Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization

Recent Progress on Perovskite Materials for VOC Gas Sensing

Volatile organic compound (VOC) gases are highly hazardous to human health, and their presence in the human breath plays an indispensable role for the early diagnosis of various diseases (cancer, renal failure, etc.). In recent times, perovskite materials have shown notable performance in the detection of VOC gases with high accuracy, fast response, recovery time, selectivity, and sensitivity, owing to their unique crystallographic structures and excellent optoelectronic properties. In this Review, we look at recent reports on perovskite-based sensors and their sensing performance toward VOC gases. Here, we focus on the sensing mechanisms of two types of perovskite materials, metal halide and metal oxide perovskites, and explain the differences in their crystal structures. We also discuss the common preparation methods used by researchers for the synthesis of these perovskite materials. Further, we elucidate various important factors influencing the sensing performance of perovskite-based sensors, such as doping, defects, morphology, temperature, humidity, and light. We conclude with the future prospects and challenges related to these perovskite-based sensors toward the detection of VOC gases.

Facile and Sustainable Synthesis of ZnO Nanoparticles: Effect of Gelling Agents on ZnO Shapes and Their Photocatalytic Performance

The present work involves investigating an unexplored soft-chemical method for synthesizing nanostructured ZnO through biopolymer gelation. Our objective was to exploit (i) the difference in the gelation mechanism of four tested biopolymers, namely, alginate, chitosan, carboxymethylcellulose (CMC), and pectin and (ii) numerous experimental parameters that govern this process in order to allow the control of the growth of nanostructured ZnO, with a view to using the prepared oxides as photocatalysts for the oxidation of the Orange G dye. So, the effect of biopolymer's nature on the microstructural, morphological, and textural properties was examined by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field-emission gun-scanning electron microscopy-high resolution (FEG-SEM) with energy-dispersive spectrometry (SEM-EDS), ultraviolet-visible (UV-vis) spectroscopy, and N₂ adsorption/desorption. As-prepared oxides were crystallized in a hexagonal wurtzite structure, with a clear difference in their morphologies. The sample prepared by using chitosan has a specific surface area of around 36.8 m²/g in the form of aggregated and agglomerated nanostructured minirods and thus shows the best photocatalytic performance with 99.3% degradation of the Orange G dye in 180 min.

Chemical VOC sensing mechanism of sol-gel ZnO pellets and linear discriminant analysis for instantaneous selectivity

This work reports on the integration of ZnO pellets for use as a virtual sensor array (VSA) of volatile organic compounds (VOCs). ZnO pellets consist of nano-powder prepared using a sol-gel technique. The microstructure of the obtained samples was characterized by XRD and TEM methods. The response to VOCs at different concentrations was measured over a range of operating temperatures (250-450 °C) using DC electrical characterization. The ZnO based sensor showed a good response towards ethanol, methanol, isopropanol, acetone and toluene vapors. We note that the highest sensitivity (0.26 ppm^-1) is obtained with ethanol while the lowest one (0.041 ppm^-1) corresponds to methanol. Consequently, the limit of detection (LOD) estimated analytically reached 0.3 ppm for ethanol and 2.0 ppm for methanol at an operating temperature of 450 °C. The sensing mechanism of the ZnO semiconductor was developed on the basis of the reaction between the reducing VOCs with the chemisorbed oxygen. We verify through the Barsan model that mainly O^- ions in the layer react with VOC vapor. Furthermore, dynamic response was investigated to construct mathematical features with distinctly different values for each vapor. Basic linear discrimination analysis (LDA) shows a good job of separating two groups by combining features. In the same way we have shown an original reason embodying the distinction between more than two volatile compounds. With relevant features and VSA formalism, the sensor is clearly selective towards individual VOCs.

