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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 MgxZn1–xO 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 MgxZn1–xO 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 MgxZn1–xO 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 MgxZn1–xO film, for a given value of Mg content.

Graphitic carbon nitride (g-C3N4) is first explored in the application of energy capture by taking advantage of strong piezoelectricity derived from intrinsically non-centrosymmetric holes, leading to in-plane piezoelectricity even in multilayers or bulk structures. In the present work, we demonstrate the successful fabrication and efficient energy harvesting of g-C3N4-based piezoelectric nanogenerators (PENGs), where a several-fold enhancement of output performance is achieved by modulating precursors through tuning the intrinsic lattice strain and crystallinity of g-C3N4. Specifically, the best output voltage and a current density up to 1672 mV and 141 nA/cm2are obtained from piezoelectric g-C3N4synthesized using blended precursors of urea and melamine. This work sheds light on the investigation of g-C3N4-based PENGs for energy harvesting. There is clearly potential to develop multifunctional self-powered g-C3N4-based devices by combining the excellent optoelectronic, piezoelectric, and mechanical properties of g-C3N4.

The advancement of nanogenerators (NGs) enables self-powered wearable devices　 to respond to environmental parameters, which encourages new application scenarios to be explored. In the work, an immersion-type triboelectric NG (TENG) is fabricated to harvest mechanical energy from running water and acts as a novel self-powered water-speed sensor by fluorinated graphite paper (FGP). The oscillograms of output currents driven by flowing water are comprised of major direct current (DC) and minor alternative current (AC) signals. The DC signals are attributed to tribovoltaic mechanisms between the FGP and water, and the AC signals are attributed to triboelectric mechanisms between the FGP and ions in water, testified by subsequent experiments in NaCl(aq). The FGP-water TENG demonstrates the output current density and voltage of 3.3 μA/cm2and 65mV at a water speed of 4.2cm/sec, showing 84 and 14-fold enhancements, respectively, compared with the GP-water TENG. The sensing of water speed by using the output current of the FGP-water TENG shows a linear calibration curve with R2larger than 0.99. In addition to water speed sensing, bubble sensing in an aquarium, and energy harvesting in dynamic water by ultrasound are also demonstrated by the FGP-based device. The new applications and mechanisms involved in the work provide new insights into the triboelectric and tribovoltaic mechanisms in water and novel ideas to monitor and utilize water resources.