Molecular thermal motion harvester for electricity conversion

Energy is an absolute necessity for human civilization. Today, no matter where the energy comes from (primary energy sources include nuclear energy, fossil energy such as oil, coal, and natural gas, or renewable energy such as wind,¹ solar,² geothermal, biomass, and hydro-power), people still find it most convenient to use electricity, a secondary form of energy.³ In the era of the Internet of Things (IoT) and 5G, energy demands are decentralized, mobile, and ubiquitous; thus, it is increasingly necessary to utilize renewable and green energy that is off-grid and portable.^4,5 Some mini- and micro-scale energy sources, such as airflow, human movement,^6–8 blood flow, ultrasound,⁹ etc., have already been explored and converted into electricity by various nano-energy generator technologies based on different schemes/mechanisms.^10–13 Most of these conversions are based on mechanical energy.

Molecular thermal motion is a special kind of dynamic motion¹⁴ that is essentially different from ordinary mechanical motion. It is a component of the internal energy of the physical system, which means that the molecules of all substances are in constant and random movement above absolute zero temperature. Brownian motion of particles is one example that is caused by the molecule thermal motion of the surrounding liquid or gaseous molecules. Molecule thermal motion contains an enormous amount of energy, taking an ideal gas as an example, the average kinetic energy of thermal motion per mole of gas molecules at room temperature (27 °C) is 3.7 kJ.¹⁴ If this form of energy could be utilized from the huge amounts of liquids and gases on the planet effectively, this would provide a new source of energy on an enormous scale. However, to our knowledge, the energy of the liquid molecular thermal motion has never been utilized as an energy source to date, perhaps due to the challenge of creating a viable device that can convert this energy to form electricity. In this work, we demonstrated that it was feasible to convert the energy of molecular thermal motion into electrical energy by using a molecular thermal motion harvester (MTMH). Because of the random collision of liquid molecules, small and slim objects^15,16 such as nano-tubes, nano-wires, nano-rods, and nano-sheets with one-ends attached to a wall or surface will undergo worm-like Brownian motion such as bending, flexing, or wriggling, when suspended in the liquid instead of moving around as a whole due to the geometry constraint.¹⁷ The factors influencing Brownian motion are the friction coefficient (interfacial tension), the viscous drag force from the surrounding fluid flow and temperature, etc.¹⁷ Recently, the Brownian motion of tethered nanowires in a liquid has been reported.^17,18 If these nanowires are made of a material that possesses piezoelectric properties, bending or deforming these nanowires will lead to piezoelectric potential. In an array of piezoelectric nanowires, each wire will develop a piezoelectric potential individually.⁹ Therefore, by designing a device that can regulate the direction of the output electric current, the thermal motion energy of the surrounding liquid molecules can be collected as electrical energy.

Here we developed nanoarray electrodes made of a piezoelectric material that can be bent/deformed/flexed readily by surrounding liquid molecules, demonstrating that the energy of liquid molecule thermal motion can be converted into electricity for the first time. The design of the novel MTMH is based on ZnO and gold-coated ZnO (Au@ZnO) nanoarray electrodes and n-octane liquid, etc. n-octane was chosen to drive MTMH because of its low dielectric constant, low toxicity, suitable boiling point, and low viscosity. The purity of n-octane is electronic grade (≥99.999%) to ensure there are no free-moving ions, which would leak the electricity from the nano-wires. ZnO was chosen as the piezoelectric material because it can form nano-whiskers of different structures such as tubes, sheets, wires, and rods at mild conditions,¹⁹ which could be deformed by the surrounding liquid.

Zn substance thickness: 1.5 mm; purity: 99.999%; Ethylenediamine, analytically pure; acetone, analytically pure; ethanol (95%), analytically pure; n-octane, purity ≥99.999%, Sigma-Aldrich.

The Zn substance was ultrasonic cleaned in ethanol, acetone, and distilled water for 10 min, and dried with nitrogen. Then it was covered with polytetrafluoroethylene membrane (Taizhou Aoke Filter Paper Factory, ϕ50, 0.45 µm) and filter paper (Taizhou Aoke Filter Paper Factory, quantitative, slow) in sequence. The Zn substance was suspended horizontally at a certain distance above the beaker containing the ethylenediamine-water solution with a concentration of 3.75 mol/l. Sealing and leaving it at room temperature for 48 h. Then it was taken out, rinsed with distilled water, and dried with nitrogen. The ZnO nanosheet array was grown on the Zn surface as piezoelectric materials. The other Zn substance was covered with filter paper, forming a ZnO nanosheet/rod hybrid array under the same growth environment.

Then octane was added dropwise to ensure that the gaps between nanoarrays were filled with octane and covered by the top electrode, which was a gold coated ZnO hybrid nanoarray that was also filled with octane. The Au coating of the upper electrode forms a Schottky barrier with ZnO underneath. The Schottky barrier can prevent electrons from escaping from ZnO nanosheets into the top electrode. The gold-coated ZnO hybrid nanoarray surface is used as the negative electrode, and Zn is used as the positive electrode of TMH. After connecting the wires, the entire device is packaged and sealed with epoxy to prevent liquid leakage.

