At low pH, the Ni ions (apparently) are coordinated more effectively by the carboxylic acid groups at the GO surface than by possible ligands in the surrounding, acidified medium. Under these conditions, you can think of the GO surface as a "sponge" extracting positively charged Ni ions from the surrounding medium and localizing them on its surface.

Raising the pH increases the concentration of OH ions, which coordinatively saturate the Ni ions and extract them from the GO surface into the basic medium. As a result, the carboxylic acid and phenol groups at the GO surface (which no longer coordinate Ni ions) are free to be deprotonated by the basic medium, and the GO surface acquires an abundance of negative charge.

At low pH, the Ni ions (apparently) are coordinated more effectively by the carboxylic acid groups at the GO surface than by possible ligands in the surrounding, acidified medium. Under these conditions, you can think of the GO surface as a "sponge" extracting positively charged Ni ions from the surrounding medium and localizing them on its surface.

Raising the pH increases the concentration of OH ions, which coordinatively saturate the Ni ions and extract them from the GO surface into the basic medium. As a result, the carboxylic acid and phenol groups at the GO surface (which no longer coordinate Ni ions) are free to be deprotonated by the basic medium, and the GO surface acquires an abundance of negative charge.

Zach Erno. Thank you so much.

Yes, I was about to say much the same thing as Zach Erno said. If true, then coordinating something like Co2+ to GO and oxidizing it to Co3+, might well give a stable non-labile coordination complex to the GO, which then would be even more permanently positively charged. You would have a graphitic anion exchange

substrate.

If the Ni+2 ions are adsorbed and probably deposited as a metal coating, the charge could change to positive. (Lucien Veleva)

Thanks Douglas Klein and L. Veleva.

The graphitic anion exchange substrate sounds quite intriguing. Could you provide review papers on that? Thanks in advance.

I presume the amount of cation adsorbed onto GO sheet can be tuned by changing the pH of the solution. What kind of instrument is suitable to confirm the coordination of cation on the GO?

I wonder how the bandgap of GO would be after coordinating it with different types of cation. Any idea on that?

I'm agree with Dr. Veleva

The positive charge of the GO might depend upon the deposition of the metal ion. If the Ni2+ ion is deposited over GO then it could be positively charge. And for band gap you have to study UV-vis DRS. There may be increase or decrease but this could be calculated by UV-vis DRS analysis. The presence of Nickel particle on GO could be known from FESEM and TEM analysis

Some colleagues already stated important details. Indeed it exists the possibility that Ni cations are coordinated towards any type of oxygen atom present in the GO by the O->Ni bond formation. Since the most external part of the new GO->Ni material is positive hence the zeta potential becomes positive. The most important part herein is the analysis of the magnitude of the new potential vs the original one to determine the neat effect of cation covering. Hope this helps.

This inspires me to ask, what is the charge on pristine monolayers of non oxidized "graphene"?

Although Ni-coordinated graphene oxide is positively-charged, its surface charge is still relatively weak (compared to negatively-charged GO) to prevent aggregation. Any idea how I can keep positively-charged GO in a stable dispersion?

James Deluca,I believe pristine graphene has no net charge on its surface.

Try metals with higher charge (Al3+ for example) instead of Ni2+. Or cationic surfactants (CTAB for example) if they are Ok for your purposes.

Thanks S. Alekseev. I ought to give it a try.

Co^3+ forms especially stable coordination complexes, which once formed are often not reacted very easily -- that is, they are very non-labile. To get Co^3+ coordinated to the oxo-groups in GO might be tricky if one starts with a Co^3+ compound, since the Co^3+ might well then already be stabily coordinated. A way to get around this might be to start with the much more labile Co^2+ and try to get it coordinated to the GO, then oxidize the Co^2+ to Co^3+, while the Co is coordinated to the oxo-groups of GO. I would guess that one could start with aquo-Co^2+, as should be what is present in dilute solutions of CoSO4 or CoCl2. One might well need to be careful that the oxidizing agagent taking Co^2+ to Co^3+ does not oxidize the graphene too much.

Ni^2+ typically forms fairly labile coordiantion complexes, so that I would imagine that the extent of coordination should be significantly dependent on pH. I would imagine that the Co^3+ once properly coordinated to GO should remain so coordinated.

Douglas, Co3+ could be stabilized only by high-field ligands. GO has only -CO2H groups and other O-containing low-field stuff. I have very big doubts it will stabilize Co3+. If we speak about inert complexes, Cr3+ will be better choice for GO, but to make complex boiling is needed. I don't know what happens with GO after it, and is there any profit comparing to Al3+, Ti4+, etc.

