ZnO Quasi-1D Nanostructures: Synthesis, Modeling, and Properties for Applications in Conductometric Chemical Sensors

One-dimensional metal oxide nanostructures such as nanowires, nanorods, nanotubes, and nanobelts gained great attention for applications in sensing devices. ZnO is one of the most studied oxides for sensing applications due to its unique physical and chemical properties. In this paper, we provide a review of the recent research activities focused on the synthesis and sensing properties of pure, doped, and functionalized ZnO quasi-one dimensional nanostructures. We describe the development prospects in the preparation methods and modifications of the surface structure of ZnO, and discuss its sensing mechanism. Next, we analyze the sensing properties of ZnO quasi-one dimensional nanostructures, and summarize perspectives concerning future research on their synthesis and applications in conductometric sensing devices

Understanding Heterostructure Chemiresistive Gas Sensing at Room Temperature

Chemiresistive sensors are the most widely investigated gas sensors due to their ease in fabrication, cost-effectiveness, simplicity of operation, and offer advances in miniaturization. Up to date, typical and well-researched resistive-type sensing materials include semiconductor metal oxides, noble metals, carbon-based nanomaterials (e.g., graphene and carbon nanotubes), and conducting polymers. Gas sensors based on a single material were found difficult to meet the practical requirements for multi-sensing properties, including sensitivity, selectivity, speed of response/recovery, stability, limit of detection, and room temperature operation. Rational design through a combination of chemically or electronically dissimilar nanomaterials is an effective route to enhancing gas sensing performance. Because the chemical composition varies with position, especially at the interface between two dissimilar materials, the newly hybridized structure is defined as a heterostructure. During the past decades, there has been significant research effort in exploring the nanocomposite heterostructures for chemiresistive room-temperature gas sensors. However, sensing mechanisms for such heterostructures are still elusive without solid analysis or direct characterization results. The objective of this dissertation study is to understand the sensing mechanisms of heterostructure-based chemiresistive gas sensors through in situ investigation and analysis under real operating conditions. Various novel heterostructures have been developed for specific types of gas sensing, with a variety of in situ/operando techniques applied to investigate the sensing mechanisms toward different gases. Firstly, nickel oxide-tungsten oxide (NiO-WO3) nanowire-based heterostructures with various component ratios were fabricated via a facile, sonication-based solution mixing method. The exhibited heterojunction effect is maximally observed for W3N1 (75 mol% WO3-25 mol% NiO) and confirmed by observation of the increase in resistance due to the formation of a diode-like p-n junction at the NiO-WO3 interface. The excellent hydrogen sulfide (H2S) sensing performance for W3N1 is attributed to the p-n junction effect, sulfurization by H2S (formation of tungsten sulfides (WS2-x), and nickel sulfides (NiS1-x)), and the ideal ratio of the NiO component in the composite. The formation of reactive semi-metallic products due to sulfurization on the sensor surface was confirmed by in situ X-ray diffraction (XRD) analyses. Operando impedance measurements and resistor-capacitor (RC) equivalent circuit analyses during gas sensing experiments were performed to evaluate the effect of grain-grain boundary or the p-n junction on the sensing performance. It was found that for pure WO3 and W3N1 samples, these contributing effects are in the same direction, resulting in a cooperative and highly sensitive performance, whereas, for other compositions, the samples exhibited competing influences, resulting in low sensitivity. Secondly, the gold doped tin oxide/reduced graphene oxide (Au-SnO2/rGO) ternary nanohybrid heterostructure was designed with improved room temperature hydrogen (H2) sensing performance. The sputtered Au nanoparticles enhanced both sensitivity and recovery of the SnO2-rGO platform. Such an enhancement was attributed to the increased surface area and the oxygen ions spillover effect of loaded Au nanoparticles. The catalytic effect of Au nanoparticles for hydrogen adsorption and desorption was then revealed through the temperature-dependent gas sensing test and the Arrhenius analysis. A better balance between sensitivity and recovery can be further achieved in the future by tuning the deposition conditions of Au nanoparticles. A prototype handheld device based on the Au-SnO2/rGO composites was finally developed for hydrogen detection. The prototype device demonstrates the potential for real-time hydrogen monitoring. The availability of such sensors will contribute to promoting a sustainable hydrogen economy, protecting public safety, and enhancing lead-acid battery safety in a wide range of applications. Thirdly, the nickel-doped tin oxide-reduced graphene oxide (Ni/SnO2-rGO) ternary nanohybrid heterostructure was prepared with enhanced room temperature sulfur dioxide (SO2) sensing performance. The Ni additives significantly improved the lower detection limit (ppb level) of the SnO2-rGO platform. The SO2 concentration calibration curve is well fitted by the Langmuir isotherm. The humidity effect on the sensing performance was also investigated. The results suggested that current nanohybrid materials still suffer from the humidity effect. Metal oxide nanocomposite doping enhanced the SO2 sensing and activated the adsorption of water molecules, which diminished the sensor response to sulfur dioxide gas. Finally, the Poly[3-(3carboxypropyl)thiophene-2,5-diyl]regioregular (PT-COOH)-GO binary nanocomposite heterostructure was prepared. The gas sensing properties were investigated toward NO2, NH3, SO2, and CO. The PT-COOH based sensors exhibited tunable sensing performance through the drain voltage modulation. PT-COOH-GO sensors indicated enhanced NO2 sensing performance with good sensitivity, recovery, and stable responses. The statistical signal analysis was conducted to obtain proof-of-concept results for gas discrimination through signal processing. This study reveals the electronic conduction gas sensing model of multi-metal oxide -nanowires-based chemiresistive gas sensors through the combination of direct current (DC) and alternating current (AC) impedance measurements. The research also suggests that two-dimensional (2D) rGO with proper modifications can be efficient gas sensing materials toward various gaseous analytes. Combining in situ characterization and critical sensing factor analyses, results from the study will offer valuable and comprehensive insights for the rational design of superior heterostructure-based chemiresistive gas sensors

