Hybridization of Zinc Oxide Tetrapods for Selective Gas Sensing Applications

Carbon nanowall-based gas sensors for carbon dioxide gas detection

Carbon nanowalls (CNWs) have attracted significant attention for gas sensing applications due to their exceptional material properties such as large specific surface area, electric conductivity, nano- and/or micro-porous structure, and high charge carrier mobility. In this work, CNW films were synthesized and used to fabricate gas sensors for carbon dioxide (CO2) gas sensing. The CNW films were synthesized using an inductively-coupled plasma (ICP) plasma-enhanced chemical vapor deposition (PECVD) method and their structural and morphological properties were characterized using Raman spectroscopy and electron microscopy. The obtained CNW films were used to fabricate gas sensors employing interdigitated gold (Au) microelectrodes. The gas sensors were fabricated using both direct synthesis of CNW films on interdigitated Au microelectrodes on quartz and also transferring presynthesized CNW films onto interdigitated Au microelectrodes on glass. The CO2gas-sensing properties of fabricated devices were investigated for different concentrations of CO2gas and temperature-ranges. The sensitivities of fabricated devices were found to have a linear dependence on the concentration of CO2gas and increase with temperature. It was revealed that devices, in which CNW films have a maze-like structure, perform better compared to the ones that have a petal-like structure. A sensitivity value of 1.18% was obtained at 500 ppm CO2concentration and 100 °C device temperature. The CNW-based gas sensors have the potential for the development of easy-to-manufacture and efficient gas sensors for toxic gas monitoring.

Mg-Containing Zn3O3 Structures for Detection of CO2: A DFT Study on CHEM Effects of SERS and Electronic Properties

Surface-enhanced Raman spectra (SERS) and electronic-structure-based properties are important tools for investigation of the molecular sensing ability of nanoparticles. The present computational study is intended to explore the sensing ability of Zn3O3 and Mg-containing Zn3O3 structures for CO2 molecules by CHEM effects of the SERS technique. Geometries of CO2-adsorbed Zn3O3, Zn2MgO3 (Mg as a substitutional impurity), and Zn3O3Mg (Mg as an interstitial impurity) structures are modeled using the B3LYP/6-31G(d,p) level of density functional theory. The Mg site of the Zn2MgO3 and Zn3O3Mg structures is preferential for the adsorption of CO2. The observed energy trends are supported by geometrical analysis, molecular orbital interactions, redshifts in CO2 vibrational modes, and topological properties. Raman activity enhancement of the CO2 symmetric vibrational mode is significant when the molecule is adsorbed at the Mg site of Zn3O3Mg. The observed Raman activity enhancement is supported by SERS spectra obtained from anharmonic calculations carried out on B3LYP/6-31G(d,p) geometries and substantiated by a larger change in the polarizability with energy corresponding to the symmetric vibrational mode of CO2. The TDDFT calculations, frequency-dependent polarizabilities, and charge transfer interactions show that Zn3O3Mg is a good substrate for sensing of CO2, with visible wavelengths, by resonance Raman effect. The trends with adsorption energy, Raman activity, and excited state properties are also substantiated by B3LYP/6-311+G(d,p) calculations.

Surface Conversion of ZnO Tetrapods Produces Pinhole-Free ZIF-8 Layers for Selective and Sensitive H2 Sensing Even in Pure Methane

As the necessary transition to a supply of renewable energy moves forward rapidly, hydrogen (H2) becomes increasingly important as a green chemical energy carrier. The manifold applications associated with the use of hydrogen in the energy sector require sensor materials that can efficiently detect H2 in small quantities and in gas mixtures. As a possible candidate, we here present a metal-organic framework (MOF, namely ZIF-8) functionalized metal-oxide gas sensor (MOS, namely ZnO). The gas sensor is based on single-crystalline tetrapodal ZnO (t-ZnO) microparticles, which are coated with a thin layer of ZIF-8 ([Zn(C4H5N2)2]) by a ZnO conversion reaction to obtain t-ZnO@ZIF-8 (core@shell) composites. The vapor-phase synthesis enables ZIF-8 thickness control as shown by powder X-ray diffraction, thermogravimetric analysis, and N2 sorption measurements. Gas-sensing measurements of a single microrod of t-ZnO@ZIF-8 composite demonstrate the synergistic benefits of both MOS sensors and MOFs, resulting in an outstanding high selectivity, sensitivity (S ≅ 546), and response times (1-2 s) to 100 ppm H2 in the air at a low operation temperature of 100 °C. Under these conditions, no response to acetone, n-butanol, methane, ethanol, ammonia, 2-propanol, and carbon dioxide was observed. Thereby, the sensor is able to reliably detect H2 in mixtures with air and even methane, with the latter being highly important for determining the H2 dilution level in natural gas pipelines, which is of great importance to the energy sector.

Defect passivation and electron band energy regulation of a ZnO electron transport layer through synergetic bifunctional surface engineering for efficient quantum dot light-emitting diodes

Zinc oxide nanoparticles (ZnO NPs) have been actively pursued as the most effective electron transport layer for quantum-dot light-emitting diodes (QLEDs) in light of their unique optical and electronic properties and low-temperature processing. However, the high electron mobility and smooth energy level alignment at QDs/ZnO/cathode interfaces cause electron over-injection, which aggravates non-radiative Auger recombination. Meanwhile, the abundant defects hydroxyl group (-OH) and oxygen vacancies (O_V) in ZnO NPs act as trap states inducing exciton quenching, which synergistically reduces the effective radiation recombination for degrading the device performance. Here, we develop a bifunctional surface engineering strategy to synthesize ZnO NPs with low defect density and high environmental stability by using ethylenediaminetetraacetic acid dipotassium salt (EDTAK) as an additive. The additive effectively passivates surface defects in ZnO NPs and induces chemical doping simultaneously. Bifunctional engineering alleviates electron excess injection by elevating the conduction band level of ZnO to promote charge balance. As a result, state-of-the-art blue QLEDs with an EQE of 16.31% and a T₅₀@100 cd m^-2 of 1685 h are achieved, providing a novel and effective strategy to fabricate blue QLEDs with high efficiency and a long operating lifetime.

Nanosensors Based on a Single ZnO:Eu Nanowire for Hydrogen Gas Sensing

Fast detection of hydrogen gas leakage or its release in different environments, especially in large electric vehicle batteries, is a major challenge for sensing applications. In this study, the morphological, structural, chemical, optical, and electronic characterizations of ZnO:Eu nanowire arrays are reported and discussed in detail. In particular, the influence of different Eu concentrations during electrochemical deposition was investigated together with the sensing properties and mechanism. Surprisingly, by using only 10 μM Eu ions during deposition, the value of the gas response increased by a factor of nearly 130 compared to an undoped ZnO nanowire and we found an H₂ gas response of ∼7860 for a single ZnO:Eu nanowire device. Further, the synthesized nanowire sensors were tested with ultraviolet (UV) light and a range of test gases, showing a UV responsiveness of ∼12.8 and a good selectivity to 100 ppm H₂ gas. A dual-mode nanosensor is shown to detect UV/H₂ gas simultaneously for selective detection of H₂ during UV irradiation and its effect on the sensing mechanism. The nanowire sensing approach here demonstrates the feasibility of using such small devices to detect hydrogen leaks in harsh, small-scale environments, for example, stacked battery packs in mobile applications. In addition, the results obtained are supported through density functional theory-based simulations, which highlight the importance of rare earth nanoparticles on the oxide surface for improved sensitivity and selectivity of gas sensors, even at room temperature, thereby allowing, for instance, lower power consumption and denser deployment.

