Amorphous TiO2 Layer Research Articles

III-V Semiconductors for Solar Vanadium Redox Flow Battery

The intermittent nature of renewable energy from wind and solar sources requires effective energy storage systems. Even though solar energy storage can be realized in the form of hydrogen gas by photoelectrochemical reaction, hydrogen is a very low-density gas requiring high-end technology for storage and transport. The low quantum yield is also far below commercialized value, which makes it difficult for large-scale application. The vanadium redox flow battery(VRB) is a true redox flow battery, which combines electrochemical conversion and storage via reduction and oxidation reactions of vanadium redox couples, V(II)/V(III) in the negative half-cell and V(IV)/V(V) in the positive half-cell.1-3 With the same active vanadium species in both electrolytes, it overcomes cross contamination problem. Due to its low cost, high capacity, high charge-discharge efficiency, and long life cycles, VRB has attracted extensive research focus. The solar VRB allows photoelectrodes to convert solar energy into electrochemical energy and direct storage of energy in positive and negative electrolyte containing V(IV)/V(V) and V(II)/V(III) redox couples, respectively. Gallium phosphide with a band gap of 2.26 eV, meets the requirement of optimal solar energy conversion efficiency in photoelectrochemical or photoelectrolysis cells.4 Our work demonstrated that photoredox reactions of vanadium electrolyte occurred on gallium phosphide under sunlight, indicating the promising use of gallium phosphide as photoactive electrodes for solar VRB. A thin amorphous TiO2 layer deposited on top of gallium phosphide by atomic layer deposition(ALD) was found beneficial to improve the long-term stability of gallium phosphide in an acidic medium, as well as facilitate electron/hole transfer on semiconductor/electrolyte interface. Thus, solar VRB would offer a promising approach for solar energy harvest, conversion and storage. (1) Sum, E.; Skyllas-Kazacos, M. Journal of Power Sources 1985, 15, 179–190. (2) Skyllas Kazacos, M.; Kazacos, G.; Poon, G.; Verseema, H. International Journal of Energy Research 2010, 34, 182–189. (3) Noack, J.; Roznyatovskaya, N.; Herr, T.; Fischer, P. Angewandte Chemie International Edition 2015, 54, 9776–9809. (4) Standing, A.; Assali, S.; Gao, L.; Verheijen, M. A.; van Dam, D.; Cui, Y.; Notten, P. H. L.; Haverkort, J. E. M.; Bakkers, E. P. A. M. Nature Communication 2015, 6, 7824.

Bio-molecular sensors based on guided mode resonance filters

In this work a low surface roughness and homogenous, high refractive index, and amorphous TiO2 layer on corrugated structures of diffractive optical element is coated by Atomic Layer Deposition (ALD) for biosensors. The design of Guided Mode Resonance Filters (GMRFs) is based on refractive indices and thicknesses of the waveguide biomolecular layers. The designed spectral shifts are calculated by Fourier Modal Method (FMM) and depend on the magnitude of the variations in refractive index of the biomolecular layer on waveguide structures. Furthermore, the sensitivity of the biomolecular sensors depends on the thickness of biomolecular layer and periodicity of the structures. The waveguide structures designed for larger periods show an enhancement in the sensitivity (nm/RIU) of the biomolecular sensor at longer wavelengths. The periodicities of nanophotonic structures are varied from 300 to 500 nm in design calculations with predominance of increase in effective index of the structure to support leaky waveguide modes.

Amorphous TiO2 Buffer Layer Boosts Efficiency of Quantum Dot Sensitized Solar Cells to over 9%

Charge recombination at an electrode/electrolyte interface is the main factor to limit the power conversion efficiency (PCE) of quantum dot sensitized solar cells (QDSCs). Herein, we present a novel and facile strategy based on successive coating of a sensitized electrode with a combination of blocking layers in appropriate sequence for suppressing the charge recombination. In this scenario, modification of the exposed surface of both TiO2 particles and QDs with an amorphous TiO2 (am-TiO2) layer via a classical TiCl4 hydrolysis treatment plays a fundamental role to enhance the effectiveness of a recombination blocking ZnS/SiO2 barrier layer. This strategy allows construction of CdSe0.65Te0.35 QD based champion QDSCs exhibiting a new PCE record of 9.28% and a certified PCE of 9.01% under full one sun illumination. The specific nature and sequence of the layering process is critical for the gain of photovoltaic performance. Control experiments indicate that the am-TiO2 is superior to a crystalline TiO2 laye...

