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Solvothermal synthesis of rice-like TiO2 nanocrystals with enhanced photocatalytic activity
Materials Letters 132 (2014) 1–3
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Solvothermal synthesis of rice-like TiO2 nanocrystals with enhanced photocatalytic activity Fei He a, Di Zhou a, Xiaoli Feng a, Chao Zhang a, Tao Li a,b,n, Guangxing Li a,b,n a
School of Chemistry & Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, Huazhong University of Science & Technology, Wuhan 430074, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 January 2014 Accepted 7 June 2014 Available online 14 June 2014
Rice-like TiO2 nanocrystals with high specific surface area are synthesized by a facile and mild solvothermal route without further thermal annealing. These nanocrystals are aggregated to form a secondary mesoporous structure. The TiO2 samples possess a large surface area of 269.9 m2/g, which is approximately 5 times higher than what is observed for P25. It is found that solvothermal temperature alters the crystallinity, surface area and porous structure of the sample. Photocatalytic activity of TiO2 nanocrystals for degradation of gaseous benzene is significantly higher than that of P25. The 1D structure and high porosity contribute to the superior photocatalytic activity. & 2014 Elsevier B.V. All rights reserved.
Keywords: Rice-like TiO2 nanocrystals Porous materials X-ray techniques Photocatalytic degradation Benzene
1. Introduction
2. Material and methods
TiO2 has been extensively researched because of high chemical stability, low cost, and low toxicity [1,2]. Generally, the performance of TiO2 photocatalyst is dependent on the surface area, mesoporosity, crystallinity and morphology [3]. Among the methods to prepare TiO2 photocatalysts, sol–gel method has been the most widely employed [4]. When TiO2 particles were obtained by this method, calcination temperature higher than 400 1C is required to realize the phase transition from amorphous to anatase [5]. However, this process will result in the increase in nanoparticle size and the decrease in specific surface area. The development of a simpler synthetic procedure including low temperature without post-calcination is highly demanded. In this work, we report a simple one-step solvothermal synthesis of rice-like TiO2 nanocrystals without the conventional heat treat process for the first time. The samples with lots of mesopores presenting between nanocrystals have a considerably large surface area. The photocatalytic properties of the products are evaluated by monitoring of photodegradation of gaseous benzene.
8 ml Tetrabutyl titanate (TBT) was added into a 56.80 g concentrated HCl solution to obtain solution A. 0.9 g Cetyltrimethyl ammonium bromide (CTAB) was added into 109.8 g distilled water to form aqueous solution B. Solution B was added into solution A and stirred for 1 h to form the aqueous TBT solution. The TBT solution and formamide were measured according to the volume ratio of 1:1 and put into a Teflon inner-liner stainless steel autoclave which was kept under 120 1C, 150 1C, 180 1C, and 200 1C for 20 h. The powder product was separated by centrifugation, washed with distilled water and absolute ethyl alcohol three times, respectively, and then dried at 333 K. The products are denoted as TX, in which X is the solvothermal temperature. XRD patterns were recorded on an X’Pert PRO X-ray diffractometer with Cu Kα radiation. TEM was taken with a Tecnai G20 transmission electron microscope using an accelerating voltage of 200 KV. BET surface area and porous structure were measured on a Quantachrome Autosorb-1-C-MS. FT-IR spectra were recorded on the Fourier transform infrared spectroscopy (Bruker EQVINOX 55). Photocatalytic activity was tested on a gas-phase batch reactor which was connected to a GC-9560 gas chromatograph. A 250 W, 365 nm UV lamp was used as a light source. TiO2 powder (0.5 g) was dispersed on a glass dish, and set on the bottom of the reactor. The Hg lamp was turned on until the concentration of CO2 remained unchanged, subsequently, 2 μL of benzene was injected into the reactor. Prior to photocatalytic oxidation, the benzene vapor was pre-adsorbed on the catalyst before illumination. Next, the UV lamp was turned on to trigger the photocatalytic reaction.
n Corresponding authors at: School of Chemistry & Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China. Tel.: þ 86 27 87543732; fax: þ 86 27 87543632. E-mail addresses: [email protected] (T. Li), [email protected] (G. Li).
http://dx.doi.org/10.1016/j.matlet.2014.06.034 0167-577X/& 2014 Elsevier B.V. All rights reserved.
