The development of low-cost and scalable sensors for the selective detection of aromatic volatile organic compounds (VOCs), such as benzene and o-xylene, remains challenging. In this work, undoped NiTiO3 nanostructures were synthesized via a sol-gel method and integrated onto p-type silicon nanowires (SiNWs) to form a heterojunction-based sensing platform, avoiding complex doping or hybridization strategies. Structural and morphological analyses confirmed the formation of rhombohedral NiTiO3 nanoparticles with an average size of similar to 41 nm. Optical and electrical characterizations revealed visible light absorption, defect-related emissions, and thermally activated conduction with an activation energy of 0.664 eV. Gas sensing measurements at 200 degrees C demonstrated selective and stable responses toward benzene and o-xylene, with negligible sensitivity to CO and NO2. This performance is attributed to effective p-n heterojunctions and improved charge transport at the NiTiO3/SiNW interface. These results demonstrate a simple, cost-effective, and scalable approach for high-performance VOC sensing at relatively low temperature.

Hydrogen monitoring with reliable, fast and cheap sensors is crucial to fully exploit the potential of this gas as an energy vector. One of the most appealing technologies for hydrogen sensing is based on MOS capacitors with palladium gate (Pd) made of nanoparticles, which acting as a catalyst for H2 promotes its dissociation. The generated atoms are then injected into the dielectric shifting the C(V) curve. In this work, we show that the use of active controls of trapped charge in dielectrics can radically accelerate the response time of palladium MOS capacitors as H2 sensors. The repeatability of the sensing is also improved, reducing significantly temporal drifts. The control strategy is based on second order sigma-delta modulation, implementing a discrete-time sliding mode control. Results show that the horizontal displacement of the C(V) curve of the capacitors can be cancelled out in real time by the application of voltage waveforms generated by the controls. Consequently, when exposed to hydrogen, a quasi-constant state operation of the capacitors is created, allowing a radical improvement of the time response as hydrogen sensors.

This paper presents the fabrication of sensors based on tungsten trioxide nanowires decorated with osmium oxide nanoparticles using the aerosol-assisted chemical vapor deposition (AACVD) technique. This methodology allows the obtention of different osmium oxide decoration loadings on the tungsten oxide nanowires. The morphological and chemical characteristics; and the structural properties of the sensing layers of the sensors were studied using different techniques such as FESEM, HR-TEM, and ToF-SIMS. The gas sensing properties were analyzed for pure tungsten trioxide sensors and tungsten trioxide loaded with osmium exposed to nitrogen dioxide, hydrogen, and ethanol, thus assessing the impact of the loading on the sensor response. A sensor array comprising pure and osmium-loaded tungsten oxide devices coupled to multivariate pattern recognition techniques is shown to perform well in gas identification and quantification tasks, offering promising implications in the field of gas sensing technology.

The capture and storage of CO2 are of growing interest in atmospheric science since greenhouse gas emission has to be reduced considerably in the near future. The present paper deals with the doping of cations on ZrO2, i.e., M-ZrO2 (M = Li+, Mg2+, or Co3+), defecting the crystalline planes for the adsorption of carbon dioxide. The samples were prepared by the sol-gel method and characterized completely by different analytical methods. The deposition of metal ions on ZrO2 (whose crystalline phases: monoclinic and tetragonal are transformed into a single-phase such as tetragonal for LiZrO2 and cubic for MgZrO2 or CoZrO2) shows a complete disappearance of the XRD monoclinic signal, and it is consistent with HRTEM lattice fringes: 2.957 nm for ZrO2 (101, tetragonal/monoclinic), 3.018 nm for tetragonal LiZrO2, 2.940 nm for cubic MgZrO2, and 1.526 nm for cubic CoZrO2. The samples are thermally stable, resulting an average size of ∼5.0-15 nm. The surface of LiZrO2 creates the oxygen deficiency, while for Mg2+ (0.089 nm), since the size of the atom is relatively greater than that of Zr4+ (0.084 nm), the replacement of Zr4+ by Mg2+ in sublattice is difficult; thus, a decrease of the lattice constant was noticed. Since the high band gap energy (ΔE > 5.0 eV) is suitable for CO2 adsorption, the samples were employed for the selective detection/capture of CO2 by using electrochemical impedance spectroscopy (EIS) and direct current resistance (DCR), showing that CoZrO2 is capable of CO2 capture about 75%. If M+ ions are deposited within the ZrO2 matrix, then the charge imbalance allows CO2 to interact with the oxygen species to form CO32- which produces a high resistance (21.04 × 106 (Ω, Ohm)). The adsorption of CO2 with the samples was also theoretically studied showing that the interaction of CO2 with MgZrO2 and CoZrO2 is more feasible than with LiZrO2, subscribing to the experimental data. The temperature effect (273 to 573 K) for the interaction of CO2 with CoZrO2 was also studied by the docking method and observed the cubic structure is more stable at high temperatures as compared to the monoclinic geometry. Thus, CO2 would preferably interact with ZrO2c (ERS = -19.29 kJ/mol) than for ZrO2m (22.4 J/mmol (ZrO2c = cubic; ZrO2m = monoclinic).

