Hydrogen evolution and capacitance behavior of Au/Pd nanoparticle-decorated graphene heterostructures

The outstanding properties of two-dimensional materials such as graphene offer the possibility to produce novel hybrid materials with boosted functionality for use in catalysis and electrochemical energy storage. The hydrogen evolution reaction and interfacial capacitance performance of monolayer graphene sheets decorated with various metal nanoparticles are studied herein. Chemical vapor deposition grown graphene monolayer was decorated with Au and/or Pd nanoparticles, either on one side or both sides, forming single- or bi metal graphene heterostructures. These asymmetrically decorated graphene nanocomposites were characterized using high-resolution scanning transmission electron microscopy and 3D electron tomography. Electrochemical characterization reveals enhanced hydrogen evolution activity and outstanding capacitance for the resultant composite materials in comparison to pristine graphene and other recently developed graphene-based energy storage devices.


Introduction
Concerns about atmospheric pollution and the sustainability of fossil fuel supplies have led to increased interest in the use of graphene (GR)-based materials for use in electrochemical energy storage and conversion technologies such as the hydrogen evolution reaction (HER). Several well constructed GR-based hybrid materials have been reported recently, showing extraordinary catalytic activity for the HER [1][2][3]. Pt with a catalytic activity (represented by Tafel slope) of ∼30 mV dec −1 at low overpotential has long been established as the best HER catalyst [4].
There has been a recent upsurge of interest in the applications of graphene-based materials in electrochemical energy storage because of their beneficial physical properties such as mechanical strength, high-specific surface area, and high electrical and thermal conductivity [13][14][15][16][17]. Most porous graphene-based materials are formed from liquid-phase exfoliation (LPE) flakes, and although they have a high-specific surface area, they have poor electrical conductivity. In comparison, graphene prepared in a truly two-dimensional form, for example by chemical vapor deposition (CVD) growth, has a high-specific surface area and high electrical conductivity [18]. However, functionalization of CVD graphene has associated challenges. Organic/water interfaces enable the assembly of low-dimensional metallic, semi-conducting, and carbon materials (graphene and carbon nanotubes). The polarizable liquid/liquid interfaces, i.e. the interfaces between two immiscible electrolyte solutions (ITIES), could allow the reversible interfacial assembly of such nanostructures [19][20][21]. Recently, we reported on the assembly of monolayer graphene at the ITIES and its in situ functionalization utilizing both spontaneous and electrochemical deposition [22,23] of Au/Pd nanoparticles (NPs), deposited either on the top or on the bottom, or asymmetrically with two different elements on either side of the graphene sheet. This results in a "sandwich" type mono-or bi-metal graphene heterostructure. These composites were named mono-(Pd/GR, GR/Pd, Au/GR) or bi-metal (Au/GR/Pd) graphene nanocomposites. Furthermore, we also demonstrated the wide range of opportunities offered by the polarizable organic/water interface and used it as a platform for the assembly of liquid-phase exfoliated MoS 2 flakes. The exfoliated MoS 2 flakes, as well as synthetically grown vertically aligned MoS 2 nanosheets, were used to construct various CVD GR-based heterostructures [24].
Herein, we report on the electrocatalytic and capacitance performance of Au and Pd metal NP-decorated graphene heterostructures using a microscale electrochemical technique. Pd functionalization on either the topside or underside of the graphene monolayer affects the catalytic efficiency for the HER. Additionally the bi-metal GR composite exhibits enhanced capacitive performance.

