Letter pubs.acs.org/NanoLett
Sodium-Ion Intercalated Transparent Conductors with Printed Reduced Graphene Oxide Networks Jiayu Wan, Feng Gu, Wenzhong Bao, Jiaqi Dai, Fei Shen, Wei Luo, Xiaogang Han, Daniel Urban, and Liangbing Hu* Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *
ABSTRACT: In this work, we report for the first time that Naion intercalation of reduced graphene oxide (RGO) can significantly improve its printed network’s performance as a transparent conductor. Unlike pristine graphene that inhibits Na-ion intercalation, the larger layer−layer distance of RGO allows Na-ion intercalation, leading to simultaneously much higher DC conductivity and higher optical transmittance. The typical increase of transmittance from 36% to 79% and decrease of sheet resistance from 83k to 311 Ohms/sq in the printed network was observed after Na-ion intercalation. Compared with Li-intercalated graphene, Na-ion intercalated RGO shows much better environmental stability, which is likely due to the self-terminating oxidation of Na ions on the RGO edges. This study demonstrated the great potential of metal-ion intercalation to improve the performance of printed RGO network for transparent conductor applications. KEYWORDS: Sodium ion, intercalation, printed RGO network, transparent conductor, two-dimensional materials
U
In this work, we report sodium-ion (Na-ion) intercalation in a printed graphene oxide network after a thermal reduction. Compared with liquid exfoliated multilayer graphene that hinders the insertion of Na-ions due to its small interlayer distance, RGO has a larger sheet size (therefore better junction contact)21 and an expanded interlayer distance that allows Naion insertion.33,34 As a result, a relative increase in the transmittance as large as 120% (from 36% to 79%), with a 270 times decrease of sheet resistance (from 83k to 311 Ohms/ sq) are achieved in sodiated RGO (Na-RGO) network. Such an intercalated network shows the best performance in RGObased transparent electrodes. Surprisingly, we found that NaRGO is much more stable than a Li-intercalated graphene, which may be attributed to the self-termination of oxidation products at the edges of RGO sheets by the reaction between Na-ion and water/CO2/O2. Thus, Na-ion intercalation of a printed RGO network is a promising approach leading to scalable applications as transparent conductors. Results and Discussion. Figure 1a−b shows a schematic illustration of Na-ion intercalation into RGO network. To carry out the electrochemical intercalation in electrolyte, a constant current source is applied between the intercalant (Na metal) and the host material (printed RGO network). Na-ions can electrochemically intercalate into RGO interlayers and RGO− RGO junctions. Due to the low electronegativity of Na metal, the accumulation of intercalated Na provides electrons (n-
ltrathin two-dimensional (2D) materials such as graphene are highly attractive for transparent conductor applications due to their high transparency and carrier mobility, which lead to an excellent combination of sheet conductance and optical transmittance in the visible range.1−5 Large-area graphene prepared by chemical vapor deposition (CVD) has shown 30 Ohm/sq and 90% transmittance, comparable with traditional indium tin oxide (ITO) electrodes;6 however, the high cost of CVD-based transparent electrodes is one of the main obstacles to replace ITO.7 Solution-based, large-scale, printed transparent conductors using liquid exfoliated graphene8−14 or reduced graphene oxides (RGO)15−22 show potentially much lower cost and have been successfully applied to a range of electronic devices such as solar cells23−25 and organic light-emitting diodes (OLED).26,27 However, the high sheet−sheet junction resistance largely limits the sheet conductance of printed network,28 similar to previous works on carbon nanotube network transparent conductors.29 Chemical doping6 and intercalation30,31 have been explored as effective methods to increase carrier density and thus lower the sheet resistance with little decrease or even an increase in the optical transmittance. Recently, we reported a method by electrochemical lithium-ion intercalation in mechanically exfoliated graphene sheets, which leads to a drastic simultaneous improvement of sheet conductivity and optical transmittance in the visible range.32 However, the Li-intercalated graphene sheets are unstable in air, and the methodology is limited by the size of exfoliated sheets, which narrows its range for practical applications. © XXXX American Chemical Society
Received: January 25, 2015 Revised: May 1, 2015
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Figure 1. (a) Schematic of Na-ion intercalation between two RGO sheets, enhancing simutaneously their optical transmittance and electrical conductivity. (b) Schematic of Na-ion intercalation in printed RGO network on transparent substrate. (c) AFM image of a printed RGO network. (d) Highly transparent and stable Na-RGO network (confined in red dotted square) in air after one month.
