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Article

In Situ Synthesis and Characterization of Graphitic Carbon Nitride/Metakaolin Composite Photocatalysts Using a Commercial Kaolin

by
Balázs Zsirka
1,*,
Orsolya Fónagy
2,*,
Veronika Vágvölgyi
1,
Tatjána Juzsakova
3,
Lajos Fodor
2 and
Csilla Őze
4
1
Research Group of Analytical Chemistry, Center for Natural Sciences, University of Pannonia, P.O. Box 158, 8201 Veszprem, Hungary
2
Research Group of Environmental and Inorganic Photochemistry, Center for Natural Sciences, University of Pannonia, P.O. Box 158, 8201 Veszprem, Hungary
3
Sustainability Solutions Research Laboratory, Research Centre for Biochemical, Environmental and Chemical Engineering, University of Pannonia, P.O. Box 158, 8201 Veszprem, Hungary
4
Department of Materials Engineering, Research Centre for Engineering Sciences, University of Pannonia, P.O. Box 158, 8201 Veszprem, Hungary
*
Authors to whom correspondence should be addressed.
Crystals 2024, 14(9), 793; https://doi.org/10.3390/cryst14090793
Submission received: 7 August 2024 / Revised: 29 August 2024 / Accepted: 1 September 2024 / Published: 7 September 2024
(This article belongs to the Section Hybrid and Composite Crystalline Materials)
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Abstract

:
Kaolin-based graphitic carbon nitride (g-CNx) composite photocatalysts were synthesized from a urea precursor using a commercial kaolin. Structural characterization by X-ray diffraction and infrared spectroscopy (FTIR) verified the successful thermal polycondensation of g-CNx along the thermal dehydroxylation of kaolinite to metakaolin at 550 °C. The g-CNx content of the composites were estimated by thermogravimetry and CHN analysis, ranging from ca. 87 m/m% to ca. 2 m/m% of dry mass. The addition of kaolin during the composite synthesis was found to have a significant effect: the yield of in situ formed g-CNx drastically decreased (from ca. 4.9 m/m% to 3.8–0.1 m/m%) with increasing kaolin content. CHN and FTIR indicated the presence of nitrogen-rich g-CNx, having a specific surface area of 50 m2/g, which synergistically increased after composite synthesis to 67–82 m2/g. Estimated optical band gaps indicated the affinity to absorb in the visible light spectrum (λ < 413 nm). Photocatalytic activity upon both UV and artificial sunlight irradiation was observed by hydroxyl radical evolution, however, without the synergistic effect expected from the favorable porosity.

1. Introduction

Increasing levels of anthropogenic impact on the environment and its subsequent degradation are major driving forces in finding novel ways to remedy the harmful effects. Various, harmful substances in the environment can be neutralized via remediation methods applying physical, chemical, bio-, or thermal processes. Catalytic degradation into harmless products is one of the most desired goals. The use of photocatalysts is particularly promising to satisfy both green chemistry and efficient degradation requirements. Semiconductor photocatalysts are excited by adsorbing a suitable portion of the electromagnetic radiation (mostly photons in the ultraviolet (UV) or visible light spectrum), resulting in an excited, negative electron in the conduction band and a positive hole in the valence band. This phenomenon can induce various redox reactions and the generation of active radicals, which can be applied to degrade a variety of polluting chemical compounds [1].
A substantial part of photocatalysis costs is the energy demand [2]. This can be decreased if the visible light spectrum of natural sunlight can be harnessed, instead of operating artificial radiation sources like UV lamps. There are a variety of semiconductors with a suitable electronic structure to be excitable by photons in the visible light region, such as sulfides (CdS, ZnS, etc.), oxides (Fe3O4, Bi2WO6, WO3, etc.) or graphitic carbon nitride (g-CNx). The latter has attracted tremendous attention from researchers in the last one and a half decades and is still of interest [3]. Graphitic carbon nitride is an n-type semiconductor material. Its metal-free, biodegradable, non-toxic nature, and particularly its narrow band gap (2.7 eV, 459 nm), make it a desired potential material for photocatalysis [4,5]. It is generally formed via the easily applied thermal polycondensation of various precursors, such as melamine, urea, or thiourea, among others [6,7]. Graphitic carbon nitride photocatalysts, and their modified derivatives, can be applied for the adsorption and visible-light-driven photocatalytic degradation of different dyes [8,9], organic compounds or antibiotics [3,10], nitrogen oxide decomposition, carbon dioxide reduction, or for evolving hydrogen from water [7,11,12], among others. Major disadvantages of g-CNx materials are generally their fast recombination of photoinduced electrons and holes, their low quantum efficiency, and that they have a low specific surface area and poor quantum yield [4,13].
There are many ways to counter the drawbacks of g-CNx, including structural modification by doping, introducing defect sites, and the surface deposition of various metals and metal oxides to create heterojunctions [3,14,15]. One of the most commonly applied methods is composite synthesis. The application of clay minerals as adsorbents and photocatalysts is quite promising in the field of environmental remediation [16], and they can also be utilized to form composite photocatalysts with g-CNx to improve its intrinsic properties [17,18]. By using non-toxic, natural minerals, such as kaolin, sustainable and environmentally friendly photocatalysts can be produced.
Kaolin-based g-CNx composites can have various purposes, including the photocatalytic evolution of CO and H2 using solar light [19,20], or the adsorption [21,22] and degradation of various polluting compounds in aqueous [23,24] and gas [25] phases. The clay mineral component is beneficial to enhance the dispersion of semiconductor catalysts and the adsorption of the substrate to improve its porosity and surface area, and to aid better recovery after use [17]. Moreover, it might contribute to the enhancement of the lifetime of photoinduced electron–hole pairs [26], thus improving the photocatalytic efficiency overall. There are several reports of kaolin-based g-CNx composite photocatalysts in the literature; however, the effect of kaolin addition and the resulting g-CNx content are often not systematically investigated and reported for the urea precursor [17,19,23,26], while the yield of g-CNx could vary during the in situ synthesis.
The aim of the current study is to synthesize kaolin-based graphitic carbon nitride composite photocatalysts with varying amounts of initial kaolin content and to investigate its effect on the properties. A commercially available kaolin product is chosen for this purpose, which might be advantageous, as the individually sourced kaolins mined at different locations are often difficult to procure by third-party research groups. The synthesis utilizes the simultaneous thermal treatment of kaolin and urea resulting in the in situ formation of g-CNx and the dehydroxylation of kaolinite to metakaolin, which could be beneficial in improving the morphology and specific surface area. The resulting g-CNx yield is determined as a function of initial kaolin content to aid similar composite catalysts’ design. In harmony with one of the recent, major aims of photocatalysis, the photoinduced catalytic efficiency of the synthesized composite photocatalysts is probed via the evolution of hydroxyl radicals (•OH) upon both UV and artificial sunlight irradiation.

