Porous Ga-In Bimetallic Oxide Nanofibers with Controllable Structures for Ultrasensitive and Selective Detection of Formaldehyde

Unraveling the Effect of Oxygen Vacancy on WO3 Surface for Efficient NO2 Detection at Low Temperature

Oxygen vacancies (VO) in metal oxide semiconductors play an important role in improving gas-sensing performance of chemiresistive gas sensors. Nonetheless, there is still a lack of clear understanding of the inherent mechanism of the influence of oxygen vacancies on gas sensing due to generally focusing on the concentration of VO. Herein, oxygen vacancies were rationally modulated in WO3 nanoflower structures via an annealing process, resulting in a transformation of VO from neutral (VO0) to a doubly ionized (VO2+) state. Density functional theory (DFT) calculations indicate that VO2+ is significantly more efficient than VO0 for NO2 detection in competition with atmospheric O2. Benefiting from a high concentration of VO2+, the WO3-450 (WO3 annealed at 450 °C) sensor exhibits excellent sensing performance with an ultrahigh sensitivity (3674.1 to 5 ppm NO2), superior selectivity, and long-term stability (one month). Furthermore, the sensor with the wide range of concentration detection not only can detect NO2 gas with parts per million (ppm) but also can detect NO2 with parts per billion (ppb) level concentration, with a high sensibility reaching 2.8 to 25 ppb NO2 and over 100 to 100 ppb NO2. This study elucidates the oxygen vacancy mediated sensing mechanism toward NO2 and provides an effective strategy for the rational design of gas sensors with high sensing performance.

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.

Essential role of lattice oxygen in hydrogen sensing reaction

Understanding the sensing mechanism of metal oxide semiconductors is imperative to the development of high-performance sensors. The traditional sensing mechanism only recognizes the effect of surface chemisorbed oxygen from the air but ignores surface lattice oxygen. Herein, using in-situ characterizations, we provide direct experimental evidence that the surface chemisorbed oxygen participated in the sensing process can come from lattice oxygen of the oxides. Further density functional theory (DFT) calculations prove that the p-band center of O serves as a state of art for regulating the participation of lattice oxygen in gas-sensing reactions. Based on our experimental data and theoretical calculations, we discuss mechanisms that are fundamentally different from the conventional mechanism and show that the easily participation of lattice oxygen is helpful for the high response value of the materials.

Facile preparation of Au-loaded mesoporous In2O3 nanoparticles with improved ethanol sensing performance

The in situ monitoring of toxic volatile organic compound gases using metal oxide-based gas sensors is still challenging. Herein, mesoporous In2O3 nanoparticles, assembled using smaller nanoparticles, were synthesized via a facile solvothermal method and used to load Au nanoparticles to prepare mesoporous Au/In2O3 for ethanol detection. The obtained In2O3 and Au/In2O3 were meticulously analysed by XRD, SEM, BET, TEM and XPS techniques. It was revealed that Au nanoparticles were uniformly distributed on mesoporous In2O3 nanoparticles. Notably, the obtained mesoporous 1% Au/In2O3 is highly sensitive to ethanol gas at an optimal working temperature of 180 °C, showing a response of 55 to 50 ppm of ethanol, which is considerably higher compared to that of In2O3 nanoparticles. The significantly enhanced sensitivity results from the electronic and chemical sensitization effects of Au nanoparticles. Moreover, the mesoporous Au/In2O3 nanoparticles also showed eminent selectivity, short response/recovery time, low detection limit, good linear relationship, superb repeatability, and wonderful long-term stability, suggesting that Au/In2O3 nanoparticles have great potential application for in situ monitoring of ethanol gas.

Improvement of the CO2 Sensitivity: HPTS-Based Sensors along with Zn@SnO2 and Sn@ZnO Additives

