Conductometric Gas Research Articles

Tin Oxide Nanowires Decorated with Ag Nanoparticles for Visible Light-Enhanced Hydrogen Sensing at Room Temperature: Bridging Conductometric Gas Sensing and Plasmon-Driven Catalysis

We demonstrate that conductometric gas sensing at room temperature with SnO2 nanowires (NWs) is enhanced by visible and supraband gap UV irradiation when and only when the metal oxide NWs are decorated with Ag nanoparticles (NPs) (diameter < 20 nm); no enhancement is observed for the bare SnO2 case. We combine the spectroscopic techniques with conductometric gas sensing to study the wavelength dependency of the sensors’ response, showing a strict correlation between the Ag-loaded SnO2 optical absorption and its gas response as a function of irradiation wavelength. Our results lead to the hypothesis that the enhanced gas response under UV–vis light is the effect of plasmonic hot electrons populating the Ag NPs surface. Finally, we discuss the chemiresistive properties of Ag-loaded SnO2 sensor in parallel with the theory of plasmon-driven catalysis, to propose an interpretative framework that is coherent with the established paradigma of these two separated fields of study.

Synthesis, characterization and hydrogen sensing properties of nanosized colloidal rhodium oxides prepared by Pulsed Laser Ablation in water

Nanosized colloidal rhodium oxide particles in water were obtained by using a nanosecond pulsed excitation, starting from a high purity rhodium target. The as-synthesized colloids show an almost spherical shape and size in the range between 2 and 12 nm. Characterization results indicate that colloidal particles are composed mainly of oxidized RhxOy phases, whose content increase after annealing at 200 °C in air and decrease under hydrogen treatment in mild conditions, suggesting a reversible behavior. The hydrogen sensing characteristics of thin films deposited by drop casting from the colloidal solution onto alumina sensor substrates provided with platinum interdigitated electrodes, were also investigated. The developed conductometric hydrogen sensor shows high response to low concentration of the target gas in air at low operating temperature and full reversibility with fast dynamics. Further, a good selectivity towards hydrogen was demonstrated, testing other simple molecules such as CO, CO2, NO2 and O2. To our knowledge, this is the first report on the use of nanosized rhodium/rhodium oxide colloids in conductometric gas sensors.

On the interest of ambipolar materials for gas sensing

Abstract Based on the electrochemical properties of a series of metallophthalocyanines this article shows that the phthalocyanine bearing four alkoxy groups and twelve fluorine atoms behaves approximately as those with eight fluorine atoms. This indicates that the electron-donating effect of one alkoxy group balances the electro-withdrawing effect of one fluorine atom. We engaged three metallophthalocyanines, namely the octafluoro copper phthalocyanine, Cu(F8Pc), an octaester metallophthalocyanine and a phthalocyanine bearing four alkoxy groups and twelve fluorine atoms, Zn(T4F12Pc), in building original conductometric transducers that are Molecular Semiconductor – Doped Insulator heterojunctions (MSDIs) in association with the highly conductive lutetium bisphthalocyanine, LuPc2. Whereas the octaester derivative and Zn(T4F12Pc) exhibited a negative response to ammonia, as expected for p-type materials, Cu(F8Pc) exhibited a particular behavior. At low humidity levels, 30 and 10% rh, the current of the Cu(F8Pc)/LuPc2 MSDI decreases, similarly to p-type devices, but at higher relative humidity values, 70% rh, the current increases under ammonia, which is the signature of a n-type behavior. This ambipolar behavior is unique amongst semiconducting sensing materials. This work opens the way to the study of ambipolar materials as sensing materials for the development of a new type of conductometric gas sensors.

Light-Activated Metal Oxide Gas Sensors: A Review.

Conductometric gas sensors facilitated by photons have been investigated for decades. Light illumination may enhance device attributes including operational temperature, sensing sensitivity and selectivity. This paper aims to provide an overview on the progress of light-activated gas sensors, with a specific focus on sensors based on metal oxides. The material systems that have been studied include pure metal oxides, heterostructures of semiconductor-metal oxides and metal-metal oxides, and metal oxides with dopant. Other reported works on the use of different nanostructures such as one-dimensional and porous nanostructures, study of sensing mechanisms and the interplay between various factors are also summarized. Possible directions for further improvement of sensing properties, through optimizing the size of nanomaterials, film thickness, light intensity and wavelength are discussed. Finally, we point out that the main challenge faced by light-activated gas sensors is their low optical response, and we have analyzed the feasibility of using localized surface plasmon resonance to solve this drawback. This article should offer readers some key and instructive insights into the current and future development of light-activated gas sensors.

