Highly sensitive ZnO nanorod- and nanoprism-based NO2 gas sensors: size and shape control using a continuous hydrothermal pilot plant

General Model for Predicting Response of Gas-Sensitive Materials to Target Gas Based on Machine Learning

Gas sensors play a crucial role in various industries and applications. In recent years, there has been an increasing demand for gas sensors in society. However, the current method for screening gas-sensitive materials is time-, energy-, and cost-consuming. Consequently, an imperative exists to enhance the screening efficiency. In this study, we proposed a collaborative screening strategy through integration of density functional theory and machine learning. Taking zinc oxide (ZnO) as an example, the responsiveness of ZnO to the target gas was determined quickly on the basis of the changes in the electronic state and structure before and after gas adsorption. In this work, the adsorption energy and electronic and structural characteristics of ZnO after adsorbing 24 kinds of gases were calculated. These computed features served as the basis for training a machine learning model. Subsequently, various machine learning and evaluation algorithms were utilized to train the fast screening model. The importance of feature values was evaluated by the AdaBoost, Random Forest, and Extra Trees models. Specifically, charge transfer was assigned importance values of 0.160, 0.127, and 0.122, respectively, ranking as the highest among the 11 features. Following closely was the d-band center, which was presumed to exert influence on electrical conductivity and, consequently, adsorption properties. With 5-fold cross-validation using the Extra Tree accuracy, the 24-sample data set achieved an accuracy of 88%. The 72-sample data set achieved an accuracy of 78% using multilayer perceptron after 5-fold cross-validation, with both data sets exhibiting low standard deviations. This verified the accuracy and reliability of the strategy, showcasing its potential for rapidly screening a material's responsiveness to the target gas.

Two-Step Visible Light Photocatalytic Dye Degradation Phenomena in Ag2O-Impregnated ZnO Nanorods via Growth of Metallic Ag and Formation of ZnO/Ag0/Ag2O Heterojunction Structures

Visible light photocatalytic activity follows the single-slope pseudo-first-order reaction kinetics in pristine ZnO nanorods, while for pure Ag₂O, a two-slope paradigm is pursued with a higher slope at a later period. For the Ag₂O-impregnated ZnO heterostructured nanorod photocatalyst, the two-step photocatalysis phenomena proceed with dye degradation rate constants emerging higher than those of individual ZnO and Ag₂O, at both time zones. Improved performance of ZnO/Ag₂O heterostructures arises initially from the reduced e^-/h⁺ recombination rate by the synergistic effect between ZnO and Ag₂O. At a later phase, metallic Ag is produced, which traps the valence electrons of Ag₂O nanoparticles and advances the e^-/h⁺ separation across the ZnO/Ag⁰/Ag₂O heterojunction structures, rendering them promptly accessible for dye degradation. At an increased Ag₂O loading, the photodegradation rate constants boost up in both time zones, and the corresponding crossover time (t_C) between the two phases steadily diminishes, leading toward a unique photocatalytic phenomenon that prevails with a superior rate constant. The optimized ZAO₂₅ heterostructure photocatalyst demonstrates ∼96.24% photodegradation of methylene blue (MB) dye within 30 min of visible light exposure, and its degradation rate constant is ∼0.24848 min^-1, which is ∼26.75 times superior than that of pristine ZnO samples. The metal-induced biphasic photocatalysis phenomena have never been reported earlier.

Hollow ZnO nanorices prepared by a simple hydrothermal method for NO2 and SO2 gas sensors

Chemoresistive gas sensors play an important role in detecting toxic gases for air pollution monitoring. However, the demand for suitable nanostructures that could process high sensing performance remains high. In this study, hollow ZnO nanorices were synthesized by a simple hydrothermal method to detect NO₂ and SO₂ toxic gases efficiently. Material characterization by some advanced techniques, such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy, demonstrated that the hollow ZnO nanorices had a length and diameter size of less than 500 and 160 nm, respectively. In addition, they had a thin shell thickness of less than 30 nm, formed by an assembly of tiny nanoparticles. The sensor based on the hollow ZnO nanorices could detect low concentration of NO₂ and SO₂ gasses at sub-ppm level. At an optimum operating temperature of 200 °C, the sensor had response values of approximately 15.3 and 4.8 for 1 ppm NO₂ and 1 ppm SO₂, respectively. The sensor also exhibited good stability and selectivity, suggesting that the sensor can be applied to NO₂ and SO₂ toxic gas detection in ambient air.

