Encapsuled nanoreactors (Au@SnO₂): a new sensing material for chemical sensors

Optimization of the hydrogen response characteristics of halogen-doped SnO2

The increasing demand for efficient sensing devices with facile low-cost fabrication has attracted a lot of scientific research effort in the recent years. In particular, the scientific community aims to develop new candidate materials suitable for energy-related devices, such as sensors and photovoltaics or clean energy applications such as hydrogen production. One of the most prominent methods to improve materials functionality and performance is doping key device component(s). This paper aims to examine in detail, both from a theoretical and an experimental point of view, the effect of halogen doping on the properties of tin dioxide (SnO₂) and provide a deeper understanding on the atomic scale mechanisms with respect to their potential applications in sensors. Density Functional Theory (DFT) calculations are used to examine the defect processes, the electronic structure and the thermodynamical properties of halogen-doped SnO₂. Calculations show that halogen doping reduces the oxide bandgap by creating gap states which agree well with our experimental data. The crystallinity and morphology of the samples is also altered. The synergy of these effects results in a significant improvement of the gas-sensing response. This work demonstrates for the first time a complete theoretical and experimental characterization of halogen-doped SnO₂ and investigates the possible responsible mechanisms. Our results illustrate that halogen doping is a low-cost method that significantly enhances the room temperature response of SnO₂.

Operando X-ray Absorption Spectroscopy Study of SnO2 Nanoparticles for Electrochemical Reduction of CO2 to Formate

Tin-based electrocatalysts exhibit a remarkable ability to catalyze CO₂ to formate selectively. Understanding the size-property relationships and exploring the evolution of the active size still lack complete understanding. Herein, we prepared SnO₂ nanoparticles (NPs) with a controllable size supported on commercial carbon spheres (SnO₂/C-n, n = 1, 2, and 3) by a simple low-temperature annealing method. The transmission electron microscopy/scanning transmission electron microscopy images and fitting results of the small-angle X-ray scattering profile confirm the increased size of SnO₂ NPs due to the increase of SnO₂ loading. The catalytic performance of SnO₂ has proved the size-dependent effect during the CO₂ reduction reaction process. The as-prepared SnO₂/C-1 displayed the maximum Faradic efficiency of formate (FE_HCOO-) of 82.7% at -1.0 V versus reversible hydrogen electrode (RHE). In contrast, SnO₂/C-2 and SnO₂/C-3 with larger particle sizes achieved lower maximum FE_HCOO- and larger overpotential. Moreover, we employed operando X-ray absorption spectroscopy to study the evolution of the oxidation state and local coordination environment of SnO₂ under working conditions. In addition to the observed shifts of the rising edge of Sn K-edge X-ray absorption near-edge structure spectra to a lower energy side as the applied voltage decreases, the decreased coordination number of Sn in the Sn-O scattering path and the presence of Sn metal contribution in the extended X-ray absorption fine structure spectra verify the reduction of SnO₂ to SnO_x and metallic Sn.

Enhanced Response for Foodborne Pathogens Detection by Au Nanoparticles Decorated ZnO Nanosheets Gas Sensor

Listeria monocytogenes is a hazardous foodborne pathogen that is able to cause acute meningitis, encephalitis, and sepsis to humans. The efficient detection of 3-hydroxy-2-butanone, which has been verified as a biomarker for the exhalation of Listeria monocytogenes, can feasibly evaluate whether the bacteria are contained in food. Herein, we developed an outstanding 3-hydroxy-2-butanone gas sensor based on the microelectromechanical systems using Au/ZnO NS as a sensing material. In this work, ZnO nanosheets were synthesized by a hydrothermal reaction, and Au nanoparticles (~5.5 nm) were prepared via an oleylamine reduction method. Then, an ultrasonic treatment was carried out to modified Au nanoparticles onto ZnO nanosheets. The XRD, BET, TEM, and XPS were used to characterize their morphology, microstructure, catalytic structure, specific surface area, and chemical composition. The response of the 1.0% Au/ZnO NS sensors vs. 25 ppm 3-hydroxy-2-butanone was up to 174.04 at 230 °C. Moreover, these sensors presented fast response/recovery time (6 s/7 s), great selectivity, and an outstanding limit of detection (lower than 0.5 ppm). This work is full of promise for developing a nondestructive, rapid and practical sensor, which would improve Listeria monocytogenes evaluation in foods.

Au modified PrFeO3 with hollow tubular structure can be efficient sensing material for H2S detection

The H₂S concentration in exhaled breath increases marginally with the progress of periodontal disease, and H₂S is considered to be one of the most important gases related to meat and seafood decomposition; however, the concentration of H₂S is low and difficult to detect in such scenarios. In this study, Au–PrFeO₃ nanocrystalline powders with high specific surface areas and porosities were prepared using an electrospinning method. Our experimental results show that loading Au on the material provides an effective way to increase its gas sensitivity. Au doping can decrease the material’s resistance by adjusting its energy band, allowing more oxygen ions to be adsorbed onto the material’s surface due to a spillover effect. Compared with pure PrFeO₃, the response of 3 wt% Au–PrFeO₃ is improved by more than 10 times, and the response time is more than 10 s shorter. In addition, the concentration of H₂S due to the decomposition of shrimp was detected using the designed gas sensor, where the error was less than 15%, compared with that obtained using a GC-MS method. This study fully demonstrates the potential of Au–PrFeO₃ for H₂S concentration detection.

