Sol-Gel Synthesis of Thermochromic VО2 Coatings

Introduction

Recently, thermochromic VО2 coatings have induced great interest as a result of their promising application in smart windows because of their unique ability to regulate solar heat gain [1-4]. V02(M) undergoes a first-order reversible phase transition at a critical temperature rc [68°C for the bulk) from a low-temperature monoclinic phase (P2i/c, Ml) to a high-temperature rutile phase {PA2/mnm, R), which is accompanied by a remarkable electric conductivity change from a semiconductor state to a metallic state and a unique optical switching from infrared (IR) transparency to IR reflectance [3, 5, 6]. The particular optical properties of V02 in response to temperature have made it promising for application in smart thermochromic windows for energy-efficient livable conditions [3, 7].

There are many techniques to prepare thermochromic VO2 coatings, such as magnetron sputtering [8], chemical vapor deposition [9, 10], hydrothermal growth [11-13], sol-gel method [14- 16], and polymer-assisted deposition [17, 18]. Among these, the sol-gel method has been demonstrated as a facile wet-chemical technique to prepare thermochromic V02 coatings because of its low cost, scalable deposition, accommodation for various substrates, and convenience in terms of heteroatom doping and multilayer deposition [1, 4, 19]. In this chapter, a comprehensive review is presented on the recent progress in the sol-gel method to synthesize effective thermochromic V02 coatings. The fundamentals of the sol-gel method are first introduced to understand the chemistry involved in the synthesis of V02 coatings by the sol-gel method. Inorganic and organic sol-gel methods, as two commonly used methods for the synthesis of V02 coatings, are recommended, with emphasis on the preparation parameter-induced variations in phase structures, morphologies, and thermochromic properties. Moreover, two effective sol-gel strategies of doping and composite construction to optimize thermochromic properties of V02 coatings are elaborated.

Fundamentals of the Sol-Gel Method

The sol-gel method is a wet-chemical way to synthesize inorganic materials (e.g., metal oxides or hydroxides) through the hydrolysis and condensation of the precursor to form a sol and finally a network structure of gel [20-23]. A sol possesses the characteristics of a colloidal suspension, which is defined by the International Union of Pure and Applied Chemistiy as the dispersion of one phase in another. In this definition, the molecules or polymolecular particles dispersed in a medium have at least in one direction a dimension roughly between 1 nm and 1 pm. Accordingly, not only the colloidal V2Os suspension in water but also the covalent- type vanadium polymers in organic solvents in this paper can be classified as sols. During a sol-gel process, a gel can form in various ways. Small changes in conditions can lead to different gel structures even from the same precursor [24-26]. In general, a gel is defined as a nonfluid 3D network that extends through a fluid phase [23]. Four types of gels were classified by Flory in 1974: lamellar gels, covalent polymer networks, networks of physically aggregated polymers, and disordered particulate gels [27]. In 1996, considering the primary purpose of the sol-gel method to synthesize inorganic solids, Kakihana [28] presented a more useful classification with five different gel types, whose features are summarized in Table 10.1 [23]. These five key types of gels can provide a prospective guidance for the structural and morphological design of the final inorganic products.

Sol-Gel Process for VO2 Coatings

In 1983, Greenberg [29] first used the sol-gel method to deposit VO2 coatings and W-, Mo-, and Nb-doped VO2 coatings on a glass substrate. The sol was obtained through the hydrolysis and condensation of vanadyl tri-isopropoxide, or ГО(ОСзН7)з, in 2- propanol. After gel formation on a glass substrate, postannealing at 400°C was conducted in a reductive atmosphere to obtain the final V02 coatings. WOCI4, МоОСЦ, and NbfT^HsCFJs were employed as the sources of W, Mo, and Nb dopants, respectively, to prepare the doped V02 coatings, whose temperature-induced metal-insulator transition (MIT) and IR switching properties were investigated in comparison to those of the undoped VO2 coatings. Since then, V02 thermochromic coatings by the sol-gel technology have been frequently reported [30-35]. In general, the sol-gel method to prepare V02 coatings includes four fundamental steps [19]:

  • 1. The formation of a stable and homogeneous sol through the hydrolysis and polycondensation of an active vanadium precursor in a liquid intermediate
  • 2. The deposition of the sol on a substrate by dip coating or spin coating
  • 3. Gel formation after a sufficient aging process
  • 4. The formation of crystalline V02 coatings through an appropriate annealing process

Type of gel

Type of bonding

Composition

Schematic

Colloidal

Van der Waals or hydrogen bonding

Metal-oxide or hydroxide particles

Metal-oxane polymers

Covalent bonding or intermolecular bonding

Inorganic polymers from the hydrolysis and condensation of molecular precursors

Metal complex

Weak intermolecular bonding

Concentrated metal complex solution

In situ polymerizable complex

Covalent and coordinate bonding

Polyesters of polyhydroxy alcohol and carboxylic acid with a metal complex

Coordinate and cross-linked polymer complex

Coordinate and intermolecular bonding

Coordinate polymers and metal salt solution

Vanadium alkoxides, vanadium oxides, vanadium salts, and molecular precursors, including V(acac)4 and VO(acac)2, are commonly used for the sol-gel synthesis of V02 thermochromic coatings [1, 10, 19]. According to the type of gel and the chemical reactions involved, the sol-gel method can be divided into inorganic [30] and organic [36]. For the organic sol-gel process, the sol forms through the hydrolysis and condensation of organic precursors, such as vanadium alkoxides, V(acac)4, and VO(acac)2. As a result, a vanadium-oxane polymer gel is obtained [26, 36, 37]. On the other hand, an inorganic process occurs on using inorganic V2Os as the precursor, where a vanadium oxide colloidal gel forms in an aqueous medium [35, 38, 39]. It is worth mentioning that an organic process can be realized even with the V205 as the initial precursor, because of a transformation reaction from V205 to an organic vanadium alkoxide and the final formation of a vanadium-oxane polymer gel [24].

 
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