Hollow ZnO nanorices with an ultrathin shell show excellent response to NO₂ and SO₂ gases.

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.

Simulating alveoli-inspired air pockets in a ZnO/NiMoO4/C3N4 catalyst filter for toluene entrapment and photodecomposition

Here, we propose an alveoli-inspired catalyst to address the susceptibility of photocatalytic air oxidation systems to fluctuations in volatile organic contaminant (VOC) loads. An alveoli structure was fabricated by covering ZnO nanorods grown on a stainless-steel mesh (SSM) with a porous NiMoO₄/C₃N₄ layer. The alveoli catalyst regulates VOC mass transfer from the air to the catalyst surface using air pockets that capture VOC molecules by diffusion driven by a concentration gradient. Air pockets act as localized reservoirs of molecules that prevent scarcity and congestion at the catalyst surface at low and high VOC loads, respectively. The presence of air pockets in the catalyst assembly and its potential to capture VOC was confirmed by a distinct bimodal adsorption configuration. A ZnO/NiMoO₄/C₃N₄@SSM (ZNC@SSM) catalyst with air pockets achieved a high degree of toluene adsorption (6.1 μmol·m^-2). Toluene selectivity of ZnO controlled the delivery of molecules to active catalyst sites, resulting in 95% toluene conversion in 90 min. Synergetic toluene adsorption in air pockets and degradation on catalytic sites helped achieve a quantum yield of 4.14 × 10^-05 molecules/photon. A figure of merit reflecting fundamental system parameters was compared with previous photocatalytic systems to evaluate the practicality of ZNC@SSM.

Hydrothermally grown 1D ZnO nanostructures for rapid detection of NO2 gas

The present paper reports novel approach of surfactant and template free aqueous hydrothermal growth of 1D ZnO nanostructures, which facilitates the generation of large scale, low cost, and moderate working temperature films with controlled morphology for NO2 gas sensor application. Gas sensing properties of 1D ZnO nanostructures were studied at various temperatures for different reducing and oxidizing gases. As-fabricated by 1D ZnO nanostructures showed the highest sensor response of 11,791% with rapid response time of 9 s and recovery time of 220 s towards 100 ppm NO2. Moreover, for 5 ppm NO2 concentration, sensor showed a significant response of 70% with an response time of 16 s and recovery time of 200 s. The sensor shows good continuous performance in terms of response, response time, and recovery time, indicating that the sensor is highly reproducible and stable as well. This study successfully employed 1D ZnO nanostructures based NO2 sensing within the higher (100 ppm) and lower exposure limit (5 ppm) of NO2 gas.

Plasmon expedited response time and enhanced response in gold nanoparticles-decorated zinc oxide nanowire-based nitrogen dioxide gas sensor at room temperature

A highly sensitive and rapidly responsive nitrogen dioxide (NO₂) gas sensor based on gold (Au) nanoparticles (NPs)-decorated zinc oxide (ZnO) nanowires (NWs) is presented. The Au NPs decoration was conducted onto ZnO NWs with and without a (3-aminopropyl)triethoxysilane (APTES) layer on their surface by using the electrostatic force. The samples without the APTES layer exhibited high NO₂ gas sensitivity (i.e. expedited response time and enhanced gas response) due to localized surface plasmon resonance (LSPR) of the Au NPs; in particular, the NO₂ gas response and the response time were increased by three times and shortened by 86%, respectively, compared with the undecorated ZnO NWs. The presence of the APTES layer improved the Au NPs attachment, but hindering the gas adsorption on the ZnO NWs surface, as proven by the observed photocurrent and gas response. Our findings imply that the response time of semiconductor gas sensors can be remarkably expedited by the LSPR effect, which is useful for developing practical gas sensors.

