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Photocatalytic performance comparison of titania hollow spheres composed of nanoplates with dominant {001} facets and nanoparticles without dominant {001} facets
Catalysis Communications 66 (2015) 46–49
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Photocatalytic performance comparison of titania hollow spheres composed of nanoplates with dominant {001} facets and nanoparticles without dominant {001} facets Jian-Wen Shi a,⁎, Chong Xie a, Chi He b, Chang Liu a, Chen Gao a, Shenghui Yang a, Jian-Wei Chen c, Guodong Li d a
Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi'an Jiaotong University, Xi'an 710049, China Department of Environmental Science and Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China c Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361021, China d State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China b
a r t i c l e
i n f o
Article history: Received 2 February 2015 Received in revised form 15 March 2015 Accepted 16 March 2015 Available online 18 March 2015 Keywords: Titania Photocatalysis {001} facets Hollow spheres PPCPs
a b s t r a c t Anatase titania hollow spheres (THS) composed of nanoplates with dominant {001} facets and nanoparticles without dominant {001} facets were successfully prepared, respectively. The physicochemical properties of the two kinds of THS were characterized systematically, and their photocatalytic performances for the mineralization of ciprofloxacin, one of PPCPs, were compared in detail. The results showed that the former presented higher photocatalytic performance due to {001} facet exposure and better crystallinity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Pharmaceutical and Personal Care Products (PPCPs) are emerging contaminants, which have attracted increasing concerns in recent years [1]. Owing to their persistence against biological degradation and natural attenuation, conventional wastewater treatment technologies, such as activated sludge and filtration, cannot efficiently remove PPCPs from water [2]. Therefore, effective alternative technologies are highly desired to deal with the pollution resulted from PPCPs. As one of advanced oxidation processes, TiO2-mediated photocatalysis has been shown to be potentially advantageous for the degradation of organic pollutants as it can lead to complete mineralization [3–5]. However, the photocatalytic performance of TiO2 is still far below the level of practical application. To improve its photocatalytic performance, many approaches have been explored. Morphology control and reactive facet exposure are considered as two very promising strategies [6,7]. It is well known that the photocatalytic performance of TiO2 can be improved by controlling its morphology. Titania hollow spheres (THS) have been considered as a potential photocatalyst, owing to the excellent properties, such as low density, excellent surface permeability, tunable pore structure, as well as high light-harvesting efficiency [8,9]. On the other hand, anatase TiO2 crystals with high-energy {001} facet ⁎ Corresponding author. E-mail address: [email protected] (J.-W. Shi).
http://dx.doi.org/10.1016/j.catcom.2015.03.016 1566-7367/© 2015 Elsevier B.V. All rights reserved.
exposure have attracted significant attention because of their high chemical activity [10]. It is widely accepted that anatase TiO2 crystals with exposed {001} facets possess higher photocatalytic activity in comparison with TiO2 crystals without {001} facets [11]. In present work, we explore a path to improve the photocatalytic performance of TiO2 by organically fusing the two factors, hollow sphere structure and {001} facet exposure together. We successfully prepared THS composed of nanoplates with dominant {001} facets. To reveal the significant improvement of {001} facet exposure for photocatalytic performance, THS composed of nanoparticles without dominant {001} facets were also prepared. Subsequently, the physicochemical properties of two samples were characterized systematically, and their photocatalytic performances for the mineralization of ciprofloxacin (abbreviated as CIP, one of PPCPs) were compared in detail. 2. Experimental 2.1. The preparation of THS composed of anatase nanoplates with dominant {001} facets THS composed of anatase nanoplates with dominant {001} facets (abbreviated as THS-1) were prepared by a modified wet-chemical method in the presence of fluorine ions [12]. In a typical synthesis, 0.72 g of titanium sulfate and 0.1 g of ammonium fluoride were mixed with 20 mL of ethanol and 10 mL of distilled water under vigorous
J.-W. Shi et al. / Catalysis Communications 66 (2015) 46–49
a clear solution, and then 0.6 g of polyethylene glycol (molecular weight 2000) and 1.8 g of urea were added under vigorous stirring at 40 °C to form a transparent solution. The solution was then transferred into an 80 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. After reaction, the product was collected, and then washed 3–4 times with distilled water, and then dried at 80 °C for 12 h. A(116) A(220)
A(204)
A(105) A(211)
A(200)
A(103) A(112) A(004)
A(101)
Intensity (a.u.)
