Facile synthesis and electrical switching properties of V2O3 powders

Materials Science and Engineering B 217 (2017) 1–6

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Facile synthesis and electrical switching properties of V2O3 powders Haining Ji, Dongqing Liu ⇑, Haifeng Cheng, Lixiang Yang, Chaoyang Zhang, Wenwei Zheng Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073, PR China

a r t i c l e

i n f o

Article history: Received 8 November 2016 Received in revised form 26 December 2016 Accepted 11 January 2017

Keywords: V2O3 Hydrothermal synthesis Phase transition Electrical properties

a b s t r a c t V2O3 powders were synthesized with mercaptoacetic acid (C2H4O2S) as reducing agent and stabilizer via a facile hydrothermal approach. The crystalline structure, surface morphology, valence state of the derived V2O3 powders were characterized via X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy. It was found that the ratio and reaction time played a duel role in the formation and morphology of the V2O3 powders. The metal-insulator transition properties of V2O3 powders were studied by the differential scanning calorimetry curve and variable temperature Raman spectra. The change in electrical resistance due to the metal-insulator transition was measured from 80 to 240 K using physical property measurement system. The results showed V2O3 samples had excellent electrical switching properties with resistance changes as large as 104. This simple and fast synthesis approach makes the V2O3 powders easily accessible for exploring their fundamental properties and potential applications in novel electronic devices. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Vanadium sesquioxide (V2O3) is a typical metal-insulator transition (MIT) material exhibiting phase transition from the metallic and paramagnetic to insulating and antiferromagnetic state [1]. Despite having a MIT temperature as low as 150 K [2,3], V2O3 with narrow hysteresis width and remarkable enhanced electrical resistance jump, is still highly desirable to better meet its burgeoning demand as electrical switches in the ultra-low temperature field [4]. Although V2O3 has already been discovered by Foexin in 1946 and a number of routes have been performed to synthesize V2O3, such as solid phase reaction [5], mechanochemical thermal reduction [6], supercritical fluid [7], thermal decomposition [8–11] or hydrothermal method [12–16]. Hydrothermal synthesis allows for unique nanoscale morphology and structure control and do not afford traditional high-temperature solid-state reactions. In addition, hydrothermal synthesis, involving water-soluble precursors at low temperatures, is cost-effective, readily scalable, and sensitive to a host of synthesis parameters, thereby enabling the isolation of desired oxides and phases. However, most of hydrothermal synthesis for the formation of V2O3 crystals need to be calcined after hydrothermal synthesis or add surfactant to ensure its dispersion [12–15,17]. Accordingly, it is still a challenge

⇑ Corresponding author. E-mail address: [email protected] (D. Liu). http://dx.doi.org/10.1016/j.mseb.2017.01.003 0921-5107/Ó 2017 Elsevier B.V. All rights reserved.

to synthesize high quality V2O3 crystals with facile and fast synthesis method for the practical applications [18]. In this work, we develop a facile and fast method for preparing V2O3 powders by one-step hydrothermal synthesis. In addition, V2O3 powders are successfully synthesized using C2H4O2S as reducing agent and stabilizer without additional surfactants by modifying the reaction parameters. Compared with previous reports [19,20], our synthesis is much shorter, easier to control, and the V2O3 powders obtained have favorable dispersibility in the absence of surfactant. Besides, the electrical properties of these V2O3 powders are also investigated. 2. Experimental 2.1. Materials Ammonium metavanadate (NH4VO3), mercaptoacetic acid (C2H4O2S) with analytical grade were purchased from Aladdin chemical reagent corporation and used without further purification. The water used to make up solutions was deionized. 2.2. Synthesis V2O3 powders were prepared by a facile one-step hydrothermal method using a vanadium source of NH4VO3 and a reducing agent of C2H4O2S without additional surfactant. In a typical procedure, 1.76 g (15 mmol) NH4VO3 and 0.46–2.76 g (5–30 mmol) C2H4O2S were dispersed in 60 mL deionized water. The mixture was stirred

