Photocatalytic Performance of Sol-Gel Prepared TiO2 Thin Films Annealed at Various Temperatures

Abstract

Titanium dioxide (TiO2) in the form of thin films has attracted enormous attention for photocatalysis. It combines the fundamental properties of TiO2 as a large bandgap semiconductor with the advantage of thin films, making it competitive with TiO2 powders for recycling and maintenance in photocatalytic applications. There are many aspects affecting the photocatalytic performance of thin film structures, such as the nanocrystalline size, surface morphology, and phase composition. However, the quantification of each influencing aspect needs to be better studied and correlated. Here, we prepared a series of TiO2 thin films using a sol-gel process and spin-coated on p-type, (100)-oriented silicon substrates with a native oxide layer. The as-deposited TiO2 thin films were then annealed at different temperatures from 400 °C to 800 °C for 3 h in an ambient atmosphere. This sample synthesis provided systemic parameter variation regarding the aspects mentioned above. To characterize thin films, several techniques were used. Spectroscopic ellipsometry (SE) was employed for the investigation of the film thickness and the optical properties. The results revealed that an increasing annealing temperature reduced the film thickness with an increase in the refractive index. Atomic force microscopy (AFM) was utilized to examine the surface morphology, revealing an increased surface roughness and grain sizes. X-ray diffractometry (XRD) and UV-Raman spectroscopy were used to study the phase composition and crystallite size. The annealing process initially led to the formation of pure anatase, followed by a transformation from anatase to rutile as the annealing temperature increased. An overall enhancement in crystallinity was also observed. The photocatalytic properties of the thin films were tested using the photocatalytic decomposition of acetone gas in a home-built solid (photocatalyst)–gas (reactant) reactor. The composition of the gas mixture in the reaction chamber was monitored using in situ Fourier transform infrared spectroscopy. Finally, all of the structural and spectroscopic characteristics of the TiO2 thin films were quantified and correlated with their photocatalytic properties using a correlation matrix. This provided a good overview of which film properties affect the photocatalytic efficiency the most.

Effect of annealing temperature on the structural, optical, morphological, and photocatalytic properties of TiO2 thin films prepared by sol-gel spin-coating and electron beam physical vapor deposition

“…To improve the photocatalytic ability of TiO 2 , researchers have attempted changing the forbidden bandwidth of TiO 2 [2], annealing to change the type of TiO 2 crystalline phase [3], doping to form defects [4], creating composite thin films [5], and modifying the surface [6]. Among them, the crystalline phase transformation of TiO 2 due to changing the annealing temperature (T a ) has been widely studied because of its crucial importance in the photocatalytic process [7]. Albetran and Low showed that TiO 2 transforms from an amorphous to an anatase phase when annealed at 600 • C and to a rutile phase when annealed at 700 • C [8].…”

“…Wang et al showed that rutile crystalline phases also possess photocatalytic ability [13]. The crystalline phase transformation, crystalline size, and photocatalytic changes of TiO 2 during the annealing process are also different for TiO 2 prepared by different preparation methods [7,[14][15][16]. Therefore, a comparison of the physical structural, optical, and photocatalytic properties of TiO 2 thin films prepared by different preparation methods during the annealing process needs to be further investigated.…”