THS-1
2.3. Characterizations
THS-2
10
20
30
40
50
60
47
70
2 Theta (degree) Fig. 1. The XRD patterns of THS-1 and THS-2 (A: anatase).
stirring. Then the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. After reaction, the product was collected, and then washed 3–4 times with distilled water, and then dried at 80 °C for 12 h. 2.2. The preparation of THS composed of anatase nanoparticles without dominant {001} facets THS composed of anatase nanoparticles without dominant {001} facets (abbreviated as THS-2) were prepared by a fluorine-free hydrothermal process. In a typical process, 3.4 mL of titanium isopropoxide, 4.8 mL of glacial acetic acid were mixed with 30 mL of ethanol to form
XRD patterns were recorded at room temperature with a Rint-2000 Rigaku diffractometer with copper Ka1 radiation. SEM images were obtained by FEI Quanta 250 equipment. TEM was carried out on JEM2100 HT. XPS analyses were tested on an Escalab 250 with aluminum Ka radiation. UV–vis absorption spectra were recorded by a V-670 spectrophotometer equipped with an integrating sphere. The nitrogen adsorption was taken at 77 K using an Autosorb-iQ analyzer, and the BJH pore diameter distribution curves were obtained from desorption branch and specific surface areas were obtained according to the BET model. 2.4. Experimental procedures of photocatalytic mineralization The photocatalytic experiment was carried out in a photo reaction system reported in our previous publication [13]. A 300 W mediumpressure mercury lamp (major emission 365 nm), positioned in the center of a water-cooled quartz jacket, was used to offer ultraviolet irradiation. At the side of quartz jacket, a 50-mL cylindrical vessel was used as the reaction bottle to load reaction solution. The distance between lamp and reaction bottle was 40 mm. In the bottom of the reaction bottle, a magnetic stirrer was equipped to keep the catalyst suspended in the
(a)
(b)
(c)
(d)
Fig. 2. SEM images of THS-1 with different magnifications (a) and (b), and SEM images of THS-2 with different magnifications (c) and (d).
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J.-W. Shi et al. / Catalysis Communications 66 (2015) 46–49
solution. Photocatalyst powder (50 mg) was added into 50 mL CIP solution with the concentration of 50 mg/L to form suspension. During photocatalysis, the suspension was stirred continuously. At given time interval, a certain volume of suspension was taken out and immediately centrifuged to eliminate solid particles. Total organic carbon (TOC) of filtrate was measured by using a Shimadzu TOC-VCPH analyzer. 3. Results and discussion Fig. 1 displays the XRD patterns of THS-1 and THS-2. All peaks can be assigned to anatase TiO2. Compared with THS-2, the peaks in the pattern of THS-1 are stronger and sharper, suggesting anatase TiO2 in THS-1 possesses higher crystallinity, which can be ascribed to its crystallographic orientation under fluoride-mediated reaction conditions. Fig. 2 shows the SEM images of THS-1 and THS-2 with different magnification. Fig. 2a displays these spheres have a diameter of about 800 nm. From those broken spheres, we can clearly see the hollow structure. Furthermore, it can be observed from Fig. 2b, the enlarged view of Fig. 2a, that the spherical shells are composed of a large number of square nanoplates with dominant {001} facets, and these nanoplates have the size between 50 and 120 nm. The two flat and square surfaces can be ascribed to {001} facets and the eight isosceles trapezoidal surfaces can be assigned to {101} facets (as illustrated in the inset in Fig. 2b) [14], which can be further approved by a single TiO2 nanoplate (as marked with black square in Fig. 2b) dissociated from spherical shells. In order to confirm the exposure of {001} facets, a single TiO2 nanoplate was analyzed by TEM. As shown in Fig. A1 (Appendix), two clearly lattice fringes with a lattice spacing of 0.20 nm, corresponding to that of the {020} and {200} facets, and the 90° angle between the two facets can be found, implying that the two square facets (top and
