Enhancing charge transfer for ZnO nanorods based triboelectric nanogenerators through Ga doping

Triboelectric nanogenerator (TENG) has been developed as a promising green energy source, but the working mechanism as regard to the surface condition in charge transfer is still under investigation. Therefore, the work is devised to study the charge transfer efficiency by varying electron concentration at the surface of ZnO nanorods (NRs) arrays through Ga doping as a means to change surface chemistry of a given material when rubbing with PDMS and nylon. The hydro-thermally grown ZnO NRs arrays remain vertically aligned upon Ga doping from 0.18 to 0.69 at%, effectively generating free electron concentrations from 2.63× 10¹⁴ to 1.80× 10¹⁸ cm⁻³, compared to 4.24 × 10¹⁶ cm⁻³ corresponding to the un-doped ZnO NRs. The triboelectric output voltage, current density and tribo-charges density of the Ga-doped ZnO NRs are remarkably enhanced by up to ~13, ~90 and ~15 times, respectively, compared to the un-doped ZnO NRs. This work verifies the concept of n-type semiconductors in favor of transferring electrons to the counter materials bound to receiving electrons such as PDMS in the triboelectric series and vice versa to the materials bound to donating electrons such as nylon. This behavior is mainly ascribed to the work function difference in proposed band diagrams with surface band bending as well as the kinetic constraints to the ease of charge transfer. This work not only deepens the understanding into the fundamental charge transfer mechanism governing the triboelectrification efficiency but provides a facile method to maximize the range of positions in the triboelectric series for a given semiconductor to optimize device design for TENG.