Figure 2. Electrochemical and XRD characterizations of Na-ion intercalation in RGO thin films. (a) Voltage profile of RGO film as positive electrode in coin cell. Inset is the schematic of RGO-Na coin cell (b) XRD of RGO film before, after Na intercalation, and plastic wrap as background.
manufacturing. Figure 1c shows an AFM image of RGO network, which exhibits a nanoscale surface smoothness, and curved lines are ripples or overlapped edges of RGO sheets. Figure 1d shows that printed RGO after sodiation is highly stable even when exposed in air for a month, which demonstrates the potential for its utilization in practical applications. We have two motivations to use Na-ion as intercalants for such novel transparent electrodes. First, Na-ions are much more cost-efficient and more abundant than Li. Second, Na-ion is expected to form a more stable barrier layer to prevent further oxidation toward better stability than Li-ions. However,
doping) and thus shifts the Fermi level up. Due to the large storage capacity of RGO for Na ions, the electron doping level is high as well. The large electron doping is expected to cause the “Pauli blocking”35−37 of incident light in the visible range. This results in a large enhancement of the optical transparency of the RGO network. On the other hand, electron doping will also improve the conductivity of individual RGO flakes. Junctions in printed RGO networks usually act as barriers for charge transport and reduce the entire conductivity of the network.29 Na-ion intercalation can provide electron pathways in junctions and greatly reducing the junction resistance. We focus on printed RGO network toward scalable nanoB
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Nano Letters Na-ion cannot intercalate into graphene as demonstrated by others before. Recently, Na ion intercalation in RGOs has been demonstrated. To prove that Na-ions can intercalate into RGO materials prepared in our lab, several characterization tests were carried out. First, a coin cell was made using free-standing RGO film and Na metal as positive and negative electrodes, respectively. Figure 2a shows the voltage profile (second cycle) of sodiation/desodiation in RGO thin film at a low current density of 25 mA/g, indicating a reversible Na-ion intercalation/deintercalation process in RGO network. From the voltage profile, the specific capacity for sodiation is 170 and 127 mAh/g for desodiation. The irreversible capacity is mainly due to the electrolyte decomposition and side reactions during Na-ion intercalation process, which is commonly observed for carbon anodes in Na-ion batteries.33,34,38 The amount of intercalated Na-ions in the RGO network is 0.028 per carbon atom, calculated from the reversible specific capacity. We also applied X-ray diffraction (XRD) on the same RGO thin film before and after sodiation to confirm the actual intercalation process. During the XRD characterization, samples were protected by a thin layer of plastic wrap to avoid chemical reactions under ambient condition. In Figure 2b, a sharp peak is shown in the RGO film before Na-ion intercalation. The peak position is at 25.35°, which corresponds to an interlayer distance of 3.49 Å. This interlayer distance is larger than that of graphite (3.35 Å). The light green shadowed area represents a peak with full width at half-maximum (FWHM) in length of 1.02 Å. After Na-ion intercalation, the XRD peak shifted to left, indicating a larger interlayer distance of 3.76 Å in average. The peak shape is also much broader than what was observed before Na-ion intercalation, with a FWHM changes to 1.67 Å (in length of interlayer distance). The enlarged interlayer distance is indicative of a successful intercalation of Na-ions between RGO layers. The peak around 18° is due to the sealing plastic wrap. The broadening of the peak may be due to the nonuniform intercalations of Na-ions in the printed RGO flakes. To demonstrate a scalable application of Na-ion intercalated RGO as high performance transparent electrodes, printable GO ink is prepared (Figure 3a). Commercial wetting agent Zonyl was added to decrease the surface energy of GO ink in water to be printable. Meyer rod coating method was applied to deposit GO network, which shows excellent uniformity (Figure 3b). GO network on glass slides were then thermally reduced to RGO (also see Figure S1), and trimmed by blade to a desired shape. Copper current collectors were deposited by thermal evaporation, then Na metal and 1 M NaPF6 in EC: DEC (1:1 = v:v) were added as the negative electrode and electrolyte, respectively. The entire device was finally sealed with epoxy in an argon-filled glovebox. The schematic and photo images of the electrochemical intercalation device39 are shown in Figure 3c. After electrically connecting the two electrodes of RGO and Na metal, RGO network can be fully intercalated by Na-ions within 10 min, as shown in Figure 3d, leading to a uniform transmittance enhancement to the Na-RGO network. The transmittance of RGO network before and after Na-ion intercalation is quantitatively illustrated by gray scale images captured by an optical microscope (operated in transmission mode), as shown in Figure 4a and b, respectively. Note that the increase of optical transmittance is very uniform across the entire printed RGO network. Figure 4c plots the transmittance change of RGO network before and after complete sodiation at wavelength of 550 nm
Figure 3. (a) A bottle of as-prepared GO ink to be added with Zonyl to tailor the surface energy for printing purpose. (b) Meyer rod coating of GO ink with excellent uniformity on glass substrate. (c) A two-terminal, lateral device with RGO network as working electrode, Na metal as the counter electrode and 1 M NaPF6 in EC: DEC (1:1 = v:v) as the electrolyte. (d) After Na ion intercalation, the RGO network becomes more transparent.