529.8
bottom facets) are {001} facets [10]. For comparison, Fig. 2c shows that these spheres in THS-2 have a diameter of about 750 nm, and are composed of a large number of nanoparticles without dominant {001} facets due to a fluorine-free preparation environment. Furthermore, these nanoparticles have the size between 15 and 25 nm. A broken sphere clearly reveals the hollow nature of these spheres (Fig. 2d). The XPS survey spectra of both THS-1 and THS-2 (Fig. A.2, Appendix) exhibit prominent peaks of titanium, oxygen and carbon. The carbon peak is attributed to the residual carbon from the sample and adventitious hydrocarbon from the XPS instrument itself. Due to the addition of ammonium fluoride used as fluorine resource, F1s peak at about 684.5 eV can be detected in THS-1. Fig. A.2b (Appendix) shows the typical high-resolution XPS spectrum of the F1s region, taken on the surface of THS-1. According to previous literatures [15], the F1s peak at 684.5 eV is due to the surface fluoride (Ti–F). No signal of F− ions in the lattice (binding energy of 688.5 eV) appears. Figs. 3a and b show the high-resolution XPS spectra of O1s for THS-1 and THS-2, respectively. Two different kinds of function oxygen are observed with the peak at 532.1 eV corresponding to OH− groups adsorbed on the surface of sample (O\H) [16] and the peak at 529.8 eV corresponding to O\Ti bond in TiO2 lattice [17]. The percentage of oxygen existing as OH− groups in total oxygen can be calculated by the integral of the areas below the two peaks, and the result is 24.47% for THS-1 and 35.94% for THS-2, indicating more OH− groups are adsorbed onto the surface of THS-2 than that onto the surface of THS-1. Fig. 4a compares the optical response properties of THS-1 and THS-2. The light absorption edges of THS-1 and THS-2 completely overlap (350–400 nm), and their band gaps are estimated 3.17 eV [18,19], which is very close to the accepted value of anatase TiO2 (3.2 eV). Compared with THS-2, THS-1 presents stronger absorption in visible
(a)
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Binding energy (eV) Fig. 3. High-resolution XPS spectra of O1s for THS-1 (a) and THS-2 (b).
Fig. 4. UV–vis absorption spectra (a), and nitrogen adsorption–desorption isotherms and pore diameter distribution curves (b) of THS-1 and THS-2.
J.-W. Shi et al. / Catalysis Communications 66 (2015) 46–49
1.0
TOC/TOC0
carries [24,25]. THS-1 presents better crystallinity than THS-2, which may be the other important reason that THS-1 displays higher photocatalytic performance than THS-2.
THS-1 THS-2
0.8
49
4. Conclusions
0.6
In this communication, anatase titania hollow spheres (THS) composed of nanoplates with dominant {001} facets and nanoparticles without dominant {001} facets were successfully prepared, respectively. The former exhibited higher photocatalytic performance, which can be ascribed to two important advantages: {001} reactive facet exposure and better crystallinity.
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Acknowledgements
Irradiation time (h) Fig. 5. Relative TOC of CIP dependence on the irradiation time during photocatalytic mineralization.
