change attribute names in serialized output - to_xml or to_json

When serializing an object, is there a way to change the name of an
attribute outputted without changing the underlying model (i.e.. I do
not want to change model attributes or methods? I have a class that
has an association called materials... I would like the tag generated
when I call to_xml to be <contents> vs <materials>. Another example
is I have a method called get_url... I would like the tag to be <url>
vs <get_url>...