Standard lab grade acetic acid I have used for 5 min to remove the oxide layer. if it not works, keep Cu substrates in FeCl3 solution for 1 min to remove the top layer of Cu.
It depends which particular CU foil you are using... there are some Cu foils from Alfa Aesar which have some coatings which must need to remove.... The best is use Acetic acid to clean Cu foil, just immerse Cu foil piece in it for 2-5 minutes and remove and rinse with water and blow dry with N2, the one with HCl and FeCl3/HCl etch the surface a bit which is not that good for growth. I hope this will help.
You can find a very complete investigation of that matter in the supplementary information associated to my article 10.1039/C8NR09841G. Many methods are available, some inefficient, for instance acetic acid is indeed inefficient if you have a coating (as mentioned by Irfan) with certain types of commercial foils. I also have tried several techniques mentioned to work in the literature, but to no avail in my case. Obtaining a cleaning technique efficient in all cases is not trivial. It really depends on what graphene quality you want to get in the end. So it is really to be tested in your specific case. Regards.
when dealing with copper foils with various types of anti-tarnish coating we recommend an optional two step process to remove the anti-tarnish, the copper oxides and any contamination such as finger prints. First use an alkaline copper surface prep formulation at ambient for 30 sec, DI rinse and dry. If it does not remove the anti-tarnish coating spike with 0.25% peroxide. If the ant-tarnish is still not removed, then add an acidic/peroxide blend with organic additives as a pretreatment step to remove the anti-tarnish first then process through the alkaline formulation without peroxide spiking. Best Wishes. High pH alkaline last treatment will provide you with minimum copper oxide thickness < 30 nm in the form of cuprous oxide which will bind with graphene via covalent bonding.