Potential-dependent surface-enhanced resonance raman spectroscopy at nanostructured TiO2 : A case study on cytochrome b5

Abstract: Nanostructured titanium dioxide (TiO2) electrodes, prepared by anodization of titanium, are employed to probe the electron-transfer process of cytochrome b5 (cyt b5) by surface-enhanced resonance Raman (SERR) spectroscopy. Concomitant with the increased nanoscopic surface roughness of TiO2, achieved by raising the anodization voltage from 10 to 20 V, the enhancement factor increases from 2.4 to 8.6, which is rationalized by calculations of the electric field enhancement. Cyt b 5 is immobilized on TiO2 under preservation of its native structure but it displays a non-ideal redox behavior due to the limited conductivity of the electrode material. The electron-transfer efficiency which depends on the crystalline phase of TiO2 has to be improved by appropriate doping for applications in bioelectrochemistry. Nanostructured TiO2 electrodes are employed to probe the electron-transfer process of cytochrome b5 by surface-enhanced resonance Raman spectroscopy. Concomitant with the increased nanoscopic surface roughness of TiO2, the enhancement factor increases, which can be attributed to the electric field enhancement. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nanoindentation study of the mechanical behavior of TiO2 nanotube arrays

Titanium dioxide (TiO2) nanotube arrays are attracting increasing attention for use in solar cells, lithium-ion batteries, and biomedical implants. To take full advantage of their unique physical properties, such arrays need to maintain adequate mechanical integrity in applications. However, the mechanical performance of TiO2 nanotube arrays is not well understood. In this work, we investigate the deformation and failure of TiO2 nanotube arrays using the nanoindentation technique. We found that the load–displacement response of the arrays strongly depends on the indentation depth and indenter shape. Substrate-independent elastic modulus and hardness can be obtained when the indentation depth is less than 2.5% of the array height. The deformation mechanisms of TiO2 nanotube arrays by Berkovich and conical indenters are closely associated with the densification of TiO2 nanotubes under compression. A theoretical model for deformation of the arrays under a largeradius conical indenter is also proposed.

Atomistic investigation into the mechanical behaviour of crystalline and amorphous TiO2 nanotubes

Titanium dioxide (TiO2) nanotubes are appealing to research communities due to their excellent functional properties. However, there is still a lack of understanding of their mechanical properties. In this work, we conduct molecular dynamics (MD) simulations to investigate the mechanical behaviour of rutile and amorphous TiO2 nanotubes. The results indicate that the rutile TiO2 nanotube has a much higher Young's modulus (∼800 GPa) than the amorphous one (∼400 GPa). Under tensile loading, rutile nanotubes fail in the form of brittle fracture but significant ductility (up to 30%) has been observed in amorphous nanotubes. This is attributed to a unique ‘repairing’ mechanism via bond reconstruction at under-coordinated sites as well as bond conversion at over-coordinated sites. In addition, it is observed that the fracture strength of rutile nanotubes is strongly dependent on their free surfaces. These findings are considered to be useful for development of TiO2 nanostructures with improved mechanical properties.

Rational design of 3D dendritic TiO2 nanostructures with favorable architectures

Controlling the morphology and size of titanium dioxide (TiO2) nanostructures is crucial to obtain superior photocatalytic, photovoltaic, and electrochemical properties. However, the synthetic techniques for preparing such structures, especially those with complex configurations, still remain a challenge because of the rapid hydrolysis of Ti-containing polymer precursors in aqueous solution. Herein, we report a completely novel approach-three- dimensional (3D) TiO2 nanostructures with favorable dendritic architectures-through a simple hydrothermal synthesis. The size of the 3D TiO2 dendrites and the morphology of the constituent nano-units, in the form of nanorods, nanoribbons, and nanowires, are controlled by adjusting the precursor hydrolysis rate and the surfactant aggregation. These novel configurations of TiO2 nanostructures possess higher surface area and superior electrochemical properties compared to nanoparticles with smooth surfaces. Our findings provide an effective solution for the synthesis of complex TiO2 nano-architectures, which can pave the way to further improve the energy storage and energy conversion efficiency of TiO 2-based devices.

