  Hydrogenation is an effective way to tune material property1-5. Traditional
techniques for doping hydrogen atoms into solid materials are very costly due
to the need for noble metal catalysis and high-temperature/pressure annealing
treatment or even high energy proton implantation in vacuum condition5-8. Acid
solution contains plenty of freely-wandering protons, but it is difficult to
act as a proton source for doping, since the protons always cause corrosions by
destroying solid lattices before residing into them. Here we achieve a facile
way to hydrogenate monoclinic vanadium dioxide (VO2) with protons in acid
solution by attaching suitable metal to it. Considering the Schottky contact at
the metal/VO2 interface, electrons flow from metal to VO2 due to workfunction
difference and simultaneously attract free protons in acid solution to
penetrate, forming the hydrogens dopants inside VO2 lattice. This metal-acid
treatment constitutes an electron-proton co-doping strategy, which not only
protects the VO2 lattice from corrosion, but also causes pronounced
insulator-to-metal transitions. In addition, the metal-acid induced hydrogen
doping behavior shows a ripple effect, and it can spread contagiously up to
wafer-size area (>2 inch) even triggered by a tiny metal particle attachment
(~1.0mm). This will stimulate a new way of simple and cost-effective atomic
doping technique for some other oxide materials.