2
F. He et al. / Materials Letters 132 (2014) 1–3
Fig. 1. XRD patterns of (a) T120, (b) T150, (c) T180 and (d) T200.
Fig. 3. (a) N2 sorption isotherms and (b) pore diameter distribution of T120, T150, T180 and T200.
Fig. 2. TEM images of T150 sample.
3. Results and discussion Fig. 1 shows that only an anatase structure (JCPDS 21-2272) of TiO2 in all samples. With increasing the solvothermal temperature, the anatase XRD peaks gradually become strong, indicating the anatase crystallinity increases and the corresponding crystallite size gradually becomes larger. With increasing the solvothermal temperature from 120 1C to 200 1C, the size of anatase phase crystallite increases from 5.7 to 9.6 nm (Table S1).
TEM image in Fig. 2a reveals that the obtained TiO2 nanocrystals exhibit rice-like shape with an average of 5 nm in width and an average of 35 nm in length. A close observation confirms that the products exhibit a mesoporous structure (Fig. 2b). The nanocrystals are connected with each other and the pores are interparticle voids. Usually, a slow hydrolysis of precursors could favor the formation of one-dimensional oxide nano-structures. The addition of formamide can increase the pH of the reaction solution and retard the hydrolysis rate of TBT during the solvothermal process. Moreover, the addition of formamide may reduce the size and the total number of CTAB surfactant micelles dramatically by a “salt-in” effect [6]. As a result, some CTAB micelles will dissolve into individual molecules and adsorb on the TiO2 seed surface, where they will act as template to form the rice-like morphology. In addition, CTAB also act as the template to form the porous structure. Fig. 3 displays the nitrogen adsorption–desorption isotherms and the pore-size distributions of the catalysts. The samples all have type IV isotherms, which are representative of mesoporous materials according to the IUPAC classification [7]. When increasing solvothermal temperature, the areas of hysteresis loops become smaller and the isotherms of the photocatalyst are shifted downward, indicating that the BET surface areas decrease [8]. T150 displays a large specific surface area (269.9 m2/g), which is approximately 5 times higher than what is observed for P25 [9]. It may be caused by the porous structure. When the solvothermal temperature increases to 200 1C, BET surface areas of the samples decrease to 177.6 m2/g (Table S1). This is because the nanopores collapsed. T150 have narrow and uniform pore sizes distribution which locates in the range of about 2–10 nm. Fig. 4 shows that the photocatalytic activity of TiO2 sample enhances when the solvothermal temperature increases from
F. He et al. / Materials Letters 132 (2014) 1–3
3
1637 cm 1 and strong broad band at 3442 cm 1 correspond to the hydroxyl group [10]. There is a significant reduction in the absorbance intensity of these superficial hydroxyl groups for the calcined sample, which is negative for the activity of these materials [11].
4. Conclusion We have synthesized rice-like TiO2 nanocrystals with a large surface area and no post-calcination purification process is required. They exhibit a remarkable photocatalytic activity which is attributed to the 1D TiO2 structure, intrinsic nanoporous and large surface area. This strategy is simple, cheap and massproductive, which may shed light on a new avenue for largescale synthesis of 1D nanomaterials for catalyst, energy and other applications.
Acknowledgments We acknowledge the National Basic Research Program of China (Grant no. 2009CB939705) and the National Natural Science Foundation of China (20973068) for financial support. We are also grateful to the Analytical and Testing Center of Huazhong University of Science and Technology, Wuhan, China.
Appendix A. Supporting information Fig. 4. (a) Comparison of the apparent rate constants and (b) CO2 concentration vs reaction time of TiO2 samples.
