Shousha Shehab, Khalil Sarah, Youssef Mostafa
Department of Nuclear and Radiation Engineering, Alexandria University, Alexandria, Egypt.
Phys Chem Chem Phys. 2020 Mar 21;22(11):6308-6317. doi: 10.1039/c9cp06660h. Epub 2020 Mar 5.
The low-temperature defect chemistry of monoclinic and tetragonal ZrO and hematite FeO is studied in the non-equilibrium state of thermochemical quenching; that is, rapid cooling starting from a certain high temperature and oxygen chemical potential. This non-equilibrium state is of great interest because many metal oxides are used at low temperatures below their growth temperatures. This paper addresses the importance of considering this non-equilibrium state rather than applying equilibrium thermodynamics as commonly used when studying point defects from first principles. Based on point defect formation energies calculated previously using density functional theory, we compare the type of dominant defects at equilibrium to those at a quenched state originating from a certain initial growth temperature and oxygen partial pressure. The comparison is facilitated by casting the dominant defects in a dominance diagram on the temperature - oxygen partial pressure plane. We consider two scenarios to model the quenched state. In the first, only electronic defects equilibrate whereas all ionic defects are frozen. Whereas, in the second, electronic defects and interstitials are allowed to equilibrate under the assumption of mobile interstitials at low temperatures. We find that new ionic charge compensation modes can appear on the dominance diagram after quenching. Additionally, purely ionic charge compensation modes consisting of vacancies and/or interstitials expand in the dominance diagram at the expense of purely electronic compensation modes. For the ZrO phases, we argue that scenario 2 is more realistic and leads to difficulty in achieving n-type doping by thermochemical quenching. For FeO, and regardless of the quenching scenario, iron vacancies occupy a wider zone of domination, which limits the performance of this oxide as a water splitting photoanode. Our study shows that by controlling the growth thermochemical conditions, it is possible to tune the Fermi level of oxides over a considerable range within the band gap by quenching. This provides an extra tool to tune the electric conductivity of metal oxides beyond traditional extrinsic doping. This work indicates that non-equilibrium thermodynamic analysis is necessary to understand and control defect chemistry at low temperatures.
在热化学淬火的非平衡状态下研究了单斜和四方氧化锆以及赤铁矿FeO的低温缺陷化学；即从某一高温和氧化学势开始的快速冷却。这种非平衡状态备受关注，因为许多金属氧化物在低于其生长温度的低温下使用。本文阐述了在从第一原理研究点缺陷时考虑这种非平衡状态而非应用常用的平衡热力学的重要性。基于先前使用密度泛函理论计算的点缺陷形成能，我们将平衡态下的主要缺陷类型与源自某一初始生长温度和氧分压的淬火态下的主要缺陷类型进行比较。通过在温度 - 氧分压平面的优势图中绘制主要缺陷来便于比较。我们考虑两种情况来模拟淬火态。第一种情况是仅电子缺陷达到平衡而所有离子缺陷被冻结。而在第二种情况中，在低温下间隙原子可移动的假设下，允许电子缺陷和间隙原子达到平衡。我们发现淬火后新的离子电荷补偿模式会出现在优势图上。此外，由空位和/或间隙原子组成的纯离子电荷补偿模式在优势图中扩展，而纯电子补偿模式则相应减少。对于ZrO相，我们认为情况2更现实，并且这导致通过热化学淬火实现n型掺杂存在困难。对于FeO，无论淬火情况如何，铁空位占据更宽的主导区域，这限制了这种氧化物作为水分解光阳极的性能。我们的研究表明，通过控制生长热化学条件，有可能通过淬火在带隙内相当大的范围内调节氧化物的费米能级。这提供了一种超越传统非本征掺杂来调节金属氧化物电导率的额外工具。这项工作表明，非平衡热力学分析对于理解和控制低温下的缺陷化学是必要的。
