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Fei Zhao, Huhu Cheng, Zhipan Zhang, Lan Jiang, and Liangti Qu* Environmental energy harvesting devices hold great promise for the next generation electronics. Nowadays, there are varieties of available energies as the candidates, such as thermal power, hydropower, nuclear energy, but their applications are suffering from the environmental pollution and/or large-scale supporting equipments. In recent years, nano-/microelectronics has provided new possibility of clean and renewable energy power sources by developing piezoelectric,[1–4] triboelectric,[5–8] and fluidic–electric[9–12] generators. However, an urgent challenge is to effectively convert these potential energies into the directly utilized power without much loss of accompanied electrical, thermal, and mechanical energy during the conversion. Ideally, the highly effective energy conversion is induced by a spontaneous process (e.g., diffusion), such that there is no temperature variation, mechanical movement, and by-produced pollutant (chemical energy loss). Graphene-based materials with tunable properties provide considerable opportunities for development of future-generation energy-transform devices.[10–12] Among them, graphene oxide (GO), rich in oxygen-related functional groups such as hydroxyl and carboxyl groups, is of great interest for the potential applications in electronics, energy-related devices, and smart systems that strongly depend on the agile regulation of oxygen content.[13–21] In this regard, we have demonstrated the unique GO-based stimuli-responsive systems due to the sensitivity of GO with the moisture.[16,20] Meanwhile, the ionic and electronic conductivities of GO vary with the alternation of relative humidity (RH).[13,15,20] These results suggest the importance of the interaction between the oxygen-rich GO and water molecules to tune the properties and functions of the final materials and devices. In this work, we present a power-generating phenomenon of single GO film (GOF) with a preformed oxygen-containing group gradient (also named as gradient GOF, g-GOF). The
F. Zhao, H. Cheng, Dr. Z. Zhang, Prof. L. Qu Beijing Key Laboratory of Photoelectronic/ Electrophotonic Conversion Materials Key Laboratory of Cluster Science Ministry of Education School of Chemistry Beijing Institute of Technology Beijing 100081, PR China E-mail: [email protected] Prof. L. Jiang Laser Micro-/Nano-Fabrication Laboratory Ministry of Education School of Chemistry Beijing Institute of Technology Beijing 100081, PR China
DOI: 10.1002/adma.201501867
Adv. Mater. 2015, 27, 4351–4357
g-GOF was prepared by a moisture/electric field co-induced gradient process of oxygen-containing groups on a vacuum-filtrating GOF, and this strategy developed here was called moisture–electric annealing (MeA). The as-prepared g-GOF is able to provide moisture-enabled voltage output of ≈35 mV with a power density of ≈4.2 mW m−2 by harvesting energy from moisture diffusion with energy conversion efficiency of up to ≈62%, which surpassed the fluidic–electric generators and were even better than those of piezoelectric generators with optimized structure. More interestingly, an extra-power-free respiratory monitor has been built by utilizing the moisture flow of human breath as a clean and renewable power source. To prepare a g-GOF, we developed the MeA method in which a constant bias (4 V) was applied between the two sides of GOF in an enclosed container with an RH of 70% (Figure S1, Supporting Information). The initial GOF was formed by filtration of an aqueous suspension of GO nanosheets through a filter membrane with pore size of 1 µm, which was sandwiched between two pieces of gold electrodes connecting with external circuit as shown in Figure 1a. During this process, the RH in the enclosed container and current intensity of GOF were monitored in real time and kept in range of 70% ± 2% and 0.01–0.1 mA cm−2, respectively. A GOF with a size of 0.5 cm × 0.5 cm was utilized in this study although it could be scaled up by simply using large-area GOFs and electrodes. The thickness of g-GOFs is ≈2.8 µm which could be readily tunable by filtrating desirable amount of GO dispersion with a certain concentration (Figure 1b). Remarkably, the cross-sectional element mapping of g-GOF demonstrated the gradient distribution of the oxygen component on a typical layered GOF (Figure 1c,d). The oxygen content gradually increases from the top side (the negative-electrode-contacting side in Figure 1a) to the bottom side (the positive-electrode-contacting side in Figure 1a). On the other hand, X-ray diffraction (XRD) patterns of g-GOF reveal that the top and the bottom sides have the distinct peak shift (Figure 1e) nonanalogous to the initial GOF, which suggests the MeA has indeed induced the change of interlayer structure. In contrast to GOF with a calculated interlayer spacing of ≈0.8 nm for both sides, the top and bottom sides of MeA-treated GOF has an interlayer spacing of 0.68 and 0.85 nm, respectively, implying a partially reduced top GO layers and a slightly oxidized bottom layers of g-GOF in consistence with the corresponding element mapping (Figure 1d). The composition through the layers of g-GOF was further determined by X-ray photoelectron spectroscopy (XPS). As shown in Figure S2 (Supporting Information), the XPS survey spectra of both the top and bottom surfaces show predominant graphitic C 1s peak at ≈284 eV and O 1s peak at ≈532 eV. The O/C atomic ratio for the bottom side of g-GOF is ≈0.52, slightly higher than that of GO starting material (≈0.45)
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Figure 1. Preparation processes and characterizations of g-GOF. a) MeA process of GOF with applied voltage under humid environment. The e− and Oδ− represented electrons and oxygen-containing ions, respectively. b) Cross-section scanning electron microscopic (SEM) image and c,d) the corresponding carbon and oxygen mapping from X-ray energy dispersive spectroscopy. e) XRD patterns of different sides of as-prepared GOF and g-GOF. f) XPS spectra of different surfaces of g-GOF. The surfaces contacting with the top and bottom electrodes in (a) are noted as Top and Bottom, respectively.
