Characterization of zinc oxide and graphitic carbon nitride nanocomposites for use as a potential photoelectrode

In this research, two nanocomposites of zinc oxide and graphitic carbon nitride were obtained in a 1:0.15 ratio for potential use as photoelectrocatalysts. Calcination and the simple reflux method were used to obtain routes for synthesizing zinc oxide and graphitic carbon nitride nanocomposites. Subsequently, Fourier transform infrared spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, and Raman spectroscopy analyses were performed, from which it was determined that there is a strong interaction between zinc oxide and graphitic carbon nitride in both nanocomposites. Nevertheless, the nanocomposite that exhibited the most significant band gap reduction was obtained by calcination, reaching 2.93 eV.


Introduction
The increasing demand and utilization of fossil fuels have generated severe environmental pollution, thus causing the need to find alternative supplies of clean and renewable energy [1]; recently, hydrogen generation has been studied as a solution to the problem described [2,3], and the use of photoelectrocatalysis (PEC) is proposed as a promising method for the production of "green hydrogen" from the water splitting [4][5][6].Most PEC processes use metal oxides as photo electrocatalysts; however, the use of these oxides presents a disadvantage due to the high electron recombination rate, wide bandgap, and low photon absorption in the ultraviolet-visible (UV-VIS) range [7].
Some studies have shown that the union of metal oxides with two-dimensional (2D) carbon-based materials dramatically improves the effectiveness and behavior of PEC processes [1,[8][9][10][11].Among these 2D materials, graphitic carbon nitride (g-C3N4) stands out because it is an organic semiconductor with a high radiation absorption rate in the UV-VIS range [12], which has led some researchers to search for a coupling between various metal oxides (especially zinc oxide) and g-C3N4 in order to enhance the photoelectrochemical properties and reduce the band gap of these oxides [8,13,14].
In the present investigation, the fabrication of two nanocomposites of zinc oxide (ZnO) and g-C3N4 was carried out; the first nanocomposite was made by calcination [13], and the second through a reflux method [14].Consequently, their Fourier transform infrared spectroscopy (FT-IR), Ultraviolet-visible diffuse reflectance spectroscopy (UV-DRS), and Raman spectroscopy analyses were performed and compared to determine the most appropriate fabrication route for subsequent use as photoelectrodes in PEC.

Methodology
The experimental methodology used to obtain zinc oxide and graphitic carbon nitride nanocomposites by calcination (C:ZnO@g-C3N4) and the reflux method (R:ZnO@g-C3N4) is detailed below.

Synthesis of zinc oxide nanoparticles
The ZnO nanoparticles were synthesized using the sol-gel method.For this, 2 g of zinc acetate dihydrate was dissolved in 15 mL of distilled water using magnetic stirring for 15 min, then a 0.5 molar solution of sodium hydroxide was added to reach a pH = 13, next, stirring was maintained and heated at 60 ºC for one hour.After this time, ethanol was added drop by drop until a neutral pH was reached (pH=7) and a white precipitate formed.Finally, this precipitate was washed, centrifuged, and lyophilized [15].

Synthesis of bulk graphitic carbon nitride
The bulk of g-C3N4 was synthesized by a high-temperature polymerization method as established in some investigations [14,16].For this synthesis, melamine was used as a precursor; 5 g was added to crucible silica and calcined at 550 ºC for 4 hours in an ambient atmosphere with a heating rate of 5 ºC/min.After cooling to room temperature, the synthesized material was ground in an agate mortar.

Fabrication route of the nanocomposite by calcination
The C:ZnO@g-C3N4 nanocomposite was obtained by mixing and grinding 1 g of the previously synthesized ZnO nanoparticles and 0.15 g of g-C3N4 in an agate mortar.Subsequently, it was calcined in a muffle at 350 ºC in an ambient atmosphere.Finally, a gray powder was obtained that was cooled to room temperature and stored for characterization [13].

Fabrication route of the nanocomposite (core-shell type) by reflux
To obtain the R:ZnO@g-C3N4 core-shell type nanocomposite, the previously synthesized g-C3N4 was chemically exfoliated to obtain monolayers.For this, 1 g of g-C3N4 was taken, added in 10 mL of sulfuric acid (98 wt.%), and stirred for 8 hours at room temperature.After this time, 100 mL of distilled water was added, and it was sonicated for one hour to exfoliate, then washed to remove the remains of the acid, centrifuged, and dried at 80 ºC for 12 hours.Finally, the powder obtained was refluxed in methanol at 65 °C for 6 hours, centrifuged, and dried at 80 °C [17].
For the formation of the nanocomposite, 0.15 g of the nanosheets were sonicated for 1 hour in 100 mL of methanol to be dispersed entirely, then 1 g of the previously synthesized ZnO nanoparticles were added, and the mixture was stirred for an additional 30 minutes.To finish, the mixture was refluxed at 65 ºC for 14 hours, centrifuged, and dried at 80 ºC [14].

