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Article

Photocatalytic Properties of Immobilised Graphitic Carbon Nitride on the Alumina Substrate

by
Milan Vukšić
1,*,
Martina Kocijan
1,*,
Lidija Ćurković
1,
Tina Radošević
2,
Damjan Vengust
3 and
Matejka Podlogar
2,4
1
Department of Material, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lučića 5, 10000 Zagreb, Croatia
2
Department for Nanostructured Materials, Jožef Stefan Institute, Jamova Cesta 39, SI-1000 Ljubljana, Slovenia
3
Advanced Materials Department, Jožef Stefan Institute, Jamova Cesta 39, SI-1000 Ljubljana, Slovenia
4
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna Pot 113, SI-1000 Ljubljana, Slovenia
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2022, 12(19), 9704; https://doi.org/10.3390/app12199704
Submission received: 20 August 2022 / Revised: 23 September 2022 / Accepted: 23 September 2022 / Published: 27 September 2022
(This article belongs to the Section Surface Sciences and Technology)
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Versions Notes

Abstract

:
Textile industries significantly impact the contamination of wastewater. Conventional wastewater treatment methods consider the most common pollutants; however, they are very expensive and commonly produce toxic by-products. In the scientific community, advanced oxidation processes appear to be the most appealing, and a majority of the published work considers heterogeneous photocatalysis for the degradation of various toxic chemicals. For convenience, the reaction is performed directly in the water environment. In this work, a metal-free graphitic carbon nitride (g-C3N4) was prepared through a simple thermal method using urea as a precursor. Prepared g-C3N4 was deposited on the surface of the alumina ceramic ring by the dip-coating method using ethylene glycol as binder. The alumina ceramic ring, as substrate, was prepared by the slip casting method. Photocatalytic properties of immobilised graphitic carbon nitride were used for degradation of methylene blue as a model pollutant under simulated solar light irradiation. The photocatalyst was characterised by XRD, FTIR, UV-Vis DRS, TGA, BET and SEM/EDX analyses. The photocatalytic degradation of MB from an aqueous solution was found to increase with increasing irradiation time. It was found that the graphitic carbon nitride immobilised on alumina ceramic is convenient for largescale environmental applications because the whole setup is cheap, nontoxic, easy to operate and offers reusability with a high removal rate of MB after three consecutive cycles.

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 carbon nitride (g-C3N4) 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. Immobilisation 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. Immobilised g-C3N4 in the form of coating is a promising solution that opens the possibility of manufacturing reactors for reusable photocatalysis 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]. It is stable at various temperature conditions and chemically very inert. In the presented study, the g-C3N4 photocatalyst was deposited on such alumina substrate. The quality of g-C3N4 coatings on ceramic substrates had a key role in efficient catalytic processes. The photocatalytic activity was evaluated by decolourisation of MB dye as well as degradation of the aromatic ring under simulated sunlight irradiation.

2. Materials and Methods

2.1. Materials

Urea (≥99%) and ethylene glycol (>99.5) were purchased from Merck KGaA (Darmstadt, Germany). Methylene blue (MB) was supplied by VWR Chemicals GmbH (Dresden, Germany). Alumina powder and Tiron® were purchased from Almatis (Ludwigshafen, Germany) and Sigma-Aldrich (Steinheim, Germany), respectively. The ultra-pure water was produced in a LaboStar® PRO water purification system (resistivity 18.2 MΩ/cm at 24.5 °C, 0.2 µm sterile filter, Siemens). The characteristics of the alumina powder are given in Table 1.

2.2. Preparation of Alumina Substrate

The alumina ceramic ring was prepared by the slip casting method and used as the substrate for the deposition of synthesized g-C3N4 catalyst. The stable alumina ceramic suspension (40 vol.%) was prepared by adding alumina powder (A 16 SG) and dispersant Tiron® into demineralised water. Afterward, all components were mixed for 2 h in the planetary ball mill (PM400, Retsch, Haan, Germany), poured into ring paster moulds, and dried at ambient temperature for 24 h. The ring-shaped green bodies were sintered applying the 4 °C min−1 heating rate till 1650 °C with a holding time of 6 h. The sintered ring-shaped alumina substrates with the following dimensions: inner diameter of 25 ± 2 mm, outer diameter of 55 ± 2 mm, and 5 ± 2 mm thickness was obtained.