Fluorescent pH-sensitive indicator dye, 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS), has become known as a preferred alternative for continuous and accurate monitoring of dissolved and/or gaseous CO2 in chemistry, medical, and biochemical research. The objective of this work is to enhance the HPTS dye's CO2 sensitivity in the presence of Zn@SnO2 and Sn@ZnO additive particles. Sol-gel synthesized metal oxide semiconductors (MOSs) were characterized using XRD, XPS, and SEM. The fluorophore dye and the MOS additives were embedded in the ethyl cellulose (EC) polymeric matrix to prepare the sensing thin films. The steady-state and decay kinetic measurements of the HPTS-based composites were obtained by PL spectroscopy for the concentration ranges of 0-100% p[CO2]. As expected, the addition of MOSs improves the sensor characteristics, specifically its CO2 sensing ability, linear response range, and relative signal change compared to the free form of HPTS. The CO2 sensitivities of the HPTS-based thin films were found at 17.6, 23.2, and 40.9 for the undoped, Zn@SnO2,-doped, and Sn@ZnO-doped forms of the HPTS, respectively. Additionally, the response and recovery times of the HPTS-based sensor agent with Sn@ZnO were measured as 10 and 460 s, respectively. The obtained results demonstrate that materials composed of HPTS with MOSs are potential candidates for CO2 sensors.

Rare-earth-doped indium oxide nanosphere-based gas sensor for highly sensitive formaldehyde detection at a low temperature

Formaldehyde (HCHO) is widely viewed as a carcinogenic volatile organic compound in indoor air pollution that can seriously threaten human health and life. Thus, there is a critical need to develop gas sensors with improved sensing performance, including outstanding selectivity, low operating temperature, high responsiveness, and short recovery time, for HCHO detection. Currently, doping is considered an effective strategy to raise the sensing performance of gas sensors. Herein, various rare earth elements-doped indium oxide (RE-In₂O₃) nanospheres were fabricated as gas sensors for improved HCHO detection via a facile and environmentally solvothermal method. Such RE-In₂O₃ nanosphere-based sensors exhibited remarkable gas-sensing performance, including a high selectivity and stability in air. Compared with pure, Yb-, Dy-doped In₂O₃ and different La ratios doped into In₂O₃, 6% La-doped In₂O₃ (La-In₂O₃) nanosphere-based sensors demonstrated a high response value of 210 to 100 ppm at 170 °C, which was around 16 times higher than that of the pure In₂O₃ sensor, and also exhibited a detection limit of 10.9 ppb, and a response time of 30 s to 100 ppm HCHO with a recovery time of 160 s. Finally, such superior sensing performance of the 6% La-In₂O₃ sensors was proposed to be attributed to the synergistic effect of the large specific surface area and enhanced surface oxygen vacancies on the surface of In₂O₃ nanospheres, which produced chemisorbed oxygen species to release electrons and provided abundant reaction sites for HCHO gas. This study sheds new light on designing nanomaterials to build gas sensors for HCHO detection.

Recent advances in determination applications of emerging films based on nanomaterials

Sensitive and facile detection of analytes is crucial in various fields such as agriculture production, food safety, clinical diagnosis and therapy, and environmental monitoring. However, the synergy of complicated sample pretreatment and detection is an urgent challenge. By integrating the inherent porosity, processability and flexibility of films and the diversified merits of nanomaterials, nanomaterial-based films have evolved as preferred candidates to meet the above challenge. Recent years have witnessed the flourishment of films-based detection technologies due to their unique porous structures and integrated physical/chemical merits, which favors the separation/collection and detection of analytes in a rapid, efficient and facile way. In particular, films based on nanomaterials consisting of 0D metal-organic framework particles, 1D nanofibers and carbon nanotubes, and 2D graphene and analogs have drawn increasing attention due to incorporating new properties from nanomaterials. This paper summarizes the progress of the fabrication of emerging films based on nanomaterials and their detection applications in recent five years, focusing on typical electrochemical and optical methods. Some new interesting applications, such as point-of-care testing, wearable devices and detection chips, are proposed and emphasized. This review will provide insights into the integration and processability of films based on nanomaterials, thus stimulate further contributions towards films based on nanomaterials for high-performance analytical-chemistry-related applications.

Gallium Oxide for Gas Sensor Applications: A Comprehensive Review

Ga₂O₃ has emerged as a promising ultrawide bandgap semiconductor for numerous device applications owing to its excellent material properties. In this paper, we present a comprehensive review on major advances achieved over the past thirty years in the field of Ga₂O₃-based gas sensors. We begin with a brief introduction of the polymorphs and basic electric properties of Ga₂O₃. Next, we provide an overview of the typical preparation methods for the fabrication of Ga₂O₃-sensing material developed so far. Then, we will concentrate our discussion on the state-of-the-art Ga₂O₃-based gas sensor devices and put an emphasis on seven sophisticated strategies to improve their gas-sensing performance in terms of material engineering and device optimization. Finally, we give some concluding remarks and put forward some suggestions, including (i) construction of hybrid structures with two-dimensional materials and organic polymers, (ii) combination with density functional theoretical calculations and machine learning, and (iii) development of optical sensors using the characteristic optical spectra for the future development of novel Ga₂O₃-based gas sensors.

