Hydrothermal Synthesis of VО2 Powders

OD VО2 Nanoparticles

0D V02 NPs with sizes much smaller than the wavelength of incident light can provide superior luminous transmittance and solar energy transmittance modulation. The nanothermochromic concept was thus proposed to indicate V02 NPs in a dielectric host, which opens new avenues in energy-efficient fenestration [55]. High-quality V02 NPs are essential to improve thermochromic performance.

Controllable hydrothermal synthesis of VO2 polymorphs and their transformation to thermochromic VO2 for the application of energysaving windows [8]. Republished with permission of Wiley

Figure 7.7 Controllable hydrothermal synthesis of VO2 polymorphs and their transformation to thermochromic VO2 for the application of energysaving windows [8]. Republished with permission of Wiley.

One-step hydrothermal method

Cao et al. synthesized a mixture of V02(M) and V02(B) NPs by a hydrothermal reaction in the V205-H2S04-N2H4H20 system, and the obtained NPs had diameters ranging from 20 to 50 nm at pH = 10. In the experiment, they believe that lower concentrations of the vanadium source and hydrazine are necessary for the formation of pure V02 [56]. V02 NPs with an average size less than 20 nm were obtained by Jin et al. by the hydrothermal treatment of a V205 • nH20 sol. at 260°C for 24 h [57]. In this experiment, N2H4 performed as both reductant and pH controller and the molar ratio of V205 to N2H4 was found to be a critical factor for the preparation of V02 NPs. In addition, a hydrothermal temperature lower than 260ЭС will lead to poor crystallinity of V02. Gao et al. reported a new phase of (NH4)2V409 that acts as an intermediate phase for the formation of V02, and a self-assembly decomposition nucleation growth process was proposed to explain the growth mechanism [58]. Lowering systematic energy is the driving force for the self-assembly of (NH4)2V409, which can be decomposed at an elevated hydrothermal temperature (300°C), and then budlike V02 nuclei are generated at the expense of (NH4)2V4C>9. Note that the common metastable phase of either V02(A) or V02(B) was not found in the hydrothermal process. Meanwhile, Jin et al. prepared Sb-doped V02 NPs with a tunable size (8-30 nm) by a one-pot hydrothermal processing, and the metal-semiconductor transformation takes place at temperatures ranging from 55' C to 68 C [59]. A convenient hydrothermal-assisted homogeneous precipitation approach was developed to tune the size of V02 from several tens to hundreds of nanometers by controlling the initial vanadium source concentration [60]. The as-prepared V02 NPs exhibit size-dependent phase transition property, and the decreased dimension of V02 particles led to an improvement in the optical properties along with a slightly increased transition temperature and wider hysteresis. Long et al. reported one-step hydrothermal synthesis of rare earth-/W-codoped V02 NPs with a reduced phase transition temperature and improved thermochromic properties compared with those of W-doped V02 [61]. High-quality crystalline V02 NPs with diameters of 25-45 nm were synthesized by Gao et al. [62], and the obtained NPs have the highest latent heat, 43 Jg-1, which is close to that of bulk V02, 51 Jg-1. After being dispersed in polyurethane and coated on polyethylene terephthalate to form a flexible film, the V02 shows a luminous transmittance (T|Um) of 45.6% and a solar energy regulate ability (Tsoi) of 22.3%.

Most recently, Jin et al. reported an original one-step hydrothermal approach by controlling the oxidizing atmosphere of the reaction with hydrogen peroxide to prepare ultrafine V02 NPs [63]. Figure 7.8a shows the schematic of the hydrothermal synthesis process, and the evolution of the synthetic route is shown below. Figure 7.8b displays the SEM image of the obtained NPs with high dispersity and an average particle size of about 30 nm. Figure 7.8c is the corresponding scheme image of the V02 flexible film. The transmittance spectra of flexible V02 films of different thicknesses are measured at 30°C and 90°C to examine the thermochromic properties, as shown in Fig. 7.8d. The flexible V02 film shows excellent thermochromic performance, with a T|Um of 54.26% and a

(a) Schematic of one-step hydrothermal synthesis assisted by

Figure 7.8 (a) Schematic of one-step hydrothermal synthesis assisted by

self-released oxidizing atmosphere; (b) SEM image of the as-prepared V02 NPs; (c) scheme of the V02 flexible film; (d) transmittance spectra of V02 flexible films with V02 NPs synthesized with 0.3 mL H202 (1#, 2#, 3#, and 4# with thicknesses of about 1.1, 2.3, 4, and 2.7 pm, respectively.). The real lines denote transmittance spectra measured at 30 C, and the dashed lines are measured at 90 C; (e) comparison of thermochromic properties in this work with those of reported ones [63]. Republished with permission of American Chemical Society.

