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Pristine Graphene Electrode in Hydrogen Evolution Reaction Aozhen Xie, Ningning Xuan, Kun Ba, and Zhengzong Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14732 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 16, 2017

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ACS Applied Materials & Interfaces

Pristine Graphene Electrode in Hydrogen Evolution Reaction †‡

Aozhen Xie,† Ningning Xuan,† Kun Ba,† and Zhengzong Sun*, ,

†

Department of Chemistry, Fudan University, Shanghai 200433, P. R. China;

‡

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and

Innovative Materials, Fudan University, Shanghai 200433, P. R. China

Keywords: flipped transfer method; pristine graphene electrode; hydrophobic; hydrogen evolution reaction; atomic barrier; semi-transparency

Abstract: Graphene, the sp2 carbonaceous two-dimensional (2D) material, is gaining more attentions in recent electrochemical studies. However, this atomic thick electrode usually suffers with surface contamination and poor electrochemical endurance. To overcome the drawbacks, we developed a PMMA-assisted, flipped transfer method to fabricate the graphene electrode with pristine surface and prolonged lifetime in hydrogen evolution reaction (HER). The HER performances of the single-layer graphene (SLG) were evaluated on various insulating and conductive substrates, including SiO2, polymers, SLG, highly oriented pyrolytic graphite (HOPG), and copper. The parallel Tafel slopes of SLG, bilayer graphene (BLG) and HOPG suggest they share the same electrochemical activities deriving from the sp2 carbon basal plane. Moreover, the atomic barriers, both for SLG and the single-layer h-BN (SLBN),

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are semi-transparent in HER for the underneath copper, providing a new perspective for the 2D materials to protect and couple with the other electrochemical catalysts.

1. Introduction In electrochemistry, graphene usually works as a high surface area platform to host different electrochemical catalysts, such as the 2D transition metal dichalcogenides (TMDs) and noble metal nanoparticles1-3. Some hetero-atom doped graphene was found to be an economically favored alternative catalyst for the HER, avoiding using any transition and noble metals completely4-6. Most of the previous reports employed the reduced graphene oxide (RGO) or porous 3D graphene as the primary electrode materials, because of their high surface areas. So far, there has been little research focused mainly on the pristine 2D graphene as the electrode materials. One major hurdle is that most traditional graphene transfer methods inevitably introduce polymer contaminations to the graphene’s surface (Figure S1), which has been thoroughly investigated and confirmed with high resolution transmission electron microscopy (HRTEM)7,8. Other than the surface contaminations, in a typical electrochemical reaction such as the HER, the water molecules and electrolyte ions are able to intercalate in-between the graphene and the underneath substrates, either through SLG’s edges or small cracks. When the HER starts and bubble are evolving, graphene can be easily peeled off from the substrates and fall apart during the reaction (Figure S2). Other transfer methods, for example, the dry transfer using PDMS9 and the self-release layer10, are specifically designed for electronic devices, which only partly


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mitigate these two problems in electrochemistry. Mass productive roll-to-roll transfer is mostly based on PMMA transfer and thus shares the similar problems11. Graphene transferred with these methods are not robust enough for HER, in which the long-term usage for a pristine graphene electrode is desirable. In response to these major challenges, we developed a flipped transfer method using Poly(methyl methacrylate) (PMMA) as an assisted adhesive layer, to get a pristine graphene surface with no polymer contaminations. Moreover, the hydrophobic PMMA layer, sitting between the graphene electrode and the hydrophilic SiO2 substrates, prevents water’s intercalation and eliminate further bubble damages during the electrochemical reactions.

2. Experimental section 2.1. Full Methods to prepare and characterize the graphene, h-BN and other 2D electrodes are given in the Supporting Information. 2.2. AFM characterization. AFM tapping mode was used and measurement was performed on Bruker Edge at room temperature. The scanning rate is 1 Hz and the tip is NCHV (42 N/m). 2.3. Raman characterizations. Raman measurements were taken on Horiba HR800 system using a 473 nm excitation laser, ×100 objective lens with ~1-µm-diameter spot size and a motorized XYZ stage. Raman spectra were processed by Horiba Labspec 6 and Origin 9.1. 2.4. Electrochemical characterizations. Electrochemical measurements were carried out with a 3-electrode cell using a CHI 660E electrochemical workstation. Linear sweep voltammetry with a 5 mV s−1 scan rate was performed in 0.5 M H2SO4



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electrolyte bubbled with nitrogen, with a Saturated calomel electrode (SCE) as the reference electrode, a platinum foil as the counter electrode and electrodes prepared as the working electrode. Polarization curves were all iR-corrected (95%). A.C. impedance measurements were performed in the same configuration at different potentials from 1 MHz to 0.1 Hz with an A.C. voltage of 10 mV.