light region (400–800 nm) and weaker absorption in UV region (200–350 nm). Fig. 4b shows that the isotherm of THS-1 is type IV with a hysteresis loop (IUPAC [20]), and the hysteresis curve belongs to type H3. Isotherms with type H3 loops have been reported for materials comprised of aggregates of platelike particles forming slitlike pores [21], which exactly coincides with our results that nanoplates assembled into hollow spheres. The isotherm of THS-2 also presents type IV with a hysteresis loop, and the hysteresis loop can be assigned to type H2, which can be ascribed to those pores formed by the accumulation of nanoparticles [22]. The BJH pore diameter distribution curves of THS-1 and THS-2 (the inset in Fig. 4b) reveal that the majority of pores in THS-2 are mainly between 4 and 10 nm. The BET specific surface areas of THS-1 and THS-2 are 21.67 and 89.77 m2 g−1, respectively. The photocatalytic activities of THS-1 and THS-2 were evaluated by the mineralization of CIP solution (Fig. 5). THS-2 shows a worse performance and only about 46% CIP was mineralized after 5 h reaction. Compared with THS-2, the photocatalytic performance of THS-1 is much higher because about 95% CIP was mineralized within the same reaction time. Compared with THS-1, more OH− groups are adsorbed onto the surface of THS-2, which can enhance the photocatalytic performance of TiO2 by trapping photogenerated holes to produce hydroxyl radicals. THS-1 and THS-2 share the same light absorption edge, and THS-2 presents stronger absorption in UV region. Furthermore, THS-2 possesses larger specific surface area in comparison with THS-1, which endows its stronger adsorption capacity. All these factors seem to prefigure that THS-2 possesses higher photocatalytic performance than THS-1. However, the mineralization experiment of CIP shows the opposite results. Therefore, it can be deduced that there must be other key factors which contribute significantly the photocatalytic performance of THS-1. Both theoretical and experimental studies have demonstrated that the catalytic reactivity of anatase TiO2 {001} facets is higher than that of the thermodynamically stable {101} surfaces [23]. THS-1 is composed of anatase nanoplates with dominant {001} facets. In contrast, THS-2 is composed of anatase nanoparticles without dominant {001} facets. The {001} facet exposure is one of the important reasons why THS-1 presents higher photocatalytic performance than THS-2. Crystallinity plays a very important role in the photocatalytic performance of TiO2. A better crystallization means the decrease of crystal defects, which are the recombination centers of photo-induced charge
This work was sponsored by the Fundamental Research Funds for the Central Universities, the Opening Project of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (2015-14), and the National Natural Science Foundation of China (51201175). The authors thank Ms. Juan Feng and Ms. Yanzhu Dai for their helps with SEM characterization carried out at the International Center for Dielectric Research (ICDR), Xi'an Jiaotong University. The valuable comments of anonymous reviewers are greatly appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2015.03.016. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
J.B. Ellis, Environ. Pollut. 144 (2006) 184. S. Esplugas, D.M. Bila, L.G.T. Krause, M. Dezotti, J. Hazard. Mater. 149 (2007) 631. J. Shi, J. Zheng, P. Wu, X. Ji, Catal. Commun. 9 (2008) 1846. N. Miranda-García, S. Suárez, B. Sánchez, J.M. Coronado, S. Malato, M. Ignacio Maldonado, Appl. Catal., B 103 (2011) 294. F.J. Rivas, F.J. Beltrán, A. Encinas, J. Environ. Manage. 100 (2012) 10. G. Cernuto, N. Masciocchi, A. Cervellino, G.M. Colonna, A. Guagliardi, J. Am. Chem. Soc. 133 (2011) 3114. W.-J. Ong, L.-L. Tan, S.-P. Chai, S.-T. Yong, A.R. Mohamed, Nanoscale 6 (2014) 1946. H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li, Y. Lu, J. Am. Chem. Soc. 129 (2007) 8406. J.H. Pan, X.W. Zhang, A.J. Du, D.D. Sun, J.O. Leckie, J. Am. Chem. Soc. 130 (2008) 11256. Y. Bai, P.-Y. Luo, P.-Q. Wang, J.-Y. Liu, Catal. Commun. 37 (2013) 45. G. Liu, H.G. Yang, J. Pan, Y.Q. Yang, G.Q. Lu, H.