2.3. Preparation of g-C3N4

The graphitic carbon nitride photocatalyst was synthesised by heating urea as a precursor in a covered crucible. In a typical synthesis, 10 g of urea were heated up to 550 °C at a rate of 5 °C/min in an air atmosphere and left for 2 h to dwell in a chamber furnace (Nabertherm, Lilienthal, Germany). The obtained g-C3N4 was cooled naturally to room temperature, and it was in the form of a light yellowish powder as shown in Figure 1.

2.4. Immobilisation of g-C3N4 on Alumina Substrate

The obtained g-C3N4 catalyst was immobilised on an alumina substrate using a procedure previously reported [24]. The prepared g-C3N4 catalyst and ethylene glycol were mixed in the appropriate ratio. The mixture was stirred for 15 min and transferred into an ultrasonic bath for 15 min at room temperature to obtain a homogeneous suspension. For the immobilisation of the catalyst on the alumina substrate, a dip-coating method was used. The alumina substrate was immersed in the catalyst suspension for 2 min and the photocatalyst adhered to the surface of the alumina substrate. Afterward, it was thermally treated in a dryer at 105 °C for 15 min. The immobilised g-C3N4 catalyst on alumina support is shown in Figure 2.

2.5. Characterisation of Prepared g-C3N4

Morphology, chemical elemental composition, and surface analysis were conducted with a scanning electron microscope with energy dispersive spectroscopy (SEM/EDXS, Verios 4G HP, Thermo Fisher). To provide information related to the specific functional groups, the Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR–ATR, Shimadzu, Kyoto, Japan) measurements were performed in the 400–4000 cm−1 range at room temperature. UV-Vis diffuse reflectance spectra (DRS) were obtained in the measurement range of 200–700 nm at room temperature. The samples were analysed with a UV-3600 spectrophotometer (Shimadzu, Kyoto, Japan) equipped with a reflectance sphere using BaSO4 as a reference. TGA analysis was performed with NETZSCH STA 449 C/6/G Jupiter—QMS 403 in the range from room temperature to 900 °C with the 5°/min heating rate. The N2 adsorption–desorption isotherm and pore-size distribution of as-prepared g-C3N4 photocatalyst was analysed using Brunauer−Emmett−Teller (BET) method (Quantachrome Nova 2000e, Anton Paar QuantaTec Inc., Graz, Austria). The crystallinity phase and composition of the prepared g-C3N4 were obtained by an X-ray diffractometer (X’Pert PRO high-resolution X-ray diffractometer; PANalytical B.V., Almelo, Netherlands) using CuKα as a radiation source (λ = 1.5406 Å) at 2θ ranging from 8° to 80° with the step of 0.033°/100 s.

2.6. Photocatalytic Tests

To examine the photocatalytic activity of the immobilised photocatalyst g-C3N4, the photodegradation of the MB dye (Table 2) with a concentration of 10 mg·L−1 was investigated under simulated solar light irradiation. Osram’s Ultra Vitalux lamp (300 W) was used as a light source. The MB dye aqueous solution and immobilised g-C3N4 photocatalyst were stirred in the dark for 30 min to achieve an adsorption-desorption equilibrium between the organic pollutant and the photocatalyst. All the photocatalytic experiments were carried out under the same conditions at room temperature. At defined intervals, 15 min, the absorbance was monitored by UV-Vis spectroscopy on a Lambda 950 UV/VIS/NIR spectrophotometer (PerkinElmer, Waltham, MA, USA) in the 200–750 nm range using a quartz cuvette (Hellma Analytics, Stuttgart, Germany) with a path length of 10 mm. Under identical experimental conditions, the activity of bare alumina substrate was also investigated and compared with immobilised photocatalyst g-C3N4 on the substrate.
The kinetics study of MB photocatalytic decolourisation and degradation is described by pseudo-first-order kinetic equation. The integrated form of the pseudo-first-order kinetic model is expressed in Equation (1) [25]:
ln ( A 0 / A t ) = k   ·   t
where A0 and At express the initial and remaining MB dye absorbance for a specific interval of time and k represents the first-order rate constant (min−1) of photocatalytic degradation of MB dye.
To investigate the reusability of immobilised g-C3N4 catalyst, recycling experiments were carried out for three cycles under the same experimental conditions. After each cycle, the immobilised g-C3N4 catalyst was washed with ultrapure water and ethanol, and then dried at 60 °C.

