Nanostructured Metal Oxide Films Research Articles

Thickness dependence of nanocrystalline tin oxide thin films in capacitive biosensor characterization

Nanocrystalline SnO2 thin films of varying thicknesses (250–50 nm) have been prepared on p-type Si by Langmuir Blodgett technique. Nanocrystalline SnO2/polyaniline (Si/SnO2/PANI) thin film heterostructure has been studied for label free, real time electrochemical detection for model immunoglobulin interactions. Sensor responses were studied using impedance methods, wherein transient capacitance gave fast response for antigen-antibody interaction in picomolar concentration range. From the study of sensor structures with varying film thicknesses, it was observed that SnO2 film with less than 200 nm thickness were effected with insulating (dielectric) properties whereas for thickness higher than 200 nm, it was n-type semiconducting characteristics. This is due to effects of charge on the surface adsorbed species as revealed by capacitance voltage characteristics, suggesting these characteristics as indicators of ionic transport effects in nanostructured metal oxide films in electrochemical cell configuration. The current study clearly states the possibility of fabricating cost effective and stable capacitive sensors with SnO2 films with thickness less than 100 nm. The study helps in optimizing the sensing method for capacitive/conductive biosensors based on nanostructured semiconductor thin films.

Flame spray pyrolysis for the one-step fabrication of transition metal oxide films: Recent progress in electrochemical and photoelectrochemical water splitting

Developing large scale deposition techniques to fabricate thin porous films with suitable opto-electronic properties for water catalysis is a necessity to mitigate climate change and have a sustainable environment. In this review, flame spray pyrolysis (FSP) technique, a rapid and scalable methodology to synthesize nanostructured transitional metal oxide films with designed functionalities, is firstly introduced. Furthermore, applications in electrochemical (EC) and photoelectrochemical (PEC) water splitting for the production of hydrogen fuel is also presented. The high combustion temperature and the aggregation of flame aerosol ensure that the FSP-made films possess high crystallinity, tunable porosity and high surface areas, making this method suitable either as catalysts for EC water splitting or as efficient semiconductor materials for PEC water splitting. Finally, a perspective on the next generation FSP engineered films with potential applications in energy storage and conversion is described.

Hydrothermal assisted growth of vertically aligned platelet like structures of WO3 films on transparent conducting FTO substrate for electrochromic performance

Controlled growth of nanostructured metal oxide films growing on the conducting substrates with good adhesion in a single step process via hydrothermal method is still challenging. In the present work, WO3 platelets were deposited on transparent conducting fluorine doped tin oxide substrates by single step hydrothermal process via tailoring the reaction time (8 h, 12 h and 16 h) and reaction temperature (120 °C and 180 °C). X-ray diffraction analysis showed that the WO3 thin films grown at 120 °C for 8 h and 12 h reaction time comprised with mixed phases of hexagonal and monoclinic system, whereas 16 h of reaction time produced single phase monoclinic system. However the WO3 films deposited at 180 °C for 8 h, 12 h and 16 h reaction time acquired monoclinic system. Raman spectrum showed the stretching and bending modes of WO3. The WO3 films deposited at 120 °C and 180 °C for different reaction time 8 h, 12 h and 16 h exhibited various surface morphology such as buds like, bricks like, platelets like and sheets with platelets like structures. Optical studies showed that the transmittance of WO3 film is decreased with increase in the reaction temperature. The WO3 film deposited at 120 °C for 12 h reaction time acquired transmittance of about 86% at 463 nm whereas the film deposited at 180 °C for 12 h reaction time showed transmittance of about 75% at 465 nm. The direct and indirect optical band gap of the deposited WO3 films varies in the range 2.5 eV– 2.9 eV and 2.6–2.75 eV respectively with varying growth parameters. The electrical properties of WO3 film deposited at 120 °C for 12 h showed relatively low resistivity (1.8 × 10−3 Ωcm) and high carrier concentration (9.3 × 1019 cm−3). The electrochromic studies carried out for the WO3 film synthesized at 120 °C for 16 h exhibits high peak current density, diffusion coefficient and fast switching kinetics with respect to other deposited films.

Sensitization of Nanocrystalline Metal Oxides with a Phosphonate-Functionalized Perylene Diimide for Photoelectrochemical Water Oxidation with a CoOx Catalyst.

