Hierarchical heterostructure of porous NiO nanosheets on flower-like ZnO assembled by hexagonal nanorods for high-performance gas sensor

A novel hierarchical heterostructure consisting of porous NiO nanosheets and flower-like ZnO assembled by hexagonal nanorods was successfully fabricated by a simple two-step hydrothermal approach. Flower-like ZnO was obtained by the first step hydrothermal method. Through the second step hydrothermal method, porous NiO nanosheets grew on the surface of flower-like ZnO to realize integration of ZnO and NiO, so the p-n heterostructure between ZnO and NiO formed. The samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX). Gas sensing test results showed that the sensor based on NiO/ZnO composite exhibited superior sensing properties to acetone. The sensor response to 100 ppm acetone was about 205.14 at the optimum working temperature of 240 °C, and the response and recovery times were about 7 and 20 s, respectively. The enhanced response might be attributed to heterojunction and larger specific surface area provided by attached porous NiO nanosheets. The rapid response and recovery characteristics and improved selectivity attributed to the porous structure and good catalytic actions of NiO nanosheets.     

Enhanced NO2 detection using hierarchical porous ZnO nanoflowers modified with graphene

Because of their potential applications in gas sensing and catalysis, reduced graphene oxide (RGO) and ZnO have been the focus of much recent attention. However, few reported materials have been produced via the combination of hierarchical ZnO structures with RGO to achieve high sensing performances. In this paper, a hydrothermal method was used to synthesize hierarchical porous ZnO nanoflowers, which were then combined with graphene to enhance their sensing performances. The rapid detection of 1 ppm NO₂ was achieved at 174 °C. The morphologies and structures of these materials were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Photoluminescence measurements and X-ray photoelectron spectroscopy were also used to investigate the mechanism of gas sensing by these materials.     

多孔分级结构金属氧化物气体传感器研究概述

多孔和分级结构因其大的表面积和有助于气体运输的特点而拥有理想的气敏性能。本文分别综述了应用于气体传感的多孔和分级结构金属氧化物的主要制备方法,并介绍了一种新的基于生物模板合成多孔分级结构金属氧化物的方法。由于其形貌中团聚较少,多孔分级结构的气敏性能较传统纳米结构的更为优秀。     

Incorporation of carbon nanotube and graphene in ZnO nanorods-based hydrogen gas sensor

In this study, ZnO nanorods (NRs) were grown using solgel/hydrothermal methods on SiO₂, carbon nanotube (CNT)/SiO₂, and graphene/SiO₂ substrates to form hydrogen gas sensing chips. Results indicate that ZnO NRs/CNT/SiO₂ structures exhibited better H₂ sensing performance than the other two types of ZnO NRs-based structures. Furthermore, multiple electrical and material characterizations show that ZnO NRs/CNT/SiO₂ structures had a stronger (002) crystalline phase, with nanorod fusion near the bottom, and more oxygen-related defects. Owing to their small size, simple fabrication, and low cost, the ZnO NRs/CNT/SiO₂based H₂ gas sensors are promising for future industrial H₂ sensing applications.     

Heterostructure nanoarchitectonics with ZnO/SnO2 for ultrafast and selective detection of CO gas at low ppm levels

Heterojunction-based gas sensors are very attractive as they substantially improve the sensing characteristics due to the effective potential barrier present at the interface. Taking the advantages of two excellent semiconducting gas sensing materials i.e., SnO₂ and ZnO, herein, we have constructed ZnO/SnO₂ heterojunction by the combination of vacuum evaporation and r.f. sputtering or atomic layer deposition techniques. The ZnO/SnO₂ heterostructure with optimized thickness of ZnO (～10 nm) shows a 6-fold enhancement in sensing response compared to bare SnO₂ films against CO gas. The sensing responses of 81 and 85 % have been obtained for ZnO/SnO₂ heterostructures with ZnO deposited by sputtering and atomic layer deposition (ALD) methods, respectively, against 91 ppm of CO gas with an estimated limit of detection of 1.67 and 0.37 ppm. The ALD ZnO/SnO₂ sample displays an extremely fast response time of 5 s. The heterostructure sensors are also highly selective towards CO gas in the presence of other interfering toxic agents. The enhanced sensing characteristics of ZnO/SnO₂ are assigned to the formation of n-n heterojunction as depicted by X-ray photoelectron spectroscopic band alignment study and the strong CO adsorption on ZnO surface as derived from density functional theory calculations.     