Two-Dimensional, Few-Layer MnPS3 for Selective NO2 Gas Sensing under Ambient Conditions

In this study, two-dimensional few-layer MnPS₃ is introduced as a selective and reversible NO₂ gas sensor in dry nitrogen (N₂) under ambient conditions. The solvent exfoliation technique is utilized to exfoliate bulk MnPS₃ into a few layers, which are further assembled as thin films by the vacuum filtration method. The films are subsequently transferred onto a sensing device and used for NO₂ sensing. Exfoliated MnPS₃ shows excellent sensitivity toward NO₂ gas with a low detection limit of a few tens of ppb at 25 °C. A sensitivity of 9530% is obtained at 35 ppm concentration of NO₂ with the theoretical limit of detection calculated to be ∼9.5 ppb. The sensor is highly selective toward NO₂ gas (with respect to interferents NO, NH₃, H₂, CO, CO₂, C₂H₂, and O₂) and is fully reversible under ambient conditions. The time constant is determined to be in the range of 30-160 s for adsorption and desorption processes. Raman spectroscopy reveals that the mechanism of sensing is based on charge transfer interactions between the sensor and analyte. This study opens up ways to fabricate gas sensors using few-layer metal phosphochalcogenides (MPX₃).

Chemistry in supercritical fluids for the synthesis of metal nanomaterials

The supercritical flow synthesis of metal nanomaterials is sustainable and scalable for the efficient production of materials.

Multifunctional properties of cotton fabrics coated with in situ synthesis of zinc oxide nanoparticles capped with date seed extract

In situ formation of zinc oxide nanoparticles (ZnO-NPs) was studied within the framework of several factors. variables examined include (i) innovation of a new capping agent; (ii) nature of the cotton fabric related to its processing; (iii) formation of Zinc hydroxide (Zn(OH)₂) due to reduction of zinc acetate with sodium hydroxide (iv) treatment of the differently processed cotton fabrics with (Zn(OH)₂) functionalized dispersion as per the exhaustion method, (v) further treatment of the cotton fabrics with (Zn(OH)₂) dispersion according to the pad-dry-cure method and (Vi) conversion of (Zn(OH)₂) to ZnO-NPs during the curing step in the latter method. Results depict that the incorporation of the bio-extract obtained from date seed waste works effectively as capping material which stabilize ZnO-NPs. Mercerized bleached cotton fabric proves to be a better candidate than mercerized loomstate cotton fabric in conferring sustainable bactericidal and UV blocking.

Aligned hierarchical Ag/ZnO nano-heterostructure arrays via electrohydrodynamic nanowire template for enhanced gas-sensing properties

Gas sensing performance can be improved significantly by the increase in both the effective gas exposure area and the surface reactivitiy of ZnO nanorods. Here, we propose aligned hierarchical Ag/ZnO nano-heterostructure arrays (h-Ag/ZnO-NAs) via electrohydrodynamic nanowire template, together with a subsequent hydrothermal synthesis and photoreduction reaction. The h-Ag/ZnO-NAs scatter at top for higher specific surface areas with the air, simultaneously contact at root for the electrical conduction. Besides, the ZnO nanorods are uniformly coated with dispersed Ag nanoparticles, resulting in a tremendous enhancement of the surface reactivity. Compared with pure ZnO, such h-Ag/ZnO-NAs exhibit lower electrical resistance and faster responses. Moreover, they demonstrate enhanced NO₂ gas sensing properties. Self-assembly via electrohydrodynamic nanowire template paves a new way for the preparation of high performance gas sensors.

TiO₂-Based Nanoheterostructures for Promoting Gas Sensitivity Performance: Designs, Developments, and Prospects

Gas sensors based on titanium dioxide (TiO₂) have attracted much public attention during the past decades due to their excellent potential for applications in environmental pollution remediation, transportation industries, personal safety, biology, and medicine. Numerous efforts have therefore been devoted to improving the sensing performance of TiO₂. In those effects, the construct of nanoheterostructures is a promising tactic in gas sensing modification, which shows superior sensing performance to that of the single component-based sensors. In this review, we briefly summarize and highlight the development of TiO₂-based heterostructure gas sensing materials with diverse models, including semiconductor/semiconductor nanoheterostructures, noble metal/semiconductor nanoheterostructures, carbon-group-materials/semiconductor nano- heterostructures, and organic/inorganic nanoheterostructures, which have been investigated for effective enhancement of gas sensing properties through the increase of sensitivity, selectivity, and stability, decrease of optimal work temperature and response/recovery time, and minimization of detectable levels.

Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions

Nanomaterials are at the leading edge of the emerging field of nanotechnology. Their unique and tunable size-dependent properties (in the range 1-100 nm) make these materials indispensable in many modern technological applications. In this Review, we summarize the state-of-art in the manufacture and applications of inorganic nanoparticles made using continuous hydrothermal flow synthesis (CHFS) processes. First, we introduce ideal requirements of any flow process for nanoceramics production, outline different approaches to CHFS, and introduce the pertinent properties of supercritical water and issues around mixing in flow, to generate nanoparticles. This Review then gives comprehensive coverage of the current application space for CHFS-made nanomaterials including optical, healthcare, electronics (including sensors, information, and communication technologies), catalysis, devices (including energy harvesting/conversion/fuels), and energy storage applications. Thereafter, topics of precursor chemistry and products, as well as materials or structures, are discussed (surface-functionalized hybrids, nanocomposites, nanograined coatings and monoliths, and metal-organic frameworks). Later, this Review focuses on some of the key apparatus innovations in the field, such as in situ flow/rapid heating systems (to investigate kinetics and mechanisms), approaches to high throughput flow syntheses (for nanomaterials discovery), as well as recent developments in scale-up of hydrothermal flow processes. Finally, this Review covers environmental considerations, future directions and capabilities, along with the conclusions and outlook.

Highly Efficient Gas Sensor Using a Hollow SnO2 Microfiber for Triethylamine Detection

Triethylamine (TEA) gas sensors having excellent response and selectivity are in great demand to monitor the real environment. In this work, we have successfully prepared a hollow SnO₂ microfiber by a unique sustainable biomass conversion strategy and shown that the microfiber can be used in a high-performance gas sensor. The sensor based on the hollow SnO₂ microfiber shows a quick response/recovery toward triethylamine. The response of the hollow SnO₂ microfiber is up to 49.5 when the concentration of TEA gas is 100 ppm. The limit of detection is as low as 2 ppm. Furthermore, the sensor has a relatively low optimal operation temperature of 270 °C, which is lower than those of many other reported sensors. The excellent sensing properties are largely attributed to the high sensitivity provided by SnO₂ and the good permeability and conductivity of the one-dimensional hollow structure. Thus, the hollow SnO₂ microfiber using sustainable biomass as a template is a significant strategy for a unique TEA gas sensor.

A novel ethanol gas sensor based on TiO2/Ag0.35V2O5 branched nanoheterostructures

Much greater surface-to-volume ratio of hierarchical nanostructures renders them attract considerable interest as prototypical gas sensors. In this work, a novel resistive gas sensor based on TiO2/Ag0.35V2O5 branched nanoheterostructures is fabricated by a facile one-step synthetic process and the ethanol sensing performance of this device is characterized systematically, which shows faster response/recovery behavior, better selectivity, and higher sensitivity of about 9 times as compared to the pure TiO2 nanofibers. The enhanced sensitivity of the TiO2/Ag0.35V2O5 branched nanoheterostructures should be attributed to the extraordinary branched hierarchical structures and TiO2/Ag0.35V2O5 heterojunctions, which can eventually result in an obvious change of resistance upon ethanol exposure. This study not only indicates the gas sensing mechanism for performance enhancement of branched nanoheterostructures, but also proposes a rational approach to design nanostructure based chemical sensors with desirable performance.

Surface Structure Modification of ZnO and the Impact on Electronic Properties

Zinc oxide (ZnO) is a widely utilized, versatile material implemented in a diverse range of technological applications, particularly in optoelectronic devices, where its inherent transparency, tunable electronic properties, and accessible nanostructures can be combined to confer superior device properties. ZnO is a complex material with a rich and intricate defect chemistry, and its properties can be extremely sensitive to processing methods and conditions; consequently, surface modification of ZnO using both inorganic and organic species has been explored to control and regulate its surface properties, particularly at heterointerfaces in electronic devices. Here, the properties of ZnO are described in detail, particularly its surface chemistry, along with the role of defects in governing its electronic properties, and methods employed to modulate the behavior of as-grown ZnO. An outline is also given on how the native and modified oxide interact with molecular materials. To illustrate the diverse range of surface modification methods and their subsequent influence on electronic properties, a comprehensive review of the modification of ZnO surfaces at molecular interfaces in hybrid photovoltaic (hPV) and organic photovoltaic (OPV) devices is presented. This is a case study rather than a progress report, aiming to highlight the progress made toward controlling and altering the surface properties of ZnO, and to bring attention to the ways in which this may be achieved by using various interfacial modifiers (IMs).