A planar organic thin film composed of a perylene diimide dye (N,N'-bis(phosphonomethyl)-3,4,9,10-perylenediimide, PMPDI) with photoelectrochemically deposited cobalt oxide (CoOx) catalyst was previously shown to photoelectrochemically oxidize water (DOI: 10.1021/am405598w). Herein, the same PMPDI dye is studied for the sensitization of different nanostructured metal oxide (nano-MOx) films in a dye-sensitized photoelectrochemical cell architecture. Dye adsorption kinetics and saturation decreases in the order TiO2 > SnO2 ≫ WO3. Despite highest initial dye loading on TiO2 films, photocurrent with hydroquinone (H2Q) sacrificial reductant in pH 7 aqueous solution is much higher on SnO2 films, likely due to a higher driving force for charge injection into the more positive conduction band energy of SnO2. Dyeing conditions and SnO2 film thickness were subsequently optimized to achieve light-harvesting efficiency >99% at the λmax of the dye, and absorbed photon-to-current efficiency of 13% with H2Q, a 2-fold improvement over the previous thin-film architecture. A CoOx water-oxidation catalyst was photoelectrochemically deposited, allowing for photoelectrochemical water oxidation with a faradaic efficiency of 31 ± 7%, thus demonstrating the second example of a water-oxidizing, dye-sensitized photoelectrolysis cell composed entirely of earth-abundant materials. However, deposition of CoOx always results in lower photocurrent due to enhanced recombination between catalyst and photoinjected electrons in SnO2, as confirmed by open-circuit photovoltage measurements. Possible future studies to enhance photoanode performance are discussed, including alternative catalyst deposition strategies or structural derivatization of the perylene dye.

In Situ Localized Growth of Porous Tin Oxide Films on Low Power Microheater Platform for Low Temperature CO Detection

This paper reports a facile method for creating a nanostructured metal oxide film on a low power microheater sensor platform and the direct realization of this structure as a gas sensor. By fast annealing the deposited liquid precursors with the microheater, a highly porous, nanocrystalline metal oxide film can be generated in situ and locally on the sensor platform. With only minimal processing, a high performance, miniaturized gas sensor is ready for use. A carbon monoxide sensor using the in situ synthesized porous tin oxide (SnO2) sensing film is made as a demonstration of this technique. The sensor exhibits a low detection limit and fast response and recovery time at a low operating temperature. This facile fabrication method is highly flexible and has great potential for large-scale gas sensor fabrication.

Investigation of sol-gel yttrium doped ZnO thin films: structural and optical properties

Nanostructured metal oxide films are extensively studied due to their numerous applications such as optoelectronic devices, sensors. In this work, we report the Y-Zn-O nanostructured films prepared by sol-gel technology from sols with different concentration of yttrium precursor, followed by post-annealing treatment. The Y doped ZnO thin films have been deposited on Si and quartz substrates by spin coating method, then treated at temperatures ranging from 300-800oC. XRD analysis reveals modification of the film structure and phases in the doped ZnO films.

Structure and physicochemical properties of nanostructured metal oxide films for use as the sensitive layer in gas sensors

The morphological features of nanostructured films of tin, zinc, indium, and cerium oxides are established. The parameters of electron traps, such as adsorbed oxygen atoms and structural defects, responsible for the sensory effect are determined. An increase in the conductance of indium oxide films upon annealing in vacuum is revealed.

Dye Sensitized Solar Cell Using Natural Dyes as Chromophores - Review

The molecular dye is an essential component of the Dye sensitized solar cell (DSSC), and improvements in efficiency over the last 15 years have been achieved by tailoring the optoelectronic properties of the dye. The most successful dyes are based on ruthenium bipyridyl compounds, which are characterized by a large absorption coefficient in the visible part of the solar spectrum, good adsorption properties, excellent stability, and efficient electron injection. However, ruthenium-based compounds are relatively expensive, and organic dyes with similar characteristics and even higher absorption coefficients have recently been reported; solar cells with efficiencies of up to 9% have been reported. Organic dyes with a higher absorption coefficient could translate into thinner nanostructured metal oxide films, which would be advantageous for charge transport both in the metal oxide and in the permeating phase, allowing for the use of higher viscosity materials such as ionic liquids, solid electrolytes or hole conductors. Organic dyes used in the DSSC often bear a resemblance to dyes found in plants, fruits, and other natural products, and several dye-sensitized solar cells with natural dyes have been reported. This paper gives an over-view of the recent works in DSSC using the natural dyes as chromophores.

Oxide Nanoparticle Thin Films Created Using Molecular Templates

We present a new generic method to synthesize nanostructured metal(II) oxide films on a substrate such as silicon. Vacuum ultraviolet (VUV) excimer lamps rupture metal organic precursors, creating volatile organic fragments, while the metal species at the surface form oxides. X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) confirm that 172 nm VUV irradiation of manganese and copper phthalocyanines yields manganese(II) and copper(II) oxides, respectively. The morphology of the precursor film provides a template from which metal oxide nanoparticles are formed, which we demonstrate for particles with dimensions 40 x 40 x 10 nm(3). Our procedure is both simple and flexible, with low thermal budget and potential for patterning.

