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{001} Facets of anatase TiO2 show high photocatalytic selectivity
Materials Letters 79 (2012) 259–262
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
{001} Facets of anatase TiO2 show high photocatalytic selectivity Jun Zhang a,⁎, Wanke Chen c, Junhua Xi a, b, Zhenguo Ji a, b, c a b c
College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, People's Republic of China State Key Lab of Silicon Materials, Zhejiang University, Hangzhou, 310018, People's Republic of China School of Electronics and Information, Hangzhou Dianzi University, Hangzhou, 310018, People's Republic of China
a r t i c l e
i n f o
Article history: Received 27 March 2012 Accepted 10 April 2012 Available online 17 April 2012 Keywords: TiO2 {001} Facets Photocatalytic selectivity
a b s t r a c t {001} Facets of anatase TiO2 show excellent photocatalytic selectivity which is confirmed by photodegradation of azo dye molecules. A novel adsorption and degradation competition between dyes on the {001} facets of TiO2 is found. The TiO2 nanosheets with a large percentage of exposed {001} facets show preferential adsorption and photo-degradation of methylene blue than methyl orange and rhodamine B. Instead of charged chemical group adsorption on the surface, the reactive oxygen atoms and separation of photoinduced holes on {001} facets are consider to be key factors for the high photocatalytic selectivity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As an essential semiconducting material, TiO2 has been studied and widely applied in photocatalysis due to its excellent photoelectrochemical properties. Most of these studies devote to enhancing the photocatalytic activity of TiO2 [1], however, less work is related to the photocatalytic selectivity of TiO2. Selective photocatalysis is a challenging but interesting field with potential applications in organics separation as well as selective degradation [2–5]. The oxidizing •OH groups generated from a photo-inducing hole have a high oxidization potential energy leading to the poor photocatalytic selectivity of TiO2 [6]. Some strategies have been developed to improve the photocatalytic selectivity of TiO2: 1) synthesizing mesoporous TiO2 to hinder the passing of molecules larger than the mesoporous [2]; and 2) tunable changing of the surface chemical groups on TiO2 to induce selective adsorption for different organics [3–5]. Unfortunately, there are no works focused on the photocatalytic selectivity from the surface state of TiO2. It is known that the adsorption photocatalytic reaction of TiO2 is strongly dependent on its surface state and grain boundary [7–10]. Recently, {001} facets of anatase TiO2 have attracted a lot of attention [11–14]. The surface energy of {001} facets has been demonstrated to be higher than that of {100} and {101} facets [11] and which induces higher reactivity [14]. However, there are other differences during photocatalytic progress between various crystal facets except photocatalytic activity such as separation of photo-induced electrons and holes [15]. In view of the different role of crystal facets on photocatalytic progress, highly selective photocatalysis may be achieved
⁎ Corresponding author. Tel.: + 86 0571 8687 8609. E-mail address: [email protected] (J. Zhang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.04.045
through simply controlling the exposure of various crystal facets of TiO2. Here, we demonstrate that {001} facets of anatase TiO2 show excellent photocatalytic selectivity.