The nanoarray micrograph was observed by SEM (JSM-7800F). The voltage and current of the devices were measured by the digital source meter (Keithley 2450).

The ZnO nanoarrays were synthesized with length, width, and height of about 266, 25 nm, and 3.41 µm [Figs. 1(a)–1(c)]. These special nanoarrays with small, specific surface areas enable them more easily susceptible to the mechanical deformation induced by molecular thermal motion. Subsequently, the ZnO nanoarrays were assembled into an MTMH device, and the fabrication process is shown in Fig. 1(d). A top electrode was made by Au@ZnO, and then soaked in n-octane, and then placed on the top of the aforementioned nanoarray. A piece of PET (polyethylene terephthalate) film was used to wrap around the two electrodes and also to maintain the distance between them. Then n-octane was added drop-wise to ensure that the gaps between nanoarrays were filled and soaked. The Au coating of the upper electrode forms a Schottky barrier with ZnO underneath.²⁰ The Schottky barrier prevents electrons from escaping from ZnO nanosheets outward. This is a key design feature that preserves piezoelectric potential, accumulates free carriers at the interface, and directs the charges outflow through the substrate connections. The gold layer on the Au@ZnO hybrid nanoarray surface is the negative electrode, and the Zn substrate of the uncoated ZnO nanoarray on the bottom is the positive electrode of MTMH. After connecting the wires, the entire device is sealed with epoxy to prevent liquid leakage. More details can be found in the experimental part in the supplementary material. The ZnO nanoarray was grown on a Zn substrate, prepared by the ethylenediamine vapor method, which is a modified process that was developed by ethylenediamine and provides an alkaline environment that is necessary for growing ZnO nanoarrays. Through multiple experiments, we have found that a gold-coated ZnO hybrid nanoarray as the top electrode and a ZnO nanosheets array as the bottom electrode are the best choices.

The Brownian motion of an object is caused not only by the random motion of the surrounding solvent molecules but also by its self-motion due to inter-particle interactions like electric force when the particles are charged.²¹ In the MTMH we made, the top electrode with Au@ZnO nano-array and the bottom electrode made of ZnO nano-array were placed facing each other. The substrate Zn is connected with copper wire, and an electrometer capable of detecting mini-scale electricity is connected as well. Driven by the thermal motion of the immersed octane molecules, the nanoarrays oscillate randomly, resulting in bending, flexing, or wriggling. These motions generate piezoelectric potentials in ZnO nanosheets with piezoelectricity. Regardless of the direction of the piezoelectric potential distribution, the output current of ZnO nanosheets had the same direction as the Schottky junction at the ZnO–Au interface, as shown in Fig. 2(a). Driven by the thermal motion of the octane, the top electrode and nanosheets oscillate randomly. Brownian motion of nanosheets, i.e., the flexes of the nanosheets on the top electrode develops piezoelectric potential. The separated nanosheets in configuration III cannot generate current. The piezoelectric potential in the nanosheet configuration I, because of the Schottky reverse bias at the contact interface between the electrode and the nanosheets, cannot generate current either. Schottky is forward-biased at the interface between the nanosheets in configuration II and the electrode, piezoelectric discharge can occur, and current can be detected in the external circuit [Figs. 2(b) and 2(c)]. The output current is the sum of the contributions from all nanosheets and exhibits a stable, unidirectional direct current output. To ensure no external interference, the device was placed in a metal-shielded box on a vibration-controlled table (TMC, 63-21554-01) while the performance of MTMH was measured. We characterized the I–V characteristics of the MTMH to confirm the formation of the Schottky junction [Fig. 2(d)]. Since the ZnO nanosheets were bent constantly by the thermal motion of the surrounding octane molecules, the current output performance of the MTMH could be directly measured without considering the factors of external forces required for piezoelectric and triboelectric nanogenerators, such as the external mechanical wave of a certain magnitude and frequency.^22,23 The output voltage and current are constant at 2.28 mV and 2.47 nA, respectively, revealing the MTMH can generate direct current output [Figs. 2(d) and 2(e)].

