ZnO/γ-Fe2O3 Charge Transfer Interface toward Highly Selective H2S Sensing at a Low Operating Temperature of 30 °C

CuO nanostructure-decorated InGaN nanorods for selective H2S gas detection

Establishing a heterostructure is one of the adequate strategies for enhancing device performance and has been explored in sensing, and energy applications. In this study, we constructed a heterostructure through a two-step process involving hydrothermal synthesis of CuO nanostructures and subsequent spin coating on MBE-grown InGaN NRs. We found that the CuO content on the InGaN NRs has a great impact on carrier injection at the heterojunction and thus the H2S gas sensing performance. Popcorn CuO/InGaN NR shows excellent gas sensing performance towards different concentrations of H2S at room temperature. The highest response is up to 35.54% to a H2S concentration of 100 ppm. Even more significantly, this response is further enhanced significantly (123.70%) under 365 nm UV light. In contrast, this composite structure exhibits negligibly low responses to 100 ppm of NO2, H2, CO, and NH3. The heterostructure band model associated with a surface reaction model is manifested to elucidate the sensing mechanism.

Chameleon-like Response Mechanism of Gold-Silver Bimetallic Nanoclusters Stimulated by Sulfur Ions and Their Application in Visual Fluorescence Sensing

Herein, 2-mercapto-5-benzimidazolesulfonate acid sodium salt dihydrate (MBZS)-protected gold-silver bimetallic nanoclusters, named MBZS-AuAg NCs, were synthesized. Interestingly, we found that MBZS-AuAg NCs solutions can exhibit different fluorescence color changes under sulfide stimulation. A series of modern analytical testing techniques were used to explore the interaction mechanism between MBZS-AuAg NCs and sulfide. Sulfide ions can not only cause MBZS-AuAg NCs to exhibit rich fluorescence color changes similar to those of a chameleon but also have four linear relationships between the response intensity and sulfide concentration. A wide-range sulfide fluorescence sensing platform was constructed based on four linear segments with different fluorescence color responses. This sensing platform can be directly used for the determination of S2- with a detection limit as low as 11 nM. The portable test paper based on MBZS-AuAg NCs can realize the visual and rapid detection of gaseous hydrogen sulfide with a detection limit of 100 ppb (v/v). The wide detection range of the proposed method not only allows it to be used as an alternative method for sulfide detection in environmental samples but also has potential applications in the rapid detection and early warning of hydrogen sulfide gas in industrial and mining scenarios.

Ultrasensitive Bio-H2S Gas Sensor Based on Cu2O-MWCNT Heterostructures

Developing a respiratory analysis disease diagnosis platform for the H2S biomarker has great significance for the real-time detection of various diseases. However, achieving highly sensitive and rapid detection of H2S gas at the parts per billion level at low temperatures is one of the most critical challenges for developing portable exhaled gas sensors. Herein, Cu2O-multiwalled carbon nanotube (MWCNT) heterostructures with excellent gas sensitivity to H2S at room temperature and a lower temperature were successfully synthesized by a facile two-dimensional (2D) electrodeposition in situ assembly method. The combination of Cu2O and MWCNTs via the principle of optimal conductance growth not only reduced the initial resistance of the material but also provided an ideal interfacial barrier structure. Compared to the response of the pure Cu2O sensor, that of the Cu2O-MWCNT sensor to 1 ppm of H2S increased nearly 800 times at room temperature, and the response time decreased by more than 500 s. In addition to the excellent sensitivity with detection limits as low as 1 ppb, the Cu2O-MWCNT sensor was extremely selective with low-temperature adaptability. The sensor had a response value of 80.6 to 0.1 ppm of H2S at -10 °C, which is difficult to achieve with sensors based on oxygen adsorption/desorption mechanisms. The sensor was used for the detection of real oral exhaled breath, confirming its feasibility as a real-time disease monitoring sensor. The Cu2O-MWCNT heterostructures maximized the advantages of the individual components and laid the experimental foundation for future applications of highly sensitive portable breath analysis platforms for monitoring H2S.

