Metal-Organic Frameworks-Derived Hierarchical Co3O4 Structures as Efficient Sensing Materials for Acetone Detection

Hierarchically porous ZnO derived from zeolitic imidazolate frameworks for high-sensitive MEMS NO2 sensor

Three-dimensional (3D) porous metal oxide nanomaterials with controllable morphology and well-defined pore size have attracted extensive attention in the field of gas sensing. Herein, hierarchically porous ZnO-450 was obtained simply by annealing Zeolitic Imidazolate Frameworks (ZIF-90) microcrystals at an optimal temperature of 450 °C, and the effect of annealing temperature on the formation of porous nanostructure was discussed. Then the as-obtained ZnO-450 was employed as sensing materials to construct a Micro-Electro-Mechanical System (MEMS) gas sensor for detecting NO2. The MEMS sensor based on ZnO-450 displays the excellent gas-sensing performances at a lower working temperature (190 °C), such as high response value (242.18% @ 10 ppm), fast response/recovery time (9/26 s) and ultralow limit of detection (35 ppb). The ZnO-450 sensor shows better sensing performance for NO2 detection than ZnO-based composites materials or commercial ZnO nanoparticles (NPs), which are attributed to its unique hierarchically structures with high porosity and larger surface area. This ZIFs driven strategy can be expected to pave a new pathway for the design of high-performance NO2 sensors.

MXene-Based Chemo-Sensors and Other Sensing Devices

MXenes have received worldwide attention across various scientific and technological fields since the first report of the synthesis of Ti3C2 nanostructures in 2011. The unique characteristics of MXenes, such as superior mechanical strength and flexibility, liquid-phase processability, tunable surface functionality, high electrical conductivity, and the ability to customize their properties, have led to the widespread development and exploration of their applications in energy storage, electronics, biomedicine, catalysis, and environmental technologies. The significant growth in publications related to MXenes over the past decade highlights the extensive research interest in this material. One area that has a great potential for improvement through the integration of MXenes is sensor design. Strain sensors, temperature sensors, pressure sensors, biosensors (both optical and electrochemical), gas sensors, and environmental pollution sensors targeted at volatile organic compounds (VOCs) could all gain numerous improvements from the inclusion of MXenes. This report delves into the current research landscape, exploring the advancements in MXene-based chemo-sensor technologies and examining potential future applications across diverse sensor types.

Enhanced CO oxidation performance over hierarchical flower-like Co3O4 based nanosheets via optimizing oxygen activation and CO chemisorption

Enhancing low-temperature activity is a focus for carbon monoxide (CO) elimination by catalytic oxidation. In this work, the hierarchical flower-like silver (Ag) modified cobalt oxides (Co3O4) nanosheets were prepared by solvothermal method and applied into catalytic CO oxidation. The doped Ag species in the form of AgCoO2 induced the prolongated surface Co-O bond and weaker bond intensity. Consequently, the oxygen activation/migration ability and redox capacity of Ag0.02Co were enhanced with more oxygen vacancies. The chemisorbed CO was preferentially converted to CO2 but not carbonates. The inhibited carbonates accumulation could avoid the coverage of active sites. According to Density functional theory (DFT) calculations, the electron transfer from AgCoO2 to Co3O4 promote electron donation ability of Co3O4 layer, benefiting for oxygen activation. Moreover, the longer Co-C and C-O bond length suggest the weakened chemisorption strength and higher active of CO molecule. The Ag modified Co3O4 exhibited more satisfactory activity at lower temperature. Typically, it realized 100% CO conversion at 90 °C, and displayed 6.3-fold higher reaction rate than pristine Co3O4 at 40 °C. Moreover, the Ag0.02Co exhibited outstanding long-term stability and water resistance. In summary, the optimized oxygen activation, CO chemisorption and interfacial electron transfer synergistically boosted the CO oxidation activity on Ag modified Co3O4.

Electro-Oxidation Reaction of Methanol over La2-xSrxNi1-y(Mn/Fe/Co)yO4+δ Ruddlesden-Popper Oxides

Solution combustion-synthesized Ruddlesden-Popper oxides La1.4Sr0.6Ni0.9(Mn/Fe/Co)0.1O4+δ were explored for the methanol electro-oxidation reaction. With optimal doping of Sr2+ in the A site and Co2+ in the B site, Ni3+ with t2g6 d x 2 - y 2 1 configuration in La1.4Sr0.6Ni0.9Co0.1O4+δ exhibited a tetragonal distortion with compression in axial bonds and elongation in equatorial bonds. This structural modification fostered an augmented overlap of d z 2 orbitals with axial O 2p orbitals, leading to a heightened density of states at the Fermi level. Consequently, this facilitated not only elevated electrical conductivity but also a noteworthy reduction in the charge transfer resistance. These effects collectively contributed to the exceptional methanol oxidation activity of La1.4Sr0.6Ni0.9Co0.1O4+δ, as evidenced by an impressive current density of 21.4 mA cm-2 and retention of 95% of initial current density even after 10 h of prolonged reaction. The presence of Ni3+ further played a pivotal role in the creation of NiOOH, a crucial intermediate species, facilitated by the presence of surface oxygen vacancies. These factors synergistically enabled efficient methanol oxidation. In summary, our present study not only yields substantial insights but also paves the way for a novel avenue to fine-tune the activity of Ruddlesden-Popper oxides for the successful electro-oxidation of methanol.

