TiO2 Top Layer Research Articles

Fabrication and application of an immobilized TiO2/chitosan layer-by-layer system loaded with Reactive Red 4 dye for the removal of phenol and its intermediates

An immobilized TiO2/chitosan loaded with Reactive Red 4 (RR4) dye has been successfully fabricated via a layer-by-layer assemblage system. The immobilized photocatalyst was prepared by adsorbing RR4 dye on a chitosan (CS) sub-layer supported by a glass plate, before being coated with TiO2 as the top layer (known as TiO2/RR4-CS/glass). This layer-by-layer assemblage system allows the dye to be in contact with the TiO2/CS interface. In addition, the leaching as well as direct oxidation of the chemisorbed dye would also be prevented. Photo-etching of the immobilized TiO2/RR4-CS system for at least 10 h increased its surface area and enhanced its photocatalytic activity due to the formation of macropores within the TiO2 top layer. Application of the immobilized TiO2/RR4-CS layer-by-layer system shows that the photocatalytic system exhibited high photocatalytic efficiencies under the irradiation of a 45 W compact fluorescent lamp for the degradation of phenol as well as its intermediates. In addition, the immobilized photocatalyst system was also reusable for at least five cycles of extended use without losing its efficiency. The immobilized TiO2/RR4-CS layer-by-layer system improved the photocatalytic activity of TiO2 due to its better charge separation and lower electron–hole pair recombination.

Extracting large photovoltages from a-SiC photocathodes with an amorphous TiO2 front surface field layer for solar hydrogen evolution

A p–i–n junction photocathode made from 110 nm (p/i) amorphous SiC and an (n) TiO2 top layer leads to an onset potential of +0.8 VRHE and a photocurrent density of 8.3 mA cm−2 at 0 VRHE using only earth abundant materials.

Enhanced photoelectrochemical and photocatalytic performance of TiO2 nanorod arrays/CdS quantum dots by coating TiO2 through atomic layer deposition

TiO2 nanorod arrays/CdS quantum dots/ALD-TiO2 nanostructures were fabricated as photocatalyst and its related properties were investigated comprehensively. Enhanced photoelectrochmical properties are observed and the degradation rate of methyl orange under visible light is enhanced by ca.156% compared to the nanorod arrays/CdS quantum dots, which is due to the enhancement of the separation of electrons-holes induced by the introduction of the ultrathin TiO2 top layer. Moreover, the stability of the TiO2 nanorod arrays/CdS quantum dots is increased by the ALD-TiO2 coating. These results suggest that the design of TiO2 nanorod arrays/CdS quantum dots/ALD-TiO2 nanostructures gives a promising strategy to improve the photoelectrochemical and photocatalytic properties in solar energy conversion, along with reduced photo-corrosion in the semiconductor-semiconductor heterojunction.

Effects of passivation on breakdown voltage and leakage current of normally-off InAlN/GaN MISHFETs—a simulation study

Effects of passivation on breakdown characteristics of normally-off InAlN/GaN MISHFETs have been investigated via physics based simulations. The product of passivation layer permittivity and thickness is found to dramatically affect breakdown voltage, due to alleviation of peak electric field at the gate edge on the drain side, leading to a much smoother field distribution in the channel. A proposed structure with a 985 nm TiO2 top and 15 nm HfO2 bottom passivation stack exhibits a breakdown at ∼750 V with leakage current ∼4 μA mm−1, showing ∼15× increase in breakdown voltage compared to the structure without the TiO2 top layer (50 V).

Cost-effective nanoporous SiO2–TiO2 coatings on glass substrates with antireflective and self-cleaning properties

Simultaneous antireflective and self-cleaning properties have been realized by depositing double-layered SiO2–TiO2 coatings onto glass substrates using a combined sol–gel dip-coating process in which the two oxide layers were deposited in succession. The first layer is composed of a hybrid methyl-functionalized nanoporous SiO2 material that provides on average a 6% anti-reflection gain. The degree of antireflectivity can be controlled by adjusting the thickness and refractive index by selecting suitable solvents and pore-forming agents in the coating mixture. The second layer is an ultrathin TiO2 nanoporous layer deposited on top of antireflective layer. The high hardness of the TiO2 top layer acts as a protection barrier toward mechanical damage and ensures the surface is self-cleaning. An average anti-reflectivity of 3.4% achieved over a spectrum range of 400–800nm for the optimal SiO2–TiO2 bilayers is obtained. It is found that a homogeneous pore size distribution in the SiO2 layer introduced by using isopropanol as the solvent can compensate for the TiO2 band edge absorption based on the optical interference that occurs between the two layers. These systems are easy to produce on a large scale at low cost and exhibit high mechanical and chemical durability. These materials are thus suitable for use as bifunctional antireflective and self-cleaning coatings for use as large substrates for solar cells.