Randomly oriented ZnO nanowires were grown directly onto alumina substrates having platinum interdigitated screen-printed electrodes via the chemical vapor deposition method using Au as catalyst. Three different Au film thicknesses (i.e., 3, 6 or 12 nm) were used in the growth of nanowires, and their gas sensing properties were studied for ethanol and NO2' as reducing and oxidizing species, respectively. ZnO nanowires grown employing the 6 nm thick layers were the less defective and showed the most stable, repeatable gas sensing properties. Despite ZnO nanowires grown employing the thickest Au layers reached the highest responses under dry conditions, ZnO nanowires grown using the thinnest Au film were more resilient at detecting NO2 in the presence of ambient moisture. The gas sensing results are discussed in light of the defects and the presence of Au impurities in the ZnO nanowires, as revealed by the characterization techniques used, such as X-ray diffraction, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy and photoluminescence spectroscopy. Promising results were obtained by the implementation of ZnO NWs directly grown over alumina substrates for the detection of ethanol and NO2, substantially ameliorating our previously reported results. (C) 2021 The Author(s). Published by Elsevier B.V.

The objective of this paper is to apply a closed-loop control based on dielectric excitation to MOX gas sensors in order to improve their response time. The control implements a feedback loop in which temperature modulations keep constant the sensor reactance, measured at constant temperature. The required fast temperature switching has been implemented on MEMS microhotplates. The mean temperature generated by the control is the new output signal. This technique is applied to an in-house sensor made of WO3 nanowires decorated with gold nanoparticles to detect NH3 and to a commercial MEMS MOX sensor (CCS801).

Here we discuss the aerosol-assisted synthesis of p-n heterojunction metal oxides and we report their gas sensing properties via a short review of the latest results achieved. In particular, we show that the decoration of one-dimensional tungsten oxide (n-type) with nanoparticles of different p-type oxides from transition metals such as Ni, Co or Ir enables achieving a chemical and electronic sensitization of the resulting hybrid metal oxide materials. This leads to remarkable differences in responsiveness to gases, showing that, to some extent, a selective detection of some major pollutant gases (NO₂, H₂S or NH₃) would be possible. Results are critically reviewed, shortcomings are identified and future research directions are given.

In this work, we reported a high-performance ethanol gas sensor based on novel Pd@ZnO core@shell nanoparticles (CSNPs). The Pd@ZnO CSNPs were synthesized by chemical method and characterized by XRD, TEM and EDS techniques. Gas sensing results demonstrated that Pd@ZnO CSNPs show high sensitivity and remarkable selectivity towards ethanol at 250 degrees C. The response value of Pd@ZnO CSNPs is 152, which is almost six times higher than the response value (27) of ZnO NPs at 250 degrees C. The mechanism of enhancement in sensing properties can be ascribed to the chemical and electronic sensitization effect of Pd NPs and also due to the unique core@shell structure. These characteristics may shed light on the development of a selective ethanol sensor based on Pd@ZnO CSNPs. (C) 2021 The Authors. Published by Elsevier B.V.