Hall effect spintronics for gas detection

We present the concept of magnetic gas detection by the extraordinary Hall effect. The technique is compatible with the existing conductometric gas detection technologies and allows the simultaneous measurement of two independent parameters: resistivity and magnetization affected by the target gas. Feasibility of the approach is demonstrated by detecting low concentration hydrogen using thin CoPd films as the sensor material. The Hall effect sensitivity of the optimized samples exceeds 240% per 104 ppm at hydrogen concentrations below 0.5% in the hydrogen/nitrogen atmosphere, which is more than two orders of magnitude higher than the sensitivity of the conductance detection.

Metal Oxides for Application in Conductometric Gas Sensors: How to Choose?

This article gives a brief description of metal oxides, acceptable for using in advanced conductometric gas sensors, as well as a consideration of approaches, that can be used to select metal oxides for the manufacture of devices intended to sensor market.

Conductometric gas sensing behavior of WS2 aerogel

The gas sensing characteristics of porous tungsten disulfide (WS2) aerogel are investigated. The sensor is fabricated by integrating WS2 aerogel onto a low power polycrystalline silicon microheater platform to provide control over the operating temperature. The WS2 aerogel is characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermal gravimetric analysis. The sensing performances of the WS2 aerogel-based sensor to NO2, O2, NH3, H2, and humidity are investigated, indicating a p-type behavior. The optimum sensing temperature is found to be about 250°C, when considering sensitivity, power consumption and response time. The role of O2 in the sensor performance is probed and is found to be helpful for enhancing the sensitivity and recovery of the sensor in H2, humidity and NH3.

Thin 2D: The New Dimensionality in Gas Sensing

Since the first report of graphene, thin two-dimensional (2D) nanomaterials with atomic or molecular thicknesses have attracted great research interest for gas sensing applications. This was due to the distinctive physical, chemical, and electronic properties related to their ultrathin thickness, which positively affect the gas sensing performances. This feature article discusses the latest developments in this field, focusing on the properties, preparation, and sensing applications of thin 2D inorganic nanomaterials such as single- or few-layer layered double hydroxides/transition metal oxides/transition metal dichalcogenides. Recent studies have shown that thin 2D inorganic nanomaterials could provide monitoring of harmful/toxic gases with high sensitivity and a low concentration detection limit by means of conductometric sensors operating at relatively low working temperatures. Promisingly, by using these thin 2D inorganic nanomaterials, it may open a simple way of improving the sensing capabilities of conductometric gas sensors.

Modification of anatase using noble-metals (Au, Pt, Ag): Toward a nanoheterojunction exhibiting simultaneously photocatalytic activity and plasmonic gas sensing

A new and original method, based on a non-aqueous sol–gel process, has been successfully established to produce quasi-spherical monodispersed TiO2 nanoparticles (NPs) and also noble metals (NM) @TiO2 heterostructures (NM=Au, Pt, Ag, 2wt%), in one-pot and at low temperature. This has been achieved by using titanium oxyacetylacetonate as new single source precursors. This system has been deeply investigated by advanced characterization techniques. By using NMR, we have demonstrated the relatively complex mechanism behind this apparently simple synthesis, mediated by the reaction of the solvent and generated species, with many separate organic and organometallic molecules identified as being involved in the mechanism. The morphology and structure of the NM@TiO2 heterostructures were investigated by advanced scanning transmission electron microscopy while the chemical state of the noble metal nanoparticles was check by X-ray photoelectron spectroscopy (XPS). Undoped and noble metal (Au, Pt, Ag) decorated quasi/spherical TiO2 nanoparticles worked also as sensing interfaces, leading to the development of a highly sensitive conductometric NO gas sensor under both dark and UV–vis light irradiation, in the first result of its kind. Furthermore, the photocatalytic activity (PCA) was also evaluated, in the gas–solid phase, by monitoring the degradation of NOx under solar-light irradiation. Au-modified TiO2 showed improved photocatalytic efficiencies, compared to unmodified TiO2.

Two-Dimensional Zinc Oxide Nanostructures for Gas Sensor Applications

Two-dimensional (2D) nanomaterials, due to their unique physical and chemical properties, are showing great potential in catalysis and electronic/optoelectronic devices. Moreover, thanks to the high surface to volume ratio, 2D materials provide a large specific surface area for the adsorption of molecules, making them efficient in chemical sensing applications. ZnO, owing to its many advantages such as high sensitivity, stability, and low cost, has been one of the most investigated materials for gas sensing. Many ZnO nanostructures have been used to fabricate efficient gas sensors for the detection of various hazardous and toxic gases. This review summarizes most of the research articles focused on the investigation of 2D ZnO structures including nanosheets, nanowalls, nanoflakes, nanoplates, nanodisks, and hierarchically assembled nanostructures as a sensitive material for conductometric gas sensors. The synthesis of the materials and the sensing performances such as sensitivity, selectivity, response, and recovery times as well as the main influencing factors are summarized for each work. Moreover, the effect of mainly exposed crystal facets of the nanostructures on sensitivity towards different gases is also discussed.

