Materials Science-Poland, 30(2), 2012, pp. 151-157 http://www.materialsscience.pwr.wroc.pl/ DOI: 10.2478/s13536-012-0010-7
A facile synthesizing method of vanadium dioxide by pyrolyzing ammonium metavanadate J. Q I 1∗ , G. N ING 2 , R. H UA 1 , M. T IAN 1 , S. L IU 1 1 Department 2 Department
of Chemical Engineering, Life Science College, Dalian Nationalities University, 18 Laohe West Road, Dalian 116600, China
of Chemical Engineering of Material, School of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, China
A facile process was developed for synthesizing vanadium dioxide (VO2 ) by pyrolyzing ammonium metavanadate (NH4 VO3 ) in nitrogen flow. The process was designed on the base of thermodynamic modeling of chemical reaction and thermal analyzing of NH4 VO3 pyrogenation in N2 gas, and optimized by experiments. X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) were applied to characterizing the product VO2 . Thermo-gravimetric analysis (TG) and DSC were used for analyzing the process of NH4 VO3 pyrogenation. The experimental results indicated that VO2 microcrystal particles were successfully synthesized. The product VO2 presents two kinds of micro morphologies, torispherical and pentagonal prism. The phase transition mainly takes place at 338.4 K and 341.9 K. The average enthalpy of the phase transition is 28.82 J/g. Keywords: vanadium dioxide, ammonium metavanadate, phase transition, microstructure, pyrogenation c Wroclaw University of Technology.
1.
Introduction
Vanadium dioxide undergoes the first-order phase transition, from a semi-conducting state to a metallic state at a temperature around 341 K [1]. The phase transition of VO2 is reversible from a monoclinic crystal structure at low temperature to a tetragonal structure at high temperature, and is accompanied by the changes in optical and electrical properties [2]. Monoclinic VO2 is highly transparent in the infrared spectral band between 2.5 and 11.5 µm, and at the radio- and microwave- frequencies. The tetragonal structure at high temperature, however, strongly attenuates the incident electromagnetic radiation at all frequencies [3]. The material is of practical interest since its transition point occurs at a relatively low temperature of about 341 K, which can be easily reached by physical warming, heat dissipation, or laser excitation at a sufficient level [4]. A great deal of attention has been paid to ∗ E-mail:
[email protected]
vanadium dioxide due to its various technological applications such as optical switching devices, optical recording materials and infrared sensors, passive solar-energy control of buildings, and switchable/tunable microwave devices [5]. In the previous research works [6–10], VO2 films were prepared by a complicated process, requiring an exact partial pressure for reducing gas under vacuum conditions and during long periods of time. There were some reports regarding synthesizing of VO2 (B) nanobelts and nanorods [11–13]. It was noted that VO2 (B) is a metastable allotropic phase of vanadium dioxide [14]. On the basis of our previous works [15, 16], the authors developed the novel and simple process to synthesize VO2 micro-crystal particles by pyrolyzing ammonium metavanadate. In this paper, we report on the synthesizing process, thermodynamic analysis and characterization of the product VO2 .
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air-dried sample were used for the DSC. SEM images were obtained by a JSM-5600LV scanning electron microscope (JEOL, Japan) operated at 15 kV.
1.3. Analysis of the process of ammonium metavanadate Thermogravimetric (TG) data of NH4 VO3 were acquired with a thermogravimetric analyzer, Mettler-Toledo TGA/SDTA 851e (Mettler-Toledo GmbH, Schwerzenbach, Switzerland) in the temperature range of 350 – 850 K at a heating rate of 5 K/min in a nitrogen atmosphere. Thermal Fig. 1. Technical process of synthesizing VO2 by properties of NH4 VO3 were measured in a nitrogen NH4 VO3 pyrogenation. atmosphere by DSC-60A (Shimadzu, Japan) in the temperature range of 350 – 850 K at a heating rate of 5 K/min. 1.1. Preparation of vanadium dioxide The VO2 particles were synthesized by controlled pyrolysis of ammonium metavanadate (NH4 VO3 ). 800 mg NH4 VO3 (99.0%, Shanghai Chemical Limited Company of China, analytical reagent) were loaded into a ceramic container. The container loaded with NH4 VO3 was placed into a tube furnace. The technical process of synthesizing VO2 is illustrated in Fig. 1. A flow of N2 (99.99%, 20 ml/min) passed through the furnace and the heating rate was set on 15 K/min. The water was used to absorb the exhausted gas from the furnace. After the temperature reached 1000 K, the temperature was kept constant for 1.5 hours. The furnace was naturally cooled to 500 K in the N2 flow and the sample was ready for characterizations.
