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Unpredictable adsorption and visible light induced decolorization of nano rutile for the treatment of crystal violet
Solid State Sciences 66 (2017) 1e6
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Unpredictable adsorption and visible light induced decolorization of nano rutile for the treatment of crystal violet Yanling Dong a, 1, Yang Liu b, 1, Dingze Lu b, 1, Feng Zheng b, Pengfei Fang b, *, Haining Zhang a, ** a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Department of Physics, Hubei Nuclear-Solid Physics Key Laboratory and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 November 2016 Received in revised form 26 January 2017 Accepted 31 January 2017 Available online 6 February 2017
Photocatalysts containing different ratios of anatase and rutile are prepared via heat treatment of Degussa P-25 titania. X-ray diffraction (XRD), Bruuauer-Emmett-Teller (BET), ultravioletevisible light diffuse reflectance spectra (DRS), Raman spectra (Raman), positron annihilation lifetime spectra (PAL) and temperature-programmed desorption (TPD) are applied to investigate the phase composition of the synthesized catalysts. Using crystal violet (CV) as the target pollutant, the unexpected visible light decolorization of rutile is observed. Despite the decreased specific surface area, the as-synthesized rutile samples exhibit much higher adsorption capability of CV than P-25 does, which in turn leads to improved photoreaction efficiency. Since the rutile samples can't absorb the visible light, the degradation under visible light irradiation is attributed to self-sensitization of CV on the surface of rutile. © 2017 Elsevier Masson SAS. All rights reserved.
Keywords: Rutile Crystal violet Adsorption Decolorization efficiency
1. Introduction Semiconductor photocatalysis is a powerful tool for pollution control [1e3]. As an important photocatalyst, TiO2 has received widespread concern [4e6]. The photocatalytic activity of TiO2 is closely related to its phase composition [7]. Although rutile (Eg ¼ 3.0 eV) can absorb more light than anatase (Eg ¼ 3.2 eV), and it is more stable than anatase [8], it is generally agreed that anatase is a better photocatalyst. There are mainly two reasons: the first, there are more adsorbed oxygen molecules on the surface of anatase [9], and it is a crucial factor during the photoreaction; the second, anatase always has a larger specific surface area [10,11], which can provide more opportunities to contact with pollutant molecules. Therefore, there are fewer researches on rutile than that on anatase in the field of photocatalysis. Except phase composition, the other factors, such as specific surface area, adsorption ability, and surface chemical condition, also have effects on the photocatalytic activity [12e15]. Rutile
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Fang), [email protected] (H. Zhang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.solidstatesciences.2017.01.011 1293-2558/© 2017 Elsevier Masson SAS. All rights reserved.
sometime can also exhibit high photocatalytic activity [16e18]. ez et al. [16] had synthesized H2-reduced TiO2 by calcinations Pa under H2 flow. It was found that rutile with low specific surface area obtained high adsorption ability and visible photoactivity. This unusual phenomenon indicates that rutile may have potential in dealing with some kinds of dyes under visible light irradiation. However, in order to make deeper research, it is necessary to evaluate the adsorption abilities and visible photoactivity of TiO2 prepared without H2 treatment. In this paper, adsorption abilities and decolorization efficiency of TiO2 containing different ratios of anatase/rutile are investigated. Different from the usual results, rutile prepared by simple heat treatment of Degussa P25 obtains the highest visible decolorization efficiency in degrading crystal violet (CV). Characterization techniques are used to study the phenomenon, and the possible mechanism is also discussed. 2. Experiment 2.1. Materials Titania P25 was product of Degussa Co. Ltd. CV was purchased from Shanghai Yuanhang Chemical Co. Ltd. P25 samples were heat treated in a conventional muffle furnace at 400, 500, 600, 700, 800,
2
Y. Dong et al. / Solid State Sciences 66 (2017) 1e6 Table 1 XRD, BET, and band-gap result of photocatalysts. Samples
WAa
W Rb
Crystallite size (nm)
SBET (m2/g)
BE (eV)c
P25 h-400 h-500 h-600 h-700 h-800 h-900
81.38% 82.10% 74.41% 46.32% 1.58% 0.00% 0.00%
18.62% 17.90% 25.59% 53.68% 98.42% 100.00% 100.00%
A 21.8 A 21.4 A 22.0 A 24.1 e e e
57.5 54.9 49.1 40 14.4 9.7 3.4
3.18 3.17 3.15 3.08 3.06 3.02 3.01
a b c
R R R R R R R
30.0 37.1 40.6 42.1 54.6 63.3 75.0
WA presents the phase content of anatase. WR presents the phase content of rutile. BE is the band gap energy of samples.
