Evaluation of sol-gel and solvothermal method on titanium dioxide and reduced graphene oxide nanocomposite

The present study compares two synthesis routes to obtain titanium dioxide and reduced graphene oxide nanocomposites that could be used as photoelectrodes in a water-splitting photoelectrocatalytic system. The nanocomposites were obtained using in-situ sol-gel and solvothermal methods as fabrication routes. Subsequently, the materials obtained were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy techniques. The results indicated a strong interaction between reduced graphene oxide and titanium dioxide nanomaterials using both synthesis processes; however, the in-situ sol-gel method exhibited more significant conservation of the aromatic rings of the graphene structure and a lower bandgap (2.45 eV), which are suitable characteristics for its potential use in photoelectrocatalytic processes.


Introduction
The current problem of environmental pollution emerging from conventional energy sources has driven the transition to optimal and environmentally friendly energy alternatives.Nowadays, obtaining green hydrogen through photoelectrocatalytic water splitting is a promising study since it mitigates the high level of recombination that occurs in photocatalysis and, therefore, improves production effectiveness.Likewise, it's a sustainable and environmentally friendly process that generates an alternative with high energy density, renewable and without emissions [1,2].
Diverse semiconductors such as titanium dioxide (TiO2) have been used over time as photoanodes in photoelectrocatalysis (PEC); however, despite its remarkable chemical stability, the wide band gap of TiO2 represents a disadvantage in the process due to the significant recombination of charge carriers and the reduced absorption region, which is only the ultraviolet light region [3,4].Some studies have shown that the combination of reduced graphene oxide (rGO) with TiO2 improves the photoelectrochemical performance by taking advantage of the adsorption and conductivity capabilities of rGO, thus implying a reduction in the band gap [3,5,6].
In the present investigation study, the comparison of two methodological routes for the synthesis of TiO2 and rGO nanocomposite was carried out, the first one through a solvothermal treatment (TiO2@rGO-ST) and the second one through an in-situ sol-gel process (TiO2@rGO-SG).In both cases, the nanocomposite was characterized using Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy (UV-DRS), techniques to suggest the appropriate synthesis method to achieve a nanocomposite according to the needs of a PEC system under the visible region.

Methodology
The synthesis processes of the graphene oxide (GO) and TiO2 nanomaterials, as well as the in-situ solgel and solvothermal methods for forming the nanocomposites of interest, are presented in detail below.

Synthesis of graphene oxide
The synthesis of GO was obtained through the modified Hummer's method [7], where the graphite is initially intercalated with anions from acids before an oxidation process and finally exfoliated to receive the GO nanomaterial.A 9:1 mixture of sulfuric acid (H2SO4) and phosphoric acid (H3PO4) was added to 1 g of pristine graphite at 10 °C and with constant stirring for 2 h.Under the same temperature and stirring conditions, 6 g of potassium permanganate (KMnO4) was added at a rate of 0.1 g per minute and then was heated at 50 °C for 3 h, counting from the first addition of KMnO4.After that time, it was carried to room temperature, and stirring was maintained for 9 h to complete 12 h of oxidation.
Subsequently, 20 mL of hydrogen peroxide (H2O2) was dissolved in 200 mL of cold distilled water and added to the previous mixture to stop oxidation.Then, with cold distilled water, a volume of 700 mL was completed, and it was allowed to decant for 24 h.After this time, the supernatant was separated and discarded, and the solid was washed with 6 successive centrifugations of 30 min at 8000 rpm and 5 °C, thus ensuring a neutral pH.Afterward, the solid was added to 500 mL of distilled water and sonicated for 90 min at 20 °C with an amplitude of 40%, pulsed 1:1 s, and finally lyophilized [8].

Synthesis of titanium dioxide
The synthesis of TiO2 was carried out by the in-situ sol-gel method, where titanium (IV) butoxide (TBT) was used as a precursor [9].The TBT was dissolved in ethanol for 15 minutes under constant stirring.Then distilled water was added drop by drop in a TBT:ethanol: water molar ratio of 1.00:20.2:1.50.Subsequently, the solution was kept for 1 h with magnetic stirring and then heated for 72 h at 60 °C in an oven.Finally, it was calcined at 600 °C for 1 h.

Synthesis of the nanocomposite by the solvothermal method
The TiO2@rGO-ST nanocomposite was synthesized by a solvothermal process with a ratio of 10% GO [10].Initially, 0.1 g of GO was added to a mixture of 20 mL of distilled water and 10 mL of ethanol and sonicated for 30 min.After this time, 1 g of TiO2 was added and sonicated for 2 h.The solution formed was placed in a Teflon-lined autoclave at 180 °C for 18 h and washed with distilled water.Finally, the gray solid obtained was dried at 70 °C for 12 h.

Synthesis of the nanocomposite by in-situ sol-gel method
Maintaining TBT as a precursor, the TBT was dissolved in ethanol for 15 minutes with constant stirring.Then distilled water was added drop by drop, in a TBT:ethanol:water ratio of 1:20.2:1.5.At the same time, a 10% amount of GO relative to TBT was dispersed in 20 mL of distilled water and 10 mL of ethanol with sonication for 90 min.Subsequently, the precursor solution was added slowly to the dispersed GO and kept under constant stirring for 2 h.The solution obtained was placed in an autoclave and maintained at 180 °C for 24 h.Finally, the solid was washed and dried at 70 °C for 12 h [11,12].

Results and discussion
UV-DRS, FTIR, and Raman spectroscopy characterized the synthesized nanomaterials and nanocomposites.Consequently, the spectra of the techniques and their respective analyses are presented below.

