Plasmon-Enhanced Hydrogen Evolution on Specific Facet of Silver

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Plasmon-Enhanced Hydrogen Evolution on Specific Facet of Silver Nanocrystals Tsung-Rong Kuo, Yi-Cheng Lee, Hung-Lung Chou, Swathi M G, ChuanYu Wei, Cheng-Yen Wen, Yi-Hsuan Chang, Xi-Yu Pan, and Di-Yan Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00652 • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 28, 2019

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Chemistry of Materials

Plasmon-Enhanced Hydrogen Evolution on Specific Facet of Silver Nanocrystals

Tsung-Rong Kuo,*ab Yi-Cheng Lee,c Hung-Lung Chou,*d Swathi M G,c Chuan-Yu Wei,e Cheng-Yen Wen,e Yi-Hsuan Chang,a Xi-Yu Pana and Di-Yan Wang*cf

a Graduate

Institute of Nanomedicine and Medical Engineering, College of

Biomedical Engineering, Taipei Medical University, Taipei 11031, Taiwan b International

Ph.D. Program in Biomedical Engineering, College of Biomedical

Engineering, Taipei Medical University, Taipei 11031, Taiwan c Department d Graduate

of Chemistry, Tunghai University, Taichung 40704, Taiwan

Institute of Applied Science and Technology, National Taiwan University

of Science and Technology, Taipei 10617, Taiwan. e

Department of Materials Science and Engineering, National Taiwan University, Taipei, 10617, Taiwan

f

Center for Science and Technology, Tunghai University, Taichung 40704, Taiwan

Corresponding Author *E-mail: [email protected] (T.-R. Kuo), [email protected] (H.-L. Chou) and [email protected] (D.-Y. Wang)

ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Hydrogen evolution reaction (HER) from electrocatalytic water splitting is a prospective technology to supply clean energy with low environmental impact for the future. In this work, plasmonic silver nanocubes (AgNCs) with (100) facet and silver nanooctahedrons (AgNOs) with (111) facet were applied as the light-harvesting catalysts for enhancing hydrogen production in the plasmon-activated HER electrochemical system. As light harvesters, AgNCs and AgNOs can efficiently absorb light ranging from ultraviolet to near-infrared to generate hot electrons for facilitating electrocatalytic HER. Both AgNCs and AgNOs revealed the light-harvesting capability to improve HER activities with laser irradiation. Moreover, the current densities of AgNOs with (111) facet were higher than those of AgNCs with (100) facet for electrocatalytic HER under irradiations with three different laser wavelengths. The density function theory (DFT) simulations revealed the adsorption energy of the surfaces followed the order Ag(111) < Ag(100), indicating that the hydrogen could be easily desorbed on the Ag(111) surface for HER. Combination of the experimental HER results and DFT simulations expressed that AgNOs with (111) facet were the excellent light harvesters in this study. Based on the DFT simulations of the H-Ag(111) and H-Ag(100) systems, the findings could be extended to other plasmon-enhanced HER electrochemical systems and could enable electrocatalysts to be designed at the atomic level.


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Introduction The issues of increasing global energy demand and mitigating climate change have fascinated extensive research interests from both academia and industry to develop sustainable and environment-friendly energy for replacement of non-renewable fossil fuels in the world today. Green energy of hydrogen has been believed as a promising chemical fuel to provide renewable and clean energy in the foreseeable future.1-7 Recently, the three key technologies included thermal, electrolytic, and photolytic processes have been applied to produce high purity hydrogen.8-11 With current technologies for hydrogen production, water electrolysis has caused much attention due to its clean and safe hydrogen production processes.12-16 The electrocatalytic hydrogen evolution reaction (HER) can result in low environmental impact, depending on the source of the electricity used. For HER application, platinum is the most active (overpotential of -33 mV at 10 mA/cm2) and stable electrocatalyst.17-18 However, the practical application of precious platinum electrocatalyst is limited for HER because of its low quantity and high price. The uses of other materials such as molybdenum-based, cobalt-based, nickel-based and iron-based electrocatalysts have been made with high HER electrocatalytic activity and excellent durability. For example, the hybrid electrocatalysts of MoS2 nanoparticles on reduced graphene oxide sheets have shown superior HER activity with a small overpotential about -100 mV, large cathodic current of 60 mA/cm2 at -0.2 V, and a Tafel slope of 41 mV/decade.19 The P/Co-FeS2 catalyst grown on titanium foil directly exhibits superior electrochemical performance of the HER with a low overpotential of -90 mV at 100 mA/cm2 and a small Tafel slope of 41 mV/decade.20 The Ni@NiO/Cr2O3 triphase material has been demonstrated as a remarkably active and stable HER electrocatalyst with a current density of 100 mA/cm2 at a -115 mV overpotential in an alkaline system.21 Although many



