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Direct solvethermal growth of hierarchical porous TiO2 nanosheets with high photocatalytic activity
Materials Letters 111 (2013) 161–164
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Direct solvethermal growth of hierarchical porous TiO2 nanosheets with high photocatalytic activity Wei Wang, Yaru Ni, Chunhua Lu n, Zhongzi Xu State Key Laboratory of Materials-Orient Chemical Engineering, College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 July 2013 Accepted 25 August 2013 Available online 31 August 2013
Hierarchical porous anatase TiO2 nanosheets exposed with the high-reactive {001} facets were synthesized by a simple hydrothermal method. Each TiO2 nanosheet is found to be composed of several much thinner {001} facets dominated nanosheets, hence the specific surface area and the percentage of the {001} facets were significantly enlarged. The thinner nanosheet also facilitates the transport of the photogenerated electrons and holes to the particle surface, resulting in the formation of a large amount of high-reactive radicals and holes for photocatalystic oxidation reaction. Remarkably, due to the special structure of as-prepared TiO2, it exhibits a superior photocatalytic degradation capacity for organic dyes than microsized anatase TiO2 sheet dominated by the {001} facets. & 2013 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Crystal growth Nanocrystals Solar energy materials
1. Introduction Owning to its high chemical stability, environmental friendly, low cost, and high photocatalytic activity, TiO2 has been widely investigated in photocatalysis [1,2]. Among the three crystalline forms (anatase, rutile and brookite) of TiO2, anatase is considered as the most reactive phase for photocatalysis. The photocatalytic efficiency of TiO2 is closely related to the size, shape, crystallinity, surface atomic structure, and degree of exposed reactive crystal facets [3,4]. In the last several years, researches on surface science have confirmed that the order of the average surface energies of anatase TiO2 is {001} (0.90 J/m2)4{100} (0.53 J/m2)4{101} {0.44 J/m2}. Both theoretical and experimental studies have found that the {001} facets are more reactive than the more thermodynamic stable {101} facets due to the unique atomic structures and the high average surface energy [5,6]. Today, a lot of works have been reported on the preparation of {001} facets dominated anatase TiO2 with different particle sizes, shapes, and optical properties [7–10]. However, most of the works are only focused on the powdered single TiO2 sheet, how to improve the specific surface area, adsorption ability, and percentage of the {001} facets of as-prepared photocatalysts still need further investigation. To solve this problem, an ideal way is to grow three-dimensional photocatalysts composed of TiO2 nanosheets with porous structure. In the present study, we introduce a simple solvethermal synthetic route to prepare hierarchical porous anatase TiO2 nanosheets. With this superiority structure, specific surface area
n
Corresponding author. Tel.: þ 86 25 83587252; fax: þ 86 25 83587220. E-mail address: [email protected] (C. Lu).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.105
and the percentage of the {001} facets are significantly enhanced. In comparison with {001} facets dominated TiO2 microcrystals, this type TiO2 gives higher photocatalytic activity in the degradation of Rhodamine (RhB). 2. Experimental In a typical synthesis process, tetrabutyl titanate (25 ml) was dropped into hydrofluoric acid (15 ml, 24 wt%) solution very slowly in order to release the generated heat (caution: during the process, it will undergo the hydrolysis of tetrabutyl titanate and further dissolve the hydrolysis product). The obtained transparent yellow solution was transferred into a Teflon-lined stainless steel vessel (60 ml) at 150 1C for 24 h (caution: a relative low temperature is necessary for the formation of the structure). After the hydrothermal reaction, the white product was centrifuged and washed with deionized water and ethanol. At last, the product was dried at 60 1C overnight. The resulting products were measured on an ARL X'TRA powder X-ray diffraction (XRD) diffractometer with Cu Kα radiation. The morphology and structure of as-prepared samples were conducted on a field emission scanning electron microscopy (FESEM) and a JEM2010 transmission electron microscopy (TEM) with an accelerating voltage of 200 kV. The surface area and pore volume of the prepared TiO2 photocatalysts were determined by the BET model using adsorption data and the BJH model based on desorption isotherms, respectively. For the photocatalytic reaction, 200 mg of photocatalyst was suspended in the RhB aqueous solution (10 5 M, 250 ml), which was then irradiated with a 500 W xenon lamp. The xenon lamp
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W. Wang et al. / Materials Letters 111 (2013) 161–164
was surrounded by the cooling water to maintain the reaction temperature. Prior to the photocatalytic reaction, the suspension was kept in the dark with magnetic stirring for 60 min to establish the adsorption–desorption equilibrium. During the measurement process, 3 ml of the solution was collected at certain time intervals (30 min) to remove the suspended photocatalyst, and the resulted solution was analyzed with the UV–visible spectrophotometer by recording the absorption band maximum (553 nm) of RhB. hydroxyl radicals ( OH) analysis was conducted in a method similar to the photocatalytic analysis except the RhB solution was replaced by terephthalic acid aqueous solution (5 10 4 M) with a concentration of NaOH (2 10 3 M). Fluorescence spectrum of the solution was recorded on a FL3-221 fluorescence spectrophotometer with excitation and emission wavelengths of 315 nm and 425 nm, respectively, at room temperature.
