I used Hummer's method to do GO synthesis. I did FTIR and XPS measurement. I didn't find carboxyl group. There are all hydroxyl group. I want to know why.
There may be possibility that you have prepared reduced graphene oxide where the -OH and -COOH groups are very less. It may be possible that you are observing the characteristics peak of water molecule adsorbed in your sample. Try to rectify it with powder XRD also. Check your procedure also for GO preparation using improved Hummer method.