Newly designed dye-sensitized solar cells fabricated with double-layered TiO2 (bottom layer)/Bx–TiO2 (top layer) combined electrodes for improved photocurrent

An unusual double-layered TiO2 (bottom layer)/Bx–TiO2 (top layer) combined electrode array was investigated to improve the photocurrent in dye-sensitized solar cells (DSSCs). A positive semiconductor, Bx–TiO2, with nanometer-sized B (1.0, 5.0, and 10.0mol%)-incorporated TiO2 prepared using a solvothermal method, was utilized as the working electrode material by coating onto the second level above the TiO2 electrode. The photocurrent and photovoltaic efficiency of the TiO2 (bottom)/Bx–TiO2 (top)-DSSC were 20.5% and 17.3% greater, respectively, than that of the double-layers of anatase TiO2–DSSC in the photocurrent–voltage (I–V) curve of the optimal electrode. This result was attributed to their energy levels of reduction (LUMO)/oxidation (HOMO) as determined by cyclic voltammetry (CV). As the LUMO level of Bx–TiO2 was located at a slightly higher level than that of pure anatase TiO2, the electrons donated from the dye were easily transferred to the surface of the TiO2 electrode without electron loss. Moreover, the recombination was also much slower in the TiO2 (bottom)/Bx–TiO2 (top)-based DSSCs than in the double-layered pure TiO2 DSSC.

Synthesis of TiO2/Pt/TiO2 multilayer films via radio frequency magnetron sputtering and their enhanced photocatalytic activity

TiO2/Pt/TiO2 (TPT) multilayered films with different thicknesses of Pt layers from about 0.75 to 12nm were prepared by radio frequency magnetron sputtering method. The as-prepared films were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, UV–visible diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The photocatalytic activity of the samples was evaluated by the photocatalytic decolorization of methyl blue aqueous solution under ultraviolet light irradiation. The photocatalytic activities of all TPT multilayer films were higher than that of the pure TiO2 films. When Pt thickness was increased to 3nm, the measured photocatalytic activity of the TPT film was highest, and exceeded that of the pure TiO2 films by a factor of more than three times. Such enhancement was ascribed to the presence of Pt layer, which inhibits the recombination of the photogenerated charge and carriers, as well as modifies the crystallinity of the TiO2 top layer.

Sol-gel thin films with anti-reflective and self-cleaning properties

AbstractSelf-cleaning photocatalytic TiO2 films are beneficial since they reduce the maintenance cost and enhance the efficiency of various optical systems, especially thermal and photovoltaic solar systems. However, the presence of a TiO2 layer on glass reduces the transmission of incident light, which leads to a decrease in efficiency. This drawback can be overcome by applying a layer of anti-reflective coating beneath the TiO2 layer. Generally, the anti-reflective layer is porous silica. The presence of the anti-reflective layer compensates for the loss of light transmittance caused by the photocatalytic TiO2 top layer. This paper reviews some of the previous and the latest fundamental studies in the literature on anti-reflective, self-cleaning and multi-functional films.

1.2 m Shielded Cassegrain Antenna for Close-Packed Radio Interferometer

ABSTRACTInterferometric millimeter observations of the cosmic microwave background and clusters of galaxies with arcminute resolutions require antenna arrays with short spacings. Having all antennas co-mounted on a single steerable platform sets limits to the overall weight. A 25 kg lightweight novel carbon-fiber design for a 1.2 m diameter Cassegrain antenna is presented. The finite element analysis predicts excellent structural behavior under gravity, wind, and thermal load. The primary- and secondary-mirror surfaces are aluminum-coated with a thin TiO2 top layer for protection. A low beam sidelobe level is achieved with a Gaussian feed-illumination pattern with edge taper, designed based on feed-horn antenna simulations and verified in a far-field beam-pattern measurement. A shielding baffle reduces interantenna coupling to below ∼-135 dB. The overall antenna efficiency, including a series of efficiency factors, is estimated to be around 60%, with major losses coming from the feed spillover and secondary blocking. With this new antenna, a detection rate of about 50 clusters yr-1 is anticipated in a 13-element array operation.

Charge build-up and re-distribution in SrTixNb1−xO3 quantum wells and TiO2–SrTiO3 heterostructure

This work examined charge build-up and re-distribution in SrTixNb1−xO3 quantum wells and TiO2–SrTiO3 heterostructure based on thermoelectric power data reported in the literature. In this work, we first identified the equations that best related charge density to the thermoelectric power. Using such equations together with a distributed model that accounted for the layered structure, we computed the charge densities in the two structures over a broad temperature range. Our findings suggested that charge migration from the quantum wells into the substrate was primarily governed by the barrier height at the interface. A very different charge density profile was observed in the heterostructure and the computed charge spread into the substrate was immense (of the order of hundreds of micrometres). By comparing such charge spread with values taken from samples when the TiO2 top layer had been removed, we arrived at the conclusion that it could not have originated from the interface and must have arisen from other sources such as oxygen vacancies (acting as donors) embedded during film deposition.

