Alexey Varezhnikov

Highly selective gas sensor arrays based on thermally reduced graphene oxide

The electrical properties of reduced graphene oxide (rGO) have been previously shown to be very sensitive to surface adsorbates, thus making rGO a very promising platform for highly sensitive gas sensors. However, poor selectivity of rGO-based gas sensors remains a major problem for their practical use. In this paper, we address the selectivity problem by employing an array of rGO-based integrated sensors instead of focusing on the performance of a single sensing element. Each rGO-based device in such an array has a unique sensor response due to the irregular structure of rGO films at different levels of organization, ranging from nanoscale to macroscale. The resulting rGO-based gas sensing system could reliably recognize analytes of nearly the same chemical nature. In our experiments rGO-based sensor arrays demonstrated a high selectivity that was sufficient to discriminate between different alcohols, such as methanol, ethanol and isopropanol, at a 100% success rate. We also discuss a possible sensing mechanism that provides the basis for analyte differentiation.

Intrinsic device-to-device variation in graphene field-effect transistors on a Si/SiO2 substrate as a platform for discriminative gas sensing

Arrays of nearly identical graphene devices on Si/SiO2 exhibit a substantial device-to-device variation, even in case of a high-quality chemical vapor deposition (CVD) or mechanically exfoliated graphene. We propose that such device-to-device variation could provide a platform for highly selective multisensor electronic olfactory systems. We fabricated a multielectrode array of CVD graphene devices on a Si/SiO2 substrate and demonstrated that the diversity of these devices is sufficient to reliably discriminate different short-chain alcohols: methanol, ethanol, and isopropanol. The diversity of graphene devices on Si/SiO2 could possibly be used to construct similar multisensor systems trained to recognize other analytes as well.

Toward new gas-analytical multisensor chips based on titanium oxide nanotube array

Reliable environmental monitoring requires cost effective but highly sensitive and selective gas sensors. While the sensitivity of the sensors is improved by reducing the characteristic dimensions of the gas-sensing material, the selectivity is often approached by combining the sensors into multisensor arrays. The development of scalable methods to manufacture such arrays based on low-dimensional structures offers new perspectives for gas sensing applications. Here we examine an approach to produce multisensor array chips based on the TiOx nanotube layers segmented by multiple Pt strip electrodes. We study the sensitivity and selectivity of the developed chip at operating temperatures up to 400 °C towards organic vapors in the ppm range. The results indicate that the titania nanotubes are a promising material platform for novel cost-effective and powerful gas-analytical multisensor units.

The Potentiodynamic Bottom-up Growth of the Tin Oxide Nanostructured Layer for Gas-Analytical Multisensor Array Chips

We report a deposition of the tin oxide/hydroxide nanostructured layer by the potentiodynamic method from acidic nitrate solutions directly over the substrate, equipped with multiple strip electrodes which is employed as a gas-analytical multisensor array chip. The electrochemical synthesis is set to favor the growth of the tin oxide/hydroxide phase, while the appearance of metallic Sn is suppressed by cycling. The as-synthesized tin oxide/hydroxide layer is characterized by mesoporous morphology with grains, 250–300 nm diameter, which are further crystallized into fine SnO2 poly-nanocrystals following heating to 300 °C for 24 h just on the chip. The fabricated layer exhibits chemiresistive properties under exposure to organic vapors, which allows the generation of a multisensor vector signal capable of selectively distinguishing various vapors.

The room-temperature chemiresistive properties of potassium titanate whiskers versus organic vapors

The development of portable gas-sensing units implies a special care of their power efficiency, which is often approached by operation at room temperature. This issue primarily appeals to a choice of suitable materials whose functional properties are sensitive toward gas vapors at these conditions. While the gas sensitivity is nowadays advanced by employing the materials at nano-dimensional domain, the room temperature operation might be targeted via the application of layered solid-state electrolytes, like titanates. Here, we report gas-sensitive properties of potassium titanate whiskers, which are placed over a multielectrode chip by drop casting from suspension to yield a matrix mono-layer of varied density. The material synthesis conditions are straightforward both to get stable single-crystalline quasi-one-dimensional whiskers with a great extent of potassium replacement and to favor the increase of specific surface area of the structures. The whisker layer is found to be sensitive towards volatile organic compounds (ethanol, isopropanol, acetone) in the mixture with air at room temperature. The vapor identification is obtained via processing the vector signal generated by sensor array of the multielectrode chip with the help of pattern recognition algorithms.

The application of nanotubular titanium dioxide for gas sensors

The work considers possibilities to employ nanotubular (NT) titanium dioxide layers in order to develop gas sensors. The NT TiO2 layers have been fabricated by electrochemical anodizing of Ti foil with subsequent dissolution of metal layer. The technique to apply TiO2 NT layer over the substrate equipped with strip coplanar electrodes is developed. It has been found that the TiO2 NTs exhibit a considerable chemiresistive effect at temperatures over 300 °C versus a number of organic vapors under ppm concentrations in air.

A new gas-analytical device concept via implementation of impedance spectroscopy

Here we discuss the concept of new gas-analytical device based on a.c. driven chemiresistive semiconducting layer. We test the approach utilizing quasi-amorphous potassium polytitanates, K2O·nTiO2·mH2O, which are characterized by high specific surface. In this work, we placed the polytitanate layer over a set of electrodes at the multielectroded chip and measured its impedance under exposure to various gas mixtures. The obtained data allow one to make an equivalent circuit for these structures where the combination of each active/reactive component can be used for selective gas identification.

The gas multisensor chip fabricated by direct electrochemical deposition of tin oxide

We report a direct synthesis of tin oxide layers onto Si/SiO2 chip equipped with Pt multiple electrodes to develop new gas-analytical multisensor arrays. The tailored local morphology of the layer makes it possible to differentiate the gas response of each layer segment and to build a gas-selective vector signal recognized by pattern recognition algorithms in frames of conventional multisensor principles. The response of the chip to alcohols is in the ppm range.

Impedance spectroscopy study of a response of potassium polytitanate based gas sensor

Quasi-amorphous potassium polytitanates represent a new type of chemical compounds described by formula K₂o·nTiO₂·mH₂O and are characterized by high specific surface. These materials are very promising as active materials in gas sensors. In this work, we studied the potassium polytitanates placed over SiO₂/Si/SiO₂ substrate equipped with multiple coplanar electrodes for the electrical response under various gases in mixture with air by impedance spectrometry.

Study of feasibility to apply the graphene as a sensitive material for chemiresistor-type gas sensors

In this study we have considered chemo-resistive gas sensors based on graphene prepared by CVD. To investigate the gas sensing properties of graphene was proposed multielectrode chip design. Gas responses were obtained at room temperature of the chip.