**1. Introduction**

Semiconductor materials are of long standing research interest due to their practical applications as photocatalysts for environmental remediation, with specific focus on titanium dioxide owing to its durability and high performance. Titanium dioxide has been used consistently as the photocatalyst of choice to address a variety of environmental problems, and has been shown to be particularly effective when utilised as a thin film coating, specifically in self-cleaning glass [1], antimicrobial applications [2], and for water-splitting to produce hydrogen [3]. A multitude of approaches are used to produce TiO2 thin films, including sol-gel [4] and hydrothermal routes [5], as well as vapour deposition methods, such as chemical vapour deposition (CVD) [6], physical vapour deposition (PVD) [7] and more recently, aerosol-assisted CVD [8] and electric field assisted CVD [9].

Of all the deposition methods mentioned, sol-gel remains one of the most popular routes for producing TiO2 thin films due to its low cost, experimental simplicity and easy scale-up ability. Sol-gel also enables direct control of particle homogeneity during the particle growth phase, and as a result it is a particularly popular strategy for simple modification of thin films for the properties listed above. There has been a great variety of research focused on process modification that encourages specific morphological control within the resulting thin films [10]. Typically, sol-gel methods have been modified with the addition of block co-polymer templating agents [11] or non-ionic surfactants, such as Triton X-100 [12]. Previously we have reported the use of Brij® surfactants in a non-aqueous sol-gel process to produce TiO2 thin films with an increased average particle size and increased surface roughness, whereby such structural changes led to an increase in the photocatalytic activity of produced TiO2 thin films [13]. The use of such non-ionic surfactants in sol-gel processing is a commonplace strategy for the direct control of particle size and shape during the growth phase for the enhancement of resulting properties [14,15]. Due to their amphiphilic nature, surfactants act as pore-directing agents that can enable the production of highly porous materials with specific pore size and structure. A wide variety of surfactants have been used in sol-gel processing for TiO2 thin film production including Brij® surfactants [14,16], Triton™ X-100 [17], Pluronic triblock copolymers [14,15] and Tween® 20 [18], which has been used in this investigation in comparison with Tween® 40.

This paper focuses on the use of Tween® 20, 40 surfactants in a modified non-aqueous sol-gel method to investigate the effect of surfactant type and concentration on the subsequent microstructure and functional properties of TiO2 thin films.
