Obtaining and Characterization of TiO2-GO Composites for Photocatalytic Applications

Titanium dioxide (TD) and graphene oxide (GO) were synthesized by sol-gel and improved Hummers method, respectively. This study shows the results of the incorporation through four different conditions (sol-gel, sol-gel and ultrasonic, annealed, and UV radiation, C1 to C4, respectively). It was observed that a homogeneous incorporation of TD on sheets of GO was obtained satisfactorily. The composites of TiO2/GO were characterized using different techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and infrared spectroscopy (IR). The photocatalytic activity of the composites was determined from the degradation of the dye azo tartrazine using UV and solar radiation. The best incorporation of TD nanoparticles on GO was obtained with condition C3 (thermal incorporation method) at a temperature of 65°C. This shows a uniformity in the size and shape of the TD as well as an excellent adherence to the sheet of GO. This addition is accomplished by ionic bonding in the presence of electrostatic Coulomb forces. The C3 composite degraded the tartrazine dye using UV radiation and sunlight. With the latter, the degradation time was three times faster than using UV light.

However, the photocatalytic activity of this material is limited by the band gap (3.2 eV for anatase and 3.0 eV for rutile), which absorbs only in the ultraviolet region of the electromagnetic spectrum resulting in a low efficiency [18].
In recent decades, researchers have focused on doped TD to shift the absorption edge to the visible and red spectrum in order to reduce the times of photodegradation. The elements that have been used are Pt, Au, Pd, Rh, Ni, Cu, Ag, Co, Cr, Fe, Mo, V, and W [18][19][20][21][22][23]. The synergy with organic materials has also been investigated as in the case of the allotropes of carbon [24][25][26][27]. Currently, interest on graphene materials has increased due to their application in the photocatalysis area [28,29]. The graphene oxide is defined as a blade of graphene functionalized with hydroxyl and epoxy groups in the basal plane and carboxyls and carbonyls at the edges [30,31]. The oxygen contained on the GO surface, as a functional group, is responsible for the incorporation of TD [27,32,33].
On the other hand, reduced graphene oxide (rGO) is defined as a sheet of functionalized graphene with only carboxyl and carbonyl groups at the edges [31]; rGO has shown advantageous enhancement of photocatalytic activity in a number of studies because it can promote charge separation and electron transfer, it can extend the light absorption range, and it possesses a high specific surface area. The incorporation of TD on the basal plane and edges of graphene oxide has been studied in the application to photocatalytic materials [18,[34][35][36][37][38][39] with very satisfactory results. This is because of the high capacity of the GO for capturing the TD photogenerated electrons, therefore interfering with recombination [35,37,[40][41][42]. Different methods have been used to support the TD on GO: sol-gel [43], thermal (temperature range from 80°C to 120°C) [34,[44][45][46], and radiation (UV and Xenon lamps) [25,42,47,48].
In this study, four different conditions were tested for incorporating TD into GO. The first used sol-gel synthesis to obtain TD that was incorporated directly into the GO. The second was by obtaining TD from sol-gel synthesis and incorporating it into the GO with the aid of ultrasound. The third was through the sonochemical synthesis of TD incorporated into the GO with the aid of thermal treatment. In the fourth test, the TD was obtained by sonochemical synthesis and incorporated into the GO with the aid of UV radiation and temperature. To determine the degree of incorporation (TiO 2 /GO), the composites were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), Raman spectroscopy, and infrared spectroscopy (IR). To corroborate the increase in photocatalytic activity of the TD nanoparticles incorporated into the GO, photodegradation experiments of the azo tartrazine dye were conducted under UV radiation and solar light.

Experimental
2.1. GO Synthesis. GO was obtained using the improved Hummers method [25,35,42,48,49], which consisted of 360 mL of H 2 SO 4 , 40 mL of H 3 PO 4 , and 2 g of graphite powder mixed under vigorous stirring at room temperature for 2 hours. Then, 18 g of KMnO 4 and 800 mL of deionized water were slowly added, being careful to keep the temperature at 60°C using a recirculator. Finally, 15 mL of H 2 O 2 (30%) was poured in. After 24 hours, the resulting solution was centrifuged and washed with deionized water and ethanol until a neutral pH was obtained. The solid obtained was dried at 65°C for 24 hours. Subsequently, the product was dispersed in water under ultrasound for 3 hours to obtain the exfoliated sheets.
2.2. Sol-Gel TD Synthesis and Incorporation into GO (Composite C1). The TD powder was obtained from 1 mole of titanium (IV) isopropoxide, 36 moles of ethanol, and 0.5 mole of hydrofluoric acid (HF). The acid was mixed with half the ethanol solution, and another half of ethanol was mixed with the isopropoxide. The first solution was poured slowly into the second with vigorous stirring. This synthesis was carried out in an inert atmosphere. No thermal treatment was applied after the synthesis. For the incorporation of the TiO 2 /GO material, 20% w/w of GO was added to the solgel solution of TD and was allowed to precipitate. Subsequently, the solvent was evaporated at room temperature.

