Materials Research Bulletin 47 (2012) 1978–1986

Facile synthesis, phase transition, optical switching and oxidation resistanceproperties of belt-like VO2(A) and VO2(M) with a rectangular cross section

Yifu Zhang a, Yanfen Huang a, Juecheng Zhang a, Weibing Wu a, Fei Niu a, Yalan Zhong a, Xinghai Liu b,*,Xin Liu c, Chi Huang a,*a College of Chemistry and Molecular Sciences, 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 18 January 2012

Received in revised form 15 March 2012

Accepted 11 April 2012

Available online 16 April 2012

Keywords:

A. Oxides

B. Chemical synthesis

D. Microstructure

D. Optical properties

A B S T R A C T

Belt-like VO2(A) with a rectangular cross section (VA-RCS) was successfully synthesized using V2O5,

H2C2O4�2H2O and H2O as the starting materials by a facile hydrothermal approach. Some synthetic

parameters, such as, the reaction time, reaction temperature and concentration of H2C2O4�2H2O, were

systematically investigated to control the fabrication of belt-like VA-RCS. The formation mechanism of

belt-like VA-RCS was proposed. Subsequently, belt-like VO2(M) with a rectangular cross section (VM-

RCS) was prepared by the irreversible transformation of VA-RCS at 700 8C for 2 h under the inert

atmosphere. The phase transition temperature (Tc) of VA-RCS and VM-RCS was evaluated by DSC test. The

optical switching properties of VA-RCS and VM-RCS were studied by the variable-temperature infrared

spectra, and it was found that the as-obtained VA-RCS and VM-RCS could be used as the optical switching

materials. Furthermore, the oxidation resistance properties of VA-RCS and VM-RCS were investigated by

TGA, indicating that they have good thermal stability and oxidation resistance below 400 8C in air.

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Materials Research Bulletin

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1. Introduction

Increasing attention has been paid on low-dimensionalmaterials in recent years, because of their novel physical andchemical properties comparing with those of bulk materials [1–4].Nanobelts as a new class of nanostructures with a rectangular crosssection (RCS) have been the subject of intensive research due totheir specific chemical and physical properties and a wide range ofpotential applications in fabricating nanoscale electronic, optical,optoelectronic, electrochemical, and electromechanical devices[5–9]. Besides, nanobelts can be an ideal system for fullyunderstanding dimensionally confined transport phenomenaand show promising applications in building nanodevices [6,8].

In the past decades, vanadium oxides and their relatedcompounds have been widely researched owing to their specificstructures, novel chemical and physical properties. These proper-ties make them have a wide range of potential applications, such ascathode materials for reversible lithium-ion batteries, catalysts,magnetic devices, gas sensors, electrochemical devices, opticalswitching devices and intelligent thermochromic windows [10–18]. Among vanadium oxides, VO2 is a representative binary

* Corresponding authors. Tel.: +86 18971088222; fax: +86 27 68754067.

E-mail addresses: liuxh@whu.edu.cn (X. Liu), chihuang@whu.edu.cn (C. Huang).

0025-5408/$ – see front matter . Crown Copyright � 2012 Published by Elsevier Ltd. A

http://dx.doi.org/10.1016/j.materresbull.2012.04.015

compound with different polymorphs, including VO2(M), VO2(R),VO2(B), VO2(A) [19], recently reported VO2(C) [20], etc. In theprevious decades, the preparations, properties and applications ofVO2(M), VO2(R) and VO2(B) have been researched abundantly.However, the study on other polymorphs of VO2, which may alsoown novel chemical and physical properties and potentialapplications, has been comparatively less reported [21,22].Recently, a little of attention has been paid to VO2(A). Theobaldfirst reported VO2(A) [19] at reaction temperatures between 220and 330 8C from a suspension of V2O3 and V2O5 with an averageoxidation number of V being 4.0 under the hydrothermal reaction.Several decades later, Oka et al. [23–25] found that VO2(A)undergoes a metal-semiconductor transition with the phasetransition temperature (Tc) at 162 8C, accompanied by a crystallo-graphic transition between a low temperature phase (LTP, P4/ncc,130 below 162 8C) and a high temperature phase (HTP, I4/m, 87above 162 8C). The phase transition behavior of W-doped VO2(A)nanorods was investigated by Ji et al. [22]. VO2(A) nanobelts wereprepared by the transformation from VO2(B) nanobelts [21] andthe reduction of [VO(O2)2]� [26] with ethanol via the hydrothermalapproach, and found that VO2(A) could be used as the opticalswitching material. Xie’s group reported [27] that the phasetransition process from VO2(B) to VO2(A) was first observedthrough a mild hydrothermal approach, using hybrid densityfunctional theory (DFT) calculations and crystallographic VO2

