2.1. Synthesis of graphene/TiO₂ nanocomposite

Graphene (G) was synthesized by the electrochemical graphite exfoliation method. First, a 1 M sulfuric acid solution was prepared to subsequently place the electrodes that constitute the electrochemical array in a beaker. A graphite bar was used as a working electrode and a copper wire as a counter electrode. The operating voltage used was 12 Volts.

The synthesis of titanium dioxide (anatase) was performed using the sol-gel method [¹³], and it is described below: To start, 20 ml of titanium butoxide was added to 200 ml of distilled water in a beaker, which generated a white precipitate that was separated from the liquid by decantation. Then, the precipitate was redispersed in distilled water containing a 0.1 M solution of nitric acid and glacial acetic acid in a 9/1 molar ratio (acetic acid/titanium butoxide). Subsequently, the graphene/TiO₂ nanocomposite was generated from the above-mentioned precursors through mixing with a 50/50 wt % ratio inside a cylindrical vial. After mixing, sonication was performed for one hour to disperse the respective phases. Figure 1 shows the complete methodology followed to obtain and characterize the TiO₂/graphene nanocomposite.

FIGURE 1 The experimental diagram to obtain graphene/TiO₂ nanocomposites. 

2.2. Sample characterization

The equipment used to identify the crystalline phases of the precursors was the X-ray diffraction system model D8 ADVANCE DAVINCI. The samples were analyzed with the conventional technique (Bragg-Brentano geometry). In general, the samples were run in a range of 2θ from 5^◦ to 80^◦, with a 0.2^◦ increase in the reading angle, every 0.5 seconds. For the analysis of the morphology of the samples, several types of equipment were used: a JEOL brand Field Emission Scanning Electron Microscope (FESEM), model JSM 7600F in the secondary electron modes and topographic mode, a Transmission Electron Microscope (TEM), Tecnai Supertwin model with field emission. The instrument used to obtain the Raman spectrum of the graphene sample is a Bruker Optics Senterra Dispersive Microscope equipped with 532 nm, 785 nm and 633 nm laser. The configuration used was: 532 nm laser, 5 mW power, and 5 seconds of acquisition time. UV-vis spectroscopy was carried out on a Perkin Elmer (Lambda 25) spectrophotometer at wavelengths of 300-1100 nm to determine the bandgap energy of TiO₂ nanoparticles.

3.1. Raman spectroscopy to confirm the exfoliation of the graphite layers

The Raman spectrums of the exfoliated graphene sample and the graphite precursor are shown in Fig. 2. The peaks obtained in the graphene sample correspond to the characteristic bands of graphene: D (1344), G (1569) D’ (2689) G’ (3220). Band D indicates the presence of defects or edges in graphene samples [¹⁴], which could be observed by SEM and TEM. The intensity of the G peak is relatively high, compared to the intensity that occurs in monolayer graphene samples, which is much lower. A slight shift to the left of the D’ band after exfoliation can be observed. The D’ band of monolayer graphene does not have sub-peaks, unlike bilayer graphene that has 4 sub-peaks and few-layer graphene and graphite that both have 2 sub-peaks [¹⁵]. The band D’ in the graphene sample spectrum resembles that of graphene of a few layers, indicating the proper exfoliation of graphite layers.

FIGURE 2 Raman spectrum of the graphene and graphite samples. 

3.2 Identification of crystalline phases present in the graphene/TiO₂ nanocomposite

The graph in Fig. 3 shows a comparison of the diffractograms obtained from anatase, graphene, and the G/TiO₂ nanocomposite. In the graphene precursor sample, the peak at 2θ = 26.4^◦ (002) is characteristic of graphite [¹⁶,¹⁷], and it may also be attributed to graphene or reduced graphene oxide. In the latter case, the peak is much wider, resembling an amorphous signal [¹⁸,¹⁹].

FIGURE 3 Comparison of the DRX patterns of graphene, anatase, and G/TiO₂ nanocomposite. 

The most relevant peak in the TiO₂ sample obtained with the sol-gel method appears at around 25.3^◦, and it is characteristic of the crystalline phase of anatase [²⁰]. Other peaks are also attributed to anatase and can be observed in the resulting diffractogram: 37.9^◦ (103), 47.7^◦ (200), 54.8^◦ (211) and 62.5^◦ (213). Both characteristic peaks of graphite and graphene, as well as the different TiO₂ anatase peaks, are still present in the diffractogram of the anatase/graphene nanocomposite. Such nanocomposite corresponds to a mixture of anatase and graphene phases, and no chemical reaction has taken place that would have ended up in an additional phase. The average crystallite size of TiO₂ nanoparticles was calculated to be around 6.14 nm by applying the Scherrer equation:

where K is a dimensionless constant with a value close to unity, λ is the X-ray wavelength, β is the full width at halfmaximum (FWHM) of the diffraction peak, and θ is the diffraction angle [²¹].