Electrospinning Derived NiO/NiFe2O4 Fiber-in-Tube Composite for Fast Triethylamine Detection under Different Humidity

Designing high-performance triethylamine gas sensors with the stable gas response and low resistance variation in air under a wide relative humidity range is expected for human health and environmental surveillance. Here, a novel porous NiO/NiFe₂O₄ fiber-in-tube nanostructure is prepared by the electrospinning process. The characterizations related to microstructure and surface morphology are carried out. Meanwhile, the gas sensing performance of the porous fiber-in-tube NiO/NiFe₂O₄ materials is evaluated and compared systematically. The results indicate that the introduction of NiO as the second component can not only reduce the baseline resistance of NiFe₂O₄ gas sensors dramatically but also optimize the gas sensing performance to a significant extent. Especially, the fabricated sensor based on the NiO/NiFe₂O₄ fiber-in-tube with a Ni/Fe molar ratio of 1.5 exhibits the best performance. The gas response while detecting 50 ppm triethylamine at 300 °C is about 3.6 times higher than that with Ni/Fe molar ratio of 0.5. Moreover, the response values become more stable, and the baseline resistance has a lower variation under a wide relative humidity range, demonstrating the excellent humidity resistance. These phenomena might be ascribed to the distinctive fiber-in-tube nanostructure as well as the heterojunction between NiFe₂O₄ and NiO.

Simple self-organization-based synthesis of gold nanoparticle-implanted ZnO aerogels with good sensing performance to gaseous ethanol

Simple fabrication of metal-modified oxide aerogels is expected but remains challenging. This work presents a sample one-pot synthesis method for gold nanoparticle (NP) implanted ZnO (Au-ZnO) aerogels just by sequentially adding (CH₃COO)₂Zn and NaBH₄solutions into a pre-prepared Au colloidal solution. The typically fabricated Au-ZnO aerogels are constituted by ZnO networks implanted with uniform Au NPs. The Au NPs had a size of about 100 nm, and the ZnO nanochains in the networks were about 10 nm in thickness. Further, the proportion of the Au NPs in the final aerogels could be tuned by using different amounts of the Zn precursors. Furthermore, a mechanism based on metal oxidation and oriented connection growth (a self-organization process) has been presented for describing the formation of such Au-ZnO aerogels. In the typical formation, the Zn²⁺ions first convert into ZnO beads, and then are self-organized to form networks wrapping the colloidal Au NPs under the effect of linker molecules, and this matches well with the observed experimental results. Most importantly, these Au-ZnO aerogels show great structurally enhanced gas sensing properties to gaseous ethanol compared with a pure ZnO film. They have a fast response (about 30 s), a high selectivity, and quantitative sensing to the target gas. This work has provided a simple preparation method for Au-ZnO aerogels, and also shows their great potential in gas sensing applications.

Room-Temperature Benzene Sensing with Au-Doped ZnO Nanorods/Exfoliated WSe2 Nanosheets and Density Functional Theory Simulations

A gold-doped zinc oxide (Au-ZnO)/exfoliated tungsten diselenide (exfoliated WSe₂) nanocomposite-based gas sensor toward benzene with high sensing properties was demonstrated. Epoxy resin was used as the matrix of the Au-ZnO/exfoliated WSe₂ nanocomposite sensor. The straw-shaped Au-ZnO was synthesized by the hydrothermal method, and WSe₂ nanosheets (NSs) were prepared via hydrothermal and liquid-phase exfoliation methods. The properties of Au-ZnO/exfoliated WSe₂ nanoheterostructures constructed by self-assembly technology have been confirmed via a series of characterization methods. The benzene-sensing performances of sensors were tested at 25 °C. Compared with Au-ZnO, WSe_2, and their composites, the Au-ZnO/exfoliated WSe₂ sensor has a significant performance improvement, including a higher response and linear fit degree, better selectivity and repeatability, and faster detection rate. The significantly enhanced sensing properties of the Au-ZnO/exfoliated WSe₂ sensor can be ascribed to the doping of Au nanoparticles, the increase in the specific surface area and adsorption sites of NSs after exfoliation, and the cooperative interface combination of the ZnO/WSe₂ heterojunction. Furthermore, the sensitivity mechanism of the composite sensor to benzene was explored by density functional theory simulations.

Yolk-Shell Nanocapsule Catalysts as Nanoreactors with Various Shell Structures and Their Diffusion Effect on the CO2 Reforming of Methane

Well-geometric-confined yolk-shell catalysts can act as nanoreactors that are of benefit for the antisintering of metals and resistance to coke formation in high-temperature reactions such as the CO₂ reforming of methane. Notwithstanding the credible advances of core/yolk-shell catalysts, the enlarged shell diffusion effects that occur under high space velocity can deactivate the catalysts and hence pose a hurdle for the potential application of these types of catalysts. Here, we demonstrated the importance of the shell thickness and porosity of small-sized Ni@SiO₂ nanoreactor catalysts, which can vary the diffusional paths/rates of the diffusants that directly affect the catalytic activity. The nanoreactor with an ∼4.5 nm shell thickness and rich pores performed the best in tolerating the shell diffusion effects, and importantly, no catalytic deactivation was observed. We further proposed a shell diffusion effect scheme by modifying the Weisz-Prater and blocker model and found that the "gas wall/hard blocker" formed on the openings of the shell pores can cause reversible/irreversible interruption of the shell mass transfer and thus temporarily/permanently deactivate the nanoreactor catalysts. This work highlights the shell diffusion effects, apart from the metal sintering and coke formation, as an important factor that are ascribed to the deactivation of a nanoreactor catalyst.

