Functionalizing nanowires with catalytic nanoparticles for gas sensing application

Metal oxide semiconducting nanowires are among the most promising materials systems for use as conductometric gas sensors. These systems function by converting surface chemical processes, often catalytic processes, into observable conductance variations in the nanowire. The surface properties, and hence the sensing properties of these devices can be altered dramatically improving the sensitivity and selectivity, by the deposition of catalytic metal nanoparticles on the nanowire's surface. This leads not only to promising sensor strategies but to a route for understanding some of the fundamental science occurring on these nanoparticles and at the metal/nanowire junction. In particular studying these systems can lead to a better understanding of the influence of the catalyst particle on the electronic structure of the nanowire and its electron transport. This report surveys results obtained so far in this area. In particular, the comparative sensing performance of single quasi-1D chemiresistors (i.e., nanowires or nanobelts) before and after surface decoration with noble metal catalyst particles show significant improvement in sensitivity toward oxidizing and reducing gases. Moreover, one finds that the sensing mechanism can depend dramatically on the degree of metal coverage of the nanowire.     

1D and 2D Field Effect Transistors in Gas Sensing: A Comprehensive Review

Rapid progress in the synthesis and fundamental understanding of 1D and 2D materials have solicited the incorporation of these nanomaterials into sensor architectures, especially field effect transistors (FETs), for the monitoring of gas and vapor in environmental, food quality, and healthcare applications. Yet, several challenges have remained unaddressed toward the fabrication of 1D and 2D FET gas sensors for real-field applications, which are related to properties, synthesis, and integration of 1D and 2D materials into the transistor architecture. This review paper encompasses the whole assortment of 1D—i.e., metal oxide semiconductors (MOXs), silicon nanowires (SiNWs), carbon nanotubes (CNTs)—and 2D—i.e., graphene, transition metal dichalcogenides (TMD), phosphorene—materials used in FET gas sensors, critically dissecting how the material synthesis, surface functionalization, and transistor fabrication impact on electrical versus sensing properties of these devices. Eventually, pros and cons of 1D and 2D FETs for gas and vapor sensing applications are discussed, pointing out weakness and highlighting future directions.     

Energetics and electronic structures of AlN nanotubes/wires and their potential application as ammonia sensors

Aluminium nitride (AlN) one-dimensional (1D) nanostructures, including crystalline nanowires, faceted nanotubes and conventional single-walled nanotubes, were investigated by means of density functional theory (DFT) using the generalized gradient approximation (GGA). While the larger diameter crystalline nanowires are the most favoured energetically of all these 1D nanostructures, the thick faceted nanotubes have comparable binding energies and can be obtained experimentally. The single-walled nanotubes have the lowest binding energies, and are less feasible experimentally. Due to the surface states at the band edges, the band gaps of all the AlN 1D nanostructures are much smaller than that of bulk AlN. The band structures of AlN nanowires can be modified by NH(3) adsorption. Consequently AlN nanowires have potential applications as gas sensors, since their electronic structures are very sensitive to NH(3) adsorption.     

Current state of the art on tailoring the MXene composition,structure, and surface chemistry

MXenes constitute a family of two-dimensional transition metal carbides, carbonitrides and nitrides. Discovered in 2011, the number of MXenes has expanded significantly and more than 20 different MXenes have been synthesized, with many more predicted from theoretical calculations. MXenes constitute an exceptional family of materials based on their availability for elemental alloying and control of surface terminations, which enables synthesis of a range of structures and chemistries. Consequently, the MXenes exhibit an unparalleled potential for tuning of the materials properties for a wide range of applications. At present, MXenes have emerged with astonishing electronic, optical, plasmonic and thermoelectric properties. This has resulted in a global surge of research around a wide variety of applications, including but not limited to energy storage, carbon capture, electromagnetic interference shielding, reinforcement for composites, water filtering, sensors, and photo-, electro- and chemical catalysis etc. In this review, we present the available state of the art tailoring of the MXene properties owing to recent advances in structural ordering and tuning of surface terminations.     