Breakthroughs in the Design of Novel Carbon-Based Metal Oxides Nanocomposites for VOCs Gas Sensing

Nowadays, the detection of volatile organic compounds (VOCs) at trace levels (down to ppb) is feasible by exploiting ultra-sensitive and highly selective chemoresistors, especially in the field of medical diagnosis. By coupling metal oxide semiconductors (MOS e.g., SnO2, ZnO, WO3, CuO, TiO2 and Fe2O3) with innovative carbon-based materials (graphene, graphene oxide, reduced graphene oxide, single-wall and multi-wall carbon nanotubes), outstanding performances in terms of sensitivity, selectivity, limits of detection, response and recovery times towards specific gaseous targets (such as ethanol, acetone, formaldehyde and aromatic compounds) can be easily achieved. Notably, carbonaceous species, highly interconnected to MOS nanoparticles, enhance the sensor responses by (i) increasing the surface area and the pore content, (ii) favoring the electron migration, the transfer efficiency (spillover effect) and gas diffusion rate, (iii) promoting the active sites concomitantly limiting the nanopowders agglomeration; and (iv) forming nano-heterojunctions. Herein, the aim of the present review is to highlight the above-mentioned hybrid features in order to engineer novel flexible, miniaturized and low working temperature sensors, able to detect specific VOC biomarkers of a human's disease

Nanostructured Metal Oxide Semiconductors towards Greenhouse Gas Detection

Climate change and global warming are two huge current threats due to continuous anthropogenic emissions of greenhouse gases (GHGs) in the Earth’s atmosphere. Accurate measurements and reliable quantifications of GHG emissions in air are thus of primary importance to the study of climate change and for taking mitigation actions. Therefore, the detection of GHGs should be the first step when trying to reduce their concentration in the environment. Throughout recent decades, nanostructured metal oxide semiconductors have been found to be reliable and accurate for the detection of many different toxic gases in air. Thus, the aim of this article is to present a comprehensive review of the development of various metal oxide semiconductors, as well as to discuss their strong and weak points for GHG detection