Fourier transform infrared spectroscopy results
Figure 1 shows the FTIR spectra of GO, TiO2, TiO2@rGO-ST, and TiO2@rGO-SG in the 400 cm -1 -4000 cm -1 range.The characteristic bands of GO are located at 3340 cm -1 and 1724 cm -1 corresponding to the stretching vibrations of the hydroxyl (-OH) and carbonyl (-C=O) groups respectively; also, in 1620 cm -1 formed by the vibrations of the non-oxidized graphitic aromatic rings, 1400 cm -1 and 1223 cm -1 , attributed to the tension of the C-OH (alcohols) and -C-O-C-(epoxy) bonds respectively [13] and finally, the peak generated by the tension of the C-O bonds at 1056 cm -1 .TiO2 presents its only characteristic absorption band at 581 cm -1 ascribed to the vibration of the Ti-O-Ti bonds [14].
It is evident through the spectra of TiO2@rGO-ST and TiO2@rGO-SG that, although the bands of the GO and TiO2 nanomaterials are maintained, the peaks associated with the oxygenated groups of GO decreased drastically, indicating a correct reduction of the GO using solvothermal and in-situ sol-gel methods [10,14,15].Likewise, it is important to mention a decrease in the band's intensity at 1620 cm -1 , pointing out a lower concentration of aromatic rings in the nanocomposites, mostly in TiO2@rGO-ST.Additionally, it can be seen by the rectangle marked in Figure 1 where the peak at 581 cm -1 associated with the Ti-O-Ti bond presents a widening and a slight shift of the mean position from 581 cm -1 to 650 cm -1 , suggesting the formation of Ti-O-C bonds.This phenomenon may be due to an interaction between the residual oxygenated groups of GO and the hydroxyl groups on the surface of the TiO2 nanoparticles, generating chemically linked nanocomposites [15,16].

Raman spectroscopy results
The Raman spectra of GO, TiO2, TiO2@rGO-ST, and TiO2@rGO-SG are shown in Figure 2; for GO, two typical peaks are observed at around 1351 cm -1 and 1580 cm -1 , corresponding to the D and G modes [13,16,17].The D band is generally ascribed to out-of-plane vibrations related to disordered carbon and structural defects in GO, and the G band arises from the E2g zone-center mode attributed to sp 2 -bonded ordered carbon atoms.Thus, it is possible to measure the degree of disorder in the GO through the ratio of the intensity of the D band to that of the G band (ID/IG) [10].In Figure 2, it can be seen that there is a decrease in ID/IG between GO and the nanocomposites.The decrease in this ratio indicates the reduction of GO and the decrease in the concentration of defects and, therefore, a recovery of the aromatic structures.Furthermore, it suggests a slight increase in the plane's average size of the sp 2 domains [10,13,18].
Additionally, the peaks at 140 cm -1 , 391 cm -1 , 513 cm -1 , and 639 cm -1 can be clearly identified, corresponding to the Raman modes of Eg, B1g, A1g, and Eg, respectively, characteristic of TiO2 [4,15,17].Finally, in the spectra of TiO2@rGO-ST and TiO2@rGO-SG, the same bands attributed to the TiO2 and rGO nanomaterials are found, indicating their presence in the nanocomposites.Compared to the peaks of TiO2 and rGO, these bands present a slight shift that indicates chemical interaction between the precursor's nanomaterials [15].

Ultraviolet-visible diffuse reflectance spectroscopy results
The UV-DRS spectra are presented in Figure 3 by plotting the transformed Kubelka-Munk function (αhʋ) 1/2 vs the photon energy (hʋ) [19].The treatment of the data allowed the calculation of the band gap of the semiconductors TiO2, TiO2@rGO-ST, and TiO2@rGO-SG, which were 3.15 eV, 2.86 eV, 2.45 eV, respectively.The reason for the narrowing of the band gap in the nanocomposites is due to the incorporation of a new energy band of rGO close to the conduction band of TiO2 [20], inferring once again, the formation of a strong interaction between TiO2 and the specific carbon sites of rGO [10].
The difference between the band gaps of TiO2@rGO-ST and TiO2@rGO-SG may be due to greater interaction between the GO and TiO2 nanomaterials in TiO2@rGO-SG than in TiO2@rGO-ST [18] or, it is possible that the in-situ sol-gel treatment manages to preserve to a greater extent the aromatic rings of the graphene structure which agrees with the FTIR and Raman results.The optimal reduction of GO in the process improves the conjugation length, thus influencing a drastic decrease in the band gap compared to the solvothermal treatment [21].
UV-DRS analysis provides relevant information about the photoelectrocatalytic proprieties of material since the narrowing of the band gap translates into the extension of the wavelength that the nanocomposite can absorb, suggesting a higher photoelectrocatalytic activity under the visible region [18,22].

Conclusions
The synthesis of titanium dioxide and reduced graphene oxide nanocomposites by in-situ sol-gel and solvothermal methods was successfully studied.The analysis of the results by Fourier transforms Infrared spectroscopy and Raman techniques demonstrated a strong interaction between reduced graphene oxide and titanium dioxide nanomaterials through both synthesis processes, with a higher preservation of the aromatic structure in the nanocomposite synthesized by in-situ sol-gel method.
Additionally, the ultraviolet-visible diffuse reflection spectra indicated a significant difference in the band gap value, being lower for the nanocomposite synthesized by the in-situ sol-gel method (2.45 eV).Finally, it is possible to suggest the in-situ sol-gel method as an appropriate technique for forming nanocomposites for future photoelectrochemical applications.