LI Xiong-Jian YANG Shui-Jin

(Hubei Key Laboratory of Pollutant Analysis & Reuse Technology, College of Chemistry and Chemical Engineering, Hubei Normal University, Huangshi435002, China)

ABSTRACT Prisms-built VO2(M) micro-nanostructures with narrow hysteresis width of 2.7 ℃ were successfully synthesized using V2O5-H2C2O4-H2O system by one-pot hydrothermal approach. The structure,composition, phase transition and optical properties were characterized by XRD, SEM, DSC, and variable-temperature UV-vis. The results revealed the prism had well-defined six facets and entire smooth surface with lengths of about 500 nm and thicknesses of around 100 nm. Several prisms were connected to each other through the apical growth. The prismatic VO2(M) showed excellent phase transition and optical switching properties that would be beneficial for highly sensitive electrical/optical devices or other applications. The possible formation mechanism of prismatic VO2(M) was proposed via time-dependent SEM images and XRD patterns.Furthermore, the influence of the amount of H2O on the final product was discussed in detail.

Keywords: VO2(M), optical properties, hysteresis width, one-pot synthesis;

1 INTRODUCTION

Vanadium dioxide (VO2) as one of the most well-known binary compounds has various polymorphs, including VO2(M)VO2(B), VO2(A), etc .[1-4]Among these polymorphs, VO2(M)has been paid much attention due to its interesting temperature-dependent optical and electrical properties induced by a fully reversible metal semiconductor transition between monoclinic VO2(M) and tetragonal VO2(R) at a critical temperatureτc(68 ℃ for bulk VO2)[5,6]. These properties give rise to a wide range of promising potential applications of VO2(M) in intelligent energy conserving windows,electrical and optical devices, laser shield, flat panel displays,and so on[7-11]. Among those, VO2(M) applied as smart optical and electrical devices is identified as one of the most attractive applications. Thermal hysteresis (ΔTc) named as the difference of phase transition temperature existing in the heating and cooling process is a key factor for evaluating sensitivity of smart optical and electrical devices[12]. Generally,the wide ΔTcdoes harm to the switching behavior and reduces the sensitivity of switching responses to temperature[13]. In view of this, it is fascinating and significative to reduce the ΔTcof VO2(M) usually depending on specific morphologies,sizes, dopants and synthetic methods.

In order to reduce the ΔTc, numerous efforts have been concentrated on synthesizing VO2(M) and doped VO2(M)nanostructures using various technologies, including microwave irradiation method, sol-gel processes, RF sputtering, hydrothermal synthesis, physical/chemical vapor deposition and so forth[5,14]. However, comparative study of these references reveals that the ΔTcfor most reported VO2(M)materials occurs in a broad temperature range of about 5～20 ℃ during reversible phase transition[15]. For example, Li et al.[16]reported the ΔTcof VO2(M) nanoparticles using interfacial defects and size effect approach exhibited an obvious hysteresis above 10 ℃. Among doped VO2(M) for reducing the ΔTc, the dopant of Ti has been regarded as the first choice relative to other elements. Du et al.[17]prepared a series of Ti-doped VO2films by polymer-assisted deposition and systematically studied their hysteresis; They found the ΔTcreduced continuously from 38.2 to 3.5 ℃ with increasing the Ti content. Nevertheless, the doping process of VO2(M) is complex and difficult to control, which would lead to the impurity and inhomogeneity of the final products.Although lots of studies have been reported for preparing pure VO2(M) usually accompanied by subsequent annealing treatment at 450～700 ℃[18,19], the direct formation of pure VO2(M) using one-step approach has been rarely reported.Recently, Ji et al.[20]reported one-pot hydrothermal synthesis of single crystal VO2(M) in the V2O5-H2C2O4-H2O system by adding a proper amount of H2SO4as pH adjustment agent. In addition, Chen et al.[21]reported that pure VO2(M) was difficult to synthesize and always mixed with VO2(B).Therefore, the development of one-step synthesis of novel VO2(M) to reduce the ΔTcis highly desirable and challenging.

