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ZnCo2O4 porous microspheres were successfully synthesized through a facile one-step solvent method using polyethylene glycol 1000 as a self-assembly template and subsequent annealing. Ag particles were then grown on their surface via a green and facile precipitation process. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The gas sensing properties of ZnCo2O4 and Ag–ZnCo2O4 were investigated at their optimum operating temperatures (99.3 and 119.1 °C), respectively. The results show that the response of Ag–ZnCo2O4 (11.18) is much higher than that of pure ZnCo2O4 materials (4.5) to 100 ppm formaldehyde at their optimum operating temperature. Meanwhile, compared to ZnCo2O4, the Ag–ZnCo2O4-based gas sensor exhibits excellent selectivity to formaldehyde over acetone, xylene, toluene, carbinol, alcohol and benzene. The results indicate that the modification via Ag particles can improve the gas sensing properties of the ZnCo2O4 hollow microspheres.

Urchin-like hierarchical Zn doped CdS powders were successfully synthesized via simple one-pot hydrothermal process. Their SEM and TEM images indicated that the hierarchical structure were assembled by single crystal nanorods with the hexagonal wurtzite phase. EDS element mapping verified that Zn ions were homogeneously distributed among the hierarchical microstructure. The performances of gas sensors based on pure and Zn doped CdS were measured and compared. The results indicated that Zn doping could enhance their responses to some volatile organic compounds and improve its selectivity to ethanol and toluene as well. The possible reasons for this enhancement were investigated. In addition, the sensor based on Zn doped CdS exhibited the ultrafast response and recovery to ethanol ( τ response 1 , τ recovery = 8 s ), indicating that the Zn doped CdS could be a promising gas sensing candidate for online monitoring of ethanol.

The undoped and 1.0–5.0 mol% La–doped In2O3 hollow microspheres have been successfully synthesized via a simple hydrothermal method without template and gas sensor have been fabricated basing on them. The nanostructures and morphologies of the maintained hollow spheres were characterized by various experimental techniques. The gas sensing properties of these hollow microspheres were investigated systematically. The results indicated that among all the samples (pure, 1.0, 3.0 and 5.0 mol% La–doped In2O3), 3.0 mol% La–doped In2O3 exhibited the highest response toward 10 ppm hydrogen sulfide (H2S) at 200 °C, having a response value of 17.8, approximately 4.8 times higher than pure In2O3. Furthermore, excellent selectivity, good repeatability and outstanding long-term stability were also achieved. The significantly enhanced sensing properties to H2S could be attributed to the changes in distribution of different oxygen components, crystallite size and specific surface area caused by La doping.

Palladium-doped SnO2 nanomaterials, with palladium in fractions from 0 to 10 mol% were hydrothermally synthesized and characterized by XRD, FESEM, TEM, and XPS. Their gas sensing properties were studied in two temperature ranges of 75–95 °C and 160–210 °C. The sensor using 5 mol% Pd-doped SnO2 exhibits temperature-dependent sensing property. NO2 can be detected at 80 °C, while H2S is preferably detected at 180 °C. The response to 10 ppm H2S is 50 times higher than that of the undoped sample. Its detection limit is 500 ppb. For NO2, the sensor exhibited strong response and a lower detection limit of 20 ppb. In view of the selective detection of H2S and NO2 by regulating the temperature, palladium-doped SnO2 has great prospects in the detection of H2S and NO2.

An ultrafast-response yttria stabilized zirconia (YSZ)-based mixed potential type gas sensor utilizing molybdate CoMoO4 sensing electrode was developed to realize the effective detection of triethylamine (TEA) at 600 °C. The gas sensing characteristic of the fabricated sensor was optimized by changing the calcination temperature of CoMoO4 sensing material and results indicated that the device attached with CoMoO4-SE sintered at 1000 °C exhibited the highest response value (−102 mV) to 100 ppm TEA and low detection limit of 100 ppb at 600 °C. Interestingly, the response and recovery times of the present sensor to 100 ppm TEA were 1 s and 10 s, which signified the ultra-fast response rate. The response value of fabricated sensor displayed the piecewise linear function to logarithm of TEA concentrations and the sensitivities were −14 mV/decade (0.1–5 ppm) and −53 mV/decade (5–200 ppm), respectively. More importantly, the present sensor also exhibited good repeatability, selectivity, slight effect of humidity and long-term stability of 20 days, indicating a great candidate for use in detection of TEA. Furthermore, the device based on mixed potential mechanism was proposed and further verified using polarization curve.

