Jang Mee Lee

Self-assembly of layered double hydroxide 2D nanoplates with graphene nanosheets: an effective way to improve the photocatalytic activity of 2D nanostructured materials for visible light-induced O2 generation

Highly efficient photocatalysts for visible light-induced O2 generation are synthesized via an electrostatically derived self-assembly of Zn–Cr-LDH 2D nanoplates with graphene 2D nanosheets. In the obtained nanohybrids, the positively charged Zn–Cr-LDH nanoplates are immobilized on the surface of negatively charged graphene nanosheets with the formation of a highly porous stacked structure. A strong electronic coupling of the subnanometer-thick Zn–Cr-LDH nanoplates with reduced graphene oxide (RGO)/graphene oxide (GO) nanosheets gives rise not only to the prominent increase of visible light absorption but also to a remarkable depression of the photoluminescence signal. The self-assembled Zn–Cr-LDH–RGO nanohybrids display an unusually high photocatalytic activity for visible light-induced O2 generation with a rate of ∼1.20 mmol h−1 g−1, which is far superior to that of the pristine Zn–Cr-LDH material (∼0.67 mmol h−1 g−1). The fact that pristine Zn–Cr-LDH is one of the most effective visible light photocatalysts for O2 production with unusually high quantum efficiency of 61% at λ = 410 nm highlights the excellent functionality of the Zn–Cr-LDH–RGO nanohybrids as visible light active photocatalysts. The Zn–Cr-LDH–RGO nanohybrid shows a higher photocatalytic activity than the Zn–Cr-LDH–GO nanohybrid, providing strong evidence for the superior advantage of the hybridization with RGO. The present findings clearly demonstrate that graphene nanosheets can be used as an effective platform for improving the photocatalytic activity of 2D nanostructured inorganic solids.

A strong electronic coupling between graphene nanosheets and layered titanate nanoplates: a soft-chemical route to highly porous nanocomposites with improved photocatalytic activity.

Strongly coupled nanocomposites of layered titanate and reduced graphene oxide (RGO) are synthesized by electrostatically derived self-assembly between negatively charged RGO nanosheets and positively charged TiO(2) nanosols, which is then followed by a phase transition of the anatase TiO(2) component into layered titanate. The resulting nanocomposite consists of thin 2D nanoplates of lepidocrocite-type layered titanate immobilized on the surface of RGO nanosheets. The composite formation with RGO nanosheets is effective not only in promoting the phase transition of anatase TiO(2) nanosols, but also in improving the thermal stability of the layered titanate, indicating the role of RGO nanosheets as an agent for directing and stabilizing layered structures. The layered-titanate-RGO nanocomposites exhibit remarkably expanded surface area with the formation of micropores and mesopores. The composite formation with RGO nanosheets gives rise to the disappearance of the reflectance edge of layered titanate in the diffuse reflectance UV-vis spectra, indicating a strong electronic coupling between the RGO and layered titanate. The strong electronic correlation between the two components is further evidenced by the visible-light-induced generation of photocurrents after the hybridization with RGO. The layered-titanate-RGO nanocomposite shows a higher activity for the photodegradation of organic molecules than uncomposited layered titanate, underscoring the usefulness of graphene hybridization in improving the photocatalyst performance of layered titanate. The experimental findings presented here clearly demonstrate that the self-assembly of metal oxide nanoparticles with RGO 2D nanosheets is quite effective not only in synthesizing porous metal-oxide-graphene nanocomposites with improved photo-induced functionality, but also in achieving strong electronic coupling between RGO and metal oxides.

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.

Mixed Colloidal Suspensions of Reduced Graphene Oxide and Layered Metal Oxide Nanosheets: Useful Precursors for the Porous Nanocomposites and Hybrid Films of Graphene/Metal Oxide

Homogeneously mixed colloidal suspensions of reduced graphene oxide, or RGO, and layered manganate nanosheets have been synthesized by a simple addition of the exfoliated colloid of RGO into that of layered MnO(2). The obtained mixed colloidal suspensions with the RGO/MnO(2) ratio of ≤0.3 show good colloidal stability without any phase separation and a negatively charged state with a zeta (ζ) potential of -30 to -40 mV. The flocculation of these mixed colloidal suspensions with lithium cations yields porous nanocomposites of Li/RGO-layered MnO(2) with high electrochemical activity and a markedly expanded surface area of around 70-100 m(2)  g(-1). Relative to the Li/RGO and Li/layered MnO(2) nanocomposites (≈116 and ≈167 F g(-1)), the obtained Li/RGO-layered MnO(2) nanocomposites deliver a larger capacitance of approximately 210 F g(-1) with good cyclability of around 95-97 % up to the 1000th cycle, thus indicating the positive effect of hybridization on the electrode performances of RGO and lithium manganate. Also, an electrophoretic deposition of the mixed colloidal suspensions makes it possible to easily fabricate uniform hybrid films composed of graphene and manganese oxide. The obtained films show a distinct electrochemical activity and a homogeneous distribution of RGO and MnO(2). The present experimental findings clearly demonstrate that the utilization of the mixed colloidal suspensions as precursors provides a facile and universal methodology to synthesize various types of graphene/metal oxide hybrid materials.