Cadmium sulfide in-situ derived heterostructure hybrids with tunable component ratio for highly sensitive and selective detection of ppb-level H2S

Herein, we reported cadmium sulfide derivatives pine needles-like CdS/CdO heterostructure hybrids synthesized by hydrothermal treatment and subsequent self-template oxidation approach. The component ratio of the CdS/CdO hybrids can be controlled specifically via tuning the annealing treatment protocol, and thereby giving rise to the optimization of morphology, electrical characteristics, and gas sensing properties of derived hybrids. As proof of concept, the pine needles-like CdS/CdO, which obtained after different annealing temperatures and durations, as sensitive material was employed to manufacture H₂S gas sensors. The sensor based on CdS/CdO hybrids (400 °C & 1 h) exhibited high sensitivity (73.5 to 5 ppm), ppb-level limit of detection (10 ppb), and excellent selectivity regardless of the interference of other gases at optimal working temperature of 200 °C. Due to the abnormal resistance variation of n-type cadmium sulfide derived hybrids while contacting with H₂S, the sensing mechanism mainly depends on the surface chemical conversion from oxide to sulfide. The pine needles-like hierarchical morphology provided an excellent scaffold for the carriers transportation and the growth of the CdO, which played a key role in resistance modulation both in air and target gas, resulting in the enhanced H₂S sensing performance ultimately.

Organic/Inorganic-Based Flexible Membrane for a Room-Temperature Electronic Gas Sensor

A room temperature (RT) H₂S gas sensor based on organic–inorganic nanocomposites has been developed by incorporating zinc oxide (ZnO) nanoparticles (NPs) into a conductivity-controlled organic polymer matrix. A homogeneous solution containing poly (vinyl alcohol) (PVA) and ionic liquid (IL) and further doped with ZnO NPs was used for the fabrication of a flexible membrane (approx. 200 μm in thickness). The sensor was assessed for its performance against hazardous gases at RT (23 °C). The obtained sensor exhibited good sensitivity, with a detection limit of 15 ppm, and a fast time response (24 ± 3 s) toward H₂S gas. The sensor also showed excellent repeatability, long-term stability and selectivity toward H₂S gas among other test gases. Furthermore, the sensor depicted a high flexibility, low cost, easy fabrication and low power consumption, thus holding great promise for flexible electronic gas sensors.

C60Br24/SWCNT: A Highly Sensitive Medium to Detect H2S via Inhomogeneous Carrier Doping

H₂S is a toxic and corrosive gas, whose accurate detection at sub-ppm concentrations is of high practical importance in environmental, industrial, and health safety applications. Herein, we propose a chemiresistive sensor device that applies a composite of single-walled carbon nanotubes (SWCNTs) and brominated fullerene (C₆₀Br₂₄) as a sensing component, which is capable of detecting 50 ppb H₂S even at room temperature with an excellent response of 1.75% in a selective manner. In contrast, a poor gas response of pristine C₆₀-based composites was found in control measurements. The experimental results are complemented by density functional theory calculations showing that C₆₀Br₂₄ in contact with SWCNTs induces localized hole doping in the nanotubes, which is increased further when H₂S adsorbs on C₆₀Br₂₄ but decreases in the regions, where direct adsorption of H₂S on the nanotubes takes place due to electron doping from the analyte. Accordingly, the heterogeneous chemical environment in the composite results in spatial fluctuations of hole density upon gas adsorption, hence influencing carrier transport and thus giving rise to chemiresistive sensing.

Type discrimination and concentration prediction towards ethanol using a machine learning-enhanced gas sensor array with different morphology-tuning characteristics

A simple microwave-assisted method was applied to synthesize zinc oxide (ZnO) with controllable hierarchical structures. In a surfactant-free solvent system, the hierarchical structure of the ZnO precursor can be regulated by the concentration of urea at normal temperature and pressure. Upon annealing, ZnO with different morphologies shows its unique response towards six kinds of gases. The response data were clustered and analyzed by principal component analysis (PCA) to provide a basis for feature extraction. The classification to six kinds of gases was conducted through a model based on linear ridge classification (LRC), support vector machine (SVM). The prediction of ethanol concentration was achieved using backpropagation (BP) neural network and extreme learning machine (ELM). The results indicate that the six confusing gases can be distinguished clearly using SVM with an accuracy more than 0.99. Furthermore, the prediction of ethanol concentration shows a prominent performance (R² > 0.98) by the ELM-based regressor, despite the nearly saturated response of the sensor array. This study explores the possibility of pattern recognition analysis based on machine learning to further improve the detection performance of the gas sensor array with different response characteristics regulated by the morphology.