Advances in Noble Metal-Decorated Metal Oxide Nanomaterials for Chemiresistive Gas Sensors: Overview

Highly sensitive gas sensors with remarkably low detection limits are attractive for diverse practical application fields including real-time environmental monitoring, exhaled breath diagnosis, and food freshness analysis. Among various chemiresistive sensing materials, noble metal-decorated semiconducting metal oxides (SMOs) have currently aroused extensive attention by virtue of the unique electronic and catalytic properties of noble metals. This review highlights the research progress on the designs and applications of different noble metal-decorated SMOs with diverse nanostructures (e.g., nanoparticles, nanowires, nanorods, nanosheets, nanoflowers, and microspheres) for high-performance gas sensors with higher response, faster response/recovery speed, lower operating temperature, and ultra-low detection limits. The key topics include Pt, Pd, Au, other noble metals (e.g., Ag, Ru, and Rh.), and bimetals-decorated SMOs containing ZnO, SnO2, WO3, other SMOs (e.g., In2O3, Fe2O3, and CuO), and heterostructured SMOs. In addition to conventional devices, the innovative applications like photo-assisted room temperature gas sensors and mechanically flexible smart wearable devices are also discussed. Moreover, the relevant mechanisms for the sensing performance improvement caused by noble metal decoration, including the electronic sensitization effect and the chemical sensitization effect, have also been summarized in detail. Finally, major challenges and future perspectives towards noble metal-decorated SMOs-based chemiresistive gas sensors are proposed.

Obtaining of ZnO/Fe2O3 Thin Nanostructured Films by AACVD for Detection of ppb-Concentrations of NO2 as a Biomarker of Lung Infections

ZnO/Fe₂O₃ nanocomposites with different concentration and thickness of the Fe₂O₃ layer were obtained by two-stage aerosol vapor deposition (AACVD). It was shown that the ZnO particles have a wurtzite structure with an average size of 51-66 nm, and the iron oxide particles on the ZnO surface have a hematite structure and an average size of 23-28 nm. According to EDX data, the iron content in the films was found to be 1.3-5.8 at.%. The optical properties of the obtained films were studied, and the optical band gap was found to be 3.16-3.26 eV. Gas-sensitive properties at 150-300 °C were studied using a wide group of analyte gases: CO, NH₃, H₂, CH₄, C₆H₆, ethanol, acetone, and NO₂. A high response to 100 ppm acetone and ethanol at 225-300 °C and a high and selective response to 300-2000 ppb NO₂ at 175 °C were established. The effect of humidity on the magnitude and shape of the signal obtained upon NO₂ detection was studied.

A novel method for the synthesis of core-shell nanoparticles for functional applications based on long-term confinement in a radio frequency plasma

A novel combined setup of a Haberland type gas aggregation source and a secondary radio frequency discharge is used to generate, confine, and coat nanoparticles over much longer time scales than traditional in-flight treatment. The process is precisely monitored using localized surface plasmon resonance and Fourier-transform infrared spectroscopy as in situ diagnostics. They indicate that both untreated and treated particles can be confined for extended time periods (at least one hour) with minimal losses. During the entire confinement time, the particle sizes do not show considerable alterations, enabling multiple well-defined modifications of the seed nanoparticles in this synthesis approach. The approach is demonstrated by generating Ag@SiO2 nanoparticles with a well-defined surface coating. The in situ diagnostics provide insights into the growth kinetics of the applied coating and are linked to the coating properties by using ex situ transmission electron microscopy and energy dispersive X-ray spectroscopy. Surface coating is shown to occur in two phases: first, singular seeds appear on the particle surface which then grow to cover the entire particle surface over 3 to 5 minutes. Afterwards, deposition occurs via surface growth which coincides with lower deposition rates. Our setup offers full control for various treatment options, which is demonstrated by coating the nanoparticles with a SiO2 layer followed by the etching of the part of the applied coating using hydrogen. Thus, complex multi-step nanofabrication, e.g., using different monomers, as well as very large coating thicknesses is possible.

Operando monitoring of a room temperature nanocomposite methanol sensor

The sensing of volatile organic compounds by composites containing metal oxide semiconductors is typically explained via adsorption-desorption and surface electrochemical reactions changing the sensor's resistance. The analysis of molecular processes on chemiresistive gas sensors is often based on indirect evidence, whereas in situ or operando studies monitoring the gas/surface interactions enable a direct insight. Here we report a cross-disciplinary approach employing spectroscopy of working sensors to investigate room temperature methanol detection, contrasting well-characterized nanocomposite (TiO2@rGO-NC) and reduced-graphene oxide (rGO) sensors. Methanol interactions with the sensors were examined by (quasi) operando-DRIFTS and in situ-ATR-FTIR spectroscopy, the first paralleled by simultaneous measurements of resistance. The sensing mechanism was also studied by mass spectroscopy (MS), revealing the surface electrochemical reactions. The operando and in situ spectroscopy techniques demonstrated that the sensing mechanism on the nanocomposite relies on the combined effect of methanol reversible physisorption and irreversible chemisorption, sensor modification over time, and electron/O2 depletion-restoration due to a surface electrochemical reaction forming CO2 and H2O.

