N-P transition sensing behaviors of ZnO nanotubes exposed to NO2 gas

Ocimum sanctum Leaf Extract-Assisted Green Synthesis of Pd-Doped CuO Nanoparticles for Highly Sensitive and Selective NO2 Gas Sensors

In view of facile, cost-effective, and environmentally friendly synthetic methods, palladium-doped copper oxide (Pd-CuO) nanoparticles have been synthesized from Ocimum sanctum (commonly known as "Tulsi") phytoextract for gas-sensing applications. The structural, morphological, and compositional properties of Pd-doped CuO nanoparticles were studied using various techniques such as XRD, FESEM, XPS, and EDX. The characterization results confirmed the doping of Pd on CuO nanoparticles, and Pd-CuO nanostructures appear as nanoflakes in FESEM analysis. The gas-sensing response of Pd (1.12 wt %)-CuO nanoflake-based sensor was measured at 5-100 ppm concentration of different gases, NO2, H2S, NH3, and H2, at 125 °C. Gas-sensing tests reveal that the sensitivity of the sensor were 81.7 and 38.9% for 100 and 5 ppm concentrations of NO2, respectively, which was significantly greater than that of pure CuO. The response and recovery times of the sensor were 72 and 98 s for 100 ppm of NO2 gas, while they were 90 and 50 s for 5 ppm NO2. The calculated limit of detection (LOD) value of the sensor is 0.8235. This appealing LOD is suitable for real-time gas detection. The gas sensor was found to exhibit excellent selectivity toward NO2 gas and repeatability and stability in humid (80%) conditions. The Pd doping in CuO nanostructures plays a significant role in escalating the sensitivity and selectivity of CuO-based NO2 gas sensor suitable to work at low operating temperatures.

Dynamic Reaction Mechanism of P-N-Switched H2-Sensing Performance on a Pt-Decorated TiO2 Surface

Pt decoration is known to be one of the most promising strategies to enhance the performance of TiO₂ hydrogen gas sensors, while the effect of Pt-decorating concentration on the sensing performance of TiO₂ and the specific interaction between Pt and TiO₂ have not been fully investigated. Here, a series of TiO₂ nanoarray thin films with differing amounts of Pt decorated (Pt/TiO₂) is fabricated, and the H₂-sensing performance is evaluated. A switch in the response from P-type to N-type is observed with increasing Pt decoration. The response additionally depends on the H₂ concentration: resistance increases in low H₂ concentrations and decreases in hydrogen concentrations higher than 40 ppm. This is explained by the competitive adsorption of hydrogen between the Pt nanoparticles (Pt NPs) and the exposed TiO₂ surface. The preference for H₂ adsorption and splitting between Pt and TiO₂ is established by DFT calculations. Humidity brings preferential adsorption of H₂O on the surface of Pt, which affects the following adsorption and splitting of H₂, thus resulting in a P-N switch of the sensing performance. The detailed dynamic reaction process is described according to the findings.

Electrically Transduced Gas Sensors Based on Semiconducting Metal Oxide Nanowires

Semiconducting metal oxide-based nanowires (SMO-NWs) for gas sensors have been extensively studied for their extraordinary surface-to-volume ratio, high chemical and thermal stabilities, high sensitivity, and unique electronic, photonic and mechanical properties. In addition to improving the sensor response, vast developments have recently focused on the fundamental sensing mechanism, low power consumption, as well as novel applications. Herein, this review provides a state-of-art overview of electrically transduced gas sensors based on SMO-NWs. We first discuss the advanced synthesis and assembly techniques for high-quality SMO-NWs, the detailed sensor architectures, as well as the important gas-sensing performance. Relationships between the NWs structure and gas sensing performance are established by understanding general sensitization models related to size and shape, crystal defect, doped and loaded additive, and contact parameters. Moreover, major strategies for low-power gas sensors are proposed, including integrating NWs into microhotplates, self-heating operation, and designing room-temperature gas sensors. Emerging application areas of SMO-NWs-based gas sensors in disease diagnosis, environmental engineering, safety and security, flexible and wearable technology have also been studied. In the end, some insights into new challenges and future prospects for commercialization are highlighted.

