One-Step Hydrothermal Synthesis, Thermochromic and Infrared Camouflage Properties of Vanadium Dioxide Nanorods

Vanadium dioxide (VO2) has attracted interest from researchers because it undergoes a metal–insulator phase transition (MIT), which is accompanied by a reversible and remarkable change in both electrical and optical properties. VO2 exhibits numerous polymorphs and thus it is essential to control the growth of specific monoclinic VO2 (M) and rutile VO2 (R) phases. In this study, we developed a cost-effective and facile method for preparing VO2 nanorods with a highly crystalline monoclinic phase by one-step hydrothermal synthesis, in which only V2O5 and H2C2O4 are used as raw materials. The phase evolution of VO2 during the hydrothermal process was studied. The obtained VO2 nanorods were thoroughly mixed with fluorocarbon resin and homogeneous emulsifier in an ethanol solution to obtain a VO2 dispersion. To prepare VO2 films, screen printing was performed with a stainless steel screen mesh mask on glasses or fabric substrate. The VO2 coating had good thermochromic performance; the infrared transmittance change was greater than 20% @1.5 μm whilst keeping the visible transmittance greater than 50%. Meanwhile, the polyester base coating on the fabric had an emissivity change of up to 22%, which provides a solution for adaptive IR camouflage.


Introduction
VO 2 is the most common thermal-induced metal-insulator transition (MIT) material, which is characterized by a subtle and reversible structural transformation from VO 2 (M) to VO 2 (R) at the critical temperature (τ c ) of~68 • C [1-3] ("M" and "R" are abbreviations for monoclinic and rutile, respectively). The MIT of VO 2 displays a significant change in its infrared optical property, which means that VO 2 (M) can be transparent to infrared solar irradiation, and that VO 2 (R) has an adverse effect on infrared irradiation at a temperature higher than τ c . Using this property, it can be observed that VO 2 possesses excellent potential for a wide range of applications, including optic switches [4][5][6][7], energy-efficient smart windows [7][8][9], and infrared camouflage purposes [10][11][12]. In addition to VO 2 (M/R), VO 2 can also adopt other kinds of polymorphs, including monoclinic VO 2 (B), triclinic VO 2 (T), tetragonal VO 2 (A), and parametricities VO 2 (P). Among them, VO 2 (M/R) is the most practical material due to its τ c value that is close to ambient temperature. Furthermore, similar to most transition metals, vanadium exists in a wide range of oxidation states, which result in the existence of several intermediate vanadium oxides during the synthesis of VO 2 . Therefore, the study of a facile and suitable method of preparing the specific VO 2 (M/R) phase presents many opportunities for optimization, with the main final goal of controlling the desirable size, morphology, and crystallinity of VO 2 to satisfy different specific demands for practical applications. For example, for the sake of good dispersion, the quantity of VO 2 (M/R) powder on the nanometer level is important; as regards the infrared modulation, the better the crystallization, the better the regulation ability. Extensive studies have been conducted to prepare VO 2 thin films or nanopowders, such as the sol-gel method [13], vapor phase transport [14], pulsed-laser deposition [15] hydrothermal treatment [16], and magnetron-sputter [17]. By contrast, hydrothermal treatment has been considered the main method used to prepare VO 2 because it functions at both a high temperature and pressure that are well-suited to control the morphology, crystallinity, and chemical stoichiometry of the final product [18]. Unfortunately, to date, hydrothermal reactions in solution have mostly yielded the metastable phases of VO 2 (B) or VO 2 (A) because the reaction time is not long enough or the reaction temperature is not high enough, as shown in Figure S1. Therefore, the one-step hydrothermal process directly synthesizing thermodynamically stable phases of VO 2 (M/R) has attracted significant attention in the research.
The growth of VO 2 (R) was first reported by the hydrothermal reaction of the V 2 O 3 -V 2 O 5 -H 2 O system at temperatures higher than 350 • C [19]. VO 2 (R) nanorods were described by Jin et al. [20] in a study for which H 2 SO 4 was used as a very effective addition for the morphology control of VO 2 . Gao et al. developed a process that could prepare single-crystal VO 2 (R) snowflake powders by the hydrothermal treatment of V 2 O 5 and oxalic acid at 240 • C for 7 days [21]. Micro-and nanocrystals of VO 2 (M)were prepared via the hydrothermal method using N 2 H 4 as a reducing agent [22]. Wang et al. [23] confirmed that a certain quantity of W dopants could promote a phase transformation from VO 2 (B) to VO 2 (M/R) during hydrothermal synthesis; however, excess content can lead to the generation of undesirable crystals and a lower purity of VO 2 . Zou et al. [24] prepared VO 2 nanoparticles via the hydrothermal treatment of NH 4 VO 3 , using N 2 H 4 ·H 2 O as a reducing agent at 340 • C for 6h. The prepared powder was VO 2 (M), including nanoparticles with particle sizes between 20 nm and 100 nm, and nanorods that were approximately 200 nm long. A novel, nested, hydrothermal reactor was proposed by Jin et al. in their study, where they separated the oxidant of hydrogen peroxide from the precursor solution; a reaction was performed at 260 • C for 24 h, and VO 2 (M) nanoparticles with an average size ∼30 nm were prepared [25]. In general, for the one-step hydrothermal synthesis, toxic hydrazine was extensively used to obtain VO 2 (M/R); however, sometimes other steps are essential to prepare the hydrothermal precursor. Thus, there still remains an opportunity to develop a facile and environmentally friendly hydrothermal route for synthesizing VO 2 (M/R). While many studies report on VO 2 nanoparticles, there is less literature concerning the one-step hydrothermal synthesis of VO 2 nanorods for the exploration of thermochromic and infrared camouflage properties.
The main purpose of the current study was to directly perform the hydrothermal synthesis of VO 2 (M/R), in which only V 2 O 5 and H 2 C 2 O 4 were used as raw materials. To prepare VO 2 films, we performed screen printing with a stainless steel screen mesh mask on glasses or a fabric substrate. The VO 2 coating had an obvious infrared modulation ability and displayed a good thermochromic performance. Moreover, the polyester base coating on fabric presented an emissivity change of up to 22%, which provides great potential for adaptive infrared camouflage applications.

