pH play important role in the size of nanoparticles.  At high pH reduction rate is high. At low pH (below 5) instead of reduction oxidation will occur.  At very high pH (above 9, 10) reduction rate will be too much high resulting an aggregation of your nanoparticles.  So that is why pH 8 is desirable in order to synthesized smaller size nanoparticles

The concentration of protons in solution could affect the reduction of Ag+ ions as well as the speciation of whatever you are using to reduce these ions. At the end the effect will be grouped in the standard reduction potential, which could favor or not the generation of small particles. 

Respected Sir, If you have authenticated result, could You send me that answer (Published articles) sir.....

I do not have any publication regarding this, it is common knowledge. If you want to get insight of your process you must know who is running the reduction process. 

Thank you sir

pH play important role in the size of nanoparticles.  At high pH reduction rate is high. At low pH (below 5) instead of reduction oxidation will occur.  At very high pH (above 9, 10) reduction rate will be too much high resulting an aggregation of your nanoparticles.  So that is why pH 8 is desirable in order to synthesized smaller size nanoparticles

For the reduction of Ag+ to Ag NPs the pH, as mentioned above by other colleagues, has to be higher than 7. In alkaline solution Ag+ cations are first converted to Ag2O ( grey solid) and then in the presence of a reducing agent, Ag2O is reduced to Ag NPs.

Obviously, the size of NPs, in all ractions, depends on reaction conditions, one of which is the pH.