Ex-situ XPS analysis of yolk-shell Sb2O3/WO3 for ultra-fast acetone resistive sensor

The preparation of fast, highly responsive and reliable gas sensing devices for the detection of acetone gas is considered to be a key challenge for the development of accurate disease diagnosis systems through exhaled respiratory gases. In the paper, yolk shell Sb₂O₃/WO₃ is synthesized and its gas sensing performance was studied by static test system. Special, the maximum response value of 1:1 Sb₂O₃/WO₃ yolk-shell (WO₃-1 YSL) sensor to 100 ppm acetone can reach as high as 50.0 at 200 ℃. And it also exhibits excellent response/recover time (4 s/5 s), low detection limit (2 ppm) and superior selectivity towards acetone. More importantly, in mixed selective gas test, the sensor shows high selectivity towards acetone. And the mechanism is analyzed by ex-situ XPS. The excellent gas-sensing performance can be attributed to unique yolk-shell structure, which facilitates the rapid transport of charge carriers from the surface to the bulk and provides more active sites for gas adsorption and desorption; the heterojunction between of Sb₂O₃ and WO₃, which promotes oxygen pre-adsorption on the surface and increasing the interfacial potential; the increased oxygen vacancies which allowing more chemisorbed oxygen to form.

Highly Sensitive Hybrid Nanostructures for Dimethyl Methyl Phosphonate Detection

Nanostructured materials synthesized by the hydrothermal and thermal reduction process were tested to detect the dimethyl methylphosphonate (DMMP) as a simulant for chemical warfare agents. Manganese oxide nitrogen-doped graphene oxide with polypyrrole (MnO2@NGO/PPy) exhibited the sensitivity of 51 Hz for 25 ppm of DMMP and showed the selectivity of 1.26 Hz/ppm. Nitrogen-doped multi-walled carbon nanotube (N-MWCNT) demonstrated good linearity with a correlation coefficient of 0.997. A comparison between a surface acoustic wave and quartz crystal microbalance sensor exhibited more than 100-times higher sensitivity of SAW sensor than QCM sensor.

Sensitive and Low-Power Metal Oxide Gas Sensors with a Low-Cost Microelectromechanical Heater

In this study, a simple and cost-effective metal oxide semiconductor (MOS) gas sensor, which can be fabricated utilizing only two photolithography steps, was designed and developed through the planar microelectromechanical systems (MEMS) technique. Ball-milled porous tin dioxide nanoparticle clusters were precisely drop-coated onto the integrated microheater region and subsequently characterized using a helium ion microscope (HIM). The spatial suspension of the silicon nitride platform over the silicon substrate provides superior thermal isolation and thus dramatically reduces the power consumption of the microheater. The well-designed microheater exhibits excellent thermal uniformity, which was verified both computationally and experimentally. The as-fabricated sensors were tested for ethanol gas sensing at various operating temperatures with different concentrations. At the optimal work temperature of ∼400 °C, our gas sensors demonstrated a respectable sensitivity to 1 ppm ethanol, which is the lower detection limit to most commercial products. Moreover, stable performance over repetitive testing was observed. The innovative sensor developed here is a promising candidate for portable gas sensing devices and various other commercial applications.

Self-template-derived ZnCo2O4 porous microspheres decorated by Ag nanoparticles and their selective detection of formaldehyde

ZnCo₂O₄ porous microspheres were successfully synthesized through a facile one-step solvent method using polyethylene glycol 1000 as a self-assembly template and subsequent annealing.

Enhanced CO Sensing Performances of PdO/WO3 Determined by Heterojunction Structure under Illumination

The CO sensing performances and mechanism of PdO/WO3-based sensors were investigated by experiments and density functional theory calculations. The CO sensing performance can be significantly enhanced by decorating WO3 with PdO, which is attributed to the catalyst (chemical sensitization) and P-N junction (electronic effect). On the one hand, PdO is an excellent catalyst used to promote the adsorption of oxygen species. On the other hand, the constructed P-N junction structure between PdO and WO3 can facilitate the migration of carriers and suppress the recombination of electrons and holes, which promote the adsorption of more oxygen species. Furthermore, the calculation results verify that decorating WO3 with PdO can significantly enhance the CO sensing response by providing more adsorption sites available for oxygen species and make more electrons to transfer from CO to PdO/WO3 configuration. Moreover, the band gap energies of the WO3 sensor can be reduced by PdO decoration, and the light absorption range in the visible light region can be expanded. More photogenerated electron-hole pairs can be produced based on the P-N junction structure, which can promote the progress of electrochemical reactions. Thus, the PdO/WO3 material can be a promising candidate to detect CO, and it can effectively utilize the UV-visible light to destruct the CO contaminant.