2.2. Characterization The X-ray diffraction (XRD) patterns were obtained by a Bruker D8 advance X-ray diffractometer using monochromatic Cu Ka radiation (l ¼ 1.5406 Å) with an accelerating voltage of 40 kV and current of 40 mA. Horiba LabRAM HR Raman spectrometer, with excitation wavelength of 488 nm was used to characterize the structure of photocatalysts. The Bruuauer-Emmett-Teller (BET) surface area was measured on a Beijing JWGB JW-BK using N2 adsorption at 196 C. The positron annihilation lifetime spectra (PAL) were measured by a conventional fast-fast coincidence system with time resolution of 300 ps. Diffusive reflectance UVevis absorption spectra (DRS) were collected on a Shimadzu UV-2550 spectrophotometer using integrating sphere attachment with barium sulfate as a reference. Temperature-programmed desorption (TPD) of CO2 was measured on a TP-5080 adsorption instrument (Tianjin Xianquan Co.). The samples were pretreated in N2 atmosphere at 400 C to remove the surface adsorbate. The CO2 adsorption process was taken at 25 C, and CO2 desorption was measured with heating rate of 10 C/min. 2.3. Decolorization evaluation The photoreaction was taken under a 160 W high-pressure Hg lamp (Shanghai Minghua Co.). The light focused onto a 200 mL of beaker filled with 100 mL of 20 mg/L CV solution and 0.10 g of photocatalyst. The beaker was put in a water tank to slow down the increase of temperature. Before irradiation, the suspension had been magnetically stirred in darkness for 1 h. Every 30 min, 1.6 mL of the suspension was collected and centrifuged to remove the photocatalyst. During the visible photoreaction, a cut-off filter (l > 420 nm) was put on the top of the beaker. The concentration of CV was measured by the UVevis absorption spectra. 3. Results and discussion 3.1. Photocatalysts characterization
Fig. 1. XRD patterns (a), Raman spectra (b), and average positron lifetime tav (c) of titania containing different ratio of rutile.
and 900 C for 3 h, respectively. The samples were denoted by h-x, where x represents calcination temperature.
XRD (Fig. 1a) and Raman spectra (Fig. 1b) were used to study the structures of different photocatalysts. In Fig. 1a, IA (101) at 25.6 is the strongest peak of anatase, and IR (110) at 27.7 is the strongest peak of rutile [19]. The phase content and particle size results (Table 1) suggest that the phase transition started at 500 C and ended above 700 C. From 500 C to 900 C, the crystallite size of rutile had grown up obviously from 40.6 nm to 75.0 nm. With the increase of heat treatment temperature, the intensities of the Raman peaks were getting stronger obviously (Fig. 1b), which is due to the formation of the dense structures during the heat treatment. The Raman shifts at 396, 515, and 636 cm1 are attributed to the B1g, A1g, and Eg modes of anatase. The Raman shifts at 449 and
Y. Dong et al. / Solid State Sciences 66 (2017) 1e6
Fig. 2. Decolorization efficiency of CV with irradiation of UVevis light (aeb) and visible light (dee); k value of UVevis light photoreaction (c) and visible light photoreaction (f).
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Y. Dong et al. / Solid State Sciences 66 (2017) 1e6
were annealed out [22]. The microstructure of rutile became more homogeneous and dense, which is in accordance with XRD, Raman, and BET results. 3.2. Unpredictable decolorization efficiency of rutile in degrading CV
Fig. 3. Decolorization efficiency by external surface area.
