Police officers absolutely will not accept a tip, gift or gratuity of any kind. To accept a gratuity of any kind would put that case and all their other cases under suspicion due to what many would consider "a bribe" or "a conflict of interest". If a police officer accepted a tip, gift or gratuity, it would be unethical; and, could possibly cost them their job.
If you really want to thank an officer for outstanding duty, donate money to a cause such as a missing-child foundation that survives on donations in that officer's name. Write a letter of commendation to the officer's station - it will be put on their record. Write a letter to the editor of your local paper thanking this officer. There are many ways to thank them without offering a gratuity.