2. Materials and Methods

2.1. Sample Preparation, Designation, and Applied Chemicals

Heat treatment was carried out in a Nabertherm L9/11/B410-type muffled furnace (Nabertherm GmbH, Lilienthal, Germany) by placing the ground compounds in a covered ceramic crucible and heating up to 550 °C with a 2 °C/min heating rate, then a 4 h isothermal section was applied to allow heat and mass transfer reactions and thermal polycondensation to proceed. Samples were then cooled down to room temperature, gently grounded using an agate mortar and pestle, weighed, and subjected to characterization.
The heat-treated kaolin sample is designated as K (550 °C), while the pristine, synthesized graphitic carbon nitride is referred to as g-CNx. Their composites have a varying amount of initial kaolin content (50, 100, 200, 300, 500 mg) beside 15 g of the urea precursor. Sample names reflect the amount of initial kaolin with increasing masses as follows: g-CNx_K-50, g-CNx_K-100, g-CNx_K-200, g-CNx_K-300, and g-CNx_K-500. The exact initial composition for each sample is given in Table A1. The estimated g-CNx content, determined by a thermal analysis and CHN analysis (see Section 3.1.2 and Table 1), is found to be varying from ca. 86.91 m/m% (g-CNx_K-50) to ca. 1.95 m/m% (g-CNx_K-500) with increasing initial kaolin contents related to dry mass.
Compounds of coumarin (Acros Organics, puriss), urea (Reanal, purity > 99.5%), and MilliQ water (conductivity < 0.056 μS/cm) were used in this investigation, while the commercially available kaolin sample was obtained from Sigma-Aldrich (lot number: BCBM2772V, Steinheim, Germany). Muscovite, quartz, and microcline were identified as mineral phases beside the dominant kaolinite phase (see Section 3.1.1, Figure A1), while the kaolinite content of the kaolin sample was calculated to be 91.2 m/m% by thermal analysis.

2.2. Analytical Methods

X-ray powder diffraction (XRD) measurements were carried out using a Philips PW 3710-type instrument (Philips Analytical B.V., Almelo, The Netherlands, CuKα radiation, λ = 1.54056 Å, 50 kV, 40 mA), in the range of 6–70° 2θ with a scanning speed of 0.02°/s and a 1 s dwell time. Calcined Al2O3 was used as the standard sample for estimating the instrumental broadening in the calculation of average crystallite sizes. Powder Diffraction File cards were utilized for the assignment of XRD reflections: graphitic carbon nitride (PDF 00-066-0813), kaolinite (PDF 00–014-0164), quartz (PDF 00-033-1161), muscovite (PDF 00-007-0025), and microcline (PDF 00-019-0926).
The infrared spectra of ground samples were recorded using a Bruker Vertex 70-type Fourier-transform infrared (FTIR) spectrometer (Bruker GmbH Rosenheim, Germany) equipped with a Bruker Diamond ATR sample compartment, which was operated at a resolution of 2 cm−1 with a room temperature DTGS detector. The final spectra were acquired by averaging 512 scans with atmospheric compensation.
Thermoanalytical measurements were carried out in a Netzsch TG-209-type thermobalance (NETZSCH-Gerätebau GmbH, Selb). Powdered samples were heated to 1000 °C with a 10 °C/min heating rate in lid-covered ceramic crucibles under dynamic argon flow (Messer, 99.998%). The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves were recorded.
Nitrogen adsorption measurements were carried out using a high-resolution Micromeritics 3Flex 3500 gas adsorption analyzer (Micromeritics Instrument Corporation, Norcross, GA USA). Samples were degassed using a Micromeritics FlowPrep 060 preparation unit (160 °C, 4 h). The specific surface area (SSA) was determined by using the Brunauer–Emmett–Teller (BET) method [27], utilizing the linear range (p/p0 = 0.05–0.45). The Barret–Joyner–Halenda (BJH) model was used to determine the mesopore volume distribution, total pore volume, and average pore diameter values, while the t-Plot method was applied to calculate micropore surface and volume. Measurements were conducted at liquid nitrogen temperature (−196 °C) in the 1.7–100 nm diameter ranges. Micropores were considered to have widths not exceeding 2 nm, mesopores to have widths between 2 and 50 nm, and macropores to have widths exceeding 50 nm.
Electron beam measurements were carried out using a FEI Talos F200X-type electron microscope (X-FEG electron source, operated at 200 kV accelerator potential; Thermo Fisher Scientific Inc., Brno, Czeh Republic). Samples were dispersed in MilliQ water and drop-dried at 60 °C onto a lacy carbon-coated copper grid. Images of the samples were recorded in transmission (TEM) and scanning (STEM) modes, while elemental composition maps were recorded based on the energy-dispersive X-ray spectra, using SuperX EDX detectors.
Optical band gaps were estimated by measuring the diffuse reflectance spectra using a Specord S600 spectrofluorometer with an integrating sphere module (Analytik Jena GmbH Co., Jena, Germany, 300–800 nm range, coaddition number: 8). Samples were drop-dried onto a flat sample carrier and dried out at room temperature to form a continuous flat surface before measurement.
The carbon, hydrogen, and nitrogen (CHN) contents of the samples were analyzed with a Carlo Erba EA 1108 CHNS-O elemental analyzer (Erba Science GmbH). The measurements were performed in tin capsules within a quartz combustion reactor at 1000 °C, using a 10 mL oxygen loop and helium (Messer, 99.996%) as the carrier gas. Calibration was made using sulfanilamide and bitumen–asphalt standards (Elemental Microanalysis Ltd., Okehampton, United Kingdom). The resultant gases were separated on a 2.5-metre stainless steel gas chromatography column and quantified with a thermal conductivity detector (TCD), with a measurement time of 10 min and a range of 100 ppm to 100%.
Photocatalytic activity investigations were performed by irradiating a continuously stirred 50 mL of 10−4 M coumarin solution with 50 mg of the catalyst sample in a closed reactor vessel using two different light sources: one in the UV range and a solar simulator. The former experimental setup utilized dual 14W λmax = 365 nm UV lamps at 14 cm distances on either side of the photoreactor, while in the latter setup, the artificial sunlight was produced by an Oriel LCS-100-type solar simulator with a 100 W Xe lamp, which illuminated a single side of the reaction vessel. In both cases, after mixing the catalyst and the test solution using a magnetic stirrer (500 rpm) and a brief ultrasound-aided homogenization (Elmasonic-P equipment, 37 kHz, 1 min), the first sample was taken after 20 min in the dark, and then the light source was turned on and sampling was completed every 60 min for 5 h. All samples were filtered through a 0.45 μm PTFE syringe filter and then measured by UV–Vis emission spectroscopy, using a Perkin Elmer LS 50B type spectrofluorometer (Perkin-Elmer Ltd., Beaconsfield, United Kingdom). The liquid samples were scanned after excitation at 332 nm in the 360–600 nm range with a scan speed of 5 nm/s. The final spectra were generated by the coaddition of 3 measurements. The theoretical background and interpretation for coumarin hydroxyl radical measurements can be found in Section 3.2.

