Jixue Lei

Flexible piezoelectric nanogenerator based on Cu2O–ZnO p–n junction for energy harvesting

In this study, a nanogenerator based on Cu2O–ZnO p–n junction has been fabricated on Cu wire substrates for harvesting mechanical energy from the environment. The flexible nanogenerator is composed of a Cu substrate, a Cu2O layer, ZnO nanorods and an outer Au-coated paper electrode; the Cu2O layer was obtained by oxidizing Cu wires directly and the ZnO nanorods were grown on the Cu2O layer using a low-temperature hydrothermal method. The existence of the Cu2O–ZnO p–n junction makes a contribution towards reducing the number of excess electrons in the ZnO, which facilitates in improving the output signal and also overcomes short circuits. An Au-coated paper electrode can involve more nanorods in the power generation process. The DC output voltage was up to 42 mV and the maximum output current density was 400 nA, which are approximately a 13-fold higher voltage and a one order of magnitude larger current in comparison to devices without a Cu2O layer, respectively. This study may provide important insight into a facile fabrication method for low-cost and high-performance energy harvesting devices.

Fabrication of p-NiO/n-ZnO heterojunction devices for ultraviolet photodetectors via thermal oxidation and hydrothermal growth processes

AbstractnIn this research work, a p-NiO/n-ZnO heterostructure was fabricated using thermal oxidation and hydrothermal growth processes. The p-NiO films were oxidized at different temperatures. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and UV–visible spectral analysis were used to characterize the p-NiO/n-ZnO heterostructure. The results indicated that the NiO films oxidized at higher temperature have wider optical band gap and lower defect density. In particular, by comparing the photoresponse properties of the UV photodetectors oxidized at different temperatures we suggest that the oxidation temperatures have a great influence on the photoresponse time. The defect density of NiO film decreases with increasing oxidation temperature. And the defect density affects the photoresponse characteristics that the decay time decreases with the decreasing of defect density as the NiO oxidation temperature increases. This work could serve as a valuable guideline for designing and improving the p-NiO/n-ZnO UV photodetectors in a low-cost and large-scale way.

Branched ZnO nanotrees on flexible fiber-paper substrates for self-powered energy-harvesting systems

In this paper, branched ZnO nanotrees (NTs) have been synthesized on flexible fiber-paper substrates by introducing a multistep hydrothermal approach for realizing high-performance piezoelectric nanogenerators. With this method, a significant enhancement in output voltage of the NGs ranging from 14 mV to 0.1 V was achieved, with a nearly 20 times enhanced power density compared to the vertically grown ZnO NWs. This is the first demonstration of fabricating branched ZnO NTs-coated fiber paper for energy harvesting devices, which may provide guidelines for designing high-performance piezoelectric energy harvesting.

Enhanced performance of wearable piezoelectric nanogenerator fabricated by two-step hydrothermal process

A simple two-step hydrothermal process was proposed for enhancing the performance of the nanogenerator on flexible and wearable terylene-fabric substrate. With this method, a significant enhancement in output voltage of the nanogenerator from ∼10u2009mV to 7u2009V was achieved, comparing with the one by conventional one-step process. In addition, another advantage with the devices synthesized by two-step hydrothermal process was that their output voltages are only sensitive to strain rather than strain rate. The devices with a high output voltage have the ability to power common electric devices and will have important applications in flexible electronics and wearable devices.

Controlled growth of ZnO nanorods on common paper substrate and their application for flexible piezoelectric nanogenerators

A simple and effective method of synthesizing nanorods (NRs) and the ability to control the size and aspect ratio of them are crucial for fabricating nanodevices. In this paper, we present a systematic study of the growth of ZnO NRs on common paper substrates using a hydrothermal approach by adjusting the growth conditions. By a slight variation of the solution concentration and the growth time, significant changes in morphology and size (aspect ratio) of the obtained ZnO NRs have been controlled. Moreover, the piezoelectric power generation from ZnO-paper nanogenerators grown with different precursor concentration and growth time are also investigated. It is found that the morphology and aspect ratio of NRs have significant influence on the piezoelectric behavior. This type of flexible piezoelectric nanogenerator will have potential applications in implantable biosensors and wearable self-powered electronic devices.

