# 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
Parameters responsible for the pH changes are substrate and surface chemistry, solvent molecules, catalyst surface charge, types of surface interactions, substrate nature, the extent of adsorption, the numerous intermediates formed during the progress of the reaction, etc. [37]. The pH value of MB initial solution was measured to be around 6.19. By measuring the whole UV-Vis spectra, it is also possible to distinguish the degradation rate of different chemical bonds. Figure 8A shows the decolourisation (exhibited at 664 nm) and mineralisation (at 292 nm) of MB under irradiation. The photodegradation rate constant (\(k\)) was calculated from the slope of the time-dependent degradation function in a logarithmic plot, as shown in Figure 8B. The overall rate for both mineralisation and decolourisation is quite different. The decolourisation pseudo-first-order rate constant (0.01359 min\({}^{-1}\)) is two times faster in comparison with rate constant of mineralisation (0.00621 min\({}^{-1}\)). The obtained linear correlation coefficient (\(R^{2}\)) of decolourisation and mineralisation was 0.98796 and 0.99041, respectively; however, the logarithmic plot clearly shows two different kinetics for degradation in both processes. As expected, mineralisation which is slower overall is even slower in the beginning. Within the last 15 min, however, the rate intensifies (green slope Figure 8A) to the same or even larger value as in decolourisation. This furthermore suggests that immobilised g-C\({}_{3}\)N\({}_{4}\) photocatalyst on alumina substrate drives decolourisation and mineralisation to faster completion. Figure 7: **(A)** Monitoring adsorption and degradation of alumina substrate (red) and immobilised g-C\({}_{3}\)N\({}_{4}\) on alumina substrate (blue), and **(B)** UV-vis absorbance spectra of initial concentration of MB dye and after 75 min photocatalysis. Developed CdO/Al2O3 nanocomposite by [PERSON] et al. [38] achieved the maximum decolourisation (97.3%) and high mineralisation (87%) in 220 minutes. In another study, copper ferrite particles were co-precipitated onto polydopamine pretreated ceramic/polyester fabrics (CPF) to develop CPF/CuFe2O4 photocatalyst. The photocatalytic activity was investigated by degradation of methylene blue (MB), where maximal degradation rate of 98.2% under visible light irradiation for 80 minutes was achieved [39]. [PERSON] et al. [40], anodised aluminium oxide membranes with different pore sizes coated with a titanium dioxide (TiO2) photocatalyst via atomic layer deposition. The photocatalytic membrane cell with active area of 2 cm2 was used for MB removal. The MB degradation around 50% was achieved in a single-pass flow-through process with possibility to increase the degradation rate by further refinements of the process. In view of this reports our \(g\)-C3N4 photocatalyst immobilised on alumina substrate demonstrated comparable decolourisation and mineralisation of MB. The photophysical potentials of pure \(g\)-C3N4 can be modified by heteroatom doping, heterojunction formation by coupling with other materials, and textural modifications to increase the surface area. The improvement or decline of the photocatalytic performance was attributed to the decreased range of the band gap and change in the surface area [41]. Reuse experiments between cycles were presented in Figure 9. The immobilised \(g\)-C3N4 was reused for several photocatalytic cycles under the same conditions. Between the different cycles, the catalysts were rinsed with ultrapure water and ethanol then dried. Decolourisation of the MB dye (664 nm) as well as mineralisation (292 nm), in comparison between three cycles, is very similar. Decolourisation efficiency of MB was 79.7%, 80.5% and 78.1% after the first, second and third cycles, respectively. Degradation of the aromatic ring was 64.3%, 65.8%, and 64.0% between the cycles. With three cycles, the removal of the aromatic ring is lower in contrast to the decolourisation.