Highly enhanced H2 gas sensing characteristics of Co:ZnO nanorods and its mechanism
TL;DR: In this paper, a mechanism for the enhanced H 2 response of Co:ZnO is proposed that involves donor-related oxygen vacancies (V O ) introduced by the Co dopants in the ZnO lattice.
Abstract: We report here excellent H 2 gas response property of Co-doped ZnO nanorods (Co:ZnO NRs). Co:ZnO NRs have been synthesized by hydrothermal method by varying the Co content from 0 to 10 mol%. It is found that Co:ZnO NRs with 8 mol% of Co exhibits the highest and faster H 2 gas response ability compared to the undoped ZnO resulting in a ∼5 fold enhancement in the gas response in the presence of air at 150 °C. While the gas response value (S (%) = [(I g -I a )/I a ] × 100, where I a is the current of the sensors in the presence of air only and I g is the current in the presence of certain H 2 concentration) to 3000 ppm H 2 for undoped ZnO NRs’sensor is 11%, an unprecedented high value of 53.7% is obtained for the Co:ZnO with 8 mol% of Co. Based on the electrical current and photoluminescence results, a mechanism for the enhanced H 2 response of Co:ZnO is proposed that involves donor-related oxygen vacancies (V O ) introduced by the Co dopants in the ZnO lattice.
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TL;DR: In this paper, the authors highlight the designs and mechanisms of different SMONs with various patterns (e.g., nanoparticles, nanowires, nanosheets, nanorods, nanotubes, nanofilms, etc.) for gas sensors to detect various hazardous gases at room temperature.
Abstract: High-precision gas sensors operated at room temperature are attractive for various real-time gas monitoring applications, with advantages including low energy consumption, cost effectiveness and device miniaturization/flexibility. Studies on sensing materials, which play a key role in good gas sensing performance, are currently focused extensively on semiconducting metal oxide nanostructures (SMONs) used in the conventional resistance type gas sensors. This topical review highlights the designs and mechanisms of different SMONs with various patterns (e.g. nanoparticles, nanowires, nanosheets, nanorods, nanotubes, nanofilms, etc.) for gas sensors to detect various hazardous gases at room temperature. The key topics include (1) single phase SMONs including both n-type and p-type ones; (2) noble metal nanoparticle and metal ion modified SMONs; (3) composite oxides of SMONs; (4) composites of SMONs with carbon nanomaterials. Enhancement of the sensing performance of SMONs at room temperature can also be realized using a photo-activation effect such as ultraviolet light. SMON based mechanically flexible and wearable room temperature gas sensors are also discussed. Various mechanisms have been discussed for the enhanced sensing performance, which include redox reactions, heterojunction generation, formation of metal sulfides and the spillover effect. Finally, major challenges and prospects for the SMON based room temperature gas sensors are highlighted.
434 citations
TL;DR: In this paper, several techniques related to the synthesis of ZnO nanostructures and their efficient performance in sensing are reviewed, such as functionalization of noble metal nanoparticles, doping of metals, inclusion of carbonaceous nanomaterials, using nanocomposites of different MO x, UV activation, and post-treatment method of high-energy irradiation on ZnOs, with their possible sensing mechanisms.
323 citations
TL;DR: In this article, the impact of Er3+ doping on the response and selectivity of SnO2-based gas sensor has been investigated in detail, and it has been observed that specific surface area of nanoparticles has increased with increase in dopant concentration.
Abstract: In the present work, impact of Er3+ doping on the response and selectivity of SnO2 based gas sensor has been investigated in detail. X-ray diffraction (XRD) results confirmed formation of a tetragonal rutile structure of undoped and erbium doped SnO2 nanoparticles. It has been observed that specific surface area of nanoparticles has increased with increase in dopant concentration. The oxidation states and presence of erbium in SnO2 lattice has been confirmed by X-ray photoelectron spectroscopy (XPS). Photoluminescence (PL) analysis revealed that concentration of oxygen vacancies increases with increase in dopant incorporation. It has been observed that 3% Er-doped SnO2 sensor exhibited enhanced sensor response and temperature dependent selectivity towards ethanol and hydrogen at 240 and 360 ℃ respectively. The enhanced sensor response of the fabricated sensor has been ascribed to large surface area, enormous oxygen vacancies and elevated surface basicity of doped nanoparticles used. The tunable dual selectivity of 3% doped sensor towards ethanol and hydrogen makes it a perfect candidate for ethanol-hydrogen sensing for ethanol steam reforming systems combined to fuel cells.
125 citations
TL;DR: In this paper, RF sputtered Ni-doped ZnO nanostructures for detection of extremely low concentration (1ppm) of hydrogen gas at moderate operating temperature of 75°C.
Abstract: We demonstrate RF sputtered Ni-doped ZnO nanostructures for detection of extremely low concentration (1 ppm) of hydrogen gas at moderate operating temperature of 75 °C. Structural, morphological, electrical and hydrogen sensing behavior of the Ni-doped ZnO nanostructures strongly depends on doping concentration. Ni doping exceptionally enhances the sensing response and reduces the operating temperature of the sensor as compared to undoped ZnO. The major role of the Ni-doping is to create more active sites for chemisorbed oxygen on the surface of sensor and, correspondingly, to improve the sensing response. The 4 at% of Ni-doped ZnO exhibits the highest response (∼69%) for 1% H 2 at 150 °C, which are ∼1.5 times higher than for the undoped ZnO. This is ascribed to lowest activation energy ∼6.47 KJ/mol. Diminishing of the relative response was observed in 6% Ni- doped ZnO due to separation of NiO phase.
