what are the requirements for an investment visa ?

You need to apply for a F type one. You should prepare the following documents:
1. Your passport (valid for at least six months and with two blank pages)
2. filled application form affixed with a recent passport size photo
3. An "Invitation Letter of Duly Authorized Unit" or "Confirmation Letter of Invitation" prescribing the visa validity, number of entries and stay period.
4. in some situations, the applicants are required to provide a copy of hotel resrevation and return air ticket booking order and financial proof.