Creative Commons licenses should not be used for works that are in the public domain since their use is already completely open. Adding a Creative Commons license to something already in the public domain would have to effect of adding restrictions to the use rather than allowing more use.
If you are aware of something that is in the public domain, there is a Creative Commons mark that can be used to identify the work as being in the public domain. This is known as the Public Domain Mark** and it's use helps others understand that it can be freely used.
** 