Reversible mechanochoromic studies on AIE-inspired Smart materials and applications on HCHO sensing

Smart fluorescent materials that respond to external stimuli have received more and more attention because of their excellent optical properties in the field of anti-counterfeiting information security and fluorescent sensing....

Electronic and morphological dual modulation of NiO by indium-doping for highly improved xylene sensing

Ultrathin indium-doped NiO nanosheets with simultaneously optimized nanostructures and electronic properties were developed to achieve a high response to a low concentration of xylene.

Selectivity in trace gas sensing: recent developments, challenges, and future perspectives

Selectivity is one of the most crucial figures of merit in trace gas sensing, and thus a comprehensive assessment is necessary to have a clear picture of sensitivity, selectivity, and their interrelations in terms of quantitative and qualitative views.

Microstructure and Optical Properties of Nanostructural Thin Films Fabricated through Oxidation of Au-Sn Intermetallic Compounds

AuSn and AuSn₂ thin films (5 nm) were used as precursors during the formation of semiconducting metal oxide nanostructures on a silicon substrate. The nanoparticles were produced in the processes of annealing and oxidation of gold–tin intermetallic compounds under ultra-high vacuum conditions. The formation process and morphology of a mixture of SnO₂ and Au@SnO_x (the core–shell structure) nanoparticles or Au nanocrystalites were carefully examined by means of spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX). The annealing and oxidation of the thin film of the AuSn intermetallic compound led to the formation of uniformly distributed structures with a size of ∼20–30 nm. All of the synthesized nanoparticles exhibited a strong absorption band at 520–530 nm, which is typical for pure metallic or metal oxide systems.

Olivine-type cadmium germanate: a new sensing semiconductor for the detection of formaldehyde at the ppb level

In this study, for the first time, olivine-structured Cd2GeO4 was identified as an excellent formaldehyde sensing material, with a low detection limit of 60 ppb.

Atomically Dispersed Au on In2O3 Nanosheets for Highly Sensitive and Selective Detection of Formaldehyde

As an important industrial chemical, formaldehyde is used in various fields but is harmful to health. Developing a convenient detection device for formaldehyde is significant. Based on atomically dispersed Au on In₂O₃ nanosheets, a formaldehyde sensor was fabricated in this work. The highly dispersed Au obtained by the ultraviolet (UV) light-assisted reduction method helps improve the sensing performance. A meager loading amount (0.01 wt %) of Au on In₂O₃ nanosheets exhibits high sensitivity toward ppb-level formaldehyde. Au acts as an electron sink and promotes the oxidation of formaldehyde. Atomically dispersed Au on In₂O₃ nanosheets decreases the activation energy and increases the number of active sites, which result in a highly efficient conversion of formaldehyde and a marked resistance change of the fabricated sensors. The selective adsorption and oxidation of formaldehyde on single atom Au's uniform sites establish excellent selectivity. Besides, the sensor exhibits short response/recovery time and excellent stability, with promising applications in formaldehyde detection.

Synthesis of SnO2 nanoparticles for formaldehyde detection with high sensitivity and good selectivity

During the detection of industrial hazardous gases, like formaldehyde (HCHO), the selectivity is still a challenging issue. Herein, an alternative HCHO chemosensor that based on the tin oxide nanoparticles is proposed, which was obtained through a facile hydrothermal method. Gas sensing performances showed that the optimal working temperature located at only 180 °C, the response value of 79 via 50 ppm HCHO was much higher than that of 35 at 230 °C. However, the compromised test temperature was selected as 230 °C, taking into account the faster response/recovery speeds than 180 °C, named 20/23versus 53/60 s, respectively. The response (35) of the SnO₂ nanoparticles-based sensor to 50 ppm of HCHO is about 400% higher than that of bulk SnO₂ sensor (9), especially when the gas concentration is 1 ppm, SnO₂ nanoparticles also has a higher sensitivity which may possibly result from more exposed active sites and small size effect for nanoparticles than for bulk ones. The gas sensor based on SnO₂ nanoparticles can be utilized as a promising candidate for practical low-temperature detectors of HCHO due to its higher gas response, excellent response-recovery properties, and perfect selectivity.

