Hole concentration modulated gas sensor for selective detection of 2-methoxy ethanol

The inhaling rate of toxic gases in daily life is increasing alarmingly; in turn, human health is in question. To resolve this dilemma, the possible remedy is the early detection and adequate regulation of VOCs in the atmosphere via sensors. Therefore, the investigation focused on developing a gas sensor for sensing several VOCs and flourished with a highly selective C₃H₈O₂ gas sensor at room temperature. In-depth structural, elemental, and morphological analysis followed by the sensing test affirmed the enhanced C₃H₈O₂ detection performance of Ag–NiO over pure NiO. The nanosized sphere formation was confirmed via XRD and TEM characterizations alongside the effect of sintered temperature on the shape and crystallite size. Moreover, the XPS examined the combined effect of sintering temperature and doping on the synthesized nanostructures and optimized the exact temperature as 500 °C because of the improved hole concentration. The Ag–NiO(500 °C) exhibited appealing sensing characteristics, specifically, a high response of 6491.57 for 100 ppm C₃H₈O₂ at room temperature. The sensor displayed a quick response and recovery (10 s,10 s) C₃H₈O₂ alongside long-term repeatability and stability; it showed practical implementation in real-time scenarios.     

Enhanced NO2 sensing properties of Zn2SnO4-core/ZnO-shell nanorod sensors

Zn₂SnO₄-core/ZnO-shell nanorods were synthesized using a two-step process: synthesis of Zn₂SnO₄ nanorods the thermal evaporation of a mixture of ZnO, SnO₂, and graphite powders, followed by atomic layer deposition (ALD) of ZnO. The nanorods were 50–250 nm in diameter and a few to a few tens of micrometers in length. The cores and shells of the nanorods were face-centered cubic-structured single crystal Zn₂SnO₄ and wurtzite-structured single crystal ZnO, respectively. The multiple networked Zn₂SnO₄-core/ZnO-shell nanorod sensors showed a response of 173–498% to NO₂ concentrations of 1–5 ppm at 300 °C. These response values are 2–5 times higher than those of the Zn₂SnO₄ nanorod sensor over the same NO₂ concentration range. The NO₂ sensing mechanism of the Zn₂SnO₄core/ZnO-shell nanorods is discussed.     

A new synergistic effect in one step sputtered ZnO/Zn2SnO4 heterojunction films for H2 sensing related to crystal structure and film compactness

In this paper, ZnO/Zn₂SnO₄ heterojunction films were one step fabricated by magnetron sputtering and the dependence of crystal structures, film compactness and H₂ sensing properties on annealing process were investigated and discussed. The results showed that three typical surface morphologies can be controlled by adjusting annealing temperatures and periods. The films annealed at the temperature of 550 °C for 6 h showed the best H₂ sensing properties. It exhibited a response (R_a/R_g) of 28.3–100 ppm H₂ at the temperature of 230 °C and the detection limit is 30.2 ppb. Meanwhile, it also showed a good selectivity and long-term stability to H₂. The H₂ sensing mechanism is attributed to the synergistic effect between ZnO (0001) signal crystal facets and ZnO/Zn₂SnO₄ heterojunction structures which enhanced the gas reactivity and resistance modulation range. On the contrary, insufficient annealing restricts the film crystallinity and the growth of hexagonal ZnO while undue annealing destroys the compactness of the films, leading to poor H₂ sensing properties.     

ZnO-capped nanorod gas sensors

This paper reports on the synthesis of pristine α-Fe₂O₃ nanorods and Fe₂O₃–ZnO core–shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core–shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5–200 ppm) at different temperatures (150–250 °C). The XRD and SEM revealed the excellent crystallinity of the Fe₂O₃–ZnO core–shell nanorods, as well as their uniformity in terms of shape and size. The Fe₂O₃–ZnO core–shell nanorod sensor showed a stronger response to ethanol than the pristine Fe₂O₃ nanorod sensor. The response (i.e., the relative change in electrical resistance R_a/R_g) of the core–shell nanorod sensor was 22.75 for 100 ppm ethanol at 200 °C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe₂O₃–ZnO core–shell nanorods was 15.96 s, which was shorter than that of the pristine nanorod sensor (22.73 s). The core–shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe₂O₃–ZnO core–shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe₂O₃–ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe₂O₃–ZnO interface.     