Environmentally Benign Structural, Topographic, and Sensing Properties of Pure and Al-Doped ZnO Thin Films

In the present research work, Zn_1-x Al _x O thin films with varying proportions of Al (x = 0.00, 0.01, 0.02, and 0.03) are prepared by a chemical sol-gel spin-coating technique. The crystal structural, morphological, and humidity-sensing properties of the synthesized Zn_1-x Al _x O thin films, with varying concentrations of Al (x = 0.00, 0.01, 0.02, and 0.03), were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM); a special humidity-controlled chamber was designed for the humidity-sensing studies. In structural and phase analyses, XRD patterns of Zn_1-x Al _x O thin films show a hexagonal wurtzite crystal structure. The average crystallite sizes of Zn_1-x Al _x O thin films were calculated and found to be ∼18.00, 22.50, 26.30, and 29.70 nm using the X-ray diffraction (XRD) pattern. The surface morphology of Zn_1-x Al _x O Al (x = 0.00, 0.01, 0.02, and 0.03) thin films obtained from AFM micrographs analysis indicates the modification of the spherical grains into nanorods, which were distributed throughout the surface of the films. The SEM image of 3 wt % Al-doped ZnO nanomaterials also shows that spherical nanoparticles changed to nanorod-like structures with a high packing density. Furthermore, increasing the Al-doping concentration from 0 to 3 wt % in ZnO NPs shows lower hysteresis loss, less aging effect, and good sensitivity in the range of 9.8-16.5 MΩ/%RH. The sensitivity of the sensing materials increased with increasing Al-doping concentration, which is very useful for humidity sensors.

Highly Selective and Sensitive Detection of Breath Isoprene by Tailored Gas Reforming: A Synergistic Combination of Macroporous WO3 Spheres and Au Catalysts

Precise detection of breath isoprene can provide valuable information for monitoring the physical and physiological status of human beings or for the early diagnosis of cardiovascular diseases. However, the extremely low concentration and low chemical reactivity of breath isoprene hamper the selective and sensitive detection of isoprene using oxide semiconductor chemiresistors. Herein, we report that macroporous WO₃ microspheres whose inner macropores are surrounded by Au nanoparticles exhibit a high response (resistance ratio = 11.3) to 0.1 ppm isoprene under highly humid conditions at 275 °C and an extremely low detection limit (0.2 ppb). Furthermore, the sensor showed excellent selectivity to isoprene over five interferants that could be exhaled by humans. Notably, the selectivity to isoprene is critically dependent on the location of Au nanocatalysts and macroporosity. The mechanism underlying the selective isoprene detection is investigated in relation to the reforming of less reactive isoprene into more reactive intermediate species promoted by macroporous catalytic reactors, which is confirmed by the analysis using a proton transfer reaction quadrupole mass spectrometer. The sensor for breath analysis has high potential for simple physical and physiological monitoring as well as disease diagnosis.

Advances of nanomaterials for air pollution remediation and their impacts on the environment

One of the most favorable environmental applications of nanotechnology has been in air pollution remediation in which different nanomaterials are used as nanoadsorbents, nanocatalysts, nanofilters, and nanosensors. The nanomaterials have the ability to adsorb several contaminants existing in the air. Also, certain semiconducting nanomaterials materials can be used for photocatalytic remediation. Air contamination control can also be achieved by nanostructured membranes with pores sufficiently small to separate various pollutants from the exhaust. Nanomaterial enabled sensors are also used for the detection of harmful gases such as hydrogen sulfide, sulphur dioxide, and nitrogen dioxide. Conversely, because of the uncertainties in addition to irregularities in size, shape as well as chemical compositions, the existence of some nanomaterials might cause harmful effects on the environment along with the health of people. Thus, concerns were expressed about the transport and conversion of nanoparticles discharged into the surroundings. This review critically examined and assessed the present literature on the application of nanomaterials in the air, together with its negative impacts. The main focus is placed on the application of carbon-based and metal-based nanomaterials for air pollution remediation. It is noted that these nanomaterials demonstrating fascinating properties for improving the environmental pollution remediation system.

Dual Transduction of H2O2 Detection Using ZnO/Laser-Induced Graphene Composites

Zinc oxide (ZnO)/laser-induced graphene (LIG) composites were prepared by mixing ZnO, grown by laser-assisted flow deposition, with LIG produced by laser irradiation of a polyimide, both in ambient conditions. Different ZnO:LIG ratios were used to infer the effect of this combination on the overall composite behavior. The optical properties, assessed by photoluminescence (PL), showed an intensity increase of the excitonic-related recombination with increasing LIG amounts, along with a reduction in the visible emission band. Charge-transfer processes between the two materials are proposed to justify these variations. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy evidenced increased electron transfer kinetics and an electrochemically active area with the amount of LIG incorporated in the composites. As the composites were designed to be used as transducer platforms in biosensing devices, their ability to detect and quantify hydrogen peroxide (H2O2) was assessed by both PL and CV analysis. The results demonstrated that both methods can be employed for sensing, displaying slightly distinct operation ranges that allow extending the detection range by combining both transduction approaches. Moreover, limits of detection as low as 0.11 mM were calculated in a tested concentration range from 0.8 to 32.7 mM, in line with the values required for their potential application in biosensors.

Metal-Oxide Based Nanomaterials: Synthesis, Characterization and Their Applications in Electrical and Electrochemical Sensors

Pure, mixed and doped metal oxides (MOX) have attracted great interest for the development of electrical and electrochemical sensors since they are cheaper, faster, easier to operate and capable of online analysis and real-time identification. This review focuses on highly sensitive chemoresistive type sensors based on doped-SnO₂, RhO, ZnO-Ca, Sm_x-CoFe_2−xO₄ semiconductors used to detect toxic gases (H₂, CO, NO₂) and volatile organic compounds (VOCs) (e.g., acetone, ethanol) in monitoring of gaseous markers in the breath of patients with specific pathologies and for environmental pollution control. Interesting results about the monitoring of biochemical substances as dopamine, epinephrine, serotonin and glucose have been also reported using electrochemical sensors based on hybrid MOX nanocomposite modified glassy carbon and screen-printed carbon electrodes. The fundamental sensing mechanisms and commercial limitations of the MOX-based electrical and electrochemical sensors are discussed providing research directions to bridge the existing gap between new sensing concepts and real-world analytical applications.