Ultra-high responsivity (>12.34 kA W-1) of Ga-In bimetallic oxide nanowires based deep-UV photodetector

This work demonstrates the application of bimetallic oxide nanowires (NWs) for deep UV-photodetectors. The Ga-In based NWs have been fabricated using a low cost and scalable electrospinning method. The fabrication process includes the synthesis of NWs on cleaned quartz substrates using electrospinning, followed by Al (100 nm)/Au (20 nm)-electrode deposition on top of the fine sheet of NWs. Due to the very large surface to volume ratio, high porosity, photon trapping and simultaneous front and back illumination, the fabricated bimetallic photodetector demostrates a record ultra-high responsivity and the photo-dark current ratio of ~12 348 AW^-1 and ~298 846 at 1 V bias respectively. The device has further revealed high detectivity ~3.27 × 10¹⁵ mHz^0.5 W^-1 and external quantum efficiency ~7.73 × 10⁶%.

Effects of Sulfuric Acid Treatment on the Performance of Ga-Al2O3 for the Hydrolytic Decomposition of 1,1,1,2-Tetrafluoroethane (HFC-134a)

HFC-134a, one of the representative hydrofluorocarbons (HFCs) used as a coolant gas, is a known greenhouse gas with high global warming potential. Catalytic decomposition is considered a promising technology for the removal of fluorinated hydrocarbons. However, systematic studies on the catalytic decomposition of HFC-134a are rare compared to those for other fluorinated hydrocarbon gases. In this study, Ga-Al2O3 and S/Ga-Al2O3 catalysts were prepared and the change in their properties post-acid treatment was investigated by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), temperature-programmed desorption of ammonia (NH3-TPD), in situ Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS). The S/Ga-Al2O3 catalyst achieved a much higher HFC-134a conversion than Ga-Al2O3, which was ascribed to the promotional effect of the sulfuric acid treatment on the Lewis acidity of the catalyst surface, as confirmed by NH3-TPD. Furthermore, the effect of hydrogen fluoride (HF) gas produced by HFC-134a decomposition on the catalyst was investigated. The S/Ga-Al2O3 maintained a more stable and higher HFC-134a conversion than Ga-Al2O3. Combining the results of the stability test and characterization, it was established that the sulfuric acid treatment not only increased the acidity of the catalyst but also preserved the partially reduced Ga species.

ZnO-Decorated In/Ga Oxide Nanotubes Derived from Bimetallic In/Ga MOFs for Fast Acetone Detection with High Sensitivity and Selectivity

The development of acetone gas sensors is desirable but challenging for both air quality monitoring and medical diagnosis. Herein, starting from bimetallic In/Ga metal-organic frameworks (MOFs) (MIL-68 (In/Ga)), a facile strategy is proposed to couple with zinc ions to design In/Ga oxide (IGO)@ZnO core-shell nanotubes for efficient acetone detection. In such a heterostructure, tiny ZnO nanoparticles are closely decorated on IGO nanotubes, which is beneficial to enlarge the specific surface area and create rich oxygen vacancies and heterojunction interfaces. Benefiting from the structural merits and synergetic effects, the IGO@ZnO-based gas sensor exhibits a low detection limitation (200 ppb), a high response, good linearity relationship between the sensing responses and wide testing acetone concentrations, and fast response and recovery time (6.8/6.1 s) with good selectivity and stability. These sensing performances strongly indicate the practical application to quantitatively detect acetone.

