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In this study, interfacial segregation in Cl−-doped ZnO (0.0, 1.0, 3.0, 4.0, and 6.0 mol%) was explored as a strategy to compensate the space charge layer to decrease the electric potential barrier height at the grain boundaries and increase the overall electrical conductivity of the system. The focus of this work was to evaluate the dopant segregation and provide the first insights into the influence of interfacial segregation on the electrical properties. By using a systematic lixiviation method, we demonstrated that in addition to the bulk solubility, the Cl− anions segregated at both the surface and grain boundaries. Impedance spectroscopy measurements showed a four orders of magnitude reduction in the total electrical resistivity in the Cl−-doped ZnO samples compared to that of undoped ZnO. The calculated value of the electric potential barrier height decreased, as well as the activation energy for conduction, which decreased from 853 meV for undoped ZnO to 168 meV for 1.2 mol% Cl−-doped ZnO.

One of the attempts to improve γ-alumina catalytic performance lies in the use of additives that prone to segregate on its interfaces. The catalytic activity depends on the acidity or basicity of the doping oxides. Li2O is one of these oxides that when is loaded on γ-alumina surface decreases the number of Lewis acid sites. However, little is known about the Li2O distribution in the γ-alumina structure and interfaces. Thus, a detailed study of the lithium oxide distribution in the nanostructure of γ-alumina is the aim of this study. Nanometric powders were synthesized by the polymeric precursor method. The nanopowders were doped with Li+ ions (0.1, 0.5, 1, 3, 5, and 10 mol%) followed by calcination at 650 °C and 750 °C for 15 h. The powders were characterized by XRD, XRF, FTIR-DRIFT, BET specific surface area (SSA) and helium pycnometry. The surface excess due to the lithium oxide segregation was determined by the selective lixiviation method which has proved to be effective in quantifying surface excess of segregated ions at oxide interfaces. The crystallite sizes of the nanopowders varied from 3.3 nm to 9.2 nm, while their SSA were between 82.0 m2/g to 119.9 m2/g. The chemical analyses suggest that the lithium segregates in the γ-alumina interfaces at 650 °C, but it volatilizes from the surface after calcination at 750 °C.

The process of CO2 and H2O adsorption on the surface of nano-oxide semiconductors is important in the overall performance of artificial photosynthesis and other applications. In this study, we explored the thermodynamics of CO2 and H2O adsorption on TiO2 as a function of surface chemistry. We applied gas adsorption calorimetry to investigate the energetics of adsorption of those molecules on the surface of anatase nanoparticles. In an attempt to increase TiO2 surface affinity to CO2 and H2O, TiO2 was doped with alkaline earth metals (MgO, CaO, SrO, and BaO) by manipulating the chemical synthesis. Adsorption studies using diffuse reflectance infrared spectroscopy at different temperatures indicate that due to the segregation of alkaline earth metals on the surface of TiO2 nanoparticles, both CO2 and subsequent H2O adsorption amounts could be increased. CO2 adsorbs in two different manners, forming carbonates which can be removed at temperatures lower than 700 °C, and a more stable linear adsorption that remains even at 700 °C. Additionally to the surface energetic effects, doping also increased specific surface area, resulting in further improvement in net gas adsorption.

Nanoparticle stability against coarsening is one of the keys to allow better exploitation of the properties of nanoscale materials. The intrinsically high interfacial energies of nanoparticles constitute the driving force for coarsening, and therefore can serve as targets to design materials with improved thermal stability. In this study, we discuss the surface engineering of TiO2 nanocatalysts for artificial photosynthesis by exploiting the spontaneous segregation of Ba2+ ions to the interfaces of TiO2 nanocrystals. Ba2+ is a strong candidate for photoelectrocatalytic reduction of CO2 and its effects on interfacial energies lead to a remarkable increase in thermal stability. By using a systematic lixiviation method, we quantified the Ba2+ content located at both the surface and at grain boundary interfaces and combined with direct calorimetric measurements of surface energies and microstructural studies to demonstrate that Ba2+ excess quantities directly impact coarsening of TiO2 nanocatalysts by creatin...

The process of CO

In this study, TiO 2 nanoparticles containing 0–10 mol% ZnO were synthesized using the polymeric precursor method. The surface excess of ZnO on the TiO 2 surface was measured by the selective lixiviation method, and the grain boundary (GB) excess was calculated considering the total amount of ZnO and its solubility in the TiO 2 bulk. The results showed that ZnO segregates on both the surface and GBs of the TiO 2 nanopowder and that the GBs are richer in ZnO at high ZnO concentrations. X-ray photoelectron spectroscopy (XPS) analysis confirmed that ZnO segregated on the TiO 2 surface. However, after acid lixiviation, the same analysis showed a considerable reduction of the surface concentration of ZnO. A systematic reduction in the crystallite size and an increase in the specific surface area of TiO 2 were observed when increasing the ZnO concentration, which confirms the nanoparticle stability provided by the interfaces enrichment with ZnO. By measuring the electrophoretic mobility, it was possible to show the changes to the surface composition of the TiO 2 nanoparticles and the pH for ZnO solubilization.