Hierarchical ZnO Nanosheet-Nanorod Architectures for Fabrication of Poly(3-hexylthiophene)/ZnO Hybrid NO2 Sensor

A facile one-step solution method has been developed here to fabricate hierarchical ZnO nanosheet-nanorod architectures for compositing with poly(3-hexylthiophene) (P3HT) for fabricating a hybrid NO2 sensor. The hierarchical ZnO nanosheet-nanorod architectures were controllably synthesized by aging the solutions containing 0.05 mol·L(-1) Zn(2+) and 0.33 mol·L(-1) OH(-) at 60 °C through a metastable phase-directed mechanism. The concentration of OH(-) played a huge role on the morphology evolution. When the [OH(-)] concentration was decreased from 0.5 to 0.3 mol·L(-1), the morphology of the ZnO nanostructures changed gradually from monodispersed nanorods (NR) to nanorod assemblies (NRA), and then to nanosheet-nanorod architectures (NS-NR) and nanosheet assemblies (NSA), depending on the formation of various metastable, intermediate phases. The formation of NS-NR included the initial formation of ZnO nanosheets/γ-Zn(OH)2 mixed intermediates, followed by the dissolution of Zn(OH)2, which served as soluble zinc source. Soluble Zn(OH)2 facilitated the dislocation-driven secondary growth of ZnO nanorod arrays on the primary defect-rich nanosheet substrates. Hybrid sensors based on composite films composed of P3HT and the as-prepared ZnO nanostructures were fabricated for the detection of NO2 at room temperature. The P3HT/ZnO NS-NR bilayer film exhibited not only the highest sensitivity but also good reproducibility and selectivity to NO2 at room temperature. The enhanced sensing performance was attributed to the formation of the P3HT/ZnO heterojunction in addition to the enhanced adsorption of NO2 by NS-NR ZnO rich in oxygen-vacancy defects.

Farming of maize-like zinc oxide via a modified SILAR technique as a selective and sensitive nitrogen dioxide gas sensor

Novel hierarchical nanostructures of metal oxides synthesized via simplistic SILAR cycles.

Synthesis of ZnO nanosheet arrays with exposed (100) facets for gas sensing applications

ZnO nanosheet (NS) arrays have been synthesized by a facile ultrathin liquid layer electrodeposition method. The ion concentration and electrode potential play important roles in the formation of ZnO NS arrays. Studies on the structural information indicate that the NSs are exposed with (100) facets. The results of Raman and PL spectra indicate that there existed a large amount of oxygen vacancies in the NSs. The gas sensing performances of the ZnO NS arrays are investigated: the ZnO NS arrays exhibited high gas selectivity and quick response/recovery for detecting NO2 at a low working temperature. High binding energies between NO2 molecules and exposed ZnO(100) facets lead to large surface reconstructions, which is responsible for the intrinsic NO2 sensing properties. In addition, the highly exposed surface and a large amount of oxygen vacancies existing in the NSs also make a great contribution to the gas sensing performance.

Zinc oxide nanostructure-modified textile and its application to biosensing, photocatalysis, and as antibacterial material

Recently, one-dimensional nanostructures with different morphologies (such as nanowires, nanorods (NRs), and nanotubes) have become the focus of intensive research, because of their unique properties with potential applications. Among them, zinc oxide (ZnO) nanomaterials has been found to be highly attractive, because of the remarkable potential for applications in many different areas such as solar cells, sensors, piezoelectric devices, photodiode devices, sun screens, antireflection coatings, and photocatalysis. Here, we present an innovative approach to create a new modified textile by direct in situ growth of vertically aligned one-dimensional (1D) ZnO NRs onto textile surfaces, which can serve with potential for biosensing, photocatalysis, and antibacterial applications. ZnO NRs were grown by using a simple aqueous chemical growth method. Results from analyses such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that the ZnO NRs were dispersed over the entire surface of the textile. We have demonstrated the following applications of these multifunctional textiles: (1) as a flexible working electrode for the detection of aldicarb (ALD) pesticide, (2) as a photocatalyst for the degradation of organic molecules (i.e., Methylene Blue and Congo Red), and (3) as antibacterial agents against Escherichia coli. The ZnO-based textile exhibited excellent photocatalytic and antibacterial activities, and it showed a promising sensing response. The combination of sensing, photocatalysis, and antibacterial properties provided by the ZnO NRs brings us closer to the concept of smart textiles for wearable sensing without a deodorant and antibacterial control. Perhaps the best known of the products that is available in markets for such purposes are textiles with silver nanoparticles. Our modified textile is thus providing acceptable antibacterial properties, compared to available commercial modified textiles.