ll rights reserved.

Fig. 1. The survey XPS spectrum of the sample, inset a core-level spectrum.

Y. Zhang et al. / Materials Research Bulletin 47 (2012) 1978–1986 1979

topology analysis. Except the above reports, there are fewreferences reported about VO2(A). Therefore, the preparations,novel morphologies, properties and applications of VO2(A) are verydeserved to be studied. Herein, we developed a facile hydrothermalapproach to synthesize belt-like VA-RCS and investigated its phasetransition, optical switching and oxidation resistance properties.Very recently, Ji et al. [28] reported that VO2(A) nanobelts weresynthesized via a two step hydrothermal method in the V2O5–H2C2O4–H2O system: (1) at 210 8C for 48 h; (2) rapidly to 260 8C for8–24 h. In our study, belt-like VA-RCS was fabricated by one-pothydrothermal approach using V2O5, H2C2O4�2H2O and H2O as thestarting materials at 280 8C for 48 h. In the literature, they focusedon that the unexpected phase transformation VO2(A) could be seenfrom VO2(R) to VO2(A) during hydrothermal treatment. Herein, wefocused on the synthesis of belt-like VA-RCS, the irreversibletransformation of VA-RCS to belt-like VM-RCS and the opticalswitching properties of VA-RCS and VM-RCS.

VO2(M) undergoes a reversible first-order metal-to-insulatortransition (MIT) with the Tc at around 68 8C. The reversiblecrystallographic transition between a low temperature monoclinicphase (M) and a high temperature tetragonal phase (R) occurs atthe Tc [29,30]. On warming through the transition, drastic changesoccur in both optical and electrical properties [29,31]. Thesefeatures make VO2(M) to have potential applications in storagemedium, optical switching devices, laser protection, intelligentenergy conserving windows, optical or electrical devices and so on[15,29,32–35]. As a result, micro and nano VO2(M) with variousmorphologies has been reported, such as, particles, spheres, rods,belts, ribbons, wires and thin films [11,36–43]. However, VO2(M)with a rectangular cross section (VM-RCS) has comparativelyrarely been reported [44,45]. In this paper, VM-RCS was synthe-sized by the irreversible transformation of VA-RCS via the thermaltreatment in the case of converting VO2(B) to VO2(M). Further-more, the optical switching properties of VM-RCS were studied bythe variable-temperature infrared spectra, and it was found thatthe as-obtained VM-RCS could be used as the optical switch atdifferent vibratory absorption bands.

2. Materials and methods

2.1. Materials

Vanadium pentoxide (V2O5), and oxalic acid dihydrate(H2C2O4�2H2O) with analytical grade were purchased fromSinopharm Chemical Reagent Co., Ltd and used without anyfurther purification.

2.2. Synthesis of belt-like VA-RCS

In a typical synthesis, 0.91 g (5 mmol) of V2O5 powder and0.63–1.89 g (5–15 mmol) of H2C2O4�2H2O were dispersed into40 mL of deionized water with magnetic stirring vigorously forabout 10 min at room temperature. After the solution becamesuspension, the mixed solution was transferred into a 60 mLstainless steel autoclave, which was sealed and maintained at 210–280 8C for 3–168 h and then cooled to room temperature naturally.The products were filtered off, washed with distilled water andabsolute ethanol several times to remove any possible residue, anddried in vacuum at 75 8C for future characterization andapplication.