Synthesis and ethanol sensing properties of CuO nanorods coated with In2O3

CuO/In₂O₃ core–shell nanorods were fabricated using thermal evaporation and radio frequency magnetron sputtering. X-ray diffraction and transmission electron microscopy showed that both the cores and shells were crystalline. The multiple networked CuO/In₂O₃ core–shell nanorod sensors showed responses of 382–804%, response times of 36–54 s and recovery times of 144–154 s at ethanol (C₂H₅OH) concentrations ranging from 50 to 250 ppm at 300 °C. These responses were 2.3–2.8 times higher than those of the pristine CuO nanorod sensor over the same C₂H₅OH concentration range. The origin of the enhanced ethanol sensing properties of the core–shell nanorod sensor is discussed.     

Highly sensitive and selective acetylene sensors based on p-n heterojunction of NiO nanoparticles on flower-like ZnO structures

Acetylene (C₂H₂) gas concentration is a key parameter in transformer monitoring. In current work, the selective acetylene sensors which based on flower-like ZnO structures with NiO nanoparticles were successfully fabricated. The NiO–ZnO composites were synthesized by two-step hydrothermal method. And various of characterization analyses had been applied to the exploration of crystal structure and the p-n heterojunction. According to the systematic gas sensitivity tests, the response of NiO–ZnO (5%) to 50 ppm C₂H₂ was 15.23 at 200 °C whereas the response of pure ZnO was 4.1 in the same condition. In addition, the response value of NiO–ZnO (5%) to 50 ppm C₂H₂ was 3.6 times to 50 ppm H₂. Such a good gas-sensing property of NiO–ZnO composites is due to p-n heterojunction and high catalytic activity of NiO.     

One step from nanofiber to functional hybrid structure: Pd doped ZnO/SnO2 heterojunction nanofibers with hexagonal ZnO columns for enhanced low-temperature hydrogen gas sensing

In this paper, a novel hybrid structure of Pd doped ZnO/SnO₂ heterojunction nanofibers with hexagonal ZnO columns was one step synthesized from electrospun precursor nanofibers. Due to the synergistic effect of hexagonal ZnO, SnO₂ and Pd, the structure exhibited excellent hydrogen (H₂) gas sensing properties. At low-temperature of 120 °C, the response (R_a/R_g) to 100 ppm H₂ gas exceeded 160, the response/recovery time was only 20 s and 6 s respectively and the limit of detection was only 0.5 ppm. Meanwhile, it also had good selectivity for H₂ gas and excellent linearity. In addition, the materials were characterized by XRD, FESEM, HRTEM, XPS, and the synthesis mechanism and gas sensing mechanism were proposed.     

Low-temperature and highly sensitivity H2S gas sensor based on ZnO/CuO composite derived from bimetal metal-organic frameworks

The bimetallic metal-organic frameworks (MOF) Zn/Cu-BTC were prepared by a facile solvothermal method in one step and used as a self-sacrificed template to obtain the ZnO/CuO composites. The composites with different Cu/Zn molar ratios were characterized by XRD, FESEM, and XPS. The ZnO/CuO composite exhibited an octahedral structure, and a p-n heterojunction may be formed between p-type CuO and n-type ZnO. To prove its functional characteristics, the ZnO/CuO composite was used as a sensing material to test its gas sensitivity. The effect of Cu/Zn molar ratios was examined, and the results showed that the optimized ZnO/CuO (1: 0.33) composite based gas sensor exhibited reasonable selectivity to 10 ppm H₂S, operated at 40 °C. The sensitivities were improved by 17.1 times and 327.8 times compared with the pristine CuO and ZnO based gas sensors, respectively. Moreover, the detection limit to H₂S of such sensors could be reduced as low as 300 ppb. The sensing mechanism has been thoroughly studied and such ZnO/CuO composite is an ideal candidate for highly sensitive detection for H₂S with low power consumption in the real application.     

