The results revealed that TiO2 powders maintained the anatase phase for calcination temperature below 600 °C, but gradually changed to the rutile phase above 800 °C.. The absorption edge
Trang 1Microwave-assisted synthesis and characterization of
Ti 1 − x V x O 2 (x = 0.0 –0.10) nanopowders
Luc Huy Hoanga,⁎ , Pham Van Haia, Pham Van Hanha, Nguyen Hoang Haib, Xiang-Bai Chenc, In-Sang Yangd
a
Faculty of Physics, Hanoi National University of Education, 136 Xuanthuy, Caugiay, Hanoi, Vietnam
b Center for Materials Science, Hanoi University of Science, 334 Nguyen Trai, Hanoi, Vietnam
c
Department of Applied Physics, Konkuk University, Chungju 380–701, Republic of Korea
d
Department of Physics and Division of Nano-Sciences, Ewha Womans University, Seoul, 120-750, Republic of Korea
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 20 April 2011
Accepted 8 June 2011
Available online 15 June 2011
Keywords:
Ti 1− x V x O 2 nanopowders
Photocatalysts
Microwave-assisted synthesis
Ti1− xVxO2(x = 0.0–0.10) nanopowders were successfully synthesized by a microwave-assisted sol–gel technique and their crystal structure and electronic structure were investigated The products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and UV–Vis spectroscopy The results revealed that TiO2 powders maintained the anatase phase for calcination temperature below 600 °C, but gradually changed to the rutile phase above 800 °C The formation of the rutile phase was completed at 1000 °C For Ti1− xVxO2(x = 0.05) powders, the phase transformation appeared
at 600 °C The absorption edge of Ti1− xVxO2(xN0) powders broadened to the visible region with increasing V concentration and a strong visible light absorption was obtained with 10% V doping V doping and subsequent coexistence of both anatase and rutile phases in our Ti1− xVxO2nanoparticles are considered to be responsible for the enhanced absorption of visible light up to 800 nm
© 2011 Elsevier B.V All rights reserved
1 Introduction
In recent years, titanium dioxide (TiO2) has been investigated with
considerable attention due to their promising applications in many
areas such as thin-film optical devices, solar cells, gas sensor, and
photocatalyst [1,2] Especially, applications for photocatalyst are
challenging topics in environmental issues In this context, the
development of TiO2sensitive to visible light was intensively studied
since the pure TiO2shows fascinating photocatalytic activities only
under UV light, which limited the use of a small UV fraction of natural
solar light (ca 3%) Many techniques have been examined to extend
the spectral response of TiO2into the visible region and enhance its
photocatalytic activity, including doping TiO2with non-metals such as
N, F ,C, B, S, I[3–6], or metals such as Cr, V, Mo, Cr, Fe, Nd[6–11], or
mixture of those elements[12,13] Among them, V-doped TiO2has
been one of the best candidates
Nowadays, there are many reports on the synthesis of V doped
TiO2by different techniques, including co-precipitation method[14],
metal ion-implantation method[15], sol–gel reaction[16]and wet
chemical method [17] Recently, microwave-assisted method has
been widely used for synthesizing nanoparticles, due to the
advantages of short reaction time, energy saving, and high reaction
rate[2,18–21] This microwave-assisted method has been employed
to synthesize TiO2[1]and TiO2doped with various ions such as La and
Zr [22], Ag [23], N [21] and so on However, to our knowledge, microwave-assisted synthesis of V-doped TiO2nanopowders has not been reported yet
In the present study, we report a simple microwave-assisted synthesis to produce Ti1− xVxO2nanopowders The influences of the preparation methods and the concentrations of vanadium doping on the structural and optical properties of Ti1− xVxO2nanopowders were examined in detail
2 Experimental 2.1 Preparation of vanadium doped TiO2powders The V-doped TiO2nanopowder–photocatalysts were prepared by the following procedures Titanium(IV) isoproxide (Ti{OCH(CH3)2}4, 97%), vanadyl acetylacetonate (98%), urea (99%) and thiourea (99%) (from Aldrich) were utilized for the synthesis Thiourea and urea were first dissolved in deionized water The desired amount of vanadyl acetylacetonate (corresponding to 1, 2, 5, 7 and 10% of vanadium) was added to the solution Titanium isopropoxide was then added drop-wise under stirring The obtained solution was heated by a microwave oven at a power of 300 W for 20 min The resulted precipitate was separated by centrifugation, then washed with deionized water for several times The products were followed by drying at 70 °C, and then calcining at 200, 400, 600, 800 and 1000 °C for 2 h in air, respectively
⁎ Corresponding author.
