Hierarchical SnO2 nanospheres: bio-inspired mineralization, vulcanization, oxidation techniques, and the application for NO sensors

High selectivity of sulfur-doped SnO2 in NO2 detection at lower operating temperatures

Resistive gas sensors based on metal oxides have aroused great interest in the sensing of NO2 gas due to their low cost, good stability, and easy fabrication. However, drawbacks such as low sensitivity and a lack of selectivity, which originate from the limited kinds of intrinsic active centers on the surface of the metal oxides that could be involved in the gas-sensing reaction, remain great challenges to overcome. To solve these problems, surface modification of SnO2 by S-doping was carried out by the sintering of flower-like SnS2. Gas-sensing tests revealed that the S-doped SnO2 showed ultra-high sensitivity to NO2 (Rg/Ra = 600 toward 5 ppm) with low optimal operating temperature (50 °C). The detection limit of the sensor was as low as 50 ppb (Rg/Ra = 11). Notably, the S-doped SnO2 showed negligible cross-responses to alcohol, acetone, HCHO, SO2, H2S, and xylene. The ultra-high sensitivity and selectivity toward NO2 were closely related to the content of the S-dopant. This phenomenon is attributed to the active role of S-dopant during the surface reactions with NO2, which was substantiated by in situ Raman characterization and DFT-based calculations. This study offers an important guide for surface modification by doping to improve the sensitivity and selectivity of metal oxides and sheds new light on material design to develop resistive gas sensors for NO2 detection.

Hydrothermally synthesized Copper Oxide (CuO) superstructures for ammonia sensing

According to environmental protection agencies (EPA), the emission threshold of NH3 in air is 1000kg/yr which is now about 20Tg/yr. Hence, there is a rapid increase in need of NH3 sensors to timely detect and control NH3 emissions. Metal oxide nanostructures such as CuO with special features are potential candidates for NH3 sensing. In the present study, morphology controlled 3-dimensional CuO superstructures were synthesized by surfactant-free hydrothermal method for NH3 detection. In addition to conventional hydrothermal method where water as solvent, a modified approach using a mixture of water and ethylene glycol (EG) was used as solvent to control the growth process. Hierarchical superstructures namely, snowflake-like, flower-like, hollow-sphere-like and urchin-like feature with particle dimensions ranging from 0.3 to 1μm were obtained by varying water/EG ratio and reaction temperature. The synthesized nanostructures exhibited morphology dependent luminescence and gas sensing properties. The surface area and pore distribution determined by BET surface analysis also largely influenced by the presence of EG in the reaction system. The average pore diameter enhanced from 6nm to 14nm by the addition of 10ml EG as solvent. The room temperature ammonia sensing behavior of all samples was studied using an indigenous gas sensing set-up. It was found that hollow-sphere like CuO nanostructures showed a maximum sensitivity of 150% towards 600ppm ammonia with a response and recovery time of 6min. The hydrothermal synthesis strategy reported here has the advantage of producing shape controlled hierarchical materials are highly suitable for various technological applications.

Defect structure and optical phonon confinement in ultrananocrystalline BixSn1−xO2 (x = 0, 0.03, 0.05, and 0.08) synthesized by a sonochemical method

Relaxation of the zone-centre optical phonon selection rule due to size effect causes interesting changes in vibrational spectra.

High-response H2S sensor based on ZnO/SnO2 heterogeneous nanospheres

Compared to SnO₂ and ZnO gas sensors, the ZnO/SnO₂ heterogeneous sensors showed exceptional electrical responses to H₂S gas at 300 °C.

Diethylamine gas sensor using V2O5-decorated α-Fe2O3 nanorods as a sensing material

V₂O₅-decorated α-Fe₂O₃ composite nanorods were synthesized successfully by electrospinning and an environmentally-friendly soak-calcination method.

Micro/Nano gas sensors: a new strategy towards in-situ wafer-level fabrication of high-performance gas sensing chips

Nano-structured gas sensing materials, in particular nanoparticles, nanotubes, and nanowires, enable high sensitivity at a ppb level for gas sensors. For practical applications, it is highly desirable to be able to manufacture such gas sensors in batch and at low cost. We present here a strategy of in-situ wafer-level fabrication of the high-performance micro/nano gas sensing chips by naturally integrating microhotplatform (MHP) with nanopore array (NPA). By introducing colloidal crystal template, a wafer-level ordered homogenous SnO2 NPA is synthesized in-situ on a 4-inch MHP wafer, able to produce thousands of gas sensing units in one batch. The integration of micromachining process and nanofabrication process endues micro/nano gas sensing chips at low cost, high throughput, and with high sensitivity (down to ~20 ppb), fast response time (down to ~1 s), and low power consumption (down to ~30 mW). The proposed strategy of integrating MHP with NPA represents a versatile approach for in-situ wafer-level fabrication of high-performance micro/nano gas sensors for real industrial applications.