Metal Oxide Nanowire and Thin-Film-Based Gas Sensors for Chemical Warfare Simulants Detection

This work concerns with metal oxide (MOX) gas sensors based on nanowires and thin films. We focus on chemical warfare agents (CWAs) detection to compare these materials from the functional point-of-view. We work with different chemicals including simulants for Sarin nerve agents, vescicant gases, cyanide agents, and analytes such as ethanol, acetone, ammonia, and carbon monoxide that can be produced by everyday activities causing false alarms. Explorative data analysis has been used to demonstrate the different sensing performances of nanowires and thin films. Within the chosen application, our analysis reveal that the introduction of nanowires inside the array composed by thin films can improve its sensing capability. Cyanide simulants have been detected at concentrations close to 1 ppm, lower than the Immediately Dangerous for Life and Health (IDLH) value of the respective warfare agent. Higher sensitivity has been obtained to simulants for Sarin and vescicant gases, which have been detected at concentrations close or even lower than 100 ppb. Results demonstrate the suitability of the proposed array to selectively detect CWA simulants with respect to some compounds produced by everyday activities.     

Discontinuously Operated Metal Oxide Gas Sensors for Flexible Tag Microlab Applications

Micromachined silicon substrates have significantly reduced the heating power consumption of metal oxide (MOX) gas sensors. Specific applications, however, require further reductions far beyond the present state-of-the-art. In this paper, we report on discontinuously operated MOX gas sensors on micromachined heater platforms and show that such sensors allow power consumption levels to be reached which are consistent with Flexible Tag Microlab (FTM) operation. Such FTMs allow gas concentrations to be measured and recorded to reveal the transport history of goods along the logistics chain for later interrogation by a wireless reader.     

One-dimensional nanostructures of ferroelectric perovskites

Nanorods, nanowires, and nanotubes of ferroelectric perovskites have recently been studied with increasing intensity due to their potential use in non-volatile ferroelectric random access memory, nano-electromechanical systems, energy-harvesting devices, advanced sensors, and in photocatalysis. This Review summarizes the current status of these 1D nanostructures and gives a critical overview of synthesis routes with emphasis on chemical methods. The ferroelectric and piezoelectric properties of the 1D nanostructures are discussed and possible applications are highlighted. Finally, prospects for future research within this field are outlined.     

Synthesis and applications of one-dimensional semiconductors

Nanoscale inorganic materials such as quantum dots (0-dimensional) and one-dimensional (1D) structures, such as nanowires, nanobelts and nanotubes, have gained tremendous attention within the last decade. Among the huge variety of 1D nanostructures, semiconducting nanowires have gained particular interest due to their potential applications in optoelectronic and electronic devices. Despite the huge efforts to control and understand the growth mechanisms underlying the formation of these highly anisotropic structures, some fundamental phenomena are still not well understood. For example, high aspect-ratio semiconductors exhibit unexpected growth phenomena, e.g. diameter-dependent and temperature-dependent growth directions, and unusual high doping levels or compositions, which are not known for their macroscopic crystals or thin-film counterparts.This article reviews viable synthetic approaches for growing high aspect-ratio semiconductors from bottom-up techniques, such as crystal structure governed nucleation, metal-promoted vapour phase and solution growth, formation in non-metal seeded gas-phase processes, structure directing templates and electrospinning. In particular new experimental findings and theoretical models relating to the frequently applied vapour-liquid-solid (VLS) growth are highlighted. In addition, the top-down application of controlled chemical etching, using novel masking techniques, is described as a viable approach for generating certain 1D structures. The review highlights the controlled synthesis of semiconducting nanostructures and heterostructures of silicon, germanium, gallium nitride, gallium arsenide, cadmium sulphide, zinc oxide and tin oxide. The alignment of 1D nanostructures will be reviewed briefly. Whilst specific and reliable contact procedures are still a major challenge for the integration of 1D nanostructures as active building blocks, this issue will not be the focus of this paper. However, the promising applications of 1D semiconductors will be highlighted, particularly with reference to surface dependent electronic transduction (gas and biological sensors), energy generation (nanomechanical and photovoltaic) devices, energy storage (lithium storage in battery anodes) as well as nanowire photonics.     