composed of a lot of hierarchical square shaped nanosheets, resulting in the formation of a three-dimensional structure which will give a high ability to absorb the pollutants for photocatalysis. A higher solvethermal temperature will result in the formation of single
3. Results and discussion As shown in Fig. 1a, all the diffraction peaks of the XRD pattern can be indexed to the anatase phase TiO2 (JCPD no. 21–1272). The diffraction peaks are very strong and sharp, indicating that the obtained TiO2 has high crystallinity. The typical FESEM image shown in Fig. 1b demonstrates that the TiO2 is uniform and flower-like. It is
Fig. 3. Nitrogen adsorption–desorption isotherm and pore size distribution curves (inset) of as-prepared TiO2.
Fig. 1. (a) XRD pattern and (b) typical FESEM image of as-prepared TiO2.
Fig. 2. (a) Typical TEM image of TiO2 and high resolution TEM images of region 1 (b), 2 (c), and 3 (d) shown in image a.
W. Wang et al. / Materials Letters 111 (2013) 161–164
163
Fig. 4. (a) SEM image of reference TiO2 (inset), photocatalytic degradation efficiency of RhB and (b) generated OH analysis under the xenon lamp irradiation with the presence of as-prepared TiO2 and reference TiO2.
nanosheet [11], which may due to the surface fusion of the {001} facets during the further crystallization process [12]. The TEM image shown in Fig. 2a indicates that each nanosheet is square shaped and with a side length of ca. 1 μm. To further investigate its structure, high resolution TEM images presented in Fig. 2b, c and d show the region 1, 2 and 3 of Fig. 2a, respectively. Fig. 2b clearly indicates that each nanosheet is composed of several much thinner nanosheets, indicating the hierarchical porous structure of as-prepared TiO2. Fig. 2c shows that the much thinner nanosheet has a thickness of ca. 5 nm and the spacing between each nanosheet is less than 1 nm. The observed lattice spacing (ca. 0.235 nm) of the nanosheets is accord with the (001) plane of anatase TiO2. The square surface has a lattice spacing of ca. 0.35 nm, corresponding to the (101) plane of anatase TiO2 (Fig. 2d) [13]. All these results indicate that the as-prepared TiO2 is composed of a lot of thinner nanosheets with exposed {001} facets. The small spacing between each nanosheet gives the hierarchical porous character and significantly enlarges the percentage of the {001} facets. Fig. 3 shows the adsorption–desorption isotherms and the corresponding pore size distribution curves (inset) of as-prepared TiO2. It gives a similar type-IV isotherms with a typical hysteresis loop belonged to type H3, which are typical characteristics of mesoporous materials [14]. BJH analysis yields pore size distribution curves with a peak at 11 nm, and the corresponding specific surface area is 73 m2/g. This result further confirms the deduction proposed in the FESEM and TEM analysis. The photocatalytic activity of as-prepared TiO2 is measured by the degradation of RhB under the xenon lamp irradiation (Fig. 4a). To illustrate the superiority of the structure, microsized TiO2 (with a particle size of ca. 2 μm, specific surface area of 4.8 m2/g) dominated by the {001} facets was synthesized according to the method reported method and was used as the reference photocatalyst (Fig. 4a, inset) [15]. It can be observed that the photocatalytic activity of TiO2 is much higher than that of reference TiO2. As discussed before, as-prepared TiO2 preserves a hierarchical porous structure which gives higher adsorption ability (surface area) than the microsized single sheet. On the other hand, the structure of each thinner nanosheet determines that the distance of the photogenerated electrons and holes move from the bulk to the particle surface is much shorter than the microsized one, thus the photogenerated electrons and holes can transport to the TiO2 surface in a short time. Bulk recombination is reduced and the electrons and holes can be applied for photocatalytic degradation of RhB efficiently. The OH radicals analysis presented in Fig. 4b can be used to confirm this deduction. A higher PL intensity means more photogenerated electrons were captured to form highreactive species [16,17]. These high-reactive species combined with the holes will favor the photocatalytic oxidation reaction.