Pentagram-Shaped Ag@Pt Core-Shell Nanostructures as High-Performance Catalysts for Formaldehyde Detection

High-performance HCHO sensors are of great importance in various application fields such as indoor air quality assessments. Herein, bimetallic Ag-Pt nanoparticles are synthesized as high-performance catalysts for ZnO-based gas sensors. Spherical aberration (Cs)-corrected transmission electron microscopy images with atomic resolution clearly indicate that the prepared nanoparticles exhibit a novel Ag@Pt core-shell nanostructure with a pentagram shape. For high-performance HCHO sensor construction, integrated micro-electrodes are first fabricated with the microelectromechanical system (MEMS) technology. Then, the hydrothermal route is used to self-assemble well-aligned ZnO nanowire arrays onto the sensing microregion. After that, the pentagram-shaped Ag@Pt nanoparticles are loaded onto the surface of ZnO nanowires with the inkjet printing technique to form MEMS sensors with Ag@Pt@ZnO as the sensing material. The thoroughly sensing experiments indicate that the Ag@Pt nanoparticles exhibit satisfied catalytic activation to HCHO molecules. The experimental observed detection limit of our sensor to HCHO reaches the parts per billion level. To elucidate the HCHO-sensing mechanism, the online mass spectrum (online MS) is utilized to analyze the components of exhaust gas stream of HCHO flowing through the Ag@Pt@ZnO material. The online MS indicates that with the Ag@Pt catalyst, HCHO molecules are partially oxidized to HCOOH molecules at low temperatures and are completely oxidized to CO₂ molecules at high temperatures.

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

In recent years, it is urgent and challenging to fabricate highly sensitive and selective gas sensors for breath analyses. In this work, Sr-doped cubic In₂O₃/rhombohedral In₂O₃ homojunction nanowires (NWs) are synthesized by one-step electrospun technology. The Sr doping alters the cubic phase of pure In₂O₃ into the rhombohedral phase, which is verified by the high-resolution transmittance electron microscopy, X-ray diffraction, and Raman spectroscopy, and is attributable to the low cohesive energy as calculated by the density functional theory (DFT). As a proof-of-concept of fatty liver biomarker sensing, ethanol sensors are fabricated using the electrospun In₂O₃ NWs. The results show that 8 wt % Sr-doped In₂O₃ shows the highest ethanol sensing performance with a high response of 21-1 ppm, a high selectivity over other interfering gases such as methanol, acetone, formaldehyde, toluene, xylene, and benzene, a high stability measured in 6 weeks, and also a high resistance to high humidity of 80%. The outstanding ethanol sensing performance is attributable to the enhanced ethanol adsorption by Sr doping as calculated by DFT, the stable rhombohedral phase and the preferred (104) facet exposure, and the formed homojunctions favoring the electron transfer. All these results show the effective structural modification of In₂O₃ by Sr doping, and also the great potency of the homojunction Sr-doped In₂O₃ NWs for highly sensitive, selective, and stable breath ethanol sensing.

Ultraviolet-induced Ostwald ripening strategy towards a mesoporous Ga2O3/GaOOH heterojunction composite with a controllable structure for enhanced photocatalytic hydrogen evolution

The design of efficient semiconductor oxide materials with heterojunction nanostructures for photocatalysis holds great promise in the fields of clean energy conversion and storage.

Gas sensing mechanisms of metal oxide semiconductors: a focus review

In recent years, gas sensors have been increasingly used in industrial production and daily life. Metal oxide semiconductor gas sensing materials are favoured for their outstanding physical and chemical properties, low cost and simple preparation methods. However, the gas sensing mechanisms of metal oxide semiconductors have not been considered by researchers, resulting in omissions and errors in the interpretation of gas sensing mechanisms in many articles. This review organizes and introduces several common gas sensing mechanisms of metal oxide semiconductors in detail and classifies them into two categories. The scope and relationship of these mechanisms are clarified. In addition, this review selects four strategies for enhancing the gas sensing properties of metal oxide semiconductors and analyses the gas sensing mechanisms to highlight the importance of the gas sensing mechanism. Finally, some perspectives for future investigations on the gas sensing mechanisms of metal oxide semiconductors are discussed as well.

Graphene quantum dot-functionalized three-dimensional ordered mesoporous ZnO for acetone detection toward diagnosis of diabetes

The development of a high-performance semiconductor oxide sensor for the accurate detection of trace disease biomarkers in exhaled breath is still a challenge that urgently needs to be addressed. Here, we proposed a self-assembly strategy and spin-coating process to create a graphene quantum dot (GQD)-functionalized three-dimensional ordered macroporous (3DOM) ZnO structure. The strong synergistic effect and the p-n heterojunction between the p-type GQDs and n-type ZnO effectively enlarged the resistance variation due to the change in oxygen adsorption. The specific 3DOM structure induced a hierarchical pore size (286 nm in macroscale and 26 nm in mesoscale) and 3D interconnection, which guaranteed high gas accessibility and fast carrier transportation. As a result, the GQD-modified 3DOM ZnO sensor exhibited a remarkably high response (Rair/Rgas = 15.2 for 1 ppm acetone), rapid response/recovery time (9/16 s), extremely low theoretical detection limit (8.7 ppb), and good selectivity towards acetone against other interfering gases. In particular, the proposed sensor could accurately distinguish trace acetone in the simulated breath of diabetic patients. These results demonstrate a high potential for the feasibility of the GQD-modified 3DOM SMO structure as a new sensing material for the possibility of noninvasive real-time diagnosis of diabetes.

