# Photocatalytic Properties of Immobilised Graphitic Carbon Nitride on the Alumina Substrate
## 3 Results and Discussion
### Photocatalytic Activity of Immobilised \(g\)-C\({}_{3}\)N\({}_{4}\) and Its Reusability  
Before performing photocatalytic tests, the adhesion of the immobilised photocatalyst on the alumina substrate was investigated. With a single dip into the dispersion, followed by heat treatment for 15 min at 105 \({}^{\circ}\)C, \(g\)-C\({}_{3}\)N\({}_{4}\) bonds to the surface of the ceramic without noticeable pealing. The colour of the disk changes evenly suggesting uniform coverage of the whole disk. Even more, such substrate withstands overnight (total duration of 15 h) magnetic stirring (350 rpm) in ultrapure water with no visual leaching of \(g\)-C\({}_{3}\)N\({}_{4}\) into the water. The stability of immobilised \(g\)-C\({}_{3}\)N\({}_{4}\) was also evident by subsequent UV-Vis analysis initially performed to evaluate catalytic activity. Rayleigh scattering from the possible small residuals, if present in a solution, would intensify an absorption spectrum in the UV part; however, as can be seen from Figure 4B, little if any such elevation was observed. SEM image (Figure 6B) poorly resolves the coverage of alumina substrate probably due to charging and inhomogeneities over the exposed ceramic surface; however, the presence of \(g\)-C\({}_{3}\)N\({}_{4}\) is clearly resolved with EDXS analysis (inset Figure 6B). The kinetic study is also presented where changes in absorbance were monitored at different time intervals and for different absorption peaks. The degradation of initial concentration (10 mg\(\cdot\) L\({}^{-1}\)) was also monitored in pure alumina substrate, the results are also present in Figure 7A (red line). Following the commonly exploited process for evaluating the degradation rate of MB, the time to reach adsorption-desorption equilibrium by exposing the sample pollutant solutions in the dark was firstly estimated. While the immobilised \(g\)-C\({}_{3}\)N\({}_{4}\) catalyst showed significant adsorption compared to pure alumina initially, MB concentration did not change much after 30 min (and around 15 for pure alumina). With the onset of photoexcitation, \(g\)-C\({}_{3}\)N\({}_{4}\) immediately starts to further reduce the amount of MB in the solution. The concentration of MB drops to around 20% of the initial value in investigated time interval of 75 minutes. Pure alumina substrate, on the other hand, does not alter the concentration of MB within the next minutes. Small variation in the intensity is probably related to systematic errors introduced by changing sample, refiling the cuvettes etc. Figure 6: (**A**) SEM image of g-C\({}_{3}\)N\({}_{4}\) powder and (**B**) SEM image of immobilised \(g\)-C\({}_{3}\)N\({}_{4}\) on alumina surface along with EDXS conducted over the area. adsorption and degradation capabilities of g-C\({}_{3}\)N\({}_{4}\) catalyst deposited on alumina substrate for MB photodegradation was investigated by UV-Vis spectroscopy. Degradation of the MB is monitored through the decrease of the absorption spectra characteristic for a given species, Figure 7B. The characteristic peaks of MB at 246 nm, 292 nm, and 664 nm correspond to the chromophore in the structure of MB, N, S substituted heterocycle structure, benzene ring, and dimethylamino group, respectively [33,34]. After 75 min of irradiation under a simulated solar light, all the absorption peaks significantly decreased, Figure 7B, which points to the fact that the chemical structure of MB starts to break. Especially noticeable is the heterocyclic part. All absorption peaks are also shifted to the lower wavelengths at 623, 287, and 241 nm, respectively. Blue shift from 664 nm to 623 nm correspondent to the demethylation of MB structure. The absorption peak at 241 nm corresponding to the aromatic compound suggests that the degradation of MB (using the g-C\({}_{3}\)N\({}_{4}\) catalyst) probably produces new metabolites [35]. The pH of the solution plays an important role in the reactive oxygen species (ROS) formation for the photocatalytic degradation of persistent organic contaminants in water [36].