J. Phys. Chem. C 2007, 111, 1119-1122
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Synthesis and Electrical Properties of Tungsten-Doped Vanadium Dioxide Nanopowders by Thermolysis Zifei Peng,* Wei Jiang, and Heng Liu Department of Chemistry, Shanghai Normal UniVersity, Guilin Road 100, Shanghai, People’s Republic of China 200234 ReceiVed: September 27, 2006; In Final Form: NoVember 14, 2006
Tungsten-doped vanadium dioxide (VO2) nanopowders were synthesized by thermolysis of (NH4)5[(VO)6(CO3)4(OH)9]‚10H2O at low temperature with, to the best of our knowledge, active white powdery tungstic acid (WPTA) used as a substitutional dopant for the first time. The change in electrical resistance due to the semiconductor-metal transition was measured from -5 to 150 °C by the four-probe method. Differential scanning calorimetry and the resistance-temperature curve of the nanopowders indicated that the phase transition temperature of VO2 powders was 67.15 °C, but for W-doped VO2, the temperature was reduced to 26.46 °C. The results indicated that WPTA was found to be exceptionally effective as a dopant for reducing the transition temperature.
Introduction Thermochromic (TC) materials, characterized by the ability of the electrical and optical properties to change reversibly with temperature, have attracted great interest and have been extensively studied.1 Vanadium dioxide (VO2) is a typical TC material. VO2 undergoes a metal-insulator transition at 68 °C, accompanied by a structural transformation between a lowtemperature monoclinic phase and a high-temperature tetragonal phase.2 Its electrical, magnetic, and optical properties in the IR region are associated with its phase transition temperature (Tc). Current understanding favors the picture of a Mott-Hubbard insulator (low-temperature phase) which is turned into a simple metal upon relaxation of a structural distortion above the transition temperature.3 Both scientific investigation and technological applications are desirable due to the temperature range of the structural transition, for uses such as temperature-sensing devices, optical-switching devices, laser protection, and solar energy control of windows.4,5 However, the phase transition temperature of VO2 should be reduced to nearly ambient temperature for an intelligent window material. It has been shown that the transition temperature of VO2 can be modified by doping. Tungsten and molybdenum are frequently used for this purpose because they produce reversely large Tc shifts for small dopant concentrations.6 Doping of VO2 thin films by sputtering, sol-gel methods, high-energy ion implantation, and introduction of dopants into the precursors have been reported in the past decades.7-9 Unfortunately, although the methods of film preparation have been investigated extensively, it has been found that these methods are not suitable for coating the surface of substrates with a large surface area and/or complex morphology. Compared with thin films, powders can overcome such shortcomings and work more effectively, and the cost is lower. Especially nanopowders can remarkably lessen the stress for the phase changes and have broader application, so more and more attention has begun to be paid to their synthetic techniques, such * Corresponding author. Telephone: ++86-21-64322513. E-mail:
[email protected].
as hydrothermal method, chemical vapor deposition, reductionhydrolysis method, liquid-phase hydrolysis method, and the evaporative decomposition of solutions (EDS) technique.10-14 However, all of these techniques are hard to put into practice, because of complex control parameters, unstable technology, and the necessity of special and expensive apparatus. Also, several engineering problems are associated with these processes. For example, metal ions were doped by bombarding, reacting with volatile salts, and heating the metals or their oxides at high temperature (>800 °C, as tungsten trioxide is inactive and its melting point is about 1473 °C). Preparation of vanadium oxide powders by thermolysis has been demonstrated by Zheng and Zhang.15 The process can be integrated into commercial production lines easily with a short cycle period and simple operation. In this paper, tungsten-doped vanadium dioxide nanopowders with reduced transition temperature were prepared by thermolysis using white powdery tungstic acid (WPTA) as the dopant. The phase transition temperature of 26.46 °C was close to room temperature. WPTA used as the dopant was found to be effective. Such doped VO2 nanopowders have the potential to be used as smart windows materials. Electrical properties of the powders were also described. Experimental Section Preparation of WPTA. White powdery tungstic acid (WPTA) was prepared by the method of Zhu and Gu.15 After a solution of sodium tungstate (Na2WO4‚2H2O, 5.0 g, 30 mL) was added into nitric acid (HNO3, 1.0 mol L-1, 45 mL), a yellowish precipitate was produced. Then the precipitate was filtered and washed with a solution of HNO3 (0.05 mol L-1, 100 mL), ethanol (95%, 20 mL), and ether (10 mL), respectively. Finally, WPTA was dried for 4 h at 100 °C. Preparation of Undoped and Tungsten-Doped Vanadium Dioxide. Vanadium dioxide was prepared by the method of thermolysis.16,17 A 9.60 mL volume of concentrated hydrochloric acid and a solution containing 1.68 g of hydrazine hydrogen chloride were added alternately into an aqueous suspension (7
10.1021/jp066342u CCC: $37.00 © 2007 American Chemical Society Published on Web 12/24/2006
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Figure 2. XPS survey spectrum of W-doped VO2. Figure 1. EDX patterns of W-doped nanopowders.
