# Enhancing the Photocatalytic Activity of Immobilized TiO2 Using Laser-Micropatterned Surfaces
###### Abstract
Various chemical and physical techniques, such as chemical vapor deposition (CVD) [12], sol-gel [13], hydrothermal synthesis [14], metal organic chemical vapor deposition (MOCVD) [15], and pulsed laser deposition (PLD) [16], have been employed for the preparation of TiO\({}_{2}\) thin films. The method of preparation significantly influences the properties of the films, with many processes requiring expensive infrastructure as well as high temperatures and pressures. Among these methods, the sol-gel process stands out as a common and advantageous approach for producing thin solid films and coatings on diverse substrates, offering benefits such as low cost, simplicity, low process temperatures, homogeneity control, and potential for large-scale production. Additionally, the sol-gel process enables the customization of material properties through the control of synthetic conditions [17]. While thin TiO\({}_{2}\) films have been employed previously as photocatalysts for water contaminant removal [18], their photocatalytic behavior is limited by a low specific surface area, resulting from agglomerations in their micro/nanostructure and random orientation. To address this limitation, numerous efforts have focused on increasing the surface area of TiO\({}_{2}\) films, often achieved through porosity control [19; 20; 21]. Various methods, including atomic layer deposition [22], sol-gel synthesis [23], hydrothermal reaction [24], and 3D printing [25], have been employed to tailor the porosity of TiO\({}_{2}\). Additionally, increasing its specific area has been pursued through the development of needle-like microstructures [26; 27] and the fabrication of aligned TiO\({}_{2}\) nanotubes [28; 29], among others. A complementary approach to achieve a large surface area consists of combining a thin film with a patterned substrate, which offers a 3D morphology. Silicon is often used for this purpose due to its abundance and extended variety of patterning techniques available. In the field of micro/nanopatterning silicon surfaces, laser structuring methods have been often implemented [30; 31]. Different wavelengths and pulse durations, such as 248 nm (nanosecond excimer) [32; 33], 800 nm (femtosecond Ti:Sapphire) [34; 35], 355/532 nm (nanosecond Nd:YAG lasers) [36; 37], etc., have been employed to create specific morphologies on silicon, while the influence of ambient media, including vacuum, water, sulfur hexafluoride (SF\({}_{6}\)), ethanol, and others, has been thoroughly investigated [30; 38; 39]. Micro/nanostructured silicon, owing to its high absorption across a wide electromagnetic spectrum, finds diverse applications, such as solar cell and photodetector development, surface-enhanced Raman spectroscopy, and others [38]. The wettability/ biomimetic [40; 41; 42] and cell growth properties [43] of micro/nanostructured silicon substrates have been extensively explored due to their unique morphology. Similarly, the large surface area of micro/nanostructured silicon makes it an attractive substrate for enhancing the photocatalytic activity of immobilized coatings. Since there is an increasing interest in using photocatalysis for water treatment, a reliable evaluation of catalytic material performance is necessary. The European Committee for Standardization, CEN TC386/WG3, has developed a standard for the assessment of the photocatalytic activity of bulk materials [44]. Evaluating photocatalytic material performance in water purification relies on measuring phenol degradation under controlled conditions with UV irradiation. However, it is important to note that this assessment cannot be extrapolated to photocatalytic surfaces. Two ISO standards, ISO 10678:2010 and ISO 10676:2010, are available for assessing the photocatalytic activity of surfaces concerning water purification, which are based on the photobleaching of methylene blue and the oxidation of dimethyl sulfoxide (DMSO), respectively [45; 46]. While these standards focus on assessing photocatalytic surfaces, they exhibit reliability shortcomings.