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for 30 min and then transferred to a 100 mL Teflon-lined stainlesssteel autoclave. The hydrothermal reaction was carried out at 260 °C for 2–30 h and then air-cooled to room temperature. The final products were collected via centrifugation, washed with deionized water and ethanol three times and dried in a vacuum drying oven at 80 °C for 10 h. 2.3. Characterization Powder XRD characterization of the prepared materials was performed using monochromatic Cu Ka radiation with a D8ADVANCE diffractometer (Bruker, Germany). The morphology and the size were obtained using a field-emission scanning electron microscope (FESEM, NOVA NanoSEM 230). The particle size distribution of V2O3 powders was evaluated statistically by counting more than 200 individual particles on SEM images using image processing software Nano Measure 1.2. The microstructure of the samples were further analyzed using a transmission electron microscopy (TEM, JEOL2010) with a LaB6 source operating at an acceleration voltage of 200 kV. Selected area electron diffraction experiments were carried out in vacuum in a JEOL 2100 transmission electron microscope working at 200 kV. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Ka radiation. The valence band XPS spectrum was measured using a Thermo ESCALAB 250XI spectrometer with a monochromated AlKa (hm = 1486.6 eV). The spectrum given here was obtained using a 500 lm diameter analysis area. The C1 s line with a binding energy of 284.6 eV was used as a reference to eliminate the charging effect. The phase transition behaviors of the resulting products were measured by differential scanning calorimetry (DSC, NETZSCH, DSC200F3) over the temperature range from 100 to 300 K using a liquid nitrogen cooling unit. The heating and cooling rates were set at 5 °C/min. Variable temperature Raman spectra were collected with a confocal microscope Raman system (LabRAM HR800, Horiba Jobin Yvon) using 532 nm excitation with a laser power of 2.5 mW and the integration time was 25 s with 5 times accumulation. To avoid the oxidization introduced by laser heating, the location of the laser on the powders was changed with the change of temperature. Temperature-dependent resistance measurements were performed by standard four-terminal method using a Physics Property Measurement System (PPMS, Quantum Design). The system equipped with DC resistivity option in a heating/cooling cycles between 80 and 240 K. Powder samples to measure the electrical properties were densely compacted into a rectangular shape.

Fig. 1. XRD patterns of the as-obtained samples with various molar ratios (NH4VO3 to C2H4O2S from 3:1 to 1:2).

peaks can be readily indexed to the rhombohedral phase of V2O3 in agreement with the literature value (JCPDS, No.65-9474, a = 4.951 Å, b = 4.951 Å c = 14.003 Å). Moreover, when the mole ratio of the raw materials increases, all the diffraction peaks can still be readily assigned to the rhombohedral crystalline phase of V2O3 and diffraction peak intensity increases, meaning the V2O3 powders crystallinity increases. When the molar ratio of NH4VO3 and C2H4O2S at 2:3 and 1:2, the V2O3 powders synthesized have high crystallinity. This is similar to the previously reported literature [3]. 3.1.2. Reaction time To deeply reveal the evolution process of the formation of V2O3, the samples were prepared under various reaction time at 2, 6, 10, 20 and 30 h investigated with the fixed molar ratio and synthesis temperature (the molar ratio of NH4VO3 and C2H4O2S at 2:3 and the reaction temperature is 260 °C). The structures, surface morphologies and particle size distribution of the as-obtained intermediate V2O3 samples were thoroughly examined by XRD and SEM. The results are shown in Figs. 2–4. When the reaction is carried out for 2 h, the diffraction peaks from the XRD pattern (Fig. 2) could be readily indexed as the rhombohedral phase of V2O3 (JCPDS, No. 65-9474) already described although the intensity of the diffraction peak is low.

