Various Active Metal Species Incorporated within Molecular Layers on

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Various Active Metal Species Incorporated within Molecular Layers on Si(111) Electrodes for Hydrogen Evolution and CO2 Reduction Reactions Takuya Masuda, Yu Sun, Hitoshi Fukumitsu, Hiromitsu Uehara, Satoru Takakusagi, Wang-Jae Chun, Toshihiro Kondo, Kiyotaka Asakura, and Kohei Uosaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00895 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 9, 2016

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The Journal of Physical Chemistry

Various Active Metal Species Incorporated within Molecular Layers on Si(111) Electrodes for Hydrogen Evolution and CO2 Reduction Reactions Takuya Masuda,a,b,* Yu Sun,c Hitoshi Fukumitsu,a Hiromitsu Uehara,d Satoru Takakusagi,d Wang-Jae Chun,e Toshihiro Kondo,f Kiyotaka Asakura,d and Kohei Uosakia,c,* a

Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan b

Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan

c

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan d

Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido 001-0021, Japan

e

Graduate School of Arts and Sciences, International Christian University, 3-10-2 Osawa Mitaka, Tokyo 181-8585, Japan f

Graduate School of Humanities and Sciences, Ochanomizu University, Tokyo 112-8610, Japan

Email_Address [email protected], [email protected] Phone: +81-29-860-4971 ACS Paragon Plus Environment

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Abstract Organic molecular layers with viologen moieties as electron transfer mediators were constructed on hydrogen-terminated Si(111) surfaces and metal catalysts for multi-electron transfer reactions were incorporated into the molecular layers by immersing the viologen-modified Si(111) electrodes in aqueous solutions containing various metal complexes (K2PdCl4, NaAuCl4 and K2PtCl4). Significant enhancements were achieved for CO2 reduction at the Au-modified Si(111) electrode and for both hydrogen evolution reaction (HER) and CO2 reduction at the Pd-modified Si(111) electrode. XPS and XAFS analysis showed that Au complexes were spontaneously reduced to metal nanoparticles during the metal insertion, and therefore, actual catalysts for CO2 reduction at the Au-modified Si(111) electrode were Au metal nanoparticles. In contrast, Pd complexes were inserted into the molecular layers and partly reduced during HER and CO2 reduction. Pd complexes and relatively small Pd nanoparticles (< 2nm) were considered to be actual catalysts for HER and CO2 reduction. Interestingly, at the Ptmodified Si(111) electrode, not only highly efficient HER but also highly selective CO2 reduction in preference to HER was achieved, despite the fact that HER is dominant at Pt pure metal electrodes even in CO2-saturated aqueous solutions. On the basis of XPS and XAFS analysis, Pt complexes were incorporated into the molecular layers and acted as confined molecular catalysts for both HER and CO2 reduction without being converted into Pt metal nanoparticles. This should be the major reason for the anomalously high selectivity of the Pt-modified Si(111) electrode for CO2 reduction, unlike to pure Pt metal electrocatalysts.

Keywords Photoelectrochemistry, Semiconductor electrodes, X-ray absorption fine structure, Confined molecular catalysts

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Introduction Achievement of photoelectrochemical multi-electron transfer reactions at semiconductor electrodes is a longstanding target not only because of their possible applications to solar-to-chemical energy conversion but also because they are scientifically very challenging.1 One of the major issues in this approach is that most semiconductors which have a suitable bandgap for solar energy conversion are thermodynamically unstable in aqueous solutions.2-4 Furthermore, those semiconductor surfaces are not catalytically active for multi-electron transfer reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and CO2 reduction reaction and require an appropriate catalyst to drive those complicated processes with high efficiency and durability.5-13 Semiconductor electrodes can be effectively protected from corrosion by coating with thin metal layers and rate of those multi-electron transfer reactions can be significantly enhanced by the catalytic effect of the deposited metals as compared to those of uncoated electrodes.6,7,14-21 However, deposition of the metal layers on semiconductor electrodes often resulted in a rather lower efficiency because of the formation of Schottky barrier.10,22 Although deposition of the metal nanoparticles on the semiconductor electrodes, instead of the continuous metal layers, allows to control the barrier height by changing the size and density of the metal nanoparticles,6,9,13,16,17,23-25 this approach often compromised the effective protection of the semiconductor surfaces from corrosion because of the lower coverage, leading to the formation of surface states which act as charge recombination centers.26,27 Chemical modification of the semiconductor surfaces with organic molecular layers before the metal deposition can avoid not only the surface corrosion but also the formation of Schottky barrier and charge recombination center because the particles of catalytic metals can be separated from the semiconductor surfaces by the organic molecular layers. Various organic molecular layers have been constructed as separation layers by silane coupling reactions on silicon oxide surface and hydrosilylation reactions on hydrogen-terminated silicon surfaces.28-37 Usually, nanoparticles of precious metal atoms are used as catalysts for the multi-electron transfer reactions. In the case of nanoparticles with a diameter of 3 nm, however, only atoms on the outermost ACS Paragon Plus Environment

