Defect-free ZnO nanorods for low temperature hydrogen sensor applications
TL;DR: In this paper, a H2 sensing mechanism based on the change in physical dimension of channel is proposed to explain the fast response and recovery times of ZnO NRs/Si/ZnONRs sensors.
Abstract: Uniformly distributed and defect-free vertically aligned ZnO nanorods (NRs) with high aspect ratio are deposited on Si by sputtering technique. X-ray diffraction along with transmission electron microscopy studies confirmed the single crystalline wurtzite structure of ZnO. Absence of wide band emission in photoluminescence spectra showed defect-free growth of ZnO NRs which was further conformed by diamagnetic behavior of the NRs. H2 sensing mechanism based on the change in physical dimension of channel is proposed to explain the fast response (∼21.6 s) and recovery times (∼27 s) of ZnO NRs/Si/ZnO NRs sensors. Proposed H2 sensor operates at low temperature (∼70 °C) unlike the existing high temperature (>150 °C) sensors.
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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: This highly hydrogen selective Pd contacted ZnO nanorods based sensor detecting low concentration even at low operating temperature of 50 °C is reported, which exhibits dual characteristics as metal contact and excellent catalyst to hydrogen molecules.
Abstract: We report highly hydrogen selective Pd contacted ZnO nanorods based sensor detecting low concentration even at low operating temperature of 50 °C. The sensor performance was investigated for various gases such as H2, CH4, H2S and CO2 at different operating temperatures from 50 °C to 175 °C for various gas concentrations ranging from 7 ppm to 10,000 ppm (1%). The sensor is highly efficient as it detects hydrogen even at low concentration of ~7 ppm and at operating temperature of 50 °C. The sensor’s minimum limit of detection and relative response at 175 °C were found 7 ppm with ~38.7% for H2, 110 ppm with ~6.08% for CH4, 500 ppm with ~10.06% for H2S and 1% with ~11.87% for CO2. Here, Pd exhibits dual characteristics as metal contact and excellent catalyst to hydrogen molecules. The activation energy was calculated for all the gases and found lowest ~3.658 kJ/mol for H2. Low activation energy accelerates desorption reactions and enhances the sensor’s performance.
112 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 paper, the E2L peak shift of copper-doped ZnO nanoflower and nanoellipsoids was investigated in low-temperature ammonia gas sensing properties.
Abstract: Copper-doped ZnO (CZO) nanoflower and nanoellipsoids were synthesized by hydrothermal method. Field emission electron microscopy and transmission electron microscopy revealed that the flower-like morphology of undoped ZnO transformed into nanoellipsoids upon incorporation of copper (Cu) in ZnO. Raman spectra of copper- doped ZnO showed E2L peak shift compared with undoped ZnO nanoflower which indicated enhanced oxygen or zinc vacancy in copper-doped ZnO. Low-temperature ammonia gas sensing properties based on copper-doped ZnO were systematically studied. Cu-doped ZnO (6 wt%) showed enhanced selectivity compared with other copper doping (wt%). Furthermore, the Cu-doped ZnO showed an excellent response and recovery time at a low concentration of ammonia (10 ppm). Cu-doped ZnO showed better long-term stability and reproducibility towards ammonia gas.
88 citations
TL;DR: In this article, the authors reported the fabrication process and the relevant sensing mechanisms of a TiO2 film based hydrogen sensor which showed an excellent performance at room temperature in air and exhibited the concentration detection limit as low as 1ppm and the response time as short as 9 s.
Abstract: Hydrogen sensors with high sensitivity, rapid response and low cost are urgently demanded in today’s industry. In this work, we reported our research on the fabrication process and the relevant sensing mechanisms of a TiO2 film based hydrogen sensor which showed an excellent performance at room temperature in air. The sensor, prepared by hydrothermal methods with a seed layer being introduced during the growth of the thin films, exhibited the concentration detection limit as low as 1 ppm and the response time as short as 9 s. Structural analysis revealed that the introduction of the seed layer resulted in a significant improvement in the quality of the TiO2 thin film, which was intimately related to the sensing properties. The dynamical response process was studied and the corresponding sensing mechanisms were analyzed in detail. It was found that, in addition to the generally accepted Schottky barrier mechanism, the adsorption of H2 molecules at (002) surface of the TiO2 thin film played a key role in achieving such a low detection limit and a short response time for the sensor.
