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1801144 (1 of 11) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Plasmon-Enhanced Electrocatalytic Properties of Rationally Designed Hybrid Nanostructures at a Catalytic Interface

Ji-Eun Lee, Filipe Marques Mota, Chi Hun Choi, Ying-Rui Lu, Ramireddy Boppella, Chung-Li Dong, Ru-Shi Liu, and Dong Ha Kim*

DOI: 10.1002/admi.201801144

approach to overcome the limits of current (photo)electrocatalysts. SPR at the inter-face of two media with dielectric constants of opposite signs constitutes electromag-netic waves coupled to the collective oscil-lations of free conductive electrons on the surface of noble nanostructures.[1–6] Upon light irradiation, localized SPR (LSPR) at the interface between nanostructured noble metals with higher curvature and dielectric materials at a resonant fre-quency is postulated to generate strong local electromagnetic fields.[7–13] This phenomenon can be strategically utilized to induce enhancement of both photo-thermal and photocatalytic effects.[11,14–19] By taking advantage of light-enhanced electromagnetic fields, plasmonic nano-structures promote redox processes in multiple reactions by constructing electro-catalytic active sites on their surfaces.[20,21] Also, the plasmonic metal nanostructures have been shown to exhibit enhanced absorption cross-sections that facilitate light harvesting, the high mobility of charge carriers, and efficient energetic electron transfer to incorporated semicon-ductors or materials near the interface.

Enhanced (photo)electrocatalysis in plasmonic architecture has been primarily ascribed to electromagnetic near-field enhancement, and alternatively to SPR-induced direct electron transfer to neighboring reactive sites. Despite the promising premises of plasmon-induced electrocatalysis, the underlying detailed mechanism remains to date controversial.[1,2,22–26]

In recent years, a promising role of plasmonic metal nanoparticles (NPs) has been demonstrated toward an improvement of the catalytic efficiency of well-designed hybrid electrocatalysts. In particular, the coupling of plasmonic functionality with the metal-based core–shell architectures in plasmon-enhanced electrocatalysis provides a sustainable route to improve the catalytic performances of the catalysts. Herein, the rationally designed AuNPs wrapped with reduced graphene oxide (rGO) spacer along with PdNPs (AuNP@rGO@Pd) as the final composite are reported. The rGO is proposed to promote the reduction of PdO, greatly enhance the conductivity, and catalytic activity of these nanohybrid structures. The plasmon-enhanced electrocatalytic performance of optimized AuNP@rGO(1)@Pd exhibits an ≈1.9- and 1.1-fold enhanced activity for the hydrogen evolution reaction and oxygen evolution reaction, respectively. The final composite also exhibits a superior stability up to 10 000 s compared with the commercial Pd/C. The mechanism of the enhanced catalytic performance is monitored through in situ X-ray absorption spectroscopy by observing the generated electron density under light irradiation. The results demonstrate that the energetic charge carriers are concentrated in the incorporated PdNPs, allowing higher catalytic performances for the overall water-splitting reaction. The conclusions herein drawn are expected to shed light on upcoming plasmon-induced electrocatalytic studies with analogous hybrid nanoarchitectures.

Dr. Y.-R. Lu, Prof. C.-L. DongDepartment of PhysicsTamkang UniversityTamsui 25137, TaiwanProf. R.-S. LiuDepartment of ChemistryNational Taiwan UniversityTaipei 10617, TaiwanProf. R.-S. LiuDepartment of Mechanical Engineering and Graduate Institute of Manufacturing TechnologyNational Taipei University of TechnologyTaipei 10608, Taiwan

Plasmonic Nanoparticles

Dr. J.-E. Lee, Dr. F. Marques Mota, C. H. Choi, Dr. R. Boppella, Prof. D. H. KimDepartment of Chemistry and Nano ScienceDivision of Molecular Life and Chemical SciencesCollege of Natural SciencesEwha Womans University52 Ewhayeodae-Gil, Seodaemun-Gu, Seoul 03760, Republic of KoreaE-mail: [email protected]. Y.-R. LuNational Synchrotron Radiation Research CenterHsinchu 30076, Taiwan

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.201801144.

1. Introduction

The heterogeneous catalysis as the conversion platform for the efficient solar light conversion into fuel has attracted a significant attention in the catalysis community. In recent years, surface plasmon resonance (SPR) has emerged as a novel

Adv. Mater. Interfaces 2018, 1801144

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Meticulous characterization and in situ or operando studies are of the primary need to consolidate necessary knowledge in this field. For example, in situ monitoring of the electron density upon light irradiation has been considered of prime importance to realizing advanced mechanistic insight.

The usage of Au nanoparticles (AuNPs) is a well-established approach to introduce visible light responsivity into electro-catalytic systems. Herein, we report for the plasmon-enhanced electrocatalytic properties and underlying fabrication process of rationally designed AuNP@rGO@Pd architectures, with AuNP core wrapped in reduced graphene oxide (rGO) subsequently decorated with Pd nanoparticles. The utilization of rGO was considered of great interest in the preparation of these core–shell nanostructures, owing to its unique physicochemical properties (e.g., quantum Hall effect, high carrier mobility, good optical transparency, and electric conductivity). The effects of the proximity of Pd and the plasmonic AuNP, and the resulting near-field plasmonic enhancement were assessed by controlling the thickness of the rGO shell.

The water splitting, a promising pathway to fulfill future energy demands in an eco-friendly manner, was herein con-sidered as a representative model reaction to assess the effi-ciency of our novel plasmon-enhanced electrocatalytic in the bifunctional system. The results were reflected in the enhancement of the activity of Pd nanoparticles in both hydrogen evolution reaction (HER) and oxygen evolution reac-tion (OER), and the optimized composite was compared with conventional commercial Pd-based catalysts dispersed in Vulcan XC 72, carbon black. In addition, the generated electron density under light irradiation was monitored for the first time through in situ X-ray absorption spectroscopy (XAS) to provide further insight into the fundamental physical and chemical prop-erties of plasmon-enhanced electrocatalysts during the reaction.