Pure α-Fe2O3 nanotubes and a series of Al2O3/α-Fe2O3 composite nanofibers were synthesized via facile electrospinning and subsequent calcination at 500 °C in air for 2 h. The morphologies and nanostructures of these samples were characterized through various experimental techniques. It was found that the composite samples were composed of crystal α-Fe2O3 and amorphous Al2O3. The composite Al2O3/α-Fe2O3 nanofibers have smaller grain size than that of pure α-Fe2O3 nanotubes prepared by the same process. Furthermore, comparative gas sensing investigation between Al2O3/α-Fe2O3 composite nanofibers and pure α-Fe2O3 nanotubes was performed to show the superior sensing properties of the composite samples. When the mass ratio of Al source and Fe source was 1:4, the sensor exhibited the highest response to 100 ppm triethylamine at 250 °C. It is worth noting that the response times were no more than 2 s at operating temperatures ranged from 175 °C to 300 °C. Finally, the related mechanism of gas sensing and the role of Al2O3 in the composite nanofibers were discussed in this paper.

A facile hydrothermal route with the subsequent annealing and chemical etching process to the synthesis of yolk-shell LaFeO3 microspheres without any template is described. Transmission electron microscopy (TEM) images of the products obtained at different hydrothermal reaction time revealed the formation mechanism of yolk-shell LaFeO3 microspheres. Gas sensing devices were fabricated from the as-prepared samples, and their gas sensing properties were tested for response to various reducing gases. The sensor based on yolk-shell LaFeO3 microspheres exhibited high response and good selectivity toward acetone at 225 °C, giving a response of 25.5–100 ppm.

In this work, an ultrasensitive and low detection limit of toluene gas sensor based on SnO 2 -decorated NiO nanostructure synthesized via a simple hydrothermal route was described. The test results demonstrated that the sensor based on SnO 2 -decorated NiO nanostructure showed excellent sensitivity and selectivity toward toluene, giving a high response of 66.2–100 ppm, which was 50 times higher than that of the pure NiO nanospheres (1.3–100 ppm). Additionally, the sensor had surpassingly low detection limit (ppb-level), showing a response of 1.2–10 ppb toluene. Besides, the sensor also exhibited anti-humidity properties in some way, giving an acceptable response (11.1–10 ppm) to toluene at 90% relative humidity. The ultrasensitive characteristics and low detection limit could be explained from the variation of the carrier concentration caused by the change of the thickness of near-surface hole accumulation layer between p-type NiO and n-type SnO 2 . Furthermore, the increase of oxygen adsorption also improved the sensing performance.

Leaf-like SnO2 hierarchical architectures were successfully synthesized via a facile template-free hydrothermal synthesis method. The observations with field emission scanning electron microscopic and transmission electron microscopy demonstrated that the uniform and independent leaf-like SnO2 architectures were obtained, and each dendrite was consisted by a main stem with two groups of highly symmetric and parallel branch. To investigate the effect of diammonium phosphate and the formation process of hierarchical SnO2 leaf-like microstructure, the controlled experiments of the hydrothermal process with different amounts of diammonium phosphate and different reaction times have been carried out systematically. Based on the experimental results, a possible formation mechanism of SnO2 hierarchical architecture was speculated. In addition, the gas sensing performances of the as-prepared products were investigated and the results revealed that the sensor based on the unique leaf-like SnO2 hierarchical architectures exhibited high response and good selectivity properties to NO2 at low working temperature.

In this work, MoO3 microsheets were successfully prepared by thermally oxidizing the MoO2 nanospheres synthesized by a hydrothermal method. The crystal structures and morphologies of precursor MoO2 and the final product were characterized by XRD, FESEM, and TEM. Gas sensors based on MoO3 were fabricated and their gas sensing properties were tested. These results revealed that they exhibited excellent sensing performance towards triethylamine (TEA) with high selectivity, good repeatability, relatively high response and short response time (

Porous α-Fe 2 O 3 microflowers, which were composed of many nanospindles assembled by large numbers of nanoparticles, were successfully synthesized by calcining the FeSO 4 (OH) precursor prepared through a simple ethanol-mediated method. Various techniques were employed to obtain the crystalline and morphological properties of the as-prepared products. The formation process of such microstructure was proposed according to the morphology and component of the products obtained at different reaction time. Moreover, the obtained α-Fe 2 O 3 was utilized as sensing materials upon exposure to various test gases. As expected, in virtue of the less-agglomerated configuration and unique porous structure, the hierarchical α-Fe 2 O 3 microflowers exhibited higher response as well as faster response/recovery time to acetone when compared with α-Fe 2 O 3 nanoparticles. Significantly, the response time was measured to be 1 s at the low operating temperature of 210 °C.