Pd-Decorated PdO Hollow Shells: A H2-Sensing System in Which Catalyst Nanoparticle and Semiconductor Support are Interconvertible

Developing a simple strategy to fabricate high-performance hydrogen sensors with long-term stability remains quite challenging. Here, we report the H₂-sensing performance of Pd-decorated PdO hollow shells (Pd/PdO HSs). In this novel system, the catalyst nanoparticles (Pd NPs) and semiconductor support (PdO) are interconvertible, which is different from traditional hydrogen-sensing systems such as Pd/TiO₂ and Pd/ZnO. This Pd/PdO system exhibits multiple unique properties. First, well-distributed Pd NPs with controllable density can be decorated on PdO support through a one-step NaBH₄ treatment during which PdO is partially reduced into Pd. Second, the decorated Pd NPs are physically inlaid in the PdO support, which not only prevents the agglomeration or detachment of Pd NPs but also enhances the electron transfer between Pd NPs and PdO. Third, Pd/PdO HSs can be reoxidized into PdO HSs once their sensing performance degrades, which repeatedly manipulates Pd/PdO HSs under the initial reduction process, leading to the reactivation of the sensing performance. With all these advantages, Pd/PdO HSs demonstrate a detection limit lower than 1 ppm, a response/recovery time to 1% H₂ of 5 s/32 s at room temperature, and a repeatable reactivation ability. The strategy presented here is convenient and time saving and has no need to prefunctionalize the PdO surface for the decoration of catalyst NPs. Moreover, the unique reactivation ability of Pd/PdO system opens a new strategy toward extending the lifetime of H₂ sensors.

Ultrasonic preparation and nanosheets supported binary metal oxide nanocomposite for the effective application towards the electrochemical sensor

Binary metal oxides (La₂O₃@SnO₂) decorated reduced graphene oxide nanocomposite was synthesized by ultrasound process in an environmentally benign solvent with a working frequency of 25 and 40 kHz (6.5 l200 H, Dakshin, India and maximum input power 210 W). Further, to enhance the electrocatalytic activity, the reduced graphene oxide (rGO) was prepared from graphene oxide by ultrasonication method. As prepared La₂O₃@SnO₂/rGO was scrutinized using XRD, TEM, EDX and quantitative test for the structural and morphology properties. As modified La₂O₃@SnO₂/rGO nanocomposite exhibits better electrochemical activity towards the oxidation of methyl nicotinate with higher anodic current compared to other modified and unmodified electrode for the detection of methyl nicotinate with larger linear range (0.035-522.9 µM) and lower limit of detection (0.0197 µM). In addition, the practical feasibility of the sensor was inspected with biological samples, reveals the acceptable recovery of the sensor in real samples.

A redox interaction-engaged strategy for multicomponent nanomaterials

The review article focuses on the redox interaction-engaged strategy that offers a powerful way to construct multicomponent nanomaterials with precisely-controlled size, shape, composition and hybridization of nanostructures.

Functionalization of Hollow Nanomaterials for Catalytic Applications: Nanoreactor Construction

Hollow nanomaterials have attracted a broad interest in multidisciplinary research due to their unique structure and preeminent properties. Owing to the high specific surface area, well-defined active site, delimited void space, and tunable mass transfer rate, hollow nanostructures can serve as excellent catalysts, supports, and reactors for a variety of catalytic applications, including photocatalysis, electrocatalysis, heterogeneous catalysis, homogeneous catalysis, etc. Based on state-of-the-art synthetic methods and characterization techniques, researchers focus on the purposeful functionalization of hollow nanomaterials for catalytic mechanism studies and intricate catalytic reactions. Herein, an overview of current reports with respect to the catalysis of functionalized hollow nanomaterials is given, and they are classified into five types of versatile strategies with a top-down perspective, including textual and composition modification, encapsulation, multishelled construction, anchored single atomic site, and surface molecular engineering. In the detailed case studies, the design and construction of hierarchical hollow catalysts are discussed. Moreover, since hollow structure offers more than two types of spatial-delimited sites, complicated catalytic reactions are elaborated. In summary, functionalized hollow nanomaterials provide an ideal model for the rational design and development of efficient catalysts.

Biocompatible Two-Dimensional Titanium Nanosheets for Multimodal Imaging-Guided Cancer Theranostics

Photothermal therapy (PTT) based on two-dimensional (2D) nanomaterials has shown significant potential in cancer treatment. However, developing 2D nanomaterial-based theranostic agents with good biocompatibility and high therapeutic efficiency remains a key challenge. Bulk titanium (Ti) has been widely used as biomedical materials for their reputable biocompatibility, whereas nanosized Ti with a biological function remains unexplored. In this work, the 2D Ti nanosheets (NSs) are successfully exfoliated from nonlayer bulk Ti and utilized as an efficient theranostic nanoplatform for dual-modal computed tomography/photoacoustic (CT/PA) imaging-navigated PTT. Besides the excellent biocompatibility obtained by TiNSs as expected, they are found to show strong absorption ability with an extinction coefficient of 20.8 L g^-1 cm^-1 and high photothermal conversion ability with an efficiency of 61.5% owing to localized surface plasmon resonances, which exceeds most of other well-known photothermal agents, making it quite promising for PTT against cancer. Furthermore, the metallic property and light-heat-acoustic transformation endow 2D Ti with the strong CT/PA imaging signal and efficient cancer therapy, simultaneously. This work highlights the enormous potential of nanosized Ti in both the diagnosis and treatment of cancer. As a paradigm, this study also paves a new avenue for the elemental transition-metal-based cancer theranostics.

