I did Zeta potential and also XPS for my ZnO nps it that i got potential of -25.6mv. but from XPS analysis it showcasing ZN2+ ions are present.. kindly explain how i have to describe.
Thank you
Gouranga Dutta
The XPS method is used to study the surface of solid samples in vacuum. When irradiated with X-rays, electrons are generated and detected from atoms on the surface.
The zeta potential is measured in a dispersed system of these particles. The disperse potential arises at the boundary of the discontinuity of the diffuse layer of nanoparticles (in solution) when they move in the process of electrophoresis of a disperse system.
In addition to Yuri Mirgorod 's good answer, please be aware that other oxidation states than +2 and 0 are really rare for Zn, so finding them in stable NPs in a stationary state would require very strong evidence. Maybe you have something intermediate in your elctrochemistry, but I doubt it constitutes the "bulk" of the nanoparticle.
Dear Gouranga Dutta
XPS is giving elemental composition and oxidation state of your compound, it won't give the surface charge of your nanoparticles. Zeta potential is giving the electrostatic interaction charge between your nanoparticles and the solvent. When you disperse the nanoparticles in a solvent it forms a layer between your nanoparticles and solvent and that layer is called the stern layer. Zeta potential is giving that layer charge and that's why we are getting hydrodynamic size from the DLS, not the actual size of the nanoparticle.
Shama Parwin
I have to intervene in your answer. The electric double layer consists of a dense adsorption layer (Stern layer) and a diffuse layer. The Stern layer has nothing to do with the zeta potential. During electrophoresis of nanoparticles with a double layer, the diffuse layer breaks along the glide plane with the rest of the solution, and the zeta potential appears. Zeta potential is measured in volts. The charge is measured in coulombs. The measurement of the zeta potential is not related to the DLS method. DLS measures the diffusion of nanoparticles, and then the diffusion is converted into particle size. If you don't know the problem, you don't need to become an expert. A dictionary of colloidal chemistry will be useful to you.
Data Yu.A.Mirgorod, R.V. Grebennikova. Glossary on Colloid Chemistry.
Yuri Mirgorod
Sir,
I agreed zeta potential is measured in volts but when we talk about the charge from a chemistry point of view, charge means positive and negative charges are carried by charge carriers, like protons and electrons. According to the question, he asked he is asking zinc oxide Zn is having +2 charge in XPS, that I m explaining to him that +2 is an oxidation state of zinc along with oxide but when he disperse the zinc oxide in water (he did not mention the solvent but zeta potential only we can measure in solution form, not in solid), so water is forming a solvent cage on the surface of the zinc oxide surface.
Shama Parwin
You don't want to listen to a professor with 50 years of experience. Keep consulting. Let them read your excuse and learn from you.
One very minor comment on 'The measurement of the zeta potential is not related to the DLS method'. This comment is absolutely true in the strictest sense. However, zeta potential is inferred from mobility (movement) in an electrical field (voltage/distance). Hence the same equipment used to measure the diffusion coefficient (mobility) of particles in the absence of an electrical field - the basis of the DLS method - can often be used to provide the basis of the zeta potential calculation by measuring the movement under an electrical field.
Back to the original question, which has been (partly?) answered above by a number of posters. XPS will measure (with some assumptions) the oxidation state of elements present on the surface (perhaps up to 10 or 15 atomic layers) of a system. In air all surfaces are fully oxidized and thus zinc will be shown in the +2 oxidation state. (In the 1970's we showed that even gold was in the +3 oxidation state on silica-supported catalysts). Zeta potential refers to the charge on the slipping plane of particles in liquid suspension as correctly stated by @Yuri Mirgorod. It is unrelated to the oxidation state of the elements on a surface. A positively charged surface (note not oxidation state) will attract the counter ions (negative) from solution and effectively bind them to the surface. This means that when a particle moves it's surrounded by a swarm of these strongly attracted counter ions. Thus a positively surface charged particle (through chemistry or chemisorption induced segregation) can provide a system where the particle surrounded by the counter ions has a negative zeta potential and will move to the positive electrode under a DC voltage.
The fallacy is found in the first sentence in the question: 'Zeta potential of ZnO NPs found -ve, means it's having -ve charge on surface...'. No! A negative zeta potential does not mean a negatively (surface) charged particle. Usually the converse is true due to the strongly absorbed counter ions.
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