Study of Chromium Modified TiO2 Nano Catalyst Under Visible Light

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Journal of Nanoscience and Nanotechnology Vol. 10, 1–5, 2010

Study of Chromium Modified TiO2 Nano Catalyst Under Visible Light Irradiation Yu-Ling Wei1 ∗ , Kai-Wen Chen1 , and H. Paul Wang2 3 1

Department of Environmental Science and Engineering, Tunghai University, Taichung City, 407, Taiwan 2 Department of Environmental Engineering, National Cheng Kung University, Tainan City, 701, Taiwan 3 Sustainable Environment Research Center, National Cheng Kung University, Tainan City, 701, Taiwan

Keywords: Nano TiO2 , Visible Light Active, Photocatalysts, Chromium-Doped TiO2 .

1. INTRODUCTION Titanium dioxide, a semi-conducting photocatalyst, has been attracting extensive research efforts due to its potential in solving serious environment pollution problems. It can decompose many organic and inorganic pollutants while irradiated with ultra violet (UV) light; however UV light only represents approximately 5% of total sun irradiation. Thus there have been increasing studies in investigating the property and function of the red-shift titanium dioxide doped with metallic compound(s) using various approaches, such as sol–gel method, chemical vapor deposition, and liquid phase deposition. Among them, sol–gel method is a low-temperature and low-cost process that can produce metal-doped titanium dioxide of high purity and homogeneity. To synthesize visible light response titanium dioxide catalyst, small amount of various noble metals or transitional metal compounds can be doped.1–6 The electrons can be effectively transferred away from hole, and ∗

Author to whom correspondence should be addressed.

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electron–hole pair recombination thus is delayed and the photocatalytic reaction rate is increased.1 Doping of transitional compounds can lower the energy level of conductance band; increasing the probability of electron excitation from valence band under the irradiation of visible light.5 Greater doping amount is always associated with more visible light absorption; however excessive doping dosage usually leads to lower photocatalytic rate.5 Only small doping amount (0.32–1.0 wt%) can result in enhanced photocatalytic reaction rate under the irradiation of UV or visible light.1 5 The objective of this study is to synthesize Cr-doped titanium dioxide with enhanced photocatalytic activity by the use of a simple modified sol–gel method in basic solution with Cr(NO3 3 as a precursor of Cr dopant. Unlike Cr(VI) oxyanion that is a human carcinogen, Cr(III) is an essential micronutrient.7 The synthesis recipe used in this study has not been previously reported. Characteristics of the nanocatalyst and photodegradation of methylene blue (MB) with the nanocatalyst are studied with various instrumental techniques.

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doi:10.1166/jnn.2010.1944

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Visible light active photocatalysts were successfully prepared by incorporating chromium into anatase TiO2 at two Cr/Ti atomic ratios (0.03% and 0.11%) by the use of a modified sol–gel process. Results show that the size of the chromium modified TiO2 particles is approximately 14–25 nm. As indicated by diffuse reflectance ultra violet/visible absorption spectra, heavier chromium dosage tends to result in greater absorption in both ultra violet and visible light. The simulation results from Cr K -edge X-ray absorption spectra suggest that Cr(0) and Cr(III), accounting for approximately 25% and 75% of total Cr, respectively, coexist in the TiO2 catalyst doped with 0.11% Cr. Cr dopant is suggested to be responsible for the phenomenon of enhanced light absorption in both ultra violet and visible regions. Further, Cr(0) can act as an electron remover because of the formation of the Schottky barrier between Cr(0) and TiO2 , thus reducing the possibility of electron hole recombination. Photo-catalytic degradation of methylene blue under irradiation of blue light (with peak flux at 460 nm wavelength and a small flux near 367 nm) was considerably enhanced under appropriate reaction time (12 and 24 h) as small amount of Cr was doped into anatase titanium dioxide catalyst. After prolonged reaction time, Cr(0) was suggested to be poisoned and/or oxidized by SO2− 4 , one of the final products of mineralizing methylene blue, thus loosing the capability of the electron hole separation.

