Remove Sensitive Data From Work Phone I've moved on from a previous job, and as part of the process I need to return my work provided phone. 

I have factory reset the phone, as well as electing to erase the SD card. There was some sensitive data on the phone (pictures, documents etc) that I want to make sure are unavailable to whoever uses the phone next. 

Can I be certain that this data cannot be recovered via an undelete application? Does the factory reset + erase sd write zeros to the drives or simply mark them as empty space?
One more hint is Nuke My Phone:


  Nuke My Phone protects your privacy by secure deleting your personal data, such a photos and videos, and returning your phone to factory settings. Now you can safely repurpose old devices or upgrade your current one without worrying about unauthorized data recovery. Nuke My Phone delivers the peace of mind that comes only from knowing your personal data cannot be compromised.


Works directly on the device.