E-mail address: hoanglhsp@hnue.edu.vn (L.H Hoang).
0167-577X/$ – see front matter © 2011 Elsevier B.V All rights reserved.
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Trang 2For comparison, pure TiO2nanopowders were also prepared with the
same procedures described above
2.2 Characterizations
The XRD patterns for all the samples were obtained from Siemens
D5500 X-ray diffractometer using CuKα (λ=1.540 Å) radiation The
Raman scattering was performed using Jobin–Yvon T64000
micro-Raman system in back scattering geometry with 532.5 nm laser
excitation The SEM was carried out using Hitachi S-4800
field-emission instrument UV–VIS diffuse reflectance spectra (DRS) were
recorded in the range of 250–1000 nm on pressed pallets of these
powder samples by JASCO V670 UV–VIS spectrophotometer
3 Results and discussion
Fig 1a shows the XRD patterns of the TiO2nanopowder before and
after calcination for 2 h in air at different temperatures The XRD patterns
show that all peaks are clearly assigned to either the anatase or the rutile
phase The anatase phase appeared even before calcination While
according to Ref.[24], anatase phase only appeared above 300 °C Thus
we believe that the microwave irradiation process itself has heating
effect It can be seen fromFig 1a that the intensities of the XRD peaks
increase with increasing calcining temperature up to 600 °C, implying an
improvement in crystallinity of TiO2anatase phase Higher temperature
calcining lead to the formation of rutile phase At 1000 °C, anatase peaks
completely disappeared while rutile peaks greatly increased
Fig 1b shows the XRD patterns of V-doped TiO2calcined at 600 °C
No characteristic peaks of vanadium oxide impurities were observed
with V doping up to 10% However, the increase of the XRD peak
positions with V concentration indicates the decrease of the lattice
parameters This can be attributed to the substitution of Ti (0.61 Å) sites
by V ions with smaller ionic radius (0.54 Å), consistent with earlier
studies [25,26] On the other hand, Ti1 − xVxO2 (x= 0.05) shows a
mixture of anatase and rutile phases, while Ti1 − xVxO2(x= 0.10) is
nearly in the rutile phase This indicates that increasing V concentration
decreases the temperature of anatase-to-rutile phase transformation
SEM micrographs of nanopowders calcined at 600 °C are shown in
Fig 2 Formation of spherical particles has been observed for all samples SEM images show particle diameter in range from 6 to 50 nm (Fig 2a) Similar morphology has also been observed for all other vanadium doped samples, there is no change in morphology when V enters into the TiO2lattice, but the particle size is slightly decreased as doping concentration increases (Fig 2b)
The space group of the tetragonal anatase TiO2belongs to D19
4h, with two formula units per primitive cell Group theory predicts the following irreducible representation of normal vibrations: 1A1+ 1A2u+ 2B1g+ 1B2u+ 3Eg+ 2Eu Among them, B1gand Egare Raman active and those of A2uand Euare infrared active[27] Raman spectra
of the TiO2 nanopowders calcined at different temperatures are shown inFig 3a The frequencies of the Raman peaks agree with those
in previous studies for anatase phase of TiO2[27], i.e., 144 cm− 1(Eg),
197 cm− 1(Eg), 399 cm− 1(B1g), 513 cm− 1(A1g), 519 cm− 1(B1g) and
639 cm− 1(Eg) With increasing the calcination temperature up to
600 °C, blue shift of the most intense Egpeak, and decrease of the peak FWHM (full width at half-maximum) are observed (inset inFig 3a) This is well known behavior of increasing nanoparticle size and is explained by phonon confinement However, when the calcination temperature reached 800 °C, new Raman bands appear in the spectrum, as seen in Fig 3(a) After calcination at 1000 °C, these bands increase in intensity and the modes of the anatase phase disappear The exhibiting dominant peaks at 238, 446 and 611 cm− 1 can be assigned to the Raman active modes of the rutile phase[28] This is in good agreement with XRD result that the transformation of anatase to rutile phase appears at calcination temperature of 800 °C and at 1000 °C it is in the rutile phase completely
