VO2 (A): Reinvestigation of crystal structure, phase transition and crystal growth mechanisms

Journal of Solid State Chemistry 213 (2014) 79–86

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VO2 (A): Reinvestigation of crystal structure, phase transition and crystal growth mechanisms Srinivasa Rao Popuri a,b,c,1, Alla Artemenko a,b, Christine Labrugere d, Marinela Miclau c, Antoine Villesuzanne a,b, Michaël Pollet a,b,n a

ICMCB, CNRS, UPR 9048, F-33608 Pessac, France University of Bordeaux, ICMCB, UPR 9048, F-33608 Pessac, France c National Institute for Research and Development in Electrochemistry and Condensed Matter, Timisoara, Plautius Andronescu Str. No. 1, 300224 Timisoara, Romania d CeCaMA, University of Bordeaux 1, ICMCB, 87 Avenue du Dr. A. Schweitzer, F-33608 Pessac, France b

art ic l e i nf o

a b s t r a c t

Article history: Received 13 September 2013 Received in revised form 26 January 2014 Accepted 28 January 2014 Available online 5 February 2014

Well crystallized VO2 (A) microrods were grown via a single step hydrothermal reaction in the presence of V2O5 and oxalic acid. With the advantage of high crystalline samples, we propose P4/ncc as an appropriate space group at room temperature. From morphological studies, we found that the oriented attachment and layer by layer growth mechanisms are responsible for the formation of VO2 (A) micro rods. The structural and electronic transitions in VO2 (A) are strongly first order in nature, and a marked difference between the structural transition temperatures and electronic transitions temperature was evidenced. The reversible intra- (LTP-A to HTP-A) and irreversible inter- (HTP-A to VO2 (M1)) structural phase transformations were studied by in-situ powder X-ray diffraction. Attempts to increase the size of the VO2 (A) microrods are presented and the possible formation steps for the flower-like morphologies of VO2 (M1) are described. & 2014 Elsevier Inc. All rights reserved.

Keywords: Hydrothermal synthesis Vanadium dioxide polymorphs Crystal growth Structural and electronic transitions X-ray photoelectron spectroscopy

1. Introduction Among the binary transition metal oxides, vanadium dioxide shows a rich phase diagram with numerous non-hydrate polymorphs like VO2 (M1), VO2 (R), VO2 (B), VO2 (A), VO2 (D), VO2 (BCC) and VO2 (N) [1,2]. These oxide materials offer a wide range of applications in the fields of high speed electronics, memory devices, electro-chromic and thermo-chromic applications, field emission displays, cathode material in batteries etc. In particular, M1 and B polymorphs have attracted much attention due to (i) the tunable, near room temperature, metal insulator transition (MIT) for VO2 (M1) and (ii) the relatively easy preparation process [2] as well as potential application as a cathode material in batteries for VO2 (B). The studies of the recently discovered polymorphs (D, BCC, N) are still at synthesis stage. In the case of VO2 (A) polymorph, the synthesis was first reported by Theobald [3];

n Corresponding author at: ICMCB, CNRS, UPR 9048, F-33608 Pessac, France. Tel.: þ 33 5 4000 31 48. E-mail address: [email protected] (M. Pollet). 1 Present address: Institute of Chemical Sciences and Centre for Advanced Energy Storage and Recovery, Heriot–Watt University, Edinburgh, EH14 4AS, United Kingdom.

http://dx.doi.org/10.1016/j.jssc.2014.01.037 0022-4596 & 2014 Elsevier Inc. All rights reserved.