Colour simulation and prediction of complex nano-structured metal oxide films: Test case: Analysis and modeling of electro-coloured anodized aluminium

An optical modeling procedure is developed to predict and model the colour of electro-coloured anodized aluminium that has been modified in pore structure for the generation of interference colours. The relation between the multi-layered, nano-sized oxide microstructure and the colour is experimentally determined and translated into an optical model that is able to predict the colour as a function of microstructural variations. The optical modeling procedure is highly sensitive to changes on the nano-scale (porosity, percentage of metal deposition, presence of barrier regrowth…), and has high potential as a predictive tool to relate the microstructure of a surface to the final colour characteristics of the metal, and as such to the used electrolytic processing conditions.

Fabrication of gold dot, ring, and corpuscle arrays from block copolymer templates via a simple modification of surface energy

We demonstrate a simple method for tuning the morphologies of as-spun micellar thin films by modifying the surface energy of silicon substrates. When a polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) copolymer dissolved in o-xylene was spin-coated onto a PS-modified surface, a dimple-type structure consisting of a thick PS shell and P2VP core was obtained. Subsequently, when the films were immersed in metal precursor solutions at certain periods of time and followed by plasma treatment, metal individual dots in a ring-shaped structure, metal nanoring, and metal corpuscle arrays were fabricated, depending on the loading amount of metal precursors. In contrast, when PS-b-P2VP films cast onto silicon substrates with a native oxide were used as templates, only metal dotted arrays were obtained. The combination of micellar thin film and surface energy modification offers an effective way to fabricate various nanostructured metal or metal oxide films.

Non-emissive colour filters for fluorescence detection

We describe a simple technique for fabricating non-emissive colour filters based on the sensitisation of a highly porous nanostructured metal-oxide film with a monolayer of dye molecules. Ultrafast electron transfer at the oxide/dye interface induces efficient quenching of photogenerated excitons in the dye, reducing the photoluminescence quantum yield by many orders of magnitude. The resultant filters exhibit much less autofluorescence than conventional colour filters (where the chromophore is dispersed in a glass or polymer host), and are a viable low cost alternative to interference filters for microfluidic devices and other applications requiring non-emissive filtering.

Evolution of potential distributions during the charging of nano-structured metal oxide films in air as response to sudden voltage application

Using metal oxide film structures, which were originally designed for gas sensing applications, we measured the charging and discharge currents and potential distributions on several metal oxide coatings after the application of an electrical potential against earth. The potential distributions show a specific charging of the surface with oxygen ions through the gas phase. The accumulated charge corresponds to that of the pseudocapacitors. Influence of air humidity has been found to be low, voltaic and temperature dependences of the charge are presented. The activation energy of discharge indicates a weak chemisorption of the charging oxygen species on the metal oxide surface.

CHEMICAL DESIGN OF COMPLEX NANOSTRUCTURED METAL OXIDES IN SOLUTION

Nanostructured materials with controlled architectures are desirable for many applications, among which, metal oxides are especially important in optics, electronics, biology, catalysis, and energy conversions. Various chemical routes have been widely investigated for the synthesis of nanostructured metal oxide particles and films. More recently, deliberately designed chemical strategies have been used to produce particles and films composed of more complex crystal structures. In this paper, we discuss some recent progresses in the design of complex nanostructures through chemical routes, emphasize particularly on metal oxides. We first review some basic concepts involved in the fabrication of complex nanostructures, including crystal nucleation and growth, shape controlling and ripening process. We then describe more recent work on the use of different methods to synthesize a wide range of complex nanostructures, including hierarchical structures, heterostructures, as well as oriented nanowires and nanotubes. Such purposely built materials are designed, and engineered to match the physical, chemical, and structural requirements of their applications.

Aerosol-Chemical Vapor Deposition Method For Synthesis of Nanostructured Metal Oxide Thin Films With Controlled Morphology

An aerosol-chemical vapor deposition (ACVD) was designed to deposit nanostructured metal oxide films with controlled morphologies. Characteristic times of the different processes governing deposition of the film were used to establish the relationship of process parameters to the resultant morphology of the film. Titanium dioxide (TiO2) films were synthesized with different morphologies: dense, columnar, granular, and branched tree-type structures. The developed ACVD process was also used to deposit columnar nickel oxide (NiO) films. The various films with well-controlled characteristics (length, morphology) were used to establish the performance in solar energy applications, such as photosplitting of water to produce hydrogen. Columnar TiO2 films of 1.6 μm length with a platinum wire counter electrode resulted in 15.58% hydrogen production efficiencies under UV light illumination, which was 2.50 times higher than dense TiO2 films with a platinum wire counter electrode. On replacing the Pt counter electro...

