In this study, the Yb1–xCaxFeO3 (0≤x≤0.3) nanocrystalline powders were prepared by sol-gel method. We used the method of quantitative analysis to research the gas-sensitive properties for Yb1–xCaxFeO3 to CO2. Also, we investigated the effects of various factors on gas sensing properties by simple variable method. The doping of Ca could not only decrease the resistance of YbFeO3, but also enhance its sensitivity to CO2. When the Ca content x=0.2, Yb1–xCaxFeO3 showed the best response to CO2. The response Rg/Ra to 5000 ppm CO2 for Yb0.8Ca0.2FeO3 at its optimal temperature of 260 °C with the room temperature humidity of 28%RH was 1.85. The response and recovery time decreased with an increase of the operating temperature for Yb0.8Ca0.2FeO3 sensor to 5000 ppm CO2. Furthermore, with an increase of CO2 concentration from 1000 to 50000 ppm, the response time of Yb0.8Ca0.2FeO3 became shorter, and meanwhile the recovery time was longer. CO2-sensing response for Yb0.8Ca0.2FeO3 increased with the increase of relative humidity. The response for Yb0.8Ca0.2FeO3 in the background of air (with the room temperature humidity of 39%RH) at 260 °C could reach 2.012 to 5000 ppm CO2, which was larger than the corresponding value (1.16) in dry air.Dynamic gas-sensing characteristics of Yb1–xCaxFeO3 (x=0, 0.1, 0.2 and 0.3) sensors to 1000, 5000 and 10000 ppm CO2 at 260 °C in the background of ambient air (with the room temperature humidity of 28%RH)Download high-res image (96KB)Download full-size image

The sensing response S of SnO2 thick-film sensor based on nano-powders annealed at 600 °C to 2000 ppm CO2 at the operating temperature 240 °C can reach 1.24 for the background of wet air with 14% of room-temperature (23 °C) relative humidity (RH), which is much larger than the corresponding value (1.048) in dry air. With an increase of humidity from 14% to 66% RH, the sensing response of SnO2 to low CO2 concentrations such as 2000–4000 ppm initially increases, peaks at about 34% RH, and then decreases. CO2-sensing response monotonically decreases with an increase of humidity at high CO2 concentrations such as 6000 and 20,000 ppm. Density functional theory calculations show that CO2 molecule cannot be adsorbed onto stoichiometric SnO2 (1 1 0) surface or SnO2 (1 1 0) surface pre-adsorbed by O2− and O− in dry air. When CO2 is introduced to SnO2 surface in wet air, CO2 reacts with O of pre-adsorbed OH−, bringing about the formation of carbonates containing (CO3)2− and the dissociation/movement of surface OH− group, accompanying the releasing of electron from CO2 to SnO2 surface. The appropriate pre-adsorption of OH− on SnO2 (1 1 0) surface promotes the sensing response of CO2.

LaFeO3 nanocrystalline powders prepared by sol–gel method with annealing at 800 °C for 4 h can sensitively detect low concentration of acetone. When exposed to 0.5 ppm acetone, the response of LaFeO3 thick film at 260 °C is 2.068 with response time of 62 s and recovery time of 107 s, respectively. The possible acetone sensing mechanisms for LaFeO3 sensor are investigated with first principles calculations. Calculated results demonstrate that acetone could release electrons to the surface of LaFeO3 (010) pre-adsorbed with oxygen species O−O− and O2−. The acetone molecule reacts with oxygen species in the following ways: (1) adsorbs on oxygen species O−O− or (2) replaces of the weakly pre-adsorbed oxygen species O2− on Fe site, accompanying the formation of oxygen molecule. These above two processes may play important roles in acetone sensing for LaFeO3. We also find that the acetone molecule can be directly adsorbed on Fe site, transferring some electrons to the LaFeO3 (010) surface. The latter processes may also provide additional contribution to acetone sensing.