however a complete study of its electronic properties is still missing. Theobald showed that VO2 (A) crystallizes in tetragonal symmetry with lattice parameters a ¼ 11.90 Å and c¼ 7.68 Å, but the space group was unknown at that time. A decade later, Oka et al. prepared flat rectangular VO2 (A) particles by hydrothermal process and studied the phase transition and some electrical properties: from temperature dependent powder X-ray diffraction (P-XRD), electrical and magnetic properties measurements, they observed a discontinuity in the evolution of the lattice parameters, resistivity and magnetic susceptibility at 435 K, which was associated with a structural phase transition [4]. Oka et al. and later Yao et al. solved the crystal structure of hydrothermally synthesized polycrystalline samples of VO2 (A); they assigned the space group P42/nmc with lattice parameters a¼ 8.433 Å, c ¼7.678 Å and Z¼16 [5,6]. However, Oka et al. reinvestigated the crystal structure of VO2 (A) by single crystal diffractometry [7] and showed that, at 298 K, VO2 (A) low temperature phase (LTP-A) adopts the P4/ncc space group with a ¼8.4403 Å, c¼ 7.666 Å, Z¼16; with this new assignment a degree of freedom is removed and the octahedrons are less distorted; at 473 K (high temperature phase—HTP-A), VO2 (A) adopts the I4/m space group with a ¼8.476 Å, c¼ 3.824 Å and Z¼8. VO2 (A) actually undergoes a progressive first-order structural transition from primitive tetragonal (LTP-A) to body-centered

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tetragonal (HTP-A) structure at 435 K, related to a slight change in V–V bond length. It is worth noting that the lack of highly crystalline VO2 (A) samples often leads to ambiguous assignment of space group [5,8,9]. In 2010, Ji et al. prepared fiber like particles of VO2 (A) using V2O5 and dihydrate oxalic acid. According to this study, a two-step hydrothermal method is necessary to obtain VO2 (A) [10]. In addition, if the molar ratio of V2O5 to oxalic acid exceeds 1:3, other lower valence vanadium oxides would be found in the final compounds. Recently, Zhang et al. obtained VO2 (A) nanobelts from two steps hydrothermal processes also by treating at 280 1C/48 h an already hydrothermally prepared VO2 (B) sample [11]. In some of the above mentioned synthesis procedures, special organic reagents, surfactants, or templates and two-step processes are involved for the fabrication of VO2 (A) micro- or nanostructures. The development of facile and single step processes for making controllable VO2 (A) architectures remains a key challenge. In addition, although a number of morphologies have been reported, the investigation of the relation between morphologies and properties is still missing. The motivations of the present article are (i) to describe a single step synthesis procedure for VO2 (A) under hydrothermal conditions and to clarify the discrepancies about the space group, (ii) to reinvestigate the phase transition, (iii) to understand the formation mechanism of VO2 (A) micro-rod particles and their conversion to flower like VO2 (M1) particles and finally (iv) to shortly report on the morphology effect on the phase transition temperature in the case of VO2 (M1).

2. Experimental section

evidencing that the synthesis conditions play an important role on the resulting phases as well as their morphologies. This temperature and time dependent phase and shape evolution suggests a possible strategy to control the microstructures without any additional reactants. 2.2. Characterization details P-XRD patterns of the final products were recorded using a PANalytical X’Pert PRO MPD diffractometer with graphite monochromatized Cu Kα radiation (λ ¼1.54178 Å). The phase purity of the products was checked by comparing the experimental P-XRD patterns to standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS). In addition, the above mentioned P-XRD diffractometer was further used with an Anton-Paar high temperature chamber (9.2  10  5 bar) to study the structural phase transformation. The morphology of the products was examined by a field emission scanning electron microscope (FESEM, JEOL JSM-6300F, 15 kV): energy dispersive X-ray spectroscopy (EDX) coupled to the FESEM was used to check for the chemical content and the homogeneity in the products. In order to study the phase transition behaviors, we carried out differential scanning calorimetry (DSC, Perkin Elmer DSC 8500) measurements under argon atmosphere over a temperature range from 300 to 500 K along heating/cooling cycles. The room temperature electronic structure of VO2 (A) was studied by means of X-ray photoemission spectroscopy (XPS) with a VG Escalab 220i XL X-Ray Photoelectron Spectrometer using aluminium monochromatic source (hυ ¼ 1486.6 eV) under ultrahigh vacuum conditions. High temperature (HT) magnetization measurements were performed using a MANICS susceptometer.