2. Experimental Anatase TiO2 nanosheets with dominant {001} facets were synthesized through a hydrothermal route as follows: 25 mL Ti(OC4H9)4 was mixed with an appropriate amount of HF (40 wt.%) in a Teflon-lined autoclave with a capacity of 100 mL then kept at 180 °C for 24 h. The precipitates were then separated from the suspension by centrifugation and dried under an infrared lamp. Samples obtained from different HF contents with 1, 2, 3 to 6 mL were correspondingly labeled as TN1, TN2, TN3 and TN4. The obtained powders were then calcined at 600 °C for 90 min to clear surface fluoride [12]. As a contrast, the fluorinated sample from hydrothermal progress with 6 mL HF was labeled as TF4. The crystal structures of the samples were measured using an X-ray diffractometer (XRD) (TD-3500, Dandong Tongda Instrument Co., Ltd.) with Cu–Kα radiation. Morphologies of the samples were characterized by high-resolution transmission electron microscope (HRTEM) (JEOL JEM 2010FEF, Japan). X-ray photoelectron spectroscopy (XPS) measurements were made on a VG Multilab2000 spectrometer using Mg Kα radiation. In the photocatalytic experiment, a 250 W high pressure mercury lamp was placed 8 cm above the liquid surface and 50 mg of a photocatalyst was added into 100 mL of mixed 2 × 10 − 5 M MB and 2 × 10 − 5 M MO or 1 × 10 − 5 M RhB aqueous solution. Before irradiation, the mixed solution was stirred in a dark environment to reach an adsorption–desorption equilibrium. During photocatalytic progress, after each appropriate time, a 3 mL solution was extracted to test the residual concentration of eyes, which was evaluated by
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J. Zhang et al. / Materials Letters 79 (2012) 259–262
measuring the change of maximum absorbance in the UV–vis spectrum (UV-3600, Shimadzu, Japan). 3. Results and discussion In the HRTEM images of sample TN4 (Fig. 1A and B), a number of nanosheets with an average length of 50 nm and thickness of 4 nm can be seen (Fig. 1A). The lattice spacing parallel to the top and bottom facets is 0.235 nm (Fig. 1B) which is the crystal face space of {002} facets, indicating that the dominant surface is {001} facets. The percentages of dominant {001} facets in all samples were calculated and shown in Table 1 [5]. Fig. 1C illustrates a comparison of the XRD patterns of the samples TN1, TN2, TN3 and TN4 and all samples are confirmed to be in a pure anatase phase after having been calcined. The surfaces of the obtained TiO2 nanosheets were fluorinated with the attendance of HF [12]. The surface chemical states of sample TF4 were analyzed by XPS. It can be found that the surface of TiO2 was fluorinated and the peak for F 1 s at 684.3 eV as a typical value for surface Ti–F species [12] was observed (Fig. 1D). Surface fluoride has a large effect on the adsorption and photocatalytic selectivity due to the positive surface [4,5]. To eliminate the influence of surface fluoride, all samples were calcined at 600 °C for 90 min to obtain a clear surface [12]. The XPS spectrum of sample TN4 (Fig. 1D) shows the peaks of F 1 s before it disappeared, indicating that surface fluoride on TiO2 was cleared fully. The photocatalytic selectivity of the TiO2 nanosheets with a different percentage of dominant {001} facets was investigated by photocatalytic degradation of a mixed MB and MO aqueous solution shown in Fig. 2A–D. For all TiO2 nanosheets, the degradation rates of MB are much faster than those of MO, confirming a preferential degradation of MB. Surprisingly, the ln(C0/C) vs. time is nonlinear and the degradation rate of MB and MO (kMB and kMO) respectively
can be obviously divided into two stages: initial rate kMB1, kMO1 and later rate kMB2, kMO2 (Table 1). The selective photocatalytic ability of the photocatalysts should be quantified as: r = kMB1 / kMO1. In a further analysis, along with the increase of percentage of dominant {001} facets, the initial degradation rates of MO (kMO1) descend while those of MB (kMB1) rise gradually, indicating that the selective photocatalytic ability (r) is enhanced correspondingly. In contrast, the percentage of dominant {001} facets of anatase TiO2 particles in P25 is very low (6%), leading to very poor photocatalytic selectivity for MB and MO (Fig. 2E). It can be inferred that {001} facets of anatase TiO2 play a key role in high selective photocatalysis. The sample TN4 has an absolute selectivity for MB for there was only 3.3% MB remaining in solution while as little as 4.5% MO decomposed after 20 min of UV–vis irradiation. In fact, photocatalytic selectivity is mainly related to selectivity [4] and adsorption ability is extremely sensitive to synthesis conditions [8,9]. Compared with bulk phase, grain boundary is more chemically sensitive and can change the physical and chemical properties [10]. The adsorption ratio of MB on TiO2 rises observably with an increase of exposed {001} facets while that of MO keeps at a low ratio (b5%). {001} Facets of anatase TiO2 prefer to adsorb MB than MO resulting to prior degradation of MB. MB in aqueous solution is decomposed preferentially with a high rate due to the saturated adsorption when a photocatalytic reaction starts. Adsorption of MB on the surface of TiO2 becomes unsaturated when residual MB in solution is under ca. 5% leading to a lower degradation rate. The difference of rate (kMB1/kMB2) is enlarged following an increase of the adsorption ratio of MB. The photocatalytic selectivity of surface fluorinated TiO2 (TF4) was also investigated (Fig. 2F). Compared to sample TN4, surface fluorinate reduces the adsorption ratio of MB as well as weakens selective photocatalysis, confirming that high photocatalytic selectivity is due to the exposure of {001} facets instead of surface modification.