Exploiting Free-Standing p-CuO/n-TiO2 Nanochannels as a Flexible Gas Sensor with High Sensitivity for H2S at Room Temperature

Hydrogen sulfide (H₂S) is an extremely hazardous gas and is harmful to human health and the environment. Here, we developed a flexible H₂S gas-sensing device operated at room temperature (25 °C) based on CuO nanoparticles coated with free-standing TiO₂-nanochannel membranes that were prepared by simple electrochemical anodization. Benefiting from the modulated conductivity of the CuO/TiO₂ p-n heterojunction and a unique nanochannel architecture, the traditional thermal energy was innovatively replaced with UV irradiation (λ = 365 nm) to provide the required energy for triggering the sensing reactions of H₂S. Importantly, upon exposure to H₂S, the p-n heterojunction is destroyed and the newly formed ohmic contact forms an antiblocking layer at the interface of CuS and TiO₂, thus making the sensing device active at room temperature. The resulting CuO/TiO₂ membrane exhibited a notable detection sensitivity for H₂S featuring a minimum detection limit of 3.0 ppm, a response value of 46.81% against 100 ppm H₂S gas, and a rapid response and recovery time. This sensing membrane also demonstrated excellent durability, long-term stability, and wide-range response to a concentration of up to 400 ppm in the presence of 40% humidity as well as outstanding flexibility and negligible change in electrical measurements under various mechanical stability tests. This study not only provides a new strategy to design a gas sensor but also paves a universal platform for sensitive gas sensing.

Plasma-induced oxygen vacancies enabled ultrathin ZnO films for highly sensitive detection of triethylamine

Metal oxide semiconductor (MOS) thin films hold great promise for electronic devices such as gas sensors. However, the low surface activity of pristine MOS often leads to inferior sensitivity and the sensitization mechanism of ultrathin MOS films has received rare attention. Herein, we report a high performance gas sensor based on plasma-etched ZnO thin films. The ultrathin ZnO films (20 nm) were deposited on SiO₂ wafers by atomic layer deposition (ALD), which enables high-throughput production of sensor devices. The ZnO sensor shows typical n-type conductivity, which is highly variable to the exposure of triethylamine (TEA). Annealing temperature of the films is found to impact the sensor response, revealing calcination at a moderate temperature, i.e. 700 °C, leads to the best response. Further treatment by Ar plasma results in a remarkable decrease of sensor working temperature from 300 °C of untreated films to 250 °C and nearly 4-fold enhancement in the sensor response to 10 ppm TEA. Notably, the plasma-treated ZnO sensor also shows decent response even at room temperature (RT), which has been seldom reported for ZnO-based sensors. Structure and mechanism investigations reveal that the superior sensor properties are derived from the abundant oxygen vacancies generated by Ar plasma etching.

Magnesium Zirconate Titanate Thin Films Used as an NO2 Sensing Layer for Gas Sensor Applications Developed Using a Sol-Gel Method

Magnesium zirconate titanate (MZT) thin films, used as a sensing layer on Al interdigitated electrodes prepared using a sol–gel spin-coating method, are demonstrated in this study. The p-type MZT/Al/SiO₂/Si structure for sensing NO₂ is also discussed. The results indicated that the best sensitivity of the gas sensor occurred when it was operating at a temperature ranging from 100 to 150 °C. The detection limit of the sensor was as low as 250 ppb. The sensitivity of the MZT thin film was 8.64% and 34.22% for 0.25 ppm and 5 ppm of NO₂ gas molecules at a working temperature of 150 °C, respectively. The gas sensor also exhibited high repeatability and selectivity for NO₂. The response values to 250, 500, 1000, 1500, 2000, 2500, and 5000 ppb NO2 at 150 °C were 8.64, 9.52, 12, 16.63, 20.3, 23, and 34.22%, respectively. Additionally, we observed a high sensing linearity in NO₂ gas molecules. These results indicate that MZT-based materials have potential applications for use as gas sensors.

New Alternating Current Noise Analytics Enables High Discrimination in Gas Sensing

Feature analysis has been increasingly considered as an important way to enhance the discrimination performance of gas sensors. In this work, a new analytical method based on alternating current noise spectrum is developed to discriminate chemically and structurally similar gases with remarkable performance. Compared with the conventional analytics based on the maximum, integral, and time of response, the noise spectrum of electrical response introduces a new informative feature to discriminate chemical gases. In experiment, three chemically and structurally similar gases, mesitylene, toluene, and o-xylene, are tested on ZnO thin film gas sensors. The result indicated that the noise analytics assisted by the support vector machine algorithm discriminated these similar gases with 94.2% in precision, about 20% higher than those obtained by conventional methods. Such a new alternating current noise analytics is very promising for application in sensors for high discrimination precision.