3. Results and discussion The detailed flipped transfer method is illustrated in Figure 1a. 10 % PMMA in anisole is spin-coated on a 300-nm SiO2/Si wafer (denoted SiO2 wafer in the following discussion) and then baked to evaporate the solvent. Another layer of PMMA/anisole is also spin-coated on the freshly grown CVD graphene/Cu as the adhesive layer. Then we flip the PMMA/graphene/Cu and gently press the two PMMA sides together. After the anisole evaporates, the graphene/Cu becomes attached to a PMMA layer siting on the SiO2 wafer. Top layer graphene and Cu is quickly etched away using the Marble Cu etchant, leaving a clean and pristine bottom graphene film above the PMMA adhesive (details in Figure S3). Using our transfer, the size of the electrode is only limited by the size of the as-grown CVD graphene, which is ideal for making electrochemical arrays. In this article, the typical size of the single or multilayer graphene transferred is around 1.5 × 1 cm2. As shown in Figure 1b, the thickness of PMMA atop the SiO2 wafer is ~1.3 µm and the roughness is surprisingly low, ~ 0.67 nm, closed to the commercial polished SiO2 wafer12. Such a flat substrate provides an ideal platform to host the graphene electrode. The uniformity and single-layer signature of the SLG were characterized with a


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transmission electron microscopy (TEM, JEM 2011) in Figure. S4.

The untreated SiO2 wafer, whose water contact angle is ~ 45°, is evidently a hydrophilic surface. After the PMMA spin-coating, the substrate becomes more hydrophobic, with a contact angle ~ 72°, as demonstrated in Figure 1b. According to a recent report by Wang et al., hydrophobic surface can effectively reduce wrinkles and cracks during the transfer process because of reduced adhesion of graphene on SiO2 induced by water molecule13. After the two PMMA layers merge into one, the SLG shows almost no wrinkles. Figure 1c presents the AFM image of height profile between the top SLG/PMMA layer and the PMMA/SiO2 substrate. The step of the PMMA adhesive (PMMA spin-coated on Cu) is ~350 nm. The roughness of SLG on PMMA increased to ~ 40 nm, which is worse than PMMA/SiO2 substrates but still superior to most other electrochemical electrodes. This means that the transferred graphene is lying flat on PMMA, which allows us to direct adopt its geometric area as the electrochemical active area in the following characterization. More significantly, the hydrophobic PMMA substrate and graphene can block the entrance for water molecules and the carried electrolyte ions, and all of the electrochemical reactions will thereby only happen on the pristine upside graphene surface.



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Figure 1. Schematic illustration of the graphene transferred on a PMMA substrate. (a) Scheme of PMMA-assisted, flipped transfer. Different from the traditional transfer methods, PMMA is employed as substrate rather than a sacrifice layer. (b) AFM image of a PMMA substrate spin-coated on the SiO2 wafer. Inset shows the surface roughness of the PMMA substrate. (c) Contact angle of water on SiO2 substrate before and after the PMMA coating. (d) AFM image the SLG/PMMA electrode. The height of SLG/PMMA adhesive layer is about 350 nm.

Besides the morphology and water-resistant merits from our graphene electrode’s configuration, the electrical properties plays a dominate role in an electrochemical reaction. With this method, large integrated graphene can be transferred without any polymer residue sitting above the graphene surface. Since PMMA is an insulator, all of the electrical signals acquired from the electrode can be attributed to the conductive SLG layer. Sheet resistances of the transferred SLG on different substrates are listed in Figure 2a. With less cracks and wrinkles, the average sheet resistance of SLG from


