Sr-Doped Cubic In2O3/Rhombohedral In2O3 Homojunction Nanowires for Highly Sensitive and Selective Breath Ethanol Sensing: Experiment and DFT Simulation Studies

Insights into Selective Sensitivity of In2O3-CuO Heterojunction Nanocrystals to CH4 over CO and H2: Experiments and First-Principles Calculations

Metal oxide semiconductor gas sensors have demonstrated exceptional potential in gas detection due to their high sensitivity, rapid response time, and impressive selectivity for identifying various sorts of gases. However, selectively distinguishing CH4 from those of CO and H2 remains a significant challenge. This difficulty primarily stems from the weakly reducing nature of CH4, which results in a low adsorption response and makes it prone to interference from stronger reducing gases in the surroundings. Herein, we synthesized In2O3-xCuO nanocomposites using a hydrothermal method to explore their gas sensing properties toward CH4, CO, and H2. Characterization tests confirmed the successful preparation of In2O3-xCuO nanocomposites with different In:Cu molar ratios and the formation of a p-n heterojunction. The gas sensing test results indicated that the In2O3-2.1CuO nanocomposites calcined at 500 °C and measured at 350 °C displayed a p-type response for CH4 and an n-type response for CO and H2, allowing for accurate differentiation of CH4 from CO and H2. Moreover, the In2O3-2.1CuO sensor also showed excellent stability and reproducibility across all three gases. First-principles calculations revealed distinct changes in the electronic structure of the In2O3-CuO heterojunction upon adsorption of CH4, CO, and H2, a finding that aligns with empirical evidence. The gas selectivity mechanism was effectively explained by variations in the energy band gap, driven by electrical behavior during the adsorption process. This work suggests a promising approach for developing selective gas sensors capable of detecting weakly reducing gases.

Gas Sensor for Efficient Acetone Detection and Application Based on Au-Modified ZnO Porous Nanofoam

Toxic acetone gas emissions and leakage are a potential threat to the environment and human health. Gas sensors founded on metal oxide semiconductors (MOS) have become an effective strategy for toxic gas detection with their mature process. In the present work, an efficient acetone gas sensor based on Au-modified ZnO porous nanofoam (Au/ZnO) is synthesized by polyvinylpyrrolidone-blowing followed by a calcination method. XRD and XPS spectra were utilized to investigate its structure, while SEM and TEM characterized its morphology. The gas sensitivity of the Au/ZnO sensors was investigated in a static test system. The results reveal that the gas-sensitive performance of porous ZnO toward the acetone can be enhanced by adjusting the loading ratio of noble Au nanoparticles. Specifically, the Au/ZnO sensor prepared by the Au loading ratio of 3.0% (Au/ZnO-3.0%) achieved a 100 ppm acetone gas response of 20.02 at the optimum working temperature of 275 °C. Additionally, a portable electronic device used a STM32 primary control chip to integrate the Au/ZnO-3.0% gas sensor with other modules to achieve the function of detecting and alarming toxic acetone gas. This work is of great significance for efficiently detecting and reducing acetone emissions.

Effectively Improved CH4 Sensing Performance of In2O3 Porous Hollow Nanospheres by Doping with Cd

Recently, due to the promising application of metal oxide semiconductors in high-performance methane (CH4) sensors, more attention has been paid to the development of feasible strategies for improving CH4 sensing performance. Herein, we present a strategy of cadmium (Cd) doping to improve the CH4 sensing property of In2O3 porous hollow nanospheres (PHNSs). The Cd-doped In2O3 PHNSs were prepared via an impregnation-calcination approach with self-made carbon nanospheres as a hard template. The samples were characterized by various techniques to evaluate their structure, morphology, surface state, composition, and band gap. When applied as a sensitive material in the CH4 sensor, the Cd-doped In2O3 PHNSs, compared with bare In2O3 PHNSs, showed some significant improvements in performance, especially a reduced operating temperature (200 °C vs 300 °C), an enhanced response (9.5 vs 2.5 for 500 ppm of CH4), a faster response speed (16 s vs 276 s), and better selectivity. In addition, the Cd-doped In2O3 sensor can also maintain a commendable long-term stability, and the range of its response amplitude within 30 days is only 6.3%. The sensitization effects of the Cd dopant on the In2O3 PHNSs are discussed.

