As a catalyst, titanium dioxide (TiO
2) was employed already very early; however, its photocatalytic application is limited due to the high recombination rate of photogenerated charge carriers and efficiency for photodegradation under abundant sunlight. The latter is due to TiO
2′s wide bandgap energy of 3.2 eV, which makes merely 5% of the sunlight energy effective for the reaction
[5]. Given the importance of water remediation and in order to employ this fascinating, cheap, and harmless compound in other applications, many strategies have been implemented
[6]. Tuning the band gap or reducing the recombination rate of the semiconductor by ion doping has been a known technique for decades. Today, catalytic limitations are being overcome by producing complex nanostructures where TiO
2 is coupled with metal or non-metal elements
[7][8]. In these composites, heterostructures are introduced, which effectively tailor the bandgap width or recombination rate of electron-hole pairs. Such TiO
2-based nanocomposites have been successfully fabricated and showed enhanced photocatalytic activity shifted into the visible, e.g., sunlight range
[9]. The combination of more components into the system to reduce the bandgap width, however, reduce overall catalytic activity in comparison to a single bandgap excitation, and to avoid this pitfall, so-called Z-scheme photocatalytic systems are being considered lately
[10]. This photocatalytic reaction can facilely be realized by introducing a metal sink into a two-semiconductor heterojunction, by using noble metal nanoparticles, for example
[11]. Very promising and mostly cheaper, however, are composites of graphene oxide/TiO
2/graphitic carbon nitride
[5][12][13].
2. Characterization
Raman and FTIR analyses were used to identify the chemical bonding in the synthetized photocatalysts.
Figure 1A shows the FTIR spectra of the prepared GO, rGO, and
g-C
3N
4 samples. The GO sample shows three characteristic absorption peaks in 1721, 1041, and 1381 cm
−1 attributed to several oxygen functional groups, i.e., C=O and C-O stretching, and C-OH bending. The stretching vibration at 1225 cm
−1 is assigned to the C-O-C of the epoxy groups. The broad stretching vibration band of the hydroxyl group, O-H, associated with absorbed water molecules and alcohol groups, is represented as a broad peak at 3380 cm
−1. Typical absorption peaks of GO disappear in the rGO spectrum. There, the absorption peak at 1617 cm
−1 is assigned to the skeleton vibration of less-oxidized graphitic materials, C=C. Comparison between the rGO spectrum and the GO spectrum shows that the reduction of GO by thermal treatment was significant. In the FTIR spectrum of the
g-C
3N
4 sample, several strong peaks were observed in the 1200–1650 cm
−1 region. Typical stretching vibrations of C-N heterocycles are represented at 1217, 1315, 1395, and 1452 cm
−1. The two bands appeared at 1541 and 1633 cm
−1, related to the stretching vibrations of C=N heterocycles, while the peaks located at 885 and 804 cm
−1 arose from the characteristic breathing mode of tri-s-triazine rings units. All are characteristic for
g-C
3N
4 material. The extra vibrational region between 3000–3300 cm
−1, although not directly related to the
g-C
3N
4 structure, is commonly found in this compound
[5][24], and can be associated with H
2O absorption and possible N-H bonds at the surface.
Figure 1B shows characteristic functional vibrations of FTIR spectra of the prepared pure TiO
2 nanoparticles, TiO
2/rGO, TiO
2/CN nanocomposites, and the rGO@TiO
2/CN hybrid photocatalyst.
Figure 1. (A) FTIR spectra of as-prepared GO, rGO, and g-C3N4 materials and (B) pure TiO2 nanoparticles, TiO2/rGO, TiO2/CN nanocomposites, and rGO@TiO2/CN hybrid photocatalyst.
