Highly Selective Photoreduction of CO2 with Suppressing H2 Evolution over Monolayer Layered Double Hydroxide under Irradiation above 600†nm.

Although progress has been made to improve photocatalytic CO2 reduction under visible light (λ>400 nm), the development of photocatalysts that can work under a longer wavelength (λ>600 nm) remains a challenge. Now, a heterogeneous photocatalyst system consisting of a ruthenium complex and a monolayer nickel-alumina layered double hydroxide (NiAl-LDH), which act as light-harvesting and catalytic units for selective photoreduction of CO2 and H2 O into CH4 and CO under irradiation with λ>400 nm. By precisely tuning the irradiation wavelength, the selectivity of CH4 can be improved to 70.3 %, and the H2 evolution reaction can be completely suppressed under irradiation with λ>600 nm. The photogenerated electrons matching the energy levels of photosensitizer and m-NiAl-LDH only localized at the defect state, providing a driving force of 0.313 eV to overcome the Gibbs free energy barrier of CO2 reduction to CH4 (0.127 eV), rather than that for H2 evolution (0.425 eV).

Kinetically Controlled Layer-by-Layer Stacking of Metal Oxide 2D Nanosheets.

An efficient chemical way to finely control the layer-by-layer stacking of inorganic nanosheets (NS) is developed by tuning the type and composition of intercalant ion, and the reaction temperature for restacking process. The finely controlled stacking of NS relies on a kinetic control of the self-assembly of NS in the presence of coordinating organic cations. A critical role of organic cations in this assembly highlights the importance of the appropriate activation energy. Of prime importance is that a fine-control of the interstratification of 2D NS is highly effective not only in tailoring its pore structure but also in enhancing its electrode activity. The present study clearly demonstrates that the kinetically controlled restacking of NS provides a facile and powerful method to tailor their stacking number and functionality.

A Crucial Role of Rh Substituent Ion in Photoinduced Internal Electron Transfer and Enhanced Photocatalytic Activity of CdS-Ti(5.2-x)/6 Rhx /2 O2 Nanohybrids.

The photocatalytic activity and photostability of CdS quantum dot (QD) can be remarkably enhanced by hybridization with Rh-substituted layered titanate nanosheet even at very low Rh substitution rate (<1%). Mesoporous CdS-Ti(5.2-x)/6 Rhx/2O2 nanohybrids are synthesized by a self-assembly of exfoliated Ti(5.2-x)/6 Rhx/2O2 nanosheets with CdS QDs. The partial substitution of Rh(3+)/Rh(4+) ions for Ti(4+) ions in layered titanate is quite effective in enhancing an electronic coupling between hybridized CdS and titanate components via the formation of interband Rh 4d states. A crucial role of Rh substituent ion in the internal electron transfer is obviously evidenced from in situ X-ray absorption spectroscopy showing the elongation of (RhO) bond under visible light irradiation. This is the first spectroscopic evidence for the important role of substituent ion in the photoinduced electron transfer of hybrid-type photocatalyst. The CdS-Ti(5.2-x)/6 Rhx/2O2 nanohybrids show much higher photocatalytic activity for H2 production and better photostability than do CdS and unsubstituted CdS-TiO2 nanohybrid. This result is ascribable to the enhancement of visible light absorptivity, the depression of electron-hole recombination, and the enhanced hole curing of CdS upon Rh substitution. The present study underscores that the hybridization with composition-controlled inorganic nanosheet provides a novel efficient methodology to optimize the photo-related functionalities of semiconductor nanocrystal.

Unique properties of 2 D layered titanate nanosheets as a building block for the optimization of the photocatalytic activity and photostability of TiO2-based nanohybrids.

In comparison with the hybridization with 0D TiO2 nanoparticle, 2D layered TiO2 nanosheets are much more effective in the improvement of the photocatalytic activity and photostability of semiconducting compounds. The 2D TiO2-Ag3PO4 nanohybrid described in this paper shows a greater decrease in the electron-hole recombination upon hybridization and a stronger chemical interaction between the components than the 0D homologue. This result confirms the benefits of 2D layered TiO2 nanosheets as a building block for efficient hybrid-type photocatalyst materials.

