In Situ Triggering and Dynamically Tracking the Phase Transition in

Jul 3, 2014 - Jing Pan,. †. Hao Wu,. †. Li Zhong, ... proposed to in situ trigger and dynamically track the phase transition of VO2 nanoparticles,...
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In Situ Triggering and Dynamically Tracking the Phase Transition in Vanadium Dioxide Ming Li,† Dengbing Li,† Jing Pan,† Hao Wu,† Li Zhong,† Qiang Wang,† and Guanghai Li*,†,‡ †

Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China ‡ University of Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: As the most widely studied thermochromic material, monoclinic vanadium dioxide (VO2 (M)) shows promising applications in the energy-saving field. Solid-state transformation, especially fast annealing, plays an important role in the production of VO2 nanomaterials. On the other hand, the fast process makes it impossible to real-time monitor the phase transition in VO2. In this paper, a differential scanning calorimetry technique is proposed to in situ trigger and dynamically track the phase transition of VO2 nanoparticles, which gives a distinguished method to identify the underlying size-dependent and defectmediated structure phase transition.

1. INTRODUCTION Monoclinic vanadium dioxide, VO2 (M), has long been regarded as a typical example possessing metal−insulator phase transition (MIT, VO2 (M)↔VO2 (R)) since it was first reported.1 MIT is fully reversible and can lead to a dramatic change in the electrical and optical properties of VO2 (M), which is thus regarded as an attractive candidate for a variety of technological applications such as field-effect devices,2−4 terahertz metamaterials,5−7 intelligent windows,8,9 memory devices,10,11 and sensors.12−14 Solid-state transformation of metastable VO2 (B) driven by thermal treatment is generally used to prepare thermodynamically stable VO2 (R) nanomaterials.15 In practice, high reaction temperature and overgrowth of VO2 (R) is a common issue, and commercially available VO2 is limited to relatively large primary particles other than nanocrystals.16,17 Recently, the findings of similar structure and formation energy between some metastable VO2 and VO2 (M/R) phases open a new avenue to synthesize VO2 (M/R) nanomaterials through solidstate transformation under mild annealing conditions. For example, paramontroseite VO2 can be transformed to VO2 (R) at 400 °C within 1 min in a N2 environment,18 VO2 (M) nanocrystals can be obtained by directly heating VO2 (P) nanocrystals at 400 °C for 40 s in air,19 and VO2 (M) is prepared via mild thermal treatment of VO2 (D) micro/ nanoparticles at 320 °C.20 On the other hand, to facilely in situ monitor the phase transition process from metastable VO2 to VO2 (M/R) is still a great challenge because it is highly sensitive not only to temperature but also to factors such as internal defects, strain, and size.21−25 © 2014 American Chemical Society

Differential scanning calorimetry (DSC) can provide a distinct insight into the phase transition, and the reversible MIT of VO2 can be easily studied and monitored. Herein, we show that the phase transition of metastable VO2 (D) can be triggered using DSC cycle scanning, and it was found that the heating cycle in the DSC measurement plays the role as a rapid annealing process and can induce the phase transition from metastable VO2 (D) to VO2 (R) and finally to VO2 (M). DSC scanning also can dynamically track the phase transition temperature of VO2, upon which to reveal the underlying nature of the phase transition.

2. EXPERIMENTAL DETAILS Metastable VO2 (D) nanoparticles were synthesized by using a hydrothermal method.26 Briefly, the starting materials are a mixture of vanadium pentoxide and oxalic acid dehydrate (with molar ratio about 1:1−2), and the surfactant includes poly(vinyl alcohol) and propylene glycol methyl ether acetate (1−2 wt %, respectively). The synthesis was performed in a 50 L autoclave at 220 °C for 36 h. After cooling to room temperature, the resulting nanoparticles were collected by centrifugation, washed alternately with copious amounts of deionized water and ethanol to remove any organic residue, and then dried in an oven at 70 °C. Received: May 16, 2014 Revised: July 2, 2014 Published: July 3, 2014 16279

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Figure 1. (a) FESEM image and (b) XRD pattern of VO2 (D) nanoparticles. The inset in panel a is the corresponding size histogram.