2.1. Synthesis Table 1 summarizes the experimental conditions for the synthesis of the different samples. Samples A to C were produced by hydrothermal method. An aqueous solution of vanadium oxide was prepared by dissolving V2O5 in double distilled water under rapid magnetic stirring. After complete dissolution, oxalic acid was added and continuously stirred during 15 min; the resulting suspension was transferred to a Teflon-lined stainless steel autoclave of 60 ml volume for the thermal treatment. Once the reaction time was completed, the autoclaves were cooled down to ambient temperature naturally; the resultant powders were collected and washed with distilled water and then with ethanol several times; the final products were finally dried in an oven at 80 1C for 6 h. For the preparation of polycrystalline VO2 (M1) spherical particles, appropriate stoichiometric amounts of highpurity V2O3 and V2O5 were sealed in evacuated quartz ampoules and slowly heated to 750 1C and then held there for 48 h. Fig. 1 shows the micrographs of the powders obtained after treatment,

3. Results and discussion 3.1. Reinvestigation of crystal structure Both LTP-A and HTP-A structures consists of a three-dimensional framework of distorted VO6 octahedra, in which the distortion is due to a slight compression of the octahedral environment along the c-axis and the shift of V atoms from the centre as shown in Fig. 1a and b, respectively. Four units of two edge-sharing VO6 octahedra are linked together by corner sharing resulting 2  2 square blocks. Along the c-axis, these blocks are stacked and share edges what results in zigzag chains of vanadium ions. The good crystallinity of the sample as well as the quality of the diffractogram is essential for an accurate determination of the structure and also to discriminate the possible space groups at low temperature. Indeed, the differences in the structures resulting from the assignment to P42/ncm [5,6] rather than P4/ncc [7] is quite subtle: in

Table 1 Details of the samples synthesis conditions. (OA: oxalic acid). Sample

Method

Precursors

Treatment

Morphology

A

Hydrothermal

Hydrothermal

C

Hydrothermal

D

Solid state reaction

þ 1.5 K min  1 250 1C/24 h  1.5 K min  1 þ 0.67 K min  1 250 1C/24 h  0.0142 K min  1 þ 1.5 K min  1 250 1C/24 h  1.5 K min  1 þ 2 K min  1 750 1C/48 h  5 K min  1

Micro-rods

B

2.6 mmol V2O5 OA/V2O5 ¼ 1.5 Water¼ 43 ml 2.6 mmol V2O5 OA/V2O5 ¼ 1.5 Water¼ 43 ml 2.6 mmol V2O5 OA/V2O5 ¼ 4.5 Water¼ 43 ml V2O3 þV2O5 sealed tube

Micro-rods micro-flowers

Micro-flowers

Micro-spheres

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Fig. 1. Crystal structures of (a) LTP-A (300 K, P4/ncc) and (b) HTP-A (623 K, I4/m) along the (1 1 0) plane. The solid black line represents the projection of the unit cell, which indicates the halving of unit cell along c-axis in HTP-A. Along the c-axis, 2  2 square blocks of two edge-sharing distorted VO6 octahedra are stacked up resulting in zigzag chains of V ions. (c) and (d) Shows the projected view of VO6 octahedra along the a-axis. The changes in the neighbouring V–V distances along c-axis are highlighted with different colours for different V–V bond lengths, at 300 K and 623 K, respectively.

both cases, vanadium ions sit in a slightly compressed octahedral environment along the c-axis and are off-centred; however, the refinement of the structure with P42/ncm leads to a slightly more compressed environment, and in addition to an antiphase rotation (tilt) of the successive octahedrons along c by ca 1.81 and a small D2 distortion with an alternating shift (up or down as referred to the apex–apex direction) of the basal oxygen ions with an average elevation angle less than 31. As shown in Fig. 2a, well crystallized samples were obtained from experimental conditions A (Table 1) and were used for structure refinement. Fig. 3 shows the room temperature P-XRD pattern of LTP-A phase with micro-rod morphology prepared at 250 1C for 24 h (sample A); all reflections can be indexed to a pure tetragonal unit cell with lattice constants a¼ 8.4347 (4) Å, c¼7.6722 (4) Å and match with space group P4/ncc (JCPDS data card 70–2716); in particular as compared to P42/ncm, this assignment perfectly accounts for the additional reflection condition at hhl with l¼2n. The inset in Fig. 3 focuses on the reflections between 2θ ¼ 21–26.61 and compares the assignment with space groups P42/ncm and P4/ncc. It is readily seen that space group P4/ncc accounts for all the reflections including the low intensity ones (2 0 0) and (0 0 2). The crystal structural details determined from Rietveld analysis using FullProf program [12] are summarized in ESI Table 1; the lattice parameters are in agreement with the values reported by Oka et al. using single crystals [7]. At 400 1C, all P-XRD reflections of VO2 (A) (HTP-A) can be indexed with space group I4/m with lattice parameters a¼8.4819 (3) Å, c¼3.8257 (2) Å. The P-XRD pattern is shown in ESI Fig. 1 and the refinement details are summarized in ESI Table 2. The lattice parameter ‘a’ slightly increases from LTP-A to HTP-A phase and lattice parameter