Ultrasensitive Acetone Gas Sensor Based on a K/Sn-Co3O4 Porous Microsphere for Noninvasive Diabetes Diagnosis

The detection of acetone in human exhaled breath is crucial for the noninvasive diagnosis of diabetes. However, the direct and reliable detection of acetone in exhaled breath with high humidity at the parts per billion level remains a great challenge. Here, an ultrasensitive acetone gas sensor based on a K/Sn-Co3O4 porous microsphere was reported. The sensor demonstrates a detection limit of up to 100 ppb, along with excellent repeatability and selectivity. Remarkably, without the removal of water vapor from exhaled breath, the sensor can accurately distinguish diabetic patients and healthy individuals according to the difference in acetone concentrations, demonstrating its great potential for diabetes diagnosis. The enhanced sensitivity of the sensor is attributed to the increased oxygen adsorption on the material surface due to K/Sn codoping and the stronger coadsorption of Sn-K atoms to acetone molecules. These findings shed light on the mechanisms underlying the sensor's improved performance.

Structural, Electronic Properties, and Relative Stability Studies of Low-Energy Indium Oxide Polytypes Using First-Principles Calculations

Materials made of indium oxide (In₂O₃) are now being used as a potential component of the next generation of computers and communication devices. Density functional theory is used to analyze the physical, electrical, and thermodynamical features of 12 low-energy bulk In₂O₃ polytypes. The cubic structure In₂O₃ is majorly used for many of the In₂O₃-based transparent conducting oxides. The objective of this study is to explore other new stable In₂O₃ polytypes that may exist. The structural properties and stability studies are performed using the Vienna ab initio simulation package code. All the In₂O₃ polytypes have semiconductive properties, according to electronic band structure investigations. The full elastic tensors and elastic moduli of all polytypes at 0 K are computed. Poisson's and Pugh's ratio confirms that all stable polytypes are ductile. The phonon and thermal properties including heat capacity are obtained for mechanically stable polytypes. For the first time, we report the Raman and infrared active modes of stable polytypes.

Sub PPM Detection of NO2 Using Strontium Doped Bismuth Ferrite Nanostructures

The present work investigates the NO₂ sensing properties of acceptor-doped ferrite perovskite nanostructures. The Sr-doped BiFeO₃ nanostructures were synthesized by a salt precursor-based modified pechini method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The synthesized materials were drop coated to fabricate chemoresistive gas sensors, delivering a maximum sensitivity of 5.2 towards 2 ppm NO₂ at 260 °C. The recorded values of response and recovery time are 95 s and 280 s, respectively. The sensor based on Bi_0.8Sr_0.2FeO_3-δ (BSFO) that was operated was shown to have a LOD (limit of detection) as low as 200 ppb. The sensor proved to be promising for repeatability and selectivity measurements, indicating that the Sr doping Bismuth ferrite could be a potentially competitive material for sensing applications. A relevant gas-sensing mechanism is also proposed based on the surface adsorption and reaction behavior of the material.

Exploitation of Schottky-Junction-based Sensors for Specifically Detecting ppt-Concentration Gases

Gas species and concentrations of human-exhaled breath correlate with health, wherein disease markers contain volatile organic compounds (VOCs) of concentrations in parts per billion. It is expected that a gas-sensing strategy possesses a gas specificity and detection limit in the parts per trillion (ppt) range; however, it is still a challenge. This investigation has exploited the Schottky junction of gas sensors for detecting the reactance signal of ppt VOC, aiming for a specific and rapid detection toward disease marker acetone. In this new sensing paradigm, formed by the engineered energy band between metal-semiconductor contact, the Schottky junction is accessed to specific modulation of different adsorbate dopings and the corresponding reactance signal is measured. Regarding the detection toward ppt concentration of acetone, this sensing paradigm possesses rapid (∼100 s) and room-temperature response, molecular specificity, and 34 ppt of detection limit. The proposed detection paradigm is demonstrated to show a high feasibility toward detection of disease marker acetone.

