studies for degradation of 2-propanol in water as a model contaminant of water using naked TiO2/MCM-41 under UV light irradiation. 2. Experimental Section 2.1. Sample Preparation. Mesoporous silica (MCM-41) as a support of TiO2 nanoparticles was prepared by a hydrothermal synthesis method in accordance with previous literature,30 using trimethoxysilane as a silica source and hexadecyltrimethylammonium bromide as a template. After washing with ionexchanged water and dried at 373 K for 12 h, the product was calcined at 823 K for 5 h in air. Anchoring of TiO2 nanoparticles on MCM-41 was carried out by a simple impregnation method from an aqueous solution of titanium(IV) ammonium oxalate. After evaporation of water, recovered white powder was dried at 373 K for 12 h and then calcined at 773 K for 5 h (denoted as TiO2/MCM-41). The content of TiO2 nanoparticles was adjusted to 10 wt %. The selective graphene coating of the TiO2 nanoparticle surface loaded on MCM-41 was performed using 2,3-dihydroxynaphthalene (DN) as a precursor. An acetone solution of DN was poured into pre-evacuated TiO2/MCM-41 at 423 K, and the thus obtained mixture was stirred at 298 K for 1 h. The powder color of TiO2/MCM-41 was immediately changed from white to orange in this step. Next, the sample (DN-TiO2/MCM-41) was filtrated, repeatedly washed with acetone, dried under vacuum, and then heat-treated at 1073 K for 4 h under a N2 flow (100 mL/min). After carbonization, samples were denoted as Gn-TiO2/MCM-41, where n describes the graphene contents determined by thermogravimetrydifferential thermal analysis (n ) 0.05, 0.15, 0.4, 0.6 wt %,

10.1021/jp105526d  2010 American Chemical Society Published on Web 08/19/2010

15050

J. Phys. Chem. C, Vol. 114, No. 35, 2010

Kamegawa et al.

Figure 1. Diffuse reflectance UV-vis spectra of (a) MCM-41, (b) TiO2/MCM-41, (c) DN-TiO2/MCM-41, and (d, e) Gn-TiO2/MCM-41 (n ) (d) 0.15, (e) 0.6).

Figure 2. Diffuse reflectance UV-vis spectra of (a) DN/MCM-41 and (b) after washing with acetone. (c) Transmission UV-vis spectrum of an ethanol solution of DN.

which were calculated on the basis of the weight of TiO2/MCM41). For characterization and comparison of photocatalytic activity, unmodified TiO2/MCM-41 was also used after heat treatment at 1073 K in air. 2.2. Catalyst Characterization. The powder XRD measurements were performed using a Rigaku RINT 2500 diffractometer with Cu KR radiation (λ ) 1.5406 Å). Diffuse reflectance UV-vis spectra were recorded at 298 K with a Shimadzu UV2450A double-beam digital spectrophotometer. The Raman spectra were obtained on a JASCO NRS-3100 laser Raman spectrophotometer. The transmission electron microscopy (TEM) image was obtained with a Hitachi Hf-2000 FE-TEM equipped with a Kevex energy-dispersive X-ray detector operated at 200 kV. Nitrogen adsorption-desorption isotherms at 77 K and water adsorption isotherms at 298 K were recorded by using a BEL-SORP max (BEL Japan, Inc.) after degassing samples under vacuum at 423 K for 2 h. The relative water adsorption capacity was calculated by the adsorbed amount of water on each sample at P/P0 ) 0.4. Thermogravimetry-differential thermal analyses for determining the contents of graphene within Gn-TiO2/MCM-41 were performed using a TG-DTA2000S (MAC Science Co. Ltd.) from RT to 1073 K at a heating rate of 10 K/min under an air flow 50 mL/min. 2.3. Photocatalytic Reactions. The photocatalytic activity of the samples was evaluated by the degradation of 2-propanol diluted in water as a probe reaction. The fixed amounts of catalysts (50 mg) and an aqueous 2-propanol solution (25 mL, 2.6 mmol/L) were charged into a quartz reaction vessel. The initial amount of 2-propanol in the reaction mixture was equivalent to 65 µmol. After stirring under dark conditions for 30 min, the solution was bubbled by oxygen for another 30 min. Next, UV light irradiation was carried out using a 100 W high-pressure Hg lamp through a water filter (UV light intensity (λ ) 360 nm), 5 mW/cm2). The progress of the reactions was monitored by gas chromatography analysis (Shimadzu GC-14B with FID and TCD detector).