3. Results and Discussion

3.1. Synthesis and Characterization

3.1.1. X-ray Diffraction (XRD) and Infrared Spectroscopy (FTIR-ATR)

As a result of prolonged heat treatment at 550 °C, the powder X-ray diffraction (XRD) pattern of K (550 °C) shows the absence of characteristic, crystalline kaolinite reflections (Figure 1 and Figure A1). This is due to the thermal dehydroxylation of the octahedral alumina sheet, resulting in the formation of an amorphous, mullite-like, metakaolin phase [28]. Three minor, mineral pollutant phases of the commercially available K sample are identified [29,30]: quartz, muscovite, and microcline. These reflections remain mostly unchanged upon calcination (Figure A1), as muscovite dehydroxylation is only expected at higher temperatures [29], while the alkali feldspar microcline and quartz can be considered as inert.
Graphitic carbon nitride is the sole identified component on the XRD pattern of the g-CNx sample (Figure 1), indicating successful synthesis. The two major reflections centered around 12.9° and 27.4° 2 theta can be assigned to the intra- and inter-layer distances of the graphitic carbon nitride structure. The 27.4° 2 theta inter-layer reflection corresponds to the (002) plane with a 3.24 Å distance between the stacked graphitic carbon layers, while the 12.9° 2 theta (210) reflection indicates the periodic presence of voids in the planar structure of the layers. The latter reflection is assigned to the gap between neighboring, hydrogen-bonded triazine units of the g-CNx sheets [31,32,33]. Taking into consideration that multiple reflections are located near the observed maxima at 27.4° 2 theta, the average number of layers in a graphitic carbon nitride crystallite is calculated to be 5.1 nm by using the Scherrer equation [34]. This corresponds to about 13 layers of g-CNx being stacked in an average crystallite. It was reported that the number of layers can influence photocatalytic activity, as nanostructured g-CNx shows better photocatalytic activity than the bulkier crystallites with a greater number of layers [3,35,36].
The kaolin-based graphitic carbon nitride composites from g-CNx_K-500 to g-CNx_K-50 show increasing intensity reflections at 12.9° and 27.4° 2 theta (Figure 1), indicating the presence and increasing quantity of graphitic carbon nitride content. In the case of g-CNx_K-500, the presence of g-CNx is not unambiguously evident by the diffraction pattern.
The Fourier-transform infrared (FTIR) adsorption bands of g-CNx correspond to the typical vibrations of crystalline, polymerized graphitic carbon nitride structure (Figure 2). Related to the carbon nitride molecular structure, various C-N and sp2 C-C vibration modes are identified in the 1700–700 cm−1 region [32,37]. The intense band at 806 cm−1 is assigned to a C-N (out-of-plane) deformation vibration, indicating the presence of triazine units [38], while the bands at 1630 cm−1 and 1563 cm−1 might be attributed to their in-plain stretching vibrations [39,40]. Various adsorption bands in the 1450–1200 cm−1 region are present due to the aromatic C-N stretching vibrations. The 3100, 3181, and 3268 cm−1 bands could be assigned to terminal N-H stretching vibrations [39]. Minor amounts of adsorbed water might be present, overlapping with the C-N vibration peaks around 3600–3000 cm−1 (stretching) and 1630 cm−1 (deformation) [41]. The observations are in agreement with the literature, and along with the XRD results, confirm the successful synthesis of graphitic carbon nitride.
The spectrum of the K (550 °C) sample is dominated by the presence of Si-O-Si vibrations (1250–400 cm−1), with the presence of some minor Al-O vibrations (900–400 cm−1). The intense band at 1035 cm−1 and the shoulders at 1200 and 1121 cm−1 are assigned to the Si-O-Si stretching vibration modes of the silica framework, while the intense 420 cm−1 band is due to its deformation (bending) mode [42]. Instead of a double, split Si-O-Si band observed in crystalline kaolinites [28], the single band maxima at 1035 cm−1 and the lack of distinctive Al-OH vibrations in the 3700–3620 cm−1 region (Figure A2) support the observation that the kaolinite mineral is completely dehydroxylated as a result of heat treatment [43,44]. Besides the split bands at 796 and 774 cm−1, the band at 691 cm−1 is a good indicator [45] of the presence of crystalline quartz, beside the majority of amorphous Si-O from the metakaolin phase (XRD, Figure 1).
The spectrum of g-CNx_K-500 shows the major bands of heat-treated K (550 °C) samples, which are assigned to the structural vibrations of the dehydroxilated aluminosilicate backbone and quartz. After a closer examination (Figure 2B inlet), adsorption bands at 1630, 1563, and 1452 cm−1 are visible on the g-CNx_K-500 spectra, corresponding to the C-C and C-N stretching vibration modes of g-CNx [32]. This complements XRD data and verifies the presence of minor amounts of graphitic carbon nitride in the K-500 composite. The Si-O-Si band shifted to 1020 cm−1, indicating a slightly different chemical environment that is most probably the connection of g-CNx to the Si-O surface for the sample. This phenomenon is also visible for g-CNx_K-300 and somewhat for g-CNx_K-200 samples, though its intensity drastically decreases with increasing g-CNx content, making it difficult to identify for g-CNx_K-100 and g-CNx_K-50 samples.