for samples with different thickness (also see in Figures S2 and S3). It is clear that all samples exhibited a drastic transmittance increase. Note that the substrate is excluded in the transmittance measurement. The percentage of relative increase in transmittance is also plotted in Figure 4c inset. For instance, RGO network with transmittance of 46.1% increased to 91.5% after sodiation, which is nearly a 100% increase compared to its original value. Wavelength dependent spectrum from 450 to 900 nm is shown in Figure 4d. The optical transmittance vs wavelength of Na-RGO is flat in the visible range, which indicates a neutral color and is beneficial for a range of applications. To further demonstrate the feasibility of Na-RGO as transparent conductors, sheet resistance of RGO network before and after Na-ion intercalation was investigated. The resistance measurement was carried out by a four probe method (schematic device image shown in the inset of Figure 5a) to eliminate the contact resistance between Cu electrodes and the RGO network. I−V curves taken from a RGO device before and after sodiation are shown in Figure 5(a), starting with a large sheet resistance at around 100k Ohm/sq since the GO network was only partially reduced at a relatively low temperature of 300 °C.40 Surprisingly, with an increased transmittance after sodiation, the sheet resistance also decreased by 300 folds, from 523k to 1467 Ohms/sq. The other typical decrease in sheet resistance is from 83k to 311 Ohms/sq (Figure 5b). It is expected that Na ion doping largely enhances the carrier density in RGO, which leads to the conductivity increase of individual RGOs.30,32 Meanwhile, Na ion intercalation will potentially improve the RGO−RGO contacts which overcomes the obstacle of conventional network conductors.29 This synergistic effect in increase of optical transmittance and decrease of sheet resistance can be qualitatively explained by heavy electron doping upon sodiation. Both theoretical and experimental work on alkali (Li, Na) metal intercalation in SiC/ C
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Figure 4. Optical transmittance of a RGO network device before and after Na-ion intercalation. (a−b) show optical microscope (transmission mode) images of the same RGO network before and after sodiation. (c) Transmittance vs thickness of RGO network before and after sodiation at 550 nm. The inset is the percentage of relative increase in transmittance at 550 nm. (d) Transmittance vs wavelength of the same RGO network before and after sodiation with a visible spectrum from 450 to 900 nm.
Figure 5. (a) Four-probe I−V measurement of a RGO network before and after sodiation. Insets are the schematic and photo images of four-probe measurement on RGO network. (b) Sheet resistance change of two different samples. (c) Transmittance vs sheet resistance plots of Na-RGO vs reported RGO network transparent conductors. (d) FOM of network transparent electrodes, including exfoliated graphene, RGO, and Na-RGO.
synergistic effect in the increase of optical transmittance and decrease of sheet resistance leads to an excellent performance of Na-RGO as a transparent conductor. This is consistent with our previous work on Li intercalation in few layer graphene.32 For fair comparison, we compare our data with other 2D network based materials including RGO and exfoliated graphene (Figure 5a). Na-RGO shows clear better performance than other materials. To quantify this comparison, the figure of
graphene interfaces n-dope the monolayer graphene, with an upshift in Fermi level.41,42 Similarly, due to the large Na ion storage capacity of RGO and low electronegativity of Na metal, intercalated Na provides electrons and n-dopes the RGO network. The large electron doping leads to a large Fermi energy upshift, which blocks optical transition in the visible range and increases the optical transmittance. Electron doping also leads to the large increase of the conductivity. This D
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Figure 6. Air stability of transparent conductor based on large-scale Na-RGO network. Photo images of Na-RGO in air after 0, 0.5, 2, and 3 h. The scale bar (white) is 2 mm.
merit (FOM, defined as ratio of electrical to optical conductivity, σdc/σopt) is calculated based on sheet resistance and optical transmittance at 550 nm (Figure 5d). Sheet resistance and optical transmittance of approximately 80−85% were used for FOM calculations. Na-RGO network has the highest FOM value. Better performance is expected with large RGO sheets, which will have a low percolation threshold and better initial performance as transparent conductor before Na ion intercalation. Air-stability of Na-ion intercalated RGO is a potential concern due to the high reactivity of Na. To evaluate the stability, the Na-RGO network was first fully sodiated in encapsulated devices. One device was then disassembled and Na-RGO on glass was exposed in ambient environment. Optical images captured by a microscope on Na-RGO network over different spots on the same device were recorded after 0, 0.5, 2, 3, and 13 h. Optical transmittance of Na-RGO on glass was obtained based on microscope images. After complete Na intercalation, transmittance at 550 nm of the RGO sample (red bar) increased from 57% to 90%. The sample shows small degradation after immediate exposure in air. Then the transmittance is stabilized over time, and even after 13 h. Our continuing observations show that Na-RGO does not change the optical transmittance, even after one month. Photo images of Na-RGO network in air for the first few hours are shown in Figure 6b. As we demonstrated before, Li-intercalated ultrathin-graphite has the highest FOM among all continuous thin film.32 However, the large diffusivity/reactivity of Li-ions in the intercalant causes poor stability and requires further device encapsulation. Graphene flakes become highly transparent after Li-ion intercalation. But intercalation compound completely changes back to the original, much lower transmittance after a few hours in air.32 The better stability of Na ion intercalated RGO than Li ion intercalated graphene flakes is explained as the following. Intercalated alkaline ions such as Li and Na are extremely reactive and thus will react with oxygen and water in ambient environmental immediately. The reactants on the edges of the 2D materials will prevent further reaction of alkaline ions, which explains the initial decrease of the optical transmittance in the first half hour. The difference between Li ion and Na ion is likely to be explained by their different diffusivity through the self-formed barrier layer. The large Naion size could lead to a much slow diffusion through the barrier
