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Early Stages of Electrochemical Oxidation of Cu(111) and Polycrystalline Cu Surfaces Revealed by in situ Raman Spectroscopy Nataraju Bodappa, Min Su, Yu Zhao, Jia-Bo Le, Wei-Min Yang, Petar Radjenovic, Jin-Chao Dong, Jun Cheng, Zhong-Qun Tian, and Jian-Feng Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b04638 • Publication Date (Web): 21 Jul 2019 Downloaded from pubs.acs.org on July 21, 2019
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Journal of the American Chemical Society
Early Stages of Electrochemical Oxidation of Cu(111) and Polycrystalline Cu Surfaces Revealed by in situ Raman Spectroscopy Nataraju Bodappa,† Min Su,† Yu Zhao,† Jia-Bo Le,† Wei-Min Yang,‡ Petar Radjenovic,† Jin-Chao Dong,† Jun Cheng,† Zhong-Qun Tian,† and Jian-Feng Li*,†,‡ †MOE
Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡Department
of Physics, Xiamen University, Xiamen 361005, China
Supporting Information Placeholder ABSTRACT: Investigating the chemical nature of the adsorbed intermediate species on well-defined Cu single crystal substrates is crucial in understanding many electrocatalytic reactions. Herein, we systematically study the early stages of electrochemical oxidation of Cu(111) and polycrystalline Cu surfaces in different pH electrolytes using in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). On Cu(111), for the first time, we identified surface OH species which convert to chemisorbed “O” before forming Cu2O in alkaline (0.01 M KOH) and neutral (0.1 M Na2SO4) electrolytes; while at the Cu(poly) surface, we only detected the presence of surface hydroxide. Whereas, in a strongly acidic solution (0.1 M H2SO4), sulfate replaces the hydroxyl/oxy species. This results improves the understanding of the reaction mechanisms of various electrocatalytic reactions.
From a technology perspective, copper metal (Cu) has received much interest due to its many industrial applications which have implications for improving the economic viability of green energy, and microelectronic technologies. Additionally, Cu and its oxides are well-known as the most active catalytic materials for carbon dioxide (CO2) and nitrate (NO3-) electrochemical reduction reactions.1-5 It was recently stated that the hydroxide ions in the electrolyte (0.1 M KOH) may participate in the early stage of CO2 reduction reaction (CO2RR) mechanism.2 Both CO2 and NO3reduction reactions occur in the early stages of Cu oxidation
(A)
potentials. Moreover, knowledge of different surface intermediate species’ guides the understanding of active sites and the rational design of catalysts.6-9 Therefore, it is important to understand the early stage mechanisms of electrochemical surface oxidation at Cu surfaces. Electrochemical oxidation of Cu(111) have mainly been investigated by conventional electrochemical methods,10-13 in situ microscopic,14-19 and spectroscopic20,21 techniques. Notably, using these techniques, several ordered adlayers such as hydroxyl,15-17,22 “oxy like”23 and sulfate/bisulfate14,24 were proposed to be on the Cu(111) surface in different pH electrolytes. However, identifying its exact chemical nature and conversion processes at different electrode potentials remains elusive. Since single crystal substrates act as model catalysts with atomic-level precision, therefore, they are attractive surfaces for studying the intermediates and reaction mechanism of catalytic reactions. Surface-enhanced Raman spectroscopy (SERS) is a powerful tool capable of identifying the vibration fingerprints of the chemical species down to the single molecule level.25,26 However, SERS is limited to use with certain materials under restricted conditions.27,28 These limitations were circumvented by the advanced Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS) technique,29 invented by our group, where ultrathin (~ 2 nm) SiO2 shell coated Au nanoparticles (NPs) are spread on a surface of interest to enhance the Raman signals. Using electrochemical SHINERS, several important catalytic reactions 30-35 were successfully investigated. (B)
50 nm
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log[(E/E0)4] (C)
Au@SiO2 NP
Cu(111)
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Figure 1. (A) Schematics of in situ electrochemical SHINERS on Cu(111) surface. (B) SEM image of the Au@SiO2 NPs on the Cu surface. The inset shows a high-resolution TEM image of the single Au@SiO2 NP. (C) Three-dimensional finite-difference time domain (FDTD) simulations of two SHINs on a Cu substrate. Scale 200 150 nm.
