Cooper Susan R, Candler Randall O, Cosby Alexia G, Johnson Darren W, Jensen Kirsten M Ø, Hutchison James E
Department of Chemistry and Biochemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403-1253, United States.
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK 2100 Copenhagen, Denmark.
ACS Nano. 2020 May 26;14(5):5480-5490. doi: 10.1021/acsnano.9b09551. Epub 2020 May 8.
Spinel iron oxide nanocrystals (NCs) have been reported to have atomic-level core and surface structural features that differ from those of the bulk material. Recent advances in a continuous growth synthesis of metal oxide NCs make it possible to prepare a series of NCs with subnanometer control of size with diameters below 10 nm that are well-suited for investigating size-dependent structure and reactivity. Here, we study the evolution of size-dependent structure in spinel iron oxide and determine how nanoscale structure influences the growth of NCs. We synthesized spinel iron oxide NCs via a continuous growth method that permits layer-by-layer control of size in order to monitor nanoscale structure over 16 core sizes between 3 and 10 nm. X-ray total scattering data were collected and analyzed with pair distribution function (PDF) analysis in order to refine quantitative structural features including cation occupancies that could be used to detect changes both in the oxidation state and the presence of tetrahedrally coordinated cation vacancies in the NCs. We find that the average iron oxidation state increases as core diameters decrease from 8 down to 3 nm. The trend in iron oxidation state can be explained by the oxidation of surface layers in the NCs. For samples exposed to air for several weeks, oxidation appears to cease when a volume equivalent to that of an ∼1.3 nm shell is converted to the more oxidized maghemite. The number of tetrahedrally coordinated cation vacancies also increases as the NC core size decreases. The correlation between the number of these vacancies and the faster growth for smaller NCs suggests that these reactive vacancies may be responsible for the rapid growth observed for nanocrystals with diameters smaller than 8 nm.
据报道，尖晶石型氧化铁纳米晶体（NCs）具有与块状材料不同的原子级核心和表面结构特征。金属氧化物纳米晶体连续生长合成技术的最新进展使得制备一系列尺寸可控在亚纳米级、直径小于10 nm的纳米晶体成为可能，这些纳米晶体非常适合研究尺寸依赖的结构和反应活性。在此，我们研究尖晶石型氧化铁中尺寸依赖结构的演变，并确定纳米级结构如何影响纳米晶体的生长。我们通过连续生长法合成了尖晶石型氧化铁纳米晶体，该方法允许逐层控制尺寸，以便监测3至10 nm之间16种核心尺寸的纳米级结构。收集了X射线全散射数据并用对分布函数（PDF）分析进行分析，以优化定量结构特征，包括阳离子占有率，这些特征可用于检测纳米晶体中氧化态的变化以及四面体配位阳离子空位的存在。我们发现，随着核心直径从8 nm减小到3 nm，平均铁氧化态增加。铁氧化态的趋势可以用纳米晶体表面层的氧化来解释。对于暴露在空气中几周的样品，当相当于约1.3 nm壳层体积的部分转化为氧化程度更高的磁赤铁矿时，氧化似乎停止。随着纳米晶体核心尺寸减小，四面体配位阳离子空位的数量也增加。这些空位数量与较小纳米晶体更快生长之间的相关性表明，这些活性空位可能是直径小于8 nm的纳米晶体快速生长的原因。
