Tunable Work Function of Mg x Zn 1- x O as a Viable Friction Material for a Triboelectric Nanogenerator.

Since the invention of triboelectric nanogenerators (TENGs), their output performance has been improved through various approaches such as material surface modification, device structure optimization, and so on, but rarely through the development of new friction materials. In this work, a magnetron sputtered Mg _x Zn _{1- x} O film is developed as a viable friction material that rubs against polydimethylsiloxane in a TENG. The work function, measured by Kelvin probe microscopy, of the Mg _x Zn _{1- x} O films can be effectively tuned by varying Mg composition, x, and exposed surface facets, which are shown to dominate the charge-transfer behavior. In addition, film thickness also plays an important role, affecting the output performance. The output voltage and total charge of a TENG with a Mg _x Zn _{1- x} O film are demonstrated to be tremendously enhanced by 55 and 90 times, respectively, compared to that of a TENG with a ZnO film. Even more intriguingly, the tribo-output polarity can be reversed by adjusting the relative work function through varying the preferred growth orientation of the Mg _x Zn _{1- x} O film, for a given value of Mg content.