Formaldehyde is a common air pollutant. In this paper, a formaldehyde sensor with a high response is fabricated based on Sn-doped three dimensionally ordered macroporous (3DOM) NiO. The sensing performances of Sn-doped 3DOM NiO for formaldehyde are investigated and the possible sensing mechanism is proposed. The results indicate that Sn doping can effectively improve the gas sensing properties of NiO. 10 at% Sn-doped 3DOM NiO exhibits the highest response (∼145) to 100 ppm formaldehyde at 225 °C, which is nearly 85 times higher than that of the pure one. With the increase of the doping concentration, the surface area of the samples increases gradually, which can provide more active sites for gas adsorption and enhance the sensing response. Moreover, the significant improvement of the response to formaldehyde can be explained by the electronic compensation mechanism. Free electrons are generated to compensate for the substitution of Sn4+ into Ni2+ sites, which decreases the hole concentration in NiO. In addition, the free electrons contribute to the formation of adsorbed oxygen, which is beneficial for the improvement of the formaldehyde sensing performance.

Novel alkali metal doped 3DOM WO3 materials were prepared using a simple colloidal crystal template method. Raman, XRD, SEM, TEM, XPS, PL, Hall and UV-Vis techniques were used to characterize the structural and electronic properties of all the products, while the corresponding sensing performances targeting ppb level NO2 were determined at different working temperatures. For the overall goal of structural and electronic engineering, the co-effect of structural and electronic properties on the improved NO2 sensing performance of alkali metal doped 3DOM WO3 was studied. The test results showed that the gas sensing properties of 3DOM WO3/Li improved the most, with the fast response–recovery time and excellent selectivity. More importantly, the response of 3DOM WO3/Li to 500 ppb NO2 was up to 55 at room temperature (25 °C). The especially high response to ppb level NO2 at room temperature (25 °C) in this work has a very important practical significance. The best sensing performance of 3DOM WO3/Li could be ascribed to the most structure defects and the highest carrier mobility. And the possible gas sensing mechanism based on the model of the depletion layer was proposed to demonstrate that both structural and electronic properties are responsible for the NO2 sensing behavior.

Yolk–shell Au/CeO2 (Y-Au/CeO2) and encapsulated Au/CeO2 (E-Au/CeO2) nanocatalysts were prepared by using silica templates. A strong metal–support interaction (SMSI) in the Au/CeO2 nanostructures induced by different pretreatment atmospheres and its influence on CO oxidation were studied. E-Au/CeO2 pretreated in O2 had the best performance, followed by Y-Au/CeO2 pretreated in O2, Y-Au/CeO2 pretreated in H2, and E-Au/CeO2 pretreated in H2. The reasons for the different activities were discussed. There were two kinds of strong metal–support interactions (SMSI) between Au and CeO2 termed as R-SMSI (pretreated in reductive atmosphere) and O-SMSI (pretreated in oxidation atmosphere). Because of the smaller size of the Au and the larger contact area, both the R-SMSI and O-SMSI of E-Au/CeO2 were larger than those of Y-Au/CeO2. The O-SMSI was accompanied by the formation of cationic Au species that were beneficial to the enhancing of activity. As expected, the activity of E-Au/CeO2 pretreated in O2 with a Au size less than 5 nm was higher than that of Y-Au/CeO2 pretreated in O2 with 25 nm Au. However, it is surprisingly found that the activity of Y-Au/CeO2 pretreated in H2 with 25 nm Au was higher than that of E-Au/CeO2 pretreated in H2 with a Au size less than 5 nm. R-SMSI resulted in the formation of a AuCe alloy that had a negative effect on the activity. Compared with E-Au/CeO2 pretreated in H2, Y-Au/CeO2 pretreated in H2 exhibited a smaller relative content of the AuCe alloy, leading to a better activity of Y-Au/CeO2 pretreated in H2.

ZnO is an important n-type semiconductor sensing material. Currently, much attention has been attracted to finding an effective method to prepare ZnO nanomaterials with high sensing sensitivity and excellent selectivity. A three-dimensionally ordered macroporous (3DOM) ZnO nanostructure with a large surface area is beneficial to gas and electron transfer, which can enhance the gas sensitivity of ZnO. Indium (In) doping is an effective way to improve the sensing properties of ZnO. In this paper, In-doped 3DOM ZnO with enhanced sensitivity and selectivity has been synthesized by using a colloidal crystal templating method. The 3DOM ZnO with 5 at. % of In-doping exhibits the highest sensitivity (∼88) to 100 ppm ethanol at 250 °C, which is approximately 3 times higher than that of pure 3DOM ZnO. The huge improvement to the sensitivity to ethanol was attributed to the increase in the surface area and the electron carrier concentration. The doping by In introduces more electrons into the matrix, which is helpful for increasing the amount of adsorbed oxygen, leading to high sensitivity. The In-doped 3DOM ZnO is a promising material for a new type of ethanol sensor.Keywords: ethanol; indium; macroporous; sensor; ZnO