610 cm1 are attributed to the Eg and A1g modes of rutile, respectively [20,21]. The h-700, h-800, and h-900 samples only exhibit rutile vibration modes. Besides, the phase transition of anatase to rutile caused a rapid collapse in the specific surface area (SBET in Table 1). PAL (Fig. 1c) is a powerful technique for probing the vacancytype defects in materials due to its sensitivity to atomic-scale defects. From 300 C to 500 C, defects such as vacancy cluster and monovacancy were probed. Above 500 C, the average positron lifetime tav sharply decreased. The decrease indicates that the monovacancy, vacancy cluster, and void among grain boundaries
Decolorization was evaluated by degrading CV. The decolorization efficiency obey the first-order relationship (eln (C/C0) ¼ kapp t). The UVevis decolorization is shown in Fig. 2a and b. The h-500 TiO2 reveals the highest UVevis decolorization. Fig. 2c and d shows the visible decolorization efficiency. Surprisingly, the samples calcined at higher temperatures (600, 700, and 800 C) exhibit much higher decolorization efficiency under visible light irradiation. The rutile h-700 TiO2 has obtained the highest decolorization efficiency, which is 8.9-fold of that of P25. Using decolorization efficiency divided by specific surface area, k/SBET, to evaluate the visible decolorization efficiency (Fig. 3), interestingly, the result suggests that rutile samples (h-700, h-800, and h-900) have a very stable k/ SBET value, which is much higher than other samples. The results were accordance with the previous reports [23]. 4. Discussion Heat treatment brought a better crystallization for TiO2. However, phase transition occurred at the same time. The grain grew up obviously, and the specific surface area decreased fast. All of these factors can influence the decolorization efficiency of samples. The h-500 sample has kept 74.41% of anatase. Calcination at 500 C has not caused the sharp decrease in surface area. It still keeps a relative
Fig. 4. (a) The adsorption of CV on the negative electricity center of TiO2 surface; (b) visible light self-photosensitized oxidation of dye; (c) DRS of the samples calcined at different temperatures.
Y. Dong et al. / Solid State Sciences 66 (2017) 1e6
5
Fig. 5. (a) CV adsorption in darkness for 1 h and specific surface area as a function of calcining temperature; (b) CO2 TPD results of titania containing different ratios of rutile.
high specific surface area (49.1 m2/g). Besides, its particle size is low (28.2 nm). Therefore, h-500 TiO2 exhibits the best UVevis decolorization efficiency. When it comes to the visible decolorization efficiency, it is reported that visible light-induced photoreactions of TiO2 occur mainly through two mechanisms: [24]. The first one is to lower the band gap of TiO2, so that it could make use of visible light. Many approaches such as metal ions, N, and C doping are employed for this purpose. The second way is through the self-photosensitized oxidation of surface adsorbed dye. CV is a kind of cationic dye, and it will be adsorbed on the negative electricity centers of TiO2 surface (Fig. 4a). The adsorbed CV will absorb visible light irradiation rather than TiO2 itself. The visible light excites the dyes, and the excited species can inject electrons into the conduction band of TiO2 (Fig. 4b). The injected electrons can combine with O2 on the surface of TiO2 to form O2e, and then through a serials of reactions to form OH in water condition. In order to test the light absorbing abilities of the samples, the DRS measurement was taken, and the results were displayed in Fig. 4c. According to the UVevis diffuse reflectance spectra (DRS), the absorption data are fitted to equation for indirect band-gap transitions: (ahn)1/2 ¼ A (hnEg). The bandgap energy (Eg) of samples can be estimated from the x-intercept of the tangents. The Eg values of P25, h-400, h-500, h-600, h-700, h800, and h-900 are 3.18, 3.17, 3.15, 3.08, 3.06, 3.02, and 3.01 eV, respectively. Besides, Eg values were also listed in Table 1. Heat treatment induced a red shift. However, none of the samples is capable to absorb the visible light, which means none of the samples can make use of the visible light to excite electro-hole pairs. Therefore, the decolorization efficiency of CV by rutile under visible light is through the self-photosensitized oxidation of dye. Adsorption of dye is a prerequisite for the electron injection. The CV adsorption result is shown in Fig. 5a. It could be observed that the adsorption does not decrease with the decreasing specific surface area, on the contrary, increases obviously. Adsorption ability of h-700 is 3.7-fold of that of P25. Considering the specific surface area of h-700 (14.4 m2/g) and P25 (57.5 m2/g), the adsorption efficiency divided by specific surface area of h-700 is 14.8-fold of that of P25. The h-800 and h-900 samples also adsorbed much more CV than that of P25, but their specific surface areas are only 9.7 and 3.4 m2/g, respectively. The result indicates that rutile has much higher adsorption of CV than that of anatase, and the high adsorption ability may have greatly enhanced the photodecomposition of CV [25].