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our method is much lower, ~ 0.5 kΩ/sqr compared to ~1.1 kΩ/sqr in the traditional transferred graphene films14. For control experiments, without the PMMA layer spin-coated on SiO2 wafer, the flipped transferred SLG suffered with a much worse sheet resistance, ~200 kΩ/sqr. Both the PMMA layers on Cu and on SiO2 wafer substantially influence the quality of the SLG (Figure S5). The same transfer was also carried out with BLG, prepared by overlapping one SLG on another. The average sheet resistance of BLG would further drop to ~ 0.25 kΩ/ sqr. Other polymers, such as the polystyrene (PS), with higher water contact angle (Figure S6) were also employed as the assisted layer for the transfer15-17. The integrity of SLG on PS is acceptable, but its average sheet resistance is higher than the PMMA’s. This indicates that the contact angle might not be the only determining factor in the transfer process. Finally, we chose PMMA as the assisted polymer layer, as the SLG/PMMA shows the lowest sheet resistance. The transferred SLG electrode on PMMA cannot be observed under optical microscope due to absence of contrast. Therefore, Raman spectroscopy was employed to distinguish the SLG from PMMA. Figure 2b demonstrates the Raman spectra for SLG/PMMA and the blank PMMA. As seen, there is little spectroscopic interference in Raman between PMMA and graphene. Nevertheless, the Raman spectrum of SLG/PMMA behaves differently from a typical SLG/SiO2. Variations like intensity, peak shift and broadening full width half maximum (FWHM) of the 2D peak can be observed in the SLG/PMMA samples18. Additional experiments were carried out for comparison by transferring the same batch of SLG on different insulating substrates,



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the PMMA, SiO2 wafer, PS and polyvinyl butyral (PVB) (Figure S7). The Raman spectra confirmed that the transferred graphene on PMMA was indeed SLG. According previous studies, the SLG/PMMA should be p-doped according to the IG/I2D ≥ 119. To demonstrate the integrity and homogeneity of the transferred graphene film, 2D peak mapping (70 × 55 µm) and its histogram are also presented in Figure 2c. Statistical result indicates there are no micrometer-scaled cracks in the SLG. And the mean and standard deviation of the Gaussian fitted normalized 2D peak intensity are ~0.65 and ~0.07, respectively. Small variation of the peak intensity verifies the homogeneity and the flatness for the transferred SLG film. The defective related signature of graphene was accessed by the intensity ratio between the D peak and the G peak (ID/IG)20,21. Mapping and histogram in Figure 2d provides a statistical result of ID/IG ratio, and the mean and standard deviation of ID/IG ratio are ~0. 07 and ~0.017, respectively. Despite of some small contributions from the PMMA around the graphene D peak region (~1,355 cm-1), which may set the ID/IG overestimated, the tiny ID/IG values testify the high-quality SLG transferred with our new method. In addition to graphene film, other 2D material thin films, such as the high-quality h-BN film grown on Cu foil were also transferred with this method (Figure S8). It is therefore a potentially general method to fabricate other 2D materials electrodes and abstract their electrochemical properties.


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Figure 2. Electrical and Raman spectroscopic characteristics of transferred graphene electrodes. (a) Statistical result of sheet resistance. It demonstrates the lower resistance with our transfer method than traditional method. (b) Raman spectra of SLG on PMMA and PMMA substrate. (c) Normalized 2D peak intensity mapping and histogram. Few cracks are detected. (d) ID/IG ratio mapping and histogram. It shows large-area, homogeneous and high-quality SLG. In both (c) and (d), the mapping sizes are 70 × 55 µm. Gaussian fittings are applied. Inset scale bars, 20 µm.

After the transfer was finished, silver paste was used to make the electrical contact, and the SLG electrode was connected as the working electrode to evaluate the HER performance using the standard three-electrode system in a 0.5 M H2SO4 electrolyte, saturated with N2. However, with the silver paste contact method, the graphene



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electrode usually has a large electrode area, which could cause a fair amount of iR drop (0.5-1 kΩ). Thus to better evaluate the HER performance, a modification was introduced to limit the graphene electrode area during the Cu etching step. Only a small area (~ 2 × 2 mm) of Cu was selectively etched away while the rest of the Cu were kept as part of the contact to the graphene. In order to do this, the SLG/Cu films were masked and protected with PMMA. Epoxy was later used to seal the exposed Cu edges from the electrolyte. With this improvement, a smaller and stable iR drop (70-150 Ω) can be obtained for all samples. The revised graphene electrode’s structure is shown in Figure S11.

