Jinjin Wu

SYNTHESIS AND GAS SENSITIVITY OF IN-DOPED ZNO NANOPARTICLES

Undoped and In-doped ZnO nanoparticles were produced by renovated hybrid induction and laser heating (HILH) in this study from Zn–In alloy, with different mole ratios, as the raw material in a flowing mixed gas atmosphere of Ar+O2. The morphological characteristics, phase microstructure, and chemical state of In-doped ZnO nanoparticles were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The change in electrical resistance of thick film based on the In-doped ZnO nanoparticles and their gas sensitivities to volatile organic compounds (VOCs), benzene, acetone, ethyl alcohol, toluene, and xylene, as a function of temperature were measured in the temperature range of 200–500 °C, and compared with the undoped thick film. The results showed that the In-doped ZnO has lower resistance and higher sensitivity than that of the undoped ZnO. This was probably due to the fact that the In3+ ions, replacing the Zn2+ ions in the ZnO lattice, resulted in an increase of the concentration of free electrons followed by an increase of the adsorbed oxygen. Among the types of In-doped ZnO, 4.58 at % In-doped ZnO had the lowest resistance, and had the highest sensitivity. On increasing the concentration of In into ZnO, its resistance increased, while the sensitivity decreased. The sensitivity of the 4.58 at % In-doped ZnO to VOCs was in the order of acetone>alcohol>xylene>toluene> benzene at an operating temperature of 420 °C.

A low temperature gas sensor based on Pd-functionalized mesoporous SnO2 fibers for detecting trace formaldehyde

The permissible limitation of formaldehyde (HCHO) is 80 ppb in an indoor environment. Hence, the rapid real-time monitoring of trace HCHO is urgent and faced as a great challenge by gas sensors based on semiconducting metal oxides. To enhance the HCHO sensing performance of gas sensors, mesoporous SnO2 fibers are used to fabricate a bare SnO2 sensor and then the sensor is functionalized with Pd nanodots by a facile dipping–annealing process. The obtained Pd-functionalized SnO2 sensor exhibits a very high response to HCHO, ultralow detection limit (50 ppb), excellent sensor selectivity over other reducing gases, and short response and recovery time to 100 ppb HCHO (53 s and 103 s, respectively) at a low working temperature of 190 °C. Herein, the Pd nanodots loaded onto SnO2 fibers serve as sensitizers or promoters, increasing the amount of adsorbates as well as molecule–ion conversion rate and simultaneously providing a new catalytic oxidization pathway of HCHO (HCHO → [CH2O]n (POM) → HCOOH → CO2 + H2O) accompanied with a promotion in the electron transfer rate, and thus improving HCHO sensing performance. The combination of the SnO2 mesoporous structure and catalytic activity of the Pd nanodots loaded could give us a very attractive sensing behavior for applications as real-time monitoring gas sensors with rapid response speed.

Mechanistic Insights into Formation of SnO2 Nanotubes: Asynchronous Decomposition of Poly(vinylpyrrolidone) in Electrospun Fibers during Calcining Process

The formation mechanism of SnO2 nanotubes (NTs) fabricated by generic electrospinning and calcining was revealed by systematically investigating the structural evolution of calcined fibers, product composition, and released volatile byproducts. The structural evolution of the fibers proceeded sequentially from dense fiber to wire-in-tube to nanotube. This remarkable structural evolution indicated a disparate thermal decomposition of poly(vinylpyrrolidone) (PVP) in the interior and the surface of the fibers. PVP on the surface of the outer fibers decomposed completely at a lower temperature (<340 °C), due to exposure to oxygen, and SnO2 crystallized and formed a shell on the fiber. Interior PVP of the fiber was prone to loss of side substituents due to the oxygen-deficient decomposition, leaving only the carbon main chain. The rest of the Sn crystallized when the pores formed resulting from the aggregation of SnO2 nanocrystals in the shell. The residual carbon chain did not decompose completely at temperatures less than 550 °C. We proposed a PVP-assisted Ostwald ripening mechanism for the formation of SnO2 NTs. This work directs the fabrication of diverse nanostructure metal oxide by generic electrospinning method.