COMPOSITES BASED ON EXFOLIATED g-C 3 N 4 AND TiO 2

The photocatalytic degradation of dangerous substances is widely studied topic and many scientific reports have been already published about this issue. Undoubtedly, titanium dioxide is the most often studied compound for photocatalysis in UV region, while graphitic carbon nitride is the most often studied photocatalyst in VIS region. Combination of the photocatalysts leads to the formation of composites with improved photocatalytic performance and better applicability. In this work, the composites of exfoliated g-C 3 N 4 and TiO 2 in anatase form were prepared by mechanical mixing of both components in weight ratio 1:1. The prepared mechanical mixtures as well as the original components were thermally treated at 300 a 400 °C and characterized using X-ray diffraction method, infrared (FTIR) spectroscopy, UV-VIS diffuse reflectance spectroscopy (DRS). The morphology of the particles was studied using scanning electron microscopy (SEM). The photocatalytic activity of the samples was tested using the method based on the 2h long UV assisted photodegradation of Acid Orange 7. The results indicated the highest photodegradation activity for a non-thermally treated mechanical mixture of g-C 3 N 4 and TiO 2 . For the composites the photocatalytic activity (PA) decreases with the temperature of the thermal treatment, the same effect was observed for original TiO 2 , while for original g-C 3 N 4 the photodegradation activity increased with the temperature of the treatment. 3 N 4 (exfoliated) decreases with the calcination temperature contrary to the exfoliated g-C 3 N 4 for which the photodegradation activity increases. The highest photodegradation activity was observed for non-calcined composite TiO 2 /g-C 3 N 4 (exfoliated) and this composite was indicated as the most promising candidate for the photocatalytic applications. The further study will be focused on the evaluation of the stability of this composite and explanation of the photodegradation mechanism.


INTRODUCTION
The photacatalysts based on g-C3N4 has attracted the attention of the scientific community mainly due to their photodegradation activity in VIS region. Although several synthesis procedures of g-C3N4 have been reported the process based on thermal polymerization of melamine is used most often and g-C3N4 in the so called bulk structure is obtained [1]. The process of the melamine thermal treatment has to be carefully controlled other way the products with different photodegradation activity are obtained [2]. Bulk g-C3N4 suffers from low photoacatalytic efficiency attributed mainly to low specific surface area and fast recombination of charge carriers. Exfoliation of the bulk g-C3N4 structure leads to the separation of the individual layers and significant increase of photocatalytic activity. The exfoliation procedures include for example the thermal treatment [3], sonication [4] or chemical treatment [5]. Other approach to improve the photodegradation activity is the doping of the g-C3N4 with metal and non-metal elements [6] or the combination of the bulk g-C3N4 with other photocatalyst for example with BiOIO3 [7], TiO2 [8], graphene oxide [9] etc. Tropová et al. 1 prepared the composites using the thermal treatment of bulk g-C3N4 with TiO2 (in anatase form) at 300 °C and tested the photodegradation activity of the prepared samples against the N2O. The highest photodegradation efficiency was observed for the composite with weight ratio of TiO2:g-C3N4 1:3. Liu et al prepared the composite of g-C3N4/TiO2 by the calcination of TiO2 Degussa P25 with urea in different weight https://doi.org/10.37904/nanocon.2019.8758 ratios. The photoactivity of prepared samples were tested using VIS light assisted photodegradation of formaldehyde.
The aim of our work was to prepare the composite of the exfoliated graphitic carbon nitride with titanium dioxide, characterization of the prepared samples by X-ray diffraction method, infrared spectroscopy and UV-VIS DRS spectroscopy. The morphology of the prepared samples was studied by scanning electron microscopy. Photocatalytic activity of the prepared samples was tested by the photodegradation of the model azo dye acid orange 7.

MATERIALS AND METHODS
The exfoliated graphitic carbon nitride was prepared using two steps procedure. In the first step, the so called bulk g-C3N4 was prepared by the thermal poly-condensation of melamine in a muffle furnace. In detail, 10g of melamine was weighted in a ceramic crucible, which was then covered with a ceramic lid. The crucible was heated with 3 °C•min -1 to the final temperature 550 °C and held 4h at this temperature. In the second step, the bulk g-C3N4 was crushed in agate mortar and put in a thin layer on ceramic plate and heated in a muffle furnace at 500 °C for 2h and then fast cooled and the sample of exfoliated g-C3N4 assigned as Tex was obtained. The composite g-C3N4/TiO2 was prepared by mechanical mixing of Tex and TiO2 (anatase form) in an agate mortar in weight ratio 1:1. The obtained mechanical mixture was split in a three equivalent portions while two of them were 2h thermally treated at 300 and 400 °C, respectively and composites 1:1-300 and 1:1-400 were obtained. Initial Tex and TiO2 were thermally treated at the same conditions as 1:1 mixture and the samples were labeled as Tex-300, Tex-400, TiO2-300 and TiO2-400 were obtained.
All the samples were characterized using X-ray diffraction technique on powder diffractometer Miniflex 600 (Rigaku) equipped with Co tube (40kV, 15mA) and 1D detector D-Tex. X-ray powder diffraction patterns were registered in the range 10 -90 °2Theta.
Infrared spectra of the samples were registered in ATR mode using Fast Fourier infrared spectrometer Nicolet iS50 equipped with ATR diamond crystal. The spectra were registered in the range 4000 -450 cm -1 and 32 spectra were averaged.
UV-VIS diffuse reflectance spectra were measured using UV-VIS spectrometer UV-2600 (Shimadzu) equipped with integrating sphere IRS-2600Plus (Shimadzu). The spectra were registered in the range 220 -800 nm. The diffuse reflectance data were further transformed using Kubelka-Munk function and Tauc's plots was used for the evaluation of indirect band gap energies.
The morphology of the particles of the composite 1:1 was studied using scanning electron microscope Aspex Explorer 4 (ThermoScientific) equipped back scattered electron detector. To prepare the samples for SEM investigation the diluted suspension of the powder sample in ethanol was spread over an aluminum pin and let dry at the laboratory temperature.
Photodegradation activity of the samples was evaluated using the method based on the UV assisted photodegradation of azo dye acid orange 7 (AO7). In a typical experiment, 0.0500 g of the sample was put to the glass bowl with 150 ml of distilled water and homogenized on the electromagnetic stirrer at 250 rpm. After this step 15 ml of the AO7 solution (c = 7.136•10-4 mol•dm -3 ) was added to the stirred suspension.So prepared stirred suspension was covered and after 30 min of stirring in the dark 2 ml of suspension was sampled using syringe and filtered using a syringe filter Chromafil PP/GF/RC-20/25. The absorbance of the filtered solution at 480 nm was measured using HELIOS  spectrometer (ThermoSpectronic) to obtain the absorbance value A0. After this step the stirred suspension was irradiated using UV light ((BLB 36 W, NARVA) for 120 min. After this period, 2 ml of the suspension was sampled, filtered and absorbance A120 was measured. The measured values of the absorbance were transformed to concentration values c0 and c120 and the ratio c120 / c0 was used for the comparison of the photodegradation activity of the individual samples.