A
B
C
D
Fig. 1. (A) Low-resolution and (B) high-resolution HRTEM images of TN4; (C) XRD patterns of TN1, TN2, TN3 and TN4, (D) XPS spectra of survey spectrum for TN4 and TF4.
J. Zhang et al. / Materials Letters 79 (2012) 259–262
261
Table 1 Information on the structure and properties of the samples prepared at different experimental conditions. Sample
Phase
P25 TN1 TN2 TN3 TN4 TF4
A, R A A A A A
a b c
a
Percentage of (001)
Surface fluorite
CMB/C0
6% 40% 62% 73% 85% 85%
No No No No No Yes
6.7% 6.8% 12.9% 24.1% 60.0% 5.0%
b
kMB1
kMB2
CMO/C0
0.04138 0.07065 0.07792 0.07911 0.17386 0.10514
– – 0.04641 0.02706 0.02302 0.01370
5.1% 1.2% 2.6% 4.3% 0.4% 0.1%
c
kMO1
kMO2
r
0.03271 0.01730 0.01166 0.00755 0.00148 0.01370
– 0.08217 0.04981 0.02005 0.04055 0.06440
1.265 4.084 6.683 10.478 117.473 7.674
A is anatase and R is rutile. CMB is absorption concentration of MB, C0 is initial concentration of MB and CMB/C0 is the absorption ratio of MB. CMO is absorption concentration of MO, C0 is initial concentration of MO and CMO/C0 is the absorption ratio of MO.
As MB and MO are cationic and anionic dyes respectively, for eliminating the influence of charged adsorption on surface, two kinds of cationic dyes (MB and RhB) were mixed to execute selective photocatalytic tests for P25 and TN4 (Fig. 3). P25 shows poor photocatalytic
selectivity for MB and RhB, while that of TN4 is kept at a high standard. The Ti―O―Ti bond angles on exposed {001} facets are very large and 2p states on the surface oxygen atoms are destabilized and reactive [13]. Adsorption on {001} facets mainly react with the
Fig. 2. Photocatalytic degradation of mixed MB and MO in aqueous solution for (A) TN1, (B) TN2, (C) TN3, (D) TN4, (E) P25 and (F) TF4.
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to adsorb on and interact with electron deficient {001} facets of anatase TiO2 which leads to a higher selective photo-degradation of MB. 4. Conclusion Except for high photocatalytic activity, {001} facets of anatase TiO2 also show excellent photocatalytic selectivity which was confirmed by photo-degradation azo dyes. It seems that the reactive oxygen atoms on {001} facets and separation of photo-induced electron and hole pairs on a different crystal face of anatase TiO2 cause the high photocatalytic selectivity of {001} facets. Investigating this is expected to open up a new strategy for photocatalytic selectivity and provide a deeper understanding of the role of {001} facets in photocatalysis. Acknowledgments This work was supported by the School Science Starting Foundation of Hangzhou Dianzi University (no. KYS205611033), the financial aids of the Chinese National Natural Science Foundation (no. 61072015), and Zhejiang Provincial Natural Science Foundation of China (no. ZZ4110503 and Q12E020007). References
Fig. 3. Photocatalytic degradation of mixed MB and RhB in aqueous solution for (A) P25 and (B) TN4.
