ABSTRACT
The ZnIn2S4/BiVO4 heterostructures were elegantly designed through assembling ZnIn2S4 nanosheets onto the surface of BiVO4 decahedrons. This composite photocatalyst exhibits efficient photocatalytic conversion of CO2 into CO with a detectable amount of CH4 in the presence of water vapor. An electron spin-resonance spectroscopy (ESR) technique and density function theory (DFT) calculation affirm the direct Z-scheme structure in ZnIn2S4/BiVO4. The larger surface photovoltage (SPV) change and the longer liquid photoluminescence (PL) lifetime of the heterostructure, compared to the individual ZnIn2S4 and BiVO4 components, demonstrate that the Z-scheme structure can effectively promote the recombination of the photogenerated holes in the valence band (VB) of the ZnIn2S4 nanosheet with the electrons in the conduction band (CB) of the decahedral BiVO4 and lead to the abundant electrons surviving in the CB of ZnIn2S4 and holes in the VB of BiVO4, thus enhancing photocatalytic CO2 reduction performance. This study may make a potential contribution to the rational construction and deep understanding of the underlying mechanism of direct Z-schemes for advanced photocatalytic activity.
ABSTRACT
Effective oxygen evolution reaction (OER) catalysts composed of Earth-abundant transition metals are crucial for sustainable energy conversion and storage. Metal-organic frameworks (MOFs) with tunable compositions are promising precursors for the fabrication of hollow and porous electrocatalysts. However, pulverous MOFs usually suffer from agglomeration during pyrolysis, greatly reducing the activity of their derived catalysts. In this work, Prussian blue analogue (PBA) arrays with hierarchical multidimensional architecture were directly grown on nickel foam (NF) using a template-oriented method. The subsequent calcination in air allowed for obtaining NixCo3-xO4 nanoplate arrays consisting of porous and hollow nanocubes. The derived bimetallic NixCo3-xO4/NF required only an overpotential of 287 mV to achieve a current density of 10 mA cm-2 in 1.0 M KOH solution, which is much lower than that of the monometallic NiO and the RuO2 benchmark. The 3D intersectional architecture of the NixCo3-xO4 nanoplates and the porous and hollow nanocube subunits contributed to the large specific surface area and reduced charge-transfer resistance of the NixCo3-xO4/NF electrode. Density functional theory (DFT) calculations and post-OER characterization revealed that the incorporated Co was the active sites and electrochemical active CoOOH intermediates were in situ formed during the OER. Our study provides a facile and efficient strategy for the rational design of MOF-derived materials towards effective and low-cost electrocatalysis.
ABSTRACT
Metal-organic frameworks (MOFs) as versatile templates for preparing transition metal compounds has received wide attention. Benefiting from their diversified spatial structure and controllable chemical constituents, they have become a research hotspot in the field of electrocatalytic water splitting. Herein, Fe2Ni-MIL-88B MOF on nickel foam (Fe2Ni MOF/NF) has been prepared through a one-pot method growth process. Compared with Fe MOF/NF and Ni MOF/NF, the interaction between Fe3+ and Ni2+ in Fe2Ni MOF/NF accelerates the electron transfer through the oxygen of the ligand, leading to increased 3d orbital electron density of Ni, which enhances the activity of the oxygen evolution reaction (OER) in alkaline solution. Fe2Ni MOF/NF provides a current density of 10 mA cm-2 at a low overpotential of 222 mV, and its Tafel slope is also very small, reaching 42.39 mV dec-1. The success of the present Fe2Ni MOF/NF catalyst is attributed to the abundant active centers, the bimetallic clusters Fe2Ni-MIL-88B, the positive coupling effect between Ni and Fe metal ions in the MOF, and synergistic effect between the MOF and NF. Besides, Fe2Ni MOF/NF possesses excellent stability over 50 h of continuous operation, providing feasibility for commercial use.
