frequency domain zero padding in OFDM
Zero padded OFDM (i.e.) adding zeros after IFFT is used to combat Inter symbol interference, suppose that there is multi path in the system that spans over L samples then atleast L zero samples are prefixed at the transmitter after IFFT  so that these can be thrown away at the receiver and then recover ISI free IFFT frame / OFDM symbol. 

The advantage of zero padding also has to do with power saving at the transmitter, the transmitter is simply switched off during the transmission of zero padded L samples duration. This has the advantage of saving power when compared to cyclic prefix OFDM which should then also transmit the cyclic prefix.

Zero padded OFDM though can leave to noise boosting and this is a disadvantage when compared to cyclic prefix OFDM. The equalization also would be non linear in case noise boosting is to be avoided. There is a lot of literature on zero padded OFDM you could refer to that as well.

Regarding the null sub carriers at the ends, these are implemented to reduce the ACLR and make the design of anti aliasing filters easier ( the cut off can be less sharp) hence save cost