Sung Hong Hahn

Influence of calcination temperature on structural and optical properties of TiO2 thin films prepared by sol-gel dip coating

TiO2 thin films were prepared by sol–gel dip coating and their structural and optical properties were examined at various calcination temperatures. The X-ray diffraction (XRD) results showed that TiO2 thin film calcined at 300 °C was amorphous, and transformed into the anatase phase at 400 °C, and further into rutile phase at 1000 °C. The phase transformation temperature has been dependent upon the concentration of catalyst HCl. The crystallite size of TiO2 thin films was increased with increasing calcination temperature. The micro-particles in the films grew by intra-agglomerate densification below 1000 °C, whereas they grew by inter-agglomerate densification above 1000 °C. The transmittance of the films calcined at 1000 and 1100 °C was reduced significantly in the wavelength range of about 300–700 nm due to the change of crystallite phase and composition in the films. The refractive index of TiO2 thin films was increased with increasing calcination temperature, and the film thickness and the porosity of TiO2 thin films were decreased.

Variation of structural and optical properties of sol-gel TiO2 thin films with catalyst concentration and calcination temperature

TiO2 thin films were prepared on quartz glass by sol-gel process and their structural and optical properties were examined at various catalyst concentrations and calcination temperatures. The as-deposited TiO2 thin films are amorphous, and they transform into the anatase phase at 400–600 °C, and into the anatase–rutile phase at 800 °C, and further into the rutile phase at 1000 °C. The temperature of phase transformation is decreased with the concentration of catalyst HCl. The crystallite size of TiO2 thin films is increased with increasing calcination temperature and catalyst concentration. The secondary particles in the TiO2 thin films grow by intra-agglomerate densification below 800 °C, whereas they grow by inter-agglomerate densification above 800 °C. The deposited TiO2 thin films have high transparency in the visible range. The transmittance of the films calcined at 800 and 1000 °C are significantly reduced in the wavelength range of 300–800 nm due to the change of crystallite phase and increased particle size. The refractive index of TiO2 thin films is increased with increasing calcination temperature and catalyst concentration. On the other hand, the porosity of TiO2 thin film is decreased.

Fabrication of Au/Graphene-Wrapped ZnO-Nanoparticle-Assembled Hollow Spheres with Effective Photoinduced Charge Transfer for Photocatalysis

Heterostructures of gold-nanoparticle-decorated reduced-graphene-oxide (rGO)-wrapped ZnO hollow spheres (Au/rGO/ZnO) are synthesized using tetra-n-butylammonium bromide as a mediating agent. The structure of amorphous ZnO hollow spheres is found to be transformed from nanosheet- to nanoparticle-assembled hollow spheres (nPAHS) upon annealing at 500 °C. The ZnO nPAHS hybrids with Au/rGO are characterized using various techniques, including photoluminescence, steady-state absorbance, time-resolved photoluminescence, and photocatalysis. The charge-transfer time of ZnO nPAHS is found to be 87 ps, which is much shorter than that of a nanorod (128 ps), nanoparticle (150 ps), and nanowall (990 ps) due to its unique structure. The Au/rGO/ZnO hybrid shows a higher charge-transfer efficiency of 68.0% in comparison with rGO/ZnO (40.3%) and previously reported ZnO hybrids. The photocatalytic activities of the samples are evaluated by photodegrading methylene blue under black-light irradiation. The Au/rGO/ZnO exhibits excellent photocatalytic efficiency due to reduced electron-hole recombination, fast electron-transfer rate, and high charge-transfer efficiency.

Comparison of optical and photocatalytic properties of TiO2 thin films prepared by electron-beam evaporation and sol–gel dip-coating

Transparent TiO2 thin films were deposited on soda lime glass by electron-beam evaporation (EBE) and sol–gel dip-coating (SGD) methods. Optical and photocatalytic properties of the EBE and SGD films were compared. The transmittance of both films decreased with increasing calcination temperature from 300 to 500 °C as a result of the growth of particles. The refractive index of the SGD films increased significantly from 1.79 to 1.93 after heating at 500 °C, whereas that of the EBE films increased slightly from 2.09 to 2.13. The EBE films had better optical property than the SGD films. On the other hand, the SGD films exhibited much better photoactivity than the EBE films. This has been explained in terms of porosity and the formation of Ti3+ ions during calcination.