Atmospheric Pressure Solvothermal Synthesis of Nanoscale SnO2 and Its Application in Microextrusion Printing of a Thick-Film Chemosensor Material for Effective Ethanol Detection

The atmospheric pressure solvothermal (APS) synthesis of nanocrystalline SnO₂ (average size of coherent scattering regions (CSR)-7.5 ± 0.6 nm) using tin acetylacetonate as a precursor was studied. The resulting nanopowder was used as a functional ink component in microextrusion printing of a tin dioxide thick film on the surface of a Pt/Al₂O₃/Pt chip. Synchronous thermal analysis shows that the resulting semiproduct is transformed completely into tin dioxide nanopowder at 400 °C within 1 h. The SnO₂ powder and the resulting film were shown to have a cassiterite-type structure according to X-ray diffraction analysis, and IR spectroscopy was used to establish the set of functional groups in the material composition. The microstructural features of the tin dioxide powder were analyzed using scanning (SEM) and transmission (TEM) electron microscopy: the average size of the oxide powder particles was 8.2 ± 0.7 nm. Various atomic force microscopy (AFM) techniques were employed to investigate the topography of the oxide film and to build maps of surface capacitance and potential distribution. The temperature dependence of the electrical conductivity of the printed SnO₂ film was studied using impedance spectroscopy. The chemosensory properties of the formed material when detecting H₂, CO, NH₃, C₆H₆, C₃H₆O and C₂H₅OH, including at varying humidity, were also examined. It was demonstrated that the obtained SnO₂ film has an increased sensitivity (the sensory response value was 1.4-63.5) and selectivity for detection of 4-100 ppm C₂H₅OH at an operating temperature of 200 °C.

MXene Ti3C2Tx-Derived Nitrogen-Functionalized Heterophase TiO2 Homojunctions for Room-Temperature Trace Ammonia Gas Sensing

In this work, MXene Ti₃C₂T_x-derived nitrogen-functionalized heterophase TiO₂ homojunctions (N-MXene) were prepared via the urea-involved solvothermal treatment with varying reaction time as the sensing layer to detect trace NH₃ gas at room temperature (20 °C). Compared with no signal for the pristine MXene counterpart, the 18 h-treated sensors (N-MXene-18) achieved a detection limit of 200 ppb with an inspiring response that was 7.3% better than the existing MXene-involved reports thus far. Also, decent repeatability, stability, and selectivity were demonstrated. It is noteworthy that the N-MXene-18 sensors delivered a stronger response, more sufficient recovery, and quicker response/recovery speeds under a humid environment than those under dry conditions, proving the significance of humidity. Furthermore, to suppress the effect of the fluctuation of humidity on NH₃ sensing during the tests, a commercial waterproof polytetrafluoroethylene (PTFE) membrane was anchored onto the sensing layer, eventually bringing about humidity-independent features. Both nitrogen doping and TiO₂ homojunctions constituted by mixed anatase and rutile phases were primarily responsible for the performance improvement with respect to pristine MXene. This work showcases the enormous potential of N-MXene materials in trace NH₃ detection and offers an alternative strategy to realize both heteroatom doping and partial oxidation of MXene that is applicable in future optoelectronic devices.

Low-voltage and fast-response SnO2nanotubes/perovskite heterostructure photodetector

One-dimensional metal-oxides (1D-MO) nanostructure has been regarded as one of the most promising candidates for high-performance photodetectors due to their outstanding electronic properties, low-cost and environmental stability. However, the current bottlenecks are high energy consumption and relatively low sensitivity. Here, Schottky junctions between nanotubes (NTs) and FTO were fabricated by electrospinning SnO₂NTs on FTO glass substrate, and the bias voltage of SnO₂NTs photodetectors was as low as ∼1.76 V, which can effectively reduce energy consumption. Additionally, for improving the response and recovery speed of SnO₂NTs photodetectors, the NTs were covered with organic/inorganic hybrid perovskite. SnO₂NTs/perovskite heterostructure photodetectors exhibit fast response/recovery speed (∼0.075/0.04 s), and a wide optical response range (∼220-800 nm). At the same time, the bias voltage of heterostructure photodetectors was further reduced to 0.42 V. The outstanding performance is mainly attributed to the formation of type-II heterojunctions between SnO₂NTs and perovskite, which can facilitate the separation of photogenerated carriers, as well as Schottky junction between SnO₂NTs and FTO, which reduce the bias voltage. All the results indicate that the rational design of 1D-MO/perovskite heterostructure is a facile and efficient way to achieve high-performance photodetectors.