2.3. Synthesis of belt-like VM-RCS

To synthesize belt-like VM-RCS, the above belt-like VA-RCS washeated in a tube furnace with 5 8C/min heating rate under a flow ofargon (99.999%) gas at 700 8C for 2 h according to our previous

reports [21,45], and cooled to room temperature in the argon flowto prevent oxidation of VO2(M).

2.4. Characterization

X-ray photoelectron spectroscopy (XPS, KRATOS, XSAM800with MgKa 1253.6 eV 16 mA � 12 kV) was used to confirm thecomposition of the sample and the oxidation state of thevanadium. X-ray powder diffraction (XRD) was carried out onD8 X-ray diffractometer equipment with Cu Ka radiation,l = 1.54060 A. The morphology of the products was observed byscanning electron microscopy (SEM, Quanta 200) and transmissionelectron microscopy (TEM, JEM-2100). The phase transitiontemperature of the samples was measured by differential scanningcalorimetry (DSC, DSC822e, METTLER TOLEDO) in a heating rate at5 8C/min with a liquid nitrogen cooling system. Optical propertiesof the samples were tested by Fourier transform infraredspectroscopy (FT-IR, NICOLET 5700) with an adapted heatingcontrolled cell. FT-IR patterns of the solid samples were measuredusing KBr pellet technique from 4000 to 400 cm�1 with aresolution of 4 cm�1. About 1 wt% of the samples and 99 wt% ofKBr were mixed homogeneously, and then the mixture waspressed to a pellet. Thermal gravimetric analysis (TGA) wasperformed on SETSYS-1750 (AETARAM Instruments). About 10 mgof the as-obtained samples was heated in an Al2O3 crucible in airatmosphere from ambient temperature to 700 8C at a constant riseof temperature (10 8C/min).

3. Results and discussion

3.1. The composition of the as-obtained belt-like VA-RCS

The surface composition of the sample and the oxidation stateof the vanadium were investigated by XPS measurement. Fig. 1shows the typical XPS spectrum of the sample obtained at 280 8Cfor 48 h, which reveals that the sample only consists of vanadiumand oxygen. The peak of C1s is owing to some carbon dioxidesabsorbed on the surface of the sample and can be disregarded [8].The core-level curve further confirms the sample consisting of Vand O elements, and the peaks centered at 516.2 eV, 523.4 eV and529.8 eV are attributed to the V2p3/2, V2p1/2 and O1s, respectively.The binding energy at 516.2 eV and 523.4 eV are the characteristicof vanadium in the +4 oxidation state, which are consistent withthe previous values of the bulk VO2 [46–48]. Besides, it had been

Fig. 2. XRD patterns for (a) the standard JPCDS plots of VO2(A) and (b) the as-

obtained VO2 (A).

Y. Zhang et al. / Materials Research Bulletin 47 (2012) 1978–19861980

established that the oxidation state of the vanadium oxides couldbe determined by the distance in binding energy (D) between theO1s and V2p3/2 level [46]. As for the as-obtained sample in this work,the D(O1s–V2p3/2) value is 13.6 eV, which well corresponded to thereported value of V4+ in the literature [46], further confirming thevanadium valence to be the +4 oxidation state.

The phase and structure of the sample were confirmed by XRDtest. Fig. 2 shows a typical XRD pattern of the sample obtained at280 8C for 48 h via the hydrothermal route. All the diffraction peaksfrom Fig. 2b, can be readily assigned to the tetragonal crystallinephase (space group: P42/ncm 138) of VO2(A), in agreement withthe literature values (JCPDS No. 42-0876, a = 8.450 A, c = 7.686 A)[23], whose plots are shown in Fig. 2a. No peaks of any otherphases, such as V2O5, V3O7, VO2(B) and V2O3, are detected,indicating that the as-obtained VO2(A) is with high purity via thecurrent synthetic route. Compared with the standard data (Fig. 2a),the (1 1 0), (2 2 0) and (3 3 0) peaks are extraordinary strongerthan other peaks, which greatly differ from the XRD data for thepowder sample (JCPDS 42-0876), indicating that as-obtainedVO2(A) may have special morphology, which can be confirmed bySEM and TEM tests discussed in the following section.