Comparison of Thermal Annealing versus Hydrothermal Treatment Effects on the Detection Performances of ZnO Nanowires

A comparative investigation of the post-electroplating treatment influence on the gas detecting performances of single ZnO nanorod/nanowire (NR/NW), as grown by electrochemical deposition (ECD) and integrated into nanosensor devices, is presented. In this work, hydrothermal treatment (HT) in a H₂O steam and conventional thermal annealing (CTA) in a furnace at 150 °C in ambient were used as post-growth treatments to improve the material properties. Herein, the morphological, optical, chemical, structural, vibrational, and gas sensing performances of the as-electrodeposited and treated specimens are investigated and presented in detail. By varying the growth temperature and type of post-growth treatment, the morphology is maintained, whereas the optical and structural properties show increased sample crystallization. It is shown that HT in H₂O vapors affects the optical and vibrational properties of the material. After investigation of nanodevices based on single ZnO NR/NWs, it was observed that higher temperature during the synthesis results in a higher gas response to H₂ gas within the investigated operating temperature range from 25 to 150 °C. CTA and HT or autoclave treatment showed the capability of a further increase in gas response of the prepared sensors by a factor of ∼8. Density functional theory calculations reveal structural and electronic band changes in ZnO surfaces as a result of strong interaction with H₂ gas molecules. Our results demonstrate that high-performance devices can be obtained with high-crystallinity NWs/NRs after HT. The obtained devices could be the key element for flexible nanoelectronics and wearable electronics and have attracted great interest due to their unique specifications.

General Strategy for Designing Highly Selective Gas-Sensing Nanoreactors: Morphological Control of SnO2 Hollow Spheres and Configurational Tuning of Au Catalysts

Catalyst-loaded hollow spheres are effective at detecting ethanol with high chemical reactivity. However, this has limited the widespread use of catalyst-loaded hollow spheres in designing highly selective gas sensors to less-reactive gases such as aromatics (e.g., xylene). Herein, we report the preparation of xylene-selective Au-SnO₂ nanoreactors by loading Au nanoclusters on the inner surface of SnO₂ hollow shells using the layer-by-layer assembly technique. The results revealed that the sensor based on SnO₂ hollow spheres loaded with Au nanoclusters on the inner surface exhibited unprecedentedly high xylene selectivity and an ultrahigh xylene response, high enough to be used for indoor air quality monitoring, whereas the sensor based on SnO₂ hollow spheres loaded with Au nanoclusters on the outer surface exhibited the typical ethanol-sensitive sensing behaviors as frequently reported in the literature. In addition, the xylene selectivity and response were optimized when the hollow shell was sufficiently thin (∼25 nm) and semipermeable (pore size = ∼3.5 nm), while the selectivity and response decreased when the shell was thick or highly gas permeable with large mesopores (∼30 nm). Accordingly, the underlying mechanism responsible for the unprecedentedly high xylene sensing performance is discussed in relation to the configuration of the loaded Au nanoclusters and the morphological characteristics including shell thickness and pore size distribution. This novel nanoreactor concept can be widely used to design highly selective gas sensors especially to less-reactive gases such as aromatics, aldehydes, and ketones.

SnO2-ZnO-Fe2O3 tri-composite based room temperature operated dual behavior ammonia and ethanol sensor for ppb level detection

In this paper, we present a novel room temperature (RT) operated SnO2-ZnO-Fe2O3 based tri-composite analyte sensor with dual behavior having detection ability of up to ∼1 ppb with a substantial % response (R) to detect ammonia and ethanol vapors. The tri-composite is synthesized via a sol-gel spin coating technique and characterized using X-ray diffraction (XRD) for structural analysis. Fourier transform infrared spectroscopy (FTIR) and Raman results are used to confirm tri-composite formation. Further, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) results are used for examining the detailed surface morphology and structural and topographical characteristics of the tri-composite. The sensing characteristics are monitored from 1 ppb to 50 ppm for ammonia detection and 1 ppb to 25 ppm for ethanol detection at RT (∼27 °C) under ∼45% relative humidity (RH) conditions. This dual sensing behavior (based on change in resistance under ammonia and ethanol exposure) of the sensor is used to differentiate and detect the presence of ammonia (resistance decreases) and ethanol (resistance increases) with high %R within a few seconds. In addition, the sensor showed excellent sensing characteristics under moist conditions (up to 85% RH) and outstanding reproducibility, and was found to be highly stable, selective and specific towards the target analytes. This work not only reports a RT operated ppb level ammonia and ethanol sensor, but also explores the novel SnO2-ZnO-Fe2O3 tri-composite along with a scientific approach towards multi-composite nanostructures to develop analyte sensors.

Enhancing ZnO nanowire gas sensors using Au/Fe2O3 hybrid nanoparticle decoration

Heterojunctions are an important strategy for designing high performance electrical sensor materials and related devices. Herein, a new type of metal-semiconductor hybrid nanoparticle has been successfully used to remarkably sensitize the surface of ZnO nanowires for detecting NO₂ with high responses over a broad temperature window ranging from room temperature to 600 °C. These hybrid nanoparticles are comprised of iron oxide nanowires with well dispersed single crystalline Au nanoparticles. The hybrid nanoparticle decorated ZnO nanowires have achieved a giant response, as high as 74 500 toward NO₂ gas, about 42 times that of Au decorated ZnO nanowire sensors. This dramatic enhancement may be attributed to the efficient charge transfer across the Au-Fe₂O₃ Schottky and Fe₂O₃-ZnO semiconductor heterojunction interfaces. Due to the incorporation of thermally-stable Fe₂O₃ nanoparticles as the support of Au nanoparticles, the working temperature of nanowire sensors was successfully extended to higher temperatures, with an increase of 200 °C, from 400 °C to 600 °C. Such a combination of semiconductor heterojunction and semiconductor-metal Schottky contact presents a new strategy for designing high performance electrical sensors with high sensitivity, stability, selectivity, and wide operation temperature window, which are potentially suitable for advanced energy systems such as automotive engines and power plants.

A stable and highly sensitive room-temperature liquefied petroleum gas sensor based on nano-cubes/cuboids of zinc antimonate

Trirutile zinc antimonate (ZnSb2O6) nano-cubes/cuboids have been fabricated by a sol-gel spin-coating method using polyethylene glycol (PEG) as the structure-directing agent. The fabricated films were characterized for surface morphology, along with structural, FT-IR and thermal analysis. The crystallite size of ZnSb2O6 is found to be 35 nm. The fabricated films have been tested for the detection of liquefied petroleum gas (LPG) and carbon dioxide (CO2) gas leakage at room temperature (27 °C). They exhibit fairly high sensitivity (1.73), low response and recovery times (∼41 and 95 s, respectively), and good reproducibility and stability (99.2%) at room temperature for the detection of LPG leakage. Based on these observations, the fabricated film has the potential to be used as a LPG sensor at room temperature.

