## 1 Introduction  
Microorganic pollutants are increasingly more commonly found throughout environmental water systems. Although small in quantity, these unnatural elements could have a harmful impact on living organisms in the ecosystem and can negatively impact the ecological balance. The textile industry plays an important role in the developing world; however, it is also associated with increased pollution. According to the latest research, textile industry releases wastewater containing harmful chemicals such as dyes, acids, alkalis, surfactants, hydrogen peroxide, etc., throughout several processing steps [1, 2]. It is responsible for generation of 20% wastewater in comparison to remaining industry sectors [3, 4]. Hence, new processing solutions have been proposed in order to reduce wastewater generation. The proposed solutions need to be improved due to generation of toxic by-products [1]. Increasingly more attractive are purification processes that consider catalysts in the advanced oxidation processes (AOPs). These are especially effective against organic macromolecules of contaminants commonly used as colouring agents. Most studied is probably methylene blue, which is still widely used despite being carcinogenic [5]. Catalytic materials that can be applied in the AOPs for environmental remediation are commonly graphene-based materials because of their characteristic high surface area, physicochemical and mechanical strength, thermal and electron mobility, etc. [6; 7; 8]. A simple and low-cost approach to preparing graphene-based materials is to synthesise graphene oxide (GO) from the chemical exfoliation of commercial graphite flakes. GO is an oxidised form of graphene material with several oxygen functional groups such as epoxy, carbonyl, carboxyl, and hydroxyl, which also assists the catalysis. The physicochemical properties of GO can significantly change because the oxidation process also introduces defects into the carbon structure [9; 10]. Another related compound is reduced graphene oxide (rGO) which has a lower content of oxygen functional groups than GO nanosheets, while the overall properties are closer to graphene. Together with graphene-based material, researchers also consider a composite which also includes nanosized semiconductors. AOPs of such compounds depend on the interaction between both [11; 12]. Two methods that are commonly considered for the purification of water, using these types of materials, are namely absorption and photocatalysis [13; 14; 15; 16]. While absorption is fast, organic pollutants cannot be completely removed from aquatic environments if they have abounded. Thus, photocatalysis is more appropriate for the thorough degradation of organic pollutants [17; 18; 19; 10]. rGO material in the TiO\({}_{2}\)-rGO nanocomposite can reduce the electron-hole recombination in TiO\({}_{2}\). Thus, the charge transfer rate of electrons increases as well as adsorbed amount of materials at the surface, which contributes to the enhancement of the photocatalytic activity of the nanocomposite compared to the pure TiO\({}_{2}\) nanoparticles [20; 21]. Many existing reactors involve suspension of prepared rGO/TiO\({}_{2}\) nanoparticles in an aqueous medium, providing large surface area for the reaction, which is very effective. Fine particles of photocatally can be used in the form of suspension or it can be immobilised on various substrates by different methods. Different immobilisation routes have been utilised to cover substrates by fine particles of photocatalyst such as glass [22], polymer [23], zeolite [24], cellulose [25], ceramic [26; 27; 28], and magnetic particles [29; 30; 31]. Immobilisation of fine particles of photocatalyst on the surface of the substrate eliminates the need for complicated separation of the photocatalyst from the treated water. The aim of this study is to optimise the suspension parameters of the spin-coating method for rGO/TiO\({}_{2}\) nanocomposite immobilisation on the glass substrate.