electrocatalysts have been developed with high HER activity, these electrocatalysts are still need to be improved to meet the HER performance of platinum. Metallic nanomaterials with surface plasmon resonance (SPR) included Au, Ag, and Cu have been generally employed as the light harvesters to extend light absorption for wide bandgap photoelectrodes.22-25 Under SPR excitation, plasmonic metal nanomaterials can produce hot electrons on their surface and then these hot electrons can be conducted into the conduction band of semiconductor photoelectrodes to obtain photoactivity for water splitting. Previous study has demonstrated that TiO2 nanowires decorated with different shapes of SPR Au nanomaterials have effectively enhanced photoactivity across entire ultraviolet-visible region for PEC water splitting.26 The photoelectrocatalytic activity of SPR Ag nanoparticles dispersed on TiO2 nanotube arrays under ultraviolet light irradiation has revealed 1.6-fold enhancement factor in comparison with pure TiO2 nanotube arrays.27 Cu nanoparticles-decorated TiO2 nanotube arrays have significantly enhanced the visible light response because of the expanded absorption in the district of visible light causing by Cu nanoparticles.28 The advancements of these studies have demonstrated that plasmonic metal nanomaterials have shown promising efficiency in improving the light photoactivity of wide bandgap semiconductor photoelectrodes for PEC water splitting. However, in electrocatalysis systems, the uses of light-harvesting cocatalysts combined with electrocatalysts are still very few cases for improvement of HER activity. A recent work of plasmon-activated HER electrochemical system with the uses of hybrids of Au nanorods and MoS2 nanosheets has proven the increase of current about ∼3-fold under laser excitation of Au localized SPR.29 However, under SPR excitation, the detailed mechanism of plasmonic metal nanomaterials as the light-harvesting cocatalysts is still unclear for electrocatalytic HER. Crystals with different facets usually have revealed different surface energy, reaction kinetics and


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catalytic activity.30 Therefore, the development of optimal plasmonic nanomaterials with a unique facet is of important for plasmon-enhanced HER electrochemical system. In the work, the light-harvesting catalysts of silver nanocrystals included AgNCs and AgNOs were successfully prepared by the polyol reaction. The structural and optical properties of the silver nanocrystals were examined by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and UV-Vis-NIR spectrophotometry. HER activities of AgNCs and AgNOs on the CFP substrates were respectively measured in sacrificial reagents (0.5 M H2SO4 and 1 M ethanol) by linear-sweep voltammetry (LSV) with a typical three-electrode system with and without laser irradiations. Furthermore, the density function theory (DFT) simulations were utilized to calculate the adsorption energies of the surfaces of Ag(111) and Ag(100), respectively.

Experimental Section Chemicals 1,5-Pentanediol (97%) and silver nitrate (99.99%) were purchased from Alfa Aesar. Copper(II) chloride dehydrate was purchased from Fisher Scientific. Polyvinylpyrrolidone (average Mw ~55,000) and ethanol were purchased from Sigma-Aldrich. All chemicals were used without further purification.

Preparations of Silver Nanocrystals Shape-controlled Ag nanocrystals included silver nanocubes (AgNCs) and silver nanooctahedra (AgNOs) were synthesized by the polyol reaction with some modifications from previous studies.31-32 Precursor of CuCl2 (0.08 g) was dissolved in 1,5-pentanediol (PD, 10 mL) following by sonication/vortex for about ~10 min.