2. Results and Discussion

2.1. Characterization of the Synthesized Materials

Figure 1a shows a schematic of the experimental procedure. The positive graphene oxide (GO(+)) was deposited onto the synthesized citrate-capped AuNPs through electrostatic inter-action using a layer-by-layer (LbL) self-assembly technique. The obtained AuNP@GO(+) nanostructures were then mixed with Pd precursors, followed by the reduction of the composite using sodium borohydride. The collected transmission electron microscopy (TEM) images of AuNP@rGO@Pd evidenced the successful incorporation of Pd into the prepared binary systems. The TEM images show that no distinct morpholog-ical evolution was noted upon Pd deposition, while similar Au particle sizes and rGO thickness are being confirmed as well. (Figures 1d,g).

In accordance with the previous reports, the synthesized Au nanoparticles were found to be in the 50 ± 5 nm range (Figure S1a, Supporting Information). The reference AuNP@Pd nanostructures where Pd nanoparticles are in direct contact with AuNP, were also analyzed through scanning electron microscopy (SEM) image (Figure S1b, Supporting Informa-tion). A single-layer positively charged GO was characterized by the diffraction patterns collected during the TEM evaluation (Figure S2, Supporting Information). The negatively and posi-tively charged GO and GO-multilayer-capped AuNP surfaces were additionally characterized by zeta-potential measurement (Figures S3a,b, Supporting Information). When compared to the pristine Au film (≈25.5°) the resulting Au film@GO(+) and Au film@[GO(+)/GO(−)]2GO(+) samples exhibited a red-shift SPR angle by (0.1° and 0.5°). This has been ascribed to a change in the refractive index of Au film with increasing thickness of


Figure 1. a) Schematic representation of the stepwise fabrication procedure of AuNP@rGO layer@Pd nanostructures. Representative TEM images of b) AuNP@GO(+), c) AuNP@rGO(1), d) AuNP@rGO(1)@Pd, e) AuNP@[GO(+)/GO(−)]2GO(+), f) AuNP@rGO(5), and g) AuNP@rGO(5)@Pd nanostructures.




GO deposit on the Au film. The GO thickness was found to be about 1 and 3.5 nm, respectively, as confirmed by the fitting result obtained using Winspall program (Figure S3c, Sup-porting Information). As shown in representative TEM images (Figure S3e,f, Supporting Information), the GO thickness of the AuNP@GO(+) and AuNP@[GO(+)/GO(−)]2GO(+) samples were found to be ≈1 and 3.5 nm, respectively, reflecting the results obtained by SPR spectroscopy of the evaluated Au film@GO(+) and Au film@[GO(+)/GO(−)]2GO(+) samples. Finally, the Raman spectroscopy (Figure S3d, Supporting Infor-mation) yielded characteristic ratios of (D and G) bands of 1.19 and 1.27 for AuNP@GO(+) and AuNP@rGO(+), respectively. A peak intensity ratio from 1.27 to 1.64 further corroborated the increasing rGO thickness in AuNP@rGO(1) and AuNP@rGO(5). Figure 1 shows the representative TEM images of the aforementioned samples.

The optical properties of the as-prepared AuNP@GO with varying GO thickness were assessed through UV–vis absorp-tion spectroscopy (Figure S4a, Supporting Information). Whereas the absorption peak of GO-capped AuNP does not dramatically increase, the LSPR bands of the prepared AuNP@GO nanostructures evidenced a red-shift when compared with those of AuNP. An increase in the thickness of the incorporated GO layers by the layer-by-layer (LbL) self-assembly method could notably increase the absorption intensity, with an amine group being observed at λ = 228 nm following incorporation of positive GO. The optical properties of Pd/C, AuNP@Pd, and AuNP@rGO@Pd nanostructures were assessed through the corresponding UV–vis absorption spectra (Figure S4b, Supporting Information). In particular, an increase of the rGO layer thickness resulted in a decrease of the LSPR band inten-sities of the Au core. Relatively similar results were attained upon the incorporation of Pd. The notable decrease in the intensity of the LSPR bands of Au could be tentatively ascribed to the well-known damping effect in the presence of Pd, which exhibits significantly lower conductivity at the relevant optical frequency range. The broad band observed with AuNP@Pd in the 200–800 nm region reasonably infers that the optical prop-erties of the core–shell structures change from core-dominant to shell-dominant.[27–29] Conversely, the continuous increase of the thickness of rGO evidenced a notable decrease in inten-sity of the characteristic LSPR band. Further characterization was provided through supplementary TEM images, energy-dispersive X-ray spectroscopy (EDS) elemental mapping, and X-ray diffraction (XRD) patterns. The representative AuNP@Pd nanostructures similar in size to AuNP@rGO@Pd were syn-thesized for the comparison purposes. EDS elemental mapping shows a homogeneous distribution of Pd nanoparticles incor-porated in the rGO layers (Figure S5, Supporting Information). In accordance with the TEM images, Pd formed a shell-like porous structure and nanobranches over the Au core (Figure 1d and Figure S5a, Supporting Information). The incorporation of a thin rGO layer and multilayer rGO onto the AuNP surface were analyzed through corresponding TEM images, as shown in Figure S5b,c, respectively. The XRD patterns clearly show the Au and Pd characteristic diffraction peaks (Figure S6, Supporting Information). In particular, Pd (111) and (200) lattice planes and Au (111) and (200) planes could be discerned and were ascribed to the face-centered-cubic (fcc) forms of

both Au and Pd. A comparison with AuNP@Pd underlines the importance of rGO on the surface of Au for the enhanced distribution of the incorporated Pd nanoparticles.[30–32]