ZnAl2O4 decorated Al-doped ZnO tetrapodal 3D networks: microstructure, Raman and detailed temperature dependent photoluminescence analysis

3D networks of Al-doped ZnO tetrapods decorated with ZnAl₂O₄ particles synthesised by the flame transport method were investigated in detail using optical techniques combined with morphological/structural characterisation. Low temperature photoluminescence (PL) measurements revealed spectra dominated by near band edge (NBE) recombination in the UV region, together with broad visible bands whose peak positions shift depending on the ZnO : Al mixing ratios. A close inspection of the NBE region evidences the effective doping of the ZnO structures with Al, as corroborated by the broadening and shift of its peak position towards the expected energy associated with the exciton bound to Al. Both temperature and excitation density-dependent PL results pointed to an overlap of multiple optical centres contributing to the broad visible band, with the peak position dependent on the Al content. While in the reference sample the wavelength of the green band remained unchanged with temperature, in the case of the composites, the deep level emission showed a blue shift with increasing temperature, likely due to distinct thermal quenching of the overlapping emitting centres. This assumption was further validated by the time-resolved PL data, which clearly exposed the presence of more than one optical centre in this spectral region. PL excitation analysis demonstrated that the luminescence features of the Al-doped ZnO/ZnAl₂O₄ composites revealed noticeable changes not only in deep level recombination, but also in the material's bandgap when compared with the ZnO reference sample. At room temperature, the ZnO reference sample exhibited free exciton resonance at ∼3.29 eV, whereas the peak position for the Al-doped ZnO/ZnAl₂O₄ samples occurred at ∼3.38 eV due to the Burstein-Moss shift, commonly observed in heavily doped semiconductors. Considering the energy shift observed and assuming a parabolic conduction band, a carrier concentration of ∼1.82 ×10¹⁹ cm^-3 was estimated for the Al-doped ZnO/ZnAl₂O₄ samples.

Optical properties of hydrothermally synthesised and thermally annealed ZnO/ZnO2 composites

ZnO/ZnO2 composites grown by hydrothermal synthesis at low temperature (180 °C) and thermally annealed at 300 °C were fully analysed by morphological, structural and optical techniques. X-ray diffraction patterns (XRD) and Raman spectroscopy clearly evidence the presence of both crystalline phases in the ZnO/ZnO2 sample. The differential scanning calorimetry analysis and thermogravimetric profiles indicate an exothermic event with a peak temperature ca. 225 °C, which is accompanied by a 8.5% weight loss, being attributed to the crystallization of ZnO from ZnO2. Upon a thermal annealing treatment at 300 °C the ZnO2 phase was completely converted into ZnO, as measured by XRD and Raman spectroscopy. Photoluminescence investigations reveal that the emission is dominated by a broad band recombination in both samples, due to the overlapping of different emitting centres, and that the peak position of the PL emission is dependent on the excitation density. The ZnO/ZnO2 sample exhibits a widening of the bandgap when compared to the one only containing ZnO, likely related to the presence of the additional ZnO2 phase and suggesting a bandgap energy of ~3.42 eV for this compound. Surface analysis revealed that the sample exhibits a surface area of 90 m2 g-1, which decreases to 30 m2 g-1 after the thermal annealing and the full conversion into ZnO. This difference in the surface area showed particular relevance in the stability of the measured optical properties. Particularly, the intensity of the photoluminescence signal was seen to be higher in the ZnO/ZnO2 sample and strongly dependent on the measurement atmosphere, highlighting its potential to be employed in the fabrication of optical-based sensing systems for environmental applications, namely in gas sensors.

An efficient room-temperature liquefied petroleum gas sensor based on trirutile copper antimonate nano-polygons

A new direction to copper antimonate nano-polygons as an efficient LPG sensing material.

Controlled sol-gel synthesis of oxygen sensing CdO : ZnO hexagonal particles for different annealing temperatures

CdO : ZnO hexagonal particles were synthesized by a sol-gel precipitation method at different annealing temperatures. A mixed crystal phase of cubic and wurtzite structures was observed from X-ray diffraction patterns. The micrographs showed hexagonal shapes of the CdO : ZnO nanocomposites particles. The energy dispersive X-ray spectroscopy mapping images showed a uniform distribution of the Cd and Zn. The CdO : ZnO nanocomposite pallet annealed at 550 °C has an electrical resistance of 0.366 kΩ at room temperature. The nanocomposites showed an excellent sensing response against oxygen gas with a sensing response of 47% at 200 °C for the CdO : ZnO particles annealed at 550 °C. The sensor response and recovery times were found to be 43s and 45s, respectively. The sensor response was due to the sorption of oxygen ions on the surfaces of the CdO : ZnO hexagonal particles.

Synthesis of ZnO Hierarchical Structures and Their Gas Sensing Properties

Firecracker-like ZnO hierarchical structures (ZnO HS1) were synthesized by combining electrospinning with hydrothermal methods. Flower-like ZnO hierarchical structures (ZnO HS2) were prepared by a hydrothermal method using ultrasound-treated ZnO nanofibers (ZnO NFs) as raw material which has rarely been reported in previous papers. Scanning electron microscope (SEM) and transmission electron microscope's (TEM) images clearly indicated the existence of nanoparticles on the ZnO HS2 material. Both gas sensors exhibited high selectivity toward H₂S gas over various other gases at 180 °C. The ZnO HS2 gas sensor exhibited higher H₂S sensitivity response (50 ppm H₂S, 42.298) at 180 °C than ZnO NFs (50 ppm H₂S, 9.223) and ZnO HS1 (50 ppm H₂S, 17.506) gas sensors. Besides, the ZnO HS2 sensor showed a shorter response time (14 s) compared with the ZnO NFs (25 s) and ZnO HS1 (19 s) gas sensors. The formation diagram of ZnO hierarchical structures and the gas sensing mechanism were evaluated. Apart from the synergistic effect of nanoparticles and nanoflowers, more point-point contacts between flower-like ZnO nanorods were advantageous for the excellent H₂S sensing properties of ZnO HS2 material.

ZnO decorated laser-induced graphene produced by direct laser scribing

A scalable laser scribing approach to produce zinc oxide (ZnO) decorated laser-induced graphene (LIG) in a unique laser-processing step was developed by irradiating a polyimide sheet covered with a Zn/ZnO precursor with a CO₂ laser (10.6 μm) under ambient conditions. The laser scribing parameters revealed a strong impact on the surface morphology of the formed LIG, on ZnO microparticles' formation and distribution, as well as on the physical properties of the fashioned composites. The ZnO microparticles were seen to be randomly distributed along the LIG surface, with the amount and dimensions depending on the used laser processing conditions. Besides the synthesis conditions, the use of different precursors also resulted in distinct ZnO growth's yields and morphologies. Raman spectroscopy revealed the existence of both wurtzite-ZnO and sp² carbon in the majority of the produced samples. Broad emission bands in the visible range and the typical ZnO near band edge (NBE) emission were detected by photoluminescence studies. The spectral shape of the luminescence signal was seen to be extremely sensitive to the employed processing parameters and precursors, highlighting their influence on the composites' optical defect distribution. The sample produced from the ZnO-based precursor evidenced the highest luminescence signal, with a dominant NBE recombination. Electrochemical measurements pointed to the existence of charge transfer processes between LIG and the ZnO particles.