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layer of nanoparticles can act as active reaction sites and ~60% of those precious metal atoms constituting the core of nanoparticles38 can be unused. In addition, the catalytic activity and selectivity of the metal nanoparticles can be significantly affected by their size, shape and oxidation state. Hence, controlling the structure of both molecular layers and catalysts in atomic and molecular scales is very important for reducing the cost and improving the catalytic activity and selectivity. In our series of work, we have constructed an ordered molecular layer on a hydrogen-terminated Si(111) surface, in which various functional moieties such as 4-ethylbenzylchloride, viologen moieties and Pt as surface protection layer, electron transfer mediators and catalysts for multi-electron transfer reactions, respectively, are precisely manipulated at the controlled positions,39,40 and briefly reported that the Pt incorporated into the molecular layer on the Si(111) electrode acted as “confined molecular catalysts” during the electrochemical HER.41 Subsequently, we have attempted to apply this concept for the CO2 reduction, but Pd and Au complexes were chosen as metal sources42 and Pt complexes were avoided to use because Pd and Au metal electrodes are efficient electrocatalysts for CO2 reduction while HER is dominant at Pt metal electrodes even in CO2-saturated solutions.43-45 As a result, high selectivity for CO2 reduction was certainly achieved at the Si(111) electrodes modified by molecular layer with viologen moiety and Pd and Au catalysts. In the present work, we constructed viologen layers on hydrogenterminated Si(111) surfaces, incorporated metal catalysts within the molecular layers by using not only Pd and Au complexes but also Pt complexes as metal sources, and examined their electrochemical and photoelectrochemical HER and CO2 reduction activity. Surprisingly, CO2 reduction proceeded in preference to HER not only at the Si(111) electrodes modified by Pd and Au complexes but also at the Si(111) electrode modified by Pt complexes, unlike pure Pt metal electrodes in CO2-saturated solutions. In order to clarify the reason for this unique catalytic property, local structure and oxidation state of those metal catalysts were investigated by x-ray photoelectron spectroscopy (XPS) and x-ray absorption fine structure (XAFS).


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Experimental Materials Ultrapure nitrogen (>99.99%) and carbon dioxide (>99.99%) were purchased from Taiyo nippon sanso. Ultrapure-grade dichloromethane, reagent-grade sodium sulphate, potassium tetrachloroplatinate (II), potassium tetrachloropalladate (II), sodium tetrachloroaurate (III), benzene, ethanol, 1-bromobutane, 1, 4-dibromobutane, 4, 4’-bipyridine from Wako, and 4-vinylbenzylchloride (4VBC) from Aldrich were used as received except that 4VBC was purified by vacuum distillation before using. Water was purified using a Milli-Q system (Yamato, WQ-500). Ag wire (99.99%) and Pt wires (99.99%) were purchased from Nilaco. Double-side polished Si(111) single-crystal wafers of 100 mm in diameter, 500 µm in thickness, and resistivity of 1-10 Ω cm (n-type; phosphorus-doped, p-type; boron-doped) were purchased by Shin-Etsu Semiconductor. Sample Preparations A Si(111) wafer was cut into 1 cm × 1 cm pieces and cleaned by acetone, followed by sequential immersion (1) in sulphuric acid/30% hydrogen peroxide aqueous solution (2:1 by volume) at 60°C for 20 min, (2) in aqueous solution of 0.5% hydrofluoric acid at room temperature (RT) for 5 min, and (3) in RCA solution (water/hydrogen peroxide/hydrochloric acid, 4:1:1 by volume) at 80°C for 20 min. The samples were thoroughly rinsed with pure water after each immersion. Finally, the samples were immersed in a deaerated aqueous solution of 40% ammonium fluoride for 5 min and rinsed with pure water to obtain monohydride-terminated (H-) Si(111) surfaces.40 Surface modification was performed as shown in Scheme 1.40,42 (i) A freshly prepared H-Si(111) substrates were illuminated with 254-nm light for 8 h in deaerated 4-vinylbenzylchloride in the Ar-filled glovebox. The substrates were first reacted with (ii) 4, 4’-bipyridine, and then alternately reacted with (iii) 1, 4-dibromobutane and (iv) 4, 4’-bipyridine. After repeating the steps (iii) and (iv) for certain times, the pyridine groups facing to air were (v) terminated with1-bromobutane to form a viologen mono and multilayer on the Si(111) surfaces, that are V2+-Si(111) surfaces. (vi) The V2+-Si(111) surfaces were immersed in an aqueous solution of 10 mM potassium tetrachloropalladate (II), potassium ACS Paragon Plus Environment

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tetrachloroplatinate (II) or sodium tetrachloroaurate (III) to introduce a metal species into the molecular layers. The Si(111) surfaces modified with Pd, Pt and Au are denoted as Pd-V2+-, Pt-V2+- and Au-V2+Si(111) electrodes, respectively. Characterization of the molecular layers after each modification step was reported previously.40 Surface concentration of the viologen moieties was estimated to ca. ~3 × 1010

mol cm-2 per layer from the redox peaks in CVs of the metal-free V2+-Si(111) electrode. The coverage

is ~75% of the geometric surface area.

Scheme 1. Surface modification steps.