In situ synthesis of porous array films on a filament induced micro-gap electrode pair and their use as resistance-type gas sensors with enhanced performances

Resistance-type metal-oxide semiconductor gas sensors with high sensitivity and low detection limit have been explored for practical applications. They require both sensing films with high sensitivity to target gases and an appropriate structure of the electrode-equipped substrate to support the sensing films, which is still challenging. In this paper, a new gas sensor of metal-oxide porous array films on a micro-gap electrode pair is designed and implemented by taking ZnO as a model material. First, a micro-gap electrode pair was constructed by sputtering deposition on a filament template, which was used as the sensor's supporting substrate. Then, the sensing film, made up of ZnO porous periodic arrays, was in situ synthesized onto the supporting substrate by a solution-dipping colloidal lithography strategy. The results demonstrated the validity of the strategy, and the as-designed sensor shows a small device-resistance, an enhanced sensing performance with high resolution and an ultralow detection limit. This work provides an alternative method to promote the practical application of resistance-type gas sensors.

Zinc Oxide Nanostructures for NO2 Gas-Sensor Applications: A Review

Because of the interesting and multifunctional properties, recently, ZnO nanostructures are considered as excellent material for fabrication of highly sensitive and selective gas sensors. Thus, ZnO nanomaterials are widely used to fabricate efficient gas sensors for the detection of various hazardous and toxic gases. The presented review article is focusing on the recent developments of NO₂ gas sensors based on ZnO nanomaterials. The review presents the general introduction of some metal oxide nanomaterials for gas sensing application and finally focusing on the structure of ZnO and its gas sensing mechanisms. Basic gas sensing characteristics such as gas response, response time, recovery time, selectivity, detection limit, stability and recyclability, etc are also discussed in this article. Further, the utilization of various ZnO nanomaterials such as nanorods, nanowires, nano-micro flowers, quantum dots, thin films and nanosheets, etc for the fabrication of NO₂ gas sensors are also presented. Moreover, various factors such as NO₂ concentrations, annealing temperature, ZnO morphologies and particle sizes, relative humidity, operating temperatures which are affecting the NO₂ gas sensing properties are discussed in this review. Finally, the review article is concluded and future directions are presented.

Urchin-like polypyrrole nanoparticles for highly sensitive and selective chemiresistive sensor application

Urchin-like polypyrrole (U_PPy) nanoparticles with various diameters were fabricated using a dual-nozzle electrospray and vapor deposition polymerization (VDP). Metal oxide nanoneedle-decorated PPy (FePPy) particles were fabricated as starting materials for deposition of a PPy layer on the metal oxide surface. The FePPy particles were prepared by heating and stirring an aqueous solution of the metal precursor and electrosprayed PPy (E_PPy) particles with nucleated sites on the surface. U_PPys with a maximized surface area were then formed by soaking in an initiator solution followed by VPD. The U_PPy particles were evaluated in various hazardous chemical gas sensors at room temperature. Because of their larger surface area, U_PPy based chemiresistive sensors exhibited greater sensitivity and ca. 10-100 times higher minimum detectable levels (MDLs) of common analytes than pristine PPy particle-based sensors. For example, the MDL of NH₃ was approximately 0.01 ppm, which is better than that observed for other conducting polymer nanostructures. Our new fabrication methodology promises to be an effective approach for fabrication of hybrid nanostructures for future sensing technologies.

A simple hydrothermal route for the preparation of HgS nanoparticles and their photocatalytic activities

HgS nanoparticles, synthesized by a hydrothermal method using PEG as stabilizing agent, show good photocatalytic behaviour.

Controllable growth of ZnO–ZnSe heterostructures for visible-light photocatalysis

ZnO–ZnSe heterostructures for visible-light photocatalysis are fabricated via a two-step CVD process.