‘c’ is nearly halved. This can be related to the de-pairing of V4 þ ions along the c axis above the transition. In the LTP-A phase, pairing of V4 þ ions leads to alternating distances between V4 þ ions along the c axis (2.767, 3.106 and 3.249 Å), shown in Fig. 1c; in the HTP phase, the de-pairing leads to equal distances between neighboring V4 þ ions (3.082 Å), shown in Fig. 1d. According to Oka et al. the structural transition is too faint making the insulator to metal transition difficult to occur [4]; however Li et al. assigned the transition in VO2 (A) to an insulator (or semiconductor) to metal transition [9]. However the V–V distance in HTP-A phase (3.082 Å) is much higher than the Goodenough critical V–V distance 2.94 Å for the metallic conduction in vanadium oxides [13]. 3.2. Crystal growth mechanism involved in the formation of rectangular microrods The high crystallinity of VO2 (A) micro-rod samples (A) is supported by the sharp diffraction peaks of P-XRD patterns (Fig. 3) as well as crystals morphology using SEM (Fig. 2a). Such rectangular microrods shape is usually due to the preferential growth in one specific crystallographic direction. In principle, however, crystal growth and morphology are governed by both intrinsic and extrinsic factors, such as crystal structure, crystal surface, interfacial energies, diffusion of the reactant species to the surface of the crystals, nature of the species in the solution, etc. The formation of the micro-rod like VO2 (A) crystals can be understood on the basis of morphology-preferential growth relationships as defined by the Bravais–Friedel–Donnay–Harker law [14]: the nanocrystals growth is such that the total surface energy

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4 μm

1 μm

10 μm

20 μm

Fig. 2. Morphology of the final products depending on the experimental preparation conditions (a) sample A: VO2 (A) micro rods; (b) sample B: VO2 (A) micro rods and VO2 (M1) micro flowers; (c) sample C: VO2 (M1) micro flowers; (d) sample D: VO2 (M1) micro spheres. Complete details of experimental preparation conditions are described in Table 1.

combination of oriented attachment type formation mechanism and layer by layer models are important in the formation of rectangular micro-rod-like morphologies of VO2 (A). The formation steps of rectangular micro-rod crystals of VO2 (A) are summarized in the scheme in Fig. 4. The dissolution of V2O5 in water gives rise to a yellow colour aqueous solution; after the addition of oxalic acid and prolongation of the stirring time, a light green/blue solution forms, which indicates that the valence of vanadium in the solution was partly reduced from V5 þ to V4 þ (V5 þ and V4 þ solutions are yellow and blue, respectively) [2]; the rectangular micro rods evolve from the micro/nano belts like crystals. The oriented attachment mechanism is clearly seen at earlier stage, while the final stage of microrods shaped crystals involve the layer by layer combination of extended belt like crystals. 3.3. Electronic structure from XPS

Fig. 3. Observed and calculated P-XRD pattern of LTP-A phase taken at room temperature. The black square box highlighted the zoomed region shown in the inset. The inset compares the reflections between 2θ ¼21–26.61 assigned using P4/ncc (70–2716) and P42/ncm (42-0876).