In this study, prism-built VO2(M) micro-nanostructures were successfully synthesized using V2O5-H2C2O4-H2O system by one-pot hydrothermal approach without any additives. The prismatic VO2(M) shows excellent phase transition with ΔTcof 2.7 ℃ and optical switching properties that will be beneficial for highly sensitive smart optical and electrical devices or other applications. Furthermore, the influences of reaction time and amount of H2O on the crystallized phase and morphologies of VO2were respectively discussed in detail. According to the experimental results, the possible formation mechanism of prismatic VO2(M) was proposed.

2 EXPERIMENTAL

2. 1 Synthesis of prismatic VO2(M)

All of the reagents used in the experiments were analytically pure. In a typical synthesis, 1.460 g vanadium pentoxide (V2O5) and 2.020 g oxalic acid dihydrate(H2C2O4·2H2O) were dispersed into deionized water (62 mL)and stirred for 30 min at room temperature. Then, the suspension was transferred into an 80 mL stainless-steel autoclave and maintained at 260 ℃ for 48 h. After being accomplished, the autoclave was cooled down to room temperature naturally. The precipitates were collected by centrifugation, washed with deionized water and ethanol several times and dried in the oven at 70 ℃ for more than 12 h.

2. 2 Characterization

The as-prepared products were characterized by powder XRD using D8 X-ray diffractometer equipment with Cu-Karadiation,λ= 1.54060 Å. The morphologies were analyzed by scanning electron microscopy (SEM, Quanta 200). The phase transition temperature of the sample was measured by differential scanning calorimetry (DSC, DSC822e, METTLER TOLEDO) with a heating ramp of 5 ℃/min. Thermal gravimetric analysis (TGA) was performed on TG 209 F1.Optical properties of the samples were tested using UV-vis reflectance spectrum by adding a heater at the sample holder on a Shimadzu UV-3600 spectrophotometer using BaSO4as reference.

3 RESULTS AND DISCUSSION

Fig.1a shows the XRD patterns of the obtained sample as well as the standard JPCDS plots of VO2(M). By comparison,all diffraction peaks of the obtained products were readily indexed to the standard phase of VO2(M) (JCPDS 43-1051)and no peaks of any other phases or impurities were detected.In addition, the strong diffraction peaks suggested a high crystallinity of VO2(M). The morphologies of as-obtained sample were investigated by SEM (Figs. 1b and 1c). The panoramic view shown in Fig.1b obviously displays that the obtained VO2(M) consisted of a large quantity of uniform prisms. From the high-magnification SEM image (Fig.1c), it can be clearly seen that these prisms possessed a well-defined six facets with entirely smooth surface, and had lengths of about 500 nm and thicknesses of around 100 nm. In addition,several prisms were connected to each other through the apical growth.

Fig.1. (a) XRD patterns and (b, c) SEM images of the as-obtained VO2(M)

To gain further understanding the formation process of prismatic VO2(M) micro-nanostructures, time-dependent investigations were carried out by extracting products at different reaction stages. The synthetic processes were ceased at definite reaction periods of 6, 12, 24 and 36 h, and the as-obtained intermediate samples were thoroughly determined by XRD and SEM, as shown in Fig.2. At the initial stage, the main diffraction peaks from the XRD pattern were indexed as monoclinic phase of VO2(B) while some weak peaks were attributed to VO2(M). It can be seen from the corresponding SEM image that the products were irregular nano-bulks with large variability in particle sizes.When the reaction was carried out for 12 h, the mixed crystalline phases mainly consisting of VO2(B) and VO2(M)were obtained, and lots of preformed prisms as well as a small number of nano-bulks were formed. After the reaction time was prolonged to 24 h, crystallized VO2(M) became the predominant phase and the incomplete prisms grew gradually.When the reaction time was extended to 36 h, the XRD pattern only displayed the diffraction peaks of VO2(M) and good micro-nanostructures assembled by several prisms were formed through the apical growth. After the reaction was prolonged further to 48 h, high crystallinity of VO2(M) with prisms-built micro-nanostructures was formed. On the basis of these experimental results, three steps,i.e., nucleation,dissolution-recrystallization and subsequent growth, were proposed to explain the whole evolution process of prism-built VO2(M) micro-nanostructures, as illustrated in Fig.2f. In terms of the first step, metastable VO2(B) phase was formed preferentially. With the reaction time going, the metastable phase dissolved into the solution system and the nucleation of VO2(M) occurred and grew, and then evolved to prismatic assemblies in the process of Ostwald ripening growth[13].