Hydrophobic, Antireflective, Self-Cleaning, and Antifogging Sol−Gel Coatings: An Example of Multifunctional Nanostructured Materials for Photovoltaic Cells

Antireflective, photocatalytic (self-cleaning), water repellent, and high water-wetting (anti fogging) properties were combined for the first time into a sol−gel coating deposited onto glass substrates. Such an original multifunctional coating was obtained by sol−gel liquid deposition of two successive oxide layers. The first coating is composed of a hybrid methyl-functionalized nanoporous SiO2 material that exhibits high transparency, high water resistance, close to null water adsorption, and fairly high mechanical stability (transversal Young Modulus: 1.5 GPa). Thickness and refractive index can be controlled by selecting proper chemical and processing conditions so as to adjust the antireflectivity properties. The second layer is an ultrathin crystalline TiO2 nanoperforated layer that was deposited on top of the previous antireflective layer. Its thickness and refractive index were adjusted around 12 nm and n ≈ 1.8 respectively. This hard TiO2 top layer acts as a protecting barrier toward mechanical ag...

Double-Layered TiO2−SiO2 Nanostructured Films with Self-Cleaning and Antireflective Properties

Dual function of self-cleaning and antireflection can be created in double-layered TiO2-SiO2 nanostructured films. The film were prepared by (1) layer-by-layer deposition of multilayered SiO2 nanoparticles with polydiallyldimethylammonium (PDDA) cations, (2) layer-by-layer deposition of multilayered titanate nanosheets with polications on PDDA/SiO2 multilayer films, and (3) burning out the polymer and converting titanate nanosheets into TiO2 by hearing at 500 degrees C. The as-prepared films, consisting of a porous SiO2 bottom layer and a dense TiO2 top layer, improved the transmittance of glass or quartz substrates, as demonstrated by transmission spectra collected at normal incidence. The photocatalytic properties of the films were studied by the change of the water contact angle together with the decay of the IR absorption of the hydrocarbon chain of octadecylphosphonic-acid-modified films under 2.6 mW cm-2 UV illumination. Both the antireflective and the photocatalytic properties of the films were dependent on the number of PDDA/nanosheet bilayers deposited. however, excellent surface wettability of the films for water was obtained, independent of the preparation conditions. The experimental findings are discussed in terms of the special structure of the double-layered nanostructured film.

A wet air oxidation process using a catalytic membrane contactor

A new process for oxidation of toxic compounds in liquids has been demonstrated. The concept is based on the same principles as catalytic wet air oxidation (CWAO), but the metal catalyst is fixed to a ceramic porous membrane in a catalytic membrane reactor of the contactor type (CMR-C). Air is flowing along the surface of the contactor, and the waste liquid is supplied from the other side of the contactor through the porous contactor wall. In this way, the gas and liquid phases are driven to contact in the porous network of the catalytic contactor separating them. Fifty percent of conversion of formic acid model solution (5 g/l) was obtained in initial reactor experiments at 150 8C and 10 bar pressure, but the observed oxidation rate was low: about 0.13 mmol/s per gPt. TEM and EDS investigations of the contactor showed that 5 � /10 nm Pt particles were evenly distributed close to the surface of the mesoporous TiO2 top layer. After the experiments, a 10 � /50 nm thick aluminium-rich amorphous deposit was observed in the porous structure. The low conversion rate has been attributed to this deposit causing deactivation by encapsulation of the catalyst and plugging of the mesoporous layer of the contactor. The deposits are believed to be caused by chemical instability of a-Al2O3 in acidic aqueous environment at elevated temperature. a-Al2O3 is present in the coarse-grained membrane support. # 2003 Elsevier Science B.V. All rights reserved.

Formation and characterization of TiO2 thin films with application to a multifunctional heat mirror

Thin films of titanium dioxide (TiO2) with desired crystal phase of anatase or rutile were formed by rf sputtering of a TiO2 target under precisely controlled process parameters. The application of TiO2 as the antireflection layers of a TiN-based heat mirror was testified. Optical calculation was done for the heat mirror, and an optimized structure with TiO2 (30nm)/TiN (30nm)/TiO2 (30nm) was deposited on SiO2 glass by sequential sputtering. Optical evaluation of the heat mirror was performed by measuring the transmittance/reflectance with a spectrophotometer in wavelengths 250–2500nm and with a FT-IR system in wavelengths 1–25μm, respectively. Furthermore, it is suggested that a better and a multifunctional performance can mostly be expected through precise control of the crystal structure of the TiO2 top layer which acts as a photocatalyst.