CO2 is a kind of lewis acid, and can be used as a probe to identify the basic sites [26,27]. In order to study the surface difference between anatase and rutile, CO2 TPD test was performed to evaluate the surface basicity. From Fig. 5b, it can be seen that when the rutile content increases, CO2 desorption temperature increases obviously. P25 (18.6% of rutile) shows a main CO2 desorption peak at 78 C, while the h-600 (53.7% of rutile) exhibits CO2 desorption peaks at higher temperature of 187 C and 358 C, and h-700 (98.4% of rutile) shows CO2 desorption peaks at 175 C, 309 C, and 390 C. The higher desorption temperature means that it is harder for CO2 to escape from the surface. The high CO2 desorption temperature for rutile suggests that rutile may have a surface with higher basicity than that of anatase. Su et al. [27] reported that the basicity of surface OH groups of rutile phase is stronger than that of anatase phase. Our research is in accordance with this result. The performance of CV adsorption is similar to that of CO2. CV is a typical cationic dye, which is easier to be adsorbed on the basic surface [24]. Therefore, the TPD result may have interpreted the adsorption enhancement of CV of the rutile photocatalysts. This may be the reason for the unpredictable visible photoactivity of nano rutile in degrading CV.
5. Conclusion Rutile showed unpredictable visible decolorization efficiency in degrading CV, which is 8.9-fold of that of P25. Although SBET of rutile sample is only 14.4 m2/g (SBET of P25 is 57.5 m2/g), its adsorption of CV is 3.7-fold of that of P25. Rutile surface has higher basicity than that of P25, which is the reason for the enhanced adsorption and visible light decolorization of CV.
Acknowledgments This work was financially supported by the National Basic Research Program of China (973 Program, Nos. 2009CB939704 and 2009CB939705), the Natural Science Foundation of China (21576216), the Experimental Technique Foundation of Wuhan University (WHU-2014-SYJS-01), the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University (Nos. LF20150681 and 20150683).
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Y. Dong et al. / Solid State Sciences 66 (2017) 1e6
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2017.01.011 References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69e96. [2] A. Mills, R.H. Davies, D. Worsley, Water purification by semiconductor photocatalysis, Chem. Soc. Rev. 22 (1993) 417e425. [3] D. Lu, P. Fang, X. Liu, S. Zhai, C. Li, X. Zhao, J. Ding, R. Xiong, A facile one-pot synthesis of TiO2-based nanosheets loaded with MnxOy nanoparticles with enhanced visible light-driven photocatalytic performance for removal of Cr(VI) or RhB, Appl. Catal. B 179 (2015) 558e573. [4] A. Fujishima, X.T. Zhang, Titanium dioxide photocatalysis: present situation and future approaches, C. R. Chim. 9 (2006) 750e760. [5] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735e758. [6] D. Lu, W. Chai, M. Yang, P. Fang, W. Wu, B. Zhao, R. Xiong, H. Wang, Visible light induced photocatalytic removal of Cr(VI) over TiO2-based nanosheets loaded with surface-enriched CoOx nanoparticles and its synergism with phenol oxidation, Appl. Catal. B 190 (2016) 44e65. [7] A.P. Rivera, K. Tanaka, T. Hisanaga, Photocatalytic degradation of pollutant over TiO2 in different crystal structures, Appl. Catal. B 3 (1993) 37e44. [8] A. Rothschild, A. Levakov, Y. Shapira, N. Ashkenasy, Y. Komem, Surface photovoltage spectroscopy study of reduced and oxidized nanocrystalline TiO2 films, Surf. Sci. 532 (2003) 456e460. [9] A. Sclafani, J.M. Herrmann, Comparison of the photoelectronic and photocatalytic activities of various anatase and rutile forms of titania in pure liquid organic phases and in aqueous solutions, J. Phys. Chem. 100 (1996) 13655e13661. [10] A. Fujishima, X.T. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008) 515e582. [11] D.T. Cromer, K. Herrington, The structures of anatase and rutile, J. Am. Chem. Soc. 77 (1955) 4708e4709. [12] Q.H. Zhang, L. Gao, J.K. Guo, Effects of calcination on the photocatalytic properties of nanosized TiO2 powders prepared by TiCl4 hydrolysis, Appl. Catal. B 26 (2000) 207e215. [13] M. Hamadanian, A. Reisi-Vanani, A. Majedi, Synthesis, characterization and effect of calcination temperature on phase transformation and photocatalytic activity of Cu, S-codoped TiO2 nanoparticles, Appl. Surf. Sci. 256 (2010) 1837e1844.
