Influence of deposition parameters on the structure and microstructure of Bi₁₂TiO₂₀ films obtained by pulsed laser deposition

Abstract

The structure, morphology and surface roughness of Bi₁₂TiO₂₀ (BTO) thin films grown on R-sapphire by pulsed laser deposition (PLD) were studied at different substrate temperatures, target-substrate distances, oxygen pressures and laser-pulse repetition rates. Although the substrate temperature seems to be the most important experimental parameter, the gas pressure and the target–substrate distance played important role on the phase formed and film thickness, with a significant effect of the laser-pulse repetition rate on the films thickness and preferred orientation of the deposited film. Single-phase γ-Bi₁₂TiO₂₀ was obtained on substrates at 650 °C, while several BTO metastable phases were observed in films deposited on substrates at temperatures between 500 and 600 °C. By the first time, thin films of pure and textured δ-Bi₁₂TiO₂₀ were successfully growth on substrates at 450 °C. When annealed, all the films deposited at lower temperatures resulted in the thermodynamically stable γ-Bi₁₂TiO₂₀.

Introduction

Semiconductor photocatalysts for solar energy conversion is an active area in the field of environmental research. Although the development of various materials, the design of efficient visible light-driven photocatalysts is still a great challenge [1], [2], [3], [4], [5], [6], [7]. Bismuth titanate (BTO) is a promising candidate to replace TiO₂ because of its chemical stability, nontoxicity and enhanced photocatalytic reactivity. Bismuth oxide occurs in different composition and structures, such as Bi₄Ti₃O₁₂ [8], [9], [10], Bi₂Ti₂O₇ [11] Bi₂Ti₄O₁₁ [12], Bi₂₀TiO₃₂ [13], [14], [15], [16] and Bi₁₂TiO₂₀ phase [3], [4], [5], [17], [18], [19], [20], [21], but there are at least three polymorphs that exhibit catalytic properties [22], [23], [24], [25] and unusual high photocatalytic activity under solar light irradiation [26], [27], [28], [29]. The Bi₁₂TiO₂₀ belongs to the sillenite family, where the bismuth coordinates to five oxygens of the octahedra together with the stereochemically active 6 s2 lone electron pair of Bi³⁺. This configuration turns this structure unique for application in photocatalysis. When synthesized as powders, two polymorphs were identified, the γ-Bi₁₂TiO₂₀ formed at room temperature and the δ-Bi₁₂TiO₂₀ that is stable only at high temperature [30], [31], [32], [33], [34]. The γ-Bi₁₂TiO₂₀ has the (BCC) body-centered cubic structure of γ-Bi₂O₃ (space group I23) while the δ-Bi₁₂TiO₂₀ have the (FCC) face-centered cubic structure of δ-Bi₂O₃ (space group Fm-3m). The γ-Bi₁₂TiO₂₀ showed the photocatalytic ability to degrade organic pollutants, offering a potentially efficient technology for the elimination of toxic waste chemicals [35], [36], [37], [38]. Nanostructured γ-Bi₁₂TiO₂₀, for example, exhibited improved photoelectrochemical activity to generate hydrogen and to degrade textile dye under UV–visible light [39]. Indeed, γ-Bi₁₂TiO₂₀ powders exhibited superior performance than the commercial TiO₂ (P25 catalyst) under visible light irradiation [40], [41] and could degrade dyes much more efficiently than the traditional N-doped TiO₂ [42].

Various methods have been used to synthesize sillenite γ-Bi₁₂TiO₂₀, such as chemical solution decomposition [21], [43], isopropanol-assisted hydrothermal synthesis [2], [41], [42], coprecipitation [36], [44], Pechini-based routes [45], electrochemical method [39], [46], sol-gel processing [47], the traditional solid-state reaction [48] and, recently, by means of the oxidant peroxo method (OPM) [49], [50], [51], [52].

Although several studies used ultrafine powders suspended in the reaction media, photocatalyst in the form of supported thin film provides an advantage over the drawbacks encountered in powder suspensions. In fact, there are only a few studies focused on the synthesis and applications of Bi₁₂TiO₂₀ thin films. [46], [53], [54], [55], [56], [57], [58]. In this context, it appears attractive to synthesize thin films of Bi₁₂TiO₂₀ with controlled structure and microstructure aiming applications in photocatalysis. This study presents the synthesis of Bi₁₂TiO₂₀ thin films growth by pulsed laser deposition (PLD) evaluating the influence of several experimental parameters, such as the control of the substrate temperature, oxygen pressure, target-substrate distance and the laser-pulse repetition frequency on the microstructural (thickness, morphology, crystallinity) and structural (orientation and phases) properties.