Precursor of polyvinylpyrrolidone (PVP, 0.2 g) was dissolved in 10 mL of PD using sonication/vortex for about ~30 min. AgNO3 precursors with two different concentrations were respectively developed for the growths of AgNCs and AgNOs. To synthesize AgNCs, precursor of AgNO3 (120 mM) was developed by dissolving 0.2 g of AgNO3 and 20 μL of CuCl2 precursor in 10 mL of PD. To synthesize AgNOs, precursor of AgNO3 (240 mM) was prepared by dissolving 0.4 g of AgNO3 and 40 μL of CuCl2 precursor in 10 mL of PD. Before the syntheis of AgNCs, the solvent of 20 mL of PD was preheated at 190 °C using temperature-controlled silicone oil bath under vigorous stirring for 10 min. For the AgNCs synthesis, the two solutions of 120 mM of AgNO3 precursor and PVP precursor were rapidly injected into the hot reaction flask with various rates. 500 μL of AgNO3 (120 mM) precursor was first injected every 60 s and then 250 μL of the PVP precursor was also injected every 30 s. AgNCs were obtained once the solution turned opaque (This reaction can take anywhere from 10 - 13 min). For the further synthesis of AgNOs, the polyol reaction was continued by the additions of AgNO3 (240 mM) precursor and PVP precursor for further 65-70 min. After the polyol reaction, the solution containing Ag nanocrystals was washed with ethanol and then centrifuged at 12 000 rpm for 20 min to separate AgNCs and AgNOs from PD solvent. The wash processes were repeated twice. After removal of the supernatant, the AgNCs and AgNOs were stored in ethanol for following experiment.

Drop-casting Sliver Nanocrystals on CFP Substrate To load sliver nanocrystals on CFP substrate, drop-casting method was applied. 100 L of silver nanocrystal solutions included AgNCs and AgNOs were respectively dropped on CFP substrates (1 cm2). Afterward, the CFP substrates loaded with AgNCs or AgNOs were placed in a vacuum oven to dry the solvent of ethanol at 37


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°C. To obtain the weight of silver nanocrystal on CFP substrate, the weight of CFP substrate before loading silver nanocrystals was subtracted from the weight of CFP after loading silver nanocrystals.

Electrochemical Measurement Electrocatalytic experiments were measured by using a standard three-electrode system with an electrochemistry workstation (Autolab, the Netherlands). Working electrodes were developed by drop-casting the catalysts of AgNCs (1.2 mg) and AgNOs (1.2 mg) on the CFP substrate (1×1 cm2), platinum rod was used as counter electrode, and Ag/AgCl electrode (Metrohm) was used as reference electrode. Before the electrocatalytic measurements, the reference was calibrated and converted to RHE. LSV was executed at 1 mV/s to obtain polarization curves. Before electrochemical measurements, to remove the impurities on the surface of silver nanocrystals, the silver nanocrystals on the CFP substrates were treated by the cyclic voltammetry procedure in a controlled potential range from -0.1 V to 0.3V in 0.5 M aqueous solution of NaBr for several cycles. Fourier-transform infrared spectroscopy (FTIR) was applied to characterize the functional groups on the surfaces of silver nanocrystals before and after NaBr treatments (Figure S1). Moreover, all polarization curves were iR-corrected. For electrocatalytic experiments, the electrolyte was composed by 0.5 M H2SO4 and 1 M ethanol. Near-infrared continuous wave laser (808 nm), green continuous wave laser (532 nm) and blue continuous wave laser (450 nm) were used as the light sources. The power densities of these lasers are 200 mW/cm2. Schematic illustration of the devices for electrochemical measurement was shown in the supporting information as Figure S2.



Computational Methods For DFT calculations, projector-augmented waves (PAW) was utilized for generalized gradient approximation (GGA).33-35 About the plane wave calculations, 500 eV was performed as cutoff energy, which was spontaneously fixed by the total energy convergence calculation for Ag(100) and Ag(111) slab systems. Bader charge simulations were utilized with the use of the software at http://theory.cm.utexas.edu/vtsttools/bader/.36-37 In this work, the simulation images were generated by XCrysden.38

