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Effect of TiO2 on the photocatalytic properties of bismuth oxide a
a
Gourav Singla & K. Singh a
School of Physics and Materials Science, Thapar University, Patiala 147004, Punjab, India Published online: 21 Jan 2014.
To cite this article: Gourav Singla & K. Singh (2014) Effect of TiO2 on the photocatalytic properties of bismuth oxide, Environmental Technology, 35:12, 1520-1524, DOI: 10.1080/09593330.2013.871583 To link to this article: http://dx.doi.org/10.1080/09593330.2013.871583
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2014 Vol. 35, No. 12, 1520–1524, http://dx.doi.org/10.1080/09593330.2013.871583
Effect of TiO2 on the photocatalytic properties of bismuth oxide Gourav Singlaa and K. Singha∗ a School
of Physics and Materials Science, Thapar University, Patiala 147004, Punjab, India
Downloaded by [University of Connecticut] at 02:51 10 October 2014
(Received 22 September 2013; accepted 27 November 2013 ) Bi2−x Tix O3+x/2 (x = 0.05, 0.10 and 0.15) photocatalysts are prepared by solid-state technique. The band gap of the prepared sample was estimated from the onset of UV–Vis absorption spectra. The photocatalytic activities of as prepared samples are investigated under UV light irradiation for the decomposition of methyl orange (MO). It has been observed that the rate of decomposition of MO increased with increasing Ti4+ content in the present samples. Moreover, it shows better photocatalytic activity than undoped Bi2 O3 and TiO2 . The lowest band gap is observed for x = 0.15 sample, i.e. 2.55 eV, which also showed the highest photocatalytic activity in the present sample. Keywords: photocatalyst, Bi12 TiO20 , band gap, solid-state reaction, methyl orange
1. Introduction Water is a basic requirement for all industrial, domestic and commercial activities. Nevertheless, the pharmaceutical, textile, acrylic fibre, pesticides and other organic chemicals manufacturing industries, etc. generate waste water containing phenolic compounds in addition to various dyes.[1] Even small concentration of these compounds in waste water causes serious threat to all species on earth. Degradation of these nonbiodegradable organic compounds is not possible by conventional biological treatment processes. However, these pollutes can be broken or degraded using a proper photocatalyst. On irradiation of light, the photocatalyst generates an electron-hole pair that generates free radicals capable of undergoing secondary reactions. Recently, many catalysts like TiO2 , ZnO, ZrO2 , WO3 and SnO2 have been used widely for the photodegradation of water contaminants.[2– 4] Among all metal oxides, TiO2 became an effective photocatalyst after the decomposition of water.[5] However, it has a disadvantage due to its large band gap (3.2 eV),[6] which greatly limit its practical applications as photocatalyst. Different doped systems could be used to improve the photoactivity with maximum utilization of spectral portion of the visible light.[7–11] On the other hand, bismuth oxide (Bi2 O3 ) is another important photocatalyst having band gap of 2.8 eV that is lower than that of TiO2 .[6] The band gap of the materials can be modified by selecting proper dopant. Thus, the selection of dopant plays an important role in lowering the optical band gap of host materials. The dopant atoms create a new energy level or defects in the band region depending
∗ Corresponding
author. Email: [email protected]
© 2014 Taylor & Francis
upon the characteristics of the dopant. Apart from bismuthbased systems, another multiple-metal oxides material, also show very interesting and promising visible-light photocatalytic applications in the treatment of pollutants.[12] Bismuth-based mixed oxides have been widely investigated as visible-light-driven photocatalyst.[13–18] Yao et al. [19,20] have reported good catalytic efficiency of TiO2 -doped Bi2 O3 . However, depending upon the concentration of TiO2 , different crystalline phases have been formed in TiO2 -doped Bi2 O3 systems. Among all possible crystalline phases in the Bi–Ti–O system, Bi12 TiO20 phase has found to possess better degradation efficiency of organic pollutants.[20–22] Therefore, it is worthwhile to investigate the effect of TiO2 doping on the photocatalytic activity of Bi2 O3 . In the present study, the composition of (1-x) Bi2 O3 (x) TiO2 (0.05 ≤ x ≤ 0.15) is used for the decomposition of methyl orange (MO) dye under UV light irradiation.