3.1.2. Thermal Analysis (TG/DTG) and CHN Measurement

The thermogravimetric analysis of g-CNx sample indicates its low adsorbed water/volatile content, based on the small mass loss step (1.34 m/m%) until 200 °C. Minor impurities might be present based on the second thermal step (200–430 °C, 0.45 m/m%), while the major decomposition of g-CNx is evident between 430 and 800 °C [46]. The pyrolytic decomposition in nitrogen atmosphere [7] is indicated by this mass loss step (Figure 3/g-CNx, DTG curve), which slowly starts after 430 °C, then speeds up after 550 °C, and reaches its maximum at around 710 °C. The process terminates at 800 °C. This clearly indicates the higher thermal stability of graphitic carbon nitride compared to its urea precursor (Figure A3A), as the latter decomposes up to 400 °C [47]. No residual mass is observed after 800 °C. Considering the water-free mass of the g-CNx sample (m = 5.193 mg) and the major thermal decomposition step (m = 5.166 mg), the purity and graphitic carbon nitride content of the sample can be estimated as 99.48 m/m% related to dry mass (Table 1), which corresponds well with the purity of the applied urea precursor (see Section 2.1).
TG/DTG curves of the heat-treated K (550 °C) sample indicate a minor water content (0.57 m/m%) and that no major thermal process is taking place at temperatures up to 550 °C (Figure A3B). This is expected as the thermal dehydroxylation of kaolinite to metakaolin [28,44] was completed during the heat treatment (see previously, Section 3.1.1). The minor mass loss between 550 and 1000 °C might be indicative of the muscovite co-mineral dehydroxylation [29].
The graphitic carbon nitride/metakaolin composite samples display the above-mentioned thermal processes (Figure 3, from g-CNx_K-50 to g-CNx_K-500), having varying amounts of adsorbed water (0.83–8.73 m/m%). Consequently, the 430–800 °C thermal mass loss step can be utilized to estimate the graphitic carbon nitride content of the samples. The results and the initial and final masses of the synthesis are summarized in Table 1 and Table A1. The estimation is given for both dry and wet mass; the dry mass is more beneficial to eliminate the effect of variable adsorbed water content, while the wet mass is more suitable for the direct comparison of data obtained under similar circumstances (e.g., with CHN analysis). The g-CNx content of the dry composites were found to be varying from 1.95 m/m% (g-CNx_K-500) to 86.91 m/m% (g-CNx_K-50). The presence of kaolin decreased the yield: a higher initial kaolin content resulted in a smaller g-CNx yield, varying from ca. 0.1% to 3.8% of the initial urea amount (Table 1). The estimation from wet mass follows a similar trend, with slightly smaller values due to the additional presence of water. Examining the case of dry g-CNx-K200 and g-CNx-K300 shows that increasing the initial kaolin content during synthesis only by 0.67 m/m% results in a significant drop in g-CNx content from 70.0 m/m% to 36.25 m/m%. Further increasing it by 1.33% causes the g-CNx content to be only 1.95 m/m%.
The composition of g-CNx was also probed by a CHN analysis (Table A2), verifying the analyzed trend from TG/DTG. The values of wet TG/DTG should be compared with a CHN analysis. The established g-CNx content values have a relatively good agreement (TG/DTG vs. CHN analysis) with an average deviation of ±1.8% (dry) and ±1.3% (wet), and the minima/maxima being 0.38%, 7.06% (dry), and 1.49%, 10.33 (wet)%. The N/C ratio was found to be relatively constant (1.7) for the pristine and composite samples, which conforms the literature on using the urea precursor [41] and indicates that nitrogen-rich g-CNx is synthesized uniformly. An exception is the g-CNx-K500 sample, where the conditions causing the small yield might have altered the composition as well, since the N/C ratio is significantly closer to the ideal value of 1.3 for g-C3N4. Higher N/C values indicate an increased nitrogen content in the g-CNx structure, which is assigned to the presence of uncondensed NH/NH2 groups and is generally reported in the literature [48,49,50].
Table 1. Comparison and estimation of g-CNx content by TG/DTG and CHN analysis. Results from TG/DTG are given for both wet and dry mass (see Section 3.1.2, Figure 3).
Table 1. Comparison and estimation of g-CNx content by TG/DTG and CHN analysis. Results from TG/DTG are given for both wet and dry mass (see Section 3.1.2, Figure 3).
by TG/DTG Analysisby CHN Analysis
Samplem/m%
(Dry Mass)		Yield from Urea (m/m%)m/m%
(Wet Mass)		Yield from Urea (m/m%)m/m%Yield from Urea (m/m%)
g-CNx98.48%4.90%98.13%4.83%95.09%4.68%
g-CNx-K5086.91%3.79%79.32%3.46%89.65%3.91%
g-CNx-K10080.81%3.10%76.83%2.94%75.34%2.89%
g-CNx-K20070.00%2.64%66.47%2.51%62.94%2.37%
g-CNx-K30036.25%1.05%33.82%0.98%36.63%1.06%
g-CNx-K5001.95%0.07%1.93%0.06%4.70%0.16%
The observed, small yield conforms with reports from the literature for the urea precursor [39,51]; however, since the g-CNx yield is not constant when kaolin is present, the worsening synthesis efficiency upon the increasing kaolin content should be taken into consideration when designing kaolin-based composite systems. The phenomenon might be explained by the adverse effect on the thermal polycondensation reaction in the presence of kaolin, a 1:1-type phyloaluminosilicate, undergoing thermal dehydroxylation [52,53], which could be influenced by the surface acidity and minor, catalytically active nature of the layer complex [54].