掺杂石墨烯的ZnO复合材料研究进展

ZnO是一种低成本且应用广泛的材料,石墨烯具有较大的比表面积以及优良的吸附、光电等特性,易于与ZnO结合,可提高ZnO的性能。掺杂石墨烯的ZnO基材料在气体检测、抗菌表面涂层、发光二极管、透明导电电极和光催化等方面都有着应用性。本文概述了近几年来石墨烯掺杂ZnO材料作为导电薄膜、传感器、光催化剂等在光电子、生物医疗、环保等不同领域内的研究与发展,提出了目前该复合材料在制备工艺复杂与可控性差,实际应用与理论有较大差距等问题,并对未来的研究趋势进行了预测和展望。     

基于金属氧化物的乙醇检测气敏材料的研究进展

金属氧化物型半导体气体传感器是目前常用的乙醇检测手段,深入研究和改进金属氧化物型半导体材料是提升传感器性能的重要方式。本文首先论述了气敏检测的机理和影响因素,并综述了近年来发展的主要金属氧化物型半导体气敏材料,重点介绍了不同微观结构的Co₃O₄、ZnO、SnO₂及掺杂金属氧化物材料、氧化物异质结等的研究和发展情况,对它们的合成方法、结构特点以及结构与乙醇气敏性能之间的关系进行了探讨。分析表明,减小材料颗粒尺寸、构建大比表面积多孔结构、掺杂和复合改性,是提升金属氧化物材料气敏性能的有效措施。此外,基于传感器微小化的趋势,以微机电系统（MEMS）工艺为基础的微型传感器成为气体传感器的发展趋势。然而,目前针对金属氧化物气敏材料的制备依然缺乏一定的理论指导,气体检测缺乏相应的机理研究,亟需物理、化学、材料等多学科的相互结合,促进乙醇等半导体气体传感器的进一步发展。     

Mechanism of Nanovoid Formation at the ZnO–Glass Interface in Planar Multilayered Structures of AlOx–ZnO–Glass

Functional porous materials require easy fabrication methods with controllability of a wide range of pore size and its density for practical applications including optical devices. The Kirkendall effect based on unbalanced material diffusion provides such a possibility in conjunction with material configurations of multilayers. This study reports a formation of nanoscale pores within ZnO films in planar multilayered structures of Al₂O₃–ZnO‐aluminosilicate glass and demonstrates the mechanism of forming relatively large nanopores in ZnO near the ZnO–glass interface via stress‐promoted Kirkendall diffusion. Experimental characterizations supported by atomic simulation reveal that an enhanced in‐plane tensile stress in the ZnO films with increasing the thickness of the neighboring Al₂O₃ films can promote the diffusivity of the Zn atoms and the pore growth in the ZnO films. The pore size and location in the intermediate ZnO layer of the Al₂O₃–ZnO–glass is alterable by simply selecting the thickness of the Al₂O₃ layer. Promoted diffusion of the Zn atoms enables to fabricate porous planar ZnO films with pore sizes up to a few hundred nm with an enhanced light scattering ability. These findings offer a promising route to produce porous planar films through in‐depth understanding of diffusivity enhancement in glass–metal oxide couples.     