Sol-Gel TD Synthesis and Ultrasound-Assisted
Incorporation of GO (Composite C2). To the sol-gel solution of TD, 20% w/w of GO was added. The solution was then placed in an ultrasonic bath for 2 hours and subsequently allowed to precipitate. The solvent was evaporated at room temperature. These samples did not undergo additional thermal treatment.

Sonochemistry TD Synthesis and Thermally Assisted
Incorporation of GO (Composite C3). TD was synthesized by the sonochemistry technique using 50 mL of deionized water, a ratio of 1 mole of titanium (IV) isopropoxide for each 36 moles of ethanol, and 0.5 mole of HF. The water was mixed with half of the ethanol and the HF acid. The other half of the ethanol was poured with the isopropoxide (this mixture was prepared under an inert atmosphere). Then, the mixture was placed in an ultrasonic bath at 60°C for 3 hours. Under these conditions, TD was obtained in the anatase phase with a nanometric size on the order of 50 nm. For the incorporation of GO, 20% w/w of the GO, deionized water, and ethanol in a 1 : 1 (v/v) ratio were added. The solution was immediately placed in an ultrasound bath for 2 hours in order to disperse the particles. After that, the dispersion was stirred and heated at 65°C for 24 hours.

Sonochemistry TD Synthesis and UV Radiation-Assisted
Incorporation of GO (Composite C4). TD was obtained by sonochemistry methodology, and then, 20% w/w of GO was added to a deionized water and ethanol solution (1 : 1 v/v). The solution was left in an ultrasonic bath for 2 hours in order to disperse the particles. After that, the solution was stirred for 24 hours at a constant temperature of 65°C under the radiation of a germicidal lamp model PLS 9W (UV 254 nm).    This addition takes place by ionic bonding in the presence of an electrostatic Coulomb force. In a contaminant degradation process, the greater the homogeneity and the incorporation of nanoparticles of TD in the GO sheets, the greater the predominance of the photocatalytic process on the possible adsorption process. The adsorption can occur due to the high specific surface of the GO [45,50]. Figure 2 shows the TEM images of the compounds and TD powder. In samples C1 and C2, the TD particles are dispersed in a less homogeneous manner in the GO than in C3 and C4. The homogeneity of the composite material is important to obtain an efficient transfer of electrons captured from the TD [41,43], reducing the recombination times and increasing the photocatalytic activity. This superposition of TD nanoparticles observed in the TEM images corroborates the XRD results, where it is not possible to observe the characteristic GO peak (2θ = 12:3°) with the crystalline plane (001) due to the agglomeration of TD in the exfoliation GO sheets. Additionally, in these images, the size of the TD nanoparticles (>20 nm) is shown.