Fast response and low temperature sensing of acetone and ethanol using Al-doped ZnO microrods

We report low temperature acetone and ethanol sensing properties of Al-doped ZnO microrods synthesized using hydrothermal technique. We observe the acetone detection at room temperature as well as ethanol and acetone detection at low temperature of 150 °C using Al-doped ZnO microrods. 3 wt% Al-doped ZnO microrods sensor exhibits the highest response of 231 toward 8100 parts per million (ppm) of ethanol at 150 °C. The response & recovery time are found to be ultrafast of 60 ms & 870 ms for ethanol and 110 ms & 330 ms for acetone of the Al-doped ZnO microrods at an operating temperature of 150 °C, respectively. In addition, sensing mechanism has explained to illuminate the improved sensing performances of Al-doped ZnO microrods. Thus it is revealed that Al-doped ZnO microrods are promising as an ultrafast gas sensor

Development of thin film photovoltaic cells based on low cost metal oxides

The major market barriers to the use of photovoltaic solar cells are high cost and long payback time of conventional technologies, based largely on the silicon material. In order to overcome the environmental problem resulting from the consumption of fossil fuels, all western countries are required to impose heavy subsidies to encourage the use of solar cells in the reduction of carbon consumption; thereby making them highly unsustainable. Therefore, it is necessary to develop solar cells based on low-cost metal oxides with large natural resources. The objective of this program is to investigate the effects of doping on the structural, optical and electrical properties of low-cost metal oxides, such as doped ZnO and copper oxides (CuO and Cu4O3). These are synthesised via sputter deposition and thermal oxidation method in air. Al doped ZnO is an n-type direct semiconductor with a band gap of around 3.5eV. Its crystalline structure is wurtzite, which is deposited widely by the RF reactive magnetron sputtering technology. In my work, the Al doped ZnO thin films were deposited by sputter with metal and ceramic targets. On the one hand, the influence of RF power on the structural, electrical and optical properties of Al doped ZnO thin films were investigated when they were deposited with metal targets. Conversely, the influence of O2 flow rate on the structural, electrical and optical properties of Al doped ZnO thin films was examined when they were deposited with ceramic targets. CuO is a p-type indirect semiconductor with a narrow band gap of 1.0-1.4eV. Its crystalline structure is monoclinic crystal system. CuO nanowires (NWs) were fabricated by the thermal oxidation method in air. It was found that CuO NWs not only grows on Cu sheets, but also on the Si, FTO, Al doped ZnO and glass substrates. For the growth of CuO NWs, the expanding parameters should meet the following requirements: growing temperature: >390°C and growing duration: ≥6hrs. The peeling-off of the CuO NWs on Cu sheets resulted from the formation of Cu8O and Cu64O between the Cu sheets and Cu2O layer. The electrical properties of a single CuO NW were measured using a nano probe station. The contact behaviour between a CuO NW and metal electrodes (Au and W) was schottky. The electrical resistivity of a CuO NW depended on the diameter of the NW. The contact behaviour between CuO NWs on Cu sheets with silver paste top electrodes was schottky as well. A simple PV cell based on CuO NWs-PCBM p-n heterojunction was fabricated, and the short circuit current, open voltage and fill factor of the PV cell was also measured. It indicated that CuO NWs can be utilized to fabricate diodes and PV cells. Copper oxides thin films were deposited by RF reactive magnetron sputtering technology. The phase structure of copper oxides thin films depended on the sputtering parameters. When the thin film was deposited without a bias power, only CuO was detected in the copper oxide thin films. The electrical properties of CuO thin films depended on the O2 fraction during the sputter process. The current-voltage (I-V) characteristics of CuO thin films with Cu electrodes demonstrated that it was influenced by the O2 fraction during the sputter process. Moreover, Cu4O3 is a p-type indirect semiconductor with narrow band gap of 1.0-1.4eV and its crystalline structure is tetragonal crystal system. When the copper oxide thin films were deposited with a bias power, only Cu4O3 phase was detected. Its structural, optical and electrical properties were studied. The optical band gap of Cu4O3 thin film was 1.37eV. Hall properties of Cu4O3 thin films were 1020cm-3, 10-2cm2·V-1·s-1 and 10-1Ω·cm. The Cu4O3-Al Abstract III doped ZnO p-n heterojunction demonstrated excellent rectifying performance, indicating that Cu4O3 is a good candidate for fabricating diodes and PV cells. In addition, Cu4O3 thin films were annealed at different temperatures in air. Furthermore, I studied the influence of annealing temperature on the structural, optical and electrical properties of Cu4O3 thin films

Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review

Metal oxide gas sensors are predominant solid-state gas detecting devices for domestic, commercial and industrial applications, which have many advantages such as low cost, easy production, and compact size. However, the performance of such sensors is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties. Lots of metal oxide nanostructures have been developed to improve the gas sensing properties such as sensitivity, selectivity, response speed, and so on. Here, we provide a brief overview of metal oxide nanostructures and their gas sensing properties from the aspects of particle size, morphology and doping. When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called “small size effect”, yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them. In view of those reasons, nanostructures with many kinds of shapes such as porous nanotubes, porous nanospheres and so on have been investigated, that not only possessed large surface area and relatively mass reactive sites, but also formed relatively loose film structures which is an advantage for gas diffusion. Besides, doping is also an effective method to decrease particle size and improve gas sensing properties. Therefore, the gas sensing properties of metal oxide nanostructures assembled by nanoparticles are reviewed in this article. The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given

Multifunctional Materials: A Case Study of the Effects of Metal Doping on ZnO Tetrapods with Bismuth and Tin Oxides

Hybrid metal oxide nano‐ and microstructures exhibit novel properties, which make them promising candidates for a wide range of applications, including gas sensing. In this work, the characteristics of the hybrid ZnO‐Bi2O3 and ZnO‐Zn2SnO4 tetrapod (T) networks are investigated in detail. The gas sensing studies reveal improved performance of the hybrid networks compared to pure ZnO‐T networks. For the ZnO‐T‐Bi2O3 networks, an enhancement in H2 gas response is obtained, although the observed p‐type sensing behavior is attributed to the formed junctions between the arms of ZnO‐T covered with Bi2O3 and the modulation of the regions where holes accumulate under exposure to H2 gas. In ZnO‐T‐Zn2SnO4 networks, a change in selectivity to CO gas with high response is noted. The devices based on individual ZnO‐T‐Bi2O3 and ZnO‐T‐Zn2SnO4 structures showed an enhanced H2 gas response, which is explained on the basis of interactions (electronic sensitization) between the ZnO‐T arm and Bi2O3 shell layer and single Schottky contact structure, respectively. Density functional theory‐based calculations provide mechanistic insights into the interaction of H2 and CO gas molecules with Bi‐ and Sn‐doped ZnO(0001) surfaces, revealing changes in the Fermi energies, as well as charge transfer between the molecules and surface species, which facilitate gas sensing

Metal Oxide Nanomaterials

This is a timely collection of recent diverse work on metal oxide nanomaterials, connecting their fundamental aspects and application perspectives in a concise fashion to give a broad view of the current status of this fascinating field. This book presents eight original research articles and two comprehensive reviews to highlight the recent development and understanding of different types of metal oxide nanoparticles and their use for applications in luminescence, photocatalysis, water–oil separation, optoelectronics, gas sensors, energy-saving smart windows, etc. It presents just the tip of the iceberg of the broad, dynamic, and active fundamental research and applications in the developing field of metal oxide nanomaterials by collecting a few examples of the latest advancements