Materials Letters 65 (2011) 3047–3050 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w e l s ev i e r c o m / l o c a t e / m a t l e t Microwave-assisted synthesis and characterization of Ti1 − xVxO2 (x = 0.0–0.10) nanopowders Luc Huy Hoang a,⁎, Pham Van Hai a, Pham Van Hanh a, Nguyen Hoang Hai b, Xiang-Bai Chen c, In-Sang Yang d a Faculty of Physics, Hanoi National University of Education, 136 Xuanthuy, Caugiay, Hanoi, Vietnam Center for Materials Science, Hanoi University of Science, 334 Nguyen Trai, Hanoi, Vietnam 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 b c a r t i c l e i n f o Article history: Received 20 April 2011 Accepted June 2011 Available online 15 June 2011 Keywords: Ti1 − xVxO2 nanopowders Photocatalysts Microwave-assisted synthesis a b s t r a c t 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 (x N 0) 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 − xVxO2 nanoparticles are considered to be responsible for the enhanced absorption of visible light up to 800 nm © 2011 Elsevier B.V All rights reserved 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 TiO2 sensitive to visible light was intensively studied since the pure TiO2 shows 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 TiO2 into the visible region and enhance its photocatalytic activity, including doping TiO2 with 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 TiO2 has been one of the best candidates Nowadays, there are many reports on the synthesis of V doped TiO2 by 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 ⁎ Corresponding author E-mail address: hoanglhsp@hnue.edu.vn (L.H Hoang) 0167-577X/$ – see front matter © 2011 Elsevier B.V All rights reserved doi:10.1016/j.matlet.2011.06.034 to synthesize TiO2 [1] and TiO2 doped 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 TiO2 nanopowders has not been reported yet In the present study, we report a simple microwave-assisted synthesis to produce Ti1 − xVxO2 nanopowders The influences of the preparation methods and the concentrations of vanadium doping on the structural and optical properties of Ti1 − xVxO2 nanopowders were examined in detail Experimental 2.1 Preparation of vanadium doped TiO2 powders The V-doped TiO2 nanopowder–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, and 10% of vanadium) was added to the solution Titanium isopropoxide was then added dropwise under stirring The obtained solution was heated by a microwave oven at a power of 300 W for 20 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 h in air, respectively 3048 L.H Hoang et al / Materials Letters 65 (2011) 3047–3050 For comparison, pure TiO2 nanopowders 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 microRaman system in back scattering geometry with 532.5 nm laser excitation The SEM was carried out using Hitachi S-4800 fieldemission 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 Results and discussion Fig 1a shows the XRD patterns of the TiO2 nanopowder before and after calcination for 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 from Fig 1a that the intensities of the XRD peaks increase with increasing calcining temperature up to 600 °C, implying an improvement in crystallinity of TiO2 anatase 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 TiO2 calcined 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 Formation of spherical particles has been observed for all samples SEM images show particle diameter in range from 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 TiO2 lattice, but the particle size is slightly decreased as doping concentration increases (Fig b) The space group of the tetragonal anatase TiO2 belongs to D 194h, 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, B1g and Eg are Raman active and those of A2u and Eu are infrared active [27] Raman spectra of the TiO2 nanopowders calcined at different temperatures are shown in Fig 3a The frequencies of the Raman peaks agree with those in previous studies for anatase phase of TiO2 [27], i.e., 144 cm − (Eg), 197 cm − (Eg), 399 cm − (B1g), 513 cm − (A1g), 519 cm − (B1g) and 639 cm − (Eg) With increasing the calcination temperature up to 600 °C, blue shift of the most intense Eg peak, and decrease of the peak FWHM (full width at half-maximum) are observed (inset in Fig 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 − 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 TiO2 calcined at 600 °C It shows that the Eg peak shifted towards higher wavenumber with increasing V concentration to 2% (inset in Fig 3b) Since this Eg mode involves mainly Ti motion, the shift and broadening of Eg peak will be associated with the substitution of V to Ti in host lattice However, the asymmetric broadening of the 144 (Eg) (inset in Fig 3b) 519 (A1g) and 639 (Eg) cm − modes 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 Fig XRD patterns of TiO2 nanopowders calcined at different temperatures (a) and TiO2 doped with V after calcined at 600 °C (b) L.H Hoang et al / Materials Letters 65 (2011) 3047–3050 3049 Fig UV–Vis absorption spectra of Ti1 − xVxO2 calcined at 600 °C for h Fig SEM images of TiO2 (a) and TiO2 doped with 1%V (b) after calcined at 600 °C observed in 10% V-doping Similar asymmetric broadening was observed in earlier Raman measurements of a rutile–anatase mixture TiO2 by V Swamy [29] Fig shows 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 t2g level 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 concentration 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 (x N 0.05) nanopowders The contribution of the mixed crystal lattice on the reduction of energy gap of TiO2 has been reported earlier Fig Raman spectra of TiO2 nanopowders calcined at different temperatures (a) and TiO2 doped with V after calcined at 600 °C (b) 3050 L.H Hoang et al / Materials Letters 65 (2011) 3047–3050 [31,32] In conclusion, V-doping and subsequent coexistence of both anatase and rutile phases in our Ti1 − xVxO2 nanoparticles 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 Conclusions Ti1 − xVxO2 (x = 0.0–0.10) nanopowders have been successfully prepared using microwave assisted methods The anatase TiO2 phased 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 TiO2 is 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 TiO2 doped 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 the financial 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, 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