Substrate temperature influenced physical properties of silicon MOS devices with TiO2 gate dielectric

DC reactive magnetron sputtering technique was employed for deposition of titanium dioxide (TiO2) films. The films were formed on Corning glass and p-Si (100) substrates by sputtering of titanium target in an oxygen partial pressure of 6x10-2 Pa and at different substrate temperatures in the range 303 673 K. The films formed at 303 K were X-ray amorphous whereas those deposited at substrate temperatures?=?473 K were transformed into polycrystalline nature with anatase phase of TiO2. Fourier transform infrared spectroscopic studies confirmed the presence of characteristic bonding configuration of TiO2. The surface morphology of the films was significantly influenced by the substrate temperature. MOS capacitor with Al/TiO2/p-Si sandwich structure was fabricated and performed currentvoltage and capacitancevoltage characteristics. At an applied gate voltage of 1.5 V, the leakage current density of the device decreased from 1.8?x?10-6 to 5.4?x?10-8 A/cm2 with the increase of substrate temperature from 303 to 673 K. The electrical conduction in the MOS structure was more predominant with Schottky emission and Fowler-Nordheim conduction. The dielectric constant (at 1 MHz) of the films increased from 6 to 20 with increase of substrate temperature. The optical band gap of the films increased from 3.50 to 3.56 eV and refractive index from 2.20 to 2.37 with the increase of substrate temperature from 303 to 673 K. Copyright (c) 2012 John Wiley & Sons, Ltd.

Influence of annealing temperature on Raman and photoluminescence spectra of electron beam evaporated TiO2 thin films

Titanium dioxide (TiO2) thin films were deposited on fused quartz substrates by electron beam evaporation method at room temperature. The films were annealed at different temperatures in ambient air. The surface morphology/roughness at different annealing temperatures were analyzed by atomic force microscopy (AFM). The crystallinity of the film has improved with the increase of annealing temperature. The effect of annealing temperature on optical, photoluminescence and Raman spectra of TiO2 films were investigated. The refractive index of TiO2 films were studied by envelope method and reflectance spectra and it is observed that the refractive index of the films was high. The photoluminescence intensity corresponding to green emission was enhanced with increase of annealing temperature. The peaks in Raman spectra depicts that the TiO2 film is of anatase phase after annealing at 300 degrees C and higher. The films show high refractive index, good optical quality and photoluminescence characteristics suggest that possible usage in opto-electronic and optical coating applications. (C) 2012 Elsevier B.V. All rights reserved.

Structural, electrical and dielectric properties of sputtered TiO2 films for Al/TiO2/Si capacitors

Metal-oxide semiconductor capacitors based on titanium dioxide (TiO2) gate dielectrics were prepared by RF magnetron sputtering technique. The deposited films were post-annealed at temperatures in the range 773-1173 K in air for 1 hour. The effect of annealing temperature on the structural properties of TiO2 films was investigated by X-ray diffraction and Raman spectroscopy, the surface morphology was studied by atomic force microscopy (AFM) and the electrical properties of Al/TiO2/p-Si structure were measured recording capacitance-voltage and current-voltage characteristics. The as-deposited films and the films annealed at temperatures lower than 773 K formed in the anatase phase, while those annealed at temperatures higher than 973 K were made of mixtures of the rutile and anatase phases. FTIR analysis revealed that, in the case of films annealed at 1173 K, an interfacial layer had formed, thereby reducing the dielectric constant. The dielectric constant of the as-deposited films was 14 and increased from 25 to 50 with increases in the annealing temperature from 773 to 973 K. The leakage current density of as-deposited films was 1.7 x 10(-5) and decreased from 4.7 X 10(-6) to 3.5 x 10(-9) A/cm(2) with increases in the annealing temperature from 773 to 1173 K. The electrical conduction in the Al/TiO2/p-Si structures was studied on the basis of the plots of Schottky emission, Poole-Frenkel emission and Fowler-Nordheim tunnelling. The effect of structural changes on the current-voltage and capacitance-voltage characteristics of Al/TiO2/p-Si capacitors was also discussed.