layer than Li-ion. Therefore, the reactants on the edges of 2D materials prevents further reaction of Na with O2, H2O and CO2 in air, and seals intercalated Na inside the sheets. Conclusion. For the first time, we demonstrate that Na-ion can intercalate into RGO network and simultaneously increase the optical transmittance (∼100%) and conductivity (∼300 times) dramatically. The intercalated RGO network shows superior performance as transparent conductor, better than any other RGO network based transparent electrodes. Surprisingly, such transparent electrodes shows excellent stability in ambient environment, much more stable than Li ion intercalated graphene. The processes including GO network printing, reduction, and electrochemical intercalation can all be potentially scaled toward practical applications. Further improvement of transmittance and sheet resistance can be achieved with GO networks with large size of individual flakes.21 Fundamental studies including air stability mechanism based on Na-ion transport through the reactant layer, charge transport through RGO−RGO layers with ion intercalation,43 and tunable work function studies with Na-RGO44−46 will be further investigated in the future. This work demonstrated the great feasibility of using metal ion intercalations in 2D materials for promising transparent conductor applications. Experimental Section. Preparation of Printable GO Ink. All chemicals used in the experiments were purchased from Sigma-Aldrich. GO powder was synthesized with modified Hummer’s method.47 Concentrated acid H2SO4/H3PO4 was mixed at a volume ratio of 9:1 (180:20 mL) and slowly added to the mixture of KMnO4 (7.2 g) and graphite flakes (1.2 g). The reaction was carried out with ice to avoid overheating. Then the solution was heated up to 50 °C and stirred for 12 h. After that, the mixture was immersed with ice (500 mL), and 30% H2O2 (3 mL) was added. H2SO4/H3PO4 and KMnO4 were washed away by DI water, and centrifugation was applied (8500 rpm for 0.5 h) until PH reached 7.0. 250 mL of 30% HCl was then added to expand the GO. Finally, the Cl-ions were eliminated with DI water and centrifuged at 8500 rpm for 0.5 h. As obtained GO solution is with concentration of 2 mg/mL. A mixed solution of Zonyl and GO (w:w = 3:10) is then obtained for coating. RGO Network Preparation. 200 μL of GO/Zonyl ink was dropped at the edge of clean glass slide followed by Meyer rod (R.D. Specialties, #10) rolled along the glass slide with GO ink. When the glass was uniformly coated by GO ink, it was then E
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Nano Letters dried on a hot plate at 50 °C for 1 min. The process was repeated until a desired multilayer GO network was obtained. GO coated glass slides were reduced in tube furnace with controlled Ar/H2 gas (95%/5%) flow. They are then thermally annealed with a heating rate of 1 °C/min and heated up to 300 °C in 5 h. The samples were cooled down with a tube furnace to room temperature after it reached 300 °C. Electrochemical Device Fabrication/Measurement. RGO networks were trimmed with a blade into desired size. The Cu electrodes as current collector/four-probe electrodes were thermally evaporated with shadow masks. PDMS reservoirs were then built on the devices. After this, the devices were transferred into a glovebox for Na metal deposition and electrolyte injection. The devices were also sealed inside the glovebox for optical and resistance measurement. Optical and Resistance Measurement of RGO/Na-RGO Network. Na-RGO network was obtained by applying potential difference in as-prepared device between RGO and Na metal. Optical transmittances of RGO/Na-RGO network were achieved by analyzing the gray scale image taken by chargecoupled device (CCD), coupled to a microscope (Nikon Eclipse Ti-U) in transmission mode. A narrow-band filter (Thorlabs Inc.) was added in the optical path to obtain transmittance at 550 nm. Spectrum data were obtained by compact spectrometer (Thorlabs) incorporated to the microscope. The sheet resistances of RGO/Na-RGO were measured by four probe method to eliminate contact resistance. Material Characterization. RGO freestanding films were obtained by thermally reducing (300 °C, 1 °C/min ramp) vacuum-filtrated GO film. RGO freestanding films were then assembled in coin cells and cycled electrochemically at a current density of 25 mA/g by a battery tester (Biologic USA). NaRGO free-standing films were then taken out by disassembling coin cells, sealed by plastic wrap and stored in an Ar-filled exchange box to avoid air contamination/side reactions before characterization. XRD patterns were obtained by C2 discover with GADDS, and Raman spectra were obtained by a Raman spectrometer (Horiba Jobin Yvon, a 633 nm He−Ne laser source).
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spectrometer for this work. We also thank Dr. Kang Xu and Dr. Arthur v. Cresce from Army Research Lab for their Na-ion battery electrolyte. We thank Dr. Colin Preston, John Panagiotopoulos and Yanan Chen for their help on this manuscript.
