Possibly around 25 or 26. But most of the time there is no peak of graphene during XRD. the best tool is Raman if your substrate dont have screening effect/
The XRD of graphene should be about ~25 2(theta). however if you start from single sheets of graphene oxide eventually this peak of diffration could be absent due to complet intercalation of TiO2. See my work synthesis of graphene -ceo2 composite at my page. I hope it can help you.
i agree but you should have this in mind it depends if you have decorated graphene or graphene oxide ? and if you have reduced the graphene oxide in the decoration process??