Experimental results show that with an increase of relative humidity, the resistance of La0.875Ca0.125FeO3 decreases at room temperature but increases at higher temperatures (140–360 °C). The humid effect at room temperature is due to the movement of H+ or H3O+ inside of the condensed water layer on the surface of La0.875Ca0.125FeO3. Regarding the humid effect at high temperatures, the density functional theory (DFT) calculations show that H2O can be adsorbed onto the La0.875Ca0.125FeO3 surface in the molecular and dissociative adsorption configurations, where the La0.875Ca0.125FeO3 surface gains some electrons from H2O or its dissociative products, consistent with our observation. Experimental results also show that CO2 sensing response at high temperatures decreases with an increase of room-temperature relative humidity. DFT calculations indicate that CO2 adsorbed onto the La0.875Ca0.125FeO3(010) surface, where high concentration oxygen adsorption occurs without water adsorption nearby, releases some electrons into the semiconductor surface, playing the role of a donor. The interaction between CO2 and the local La0.875Ca0.125FeO3(010) surface with pre-adsorption of H2O nearby results in some electron transfer from the La0.875Ca0.125FeO3 surface to CO2, which is responsible for the weakening of CO2 response at high temperatures for La0.875Ca0.125FeO3 with an increase of room-temperature relative humidity.

We compared the CO2 sensing properties of La1−xBaxFeO3 packed powder and thick film sensors. The thick film sensor exhibits a larger response to CO2 gas with lower working temperature and shorter response and recovery times than the packed powder sensor because the former exposes a larger surface, absorbs the oxygen and interacts with CO2 gas more easily than the latter. However, the resistance of the thick film sensor is much larger than the packed powder sensor. La0.8Ba0.2FeO3 sensor exhibits the largest CO2 response among all La1−xBaxFeO3 packed powder and thick film sensors investigated. The response for the La0.8Ba0.2FeO3 thick film sensor to 2000 ppm CO2 at 150 °C is 2.29 and the response and recovery times are 14 s and 8 s, respectively. In addition, the CO2 responses for these two types of La0.8Ba0.2FeO3 sensors decrease with increase in the humidity. The possible sensing mechanisms of La1−xBaxFeO3 sensors are discussed.

LaCrO3 nanocrystalline powders with perovskite structure were prepared by sol–gel method with annealing at 700 °C, 900 °C for 4 h, respectively. At room temperature, LaCrO3 nanocrystalline powders crystallizes as the orthorhombic structure. At high temperatures above the transition temperature from orthorhombic to rhombohedral, when exposed to CO2 the optimum operating temperature for sensing is 360 °C and the resistance of the rhombohedral LaCrO3 sensor decreases, which is different from other p-type semiconductors. First principles calculations (GGA + U) were performed to investigate the possible CO2 sensing mechanism for rhombohedral LaCrO3 sensor. The calculation results show that when exposed to CO2, the C atom of CO2 molecule chemically binds with the surface O atom of rhombohedral LaCrO3 (012) surface, meanwhile the two O atoms of CO2 molecule chemically bind with the adjacent surface Cr atoms on LaCrO3 (012) surface. For this case, the CO2 molecule captures electrons from the LaCrO3 (012) surface, which is consistent with the experimental observation.

Nanocrystalline (Na1−xKx)0.5Bi0.5TiO3 (x = 0.1, 0.16, 0.20, 0.25) plates exhibit ferromagnetism at room temperature. The reduction of ferromagnetism for the (Na1−xKx)0.5Bi0.5TiO3 plates over time and the high temperature air-annealing indicates that the observed ferromagnetism is connected with the cation vacancies at/near the surface of nanograins. The density functional theory calculation with the local density approximation plus on-site effect method on the magnetism of the (Na5/6K1/6)0.5Bi0.5TiO3 supercell shows that Na vacancies can introduce a nonzero magnetic moment. The (Na1−xKx)0.5Bi0.5TiO3 (x = 0.1, 0.16, 0.20, 0.25) plates when annealed at 900 °C for 1 hour present d0 multiferroicity with the coexistence of ferromagnetism and ferroelectricity at room-temperature. A room-temperature magnetodielectric effect is observed in (Na1−xKx)0.5Bi0.5TiO3 plates and the appropriate substitution of potassium can increase the magnetodielectric effect. In addition, electric field treatment leads to an enormous enhancement of saturation magnetization for (Na1−xKx)0.5Bi0.5TiO3 multiferroic plates, resulting in a strong magnetoelectric coupling.