Result and Discussion Characterizations of Silver Nanocubes and Silver Nanooctahedra The AgNCs and AgNOs were prepared by polyol reaction and then characterized by SEM. As shown in Figure 1a and 1b, AgNCs (edge lengths of ∼75 nm) and AgNOs (edge lengths of ∼275 nm) were successfully obtained in high yield (>95%). To further investigate the crystal structures of AgNCs and AgNOs, high resolution transmission electron microscopy (HRTEM) was applied. In HRTEM image of Figure 1c, the AgNCs exhibited clear lattice fringes with an interplanar distance of 0.20 nm corresponding to the (200) plane of face-centered cubic (fcc) silver. The AgNOs exhibited an interplanar distance of 0.23 nm corresponding to the (111) plane of fcc silver as shown in the HRTEM image of Figure 1d. The selected area electron diffraction (SAED) patterns also showed the single crystalline nature of AgNCs and AgNOs as shown in the insets of Figure 1c and 1d, respectively. The absorption spectra of AgNCs and AgNOs were examined by UV/Vis/NIR spectrophotometry after their colloidal solutions were repeatedly washed with ethanol for the removal of excess PVP. The absorption spectra showed the differences in localized surface plasmon resonance (LSPR) modes for the AgNCs (Figure 2a) and


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AgNOs (Figure 2b). For AgNCs with edge lengths about ~75 nm, there were six strong LSPR as shown in Figure 2a. In the lowest frequency, the modes (first and second) at ~510 nm and ~650 nm were respectively attributed to quadrupolar and dipolar LSPR modes. The sharpest and the strongest LSPR mode of quadrupolar mode was revealed in AgNCs. In Figure 2b, AgNOs with edge lengths of ∼275 nm showed several strong LSPR modes in district from 300 to 900 nm. For AgNOs, higher frequency LSPR modes included hexapolar and higher-order modes were revealed at absorption ~800 nm. According to the results of absorption spectra, the shape-controlled syntheses of AgNCs and AgNOs by the polyol reaction were successful developed. Moreover, with the excellent property of LSPR and strong absorption from 300 to 900 nm, which covers entire visible light region, AgNCs and AgNOs are promising for the uses as the light-harvesting cocatalysts in plasmon-enhanced HER electrochemical system.

Light-harvesting Capability of AgNCs and AgNOs To investigate the light-harvesting capability, AgNCs and AgNOs were respectively loaded on carbon fiber paper (CFP, 1 cm2) by drop-casting method. In this work, the CFP was applied as a substrate due to its thermal stability and electrical conductivity. HER activities of AgNCs (1.2 mg) and AgNOs (1.2 mg) on CFP substrate were separately estimated in the electrolyte (0.5 M H2SO4 and 1 M ethanol) with and without 808 nm laser irradiation by LSV with a typical three-electrode system. In the polarization curve of Figure 3a, the blank substrate of CFP revealed negligible HER activity in the measured voltages form 0 to -0.6 V with and without 808 nm laser irradiation, indicating little effect on the HER performance of the loaded samples. The polarization curve of the commercial silver electrode was shown in the supporting information as Figure S3. After AgNCs loaded on CFP substrate, the



current densities were 24.8 and 18.9 mA/cm2 at -0.4 V versus RHE with and without 808 nm laser irradiation. Moreover, with AgNOs loaded on CFP substrate, the current densities were 13.3 and 7.6 mA/cm2 at -0.4 V versus the RHE with and without 808 nm laser irradiation. Here, the area of CFP (1 cm2) was utilized to calculate the current density. With 808 nm laser irradiations, the increases of current densities of AgNCs and AgNOs demonstrated their light-harvesting capabilities for plasmon-enhanced HER electrochemical system. Furthermore, before and after the uses of AgNCs and AgNOs as the catalysts with laser irradiations for hydrogen production, SEM was used to characterize their morphologies and distribution on the CFP substrate. As shown in Figure 3b and 3c, there are no significant changes of morphology and distribution for the catalysts of AgNCs and AgNOs after hydrogen production with 808 nm laser irradiations. The results of SEM characterization indicated that AgNCs and AgNOs were both stable and suitable for the uses as the light-harvesting catalysts in plasmon-enhanced HER electrochemical system.

