Gas sensing properties of conducting polymer/Au-loaded ZnO nanoparticle composite materials at room temperature

In this work, a new poly (3-hexylthiophene):1.00 mol% Au-loaded zinc oxide nanoparticles (P3HT:Au/ZnO NPs) hybrid sensor is developed and systematically studied for ammonia sensing applications. The 1.00 mol% Au/ZnO NPs were synthesized by a one-step flame spray pyrolysis (FSP) process and mixed with P3HT at different mixing ratios (1:1, 2:1, 3:1, 4:1, and 1:2) before drop casting on an Al₂O₃ substrate with interdigitated gold electrodes to form thick film sensors. Particle characterizations by X-ray diffraction (XRD), nitrogen adsorption analysis, and high-resolution transmission electron microscopy (HR-TEM) showed highly crystalline ZnO nanoparticles (5 to 15 nm) loaded with ultrafine Au nanoparticles (1 to 2 nm). Film characterizations by XRD, field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy, and atomic force microscopy (AFM) revealed the presence of P3HT/ZnO mixed phases and porous nanoparticle structures in the composite thick film. The gas sensing properties of P3HT:1.00 mol% Au/ZnO NPs composite sensors were studied for reducing and oxidizing gases (NH₃, C₂H₅OH, CO, H₂S, NO₂, and H₂O) at room temperature. It was found that the composite film with 4:1 of P3HT:1.00 mol% Au/ZnO NPs exhibited the best NH₃ sensing performances with high response (approximately 32 to 1,000 ppm of NH₃), fast response time (4.2 s), and high selectivity at room temperature. Plausible mechanisms explaining the enhanced NH₃ response by composite films were discussed.     

Hole concentration modulated gas sensor for selective detection of 2-methoxy ethanol

The inhaling rate of toxic gases in daily life is increasing alarmingly; in turn, human health is in question. To resolve this dilemma, the possible remedy is the early detection and adequate regulation of VOCs in the atmosphere via sensors. Therefore, the investigation focused on developing a gas sensor for sensing several VOCs and flourished with a highly selective C₃H₈O₂ gas sensor at room temperature. In-depth structural, elemental, and morphological analysis followed by the sensing test affirmed the enhanced C₃H₈O₂ detection performance of Ag–NiO over pure NiO. The nanosized sphere formation was confirmed via XRD and TEM characterizations alongside the effect of sintered temperature on the shape and crystallite size. Moreover, the XPS examined the combined effect of sintering temperature and doping on the synthesized nanostructures and optimized the exact temperature as 500 °C because of the improved hole concentration. The Ag–NiO(500 °C) exhibited appealing sensing characteristics, specifically, a high response of 6491.57 for 100 ppm C₃H₈O₂ at room temperature. The sensor displayed a quick response and recovery (10 s,10 s) C₃H₈O₂ alongside long-term repeatability and stability; it showed practical implementation in real-time scenarios.     

Gas sensing properties of asymmetrically reduced Na1/2Bi1/2TiO3–based ceramics

We report the gas sensing properties of a type of new materials, Na_1/2Bi_1/2TiO₃ (NBT)-based ceramics. After the NBT-based ceramics were asymmetrically reduced and coated with Au electrodes, the materials exhibit relatively large electrical responses when exposed to oxygen and some oxidizable gases at a relatively low temperature (≤300 °C). An electric voltage ～60 mV is measured in the mixture of O₂ and N₂ (1% O₂). In oxidizable gases, a negative response can be obtained. The measured voltages are ?45 mV and ?98 mV in the mixtures of H₂/air (1000 ppm H₂) and C₂H₅OH/air (1000 ppm C₂H₅OH), respectively. The electrical responses are proportional to the logarithm of the concentrations of the analyzed gases. Also, the electrical responses to oxygen and oxidizable gases have opposite signs, and the model of mixed-potential is proposed to explain the gas sensing phenomenon. This study provides a new material and a simple design for gas sensors. The proposed gas sensor comprises a reduced NBT-based ceramic wafer with the same electrodes on the opposite surfaces. Additional components in traditional gas sensors, such as sensing or reference electrode, are unnecessary.     

