Exsolution-Driven Surface Transformation in the Host Oxide

Abstract

Exsolution synthesizes self-assembled metal nanoparticle catalysts via phase precipitation. An overlooked aspect in this method thus far is how exsolution affects the host oxide surface chemistry and structure. Such information is critical as the oxide itself can also contribute to the overall catalytic activity. Combining X-ray and electron probes, we investigated the surface transformation of thin-film SrTi0.65Fe0.35O3 during Fe0 exsolution. We found that exsolution generates a highly Fe-deficient near-surface layer of about 2 nm thick. Moreover, the originally single-crystalline oxide near-surface region became partially polycrystalline after exsolution. Such drastic transformations at the surface of the oxide are important because the exsolution-induced nonstoichiometry and grain boundaries can alter the oxide ion transport and oxygen exchange kinetics and, hence, the catalytic activity toward water splitting or hydrogen oxidation reactions. These findings highlight the need to consider the exsolved oxide surface, in addition to the metal nanoparticles, in designing the exsolved nanocatalysts.

Fast Surface Oxygen Release Kinetics Accelerate Nanoparticle Exsolution in Perovskite Oxides

“…In the modeling, we set d * = 2 nm. This is because we have recently shown that the Fe atoms in the surface exsolved Fe 0 nanoparticles on STF are mainly extracted from a very thin layer (∼2 nm) near the STF surface, while the bulk of the STF film was not affected . Finally, c is a constant coefficient that accounts for the stoichiometric relationship between the metal cations and oxygen anions.…”

“…To examine this hypothesis, we chose single-crystalline thin-film SrTi 0.65 Fe 0.35 O 3 (STF) as a model system. Previous studies have shown that STF is a promising electrode material in solid oxide fuel cells and electrolysis cells − that can exsolve metallic iron (Fe 0 ) nanoparticles as catalysts to enhance water splitting kinetics . In this study, we begin by presenting an analytical kinetic model, which shows that the oxygen release kinetics in thin-film STF are dependent on both the film thickness and surface reactivity.…”

“…While previous TEM studies revealed the exsolution kinetics of individual nanoparticles, , NAP-XPS can provide a unique platform to uncover the statistical exsolution behaviors of a large number of nanoparticles. With the typical X-ray beam size (>1500 μm 2 ) and surface particle density , (>10 10 cm –2 ), we can simultaneously probe the average exsolution behaviors of more than 10 6 nanoparticles, which yields better statistics. Moreover, while previous TEM studies were limited to polycrystalline samples, , NAP-XPS can be used to investigate exsolution on single-crystalline samples with well-controlled thickness and orientation, which are essential to quantitatively examine the role of oxygen release kinetics in exsolution.…”

“…Moreover, while previous TEM studies were limited to polycrystalline samples, , NAP-XPS can be used to investigate exsolution on single-crystalline samples with well-controlled thickness and orientation, which are essential to quantitatively examine the role of oxygen release kinetics in exsolution. Due to the aforementioned unique advantages, NAP-XPS has gained increasing interest in the field of metal exsolution and has been employed in several studies to investigate the exsolution mechanisms. ,,,− A common practice to date is to use NAP-XPS to probe exsolution while varying the sample temperatures ,, or electrochemical biases. , However, such an experimental protocol cannot be used to investigate the kinetic aspects of exsolution as the thermodynamic driving forces for exsolution kept changing during the course of those experiments. ,,,, To bridge this gap, here we develop a novel experimental method for in situ probing materials’ exsolution kinetics under a constant reduction condition using time-resolved NAP-XPS. As will be elaborated below, this unique experimental approach can not only quantify the concentration of exsolved metallic species as a function of reduction time, but also reveal the prereduction time required to onset metal exsolution with great precision.…”

The impact of Mn nonstoichiometry on the oxygen mass transport properties of La0.8Sr0.2Mn y O3±δ thin films

“…Variation of cation stoichiometry is known to take place in perovskite thin films in many different circumstances, such as structural defects [26,67], strain relaxation [58,68] or near free surfaces [69,70]. For instance, cation segregation or exsolution were observed to generate regions with altered B/A ratio, which may affect the mobility and concentration of oxygen vacancies [71][72][73]. However, here we show that antisite defects may provide an efficient way to mitigate cationic nonstoichiometry effects on oxygen mass transport properties and that they should be considered when dealing with variation of the B/A ratio.…”