Shouqin Tian

Interface Bonds Determined Gas-Sensing of SnO2-SnS2 Hybrids to Ammonia at Room Temperature.

Unique gas-sensing properties of semiconducting hybrids that are mainly related to the heterogeneous interfaces have been considerably reported. However, the effect of heterogeneous interfaces on the gas-sensing properties is still unclear, which hinders the development of semiconducting hybrids in gas-sensing applications. In this work, SnO2-SnS2 hybrids were synthesized by the oxidation of SnS2 at 300 °C with different times and exhibited high response to NH3 at room temperature. With the increasing oxidation time, the relative concentration of interfacial Sn bonds, O-Sn-S, among the total Sn species of the SnO2-SnS2 hybrids increased first and then decreased. Interestingly, it can be found that the response of SnO2-SnS2 hybrids to NH3 at room temperature exhibited a strong dependence on the interfacial bonds. With more chemical bonds at the interface, the lower interface state density and the higher charge density of SnO2 led to more chemisorbed oxygen, resulting in a high response to NH3. Our results revealed the real roles of the heterogeneous interface in gas-sensing properties of hybrids and the importance of the interfacial bonds, which offers guidance for the material design to develop hybrid-based sensors.

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.

Surface doping of La ions into ZnO nanocrystals to lower the optimal working temperature for HCHO sensing properties

Lowering the working temperature without sacrificing other good gas-sensing properties is of particular interest to gas sensors for an excellent performance. In this work, La surface doped ZnO nanocrystals were successfully prepared by a facile thermal treatment with lanthanum nitrate (La(NO3)3) solution injected into ZnO thick films, which exhibited a remarkable decrease in the optimal working temperature for formaldehyde (HCHO) sensing properties. This was probably attributed to the formation of surface LaZn defects in the ZnO nanocrystals which was evidenced by XRD, XPS results and DFT calculations. The surface LaZn defects can introduce a shallower donor level than oxygen vacancies, and probably facilitate the charge transfer from oxygen species to ZnO for producing chemisorbed oxygen species more easily. This was in good agreement with the DFT results that the absorption energy of oxygen molecules on the surface of La doped ZnO was only -10.61 eV, much lower than that of pure ZnO. Moreover, the optimal working temperature of the La doped ZnO based sensor was significantly decreased from 350 to 250 °C without sacrificing the high and quick response to HCHO gas as the content of surface LaZn defects was increased gradually. Therefore, the behavior of the surface LaZn defects in the optimal working temperature revealed a HCHO response mechanism in ZnO, which can provide new insights into the enhanced HCHO sensing performance of gas sensors.

The atomic origin of high catalytic activity of ZnO nanotetrapods for decomposition of ammonium perchlorate

Distinct from the common well faceted ZnO nanorods (R-ZnO), ZnO nanotetrapods (T-ZnO) exhibited a remarkable catalytic activity for the thermal decomposition of ammonium perchlorate (AP): the activation energy at high temperature decomposition (HTD) was significantly decreased to 111.9 kJ mol−1, much lower than 162.5 kJ mol−1 for pure AP and 156.9 kJ mol−1 for AP with R-ZnO. This was attributed to more abundant atomic steps on the surface of T-ZnO than that of R-ZnO, as evidenced by HRTEM and density function theory (DFT) calculations. It was shown that the initiation step of perchloric acid (PA) decomposition happened much faster on stepped T-ZnO edges, resulting in the formation of active oxygen atoms from HClO4. The formed oxygen atoms would subsequently react with NH3 to produce HNO, N2O and NO species, thus leading to an obvious decrease in the activation energy of AP decomposition. The proposed catalytic mechanism was further corroborated by the TG-IR spectroscopy results. Our work can provide atomic insights into the catalytic decomposition of AP on ZnO nanostructures.

Hierarchical ZnO hollow microspheres with exposed (001) facets as promising catalysts for the thermal decomposition of ammonium perchlorate

Hierarchical porous ZnO hollow microspheres assembled from nanorods with exposed (001) facets on their external surface were prepared in one pot by a simple low-temperature wet chemical method without templates. The formation mechanism based on their chemical self-transformation was proposed. Importantly, these ZnO hollow microspheres exhibited better catalytic activity for the thermal decomposition of ammonium perchlorate (AP) than the dispersed ZnO nanorods by lowering its decomposition temperature from 409 °C to 308 °C and decreasing its activation energy from 150 ± 14 kJ mol−1 to 63 ± 7 kJ mol−1. This is attributed to their hierarchical porous structure with larger surface area and exposed (001) facets dominant on their external surface, which can facilitate the adsorption of HClO4 and NH3 gases from AP and the formation of active oxygen. The active oxygen will promote the oxidation reaction of NH3 more completely in AP decomposition, thus leading to a significant decrease in decomposition temperature and activation energy. Therefore, this work could provide a new insight into the thermal decomposition mechanism of AP catalyzed by hierarchical micro/nanostructures of metal oxides.

Controlled surface modification of various substrates with SnO2 nanoparticles

We describe a simple, cost-effective approach for surface modification of various substrates with SnO2 nanoparticles by controlled hydrolysis, which is not confined on specific chemical states of the surface. Specifically, these SnO2 nanoparticles were loaded onto the surfaces of carbon-based materials (graphene), ionic crystals (tin sulfide), polymer materials (silk fiber) and biological materials (yeast) with uniform distribution, despite the great differences in surface chemistries. Moreover, the formation mechanism of the SnO2 nanoparticles has been discussed, confirming that the process described can be easily implemented and adapted to other systems. These as-synthesized nanocomposites are expected to have wide applications in the fields of gas sensing, photocatalysis and biomaterials.