Machine Learning-Assisted Volatile Organic Compound Gas Classification Based on Polarized Mixed-Potential Gas Sensors

The performance of electrochemical gas sensors depends on the reactions at the three-phase boundary. In this work, a mixed-potential gas sensor containing a counter electrode, a reference electrode, and a sensitive electrode was constructed. By applying a bias voltage to the counter electrode, the three-phase boundary can be polarized. The polarization state of the three-phase boundary determined the gas-sensitive performance. Taking 100 ppm ethanol vapor as an example, by regulating the polarization state of the three-phase boundary, the response value of the sensor can be adjusted from -170 to 40 mV, and the sensitivity can be controlled from -126.4 to 42.6 mV/decade. The working temperature of the sensor can be reduced after polarizing the three-phase boundary, lowering the power consumption from 1.14 to 0.625 W. The sensor also showed good stability and short response-recovery time (3 s). Based on this sensor, the Random Forest algorithm reached 99% accuracy in identifying the kind of VOC vapors. This accuracy was made possible by the ability to generate several signals concurrently. The above gas-sensitive performance improvements were due to the polarized three-phase boundary.

Gas-phase synthesis of nanoparticles: current application challenges and instrumentation development responses

Nanoparticles constitute fundamental building blocks required in several fields of application with current global importance. To fully exploit nanoparticle properties specifically determined by the size, shape, chemical composition and interfacial configuration, rigorous nanoparticle growth and deposition control is needed. Gas-phase synthesis, in particular magnetron-sputtering inert-gas condensation, provides unique opportunities to realise engineered nanoparticles optimised for the desired use case. Here, we provide an overview of recent nanoparticle growth experiments via this technique, how the latter can meet application-specific requirements, and what challenges might impede the wide-spread adoption for scalable industrial synthesis. More specifically, we discuss the timely topics of energy, catalysis, and sensing applications enabled by gas-phase synthesised nanoparticles, as well as recently emerging advances in neuromorphic devices for unconventional computing. Having identified the most relevant challenges and limiting factors, we outline how advances in nanoparticle source instrumentation and/or in situ diagnostics can address current shortcomings. Eventually we identify common trends and directions, giving our perspective on the most promising and impactful applications of gas-phase synthesised nanoparticles in the future.

Entrapped Molecule-Like Europium-Oxide Clusters in Zinc Oxide with Nearly Unaffected Host Structure

Nanocrystalline ZnO sponges doped with 5 mol% EuO_1.5 are obtained by heating metal-salt complex based precursor pastes at 200-900 °C for 3 min. X-ray diffraction, transmission electron microscopy, and extended X-ray absorption fine structure (EXAFS) show that phase separation into ZnO:Eu and c-Eu₂ O₃ takes place upon heating at 700 °C or higher. The unit cell of the clean oxide made at 600 °C shows only ≈0.4% volume increase versus undoped ZnO, and EXAFS shows a ZnO local structure that is little affected by the Eu-doping and an average Eu³⁺ ion coordination number of ≈5.2. Comparisons of 23 density functional theory-generated structures having differently sized Eu-oxide clusters embedded in ZnO identify three structures with four or eight Eu atoms as the most energetically favorable. These clusters exhibit the smallest volume increase compared to undoped ZnO and Eu coordination numbers of 5.2-5.5, all in excellent agreement with experimental data. ZnO defect states are crucial for efficient Eu³⁺ excitation, while c-Eu₂ O₃ phase separation results in loss of the characteristic Eu³⁺ photoluminescence. The formation of molecule-like Eu-oxide clusters, entrapped in ZnO, proposed here, may help in understanding the nature of the unexpected high doping levels of lanthanide ions in ZnO that occur virtually without significant change in ZnO unit cell dimensions.

