Characterization of the samples. To demonstrate the surface topography, field-emission scanning electron microscopy (FE-SEM) images of the TTOC-2 nanocomposites at different magnification are presented in Figures 1a and 1b. The FE-SEM micrographs show that the as-synthesized compound is composed of irregular spherical-shaped NPs with diameters ranging from ~15 to ~75 nm and a mean diameter of 43 nm (Figure 1d). The chemical composition of the TTOC-2 nanocomposite was confirmed from its energy-dispersive X-ray spectrum (EDS) (Figure 1c). The inset FE-SEM image in Figure 1c was used for EDS analysis, and the elemental composition is presented in the table (inset). The EDS spectrum affirms the presence of carbon species with Ti and O elements. The precursor CNT was not observed in the FE-SEM analysis; however, the EDS studies confirm the presence of carbon in TTOC-2, indicating breakdown of the CNTs.
Transmission electron microscopy (TEM) analysis was conducted to further analyze the state of the CNTs in the TTOC-2 sample; images at three different magnifications are shown in Figures 2a–2c. The formation of a thin carbon layer (marked with a red dashed line) was clearly observed. High-resolution TEM (HR-TEM) analysis was subsequently used to observe the crystal lattice (Figure 2d), which confirms the existence of two different conjoint planes. The estimated d-spacing value of 0.41 nm matches the interplanar spacing of graphitic carbon1, and the d-spacing (0.35 nm) is assigned to the (101) facet of anatase TiO219.
FE-SEM images of the TTOC-1 nanocomposite are shown in Supplementary Figures S2a and S2b. The densely packed NPs were observed to uniformly coat the CNT surface. FE-SEM images of TTOC-3 are presented in Supplementary Figures S2c and S2d. CNTs are clearly observed to homogeneously decorate the nanocomposite surface. The morphology of the TTOC-1 nanocomposite is similar to that of the TTOC-3 nanocomposite even though the amount of CNTs in the composites differs. The EDS spectra of TTOC-1 and TTOC-3 are shown in Supplementary Figures S3a and S3b, respectively. The respective FE-SEM images used for the EDS analysis and the tables detailing the elemental compositions are shown in the insets of the figures. Both spectra indicate the presence of O, Ti, and C.
The TTOC-1 and TTOC-2 nanocomposites were studied by TEM. Supplementary Figure S4a shows TEM images of TTOC-1. The spherical NPs with a mean diameter of 51 nm are superimposed on the CNTs. In the HR-TEM image of (Supplementary Figure S4b), the lattice d-spacing of 0.35 nm is assigned to the (101) plane of TiO2 NPs and that of 0.41 nm is ascribed to the graphitic carbon of CNTs. A TEM image of TTOC-3 is shown in Supplementary Figure S4c; the growth of TiO2 NPs with a mean diameter of 47 nm is clearly observed. The same lattice fringe spacings observed for TTOC-1, 0.35 nm and 0.41 nm, are also observed for TTOC-3 and are attributed to the TiO2 NPs and the CNTs, respectively (HR-TEM image, Supplementary Figure S4d).
The structure and phase purity of the photocatalysts was studied by X-ray diffraction (XRD). The XRD plots of the nanocomposites are displayed in Figure 3. For comparison, the XRD patterns of commercial anatase TiO2 (CTiO2) and pristine CNTs are also presented. The two characteristic peaks of the CNTs, centered at 2θ angles of 25.28° and 44.27°, are indexed to their (002) and (100) crystal planes, respectively20. The first broad peak corresponds to interlayer stacking, and the second, weaker peak is attributed to the interplanar stacking of aromatic systems. The pattern of the commercial TiO2 shows diffraction peaks at 25.28°, 37.56°, 47.77°, 53.72°, 54.73°, 62.51°, 68.65°, 70.07°, 74.79°, and 82.53°, which can be assigned to the (101), (004), (200), (105), (211), (204), (116), (220), (215), and (224) planes, respectively. They are all signature peaks of anatase tetragonal TiO2 (JCPDS card No. 96-500-0224) with space group I41/amd. The diffraction patterns of nanocomposites TTOC-1 and TTOC-3 are well indexed to a combination of anatase TiO2 and CNTs. The presence of a carbon species (2θ = 25.28° and 44.27°) and anatase TiO2 was also observed in the TTOC-2 nanocomposite. The characteristic peaks of rutile TiO2 were absent. The most common forms of TiO2 are the rutile and anatase polymorphs. According to the literature, anatase exhibits greater photocatalytic activity than rutile. Notably, the synthesized nanocomposite contains pure anatase TiO2. The identification of the most intense diffraction peak of carbon (2θ = 25.28°) in the patterns of the nanocomposites was difficult because of overlap with the most intense diffraction peak of TiO2. However, the asymmetry and broadening of the 25.28° peak of TiO2 in the nanocomposites reveal the effect of CNT/carbon species on the diffraction pattern of TiO2. The peaks in the black-dotted rectangle clearly indicate the presence of carbon species (2θ = 44.27°; (100) plane) in all of the prepared nanocomposites.
