The Role of Zn Ions in the Structural, Surface, and Gas-Sensing Properties of SnO2:Zn Nanocrystals Synthesized via a Microwave-Assisted Route.

Although semiconducting metal oxide (SMOx) nanoparticles (NPs) have attracted attention as sensing materials, the methodologies available to synthesize them with desirable properties are quite limited and/or often require relatively high energy consumption. Thus, we report herein the processing of Zn-doped SnO2 NPs via a microwave-assisted nonaqueous route at a relatively low temperature (160 °C) and with a short treatment time (20 min). In addition, the effects of adding Zn in the structural, electronic, and gas-sensing properties of SnO2 NPs were investigated. X-ray diffraction and high-resolution transmission electron microscopy analyses revealed the single-phase of rutile SnO2, with an average crystal size of 7 nm. X-ray absorption near edge spectroscopy measurements revealed the homogenous incorporation of Zn ions into the SnO2 network. Gas sensing tests showed that Zn-doped SnO2 NPs were highly sensitive to sub-ppm levels of NO2 gas at 150 °C, with good recovery and stability even under ambient moisture. We observed an increase in the response of the Zn-doped sample of up to 100 times compared to the pristine one. This enhancement in the gas-sensing performance was linked to the Zn ions that provided more surface oxygen defects acting as active sites for the NO2 adsorption on the sensing material.

Unveiling the efficiency of microwave-assisted hydrothermal treatment for the preparation of SrTiO3 mesocrystals.

Material processing has become essential for the proper control, tuning and consequent application of the properties of micro/nanoparticles. In this case, we report herein the capability of the microwave-assisted hydrothermal (MAH) method to prepare the SrTiO3 compound, as a case study of inorganic compounds. Analyses conducted by X-ray diffraction, X-ray photoelectron and X-ray absorption spectroscopies confirmed that the MAH route enables the formation of pristine SrTiO3. The results indicated that the combination of thermal and non-thermal effects during the MAH treatment provides ideal conditions for an efficient and rapid synthesis of pristine SrTiO3 mesocrystals. Scanning electron microscopy images revealed a cube-like morphology (of ca. 1 µm) formed via a self-assembly process, influenced by the MAH time. Additionally, photoluminescence measurements revealed a broad blue emission related to intrinsic defects, which decreased with the MAH synthesis time.

Insights into the photocatalytic performance of Bi2O2CO3/BiVO4 heterostructures prepared by one-step hydrothermal method.

This paper describes the synthesis of Bi2O2CO3/BiVO4 heterostructures through a one-step method based on the difference in solubility between two semiconductors that possess a metal in common. The as-synthesized Bi2O2CO3/BiVO4 heterostructures were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 physisorption, X-ray photoelectron spectroscopy (XPS) and time resolved photoluminescence spectroscopy (TRPL). The role of the heterojunction formed was evaluated by methylene blue (MB) dye and amiloride photodegradation. The formation of the heterostructure was observed indirectly by the great increase in the thermal stability of the Bi2O2CO3 phase when compared to its pure phase. The amount of heterojunctions formed between the Bi2O2CO3 and BiVO4 was tuned by vanadium precursor concentration. The proposed strategy was efficient for obtaining Bi2O2CO3/BiVO4 heterostructures with enhanced photocatalytic performance when compared to their isolated phases, MB and amiloride photodegradation occurred mainly by the action of ˙OH radicals, i.e. by an indirect mechanism. Based on TRPL spectroscopy and VB-XPS results, an enhancement of photoactivity related to an increase in the spatial separation of photo-generated electron/hole pairs was observed due to the formation of a type-II heterostructure.

A building blocks strategy for preparing photocatalytically active anatase TiO2/rutile SnO2 heterostructures by hydrothermal annealing.

Effective heterostructures for photocatalysis need to present good electron-hole mobility among phases, a feature that is only attained with satisfactory interface quality. For very small sizes and with a stable colloidal state, the oriented attachment mechanism can be used to prepare suitable structures by means of a building blocks strategy, whereby preformed nanoparticles are used to control the desired amount of each phase in the heterostructure. Here, we show the success of this strategy for anatase TiO2/rutile SnO2 heterostructures, applying conventional hydrothermal annealing, where successive collisions among particles increase the probability of oriented attachment. The photodegradation of Rhodamine B in water under UV radiation was used as a probe reaction to evaluate formation of the heterostructure, as indicated by an increase in photoactivity. Increased heterostructure photoactivity was related to heterojunction formation and charge separation. The increased lifetimes of the photogenerated charges, due to heterojunction formation, enabled them to reach the oxides surface and promote photocatalytic reactions. The insights presented here may be used in a rationalized synthesis method to obtain heterostructures from preformed nanocrystals.

In situ Transmission Electron Microscopy observation of Ag nanocrystal evolution by surfactant free electron-driven synthesis.

The study of the interaction of electron irradiation with matter and the response of the material to the passage of electrons is a very challenging problem. However, the growth mechanism observed during nanostructural evolution appears to be a broad and promising scientific field in nanotechnology. We report the in situ TEM study of nanostructural evolution of electron-driven silver (Ag) nanocrystals through an additive-free synthetic procedure. Observations revealed the direct effect of the electron beam on the morphological evolution of Ag nanocrystals through different mechanisms, such as mass transport, site-selective coalescence, and an appropriate structural configuration after coalescence leading to a more stable configuration. A fundamental understanding of the growth and formation mechanisms of Ag nanocrystals, which interact with the electron beam, is essential to improve the nanocrystal shape-control mechanisms as well as the future design and study of nanomaterials.