Electronic and Simple Oscillatory Conduction in Ferrite Gas Sensors: Gas-Sensing Mechanisms, Long-Term Gas Monitoring, Heat Transfer, and Other Anomalies

The early detection and warning of the presence of hazardous gases have been well studied. We present a study that focuses on some fundamental properties of gas sensors for liquefied petroleum gas (LPG) using spinel nanoferrites, namely, CoSm_0.1Fe_1.9O₄, CoCe_0.1Fe_1.9O₄, MgCe_0.1Fe_1.9O₄, and MgFe₂O₄. A highly sensitive and selective response of 846.34 at 225 °C toward 10,000 ppm concentration of LPG was recorded. Other flammable gases tested were hydrogen, methane, propane, and butane. Electronic conduction of LPG sensors near saturation showed simple electrical oscillations that can be attributed to the self-dissociation of water molecules physically adsorbed on the surface of the chemisorbed oxygen species due to proton transfer. The oscillatory behaviors follow fluctuations in the operating temperature attributed to heat transfer between the physisorbed water molecules and the hot sensor surface. This depends on the LPG concentration because higher LPG concentration gives rise to greater heat transfer from the sensors. The adsorption and desorption of these water molecule multilayers take a few hundreds of seconds at low concentrations, while the adsorption formation process takes longer at higher concentrations. Other parameters such as LPG exposure time, bias voltage, relative humidity, ambient conditions, operating temperatures, and temperature of the gas not only affect electrical oscillations and thermal fluctuations but also switch the dominant charge carriers from p- to n-type or vice versa. The type of sensor surface, either p- or n-type, did not appear to affect the oscillatory behavior, while the exposure time, short or long, determined the appearance and further behavior of the oscillations. The long-time exposure to 10,000 ppm concentration resulted in the resistance gradually decreasing due to the lack of oxygen supply, while at 5000 ppm, this was constant, stable, and oscillated indefinitely. Changing the dry air to argon gas as a carrier and for dilution of the hazardous gas prevented the electrical oscillations and thermal fluctuations and significantly lowered the response values. Both the inert ambient (argon gas) and changing operating temperature flipped the dominant charge carriers of these sensors. The concentration of these chemisorbed oxygen species governs the charge space and depletion layers. In addition, the spinel nanoferrites used contained higher oxygen vacancies than the lattice oxygen and chemisorbed oxygen. When using dry air, the oscillations were observed at 3000 ppm concentration, while using argon gas, they were observed at 7000 ppm concentration. The room-temperature LPG responses were about 35 and 80 under 45% relative humidity using dry air and argon gas, respectively. These room-temperature measurements showed electrical oscillations but did not show any thermal fluctuations or heat transfer phenomena. This study presents a deeper insight into the fundamentals of gas-sensing mechanisms and energy costs involved.

Influence of Humidity on NO2-Sensing and Selectivity of Spray-CVD Grown ZnO Thin Film above 400 °C

Thin films are being used more and more in gas sensing applications, relying on their high surface area to volume ratio. In this study, ZnO thin film was produced through a thermal aerosol spraying and chemical vapor deposition (spray-CVD) process at 500 °C using zinc acetate as a precursor. The phase identification and the morphologies of the film were investigated by XRD and SEM, respectively. Gas-sensing properties of the ZnO thin film were evaluated toward NO2, CO, and NO at a moderate temperature range (400–500 °C) in dry and humid air (relative humidity = 2.5, 5, 7.5, and 10% RH). The obtained results show good sensor signal for both NO2 (R/R0 = 94%) and CO (92%) and poor sensor signal to NO (52%) at an optimum temperature of 450 °C in dry air. The response and recovery times decrease with the increase of NO2 concentration. In the presence of humidity (10% of RH), the sensor is more than twice as sensitive to NO2 (70%) as CO (29%), and accordingly, exhibits good selectivity toward NO2. As the amount of humidity increases from 2.5 to 10% RH, the selectivity ratio of ZnO thin film to NO2 against CO increases from 1 to 2.4. It was also observed that the response and the recovery rates decrease with the increase of relative humidity. The significant enhancement of the selectivity of ZnO thin film toward NO2 in the presence of humidity was attributed to the strong affinity of OH species with NO2.

Gas sensors using ordered macroporous oxide nanostructures

Detection and monitoring of harmful and toxic gases have gained increased interest in relation to worldwide environmental issues. Semiconducting metal oxide gas sensors have been considered promising for the facile remote detection of gases and vapors over the past decades. However, their sensing performance is still a challenge to meet the demands for practical applications where excellent sensitivity, selectivity, stability, and response/recovery rate are imperative. Therefore, sensing materials with novel architectures and fabrication processes have been pursued with a flurry of research activity. In particular, the preparation of ordered macroporous metal oxide nanostructures is regarded as an intriguing candidate wherein ordered aperture sizes in the range from 50 nm to 1.5 μm can increase the chemical diffusion rate and considerably strengthen the performance stability and repeatability. This review highlights the recent advances in the fabrication of ordered macroporous nanostructures with different dimensions and compositions, discusses the sensing behavior evolution governed by structural layouts, hierarchy, doping, and heterojunctions, as well as considering their general principles and future prospects. This would provide a clear scale for others to tune the sensing performance of porous materials in terms of specific components and structural designs.