but much higher than that of the top surface (≈0.22), demonstrating the distribution variation of oxygen-containing groups from top to bottom as shown in Figure 1a. Moreover, the high resolution C 1s spectra (Figure 1f) confirmed the presence of C O bands (286.6 eV) and C O bands (288.5 eV) on both sides of the g-GOF.[14] In contrast with the corresponding spectra of initial GOF sample (Figure S3, Supporting Information), the top side had the weakened C O and C O peaks, indicating that the oxygen-containing groups on the top layers of g-GOF were partially removed. In the meanwhile, the signals from bottom side presented the stronger C O and C O peaks, implying the increasing O/C atomic ratio of bottom side probably associated with the further oxidation. These results also explained the inconsistent interlayer spacing change of two sides of g-GOF observed by XRD.
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Along with the basic principle of electric generators, the middle layer (i.e., the g-GOF used in this study) served as both dielectric layer and functional layer in a GO-based moisture– electric energy transformation (MEET) device7 as shown in Figure 2a. When the device was exposed to moisture (e.g., ΔRH = 30%) and inserted in a test circuit, the voltage and current output were observed. Subsequently, when the environmental RH went back to the initial state, the signals of reverse current with negative voltage were collected. Accordingly, the device reverts to the original state and a whole MEET cycle was finished. Based on the operation procedure mention above, the g-GOF-based MEET device could provide voltage and current output of ≈20 mV and ≈5 µA cm−2 within 3 s under a ΔRH of 30% as demonstrated in Figure 2b,c. Notably, unlike other kinds of electric generators, the output voltage and current
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COMMUNICATION Figure 2. Operation of MEET process. a) Schematic illustration of a MEET cycle. The V and A represented the induced voltage and current along the direction of arrows, respectively. b) Voltage and c) current output cycles of a prototype MEET device of g-GOF in response to the intermittent and periodic RH variation (ΔRH = 30%). Experimental data of single d) voltage and e) current output cycle from (b) and (c) matched with ΔRH. f) Thickness change ratio, i.e., thickness change (Δd) divided by original thickness (d), and g) resistance alteration ratio, i.e., resistance alteration (ΔR) divided by original resistance value (R) of g-GOF matched with ΔRH. For clarity in interpretation, ON and OFF in (d)–(g) represent the injection and ejection of moisture in experimental environment, respectively.
pulses of g-GOF-based MEET device show an asymmetric behavior that the peak value on the positive pulses is always significantly higher than that on the negative pulses. The unique output behavior of the single cycle was investigated corresponding to the ΔRH. As shown in Figure 2d, the induced
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positive voltage rapidly rose up once the RH increased, and returned to zero prior to the reduction of RH. Meanwhile, during the RH decrease, the voltage evolved negatively and remained for several seconds after the environmental moisture was removed completely. The identical phenomenon arose in
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the case of current output as shown in Figure 2e. Therefore, the MEET process should be directly related to the interaction between water molecules and g-GOF. According to our previous investigations, water molecules could insert the interlayer space of GO quickly, but the desorption process would take more time.[16,20] The adsorption and desorption behavior of water molecules was reflected by the moisture induced change of thickness and resistance.[13,15] As shown in Figure 2f,g, the thickness and resistance change of g-GOF closely correlate with RH variation. Once moisture is on, the increase of thickness and the decrease of resistance of g-GOF are observed. On the contrary, a slight hysteresis appears during RH reduction due to a relatively slow water release from g-GOF (Figure 2f,g). Along with the analysis mentioned above, the blue dashed line in Figure 2d–g labeled as ON represented the injection
of moisture and the moment that ΔRH, voltage, current density, thickness change ratio (Δd/d), and resistance alteration ratio (ΔR/R) begin to increase, indicating the moisture adsorption induced positive electric output. At the time of moisture ejection marked by OFF (red dashed line), the ΔRH, Δd/d and ΔR/R started to decrease and the negative output appeared, suggesting the moisture desorption induced negative electric output. Several seconds later after ΔRH return to zero, the electric output stopped when the Δd/d and ΔR/R gently reduced to zero, implying the desorption induced electronic output was dominated by the release of residual water molecules in g-GOF. Moreover, when the injection of moisture was maintained after the presence of positive plus, the negative signals appeared once the moisture ejection process began (Figure S4, Supporting Information), which was further substantiated that
Figure 3. Harvesting the body energy hidden in the respiratory moisture-tide and self-powered monitoring of body condition. a) MEET device that is responsive to human breathing increased and decreased RH with exhale and inhale, respectively. b) The g-GOF was sandwiched in two gold electrodes with vents to construct the MEET device. c) Voltage and d) current output generated by respiratory moisture-tide with a ΔRH of ≈21%. e) Self-powered monitoring of respiratory frequency relating to heart rate after different intensity of exercise.
