Morphology-Controlled Aluminum-Doped Zinc Oxide Nanofibers for Highly Sensitive NO2 Sensors with Full Recovery at Room Temperature

Room-temperature (RT) gas sensitivity of morphology-controlled free-standing hollow aluminum-doped zinc oxide (AZO) nanofibers for NO2 gas sensors is presented. The free-standing hollow nanofibers are fabricated using a polyvinylpyrrolidone fiber template electrospun on a copper electrode frame followed by radio-frequency sputtering of an AZO thin overlayer and heat treatment at 400 degrees C to burn off the polymer template. The thickness of the AZO layer is controlled by the deposition time. The gas sensor based on the hollow nanofibers demonstrates fully recoverable n-type RT sensing of low concentrations of NO2 (0.5 ppm). A gas sensor fabricated with Al2O3-filled AZO nanofibers exhibits no gas sensitivity below 75 degrees C. The gas sensitivity of a sensor is determined by the density of molecules above the minimum energy for adsorption, collision frequency of gas molecules with the surface, and available adsorption sites. Based on finite-difference time-domain simulations, the RT sensitivity of hollow nanofiber sensors is ascribed to the ten times higher collision frequency of NO2 molecules confined inside the fiber compared to the outer surface, as well as twice the surface area of hollow nanofibers compared to the filled ones. This approach might lead to the realization of RT sensitive gas sensors with 1D nanostructures

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

Nanostructured zinc oxide gas sensors by successive ionic layer adsorption and reaction method and rapid photothermal processing, Thin Solid Films

Abstract Undoped and Sn, Ni-doped nanostructured ZnO thin films were deposited on glass substrates using a successive ionic layer adsorption and reaction (SILAR) method at room temperature. The SILAR deposited zinc oxide films have been rapid photothermal processing (RPP) at various temperatures to study the effect of annealing on the sensing properties. Structural, electrical and sensing properties were investigated by means of X-ray diffraction (XRD), Energy Dispersive X-ray spectroscopy, scanning electron microscopy, electrical resistivity, and sensitivity measurements. Microstructures of the deposited films were studied for different concentrations of dopants and zinc-complex solution and temperatures. The results of influence of growth processes, doping, and RPP on phase structure, surface morphology, particles size and resistivity values are presented and discussed. The average grain size determined from XRD patterns was 240, 220 and 265 Å for ZnO, Sn-ZnO and Ni-ZnO films, respectively. Moreover, electrical characterization of the sensors prepared from SILAR deposited nanostructured zinc oxide thin film has been carried out. The variation in resisitivity of the ZnO film sensors was obtained with doping and post-deposition rapid photothermal processing in vacuum and N 2 ambient. Electrical resistivity measurements showed semiconducting nature with room temperature resistivity 1.5 × 10 5 , 6.1 × 10 2 , 70 Ω cm for as-deposited ZnO, 4 at.% Ni-ZnO and 4 at.% Sn-ZnO, respectively. These values decreased to 1 × 10 4 , 2×10 2 , 30 Ω cm for RPP annealed films. The types of doping and temperatures of RPP were found to have an important role in determining the sensitivity and resolution of the NO 2 , NH 3 ZnO-based sensors. While the nanostructured ZnO sensor showed higher ammonia sensitivity than that of NO 2 , an enhanced NO 2 sensitivity was noticed with the ZnO films doped with 4 at.% Sn and higher NH 3 sensitivity was obtained by 4 at.% Ni doping of zinc oxide thin films

Synthesis And Characterization Of Functional Nanostructured Zinc Oxide Thin Films

A novel aqueous relatively low-temperature thin film growth technique has been developed to fabricate a new generation of smart and functional nanostructured metal oxide thin film materials. This chemical synthetic route uses stable and inexpensive metal inorganic salts and environment-friendly solvents. The nanomaterials are obtained to analyze the physical and structural requirements of their applications in gas sensors and solar cells. The rapid photothermal processing at the 650 °C of nanostructured zinc oxide leads to the suppression of deep-defect-level emission and improvement of near-band edge emission. The responses of the sensing elements when exposed to 100 ppm ammonia at temperatures between 20 °C and 300 °C have been assessed. copyright The Electrochemical Society