Ultrafast Response and High Selectivity of Diethylamine Gas Sensors at Room Temperature Using MOF-Derived 1D CuO Nano-Ellipsoids

Environmental and health monitoring requires low-cost, high-performance diethylamine (DEA) sensors. Materials based on metal-organic frameworks (MOFs) can detect hazardous gases due to their large specific surface area, many metal sites, unsaturated sites, functional connectivity, and easy calcination to remove the scaffold. However, developing facile materials with high sensitivity and selectivity in harsh environments for accurate DEA detection at a low detection limit (LOD) at room temperature (RT) is challenging. In this study, p-type semiconducting porous CuOx sensing materials were synthesized using a simple solvothermal process and annealed in an argon atmosphere at three different temperatures (x = 400, 600, and 800 °C). Significant variations in particle size, specific area, crystallite size, and shape were noticed when the annealing temperature was elevated. Cu-MIL-53 annealed at 400 °C (CuO-400) has a typical nanoellipsoid (NEs) shape with a length of 61.5 nm and a diameter of 33.2 nm. Surprisingly, CuO-400 NEs showed an excellent response to DEA with an ultra-LOD (Rg/Ra = 7.3 @ 100 ppb, 55% relative humidity), excellent selectivity and sensitivity (Rg/Ra = 236 @ 15 ppm), exceptional long-term stability and repeatability, and a fast response/recovery period at RT, outperforming most previously reported materials. CuO-400 NEs have outstanding gas-sensing characteristics due to their high porosity, 1D nanostructure, unsaturated Cu sites (Cu+ and Cu2+), large specific surface area, and numerous oxygen vacancies. This study presents a generic approach to produce future CuO derived from Cu-MOFs-sensitive materials, revealing new insights into the design of effective sensors for environmental monitoring at RT.

Metal-organic framework-derived metal oxides for resistive gas sensing: a review

Gas sensors with exceptional sensitivity and selectivity are vital in the real-time surveillance of noxious and harmful gases. Despite this, traditional gas sensing materials still face a number of challenges, such as poor selectivity, insufficient detection limits, and short lifespan. Metal oxides, which are derived from metal-organic framework materials (MOFs), have been widely used in the field of gas sensors because they have a high surface area and large pore volume. Incorporating metal oxides derived from MOFs into gas sensors can improve their sensitivity and selectivity, thus opening up new possibilities for the development of innovative, high-performance gas sensors. This article examines the gas sensing process of metal oxide semiconductors (MOS), evaluates the advances made in the research of different structures of MOF-derived metal oxides in resistive gas sensors, and provides information on their potential applications and future advancements.

Low Detection Limit and High Sensitivity 2-Butanone Gas Sensor Based on ZnO Nanosheets Decorated by Co Nanoparticles Derived from ZIF-67

2-butanone has been certified to cause potential harm to the human body, environment, etc. Therefore, achieving a method for the high sensitivity and low limit detection of 2-butanone is of great significance. To achieve this goal, this article uses ZIF-67 prepared by a precipitation method as a cobalt source, and then prepares cobalt-modified zinc oxide nanosheets through a hydrothermal method. The microstructure of the materials was observed by SEM, EDS, TEM, HRTEM, XPS and XRD. The test data display that the sensor ZC2 can produce a high response (2540) to 100 ppm 2-butanone at 270 °C, which is 21 times higher than that of pure ZnO materials. Its detection limit is also optimized to 24 ppb. The sensor (ZC2) also excels in these properties: selectivity, repeatability and stability over 30 days. Further analysis indicates that the synergistic and catalytic effects of p-n heterojunction are the key sources for optimizing the performance of sensors for detecting 2-butanone.

A Novel Platform of MOF for Sonodynamic Therapy Advanced Therapies

Metal-organic frameworks (MOFs) combined with sonodynamic therapy (SDT) have been introduced as a new and efficient treatment method. The critical advantage of SDT is its ability to penetrate deep tissues and concentrate energy on the tumor site to achieve a non-invasive or minimally invasive effect. Using a sonosensitizer to generate reactive oxygen species (ROS) under ultrasound is the primary SDT-related method of killing tumor cells. In the presence of a sonosensitizer, SDT exhibits a more lethal effect on tumors. The fast development of micro/nanotechnology has effectively improved the efficiency of SDT, and MOFs have been broadly evaluated in SDT due to their easy synthesis, easy surface functionalization, high porosity, and high biocompatibility. This article reviews the main mechanism of action of sonodynamic therapy in cancer treatment, and also reviews the applications of MOFs in recent years. The application of MOFs in sonodynamic therapy can effectively improve the targeting ability of SDT and the conversion ability of reactive oxygen species, thus improving their killing ability on cancer cells. This provides new ideas for the application of micro/nano particles in SDT and cancer therapy.