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These results experimentally revealed that the close relevance of heart rate with respiratory frequency with a coefficiency of ≈4.1 for different body condition (Figure S15, Supporting Information). This simple demonstration has indeed presented the great potential of g-GOF device in self-powered body condition monitor toward clinical diagnosis. Based on the analysis above, a proposed mechanism of MEET process was illustrated by a schematic in Figure 4. The g-GOF adsorbs water molecules upon exposure to moisture due to the strong hydrophilicity of oxygen-containing groups (e.g., OH, COOH). With the accumulation of water molecules in the O-rich part of g-GOF, a regional solvation effect will weaken the Oδ− Hδ+ bond in oxygen-containing groups, thus releasing free H+.[13,15] The g-GOF possessed a gradient distribution of oxygen-containing groups (Figure 4a), which accordingly led to the concentration gradient of H+ (Figure 4b). The induced potential and free electron movement of external circuit were thereupon generated once the diffusion of H+ occurred due to the drive of concentration gradient (Figure 4c). Meanwhile, the induced electric field tends to force the H+ drift back to high concentration side. Therefore, when the diffusion and drift effect reached a dynamic balance, the induced potential was stabilized, and hence free electron movement of external circuit was terminated. In contrast, the desorption of moisture accelerated the recombination of negatively charged groups (e.g., COO−) and H+, weakening the diffusion driving effect of concentration gradient. As a result, H+ drifted back and hence induced negative potential with a consequent reverse electron movement (Figure 4d). After the release of residual water molecules, the g-GOF returned to the initiate state.
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adsorption and desorption of moisture enabled electric output of g-GOF rather than the metal electrode contact and the concentration gradient of GO as demonstrated and discussed in detail in Figures S5–S13 (Supporting Information). The breath of human and other terrestrial animals is one of the most steady and constant resources of moisture tide. Generally, the exhale and inhale of adults through nasal cavity can cause an RH variation. We therefore built a g-GOF based power generator (Figure 3a,b) to sensitively harvest the energy of breath. As shown in Figure 3c,d, the calm breathing from a healthy man (ΔRH = 21%) could provide the voltage and current output of ≈18 mV and ≈5.7 µA cm−2, respectively. This power generator can indeed serve as a respiratory energy collector to convert body-provided energy to electric power, indicating a promising potential of g-GOF in energy relative applications. Beyond the power generation, g-GOF could also work as the extra-power-free device to monitor the body condition. For instance, by counting the number of voltage output pulses, we can monitor the respiratory frequency in relevance to the heartbeat of a healthy man after exercise with different intensity (Figure 3e). Before jogging, the man’s calm breathing was 13 times per minute and caused a ΔRH of ≈21% (Figure S14, Supporting Information), corresponding to a heart rate of 58 times per minute. After jogging for 5 min, the breathing was accelerated to 18 times per minute with incidental ΔRH of ≈29%, and the heart rate was elevated to 79 times per minute. Moreover, when the jogging time was extended to 60 min, the signals representing rapid breathing (28 times per minute) were observed according to a fast heartbeat of 118 times per minute, accompanied with the breath induced ΔRH of up to ≈48%.
Figure 4. MEET process of g-GOF. a) Schematic illustration of the gradient distribution of oxygen-containing groups (e.g., carboxyl and hydroxy) in g-GOF. b) Free hydrogen ions (H+) were ionized by adsorbed water from oxygen-containing groups and a density gradient of H+ was established. c) When the free H+ diffused from high density side to low density side, the potential was induced and the electrons moved from bottom electrode to top electrode along external circuit. d) With the desorption of moisture, the recombination of H+ with negatively charged O-functional groups reduced the induced potential, resulting in reverse electrons flow in external circuit.
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Figure 5. Importance of oxygen-containing group gradient in g-GOF based MEET. a) Photographs of UV reduced GO (UV-rGO) dispersion (0.5 mg mL−1) with different UV-irradiation time, which were noted as A, B, and C. The different UV irradiation caused variant O/C ratios of 0.46, 0.27, and 0.2 for A, B, and C, respectively. The UV-rGO dispersions (10 mL) of A, B, and C were filtrated in sequence to prepare GOF with oxygen-containing group steps. b) Voltage and c) current output of UV-rGO based GOF with oxygen-containing group steps. d) Photographs of GO dispersions (0.5 mg mL−1) prepared by hummers’ oxidation (HO-GO) for different time which were noted as D, E, and F. The different degree of oxidation also caused variant O/C ratios of 0.46, 0.35, and 0.21 for D, E, and F. The HO-GO dispersions (10 mL) of D, E, and F were filtrated successively to prepare GOF with oxygen-containing group steps. e) Voltage and f) current output of HO-GO based GOF with oxygen-containing group steps. The electric outputs in (b), (c), (e), and (f) were tested with ΔRH = 30%.
