In Situ Scanning Tunnelling Microscopy in Ionic Liquids: Prospects and Challenges

Abstract

In this paper devoted to Professor Dieter Kolb's 65th birthday the prospects and challenges of ionic liquids for fundamental investigations at the interface electrode/electrolyte are discussed. Ionic liquids consist solely of mainly organic cations and anions and they have wide electrochemical windows of up to 6 V (approximately ± 3 V vs. NHE) combined with wide thermal windows of up to 300ºC and extremely low vapour pressures between 10-11 and 10-10 mbar around room temperature. Thus, thermodynamically they give access to many elements and compounds at variable temperature which due to their reactivity cannot be electrodeposited in aqueous solutions. Apart from a discussion of electrochemical windows examples to be covered in this paper are the local probe deposition of silicon, aluminium and tantalum on Au(111). There is an incredibly high number of possible liquids (between 1012 and 1018 liquids, binary and ternary mixtures have been predicted) but also one major challenge for fundamental physicochemical studies, especially with the in situ STM: purity. It is tough to purify ionic liquids as hitherto they can neither be distilled with considerable rates without decomposition nor recrystallized nor sublimed. It will be shortly discussed that even apparently ultrapure ionic liquids can contain low amounts of inorganic impurities leading to inexpected behaviour on the single crystalline surface of Au(111). Due to their importance this paper focuses soleley on the third generation of ionic liquids, i.e. air and water stable ones.