3. Results and Discussion

3.1. Catalyst Characterisation

Alumina substrate densification is well covered in the literature, and a process for the production of various alumina objects and crucibles has been commercialised within our institution, yielding a fully densified corundum (α-Al2O3) phase. Synthesis of g-C3N4 is subtler especially if we consider small changes, which could influence the electronic structure and with-it photocatalytic activity.
The crystal structure was studied first by XRD, and the corresponding pattern of g-C3N4 is presented in Figure 3A. The diffractogram shows two characteristic diffraction peaks at 13.3° (100) and 27.5° (002), which correspond to the in-plane spacing of repeated tri-s-triazine rings and the interlayer stacking reflection, respectively [26]. The spectrum is broad due to the small crystal size and reflects the sheet-like (2D) morphology of the material. This is evident because the (002) peak corresponding to interlayer stacking dominates the spectrum. Equivalently, because these planes represent the majority of the total and are parallel with the XRD sample holder, they provide more signals in a standard X-ray powder diffraction configuration setup. A slight elevation of the whole spectra could point out to a presence of less crystalline phase; however, if the 2D sheet is very thin, similar features would also be found.
The decomposition of g-C3N4 powder measured by TGA analysis nicely follows expected temperature dependence (Figure 3B). There are no bumps that would indicate the presence of impurities [27]. According to evolved gas analysis (not shown), a small decrease in mass at 100 °C correspond to water loss, while in the degradation region also CO2 and N2 were detected. The degradation is completed at around 700 °C. Optical properties of the prepared g-C3N4 were characterised by UV-visible diffuse reflection spectrum (UV-vis DRS) in Figure 4A. This semiconductor catalyst is supposed to be effective in simulated solar irradiation because g-C3N4 absorbs light from the UV to the visible region, with slight variations with respect to the doping or shape of the crystals [28]. Utilizing Kubelka–Munk transformation (Figure 4B), the Eg value of 2.62 eV was estimated, which is similar to other reports and corresponds to an absorption edge starting in blue at 473 nm and extending to the UV region.
The fingerprint vibrational spectrum, which corresponds to specific chemical bonding in molecules was measured with FTIR, as shown in Figure 5. Normal modes of bond vibrations for g-C3N4 can be easily identified. Most characteristics are those in the range between 1100–1700 cm−1. The strong peaks observed at approximately 1228, 1311, 1393, and 1456 cm−1 correspond to the aromatic C–N stretching vibrations, whereas the heterocycles C=N stretching vibrations are found at 1539 and 1627 cm−1 [29,30]. The typical peak of 805 cm−1 represents the tri-s-triazine structure. The broad peaks located at 3000–3300 cm−1 are assigned to the stretching of N–H and C-H bonds.
In contrast to other methods, the FTIR spectrum also revealed changes in the g-C3N4 sample after catalytic treatment. The sample which was repeatedly exposed to the reaction (as revealed later) exhibited few differences. Most apparent is the loss of sharpness of individual peaks. It is clearly evident in the extra vibrational region between 3000–3300 cm−1, although not directly related to the g-C3N4 structure, it is commonly found in this compound, [29,30] and can be associated with H2O absorption and possible N-H bonds at the surface. In this particular region also C-H vibrations are revealed [30,31] and could similarly be associated with the surface changes.
Microscopic analysis of the prepared photocatalyst is presented in Figure 6A. g-C3N4. appear curled, banded, and twisted and range in sizes from 0.5 to 1.5 µm. Thickness is hard to determine; however, it is much less than 100 nm. EDX which was also performed show the right ratio between carbon and nitrogen within expectation (inset Figure 6 B).
The results of N2 adsorption–desorption isotherm reveal the surface area of around 88.5 m2/g. With the help of Barret−Joyner−Halenda (BJH) method, the pore diameter was also estimated at 25.8 nm. The obtained result is higher than to the literature findings, based on similar preparation procedures for the g-C3N4 [31,32].