To further demonstrate the mechanism that the gradient of oxygen-containing groups induced the MEET under moisture, we established a GOF consisting of three layers of GO with different oxygen content (Figure 5a). GO with an O/C ratio of 0.46 was controllably reduced as exposed to the UV irradiation for certain periods.[14] The UV-reduced GO with O/C ratio of 0.46, 0.27, and 0.2 were filtrated successively to form a GOF with gradient steps of O-related groups. The obtained GOF based MEET device presented a power-generating behavior similar to that of g-GOF-based one (Figure 5b,c). Additionally, GOFs with the steps of oxygen-containing groups can also be established by filtration of GO dispersions with different oxidation degree (Figure 5d), where the O/C ratios were intended to 0.46, 0.35, and 0.21. The output of voltage (≈0.2 mV) and current (10 nA cm−2) were comparable with UV-rGO-based MEET device (Figure 5e,f). These results demonstrate the mechanism that the gradient of oxygen-containing groups within the GOF indeed induced the MEET effect under moisture. Because of the limited gradient interfaces of oxygen-containing group within the GOF, the low output voltage and current for the GOF is understandable. Since the MEET was based on a diffusion-drift balance, the value of induced potential (Ui) could be expressed by:[22] Ui =
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qD dc σ ∫ dx
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(1)
where q, D, σ, dc/dx are electric quantity of elementary charge, diffusion coefficient of H+, ionic conductivity of water, and concentration gradient of H+ along an x-axis perpendicular to surface of GOF (see thermodynamic analysis of MEET in the Supporting Information for details). It means that Ui directly depends on the concentration gradient of H+, and hence the gradient of oxygen-containing groups. This conclusion is consistent with the experimental data from GOF with oxygen-containing group steps, in which the gradient of oxygen-containing groups was located at the interfaces of GO steps. Considering that there is no additional energy involved in one MEET cycle, the chemical potential drop of water diffusion process (equals to minus Gibbs free energy, −ΔGM, also see calculation of MEET efficiency in the Supporting Information for details) is the sole energy source for the MEET, we can evaluate the MEET efficiency based on the ratio of ΔGM to output power (WM). It is found that the MEET efficiency of g-GOF generator is up to ≈62%, which is ≈3 times higher than that of piezoelectric zinc oxide (ZnO),[1] ≈61 times higher than that of fluidic–electric chemical vapor deposition graphene (CVD-G)[12] and hundreds of times higher than the fluidic–electric carbon nanotube (CNT).[9] Meanwhile, without the systematic optimization of the devices performance in this preliminary study, the power density is up to 4.2 mW m−2 at the output voltage of 30 mV, which is ≈50 times higher than that of piezoelectric gallium nitride (GaN),[23]
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by NSFC (Nos. 21325415, 21174019), National Basic Research Program of China (2011CB013000), and Beijing Natural Science Foundation (2152028). Received: April 19, 2015 Revised: May 12, 2015 Published online: June 18, 2015
[1] Z. L. Wang, J. Song, Science 2006, 312, 242. [2] X. Wang, J. Song, J. Liu, Z. L. Wang, Science 2007, 316, 102.
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[3] Y. Qin, X. Wang, Z. L. Wang, Nature 2008, 451, 809. [4] R. Yang, Y. Qin, L. Dai, Z. L. Wang, Nat. Nanotechnol. 2009, 4, 34. [5] S. Wang, L. Lin, Z. L. Wang, Nano Lett. 2012, 12, 6339. [6] F. R. Fan, L. Lin, G. Zhu, W. Wu, R. Zhang, Z. L. Wang, Nano Lett. 2012, 12, 3109. [7] S. Wang, L. Lin, Y. Xie, Q. Jing, S. Niu, Z. L. Wang, Nano Lett. 2013, 13, 2226. [8] G. Zhu, J. Chen, T. Zhang, Q. Jing, Z. L. Wang, Nat. Commun. 2014, 5, 3426. [9] S. Ghosh, A. Sood, N. Kumar, Science 2003, 299, 1042. [10] P. Dhiman, F. Yavari, X. Mi, H. Gullapalli, Y. Shi, P. M. Ajayan, N. Koratkar, Nano Lett. 2011, 11, 3123. [11] W. Guo, C. Cheng, Y. Wu, Y. Jiang, J. Gao, D. Li, L. Jiang, Adv. Mater. 2013, 25, 6064. [12] J. Yin, X. Li, J. Yu, Z. Zhang, J. Zhou, W. Guo, Nat. Nanotechnol. 2014, 9, 378. [13] W. Gao, N. Singh, L. Song, Z. Liu, A. L. M. Reddy, L. Ci, R. Vajtai, Q. Zhang, B. Wei, P. M. Ajayan, Nat. Nanotechnol. 2011, 6, 496. [14] F. Zhao, J. Liu, X. Huang, X. Zou, G. Lu, P. Sun, S. Wu, W. Ai, M. Yi, X. Qi, ACS Nano 2012, 6, 3027. [15] S. Kim, S. Zhou, Y. Hu, M. Acik, Y. J. Chabal, C. Berger, W. de Heer, A. Bongiorno, E. Riedo, Nat. Mater. 2012, 11, 544. [16] H. Cheng, J. Liu, Y. Zhao, C. Hu, Z. Zhang, N. Chen, L. Jiang, L. Qu, Angew. Chem. Int. Ed. 2013, 52, 10482. [17] J. Liu, Z. Yin, X. Cao, F. Zhao, L. Wang, W. Huang, H. Zhang, Adv. Mater. 2013, 25, 233. [18] Y. Zhao, J. Liu, Y. Hu, H. Cheng, C. Hu, C. Jiang, L. Jiang, A. Cao, L. Qu, Adv. Mater. 2013, 25, 591. [19] Y. Meng, Y. Zhao, C. Hu, H. Cheng, Y. Hu, Z. Zhang, G. Shi, L. Qu, Adv. Mater. 2013, 25, 2326. [20] H. Cheng, Y. Hu, F. Zhao, Z. Dong, Y. Wang, N. Chen, Z. Zhang, L. Qu, Adv. Mater. 2014, 26, 2909. [21] F. Zhao, H. Cheng, Y. Hu, L. Song, Z. Zhang, L. Jiang, L. Qu, Sci. Rep. 2014, 4, 122. [22] T. Engel, P. J. Reid, Physical Chemistry, Pearson Education Deutschland GmbH, Germany 2006. [23] C. H. Wang, W. S. Liao, Z. H. Lin, N. J. Ku, Y. C. Li, Y. C. Chen, Z. L. Wang, C. P. Liu, Adv. Energy Mater. 2014, 4, 1400392. [24] A. E. Cohen, S. Ghosh, A. Sood, N. Kumar, Science 2003, 300, 1235.