oxygen atoms, and when the surface Ti―O―Ti bond is broken, the reactive Ti―O − preferential bonds with ionization atom S + in MB or N + in MB and RhB than O − in MO. Furthermore, many reports show that under irradiation, photo-produced electrons move to the {101} facets while photo-produced holes move to the {001} facets of anatase TiO2 [15]. The ionization energy of MB (5.3 eV) is much lower than that of RhB (6.7 eV), which makes MB play as a better electron donor than RhB [3]. The good electron donor (MB) is inclined
[1] Zhang J, Pan C, Fang P, Wei J, Xiong R. ACS Appl Mater Interfaces 2010;2:1173. [2] Yasuhiro S, Naoya S, Takayuki H. J Am Chem Soc 2005;127:12820. [3] Gopalakrishnan K, Joshi HM, Kumar P, Panchakarla LS, Rao CNR. Chem Phys Lett 2011;511:304. [4] Liu S, Yu J, Jaroniec M. J Am Chem Soc 2010;132:11914. [5] Xiang Q, Yu J, Jaroniec M. Chem Commun 2011;47:4532. [6] Konstantinou IK, Albanis TA. Appl Catal B Environ 2004;49:1. [7] Fujishima A, Zhang X, Tryk DA. Surf Sci Rep 2008;63:515. [8] Straumal BB, Mazilkin A, Protasova S, Myatiev A, Straumal P, Goering E, et al. Phys Status Solidi B 2011;248:1581. [9] Straumal BB, Myatiev AA, Straumal PB, Mazilkin AA, Protasova SG, Goering E, et al. JETP Lett 2010;92:396. [10] Straumal BB, Protasova SG, Mazilkin AA, Myatiev AA, Straumal PB, Schütz G, et al. J Appl Phys 2010;108:073923. [11] Lazzeri M, Vittadini A, Selloni A. Phys Rev B 2001;63:155409. [12] Yang HG, Sun CH, Qiao SZ, Zou J, Liu G, Smith SC, et al. Nature 2008;453:638. [13] Selloni A. Nat Mater 2008;7:613615. [14] Han X, Kuang Q, Jin M, Xie Z, Zheng L. J Am Chem Soc 2009;131:3152. [15] Taguchi T, Saito Y, Sarukawa K, Ohnoz T, Matsumura M. New J Chem 2003;27: 1304.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
{001} Facets of anatase TiO2 show high photocatalytic selectivity Jun Zhang a,⁎, Wanke Chen c, Junhua Xi a, b, Zhenguo Ji a, b, c a b c
College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, People's Republic of China State Key Lab of Silicon Materials, Zhejiang University, Hangzhou, 310018, People's Republic of China School of Electronics and Information, Hangzhou Dianzi University, Hangzhou, 310018, People's Republic of China
a r t i c l e
i n f o
Article history: Received 27 March 2012 Accepted 10 April 2012 Available online 17 April 2012 Keywords: TiO2 {001} Facets Photocatalytic selectivity
a b s t r a c t {001} Facets of anatase TiO2 show excellent photocatalytic selectivity which is confirmed by photodegradation of azo dye molecules. A novel adsorption and degradation competition between dyes on the {001} facets of TiO2 is found. The TiO2 nanosheets with a large percentage of exposed {001} facets show preferential adsorption and photo-degradation of methylene blue than methyl orange and rhodamine B. Instead of charged chemical group adsorption on the surface, the reactive oxygen atoms and separation of photoinduced holes on {001} facets are consider to be key factors for the high photocatalytic selectivity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As an essential semiconducting material, TiO2 has been studied and widely applied in photocatalysis due to its excellent photoelectrochemical properties. Most of these studies devote to enhancing the photocatalytic activity of TiO2 [1], however, less work is related to the photocatalytic selectivity of TiO2. Selective photocatalysis is a challenging but interesting field with potential applications in organics separation as well as selective degradation [2–5]. The oxidizing •OH groups generated from a photo-inducing hole have a high oxidization potential energy leading to the poor photocatalytic selectivity of TiO2 [6]. Some strategies have been developed to improve the photocatalytic selectivity of TiO2: 1) synthesizing mesoporous TiO2 to hinder the passing of molecules larger than the mesoporous [2]; and 2) tunable changing of the surface chemical groups on TiO2 to induce selective adsorption for different organics [3–5]. Unfortunately, there are no works focused on the photocatalytic selectivity from the surface state of TiO2. It is known that the adsorption photocatalytic reaction of TiO2 is strongly dependent on its surface state and grain boundary [7–10]. Recently, {001} facets of anatase TiO2 have attracted a lot of attention [11–14]. The surface energy of {001} facets has been demonstrated to be higher than that of {100} and {101} facets [11] and which induces higher reactivity [14]. However, there are other differences during photocatalytic progress between various crystal facets except photocatalytic activity such as separation of photo-induced electrons and holes [15]. In view of the different role of crystal facets on photocatalytic progress, highly selective photocatalysis may be achieved
⁎ Corresponding author. Tel.: + 86 0571 8687 8609. E-mail address: [email protected] (J. Zhang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.04.045
through simply controlling the exposure of various crystal facets of TiO2. Here, we demonstrate that {001} facets of anatase TiO2 show excellent photocatalytic selectivity.