Fabrication of flexible nanogenerator with enhanced performance based on p-CuO/n-ZnO heterostructure

A p-CuO/n-ZnO heterostructure was synthesized on an indium tin oxide coated flexible substrate (polyethylene terephthalate) by a simple and low-cost hydrothermal method. Scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction were used to characterize the as-synthesized heterostructure. The results indicated that flower-like CuO nanostructure conformably formed on the ZnO nanorods. Furthermore, a nanogenerator based on the p-CuO/n-ZnO heterostructure was fabricated for harvesting energy from environment, and the output performance of the device was investigated. A larger output current of ~100xa0nA was obtained in comparison with ZnO based nanogenerator, which could be ascribed to the formation of p–n heterojunctions in the CuO/ZnO composites, reducing the screening effect of excessive electrons. This strategy may provide a highly promising platform for realizing a high-performance flexible piezoelectric energy harvester by new composite piezoelectric material.

Piezoelectric nanogenerator with 3D-ZnO micro-thornyballs prepared by chemical vapour deposition

Piezoelectric nanogenerators have been intensively developed in terms of their materials and applications; however, only modest structural progress has been made due to limitations in the growth mechanisms of nano-materials. In this work, a piezoelectric nanogenerator based on ZnO micro-thornyballs (ZMTBs) was introduced. ZMTBs were synthesized by chemical vapor deposition method without the presence of any seed layers or substrates. Electrical characterization was subsequently performed to reveal the characteristics of the contacts formed between the ZnO micro-thornyballs and the copper electrode. The electric output ability of the ZNTTs nanogenerators has been studied in reference to the experiment and the numerically calculation.

A vibration-driven nanogenerator fabricated on common paper substrate for harvesting energy from environment

There are numerous sources of mechanical energy in our environment, such as ultrasonic waves, body movement, and irregular air flow/vibration. Here, we present a simple, cost-effective approach for fabricating a flexible nanogenerator and apply it to harvest energy from environmental mechanical vibrations. The nanogenerator was based on ZnO nanorods grown on common paper substrate using a low-temperature hydrothermal method. Piezoelectric currents were measured by attaching the nanogenerator on the surface of a cantilever and a wind-up drum, respectively. At the same time, the vibrations of the cantilever and wind-up drum could also be characterized by the corresponding output signals. This is a practical and versatile technology with the potential for converting a variety of environment energy into electric energy, and also with the application for pre-warning of emergency, such as earthquake and burgling.

Piezoelectric effect of 3-D ZnO nanotetrapods

In this work the piezoelectric effect of 3-D ZnO nanotetrapods (ZNTs) is studied by the finite element method. The results show that the nanogenerator based on ZnO nanotetrapods has a number of advantages over the generators based on traditional hexagonal nanorods due to the unique tetrapod structure. Different from double-terminal sensors, the sensors based on ZnO nanotetrapods can give multiple responses to a single signal at the same time. Therefore, ZnO nanotetrapods could be designed as multiterminal strain sensors for enhancing sensitivity and directivity. Here we demonstrate that lateral bending of ZnO nanotetrapods (ZNTs) results in higher output piezopotentials than vertical compression. Effect of geometric size on piezopotential is also studied. The results provide guidance for optimizing the output of piezoelectric nanogenerators and designing high-performance strain sensors.

ZnO nanorods array/BaTiO3 coating layer composite structure nanogenerator

In this study, we fabricate a nanogenerator device based on a composite structure formed by ZnO nanorods array/BaTiO3 coating layer for harvesting mechanical energy from environment. The whole device is composed of a Si substrate, a ZnO nanorods array, a BaTiO3 coating layer and a top Cu foil electrode. The ZnO nanorods were grown by a hydrothermal synthesis technique and the BaTiO3 thin layer was coated on the surface of the ZnO nanorods by rf magnetron sputtering process. It is a simple and low-cost growth method to make the ZnO nanorods array act as a template to increase the area of the subsequently deposited BaTiO3 layer. The device produced an enhanced average current of 105xa0nA, which was 7 times larger than the devices with only piezoelectric ZnO nanorods array and 3.5 times larger than the devices with only piezoelectric BaTiO3 thin film, respectively. The possible reasons contributing to the enhancement of the output current were discussed.