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≈21 times higher than that of fluidic–electric CNT[24] and ≈18 times higher than that of fluidic–electric CVD-G (also see Table S1, Supporting Information).[12] We believe the power density could be further improved by optimizing the assembly of GO, the regulation of oxygen groups and scaling up of film size. In summary, GOF with gradient oxygen-containing groups has been fabricated by an MeA method. This strategy provides a simple but effective approach for the construction of the g-GOF, through which the MEET devices have been achieved to have a high sensitivity to moisture variation (ΔRH of less than 5%), and possess a high energy-conversion efficiency of ≈62% and a power density of ≈4.2 mW m−2 under the tidal moisture. Even the respiratory moisture of human can be converted into electric power, and more impressively, the g-GOF device can ingeniously track the body conditions in a real time manner without any external power supply promising for energy-generating and sensing applications. Although the g-GOF MEET device only produces intermittent power in the preliminary study, it could be combined with energy-storage unit for sustained use.
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Fei Zhao, Huhu Cheng, Zhipan Zhang, Lan Jiang, and Liangti Qu* Environmental energy harvesting devices hold great promise for the next generation electronics. Nowadays, there are varieties of available energies as the candidates, such as thermal power, hydropower, nuclear energy, but their applications are suffering from the environmental pollution and/or large-scale supporting equipments. In recent years, nano-/microelectronics has provided new possibility of clean and renewable energy power sources by developing piezoelectric,[1–4] triboelectric,[5–8] and fluidic–electric[9–12] generators. However, an urgent challenge is to effectively convert these potential energies into the directly utilized power without much loss of accompanied electrical, thermal, and mechanical energy during the conversion. Ideally, the highly effective energy conversion is induced by a spontaneous process (e.g., diffusion), such that there is no temperature variation, mechanical movement, and by-produced pollutant (chemical energy loss). Graphene-based materials with tunable properties provide considerable opportunities for development of future-generation energy-transform devices.[10–12] Among them, graphene oxide (GO), rich in oxygen-related functional groups such as hydroxyl and carboxyl groups, is of great interest for the potential applications in electronics, energy-related devices, and smart systems that strongly depend on the agile regulation of oxygen content.[13–21] In this regard, we have demonstrated the unique GO-based stimuli-responsive systems due to the sensitivity of GO with the moisture.[16,20] Meanwhile, the ionic and electronic conductivities of GO vary with the alternation of relative humidity (RH).[13,15,20] These results suggest the importance of the interaction between the oxygen-rich GO and water molecules to tune the properties and functions of the final materials and devices. In this work, we present a power-generating phenomenon of single GO film (GOF) with a preformed oxygen-containing group gradient (also named as gradient GOF, g-GOF). The
F. Zhao, H. Cheng, Dr. Z. Zhang, Prof. L. Qu Beijing Key Laboratory of Photoelectronic/ Electrophotonic Conversion Materials Key Laboratory of Cluster Science Ministry of Education School of Chemistry Beijing Institute of Technology Beijing 100081, PR China E-mail: [email protected] Prof. L. Jiang Laser Micro-/Nano-Fabrication Laboratory Ministry of Education School of Chemistry Beijing Institute of Technology Beijing 100081, PR China
DOI: 10.1002/adma.201501867
Adv. Mater. 2015, 27, 4351–4357
g-GOF was prepared by a moisture/electric field co-induced gradient process of oxygen-containing groups on a vacuum-filtrating GOF, and this strategy developed here was called moisture–electric annealing (MeA). The as-prepared g-GOF is able to provide moisture-enabled voltage output of ≈35 mV with a power density of ≈4.2 mW m−2 by harvesting energy from moisture diffusion with energy conversion efficiency of up to ≈62%, which surpassed the fluidic–electric generators and were even better than those of piezoelectric generators with optimized structure. More interestingly, an extra-power-free respiratory monitor has been built by utilizing the moisture flow of human breath as a clean and renewable power source. To prepare a g-GOF, we developed the MeA method in which a constant bias (4 V) was applied between the two sides of GOF in an enclosed container with an RH of 70% (Figure S1, Supporting Information). The initial GOF was formed by filtration of an aqueous suspension of GO nanosheets through a filter membrane with pore size of 1 µm, which was sandwiched between two pieces of gold electrodes connecting with external circuit as shown in Figure 1a. During this process, the RH in the enclosed container and current intensity of