3.2. Photocatalytic Activity of Immobilised g-C3N4 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 °C, g-C3N4 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-C3N4 into the water. The stability of immobilised g-C3N4 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-C3N4 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·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-C3N4 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-C3N4 immediately starts to further reduce the amount of MB in the solution. The concertation 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.
Adsorption and degradation capabilities of g-C3N4 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-C3N4 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]. 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 (R2) 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-C3N4 photocatalyst on alumina substrate drives decolourisation and mineralisation to faster completion.
Developed CdO/Al2O3 nanocomposite by Janani 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]. Berger 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. The obtained results indicate the high photocatalytic stability of the immobilised g-C3N4 on alumina substrate and point towards significant reusability of such setup, which is highly adoptable for various device architectures.
The possible mechanism involved in the degradation of MB dye of the immobilised g-C3N4 is given schematically in Figure 10. Proposed mechanism presents the explanation of separation of photo-induced electron-hole pairs. When the energy photon (hν) is absorbed on the g-C3N4 surface, the electrons (e−) in valence band (VB) absorb photons to get excited and move towards conductive band (CB) leaving positive holes (h+) in VB. Furthermore, electron-hole pairs are produced which play a key role in oxidation and reduction processes. The photocatalytic efficiency of a photocatalyst depends on several crucial factors such as adsorption ability, morphological structure, and charge separation of photo-induced electron-hole pairs [42]. From the oxidation and reduction process hydroxyl (•OH )and superoxide radical (O2•−) are obtained, respectively, which react with MB dye pollutant to transform MB into non-hazardous compounds such as water and carbon dioxide [43]. A lot of research efforts were put into narrowing the energy bandgap while reducing electron-hole pair’s recombination by creating heterojunctions or doping metals into g-C3N4 as electron entrapment sites, generating highly oxidized holes to efficiently separate holes and electrons; therefore, photocatalytic performance improves [41].

4. Conclusions

The g-C3N4 catalyst was successfully prepared by thermal treatment using urea as a precursor at 550 °C in an air chamber and immobilised on an alumina ring substrate. The photocatalytic activity of the immobilised g-C3N4 catalyst was verified by the photodegradation of methylene blue (MB) dye in an aqueous solution under simulated solar irradiation. The results showed that immobilised g-C3N4 catalyst shows relatively high adsorption capacity of MB dye in the first 15 min. Further results of the photocatalytic activity show that decolourisation is more than two times faster than mineralisation. After three repetitions of the photocatalytic test, the photodegradation efficiency of MB was still high with negligible deterioration. These results confirmed the stability of the immobilised g-C3N4 catalyst on the alumina substrate and prove its usefulness due to its simple preparation method, cost-effectiveness, and environmental friendliness.

Author Contributions

Conceptualization, M.V., M.K., D.V. and M.P.; methodology, M.V.; software, M.V.; validation, M.V., M.K. and M.P.; formal analysis, M.V.; investigation, M.V. and M.K.; resources, M.P.; data curation, M.V.; writing—original draft preparation, M.V. and M.K.; writing—review and editing, L.Ć., T.R., D.V. and M.P.; visualization, M.K. and M.P.; supervision, M.P.; project administration, M.P.; funding acquisition, M.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The financial support of the Slovenian Research Agency is gratefully acknowledged (Project No. L2-1830 and Program No. P2-0084).