3. Results and discussion 3.1. Hydrothermal synthesis Molar ratio of precursors, synthesis pressure (sometimes referred to as fill ratio), temperature and reaction time may influence the physicochemical properties of a particular material system. In this work, two major synthesis parameters, including molar ratio of raw materials and reaction time, have been investigated. 3.1.1. Molar ratio of raw materials As shown in Fig. 1, the phases of the V2O3 samples obtained in the different mole ratio of raw materials, as NH4VO3 to C2H4O2S from 3:1 to 1:2, were examined by XRD. Samples synthesized with the molar ratio of NH4VO3 and C2H4O2S at 3:1 and 2:1 can yield V2O3 crystals with weak diffraction peak, indicating the crystallinity is bad. When the molar ratio is 1:1, all the diffraction

Fig. 2. XRD patterns of as-obtained samples with different reaction time.

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Fig. 3. SEM images of as-obtained samples with different reaction times: (a) 2 h; (b) 6 h; (c) 10 h; (d) 20 h; (e) 30 h.

Fig. 4. Particle size distribution of as-obtained samples from Fig 3(a)–(e).

With the reaction time extends to 6 h, the intensity of the diffraction peak increases. When the reaction time continues to increase, the intensity of diffraction peak is almost constant. This shows that the finely crystalline powders can be prepared in just 6 h. Compared with previous reports [16,21–23], our synthesis is much shorter. Fig. 3 demonstrates the typical FESEM images which illustrate the particle morphology and size change with increasing reaction time during the growth process of the V2O3 powders. When the reaction time is 2 h, the obtained products are composed of small

particles and polyhedron powders (Fig. 3a). When the reaction time increased to 6 h, small particles disappeared and the polyhedron powders became larger (Fig. 3b). It is clearly seen from 3c-3e that polyhedron powders increase gradually with the increase of hydrothermal reaction time. Fig. 4 graphically expresses the detailed particle size distributions of V2O3 powders obtained from Fig 3(a-e) SEM images. The average particle sizes of V2O3 powders are 0.26 lm, 0.33 lm, 0.37 lm, 0.39 lm, and 0.42 lm, respectively. From the result, we find that the particles gradually grow up, and particle size distributions become narrower gradually with

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the increase of reaction time, and the polyhedral powders with fine dispersion can be obtained as low as 6 h. Fast synthesis of V2O3 powders mainly due to the strong reduction of the thiol groups, thiol groups are easily oxidized to disulfide bonds [24], meanwhile NH4VO3 was reduced to V2O3. Fine dispersion may be caused by interactions of vanadyl with mercaptoacetic acid [25]. 3.2. Characterization To determine the chemical component and valence state of vanadium atom of the samples, XPS scan surveys were performed on V2O3 powders. Figs. 5a shows typical survey spectrum for vanadium oxides obtained through 6 h hydrothermal reaction. The photoelectron peaks of the main elements V, O and C can be seen clearly. The C1s peak, taken at 285.0 eV, which was used as binding energy reference. From Fig. 5b and c, the binding energy (515.8 eV) of V2p3/2 and the binding energy difference (4 = 14.2 eV) between the O1 s and V2p3/2 level are fairly consistent with the reported value for V3+, further confirming that the vanadium in the asobtained sample is of +3 valence state [26–29]. Fig. 5d shows the valence band XPS spectrum for V2O3 samples. In the valence band XPS spectrum, two peaks are observed: a low binding energy peak at about 0.8 eV from the Fermi edge due to the V3d electrons not involved in the V-O bonding, and a broad band at about 6.3 eV. This broad band is mainly due to the O2p states, which are hybridized with the V3d states, and it is commonly referred to as the O2p band [30,31].And One can see a non-zero spectrum weight at the Fermi level, indicating its metallic behavior at room temperature, consistent with previous reports [32,33]. Analysis of the results mentioned above, in agreement with previous XPS studies [26–33] on