GOF were monitored in real time and kept in range of 70% ± 2% and 0.01–0.1 mA cm−2, respectively. A GOF with a size of 0.5 cm × 0.5 cm was utilized in this study although it could be scaled up by simply using large-area GOFs and electrodes. The thickness of g-GOFs is ≈2.8 µm which could be readily tunable by filtrating desirable amount of GO dispersion with a certain concentration (Figure 1b). Remarkably, the cross-sectional element mapping of g-GOF demonstrated the gradient distribution of the oxygen component on a typical layered GOF (Figure 1c,d). The oxygen content gradually increases from the top side (the negative-electrode-contacting side in Figure 1a) to the bottom side (the positive-electrode-contacting side in Figure 1a). On the other hand, X-ray diffraction (XRD) patterns of g-GOF reveal that the top and the bottom sides have the distinct peak shift (Figure 1e) nonanalogous to the initial GOF, which suggests the MeA has indeed induced the change of interlayer structure. In contrast to GOF with a calculated interlayer spacing of ≈0.8 nm for both sides, the top and bottom sides of MeA-treated GOF has an interlayer spacing of 0.68 and 0.85 nm, respectively, implying a partially reduced top GO layers and a slightly oxidized bottom layers of g-GOF in consistence with the corresponding element mapping (Figure 1d). The composition through the layers of g-GOF was further determined by X-ray photoelectron spectroscopy (XPS). As shown in Figure S2 (Supporting Information), the XPS survey spectra of both the top and bottom surfaces show predominant graphitic C 1s peak at ≈284 eV and O 1s peak at ≈532 eV. The O/C atomic ratio for the bottom side of g-GOF is ≈0.52, slightly higher than that of GO starting material (≈0.45)
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Figure 1. Preparation processes and characterizations of g-GOF. a) MeA process of GOF with applied voltage under humid environment. The e− and Oδ− represented electrons and oxygen-containing ions, respectively. b) Cross-section scanning electron microscopic (SEM) image and c,d) the corresponding carbon and oxygen mapping from X-ray energy dispersive spectroscopy. e) XRD patterns of different sides of as-prepared GOF and g-GOF. f) XPS spectra of different surfaces of g-GOF. The surfaces contacting with the top and bottom electrodes in (a) are noted as Top and Bottom, respectively.
but much higher than that of the top surface (≈0.22), demonstrating the distribution variation of oxygen-containing groups from top to bottom as shown in Figure 1a. Moreover, the high resolution C 1s spectra (Figure 1f) confirmed the presence of C O bands (286.6 eV) and C O bands (288.5 eV) on both sides of the g-GOF.[14] In contrast with the corresponding spectra of initial GOF sample (Figure S3, Supporting Information), the top side had the weakened C O and C O peaks, indicating that the oxygen-containing groups on the top layers of g-GOF were partially removed. In the meanwhile, the signals from bottom side presented the stronger C O and C O peaks, implying the increasing O/C atomic ratio of bottom side probably associated with the further oxidation. These results also explained the inconsistent interlayer spacing change of two sides of g-GOF observed by XRD.
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Along with the basic principle of electric generators, the middle layer (i.e., the g-GOF used in this study) served as both dielectric layer and functional layer in a GO-based moisture– electric energy transformation (MEET) device7 as shown in Figure 2a. When the device was exposed to moisture (e.g., ΔRH = 30%) and inserted in a test circuit, the voltage and current output were observed. Subsequently, when the environmental RH went back to the initial state, the signals of reverse current with negative voltage were collected. Accordingly, the device reverts to the original state and a whole MEET cycle was finished. Based on the operation procedure mention above, the g-GOF-based MEET device could provide voltage and current output of ≈20 mV and ≈5 µA cm−2 within 3 s under a ΔRH of 30% as demonstrated in Figure 2b,c. Notably, unlike other kinds of electric generators, the output voltage and current
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COMMUNICATION Figure 2. Operation of MEET process. a) Schematic illustration of a MEET cycle. The V and A represented the induced voltage and current along the direction of arrows, respectively. b) Voltage and c) current output cycles of a prototype MEET device of g-GOF in response to the intermittent and periodic RH variation (ΔRH = 30%). Experimental data of single d) voltage and e) current output cycle from (b) and (c) matched with ΔRH. f) Thickness change ratio, i.e., thickness change (Δd) divided by original thickness (d), and g) resistance alteration ratio, i.e., resistance alteration (ΔR) divided by original resistance value (R) of g-GOF matched with ΔRH. For clarity in interpretation, ON and OFF in (d)–(g) represent the injection and ejection of moisture in experimental environment, respectively.
