Yongcai Guo

UV assisted ultrasensitive trace NO2 gas sensing based on few-layer MoS2 nanosheet–ZnO nanowire heterojunctions at room temperature

Nitrogen dioxide (NO2) is a hazardous gas species that could impose a great threat on environmental protection and human health even at very low doses. Thus, it is of great importance to selectively detect trace NO2 gas below the ppm level. This has been a serious challenge so far, especially in the presence of other interfering gases. Herein, we report ultrasensitive, room-temperature and UV light-assisted NO2 gas sensing based on few-layer MoS2 nanosheet/ZnO nanowire composites serving as the sensing layer. A series of characterization techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), Raman, energy-dispersive X-ray spectroscopy (EDS), UV-Visual and ultraviolet photoelectron spectroscopy (UPS), were employed to explore the componential and structural properties of the obtained sensitive materials. Initially, the as-prepared MoS2/ZnO composites showed a tiny response (40%) and an incomplete recovery to 10 ppm NO2 gas in dark condition. While under UV illumination, the sensing response attained 8.4 and 188, toward 50 ppb and 200 ppb NO2 with a full recovery. Meanwhile, a sensitivity of 0.93 ppb−1, a detection limit of 50 ppq (10−15), and excellent repeatability and selectivity were also achieved. The experimental results were better than most previous work on NO2 detection to the best of our knowledge. Two main aspects are responsible for the outstanding performance. One is that a mass of photo-excited electron–hole pairs participated in the reaction with NO2 molecules under UV illumination. The other lies in numerous p–n MoS2/ZnO nanojunctions, favorable for the extension of the depletion region and the separation of the charge carrier. Additionally, long-term stability, as well as the effect of film thickness and carrier gas species on sensor performance, were simply investigated. We combined p–n nanojunctions with the UV illumination method, providing an alternative strategy to realize room-temperature operation and high sensitivity in the field of gas sensors.

Heat-pulse assisted NH3 gas sensing based on cuprous oxide nanoparticles anchored on reduced graphene oxide nanosheets

In this report, reduced graphene oxide (RGO)–cuprous oxide (Cu2O) nanocomposites are prepared as sensing layer via a combination of hydrothermal method and airbrush technology for NH3 gas detection at low temperature (≤u2009100xa0°C). A variety of characterization techniques such as SEM, TEM, XRD, FTIR and XPS were employed to probe morphological and componential properties of the obtained nanocomposites. By introducing a 70xa0°C heat pulse with duration period of 5xa0s (i.e., 5 s@70xa0°C) upon the beginning of NH3 desorption, it was noteworthy that the as-prepared sensors eventually showed a full and swift recovery within 26xa0s, which was considerably improved in comparison to a partial and sluggish one (77% recovery within 10xa0min) in absence of this treatment. Moreover, a good repeatability was achieved toward seven consecutive 150xa0ppm NH3 exposures, accompanied with a negligible baseline drift. Temperature-dependent sensing performances demonstrated that RGO–Cu2O sensors exhibited an enhanced sensing response one order of magnitude larger than pure RGO counterparts at each temperature (25, 60, and 100xa0°C), wherein 60xa0°C was considered as the optimal operation temperature. A modest selectivity toward NH3 was revealed against numerous interference gases.

UV light activated NO2 gas sensing based on Au nanoparticles decorated few-layer MoS2 thin film at room temperature

Nitrogen dioxide (NO2) plays a key role in environmental protection and human health. Recently, molybdenum disulfide (MoS2) representative of novel 2D materials has been utilized to detect NO2 gas, still hindered by the weak response, high operation temperature, as well as poor recovery characteristics. Herein, we report a UV light-assisted, room-temperature, recoverable, sensitive and selective NO2 gas sensing based on few-layer MoS2 nanosheet-Au nanoparticle composites serving as the sensing layer. In the dark condition, the as-prepared MoS2-Au sensor showed a response of 10% toward 2.5u2009ppm NO2 more than two times larger than MoS2 one, arising from more reaction sites (spillover effect and produced interfaces) and smaller baseline resistance after Au incorporation. Undesirably, all sensors exhibited an incomplete recovery. When MoS2-Au sensors were exposed to NO2 gas under UV illumination, better performance in terms of three-time enhanced response, full recovery and favorable repeatability was achieved in comparison with that in the dark case. Two aspects were responsible for these phenomena. On one hand, additional photoinduced charge carriers ensured sufficient gas-solid interactions between sensing layer and target molecules, thereby resulting in a larger response. The other aspect lay in effective separation of these charge carriers at MoS2/Au interfaces, contributing to recoverable and repeatable reactions. MoS2-Au composites were adopted for NO2 detection under UV illumination and exhibited a promising capacity of UV photodetector as well as an inspiring room-temperature gas detection, which provided an alternative strategy to design a single optoelectronic device with multi-functions.

Occurrence and suppression of transition behavior of reduced graphene oxide thin film for gas sensing

In this paper, we prepared reduced graphene oxide (RGO) thin film by simple airbrush technology and then studied its gas-sensing behaviors at room temperature. On exposure to dynamic NO2 gas, RGO thin film showed a time-resolved n–p transition process (n-type switched to p-type), due to its weak n-type properties arising from hydrazine-reduction method and strong electron-accepting NO2 molecules. As RGO amount was added, the time taken to run through this transition became longer. On consecutive exposures to NO2 and NH3 gases, p–n transition (p-type switched to n-type) emerged as well. To suppress these transition behaviors, CuCl was incorporated into RGO material, resulting in opposite p-type characteristics for the composite film as well as no occurrence of p–n transition during the sensing tests. Compared to pure RGO counterpart, RGO/CuCl film had a six-fold response enhancement and a likewise good repeatability toward 10xa0ppm NH3 within four cyclic periods. We expect that the proposed research on transition behaviors and relevant suppression methods will shed new light on gas-sensing mechanisms and improve operation stability of RGO based sensors.

Detecting devices in dynamic, module, and time sharing

In the research and development of multi-parameter precision detecting system, various parameters need to be measured (such as length, diameter, surface roughness and so on). We not only measure some parameters statically but also measure others dynamically and calculate others (such as volume, density) using some detected parameters. At the same time, this system need higher precision and higher measuring speed. We propose a new detecting idea for this system--detecting devices in dynamic, module and time-sharing. And we design and optimize multi-parameter high precision measurement system employing the method. This idea includes three parts: the first part is dynamical part. We can make system more stability and more continuity in high detecting speed. The second part is module part. We can settle on a solution to measure similar parameters and make system structure more reasonable and reduce error factors. The third is time-sharing part. We solve the problem how to allot time to every parameter and make every measurement part and its software tie in. The detecting idea has been employed to design and optimize several multi-parameter precision detecting systems. Now these systems are running successfully in workshop.