Luong Jason, Wang Xin, Tsung Alicia, Humphrey Nicholas, Guo Huiming, Lam Benjamin X, Mallikarjun Sharada Shaama, Bowman William J
Department of Materials Science and Engineering, University of California, Irvine, Irvine, California92697, United States.
Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California90089, United States.
ACS Appl Nano Mater. 2023 Jan 20;6(3):1620-1630. doi: 10.1021/acsanm.2c04537. eCollection 2023 Feb 10.
Potential applications of the earth-abundant, low-cost, and non-critical perovskite CaTi Fe O in electrocatalysis, photocatalysis, and oxygen-transport membranes have motivated research to tune its chemical composition and morphology. However, investigations on the decomposition mechanism(s) of CaTi Fe O under thermochemically reducing conditions are limited, and direct evidence of the nano- and atomic-level decomposition process is not available in the literature. In this work, the phase evolution of CaTi Fe O ( = 0-0.4) was investigated in a H-containing atmosphere after heat treatments up to 600 °C. The results show that CaTi Fe O maintained a stable perovskite phase at low Fe contents while exhibiting a phase decomposition to Fe/Fe oxide nanoparticles as the Fe content increases. In CaTiFeO and CaTiFeO, the phase evolution to Fe/Fe oxide was greatly influenced by the temperature: Only temperatures of 300 °C and greater facilitated phase evolution. Fully coherent Fe-rich and Fe-depleted perovskite nanodomains were observed directly by atomic-resolution scanning transmission electron microscopy. Prior evidence for such nanodomain formation was not found, and it is thought to result from a near-surface Kirkendall-like phenomenon caused by Fe migration in the absence of Ca and Ti co-migration. Density functional theory simulations of Fe-doped bulk models reveal that Fe in an octahedral interstitial site is energetically more favorable than in a tetrahedral site. In addition to coherent nanodomains, agglomerated Fe/Fe oxide nanoparticles formed on the ceramic surface during decomposition, which altered the electrical transport mechanism. From temperature-dependent electrical conductivity measurements, it was found that heat treatment and phase decomposition change the transport mechanism from thermally activated p-type electronic conductivity through the perovskite to electronic conduction through the iron oxide formed by thermochemical decomposition. This understanding will be useful to those who are developing or employing this and similar earth-abundant functional perovskites for use under reducing conditions, at elevated temperatures, and when designing materials syntheses and processes.
在地壳中储量丰富、成本低廉且无关键元素的钙钛矿CaTiFeO在电催化、光催化和氧传输膜方面的潜在应用推动了对其化学成分和形态进行调控的研究。然而，关于CaTiFeO在热化学还原条件下的分解机制的研究有限，且文献中尚无纳米级和原子级分解过程的直接证据。在这项工作中，研究了CaTiFeO（x = 0 - 0.4）在含H气氛中经高达600°C热处理后的相演变。结果表明，在低Fe含量时CaTiFeO保持稳定的钙钛矿相，而随着Fe含量增加则表现出向Fe/Fe氧化物纳米颗粒的相分解。在CaTiFeO₃和CaTiFeO₄中，向Fe/Fe氧化物的相演变受温度影响很大：仅300°C及以上的温度促进相演变。通过原子分辨率扫描透射电子显微镜直接观察到了完全相干的富Fe和贫Fe钙钛矿纳米畴。此前未发现这种纳米畴形成的证据，据认为这是由在没有Ca和Ti共同迁移的情况下Fe迁移导致的近表面类柯肯达尔现象引起的。对Fe掺杂体相模型的密度泛函理论模拟表明，八面体间隙位置的Fe在能量上比四面体位置的Fe更有利。除了相干纳米畴外，分解过程中在陶瓷表面形成了团聚的Fe/Fe氧化物纳米颗粒，这改变了电传输机制。通过与温度相关的电导率测量发现，热处理和相分解将传输机制从通过钙钛矿的热激活p型电子传导转变为通过热化学分解形成的氧化铁的电子传导。这种认识对于那些正在开发或使用这种及类似的在地壳中储量丰富的功能性钙钛矿以在还原条件、高温下使用以及设计材料合成和工艺的人将是有用的。