1.2. Characterization sample
of
the
product
The phase and crystallinity of VO2 were characterized using a XRD-6000 X-ray diffractometer (Shimadzu, Japan) in a reflection mode with Cu Kα(λ = 0.154 nm) radiation and a graphite monochromator. The 2θ scanning rate was 4 deg/min. Thermal properties of VO2 were measured in a nitrogen atmosphere by DSC 204 (Netzsch, Germany) in the temperature range of 305–375 K. A heating rate of 5 K/min and the
2.
Results and Discussion
2.1. Thermodynamic analysis of synthesizing VO2 by NH4 VO3 pyrogenation in N2 gas Thermodynamic properties of the pure substances in their standard states, including standard enthalpies of formation ∆f Hmo , standard Gibbs free energy of formation ∆f Gom and absolute o at 298.15 K, were taken from the entropies Sm literature [17]. Enthalpies of formation allow us to calculate the enthalpies of any reaction, provided that we know ∆f Hm◦ (T ) values for all the reactants and products [18]. The ∆r Hm◦ (T ) for any reaction is the difference between the sum of ∆r Hm◦ (T ) values for all the products and the sum of the ∆r Hm◦ (T ) values for all the reactants. The relationship is shown in equation 1. In the same way, if the absolute entropies of all the substances in a chemical reaction are known, it is then a simple matter to calculate the entropy change in the reaction. The relationship is shown in equation 2. The thermodynamic parameters for the reactions at other temperatures except 298.15 K were calculated with the equations 3, 4, 5, where “ν B ” are the stoichiometric coefficients of the substances participating in the reactions,
A facile synthesizing method of vanadium dioxide by pyrolyzing ammonium metavanadate
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◦ “T” denotes the reaction temperature, “m” the Table 1. ∆r Gm (T) for reactions 7, 8, 9 at the indicated temperatures. molar value, “◦ ” indicates that the reactants and products are in their standard state, “r” denotes the Temperture ∆r G◦m (T) (kJ/mol) parameters of reaction processes and “f” the data ◦ [K] Reaction (7) Reaction (8) Reaction (9) in the formation process of the substances, Cp, m is 298.15 87.36 -585.32 -491.92 heat capacity. In addition, the correlation between ◦ heat capacity Cp, m and temperature T is shown by 400 49.18 -586.88 -494.38 equation 6, where the values of a, b, c, d, e for 500 5.09 -603.40 -514.25 the pure substances in the reactions were given in 511.37 0 -605.24 -516.55 Ref. [17]. 600 -40.38 -619.13 -534.90 ◦ ◦ ◦ 700 -87.67 -633.68 -556.63 ∆r Hm (T ) = ∑ ∆f Hm (T )(products) − ∑ ∆f Hm (T )(reactants) (1) 800 -137.20 -646.68 -579.93 900 -189.20 -657.68 -605.38 1000 -244.10 -666.21 -633.70 ◦ ◦ ◦ ∆r Sm (T ) = ∑ Sm (T )(products) − ∑ Sm (T )(reactants) (2) 1100 -302.40 -671.80 -665.68 1119.80 -314.40 -672.51 -672.51 Z T 1200 -364.30 -673.94 -702.12 ◦ ∆r Hm◦ (T) = ∆r Hm◦ (298.15K) + v C dT ∑ B P, m
298.15K
(3)
in 8 and 9. Firstly, we calculated ∆r Hm◦ (298.15 K) ◦ v C ∑ ◦ B P, m ◦ ◦ and ∆r Sm ∆r Sm dT (298.15 K) for the reactions 8 and 9 (T) = ∆r Sm (298.15K) + T 298.15K by equations 1 and 2. Secondly, we calculated (4) ◦ ◦ ∆r Hm◦ (T ) and ∆r Sm (T ) by substituting CP, m in equations 3 and 4 for the correlation equations 6. ◦ ∆r G◦m (T) = ∆r Hm◦ (T ) − T ∆r Sm (5) The integrating process in equations 3 and 4 (T) included two parts: (1) for the temperature range ◦ where H O from 298.15 K to 373.15 K, CP, 2 m was assumed to be liquid H2 O(l), and (2) for Cp, m = a + bT + cT 2 + dT 3 + eT 4 (6) the temperature range from 373.15 K to 1000 K, ◦ CP, m where H2 O was assumed to be gaseous H2 O(g). The phase transition enthalpy for H2 O was considered in the calculations for equations 3 2NH4 VO3 = V2 O5 + 2NH3 + H2 O (7) and 4. Finally, Gibbs free energy ∆r G◦m (T) for the reactions was calculated using equation 5. The results are shown in Table 1. A negative value of ∆r G◦m (T) for a reaction V2 O5 + 2NH3 + O2 = 2VO2 + N2 + 3H2 O (8) means that the process is spontaneous. The data in Table 1 show that the reactions from 7 to 9 are spontaneous in a standard state over 2VO2 + 2NH3 + O2 = V2 O3 + N2 + 3H2 O (9) 511.17 K. Actual reactions always take place in a nonstandard state. The relationship between The decomposition of NH4 VO3 is described ∆ G r m (T) and pressure quotient (Q) for the reactions by reaction 7. The subsequent reactions in the in the nonstandard state is shown in equation 10. process of NH4 VO3 pyrogenation are shown The pressure quotients Q for the reactions from 7 Z T
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Table 2. Q ranges for synthesizing VO2 by NH4 VO3 pyrolysis.
