Sung Yeun Choi

Pore architecture affects photocatalytic activity of periodic mesoporous nanocrystalline anatase thin films

We report the photocatalytic activity of periodic mesoporous nanocrystalline anatase thin films (denoted meso-nc-TiO2) using Methylene Blue (denoted MB) as a probe of pore architecture effects on reactivity. Specifically, 2D hexagonal and 3D cubic mesoporous nanocrystalline anatase thin films (denoted h-meso-nc-TiO2 and c-meso-nc-TiO2 respectively) annealed at different temperatures were investigated to reveal the effects of different pore architectures on the photocatalytic activity. The adsorption behavior of MB on the films annealed at the same temperature signaled that c-meso-nc-TiO2 has a larger accessible surface area but a lower adsorption surface affinity, compared to h-meso-nc-TiO2. In the case of the solid-state photodegradation of MB, the most efficacious photocatalyst was found to be c-meso-nc-TiO2 annealed at 450 °C. For MB in solution, a 400 °C annealed c-meso-nc-TiO2 was established to have the optimum photocatalytic activity among the samples investigated. The observed superior photocatalytic activity of c-meso-nc-TiO2 relative to both h-meso-nc-TiO2 and nc-TiO2 is believed to originate from the higher photoactivity of anatase nanocrystallites comprising the more open cubic framework. as well as geometrical advantages, such as a larger surface area and less obstructed 3D diffusion paths of guest molecules. It is concluded that the photocatalytic efficiency of periodic mesoporous nanocrystalline anatase thin films depends on the pore architecture.

Slow photons in the fast lane in chemistry

A driving force in the rapidly developing field of photonic crystals has been the photonic bandgap, a range of energies where the propagation of light is completely forbidden. The photonic bandgap allows the design of photonic lattices that localize, guide and bend light at sub-micron length scales, providing opportunities for the creation of miniature optical devices and integrated optical circuits to help drive the revolution in photonics. A less well known attribute of photonic crystals is their theoretical ability to slow light to a velocity of zero. This phenomenon can be achieved at the high and low energy edges of photonic stopgaps where the photonic bands are flat and light exists as a standing wave commensurate with the photonic lattice and travels at a group velocity of zero, referred to as “slow photons” herein. It has been shown theoretically that the probability of harvesting slow photons scales inversely with their group velocity. This means that a number of well known photon driven processes and devices in chemistry and physics can be enhanced by capturing this unique property of slow photons. In this paper we will look at slow photons mainly through the eye of chemistry and highlight some recent developments in this exciting and emerging field that demonstrate the potential of slow photons in materials chemistry and nanochemistry.

Thermal plasma synthesis of tungsten bronze nanoparticles for near infra-red absorption applications

A novel synthesis concept based on inductively coupled thermal plasma technology is presented for the continuous high throughput production of tungsten bronze nanoparticles with high purity and tunable composition. Tungsten bronzes in nanoparticle form are of renewed technical interest for use in applications such as IR curing of coatings and heat shielding filters since they exhibit a high extinction coefficient in the near IR region but with little impact on transparency or visible color.

Large scale production of high aspect ratio graphite nanoplatelets with tunable oxygen functionality

High aspect ratio graphite nanoplatelets with tunable oxygen functionality are prepared using a two-step scalable process consisting of expansion by thermal plasma processing followed by exfoliation with ultrasonic processing. The result of this process is graphite nanoplatelets in high yield with an average particle size distribution ranging from monolayer graphene up to 10 nm in thickness, 1 to 30 μm in length/width, and a tunable carbon to oxygen (C/O) ratio from 50 to 200 without requiring any special separation or isolation techniques such as sedimentation or centrifugation. It has been demonstrated for the first time that an intermediate level of oxygen functionalization improves the ability to exfoliate expanded graphite without having a detrimental effect on electrical conductivity.

Towards flexible inorganic ?mesomaterials?: one-pot low temperature synthesis of mesostructured nanocrystalline titaniaElectronic supplementary information (ESI) available: IR spectra and SEM image of mesoporous nanocrystalline titania. See http://www.rsc.org/suppdata/cc/b4/b403607g/

We hereby report a simple route for the low temperature synthesis of mesoporous nanocrystalline titania involving brief hydrothermal treatment of butanolic precursors and non-ionic tri-block-copolymer surfactant at 100 degrees C, followed by evaporation induced self assembly to make a crack-free flexible film. At no time in the film-forming process is a temperature of more than 120 degrees C reached, thereby permitting the use of substrates that are not stable to higher temperatures.

Slow Photons in TiO2 Inverse Opals: Optical Amplification and Effect of Disorder on the Photocatalytic Efficiency

Nanocrystalline TiO2 (nc-TiO2) in the anatase phase is widely used for photo-degrading organic pollutants in a variety of environmental applications. Herein we show that slow photons in photonic crystals fashioned from nc-TiO2 can optically amplify the photocatalytic efficiency as a result of the longer path length of light and increased probability in anatase absorption, and that the optical amplification is tolerant to some degree of disorder. We investigated the photodegradation of adsorbed methylene blue on inverse TiO2 opals (i-nc-TiO2-o) with different stop-band energies under monochromatic and white light irradiation. By using template spheres with different diameters, the energy of the slow photons was tuned in-and-out of the anatase electronic absorption, thereby allowing the systematic study of the effects of photonic structure on the photo-degradation efficiency of TiO2. Under monochromatic irradiation at 370 nm, a remarkable twofold enhancement was observed for i-nc-TiO2-o with stop-band at 345 nm, as a result of slow photon coupling at 370 nm. Under white light (>300 nm) irradiation, an increase in the photo-degradation efficiency was observed when the stop-band moves from 370 to 300 nm, as a result of slow photon coupling and the suppression of stop-band reflection by the anatase absorption. By optimizing the energy of the photonic stop-band with respect to the semiconductor electronic band gap, we effectively harvested slow photons in the dielectric part of the material to give optically amplified photochemistry. Furthermore, we studied the effect of structural disorder on the photocatalytic efficiency of the inverse opals by introducing different fractions and sizes of guest spheres into the opal template. We found that half of the enhancement originally achieved by the inverse opal made from monodispersed spheres is conserved when the domain size of the host spheres remains above a critical threshold. Such a high tolerance to structural disorder provides strong support for the potential use of inverse TiO2 opals in environmental cleanup and water treatment applications.

Evolution of nanocrystallinity in periodic mesoporous anatase thin films

Herein we report the first kinetic study of the intrachannel wall phase-transition of amorphous titania to nanocrystalline anatase for periodic mesoporous titania thin films, monitored by time-resolved in situ high-temperature X-ray diffraction. Structural transformations associated with the phase transition are further probed by high-resolution scanning electron microscopy and transmission electron microscopy. The model found to be most consistent with the kinetic data involves 1D diffusion-controlled growth of nanocrystalline anatase within the spatial confines of the channel walls of the mesostructure. The observation of anisotropic, rod-shaped anatase nanocrystals preferentially aligned along the channel axis implies that the framework of the liquid-crystal-templated mesostructure guides the crystal growth.