Oxygen Vacancy and Interface Effect Adjusted Hollow Dodecahedrons for Efficient Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) with special morphologies provide the geometric morphology and composition basis for the construction of platforms with excellent catalytic activity. In this work, cobalt-cerium composite oxide hollow dodecahedrons (Co/Cex-COHDs) with controllable morphology and tunable composition are successfully prepared via a high-temperature pyrolysis strategy using Co/Ce-MOFs as self-sacrificial templates. The construction of the hollow structure can expose a larger surface area to provide abundant active sites and pores to facilitate the diffusion of substances. The formation and optimization of phase interface between Co3O4 and CeO2 regulate the electronic structure of the catalytic site and form a fast channel favorable to electron transport, thereby enhancing the electrocatalytic oxygen evolution activity. Based on the above advantages, the optimized Co/Ce0.2-COHDs obtained an enhanced oxygen evolution reaction (OER) performance.

Biotemplate synthesis of a Co3O4 microtube sensor for fast triethylamine detection

A low operating temperature and short response/recovery time are essential factors for sensors. Hence, it is necessary to create a sensor that can quickly detect target gas at a relatively low temperature. In this work, Co₃O₄ microtube based sensors were fabricated by a bio-template method using absorbent cotton. Co₃O₄ microtube sensors prepared in different concentrations of a salt solution displayed different sensitivities to triethylamine (TEA). The Co₃O₄-0.10 microtube sensor exhibited excellent sensitivity to TEA at 160 °C. The response of the Co₃O₄-0.10 sensor to 100 ppm TEA gas was 31.27 and the detection limit of TEA was 50 ppb. Meanwhile, the Co₃O₄-0.10 sensor also showed a short response/recovery time, such as 95 s/38 s to 100 ppm TEA, high selectivity, a good linear relationship (R² = 0.994 for 1-100 ppm TEA and R² = 0.991 for 50-1000 ppb TEA gas), fine repeatability and long-term stability, and strong humidity resistance. Thus, Co₃O₄ microtube based sensors prepared using a bio-template method have potential application prospects for the detection of TEA gas.

Preparation of MOF-derived ZnO/Co3O4 nanocages and their sensing performance toward H2S

We report a type of micro-electro-mechanical system (MEMS) H₂S gas sensors with excellent sensing performance at the ppb level (lowest detection limit is 5 ppb). The sensors were fabricated with ZnO/Co₃O₄ sensing materials derived from Zn/Co-MOFs by annealing at a suitable temperature of 500 °C. ZnO/Co₃O₄-500 exhibits the highest response when exposed to 10 ppb H₂S gas at 120 °C, and the response/recovery times are 10 s/21 s. Moreover, it exhibits outstanding selectivity, long-term stability (retained 95% response after 45 days), and moisture resistance (only a minor fluctuation of 2% even at 90% RH). This can be ascribed to the fact that ZnO/Co₃O₄-500 has regular morphology, abundant oxygen vacancies (52.8%) and high specific surface area (96.5 m² g^-1). This work provides not only a high performance H₂S MEMS gas sensor but also a systematic study of the effect of the annealing temperature on the sensing performance of ZnO/Co₃O₄ sensing materials derived from bimetal organic frameworks.

Metal-organic frameworks-derived In2O3/ZnO porous hollow nanocages for highly sensitive H2S gas sensor

The detection of hydrogen sulfide (H₂S) is critical because of its potential harm and widespread presence in the oil and gas sectors. The zeolitic imidazolate framework-8 (ZIF-8) derived ZnO nanostructures manufactured as gas sensors have exceptional sensitivity and selectivity for H₂S gas. In/Zn-ZIF-8 template material was synthesized by a simple one-step co-precipitation method followed by thermal annealing in air. The heat treatment resulted in In₂O₃/ZnO nanostructures with mixed heterostructures. The crystal structure (XRD), morphology (SEM/TEM), chemical state (XPS), surface area (BET), etc were investigated to ascertain the nature of the as-prepared material. SEM imagery revealed that the as-prepared In₂O₃/ZnO sensitive material had a microstructure of porous hollow nanocages with an average particle size of about 200 nm, which is beneficial to the diffusion and adsorption of gas molecules. The gas sensing performance test results of the In₂O₃/ZnO hollow nanocages show that their response to H₂S gas is significantly improved 67.5 @50 ppm H₂S (about 11 times that of pure ZnO nanocages) at an optimal temperature of 200 °C, better selectivity, lower theoretical detection limit and good linearity between gas concentration and response values. The enhanced gas sensing feat to H₂S gas is mainly attributed to the formation of n-n heterojunction and the wide surface area of the newly formed In₂O₃/ZnO porous hollow nanocages.

ZIF-X (8, 67) based nanostructures for gas-sensing applications

ZIF-8 and ZIF-67 are the most investigated zeolitic imidazolate frameworks (ZIFs) materials that have aroused enormous scientific interests in numerous areas of application including electrochemistry, gas storage, separation, and sensors by reason of their fascinating structural properties. Recently, there is a rapidly growing demand for chemical gas sensors for the detection of various analytes in widespread applications including environmental pollution monitoring, clinical analysis, wastewater analysis, industrial applications, food quality, consumer products, and automobiles. In general, the key to the development of superior gas sensors is exploring innovative sensing materials. ZIF-X (8, 67) based nanostructures have demonstrated great potential as ideal sensing materials for high-performance sensing applications. In this review, the general properties and applications of ZIF-X (8, 67) including gas storage and gas adsorption are first summarized, and then the recent progress of ZIF-X (8, 67) based nanostructures for gas-sensing applications and the structure-property correlations are summarized and analyzed.