Study of Chromium Modified TiO2 Nano Catalyst Under Visible Light Irradiation

To prepare Cr-doped TiO2 , pre-calculated amount of Cr(NO3 3 · 9H2 O salt (Cr/Ti atomic ratio: 0.1% and 0.5%) is dissolved in 75 mL 1-propanol, followed by the addition of 15 mL Ti(OC3 H7 4 with concomitant stirring for ca. 5 min, followed by drop-by-drop addition of 5-mL 1.0 N NaOH (pH 13.6) in 2–3 min span. This total solution was then thoroughly mixed for 24 h with a stirring speed of 250 rpm, and it turned into dry gel after being placed still at 25  C for 5 days and dried at 70  C for 24 hours. The dry gel was calcined from 30  C to 500  C with a temperature rising rate of 5  C/min; the temperature was held at 250  C and 500  C each for 30 min. The calcined material was washed with de-ionized water, dried at 70  C for 24 h, and ground to produce Cr-doped TiO2 nanocatalyst (note that although catalysts with Cr/Ti atomic ratios much greater than 0.5% were also studied, their photocatalytic activities were only comparable to pure TiO2 , thus the results are not presented in present study). The 0.1% and 0.5% nanocatalysts, termed as “L-catalyst” and “H-catalyst,” respectively were studied with inductively coupled plasma-atomic emission spectrometer (ICP-AES), N2 -based Brunauer-Emmett-Teller (BET) surface area analyzer, X-ray diffraction (XRD), scanning electron microscopy (SEM) with electron dispersive X-ray (EDX), transmission electron microscopy (TEM), diffuse reflectance ultraviolet/visible (DR-UV/Vis) spectrometer, and synchrotron-based X-ray absorption spectroscopy (XAS, recorded in NSRRC). All Cr K-edge (5989 eV) XAS spectra were recorded on wiggler C (BL-17C) beamline at the National Synchrotron Radiation Research Center (NSRRC) of Taiwan. During the XAS experimentation, the facility was operated at ring storage energy of 1.5 GeV, beam current of 120–200 mA, and energy resolution of 1.9 × 10−4 for the operated beamline. The energy resolution is defined as the average ratio between preset tunable energy increment step (approximately 0.3–6 eV) and scanning energy range (5789–6989 eV). All XAS data were reduced with WinXAS 3.1 software.8 Using Cr as an example, this software includes a function of simulating a sample X-ray absorption near-edge structure (XANES) spectrum to quantitatively determine Cr species fraction in a multiCr-species sample based on their fingerprints in the near edge region by linearly combining a set of XANES spectra from reference compounds,8 such as Cr(0), Cr(OH)3 , Cr2 O3 , and CrO3 in present study. The simulation process is based on the least-squares procedure, and two parameters (i.e., species fraction and energy correction) are calculated for each Cr reference compound.8 Upon finishing the simulation refinement process, any reference showing negative fraction or unreasonable energy shift is usually not present in the simulated sample.8 The photocatalytic degradation experiment of MB was performed in a batch-type reactor. The reactor system contains a cylindrical quartz chamber that is seated on a 2

magnetic stirring device and surrounded by a set of light source tubes. The entire reactor system was shielded with a stainless steel cover and placed in a dark room. In present study, the photocatalytic degradation reaction of MB was carried out under irradiation of eight 10-watt blue light tubes (peak flux at wavelength 450 nm, but also having a very small light flux near 367 nm) for 12, 24, and 36 h. One liter aqueous MB solution (concentration: 15 mg/L) containing 0.1 g Cr-doped TiO2 was placed in the quartz chamber and constantly stirred during the reaction period. Prior to and after the reaction, MB concentration in the solution was determined by measuring the level of light absorption at 664 nm with a spectrophotometer.