Furthermore, it is well known that the {001} facet of anatase TiO2 has a higher oxidation ability than the {101} facets [18]. The hierarchical porous TiO2 can be considered as several {001} facets dominated single nanosheet used for photocatalysis, hence a much larger percentage of the {001} facets is obtained than the reference TiO2, which is another reason for its higher photocatalytic activity.
4. Conclusions We develop a simple hydrothermal route for the synthesis of hierarchical porous anatase TiO2 nanosheets dominated by the high-reactive {001} facets. This superiority structure can significantly enhance the specific surface area and the percentage of the {001} facets. The distance of the photogenerated electrons and holes transport from the bulk to the surface was also reduced. Hence more high-reactive species can be used for photocatalytic oxidation reaction. Based on these excellent properties, the photocatalytic results demonstrate that the as-prepared TiO2 is superior to the microsized anatase TiO2 sheet. This type of TiO2 can be used as a high-reactive photocatalyst to solve environmental pollutants.
Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant no. 20901040/B0111), the Key University Science Research Project of Jiangsu Province (No. 10KJA430016), the Innovation Foundation for Graduate Students of Jiangsu Province China (CXLX11_0346), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] Fujishima A, Zhang X, Tryk DA. Surface Science Reports 2008;63:515–82. [2] Thompson TL, Yates JT. Chemical Reviews 2006;106:4428–53. [3] Yang HG, Sun CH, Qiao SZ, Zou J, Liu G, Smith SC, et al. Nature 2008;453: 638–641. [4] Guo W, Zhang F, Lin C, Wang ZL. Advanced Materials 2012;24:4761–4. [5] Liu G, Yu JC, Lu GQ, Cheng H-M. Chemical Communications 2011;47:6763–83. [6] Wen CZ, Jiang HB, Qiao SZ, Yang HG, Lu GQ. Journal of Materials Chemistry 2011;21:7052–61. [7] Liu N, Zhao Y, Wang X, Peng H, Li G. Materials Letters 2013;102–103:53–5. [8] Ma XY, Chen ZG, Hartono SB, Jiang HB, Zou J, Qiao SZ, et al. Chemical Communications 2010;46:6608–10. [9] Wang H, Wang B-L, Ma S-Y. Chinese Chemical Letters 2013;24:260–3. [10] Xiang Q, Yu J, Jaroniec M. Physical Chemistry Chemical Physics 2011;13: 4853–4861. [11] Wang W, Lu CH, Ni YR, Su MX, Xu ZZ. Applied Catalysis B: Environmental 2012;127:28–35.
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[12] Yang XH, Li Z, Sun CH, Yang HG, Li CZ. Chemistry of Materials 2011;23: 3486–3494. [13] Dai Y, Cobley CM, Zeng J, Sun Y, Xia Y. Nano Letters 2009;9:2455–9. [14] Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquerol J, et al. Pure and Applied Chemistry Chimie Pure et Appliquée 1985;57:603–19. [15] Pan J, Liu G, Lu GQM, Cheng H-M. Angewandte Chemie International Edition 2011;50:2133–7.