High Annealing Stability of InAlZnO Nanofiber Field-Effect Transistors with Improved Morphology by Al Doping

In₂O₃ nanofibers usually suffer a high off-current and consequent low on/off current ratio, as well as a large negative threshold voltage (V_th). Furthermore, regarding Zn doped binary-cation In₂O₃ nanofibers, severe thermal diffusion of Zn elements can result in deteriorated electrical performance when annealed at high temperature. Here, we applied an electrospinning technique to obtain ternary-cation IAZO nanofibers with controllable V_th and chemical stoichiometry. The presence of the Al element in IAZO nanofibers can lead to more superior microstructure with improved uniformity, lower surface defect, and superior metal-oxide-metal lattice at high annealing temperature. Consequently, our Al-doped ternary-cation IAZO devices exhibited an improved on/off current ratio of 10⁷ and a high electron mobility of ∼10 cm² V^-1 s^-1. Moreover, the electron mobility can be increased to 30 cm² V^-1 s^-1 in our low-voltage operated FETs with high-k AlO_x as the dielectric layer, which can be envisioned to exhibit vast implications for high-performance transparent electronics.

Inorganic-Diverse Nanostructured Materials for Volatile Organic Compound Sensing

Environmental pollution related to volatile organic compounds (VOCs) has become a global issue which attracts intensive work towards their controlling and monitoring. To this direction various regulations and research towards VOCs detection have been laid down and conducted by many countries. Distinct devices are proposed to monitor the VOCs pollution. Among them, chemiresistor devices comprised of inorganic-semiconducting materials with diverse nanostructures are most attractive because they are cost-effective and eco-friendly. These diverse nanostructured materials-based devices are usually made up of nanoparticles, nanowires/rods, nanocrystals, nanotubes, nanocages, nanocubes, nanocomposites, etc. They can be employed in monitoring the VOCs present in the reliable sources. This review outlines the device-based VOC detection using diverse semiconducting-nanostructured materials and covers more than 340 references that have been published since 2016.

Au Nanoparticles Decorated Mesoporous SiO2 -WO3 Hybrid Materials with Improved Pore Connectivity for Ultratrace Ethanol Detection at Low Operating Temperature

Semiconducting metal oxides-based gas sensors with the capability to detect trace gases at low operating temperatures are highly desired in applications such as wearable devices, trace pollutant detection, and exhaled breath analysis, but it still remains a great challenge to realize this goal. Herein, a multi-component co-assembly method in combination with pore engineering strategy is proposed. By using bi-functional (3-mercaptopropyl) trimethoxysilane (MPTMS) that can co-hydrolyze with transition metal salt and meanwhile coordinate with gold precursor during their co-assembly with PEO-b-PS copolymers, ordered mesoporous SiO₂ -WO₃ composites with highly dispersed Au nanoparticles of 5 nm (mesoporous SiO₂ -WO₃ /Au) are straightforward synthesized. This multi-component co-assembly process avoids the aggregation of Au nanoparticles and pore blocking in conventional post-loading method. Furthermore, through controlled etching treatment, a small portion of silica can be removed from the pore wall, resulting in mesoporous SiO₂ -WO₃ /Au with increased specific surface area (129 m²  g^-1 ), significantly improved pore connectivity, and enlarged pore window (>4.3 nm). Thanks to the presence of well-confined Au nanoparticles and ε-WO₃ , the mesoporous SiO₂ -WO₃ /Au based gas sensors exhibit excellent sensing performance toward ethanol with high sensitivity (R_a /R_g = 2-14 to 50-250 ppb) at low operating temperature (150 °C).