3. RESULTS AND DISCUSSION With the usage of microwave-assisted acid digester to extract total Cr from Cr-doped titania into acidic solution and ICP-AES for measuring Cr concentration in the solution, actual Cr contents in the L-catalyst and H-catalyst are determined to be only 0.03% and 0.11%, respectively. Obviously most Cr was washed out during catalyst preparation. This is not unpredictable because Cr(NO3 3 salt, the Cr dopant precursor, is water soluble. However, such low Cr content in the catalysts imposes a great difficulty on the XAS data reduction. Cr content in L-catalyst is too low to give usable XAS data, thus only H-catalyst was discussed regarding Cr molecular environment. BET results indicate that the surface areas are 72 (L-catalyst) and 76 m2 g−1 (H-catalyst), corresponding to 22 and 21 nm particle diameter based on the calculation assuming spherical shape for the catalysts. For pure TiO2 , the BET surface area is 78 m2 g−1 , corresponding to 20 nm in size. Figure 1 presents the XRD results from pure TiO2 , L-catalyst, and H-catalyst. Only the presence of diffraction 300 A

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2θ (degree) Fig. 1. XRD results from pure TiO2 , TiO2 doped with 0.03% at. Cr (L-catalyst), and TiO2 doped with 0.11% at. Cr (H-catalyst).

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Fig. 2. SEM morphologies from: (a) TiO2 doped with 0.03% atomic Cr (L-catalyst) and (b) TiO2 doped with 0.11% atomic Cr (H-catalyst).

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Wavelength (nm) Fig. 4. DR-UV/Vis results from pure TiO2 , TiO2 doped with 0.03% at. Cr (L-catalyst), and TiO2 doped with 0.11% at. Cr (H-catalyst).

absorption in both visible and UV regions for both Crdoped TiO2 is not a result of baseline drifting from instrumental instability because the absorption spectra can be well experimentally repeated. Despite the enhanced light absorption in both UV and visible regions resulting from Cr doping in TiO2 , the change in band gap of TiO2 incurred by Cr doping is negligible. Figure 5 shows the results from MB photocatalytic degradation using various catalysts. MB degradation is enhanced within 24-h reaction time by doping Cr in TiO2 at the levels presented in this study. To explain this fact, it is suggested that Cr(0) in the Cr/TiO2 catalyst act as electron remover from hole to lessen electron hole recombination process during the photocatalytic degradation of MB; note that the existence of Cr(0) is evidenced with Cr k-edge XANES (in Fig. 6). A Schottky barrier between Cr(0) and TiO2 is suggested to have formed; the electrons generated under light irradiation flow into Cr(0) from the TiO2 conduction band.9–11 The net charge on the Cr(0)-containing TiO2 is kept invariant despite the flow of electron into Cr(0). Pure TiO2 also has the capability of photo-degrading MB because the light source in present study contains a small flux near 367 nm. Figure 5 indicates 1.0 pure TiO2 H-catalyst L-catalyst

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Irradiation time (h) Fig. 5. Methylene blue photocatalytic degradation using pure TiO2 , TiO2 doped with 0.03% at. Cr (L-catalyst), and TiO2 doped with 0.11% at. Cr (H-catalyst).

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patterns from anatase TiO2 is confirmed. The characteristic peak locates between 25.28–25.48 degree. Based on the calculation with Scherrer formula from the information of half-height width of 2 peak (at 25.28–25.48 ), the particle diameters are determined to be 15 nm (for pure TiO2 ), 14 nm (L-catalyst), and 14 nm (H-catalyst), respectively. The difference in particle size resulting from the BET surface area measurement and the XRD diffraction patterns is due to their difference in the assumption of particle shape; spherical shape for BET method while cubic shape for the XRD method. No diffraction pattern associated with Cr is present in the XRD patterns from Cr-doped TiO2 in Figure 1, mostly due to the extremely low content of Cr in the samples. Figure 2 shows the SEM morphologies from L-catalyst and H-catalyst, as well as the EDS mapping result from H-catalyst. The particles are approximately 25 nm in size, and they agglomerate. EDS mapping (not shown here) result from H-catalyst shows that detectable elements are Ti and oxygen; Cr peak is non-detectable due to its small content. Figure 3 depicts TEM morphologies from L-catalyst and H-catalyst. It reveals that the particle is considerably uniform in size, and both catalysts is approximately 20–25 nm in size. Figure 4 presents the DR-UV/Vis results from pure TiO2 , L-catalyst, and H-catalyst. In visible region, light absorption was observed only for Cr-doped TiO2 with catalyst containing more Cr having greater absorption level. In UV region, enhancement in absorption is also observed for both Cr-doped catalysts, as compared with pure TiO2 ; H-catalyst has the greatest level. The observation of greater