Materials and Synthesis
In the experiments, all of the chemicals, including V 2 :1 and 3:1, the samples were labeled as S1, S2, S3, S4, S5, and S6, respectively. The obtained precursor solution was transferred into a 100 mL PPL material-lined stainless-steel autoclave, and the reactor was sealed. The hydrothermal temperature was set to 280 • C and the reaction time was 48 h. The samples were cooled to room temperature in air at the end of the reaction process. A black precipitate was collected by centrifugation and washed three times with deionized water and ethanol in turn. The final black powder solution was obtained after drying in a vacuum oven at 60 • C for 12 h. There was no subsequent annealing treatment.

Preparation of VO 2 Coating on Polyester Fabric Surface
A total of 1 g of VO 2 powder and 0.1 mL of Disperbyk-110 dispersant were mixed together in 10 mL of ethanol and sonicated for 30 min, and then a high shear emulsifier was added to completely disperse the VO 2 powder. A total of 2 g of resin (fluorocarbon resin, PVP, aqueous polyurethane) was used to obtain the ink for printing. The screen-printing technique was used to print the VO 2 coating evenly on the polyester fabric. The screen used for the printing process was 200 mesh to ensure that the VO 2 nanopowders could be smoothly printed on the surface of the base fabric. The printed fabric was dried on a constant-temperature heating table at 50 • C for 20 min to remove the solvent to form a uniform and firm coating.

Preparation of VO 2 Coating on Quartz Surface
A spin coater was used to prepare VO 2 optical coatings on the surface of quartz glass using the same ink as mentioned above. The spin-coater speed was set at 400 r/min, acceleration at 100 r/s, and spin-coating time at 65 s. The coatings were prepared with different thicknesses from one to four spin coatings.