V2O3 denotes an oxidization compatible with a V3+ state and shows that the samples are well identified as a V2O3 compound. The morphology and crystallinity of the V2O3 powders are further observed by TEM, and the results are shown in Fig. 6. TEM image (Fig. 6a) of samples shows that the morphology of V2O3 powders is polyhedron and the size of V2O3 powders is between 0.24 lm and 0.63 lm, which is consistent with the SEM images (Fig. 3b). HRTEM image (Fig. 6b) of V2O3 powders indicates that the inter-plane distance for the nanocrystals is 0.366 nm, which matches well with the data for (0 1 2) orientation in JCPDS# 659474, this further confirms that the formed phase of the powders is rhombohedral; the corresponding SAED pattern (Fig. 6c) shows that the as-synthesized powders are single crystals indexed to V2O3, which is consistent with XRD patterns. 3.3. Phase transition When the phase transition of V2O3 occurs, it respectively exhibits a noticeable endothermal and exothermal profile in the heating and cooling differential scanning calorimetry (DSC) curves, which corresponds to the first phase transition of V2O3. Fig. 7 shows the typical DSC curves of V2O3 with heating and cooling cycles. The Tc of V2O3 is about 110 °C in the heating cycle, whereas it is about 132 °C in the cooling cycle, with a temperature hysteresis width of 22 °C, in agreement with the previous reports [20,34,35]. The result reveals that V2O3 possess phase transition properties, which indicates the as-obtained V2O3 have potential application in electronic devices. Furthermore, the metal-insulator transition of V2O3 is directly detected using the variable temperature Raman spectra in Fig. 8. The Raman spectra of V2O3 powers at low temperature (90 K) dis-

Fig. 5. XPS of the as-prepared V2O3 powders through 6 h hydrothermal reaction: (a) survey spectrum; (b) core-level spectrum of V2p; (c) core-level spectrum of O1s; (d) valence band spectrum.

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Fig. 6. (a) TEM image, (b) HRTEM image and (c) the SAED pattern of the as-prepared V2O3 powders through 6 h hydrothermal reaction.

appears again. The above results indicate that the metal-insulator transition occurs in V2O3 powders. Therefore, variable temperature Raman spectra offers an alternative tool to verify the phase transition of the V2O3 powders.

3.4. Electrical properties

Fig. 7. DSC curves of the as-prepared V2O3 powders through 6 h hydrothermal reaction.

As mentioned above, phase transition is one of the most critical intrinsic characteristics of V2O3 which often reflected in the variation of resistance. Fig. 9 shows the temperature-dependent resistance of the rhombohedral V2O3 powders in temperature range of 80–240 K, measured in the process of both cooling and heating. Thermal hysteresis is observed in the V2O3 samples, which indicates a first order phase transition. The critical temperature is determined to be about 152 K, which is consistent with DSC results. At high temperature, V2O3 exhibit a metallic state with lower resistance; but they transit into insulating state when the temperature decreases below the critical point characterized by abrupt jump of resistance. The resistance changes little with temperature between 80 and 110 K in the cooling process. It should be noted that the resistance increases by 4 order of magnitude as the temperature decreased. The obtained V2O3 powders have the larger resistance jump than previously reported results of 103 resistance change [15,38]. The abrupt and large change of the resistance indicates that the V2O3 powders is stoichiometric and highly crystalline. The above results show that V2O3 has excellent electrical switching properties, similar as the results obtained that the metal-insulator transition in V2O3 powders via variable temperature Raman spectra.

Fig. 8. Variable temperature Raman spectra of the as-prepared V2O3 powders through 6 h hydrothermal reaction.

plays peaks at 187(Bg), 234(Ag), 280(Ag), 325(Ag), 340(Bg), and 522 (Ag) cm1, which confirm the characteristic vibration modes for the insulator phase V2O3 powers [36,37]. When the temperature rises to 180 K, Raman spectra with peaks at 207, 245, and 507 cm1 can be assigned to the Eg, A1g, and A1g modes of the metallic phase V2O3 powers [36,37]. However, when the temperature is again reduced to 90 K, Raman peaks of the insulator phase V2O3 powers

Fig. 9. Temperature-dependent resistance curves of the as-prepared V2O3 powders through 6 h hydrothermal reaction.

