Evaluation of gas-sensing properties of ZnO nanostructures electrochemically doped with Au nanophases

CuO-decorated ZnO nanotube-based sensor for detecting CO gas: a first-principles study

Understanding the effect of copper oxide (CuO)-decorated zinc oxide nanotube on carbon monoxide (CO) adsorption is crucial for designing a high-performance CO gas sensor. In this work, CO sensing properties of copper oxide-decorated zinc oxide (CuO-ZnO) nanotube are studied theoretically by employing first-principles density functional theory for the first time. The stability, adsorption mechanism, density of states, and change in electrical conductivity are studied. The results of calculating the adsorption energy show strong chemical adsorption of CO on CuO-ZnO nanotubes. The adsorption energy of CO on CuO-ZnO nanotube is calculated as 7.5 times higher than that on ZnO nanotube. The results of the Mulliken charge analysis reveal that electron transfer occurs from CO molecules to CuO-ZnO nanotubes. Additionally, the electrical conductivity of CuO-ZnO nanotubes significantly changes after adsorption of CO at room temperature. According to these studies, CuO-ZnO nanotube sensors can be used for the detection of CO gas. The results are in excellent agreement with the reported experimental results.

Enhanced NO2-Sensing Properties of Au-Loaded Porous In2O3 Gas Sensors at Low Operating Temperatures

NO2-sensing properties of semiconductor gas sensors using porous In2O3 powders loaded with and without 0.5 wt% Au (Au/In2O3 and In2O3 sensors, respectively) were examined in wet air (70% relative humidity at 25 °C). In addition, the effects of Au loading on the increased NO2 response were discussed on the basis of NO2 adsorption/desorption properties on the oxide surface. The NO2 response of the Au/In2O3 sensor monotonically increased with a decrease in the operating temperature, and the Au/In2O3 sensor showed higher NO2 responses than those of the In2O3 sensor at a temperature of 100 °C or lower. In addition, the response time of the Au/In2O3 sensor was much shorter than that of the In2O3 sensor at 30 °C. The analysis based on the Freundlich adsorption mechanism suggested that the Au loading increased the adsorption strength of NO2 on the In2O3 surface. Moreover, the Au loading was also quite effective in decreasing the baseline resistance of the In2O3 sensor in wet air (i.e., increasing the number of free electrons in the In2O3), which resulted in an increase in the number of negatively charged NO2 species on the In2O3 surface. The Au/In2O3 sensor showed high response to the low concentration of NO2 (ratio of resistance in target gas to that in air: ca. 133 to 0.1 ppm) and excellent NO2 selectivity against CO and ethanol, especially at 100 °C.

Improvements in the Performance of a Visible-NIR Photodetector Using Horizontally Aligned TiS3 Nanoribbons

We report the fabrication and characterization of visible and near-infrared-resistive photodetector using horizontally aligned titanium tri sulfide (TiS3) nanoribbons. The fabrication process employed micro-electromechanical system, photolithography and dielectrophoretic (DEP) methods. The interdigitated electrodes (IDE) fingers were fabricated using photolithography and thin-film metallization techniques onto the Si/SiO2 substrate, and then TiS3 nanoribbons were horizontally aligned in between IDE using DEP. The fabricated device was first characterized in absence of light and then, the photodetector-based characteristics were obtained by illuminating it with fiber-coupled laser beam. These characteristics were optimized by varying wavelength and power density of the laser beam. The present photodetector shows a maximum responsivity of 5.22 × 102 A/W, quantum efficiency of 6.08 × 102, and detectivity of 1.69 × 109 Jones. The switching times, i.e., response and recovery times were found to be 1.53 and 0.74 s, respectively, with 1064 nm wavelength and 3.4 mW/mm2 power density of the laser beam. Also, the effect of O2 adsorption on nanoribbons has been studied and it is found that adsorbed O2 acts as electron acceptor and decreases the conductivity of the photodetector. Experimentally, it is found that the photoresponse of the horizontally aligned TiS3 nanoribbons is better than that of a randomly oriented TiS3 nanoribbon-based photodetector. Finally, the performance of the present photodetector was compared to that of the previous ones that were found to outperform the reported ones. The additional advantages of the photodetector include excellent stability and portability from which it may be concluded that TiS3 nanoribbons can be a promising candidate for application in nanoscale electronic and optoelectronic devices.

