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Current Applied Physics 12 (2012) 875e879

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Current Applied Physics

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Belt-like VO2(M) with a rectangular cross section: A new route to prepare,the phase transition and the optical switching properties

Yifu Zhang a, Meijuan Fan b, Fei Niu a, Weibing Wu a, Chi Huang a,*, Xinghai Liu b,*, Houbin Li b, Xin Liu c

aCollege of Chemistry and Molecular Sciences, Engineering Research Center of Organosilicon Compound and Material, Ministry of Education of China, Wuhan University,Wuhan 430072, PR Chinab School of Printing and Packaging, Wuhan University, Wuhan 430079, PR Chinac School of Life Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China

a r t i c l e i n f o

Article history:Received 17 October 2011Received in revised form29 November 2011Accepted 30 November 2011Available online 8 December 2011

Keywords:Vanadium oxidesVO2(M)Phase transitionThermal treatmentOptical switch

* Corresponding authors. Tel.: þ86 27 68752701; faE-mail addresses: chihuang@whu.edu.cn (C. Huang

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2011.11.024

a b s t r a c t

Belt-like VO2(M) with a rectangular cross section was first synthesized by the irreversible transformationof VO2(A) at the elevated temperatures under the inert atmosphere to the best of our knowledge. The as-obtained samples were characterized by a combination of techniques including XRD, SEM and TEM. Theprocesses of converting VO2(A) to VO2(M) were briefly discussed. The as-obtained VO2(M) has belt-likemorphology with a rectangular cross section with typical lengths up to several tens of micrometers,widths ranging from tens of nanometers to several micrometers, and thicknesses about 60e150 nm. Themorphology and size of the VO2(M) were dependent on that of the precursor VO2(A). The phase tran-sition properties of VO2(M) were investigated by DSC, indicating that it exhibited a strong phase tran-sition at 67.9 �C in the heating cycle and 61.1 �C in the cooling cycle. Furthermore, the optical switchingproperty of VO2(M) was studied by the variable-temperature infrared spectra, and it was found that theas-obtained VO2(M) could be used as the optical switch.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

In the past decades, vanadium oxides and their derivedcompounds as functional materials had been extensively studiedowing to their layered structures, specific chemical and physicalproperties, which make them possess a wide range of practical andpotential applications, such as, catalysts, cathode materials forreversible lithium batteries, gas sensors, intelligent thermochromicwindows, optical switching devices, laser shield, uncooled infrareddetectors, optical or electrical modulators and so on [1e9]. Of thefamily of vanadium oxides, a great deal of attention has been paid toVO2(M) [10e17] since a fully reversible first-ordermetal-to-insulatortransition (MIT) of VO2 was found in 1959 by Morin [18]. Themonoclinic vanadium dioxide VO2(M) undergoes the MIT with thephase transition temperature (Tc) at about 68 �C, accompanied bya crystallographic transition between a low temperature monoclinicphase (M) and a high temperature tetragonal phase (R) [17e19].Drastic changes occur in both electrical and optical properties beforeand after Tc. For example, the change in electrical resistivity in theorder of 105 and its infrared transmission characteristics changing

x: þ86 27 68754067.), liuxh@whu.edu.cn (X. Liu).

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dramatically over the phase transition [18,20]. Moreover, the Tc ofVO2(M) can be tuned by dopingwithW,Mo, Nb, F atoms, etc. or theirmixtures [8,15e17,21,22]. These features make VO2(M) to be consid-ered for applications in optical switching devices, temperature-sensing devices, laser protection, intelligent energy conservingwindows, optical or electrical devices and so on [1,8,15,23e25].

Up to now, a lot of methods [11,12,14e17,21,26e28], such as,chemical vapor deposition, pulse laser deposition, magnetronsputtering, vacuum evaporation, pyrolysis of precursor powders,reactive thermal, sol-gel process, hydrothermal method, etc. havebeen explored to prepare VO2(M) materials. For examples, VO2(M)particles were fabricated by thermolysis of a complex precursor(NH4)5[(VO)6(CO3)4(OH)9]$10H2O at 450 �C by Zheng [27]. VO2(M)nanobelts [11], nanorods [12] and nanosheets [26] were preparedby the thermal conversion of the metastable VO2(B). Recently,VO2(M) had been synthesized via the hydrothermal reaction ofV2O5 and oxalic acid [15,17,28]. The one-dimensional VO2(M) wassuccessfully synthesized using NH4VO3 and N5H37W6O24$H2O asthe precursors under hydrothermal condition and subsequentcalcinations reported by Li and his coworkers [16]. However, thereare not reports about the synthesis of VO2(M) using the precursorVO2(A). Various micro- and nano-structures of VO2(M), such asparticles, belts, ribbons, rods, wires, microspheres and thin films,have been synthesized by the above methods. However, the

Fig. 1. XRD patterns of the precursor VO2(A) and the resulting products obtained bythermal treatment of VO2(A) with different annealing temperatures for 2 h.