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 12 09704 g001 550
Figure 1. The obtained light yellowish bulk of g-C3N4.
Figure 1. The obtained light yellowish bulk of g-C3N4.
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Figure 2. Picture of immobilised g-C3N4 catalyst on alumina substrate.
Figure 2. Picture of immobilised g-C3N4 catalyst on alumina substrate.
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Figure 3. (A) X-ray diffractogram and (B) TGA analysis of g-C3N4.
Figure 3. (A) X-ray diffractogram and (B) TGA analysis of g-C3N4.
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Applsci 12 09704 g004 550
Figure 4. (A) UV-Visible diffuse reflectance spectra and (B) Kubelka–Munk transferred diffuse reflectance spectra of g-C3N4.
Figure 4. (A) UV-Visible diffuse reflectance spectra and (B) Kubelka–Munk transferred diffuse reflectance spectra of g-C3N4.
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Figure 5. FTIR patterns of g-C3N4.
Figure 5. FTIR patterns of g-C3N4.
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Figure 6. (A) SEM image of g-C3N4 powder and (B) SEM image of immobilised g-C3N4 on alumina surface along with EDXS conducted over the area.
Figure 6. (A) SEM image of g-C3N4 powder and (B) SEM image of immobilised g-C3N4 on alumina surface along with EDXS conducted over the area.
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Figure 7. (A) Monitoring adsorption and degradation of alumina substrate (red) and immobilised g-C3N4 on alumina substrate (blue), and (B) UV-vis absorbance spectra of initial concentration of MB dye and after 75 min photocatalysis.
Figure 7. (A) Monitoring adsorption and degradation of alumina substrate (red) and immobilised g-C3N4 on alumina substrate (blue), and (B) UV-vis absorbance spectra of initial concentration of MB dye and after 75 min photocatalysis.
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Figure 8. (A) Decolourisation (664 nm) and mineralisation (292 nm) of MB as a function of irradiation time under simulated solar irradiation and (B) plotted first-order rate constant (k).
Figure 8. (A) Decolourisation (664 nm) and mineralisation (292 nm) of MB as a function of irradiation time under simulated solar irradiation and (B) plotted first-order rate constant (k).
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Figure 9. Effect of recycling test of immobilised g-C3N4 on alumina substrate for photocatalytic degradation of MB.
Figure 9. Effect of recycling test of immobilised g-C3N4 on alumina substrate for photocatalytic degradation of MB.
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Figure 10. The possible mechanism of the photocatalytic degradation of MB at immobilised g-C3N4 catalyst on the alumina substrate when exposed under simulated solar irradiation.
Figure 10. The possible mechanism of the photocatalytic degradation of MB at immobilised g-C3N4 catalyst on the alumina substrate when exposed under simulated solar irradiation.
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Table
Table 1. The properties of the alumina powder provided by the manufacturer.
Table 1. The properties of the alumina powder provided by the manufacturer.
Chemical Composition (%)
Na2OFe2O3MgOSiO2CaOAl2O3
0.070.020.050.030.02rest
Physical and Ceramic Properties
BET
(m2 g−1)		d90
(μm)		d50
(μm)		Green dens.
(g cm−3)		Sintered dens.
(g cm−3)		Shrinkage
%
8.920.52.2 ± 0.13.93 ± 0.0518.5 ± 0.7
Table
Table 2. Physico-chemical properties of the understudied organic pollutant MB dye.
Table 2. Physico-chemical properties of the understudied organic pollutant MB dye.
Organic pollutantMethylene blue
IUPAC name3,7-bis(dimethylamino)-phenothiazin-5-ium chloride
Chemical formulaC16H18ClN3S
Molar mass/g·mol−1319.85
Chemical structure Applsci 12 09704 i001
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Vukšić, M.; Kocijan, M.; Ćurković, L.; Radošević, T.; Vengust, D.; Podlogar, M. Photocatalytic Properties of Immobilised Graphitic Carbon Nitride on the Alumina Substrate. Appl. Sci. 2022, 12, 9704. https://doi.org/10.3390/app12199704

AMA Style

Vukšić M, Kocijan M, Ćurković L, Radošević T, Vengust D, Podlogar M. Photocatalytic Properties of Immobilised Graphitic Carbon Nitride on the Alumina Substrate. Applied Sciences. 2022; 12(19):9704. https://doi.org/10.3390/app12199704

Chicago/Turabian Style

Vukšić, Milan, Martina Kocijan, Lidija Ćurković, Tina Radošević, Damjan Vengust, and Matejka Podlogar. 2022. "Photocatalytic Properties of Immobilised Graphitic Carbon Nitride on the Alumina Substrate" Applied Sciences 12, no. 19: 9704. https://doi.org/10.3390/app12199704

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