2. Experimental details The oxides of Bi2 O3 and TiO2 were taken in appropriate stoichiometric amounts to prepare the composition of (1x) Bi2 O3 (x) TiO2 for x = 0.05, 0.10 and 0.15 by using solid-state reaction technique. The purity of the oxides was > 99.9%. Appropriate quantities of required constituent oxides of high-purity fine powders were thoroughly mixed in the presence of acetone for 2 h, using mortar and pestle and dried in the air. This mixture was heated at 700◦ C for 4 h in the air using recrystallized alumina crucible and slowly cooled to room temperature. The calcined powder was ground again for 2 h and heated at 790◦ C for
Downloaded by [University of Connecticut] at 02:51 10 October 2014
Environmental Technology 12 h in the air followed by furnace cooling to avoid the volatilization of Bi2 O3 . The optical properties of samples were studied in absorbance mode using a spectrophotometer (Champion) in the 200–600 nm wavelength range at room temperature. Fourier transform infrared (FT-IR) spectra were collected using Perkin Elmer RZX spectrometer in the range 400–4000 cm−1 . Photocatalytic properties of as-prepared samples were evaluated using MO as a model organic compound. The decolourization of MO solution was carried out in a glass vessel with constant magnetic stirring in the presence of 125 W UV lamp as a light source. Reaction suspension was prepared by adding the prepared samples (1.5 g/l) into a 50-ml of aqueous MO solution. To eliminate the effect of lamp’s heat, water was passed through the vessel surrounding the external chamber of the solution to maintain the temperature at around 25◦ C. Prior to irradiation, the suspensions were ultrasonically sonicated for 10 min and then magnetically stirred in a dark condition for 30 min to establish adsorption/degradation equilibrium. The suspensions were then irradiated under the UV light. After specific time intervals, samples were withdrawn from the bulk solution. The concentration of aqueous MO was determined by measuring the absorbance at 463 nm. The photocatalytic activity of the catalyst was evaluated by measuring the absorbance of aqueous MO at 463 nm as a function of illumination time.
3. Results and discussion 3.1. Absorption spectra It is confirmed by X-ray diffraction (XRD) pattern that the Bi2−x Tix O3+x/2 system exhibits a single-phase Bi12 TiO20 (BTO) in x = 0.15. Contrary to this, samples with x < 0.15 have a mixed phase viz. BTO and monoclinic α -Bi2 O3 as secondary phase.[23,24] Figure 1 shows the diffuse reflectance spectrum of the prepared Bi2−x Tix O3+x/2 (0 < x ≤ 0.15), and the values of optical band gaps obtained are listed in Table 1. The band gap energy is estimated from the onset of absorption, which shifts towards the visible region up to a particular dopant concentration. This indicates that the prepared samples are sensitive to visible light. In metal oxides, the valence band maximum (VBM) mainly consists of O(2p) orbital and the conduction band minimum mainly consists of M(nS) orbital.[25] It is interesting to note that undoped Bi2 O3 is a direct semiconductor. The doping of TiO2 leads towards the indirect band gap. Mizoguchi et al. [26] examined the mixed-valence state of Bi and pointed out that the Bi 6 s level lies above the O 2 p level and hybridized to form an occupied valence band (VB).[27] In case of Bi2−x Tix O3+x/2, visible absorption spectra can be attributed to Ti4+ by positioning a new energy band between the Bi3+ 6s and Bi3+ 6p band. Therefore, it has been observed that in Bi2−x Tix O3+x/2 electron excitation may be due to hybridized VB in the Ti 3d level. Because of this hybridization of the band, VB shifted
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Figure 1. UV–Visible diffuse-reflectance spectra for various members of the solid solution.
Table 1. Comparison of the optical band gap and apparent reaction rate constant for Bi2−x Tix O3+x/2 (0.05 ≤ x ≤ 0.15). x Pure TiO2 Pure Bi2 O3 0.05 0.10 0.15
Optical band gap (Eg ) (eV)
Apparent reaction rate constant (k/min)
3.2 [6] 2.8 [6] 2.62 2.57 2.55
0.0020 0.0035 0.0109 0.0145 0.0257
upwards narrowing the band gap. In addition, it has been observed that the optical energy band gap decreases with respect to pure monoclinic α-Bi2 O3 (Ebg = 2.85 eV) [28] by increasing TiO2 concentration up to x = 0.15. This nonlinear relationship between the composition and band gap energy might be arisen due to the substitution of Bi3+ (1.17 Å) by smaller Ti4+ (0.42 Å) cation. Therefore, the relative shift in the absorption edge of the sample also depends strongly on the difference between the ionic radius of the dopant and the host cations, as well as on the chemical nature of the dopants. Therefore, these two effects suggest that initially the decrement in the band gap energy is due to the incorporation of O2− anions in the vacant sites whose effect on lattice parameters is discussed in our earlier report.[23] This behaviour is very similar to the previously reported results,[23,29] where the authors observed a shift in the UV absorption spectra towards higher wavelength with the increase in oxygen anion (valence electrons) content. These extra incorporated NBO ions contribute to VBM. The non-bridging orbitals have higher energies than bridging orbitals.[30] Increase in concentration of the NBO ions results in the shifting of VBM to higher energy and reduce the band gap.[25]
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Hence, the large reduction of the fundamental band gap with the increase in Ti4+ concentration up to x = 0.15 can be attributed to easily charge transfer between the VBM and higher energy bands. Thus, absorption spectrum of the present samples indicates that variation in band gap value is inversely proportional to variation in lattice parameter with composition in Bi2−x Tix O3+x/2 .