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ASSOCIATED CONTENT
S Supporting Information *
Supporting Information Available: RGO network characterization and further microscope images of transmittance increase from RGO to Na-RGO. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b00300.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
J.W., F.G., and W.B. contributed equally to the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.H. acknowledges the startup support by University of Maryland, College Park. The authors acknowledge the support of the Maryland Nanocenter and its Fablab, Nisplab and surface analysis center. We thank Dr. Jeremy Munday and their group from ECE department for kindly sharing the Microscope and F
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Sodium-Ion Intercalated Transparent Conductors with Printed Reduced Graphene Oxide Networks Jiayu Wan, Feng Gu, Wenzhong Bao, Jiaqi Dai, Fei Shen, Wei Luo, Xiaogang Han, Daniel Urban, and Liangbing Hu* Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *
ABSTRACT: In this work, we report for the first time that Naion intercalation of reduced graphene oxide (RGO) can significantly improve its printed network’s performance as a transparent conductor. Unlike pristine graphene that inhibits Na-ion intercalation, the larger layer−layer distance of RGO allows Na-ion intercalation, leading to simultaneously much higher DC conductivity and higher optical transmittance. The typical increase of transmittance from 36% to 79% and decrease of sheet resistance from 83k to 311 Ohms/sq in the printed network was observed after Na-ion intercalation. Compared with Li-intercalated graphene, Na-ion intercalated RGO shows much better environmental stability, which is likely due to the self-terminating oxidation of Na ions on the RGO edges. This study demonstrated the great potential of metal-ion intercalation to improve the performance of printed RGO network for transparent conductor applications. KEYWORDS: Sodium ion, intercalation, printed RGO network, transparent conductor, two-dimensional materials
U
In this work, we report sodium-ion (Na-ion) intercalation in a printed graphene oxide network after a thermal reduction. Compared with liquid exfoliated multilayer graphene that hinders the insertion of Na-ions due to its small interlayer distance, RGO has a larger sheet size (therefore better junction contact)21 and an expanded interlayer distance that allows Naion insertion.33,34 As a result, a relative increase in the transmittance as large as 120% (from 36% to 79%), with a 270 times decrease of sheet resistance (from 83k to 311 Ohms/ sq) are achieved in sodiated RGO (Na-RGO) network. Such an intercalated network shows the best performance in RGObased transparent electrodes. Surprisingly, we found that NaRGO is much more stable than a Li-intercalated graphene, which may be attributed to the self-termination of oxidation products at the edges of RGO sheets by the reaction between Na-ion and water/CO2/O2. Thus, Na-ion intercalation of a printed RGO network is a promising approach leading to scalable applications as transparent conductors. Results and Discussion. Figure 1a−b shows a schematic illustration of Na-ion intercalation into RGO network. To carry out the electrochemical intercalation in electrolyte, a constant current source is applied between the intercalant (Na metal) and the host material (printed RGO network). Na-ions can electrochemically intercalate into RGO interlayers and RGO− RGO junctions. Due to the low electronegativity of Na metal, the accumulation of intercalated Na provides electrons (n-
ltrathin two-dimensional (2D) materials such as graphene are highly attractive for transparent conductor applications due to their high transparency and carrier mobility, which lead to an excellent combination of sheet conductance and optical transmittance in the visible range.1−5 Large-area graphene prepared by chemical vapor deposition (CVD) has shown 30 Ohm/sq and 90% transmittance, comparable with traditional indium tin oxide (ITO) electrodes;6 however, the high cost of CVD-based transparent electrodes is one of the main obstacles to replace ITO.7 Solution-based, large-scale, printed transparent conductors using liquid exfoliated graphene8−14 or reduced graphene oxides (RGO)15−22 show potentially much lower cost and have been successfully applied to a range of electronic devices such as solar cells23−25 and organic light-emitting diodes (OLED).26,27 However, the high sheet−sheet junction resistance largely limits the sheet conductance of printed network,28 similar to previous works on carbon nanotube network transparent conductors.29 Chemical doping6 and intercalation30,31 have been explored as effective methods to increase carrier density and thus lower the sheet resistance with little decrease or even an increase in the optical transmittance. Recently, we reported a method by electrochemical lithium-ion intercalation in mechanically exfoliated graphene sheets, which leads to a drastic simultaneous improvement of sheet conductivity and optical transmittance in the visible range.32 However, the Li-intercalated graphene sheets are unstable in air, and the methodology is limited by the size of exfoliated sheets, which narrows its range for practical applications. © XXXX American Chemical Society
Received: January 25, 2015 Revised: May 1, 2015
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Figure 1. (a) Schematic of Na-ion intercalation between two RGO sheets, enhancing simutaneously their optical transmittance and electrical conductivity. (b) Schematic of Na-ion intercalation in printed RGO network on transparent substrate. (c) AFM image of a printed RGO network. (d) Highly transparent and stable Na-RGO network (confined in red dotted square) in air after one month.