is minimum for a given volume; thus, the crystallites are bounded by faces located at distances proportional to their surface energies. Under equilibrium conditions, greater inter-planar distance (dhkl) planes are thus expected to have lowest surface energies, and thereby a larger probability of growth in that direction. In the case of LTP-A, [1 1 0] direction offers the largest inter-planar distance (d110 ¼6 Å) and the shortest V–O distance [15]. Such conditions can initially favor the preferential growth of VO2 (A) samples along the [1 1 0] direction and result in belt-like nano-structures [9]. In order to gain more insight in the growth steps involved in the formation of VO2 (A) microrods, transmission electron micrographs of VO2 (A) were made (ESI Fig. 2). The TEM image in ESI Fig. 2b shows an individual VO2 (A) micro-rod that consists of an interconnection of a large quantity of uniform nanostructures; this results in a single rectangular micro-rod. This evidences that the

XPS measurements were carried out to gain insight in the electronic structure. Fig. 5a shows the XPS survey spectrum, valence band region and core level spectra for V and O elements in VO2 (A). No peaks of impurity elements other than atmospheric carbon were observed in the survey spectrum. The binding energy obtained in our XPS analysis was calibrated with respect to reference C 1s line to 284.4 eV. The high resolution XPS valence band region (inset of Fig. 5a) consists of V 3d and O 2p bands in the energy ranges between 0–2 and 3–9 eV, respectively. We found that the binding energies of the V 2p3/2, V 2p1/2 and O 1s peaks are centered at 516.4, 523.9 and 530.05 eV (Fig. 5b), which are characteristic energies of the V4 þ oxidation state and are in a good agreement with most of the reported values. As proposed by Mendialdua et al. [16], the difference in binding energies between oxygen and vanadium core levels, ΔE¼ 13.65 eV is in agreement with the þ4 oxidation state of vanadium. 3.4. Phase transition from LTP-A to HTP-A For the first time, Oka et al. observed a phase transition at 435 K for VO2 (A) upon heating using differential thermal analysis

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At 25oC, V 2 O5 + H 2 O

V5+

At 25oC V 2 O5 + H 2 O + C2H4O4.2H2O

Micro/ Nano belts

Oriented attachment mechanism

83

Layer by layer combination

V5+/4+

Fig. 4. Schematic representation of possible mechanism steps leading to the rectangular microrods shaped crystals of VO2 (A) prepared at 250 1C (Sample A).

observed electronic transition in DSC/HT magnetic measurements and reported structural phase transition (from LTP-A to HTP-A), we studied the structural phase transition using in-situ P-XRD measurements between 300 and 523 K, and the results are shown in Fig. 7. During the heating cycle, the profile of the peaks change drastically and there is a continuous disappearance of several reflections like (2 1 1) and (3 1 1). Contrariwise to Oka et al., we did not find any flattening at the top of the (6 0 0) peaks; this was attributed to the irregularity along the a axis [4]. Interestingly, the complete structural transformation occurs at much higher temperatures, around 483 K (inset in Fig. 7), as compared to the transition temperature obtained using DSC (437 K); this strongly suggests, in opposition to the situation for VO2 (M1), a much weaker coupling of the electronic transition and the structural phase transition. The intrinsic transport properties are still missing to definitely conclude on this point and alternative effects such as the phase coexistence, strain effects due to microstructures etc. that often play an important role cannot be excluded yet. For example, in the case of phase coexistence, the structural phase transition in LTP-A may be initiated at particular nucleation sites from which HTP-A phase domains evolve and extend much above the transition temperature. This kind of transformations often leads to spatial inhomogeneities and strongly influences the properties of the materials [17]. During the cooling cycle (not shown here), the transition from HTP-A to LTP-A begins at about 403 K and evidences the reversible character of the structural phase transition. 3.5. Phase transition from HTP-A to VO2 (M1) Fig. 5. (a) Surface XPS survey spectrum of VO2 (A) at room temperature (Inset: XPS spectra of V 3d and O 2p at low binding energy region); (b) core level XPS spectra of the V 2p and O 1s region.