Grain-Boundary-Induced Drastic Sensing Performance Enhancement of Polycrystalline-Microwire Printed Gas Sensors

The development of materials with high efficiency and stable signal output in a bent state is important for flexible electronics. Grain boundaries provide lasting inspiration and a promising avenue for designing advanced functionalities using nanomaterials. Combining bulk defects in polycrystalline materials is shown to result in rich new electronic structures, catalytic activities, and mechanical properties for many applications. However, direct evidence that grain boundaries can create new physicochemical properties in flexible electronics is lacking. Here, a combination of bulk electrosensitive measurements, density functional theory calculations, and atomic force microscopy technology with quantitative nanomechanical mapping is used to show that grain boundaries in polycrystalline wires are more active and mechanically stable than single-crystalline wires for real-time detection of chemical analytes. The existence of a grain boundary improves the electronic and mechanical properties, which activate and stabilize materials, and allow new opportunities to design highly sensitive, flexible chemical sensors.

Exploring the potential of electroless and electroplated noble metal-semiconductor hybrids within bio- and environmental sensing

Over the last two decades, the rapid development and widespread application of nanomaterials has significantly influenced research in various fields, including analytical chemistry and biosensing technologies. In particular, the simple functionalization and tuning of noble metal nanoparticle (NP) surface chemistry resulted in the development of a series of novel biosensing platforms with quick read-out and enhanced capabilities towards specific analyte detection. Moreover, noble metal NPs possess a number of unique properties, viz. high surface-to-volume ratio and excellent spectral, optical, thermal, electrical and catalytic characteristics. This manuscript provides an elaborate review on galvanic noble metal NPs deposited onto semiconductor surfaces, from the preparation stage towards their application in biosensors and gas sensing. Two types of deposition approaches, viz. galvanic displacement/electroless and conventional electroplating, are introduced and compared. Furthermore, the analytical merit of hybrid nanomaterials towards the improvement of sensing abilities is highlighted. Finally, some limitations and challenges related to progress in the development and application of analytical devices based on electroless and electroplated noble metal NPs-semiconductor hybrids (NMNPsHs) in biochemical and environmental sensing are discussed.

A general one-pot strategy for the synthesis of Au@multi-oxide yolk@shell nanospheres with enhanced catalytic performance

By integrating redox self-assembly and redox etching processes, we report a general one-pot strategy for the synthesis of Au@multi-M x O y (M = Co, Ce, Fe, and Sn) yolk@shell nanospheres. Without any additional protecting molecule or reductant, the whole reaction is a clean redox process that happens among the inorganic metal salts in an alkaline aqueous solution. By using this method, Au@Co3O4/CeO2 (Au@Co-Ce), Au@Co3O4/Fe2O3 (Au@Co-Fe), and Au@CeO2/SnO2 (Au@Ce-Sn) yolk@shell nanospheres with binary oxides as shells, Au@Co3O4/CeO2/Fe2O3 (Au@Co-Ce-Fe) yolk@shell nanospheres with ternary oxides as shells and Au@Co3O4/CeO2/Fe2O3/SnO2 (Au@Co-Ce-Fe-Sn) yolk@shell nanospheres with quaternary oxides as shells can be obtained. Subsequently, the catalytic CO oxidation was selected as the catalytic model, and the Au@Co-Ce system was chosen as the catalyst. It was found that the catalytic activity of Au@Co-Ce yolk@shell nanospheres can be optimized by altering the relative proportion of Co and Ce oxides.

Recent Advances of SnO2-Based Sensors for Detecting Fault Characteristic Gases Extracted From Power Transformer Oil

Tin oxide SnO₂-based gas sensors have been widely used for detecting typical fault characteristic gases extracted from power transformer oil, namely, H₂, CO, CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆, due to the remarkable advantages of high sensitivity, fast response, long-term stability, and so on. Herein, we present an overview of the recent significant improvement in fabrication and application of high performance SnO₂-based sensors for detecting these fault characteristic gases. Promising materials for the sensitive and selective detection of each kind of fault characteristic gas have been identified. Meanwhile, the corresponding sensing mechanisms of SnO₂-based gas sensors of these fault characteristic gases are comprehensively discussed. In the final section of this review, the major challenges and promising developments in this domain are also given.

Dual Role of Multiroom-Structured Sn-Doped NiO Microspheres for Ultrasensitive and Highly Selective Detection of Xylene

Sn-doped NiO multiroom spheres with unique microreactor morphology were prepared by facile ultrasonic spray pyrolysis of a solution containing tin oxalate, nickel nitrate, and dextrin and subsequent heat treatment. The multiroom structure was formed by phase segregation between the molten metal source and liquidlike dextrin and sequent decomposition of dextrin during spray pyrolysis, which played the dual roles of enhancing gas response and selectivity. The response (resistance ratio) of the Sn-doped NiO multiroom spheres to 1 ppm p-xylene was as high as 65.4 at 300 °C, which was 50.3 and 9.0 times higher than those of pure NiO multiroom spheres and Sn-doped NiO dense spheres, respectively. In addition, the Sn-doped NiO multiroom sensors showed a high selectivity to xylene. The unprecedented high response that enables the sensing of sub-ppm xylene was explained by the high gas accessibility of the multiroom structures and the Sn-doping-induced change in oxygen adsorption as well as the charge carrier concentration, whereas the high xylene selectivity was attributed to the decomposition/re-forming of xylene into smaller or more active species within the unique multiroom structure of Sn-doped NiO microreactors characterized by high catalytic activities. The multiroom oxide spheres can be used as a new and generalized platform to design high-performance gas sensors.

