Low-Temperature Synthesis of Vanadium Dioxide Thin Films by Sol-Gel Dip Coating Method

CENE Center of Excellence in Nanotechnology for Energy, Prince of Songkla University, Hat Yai 90112, !ailand CEMME Center of Excellence in Metal and Materials Engineering, Prince of Songkla University, Hat Yai 90112, !ailand Department ofMining andMaterials Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai 90112,!ailand Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai 90112, !ailand National Metal and Materials Technology Center, !ailand Science Park, Pathum !ani 12120, !ailand


Introduction
Vanadium oxide presents a partly-filled 3d orbital, leading to the formation of several oxides such as VO, V 2 O 3 , V 4 O 7 , VO 2 , V 2 O 5 , and V 6 O 13 , which all belong to the series of Magneli (V n O 2n-1 ) and Wadsley (V 2n O 5n-2 ) systems [1][2][3]. Among these oxides, monoclinic vanadium dioxide (VO 2 (M)) has gained most attention owing to its fascinating thermochromic property that can switch a phase structure reversibly between a monoclinic (semiconductor) and a rutile (metal) phase at about 68°C [3,4] in response to external stimuli, leading to dramatic changes in the electrical and optical properties. e change of optical property from infrared-transmitting at low temperature to infraredreflecting at temperatures higher than the monoclinic-rutile phase transition temperature, while maintaining the visible transmittance, makes the vanadium dioxide a promising candidate as thermochromic coating for a variety of devices such as smart windows, tunable absorbers and emitters, and thermal rectification [4,5]. e thermochromic VO 2 (M) thin films have been prepared by several methods. e most typical methods include physical vapor deposition, chemical vapor deposition, and sol-gel method. e physical and chemical vapor depositions can precisely control the stoichiometry of films by monitoring oxygen partial pressure. However, the complex deposition processes and expensive equipment significantly limit their practical applications [6,7]. e solgel method has been widely employed for material investigation because of several advantages. For instance, it can be coated on complex shape, large substrate. e process is of low cost, and there is easy control of reaction kinetics and atomic doping. In addition, lower calcination temperature is typically required to obtain crystalline structure [8][9][10]. In a typical sol-gel process for VO 2 preparation, a V 5+ -containing precursor is firstly prepared, and then the precursor is deposited onto a clean, hydrophilic substrate through dip or spin coating. e obtained sample is dried and calcined typically at ≥600°C [11][12][13] to transform an amorphous phase to a desired crystalline structure. Meanwhile, part of V 5+ ions was reduced to V 4+ ions [8,11].
For commercial applications, such as smart window coating, glass is the desirable substrate. e soda lime glass is the most common and cheapest one [14,15]. Considering that, for wet-chemical processes, the crystalline VO 2 phase is typically formed by a high-temperature treatment process.
is would restrict the calcination of VO 2 thin films on the soda lime glass substrate which is generally tempered at approximately 600 o C. e glass begins to soften at this temperature. e strength of the tempered glass would be weakened after VO 2 thin film deposition at temperatures higher than its softening temperature. erefore, for soda lime glass panel substrates, low-temperature deposition (<600°C) is of significant importance for the VO 2 thin film preparation under inert atmosphere, e.g., vacuum or N 2 /Ar/ H 2 /CO/CO 2 [7,8,10,12,14,16,17]. Moreover, most of the methods reported in literatures employed low vacuum pressure of tube furnace, which utilizes high power of the turbo and diffusion vacuum pumps [10,12,14,17]. In some cases, intermediate gases were purged in the tube furnace, during calcination, to prevent film's surface from oxidation in the atmosphere [7,8,12,14,16,17].
In this work, the outstanding VO 2 (M) phase was prepared via sol-gel dip coating on a soda lime glass slide at low calcination temperature of 500 o C by using only a turbo pump to generate low vacuum pressure of 2 × 10 −3 mbar in a tube furnace. Moreover, no intermediate gas was purged during a reducing step. Synthesis conditions, which include vacuum pressure, calcination temperature, and steps in the vacuum tube furnace, were investigated. Electrical resistance and optical properties of the prepared vanadium dioxide film were characterized.

