A novel chemiresistive-type sensor for detecting sub-ppm NO2 has been fabricated using AuPt bimetal-decorated SnSe2 microflowers, which was synthesized by the hydrothermal treatment followed by in ...situ chemical reduction of the bimetal precursors on the surface of the petals of the microflowers. The as-prepared sensor registers a superior performance in detection of sub-ppm concentration of NO2. Functionalized by the AuPt bimetal, the SnSe2 microflower-based sensor shows a response of approximately 4.62 to 8 ppm NO2 at 130 °C. It is significantly higher than those of the sensors using the pristine SnSe2 (∼2.29) and the modified SnSe2 samples by a single metal, either Au (∼3.03) or Pt (∼3.97). The sensor demonstrates excellent long-term stability, signal repeatability, and selectivity to some typical interfering gaseous species including ammonia, acetone, formaldehyde, ethanol, methanol, benzene, CO2, SO2, and CO. The remarkable improvement of the sensitive characteristics could be induced by the electronic and chemical sensitization and the synergistic effect of the AuPt bimetal. Density functional theory (DFT) is implemented to calculate the adsorption states of NO2 on the sensing materials and thus to possibly reveal the sensing mechanism. The significantly enhanced response of the SnSe2-based sensor decorated with AuPt bimetallic nanoparticles has been found to be possibly caused by the orbital hybridization of O, Au, and Pt atoms leading to the redistribution of electrons, which is beneficial for NO2 molecules to obtain more electrons from the composite material.
•This article reports for the first time the percolation effect of rGO on the ammonia sensing properties of rGO-SnO2 composite.•The sensor using rGO-SnO2 composites exhibited a switch from an n-type ...semiconductor response behavior to a p-type semiconductor behavior as the rGO content increased from 0.1wt% to 1wt%.•A physical model for prediction of the critical weight ratio of rGO in the composite was developed. The calculated result was reasonably consistent with the experimental one.
Reduced graphene oxide (rGO) was added to SnO2 to implement a room temperature chemoresistive ammonia sensor. The percolation effect of rGO on the ammonia sensing properties of SnO2 based sensor was observed. rGO was added physically to SnO2 followed with a magnetic stirring. The sensor using rGO-SnO2 composites exhibited a switch from an n-type semiconductor response behavior to a p-type semiconductor behavior as the rGO content increased from 0.1wt% to 1wt%. The p-type response to ammonia indicated an enhanced sensitivity, better signal stability and faster response/recovery speeds compared to the n-type response. The p-type response can be due to the p-type rGO in the composite and the enhanced room temperature n-type response of SnO2 could be assisted by the added rGO which facilitated the redox reactions of ammonia with oxygen in air. A physical model for prediction of the critical weight ratio of rGO in the composite was developed. The calculated results were reasonably consistent with the experimental ones.
UV activated metal oxides for chemiresistive-type gas sensors have been recently studied aiming to lower the working temperature and thus lower the power consumption. In this work, the mesoporous ...hollow ZnO microspheres were prepared by template-assisted method and examined for VOCs detection with UV LED illumination at lower temperatures than 150°C. The as-synthesized ZnO based sensor indicated an excellent response and selectivity to different concentrations of ethanol (10–1000ppm) with low-powered UV LED (2mW) at 80°C. The response time is only ∼6s while the recovery is a little sluggish (∼94s). The enhanced sensing performance could be attributed to the mesoporous hollow microstructure of the synthesized ZnO and UV activated differential photoctalytic oxidation reactions at the shallow oxide surface regions. The repeatability and long term stability of the sensor and the sensor response were also investigated. A comparison with the conventional thermal-activated gas sensing of the hollow ZnO was also conducted and the photo-activated gas sensing mechanism was discussed.
Two-dimensional (2D) nanomaterials have attracted a large amount of attention regarding gas sensing applications, because of their high surface-to-volume ratio and unique chemical or physical gas ...adsorption capabilities. As an important research method, theoretical calculations have been massively applied in predicting the potentially excellent gas sensing properties of these 2D nanomaterials. In this review, we discuss the contributions of theoretical calculations in the study of the gas sensing properties of 2D nanomaterials. Firstly, we elaborate on the gas sensing mechanisms of 2D layered nanomaterials, such as the traditional charge transfer mechanism, and a standard for distinguishing between physical and chemical adsorption, from the perspective of theoretical calculations. Then, we describe how to conduct a theoretical analysis to explain or predict the gas sensing properties of 2D nanomaterials. Thirdly, we discuss three important methods that have been applied in order to improve the gas sensing properties, that is, defect functionalization (vacancy, edge, grain boundary, and doping), heterojunctions, and electric fields. Among these strategies, theoretical calculations play a very important role in explaining the mechanisms underlying the enhanced gas sensing properties. Finally, we summarize both the advantages and limitations of the theoretical calculations, and present perspectives for further research on the 2D nanomaterials-based gas sensors.
