Superior response in ZnO nanogenerator via interfaced heterojunction for novel smart gas purging system

Highlights

• Fast, direct and scalable method to synthesis Heterojunction.

• Fabricated nanogenerator and compared with intrinsic nanogenerator.

• Improved the performance by 3–4 folds.

• Fabricated the Smart purging system for Chemical labs and food industry.

Abstract

A novel direct way to synthesis Zinc oxide-tin dioxide heterojunction (ZTH) using a simple grinding technique without employing successive processes has been introduced. A facile, rapid and economical fabrication strategy was studied using various characterization tools. The underlining mechanism of adhesion was studied using finite elemental simulation tools. The heterojunction was further processed into PDMS based composite for the realization of nanogenerator. The ZTH showed a 4 fold improved response compared to intrinsic ZnO which is based on novel n–n bulk heterojunction principle. The peak to peak voltage was around 12 V with a current of 60–70 nA. The calculated power density was 1.7 mW/m² and load sensitivity was around 1.5 V/N. The device was further demonstrated to charge the commercial capacitors and is used for biomechanical energy harvesting. Finally, a novel piezoelectric smart gas purging system (P-SMGP) was demonstrated using the device which may find applications in chemistry lab and food industry.

Introduction

Over the past decade, the fabrication of nanostructured metal oxide semiconductor has been the area of intense research due to their exclusive electronic, optical and chemical properties. Such properties have led to a nanotech oriented metamorphosis from materials level to functional devices stage. The use of metal oxides has been used in various devices like bio and chemosensor, transistors, LASER and energy harvesting. Energy harvesting using piezoelectric materials is one such key areas of research in the past few years [1], [2], [3], [4], [5], [6], [7], [8]. The use of Zinc oxide in energy harvesting is widespread due to its relatively superior piezoelectric coefficient, low impedance characteristics and charge transfer characteristics. Additionally, its rich diversity of nanostructure and extremely cost-effective deposition techniques made it easier to use at bigger scale. Such material properties have been extensively exploited towards augmentation using various strategies like doping, annealing, nano-structure growth optimization, junction engineering, and interface engineering [8], [9], [10], [11]. A heterojunction is a modest approach towards the enhancement of piezo potential by several folds without employing the crucial chemical procedures of doping and high-temperature annealing procedures. Moreover, large band gap metal oxides usually show a poor doping efficiency over a period of time because of the deep acceptor energy level [12], [13]. Besides, heterojunction technique is relatively stable over a period compared to doping and annealing, is economical and user friendly as articulated in previous reports [14]. In heterojunction enhancement, as the name suggests, the presence of junction of different band structures of metal oxide at the interface facilitates improved charge transport characteristics compared to individual oxides. This led to remarkable discoveries in the field of transistors, lasers and solar cells [15], [16]. However there is limited number of reports on heterojunction based devices in piezoelectric enhancement. In this area all the previous heterojunction reports are based on simple p–n junction concept where a p-type material is coupled with the n-type semiconductor to reduce the number of free charge carriers. This leads to a reduction of screening of piezo potential thus giving rise to increased overall output [9], [14], [17], [18]. Here to the best of our knowledge we are introducing for the first time the use of two n-type heterostructure of different electron affinity and ionization potential to improve the piezoelectric output. The use of ZnO:TiO₂, ZnO:WO₃ and ZnO:SnO₂ have been widely used in the field of photocatalysis and gas sensing [19], [20], [21]. When a load acts on the piezoelectric material i.e. ZnO the charge formation occurs due to non-centrosymmetric lattice structure. This leads to the movement of electrons from valence band to conduction band leaving few holes behind. Now these highly energetic electrons move from high energy conduction band edge to low energy conduction band edge of another semiconductor. Instantaneously the holes are introduced in the reverse direction for suitable valance band offset. This increases the rate of charge separation and reduces the rate of electron–hole pair recombination [15]. The fabrication is done in core–shell or surface contact strategy however the mechanism remains the same. Among the several metal oxides, tin dioxide is a well-known multifunctional, large band gap and inexpensive semiconducting materials which finds numerous applications in the field of solar cell, sensors, batteries and field emission etc. [22]. Moreover being a superior electron acceptor than titanium oxide makes it a better candidate for the heterostructure based devices. In this context various strategies have been executed to produce a ZnO:SnO₂ heterostructure like calcination, electrospinning, solvothermal, electrochemical and sol–gel etc. in which every technique is a double step process because single step process leads to impurities of Zn₂SnO₄ due to calcination as reported before [23]. Here we report for the very first time the fabrication of single step heterojunction fabrication process without the involvement of extreme chemical and physical procedures. The process is simple grinding in which both the oxides are ground together in certain proportion for 10–15 min in order to favor the adhesion of SnO₂ nanoparticles to that of ZnO particles. The adhesion mechanism is explained in detail using simulation studies. The adhesion was verified using EDS and DRS analysis. The ratio of different concentrations was taken into account and optimized for the best output. The device current and potential were measured by the force of 2N. The device was further tested to charge different commercial capacitors to store the energy. The load resistance analysis was performed to calculate the peak power and power density. The device was tested at various forces to calculate the force sensitivity of the device. Finally the device was tested under simple biomechanical forces.