[en] Porous TiO₂ nanotablets were fabricated by calcining the MIL-125 metal–organic framework (MOF) precursors formed by a solvothermal method. Sensing results indicated that this kind of MOF-derived TiO₂ nanotablets exhibited excellent ethanol sensing properties, including high response (46.12–500 ppm), relatively low operating temperature (250 °C), and low detection limit (0.417 ppm). The response of TiO₂ nanotablets to 500 ppm ethanol at 250 °C was about 4.18 times higher than that of commercial TiO₂ powder at the optimum operating temperature of 275 °C. Moreover, TiO₂ nanotablets also displayed high stability and ethanol selectivity. The special porous structure, high valence state of absorbed oxygen species, and formation of rutile–anatase n–n junctions can enhance the resistance modulation of MOF-based TiO₂ nanotablets, contributing to their excellent ethanol sensing properties.

[en] Porous ternary Zn₂SnO₄ nanofibers with a high surface-to-volume ratio were fabricated through an electrospinning technique. UV-activated ethanol sensing responses at low temperatures were revealed using these porous Zn₂SnO₄ nanofibers as a sensing active layer. The ethanol response was up to 32.5, and the calculated detection limit was as low as 1.6 ppm at a low temperature of 130 °C. The sensor exhibited good ethanol selectivity and stability under UV irradiation. The photoinduced electrons reacted with the absorbed oxygen molecules to form active O⁻ species [O⁻(hν)], which contributed to the enhanced resistance modulation and low-temperature ethanol response of Zn₂SnO₄ nanofibers.

[en] Highlights: • Porous ZnO–SnO₂–Zn₂SnO₄ nanofibers were constructed by electrospinning technique. • Multiple n−n heterojunctions were formed in the ZnO–SnO₂–Zn₂SnO₄ nanofibers. • Light absorption and charge separation rate were enhanced in heterojunction nanofibers. • ZnO–SnO₂–Zn₂SnO₄ nanofibers exhibited enhanced ethanol sensing properties. • The enhanced sensing mechanism for UV-activated ethanol response was proposed. -- Abstract: An electrospinning technique followed by subsequent heat treatment with a high ramp rate (10 °C/min) was developed to construct porous ZnO–SnO₂–Zn₂SnO₄ heterojunction nanofibers. Ethanol sensing results under UV light irradiation showed that the ZnO–SnO₂–Zn₂SnO₄ nanofibers exhibited much higher response and lower operating temperature in comparison with the pure Zn₂SnO₄ nanofibers prepared with a low ramp rate of 2 °C/min. The response of ZnO–SnO₂–Zn₂SnO₄ nanofibers to 200 ppm ethanol was up to 121.0 at 130 °C, which was 3.7 times higher than that of pure Zn₂SnO₄ nanofibers. The ethanol response of the ZnO–SnO₂–Zn₂SnO₄ sensor can still up to 6.0–200 ppm under a high relative humidity of 82% at 130 °C. In addition, ZnO–SnO₂–Zn₂SnO₄ heterojunction nanofibers also exhibited good ethanol reproducibility and selectivity. The enhanced sensing characteristics were mainly attributed to the synergetic effects of several factors including the formation of multiple n−n heterojunctions, increased oxygen species adsorption capacity, enhanced light absorption, reduced recombination rate of photoinduced electrons and holes, as well as increased specific surface area and porosity of ZnO–SnO₂–Zn₂SnO₄ nanofibers.

[en] Highlights: • A new kind of Pd decorated Bi_2WO_6 hierarchical microarchitecture was synthesized. • Pd nanoparticles remarkably improved the photocatalytic activity of Bi_2WO_6. • The photo-generated holes and ·O_2"− played a crucial role in the degradation of RhB. • The photocatalytic enhancement mechanism of the Pd-Bi_2WO_6 composites was proposed. - Abstract: A new kind of hierarchical Pd-Bi_2WO_6 architecture decorated with different molar ratios of Pd to Bi, has been fabricated by a hydrothermal process, followed by a chemical deposition method. The photocatalytic activities of the pure Bi_2WO_6 and Pd-Bi_2WO_6 nanocatalyst were examined in the degradation of Rhodamine B (RhB) dyes and phenol under visible light. The photocatalytic results showed that the Pd-Bi_2WO_6 nanocomposites possessed observably enhanced photocatalytic activities. Particularly, the 2.0% Pd loaded Bi_2WO_6 had the highest photocatalytic activity, exhibiting a nearly complete degradation of 30 mg/L RhB and 10 mg/L phenol within only 50 and 60 min, respectively. In addition, the trapping experiment results indicated that the photo-generated holes (h"+) and ·O_2"− played a crucial role in the degradation of RhB. According to the experimental results, the photocatalytic degradation mechanism of Pd-Bi_2WO_6 was also proposed. The enhanced photocatalytic activities were ascribed to the combined effects of the highly efficient separation of electrons and holes, improved visible light utilization and increased BET specific surface areas of the Pd-Bi_2WO_6 nanocomposites.

[en] Highlights: • Co/N-CNT/PCSx were fabricated by one-pot annealing of Co salt, melamine and PVP. • Co/N-CNT/PCSx possessed a Co/N-doped CNTs-grafted porous carbon sheets structure. • Co/N-CNT/PCS800 exhibited excellent electrocatalytic performance for ORR. -- Abstract: The development of high performance and low-cost non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) to replace platinum-based catalysts is significant in facilitating the commercialization of proton exchange membrance fuel cells (PEMFCs). In this work, a low-cost and effective approach is employed for producing Co/N-doped carbon nanotubes/porous carbon sheets (Co/N-CNT/PCS) catalysts by one-pot annealing of Co salt, melamine and polyvinylpyrrolidone (PVP). The prepared catalysts exhibit a N-doped carbon nanotubes-grafted porous carbon sheets structure, in which Co nanoparticles (Co NPs) encapsulated in bamboo-like CNTs. When compared with a commercial platinum carbon (Pt/C) catalyst in an alkaline solution, one such catalyst (Co/N-CNT/PCS800) displays comparable ORR electrocatalytic activity as well as improved long-team stability and methanol tolerance, arising from the combined effect of high specific surface area, large pore volume and the cooperation of pyridinic-N and graphitic-N. Moreover, given the scalability and economy of this synthesis, Co/N-CNT/PCS catalysts produced in this way are likely to emerge as a major contender as an ORR catalysts in PEMFCs.

[en] Highlights: • The ZnO{0001}/graphene composites are successfully prepared by changing the density of sodium citrate in the reaction. • The ZnO{0001}/graphene composites exhibit high selectivity and response to NO₂ gas at room temperature. • The ZnO{0001} surface shows the metallic property, promotes charge transfer at interface between ZnO and graphene and enhances NO₂ detection sensitivity. Metal oxides with high-energy facets usually have more active surface sites to facilitate gas adsorption. In this work, the ZnO/graphene composites are synthesized hydrothermally and the morphology of ZnO nanoparticles can be changed from nanorods to nanoplates with the high-energy {0001} polar surface exposed by changing the density of sodium citrate in the reaction. The ZnO{0001}/graphene composites have better sensing properties to NO₂ at room temperature (RT). Based on first-principles calculation, the adsorption energy of NO₂ on the ZnO{0001} facet is more negative than that on the {01$\overline{1}$0} surface indicating stronger interactions between the former and NO₂. Furthermore, the ZnO{01$\overline{1}$0} surface shows semiconducting characteristics, whereas the ZnO{0001} polar surface is metallic. The metallic characteristics promote charge transfer at the interface between ZnO and graphene and enhance the NO₂ detection sensitivity.