Self-assembled diphenylalanine nanowires for cellular studies and sensor applications

In this paper we present a series of experiments showing that vertical self-assembled diphenylalanine peptide nanowires (PNWs) are a suitable candidate material for cellular biosensing. We grew HeLa and PC12 cells onto PNW modified gold surfaces and observed no hindrance of cell growth caused by the peptide nanostructures; furthermore we studied the properties of PNWs by investigating their influence on the electrochemical behavior of gold electrodes. The PNWs were functionalized with polypyrrole (PPy) by chemical polymerization, therefore creating conducting peptide/polymer nanowire structures vertically attached to a metal electrode. The electroactivity of such structures was characterized by cyclic voltammetry. The PNW/PPy modified electrodes were finally used as amperometric dopamine sensors, yielding a detection limit of 3,1 microM.     

Metal Chelation Assisted In Situ Migration and Functionalization of Catalysts on Peapod‐Like Hollow SnO2 toward a Superior Chemical Sensor

Rational design of nanostructures and efficient catalyst functionalization methods are critical to the realization of highly sensitive gas sensors. In order to solve these issues, two types of strategies are reported, i.e., (i) synthesis of peapod‐like hollow SnO₂ nanostructures (hollow 0D‐1D SnO₂) by using fluid dynamics of liquid Sn metal and (ii) metal–protein chelate driven uniform catalyst functionalization. The hollow 0D‐1D SnO₂ nanostructures have advantages in enhanced gas accessibility and higher surface areas. In addition to structural benefits, protein encapsulated catalytic nanoparticles result in the uniform catalyst functionalization on both hollow SnO₂ spheres and SnO₂ nanotubes due to their dynamic migration properties. The migration of catalysts with liquid Sn metal is induced by selective location of catalysts around Sn. On the basis of these structural and uniform functionalization of catalyst benefits, biomarker chemical sensors are developed, which deliver highly selective detection capability toward acetone and toluene, respectively. Pt or Pd loaded multidimensional SnO₂ nanostructures exhibit outstanding acetone (R _air/R _gas = 93.55 @ 350 °C, 5 ppm) and toluene (R _air/R _gas = 9.25 @ 350 °C, 5 ppm) sensing properties, respectively. These results demonstrate that unique nanostructuring and novel catalyst loading method enable sensors to selectively detect biomarkers for exhaled breath sensors.     

Rutile structured SnO2 nanowires synthesized with metal catalyst by thermal evaporation method

One-dimensional (1-D) nanostructures such as tubes, rods, wires, and belts have attracted considerable research activities owing to their strong application potential as components for nanosize electronic or optoelectronic devices utilizing superior optical and electrical properties. Characterizing the mechanical properties of nanostructure is of great importance for their applications in electronics, optoelectronics, sensors, actuators. Wide-bandgap SnO2 semiconducting material (Eg = 3.6 eV at room temperature) is one of the attractive candidates for optoelectronic devices operating at room temperature, gas sensors, and transparent conducting electrodes. The synthesis and gas sensing properties of semiconducting SnO2 nanomaterials have became one of important research issues since the first synthesis of SnO2 nanobelts. Considering the important application of SnO2 in sensors, these structures are not only ideal systems for fundamental understanding at the nanoscale level, but they also have potential applications as nanoscale sensors, resonator, and transducers. The structured SnO2 nanorods have been grown on silicon substrates with Au catalytic layer by thermal evporation process over 800 degrees C. The resulting sample is characterized and analyzed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDS). The morphology and structural properties of SnO2 nanowires were measured by scanning electron microscopy and high-resolution transmission electron microscopy. The mean diameter of the SnO2 nanorods grown on Au coated silicon (100) substrate is approximately 80 nm. In addition, X-ray diffraction measurements show that SnO2 nanorods have a rutile structure. The formation of SnO2 nanowires has been attributed to the vapor-liquid-solid (VLS) growth mechanisms depending on the processing conditions. We investigated the growth behavior of the SnO2 nanowires by variation of the growth conditions such as gas partial pressure and temperature.     