Fig 3(b) presents the Raman spectra of V-doped TiO2calcined at
600 °C It shows that the Egpeak shifted towards higher wavenumber with increasing V concentration to 2% (inset inFig 3b) Since this Eg mode involves mainly Ti motion, the shift and broadening of Egpeak will be associated with the substitution of V to Ti in host lattice However, the asymmetric broadening of the 144 (Eg) (inset inFig 3b)
519 (A1g) and 639 (Eg) cm− 1modes were observed with V-doping above 5% This asymmetric broadening was found to be due to the contribution of B1g, Eg and A1g mode of rutile phase, as clearly
Trang 3observed in 10% V-doping Similar asymmetric broadening was
observed in earlier Raman measurements of a rutile–anatase mixture
TiO2by V Swamy[29]
Fig 4shows the UV–vis spectra of the Ti1− xVxO2(x = 0.0–0.10) nanopowders The absorption edge of our TiO2 samples appeared around 405 nm (3.06 eV), which is red-shifted compared with the intrinsic bandgap of pure anatase TiO2(3.20 eV) With V-doping, extra broad absorption bands occurred at wavelengths of about 500 and
670 nm The broad absorption can be attributed to the charge transfer between valence band (VB) to the t2glevel of vanadium, which lies just below the conduction band[30]
Our absorption study shows that the V-doping shifts the absorption edge from UV to visible regimes; in addition the light absorption in the visible region increases with increasing V concen-tration Strong absorption in the visible range up to 800 nm is observed with 10% V-doping XRD and Raman studies revealed that, two crystallite structures, anatase and rutile, coexist in the Ti1 − xVxO2
(xN0.05) nanopowders The contribution of the mixed crystal lattice
on the reduction of energy gap of TiO2 has been reported earlier
Fig 2 SEM images of TiO 2 (a) and TiO 2 doped with 1%V (b) after calcined at 600 °C.
Fig 4 UV–Vis absorption spectra of Ti 1− x V x O 2 calcined at 600 °C for 2 h.
Trang 4[31,32] In conclusion, V-doping and subsequent coexistence of both
anatase and rutile phases in our Ti1− xVxO2nanoparticles prepared by
micro-wave assistance have enhanced the absorption of visible light
up to 800 nm Accordingly, improved photo-activity of the Ti1 − xVxO2
for the visible-light would be expected
4 Conclusions
Ti1 − xVxO2 (x = 0.0–0.10) nanopowders have been successfully
prepared using microwave assisted methods The anatase TiO2phased is
formed after microwave process and the crystalline quality is improved
after further calcining in air The anatase-to-rutile phase transition
temperature of the pure TiO2is about 800 °C but decreased to 600 °C
with 5% V-doping The evidences of incorporation of V in Ti sites have been
found from XRD and Raman results XRD and Raman studies revealed that,
two crystallite structures, anatase and rutile, coexist with V-doping higher
than 5% The strong visible light absorption was found in the TiO2doped
with 10% V V-doping and subsequent coexistence of both anatase and
rutile phase in our Ti1− xVxO2 nanoparticles are considered to be
responsible for the enhanced absorption of visible light up to 800 nm
Acknowledgements
The authors would like to thank the key project QGTD 10.29 of
Vietnam National University, Ha noi; Vietnam's National Foundation
for Science and Technology Development (NAFOSTED), grant
103.02.2010.04 and the National Research Foundation of Korea
Grant 2009-0063320 for thefinancial support
References
[1] Carp O, Huisman CL, Reller A Prog Solid State Chem 2004;32:33–177.