ZnO nanopyramids (NPys) with exposed crystal facets of {101̅1} were synthesized via a one-step solvothermal method, having a uniform size with a hexagonal edge length of ∼100 nm and a height of ∼200 nm. Technologies of XRD, TEM, HRTEM, Raman, PL, and XPS were used to characterize the morphological and structural properties of the products, while the corresponding gas sensing properties were determined by using ethanol as the target gas. For the overall goal of defect engineering, the effect of aging temperature on the gas sensing performance of the ZnO NPys was studied. The test results showed that, at the aging temperature of 300 °C, the gas sensing property has been improved to the best, with the fast response-recovery time and the excellent selectivity, because the ZnO300 has the most electron donors for absorbing the largest content of O2–. Model of defect redistribution was used to explicate the changing of the surface defects at different aging temperatures. The findings showed that, in addition to VO, Zni was the dominant defect of the {101̅1} crystal facet. The gas sensing performance of the ZnO NPys was determined by the contents of VO and Zni, with all of the defects redistributed on the surface. All of the results will be noticeable for the improvement of the sensing performance of materials with special crystal facet exposing.Keywords: crystal facet; gas sensing; nanopyramid; surface defect; ZnO

It was found that a modified natural sepiolite material could tremendously catalyze the chemiluminescence (CL) emission of the luminol–H2O2 system. A variety of characterization methods, including FTIR, XRD, SEM, BET and CL spectroscopy, were utilized to investigate the possible CL enhancement mechanism. Moreover, a modified natural sepiolite was used for the luminol-driven CL detection of H2O2. Under the optimum conditions, the CL intensity was proportional to the concentration of H2O2 in the range from 0.01 to 8 μM. The detection limit (S/N = 3) was 8.8 nM and the relative standard deviation (RSD) for twelve repeated measurements of 1.0 μM H2O2 was 2.5%. The proposed method was also successfully applied to detect H2O2 in tap and rain water samples with recoveries of 98–104%. Thus, this method can be used as a sensitive detection tool for the analysis of H2O2.

Au-based nanocatalysts are usually capped with surfactant and cannot be directly used. Thermal annealing is an effective method for surface cleaning. However, the effect of Au species on activity in the thermal annealing process has not previously been researched. We studied the effects of surfactant and Au species on catalytic performance using Au@porous SiO2 (Au@pSiO2). It was found that Au@pSiO2 annealed at different temperatures showed different performances toward the reduction of 4-nitrophenol even though the size of Au was maintained. The activity of the annealed Au@pSiO2 was higher than that of the untreated sample. The sample annealed at 500 °C had the best performance, and the catalytic activity was higher than that of the Au-based catalysts reported in the literature. It was concluded that the cationic Au species and the surfactant poly(vinyl pyrrolidone) (PVP) had a combined effect on catalytic performance. The removal of the surfactant PVP from the surface of the Au NPs during the thermal annealing process enhanced the activity, and the cationic Au species played a vital role in catalytic performance. The results are important in relation to surface cleaning and the determination of the pre-treatment conditions of catalysts. Surprisingly, there was an induction period for the untreated Au@pSiO2 in the catalytic process because PVP blocked the adsorption and migration of 4-nitrophenol on the surface of the Au. The disappearance of the induction period for the annealed samples can be attributed to the removal of PVP.

Three-dimensionally ordered macroporous (3DOM) Pd–LaMnO3 self-regeneration catalysts were successfully prepared. After reduction treatment at 500 °C, the catalyst exhibited the best catalytic activity for methane combustion. The excellent catalytic performance was attributed to the ordered porous structure, the large surface area, and the strong interaction between segregated Pd and the LaMnO3 substrate.

Perovskite-type metal oxides have been regarded as promising materials for solar cells and catalysts. However, they suffer from a major challenge due to their low specific surface area. In this work, high specific surface area LaMO3 (M = Co, Mn) hollow spheres were synthesized by a hard template method. The influence of the reactant ratios on the properties of the products was investigated. The formation of La2O3, Co3O4 or MnO2 prevented the growth of LaMO3, which resulted in variations in the composition, morphology, specific surface area, surface chemistry and catalytic activity of the products. An acid washing process could remove La2O3 and Co3O4, which led to the enhancement of the specific surface area of LaCoO3. Due to the high reactant concentration and the slow heating rate, multishelled LaMnO3 hollow spheres with a high specific surface area of 42.6 m2 g−1 were formed, which showed the best catalytic activity in methane combustion.