The surface composition of the nanocomposites was evaluated by X-ray photoelectron spectroscopy (XPS); the results are shown in Figures 4, Supplementary Figures S5, and S6 for TTOC-2, TTOC-1, and TTOC-3, respectively. The survey spectrum affirms the presence of Ti, C, and O in the TTOC-2 (Figure 4a), TTOC-1 (Supplementary Figure S5a), and TTOC-3 (Supplementary Figure S6a) nanocomposites. The Ti-2p XPS spectra of all of the nanocomposites (Figures 4b, Supplementary Figures S5b, and S6b) show two peaks at 457 and 463 eV. However, the Ti-2p XPS signals of Ti4+ should be located at ~459.0 and ~464.5 eV. This shift in the binding energy of Ti4+ to lower energies suggests the presence of Ti3+ dopant. Deconvolution of the Ti-2p peaks reveals the presence of both Ti4+ and Ti3+ in the samples. The signals at 463.08 and 457.73 eV are assigned to Ti-2p1/2 and Ti-2p3/2 of Ti3+, whereas the modes at 464.02 and 459.09 eV are attributed to the Ti-2p1/2 and Ti-2p3/2 of Ti4+, respectively9. A Gaussian fitting of the Ti-2p peaks was used to estimate the Ti3+ content of the nanocomposites quantitatively. Small shoulders of the peaks associated with Ti3+ species compared with the peaks associated with Ti4+ species are noticeable in the spectra of all of the nanocomposites. The calculated Ti4+:Ti3+ ratio for TTOC-1, TTOC-2, and TTOC-3 is 1:0.55, 1:0.74, and 1:0.57, respectively. These results indicate that Ti3+ is more prevalent in TTOC-2 than in TTOC-1 and TTOC-3. The large Ti3+ content in TTOC-2 confirms the high stability of the produced Ti3+ ions. The stability of Ti3+ increases because of the presence of a carbon layer around the Ti3+/TiO2 particles in the TTOC-2 nanocomposite. In TTOC-1 and TTOC-3, Ti3+ ions are located on or near the surface of TiO2, enabling their easy oxidation to Ti4+ and thereby reducing the peak area of Ti3+ ions. The C-1s and O-1s fitting XPS spectra of all the nanocomposites discussed in the Supplementary Section 2 (Supplementary Figures S5 and S6).
The functional groups investigation of the nanocomposites by Fourier transform infrared (FTIR) spectroscopy confirmed the presence of carbon species and TiO2 in all the prepared nanocomposites; this is discussed in the Supplementary Section 3 (Supplementary Figure S7).
UV–vis absorbance was employed to investigate the optical features of the nanocomposites. Figures 5a–d shows the UV–vis absorbance of pristine CNT, TTOC-1, TTOC-2, and TTOC-3, respectively. The pristine CNT shows a characteristic absorption peak at 263 nm. The absorption edge of TTOC-1, TTOC-2, and TTOC-3 nanocomposites was observed at 718, 761, and 738 nm, respectively. The absorption-edge wavelength (λg) was calculated from the intercept between the abscissa coordinate and the tangent of the absorption curve. The absorbance of all the nanocomposites shows almost full-range coverage of VL wavelengths. Among them, the TTOC-2 nanocomposite exhibits the highest λg. The extended VL absorption range might be dependent on the Ti3+ oxygen/vacancy states on the TiO2 surface. The carbon layer improves the stability of the Ti3+ in the TTOC-2 composite compared with that in the TTOC-1 and TTOC-3 composites.
The bandgaps of the nanocomposites and pristine CNT were calculated from plots (αhʋ)2 vs. E, where the intercept to the E axis denotes Eg where (αhʋ)2 = 0. Supplementary Figures S8a–S8d show the Tauc plots of pristine CNT, TTOC-1, TTOC-2, and TTOC-3. The calculated bandgaps of the CNTs and the TTOC-1, TTOC-2, and TTOC-3 nanocomposites are 3.47, 2.08, 1.93, and 2.04 eV, respectively. The bandgap of the nanocomposites is dramatically lowered by the introduction of carbonaceous species and Ti3+ ions. This smaller bandgap makes the nanocomposites applicable in the visible range, which is one of the criteria for a good photocatalyst. In the case of a small bandgap, low-energy light is sufficient to excite valence-band (VB) electrons into the conduction band (CB). The lowest bandgap of TTOC-2 among the nanocomposites is attributed to the formation of a carbon layer.