mL) containing 5.46 g of vanadium pentoxide. After warming under stirring, a blue solution was formed. Then the solution was filtered and a clear vanadyl chloride (VOCl2) solution (pH ≈1) was obtained. The VOCl2 solution was dropped slowly into 20 mL of an aqueous suspension containing 15 g of ammonium bicarbonate (NH4HCO3) with stirring while a flow of CO2 gas was introduced to prevent oxidation to VO2+ until the addition of VOCl2 solution was completed and violet crystals were produced. The crystals were filtered and washed with a saturated NH4HCO3 solution until no chloride could be detected; they were finally dried by a small amount of ethanol and ether, respectively. The precursor was spread in 8 mm × 80 mm cymbiform crucibles, heated in a tube furnace with 10 °C min-1 heating rate under a flow of nitrogen gas at 450-550 °C for 0.5-1.0 h, and cooled to room temperature in the nitrogen flow to prevent oxidation of vanadium dioxide. Nanopowders of VO2 were obtained through thermal decomposition. The doped VO2 was prepared in the same way. WPTA was added to the supersaturated solution of NH4HCO3 and stirred before the blue solution of VOCl2 was dropped into them. Analyses. The chemical composition of as-obtained samples was revealed by use of an energy-dispersive X-ray spectrometer (EDX) attached to a scanning electron microscope (SEM; JEOL Model JSM 6460). EDX analyses were employed to provide the chemical compositions of selected crystals. The valence state of tungsten was studied by X-ray photoelectron spectroscopy (XPS; Perkin-Elmer ESCA, PHI 5000C). The structure of the samples was determined by X-ray diffraction (XRD; Rigaku D/max-2200 and Cu KR radiation at 1.540 56 Å). The temperature was controlled using an aluminum temperature cell directed by RS resistive heaters. The microstructure and crystallinity were obtained by a transmission electron microscope (TEM; JEOL Model JEM 0200CX). Differential scanning calorimetry (DSC) experiments were performed using a Rigau differential thermal analyzer under nitrogen flow in the range of 0-200 °C with a heating rate of 5 °C min-1. The transition behavior observed in the resistance values of the samples were measured from -5 to 150 °C using the four-probe method. The powders were compacted with tablet machines and a silver paster was coated in a coplanar geometry to serve as the electrodes. Results and Discussion In Figure 1 and Table 1, EDX measurements showed only the expected amount of tungsten. The peaks that represent W,
Figure 3. W(4f) peaks of the sample containing WPTA (x ) 2 atom %).
TABLE 1: Chemical Compositions of W-Doped Nanopowders element
wt %
atom %
OK WM VK
34.38 12.00 53.61
65.78 02.00 32.22
V, and O elements were detected in the EDX pattern. Therefore, it was also revealed by the results that the W-doped nanopowders had a composition of V0.98W0.02O2. XPS analysis results of the surface composition are shown in Figure 2. The XPS results indicate that there are only four elements, carbon, vanadium, tungsten, and oxygen, in the spectrum where the carbon peak is from surface contamination. Figure 3 shows the W(4f) peaks of the sample containing WPTA (2 atom % WPTA) with binding energies of W(4f7/2) and W(4f5/2) at 35.6 and 37.6 eV, respectively. According to the standard binding energy, it is shown that there is a little tungsten in the sample, and the existing form of tungsten ions in these nanopowders is W6+. The XPS compositional results indicate the W-doped VO2 purity. The XRD patterns of the undoped and doped powders are shown in Figure 4, together with the standard powder pattern for polycrystalline VO2 (monoclinic) (PDF 44-0252). Figure 4 exhibits the well-known Bragg peaks of the monoclinic structure (P21/c14) VO2 low-temperature phase, and the Miller indices in the monoclinic system are marked. This phase was well crystallized, and no other vanadium oxides were observed. Some preferred orientation in the (011) direction was observed for the monoclinic VO2 samples, and the XRD pattern of the W-doped material indicated the monoclinic structure. In addi-
Properties of Tungsten-Doped VO2 Nanopowders
Figure 4. XRD patterns of the samples. Inset shows the XRD patterns of W-doped VO2 below and above Tc.