Carbon monoxide (CO) optical gas sensor based on ZnO thin films

Optical gas sensors provide an alternative over the conventional conductometric gas sensors. Carbon monoxide (CO) gas is generally regarded as one of the most dangerous air pollutants, hence, a room temperature operated CO gas sensor using ZnO sensing film deposited on Au coated prisms has been developed using an indigenously developed surface plasmon resonance (SPR) measurement setup. This system is found to be highly sensitive with very fast response towards a wide concentration range (0.5–100ppm) of CO gas at room temperature and hence pave way for commercial application of an efficient optical CO gas sensor.

Room Temperature Detection of Hydrogen Gas Using Graphene Based Conductometric Gas Sensor

Room Temperature Detection of Hydrogen Gas Using Graphene Based Conductometric Gas Sensor

Electrospun Ni-doped SnO2 nanofiber array for selective sensing of NO2

Ni-doped SnO2 nanofiber array was prepared from a modified electrospinning technique followed with hot-press and calcination, which impart the nanofiber with high crystallinity as well as good adhesion to the substrate. The nanofiber array consists of well-aligned straight fibers with an average diameter of ∼200nm and length of several millimeters. Each fiber consists of rutile-structured polycrystals with an average grain size of ∼5nm. Conductometric gas sensors were developed from the nanofiber array using magnetron sputtering. These sensors show excellent performance for NO2 detection at 250°C, including high response (90.3 under 20ppm), fast response and recovery (40 s/18s), and above all, excellent selectivity. Beneficial for electron transport, the nanofiber array offers better device performance over random nanofibers on sensing response and time for recovery. In addition, the NO2 sensing mechanism was probed with detailed DRIFT and XPS analysis.

Catalysis-Based Cataluminescent and Conductometric Gas Sensors: Sensing Nanomaterials, Mechanism, Applications and Perspectives

Gas environment detection has become more urgent and significant, for both industrial manufacturing and environment monitoring. Gas sensors based on a catalytically-sensing mechanism are one of the most important types of devices for gas detection, and have been of great interest during the past decades. However, even though many efforts have contributed to this area, some great challenges still remain, such as the development of sensitively and selectively sensing catalysts. In this review, two representative catalysis-based gas sensors, cataluminescent and conductometric sensors, the basis of optical and electric signal acquisition, respectively, are summarized comprehensively. The current challenges have been presented. Recent research progress on the working mechanism, sensing nanomaterials, and applications reported by our group and some other researchers have been discussed systematically. The future trends and prospects of the catalysis-based gas sensors have also been presented.

Direct Electrodeposition of Thin Metal Films on Functionalized Dielectric Layer and Hydrogen Gas Sensor

Harnessing cathodic hydrogen atom generation, metals such as Pt, Pd, Cu, Au and Ni are directly electrodeposited on functionalized dielectric layers of 6 nm thick silicon oxide formed by thermal oxidation of c-Si substrate. Modifying the oxide layer with functional molecules and Au nanoparticles by simple wet chemistry, we can enhance electrodeposition efficiently and thereby electroplate nanoparticles of Pd and Pt to obtain such thin metal films. In particular, Au nanoparticles on amine-modified silicon oxides remarkably enhance electrodeposition of metal nanoparticles to create sturdy metal films on the dielectric layer, n+-Si/SiO2-NH2. Such enhancement is ascribed to good affinity of Au nanoparticles with electrodeposited metal as well as enhanced current density across the silicon oxide. For an example of potential applications, we show one-step electrodeposition Pd film on dielectric layers to fabricate conductometric hydrogen gas sensor without transferring Pd film.

