Recrystallization of Single-Crystalline VO2 Microtube Arrays on V2O5 Substrate

Single-crystalline VO2 microtube arrays on V2O5 substrate were fabricated through a thermal oxidation route based on resistive heating V foil in air. Four sheets of as-fabricated single-crystalline VO2 microtube arrays on V2O5 substrate were then, respectively, heated to approximately 855 °C and 1660 °C to melt V2O5 or VO2. Thereafter, the melted V2O5 or VO2 was cooled rapidly or slowly to recrystallize the liquid V2O5 or VO2. The morphologies and phases of the recrystallization products were characterized by scanning electron microscopy and X-ray diffraction. This study proposes that the peak temperature of heating and the cooling rate are responsible for the recrystallization products of single-crystalline VO2 microtube arrays on V2O5 substrate.


Introduction
VO2 is characterized by a metal-insulator transition (MIT) [1], which Morin discovered in 1959 [2]. With an increased temperature to 68 °C from room temperature, a low-temperature monoclinic phase (insulator) of VO2 transforms into its high-temperature tetragonal rutile phase (metal). Accompanying the MIT are the large changes in optical, electrical, and magnetic properties of VO2. Based on the MIT, VO2 can be applied in several modern devices, such as optical switching [3], smart window [4,5], oscillators, [6], and field-effect transistors [7,8]. Previous reports showed that the MITs of VO2 are strongly related to its crystal quality and morphology, which are generally determined by the fabrication methods. Taking previous reports into consideration, many researchers have focused on the fabrication method of VO2 in recent years. These fabrication methods primarily include hydrothermal synthesis [9][10][11][12], chemical vapor deposition [13], sol-gel deposition [14], vapor transport method [15], and so on. However, these methods suffer from many disadvantages, such as time-consuming, complex operation, expensive equipment, poor crystal quality, and waste generation. Thus, discovering an easy method to fabricate high crystal quality VO2 is necessary.
Recently, a simple, fast, green, and low-cost fabrication method is developed for single-crystalline VO2 microtube arrays on V2O5 substrate through a thermal oxidation route based on resistive heating V foil in air [16]. However, the recrystallization characteristic of the single-crystalline VO2 microtube arrays on V2O5 substrate remains unclear. At present, at least 52 stable and metastable phases of the vanadium-oxygen system have been obtained [17]. These phases may transform from one-to-another via recrystallization [18,19], redox reaction [20], or thermal treatment [21]. For a further understanding of the microstructure and phase evolution characteristics of single-crystalline VO2 microtube arrays on V2O5 substrate during heating and cooling, a recrystallization study of single-crystalline VO2 microtube arrays on V2O5 substrate is carried out in this work.

Materials and Methods
Four bars of commercially available pure V foil (99.9 wt.%) with thicknesses of 0.2 mm, widths of 3 mm, and lengths of 20 mm were used in this work. First, these V foils were gradually heated to a temperature higher than 1700 °C by direct current (< 40 A) in several tens of seconds with a GW Instek PSB-2400L power supply followed by cooling rapidly to room temperature in a few seconds to fabricate single-crystalline VO2 microtube arrays on V2O5 substrate (VOx), which is located on the surface of each V foil ( Figure 1). The detailed fabrication process of VOx can be found elsewhere [16]. Then, each V foil was resistively heated again with a direct current to reach temperatures of approximately 855 °C or 1660 °C to melt the as-fabricated V2O5 substrate or single-crystalline VO2 microtubes. Then, the liquid V2O5 or VO2 were cooled at a rapid rate or a slow rate by controlling the direct current manually to recrystallize the liquid V2O5 or VO2. The fabrication temperature was monitored by a pyrometer (LumaSense IGAR 6 Advanced with a temperature range of 250-2000 °C, a resolution of 0.1 °C, and a sampling rate of 60 s −1 ). The obtained products were detached from V foil for morphology observation and microstructure analysis. The morphologies of obtained products were observed using a Hitachi TM3030 scanning electron microscope (SEM). The microstructures and phases of obtained products (without any further treatment) were examined using a Rigaku Ultima IV X-ray diffractometer at room temperature.