2.2. Electrocatalytic Evaluation

The electrochemical properties of the prepared hybrids were assessed toward their potential applications in the electro-catalysis under dark and light conditions. The cyclic voltam-mograms (CV) for AuNP@rGO with varying rGO thickness were obtained in N2-saturated 0.1 KOH solution at a scan rate of 20 mV s−1 (Figures S7a and S8a, Supporting Information). The LSV curves for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were measured at a scan rate of 5 mV s−1 under N2- and O2-saturated 0.1 m KOH solution, respectively. The lower potentials could be found with AuNP@rGO(1) at a fixed current density of −0.1 mA cm−2 upon light irradiation suggesting an improved activity for HER (Figure S7b, Supporting Information). The analogous results could be found for AuNP@rGO(5) (Figure S8b, Supporting Information). The commercial Pd/C reference sample did not reveal an improved electrocatalytic performance under light illumination (Figure S9b, Supporting Information). Similarly, under OER-relevant operating conditions at a fixed current density (−0.1 mA cm−2), the prepared AuNP@rGO architectures revealed lower potential values under light (Figures S7c and S8c, Supporting Information).

Following the Pd incorporation, the CVs of AuNP@Pd and AuNP@rGO@Pd nanostructures were measured in nitrogen-saturated 0.1 m KOH solution at a scan rate of 20 mV s−1 under dark and light illumination condition (Figure S10, Supporting Information). When compared with AuNP@Pd, the presence of conductive rGO markedly accelerated the energy conversion of the resulting architectures. The HER electrocatalytic activities of the prepared AuNP@Pd, AuNP@rGO(1)@Pd, and AuNP@rGO(5)@Pd samples were further assessed through LSV curves under dark and illuminated conditions in the N2-saturated 0.1 m KOH solution; finally, the prepared samples were com-pared with a commercial 30 wt% Pd on carbon black reference (Figure 2a). With both AuNP@rGO@Pd samples with varying rGO thickness, the enhanced electrocatalytic properties could be obtained under the light illumination. The onset potential for HER observed at −0.074 V with AuNP@rGO(1)@Pd before the illumination was dramatically decreased to −0.030 V upon the light irradiation. Similarly, at a current density of −10 mA cm−2, AuNP@rGO(1)@Pd yielded a significantly lower overpoten-tial of −0.291 V compared with the corresponding value in the dark (−0.342 V). The enhanced electrocatalytic performances under light irradiation suggested an enhanced electrocatalytic activity for HER with these materials. Under light illumina-tion, AuNP@rGO(1)@Pd further demonstrated enhanced cata-lytic properties compared with both AuNP@Pd and AuNP@rGO(5)@Pd, showing not only the importance of the incorpo-rated rGO, but also the impact of tuning the thickness of the rGO shell. However, in comparative terms, Pd/C revealed a superior onset potential (0.004 V) and a lower overpotential at the fixed −10 mA cm−2 current density (−0.213 V) (Figure S9b, Supporting Information). Even though the similar loadings





could be found in the compared samples, the incorporation of Pd in the commercial sample was believed to result in a slightly different size of the metal nanoparticles. For the HER performance, Pd is well-known for its high affinity for the H* binding energy over the Pd (111) lattice planes than Au.[33] A variation in the average particle size may therefore be ascribed to a corresponding change in the resulting catalytic activity of these Pd-based electrocatalysts. To evaluate the HER activity of these hybrids, corresponding Tafel slopes were determined by fitting the polarization data under dark and light illumina-tion conditions (Figure 2c). The found values for AuNP@Pd, AuNP@rGO(1)@Pd, and AuNP@rGO(5)@Pd are 109.85, 162.14, and 118.62 mV dec−1 under dark condition, respectively. Upon light illumination, the calculated Tafel slopes attained 97.69, 86.27, and 108.06 mV dec−1, respectively, revealing in each case a significantly higher HER activity. In particular,

AuNP@rGO(1)@Pd demonstrated similar activity, when com-pared with the commercial Pd/C reference (87.47 mV dec−1).

The OER electrocatalytic activities of these hybrids were then assessed in O2-saturated 0.1 m KOH solution under dark and illuminated conditions. Similarly, the LSV curves were measured at a low scan rate of 5 mV s−1 at 25 °C (Figure 2b). For OER, the 0.1 m KOH electrolyte was purged with O2 gas for at least 30 min to saturate the amount of dissolved oxygen. The catalytic evaluation was additionally carried out under the nitrogen flow for comparison purposes. The results were in an agreement with the previously reported values of the literature and discussed in Figure S11 (Supporting Information).[34] The onset potential of AuNP@rGO(1)@Pd in the dark condition was found at 1.533 V with the sample evidencing a positive shift to 1.515 V upon light illumination. The observed potential of 1.802 and 1.821 V at 10 mA cm−2 for Pd/C and AuNP@Pd,


Figure 2. a) LSVs of AuNP@Pd and AuNP@rGO@Pd nanostructures measured in nitrogen-saturated 0.1 m KOH solution at 25 °C. The scan rate was 5 mV s−1. b) LSVs of AuNP@Pd and AuNP@rGO@Pd nanostructures measured in oxygen-saturated 0.1 m KOH solution at 25 °C. The scan rate was 5 mV s−1. c) HER Tafel plots, and d) OER Tafel plots (log j versus the potential for linear voltammetry). Durability tests of the catalysts by chronoamperometric measurement under (e) HER-relevant conditions at −0.2 V for 5 000 s and (f) OER-relevant conditions at 1.75 V for 5 000 s. (The electrochemistry performance was measured under dark and light illumination conditions.)