The primary characteristic of nanopowders is the high surface area and consequently high fraction of atoms on the interfaces, which changes the energy of the system. The additive distribution in the nanopowder interfaces is a fundamental aspect to control the energy, particle size, and final properties of nanopowders. In this work, the surface excess was determined using a selective lixiviation method, where a low-water-soluble oxide, SnO2, was used as the matrix, and a high-water-soluble oxide, ZnO, was used as the additive. The XPS analysis confirmed that ZnO segregated on SnO2 surfaces. However, after acid lixiviation the same analysis showed an undetectable surface concentration of ZnO. The evaluation of the nanostructure change and surface composition enables us to calculate the heat of segregation for the grain boundary(∆Heg^gb = -47.2 kJ mol-1)and surface(∆Hseg^s=-36.4 kJ.mol-1) and the interface energy reduction because of segregation. At low ZnO concentrations, the additive solubilizes in the bulk and promotes particle growth. However, the segregation to the grain boundary and surface determines the relative stability of each interface, which promotes hard agglomeration and particle size stabilization at intermediated ZnO amounts. At high ZnO concentrations, the surface segregation stabilizes the solid-gas interface and decreases the agglomeration and final particle size. This article is protected by copyright. All rights reserved.

Ostrich eggshells are a potentially abundant and a high purity and low cost source of calcium to produce β-Tricalcium Phosphate (β-TCP) and Hydroxyapatite (HA), important calcium phosphates used as biomaterials. Here, we use a wet precipitation procedure to synthesize these phosphates using ostrich eggshells as a source of calcium ions. The biphasic precipitated powder, calcined at 800ºC, is a mixture of both Hydroxyapatite and β-Tricalcium Phosphate, also known as the biomaterial Biphasic Calcium Phosphate (BCP). Physico-chemical properties of the final powder product show water and CO32- groups absorbed in the particles surface, 0.1 to 100 µm particles size distribution and 11.70 m2/g of specific surface area.

MgAl 2 O 4 is a stable catalyst support with potential for replacing gamma-alumina in several applications. However, synthesis of magnesium spinel requires elevated temperatures to avoid phase separation (in MgO and Al 2 O 3 ) at low temperatures, leading to coarsening and reduction of active surface area. In this work, nano CaO-doped and undoped magnesium aluminate were successfully prepared by Ultrasonic Spray Pyrolysis (USP), using a simple adapted experimental set-up operating at 1100 °C. During the process, the particles stay at high temperatures for a short period of time, allowing phase stability and limited coarsening. The influence of calcium oxide on the particles morphology and structure was investigated via X-ray diffraction, N 2 adsorption, X-ray fluorescence, scanning electron microscopy and transmission electron microscopy. The spinel nanopowders were obtained as spherical porous agglomerates of ~1 µm. The resulting powder showed low crystallite sizes in the 5–10 nm range and high specific surface area from 110.0 to 76.6 m 2 g −1 .

Flash sintering (FS) is a current-assisted sintering technique able to densify ceramics in short periods of time (just a few seconds) at temperatures significantly lower than in conventional sintering processes. FS technique was firstly reported for yttrium-stabilized zirconia and later it had been proved successful for a large range of oxide materials that present ionic conduction by oxygen vacancies. This paper describes the use of FS on a sodium ion conductor based on a model compound, the beta-alumina. Different electrode materials have been tested, i.e., silver and platinum. The impact of the electrode reaction on the current flow, and thus, on the sintering efficiency is shown for the first time. It appears that the densification by FS can only be possible if the current collectors, i.e., the electrodes, are specifically designed to enable reversible electrochemical reactions at the interfaces between the electrodes and the ionic compound, insuring the current flow through the powder compact.