pulses of g-GOF-based MEET device show an asymmetric behavior that the peak value on the positive pulses is always significantly higher than that on the negative pulses. The unique output behavior of the single cycle was investigated corresponding to the ΔRH. As shown in Figure 2d, the induced
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positive voltage rapidly rose up once the RH increased, and returned to zero prior to the reduction of RH. Meanwhile, during the RH decrease, the voltage evolved negatively and remained for several seconds after the environmental moisture was removed completely. The identical phenomenon arose in
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the case of current output as shown in Figure 2e. Therefore, the MEET process should be directly related to the interaction between water molecules and g-GOF. According to our previous investigations, water molecules could insert the interlayer space of GO quickly, but the desorption process would take more time.[16,20] The adsorption and desorption behavior of water molecules was reflected by the moisture induced change of thickness and resistance.[13,15] As shown in Figure 2f,g, the thickness and resistance change of g-GOF closely correlate with RH variation. Once moisture is on, the increase of thickness and the decrease of resistance of g-GOF are observed. On the contrary, a slight hysteresis appears during RH reduction due to a relatively slow water release from g-GOF (Figure 2f,g). Along with the analysis mentioned above, the blue dashed line in Figure 2d–g labeled as ON represented the injection
of moisture and the moment that ΔRH, voltage, current density, thickness change ratio (Δd/d), and resistance alteration ratio (ΔR/R) begin to increase, indicating the moisture adsorption induced positive electric output. At the time of moisture ejection marked by OFF (red dashed line), the ΔRH, Δd/d and ΔR/R started to decrease and the negative output appeared, suggesting the moisture desorption induced negative electric output. Several seconds later after ΔRH return to zero, the electric output stopped when the Δd/d and ΔR/R gently reduced to zero, implying the desorption induced electronic output was dominated by the release of residual water molecules in g-GOF. Moreover, when the injection of moisture was maintained after the presence of positive plus, the negative signals appeared once the moisture ejection process began (Figure S4, Supporting Information), which was further substantiated that
Figure 3. Harvesting the body energy hidden in the respiratory moisture-tide and self-powered monitoring of body condition. a) MEET device that is responsive to human breathing increased and decreased RH with exhale and inhale, respectively. b) The g-GOF was sandwiched in two gold electrodes with vents to construct the MEET device. c) Voltage and d) current output generated by respiratory moisture-tide with a ΔRH of ≈21%. e) Self-powered monitoring of respiratory frequency relating to heart rate after different intensity of exercise.
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These results experimentally revealed that the close relevance of heart rate with respiratory frequency with a coefficiency of ≈4.1 for different body condition (Figure S15, Supporting Information). This simple demonstration has indeed presented the great potential of g-GOF device in self-powered body condition monitor toward clinical diagnosis. Based on the analysis above, a proposed mechanism of MEET process was illustrated by a schematic in Figure 4. The g-GOF adsorbs water molecules upon exposure to moisture due to the strong hydrophilicity of oxygen-containing groups (e.g., OH, COOH). With the accumulation of water molecules in the O-rich part of g-GOF, a regional solvation effect will weaken the Oδ− Hδ+ bond in oxygen-containing groups, thus releasing free H+.[13,15] The g-GOF possessed a gradient distribution of oxygen-containing groups (Figure 4a), which accordingly led to the concentration gradient of H+ (Figure 4b). The induced potential and free electron movement of external circuit were thereupon generated once the diffusion of H+ occurred due to the drive of concentration gradient (Figure 4c). Meanwhile, the induced electric field tends to force the H+ drift back to high concentration side. Therefore, when the diffusion and drift effect reached a dynamic balance, the induced potential was stabilized, and hence free electron movement of external circuit was terminated. In contrast, the desorption of moisture accelerated the recombination of negatively charged groups (e.g., COO−) and H+, weakening the diffusion driving effect of concentration gradient. As a result, H+ drifted back and hence induced negative potential with a consequent reverse electron movement (Figure 4d). After the release of residual water molecules, the g-GOF returned to the initiate state.
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adsorption and desorption of moisture enabled electric output of g-GOF rather than the metal electrode contact and the concentration gradient of GO as demonstrated and discussed in detail in Figures S5–S13 (Supporting Information). The breath of human and other terrestrial animals is one of the most steady and constant resources of moisture tide. Generally, the exhale and inhale of adults through nasal cavity can cause an RH variation. We therefore built a g-GOF based power generator (Figure 3a,b) to sensitively harvest the energy of breath. As shown in Figure 3c,d, the calm breathing from a healthy man (ΔRH = 21%) could provide the voltage and current output of ≈18 mV and ≈5.7 µA cm−2, respectively. This power generator can indeed serve as a respiratory energy collector to convert body-provided energy to electric power, indicating a promising potential of g-GOF in energy relative applications. Beyond the power generation, g-GOF could also work as the extra-power-free device to monitor the body condition. For instance, by counting the number of voltage output pulses, we can monitor the respiratory frequency in relevance to the heartbeat of a healthy man after exercise with different intensity (Figure 3e). Before jogging, the man’s calm breathing was 13 times per minute and caused a ΔRH of ≈21% (Figure S14, Supporting Information), corresponding to a heart rate of 58 times per minute. After jogging for 5 min, the breathing was accelerated to 18 times per minute with incidental ΔRH of ≈29%, and the heart rate was elevated to 79 times per minute. Moreover, when the jogging time was extended to 60 min, the signals representing rapid breathing (28 times per minute) were observed according to a fast heartbeat of 118 times per minute, accompanied with the breath induced ΔRH of up to ≈48%.