A simple numerical model for the charge transport and recombination properties of dye-sensitized solar cells: A comparison of transport-limited and transfer-limited recombination

A simple model is presented for the charge transport and recombination characteristics of dye-sensitized solar cells (DSCs), based on solving the continuity equation for electrons using numerical methods. We analyze two models, in which the recombination kinetics are determined by either the electron transport kinetics in the nanostructured metal oxide film or the electron transfer rate across the metal oxide–electrolyte solution interface. It is found that transport-limited recombination results in ideal diode behaviour, in which case the open-circuit voltage is proportional to the logarithm of the light intensity with a slope of 26 mV, while the transfer-limited model allows for a different slope that depends on the trap distribution parameter, α, and the electron transfer coefficient, b.

Amplified electrochemical DNA-sensing of nanostructured metal oxide films deposited on disposable graphite electrodes functionalized by chemical vapor deposition

Metal oxide nanostructures offer interesting possibilities to design functional surfaces for bio-sensing applications, for instance, through higher surface area leading to enhanced immobilization of biomolecules, which increases the detection limit. Herein, an amplified electrochemical sensing method has been presented for the detection of DNA based on the readout resulting from chemical oxidation of guanine on nanoscaled metal oxides (TiO 2, SnO 2 and Fe 3O 4) obtained by chemical vapor deposition (CVD) onto pencil graphite electrode (PGE) as electrochemical transducer. The proposed strategy is suitable to produce cost-effective disposable sensor elements enabling quantitative detection of nanomolar concentrations of DNA. When preparing these metal oxide surfaces by CVD onto PGEs, the various experimental conditions; such as, the effect of different concentrations of 20 mer-bases DNA oligonucleotide (ODN20), and the surface pretreatment steps were studied to obtain better surface properties for DNA immobilization. The detection limit estimated for signal-to-noise ratios >3 corresponds to 21.3, 53.9 and 45.8 nmole/ml ODN20 concentrations for PGEs modified with TiO 2, SnO 2 and Fe 3O 4 films, respectively. The electrochemical detection of DNA onto metal oxide@PGEs is discussed together with the application potential.

Batch fabrication of metal oxide sensors on micro-hotplates

We report the parallel fabrication of miniaturized chemical sensors by the direct integration of nanostructured transition metal oxide films onto micro-hotplate platforms based on micromachined suspended membranes. This has been achieved by local deposition on a 10 × 10 membrane wafer of a supersonic cluster beam through a microfabricated auto-aligning silicon shadow mask. The sensing properties of the obtained devices were tested with respect to various gaseous species. For reducing and oxidizing species such as ethanol and NO2, very good performance in terms of linearity and sensitivity was observed. These results demonstrate the feasibility of the coupling of a bottom-up nanofabrication technique such as supersonic cluster beam deposition to a top-down microfabricated platform for a direct and parallel integration methodology of nanomaterials in MEMS.

Electrodeposition synthesis and electrochemical properties of nanostructured γ-MnO 2 films

The thin films of carambola-like γ-MnO 2 nanoflakes with about 20 nm in thickness and at least 200 nm in width were prepared on nickel sheets by combination of potentiostatic and cyclic voltammetric electrodeposition techniques. The as-prepared MnO 2 nanomaterials, which were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), were used as the active material of the positive electrode for primary alkaline Zn/MnO 2 batteries and electrochemical supercapacitors. Electrochemical measurements showed that the MnO 2 nanoflake films displayed high potential plateau (around 1.0 V versus Zn) in primary Zn/MnO 2 batteries at the discharge current density of 500 mA g −1 and high specific capacitance of 240 F g −1 at the current density of 1 mA cm −2. This indicated the potential application of carambola-like γ-MnO 2 nanoflakes in high-power batteries and electrochemical supercapacitors. The growth process for the one- and three-dimensional nanostructured MnO 2 was discussed on the basis of potentiostatic and cyclic voltammetric techniques. The present synthesis method can be extended to the preparation of other nanostructured metal-oxide films.

Fabrication of Device Nanostructures Using Supercritical Fluids

AbstractSupercritical fluids including carbon dioxide offer a combination of properties that are uniquely suited for device fabrication at the nanoscale. Liquid-like densities, favorable transport properties, and the absence of surface tension enable solution-based processing in an environment that behaves much like a gas. These characteristics provide a means for extending “top-down” processing methods including metal deposition, cleaning, etching, and surface modification chemistries to the smallest device features. The interaction of carbon dioxide with polymeric materials also enables complete structural specification of nanostructured metal oxide films using a “bottom-up” approach in which deposition reactions are conducted within sacrificial, pre-organized templates dilated by the fluid. The result is high-fidelity replication of the template structure in a new material. In particular, block copolymer templates yield well-ordered porous silica and titania films containing spherical or vertically aligned pores that can serve as device substrates for applications in microelectronics, detection arrays, and energy conversion. Finally, the synthesis of nanoparticles and nanowires in supercritical fluids is developing rapidly and offers promise for the efficient production of well-defined materials. In this review, we summarize these developments and discuss their potential for nextgeneration device fabrication.