120 1C to 150 1C. T150 has the best photocatalytic activity whose k reaches the highest value of 14.8 10 3 min 1, which is significantly higher than that of P25. Note that the amount of CO2 produced with T150 is obviously higher than other samples. This is due to large surface area, small crystallite size and good crystallization. With increasing the solvothermal temperature to 200 1C, k decreases because the BET surface area decreases rapidly. Increasingly, when the samples are calcined, the photocatalytic activity decreases obviously (Fig. S1), which is attributed to the following fact that: on one hand, the porous structures have been destroyed; on the other hand, the great loss of surface hydroxyl groups. As shown in Fig. S2, FT-IR spectroscopy of the uncalcined sample prepared with and without CTAB is identical, indicating that CTAB have been cleared away. The relatively sharp band at
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.06.034. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Lee JH, Kim HE, Shin KH, Koh YH. Mater Lett 2011;65:1519–21. Wang Y, Chen GH, Shen QH, Zhang FM, Chen GL. Mater Lett 2014;116:27–30. Chen DH, Huang FZ, Cheng YB, Caruso RA. Adv Mater 2009;21:2206–10. Miki T, Nishizawa K, Suzuki K, Kato K. Mater Lett 2004;22–23:2751–3. Lin J, Lin Y, Liu P, Meziani MJ, Allard LF, Sun YP. J Am Chem Soc 2002;124:11514–8. Sun ZQ, Kim JH, Zhao Y, Bijarbooneh F, Malgras V, Lee YM, et al. J Am Chem Soc 2011;133:19314–7. Brunauer S, Deming LS, Deming WE, Teller E. J Am Chem Soc 1940;62: 1723–32. He F, Li JL, Li T, Li GX. Chem Eng J 2014;237:312–21. Ye MM, Chen ZL, Wang WS, Shen JM, Ma J. J Hazard Mater 2010;184:612–9. Guo H, Tian DX, Liu LX, Wang YP, Guo Y, Yang XJ. J Solid State Chem 2013;201:137–43. Szczepankiewicz SH, Colussi AJ, Hoffmann MR. J Phys Chem B 2000;104: 9842–50.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Solvothermal synthesis of rice-like TiO2 nanocrystals with enhanced photocatalytic activity Fei He a, Di Zhou a, Xiaoli Feng a, Chao Zhang a, Tao Li a,b,n, Guangxing Li a,b,n a
School of Chemistry & Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, Huazhong University of Science & Technology, Wuhan 430074, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 January 2014 Accepted 7 June 2014 Available online 14 June 2014
Rice-like TiO2 nanocrystals with high specific surface area are synthesized by a facile and mild solvothermal route without further thermal annealing. These nanocrystals are aggregated to form a secondary mesoporous structure. The TiO2 samples possess a large surface area of 269.9 m2/g, which is approximately 5 times higher than what is observed for P25. It is found that solvothermal temperature alters the crystallinity, surface area and porous structure of the sample. Photocatalytic activity of TiO2 nanocrystals for degradation of gaseous benzene is significantly higher than that of P25. The 1D structure and high porosity contribute to the superior photocatalytic activity. & 2014 Elsevier B.V. All rights reserved.
Keywords: Rice-like TiO2 nanocrystals Porous materials X-ray techniques Photocatalytic degradation Benzene
1. Introduction
2. Material and methods
TiO2 has been extensively researched because of high chemical stability, low cost, and low toxicity [1,2]. Generally, the performance of TiO2 photocatalyst is dependent on the surface area, mesoporosity, crystallinity and morphology [3]. Among the methods to prepare TiO2 photocatalysts, sol–gel method has been the most widely employed [4]. When TiO2 particles were obtained by this method, calcination temperature higher than 400 1C is required to realize the phase transition from amorphous to anatase [5]. However, this process will result in the increase in nanoparticle size and the decrease in specific surface area. The development of a simpler synthetic procedure including low temperature without post-calcination is highly demanded. In this work, we report a simple one-step solvothermal synthesis of rice-like TiO2 nanocrystals without the conventional heat treat process for the first time. The samples with lots of mesopores presenting between nanocrystals have a considerably large surface area. The photocatalytic properties of the products are evaluated by monitoring of photodegradation of gaseous benzene.
