Luís F. da Silva

A novel ozone gas sensor based on one-dimensional (1D) α-Ag2WO4 nanostructures

This paper reports on a new ozone gas sensor based on α-Ag₂WO₄ nanorod-like structures. Electrical resistance measurements proved the efficiency of α-Ag₂WO₄ nanorods, which rendered good sensitivity even for a low ozone concentration (80 ppb), a fast response and a short recovery time at 300 °C, demonstrating great potential for a variety of applications.

An easy method of preparing ozone gas sensors based on ZnO nanorods

One-dimensional (1D) ZnO nanorod-like structures were successfully grown via a hydrothermal method to be used as an ozone gas sensor. X-ray diffraction measurements (XRD) and field-emission scanning electron microscopy (FE-SEM) analysis revealed a preferential growth of the nanorod-like structures along the [002] direction. Electrical resistance measurements indicated a good sensitivity to different ozone concentrations (0.06–1.19 ppm) as well as long-term stability over a 6 month period at 250 °C. In addition, it was observed that the nanorods had a good sensitivity to ozone at room temperature when the sensor was exposed to UV illumination. This study provides an easy and efficient way to obtain 1-D ZnO nanorods exhibiting remarkable properties for applications as ozone gas sensing materials.

One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structures

Nanostructured semiconducting oxides have been used as resistive gas sensors of toxic and non-toxic gases, but little emphasis has been placed on ozone sensing. Here we present a new ozone gas sensor based on hierarchical NiCo2O4 cubic structures synthesized via a facile urea-assisted co-precipitation method and annealed at 450 °C, which showed a low detection level. Ozone detection was carried out through electrical measurements with an optimized performance at 200 °C, with fast response (∼32 s) and recovery (∼60 s) time with suitable concentration range (from 28 to 165 ppb) for technological applications. Furthermore, NiCO2O4 platelets are selective to ozone compared to other oxidizing and reducing gases. The low detection level can be attributed to the coexistence of 3D structures based on hexagonal platelet-like and porous flower-like shape, which were revealed by field emission scanning electron microscopy (FE-SEM). In summary, NiCo2O4 is promising for detection of sub-ppb levels of ozone gas.

Hierarchical growth of ZnO nanorods over SnO2 seed layer: insights into electronic properties from photocatalytic activity

The use of nanostructured heterojunctions has been a promising option for hindering the charge recombination and thus enhancing the photocatalytic performance of catalysts. Here we present a simple strategy to hierarchically grow heterostructures using a hydrothermal treatment route. A buffer SnO2 film was produced by a sol–gel derived method, resulting in a film of approximately 100 nm composed of 5–10 nm nanoparticles. X-ray diffraction and scanning electron microscopy revealed preferential growth of the nanorod-like structures along the c-axis perpendicular to the SnO2 film, with an average nanorod diameter and length of approximately 160 nm and 1.5 μm, respectively. The photoluminescence spectra of ZnO–SnO2 revealed a reduction in UV emission compared to individual ZnO nanorods, indicating that the recombination of the photogenerated carriers was inhibited in the heterojunction. This behavior was confirmed by evaluating the photocatalytic performance of such films against methylene blue degradation, showing that the as-prepared ZnO–SnO2 heterojunction was superior to the individual semiconductors, ZnO and SnO2.

An Understanding of the Photocatalytic Properties and Pollutant Degradation Mechanism of SrTiO3 Nanoparticles

Strontium titanate nanoparticles have attracted much attention due to their physical and chemical properties, especially as photocatalysts under ultraviolet irradiation. In this paper, we analyze the effect of heating rate during the crystallization process of SrTiO3 nanoparticles in the degradation of organic pollutants. The relationship between structural, morphological and photocatalytic properties of the SrTiO3 nanoparticles was investigated using different techniques. Transmission electron microscopy and N2 adsorption results show that particle size and surface properties are tuned by the heating rate of the SrTiO3 crystallization process. The SrTiO3 nanoparticles showed good photoactivity for the degradation of methylene blue, rhodamine B and methyl orange dyes, driven by a nonselective process. The SrTiO3 sample with the largest particle size exhibited higher photoactivity per unit area, independent of the molecule to be degraded. The results pointed out that the photodegradation of methylene blue dye catalyzed by SrTiO3 is caused by the action of valence band holes (direct pathway), and the indirect mechanism has a negligible effect, i.e. degradation by O2−• and •OH radicals attack.