3. RESULTS AND DISCUSSION Figure 1a shows the FESEM image of the obtained VO2 nanoparticles and the corresponding size distribution histogram. The nanoparticles have a nearly spherical shape with most sizes less than 100 nm and an average size of about 70 nm. The corresponding XRD analysis (Figure 1b) indicates that all the diffraction peaks can be assigned to the newly reported phase of metastable VO2 notated as VO2 (D).20,27 According to recent theoretical studies, a semblable lattice structure and adjacent formation energy between VO2 (D) and VO2 (R) give a low barrier to trigger the phase transition between them with a critical transition temperature of about 600 K, which is consistent with the experiment studies.20,28 We found that the phase transition temperature can be further reduced as the size of VO2 (D) particles reaches nanoscale,26 and the phase transition from VO2 (D) to VO2 (R) can be realized by annealing treatment for a few minutes at temperatures as low as 200 °C. This rapid solid-state transformation provides us an opportunity to in situ trigger and dynamically track the phase transition of VO2 (D) and VO2 (M) by DSC cycle scanning. Figure 2 shows typical DSC cycles of VO2 nanoparticles with the maximum setting temperature of 400 °C (thermal cycle between −20 °C and 400 °C). The endothermic peaks in the heating cycles correspond to the phase transition from VO2 (M) to VO2 (R), and the exothermic peaks in the cooling cycles correspond to the phase transition from VO2 (R) to VO2 (M). The peak intensity gradually increases and the peak temperature (Tc, MIT temperature) shifts from 61 °C to 68.8 °C and from 49.7 °C to 53.4 °C with increasing cycle numbers from the first to the seventh in heating and cooling cycles, respectively, indicating that Tc depends strongly on cycle number in the DSC scanning. Note that the first two cycles have a relatively large increase in Tc, and after five cycles, the Tc reaches 68.2 °C in the heating cycle, which is close to that of bulk VO2 (M). The inset in Figure 2 shows two endothermic peaks situated at 133 °C and 197 °C in the first heating cycle for the VO2 (D) nanoparticles, and no corresponding exothermic peak appears in cooling cycle. These two endothermic peaks are attributed to the phase transition from VO2 (D) to VO2 (R), as our further experiments show that the VO2 (D) nanoparticles can be transformed to VO2 (M) after annealing treatment at temperatures as low as 200 °C in air (Figure S1, Supporting Information). The fact that the endothermic peaks disappear in the second heating cycle indicates that the transformation from VO2 (D) to VO2 (M) is irreversible and is completed in the first heating process. These results indicate that the phase transition from metastable VO2

Figure 2. DSC curves of VO2 (M) nanoparticles at different cycles with the maximum setting temperature of 400 °C and scanning rate of 10 °C/min. The inset is the DSC curves of VO2 (D) nanoparticles at the first two cycles.

(D) to the thermodynamically stable VO2 (R) phase can be easily triggered in the heating process. The maximum setting temperature also affects the phase transition of the VO2 nanoparticles: the higher the maximum setting temperature, the higher the Tc. For example, Tc increases from about 39.8 °C to about 51.5 °C in the first DSC peak heating cycle when the maximum setting temperature increases from 300 °C to 350 °C (Figure S2, Supporting Information). Figure S2a shows that Tc in the first cycle is at about 51.5 °C and 39.8 °C, respectively, for the heating and cooling cycle with the maximum setting temperature of 350 °C, which is well below that with the maximum setting temperature of 400 °C, and reaches the values of the first cycle in Figure 2 after seven cycles. This result indicates that more cycles at a low setting temperature equals less cycles at a high setting temperature. The same tendency in Tc variation occurs for VO2 (M) nanoparticles in DSC cycle scanning at the maximum setting temperature of 300 °C, except that the first endothermic and exothermic peaks corresponding to MIT appear after the fourth cycle, as shown in Figure S2b. Note that Tc with values as low as 39.8 °C and 31 °C, respectively, in the heating and 16280