Synthesis of flower-like porous ZnO and their ultrahigh acetone sensing properties

Flower-like porous ZnO was successfully synthesized by a simple hydrothermal method followed by calcination. The morphologies of the as-prepared materials were characterized by scanning electron microscopy (SEM) and the crystal structures were determined by X-ray diffraction. It can be seen in SEM images that each flower-like ZnO unit is composed of randomly arranged ZnO thin flakes which makes the materials extremely porous. Meanwhile, there are numerous through-holes distributed on the surface of ZnO flakes. The gas-sensing properties of the as-prepared materials were investigated, and the results indicate the ultrahigh sensing properties of flower-like porous ZnO to acetone. The response of flower-like porous ZnO sensors to 50 ppm acetone is about 97.8 at the optimum operating temperature of 280 °C. The response and recovery times to 50 ppm acetone are about 2 and 23 s, respectively. Moreover, even at low concentrations of 0.25, 1 and 10 ppm acetone high responses can be observed with the values of 6.7, 15.8 and 30.1. In addition, the as-synthesized flower-like ZnO shows excellent selectivity to acetone and the response to 50 ppm acetone (97.8) is about 4.43 times larger than ethanol (22.1) at the same concentration, which can successfully distinguish acetone and ethanol.     

Cactus-inspired GO/ZnO sensors for fast and robust acetone sensing properties

Fabrication of fast, high-selectivity and reliable gas sensors for real-time monitoring of acetone in exhaled gases remains a key challenge for the development of accurate diabetes diagnosis systems. Inspired by the cactus, in this paper, acetone gas sensor using graphene oxide (GO) with porous zinc oxide (ZnO) nanotube clusters was introduced by room temperature liquid phase method. Porous ZnO nanotubes with lengths of 1～2 μm formed nanoclusters and were uniformly dispersed on the GO sheets, presenting a specific “cactus-like” three-dimensional structure, which supplied more channels and active sites for the diffuse and adsorption of acetone molecules. The cactus-like GO/ZnO sensor exhibits a high response value of 54.3 (5.9-fold improvement compared to ZnO), fast response/recovery time of 3.8/2.9 s (15.7/19.8 s for ZnO), and robust stability to 50 ppm acetone at 180 °C. Special, the sensor can detect even acetone down to 0.1 ppm with a remarkable response (2.1). Moreover, the adsorption energies of different gases were calculated by density functional theory, which further confirmed the good acetone selectivity of the cactus-like GO/ZnO sensor. The appreciable gas sensing performances are mainly attributed to the unique cactus-like nanostructure with abundant holes and high specific surface area, as well as the heterojunctions between GO and ZnO. This unique biomimetic structure provides a promising strategy for designing acetone gas sensors with high practicality.     

Preparation of Nanostructured ZnO Thin Films Using Magnetron Sputtering for the Gas Sensors Applications

ZnO is one of the most promising transparent conducting oxide materials, which widely used in thin film gas sensors. In this research, the dependence of the thermal oxidation time on structural, morphological and gas sensing properties of ZnO thin films is investigated. ZnO nanostructures are synthesized by using DC magnetron sputtering for deposition of pure zinc layers on glass substrates and then thermal oxidation of deposited zinc layers to produce zinc oxide (ZnO) thin films. Obtained results from X-ray diffraction revealed that the degree of crystallinity and the average grain size of the ZnO deposited thin films enhance with increasing the thermal oxidation time. Surface topography and growth behavior of ZnO thin films have important role in optimization of gas sensing properties of these films. In this study, scanning electron microscopy and atomic force microscopy have been used to investigate the effective parameters related to the surface topography of the films. Obtained results from these analyzes revealed that the surface topography of ZnO deposited samples strongly depend on thermal oxidation time. Also the effect of thermal oxidation time on the performance of ZnO gas sensors is investigated. The results indicated that the ethanol gas sensing properties of ZnO samples improve with decreasing the size of grains.     