was clearly observed even after washing with acetone repeatedly (Figure 1, spectrum c) and assigned to the ligand-to-metal charge transfer (CT) of surface complexes formed between the surface Ti atoms and DN (surface-attached ligands). This optical change was known to be responsible for the excitation of electrons from the chelating ligand (e.g., aromatic substrates with hydroxyl or carboxyl binding groups) into the continuum conduction band of TiO2 nanoparticles.31-35 For instance, the structure model as well as the light absorption properties of surface complexes between 1,2-dihydroxybenzene (catechol) and TiO2 were clearly shown in a previously published paper.31 Considering the quite similar molecular structure of DN compared to catechol, the formation of DN-TiO2 surface complexes presumably occurred through the similar process, that is, the dehydration between Ti-OH with the hydroxyl groups in DN. On the other hand, no optical change was observed in the case of pure MCM-41 after the same treatment with an acetone solution of DN. As shown in Figure 2, spectrum a, DN supported on pure MCM-41 (DN/MCM-41) exhibited the almost same absorption band of DN in ethanol (Figure 2, spectrum c). These absorption bands completely disappeared after washing with acetone (Figure 2, spectrum b), suggesting that DN was easily removed from the surface of MCM-41 in contrast to the surface of TiO2. These results indicated that the DN as a precursor of graphene was selectively anchored on TiO2 nanoparticles supported on MCM-41 through the formation of stable surface complexes, which can be converted to graphene in the same places during the heat treatment in an inert atmosphere (Figure 3). Generally, alcohols and aromatic substrates were used as carbon sources to render carbon materials with an amorphous or graphite-like structure for desired forms.36,37 In fact, DNTiO2/MCM-41 became a gray powder after heat treatment for carbonization at 1073 K under a N2 flow. As shown in Figure 1, spectra d and e, a broad background absorption was observed in the whole range of visible light, attributed to the transformation of DN-TiO2 surface complexes to graphene over TiO2. The band-gap energy of the samples was estimated by the method reported in a previously published paper through a plot of the modified Kubelka-Munk function versus the energy of exciting light.38,39 The band-gap energy was determined as ca. 3.16 and 3.14 eV for TiO2/MCM-41 and G0.15-TiO2/MCM-41, respectively, whereas there are difficulties in the case of G0.6TiO2/MCM-41 due to the strong background absorption of formed graphene in the visible light region. These results suggested that the band-gap narrowing due to the substitutional doping of carbon within the TiO2 hardly occurred in our modification method.

3. Results and Discussion 3.1. Characterization of Prepared Catalysts. As shown in Figure 1, spectra a and b, TiO2/MCM-41 showed the absorption band below 380 nm corresponding to the band-gap energy of TiO2 nanoparticles loaded on MCM-41. When TiO2/MCM-41 was treated with an acetone solution of DN as a precursor of graphene (denoted as DN-TiO2/MCM-41), the powder color changed from white to orange, whereas the original DN solution exhibited no absorption in the visible light region (Figure 2, spectrum c). This visible light absorption band below 600 nm

Graphene Coating of TiO2 NPs on Mesoporous Silica

J. Phys. Chem. C, Vol. 114, No. 35, 2010 15051

Figure 3. Schematic diagram of the procedures for the preparation of Gn-TiO2/MCM-41.

Figure 4. Raman spectra of Gn-TiO2/MCM-41 (n ) (a) 0.05, (b) 0.15, (c) 0.4, and (d) 0.6).