Electrochemical performance of poly(vinyl alcohol)-based solid polymer electrolyte for lithium polymer batteries.

Solid polymer electrolytes (SPEs) are an excellent alternative to liquid electrolytes due to their non-volatility, low toxicity, and high energy density. In this study, a SPE having the ion transport mechanism decoupled from segmental motion of a polymer based on poly(vinyl alcohol) (PVA) containing the salt lithium trifluoromethane sulfonate (LiCF3SO3, LiTf) has been prepared to overcome the low ionic conductivity of traditional SPEs at room temperature. PVA has a high glass transition temperature (358 K) and good mechanical properties, and despite being atactic, it can crystallize, especially if highly hydrolyzed. From an ac impedance analysis, it was found that the ionic conductivity of the PVA-based SPE increased with increasing salt concentration. In particular, a dramatic increase was observed between 40 and 50 wt% of salt. The ionic conduction mechanism of the PVA-based SPE is proposed based on intensive study using FT-IR spectroscopic measurements, XRD and AFM. Through measurements of linear sweep voltammetry (LSV) and cyclic voltammetry (CV), it is also found that the SPE with PVA and LiCF3SO3 has good electrochemical stability.

Organic Intercalant-Free Liquid Exfoliation Route to Layered Metal-Oxide Nanosheets via the Control of Electrostatic Interlayer Interaction.

A scalable organic intercalant-free liquid exfoliation route to 2D nanosheets (NSs) of layered transition-metal oxides (TMOs) is developed by employing hydronium-intercalated derivatives as precursors. The replacement of interlayer alkali metal ions with larger hydronium ions via acid treatment makes possible the efficient liquid exfoliation of TMOs without any assistance of organic intercalant cations. Not only a weakening of interlayer electrostatic interaction upon hydronium intercalation but also an efficient solvation of deintercalated hydronium ions via hydrogen bonding with polar solvents is mainly responsible for the high efficacy of hydronium-intercalated TMOs as precursors for liquid exfoliation. The nature of the solvent employed also has a profound effect on the exfoliation yield of these TMO NSs; viscosity, surface tension, density, and Hansen solubility parameter as well as the capability to solvate the exfoliated NSs and hydronium ions are crucial factors for determining the exfoliation efficiency of the hydronium-intercalated precursor. All the obtained Ti1- xO2, MnO2, and RuO2 NSs show highly anisotropic 2D morphologies and distinct negative surface charges with a zeta potential of -30 to -50 mV. Such distinct surface charges of these NSs render them versatile hybridization matrices for the synthesis of novel nanohybrids with enhanced functionalities. The hybridization with the liquid-exfoliated TMO NSs is quite effective in improving the photocatalytic activity of CdS and the electrode functionalities of graphene and graphene-layered double hydroxide nanohybrids. The present study underscores the usefulness of the present liquid exfoliation method in synthesizing organic-free TMO NSs and their nanohybrids as well as in widening the application field of exfoliated TMO NSs.

Intercalative hybridization of layered double hydroxide nanocrystals with mesoporous g-C3N4 for enhancing visible light-induced H2 production efficiency.

Efficient visible light active hybrid photocatalysts for H2 production can be synthesized by the intercalative hybridization of Zn-Cr-layered double hydroxide (Zn-Cr-LDH) with a mesoporous g-C3N4 lattice. Small Zn-Cr-LDH nanocrystals with a size of ∼6 nm are immobilized in the mesopores of g-C3N4. Beyond an optimal LDH/g-C3N4 molar ratio of 0.3, a further increase in the LDH content leads to the surface deposition of LDH crystals on the g-C3N4 material as well as the intercalative immobilization of LDH into its mesopores, indicating the controllability of the LDH deposition site. The Zn-Cr-LDH-g-C3N4 nanohybrids exhibit smaller surface areas than the pristine g-C3N4, confirming the intercalative stabilization of Zn-Cr-LDH nanocrystals in the mesopore of g-C3N4. The hybridization between Zn-Cr-LDH and g-C3N4 is effective in enhancing visible light absorptivity and also in depressing electron-hole recombination, which is attributable to an efficient electronic coupling between both the hybridized components. The present Zn-Cr-LDH-g-C3N4 nanohybrid exhibits promising photocatalytic activities for visible light-induced H2 production at a rate of 155.7 µmol g-1 h-1, which is much superior to that of the pristine g-C3N4 (21.7 µmol g-1 h-1). The present study underscores that the intercalative immobilization of Zn-Cr-LDH crystals in the limited space of a mesopore is quite useful in improving the visible light active photocatalyst functionality of mesoporous carbon nitride.