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 decorated luminescent graphene as a potential gas sensor at room temperature

We present a simplistic single step synthesis and a detailed study of the remarkable room temperature gas sensing and photoluminescence (PL) properties of zinc oxide (ZnO) decorated graphene oxide sheets (GrO). Investigation of opto-electronic properties reveal near UV to blue PL and semiconducting behavior of ZnO–GrO sheets. ZnO nano-crystallites serve the dual purpose of acting as a nano-spacer between dried graphene sheets as well as a primary sensing transducer for the gas sensing applications. PL has been used as a tool to study the defects associated with the surface of the nanocrystallite’s trap levels and/or acceptor–donor recombinations. Time-resolved PL was used to determine free carrier or exciton lifetimes, a vital parameter related to quality of composite and device performance. Results are presented for the detection of common industrial toxins like CO, NH₃ and NO for concentrations as low as 1 ppm at room temperature. A large sensor response and quick recovery time was observed at room temperature with preferred selectivity towards electron donor gases like CO and NH₃.     

Synthesis and optimization strategies of nanostructured metal oxides for chemiresistive methanol sensors

Thanks to the merits such as high specific surface areas, superior electronic conduction and unique gas diffusion path derived from the nanoscales, the demand for detecting methanol has contributed to the rapid expansion of gas sensors based on metal oxide nanostructures. In this review, the “process-structure-performance” correlations of metal oxide nanostructures utilized in the detection of methanol are analyzed. The sensing mechanisms of nanostructured metal oxides operated at different temperatures for methanol monitoring are first introduced. Subsequently, various synthesis processes (e.g. hydrothermal, sol-gel and electrospinning) utilized to modulate the structure and morphology of metal oxide nanostructures are discussed. Given the limitations that existed in methanol gas sensors, numerous optimization strategies including doping, surface modifications, newly designed structures and morphologies, the self-doping defects are enumerated to dramatically enhance the sensing properties represented by the improvement of sensitivity, the reduction of working temperature, the decrease of detection limit, etc. Additionally, the challenges and future research directions of advanced methanol sensors based on metal oxide nanostructures are proposed. It is ultimately expected that this review will help break the bottleneck of nanostructured metal oxides gas sensors in the field of methanol detection, and further promote the actual application of chemiresistive methanol sensors.     

Recent progress in zinc oxide nanomaterials and nanocomposites: From synthesis to applications

The global market of ZnO grows at an annual rate of 4.03%. ZnO has been used in a wide array of applications owing to its unique chemical, physical, and biological properties. In general, the properties of ZnO, such as band gap, crystallite size, and morphology, depend on the synthesis method and parameters employed. In this review, recent progress in the research on ZnO is presented. This review focuses on the latest advancements in pristine ZnO, doped ZnO, ZnO-based nanocomposites with other metal oxides, carbon-based materials, and spinels. The effect of the synthesis method and conditions on the properties of ZnO is discussed. In particular, recent studies on ZnO prepared through precipitation method, green synthesis, green combustion method, hydrothermal method, microwave-assisted hydrothermal, and sol–gel synthesis are reviewed. The effect of dopants and metal oxides on ZnO characteristics is also laid out. This review aims to provide the readers with a thorough understanding of structure–property relationships achieved by varying the synthesis parameters of ZnO, which will be beneficial for the fabrication of high-performance ZnO-based materials for photocatalytic, biological, gas sensing, and flexible electronic applications.     

乙醇缩合制高碳醇(C6+醇)多相催化剂研究进展

以乙醇为原料制备其他高附加值化工产品近期得到了广泛关注,特别是乙醇基于Guerbet反应缩合制备高碳醇(C₆₊)的过程更成为这一领域的研究热点。本文着重报道了近年来乙醇转化为高碳醇过程的几种典型的多相催化剂,包括羟基磷灰石(HAP)、类水滑石(LDHs)以及各种负载型金属催化剂的研究开发现状,分析比较了各类催化剂的优势与不足,结合反应机理针对催化剂对高碳醇选择性不高的原因进行了讨论,对理想催化剂的设计和改进进行了展望,指出应该通过促进Guerbet反应中的羟醛缩合过程增加高碳醇的选择性。金属负载型催化剂对该反应控制步骤的加速作用显著,而金属位点和载体表面酸碱活性位点的精细调控以及特定催化剂形貌结构的控制合成则是该领域未来的研究重点。     