Bimetallic CuPd alloy nanoparticles decorated ZnO nanosheets with enhanced photocatalytic degradation of methyl orange dye

Photocatalytic technology is widely explored as a promising alternative for water treatments. However, low photocatalytic efficiency and selectivity usually limit its practical application. Herein, we develop the synthesis of two-dimensional zinc oxide (ZnO) nanosheets decorated with copper (Cu)-palladium (Pd) bimetallic nanoparticles (NPs) for the degradation of organic dyes in an aqueous solution. Compared to pristine ZnO nanosheets, the prepared CuPd/ZnO composites exhibited superior performance for the photocatalytic degradation of organic dyes under visible-light irradiation. The remarkable improvement of degradation activity was attributable to the enhanced separation and transfer efficiency of photogenerated charge carriers. The highest catalytic efficiency of CuPd/ZnO nanocomposite with the CuPd content of 0.5 wt% exhibited 95.3% removal of methyl orange (MO) (40 mg/L) within 45 min. From the experimental data, we believe this study provides a new avenue for the design and fabrication of high-performance photocatalysts capable of water treatments.

First-Principles Insight into a B4C3 Monolayer as a Promising Biosensor for Exhaled Breath Analysis

Nanomaterial-based room temperature gas sensors are used as a screening tool for diagnosing various diseases through breath analysis. The stable planar structure of boron carbide (B₄C₃) is utilized as a base material for adsorption of human breath exhaled VOCs, namely formaldehyde, methanol, acetone, toluene along, with interfering gases of carbon dioxide and water. The adsorption energy, charge density, density of states, energy band gap variation, recovery time, sensitivity, and work function of adsorbed molecules on pristine B₄C₃ are analyzed by density functional theory. The computed adsorption energies of VOC are in the range of - 0.176 to - 0.238 eV, and a larger interaction distance validate the physisorption behavior of these VOCs biomarkers on pristine boron carbide monolayer. Minute changes are determined from the electronic band structure of all adsorbed systems conserving the semiconducting nature of the B₄C₃ monolayer. The band gap variation upon adsorption of VOCs and interfering gases is examined between 0.05 and 0.52%. The 13.63 × 10^-9 s recovery time of methanol is slower among VOCs, and 0.556 × 10^-9 s of carbon dioxide (CO₂) is faster for desorption. The results reveal that boron carbide can be utilized as a biosensor at room temperature for the analysis of exhaled VOCs from human breath.

Nanosensors Based on a Single ZnO:Eu Nanowire for Hydrogen Gas Sensing

Fast detection of hydrogen gas leakage or its release in different environments, especially in large electric vehicle batteries, is a major challenge for sensing applications. In this study, the morphological, structural, chemical, optical, and electronic characterizations of ZnO:Eu nanowire arrays are reported and discussed in detail. In particular, the influence of different Eu concentrations during electrochemical deposition was investigated together with the sensing properties and mechanism. Surprisingly, by using only 10 μM Eu ions during deposition, the value of the gas response increased by a factor of nearly 130 compared to an undoped ZnO nanowire and we found an H2 gas response of ∼7860 for a single ZnO:Eu nanowire device. Further, the synthesized nanowire sensors were tested with ultraviolet (UV) light and a range of test gases, showing a UV responsiveness of ∼12.8 and a good selectivity to 100 ppm H2 gas. A dual-mode nanosensor is shown to detect UV/H2 gas simultaneously for selective detection of H2 during UV irradiation and its effect on the sensing mechanism. The nanowire sensing approach here demonstrates the feasibility of using such small devices to detect hydrogen leaks in harsh, small-scale environments, for example, stacked battery packs in mobile applications. In addition, the results obtained are supported through density functional theory-based simulations, which highlight the importance of rare earth nanoparticles on the oxide surface for improved sensitivity and selectivity of gas sensors, even at room temperature, thereby allowing, for instance, lower power consumption and denser deployment.