Pd-doped TiO2@polypyrrole core-shell composites as hydrogen-sensing materials

A core-shell composite consisting of polypyrrole (PPy) nanofibers and TiO₂ was synthesized by using PPy nanofibers as the core and TiO₂ as the shell. The TiO₂@PPy composite substrate was doped with Pd nanoparticles via chemical reduction. The resulting Pd–TiO₂@PPy nanocomposite was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Brunauer–Emmett–Teller (BET) adsorption analysis before it was utilized to fabricate a hydrogen sensor. Compared with sensors based on TiO₂@PPy or PPy, the Pd–TiO₂@PPy sensor was highly sensitive and selective to hydrogen gas, exhibiting a fast response time in air at room temperature. The Pd–TiO₂@PPy-based sensor exhibited a sensitivity of 8.1% toward 1 vol% of H₂ gas, which is much larger than the sensitivities of sensors based on only TiO₂@PPy and PPy nanofibers. The excellent reproducibility, stability and selectivity of the Pd–TiO₂@PPy nanocomposite make it a high potential candidate for hydrogen sensors.     

Enhanced H2S sensing performance of TiO2-decorated α-Fe2O3 nanorod sensors

Pristine and TiO₂ nanoparticle-decorated Fe₂O₃ nanorods were synthesized via thermal oxidation of Fe thin foils, followed by the solvothermal treatment with titanium tetra isopropoxide (TTIP) and NaOH for TiO₂ nanoparticle-decoration. Subsequently, gas sensors were fabricated by connecting the nanorods with metal conductors. The structure and morphology of the pristine and TiO₂ nanoparticle-decorated Fe₂O₃ nanorods were examined via X-ray diffraction and scanning electron microscopy, respectively. The gas sensing properties of the pristine and TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensors with regard to H₂S gas were examined. The TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensor showed a stronger response to H₂S than the pristine Fe₂O₃ nanorod sensor. The responses of the pristine and TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensors were 2.6 and 7.4, respectively, when tested with 200 ppm of H₂S at 300 °C. The TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensor also showed a faster response and recovery than the sensor made from pristine Fe₂O₃ nanorods. Both sensors showed selectivity for H₂S over NO₂, SO₂, NH₃, and CO. The enhanced sensing performance of the TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensor compared to that of the pristine Fe₂O₃ nanorod sensor might be due to enhanced modulation of the conduction channel width, the decorated nanorods’ increased surface-to-volume ratios and the creation of preferential adsorption sites via TiO₂ nanoparticle decoration. The dominant sensing mechanism in the TiO₂ nanoparticle-decorated Fe₂O₃ nanorod sensor is discussed in detail.     

Realization of low-temperature and selective NO2 sensing of SnO2 nanowires via synergistic effects of Pt decoration and Bi2O3 branching

Metal oxide semiconductors with branched structures, such as branched nanowires (b-NWs), have promising properties for being used in gas sensors. In this work, we synthesized Pt-decorated Bi₂O₃-branched SnO₂ nanowires (NWs). NO₂ sensing studies revealed the superior capacity of a Pt-decorated Bi₂O₃-branched SnO₂ NWs gas sensor relative to pristine and branched SnO₂ gas sensors, and it worked at near room temperature (50 °C). The increased sensing capacity was related to the synergistic effects of Pt decoration and Bi₂O₃ branching, particularly the morphology of the gas sensor with branched structures, the promising effects of Pt as a noble metal with good catalytic activity, and the generation of homo- and heterojunctions in the Pt-decorated Bi₂O₃-branched SnO₂ NWs gas sensor. The results obtained in this work are useful for design and development of NO₂ gas sensors using a simple strategy, which can be easily extended to various metal oxides.     

Highly sensitive gas sensors on low-cost nanostructured polymer substrates

SnO₂ thin-film gas sensors have been successfully fabricated on nanospiked polyurethane polymer surfaces, which are replicated by a low-cost soft nanolithography method from silicon nanospike structures formed with femtosecond laser irradiations. Measurements revealed significant response to carbon monoxide (CO) gas at room temperature, which is considerably different from the sensors of SnO₂ thin films coated on smooth surfaces that show no response to CO gas at room temperature. The high area/volume ratio and sharp structures of the nanospikes enhance the sensitivity of SnO₂ at room temperature. This will greatly decrease the electrical power consumption of the gas sensor and the cost for calibrations, and has great potential for application in other sensing systems.     

One-pot synthesis of zinc oxide - polyaniline nanocomposite for fabrication of efficient room temperature ammonia gas sensor

Development of efficient room temperature ammonia (NH₃) gas sensor from one pot synthesized zinc oxide (ZnO) – polyaniline (PANI) nanocomposite is reported in the present article. Prior to gas sensing study, the material is characterized to understand the structural, morphological, compositional, optical and thermal properties. Structural and morphological studies indicate good incorporation of ZnO particles in PANI matrix. The gas sensing efficiency of ZnO-PANI nanocomposite is examined at room temperature for ethanol (C₂H₅OH), methanol (CH₃OH) and NH₃ gas. The results confirm that ZnO-PANI nanocomposite to be highly selective for NH₃ with fast response time and better stability. The response and recovery times are observed to be significantly dependent on NH₃ concentration and the lowest detectivity limit of the sensor for NH₃ is found 10 ppm. ZnO-PANI nanocomposite shows better gas sensing efficiency as compared to the sensors developed from single phase PANI film.     