Quantifying Geometric Strain at the PbS QD-TiO2 Anode Interface and Its Effect on Electronic Structures

Quantum dots (QDs) show promise as the absorber in nanostructured thin film solar cells, but achieving high device efficiencies requires surface treatments to minimize interfacial recombination. In this work, lead sulfide (PbS) QDs are grown on a mesoporous TiO2 film with a crystalline TiO2 surface, versus one coated with an amorphous TiO2 layer by atomic layer deposition (ALD). These mesoporous TiO2 films sensitized with PbS QDs are characterized by X-ray and electron diffraction, as well as X-ray absorption spectroscopy (XAS) in order to link XAS features with structural distortions in the PbS QDs. The XAS features are further analyzed with quantum simulations to probe the geometric and electronic structure of the PbS QD-TiO2 interface. We show that the anatase TiO2 surface structure induces PbS bond angle distortions, which increases the energy gap of the PbS QDs at the interface.

Enhancing the photocatalytic activity of CdS nanorods for selective oxidation of benzyl alcohol by coating amorphous TiO2 shell layer

One dimension (1D) core–shell structural CdS@TiO2 was fabricated by a modified sol–gel method. The thickness of amorphous TiO2 (a-TiO2) layer was adjusted by changing the amount of ammonia. Although a-TiO2 shell layer diminishes the visible light absorption in a certain extent, it largely increases the surface area of photocatalyst and effectively promotes the separation rate of photogenerated electron–hole pairs. It was revealed that a-TiO2 shell layer can effectively enhance the photocatalytic activity of CdS NRs for benzyl alcohol (BA) to benzaldehyde (BAD) transformation. Moreover, the core–shell structural CdS@a-TiO2 showed high stability and reusability for the selective oxidation of BA.

Facile synthesis of CdS@TiO2 core–shell nanorods with controllable shell thickness and enhanced photocatalytic activity under visible light irradiation

Amorphous TiO2 layers with a controllable thickness from 3.5 to 40nm were coated on the one-dimensional CdS nanorods surface under mild conditions. Compared to the bare CdS nanorods, the as-prepared CdS@TiO2 nanorods exhibit enhanced photocatalytic activities for phenol photodecomposition under visible light irradiation. The improved photoactivity is ascribed to the efficient separation of photogenerated electron and hole charge carriers between CdS cores and TiO2 shells. This study promises a simple approach to fabricating CdS@TiO2 core–shell structure nanocomposites, and can be applied for other semiconductor cores with TiO2 shells.

Intermixing and phase transformations in Al/Ti multilayer system induced by picosecond laser beam

Multilayer structures, consisting of 15 alternate Ti and Al thin films, and covered with thicker Ti layer, were deposited on a Si substrate to a total thickness of 900nm. Laser treatment was performed in air by defocused Nd:YAG laser pulses (150ps) with energies of 7 and 10mJ covering an area of 3mm in diameter (fluences were 0.1 and 0.14Jcm−2 respectively). Laser beam was scanned over the 5×5mm surface area. Characterizations were done by Auger electron spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and transmission electron microscopy. Nano-hardness measurements were performed by Vicker's method with applied load of 5mN.Obtained results show that irradiation with picosecond laser pulses leads to the formation of Ti2O3 in the top Ti layer. This process is more pronounced for higher energy and/or higher number of applied laser pulses. On the surface of all samples very thin amorphous TiO2 layer was formed. Laser irradiation induces surface melting and transformation of relatively flat surface into mosaic shaped one for samples treated with higher energy and higher number of applied laser pulses. Irradiation at smaller energy did not induce any noticeably mixing between Al and Ti beneath the covered Ti layer. Some mixing of Al and Ti appeared in the sample irradiated with higher energy of laser pulses.