Effect of Surface Area on Plasmon-Enhanced Hydrogen Production From the SEM images of Figure 3b and 3c, with the same weight (1.2 mg), AgNCs and AgNOs exhibited different surface coverage ratios on the CFP substrate. In the plasmon-enhanced HER electrochemical system, hydrogen gas was generated on the surface of light harvesters of silver nanocrystals. To investigate the effect of surface area on plasmon-enhanced hydrogen production, different weights of AgNCs included 0.08 mg, 0.5 mg, 1.2 mg, 2.2 mg, 3.1 mg and 3.6 mg were separately applied as the light-harvesting catalysts on the CFP substrate for electrocatalytic HER. The SEM images of 0.08 mg, 0.5 mg, 2.2 mg, 3.1 mg and 3.6 mg of AgNCs on the CFP substrate were shown in the supporting information as Figure S4. The surface areas of AgNCs were calculated to be 6.1 cm2, 38.1 cm2, 91.4 cm2, 167.6 cm2, 236.2 cm2 and


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274.3 cm2 for 0.08 mg, 0.5 mg, 1.2 mg, 2.2 mg, 3.1 mg and 3.6 mg, respectively. (See supporting information for details of calculation) The surface areas of AgNCs were applied to calculate their current densities at -0.4 V versus the RHE for plasmon-enhanced hydrogen production. As show in Figure 4a, the current densities of 0.08 mg, 0.5 mg, 1.2 mg, 2.2 mg, 3.1 mg and 3.6 mg of AgNCs were respectively 0.273 mA/cm2, 0.269 mA/cm2, 0.271 mA/cm2, 0.208 mA/cm2, 0.172 mA/cm2 and 0.148 mA/cm2 under 808 nm laser irradiations. The data of surface areas of different masses of AgNCs and their current densities under 808 nm laser irradiations have been tabulated as shown in Table S1. No significant changes of the current densities were observed with the mass of AgNCs from 0.08 mg to 1.2 mg. The SEM images showed that the light-harvesting catalysts of 0.08 mg and 0.5 mg of AgNCs were partially covered on the CFP substrate. With 1.2 mg of AgNCs, most of the surface of the CFP substrate was covered by AgNCs as shown in Figure 3b. In these stages, the laser energy can be efficiently absorbed by AgNCs and then transferred for hydrogen production. With 2.2 mg, 3.1 mg and 3.6 mg of AgNCs, the current densities decreased from 0.208 mA/cm2 to 0.148 mA/cm2. In these cases, the SEM images showed that the multilayers of AgNCs were revealed on the CFP substrate. The results of the decreases of current densities can be attributed to that the laser cannot penetrate into the inner layers of AgNCs for electrocatalytic HER. Furthermore, the electron transfer can be impeded by the defects between multilayers of AgNCs to result in the decrease of the current densities. Overall, single layer of AgNCs on the CFP substrate showed the best performance for the plasmon-enhanced HER electrochemical system. The i-t curve of AgNCs at -0.4 V with and without 808 nm laser irradiations was shown in Figure 4b. Initially, steady current density of AgNCs was observed without laser irradiation. With 808 nm laser irradiation, the current density immediately



increased and then reached the steady state. When the laser was turned off, the current density rapidly decreased, indicating good reproducibility of the process. The repeatable laser response showed that AgNCs could be a suitable light-harvesting catalyst in the application for SPR enhanced electrocatalytic activity of HER.

Facet-Dependent Catalytic Activities of AgNCs and AgNOs To investigate the facet-dependent catalytic activities of silver nanocrystals, AgNCs with (100) facet and AgNOs with (111) facet were separately applied as the catalysts in plasmon-enhanced HER electrochemical system. Here, the surface areas of AgNCs (91.4 cm2, 1.2 mg) and AgNOs (30.9 cm2, 1.2 mg) were used to calculate the current densities. The current densities of AgNCs and AgNOs at -0.4 V versus the RHE without laser irradiations were provided in supporting information (Table S2). As shown in Figure 5, the current densities of AgNCs were 0.257 mA/cm2, 0.258 mA/cm2 and 0.271 mA/cm2 with 450 nm, 532 nm and 808 nm laser irradiations, respectively. For AgNOs, the current densities were separately 0.420 mA/cm2, 0.433 mA/cm2 and 0.430 mA/cm2 with 450 nm, 532 nm and 808 nm laser irradiations. No significant changes on the temperatures of CFP substrates loaded with AgNCs and AgNOs were found under laser irradiations for electrocatalytic HER (Figure S5). Therefore, as the light harvesters, AgNCs and AgNOs can efficiently absorb light ranging from ultraviolet to near-infrared to generate hot electrons for facilitating electrocatalytic HER.39-42 Furthermore, with the same laser power density, AgNCs revealed similar current densities with three different laser irradiations. The same phenomenon was also observed for the uses of AgNOs as the catalysts. However, in comparison with AgNCs and AgNOs, the current densities of AgNOs were higher than that of AgNCs with laser irradiation. With the 808 nm laser irradiation, the current density of AgNOs (0.430 mA/cm2) was 1.59 time higher than that of AgNCs


