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highly efficient moisture triggered nanogenerator based on graphene quantum dots Yaxin Huang, Huhu Cheng, Gaoquan Shi, and Liangti Qu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12542 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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ACS Applied Materials & Interfaces

Highly efficient moisture triggered nanogenerator based on graphene quantum dots Yaxin Huang,† Huhu Cheng,†,‡ Gaoquan Shi‡ and Liangti Qu†,ǁ,*

† Key Laboratory for Advanced Materials Processing Technology, Ministry of

Education of China; State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China.

‡ Department of Chemistry, Tsinghua University, Beijing 100084, PR China.

ǁ Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials,

School of Chemistry and Chemical Engineering, Beijing Institute of Technology,

Beijing 100081, PR China.

ACS Paragon Plus Environment


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ABSTRACT A high-performance moisture triggered nanogenerator is fabricated by using graphene quantum dots (GQDs) as the active material. GQDs are prepared by direct oxidation and etching of natural graphite powder, which have small sizes of 2-5 nm and abundant

oxygen-containing

functional

groups.

After

the

treatment

by

electrochemical polarization, the GODs-based moisture triggered nanogenerator can deliver a high voltage up to 0.27 V under 70% relative humidity variation, and a power density of 1.86 mW cm−2 with an optimized load resistor. The latter value is much higher than the moisture-electric power generators reported previously. The GQDs moisture triggered nanogenerator is promising for self-power electronics and miniature sensors.

KEYWORDS:

moisture

triggered

nanogenerator;

graphene

quantum

dots;

electrochemical treatment; ion concentration gradient; moisture-electric energy transformation.

Electric power is indispensable in our daily life from wearable electronics to electric vehicles.

Benefiting

from

the

development

of

advanced

materials

and

nanotechnologies, many energy generating forms including hydroelectricity, thermal power, wind energy and solar energy have been developed to meet these requirements. Considering their high costs and low efficiencies, new energy harvesting manners are still desirable. Therefore, various techniques have been explored

to

generate

force

intrigued

piezoelectricity,1,2


friction

triggered

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triboelectricity,3,4 temperature

gradient tempted

thermoelectricity5,6 and

ion

concentration gradient induced electricity.7-12 Among them, ion concentration gradient induced electricity is a novel form to generate electricity from environmental relative humidity variation, which is based on the moisture-electric energy transformation (MEET) process,7 and has aroused increasingly interest due to the high efficiency and ubiquitous stimuli source. The principal mechanism of MEET process is dependent on the reversible change between hydration and dehydration status, which results in the preformed ionizable groups with gradient distribution to release out free hydrated ions when exposed to moisture. Free hydrated ions will directionally migrate from high concentration region to low concentration region under the drive of concentration gradient, thus leading to the output of electric energy. Rooted in MEET method, a series of electricity generators (EGs) has been developed in the form of films,7 foams8 and fibers9 utilizing graphene oxide (GO) sheets as a building block, which exhibits high efficiency to generate power in response to environmental humidity change. However, the compact stacking structure, large size and slight active sites of GO sheets severely hinder the adsorption of moisture and transportation of hydrated ions, which dramatically impede the performance of power generation. Graphene quantum dots (GQDs), consisting of nanometer-sized fragments of single- or few-layered graphene sheets, show unique properties relevant to quantum-confinements and edge effects.13-15 The intrinsic small size, high specific surface area and abundant edge sites endow GQDs with excellent electrical and optical properties, which have been extensively investigated in photovoltaic, catalysis,



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bioimaging and sensor aspects.16-19 These inherent superior characteristics provide an avenue to overcome the aforementioned obstacles during MEET process, thus offering exciting opportunities to promote EG’s output performance. Herein, we demonstrate a graphene quantum dots-based electricity generator (GQDs-EG) to harvest energy from environmental humidity change via MEET methodology. A single GQDs-EG unit can produce a peak voltage output of ca 0.27 V with power density of ca. 1.86 mW cm-2 when loaded with a 50 MΩ resistor under 70% environmental relative humidity variation (RH), which is prominently higher than those moisture-electric EGs based on GO sheets reported previously. The remarkable performance enhancement could be attributed to the small size, enormous oxygen containing functional groups and accessible ionic conductivity of GQDs. We prepared GQDs suspensions by oxidation of natural graphitic powder using modified Hummers method.20-22 The obtained suspensions were further added dropwise to an Au interdigital electrodes (IDE) to fabricate the GQDs based device (Fig. 1a, b). Oxygen-containing functional groups (O-groups) gradient within the device is established along the planer direction perpendicular to the comb via moisture assisted electrochemical treatment (ECT) methodology we developed recently.7 During ECT process, a constant bias voltage (18 V) was applied to the device, which was placed in a closed RH controllable container with high humidity (Fig. 1c). Due to the different electrical potential of the anode and cathode, the GQDs approaching to anode are partially oxidized while the opposite part near to cathode experiences reduction process, thus resulting in O-groups gradient across the comb