Figure 2. Electrochemical and XRD characterizations of Na-ion intercalation in RGO thin films. (a) Voltage profile of RGO film as positive electrode in coin cell. Inset is the schematic of RGO-Na coin cell (b) XRD of RGO film before, after Na intercalation, and plastic wrap as background.
manufacturing. Figure 1c shows an AFM image of RGO network, which exhibits a nanoscale surface smoothness, and curved lines are ripples or overlapped edges of RGO sheets. Figure 1d shows that printed RGO after sodiation is highly stable even when exposed in air for a month, which demonstrates the potential for its utilization in practical applications. We have two motivations to use Na-ion as intercalants for such novel transparent electrodes. First, Na-ions are much more cost-efficient and more abundant than Li. Second, Na-ion is expected to form a more stable barrier layer to prevent further oxidation toward better stability than Li-ions. However,
doping) and thus shifts the Fermi level up. Due to the large storage capacity of RGO for Na ions, the electron doping level is high as well. The large electron doping is expected to cause the “Pauli blocking”35−37 of incident light in the visible range. This results in a large enhancement of the optical transparency of the RGO network. On the other hand, electron doping will also improve the conductivity of individual RGO flakes. Junctions in printed RGO networks usually act as barriers for charge transport and reduce the entire conductivity of the network.29 Na-ion intercalation can provide electron pathways in junctions and greatly reducing the junction resistance. We focus on printed RGO network toward scalable nanoB
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Nano Letters Na-ion cannot intercalate into graphene as demonstrated by others before. Recently, Na ion intercalation in RGOs has been demonstrated. To prove that Na-ions can intercalate into RGO materials prepared in our lab, several characterization tests were carried out. First, a coin cell was made using free-standing RGO film and Na metal as positive and negative electrodes, respectively. Figure 2a shows the voltage profile (second cycle) of sodiation/desodiation in RGO thin film at a low current density of 25 mA/g, indicating a reversible Na-ion intercalation/deintercalation process in RGO network. From the voltage profile, the specific capacity for sodiation is 170 and 127 mAh/g for desodiation. The irreversible capacity is mainly due to the electrolyte decomposition and side reactions during Na-ion intercalation process, which is commonly observed for carbon anodes in Na-ion batteries.33,34,38 The amount of intercalated Na-ions in the RGO network is 0.028 per carbon atom, calculated from the reversible specific capacity. We also applied X-ray diffraction (XRD) on the same RGO thin film before and after sodiation to confirm the actual intercalation process. During the XRD characterization, samples were protected by a thin layer of plastic wrap to avoid chemical reactions under ambient condition. In Figure 2b, a sharp peak is shown in the RGO film before Na-ion intercalation. The peak position is at 25.35°, which corresponds to an interlayer distance of 3.49 Å. This interlayer distance is larger than that of graphite (3.35 Å). The light green shadowed area represents a peak with full width at half-maximum (FWHM) in length of 1.02 Å. After Na-ion intercalation, the XRD peak shifted to left, indicating a larger interlayer distance of 3.76 Å in average. The peak shape is also much broader than what was observed before Na-ion intercalation, with a FWHM changes to 1.67 Å (in length of interlayer distance). The enlarged interlayer distance is indicative of a successful intercalation of Na-ions between RGO layers. The peak around 18° is due to the sealing plastic wrap. The broadening of the peak may be due to the nonuniform intercalations of Na-ions in the printed RGO flakes. To demonstrate a scalable application of Na-ion intercalated RGO as high performance transparent electrodes, printable GO ink is prepared (Figure 3a). Commercial wetting agent Zonyl was added to decrease the surface energy of GO ink in water to be printable. Meyer rod coating method was applied to deposit GO network, which shows excellent uniformity (Figure 3b). GO network on glass slides were then thermally reduced to RGO (also see Figure S1), and trimmed by blade to a desired shape. Copper current collectors were deposited by thermal evaporation, then Na metal and 1 M NaPF6 in EC: DEC (1:1 = v:v) were added as the negative electrode and electrolyte, respectively. The entire device was finally sealed with epoxy in an argon-filled glovebox. The schematic and photo images of the electrochemical intercalation device39 are shown in Figure 3c. After electrically connecting the two electrodes of RGO and Na metal, RGO network can be fully intercalated by Na-ions within 10 min, as shown in Figure 3d, leading to a uniform transmittance enhancement to the Na-RGO network. The transmittance of RGO network before and after Na-ion intercalation is quantitatively illustrated by gray scale images captured by an optical microscope (operated in transmission mode), as shown in Figure 4a and b, respectively. Note that the increase of optical transmittance is very uniform across the entire printed RGO network. Figure 4c plots the transmittance change of RGO network before and after complete sodiation at wavelength of 550 nm
Figure 3. (a) A bottle of as-prepared GO ink to be added with Zonyl to tailor the surface energy for printing purpose. (b) Meyer rod coating of GO ink with excellent uniformity on glass substrate. (c) A two-terminal, lateral device with RGO network as working electrode, Na metal as the counter electrode and 1 M NaPF6 in EC: DEC (1:1 = v:v) as the electrolyte. (d) After Na ion intercalation, the RGO network becomes more transparent.
