Fig. 1: The electron–proton synergistic doping for WO₃ film.

a The schematic diagram of electron–proton synergistic doping route. The whole system is immersed into an acid solution. The purple bubbles are free protons and the yellow column is a low-work function metal particle. b The mechanism of electron–proton synergistic doping effect: electrons flowing from metal with a higher Fermi level (i.e., lower work function W_m) to oxide with a lower Fermi level (i.e., higher work function W_s) at the interface. c The work function values for Zn and WO₃. d Experiments prove that Zn and WO₃ can realize the synergistic effect, which can be observed by eyesight due to the electrochromic effect. e Schematic depiction: this synergistic effect has a good spatial selection. f Sharp zinc pen was used to write “WO₃” in WO₃ film in acid solution. g This synergistic effect is reversible by 300 °C annealing in air.