Selectivity of volatile organic compounds on the surface of zinc oxide nanosheets for gas sensors

The detection of volatile organic compounds by gas sensors is of great interest for environmental quality monitoring and the early-stage and noninvasive diagnosis of diseases. Experiments found hexane, toluene, aniline, butanone, acetone, and propanol gases in the exhaled breath of patients suffering from COVID-19, lung cancer, and diabetes. However, no studies are available to systematically elucidate the selectivity of these gases on nanosheets of zinc oxide for chemiresistive and direct thermoelectric gas sensors. Therefore, this work performed the elucidation by studying the electronic, electrical, and thermal properties of the bilayered ZnO nanosheets with polar (0001) and non-polar (112̄0) surfaces under the adsorption of the gases. The interaction between the gases and the nanosheets belongs to two groups: electrostatic attraction and charge exchange. The second one occurs due to the peak resonance of the same type of orbitals between the substrates and the gases along the surface normal and the first one for the other cases. The characteristics of the Seebeck coefficient exhibited distinct selectivity of butanone and acetone.

Controllable construction of ZnFe2O4-based micro-nano heterostructure for the rapid detection and degradation of VOCs

Micro-nano heterogeneous oxides have received extensive attention due to their distinctive physicochemical properties. However, it is a challenge to prepare the hierarchical multicomponent metal oxide nanomaterials with abundant heterogeneous interfaces in a controllable way. In this work, the effective construction of the heterogeneous structure of the material is achieved by regulating the ratio of metal salts under thermal solvent condition. Three-dimensional spheres (ZnFe₂O₄) constructed by zero-dimensional ultra-small nanoparticles, in particular three-dimensional hollow sea urchin spheres (ZnO/ZnFe₂O₄) constructed by one-dimensional nanorods and three-dimensional hydrangeas (α-Fe₂O₃/ZnFe₂O₄) assembled by two-dimensional nanosheets were obtained. The two composite materials contain a large number of heterojunctions, which enhances the sensitivity of material to volatile organic compounds gas. Among them, the α-Fe₂O₃/ZnFe₂O₄ composite shows the best sensing performance for VOCs. For example, its response to 100 ppm acetone reaches 142 at 170 °C with the response time shortened to 3 s and the detection limit falling to 10 ppb. Meanwhile, the composite material presents a degradation rate of more than 90% for VOCs at a flow rate of 20 mL/min at 170 °C. In addition, the sensing and sensitivity mechanism of the composite material are studied in detail by combining GC-MS, XPS with UV diffuse reflectance tests.

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

Monitoring volatile organic compounds (VOCs) in harsh environments, especially for safety applications, is a growing field that requires specialized sensor structures. In this work, we demonstrate the sensing properties toward the most common VOCs of columnar Al₂O₃/ZnO heterolayer-based sensors. We have also developed an approach to tune the sensor selectivity by changing the thickness of the exposed amorphous Al₂O₃ layer from 5 to 18 nm. Columnar ZnO films are prepared by a chemical solution method, where the exposed surface is decorated with an Al₂O₃ nanolayer via thermal atomic layer deposition at 75 °C. We have investigated the structure and morphology as well as the vibrational, chemical, electronic, and sensor properties of the Al₂O₃/ZnO heterostructures. Transmission electron microscopy (TEM) studies show that the upper layers consist of amorphous Al₂O₃ films. The heterostructures showed selectivity to 2-propanol vapors only within the range of 12-15 nm thicknesses of Al₂O₃, with the highest response value of ∼2000% reported for a thickness of 15 nm at the optimal working temperature of 350 °C. Density functional theory (DFT) calculations of the Al₂O₃/ZnO(1010) interface and its interaction with 2-propanol (2-C₃H₇OH), n-butanol (n-C₄H₉OH), ethanol (C₂H₅OH), acetone (CH₃COCH₃), hydrogen (H₂), and ammonia (NH₃) show that the molecular affinity for the Al₂O₃/ZnO(1010) interface decreases from 2-propanol (2-C₃H₇OH) ≈ n-butanol (n-C₄H₉OH) > ethanol (C₂H₅OH) > acetone (CH₃COCH₃) > hydrogen (H₂), which is consistent with our gas response experiments for the VOCs. Charge transfers between the surface and the adsorbates, and local densities of states of the interacting atoms, support the calculated strength of the molecular preferences. Our findings are highly important for the development of 2-propanol sensors and to our understanding of the effect of the heterojunction and the thickness of the top nanolayer on the gas response, which thus far have not been reported in the literature.