VO 2 Nanorods
(a) (b) Figure 1. XRD data of (a) S1 (reactants ratio 0.5:1) and (b) S2 (reactants ratio 1:1) samples. Generally, the S1 sample indicates that the low concentration of oxalic acid i sufficient to completely reduce V2O5, and some unreacted V2O5, as well as interme products of V4O9 and V3O7, coexist in S1, while VO2 (M) is evident in S2, but the i mediate product of V6O13 remains present. Therefore, a higher concentration of oxal id is necessary to obtain pure VO2. Figure 2 presents the XRD data for the hydrothermal product when the precu ratio between C2H2O4.2H2O and V2O5 is greater than 1:1; all of the products are VO2 The diffraction peaks at 2θ = 27.796°, 37.089°, 42.079°, 55.451°, 64.972°, and 70.256 the four samples corresponded to the (011), (200), (210), (220), (031), and (202) crysta planes of VO2(M), respectively, which indicates the purity of the obtained sample contrast, the diffraction peak of the S4 sample was higher than that of the other thr samples, implying the good crystalline properties of S4. Meanwhile, as can be obse from S5 and S6, the much higher concentration of oxalic acid is not beneficial to the ty of VO2 and leads to the poor crystallinity of the samples. Overall, the main XRD p of these four samples are basically consistent. This kind of consistency is also reflect their morphology, as presented in the SEM in Figure 3. All of four samples (S2-S5) sented short, rod-like structures, but some differences still remained. For S3, there  Generally, the S1 sample indicates that the low concentration of oxalic acid is not sufficient to completely reduce V 2 O 5 , and some unreacted V 2 O 5 , as well as intermediate products of V 4 O 9 and V 3 O 7 , coexist in S1, while VO 2 (M) is evident in S2, but the intermediate product of V 6 O 13 remains present. Therefore, a higher concentration of oxalic acid is necessary to obtain pure VO 2 . Figure 2 presents the XRD data for the hydrothermal product when the precursor ratio between C 2 H 2 O 4 ·2H 2 O and V 2 O 5 is greater than 1:1; all of the products are VO 2 (M). , respectively, which indicates the purity of the obtained samples. By contrast, the diffraction peak of the S4 sample was higher than that of the other three of samples, implying the good crystalline properties of S4. Meanwhile, as can be observed from S5 and S6, the much higher concentration of oxalic acid is not beneficial to the purity of VO 2 and leads to the poor crystallinity of the samples. Overall, the main XRD peaks of these four samples are basically consistent. This kind of consistency is also reflected in their morphology, as presented in the SEM in Figure 3. All of four samples (S2-S5) presented short, rod-like structures, but some differences still remained. For S3, there were obviously some larger micron-scale layers and lumps of crystals, and the particle size was not uniform across the sample. A small amount of flakes close to the micron scale were also observed in S5 and S6, suggesting that excessive oxalic acid was not beneficial to forming uniform samples. The nanorods present in S4 were much more uniform and presented good dispersity properties.
obviously some larger micron-scale layers and lumps of crystals, and the particle size was not uniform across the sample. A small amount of flakes close to the micron scale were also observed in S5 and S6, suggesting that excessive oxalic acid was not beneficial to forming uniform samples. The nanorods present in S4 were much more uniform and presented good dispersity properties.   Figure 4 presents the DSC data obtained for samples of S1-S6. Distinct endothermic and exothermic peaks were observed during the heating and cooling cycles, respectively, for samples S2-S6. The thermal hysteresis indicates fully reversible and first-order phase-transition features. The phase-transition temperatures and enthalpies of the samples were determined by particular software (NETZSCH Proteus Thermal Analysis). As shown in Figure 4b, τc is mainly concentrated at approximately ~70 °C . It can be observed that sample S4 presents the highest τc of 72.2 °C , indicating the good crystallinity, obviously some larger micron-scale layers and lumps of crystals, and the particle size was not uniform across the sample. A small amount of flakes close to the micron scale were also observed in S5 and S6, suggesting that excessive oxalic acid was not beneficial to forming uniform samples. The nanorods present in S4 were much more uniform and presented good dispersity properties.   Figure 4 presents the DSC data obtained for samples of S1-S6. Distinct endothermic and exothermic peaks were observed during the heating and cooling cycles, respectively, for samples S2-S6. The thermal hysteresis indicates fully reversible and first-order phase-transition features. The phase-transition temperatures and enthalpies of the samples were determined by particular software (NETZSCH Proteus Thermal Analysis). As shown in Figure 4b, τc is mainly concentrated at approximately ~70 °C . It can be observed that sample S4 presents the highest τc of 72.2 °C , indicating the good crystallinity,  Figure 4 presents the DSC data obtained for samples of S1-S6. Distinct endothermic and exothermic peaks were observed during the heating and cooling cycles, respectively, for samples S2-S6. The thermal hysteresis indicates fully reversible and first-order phasetransition features. The phase-transition temperatures and enthalpies of the samples were determined by particular software (NETZSCH Proteus Thermal Analysis). As shown in Figure 4b, τ c is mainly concentrated at approximately~70 • C. It can be observed that sample S4 presents the highest τ c of 72.2 • C, indicating the good crystallinity, which is consistent with the XRD data. The underlying mechanism was mainly related to the defects that can act as the nucleation site to trigger the phase transition from VO 2 (M) to VO 2 (R) [26]. S4 presents relatively fewer defects and thus produces a higher τ c value. The highest phase-change enthalpy (∆H) for S4 also indicates that it presents the best crystallinity (Figure 4b). The higher enthalpy presented during the phase transition indicates that the infrared modulation property of sample S4 will considerably change, which is reflected by Nanomaterials 2022, 12, 3534 6 of 10 the fact that its emissivity changes more rapidly than the other samples. Therefore, in the subsequent experiment, sample S4 was selected for the preparation of a VO 2 coating. which is consistent with the XRD data. The underlying mechanism was mainly related to the defects that can act as the nucleation site to trigger the phase transition from VO2(M) to VO2(R) [26]. S4 presents relatively fewer defects and thus produces a higher τc value. The highest phase-change enthalpy (ΔH) for S4 also indicates that it presents the best crystallinity (Figure 4b). The higher enthalpy presented during the phase transition indicates that the infrared modulation property of sample S4 will considerably change, which is reflected by the fact that its emissivity changes more rapidly than the other samples. Therefore, in the subsequent experiment, sample S4 was selected for the preparation of a VO2 coating.