3.3. Morphology of the graphene/TiO₂ nanocomposite

Figure 4 shows the images obtained with the SEM of the synthesized composite. With high amplification (Fig. 4a), a graphene base with some TiO₂ particles deposited on the surface can be observed, and some titania particles are below the graphene layers. The observed morphology is similar to that reported in the literature and corresponds to the physical mixture between the graphene and titania phases [²²,²³]. The EDS analysis with elemental mapping mode allows to distinguish the areas where each element is located. In Fig. 4b, the carbon is colored in red and titanium in green, the resulting mixture of both elements and, consequently, of both colors is shown in yellow. It can be noted that the most present color in the sample is yellow (a mixture of titanium and carbon), followed by areas where only green (titanium) is seen, and finally, some red dots (carbon). Besides a measurement of the green (titanium) and red (carbon) zones areas was carried out and nanometric particles of titanium (42 nm) and carbon (31 nm) were measured.

FIGURE 4 FESEM images showing (a) the obtained G/TiO₂ nanocomposite at low magnification and (b) EDS maping of the same area showing the nano-scale dispersion and mixing of the TiO₂ and graphene phases that constitute de nanocomposite. 

The titania-graphene composite mixed by agitation and ultrasound was also analyzed with a Transmission Electron Microscope to observe the interaction between the crystalline phases of the precursors. In Fig. 5a an image is presented at very high amplification of the titaniagraphene composite, where different crystalline planes can be seen, as well as some amorphous areas. By performing a Fourier transform to this image, the pattern of dots and rings showed in Fig. 5b is obtained, from which the different crystalline planes of the sample are indexed. From the rings displayed in the image and determining the interplanar distance to later compare with the databases, the presence of the planes (101), (103), (200), and (211), which correspond to the TiO₂ (anatase) precursor is confirmed. The TiO₂ nanoparticles have an average length of 6 nm and an average width of 4 nm according to measurements. The graphene diffraction points highlighted in the Fourier transform of Fig. 5c corresponds to an interplanar distance of approximately 0.344 nm, which can be attributed to turbostrophic graphite according to the literature [²⁴,²⁵], that is to say, disordered graphite product of the electrochemical exfoliation process, which has an interplanar spacing greater than that of the ordered graphite (0.335 nm). This means than in the highlighted zone, few-layer graphene is present, alongside a TiO₂ (anatase) nanoparticle with an interplanar spacing of 0.351 nm (101).

FIGURE 5 TEM images of the anatase and graphene composite mixed with ultrasound (a) general view of the nanocomposite at very high magnifications (b) ring pattern of the anatase crystalline planes, (c) point pattern corresponding to the graphene and TiO₂ (anatase) phases. 

TiO₂ nanoparticles were also measured from several TEM micrographs, and the average crystallite size was calculated to be 6.61 nm, as shown in Fig. 6, which is close to the value obtained from the Scherrer equation (6.14 nm). This extremely small TiO₂ particle size, coupled with an enhanced transport of electrons on the highly conductive graphene phase, is expected to halt the recombination process of electrons and holes.

FIGURE 6 TiO₂ statistical particle size distribution from TEM images measurements. 

3.4. Bandgap energies of TiO₂ and graphene/TiO₂ nanocomposite

The bandgap energies of TiO₂ nanoparticles and G/TiO₂ nanocomposite were calculated using Tauc-relation [²⁶,²⁷], as shown in Fig. 7, where α is the absorption coefficient, and hν is the photon energy, which is calculated with Eq. (2). In this equation, λ is the wavelength in nanometers, h is the Planck’s constant (6.626 × 10⁻³⁴ Js), and c is the speed of light (3×10⁸ m/s).

FIGURE 7 Bandgap energies of TiO₂ nanoparticles and G/TiO₂ nanocomposite calculated using Tauc-relation. 

The value of hν extrapolated to α = 0 gives and absorbs energy, which corresponds to a bandgap. For the TiO₂ nanoparticles, the bandgap was calculated to be 3.54 eV, which is higher than 3.2 eV for bulk TiO₂ (anatase) [²⁸]. This increase in the bandgap value occurs because the synthesized TiO₂ nanoparticles have a relatively smaller size compared to bulk TiO₂, and it is known that the decrease in particle size produces a larger bandgap [²⁹]. The bandgap energy of G/TiO₂ nanocomposite was calculated to be 3.6 eV, which is practically the same bandgap value of the anatase phase. This similarity in bandgap values occurs presumably due to the high concentration of TiO₂ nanoparticles surrounding the graphene flakes caused by electrostatic interactions. However it can be expected that in future studies, different concentration ratios of TiO₂ and graphene in the nanocomposite will significantly decrease the combined bandgap, as this concentration ratio dependence on the bandgap value has been demonstrated in other graphene/TiO₂ nanocomposites [³⁰-³²].

3.5. A theoretical model of graphene/TiO₂ nanocomposite

Figure 8 shows a graphene/TiO₂ nanocomposite model that was developed from the TEM images. Anatase nanoparticles appear to aggregate around graphene flakes and also on the surface. This attraction between the two phases may be due to the positive charge in graphene and the negative charge in TiO₂ particles, generating a weak bond, possibly Van der Waals, that does not generate a chemical reaction but does produce an aggregation of both nanomaterials.

FIGURE 8 Graphene/TiO₂ nanocomposite model developed from TEM data.