A device was made without connecting the electrode and ZnO nanosheets. The I–V curve of this generator indicates that there is no contact between the Au electrode and the ZnO nanosheet. As expected, no noticeable electrical signal output was detected. This further confirms that the signal is from molecular thermal motion rather than other possible factors (Fig. S1). The switching polarization is used to confirm whether the device’s tiny output signal is caused by the piezoelectric effect or other possible factors such as instrument noise.²⁴ That is the positive electrode probe of the meter is sequentially connected to the positive electrode and the negative electrode of the MTMH, respectively, and then reverses operation on the negative probe simultaneously. The reverse connection led to an opposite output of voltage and current vs the forward connection, with a comparable intensity [Figs. 3(a) and 3(b)]. This proves that the output of the MTMH is not caused by instrument noise. Due to the presence of Schottky, linear superposition is often applied to confirm electricity output as well.²⁵ MTMHs were connected in both forward and reverse-direction series to examine the liner superposition of voltage. Under the same conditions, the stable output voltages of the MTMHs were also demonstrated. MTMH I and II, fabricated and tested by the same method, were selected for the parallel and series measurements. MTMH I and II have acceptable differences in electricity output because the substrates are not always perfectly flat, and the contact between the top and bottom nanoarrays is not coincidental. When MTMH I and II are connected in series, the voltage output signal is enhanced, and the output value is 3.4 mV as expected [Fig. 3(c)]. Instead, the output signal is attenuated, and the value is 0.7 mV. When MTMHs I and II are connected in parallel, the current also conforms to the linear superposition [Fig. 3(d)]. In addition, the switching polarization output signal of these series and parallel MTMH devices is also observed by the same measurement method, which also presents the characteristic of linear superposition (Fig. S2). Therefore, based on the above switching-polarity and linear superposition tests, it is further confirmed that the electricity output was induced by the molecular thermal motion.

It is known that the thermal motion of molecules is proportional to the absolute temperature. Theoretically, the higher the temperature, the faster and more violent the octane molecules moves, which causes more Brownian motion of the ZnO nanoarrays, resulting in increased bending and flexing of these arrays. For piezoelectric materials, the generated piezoelectric potential is proportional to the magnitude and frequency of the deformation. Therefore, five different temperature values were chosen to investigate the effects of temperature on the output performance. The same device is placed at different temperatures and tested under the same conditions. As predicted, the output performance of the MTMH increases with temperature (Fig. 4). In addition, the switching polarization of output voltage and current was further observed at different temperatures (Fig. S3). The results of the experiment further verify that the electricity output of the thermal motion generator is indeed from molecular thermal motion.

In addition, n-octane, we have also tested the MTMH in several other liquids, such as cyclohexane and n-heptane and it could be found that there were similar behaviors (for detailed measurement results please see the supplementary material Note S1), which proves the universality of the MTMH (Figs. S4 and S5). Based on the mechanism of molecular thermal motion as an energy source, MTMH can be further modified, such as (1) other suitable solvent molecules or mixtures; (2) other piezoelectric materials such as lead zirconate titanate (PZT) and barium titanate have higher piezoelectric coefficients; and (3) the design of MTMHs with different structures (reducing internal resistance, using more flexible substrates, etc.).²⁶ Therefore, it could be expected that we can improve the energy conversion efficiency significantly. We believe that an improved MTMH can be developed not only to collect molecular thermal motion but also to collect external energy sources like waves. In addition, after modification, not only piezoelectric materials but also triboelectric materials, micro-fabricated coils, electromagnetic vibration generators,²⁷ or other micro- or nano-scale devices,²⁸ can be created to generate electricity, by harvesting molecular thermal motion. Therefore, we expect various types of MTMHs to be developed in the future. MTMH, like other micro-nanogenerators, has many potential applications, such as flexible electronics, sensors, and implantable medical devices.²⁹ 

In summary, the results reported here indicated that the energy of the thermal motion of octane can be converted into electrical energy through the device based on the piezoelectric properties of ZnO and a nano-array structure. Its output voltage and current can reach 2.28 mV and 2.47 nA at room temperature, respectively. With the increase in temperature, the output currents and voltages of the MTMH also increased. Two kinds of liquids n-octane, cyclohexane, and n-heptane can be used to drive this MTMH. The advantages of MTMH are obvious; for example, after the proper solvent is packaged, no additional energy source is required as long as the ambient temperature is above absolute zero and there is a constant thermal exchange with the surrounding. The generated electrical energy of MTMH is continuous, steady, and clean without any negative impact on the environment compared to fossil and nuclear energy. With the advancement of this technology, we expect, with a single device, not only to generate micro-watt level energy but also to provide novel thoughts for the watt- or even kilowatt-scale energy supply by making large-sized generators. Larger levels of energy supply can be solved by for example, electrolytic hydrogen production. The MTMH technology can be applied potentially to many fields such as home, personal care, outdoor sports, etc.

The additional experimental details, figures, and data are included in the supplementary material.

We are grateful to Ms. Z.-Y. Li for her assistance of drawing scientific figures.

The authors have no conflicts to disclose.

Yucheng Luan: Conceptualization (lead); Data curation (equal); Funding acquisition (lead); Investigation (lead); Methodology (lead); Project administration (lead); Resources (lead); Writing – original draft (lead). Fengwei Huo: Data curation (equal); Formal analysis (equal); Writing – review & editing (equal). Mengshi Lu: Formal analysis (equal); Writing – review & editing (equal). Wei Li: Data curation (equal); Formal analysis (equal); Writing – review & editing (equal). Tonghao Wu: Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.