Figure 4 shows Raman spectra of samples after carbonization at 1073 K under a N2 flow. Each sample exhibited two broad bands at around 1350 and 1580 cm-1, which were ascribed to disordered amorphous carbon (D band) and graphitic sp2 carbon (G band), respectively.40,41 The position and intensity ratio of these two bands were scarcely changed by increasing the graphene contents. The preparation of mesoporous silica evenly coated with 1-2 graphene sheets by using 2,3-dihydroxynaphthalene as a carbon source has also been reported.42 According to the results of previously published papers and the above characterizations, graphene might be selectively formed on the surfaces of TiO2 nanoparticles loaded on MCM-41 via formation of DN-TiO2 surface complexes (Figure 3). In the nitrogen adsorption-desorption measurement of TiO2/ MCM-41 calcined at 1073 K in air and G0.15-TiO2/MCM-41 (Figure 5A), the typical type IV isotherm was observed and the BET surface areas as well as pore volumes estimated from BJH analysis of the isotherms (shown in parentheses) were determined to be 782 m2/g (0.46 cm3/g) and 710 m2/g (0.49 cm3/g), respectively. The BET surface area as well as pore volume was almost same in the series of Gn-TiO2/MCM-41. TEM images of G0.15-TiO2/MCM-41 recorded along two different directions are also shown in Figure 6. In support of the pore size distribution curves (Figure 5B), TEM images showed the uniformity and provide direct evidence for the presence of ordered hexagonal arrays aligned in the one-dimensional channel even after carbonization treatment.

Figure 5. (A) Nitrogen adsorption/desorption isotherms and (B) pore size distribution curve of (a) TiO2/MCM-41 and (b) G0.15-TiO2/MCM41.

Figure 6. TEM images of G0.15-TiO2/MCM-41: (a) cross-sectional projection, (b) longitudinal projection.

The XRD pattern of TiO2/MCM-41 calcined at 1073 K in air and Gn-TiO2/MCM-41 modified with different amounts of graphene over TiO2 nanoparticles is shown in Figure 7. The typical XRD pattern due to the hexagonal arrangement of mesopores was clearly observed in the region of 2θ < 10° (Figure 7A). The XRD peak intensity and position of Gn-TiO2/ MCM-41 were scarcely changed even after carbonization at 1073 K (Figure 7A, patterns b-e), showing the correspondence with the results of nitrogen adsorption-desorption measurements. Moreover, the diffraction peaks assigned to TiO2 anatase phase were observed in the region of 2θ from 20° to 60° (Figure 7B). The crystal phase transformation of TiO2 from anatase to rutile hardly proceeded even in the case of TiO2/MCM-41 after calcinations at 1073 K in air, showing that the presence of graphene over TiO2 nanoparticles did not affect the crystallization of TiO2 nanoparticles loaded on MCM-41 in these systems.

15052

J. Phys. Chem. C, Vol. 114, No. 35, 2010

Kamegawa et al.

Figure 9. Effect of the contents of graphene on photocatalytic activities for degradation of 2-propanol (reaction time ) 6 h), water adsorption capacities, and adsorption properties of 2-propanol.

Figure 7. (A) Low-angle and (B) high-angle XRD patterns of (a) TiO2/ MCM-41 and (b-e) Gn-TiO2/MCM-41 (n ) (b) 0.05, (c) 0.15, (d) 0.4, and (e) 0.6).

Figure 8. Reaction time profiles for the degradation of 2-propanol diluted in water over G0.15-TiO2/MCM-41 under UV light irradiation.

3.2. Photocatalytic Reactions. Figure 8 shows the reaction time profiles of the photocatalytic degradation of 2-propanol diluted in water on G0.15-TiO2/MCM-41. This reaction was performed as the test reaction for investigating the effects of graphene coating of TiO2 nanoparticles loaded on MCM-41. Under UV light irradiation of the mixture of a photocatalyst and an aqueous solution of 2-propanol, the concentration of 2-propanol was gradually decreased with increasing the UV irradiation time. Diluted 2-propanol in water was decomposed into CO2, H2O, and the intermediate substance, such as acetone. Formed acetone is also finally decomposed into CO2 and H2O, which was similar to the reaction scheme observed in the previously reported literature.5,12 It was found that the recovered catalysts could be recycled without significant loss of the original catalytic activity. Among the prepared photocatalysts, 2-propanol was most efficiently decomposed on G0.15-TiO2/MCM-41, as shown in Figure 9. The adsorption capacity of water as well as 2-propanol was also investigated to clarify the effect of the graphene coating.

