Collocations for solemn - adj.

be, look, seem, sound
She usually had a smile on her face, but now she looked solemn.

become, grow
His face grew solemn.

remain

extremely, fairly, very, etc.
He addressed them all in very solemn tones.
Her mood was rather solemn.

most
Our most solemn duty is to protect the public.

Solemn is used with these nouns:
ceremony, declaration, duty, eye, face, oath, occasion, procession, promise, ritual, tone, undertaking, vow, word