Gas sensing based on metal-organic frameworks: Concepts, functions, and developments

Effective detection of pollutant gases is vital for protection of natural environment and human health. There is an increasing demand for sensing devices that are equipped with high sensitivity, fast response/recovery speed, and remarkable selectivity. Particularly, attention is given to the designability of sensing materials with porous structures. Among diverse kinds of porous materials, metal-organic frameworks (MOFs) exhibit high porosity, high degree of crystallinity and exceptional chemical activity. Their strong host-guest interactions with guest molecules facilitate the application of MOFs in adsorption, catalysis and sensing systems. In particular, the tailorable framework/composition and potential for post-synthetic modification of MOFs endow them with widely promising application in gas sensing devices. In this review, we outlined the fundamental aspects and applications of MOFs for gas sensors, and discussed various techniques of monitoring gases based on MOFs as functional materials. Insights and perspectives for further challenges faced by MOFs are discussed in the end.

Enhanced NO2 gas-sensing performance by core-shell SnO2/ZIF-8 nanospheres

Timely detection of harmful, poisonous and air pollutant gases is of vital importance to the protection of human beings from exposure to rigorous gases. The development of gas-sensing devices based on sphere-like porous SnO₂/ZIF-8 nanocomposites is required to overcome this challenge. Nanostructures with high surface area, more porosity and hollow interior provide plenty of active cites for high responses in metal oxide gas sensors. The engineered gas sensors have excellent sensing sensitivity (164), rapid response and recovery times (60, 45 s), and favorable selectivity for NO₂ gases under 300 °C. Consequently, NO₂ gas sensors based on core-shell SnO₂/ZIF-8 nanospheres are regarded viable capacity industrial applicants.

Mesoporous Ti0.5Cr0.5N for trace H2S detection with excellent long-term stability

Efficient, accurate and reliable detection and monitoring of H₂S is of significance in a wide range of areas: industrial production, medical diagnosis, environmental monitoring, and health screening. However the rapid corrosion of commercial platinum-on-carbon (Pt/C) sensing electrodes in the presence of H₂S presents a fundamental challenge for fuel cell gas sensors. Herein we report a solution to the issue through the design of a sensing electrode, which is based on Pt supported on mesoporous titanium chromium nitrides (Pt/Ti_0.5Cr_0.5N). Its desirable characteristics are due to its high electrochemical stability and strong metal-support interactions. The Pt/Ti_0.5Cr_0.5N-based sensors exhibit a much smaller attenuation (1.3%) in response to H₂S than Pt/C-sensor (40%), after 2 months sensing test. Furthermore, the Pt/Ti_0.5Cr_0.5N-based sensors exhibit negligible cross response to other interfering gases compared with hydrogen sulfide. Results of density functional theory calculation also verify the excellent long-term stability and selectivity of the gas sensor. Our work hence points to a new sensing electrode system that offers a combination of high performance and stability for fuel-cell gas sensors.

Heterostructured CoP·CoMoP nanocages as advanced electrocatalysts for efficient hydrogen evolution over a wide pH range

A sustainable and environmental-friendly method to produce hydrogen with high purity is the electrochemical water splitting, but its commercialization is challenged due to lack of cost-effective electrocatalysts for hydrogen evolution reaction (HER) over a wide pH range. Herein, a series of CoP·xCoMoP heterostructured nanocages (NCs) were prepared via a dissolution-regrowth and subsequent phosphorization process using metal-organic frameworks (MOFs) as template. The three-dimensional (3D) architecture of CoP·xCoMoP is constituted by the heterostructured nanosheets composed with CoP and CoMoP phase. These noble-metal-free earth-abundant transition metal phosphide (TMP) catalysts show a pH-universal HER activity with high efficiency. Under the optimal atom ratio of Co and Mo (6:5), CoP·5CoMoP NC catalysts can deliver a current density of 10 mA cm^-2 at the overpotential of 72 mV with a Tafel slope of 60.3 mV dec^-1 in 1.0 M KOH solution. The same current output requires overpotential of 44 mV in 0.5 M H₂SO₄ solution and 151 mV in1.0 M phosphate buffered solution (PBS), respectively. The superior HER activity of CoP·5CoMoP NC catalysts can be comparable to or even better than most of noble metal-free HER electrocatalysts reported recently. In addition, CoP·5CoMoP NC catalysts also show a fairly high HER stability over a wide pH range, and their HER activity can be well kept without significant loss for long-term electrolysis. The 3D CoP·5CoMoP heterostructured catalysts hold promise as efficient and low-cost catalysts for water splitting devices over a wide pH range.