92 citations
TL;DR: In this article, a facile hydrothermal method combined with calcination was used to synthesize unique Zn2SnO4-ZnO hierarchical structures for TEA gas sensors.
Abstract: In this work, unique Zn2SnO4–ZnO hierarchical structures composed of one-dimensional Zn2SnO4 nanowires and two-dimensional ZnO nanosheets were successfully synthesized via a facile hydrothermal method combined with calcination. The Zn2SnO4 nanowires bridged across the ZnO nanosheets, which played an important role in electron transmission. Compared with pure Zn2SnO4 nanowires and pure ZnO nanosheets, the obtained Zn2SnO4–ZnO hierarchical structures exhibited improved gas-sensing performance toward triethylamine (TEA) in terms of a low operating temperature, high sensor response, and good selectivity. Gas-sensing test results revealed that the sensor response of Zn2SnO4–ZnO sensor reached 175.5 toward 100 ppm TEA at an optimum operating temperature of 200 °C, which is approximately 47.4 and 30.8 times higher than that of pure Zn2SnO4 and pure ZnO, respectively. In addition, the Zn2SnO4–ZnO hierarchical structures exhibited good selectivity and long-term stability to TEA, suggesting their potential application in advanced TEA gas sensors. The improved sensing properties of the Zn2SnO4–ZnO hierarchical structure could mainly be attributed to their large specific surface areas, unique bridged hierarchical microstructure, and appropriate energy band structure.
87 citations
References
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TL;DR: There are an immense number of sensors reported in the literature for hydrogen detection and in this article these sensors are classified into eight different operating principles, such as measuring range, sensitivity, selectivity and response time.
Abstract: Hydrogen sensors are of increasing importance in connection with the development and expanded use of hydrogen gas as an energy carrier and as a chemical reactant. There are an immense number of sensors reported in the literature for hydrogen detection and in this work these sensors are classified into eight different operating principles. Characteristic performance parameters of these sensor types, such as measuring range, sensitivity, selectivity and response time are reviewed and the latest technology developments are reported. Testing and validation of sensor performance are described in relation to standardisation and use in potentially explosive atmospheres so as to identify the requirements on hydrogen sensors for practical applications.
1,217 citations
TL;DR: ZnO nanorod arrays were fabricated using a hydrothermal method and found that, while the defect emission can be significantly reduced by annealing at 200 degrees C, the rods still have large defect concentrations as confirmed by their low positron diffusion length and short PL decay time constants.
Abstract: ZnO nanorod arrays were fabricated using a hydrothermal method. The nanorods were studied by scanning electron microscopy, photoluminescence (PL), time-resolved PL, X-ray photoelectron spectroscopy, and positron annihilation spectroscopy before and after annealing in different environments and at different temperatures. Annealing atmosphere and temperature had significant effects on the PL spectrum, while in all cases the positron diffusion length and PL decay times were increased. We found that, while the defect emission can be significantly reduced by annealing at 200 °C, the rods still have large defect concentrations as confirmed by their low positron diffusion length and short PL decay time constants.
722 citations
TL;DR: In this paper, a C-axis vertically aligned ZnO nanorod arrays were synthesized using a simple hydrothermal route, with a diameter of 30-100nm and a length of about several hundred nanometres.
Abstract: C-axis vertically aligned ZnO nanorod arrays were synthesized on a ZnO thin film through a simple hydrothermal route. The nanorods have a diameter of 30–100 nm and a length of about several hundred nanometres. The gas sensor fabricated from ZnO nanorod arrays showed a high sensitivity to H2 from room temperature to a maximum sensitivity at 250 °C and a detection limit of 20 ppm. In addition, the ZnO gas sensor also exhibited excellent responses to NH3 and CO exposure. Our results demonstrate that the hydrothermally grown vertically aligned ZnO nanorod arrays are very promising for the fabrication of cost effective and high performance gas sensors.
662 citations
TL;DR: In this paper, defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths, while green emission was not significantly affected by annealing.
Abstract: Defects in three different types of ZnO nanostructures before and after annealing under different conditions were studied. The annealing atmosphere and temperature were found to strongly affect the yellow and orange-red defect emissions, while green emission was not significantly affected by annealing. The defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths. The yellow emission in samples prepared by a hydrothermal method is likely due to the presence of OH groups, instead of the commonly assumed interstitial oxygen defect. The green and orange-red emissions are likely due to donor acceptor transitions involving defect complexes, which likely include zinc vacancy complexes in the case of orange-red emissions.
633 citations
TL;DR: In this article, a sputter-depositing clusters of Pd on the surface of a ZnO nanorod was used to detect hydrogen in the presence of air or pure O2.
Abstract: The sensitivity for detecting hydrogen with multiple ZnO nanorods is found to be greatly enhanced by sputter-depositing clusters of Pd on the surface. The resulting structures show a change in room- temperature resistance upon exposure to hydrogen concentrations in N2 of 10–500ppm of approximately a factor of 5 larger than without Pd. Pd-coated ZnO nanorods detected hydrogen down to 2.6% at 10ppm and >4.2% at 500ppm H2 in N2 after a 10min exposure. There was no response at room temperature to O2. Approximately 95% of the initial ZnO conductance after exposure to hydrogen was recovered within 20s by exposing the nanorods to either air or pure O2. This rapid and easy recoverability make the Pd-coated nanorods suitable for practical applications in hydrogen-selective sensing at ppm levels at room temperature with <0.4mW power consumption.
541 citations