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Study of Chromium Modified TiO2 Nano Catalyst Under Visible Light Irradiation

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Fig. 6. XANES simulation results from the XANES spectrum for TiO2 doped with 0.11% at. Cr (H-catalyst) using various Cr reference spectra.

that the MB degradation level, over 12–24 h, is in the following order: L-catalyst > H-catalyst > pure TiO2 . Similar trend has been previously reported that there exists an optimum doping amount of Cr and Fe for best photocatalytic performance under the irradiation of UV or visible light.1 5 The optimum dosages of Cr and Fe were 0.32–1.0 wt% depending on the type of target pollutants.1 5 The decrease in reaction rate constants by the use of TiO2 excessively doped with Cr has been attributed to the introduction of electron hole recombination center to the catalyst despite an enhanced visible light absorption.5 Thus, although the H-catalyst has greater UV/Vis light absorption than the L-catalyst (see Fig. 4), it is less effective in photocatalytic degradation of MB for 12–24 h reaction time. Figure 5 also shows that the MB degradation level measured at 36-h reaction time for all three catalysts is not different. This fact is suggested to be due to a poisoning and/or oxidation of the Cr dopant by the reaction product SO2− that is a 4 strong oxidant with +6 oxidation state for the S, leading to a decrease in separating electrons from holes. The final oxidation product, Cr2 O2− 7 , of Cr dopant is an oxyanion that would lose the capability in separating electrons from holes. Note that irreversible adsorption of fractional SO2− 4 , a product of MB mineralization by the use of TiO2 photocatalyst, has been reported in previous studies.12–15 We thus suggested that SO2− 4 might have poisoned and/or oxidized the Cr dopant that located on TiO2 surface. Regarding the MB degradation products, previous studies have shown that almost complete mineralization of MB into the + − final products CO2 , H2 O, SO2− 4 , H2 S, SO2 , NH4 , and NO3 in aqueous solution with TiO2 photocatalyst under UV/VIS irradiation is readily achieved because the generated OH free radicals are such a strong oxidant that they can open the aromatic rings of MB and further mineralize the intermediates into the final products.12–16 The mineralization of MB consists of a few chemical steps involving sequential attacks of OH free radicals on a series of aromatic intermediates, including phenyl-methyl-amine radicals, phenolic metabolites, and hydroxyquinone. Once the aromatic ring is opened, the intermediates are further oxidized into the final reaction products.12–16 Detailed reaction scheme 4

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of MB photodegradation by the use of TiO2 photocatalyst has been well formulated.13 15 16 Figure 6 presents XANES simulation results from H-catalyst using various Cr reference spectra (Cr(0), Cr(OH)3 , Cr2 O3 , and CrO3 . The Cr speciation in the H-catalyst consists of approximately 25% Cr(0) and 75% Cr(III). The formation of Cr(0) might result from the dissociation of Cr(OH)3 with a release of H2 O and O2 to oxidize the residual carbon during the calcination step; note that the Cr(OH)3 was generated by hydrolysis reaction of Cr(NO3 3 under the addition of NaOH. The observation here is different from that from the TiO2 doped with considerable amount of Cr (for example, 3–10 at% Cr/Ti which are not presented here due to their lack of photocatalytic activity); Cr(VI) is clearly observed in the 3–10% Cr-TiO2 catalysts. It is suggested that Cr(0) in the catalysts act as an electron remover from hole to lessen electron hole recombination process during the photocatalytic degradation of MB.