Reduced Graphene Oxide-Coated Si Nanowires for Highly Sensitive and Selective Detection of Indoor Formaldehyde

Although significant developments have been made in the low-concentration formaldehyde monitoring in indoor air by using gas sensors, they still suffer from insufficient performance for achieving ppb-level detection. In this work, <100> oriented Si nanowires (SiNWs) with high specific surface area were prepared via metal-assisted chemical etching method (MACE), and then were uniformly coated with graphene oxide (GO) followed by the subsequent reductive process in H₂/Ar atmosphere at 800 °C to obtain reduced graphene oxide (RGO). The RGO coating (RGO@n-SiNWs) obviously enhances SiNWs sensitivity to low-concentration formaldehyde, benefiting from the increased specific surface area, the sensitization effect of RGO, and the formation of p-n junction between SiNWs and RGO. Specifically, RGO@n-SiNWs exhibits a high response of 6.4 to 10 ppm formaldehyde at 300 °C, which is about 2.6 times higher than that of pristine SiNWs (~ 2.5). Furthermore, the RGO@n-SiNWs show a high response of 2.4 to 0.1 ppm formaldehyde which is the largest permissive concentration in indoor air, a low detection limit of 35 ppb obtained by non-linear fitting, and fast response/recovery times of 30 and 10 s. In the meanwhile, the sensor also shows high selectivity over other typical interfering gases such as ethanol, acetone, ammonia, methanol, xylene, and toluene, and shows a high stability over a measurement period of 6 days. These results enable the highly sensitive, selective, and stable detection of low-concentration formaldehyde to guarantee safety of indoor environment.

Realizing the Control of Electronic Energy Level Structure and Gas-Sensing Selectivity over Heteroatom-Doped In2O3 Spheres with an Inverse Opal Microstructure

Understanding the effect of substitutional doping on gas-sensing performances is essential for designing high-activity sensing nanomaterials. Herein, formaldehyde sensors based on gallium-doped In₂O₃ inverse opal (IO-(Ga _xIn_{1- x})₂O₃) microspheres were purposefully prepared by a simple ultrasonic spray pyrolysis method combined with self-assembled sulfonated polystyrene sphere templates. The well-aligned inverse opal structure, with three different-sized pores, plays the dual role of accelerating the diffusion of gas molecules and providing more active sites. The Ga substitutional doping can alter the electronic energy level structure of (Ga _xIn_{1- x})₂O₃, leading to the elevation of the Fermi level and the modulation of the band gap close to a suitable value (3.90 eV), hence, effectively optimizing the oxidative catalytic activity for preferential CH₂O oxidation and increasing the amount of adsorbed oxygen. More importantly, the gas selectivity could be controlled by varying the energy level of adsorbed oxygen. Accordingly, the IO-(Ga_0.2In_0.8)₂O₃ microsphere sensor showed a high response toward formaldehyde with fast response and recovery speeds, and ultralow detection limit (50 ppb). Our findings finally offer implications for designing Fermi level-tailorable semiconductor nanomaterials for the control of selectivity and monitoring indoor air pollutants.

Tailoring energy level and surface basicity of metal oxide semiconductors by rare-earth incorporation for high-performance formaldehyde detection

The elevated Fermi level and increased surface basicity of 5Y-In₂O₃ led to the improvement of response and selectivity towards formaldehyde.

Amorphous Europium Hexaboride: A Potential Room Temperature Formaldehyde Sensing Material

Amorphous EuB₆ was successfully prepared having a high specific surface area (221.3 m² g^-1) via the reaction between EuCl₃ and B₂H₆ in the presence of liquid plasma in an ionic liquid environment. The material exhibits an immediate, lasting, and highly selective response toward formaldehyde at room temperature with a detection limit of 50 ppb. The good sensing performance of the amorphous EuB₆ material is attributed to the strong interaction between formaldehyde and the increased number of accessible electron-rich surface Eu sites.

