Speciation of Adsorbed Phosphate at Gold Electrodes: A Combined

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Speciation of Adsorbed Phosphate at Gold Electrodes: A Combined Surface-Enhanced Infrared Absorption Spectroscopy and DFT Study Momo Yaguchi,†,‡ Taro Uchida,§ Kenta Motobayashi,† and Masatoshi Osawa*,† †

Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan § Center for Energy and Environmental Science, Shinshu University, Nagano 390-8621, Japan ‡

S Supporting Information *

ABSTRACT: Despite the significance of phosphate buffer solutions in (bio)electrochemistry, detailed adsorption properties of phosphate anions at metal surfaces remain poorly understood. Herein, phosphate adsorption at quasi-Au(111) surfaces prepared by a chemical deposition technique has been systematically investigated over a wide range of pH by surface-enhanced infrared absorption spectroscopy in the ATR configuration (ATRSEIRAS). Two different pH-dependent states of adsorbed phosphate are spectroscopically detected. Together with DFT calculations, the present study reveals that pKa for adsorbed phosphate species at the interface is much lower than that for phosphate species in the bulk solution; the dominant phosphate anion, H2PO4− at 2 < pH < 7 or HPO42− at 7 < pH < 12, undergoes deprotonation upon adsorption and transforms into the adsorbed HPO4 or PO4, respectively. This study leads to a conclusion different than earlier spectroscopic studies have reached, highlighting the capability of the ATR-SEIRAS technique at electrified metal− solution interfaces.

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mass transport between the thin-layer and the reservoir, negative absorption bands of the species in the bulk solution are superposed on positive absorption bands of the adsorbed species,39,40 and obscure spectral features of adsorbates. Although solution background can be subtracted using a spectrum acquired with s-polarization,39,40 this process can cause artifacts in practice when the adsorbate bands and the solution bands are highly overlapped and the adsorbate bands are much weaker than the solution bands. Moreover, for electrode processes involving proton transfer, the limited mass transport alters pH in the thin-layer, which is another problem associated with the IRAS technique. Surface-enhanced Raman scattering (SERS) is also a powerful technique and has higher surface specificity than IRAS, but the electrode applicable is practically limited to coinage metals. In addition, roughening metal surfaces by repeated oxidation−reduction cycles, required for achieving desired enhancement effects, complicates the system. It should also be noted that earlier IRAS30,32−34,36,37 and SERS35 studies on phosphate adsorption had interpreted spectral information based on symmetry considerations, which is useful but not convincing. Indeed, mono-,35 bi-,36 and tridentate34 binding fashions of the adsorbed PO4 were proposed from nearly identical spectra. Herein, we have studied the adsorption of phosphate species at Au electrodes over a wide range of pH by surface-enhanced infrared absorption spectroscopy in an attenuated total

ontrolling pH is essential in facilitating electrocatalysis involving proton-coupled electron transfer.1−5 Phosphate buffer solutions are available over a wide range of pH and are the most prevalent electrolytes employed in practical applications (phosphoric acid6,7 or microbial8 fuel cells and biosensors 9,10 ) and fundamental researches in (bio)electrochemistry. Recent studies, however, have demonstrated that specific adsorption of phosphate anions at metal surfaces impacts electrocatalysis11−14 and adsorption or redox behaviors of proteins.15,16 Efforts have been made to rationalize the phosphate adsorption at electrochemical interfaces via various approaches such as conventional electrochemical techniques,17−23 radiotracer method,24 scanning tunneling microscopy,25,26 and spectroelectrochemical techniques.27−37 Nevertheless, the consensus on the geometry and state of adsorbed phosphate species (HnPO4, n = 0−3) has not been reached yet, partly because of the complexity arising from the polyprotic nature of phosphate species (eqs 1−3)38 H3PO4 ⇌ H 2PO4 − + H+

H 2PO4 − ⇌ HPO4 2 − + H+ HPO4 2 − ⇌ PO4 3 − + H+

pK a1 = 2.16

pK a2 = 7.21 pK a3 = 12.32

(1) (2) (3)

Owing to high molecular specificity, infrared reflection absorption spectroscopy (IRAS) has been widely employed in monitoring phosphate adsorption on poly-28−30 and singlecrystal31−34,36,37 electrodes assembled to thin-layer cells. However, since adsorption of a species onto the electrode reduces its concentration in the thin-layer due to the limited © 2016 American Chemical Society

