# Photocatalytic Properties of Immobilised Graphitic Carbon Nitride on the Alumina Substrate
## 1 Introduction  
Pollution in oceans, lakes and rivers has become a global environmental burden. The presence of manmade organic species such as fertilizers, pesticides, and pharmaceuticals were found throughout all aquatic ecosystems [1]. One of the most frequent forms of pollution worldwide is dye contamination most abundantly generated by the industrial sector responsible to produce leather, textile, and cosmetics. Daily release of various raw materials such as cotton, synthetic fibres, and dyes pollutes the ecosystem. Bigger fragments are increasingly more often isolated by filtering and subsequently discarded in the land fields. Dyes and pigments remain due to their small molecular size and complex chemical structure [2, 3]. The pollutants are abundant in water systems, where they degrade to simpler molecules by very complex physical and chemical mechanisms. One of those mechanisms is photolysis, where light irradiates a given volume of the mixture, thus providing a constant inflow of energy, which is responsible for scissoring chemical bonds. On the laboratory scale, the process under consideration can be greatly enhanced. In the experiment, photocatalysts are usually considered. Heterogeneous photocatalysis uses solid-state catalyst immersed in 'polluted' water. Among them, graphitic carbonnitride (g-C\({}_{3}\)N\({}_{4}\)) gains considerable attention [4; 5; 6]. To evaluate the catalytic activity of heterogeneous catalysts it is convenient to disperse unsupported, bulk catalyst in a powder form in the targeted solution. In this way, it is possible to quickly gain information about its activity. However, from the application point of view, the catalyst should be immobilised on a suitable substrate such as ceramic [7; 8], polymers [9; 10], and zeolite [11; 12]. Robust substrate with immobilised photocatalysts provide a stable system during a dynamic exchange of solution. Without any post-treatment, e.g., such as filtration or sedimentation to separate solution from the catalyst, they can be used sequentially multiple times. Im-mobilisation of the given catalyst also requires new research with respect to its physical and chemical properties. While the most important parameter such as its bandgap energy is not expected to change, the construction of a more complex setup brings about many other discrepancies. Among them, for example, is the scattering of the harvested light. Differences appear between for example isolated dispersed particles in sizes of a few 100 nm or deposited ones, because the former favour the light scattering over the desired absorption [13]. There are a lot of new parameters that must be considered when immobilising a given photocatalyst [14]. These issues are being continuously addressed. Novel insight is needed to prepare immobilised photocatalysts deposited together with noble metals, which were proven many times to improve activity [15; 16], morphologically modified photocatalysts [17; 18], and new strategies for the preparation of composites [19; 20; 21]. Graphitic carbon nitride has excellent properties with great potential to be used as a metal-free catalyst in wastewater treatment. The published scientific papers pursued the research in suspensions which has significant drawbacks such as toxicity of the nanoparticles, the uncontrolled release into the environment needs to be closely monitored. Many existing slurry-type reactors use nanoparticles suspension where separation and reuse of catalyst from the final product are difficult to achieve. Imombilised _g_-C\({}_{3}\)N\({}_{4}\) in the form of coating is a promising solution that opens the possibility of manufacturing reactors for reusable photocatalysts with reduced erosion of nanoparticles into the environment. Ceramic substrates are widely used due to their chemical stability, and very good mechanical properties. Alumina stands out among them. It is environmentally benign, inexpensive and can be easily envisioned even in high concentrated effluents [22; 23].