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(Fig. 1d). When exposed to moisture, free hydrated ions (H+) are dissociated from O-groups and conformably form gradient in accordance with original O-groups gradient (Fig. 1e). Hydrated ions will spontaneously diffuse from anode to cathode under the influence of concentration distinction (Fig. 1f), thus leading to voltage and current output when connected with external circuit.

Figure 1. Schematic illustration of the GQDs-EG. (a) The Au IDM deposited onto the PET substrate via vacuum ion sputtering method. (b) Drop casting of GQDs suspensions to fabricate the GQDs based device. (c) The ECT process to construct O-groups gradient using a source meter. (d) The close-up view of the O-groups gradient. (e, f) The free hydrated ions (H+) are ionized when exposed to moisture and subsequently migrate from the oxidized side to the reduced side to reach the final equilibrium under the influence of ions concentration gradient.

As clearly depicted in Fig. 2a, b, the TEM image displays that the GQDs are well monodispersed in deionized water on grid implying a uniform size of ca. 2-5 nm. The inset high-resolution TEM image presents clear stripe, indicating the lattice parameter



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of 0.24 nm, which is in good accordance with the (002) crystal plane of graphene (Fig. S1, Supporting information).23 The corresponding AFM image (Fig. S1) exhibits a typical topographic height of 0.5-1.5 nm with average height of 1 nm, suggesting that most of GQDs are composed of 1-3 graphene layers.24 The nano-scale lateral size and few-layered height imply a huge surface area and abundant active sites (e.g. edges and defects), which are beneficial for adsorption of moisture. Moreover, the FTIR spectrum confirms obvious absorption peak of carboxyl and hydroxyl groups of GQDs (Fig. 2c). This hydrophilic functional groups greatly facilitate the adsorption of moisture and favor the establishment of O-groups gradient. In addition, XRD pattern shows a broad and week peak centered at around 24.4°, corresponding to a larger interlayer spacing of 0.36 nm, which is probably attributed to the existence of abundant oxygen containing functional groups on the defects and edges of GQDs.20, 22 Further Raman spectrum (Fig. S1) denotes obvious disorder D band centered at 1357 cm-1 and crystalline G band centered at 1587 cm-1. The intensity ratio ID/IG is about 0.94, implying plenty of defects (active sites) on the GQDs.25 A typical SEM image of GQDs-EG device is shown in Fig. 2d and Fig. 2e. GQDs are almost fully filled in the gaps and uniformly covered on Au IDE, and the corresponding thickness of as-prepared GQDs film was about 0.5µm (Fig. 2f). The as-fabricated GQDs film was continuous and abundant with oxygen containing functional groups (Fig. S4). Additionally, to construct the ions concentration gradient across the device, mild ECT process was employed with assistance of moisture. During the electrochemical treatment process, a bias voltage of 18 V was applied to


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the device with high environmental humidity. In this case, one side of GQDs approaching to anode is subjected to oxidation due to relatively high electric potential, whereas the other side near to cathode is slightly reduced, thus leading to the construction of oxygen containing functional groups gradient. Therefore, the X-ray energy dispersive spectroscopy (EDS) profile of line scan (Fig. 2g) between the combs notably reveals that the chemical content variation of O/C atomic ratio faded away from anode to cathode, further validating the successful establishment of O-groups gradient.

Figure 2. Characterizations of GQDs and as-fabricated EG device. (a) TEM image of GQDs. The insert is the corresponding high-resolution TEM image. (b) Size distribution of GQDs. (c) FT-IR spectrum of GQDs. (d) SEM image of Au IDE (light grey) covered with GQDs film on a PET substrate. (e) Zoom-in SEM image of GQDs film. (f) Cross-sectional SEM image of GQDs film. (g) Line scan EDS spectrum of O/C atomic ratio after polarization at 18 V voltage. Scale bar: d, 500 µm; e, f, 2 µm; g, 500 nm.