-M. Cheng, Chem. Rev. 114 (2014) 9559. S. Liu, J. Yu, M. Jaroniec, J. Am. Chem. Soc. 132 (2010) 11914. J.-W. Shi, X. Zong, X. Wu, H.-J. Cui, B. Xu, L. Wang, M.-L. Fu, ChemCatChem 4 (2012) 488. H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, G. Liu, S.C. Smith, H.M. Cheng, G.Q. Lu, Nature 53 (2008) 638. H.G. Yang, G. Liu, S.Z. Qiao, C.H. Sun, Y.G. Jin, S.C. Smith, J. Zou, H.M. Cheng, G.Q. Lu, J. Am. Chem. Soc. 131 (2009) 4078. P. Górska, A. Zaleska, E. Kowalska, T. Klimczuk, J.W. Sobczak, E. Skwarek, W. Janusz, J. Hupka, Appl. Catal., B 84 (2008) 440. H. Li, D. Wang, H. Fan, P. Wang, T. Jiang, T. Xie, J. Colloid Interface Sci. 354 (2011) 175. L. Wu, J.C. Yu, X. Fu, J. Mol. Catal. A 244 (2006) 25. J.-W. Shi, J.-W. Chen, H.-J. Cui, M.-L. Fu, H.-Y. Luo, B. Xu, Z.-L. Ye, Chem. Eng. J. 195–196 (2012) 226. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169. H. Liu, L. Zhang, N.A. Seaton, J. Colloid Interface Sci. 156 (1993) 285. M. Liu, L. Piao, W. Lu, S. Ju, L. Zhao, C. Zhou, H. Li, W. Wang, Nanoscale 2 (2010) 1115. H.G. Yu, J.G. Yu, B. Cheng, J. Lin, J. Hazard. Mater. 147 (2007) 581. J.G. Yu, G.H. Wang, B. Cheng, M.H. Zhou, Appl. Catal., B 69 (2007) 171.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Photocatalytic performance comparison of titania hollow spheres composed of nanoplates with dominant {001} facets and nanoparticles without dominant {001} facets Jian-Wen Shi a,⁎, Chong Xie a, Chi He b, Chang Liu a, Chen Gao a, Shenghui Yang a, Jian-Wei Chen c, Guodong Li d a
Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi'an Jiaotong University, Xi'an 710049, China Department of Environmental Science and Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China c Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361021, China d State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China b
a r t i c l e
i n f o
Article history: Received 2 February 2015 Received in revised form 15 March 2015 Accepted 16 March 2015 Available online 18 March 2015 Keywords: Titania Photocatalysis {001} facets Hollow spheres PPCPs
a b s t r a c t Anatase titania hollow spheres (THS) composed of nanoplates with dominant {001} facets and nanoparticles without dominant {001} facets were successfully prepared, respectively. The physicochemical properties of the two kinds of THS were characterized systematically, and their photocatalytic performances for the mineralization of ciprofloxacin, one of PPCPs, were compared in detail. The results showed that the former presented higher photocatalytic performance due to {001} facet exposure and better crystallinity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Pharmaceutical and Personal Care Products (PPCPs) are emerging contaminants, which have attracted increasing concerns in recent years [1]. Owing to their persistence against biological degradation and natural attenuation, conventional wastewater treatment technologies, such as activated sludge and filtration, cannot efficiently remove PPCPs from water [2]. Therefore, effective alternative technologies are highly desired to deal with the pollution resulted from PPCPs. As one of advanced oxidation processes, TiO2-mediated photocatalysis has been shown to be potentially advantageous for the degradation of organic pollutants as it can lead to complete mineralization [3–5]. However, the photocatalytic performance of TiO2 is still far below the level of practical application. To improve its photocatalytic performance, many approaches have been explored. Morphology control and reactive facet exposure are considered as two very promising strategies [6,7]. It is well known that the photocatalytic performance of TiO2 can be improved by controlling its morphology. Titania hollow spheres (THS) have been considered as a potential photocatalyst, owing to the excellent properties, such as low density, excellent surface permeability, tunable pore structure, as well as high light-harvesting efficiency [8,9]. On the other hand, anatase TiO2 crystals with high-energy {001} facet ⁎ Corresponding author. E-mail address: [email protected] (J.-W. Shi).
http://dx.doi.org/10.1016/j.catcom.2015.03.016 1566-7367/© 2015 Elsevier B.V. All rights reserved.