Bioinspired Cocatalysts Decorated WO3 Nanotube Toward Unparalleled Hydrogen Sulfide Chemiresistor

Herein, we incorporated dual biotemplates, i.e., cellulose nanocrystals (CNC) and apoferritin, into electrospinning solution to achieve three distinct benefits, i.e., (i) facile synthesis of a WO₃ nanotube by utilizing the self-agglomerating nature of CNC in the core of as-spun nanofibers, (ii) effective sensitization by partial phase transition from WO₃ to Na₂W₄O₁₃ induced by interaction between sodium-doped CNC and WO₃ during calcination, and (iii) uniform functionalization with monodispersive apoferritin-derived Pt catalytic nanoparticles (2.22 ± 0.42 nm). Interestingly, the sensitization effect of Na₂W₄O₁₃ on WO₃ resulted in highly selective H₂S sensing characteristics against seven different interfering molecules. Furthermore, synergistic effects with a bioinspired Pt catalyst induced a remarkably enhanced H₂S response ( R_air/ R_gas = 203.5), unparalleled selectivity ( R_air/ R_gas < 1.3 for the interfering molecules), and rapid response (<10 s)/recovery (<30 s) time at 1 ppm of H₂S under 95% relative humidity level. This work paves the way for a new class of cosensitization routes to overcome critical shortcomings of SMO-based chemical sensors, thus providing a potential platform for diagnosis of halitosis.

High temperature gas sensing performances of silicon carbide nanosheets with an n–p conductivity transition

Fast and effective detecting of flammable and explosive gases in harsh environments (high temperature, corrosion atmosphere) is crucial for preventing severe accidents for the chemical industry, fuel cell applications and engine tests. Silicon carbide material is reported to be a good candidate for gas sensing devices applied in extreme conditions. Herein, high-temperature available silicon carbide nanosheets (SiC NSs) were synthesized from graphene oxide (GO) via a catalyst-free carbothermal method. The structure and composition of SiC NSs under different reaction conditions are carefully characterized. The received SiC NSs were firstly utilized as gas sensing materials for hazardous gases (acetone, ethanol, methanol and ammonia) at a high temperature (500 °C). Importantly, the SiC NSs sensors exhibited a fast response (8–39 s) and recovery (12–69 s) towards detecting gases. Besides, an n–p conductivity transition phenomenon is found and studied. This paper firstly proves that such SiC NSs has the potential to be used in gas sensing fields.

Novel silicon carbide nanosheets were synthesized by a carbothermal reduction reaction. We studied their high-temperature gas sensing properties and the mechanism of n–p conductivity transition during gas sensing tests.

Evaluation of gas-sensing properties of ZnO nanostructures electrochemically doped with Au nanophases

A one-step electrochemical method based on sacrificial anode electrolysis (SAE) was used to deposit stabilized gold nanoparticles (Au NPs) directly on the surface of nanostructured ZnO powders, previously synthesized through a sol-gel process. The effect of thermal annealing temperatures (300 and 550 °C) on chemical, morphological, and structural properties of pristine and Au-doped ZnO nancomposites (Au@ZnO) was investigated. Transmission and scanning electron microscopy (TEM and SEM), as well as X-ray photoelectron spectroscopy (XPS), revealed the successful deposition of nanoscale gold on the surface of spherical and rod-like ZnO nanostructures, obtained after annealing at 300 and 550 °C, respectively. The pristine ZnO and Au@ZnO nanocomposites are proposed as active layer in chemiresistive gas sensors for low-cost processing. Gas-sensing measurements towards NO2 were collected at 300 °C, evaluating not only the Au-doping effect, but also the influence of the different ZnO nanostructures on the gas-sensing properties.

Regulable switching from p- to n-type behavior of ordered nanoporous Pt-SnO2 thin films with enhanced room temperature toluene sensing performance

In this work, a nanoporous SnO₂ sensing film is fabricated in situ on a sensing device using a block polymer template and is applied as a chemiresistive gas sensor. The ordered film is capable of detecting 10 ppm toluene at room temperature and shows good stability.

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.