for samples with different thickness (also see in Figures S2 and S3). It is clear that all samples exhibited a drastic transmittance increase. Note that the substrate is excluded in the transmittance measurement. The percentage of relative increase in transmittance is also plotted in Figure 4c inset. For instance, RGO network with transmittance of 46.1% increased to 91.5% after sodiation, which is nearly a 100% increase compared to its original value. Wavelength dependent spectrum from 450 to 900 nm is shown in Figure 4d. The optical transmittance vs wavelength of Na-RGO is flat in the visible range, which indicates a neutral color and is beneficial for a range of applications. To further demonstrate the feasibility of Na-RGO as transparent conductors, sheet resistance of RGO network before and after Na-ion intercalation was investigated. The resistance measurement was carried out by a four probe method (schematic device image shown in the inset of Figure 5a) to eliminate the contact resistance between Cu electrodes and the RGO network. I−V curves taken from a RGO device before and after sodiation are shown in Figure 5(a), starting with a large sheet resistance at around 100k Ohm/sq since the GO network was only partially reduced at a relatively low temperature of 300 °C.40 Surprisingly, with an increased transmittance after sodiation, the sheet resistance also decreased by 300 folds, from 523k to 1467 Ohms/sq. The other typical decrease in sheet resistance is from 83k to 311 Ohms/sq (Figure 5b). It is expected that Na ion doping largely enhances the carrier density in RGO, which leads to the conductivity increase of individual RGOs.30,32 Meanwhile, Na ion intercalation will potentially improve the RGO−RGO contacts which overcomes the obstacle of conventional network conductors.29 This synergistic effect in increase of optical transmittance and decrease of sheet resistance can be qualitatively explained by heavy electron doping upon sodiation. Both theoretical and experimental work on alkali (Li, Na) metal intercalation in SiC/ C
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Figure 4. Optical transmittance of a RGO network device before and after Na-ion intercalation. (a−b) show optical microscope (transmission mode) images of the same RGO network before and after sodiation. (c) Transmittance vs thickness of RGO network before and after sodiation at 550 nm. The inset is the percentage of relative increase in transmittance at 550 nm. (d) Transmittance vs wavelength of the same RGO network before and after sodiation with a visible spectrum from 450 to 900 nm.
Figure 5. (a) Four-probe I−V measurement of a RGO network before and after sodiation. Insets are the schematic and photo images of four-probe measurement on RGO network. (b) Sheet resistance change of two different samples. (c) Transmittance vs sheet resistance plots of Na-RGO vs reported RGO network transparent conductors. (d) FOM of network transparent electrodes, including exfoliated graphene, RGO, and Na-RGO.
synergistic effect in the increase of optical transmittance and decrease of sheet resistance leads to an excellent performance of Na-RGO as a transparent conductor. This is consistent with our previous work on Li intercalation in few layer graphene.32 For fair comparison, we compare our data with other 2D network based materials including RGO and exfoliated graphene (Figure 5a). Na-RGO shows clear better performance than other materials. To quantify this comparison, the figure of
graphene interfaces n-dope the monolayer graphene, with an upshift in Fermi level.41,42 Similarly, due to the large Na ion storage capacity of RGO and low electronegativity of Na metal, intercalated Na provides electrons and n-dopes the RGO network. The large electron doping leads to a large Fermi energy upshift, which blocks optical transition in the visible range and increases the optical transmittance. Electron doping also leads to the large increase of the conductivity. This D
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Figure 6. Air stability of transparent conductor based on large-scale Na-RGO network. Photo images of Na-RGO in air after 0, 0.5, 2, and 3 h. The scale bar (white) is 2 mm.
merit (FOM, defined as ratio of electrical to optical conductivity, σdc/σopt) is calculated based on sheet resistance and optical transmittance at 550 nm (Figure 5d). Sheet resistance and optical transmittance of approximately 80−85% were used for FOM calculations. Na-RGO network has the highest FOM value. Better performance is expected with large RGO sheets, which will have a low percolation threshold and better initial performance as transparent conductor before Na ion intercalation. Air-stability of Na-ion intercalated RGO is a potential concern due to the high reactivity of Na. To evaluate the stability, the Na-RGO network was first fully sodiated in encapsulated devices. One device was then disassembled and Na-RGO on glass was exposed in ambient environment. Optical images captured by a microscope on Na-RGO network over different spots on the same device were recorded after 0, 0.5, 2, 3, and 13 h. Optical transmittance of Na-RGO on glass was obtained based on microscope images. After complete Na intercalation, transmittance at 550 nm of the RGO sample (red bar) increased from 57% to 90%. The sample shows small degradation after immediate exposure in air. Then the transmittance is stabilized over time, and even after 13 h. Our continuing observations show that Na-RGO does not change the optical transmittance, even after one month. Photo images of Na-RGO network in air for the first few hours are shown in Figure 6b. As we demonstrated before, Li-intercalated ultrathin-graphite has the highest FOM among all continuous thin film.32 However, the large diffusivity/reactivity of Li-ions in the intercalant causes poor stability and requires further device encapsulation. Graphene flakes become highly transparent after Li-ion intercalation. But intercalation compound completely changes back to the original, much lower transmittance after a few hours in air.32 The better stability of Na ion intercalated RGO than Li ion intercalated graphene flakes is explained as the following. Intercalated alkaline ions such as Li and Na are extremely reactive and thus will react with oxygen and water in ambient environmental immediately. The reactants on the edges of the 2D materials will prevent further reaction of alkaline ions, which explains the initial decrease of the optical transmittance in the first half hour. The difference between Li ion and Na ion is likely to be explained by their different diffusivity through the self-formed barrier layer. The large Naion size could lead to a much slow diffusion through the barrier