Raman spectra of pure TiO
2 nanoparticles, GO, thermally treated rGO, and the prepared rGO@TiO
2/CN hybrid material are displayed in
Figure 2A. The typical bands of anatase TiO
2 are located at 147, 398, 517, and 641 cm
−1, which are attributed to the Eg
(1), B
1g, A
1g + B
1g, and E
g(2), respectively. The Raman spectrum of synthesized GO showed two typical peaks at the wavenumber of 1356 cm
−1 and 1607 cm
−1, which corresponds to the D and G bands. The D band is associated with the in-plane sp
3 defects, while the G band represents in-plane vibrations of ordered sp
2-bonded carbon atoms in a two-dimensional hexagonal lattice. The D and G bands, after the hydrothermal reduction process, are shifted to the lower wavenumber at 1336 cm
−1 and 1591 cm
−1. In the case of rGO, the D band got slightly larger with the reduction process, which increased the level of defects in the material, while the shifted G band indicates the re-graphitization process
[25]. The Raman spectrum of the rGO@TiO
2/CN hybrid sample contained four characteristic bands of anatase phase of TiO
2 and two typical bands of rGO (D and G) The D and G bands in the prepared rGO@TiO
2/CN sample were found in the unchanged position as in the rGO sample, which suggests a reduction level similar to the pure GO reduction. As shown Raman spectrum of
g-C
3N
4, a broad peak was observed between 1200–1800 cm
−1, which is similar to other reports
[13][26]. Although two characteristic peaks, corresponding to D and G bands, should be resolved in this region, the obtained spectrum did not clearly express this feature. This was either because the excitation energy (633 nm excitation) was not optimal for the material
[27], which could include defects, or because it was less crystalline. The intensity ratios of the D and G bands (
ID/
IG), specifically, show the concentration of the sp
3 hybridized defects with respect to the sp
2-hybridized graphene domains. The hydrothermally prepared hybrid sample shows a higher intensity ratio (
ID/
IG = 1.10) in comparison with the synthesized GO sample (
ID/
IG = 1.02). According to
[5], this suggests an interplay between TiO
2 intercalation and variation in defect concentration. Thus, Raman results better prove chemical interaction between graphene material and TiO
2 nanoparticles.
Figure 2. (A) Raman spectra of TiO2, GO, rGO, g-C3N4, and rGO@TiO2/CN nanocomposite, and (B) XRD patterns of the prepared materials.
Figure 2B shows the X-ray powder diffraction pattern of the prepared GO, rGO,
g-C
3N
4, pure TiO
2 nanoparticles, TiO
2/rGO and TiO
2/CN nanocomposites, and rGO@TiO
2/CN hybrid samples. The GO pattern shows the strong diffraction peak at 2
θ = 11.5° (001), which corresponds to an interlayer spacing of 0.76 nm and indicates the presence of oxygen functional groups in the GO sample
[28]. In the rGO pattern, the broad diffraction peak is located at 2
θ = 23.8° (002), which corresponds to an interlayer spacing of 0.74 nm. The decreased interlayer confirms the successful reduction of GO to rGO by applied hydrothermal treatment
[19]. The
g-C
3N
4 pattern contains two characteristic diffraction peaks according to JCPDS card no. 87-1526. The weaker peak located at 2
θ = 12.8° (100) corresponds to an in-plane structural repeating of tri-s-triazine units and represents an interlayer spacing value of 0.69 nm. The sharp diffraction peak presented at 2
θ = 27.5° (002) corresponds to the interlayer stacking of the conjugated aromatic segments with an interlayer spacing of 0.32 nm
[20]. The nanoparticles synthesized by the simple hydrothermal method showed a crystalline nature, with 2
θ peaks lying at 25.29° (101), 37.96° (004), 48.03° (200), 54.89° (105), 62.74° (204), 68.78° (116), 70.23° (220), and 75.35° (215). According to JCPDS card no. 21-1272, all the diffraction peaks of TiO
2 sample can be indexed to a pure anatase phase. These peaks are also observed in the prepared TiO
2/rGO, TiO
2/CN, and rGO@TiO
2/CN samples. Furthermore, in all the composite mixtures, XRD analysis failed to detect clear
g-C
3N
4 or rGO presence, mostly due to a low mass percentage of the later, but also because carbon-based compounds lack long range order, which would be similar to TiO
2. Using the Scherrer equation, the crystalline domain size of the TiO
2 particle for all prepared samples was also estimated. The obtained crystallite size using (101) peak and a shape factor of 0.9 showed similar values around 7 nm (
Table 1) for all samples.
Table 1. Calculated crystallite size (D, nm) of TiO2 samples using the Scherrer equation.