Y. Zhang et al. / Current Applied Physics 12 (2012) 875e879876

rectangular cross section as a novel morphology for VO2(M) hascomparatively rarely been reported. Guiton and his coworkers [29]reported that single-crystalline, well-faceted VO2(M) nanowireswith rectangular cross sections by a vapor transport method in2005. After that, VO2(M) with a rectangular cross section has notbeen reported. Therefore, although some methods to prepareVO2(M) have been reported in references, the new approach as wellas the belt-like morphology with a rectangular cross section forVO2(M) are still worthy of development.

Herein, we developed a new precursor VO2(A) to synthesizebelt-like VO2(M) with a rectangular cross section by the thermaltreatment under the inert atmosphere. The processes of thetransformation from VO2(A) to VO2(M) were briefly discussed inthis paper. To the best of our knowledge, it was the first time toprepare belt-like VO2(M) with a rectangular cross section using theprecursor VO2(A). Furthermore, the optical switching property ofthe as-obtained VO2(M) was studied by the variable-temperatureinfrared spectra, and it was found that the VO2(M) could be usedas the optical switch at different vibratory absorption bands.

2. Experimental

2.1. Synthesis of the precursor VO2(A)

All reagents used in the experiments were of analytical gradeand used without any further purification. In a typical synthesis,0.90 g of bulk V2O5 was dispersed in 35mL of redistilled water withmagnetic stirring. Then 4 mL of H2O2 (30 wt%) and 2 mL of absoluteethanol were successively added into the above solution. Thesolution was stirred for 1 h at room temperature to get goodhomogeneity. After mixing, the mixture was transferred intoa 60 mL stainless steel autoclave, then sealed and maintained at280 �C for 96 h. When the reaction was finished, the blue-blackprecipitate was filtered off, washed with distilled water andanhydrous alcohol several times, respectively, and dried in vacuumat 70 �C for future characterization and application.

2.2. Synthesis of belt-like VO2(M)

To synthesize belt-like VO2(M), the as-obtained VO2(A) washeated in a tube furnace with 5 �C/min heating rate under a flow ofargon (99.999%) gas at 400e700 �C for 2 h, and cooled to roomtemperature in the argon flow to prevent oxidation of VO2(M).

2.3. Characterization

X-ray powder diffraction (XRD) was carried out on D8 X-raydiffractometer equipment with Cu Ka radiation, l ¼ 1.54060 Å. Themorphology of the products was observed by scanning electronmicroscopy (SEM, Quanta 200) and transmission electron micros-copy (TEM, JEM-100CXII). The phase transition temperature of thesamples was measured by differential scanning calorimetry (DSC,DSC822e, METTLER TOLEDO) in a heating rate at 5 �C/min witha liquid nitrogen cooling system. Optical properties of the sampleswere tested by Fourier transform infrared spectroscopy (FT-IR,NICOLET 5700) with an adapted heating controlled cell. FT-IRpatterns of the solid samples were measured using KBr pellettechnique from 4000 to 400 cm�1 with a resolution of 4 cm�1.About 1 wt% of the samples and 99 wt% of KBr were mixedhomogeneously, and then the mixture was pressed to a pellet.

3. Results and discussion

The phases and structures of the samples were characterized byXRD measurements, as shown in Fig. 1. All the diffraction peaks

from Fig. 1b, can be readily indexed to the tetragonal crystallinephase (space group: P42/ncm 138) of VO2(A), in agreement with thereported values (JCPDS No. 42-0876, a ¼ 8.450 Å, c ¼ 7.686 Å) [30],whose plots are shown in Fig. 1a. These results reveal that theprecursor VO2(A) is successfully synthesized at 280 �C for 96 h. Inaddition, no peaks of any other phases, such as V2O3 and V2O5, aredetected from the XRD pattern, indicating the precursor VO2(A)with high purity, which is very important to prepare the pure phaseof VO2(M). However, there are no references reported about thetransformation from VO2(A) to VO2(M) by thermal treatment to thebest of our knowledge. Therefore, the as-obtained VO2(A) wasexplored as a new precursor to synthesize VO2(M) in this paper.Luckily, we found that VO2(M) could be also converted from VO2(A)as is the case of VO2(B) [11,26].