Infrared Camouflage and Thermochromic Properties
VO2 coating is printed on the surface of polyester fabric using screen-printing technology. Polyester fabric is, at present, the most widely used fabric material for the factory production of camouflage nets because it is a high-strength fiber with good strength and toughness properties that is not easily damaged, and it is heat resistant and not easily deformed. Screen-printing technology is a convenient and practical printing technology that can use various types of inks, such as oil-based, water-based, synthetic-resin emulsion type, and powder; at the same time, it is not limited by the shape of the substrate's surface and the size of the area, which exhibits a high operability quality and is suitable for a wide range of purposes and uses.
In the current paper, three kinds of resins commonly used to prepare infrared stealth coatings were used to prepare the coatings: aqueous polyurethane (PU) [27], PVP [28], and fluorocarbon resin [29]. The advantage of the three resins is that they are all infrared transparent resins. The resins do not affect the infrared properties of the prepared coatings, and the sample powders can be well dispersed in the three resins. However, PU and PVP are waterborne resins, and the coating stability following film formation is not as good as that of fat-soluble fluorocarbon resin. The SEM images of the printed coating are presented in Figure 5a-c. It can be observed from the figure that the coating surface of water-based polyurethane is rough and cannot completely cover the fibers of the polyester base cloth; the coating surface of the PVP resin is smooth but there are a lot of bubbles present. The coating surface of the fluorocarbon resin is more delicate, and the VO2 powder is evenly adhered to the fiber surface. Therefore, the fluorocarbon resin was finally elected as the film-forming substance for the coating preparation. Figure 5c,d present the SEM images of the surface and cross-section of the coating formed by printing once and twice, respectively. The surface fibers of the twice-printed coating are completely covered by the coating, and the surface is fine without cracks and bubbles. From the cross-sectional view, it can be observed that the average thickness of the coating is approximately 10 μm for the once-printed coating and approximately 20 μm for the twice-printed coating, doubling the thickness.