3.2. FT-IR spectra To further access the chemical composition of the samples and, in particular, to identify the chemical bonding of the species that were not directly evident from the XRD results, FT-IR measurements were performed on the samples. As shown in the IR transmission spectra (Figure 2) of the powders, the main intensive peaks at 462, 529, 590, 662 and 1383 cm−1 correspond to the formation of sillenite phase BTO having Bi–O–Bi vibration modes as reported by many authors.[31,32] The obtained IR spectrum is in good agreement with earlier reports on this system. The major bands associated with Bi12 TiO20 crystals are observed in the present samples. The large bandwidth of this band is associated with polycrystalline characteristic of the samples.[33] The IR absorption band at 817 cm−1 seen from the spectrum is attributed to Bi2 O3 . At later stage, this band shifts to high wave number that may be due to the effect of the formation of Bi12 TiO20 crystals. The transmission bands at 462 cm−1 specific to the vibrations of Bi–O bonds in BiO6 polyhedral octahedral units and vibration absorption bands of the Bi–O-Bi at 840 cm−1 were clearly observed in the FT-IR spectrum for all compositions in units BiO3 , which decrease with increase in dopant concentration that is also clear from XRD results.[23] It has been observed that the octahedral site is more polarizable than the tetrahedral site.[34] The BiO6 polyhedral in Bi2−x Tix O3+x/2 may serve as active electron donor, which enhancing the electrons transfer to O2 and eliminating the recombination of photo-generated
Figure 2. 0.15).
FT-IR spectra of Bi2−x Tix O3+x/2 (x = 0.05, 0.10 and
electron-hole pairs creates favourable condition for higher photocatalytic activity. 3.3. Photocatalytic activity The photocatalytic activity of the catalyst can be improved to separate the photo-generated hole and electron pairs.[19, 20] It has been found that some oxides, such as Bi2 O3 , play a role of active oxygen donors in photo-oxidation process.[35] The results of photocatalytic activity of TiO2 doped Bi2 O3 are shown in Figure 3. It can be seen that because of higher band gap, undoped Bi2 O3 and absolute TiO2 have less degradation efficiency as compared with the present samples (Table 1). The dependence of the photocatalytic activity on the concentration of dopant is also shown in Figure 3. When the concentration of dopant (TiO2 ) was increased from x = 0.05 to 0.15, the absorbance of MO solution decreases slowly at 463 nm with increasing illumination time. The photocatalytic decolourization of MO seems to be pseudo-first-order reaction and its kinetic may be expressed as ln(A0 /A) = kt, where k is the apparent reaction rate constant, A0 is the initial absorbance of aqueous MO, t is the reaction time and A is the absorbance of aqueous MO at the reaction time of t.[36] The calculated apparent rate constant of each sample is listed in Table 1. The high photoreactivity of the prepared polycrystalline Bi2−x Tix O3+x/2 (0.05 ≤ x ≤ 0.15) may be related to two factors, the position of the valence band of the photocatalyst and the mobility of photo-generated carriers.[37] Such a kind of high mobility of photo-generated holes exists in Bi12 TiO20 ,[27] which play a crucial role in enhancing the high activity of the photocatalyst. Moreover, the existence of Bi–O polyhedra, in formed Bi12 TiO20 crystals, serves as active electron donor sites, which enhances the electron transfer to O2 and responsible to eliminate the recombination of electron-hole pairs.[19] Since it has been reported
Figure 3. Degradation profile of methyl orange in the presence Bi2−x Tix O3+x/2 (x = 0.05, 0.10 and 0.15), undoped Bi2 O3 and absolute TiO2 under UV light irradiation as a function of UV irradiation time.
Environmental Technology
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that the photoreactivities of doped catalyst are related to the dopant trap sites because of the trapping behaviour of Ti4+ ,[19,38] the charge trapping sites increases with the increase in dopant concentration TiO2 .[39] It enhances the resistance to recombination of electron-hole pairs by transferring the trapped charges to the interface. This initiate the photoreactions which leads to increase the photocatalytic activity of the present samples.
4. Conclusion In summary, Bi2−x Tix O3+x/2 (0.05 ≤ x ≤ 0.15) was prepared by the solid-state reaction method and their structural, optical and photocatalytic properties were studied. Fifteen percent of TiO2 contained single phase, i.e. Bi12 TiO20 phase. The band gap of Ti-doped Bi2 O3 decreased slightly with the increase in Ti content and was attributed to the introduction of oxygen ion. The photocatalytic activity of the present samples is higher than undoped Bi2 O3 and TiO2 . The band gap of sample x = 0.15 is lowest, i.e. 2.55 eV. The highest photocatalytic activity is observed for x = 0.15 sample.