[14] Y. Tanaka, M. Suganuma, Effects of heat treatment on photocatalytic property of sol-gel derived polycrystalline TiO2, J. Sol-Gel Sci. Technol. 22 (2001) 83e89. [15] J.F. Porter, Y.G. Li, C.K. Chan, The effect of calcination on the microstructural characteristics and photoreactivity of Degussa P-25 TiO2, J. Mater. Sci. 34 (1999) 1523e1531. [16] C.A. P aez, D. Poelman, J.P. Pirard, B. Heinrichs, Unpredictable photocatalytic ability of H2-reduced rutile-TiO2 xerogel in the degradation of dye-pollutants under UV and visible light irradiation, Appl. Catal. B 94 (2010) 263e271. [17] Q. Gao, X.M. Wu, Y.M. Fan, X.Y. Zhou, Low temperature fabrication of nanoflower arrays of rutile TiO2 on mica particles with enhanced photocatalytic activity, J. Alloy. Compd. 579 (2013) 322e329. [18] M. Krivec, R.A. Segundo, J.L. Faria, A.M.T. Silva, G. Dra zi c, Low-temperature synthesis and characterization of rutile nanoparticles with amorphous surface layer for photocatalytic degradation of caffeine, Appl. Catal. B. Environ. 140e141 (2013) 9e15. [19] T. Sreethawong, Y. Suzuki, S. Yoshikawa, Synthesis, characterization, and photocatalytic activity for hydrogen evolution of nanocrystalline mesoporous titania prepared by surfactant-assisted templating solegel process, J. Solid State Chem. 178 (2005) 329e338. [20] J. Zhang, M.J. Li, Z.C. Feng, J. Chen, C. Li, UV Raman spectroscopic study on TiO2. I. Phase transformation at the surface and in the bulk, J. Phys. Chem. B 100 (2006) 927e935. [21] J. Zhang, Q. Xu, M.J. Li, Z.C. Feng, C. Li, UV Raman spectroscopic study on TiO2. II. Effect of nanoparticle size on the outer/inner phase transformations, J. Phys. Chem. C 113 (2009) 1698e1704. [22] F. Zheng, Y. Liu, Z. Liu, Y.Q. Dai, S.J. Wang, Study on defect properties of nanocrystalline TiO2 during phase transition by positron annihilation lifetime, J. Cryst. Growth 353 (2012) 55e58. [23] J.M. Wu, T.W. Zhang, Photodegradation of rhodamine B in water assisted by titania films prepared through a novel procedure, J. Photoch Photobio A 162 (2004) 171e177. [24] C.C. Chen, W.H. Ma, J.C. Zhao, Semiconductor-mediated photodegradation of pollutants under visible-light irradiation, Chem. Soc. Rev. 39 (2010) 4206e4219. [25] X.M. Song, J.M. Wu, M. Yan, Photocatalytic degradation of selected dyes by titania thin films with various nanostructures, Thin Solid Films 517 (2009) 4341e4347. [26] T.L. Thompson, O. Diwald, J.T. Yates Jr., CO2 as a probe for monitoring the surface defects on TiO2 (110) temperature-programmed desorption, J. Phys. Chem. B 107 (2003) 11700e11704. [27] W.G. Su, J. Zhang, Z.C. Feng, T. Chen, P.L. Ying, C. Li, Surface phases of TiO2 nanoparticles studied by UV Raman spectroscopy and FT-IR spectroscopy, J. Phys. Chem. C 112 (2008) 7710e7716.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Unpredictable adsorption and visible light induced decolorization of nano rutile for the treatment of crystal violet Yanling Dong a, 1, Yang Liu b, 1, Dingze Lu b, 1, Feng Zheng b, Pengfei Fang b, *, Haining Zhang a, ** a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Department of Physics, Hubei Nuclear-Solid Physics Key Laboratory and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 November 2016 Received in revised form 26 January 2017 Accepted 31 January 2017 Available online 6 February 2017
Photocatalysts containing different ratios of anatase and rutile are prepared via heat treatment of Degussa P-25 titania. X-ray diffraction (XRD), Bruuauer-Emmett-Teller (BET), ultravioletevisible light diffuse reflectance spectra (DRS), Raman spectra (Raman), positron annihilation lifetime spectra (PAL) and temperature-programmed desorption (TPD) are applied to investigate the phase composition of the synthesized catalysts. Using crystal violet (CV) as the target pollutant, the unexpected visible light decolorization of rutile is observed. Despite the decreased specific surface area, the as-synthesized rutile samples exhibit much higher adsorption capability of CV than P-25 does, which in turn leads to improved photoreaction efficiency. Since the rutile samples can't absorb the visible light, the degradation under visible light irradiation is attributed to self-sensitization of CV on the surface of rutile. © 2017 Elsevier Masson SAS. All rights reserved.