Sample ID |
D, nm |
TiO2 |
7.6 |
TiO2/rGO |
6.8 |
TiO2/CN |
7.5 |
rGO@TiO2/CN |
6.7 |
As revealed by the SEM (Figure 3A), pure TiO2 crystals precipitated in the form of spheres. It was found that the particle size was smaller than 10 nm, which is consistent with the calculated crystallite size of 7.6 nm (Table 1). The SEM analysis of pure rGO and g-C3N4 materials pointed out to their known high specific surface area (Figure 3B,C), while imaging of the composite TiO2/rGO, TiO2/CN, and rGO@TiO2/CN composite samples revealed similar TiO2 crystals precipitated at the surface of the hybrid system (Figure 3D–F). Obviously, bigger aggregates in the composite samples arise most probably during the drying sequence and as is shown later, do not diminish degradation process. Figure 4 shows the TEM image of the rGO@TiO2/CN hybrid material, where the TiO2 component is undoubtedly resolved, while carbon containing compounds are not so clear. However, the sheet of rGO and fiber-like g-C3N4 are obviously present, and EDS shows Ti, O, C, and N over the region of this hybrid material. The average size of anatase TiO2 nanoparticles is around 6 nm.
Figure 3. SEM images of (A) pure TiO2 nanoparticles, (B) rGO, (C) g-C3N4, (D) TiO2/rGO, (E) TiO2/CN, and (F) rGO@TiO2/CN.
Figure 4. TEM image of rGO@TiO2/CN nanocomposite and inset of anatase TiO2 nanoparticles.
The specific surface area of the rGO@TiO
2/CN hybrid material was determined by the Brauner-Emmett-Teller (BET) method, and it was 185.82 m
2/g. The obtained result is higher than to the literature findings, based on similar preparation procedures for the rGO@TiO
2/CN hybrid
[29].
The surface chemical composition and oxidation state of the as-prepared rGO@TiO
2/CN hybrid material were determined by XPS analysis, as shown in
Figure 5.
Figure 5A shows the full spectrum of characterized photocatalyst; Ti, O, C, and N peaks can be observed. Moreover, the peak intensities of Ti and O elements are clearly stronger than that of C and N, which corresponds to the obtained XRD results. The high-resolution spectrum of Ti 2p is shown in
Figure 5B. The Ti 2p spectrum contains two symmetric peaks centered at 465.0 eV and 459.3 eV, which correspond to the Ti 2p
1/2 and Ti 2p
3/2. The obtained binding energy values correspond with Ti
4+ in the TiO
2 chemical environments. The peak was determined at 456.7 eV, which can be attributed to the Ti-N bond due to the solid adhesion of TiO
2 on the
g-C
3N
4 surface
[18][30]. Based on this,
Figure 5C shows the O 1s spectrum. The main peak is located at 530.5 eV, assigned to oxygen bonded to titanium (Ti-O-C), and a smaller peak around 531.3 eV, ordinarily attributed to hydroxyl groups in the structure of TiO
2.
Figure 5D represents the N 1s spectrum, which can be fitted into two peaks with binding energies centered at 399.3 eV and 400.5 eV, assigned to the sp
2-hybridized nitrogen atom in the C-N=C group of triazine rings and sp
3-hybridized tertiary nitrogen N-(C)
3 in the heptazine unit
[5]. The peak located at 401.4 eV, attributed to the C-N-H bond, which signifies the covalent bonding between rGO and
g-C
3N
4, was not determined
[30]. As shown in
Figure 5E, the C1s spectrum could be divided into four peaks. The binding energies of the observed peaks at 284.5 and 287.2 eV can be assigned to the sp
2-hybridized carbon atom in the C-C bond and oxygen-containing epoxy/hydroxyl groups bond in the rGO material. The observed peaks located at 285.9 eV and 288.6 eV correspond to the sp
2-bonded carbon in N-containing (C-N-C) and (N)
2-C=N in the aromatic ring of the
g-C
3N
4 material
[5][30]. Overall, the analysis of XPS results for the as-prepared rGO@TiO
2/CN hybrid material reveals successful interactions of
g-C
3N
4 and rGO with TiO
2 nanoparticles and the pure form of the prepared hybrid sample.
Figure 5. High-resolution XPS spectra of the as-prepared rGO@TiO2/CN sample: (A) fully scanned spectrum, (B) Ti 2p, (C) O 1s, (D) N 1s, (E) C 1s, and (F) band gap energies (Eg) of prepared pure TiO2 nanoparticles, TiO2/rGO, and TiO2/CN nanocomposites, and rGO@TiO2/CN hybrid.