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Figure 2. (A) CVs of Cu(111) in 0.01 M KOH with 0.8 V (black line) and 0.4 V (red line, 4 times enlarged) oxidation limit. = 0.05 V s-1. (B) In situ SHINERS spectra of Cu(111) oxidation in 0.01 M KOH/water. (C) Normalized Raman intensities were plotted together with anodic scan Cu(111) CV. very well with the characteristic A1/C1 & A2/C2 process in Cu(111) CV and is assigned to an adsorbed “O” species. The DFT calculated the vibrational frequency for a Cu-Oad stretch appears at 606 cm-1, which matches closely with the experiments. The corresponding cathodic process (Oad to OH) was also clearly evident (Figure S8B). The DFT calculations also confirm the reaction feasibility (for details, see computational details in the SI). The normalized Raman intensities plotted together with the anodic scan of the CV (Figure 2C) shows that most of the OH converted into adsorbed “O” in 0 to 0.5 V region. At 0.55 V, the adsorbed “O” peak almost diminished completely and start to see various new peaks at 92, 145, 222, 450, 528, and 627 cm-1. These peaks were concluded to be surface Cu2O phase (Figure S9).39,40 In particular, lower frequency bands such as 92 and 144 cm-1 are characteristic to the Cu2O phase. This result provides a new way of understanding early stage Cu(111) oxidation compared to earlier in situ STM studies, where only ordered hydroxyl adlayers were proposed from the onset of HER to Cu2O formation potential.15,23 But our in situ SHINERS results reveal that after the formation of OH, it further converts into the chemisorbed O species starting from 0 V and eventually forms Cu2O at > 0.55 V. Combining in situ SHINERS and CV experiments, the early stages of surface oxidation of Cu(111) can be represented as follows: Cu + H2O Cu-OHsurf + eE 0.55 V (Cu2O)surf
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In this work, for the first time, we systematically investigated the early stages of electrochemical oxidation and reduction processes on Cu(111) with different pH electrolytes ranging from alkaline, neutral to acidic conditions using the in situ SHINERS approach combined with cyclic voltammetry (CV). To complement Cu(111) studies, Cu(poly) surface oxidation was also studied in an alkaline electrolyte. The assignment of the chemical nature of the surface intermediate species was done using isotopic experiments together with DFT calculations. Further, spectroscopic results were correlated to the observed CV features. Figure 1A shows the schematics of the in situ SHINERS approach used to study the electrochemical oxidation/reduction reactions of Cu(111) and Cu(poly) surfaces. Figure 1B shows the SEM image of the Au@SiO2 NPs assembled Cu surface. The inset shows a high-resolution TEM image of a single Au@SiO2 NP. Figure 1C is the result of FDTD simulations showing the NP hotspot regions of high enhancement. A linearly polarized plane wave (638 nm) with an electric field amplitude of 1 V m-1 was illuminated on Au@SiO2 NPs modified Cu(111) substrate. As the electric field distribution shows (Figure 1C), the electromagnetic field intensity was dominated in the NP-Cu(111) substrate junctions. The NP modified substrate was subsequently used in spectroelectrochemical experiments.36 All potentials are presented against the reversible hydrogen electrode. Figure 2A shows CVs of the Cu(111) surface in 0.01 M KOH solution. In the first cycle, we observed a pair of anodic (A1, A2) and cathodic current peaks (C1, C2) at 0.05 V, 0.15 V, and 0.05 V, -0.03 V respectively. These peak positions and their shapes were very similar to the previous reports11,15,38,40 which are characteristic of a Cu(111) surface structure. Additional details on the CV shapes are given in the supporting information (Page S3 and Figures S1S4). Since the SiO2 shell on the Au NP remains stable in a 0.01 M KOH (pH = 12) solution (Figure S5), we carried out the in situ SHINERS study of Cu(111) oxidation in 0.01 M KOH solution (Figure 2B). The spectra show prominent intermediate species bands at 535 and 680 cm-1. These two peaks were sustained even at the hydrogen evolution reaction potential (-0.45 V) (Figure S6). The 680 cm-1 peak is replaced by a peak at 667 cm-1 as the potential steps from -0.45 V to 0.1 V. Both 535 and 680 cm-1 peaks redshifted to 497 and 640 cm-1, respectively, in deuterated water (D2O) electrolyte (Figure S7). Density functional theory (DFT) calculations suggest the 680 cm-1 band belongs to the bending mode of free Cu-OH. 535 cm-1 belongs to a top site OH stretching mode, where OH participates in hydrogen bonding with another OH (Figure S13, Table S2). Therefore, the two peaks belong to the OHsurf adlayer. As the potential steps to 0.05 V, a broad peak can be observed at 570-633 cm-1 which is unshifted in D2O (Figure S7), matching
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Figure 3. (A) CV of Cu(poly) in 0.01 M KOH. = 0.05 V s-1. (B) In situ SHINERS spectra of Cu(poly) surface in 0.01 M KOH/H2O. Normalized Raman intensities of 700 cm-1 peak were plotted for both oxidation and reduction in Figure 3A. Figure 3A shows the CV of Cu(poly) surface in 0.01 M KOH solution. The CV has an oxidation and reduction peak potential at 0.65 and 0.3 V, respectively. The in situ SHINERS spectra of oxidation (Figure 3B) and reduction (Figure S10) shows a welldefined peak at 706 cm-1 and a small peak at 525 cm-1. These two