Results and Discussion
This section describes the results obtained through FT-IR, Raman, and UV-DRS spectroscopy.

Fourier transform infrared spectroscopy results
Figure 1(a) shows the FT-IR spectra obtained for ZnO, bulk g-C3N4, and C:ZnO@g-C3N4 nanocomposite.For ZnO, a characteristic peak of the stretching vibration of the bond between zinc and oxygen is observed at 450 cm -1 [18].The FT-IR spectrum of bulk g-C3N4 shows the peaks at 3300 cm -1 , 1637 cm -1, and 1243 cm -1 corresponding to the stretching vibrations of the terminal amino group (N-H) and the bonds between carbon and nitrogen (C=N and C-N), respectively.Furthermore, the peaks 1404 cm -1 , 1450 cm -1 , and 1560 cm -1 associated with heptazine-derived repeating units are clearly defined.
Likewise, a peak is seen at 804 cm -1 , belonging to the out-of-plane breathing vibration characteristic of the s-triazine ring system [13,14,16,19].In the case of the C:ZnO@g-C3N4 nanocomposite, it could be observed that the main peaks of both ZnO and g-C3N4 were maintained.However, it is essential to mention that the peaks associated with g-C3N4 shifted slightly, possibly due to the weakening of the bond strength of the C=N and C-N.
The FT-IR spectra of the exfoliated g-C3N4 and the R:ZnO@g-C3N4 nanocomposite is shown in Figure 1(b); in this graph, it is evident that the chemical exfoliation of the g-C3N4 causes modifications in its structure.The spectrum shows how, after exfoliation, the peak located at 1404 cm -1 disappears, and the other peaks shift to higher wave numbers.In Figure 1(b), it is easily observed that the R:ZnO@g-C3N4 nanocomposite exhibits the characteristic peak of ZnO and the peaks attributed to the exfoliated g-C3N4, which indicates that the coupling of the two nanomaterials was effective [14].
The Raman spectra for bulk g-C3N4 were difficult to obtain, given that this compound emits fluorescence which overlaps the peaks belonging to the Raman spectrum; however, the peaks 478 cm -1 , 753 cm -1 , 1159 cm -1 , 1235 cm -1 , and 1310 cm -1 that are attributed to the stretching vibration of aromatic C-N heterocycle could be observed; additionally, the peaks at 708 cm -1 and 1000 cm -1 are shown to belong to the different types of ring breathing modes of s-triazine [21].When comparing the Raman spectrum of bulk g-C3N4 with exfoliated g-C3N4, variations caused by chemical exfoliation can be observed; the main variation observed is the shift of the peak located at 478 cm -1 towards higher values (approximately 540 cm -1 ) and the disappearance of the peaks located at 708 cm -1 and 1235 cm -1 .
In the spectrum of C:ZnO@g-C3N4 nanocomposite, the main peaks of ZnO (99 cm -1 , 331 cm -1 , and 434 cm -1 ) and bulk g-C3N4 (478 cm -1 , 708 cm -1 , 753 cm -1 , 1000 cm -1 , and 1235 cm -1 ) are evident, but with a lower intensity.The integration of the peaks of both precursors suggests a union between them.The Raman spectrum of R:ZnO@g-C3N4 shows a combination of the main peaks of Zno and the exfoliated g-C3N4, nevertheless with a lower intensity.It is important to mention that there is a shift of the peaks located at 99 cm -1 and 230 cm -1 (associated with ZnO).
In the analysis of the Bulk g-C3N4 (2.49 eV) and the exfoliated g-C3N4 band gap (2.82 eV), an increase of 0.33 eV is observed in the band gap of the exfoliated g-C3N4 that can presumably be attributed to the decrease of the conjugation length and the strong quantum confinement effect due to the single layer structure generated by the chemical exfoliation of g-C3N4 [17]; among the two nanocomposites fabricated, the one that exhibited a greater decrease in the band gap was C:ZnO@g-C3N4 nanocomposite (2.93 eV), which when compared with the band gap of zinc oxide showed a decrease of 0.32 eV.

Conclusions
It was proven that it is possible to fabricate zinc oxide and graphitic carbon nitride nanocomposites by simple calcination and the reflux method; consequently, the synthesized nanocomposites showed a strong interaction between zinc oxide and graphitic carbon nitride both in bulk and exfoliated.However, the composite that presented the greatest reduction in the band gap compared to zinc oxide was the nanocomposite obtained by route calcination (2.93 eV); the above suggests that the most effective route for the fabrication of zinc oxide and graphitic carbon nitride nanocomposites is simple calcination.