Low-Voltage-Driven Sensors Based on ZnO Nanowires for Room-Temperature Detection of NO2 and CO Gases

Herein, we report the synthesis of pristine and Au-functionalized ZnO nanowires (NWs) for low power consumption (self-heated) gas sensors at room temperature. The ZnO NWs were produced via a vapor-liquid-solid growth technique, and Au layers with different thicknesses were sputter-deposited on the ZnO NWs, followed by subsequent annealing. Microscopic characterization methods demonstrated that ZnO NWs were successfully formed. Pristine ZnO NW gas sensors showed the best sensitivity toward either CO or NO₂ gases at 300 and 350 °C, respectively. Also, the sensitivities of pristine ZnO NW gas sensors were tested toward NO₂ gas under different applied voltages; the sensors revealed a good response and selectivity under an applied voltage of 7 V. Au-functionalized ZnO NW gas sensors exhibited the best response for CO gas at an applied voltage of 7 V and showed a much higher response relative to the pristine ZnO NWs. The sensing mechanisms for pristine and functionalized gas sensors are comprehensively discussed.

Enhanced room temperature ferromagnetism and green photoluminescence in Cu doped ZnO thin film synthesised by neutral beam sputtering

The Cu (3 to 15 at%) is incorporated into ZnO thin film by atomic beam co-sputtering has been investigated for enhancement in room temperature ferromagnetism and green photo-luminance. These Cu-ZnO thin films examined with Raman spectroscopy, X-Ray Diffraction (XRD), UV-Visible spectroscopy, Hall measurement, magnetic force microscopy (MFM) and magnetic hysteresis. Raman spectroscopy, XRD confirms wurtzite structure and improvement in the crystallinity of ZnO upto 7% Cu. Further increase in Cu concentration results in growth in Cu nanoparticles. On increasing Cu concentration, there is decrement in transparency and increase in band gap with increase in n-type carrier concentration as confirmed from UV-Visible and Hall measurement studies. Magnetic measurement exhibited unique feature of room temperature ferromagnetic ordering in undoped and doped sample upto 3% Cu. The enhancement in magnetic moment as well as green emission in photoluminescence response with increase in Cu doping indicates that generation of large defects in ZnO by Cu doping, which can be attributed to combined effect of the presence of oxygen vacancies and/or structural inhomogeneity as well as formation of bound magnetic polarons. Importantly, synthesised Cu doped ZnO thin films can be used as spin LEDs and switchable spin-laser diodes.

Probing surface states in C60 decorated ZnO microwires: detailed photoluminescence and cathodoluminescence investigations

ZnO microwires synthesised by the flame transport method and decorated with C₆₀ clusters were studied in detail by photoluminescence (PL) and cathodoluminescence (CL) techniques. The optical investigations suggest that the enhanced near band edge recombination observed in the ZnO/C₆₀ composites is attributed to the reduction of the ZnO band tail states in the presence of C₆₀. Well-resolved free and bound excitons recombination, as well as 3.31 eV emission, are observed with increasing amount of C₆₀ flooding when compared with the ZnO reference sample. Moreover, a shift of the broad visible emission to lower energies occurs with increasing C₆₀ content. In fact, this band was found to be composed by two optical centres peaked in the green and orange/red spectral regions, presenting different lifetimes. The orange/red band exhibits faster lifetime decay, in addition to a more pronounced shift to lower energies, while the peak position of the green emission only shows a slight change. The overall redshift of the broad visible band is further enhanced by the change in the relative intensity of the mentioned optical centres, depending on the excitation intensity and on the C₆₀ flooding. These results suggest the possibility of controlling/tuning the visible emission outcome by increasing the C₆₀ amount on the ZnO surface due to the surface states present in the semiconductor. An adequate control of such phenomena may have quite beneficial implications when sensing applications are envisaged.

High-Sensitive Ammonia Sensors Based on Tin Monoxide Nanoshells

Ammonia (NH₃) is a harmful gas contaminant that is part of the nitrogen cycle in our daily lives. Therefore, highly sensitive ammonia sensors are important for environmental protection and human health. However, it is difficult to detect low concentrations of ammonia (≤50 ppm) using conventional means at room temperature. Tin monoxide (SnO), a member of IV⁻VI metal monoxides, has attracted much attention due to its low cost, environmental-friendly nature, and higher stability compared with other non-oxide ammonia sensing material like alkaline metal or polymer, which made this material an ideal alternative for ammonia sensor applications. In this work, we fabricated high-sensitive ammonia sensors based on self-assembly SnO nanoshells via a solution method and annealing under 300 °C for 1 h. The as fabricated sensors exhibited the response of 313%, 874%, 2757%, 3116%, and 3757% (∆G/G) under ammonia concentration of 5 ppm, 20 ppm, 50 ppm, 100 ppm, and 200 ppm, respectively. The structure of the nanoshells, which have curved shells that provide shelters for the core and also possess a large surface area, is able to absorb more ammonia molecules, leading to further improvements in the sensitivity. Further, the SnO nanoshells have higher oxygen vacancy densities compared with other metal oxide ammonia sensing materials, enabling it to have higher performance. Additionally, the selectivity of ammonia sensors is also outstanding. We hope this work will provide a reference for the study of similar structures and applications of IV⁻VI metal monoxides in the gas sensor field.

Realizing the Control of Electronic Energy Level Structure and Gas-Sensing Selectivity over Heteroatom-Doped In2O3 Spheres with an Inverse Opal Microstructure

Understanding the effect of substitutional doping on gas-sensing performances is essential for designing high-activity sensing nanomaterials. Herein, formaldehyde sensors based on gallium-doped In₂O₃ inverse opal (IO-(Ga _xIn_{1- x})₂O₃) microspheres were purposefully prepared by a simple ultrasonic spray pyrolysis method combined with self-assembled sulfonated polystyrene sphere templates. The well-aligned inverse opal structure, with three different-sized pores, plays the dual role of accelerating the diffusion of gas molecules and providing more active sites. The Ga substitutional doping can alter the electronic energy level structure of (Ga _xIn_{1- x})₂O₃, leading to the elevation of the Fermi level and the modulation of the band gap close to a suitable value (3.90 eV), hence, effectively optimizing the oxidative catalytic activity for preferential CH₂O oxidation and increasing the amount of adsorbed oxygen. More importantly, the gas selectivity could be controlled by varying the energy level of adsorbed oxygen. Accordingly, the IO-(Ga_0.2In_0.8)₂O₃ microsphere sensor showed a high response toward formaldehyde with fast response and recovery speeds, and ultralow detection limit (50 ppb). Our findings finally offer implications for designing Fermi level-tailorable semiconductor nanomaterials for the control of selectivity and monitoring indoor air pollutants.