exposure have attracted significant attention because of their high chemical activity [10]. It is widely accepted that anatase TiO2 crystals with exposed {001} facets possess higher photocatalytic activity in comparison with TiO2 crystals without {001} facets [11]. In present work, we explore a path to improve the photocatalytic performance of TiO2 by organically fusing the two factors, hollow sphere structure and {001} facet exposure together. We successfully prepared THS composed of nanoplates with dominant {001} facets. To reveal the significant improvement of {001} facet exposure for photocatalytic performance, THS composed of nanoparticles without dominant {001} facets were also prepared. Subsequently, the physicochemical properties of two samples were characterized systematically, and their photocatalytic performances for the mineralization of ciprofloxacin (abbreviated as CIP, one of PPCPs) were compared in detail. 2. Experimental 2.1. The preparation of THS composed of anatase nanoplates with dominant {001} facets THS composed of anatase nanoplates with dominant {001} facets (abbreviated as THS-1) were prepared by a modified wet-chemical method in the presence of fluorine ions [12]. In a typical synthesis, 0.72 g of titanium sulfate and 0.1 g of ammonium fluoride were mixed with 20 mL of ethanol and 10 mL of distilled water under vigorous
J.-W. Shi et al. / Catalysis Communications 66 (2015) 46–49
a clear solution, and then 0.6 g of polyethylene glycol (molecular weight 2000) and 1.8 g of urea were added under vigorous stirring at 40 °C to form a transparent solution. The solution was then transferred into an 80 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. After reaction, the product was collected, and then washed 3–4 times with distilled water, and then dried at 80 °C for 12 h. A(116) A(220)
A(204)
A(105) A(211)
A(200)
A(103) A(112) A(004)
A(101)
Intensity (a.u.)
THS-1
2.3. Characterizations
THS-2
10
20
30
40
50
60
47
70
2 Theta (degree) Fig. 1. The XRD patterns of THS-1 and THS-2 (A: anatase).
stirring. Then the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. After reaction, the product was collected, and then washed 3–4 times with distilled water, and then dried at 80 °C for 12 h. 2.2. The preparation of THS composed of anatase nanoparticles without dominant {001} facets THS composed of anatase nanoparticles without dominant {001} facets (abbreviated as THS-2) were prepared by a fluorine-free hydrothermal process. In a typical process, 3.4 mL of titanium isopropoxide, 4.8 mL of glacial acetic acid were mixed with 30 mL of ethanol to form
XRD patterns were recorded at room temperature with a Rint-2000 Rigaku diffractometer with copper Ka1 radiation. SEM images were obtained by FEI Quanta 250 equipment. TEM was carried out on JEM2100 HT. XPS analyses were tested on an Escalab 250 with aluminum Ka radiation. UV–vis absorption spectra were recorded by a V-670 spectrophotometer equipped with an integrating sphere. The nitrogen adsorption was taken at 77 K using an Autosorb-iQ analyzer, and the BJH pore diameter distribution curves were obtained from desorption branch and specific surface areas were obtained according to the BET model. 2.4. Experimental procedures of photocatalytic mineralization The photocatalytic experiment was carried out in a photo reaction system reported in our previous publication [13]. A 300 W mediumpressure mercury lamp (major emission 365 nm), positioned in the center of a water-cooled quartz jacket, was used to offer ultraviolet irradiation. At the side of quartz jacket, a 50-mL cylindrical vessel was used as the reaction bottle to load reaction solution. The distance between lamp and reaction bottle was 40 mm. In the bottom of the reaction bottle, a magnetic stirrer was equipped to keep the catalyst suspended in the
(a)
(b)
(c)
(d)
Fig. 2. SEM images of THS-1 with different magnifications (a) and (b), and SEM images of THS-2 with different magnifications (c) and (d).