compared with 1.783 V for the AuNP@rGO(1)@Pd nanostruc-ture reveals that the composite is very active for OER under dark. In addition, under light illumination the potential of our material could be further reduced to 1.772 V. The poor perfor-mance of AuNP@rGO(5)@Pd was tentatively ascribed to an obstruction in the direct charge transfer between Pd nanopar-ticles and Au, in the presence of a thick-rGO layer. As during OER, the anode operates at inherently oxidizing potentials, the possibility of agglomeration of Pd nanoparticles on the rGO surface during the reaction is to be further considered. In contrast, AuNP@rGO(1)@Pd based materials showed reason-able activity and stability under the similar operating conditions owing to the key role of a thin-rGO layer to prevent the leaching and aggregation of Pd nanoparticles. It should be noted that the incorporation of rGO could possibly play as a key strategy to inhibit aggregation of Pd. The electron transfer is equally facilitated as proposed in the schematic representation of the proposed mechanism. However, with an excessive amount of rGO in the catalyst, those AuNP nanoparticles are enclosed by thick rGO layer, which not only is believed to decrease light exposure of the Au surface, but further offers a hindered transfer of generated hot electrons.[35]

The OER kinetics for Pd/C, AuNP@Pd, and AuNP@rGO@Pd under dark and light irradiation were further assessed by recording Tafel polarization curves at a slow scan rate (5 mV s−1) (Figure 2d). The found values for Pd/C, AuNP@Pd, AuNP@rGO(1)@Pd, and AuNP@rGO(5)@Pd under dark conditions are 54.63, 49.41, 47.23, and 75.23 mV dec−1, respectively. Upon light illumination, the calculated values for the above-listed materials attained 54.89, 49.01, 43.08, and 73.33 mV dec−1, respectively. The results underline the enhanced catalytic activity attained with our AuNP@rGO@Pd nanohybrids under light irradiation. Most importantly, AuNP@rGO(1)Pd revealed superior catalytic activity under the light when compared with the commercial Pd/C. By tuning the light illumination condi-tion, the AuNP@rGO(1)@Pd exhibits a small overpotential for OER activity, compared with the dark condition. The finding is consistent with what was observed in the HER and OER, a photo-electrocatalysis of AuNP@rGO(1)@Pd nanostructure in the light illumination condition.

The durability of AuNP@rGO(1)@Pd was assessed under HER and OER conditions for 5000 s at −0.2 and 1.75 V and was compared with both AuNP@Pd and a commercial Pd/C reference (Figures 2e,f). In each case, the catalysts were evaluated under dark conditions up to 2500 s, after which light was shed for the remaining duration of the test. In accordance with the results observed in Figures 2a,c, AuNP@rGO(1)@Pd unveiled enhanced catalytic properties and attained higher current densities upon light irradiation. For both HER and OER-relevant operating conditions, our AuNP@rGO(1)@Pd showed a high stability up to 5000 s in each case. Additional experiments were carried out up to 10 000 s (Figure S12, Supporting Information) to further investigate the stability of the material. In this case, the current densities of AuNP@rGO(1)@Pd after chronoamperometric (CA) experiments were 74.6% and 69.9% of the initial values for HER and OER performance. The above relative currents were notably superior to those found for AuNP@Pd, i.e., 67% and 36.7%, respectively. It is suggested that the incorporation of wrapped-rGO thin

layers plays a critical role for a higher electrocatalytic stability compared to the evaluated AuNP@Pd counterparts.

In particular, the stability of our new hybrid architecture reported herein for the first time is strikingly advantageous when compared with a commercial Pd/C reference that equally exhibited nearly 37% decrease in the attained cur-rent density over the same time period. The poor stability of Pd/C catalyst is well reported in the literature and remains a drawback to the application of this material in electrocatalytic water splitting reaction.[24,35] This limitation based on a weak metal-C interaction on analogous metal-supported systems using carbon black and carbon nanotubes appears to be the apparently advantageous dispersion of Pd on rGO. With a dra-matically superior stability under OER conditions, this result may serve as a distinct advantage of these materials compared to commercial Pd-based counterparts.

2.3. Insight into the Mechanism

While the highly active AuNP@rGO@Pd nanoarchitectures and the underlying fabrication process herein reported signifi-cantly add to the promising number of plasmon-induced photo-electrocatalytic-based reports, this study further intends to explore the mechanistic details, which are relatively unexplored in the literature. The assumed synergism between the AuNPs, rGO, and Pd and the possible charge transfer mechanism in the AuNP@rGO@Pd system are in this sense discussed below.

Electrochemical impedance spectroscopy (EIS) measure-ments were first conducted to further assess the photo-induced charge transfer behavior of these hybrids under light illumi-nation (Figure 3). The calculated electrochemical resistance decreased in the order of AuNP@rGO(5)@Pd > Pd/C > AuNP@Pd > AuNP@rGO(1)@Pd, under both dark and light illumination. However, the collected Nyquist plots highlight a decrease in the charge transfer resistance of the core–shell architectures upon light irradiation toward an enhanced charge-transfer capacity during the electrochemical process. Most importantly, our best AuNP@rGO(1)@Pd hybrid revealed a superior performance compared to the commercial Pd/C reference.

The superior charge transport properties of rGO were well reflected upon incorporation in the resulting AuNP@rGO(1)@Pd architecture, evidencing smaller charge transfer resistance. In particular, a four-fold enhancement in the conductivity of AuNP@rGO(1)@Pd compared with AuNP@Pd upon light illumination underlines the excellent conductivity and electron-accepting nature of rGO. This is believed to be of prime importance and to pave the way to enhance the direct injection of plasmon-induced hot electrons to the neighboring Pd particles for both HER and OER reactions. Surprisingly, however, a continuous increase of the amount of deposited rGO with AuNP@rGO(5)@Pd revealed an increasing charge transfer resistance. The results have been tentatively ascribed to the decrease in the exposed and conductive Au surface; this result is also in accordance with the previous literature reports that evidenced a decrease in the conductivity of thicker rGO stacking layers due to the mechanical damage on the graphene sheet with cracks by stacking the films on the Au surface.[36]





Consequently, the multilayers are not directly affected by the adhesion with the AuNP surface. Most importantly, lower charge-transfer capacity hindering the efficient transfer of hot electrons from AuNP may be tentatively ascribed to the poor performance of AuNP@rGO(5)@Pd, compared with its rGO(1)-based counterpart, for both HER, and in particular, OER.