layer than Li-ion. Therefore, the reactants on the edges of 2D materials prevents further reaction of Na with O2, H2O and CO2 in air, and seals intercalated Na inside the sheets. Conclusion. For the first time, we demonstrate that Na-ion can intercalate into RGO network and simultaneously increase the optical transmittance (∼100%) and conductivity (∼300 times) dramatically. The intercalated RGO network shows superior performance as transparent conductor, better than any other RGO network based transparent electrodes. Surprisingly, such transparent electrodes shows excellent stability in ambient environment, much more stable than Li ion intercalated graphene. The processes including GO network printing, reduction, and electrochemical intercalation can all be potentially scaled toward practical applications. Further improvement of transmittance and sheet resistance can be achieved with GO networks with large size of individual flakes.21 Fundamental studies including air stability mechanism based on Na-ion transport through the reactant layer, charge transport through RGO−RGO layers with ion intercalation,43 and tunable work function studies with Na-RGO44−46 will be further investigated in the future. This work demonstrated the great feasibility of using metal ion intercalations in 2D materials for promising transparent conductor applications. Experimental Section. Preparation of Printable GO Ink. All chemicals used in the experiments were purchased from Sigma-Aldrich. GO powder was synthesized with modified Hummer’s method.47 Concentrated acid H2SO4/H3PO4 was mixed at a volume ratio of 9:1 (180:20 mL) and slowly added to the mixture of KMnO4 (7.2 g) and graphite flakes (1.2 g). The reaction was carried out with ice to avoid overheating. Then the solution was heated up to 50 °C and stirred for 12 h. After that, the mixture was immersed with ice (500 mL), and 30% H2O2 (3 mL) was added. H2SO4/H3PO4 and KMnO4 were washed away by DI water, and centrifugation was applied (8500 rpm for 0.5 h) until PH reached 7.0. 250 mL of 30% HCl was then added to expand the GO. Finally, the Cl-ions were eliminated with DI water and centrifuged at 8500 rpm for 0.5 h. As obtained GO solution is with concentration of 2 mg/mL. A mixed solution of Zonyl and GO (w:w = 3:10) is then obtained for coating. RGO Network Preparation. 200 μL of GO/Zonyl ink was dropped at the edge of clean glass slide followed by Meyer rod (R.D. Specialties, #10) rolled along the glass slide with GO ink. When the glass was uniformly coated by GO ink, it was then E
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Nano Letters dried on a hot plate at 50 °C for 1 min. The process was repeated until a desired multilayer GO network was obtained. GO coated glass slides were reduced in tube furnace with controlled Ar/H2 gas (95%/5%) flow. They are then thermally annealed with a heating rate of 1 °C/min and heated up to 300 °C in 5 h. The samples were cooled down with a tube furnace to room temperature after it reached 300 °C. Electrochemical Device Fabrication/Measurement. RGO networks were trimmed with a blade into desired size. The Cu electrodes as current collector/four-probe electrodes were thermally evaporated with shadow masks. PDMS reservoirs were then built on the devices. After this, the devices were transferred into a glovebox for Na metal deposition and electrolyte injection. The devices were also sealed inside the glovebox for optical and resistance measurement. Optical and Resistance Measurement of RGO/Na-RGO Network. Na-RGO network was obtained by applying potential difference in as-prepared device between RGO and Na metal. Optical transmittances of RGO/Na-RGO network were achieved by analyzing the gray scale image taken by chargecoupled device (CCD), coupled to a microscope (Nikon Eclipse Ti-U) in transmission mode. A narrow-band filter (Thorlabs Inc.) was added in the optical path to obtain transmittance at 550 nm. Spectrum data were obtained by compact spectrometer (Thorlabs) incorporated to the microscope. The sheet resistances of RGO/Na-RGO were measured by four probe method to eliminate contact resistance. Material Characterization. RGO freestanding films were obtained by thermally reducing (300 °C, 1 °C/min ramp) vacuum-filtrated GO film. RGO freestanding films were then assembled in coin cells and cycled electrochemically at a current density of 25 mA/g by a battery tester (Biologic USA). NaRGO free-standing films were then taken out by disassembling coin cells, sealed by plastic wrap and stored in an Ar-filled exchange box to avoid air contamination/side reactions before characterization. XRD patterns were obtained by C2 discover with GADDS, and Raman spectra were obtained by a Raman spectrometer (Horiba Jobin Yvon, a 633 nm He−Ne laser source).
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spectrometer for this work. We also thank Dr. Kang Xu and Dr. Arthur v. Cresce from Army Research Lab for their Na-ion battery electrolyte. We thank Dr. Colin Preston, John Panagiotopoulos and Yanan Chen for their help on this manuscript.
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ASSOCIATED CONTENT
S Supporting Information *
Supporting Information Available: RGO network characterization and further microscope images of transmittance increase from RGO to Na-RGO. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b00300.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
J.W., F.G., and W.B. contributed equally to the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.H. acknowledges the startup support by University of Maryland, College Park. The authors acknowledge the support of the Maryland Nanocenter and its Fablab, Nisplab and surface analysis center. We thank Dr. Jeremy Munday and their group from ECE department for kindly sharing the Microscope and F
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