Rapid room-temperature synthesis of nanosheet-assembled ZnO mesocrystals with excellent photocatalytic activity

We have investigated the growth of ZnO in a simple alkaline solution by chemical precipitation. ZnO nano(micro)structures with well-defined morphologies have been realized by tuning the Zn2+ : OH− ratio and the amount of the complexing agent NH4F. Importantly, we have demonstrated a fast spontaneous room-temperature formation of ZnO mesocrystals constructed with nanosheet subunits. The growth of ZnO involves the phase transformation from two intermediate compounds ZnF(OH) and Zn(OH)2. The nanosheets are observed to be ZnO (100) facets from the side- and top-view high-resolution TEM measurements. By comparing the arrangement of the (100) nanosheets in the mesocrystal with the organization of the {100} planes in the wurtzite crystal structure of ZnO, we identify for the first time the assembly of 2D nanosheets into 3D architectures by epitaxy. This unique self-assembly by epitaxy effectively separates the nanosheets in an ordered manner, producing a robust, geometrically ideal photocatalyst that is easily separable and recyclable. Being constructed of reactive (100) facets, the microstructured ZnO exhibits much higher photocatalytic efficiency than its nanoparticle counterpart. The defective nature of the (100) facets confirmed by the photoluminescence study explains the visible photocatalysis of the nanosheet assemblies.

Near-infrared photoluminescence from ZnO

Understanding the defect physics of ZnO is crucial in controlling its properties for various applications. We report the observation of an interesting 1.64 eV near-infrared (NIR) photoluminescence from ZnO and its evolution with annealing temperature. Based on a recent calculation on the transition levels of native point defects of ZnO [A. Janotti and C. G. Van de Walle, Phys. Rev. B 76, 165202 (2007)], the NIR emission can be successfully explained by the donor-acceptor transition between VO and VZn and/or the radiative recombination of shallowly trapped electrons with deeply trapped holes at Oi.

Hydrothermally controlled ZnO nanosheet self-assembled hollow spheres/hierarchical aggregates and their photocatalytic activities

We synthesized ZnO nano-micro structures, including microrods, nanosheets, a nanosheet self-assembled hollow sphere (NSHS), and a hollow hierarchical aggregate (HHA) by changing the concentration of trisodium citrate in a hydrothermal process. The NSHS was formed by the self-assembly of 2D nanosheets on the surface of a formaldehyde bubble, while the HHA was obtained by the self-assembly and hierarchical arrangement of NSHS. The photocatalytic activities of the ZnO structures were evaluated from the degradation of methylene blue under visible and black light irradiation. The NSHS showed better photocatalytic performance due to its excellent photon absorbance, low defect density and low emission quantum yield compared with the HHA. Furthermore, the size and number of the 2D nanosheets hydrothermally grown on the NSHS bubble template were controlled by changing the growth temperature.

From Zn(OH)2 to ZnO: a study on the mechanism of phase transformation

Phase transformation from Zn(OH)2 to ZnO has been studied by monitoring an intermediate growth stage. SEM observations reveal that the phase transformation takes place in three ways, i.e. dissolution–reprecipitation, in situcrystallization and solid–solid phase transformation.

Effect of calcination on the structural and optical properties of M/TiO2 thin films by RF magnetron co-sputtering

M/TiO2 (M=metal; Ag, Cu) thin films on quartz glass were prepared by RF magnetron co-sputtering process and the calcination effects on their optical and structural properties were investigated. The as-deposited M/TiO2 films are amorphous, and they are the anatase phase at 300–700 °C. The Ag-doped films have the anatase phase at 900 °C, whereas the Cu-doped films contain the rutile phase at 900 °C, implying that metal doping affects the temperature of phase transformation from anatase to rutile. The crystallite size of the anatase phase and the agglomerates of primary particles of the metal-doped TiO2 thin films increase with increasing calcination temperature. The X-ray photoelectron spectroscopy (XPS) results show that doped-silver is present in the metal state, while doped-copper exists in the mixed metal/oxide state. The as-deposited thin films have high transparency in the visible range. The transmittance of Cu/TiO2 thin films calcined at 900 °C is considerably reduced due to the crystal phase transformation, the variation of film composition and the optical scattering effect resulting from increased crystallite size. The absorption edge of transmittance is red-shifted at 900 °C due to the change of the optical band gap energy.

Synthesis of hierarchical rose bridal bouquet- and humming-top-like TiO2 nanostructures and their shape-dependent degradation efficiency of dye

Novel hierarchical rose bridal bouquet- and humming-top-like nanostructured TiO(2) were successfully prepared by the simple process with the hydrothermal temperature as the morphology-controlling factor. The gradual transformation from layered titanate to brookite phase was well consistent with the formation mechanism of the hierarchical morphologies. The three-dimensional flower bouquets built from the bunches of roses with surrounding fern fronds displayed the best adsorptivity and completely degraded methylene blue within 60 min under UV irradiation, whereas the humming-top geometry composed of anisotropically elongated spindle-like crystallites was detrimental to the dye photodegradation.