Nano Research DOI 10.1007/s12274-015-0754-6 Nano Res 1 Flow-induced voltage generation of graphene network Junchao Lao 1,2, Yijia He 2,3, Xiao Li 3, Fuzhang Wu 1,2, Tingting Yang 2,3, Miao Zhu 2,3, Yangyang Zhang 3, Pengzhan Sun 3, Zhen Zhen 2,3, Baochang Cheng 1, and Hongwei Zhu 2,3 ( ) Nano Res., Just Accepted Manuscript DOI 10.1007/s12274-015-0754-6 http://www.thenanoresearch.com on March 4, 2015 Tsinghua University Press 2015 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.
Flow-induced Voltage Generation of Graphene Network Junchao Lao 1,2, Yijia He 2, Xiao Li 2, Fuzhang Wu 1,2, Tingting Yang 2, Miao Zhu 2, Yangyang Zhang 2, Pengzhan Sun 2, Zhen Zhen 2, Baochang Cheng 1, Hongwei Zhu 2 * 1 Nanchang University, China 2 Tsinghua University, China V=730 mv Voltages of up to several hundred millivolts are generated under ambient conditions from graphene network in response to the movement of droplets of ionic solution over a graphene strip.
Nano Research DOI (automatically inserted by the publisher) Research Article Flow-induced voltage generation of graphene network Junchao Lao 1,2, Yijia He 2,3, Xiao Li 3, Fuzhang Wu 1,2, Tingting Yang 2,3, Miao Zhu 2,3, Yangyang Zhang 3, Pengzhan Sun 3, Zhen Zhen 2,3, Baochang Cheng 1, and Hongwei Zhu 2,3 ( ) These authors contributed equally to this work. Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2015 ABSTRACT We report a voltage generator based on graphene network (GN). In response to the movement of a droplet of ionic solution over a GN strip, a voltage of several hundred millivolts is observed under ambient conditions. In the voltage-generating process, the unique structure of GN plays an important role in improving the rate of electron transfer. Given their excellent mechanical properties, GN may find applications for harvesting vibration energy in various places such as raincoat, umbrella and windows that are exposed to rain. KEYWORDS Graphene, flow, ionic solution, flexible, energy-harvesting 1. Introduction Since the first successful synthesis in 2004, graphene has been a subject of considerable interest due to its unique structure and excellent properties [1]. There are increasing devices based on graphene that have been produced, such as biosensor [2-3], strain sensor [4-6], photoelectric sensor [7-10], solar cell [11-13], and so on. In our case, we consider graphene as a potential solution to energy crisis, one of the most urgent and serious problems facing the human society. Traditional energy sources rely on fossil fuels which are known unsustainable and posing many environment issues. With the deployment of clean technologies, more options become available in the supply of renewable energy. It is worth noting that devices based on nanomaterials have been realized for energy harnessing. For instance, graphene-si solar cells were reported [14-15] and carbon nanotubes was found to be able to generate an electric current in flowing liquids [16-17]. With regard to graphene, there have been active studies exploring the similar properties [18-19]. Recently, graphene has been reported to generate induced voltage by moving a droplet of ionic liquid along the material [20]. The induced voltage was ~17mV when the ionic droplet dropped onto a monolayer graphene. All of these studies demonstrated the great potential of Address correspondence to H. W. Zhu, hongweizhu@tsinghua.edu.cn
2 Nano Res. carbon-materials in energy generation. A new form of graphene, graphene network (GN) or woven fabrics [21,22] could be fabricated on copper meshes by chemical vapor deposition (CVD), a common method for the synthesis of 2D materials [23]. As we use copper mesh instead of copper foil as the substrate, GN materials are fabricated in 3D network but not thin film (Figure 1a,b). Previously, 3D graphene networks have been produced mainly on nickel foams by CVD [24,25] and widely applied in surpercapacitor and absorbing materials [25-27]. In our work, the process of fabricating GN is as the same as that for monolayer or few-layer graphene on copper foil [21,22]. Based on graphene with woven fabrics structure, we have successfully assembled GN-Si solar cell [28], thin film supercapacitor [29] and torsion sensors [30]. In this work, we find that GN shows excellent properties in generating larger voltage by moving a droplet of ionic liquid. Thanks to its special criss-cross configuration (Figure S1 in the Electronic Supplementary Material (ESM)), GN possesses flexible structure and high mechanical strength. In addition, they can be prepared through a simple method with low cost. These attributes afford GN the flexibility for application of energy harvesting in harsh environments. 