By heating you can remove hydroxyl groups, if it is an oxide material you can heat in ordinary air atmosphere, if it's a metal sample you must heat in specified atmosphere like nitrogen and argon. Mostly 200 C is enough for removing those OH groups 

I would try to put them in some reducing atmosphere (H2?) or disperse them in some liquid quich contains some suitable reduction agent.

If surface hydroxyl group appears from the aqueous phase only, it can be removed by heat-treating at  >200oC .

Try reducing with hydrazine, since after reduction reaction the by products are N2 and H2O.

By heating you can remove hydroxyl groups, if it is an oxide material you can heat in ordinary air atmosphere, if it's a metal sample you must heat in specified atmosphere like nitrogen and argon. Mostly 200 C is enough for removing those OH groups 

I would say yea and nay. Heating under reduced presure at low temeperatures may do the trick (P< 1 mBar and T< 100°C). However at high temperatures you will have competing processes like oxidation of the solid beneath. I would suggest that depending on the nature of the solid (nano-silicates) or nano-metals with oxidation potential you may use T below 100°C under reduced pressure.

However a word of caution, metallic nanoparticles an hydrazine are pasionate lovers.  The smaller the particle, the faster is the reaction. Send a student to do the mixture. A well trained student, master in explosions and fires, and get an insurance for god's sake.

An yea, N2 atmosphere is a good habit to keep oxidation off your material.

I agree with the idea of thermal treatment of your samples under Ar ambience at a temperature ~200 oC.

heating at 80 0C  for 10 hrs and thereafter  washing with 18  megaohm water for 6-7 times is helpful in removing the hydroxyl group from the surface of  the nanoparticles 

Can you please mention clearly what kind of nanoparticle it is and thermal behavior of that nanoparticle?  Actually, there are several ways of removing OH- from nanoparticle surface. The suitable way will depend on the type of nanoparticle. As for example, if the nano particle is dissolve in aqueous suspension and if we can make sure the existance of H+ that will produce H20. But question arises what will happen to the nanoparticle surface when OH- will remove? does it leave a + bond or not?

So it is very difficult to answer unitl knowing the surface chemistry of the nanoparticle and their purpose of synthesis.

I can also suggest low power microwave treatment, preferrably in an inert gas flow. Especially if high temperatures are undesirable.

It is a pity, the material of your nanoparticles is not mentioned. Giving a general advice is sometimes not much help. I can only say that the heating in air or other atmospheres will not necessarily work. You can see, for exapmle, here

http://pubs.acs.org/doi/pdf/10.1021/la00075a004

that after 180-200°C in vacuum, the surface of silica is fully hydroxylated and you get rid only of adsorbed water. As far as I know, not only silica behaves this way.

If you really want to avoid hydroxylation of the surface, you probably should consider non-aqueous synthesis mathods, such as the one described here:

http://pubs.acs.org/doi/abs/10.1021/la800470e

Have you think about chemical functionalization of the nanoparticle´s surface?Good luck

Dear Professor Mehta:

Only now, a see this very interesting question, which exhibits a great set of information and contributions. But I would like to add a bit  of ideas, if nanoparticle be oxide.

I think that a solution is case sensitive. In this sense, structure, material and size of nanoparticle can affect the sorption phenomena, then chemisorption or physiosorption (Van Der Waals) can be carried out. This event can be expected since major fraction or all defects, in a nanoparticle, are positioned at surface.

When physio-soption is responsable by "the hydroxilation" and this is the single chemical specie adsorbed in a general way using a flux of carrier gas with small help of increasing of the temperature leads to clean surface. Similar approach can be conducted with vacuo and temperature.

Otherwise, if phenomenon in question exhibits some kind of chemisorption, calcination under inert carrier-gas and temperatures at around 150 oC can not be sufficient. Sometimes, particular name to OH groups and its bond energy are given and referred as hidratation water and crystallization water (OH groups develop chemical bonds with primary characteristics), but these terms are proper only when the crystalline structure is involved. At surface phenomena, chemi and physiosorption a priori should be adequate and reliable. Thus, it is possible that exist one or more site at surface with very great affinity by a molecule or molecular site or vapor meaning a kind or event with characteristic of chemisorption. In this case, a calcination with carried gas as N2 and temperatures at around 100-150 oC  can be not sufficient to clean the surface.  As a strategy to maintain the parameter temperature as low as viable is possible to use a functional carrier-gas as a mixture of N2 and O2, instead of pure and inert N2. Depending on the class of oxide and cationic species, a integral atmosphere of O2 can be further considered.  In this sense, also, a mixture of O2 and O3 can be engineered exploiting the possibility of decomposition O3 in O2 and O-.

Kind regards

Marcos Nobre

Interesting comments, I have been working in the synthesis of magnetic nanoparticles by co-precipitation route, as it is known this method used N2 or Ar atmosphere to avoid oxidation of Fe3O4 to Fe2O3 during the synthesis; besides researchers used surfactants containing hydroxides to give stability, however sometimes the used of OH- groups form phases like goethite --- FeOOH in the surface of the nanoparticles, can this phase formation be a consequence of hydroxilation?, and if it is, should I avoid this by increasing the temperature of the synthesis up to 200º C N2 atmosphere or I have necessarily to dry the sample and then calcined at that temperature?  Plz. I wait some comments! Thanks!

Regards

Juan Ramos

I do not know your case but with my gold nanoparticles based electrodes, I succeed in removal of OH by applying -1.2 V potential, constantly for certain timing. Maybe you can give your electrode a try. just setup a normal standard three electrochemical electrode system with ref, counter, and working electrode.

Removing of -OH group there are so many method but you have to look which kind of product are you looking on that way you can chose  

Application of Iron Oxide Nanomaterials for the Removal of Heavy Metals

UV/Ozone Activation Treatment for Wafer Bonding..

Completely agree that the key thing here is the kind of particle you are producing and the "matrix" that it will be sitting in.

However, I would be very wary of calling the residual -OH groups a "dead layer".  These hydroxyls can be highly active and in the wrong setting will really mess up you day.

The two papers below might be of use in understanding this phenomenon (and there are some references to the manufacture of the particles used in these two papers).

Article The Effect of Ormosil Nano-Particles on the Toughness of a P...

Article Organic-inorganic hybrid nanoparticles: Adsorption character...

Different organic functional groups have different binding ability with the surface atoms of inorganic nanoparticles. For example, the carboxylic group has strong binding with surface atom of FexOy nanoparticle or FePt nanoparticle, and the organics cannot be completely removed even the nanoparticles are annealing at 500 C, a temperature which exceed the boiling point of the surfactant. (DOI: 10.1126/science.287.5460.1989) For your case, thermal characterization can be performed using TGA/DSC to check if the material with hydroxil groups will be desorbed or decomposed at elevated temperature.

Depending on the nature of the nanoparticles that you are trying to produce, then you might want to look at a Stober process, potentially with a range of different precursors.

However, I would be wary of referring to hydroxyl groups as "dead" -in some systems they bond extremely well, to the extent that they actually give rise to problems with the mechanical properties of materials.  This can be turned to your advantage though, so you can use a compatibiliser, similar to a 'size' applied to glass fibres for GFRP composites.  The trick here is to be mindful of the chemistry of the surface and of the matrix that the nanoparticles will be used with.