With linear sweep voltammetry, the polarization curves of the SLG/SiO2, SLG/PMMA, as well as the background, the PMMA/SiO2 are presented in Figure 3a. As our samples’ surface roughness is negligible, the current can be normalized to the geometric area of the electrode. The insulating PMMA substrate simplifies the analysis of the electrochemical performance since all electrochemical reactions only occurred at the clean and conductive surface of graphene. For comparison, a SLG/SiO2 electrode was prepared using traditional transferred method, whose surface was covered with PMMA residues. The PMMA contamination clearly deteriorates the performance of graphene, leading to only half of the current density relative to the clean SLG/PMMA electrode, at the same overpotential η = 0.9 V. Besides, long-term HER endurance test also favors the flipped transferred SLG/PMMA electrode, shown in Figure 3b. As the HER was test on SLG/SiO2 electrode, its performance declined


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quickly, with a current retention of only 8.1 % in 10 h. For the SLG/PMMA electrode, the current maintains stable for over 20 h without any declination. In this prolonged HER test, there are bubble accumulation and bubble release processes22-24. This leads to a current fluctuation around 100%, causing spikes and dips in the chronoamperometric i-t curves. In addition, in a Raman comparison for our SLG/PMMA electrode before and after the HER test, there is no sign of any rise of D peak (Figure S9). Therefore, the H2 bubbles generated in the HER cause neither chemical nor physical damages to the graphene film. Furthermore, oxygen evolution reaction (OER) was also carried out with SLG/PMMA electrode in 1 M NaOH solution, showing great electrochemical stability under oxidized conditions, lasting over 4 h without any performance declination, at a positive potential of 2.5 V versus RHE (Figure S10). This suggests our SLG/PMMA electrode is a stable and versatile electrochemical electrode within water’s redox window.

A series of carbonaceous materials, the SLG, BLG, HOPG and glassy carbon (GC), were assessed with their LSV polarization curves, in Figure 3c. All the electrode surfaces were freshly prepared to prevent potential contamination. The overpotentials to drive a current of 0.1 mA/cm2 (η@ 0.1 mA/cm2) for SLG, BLG, HOPG and GC are 0.72, 0.71, 0.64 and 0.76 V, respectively. The multilayered formed graphene, the bulk HOPG possesses the smallest η@ 0.1 mA/cm2, while the SLG and BLG’s are relatively larger. The sp2-sp3 hybridized GC displays the largest η@ 0.1 mA/cm2.This trend is against the reported electrochemical activities in other 2D materials, such as



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the MoS2 and WS2, whose thinner ones usually display better electrochemical performance

25,26

. However, it is worth noted that our PMMA substrate’s p-doping

effect might withdraw the electrons from the HER catalysis, which might account for the incremental of η@ 0.1 mA/cm2. The SLG/SiO2 represents high electrocatalytic activity toward a more positive onset potential (0.44 V) and η@ 0.1 mA/cm2 (0.63 V), suggesting the SLG’s electrochemical properties are greatly influenced by the substrates or doping. Another noticeable difference is that the reported HER characterizations are usually carried out by loading powder-like active materials on GC electrode, while our graphene is in a single-layer form, acting as both the active material and the current conductor. The different electrode’s structure might cause further differentiations in the LSV curves. The HOPG, with the lowest resistance, apparently has the lowest onset overpotential and the η@ 0.1 mA/cm2. However, their Tafel slopes are similar for different carbonaceous materials, in Figure 3b. Especially for the almost parallel Tafel plots of SLG, BLG and HOPG, whose slopes are 147, 142, 122 mV/dec, respectively, recorded with errors in Table S1. The Tafel slopes provide further insight for these electrodes’ HER catalytic activities. The parallel Tafel slopes imply the HER may follow similar mechanism for all the sp2 carbonaceous materials with the same catalytic sites or chemical environments. The impedance measurement was carried out at η = 0.9 V (Figure S13). The decreasing size of semicircle confirms that the impedance is getting smaller substantially as the graphene layer number increases, from SLG, BLG to multilayered HOPG27. In carbonaceous materials, the pristine sp2 usually have larger activation energy in HER


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than the other electroactive sites, such as the defects, heteroatoms and edges. The mechanism and kinetics of metal-like HOPG electrode have been well studied in HER28. The SLG and BLG share the same pristine sp2 surface with the HOPG, yet have higher resistance, confirmed with their sheet resistance and impedance measurement. The electrode’s own impedance plays an important role in the kinetics. The more layers of graphene, the more conductive the electrodes are, and the faster kinetics happens on the electrodes29-30. That’s probably the reason why the HER performance sequence follows HOPG>BLG>SLG in our experiment31. Judging from their onset overpotentials, Tafel slopes and charge transfer resistances, the HER activity for pure sp2 carbonaceous materials is indeed far behind the platinum or some reported transitional metal compound32. Nevertheless, since these carbonaceous materials are often synchronously used with other catalysts, it is useful to understand their native HER performance. It is worth mentioning that to maximize the electrode-electrolyte interface, our 2D electrode could be rendered into 3D geometry through adding other nano-building blocks on the platform. One possible route is to use the electrophoretic deposition33.