Funding The authors are very grateful to the DRDO (Defence Research and Development Organization), New Delhi, India to provide the financial grant under vide Letter No. ERIP/ER/1103976/M/ 01/1411. The authors also thanks SAIF (Punjab University) for FTIR spectroscopy. One of the author also thanks Dr. O P Pandey, Jagdeep Kaur and Bhupinder Thakur for their contribution to this work.
References [1] Popa VI, Stingu AP, Volf I. Lignins and polyphenols in bioremediation. In: Fulekar MH, editor. Bioremediation technology. Dordrecht: Springer; 2010. p. 100–134. [2] Alinsafi A, Evenou F, Abdulkarim EM, Pons MN, Zahraa O, Benhammou A, Yaacoubi A, Nejmeddine A. Treatment of textile industry wastewater by supported photocatalysis. Dyes Pigm. 2007;74:439–445. [3] Kansal SK, Singh M, Sud D. Studies on photodegradation of two commercial dyes in aqueous phase using different photocatalysts. J Hazard Mater. 2007;141:581–590. [4] Abdollahi Y, Abdullah AH, Zainal Z, Yusof NA. Photodegradation of p-cresol by zinc oxide under visible light. Int J Appl Sci Technol. 2011;1:99–105. [5] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature. 1972;238:37–38. [6] Xu Y, Schoonen MAA. The absolute energy positions of conduction and valence bands of selected semiconducting minerals. Am Mineral. 2000;85:543–556. [7] Andronic L, Enesca A, Vladuta C, Duta A. Photocatalytic activity of cadmium doped TiO2 films for photocatalytic degradation of dyes. Chem Eng J. 2009;152:64–71. [8] Narayana RL, Matheswaran M, Aziz AA, Saravanan P. Photocatalytic decolourization of basic green dye by pure and Fe, Co doped TiO2 under daylight illumination. Desalination. 2011;269:249–253.
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[9] Abdollahi Y, Abdullah AH, Gaya UI, Zainal Z, Yusof NA. Enhanced photodegradation of o-cresol in aqueous Mn (1%)-doped ZnO suspensions. Environ Technol. 2011;33: 1183–1189. [10] Abdollahi Y, Abdullah AH, Zakaria A, Zainal Z, Masoumi HRF, Yusof NA. Photodegradation of p-cresol in Aqueous Mn (1%)-doped ZnO suspensions. J Adv Oxid Technol. 2012;15:146–152. [11] Liu S, Li C, Yu J, Xiang Q. Improved visible-light photocatalytic activity of porous carbon self-doped ZnO nanosheetassembled fowers. Cryst Eng Comm. 2011;13:2533–2541. [12] Kong L, Chen H, Hua W, Zhang S, Chen J. Mesoporous bismuth titanate with visible-light photocatalytic activity. Chem Commun. 2008;40:4977–4979. [13] Choi T, Lee S, Choi YJ, Kiryukhin V, Cheong SW. Switchable ferroelectric diode and photovoltaic effect in BiFeO3 . Science. 2009;324:63–66. [14] Zhao Y, Xie Y, Zhu X, Yan S, Wang S. Surfactant-free synthesis of hyperbranched monoclinic bismuth vanadate and its applications in photocatalysis, gas sensing, and lithiumion batteries. Chem Eur J. 2008;14:1601–1606. [15] Tang J, Zou Z, Ye J. Efficient photocatalytic decomposition of organic contaminants over CaBi2 O4 under visible-light irradiation. Angew Chem Int Ed. 2004;43:4463–4466. [16] Tang J, Zou Z, Ye J. Efficient photocatalysis on BaBiO3 driven by visible light. J Phys Chem C. 2007;111: 12779–12785. [17] Kudo A, Hijii S. H2 or O2 evolution from aqueous solutions on layered oxide photocatalysts consisting of Bi3+ with 6s2 configuration and d0 transition metal ions. Chem Lett. 1999;10:1103–1104. [18] Shimodaira Y, Kato H, Kobayashi H, Kudo A. Photophysical properties and photocatalytic activities of bismuth molybdates under visible light irradiation. J Phys Chem B. 2006;110:17790–17797. [19] Yao WF, Wang H, Xu XH, Zhou JT, Yang XN, Zhang Y, Shang SX. Photocatalytic property of bismuth titanate Bi2 Ti2 O7 . Appl Catal A Gen. 2004;259:29–33. [20] Yao WF, Wang H, Xu XH, Cheng XF, Huang J, Shang SX, Yang XN, Wang M. Photocatalytic property of bismuth titanate Bi12 TiO20 crystals. Appl Catal A Gen. 2003;243:185–190. [21] Zhang H, Lu M, Liu S, Xiu Z, Zhou G, Zhou Y, Qiu Z, Zhang A, Ma Q. Preparation and photocatalytic properties of sillenite Bi12 TiO20 films. Surf Coat Technol. 