To shed additional light on the mechanistic details herein discussed, the distribution of hot-spots was investigated by finite-difference time-domain (FDTD) simulation. Figure 4 shows the FDTD along the x and y directions of the simulated electromagnetic near-field distributions using plane wave

source at 532 nm wavelength for all synthesized hybrids. In each case, the generated electromagnetic fields were well discerned with similarly prominent hot-spots distributed in the perimeter of the evaluated hybrid architectures. The electron transfer between the metal and graphene levels is driven by the work function difference as well as by the chemical interac-tion between the two components. With the incorporation of graphene on the Au surface, the surface electric field is con-sistent with the changes in the metal work function due to the increased interface dipole.[35,37,38] Upon Pd incorporation, the results reflect an enhancement of the near electromagnetic field in the vicinity of the AuNP, increasing the photon rate perceived by neighboring Pd active centers.

The maximum |E/E0|2 value field distributions for AuNP, AuNP@rGO(1), and AuNP@rGO(5) were 47.5, 51.4, and 53.6, respectively. The corresponding values for Pd-incorporated counterparts attained 448, 547, and 167 for AuNP@Pd, AuNP@rGO(1)@Pd, and AuNP@rGO(5)@Pd, respectively. In addition, we further conducted the FDTD simulation based on a model that resembles the actual nanoparticles distribution to confirm the catalytic enhancement at a pilot scale. The maximum |E/E0|2 value for concentrated nanostructures of AuNP, AuNP@rGO(1), and AuNP@rGO(5) were 438, 895, and 640, respec-tively (Figure S13, Supporting Information). The results clearly reflect the effect of the thin rGO layers along with the single nanoparticle system. The highest electromagnetic enhance-ment is localized at the surface of Au neighboring PdNPs incor-porated in the rGO layers with the strongest electromagnetic field being found for AuNP@rGO(1)@Pd. The results clearly reflect the effect of incorporating Pd nanoparticles along with the impact of varying rGO layer thickness. The incorporation of rGO was confirmed to lead to a stronger electromagnetic field upon comparison with AuNP@Pd, in which a direct contact is established between both metallic components. In addition, it


Figure 3. Electrochemical impedance spectra of Pd/C, AuNP@Pd, AuNP@rGO(1)@Pd, and AuNP@rGO(5)@Pd nanostructures under dark and light illumination conditions.

Figure 4. Finite-difference time-domain (FDTD) results of near-field electromagnetic field distributions without Pd nanoparticles, respectively. a) AuNP, b) AuNP@rGO(1), and c) AuNP@rGO(5); with Pd nanoparticles d) AuNP@Pd, e) AuNP@rGO(1)@Pd, and f) AuNP@rGO(5)@Pd nanostructures.




was observed that varying the gap-distance between the Pd and the AuNP core through fine-tuning of the rGO shell thickness dramatically changed the resulting electromagnetic field.

In addition to the above discussed enhanced conductivity and facilitated charge transfer of plasmon-induced hot electrons, the maximum near-field enhancement was attained with our most active AuNP@rGO(1)@Pd material. Both phenomena are expected to be responsible for the enhanced catalytic properties of this nanohybrid material.

In situ XAS was carried out in the dark and under light irradiation, as shown in Figure 5. The evaluated AuNP@Pd, AuNP@rGO(1), AuNP@rGO(1)@Pd, and AuNP@rGO(5)@Pd hybrids yielded rather a similar Au L3-edge XAS spectrum profiles (Figure 5a). Each architecture produced a characteristic Au0 state, suggesting a significant contribution of metallic-like Au in the synthesized nanoparticles constituting the core of these hybrid materials. For AuNP@Pd, AuNP@rGO(1), AuNP@rGO(1)Pd, and AuNP@rGO(5)@Pd nanostructures, a fraction of Au2+/Au3+ was reduced to Au0 via the introduc-tion of graphene. For the AuNP@rGO(5) nanostructure, a significant fraction of Au0 was oxidized to Au2+/Au3+ due to the larger amount of rGO deposited onto the AuNP surface (Figure S15, Supporting Information).[39] All nanostructures unveiled an increasing intensity under light illumination, suggesting an effective electron transfer from the Au 5d orbital, in which in situ measurement was conducted to reveal the light illumination effects.

All characterized samples yielded comparable Pd K-edge XAS spectra characterized by an absorption edge at three maxima found at ≈24.37, 24.39, and 24.43 keV (Figure 5b). The absorption threshold resonance appearing between 24.36 and 24.38 keV corresponds to the electronic transitions that arise from 1s to unoccupied 4p states above the Fermi level. Conversely, the second and third peaks are attributed to 1s to dp, and 1s to dsp transitions, respectively. The region intensity is sensitive to the change in electron occupancy of valence orbital and can be used to estimate the density of unoccupied states and corresponding change in the state of Pd species, whereas the energy shift is related to a change in the effective number of valence electrons at the absorbing site. AuNP@Pd revealed a relatively high fraction of oxidized Pd2+ species of 24.37 keV. In contrast, the Pd phase found in both AuNP@rGO@Pd hybrid structures is primarily metallic, accentuating the importance of rGO to reduce the presence of oxidized PdO species in these materials. The results agree with the previous reports in analo-gous electrocatalytic systems upon rGO incorporation.[35] Aside from promoting the reduction of PdO and greatly enhancing the conductivity of the nanohybrid structures, when inserted into the nanostructures, rGO is believed to further hinder the formation of AuPd alloy, thereby enhancing the catalytic activity of Au–Pd supported on the rGO hybrids. Upon light illumi-nation, no significant variation could be observed in AuNP@rGO(5)@Pd and AuNP@Pd, whereas the decreased intensity of the Pd K-edge in the AuNP@rGO(1)@Pd nanostructures was clearly noted. The decreasing intensity under light illumi-nation suggested a plasmonic hot electron transfer from the Au to the Pd, in which in situ measurement was conducted to reveal the light illumination effects.[26,40] The reduction in peak intensity suggests a decreased number of available unoccupied

