can an etched surface of silicon be approximated as a negatively charged planar polyion?

Answer 1

Sure. You can approximate a cow as a sphere of water too if you want (ask any physicist).

Since the etched surface of a metal IS a negatively charged surface, that particular approximation is indeed good.

A bulk metal is neutral, but the electrons on the surface are not surrounded by nucleii on all sides like the ones in the center, so the surface is more negative. In addition, the surface of even a freshly etched metal will be at least partially oxidized, and them oxygens loves them some electrons, making the surface even more negative.