measurements; however, they did not observe it on cooling cycle [4]. On the contrary, some recent studies report signature of the phase transition of VO2 (A) during heating as well as cooling cycles [9]. In order to clarify this issue and to study the electronic phase transition, we performed the DSC and HT magnetic measurements around the reported phase transition temperature; results are shown in Fig. 6a and b, respectively. The hysteresis (40 K) between heating and cooling cycles indicates the first-order nature of the transition; such large hysteresis is one of the largest among the reported hysteresis in VO2 (A) micro or nano structures. Such striking features can be associated with a first order martensitic type structural transformation as in the case of VO2 (M1). This kind of structural transformations takes place at two fixed temperatures, only if a single interface between the two transformable phases is involved, like in case of single crystals. On the other hand, in the case of polycrystalline samples, this transition can be extended over a range of temperatures due to existence of several interfaces. In order to understand the relation between the

In order to study the phase transition process from HTP-A to VO2 (M1), we have extended the in-situ P-XRD studies from 523 to 1023 K; the corresponding patterns are shown in Fig. 8. The HTP phase of VO2 (A) is stable until 673 K and the phase transformation from HTP-A to rutile occurs between 773 and 873 K. In this temperature range, we noticed phase coexistence; the phase transformation is complete at 973 K. The P-XRD pattern acquired at room temperature at the end of the temperature experiment only evidences the presence VO2 (M1) phase; this confirms the irreversible phase transformation from HTP-A to VO2 (R), and the reversible phase transformation from VO2 (R) to VO2 (M1). The inset in Fig. 8 shows the morphology of VO2 (M1) particles after the in-situ high temperature P-XRD studies under vacuum. Their morphology is quite similar to as synthesized VO2 (A) microrods, with minor breaking of microrods when the transformation into VO2 (M1) occurs. The irreversible phase transformation between HTP-A and VO2 (M1/R) and breaking of microrods can be explained by the drastic differences in crystal structures of the two phases. The crystal structure of VO2 (M1/R) contains a 3D pattern of VO6 octahedra, whose 4-fold axes are aligned along two perpendicular directions, whereas in HTP-A VO6 octahedra are aligned only along one direction; in addition there is drastic

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Fig. 8. In-situ P-XRD study of phase transformation process from HTP-A-VO2 (R)-VO2 (M1). (inset: Morphology of VO2 (M1) sample obtained after high temperature in-situ P-XRD study).

Fig. 6. (a) DSC and (b) HT magnetic measurement curves of VO2 (A) during heating and cooling cycles.

Fig. 9. P-XRD pattern of final product obtained after slow cooling of autoclave for 264 h. Here multiple crystalline phases are noticed: (1) VO2 (A); (2) VO2 (M1); (3) V6O13 and (4) V3O7.H2O. The possible formation steps for the flower-like morphologies of VO2 (M1) are arranged in the inset: From nanocrystals, thickness of the flower is increased vertically through a layer by layer process (a) and the flower's petals are growing in horizontal way, possibly resulting in an increase crystal size (b).

changes observed during the in-situ electron microscopy phase transformation study from VO2 (B) to VO2 (M1) [18]. 3.6. Crystal growth of VO2 (A) and formation mechanisms

Fig. 7. In-situ high temperature P-XRD patterns of VO2 (A) during heating cycle from RT to 523 K (The inset highlights the disappearance of the (2 1 1) reflection during the structural phase transition).

change in the density (LTP-A: 4.037 g cm  3, HTP-A: 4.003 g cm  3, VO2 (M1): 4.67 g cm  3) which increases by ca. 14% from HTP-A to VO2 (M1). These changes can account both for the irreversibility of the transformation and the breaking of the particles during the phase transformations. This is also consistent with the morphology

Even though few reports are available on the preparation of VO2 (A) micro/nano structures, none of them focuses on the crystal size increase under hydrothermal conditions. This may be due to difficulty to stabilize single phase VO2 samples, because of the formation of a variety of stable vanadium oxide crystalline phases in a narrow range of preparation conditions. Here we attempted to increase the crystal size by slowly cooling the autoclave from reaction temperature to room temperature. The complete details are summarized as experimental condition B in Table 1. After slowly cooling the autoclave for 264 h, we studied the crystal structure using P-XRD patterns (Fig. 9). From morphological studies, we noticed 3-fold increase in size of the VO2 (A) rectangular microrods (Fig. 1b) as compared to the natural cooling (Fig. 1a). A careful analysis of the P-XRD pattern after slow cooling reveals the presence