Methods
High-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) imaging was performed on an FEI probe-corrected Titan G2 80-200 S/TEM with a high brightness X-FEG electron source and Super-X energy dispersive X-ray (EDX) silicon drift detectors (SDDs). The microscope was operated at an accelerating voltage of 80 kV with a beam current of 260 pA, a convergence angle of 21 mrad and a HAADF acceptance inner angle of 50 mrad. EDX spectrum images were acquired in the Bruker Esprit software using the Titan's Super-X detector system, with a total per pixel dwell time of 6 ms and an image size of 512 × 512 pixels. All EDX elemental maps have been filtered using a 3pixel smoothing window in Esprit. A tilt series for HAADF-STEM tomography was acquired using Xplore3D acquisition software at angular increments of 1 • from −50 • to +65 • . Spatial image alignment was performed using cross-correlation of HAADF images in FEI's Inspect3D software package. Tilt axis alignment was also undertaken in Inspect3D followed by the use of a simultaneous reconstruction technique (SIRT) algorithm to perform the reconstruction with 20 iterations. Segmentation of Au and Pd voxels was performed manually in Avizo; the segmentation was guided via comparison to the EDX spectrum images acquired of the same sample area. Visualization of the 3D reconstructions was also performed in the Avizo software package.
Raman spectroscopy was measured with an inVia Renishaw spectrometer (Renishaw PLC, UK), using a 532 nm laser excitation wavelength and a ×100 objective, resulting in a laser spot size of ca. 0.8 m 2 . The atomic force microscopic (AFM) characterization was carried out using a Bruker MultiMode 8 equipment (operated in "Peak Force" tapping mode) on samples transferred to a Si/SiO 2 wafer. The CVD graphene layers on a Cu foil were spin-coated with 3% 950 K polymethyl methacrylate (PMMA) solution in anisole (MicroChem Corp.) at 3000 rpm for 60 s using a SPIN200i-NPP spin processor (SPS-Europe B.V., The Netherlands) in preparation for transfer.
Aqueous droplets of either 6 M LiCl or 0.1 M HCl in 6 M LiCl used for the electrochemical measurements were formed using a borosilicate micropipette (ca. 1 m internal tip diameter) and a pneumatic microinjector (PV820 Pneumatic PicoPump, WPI, USA). The motion of the micropipette was controlled electronically using MX7630 micromanipulator and MC1000e motion controller (Siskiyou, OR, USA). An Ag/AgCl reference electrode (RE) and Pt counter electrode (CE) were embedded in the micropipette [28,29]. In this three-electrode configuration, the localized HER and capacitance of the interfacial area between the droplet and sample surface were measured using a PGSTAT302N potentiostat (Metrohm Autolab B.V., The Netherlands). Electrochemical measurements were performed at ambient temperature (22-24 • C). The potential, which is measured against Ag/AgCl reference electrode in 6 M LiCl (E Ag/AgCl (6 M LiCl)), is shifted by ca. +0.19 V with respect to the standard hydrogen electrode (SHE) scale. All the potentials (E RHE ) reported in this work are plotted on the reversible hydrogen electrode (RHE) scale, calculated from Eq. (1).
where E RHE is the potential of the RHE, E measured is the measured potential, and E Ag/AgCl (6 M LiCl) is the potential of the Ag/AgCl reference electrode in 6 M LiCl, shifted by ca. +0.19 V with respect to the standard hydrogen electrode scale. A schematic of the microdroplet electrochemical setup and an optical micrograph of a contacted droplet on a Pd/GR sample can be seen in Scheme 1A and B, respectively.

Preparation and transfer of the pristine and decorated GR layers
NP-decorated CVD GR layers were prepared by metal NP deposition on the monolayer GR assembled at the ITIES, as reported previously [22,23]. Briefly, three kinds of functionalization of graphene were carried out: topside (Scheme 2Aa), underside (Scheme 2Ab), and dual/asymmetric (Scheme 2Ac) decoration.