Facile preparation of Silicon/ZnO thin film heterostructures and ultrasensitive toxic gas sensing at room temperature: Substrate dependence on specificity

Two types of silicon-Zinc oxide (ZnO) heterostructures were prepared simply by depositing (drop casting) chemically prepared ZnO nanoparticles onto single crystalline (p-type) silicon substrates (Si) as well as electrochemically prepared p-type porous silicon (PS). ZnO nanoparticles and PS/ZnO structures were characterized structurally by various techniques. By depositing in-plane gold contacts on the heterostructures, gas sensors were fabricated and characterized electrochemically by dc and ac impedance measurements. The PS/ZnO sensors showed specific response at room temperature for NO₂ with increase in current and no significant response for other reducing and oxidizing gases. The sensor is sensitive to 200 ppb NO₂ at 25 °C with 35% change in current and 50 s response time. Temperature dependent studies of sensor in the range of 25-100 °C have shown maximum sensitivity at 40 °C (50% change for 200 ppb) with decreasing sensitivity thereafter (23% change at 60 °C), indicating the suitability of the sensor till 60 °C. Alternatively Si/ZnO heterostructures showed maximum response with NO₂, along with lesser specific responses for SO₂ and NH₃. Detailed multifrequency impedance studies with temperature suggested the role of space charge layers at various interfaces in the charge transport properties of PS/ZnO and Si/ZnO heterostructures resulting in their specific gas sensing properties.

Surface-plasmon-enhanced ultraviolet emission of Au-decorated ZnO structures for gas sensing and photocatalytic devices

Pure and Au-decorated sub-micrometer ZnO spheres were successfully grown on glass substrates by simple chemical bath deposition and photoreduction methods. The analysis of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, energy-dispersive X-ray spectroscopy (EDS), UV-vis absorption, and photoluminescence (PL) spectra results were used to verify the incorporation of plasmonic Au nanoparticles (NPs) on the ZnO film. Time-resolved photoluminescence (TRPL) spectra indicated that a surface plasmonic effect exists with a fast rate of charge transfer from Au nanoparticles to the sub-micrometer ZnO sphere, which suggested the strong possibility of the use of the material for the design of efficient catalytic devices. The NO₂ sensing ability of as-deposited ZnO films was investigated with different gas concentrations at an optimized sensing temperature of 120 °C. Surface decoration of plasmonic Au nanoparticles provided an enhanced sensitivity (141 times) with improved response (τ_Res = 9 s) and recovery time (τ_Rec = 39 s). The enhanced gas sensing performance and photocatalytic degradation processes are suggested to be attributed to not only the surface plasmon resonance effect, but also due to a Schottky barrier between plasmonic Au and ZnO structures.

Sensors based on mesoporous SnO2-CuWO4 with high selective sensitivity to H2S at low operating temperature

Development of new sensitive materials by different synthesis routes in order to emphasize the sensing properties for hazardous H₂S detection is one of a nowadays challenge in the field of gas sensors. In this study we obtained mesoporous SnO₂-CuWO₄ with selective sensitivity to H₂S by an inexpensive synthesis route with low environmental pollution level, using tripropylamine (TPA) as template and polyvinylpyrrolidone (PVP) as dispersant/stabilizer. In order to bring insights about the intrinsic properties, the powders were characterized by means of a variety of complementary techniques such as: X-Ray Diffraction, XRD; Transmission Electron Microscopy, TEM; High Resolution TEM, HRTEM; Raman Spectroscopy, RS; Porosity Analysis by N₂ adsorption/desorption, BET; Scanning Electron Microscopy, SEM and X-ray Photoelectron Spectroscopy, XPS. The sensors were fabricated by powders deposition via screen-printing technique onto planar commercial Al₂O₃ substrates. The sensor signals towards H₂S exposure at low operating temperature (100°C) reaches values from 10⁵ (for SnWCu600) to 10⁶ (for SnWCu800) over the full range of concentrations (5-30ppm). The recovery processes were induced by a short temperature trigger of 500°C. The selective sensitivity was underlined with respect to the H₂S, relative to other potential pollutants and relative humidity (10-70% RH).