Structural and electronic analysis of the atomic scale nucleation of Ag on α-Ag2WO4 induced by electron irradiation.

In this work, we utilise a combination of theory, computation and experiments to understand the early events related to the nucleation of Ag filaments on α-Ag2WO4 crystals, which is driven by an accelerated electron beam from an electron microscope under high vacuum. The growth process and the chemical composition and elemental distribution in these filaments were analysed in depth at the nanoscale level using TEM, HAADF, EDS and XPS; the structural and electronic aspects were systematically studied in using first-principles electronic structure theory within QTAIM framework. The Ag nucleation and formation on α-Ag2WO4 is a result of the order/disorder effects generated in the crystal by the electron-beam irradiation. Both experimental and theoretical results show that this behavior is associated with structural and electronic changes of the [AgO2] and [AgO4] clusters and, to a minor extent, to the [WO6] cluster; these clusters collectively represent the constituent building blocks of α-Ag2WO4.

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.

Potentiated electron transference in α-Ag2WO4 microcrystals with Ag nanofilaments as microbial agent.

This study is a framework proposal for understanding the antimicrobacterial effect of both α-Ag2WO4 microcrystals (AWO) synthesized using a microwave hydrothermal (MH) method and α-Ag2WO4 microcrystals with Ag metallic nanofilaments (AWO:Ag) obtained by irradiation employing an electron beam to combat against planktonic cells of methicillin-resistant Staphylococcus aureus (MRSA). These samples were characterized by X-ray diffraction (XRD), FT-Raman spectroscopy, ultraviolet visible (UV-vis) measurements, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HRTEM). The results reveal that both AWO and AWO:Ag solutions have bacteriostatic and bactericidal effects, but the irradiated sample is more efficient; i.e., a 4-fold of the MRSA planktonic cells as compared to the nonirradiated sample was observed. In addition, first principles calculations were performed to obtain structural and electronic properties of AWO and metallic Ag, which provides strong quantitative support for an antimicrobacterial mechanism based on the enhancement of electron transfer processes between α-Ag2WO4 and Ag nanoparticles.

Long-range and short-range structures of cube-like shape SrTiO3 powders: microwave-assisted hydrothermal synthesis and photocatalytic activity.

We report herein a detailed study on the influence of microwave-assisted hydrothermal (MAH) treatment time on both long and short range structures around Ti atoms of SrTiO3 powders. Few studies have been carried out on short-order structural properties as well as the relationship between the local order and the SrTiO3 photocatalytic properties. We use X-ray diffraction to determine the long-range structure, while the local environment around the Ti atom is probed with X-ray absorption spectroscopy and the vibration frequencies are investigated by Raman spectroscopy. The faster crystallization of SrTiO3 powders provided by the MAH system resulted in large distortions of Ti-O bond lengths which remain unchanged even for a longer MAH treatment time. Despite the long-range structure being associated with ideal TiO6 clusters, X-ray absorption spectroscopy measurements identified the presence of undercoordinated TiO5 clusters. Compared with the reference bulk SrTiO3, the hierarchical SrTiO3 cube-like shape showed enhanced photocatalytic activity, which was associated with the presence of these TiO5 clusters. Field emission scanning electron microscopy (FE-SEM) revealed that the superstructures based on a cube-like shape are formed by an assembly process, becoming well defined as a function of MAH treatment time.

Indirect doping of microstructures fabricated by two-photon polymerization with gold nanoparticles.

Nanoplasmonics and metamaterials sciences are rapidly growing due to their contributions to photonic devices fabrication with applications ranging from biomedicine to photovoltaic cells. Noble metal nanoparticles incorporated into polymer matrix have great potential for such applications due to their distinctive optical properties. However, methods to indirectly incorporate metal nanoparticles into polymeric microstructures are still on demand. Here we report on the fabrication of two-photon polymerized microstructures doped with gold nanoparticles through an indirect doping process, so they do not interfere in the two-photon polymerization (2PP) process. Such microstructures present a strong emission, arising from gold nanoparticles fluorescence. The microstructures produced are potential candidates for nanoplasmonics and metamaterials devices applications and the nanoparticles production method can be applied in many samples, heated simultaneously, opening the possibility for large scale processes.

Ion-sensing properties of 1D vanadium pentoxide nanostructures.

The application of one-dimensional (1D) V2O5·nH2O nanostructures as pH sensing material was evaluated. 1D V2O5·nH2O nanostructures were obtained by a hydrothermal method with systematic control of morphology forming different nanostructures: nanoribbons, nanowires and nanorods. Deposited onto Au-covered substrates, 1D V2O5·nH2O nanostructures were employed as gate material in pH sensors based on separative extended gate FET as an alternative to provide FET isolation from the chemical environment. 1D V2O5·nH2O nanostructures showed pH sensitivity around the expected theoretical value. Due to high pH sensing properties, flexibility and low cost, further applications of 1D V2O5·nH2O nanostructures comprise enzyme FET-based biosensors using immobilized enzymes.