Metal-organic frameworks for advanced transducer based gas sensors: review and perspectives

The development of gas sensing devices to detect environmentally toxic, hazardous, and volatile organic compounds (VOCs) has witnessed a surge of immense interest over the past few decades, motivated mainly by the significant progress in technological advancements in the gas sensing field. A great deal of research has been dedicated to developing robust, cost-effective, and miniaturized gas sensing platforms with high efficiency. Compared to conventional metal-oxide based gas sensing materials, metal-organic frameworks (MOFs) have garnered tremendous attention in a variety of fields, including the gas sensing field, due to their fascinating features such as high adsorption sites for gas molecules, high porosity, tunable morphologies, structural diversities, and ability of room temperature (RT) sensing. This review summarizes the current advancement in various pristine MOF materials and their composites for different electrical transducer-based gas sensing applications. The review begins with a discussion on the overview of gas sensors, the significance of MOFs, and their scope in the gas sensing field. Next, gas sensing applications are divided into four categories based on different advanced transducers: chemiresistive, capacitive, quartz crystal microbalance (QCM), and organic field-effect transistor (OFET) based gas sensors. Their fundamental concepts, gas sensing ability towards various gases, sensing mechanisms, and their advantages and disadvantages are discussed. Finally, this review is concluded with a summary, existing challenges, and future perspectives.

Efficient detection of hazardous H2S gas using multifaceted Co3O4/ZnO hollow nanostructures

The rapid increases in environmental hazardous gases have laid dangerous effects on human health. The detection of such pollutants gases is mandatory using various optimal techniques. In this paper, porous multifaceted Co₃O₄/ZnO nanostructures are synthesized by pyrolyzing sacrificial template of core-shell double zeolitic imidazolate frameworks (ZIFs) for gas sensing applications. The fabricated exhibit superior gas sensor response, high selectivity, fast response/recovery times, and remarkable stability and sensitivity to H₂S gas. In particular, the multifaceted Co₃O₄/ZnO nanostructures show a maximum response of 147 at 100 ppm of H₂S under optimum conditions. The remarkable gas sensing performances are mainly ascribed to high porosity, wide surface area multifaceted nanostructures, presence of heterojunctions and catalytic activity of ZnO and Co₃O₄, which are beneficial for H₂S gas sensors industry.

Bibliometric analysis and recent trends on MXene research - A comprehensive review

MXene, identified as a high performance material with superior properties, has gained significant importance in the field of applications including energy storage, photo catalysis, sensing of components and environmental pollution control. This review article is a comprehensive study on scientometric review on the research studies involving MXene and its derivatives for various applications. The aim of this study is to identify the areas of priority focused during the study period (2012-2020) and evaluate the impact of the studies in terms of different parameters. Using the suitable key words, a total of 3332 documents are identified and screened with respect to yearly count of literature, type of literature, language of publication, authors, Web of science (WoS) categories, most cited literature, author contribution, name of the affiliated institution, country of author affiliation, journals and key words. In addition, collaboration behavior and citation network are reviewed using the mapping tool. The total local citation score (TLCS) and total global citation score (TGCS) are evaluated. Based on the review data, the developments in the field of MXene applications are presented with more focus on sensing applications and photocatalysis. The top two contributing countries in the chosen field of MXene research are China and USA. Based on the number of documents published, ACS Applied Materials & Interfaces and Journal of Materials Chemistry "A" are identified as the best two journals.

Boosting selective H2 sensing of ZnO derived from ZIF-8 by rGO functionalization

H2 sensors have attracted considerable attention for safety warning of traditional industries and energy storing systems. This ZnO/rGO composite demonstrated excellent H2 sensing performances, good baseline stability and excellent selectivity.

Efficient and portable cellulose-based colorimetric test paper for metal ion detection

Fabrication of metal ion detection materials generally involved problems such as high cost and complicated processes of pretreatment and operation. Herein, a novel colorimetric test paper for metal ions detection was developed based on functionalized cellulose fibers. Acetoacetyl groups were introduced on cellulose fibers by a surface esterification process. The obtained cellulose acetoacetate (CAA) fibers were made into CAA paper via a paper-making process. The CAA paper possessed robust mechanical property, thermal stability selectivity and rapid response to Fe³⁺ and Cu²⁺ ions, with an obvious naked-eye color change within 5 s. The mechanism of this visual recognition for metal ions due to that the acetoacetyl groups coordination chelated with metal ion to form six-membered ring structure, further leading to the color change of the materials. It provided a facile and universal method to prepare efficient and portable cellulose-based test paper, which has great potential in metal ion detection field.

Ethanol Sensing Properties and First Principles Study of Au Supported on Mesoporous ZnO Derived from Metal Organic Framework ZIF-8

It is of great significance to develop ethanol sensors with high sensitivity and low detection temperature. Hence, we prepared Au-supported material on mesoporous ZnO composites derived from a metal-organic framework ZIF-8 for the detection of ethanol gas. The obtained Au/ZnO materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (SEM), field emission transmission electron microscopy (TEM) and nitrogen adsorption and desorption isotherms. The results showed that the Au/ZnO-1.0 sample maintains a three-dimensional (3D) dodecahedron structure with a larger specific surface area (22.79 m² g⁻¹) and has more oxygen vacancies. Because of the unique ZIF structure, abundant surface defects and the formation of Au-ZnO Schottky junctions, an Au/ZnO-1.0 sensor has a response factor of 37.74 for 100 ppm ethanol at 250 °C, which is about 6 times that of pure ZnO material. In addition, the Au/ZnO-1.0 sensor has good selectivity for ethanol. According to density functional theory (DFT) calculations, the adsorption energy of Au/ZnO for ethanol (−1.813 eV) is significantly greater than that of pure ZnO (−0.217 eV). Furthermore, the adsorption energy for ethanol is greater than that of other gases.

