I feel thiourea is better. Though we have used Na2S and thiourea both in different synthesis, we have not studied the relative merit or demerit.  However, the possibility of Na getting incorporated in to the product cannot be ruled out. Though no noticeable Na was found ( I don't remember the limits now as it was done years ago), still it can be in smaller concentrations. We have also used the method of passing H2S through the metal precursor ( eg. cadmium nitrate) solution. This may be the best source.

Thank you very much Dr. Manjunatha Pattabi, for helpful answer.

Thank you. You can have some of our papers on CdS if you give your mail id or write to [email protected].