2008;202:4930–4934. [22] Xu S, Shangguan W, Yuan J, Shi J, Chen M. Photocatalytic properties of bismuth titanate Bi12 TiO20 prepared by co-precipitation processing. Mat Sci Eng B Solid. 2007;137:108–111. [23] Singla G, Jha PK, Gill JK, Singh K. Structural, thermal and electrical properties of Ti4+ substituted Bi2 O3 solid systems. Ceram Int. 2012;38:2065–2070. [24] Singla G, Singh K. Dielectric properties of Ti substituted Bi2−x Tix O3+x/2 ceramics. Ceram Int. 2013;39:1785–1792. [25] Pal I, Aggarwal A, Sanghi S. Spectral analysis and structure of Cu2+ doped cadmium bismuth borate glasses. Indian J Pure Appl Phys. 2012;50:237–244. [26] Mizoguchi H, Ueda K, Kawazoe H, Hosono H, Omata T, Fujitsu S. New mixed-valence oxides of bismuth: Bi1−x Yx O1.5+δ (x = 0.4). J Mater Chem. 1997;7: 943–946. [27] Zhou J, Zou Z, Ray AK, Zhao XS. Preparation and characterization of polycrystalline bismuth titanate Bi12 TiO20 and its photocatalytic properties under visible light irradiation, Ind. Eng Chem Res. 2007;46:745–749.
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[28] Dolocan V. Transmission spectra of bismuth trioxide thin films. Appl Phys. 1978;16:405–407. [29] Shan W, Walukiewicz W, Ager JW, Yu KM, Wu J, Haller EE, Nabetani Y, Mukawa T, Ito Y, Matsumoto T. Effect of oxygen on the electronic band structure in ZnOx Se1−x alloys. Appl Phys Lett. 2003;83:299–301. [30] Jose R, Suzuki T, Ohishi Y. Thermal and optical properties of TeO2 –BaO–SrO–Nb2 O5 based glasses: new broadband Raman gain media. J Non-Cryst Solids. 2006;352: 5564–5571. [31] Carvalho JF, Franco RWA, Magon CJ, Nunes LAO, Pellegrini F, Hernandes AC. Vanadium characterization in BTO:V sillenite crystals. Mater Res. 1999;2:87–91. [32] Feng YW, Hong W, Xia SS, Hong XX, Na YX, Dong W, Min W. Preparation and characterization of bismuth titanate Bi12 TiO20 nanocrystals. J Mater Sci Lett. 2002;21: 1803–1805. [33] Vasconcelos IF, Pimenta MA, Sombra ASB. Optical properties of Bi12 SiO20 (BSO) and Bi12 TiO20 (BTO) obtained by mechanical alloying. J Mater Sci. 2001;36: 587–592.
[34] Kumar PM, Badrinarayanan S, Sastry M. Nanocrystalline TiO2 studied by optical, FTIR and X-ray photoelectron spectroscopy: correlation to presence of surface states. Thin Solid Films. 2000;358:122–130. [35] Weng LT, Delmon B. Phase cooperation and remote control effects in selective oxidation catalysts. Appl Catal A. 1992;81:141–213. [36] Yu J, Zhao X, Zhao Q. Effect of surface structure on photocatalytic activity of TiO2 thin films prepared by sol–gel method. Thin Solids Films. 2000;379:7–14. [37] Sato J, Kobayashi H, Inoue Y. Photocatalytic activity for water decomposition of indates with octahedrally coordinated d10 configuration. II. Roles of geometric and electronic structures. J Phys Chem B. 2003;107:7970–7975. [38] Kaiser A, Feighery AJ, Fagg DP, Irvine JTS. Electrical characterization of highly titania doped YSZ. Ionics. 1998;4:215–219. [39] Thanabodeekij N, Gulari E, Wongkasemjit S. Bi12 TiO20 synthesized directly from bismuth (III) nitrate pentahydrate and titanium glycolate and its activity. Powder Technol. 2005;160:203–208.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Effect of TiO2 on the photocatalytic properties of bismuth oxide a
a
Gourav Singla & K. Singh a
School of Physics and Materials Science, Thapar University, Patiala 147004, Punjab, India Published online: 21 Jan 2014.