Gas sensing materials roadmap

Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.

General Strategy to Fabricate Porous Co-Based Bimetallic Metal Oxide Nanosheets for High-Performance CO Sensing

Two-dimensional (2D) porous bimetallic oxide nanosheets are attractive for high-performance gas sensing because of their porous structures, high surface areas, and cooperative effects. Nevertheless, it is still a huge challenge to synthesize these nanomaterials. Herein, we report a general strategy to fabricate porous cobalt-based bimetallic oxide nanosheets (Co-M-O NSs, M = Cu, Mn, Ni, and Zn) with an adjustable Co/M ratio and the homogeneous composition using metal-organic framework (MOF) nanosheets as precursors. The obtained Co-M-O NS possesses the porous nanosheet structure and ultrahigh specific surface areas (146.4-220.7 m² g^-1), which enhance the adsorption of CO molecules, support the transport of electrons, and expose abundant active sites for CO-sensing reaction. As a result, the Co-M-O NS exhibited excellent sensing performances including high response, low working temperature, fast response-recovery, good selectivity and stability, and ppb-level detection limitation toward CO. In particular, the Co-Mn-O NS showed the highest response of 264% to 100 ppm CO at low temperature (175 °C). We propose that the excellent sensing performance is ascribed to the specific porous nanosheet structure, the relatively highly active Co³⁺ ratio resulting from cation substitution, and large amounts of chemisorbed oxygen species on the surface. Such a general strategy can also be introduced to design noble-metal-free bimetallic metal oxide nanosheets for gas sensing, catalysis, and other energy-related fields.

Hierarchical hollow BiFeO3 microcubes with enhanced acetone gas sensing performance

Metal oxide-based gas sensors have drawn tremendous research interests owing to their various compositions and selective and improved performance. However, the development of a targeted metal oxide with controlled microstructures via a facile preparation procedure is still a challenge. In this work, hierarchical BiFeO3 nano-microstructures are successfully developed through the post-modification of Bi3+ encapsulation with Fe-based Prussian blue microcubes followed by a sequential annealing strategy. The microstructures of the hierarchical BiFeO3 architectures can be effectively modulated by tuning various thermolysis temperatures. Among them, the hierarchical hollow BiFeO3 microcubes assembled from ultrathin nanosheets exhibit optimum acetone selective sensing performances with a gas response value (Ra/Rg) of 5.2 at 240 °C, rapid response/recovery times (10 s/9 s), and excellent long-term stability (for at least 30 days). The high and reproducible acetone-sensing properties are mainly attributed to the unique interior loose and porous structures with good permeability. The corresponding acetone sensing mechanism relying on the microstructure of BiFeO3 was also discussed. This work highlights the key role of morphological evolution in the fabrication of multi-functional multimetal oxides, and thus offers new opportunities for the rational design of novel gas sensing materials.

Zinc Oxide-Based Acetone Gas Sensors for Breath Analysis: A Review

Acetone is one of the toxic, explosive, and harmful gases. It may cause several health hazard issues such as narcosis and headache. Acetone is also regarded as a key biomarker to diagnose several diseases as well as monitor the disorders in human health. Based on clinical findings, acetone concentration in human breath is correlated with many diseases such as asthma, halitosis, lung cancer, and diabetes. Thus, its investigation can become a new approach for health monitoring. Better management at the early stages of such diseases has the potential not only to reduce deaths associated with the disease but also to reduce medical costs. ZnO-based sensors show great potential for acetone gas due to their high chemical stability, simple synthesis process, and low cost. The findings suggested that the acetone sensing performance of such sensors can be significantly improved by manipulating the microstructure (surface area, porosity, etc.), composition, and morphology of ZnO nanomaterials. This article provides a comprehensive review of the state-of-the-art research activities, published during the last five years (2016 to 2020), related to acetone gas sensing using nanostructured ZnO (nanowires, nanoparticles, nanorods, thin films, etc). It focuses on different types of nanostructured ZnO-based acetone gas sensors. Furthermore, several factors such as relative humidity, acetone concentrations, and operating temperature that affects the acetone gas sensing properties- sensitivity, long-term stability, selectivity as well as response and recovery time are discussed in this review. We hope that this work will inspire the development of high-performance acetone gas sensors using nanostructured materials.