Fe-Doped ZnO/Reduced Graphene Oxide Nanocomposite with Synergic Enhanced Gas Sensing Performance for the Effective Detection of Formaldehyde

Here, we report the synthesis of Fe-doped ZnO/reduced graphene oxide (rGO) nanocomposites for gas sensing applications via a one-pot hydrothermal process. A wide range of characterization techniques were used to confirm the successful fabrication of the nanocomposite material and to determine the surface area, the structural and morphological properties, the chemical composition, and the purity of the samples, such as Brunauer-Emmett-Teller, X-ray diffraction, Fourier transform infrared, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, UV-vis spectroscopy, and X-ray photoelectron spectroscopy techniques. The gas sensing performance to formaldehyde was studied thoroughly in a temperature-controlled test chamber. Compared to that of the bare ZnO and ZnO/rGO nanocomposites, the as-prepared 5 atom % Fe-doped ZnO/rGO nanocomposites presented significantly enhanced gas sensing performance to formaldehyde at relatively low temperatures. Whereas most formaldehyde sensors operate at 150 °C and can detect as low as 100 ppm concentrations, the presented sensor can detect 5 ppm formaldehyde at 120 °C. Its fast response-recovery time, high stability, and high selectivity make it an ideal sensor; however, it can exhibit degenerative gas sensing performance at elevated relative humidity. The enhanced gas sensing mechanism was explained as the synergic effect of rGO and Fe doping. The results demonstrate that Fe doping and decorating the nanocomposite with rGO are promising approaches for achieving a superior gas sensing performance for the development of ZnO gas sensors for the detection of formaldehyde.

Impedance Spectroscopy-Based Reduced Graphene Oxide-Incorporated ZnO Composite Sensor for H2S Investigations

Electrochemical impedance spectroscopy (EIS) has been applied to measure the H₂S gas response of the sensor fabricated on reduced graphene oxide (rGO)-incorporated nano-zinc oxide (n-ZnO) composites. These nanocomposites were prepared by a facile one-step solution route at room temperature. The structural, surface morphological, and elemental analyses of the composite material have been investigated. EIS was carried out to study the H₂S gas-sensing properties of fabricated sensors. The developed sensor showed an optimal H₂S gas response to various concentrations ranging from 2 to 100 ppm at 90 °C. The H₂S gas-sensing performances of pure n-ZnO and various concentrations of rGO-incorporated n-ZnO were evaluated. The H₂S gas-sensing results showed that n-ZnO/rGO composites exhibited high response when compared to pure n-ZnO. The enhanced H₂S response was speculated to be ascribed due to two factors. First, rGO creates reactive sites for H₂S molecule adsorption. Second, rGO has great electrical conductivity compared to n-ZnO that enables the active transport of electrons from H₂S gas on interaction with the sensing layer, resulting in enhanced gas response at 90 °C temperatures.

Electrospun CuO-ZnO nanohybrid: Tuning the nanostructure for improved amperometric detection of hydrogen peroxide as a non-enzymatic sensor

Hydrogen peroxide (H₂O₂) is a by-product of some biochemical processes which is catalyzed by enzymes such as glucose oxidase (GOx), cholesterol oxidase (ChoOx), etc and its overproduction in living cells can trigger cancer growth and various diseases. Therefore, H₂O₂ sensing is of great importance in the determination of diseases as well as industries and environmental health plans. We produced ZnO-CuO nanofibers by electrospinning method for non-enzymatic electrochemical H₂O₂ sensing. The sensing properties of the carbon paste electrode (CPE) modified with ZnO (0.3 wt%)/CuO (0.7 wt%) nanofibers (named as ZnO3-CuO7) for detection of H₂O₂ were explored in phosphate-buffered saline (PBS) at pH ∼ 7.4 solution. The ZnO3-CuO7 nanofiber exhibited the lowest charge transfer resistance and the highest electrocatalytic performance among other modified electrodes for detection of H₂O₂ and considered as an optimized sample. The effect of scan rate and H₂O₂ concentration in the reduction process were also investigated by cyclic voltammetry (CV) and the mechanism for the electrochemical reaction of H₂O₂ at the surface of the optimized electrode was studied. The diffusion coefficient of H₂O₂ and the catalytic rate constant were evaluated by chronoamperometry as 1.65 × 10^-5 cm² s^-1 and 6 × 10³ cm³ mol^-1 s^-1, respectively. Furthermore, amperometric detection of H₂O₂ with a low detection limit of 2.4 µM and a wide linear range of 3 to 530 µM were obtained. Meanwhile, the optimized electrode displayed no recognizable response towards some biomolecules such as ascorbic acid, uric acid, dopamine and glucose. The obtained results confirmed that the modified electrode shows high sensitivity and selectivity as a H₂O₂ biosensor with improved reproducibility and stability.

