Al2O3/ZnO Heterostructure-Based Sensors for Volatile Organic Compounds in Safety Applications

Synergistic effect of ZnO-ZnFe2O4 heterostructures for enhanced surface catalytic activity in Cr(VI) reduction, green H2 generation and CO sensing: an experimental study supported by DFT

Increasing energy demand is an indication of progress, but it necessitates careful management of environmental pollution for maintaining a healthy life and ensuring a better planet for future generations. Heterostructure material-based catalysts have emerged as a comprehensive solution to combat the diverse challenges related to energy and environment. Herein, an n-n-type ZnO-ZnFe2O4 heterostructure was synthesized via a simple reflux followed by a co-precipitation technique for the same. Detailed photocatalytic and gas sensing studies revealed that a 50% ZnO-50% ZnFe2O4-based sample (ZZF-11) showed the highest Cr(VI) degradation with a rate constant of ∼159 × 10-4 s-1, which was ∼23 times higher than that of pristine ZnO and 6.4 times higher than that of pristine ZnFe2O4. Additionally, the ZZF-11 sample produced ∼550 μmol g-1 of H2 within a 300 minute interval via a photocatalytic water-splitting reaction. The ZZF-11 sensor also showed a significantly high response to 1 ppm CO gas (S = 29.4%) compared to all other pure and composite samples. The formation of the heterostructure and transfer of charges through the interface played an important role here. The most possible mechanism for the enhanced surface catalytic performance of ZZF-11 was critically analysed by corroborating the experimental results with DFT results. This study demonstrates a unified pathway to enhance the various surface catalytic processes by tuning different parameters of the heterostructure material to simultaneously overcome environment- and energy-related issues.

Intertwining Density Functional Theory and Experiments in the Investigation of Gas Sensing Mechanisms: A Review

In this review, we present the last ten years of progress in evaluation of gas sensing mechanisms. We focus mostly on the studies joining theoretical modeling of gas adsorption by density functional theory method with advanced experimental characterization of sensing materials. We provide the background about important aspects that should be taken into account during the design of the effective sensing device and an overview of the most recently studied sensing materials and analytes. Using the exemplary works, we next show how theory and experiment intertwine in revealing how the sensing mechanism serves to improve the device performance. In the end, we summarize the progress already made despite the existing difficulties, and provide an outlook for future methodological development.

Stoichiometric-Ratio-Controlled Fe and Ni Non-Noble Metal Catalysts Supported on γ-Al2O3 for Turquoise Hydrogen and Carbon Nanotubes Production

Herein, we synthesized a series of catalysts comprising iron (Fe), and nickel (Ni) supported on γ-Al2O3 nano-powder (Fe-Ni/γ-Al2O3) by controlling the stoichiometric ratio of the metals through the facile co-precipitation method. The ratio of Fe and Ni on the γ-Al2O3 support varied from 0 to 70 weight percent (wt %). The freshly prepared catalysts phase, structure, and crystallinity exhibited variability as the Fe and Ni stoichiometric ratios were altered. The catalyst demonstrated effective performance in methane cracking, producing turquoise hydrogen and carbon nanotubes (CNTs) using a temperature-programmed reactor coupled with mass spectrometry. It was observed that the Fe3Ni4 catalyst, comprising 30 % Fe and 40 % Ni, exhibited a maximum methane conversion rate of 85 % and a hydrogen yield of 72.55 %. Moreover, the values of turnover frequency (2.38 min-1) indicated that the Fe3Ni4 had a better production rate and was consistent with the conversion process throughout the reaction. The structural attributes of the spent catalysts were examined, revealing variations in the lateral length, uniformity, and diameters (~33 to 56 nm) of the produced Carbon Nanotubes (CNTs) when transitioning from catalyst Fe0Ni7 to Fe7Ni0. The investigation underscored the significance of metal stoichiometrically controlled catalysts and their catalytic efficacy in methane cracking applications.

Interface Defects and Carrier Regulation in MOF-Derived Co3O4/In2O3 Composite Materials for Enhanced Selective Detection of HCHO

Gas sensors for real-time monitoring of low HCHO concentrations have promising applications in the field of health protection and air treatment, and this work reports a novel resistive gas sensor with high sensitivity and selectivity to HCHO. The MOF-derived hollow In2O3 was mixed with ZIF-67(Co) and calcined twice to obtain a hollow Co3O4/In2O3 (hereafter collectively termed MZO-6) composite enriched with oxygen vacancies, and tests such as XPS and EPR proved that the strong interfacial electronic coupling increased the oxygen vacancies. The gas-sensitive test results show that the hollow composite MZO-6 with abundant oxygen vacancies has a higher response value (11,003) to 10 ppm of HCHO and achieves a fast response/recovery time (11/181 s) for HCHO at a lower operating temperature (140 °C). The MZO-6 material significantly enhances the selectivity to HCHO and reduces the interference of common pollutant gases such as ethanol, acetone, and xylene. There is no significant fluctuation of resistance and response values in the 30-day long-term stability test, and the material has good stability. The synergistic effect of the heterostructure and oxygen vacancies altered the formaldehyde adsorption intermediate pathway and reduced the reaction activation energy, enhancing the HCHO responsiveness and selectivity of the MZO-6 material.