Single-Crystalline VO2 Microtube Arrays on V2O5 Substrate
The SEM images of the typical morphology of as-fabricated VOx is depicted in Figure 2a. The single-crystalline VO2 are straight, hollow, rod-like, rectangular cross sectional, and nearly vertically aligned on the V2O5 substrate. The temperature vs. time curves (Figure 2b) during fabrication clearly indicates that the peak temperature is 1816 °C, the average heating rate is 25 °C/s, the average cooling rate is 310 °C/s, the crystallization temperatures of VO2, and V2O5 are 806 °C and 304 °C, respectively. The X-ray diffraction (XRD) pattern (Figure 2c) of as-fabricated VOx demonstrates that only VO2 and V2O5 presented. Apparently, the diffraction peaks of (200) and (211) of VO2 are higher than the others, which indicate a preferential growth of single-crystalline VO2 microtubes. Our previous transmission electron microscopy observations demonstrated that the as-fabricated microtube is in the [100] growth direction of the M1 phase VO2 at room temperature with a thin layer of V2O5 on its surface [16].

Recrystallization of VOx via Heating with Low Peak Temperature and Cooling Rapidly
One sheet of as-fabricated VOx was gradually heated to a relatively low peak temperature with a heating rate of 40 °C/s to melt the V2O5 substrate, followed by cooling rapidly with a cooling rate about 169 °C/s by stopping resistive heating to recrystallize the liquid V2O5. The SEM image of obtained recrystallization product is depicted in Figure 3a. Hollow, rod-like, and nearly vertically aligned products still exist on the surface of a substrate. The microtubes shown in Figure 3a are covered with a thin layer, which are different from as-fabricated single-crystalline VO2 microtubes. The corresponding temperature vs. time curve is shown in Figure 3b with a peak temperature of 855 °C and a crystallization temperature of 298 °C. The XRD pattern (Figure 3c) of the obtained product shows the existence of VO2 and V2O5. When the temperature of as-fabricated VOx is heated between 678 °C (the melting point of bulk V2O5 [22]) and 1542 °C (the melting point of bulk VO2 [22]), only V2O5 substrate is melted into liquid and resulted in the single-crystalline VO2 microtubes immersed in liquid V2O5. When the VOx is rapidly cooled down to 298 °C, a part of the liquid V2O5 recrystalized on the surface of single-crystalline VO2 microtubes and other parts of liquid V2O5 recrystalized as the substrate of the single-crystalline VO2 microtube arrays again. The recrystallization process of liquid V2O5 is exothermic, which maintains the recrystallization starting temperature of V2O5 for a short time. As a result, a platform at 298 °C appears in the temperature vs. time curve-marked by an arrow in Figure 3b.

Recrystallization of VOx via Heating with Low Peak Temperature and Cooling Slowly
One sheet of as-fabricated VOx was heated to relatively low peak temperature with a heating rate of 11 °C/s to melt the V2O5 substrate, followed by slow cooling with a cooling rate of 24 °C/s. The direct current of resistive heating is gradually decreased, thereby inducing the liquid V2O5 to recrystallize. The SEM image of the obtained recrystallization product is depicted in Figure 4a. Hollow, rod-like products still exist, which are randomly aligned on the surface of a substrate. The microtubes shown in Figure 4a are covered with a thick layer, which are different from as-fabricated single-crystalline VO2 microtubes. The corresponding temperature vs. time curve is shown in Figure  4b with a peak temperature of 855 °C and a crystallization temperature of 298 °C. The XRD pattern (Figure 4c) of the obtained product demonstrates that only VO2 and V2O5 are presented. Only V2O5 substrate is melted into liquid. The single-crystalline VO2 microtubes are immersed in liquid V2O5 when heated. When the VOx is slowly cooled down to 299 °C, a part of the liquid V2O5 recrystallized on the surface of single-crystalline VO2 microtubes and other parts of the liquid V2O5 recrystallized as the substrate of the single-crystalline VO2 microtube arrays again. Due to the slow cooling, some VO2 microtubes cannot erect on the substrate, which results in a random distribution on the surface of the V2O5 substrate.