Recent Advances in Electrochemical Sensors for Detecting Toxic Gases: NO₂, SO₂ and H₂S

Toxic gases, such as NOx, SOx, H₂S and other S-containing gases, cause numerous harmful effects on human health even at very low gas concentrations. Reliable detection of various gases in low concentration is mandatory in the fields such as industrial plants, environmental monitoring, air quality assurance, automotive technologies and so on. In this paper, the recent advances in electrochemical sensors for toxic gas detections were reviewed and summarized with a focus on NO₂, SO₂ and H₂S gas sensors. The recent progress of the detection of each of these toxic gases was categorized by the highly explored sensing materials over the past few decades. The important sensing performance parameters like sensitivity/response, response and recovery times at certain gas concentration and operating temperature for different sensor materials and structures have been summarized and tabulated to provide a thorough performance comparison. A novel metric, sensitivity per ppm/response time ratio has been calculated for each sensor in order to compare the overall sensing performance on the same reference. It is found that hybrid materials-based sensors exhibit the highest average ratio for NO₂ gas sensing, whereas GaN and metal-oxide based sensors possess the highest ratio for SO₂ and H₂S gas sensing, respectively. Recently, significant research efforts have been made exploring new sensor materials, such as graphene and its derivatives, transition metal dichalcogenides (TMDs), GaN, metal-metal oxide nanostructures, solid electrolytes and organic materials to detect the above-mentioned toxic gases. In addition, the contemporary progress in SO₂ gas sensors based on zeolite and paper and H₂S gas sensors based on colorimetric and metal-organic framework (MOF) structures have also been reviewed. Finally, this work reviewed the recent first principle studies on the interaction between gas molecules and novel promising materials like arsenene, borophene, blue phosphorene, GeSe monolayer and germanene. The goal is to understand the surface interaction mechanism.

Multifunctional Screen-Printed TiO2 Nanoparticles Tuned by Laser Irradiation for a Flexible and Scalable UV Detector and Room-Temperature Ethanol Sensor

Recently, multifunctional devices printed on flexible substrates, with multisensing capability, have found new demand in practical fields of application, such as wearable electronics, soft robotics, interactive interfaces, and electronic skin design, revealing the vital importance of precise control of the fundamental properties of metal oxide nanomaterials. In this paper, a novel low-cost and scalable processing strategy is proposed to fabricate all-printed multisensing devices with UV- and gas-sensing capabilities. This undertaken approach is based on the hierarchical combination of the screen-printing process and laser irradiation post-treatment. The screen-printing is used for the patterning of silver interdigitated electrodes and the active layer based on anatase TiO₂ nanoparticles, whereas the laser processing is utilized to fine-tune the UV and ethanol-sensing properties of the active layer. Different characterization techniques demonstrate that the laser fluence can be adjusted to optimize the morphology of the TiO₂ film by increasing the contribution from volume porosity, to improve its electrical properties and enhance its UV photoresponse and ethanol-sensing characteristics at room temperature. Furthermore, results of the UV and ethanol-sensing investigation show that the optimized UV and ethanol sensors have good repeatability, relatively fast response/recovery times, and excellent mechanical flexibility.

Improving gas sensing by CdTe decoration of individual Aerographite microtubes

Novel gas sensors have been realized by decorating clusters of tubular Aerographite with CdTe using magnetron sputtering techniques. Subsequently, individual microtubes were separated and electrically contacted on a SiO₂/Si substrate with pre-patterned electrodes. Cathodoluminescence, electron microscopy and electrical characterization prove the successful formation of a polycrystalline CdTe thin film on Aerographite enabling an excellent gas response to ammonia. Furthermore, the dynamical response to ammonia exposure has been investigated, highlighting the quick response and recovery times of the sensor, which is highly beneficial for extremely short on/off cycles. Therefore, this gas sensor reveals a large potential for cheap, highly selective, reliable and low-power gas sensors, which are especially important for hazardous gases such as ammonia.

Electrospun Janus-like pellicle displays coinstantaneous tri-function of aeolotropic conduction, magnetism and luminescence

A new Janus-like pellicle with top-bottom structure, functionalized by conductive aeolotropism, magnetism and luminescence (defined as a CML Janus-like pellicle), is conceived and constructed via electrospinning by combining microcosmic with macroscopic partitions. [PANI/PMMA]//[Eu(BA)₃phen/PMMA] and [Fe₃O₄/PMMA]//[Tb(BA)₃phen/PMMA] Janus-like microribbons are selected as building units to construct a conductive aeolotropism-luminescence layer (CL layer) and magnetism-luminescence layer (ML layer), and the two layers are combined to form a CML Janus-like pellicle. Macroscopic partition is achieved by designing the Janus-like structure of the pellicle, while Janus-like microribbons are used for the microcosmic partition by separating rare earth luminescent compounds from dark-colored magnetic Fe₃O₄ NPs and conductive PANI. The CML Janus-like pellicle has stronger luminescence compared to the contrast samples. The magnetism of the CML Janus-like pellicle can be adjusted by changing the doping amount of Fe₃O₄ NPs. The CML Janus-like pellicle can achieve a strong and variable conductive aeolotropism via changing the doping amount of PANI and the highest conductive aeolotropism ratio can reach ca. 10⁸ times when the PANI content is 70%. Microcosmic and macroscopic partitions are simultaneously integrated into the CML Janus-like pellicle, which results in almost no detrimental mutual influences between the two layers, and the overall performances of the CML Janus-like pellicle are greatly improved.

Janus-like pellicle with conductive aeolotropism, magnetism and luminescence is designed and constructed via electrospinning by combining microcosmic with macroscopic partitions.

A review on the laser-assisted flow deposition method: growth of ZnO micro and nanostructures

A newly developed LAFD method was revealed to be effective in producing ZnO crystals with different morphologies, evidencing a high crystalline and optical quality.