The topside decoration of graphene (GR) was performed using a simple galvanic displacement for the nucleation of Au or Pd NPs (hereafter, Au/GR or Pd/GR) [30]. The underlying copper foil below graphene acts as an electron donor and transfers electrons to the tetrachloro-metal complex on the other side (top) [23]. Another simple, spontaneous Pd NP deposition technique was applied at the interface-assembled CVD GR to decorate the underside of graphene. An interfacial redox reaction between bis(pentamethylcyclopentadienyl) iron(II) (DecMFc), which is the organic-phase electron donor, and PdCl 4 2− contained in the aqueous phase creates GR/Pd layers [22]. The combination of these two methods (the topside and underside decoration) was applied to obtain an asymmetric functionalization of the graphene layer at the ITIES, producing bi-facially decorated monolayer graphene with Au NPs on the topside and Pd NPs on the underside of graphene, respectively (hereafter, Au/GR/Pd). Scheme 2B depicts a three-dimensional (3D) illustration of Au/GR/Pd "sandwich" heterostructure.
In the case of topside decoration of graphene (also the first step of the Au/GR/Pd preparation), 1 mM HAuCl 4 or (NH 4 ) 2 PdCl 4 aqueous solution was placed on top of a CVD GR layer lying on Cu foil (GR/copper); then, after 1 min, the Au or Pd NPs were spontaneously deposited, respectively. The as-formed Au/GR/copper or Pd/GR/copper stacks were subsequently placed in 0.5 M ammonium persulfate solution to remove the underlying copper foil by etching. The resulting Au/GR or Pd/GR layers were cleaned three times by transfer to deionized water. These topside functionalized graphene monolayers were then transferred either onto an Si/SiO 2 substrate for electrochemical measurements or, in the case of Au/GR, onto the surface of an aqueous solution containing 1 mM (NH 4 ) 2 PdCl 4 salt to perform the second step of Au/GR/Pd preparation. Briefly, 2 mM DecMFc (in the DCE/5-nonanone mixture) was layered carefully on top of the free-standing Au/GR film floating on the aqueous solution layer in order to induce the deposition of Pd nanoparticles on the underside of the graphene monolayer yielding an Au/GR/Pd sandwich. This asymmetrically functionalized graphene/metal NP heterostructure was then transferred onto Si/SiO 2 wafers for electrochemical measurements. Some of the graphene monolayers were also transferred from the Cu foil onto the surface of aqueous solutions containing the PdCl 4 2− complex for the underside decoration to give GR/Pd. The transfer from Cu foil was carried out by spin-coating the CVD GR with ca. 100 m PMMA thick film on the Cu foil [22], resulting in a PMMA/GR/Cu stack. The Cu foil was etched from underneath the PMMA/GR stack using a 0.5 M ammonium persulfate solution, and the remaining PMMA/GR stack was "fished out" with a Si/SiO 2 wafer, and then cleaned three times by deionized water. This PMMA/GR stack on a Si/SiO 2 wafer was then transferred onto the surface of an aqueous phase, containing the 1 mM (NH 4 ) 2 PdCl 4 salt, and carefully covered by a DecMFc-free organic phase (DCE/5-nonanone mixture, 1:4 by volume ratio). After a while, the PMMA/GR stack became invisible as the PMMA was dissolved from the top of GR by the organic solvent mixture. The organic phase was then replaced with a fresh batch containing 2 mM DecMFc, to perform the underside functionalization. These underside functionalized CVD GR layers were then transferred onto Si/SiO 2 wafers for electrochemical measurements. In all cases of the underside decoration, 0.1 M TBAClO 4 and 0.1 M LiClO 4 supporting electrolytes were used in the organic (org) and aqueous (aq) phases, respectively.

Transmission electron microscopy characterization of decorated graphene layers
The HAADF-STEM image (Fig. 1A) shows the Au/GR/Pd sandwich heterostructure. The contrast of this imaging technique scales with atomic number, so the heavier Au NPs are visualized as regions of high brightness while the lighter Pd NPs are visualized as regions of medium brightness (gray tones). Graphene monolayer can also be seen between the topside and underside-deposited particles as a faint, very low brightness membrane.
