Realization of a room-temperature/self-powered humidity sensor, based on ZnO nanosheets

Abstract

In this paper, we report the first use of ZnO nanosheet-based direct-current nanogenerators (NG), as a room-temperature self-powered humidity sensor. It has been shown that the networked ZnO nanosheets are structurally stable under external mechanical pressures; hence these 2D nanostructures can be proposed as an emerging/attractive candidate in the field of mechanical energy harvesting. Also, benefiting from the high surface adsorption activity and large surface-to-volume ratio of ZnO nanosheets, we propose the fabricated NG as a promising self-powered gas sensor. Our study demonstrates short circuit and open circuit sensitivities of about 43.4% and −2.96%, because of humidity exposure. It is notable that the achieved output short circuit sensitivity is more than 10 times higher than the maximum sensitivity of a recently reported self-powered humidity sensor, based on ZnO nanorods. High sensitivity and stability, as well as direct-current output current are attractive properties of the realized humidity sensor, which can be proposed as a promising self-powered sensor.

Introduction

It is well established that metal-oxide nanostructures have high surface to volume ratio, stability and surface adsorption activity, which entitles them as attractive candidates for gas sensors [1], [2]. Among metal-oxides, ZnO nanostructures benefit from coupled semiconducting/piezoelectric properties which can be applied to realize a new generation of self-powered gas sensors. Since 2006, ZnO nanowires (NW) have been introduced as active components of piezoelectric nanogenerators (NGs), which convert mechanical energy to electricity [3], [4], [5], [6], [7], [8], [9], [10], [11]. Piezoelectric NGs can potentially convert different mechanical sources, including: (i) mechanical-movement energy, such as body or muscle movement [12], [13], [14], [15], [16], [17]; (ii) vibration energy, from acoustic or ultrasonic waves [3], [4], [5], [6]; (iii) and hydraulic energy, such as the flow of body fluids or blood, or dynamic fluids in nature [18]. As the most ideal application of NGs, it is desired that piezoelectric NGs will be applicable to power implantable devices such as pacemaker [19], remote patient monitoring or environmental monitoring. In this line of research, “self-powered devices” were firstly proposed by Xu et al. [11], when they fabricated sensors, powered by piezoelectric NGs. The concept of self-powered devices is harvesting of mechanical energy from ambient sources to power electrical devices, without the need for batteries. Since then, numerous groups reported different self-powered devices, including self-powered pH sensors [11], [20] and self-powered UV-detectors [21]. Kim et al. have shown that external surface charges on ZnO NWs can influence on the piezoelectric behavior of the ZnO NWs and nanotubes [22]. Regarding this, Xue et al. reported the application of NGs based on ZnO nanowires, as a self-powered active gas sensor, and other groups have pursued similar studies on ZnO nanowires/nano-arrays [23], [24], [25].

On the other hand, 2D semiconducting nanomaterials such as nanosheets, nanoplates, and nanowalls, have attracted much attention due to their novel and attractive physical/chemical properties including nanometer scale thickness, high surface-to-volume ratio, and high mechanical durability [26]. In spite of these interesting advantages, there are few reports on 2D nanostructure-based devices, especially in the field of piezoelectric NGs [26], [27], [28]. The other worthy point about the ZnO nanosheet-based NGs, is their direct-current signal generation which is very crucial due to possibility of direct utilization of their generated power for nanodevices. However, most of the previously reported NGs generate alternating current (AC)-type output, requiring a rectification circuit, which increases the total size of the power package and dissipates power.

In this paper, combining the semiconducting/piezoelectric properties of ZnO nanosheets, we propose a direct-current ZnO nanosheet-based NG as an efficient self-powered gas sensor. To the best of our knowledge, there has been no report on 2D nanosheet-based direct-current self-powered gas sensors. In addition to large surface-to-volume ratio and high surface adsorption activity, high mechanical and chemical stability are among other worthy properties of ZnO nanosheets, which can entitle the realized NG as an efficient gas sensor. ZnO nanosheets are synthesized by hydrothermal method, which is simple equipment, low temperature, cost effective and catalyst-free method, and also benefits from large area uniform production and environmental friendliness. High output sensitivity and efficient direct-current energy scavenging are proved as the benefits of the proposed self-powered gas sensor which is able to actively detect various gases, without requiring external power.