PdO/PdO2 functionalized ZnO : Pd films for lower operating temperature H2 gas sensing

Noble metals and their oxide nano-clusters are considered to be the most promising candidates for fabricating advanced H₂ gas sensors. Through this work, we propose a novel strategy to grow and modulate the density of PdO/PdO₂ nanoparticles uniformly on nanostructured Pd-doped ZnO (ZnO : Pd) films by a one-step solution approach followed by thermal annealing at 650 °C, and thus to detect ppm-level H₂ gas in a selective manner. The gas sensing properties of such hybridized materials showed that the PdO-functionalized ZnO samples offer significantly improved H₂ gas sensing properties in an operating temperature range of 25-200 °C. The deposition of ZnO : Pd films via a simple synthesis from chemical solutions (SCS) approach with an aqueous bath (at relatively low temperatures, <95 °C) is reported. Furthermore, the functionalization of palladium oxide nanoclusters by a simple but highly effective approach on ZnO : Pd film surfaces was performed and is reported here for the first time. The morphological, structural, vibrational, optical, chemical, and electronic properties were studied in detail and the mixed phases of palladium oxide nanoclusters on the ZnO surface were found. Sensor studies of the ZnO : Pd samples (in the range of 25-350 °C operating temperature) showed good selectivity to H₂ gas, especially in the range of higher temperatures (>150 °C, up to 350 °C); however, the PdO/PdO₂ mixed phases of the nanocluster-modified surface ZnO : Pd films showed a much better selectivity to H₂ gas, even at a lower operating temperature, in the range of 25-150 °C. For such PdO-functionalized ZnO : Pd films, even at room temperature, a gas response of ∼12.7 to 1000 ppm of H₂ gas was obtained, without response to any other reducing gases or tested vapors. The large recovery time of the samples at room temperatures (>500 s) can be drastically reduced by applying higher bias voltages. Furthermore, we propose and discuss the gas sensing mechanism for these structures in detail. Our study demonstrates that surface functionalization with PdO/PdO₂ mixed phase nanoclusters-nanoparticles (NPs) is much more effective than only the Pd doping of nanostructured ZnO films for selective sensing applications. This approach will pave a new way for the controlled functionalization of PdO/PdO₂ nanoclusters on ZnO : Pd surfaces to the exact detection of highly explosive H₂ gas under various atmospheres by using solid state gas sensors.

Cataluminescence sensing of carbon disulfide based on CeO2 hierarchical hollow microspheres

Material morphology-dependent cataluminescence (CTL) sensing characteristic and application are presented in this work. Hierarchical hollow microspheres CeO₂ were synthesized via the hydrothermal reaction of glucose and N, N-dimethyl-formamide (Glu-DMF). SEM, XRD, TEM, HRTEM and BET were used to characterize the prepared CeO₂ materials. Compared with CeO₂ cubics (CeO₂ Cubs), CeO₂ hierarchical hollow microspheres (CeO₂ HMs) show an enhanced CTL response to carbon disulfide. The response and recovery times of CeO₂ HMs-based CTL sensor towards carbon disulfide are about 8 s and 20 s, respectively. CeO₂ HMs exhibits a linear CTL response to carbon disulfide in the concentration range of 0.50~10 μg•mL^-1 with an excellent sensitivity and selectivity. These results suggest that CeO₂ HMs will be a highly promising CTL sensing material for the detection and monitoring carbon disulfide. Graphical abstract CeO₂ hierarchical hollow microspheres (CeO₂ HMs) were synthesized via the hydrothermal reaction of glucose and N, N-dimethyl-formamide (Glu-DMF). Meanwhile, the prepared CeO₂ HMs shows commendable CTL response towards carbon disulfide. Due to the excellent analytical performance of designed CeO₂ HMs-based sensor for carbon disulfide, it has potential application value in various locations.

Regulation of Charge Carrier Dynamics in ZnO Microarchitecture-Based UV/Visible Photodetector via Photonic-Strain Induced Effects

A feasible, morphological influence on photoresponse behavior of ZnO microarchitectures such as microwire (MW), coral-like microstrip (CMS), fibril-like clustered microwire (F-MW) grown by one-step carrier gas/metal catalyst "free" vapor transport technique is reported. Among them, ZnO F-MW exhibits higher photocurrent (I_Ph ) response, i.e., I_{Ph/ZnO F-MW} > I_{Ph/ZnO CMS} > I_{Ph/ZnO MW} . The unique structural alignment of ZnO F-MW has enhanced the I_Ph from 14.2 to 186, 221, 290 µA upon various light intensities such as 0 to 6, 11, 17 mW cm^-2 at λ_{405 nm} . Herein, the nature of the as-fabricated ZnO photodetector (PD) is also demonstrated modulated by tuning the inner crystals piezoelectric potential through the piezo-phototronic effect. The I_Ph response of PD decreases monotonically by introducing compressive strain along the length of the device, which is due to the synergistic effect between the induced piezoelectric polarization and photogenerated charge carriers across the metal-semiconductor interface. The current behavior observed at the two interfaces acting as the source (S) and drain (D) is carefully investigated by analyzing the Schottky barrier heights (Φ_SB ). This work can pave the way for the development of geometrically modified strain induced performances of PD to promote next generation self-powered optoelectronic integrated devices and switches.

Sensing performances of pure and hybridized carbon nanotubes-ZnO nanowire networks: A detailed study

In this work, the influence of carbon nanotube (CNT) hybridization on ultraviolet (UV) and gas sensing properties of individual and networked ZnO nanowires (NWs) is investigated in detail. The CNT concentration was varied to achieve optimal conditions for the hybrid with improved sensing properties. In case of CNT decorated ZnO nanonetworks, the influence of relative humidity (RH) and applied bias voltage on the UV sensing properties was thoroughly studied. By rising the CNT content to about 2.0 wt% (with respect to the entire ZnO network) the UV sensing response is considerably increased from 150 to 7300 (about 50 times). With respect to gas sensing, the ZnO-CNT networks demonstrate an excellent selectivity as well as a high gas response to NH₃ vapor. A response of 430 to 50 ppm at room temperature was obtained, with an estimated detection limit of about 0.4 ppm. Based on those results, several devices consisting of individual ZnO NWs covered with CNTs were fabricated using a FIB/SEM system. The highest sensing performance was obtained for the finest NW with diameter (D) of 100 nm,  with a response of about 4 to 10 ppm NH₃ vapor at room temperature.

Micropatternable Double-Faced ZnO Nanoflowers for Flexible Gas Sensor

Micropatternable double-faced (DF) zinc oxide (ZnO) nanoflowers (NFs) for flexible gas sensors have been successfully fabricated on a polyimide (PI) substrate with single-walled carbon nanotubes (SWCNTs) as electrode. The fabricated sensor comprises ZnO nanoshells laid out on a PI substrate at regular intervals, on which ZnO nanorods (NRs) were grown in- and outside the shells to maximize the surface area and form a connected network. This three-dimensional network structure possesses multiple gas diffusion channels and the micropatterned island structure allows the stability of the flexible devices to be enhanced by dispersing the strain into the empty spaces of the substrate. Moreover, the micropatterning technique on a flexible substrate enables highly integrated nanodevices to be fabricated. The SWCNTs were chosen as the electrode for their flexibility and the Schottky barrier they form with ZnO, improving the sensing performance. The devices exhibited high selectivity toward NO₂ as well as outstanding sensing characteristics with a stable response of 218.1, fast rising and decay times of 25.0 and 14.1 s, respectively, and percent recovery greater than 98% upon NO₂ exposure. The superior sensing properties arose from a combination of high surface area, numerous active junction points, donor point defects in the ZnO NRs, and the use of the SWCNT electrode. Furthermore, the DF-ZnO NF gas sensor showed sustainable mechanical stability. Despite the physical degradation observed, the devices still demonstrated outstanding sensing characteristics after 10 000 bending cycles at a curvature radius of 5 mm.