Designing Binary Ru-Sn Oxides with Optimized Performances for the Air Electrode of Rechargeable Zinc-Air Batteries

Because of the sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), binary ruthenium-tin oxides synthesized by a hydrothermal method with postannealing at 450 °C for 2 h are first proposed as bifunctional catalysts for these two reactions on the air electrode of rechargeable zinc-air batteries. The binary Ru-Sn oxides in various compositions show the typical oxide solid solution in the rutile phase. Among all binary Ru-Sn oxides, RuSn73 (70 atom % RuO₂ and 30 atom % SnO₂) and RuSn37 (30 atom % RuO₂ and 70 atom % SnO₂) show the highest catalytic activities toward the OER and ORR, respectively. Consequently, a novel design of the air electrode consisting of a RuSn37 coating on the carbon paper and a Ti mesh coated with RuSn73 (denoted RuSn(37-C|73-Ti)) is proposed to possess the optimal charge-discharge performances. A unique cell employing such an air electrode has been demonstrated to exhibit a very low charge-discharge cell voltage gap of 0.75 V at 10 mA cm^-2. This cell with a peak power density of 120 mW cm^-2 at the current density of 235 mA cm^-2 also shows an outstanding charge-discharge stability over 80 h. This cell also exhibits an exceptionally high charge rate capability at 150 mA cm^-2 with a low charging voltage of 2.0 V.

Synthesis and Characterization of Highly Sensitive Hydrogen (H₂) Sensing Device Based on Ag Doped SnO₂ Nanospheres

In this paper, pure and Ag-doped SnO₂ nanospheres were synthesized by hydrothermal method and characterized via X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectra (XPS), respectively. The gas sensing performance of the pure, 1 at.%, 3 at.%, and 5 at.% Ag-doped SnO₂ sensing devices toward hydrogen (H₂) were systematically evaluated. The results indicated that compared with pure SnO₂ nanospheres, Ag-doped SnO₂ nanospheres could not only decrease the optimum working temperature but also significantly improve H₂ sensing such as higher gas response and faster response-recovery. Among all the samples, the 3 at.% Ag-doped SnO₂ showed the highest response 39 to 100 μL/L H₂ at 300 °C. Moreover, its gas sensing mechanism was discussed, and the results will provide reference and theoretical guidance for the development of high-performance SnO₂-based H₂ sensing devices.

Gas-sensing enhancement methods for hydrothermal synthesized SnO2-based sensors

Gas sensing for hydrothermal synthesized SnO₂-based gas sensors can be enhanced in three ways: structural improvement, composition optimization, and processing improvement. There have been zero-dimensional, one-dimensional, and three-dimensional structures reported in the literature. Controllable synthesis of different structures has been deployed to increase specific surface area. Change of composition would intensively tailor the SnO₂ structure, which affected the gas-sensing performance. Furthermore, doping and compounding methods have been adopted to promote gas-sensing performance by adjusting surface conditions of SnO₂ crystals and constructing heterojunctions. As for processing area, it is very important to find the optimal reaction time and temperature. In this paper, a gas-solid reaction rate constant was proposed to evaluate gas-sensing properties and find an excellent hydrothermal synthesized SnO₂-based gas sensor.

NiO/NiWO4 Composite Yolk-Shell Spheres with Nanoscale NiO Outer Layer for Ultrasensitive and Selective Detection of Subppm-level p-Xylene

NiO/NiWO₄ composite yolk-shell spheres with a nanoscale NiO outer layer were prepared using one-pot ultrasonic spray pyrolysis and their gas sensing characteristics were studied. The NiO/NiWO₄ yolk-shell spheres exhibited an extremely high response to 5 ppm p-xylene (ratio of resistance to gas and air = 343.5) and negligible cross-responses to 5 ppm ethanol, ammonia, carbon monoxide, hydrogen, and benzene, whereas pure NiO yolk-shell spheres showed very low responses and selectivity to all the analyte gases. The detection limit for p-xylene was as low as 22.7 ppb. This ultrasensitive and selective detection of p-xylene is attributed to a synergistic catalytic effect between NiO and NiWO₄, high gas accessibility with large specific surface area, and increased chemiresistive variation due to the formation of a heterojunction. The NiO/NiWO₄ yolk-shell spheres with a thin NiO outer layer can be used to detect subppm-level p-xylene in a highly sensitive and selective manner for monitoring indoor air pollution.

Single and Networked ZnO-CNT Hybrid Tetrapods for Selective Room-Temperature High-Performance Ammonia Sensors

Highly porous hybrid materials with unique high-performance properties have attracted great interest from the scientific community, especially in the field of gas-sensing applications. In this work, tetrapodal-ZnO (ZnO-T) networks were functionalized with carbon nanotubes (CNTs) to form a highly efficient hybrid sensing material (ZnO-T-CNT) for ultrasensitive, selective, and rapid detection of ammonia (NH₃) vapor at room temperature. By functionalizing the ZnO-T networks with 2.0 wt % of CNTs by a simple dripping procedure, an increase of 1 order of magnitude in response (from about 37 to 330) was obtained. Additionally, the response and recovery times were improved (by decreasing them from 58 and 61 s to 18 and 35 s, respectively). The calculated lowest detection limit of 200 ppb shows the excellent potential of the ZnO-T-CNT networks as NH₃ vapor sensors. Room temperature operation of such networked ZnO-CNT hybrid tetrapods shows an excellent long-time stability of the fabricated sensors. Additionally, the gas-sensing mechanism was identified and elaborated based on the high porosity of the used three-dimensional networks and the excellent conductivity of the CNTs. On top of that, several single hybrid microtetrapod-based devices were fabricated (from samples with 2.0 wt % CNTs) with the help of the local metal deposition function of a focused ion beam/scanning electron microscopy instrument. The single microdevices are based on tetrapods with arms having a diameter of around 0.35 μm and show excellent NH₃ sensing performance with a gas response (I_gas/I_air) of 6.4. Thus, the fabricated functional networked ZnO-CNT hybrid tetrapods will allow to detect ammonia and to quantify its concentration in automotive, environmental monitoring, chemical industry, and medical diagnostics.