Infrared Camouflage and Thermochromic Properties
VO 2 coating is printed on the surface of polyester fabric using screen-printing technology. Polyester fabric is, at present, the most widely used fabric material for the factory production of camouflage nets because it is a high-strength fiber with good strength and toughness properties that is not easily damaged, and it is heat resistant and not easily deformed. Screen-printing technology is a convenient and practical printing technology that can use various types of inks, such as oil-based, water-based, synthetic-resin emulsion type, and powder; at the same time, it is not limited by the shape of the substrate's surface and the size of the area, which exhibits a high operability quality and is suitable for a wide range of purposes and uses.
In the current paper, three kinds of resins commonly used to prepare infrared stealth coatings were used to prepare the coatings: aqueous polyurethane (PU) [27], PVP [28], and fluorocarbon resin [29]. The advantage of the three resins is that they are all infrared transparent resins. The resins do not affect the infrared properties of the prepared coatings, and the sample powders can be well dispersed in the three resins. However, PU and PVP are waterborne resins, and the coating stability following film formation is not as good as that of fat-soluble fluorocarbon resin. The SEM images of the printed coating are presented in Figure 5a-c. It can be observed from the figure that the coating surface of water-based polyurethane is rough and cannot completely cover the fibers of the polyester base cloth; the coating surface of the PVP resin is smooth but there are a lot of bubbles present. The coating surface of the fluorocarbon resin is more delicate, and the VO 2 powder is evenly adhered to the fiber surface. Therefore, the fluorocarbon resin was finally elected as the film-forming substance for the coating preparation. Figure 5c,d present the SEM images of the surface and cross-section of the coating formed by printing once and twice, respectively. The surface fibers of the twice-printed coating are completely covered by the coating, and the surface is fine without cracks and bubbles. From the cross-sectional view, it can be observed that the average thickness of the coating is approximately 10 µm for the once-printed coating and approximately 20 µm for the twice-printed coating, doubling the thickness. Figure 6a-c presents the values of the surface's normal, integrated emissivity of the polyester base fabric, and the once-and twice-coated films using an IR-2 dual-band emissivity tester. The emissivity-modulation ability of the coating increased significantly with the increase in the coating's thickness. The maximum change in the emissivity of the once-printed coating was 0.1, reaching 10.6%; the maximum change in the emissivity of the twice-printed coating was 0.19, reaching 22%; and the surface of the twice-printed coating was smoother and flatter; therefore, its room-temperature emissivity was lower than that of the polyester-based fabric.  6a-c presents the values of the surface's normal, integrated emissivity of the polyester base fabric, and the once-and twice-coated films using an IR-2 dual-band emissivity tester. The emissivity-modulation ability of the coating increased significantly with the increase in the coating's thickness. The maximum change in the emissivity of the once-printed coating was 0.1, reaching 10.6%; the maximum change in the emissivity of the twice-printed coating was 0.19, reaching 22%; and the surface of the twice-printed coating was smoother and flatter; therefore, its room-temperature emissivity was lower than that of the polyester-based fabric. The thermal infrared camouflage capability of the coating was characterize infrared thermography. Figure 6d presents the optical photographs of the polyes fabric, once-printed coating, as well as the twice-printed coating, with three placed on the same constant-temperature heating table. The figure presents the thermal image at room temperature, and the uncoated sample has almost th thermal radiation intensity as the coated sample. When the temperature reache the coated sample has a significantly lower thermal radiation intensity and ca better with the background, achieving the effect of adaptive camouflage. Figure 7 exhibits the optical transmittance curves of the coatings with d thicknesses formed by spin coating on the surface of quartz glass using the selected inks, in which the solid and dashed lines indicate the low-an temperature cases, respectively. The four samples were spin-coated 1-4 times, nu 1-4 and in order, and the transmittance curves present that the low-temperatu mittance decreases with the increase in the coating's thickness, from approxima to less than 60%, but the optical-transmittance-modulation ability of the coating uously increases with the increase in the thickness. The amount of infrared t tance changes for samples 1, 2, 3, and 4 were 14%, 22%, 27%, and 29% @1.5 μm tively, while maintaining the visible transmittance at a value greater than 50%. R et al. also prepared VO2 films in their study. The infrared transmittance change films printed once and twice were 20% and 23%, respectively [30]. The thermo properties of the films prepared in our study are comparable to those presente literature. The thermal infrared camouflage capability of the coating was characterized using infrared thermography. Figure 6d presents the optical photographs of the polyester base fabric, once-printed coating, as well as the twice-printed coating, with three samples placed on the same constant-temperature heating table. The figure presents the infrared thermal image at room temperature, and the uncoated sample has almost the same thermal radiation intensity as the coated sample. When the temperature reaches 96 • C, the coated sample has a significantly lower thermal radiation intensity and can blend better with the background, achieving the effect of adaptive camouflage. Figure 7 exhibits the optical transmittance curves of the coatings with different thicknesses formed by spin coating on the surface of quartz glass using the above-selected inks, in which the solid and dashed lines indicate the low-and high-temperature cases, respectively. The four samples were spin-coated 1-4 times, numbered 1-4 and in order, and the transmittance curves present that the low-temperature transmittance decreases with the increase in the coating's thickness, from approximately 80% to less than 60%, but the optical-transmittance-modulation ability of the coating continuously increases with the increase in the thickness. The amount of infrared transmittance changes for samples 1, 2, 3, and 4 were 14%, 22%, 27%, and 29% @1.5 µm, respectively, while maintaining the visible transmittance at a value greater than 50%. Ru Chen et al. also prepared VO 2 films in their study. The infrared transmittance changes in the films printed once and twice were 20% and 23%, respectively [30]. The thermochromic properties of the films prepared in our study are comparable to those presented in the literature.

Conclusions
The VO 2 nanorods were synthesized by a one-step hydrothermal method using only V 2 O 5 and H 2 C 2 O 4 as the raw materials, and the optimum mole ratio between C 2 H 2 O 4 and V 2 O 5 was 2:1. Three different polymers were used to prepare the VO 2 ink that was used to print the coating onto the surface of the polyester base cloth, and fluorocarbon resin was identified as the best dispersant. The flexible VO 2 coating can adaptively change the surface's emissivity property, and the changes in emissivity could reach 0.19, which presented good adaptability to the environment and was expected to be used for producing military tents, camouflage nets, and other equipment. The VO 2 coating on the glass substrate also displayed a good thermochromic performance; the amount of infrared transmittance change was greater than 20% @1.5 µm while maintaining the visible transmittance value at a figure greater than 50%.