Fig. 3. Typical SEM images of the as-obtained VO2(A): (a)

3.2. The morphology of the as-obtained belt-like VA-RCS

The morphology and size of the typical sample wereinvestigated by SEM and TEM measurements. Fig. 3 depicts therepresentative SEM images of VO2(A) obtained at 280 8C for 96 h. Itcan be seen that the as-obtained VO2(A) predominantly consists ofa large quantity of uniform micro- and nano-structures with well-defined facets. It is noted that the formation of highly facetedmicrobelts structures with approximately rectangular crosssections (RCS) wherein the widths exceed the thicknesses, asevidenced from the high-resolution SEM image (Fig. 3 b),indicating the as-obtained VO2(A) has belt-like morphology. Thetypical TEM image (Fig. S1, Supplementary data) also reveals thatVO2(A) has the belt-like morphology, which is well consistent withthe results of SEM analyses. It can be observed from Fig. 3 and Fig.S1 (Supplementary data), that the belt-like VA-RCS consists of alarge quantity of uniform micro- and nano-structures with typicallengths up to several tens of micrometers, widths ranging fromseveral hundred nanometers to several micrometers, and thick-nesses about 100–300 nm, which leads to the formation of beltswith an ultrahigh-aspect-ratio. The phase and morphology of thefinal products are sensitively dependent on the reaction condi-tions, such as, the reaction temperature, the concentration ofH2C2O4 and the reaction time through a series of designedexperiments, which will be discussion in detail in the followingsections.

3.2.1. The reaction time and the formation mechanism of VA-RCS

To reveal the evolution process of the formation of VA-RCS, thereaction time was changed with other parameters unchanged inour designed experiments (0.91 g of V2O5 powder, 1.26 g ofH2C2O4�2H2O, 40 mL of H2O and the reaction temperature is280 8C). The synthetic processes were ceased at definite reactionperiods of 3, 6, 12, 24, 48, 96 and 168 h, and the as-obtainedintermediate samples were thoroughly determined by XRD andSEM tests, which are shown in Figs. 4 and 5, respectively. When thereaction was carried out for 3 h, the diffraction peaks from the XRDpattern (Fig. 4a) could be readily indexed as the monocliniccrystalline phase (space group C2/m) of VO2(B), which correspondsto the VO2(B) (JCPDS, No. 31-1438) already described in theliterature [49]. It can be seen from the SEM image (Fig. 5a) that theas-obtained VO2(B) has the belt-like morphology with a rectangu-lar cross section in the nanoscale. Therefore, VO2(B) nanobelts witha rectangular cross section (VB-RCS) could be synthesized at thecurrent synthetic conditions, which is a new route for the synthesis

a lower magnification and (b) a higher magnification.

Fig. 4. Top: XRD patterns of the resulting samples obtained with different reaction

time; below: enlarged XRD patterns of (a–c), insert a narrow diffraction peak figure.

Y. Zhang et al. / Materials Research Bulletin 47 (2012) 1978–1986 1981

of VB-RCS. With the reaction time extended to 6 h, the mixturemainly consisted of VO2(B) and VO2(A) was obtained (Fig. 4b) andwe could occasionally observe some micro-structured belts, asdepicted in Fig. 5b. A lot of nanobelts and irregular fragments canbe also seen in Fig. 5b, which may be ascribed to VO2(B) nanobeltsdescribed in Fig. 5a. However, when the heating time wasincreased to 12, 24, 48, 96 and 168 h, the phase of VO2(A) wassynthesized, as examined by XRD measurements (Fig. 4c–g). After12 h, the as-synthesized VO2(A) mainly consists of uniform beltswith RCS (Fig. 5c), whose length is up to tens of micrometers, whichcorresponds to the results of Fig. 3. However, we can also observelots of irregular nanowires and nanosheets in Fig. 5c. Thesefragments in nanoscales are decreased with the reaction timeincreased to 48 h, as shown in Fig. 5d. When the reaction time isextended to 96 h, the SEM images (Fig. 3) reveal that the good VA-RCS with few fragments is formed. The above results indicate that

the fragments can be dissolved into the solution system to help thegrowth formation of VO2(A).