High selectivity and response H2 sensors based on ZnO@ZIF-71@Ag nanorod arrays

Hydrogen (H₂) is widely used in industrial and medical, however its flammable and explosive nature requires economical and effective monitoring to ensure safety. In this work, ZnO@ZIF-71@Ag nanorod arrays were synthesized to provide an effective adsorption response to H₂ through size effect and high catalytic activity by immobilizing Ag nanoparticles (NPs) into the pores of ZIF-71. The results of the gas sensitivity tests showed that the nanorod arrays were significantly more selective towards H₂. Moreover, the response of ZnO@ZIF-71@Ag to 50 ppm H₂ was 11 times higher than that of ZnO@ZIF-71 at a lower operating temperature (150°C). The size of the Ag NPs was demonstrated to be below 10 nm by TEM characterization, suggesting that Ag in the form of quantum dots (QDs) to bring an unignorable catalytic effect for breaking hydrogen molecule (H₂) into highly active atoms ([H]). In addition, the result of Density Function Theory (DFT) calculation revealed that the adsorption energy of Ag-catalyzed [H] (−8.255 eV) was much higher than that of H₂ (−4.222 eV) on ZnO (100), which results in elevated charge transfer to promote hydrogen sensing performance of ZnO@ZIF-71@Ag. In this study, a novel hydrogen sensor based on pore sieving and catalytic sensing mechanisms was obtained, which provides a new reference for the development of hydrogen sensors.     

Highly sensitive H2S gas sensors based on CuO-coated ZnSnO3 nanorods synthesized by thermal evaporation

ZnSnO₃ one-dimensional (1D) nanostrutures were synthesized by thermal evaporation. The morphology, crystal structure and sensing properties of the CuO-coated ZnSnO₃ nanostructures to H₂S gas at 100 °C were examined. Transmission electron microscopy and X-ray diffraction revealed both the ZnSnO₃ nanorods and CuO nanoparticles to be single crystals. The diameters of the CuO nanoparticles on the nanorods ranged from a few to a few tens of nanometers. The gas sensors fabricated from multiple networked CuO-coated ZnSnO₃ nanorods exhibited enhanced electrical responses to H₂S gas compared to the uncoated ZnSnO₃ nanorod sensors, showing 61.7-, 49.9-, and 31.3-fold improvement at H₂S concentrations of 25, 50, and 100 ppm, respectively. The response time of the nanorod sensor to H₂S gas was reduced by the CuO coating but the recovery time was similar. The mechanism for the enhanced H₂S gas sensing properties of ZnSnO₃ nanorods by the CuO coating is discussed.     

C2H2 gas sensor based on Ni-doped ZnO electrospun nanofibers

Pure and Ni-doped ZnO nanofibers were synthesized using the electrospinning method. The morphology, crystal structure and optical properties of the nanofibers were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy, respectively. It is found that Ni doping does not change the morphology and crystal structures of the nanofibers, and the ultraviolet emissions of ZnO nanofibers present red shift with increasing Ni doping concentration. C₂H₂ sensing properties of the sensors based on the nanofibers were investigated. The results show that the C₂H₂ sensing properties of ZnO nanofibers are effectively improved by Ni doping, and 5 at% Ni-doped ZnO nanofibers exhibit a maximum sensitivity to C₂H₂ gas.     

Nanoseed-assisted rapid formation of ultrathin ZnO nanorods for efficient room temperature NO2 detection

The influence of ZnO nanoseeds on the formation of ZnO nanorods from ε-Zn(OH)₂ in NaOH solution at 80 °C was investigated, using ZnO nanoparticles with a diameter of 4–10 nm as the seeds. The experimental results indicated that the presence of ZnO nanoseeds promoted the rapid heterogeneous formation of ultrathin ZnO nanorods. Compared with the ZnO submicron rods with a diameter of 0.5–1.0 µm, the ultrathin ZnO nanorods with a diameter of 10–15 nm were found to be more sensitive for detecting NO₂ at room temperature owing to their higher variation of channel conduction to the diameter.     

