Dear,

Capping agents cover the refine surface of nanoparticles and prevent them from coagulation or aggregation. For metal oxide use CTAB or Twin 80 as surfactant.

 Zinc oxide nanoparticles were synthesized using chemical method in alcohol base. During synthesis three capping agents, i.e. triethanolamine (TEA), oleic acid and thioglycerol, were used and the effect of concentrations was analyzed for their effectiveness in limiting the particle growth. 

http://www.sciencedirect.com/science/article/pii/S002223130900088X

may give some idea

Dear panneer, yes capping agents are material dependent,even selection capping agent depend on the method of synthesis,nature of metal oxide, and conditions  you are going to apply also,but in your case  use of  CTAB,TITRON X-100,PVP OR POLYOLS will be the right choice, capping agent more probably increases stability of  NP'S ,dec aggregation, keeping NP away from one another,even it effects morphology and  size of NP .I suggest you to study some publications related to your question may it will help you better.

THANK YOU...

DEAR Muhammad,

You said one of the capping agent is PVP. so how should be the capping agent.PVP solution or PVP in the form of solid? what is the difference between having ,the capping agent in the form of solution or solid .

THANK YOU.

DEAR KARTHIk

Am gng to prepare metal oxide nanoparticles..

The purpose of adding capping agent is to prevent  agglomeration and attaining nano size.Surfactants(capping agents) can be classified as cationic,anionic and non-ionic.You can chose suitable one

Capping agents are used in solution form.

Dear Panneer,Capping agent is always used in form of solution, solvent is mostly D.I water but other can also be used, just it should be noted that solvent used must remove easly after formation of nanoparticles.

Regard.

Muhammad Imran khattak 

thank you..

Very good suggestions are given Karthik and others

Capping agents/ligands are needed to control nanoparticle growth and agglomeration during synthesis.