Through the above experimental results, we know that VB-RCSis the intermediate product to synthesize VA-RCS. After 3 h, belt-like VB-RCS in nanoscale is formed, and then VB-RCS is slowlyconverted to VA-RCS with prolonging the reaction time, which iswell in agreement with Ref. [21]. The pure phase of VO2(A) can besynthesized after 12 h. The basic reaction we employed for thesynthesis of the VO2(A) in our hydrothermal synthesis can beformulated in Eq. (1):

V2O5þ 2H2C2O4 ! 2VO2þ 3CO2þ CO þ 2H2O (1)

However, the growth formation of VA-RCS needs much moretime. Although the exact growth mechanism of VA-RCS is not clearunder this hydrothermal condition at the present stage, a possiblegrowth mechanism is proposed as the ‘‘Reaction-Transformation-Dissolution-Recrystallization’’ (RTDR) mechanism, which mainlycontains four steps as follows: (1) VO2(B) nanobelts is fast formed viathe hydrothermal reaction between V2O5 and H2C2O4, as shown inFigs. 4a and 5a. The VB-RCS can be obtained with short time due tothe high hydrothermal reaction temperature (280 8C). (2) VO2(B) istransformed to VO2(A), as depicted in Figs. 4b–c and 5b, in agreementwith Ref. [21]. (3) The irregular and broken fragments of VO2(A) isdissolved into the solution to help the growth of VA-RCS, which canbe clearly observed from Fig. 5b–d. (4) The VA-RCS is continuing togrow and the fragments of becomes fewer and fewer (Figs. 5b–d and3). In a word, the formation of VA-RCS can be described as thetransformation, dissolution and recrystallization process.

3.2.2. The reaction temperature

The reaction temperature is a significant factor for synthesizingVA-RCS. To investigate the effect of temperature on preparing VA-RCS, a series of experiments were carried out at differenttemperatures from 210 to 280 8C with other parameters constant(0.91 g of V2O5 powder, 1.26 g of H2C2O4�2H2O, 40 mL of H2O andthe reaction time is 48 h). Fig. S2 (Supplementary data) and Fig. 6respectively represent the corresponding XRD patterns and SEMimages. The results indicate that the reaction temperature at 260–280 8C is favorable for the fabrication of VA-RCS by our designedexperiments. When the reaction temperature was decreased to210 8C, only the pure phase of VO2(B) [49] is obtained, as shown inFig. S2a, and the SEM image (Fig. 6a) reveals that the sampleconsists of lots of VO2(B) nanobelts and some fragments. With thereaction temperature increased, more and more VO2(A) wassynthesized, as depicted in Fig. S2b–d. When the temperature risesto 240 8C (Fig. S2b), the mixture of VO2(A) and VO2(B) is formed.However, the pure phase of VO2(A) is prepared with the reactiontemperature increasing to 260 8C (Fig. S2c) or 280 8C (Fig. S2d). Theas-obtained VO2(A) consists of lots of micro-belts and somefragments in nanoscale, which well agrees with the morphology ofVA-RCS obtained at 280 8C.