4. CONCLUSIONS The study observes the phenomenon of absorption in visible-light region for all Cr-doped TiO2 ; more Cr dosage leads to greater absorption level. Moreover, enhanced absorption is also observed in UV region for both Crdoped catalysts, as compared with pure TiO2 . However, TiO2 doped with lower Cr dosage (Cr/Ti ratio of 0.03 at%) shows greater activity in photocatalytica degradation of MB than that doped with 0.11 at% Cr. Results from XANES simulation reveal that fraction of the doped Cr(III) has been chemically reduced to Cr(0) that acts as an electron acceptor from hole because the Schottky barrier is suggested to form between Cr(0) and TiO2 , thus lessening the possibility of electron hole recombination. This may explain why the degradation level of MB is enhanced using TiO2 doped with Cr at low level. Various approaches to grow the photocatalyst on recoverable ceramic and glass substrate will be investigated in future although centrifuging process can readily recover the powder photocatalyst in this study. Acknowledgments: The authors thank Professor J.-F. Lee and the staff of NSRRC of Taiwan for their assistance in the XAS experiment. Special thanks are extended to Mr. Jing-Qiang Yang of Tunghai University for his graphing work and XAS data processing.

References and Notes 1. V. Vamathevan, H. Tse, R. Amal, G. Low, and S. McEvoy, Catal. Today 68, 201 (2001). 2. S. X. Liu, Z. P. Qu, X. W. Han, and C. L. Sun, Catal. Today 93–95, 877 (2004). 3. E. P. Reddy, B. Sun, and P. G. Smirniotis, J. Phys. Chem. B 108, 17198 (2004).

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4. M. A. Barakat, H. Schaeffer, G. Hayes, and S. Ismat-Shah, Appl. Catal. B: Environment 57, 23 (2005). 5. C. C. Pan and J. C. S. Wu, Mater. Chem. Phys. 100, 102 (2006). 6. Y. Mizukoshi, Y. Makise, T. Shuto, J. Hu, A. Tominaga, S. Shironita, and S. Tanabe, Ultrason. Sonochem. 14, 387 (2007). 7. M. D. Szulczewski, P. A. Helmke, and W. F. Bleam, Environ. Sci. Technol. 31, 2954 (1997). 8. T. Ressler, J. Synchrotron Rad. 5, 118 (1998). 9. I. M. Arabatzis, T. Stergiopoulos, D. Andreeva, S. Kitova, S. G. Neophytides, and P. Falaras, J. Catal. 220, 127 (2003). 10. U. Siemon, D. Bahnemann, J. J. Testa, D. Rodríguez, M. I. Litter, and N. Bruno, J. Photoch. Photobio. A 148, 247 (2002).

11. A. L. Linsebigler, G. Lu, and J. T. Yates, Jr., Chem. Rev. 95, 735 (1995). 12. J. Luan, H. Cai, S. Zheng, X. Hao, G. Luan, X. Wu, and Z. Zou, Mater. Chem. Phys. 104, 119 (2007). 13. H. Lachheb, E. Puzenat, A. Jouas, M. Ksibi, E. Elaloui, C. Guillard, and J.-M. Herrmann, Appl. Catal. B: Envrion. 39, 75 (2002). 14. J.-M. Herrmann, C. Guillard, and P. Pichat, Catal. Today 17, 7 (1993). 15. A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, and J.-M. Herrmann, Appl. Catal. B: Environ. 31, 145 (2001). 16. T. Zhang, T. Oyama, A. Aoshima, H. Hidaka, J. Zhao, and N. Serpone, J. Photoch. Photobio. A 140, 163 (2001).

Received: 14 September 2007. Accepted: 11 April 2009.

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