Pd states induced by light irradiation along with a possible electron transfer from the plasmonic Au to Pd active centers. The incorporation of rGO in our hybrid nanostructures was further investigated through the analysis of both C K-edge and O K-edge spectra (Figure 5c and Figure S14, Supporting Infor-mation). The O K-edge spectra at 532.8 eV have π* character, and strongly depend on the chemical environment of the bond. The band position can be assigned to the π* transitions of CO states that are associated with carboxylic groups. Conversely, the broad peak at 537–542.5 eV is attributed to the


Figure 5. In situ X-ray absorption spectroscopy (XAS) of Pd/C, AuNP@Pd, AuNP@rGO(1)@Pd, and AuNP@rGO(5)@Pd under dark and light illumination conditions a) Au L3-edge, b) Pd K-edge, and c) C K-edge.




σ* transitions of OH, CO, or CO groups of functional-ized rGO. The almost identical spectral profile of both in situ O K-edge in the dark and under light illumination suggested that the oxygen is not an active site. The C K-edge reveals the elec-tronic transition from the C 1s core level to the 2p unoccupied states. The peak at ≈285 eV is attributed to electron transition from C 1s to the unoccupied π* states (C 2px of 2py orbitals) of graphitic CC bond. The peaks at around 289 and 291 eV are assigned to the σ* excitation from C 1c to the unoccupied σ* state (C 2pz orbital) of the CC bond. The decrease in inten-sity of C K-edge for AuNP@rGO(1) and AuNP@rGO(1)@Pd in the illuminated condition compared with those in the dark suggesting a decrease of unoccupied states, which is associ-ated with electron transfer from plasmonic Au. Notably, the less deviation of the spectra of AuNP@rGO(1)@Pd collected in the dark and illuminated conditions corroborates an efficient change transfer to Pd through the thin rGO layer, as above revealed by Pd K-edge. The conclusions herein drawn reflect an enhanced electronic effect for efficient charge transfer and improved catalytic properties.

Figure 6 shows a schematic of the overall proposed plasmon-induced photo-electrochemical mechanism in line with the above discussion, and the catalytic performance displayed by the evaluated materials. The incorporated AuNPs served as the ideal platform to induce visible-light-responsivity in the catalyst system. The plasmon-induced electrocatalytic systems have received remarkable attention in recent years. However, relevant questions that might clarify the mechanistic details of light-responsive electrocatalytic systems remain relatively unanswered to date. Here, we demonstrated efficient elec-tron transfer between the Au metal core and the rGO layers through both EIS and XAS results and used FDTD simulation results to detail a photothermal effect as an electromagnetic field distribution. The electron transfer between the metal and graphene levels is driven by the work function difference, as well as by the chemical interaction between graphene and metal. Graphene provides high specific surface area and excel-lent nanoparticles dispersion.[35,37,38] The AuNPs absorbs reso-nant photons, which can be transferred to unoccupied energy states in the graphene. Upon graphene incorporation the surface electric field is consistent with the changes of metal

work function due to the increased interface dipole.[41,42] On the other hand, the electronic structure of graphene is chemi-sorbed on incorporated Pd nanoparticles, leading to a strong binding energy, whereas adsorption on Au leads to a weaker bonding. The proposed mechanistic rationality then suggests an electron transfer from Au to Pd upon visible light irradia-tion, through the electronic structures of graphene, when the Fermi energies are equivalent.[15,42–45] Consequently, plasmonic photothermal and photo-electrocatalytic properties of AuNP can be beneficial to enhance the catalytic activity. In this con-text, as demonstrated by the above results, the photo-induced temperature increase of the AuNP provides heat to an adjacent reactant, and enhancement of the near electromagnetic field in the vicinity of the AuNP increases the photon rate seen by an adjacent reactant, or otherwise a photo-induced hot electron is transferred to a nearby reactant active site.[10,11]

The well-organized heterogeneous core–shell nanostruc-tures have a high mobility of charge carriers and high absorp-tion cross-section, in which Au core nanostructures absorb resonant photons, and the energetic electrons formed by the SPR excitation are efficiently transferred to the Pd active sites. The active metal is in close proximity to the plasmonic metal nanoparticles, and the near-field plasmonic enhancement increases as the thickness of the insulator layer decreases. Upon the plasmon excitation due to visible-light absorption, a hot electron can be transferred to the reactive sites in the catalyst through the direct transfer mechanism. Consequently, the plasmonic photothermal and photo-electrocatalytic properties of AuNP can enhance catalytic activity.