3.1.3. Nitrogen Adsorption Measurements and Transmission Electron Microscopy with Energy Dispersive X-ray Analysis (TEM-EDX)

The specific surface area (SSA) of photocatalysts is an important feature, as it generally influences their catalytic performance; a higher SSA value could result in higher photocatalytic activity [55,56].
The low SSA value (Table 2, 8 m2/g) of heat-treated K (550 °C) samples is within the lower-end of the characteristic SSA values of kaolins [57,58]. The 50 m2/g SSA of the synthesized graphitic carbon nitride is slightly higher than the typical 10–30 m2/g SSA range [59,60], though this conforms the reports for porous, urea-derived g-CNx [39]. The higher SSA value was foreshadowed by the presence of intense, terminal N-H vibrations in the FTIR-spectra typical of smaller, more fragmented units of g-CNx, and the low-stacked crystallites indicated by XRD calculations (see Section 3.1.1). The pore volume and size of the samples follow a similar trend as the SSA values, while the heat-treated kaolin sample is dominated by larger pores than the g-CNx sample.
Conforming literature reports [19], the SSA of g-CNx/metakaolin composite samples are increased by 34%–64%, compared to the pure g-CNx (Table 2). Pore surface area and volume generally increase when the g-CNx content of the composites are dominant (g-CNx_K-50 to g-CNx_K-200, mg-CNx > 70 m/m%, see Table 1). The highest SSA and total pore volume values belong to the dry g-CNx_K-100 sample having approx. 80.8% g-CNx content. In case of g-CNx_K-300 sample, where the dry g-CNx content is around 36 m/m%; a ca. 6% reduction in the micro surface area and volume is observed, along with an overall ca. 109% increase in total pore volume and a ca. 34% improvement in SSA. When the g-CNx content is almost negligible (ca. 2%, g-CNx_K-500), the porosity parameters show only a minor improvement for micropore surface and volume compared to the K (550 °C) sample. The observations suggest that the kaolin-based composites display a synergistic effect in the improvement of catalytically important specific surface and porosity parameters. The metakaolin content plays an important role, as even a ca. 2% g-CNx content can have an advantageous, though minor, effect for the improvement of porosity.
The synthesized g-CNx sample shows a two-dimensional, wavy fabric-like morphology in Figure 4A, which is characteristic of graphitic carbon nitride [61]. The observed porous, flaky carbon nitride polymeric fabric is typical of urea-derived g-CNx [62], and significantly contributes to the higher BET-SSA value of the sample. Individual crystallites are difficult to clearly identify; however, the high-resolution TEM images indicate the presence of 25 to 30-layered g-CNx crystallites (Figure A4), which is higher than the average value of 13 indicated by the XRD results (see Section 3.1.1). The difference can be explained by the nature of the chemical information from the two different methods, as XRD provides an average of the irradiated sample portion, while the TEM measurement necessarily probes a significantly lower amount of samples [63]. Therefore, extrapolation and generalization from a TEM-like nanoanalysis for the average of the whole sample should be cautiously applied. Images of g-CNx_K-50 and g-CNx_K-300 composite samples (Figure 4B,C) indicate slight morphological changes [26] and the presence of pseudo-hexagonal and fragmented shapes with varying sizes from a few to sub micrometer ranges among the g-CNx crystallites. Morphological changes of g-CNx, and its dispersion among the metakaolin phase, might be responsible for the synergistic SSA improvement observed for the composites.
TEM-EDX elemental maps (Figure 5) confirm the composition of g-CNx and verify the presence of the aluminosilicate in both composite samples having a ca. 13.1 m/m% and 63.8 m/m% dry metakaolin phase (Table 1). Heat treatment above dehydroxylation temperature might result in morphological changes of 1:1-type phylloaluminosilicates [54], which could explain the presence of aluminosilicates having halloysite-like, tubular, or irregular, fragmented morphology, beside the typical pseudo-hexagonal shapes of kaolinite (Figure 4 and Figure 5). Occasionally, an alumina-bearing phase could also be identified (Figure 5, g-CNx_K-300 Al vs. Si maps), which is attributed to a co-mineral phase of the natural kaolin sample.

3.2. Photoinduced Catalytic Activity Measurement

The photocatalytic activity of the graphitic carbon nitride sample and its composite derivatives were tested by using the coumarin method. Coumarin is a direct hydroxyl radical (•OH) scavenger, which forms hydroxylated derivatives in the process, 7-hydroxy-coumarin among them. The amount of 7-hydroxy-coumarin can be determined by the 449 nm emission maxima by UV–Vis fluorescence spectroscopy, which is proportional to the amount of photogenerated •OH [64,65]. The determination of photogenerated radicals (e.g., •OH, O2•−, HO2•), responsible for the oxidative degradation of various chemical compounds, offers a more generalized and therefore advantageous way of characterization than the degradation or discoloration of a test compound. The latter method also carries an increased risk of incorrect analysis [8] (e.g., reversible spectral changes due to pH change and the modification of the chromophore groups without proper degradation [66]), especially when dye discoloration tests are applied for visible-light photocatalysts. The electronic structure of g-CNx results that direct •OH formation is thermodynamically unfavorable; however, •OH could be indirectly formed from photogenerated O2•− and H2O2 [67,68]. It is noted that hydroxyl radical formation is enhanced when using crystalline g-CNx [69].