Critical Role of the Chemical Environment of Interlayer Na Sites: An Effective Way To Improve the Na Ion Electrode Activity of Layered Titanate.

The chemical environments of the interlayer Na sites of layered titanate are finely controlled by the intercalation of n-alkylamine with various alkyl chain lengths to explore an effective way to improve its electrode functionality for sodium-ion batteries (SIBs). The n-alkylamine intercalation via ion-exchange and exfoliation-restacking routes allows the modification of in-plane structures of layered titanate to be tuned. Among the present n-alkylamine-intercalates, the n-pentylamine-intercalated titanate shows the largest discharge capacity with the best rate characteristics, underscoring the critical role of optimized intracrystalline structure in improving the SIB electrode performance of layered titanate. The creation of turbostratic in-plane structure degrades the SIB electrode performance of layered titanate, indicating the detrimental effect of in-plane structural disorder on electrode activity. 23Na magic-angle spinning nuclear magnetic resonance spectroscopy demonstrates that the n-alkylamine-intercalated titanates possess two different interlayer Na+ sites near ammonium head groups/titanate layers and near alkyl chains. The intercalation of long-chain molecules increases the population of the latter site and the overall mobility of Na+ ions, which is responsible for the improvement of electrode activity upon n-alkylamine intercalation. The present study highlights that the increased population of interlayer metal sites remote from the host layers is effective in improving the electrode functionality of layered metal oxide for SIBs and multivalent ion batteries.

A facile exfoliation-crystal growth route to multicomponent Ag2CO3/Ag-Ti5NbO14 nanohybrids with improved visible light photocatalytic activity.

Multicomponent Ag2CO3/Ag-layered Ti5NbO14 nanohybrids are synthesized by the crystal growth of silver carbonate on the surface of exfoliated layered titanoniobate 2D nanosheets. In the obtained nanohybrids, the spherical Ag2CO3 nanoparticles with a size of 5-10 nm are immobilized on the surface of the titanoniobate nanosheets with partial formation of neutral Ag metal caused by electron transfer from anionic titanoniobate nanosheets to silver cations. An electronic coupling between Ag2CO3/Ag and Ti5NbO14 nanosheets leads to a remarkable enhancement of visible light absorption and a significant depression of electron-hole recombination. The present Ag2CO3/Ag-layered Ti5NbO14 nanohybrids show much higher visible light photocatalytic activity than the unhybridized Ag2CO3, underscoring the beneficial effect of hybridization with metal oxide nanosheets on the photocatalytic activity of silver oxosalts. Before and after the photoreaction, the crystal structure and crystal morphology of the Ag2CO3/Ag-layered Ti5NbO14 nanohybrids remain unchanged, highlighting the excellent photostability of these materials. All the present experimental findings clearly demonstrate the usefulness of the exfoliation-crystal growth method in exploring novel efficient visible light active photocatalysts.

Unique Advantages of Exfoliated 2D Nanosheets for Tailoring the Functionalities of Nanocomposites.

Hybridization with exfoliated two-dimensional (2D) nanosheets provides a very effective and powerful way not only to control the physicochemical properties of hybridized species but also to explore nanocomposites with novel functionalities. Deliberate coupling between the hybridized species is critically important in maximizing the effect of hybridization on the physicochemical properties and functionality of hybridized components. The very small thickness and extremely large surface of exfoliated 2D nanosheets render these materials ideal candidates for achieving a strong coupling with diverse guest species. This Perspective focuses on the unique characteristics of exfoliated 2D nanosheets as building blocks for designing hybrid materials. Several intriguing examples of strong interaction between exfoliated 2D nanosheets and hybridized species are summarized with an emphasis on the effective control of electronic, optical, structural, and morphological characteristics. An outlook on the future research directions is provided along with new strategies to maximize the coupling in the 2D nanosheet-based hybrid materials.