As it suggested surfactants works well in most of the time. But if you use positively charged surfactant instead of Ni, you make an other colloid. In this situation, you stabilize the negatively charged GO with surfactants.

you can use negatively charged or neutral surfactants for your GO/Ni system.

Stability of colloid sol depends on ionic concentration and in many cases on pH as well. So I suggest you to try playing with cation concentration and/or the pH as well. but do not forget, that many metal ion forms hydroxide at high pH. different source of cation could be your goal too.

you can also make positively charged GO with surface modification. If you change -OH groups (some of them or most of them) to -NH2 groups for example you can tune the zeta potential and the charge. But it needs organic chemistry route and cleaning the product from by-products and reactants, absolve solvent changes are could be difficult.

If the resistivity of pristine graphene is well below 200 Ohms based upon readings from a simple resistivity meter in the bulk powder, does the resistivity and inverse conductivity decrease when structures are built and measured at the submesoporous (microporous)size regimes as indicated in technical papers such as Boon-Hong Wee indicates with the graphene oxide over conductive organic polymer?? The measurements of resistivity appear extremely high and appear to exhibit low conductivity?

S Alekseev: Co^3+ coordination to carboxylate or OH^- is in fact well known -- I am less sure about neutral carboxylic acids. Werner's first inorganic enantiomers were {Co[(OH)2Co(NH3)4]3}^6+ involved OH^-, especially on the central Co^3+. Also trisoxalato-cobaltate (III) is very well known.

Could someone tell me the value of achieving coordinated metal hydroxides to graphene oxide. Unless low valent metals can be coordinated with pristine graphene it will exhibit no value. Please correct my understanding? oxides of graphene with oxides or hydroxides of metals derived from colloids, I am assuming metal chloride derivations?? Is not the holy grail so to speak ultimate conductivity with pristine graphene and low valent amorphous metals? Please compare and contrast?

James, your are absolutely right that the conductivity of pristine graphene is high. However, reduced graphene oxide, which can be obtained by reducing oxygenated graphene, has higher resistance due to the structural defects. If not mistaken, the substrate on which the pristine graphene is deposited will also affect its conductivity.

What about the thickness of the graphene layers. Does this affect conductivity?

Not only does the thickness and oxygenated structural impurities matter for electrical conductivity, but also the types of boundaries which occur matter. The zig-zag boundary ends up putting a certain amount of density of states right at the Fermi level -- the amount being proportional to the amount of that edge: 1/3 extra orbital (localized near the edge & at the Fermi level) per unit cell of such edge. Other edges give other extents of such additional density -- including 0 for the arm-chair edge. There are different ways to understand such edge-localized conductive orbitals -- including via a simple resonating valence-bond argument. The number of such orbitals per unit cell of edge has been argued to be given by

u = {[n(*,2) + 2 n(*,1)] - [n(o,2) + 2 n(o,1)]} / 3

where n(*,d) is the number of "starred" degree-d pi-centers per unit cell of edge and

n(o,d) is the number of "unstarred" degree-d pi-centers per unit cell of edge. And this is verified for a few cases with quantum computations, or expt.

I would not imagine that coordinating Ni^2+ or Co^3+ to carboxylate (impurity) groups in graphene or GO would enhance the conductivity. Metal salts (or coordination complexes) very seldom manifest metallic conductivity. I would imagine the most noticable effect would be on the modification of the pi-electron network of graphene. If the impurities are randomly placed, I would expect this to hinder conductivity. On the other hand, if the "impurities"are non-randomly placed at the right spots (and now perhaps better called "decorations"), then the conductivity can be enhanced -- this particularly can happen when there is an imbalance in an effective "removal" of starred & unstarred C atoms from the pi-network.

It is effort only:

Graphene oxide, a yellow solid with C: O ratio between 2.1 and 2.9, is a non stoichiometric compoud with interstitial sites of 5.0 nm2 having some sp2 carbon that retains the layer structure of graphite but with a much larger and irregular spacing. Some eletrons get trapped in these sites or in between the layers and make it negatively charged sol. When doped in nickel nitrate solution, Ni(II) ions get replaced by occluding electrons and in the process, some extra Ni(II) are trapped in these interstices to make it a positively charged sol.

Dear Manohar, how would this doping "interstitial between platelets?" improve conductivity in the bulk? For example what would be an approximate qualitative resistivity if the composition from a slurry of your suggested composition was applied to a substrate exhbiting several microns of thickness?

Sir, no doubt, both (G t) and (GO) are highly conducting (cf. n-type), but we are discussing

Ni (II) doped positive sol. The cation doped sols do conduct (may be p-type / movement of holes); of course to a lesser

extent than those of the normal n- type conductors