[16] Wang W, Lu CH, Su MX, Ni YR, Xu ZZ. Chinese Journal of Catalysis 2012; 33:629–36. [17] Zhang J, Xi J, Ji Z. Journal of Materials Chemistry 2012;22:17700–8. [18] Ye L, Liu J, Tian L, Peng T, Zan L. Applied Catalysis B: Environmental 2013;134– 135:60–5.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Direct solvethermal growth of hierarchical porous TiO2 nanosheets with high photocatalytic activity Wei Wang, Yaru Ni, Chunhua Lu n, Zhongzi Xu State Key Laboratory of Materials-Orient Chemical Engineering, College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 July 2013 Accepted 25 August 2013 Available online 31 August 2013
Hierarchical porous anatase TiO2 nanosheets exposed with the high-reactive {001} facets were synthesized by a simple hydrothermal method. Each TiO2 nanosheet is found to be composed of several much thinner {001} facets dominated nanosheets, hence the specific surface area and the percentage of the {001} facets were significantly enlarged. The thinner nanosheet also facilitates the transport of the photogenerated electrons and holes to the particle surface, resulting in the formation of a large amount of high-reactive radicals and holes for photocatalystic oxidation reaction. Remarkably, due to the special structure of as-prepared TiO2, it exhibits a superior photocatalytic degradation capacity for organic dyes than microsized anatase TiO2 sheet dominated by the {001} facets. & 2013 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Crystal growth Nanocrystals Solar energy materials
1. Introduction Owning to its high chemical stability, environmental friendly, low cost, and high photocatalytic activity, TiO2 has been widely investigated in photocatalysis [1,2]. Among the three crystalline forms (anatase, rutile and brookite) of TiO2, anatase is considered as the most reactive phase for photocatalysis. The photocatalytic efficiency of TiO2 is closely related to the size, shape, crystallinity, surface atomic structure, and degree of exposed reactive crystal facets [3,4]. In the last several years, researches on surface science have confirmed that the order of the average surface energies of anatase TiO2 is {001} (0.90 J/m2)4{100} (0.53 J/m2)4{101} {0.44 J/m2}. Both theoretical and experimental studies have found that the {001} facets are more reactive than the more thermodynamic stable {101} facets due to the unique atomic structures and the high average surface energy [5,6]. Today, a lot of works have been reported on the preparation of {001} facets dominated anatase TiO2 with different particle sizes, shapes, and optical properties [7–10]. However, most of the works are only focused on the powdered single TiO2 sheet, how to improve the specific surface area, adsorption ability, and percentage of the {001} facets of as-prepared photocatalysts still need further investigation. To solve this problem, an ideal way is to grow three-dimensional photocatalysts composed of TiO2 nanosheets with porous structure. In the present study, we introduce a simple solvethermal synthetic route to prepare hierarchical porous anatase TiO2 nanosheets. With this superiority structure, specific surface area
n
Corresponding author. Tel.: þ 86 25 83587252; fax: þ 86 25 83587220. E-mail address: [email protected] (C. Lu).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.105
and the percentage of the {001} facets are significantly enhanced. In comparison with {001} facets dominated TiO2 microcrystals, this type TiO2 gives higher photocatalytic activity in the degradation of Rhodamine (RhB). 2. Experimental In a typical synthesis process, tetrabutyl titanate (25 ml) was dropped into hydrofluoric acid (15 ml, 24 wt%) solution very slowly in order to release the generated heat (caution: during the process, it will undergo the hydrolysis of tetrabutyl titanate and further dissolve the hydrolysis product). The obtained transparent yellow solution was transferred into a Teflon-lined stainless steel vessel (60 ml) at 150 1C for 24 h (caution: a relative low temperature is necessary for the formation of the structure). After the hydrothermal reaction, the white product was centrifuged and washed with deionized water and ethanol. At last, the product was dried at 60 1C overnight. The resulting products were measured on an ARL X'TRA powder X-ray diffraction (XRD) diffractometer with Cu Kα radiation. The morphology and structure of as-prepared samples were conducted on a field emission scanning electron microscopy (FESEM) and a JEM2010 transmission electron microscopy (TEM) with an accelerating voltage of 200 kV. The surface area and pore volume of the prepared TiO2 photocatalysts were determined by the BET model using adsorption data and the BJH model based on desorption isotherms, respectively. For the photocatalytic reaction, 200 mg of photocatalyst was suspended in the RhB aqueous solution (10 5 M, 250 ml), which was then irradiated with a 500 W xenon lamp. The xenon lamp
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was surrounded by the cooling water to maintain the reaction temperature. Prior to the photocatalytic reaction, the suspension was kept in the dark with magnetic stirring for 60 min to establish the adsorption–desorption equilibrium. During the measurement process, 3 ml of the solution was collected at certain time intervals (30 min) to remove the suspended photocatalyst, and the resulted solution was analyzed with the UV–visible spectrophotometer by recording the absorption band maximum (553 nm) of RhB. hydroxyl radicals ( OH) analysis was conducted in a method similar to the photocatalytic analysis except the RhB solution was replaced by terephthalic acid aqueous solution (5 10 4 M) with a concentration of NaOH (2 10 3 M). Fluorescence spectrum of the solution was recorded on a FL3-221 fluorescence spectrophotometer with excitation and emission wavelengths of 315 nm and 425 nm, respectively, at room temperature.