2. Results and Discussion 2.1 Flow-induced voltage in GN The experimental set-up is presented in Figure 1c. A droplet of 0.6M NaCl (60 µl in volume) aqueous solution was sandwiched between GN and PDMS substrate, which was placed 1 mm above the GN. A typical voltage response to the a droplet moving on the GN is shown in Figure 2a. When the movement of the droplet on GN from the left to right end of the strip was accelerated, the induced voltage simultaneously increased while deceleration was accompanied by the decreased voltage. Furthermore, when the droplet moved back from right to left, the typical voltage was found to be negative. In other words, the voltage response synchronized with droplet movement. In the following test, the droplet was accelerated to 3 cm s -1 and a voltage of ~ 3 mv was produced (Figure 2b). When the speed was lowered, response voltage became only about 1.5 mv (Figure 2c). In order to simulate the rain droplet, different solutions were dropped onto GN at an angle of 70 from a height of 15 cm, as shown in Figure 2d. Under these given conditions, the largest voltage (~100 mv) was produced by 0.6 M CuCl2 droplet. This was because droplets of CuCl2 solution with a larger mass had higher level kinetic energy compared to others though all of them had the same velocity. In order to investigate the influence of structure on electricity generation, two additional experiments were designed. One was that we adopted copper mesh (150-mesh) as the substrate to fabricate GN (Figure 3a). The other one was that we stacked a piece of GN (100-mesh) on another one to assemble double-layers-gn (DL-GN). Then we took them as two different substrates and moved a droplet of 0.6 M NaCl aqueous solution along them. As seen in Figure 3b, when the droplet is moved at the same velocity (~3 cm s -1 ), the induced voltage is only ~0.2 mv. To further explore the performances of GN with different structures in energy-harvesting, the liquid dropping experiment becomes necessary in the tests. On the basis of previous experiments, we chose CuCl2 solution as ion liquid for the dropping experiment. In this test, we set the same conditions that droplet was dropped from 15 cm height and the contact angle was 70. As shown in Figure 3c, the 100-mesh GN and DL-GN exhibit the better performance than 150-mesh GN. www.editorialmanager.com/nare/default.asp
Nano Res. 3 2.2 Mechanism According to classic electro-kinetic theory, when a solid surface is in touch with an ionic solution, a layer of ions, either cations or anions, will be adsorbed to the surface due to an electrochemical interaction. A second layer comprising counter-ions will also be attracted to the first layer via Coulomb force, thereby forming an electrical double layer. According to Yin, et al. [20], Na + was adsorbed on the surface of GN and formed a Na + layer. Due to the rough surface of GN, the Na + layer is raised and falls on the upper surface of GN (Figure 4a). When a droplet moves from right to left on the raised grain, some positive charges adsorbed on the bulge still stay at the original positions and others sandwiched between two bulges will move forward to the right bulge. As a result, the concentration of hydrated Na + ions increases on the upper surface of some bulges. In other word, the level of ion on the upper surface between the front and the rear increases and gives rise to more Na + ions in this process (Figure 4a). However, this theory needs a prerequisite that the droplet moves on a surface under the sliding condition. In this sliding state, one can consider that one droplet was divided into smaller droplets on the surface of GN due to its roughness. When a droplet is in a rolling condition, only the level of ions on the front end and the rear end will change whereas others do not contribute to the current. As we know, the value of current can be evaluated from the rate of transferred electrons dq/dt, so the induced voltage is given by V = RdI = -LRsq dq/dt where Rsq is the square resistance of GN and L is the length of droplet. Rsq of both 150-mesh GN and DL-GN was ~5 kω, and Rsq (100-mesh GN) was ~13 kω. Compared with Yin, et al. s work [20], one 0.6 M NaCl droplet moving on the graphene (Rsq=1.09 kω) could generate a voltage of ~0.2 mv at a velocity of ~3 cm s -1, as for our 100-mesh GN (Rsq= 13 kω), ~2.5 mv of voltage could be produced. However, the GN has only 60% efficient contact surface compared with the graphene film. Theoretically, the current values produced by moving droplet on GN strips and graphene strips should be the same though GN has lower efficient contact surface. Therefore only the increment of the rate of