Graphical abstract, Most flash sintering experimental set-ups use dog bone-shaped specimen and DC current, which results in heterogeneously distributed densification and grain growth throughout the sample. This is the reason why only the sample's core characteristics after flash are taken into account. On the other hand, some recent procedures suggest the use of cylindrical pellets, which have some advantages compared to the traditional mode as the use of easily conformed samples and its final uniformity. Our new experimental set-up offers the possibility of atmosphere control and pressure application. Also the electrodes material change can be easily made when necessary. Shrinkage measurements and impedance spectroscopy are realized in situ and experimental parameters, as oven heating for example, can be varied to control microstructure changes. Advantages:•Sample can be entirely recovered at the end of the experiment and can be analyzed throughout its entire extension, including regions in contact with the electrodes that may present some differences from pellets inner part.•The use of AC current enables the study of different frequencies effects.•Experimental set-up can be adapted to different kinds of electrolytes (samples), easily changing electrode's material and atmosphere.

A beta-alumina de sódio é uma cerâmica condutora de íons Na+ utilizada como eletrólito sólido em baterias de sódio para armazenamento de energias intermitentes como energia solar e eólica. Devido ao alto teor de sódio, esse material é instável a altas temperaturas, podendo sofrer variações de composição durante a etapa de sinterização convencional que utiliza altas temperaturas por longos períodos de tempo. A sinterização flash é uma técnica de sinterização ativada por corrente elétrica que proporciona a densificação de compactos cerâmicos em poucos segundos, a temperaturas notavelmente mais baixas que as convencionais. Uma vez obrigatória a passagem de corrente elétrica através da amostra, a sinterização flash de qualquer material condutor parece bastante razoável. Não obstante, até o presente momento a maioria dos trabalhos publicados sobre o assunto aborda apenas condutores de vacância de oxigênio ou semicondutores, materiais compatíveis com eletrodos de platina (Pt). Nesse trabalho a sinterização flash de um condutor catiônico foi estudada utilizando-se a beta-alumina como material modelo. A beta-alumina foi sintetizada pelo método dos precursores poliméricos, caracterizada e então submetida à sinterização flash. O material de eletrodo padrão (platina) provou ser um eletrodo bloqueador em contato com a beta-alumina. O sucesso da sinterização flash foi determinado pela troca do material de eletrodo por prata (Ag), o que possibilitou uma reação eletroquímica reversível nas interfaces eletrodo-cerâmica e possibilitou a obtenção de um material densificado com morfologia e composição química homogêneas. Devido à metaestabilidade da beta-alumina, a atmosfera dos experimentos precisou ser alterada para manter a integridade desse material rico em um metal alcalino (Na+). A sinterização flash de um condutor catiônico é apresentada pela primeira vez na literatura e ressalta a importância da reação de eletrodo, que é um fator limitante para o sucesso da sinterização flash e precisa ser estudada e adaptada para cada tipo de material. Sodium beta-alumina is a Na+ ions conductor ceramic used as solid electrolyte in sodium batteries for energy storage in solar and eolic plants. Due to its high content of sodium, this material is instable at high temperatures and can suffer decomposition during conventional sintering (high temperatures for long periods of time). Flash sintering is an electric current activated sintering technique which promotes the densification of ceramic powder compacts in a few seconds at significantly lower temperatures than conventional sintering. Since in flash sintering the electric current flows through the sample, the flash sintering of any conductor material seems to be reasonable. Nevertheless, up to this date all published papers concern flash sintering of oxygen vacancies conductors or semi-conductors, materials which are compatible with platinum (Pt) electrodes. In this work the flash sintering of the cationic conductor betaalumina was studied. Beta-alumina was synthetized by the polymeric precursors method, characterized and then it was submitted to flash sintering. Platinum electrode was shown to be a blocking electrode for beta-alumina. Flash sintering success was determined by the change in electrode\'s material to silver (Ag), which made possible the reversible electrochemical reaction at the electrode-electrolyte interfaces and preserved chemical composition and morphology after flash. Due to this electrolyte metastability, atmosphere had to be changed in order to preserve the chemical composition of an alkali-rich (Na+) material such as beta-alumina. The flash sintering of a cationic conductor is shown for the first time in the literature and highlights the relevance of electrode reaction, which is a limiting point to the success of a flash sintering and needs to be well studied and adapted to each type of material.

The intense violet color and the high catalyst activity of Cr-doped SnO2nanoparticles have motivated several authors to understand the solid solution formation and the oxidation state of chromium ions after synthesis. Recent work has demonstrated the ability of surface segregation in chromium-doped tin oxide system but the oxidation state is still misunderstood. Calcium oxide addition changes the color of (Cr,Ca) codoped nanocrystalline tin dioxide pigments from violet to yellow simultaneously to a high particle size stabilization demonstrating that co-segregation could be associated to color change due to chemical environment change of chromium ions and specific surface area increase. High solubility of Cr+6and Ca+2allow us to determine the surface excess of both cations by ionic chromatography and the color change after surface solubilization.