8 ml Tetrabutyl titanate (TBT) was added into a 56.80 g concentrated HCl solution to obtain solution A. 0.9 g Cetyltrimethyl ammonium bromide (CTAB) was added into 109.8 g distilled water to form aqueous solution B. Solution B was added into solution A and stirred for 1 h to form the aqueous TBT solution. The TBT solution and formamide were measured according to the volume ratio of 1:1 and put into a Teflon inner-liner stainless steel autoclave which was kept under 120 1C, 150 1C, 180 1C, and 200 1C for 20 h. The powder product was separated by centrifugation, washed with distilled water and absolute ethyl alcohol three times, respectively, and then dried at 333 K. The products are denoted as TX, in which X is the solvothermal temperature. XRD patterns were recorded on an X’Pert PRO X-ray diffractometer with Cu Kα radiation. TEM was taken with a Tecnai G20 transmission electron microscope using an accelerating voltage of 200 KV. BET surface area and porous structure were measured on a Quantachrome Autosorb-1-C-MS. FT-IR spectra were recorded on the Fourier transform infrared spectroscopy (Bruker EQVINOX 55). Photocatalytic activity was tested on a gas-phase batch reactor which was connected to a GC-9560 gas chromatograph. A 250 W, 365 nm UV lamp was used as a light source. TiO2 powder (0.5 g) was dispersed on a glass dish, and set on the bottom of the reactor. The Hg lamp was turned on until the concentration of CO2 remained unchanged, subsequently, 2 μL of benzene was injected into the reactor. Prior to photocatalytic oxidation, the benzene vapor was pre-adsorbed on the catalyst before illumination. Next, the UV lamp was turned on to trigger the photocatalytic reaction.
n Corresponding authors at: School of Chemistry & Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China. Tel.: þ 86 27 87543732; fax: þ 86 27 87543632. E-mail addresses: [email protected] (T. Li), [email protected] (G. Li).
http://dx.doi.org/10.1016/j.matlet.2014.06.034 0167-577X/& 2014 Elsevier B.V. All rights reserved.
2
F. He et al. / Materials Letters 132 (2014) 1–3
Fig. 1. XRD patterns of (a) T120, (b) T150, (c) T180 and (d) T200.
Fig. 3. (a) N2 sorption isotherms and (b) pore diameter distribution of T120, T150, T180 and T200.
Fig. 2. TEM images of T150 sample.
3. Results and discussion Fig. 1 shows that only an anatase structure (JCPDS 21-2272) of TiO2 in all samples. With increasing the solvothermal temperature, the anatase XRD peaks gradually become strong, indicating the anatase crystallinity increases and the corresponding crystallite size gradually becomes larger. With increasing the solvothermal temperature from 120 1C to 200 1C, the size of anatase phase crystallite increases from 5.7 to 9.6 nm (Table S1).