Novel Self-Assembly Route Assisted Ultra-Fast Trace Volatile Organic Compounds Gas Sensing Based on Three-Dimensional Opal Microspheres Composites for Diabetes Diagnosis

The development of ultra-fast response semiconductor gas sensors for high-accuracy detection of trace volatile organic compounds in human exhaled breath still remains a challenge. Herein, we propose a novel self-assembly synthesis concept for preparing intricate three-dimensional (3D) opal porous (OP) SnO₂-ZnO hollow microspheres (HM), by employing sulfonated polystyrene (S-PS) spheres template-assisted ultrasonic spray pyrolysis. The high gas accessibility of the unique opal hollow structures resulted in the existence of 3D interconnection and bimodal (mesoscale and macroscale) pores, and the n-n heterojunction-induced change in oxygen adsorption. The 3D OP SnO₂-ZnO HM sensor exhibited high response and ultra-fast dynamic process (response time ∼4 s and recovery time ∼17 s) to 1.8 ppm acetone under highly humid ambient condition (98% relative humidity), and it could rapidly identify the states of the exhaled breath of healthy people and simulated diabetics. In addition, the rational structure design of the 3D OP SnO₂ HM enables the ultra-fast detection (within 1 s) of ethanol in simulation drunk driving testing. Our results obtained in this work provided not only a facile self-assembly approach to fabricate metal oxides with 3D OP HM structures but also a new methodology for achieving noninvasive real-time exhaled breath detection.

Revealing the Relationship between Energy Level and Gas Sensing Performance in Heteroatom-Doped Semiconducting Nanostructures

The cation substitutional doping of metal oxide semiconductors plays pivotal roles in improving the gas sensing performances, but the doping effect on surface sensing reaction is still not well understood. In this study, indium oxides doped with various heteroatoms are investigated to obtain in-depth understanding of how doping (or the resulting change in the electronic structure) alters the surface-absorbed oxygen chemistry and subsequent sensing process. The experimental results reveal that energy level of In₂O₃ can be modulated by introduction of these dopants, some of which (e.g., Al, Ga, and Zr) lead to the elevation of Fermi level, whereas others (e.g., Ti, V, Cr, Mo, W, and Sn) bring about relative drop in Fermi level. However, only the former can improve the response to formaldehyde, indicating a strong link between Fermi level and sensing properties. Mechanistic study suggests that the elevation of Fermi level increases energy level difference between oxide semiconductor and oxygen molecules and facilitates the surface absorption of oxygen species, resulting in superior formaldehyde sensing activity. Especially, Al-doped In₂O₃ exhibits remarkably enhanced sensing performances toward formaldehyde at low working temperature (150 °C) with high response, good selectivity, ultralow limit of detection (60 ppb), and short response time (2-23 s). Our findings not only promote the understanding of sensing reaction process and its correlation with the semiconductor electronic structure but also offer a general guideline for large-scale screening of promising oxide semiconductor-based sensing materials for gas detection.

Interfacial Doping of Heteroatom in Porous SnO2 for Highly Sensitive Surface Properties

The design and synthesis of heteroatom-doping porous materials with unique surface/interfaces are of great significance for enhancing the sensitive surface performance in the fields of catalytic energy, especially gas sensor, CO oxidation, and ammonium perchlorate decomposition. Usually, the template method followed by a high-temperature calcination process is considered as the routes of choice in preparing ion-doped porous materials, but it requires extra templates and will undergo complicated steps. Here, we present a simple fusion/diffusion-controlled intermetallics-transformation method to synthesize various heteroatom-doping porous SnO₂ only by changing the species of intermetallics. By this new method, Ni-doped popcornlike SnO₂ with plenty of ∼30 nm pores and two kinds of Cu-doped SnO₂ nanocages was successfully constructed. Phase-evolution investigations demonstrated that growth kinetics, diffusion, and solubility of the intermediates are highly related to the architecture of final products. Moreover, low-solid-solution limit of MO _x (M: Ni, Cu) in SnO₂ made the ion dope close to the surface to form a special surface/interfaces structure, and selective removal of MO _x produce abundant pores to increase the surface area. As a consequence, Ni-doped composite exhibits higher sensitivity in formaldehyde detection with a relative low-operating temperature in a short response time (i.e., 23.7-50 ppm formaldehyde, 170 °C, and 5 s) and Cu-doped composites show excellent activity in decreasing the catalytic temperature of CO oxidation and ammonium perchlorate decomposition. The fusion/diffusion-controlled intermetallics-transformation method reported in this work could be readily adopted for the synthesis of other active heteroatom-doping porous materials for multipurpose uses.