In the cases of both topside and underside decorations, the STEM-HAADF images at higher magnification reveal that the nanostructures consist of smaller NPs [23]. The size distribution of the Au and Pd NPs on the top of graphene was ca. 10-90 nm (Au/GR) and 20-80 nm (Pd/GR), respectively. Similarly, in the case of Pd decoration of the underside of CVD GR (GR/Pd), some small roundshaped individual Pd particles were found with sizes of 10-80 nm (Supplementary Figure 2). Elemental analysis was carried out using EDX spectroscopy (EDXS), which was subsequently used to guide the segmentation of a 3D reconstruction from a HAADF-STEM tilt series [31,32]. The STEM-EDXS chemical map (Fig. 1B) and a segmented 3D visualization of a HAADF-STEM tomography reconstruction are shown from the top- (Fig. 1C) and side-views (Fig. 1D), where the green and red colors correspond to the Au and Pd NPs, respectively. Some parts of the sample are decorated only on the topside by Au or the underside by Pd NPs, respectively, but two parts show clearly the "sandwich" heterostructure. The red circular structure with the green spot in the middle (Fig. 1B) corresponds to Pd aggregates underneath the GR layer with an Au nanostructure on the top, which is confirmed by the side-view 3D STEM-EDXS image (Fig. 1D). More evidence for the dual-decorated CVD GR can be found in previous reports [22,23]. For example, the graphene-based metal nanoclusters were previously characterized using atomic force and electron microscopy techniques (SEM and STEM), and the location of the metal nanostructures underneath the graphene layer has been identified from the position shift of graphene Raman modes (G, 2D) toward higher frequencies and the intensity changes of the Raman modes (I D /I G , I 2D /I G ) [22]. The dual-decorated "bimetal-graphene sandwiches" were previously studied using Raman spectroscopy, X-ray photoelectron spectroscopy, STEM, and electron energy loss spectroscopy (EELS) [23]. Additionally, the aligned HAADF-STEM tilt series and a rotating view of the segmented reconstruction for the asymmetrically dual-decorated Au/GR/Pd heterostructures can be found in Supplementary Information (Movie S1, S2). In the case of Movie S1, the heavier (and therefore brighter) Au NPs can be seen on the top, while the moderately heavy Pd NPs are at the bottom of the very light graphene support, which appears as a "shadow" during the rotation in the HAADF-STEM imaging (different tilt series of STEM-HAADF, and STEM-EDXS maps are also shown in Supplementary Figure 3). The 3D surface visualization of the segmented Au/GR/Pd analyzed by HAADF-STEM electron tomography (Movie S2) shows the surface visualization of the segmented Au NPs (green) on the topside and Pd NPs (red) on the underside of graphene layer.

Electrochemical investigation of decorated graphene heterostuctures
Electrochemical measurements using micro-droplets [28,29] were performed to explore the chemical and physical properties of these metal NP/GR heterostructures. Specifically, we have investigated the HER catalytic performance and the interfacial capacitance. The micro-droplets were 30-50 m in diameter, and 3 individual droplets for each specimen were used for the analysis of each measurement. In order to assess the catalytic activity toward the HER, polarization curves of all the samples (GR, Pd/GR, GR/Pd, Au/GR, and Au/GR/Pd) were measured using aqueous droplets containing 0.1 M HCl and 6 M LiCl (Fig. 2A). It is well established that pristine GR is a poor HER catalyst, and only a small current density was also observed for Pd/GR or GR/Pd. However, the Au-decorated GR composites seem to be a more promising system. The Tafel slopes were estimated from the linear portions of the polarization curves ( Fig. 2A) using Eq. (2): where Á is the overpotential, j is the current density, b is the Tafel slope and a is the intercept (log j 0 , where j 0 is the exchange current density  [33]. In this case, the small rGO flakes could cause the higher activity due to increased density of rGO edges, similar to the case of ball-milled graphite, with more exposed edges showing a much better increased electro-catalytic performance for the oxygen reduction reaction (ORR) [34]. Decoration of rGO using either Pd, or Au, or both NPs has also been reported by Darabdhara et al. [12]. In the case of both Pd and Au NPs, 29.0 mV dec −1 Tafel slope was found, and additionally, an excellent electrocatalytic activity was observed even after 5000 sweep cycles. Furthermore, the Tafel analysis allows us to extract the transfer coefficient (˛), which is used for the kinetic evaluation of the HER process [35,36]. Fitting of the experimental data (using overpotential values calculated from linear parts of the Tafel plots) demonstrated a difference between the transfer coefficients of the various metal NP-graphene heterostructures. In the case of the pristine monolayer graphene, ˛ is 0.34, while for the heterostructures, ˛ was equal to 0.38, 0.47, 0.53, and 0.56 for Pd/GR, GR/Pd, Au/GR, and Au/GR/Pd, respectively. An inverse correlation is exhibited between the value of the Tafel slope and ˛, which is to be expected according to the Butler-Volmer equation [36]. The difference in the observed current densities, Tafel slopes, and ˛ between the Pd/GR and GR/Pd as a function of scan rate (Fig. 2B) shows that there is a higher HER activity when the Pd NPs are lying under the monolayer GR as opposed to on top. Although there is the possibility of the Pd NPs under GR being exposed due to defects in GR, significant proton transfer is likely to have occurred in order to produce the observed enhanced HER activity [37]. Relating to the three reaction mechanisms in acidic media for the HER [38], the first is an initial discharge step (b ≈ 120 mV dec −1 ), followed by either an electrochemical desorption step (b ≈ 40 mV dec −1 ) or a recombination step (b ≈ 30 mV dec −1 ). Our Tafel slopes are in the range of 154 and 105 mV dec −1 for the studied heterostructures, which suggest that the process is a combination of the initial discharge and electrochemical desorption steps. To conclude this part, while quite diverse HER activities are observed for the various graphene-based metal nanocomposites, the additional Aumodified GR hybrids show more promising catalytic performance for HER compared to the Pd-only modified composites.
The capacitive performance of the metal NP/GR heterostructures was electrochemically tested by the determination of the electric double-layer capacitance (C) for each sample using the cyclic voltammetric measurements in a 6 M LiCl aqueous solution following Eq. (3): where is the scan rate, I is the measured current, E is the applied potential and E max and E min are the maximum and minimum potentials applied during the voltammetric scan, respectively. The values of C are directly proportional to electrochemically active surface areas of the samples and were determined from cyclic voltammetry (CV) measurements in the potential window −0.4 V to 0.4 V in 6 M LiCl aqueous solution at various scan rates. The specific capacitance (C sp , in mF cm −2 ) values were calculated from C and geometric surface area values. The dependence of the C sp on scan rate is depicted in Fig. 3A, showing the previously observed [15,39] inverse correlation with the increasing scan rate, arising from the diffusion-limited supply of ions. At 100 mV s −1 , the calculated specific capacitance for Au/GR/Pd (3.84 mF cm −2 ) is approximately 3, 6, 70, and 870 times larger than that for Au/GR (1.15 mF cm −2 ), GR/Pd (0.70 mF cm −2 ), Pd/GR (56 F cm −2 ), and GR (4.4 F cm −2 ), respectively (Fig. 3A). The C sp decreases for example by 32.8% when the scan rate increases from 100 mV s −1 up to 3 V s −1 in the case of the pristine graphene, and similar downward trend is also observed for the other heterostructures. Considering novel hybrid materials for application in an electrochemical energy storage device, one of the most important behaviors for effective use is the cycling stability. Ideal supercapacitors should possess extremely high stability with minimal degradation in performance with repeated charge/discharge cycles, while pseudocapacitors typically sacrifice cycling stability for increased energy density [15]. Fig. 3B shows the cycling stability of the Au/GR/Pd heterostructure over 100 CV cycles. There is a large increase (∼400%) when comparing the C sp values of the 100th and 1st cycles. This increased capacitance with continuous cycling merits careful consideration. It has been reported previously for reduced graphene oxide films, MoS 2 films and particularly for transition metal oxide films and was attributed to the partial exfoliation with continued ion