Seong Ju Hwang

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 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.

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

Electrophoretic deposition of Ca2Nb3O10− nanosheets synthesized by soft-chemical exfoliation

Ca2Nb3O10− (CNO−) nanosheets exfoliated from a KCa2Nb3O10 ceramic were deposited on a Pt/Ti/SiO2/Si substrate at room temperature (RT) using the electrophoresis method, and subsequently annealed at various temperatures to remove organic defects. The inter-planar distance of the (001) plane of the CNO film deposited at RT was large (1.68 nm). This distance gradually decreased to 1.44 nm when the annealing temperature was increased to 600 °C due to the removal of organic defects. For CNO films annealed at temperatures higher than 600 °C, a CaNb2O6 secondary phase was formed, indicating that annealing should be conducted at 600 °C to obtain a homogeneous CNO film. Moreover, the CNO film annealed at 600 °C showed a large er value of 65 with a low tan δ of 0.01 at 1.0 MHz. This film also exhibited a high breakdown electric field of 0.43 MV cm−1 and a very low leakage current density of 3 × 10−8 A cm−2 at 0.4 MV cm−1.