The function of materials that have been coated with an ionic liquid can be altered by applying an electric field to shuttle ions in and out. The technique has been used to make materials that have switchable properties. See Letter p.124
The search for functional materials has been at the forefront of research spanning natural philosophy to engineering for centuries. Usually, this discovery process is planned — raw materials are cooked in a furnace, and the properties of the resulting products are studied. But once in a while, desirable properties can emerge unexpectedly from an existing material when its composition is subtly changed or when small quantities of impurities are added. The trick is to work out which impurities to throw into the mix (and how this should be done), and to understand the mechanisms behind the properties that emerge. On page 124, Lu et al.1 report a remarkable advance in this direction. The authors used an electric field to carefully manipulate a material's composition and concentration of ionic impurities, causing it to switch between multiple phases that have distinct electronic, optical and magnetic properties.
Lu and colleagues grew a thin film of a ceramic material (strontium cobalt oxide, SrCoO2.5) on the surface of a substrate using a technique called pulsed laser deposition. Usually, to stimulate the diffusion of ions into such a material, extreme heat would be applied. The authors instead added an ionic liquid to the material's surface (Fig. 1). This gel-like material is both an electrical insulator and an ionic conductor, and it contains dissolved oxide ions (O2) and hydrogen ions (H+). Ionic liquids have been previously used as a means to accumulate charge at the surfaces of semiconductors to tune their electrical conductivity2 and as a medium in which to store ions (such as oxide ions) for insertion into solids3.
The researchers applied a voltage across the ionic liquid using a pair of electrodes. They show that, depending on the polarity of the voltage, either the oxide ions or the hydrogen ions are driven into the ceramic material. Furthermore, they demonstrate that reversing the voltage extracts the relevant ions in each case, implying that the process is reversible.
The beauty of Lu and collaborators' approach is twofold. First, extreme heat is not needed because the driving force for ionic motion is an electric field rather than thermal energy. Second, the ionic transfer occurs at near room temperature, which allows the distance moved by the ions to be controlled down to the atomic scale. This enabled the authors to carefully vary the composition of the SrCoO2.5, either increasing its oxygen concentration by almost 20% (SrCoO3−δ, where δ refers to a small fraction of oxygen vacancies in the material), or hydrogenating it (HSrCoO2.5).
The authors studied the three phases of their material using a combination of photon spectroscopy, X-ray diffraction and magnetic probing. They demonstrate that each phase has distinct electronic, optical and magnetic properties. For example, SrCoO2.5 transmits infrared light, SrCoO3 is opaque and HSrCoO2.5 transmits both infrared and visible light. One can envisage many applications for the authors' material system, including 'smart' windows that change colour when electronically stimulated, or materials whose magnetism (the alignment of electron magnetic moments) can be switched on or off by the insertion or extraction of ions.
Although such ion-mediated transformation techniques have been the subject of previous studies4,5, Lu and colleagues' results call attention to a hitherto unexplored approach to functional-material design: the use of ionic liquids. For decades, researchers have studied these liquids in the context of energy storage6. It is now clear that they have more to offer the broader natural-science and engineering communities. Because ionic liquids can sustain electric fields without the need for a large current to be passed through them, heat generation (and therefore energy loss) is minimized. As the authors have shown, these systems can be used to selectively propagate ions into and out of materials to create designer electronic or magnetic phases that do not otherwise form. Because the authors' technique is relatively simple to implement, with a plethora of materials to choose from, it could be easily applied by other research groups to the materials with which they work.
Lu and colleagues have presented initial results, but further work is required to nurture this fledgling field. For instance, it would be useful to know whether the phases created using the authors' approach are stable, with respect to both time and temperature. This stability could be essential for practical applications — or perhaps instability could be exploited in some way. Another concern is whether there are fatigue issues (such as strain) when relatively large ions — for example, oxide ions — are repeatedly shuttled in and out of materials. The authors' study also poses challenges for theorists to predict the types of ion that will bring about the greatest changes in electromagnetic properties, and the choices of liquid that will minimize the energy cost. But for now, the race is on to see what other fascinating phenomena can be discovered using these ion-mediated transformation techniques.