transferred electrons per unit area can cause this result. As a consequence, the bugles make a difference in the electricity generation and improve the utilization of ions. According to the previous data, we put forward two conjectures. One is that the droplet moves on the substrate in two states: rolling and sliding. The other one is that the droplet is on partial wetting state (mixed state: Wenzel and Cassie-Baxter Figure 4c). The first model can be understood as bowling (Figure 4b). In this experiment, 150-mesh GN and DL-GN have more intensive holes than 100-mesh GN. When the droplet moves forward, the droplet on the 150-mesh GN is mainly in Cassie-Baxter state. The contact angles on 100-mesh GN and 150-mesh GN are shown in Figure S2a and S2b in the ESM. In this situation, a plenty of bulges on the GN not only make no differences in this process but also reduce the effective contact area. But for DL-GN, during the assembling process, they could produce a lot of disordered structures which have effects on the contact angle, and this is why we find that contact angles are not the same for the same substrate (Figure S2c in the ESM). These two factors of both the square resistance of GN and the motion of droplet cause the lower induced voltage. In a similar way, the droplet on the 100-mesh GN is mainly in Wenzel state, and more ions are working as presented in Figure 4a. As for the dropping experiment, V100 is double of V150 because the 100-mesh GN has larger Rsq and the www.thenanoresearch.com www.springer.com/journal/12274 Nano Research
4 Nano Res. droplet is all in Wenzel state when it drops on GN. In account of the same contact mode, Rsq is the main factor for generating voltage. Even though the Rsq of DL-GN is ~5 kω, we still get the same high voltage of ~120 mv (Figure 3c). The double-layers structure provides larger efficient contact surface. 2.3 Potential Applications All these experiments prove that this special structure has huge effects on the inducement of voltage, no matter whether droplet is moving or dropping. In our previous work, GN and flexible polymer substrates have been fabricated into integrated electrodes in thin-film supercapacitors (Figure S3 in the ESM). The supercapacitor exhibited remarkable electrochemical performances, even though they have been transformed into diverse shapes [29]. These studies demonstrated that GN had a great capacitive performance which was beneficial to not only energy-storage but also electricity generation and energy-harvesting via this device. There are many energy resources coming from nature, such as solar energy from the sun, gasoline from fossil fuel, and wind from the air motion. Besides, those regular energy resources, for example, the rain droplets from the sky can also be converted into a resource of electricity via flow-induced voltage generating of GN structure. The devices can be attached on many places exposed to the rain, such as umbrella, raincoat and windows and generate voltages of several hundred millivolts in a light rain condition (Figure S4 in the ESM). The great significance in this research is that we find a way to convert disordered energy into ordered energy. 3. Conclusions In summary, GN has an excellent performance in voltage generation. The special network structure of GN is distinctly different from graphene film while excellent properties are retained under many harsh conditions. The criss-cross configuration plays an essential role in the energy conversion. Thanks to its flexible and mechanical properties, GN can be attached on many places. Besides, the low cost and the ease to manufacture further strengthen the applicability of this network material in energy-harvesting. For more instructions, please see our web page at http://www.thenanoresearch.com/. Acknowledgements This work was supported by Beijing Science and Technology Program (No. D141100000514001) and National Science Foundation of China (No. 51372133). Electronic Supplementary Material: Supplementary material (Materials and methods, SEM and contact angle measurements, photographs of potential applications) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-***-****-* (automatically inserted by the publisher). References [1] Geim, A. K., Graphene: Status and Prospects. Science. 2009, 324, (5934), 1530-1534. [2] Hong, W.; Bai, H.; Xu, Y.; Yao, Z.; Gu, Z.; Shi, G., Preparation of gold nanoparticle/graphene composites with controlled weight contents and their application in biosensors. The Journal of Physical Chemistry C. 2010, 114, (4), 1822-1826. [3] Choi, B. G.; Park, H.; Park, T. J.; Yang, M. H.; Kim, J. S.; Jang, S.; Heo, N. S.; Lee, S. Y.; Kong, J.; Hong, W. H., Solution Chemistry of Self-assembled graphene nanohybrids for high-performance flexible biosensors. ACS Nano. 2010, 4, (5), 2910-2918. [4] Li, X.; Zhang, R.; Yu, W.; Wang, K.; Wei, J.; Wu, D.; Cao, A.; Li, Z.; Cheng, Y.; Zheng, Q.; Ruoff, R. S.; Zhu, H., Stretchable and highly sensitive graphene-on-polymer strain sensors. Scientific Reports. 