Figure 5. Transmission electron micrographs of W-doped VO2 nanopowders.
tion, WPTA peaks were not observed in the XRD pattern. The explanation could be that WPTA in doped powders played the role of the solute donor which forms a solution with VO2. The mean sizes of crystallites (L) can be calculated approximately using the Scherrer formula. As a result, L obtained with Bragg reflection at 2θ ) 27.98° corresponding to the (011) reflection is about 60 nm. The thermal transition was observed from a change of the monoclinic to the tetragonal structure during the heating process in the X-ray diffraction parameters. These findings are consistent with EDX and XPS results. It was observed that the diffractions between 20 and 30 °C were different in the inset of Figure 4. The exact transition temperature could not be ascertained here, as the sample temperature could only be measured accurately to 5 °C. The transformation of the diffraction peak removed the (011) plane from the monoclinic phase. At the same time, a new plane due to the (110) tetragonal plane appeared at slightly smaller 2θ. The sample annealed at 500 °C for 0.5 h was placed in absolute alcohol and dispersed by ultrasound for 15 min. The morphology of the powders was directly confirmed by the TEM. Figure 5 shows some spherical particles with weak agglomeration and narrow size distribution. As expected, the observed dimensions of each isolated domain of grain are in good agreement with the values of the mean sizes of crystallites calculated from the X-ray diffraction data. When the phase transition of VO2 occurred, it exhibited a noticeable endothermal profile in the DSC curve. The temperature of this endothermal profile corresponds to that of the VO2 phase change. Figure 6 shows the DSC curves of undoped and doped VO2 powders between 0 and 200 °C. It can be seen that there is an endothermic phase transition in each of the DSC curves. The phase transition temperature of VO2 powders prepared by thermolysis was 67.15 °C, which was slightly lower
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Figure 6. DSC curves of VO2 and W-doped VO2 nanopowders.
Figure 7. Effect of WPTA concentration on the phase transition temperature of the samples.
Figure 8. Typical transition behavior of the electrical resistance of VO2 (a) and W-doped VO2 nanopowders (b).
than that reported in the literature.13 However, for tungstendoped VO2, the temperature was reduced to 26.46 °C. By atmospheric press chemical vapor deposition (APCVD), Manning and Parkin18,19 got VO2 with Tc ) 29 °C for a 1.9 atom % tungsten-doped thin film to as low as 5 °C for a 3.0 atom % tungsten-doped VO2 thin film. Figure 7shows that the decrease in transition temperature was linear with tungsten atom percent incorporation. The variation in the electrical resistance of the nanopowders is shown in Figure 8 as a function of increasing and decreasing temperature, where it can be seen that the resistance value fell abruptly around 70 (a) and 30 °C (b). A heating-cooling hysteresis of about 8 °C was observed. The transition temperature of W-doped VO2 was lower than that of VO2, and also the resistance for W-doped VO2 was smaller than that of the
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TABLE 2: Electrical Characteristics of the S-M Transition in VO2 and W-Doped VO2 Compacted Powders sample designation VO2 W-doped VO2
transition temp, Tc (°C) heating cooling 71.4 29.2
62.2 20.7
latter. The transition temperature Tc of compacted powders seems to be shifted to a higher value. This could be due to the fact that the samples were heated from the supports and that a significant thermal gradient was probably involved in our experiments. The measured resistances of VO2 and W-doped VO2 were different. This clearly shows that VO2 is more resistive (semiconductor) than W-doped VO2 at the same temperature. These high values may be due to the formation of some insulator phase, even though X-ray diffraction of the film showed only the peaks of VO2, or alternatively the resistance of the samples may be dominated by the high resistance of the crystallite boundaries. The semiconductormetal (S-M) transition temperature Tc is defined for clarity as that corresponding to the peak in the curve obtained by plotting