composed of a lot of hierarchical square shaped nanosheets, resulting in the formation of a three-dimensional structure which will give a high ability to absorb the pollutants for photocatalysis. A higher solvethermal temperature will result in the formation of single
3. Results and discussion As shown in Fig. 1a, all the diffraction peaks of the XRD pattern can be indexed to the anatase phase TiO2 (JCPD no. 21–1272). The diffraction peaks are very strong and sharp, indicating that the obtained TiO2 has high crystallinity. The typical FESEM image shown in Fig. 1b demonstrates that the TiO2 is uniform and flower-like. It is
Fig. 3. Nitrogen adsorption–desorption isotherm and pore size distribution curves (inset) of as-prepared TiO2.
Fig. 1. (a) XRD pattern and (b) typical FESEM image of as-prepared TiO2.
Fig. 2. (a) Typical TEM image of TiO2 and high resolution TEM images of region 1 (b), 2 (c), and 3 (d) shown in image a.
W. Wang et al. / Materials Letters 111 (2013) 161–164
163
Fig. 4. (a) SEM image of reference TiO2 (inset), photocatalytic degradation efficiency of RhB and (b) generated OH analysis under the xenon lamp irradiation with the presence of as-prepared TiO2 and reference TiO2.
nanosheet [11], which may due to the surface fusion of the {001} facets during the further crystallization process [12]. The TEM image shown in Fig. 2a indicates that each nanosheet is square shaped and with a side length of ca. 1 μm. To further investigate its structure, high resolution TEM images presented in Fig. 2b, c and d show the region 1, 2 and 3 of Fig. 2a, respectively. Fig. 2b clearly indicates that each nanosheet is composed of several much thinner nanosheets, indicating the hierarchical porous structure of as-prepared TiO2. Fig. 2c shows that the much thinner nanosheet has a thickness of ca. 5 nm and the spacing between each nanosheet is less than 1 nm. The observed lattice spacing (ca. 0.235 nm) of the nanosheets is accord with the (001) plane of anatase TiO2. The square surface has a lattice spacing of ca. 0.35 nm, corresponding to the (101) plane of anatase TiO2 (Fig. 2d) [13]. All these results indicate that the as-prepared TiO2 is composed of a lot of thinner nanosheets with exposed {001} facets. The small spacing between each nanosheet gives the hierarchical porous character and significantly enlarges the percentage of the {001} facets. Fig. 3 shows the adsorption–desorption isotherms and the corresponding pore size distribution curves (inset) of as-prepared TiO2. It gives a similar type-IV isotherms with a typical hysteresis loop belonged to type H3, which are typical characteristics of mesoporous materials [14]. BJH analysis yields pore size distribution curves with a peak at 11 nm, and the corresponding specific surface area is 73 m2/g. This result further confirms the deduction proposed in the FESEM and TEM analysis. The photocatalytic activity of as-prepared TiO2 is measured by the degradation of RhB under the xenon lamp irradiation (Fig. 4a). To illustrate the superiority of the structure, microsized TiO2 (with a particle size of ca. 2 μm, specific surface area of 4.8 m2/g) dominated by the {001} facets was synthesized according to the method reported method and was used as the reference photocatalyst (Fig. 4a, inset) [15]. It can be observed that the photocatalytic activity of TiO2 is much higher than that of reference TiO2. As discussed before, as-prepared TiO2 preserves a hierarchical porous structure which gives higher adsorption ability (surface area) than the microsized single sheet. On the other hand, the structure of each thinner nanosheet determines that the distance of the photogenerated electrons and holes move from the bulk to the particle surface is much shorter than the microsized one, thus the photogenerated electrons and holes can transport to the TiO2 surface in a short time. Bulk recombination is reduced and the electrons and holes can be applied for photocatalytic degradation of RhB efficiently. The OH radicals analysis presented in Fig. 4b can be used to confirm this deduction. A higher PL intensity means more photogenerated electrons were captured to form highreactive species [16,17]. These high-reactive species combined with the holes will favor the photocatalytic oxidation reaction.
