α-MnO2 Nanowire-Anchored Highly Oxidized Cluster as a Catalyst for Li-O2 Batteries: Superior Electrocatalytic Activity and High Functionality.

An effective chemical way to optimize the oxygen electrocatalyst and Li-O2 electrode functionalities of metal oxide can be developed by the control of chemical bond nature with the surface anchoring of highly oxidized selenate (SeO4 2- ) clusters. The bond competition between (Se6+ -O) and (Mn-O) bonds is quite effective in stabilizing Jahn-Teller-active Mn3+ state and in increasing oxygen electron density of α-MnO2 nanowire (NW). The selenate-anchored α-MnO2 NW shows excellent oxygen electrocatalytic activity and electrode performance for Li-O2 batteries, which is due to the improved charge transfer kinetics and reversible formation/decomposition of Li2 O2 . The present study underscores that the surface anchoring of highly oxidized cluster can provide a facile, effective way of improving the oxygen electrocatalyst and electrochemical performances of nanostructured metal oxide in Li-O2 cells.

Porous Hybrid Network of Graphene and Metal Oxide Nanosheets as Useful Matrix for Improving the Electrode Performance of Layered Double Hydroxides.

Mesoporous hybrid network of reduced graphene oxide (rG-O) and layered MnO(2) nanosheets could act as an efficient immobilization matrix for improving the electrochemical activity of layered double hydroxide (LDH). The control of MnO(2) /rG-O ratio is crucial in optimizing the porous structure and electrical conductivity of the resulting hybrid structure. The immobilization of Co-Al-LDH on hybrid MnO(2) /rG-O network is more effective in enhancing its electrode activity compared with that of on pure rG-O network. The Co-Al-LDH-rG-O-MnO(2) nanohybrid deliveres a greater specific capacitance than does MnO(2) -free Co-Al-LDH-rG-O nanohybrid. The beneficial effect of MnO(2) incorporation on the electrode performance of nanohybrid is more prominent for higher current density and faster scan rate, underscoring the significant enhancement of the electron transport of Co-Al-LDH-rG-O. This is supported by electrochemical impedance spectroscopy. The present study clearly demonstrates the usefulness of the porously assembled hybrid network of graphene and metal oxide nanosheets as an effective platform for exploring efficient LDH-based functional materials.

Surface Optimization of Noble-Metal-Free Conductive [Mn1/4Co1/2Ni1/4]O2 Nanosheets for Boosting Their Efficacy as Hybridization Matrices.

Conductive 2D nanosheets have evoked tremendous scientific efforts because of their high efficiency as hybridization matrices for improving diverse functionalities of nanostructured materials. To address the problems posed by previously reported conductive nanosheets like poorly-interacting graphene and cost-ineffective RuO2 nanosheets, economically feasible noble-metal-free conductive [MnxCo1-2xNix]O2 oxide nanosheets are synthesized with outstanding interfacial interaction capability. The surface-optimized [Mn1/4Co1/2Ni1/4]O2 nanosheets outperformed RuO2/graphene nanosheets as hybridization matrices in exploring high-performance visible-light-active (λ >420 nm) photocatalysts. The most efficient g-C3N4-[Mn1/4Co1/2Ni1/4]O2 nanohybrid exhibited unusually high photocatalytic activity (NH4 + formation rate: 1.2 mmol g-1 h-1), i.e., one of the highest N2 reduction efficiencies. The outstanding hybridization effect of the defective [Mn1/4Co1/2Ni1/4]O2 nanosheets is attributed to the optimization of surface bonding character and electronic structure, allowing for improved interfacial coordination bonding with g-C3N4 at the defect sites. Results from spectroscopic measurements and theoretical calculations reveal that hybridization helps optimize the bandgap energy, and improves charge separation, N2 adsorptivity, and surface reactivity. The universality of the [Mn1/4Co1/2Ni1/4]O2 nanosheet as versatile hybridization matrices is corroborated by the improvement in the electrocatalytic activity of hybridized Co-Fe-LDH as well as the photocatalytic hydrogen production ability of hybridized CdS.

Synergetic Advantages of Atomically Coupled 2D Inorganic and Graphene Nanosheets as Versatile Building Blocks for Diverse Functional Nanohybrids.

2D nanostructured materials, including inorganic and graphene nanosheets, have evoked plenty of scientific research activity due to their intriguing properties and excellent functionalities. The complementary advantages and common 2D crystal shapes of inorganic and graphene nanosheets render their homogenous mixtures powerful building blocks for novel high-performance functional hybrid materials. The nanometer-level thickness of 2D inorganic/graphene nanosheets allows the achievement of unusually strong electronic couplings between sheets, leading to a remarkable improvement in preexisting functionalities and the creation of unexpected properties. The synergetic merits of atomically coupled 2D inorganic-graphene nanosheets are presented here in the exploration of novel heterogeneous functional materials, with an emphasis on their critical roles as hybridization building blocks, interstratified sheets, additives, substrates, and deposited monolayers. The great flexibility and controllability of the elemental compositions, defect structures, and surface natures of inorganic-graphene nanosheets provide valuable opportunities for exploring high-performance nanohybrids applicable as electrodes for supercapacitors and rechargeable batteries, electrocatalysts, photocatalysts, and water purification agents, to give some examples. An outlook on future research perspectives for the exploitation of emerging 2D nanosheet-based hybrid materials is also presented along with novel synthetic strategies to maximize the synergetic advantage of atomically mixed 2D inorganic-graphene nanosheets.

