Topotactic Transformation of Solvated MgCr-LDH Nanosheets to Highly Efficient Porous MgO/MgCr 2 O 4 Nanocomposite for Photocatalytic H 2 Evolution Susanginee Nayak, † Amaresh C. Pradhan, ‡ and K. M. Parida* ,† † Centre for Nano Science and Nano Technology, Institute of Technical Education and Research, Siksha ‘O’ Anusandhan Deemed to be University, Bhubaneswar-751030, Odisha, India ‡ UNAM-National Nanotechnology Research Center, Bilkent University, Ankara 06800, Turkey * S Supporting Information ABSTRACT: The hybrid structure of nanoparticles (NPs) with nano- sheets 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, MgCr 2 O 4 spinel NPs decorated on highly porous MgO nanosheets forming MgO/MgCr 2 O 4 (x) nano- composites 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 H 2 O, CO 2 , and NH 3 under moderate temperature during the heat treatment process. The synergistic effect of much wider band gap MgO nanosheets and narrow band gap MgCr 2 O 4 NPs added increased stability due to the stronger bonding coordination of MgCr 2 O 4 NPs with MgO nanosheets. The obtained MgO/MgCr 2 O 4 (x) nanocomposites possess large specific surface areas, highly porous structure, and excellent interface between MgCr 2 O 4 NPs and MgO nanosheets, which proved from N 2 sorption isotherm, TEM, HR-TEM study. With metallic ratio of MgCr3:1, MgO/MgCr 2 O 4 (MgCr3:1) nanocomposites exhibit highest H 2 evolution rate of 840 μmolg -1 2h -1 , which was 2 times higher than that of pure MgCr 2 O 4 (420 μmolg -1 2h -1 ). The LSV measurement study of MgO/MgCr 2 O 4 (MgCr3:1) nanocomposite shows an enhancement of light current density of 0.22 μA/ cm 2 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/MgCr 2 O 4 (MgCr3:1) nanocomposite, which facilitates the flow of charge carriers. The EIS and Bode phase plot of MgO/MgCr 2 O 4 (MgCr3:1) nanocomposite signifies the lower interfacial charge transfer resistance and higher lifetime of electrons (2.7 ms) for enhanced H 2 production. Lastly, the enhanced photocatalytic H 2 production activity and long-term stability of MgO/MgCr 2 O 4 (MgCr3:1) could be attributed to maximum specific surface area, porous structure, close intimacy contact angle between two cubic phases of MgCr 2 O 4 NPs and MgO nanosheets, abundant oxygen vacancies sites, reduced charge transfer resistance and suitable band edge potential to drive the thermodynamic energy for H 2 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 H 2 production utilizing abundant solar energy. ■ INTRODUCTION Two-dimensional (2D) layered nanostructured materials triggered hot research spot for photocatalytic H 2 production with the primary goal being improved charge carrier separation, transport characteristics, short lateral diffusion length, and morphological aspects to improve apparent conversion efficiency. 1-5 Among various Cr-based multi- component spinel-type metal oxides, magnesium chromite, symbolized as MgCr 2 O 4 , materials have received special attention due to their unique crystalline structure and remarkable properties toward magnetism, 6-9 interconnection material for solid fuel cells, 10 high temperature ceramics, 11 humidity sensor elements, 12 catalysts support, 13 strengthening agents, 14 and combustion catalysts. 15-17 Recent reports have highlighted the potential of MgCr 2 O 4 as an efficient photo- catalyst. 18 MgCr 2 O 4 is a narrow band gap semiconductor with band gap lying within 1.3 to 1.8 eV and crystallizes in normal cubic spinel structure in which the bivalent Mg-ions occupy the tetrahedral sites and the trivalent Cr-ions hold the octahedral sites. 19,20 The heterogeneous cationic substitution could lead to multiple valence states at these multimetal centers and reduce the activation energy for electron transport between cations and endow desirable electronic conductiv- Received: June 1, 2018 Published: June 27, 2018 Article pubs.acs.org/IC Cite This: Inorg. Chem. 2018, 57, 8646-8661 © 2018 American Chemical Society 8646 DOI: 10.1021/acs.inorgchem.8b01517 Inorg. Chem. 2018, 57, 8646-8661 Downloaded via BILKENT UNIV on February 25, 2019 at 18:03:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.