A novel atomic force microscope operating in liquid for in situ investigation of electrochemical preparation of porous alumina.

A novel atomic force microscope (AFM) operating in liquid is described in this article. The specially designed AFM probe involves a tip attached to a cantilever, a tip holder, and a circular Plexiglas window. When the probe dives into the fluid, a circular meniscus is established around the Plexiglas window, preventing the tip from being affected or destroyed by surface tension of the liquid. In this setup, the whole scanning probe and the sample can completely dive into fluid. Meanwhile, the probe tip scans over the sample surface when the instrument works. These advantages enable the instrument to scan comparatively large or heavy samples with a high speed. The highest scan rate is about 30 lines/s or 14 s for a 400 x 400-pixel, 3 x 3 microm image. Using the new AFM, we carry out in-situ investigation of the formation processes of porous alumina during electrochemical anodic oxidation. A lead ring and an aluminum foil serve as cathode and anode, respectively. They are entirely immersed in the bath electrolyte, which is oxalic acid solution. During anodic oxidation, the AFM images of the sample surface are successively acquired without elevating the sample out of the solution. Experiments reveal that electrochemical reactions take place soon after the power supply is switched on, and with the progression of anodization, nanostructures of porous alumina gradually occur on the aluminum substrate, finally yielding ordered arrays of nanopores. As a typical example of applications, the results of this work show that the new AFM is an ideal and powerful tool for in-situ observation and study of materials or samples in aqueous solutions.