Tsoi of 12.34% (Fig. 7.8e). Li et al. developed a Ti02 seed-assistant epitaxial growth of Mo-doped V02(M)/Ti02 composite NPs by a one-step hydrothermal method [64]. They found that the size of the composite NPs can be adjusted from 100 nm to about 20 nm, the Гс can be tuned to room temperature (about 25°C), and the photochromic properties can be optimized by changing the content of the Ti02 seed.

Hydrothermal method combined with annealing

The strategy of combining the hydrothermal method with annealing involves hydrothermal synthesis of the metastable phase of nanos- tructured V02 and a subsequent annealing treatment. Some newly reported metastable phases of V02, such as V02(D) and V02(P), can be transformed to V02(M/R) by being exposed to annealing at a low temperature for a short time. The resulting V02 has nearly the same morphology as V02(D) or V02(P). Having been exposed to a lower annealing temperature for a shorter thermal treatment time, the obtained V02 nanostructures show remarkably different phase transition and thermochromic performance.

Chung et al. synthesized organically functionalized highly monodisperse VOA nanocrystals 7-10 nm in size. After undergoing thermal annealing at 350°C under an air/N2 flow for 5 min, the V02 films exhibited good thermochromic properties (Tum = 49%-66% and Tsoi = 7.4%-10.7%) [65]. Li et al. reported the fabrication of V02 NPs with an average diameter of about 80 nm by mildly annealing V02(D) NPs, as shown in Fig. 7.9a.

They found that V02 NPs can be obtained at an annealing temperature range of 300 C-450°C (Fig. 7.9c). The annealing conditions have a great effect on the Tc: with increasing annealing temperature, annealing time, and vacuum conditions, the Tc increases but the effect of the annealing temperature is the most obvious (Fig. 7.9d). After mixing with poly(4-vinylphenol) (PVP) in alcohol, a stable V02 NP dispersion is made, which can be spin- coated on glass or flexible substrates to form a film. The V02 NP film shows an excellent thermochromic performance, with high visible transmittance and IR modulation (Fig. 7.9b). Li et al. also found that after mixing with epoxy resin, V02 NP films have not only high stability at a high temperature and in an acid environment but also improved transmission in visible and modulation properties in the IR region [66]. On the basis of the V02 NP film, an active and dynamic IR switching device was built by integrating V02 with some transparent conductive substrates, such as silver nanowire and indium tin oxide [67, 68]. These devices utilize the advantages of the thermal-sensitive performance of V02 and the joule heat produced by a transparent electrode to achieve both thermochromic and optical switching performances and provide a new insight into the applications of transparent conductive electrodes in the thermochromic field. High-quality single-domain V02 nanocrystals with precisely tailored sizes were obtained within an ultrashort annealing time of 40 s at 400°C in air [44]. The author suggested that the protection of surface-adsorbed oleylamine helps to realize V02

(a) FESEM image of the V0 NPs; the inset is a photograph of the large-scale production of NPs;

Figure 7.9 (a) FESEM image of the V02 NPs; the inset is a photograph of the large-scale production of NPs; (b) optical transmittance and the corresponding photographs (the topmost photo is a bare glass substrate) of the V02(M) NP films with thicknesses of 463 nm (curve 1), 730 nm (curve 2), and 1175 nm (curve 3); (c) XRD patterns of V02 NPs obtained at different annealing temperatures; (d) dependence of the phase transition temperature of V02NPs on annealing conditions [37]. Republished with permission of Royal Society of Chemistry.

nanocrystals without destroying their original size and appearance. Meanwhile, the increase of particle size from 10 to 30 nm leads to a higher transition temperature and remarkably broadened phase transition hysteresis, which is related to the defect density in V02.

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