Oxygen vacancy-enriched Zn2SnO4 for aliphatic alcohol sensing and enhanced selectivity towards n-butanol

The sensitive detection of toxic flammable volatile organics using low cost efficient sensors is important for ensuring both indoor and outdoor safety. It is essential for chemical sensors to exhibit a significantly stronger response to target analytes compared to equivalent amounts of analogous competing chemicals. In line with this importance, current work evaluated the performance of Zn2SnO4, a n-type semiconducting metal oxide, for sensing n-butanol in comparison to methanol, ethanol, and propanol vapours. These vapours fall within the category of aliphatic alcohols but vary in characteristics such as molecular weight, vapour pressure, volatility, and diffusivity. In this work we have explored the sensor's performance by adjusting the operating temperature over the range of 225-300 °C while detecting 1000 ppm of each of these vapours. Efforts were made to establish a correlation between the sensor's responses with the interactions of these vapours on the sensor's surface. Prior to assessing the sensing characteristics of the solid-state-route-derived Zn2SnO4, its structural characteristics, including phase purity, crystalline structure, bonding patterns, morphology, and defect characteristics, were studied. This comprehensive analysis sheds light on the potential of Zn2SnO4 as an effective sensor for detecting n-butanol.

Tuning the Selectivity of Metal Oxide Gas Sensors with Vapor Phase Deposited Ultrathin Polymer Thin Films

Metal oxide gas sensors are of great interest for applications ranging from lambda sensors to early hazard detection in explosive media and leakage detection due to their superior properties with regard to sensitivity and lifetime, as well as their low cost and portability. However, the influence of ambient gases on the gas response, energy consumption and selectivity still needs to be improved and they are thus the subject of intensive research. In this work, a simple approach is presented to modify and increase the selectivity of gas sensing structures with an ultrathin polymer thin film. The different gas sensing surfaces, CuO, Al₂O₃/CuO and TiO₂ are coated with a conformal < 30 nm Poly(1,3,5,7-tetramethyl-tetravinyl cyclotetrasiloxane) (PV4D4) thin film via solvent-free initiated chemical vapor deposition (iCVD). The obtained structures demonstrate a change in selectivity from ethanol vapor to 2-propanol vapor and an increase in selectivity compared to other vapors of volatile organic compounds. In the case of TiO₂ structures coated with a PV4D4 thin film, the increase in selectivity to 2-propanol vapors is observed even at relatively low operating temperatures, starting from >200 °C. The present study demonstrates possibilities for improving the properties of metal oxide gas sensors, which is very important in applications in fields such as medicine, security and food safety.

Specific Alcohol-Responsive Photonic Crystal Sensors Based on Host-Guest Recognition

A photonic crystal material based on β-cyclodextrin (β-CD) with adsorption capacity is reported. The materials ((A-β-CD)-AM PC) consist of 3D poly (methyl methacrylate) (PMMA) colloidal microsphere arrays and hydrogels supplemented with β-cyclodextrin modified by acryloyl chloride. The prepared materials are then utilized for VOCs gas sensing. The 3D O-(A-β-CD)-AM PC was used to detect toluene, xylene, and acetone and the response was seen as the red-shift of the reflection peak. The 3D I-(A-β-CD)-AM PC was used to detect toluene, xylene, and acetone which occurred redshifted, while methanol, ethanol, and propanol and the peaks' red-shifting was observed. However, among these, methanol gave the largest red-shift response The sensor has broad prospects in the detection of alcohol and the detection of alcohol-loaded drug releases in the future.

Metal-Doped Al12N12X (X = Na, Mg, K) Nanoclusters as Nanosensors for Carboplatin: Insight from First-Principles Computation

This theoretical study focuses on the adsorption, reactivity, topological analysis, and sensing behavior of metal-doped (K, Na, and Mg) aluminum nitride (Al12N12) nanoclusters using the first-principle density functional theory (DFT). All quantum chemical reactivity, natural bond orbital (NBO), free energies (ΔG, ΔH), and sensor parameters were investigated using the ωB97XD functional with the 6-311++G(d,p) basis set. The trapping of carboplatin (cbp) onto the surfaces of doped Al12N12 was studied using four functionals PBE0-D3, M062X-D3, ωB97XD, and B3LYP-D3 at the 6-311++G(d,p) basis set. Overall, the substantial change in the energy gap of the surfaces after the adsorption process affects the work function, field emission, and the electrical conductivity of the doped clusters, hence making the studied surfaces a better sensor material for detecting carboplatin. Higher free energies of solvation were obtained in polar solvents compared to nonpolar solvents. Moreover, negative solvation energies and adsorption energies were obtained, which therefore shows that the engineered surfaces are highly efficient in trapping carboplatin. The relatively strong adsorption energies show that the mechanism of adsorption is by chemisorption, and K- and Na-doped metal clusters acted as better sensors for carboplatin. Also, the topological analysis in comparison to previous studies shows that the nanoclusters exhibited very high stability with regard to their relevant binding energies and hydrogen bond interactions.