3.2.3. The concentration of H2C2O4�2H2O

The concentration of the H2C2O4�2H2O also has importantinfluence on the phase and morphology of the resulting products.Keeping other parameters constant (0.91 g of V2O5 powder, 40 mLof H2O and at 280 8C for 48 h), only the quantity of H2C2O4�2H2O(0.63, 1.26 and 1.89 g) was considered as the changeableparameter. The as-obtained samples were characterized by XRDand SEM tests, as shown in Fig. S3 (Supplementary data) and Fig. 7,respectively. When 0.63 g of H2C2O4�2H2O was used, the mixtureof V6O13 and VO2(A) was obtained, as shown in Fig. S3a. The SEMimage (Fig. 7 a) describes the mixture consists of a lot of nanobeltsand some fragments. With the increasing of H2C2O4�2H2O to 1.26 g,VA-RCS was formed, detail information observed in Figs. 2, 5d and

Fig. 5. SEM images of the resulting samples obtained with different reaction time: (a) 3 h; (b) 6 h; (c) 12 h; (d) 48 h.

Y. Zhang et al. / Materials Research Bulletin 47 (2012) 1978–19861982

Fig. S3b. However, a lower valance vanadium oxide (V2O3) isdetected with adding 1.89 g of H2C2O4�2H2O to the hydrothermalsystem, as shown in Fig. S3c, indicating that the reducibility isincreased with the increasing of the quantity of H2C2O4�2H2O. Atthis condition, some microspheres and broken fragments areobserved in Fig. 7b, indicating that the morphology of the samplebecomes irregular. Therefore, the phase and morphology of thefinal products can be controlled by the quantity of H2C2O4�2H2O

Fig. 6. SEM images of the resulting samples obtained with

and the mol ratio of V2O5/H2C2O4�2H2O = 1/2 is favorable for thepreparation of VA-RCS under our current experiments.

3.3. Irreversible transformation from VA-RCS to VM-RCS

Recently, VO2(M) has attracted much attention because it canbe considered for applications in wild fields. In this paper, we havesuccessfully transformed the as-obtained VA-RCS to VM-RCS by

different reaction temperatures: (a) 210 8C; (b) 260 8C.

Fig. 7. SEM images of the resulting samples obtained with different mol ratios of V2O5/H2C2O4�2H2O: (a) 1:1; (b) 1:3.

Y. Zhang et al. / Materials Research Bulletin 47 (2012) 1978–1986 1983

thermal treatment with VA-RCS at 700 8C for 2 h under the inertatmosphere based on our previous reports [21,45]. Fig. 8 shows thetypical XRD pattern of VM-RCS. All diffraction peaks from Fig. 8 canbe readily indexed to the monoclinic crystalline phase (spacegroup: P21/c) of VO2(M), in agreement with the reported values(JCPDS No. 72-514, a = 5.743 A, b = 4.517 A, c = 5.375 A) [50]. Nopeaks of any other phases are detected, indicating the as-obtainedVM-RCS with high purity.

The morphology of VM-RCS was investigated by SEM test, asshown in Fig. 9. The SEM image illustrates that the as-obtainedVO2(M) predominantly consists of a large quantity of uniformmicro-structures with belt-like morphology and the rectangularcross sections can be clearly observed, which reveals that the as-obtained VM-RCS keeps the orinigal shape of VA-RCS.

3.4. The phase transition and optical switching properties of the as-

obtained belt-like VA-RCS and VM-RCS

When the phase transition of VO2(A) or VO2(M) occurs, theyrespectively exhibits a noticeable endothermal and exothermalprofile in the heating and cooling DSC curves, which corresponds tothe phase transition of VO2(A) or VO2(M). Fig. 10a shows thetypical DSC curves of VA-RCS with three heating and cooling cycles.The Tc of VA-RCS is about 150 8C in the heating cycle, which is

Fig. 8. Typical XRD patterns of VM-RCS.

about 12 8C lower than that of VO2(A) in the literature value [25].We think that it may be due to the size effect and the morphologyof VA-RCS. The Tc of VA-RCS is about 112 8C in the cooling cycle,which is owing to the hysteresis behavior of VO2(A). The heatingand cooling curves obtained with different cycles are basicallycoincided, indicating that the phase transition of the as-obtainedVA-RCS has comparatively good reversibility.