UV-black rutile TiO2: An antireflective photocatalytic nanostructure

This work presents an experimental study on the specific quantitative contributions of antireflective and effective surface areas on the photocatalytic and antibacterial properties of rutile TiO2 nanospikes. They are studied when continuously distributed over the whole surface and when integrated into well-defined microstructures. The nanospikes were produced following MeV ion beam irradiation of bulk rutile TiO2 single crystals and subsequent chemical etching. The ion beam irradiation generated embedded isolated crystalline nanoparticles inside an etchable amorphous TiO2 layer, and nanospikes fixed to the not etchable TiO2 bulk substrate. The produced nanospikes are shown to resist towards aggressive chemical environments and act as an efficient UV antireflective surface. The photocatalytic activity experiments were performed under the ISO 10678:2010 protocol. The photonic and quantum efficiency are reported for the studied samples. The combined micro- and nanostructured surface triples the photonic efficiency compared to the initial flat surface. Results also revealed that the antireflective effect, due to the nanostructuring, is the dominating factor compared to the increase of surface area, for the observed photocatalytic response. The obtained results may be taken as a general strategy to design and precisely evaluate photoactive nanostructures.

One-step synthesis of SnOx nanocrystalline aggregates encapsulated by amorphous TiO2 as an anode in Li-ion battery

SnOx nanocrystalline aggregates (NAs) encapsulated by an amorphous TiO2 layer have been successfully designed by a one-step flame spray pyrolysis (FSP) method. As an anode in LIBs, the SnOx NAs@TiO2 electrode exhibits a capacity of 350 mA h g−1 after 300 cycles at a current density of 200 mA g−1, which is much superior to pure SnOx NAs and TiO2 NAs.

Photocatalytic acetaldehyde oxidation in air using spacious TiO2 films prepared by atomic layer deposition on supported carbonaceous sacrificial templates

Supported carbon nanosheets and carbon nanotubes served as sacrificial templates for preparing spacious TiO2 photocatalytic thin films. Amorphous TiO2 was deposited conformally on the carbonaceous template material by atomic layer deposition (ALD). Upon calcination at 550°C, the carbon template was oxidatively removed and the as-deposited continuous amorphous TiO2 layers transformed into interlinked anatase nanoparticles with an overall morphology commensurate to the original template structure. The effect of type of template, number of ALD cycles and gas residence time of pollutant on the photocatalytic activity, as well as the stability of the photocatalytic performance of these thin films was investigated. The TiO2 films exhibited excellent photocatalytic activity toward photocatalytic degradation of acetaldehyde in air as a model reaction for photocatalytic indoor air pollution abatement. Optimized films outperformed a reference film of commercial PC500.

On the electrophoretic and sol–gel deposition of active materials on aluminium rod current collectors for three-dimensional Li-ion micro-batteries

Electrophoretic deposition of titanium oxide particles, as well as sol–gel synthesis of thin films of TiO2 employing a titanium isopropoxide precursor solution, were studied as possible deposition routes for the coating of aluminium pillar current collectors intended for three-dimensional Li-ion micro-batteries. While electrophoresis of TiO2 particles was homogeneously covering the two-dimensional aluminium substrates, it was difficult to conformally coat the three-dimensional current collectors with this technique. The sol–gel approach, on the other hand, gave rise to thin and amorphous TiO2 layers on the Al rod based current collectors. The latter could be cycled for 100cycles indicating that such straightforward sol–gel approaches may be used for the manufacturing of 3D electrodes for Li-ion micro-batteries.

Synthesis of amorphous TiO2 modified ZnO nanorod film with enhanced photocatalytic properties

Amorphous TiO2 modified ZnO nanorod films were synthesized via multi-step processes: ZnO nanorod films were prepared by a wet chemical method. Amorphous TiO2 was then anchored on the tops and sides of the nanorods through immersion in tetrabutyltitanate solution for hydrolysis. The as-prepared samples were characterized for the phase structure, chemical state and surface morphology as well as optical absorption using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and ultraviolet–visible (UV–vis) spectrophotometer. The results showed that the nanorod films were covered by amorphous TiO2 layers, and their visible light absorption ability was strengthened. The photocatalytic studies revealed that TiO2 modified films exhibited enhanced photocatalytic efficiency for decomposition of methyl orange under ultraviolet–visible excitation, which might be attributed to the increased UV–vis light absorption and the separation of the charge carrier and prolonged electron lifetime due to the interface between TiO2 and ZnO.