A highly sensitive mixed potential type gas sensor based on stabilized zirconia (YSZ) and CdMoO4 sensing electrode (SE) was developed and used for detection of acetone at 625 °C. By comparing the sensing performance for different devices fabricated, the sensor utilizing CdMoO4-SE exhibited the highest response value (−133.5 mV) to 100 ppm acetone at 625 °C, and even could achieve low detection limit of 500 ppb at 625 °C. The sensor attached with CdMoO4-SE displayed high sensitivity of −84 mV/decade to acetone in the range of 5–300 ppm at 625 °C. The present device also showed good repeatability, selectivity to certain deleterious gases, moisture resistance and acceptable drifts in 10 days measured period at 625 °C, demonstrating great potential for practical application in acetone sensing detection. Additionally, the sensor involving mixed potential mechanism was proposed and further clarified by polarization curve.

Indium tungsten oxide ellipsoidal nanospheres were prepared with different In/W ratios by using a simple hydrothermal method without any surfactant for the first time. Sensors based on different In/W ratios samples were fabricated, and one of the samples exhibited better response to methanol compared with others. High content of defective oxygen (Ov) and proper output proportion of In to W might be the main reasons for the better gas sensing properties. The length of the nanosphere was about 150-200 nm, and the width was about 100 nm. Various techniques were applied to investigate the nanospheres. Sensing characteristics toward methanol were investigated. Significantly, the sensor exhibited ultrafast response to methanol. The response time to 400 ppm methanol was no more than 2 s and the recovery time was 9 s at 312 °C. Most importantly, the humidity almost had no effect on the response of the sensor fabricated here, which is hard to achieve in gas-sensing applications.

Mixed potential type solid-state electrochemical gas sensors using yttria- stabilized zirconia (YSZ) and columbite composite oxide sensing electrodes (SEs) prepared via a facile sol-gel method were developed for detection of sub-ppm acetone at 600 °C. In the comparison of different devices, the sensor attached with CdNb2O6-SE displayed the highest response of −25 mV to 5 ppm acetone and even could achieve low detection limit of 200 ppb at 600 °C. The fabricated sensor using CdNb2O6-SE exhibited rapid response and recovery times to 2 ppm acetone at 600 °C, which are 9 and 21 s, respectively. Additionally, the present device also showed good repeatability, selectivity, wet-resistance and stability in 30 day measured periods at 600 °C. Moreover, the sensing mechanism related to mixed potential was proposed and analyzed quantitatively and further demonstrated by polarization curve.

The NO2 sensing performance of a stabilized zirconia (YSZ)-based mixed potential type gas sensor utilizing a Co3V2O8 sensing electrode (SE) was improved by the addition of a noble metal. Among the different types of noble metal (Au, Pt, Pd and Rh), the sensor attached with Co3V2O8-SE loaded with Rh exhibited a noticeable improvement in NO2 response and the maximum response value was obtained when the loading mass fraction of Rh was 3 wt%. Results showed that the response for the sensor utilizing 3 wt% Rh/Co3V2O8-SE was 113.5 mV to 50 ppm NO2 and the sensitivity to 10–300 ppm NO2 was 85 mV per decade at the operating temperature of 650 °C, which were enhanced by 77.5 mV and 39 mV per decade compared to those of a sensor attached with Co3V2O8-SE, respectively. It is noteworthy that the response for each of sensor displayed a good linear relationship to the logarithm of NO2 concentration in the ranges of 10–300 ppm at 650 °C. Additionally, the sensor attached with 3 wt% Rh/Co3V2O8-SE also exhibited a low detection limit of 500 ppb and good selectivity to NO2 at 650 °C. The improvement of sensing characteristics for a sensor using 3 wt% Rh/Co3V2O8-SE may be attributed to enhanced electrochemical catalytic reaction activity to NO2 and a mixed potential mechanism was further verified by polarization curve.

Hollow sphere composites were synthesized by a template-free hydrothermal method from MoO3 and In2O3. The spheres have a typical size of 800 ± 50 nm and were characterized by XRD, FESEM, TEM, XPS. Gas sensors based on samples with different Mo/In composite ratios were fabricated and their gas sensing properties were studied. The results show that a Mo:In ratio of 1:1 in the composite gives the highest response, typically at a working temperature of 250 °C. The response increases to 38 when exposed to 100 ppm acetone at 250 °C. This is 13.6 times better than when using pure MoO3. The sensor shows improved selectivity, response, repeatability and long-term stability. Typical features include a large specific surface area, and high levels of chemisorbed oxygen and defective oxygen sites. The N-N heterojunction theory was used to explain the improvement of gas sensing performance.