Temperature-Dependent Abnormal and Tunable p-n Response of Tungsten Oxide--Tin Oxide Based Gas Sensors

We observed the sensing response of temperature-dependent abnormal p-n transitions in WO3-SnO2 hybrid hollow sphere based gas sensors for the first time. The sensors presented a normal n-type response to ethanol at elevated temperatures but abnormal p-type responses in a wide range of operation temperatures (room temperature to about 95 °C). By measuring various reducing gases and applying complex impedance plotting techniques, we demonstrated the abnormal p-type sensing behavior to be a pseudo-response resulting from the reaction between target gas and adsorbed water on the material surface. The temperature-controlled n-p switch is ascribed to the competition of intrinsic and extrinsic sensing behaviors, which resulted from the reaction of target gas with adsorbed oxygen ions and protons from adsorbed water, respectively. The former can modulate the intrinsic conductivity of the sensor by changing the electron concentration of the sensing materials, while the latter can regulate the conduction of the water layer, which contributes to the total conductivity as an external part. The hollow and hybrid nanostructures facilitated the observation of extrinsic sensing behaviors due to its large-area active sites and abundant oxygen vacancies, which could enhance the adsorption of water. This work might give new insight into gas sensing mechanisms and opens up a promising way to develop practical temperature and humidity controllable gas sensors with little power consumption based on the extrinsic properties.

Temperature and thickness dependence of the sensitivity of nitrogen dioxide graphene gas sensors modified by atomic layer deposited zinc oxide films

The Chemical Vapor Deposition (CVD) grown graphene nitrogen dioxide (NO₂) gas sensors modified by zinc oxide (ZnO) thin films via atomic layer deposition (ALD) were fabricated and their sensitivity dependence on the temperature and ZnO film thickness was investigated.

Spinel ZnFe2O4 nanoparticle-decorated rod-like ZnO nanoheterostructures for enhanced gas sensing performances

Spinel ZnFe₂O₄ nanoparticle-decorated rod-like ZnO nanoheterostructures were one-pot synthesized for detecting n-butanol, ethanol, acetone, methanol and formaldehyde.

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.

Piezoelectric effects and electromechanical theories at the nanoscale

Considerable effort has been made to study the piezoelectric effect on the nanoscale, which serves as a physical basis for a wide range of smart nanodevices and nanoelectronics. This paper reviews recent progress in the research on the piezoelectric properties and electromechanical effects of piezoelectric nanomaterials (PNs). The review begins with an introduction to existing PNs which exhibit a diverse range of atomic structures and configurations. The nanoscale measurement of their effective piezoelectric coefficients (EPCs) is summarised with an emphasis on the major factors determining the piezoelectric properties of PNs. The paper concludes with a review of the electromechanical theories that are able to capture the small-scale effects on PNs, which include the surface piezoelectricity, flexoelectricity and Eringen's nonlocal theory. In contrast to the classical theories, two types of EPCs are defined, which were found to be size-dependent and loading condition-selective.

Hydrothermal synthesis and acetylene sensing properties of variety low dimensional zinc oxide nanostructures

Various morphologies of low dimensional ZnO nanostructures, including spheres, rods, sheets, and wires, were successfully synthesized using a simple and facile hydrothermal method assisted with different surfactants. Zinc acetate dihydrate was chosen as the precursors of ZnO nanostructures. We found that polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), glycine, and ethylene glycol (EG) play critical roles in the morphologies and microstructures of the synthesized nanostructures, and a series of possible growth processes were discussed in detail. Gas sensors were fabricated using screen-printing technology, and their sensing properties towards acetylene gas (C2H2), one of the most important arc discharge characteristic gases dissolved in oil-filled power equipments, were systematically measured. The ZnO nanowires based sensor exhibits excellent C2H2 sensing behaviors than those of ZnO nanosheets, nanorods, and nanospheres, indicating a feasible way to develop high-performance C2H2 gas sensor for practical application.

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

Continuous hydrothermal flow synthesis of crystalline ZnO nanorods and prisms is reported via a new pilot-scale continuous hydrothermal reactor (at nominal production rates of up to 1.2 g/h). Different size and shape particles of ZnO (wurtsite structure) were obtained via altering reaction conditions such as the concentration of either additive H2O2 or metal salt. Selected ZnO samples (used as prepared) were evaluated as solid oxide gas sensors, showing excellent sensitivity toward NO2 gas. It was found that both the working temperature and gas concentration significantly affected the NO2 gas response at concentrations as low as 1 ppm.