Electrochemical, XPS and XAFS Measurements (Photo)electrochemical

measurements

were

performed

at

RT

in

a

three-electrode

photoelectrochemical cell equipped with a quartz window. A Ag/AgCl electrode, a Pt wire and the Si (111) sufaces modified with molecular layers were used as a reference, a counter, and a working ACS Paragon Plus Environment

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electrode, respectively. The potential of the working electrode was controlled by a Potentiostat (Hokuto Denko, HA-501G) and a function generator (Hokuto Denko, HB-111) was used to provide an external potential to obtain I-V curves, which were recorded by an X-Y recorder (Graphtec, WX-1200). A 500 W xenon lamp (Ushio, UXL-500-D) through an IR cut filter (Toshiba, IRA-20) and a UV cut filter (Sigma Koki) was used for photoelectrochemical measurements. FT-IR measurements were carried out by using FTS-30 spectrometer (Bio-Rad) purged by gas nitrogen to remove the atmospheric water and CO2, equipped with an HgCdTe detector cooled by liquid nitrogen, in a three-electrode homemade cell with a CaF2 window. The incident angle was 65° with respect to the surface normal at the electrode surface. All the spectra were measured by integrating 32 interferrograms with a resolution of 2 cm-1 under illumination with a spectrum measured in dark before illumination as a reference. XPS spectra were obtained using a Thermo Scientific K-Alpha with monochromic Al Kα radiation. Photoelectrons were collected at 0° from the surface normal (i.e. take-off angel of 90°). XAFS measurements were performed at BL9A and 12C of the Photon Factory (PF) and NW 10A of the Photon Factory-Advanced Ring (PF-AR) of High Energy Accelerator Research Organization. The storage rings of the PF and PF-AR were operated at 2.5 GeV and 6.5 GeV, respectively. XAFS spectra at the Pd K, Pt LIII and Au LIII absorption edges were recorded in air at RT in polarization dependent total reflection fluorescence (PTRF) configuration46,47 at the s-polarization. PTRF XAFS spectra at the Pt LIII absorption edge were also measured in an Ar-saturated 0.1 M Na2SO4 solution at various potentials using a home-made in situ spectroelectrochemical cell.48 X-rays were monochromatized using a Si(311) double-crystal monochromator for the Pd K absorption edge at the PF-AR and Si(111) doublecrystal monochromator for the Pt LIII and Au LIII absorption edges at the PF. The fluorescence signals were detected using a 19-element pure Ge solid-state detector (GL0110S; Canberra, USA). The EXAFS analyses were carried out using REX 2000 (Rigaku, Japan). All the parameters obtained by curve fitting analyses of the EXAFS were summarized in the Supporting Information.


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Results and Discussion Electrochemical Properties of the Pd-V2+-, Pt-V2+- and Au-V2+-Si(111) Electrodes in Dark and under Illumination Figure 1 shows I-V curves of the Pd-V2+-, Pt-V2+- and Au-V2+-p-type Si(111) electrodes in Ar- and CO2-saturated 0.1 M Na2SO4 solutions under photoillumination. As a comparison, those of the n-type Si(111) electrodes in dark are also shown. The pH of the Ar-saturated 0.1 M Na2SO4 solution was adjusted to 4.4 by adding H2SO4, since the pH of 0.1 M Na2SO4 changed from 5.9 to 4.4 by CO2 saturation. Both at the Pd-V2+- and Pt-V2+-p-type Si(111) electrodes in the Ar-saturated solution under illumination, cathodic photocurrents due to the HER started to flow at around 0 V and increased monotonically as potential became more negative.39,40 This is reasonable because both Pd and Pt metal electrodes are very good electrocatalysts for the HER.49 Surprisingly, not only at the Pd-V2+-p-type Si(111) electrode but also at the Pt-V2+-p-type Si(111) electrode, photocurrents in the CO2-saturated solutions were clearly larger than those in the Ar-saturated solutions in the potential range between 0 and -0.4 V. This enhancement of cathodic currents should be due to the CO2 reduction since pH of these solutions were adjusted to be the same. It is very interesting that higher selectivity for CO2 reduction in preference to HER was achieved at the Pt-V2+-p-type Si(111) electrode, because Pt is known to be the most efficient electrocatalyst for the HER and its selectivity for the CO2 reduction is very low even in CO2-saturated solutions.45 To detect the products of CO2 reduction, in situ FT-IR measurements were performed at the p-type Pt-V2+-Si(111) electrodes in the CO2-saturated 0.1 M Na2SO4 D2O solution with thin-layer configuration.50 Figure 2 shows IR spectra (s-polarization) in 1100-4000 cm-1 region by keeping the potential at -0.4 V for different periods of time under illumination. Very strong negative going peaks were observed at 1200 and 2350 cm-1 corresponding to bending and stretching of OD, respectively. A pair of positive going peaks at 1365 and 1625 cm-1 corresponding to the symmetric and asymmetric stretching of carboxylate group were also observed, indicating that one of the major products of CO2 reduction is formic acid/formate ions at the Pt-V2+-Si(111) electrode as was at the PdACS Paragon Plus Environment