A general strategy for nanohybrids synthesis via coupled competitive reactions controlled in a hybrid process

A new methodology based on core alloying and shell gradient-doping are developed for the synthesis of nanohybrids, realized by coupled competitive reactions, or sequenced reducing-nucleation and co-precipitation reaction of mixed metal salts in a microfluidic and batch-cooling process. The latent time of nucleation and the growth of nanohybrids can be well controlled due to the formation of controllable intermediates in the coupled competitive reactions. Thus, spatiotemporal-resolved synthesis can be realized by the hybrid process, which enables us to investigate nanohybrid formation at each stage through their solution color changes and TEM images. By adjusting the bi-channel solvents and kinetic parameters of each stage, the primary components of alloyed cores and the second components of transition metal doping ZnO or Al2O3 as surface coatings can be successively formed. The core alloying and shell gradient-doping strategy can efficiently eliminate the crystal lattice mismatch in different components. Consequently, varieties of gradient core-shell nanohybrids can be synthesized using CoM, FeM, AuM, AgM (M = Zn or Al) alloys as cores and transition metal gradient-doping ZnO or Al2O3 as shells, endowing these nanohybrids with unique magnetic and optical properties (e.g., high temperature ferromagnetic property and enhanced blue emission).

Three-dimensional mesoporous graphene aerogel-supported SnO2 nanocrystals for high-performance NO2 gas sensing at low temperature

A facile and cost-efficient hydrothermal and lyophilization two-step strategy has been developed to prepare three-dimensional (3D) SnO2/rGO composites as NO2 gas sensor. In the present study, two different metal salt precursors (Sn(2+) and Sn(4+)) were used to prepare the 3D porous composites. It was found that the products prepared from different tin salts exhibited different sensing performance for NO2 detection. The scanning electron microscopy and transmission electron microscopy characterizations clearly show the macroporous 3D hybrids, nanoporous structure of reduce graphene oxide (rGO), and the supported SnO2 nanocrystals with an average size of 2-7 nm. The specific surface area and porosity properties of the 3D mesoporous composites were analyzed by Braunauer-Emmett-Teller method. The results showed that the SnO2/rGO composite synthesized from Sn(4+) precursor (SnO2/rGO-4) has large surface area (441.9 m(2)/g), which is beneficial for its application as a gas sensing material. The gas sensing platform fabricated from the SnO2/rGO-4 composite exhibited a good linearity for NO2 detection, and the limit of detection was calculated to be as low as about 2 ppm at low temperature. The present work demonstrates that the 3D mesoporous SnO2/rGO composites with extremely large surface area and stable nanostructure are excellent candidate materials for gas sensing.

Occurrence of non-equilibrium orthorhombic SnO2 phase and its effect in preferentially grown SnO2 nanowires for CO detection

Tin oxide nanowires (NWs) were deposited successfully on silicon substrate. Depending on the distance between the source and substrate, NWs exhibit either a single phase structure or a mixed phase structure, the latter exhibits enhanced sensitivity to CO detection.

Gas sensing of SnO2 nanocrystals revisited: developing ultra-sensitive sensors for detecting the H2S leakage of biogas

As a typical mode of energy from waste, biogas technology is of great interest to researchers. To detect the trace H2S released from biogas, we herein demonstrate a high-performance sensor based on highly H2S-sensitive SnO2 nanocrystals, which have been selectively prepared by solvothermal methods using benzimidazole as a mineralization agent. The sensitivity of as-obtained SnO2 sensor towards 5 ppm H2S can reach up to 357. Such a technique based on SnO2 nanocrystals opens up a promising avenue for future practical applications in real-time monitoring a trace of H2S from the leakage of biogas.

Plate-like p-n heterogeneous NiO/WO₃ nanocomposites for high performance room temperature NO₂ sensors

Plate-like heterogeneous NiO/WO₃ nanocomposites have been successfully prepared by annealing Ni(OH)₂ and H₂WO₄ in air. These NiO/WO₃ nanocomposites have shown excellent sensitivity towards NO₂ and ultrafast response at room temperature due to their p-n heterogeneous characteristics.