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To quantitatively evaluate the response to the environmental change, the as-fabricated gradient GQDs-EG was connected with external test circuit within a sealed RH controlling system (Fig. S5). Based on the fundamental electricity generation principle mechanism, the GQDs are acted as both dielectric layer and functional layer during a MEET process as showed in Fig. 3a. O-groups gradient within the GQDs film (denoted as g-GQDs) is established via electrochemical treatment (Fig. 3a-i). The g-GQDs spontaneously absorb water molecules when exposed to moisture due to their hydrophilic oxygen containing functional groups (e.g., —COOH). A localized hydrolysis effect will weaken the O—H bond, leading to the release of free movable H+, and thus a density gradient of H+ is established (Fig. 3a-ii). Free H+ spontaneously diffuses from rich oxygen-groups side to poor oxygen-groups side under the drive of concentration gradient, resulting in an external current and potential (Fig. 3a-iii). When desorption of water molecules, free H+ will drift back and recombines with the unmovable electronegative oxygen-groups and hence induces a reverse current and potential (Fig. 3a-iv). Subsequent release of residual water molecules made g-GQDs back to its initiate state, thus a whole MEET cycle completed. When exposed to moisture, the voltage and current signals are instantly observed. Subsequently, after withdrawing of moisture, the humidity gradually comes back to its original state and a reverse voltage and current signal are collected, thus reverting to its initial status and achieving a complete cycle. Impressively, the gradient GQDs-EG could intermittently produce voltage and current density about 150 mV and around 16.8 mA cm-2 under a RH variation of 50%


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(Fig. 3b and Fig. 3c). The impact of flow gas has been excluded (Fig. S11). It was interesting that the voltage signal went up rapidly and kept ascending for a while to the maximum even withdrawing the moisture stimuli when exposed to moisture. We regard this delaying phenomena as the influence of excessive moisture amount and comparatively slow ionic migration rate. Subsequently, the signal descended and reversed for a relatively long time to its original state associated with diminishing of humidity (Fig. 3d). The uniform behavior was observed in the case of current output (Fig. 3e). Particularly, the asymmetric behavior that the positive pulse signal is much higher than the negative pulse signal is coincident with the fast hydration and slow dehydration process, which are resulted from abundant O-groups of GQDs. Moreover, The GQDs-EG can retain 90% of its original performance after 150 cycles, implying excellent cycle stability (Fig. 3f and Fig. S7, 10-11). We further investigate the relationship between the output performance of device and environmental relative humidity variation. Along with the increase of humidity, the voltage output accordingly ascended and reached to 270 mV upon 70% RH variation (Fig. 3g). The current signal presented the analogous behavior (Fig. S7). Moreover, we connected our device with different external resistors to investigate the output performance of the GQDs-EG. When the external resistance was increased from 1Ω to 1GΩ, the voltage increased from 0 to about 250 mV, whereas the current density decreased from 24 mA cm-2 to nearly zero, indicating a maximum output power density up to 1.86 mW cm-2 at an optimized load resistance of 50 MΩ, which was much higher than previously reported GO foam based EG (Fig. 3h and Fig. 3i).8



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Compared with other graphene based nanogenerators,

26,27

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the moisture triggered

nanogenerator which is based on a gaseous stimuli process, demonstrates a high output performance due to the spontaneous chemical diffusion process. The remarkably promotion is mainly ascribed to the plentifully active O-groups sites and adequate adsorption of moisture, which are primarily derived from nano-sized dimension and the much more available active sites.10 However, the stimuli-response behavior can only output intermittent power and the continuous electric output is still absent and can be improved by reasonable design of functional materials and stimuli manners for practical use.

Figure 3. (a) Schematic diagram of a MEET cycle. (i) gradient distribution of O-groups (e.g., carboxyl) in g-GQDs film; (ii) release of free hydrogen ions (H+) from O-groups upon absorption of H2O; (iii) induced current and potential from the diffusion of free H+; (iv) drift back of free H+ with the desorption of H2O. (b, c) Voltage and current density output cycles of a typical gradient GQDs-EG device in


