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The detection and identification of molecular traces in the environment as well as in the human breath are very significant for the development of a healthy society.

Two-dimensional nanosheets of semiconductor metal oxides are considered as promising for use in gas sensors, because of the combination of a large surface-area, high thermal stability and high sensitivity, due to the chemisorption mechanism of gas detection. In this work, 2D SnO2 nanosheets were synthesized via the oxidation of template SnS2 nanosheets obtained by surfactant-assisted one-pot solution synthesis. The 2D SnO2 was characterized using transmission and scanning electron microscopy (TEM, SEM), X-ray diffraction (XRD), low-temperature nitrogen adsorption, X-ray photoelectron spectroscopy (XPS) and IR spectroscopy. The sensor characteristics were studied when detecting model gases CO and NH3 in dry (RH25 = 0%) and humid (RH25 = 30%) air. The combination of high specific-surface-area and increased surface acidity caused by the presence of residual sulfate anions provides a high 2D SnO2 sensor’s signal towards NH3 at a low temperature of 200 °C in dry air, but at the same time causes an inversion of the sensor response when detecting NH3 in a humid atmosphere. To reveal the processes responsible for sensor-response inversion, the interaction of 2D SnO2 with ammonia was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in dry and humid air at temperatures corresponding to the maximum “positive” and maximum “negative” sensor response.

Two-dimensional nanosheets of semiconductor metal oxides are considered as promising for use in gas sensors, because of the combination of a large surface-area, high thermal stability and high sensitivity, due to the chemisorption mechanism of gas detection. In this work, 2D SnO

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.

Intermetallic compound (IMC) formed on (001) and (111) Cu single crystal has strong orientation and texture characteristics. The special interface morphology and uniform orientation provide a direction for the regulation of IMC growth behavior. In this study, the significant effect of orientation on Cu6Sn5 growth behavior in isothermal aging process was investigated. Synchrotron radiation technology and high pressure air blowing methods were employed to obtain the growth kinetics and the morphology of Cu6Sn5 during the heat preservation stage. The results indicate that scallop-like Cu6Sn5 grains were formed in heat preservation stage. These grains exhibited an abnormal coarsening behavior that their average diameter at a high temperature of 300 °C was smaller than that at a low temperature of 250 °C after aging for 30 min. Besides, the growth orientation and kinetic controlling factor of these interfacial Cu6Sn5 also changed with the increase of reaction time, which was closely related to the appearance of the abnormal coarsening phenomenon. These results are significant and meaningful to the electronic packaging industry.

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 NO2 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 NO2. 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.

RGO-SnO2 heterojunction has been extensively studied as an efficient sensitive material, while its sensing mechanism in surface chemical system is still needed to be concerned. The aim of this work is to evaluate the effects of rGO and materials processing on surface sites of nanocrystalline tin dioxide and room-temperature sensitivity to NO2. Gas-sensitive heterojunction of tin dioxide with 2-dimensional rGO has been synthesized by a one-step hydrothermal method. Contrastive researches about composition, microstructure and surface species of hydrothermal-obtained pristine SnO2, rGO-SnO2 hybrids were performed. Further reduction of rGO in hydrothermal composite with a post-treatment at over 100 °C has been found to affect the hydrophilicity and electrical conduction of rGO-SnO2 composite. Sensitivity of rGO-SnO2 to 2–8 ppm NO2 with fast response was demonstrated at room temperature, while the abrupt change of conductivity type of hydrothermal-obtained rGO-SnO2 heterojunction from $p$ -type to $n$ -type was observed after post-anneal treatment. The conductive type conversion was discussed to be closely related to the higher reduction degree of rGO and enhanced hydrophobic surface of rGO-SnO2 heterojunction.

Au modified TiO2/In2O3 hollow nanospheres were synthesized by the hydrolysis method using the carbon nanospheres as a sacrificial template. Compared to pure In2O3, pure TiO2, and TiO2/In2O3 based sensors, the Au/TiO2/In2O3 nanosphere-based chemiresistive-type sensor exhibited excellent sensing performances to formaldehyde at room temperature under ultraviolet light (UV-LED) activation. The response of the Au/TiO2/In2O3 nanocomposite-based sensor to 1 ppm formaldehyde was about 5.6, which is higher than that of In2O3 (1.6), TiO2 (2.1), and TiO2/In2O3 (3.8). The response time and recovery time of the Au/TiO2/In2O3 nanocomposite sensor were 18 s and 42 s, respectively. The detectable formaldehyde concentration could go down as low as 60 ppb. In situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) was used to analyze the chemical reactions on the surface of the sensor activated by UV light. The improvement in the sensing properties of the Au/TiO2/In2O3 nanocomposites could be attributed to the nanoheterojunctions and electronic/chemical sensitization of the Au nanoparticles.

