RESUMO
Si-doped WO3 films were sputtered at room temperature and then annealed in air at 500 °C. The Si doping resulted in structural distortion from space group P21/n to Pc. A high density of pores with a diameter of â¼20 nm was observed in the films, which is ideal for gas sensing applications because of the easy diffusion of gas. Isoprene sensitivity, which is defined as the resistance ratio measured in pure air and in air containing 5 ppm isoprene, was greatly improved by the Si doping. The films containing 6.3 at% Si showed the highest sensitivity of 7.7 at a working temperature of 325 °C. However, despite a lower sensitivity of 6.9 measured at 350 °C, the films exhibited better gas selectivity for isoprene over a range of reference gases, including methanol, ethanol, acetone, CO and CO2. The response and recovery times of the films were very short, being less than 1.5 and 3.0 seconds, respectively. Detailed characterization with a range of techniques verified that the increase in gas sensitivity in the Si-doped films was related to better oxygen adsorbability as a consequence of an increase in positively-charged oxygen vacancies introduced by the aliovalent substitution of W6+ by Si4+.
RESUMO
BiFe1-2xMnxMgxO3 (BFMM, x = 0-8%) was mixed with exfoliated g-C3N4 (GCN) to form a composite for establishing an S-scheme heterojunction for photodegradation. BFMM was synthesized by sol-gel method, and showed a decreased band gap from 2.24 eV to 1.75 eV as x increased from 0% to 7%, allowing a more efficient absorption of sunlight. GCN was prepared by thermal polymerization of melamine and then exfoliated to form nanosheets by sulfur acid in order to increase the specific surface area and thus increase reaction sites. A composite with a weight ratio of BFMM/GCN equal to 1 : 3 was prepared by sintering the powder mixture at 300 °C. Such a composite showed a greatly improved efficiency in photodegradation of methylene blue, which was over 6 times faster than pristine BiFeO3, and the Mn/Mg co-doping improved the efficiency by 48%. The Mott-Schottky plots showed that both GCN and BFMM are n-type semiconductors with flat-band potentials of -0.79 and +0.11 V (vs. NHE), respectively. So, the band alignment allowed the S-scheme to work, leading to an efficient separation of photogenerated electrons and holes, which was confirmed by the greatly increased photocurrents measured with the composites.
RESUMO
Eu1-xBaxTi1-yMyO3 (M = Co or Ni) was sintered at 1400 °C under a reduction atmosphere. X-ray photoelectron spectroscopy revealed the mixed valences of Eu2+/Eu3+ and Ti4+/Ti3+ in EuTiO3 and Eu0.7Ba0.3TiO3, as well as some oxygen vacancies required to keep the charge neutrality. The co-doping of Co2+/Ni2+ in Eu0.7Ba0.3TiO3 resulted in the disappearance of oxygen vacancies, as a result of a reduction in Ti3+ numbers and an increase in Eu3+ numbers. On the other hand, Ba2+ doping led to an increased lattice parameter due to its larger ionic size than Eu2+, whereas the Co2+/Ni2+ co-doping resulted in smaller lattice parameters because of the combined effects of ionic size and variation in the oxygen-vacancy numbers. Eu0.7Ba0.3TiO3 exhibited a clear ferroelectricity, which persisted in the Co2+/Ni2+ co-doped samples until the doping levels of y = 0.05 and 0.10, respectively. Eu0.7Ba0.3TiO3 remained to be antiferromagnetic with a reduced transition temperature of 3.1 K, but co-doping of Co2+/Ni2+ turned the samples from antiferromagnetic to ferromagnetic with transition temperatures of 2.98 K and 2.72 K, respectively. The cause for such a transition could not be explained by the larger lattice volume, oxygen vacancies and mixed valences of Eu2+/Eu3+, which were proposed in previous works. Instead, it was more likely to arise from a large asymmetric distortion of the Eu-O polyhedron introduced by the aliovalent doping, which promotes the admixture of Eu 5d and 4f states.