to 9 are given by equations 11 and 12. ∆r Gm (T) = ∆r G◦m (T) + RT ln Q
(10)
2 P PH3 2 O PN2 PNH 3 H2 O Q = Q = (8) (9) 2 P P◦ (P◦ )3 PNH 3 O2 (K) (Pa / Pa ) (Pa / Pa ) 600 <3.28×103 3.70×1046 ∼7.98×1053 700 <3.49×106 3.45×1041 ∼1.94×1047 800 <9.02×108 7.36×1037 ∼1.68×1042 900 <9.56×1010 1.37×1035 ∼1.49×1038 1000 <5.66×1012 1.27×1033 ∼6.32×1034 1100 <2.29×1014 4.09×1031 ∼7.98×1031 Requirements: ∆r G◦m (T) < 0 for Reaction 7 and 8, ∆r G◦m (T) > 0 for Reaction 9
T
Q(7) =
2 P PNH 3 H2 O (P◦ )3
Q(8) = Q(9) =
PH3 2 O PN2 2 P P◦ PNH 3 O2
(11)
(12)
where R is the universal gas constant, PH2 O , PN2 , PNH3 are the partial pressures of H2 O and N2 , NH3 , respectively, and P◦ is the standard atmospheric pressure (101325 Pa). Table 1 also shows that when the reaction temperature is lower than 1119.8 K, ∆r G◦m (T) for reaction 8 is smaller than that for reaction 9. Equation 10 shows that ∆r Gm (T) for reaction 9 can be adjusted by increasing the pressure quotient Q(9) to a value greater than zero, and at same time ∆r Gm (T) for reaction 8 will be < 0. These are the requirements for forming pure VO2 free of V2 O3 . When the reaction temperature is higher than 1119.8 K, ∆r G◦m (T) of the reaction 8 is greater than that of the reaction 9. There is no way to meet the requirements of forming pure VO2 . Between 511.37 K and 1119.8 K the critical values of the pressure quotient, Q7 , Q8 and Q9 , where ∆r Gm (T) = 0, for the reactions 7, 8 and 9, respectively, were calculated using equation 10. When the pressure quotient Q(7) < Q7 and Q(8) is between Q9 and Q8 , then ∆r Gm (T) < 0 for reaction 7, ∆r Gm (T) < 0 for reaction 8 and ∆r Gm (T) > 0 for reaction 9. On this basis, the ranges of the pressure quotients Q(7) and Q(8) for synthesizing pure VO2 were deduced and shown in Table 2. In the present work, the temperature was designed at 1000 K, between 511.37 K and 1119.8 K. Table 2 shows that at 1000 K, when the pressure quotient Q(7) <5.66×1012 and Q(8) = Q(9) = 1.27×1033 – 6.32×1034 , the reactions 7 and 8 were spontaneous and the reaction 9 was not. This is the reason why pure VO2 crystal particles could be produced in these conditions.
Q(7) =
2.2. The structure, morphology and phase transition of the product VO2 The XRD pattern of the product sample (a) and a standard PDF card data (b) are shown in Fig. 2. By comparing the results with standard JCPDS card (PDF ID number 43-1051) of XRD data documents, the sample was matched to monoclinic phase group C2/m with the lattice constants a = ˚ , b = 4.517 A, ˚ and c = 5.375 A. ˚ No impurity 5.743 A peaks were found in the diffraction pattern. VO2 exhibits a first order semiconductor-to-metal transition at the temperature of about 341 K, accompanied by a crystallographic transition from a low temperature monoclinic phase space group ˚ ,b= C2/m with the lattice constants a = 5.743 A ˚ and c = 5.375 A ˚ to a high temperature 4.517 A, tetragonal rutile structure, space group P42/mnm ˚ and c = with typical lattice constants a = 4.530 A ˚ 2.869 A. [19] Scanning electron microscopy (SEM) results showed that the grain size distribution of the VO2 microcrystal is in the range from several micrometers to several dozen micrometers as can be seen in Fig. 3 and Fig. 4. The morphologies of the VO2 particles are spherical and pentagonal prism. Differential scanning calorimetric (DSC) studies indicated that the phase transition is an endothermic process. After fitting the multiple
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A facile synthesizing method of vanadium dioxide by pyrolyzing ammonium metavanadate
Fig. 4. SEM image (3500×) of the VO2 product.