[2] Murugan A, Samuel V, Ravi V Mater Lett 2006;60:479–80.
[3] Burda C, Lou Y, Chen X, Samia AC, Stout J, Gole JL Nano Lett 2003;3:1049–51 [4] Li D, Haneda H, Hishita S, Ohashi N Chem Mater 2005;17:2596–602.
[5] Reddy KM, Baruwati B, Jayalakshmi M, Rao MM, Manorama SV J Solid State Chem 2005;178:3352–8.
[6] Zaleska A Recent Patents Eng 2008;2:157–64.
[7] Zhu J, Deng Z, Chen F, Zhang J, Chen H, Anpo M, et al Appl Catal B 2006;62:329–35 [8] Wu JS, Chen CH J Photochem Photobiol A 2004;163:509–15.
[9] Fuerte M, Maira AJ, Arias AM, Garcia MF, Conesa JC J Chem Commun 2001;24: 2718–9.
[10] Carneiro JO, Teixeira V, Portinha A, Dupak L, Magalhaes A, Coutinho P Vacuum 2005;78:37–46.
[11] Yamashita H, Harada M, Misaka J J Synchrotron Radiat 2001;8:569–71 [12] Rane KS, Mhalsiker R, Yin S, Sato T, Cho K, Dunbar E, et al J Solid State Chem 2006;179:3033–44.
[13] Ding J, Yuan Y, Xu J, Deng J, Guo J J Biomed Nanotechnol 2009;5:1–7 [14] Martin ST, Morrison CL, Hoffmann MR J Phys Chem 1994;98:13695–704 [15] Anpo M, Ichihashi Y, Takeuchi M, Yamashita H Res Chem Intermed 1998;24: 143–9.
[16] Bettinelli M, Dallacasa V, Falcomer D, Fornasiero P, Gombac V, Montini T, et al.
J Hazard Mater 2007;146:529–34.
[17] Songara S, Patra MK, Manoth M, Saini L, Gupta V, Gowd GS, et al J Photochem Photobiol A: Chem 2010;209:68–73.
[18] Ela SE, Cogal S, Icli S Inorg Chim Acta 2009;362:1855–8.
[19] Krishnakumar T, Jayaprakash R, Pinna N, Singh VN, Mehta BR, Phani AR Mater Lett 2009;63:242–5.
[20] Hoang LH, Hai PV, Hai NH, Vinh PV, Chen XB, Yang IS Mater Lett 2010;64:962–5 [21] Peng YP, Lo SL, Ou HH, Lai SW J Haz Mater 2010;183:754–8.
[22] Shojaie AF, Loghmani MH Chem Eng J 2010;157:263–9.
[23] Li X, Wang L, Lu X J Haz Mater 2010;177:639–47.
[24] Kim DJ, Hahn SH, Oh SH, Kim EJ Mater Lett 2002;57:355–60.
[25] Songara S, Patra MK, Manoth M, Saini L, Gupta V, Gowd GS, et al J Photochem Photobiol A 2010;209:68–73.
[26] Zhou M, Huang F, Wang X, du Plessis J, Murphy AB, Caruso RA Aust J Chem 2007;60:533–40.
[27] Ohsaka T, Izumi F, Fujiki Y J Raman Spectrosc 1978;7:321–3.
[28] Ma HL, Yang JY, Dai Y, Zhang YB, Lu B, Ma GH Appl Surf Sci 2007;253:7497–500 [29] Swamy V Phys Rev B 2008;77:195414–7.
[30] Tian B, Li C, Gu F, Jiang H, Hu Y, Zhang J Chem Eng J 2009;151:220–7 [31] Tseng YH, Kuo CS, Huang CH, Li YY, Chou PW, Cheng CL, et al Nanotechnology 2006;17:2490–7.
[32] Hsu CC, Wu NL J Photochem Photobiol A 2005;172:269–74.