The reduced graphene oxide (rGO) encapsulated Co3O4 nanocrystals were fabricated by using the electrospinning technology. The sample shows nanofiber morphology and the Co3O4 nanocrystals were wrapped ...by rGO thin layers. The as-electrospun rGO–Co3O4 nanofibers based chemoresistive sensor exhibited a p-type semiconductor behavior in ambient conditions and showed an excellent sensitivity with a fast response and recovery to different concentrations of ammonia from 5 to 100ppm at room temperature. The sensor displays selectivity to several potential interferrents such as methanol, ethanol, formaldehyde, acetone, benzene, and methylbenzene. It may be attributed to the unique hierarchical wrapping microstructure and the selective NH3 adsorption at both the wrapping layer of rGO and the polarized CCo3+ covalent centers within the nanofibers.
•2D nanostructured rGO/WS2 heterojunctions has been successfully synthesized by one-step hydrothermal methods.•The rGO/WS2 based chemiresitive-type sensor shows an excellent sensitivity, selectivity ...and stability to 10–50 ppm NH3 at room temperature.•The enhanced sensitivity was attributed to the introduced hydroxyls by rGO nanosheets and extra acid centers by WS2 nanoflakes.
Hybrid of the two dimensional nanostructured reduced graphene oxide (rGO) and WS2 has been investigated for a room temperature ammonia sensor. The formed rGO/WS2 heterojunctions prepared by one-step hydrothermal synthesis indicated a good sensitivity to different concentrations of ammonia from 10 ppm to 50 ppm at room temperature. The WS2 nanoflakes doped in the heterojunction plays significant role in the enhanced response through the introduction of more hydroxyls in rGO and the extra Lewis acid active centers. The sensor also shows an excellent selectivity to NO2, alcohols, formaldehyde, acetone and benzene and a good long term stability indicating a potential to be employed as a room temperature NH3 sensor.
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•The photo-energy actived gas sensor was developed for the detection of NO2 by using the SnS2 nanosheets as the gas sensing material.•The gas sensor exhibits a high response and an ...excellent selectivity to 8 ppm NO2 under the green light illumination at room temperature.•The intrinsic mechanism of the SnS2 based sensor could be attributed to the photo-generated electron-hole pairs.
The visible-light activated gas sensor has been successfully fabricated for detecting NO2 by using two dimensional (2D) structured SnS2 nanosheet as the chemiresistive sensing material. Under the light illumination, the SnS2 nanosheet based chemiresistive gas sensor exhibits a high response and excellent selectivity to NO2 at room temperature. Influences of light wavelength, light intensity, operating temperatures and humidity on the sensing characteristics are investigated in details. It suggests that photo-energy activation can effectively activate the SnS2 sensor and the green light is the most effective to achieve superior sensing property of the SnS2 sensor at room temperature in terms of the excellent sensitivity and a better response/recovery speeds. The sensor also demonstrated an excellent selectivity to NO2 over several possible interferants such as SO2, CO2, NH3, acetone, methanol, ethanol and formaldehyde, and a good stability for about six months activated by the green light. The sensing mechanism is intimately related to the extra photo-generated electrons which subsequently attract more NO2 at its 2D surface. A simple Langmuir - Hinshelwood dynamics model is proposed to explain the effect of the visible light irradiation on the absorption/desorption speed of gas molecules.
Reduced graphene oxide (rGO) decorated TiO2 microspheres are prepared by using the hydrothermal method. The as-synthesized sample shows that TiO2 particles are connected by the rGO nanosheets while ...some TiO2 particles are found to be partly covered by rGO sheets. The rGO-decorated TiO2 based chemoresistive-type sensor shows good sensitivity and excellent selectivity to different concentrations of ammonia from 5ppm to 50ppm at room temperature. However, the response and recovery speeds of the sensor to ammonia are sluggish and need further optimization.
•MOX sensors signal pre-processing methods compared for machine learning approach.•Statistical shape analysis (SSA) pre-processing is introduced and most effective.•SSA pre-processing reduces effects ...of signal/baseline drift and fluctuations.•The approach tested in synthetic air media and realistic air conditions.•Commercial and laboratory made sensors are used for methane, propane and CO detection.
Development of new signal processing approaches is essential for improvement of the reliability of metal oxide gas sensor performance in real atmospheric conditions. Advantages statistical shape analysis (SSA) method are presented in comparison to previously reported signal pre-processing techniques – principal component analysis (PCA), discrete wavelet transform (DWT), polynomial curve fitting (PCF) – used in combination with machine learning (ML) algorithm for improvement of detection selectivity. An enhanced identification of chemically related gases (methane and propane) at a concentration range of 40−200 ppm under variable real atmospheric conditions has been demonstrated using working temperature modulated metal oxide gas sensors. Laboratory samples of sensors based on nanocrystalline SnO2 modified with Au and Pd were used. The proposed data pre-processing algorithm is less sensitive to sensor response and baseline drift and fluctuations compared to other methods during two months of continuous operation and work with periods of inactivity. The collected dataset and signal processing code are made public. The advantages of SSA signal pre-processing method are also demonstrated with the use of independent publicly available dataset for the task of CO selective quantitative detection in the air with variable humidity in the 2.2−20 ppm concentrations range.