Fig. 1 cef shows the XRD patterns of the samples obtained atdifferent annealing temperatures ranging from 400 to 700 �C for2 h, which reveals that the processes of converting VO2(A) toVO2(M). When VO2(A) was treated at 400 �C for 2 h, only purephase of VO2(A) was obtained, as depicted in Fig. 1c, indicating thatVO2(A) cannot be transformed to VO2(M) or other phases of VO2 atthis condition. However, more andmore VO2(M) is formedwith the

Fig. 2. SEM images of the as-obtained VO2(M), insert higher magnification images.

Y. Zhang et al. / Current Applied Physics 12 (2012) 875e879 877

heating temperature increasing. The mixture contained differentphase of VO2 was obtained at 500 and 600 �C for 2 h, as shown inFig. 1d and e. Finally, the pure phase of VO2(M) was synthesized at700 �C for 2 h. All the peaks from Fig. 1f can be readily indexed tothe monoclinic crystalline phase (space group: P21/c, No. 14) ofVO2(M) in agreement with the literature values (JCPDS, No. 72-0514, a ¼ 5.743 Å, b ¼ 4.517 Å, c ¼ 5.375 Å) [31], whose plots areshown in Fig. 1g. No impurity phases, such as V3O7, V6O13, V2O3,V2O5, VO2(B) and VO2(A), are detected, indicating that the as-obtained VO2(M) is with high purity by the current syntheticroute, well in agreement with the pure phase of VO2(A), as shownin Fig. 1b.

The morphology of the samples was investigated by SEM andTEM tests. Fig. 2 represents the typical SEM images, which illustratethat the as-obtained VO2(M) predominantly consists of a largequantity of uniform nano- and micro-structures. We can occa-sionally see some rectangular cross sections, which are clearlyobserved from the higher magnification images inserted in Fig. 2,indicating the sample has belt-like morphology. The representativeTEM images (Fig. 3) further confirm the results of SEM analyses. Itcan be seen from Figs. 2 and 3, that the belt-like VO2(M) witha rectangular cross section consists of a large quantity of uniformnano- or micro-structures with typical lengths up to several tens ofmicrometers, widths ranging from tens of nanometers to severalmicrometers, and thicknesses about 60e150 nm.

The results based on the above analyses are well consistent withthe morphology of the precursor of VO2(A), whose SEM and TEMimages are represented in Figs. S1 and S2 (Supplementary data).However, the surface of as-synthesized VO2(M) is very roughcompared Figs. 2 and 3 with Figs. S1 and S2, as well as that there are

Fig. 3. TEM images of the as-obtained VO2(M).

more small particles in VO2(M) (Fig. 2) than that of VO2(A) (Fig. S1).The explanation could be due to the growth, transformation oraggregation by surface and boundary diffusion during thermaltreatment. Thus, themorphology of the VO2(M) is dependent on thatof the VO2(A), in agreement with the literature [11]. To further verifythis result, the precursor VO2(A) with different morphologies wascarried out to transform to VO2(M), and the corresponding SEMimageswere shown in Fig. S3 (Supplementarydata). Consequentially,themorphologies of VO2(M) is almost the same as that of VO2(A), too.In addition, it’s noted that the contrast grade in Fig. 3 (TEM image) isvery deep compared with Fig. S2, which is further proved the as-obtained sample is VO2(M) according to the recent report [32].

When the phase transition of VO2(M) occurs, it respectivelyexhibits noticeable endothermal and exothermal profiles in theheating and cooling DSC curves, which corresponds to the phasetransition of VO2(M). Fig. 4 shows the representative DSC curves ofVO2(M) obtained by calcining VO2(A) with heating and coolingcycles. The Tc of VO2(M) is about 67.9 �C in the heating cycle, whichis very in agreement with the Morin’s report [18]. While the Tc isabout 61.1 �C in the cooling cycle, which is due to the hysteresisbehavior of VO2(M).

Based on the DSC results of the as-obtained VO2(M), itundergoes a noticeable endothermic peak at about 67.9 �C in theheating cycle and a noticeable exothermal at about 61.1 �C in the

Fig. 4. DSC curve of the as-obtained VO2(M).

Fig. 5. Variable-temperature infrared spectra of the as-prepared VO2(M): (a) All of IR curves with different temperatures; (b) Selected some typical IR curves from (a) to clearlyreveal the process of the phase transition of VO2(M) before and after Tc; (c) Selected two IR curves from (a), one is below Tc, while the other is up Tc; (d) Selected two IR curves belowTc from (a), one is from the heating process, while the other is from the cooling process.

