New Approaches to Preparation of SnO2-Based Varistors — Chemical Synthesis, Dopants, and Microwave Sintering

Tin oxides have applications such as sensors, solar cells, transistors, and varistors, which are being studied to replace ZnO varistors due to similar electrical properties, simpler microstructure, no formation of secondary phases, and lower concentration of agent modifiers to promote the varistor characteristics and densification. Varistors are ceramic with a high concentration of structural and electronics defects. The type and the amount of defects are related with agent modifiers and processing steps employed. The study in materials processing aims to improve the ceramics properties. Chemical synthesis ensures the homogeneous distribution of dopants used to promote electrical and structural properties. Microwave sintering appears as processing to optimize time and sintering temperature. Varistor application is linked to its breakdown voltage, which should be larger than the operating voltage. In an operating range of 1 kV to 1 MV, the varistors are used in electricity transmission networks. In a range of 24–1000 V, the application occurs in electronics and appliances and in a range smaller than 24 V, as protective of automotive electronics and computers. This chapter aims to provide information on new processing steps for the production of SnO2 varistors and to show the possibility to get electrical properties with non-ohmic characteristic for technological applications

Connecting morphology and photoluminescence emissions in β-Ag2MoO4 microcrystals

This work elucidates the morphology-photoluminescence (PL) emission relationships, based on experimental and calculated results, on β-Ag2MoO4 samples synthesized by microwave-assisted hydrothermal method. It was shown that the solvent (water and ammonia) and temperature (120, 130, 140, and 150 °C) play a crucial role in the morphology and PL emissions. A crystal structure model, composed by the local coordination of both Ag and Mo atoms at bulk and exposed surfaces, was built, and the possible mechanism along the synthesis progress was proposed and analyzed. This study provides an idea for the preparation and development of β-Ag2MoO4 based materials with desirable properties

Microwave-Driven Hexagonal-to-Monoclinic Transition in BiPO4: An In-Depth Experimental Investigation and First-Principles Study

Present theoretical and experimental work provides an in-depth understanding of the morphological, structural, electronic, and optical properties of hexagonal and monoclinic polymorphs of bismuth phosphate (BiPO4). Herein, we demonstrate how microwave irradiation induces the transformation of a hexagonal phase to a monoclinic phase in a short period of time and, thus, the photocatalytic performance of BiPO4. To complement and rationalize the experimental results, first-principles calculations have been performed within the framework of density functional theory. This was aimed at obtaining the geometric, energetic, and structural parameters as well as vibrational frequencies; further, the electronic properties (band structure diagram and density of states) of the bulk and corresponding surfaces of both the hexagonal and monoclinic phases of BiPO4 were also acquired. A detailed characterization of the low vibrational modes of both the hexagonal and monoclinic polymorphs is key to explaining the irreversible phase transformation from hexagonal to monoclinic. On the basis of the calculated values of the surface energies, a map of the available morphologies of both phases was obtained by using Wulff construction and compared to the observed scanning electron microscopy images. The BiPO4 crystals obtained after 16–32 min of microwave irradiation provided excellent photodegradation of Rhodamine B under visible-light irradiation. This enhancement was found to be related to the surface energy and the types of clusters formed on the exposed surfaces of the morphology. These findings provide details of the hexagonal-to-monoclinic phase transition in BiPO4 during microwave irradiation; further, the results will assist in the design of electronic devices with higher efficiency and reliability

Behavior of Bi2S3 under ultrasound irradiation for Rhodamine B dye degradation

In this study, we have demonstrated that Bi2S3 semiconductor under ultrasound behaves as a piezoelectric material, facilitating the transfer of electrons and holes, thus enabling the formation of oxidizing agents. In the presence of light, Bi2S3 showed no photocatalytic activity, resulting in Rhodamine B (RhB) degradation of 2.5%, while the ultrasound application led to a degradation of 40.6%. Herein, we propose a possible mechanism to explain this behavior based on the local charge polarization in [BiS7] clusters

Zinc-substituted Ag2CrO4: A material with enhanced photocatalytic and biological activity

In the past years, new environmentally-friendly photocatalysts have been reported, but the realization of efficient visible-light driven photocatalyst with highly active bactericidal and fungicidal activity is still challenging. This work is a joint experimental and theoretical study on the structural, electronic, and optical properties of Ag2CrO4:Zn2+ (ACOxZn, x = 1%, 2%, and 4%) solid solutions for photocatalytic, bactericidal, and fungicidal activity. For the first time, synthesis of these innovative and multifunctional materials were performed through the cation exchange of zinc and silver using a simple, fast, and cheap co-precipitation method. Powder X-ray diffraction measurements revealed the long range order of the materials. X-ray photoelectron spectroscopy provided information about the surface of the samples demonstrating that they were pure. The materials showed short-range order as verified by FT-Raman spectroscopy. Additionally, ultraviolet-visible diffuse reflectance spectra and photoluminescence spectroscopy were used to examine the electronic properties which corroborated with the increasing photocatalytic activity for the degradation of Rhodamine B and bactericidal activity against Staphylococcus aureus and Candida albicans. Field emission scanning electron microscopy images showed different types of particles with different facets and sizes. Theoretical results based on density functional theory calculations complement the experimental results to rationalize the effects of the incorporation of Zn cations in the ACO host lattice

Rational Design of W-Doped Ag3PO4 as an Efficient Antibacterial Agent and Photocatalyst for Organic Pollutant Degradation

International audienceBacterial and organic pollutants are major problems with potential adverse impacts on human health and the environment. A promising strategy to alleviate these impacts consists in designing innovative photocatalysts with a wider spectrum of application. In this paper, we report the improved photocatalytic and antibacterial activities of chemically precipitated Ag3PO4 microcrystals by the incorporation of W at doping levels 0.5, 1, and 2 mol %. The presence of W directly influences the crystallization of Ag3PO4, affecting the morphology, particle size, and surface area of the microcrystals. Also, the characterization via experimental and theoretical approaches evidenced a high density of disordered [AgO4], [PO4], and [WO4] structural clusters due to the substitution of P5+ by W6+ into the Ag3PO4 lattice. This leads to new defect-related energy states, which decreases the band gap energy of the materials (from 2.27 to 2.04 eV) and delays the recombination of e'-h(center dot) pairs, leading to an enhanced degradation process. As a result of such behaviors, W-doped Ag3PO4 (Ag3PO4:W) is a better visible-light photocatalyst than Ag3PO4, demonstrated here by the photodegradation of potential environmental pollutants. The degradation of rhodamine B dye was 100% in 4 min for Ag3PO4:W 1%, and for Ag3PO4, the obtained result was 90% of degradation in 15 min of reaction. Ag3PO4:W 1% allowed the total degradation of cephalexin antibiotic in only 4 min, whereas pure Ag3PO4 took 20 min to achieve the same result. For the degradation of imidacloprid insecticide, Ag3PO4:W 1% allowed 90% of degradation, whereas Ag3PO4 allowed 40%, both in 20 min of reaction. Moreover, the presence of W-dopant results in a 16-fold improvement of bactericidal performance against methicillin-resistant Staphylococcus aureus. The outstanding results using the Ag3PO4:W material demonstrated its potential multifunctionality for the control of organic pollutants and bacteria in environmental applications