Oggi is correct; however, doing so MAY create a potential tax exposure problem for you.
You see, I believe if you don't charge interest, the IRS can still tax you on what they figure you should have earned in interest. The IRS has many "interest rules" regarding making personal loans so the bottom line is that you should talk with your CPA or Tax Advisor on the best way to proceed.
In fact, if one follows IRS guidelines the setting of a loan rate actually depends on:
1) The month your promissory note is created, and
2) The term of the loan: Short-term (3 years or less), Mid-term (> 3 and <=9 years), Long-term (> 9 years).
The setting of what rate is appropriate is "controlled" by the IRS via their monthly Index of Applicable Federal Rates (AFR) Rulings, which you can see here: http://www.irs.gov/app/picklist/list/federalRates.html