48
J.-W. Shi et al. / Catalysis Communications 66 (2015) 46–49
solution. Photocatalyst powder (50 mg) was added into 50 mL CIP solution with the concentration of 50 mg/L to form suspension. During photocatalysis, the suspension was stirred continuously. At given time interval, a certain volume of suspension was taken out and immediately centrifuged to eliminate solid particles. Total organic carbon (TOC) of filtrate was measured by using a Shimadzu TOC-VCPH analyzer. 3. Results and discussion Fig. 1 displays the XRD patterns of THS-1 and THS-2. All peaks can be assigned to anatase TiO2. Compared with THS-2, the peaks in the pattern of THS-1 are stronger and sharper, suggesting anatase TiO2 in THS-1 possesses higher crystallinity, which can be ascribed to its crystallographic orientation under fluoride-mediated reaction conditions. Fig. 2 shows the SEM images of THS-1 and THS-2 with different magnification. Fig. 2a displays these spheres have a diameter of about 800 nm. From those broken spheres, we can clearly see the hollow structure. Furthermore, it can be observed from Fig. 2b, the enlarged view of Fig. 2a, that the spherical shells are composed of a large number of square nanoplates with dominant {001} facets, and these nanoplates have the size between 50 and 120 nm. The two flat and square surfaces can be ascribed to {001} facets and the eight isosceles trapezoidal surfaces can be assigned to {101} facets (as illustrated in the inset in Fig. 2b) [14], which can be further approved by a single TiO2 nanoplate (as marked with black square in Fig. 2b) dissociated from spherical shells. In order to confirm the exposure of {001} facets, a single TiO2 nanoplate was analyzed by TEM. As shown in Fig. A1 (Appendix), two clearly lattice fringes with a lattice spacing of 0.20 nm, corresponding to that of the {020} and {200} facets, and the 90° angle between the two facets can be found, implying that the two square facets (top and
529.8
bottom facets) are {001} facets [10]. For comparison, Fig. 2c shows that these spheres in THS-2 have a diameter of about 750 nm, and are composed of a large number of nanoparticles without dominant {001} facets due to a fluorine-free preparation environment. Furthermore, these nanoparticles have the size between 15 and 25 nm. A broken sphere clearly reveals the hollow nature of these spheres (Fig. 2d). The XPS survey spectra of both THS-1 and THS-2 (Fig. A.2, Appendix) exhibit prominent peaks of titanium, oxygen and carbon. The carbon peak is attributed to the residual carbon from the sample and adventitious hydrocarbon from the XPS instrument itself. Due to the addition of ammonium fluoride used as fluorine resource, F1s peak at about 684.5 eV can be detected in THS-1. Fig. A.2b (Appendix) shows the typical high-resolution XPS spectrum of the F1s region, taken on the surface of THS-1. According to previous literatures [15], the F1s peak at 684.5 eV is due to the surface fluoride (Ti–F). No signal of F− ions in the lattice (binding energy of 688.5 eV) appears. Figs. 3a and b show the high-resolution XPS spectra of O1s for THS-1 and THS-2, respectively. Two different kinds of function oxygen are observed with the peak at 532.1 eV corresponding to OH− groups adsorbed on the surface of sample (O\H) [16] and the peak at 529.8 eV corresponding to O\Ti bond in TiO2 lattice [17]. The percentage of oxygen existing as OH− groups in total oxygen can be calculated by the integral of the areas below the two peaks, and the result is 24.47% for THS-1 and 35.94% for THS-2, indicating more OH− groups are adsorbed onto the surface of THS-2 than that onto the surface of THS-1. Fig. 4a compares the optical response properties of THS-1 and THS-2. The light absorption edges of THS-1 and THS-2 completely overlap (350–400 nm), and their band gaps are estimated 3.17 eV [18,19], which is very close to the accepted value of anatase TiO2 (3.2 eV). Compared with THS-2, THS-1 presents stronger absorption in visible
(a)
1.0
(a)
THS-1 THS-2
Intensity (a.u.)
Intensity (a.u.)
0.8
532.1
0.6 0.4 0.2 391 nm
0.0 200 540
538
536
534
532
530
528
300
400
526
500
600
700
800
Wavelength (nm)
Binding energy (eV) 200
140 120 100 80 60
538
536
534
532
530
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526
THS-1 THS-2
0.04 0.03 0.02 0.01 0.00 0
2
4
6
8
10 12 14 16 18 20
Pore size (nm)
40 20 0 0.0
540
(cm 3/g nm )
3
Intensity (a.u.)
