Date Published: March 30, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Wenyu Yuan, Laifei Cheng, Yurong An, Shilin Lv, Heng Wu, Xiaoli Fan, Yani Zhang, Xiaohui Guo, Junwang Tang.
TiO2 is an ideal photocatalyst candidate except for its large bandgap and fast charge recombination. A novel laminated junction composed of defect‐controlled and sulfur‐doped TiO2 with carbon substrate (LDC‐S‐TiO2/C) is synthesized using the 2D transition metal carbides (MXenes) as a template to enhance light absorption and improve charge separation. The prepared LDC‐S‐TiO2/C catalyst delivers a high photocatalytic H2 evolution rate of 333 µmol g−1 h−1 with a high apparent quantum yield of 7.36% at 400 nm and it is also active even at 600 nm, resulting into a 48 time activity compared with L‐TiO2/C under visible light irradiation. Further theoretical modeling calculation indicates that such novel approach also reduces activation energy of hydrogen production apart from broadening the absorption wavelength, facilitating charge separation, and creating a large surface area substrate. This synergic effect can also be applied to other photocatalysts’ modification. The study provides a novel approach for synthesis defective metal oxides based hybrids and broaden the applications of MXene family.
A sustainable society heavily relies on clean and abundant energy supply.1 Hydrogen generation through the splitting of water by photocatalysis has been considered as a promising solution to the current energy and environmental dilemma.2 In the past decades, various semiconductors, such as TiO2,3 CdS,4 ZnO,5 C3N4,6 and WO3,7 were used to water splitting by harnessing solar energy.8 Among these materials, TiO2, a kind of semiconductor with a band gap of 3.0–3.2 eV, has been considered as one of promising candidates for photocatalytic H2 generation owing to its excellent stability and low cost.9
The synthetic procedures for the LDC‐S‐TiO2/C are illustrated in Figure1. First, 2D Ti3C2MXenes were synthesized via the selectively etching of Ti3AlC2MAX phases. The morphology of Ti3C2 is shown in Figure S1 in the Supporting Information. A laminated structure can be obtained after selectively etching. After exfoliation, the d‐spacing of (002) plane was enlarged to 1.03 nm. Wherein, sulfur reactant was impregnated and adhered on the surface of Ti3C2MXenes by a melt‐diffusion process. Then, the L‐S‐TiO2/C hybrids were fabricated via the mild CO2 oxidation of S‐Ti3C2 mixture. Finally, the LDC‐S‐TiO2/C hybrids were synthesized by the air oxidation of L‐S‐TiO2/C hybrids. As shown in Figure 1, the TiO2 nanoparticles with uniform particle size are anchored on ultrathin carbon layers. The structure is stable during air oxidation process because the structure has not changed obviously (Figure S2, Supporting Information). The average size of TiO2 nanoparticles (NPs) is ≈50 nm. However, the excess generated carbon in L‐S‐TiO2/C hybrids can shield light at the surface of TiO2.27 To reduce the amounts of carbon and increase defect concentration, the further oxidation process in air atmosphere was carried out. After the air oxidation, more TiO2 nanoparticles have been exposed.
In conclusion, we demonstrated a novel approach for the synthesis of laminated defect controlled S‐doped TiO2 on carbon substrate (LDC‐S‐TiO2/C) involving an S impregnation of Ti3C2MXenes and the subsequent two‐step oxidation processes. This novel method can simultaneously achieve the doping of TiO2 and the defect‐engineered carbon substrate. The H2 evolution rate under visible light irradiation can reach up to 333 µmol g−1 h−1, in addition, a high AQY of 7.36% at 400 nm can be realized, owing to the synergistic effect of porous carbon substrate and S doping. Specially, the porous carbon substrate provides the pathway for electrons separation, leading to high charge separation efficiency with a large surface area. More importantly, laminated carbon substrate as a cocatalyst can significantly reduce the ∆GH*, provide more active sites, and shorten the diffusion path of electrons, which accelerates the photocatalytic hydrogen production. In parallel, sulfur doping can reduce the band gap of TiO2, leading to an outstanding response from UV to visible light. The hybrid is also composed of earth abundant elements, thus expecting as a low cost and efficient photocatalyst. All these together shed a new insight in material design strategy for highly active laminated hybrid photocatalysts for solar energy conversion and environmental purification.
The Preparation of Ti3C2: Typically, 1 g Ti3AlC2 was added into 10 mL hydrofluoric acid (HF) (40 wt%). The solution was stirred for 48 h at 45 °C. After HF etching, the Ti3C2 was gained by centrifugation and washing with deionized water until pH ≈ 7. After filtration, the Ti3C2 powder was vacuum dried at 60 °C for 12 h.
The authors declare no conflict of interest.