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(0.271 mA/cm2). The results indicated that AgNOs with (111) facet exhibited higher catalytic activity in comparison to AgNCs with (100) facet in plasmon-enhanced HER electrochemical system.

Density Function Theory Simulation To investigate the different catalytic activities between AgNCs(100) and AgNOs(111), the DFT was applied to simulate the interactions between hydrogen and Ag(111) and Ag(100). For modeling the Ag(100) and Ag(111) slab surface, slabs containing four layers with sixteen atoms per layer in Ag(100) and Ag(111) systems were adopted as shown in Figure 6. The surface was built with a slab by the periodic boundary layers in three dimensions and models were divided from their graphics in the perpendicular direction for the surface with a 14 Å vacuum layer. The layer in bottom was set to the bulk coordinates; whol atomic relaxations were approved for the top three layers. For the simulations, a 3 × 3 × 1 k-Point mesh was performed in the 4 × 4 super cell. To search the appropriate lattice constant, the hydrogen adsorption was utilized on Ag(100) and Ag(111) slab systems with the steady construction as an example to performance calculations in 8.667 × 8.667 × 21.076 Å3 and 8.667 × 8.667 × 20.128 Å3 systems, respectively. The cell contained with atoms were permitted for the relaxation while the strength on unconstrained atoms was fewer than 0.02 eV/Å. The adsorption energy, Eads, was explained by the total interactions between the adsorption molecule and slab system, and therefore equation was given as Eads  Etotal  E Ag (111) 

 

1 EH 2 , where Etotal, E Ag (111) and EH 2 were the total energy 2

of the system, Ag(111) slab system energy, and H2 molecule energy, respectively. The negative sign of Eads was attributed to the energy gain of the system due to molecular adsorption.



The energy of H adsorption on Ag(111) and Ag(100) surfaces was calculated, and the relevant values matched with previous simulations as shown in Table 1.43-48 The most sites revealed adsorption energies for 0.35 (hollow site) and 0.16 eV (hcp site), for the (100) and (111) surfaces, respectively. The contour maps of Figure 6c and 6d were accorded to slide models of Ag(100) and Ag(111), respectively. Figure 6c and 6d showed the electron density contour of plane cut through the one Ag atom and Ag atom at center site. In this contour, the diversity of the orbital of Ag atom and Ag atom can be easily observed. Therefore, the electron donation was a significant factor for influence of the adsorption energy of gaseous molecule (i.e. H or H2) on an Ag-Ag surface. Furthermore, DFT simulations were calculated based on hydrogen atom, Ag(100) and Ag(111) slab systems as shown in Figure 7a and 7b. The contour maps of Figure 7c and 7d were corresponded to slide models of hydrogen atom and Ag(100) and Ag(111) slab, respectively. Figure 7c revealed the electron density contour of plane cut through the Ag atom and hydrogen atom at center site. In the contour, we can clear observe the difference between the charge distribution of Ag atom and hydrogen atom. From Table 1 which provided quantitative information for the adsorption energies of Ag(111) and Ag(100) systems, and it can be observed that the adsorption energy of the surfaces follows the order Ag(111) < Ag(100), indicating that the hydrogen was easily desorbed on the Ag(111) surface for hydrogen evolution reaction. Adsorption of hydrogen has been considered in order to reveal the electronic effect. The different binding energies associated with adsorbate at surface irregularities markedly influenced the chemical properties of transition metal surfaces. Catalytic activity and selectivity were influenced by them as well as the rate of activation, a very important phenomenon in chemical technology. Based on the DFT simulations, we proposed that the interactions between hydrogen and Ag(111) and Ag(100) may