Pd-Doping-Induced Oxygen Vacancies in One-Dimensional Tungsten Oxide Nanowires for Enhanced Acetone Gas Sensing

Metal oxide semiconductors (MOS) with different nanostructures have been widely used as gas sensing materials due to the tunable interface structures and properties. However, further improvement of the sensing sensitivity and selectivity is still challenging in this area. Constructing appropriate heterogeneous interface structures and oxygen vacancies is one of the important strategies to tune the sensing properties of MOS. In the present study, interfacial heterostructures in Pd_xW₁₈O₄₉ nanowires (Pd_xW₁₈O₄₉ NWs) were fabricated and manipulated by doping different Pd contents through a simple hydrothermal process. Relevant characterization proved that the structure and composition of the one-dimensional (1D) nanomaterial can be effectively changed by Pd doping. It was found that the oxygen vacancy concentration increases first with the increase of Pd content, and when the Pd content increases to 7.18% (Pd_7.18%W₁₈O₄₉ NWs), the oxygen vacancy content reaches the maximum (52.5%). If the Pd content continues to increase, the oxygen vacancy ratio decreases. The gas sensing investigations illustrated that the Pd_xW₁₈O₄₉ NWs exhibited enhanced sensing properties than pure W₁₈O₄₉ NWs toward acetone. Among the as-prepared catalysts, the Pd_7.18%W₁₈O₄₉ NWs showed the best sensing response and the fastest response-recovery speeds (5 and 10 s, respectively) at a working temperature of 175 °C. In addition, this 1D nanostructure with fabricated heterostructures also delivers a good sensing selectivity and a wide detection range from 100 ppb to 300 ppm, with maintaining excellent performance in the presence of high concentrations of ethanol and carbon dioxide. The excellent gas sensing behavior could be attributed to the generated oxygen vacancies and the heterostructures upon Pd doping. This study offers a novel strategy for the design of high-performance gas sensors for ppb-level acetone sensing.

Metal-organic framework derived hollow CoFe@C composites by the tunable chemical composition for efficient microwave absorption

Controlling the composition and microstructure of nanomaterials is still a significant challenge in developing high-performance microwave absorption (MA) materials. Herein, metal-organic framework (MOF)-derived hollow CoFe@C nanoboxes are designed and prepared through the facile regulating the mass ratios of ZIF-67/PFC and a thermal annealing treatment. The CoFe@C composite can achieve an excellent MA performance, which have two high reflection loss (RL) values at different thickness. A RL value of -31.0 dB is obtained at 11.84 GHz with a matching thickness of 2.4 mm, and a RL value can reach -44.1 dB (4.08 GHz) at a matching thickness of 5.8 mm, and a correspondingly wide absorbing bandwidth (5.20 GHz, from 9.7 to 14.9 GHz) is simultaneously obtained at a matching thickness of 2.3 mm. The magnetic loss, interfacial polarization and hollow structure are the main reasons for their excellent MA capability. This work provides a research idea for the development of the efficient MOF-based MA materials in practical application.

Machine Learning-Aided Crystal Facet Rational Design with Ionic Liquid Controllable Synthesis

Crystallographic facets in a crystal carry interior properties and proffer rich functionalities in a wide range of application areas. However, rational prediction, on-demand customization, and accurate synthesis of facets and facet junctions of a crystal are enormously desirable but still challenging. Herein, a framework of machine learning (ML)-aided crystal facet design with ionic liquid controllable synthesis is developed and then demonstrated with the star-material anatase TiO₂ . Aided by employing ML to acquire surface energies from facet junction datasource, the relationships between surface energy and growth conditions based on the Langmuir adsorption isotherm are unveiled, enabling to develop controllable facet synthetic strategies. These strategies are successfully verified after applied for synthesizing TiO₂ crystals with custom crystal facets and facet junctions under tuning ionic liquid [bmim][BF₄ ] experimental conditions. Therefore, this innovative framework integrates data-intensive rational design and experimental controllable synthesis to develop and customize crystallographic facets and facet junctions. This proves the feasibility of an intelligent chemistry future to accelerate the discovery of facet-governed functional material candidates.

A novel high sensitive Cd-MOF fluorescent probe for acetone vapor in air and picric acid in water: Synthesis, structure and sensing properties

A novel three-dimensional luminescence Cd-MOF sensor with the molecular formula {[(CH₃)₂NH₂]₂ Cd₃(ptptc)₂} (complex 1) has been synthesized by using terphenyl-3,3',5,5'-tetracarboxylic acid (H₄ptptc) and Cd(NO₃)₂·4H₂O under solvothermal conditions. Single crystal X-ray diffraction analysis shows that complex 1 crystallizes in the monoclinic system C2/c space group and consists of one-dimensional channels. Complex 1 exhibits characteristic fluorescence emission (λ_em = 380 nm) both in solid state and solvents upon excitation at 300 nm. Real-time fluorescence quenching of complex 1 was observed in the fluorescence sensing of acetone vapor and picric acid. Intriguingly, ppm scale detection limit for acetone vapor in air and nano-mole scale detection limit for picric acid in water were observed. Moreover, good reusability and liner/nonlinear relationships were observed in the fluorescent titration.