Furthermore, it is well known that the {001} facet of anatase TiO2 has a higher oxidation ability than the {101} facets [18]. The hierarchical porous TiO2 can be considered as several {001} facets dominated single nanosheet used for photocatalysis, hence a much larger percentage of the {001} facets is obtained than the reference TiO2, which is another reason for its higher photocatalytic activity.
4. Conclusions We develop a simple hydrothermal route for the synthesis of hierarchical porous anatase TiO2 nanosheets dominated by the high-reactive {001} facets. This superiority structure can significantly enhance the specific surface area and the percentage of the {001} facets. The distance of the photogenerated electrons and holes transport from the bulk to the surface was also reduced. Hence more high-reactive species can be used for photocatalytic oxidation reaction. Based on these excellent properties, the photocatalytic results demonstrate that the as-prepared TiO2 is superior to the microsized anatase TiO2 sheet. This type of TiO2 can be used as a high-reactive photocatalyst to solve environmental pollutants.
Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant no. 20901040/B0111), the Key University Science Research Project of Jiangsu Province (No. 10KJA430016), the Innovation Foundation for Graduate Students of Jiangsu Province China (CXLX11_0346), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] Fujishima A, Zhang X, Tryk DA. Surface Science Reports 2008;63:515–82. [2] Thompson TL, Yates JT. Chemical Reviews 2006;106:4428–53. [3] Yang HG, Sun CH, Qiao SZ, Zou J, Liu G, Smith SC, et al. Nature 2008;453: 638–641. [4] Guo W, Zhang F, Lin C, Wang ZL. Advanced Materials 2012;24:4761–4. [5] Liu G, Yu JC, Lu GQ, Cheng H-M. Chemical Communications 2011;47:6763–83. [6] Wen CZ, Jiang HB, Qiao SZ, Yang HG, Lu GQ. Journal of Materials Chemistry 2011;21:7052–61. [7] Liu N, Zhao Y, Wang X, Peng H, Li G. Materials Letters 2013;102–103:53–5. [8] Ma XY, Chen ZG, Hartono SB, Jiang HB, Zou J, Qiao SZ, et al. Chemical Communications 2010;46:6608–10. [9] Wang H, Wang B-L, Ma S-Y. Chinese Chemical Letters 2013;24:260–3. [10] Xiang Q, Yu J, Jaroniec M. Physical Chemistry Chemical Physics 2011;13: 4853–4861. [11] Wang W, Lu CH, Ni YR, Su MX, Xu ZZ. Applied Catalysis B: Environmental 2012;127:28–35.
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[12] Yang XH, Li Z, Sun CH, Yang HG, Li CZ. Chemistry of Materials 2011;23: 3486–3494. [13] Dai Y, Cobley CM, Zeng J, Sun Y, Xia Y. Nano Letters 2009;9:2455–9. [14] Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquerol J, et al. Pure and Applied Chemistry Chimie Pure et Appliquée 1985;57:603–19. [15] Pan J, Liu G, Lu GQM, Cheng H-M. Angewandte Chemie International Edition 2011;50:2133–7.
[16] Wang W, Lu CH, Su MX, Ni YR, Xu ZZ. Chinese Journal of Catalysis 2012; 33:629–36. [17] Zhang J, Xi J, Ji Z. Journal of Materials Chemistry 2012;22:17700–8. [18] Ye L, Liu J, Tian L, Peng T, Zan L. Applied Catalysis B: Environmental 2013;134– 135:60–5.