To cite this article: Gourav Singla & K. Singh (2014) Effect of TiO2 on the photocatalytic properties of bismuth oxide, Environmental Technology, 35:12, 1520-1524, DOI: 10.1080/09593330.2013.871583 To link to this article: http://dx.doi.org/10.1080/09593330.2013.871583
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2014 Vol. 35, No. 12, 1520–1524, http://dx.doi.org/10.1080/09593330.2013.871583
Effect of TiO2 on the photocatalytic properties of bismuth oxide Gourav Singlaa and K. Singha∗ a School
of Physics and Materials Science, Thapar University, Patiala 147004, Punjab, India
Downloaded by [University of Connecticut] at 02:51 10 October 2014
(Received 22 September 2013; accepted 27 November 2013 ) Bi2−x Tix O3+x/2 (x = 0.05, 0.10 and 0.15) photocatalysts are prepared by solid-state technique. The band gap of the prepared sample was estimated from the onset of UV–Vis absorption spectra. The photocatalytic activities of as prepared samples are investigated under UV light irradiation for the decomposition of methyl orange (MO). It has been observed that the rate of decomposition of MO increased with increasing Ti4+ content in the present samples. Moreover, it shows better photocatalytic activity than undoped Bi2 O3 and TiO2 . The lowest band gap is observed for x = 0.15 sample, i.e. 2.55 eV, which also showed the highest photocatalytic activity in the present sample. Keywords: photocatalyst, Bi12 TiO20 , band gap, solid-state reaction, methyl orange
1. Introduction Water is a basic requirement for all industrial, domestic and commercial activities. Nevertheless, the pharmaceutical, textile, acrylic fibre, pesticides and other organic chemicals manufacturing industries, etc. generate waste water containing phenolic compounds in addition to various dyes.[1] Even small concentration of these compounds in waste water causes serious threat to all species on earth. Degradation of these nonbiodegradable organic compounds is not possible by conventional biological treatment processes. However, these pollutes can be broken or degraded using a proper photocatalyst. On irradiation of light, the photocatalyst generates an electron-hole pair that generates free radicals capable of undergoing secondary reactions. Recently, many catalysts like TiO2 , ZnO, ZrO2 , WO3 and SnO2 have been used widely for the photodegradation of water contaminants.[2– 4] Among all metal oxides, TiO2 became an effective photocatalyst after the decomposition of water.[5] However, it has a disadvantage due to its large band gap (3.2 eV),[6] which greatly limit its practical applications as photocatalyst. Different doped systems could be used to improve the photoactivity with maximum utilization of spectral portion of the visible light.[7–11] On the other hand, bismuth oxide (Bi2 O3 ) is another important photocatalyst having band gap of 2.8 eV that is lower than that of TiO2 .[6] The band gap of the materials can be modified by selecting proper dopant. Thus, the selection of dopant plays an important role in lowering the optical band gap of host materials. The dopant atoms create a new energy level or defects in the band region depending
∗ Corresponding
author. Email: [email protected]
© 2014 Taylor & Francis
upon the characteristics of the dopant. Apart from bismuthbased systems, another multiple-metal oxides material, also show very interesting and promising visible-light photocatalytic applications in the treatment of pollutants.[12] Bismuth-based mixed oxides have been widely investigated as visible-light-driven photocatalyst.[13–18] Yao et al. [19,20] have reported good catalytic efficiency of TiO2 -doped Bi2 O3 . However, depending upon the concentration of TiO2 , different crystalline phases have been formed in TiO2 -doped Bi2 O3 systems. Among all possible crystalline phases in the Bi–Ti–O system, Bi12 TiO20 phase has found to possess better degradation efficiency of organic pollutants.[20–22] Therefore, it is worthwhile to investigate the effect of TiO2 doping on the photocatalytic activity of Bi2 O3 . In the present study, the composition of (1-x) Bi2 O3 (x) TiO2 (0.05 ≤ x ≤ 0.15) is used for the decomposition of methyl orange (MO) dye under UV light irradiation.
2. Experimental details The oxides of Bi2 O3 and TiO2 were taken in appropriate stoichiometric amounts to prepare the composition of (1x) Bi2 O3 (x) TiO2 for x = 0.05, 0.10 and 0.15 by using solid-state reaction technique. The purity of the oxides was > 99.9%. Appropriate quantities of required constituent oxides of high-purity fine powders were thoroughly mixed in the presence of acetone for 2 h, using mortar and pestle and dried in the air. This mixture was heated at 700◦ C for 4 h in the air using recrystallized alumina crucible and slowly cooled to room temperature. The calcined powder was ground again for 2 h and heated at 790◦ C for
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Environmental Technology 12 h in the air followed by furnace cooling to avoid the volatilization of Bi2 O3 . The optical properties of samples were studied in absorbance mode using a spectrophotometer (Champion) in the 200–600 nm wavelength range at room temperature. Fourier transform infrared (FT-IR) spectra were collected using Perkin Elmer RZX spectrometer in the range 400–4000 cm−1 . Photocatalytic properties of as-prepared samples were evaluated using MO as a model organic compound. The decolourization of MO solution was carried out in a glass vessel with constant magnetic stirring in the presence of 125 W UV lamp as a light source. Reaction suspension was prepared by adding the prepared samples (1.5 g/l) into a 50-ml of aqueous MO solution. To eliminate the effect of lamp’s heat, water was passed through the vessel surrounding the external chamber of the solution to maintain the temperature at around 25◦ C. Prior to irradiation, the suspensions were ultrasonically sonicated for 10 min and then magnetically stirred in a dark condition for 30 min to establish adsorption/degradation equilibrium. The suspensions were then irradiated under the UV light. After specific time intervals, samples were withdrawn from the bulk solution. The concentration of aqueous MO was determined by measuring the absorbance at 463 nm. The photocatalytic activity of the catalyst was evaluated by measuring the absorbance of aqueous MO at 463 nm as a function of illumination time.