We investigated the CO sensing mechanism of SnO2 (110) surface by density functional theory calculation. The CO sensing mechanism of SnO2 surface strongly depends upon the concentration of oxygen in the ambient atmosphere. For very high oxygen concentration where oxygen species O2– or O– are not adsorbed on the stoichiometric SnO2 (110) surface, there is the direct interaction between CO and the stoichiometric surface through the CO adsorption on Sn site or formation of CO2, accompanying the release of electrons to the surface. For the considerable high oxygen concentration, the oxygen species O2– and O– adsorbed on the oxygen-deficient SnO2 (110) surface grab electrons mainly from Sn atoms of SnO2 (110) surface. When SnO2 (110) surface is exposed to CO reducing gas, the interactions between CO and preadsorbed oxygen species (O2–, O–) as well as some lattice atoms at certain sites on SnO2 surface lead to the releasing of electrons back to semiconductor SnO2. At very low oxygen concentration, the structural reconstruction is induced by the direct interaction between CO and SnO2 subreduced surface with the removing of 2-fold-coordinated bridging oxygen rows, accompanying the electron transfer from CO to the surface without the formation of CO2 in the sensing response process.

•Nano-BiFeO3/paraffin composite shows a ferromagnetic resonance.•Nano-BiFeO3/paraffin composite shows a magneto-permittivity resonance.•Resonance of negative imaginary permeability in BiFeO3 is a sample-size resonance.•Nano-BiFeO3/paraffin composite with large thickness shows a sample-size resonance.In the present work, we demonstrate that ferromagnetic resonance and magneto-permittivity resonance can be observed in appropriate microwave frequencies at room temperature for multiferroic nano-BiFeO3/paraffin composite sample with an appropriate sample-thickness (such as 2 mm). Ferromagnetic resonance originates from the room-temperature weak ferromagnetism of nano-BiFeO3. The observed magneto-permittivity resonance in multiferroic nano-BiFeO3 is connected with the dynamic magnetoelectric coupling through Dzyaloshinskii–Moriya (DM) magnetoelectric interaction or the combination of magnetostriction and piezoelectric effects. In addition, we experimentally observed the resonance of negative imaginary permeability for nano BiFeO3/paraffin toroidal samples with longer sample thicknesses D=3.7 and 4.9 mm. Such resonance of negative imaginary permeability belongs to sample-size resonance.

Experimental results show that with an increase of relative humidity, the resistance of La0.875Ca0.125FeO3 decreases at room temperature but increases at higher temperatures (140–360 °C). The humid effect at room temperature is due to the movement of H+ or H3O+ inside of the condensed water layer on the surface of La0.875Ca0.125FeO3. Regarding the humid effect at high temperatures, the density functional theory (DFT) calculations show that H2O can be adsorbed onto the La0.875Ca0.125FeO3 surface in the molecular and dissociative adsorption configurations, where the La0.875Ca0.125FeO3 surface gains some electrons from H2O or its dissociative products, consistent with our observation. Experimental results also show that CO2 sensing response at high temperatures decreases with an increase of room-temperature relative humidity. DFT calculations indicate that CO2 adsorbed onto the La0.875Ca0.125FeO3(010) surface, where high concentration oxygen adsorption occurs without water adsorption nearby, releases some electrons into the semiconductor surface, playing the role of a donor. The interaction between CO2 and the local La0.875Ca0.125FeO3(010) surface with pre-adsorption of H2O nearby results in some electron transfer from the La0.875Ca0.125FeO3 surface to CO2, which is responsible for the weakening of CO2 response at high temperatures for La0.875Ca0.125FeO3 with an increase of room-temperature relative humidity.