Preparation,characterization of 1D ZnO nanorods and their gas sensing properties

The 1D ZnO nanorods (NR's) were grown with Zinc (Zn) ion precursor concentration variation on seed layer glass substrate by the low temperature hydrothermal method and utilized for nitrogen dioxide (NO₂) gas sensing application. Zn ion precursor concentration varied as 0.02, 0.03, 0.04, 0.05 and 0.06 M and thin films were characterized for structural, morphological, optical, electrical, surface defect study and gas sensing properties. All the film showed dominant orientation along the (002) direction, the intensity of the peak vary with the length of the nanorods. SEM cross images confirmed that nanorods had vertical alignment perpendicular to the plane of the substrate surface. The PL intensity of oxygen vacancy related defects for prepared samples was found to be linearly proportional to gas sensing phenomena. This result in good agreement with the theoretical postulation that, oxygen vacancies plays the important role for adsorption sites to NO₂ molecule. The gas sensing performance was studied as a function of operating temperature, Zn ion precursor concentration variation, and gas concentration. The maximum gas response is 113.32–100 ppm NO₂ gas at 150 °C for 0.05 M sample out of all prepared samples. Additionally, ZnO thin film sensor has potential to detect NO₂ as low as 5 ppm.     

Thermal decomposition synthesis of single-crystalline porous ZnO nanoplates self-assembled by tiny nanocrystals and their pore-dependent magnetic properties

High quality porous ZnO nanoplates with a pure crystal phase of hexagonal wurtzite structure were fabricated by thermal decomposition route for the first time. Electron microscopy investigations show that each porous ZnO nanoplates has a single-crystalline nature, which is self-assembled by tiny nanocrystals. The size and pore density of the ZnO nanoplates can be tuned by only changing the amount of zinc source. Magnetic investigations show that the magnetic phase can be converted from paramagnetism to ferromagnetism at room temperature, by increasing the pore density of ZnO nanoplates. The ZnO nanoplates with highest pore density show a d° room-temperature ferromagnetic characteristic, and the saturation magnetization reaches 26 memu/g. Several experimental evidences, including XPS, PL and ESR spectra, demonstrate that the defects of singly charged oxygen vacancies related to the pore density contribute to the long-range ferromagnetic ordering in the dopant-free porous ZnO nanoplates. This finding suggests that the pore-dependent ferromagnetism can be manipulated by tuning the surface-volume ratio, which is significant for the understanding and exploration of diluted magnetic semiconductors.     

Ar plasma treatment on ZnO–SnO2 heterojunction nanofibers and its enhancement mechanism of hydrogen gas sensing

In this study, ZnO–SnO₂ heterojunction nanofibers were fabricated using electrospinning and treated by Ar plasma. The post treatment demonstrated the ability to regulate adsorbed oxygen of nanofibers and thus affecting the gas sensing performance. The results revealed that the gas sensing performances increased with the increase of plasma treatment time at first, then showed a downward trend. The setting for the best H₂ gas performance was 20 min of plasma treatment, under which the response of the sample was 80% higher and the response time was two thirds shorter than those of the untreated sample. The explanation can be that appropriate plasma treatment can increase the oxygen vacancy on the surface of heterojunction nanofibers as well as the resistance modulation range in the air; however, excessive plasma treatment can result in the reduction of the resistance modulation range in the air by reducing the metal oxide to metal.     

Electrochemical and Galvanic Fabrication of a Magnetoelectric Composite Sensor Based on InP

ABSTRACT: A process chain for a magnetoelectric device based on porous InP will be presented using only electrochemical, photochemical, and purely chemical etching and the galvanic deposition of metals in high aspect-ratio structures. All relevant process steps starting with the formation of a self-ordered array of currentline-oriented pores, followed by the membrane fabrication and a post-etching step as well as the galvanic metal filling of membrane structures are presented and discussed. The resistivity of a porous InP structure could be drastically increased and thus the piezoelectric performance of the porous InP structure. The developed galvanic Ni filling process is capable to homogeneously fill high aspect-ratio membranes.