Origin of the efficient catalytic thermal decomposition of ammonium perchlorate over (2−1−10) facets of ZnO nanosheets: surface lattice oxygen

ZnO nanocrystals as catalysts have been widely employed in the catalytic thermal decomposition of ammonium perchlorate (AP). However, the catalytic mechanism is still controversial and the role of surface lattice oxygen is always ignored. Herein, a classical catalytic mechanism based on the surface lattice oxygen was proposed to reveal AP decomposition promoted by ZnO nanosheets. ZnO and ZnS nanosheets, both of which have the same wurtzite structure and the same (2−1−10) exposed facets, have been synthesized by the calcination of a ZnS(en)0.5 precursor in different conditions. ZnO nanosheets with a smaller surface area showed a better catalytic activity than ZnS nanosheets because the surface lattice oxygen of the ZnO nanosheets can react with NH3 (an intermediate of AP thermal decomposition) to generate oxygen vacancies that can subsequently be recovered, while the surface lattice sulfur of the ZnS nanosheets did not react with NH3. The generation and replenishment of oxygen vacancies on the (2−1−10) exposed facets of the ZnO nanosheets were confirmed by XPS and FTIR results, and thus revealed the origin of the efficient catalytic AP decomposition over (2−1−10) facets of ZnO nanosheets as surface lattice oxygen. Therefore, this work could provide a new insight into the catalytic mechanism of metal oxides to promote AP decomposition.

Low-temperature solution synthesis of a ZnO nanorod array with a mesoporous surface mediated by cadmium ions

A ZnO nanorod (NR) array grown on a substrate by a wet chemical method usually exhibits a single crystalline nature and smooth surface which cannot adsorb a large amount of dye molecules. Thus, surface modification is necessary for a ZnO NR array in its photocatalysis application. In this work, a mesoporous structure has been introduced into the surface of a ZnO NR array via a facile chemical bath method mediated by cadmium (Cd) ions at 80 °C and subsequent calcination. During the growth process of the ZnO NR array, Cd ions can react with NH3 molecules and OH− ions in the growth solution to form Cd complexes and prefer to adsorb on the neutral {10-10} surface of ZnO NRs rather than on the positive (0001) facets due to face-selective electrostatic interactions. This will probably cause the formation of Zn(C2H3O2)2(C2H8N2) which can be decomposed into ZnO nanoparticles after calcination, leading to the production of mesopores on the {10-10} surface of ZnO NRs. Also, the pore density and aspect ratio of the ZnO NR array with mesoporous surface are controlled by adjusting the Cd ion concentration and reaction temperature. The mesoporous ZnO NR array exhibits enhanced photocatalytic properties for the degradation of a methylene blue (MB) aqueous solution compared with the pristine ZnO NR array. Therefore, our finding indicates that this method may be employed to synthesize porous nanostructures on devices.

Mesoporous ZnO nanorods array with a controllable area density for enhanced photocatalytic properties

Usually, a ZnO nanorods array exhibits a relatively small effective surface area due to its smooth surface and large area density (the number of ZnO nanorods per unit area). In this work, a mesoporous ZnO nanorods array with a small area density and a large effective surface area was successfully synthesized on the surface of fluorine-doped tin dioxide (FTO) glass using a facile solution process, with ethylene glycol (EG) and water serving as the mixed solvent and cadmium ions serving as an additives. The area density, aspect ratio and specific surface area of mesoporous ZnO nanorods array can be controlled by adjusting the concentration of cadmium ions in the EG-H2O mixed solution. The obtained ZnO nanorods array was applied as the photocatalyst for the photodegradation of methylene blue (MB) and showed a good catalytic performance that was dependent on the area density, rather than the specific surface area. This may be because a smaller area density of nanorods array can facilitate the diffusion of MB molecules and thus provide a larger effective surface area for MB adsorption, despite a large difference in their specific surface area. Therefore, this work can provide a guidance for synthesizing nanostructures with good photocatalytic activity on the devices.

Deep-red emitting zinc and aluminium co-doped copper indium sulfide quantum dots for luminescent solar concentrators

Pure CuInS2 quantum dots (CIS QDs) exhibited poor photoluminescent (PL) performance due to more dangling bonds on their surface and thus needed a shell to form core@shell structured QDs. However, conventional shells had a higher band gap than CIS QDs so that the core@shell structured QDs showed a blue-shifted PL peak below 700 nm and narrower light absorption range. Herein, Zn and Al co-doped CIS QDs were synthesized by a facile cation exchanged method which can significantly improve the PL property emitting at ∼740 nm compared to pure CIS QDs. The enhancement of the PL property was probably attributed to ZnCu and AlCu defects in the CIS QDs which could cause electron-hole radiative recombination via defect levels, reduce the number of dangling bonds and thus minimize the nonradiative recombination through surface electron trapping. Furthermore, the PL intensity was controlled by doping time and reached the maximum at 30 min. The obtained deep-red emitting CIS QDs were employed to fabricate semi-transparent luminescent solar concentrators (LSCs) which exhibited an excellent optical efficiency of 6.97%. The success in using such Zn and Al co-doped CIS QDs could pave the way to realize high efficiency and environment-friendly LSCs for building integrated photovoltaics.