Hydrothermal Synthesis of Thermochromic VО2 for Energy-Efficient Windows

Introduction

A pioneer hydrothermal study performed by a British geologist, Sir Roderick Murchison, in the middle of the 19th century described the action of water at elevated temperatures and pressures in bringing about changes in the earth’s crust and leading to the formation of various rocks and minerals. Much of the early hydrothermal syntheses were concerned with the development of an autoclave that can realize high-temperature and high-pressure environments and the growth of particular minerals similar to natural ones in

(a) Branches of hydrothermal technology in the 21st century

Figure 7.1 (a) Branches of hydrothermal technology in the 21st century;

(b) Pressure-temperature map of materials processing techniques [3]. Republished with permission of Elsevier.

Temperature (X)

a laboratory. Hydrothermal synthesis has been widely accepted since the 1960s, and almost all inorganic species can be obtained by using this method. Nowadays, hydrothermal technology has found its unique place in different branches of science and technology, involving highly interdisciplinary research fields, such as geotechnology, biotechnology, nanotechnology, and advanced materials technology, as schematically illustrated in Fig. 7.1a [1].

Compared with other processing methods for the synthesis of advanced materials, like physical vapor deposition, sol-gel, laser ablation, plasma synthesis, and microwave techniques, hydrothermal processing facilitates the fabrication of even the toughest or the most complex materials with desired physicochemical properties. The hydrothermal method has several advantages over other conventional ones, for example, it enables energy saving, is simple and cost effective, allows better nucleation control, is pollution free, involves higher dispersion and a higher rate of reaction, allows better shape control, and involves a relative lower operation temperature in an appropriate solvent. Hydrothermal processing involves a slightly longer reaction time compared with the vapor deposition process or milling, but it provides highly crystalline products with better controlled sizes and shapes [2]. Figure 7.1b shows the pressure-temperature map of various material processing techniques, in which hydrothermal processing can be considered as environmentally benign [3].

The principle of hydrothermal synthesis involves a process of dissolution-supersaturation and subsequent crystallization in which the temperature, pressure, and time are the three most common parameters to be considered. Temperature plays an important role in the kinetics of product formation as well as on the thermodynamic stability of the resultant phase. Pressure is essential for the solubility, the supersaturation range directing the crystallization process, as well as the thermodynamic stability of the resulting phase. Time is also an important parameter because the synthesis of kinetically stable phases is favored in a short-term process while the thermodynamically stable phases are generally formed in longterm experiments within a given temperature- pressure regime [4]. Rational design of a hydrothermal product requires understanding of the nucleation and crystal growth process under isothermal and isobar conditions [5, 6]. The fluid consisting only of water with solid precursor materials at the beginning of the hydrothermal reaction (point I in Fig. 7.2) will become denser with time and even above the solubility limit of the precursor materials (point II, supersaturation). A glassy precursor can be dissolved at a faster rate than a crystalline material with the same composition. The supersaturation zone is wider with a glassy starting material than that with a crystalline one, which is pressure dependent. At a certain level of supersaturation (marked as point III), spontaneous crystallization will finally take place, leading to a decrease in the concentration of the hydrothermal fluid as the experiment continues [4]. The hydrothermal technique has been popularly utilized for the crystallization of nanomaterials, and the growth mechanism follows either Ostwald ripening (OR) or oriented attachment (OA) or both. The OR mechanism describes the growth of large particles at the expense of smaller ones due to the lowering of the surface energy. On the other hand, for the OA mechanism, as the surface energy is low or the solubility of the material is weak, the adjacent primary particles spontaneously self-organize into chains of nanoparticles (NPs) with a common crystallographic orientation, followed by joining of these particles via the fusion process. It is worth noting that the OA mechanism was first found in the synthesis of Ti02 particles by using the hydrothermal process [7].

Hydrothermal technology occupies a unique place in the fabrication of vanadium dioxide (V02) nanostructured materials owing

Isothermal-isobar hydrothermal processing

Figure 7.2 Isothermal-isobar hydrothermal processing. At (I), only water and precursor materials are present. Between (I) and (II), time-dependent precursor material dissolution occurs. Note that the region of supersaturated fluid is often extended at higher pressures. Nucleation occurs spontaneously if point (III) is achieved. A subsequent crystal growth process may only take place in the field of supersaturated solution [4]. Republished with permission of Springer.

to its advantages over other conventional ones mentioned above. In this chapter, we describe the hydrothermal synthesis of VO2 polymorphs and the thermochromic property of V02 with various nanostructures, like NPs, nanowires, and nanosheets. Note that the term "hydrothermal” throughout this chapter refers to any chemical reaction in supercritical or near-supercritical conditions, whether it is aqueous or nonaqueous.

 
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