RESULTS AND DISCUSSION
The SEM image demonstrating the morphology of the 1:1 composite is shown in Figure 1. The strong agglomeration of the g-C3N4 flakes was observed, TiO2 particles were homogenously distributed over the surface of the g-C3N4 particles. The registered X-ray diffraction patterns are shown in Figure 1b. The X-ray diffraction technique revealed that the samples in TiO2 series were composed of anatase as a sole phase as well as the samples in Tex series were composed of g-C3N4, no impurities were detected by the X-ray diffraction method in the samples in both series. Comparing the profiles of the (101) diffraction peak of anatase the increasing of the intensity and sharpening of this peak with temperature was observed. This fact demonstrates the growing of the anatase crystallite size with temperature. The X-ray diffraction patterns of Tex were not affected by its thermal treatment as evident from Figure 2 what reflects the fact that the Tex sample had been already subjected to higher temperatures (550 and 500 °C, respectively) during its preparation and thus its treatment at the temperatures 300 and 400 °C did not influence its structure. The run of the diffraction patterns registered for the composites 1:1 reflects the behavior of the individual components TiO2 and Tex discussed above.
An example of Tauc´s plot for the non-thermally treated samples is shown in Figure 2. Band gap energies evaluated from the Tauc´s plot for all of the studied samples are listed in Table 1. The run of the curves obtained for the original non-thermally treated TiO2 and Tex (Figure 2a) indicate the monophase composition and the estimated values of the band gaps listed in Table 1 represent the typical values for both TiO2 and g-C3N4. The same situation is for the samples TiO2-400 and Tex-400 (Figure 2b). The separation of two curves belonging to both components in the case of composite 1:1-400 (Figure 2b) is more distinct in comparison to the composite 1:1 (Figure 2a). Comparing the values listed in Table 1 it is observable that the band gap energy (Eg) values for TiO2 decreased with the temperature contrary to the trend observed for Tex for which the Eg values increased with the temperature. The decreasing of the Eg value for TiO2 can be related to the growing of the crystallite at the elevated temperatures, while the growing of Eg values with temperature observed for Tex could be related to more effective separation of the individual layers of Tex particles due to the additional exfoliation of its structure. In the case of the composites the completely opposite trend of Eg values for both components was observed, the estimated values for TiO2 grows with the temperature while the Eg values estimated for Tex decrease with the temperature. This fact signalizes the interaction between both components resulting in different optical properties of the composites in comparison to the pristine materials.   (Figure 3b) shows the set of the absorption peaks in the region of 1200-1700 cm -1 , which are attributed to the stretching mode of aromatic CN heterocycles. The presence of C-H out-of-plane bonds in aromatic domains are proved by peak at 890 cm -1 and a sharp peak with the maximum at 808 cm -1 belongs to the breathing mode of s-triazine units. Peaks in the region above 3000 cm -1 evidenced the stretching vibrations of N-H bonds. FTIR spectra of the 1:1 composites (Figure 3c) and represents the superposition of the spectra of both components TiO2 and Tex.
The photodegradation activity (PA) of the studied samples are compared in Figure 4.

CONCLUSION
The composite TiO2/g-C3N4 (exfoliated) was prepared by mechanical mixing of both components in weight ratio 1:1. The composites were thermally treated at 300 and 400 °C. Characterization of the samples using XRD method as well as the IR spectroscopy revealed there was not any significant effect of the calcination temperature on the structure and character of the prepared composites. The observation of the particles of non-thermally treated composite using SEM showed strong agglomeration of g-C3N4 nanoflakes and homogenously distributed particles of TiO2 over these agglomerates. The photodegradation activity of the TiO2 and the composites TiO2/g-C3N4 (exfoliated) decreases with the calcination temperature contrary to the exfoliated g-C3N4 for which the photodegradation activity increases. The highest photodegradation activity was observed for non-calcined composite TiO2/g-C3N4 (exfoliated) and this composite was indicated as the most promising candidate for the photocatalytic applications. The further study will be focused on the evaluation of the stability of this composite and explanation of the photodegradation mechanism.