Platinum-Supported Cerium-Doped Indium Oxide for Highly Sensitive Triethylamine Gas Sensing with Good Antihumidity

Triethylamine is extremely harmful to human health, and chronic inhalation can lead to respiratory and hematological diseases and eye lesions. Hence, it is essential to develop a triethylamine gas-sensing technology with high response, selectivity, and stability for use in healthcare and environmental monitoring. In this work, a simple and low-cost sensor based on the Pt- and Ce-modified In₂O₃ hollow structure to selectively detect triethylamine is developed. The experimental results reveal that the sensor based on 1% Pt/Ce₁₂In exhibits excellent triethylamine-sensing performance, including its insusceptibility to water, reduced operating temperature, enhanced response, and superior long-term stability. This work suggests that the enhancement of sensing performance toward triethylamine can be attributed to the high relative contents of O_V and O_C, large specific surface area, catalytic effect, the electronic sensitization of Pt, and the reversible redox cycle properties of Ce. This sensor represents a unique and highly sensitive means to detect triethylamine, which shows great promise for potential applications in food safety inspection and environmental monitoring.

Electrospun Yb-Doped In2O3 Nanofiber Field-Effect Transistors for Highly Sensitive Ethanol Sensors

Enhancing the reliability and sensitivity of gas sensors based on FETs has been of extensive concern for their practical application. However, few reports are available on nanofiber FET gas sensors fabricated by the electrospinning process. In this work, ethanol gas sensors based on Yb-doped In₂O₃ (InYbO) nanofiber FETs are fabricated by a simple and fast electrospinning method. The optimized In₂O₃ nanofiber FETs with a doping concentration of 4 mol % show a better electrical performance, including a high mobility of 6.67 cm²/Vs, an acceptable threshold voltage of 3.27 V, and a suitable on/off current ratio of 10⁷, especially the enhanced bias-stress stability. When employed in ethanol gas sensors, the gas sensors exhibit enhanced stability and improved sensitivity with a high response of 40-10 ppm, which is remarkably higher than that of previously reported ethanol gas sensors. Moreover, the InYbO nanofiber FET sensors also demonstrate a low limit of detection of 1 ppm and improved sensing performance ranging from sensitivity to the ability of selectivity. This work opens up a new prospect to achieve highly sensitive, selective, and reliable ethanol gas sensors using electrospun Yb-In₂O₃ nanofiber FETs with improved stability.

Modulating electrical and photoelectrical properties of one-step electrospun one-dimensional SnO2 arrays

One-dimensional nanostructured SnO₂ has attracted intense research interest due to its advantageous properties, including a large surface-to-volume ratio, high optical transparency and typical n-type properties. However, how to fabricate high-performance and multifunctional electronic devices based on 1D nanostructured SnO₂ via low-cost and efficient preparation techniques is still a huge challenge. In this work, a low-cost, one-step electrospun technology was employed to synthesize the SnO₂ nanofiber (NF) and nanotube (NT) arrays. The electrical and photoelectrical parameters of SnO₂ NTs-based devices were effectively controlled through simple changes to the amount of Sn in the precursor solution. The optimal 0.2 SnO₂ NTs-based field effect transistors (FETs) with 0.2 g SnCl₂*4H₂O per 5 ml in the precursor solution exhibit a high saturation current (∼9 × 10^-5 A) and a large on/off ratio exceeding 2.4 × 10⁶. Additionally, 0.2 SnO₂ NTs-based FET also exhibit a narrowband deep-UV photodetectivity (240-320 nm), including an ultra-high photocurrent of 307 μA, a high photosensitivity of 2003, responsibility of 214 A W^-1 and detectivity of 2.19 × 10¹³ Jones. Furthermore, the SnO₂ NTs-based transparent photodetectors were as well be integrated with fluorine-doped tin oxide glass and demonstrated a high optical transparency and photosensitivity (∼199). All these results elucidate the significant advantages of these electrospun SnO₂ NTs for next-generation multifunctional electronics and transparent photonics.