Arrays of Ultrathin CdS Nanoflakes with High-Energy Surface for Efficient Gas Detection

Shape-Controlled Growth and In Situ Characterization of CdS Nanocrystals via Liquid Cell Transmission Electron Microscopy

Controlling the growth, structure, and shape of CdS nanocrystals is crucial for harnessing their unique physicochemical properties across diverse applications. This control can be achieved by introducing chemical additives into the synthesis reaction mixture. However, precise manipulation of nanocrystal synthesis necessitates a thorough understanding of the formation mechanisms under various chemical conditions, a task that remains challenging. In this study, we employed in situ liquid cell transmission electron microscopy (TEM) to investigate the growth mechanisms of CdS nanocrystals in a reaction solution of cadmium chloride and thiourea, with sodium citrate serving as a structure-directing agent. We observed that CdS nanocrystals evolve through two distinct growth modes: (1) in the absence of sodium citrate, spherical nanocrystals isotropically transform into CdS nanocubes, and (2) in the presence of sodium citrate, cuboid nanocrystals preferentially extend along the {011} direction and anisotropically into CdS triangular nanoplates. Theoretical analysis has confirmed that the adsorption energy of sodium citrate on different crystal facets significantly influences the morphology of the CdS nanocrystals. Our findings not only provide a method for synthesizing CdS nanocrystals based on electron beam induction but also elucidate the intricate nanoscale growth mechanisms, offering insights that could inform the future rational design of nanocrystals with tailored morphologies.

II-VI Semiconductor-Based Conductometric Gas Sensors: Is There a Future for These Sensors?

A review of the state of research in the development of conductometric gas sensors based on II-VI semiconductors is given. It was shown that II-VI compounds indeed have properties that are necessary for the development of highly efficient gas sensors. In this case, to achieve the required parameters, all approaches developed for metal oxides can be used. At the same time, during a detailed review, it was concluded that sensors based on II-VI compounds have no prospects for appearing on the gas sensor market. The main obstacle is the instability of the surface state, which leads to poor reproducibility of parameters and drift of sensor characteristics during operation.

CdS Micrometer Hollow Spheres for Detecting Alcohols Except Methanol with Strong Anti-interference Ability

Cadmium sulfide micrometer hollow spheres (CdS MHs) were fabricated by a hydrothermal method. The performance of the CdS MHs sensor was evaluated by detecting volatile organic compounds such as methanol, ethanol, 1-propanol, isopropanol, n-butanol, iso-butyl alcohol, iso-amyl alcohol, acetone, and xylene. It was found that the optimum working temperature of the CdS MHs sensor is 190 °C. The response of the CdS MHs can reach 27.4-100 ppm ethanol and reach 84.55-100 ppm isopropanol. Comparing the response to pure 5 ppm isopropanol (iso-amyl alcohol) with the mixture of 5 ppm isopropanol (iso-amyl alcohol) and 50 ppm acetone or 5 ppm isopropanol (iso-amyl alcohol) and 50 ppm methanol, the relative deviation was -1.33% (-7.11%) or -6.19% (9.20%). It suggested that the CdS MHs sensor had a strong anti-interference ability to methanol and acetone and is suitable for detecting alcohols except methanol. Therefore, the CdS MHs sensor had good response and is a promising alcohol detection material.

Ultra-High Response Detection of Alcohols Based on CdS/MoS2 Composite

Hybrid CdS/MoS2 with branch and leaf shaped structures are successfully synthesized by hydrothermal method. It is applied to detect volatile organic compounds, a basic source of indoor air pollution with deleterious effects on the human health. The sensor based on CdS/MoS2 displays an outstanding response to alcohols among numerous gases. Their response to 100 ppm ethanol and isopropanol reaches 56 and 94, respectively. Benefiting from the dendrite-like biomimetic structure and synergy effect of CdS and MoS2, the sensor exhibits higher response than traditional gas sensor. For multiple alcohols, the limit of detection reached ppb level. In addition, by comparing the response of ethanol, isopropanol, isoamyl alcohol and their mixtures with acetone and methanal, a strong resistance interference is observed. This work proved that the modified detector holds broad promise in the detection of alcohols.

Light Activation of Nanocrystalline Metal Oxides for Gas Sensing: Principles, Achievements, Challenges

The review deals with issues related to the principle of operation of resistive semiconductor gas sensors and the use of light activation instead of thermal heating when detecting gases. Information on the photoelectric and optical properties of nanocrystalline oxides SnO2, ZnO, In2O3, and WO3, which are the most widely used sensitive materials for semiconductor gas sensors, is presented. The activation of the gas sensitivity of semiconductor materials by both UV and visible light is considered. When activated by UV light, the typical approaches for creating materials are (i) the use of individual metal oxides, (ii) chemical modification with nanoparticles of noble metals and their oxides, (iii) and the creation of nanocomposite materials based on metal oxides. In the case of visible light activation, the approaches used to enhance the photo- and gas sensitivity of wide-gap metal oxides are (i) doping; (ii) spectral sensitization using dyes, narrow-gap semiconductor particles, and quantum dots; and (iii) addition of plasmon nanoparticles. Next, approaches to the description of the mechanism of the sensor response of semiconductor sensors under the action of light are considered.