S. Rao Popuri et al. / Journal of Solid State Chemistry 213 (2014) 79–86

of multiple crystalline phases along with VO2 (A), such as VO2 (M1) (JCPDS card no: 01-082-0661), V3O7  H2O (JCPDS card no: 01-0852401) and V6O13 (JCPDS card no: 01-075-1140). The calculated weight fractions of VO2 (A), VO2 (M1), V3O7  H2O and V6O13 phases are about 0.606 and 0.330, 0.041 and 0.021, respectively. These observations are in agreement with the morphological studies, in which we noticed the presence of different morphologies. Under similar hydrothermal preparation conditions, it was reported that VO2 (M1) phase appears mainly as snowflake or flower-like, VO2 (A) phase as fibers or micro rods-like and V3O7.H2O phase as nanowires; in our case, we do see similar morphologies, i.e. flowerlike, microrod and nanowires, which may correspond to VO2 (M1), VO2 (A) and V3O7.H2O respectively [19]. Although the crystallographic transformation process from A to M1 is not clearly known, our observations indicate the existence of step wise transformation mechanisms from VO2 (A) to VO2 (M1) through V6O13, under hydrothermal conditions. These observations receive further support from recent reports dealing with the transformation process of VO2 (A) or VO2 (B) to VO2 (M1) under reduced atmospheres, in which the occurrence of V6O13 during the transformation was noticed [20,21]. Similar kind of transformation processes are well explained for several vanadium oxides, using cooperative movements of octahedra or crystallographic shear mechanisms. For example, under soft reducing conditions, V2O5 can only yield V6O13 and VO2 (B) phases, but not VO2 (A). On the other hand, the transformation from VO2 (B) to VO2 (A) is possible through a crystallographic shear mechanism, which was proposed by Galy et al., and the transition from V6O13 to VO2 (M1) can be described by the cooperative movement of V–O octahedra [22,23]. In the case of VO2 (M1) flower-like morphology formation, mainly two kinds of mechanisms are proposed: (i) starting from VO2 nano crystals, similar growth along the six low-energy facets of (2 0 1̄ ) planes, in a layer by layer manner, results in flower like morphology [24]; (ii) breaking of VO2 (A) microrods into VO2 (M1) embryo, followed by recrystallization results also in VO2 (M1) flower-like morphology [19]. In a qualitative way, our observations support the formation of VO2 (M1) flower like crystals from nanocrystals, formed by nucleation process and size increase through a layer by layer process. The qualitative understanding of the overall layer by layer process behind the flower-like morphology is summarized in the inset of Fig. 9. The petals of these flowers are growing in longitudinal way, resulting in an increase in crystal size. Even though the complete formation mechanism is out of the scope of this article, we propose that the layer by layer formation process is the main mechanism behind the formation of VO2 (M1) flower-like crystals. An in-situ formation mechanism study is needed to further confirm these observations. 3.7. Effect of morphology on the phase transition in VO2 (M1) The formation of single phase VO2 (M1) with controlled morphology has always been one of the main tasks towards appropriate applications. The insulator to metal transition temperature (340 K) of VO2 (M1) has important implications but requires to be slightly decreased to room temperature. This is usually done by elemental doping in VO2 (M1). However, such kind of doping will often decrease also the magnitude of changes in electric and optical properties across the transition [25]. Another possibility that was recently explored for the tuning of transition temperature to room temperature is the reduction of particle size [26,27]. For example, Lopez et al. have observed that the optical contrast between semiconducting VO2 (M1) and metallic VO2 (R) nanoparticles is dramatically enhanced in the visible region, exhibiting size-dependent multipole optical resonance and transition temperatures [28]. On the other hand,