2. Experimental Anatase TiO2 nanosheets with dominant {001} facets were synthesized through a hydrothermal route as follows: 25 mL Ti(OC4H9)4 was mixed with an appropriate amount of HF (40 wt.%) in a Teflon-lined autoclave with a capacity of 100 mL then kept at 180 °C for 24 h. The precipitates were then separated from the suspension by centrifugation and dried under an infrared lamp. Samples obtained from different HF contents with 1, 2, 3 to 6 mL were correspondingly labeled as TN1, TN2, TN3 and TN4. The obtained powders were then calcined at 600 °C for 90 min to clear surface fluoride [12]. As a contrast, the fluorinated sample from hydrothermal progress with 6 mL HF was labeled as TF4. The crystal structures of the samples were measured using an X-ray diffractometer (XRD) (TD-3500, Dandong Tongda Instrument Co., Ltd.) with Cu–Kα radiation. Morphologies of the samples were characterized by high-resolution transmission electron microscope (HRTEM) (JEOL JEM 2010FEF, Japan). X-ray photoelectron spectroscopy (XPS) measurements were made on a VG Multilab2000 spectrometer using Mg Kα radiation. In the photocatalytic experiment, a 250 W high pressure mercury lamp was placed 8 cm above the liquid surface and 50 mg of a photocatalyst was added into 100 mL of mixed 2 × 10 − 5 M MB and 2 × 10 − 5 M MO or 1 × 10 − 5 M RhB aqueous solution. Before irradiation, the mixed solution was stirred in a dark environment to reach an adsorption–desorption equilibrium. During photocatalytic progress, after each appropriate time, a 3 mL solution was extracted to test the residual concentration of eyes, which was evaluated by
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J. Zhang et al. / Materials Letters 79 (2012) 259–262
measuring the change of maximum absorbance in the UV–vis spectrum (UV-3600, Shimadzu, Japan). 3. Results and discussion In the HRTEM images of sample TN4 (Fig. 1A and B), a number of nanosheets with an average length of 50 nm and thickness of 4 nm can be seen (Fig. 1A). The lattice spacing parallel to the top and bottom facets is 0.235 nm (Fig. 1B) which is the crystal face space of {002} facets, indicating that the dominant surface is {001} facets. The percentages of dominant {001} facets in all samples were calculated and shown in Table 1 [5]. Fig. 1C illustrates a comparison of the XRD patterns of the samples TN1, TN2, TN3 and TN4 and all samples are confirmed to be in a pure anatase phase after having been calcined. The surfaces of the obtained TiO2 nanosheets were fluorinated with the attendance of HF [12]. The surface chemical states of sample TF4 were analyzed by XPS. It can be found that the surface of TiO2 was fluorinated and the peak for F 1 s at 684.3 eV as a typical value for surface Ti–F species [12] was observed (Fig. 1D). Surface fluoride has a large effect on the adsorption and photocatalytic selectivity due to the positive surface [4,5]. To eliminate the influence of surface fluoride, all samples were calcined at 600 °C for 90 min to obtain a clear surface [12]. The XPS spectrum of sample TN4 (Fig. 1D) shows the peaks of F 1 s before it disappeared, indicating that surface fluoride on TiO2 was cleared fully. The photocatalytic selectivity of the TiO2 nanosheets with a different percentage of dominant {001} facets was investigated by photocatalytic degradation of a mixed MB and MO aqueous solution shown in Fig. 2A–D. For all TiO2 nanosheets, the degradation rates of MB are much faster than those of MO, confirming a preferential degradation of MB. Surprisingly, the ln(C0/C) vs. time is nonlinear and the degradation rate of MB and MO (kMB and kMO) respectively