Fluctuation-enhanced and conductometric gas sensing with nanocrystalline NiO thin films: A comparison

Nanocrystalline thin films of NiO were prepared by advanced reactive gas deposition, and their responses to formaldehyde, ethanol and methane gases were studied via fluctuation-enhanced and conductometric methods Thin films with thicknesses in the 200–1700-nm range were investigated in as-deposited form and after annealing at 400 and 500°C. Morphological and structural analyses showed porous deposits with NiO nanocrystals having face-centered cubic structure. Quantitative changes in frequency-dependent resistance fluctuations as well as in DC resistance were recorded upon exposure to formaldehyde, ethanol and methane at 200°C. The response to formaldehyde was higher than that to ethanol while the response to methane was low, which indicates that the NiO films exhibit significant selectivity towards different gaseous species. These results can be reconciled with the fact that formaldehyde has a nucleophilic group, ethanol is an electron scavenger, and methane is hard to either reduce or oxidize. The gas-induced variations in DC resistance and resistance fluctuations were in most cases similar and consistent.

Anatase TiO2 hierarchical microspheres consisting of truncated nanothorns and their structurally enhanced gas sensing performance

Anatase TiO2 hierarchical microspheres were synthesized by a facile hydrothermal method. Characterization results indicated that the microspheres were consisted of massively aggregative nanothorns with truncated tips, which had exposed {001} and {101} crystal facets, and there were abundant mesoporous structures in the microspheres. A possible growth model of the TiO2 hierarchical microspheres was put forward based on a series of experimental analysis. When used for gas sensing, it was found that the TiO2 hierarchical microspheres showed a distinct high sensitivity towards acetone, the optimal response to 100 ppm acetone was 14.6, which was much higher than that of other materials reported in previous works. Moreover, the TiO2 hierarchical microspheres exhibited a fast response/recovery speeds (<10 s), a low detection limit (the response still reaches 6.1 even at a low acetone concentration of 10 ppm) and an excellent selectivity. Through systematical analysis, it can be concluded that the enhancement of gas sensing performance of the TiO2 hierarchical microspheres was ascribed to its unique structural features (perfect hierarchical mesoporous structure as well as specific crystal facets exposing), which could be described as structurally enhanced gas sensing performance. The present results encourage us to further investigate crystal facets-dependent gas sensing properties of TiO2 for the designing of conductometric gas sensors with better sensing performance.

Integrated Strategy toward Self-Powering and Selectivity Tuning of Semiconductor Gas Sensors

Inorganic conductometric gas sensors struggle to overcome limitations in high power consumption and poor selectivity. Herein, recent advances in developing self-powered gas sensors with tunable selectivity are introduced. Alternative general approaches for powering gas sensors were realized via proper integration of complementary functionalities (namely, powering and sensing) in a singular heterostructure. These solar light driven gas sensors operating at room temperature without applying any additional external powering sources are comparatively discussed. The TYPE-1 gas sensor based on integration of pure inorganic interfaces (e.g., CdS/n-ZnO/p-Si) is capable of delivering a self-sustained sensing response, while it shows a nonselective interaction toward oxidizing and reducing gases. The structural and the optical merits of TYPE-1 sensor are investigated giving more insight into the role of light activation on the modulation of the self-powered sensing response. In the TYPE-2 sensor, the selectivity of...

Metal Oxide Nanocomposites: Advantages and Shortcomings for Application in Conductometric Gas Sensors

The features of conductometric gas sensors based on metal oxide composites are considered. The methods of the composites forming and the advantages of their using in the development of gas sensors are discussed. It is given the analysis of the factors that reduce the effectiveness of the composites using in conductometric gas sensors, which should be taken into account while designing and fabricating sensors based on metal oxide composites. The mechanisms explaining the operation of conductometric gas sensors based on metal oxide composites are also discussed.

Characterization of H2S gas sensor based on CuFe2O4 nanoparticles

Copper ferrite (CuFe2O4) nanoparticles were prepared by the sol-gel auto-combustion method and used to construct a conductometric gas sensor in this work. The as-prepared CuFe2O4 nanoparticles were annealed at 500 °C and 750 °C. X-ray diffraction measurements as well as transmission electron microscopy were used to identify the crystal structure of as-prepared and annealed nanoparticles. The results reveal growth of nanoparticle size crystal structure as well as phase transition from cubic structure to tetragonal symmetry upon annealing. Fourier Transform Infra-Red Spectroscopy (FTIR) measurements of the CuFe2O4 nanoparticles revealed the tetrahedral and octahedral absorption bands that are characteristic of the spinel ferrite. Nanoparticle powder was pressed in a form of a pellet to form the gas sensor device. The pellet was placed between a copper sheet and a stainless steel grid for the bottom and top electrodes, respectively. The results revealed that the produced CuFe2O4 nanoparticles are sensitive to both H2S and H2, but with higher sensitivity to H2S at low temperatures. The gas sensitivity of the sensors could be investigated in terms of the large number of reactive surface sites due to the large surface area of the nanoparticles as well as the adsorption of oxygen species on the surface of nanoparticles.