Effect of thermal treatment on TiO2 nanorod electrodes prepared by the solvothermal method for dye-sensitized solar cells: Surface reconfiguration and improved electron transport

Solvothermal synthesis is considered a novel method of preparing the photoanode in dye-sensitized solar cells (DSSCs), which can directly synthesize material with good crystallinity at low temperatures without thermal treatment. However, how thermal treatment influences the properties of the materials synthesized by this method is still unclear, especial at the microscopic level. In this study, we applied TiO2 nanorod arrays prepared by the solvothermal method to DSSCs. X-ray Diffraction (XRD) and Raman results indicate that the crystal structure of TiO2 nanorods did not change after thermal treatment. However, the photovoltaic performance improved by 39%. Detailed analysis of high-resolution transmission electron microscopy (HRTEM) results demonstrate that a surface reconfiguration occurred, shifting one thin amorphous TiO2 layer to tiny crystallite spheres. The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) measurements further confirm this morphology change, and the surface states also become more suitable for dye absorption, which leads to a significant improvement in efficiency. Moreover, good electrical transport is observed due to the low concentration of surface defects. Therefore, we believe the performance improvement comes from crystalline surface and surface chemical bonding improvements. Our results could be useful in photoelectrical applications of the solvothermal synthesis method.

High open circuit voltages of solar cells based on quantum dot and dye hybrid-sensitization

A type of solar cell based on quantum dot (QD) and dye hybrid-sensitized mesoporous TiO2 film electrode was designed and reported. The electrode was consisted of a TiO2 nanoparticle (NP) thin film layer sensitized with CdS quantum dot (QD) and an amorphous TiO2 coated TiO2 NP thin film layer that sensitized with C106 dye. The amorphous TiO2 layer was obtained by TiCl4 post-treatment to improve the properties of solar cells. Research showed that the solar cells fabricated with as-prepared hybrid-sensitized electrode exhibited excellent photovoltaic performances and a fairly high open circuit voltage of 796 mV was achieved.

Photocatalytic Activity of the Oxide Layer Formed on NiTi Surface through Thermal Oxidation Process

In the present study, we attempted to prepare a photocatalytic titanium dioxide (TiO2) layer on nickel-titanium (NiTi) alloy surface through a simple thermal oxidation process in air. At 723K, an amorphous TiO2 layer including a slight amount of Ni was formed on the surface. Above 873K, the TiO2 layer crystallized into a rutile phase. At 1023K, a complex oxide of NiTiO3 was also formed. The methylene blue degradation test for evaluating photocatalytic activity showed that the formed TiO2 layers act as photocatalyst under ultraviolet (UV) light illumination, and its activity is superior to that of the surface layer formed by oxidizing a pure Ti substrate. The adhesion strength of the surface layers formed at 723K on NiTi alloy was higher than that of a commercially available TiO2-coating. We conclude that thermal oxidation as a surface modification technique is expected to make NiTi alloys give photocatalytic activity. [doi:10.2320/matertrans.M2014114]

Surface-enhanced Raman scattering of amorphous TiO2 thin films by gold nanostructures: Revealing first layer effect with thickness variation

In this paper, amorphous titanium dioxide (TiO2) thin films have been deposited on a commercially available Klarite substrate using the sol-gel process to produce surface-enhanced Raman scattering (SERS). The substrate consists of square arrays of micrometer-sized pyramidal pits in silicon with a gold coating. Several thin TiO2 layers have been deposited on the surface to study the influence of film thickness. Ultimately, we obtained information on SERS of an amorphous TiO2 layer by gold nanostructures, whose range is less than a few nanometers. Mechanisms responsible for the enhancement are the product of concomitant chemical and electromagnetic effects with an important contribution from plasmon-induced charge transfer.