Layered SnSe2 flakes and SnSe2/SnO2 heterojunction-based chemiresistive-type sensors demonstrate good gas sensing properties at low temperatures. The pure SnSe2-based chemiresistive-type sensor exhibited good response and recovery to both NO2 and NH3 while also showing a moderate response to hydrogen, CO, C3H6 and several typical volatile organic vapors, such as methanol, ethanol, acetone and formaldehyde, near room temperature, indicating that it possesses poor selectivity. However, compared to pristine SnSe2, a SnSe2/SnO2 heterojunction-based chemiresistive-type sensor prepared by the thermal oxidation method showed greatly improved sensitivity and superior selectivity toward NO2 at 120 °C. Such impressive improvements in the sensing performance can be attributed to the formation of SnSe2/SnO2 heterojunctions. The density functional theory (DFT) calculations revealed that there was a lower adsorption energy and greater Bader charge value between the adsorbed NO2 and SnSe2/SnO2 heterojunction materials.

With the miniaturization and multi-functionalization of modern microelectronic devices, the electrical, mechanical and thermal loads carried by solder joints become heavier and the reliability problems induced by solder connection also becomes prominent accordingly. It is one of the effective measures to improve the reliability of solder joints by adding nanoparticles into solders. In this research, Sn–xTiO2 (x = 0, 0.3 wt%, 0.6 wt%, 0.9 wt% and 1.2 wt%) composite solders were fabricated by mechanical rolling method and their physical, and welding properties on Cu substrate were investigated, respectively. The research results indicate that adding TiO2 nanoparticles has little impact on the melting point of the composite solders, but when the addition amount is 0.3 wt%, an optimized wettability can be obtained. Solder resistivity distinctly increases when the weight percentage of TiO2 nanoparticles exceeds 0.6 wt%. Besides, TiO2 nanoparticles can also refine the interfacial grains between solder and substrate, thus improving the linking property of solder joints. Particularly, grain refinement phenomenon is most obvious when the addition amount of TiO2 nanoparticles is 0.3 wt%. The average thickness of intermetallic compound (IMC) layer reduces from 2.80 to 2.45 μm, and the average grain diameter reduces from 1.43 to 1.04 μm. TiO2 nanoparticles are mainly distributed on the surface of Cu6Sn5 grains, and high temperature makes it difficult for TiO2 nanoparticles to adhere to the grain surface, which weakens the effect of TiO2 nanoparticles on the inhibition of IMC growth. The inhibition rate decreases from 37.5% at 250 °C to 9.36% at 300 °C.

The room temperature gas sensing properties of the hollow SnO2/ZnO heterojunction-based chemiresistive sensors under ultraviolet ${\left ({UV}\right)}$ radiation were investigated. The hollow SnO2/ZnO heterojunctions were fabricated using the template-assisted process followed with a hydrothermal treatment. The sensor using this core-shell SnO2/ZnO heterojunctions showed a much higher response to formaldehyde under UV illumination compared to that without UV. Moreover, the response of this hollow heterojunctions-based sensor to formaldehyde is higher than the respective pure SnO2 and ZnO hollow microspheres-based sensors under same conditions indicating the synthetic effect of the formed heterojunctions on the enhanced sensitivity. More strikingly, the response of the SnO2/ZnO heterojunctions-based sensors showed a much smaller response to ethanol, methanol, and toluene indicating an excellent selectivity whereas there is a large influence from ethanol if the sensor worked under the conventional heating method. It could be attributed to the lower oxidizing ability of the adsorbed, ionized oxygen with a negative monovalence on the oxide surface and thus a selective photo-catalytic conversion of formaldehyde over the other possible interferents. The long-term stability of the sensor operated under UV light was also studied.

This paper investigated the morphologies of intermetallic compounds (IMCs) forming between pure Sn solders and different polycrystalline copper substrates with microstructures of fiber structure, equiaxed crystal and columnar crystal respectively as well as the effect of Cu microstructures on IMC growth kinetics. The results showed that when multiple IMC grains could be accommodated in a single grain area of substrates, the IMC growth behavior on some grains of polycrystalline substrates was similar to that on single crystalline substrate of same orientation. The difference on polycrystalline Cu substrates was that IMCs with different morphologies appeared alternately. Furthermore, the IMC distribution at soldering interface was stripy on fiber structure and blocky on equiaxed crystal substrates. With increasing of reflow temperature or time, IMCs with different morphologies followed their own growth behavior. In addition, the grain growth rate of IMCs on columnar crystal substrate was higher than that on the other two substrates due to different Cu supply and diffusion abilities.