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4. Conclusion In conclusion, without additional surfactant, single crystal uniform V2O3 powders have been successfully synthesized by a facile and fast hydrothermal approach. V2O3 powders were obtained in only 6 h using mercaptoacetic acid as reducing agent and stabilizer. The synthetic V2O3 powders exhibit high crystallinity and feature a pure rhombohedral phase and composition. V2O3 powders obtained exhibit reversible phase transition properties with a critical temperature at 132 °C and a hysteresis width of 22 °C. V2O3 powders have excellent electrical switching properties with resistance changes as large as 104. This present work provides a fast and facile method for preparing pure phase V2O3 powders and makes V2O3 a desirable material to meet the demand of electrical switching applications in the ultra-low temperature field. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51502344). References [1] D. Adler, J. Feinleib, Semiconductor-to-metal transition in V2O3, Phys. Rev. Lett. 12 (1967) 146–155. [2] F.A. Chudnovskii, V.N. Andreev, V.S. Kuksenko, V.A. Piculin, D.I. Frolov, P.A. Metcalf, J.M. Honig, Acoustic emission during metal-insulator phase transition in V2O3, J. Solid State Chem. 133 (1997) 430–433. [3] Z. Zhan, X. Liu, H. Li, M. Fan, C. Huang, Facile synthesis of V2O3 nanobelts by the transformation of VO2 (B) with controllable phase transition property, Mater. Lett. 165 (2015) 214–216. [4] Y. Sun, B. Qu, S. Jiang, C. Wu, B. Pan, Y. Xie, Highly depressed temperatureinduced metal-insulator transition in synthetic monodisperse 10-nm V2O3 pseudocubes enclosed by 012 facets, Nanoscale 3 (2011) 2609–2614. [5] K.F. Zhang, Preparation and characterization of V2O3 nanopowder by solid phase reaction, Chin. J. Inorg. Chem. 21 (2005) 1090–1092. ˇ aplovicˇová, M. Škrátek, A. Dvurecˇenskij, M. Majerová, R. [6] P. Billik, A. Cigánˇ, M. C Bystricky´, P. Antal, J. Manˇka, V2O3 nanocrystals prepared by mechanochemicalthermal reduction and their magnetic properties, Mater. Lett. 110 (2013) 24– 26. [7] X. Liu, Y. Zhang, S. Yi, C. Huang, J. Liao, H. Li, D. Xiao, H. Tao, Preparation of V2O3 nanopowders by supercritical fluid reduction, J. Supercrit. Fluids 56 (2011) 194–200. [8] Y. Ishiwata, T. Shiraishi, N. Ito, S. Suehiro, T. Kida, H. Ishii, Y. Tezuka, Y. Inagaki, T. Kawae, H. Oosato, Metal-insulator transition sustained by Cr-doping in V2O3 nanocrystals, Appl. Phys. Lett. 100 (2012) 043103. [9] Y. Ishiwata, S. Suehiro, T. Kida, H. Ishii, Y. Tezuka, H. Oosato, E. Watanabe, D. Tsuya, Y. Inagaki, T. Kawae, Spontaneous uniaxial strain and disappearance of the metal-insulator transition in monodisperse V2O3 nanocrystals, Phys. Rev. B: Condens. Matter 86 (2012) 6335. [10] C. Zheng, X. Zhang, S. He, Q. Fu, D. Lei, Preparation and characterization of spherical V2O3 nanopowder, J. Solid State Chem. 170 (2003) 221–226. [11] Y. Ishiwata, E. Takahashi, K. Akashi, M. Imamura, J. Azuma, K. Takahashi, M. Kamada, H. Ishii, Y.F. Liao, Y. Tezuka, Impurity-induced first-order phase transitions in highly crystalline V2O3 nanocrystals, Adv. Mater. Interfaces 2 (2015) 1500132. [12] H.J. Song, M. Choi, J.C. Kim, S. Park, W.L. Chan, S.H. Hong, D.W. Kim, Lielectroactivity of thermally-reduced V2O3 nanoparticles, Mater. Lett. 180 (2016) 243–246.

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