Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

A surface science perspective on TiO2 photocatalysis

This Review summarizes recent developments in the field of responsive photonic crystal structures, including principles for design and fabrication and many strategies for applications, for example as optical switches or chemical and biological sensors. A number of fabrication methods are now available to realize responsive photonic structures, the majority of which rely on self-assembly processes to achieve ordering. Compared with microfabrication techniques, self-assembly approaches have lower processing costs and higher production efficiency, however, major efforts are still needed to further develop such approaches. In fact, some emerging techniques such as spin coating, magnetic assembly, and flow-induced self-assembly have already shown great promise in overcoming current challenges. When designing new systems with improved performance, it is always helpful to bear in mind the lessons learnt from natural photonic structures.

Responsive photonic crystals.

TiO2 nanocrystals with tunable bulk/surface defects were synthesized and characterized with TEM, XRD, BET, positron annihilation, and photocurrent measurements. The effect of defects on photocatalytic activity was studied. It was found for the first time that decreasing the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals could significantly improve the separation efficiency of photogenerated electrons and holes, thus significantly enhancing the photocatalytic efficiency.

Tuning the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals leads to high photocatalytic efficiency.

Mesoporous Anatase TiO2 Beads with High Surface Areas and Controllable Pore Sizes: A Superior Candidate for High‐Performance Dye‐Sensitized Solar Cells

Herein a novel synthetic route is described for the production of thermally stable, structurally well-defined two-dimensional (2D) hexagonal mesoporous nanocrystalline anatase (meso-nc-TiO2), with a large pore diameter, narrow pore-size distribution, high surface area, and robust inorganic walls comprised of nanocrystalline anatase The synthetic approach involves the evaporation-induced co-assembly of a non-ionic amphiphilic triblock-copolymer template and titanium tetraethoxide, but with a pivotal change in the main solvent of the system, where the commonly used ethanol is replaced with 1-butanol This seemingly minor modification in solvent type from ethanol to 1-butanol turns out to be the key synthetic strategy for achieving a robust, structurally well-ordered meso-nc-TiO2 material in the form of either thick or thin films The beneficial “solvent” effect originates from the higher hydrophobicity of 1-butanol than ethanol, enhancing microphase separation and templating, lower critical micelle concentration of the template in 1-butanol, and the ability to increase the relative concentration of the inorganic precursor to template in the co-assembly synthesis Moreover, thin films with dimensions of several centimeters that are devoid of cracks down to the length scale of the mesostructure itself, having high porosity, well-defined mesostructural features, and semi-crystalline pore walls were straightforwardly and reproducibly obtained as a result of the physicochemical property advantages of 1-butanol over ethanol within our synthesis scheme

Thermally Stable Two‐Dimensional Hexagonal Mesoporous Nanocrystalline Anatase, Meso‐nc‐TiO2: Bulk and Crack‐Free Thin Film Morphologies

Slow photon, or light with reduced group velocity, is a unique phenomenon found in photonic crystals that theoreticians have long suggested to be invaluable for increasing the efficiency of light-driven processes. This thesis demonstrates experimentally the feasibility of using slow photons to optically amplify photochemistry of both organic and inorganic systems. The effect of photonic properties on organic photochemistry was investigated by tracing out the wavelength-dependent rate of photoisomerization of azobenzene anchored on silica opals. The application of slow photons to inorganic photochemical processes was realized by molding nanocrystalline titania into an inverse opal structure and investigating its photodegradation efficiency in relation to the photonic properties. Changes in the photodegradation efficiency were directly linked to modifications of the electronic band gap absorption as a result of the photonic properties. The highest enhancement of twofold was achieved when the energy of the slow photons overlaps with the electronic band gap absorption, such that the loss of light due to photonic stop-band reflection was significantly reduced. In addition, the strength of slowphoton amplification with respect to the macroscopic structural order was studied by

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Amplified photochemistry with slow photons

Herein we report a novel self-assembly synthesis, structural and optical characterization of mesoporous Bragg stacks (MBS) composed of spin-coated multilayer stacks of mesoporous TiO2 and mesoporous SiO2. Investigation of the optical response of MBS to the infiltration of alcohols and alkanes into its pores reveals better sensitivity and selectivity than conventional Bragg reflectors. Furthermore, we demonstrate that the chemical sensing ability can be tuned via layer thickness, composition and surface properties.

Mesoporous bragg stack color tunable sensors.

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.

Slow photons in the fast lane in chemistry

We report for the first time on the electrochromic performance of viologen-modified periodic mesoporous nanocrystalline anatase, denoted meso-nc-TiO2-V(2+). Electrodes fashioned from meso-nc-TiO2-V(2+) were found to display enhanced color contrast yet have similar conduction band edge energy level and electron percolation ability as electrodes made from viologen-modified nanocrystalline titania, nc-TiO2-V(2+). This performance can be attributed to the uniform and ordered mesopore architecture, the nanocrystalline anatase wall structure, and the large accessible surface area for tethering viologen molecules.

Sung Yeun Choi

Papers

Thermally Stable Two‐Dimensional Hexagonal Mesoporous Nanocrystalline Anatase, Meso‐nc‐TiO2: Bulk and Crack‐Free Thin Film Morphologies

Amplified photochemistry with slow photons

Mesoporous bragg stack color tunable sensors.

Slow photons in the fast lane in chemistry

Electrochromic Performance of Viologen-Modified Periodic Mesoporous Nanocrystalline Anatase Electrodes