Sol-gel-synthesized titania-vanadia nanocrystal films for triple-functional window coatings

Abstract

In this study we employed mixtures of titanium and vanadium sols, with various ratios of titanium to vanadium, in a polyvinylpyrrolidone solution to generate precursor titania-vanadia composites. Titania-vanadia nanocrystals (TVNCs) were then obtained after calcination at 700 °C for 3 h. Acanthosphere-, actinia-, and gear wheel-like structures appeared on the surfaces of the TVNCs when the ratio of titanium to vanadium sols was 2, 1, and 0.5, respectively. The acanthosphere-like structures decreased the surface contact area sufficiently to result in superhydrophobicity. Falling water droplets rebounded completely from the film, thereby potentially preventing fouling of such materials. The superhydrophobicity of the film remained stable after 180 min of UV irradiation, suggesting excellent self-cleaning ability. The acanthosphere-like structures enhanced the visible transmittance and near-infrared switching efficiency, similar to the behavior of an antireflection coating. In addition, the ability to photocatalytically degrade organic component under irradiation with UV and visible light was enhanced because of the TVNCs. Such triple-functional (self-cleaning, photocatalytic, thermochromic) nanomaterials might have applications in energy-saving smart windows.

Introduction

Titanium dioxide (TiO₂) thin films have been studied extensively because of their applications in many areas, including self-cleaning [1], water-splitting [2], [3], and gas-sensing [4] materials. Most research has focused on improving the photocatalytic efficiency to visible/solar light through the effects of doping or the forming of composites [5], [6], [7]. Because titania can degrade organic waste photocatalytically into carbon dioxide and mineral acids in sunlight and reach a superhydrophilic state, it is the material of choice for self-cleaning windows and surfaces. In addition, TiO₂ is nontoxic, chemically stable, inexpensive, and capable of extensive photocatalytic cycling; thus, it is a material with great potential to improve water quality though solar-driven processing.

Vanadium dioxide (VO₂) thin films find practical use as smart window coatings because of their thermochromic properties [8], [9], [10], [11]. In the monoclinic form, VO₂ displays a thermally induced and completely reversible semiconductor-to-metal phase transition temperature (T_p) to the tetragonal (rutile) form at 68 °C. This transition affects remarkably both the electrical and optical properties of VO₂ in the near-IR, where the monoclinic form behaves as a semiconductor and does not reflect much solar energy and the tetragonal form behaves as a semimetal, reflecting a much wider range of wavelengths [12]. The value of T_p can be decreased to more moderate temperatures when using dopants [13], opening up the prospect of using such materials in architectural glazing to allow heat into a building at temperatures below T_p and reflect heat away at temperatures above T_p. Such intelligent coatings should decrease the need for internal heating and air-conditioning within buildings, leading to savings in energy costs [14].

The natural world is replete with biominerals that exhibit periodicity, structural hierarchy, and complexity. For example, corals display complex morphologies and high superficial areas, allowing symbiotic photosynthetic unicellular algae to efficiently harvest photons and, thereby, produce energy and nutrients. These complex architectures are formed spontaneously under far-from-equilibrium conditions by consecutive deposition of biopolymers and inorganic precursors in aqueous media [15]. The fabrication of inorganic surface nanoarchitectures having high superficial areas has been studied extensively using zinc oxide (ZnO), because of its facile growth on solid surfaces [16]. Surface arrays of ZnO nanowires have been applied to fabricate nanolaser devices [17], photoelectrochemical cells [18], nanopiezotronics [19], gas sensors [20], and field emission [21] and optoelectronic materials [22]. The ability to synthesize such high-superficial-area nanoarchitectures is much coveted for TiO₂ or VO₂ nanocrystals. In contrast to ZnO nanoarray systems, however, syntheses of high-superficial-area nanostructures of TiO₂ or VO₂ nanocrystals have been rare. Although methodologies using structural templates [23], metal–organic chemical vapor deposition [24], high-temperature post-annealing processes, [25] and hydrothermal synthesis [26] have been developed, the need for expensive vacuum equipment or furnace heating considerably lessens their versatility.

Although the syntheses of titania-vanadia systems have been studied extensively, the role of the active phase in the modification of the support's structural properties has received little attention. The ability to fabricate such structurally graded, high-superficial-area nanostructures would revolutionize the field of functional surfaces, pushing the limits of current technologies in catalysis, conversion of energies and materials, and ultrasensitive diagnosis, to name a few. In this paper, we focus on the structural properties of a titania-vanadia nanocrystals (TVNC) system prepared using the sol–gel method. The TVNCs were generated from mixtures of titanium and vanadium sols at various ratios and subsequent calcination. The various structures of the TVNCs could be regulated by varying the ratio of the titanium and vanadium sols. We studied the influence of the binary mixtures on the structural characteristics of the resulting titania-vanadia systems. Films of the as-prepared composite crystals on glass exhibited remarkable hydrophobicity, with potential for self-cleaning. Furthermore, these specific structures increased the visible transmittance of the film. Such films performed triple functions: thermochromism [from the VO₂(M) layer] for solar energy modulation, self-cleaning, and photocatalysis. Thus, the as-obtained TVNC coatings appear to have applicability in smart windows.