hollow vs hollow out

hollow is a noun but hollow out is not a noun.

Word	Noun	Adjective	Verb	Adverb
hollow	Yes	Yes	Yes	No
hollow out	No	No	Yes	No

As verbs, hollow and hollow out are synonyms defined as:

Other synonyms of hollow include core out.

hollow (noun)	hollow out (noun)
a depression hollowed out of solid matter
a small valley between mountains
a cavity or space in something

hollow (adjective)	hollow out (adjective)
not solid; having a space or gap or cavity
lacking in substance or character
devoid of significance or force
as if echoing in a hollow space

hollow (verb)	hollow out (verb)
remove the interior of	remove the interior of
remove the inner part or the core of