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response to the intermittent and periodic relative humidity variation. (d, e) A typical single cycle voltage and current density output selected from (b, c) marked with dashed lines. (f) The cycle stability of voltage output in response to the change of RH. (g) Voltage output performance versus the relative humidity variations. (h, i) Output voltage (red curve, h), current density (blue curve, h) and power density (i) of the device loaded with different resistances. The inset of (i) is a sketch map for measuring the output characteristics (voltage, current density and power density) of the device. Adsorption and transportation of moisture within functional layer have a great influence on the power generation performance. In comparison with GO assembly EGs, moisture is more easily absorbed at the edges and defects of GQDs and diffuses through the functional layer along thickness direction, mainly owing to the tiny sizes (smaller than 20 nm), large specific surface area and abundant O-groups of GQDs (Fig. 4a). By contrast, GO sheets are several to dozens of micrometers in their lateral dimensions (Fig. S17), which is about two orders of magnitude larger than those of GQDs. The larger transverse dimension and fewer active sites (O-groups) make GO membrane has longer transportation distance and weaker ability of adsorbing moisture. Thus, an assembled GO membrane shows much worse performance of MEET induced power generation than that of the GQDs-based counterpart. To quantitatively assess the performance of GO assembly, we fabricated GO sheets device by simply substituting GODs suspensions with GO solutions under same experimental condition. As shown in Fig. 4b, the GO sheets EG could intermittently produce voltage and current density about 30 mV and around 1 mA cm-2 under a 50%



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RH variation, the corresponding instantaneous output power density was 0.03 mW cm-2, which was about eighty times smaller than GQDs based device (Fig. S18). To the best of our knowledge, GQDs-EG was superior to other moisture induced electricity generation device (Fig. 4c and Table S1).7-11, 28-31 To clarify the basic mechanism of remarkable improvement performance of GQDs-EG, XPS survey was further conducted to determine the accurate composition of GQDs and GO. It shows the predominant graphitic C 1s peak at =284 eV and O 1s peak at 532 eV for both GQDs and GO, and the calculated O/C atomic ratio is about 0.72 and 0.46, respectively (Fig. S19). The higher O/C ratio suggests more O-groups existence in GQDs than GO in accordance with larger interlayer spacing (Fig. S1), which affords more active sites to adsorption of moisture. Moreover, the high resolution C 1s spectra (Fig. 4d) confirms the presence of C–C/C=C (284.4 eV), C– OH (285.7 eV), C–O–C (286.9 eV), C=O/COOH (288.4 eV), and the relative ratio of peak intensity presents more carboxyl group existence in GQDs, whose dissociation upon moisture can provide more free hydrated ions (H+), thus leading to great improvement of EG performance. On the other hand, ionic conductivity of the functional layer (GQDs) within the device plays an important role in electricity generation performance.32 Hence, electrochemical impedance spectroscopy (EIS) was conducted to assess the ionic conductivity of as-prepared GQDs and GO based device under high humidity condition (95% RH). In solid electrolyte system, the corresponding equivalent circuit for our devices is typically represented by electrode resistance (Rs) in series with a parallel combination of electrolyte resistance (Re) and


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double layer capacitance (represented by Constant Phase Element (CPE)).8,

32

It

presents a segment of a circle at high frequency region and a slanted line at lower frequencies on both GQDs and GO based devices (Fig. 4e). In contrast, EIS of GQDs based device shows smaller ionic resistance than GO based device. The lower ionic resistance of GQDs-EG device implies more accessible for hydrated ions to transport in GQDs-EG than GO based device, thus resulting in huge elevation of power generation performance.8, 10



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Figure 4. (a) Schemes of moisture transportation behavior on GQDs film (left) and GO film (right). Compared with the small sized GQDs, moisture through large sized GO sheet is susceptible to plugging due to the long transport distance. (b) Voltage output cycles of a typical gradient GO-EG in response to the intermittent and periodic relative humidity variation under 50% RH variation. (c) Power density and voltage output performance of GQDs EG in comparison with other moisture-induced EGs. (d) XPS high-resolution C 1s spectrum of GQDs and GO. (e) EIS of GQDs and GO based EGs obtained at high frequency region. The inset shows the equivalent circuit.

In summary, we developed a high power density moisture-induced generator based on GQDs. The as-fabricated EG device could generate voltage and current density of 270 mV and 27.7 mA cm-2, respectively. The calculated power density is 1.68 mW cm-2, which is the highest among moisture/evaporation induced EGs. The great improvement is attributed to the inherently small size, abundant O-groups and high ionic conductivity of GQDs. Due to the ubiquity of moisture/evaporation in natural, simple fabrication process and high power generation performance, this study will promote a novel avenue to harvest ambient energy and has potential applications in low-cost, green, self-powered devices and systems.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

Preparation details, additional photographs, SEM images, EDS, AFM images and supplementary experimental results.

AUTHOR INFORMATION Corresponding Author *Email : [email protected]

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT

This work was supported by the National Key R&D Program of China (2017YFB1104300), NSFC (No. 21325415, 51673026, 51433005), Beijing Natural Science Foundation (2152028), and Beijing Municipal Science and Technology Commission (Z161100002116022).



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