If a platinum STM tip can be employed to electrochemically deposit silver nanostructures on graphite surfaces, can this method be extended to the electrochemical synthesis of materials on graphite surfaces? The answer is "yes" in at least one specific case: The oxidative polymerization of aniline to form nanostructures of the electronically conductive polymer, polyaniline (PANI):

The in-situ STM image shown above left shows four nanostructures - two pits and two protrusions. All four were produced by the application of two sample-positive voltage pulses of +6.0V x 0.005 ms followed by +3.0V x 0.045 ms to a graphite surface immersed in an HCl solution containing 10 mM aniline. In an aniline-free HCL solution, in contrast, only pits are produced (see, for example, the image at right - can you find 7 places where a pulse was applied?). In the image at left, the first nanostructure produced was the one at far right. Between the first and the second attempt, only a few seconds were permitted to elapse whereas between the second and third (another protrusion) the wait time was about five minutes. However the fourth was produced only a few seconds after the third and again a pit was generated (feature at far left). Our interpretation of this result is that the precursor aniline involved in the formation of these nanostructures exists as a adsorbed monolayer on the graphite surface, and that this "reservoir" is substantially depleted in the vicinity of a pit when a PANI nanostructure is generated. For this reason - until the aniline monolayer is regenerated by the diffusive flux of aniline on the surface and by readsorption from solution - PANI nanostuctures can not be generated.

PANI particles can be electrochemically switched between oxidized and reduced states. This "switching" involves the influx and efflux of charge compensating ions (and associated solvent) according to the reaction:

If we observe, usingin-situ STM, a single PANI particle as the potential of the surface is stepped between oxidizing and reducing potentials, the particle become larger (by about 30%) as it is oxidized, and smaller as it is reduced in qualitative agreement with expectations. These volume-applied potential correlations are plotted below:

A PANI particle can function as a nano-scale actuator!