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.

Sonochemical Synthesis of a Zinc Oxide Core-Shell Nanorod Radial p-n Homojunction Ultraviolet Photodetector

We report for the first time on the growth of a homogeneous radial p-n junction in the ZnO core-shell configuration with a p-doped ZnO nanoshell structure grown around a high-quality unintentionally n-doped ZnO nanorod using sonochemistry. The simultaneous decomposition of phosphorous (P), zinc (Zn), and oxygen (O) from their respective precursors during sonication allows for the successful incorporation of P atoms into the ZnO lattice. The as-formed p-n junction shows a rectifying current-voltage characteristic that is consistent with a p-n junction with a threshold voltage of 1.3 V and an ideality factor of 33. The concentration of doping was estimated to be N_A = 6.7 × 10¹⁷ cm^-3 on the p side from the capacitance-voltage measurements. The fabricated radial p-n junction demonstrated a record optical responsivity of 9.64 A/W and a noise equivalent power of 0.573 pW/√Hz under ultraviolet illumination, which is the highest for ZnO p-n junction devices.

Solid-State Method Synthesis of SnO₂-Decorated g-C₃N₄ Nanocomposites with Enhanced Gas-Sensing Property to Ethanol

SnO₂/graphitic carbon nitride (g-C₃N₄) composites were synthesized via a facile solid-state method by using SnCl₄·5H₂O and urea as the precursor. The structure and morphology of the as-synthesized composites were characterized by the techniques of X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), thermogravimetry-differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), and N₂ sorption. The results indicated that the composites possessed a two-dimensional (2-D) structure, and the SnO₂ nanoparticles were highly dispersed on the surface of the g-C₃N₄ nanosheets. The gas-sensing performance of the samples to ethanol was tested, and the SnO₂/g-C₃N₄ nanocomposite-based sensor exhibited admirable properties. The response value (Ra/Rg) of the SnO₂/g-C₃N₄ nanocomposite with 10 wt % 2-D g-C₃N₄ content-based sensor to 500 ppm of ethanol was 550 at 300 °C. However, the response value of pure SnO₂ was only 320. The high surface area of SnO₂/g-C₃N₄-10 (140 m²·g^-1) and the interaction between 2-D g-C₃N₄ and SnO₂ could strongly affect the gas-sensing property.

Nanomechanics of individual aerographite tetrapods

Carbon-based three-dimensional aerographite networks, built from interconnected hollow tubular tetrapods of multilayer graphene, are ultra-lightweight materials recently discovered and ideal for advanced multifunctional applications. In order to predict the bulk mechanical behaviour of networks it is very important to understand the mechanics of their individual building blocks. Here we characterize the mechanical response of single aerographite tetrapods via in situ scanning electron and atomic force microscopy measurements. To understand the acquired results, which show that the overall behaviour of the tetrapod is governed by the buckling of the central joint, a mechanical nonlinear model was developed, introducing the concept of the buckling hinge. Finite element method simulations elucidate the governing buckling phenomena. The results are then generalized for tetrapods of different size-scales and shapes. These basic findings will permit better understanding of the mechanical response of the related networks and the design of similar aerogels based on graphene and other two-dimensional materials.

Aerographite is a highly porous and lightweight carbon material obtained from hollow tubular tetrapod building units. Here, the authors present a comprehensive investigation of tetrapod deformation mechanisms which are at the core of aerographite nanomechanical properties.

Localized Synthesis of Iron Oxide Nanowires and Fabrication of High Performance Nanosensors Based on a Single Fe2 O3 Nanowire

A composed morphology of iron oxide microstructures covered with very thin nanowires (NWs) with diameter of 15-50 nm has been presented. By oxidizing metallic Fe microparticles at 255 °C for 12 and 24 h, dense iron oxide NW networks bridging prepatterned Au/Cr pads are obtained. X-ray photoelectron spectroscopy studies reveal formation of α-Fe₂ O₃ and Fe₃ O₄ on the surface and it is confirmed by detailed high-resolution transmission electron microscopy and selected area electron diffraction (SAED) investigations that NWs are single phase α-Fe₂ O₃ and some domains of single phase Fe₃ O₄ . Localized synthesis of such nano- and microparticles directly on sensor platform/structure at 255 °C for 24 h and reoxidation at 650 °C for 0.2-2 h, yield in highly performance and reliable detection of acetone vapor with fast response and recovery times. First nanosensors on a single α-Fe₂ O₃ nanowire are fabricated and studied showing excellent performances and an increase in acetone response by decrease of their diameter was developed. The facile technological approach enables this nanomaterial as candidate for a range of applications in the field of nanoelectronics such as nanosensors and biomedicine devices, especially for breath analysis in the treatment of diabetes patients.

Hybridization of Zinc Oxide Tetrapods for Selective Gas Sensing Applications

In this work, the exceptionally improved sensing capability of highly porous three-dimensional (3-D) hybrid ceramic networks toward reducing gases is demonstrated for the first time. The 3-D hybrid ceramic networks are based on doped metal oxides (Me_xO_y and Zn_xMe_1-xO_y, Me = Fe, Cu, Al) and alloyed zinc oxide tetrapods (ZnO-T) forming numerous junctions and heterojunctions. A change in morphology of the samples and formation of different complex microstructures is achieved by mixing the metallic (Fe, Cu, Al) microparticles with ZnO-T grown by the flame transport synthesis (FTS) in different weight ratios (ZnO-T:Me, e.g., 20:1) followed by subsequent thermal annealing in air. The gas sensing studies reveal the possibility to control and change/tune the selectivity of the materials, depending on the elemental content ratio and the type of added metal oxide in the 3-D ZnO-T hybrid networks. While pristine ZnO-T networks showed a good response to H₂ gas, a change/tune in selectivity to ethanol vapor with a decrease in optimal operating temperature was observed in the networks hybridized with Fe-oxide and Cu-oxide. In the case of hybridization with ZnAl₂O₄, an improvement of H₂ gas response (to ∼7.5) was reached at lower doping concentrations (20:1), whereas the increase in concentration of ZnAl₂O₄ (ZnO-T:Al, 10:1), the selectivity changes to methane CH₄ gas (response is about 28). Selectivity tuning to different gases is attributed to the catalytic properties of the metal oxides after hybridization, while the gas sensitivity improvement is mainly associated with additional modulation of the electrical resistance by the built-in potential barriers between n-n and n-p heterojunctions, during adsorption and desorption of gaseous species. Density functional theory based calculations provided the mechanistic insights into the interactions between different hybrid networks and gas molecules to support the experimentally observed results. The studied networked materials and sensor structures performances would provide particular advantages in the field of fundamental research, applied physics studies, and industrial and ecological applications.