Fig. 2. (a) X-ray diffraction pattern of the VO2 product, (b) JCPDF file data of VO2 (PDF number 43-1051).
Fig. 5. DSC curve of the VO2 product.
transition parameters. It is consistent with the recently published literature [20].
2.3. The analysis pyrogenation in N2 Fig. 3. SEM image (1000×) of the VO2 product.
peaks it was stated that the phase transition takes place mainly at 341.9 K and 338.4 K. The integration takes place between 315 K and 355 K, and the average enthalpy of the phase transition is 22.82 J/g as shown in Fig. 5. In Fig. 4 two kinds of VO2 morphologies can be recognized and in Fig. 5 two peaks can be found. This indicates that the grain morphologies may be the main factor to affect the VO2 phase
of
NH4 VO3
The thermogravimetric (TG) analysis of NH4 VO3 showed that the first weight loss occurs between 436.4 K and 523.6 K, the second is between 557.9 K and 619.6 K and the third is between 633.4 K and 674.7 K as it is illustrated in Fig. 6. As can be seen in Fig. 7, the differential scanning calorimetric (DSC) curves of NH4 VO3 exhibit two endothermic peaks and one exothermic peak.There is an endothermic peak at 487.2 K, an endothermic peak at 587.7 K and an exothermic peak at 650.9 K. Based on the TG results, we can
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V2 O3 formation should be between 1.27×1033 and 6.32×1034 .
3.
Fig. 6. Thermogravimetric curve of the NH4 VO3 reactant in nitrogen.
Conclusions
A facile process was developed for synthesizing VO2 microparticles by pyrolyzing NH4 VO3 in a tube furnace from room temperature to 1000 K with a rapid heating rate of 15 K/min in N2 (containing about 0.01% O2 ) at a flow rate of 5 ml/min. The sample was then kept at 1000 K for 1.5 h. The conditions of the pyrolysis of NH4 VO3 were designed by the thermodynamic modeling of chemical reaction process and the analysis of the process of NH4 VO3 pyrogenation in N2 gas, and then optimized by the experiments. The experimental results indicate that the designed process is a convenient and low cost method to synthesize VO2 crystal particles. As prepared, the VO2 particles have two types of morphologies, spherical and pentagonal prism. The phase transition takes place mainly at 341.9 K and 338.4 K. The average enthalpy of the phase transition is 22.82 J/g.
Acknowledgements Fig. 7. DSC curve of the NH4 VO3 reactant in nitrogen.
This work was financially supported by Fundamental Research Fund for the Central Universities (No. DC10030111) and the Research Fund for the Doctoral Program of Dalian Nationalities University (No. 20086105).
state that NH4 VO3 decomposition is the result of three processes, two endothermic processes lead to losing of NH3 and H2 O whereas the exothermic References process causes the resultant V2 O5 formation. As the result of NH4 VO3 decomposition [1] SOLTANI M., CHAKER M., HADDAD E., KRUZELECKY R.V., Appl. Phys. Lett., 85 (2004), process, N2 gas and VO2 were obtained. The 1958. pyrolysis of NH4 VO3 was carried out in a tube [2] HANLON T. J., WALKER R.E., COATH J.A., RICHARDSON M.A., Thin Solid Films, 405 (2002), furnace where the sample was heated from room 234. temperature to 1000 K with a rapid heating rate [3] HOOD P.J., DE NATALE J.F., J. Appl. Phys., 70 of 15 K/min. The process was conducted in N2 (1991), 376. atmosphere at the flow rate of 5 ml/min. The N2 [4] LIU H., VASQUEZ O., SANTIAGO V.R., DIAZ L., FERNANDEZ F.E., J. Lumin., 108 (2004), 233. contained trace amount of O2 (about 0.01%). The [5] YI X., CHEN C., LIU L., WANG Y., XIONG B., key aspect of the process is to maintain appropriate Infrared Phys. Technol., 44 (2003), 137. pressure quotients. When the temperature in the [6] WANG H., YI X., CHEN S., FU X., Sens. Actuators furnace reaches 1000 K, the pressure quotient A: Phys., 122 (2005), 108. for NH4 VO3 decomposition should be lower than [7] CHEN S., MA H., YI X., XIONG T., WANG H., KE C., Sens. Actuators A: Phys.,115 (2004), 28. 5.66×1012 , and the pressure quotients for VO2 and
A facile synthesizing method of vanadium dioxide by pyrolyzing ammonium metavanadate
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Received 2012-02-11 Accepted 2012-05-11