•Au@CeO2 YSNs were prepared by selective etching approach.•Au@CeO2 YSNs had high catalytic stability and activity.•The activity could be tailored by Au size and porous structure.Au@CeO2 yolk–shell nanoparticles (YSNs) were fabricated by a selective etching method. The oxidation activity of CO on the YSNs was evaluated, and the experimental results showed that CO was completely oxidized below 160 °C. Moreover, the catalytic activity of Au@CeO2 YSNs could be tailored by tuning the porous structure, Au size and the distance between the Au core and the CeO2 shell. High surface area, small Au nanoparticles and big pore size could enhance the catalytic activity. Moreover, because the CeO2 shell could prevent the Au core from aggregating, the Au@CeO2 YSNs kept good catalytic stability.

Unique porous ZnO polygonal nanoflakes were synthesized by the microwave hydrothermal method. The structural properties of the products were investigated by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution TEM techniques. In situ diffuse reflectance infrared Fourier transform spectroscopy technique was employed to investigate the mechanism of NO2 sensing. Free nitrate ions, nitrate ions, and nitrite anions were the main adsorbed species. N2O was formed via NO– and N2O2– that were stemmed from NO. Comparative tests for gas sensing between gas sensors based on the as-prepared porous ZnO nanoflakes and purchased ZnO nanoparticles clearly showed that the former exhibited more excellent NO2 sensing performances. Photoluminescence and X-ray photoelectron spectroscopy spectra further proved that the intensities of donors (oxygen vacancy (VO) and/or zinc interstitial (Zni)) and surface oxygen species (O2– and O2), which were involved in the mechanism of gas sensing led to the different gas-sensing properties.

Three-dimensionally ordered macroporous (3DOM) Pd–LaMnO3 self-regeneration catalysts were successfully prepared. After reduction treatment at 500 °C, the catalyst exhibited the best catalytic activity for methane combustion. The excellent catalytic performance was attributed to the ordered porous structure, the large surface area, and the strong interaction between segregated Pd and the LaMnO3 substrate.

Au-based nanocatalysts are usually capped with surfactant and cannot be directly used. Thermal annealing is an effective method for surface cleaning. However, the effect of Au species on activity in the thermal annealing process has not previously been researched. We studied the effects of surfactant and Au species on catalytic performance using Au@porous SiO2 (Au@pSiO2). It was found that Au@pSiO2 annealed at different temperatures showed different performances toward the reduction of 4-nitrophenol even though the size of Au was maintained. The activity of the annealed Au@pSiO2 was higher than that of the untreated sample. The sample annealed at 500 °C had the best performance, and the catalytic activity was higher than that of the Au-based catalysts reported in the literature. It was concluded that the cationic Au species and the surfactant poly(vinyl pyrrolidone) (PVP) had a combined effect on catalytic performance. The removal of the surfactant PVP from the surface of the Au NPs during the thermal annealing process enhanced the activity, and the cationic Au species played a vital role in catalytic performance. The results are important in relation to surface cleaning and the determination of the pre-treatment conditions of catalysts. Surprisingly, there was an induction period for the untreated Au@pSiO2 in the catalytic process because PVP blocked the adsorption and migration of 4-nitrophenol on the surface of the Au. The disappearance of the induction period for the annealed samples can be attributed to the removal of PVP.

Formaldehyde is a common air pollutant. In this paper, a formaldehyde sensor with a high response is fabricated based on Sn-doped three dimensionally ordered macroporous (3DOM) NiO. The sensing performances of Sn-doped 3DOM NiO for formaldehyde are investigated and the possible sensing mechanism is proposed. The results indicate that Sn doping can effectively improve the gas sensing properties of NiO. 10 at% Sn-doped 3DOM NiO exhibits the highest response (∼145) to 100 ppm formaldehyde at 225 °C, which is nearly 85 times higher than that of the pure one. With the increase of the doping concentration, the surface area of the samples increases gradually, which can provide more active sites for gas adsorption and enhance the sensing response. Moreover, the significant improvement of the response to formaldehyde can be explained by the electronic compensation mechanism. Free electrons are generated to compensate for the substitution of Sn4+ into Ni2+ sites, which decreases the hole concentration in NiO. In addition, the free electrons contribute to the formation of adsorbed oxygen, which is beneficial for the improvement of the formaldehyde sensing performance.