Peaks in XRD arise due to reflection of x-rays from different lattice planes in a crystal. XRD peaks shift whenever the distance between the lattice planes changes. In a composite of metal oxide with reduced graphene oxide, this lattice spacing changes.

Your metal oxide may be partially reduced by RGO and so the metal ions tend to have a low average valence. As a result, the metal ion radius increases and the lattice of metal oxide expands, usually leading to your XRD peaks shifting to lower angles.

Without knowing the exact details like what is the metal oxide and how the composite has been prepared....it is difficult to say.

It is based on Bragg's law....as lattice spacing (d) changes, the angle changes leading to shifts in the peaks.

Can you please provide some papers on it?

oxide of copper or zinc.

It also depends on the preparation conditions. Here is some additional information which can be of help to you.

Check if RGO can reduce your oxide based on their redox potentials - unlikely

Also you might want to check the composition of your composite i.e., elemental analysis etc

if i am preparing composite  by sonication, then what can be the possible reason of peaks shifting?

Just check if the result is different when you do the sonication without RGO? All other experimental parameters being the same.

Also can you give me all the experimental conditions involved in the experiment like sonicator power, solvent conditions like composition, pH etc?

Just check if your material is stable under the experimental conditions of your experiment?

I don't know the experimental conditions, but my feeling is that the conditions are not  harsh for RGO to be incorporated in the lattice.