Voltage Dependence of Nanopattern Morphology and Size in Electropolished Monocrystalline Aluminum: An Experimental Study

Abstract

The crystallographic orientation dependence of nanopattern morphology and size of electropolished Al at a fixed voltage (40 V) has been revealed and established. However, systematic experimental studies, with samples’ crystallographic orientation exactly determined, on the effects of voltage on nanopattern morphology and size has been largely lacking. Here, miscut monocrystalline Al samples are electropolished in a common electrolyte at voltages 20–60 V. Various nanopatterns (pits, pits plus stripes, stripes, dots) with distinct morphologies emerge with increasing voltage. The findings that morphologies transform gradually with misorientation angle were explained from the perspective of surface structure sensitivity. Nanopattern sizes remain largely constant with misorientation angles but increase with voltages till about 40 V, level off and decrease at higher voltage. A reported model was reexamined to explain the observed voltage dependence of nanopattern size by considering the overlooked effects of voltage on dissolution and adsorption. These findings reveal the voltage and crystallographic orientation dependence of nanopattern morphology and size, and provide critical data for theoretical investigation into the nanopattern formation mechanism of electropolished Al.

Voltage Dependence of Nanopattern Morphology in Electropolished Aluminum: A Theoretical Study

“…On the one hand, during electropolishing of Al in the perchloric acid-ethanol based solution, ethanol, water, butyl cellusolve as well as solvated ions like OH − and perchlorate/chloride ions from perchloric acid in the electrolyte are all possible adsorbed species on Al surface. Because of high volume percentage of ethanol (70% in this and previous studies 1,2 ) and the essential role of ethanol for effective electropolishing in this type of solution (the permanent dipole moments of ethanol (5.643 × 10 −30 J/(V m) −1 ) are numerically close to that of water (6.17 × 10 −30 J/(V m) −1 )), the dominating adsorption species have been considered to be ethanol. 3,7 On the other hand, as reported in the literature and discussed in our experimental paper, 2 oxide film of several nanometer thickness forms during electropolishing of Al in the perchloric acid-ethanol based solution.…”

“…In our previous studies, 1,2 the crystallographic orientation and voltage dependence of nanopattern morphology and size of electropolished Al with known crystallographic orientation was experimentally revealed and established for Al electropolished in the mixture of perchloric acid-ethanol electrolytes with rather high volume percentage of ethanol. In the literature, several theoretical models have been proposed to explain the mechanism of nanopattern formation and predict existing regions of various nanopattern morphologies.…”

“…However, this model did not incorporate the effects of interfacial energy and surface structure anisotropy, and thus failed to predict the coexistence of different types of nanopattern morphologies. 2 Guo and Johnson 4 extended the dissolution-adsorption model by including the effect of interfacial energy. The introduction of interfacial energy adds additional stability to the interface by restraining areas of high surface curvature, and modifies the parameters (voltage, interfacial energy) region where stable patterns can exist.…”

“…Moreover, we already found that their predictions of existing regions of various nanopattern morphologies disagree strongly with our experimental results of monocrystalline Al electropolished in a perchloric acid-ethanol based solution. 2 Therefore, we propose that an improved model needs to be developed by explicitly considering the calculated anisotropic interfacial energy of Al/electrolyte interface for Al planes with different orientations. The new model should correctly predict existing regions of various nanopattern morphologies.…”