Keywords: Rutile Crystal violet Adsorption Decolorization efficiency
1. Introduction Semiconductor photocatalysis is a powerful tool for pollution control [1e3]. As an important photocatalyst, TiO2 has received widespread concern [4e6]. The photocatalytic activity of TiO2 is closely related to its phase composition [7]. Although rutile (Eg ¼ 3.0 eV) can absorb more light than anatase (Eg ¼ 3.2 eV), and it is more stable than anatase [8], it is generally agreed that anatase is a better photocatalyst. There are mainly two reasons: the first, there are more adsorbed oxygen molecules on the surface of anatase [9], and it is a crucial factor during the photoreaction; the second, anatase always has a larger specific surface area [10,11], which can provide more opportunities to contact with pollutant molecules. Therefore, there are fewer researches on rutile than that on anatase in the field of photocatalysis. Except phase composition, the other factors, such as specific surface area, adsorption ability, and surface chemical condition, also have effects on the photocatalytic activity [12e15]. Rutile
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Fang), [email protected] (H. Zhang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.solidstatesciences.2017.01.011 1293-2558/© 2017 Elsevier Masson SAS. All rights reserved.
sometime can also exhibit high photocatalytic activity [16e18]. ez et al. [16] had synthesized H2-reduced TiO2 by calcinations Pa under H2 flow. It was found that rutile with low specific surface area obtained high adsorption ability and visible photoactivity. This unusual phenomenon indicates that rutile may have potential in dealing with some kinds of dyes under visible light irradiation. However, in order to make deeper research, it is necessary to evaluate the adsorption abilities and visible photoactivity of TiO2 prepared without H2 treatment. In this paper, adsorption abilities and decolorization efficiency of TiO2 containing different ratios of anatase/rutile are investigated. Different from the usual results, rutile prepared by simple heat treatment of Degussa P25 obtains the highest visible decolorization efficiency in degrading crystal violet (CV). Characterization techniques are used to study the phenomenon, and the possible mechanism is also discussed. 2. Experiment 2.1. Materials Titania P25 was product of Degussa Co. Ltd. CV was purchased from Shanghai Yuanhang Chemical Co. Ltd. P25 samples were heat treated in a conventional muffle furnace at 400, 500, 600, 700, 800,
2
Y. Dong et al. / Solid State Sciences 66 (2017) 1e6 Table 1 XRD, BET, and band-gap result of photocatalysts. Samples
WAa
W Rb
Crystallite size (nm)
SBET (m2/g)
BE (eV)c
P25 h-400 h-500 h-600 h-700 h-800 h-900
81.38% 82.10% 74.41% 46.32% 1.58% 0.00% 0.00%
18.62% 17.90% 25.59% 53.68% 98.42% 100.00% 100.00%
A 21.8 A 21.4 A 22.0 A 24.1 e e e
57.5 54.9 49.1 40 14.4 9.7 3.4
3.18 3.17 3.15 3.08 3.06 3.02 3.01
a b c
R R R R R R R
30.0 37.1 40.6 42.1 54.6 63.3 75.0
WA presents the phase content of anatase. WR presents the phase content of rutile. BE is the band gap energy of samples.
2.2. Characterization The X-ray diffraction (XRD) patterns were obtained by a Bruker D8 advance X-ray diffractometer using monochromatic Cu Ka radiation (l ¼ 1.5406 Å) with an accelerating voltage of 40 kV and current of 40 mA. Horiba LabRAM HR Raman spectrometer, with excitation wavelength of 488 nm was used to characterize the structure of photocatalysts. The Bruuauer-Emmett-Teller (BET) surface area was measured on a Beijing JWGB JW-BK using N2 adsorption at 196 C. The positron annihilation lifetime spectra (PAL) were measured by a conventional fast-fast coincidence system with time resolution of 300 ps. Diffusive reflectance UVevis absorption spectra (DRS) were collected on a Shimadzu UV-2550 spectrophotometer using integrating sphere attachment with barium sulfate as a reference. Temperature-programmed desorption (TPD) of CO2 was measured on a TP-5080 adsorption instrument (Tianjin Xianquan Co.). The samples were pretreated in N2 atmosphere at 400 C to remove the surface adsorbate. The CO2 adsorption process was taken at 25 C, and CO2 desorption was measured with heating rate of 10 C/min. 2.3. Decolorization evaluation The photoreaction was taken under a 160 W high-pressure Hg lamp (Shanghai Minghua Co.). The light focused onto a 200 mL of beaker filled with 100 mL of 20 mg/L CV solution and 0.10 g of photocatalyst. The beaker was put in a water tank to slow down the increase of temperature. Before irradiation, the suspension had been magnetically stirred in darkness for 1 h. Every 30 min, 1.6 mL of the suspension was collected and centrifuged to remove the photocatalyst. During the visible photoreaction, a cut-off filter (l > 420 nm) was put on the top of the beaker. The concentration of CV was measured by the UVevis absorption spectra. 3. Results and discussion 3.1. Photocatalysts characterization
Fig. 1. XRD patterns (a), Raman spectra (b), and average positron lifetime tav (c) of titania containing different ratio of rutile.