Synthesis of Vanadium Oxide in Films.
e vanadium oxide thin films were prepared by the sol-gel process, in which a precursor solution was prepared according to the literatures [17,18]. In a typical synthesis method, 0.3 g V 2 O 5 powder (99.5%, Sigma-Aldrich Co., Ltd.) was dissolved in 30 mL 30% H 2 O 2 (35%, Chem-Supply Co., Ltd.) with rigorous stirring for 30 min. At this step, the reaction was strongly exothermic, and the solution's color changed from yellow to dark red sol upon a continuous stirring. After the two chemicals were mixed and aged for 2 days at room temperature, thin films were obtained by dip coating method using a dip coater (Model DC-150) performed at a dipping time of 2 min and a withdrawal rate of 6 mm/min on a glass slide substrate. e substrates were cleaned by means of ultrasonication for 30 min in deionized water, 20% acetone, 70% iso-propanol, and 20% ethanol, respectively. e as-deposited thin films were dried in ambient air for 24 h. e dried thin films were calcined at room temperature, 400, 450, and 500°C, in vacuum tube furnace at pressures of 2 × 10 −2 and 2 × 10 −3 mbar. To investigate an effect of heat treatment, calcination was conducted by 1-and 2-step calcination, in which each step employed a heating rate of 2°C/min and soaking time of 3 h. For the 1-step calcination, the thin films were heated in a tube furnace to 400, 450, and 500°C, soaked at this temperature for 3 h, and cooled down to room temperature. e 2-step calcination was carried out in a tube furnace at 500°C using the same heating and cooling cycles, noted that the glass slide substrate employed in this work began to soften at a temperature of ∼500°C; thus, calcination temperature did not exceed 500°C. Process parameters for film deposition are provided in Table 1.

in Film Characterization.
Crystal structure characterization of the thin films was performed by using an X-ray diffractometer (XRD; Rigaku TTRAX III). Morphology and thickness were studied by using a scanning electron microscope (SEM; Quanta 400 FEI) performed in a top and tilted view, respectively. Films' compositions, as well as an elemental mapping, were analyzed by using an energy dispersive X-ray spectrometer (EDS). Oxidation states of the elements were analyzed by using an X-ray photoelectron spectrophotometer (XPS; AXIS Ultra DLD). e binding energy was calibrated with carbon (C1s � 284.8 eV).
Electrical property of the thin film was determined by an AC four-point probe method. e measurement was conducted in a closed chamber with controlled temperature. During heating and cooling cycles, the film temperature was raised from room temperature to 100 o C and subsequently reduced to room temperature step by step. e metal-toinsulator (MIT) phase transition temperature is defined as the average of the transition temperature (T c ) in the electrical property. e T c is defined as follows [19]: (1) Transparency of the film coated on the soda lime glass was determined at 25 and 80°C by using a UV-Vis-NIR spectrophotometer (Shimadzu UV-3600 Plus) in a wavelength range 200-2500 nm. e bare soda lime glass was employed as a blank sample. Temperatures were controlled at 25 and 80°C with applied current electricity.

Synthesis of Vanadium Oxide in Films by Sol-Gel Dip
Coating. It has been realized that it is difficult to fabricate pure monoclinic phase VO 2 thin film due to its instability under a general synthesis process. It is most likely that more stable vanadium oxides, such as V 2 O 5 , V 2 O 3 , and V 6 O 13 , are also present. ese oxides block the reversible phase transition from metallic state to insulating state of the monoclinic VO 2 . Among all of the V-O system, the V 2 O 5 is a highly stable phase [20,21]. us, in order to investigate the optimum synthesis condition, the presence of crystalline phases was initially considered. Figure 1 shows XRD patterns of the vanadium oxide thin films after calcined at various conditions summarized in Table 1.
For the 1-step calcination (Figure 1(a)), the temperatures were controlled at 400, 450, and 500 o C, respectively, and the pressure was fixed at 2 × 10 −2 mbar. XRD analysis of the dried thin film (denoted as "room temperature") revealed amorphous nature signified by the absence of diffraction peak. XRD patterns of the thin films calcined at 400 and   is reduction condition at a temperature above 400 o C and a pressure below 2 Pa (2 × 10 −2 mbar) which caused the conversion of V 2 O 5 thin films to VO 2 has been reported by Ningyi et al. [22]. It was observed that the reduction process followed the sequence as V 2 O 5 ⟶V 3 O 7 ⟶V 4 O 9 ⟶V 6 O 13 ⟶VO 2 , namely, from V n O 2n+1 (n � 2-4, 6) to VO 2 . erefore, lower pressure favoured VO 2 formation caused by the oxygen deficiency [22,23]. To further investigate the effect of calcination time, the calcination soaking time was increased from 3 to 24 h. It was found that the thin film consisted of only V 2 O 3 phase. Figure 1(c) shows XRD patterns of the thin films prepared by means of 1-step and 2-step calcination in the vacuum tube furnace at 500 o C with the pressure of 2 × 10 −3 mbar. It is evident that only diffraction peaks of the monoclinic VO 2 are present in the film prepared by 2-step calcination caused by the reduction process of V 2 O 5 and V 6 O 13 .
SEM surface analysis of the VO 2 thin film prepared by the 2-step calcination process at 500 o C with the pressure of 2 × 10 −3 mbar (Figures 2(a) and 2(b)) reveals very fine VO 2 grains with irregular shape of ∼220-380 nm in size surrounded by an amorphous phase of anhydrous vanadium oxide from gelation [24]. Film's thickness determined from a cross-sectional image (Figure 2(c)) is approximately 69-74 nm. Elemental composition analyzed by EDS (Figure 2(d)) reveals that the sample comprises of Si, O, Na, Ca, V, Mg, and Al at the amount of 30.1, 51.0, 8.5, 5.7, 1.5, 2.6, and 0.6 At.%, respectively. e V and O correspond to the VO 2 thin film, and the others (and part of O) correspond to the glass slide substrate. e elemental mappings of Si from the glass substrate and V from the VO 2 film (Figures 2(e) and 2(f )) reveal homogeneous distribution of the VO 2 grains in the thin film.
Typical surface XPS spectra of the VO 2 sample are depicted in Figure 3. Detail-scan spectra were employed to investigate the valence electron of the V and O and to confirm phase purity of the thin films. According to the standard binding energy, a typical two-peak structure (2p3/2 and 2p1/2) due to the spin-orbit splitting was observed [19]. e peaks at 516.1 and 523.8 eV shown in Figure 3(a) correspond to binding energies for V 4+ 2p3/2 and V 4+ 2p1/2 , respectively. e peaks at 517.6 eV and 525.2 eV correspond to V 5+ 2p3/2 and V 5+ 2p1/2 , respectively. is result indicated atmospheric oxidation of a small amount of V 5+ at the surface of the sample. e binding energy of O 1s appears at 530.0, 531.8, and 532.8 eV, which correspond to the V-O bonding, O--H bonding, and part of physically adsorbed H 2 O, respectively, as shown in Figure 3(b). Besides these major energy levels, a number of less intense satellite peaks are also present. ese V 2p satellite peaks are attributed to the V 2 O 3 .
e XPS composition analysis result confirms the presence of VO 2 , as well as a small amount of V 2 O 5 and V 2 O 3 [25,26].