Influence of Zn-substitution on structural,morphological, electrical,and gas sensing properties of ZnxAl2O4 (x = 0.1 to 0.5) synthesized by a sol-gel auto-combustion method

The nanosphere decorated needle-like morphology of zinc-substituted aluminate having general formula Zn_xAl₂O₄ (x = 0.1, 0.2, 0.3, 0.4, and 0.5) (ZAN) samples were synthesized by a sol-gel auto-combustion method. The phase formation and stability temperature were confirmed by TG-DTA analysis. XRD study confirmed the formation of a cubic spinel structure of ZAN samples. The effect of Zn-substitution on structural and morphological properties of aluminate were investigated using X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FESEM), and Energy dispersive X-ray analysis (EDAX). The D.C. electrical resistivity study of ZAN samples revealed that resistance decreased with increasing temperature confirmed semiconducting nature. Nanosphere existing on micro-needles of zinc-substituted aluminate gas sensor revealed sensing to several analyte gases such as H₂S, Cl₂, CH₃OH, SO₂, and NO₂ working at room temperature to 300 ^°C. The Zn_0·4Al₂O₄ compositional gas sensor produced the highest response at operating temperature 200 ^°C to 100 ppm H₂S. The results revealed that the prepared nanosphere decorated needles of the ZAN sensor was sensitive and selective to H₂S gas.     

Surface bound nanostructures of ternary r-GO / Mn3O4/V2O5 system for room temperature selectivity of hydrogen gas

Room temperature detection of highly sensitive Hydrogen (H₂) gas sensing material preparation was taken as a major objective in this present work. Herein, a novel one pot hydrothermal method is proposed for the synthesis of ternary r-GO decorated Manganese oxide (Mn₃O₄) and Vanadium pentoxide (V₂O₅) nanocomposite. The significant electrical conductivity of r-GO plays an important role here to enhance the sensing property. Tunable band bending features of metal oxides over the r-GO surface makes the composite works at room temperature with high selectivity of H₂. The optical, structural and morphological characteristics were analyzed by UV–Visible spectroscopy (UV–Vis), X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray Photoelectron spectroscopy (XPS), Scanning electron microscope (SEM), and High Resolution-Transmission Electron Microscope (HR-TEM). The sensing results reveals that the present nanocomposite is selectively sensitive towards H₂ with sensitivity value of (175%) at room temperature with response time (82 s) and recovery time (92 s). To investigate the low detection limit gas concentration was varied in the range from 20 ppm to 50 ppm. The synergetic sensing performance and stability of this nanocomposite could be due to the formation of metal oxides with perspicuous nanostructures as heterojunction decorated over the r-GO stack layer.     

Solution processed,vertically aligned,AZO nanocolumn array for chemiresistive sensor application with UV-enhanced sensitivity

In this work, a wide and highly sensitive chemiresistive sensor has been developed based on the AZO nanocolumn array film. This is meant for the room detection of H₂O₂ under UV illumination. A cost-effective one step multi-layers growth process was adopted for the synthesis of the AZO nanocolumn array. The experimental studies were done by scanning electron microscopy (SEM), transmission and electron microscopy (TEM).Then X-ray diffraction confirmed that the AZO column array was closely packed, connected, vertically aligned, and polycrystalline, with a high surface area. This structure ensures better electrical conduction over random and separated nanostructures. The hall-effect measurement indicates that the AZO film was n-type, with high conductivity (3.60 × 10³ Ωcm), high carrier density (11.3 × 10²⁰cm⁻³) and with acceptable mobility (0.95 cm²/Vs). The x-ray photoemission spectroscopy suggests that the AZO film consists of a large amount of adsorbed oxygen-related species at the sheath layer of the thin-film, which is vital for sensors. By the UV light activation, sensors based on the AZO nanocolumn array exhibited enhanced H₂O₂ detection properties at room temperature. At a concentration from 15 μM to 30 mM, H₂O₂ sensitivity evaluated by relative response was remarkably increased from 15% to 36%. The operation under ambient conditions and wide range sensing shows that this chemiresistive AZO sensor is adequate for biomedical and environmental applications.     

