Elsevier

Nano Energy

Volume 32, February 2017, Pages 125-129
Nano Energy

Hydroelectric generator from transparent flexible zinc oxide nanofilms

https://doi.org/10.1016/j.nanoen.2016.11.050Get rights and content

Highlights

  • Transparent flexible ZnO nanofilms can efficiently harvest wave energy based on waving potential.

  • It is the first time that waving potential is observed in materials other than atomically thick graphene.

  • Deep insights into the influence of material's electrical conductivity on the induced waving potential was given.

Abstract

Harvesting wave energy based on waving potential, a newly found electrokinetic effect, is attractive but limited mainly to monolayer graphene. Here we demonstrate that moving a transparent flexible ZnO nanofilm across the surface of ionic solutions can generate electricity. The generated electricity increases linearly with the moving velocity with an open-circuit voltage up to tens of millivolt and a short-circuit current at the order of microampere. The harvested electricity can be efficiently scaled up through series and parallel connections. Theoretical simulations show that it is the proper electrical property that endows the ZnO nanofilm with the outstanding capacity in harvesting the wave energy.

Introduction

In the pursuit of high-efficient energy conversion technology, nanomaterials offer much promise due to their exceptional sensitivity to external stimulations [1], [2], [3], [4], [5]. The great achievements in nanogenerators based on piezoelectric and triboelectric effect have stimulated tremendous passions in the exploration of environmental energy harvesting by nanomaterials [1], [6], [7], [8], [9], [10]. Ceaseless waving seawater covers more than 70% of the surface of the earth, thus possessing inexhaustible energy [11]. Capturing the wave energy could have special importance for powering intelligent devices in remote ocean. However, the effective strategy is underexplored [12], [13], and traditional approaches based on electromagnetic effect face socioeconomic challenges and also bring up biophysical impacts on the marine environment [14]. Use of nanomaterials to harvest wave energy is an emerging trend with promising prospect in terms of cost and efficiency [15], [16], [17], [18].

Recently, we found that a monolayer graphene sheet placed across the surface of waving water can generate electricity, which is referred to as waving potential, providing a novel electrokinetic pathway to harvest wave energy [19], [20]. The waving potential is created by moving boundary of the electrical double layer (EDL) formed at the interface of graphene and ionic solutions. However, the waving potential drops sharply when the layer number of graphene increases to two and three, and almost vanishes for other materials including graphite, gold, copper and silicon with thickness over tens of nanometers [19], hindering the applications of waving potential. Therefore, cost-efficient, transparent and flexible substitutions of graphene is highly desired [19], [20].

Zinc oxide (ZnO) is a wide band gap (~3.37 eV) semiconducting material with outstanding electrical and optical properties [21]. The hexagonal wurtzite structure endows ZnO with a notable piezoelectric property, giving rise to enormous applications, especially in piezoelectric nanogenerators [1], [6], [22]. ZnO nanofilms can now be prepared facilely by sol-gel techniques, pulsed laser deposition and filtered cathodic vacuum arc in large area at low cost [23], [24], [25], [26], [27]. These processes can be conducted at room temperature and [27], thus, ZnO can be deposited directly onto plastic and flexible substrates. Moreover, ZnO nanofilms show good transparence over the entire visible spectrum. Given these advantages, ZnO nanofilms would be a promising candidate if showing notable waving potential effect [28], [29].

Here we demonstrate that the ZnO nanofilm prepared through filtered cathodic vacuum arc can generate voltage up to tens of millivolts when moving across the interface of ionic solution and air. The influences of interval time between two moving cycles, velocity, ions concentration, have been comprehensively investigated, showing the versatility of ZnO nanofilms in harvesting ambient energy.

Section snippets

Device fabrication and measurements

The ZnO nanofilm used in this work was sputtering deposited by filtered cathodic vacuum arc at room temperature [27]. Polyester terephthalate (PET) was selected as the substrates for its high transparency and flexibility. The PET substrates are cut into 4×8 cm2 size and the thickness of ZnO film was controlled to be around 50 nm by adjusting the deposition time. The oxygen pressure was kept at 1×10−4 Torr during the deposition to maintain a square resistance of the obtained ZnO film around 20

Results and discussions

Fig. 1a and b show the photographs of a ZnO nanofilm sample, highlighting its transparence and flexibility. Fig. 1c shows a schematic illustration of the experimental setup. The ZnO sample was inserted into and then pulled out from 0.6 M NaCl solution (~ 3.5 wt% as seawater) periodically at a constant velocity, which is controlled by a variable-speed motor (Fig. S2). The typical electric voltage signal recorded during one inserting-pulling cycle at a velocity of 13.5 cm/s was shown in Fig. 1d.

Conclusions

In summary, we demonstrate for the first time that moving a transparent flexible ZnO nanofilm across the surface of ionic solutions can produce voltage up to tens of millivolt. The waving potential in the ZnO nanofilm not only inherits the attractive properties previously observed in monolayer graphene, but also shows a gradually decay behavior, giving rise to a wide time window for capturing the electricity. The exponential dependence of the induced voltage on the interval time between two

Acknowledgments

This work was supported by 973 program (2013CB932604, 2012CB933403), the National NSF (51472117, 91023026, 11172124, 51375240, 51002076) of China, Jiangsu Province NSF (BK20130781, BK2011722), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (0414K01), the NUAA Fundamental Research Funds (NP2015203) and the Funding of Jiangsu Innovation Program for Graduate Education (CXZZ13_0150). S. P. Lau acknowledges the support from the PolyU grant (1-ZVGH).

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