The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors

This review paper discusses the influence of morphology and crystallographic structure on gas-sensing characteristics of metal oxide conductometric-type sensors. The effects of parameters such as film thickness, grain size, agglomeration, porosity, faceting, grain network, surface geometry, and film texture on the main analytical characteristics (absolute magnitude and selectivity of sensor response (S), response time (τ_res), recovery time (τ_rec), and temporal stability) of the gas sensor have been analyzed. A comparison of standard polycrystalline sensors and sensors based on one-dimension structures was conducted. It was concluded that the structural parameters of metal oxides are important factors for controlling response parameters of resistive type gas sensors. For example, it was shown that the decrease of thickness, grain size and degree of texture is the best way to decrease time constants of metal oxide sensors. However, it was concluded that there is not universal decision for simultaneous optimization all gas-sensing characteristics. We have to search for a compromise between various engineering approaches because adjusting one design feature may improve one performance metric but considerably degrade another.     

One Dimensional Silver‐based Nanomaterials: Preparations and Electrochemical Applications

One dimensional (1D) silver‐based nanomaterials have a great potential in various fields because of their high specific surface area, high electric conductivity, optoelectronic properties, mechanical flexibility and high electro‐catalytic efficiency. In this Review, the preparations of 1D silver‐based nanomaterials is classified by structure composed of simple silver nanowires/rods/belts/tubes, core‐shells, and hybrids. The latest applications based on 1D silver nanomaterials and their composite materials are summarized systematically including electrochemical capacitors, lithium‐ion/lithium‐oxygen batteries, electrochemical sensors and electrochemical catalysis. The preparation process, tailored material properties and electrochemical applications are discussed.     

Water-soluble organo-silica hybrid nanowires

There has been growing interest in the past decade in one-dimensional (1D) nanostructures, such as nanowires, nanotubes or nanorods, owing to their size-dependent optical and electronic properties and their potential application as building blocks, interconnects and functional components for assembling nanodevices. Significant progress has been made; however, the strict control of the distinctive geometry at extremely small size for 1D structures remains a great challenge in this field. The anisotropic nature of cylindrical polymer brushes has been applied to template 1D nanostructured materials, such as metal, semiconductor or magnetic nanowires. Here, by constructing the cylindrical polymer brushes themselves with a precursor-containing monomer, we successfully synthesized hybrid nanowires with a silsesquioxane core and a shell made up from oligo(ethylene glycol) methacrylate units, which are soluble in water and many organic solvents. The length and diameter of these rigid wires are tunable by the degrees of polymerization of both the backbone and the side chain. They show lyotropic liquid-crystalline behaviour and can be pyrolysed to silica nanowires. This approach provides a route to the controlled fabrication of inorganic or hybrid silica nanostructures by living polymerization techniques.     

Large Single-Crystal Cu Foils with High-Index Facets by Strain-Engineered Anomalous Grain Growth

The rich and complex arrangements of metal atoms in high-index metal facets afford appealing physical and chemical properties, which attracts extensive research interest in material science for the applications in catalysis and surface chemistry. However, it is still a challenge to prepare large-area high-index single crystals in a controllable and cost-efficient manner. Herein, entire commercially available decimeter-sized polycrystalline Cu foils are successfully transformed into single crystals with a series of high-index facets, relying on a strain-engineered anomalous grain growth technique. The introduction of a moderate thermal-contact stress upon the Cu foil during the annealing leads to the formation of high-index grains dominated by the thermal strain of the Cu foils, rather than the (111) surface driven by the surface energy. Besides, the designed static gradient of the temperature enables the as-formed high-index grain seed to expand throughout the entire Cu foil. The as-received high-index Cu foils can serve as the templates for producing high-index single-crystal Cu-based alloys. This work provides an appealing material basis for the epitaxial growth of 2D materials, and the applications that require the unique surface structures of high-index metal foils and their alloys.     