The carbon materials often exhibited hydrophobicity and work as a good adsorbent of organic molecules. As shown in Figure 9, with increases in the graphene content of samples, the relative water adsorption capacity, determined from the water adsorption isotherms at 298 K, was decreased as compared with unmodified TiO2/MCM-41. Correspondingly, the adsorption capacity of 2-propanol diluted in water, which was evaluated by an adsorption test of 2-propanol in the dark, was improved by increasing the graphene content (Figure 9). Considering the correlation of the adsorption behavior of water and 2-propanol, the surface hydrophobic properties strongly affected the adsorption property of substances. The graphene selectively formed on the TiO2 surface exhibited the good affinity of organic molecules and works as a good adsorbent for providing high concentration environments of target substances near the TiO2 surface anchored on MCM-41. However, the photocatalytic activity was decreased passing through the maximum in G0.15TiO2/MCM-41 and then decreasing in the case of samples at higher graphene contents. These results indicated that the suitable amount of graphene coating led to the enhancement of photocatalytic activities of TiO2/MCM-41 due to the appropriate adsorption properties of organics without the diffusion limitations to the catalytically active site. If adsorbed substances are tightly bound to adsorbent supports, they may not be involved in photodecomposition reactions. Moreover, further loading of graphene reduced the photocatalytic activity of TiO2/MCM-41, although the adsorption of organics was enhanced corresponding to the graphene content. The graphene selectively covered on the TiO2 surface was presumably shielding the incident light at higher graphene contents and thus minimizing the photoabsorption of TiO2.16,29 As a consequence, the suitable amount of graphene coating was effective for improving the functions of TiO2/MCM-41. 4. Conclusions Selective graphene coating of TiO2 nanoparticles anchored on MCM-41 was successfully achieved via formation of DN-TiO2 surface complexes. After carbonization at 1073 K under a N2 flow, TiO2 nanoparticles anchored on MCM-41 have an anatase crystalline structure regardless of the graphene content and MCM-41 also maintained the pore structure as well as the high surface area. Gn-TiO2/MCM-41 modified with a suitable amount of graphene exhibited higher photocatalytic performances compared with unmodified samples. The enhancement of photocatalytic activity was attributed to improving the adsorption properties of 2-propanol without adverse effects on

Graphene Coating of TiO2 NPs on Mesoporous Silica TiO2 of the diffusion limitations of the substances and the incident light absorption. Acknowledgment. The present work was supported by a Grant-in-Aid for Scientific Research (KAKENHI) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. 21760630). The authors appreciate Dr. Eiji Taguchi and Prof. Hirotaro Mori at the Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, for assistance with TEM measurements. References and Notes (1) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (2) Kamat., P. V. Chem. ReV. 1995, 93, 267. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (4) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (5) Kuwahara, Y.; Kamegawa, T.; Mori, K.; Yamashita, H. Curr. Org. Chem. 2010, 14, 616. (6) Palmisano, G.; Augugliaro, V.; Pagliaro, M.; Palmisano, L. Chem. Commun. 2007, 3425. (7) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (8) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (9) Yamashita, H.; Honda, M.; Harada, M.; Ichihashi, Y.; Anpo, M.; Hirao, T.; Itoh, N.; Iwamoto, N. J. Phys. Chem. B 1998, 102, 10707. (10) Yamashita, H.; Kawasaki, S.; Ichihashi, Y.; Harada, M.; Takeuchi, M.; Anpo, M.; Stewart, G.; Fox, M. A.; Louis, C.; Che, M. J. Phys. Chem. B 1998, 102, 5870. (11) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808. (12) Yamashita, H.; Nose, H.; Kuwahara, Y.; Nishida, Y.; Yuan, S.; Mori, K. Appl. Catal., A 2008, 350, 164. (13) Xu, Y.; Langford, C. H. J. Phys. Chem. B 1997, 101, 3115. (14) Ikeda, S.; Kobayashi, H.; Ikoma, Y.; Harada, T.; Torimoto, T.; Ohtani, B.; Matsumura, M. Phys. Chem. Chem. Phys. 2007, 9, 6319. (15) Inumaru, K.; Kasahara, T.; Yasui, M.; Yamanaka, S. Chem. Commun. 2005, 2131. (16) Tsumura, T.; Kojitani, N.; Izumi, I.; Iwashita, N.; Toyoda, M.; Inagaki, M. J. Mater. Chem. 2002, 12, 1391.