In this paper, the water droplet experimental method was used to in situ study the electrochemical migration and related behaviors of Sn-based lead-free solder. Results showed that the electrochemical migration short-circuit time decreased with voltage increasing. Electrochemical migration was difficult to occur when the voltage was less than 1.5 V. However, when the voltage was above 5 V, metal deposits were generated in a short time and caused a bridge between two electrodes, which led to short circuit. As spacing increased, the short-circuit time prolonged. The three-dimensional pattern between voltage, spacing and short-circuit time had been constructed, which is beneficial to estimate the short-circuit time value under a combination of any voltage and spacing within a certain range. When changed the concentration of NaCl solution, it was founded that dendrites which surface covered much precipitates were formed at a low concentration. At medium concentration, there was no dendrites but only formed a large amount of precipitates. At a high concentration, the dendrites which surface with less precipitates were showed again. The formation mechanisms of dendrites under different NaCl solution concentrations were analyzed. In addition, the effects of different materials on electrochemical migration behavior were also studied, founding that electrochemical migration products of Sn, Sn0.7Cu and Sn3Ag were mainly Sn and Sn oxides. The electrochemical migration products of Sn9Zn contain much Zn element apart from Sn and Sn oxides. At a lower voltage, Sn9Zn solder had better electrochemical migration resistance than Sn, Sn0.7Cu and Sn3Ag, but this phenomenon was not obvious at a higher voltage.

Mesoporous ZnSe/ZnO heterojunctions were prepared by the in situ thermal oxidation of ZnSe at elevated temperatures in air. The partial replacement of selenium by oxygen in the ZnSe nanoflakes led to a highly porous microstructure with 8.2 nm mesopores distributed fairly uniformly within the formed heterojunction sample. The as-fabricated mesoporous ZnSe/ZnO heterojunction-based sensor exhibits an approximately 7.3-fold significantly higher response than those of both pristine ZnO- and ZnSe-based sensors when exposed to 8 ppm NO2 at 200 °C. This sensor also demonstrates excellent selectivity for methanol, ethanol, acetone, benzene, methylbenzene, ammonia, and formaldehyde. The significantly enhanced response of the ZnSe/ZnO-based sensor is due to the mesoporous microstructure during thermal oxidation of ZnSe to ZnO, yielding more active sites and the accumulation of electrons in the ZnO nanocrystals transferred between ZnSe/ZnO interfaces.

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.

The newly invented triboelectric nanogenerator (TENG) shows a significant potential to be a powerful technology for converting mechanical energy into electricity as a complementary usage for solving our future energy demanded for micro- and macro- grid. However, packaging and robustness are critical factors for TENG toward practical applications since the performance of TENG is affected by environmental conditions such as humidity. Here, a full-packaged waterwheel-like rolling triboelectric–electromagnetic hybrid nanogenerator (TEHNG) for harvesting mechanical energy in harsh environment has designed. The integrated silicone rubber-magnet rods and electrodes laminated by a layer of nylon film, which worked as the triboelectric material, are used to fabricate a rolling friction free-standing triboelectric layer mode triboelectric nanogenerator (RFS-TENG) and combining with copper coils subtly, electromagnetic generator (EMG) is achieved. The rolling friction causes almost no damage to the triboelectric layers and there is no obvious normalized current decay found after 3-days continuous working at 0.5 Hz. Thus, this novel design makes the TEHNG easier to be packaged, and prevent from water interference revealing a superior robustness. It is also found that the developed TEHNG broadens the working frequency range as TENG can work more effectively for converting the mechanical energy at lower-frequencies less than 5 Hz energy scavenging while EMG can work much better at higher-frequencies above 5 Hz. Moreover, combining with hand shaking and water wheel, the device can be demonstrated to collect the energy of handy and water-flow flexibly. Furthermore, using current output signals of the EMG, a self-powered wireless frequency and revolution sensor systems are also demonstrated. This work presents a new platform of hybrid generator for harvesting mechanical energy in harsh environments with several desirable features such as the broadened frequency bands, better portability and for building up self-powered wireless sensor systems.

The electrical conductivity of amorphous carbon/reduced graphene oxide(rGO)-wrapped Co3O4 nanofibers prepared by the electrospinning process is for the first time finely tuned creatively by the thermal-etching process during which the amorphous carbon is removed and the further growth of Co3O4 nanocrystals within amorphous carbon occurs through thermal oxidation. The influence of electrical conductivity on the room-temperature ammonia sensing properties of these ternary composite material based sensors can thus be investigated for the first time. The fabricated chemiresistive-type sensor using the ternary sample thermally oxidized at 390 °C in a controlled O2 environment shows a good sensitivity and fast response (∼20 s: 50 ppm) to different concentrations of ammonia from 1–100 ppm at room temperature. The sensor demonstrates an excellent selectivity to several possible interferents such as methanol, ethanol, formaldehyde, methylbenzene, benzene, acetone and water and a brilliant long-term stability for about four years. Density functional theory (DFT) calculations are performed to study the coupling effect at the interface of graphene/Co3O4 (110B) which plays a crucial role in the sensing performance to NH3. The underlying chemical sensing process is examined using in situ Fourier transform infrared spectroscopy (FTIR) which shows a water-mediated catalytic oxidation transducing mechanism.