•The WS2 sensor under the light illumination exhibits much improved sensitivity and selectivity to NH3 at low temperature than that in the dark.•The enhanced mechanism could be attributed to the ...electrons transfers from NH3 to WS2 and the amount of photo-induced oxygen ions at the surface.•The LED light can be powered by the TENGs. It could completely replace the DC-powered LED, achieving the TENGs-powered-light enhanced gas sensor.
The triboelectrical nanogenerators (TENGs)-powered light enhanced gas sensor has been successfully developed for detection of ammonia by using WS2 microflakes as the chemiresistive sensing material. The layered WS2 microflakes based chemiresistive gas sensor under the light illumination exhibits much improved sensitivity and selectivity to NH3 at low temperature than that in the dark. Within the light wavelengths range from visible to UV, the UV and near infrared light at 940nm are more effective in enhancing the sensitivities of the sensor to ammonia. The light enhanced sensing mechanism of the WS2 gas sensor could be attributed to the electrons transfers from the NH3 molecules to WS2 and the amount of photo-induced oxygen ions at the activated surface of the WS2 microflakes. Furthermore, the triboelectronic nangenerator using nanosized NaNbO3-incorporated PDMS as the tribo layers was designed and fabricated to yield the pulsed electricity for powering the lights. Interestingly, the results demonstrated for the first time that the generated pulsed power could completely replace the direct current (DC) power supply for the light in enhancing the response of the chemical sensor. Thus, it indicates a significant potential to replace the battery for powering a light-activated chemical gas sensor at low or even room temperatures.
The room temperature gas sensing properties of the Co3O4 intercalated reduced graphene oxide (rGO) based thick film semiconductor sensors were investigated. The Co3O4–rGO composite based sensors ...showed a much higher response to NO2 at room temperature compared to the rGO based sensors. However, with an increase in the rGO concentration from 5wt% to 30wt%, the response showed a decreasing tendency. The sensor response to NO2 was not fully recovered within the measurement time (∼20min) due to the much strong adsorption of NO2 at the defective sites of rGO. In contrast, the sensors using rGO showed a fast response and full recovery to methanol. This has been proposed to be exclusively due to the interaction of methanol with the sp2 bonding of the carbon. Similarly, with Co3O4 intercalated rGO, the response was significantly enhanced and the response/recovery time was within 1–2min. Two possible reasons have been discussed including the increased surface area of the rGO thick film by the intercalation of Co3O4 nanocrystals and the Co3+-carbon coupling effect for the rapid response.
The nature of the constituent components of composite materials can significantly affect the character of their interaction with the gas phase. In this work, nanocrystalline In2O3 was synthesized by ...the chemical precipitation method and was modified using reduced graphene oxide (rGO). The obtained composites were characterized by several analysis techniques—XRD, TEM, SEM, FTIR and Raman spectroscopy, XPS, TGA, and DRIFTS. The XPS and FTIR and Raman spectroscopy results suggested the formation of interfacial contact between In2O3 and rGO. The results of the gas sensor’s properties showed that additional UV illumination led to a decrease in resistance and an increase in sensor response at room temperature. However, the presence of humidity at room temperature led to the disappearance of the response for pure In2O3, while for the composites, an inversion of the sensor response toward ammonia was observed. The main reason may have been the formation of NH4NO3 intermediates with further hydrolysis and decomposition under light illumination with the formation of nitrite and nitrate species. The presence of these species was verified by in situ DRIFT spectroscopy. Their strong electron-accepting properties lead to an increase in resistance, which possibly affected the sensor signal’s inversion.
•3D Hierarchical structured PdO/PdO2-SnO2:Sb materials were uniformly deposited on microhotplate (MHP) gas sensor array incorporated with Al electrodes via advanced electrohydrodynamic inkjet ...printing method.•The sensitivity of the SnO2:Sb material is regulated by doping different concentrations of PdO/PdO2, and identification of benzene, ethanol, acetone, formaldehyde, and ethanol-acetone mixed gas (volume ratio 1:1) is achieved by two-step principal component analysis (PCA).•Simulation calculations on the I–V characteristic of the MHP gas sensors indicate the gas sensing properties are determined not only by gas sensing materials but also by the electrodes. A possible mechanism based on the Poole-Frenkel effect for electrode sensing behavior is proposed.