Au-loaded mesoporous WO3 was synthesized by nanocasting method. Mesoporous structure provided a large specific surface area, and the noble metal (Au) improved the catalytic efficiency. The above two characteristics played crucial roles in enhancing the performance of the gas sensors. The as-synthesized materials were deduced by X-ray diffraction (XRD), Brunauer-Emmet-Teller (BET), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The obtained materials showed ordered mesoporous structure with excellent crystallinity and the pore size was about 10.8 nm which matched with Barrett-Joyner-Halenda analysis. Static test system was employed to measure volatile organic compounds (VOCs) (such as methanol, ethanol, isopropanol and n-butanol) sensing properties for the sensors using mesoporous WO3 and different weight ratios of Au-loaded (0.2%, 0.5% and 1.0%) mesoporous WO3. The sensors loaded with Au exhibited much higher sensitivity and selectivity to n-butanol in this work. Furthermore, Au-loaded materials showed lower operating temperature.

ZnO nanorod arrays decorated with NiO nanosheets on FTO substrates were prepared via a simple ultrasonic spray pyrolysis process combined with chemical bath method. The synthesized samples were characterized and analyzed by scanning electron microscopy and X-ray diffraction. The hierarchical and porous morphologies of ZnO/NiO core–shell nanoheterojunctions could be controlled by changing the growth time of NiO sheets. The “oriented attachment” and “self-assembly” crystal growth mechanisms were proposed to explain the formation of the ZnO/NiO nanostructures. Sensors based on the ZnO/NiO heterojunction nanostructure were fabricated and investigated for their ethanol-sensing properties. The result indicated that response was about 180% toward 100 ppm ethanol at an operating temperature of 200 °C. The growth approach in this work offers a new technique for the design and synthesis of transition metal oxide hierarchical nanoarrays which are promising for gas sensing applications.

NaGdF4:Yb, Er nanocrystals doped with Fe3+ were synthesized by a facile hydrothermal method. The upconversion emissions in a broad range from 200 to 700 nm were measured under 980 nm laser excitation. The luminescence intensity can be improved greatly by Fe3+ doping. In addition to the typical upconversion emission of Er3+ ions, the violet emission from Gd3+ is explained by the energy transfer from Er3+ to Gd3+. Powder X-ray diffraction patterns reveal that Fe3+ can enter crystal sites interstitially and substitutionally. The effects of Fe3+ doping on morphology, crystal growth and emission properties of particles are discussed in detailed. Furthermore, these nanocrystals exhibit paramagnetic properties and have the potential applications in bimodal bioimaging in virtue of their upconversion luminescent and paramagnetic properties.

The great demand for gas sensors in practical applications has stimulated tremendous attention in this area due to its important significance in real life. A facile synthesis of WO3 nanoplates and their subsequent Sn doping strategy by using a hydrothermal method was investigated to enhance gas sensing performance for NO2 gas, one of the gases toxic to human beings and the environment. Various techniques were used to characterize all the products. The morphology characterizations demonstrated that all the samples exhibited a similar nanoplate structure with or without Sn doping. The gas sensing properties of the sensors based on different doping concentrations (0, 1, 2 and 5wt%) have been systematically investigated. The sensor based on the 2wt% Sn-doped WO3 nanoplates showed the maximum response to NO2 (55-100ppb NO2). Furthermore, the introduction of Sn ions into the sensing materials of WO3 resulted in shorter response and recovery times. This finding could be attributed to the increased number of oxygen vacancies on the surface of the sensing material and the resistance of the gas sensors. The results provide a new doping strategy to fabricate high performance NO2 gas sensors.

MoO3 gas-sensing materials with different Ni2+ doping concentration were prepared by a simple one-step solvothermal method. The phase structure, micro-structure and morphology of the as-prepared samples were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscope (FESEM) and the transmission electron microscope (TEM) etc., respectively. The morphology can be changed by adjusting the doping concentration of Ni2+. Among them, the 5 mol% Ni-doped MoO3 sample with a pompon-like morphology improved gas accessibility significantly. Prominently, the response of the sensors on xylene was increased 18 times from 3.48 to 62.6, the response time was about 1 s and the best selectivity was obtained at the optimum temperature of 250 °C. These breakthroughs are attributed to the increase of surface-active sites and the improvement of micro-morphology caused by Ni2+ doping.