Tungsten trioxide nanowires were grown employing aerosol assisted chemical vapor deposition (AACVD) and subsequently decorated with different loading levels of iridium oxide nanoparticles. AACVD has been already demonstrated to be a useful tool to load different ranges of nanoparticles on top of an already grown layer. This procedure enables growing the gas sensitive nanomaterials directly onto application substrates for the development of chemo-resistive gas sensors. The morphology and composition of the different materials were characterized via different techniques. It was found that iridium oxide loading resulted in remarkable changes in the morphology and defects of tungsten oxide nanowires. The gas sensing properties of such layers were studied towards ethanol or ammonia vapors, hydrogen, hydrogen sulfide, and nitrogen dioxide. The optimization of the operating temperature and the level of iridium oxide loading results in an improvement in the responsiveness and selectivity towards the species tested. In particular, a dramatically high increase in the response towards nitrogen dioxide is achieved. The mechanisms of gas sensing are discussed in detail.

A two-step procedure was implemented to obtain tungsten oxide nanowires (WO3) doped with cobalt oxide nanoparticles employing W(CO)(6) and the metal-organic precursor Co(acac)(2). In the first step, tungsten oxide nanowires were grown at 400 degrees C using an aerosol assisted chemical vapor deposition system (AA-CVD) and subsequently annealed at 500 degrees C for 2 h in dry air. Then, cobalt oxide loading (at different dose levels) of the nanowires was performed via a second AA-CVD process. These hybrid nanomaterials were grown on top of commercial alumina substrates that comprised interdigitated electrodes and a heating element. The response of these nanomaterials toward H2S, ethanol H-2 and ammonia is investigated and discussed. Co-loaded tungsten oxide is well suited for detecting ammonia under dry conditions and hydrogen sulfide in humid environments.

We introduce the first control of surface potential (SP) in MOX gas sensors using UV light (367 nm). The control is based on sigma-delta modulation implementing a sliding mode control on the surface potential of the sensor. Measurements are presented using a sensor made of WO 3 nanowires under mild heating at 125 °C. Without control, the sensor shows no response to ethanol. Under control, the detection limit is beyond 2 ppm with a response time below 5 minutes. For NO 2 , the response time is reduced by a factor of 10. By operating the sensor at constant SP it is possible to improve both dynamics and sensitivity. This new methodology applies generally to a wide range of gas sensitive materials and transducing elements.

The properties of multi-wall carbon nanotubes decorated with iridium oxide nanoparticles (IrOx-MWCNTs) are studied to detect harmful gases such as nitrogen dioxide and ammonia. IrOx nanoparticles were synthetized using a two-step method, based on a hydrolysis and acid condensation growth mechanism. The metal oxide nanoparticles obtained were employed for decorating the sidewalls of carbon nanotubes. Iridium-oxide nanoparticle decorated carbon nanotube material showed higher and more stable responses towards NH3 and NO2 than bare carbon nanotubes under different experimental conditions, establishing the optimal operating temperatures and estimating the limits of detection and quantification. Furthermore, the nanomaterials employed were studied using different morphological and compositional characterization techniques and a gas sensing mechanism is proposed.

Here, we report the hydrothermal synthesis of flower-shaped ZnO nanostructures and investigated their morphology-dependent gas sensing properties. Scanning electron microscope (SEM) study confirmed the formation of two kinds of floral structures. At short reaction time, flower-like structures (2-3 mu m in size) composed of nanoparticles are formed, whereas floral assemblies ((similar to) 5 mu m) of nanorods are formed at long reaction time. X-ray diffraction (XRD) confirmed the formation of the hexagonal wurtzite structure of ZnO. The average crystallite size of prepared nanoflowers and nanorods were found to be 21 nm and 43 nm, respectively. These results are supported by transmission electron microscopy (TEM). The band gap of ZnO nanostructures was calculated from the UV-vis absorption spectrum and found to be 3.0 eV and 3.19 eV for ZnO nanoflowers and nanorods, respectively. Broad absorption peak in the visible region of photoluminescence (PL) spectra confirmed the presence of oxygen vacancies in both specimens. Furthermore, morphology dependent gas sensing property was investigated for ethanol, benzene, carbon monoxide, and nitrogen dioxide at different operating temperatures and concentrations. Although both morphologies have shown good sensitivity and selectivity towards NO2 at ppb, the response of nanoflower was higher than that of nanorods, which was attributed to its relatively higher surface area and amount of surface defects.