One-Step Synthesis of Co-Doped In2O3 Nanorods for High Response of Formaldehyde Sensor at Low Temperature

Uniform and monodisperse Co-doped In₂O₃ nanorods were fabricated by a facile and environmentally friendly hydrothermal strategy that combined the subsequent annealing process, and their morphology, structure, and formaldehyde (HCHO) gas sensing performance were investigated systematically. Both pure and Co-doped In₂O₃ nanorods had a high specific surface area, which could offer abundant reaction sites to gas molecular diffusion and improve the response of the gas sensor. Results revealed that the In₂O₃/1%Co nanorods exhibited a higher response of 23.2 for 10 ppm of HCHO than that of the pure In₂O₃ nanorods by 4.5 times at 130 °C. More importantly, the In₂O₃/1%Co nanorods also presented outstanding selectivity and long-term stability. The superior gas sensing properties were mainly attributed to the incorporation of Co, which suggested the important role of the amount of oxygen vacancies and adsorbed oxygen in enhancing HCHO sensing performance of In₂O₃ sensors.

Manipulating the Defect Structure (VO) of In2O3 Nanoparticles for Enhancement of Formaldehyde Detection

Because defects such as oxygen vacancies (V_O) can affect the properties of nanomaterials, investigating the defect structure-function relationship are attracting intense attention. However, it remains an enormous challenge to the synthesis of nanomaterials with high sensing performance by manipulating V_O because understanding the role of surface or bulk V_O on the sensing properties of metal oxides is still missing. Herein, In₂O₃ nanoparticles with different contents of surface and bulk V_O were obtained by hydrogen reduction treatment, and the role of surface or bulk V_O on the sensing properties of In₂O₃ was investigated. The X-ray diffraction, ultraviolet-visible spectrophotometer, electron paramagnetic resonance, photoluminescence, Raman, X-ray photoelectron spectroscopy, Hall analysis, and the sensing results indicate that bulk V_O can decrease the band gap and energy barrier and increase the carrier mobility, hence facilitating the formation of chemisorbed oxygen and enhancing the sensing response. Benefiting from bulk V_O, In₂O₃-H10 exhibits the highest response, good selectivity, and stability for formaldehyde detection. However, surface V_O does not contribute to the improvement of formaldehyde-sensing performance, and the black In₂O₃-H30 with the highest content of surface V_O exhibits the lowest response. Our work provides a novel strategy for the synthesis of nanomaterials with high sensing performance by manipulating V_O.

Ultrathin In2O3 Nanosheets with Uniform Mesopores for Highly Sensitive Nitric Oxide Detection

Nitric oxide (NO_x, including NO and NO₂) is one of the most dangerous environmental toxins and pollutants, which mainly originates from vehicle exhaust and industrial emission. The development of sensitive NO_x gas sensors is quite urgent for human health and the environment. Up to now, it still remains a great challenge to develop a NO_x gas sensor, which can satisfy multiple application demands for sensing performance (such as high response, low detection temperature, and limit). In this work, ultrathin In₂O₃ nanosheets with uniform mesopores were successfully synthesized through a facile two-step synthetic method. This is a success due to not only the formation of two-dimensional (2D) nanosheets with an ultrathin thickness of 3.7 nm based on a nonlayered compound but also the template-free construction of uniform mesopores in ultrathin nanosheets. The sensors based on the as-obtained mesoporous In₂O₃ ultrathin nanosheets exhibit an ultrahigh response (R_g/R_a = 213) and a short response time (ca. 4 s) toward 10 ppm NO_x, and a quite low detection limit (10 ppb NO_x) under a relatively low operating temperature (120 °C), which well satisfies multiple application demands. The excellent sensing performance should be mainly attributed to the unique structural advantages of mesopores and 2D ultrathin nanosheets.