# Zinc Oxide-Based Rotational-Linear Triboelectric Nanogenerator
## 1 Introduction  
Triboelectric harvesting has been identified as a very promising technology for the conversion of mechanical energy to electrical energy due to its low cost, abundance of available materials and high conversion efficiency even at low operating frequencies [1, 2, 3, 4, 5]. The operation of triboelectric generators (TENGs) is based on the charge exchange between two surfaces that are in contact (triboelectrication) and the subsequent electrostatic induction when these surfaces are set in relative motion. A major category of TENGs focuses on the conversion of rotational energy that could be induced by air and water flow, car wheels, etc. In general, rotating TENGs can be classified, according to their design, into two categories: (a) disk type and (b) cylindrical type. Further classification can be performed based on triboelectric materials (dielectric-dielectric or dielectric-conductor) and the operation mode (sliding, contact separation, non-contact). One of the first approaches of a segmented disk TENG was presented by [PERSON] et al. [6]. The tribogenerator was based on sliding triboelectrication and periodic overlapping of the segmented disks due to rotation. Large output currents were obtained at a high frequency (117.6 \(\mu\)A and 29.0 mA/cm\({}^{2}\) of 66.7 Hz at a rotating speed of 1000 rpm). An updated version of the disk generator was later presented by the same group [7] in order to improve the operational characteristics, the longevity and the stability of the device. The new design was based on a freestanding triboelectric layer on the rotational disk, while the electrodes were located onto the stationary disk. [PERSON] et al. [8] presented a direct-current triboelectric nanogenerator (DC-TENG) based on a rotating-disk design. It consisted of two disks and two pairs of carbon fiber flexible brushes that contacted the electrodes. Due to its design, the tribogenerator was serving as a constant current source for direct driving electronic devices. [PERSON] et al. [9] demonstrated a cylindrical rotating triboelectric nanogenerator (TENG) based on sliding electrification. The TENG was based on a core-shell structure with an alternative strip on the surface. During rotation, the relative sliding between the contact surfaces of the core and the shell results in an \"in-plane\" lateral polarization that drives the external current. A power density of 36.9 W/m\({}^{2}\) was obtained by a rotating TENGwith 8 strip units at a rotation rate of 1000 rpm. [PERSON] et al. [10] presented a non-contact cylindrical rotating TENG for harvesting mechanical energy from water flow such as that in pipes and sewers. The generator was operated in freestanding mode between two rotating interfaces to minimize abrasion of the electrodes. When placed in water flow, the output voltage and the current of the TENG reached 1670 V and 13.4 uA, respectively. An eccentric-type TENG, consisting of a fixed metal electrode, a rotor-independent layer and a base, was proposed by [PERSON] et al. [11]. Due to its design, the TENG could effectively reduce the wear of the friction layer and improve the durability of devices. A rolling friction contact-separation mode triboelectric generator was presented by [PERSON] et al. [12] that was fabricated for harvesting vertical rotation energy by utilizing the integrated cylindrical surface with the conjunction of rolling contact electrification and electrostatic induction. Several rotational TENGs have also been presented for harvesting wind energy [13]. For instance, [PERSON] et al. [14] developed a continuously rotating TEG, aiming to operate at marginal wind velocities by maximizing the efficiency of the sliding contact, which was a one-dimensional fiber rather than a 2D contact area. A ring-type TENG for rotational energy harvesting was presented by [PERSON] et al. [15]. The device could also be used as a self-powered rotational speed sensor. These are only some examples of the various designs that have been presented for harvesting rotational energy.