An investigation into the influence of zinc precursor on the microstructural, photoluminescence, and gas-sensing properties of ZnO nanoparticles

This paper describes the effect of different zinc precursors, acetate, oxide, and nitrate, on the structure, microstructure, photoluminescence, and ozone gas-sensing properties of zinc oxide (ZnO) nanoparticles. Transmission and scanning electron microscopy, and BET surface area show a dependence of the particle size and surface area with the precursor type. The ZnO sample synthesized from zinc nitrate shows the best photoluminescence (PL) emission. Although electron paramagnetic resonance shows in all samples the presence of a g-signal attributed to oxygen vacancies, it is not possible to correlate the presence of these defects with PL emission behavior. Furthermore, ZnO sample synthesized from zinc nitrate also shows the best ozone gas-sensing response, however, our results do not allow correlating the best PL emission in the visible region with the best sensor response to ozone gas.

Fingerprints of short-range and long-range structure in BaZr1−xHfxO3 solid solutions: an experimental and theoretical study

A microwave-assisted hydrothermal method was applied to synthesize BaZr1-xHfxO3, (BZHO) solid solutions at a low temperature, 140 °C, and relatively short times, 160 min. The detailed features of the crystal structure, at both short and long ranges, as well as the crystal chemistry doping process, are extensively analysed. X-ray diffraction measurements and Raman spectroscopy have been used to confirm that pure and Hf-doped BZO materials present a cubic structure. Extended X-ray absorption fine structure (EXAFS) spectra indicate that Hf(4+) ions have replaced the Zr(4+) ions on the 6-fold coordination and a subsequent change on the Ba(2+) 12-fold coordination can be sensed. X-ray absorption near-edge structure (XANES) spectroscopy measurements reveal a local symmetry breaking process, associated to overlap of the 4d-2p and 5d-2p orbitals of Zr-O and Hf-O bonds, respectively. Field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HRTEM) show the mesocrystalline nature of self-assembled BZHO nanoparticles under a dodecahedron shape. In addition first principle calculations were performed to complement the experimental data. The analysis of the band structures and density of states of the undoped BZO and doped BZHO host lattice allow deep insight into the main electronic features. The theoretical results help us to find a correlation between simulated and experimental Raman modes and allow a more substantial interpretation of crystal structure.

Ozone sensing properties of nickel phthalocyanine:ZnO nanorod heterostructures

We report on the chemiresistive gas sensing characteristics of ZnO nanorods (NRs) modified by a thin layer of nickel phthalocyanine (NiPc). Ozone detection was carried out through electrical measurements with an optimized performance at 250°C, good reproducibility and suitable concentration range (from 80 to 890 ppb) for technological applications. The hybrid NiPc:ZnO films had superior performance to pure ZnO nanorods in terms of response time and sensitivity. The response times were 22 s and 26 s, respectively, whereas the ratio of resistances under ozone and air was 3.27 for NiPc:ZnO films and 2.56 for the pure ZnO NRs. The improvement in response time is attributed to the large surface area generated with the coating of the ZnO nanorods with the NiPc layer. Significantly, images taken with field-emission scanning electron microscopy (FE-SEM) indicated that the ZnO nanorods were fully covered with NiPc. X-ray diffraction measurements (XRD) revealed a preferential growth of the nanorod-like structures along the [100] direction. In summary, a successful approach has been developed to functionalize ZnO nanorods, which is promising for detection of ppb levels of ozone gas.