3. Conclusion

The rational design of the core–shell nanoparticles of AuNP cores wrapped in rGO and decorated with PdNPs was demon strated to enhance the catalytic efficiency in both hydrogen evolution reac-tion (HER) and oxygen evolution reaction (OER) under light illu-mination. The material characterization and enhanced catalytic activity of both H2 and O2 generations using visible light dem-onstrate the advantageous incorporation of rGO with tuned layer thickness. In contrast, the direct deposition of Pd NP on the Au core


Figure 6. Schematic of the mechanism of plasmon-enhanced photo-electrocatalytic activity.




resulted in inferior catalytic performances. The SPR-enhanced electrocatalytic performance of our optimized AuNP@rGO(1)@Pd architecture exhibited ≈1.9- and 1.1-fold activity in the HER and OER, respectively. In addition, remarkable stability under (HER and OER)-relevant conditions was attained up to 10 000 s in comparison with a commercial Pd/C reference. This work fur-ther reflects a significant contribution to the current state-of-the-art techniques to separate the optical and catalytic functions of the evaluated hybrid nanomaterials. Here, we have established that the flow of energy was strongly biased toward the excitation of the energetic charge carriers in the Pd active sites. Thus, the gener-ated hot electrons from the Au core to Pd during the proposed plasmon-enhanced electrocatalytic mechanism provide a sustain-able route to high-value catalytic activity. The proposed material is believed to have extended multifunction in numerous applica-tions for the efficient solar-to-energy conversion in electrocata-lyst systems. The systematically designed plasmonic core–shell nanostructures catalysts exhibited a significant enhancement in the plasmon-enhanced electrocatalytic properties for the overall water-splitting reaction. The proposed mechanisms for such enhancement are near-field enhancement by plasmonic excita-tion of Au, and graphene-mediated hot electron transfer to the palladium nanoparticles for efficient water-splitting reactions. However, a significant amount of work is still required across interdisciplinary fields, before the successful commercialization of artificial photo-electrocatalysis technology for a greener future is achieved.

4. Experimental SectionMaterials: Graphite, gold chloride hydrate (HAuCl4), sodium

tetrachloropalladate (Na2PdCl4), sodium citrate, sodium borohydride (NaBH4), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide methiodide (EDC), ethylenediamine, hydrazine, and a commercial 30 wt% palladium on carbon black were purchased from Sigma-Aldrich. Sulfuric acid (H2SO4), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), and hydrochloric acid (HCl) were purchased from DAEJUNG chemicals.

Preparation of Gold Nanoparticles (AuNPs): First, HAuCl4 (0.01 wt%) was added to deionized water (200 mL) in a 500 mL round-bottom flask, and the solution was brought to boil on a hot plate under vigorous stirring. Sodium citrate (1.4 mL, 1 wt%) was then rapidly added, and the solution was boiled for an additional 30 min period under stirring. The color of the solution finally turned to pink.

Preparation of GO(−) and GO(+): Graphite powder (5 g) and NaNO3 (2.5 g) were added to H2SO4 (115 mL) in a 500 mL flask with stirring until the reactants were completely dissolved for 1 h at room temperature. The flask was placed in an ice bath, and KMnO4 (15 g) was slowly added, ensuring that the temperature remained below 10 °C. The solution was heated to 35 °C and reacted for 2 h. After that, the flask was placed in an ice bath, and DI water (230 mL) was carefully added to the mixture. This mixture, which was dark brown in color, was further stirred for 15 min, after which heating was stopped, and the mixture was diluted with H2O (700 mL) and H2O2 (12.5 mL), then left for 2 h. The resultant light brown color solution was then washed repeatedly to remove precipitates. The product was centrifuged, and washed several times with DI water in order to reach pH (6–7) and to remove any metal residues. The obtained GO powder (50 mg) was added to deionized water (100 mL) and sonicated for 4 h to exfoliate the GO sheets. The GO solution was centrifuged to remove any un-exfoliated graphite oxide at 3000 rpm for 30 min. To construct graphene-based multilayer nanocomposites, the surface functionalization technique was employed

by amine functionalization of GO sheets to change the negative surface charge to positive. Next, EDC (600 mg) was added to the GO solution (50 mL) under vigorous stirring. Ethylenediamine (4 mL) was then added immediately, and the solution was stirred for 4 h at room temperature. The synthesized suspension was dialyzed for 1 day in DI water, to remove the remaining reagents and byproducts. The resultant solution was at neutral pH.

Preparation of AuNP@rGO Layer: For the preparation of AuNP@GO(+) solution, a layer-by-layer (LbL) self-assembly technique was employed. First, citrate-capped-AuNP and positive charged GO nanosheets were slowly stirred in aqueous suspension, and then the mixture was centrifuged to separate and purify the overall composite AuNP@GO(+). For the second layer, negatively charged GO nanosheet solution was added into AuNP@GO(+) solution, and then the mixture solution was slowly stirred. The mixture was centrifuged to separate and purify the composite AuNP@[GO(+)GO(−)]. The aforementioned reaction steps were repeated to synthesize GO-multilayer-capped AuNP. For the preparation of AuNP@rGO solution, 30 µL of hydrazine (35 wt% in water) and 60 µL of ammonia solution were added into 12 mL of AuNP@GO solution. The mixture solution was initially stirred at 95 °C for 30 min. The color of the solution changed from pink to purple color.

Preparation of AuNP@rGO@Pd Nanoparticles: The Na2PdCl4 solution (3 × 10−3 m, 2.6 mL) was incubated in 12 mL of AuNP@GO(+) solution for 12 h at room temperature. The red precipitate was redispersed by ultrasonication and washed with water by centrifugation. The metal ions attached to the AuNP@GO(+) nanoparticles were reduced by a sodium borohydride (600 µL, 0.1 m) solution with stirring for 30 min. Finally, the mixture was centrifuged, to separate and purify the final composite AuNP@rGO@Pd.

FDTD Simulation: FDTD simulation by Lumerical software was used for the calculation of the near-field electromagnetic field distributions of the metal nanostructures array with a plane wave source at 532 nm. In the simulation domain, a perfectly matched layer (PML) was used for the z-direction to absorb all of the propagating fields, while periodic boundary conditions were used for the x and y directions. The refractive indices of rGO, Au, and Pd were used from the data of Falkovsky and Palik, respectively. The size of the mesh was 0.2 nm for all directions, with plane wave input source of 200–1200 nm.