Recrystallization of VOx via Heating with High Peak Temperature and Cooling Rapidly
One sheet of as-fabricated VOx was gradually heated to a relatively high peak temperature with an average heating rate of 46 °C/s to melt the V2O5 substrate and the single-crystalline VO2 microtubes. Subsequently, rapid cooling with an average cooling rate of 288 °C/s, by stopping resistive heating, was performed to recrystallize the liquid V2O5 and VO2. The SEM image of obtained recrystallization product is depicted in Figure 5a. Hollow, rod-like products that are nearly vertically aligned on the surface of a substrate still exist. Apparently, the microtubes shown in Figure  5a are the same as the as-fabricated single-crystalline VO2 microtubes. The corresponding temperature vs. time curve is shown in Figure 5b with a peak temperature of 1663 °C, two crystallization temperatures of 826 °C and 299 °C. The XRD pattern (Figure 5c) of the obtained product demonstrates that only VO2 and V2O5 are present. When the temperature of as-fabricated VOx is heated to higher than 1542 °C, both the V2O5 substrate and single-crystalline VO2 microtubes are melted into liquid, which results in a liquid mixture of VO2 and V2O5. When the VOx is rapidly cooled down to 826 °C, liquid VO2 recrystallizes to form single-crystalline VO2 microtube arrays standing in liquid V2O5. The crystallization process of liquid VO2 is exothermic, which maintains the crystallization starting temperature of VO2 for a short time. As a result, a platform at 826 °C appears in the temperature vs. time curve-marked by an arrow in Figure 5b. After completion of the VO2 recrystallization, the temperature decreases again until 299 °C is reached. The liquid V2O5 is recrystalized to form a substrate of single-crystalline VO2 microtube arrays again. As a result, a platform at 299 °C appears in the temperature vs. time curve-marked by an arrow in Figure 5b.

Recrystallization of VOx via Heating with High Peak Temperature and Cooling Slowly
One sheet of as-fabricated VOx was gradually heated to a relatively high peak temperature with an average heating rate of 87 °C/s to melt the V2O5 substrate and the single-crystalline VO2 microtubes. Subsequently, slow cooling with an average cooling rate of 49 °C/s was performed by gradually decreasing the direct current of resistive heating to recrystallize the liquid V2O5 and VO2. The SEM image of the obtained recrystallization product is depicted in Figure 6a (top view) and Figure 6b (side view). Some plate-shaped solids exist on the surface of the V foil, although several microtubes (marked with two red ellipses in Figure 6b) can be found embedded in the plate-shaped solids. The corresponding temperature vs. time curve is shown in Figure 6c with a peak temperature of 1661 °C and V2O5 crystallization temperature of 303 °C. A hump (marked with a blue arrow) around 860 °C can be identified in the temperature vs. time curve, which means that VO2 recrystallization may occur. However, the XRD pattern (Figure 6d) of the obtained product indicates that only V2O5 can be identified. When the temperature of as-fabricated VOx is heated to higher than 1542 °C, both V2O5 substrate and single-crystalline VO2 microtubes are melted into liquid, resulting in a liquid mixture of VO2 and V2O5. As the liquid VOx is slowly cooled down, a large amount of VO2 was re-oxidized to form V2O5 above the crystallization temperature of VO2 along the following route: 4VO2 + O2 →2V2O5. With a gradually decreased temperature of 860 °C, a small amount of liquid VO2 recrystallizes to form several microtubes. As a result, a hump appears in the temperature vs. time curve around 860 °C. However, the XRD peaks of VO2 are too weak to be identified in Figure 6d. With a further decreased temperature to 303 °C, liquid V2O5 recrystalized to form plate-shaped solids.

Conclusions
When single-crystalline VO2 microtube arrays on V2O5 substrate are heated to a relatively low peak temperature to melt the V2O5 substrate, the recrystallization products are still single-crystalline VO2 microtube arrays on V2O5 substrate through slow or rapid cooling. However, the single-crystalline VO2 microtubes are coated with a V2O5 layer, which is apparently different from as-fabricated single-crystalline VO2 microtubes. These products may be applied as V2O5 arrays for gas sensing, catalysts, and so on. The single-crystalline VO2 microtube arrays on V2O5 substrate are heated to a relatively high peak temperature to melt the V2O5 substrate and the single-crystalline VO2 microtubes. The recrystallization products retain the single-crystalline VO2 microtube arrays on V2O5 substrate after rapid cooling because of the sequential recrystallization of VO2 microtube and V2O5 substrate. However, the recrystallization product is mostly plate-shaped V2O5 by slowly cooling, due to the re-oxidization of VO2 at high temperatures. Thus, the peak temperature of the heating and cooling rate are two key factors for recrystallization products of the single-crystalline VO2 microtube arrays and V2O5 substrate.

Conflicts of Interest:
The authors declare no conflict of interest.