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V2+-Si(111) electrodes.42 The growth of these peaks with time is more clearly seen in Fig. 2 (b), which shows IR spectra in 1250-1750 cm-1 region. No significant difference was noticed between the p- and spolarized spectra, showing the peaks are not of adsorbed intermediates but essentially of solution species. At the Au-V2+-p-type Si(111) electrode under illumination, on the other hand, photocurrents did not increase much in the relatively positive potential region and steeply increased from -0.55 V and -0.8 V in the CO2-saturated and Ar-saturated solutions due to the HER and CO2 reduction, respectively. The current response for each n-type Si(111) electrode in dark is similar to that for the corresponding p-type Si(111) electrode under illumination although much smaller currents were observed at more negative potentials.51 Thus, anomalously high selectivity for the CO2 reduction in preference to the HER was achieved both at the Pt-V2+-n-type Si(111) electrode in dark and at the Pt-V2+-p-type Si(111) electrode under illumination. Figure 1 (d) shows the ratio between photocurrents in the CO2-saturated and Ar-saturated solutions (ICO2/IAr) at the Pd-V2+-, Pt-V2+- and Au-V2+-p-type Si(111) electrodes as a function of potential, together with a typical CV of a metal-free V2+-p-type Si(111) electrode under illumination. In the CV of the V2+-p-type Si(111) electrode, two characteristic redox waves were observed at around -0.25 and -0.7 V corresponding to the two-step one-electron redox reactions of viologen moieties, i.e., V2+/V•+ and V•+/V••, respectively.52 One may consider that potential dependencies of photocurrent and ICO2/IAr can be related to the reduced state of viologen moieties. At the Pd-V2+- and Pt-V2+-Si(111) electrodes, large photocurrents started to flow in the Ar-saturated solutions as soon as the reduction of V2+ to V•+ started, confirming that the viologen moieties served as electron transfer mediators for the HER.53 In contrast, photoccurents started to flow from the potential slightly more positive than that of V2+/V•+ in the CO2 saturated solutions, suggesting the direct pathways from the Si electrode to the metal catalysts in parallel to the viologen-mediated pathways. In the relatively positive potential region between 0 and -0.4 V, ICO2/IAr was larger than unity, i.e., ICO2 was larger than IAr, showing that the CO2 reduction took place in preference to the HER. As potential became more negative than -0.2 V, ICO2/IAr gradually decreased probably due to the adsorption of intermediate species of the CO2 reduction on reaction active sites, and ACS Paragon Plus Environment

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ICO2 eventually became smaller than IAr at the potentials more negative than -0.5 V, indicating that the HER was also inhibited by the intermediate species. In contrast, at the Au-V2+-Si(111) electrode, only small photocurrents were observed in both solutions in the potential region where the viologen moieties were in V2+ and V•+ states, and photocurrent significantly increased in the CO2-saturated solution as soon as V•+ started to be reduced to V••. This result indicates that electron transfer from the Si(111) electrode to the Au catalyts was mediated by the interconversion of viologen moieties between V•+ and V•• at the Au-V2+-Si(111) electrodes while it was mediated by that between V2+ and V•+ at the Pd-V2+- and Pt-V2+-Si(111) electrodes. It is known that while Pd and Au metal electrodes are relatively efficient electrocatalysts for the CO2 reduction, the HER is dominant at Pt metal electrodes.45 In the present system, however, the highest ICO2/IAr was obtained at the Pt-V2+-Si(111) electrode on contrary to the electrocatalytic properties of pure metal electrodes. In order to clarify the reason for this anomalously high selectivity for the CO2 reduction at the Pt-modified Si(111) electrode, oxidation states of metal catalysts at these Si(111) electrodes were determined by XPS and XAFS.

(b) 0

n-type in Ar n-type in CO2

-0.2

p-type in Ar

-0.4 -0.6 -0.8

p-type in CO2

-1

-0.8

-0.6

-0.4

-0.2

Potential / V vs. Ag/AgCl

0

Current Density / mA cm-2

(a) Current Density / mA cm-2

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

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0

n-type in Ar n-type in CO2

-0.2

p-type in Ar

-0.4 -0.6 -0.8

p-type in CO2

-1

-0.8

-0.6

-0.4

-0.2

0



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(d) 6

n-type in Ar

0

-0.4 -0.6 p-type in Ar

-0.8

-1

-0.8

4

-0.4

V2+/V・+

3

-5

V・+/V・・

2 1

p-type in CO2

-0.6

0

5

n-type in CO2

-0.2

ICO2/IAr

Current Density / mA cm-2

(c)

-0.2

0

0


-10 -1

-0.8

-0.6

-0.4

-0.2

0

Current density / µA cm-2


Figure 1. LSVs of the (a) Pd-V2+-, (b) Pt-V2+- and (c) Au-V2+-p-type Si(111) electrodes in 0.1 M Na2SO4 solutions saturated with Ar and CO2 under illumination together with those of the n-type Si(111) electrodes in dark. Scan rate: 1 mV/s. (d) Ratio between photocurrents at the (i) Pd-V2+- (blue), (ii) Pt-V2+- (red) and (iii) Au-V2+-p-type Si(111) (black) electrodes in the CO2-saturated solution and those in the Ar-saturated solution (ICO2/IAr) as a function of potential, together with a CV of the metalfree V2+-p-type Si(111) electrode measured in 0.1 M Na2SO4 solutions saturated with Ar with a scan rate of 50 mVs-1.