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to initiate the phase transition from VO2 (M) to VO2 (R),26,29,36 leading to a narrower hysteresis width. Because the defect concentration in VO2 (M) nanoparticles will be substantially reduced at a high setting temperature in DSC cycle scanning, the size effect will dominate, resulting in broad hysteresis, which is consistent with our recent report.26 The method based on the DSC cycle to trigger and track the phase transition of VO2 (D) and VO2 (M) nanoparticles also can be used to study the phase transition of metastable VO2 (P) nanoparticles, as illustrated in Figure 4 (the corresponding

cooling cycle can be realized, providing an alternative way to regulate the phase transition temperature of VO2 (M) nanoparticles other than the traditional doping routine. The stepwise increase in Tc shown in Figures 2 and S2 is attributed to both defect and size effect in the reversible phase transition of VO2 (M)↔VO2 (R).26 Figure S3, Supporting Information, displays the twin and grain boundary defects in VO2 (M) obtained by annealing of VO2 (D). These defects are inevitable during the solid-state phase transition,29 which result in poor crystallization of VO2 (M) nanoparticles, as confirmed by the relatively weak endothermic and exothermic peaks intensities in the first few cycles (Figures 2 and S2). The defects in VO2 (M) can act as initiation sites during the phase transition and lead to a lower Tc in DSC measurement. Moreover, from Figure S4, Supporting Information, we can conclude that the lower annealing temperature gives a smaller crystalline size. Thus, a lower maximum setting temperature and less cycles have the same effect as the lower annealing temperature which results in a smaller crystalline size of VO2 (M) and a decreased Tc. The dependence of Tc on VO2 (M) crystalline size has been reported in the literature,30−33 and early study has shown that Tc decreases with decreasing VO2 (M) nanoparticle size.34 From these results, we deduce that the cycle number-dependent Tc is closely related to both size and defect effect. Besides the phase transition temperature, the thermal hysteresis is another important factor from technological and basic research perspectives.32 The hysteresis width also depends critically on both crystallinity and grain size of VO2 (M).35 Figure 3 shows the dependence of the thermal hysteresis on the

Figure 4. DSC curves of VO2 (P) nanoparticles at different cycles between −20 °C and 350 °C at a ramp rate of 10 °C/min.

SEM image and XRD pattern of VO2 (P) nanoparticles are shown in Figure S5, Supporting Information), in which the endothermic and exothermic peaks corresponding to the reversible phase transition VO2 (M)↔VO2 (R) appear only after eight cycles, indicating that the phase transition from VO2 (P) to VO2 (R) requires more thermal accumulation and longer time because the formation energy difference between VO2 (P) and VO2 (R) is much larger than that between VO2 (D) and VO2 (R).18,20

Figure 3. Dependence of hysteresis width on the DSC cycle at different maximum setting temperatures.

DSC cycle, in which the hysteresis width increases with increasing DSC cycles and finally reaches a constant value. A definite increase in the hysteresis width was observed with the maximum setting temperature of 400 °C. The irregular dependence of the hysteresis width on cycle number at the setting temperature of 300 °C is considered to be due to the competition between size and defect effects.26 Figure 3 shows that the hysteresis width at the setting temperature of 300 °C and 350 °C is generally narrower than that at 400 °C in the same cycle, demonstrating a defect effect-dominated phase transition at low setting temperature. In our previous study, we found that a low annealing temperature always results in a high defect concentration in VO2 (M) nanoparticles, such as grain boundaries, dislocations, and other structural imperfections, and these defects are not observed in VO2 (D) nanoparticles.26 The low maximum setting temperature equals a low annealing temperature, and the resulting defects serve as nucleation sites

4. CONCLUSIONS In summary, an effective strategy based on DSC has been developed to in situ trigger and dynamically track the phase transition of metastable VO2 and reversible MIT of VO2 (M)↔ VO2 (R), upon which to identify the underlying nature of the phase transition. A stepwise increase in MIT temperature and thermal hysteresis width was found upon the DSC cycle of VO2 nanoparticles and is attributed to the competition of defect and size effects. Our method also can be used to study the structural phase transition behaviors of other metastable nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

XRD pattern of VO2 (D) nanoparticles after annealing treatment at 200 °C in air; DSC cycle curves of VO2 with 16281

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maximum setting temperatures of 350 °C and 300 °C; HRTEM image of VO2 (M) nanoparticles; XRD patterns of VO2 (M) nanoparticles obtained by annealing VO2 (D) at different temperatures; FESEM image and XRD pattern of VO2 (P) nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*Tel:+86-551-65591437. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2009CB939903) and the National Natural Science Foundation of China (grant 51372250).

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