can be obviously divided into two stages: initial rate kMB1, kMO1 and later rate kMB2, kMO2 (Table 1). The selective photocatalytic ability of the photocatalysts should be quantified as: r = kMB1 / kMO1. In a further analysis, along with the increase of percentage of dominant {001} facets, the initial degradation rates of MO (kMO1) descend while those of MB (kMB1) rise gradually, indicating that the selective photocatalytic ability (r) is enhanced correspondingly. In contrast, the percentage of dominant {001} facets of anatase TiO2 particles in P25 is very low (6%), leading to very poor photocatalytic selectivity for MB and MO (Fig. 2E). It can be inferred that {001} facets of anatase TiO2 play a key role in high selective photocatalysis. The sample TN4 has an absolute selectivity for MB for there was only 3.3% MB remaining in solution while as little as 4.5% MO decomposed after 20 min of UV–vis irradiation. In fact, photocatalytic selectivity is mainly related to selectivity [4] and adsorption ability is extremely sensitive to synthesis conditions [8,9]. Compared with bulk phase, grain boundary is more chemically sensitive and can change the physical and chemical properties [10]. The adsorption ratio of MB on TiO2 rises observably with an increase of exposed {001} facets while that of MO keeps at a low ratio (b5%). {001} Facets of anatase TiO2 prefer to adsorb MB than MO resulting to prior degradation of MB. MB in aqueous solution is decomposed preferentially with a high rate due to the saturated adsorption when a photocatalytic reaction starts. Adsorption of MB on the surface of TiO2 becomes unsaturated when residual MB in solution is under ca. 5% leading to a lower degradation rate. The difference of rate (kMB1/kMB2) is enlarged following an increase of the adsorption ratio of MB. The photocatalytic selectivity of surface fluorinated TiO2 (TF4) was also investigated (Fig. 2F). Compared to sample TN4, surface fluorinate reduces the adsorption ratio of MB as well as weakens selective photocatalysis, confirming that high photocatalytic selectivity is due to the exposure of {001} facets instead of surface modification.
A
B
C
D
Fig. 1. (A) Low-resolution and (B) high-resolution HRTEM images of TN4; (C) XRD patterns of TN1, TN2, TN3 and TN4, (D) XPS spectra of survey spectrum for TN4 and TF4.
J. Zhang et al. / Materials Letters 79 (2012) 259–262
261
Table 1 Information on the structure and properties of the samples prepared at different experimental conditions. Sample
Phase
P25 TN1 TN2 TN3 TN4 TF4
A, R A A A A A
a b c
a
Percentage of (001)
Surface fluorite
CMB/C0
6% 40% 62% 73% 85% 85%
No No No No No Yes
6.7% 6.8% 12.9% 24.1% 60.0% 5.0%
b
kMB1
kMB2
CMO/C0
0.04138 0.07065 0.07792 0.07911 0.17386 0.10514
– – 0.04641 0.02706 0.02302 0.01370
5.1% 1.2% 2.6% 4.3% 0.4% 0.1%
c
kMO1
kMO2
r
0.03271 0.01730 0.01166 0.00755 0.00148 0.01370
– 0.08217 0.04981 0.02005 0.04055 0.06440
1.265 4.084 6.683 10.478 117.473 7.674
A is anatase and R is rutile. CMB is absorption concentration of MB, C0 is initial concentration of MB and CMB/C0 is the absorption ratio of MB. CMO is absorption concentration of MO, C0 is initial concentration of MO and CMO/C0 is the absorption ratio of MO.