Post-deposition annealing influenced structural and electrical properties of Al/TiO2/Si gate capacitors

Titanium dioxide (TiO2) thin films are deposited on unheated p-Si (100) and quartz substrates by employing DC reactive magnetron sputtering technique. The effect of post-deposition annealing in air at temperatures in the range 673-973 K on the structural, electrical, and dielectric properties of the films was investigated. The chemical composition of the TiO2 films was analyzed with X-ray photoelectron spectroscopy. The surface morphology of the films was studied by atomic force microscope. The optical band gap of the as-deposited film was 3.50 eV, and it increased to 3.55 eV with the increase in annealing temperature to 773 K. The films annealed at higher temperature of 973 K showed the optical band gap of 3.43 eV. Thin film capacitors were fabricated with the MOS configuration of Al/TiO2/p-Si. The leakage current density of the as-deposited films was 1.2 x 10(-6) A/cm(2), and it decreased to 5.9 x 10(-9) A/cm(2) with the increase in annealing temperature to 973 K. These films showed high dielectric constant value of 36. (C) 2013 Elsevier Ltd. All rights reserved.

Influence of sputter power on structural and electrical properties of TiO2 films for Al/TiO2/Si gate capacitors

Titanium dioxide (TiO2) thin films were deposited onto p-Si substrates held at room temperature by reactive Direct Current (DC) magnetron sputtering at various sputter powers in the range 80-200W. The as-deposited TiO2 films were annealed at a temperature of 1023K. The post-annealed films were characterized for crystallographic structure, chemical binding configuration, surface morphology and optical absorption. The electrical and dielectric properties of Al/TiO2/p-Si structure were determined from the capacitance-voltage and current-voltage characteristics. X-ray diffraction studies confirmed that the as-deposited films were amorphous in nature. After post-annealing at 1023K, the films formed at lower powers exhibited anatase phase, where as those deposited at sputter powers >160W showed the mixed anatase and rutile phases of TiO2. The surface morphology of the films varied significantly with the increase of sputter power. The electrical and dielectric properties on the air-annealed Al/TiO2/p-Si structures were studied. The effect of sputter power on the electrical and dielectric characteristics of the structure of Al/TiO2/p-Si (metal-insulator-semiconductor) was systematically investigated. Copyright (c) 2014 John Wiley & Sons, Ltd.

Effect of film thickness and annealing on optical properties of TiO2 thin films and electrical characterization of MOS capacitors

Titanium dioxide (TiO2) thin films were deposited on glass and silicon (100) substrates by the sol-gel method. The influence of film thickness and annealing temperature on optical transmittance/reflectance of TiO2 films was studied. TiO2 films were used to fabricate metal-oxide-semiconductor capacitors. The capacitance-voltage (C-V), dissipation-voltage (D-V) and current-voltage (I-V) characteristics were studied at different annealing temperatures and the dielectric constant, current density and resistivity were estimated. The loss tangent (dissipation) increased with increase of annealing temperature.

One-pot synthesis of a TiO2-CdS nano-heterostructure assembly with enhanced photocatalytic activity

An unprecedented morphology of a titanium dioxide (TiO2) and cadmium sulfide (CdS) self-assembly obtained using a `truly' one-pot and highly cost effective method with a multi-gram scale yield is reported here. The TiO2-CdS assembly, comprising of TiO2 and CdS nanoparticles residing next to each other homogeneously self-assembling into `woollen knitting ball' like microspheres, exhibited remarkable potential as a visible light photocatalyst with high recyclability.

Probing Ultrafast Photoinduced Electron Transfer to TiO2 from CdS Nanocrystals of Varying Crystallographic Phase Content

The photoinduced electron transfer processes in a nanoheterostructured semiconductor assembly are complex and depend on various parameters Of the constituents of the assembly. We present here the ultrafast electron transfer characteristics of an assembly comprised of a Wide band semiconductor, titanium dioxide (TiO2), attached to light-harvesting cadmium sulfide (CdS) nanotrystals of varying crystallographic phase content. Quantitative analysis of Synchrotron high-resolution X-ray. diffraction data of CdS nanocrystals precisely reveals the presence of both wurtzite and zinc blende phases in varying amounts. The,estimated content of crystal phases is observed to be strongly dependent on an important synthesis parameter, viz., the ratio of the two solvents. The biphasit nature of CdS influences directly the shape of the nanocrystal at long reaction times as well as the transfer of the photoexcited electrons from the CdS to TiO2 as obtained from transient absorption spectroscopy. A higher amount of zinc blende Phase is observed to be beneficial for fast electron transfer across the CdS-TiO2 interface. The electron transfer rate constant differs by one order of magnitude between the CdS nanocryStals and varies linearly with the fraction of the phases.