(b) 10 s 5 min 15 min 30 min 60 min

10 s 5 min 15 min

Absorbance

(a) Absorbance

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


30 min 60 min

0.02

0.002

1000 1500 2000 2500 3000 3500 4000 Wavenumber / cm -1

1250

1375 1500 1625 Wavenumber / cm -1

1750

Figure 2. IR spectra (s-polarization) at the p-type Pt-V2+-Si(111) electrode in the CO2-saturated 0.1 M Na2SO4 (D2O) solution at -0.4 V for different time duration in (a) 1100-4000 cm-1 region and (b) 13001700 cm-1 regions with background correction.


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Characterization of Actual Catalytic Species at the Pd-V2+-, Pt-V2+- and Au-V2+- Si(111) Electrodes Figure 3 shows XPS spectra of the Pd-V2+-, Pt-V2+-, and Au-V2+-n-type Si(111) electrodes before and after the electrolysis at various potentials in the Ar-saturated and CO2-saturated 0.1 M Na2SO4 solutions. At a pristine Pd-V2+-Si(111) electrode, a pair of peaks due to the Pd 3d5/2 and 3d3/2 was observed at 343.6 and 338.3 eV in the XPS spectrum (green curve, Fig. 3 (a)), respectively. The peak positions are in good agreement with a previous report for K2PdCl4,54 showing that the PdCl42- was successfully inserted into the molecular layers. Each peak has a small shoulder at its lower binding energy side, suggesting that the Pd complexes are slightly reduced during the immersion in an aqueous solution of K2PdCl4 or some of Cl is substituted by solution species. Although the XANES spectrum of the Pd-V2+Si(111) electrode (Fig. 4A (a)) was distinct from both those of K2PdCl4 pellet and Pd foil, the EXAFS oscillation (Fig. 4B (a)) is similar to that of K2PdCl4 pellet, confirming that the PdCl42- is the dominant species inserted into the molecular layer. Curve fitting of the Fourier transform showed that Pd-Cl bonds were partly replaced with Pd-O bonds as summarized in Table S1. In the XPS spectrum (red curve, Fig. 3 (a)) after the electrolysis at -0.8 V in the Ar-saturated solution for the HER, fraction of the small shoulders at the lower binding energy slightly increased with respect to the primary complexes, showing the further reduction or ligand exchange of the Pd complexes during the HER. Accordingly, the EXAFS oscillation (Fig. 4B (b)) was substantially changed and formation of the Pd-Pd bond was identified by curve fitting of the Fourier transform (Fig. 4C (b)), showing that Pd complexes and relatively small nanoparticles co-exist at the Pd-V2+-Si(111) electrode under the HER operation. More pronounced shift occurred in the XPS spectrum (blue curve, Fig. 3 (a)) after a freshly prepared Pd-V2+-Si(111) electrode was kept at -0.8 V in the CO2-saturated solution. Peaks corresponding to the Pd 3d5/2 and 3d3/2 of Pd metal were predominantly observed at 335.6 and 340.8 eV, indicating that major catalytic species for the CO2 reduction was Pd metal nanoparticles while Pd complexes seem to be the majority for the HER. The formation of Pd-Pd bond was more clearly observed in the EXAFS oscillation (Fig. 4B (c)) and its Fourier transform (Fig. 4C (c)).


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Relationship between the size of metal nanoparticles and coordination number was evaluated in the literatures.55-57 As summarized in the Table S1, the coordination numbers of the Pd-Pd bonds obtained by curve fittings are 5.1 and 6.0 at the Pd-V2+-Si(111) electrode after the HER (Fig. 4C (b)) and CO2 reduction (Fig. 4C (c)), respectively. This suggests that relatively small nanoparticles (< 2 nm) were formed during the reactions.

(a)

(b) Pd 3d3/2

K2PdCl43d5/2

K2PdCl43d3/2

346

344

342

Pd 3d5/2

340

338

336

Au 4f5/2

Normalized Intensity


334

NaAuCl44f7/2

332

92

90

Binding Energy / eV

Au 4f7/2

NaAuCl44f7/2

88

86

84

82

80

Binding Energy / eV

(c) Pt 4f5/2


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


K2PtCl44f7/2

80

78

76

Pt 4f7/2

K2PtCl44f7/2

74

72

70

68

Binding Energy / eV

Figure 3. Normalized XP spectra of the (a) Pd-V2+-, (b) Pt-V2+- and (c) Au-V2+-n-type Si(111) electrodes in the Pd 3d, Pt 4f and Au 4f regions, respectively, before (green) and after keeping the potential at -0.8 V in the Ar-saturated (red) and CO2-saturated (blue) 0.1 M Na2SO4 solutions for 30 min.


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The Journal of Physical Chemistry (B)

(A)

Pd-Cl

K2PdCl4

K2PdCl4

K2PdCl4

(a)

(b)

Pd-O

Fourier Transform

(a)

k2 χ (k)


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

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(b)

(c)

(c)

Pd-Cl

(a)

x2 Pd-O

Pd-Pd

(b)

(c)

x4 Pd-O

Pd-Pd x4

Pd foil

24300

Pd-Pd

Pd foil

Pd foil

24350

24400

Photon Energy / eV

24450

0

2

4

6

8

10

12

k / Angstrom-1

0

1

2

3

4

5

6

R / Angstrom

Figure 4. (A) XANES spectra, (B) EXAFS oscillation and (C) Fourier transform at the Pd K absorption edge of the Pd-V2+-Si(111) electrode (a) immediately after the metal incorporation and after keeping the potential at -0.8 V in (b) the Ar-saturated and (c) CO2-saturated solutions of 0.1 M Na2SO4 for 30 min. Results of a K2PdCl4 pellet dilluted in a fine boron nitride powder and Pd foil are also shown as standard spectra.