3.2.1. Band Gap Estimation

The optical band gap is a crucial attribute for a photocatalyst, as it determines the minimal required photon energy for excitation. The optical band gap values were calculated using the Tauc method [70] using the Kubelka–Munk transformed absorption spectra of the samples. The Tauc plots are summarized in Figure A5, while the numerical values of the estimated optical band gaps are given in Table 3.
The estimated band gap of the g-CNx sample is higher than the generally reported ca. 2.7 eV for bulk graphitic carbon nitrides [4]. Similarly, higher band gap shifts reported in the literature [71,72] are attributed to the differences in size and morphology, as the quantum confinement effect might be more prominent in nanosized g-CNx than in their bulk versions. The observed smaller layer sizes and higher SSA values of the g-CNx reported in this study (see Section 3.1.1 and Section 3.1.3) could also support the explanation for the observed shift of the band gap to 2.98 eV.
The kaolin-based graphitic carbon nitride composites have similar, though slightly higher, shifted band gap values than the pristine g-CNx (3.002–3.004 eV). Although the intensity is very low, even the g-CNx_K-500 sample, containing only ca. 2% g-CNx, displays a minor affinity for visible light adsorption (Table 1, Figure A5). In harmony with the literature [8,23], the composites samples are expected to adsorb a narrow spectral region of sunlight (λ < 413/408 nm), thus they could potentially be viable visible-light photocatalysts.

3.2.2. UV Light Irradiation

UV light irradiation investigation aims to provide a general overview about the application potential of current g-CNx composite photocatalysts, simulating a possible industrial application environment where efficient UV light sources are applied.
The amount of photogenerated hydroxyl radicals, closely correlating with the photoinduced catalytic activity, upon a λmax = 365 nm ultraviolet light irradiation are summarized in Figure 6A. A typical emission spectrum of g-CNx and the read-out process is illustrated in Figure A6. The amount of formed •OH radicals can be considered low, which is in harmony with reports from the literature for pristine g-CNx [69,73].
The minor amount of hydroxyl radicals observed for K (550 °C) sample can be related to the minor photocatalytic property of heat-treated phylloaluminosilicate, as reported previously by our group [54]. Despite the observed synergistic effect of increased SSA (see Section 3.1.3), and thus the expected photocatalytic enhancement [19,26], the amount of hydroxyl radicals photogenerated on the surface of graphitic carbon nitride composite samples are less than that of pristine g-CNx. The photocatalytic activity decreases in relation to the g-CNx content of the composites; however, minor variations might be related to the differences in sample properties (e.g., crystallinity [69] or surface acidity) of the semiconductor g-CNx phase, critically influencing the •OH radical generation potential [74,75] and thus the estimated photocatalytic performance. It is interesting to note that the g-CNx-containing samples display a spectral change, appearing as a minor radical formation during the adsorption phase (see Figure A6, 0 min curve), even before UV (or later solar) light irradiation has commenced. The observations are most probably related to the applied, low frequency ultrasonic homogenization, where minor amounts of •OH radicals are formed [76,77] and they react with the coumarin; however, this is not related to the photocatalytic activity of the sample itself.

3.2.3. Artificial Solar Light Irradiation

Based on the optical band gaps and spectral emissions of the solar simulator (Table 3 and Figure A7), approximately 8.5–9.3% (band gaps of 408 nm and 413 nm, respectively) of the produced artificial sunlight intensity can be harnessed by the composite photocatalysts.
The synthesized g-CNx and its kaolin-based composite samples are catalytically active upon solar irradiation (Figure 6B), conforming what has been published in the literature [3,4,26]. The efficiency of visible-light photocatalysts are expected to be lower, due to irradiation with less energetic photons [8,78]. In harmony with this principle, the observed amounts of hydroxyl radicals are reduced by 25–80% when a solar simulator is applied (Figure 6A vs. Figure 6B). Similar to UV irradiation, the most efficient photocatalyst is the pristine g-CNx sample, followed by g-CNx_K50, having the highest graphitic carbon nitride content. The efficiencies for the g-CNx_K100 to g-CNx_K500 samples show similar, but non-tendentious, values after 300 min (Δ intensity between 5.0 and 7.9), with g-CNx_K500 being the lowest and g-CNx_K300 the highest among them. The heat-treated kaolin sample shows a 44% reduction in the generated, minor amount of 7-hydroxil coumarin concentration. It should be noted that only ca. 2 m/m% g-CNx of the g-CNx_K500 sample resulted in a more than two-fold increase in photocatalytic activity, underlying a possible advantage of surface modification. The observed photoactivity and the irradiation with visible light could be an advantage, as more energetic irradiation could cause structural alterations and degradation to g-CNx [79,80].

4. Conclusions

Understanding synthesis parameters is fundamental in catalyst design. It was shown that the kaolin content has a major impact on the in situ yield of g-CNx and therefore the composition of kaolin-based composites. Kaolin-based graphitic carbon nitride composite photocatalysts were synthesized via an in situ thermal polymerization at 550 °C from the urea precursor and along with the thermal dehydroxylation of a commercially available kaolin. The successful synthesis of g-CNx was verified by material characterization methods (XRD, IR), while the g-CNx content of the composites was estimated by a thermal analysis (TG/DTG) and CHN analysis, ranging from ca. 2 m/m% to 87 m/m% of dry mass. Applying varying amounts of untreated kaolin mineral (50, 100, 200, 300, and 500 mg) among the urea precursor, the thermal polycondensation and subsequent yield of g-CNx was found to be decreasing (from ca. 4.9 m/m% to ca. 0.1 m/m%) with an increasing kaolin content of the composites. A minor increase from 1.33 m/m% to 2.00 m/m% of the initial kaolin precursor resulted in a further drop of yielded dry g-CNx content from ca. 36.25 m/m% to ca. 1.95 m/m%. The initial kaolin mass-dependent, varying yield should be taken into consideration when designing the in situ synthesis of kaolin-based g-CNx composites from the urea. A morphological characterization revealed a synergistic effect of specific surface area increments from 8 m2/g and 50 m2/g of the heat-treated kaolin and the g-CNx sample to 67–82 m2/g of the composite samples. Their optical band gaps were estimated to be around 3.002–3.040 eV, indicating an affinity for visible light adsorption below λ < 413 nm. The photoinduced catalytic performance was investigated by the hydroxyl radical generation potential upon UV and visible-light irradiation. Photoactivity in both spectral regions is observed; however, no synergistic effect of the composites is revealed, despite their advantageous porosity values. Despite the non-synergism, the observations indicate the potential of kaolin-based graphitic carbon nitride composite photocatalysts in the remediation of environmental pollutants by photocatalysis.

