Effect of hydrogen on the local chemical bonding states and structure of amorphous alumina by atomistic and electrostatic modeling and first principles calculations of auger parameter shifts

This study discloses the effect of hydrogen impurities on the local chemical bonding states and structure of amorphous alumina films by predicting measured Auger parameter shifts using a combination of atomistic and electrostatic modeling. Different amorphous alumina polymorphs with variable H-content and density, as grown by atomic layer deposition, were successfully modeled using a universal machine learning interatomic potential. The annealing of highly defective crystalline hydroxide structures with experimental H-contents at the corresponding atomic layer deposition temperatures led to excellent agreement between theory and experiment in the density and structure of the resulting amorphous alumina polymorphs. The measured Auger parameter shifts of Al cations in such polymorphs were accurately predicted with respect to the H content by assuming that all H atoms are present in the form of hydroxyl ligands in the randomly interconnected 4-fold, 5-fold, and 6-fold nearest-coordination spheres of Al. As revealed by a combination of atomistic and electrostatic modeling, the measured Auger shifts with an increase in the H content and an accompanying decrease in the oxide density depend on the complex correlations between local coordination, bond lengths, bond angles, and ligand type(s) around the core-ionized atoms. Moreover, cryogenic X-ray photoelectron spectroscopy is suggested to offer new insights into the local chemical and structural building blocks of crystalline and amorphous oxides by reducing thermal noise. These findings and fundamental knowledge contribute to advancing the design of e.g. hydrogen oxide barrier films, oxide membranes for H separation, H storage materials, and fuel cells for a hydrogen-based economy.

本研究通过结合原子尺度建模与静电建模，预测实验测得的俄歇参数（Auger parameter）位移，揭示了氢杂质对非晶氧化铝薄膜局部化学键合状态与微观结构的影响。本研究采用通用机器学习原子间势，成功建模了由原子层沉积（atomic layer deposition, ALD）制备的、氢含量与密度各异的多种非晶氧化铝相。在对应原子层沉积温度下，对具有实验测得氢含量的高缺陷结晶氢氧化物结构进行退火处理，所得非晶氧化铝相的密度与结构在理论与实验间实现了极佳吻合。通过假设所有氢原子均以羟基配体形式存在于铝原子随机连接的4配位、5配位与6配位最近邻配位球中，我们可基于氢含量精准预测此类相内铝阳离子的实验俄歇参数位移。结合原子尺度与静电建模的结果表明，随着氢含量升高以及氧化物密度随之降低，实验测得的俄歇位移取决于核心电离原子周围的局部配位环境、键长、键角与配体种类之间的复杂关联。此外，低温X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）可通过抑制热噪声，为解析结晶与非晶氧化物的局部化学与结构基元提供全新视角。本研究的发现与基础认知，可为氢氧化物阻隔膜、氢气分离用氧化物膜、储氢材料以及面向氢能经济的燃料电池等器件的设计研发提供助力。