intercalation/deintercalation increasing the active surface area, which was termed "electroactivation" [17,39,40]. Increased capacitance with continuous cycling has also been found for porous-type rGO electrodes, where rGO was decorated with flower-like MnO 2 nanostructures fabricated by electrodeposition [41]. These materials showed a dramatically increased specific capacitance using galvanostatic charge and discharge for about 1300 cycles. In this material, the rGO sheets can move to adjust to the different electrolyte ions, and the long time charging and discharging may also help the ions access all the graphene sheets to utilize the larger surface area. In contrast, our work relates to a CVD grown "two-dimensional" graphene layer with electro-deposited metal nanoparticles, and for this type of material, increased capacitance with continuous cycling has not been reported previously. One report for a CVD grown graphene foam ("three-dimensional") decorated by NiO nanoparticles, forming a NiO/3D graphene nanocluster, found that the specific capacitance increased by about 15% during the first 200 cycles, which was attributed to the "activation" process that allows the trapped ions to gradually diffuse out [42]. Therefore, in the case of our Au/GR/Pd heterostructure, the enhanced capacitive behavior could be explained by penetration of the electrolyte ions (Li + , Cl − ) between the metal NPs and graphene layer modifying the surface, and a subsequent increase in the accessible electroactive area upon repeated cycling. This hypothesis is further supported by our observation that the highest C sp was observed for the slowest cycling rate. Further investigations such as the exploration of different metal NPs, modified particle size and density and long-term stability studies will be required to optimize these heterostructures for specific applications.

Conclusions
In summary, either one or both sides of graphene monolayers were decorated with metal NPs to construct single-metal or bi-metal graphene heterostructures. A micro-droplet setup was employed to investigate the electrochemical performance of the GR-based hybrid materials as HER catalysts and electric double layer capacitors.
The different composites showed diverse catalytic activities, with the best Tafel slope for the HER found to be 104.7 mV dec −1 for the bi-metal Au/GR/Pd sandwich structure. This synthesis approach represents a unique and controllable platform to prepare asymmetrically functionalized 2D materials, which can provide important insights to optimize materials for applications in energy conversion and storage.
Functionalized CVD GR-metal nanocomposites show high capacitance values, i.e. 870 times larger in the case of Au/GR/Pd heterostructure, than that measured for pristine graphene and are also comparable with the capacitive activities of some recently reported energy storage device based on liquid-phase exfoliated MoS 2 /GR heterostuctures [24,39,43]. Although all these measured capacitances are likely to be overestimated due to the increased surface area (real versus geometric) [24,43], herein we make a relative comparison between different heterostuctures with CVD graphene as the baseline. Additionally, cycling stability measurements demonstrated around a 400% increase in the specific capacitance, representing promising application for supercapacitor electrodes. The increase is suggested to be due to the electroactive area change, which most probably relates to increased surface area generated by electrolyte ions during the charge/discharge cycles [24,[39][40][41][42].
Improved understanding of the electro-catalytic nature of bifacially functionalized graphene nanostructures is an essential step toward their exploitation in biphasic catalysis, such as HER or ORR at the ITIES [20,44,45]. Moreover, the interface-assembled fabrication of graphene composites is a promising platform for the synthesis of heterostructures via topside decoration using 2D materials exfoliated in the organic phase. This could potentially provide an alternative to the conventionally employed procedures, i.e. the use of a solid substrate [1,40,46].

Supplementary material
The Supplemental Information shows further spectroscopic and microscopic characterizations; the supplemental movies showing the HAADF-STEM and 3D electron tomography tilt series (Movie S1, S2) are provided.