The surface area and pore size were assessed by N2 absorption–desorption isotherm analysis. The Brunauer–Emmett–Teller (BET) surface area and pore size of the pristine CNTs are 255.39 m2/g and 79.79 Å, respectively. The high surface area of the CNTs decreased in all of the TTOC samples, suggesting the formation of composites.
The BET surface area of the TTOC-1, TTOC-2, and TTOC-3 nanocomposites was 23.53, 29.95, and 24.86 m2/g, respectively. The reason for the relatively greater surface area of TTOC-2 compared with those of TTOC-1 and TTOC-2 is the formation of small Ti3+/TiO2 NPs. The carbon layer is the driving force for the formation of small-sized NPs. The average pore size of the TTOC-1, TTOC-2, and TTOC-3 nanocomposites was 264.00, 401.23, and 394.79 Å, respectively. The pore size depends on the Ti3+ ions; that is, a high content of Ti3+ indicates a large pore content. The obtained results are consistent with this relationship. Large pores of a catalyst promote the adsorption of organic molecules on its surface during photodegradation, thus increasing its photocatalytic activity21. The surface area and pore size values are presented in Table 1. The adsorption–desorption isotherms of the pristine CNTs and the TTOC-1, TTOC-2, and TTOC-3 nanocomposites showed in Supplementary Figures S9a–9d, respectively and discussed in Supplementary Section 4.
Table 1. BET surface area and pore size of the pristine CNTs and the nanocomposites.
Surface area (m²/g)
Pore size (Å)
Visible-light-driven photocatalytic performance of the nanocomposites. The photocatalytic performance of the TTOC-1, TTOC-2, and TTOC-3 nanocomposites and pristine CNTs was evaluated on the basis of MB degradation under irradiation by a Xe lamp. MB was chosen because it is persistent and widely used in various industries. For comparison, a blank reaction without a catalyst (WC) was also conducted. Figure 6a shows the degradation percentage over the illumination time; the first-order kinetics of the reaction are represented in Figure 6b. The MB degradation percentage was approximately 83%, 98%, and 93% for the TTOC-1, TTOC-2, and TTOC-3 nanocomposites, respectively, after 25 min of VL irradiation. The self-deterioration of MB was trivial under irradiation of VL. In addition, the pristine CNTs showed no noticeable photocatalytic activity. The change in MB concentration under dark conditions was also measured at regular time intervals. During the adsorption–desorption period, the reduction of the MB concentration after 25 min was negligible. The reaction rate of the MB decomposition on the pristine CNTs and the TTOC-1, TTOC-2, and TTOC-3 nanocomposites was 0.0083, 0.07, 0.15, and 0.10 min−1, which is 166%, 1400%, 3000%, and 2000% greater, respectively, than the rate of the WC reaction (0.005 min−1). All of the prepared nanocomposites showed greater photocatalytic activity because of the coexistence of Ti3+ and carbon species along with TiO2. The bandgap of Ti3+-TiO2 differs from that of pure TiO2 and can utilize a wide wavelength range of VL radiation for exciting the VB electrons. In addition, the carbon species increased the adsorption of pollutants and reduced the e−/h+ recombination rate, thus enhancing the photodegradation efficiency. Because of its greater CNT content, TTOC-3 exhibited greater photocatalytic activity than TTOC-1. However, the photocatalytic performance of TTOC-2 was better than that of TTOC-1 and TTOC-3. In TTOC-2, the Ti3+ stability is improved by the presence of a carbon layer around the Ti3+-TiO2 NPs. The carbon layer also substantially decreases the particle size and simultaneously increases the specific surface area. A high specific surface area enhances the photodegradation efficiency because the reaction occurs at the surface. Discoloration images of an MB solution in the presence of the TTOC-1, TTOC-2, and TTOC-3 nanocomposites are presented in Supplementary Figures S10a, S10b, and S10c, respectively. The results confirm the excellent changes in MB concentration within short periods.
In addition, the degradation activity of TTOC-2 toward MO and RhB dyes was investigated under similar experimental conditions. The photodegradation ratio (Ct/C0) over the illumination time is plotted in Supplementary Figure S11a. Within 25 min, only 28% of the MO was degraded, whereas 71% of the RhB was degraded. The linear relation of ln(C0/Ct) vs illumination time (Supplementary Figure S11b) confirms the first-order reaction kinetics. The rate constants of the RhB and MO degradation reactions were 0.0452 and 0.0147 min−1, which are 904% and 307% higher than the rate constants of the corresponding reactions without a catalyst. The degradation rate of RhB is greater than that of MO because the azo bond of MO is more difficult to rupture than the C=N bond of RhB22,23.