Influence of H2O,CO2 and various combustible gases on the characteristics of a limiting current-type oxygen sensor

Current-voltage characteristics of limiting current-type oxygen sensors were investigated. The sensor showed a two-stage current plateau in current-voltage characteristics in H₂O–O₂–N₂ and CO₂–O₂–N₂ mixtures. The sensor current in the first stage corresponded to O₂ concentration and was practically independent of H₂O and CO₂ concentration in the gas mixtures. The sensor current in the second stage increased linearly with the H₂O or CO₂ concentration, for a sensor with high electrode activity. The behavior of the sensor suggests that the deoxidization of H₂O or CO₂ occurs at the sensor cathode. For nonequilibrium gas mixtures containing combustible gas and O₂, the sensor current in the first stage decreased linearly with combustible gas concentration. The decrease of the sensor current differed from that corresponding to the O₂ concentration consumed by the reaction of these gases in the ambient gas, depending on the kind of combustible gas. The reduction of the sensor current is explained by a model assuming that the reaction of these gases occurs at the cathode, and the diffusion of the combustible gas in the porous coating is a rate-limiting step.     

Single wall carbon nanotubes loaded with Pd and NiPd nanoparticles for H2 sensing at room temperature

Pd and bimetallic Ni₅₀Pd₅₀ nanoparticles protected by polyvinylpyrrolidone (PVP) have been synthesized by the reduction-by-solvent method and deposited on single wall carbon nanotubes (SWCNTs) to be tested as H₂ sensors. The SWCNTs were deposited by drop casting from different suspensions. The Pd nanoparticles-based sensors show a very reproducible performance with good sensitivity and very low response times (few seconds) for different H₂ concentrations, ranging from 0.2% to 5% vol. H₂ in air at atmospheric pressure. The influence of the metal nanoparticle composition, the quality of SWCNTs suspension and the metal loading have been studied, observing that all these parameters play an important role in the H₂ sensor performance. Evidence for water formation during the H₂ detection on Pd nanoparticles has been found, and its repercussion on the behaviour of the assembled sensors is discussed.The sensor preparation procedure detailed in this work has proven to be simple and reproducible to prepare cost-effective and highly efficient H₂ sensors that perform very well under real application conditions.     

One-step preparation of Ag nanoparticle–decorated coordination polymer nanobelts and their application for enzymeless H2O2 detection

In this paper, we report on the one-step preparation of Ag nanoparticle (AgNP)-decorated coordination polymer nanobelts, carried out by direct mixing of an aqueous AgNO₃ solution and a N,N-dimethylformamide (DMF) solution of 4,4′-bipyridine (BPD) at room temperature. The construction of an enzymeless H₂O₂ sensor using such nanocomposites with a detection limit of 0.9 μM is also demonstrated.     

Semi shield driven p-n heterostructures and their role in enhancing the room temperature ethanol gas sensing performance of NiO/SnO2 nanocomposites

The demand for the development of gas sensors operable at room temperature is increasing due to the uncountable drawbacks of high temperature gas sensors. This contribution describes the fabrication of room temperature ethanol sensor. The synthesis of NiO semi shielded SnO₂ (NiO/SnO₂) nanocomposites (NCs) was done via a simple two-step process, started with co-precipitation technique and then followed by sol-gel method. High resolution electron microscope (HRTEM) results indicated the semi shielding of NiO on SnO₂ nanoparticles (NPs). Surface morphological studies of the fabricated sensors show the porous nature of the samples which further helps in enhanced sensing response. X-ray photoelectron spectroscope (XPS) results of NiO/SnO₂ NCs revealed the valence states of Ni (+2) and Sn (+4). Excellent gas sensing response of the NiO/SnO₂ sensor towards ethanol at room temperature was observed from the gas sensing studies. The response of NiO/SnO₂ (∼140) was nearly 9 times higher than SnO₂ sensor (∼15) and nearly 11 times higher than NiO sensor (12.98) towards 100 ppm ethanol at room temperature. The observed response and recovery times of NiO/SnO₂ were 23 s and 13 s respectively. The p-n heterostructure formed between p-NiO and n-SnO₂, and high chemical sensitization and catalytic activity of the NiO are the main contributors for the excellent sensing performance of NiO/SnO₂ sensor.     