High performance H2 sensor based on rGO-wrapped SnO2–Pd porous hollow spheres

Hydrogen (H₂) sensors based on metal oxide semiconductors (MOS) are promising for many applications such as a rocket propellant, industrial gas and the safety of storage. However, poor selectivity at low analyte concentrations, and independent response on high humidity limit the practical applications. Herein, we designed rGO-wrapped SnO₂–Pd porous hollow spheres composite (SnO₂–Pd@rGO) for high performance H₂ sensor. The porous hollow structure was from the carbon sphere template. The rGO wrapping was via self-assembly of GO on SnO₂-based spheres with subsequent thermal reduction in H₂ ambient. This sensor exhibited excellently selective H₂ sensing performances at 390 °C, linear response over a broad concentration range (0.1–1000 ppm) with recovery time of only 3 s, a high response of ～8 to 0.1 ppm H₂ in a minute, and acceptable stability under high humidity conditions (e. g. 80%). The calculated detection limit of 16.5 ppb opened up the possibility of trace H₂ monitoring. Furthermore, this sensor demonstrated certain response to H₂ at the minimum concentration of 50 ppm at 130 °C. These performances mainly benefited from the special hollow porous structure with abundant heterojunctions, the catalysis of the doped-PdO_x, the relative hydrophobic surface from rGO, and the deoxygenation after H₂ reduction.     

Sm2O3-doped ZnO beech fern hierarchical structures for nitroaniline chemical sensor

Sm₂O₃-doped ZnO hierarchical composites were prepared via a facile, simple hydrothermal method using Zn(CH₃COO)_2.2H₂O and Sm(NO₃)₃0.6H₂O as precursors. Growth temperature, time, and pH for the reaction were 7 h, 155 °C and 9.5, respectively. Needle-shaped and compound leaf-shaped structures with ovate or triangular-ovate outlines similar to those of Beech Fern (Phegopteris hexagonoptera) for hierarchical composites were formed. Further, the synthesized Sm₂O₃-doped ZnO hierarchical composites were used as efficient electron mediators for the preparation of highly sensitive, fast and reliable nitroaniline electrochemical sensors. A sensitivity of 1.71 μA μM⁻¹ cm⁻² with a very low experimental detection limit of 15.6 μM was reported for Sm₂O₃-doped ZnO hierarchical composites modified Ag electrode. Thus, as synthesized Sm₂O₃-doped ZnO hierarchical composites are potential and efficient electron mediators for the fabrication of sensitive, reliable and reproducible chemical sensors.     

Patterned Zn-seeds and selective growth of ZnO nanowire arrays on transparent conductive substrate and their field emission characteristics

A method for the patterned growth of ZnO nanorods with better field emission properties is presented that combines nanoimprinting, electroplated Zn seeds and aqueous solution growth of ZnO. A patterned Zn layer over large area was prepared using the poly(dimethylsiloxane) (PDMS) template to pattern PMMA masking layer without residual layer. ZnO nanorods were selectively deposited on the Zn seeds instead of growing on the bare ITO regions due to the preferred growth on Zn seeds/ITO substrate. The diameter of ZnO nanorods was decided by the concentration of reactants of zinc nitrate and hexamethyltetramine (C₆H₁₂N₄) (0.025–0.1 M) at low temperature. This approach provides the capability of creating patterned 1D ZnO micro/nanopatterns at low temperature, ambient condition, and low cost with large area on flexible devices.     

Preparation of hollow SnO2/ZnO cubes for the high-performance detection of VOCs

Detecting volatile organic compounds is essential to improving the environment and human health. This study prepared a novel composite of hollow SnO₂/ZnO cubes using a self-template hydrothermal method followed by a calcination process. The morphology and structure of the composites were characterized using a series of analysis techniques, and the formation mechanism of a hollow cube-like structure was explored. Compared to the hollow SnO₂ cube sensor, the hollow SnO₂/ZnO cube sensor exhibited a strong response (148–100 ppm formaldehyde), fast response/recovery time (15 s/25 s), good linearity (R² = 0.995), good repeatability, and excellent stability. The superior gas sensing property of the hollow SnO₂/ZnO cubes was attributed to the combined advantages of hollow structures and heterojunctions.     

