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[Display omitted] • Triboelectric and tribovoltaic effects of novel Cu x O-based nanogenerators are investigated. • Triboelectric series of Cu x O are adjusted by impurity and valence. • The Cu x O/NaCl(aq) tribovoltaic nanogenerators exhibit a high-power density of 4900 nW/cm2. • Relative positions of Fermi levels for various liquids are determined for the first time. • Rectifier/capacitor-free self-powered electroplating is demonstrated by solid–liquid tribovoltaic nanogenerators. Cu x O is a promising p-type semiconductor, but its triboelectric properties are unexplored. This study investigates both the triboelectric properties of Cu x O and the solid–liquid tribovoltaic effects between Cu x O and various liquids. Cu x O submicron rods are synthesized on Cu foams, doped with Li, and subjected to oxidant reactions. Li doping and valence changes increase Cu x O's work function, as shown by Kelvin probe force microscopy. Solid-solid triboelectric measurements reveal systematic changes in triboelectric voltage and current, with Cu x O negatively shifting in the triboelectric series. On the other hand, the Cu x O/liquid tribovoltaic effect conducted in an immersion-type device generates a stable direct current. The magnitudes of the tribovoltaic current and voltage systematically vary with the energy level differences between Cu x O and the liquids. From the direction of tribovoltaic current and magnitude of tribovoltaic voltage, the relative positions of fermi levels for several liquids are determined. Among all, the best Cu x O/NaCl (aq) tribovoltaic nanogenerators exhibit a high-power density of 4900 nW/cm2, and the generated DC is applied for novel rectifier/capacitor-free self-powered electroplating. This work demonstrates a viable method to adjust the triboelectric properties of Cu x O and proposes a method to determine the relative energy levels of liquids for the first time. [ABSTRACT FROM AUTHOR]

Abstract Pseudocapacitive materials are promising to increase specific capacity of carbon-based electrodes, but suffer from poor rate capability. In the work, novel Li:CuO/reduced graphene oxide flexible electrodes demonstrate enhanced electrical conductivity and specific capacitance obtained via the incorporation of Li into CuO particles. The degree of capacitance enhancement increases with scan rate, reaching a maximum value of 40% at a high scan rate of 500 mV/s. Galvanostatic charge/discharge measurements show high volumetric capacitance, energy density, and power density. Cyclic life tests show excellent stability over 10,000 cycles, with a capacitance retention of 93.2% at a scan rate of 100 mV/s. Graphical abstract High-performance Li:CuO/RGO flexible electrodes for supercapacitors are demonstrated for the first time. Li doping enhances electrical conductivity, energy density, and rate capability of CuO/RGO electrodes, opening up the opportunity to fabricate low-cost and excellent supercapacitor electrodes. Unlabelled Image Highlights • Li-doped CuO were synthesized to improve the performance of RGO-based supercapacitors. • Conductivity, energy density, and rate capability of CuO/RGO electrodes were enhanced by Li doping. • The Li:CuO/RGO electrode demonstrates high volumetric capacitance and energy density. • Cyclic life tests show excellent stability over 10,000 cycles, with a capacitance retention of 93.2%. [ABSTRACT FROM AUTHOR]

Oxide semiconductors are promising for gas sensing, but low-concentration gas sensing at room temperature (RT) remains a challenge. In this work, novel Li- and Na-doped CuO single-crystalline nanowires (NWs) and nanobelts (NBs) are synthesized using a low-cost process via stress-assisted growth and in-situ doping. The results show that the dopants induce a morphological transformation from NWs to NBs. Tauc's plots show an increasing redshift in the optical band gap with increasing Li or Na concentration, with a maximum shift of 0.075 eV from 1.2 eV for Li-doped CuO. The devices made of doped one-dimensional (1D) nanostructures (NSs) demonstrate enhanced conductivity and RT gas sensing sensitivity. Li-doped CuO NSs exhibit the greatest improvement in conductivity (10 4 -fold), and enhanced gas sensitivity and selectivity to ethanol at a low concentration of 2 ppm at RT. A Mulliken charge-related mechanism is proposed to explain the excellent sensing performance. [ABSTRACT FROM AUTHOR]

ZnO/CuO p - n heterojunctions have attracted much attention for device applications, but coherent junctions, which are crucial for controlling electrical properties, still remain a challenge due to different crystal structure. In this work, CuO single-crystalline dots are coherently synthesized on ZnO nanorods by using a proposed two-step process. Transmission electron microscopy images confirm the formation of CuO coherent dots on single-crystalline ZnO nanorods upon annealing the nanorods covered with a poly-crystalline Cu x O layer. The coherent dots exhibit two types of epitaxial orientations: CuO [002] ǀǀ ZnO [ 10 1 ¯ 1 ], CuO [111] ǀǀ ZnO [0002], and CuO [002] ǀǀ ZnO [ 10 1 ¯ 1 ¯ ], CuO [111] ǀǀ ZnO [ 000 2 ¯ ]. As the thickness of the as-deposited Cu x O layer increases from 10 to 30 nm, the aspect ratio of the resulting CuO dots decreases from 0.43 to 0.21, approaching a film-like morphology. This work provides a route to prepare CuO coherent single-crystalline structures on ZnO, which is one step further toward fabricating excellent CuO/ZnO nanodevices. [ABSTRACT FROM AUTHOR]

We propose a simple system to investigate the influence of microstructure on the resistive switching behavior via bi-crystal CuO nanowires. CuO nanowires are prepared by thermally oxidizing transmission electron microscopy copper grids in air. Single-crystal and bi-crystal CuO nanowires can be selectively obtained by adjusting the temperature. The devices made of single-crystal nanowires follow Ohm’s law, with a high resistance, within the sweeping voltage range of 0–4 V, whereas those made of bi-crystal nanowires exhibit threshold and memory resistive switching behaviors, which are due to the enrichment of copper ions in the grain boundaries of bi-crystal CuO nanowires providing sources for the formation of conductive filaments. Moreover, the bi-crystal nanowires with higher defect densities in grain boundaries result in lower threshold voltages of switching from high to low resistance states. The threshold resistive switching behavior can be turned into memory resistive switching behavior by increasing the thickness of the device electrodes or reducing the compliance current. The endurance of memory resistive switching through the pre-defined conduction paths in the single grain boundaries of bi-crystal CuO nanowires is at least 1000 cycles without any performance deterioration. This high reliability is ascribed to the single conductive filaments. [ABSTRACT FROM AUTHOR]