Molecular-Level Control of the Intersheet Distance and Electronic Coupling between 2D Semiconducting and Metallic Nanosheets: Establishing Design Rules for High-Performance Hybrid Photocatalysts.

Hybridization with conductive nanospecies has attracted intense research interest as a general effective means to improve the photocatalytic functionalities of nanostructured materials. To establish universal design rules for high-performance hybrid photocatalysts, correlations between versatile roles of conductive species and interfacial interaction between hybridized species are systematically investigated through fine-control of intersheet distance between photocatalytically active TiO2 and metallic reduced graphene oxide (rGO)/RuO2 nanosheets. Molecular-level tailoring of intersheet distance and electronic coupling between 2D nanosheets can be successfully achieved by restacking of colloidal nanosheet mixture with variable-sized organic intercalants. While the shortest intersheet distance between restacked TiO2 and rGO nanosheets leads to the highest visible-light-driven photocatalytic activity, the best UV-vis photocatalyst performance occurs for moderate intersheet spacing. These results highlight the greater sensitivity of photoinduced electronic excitation to the intersheet distance than that of interfacial charge transfer. The rGO nanosheet can function as effective charge transport pathway and cocatalyst within ≈1.7 nm distance from the semiconducting nanosheet, and as efficient stabilizer for hybridized photocatalyst within ≈1.8 nm. The present study underscores that the intercalative restacking of colloidal nanosheet mixture with intercalants enables molecular-level control of distance between 2D inorganic/graphene nanosheets, which provides a rational design strategy for high-performance hybrid photocatalysts.

Lattice Engineering to Simultaneously Control the Defect/Stacking Structures of Layered Double Hydroxide Nanosheets to Optimize Their Energy Functionalities.

An effective lattice engineering method to simultaneously control the defect structure and the porosity of layered double hydroxides (LDHs) was developed by adjusting the elastic deformation and chemical interactions of the nanosheets during the restacking process. The enlargement of the intercalant size and the lowering of the charge density were effective in increasing the content of oxygen vacancies and enhancing the porosity of the stacked nanosheets via layer thinning. The defect-rich Co-Al-LDH-NO3- nanohybrid with a small stacking number exhibited excellent performance as an oxygen evolution electrocatalyst and supercapacitor electrode with a large specific capacitance of ∼2230 F g-1 at 1 A g-1, which is the largest capacitance of carbon-free LDH-based electrodes reported to date. Combined with the results of density functional theory calculations, the observed excellent correlations between the overpotential/capacitance and the defect content/stacking number highlight the importance of defect/stacking structures in optimizing the energy functionalities. This was attributed to enhanced orbital interactions with water/hydroxide at an increased number of defect sites. The present cost-effective lattice engineering process can therefore provide an economically feasible methodology to explore high-performance electrocatalyst/electrode materials.

Interstratified heterostructures of metal hydroxide nanoclusters and MoS2 monolayers with improved electrode performance.

Interstratified 2D nanohybrids of chromium hydroxide-molybdenum disulfide with improved electrode functionality are synthesized by the self-assembly of anionic monolayered MoS2 nanosheets with cationic chromium hydroxide nanoclusters. The intercalative hybridization of MoS2 with chromium hydroxide nanoclusters leads to a significant increase of basal spacing as well as to the formation of an open porous stacking structure. This is the first example of metal hydroxide nanocluster-pillared transition metal dichalcogenide (TMD) hybrid materials. According to extended X-ray absorption fine structure analysis, open tetrameric chromium hydroxide nanoclusters are stabilized in-between metallic 1T'-MoS2 monolayers. In comparison with the pristine MoS2 material, the chromium hydroxide-pillared molybdenum disulfide nanohybrids show remarkably improved charge storage capacity with excellent rate performance for lithium ion batteries, highlighting the beneficial effect of pillaring with metal hydroxides on the electrode performance of MoS2. The improvement of electrode functionality upon hybridization is attributable to the increase of basal spacing, the stabilization of metallic 1T'-MoS2 content, the improvement of charge transfer kinetics, and the stabilization of the open porous structure upon electrochemical cycling. The present study clearly demonstrates that an electrostatically-driven self-assembly between exfoliated TMD nanosheets and cationic inorganic nanoclusters can provide an effective way of synthesizing heterostructured hybrid electrode materials with improved performance.