Electrochemical Measurements: The cathodic and anodic reactions were performed in a three-electrode system, using a potentiostat with a platinum foil as the counter electrode, and saturated calomel electrode (SCE, in saturated KCl) electrode as the reference. The overall potential was calibrated with respect to a reversible hydrogen electrode (RHE). The working electrode (glassy carbon electrode, GCE) was prepared by drop-casting (3 µL) nanostructure-containing suspension and air-dried. A GC disk electrode with a diameter of 3 mm was used as the support. The catalyst inks were prepared by ultrasonic dispersal of 5 mg of catalyst in a solution containing 50 µL of Nafion solution and a solvent mixture of isopropyl alcohol (IPA) (250 µL) and water (250 µL). The cyclic voltammograms (CV) were recorded from 0.05 to 1.05 V versus RHE in N2-saturated 0.1 KOH solution, at a scan rate of 20 mV s−1. The linear sweep voltammetry (LSV) curves for HER were measured from 0.15 to −0.55) V at a rotating disk of 1600 rpm and a scan rate of 5 mV s−1 in N2-saturated 0.1 m KOH solution. The LSVs for OER were recorded from 0.9 to 2.0 V versus RHE at a rotating disk of 1600 rpm and a scan rate of 5 mV s−1 in O2-saturated 0.1 m KOH solution. Specifically, the electrolyte saturated with O2 was prepared by purging O2 gas into the 0.1 m KOH electrolyte for 30 min for kinetic reaction. Afterward, the two different electrolytes were adopted with a presence of O2 or absence of O2, which LSV polarization curves could be measured for the kinetic and efficiency of the O2 evolution reaction. All of the potentials and voltages were iR uncorrected unless noted. The chronoamperometry results were recorded for the time duration of 5000 s and 10 000 s at −0.2 V versus RHE potential bias for HER, and 1.75 V versus RHE potential bias for OER, in the nitrogen-saturated 0.1 m KOH solution for HER, and the oxygen-saturated 0.1 m KOH solution for OER.

Electrochemical impedance spectroscopy (EIS) spectra were recorded at open circuit potential bias with the frequency ranging from (100 kHz





to 0.1 Hz) in the nitrogen-saturated containing 0.1 m KOH solution. The commercially available carbon supported Pd catalysts (Sigma-Aldrich) was used as the reference. The actual composition of each sample was checked by inductively coupled plasma optical emission spectrometry (ICP-OES), and yielded the following results (by Au and Pd wt%): Pd/C (31.4%), AuNP@Pd (Au: 21.1%, Pd: 78.9%), AuNP@rGO(1)@Pd (Au: 58%, Pd: 42%), and AuNP@rGO(5)@Pd (Au: 30.5%, Pd: 69.5%). The loading amount of Pd was as follows: Pd/C (0.021 mg), AuNP@Pd (Au: 0.025 mg, Pd: 0.094 mg), AuNP@rGO(1)@Pd (Au: 0.027 mg, Pd: 0.019 mg), and AuNP@rGO(5)@Pd (Au: 0.02 mg, Pd: 0.022 mg), when 3 µL nanostructure-containing catalyst ink was drop cast on the GCE. The electrocatalytic performance under light irradiation was carried out using a Xe lamp (300 W) fitted with a cut-off filter (>420 nm).

Instruments and Characterization: TEM measurements were carried out by JEOL JSM2100-F microscopy operated at 10 kV. UV–vis absorption spectra were recorded on a Varian Technologies Cary 5000. The metal nanostructures were observed by JEOL JSM6700-F SEM. The electrochemical study was performed on an Autolab ECO Chemie PGSTAT302N potentiostat. Atomic force microscopy (AFM) was carried out in tapping mode to study the surface morphologies of the metal nanostructured on PDMS substrate (Digital Instruments Dimension 3100). The Raman spectra were obtained by Raman spectrometer from HORIBA Jobin Yvon at an excitation wavelength of 633 nm laser, and a Nikon microscope with a 50× objective lens with the numerical aperture (NA) of 0.75. Zeta potentials were recorded on a ZETASIZER 3000/MALVERN. Powder XRD patterns of all samples were recorded by XRD spectrometry (Rigaku, D/max 2000vk/pc) operated at 40 kV and 30 mA. SPR measurements were carried out by Resonant Technologies GmbH/RT2005 SPR spectrometry. The actual composition of each sample was checked by ICP-OES (OPTIMA 8300/full wavelength coverage between 163 and 782 nm).

SPR Measurements: SPR measurements were performed using a Kretschmann configuration setup. The sample and the detector were mounted on a two-axis (θ – 2θ) coaxial goniometer, which enabled precise tuning of the SPR angle of the incident light and the detector. The Au thin film was illuminated through a prism with a linearly p-polarized, frequency modulated laser (HeNe, λ = 632.8 nm, power = 10 mW). The intensity of the reflected light was recorded by a photodiode detector connected to a lock-in amplifier. The SPR measurements were conducted with scan modes.

X-Ray Absorption Spectroscopy Measurement: The XAS was performed at National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. XAS at Au L3-edge and Pd K-edge were carried out at BL17C and BL01C, respectively. C and O K-edges were conducted at BL20A. A 1.5 AM solar simulator was used to record the XAS spectra under illuminated condition.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis study was supported by National Research Foundation of Korea Grant funded by the Korean Government (2017R1A2A1A05022387) and by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (2018M3D1A1058536). The authors would like to thank the Ministry of Science and Technology of Taiwan (contract nos. MOST 107-2113-M-002-008-MY3 and 104-2112-M-032-008-MY3) for financially supporting this research.

Conflict of InterestThe authors declare no conflict of interest.

Keywordsheterogeneous catalysis, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), photo-electrocatalysis, plasmonic nanoparticles

Received: July 27, 2018Revised: October 27, 2018

Published online:

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