A beneficial role of exfoliated layered metal oxide nanosheets in optimizing the electrocatalytic activity and pore structure of Pt-reduced graphene oxide nanocomposites

Mesoporous nanocomposites of Pt-reduced graphene oxide (RGO)-layered titanate are synthesized by the reaction of a mixture of exfoliated layered titanate nanosheets, graphene oxide nanosheets, and H2PtCl6 with NaBH4 to investigate the effect of layered metal oxide nanosheets on the electrocatalyst performance of Pt–RGO nanocomposites. The obtained ternary nanocomposites are composed of a porous stacking assembly of layered titanate/RGO nanosheets with well-dispersed Pt nanocrystals whose particle size becomes smaller with the increase of titanate content. The incorporation of layered titanate nanosheets into the Pt–RGO nanocomposite induces a marked increase of surface area through the frustration of a strong π–π interaction between RGO nanosheets by the intervention of titanate nanosheets. In addition, the incorporation of layered titanate nanosheets gives rise to a remarkable improvement of electrocatalytic activity for oxygen reduction reaction (ORR). Of prime importance is that the present nanocomposites show a more positive onset potential for ORR than commercial Pt/carbon catalysts, underscoring a powerful role of titanate nanosheets in optimizing the electrocatalyst performance of Pt–RGO nanocomposites. This is the first report showing the usefulness of metal oxide nanosheets as an effective additive for enhancing the electrocatalyst performance of platinum nanoparticles. The observed enhancement of electrocatalytic activity upon the addition of titanate nanosheets is attributable to the decrease of Pt crystal size, the increase of surface area, and the increase of surface hydrophilicity. The present findings clearly demonstrate that the incorporation of layered titanate nanosheets is quite effective in improving the electrocatalytic functionality of Pt–RGO nanocomposites.

Highly Efficient Visible Light-Induced O2 Generation by Self-Assembled Nanohybrids of Inorganic Nanosheets and Polyoxometalate Nanoclusters

Unusually high photocatalytic activity of visible light-induced O2 generation can be achieved by electrostatically-derived self-assembly between exfoliated Zn-Cr-LDH 2D nanosheets and POM 0D nanoclusters (W7O246− and V10O286−) acting as an electron acceptor. This self-assembly can provide a high flexibility in the control of the chemical composition and pore structure of the resulting LDH-based nanohybrids. The hybridization with POM nanoclusters remarkably enhances the photocatalytic activity of the pristine Zn-Cr-LDH, which is attributable to the formation of porous structure and depression of charge recombination. Of prime interest is that the excellent photocatalytic activity of the as-prepared Zn-Cr-LDH-POM nanohybrid for visible light-induced O2 generation can be further enhanced by calcination at 200 °C, leading to the very high apparent quantum yield of ∼75.2% at 420 nm. The present findings clearly demonstrate that the self-assembly of LDH–POM is fairly powerful in synthesizing novel LDH-based porous nanohybrid photocatalyst for visible light-induced O2 generation.

Graphene Nanosheets as a Platform for the 2D Ordering of Metal Oxide Nanoparticles: Mesoporous 2D Aggregate of Anatase TiO2 Nanoparticles with Improved Electrode Performance

Graphene nanosheets are successfully applied as an effective platform for the 2D ordering of metal oxide nanoparticles. Mesoporous 2D aggregates of anatase TiO(2) nanoparticles are synthesized by the heat treatment of the uniformly hybridized nanocomposite of layered titanate-reduced graphene oxide (RGO) at elevated temperatures. The precursor layered titanate-RGO nanocomposite is prepared by self-assembly of anionic RGO nanosheets and cationic TiO(2) nanosols. The calcination of the as-prepared layered titanate-RGO nanocomposite at 500 °C induces a structural and morphological change of layered titanate nanoplates into anatase TiO(2) nanoparticles without significant modification of the RGO nanosheet. Increasing the heating temperature to 600 °C gives rise to elimination of the RGO component, leading to the formation of sheetlike porous aggregates of RGO-free TiO(2) nanoparticles. The nanocomposites calcined at 500-700 °C display promising functionality as negative electrodes for lithium ion batteries. Among the present calcined derivatives, the 2D sheet-shaped aggregate of TiO(2) nanoparticles obtained from calcination at 600 °C delivers the greatest specific discharge capacity with good capacity retention for all current density conditions applied. Such superior electrode performance of the nanocomposite calcined at 600 °C is attributable both to the improved stability of the crystal structure and crystal morphology of titania and to the enhancement of Li(+) ion transport through the enlargement of mesopores. The present findings clearly demonstrate the usefulness of RGO nanosheets as a platform for 2D-ordered superstructures of metal oxide nanoparticles with improved electrode performance.

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.

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.

A Linker‐Mediated Self‐Assembly Method to Couple Isocharged Nanostructures: Layered Double Hydroxide–CdS Nanohybrids with High Activity for Visible‐Light‐Induced H2 Generation

The electrostatically derived self-assembly of cationic Zn-Cr-layered double hydroxide (LDH) nanosheets and cationic CdS quantum dots (QDs) with anionic linkers leads to the formation of strongly coupled Zn-Cr-LDH-CdS nanohybrids. The hybridization with Zn-Cr-LDH leads to significant enhancement of the photocatalytic activity of CdS for visible-light-induced H2 generation, a property that is attributed to the depression of electron-hole recombination. In comparison with a direct hybridization method between oppositely charged species, this linker-mediated method provides greater flexibility in controlling the chemical composition and electronic coupling of the nanohybrids. The present hybridization strategy provides a useful method not only to couple two kinds of isocharged nanostructured materials, but also to explore efficient hybrid-type photocatalysts.