Effect of pH and point of zero charge (PZC) on photocatalytic degradation. The determination of the pH at the point-of-zero-charge (pHPZC) and an evaluation of the effect of pH on photodegradation are critical. The pH of the mixture affects the solubility of dyes and the surface chemistry of the adsorbent. The pHPZC demonstrates a sample's surface charge. The drift method was used for pHPZC calculation in the pH range between 2 and 12. HCl and NaOH were employed to control the pH of the solution. Supplementary Figure S12 shows a graph of (pHi − pHf) vs pHi, where pHi and pHf and the initial and final pH, respectively. The measured pHPZC (where the final pH is equal to the initial value) was 9.31, 9.92, and 9.63 for TTOC-1, TTOC-2, and TTOC-3, respectively. These results imply that the nanocomposites are cationic at pH levels below the pHPZC and anionic at pH levels greater than the pHPZC. To verify this speculation, the effect of pH on the degradation of MB by TTOC-2 was evaluated in the range 2 ≤ pH ≤ 12 under similar experimental conditions; the results are presented in Supplementary Figure S13. Superior photodegradation was observed at pH 12, whereas the worst performance was obtained at pH 2 (Supplementary Figure S13a). The photodegradation ratio clearly increases with increasing pH because MB is cationic at pH values greater than 5.8 (pKa = 5.8). In a basic medium, the electrostatic attraction between the cationic MB and the catalyst's negative surface increases. The opposite effect is observed in an acidic medium, and the photocatalytic efficiency is decreased. The results of kinetics studies of the effect of pH on MB degradation are shown in Supplementary Figure S13b. The linear relation between ln(C0/Ct) and irradiation time confirms first-order kinetics. The rate constant of the reaction at pH 2, 5, 9, and 12 was 0.0163, 0.0314, 0.0754, and 0.1454 min−1, respectively. The rate constant of the reaction at pH 12 was 892% greater than that of the reaction at pH 2. The superior photodegradation of the as-synthesized nanocomposites at high pH levels also confirms the presence of negative surface groups when the nanocomposites are in a basic medium. Discoloration images of MB solution at pH 2 (after 25) and pH 12 (after 20 min) are presented in Supplementary Figures S10d and S10e, respectively.
In addition to the efficiency of a photocatalyst, its stability and reusability are also essential parameters for evaluating its performance. Reusability experiments were performed using recovered nanocomposites. The degradation ratio over five consecutive cycles is presented in Supplementary Figure S14a. No significant changes were observed throughout the runs. The degradation efficiencies after five cycles were 81.0%, 96.0%, and 90.2% for TTOC-1, TTOC-2, and TTOC-3, respectively. The XRD patterns of the three nanocomposites were collected (Supplementary Figure S14b) after five consecutive runs and showed no obvious differences from the patterns of the fresh samples. The XRD crystal planes of the samples before reaction and after the reaction are consistent. The low-intensity characteristic peaks of the (100) plane of carbon species (2θ = 44.27°) are also observed. These results indicate excellent reusability and stability of the nanocomposites.
The photodegradation activity of the as-synthesized nanocomposites is compared with that of various reported catalysts in Supplementary Table S1. This comparison demonstrates that our composites show substantial photocatalytic activity under VL.
LC–MS analysis. The intermediate products in the MB degradation were examined through liquid chromatography–mass spectrometry (LC–MS) experiments. The mass spectra of an aqueous MB solution after photocatalytic reaction (25 min) with the TTOC-2 nanocomposite are presented in Supplementary Figure S15. The characteristic m/z 284 of MB was hardly observed in the mass spectrum collected after photodegradation. The high-energy electrons and OH• are responsible for the deterioration of MB. Structures of the MB degradation intermediate products were proposed on the basis of their m/z ratios. The direct hydroxylation of MB is dominant, and products at m/z ratios of 338 and 384, which are greater than that of MB, were obtained. The other hydroxylated and demethylated products were detected at m/z ratios of 325, 265, and 255. The degradation products at m/z ratios of 295 and 312 were obtained through rupture of the C– C bonds by OH• radicals (oxidation reaction). The successful rupture of the MB molecule was supported by the presence of smaller m/z peaks at 110, 168, 177, and 217. The presence of the peaks at m/z ratios of 145 and 102 indicates the complete breaking up of aromatic rings in MB. The smaller fragmented products lead to total degradation of the MB molecules into nontoxic acids (e.g., acetic acid and oxalic acid) or to mineralized inorganic substances (e.g., CO2, H2O, SO42−, and NO3−). A possible pathway of MB degradation based on the intermediate products obtained in the mass spectra is displayed in Supplementary Figure S16. The results thus confirm the effectiveness of the as-prepared nanocomposites as photocatalysts for MB dye degradation.
CB and VB edge potential was estimated to illustrate the photocatalytic mechanism of as-synthesized photocatalyst. Proposed mechanism path of photocatalytic performance was discussed in Supplementary Section 5. The schematic reaction mechanism with redox couples and energy band positions is shown in Figure 7.