Synthesis of Ag3PO4–ZnO nanorod composites with high visible-light photocatalytic activity

Ag₃PO₄ nanoparticles with 50–100 nm in size distributed on the surface of ZnO nanorods with ca. 20 nm in diameter and 1–2 μm in length have been synthesized by a facile method. The Ag₃PO₄–ZnO nanorod composites had much higher photocatalytic activity toward degradation of Rhodamine B (RhB) under visible light irradiation than pure ZnO nanorods, and had better recyclability and stability than pure Ag₃PO₄ nanoparticles. The Ag₃PO₄–ZnO nanorod composite with the molar ratio of Ag₃PO₄:ZnO = 1:40 exhibited the highest photodegradation efficiency of RhB (93%), which was 1.5 times of pure ZnO nanorods.     

Hydrothermally grown ZnO nanorods arrays for selective NO2 gas sensing: Effect of anion generating agents

Vertically aligned ZnO nanorods (ZNRs) arrays with various aspect ratios were deposited by using a simple and inexpensive hydrothermal route at relatively low temperature of 90 °C. The influence of hydroxide anion generating agents in the solution on the growth of ZNRs arrays was studied. Hexamethylenetetramine (HMTA) and ammonia were used as hydroxide anion generating agents while polyethyleneimine (PEI) as structure directing agent. The combined effect of these three agents plays a crucial role in the growth of ZNRs arrays with respect to their rod length and diameter, which controls the aspect ratio. The deposited ZNRs exhibited hexagonal wurtize crystal structure with preferred orientation along (002) plane. The highly crystalline nature and pure phase formation of ZNRs was confirmed from FT-Raman studies. The maximum gas response (R_g/R_a) of 67.5 was observed for high aspect ratio ZNRs, deposited with combination of HMTA, ammonia as well as PEI. The enhancement in gas response can be attributed to high surface area (45 cm²/g) and desirable surface accessibility in high aspect ratio ZNRs. Fast response–recovery characteristics, especially a much quicker gas response time of 32 s and recovery time of 530 s were observed at 100 ppm NO₂ gas concentration.     

Zinc oxide obtained by the solvothermal method with high sensitivity and selectivity to nitrogen dioxide

The effect of heat treatment using [Zn(H₂O)(O₂C₅H₇)₂] precursor ethylene glycol solution (synthesis temperature 125–185 °C, holding time 2–6 h) on the characteristics of the obtained nanocrystalline zinc oxide powder was studied. Under all synthesis conditions crystalline ZnO was formed with a wurtzite structure and an average crystallite size of 8–37 nm. The thermal behaviour and microstructure of the nanostructured ZnO powder were studied. The sensory properties of the obtained films in terms of the detection of hydrogen, methane, carbon monoxide and nitrogen dioxide were studied. The high sensitivity and selectivity of a thick ZnO film (synthesis temperature 145°С, holding time 6 h) when detecting NO₂ was established. It was found, that humidity had almost no effect on response value for NO₂ detection.     

Green material: ecological importance of imperative and sensitive chemi-sensor based on Ag/Ag2O3/ZnO composite nanorods

In this report, we illustrate a simple, easy, and low-temperature growth of Ag/Ag₂O₃/ZnO composite nanorods with high purity and crystallinity. The composite nanorods were structurally characterized by field emission scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy which confirmed that synthesized product have rod-like morphology having an average cross section of approximately 300 nm. Nanorods are made of silver, silver oxide, and zinc oxide and are optically active having absorption band at 375 nm. The composite nanorods exhibited high sensitivity (1.5823 μA.cm⁻².mM⁻¹) and lower limit of detection (0.5 μM) when applied for the recognition of phenyl hydrazine utilizing I-V technique. Thus, Ag/Ag₂O₃/ZnO composite nanorods can be utilized as a redox mediator for the development of highly proficient phenyl hydrazine sensor.