Regulating Crystal Structure and Atomic Arrangement in Single-Component Metal Oxides through Electrochemical Conversion for Efficient Overall Water Splitting.

Single-component transition-metal oxide (TMO: FeOx, NiOx, or CoOx) nanosheets grown on nickel foam (NF) were electrochemically optimized with Li ion (Na ion)-induced conversion reaction for bifunctional electrocatalysis. The optimum FeOx/NF-Li electrocatalyst exhibits low overpotentials of 239 mV for hydrogen evolution reaction and 276 mV for oxygen evolution reaction at a current density of 100 mA cm-2. A two-electrode water splitting cell using FeOx/NF-Li as both anode and cathode requires only 1.60 V to achieve a current density of 10 mA cm-2. The impressive water splitting performance of the FeOx/NF-Li electrode is revealed to be attributed to Li-induced electrochemical conversion, which alters the crystal structure, creating more active sites for electrocatalytic reactions, as well as introduces O vacancies increasing the electron density and the intrinsic conductivity. More importantly, the atomic arrangement is regulated from tetrahedral Fe(Td) to octahedral Fe(Oh) coordination, which acts as catalytically active sites with reduced Gibbs free energy for the rate-determining steps. This electrochemical conversion reaction can be extended to other TMOs (i.e., NiOx/NF and CoOx/NF) for promoted electrocatalytic water splitting performances. This study provides an in-depth understanding on the nature of atomic and electronic structure evolution to promote the electrocatalytic activity.

Synergy of Dopants and Defects in Graphitic Carbon Nitride with Exceptionally Modulated Band Structures for Efficient Photocatalytic Oxygen Evolution.

Electronic structure greatly determines the band structures and the charge carrier transport properties of semiconducting photocatalysts and consequently their photocatalytic activities. Here, by simply calcining the mixture of graphitic carbon nitride (g-C3 N4 ) and sodium borohydride in an inert atmosphere, boron dopants and nitrogen defects are simultaneously introduced into g-C3 N4 . The resultant boron-doped and nitrogen-deficient g-C3 N4 exhibits excellent activity for photocatalytic oxygen evolution, with highest oxygen evolution rate reaching 561.2 µmol h-1 g-1 , much higher than previously reported g-C3 N4 . It is well evidenced that with conduction and valence band positions substantially and continuously tuned by the simultaneous introduction of boron dopants and nitrogen defects into g-C3 N4 , the band structures are exceptionally modulated for both effective optical absorption in visible light and much increased driving force for water oxidation. Moreover, the engineered electronic structure creates abundant unsaturated sites and induces strong interlayer C-N interaction, leading to efficient electron excitation and accelerated charge transport. In the present work, a facile approach is successfully demonstrated to engineer the electronic structures and the band structures of g-C3 N4 with simultaneous introduction of dopants and defects for high-performance photocatalytic oxygen evolution, which can provide informative principles for the design of efficient photocatalysis systems for solar energy conversion.

Bifunctional Modification of Graphitic Carbon Nitride with MgFe2O4 for Enhanced Photocatalytic Hydrogen Generation.

To gain high photocatalytic activity for hydrogen evolution, both charge separation efficiency and surface reaction kinetics must be improved, and together would be even better. In this study, the visible light photocatalytic hydrogen production activity of graphitic carbon nitride (g-C3N4) was greatly enhanced with MgFe2O4 modification. It was demonstrated that MgFe2O4 could not only extract photoinduced holes from g-C3N4, leading to efficient charge carrier separation at the g-C3N4/MgFe2O4 interface, but also act as an oxidative catalyst accelerating the oxidation reaction kinetics at g-C3N4 surface. This dual function of MgFe2O4 thus contributed to the great improvement (up to three-fold) in photocatalytic activity for hydrogen generation over g-C3N4/MgFe2O4 as compared to pristine g-C3N4, after loading Pt by photoreduction method. It was revealed that in the Pt/g-C3N4/MgFe2O4 system, the photoinduced electrons and holes were entrapped by Pt and MgFe2O4, respectively, giving rise to the promoted charge separation; moreover, as evidenced by electrochemical analysis, the electrocatalysis effect of MgFe2O4 benefited the oxidation reaction at g-C3N4 surface.