High-performance, low-power microhotplate (MHP) gas sensors loaded with 3D hierarchical structured PdO/PdO2-SnO2:Sb materials on a pair of Al gas sensing electrodes were fabricated through advanced high-accuracy electrohydrodynamic printing method. The PdO/PdO2-SnO2:Sb materials with various PdO/PdO2 concentrations were initially synthesized via a mild and simple hydrothermal reaction. Their micromorphology and chemical composition were characterized by SEM, XRD, TEM, and XPS, which proved the successful synthesis. The gas sensing performance of the sensors to benzene, ethanol, acetone, formaldehyde, and ethanol-acetone mixed gas (volume ratio 1:1) was tested. The sensing property of the materials is regulated with different content of PdO/PdO2. The identification of the VOCs is realized by two-step principal component analysis (PCA). Simulation calculations on the I–V characteristic of the MHP gas sensors reveal that carriers have to get through Al2O3 barriers by Poole-Frenkel emission, which significantly affects the carrier conduction as well as the gas sensing properties. The gas sensing mechanism is analyzed from the aspects of gas sensing material and electrodes.
It is well known that the photovoltaic effect produces a direct current (DC) under solar illumination owing to the directional separation of light‐excited charge carriers at the p–n junction, with ...holes flowing to the p‐side and electrons flowing to the n‐side. Here, it is found that apart from the DC generated by the conventional p–n photovoltaic effect, there is another new type of photovoltaic effect that generates alternating current (AC) in the nonequilibrium states when the illumination light periodically shines at the junction/interface of materials. The peak current of AC at high switching frequency can be much higher than that from DC. The AC cannot be explained by the established mechanisms for conventional photovoltaics; instead, it is suggested to be a result of the relative shift and realignment between the quasi‐Fermi levels of the semiconductors adjacent to the junction/interface under the nonequilibrium conditions, which results in electron flow in the external circuit back and forth to balance the potential difference between two electrodes. By virtue of this effect, the device can work as a high‐performance broadband photodetector with extremely high sensitivity under zero bias; it can also work as a remote power source providing extra power output in addition to the conventional photovoltaic effect.
An alternating current (AC) photovoltaic effect, different from known photovoltaic effects, that produces a large AC at a p–n junction is demonstrated. It is suggested that this new effect is due the relative shift and realignment between the quasi‐Fermi levels of the semiconductors adjacent to the junction/interface under the nonequilibrium conditions, which results in electron flow in the external circuit back and forth to balance the potential difference between two electrodes.
Collecting mechanical energy in the surrounding environment to power small electronic devices is an ideal method as a green and clean power source. Based on the working mechanism and the advantages ...of triboelectric nanogenerator (TENG), a robust rolling friction contact-separation mode TENG (CS-TENG) has been fabricated for harvesting vertical rotation energy by utilizing the integrated cylindrical surface with the conjunction of rolling contact electrification and electrostatic induction. The output current of the CS-TENG can maintain 96% (origin: 1.72 μA, final: 1.66 μA) after two days continuous work a rotation speed of 300 r/min. Furthermore, an electromagnetic and triboelectric hybrid contact-separation mode nanogenerator (HCS-TENG) has been further designed by coupling magnets in the rolling cylinders with copper coils as an electromagnetic generator (EMG) in the acrylic cylinder to implement the multi-functional properties. The gear transmission structure makes the device more facile to be installed on the rotation objects. The output performances of CS-TENG and EMG under various rotation speeds were systematically studied. It shows that the CS-TENG with six units can deliver an output power of 0.15 mW/cm2 and the 100 μF capacitor can be charged to 6 V in less than 1 min by the hybridized nanogenerator at a rotating rate of 700 r/min. In addition, utilizing the output signal's intrinsic characteristics of CS-TENG, a self-powered rotation speed and displacement sensor using the dual cylinder structure TENG has been achieved for these rotating mobile devices that are inconvenient to get an additional power supply in the grimmest circumstances such as moon car or other celestial and tough environment detection equipment. This multifunctional TENG device reveals a significantly potential application in the grimmest circumstances as power source, self-powered electronics and sensor systems.
Based on rolling contact electrification, electrostatic induction and Faraday's Law a hybrid contact-separation mode nanogenerator (HCS-TENG) is attained. The output performances of HCS-TENG in various rotation speeds have been studied systematically, it can greatly broaden frequency band in mechanical energy harvesting and exhibit the ultra-robustness. In addition, utilizing the output signal's intrinsic characteristics of CS-TENG, a self-powered multifunctional sensor has been achieved. Display omitted
•An ultra-robustness HCS-TENG was got by rolling contact electrification and Faraday's Law.•The device gets the high output power, good energy-harvesting capacity in a wider frequency range.•A self-powered multifunctional sensors can be attained by the output signal of TENG.
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