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Figure 3. Characterization of HER performance. (a) Comparison of polarization curves between SLG electrode from new transfer, traditional transfer and PMMA substrate. (b) Comparison of chronoamperometric i-t curves for SLG/PMMA and SLG/SiO2 wafer. (c) Polarization curves of SLG, BLG, HOPG and GC. The dash line indicates the current density = 0.1 mA/cm2. (d) The corresponding Tafel plots for different carbonaceous materials same as (c).

As an insulator, PMMA may only provide weak p-doping to the graphene electrode. If we change the substrates from an insulator to a metal, the doping or electrical coupling effect between graphene and the metal would work quite differently. To acquire that kind of strong graphene-metal interaction, pristine SLG directly grown on


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Cu serves as the SLG/Cu electrode. To extend our studies with other 2D materials, the h-BN grown on Cu substrate (SLBN/Cu) is also adopted directly to study the insulating atomic barrier covering Cu in HER. The reaction’s schemes are shown in Figure 4a. Polarization curves of SLG/PMMA, SLBN/PMMA, SLG/Cu, SLBN/Cu and Cu are displayed in Figure 4b. Cu has the lowest onset potential ~ 0.52 V, with the best HER catalytic performance in all of the electrodes, while SLBN/PMMA barely shows any cathodic current. Interestingly, SLBN/Cu does substantially catalyze the HER, despite the insulating SLBN layer. Moreover, it demonstrates almost the same HER capability as the SLG/Cu (Figure 4b). In order to eliminate the possible open Cu area in SLG/Cu and SLBN/Cu electrodes, we performed oxidation tests by putting the electrode at 180 °C in air for several minutes. No Cu oxidation color appeared after the tests, which proves the full coverage of graphene or h-BN film on Cu (Figure S14). Atomic barriers, such as graphene and h-BN, can hinder or stop small molecules and electrolyte ions from penetrating through them to interact with the metal electrode directly, while keep “semi-transparent” to the electrons. This is in accordance with previous STM studies of h-BN on Cu under low bias potentials34. The underlying Cu could potentially modify the electronic band structure of graphene. Besides, since SLG is a semimetal, the electrical contact barrier between SLG and Cu should be minimized, in a form of a typical ohmic contact. Compared with the in-plane resistance of SLG, the smaller resistance of Cu and the SLG/Cu interface could facilitate the HER performance. Finally, HER performance of graphene on different substrates is summarized in Figure S12 and Table S2.



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Figure 4. Influence of atomic barriers in HER. (a) Scheme of bare Cu electrode (left) and with graphene or h-BN terminated Cu electrode (right) in HER. (b) Polarization curves of Cu, SLG/PMMA, SLBN/PMMA, SLG/Cu and SLBN/Cu.

4. Conclusion The two dimensional structured graphene electrodes just start to join the big electrochemistry family. Our PMMA-assisted, flipped method offers a pristine and robust way to prepare them and other 2D material-based electrodes. The common sp2 nature in SLG, BLG and HOPG may share the common electrochemical catalytic pathways in HER, while SLG and BLG might provoke more catalytic possibilities upon further electrical coupling or chemical doping. On the other hand, the conductive graphene and the insulating h-BN films, the atomic barriers on Cu demonstrated “semi-transparency” to electrons in HER. This could stimulate potential applications to protect metal catalysts from corrosion or poisoning using the 2D materials.

Associated content


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Supporting information Demonstration of disadvantages of traditional PMMA transfer, optical images of our transfer process, contact angle measurement, Raman characterization, electrochemical performance and oxidation of SLG or single-layer BN-terminated Cu. This material is available free of charge via the Internet at http://pubs.acs.org.

Author information Corresponding Authors *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements The authors thank Prof. Wenbin Cai for helpful discussion and advice. Funding was provided by the National Natural Science Foundation of China (21301032), National Key Research and Development Program of China (2016YFA0203900).



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Pristine Graphene for HER


Pristine Graphene Electrode in Hydrogen ... - ACS Publications

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