An organic–inorganic heterojunction with an n-type behavior to detect ammonia in a humid atmosphere.

Here we review recent results on the synthesis of single-crystalline metal oxide nanomaterials and their integration in resistive gas sensing devices. Emphasis is put on the integration of such nanomaterials in a wide spectrum of transducing platforms, ranging from standard ceramic to silicon MEMS or flexible polymeric. Some strategies for achieving higher selectivity are discussed, which are followed by a discussion on the development of advanced operation techniques for achieving more stable and faster response to gases. Finally, some aspects that remain open for research are identified

Indium oxide octahedral nanopowders were obtained from an ionic precursor compound after an oxidation process conducted under a low-oxygen atmosphere. This method was found to produce contamination-free indium oxide nanomaterial with very similar morphological and crystalline properties to the one produced by vapor-phase transport, but at significantly lower temperatures and higher yield. The as-synthesized indium oxide was mixed to an organic vehicle and microdrop deposited to form a film bridging the interdigitated silver electrodes patterned on top of a flexible, polyimide (Kapton®), substrate. The gas sensing properties of the flexible chemoresistors towards ammonia vapors, hydrogen, and nitrogen dioxide were investigated. It was found that these sensors were remarkably sensitive to nitrogen dioxide at a low operating temperature of 150 °C. These results are consistent with the performance of vapor-phase transport synthesized indium oxide octahedra sensors on rigid, ceramic substrates. Therefore, the results presented here pave the way for the mass production of inexpensive gas sensors onto flexible substrates via additive manufacturing.

Controls of surface potential have been proposed to accelerate the time response of MOX gas sensors. These controls use temperature modulations and a feedback loop based on first-order sigma-delta modulators to keep constant the surface potential. Changes in the surrounding gases, therefore, must be compensated by average temperature produced by the control loop, which is the new output signal. The purpose of this paper is to present a second order sigma-delta control of the surface potential for gas sensors. With this new control strategy, it is possible to obtain a second order zero of the quantization noise in the output signal. This provides a less noisy control of the surface potential, while at the same time some undesired effects of first order modulators, such as the presence of plateaus, are avoided. Experiments proving these performance improvements are presented using a gas sensor made of tungsten oxide nanowires. Plateau avoidance and second order noise shaping is shown with ethanol measurements.

Here, we show that the aerosol assisted chemical vapor deposition process is suitable for growing single crystalline tungsten oxide nanowires loaded with nickel oxide nanoparticles. This method allows achieving of a wide range of nickel oxide loadings on tungsten oxide nanowires with high effectivity. It also allows for the direct growth of these nanomaterials onto application substrates for developing resistive metal oxide gas sensors. Different morphological and compositional analysis tools have been employed for the characterization of the nanomaterials, finding that nickel oxide nanoparticles are homogeneously distributed over the tungsten oxide nanowire surface. However, at high Ni loadings, even though the size of the nanoparticles remains unchanged, they form agglomerates, especially at the tips of tungsten nanowires. The gas sensing properties of the different nanomaterials grown towards methane, ethanol, nitrogen dioxide and hydrogen sulfide have been studied. The optimized loading of tungsten oxide with nickel oxide nanoparticles has a positive effect on increasing the sensitivity and selectivity of the resulting nanomaterial to hydrogen sulfide, reaching a five-fold increase in the response towards this species. This improvement is attributed to combined chemical and electronic sensitization effects. Therefore, these nanomaterials show good potential for developing inexpensive resistive sensors able to monitor the presence of hydrogen sulfide in the environment.