Abstract
A highly efficient and elaborately structured visible-light-driven catalyst composed of mesoporous TiO2 (MT) doped with Ag+-coated graphene (MT-Ag/GR) has been successfully fabricated by a sol–gel and solvothermal method. The as-prepared catalyst has been investigated by X-ray diffraction (XRD), photoluminescence spectroscopy (PL), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). Inexpensive, stable MT was coupled with hole-accepting graphene (GR) and electron-trapping silver induced higher activities than those achieved with pure MT, MT-Ag, and MT/GR in the degradation of methylene blue (MB) in solution. Although the Ag dopant and graphene support were responsible for narrowing the band gap of TiO2 and shifting its optical response, respectively, they acted synergistically in shifting absorbance from the ultraviolet (UV) to the visible-light region with a smaller band-gap energy. Meanwhile, they also served to lower the photo-induced electron and hole recombination rate, and increased the specific surface area and the concentrations of Ti3+ ions and hydroxyl groups. In degradation studies, the effects of catalyst amount, pH, and initial MB concentration have been examined as operational parameters. A photocatalytic mechanism for the action of MT-Ag/GR is proposed, and possible reasons for the enhancement in visible-light photocatalytic efficiency are discussed.
3.1.6. N2 adsorption/desorption isotherm analysis. As can be seen from Fig. 6a, the isotherms of all the samples were of the typical Langmuir type IV form, suggesting that mesopores were widely distributed therein.29,30 Compared to the isotherm of MT-Ag, with a typical H3 hysteresis loop, the isotherms of the other samples presented H2 hysteresis loops. Meanwhile, the hysteresis loop of MT/GR was also obviously different from those of pure MT and MT-Ag/GR. This indicated that the mesoporous structure of MT was changed by Ag or GR modification, consistent with the shapes of the pore size distribution curves (Fig. 7b). All pore sizes of the samples were in the range 9–50 nm, indicating the presence of mesopores with a slit-shaped structure. By incorporating Ag or GR, the surface area and pore volume of MT were increased and its pore size decreased, as determined by N2 physisorption experiments as listed in Table 1. MT-Ag/GR showed the highest surface area and pore volume, but the lowest pore size. The porosity difference may be attributed either to the stacking structure or the introduction of GR and Ag, prevented the agglomeration of MT particles.23
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| Fig. 7 N2 adsorption–desorption isotherms (a) and pore size distributions (b) of the samples. | |
Table 1 Specific surface areas, total pore volumes, and pore sizes of pure MT, MT-Ag, MT/GR, and MT-Ag/GR
Samples |
SBET (m2 g−1) |
Vt (cm3 g−1) |
Pore radius (nm) |
˙OH concentration (%) |
Ti3+ concentration (%) |
MT |
129.729 |
0.206 |
22.620 |
11.7 |
4.6 |
MT-Ag |
169.321 |
0.211 |
18.303 |
13.6 |
9.7 |
MT/GR |
194.540 |
0.306 |
15.597 |
14.9 |
11.3 |
MT-Ag/GR |
242.272 |
0.425 |
11.099 |
16.5 |
14.5 |