As for VM-RCS, as shown in Fig. 9b, the Tc is about 66 8C in theheating cycle, which is about 2 8C lower than the literature value[29], while the Tc is about 57 8C in the cooling cycle, which is owingto the hysteresis behavior of VO2(M). The heating and coolingcurves obtained with different cycles are well coincided, indicatingthat the phase transition of the as-obtained VM-RCS has goodreversibility, which is better than that of VA-RCS. Compared Fig. 9awith b, the phase transition heat of VM-RCS is much more than thatof VA-RCS. The explanation can be the differences between thestructures of VO2(A) and VO2(M) before and after their phasetransition.

According to the DSC results of the as-obtained VA-RCS and VM-RCS, they respectively undergo a noticeable endothermic peak inthe heating cycle and a noticeable exothermal in the cooling cycle.

Fig. 9. Typical SEM image of VM-RCS.

Fig. 10. DSC curves with three cycles for (a) VA-RCS and (b) VM-RCS; variable-temperature infrared spectra for VA-RCS (c, e and g) and VM-RCS (d, f and h): (c) and (d) all of IR

curves with different temperatures; (e) and (f) selected some typical IR curves from (c) or (d) to clearly reveal the process of the phase transition of VA-RCS or VM-RCS before

and after Tc; (g) and (h) selected three IR curves from (c) or (d), two are below Tc (one is from the heating process, while the other is from the cooling process), and the other is

up Tc.

Y. Zhang et al. / Materials Research Bulletin 47 (2012) 1978–19861984

It was reported that some physical properties (e.g.: optical,electrical, magnetic and so on) has drastically changed when thereversible phase transition of VO2(A) or VO2(M) occurs. Therefore,in this paper, we further developed the as-obtained VA-RCS and

VM-RCS as the optical switching devices. The optical switchingproperties of VA-RCS and VM-RCS were investigated by a series ofvariable-temperature infrared spectra of heating and cooling, asshown in Fig. 10c–h.

Fig. 11. TGA analyses of the as-synthesized VA-RCS and VM-RCS in the air

atmosphere.

Y. Zhang et al. / Materials Research Bulletin 47 (2012) 1978–1986 1985

Fig. 10c shows all the curves obtained with the variable-temperatures IR tests. It can be clearly seen from Fig. 10c, that theas-obtained VA-RCS has the optical switching property at differentvibratory absorption bands, revealing that it has potentialapplications in optical switching devices at the vibratory absorp-tion bands from 680 to 660 cm�1 and from 580 to 550 cm�1. A clearview can be observed from two typical curves below and up Tc, asshown in Fig. 10g, whose enlarged figure is represented inSupplementary data (Fig. S4). Fig. 10e describes the process of thephase transition of VA-RCS before and after Tc, indicating its phasetransition is at around 155 8C in the heating cycle and at about105 8C in the cooling cycle, which is consistent with the results ofDSC. The two IR curves below Tc (one is from the heating process,while the other is from the cooling process), as shown in Fig. 10g,are basically coincided, indicating that the phase transition of VA-RCS has good reversibility. Those optical properties of VA-RCSverify that it is beneficial for the development and application of anoptical switching material.

In the case of VM-RCS, the variable-temperatures IR curvesreveal that its Tc is at around 68 8C in the heating cycle and at about53 8C in the cooling cycle, as shown in Fig. 10f. The opticalproperties of VM-RCS, as depicted in Fig. 10d, f and g, also verifythat it is an ideal candidate for the optical switching materials. Theoptical transmission of VM-RCS below Tc is much higher than thatup Tc, suggesting that VM-RCS has good thermochromic property,which is much better than that of VA-RCS, compared Fig. 10g withh. In addition, VM-RCS has better reversibility in the phasetransition. Besides, the rate of optical transmission of VM-RCS islarger than that of VA-RCS. Based on the above results, both VM-RCS and VA-RCS can be used as the optical switching materials intheir potential applications. However, the Tc of VM-RCS is at about68 8C, while VA-RCS is at about 155 8C, indicating that these twomaterials can be applied to different fields.