Supramolecular complexation for environmental control

Supramolecular complexes offer a new and efficient way for the monitoring and removal of many substances emanating from technical processes, fertilization, plant and animal protection, or e.g. chemotherapy. Such pollutants range from toxic or radioactive metal ions and anions to chemical side products, herbicides, pesticides to drugs including steroids, and include degradation products from natural sources. The applications involve usually fast and reversible complex formation, due to prevailing non-covalent interactions. This is of importance for sensing as well as for separation techniques, where the often expensive host compounds can then be reused almost indefinitely. Immobilization of host compounds, e.g. on exchange resins or on membranes, and their implementation in smart new materials hold particular promise. The review illustrates how the design of suitable host compounds in combination with modern sensing and separation methods can contribute to solve some of the biggest problems facing chemistry, which arise from the everyday increasing pollution of the environment.

Free-standing ZnO-CuO composite nanowire array films and their gas sensing properties

A modified hydrothermal method was developed to synthesize ZnO-CuO composite nanostructures. A free-standing film made of ZnO-CuO nanostructures was assembled on the surface of the hydrothermal solution with a smooth surface on one side and a spherical surface on the other side. The structure, growth mechanism and the optical properties of the composite nanostructures were studied. Structural characterizations indicate that the composite nanostructure mainly consisted of two single-crystal phases of CuO and ZnO. The sensitivity for CO gas detection was significantly improved for the composite CuO-ZnO nanostructure film. This method offers a possible route for the fabrication of free-standing nanostructure films of different functional composite oxides.

Scalable network electrical devices using ZnO nanowalls

We report the fabrication and electrical characteristics of scalable nanowall network devices and their gas sensor applications. For the network device fabrications, two-dimensional ZnO nanowall networks were grown on AlN/Si substrates with a patterned SiO(2) mask layer using selective-area metal-organic vapor-phase epitaxy. The ZnO nanowalls with c-axis orientation were heteroepitaxially grown on AlN/Si substrates, and were single-crystalline, as determined by x-ray diffraction and transmission electron microscopy. The electrical conductivity of the nanowall networks was measured as a function of nanowall dimensions. The conductance increased linearly with the channel width for widths larger than 1 µm, but saturated at 36 µS for widths below 1 µm. This conductance scaling behavior is explained by enhanced conduction through the regions near the edge of the patterned growth areas, where the density of the networks was higher. Gas sensor applications were investigated using the nanowall network devices, and highly sensitive gas detection was demonstrated.

A synthesis and sensing application of hollow ZnO nanofibers with uniform wall thicknesses grown using polymer templates

A novel approach is applied to fabricating hollow ZnO nanofibers (ZNFs) with uniform wall thicknesses. In this approach, polymers synthesized by electrospinning are used as sacrificial templates and ZnO is subsequently deposited on these templates using atomic layer deposition, which makes ZnO uniformly cover the round surface of the polymer nanofibers. Heat treatments result in the selective removal of the polymer templates and the formation of hollow ZNFs with very uniform wall thicknesses. To test a potential use of hollow ZNFs in chemical gas sensors, their sensing properties with regard to O(2), NO(2), and CO are investigated in a comparative manner with those of normal ZnO nanofibers. The excellent sensing properties observed in the hollow ZNF sensor are ascribed to (1) the more pronounced change in resistance due to the presence of nanograins and (2) the doubling of the surface-to-volume ratio due to the generation of inner surfaces.

Solution synthesis of one-dimensional ZnO nanomaterials and their applications

Recently, one-dimensional (1D) ZnO nanomaterials (NMs) have been extensively studied because both their functional properties and highly controllable morphology make them important building blocks for understanding nanoscale phenomena and realizing nanoscale devices. Compared with high temperature (>450 degrees C) vapor phase methods, solution-based synthesis methods can be conducted at low temperatures (25-200 degrees C) allowing for compatibility with many organic substrate materials and offer additional advantages such as straightforward processing, low cost, and ease of scale up. Although there exist several review articles in the literature regarding the synthesis and applications of 1D ZnO NMs, those focusing on solution-based synthesis methods are lacking. Thus, this review focuses mainly on 1D ZnO NMs synthesized by solution-based processing. Firstly, 1D ZnO non-patterned, nanoparticle-seeded synthesis and its associated solution growth kinetics are discussed. Next, synthesis of vertically-aligned ZnO nanorod arrays with controlled pattern and density on various substrates is reviewed. Finally, important applications of 1D ZnO NMs are highlighted including sensors, field emission devices, photodetectors, optical switches, and solar cells.