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bands were red-shifted to 670 cm-1 and 497 cm-1 respectively in the D2O electrolyte (Figure S11). Together with DFT calculations, we confirm that both bands belonged to the surface hydroxyl species (Table S2). The correlation between normalized Raman peak intensity of 706 cm-1 and CV shows that the Cu(poly) surface has a maximum OH coverage at < 0 V (Figure 3A). Notably, we did not observe any conversion of OH to “O” on Cu(poly) surface. The conversion of OH to “O” depends on thermodynamics (stability of OH layer) and kinetics (activation barrier of the OH to O step). Ambient X-ray photoelectron spectroscopic studies reveal that the formation of the OH layer varies significantly by changing the orientation of the Cu surface.42 In particular, Cu(110) forms a mixture of OH and H2O layer after exposing to an ambient condition. In contrast, Cu(111) remains clean and adsorbate free under identical conditions. This suggests that the OH layer is not
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stable on Cu(111) and probably, therefore, we observe OH to “O” conversion. In situ STM studies evident that Cu(poly) surface undergoes surface reconstruction to Cu(100) in alkaline conditions at -0.133 V.43 So, on Cu(poly) surface, (100) exists predominantly compared to (111) makes their difference in the Raman spectra. In CV of Cu(111) in 0.1 M Na2SO4 (pH = 7), we observed a broad anodic peak (A4) at -0.08 V with a corresponding cathodic peak (C4) at -0.15 V before Cu2O formation (Figure 4A). In the in situ SHINERS spectra of a neutral solution, we observed similar peak patterns such as observed in alkaline solution (Figure 4B). But, the Cu-OH peak Stark shifted to lower wavenumber (655 cm1). This change could be due to the strong interaction of the sulfate anion and/or the differences in the surface charge density.41 The normalized intensities of the 570 and 655 cm-1 peaks also match
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Figure 4. CVs of Cu(111) in (A) 0.1 M Na2SO4 (pH = 7) = 0.05 V s-1 and (C) 0.1 M H2SO4 (pH = 0.7); = 0.01 V s-1. In situ SHINERS spectra of Cu(111) in (B) 0.1 M Na2SO4 (pH = 7) and (D) 0.1 M H2SO4 (pH = 0.7). Normalized Raman intensities of intermediate peaks were plotted in Figure 4A & 4C. well with the A4/C4 process observed in the CV demonstrating conversion of OH to “O” species (Figure 4A). Additionally, a Raman band at 436 cm-1 corresponding to a hollow site Cu-O stretch, from the DFT, is present at > 0.05 V. The broadness of OH peak on Cu(111) in alkaline solution compared to Cu(poly) in KOH (Figure 3B), and Cu(111) in 0.1 M Na2SO4 (Figure 4B) solution could be due to the OH- induced Cu(111) surface reconstruction.13 Although we observe a small sulfate desorption peak at 0.0 V in the CV (Figure S12), we did not observe any adsorbed sulfate band in the Raman spectra. From the Pourbaix diagram, it is known that copper oxides are unstable in acidic solution (pH = 1).44 Therefore, in the CVs of Cu(111) in 0.1 M H2SO4, surface oxidation/reduction peak currents were not observed as in alkaline and neutral pH electrolytes (Figure 4C).45 In the in situ SHINERS spectra, a Raman band at 973 cm-1 appeared starting from -0.39 V and its intensity continuously increased during the positive potential excursion until 0.1 V (Figure 4D). The 0.1 M H2SO4 solution spectrum gives two bands at 980 and 1050 cm-1 corresponding to sulfate and bisulfate anions (Figure 4D). Compared with this, the band at 973 cm-1 is ascribed to the adsorbed SO42- vibration.46 The potential-dependent Raman
intensity growth of the 973 cm-1 band matches with the CV features indicates that the A5/C5 features are related to sulfate adsorption/desorption process. This result agrees with proposed species in the in situ STM studies.13 We did not observe any hydroxy species on Cu(111) in 0.1 M H2SO4 solution. However, OH to “Oad” conversion peak currents were evidenced in the CVs of Cu(111) at pH ≥ 4.7, similar to a neutral solution (Figure S12). In summary, we applied in situ SHINERS to study the early stage mechanisms of the electrochemical oxidation of Cu(111) and Cu(poly) surfaces in different pH electrolytes. For the first time, we show the existence of different forms of surface OH species and its conversion to chemisorbed O species in alkaline and neutral solutions before converting into Cu2O. However, on Cu(poly) surface, we only detect the presence of surface hydroxide. Whereas, in a strongly acidic solution, sulfate replaces the surface hydroxyl/oxy species. Our results reveal and provide direct evidence of the chemical nature of the proposed species as well as their existence and conversion at different potentials which corroborates the long-standing speculation during the early stages of Cu(111) oxidation. This is an important step towards
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understanding the involvement of hydroxyl/oxy surface species in many electrocatalytic reaction mechanisms.
ASSOCIATED CONTENT Supporting Information Experimental section, supporting results, and in situ SHINERS spectra for both oxidation and reduction Cu(111), theoretical calculations are presented. The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Corresponding Author
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21802111, 21775127, 21522508, and 21521004), the Fundamental Research Funds for the Central Universities (20720190044), the China Postdoctoral Science Foundation (2017M622059), and the State Key Laboratory of Fine Chemicals (KF1702). N. B. acknowledges the iChEM fellowship from Xiamen University.
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