At a pristine Au-V2+-Si(111) electrode immediately after the immersion of the V2+-Si(111) surface in an aqueous solution containing NaAuCl4, two pairs of peaks were observed in the XPS spectrum (green curve, Fig. 3 (b)). A pair of peaks observed at 84.2 and 87.8 eV can be assigned to the Au 4f7/2 and 4f5/2 for metallic Au,58 implying that certain amount of Au complexes was already reduced to form Au metal nanoparticles at this stage. In fact, the XANES spectrum (Fig. 5A (a)) and EXAFS oscillation (Fig. 5B (a)) of the Au-V2+-Si(111) electrode were similar to those of Au foil, rather than those of NaAuCl4 pellet, e.g., a peak, so-called white line, observed at 11917 eV in the XANES spectrum of NaAuCl4 pellet (top curve, Fig. 5A) was absent in that of the Au-V2+-Si(111) electrode (Fig. 5A (a)). This clearly shows the reduction of Au complexes to Au metal nanoparticles during the metal insertion, since the white line is corresponding to the electronic transition from the 2p3/2 core level to unoccupied 5d states.59-61 The ACS Paragon Plus Environment

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formation of Au-Au bond was also observed in the Fourier transform (Fig. 5C (a)). Another pair of peaks observed at 85.4 and 89 eV in the XPS spectrum (green curve, Fig. 3 (b)) were 1.2 eV higher than those for Au metal but much lower than those for NaAuCl4 used as a metal source.58 This component was probably due to the adsorption of Cl species on the surface of the Au nanoparticles. Similar peak shift was reported for Au films after the exposure to chlorine gas, due to the formation of AuCl (I) or AuCl-like species.58 Thus, in the present case, Au nanoparticles terminated by Cl species were formed onto the molecular layers by just soaking the V2+-Si(111) surfaces in an aqueous solution containing NaAuCl4. In contrast to the XPS spectrum, Au-Cl bond was hardly detected in the EXAFS oscillations (Fig. 5B (a)) and its Fourier transform (Fig. 5C (a)), probably because XAFS was obtained as an average over all the atoms within the Au nanoparticles including core, while XPS is sensitive to the surface atoms.62 The coordination number of the Au-Au bonds obtained by curve fitting of the Fourier transform (Fig. 5C (a)) is 7.1 as summarized in Table 2, suggesting the formation of relatively small Au nanoparticles (< 2nm).55-57 After using freshly-prepared Au-V2+-Si(111) electrodes for the HER and CO2 reduction, peaks due to the metallic Au became dominant in the XPS spectra (red and blue curves, Fig. 3 (b)), suggesting the desorption of Cl species from the Au nanoparticles in the early stage of the HER and CO2 reduction. Loss of the terminal Cl species from Au nanoparticles was not detected in the EXAFS oscillations (Fig. 5B (b)) and Fourier transform (Fig. 5C (b)) after the HER. At this stage, the coordination number of the Au-Au bonds became 12 which is the same as bulk Au, suggesting the aggregation of the Au nanoparticles during the reactions. Thus, the actual catalysts for the HER and CO2 reduction are Au metallic nanoparticles at the Au-V2+-Si(111) electrode.


15

The Journal of Physical Chemistry (A)

(B) NaAuCl4

NaAuCl

(a)

(a)

(b)

(b)

Au foil

Au foil

11900 11910 11920 11930 11940 11950

Photon Energy / eV

0

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Au-Cl

NaAuCl4

Fourier Transform

4

k2 χ (k)


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

Page 16 of 35

Au-Au

(a)

Au-Au

(b)

Au-Au

Au foil

0

1

2

3

4

5

6

R / Angstrom

Figure 5. (A) XANES spectra, (B) EXAFS oscillation and (C) Fourier transform at the Au LIII absorption edge of the Au-V2+-Si(111) electrode (a) immediately after the metal incorporation and after keeping the potential at at -0.8 V in (b) the Ar-saturated solution of 0.1 M Na2SO4 for 30 min. Results of a NaAuCl4 pellet dilluted in a fine boron nitride powder and Au foil are also shown as standard spectra.

At a pristine Pt-V2+-Si(111) electrode, a pair of peaks corresponding to the Pt 4f7/2 and 4f5/2 for K2PtCl4 were observed at 73.2 and 76.5 eV in the XPS spectrum (green curve, Fig. 3 (c)).63 Although the position of white line slightly shifted to the higher photon energy, the shape of the XANES spectrum (Fig. 6A (a)) and EXAFS oscillation (Fig. 6B (a)) were almost the same as those of K2PtCl4 pellet measured as a standard, showing that the Pt complexes, i.e., PtCl42-, were successfully incorporated into the molecular layer on the Si(111) electrode without undergoing the reduction or ligand exchange. One may expect that the peaks in the XPS spectrum shifted to lower binding energies and white line intensity decreased after the electrochemical HER and CO2 reduction due to the reduction of the Pt complexes to Pt metal64,65 because the redox potentials of most Pt complexes is much more positive than the potentials where the HER and CO2 reduction take place.41 However, the position of peaks in the XPS spectrum (red curve, Fig. 3 (c)) after the HER was almost the same as that for the pristine Pt-V2+Si(111) electrode (green curve, Fig. 3 (c)) and the white line intensity rather increased in the XANES spectrum (Fig. 6A (b), on contrary to the expectations. These changes can be attributed to the ligand exchange of PtCl42- with oxygenated species such as hydroxyl groups and water molecules during the