Sensitive detection of hydrocarbon gases using electrochemically Pd-modified ZnO chemiresistors

Pristine and electrochemically Pd-modified ZnO nanorods (ZnO NRs) were proposed as active sensing layers in chemiresistive gas sensors for hydrocarbon (HC) gas detection (e.g., CH₄, C₃H_8, C₄H₁₀). The presence of Pd nanoparticles (NPs) on the surface of ZnO NRs, obtained after the thermal treatment at 550 °C, was revealed by morphological and surface chemical analyses, using scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The effect of the Pd catalyst on the performance of the ZnO-based gas sensor was evaluated by comparing the sensing results with those of pristine ZnO NRs, at an operating temperature of 300 °C and for various HC gas concentrations in the range of 30-1000 ppm. The Pd-modified ZnO NRs showed a higher selectivity and sensitivity compared to pristine ZnO NRs. The mean sensitivity of Pd-modified ZnO NRs towards the analyzed HCs gases increased with the length of the hydrocarbon chain of the target gas molecule. Finally, the evaluation of the selectivity revealed that the presence or the absence of metal nanoparticles on ZnO NRs improves the selectivity in the detection of specific HCs gaseous molecules.

DFT insights into the electronic properties and adsorption of NO2 on metal-doped carbon nanotubes for gas sensing applications

Theoretical and computational chemistry contributes to the future chemistry for building gas sensors.

Highly selective NH3 gas sensor based on Au loaded ZnO nanostructures prepared using microwave-assisted method

ZnO nanorods synthesized using microwave-assisted approach were functionalized with gold (Au) nanoparticles. The Au coverage on the surface of the functionalized ZnO was controlled by adjusting the concentration of the Au precursor. According to X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) results, it was confirmed that Au form nanoparticles loaded on the surface of ZnO. The small Au loading level of 0.5wt% showed the highest response of 1600-100ppm of NH3 gas at room temperature (RT) whereas further increase of Au loading level resulted in poor detection of NH3. All Au loaded ZnO (Au/ZnO) based sensors exhibited very short recovery and response times compared to unloaded ZnO sensing materials. The responses of ZnO and Au/ZnO based sensors (0.5-2.5wt%) to other flammable gases, including H2, CO and CH4, were considerably less, demonstrating that Au/ZnO based sensors were highly selective to NH3 gas at room temperature. Spill over mechanism which is the main reason for the observed enhanced NH3 response with 0.5 Au loading level is explained in detail.

Microwave solvothermal synthesis and characterization of manganese-doped ZnO nanoparticles

Mn-doped zinc oxide nanoparticles were prepared by using the microwave solvothermal synthesis (MSS) technique. The nanoparticles were produced from a solution of zinc acetate dihydrate and manganese(II) acetate tetrahydrate using ethylene glycol as solvent. The content of Mn(2+) in Zn1- x Mn x O ranged from 1 to 25 mol %. The following properties of the nanostructures were investigated: skeleton density, specific surface area (SSA), phase purity (XRD), lattice parameters, dopant content, average particle size, crystallite size distribution, morphology. The average particle size of Zn1- x Mn x O was determined using Scherrer's formula, the Nanopowder XRD Processor Demo web application and by converting the specific surface area results. X-ray diffraction of synthesized samples shows a single-phase wurtzite crystal structure of ZnO without any indication of additional phases. Spherical Zn1- x Mn x O particles were obtained with monocrystalline structure and average particle sizes from 17 to 30 nm depending on the content of dopant. SEM images showed an impact of the dopant concentration on the morphology of the nanoparticles.