3. Results and discussion 3.1. Absorption spectra It is confirmed by X-ray diffraction (XRD) pattern that the Bi2−x Tix O3+x/2 system exhibits a single-phase Bi12 TiO20 (BTO) in x = 0.15. Contrary to this, samples with x < 0.15 have a mixed phase viz. BTO and monoclinic α -Bi2 O3 as secondary phase.[23,24] Figure 1 shows the diffuse reflectance spectrum of the prepared Bi2−x Tix O3+x/2 (0 < x ≤ 0.15), and the values of optical band gaps obtained are listed in Table 1. The band gap energy is estimated from the onset of absorption, which shifts towards the visible region up to a particular dopant concentration. This indicates that the prepared samples are sensitive to visible light. In metal oxides, the valence band maximum (VBM) mainly consists of O(2p) orbital and the conduction band minimum mainly consists of M(nS) orbital.[25] It is interesting to note that undoped Bi2 O3 is a direct semiconductor. The doping of TiO2 leads towards the indirect band gap. Mizoguchi et al. [26] examined the mixed-valence state of Bi and pointed out that the Bi 6 s level lies above the O 2 p level and hybridized to form an occupied valence band (VB).[27] In case of Bi2−x Tix O3+x/2, visible absorption spectra can be attributed to Ti4+ by positioning a new energy band between the Bi3+ 6s and Bi3+ 6p band. Therefore, it has been observed that in Bi2−x Tix O3+x/2 electron excitation may be due to hybridized VB in the Ti 3d level. Because of this hybridization of the band, VB shifted
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Figure 1. UV–Visible diffuse-reflectance spectra for various members of the solid solution.
Table 1. Comparison of the optical band gap and apparent reaction rate constant for Bi2−x Tix O3+x/2 (0.05 ≤ x ≤ 0.15). x Pure TiO2 Pure Bi2 O3 0.05 0.10 0.15
Optical band gap (Eg ) (eV)
Apparent reaction rate constant (k/min)
3.2 [6] 2.8 [6] 2.62 2.57 2.55
0.0020 0.0035 0.0109 0.0145 0.0257
upwards narrowing the band gap. In addition, it has been observed that the optical energy band gap decreases with respect to pure monoclinic α-Bi2 O3 (Ebg = 2.85 eV) [28] by increasing TiO2 concentration up to x = 0.15. This nonlinear relationship between the composition and band gap energy might be arisen due to the substitution of Bi3+ (1.17 Å) by smaller Ti4+ (0.42 Å) cation. Therefore, the relative shift in the absorption edge of the sample also depends strongly on the difference between the ionic radius of the dopant and the host cations, as well as on the chemical nature of the dopants. Therefore, these two effects suggest that initially the decrement in the band gap energy is due to the incorporation of O2− anions in the vacant sites whose effect on lattice parameters is discussed in our earlier report.[23] This behaviour is very similar to the previously reported results,[23,29] where the authors observed a shift in the UV absorption spectra towards higher wavelength with the increase in oxygen anion (valence electrons) content. These extra incorporated NBO ions contribute to VBM. The non-bridging orbitals have higher energies than bridging orbitals.[30] Increase in concentration of the NBO ions results in the shifting of VBM to higher energy and reduce the band gap.[25]
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Hence, the large reduction of the fundamental band gap with the increase in Ti4+ concentration up to x = 0.15 can be attributed to easily charge transfer between the VBM and higher energy bands. Thus, absorption spectrum of the present samples indicates that variation in band gap value is inversely proportional to variation in lattice parameter with composition in Bi2−x Tix O3+x/2 .