Ultrafast Response/Recovery and High Selectivity of the H2S Gas Sensor Based on α-Fe2O3 Nano-Ellipsoids from One-Step Hydrothermal Synthesis

Ultrafast response/recovery and high selectivity of gas sensors are critical for real-time and online monitoring of hazardous gases. In this work, α-Fe₂O₃ nano-ellipsoids were synthesized using a facile one-step hydrothermal method and investigated as highly sensitive H₂S-sensing materials. The nano-ellipsoids have an average long-axis diameter of 275 nm and an average short-axis diameter of 125 nm. H₂S gas sensors fabricated using the α-Fe₂O₃ nano-ellipsoids showed excellent H₂S-sensing performance at an optimum working temperature of 260 °C. The response and recovery times were 0.8 s/2.2 s for H₂S gas with a concentration of 50 ppm, which are much faster than those of H₂S gas sensors reported in the literature. The α-Fe₂O₃ nano-ellipsoid-based sensors also showed high selectivity to H₂S compared to other commonly investigated gases including NH₃, CO, NO₂, H₂, CH₂Cl₂, and ethanol. In addition, the sensors exhibited high-response values to different concentrations of H₂S with a detection limit as low as 100 ppb, as well as excellent repeatability and long-term stability.

Red Phosphorus: An Elementary Semiconductor for Room-Temperature NO2 Gas Sensing

Black and blue phosphorus (both allotropes of elementary phosphorus) have recently been widely explored as an active material for electronic devices, and their potential in gas sensing applications has been demonstrated. On the other hand, amorphous red phosphorus (a-RP), a much cheaper and readily available phosphorus allotrope, has seldom been investigated as an electronic material, and its gas sensing properties have never been studied. In this work we have investigated these properties of a-RP by combining experimental characterizations with theoretical calculations. We found that a-RP exhibited an amphoteric character for detecting both commonly regarded reducing and oxidizing gas molecules, featuring a negative correlation between the electrical resistance of a-RP and the gas concentration. Interestingly, the a-RP based sensors appear to be particularly suitable for room-temperature NO₂ detection, exhibiting excellent sensitivity and selectivity, as well as fast temporal response and recovery. A unique sensing feature of a-RP toward NO₂ was identified, which is associated with the expansion of P-P bonds upon NO₂ chemisorption. Based on density functional theory calculations we proposed a physiochemical model to elaborate the synergistic effects of the P-P bond expansion and Langmuir isotherm adsorption on the electronic properties and gas sensing processes of a-RP.

Facile synthesis of size-controlled Fe2O3 nanoparticle-decorated carbon nanotubes for highly sensitive H2S detection

Hydrogen sulfide (H₂S) is one of the most plentiful toxic gases in a real-life and causes a collapse of the nervous system and a disturbance of the cellular respiration. Therefore, highly sensitive and selective H₂S gas sensor systems are becoming increasingly important in environmental monitoring and safety. In this report, we suggest the facile synthesis method of the Fe₂O₃ particles uniformly decorated on carbon nanotubes (Fe₂O₃@CNT) to detect H₂S gas using oxidative co-polymerization (pyrrole and 3-carboxylated pyrrole) and heat treatment. The as prepared Fe₂O₃@CNT-based sensor electrode is highly sensitive (as low as 1 ppm), selective and stable to H₂S gas at 25 °C, which shows promise for operating in medical diagnosis and environment monitoring. Excellent performance of the Fe₂O₃@CNT is due to the unique morphology of the nanocomposites made from uniformly dispersed Fe₂O₃ nanoparticles on the carbon surface without aggregation.

Fe₂O₃ uniformly dispersed on carbon nanotubes are synthesized using facile oxidative co-polymerization of monomers followed by heat treatment to apply electrode materials for a highly sensitive H₂S chemical sensor system.

Porous CeO2 nanospheres for a room temperature triethylamine sensor under high humidity conditions

The porous CeO₂ nanospheres showed an enhanced triethylamine sensing performance at 98% of relative humidity in terms of sensitivity, selectivity, repeatability, and response time.