A flexible, ultra-sensitive chemical sensor with 3D biomimetic templating for diabetes-related acetone detection

The structural features of biological organisms have evolved through natural selection to provide highly tailored functions, inspiring numerous biomimetic and biological design strategies. A wide scope of untapped potential lies in harnessing the nanoscale architectural properties of natural biological materials to develop high-performance sensors. Herein, we report the development of an ultrasensitive chemical sensor that is based on the three-dimensional (3D) biomimetic templating of a structurally hierarchical butterfly wing. In conjunction with graphene sheet coating strategies, the porous 3D architecture enables highly selective detection of diabetes-related volatile organic compounds (VOCs), including a rapid response time (≤1 s), a low limit of detection (20 ppb), and superior mechanical properties. Taken together, the findings in this work demonstrate the promise of incorporating natural biological materials into high-performance sensors, with excellent potential for wearable and flexible sensors.

Hollow ZnSnO3 Cubes with Controllable Shells Enabling Highly Efficient Chemical Sensing Detection of Formaldehyde Vapors

In structural hierarchy, inherently hollow nanostructured materials preferentially possessing high surface area demand attention due to their alluring sensing performances. However, the activity of hollow and structural hierarchy nanomaterials generally remains suboptimal due to their hollow space structure and large lateral size, which greatly hamper and limit the availability of inner space active sites. Here, hollow ZnSnO₃ cubes with a controllable interior structure were successfully prepared through a simple and low-cost coprecipitation approach followed with a calcination process. The solid-, single-, double-, and multishelled ZnSnO₃ hollow cubes could be selectively tailored by repeated addition of alkaline solution. The multishelled architecture displayed outstanding sensing properties for formaldehyde vapors due to large specific surface area, less agglomerations, abundant interfaces, thin shells, and high proportion porous structure, which act synergistically to facilitate charge transfer and promote target gas adsorption.

A Generalized Method for the Synthesis of Ligand-Free M@SiO2 (M = Ag, Au, Pd, Pt) Yolk-Shell Nanoparticles

A universal method is reported for the synthesis of ligand free noble metal M@SiO₂ (M = Ag, Au, Pd, Pt) yolk-shell nanoparticles (YSNs). Mesoporous hollow silica shells (mHSS) are used as smart nanoreactors for the synthesis of noble metal yolk-shell nanoparticles. The nanocavity of a mHSS and anionic metal ions play a critical role in the formation of yolk-shell nanoparticles. The synthesis mechanism can be tuned by simply varying the pH of the noble metal precursor aqueous solution. A critical pH ≥ 4 is required for the formation of YSNs. The anionic metal ions can pass freely through the mesopores of mHSS and eventually lead to the formation of YSNs, whereas the cationic metal ions can show strong interaction with the surface of mHSS which hinders the formation of YSNs. The syntheses of YSNs are achieved without using any external capping ligands and reducing agents in the reaction.

Nanoscale PdO Catalyst Functionalized Co3O4 Hollow Nanocages Using MOF Templates for Selective Detection of Acetone Molecules in Exhaled Breath

The increase of surface area and the functionalization of catalyst are crucial to development of high-performance semiconductor metal oxide (SMO) based chemiresistive gas sensors. Herein, nanoscale catalyst loaded Co₃O₄ hollow nanocages (HNCs) by using metal-organic framework (MOF) templates have been developed as a new sensing platform. Nanoscale Pd nanoparticles (NPs) were easily loaded on the cavity of Co based zeolite imidazole framework (ZIF-67). The porous structure of ZIF-67 can restrict the size of Pd NPs (2-3 nm) and separate Pd NPs from each other. Subsequently, the calcination of Pd loaded ZIF-67 produced the catalytic PdO NPs functionalized Co₃O₄ HNCs (PdO-Co₃O₄ HNCs). The ultrasmall PdO NPs (3-4 nm) are well-distributed in the wall of Co₃O₄ HNCs, the unique structure of which can provide high surface area and high catalytic activity. As a result, the PdO-Co₃O₄ HNCs exhibited improved acetone sensing response (R_gas/R_air = 2.51-5 ppm) compared to PdO-Co₃O₄ powders (R_gas/R_air = 1.98), Co₃O₄ HNCs (R_gas/R_air = 1.96), and Co₃O₄ powders (R_gas/R_air = 1.45). In addition, the PdO-Co₃O₄ HNCs showed high acetone selectivity against other interfering gases. Moreover, the sensor array clearly distinguished simulated exhaled breath of diabetics from healthy people's breath. These results confirmed the novel synthesis of MOF templated nanoscale catalyst loaded SMO HNCs for high performance gas sensors.