dR/d(1/T) versus T. The full width at half-maximum of this peak is taken as the transition width ∆Tc. The reversible S-M transition in VO2 is accompanied by a temperature hysteresis ∆Th, which is defined as the difference between the Tc values measured during the heating and cooling cycles. Upon heating above room temperature, a rather sharp S-M transition occurred at ∼68 °C, while an equally sharp metal-semiconductor (MS) transition was at ∼60 °C upon cooling. The electrical behavior of the two samples at the S-M transition is summarized in Table 2. Begishev et al.20 and Griffiths and Eastwood21 indicated that the transition behavior depends on both structural and compositional factors. The broadening of the transition behavior for doped samples was attributed to an inhomogeneous distribution of transition temperature because of the spatial inhomogeneity in the distribution of the factors leading to a change in transition temperature.20 The existence of the substantial amorphous phase caused the broadening of the transition behavior. The crystal imperfection also affected the transition temperature. The observed electrical characteristics of the S-M transition in the two samples may be understood from their microstructures as revealed by XRD, TEM, and DSC. Conclusions Well-crystallized and narrow size distribution nanopowders of the monoclinic polymorphs of vanadium dioxide and tungsten-doped vanadium dioxide were successfully synthesized by a thermolysis method with white powdery tungstic acid used as a substitutional dopant for the first time. It was shown that the existing form of tungsten ions in the nanopowders was W6+, and a significant direct correlation with Tc was confirmed. The
hysteresis width, ∆Th (°C) 9.2 8.5
transition width, ∆Tc (°C) heating cooling 16.4 15.0
13.4 10.7
transition temperature and sharpness of the transition behavior depended on the both structural and compositional factors. The semiconductor-metal transition was observed in the X-ray diffraction parameters from the change of the monoclinic to the tetragonal structure during the heating process. The electrical properties of the powders changed remarkably during heating and cooling between -5 and 150 °C. According to the DSC and resistance-temperature curves, the transition temperature of the W-doped powders was reduced to 26.46 °C accompanied by a heating-cooling hysteresis of about 8 °C. Such a low Tc is beneficial for the development and application of thermochromic materials. Acknowledgment. This work was supported by Shanghai Leading Academic Discipline Project (Project No. T402). We thank Prof. Lehan Wei for the supply of a constant current source. We acknowledge Mrs. Jinping Huang for her assistance with the XRD analysis. References and Notes (1) Guinneton, F.; Sauques, L.; Valmalette, J. C. Thin Solid Films 2004, 446, 287. (2) Morin, F. J. Phys. ReV. Lett. 1959, 3, 34. (3) Cavalleri, A.; Chong, H. W.; et al. Phys. ReV. Lett. 2001, 87, 237401. (4) Adler, D. J. ReV. Mod. Phys. 1968, 40, 714. (5) Yin, D. C.; Xu, N. K.; et al. J. Phys. D 1996, 29, 10511. (6) Goodenough, J. B. J. Solid State Chem. 1971, 3, 490. (7) Livage, J. J. Coord. Chem. ReV. 1999, 391, 190-192. (8) Deki, S.; Aoi, Y.; Kajinami, A. J. Mater. Sci. 1997, 32, 4269. (9) Lopez, R.; Boatner, L. A.; et al. Appl. Phys. Lett. 2001, 79, 31613163. (10) Takei, H. Jpn. J. Appl. Phys. 1968, 7, 827. (11) Valmalette, J. C.; Gavarri, J. R. Sol. Energy Mater. Sol. Cells 1994, 33, 135. (12) Guinneton, F.; Sauques, L.; et al. J. Phys. Chem. Solids 2001, 62, 1229. (13) Xu, C. L.; Ma, L.; Liu, X.; et al. J. Mater. Res. Bull. 2004, 39, 881. (14) Lawton, S. A.; Thby, E. A. J. Am. Ceram. Soc. 1995, 78, 104. (15) Zhu, S. S.; Gu, Y. D. Gaodeng Xuexiao Huaxue Xuebao (Chem. J. Chin. UniV.) 1982, 3, 137(in Chinese). (16) Zheng, C. M.; Zhang, J. L.; Luo, G. B.; et al. J. Mater. Sci. 2000, 35, 3425. (17) Thomas, C. W.; Li, P. J.; et al. J. Chem. Soc., Chem. Commun. 1986, 1597. (18) Manning, T. D.; Parkin, I. P. J. Mater. Chem. 2004, 14, 2554. (19) Manning, T. D.; Parkin, I. P.; Pemble, M. E.; Sheel, D.; Vernardou, D. Chem. Mater. 2004, 16, 744. (20) Begishev, A. R.; Galiev, G. B.; et al. SoV. Phys. Solid State 1978, 20, 951. (21) Griffiths, C. H.; Eastwood, H. K. J. Appl. Phys. 1974, 45, 2201.