3.2. FT-IR spectra To further access the chemical composition of the samples and, in particular, to identify the chemical bonding of the species that were not directly evident from the XRD results, FT-IR measurements were performed on the samples. As shown in the IR transmission spectra (Figure 2) of the powders, the main intensive peaks at 462, 529, 590, 662 and 1383 cm−1 correspond to the formation of sillenite phase BTO having Bi–O–Bi vibration modes as reported by many authors.[31,32] The obtained IR spectrum is in good agreement with earlier reports on this system. The major bands associated with Bi12 TiO20 crystals are observed in the present samples. The large bandwidth of this band is associated with polycrystalline characteristic of the samples.[33] The IR absorption band at 817 cm−1 seen from the spectrum is attributed to Bi2 O3 . At later stage, this band shifts to high wave number that may be due to the effect of the formation of Bi12 TiO20 crystals. The transmission bands at 462 cm−1 specific to the vibrations of Bi–O bonds in BiO6 polyhedral octahedral units and vibration absorption bands of the Bi–O-Bi at 840 cm−1 were clearly observed in the FT-IR spectrum for all compositions in units BiO3 , which decrease with increase in dopant concentration that is also clear from XRD results.[23] It has been observed that the octahedral site is more polarizable than the tetrahedral site.[34] The BiO6 polyhedral in Bi2−x Tix O3+x/2 may serve as active electron donor, which enhancing the electrons transfer to O2 and eliminating the recombination of photo-generated
Figure 2. 0.15).
FT-IR spectra of Bi2−x Tix O3+x/2 (x = 0.05, 0.10 and
electron-hole pairs creates favourable condition for higher photocatalytic activity. 3.3. Photocatalytic activity The photocatalytic activity of the catalyst can be improved to separate the photo-generated hole and electron pairs.[19, 20] It has been found that some oxides, such as Bi2 O3 , play a role of active oxygen donors in photo-oxidation process.[35] The results of photocatalytic activity of TiO2 doped Bi2 O3 are shown in Figure 3. It can be seen that because of higher band gap, undoped Bi2 O3 and absolute TiO2 have less degradation efficiency as compared with the present samples (Table 1). The dependence of the photocatalytic activity on the concentration of dopant is also shown in Figure 3. When the concentration of dopant (TiO2 ) was increased from x = 0.05 to 0.15, the absorbance of MO solution decreases slowly at 463 nm with increasing illumination time. The photocatalytic decolourization of MO seems to be pseudo-first-order reaction and its kinetic may be expressed as ln(A0 /A) = kt, where k is the apparent reaction rate constant, A0 is the initial absorbance of aqueous MO, t is the reaction time and A is the absorbance of aqueous MO at the reaction time of t.[36] The calculated apparent rate constant of each sample is listed in Table 1. The high photoreactivity of the prepared polycrystalline Bi2−x Tix O3+x/2 (0.05 ≤ x ≤ 0.15) may be related to two factors, the position of the valence band of the photocatalyst and the mobility of photo-generated carriers.[37] Such a kind of high mobility of photo-generated holes exists in Bi12 TiO20 ,[27] which play a crucial role in enhancing the high activity of the photocatalyst. Moreover, the existence of Bi–O polyhedra, in formed Bi12 TiO20 crystals, serves as active electron donor sites, which enhances the electron transfer to O2 and responsible to eliminate the recombination of electron-hole pairs.[19] Since it has been reported
Figure 3. Degradation profile of methyl orange in the presence Bi2−x Tix O3+x/2 (x = 0.05, 0.10 and 0.15), undoped Bi2 O3 and absolute TiO2 under UV light irradiation as a function of UV irradiation time.
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that the photoreactivities of doped catalyst are related to the dopant trap sites because of the trapping behaviour of Ti4+ ,[19,38] the charge trapping sites increases with the increase in dopant concentration TiO2 .[39] It enhances the resistance to recombination of electron-hole pairs by transferring the trapped charges to the interface. This initiate the photoreactions which leads to increase the photocatalytic activity of the present samples.
4. Conclusion In summary, Bi2−x Tix O3+x/2 (0.05 ≤ x ≤ 0.15) was prepared by the solid-state reaction method and their structural, optical and photocatalytic properties were studied. Fifteen percent of TiO2 contained single phase, i.e. Bi12 TiO20 phase. The band gap of Ti-doped Bi2 O3 decreased slightly with the increase in Ti content and was attributed to the introduction of oxygen ion. The photocatalytic activity of the present samples is higher than undoped Bi2 O3 and TiO2 . The band gap of sample x = 0.15 is lowest, i.e. 2.55 eV. The highest photocatalytic activity is observed for x = 0.15 sample.
Funding The authors are very grateful to the DRDO (Defence Research and Development Organization), New Delhi, India to provide the financial grant under vide Letter No. ERIP/ER/1103976/M/ 01/1411. The authors also thanks SAIF (Punjab University) for FTIR spectroscopy. One of the author also thanks Dr. O P Pandey, Jagdeep Kaur and Bhupinder Thakur for their contribution to this work.
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