3.5. The oxidation resistance properties of VA-RCS and VM-RCS

The as-obtained VA-RCS and VM-RCS can be applied to theoptical switching materials. It has been reported that V2O3 can beeasily oxidized in the air atmosphere [51], however, the oxidationresistance of VO2 has not been reported in the literatures.Therefore, in this paper, the oxidation resistance properties ofVA-RCS and VM-RCS were investigated by TG with the flowing air,as shown in Fig. 11. In addition, the oxidation resistance propertyof VO2(B) nanobelts was also examined to compare VA-RCS andVM-RCS with VO2(B) about the oxidation stability. The detailinformation of the synthesis of VO2(B) nanobelts is shown inSupplementary data (Fig. S5). Fig. S5 reveals that VO2(B) begins tobe oxidized by O2 at 352 8C and finished at 466 8C. According to thecurve of VA-RCS, before 400 8C, the weight loss (ca. 1.26%)corresponds to the loss of water absorbed on its surface. VA-RCSbegins to be oxidized by O2 at 400 8C and finished at 643 8C. Theweight gain is ca. 9.66% in its oxidative process. As for VM-RCS, theoxidation starts at 428 8C, which is higher than that of VA-RCS. Theweight gain is ca. 9.80% from 428 8C to 643 8C. These resultsindicate that the as-obtained VA-RCS and VM-RCS have betterthermal stability than that of VO2(B) nanobelts. The weight gainvalues of VA-RCS and VM-RCS are well corresponding to theoxidation of the bulk VO2 to V2O5 (9.64%), as represented in Eq. (2):

4VO2þ O2 ! 2 V2O5 (2)

Fig. S6 (Supplementary data) shows the heat flow curves ofVA-RCS and VM-RCS. The sharp endothermic peak at about680 8C is the melting point of V2O5, which further confirms theoxidative product is V2O5. Based on the above results, both VA-RCS and VM-RCS have good thermal stability and oxidation

resistance below 400 8C in air and even better than that ofVO2(B) nanobelts, which can be applied to the optical switchingmaterials in air atmosphere.

4. Conclusion

(1) Belt-like VA-RCS was successfully synthesized using V2O5,H2C2O4�2H2O and H2O as the starting materials by a facile one-pot hydrothermal approach. The influence of syntheticparameters, such as, the reaction time, reaction temperatureand concentration of H2C2O4�2H2O, was systematically inves-tigated to control the preparation of belt-like VA-RCS. The‘‘RTDR’’ mechanism was proposed to explain the formation ofVA-RCS.

(2) Belt-like VM-RCS was successfully prepared by the irreversibletransformation of VA-RCS at 700 8C for 2 h under the argonatmosphere. The morphology and size of VM-RCS weredependent on that of VA-RCS.

(3) The Tc of VA-RCS and VM-RCS was evaluated by DSC test. VA-RCS exhibits a phase transition at 150 8C in the heating cycleand 112 8C in the cooling cycle. VM-RCS exhibits a strong phasetransition at 66 8C in the heating cycle and 57 8C in the coolingcycle.

(4) The optical switching properties of VA-RCS and VM-RCS werestudied by the variable-temperature infrared spectra. Both VA-RCS and VM-RCS could be used as the optical switchingmaterials.

(5) Both the phase transition and the optical switching propertiesof VA-RCS and VM-RCS have good reversibility.

(6) The oxidation resistance properties of VA-RCS and VM-RCSwere investigated by TGA, indicating that they have goodthermal stability and oxidation resistance below 400 8C inair.

Acknowledgements

This work was partially supported by the Fourth Installmentof Science and Technology Development 2010 Program ofSuzhou (SYG201005), the Fundamental Research Funds for theCentral Universities, Independent Research Projects of WuhanUniversity (217274721) and Luojia Young Scholars Program(217273483).

Y. Zhang et al. / Materials Research Bulletin 47 (2012) 1978–19861986

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the

online version, at http://dx.doi.org/10.1016/j.materresbull.2012.04.015.

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