Ag-Modified Porous Perovskite-Type LaFeO3 for Efficient Ethanol Detection

Perovskite (ABO₃) nanosheets with a high carrier mobility have been regarded as the best candidates for gas-sensitive materials arising from their exceptional crystal structure and physical–chemical properties that often exhibit good gas reactivity and stability. Herein, Ag in situ modified porous LaFeO₃ nanosheets were synthesized by the simple and efficient graphene oxide (GO)-assisted co-precipitation method which was used for sensitive and selective ethanol detection. The Ag modification ratio was studied, and the best performance was obtained with 5% Ag modification. The Ag/LaFeO₃ nanomaterials with high surface areas achieved a sensing response value (R_g/R_a) of 20.9 to 20 ppm ethanol at 180 °C with relatively fast response/recovery times (26/27 s). In addition, they showed significantly high selectivity for ethanol but only a slight response to other interfering gases. The enhanced gas-sensing performance was attributed to the combination of well-designed porous nanomaterials with noble metal sensitization. The new approach is provided for this strategy for the potential application of more P-type ABO₃ perovskite-based gas-sensitive devices.

Insights into ZnO-based doped porous nanocrystal frameworks

Colloidal nanocrystals play a vital role in several applications. The doping of cations in the nanocrystal matrix enhances the optical, electrical, and magnetic properties. The number and well-defined distribution of the dopant are crucial to protect the nanocrystal from clustering. The XRD, XPS, and XAS instruments reveal the change in the lattice parameters, chemical states, and local coordination environment information. In addition of detecting the position and distribution of the dopant, the 4D-STEM detector mode gathers all types of real-space atomic-resolution images by collecting all diffraction datasets from each electron probe with high-speed and efficient detection. Dopant-host ligand type, reactions conditions, and reaction time optimization during synthesis are critical for the host and dopant reactivity balance. Pearson's hard/soft acids/bases theory would be a base for balancing the solubility of the dopant-host in the given solvents/surfactant. In addition, tuning the colloidal nanocrystals to secondary structures, which enhances the mass-/ions transport, can contribute a combination of properties that do not exist in the original constituents.

Low-operating temperature and remarkably responsive methanol sensors using Pt-decorated hierarchical ZnO structure

Highly responsive methanol sensors working at low temperatures are developed using hierarchical ZnO nanorods decorated by Pt nanoparticles. The sensing materials are fabricated following a 3-step process: electrospinning of ZnO nanofibers, hydrothermal growth of hierarchical ZnO nanorods on the nanofibers and UV-assisted deposition of Pt nanoparticles. The morphology, structure and properties of the materials are examined by field-effect scanning electron microscopy, transmission electron microscope, x-ray diffraction, x-ray photoelectron spectroscopy, UV-Vis absorption spectroscopy, and electrical measurements. The methanol sensing performance is investigated at different working temperatures in the range of 110 °C-260 °C. It is observed that the surface modification of the ZnO hierarchical nanorods by Pt nanoparticles results in a remarkable enhancement of the sensing response toward methanol, which can reach approximately 19 500 times higher than that of the unmodified ZnO nanorods-based sensor. In addition, this modification enables lower working temperatures with an optimum range of 140 °C-200 °C. Based on the achieved results, a methanol sensing mechanism of the Pt/ZnO structure is proposed.

Effect of Ag Addition on the Gas-Sensing Properties of Nanostructured Resistive-Based Gas Sensors: An Overview

Nanostructured semiconducting metal oxides (SMOs) are among the most popular sensing materials for integration into resistive-type gas sensors owing to their low costs and high sensing performances. SMOs can be decorated or doped with noble metals to further enhance their gas sensing properties. Ag is one of the cheapest noble metals, and it is extensively used in the decoration or doping of SMOs to boost the overall gas-sensing performances of SMOs. In this review, we discussed the impact of Ag addition on the gas-sensing properties of nanostructured resistive-based gas sensors. Ag-decorated or -doped SMOs often exhibit better responsivities/selectivities at low sensing temperatures and shorter response times than those of their pristine counterparts. Herein, the focus was on the detection mechanism of SMO-based gas sensors in the presence of Ag. This review can provide insights for research on SMO-based gas sensors.