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HER.41 Corresponding changes were actually observed in the EXAFS oscillation (Fig. 6B and C (b)), e.g., formation of Pt-O bond was identified but no Pt-Pt bond was detected by curve fitting of the Fourier transform. We have not concluded whether [Pt(H2O)4]2+ or [Pt(OH)4]2- is formed because XAFS cannot distinguish one from the other. One may concern the stability of these complexes. However, at least, the formation of [Pt(H2O)4)]2+ was identified by NMR66, UV/visible spectroscopy67 and EXAFS analyses68-70 and white line intensity increased due to this ligand exchange reaction. Thus, it was confirmed that the Pt complexes incorporated within the viologen layers on the Si(111) electrodes acted as confined molecular catalysts for the HER, as we previously reported.41 In the XPS spectrum (blue curve, Fig. 3 (c)) after the CO2 reduction, the peaks shifted to lower binding energies by ca. 1.2 eV. This implies that the Pt complexes are in more reduced state after the CO2 reduction. However, the peak positions were still too high to be assigned to Pt metal. Another possible explanation is the ligand substitution of the Pt complex with the intermediate species resulted from the CO2 reduction. In fact, white line intensity became rather larger than those of both metal and original metal complexes in the XANES spectrum (Fig. 6A (c)) and no Pt-Pt bond was detected in the EXAFS oscillation (Fig. 6B (c) and Fourier transform (Fig. 6C(c)), suggesting that the formation of Pt nanoparticles were avoided during the CO2 reduction as is the case of the HER. Thus, the incorporated Pt complexes served as molecular catalysts not only for the HER but also for the CO2 reduction and this may be the main reason for the anomalously high CO2 reduction selectivity of the Pt-V2+-Si(111) electrode, unlike metallic Pt electrodes.


17

The Journal of Physical Chemistry (A)

(B) K2PtCl4

Pt-Cl

K2PtCl4

K2PtCl4

Fourier Transform

(a) (b)

(a)

k2 χ (k)


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

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(b)

(c)

(c)

Pt-Cl

(a)

x2 Pt-O

(b)

x2 Pt-O

(c)

x2

Pt foil

Pt foil

Pt-Pt

Pt foil

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Photon Energy / eV

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R / Angstrom

Figure 6. (A) XANES spectra, (B) EXAFS oscillation and (C) Fourier transform at the Pt LIII absorption edge of the Pt-V2+-Si(111) electrode (a) immediately after the metal incorporation and after keeping the potential at -0.8 V in (b) the Ar-saturated and (c) CO2-saturated solutions of 0.1 M Na2SO4 for 30 min. Results of a K2PtCl4 pellet dilluted in a fine boron nitride powder and Pt foil are also shown as standard spectra.

In order to confirm that the Pt complexes incorporated within the molecular layer acted as molecular catalysts without being reduced to metal nanoparticles, in situ XAFS measurements at the Pt LIII absorption edge of the Pt-V2+-Si(111) electrode were carried out in the Ar-saturated 0.1 M Na2SO4 solution at various potentials (Fig. 7). When the Pt-V2+-Si(111) electrode was immersed in 0.1 M Na2SO4 solution, white line intensity increased as potential goes more negative (Fig. 7A), as observed in ex situ XANES spectra of the Pt-V2+-Si(111) electrode measured in air after the HER and CO2 reduction (Fig. 6A). Although the EXAFS oscillation measured in air immediately after the metal incorporation (Fig. 7B (a)) is almost identical to that of the K2PtCl4 pellet, the oscillation period significantly changes as potential goes more negative (Fig. 7B (b-e)). The coordination numbers of Pt-Cl and Pt-O bonds estimated from the curve fittings of Fourier transform clearly shows the substitution of Cl- with oxygenated species (Fig. 8). This potential dependent change of the EXAFS oscillations was also wellACS Paragon Plus Environment

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reproduced by FEFF simulations for various possible structure, [PtCl4], [PtCl3O], [PtCl2O2], [PtCl3O], and [PtO4] as shown in Fig. S4. Thus, it was shown the Pt-Cl bonds were gradually replaced by oxygenated species as potential was made more negative and Pt nanoparticles were not formed even when the potential was made much more negative than the redox potential of Pt complexes, e.g., 0.51 V vs. Ag/AgCl for PtCl42-.