532.1
160
(b) 0.05
dV/dD pore volume
(b) Volume adsorbed (cm /g)
529.8
180
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Binding energy (eV) Fig. 3. High-resolution XPS spectra of O1s for THS-1 (a) and THS-2 (b).
Fig. 4. UV–vis absorption spectra (a), and nitrogen adsorption–desorption isotherms and pore diameter distribution curves (b) of THS-1 and THS-2.
J.-W. Shi et al. / Catalysis Communications 66 (2015) 46–49
1.0
TOC/TOC0
carries [24,25]. THS-1 presents better crystallinity than THS-2, which may be the other important reason that THS-1 displays higher photocatalytic performance than THS-2.
THS-1 THS-2
0.8
49
4. Conclusions
0.6
In this communication, anatase titania hollow spheres (THS) composed of nanoplates with dominant {001} facets and nanoparticles without dominant {001} facets were successfully prepared, respectively. The former exhibited higher photocatalytic performance, which can be ascribed to two important advantages: {001} reactive facet exposure and better crystallinity.
0.4
0.2
0.0 -1
0
1
2
3
4
5
Acknowledgements
Irradiation time (h) Fig. 5. Relative TOC of CIP dependence on the irradiation time during photocatalytic mineralization.
light region (400–800 nm) and weaker absorption in UV region (200–350 nm). Fig. 4b shows that the isotherm of THS-1 is type IV with a hysteresis loop (IUPAC [20]), and the hysteresis curve belongs to type H3. Isotherms with type H3 loops have been reported for materials comprised of aggregates of platelike particles forming slitlike pores [21], which exactly coincides with our results that nanoplates assembled into hollow spheres. The isotherm of THS-2 also presents type IV with a hysteresis loop, and the hysteresis loop can be assigned to type H2, which can be ascribed to those pores formed by the accumulation of nanoparticles [22]. The BJH pore diameter distribution curves of THS-1 and THS-2 (the inset in Fig. 4b) reveal that the majority of pores in THS-2 are mainly between 4 and 10 nm. The BET specific surface areas of THS-1 and THS-2 are 21.67 and 89.77 m2 g−1, respectively. The photocatalytic activities of THS-1 and THS-2 were evaluated by the mineralization of CIP solution (Fig. 5). THS-2 shows a worse performance and only about 46% CIP was mineralized after 5 h reaction. Compared with THS-2, the photocatalytic performance of THS-1 is much higher because about 95% CIP was mineralized within the same reaction time. Compared with THS-1, more OH− groups are adsorbed onto the surface of THS-2, which can enhance the photocatalytic performance of TiO2 by trapping photogenerated holes to produce hydroxyl radicals. THS-1 and THS-2 share the same light absorption edge, and THS-2 presents stronger absorption in UV region. Furthermore, THS-2 possesses larger specific surface area in comparison with THS-1, which endows its stronger adsorption capacity. All these factors seem to prefigure that THS-2 possesses higher photocatalytic performance than THS-1. However, the mineralization experiment of CIP shows the opposite results. Therefore, it can be deduced that there must be other key factors which contribute significantly the photocatalytic performance of THS-1. Both theoretical and experimental studies have demonstrated that the catalytic reactivity of anatase TiO2 {001} facets is higher than that of the thermodynamically stable {101} surfaces [23]. THS-1 is composed of anatase nanoplates with dominant {001} facets. In contrast, THS-2 is composed of anatase nanoparticles without dominant {001} facets. The {001} facet exposure is one of the important reasons why THS-1 presents higher photocatalytic performance than THS-2. Crystallinity plays a very important role in the photocatalytic performance of TiO2. A better crystallization means the decrease of crystal defects, which are the recombination centers of photo-induced charge
This work was sponsored by the Fundamental Research Funds for the Central Universities, the Opening Project of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (2015-14), and the National Natural Science Foundation of China (51201175). The authors thank Ms. Juan Feng and Ms. Yanzhu Dai for their helps with SEM characterization carried out at the International Center for Dielectric Research (ICDR), Xi'an Jiaotong University. The valuable comments of anonymous reviewers are greatly appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2015.03.016. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
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