TiO2/Cu2O/CuO Multi-Nanolayers as Sensors for H2 and Volatile Organic Compounds: An Experimental and Theoretical Investigation

TiO₂/Cu₂O/CuO multi-nanolayers highly sensitive toward volatile organic compounds (VOCs) and H₂ have been grown in various thicknesses by a cost-effective and reproducible combined spray-sputtering-annealing approach. The ultrathin TiO₂ films were deposited by spray pyrolysis on top of sputtered-annealed Cu₂O/CuO nanolayers to enhance their gas sensing performance and improve their protection against corrosion at high operating temperatures. The prepared heterostructures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and ultraviolet visible (UV-vis) and micro-Raman spectroscopy. The gas sensing properties were measured at several operating temperatures, where the nanolayered sensors with oxide thicknesses between 20 and 30 nm (Cu₂O/CuO nanolayers) exhibited a high response and an excellent selectivity to ethanol vapor after thermal annealing the samples at 420 °C. The results obtained at an operating temperature of 350 °C demonstrate that the CuO/Cu₂O nanolayers with thicknesses between 20 and 30 nm are sensitive mainly to ethanol vapor, with a response of ∼150. The response changes from ethanol vapors to hydrogen gas as the thickness of the CuO/Cu₂O nanolayers changes from 50 to 20 nm. Density functional theory-based calculations were carried out for the geometries of the CuO(1̅11)/Cu₂O(111) and TiO₂(111)/CuO(1̅11)/Cu₂O(111) heterostructures and their sensing mechanism toward alcohols of different chain lengths and molecular hydrogen. The reconstructed hexagonal Cu₂O(111) surface and the reconstructed monoclinic CuO(1̅11) and TiO₂(111) facets, all of which terminate in an O layer, lead to the lowest surface energies for each isolated material. We studied the formation of the binary and ternary heteroepitaxial interfaces for the surface planes with the best-matching lattices. Despite the impact of the Cu₂O(111) substrate in lowering the atomic charges of the CuO(1̅11) adlayer in the binary sensor, we found that it is the different surface structures of the CuO(1̅11)/Cu₂O(111) and TiO₂(111)/CuO(1̅11)/Cu₂O(111) devices that are fundamental in driving the change in the sensitivity response observed experimentally. The experimental data, supported by the computational results, are important in understanding the use of the multi-nanolayered films tested in this work as reliable, accurate, and selective sensor structures for the tracking of gases at low concentrations.

A high-response formaldehyde sensor based on fibrous Ag-ZnO/In2O3 with multi-level heterojunctions

Timely detection of formaldehyde is pivotal because formaldehyde is slowly released from the indoor decorative materials, jeopardizing our healthy. Herein, a high-response formaldehyde gas sensor based on Ag-ZnO/In₂O₃ nanofibers was successfully fabricated. Compared with all the control samples, the hybrid exhibits superior sensitivity (0.65 ppm^-1), excellent selectivity (≥ 12.5) and durable stability (the deviation value ≤ 3%). Particularly, an ultra-high response value of about 186 towards 100 ppm of formaldehyde at 260 °C was achieved, heading the list of outstanding candidates. Additionally, the limit of detection is as low as 9 ppb. The enhanced gas sensing properties can be mainly attributed to multi-level heterojunctions (n-n heterojunction and Ohmic junction) and the "spill-over" effect of Ag, ultimately increasing the adsorption of gas molecules on the surface of sensing material. This work verifies that proper design of multi-level heterojunctions significantly upgrade the sensing performance.