Figure 4. MEET process of g-GOF. a) Schematic illustration of the gradient distribution of oxygen-containing groups (e.g., carboxyl and hydroxy) in g-GOF. b) Free hydrogen ions (H+) were ionized by adsorbed water from oxygen-containing groups and a density gradient of H+ was established. c) When the free H+ diffused from high density side to low density side, the potential was induced and the electrons moved from bottom electrode to top electrode along external circuit. d) With the desorption of moisture, the recombination of H+ with negatively charged O-functional groups reduced the induced potential, resulting in reverse electrons flow in external circuit.
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Figure 5. Importance of oxygen-containing group gradient in g-GOF based MEET. a) Photographs of UV reduced GO (UV-rGO) dispersion (0.5 mg mL−1) with different UV-irradiation time, which were noted as A, B, and C. The different UV irradiation caused variant O/C ratios of 0.46, 0.27, and 0.2 for A, B, and C, respectively. The UV-rGO dispersions (10 mL) of A, B, and C were filtrated in sequence to prepare GOF with oxygen-containing group steps. b) Voltage and c) current output of UV-rGO based GOF with oxygen-containing group steps. d) Photographs of GO dispersions (0.5 mg mL−1) prepared by hummers’ oxidation (HO-GO) for different time which were noted as D, E, and F. The different degree of oxidation also caused variant O/C ratios of 0.46, 0.35, and 0.21 for D, E, and F. The HO-GO dispersions (10 mL) of D, E, and F were filtrated successively to prepare GOF with oxygen-containing group steps. e) Voltage and f) current output of HO-GO based GOF with oxygen-containing group steps. The electric outputs in (b), (c), (e), and (f) were tested with ΔRH = 30%.
To further demonstrate the mechanism that the gradient of oxygen-containing groups induced the MEET under moisture, we established a GOF consisting of three layers of GO with different oxygen content (Figure 5a). GO with an O/C ratio of 0.46 was controllably reduced as exposed to the UV irradiation for certain periods.[14] The UV-reduced GO with O/C ratio of 0.46, 0.27, and 0.2 were filtrated successively to form a GOF with gradient steps of O-related groups. The obtained GOF based MEET device presented a power-generating behavior similar to that of g-GOF-based one (Figure 5b,c). Additionally, GOFs with the steps of oxygen-containing groups can also be established by filtration of GO dispersions with different oxidation degree (Figure 5d), where the O/C ratios were intended to 0.46, 0.35, and 0.21. The output of voltage (≈0.2 mV) and current (10 nA cm−2) were comparable with UV-rGO-based MEET device (Figure 5e,f). These results demonstrate the mechanism that the gradient of oxygen-containing groups within the GOF indeed induced the MEET effect under moisture. Because of the limited gradient interfaces of oxygen-containing group within the GOF, the low output voltage and current for the GOF is understandable. Since the MEET was based on a diffusion-drift balance, the value of induced potential (Ui) could be expressed by:[22] Ui =
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(1)
where q, D, σ, dc/dx are electric quantity of elementary charge, diffusion coefficient of H+, ionic conductivity of water, and concentration gradient of H+ along an x-axis perpendicular to surface of GOF (see thermodynamic analysis of MEET in the Supporting Information for details). It means that Ui directly depends on the concentration gradient of H+, and hence the gradient of oxygen-containing groups. This conclusion is consistent with the experimental data from GOF with oxygen-containing group steps, in which the gradient of oxygen-containing groups was located at the interfaces of GO steps. Considering that there is no additional energy involved in one MEET cycle, the chemical potential drop of water diffusion process (equals to minus Gibbs free energy, −ΔGM, also see calculation of MEET efficiency in the Supporting Information for details) is the sole energy source for the MEET, we can evaluate the MEET efficiency based on the ratio of ΔGM to output power (WM). It is found that the MEET efficiency of g-GOF generator is up to ≈62%, which is ≈3 times higher than that of piezoelectric zinc oxide (ZnO),[1] ≈61 times higher than that of fluidic–electric chemical vapor deposition graphene (CVD-G)[12] and hundreds of times higher than the fluidic–electric carbon nanotube (CNT).[9] Meanwhile, without the systematic optimization of the devices performance in this preliminary study, the power density is up to 4.2 mW m−2 at the output voltage of 30 mV, which is ≈50 times higher than that of piezoelectric gallium nitride (GaN),[23]
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by NSFC (Nos. 21325415, 21174019), National Basic Research Program of China (2011CB013000), and Beijing Natural Science Foundation (2152028). Received: April 19, 2015 Revised: May 12, 2015 Published online: June 18, 2015
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≈21 times higher than that of fluidic–electric CNT[24] and ≈18 times higher than that of fluidic–electric CVD-G (also see Table S1, Supporting Information).[12] We believe the power density could be further improved by optimizing the assembly of GO, the regulation of oxygen groups and scaling up of film size. In summary, GOF with gradient oxygen-containing groups has been fabricated by an MeA method. This strategy provides a simple but effective approach for the construction of the g-GOF, through which the MEET devices have been achieved to have a high sensitivity to moisture variation (ΔRH of less than 5%), and possess a high energy-conversion efficiency of ≈62% and a power density of ≈4.2 mW m−2 under the tidal moisture. Even the respiratory moisture of human can be converted into electric power, and more impressively, the g-GOF device can ingeniously track the body conditions in a real time manner without any external power supply promising for energy-generating and sensing applications. Although the g-GOF MEET device only produces intermittent power in the preliminary study, it could be combined with energy-storage unit for sustained use.
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