Nanostructured Zinc Oxide Gas Sensors By Successive Ionic Layer Adsorption And Reaction Method And Rapid Photothermal Processing

Undoped and Sn, Ni-doped nanostructured ZnO thin films were deposited on glass substrates using a successive ionic layer adsorption and reaction (SILAR) method at room temperature. The SILAR deposited zinc oxide films have been rapid photothermal processing (RPP) at various temperatures to study the effect of annealing on the sensing properties. Structural, electrical and sensing properties were investigated by means of X-ray diffraction (XRD), Energy Dispersive X-ray spectroscopy, scanning electron microscopy, electrical resistivity, and sensitivity measurements. Microstructures of the deposited films were studied for different concentrations of dopants and zinc-complex solution and temperatures. The results of influence of growth processes, doping, and RPP on phase structure, surface morphology, particles size and resistivity values are presented and discussed. The average grain size determined from XRD patterns was 240, 220 and 265 Å for ZnO, Sn-ZnO and Ni-ZnO films, respectively. Moreover, electrical characterization of the sensors prepared from SILAR deposited nanostructured zinc oxide thin film has been carried out. The variation in resisitivity of the ZnO film sensors was obtained with doping and post-deposition rapid photothermal processing in vacuum and N2 ambient. Electrical resistivity measurements showed semiconducting nature with room temperature resistivity 1.5 × 105, 6.1 × 102, 70 Ω cm for as-deposited ZnO, 4 at.% Ni-ZnO and 4 at.% Sn-ZnO, respectively. These values decreased to 1 × 104, 2 × 102, 30 Ω cm for RPP annealed films. The types of doping and temperatures of RPP were found to have an important role in determining the sensitivity and resolution of the NO2, NH3 ZnO-based sensors. While the nanostructured ZnO sensor showed higher ammonia sensitivity than that of NO2, an enhanced NO2 sensitivity was noticed with the ZnO films doped with 4 at.% Sn and higher NH3 sensitivity was obtained by 4 at.% Ni doping of zinc oxide thin films. © 2007 Elsevier B.V. All rights reserved

Investigation Of Zinc Oxide Nanostructures Prepared By Chemical Deposition And Rapid Photo-Thermal Processing Technology

A novel and efficient low-temperature aqueous chemical method has been used to synthesize a new generation of smart and functional zinc oxide nanomaterials. The ZnO nanostructures are obtained and the physical and structural requirements of their applications in chemical sensors and solar cells are analyzed. Post-growth rapid photothermal annealing of nanostructures at 650°C in an atmosphere of N2 leads to the decrease in structure defects as compared to the as-grown nanostructures. The sensing behaviour of the nanostructured elements when exposed to 100 ppm ammonia is investigated. © 2007 IEEE

Nanostructured Zinc Oxide Films Synthesized By Successive Chemical Solution Deposition For Gas Sensor Applications

Nanostructured ZnO thin films have been deposited using a successive chemical solution deposition method. The structural, morphological, electrical and sensing properties of the films were studied for different concentrations of Al-dopant and were analyzed as a function of rapid photothermal processing temperatures. The films were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron and micro-Raman spectroscopy. Electrical and gas sensitivity measurements were conducted as well. The average grain size is 240 and 224 Å for undoped ZnO and Al-doped ZnO films, respectively. We demonstrate that rapid photothermal processing is an efficient method for improving the quality of nanostructured ZnO films. Nanostructured ZnO films doped with Al showed a higher sensitivity to carbon dioxide than undoped ZnO films. The correlations between material compositions, microstructures of the films and the properties of the gas sensors are discussed. © 2008 Elsevier Ltd. All rights reserved