Topotactic Transformation of Solvated MgCr-LDH Nanosheets to Highly Efficient Porous MgO/MgCr2O4 Nanocomposite for Photocatalytic H2 Evolution.

The hybrid structure of nanoparticles (NPs) with nanosheets has the advantage of both anisotropic properties of NPs and large specific surface areas of nanosheets, which is desirable for many technological applications. In this study, MgCr2O4 spinel NPs decorated on highly porous MgO nanosheets forming MgO/MgCr2 O4( x) nanocomposites were synthesized by a one pot coprecipitation method followed by a heat treatment process of the solvated wet gel of MgCr-LDH with polar solvent N, N-dimethylformamide (DMF) at 400 °C. This novel synthetic methodology generates materials consisting of porous metal oxides nanosheets adhered with spinel phase NPs due to the slow generation of gases such as H2O, CO2, and NH3 under moderate temperature during the heat treatment process. The synergistic effect of much wider band gap MgO nanosheets and narrow band gap MgCr2O4 NPs added increased stability due to the stronger bonding coordination of MgCr2O4 NPs with MgO nanosheets. The obtained MgO/MgCr2 O4( x) nanocomposites possess large specific surface areas, highly porous structure, and excellent interface between MgCr2O4 NPs and MgO nanosheets, which proved from N2 sorption isotherm, TEM, HR-TEM study. With metallic ratio of MgCr3:1, MgO/MgCr2O4(MgCr3:1) nanocomposites exhibit highest H2 evolution rate of 840 μmolg-12h-1, which was 2 times higher than that of pure MgCr2O4(420 μmolg-12h-1). The LSV measurement study of MgO/MgCr2O4 (MgCr3:1) nanocomposite shows an enhancement of light current density of 0.22 μA/cm2 at potential bias of -1.1 V. The Mott-Schottky analysis suggested the band edge positions of the n-type constituents and formation of n-n type heterojunctions in MgO/MgCr2O4 (MgCr3:1) nanocomposite, which facilitates the flow of charge carriers. The EIS and Bode phase plot of MgO/MgCr2O4 (MgCr3:1) nanocomposite signifies the lower interfacial charge transfer resistance and higher lifetime of electrons (2.7 ms) for enhanced H2 production. Lastly, the enhanced photocatalytic H2 production activity and long-term stability of MgO/MgCr2O4(MgCr3:1) could be attributed to maximum specific surface area, porous structure, close intimacy contact angle between two cubic phases of MgCr2O4 NPs and MgO nanosheets, abundant oxygen vacancies sites, reduced charge transfer resistance and suitable band edge potential to drive the thermodynamic energy for H2 production. This work highlighted an effective strategy for the synthesis of cost-effective 2D porous heterojunctions nanocomposite photocatalyst for promising applications in the field of clean H2 production utilizing abundant solar energy.