Niobium-doped ZnO: an excellent multifunctional energy material

Doping provides an interesting pathway for designing highly electroactive fillers in metal oxides for application in self-powered energy materials. The presence of dopants in metal oxides such as zinc oxide (ZnO) modifies their energy storage and harvesting performance. Thus, here we produced niobium (Nb)-doped ZnO or Zn_1-xNb_xO (x = 0, 0.01, 0.02, and 0.03) nanorods using a facial hydrothermal method. Interestingly, Nb doping in ZnO improved the crystallinity and induced the growth of uniaxial nanorods, particularly for 2 wt% Nb-doped ZnO (Zn_0.98Nb_0.02O). This composition showed maximum energy storage density (37 J cm^-3), energy storage efficiency (82.23%), and dielectric constant (880), with low dielectric loss, outperforming undoped ZnO and the other Nb-doped variants, thereby indicating its good capacitance behavior. In terms of its piezoelectric properties, Zn_0.98Nb_0.02O showed an exceptional piezoelectric coefficient (d₃₃ = 513 pm V^-1), output voltage (V_oc = 18 V) and power density (6.75 µWcm^-2). The piezoelectric nanogenerator (PENG) based on this material displayed excellent durability corresponding for 216000 cycles, storing around 1.68 μJ of energy for a 2.2 μF capacitor. In terms of energy-harvesting applications, this material displayed appreciable output voltage for various biomechanical movements like finger tapping, hand clutching, clapping and wrist pressing. Thus, the fabrication of Nb-doped ZnO lays the groundwork for the development of excellent energy storage and conversion devices.