Electrical Properties of VO 2 (M)
in Films. Figure 4(a) illustrates the temperature dependence of electrical resistance of the VO 2 thin films on the soda lime glass prepared by the 2-step calcination process at 500 o C with the pressure of 2 × 10 −3 mbar. e hysteresis loop indicates the metal-to-insulator transformation and the quality of the thin film. It was observed that surface resistance of the thin film decreased upon the heating process and increased upon the cooling process. e transition temperature, determined according to equation (1), was approximately 68 o C (T c � (80 o C + 56 o C)/2). Furthermore, resistivity profile of the VO 2 thin films was nearly flat above the room temperature (semiconductor phase), indicating that the transition occurs at 68 o C (metal-insulator transition phase) and constant at 80 o C (metal phase), respectively [7,27]. e VO 2 thin films were transformed from monoclinic to tetragonal (rutile) phase at the transition temperature. Figure 4(b) illustrates the optical transmittance spectra measured at 25 and 80 o C of the VO 2 thin films. Visible optical transmittance remains at ∼40-50% when the sample's temperature changes from 25 to 80 o C. However, for a near infrared (NIR) region wavelength range of 800-2500 nm, the optical transmittance measured at 25 o C is clearly higher than that measured at 80 o C. Moreover, the optical transmittance increases with increasing wavelength at 25 o C, while it reduces with increasing wavelength at 80 o C, indicating phase transition when the temperature is raised from 25 to 80 o C.

Conclusions
A simple and effective synthesis method for the preparation of the vanadium dioxide thin films on the glass slide substrate has been demonstrated by a sol-gel dip coating at low temperature of 500 o C in a vacuum tube furnace at a pressure of 2 × 10 −3 mbar. e calcination was performed by 2 steps, in which thin films were heated in a tube furnace to 500 o C at a heating rate of 2 o C/min, soaked for 3 h, and cooled down to room temperature, and the same process was repeated. e analyses revealed that the thin films consisted of monoclinic VO 2 phase and a small amount of V 2 O 5 and V 2 O 3 phases. e very fine VO 2 grains with irregular shape of ∼220−380 nm in size were homogeneously distributed on the surface. e film's thickness was in a range of 69−74 nm. e monoclinic to tetragonal (rutile) phase transition temperature was 68 o C. e thin films exhibited good thermochromic properties. e low-temperature synthesis would be beneficial for thermochromic thin film coating on a conventional soda lime glass panel. In addition, as no intermediate gas was employed during calcination, the process would be suitable for large-scale fabrication.

Data Availability
e data used to support the finding of this study are included within the article.