I had skype calls with few professors which led to a few acceptances. They mainly asked why I am interested in their program, and why do you want to pursue PhD. They also discuss about future goal of their research since it is usually not on their website. All I did was making sure I understood their research thoroughly, be able to discuss about mine, as well as understanding about the program. You can ask about their research project, how do they operate their lab, the environment in lab, or funding, rotation required in the program.
I wore a shirt, so just business casual. Make sure you have steady wifi, no disturbance. Be yourself and excited about the science. You will be fine. Good luck!
For the record: I'm chem PhD applicant this cycle