Inorganic-Diverse Nanostructured Materials for Volatile Organic Compound Sensing

Environmental pollution related to volatile organic compounds (VOCs) has become a global issue which attracts intensive work towards their controlling and monitoring. To this direction various regulations and research towards VOCs detection have been laid down and conducted by many countries. Distinct devices are proposed to monitor the VOCs pollution. Among them, chemiresistor devices comprised of inorganic-semiconducting materials with diverse nanostructures are most attractive because they are cost-effective and eco-friendly. These diverse nanostructured materials-based devices are usually made up of nanoparticles, nanowires/rods, nanocrystals, nanotubes, nanocages, nanocubes, nanocomposites, etc. They can be employed in monitoring the VOCs present in the reliable sources. This review outlines the device-based VOC detection using diverse semiconducting-nanostructured materials and covers more than 340 references that have been published since 2016.

Ni-Doped ZnS Nanospheres Decorated with Au Nanoparticles for Highly Improved Gas Sensor Performance

Novel Ni-doped wurtzite ZnS nanospheres decorated with Au nanoparticles (Au NPs⁻ZnS NSs) have been successfully fabricated using a simple method involving vacuum evaporation followed by an annealing process. This transition metal-doped gas sensor had high responsivity, extremely fast response and recovery time, and excellent selectivity to formaldehyde at room temperature. The response and recovery time are only 29 s and 2 s, respectively. Since ZnS is transformed into ZnO at a high temperature, superior room temperature-sensing performance can improve the stability and service life of the sensor. The improvement in sensing performance could be attributed to the reduced charge-transfer distance resulting from the creation of a local charge reservoir layer, and the catalytic and spillover effect of Au nanoparticles. The rough and porous spherical structure can also facilitate the detection and diffusion of gases. The as-prepared Au NPs⁻ZnS NSs are considered to be an extremely promising candidate material for gas sensors, and are expected to have other potential applications in the future.

Carrier Mobility-Dominated Gas Sensing: A Room-Temperature Gas-Sensing Mode for SnO2 Nanorod Array Sensors

Adsorption-induced change of carrier density is presently dominating inorganic semiconductor gas sensing, which is usually operated at a high temperature. Besides carrier density, other carrier characteristics might also play a critical role in gas sensing. Here, we show that carrier mobility can be an efficient parameter to dominate gas sensing, by which room-temperature gas sensing of inorganic semiconductors is realized via a carrier mobility-dominated gas-sensing (CMDGS) mode. To demonstrate CMDGS, we design and prepare a gas sensor based on a regular array of SnO₂ nanorods on a bottom film. It is found that the key for determining the gas-sensing mode is adjusting the length of the arrayed nanorods. With the change in the nanorod length from 340 to 40 nm, the gas-sensing behavior changes from the conventional carrier-density mode to a complete carrier-mobility mode. Moreover, compared to the carrier density-dominating gas sensing, the proposed CMDGS mode enhances the sensor sensitivity. CMDGS proves to be an emerging gas-sensing mode for designing inorganic semiconductor gas sensors with high performances at room temperature.

Synthesis of TiO2–ZnS nanocomposites via sacrificial template sulfidation and their ethanol gas-sensing performance

TiO₂–ZnS core–shell composite nanorods were synthesized by using ZnO as a sacrificial shell layer in a hydrothermal reaction.

Highly enhanced response of MoS2/porous silicon nanowire heterojunctions to NO2 at room temperature

Molybdenum disulfide/porous silicon nanowire (MoS₂/PSiNW) heterojunctions with different thicknesses as highly-responsive NO₂ gas sensors were obtained in the present study. Porous silicon nanowires were fabricated using metal-assisted chemical etching, and seeded with different thicknesses. After that, MoS₂ nanosheets were synthesized by sulfurization of direct-current (DC)-magnetic-sputtering Mo films on PSiNWs. Compared with the as-prepared PSiNWs and MoS₂, the MoS₂/PSiNW heterojunctions exhibited superior gas sensing properties with a low detection concentration of 1 ppm and a high response enhancement factor of ∼2.3 at room temperature. The enhancement of the gas sensitivity was attributed to the layered nanostructure, which induces more active sites for the absorption of NO₂, and modulation of the depletion layer width at the interface. Further, the effects of the deposition temperature in the chemical vapor deposition (CVD) process on the gas sensing properties were also discussed, and might be connected to the nucleation and growth of MoS₂ nanosheets. Our results indicate that MoS₂/PSiNW heterojunctions might be a good candidate for constructing high-performance NO₂ sensors for various applications.

Molybdenum disulfide/porous silicon nanowire (MoS₂/PSiNW) heterojunctions with different thicknesses as highly-responsive NO₂ gas sensors were obtained in the present study.

Wafer-Scale Synthesized MoS2/Porous Silicon Nanostructures for Efficient and Selective Ethanol Sensing at Room Temperature

This paper presents the performance of a highly selective ethanol sensor based on MoS₂-functionalized porous silicon (PSi). The uniqueness of the sensor includes its method of fabrication, wafer scalability, affinity for ethanol, and high sensitivity. MoS₂ nanoflakes (NFs) were synthesized by sulfurization of oxidized radio-frequency (RF)-sputtered Mo thin films. The MoS₂ NFs synthesis technique is superior in comparison to other methods, because it is chip-scalable and low in cost. Interdigitated electrodes (IDEs) were used to record resistive measurements from MoS₂/PSi sensors in the presence of volatile organic compound (VOC) and moisture at room temperature. With the effect of MoS₂ on PSi, an enhancement in sensitivity and a selective response for ethanol were observed, with a minimum detection limit of 1 ppm. The ethanol sensitivity was found to increase by a factor of 5, in comparison to the single-layer counterpart levels. This impressive response is explained on the basis of an analytical resistive model, the band gap of MoS₂/PSi/Si, the interface formed between MoS₂ and PSi, and the chemical interaction of the vapor molecules and the surface. This two-dimensional (2D) composite material with PSi paves the way for efficient, highly responsive, and stable sensors.