Design and Synthesis of Cu@CuS Yolk-Shell Structures with Enhanced Photocatalytic Activity

Non-spherical Cu@CuS yolk-shell structures are successfully obtained using Cu₂O cube templates in a process combining rapid surface sulfidation followed by disproportionation of the Cu₂O core upon treatment with a hydrochloric acid solution. By employing the above method, Cu@CuS yolk-shell structures with different morphologies, including octahedral, truncated octahedral, and cuboctahedral shapes, can be synthesized. The void space within the hollow structures provides a unique confined space, where the metallic copper present in the core of a shell can be protected from agglomeration and oxidation. Furthermore, the presence of metal copper in these hollow structures contributes to improvement in the photocatalytic properties of these materials. The application of these Cu@CuS structures indeed shows clearly improved photocatalytic performance.

New insights and perspectives into biological materials for flexible electronics

Materials based on biological materials are becoming increasingly competitive and are likely to be critical components in flexible electronic devices.

Visible photoassisted room-temperature oxidizing gas-sensing behavior of Sn2S3 semiconductor sheets through facile thermal annealing

Well-crystallized Sn₂S₃ semiconductor thin films with a highly (111)-crystallographic orientation were grown using RF sputtering. The surface morphology of the Sn₂S₃ thin films exhibited a sheet-like feature. The Sn₂S₃ crystallites with a sheet-like surface had a sharp periphery with a thickness in a nanoscale size, and the crystallite size ranged from approximately 150 to 300 nm. Postannealing the as-synthesized Sn₂S₃ thin films further in ambient air at 400 °C engendered roughened and oxidized surfaces on the Sn₂S₃ thin films. Transmission electron microscopy analysis revealed that the surfaces of the Sn₂S₃ thin films transformed into a SnO₂ phase, and well-layered Sn₂S₃-SnO₂ heterostructure thin films were thus formed. The Sn₂S₃-SnO₂ heterostructure thin film exhibited a visible photoassisted room-temperature gas-sensing behavior toward low concentrations of NO₂ gases (0.2-2.5 ppm). By contrast, the pure Sn₂S₃ thin film exhibited an unapparent room-temperature NO₂ gas-sensing behavior under illumination. The suitable band alignment at the interface of the Sn₂S₃-SnO₂ heterostructure thin film and rough surface features might explain the visible photoassisted room-temperature NO₂ gas-sensing responses of the heterostructure thin film on exposure to NO₂ gas at low concentrations in this work.

Template-Engaged Solid-State Synthesis of Barium Magnesium Silicate Yolk@Shell Particles and Their High Photoluminescence Efficiency

This study presents a new synthetic method for fabricating yolk@shell-structured barium magnesium silicate (BMS) particles through a template-engaged solid-state reaction. First, as the core template, (BaMg)CO3 spherical particles were prepared based on the coprecipitation of Ba(2+) and Mg(2+) . These core particles were then uniformly shelled with silica (SiO2 ) by using CTAB as the structure-directing template to form (BaMg)CO3 @SiO2 particles with a core@shell structure. The (BaMg)CO3 @SiO2 particles were then converted to yolk@shell barium magnesium silicate (BMS) particles by an interfacial solid-state reaction between the (BaMg)CO3 (core) and the SiO2 (shell) at 750 °C. During this interfacial solid-state reaction, Kirkendall diffusion contributed to the formation of yolk@shell BMS particles. Thus, the synthetic temperature for the (BaMg)SiO4 :Eu(3+) phosphor is significantly reduced from 1200 °C with the conventional method to 750 °C with the proposed method. In addition, the photoluminescence intensity of the yolk@shell (BaMg)SiO4 :Eu(3+) phosphor was found to be 9.8 times higher than that of the conventional (BaMg)SiO4 :Eu(3+) phosphor. The higher absorption of excitation light by the structure of the yolk@shell phosphor is induced by multiple light-reflection and -scattering events in the interstitial void between the yolk and the shell. When preparing the yolk@shell (BaMg)SiO4 :Eu(3+) phosphor, a hydrogen environment for the solid-state reaction results in higher photoluminescence efficiency than nitrogen and air environments. The proposed synthetic method can be easily extended to the synthesis of other yolk@shell multicomponent metal silicates.

Co3O4-SnO2 Hollow Heteronanostructures: Facile Control of Gas Selectivity by Compositional Tuning of Sensing Materials via Galvanic Replacement

Co3O4 hollow spheres prepared by ultrasonic spray pyrolysis were converted into Co3O4-SnO2 core-shell hollow spheres by galvanic replacement with subsequent calcination at 450 °C for 2 h for gas sensor applications. Gas selectivity of the obtained spheres can be controlled by varying the amount of SnO2 shells (14.6, 24.3, and 43.3 at. %) and sensor temperatures. Co3O4 sensors possess an ability to selectively detect ethanol at 275 °C. When the amount of SnO2 shells was increased to 14.6 and 24.3 at. %, highly selective detection of xylene and methylbenzenes (xylene + toluene) was achieved at 275 and 300 °C, respectively. Good selectivity of Co3O4 hollow spheres to ethanol can be explained by a catalytic activity of Co3O4; whereas high selectivity of Co3O4-SnO2 core-shell hollow spheres to methylbenzenes is attributed to a synergistic effect of catalytic SnO2 and Co3O4 and promotion of gas sensing reactions by a pore-size control of microreactors.