Optical and Morphological Properties of Sol Gel derived Titanium dioxide Films

Titanium oxide (Titania) thin films were synthesized on different substrates via the sol-gel dip-coating method using alkoxide solution. Some selected samples were also prepared with different percentage of Lead (Pb). The influence of Pb addition in precursor sol on the optical properties of titanium dioxide thin films was studied. The optical transmittance in the visible region has increased with increase in weight percentage of lead. The refractive index was slightly decreased with Pb addition. Crystallization of these coatings was achieved through thermal annealing at temperatures above 400 degrees C. The structural properties and surface morphology of the crystallized coatings were studied by Scanning Electron Microscopy. Increase in average grain size from 250 nm to 350 nm with increase in Pb concentration is observed. Films were appeared to more coarse with increase in Pb addition. An increase in Pb addition resulted increase in average roughness from 12 nm to 25 nm.

Conduction through thin titanium dioxide films

Conduction through TiO₂ films of thickness 100 to 450 Å have been investigated. The samples were prepared by either anodization of Ti evaporation of TiO₂, with Au or Al evaporated for contacts. The anodized samples exhibited considerable hysteresis due to electrical forming, however it was possible to avoid this problem with the evaporated samples from which complete sets of experimental results were obtained and used in the analysis. Electrical measurements included: the dependence of current and capacitance on dc voltage and temperature; the dependence of capacitance and conductance on frequency and temperature; and transient measurements of current and capacitance. A thick (3000 Å) evaporated TiO₂ film was used for measuring the dielectric constant (27.5) and the optical dispersion, the latter being similar to that for rutile. An electron transmission diffraction pattern of a evaporated film indicated an essentially amorphous structure with a short range order that could be related to rutile. Photoresponse measurements indicated the same band gap of about 3 ev for anodized and evaporated films and reduced rutile crystals and gave the barrier energies at the contacts.

The results are interpreted in a self consistent manner by considering the effect of a large impurity concentration in the films and a correspondingly large ionic space charge. The resulting potential profile in the oxide film leads to a thermally assisted tunneling process between the contacts and the interior of the oxide. A general relation is derived for the steady state current through structures of this kind. This in turn is expressed quantitatively for each of two possible limiting types of impurity distributions, where one type gives barriers of an exponential shape and leads to quantitative predictions in c lose agreement with the experimental results. For films somewhat greater than 100 Å, the theory is formulated essentially in terms of only the independently measured barrier energies and a characteristic parameter of the oxide that depends primarily on the maximum impurity concentration at the contacts. A single value of this parameter gives consistent agreement with the experimentally observed dependence of both current and capacitance on dc voltage and temperature, with the maximum impurity concentration found to be approximately the saturation concentration quoted for rutile. This explains the relative insensitivity of the electrical properties of the films on the exact conditions of formation.

真空中有机污染物对TiO2/SiO2介质膜损伤阈值的影响

本文主要研究了真空中有机污染物对薄膜阈值的影响。采用电子枪蒸镀的方式制备了由TiO2/SiO2两种材料组合而成的1064nm高反膜，分别在大气和真空中对其损伤阈值进行了测量。发现在真空中污染物易黏附于光学元件表面，导致损伤；真空中的氧分子会加速损伤过程，根据破斑形貌的分析认为是由于在激光作用下氧分子与有机污染物发生反应造成的；真空中的有机污染物被去除后，真空和大气中的损伤阈值差别不大，破斑形貌相似。结果表明有机污染物是真空中损伤阈值降低的主要原因。