Lao-tzu vs Lao-zi

Lao-tzu and Lao-zi both are nouns.

Word	Noun	Adjective	Verb	Adverb
Lao-tzu	Yes	No	No	No
Lao-zi	Yes	No	No	No

As nouns, Lao-tzu and Lao-zi are synonyms defined as:

Lao-tzu and Lao-zi: Chinese philosopher regarded as the founder of Taoism (6th century BC)

Other synonyms of Lao-tzu include Lao-tse, Laozi.

Lao-tzu (noun)	Lao-zi (noun)
Chinese philosopher regarded as the founder of Taoism (6th century BC)	Chinese philosopher regarded as the founder of Taoism (6th century BC)