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be attributed to the hydrogen atom of the H-Ag(111) and H-Ag(100). This interaction was expected to explain the electronic effect to H-Ag(111) and H-Ag(100); adsorption energy was an important controlling parameter to ensure H-Ag(111) attached well on Ag(111) nanocrystals. Though the results presented herein were based on the H-Ag(111) and H-Ag(100) systems, the findings could be extended to other relevant hydrogen evolution reaction (HER) systems and would enable electrocatalysts to be tailored at the atomic level. A better understanding of this interaction will facilitate the development of better electrocatalysts conjugates for plasmon-enhanced HER electrochemical system.

Conclusions The silver nanocrystals as light harvesters demonstrated a wide range of SPR absorption from ultraviolet to near-infrared light, resulting in plasmon-enhanced HER. A single layer of silver nanocrystals on the CFP substrate exhibited obvious increase of current density in the plasmon-enhanced HER electrochemical system. With repeatable laser response, AgNCs showed their light-harvesting capabilities in SPR enhanced HER. Furthermore, AgNOs with (111) facet expressed higher current densities than those of AgNCs with (100) facet for electrocatalytic HER under laser irradiation. The DFT simulations evidenced that the adsorption energy of the surfaces followed the order Ag(111) < Ag(100), indicating that the hydrogen was easily desorbed on the Ag(111) surface for hydrogen evolution reaction. Not only the experimental results, but also DFT simulations approved that AgNOs with (111) facet were the better electrocatalysts than that of AgNCs with (100) facet in the plasmon-enhanced HER electrochemical system.

Supporting Information



The Supporting Information is available free of charge on the ACS Publications website. SEM images of different weights of AgNCs on the CFP substrate and calculation of surface area for AgNCs.

Conflict of Interest The authors declare no competing financial interest.

Acknowledgements We are grateful to the NCHC and NTUST for computer time and facilities. We would like to acknowledge Mr. Chi-Ming Lee for his excellent technical support at TMU Core Facility. This work was supported by TMU-NTUST-107-1, MOST 107-2113-M-038-004, MOST 107-2622-E-038-001-CC2, Taipei Medical University, National Taiwan University of Science and Technology and Tunghai University.


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Figures and Captions

Figure 1. SEM images of (a) AgNCs and (b) AgNOs. HRTEM images of (c) AgNCs and (d) AgNOs. The SAED patterns of AgNCs (inset c) and AgNOs (inset d).



Figure 2. UV-/Vis/NIR spectra of (a) AgNCs and (b) AgNOs.

Figure 3. (a) Polarization curves obtained from CFP, AgNCs and AgNOs with and without 808 nm laser irradiation. The area of CFP (1 cm2) was utilized to calculate the current density. SEM images of (a) AgNCs and (b) AgNOs after hydrogen production.

Figure 4. (a) The current densities of AgNCs at -0.4 V versus the RHE with 808 nm laser irradiations. (b) i-t curves of AgNCs (1.2 mg) at -0.4 V versus the RHE with and without 808 nm laser excitation. The surface areas of AgNCs with different weights were applied to calculate their current densities for plasmon-enhanced hydrogen production.


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Figure 5. The current densities of AgNCs and AgNOs at -0.4 V versus the RHE with different laser irradiations. The surface areas of AgNCs (91.4 cm2, 1.2 mg) and AgNOs (30.9 cm2, 1.2 mg) were used to calculate the current densities.



Figure 6. The model of (a) top-view of Ag(100), (b) top-view of Ag(111), (c) the corresponding contour plot of Ag(100) slab and (d) the corresponding contour plot of Ag(111) slab, respectively.


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Figure 7. The model of (a) top-view of H-Ag(100), (b) top-view of H-Ag(111), (c) the contour plot of H-Ag(100) and (d) the contour plot of H-Ag(111), respectively.



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Table 1. Adsorption energies and optimized geometries calculation of hydrogen in Ag(111) and Ag(100) systems.

Systems

Eads (eV)

Eads (eV)

Ag(111)

-0.16

-0.1245

Ag(100)

-0.35

-0.3045

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Plasmon-Enhanced Hydrogen Evolution on Specific Facet of Silver

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