Fig.2. (a) XRD patterns and (b～e) SEM images of the samples obtained at different growth stages,(f) formation mechanism of prism-built VO2(M) micro-nanostructures

In addition, the amount of H2O was found to be another important controlling factor on the phases of final products.Fig.3a shows the corresponding XRD patterns of the samples produced with different amounts of H2O. When the addition of H2O was 50 mL, all diffraction peaks of samples were well indexed to the VO2(A) phase (JCPDS 42-0876)[15]. With H2O addition increasing to 54 mL, some weak peaks attributed to VO2(M) appeared. As increasing H2O to 60 mL, the phase suffered an important evolution, forming predominant VO2(M) coupled with small amount of residual VO2(A).Interestingly, with the continuous increase of H2O to 68 mL,the peak intensities of VO2(M) weakened and resulted in the formation of hydrated vanadium oxide phase, named,VO2(M)·xH2O[10,12]. Further increasing H2O to 72 mL resulted in the entire formation of VO2(M)·xH2O. The phase and structure of the VO2(M)·xH2O were confirmed by thermal analysis of TG in flowing N2atmosphere as well as XRD, as shown in Figs. 3b and 3c, respectively. The TG plot exhibits a weight loss of 3.09% at the range of 200～450 ℃andxcan be calculated as 0.147. After annealing at 500 ℃for 2 h in flowing N2atmosphere, all diffraction peaks were indexed to VO2(M), indicating the formation of crystallized VO2(M) without any other parasitic phases.

Fig.3. (a) XRD patterns of the products synthesized from different amounts of H2O at 260 ℃ for 48 h, (b) TG curve of VO2(M)·xH2O in flowing N2 atmosphere, (c) XRD pattern of as-obtained VO2(M)·xH2O after heat treatment at 500 ℃ in flowing N2 atmosphere

The reversible phase transition of VO2(M) micro-nanostructures was investigated by DSC analysis. Fig.4a shows the representative DSC curves of VO2(M) with heating and cooling curves. The endothermic and exothermic peaks,implying the phase transition between VO2(M) and VO2(R),appeared at 56.5 and 53.8 ℃, respectively. Furthermore, it displayed good cycle stability with thermal hysteresis width of 2.7 ℃ (ΔTc). Table 1 shows the comparison of DSC results in the references and this work. The obtained prism-built VO2(M) displayed much more narrowed ΔTccompared to that reported in literatures. This indicates that the prism-built VO2(M) showed high sensitivity to temperature. The showing narrow hysteresis width is due to the preferred crystallographic orientation which greatly influences the width and switching temperature in the process of reversible phase transition[22].

Variable-temperature UV-vis spectra were recorded to investigate the optical properties of obtained VO2(M), as shown in Fig.4b. It reveals that the prism-built VO2(M) has good optical switching properties in visible region during phase transition while almost no changes in ultraviolet region.Above the transition temperature, the UV-vis curves at 80 and 120 ℃ coincided well with respect to no changes in phase transition. As VO2(M) transforms to VO2(R), the reflectivity in the 1500～2100 nm region decreased as a result of the formation of metal phase VO2(R), leading to screening from the electrons delocalized at the surface of sample[31]. In these regards, VO2(M) can be applied as intelligent materials.

Fig.4. (a) DSC curves with two cycles and (b) variable-temperature UV-vis spectra of as-obtained VO2(M)

Table 1. Comparison of DSC Results in Literatures and This Work

4 CONCLUSION

In conclusion, prisms-built VO2(M) micro-nanostructures with narrow hysteresis width of 2.7 ℃ were successfully synthesized using V2O5-H2C2O4-H2O system by one-pot hydrothermal approach. The reaction time and amount of H2O were the key factors for the synthesis of VO2(M). The prepared VO2(M) consisted of numerous prisms connected to each other through the apical growth. The prisms had well-defined six facets and entire smooth surface with lengths of about 500 nm and thicknesses of around 100 nm. The prismatic VO2(M) showed excellent phase transition and optical switching properties that would be beneficial for highly sensitive electrical/optical devices or other applications.