and 900 C for 3 h, respectively. The samples were denoted by h-x, where x represents calcination temperature.
XRD (Fig. 1a) and Raman spectra (Fig. 1b) were used to study the structures of different photocatalysts. In Fig. 1a, IA (101) at 25.6 is the strongest peak of anatase, and IR (110) at 27.7 is the strongest peak of rutile [19]. The phase content and particle size results (Table 1) suggest that the phase transition started at 500 C and ended above 700 C. From 500 C to 900 C, the crystallite size of rutile had grown up obviously from 40.6 nm to 75.0 nm. With the increase of heat treatment temperature, the intensities of the Raman peaks were getting stronger obviously (Fig. 1b), which is due to the formation of the dense structures during the heat treatment. The Raman shifts at 396, 515, and 636 cm1 are attributed to the B1g, A1g, and Eg modes of anatase. The Raman shifts at 449 and
Y. Dong et al. / Solid State Sciences 66 (2017) 1e6
Fig. 2. Decolorization efficiency of CV with irradiation of UVevis light (aeb) and visible light (dee); k value of UVevis light photoreaction (c) and visible light photoreaction (f).
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Y. Dong et al. / Solid State Sciences 66 (2017) 1e6
were annealed out [22]. The microstructure of rutile became more homogeneous and dense, which is in accordance with XRD, Raman, and BET results. 3.2. Unpredictable decolorization efficiency of rutile in degrading CV
Fig. 3. Decolorization efficiency by external surface area.
610 cm1 are attributed to the Eg and A1g modes of rutile, respectively [20,21]. The h-700, h-800, and h-900 samples only exhibit rutile vibration modes. Besides, the phase transition of anatase to rutile caused a rapid collapse in the specific surface area (SBET in Table 1). PAL (Fig. 1c) is a powerful technique for probing the vacancytype defects in materials due to its sensitivity to atomic-scale defects. From 300 C to 500 C, defects such as vacancy cluster and monovacancy were probed. Above 500 C, the average positron lifetime tav sharply decreased. The decrease indicates that the monovacancy, vacancy cluster, and void among grain boundaries
Decolorization was evaluated by degrading CV. The decolorization efficiency obey the first-order relationship (eln (C/C0) ¼ kapp t). The UVevis decolorization is shown in Fig. 2a and b. The h-500 TiO2 reveals the highest UVevis decolorization. Fig. 2c and d shows the visible decolorization efficiency. Surprisingly, the samples calcined at higher temperatures (600, 700, and 800 C) exhibit much higher decolorization efficiency under visible light irradiation. The rutile h-700 TiO2 has obtained the highest decolorization efficiency, which is 8.9-fold of that of P25. Using decolorization efficiency divided by specific surface area, k/SBET, to evaluate the visible decolorization efficiency (Fig. 3), interestingly, the result suggests that rutile samples (h-700, h-800, and h-900) have a very stable k/ SBET value, which is much higher than other samples. The results were accordance with the previous reports [23]. 4. Discussion Heat treatment brought a better crystallization for TiO2. However, phase transition occurred at the same time. The grain grew up obviously, and the specific surface area decreased fast. All of these factors can influence the decolorization efficiency of samples. The h-500 sample has kept 74.41% of anatase. Calcination at 500 C has not caused the sharp decrease in surface area. It still keeps a relative
Fig. 4. (a) The adsorption of CV on the negative electricity center of TiO2 surface; (b) visible light self-photosensitized oxidation of dye; (c) DRS of the samples calcined at different temperatures.
