(558h) Multiple Applications of 2D Titanium Carbide (MXene) in Photoelectrochemical Water Splitting: Cocatalysts, Surface Modifications and Matrix for Chemical Transformation

Transition metal carbides and carbonitrides (MXenes), a newly explored family of 2D materials has expanded rapidly since discovered in 2011. Due to their unique in-plane anisotropic structures, excellent metallic conductivity and hydrophilicity, MXenes have shown attractive applications in energy storage and biology fields. As the most studied MXenes, 2D titanium carbides (Ti₃C₂T_x MXene) can be readily obtained by etching the Al layer of MAX phase Ti₃AlC₂ with HF. The T_x was the surface terminated functional groups such as –O, –F, or –OH. Although the Ti₃C₂T_x MXene has been used in photocatalysis, the potential applications of the Ti₃C₂T_x MXene in photoelectrochemical (PEC) water splitting have not been rigorously investigated. The present study focuses on the multiple applications of 2D Ti₃C₂T_x MXene in PEC water splitting, which has been carried out from the following three aspects:

1. As co-catalysts, the 2D Ti₃C₂T_x MXene can facilitate the charge transfer of the photogenerated carriers and reduce the charge recombination of the BiVO₄ photoanode. A composite photoanode of Ti₃C₂T_x/BiVO₄ was fabricated by spin coating thin Ti₃C₂T_x MXene flakes onto the surface of a BiVO₄ film grown on the FTO substrate. The coating of thin Ti₃C₂T_x flakes onto the BiVO₄ film would boost the photocurrent density from 2.95 to 3.45 mA cm^−2, exhibiting a photoconversion efficiency of 0.78% and a surface charge separation efficiency of 73%.
2. For surface modifications, the oxygen vacancies enriched Ti₃C₂T_x flakes can be obtained by reducing Ti₃C₂T_x with H₂/Ar gas (denoted as H:Ti₃C₂T_x). The H:Ti₃C₂T_x/Cu₂O, Ti₃C₂T_x/Cu₂O as well as Au/Cu₂O composite photocathodes are fabricated through a facile dip-coating method. The H:Ti₃C₂T_x/Cu₂O and Ti₃C₂T_x/Cu₂O yield a photocurrent density of -5.41 and -4.45 mA cm^-2 respectively, which is higher than the bare Cu₂O and Au/Cu₂O (-4.31 mA cm^-2). The results suggest that the Ti₃C₂T_x is more effective than noble metal Au for improving the PEC performance of Cu₂O photocathode. The superior performance of H:Ti₃C₂T_x/Cu₂O is attributed to enriched oxygen vacancies in the H:Ti₃C₂T_x which can enhance the conductivity, light-harvesting efficiency and charge-transfer ability of fabricated photocathode.
3. As matrix for chemical transformation, the {001} facets exposed TiO₂ flakes were derived from the delaminated Ti₃C₂T_x MXene (D-Ti₃C₂T_x) without adding extra HF or F ions by a facile hydrothermal synthesis because the surface of D-Ti₃C₂T_x was natively terminated by -F functional groups. The Sn-doped α-Fe₂O₃ nanorods were embedded with the {001} facets exposed TiO₂ flakes (denoted as E-Sn-Fe₂O₃/TiO₂). The Sn-doped hematite/surface-covered TiO₂ (denoted as S-Sn-Fe₂O₃/TiO₂) and Sn-doped hematite (denoted as Sn-Fe₂O₃) were also fabricated for comparison. Under simulated sunlight, the E-Sn-Fe₂O₃/TiO₂ yielded a photocurrent density of 0.85 mA cm^-2 at 1.23 V (vs. RHE) compared with 0.72 mA cm^-2 and 0.42 mA cm^-2 for S-Sn-Fe₂O₃/TiO₂ and Sn-Fe₂O₃, respectively. The E-Sn-Fe₂O₃/TiO₂ achieved an IPCE value of 18.6% at 380 nm (15.8% for S-Sn-Fe₂O₃/TiO₂ and 10.4% for Sn-Fe₂O₃) and exhibited enhanced IPCE values at all measured wavelengths compared with the other two photoanodes. The unique architecture of embedded TiO₂ can improve the interfacial contact between the α-Fe₂O₃ and TiO₂ as well as the light harvesting of the α-Fe₂O₃ compared with the surface-covered TiO₂, contributing to the better charge transport ability and enhanced PEC performance.