85

Fig. 10. DSC curves of VO2 (M1) samples with different morphologies obtained by thermal annealing from VO2 (A) (Sample A: microrods), hydrothermal process (Sample C: micro-flowers) and solid state reaction (Sample D: micro-spheres). (No baseline corrections were made.).

morphology dependent electrical properties are reported in the case of VO2 nanostructures, whereas morphology dependent phase transition behavior has not been specifically studied till now [29]. By taking the advantage of several morphologies of VO2 (M1) synthesized in the present study, we studied the effect of morphology on phase transition behavior of VO2 (M1). Here, we have selected spherical shape particles (Fig. 1d); micro-rod (inset in Fig. 8) and flower-like (Fig. 1c) morphologies of VO2 (M1) microstructures prepared using solid state, annealing and hydrothermal processes, respectively. Fig. 10 shows the DSC studies related to these three types of VO2 (M1) morphologies. The hysteresis between cooling and heating cycles in all three cases supports the first-order nature of the structural phase transition in VO2 (M1). The hysteresis magnitude appears sensitive to the particles morphology and it increases from spherical (5 1C) to micro-rod (7.5 1C) and to flower-like (15 1C) morphologies. In the case of spherical particles (Sample D in Table 1), the phase transition temperature matches nicely with the values reported for bulk samples. For the micro-rod morphology, there is a small increase of the transition temperature during heating and a decrease during cooling; the heat peaks are slightly broader than in previous case. Similar observations are reported by Lopez et al. in the case of nanoscale VO2 (M1) [30]. In addition, we noticed a shoulder in DSC curve during both the cooling heating cycles. These observations might be explained on the basis of polydispersive particle size (inset of Fig. 8). In the case of flower-like morphology (Sample C in Table 1), we observed a small increase of the transition temperature during heating and a large decrease during cooling. For this sample, the heat peaks are nicely broadened what suggests some inhomogeneity of the sample. The P-XRD studies actually reveal that the samples with flower-like morphology offers the largest broadening of the {0 1 1} reflection (0.16741) as compared to spherical (0.04661) and micro-rod (0.13061) VO2 (M1) morphologies. These observations are consistent with the finite size effects on the transition temperatures; however, the large effect of finite size during cooling cycle is still an open question [27].

4. Conclusions A single step, template free hydrothermal synthesis was used to prepare crystalline VO2 (A) polymorph. The space group at room temperature is P4/ncc. The reversible phase transition from LTP-A to HTP-A with a large hysteresis and the irreversible phase

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transition from HTP-A to VO2 (M1) were studied. In the case of VO2 (A), high-temperature in-situ P-XRD studies revealed the progressive nature of the structural phase transition; a weak coupling between structural and electronic phase transition is proposed. The formation mechanism behind rectangular micro-rod and flower-like morphologies of A and M1 polymorphs are related to a layer by layer formation mechanism. Further studies related to intrinsic electrical properties of VO2 (A) are in progress to clarify the exact nature of the electronic transition. Acknowledgments We would like to thank I. Bucur (INCEMC), A. Brull (ICMCB), S. Gomez (ICMCB), D Denux (ICMCB), O. Nguyen (ICMCB), S. Fourcade (ICMCB) for their assistance during sample preparation and characterizations. S.R.P., M.M., A.V., and M.P. gratefully acknowledge financial support from European Community's Marie Curie Initial Training Network (ITN) Seventh Framework Programme—SOPRANO FP7/ 2007-2013 (Grant agreement no. 214040). A.A. gratefully acknowledges financial support from European Community's Marie Curie Incoming International Fellowship (IIF) Seventh Framework Programme—EPREXINA FP7/2007-2013 (Grant agreement no. 255662). Appendix A. Supplementary information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2014.01.037. References [1] J. Xie, C. Wu, S. Hu, J. Dai, N. Zhang, J. Feng, J. Yang, Y. Xie, Phys. Chem. Chem. Phys. 14 (2012) 4810–4816. [2] S.R. Popuri, M. Miclau, A. Artemenko, C. Labrugere, A. Villesuzanne, M. Pollet, Inorg. Chem. 52 (9) (2013) 4780–4785. [3] F. Theobald, J. Less-Common Met. 53 (1977) 55–71.

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