Y. Zhang et al. / Current Applied Physics 12 (2012) 875e879878

cooling cycle. It was reported that the drastic change occurs inoptical properties, accompanied by the phase transition of VO2(M/R). Therefore, in this paper, the as-obtained VO2(M)was explored asan optical switching device. The optical switching property ofVO2(M) was investigated by a series of variable-temperatureinfrared spectra of the heating and cooling cycles, as depicted inFig. 5. Fig. 5a shows all the curves obtained with the variable-temperatures IR tests. It can be clearly observed from Fig. 5a, thatVO2(M) has the optical switching property. To get a noticeableobservation, two typical results below (25 �C) and up Tc (96 �C)were selected to make Fig. 5c, which reveals that the opticaltransmission of VO2(M) at 25 �C is higher than that at 96 �C, sug-gesting that VO2(M) converted from VO2(A) has good thermo-chromic property. Furthermore, Fig. 5a and cboth reveal thatVO2(M) has potential applications in optical switching devices atthe vibratory absorption bands from 1000 to 800 cm�1 due to itslarge transmission charges. Fig. 5d represents two IR curves belowTc from Fig. 5a: one is from the heating process, while the other isfrom the cooling process. These spectra have the same opticaltransmission, indicating that the phase transition of the as-obtained VO2(M) has good reversibility. Fig. 5b describes theprocess of the phase transition of VO2(M) before and after Tc,indicating its Tc is at about 69 �C in the heating cycle and at about61 �C in the cooling cycle, which agrees with the results of DSCwell.Those properties of VO2(M) discussed from Fig. 5 verify that it isbeneficial for the development and application of an opticalswitching material.

The low temperature (T < Tc ¼ 68 �C) spectrum shows vibra-tional bands characteristic of the semiconducting phase in the 1000

to 400 cm�1 wavenumber range. It has been reported [25,26,33,34]that the optical spectra are usually explained as that the observedbands are related to the electromagnetic resonance between inci-dent photons (with a specific wavenumber), and variation ofchemical bond polarization associated with a specific vibrationmode. At low temperature (T< Tc ¼ 68 �C) the electrons involved inthe V4þeV4þ bonds between VO6 octahedra are localized. However,these electrons are delocalized at high temperature (T> Tc¼ 68 �C).In the metal state (T > Tc), this delocalization involves a screeningeffect for the incident photons, which occurs at the surface of thesample [25]. As a result, no vibrational absorption bands can beobserved. In this circumstance, the transmittance drasticallydecreases. The variable-temperature infrared spectra confirm thestrong reversible metal-semiconductor phase transition at around68 �C (Figs. 4 and 5). However, the heating and cooling curves areasymmetric in Fig. 5a and b, which indicates the hysteresis behaviorin the sample, in agreement with the DSC observation.

4. Conclusion

In conclusion, belt-like VO2(M) with a rectangular cross sectionhad been successfully by the irreversible transformation of VO2(A)at the elevated temperature under the inert atmosphere. To thebest of our knowledge, it was the first time to prepare belt-likeVO2(M) with a rectangular cross section using the precursorVO2(A). The processes of the transformation fromVO2(A) to VO2(M)were briefly discussed, and the pure phase of VO2(M) wassynthesized at 700 �C for 2 h. The as-obtained VO2(M) has belt-likemorphology with a rectangular cross section consisting of a large

Y. Zhang et al. / Current Applied Physics 12 (2012) 875e879 879

quantity of uniform nano- or micro-structures with typical lengthsup to several tens of micrometers, widths ranging from tens ofnanometers to several micrometers, and thicknesses about60e150 nm. The morphology and size of the VO2(M) were depen-dent on that of the precursor VO2(A). The phase transition prop-erties of VO2(M) was investigated by DSC, indicating that itexhibited a strong phase transition at 67.9 �C in the heating cycleand 61.1 �C in the cooling cycle. Furthermore, the optical switchingproperty of VO2(M) was studied by the variable-temperatureinfrared spectra, and it was found that the as-obtained VO2(M)could be used as the optical switch at different vibratory absorptionbands.

Acknowledgment

This work was partially supported by the Fourth Installment ofScience and Technology Development 2010 Program of Suzhou(SYG201005), the Fundamental Research Funds for the CentralUniversities, Independent Research Projects of Wuhan University(217274721) and Luojia Young Scholars Program (217273483).

Appendix. Supplementary material

Supplementary material related to this article can be foundonline at doi:10.1016/j.cap.2011.11.024.

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