BiFe 0.95− x Mn 0.05 Zr x O 3 thin films were prepared on ITO/glass substrates using a sol–gel method. The P–E hysteresis loops before and after the high electric field treatment were measured to investigate the deaging effect of Zr 4+ ions on BiFe 0.95 Mn 0.05 O 3 thin films. It was found that the BiFe 0.95 Mn 0.05 O 3 thin film shows typical double P–E hysteresis loops before the cycling of high electric field, indicating that the film was aged. The double-loop feature fades away gradually with the increase of zirconium doping content from 1 to 3 mol%, demonstrating that Zr 4+ ions indeed have deaging effect on the aged BiFe 0.95 Mn 0.05 O 3 film. Such deaging effect of Zr 4+ ions can be further proved by comparing the asymmetry of the coercive field before and after the cycling of high electric field as well as the charge retaining capability.

The BiFe0.95Mn0.05O3 films were fabricated on Pt/Ti/SiO2/Si and Bi2O3/Pt/Ti/SiO2/Si substrates using a sol–gel process. Saturated square P–E hysteresis loops can be observed at frequencies ranging from 1 to 16.7 kHz for both films. The BiFe0.95Mn0.05O3 film on Bi2O3/Pt/Ti/SiO2/Si exhibits a larger 2Pr (∼156 μC/cm2), smaller 2Ec (∼510 kV/cm), more symmetric P–E loops, stronger charge retaining capability (the loss of ΔP is only 2% after 104 s) and fatigue resistance (no loss of ΔP is observed after 1010 switching cycles) compared to the film deposited directly on Pt/Ti/SiO2/Si. More importantly, the former can be uniformly polarized using a piezoelectric-mode atomic force microscopic system and exhibits a larger piezoelectric coefficient (∼64 pm/V). These results should be due to the Bi2O3 buffer layer, which can favor the grain growth and hence elimination of defect complexes formed between the negatively charged defects such as ( Mn Fe 3 + 2 + ) ′ or ( Fe Fe 3 + 2 + ) ′ and oxygen vacancies.

The Yb3+ and Tm3+ co-doped 55.98ZrF4-28BaF2-2.5LaF3-4AlF3-7NaF-2.5YbF3-0.02TmF3 amorphous fluoride film was prepared by pulsed laser deposition. The spectroscopic properties and energy transfer analysis of this film were studies in detail. Ultraviolet and visible upconversion emissions were observed under the infrared excitation at 980 nm. In comparison with that of its target, the upconversion emissions of the film in the visible and ultraviolet range were greatly enhanced. The possible energy transfer mechanism of the emissions was given to understand the upconversion process. This kind of thin films has potential applications for the integrated optical waveguide amplifier and ultraviolet laser.

Two crosslinkable second-order nonlinear optical polymers were prepared by copolymerization of 2,3,4,5,6-pentafluorostyrene, styrene (St), glycidyl methacrylate (GMA) and 1-(4-nitrophenyl)-2-(4-{[2-(methacryloyloxy) ethyl] ethylamino}-phenyl) diazene (DR1M) via the sealed-tube reaction technique. These polymers were characterized using 1H, 13C and 19F NMR spectroscopy, gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The crosslinkable polymers have high molecular weights, good organosolubility, excellent film-forming properties and high glass transition (106–110 °C) and thermal decomposition temperatures (290–350 °C) after being crosslinked. Furthermore, the polymer films possess not only high values (12–16 pm V−1) of electro-optical coefficient (r33) at 1.3 µm wavelength but also low optical loss (1.7 dB cm−1) at 1.55 µm wavelength, which is of interest for applications in electro-optical devices. Copyright © 2004 Society of Chemical Industry

Ca2Al2SiO7:Eu3+ was prepared by the sol-gel method. Through the emission spectrum of Eu3+ ion, the fluorescence parameters such as Omega(i) (i = 2,4) and radiative transition probabilities of D-5(0)-F-7(j) were calculated. The Pb2+ ion with bigger radius has an effect on the fluorescence spectra of Eu3+ which can be explained by the structure of the matrix. Simultaneously, the energy transfers between mercury-like ions (Pb2+ and Bi3+) and Eu3+ ion were observed. The D-5(4) and D-5(2) energy levels of Eu3+ are the resonance ones for Pb2+ ion.

A heterogenous core/shell strategy has been introduced to induce the growth of hexagonal phase NaREF4 shells. During the core-mediated hetero-shell growth process, the cores significantly affect the crystalline phase of shells. Using heterogeneous cubic nanocrystals as cores, the resulting sub-30 nm hybrid nanoparticles possessed both hexagonal phase shells and good water solubility.