J. Phys. Chem. C, Vol. 114, No. 35, 2010 15053 (17) Williams, G.; Seger, B.; Kamat, P. V. ACS Nano 2008, 2, 1487. (18) Torimoto, T.; Okawa, Y.; Takeda, N.; Yoneyama, H. J. Photochem. Photobiol., A 1997, 103, 153. (19) Kuwahara, Y.; Kamegawa, T.; Mori, K.; Yamashita, H. Chem. Commun. 2008, 4783. (20) Kuwahara, Y.; Maki, K.; Matsumura, Y.; Kamegawa, T.; Mori, K.; Yamashita, H. J. Phys. Chem. C 2009, 113, 1552. (21) Kasahara, T.; Inumaru, K.; Yamanaka, S. Microporous Mesoporous Mater. 2004, 76, 123. (22) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (23) Loh, K. P.; Bao, Q.; Ang, P. K.; Yang, J. J. Mater. Chem. 2010, 20, 2277. (24) Xu, C.; Wang, X.; Zhu, J. J. Phys. Chem. C 2008, 112, 19841. (25) Wang, X.; Zhi, L. J.; Mullen, K. Nano Lett. 2008, 8, 323. (26) Rao, C. N. R.; Sood, A. K.; Voggu, R.; Subrahmanyam, K. S. J. Phys. Chem. Lett. 2010, 1, 572. (27) Lightcap, I. V.; Kosel, T. H.; Kamat, P. V. Nano Lett. 2010, 10, 577. (28) Kamat, P. V.; Bedja, I.; Hotchandanit, S. J. Phys. Chem. 1994, 98, 9137. (29) Tsubota, T.; Ono, A.; Murakami, N.; Ohno, T. Appl. Catal., B 2009, 91, 533. (30) Lim, M. H.; Stein, A. Chem. Mater. 1999, 11, 3285. (31) Moser, J.; Punchihewa, S.; Infelta, P. P.; Gra¨tzel, M. Langmuir 1991, 7, 3012. (32) Ikeda, S.; Abe, C.; Torimoto, T.; Ohtani, B. J. Photochem. Photobiol., A 2003, 160, 61. (33) Kim, S.; Choi, W. J. Phys. Chem. B 2005, 109, 5143. (34) Jankovic´, I. A.; Sˇaponjic´, Z. V.; Cˇomor, M. I.; Nedeljkovic´, J. M. J. Phys. Chem. C 2009, 113, 12645. (35) Tae, E. L.; Lee, S. H.; Lee, J. K.; Yoo, S. S.; Kang, E. J.; Yoon, K. B. J. Phys. Chem. B 2005, 109, 22513. (36) Kyotani, T.; Nagai, T.; Inoue, S.; Tomita, A. Chem. Mater. 1997, 9, 609. (37) Meng, Y.; Gu, D.; Zhang, F. Q.; Shi, Y. F.; Cheng, L.; Feng, D.; Wu, Z. X.; Chen, Z. X.; Wan, Y.; Stein, A.; Zhao, D. Y. Chem. Mater. 2006, 18, 4447. (38) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908. (39) Tauc, J.; Grigorovici, R.; Vanuc, A. Phys. Status Solidi 1996, 15, 627. (40) Chu, P. K.; Li, L. Mater. Chem. Phys. 2006, 96, 253. (41) Eklund, P. C.; Holden, J. M.; Jishi, R. A. Carbon 1995, 33, 959. (42) Nishihara, H.; Fukura, Y.; Inde, K.; Tsuji, K.; Takeuchi, M.; Kyotani, T. Carbon 2008, 46, 48.

JP105526D