Author Contributions

B.Z.: Investigation, Formal analysis, Data Curation, Writing—Original Draft, Writing—Review and Editing, Visualization; O.F.: Conceptualization, Funding acquisition, Formal analysis, Data Curation, Writing—Original Draft, Writing—Review and Editing, Visualization, Investigation; V.V.: Investigation; T.J.: Investigation; L.F.: Investigation; and C.Ő.: Investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the ÚNKP-23-4 New National Excellence Program of the Ministry for Culture and Innovation from the source of the National Research, Development and Innovation Fund (O.F.). B. Zs. acknowledges the support of the TKP2021-NVA-10 project with support provided by the Ministry of Culture and Innovation of Hungary from the National Research, Development and Innovation Fund, financed under the 2021 Thematic Excellence Programme funding scheme.

Data Availability Statement

Data are contained within the article and Appendix A, or might be requested from the corresponding authors.

Acknowledgments

Measurements and aid for powder X-ray diffraction (Department of Materials Engineering, Éva Makó) and TEM microscopic (Nanolab, Péter Pekker) instrumentation at the University of Pannonia are gratefully acknowledged.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Crystals 14 00793 g0a1
Figure A1. Structural changes of untreated K sample upon isothermal 550 °C heat treatment (K 550 °C) by powder X-ray diffraction measurements. Designations: k: kaolinite, m: muscovite, q: quartz, and f: microcline.
Figure A1. Structural changes of untreated K sample upon isothermal 550 °C heat treatment (K 550 °C) by powder X-ray diffraction measurements. Designations: k: kaolinite, m: muscovite, q: quartz, and f: microcline.
Crystals 14 00793 g0a1
Crystals 14 00793 g0a2
Figure A2. Structural changes of untreated K sample upon isothermal 550 °C heat treatment (K 550 °C) by Fourier-transform Infrared spectroscopy using the Attenuated Total Reflection measurement mode.
Figure A2. Structural changes of untreated K sample upon isothermal 550 °C heat treatment (K 550 °C) by Fourier-transform Infrared spectroscopy using the Attenuated Total Reflection measurement mode.
Crystals 14 00793 g0a2
Crystals 14 00793 g0a3
Figure A3. Thermogravimetric measurements of: (A) the urea precursor used for graphitic carbon nitride synthesis and (B) the heat-treated K (550 °C) sample in Ar atmosphere. Note: The heating rate of (A) is 2 °C/min, matching the synthesis conditions and clearly indicating that all of the urea precursors are decomposed at 550 °C, where the thermal polycondensation of graphitic carbon nitride can proceed.
Figure A3. Thermogravimetric measurements of: (A) the urea precursor used for graphitic carbon nitride synthesis and (B) the heat-treated K (550 °C) sample in Ar atmosphere. Note: The heating rate of (A) is 2 °C/min, matching the synthesis conditions and clearly indicating that all of the urea precursors are decomposed at 550 °C, where the thermal polycondensation of graphitic carbon nitride can proceed.
Crystals 14 00793 g0a3
Table A1. Measured masses of g-CNx and its composites before and after calcination during laboratory synthesis.
Table A1. Measured masses of g-CNx and its composites before and after calcination during laboratory synthesis.
Initial MassesAfter Calcination
SampleKaolin (g)Urea (g)Total
Mass (g)		Kaolin (m/m%)Total
Mass (g)
g-CNx015.000915.00090.00%0.7383
g-CNx-K500.050115.000415.05050.33%0.6543
g-CNx-K1000.100315.000815.10110.67%0.5745
g-CNx-K2000.200115.000915.20101.33%0.5658
g-CNx-K3000.300215.000515.30072.00%0.4360
g-CNx-K5000.500215.000515.50073.33%0.5038
Table A2. CHN analysis results for g-CNx and its kaolin-based composites.
Table A2. CHN analysis results for g-CNx and its kaolin-based composites.
SampleN (m/m%)C (m/m%)H (m/m%)∑CHN (m/m%)N/C
g-CNx-K5002.311.670.724.701.38
g-CNx-K30022.4313.161.0536.631.70
g-CNx-K20039.0022.541.4162.941.73
g-CNx-K10046.7327.091.5375.341.73
g-CNx-K5055.8632.111.6889.651.74
g-CNx59.0034.291.8195.091.72
Crystals 14 00793 g0a4
Figure A4. Visual estimation of average carbon layers in the g-CNx sample. Note: Red lines highlight (A) 30 and (B) 25 layers in the indicated sections of the HR-TEM images.
Figure A4. Visual estimation of average carbon layers in the g-CNx sample. Note: Red lines highlight (A) 30 and (B) 25 layers in the indicated sections of the HR-TEM images.
Crystals 14 00793 g0a4
Crystals 14 00793 g0a5
Figure A5. A Tauc plot of investigated samples for the estimation of optical band gap by diffuse reflectance measurement. The inset highlights the small intensity Kubelka–Munk plot of g-CNx_K-500 and K (550 °C) samples.
Figure A5. A Tauc plot of investigated samples for the estimation of optical band gap by diffuse reflectance measurement. The inset highlights the small intensity Kubelka–Munk plot of g-CNx_K-500 and K (550 °C) samples.
Crystals 14 00793 g0a5
Crystals 14 00793 g0a6
Figure A6. Typical (A) emission spectra evolution of photogenerated 7-hydroxy-coumarin (A) and (B) the corresponding intensity change plot at 449 nm, related to the photogenerated amount of •OH radicals. The experimental data are given for sample g-CNx upon UV light irradiation.
Figure A6. Typical (A) emission spectra evolution of photogenerated 7-hydroxy-coumarin (A) and (B) the corresponding intensity change plot at 449 nm, related to the photogenerated amount of •OH radicals. The experimental data are given for sample g-CNx upon UV light irradiation.
Crystals 14 00793 g0a6
Crystals 14 00793 g0a7
Figure A7. Spectral emission of the applied solar simulator. Note: The designated area indicates the spectral region, where all of the g-CNx composite samples could absorb photons, based on their highest band gap values (λ < 408 nm, see Table 3). This area is ca. 8.5% of the overall spectral intensity of the produced emission.
Figure A7. Spectral emission of the applied solar simulator. Note: The designated area indicates the spectral region, where all of the g-CNx composite samples could absorb photons, based on their highest band gap values (λ < 408 nm, see Table 3). This area is ca. 8.5% of the overall spectral intensity of the produced emission.
Crystals 14 00793 g0a7