Y. Dong et al. / Solid State Sciences 66 (2017) 1e6
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Fig. 5. (a) CV adsorption in darkness for 1 h and specific surface area as a function of calcining temperature; (b) CO2 TPD results of titania containing different ratios of rutile.
high specific surface area (49.1 m2/g). Besides, its particle size is low (28.2 nm). Therefore, h-500 TiO2 exhibits the best UVevis decolorization efficiency. When it comes to the visible decolorization efficiency, it is reported that visible light-induced photoreactions of TiO2 occur mainly through two mechanisms: [24]. The first one is to lower the band gap of TiO2, so that it could make use of visible light. Many approaches such as metal ions, N, and C doping are employed for this purpose. The second way is through the self-photosensitized oxidation of surface adsorbed dye. CV is a kind of cationic dye, and it will be adsorbed on the negative electricity centers of TiO2 surface (Fig. 4a). The adsorbed CV will absorb visible light irradiation rather than TiO2 itself. The visible light excites the dyes, and the excited species can inject electrons into the conduction band of TiO2 (Fig. 4b). The injected electrons can combine with O2 on the surface of TiO2 to form O2e, and then through a serials of reactions to form OH in water condition. In order to test the light absorbing abilities of the samples, the DRS measurement was taken, and the results were displayed in Fig. 4c. According to the UVevis diffuse reflectance spectra (DRS), the absorption data are fitted to equation for indirect band-gap transitions: (ahn)1/2 ¼ A (hnEg). The bandgap energy (Eg) of samples can be estimated from the x-intercept of the tangents. The Eg values of P25, h-400, h-500, h-600, h-700, h800, and h-900 are 3.18, 3.17, 3.15, 3.08, 3.06, 3.02, and 3.01 eV, respectively. Besides, Eg values were also listed in Table 1. Heat treatment induced a red shift. However, none of the samples is capable to absorb the visible light, which means none of the samples can make use of the visible light to excite electro-hole pairs. Therefore, the decolorization efficiency of CV by rutile under visible light is through the self-photosensitized oxidation of dye. Adsorption of dye is a prerequisite for the electron injection. The CV adsorption result is shown in Fig. 5a. It could be observed that the adsorption does not decrease with the decreasing specific surface area, on the contrary, increases obviously. Adsorption ability of h-700 is 3.7-fold of that of P25. Considering the specific surface area of h-700 (14.4 m2/g) and P25 (57.5 m2/g), the adsorption efficiency divided by specific surface area of h-700 is 14.8-fold of that of P25. The h-800 and h-900 samples also adsorbed much more CV than that of P25, but their specific surface areas are only 9.7 and 3.4 m2/g, respectively. The result indicates that rutile has much higher adsorption of CV than that of anatase, and the high adsorption ability may have greatly enhanced the photodecomposition of CV [25].
CO2 is a kind of lewis acid, and can be used as a probe to identify the basic sites [26,27]. In order to study the surface difference between anatase and rutile, CO2 TPD test was performed to evaluate the surface basicity. From Fig. 5b, it can be seen that when the rutile content increases, CO2 desorption temperature increases obviously. P25 (18.6% of rutile) shows a main CO2 desorption peak at 78 C, while the h-600 (53.7% of rutile) exhibits CO2 desorption peaks at higher temperature of 187 C and 358 C, and h-700 (98.4% of rutile) shows CO2 desorption peaks at 175 C, 309 C, and 390 C. The higher desorption temperature means that it is harder for CO2 to escape from the surface. The high CO2 desorption temperature for rutile suggests that rutile may have a surface with higher basicity than that of anatase. Su et al. [27] reported that the basicity of surface OH groups of rutile phase is stronger than that of anatase phase. Our research is in accordance with this result. The performance of CV adsorption is similar to that of CO2. CV is a typical cationic dye, which is easier to be adsorbed on the basic surface [24]. Therefore, the TPD result may have interpreted the adsorption enhancement of CV of the rutile photocatalysts. This may be the reason for the unpredictable visible photoactivity of nano rutile in degrading CV.
5. Conclusion Rutile showed unpredictable visible decolorization efficiency in degrading CV, which is 8.9-fold of that of P25. Although SBET of rutile sample is only 14.4 m2/g (SBET of P25 is 57.5 m2/g), its adsorption of CV is 3.7-fold of that of P25. Rutile surface has higher basicity than that of P25, which is the reason for the enhanced adsorption and visible light decolorization of CV.
Acknowledgments This work was financially supported by the National Basic Research Program of China (973 Program, Nos. 2009CB939704 and 2009CB939705), the Natural Science Foundation of China (21576216), the Experimental Technique Foundation of Wuhan University (WHU-2014-SYJS-01), the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University (Nos. LF20150681 and 20150683).
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