Characteristics of stabilized spinel cathode powders obtained by in-situ coating method

TiO2-coated LiMn2O4 cathode powders are prepared using an in situ spray pyrolysis process. The TiO2-coated LiMn2O4 powders have spherical aggregated structures of nanometer-sized primary particles. The mean sizes of the primary and secondary TiO2-coated LiMn2O4 powders are 55 and 880 nm, respectively. The transmission electron microscopy images of the LiMn2O4 primary particles show a single-crystalline and well-faceted structure. The single-crystalline LiMn2O4 primary particles are uniformly coated with an amorphous TiO2 layer. An immediate reaction of titanium tetraisopropoxide with oxygen forms a small flame at the exit of an alumina tube located in the center part of a quartz reactor. Sudden collisions between TiO2 vapor and submicron-sized composite powders of the Li and Mn components occur to form the TiO2-coated cathode powders. The discharge capacities of the TiO2-coated LiMn2O4 cathode powders are 126 and 109 mAh g−1 in the first and 170 cycles at a current density of 1 C. The capacity retentions of the pure and TiO2-coated LiMn2O4 powders are 69% and 86% of the initial capacity after 170 cycles.

Formation of a crystalline nanotube–nanoparticle hybrid by post water-treatment of a thin amorphous TiO2 layer on a TiO2 nanotube array as an efficient photoanode in dye-sensitized solar cells

An atomic layer deposited, conformal thin layer (∼15 nm) of amorphous TiO2 on a primary TiO2 NT array is effectively converted into partly crystalline nanoparticles on the inside walls of TiO2 nanotubes by water mediated dissolution–precipitation phenomena without disturbing the original state of the TNTAs. This method helps to achieve the exclusive conversion of the secondary shell of amorphous TiO2 layer coated by ALD into nanoparticles and the preservation of original crystalline nanotube (core) properties such as mechanical stability and the charge transport properties. As a consequence, the resultant core–shell type nanotube–nanoparticle composite showed higher photoanode performance (25% increment in efficiency) than conventional untreated TiO2 NT array photoanode.

Surface-assisted laser desorption–ionization mass spectrometry on titanium dioxide (TiO2) nanotube layers

The paper reports on the use of a titanium oxide (TiO(2)) nanotube layer as a sensitive substrate for surface-assisted laser desorption-ionization mass spectrometry (SALDI-MS) of peptides and small molecules. The nanotube layers were prepared by electrochemical anodization of titanium foil. The optimized TiO(2) nanotubes morphology coupled to a controlled surface chemistry allowed desorption-ionization (D/I) of a peptide mixture (Mix1) with a detection limit of 10 femtomoles for the neurotensin peptide. The performance of the TiO(2) nanotubes for the D/I of small molecules was also tested for the detection of sutent, a small tyrosine kinase inhibitor, and verapamil. A detection limit of 50 fmol was obtained for these molecules, as compared to 500 fmol using classical matrix-assisted laser desorption-ionization mass spectrometry (MALDI-MS). Both amorphous and anatase TiO(2) layers displayed a comparable performance for D/I of analyte molecules. In a control experiment, we have performed D/I of analyte molecules on a flat TiO(2) layer. The absence of signal emphasizes the role of the nanostructured substrate in the D/I process.

Mesoporous Anatase Titania Hollow Nanostructures though Silica‐Protected Calcination

AbstractThe crystallization of nanometer‐scale materials during high‐temperature calcination can be controlled by a thin layer of surface coating. Here, a novel silica‐protected calcination process for preparing mesoporous hollow TiO2 nanostructures with a high surface area and a controllable crystallinity is presented. This method involves the preparation of uniform silica colloidal templates, sequential deposition of TiO2 and then SiO2 layers through sol–gel processes, calcination to transform amorphous TiO2 to crystalline anatase, and finally etching of the inner and outer silica to produce mesoporous anatase TiO2 shells. The silica‐protected calcination step allows crystallization of the amorphous TiO2 layer into anatase nanocrystals, while simultaneously limiting the growth of anatase grains to within several nanometers, eventually producing mesoporous anatase shells with a high surface area (∼311 m2 g−1) and good water dispersibility upon chemical etching of the silica. When used as photocatalysts for the degradation of Rhodamine B under UV irradiation, the as‐synthesized mesoporous anatase shells show significantly enhanced photocatalytic activity with greater enhancement for samples calcined at higher temperatures thanks to their improved crystallinity.