The role of surface oxidation and Fe-Ni synergy in Fe-Ni-S catalysts for CO2 hydrogenation

Increasing carbon dioxide (CO₂) emissions, resulting in climate change, have driven the motivation to achieve the effective and sustainable conversion of CO₂ into useful chemicals and fuels. Taking inspiration from biological processes, synthetic iron-nickel-sulfides have been proposed as suitable catalysts for the hydrogenation of CO₂. In order to experimentally validate this hypothesis, here we report violarite (Fe,Ni)₃S₄ as a cheap and economically viable catalyst for the hydrogenation of CO₂ into formate under mild, alkaline conditions at 125 °C and 20 bar (CO₂ : H₂ = 1 : 1). Calcination of violarite at 200 °C resulted in excellent catalytic activity, far superior to that of Fe-only and Ni-only sulfides. We further report first principles simulations of the CO₂ conversion on the partially oxidised (001) and (111) surfaces of stoichiometric violarite (FeNi₂S₄) and polydymite (Ni₃S₄) to rationalise the experimentally observed trends. We have obtained the thermodynamic and kinetic profiles for the reaction of carbon dioxide (CO₂) and water (H₂O) on the catalyst surfaces via substitution and dissociation mechanisms. We report that the partially oxidised (111) surface of FeNi₂S₄ is the best catalyst in the series and that the dissociation mechanism is the most favourable. Our study reveals that the partial oxidation of the FeNi₂S₄ surface, as well as the synergy of the Fe and Ni ions, are important in the catalytic activity of the material for the effective hydrogenation of CO₂ to formate.

Inkjet Printed ZnO Sensors for Early Prostate Cancer Detection by Means of Urine Odor Analysis

In the second half of the 20th century, inkjet printing has rapidly evolved from a mere digital image reproduction technique into a highly versatile materials deposition technology. Recently, inkjet production of metal oxide-based gas sensors (MOS) has emerged, and inkjet sensors have been proposed for different applications. In this context, this paper proposes the adoption of inkjet MOS sensors for urine analysis for the purpose of developing an innovative tool for the early prostate cancer (KP) diagnosis. Based on the results reported in the scientific literature, proving the correlation between urine color and odor alterations and cancer, the study focuses on the analysis by means of MOS sensor array of odors emanated from urine samples from prostate cancer patients versus healthy donors. The results achieved in terms of diagnostic accuracy, sensitivity, and specificity (i.e., about 80%) proved the potentialities of inkjet MOS sensor for the challenging purpose of detection of prostate cancer in its early stages. Resulting significantly more powerful than current KP diagnostic protocol, the approach here presented might become, after validation, a rapid, non-invasive and low-cost screening tool, capable to provide a solution to the problems of patients’ overtreatment and high health spending associated to procedures currently adopted.

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.

ZnO/CuSCN Nano-Heterostructure as a Highly Efficient Field Emitter: a Combined Experimental and Theoretical Investigation

We report the synthesis of two-dimensional porous ZnO nanosheets, CuSCN nanocoins, and ZnO/CuSCN nano-heterostructure thin films grown on fluorine-doped tin oxide substrates via two simple and low-cost solution chemical routes, i.e., chemical bath deposition and successive ionic layer adsorption and reaction methods. Detail characterizations regarding the structural, optoelectronic, and morphological properties have been carried out, which reveal high-quality and crystalline synthesized materials. Field emission (FE) investigations performed at room temperature with a base pressure of 1 × 10^-8 mbar demonstrate superior FE performance of the ZnO/CuSCN nano-heterostructure compared to the isolated porous ZnO nanosheets and CuSCN nanocoins. For instance, the turn-on field required to draw a current density of 10 μA/cm² is found to be 2.2, 1.1, and 0.7 V/μm for the ZnO, CuSCN, and ZnO/CuSCN nano-heterostructure, respectively. The observed significant improvement in the FE characteristics (ultralow turn-on field of 0.7 V/μm for an emission current density of 10 μA/cm² and the achieved high current density of 2.2 mA/cm² at a relatively low applied electric field of 1.8 V/μm) for the ZnO/CuSCN nano-heterostructure is superior to the isolated porous ZnO nanosheets, CuSCN nanocoins, and other reported semiconducting nano-heterostructures. Complementary first-principles density functional theory calculations predict a lower work function for the ZnO/CuSCN nano-heterostructure (4.58 eV), compared to the isolated ZnO (5.24 eV) and CuSCN (4.91 eV), validating the superior FE characteristics of the ZnO/CuSCN nano-heterostructure. The ZnO/CuSCN nanocomposite could provide a promising class of FE cathodes, flat panel displays, microwave tubes, and electron sources.