Effect of synthesis condition and annealing on the sensitivity and stability of gas sensors made of Zn-doped <Emphasis Type="Italic">γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> particles

Gas sensors made of flame-synthesized Zn-doped γ-Fe₂O₃ nanoparticles were found to have high sensitivity and high aging resistance. Zinc-doped γ-Fe₂O₃ nanoparticles and microparticles were synthesized by flame spray pyrolysis (FSP). Gas sensors were fabricated with as-synthesized particles, and with particles that had been annealed. The sensors’ response to acetone vapor and H₂ was measured as fabricated, and measured again after the sensors were aged for three days. The sensors made from as-synthesized particles showed a gas sensing sensitivity 20 times higher than the literature value. However, sensors made of microparticles lost their sensing ability after three days of aging; sensors made of nanoparticles retained their gas sensing capability after aging. Sensors made of annealed particles did not have significant gas sensing capabilities. Analysis using the William and Hall method showed that the microstrains decreased significantly in both H₂/O₂ and H₂/Air flame synthesized particles after annealing. The results showed that sensors made of flame-synthesized particles have much higher sensitivity than sensors made of particles previously reported. Especially, sensors made of flame-synthesized nanoparticles are resistant towards aging. This aging resistance may be attributed to the particles’ ability to retain their microstrains.     

Highly sensitive H2S gas sensors based on CuO-coated ZnSnO3 nanorods synthesized by thermal evaporation

ZnSnO₃ one-dimensional (1D) nanostrutures were synthesized by thermal evaporation. The morphology, crystal structure and sensing properties of the CuO-coated ZnSnO₃ nanostructures to H₂S gas at 100 °C were examined. Transmission electron microscopy and X-ray diffraction revealed both the ZnSnO₃ nanorods and CuO nanoparticles to be single crystals. The diameters of the CuO nanoparticles on the nanorods ranged from a few to a few tens of nanometers. The gas sensors fabricated from multiple networked CuO-coated ZnSnO₃ nanorods exhibited enhanced electrical responses to H₂S gas compared to the uncoated ZnSnO₃ nanorod sensors, showing 61.7-, 49.9-, and 31.3-fold improvement at H₂S concentrations of 25, 50, and 100 ppm, respectively. The response time of the nanorod sensor to H₂S gas was reduced by the CuO coating but the recovery time was similar. The mechanism for the enhanced H₂S gas sensing properties of ZnSnO₃ nanorods by the CuO coating is discussed.     

Room temperature operating sensitive and reproducible ammonia sensor based on PANI/hematite nanocomposite

ABSTRACT

A facile, ultra-sensitive, quickly recoverable, and room temperature operating ammonia sensor was developed by using polyaniline (PANI) and hematite (α-Fe₂O₃) hybrid nanocomposite. The hematite nanoparticles were obtained by template-free hydrothermal process. The PANI/α-Fe₂O₃ nanocomposite was synthesized by in situ chemical oxidative polymerization process of aniline in presence of α-Fe₂O₃ nanoparticles. The structural and morphological study and compositional analysis of PANI/α-Fe₂O₃ were performed by Fourier transform infrared spectroscopy, X-ray powder diffraction analysis, and scanning electron microscopy. The PANI/α-Fe₂O₃ sensor showed excellent reproducibility, ultra-fast response, and excellent sensitivity (46.72%) as compared to PANI (29.72%) sensor towards ammonia gas at room temperature.     

Three-terminal CNTs gas sensor for N2 detection

A novel three-terminal gas sensor was fulfilled by utilizing the vertically aligned carbon nanotubes (CNTs) mat. Carbon nanotubes were synthesized by thermal chemical vapor deposition (thermal CVD) at 700 °C under C₂H₂ gas flow rate of 30 sccm. Upon exposure to a with and without N₂ environment at the room temperature of 25 °C, the electrical resistance of as-made devices was found to increase and to return back, respectively. Compared to a low bias one, the sensitivity increased when applying a high source drain bias voltage. Furthermore, the device became more sensitive for N₂ detection by applying a negative gate voltage. It was concluded that the alteration of free holes concentration in the CNTs mat played the major mechanism for the N₂ gas detection.     

Ti2CTx MXene: A novel p-type sensing material for visible light-enhanced room temperature methane detection

The development of semiconductor-based room-temperature methane (CH₄) gas sensors is appealing but challenging. Herein, we report a CH₄ gas sensor operating at room temperature based on Ti₂CT_x MXene, a novel p-type sensing material, achieving high-performance CH₄ detection with visible light assistance. The Ti₂CT_x MXene based device showed more than seven-fold improvement for CH₄ detection under visible-light irradiation, and the response/recovery times were also sharply decreased. The excellent CH₄ sensing performance at room temperature could be attributed to the visible-light photocatalytic CH₄ oxidation activity of the Ti₂CT_x sensing material. CH₄ oxidation was revealed by photocatalytic measurement, O₂-TPD and in-situ IR spectroscopies. The present work demonstrates the novel application of Ti₂CT_x MXene as a promising p-type sensing material for methane detection at room temperature. Moreover, the concept of “photocatalysis-enhanced gas sensing” can be employed in room-temperature gas sensors based on other novel semiconductors.