2012, 2. [5] Wang, Y.; Yang, R.; Shi, Z.; Zhang, L.; Shi, D.; Wang, E.; Zhang, G., Super-elastic graphene ripples for flexible strain sensors. ACS Nano. 2011, 5, (5), 3645-3650. [6] Boland, C. S.; Khan, U.; Backes, C.; O Neill, A.; McCauley, J.; Duane, S.; Shanker, R.; Liu, Y.; Jurewicz, I.; Dalton, A. B.; Coleman, J. N., Sensitive, high-strain, high-rate bodily motion sensors based on graphene rubber composites. ACS Nano. 2014, 8, (9), 8819-8830. www.editorialmanager.com/nare/default.asp
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6 Nano Res. (a) (b) (c) V Figure 1. Preparation of GN. (a) GN prepared from CVD and transferred onto PDMS, then cut into strips. (b) Corresponding photographs. (c) Schematic of the fluid induced voltage generation test. www.editorialmanager.com/nare/default.asp
Nano Res. 7 (a) Voltage(mV) (c) Voltage (mv) 3 2 1 0-1 -2-3 0 5 10 0.5 0.0-0.5-1.0-1.5 Time (s) 1.5 Voltage 1.0 Velocity 2 4 6 8 Time (s) ~ 2 cm/s (b) Voltage (mv) (d) Voltage (mv) 4 3 2 1 0-1 100 75 50 25 0 2 4 6 h=15cm 0.6M NaCl Time (s) 70 0.6M CaCl 2 ~ 3 cm/s Voltage Velocity 0.6M CuCl 2 Figure 2. Voltage generation in 100-mesh GN by flowing and dropping the droplets of ionic solution. (a) Voltage induced by flowing the droplet of 0.6M NaCl in opposite directions. (b,c) Voltage induced by moving the droplets of 0.6M NaCl: ~3 mv and ~1.5 mv with the velocity ~3 cm s -1 and ~2 cm s -1. (d) Voltage induced by dropping droplets of different solutions at an angle of 70 from a height of 15 cm. www.thenanoresearch.com www.springer.com/journal/12274 Nano Research
8 Nano Res. (a) 100-mesh GN 150-mesh GN DL-GN 100 μm 100 μm 100 μm (b) 100-mesh GN 3 mv (x0.1) (c) 120 h=15cm 150-mesh GN 0.2 mv Voltage (mv) 80 40 70 DL-GN 0.2 mv 0 100-mesh GN 150-mesh GN DL-GN Figure 3. Comparative tests. (a) Optical photographs of 100-mesh, 150-mesh GN and DL-GN samples on PDMS. (b) Voltage induced by flowing droplets of 0.6M NaCl with the velocity ~3 cm s -1. (c) Voltage induced by dropping droplets of 0.6M CuCl 2 at an angle of 70 from a height of 15 cm. www.editorialmanager.com/nare/default.asp
Nano Res. 9 (a) (b) left right graphene ribbons cation anion Sliding sliding Bowling rolling (c) Wenzel Cassie-Baxter Figure 4. Mechanism for the flowing voltage. (a) Transferring process of ions and electrons. (b) Bowling model mimic the movement of droplet. (c) Wenzel and Cassie-Baxter models. www.thenanoresearch.com www.springer.com/journal/12274 Nano Research
Nano Res. Electronic Supplementary Material Flow-induced voltage generation of graphene network Junchao Lao 1,2, Yijia He 2,3, Xiao Li 3, Fuzhang Wu 1,2, Tingting Yang 2,3, Miao Zhu 2,3, Yangyang Zhang 3, Pengzhan Sun 3, Zhen Zhen 2,3, Baochang Cheng 1, and Hongwei Zhu 2,3 ( ) These authors contributed equally to this work. Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher) MATERIALS AND METHODS Synthesis of graphene network: Copper meshes (100/150-mesh,) were clean, tailored and pre-treated as reported previously. When the temperature reached 1000 o C, H2 was turned to 3 ml/min. CH4 (30 ml/min) was introduced to the reactor at ambient pressure. While Ar was turned off for 80 s, and then turned up to 200 ml/min. After 20 min of growth, the copper mesh with graphene was rapidly cooled down to room temperature. Copper mesh was then etched away using an aqueous solution of FeCl3 (0.5 mol/l) and HCl (0.5 mol/l). Then we transferred GN onto PDMS, and incised it into GN strips with a dimension of ~5 70 mm 2. Voltage measurement: The voltage signal was recorded in real time using a Keithley2601s MultiMeter, which was controlled by a LabTracer system with a sampling rate of 50/s. Address correspondence to H. W. Zhu, hongweizhu@tsinghua.edu.cn www.thenanoresearch.com www.springer.com/journal/12274 Nano Research
Nano Res. Figure S1. SEM images of graphene grown on copper mesh. (a) Graphene domains. (b) Graphene film. (c) Graphene network (GN) transferred onto a PDMS substrate. Figure S2. Contact angle test with 0.6 M NaCl droplet. (a) 100-mesh GN. (b) 150-mesh GN. (c) DL-GN. www.editorialmanager.com/nare/default.asp
Nano Res. Figure S3. Flexible GN. GN can be (a) folded and (b) twisted in many shapes. Figure S4. Potential applications. (a) On umbrella. (b) On window. (c) On raincoat. www.thenanoresearch.com www.springer.com/journal/12274 Nano Research