Hope Michael A, Griffith Kent J, Cui Bin, Gao Fang, Dutton Siân E, Parkin Stuart S P, Grey Clare P
Department of Chemistry , University of Cambridge , Lensfield Road , Cambridge CB2 1EW , U.K.
Max Planck Institute of Microstructure Physics , Halle (Saale) D-06120 , Germany.
J Am Chem Soc. 2018 Dec 5;140(48):16685-16696. doi: 10.1021/jacs.8b09513. Epub 2018 Nov 21.
Metallization of initially insulating VO via ionic liquid electrolytes, otherwise known as electrolyte gating, has recently been a topic of much interest for possible applications such as Mott transistors and memory devices. It is clear that the metallization takes place electrochemically, and, in particular, there has previously been extensive evidence for the removal of small amounts of oxygen during ionic liquid gating. Hydrogen intercalation has also been proposed, but the source of the hydrogen has remained unclear. In this work, solid-state magic angle spinning NMR spectroscopy (H, H, O, and V) is used to investigate the thermal metal-insulator transition in VO, before progressing to catalytically hydrogenated VO and electrochemically metallized VO. In these experiments electrochemical metallization of bulk VO particles is shown to be associated with intercalation of hydrogen, the degree of which can be measured with quantitative H NMR spectroscopy. Possible sources of the hydrogen are explored, and by using a selectively deuterated ionic liquid, it is revealed that the hydrogenation is due to deprotonation of the ionic liquid; specifically, for the commonly used dialkylimidazolium-based ionic liquids, it is the "carbene" proton that is responsible. Increasing the temperature of the electrochemistry is shown to increase the degree of hydrogenation, forming first a less hydrogenated metallic orthorhombic phase then a more hydrogenated insulating Curie-Weiss paramagnetic orthorhombic phase, both of which were also observed for catalytically hydrogenated VO. The NMR results are supported by magnetic susceptibility measurements, which corroborate the degree of Pauli and Curie-Weiss paramagnetism. Finally, NMR spectroscopy is used to identify the presence of hydrogen in an electrolyte gated thin film of VO, suggesting that electrolyte breakdown, proton intercalation, and reactions with decomposition products within the electrolyte should not be ignored when interpreting the electronic and structural changes observed in electrochemical gating experiments.
通过离子液体电解质使初始绝缘的VO金属化，即所谓的电解质门控，最近因其在诸如莫特晶体管和存储器件等可能应用中备受关注。很明显，金属化是通过电化学方式发生的，特别是此前有大量证据表明在离子液体门控过程中有少量氧被去除。也有人提出了氢嵌入的观点，但氢的来源仍不清楚。在这项工作中，固态魔角旋转核磁共振光谱（H、H、O和V）被用于研究VO中的热金属 - 绝缘体转变，之后研究催化氢化的VO和电化学金属化的VO。在这些实验中，块状VO颗粒的电化学金属化被证明与氢的嵌入有关，其程度可以用定量H核磁共振光谱测量。探索了氢的可能来源，通过使用选择性氘代的离子液体，发现氢化是由于离子液体的去质子化；具体而言，对于常用的基于二烷基咪唑鎓的离子液体，是“卡宾”质子起作用。结果表明，提高电化学温度会增加氢化程度，首先形成氢化程度较低的金属正交相，然后形成氢化程度更高的绝缘居里 - 外斯顺磁正交相，这两种相在催化氢化的VO中也被观察到。核磁共振结果得到了磁化率测量的支持, 磁化率测量证实了泡利顺磁性和居里 - 外斯顺磁性的程度。最后，核磁共振光谱用于识别VO电解质门控薄膜中氢的存在，这表明在解释电化学门控实验中观察到的电子和结构变化时，不应忽视电解质分解、质子嵌入以及与电解质内分解产物的反应。