TEM image in Fig. 2a reveals that the obtained TiO2 nanocrystals exhibit rice-like shape with an average of 5 nm in width and an average of 35 nm in length. A close observation confirms that the products exhibit a mesoporous structure (Fig. 2b). The nanocrystals are connected with each other and the pores are interparticle voids. Usually, a slow hydrolysis of precursors could favor the formation of one-dimensional oxide nano-structures. The addition of formamide can increase the pH of the reaction solution and retard the hydrolysis rate of TBT during the solvothermal process. Moreover, the addition of formamide may reduce the size and the total number of CTAB surfactant micelles dramatically by a “salt-in” effect [6]. As a result, some CTAB micelles will dissolve into individual molecules and adsorb on the TiO2 seed surface, where they will act as template to form the rice-like morphology. In addition, CTAB also act as the template to form the porous structure. Fig. 3 displays the nitrogen adsorption–desorption isotherms and the pore-size distributions of the catalysts. The samples all have type IV isotherms, which are representative of mesoporous materials according to the IUPAC classification [7]. When increasing solvothermal temperature, the areas of hysteresis loops become smaller and the isotherms of the photocatalyst are shifted downward, indicating that the BET surface areas decrease [8]. T150 displays a large specific surface area (269.9 m2/g), which is approximately 5 times higher than what is observed for P25 [9]. It may be caused by the porous structure. When the solvothermal temperature increases to 200 1C, BET surface areas of the samples decrease to 177.6 m2/g (Table S1). This is because the nanopores collapsed. T150 have narrow and uniform pore sizes distribution which locates in the range of about 2–10 nm. Fig. 4 shows that the photocatalytic activity of TiO2 sample enhances when the solvothermal temperature increases from
F. He et al. / Materials Letters 132 (2014) 1–3
3
1637 cm 1 and strong broad band at 3442 cm 1 correspond to the hydroxyl group [10]. There is a significant reduction in the absorbance intensity of these superficial hydroxyl groups for the calcined sample, which is negative for the activity of these materials [11].
4. Conclusion We have synthesized rice-like TiO2 nanocrystals with a large surface area and no post-calcination purification process is required. They exhibit a remarkable photocatalytic activity which is attributed to the 1D TiO2 structure, intrinsic nanoporous and large surface area. This strategy is simple, cheap and massproductive, which may shed light on a new avenue for largescale synthesis of 1D nanomaterials for catalyst, energy and other applications.
Acknowledgments We acknowledge the National Basic Research Program of China (Grant no. 2009CB939705) and the National Natural Science Foundation of China (20973068) for financial support. We are also grateful to the Analytical and Testing Center of Huazhong University of Science and Technology, Wuhan, China.
Appendix A. Supporting information Fig. 4. (a) Comparison of the apparent rate constants and (b) CO2 concentration vs reaction time of TiO2 samples.
120 1C to 150 1C. T150 has the best photocatalytic activity whose k reaches the highest value of 14.8 10 3 min 1, which is significantly higher than that of P25. Note that the amount of CO2 produced with T150 is obviously higher than other samples. This is due to large surface area, small crystallite size and good crystallization. With increasing the solvothermal temperature to 200 1C, k decreases because the BET surface area decreases rapidly. Increasingly, when the samples are calcined, the photocatalytic activity decreases obviously (Fig. S1), which is attributed to the following fact that: on one hand, the porous structures have been destroyed; on the other hand, the great loss of surface hydroxyl groups. As shown in Fig. S2, FT-IR spectroscopy of the uncalcined sample prepared with and without CTAB is identical, indicating that CTAB have been cleared away. The relatively sharp band at
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.06.034. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Lee JH, Kim HE, Shin KH, Koh YH. Mater Lett 2011;65:1519–21. Wang Y, Chen GH, Shen QH, Zhang FM, Chen GL. Mater Lett 2014;116:27–30. Chen DH, Huang FZ, Cheng YB, Caruso RA. Adv Mater 2009;21:2206–10. Miki T, Nishizawa K, Suzuki K, Kato K. Mater Lett 2004;22–23:2751–3. Lin J, Lin Y, Liu P, Meziani MJ, Allard LF, Sun YP. J Am Chem Soc 2002;124:11514–8. Sun ZQ, Kim JH, Zhao Y, Bijarbooneh F, Malgras V, Lee YM, et al. J Am Chem Soc 2011;133:19314–7. Brunauer S, Deming LS, Deming WE, Teller E. J Am Chem Soc 1940;62: 1723–32. He F, Li JL, Li T, Li GX. Chem Eng J 2014;237:312–21. Ye MM, Chen ZL, Wang WS, Shen JM, Ma J. J Hazard Mater 2010;184:612–9. Guo H, Tian DX, Liu LX, Wang YP, Guo Y, Yang XJ. J Solid State Chem 2013;201:137–43. Szczepankiewicz SH, Colussi AJ, Hoffmann MR. J Phys Chem B 2000;104: 9842–50.