•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.
Two-dimensional (2D) semiconductors exhibit unique electronic and optical properties arising from the atomic-scale thickness and two-dimensional electronic structure. However, it is usually limited ...by an intrinsically flat morphology of 2D materials. Here, we report an effect of spontaneous folding of quasi-2D CdTe nanosheets stimulated by ligand exchange. We show that initially flat CdTe nanosheets with 100–200 nm lateral size and 5–6 ML thickness are uniformly rolled up when oleic acid is replaced by thiol-containing ligands. Detailed study shows nanosheet folding along the 110 direction forming multiwall scroll-like structures with the diameter being dependent on sheet thickness. A pronounced red shift of the exciton transitions of CdTe nanosheets is found due to thickness increase and strain appearance under thiol attachment. The folding mechanism is likely related to misfit strain at CdTe (001) basal planes as ultrathin CdS layer is formed. Possibility to precisely tune the nanostructure shape simply by ligand-induced strain can evolve into new synthetic strategies to control a spatial morphology of 2D materials.
Colloidal two-dimensional cadmium chalcogenides nanoplatelets have recently emerged as a class of semiconductor nanoparticles with the narrowest emission and absorption excitonic bands that are of ...interest for optical applications. Here, we have developed a synthesis protocol for 2.5-monolayer-(ML) thick CdSe nanosheets as a single population. We found that a two-step synthesis in the presence of water promoted the growth of atomically thin nanosheets with high structural and morphological perfection. Using a seeded-growth technique, we extended the lateral size of nanosheets up to 400 nm, which led to the formation of multiwall rolled-up nanostructures. Ligand exchange of native oleic acid, attached to Cd-rich (001) planes, with achiral thioglycolic acid and chiral N-acetylcysteine retains a scroll-like morphology of nanosheets, in contrast to a thicker 3.5 ML population. A reorientation from the 110 to 100 folding direction was found during the change from an achiral to a chiral ligand. In the case of ligand exchange with chiral N-acetyl-l- or d-cysteine, we demonstrated that 2.5 ML CdSe nanosheets with 400 nm lateral size have circular dichroism with a dissymmetry g factor up to 3 × 10–3. Strong circular dichroism found for colloidal CdSe nanosheets makes them a promising candidate for polarization-enabled applications, while the growth protocol of the thinnest CdSe nanosheets enriches the known synthesis methods of a set of CdSe nanoplatelet populations.
Novel ZnSe/NiO heterostructure nanocomposites were successfully prepared by one-step hydrothermal method. The ZnSe/NiO-based sensor exhibits a response of ~ 96.47% to 8 × 10
−6
NO
2
at 140 °C, which ...is significantly higher than those of intrinsic ZnSe-based (no response) and NiO-based (~ 19.65%) sensors. The theoretical detection limit (LOD) of the sensor is calculated to be 8.91 × 10
−9
, indicating that the sensor can be applied to detect the ultralow concentrations of NO
2
. The effect of NiO content on the gas-sensing performance of the nanocomposites was investigated in detail. The optimal NiO content in the nanocomposite is determined to be 15.16% to achieve the highest response. The as-fabricated sensor also presents an excellent selectivity to several possible interferents such as methanol, ethanol, acetone, benzene, ammonia and formaldehyde. The enhanced sensing performance can be attributed to the formation of p–p heterostructures between ZnSe and NiO, which induces the charge transfer across the interfaces and yields more active sites.
Raman surface vibration modes have been measured for SnO2 nanocrystalline powders with grain sizes of 3−36 nm and a specific surface area up to 180 m2 g-1, which were prepared by four different ...routes of chemical synthesis. The influence on these surface vibration modes of the treatment temperature, the crystallite size, and the specific surface area has been studied and bands at 245, 257, 286, 310−350, and 400−700 cm-1 have been identified. The 400−700 cm-1 band intensity has been found proportional to the surface active area. Likewise, the correlation of the 400−700 cm-1 band intensity with the sensing mechanisms have been analyzed from the sensor response of the prepared thick-film gas sensors against reducing CO and oxidizing NO2 species diluted in a N2 carrier. The influence of the nanostructure surface on the sensor signal exhibits opposite trends for CO than for NO2 detection. As the Raman surface vibration modes, 400−700 cm-1, band intensity increases, the sensor response for CO increases too, while that of NO2 diminishes, giving an excellent inverse correlation between the sensor response for CO and NO2. This correlation is fulfilled for all the samples except those that are distorted by the presence of an excess of contamination caused by OH- groups together with Cl- ions introduced by the chemical synthesis procedure.