Results and Discussion
To observe with greater detail the incorporation of the TD nanoparticles on the surface of the GO laminae clusters, an incorporation with a minimal quantity of TD was realized (C3), and this was analyzed by HR-SEM in STEM mode. As shown in Figure 3, the sample was taken in a bright field. It can be observed how the TD nanoparticles adhere to the GO sheet. This union takes place by electrostatic attraction between TD and the surface of the GO sheets. The TD becomes incorporated onto the GO sheets until they are covered; this has also been observed by other investigators [51,52]. In contrast to what has been reported in the literature, the incorporation for composite C3 was obtained at a temperature of 65°C, the lowest reported to date. The size of the GO sheets was 2 μm on average.
Elemental analysis by EDS was performed to corroborate the incorporation of TD on the surface of the GO sheets. The images were taken with the detector of secondary electrons (SE) and, by mapping EDS, finding representative elements of the sample: carbon, titanium, and oxygen ( Figure 4). With this analysis, the presence of TD is confirmed on the GO sheets, and the oxygen found could be attributed to the functional groups present in the GO. Additionally, Figures 4(a) The characteristic bands of TD and those of GO were found by Raman spectroscopy in the four syntheses, as shown in Figure 5. For TD, four modes of the anatase phase are observed: two modes of E g at 156 and 637 cm -1 , one mode of B 1g at 402 cm -1 , and one mode of A 1g at 512 cm -1 , corresponding to that reported in the literature [50,51]. For the GO, three modes were identified: the band D at 1336 cm -1 , attributed to the defects and the disorder of the sheet; G mode at 1603 cm -1 assigned to the sp 2 hybridization plane of the vibrations of the carbon atoms; and the 2D mode at   International Journal of Photoenergy 2936 cm -1 , with a low intensity which is related to the Raman band second-order effect of phonons [46,[52][53][54][55]. The intensity of the GO bands (D, G, and 2D) is related to the homogeneity and the incorporation of the composites, as is observed in Figure 6. The composites C3 and C4 show the lowest intensities of the GO bands, but they are present in the material. This can be observed in the increase added to each plot. Figure 6 shows the IR spectra of the four composite syntheses of the TiO 2 /GO material. The band at 3318 cm -1 and 1632 cm -1 is attributed to the stretching of the hydroxyl groups (H 2 O, C-OH), at 1365 cm -1 to the carboxyl group (-COOH), at 1632 cm -1 confirms vibrational stretch (C=C), at 1208 cm -1 to the epoxy functional groups (-C-O-C-), at 1105 cm -1 to the alkoxy functional groups (RO-), at 1082 cm -1 attributed to vibration (Ti-O-C), and at the 900 cm -1 to 500 cm -1 attributed to vibration titanium-oxygen (Ti-O and Ti-O-Ti) [46,48,53,56]. All of these vibrational modes show a strong chemical interaction between the TD and GO in the synthesis of the composite.

Photocatalytic
Activity. The photocatalytic activity of the four composites (C1, C2, C3, and C4) was evaluated through the degradation of the tartrazine dye. Figure 7 shows the degradation curves of the dye in the presence of UV radiation at a normalized concentration as a function of time. It is observed that at 60 minutes the TD powder degraded almost 1% of the dye, very similar to the photolysis results and indicating that the presence of TD is not enough to achieve a degradation of this dye despite what has been reported in the literature [56].
With respect to the results of the composites, we can observe that with C1 and C2 a degradation close to 3% was achieved. With C3, a degradation of more than 23% was achieved, and the tartrazine degraded 10% with C4. These results are consistent with the characterization realized for the four composites, where C3 demonstrated having a greater homogeneity in the incorporation of the two materials (TiO 2 /GO).
We conducted two control experiments: adsorption and photolysis. Regarding the first adsorption percentage was 1%; this percentage confirmed that the process is actually being carried out which is photocatalysis. Regarding the second control experiment, it is confirmed that the UV radiation has little influence on the degradation of the dye.

International Journal of Photoenergy
The increase in the percentage of degradation is due to the incorporation of GO sheets; the function of this material is to be an electron acceptor recombination from TiO 2 . This process influences time photocatalysis degradation. Figure 8 shows the degradation curves of the dye normalized as a function of time and with solar radiation. The experiments were realized under the various solar conditions that are present in the day, with better results in comparison with    International Journal of Photoenergy the experiments using UV radiation. For the photolysis, no change was observed in the initial concentration of tartrazine. For the TD powder, a degradation of 15% was obtained, 25% for C1, 35% for C2, 63% for C3, and 45% for C4; all the experiments had a duration of 240 min. These results are related to the UV radiation experiments, where it is observed that the best degradation results are obtained for the C3 composite.
With the C3 sample and the solar radiation, the degradation of the dye tripled, demonstrating that the incorporation of TD and GO increases the photocatalytic activity of the TD. Also, it results in a degradation of the azo dye without a need to incorporate donor species to decompose the molecule.

Conclusions
The characterization of the different TiO 2 /GO composites demonstrated that a material with a greater degree of homogeneity of TD on the GO sheets was obtained with incorporation conditions of C3 (ultrasound and 65°C temperature). It should be pointed out that the temperature that allowed this incorporation is the lowest reported, as far as we know. It was possible to corroborate the results of the characterization with photodegradation experiments of the tartrazine. A greater degradation of the dye was found with the C3 composite. A degradation of 20% was obtained under UV radiation conditions and 63% with solar radiation. This demonstrated that the incorporation with GO increases the photocatalytic activity of pure TD. The heterogeneous photocatalysis with TiO 2 /GO composites demonstrated it to be viable for the degradation of azo dyes, which has been reported to be difficult to degrade using solar light.

Data Availability
No data were used to support this study.

Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.