Self‐Formed Channel Devices Based on Vertically Grown 2D Materials with Large‐Surface‐Area and Their Potential for Chemical Sensor Applications

2D layered materials with sensitive surfaces are promising materials for use in chemical sensing devices, owing to their extremely large surface‐to‐volume ratios. However, most chemical sensors based on 2D materials are used in the form of laterally defined active channels, in which the active area is limited to the actual device dimensions. Therefore, a novel approach for fabricating self‐formed active‐channel devices is proposed based on 2D semiconductor materials with very large surface areas, and their potential gas sensing ability is examined. First, the vertical growth phenomenon of SnS₂ nanocrystals is investigated with large surface area via metal‐assisted growth using prepatterned metal electrodes, and then self‐formed active‐channel devices are suggested without additional pattering through the selective synthesis of SnS₂ nanosheets on prepatterned metal electrodes. The self‐formed active‐channel device exhibits extremely high response values (>2000% at 10 ppm) for NO₂ along with excellent NO₂ selectivity. Moreover, the NO₂ gas response of the gas sensing device with vertically self‐formed SnS₂ nanosheets is more than two orders of magnitude higher than that of a similar exfoliated SnS₂‐based device. These results indicate that the facile device fabrication method would be applicable to various systems in which surface area plays an important role.     

Effect of annealing temperature on the characteristics and sensing properties of WO<Subscript>3</Subscript> nanowires

A uniform WO₃ nanowire structure was prepared by two-step thermal oxidation method on Si substrate. WO₃ nanowires show different morphology and crystal structures after annealing at different temperatures. The influence of annealing temperature on WO₃ nanowires was investigated by SEM, TEM and XRD. Higher crystallization property and lower surface state was obtained with higher annealing temperature. The gas sensing properties of the WO₃ nanowires with various annealing temperatures to NO₂ with the concentration ranging from 1 to 4 ppm were examined at different temperatures ranging from room temperature to 200 °C. The results indicate that WO₃ nanowires can greatly lower the working temperature of sensors and sensors based on WO₃ nanowires show p-type or n-type sensing behaviors depending on annealing temperatures. Possible sensing mechanism of p-type WO₃ nanowires and the influence of annealing temperature on sensing types was explained. This work might supply new ideas about gas sensing mechanisms and open a new way to develop p-type WO₃ sensing materials.     

Schottky-Contacted Nanowire Sensors

The progress of the Internet-of-Things in the past few years has necessitated the support of high-performance sensors. Schottky-contacted nanowire sensors have attracted considerable attention owing to their high sensitivity and fast response time. Their progress is reviewed here, based on several kinds of important nanowires, for applications such as bio/chemical sensors, gas sensors, photodetectors, and strain sensors. Although Schottky-contacted nanowire sensors deliver excellent performance in these fields, they can be further improved by various methods, including defect engineering, surface modification, the piezotronic effect, and the piezophototronic effect, all of which are discussed here. With regard to practical applications, further efforts are required to address challenges such as the stability, selectivity, ultrafast response, multifunctionality, flexibility, distributed energy supply, and sustainability of Schottky-contacted nanowire sensors. Finally, future perspectives and solutions are discussed.     

Synthesis of platinum nanowire networks using a soft template

Platinum nanowire networks have been synthesized by chemical reduction of a platinum complex using sodium borohydride in the presence of a soft template formed by cetyltrimethylammonium bromide in a two-phase water-chloroform system. The interconnected polycrystalline nanowires possess the highest surface area (53 +/- 1 m2/g) and electroactive surface area (32.4 +/- 3.6 m2/g) reported for unsupported platinum nanomaterials; the high surface area results from the small average diameter of the nanowires (2.2 nm) and the 2-10 nm pores determined by nitrogen adsorption measurements. Synthetic control over the network was achieved simply by varying the stirring rate and reagent concentrations, in some cases leading to other types of nanostructures including wormlike platinum nanoparticles. Similarly, substitution of a palladium complex for platinum gives palladium nanowire networks. A mechanism of formation of the metal nanowire networks is proposed based on confined metal growth within a soft template consisting of a network of swollen inverse wormlike micelles.