76 citations
References
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TL;DR: Hydrogen sensors and hydrogen-activated switches were fabricated from arrays of mesoscopic palladium wires fabricated by electrodeposition onto graphite surfaces and transferred onto a cyanoacrylate film.
Abstract: Hydrogen sensors and hydrogen-activated switches were fabricated from arrays of mesoscopic palladium wires. These palladium "mesowire" arrays were prepared by electrodeposition onto graphite surfaces and were transferred onto a cyanoacrylate film. Exposure to hydrogen gas caused a rapid (less than 75 milliseconds) reversible decrease in the resistance of the array that correlated with the hydrogen concentration over a range from 2 to 10%. The sensor response appears to involve the closing of nanoscopic gaps or "break junctions" in wires caused by the dilation of palladium grains undergoing hydrogen absorption. Wire arrays in which all wires possessed nanoscopic gaps reverted to open circuits in the absence of hydrogen gas.
1,315 citations
TL;DR: In this paper, it was shown that room-temperature ferromagnetism has been observed in nanoparticles of nonmagnetic oxides such as (1), (2), (3), (4), (5), (6), (7,30), and (6,30).
Abstract: Room-temperature ferromagnetism has been observed in nanoparticles $(7--30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.2em}{0ex}}\mathrm{diam})$ of nonmagnetic oxides such as ${\mathrm{CeO}}_{2}$, ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, $\mathrm{ZnO}$, ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{SnO}}_{2}$. The saturated magnetic moments in ${\mathrm{CeO}}_{2}$ and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ nanoparticles are comparable to those observed in transition-metal-doped wideband semiconducting oxides. The other oxide nanoparticles show somewhat lower values of magnetization but with a clear hysteretic behavior. Conversely, the bulk samples obtained by sintering the nanoparticles at high temperatures in air or oxygen became diamagnetic. As there were no magnetic impurities present, we assume that the origin of ferromagnetism may be the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of nanoparticles. We suggest that ferromagnetism may be a universal characteristic of nanoparticles of metal oxides.
1,239 citations
TL;DR: The results provide a new route for modulating the optical properties of two-dimensional semiconductors and the strong and stable PL from defects sites of MoS2 may have promising applications in optoelectronic devices.
Abstract: We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temperature annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temperature PL measurements. First-principle calculations reveal a strong binding energy of ∼2.395 eV for an oxygen molecule adsorbed on a S vacancy of MoS2. The chemically adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physically adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect enginee...
953 citations
TL;DR: A range of remarkable characteristics of ZnO nanostructures are presented, organized into sections describing the mechanical, electrical, optical, magnetic, and chemical sensing properties.
Abstract: This article provides a comprehensive review of the current research activities that focus on the ZnO nanostructure materials and their physical property characterizations. It begins with the synthetic methods that have been exploited to grow ZnO nanostructures. A range of remarkable characteristics are then presented, organized into sections describing the mechanical, electrical, optical, magnetic, and chemical sensing properties. These studies constitute the basis for developing versatile applications of ZnO nanostructures.
758 citations
TL;DR: In this article, a single ZnO nanowire-based nanoscale sensor fabricated using focused ion beam (FIB/SEM) instrument was used to determine the deep levels related to defects and evaluate the effect of thermal treatment in H2 atmosphere on the emission from ZnOs.
Abstract: In this work, we report on a single ZnO nanowire-based nanoscale sensor fabricated using focused ion beam (FIB/SEM) instrument. We studied the diameter dependence of the gas response and selectivity of ZnO nanowires (NWs) synthesized by chemical vapor phase growth method. The photoluminescence (PL) measurements were used to determine the deep levels related to defects which are presented in the ZnO nanomaterial as well as to evaluate the effect of thermal treatment in H2 atmosphere on the emission from ZnO nanowires. We show that sample annealed in hydrogen leads to passivation of recombination centers thus modifying the NWs properties. We studied the gas response and selectivity of these ZnO nanowires to H2 ,N H 3, i-Butane, CH4 gases at room temperature. Our results indicated that zinc oxide NWs hold a high promise for nanoscale sensor applications due to its capability to operate at room-temperature and its ability to tune the gas response and selectivity by the defect concentration and the diameter of ZnO nanowire. A method is proposed to reduce the nanosensor’s recovery time through the irradiation with an ultraviolet radiation pulse. The sensing mechanisms of ZnO nanowires will be discussed.
416 citations