As MB and MO are cationic and anionic dyes respectively, for eliminating the influence of charged adsorption on surface, two kinds of cationic dyes (MB and RhB) were mixed to execute selective photocatalytic tests for P25 and TN4 (Fig. 3). P25 shows poor photocatalytic
selectivity for MB and RhB, while that of TN4 is kept at a high standard. The Ti―O―Ti bond angles on exposed {001} facets are very large and 2p states on the surface oxygen atoms are destabilized and reactive [13]. Adsorption on {001} facets mainly react with the
Fig. 2. Photocatalytic degradation of mixed MB and MO in aqueous solution for (A) TN1, (B) TN2, (C) TN3, (D) TN4, (E) P25 and (F) TF4.
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J. Zhang et al. / Materials Letters 79 (2012) 259–262
to adsorb on and interact with electron deficient {001} facets of anatase TiO2 which leads to a higher selective photo-degradation of MB. 4. Conclusion Except for high photocatalytic activity, {001} facets of anatase TiO2 also show excellent photocatalytic selectivity which was confirmed by photo-degradation azo dyes. It seems that the reactive oxygen atoms on {001} facets and separation of photo-induced electron and hole pairs on a different crystal face of anatase TiO2 cause the high photocatalytic selectivity of {001} facets. Investigating this is expected to open up a new strategy for photocatalytic selectivity and provide a deeper understanding of the role of {001} facets in photocatalysis. Acknowledgments This work was supported by the School Science Starting Foundation of Hangzhou Dianzi University (no. KYS205611033), the financial aids of the Chinese National Natural Science Foundation (no. 61072015), and Zhejiang Provincial Natural Science Foundation of China (no. ZZ4110503 and Q12E020007). References
Fig. 3. Photocatalytic degradation of mixed MB and RhB in aqueous solution for (A) P25 and (B) TN4.
oxygen atoms, and when the surface Ti―O―Ti bond is broken, the reactive Ti―O − preferential bonds with ionization atom S + in MB or N + in MB and RhB than O − in MO. Furthermore, many reports show that under irradiation, photo-produced electrons move to the {101} facets while photo-produced holes move to the {001} facets of anatase TiO2 [15]. The ionization energy of MB (5.3 eV) is much lower than that of RhB (6.7 eV), which makes MB play as a better electron donor than RhB [3]. The good electron donor (MB) is inclined
[1] Zhang J, Pan C, Fang P, Wei J, Xiong R. ACS Appl Mater Interfaces 2010;2:1173. [2] Yasuhiro S, Naoya S, Takayuki H. J Am Chem Soc 2005;127:12820. [3] Gopalakrishnan K, Joshi HM, Kumar P, Panchakarla LS, Rao CNR. Chem Phys Lett 2011;511:304. [4] Liu S, Yu J, Jaroniec M. J Am Chem Soc 2010;132:11914. [5] Xiang Q, Yu J, Jaroniec M. Chem Commun 2011;47:4532. [6] Konstantinou IK, Albanis TA. Appl Catal B Environ 2004;49:1. [7] Fujishima A, Zhang X, Tryk DA. Surf Sci Rep 2008;63:515. [8] Straumal BB, Mazilkin A, Protasova S, Myatiev A, Straumal P, Goering E, et al. Phys Status Solidi B 2011;248:1581. [9] Straumal BB, Myatiev AA, Straumal PB, Mazilkin AA, Protasova SG, Goering E, et al. JETP Lett 2010;92:396. [10] Straumal BB, Protasova SG, Mazilkin AA, Myatiev AA, Straumal PB, Schütz G, et al. J Appl Phys 2010;108:073923. [11] Lazzeri M, Vittadini A, Selloni A. Phys Rev B 2001;63:155409. [12] Yang HG, Sun CH, Qiao SZ, Zou J, Liu G, Smith SC, et al. Nature 2008;453:638. [13] Selloni A. Nat Mater 2008;7:613615. [14] Han X, Kuang Q, Jin M, Xie Z, Zheng L. J Am Chem Soc 2009;131:3152. [15] Taguchi T, Saito Y, Sarukawa K, Ohnoz T, Matsumura M. New J Chem 2003;27: 1304.