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Crystals 14 00793 g001
Figure 1. Powder X-ray diffraction patterns of the heat-treated kaolin sample (K (550 °C)), graphitic carbon nitride synthesized from the urea precursor (g-CNx), and its composites with varying g-CNx/metakaolin contents (from g-CNx_K-500 to g-CNx_K-50). Dashed lines indicate reflections of graphitic carbon nitride (yellow) and the most intense reflections for quartz (black). Miller indices are given in brackets.
Figure 1. Powder X-ray diffraction patterns of the heat-treated kaolin sample (K (550 °C)), graphitic carbon nitride synthesized from the urea precursor (g-CNx), and its composites with varying g-CNx/metakaolin contents (from g-CNx_K-500 to g-CNx_K-50). Dashed lines indicate reflections of graphitic carbon nitride (yellow) and the most intense reflections for quartz (black). Miller indices are given in brackets.
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Crystals 14 00793 g002
Figure 2. Infrared (FTIR-ATR) spectra of the (A) heat-treated kaolin sample (K (550 °C)), graphitic carbon nitride synthesized from the urea precursor (g-CNx), and its composites with varying g-CNx/metakaolin contents (from g-CNx_K-500 to g-CNx_K-50) in the 4000–400 cm−1 range. (B) Inlet for the increased magnification of the K (550 °C) and g-CNx_K-500 samples in the 1200–400 cm−1 region.
Figure 2. Infrared (FTIR-ATR) spectra of the (A) heat-treated kaolin sample (K (550 °C)), graphitic carbon nitride synthesized from the urea precursor (g-CNx), and its composites with varying g-CNx/metakaolin contents (from g-CNx_K-500 to g-CNx_K-50) in the 4000–400 cm−1 range. (B) Inlet for the increased magnification of the K (550 °C) and g-CNx_K-500 samples in the 1200–400 cm−1 region.
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Figure 3. Thermogravimetric measurements of graphitic carbon nitride synthesized from the urea precursor (g-CNx) and its composites with varying g-CNx/metakaolin content (from g-CNx_K-500 to g-CNx_K-50).
Figure 3. Thermogravimetric measurements of graphitic carbon nitride synthesized from the urea precursor (g-CNx) and its composites with varying g-CNx/metakaolin content (from g-CNx_K-500 to g-CNx_K-50).
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Figure 4. Transmission Electron Microscopic (TEM) images of (A): g-CNx, (B): g-CNx_K-50, and (C): g-CNx_K-300 samples.
Figure 4. Transmission Electron Microscopic (TEM) images of (A): g-CNx, (B): g-CNx_K-50, and (C): g-CNx_K-300 samples.
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Figure 5. STEM high-angle annular dark-field images and corresponding EDX elemental maps for g-CNx, g-CNx_K-50, and g-CNx_K-300 samples.
Figure 5. STEM high-angle annular dark-field images and corresponding EDX elemental maps for g-CNx, g-CNx_K-50, and g-CNx_K-300 samples.
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Crystals 14 00793 g006
Figure 6. Intensity changes of 7-hydroxy-coumarin emission intensity at 449 nm, indicating the photogenerated amount of •OH radicals upon (A) λmax = 365 nm UV and (B) artificial solar light irradiation.
Figure 6. Intensity changes of 7-hydroxy-coumarin emission intensity at 449 nm, indicating the photogenerated amount of •OH radicals upon (A) λmax = 365 nm UV and (B) artificial solar light irradiation.
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Table 2. Specific surface area (SSA), micropore surface area (Smicro), micropore volume (Vmicro), total pore volume (Vtotal), and average pore size (D) of the samples, determined by nitrogen adsorption.
Table 2. Specific surface area (SSA), micropore surface area (Smicro), micropore volume (Vmicro), total pore volume (Vtotal), and average pore size (D) of the samples, determined by nitrogen adsorption.
SamplesBET-SSA (m2/g)Smicro (m2/g)Vtotal (cm3/g)Vmicro (cm3/g)Daverage (nm)
K (550 °C)80.40.06960.0001532.4
g-CNx_K-50081.10.04790.0004328.6
g-CNx_K-300678.70.49900.0038232.9
g-CNx_K-2007613.00.42370.0057625.4
g-CNx_K-1008212.80.58160.0056630.1
g-CNx_K-507112.50.41260.0055625.2
g-CNx509.20.23900.0040621.9
Table 3. Optical band gap of g-CNx and its composites estimated by diffuse reflectance measurements.
Table 3. Optical band gap of g-CNx and its composites estimated by diffuse reflectance measurements.
Band Gap
SampleeVnm
K (550 °C)--
g-CNx_K-5003.040408
g-CNx_K-3003.002413
g-CNx_K-2003.005413
g-CNx_K-1003.002413
g-CNx_K-503.004413
g-CNx2.98416
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Zsirka, B.; Fónagy, O.; Vágvölgyi, V.; Juzsakova, T.; Fodor, L.; Őze, C. In Situ Synthesis and Characterization of Graphitic Carbon Nitride/Metakaolin Composite Photocatalysts Using a Commercial Kaolin. Crystals 2024, 14, 793. https://doi.org/10.3390/cryst14090793

AMA Style

Zsirka B, Fónagy O, Vágvölgyi V, Juzsakova T, Fodor L, Őze C. In Situ Synthesis and Characterization of Graphitic Carbon Nitride/Metakaolin Composite Photocatalysts Using a Commercial Kaolin. Crystals. 2024; 14(9):793. https://doi.org/10.3390/cryst14090793

Chicago/Turabian Style

Zsirka, Balázs, Orsolya Fónagy, Veronika Vágvölgyi, Tatjána Juzsakova, Lajos Fodor, and Csilla Őze. 2024. "In Situ Synthesis and Characterization of Graphitic Carbon Nitride/Metakaolin Composite Photocatalysts Using a Commercial Kaolin" Crystals 14, no. 9: 793. https://doi.org/10.3390/cryst14090793

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