ZIF-67-derived Co3O4 hollow nanocage with efficient peroxidase mimicking characteristic for sensitive colorimetric biosensing of dopamine

Co₃O₄ hollow nanocages (Co₃O₄ HNCs) were prepared by simple calcination with ZIF-67 as the precursor. Compared with ordinary nano-sized Co₃O₄, skeletal Co₃O₄ HNCs have a larger specific surface area and porosity, lead to better dispersion, which can expose more catalytic active sites, and obtain higher catalytic activity. Experiments indicate that Co₃O₄ HNCs are used as a catalyst to make H₂O₂ generate O₂. At the same time, Co₃O₄ HNCs act as bridge to accelerate the electrons transfer from the chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) to the dissolved oxygen and efficiently obtain blue oxidized TMB (oxTMB) at low concentration of H₂O₂. Steady-state kinetic analysis shows a lower K_m and a higher V_max value than other materials, indicating its excellent affinity and high catalytic efficiency. Based on the inhibitory effect of dopamine (DA) on TMB oxidation in the system, a sensitive, visual colorimetric biosensing method is developed. The calibration curve of DA has a good linear response at both high and low concentrations. Compared with other system, it has the unique advantage of very low detection limit, while retaining a wide detection range, and realizes the accurate detection of actual samples with different concentrations.

Lattice expansion and oxygen vacancy of α-Fe2O3 during gas sensing

Identifying the nature of gas-sensing material under the real-time operating condition is very critical for the research and development of gas sensors. In this work, we implement in situ Raman and XRD to investigate the gas-sensing nature of α-Fe₂O₃ sensing material, which derived from Fe-based metal-organic gel (MOG). The active mode of α-Fe₂O₃ as gas-sensing material originate from the thermally induced lattice expansion and the changes of surface oxygen vacancy of α-Fe₂O₃ could be reflected from the further monitored Raman scattering signals during acetone gas sensing. Meanwhile, the prepared α-Fe₂O₃ gas sensor exhibits excellent gas-sensing performance with high response value (R_a/R_g = 27), rapid response/recovery time (1 s/80 s) for 100 ppm acetone gas, and broad response range (5 - 900 ppm) at 183 °C. Strategies described herein could provide a promising approach to obtain gas-sensing materials with excellent performance and unveil the gas-sensing nature for other metal-oxide-based chemiresistors.

Catalytic Activation of Cobalt Doping Sites in ZIF-71-Coated ZnO Nanorod Arrays for Enhancing Gas-Sensing Performance to Acetone

Developing acetone gas sensors with high sensitivity is crucially important for many applications including nonevasive diagnosis of diabetes. In the present work, cobalt doping is used to catalyze acetone gas-sensing reactions and hence to promote the sensitivity of acetone gas sensors. In order to achieve this, ZIF-71 metal-organic framework (MOF) is synthesized onto ZnO nanorod arrays with various concentrations of Co doping to form composite ZnO@ZIF-71(Co) sensors, which are then evaluated as sensing materials for acetone detection. Such sensors are shown to be sensitive to a trace amount of acetone (50 ppb) and have a massively enhanced response of about 100 times that for the undoped sensor at an optimal Co/Zn ratio and operating temperature. Fourier-transform infrared spectroscopy and temperature-programmed desorption with density functional theory calculations are also made to assist in elucidating the catalytic gas-sensing mechanism for the Co-doped composite sensors ZnO@ZIF-71(Co). It demonstrated that the introduced Co site in ZIF-71(Co) can activate oxygen catalytically and increase active oxygen released to the ZnO surface. Meanwhile, the Co sites also promote the decomposition of acetone. These two steps together affect the catalytic oxidation of gases and finally enhance the sensitivity. This work introduces the catalytic effect of the MOF into the gas-sensing mechanism and provides an idea for broadening the application of MOF catalysis.

Development of Co(OH)xF2−x Nanosheets for Acetone Gas Sensor Applications: Material Characterization and Sensor Performance Evaluation

This study reports the employment of Co(OH)xF2−x nanosheets, a new material in the sensor field, for gas sensor applications. We synthesize Co(OH)xF2−x nanosheets via a hydrothermal route using SiO2 sphere templates. Our material characterization confirms that the material is a densely clustered Co(OH)xF2−x nanosheet with an amorphous microstructure with some short-range ordering. Sensors based on the nanosheets demonstrate a high response of 269% toward 4.5 ppm of acetone gas at an operation temperature of 200 °C and a very low minimum detection limit of 40 ppb. It functions effectively up to a temperature below 300 °C, above which F is found to start to evaporate. Our discussion suggests that an excellent sensor performance arises from the high catalytic function of F incorporated in a high concentration in the material as well as the high specific surface area due to the morphology of densely clustered nanosheets.

The properties of MOF-Zn2(EBNB)2(BPY)2·2H2O and its basic study of loading methadone

The ligands of (E)-bis(p-3-nitrobenzoic acid) vinyl (C₁₆H₁₀N₂O₈) were synthesized in three steps, and then the MOF-Zn₂(EBNB)₂(BPY)₂·2H₂O was synthesized by solvothermal method. This structure was characterized by X-ray single crystal diffraction, SEM and TG. The drug loading and in vitro release of Zn₂(EBNB)₂(BPY)₂·2H₂O were also studied with Methadone as model drug. The results show that the highest loading amount of Zn₂(EBNB)₂(BPY)₂·2H₂O to Methadone was 0.256 g/g, and the drug delivery system was a two-phase mode. The results of in vitro cytotoxicity test show that Zn₂(EBNB)₂(BPY)₂·2H₂O has good biocompatibility.