(A)

(B) K2PtCl4

K2PtCl4

(a)

(a) (b) (c)

Fourier Transform

(b) (c)

k2 χ (k)


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


(d)

(d)

(e)

K2PtCl4

Pt-Cl

(a)

Pt-Cl Pt-O

(b)

(d)

x2 x2

(c)

Pt-O

(d)

Pt-O

(e)

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x2 x2 x2

Pt foil

Pt foil

Pt-Pt

Pt foil

11550 11560 11570 11580 11590

Photon Energy / eV

0

2

4

6

8

10

12

k / Angstrom-1

0

1

2

3

4

5

6

R / Angstrom

Figure 7. (A) XANES spectra, (B) EXAFS oscillation and (C) Fourier transform at the Pt LIII absorption edge of the Pt-V2+-Si(111) electrode (a) measured in air immediately after the metal incorporation and in the Ar-saturated aqeuous solution of 0.1 M Na2SO4 at (b) open circuit potential, (c) -0.4 V, (d) -0.6 V, and (e) -0.8 V. Results of a K2PtCl4 pellet dilluted in a fine boron nitride powder and Pt foil are also shown as standard spectra.


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Coordination number


Page 20 of 35

5 4 3

Pt-O

2 1

Pt-Cl

0 in air

OCP

-0.4 V -0.6 V -0.8 V

Figure 8. Coorination numbers of Pt-Cl and Pt-O bonds estimated from curve fitting of Fourier transform at the Pt LIII absorption edge of the Pt-V2+-Si(111) electrode measured in air immediately after the metal incorporation and in the Ar-saturated aqeuous solution of 0.1 M Na2SO4 at various potentials.

Considering the redox potentials of the three metal complexes used as metal sources in the present study, 0.40, 0.51 and 0.78 V vs. Ag/AgCl for PdCl42-, PtCl42- and AuCl4-, respectively, AuCl4- is most susceptible to reduction to metal. Nevertheless, it is surprising that Au nanoparticles were formed on the molecular layer immediately after the immersion of the V2+-Si(111) surfaces in NaAuCl4 solutions, without any electrochemical treatments. Previously, it was proposed that Au nanoclusters were formed from Au complexes via photo-induced multistep reactions in the presence of ethylene glycol as a reductant;71,72 Au(III)Cl4- was first photo-excited, and then reduced to form Au(II)Cl3-. Au cations resulted from the disproportionation of Au(II)Cl3- was further reduced by ethylene glycol to form Au nanoclusters. In the present study, metal insertion was carried out under room light and the viologen moieties of the V2+-Si(111) surfaces or trace amount of contamination in the solutions may serve as a reductant to facilitate the formation of Au nanoparticles. Oxidation states and local structures of Pd at the Pd-V2+-Si(111) electrodes were changed as expected; Pd complexes were inserted into the molecular layers of viologen moieties by ion exchange reaction, and the complexes were reduced to metal nanoparticles during the HER and CO2 reduction. Although


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the Si(111) electrodes were kept at the potentials much more negative than the redox potential of PdCl42for 30 min, a substantial amount of Pd complexes was detected in the XPS after both HER and CO2 reduction (red and blue curves, Fig. 2 (a)). Probably, the migration and aggregation of Pd complexes to form nanoparticles were significantly inhibited by the surrounding molecular layers. In the case of Pt, Pt complexes were also successfully inserted into the molecular layer by ion exchange reaction, but no evidence for the formation of metal nanoparticles, such as peaks assignable to the Pt metal, decrease in the white line intensity, and Fourier peak of the Pt-Pt bonds were observed in the XPS, XANES spectra and EXAFS, respectively. It is speculated that the formation of Pt nanoparticles is inhibited not only by the surrounding molecular layers but also by the formation of relatively stable Pt(OHx)4 species during the HER and CO2 reduction.


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Conclusion We constructed organic molecular layers of viologen moieties on hydrogen-terminated Si(111) surfaces, incorporated metal catalysts within the molecular layers by using Pd, Au and Pt complexes as metal sources and determined active catalytic species for the HER and CO2 reduction by means of XPS and XAFS. Significant enhancements were achieved for the CO2 reduction at the Au-modified Si(111) electrode and for both HER and CO2 reduction at the Pd-modified Si(111) electrode. Interestingly, not only highly efficient HER but also highly selective CO2 reduction in preference to HER was achieved at the Pt-V2+-Si(111) electrode despite the fact that HER is dominant at Pt pure metal electrodes even in CO2-saturated conditions. At the Pd-modified Si(111) electrode, Pd complexes and nanoparticles were considered to be catalytic species for the HER and CO2 reduction. At the Au-modified Si(111) electrode, Au metal nanoparticles were formed during the metal insertion process, and therefore actual catalytic species for the CO2 reduction was Au nanoparticles. At the Pt-modified Si(111) electrode, Pt complexes were successfully incorporated into the molecular layers and acted as confined molecular catalysts for both HER and CO2 reduction without being converted into Pt metal nanoparticles. Thus, we demonstrated that a unique catalytic property can be obtained by our concept, confined molecular catalysts, unlike to its pure metallic state.


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ACKNOWLEDGMENTS The present work was partially supported by World Premier International Research Center Initiative on Materials Nanoarchitectonics (WPI-MANA) and the Development of Environmental Technology using Nanotechnology from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. T.M. acknowledges the Japan Science and Technology Agency, PRESTO, for financial support. Synchrotron radiation experiments were performed as projects approved by the Photon Factory Program Advisory Committee (PAC Nos. 2011G184, 2011G594 and 2013G087).

Supporting Information Available Curve fitting results of the Fourier Transform of the EXAFS oscillations. This material is available free of charge via the Internet at http://pubs.acs.org.


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Various Active Metal Species Incorporated within Molecular Layers on

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