Received: June 16, 2016 Accepted: July 25, 2016 Published: July 25, 2016 3097

DOI: 10.1021/acs.jpclett.6b01342 J. Phys. Chem. Lett. 2016, 7, 3097−3102

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Figure 1. SEIRA spectra of phosphate species adsorbed on an Au thin-film electrode recorded during a potential sweep from 0.2 to 1.7 (black) and back to 0.2 V (blue) vs RHE at 10 mV s−1 in 0.1 M phosphate buffer electrolytes with pH of (a) 5.1 and (b) 10.4. Spectra were offset for clarity. Vibrational frequency (black) and integrated intensity (red) of each band observed in the SEIRA spectra in the forward sweep are shown as a function of applied potential for pH (c) 5.1 and (d) 10.4.

reflection mode (ATR-SEIRAS)41−43 and DFT calculations. ATR-SEIRAS enables probing and characterizing submonolayer adsorbates without being interfered by the bulk solution. Only the molecular vibrations that have a transition dipole moment perpendicular to the electrode surface are active in SEIRAS.44 The surface selection rule allows us to elucidate the orientation of adsorbed species. Gold serves as an ideal model not only due to its wide double layer region and weak interactions with OH species but also in relevance to (bio)electrochemistry. We find that one or two bands appear in the spectral range between 950 and 1150 cm−1, where P−O stretching vibrations of phosphate species are expected, and that their intensity and frequency are dependent on pH and applied potential. Corroborated by the DFT calculations, these bands are ascribed to either HPO4 or PO4 species bound via three oxygen atoms on Au surfaces. The advantages of ATR-SEIRAS for the studies at electrode− electrolyte interfaces are explicitly described. Experimental details of SEIRAS measurements are described in Supporting Information (SI). The working electrode used was an Au thin-film chemically deposited on a Si ATR prism by a “double-deposition” method: An as-prepared Au thin-film by a chemical deposition technique45 was dissolved with aqua regia, followed by a second Au chemical deposition45 to obtain a film of ca. 75 nm-thickness (AFM images of the film at each step are shown in Figure S1). The double-deposition method greatly restrains the Au film delamination from the Si substrate (Cai, W.-B., Personal Communication, Fudan University, China, 2014), which is crucial in SEIRAS measurements under alkaline conditions. The cyclic voltammograms (CVs) of the Au thin-film recorded in 0.1 M H2SO4 resembled that of the Au(111) single-crystal surfaces (Figure S2), suggesting that (111) facets were preferably exposed. Although moderately

(111) orientated SEIRAS-active Au thin-films have been prepared by physical vapor deposition,46,47 chemical deposition methods have some advantages over physical deposition methods including convenience and cost-effectiveness.45 SEIRA spectra of the Au thin-film electrode were acquired sequentially at 0.78 s intervals during a potential cycle from 0.2 to 1.7 and back to 0.2 V vs the reversible hydrogen electrode (RHE) at 10 mV s−1, while the electrolyte solution, a 0.1 M phosphate buffer solution with a pH value in between 2 and 12, was agitated by Ar bubbling to avoid or minimize the change in local pH at the interface. A single-beam spectrum collected at 0.2 V, where no phosphate adsorption occurs,35 was used as the reference for all measurements. The 3D plots of the results revealed that the spectral features and band intensity of adsorbed phosphate species are strongly dependent on pH and applied potential (Figure S3). For detailed analysis, selected SEIRA spectra at pH 5.1 and pH 10.4 in the spectral range of 900−1200 cm−1 are rearranged in Figure 1a and b, respectively. The spectral range below 900 cm−1 is not accessible due to the strong absorption by the Si prism. The SEIRA spectra bear no resemblance to the normal IR spectra of the bulk phosphate solutions at the corresponding pH values (Figure S4 and Table S1), confirming that the spectral features were solely arising from the adsorbed phosphate species and free from the solution background. At pH 5.1 where H2PO4− predominantly exists in the bulk solution, two upward bands emerge at 985 and 1092 cm−1 beginning at 0.5 V and grow as the potential is made more positive up to 1.2 V with a blue shift at a rate of 18−20 cm−1 V−1 (Figure 1c). The band intensity declines drastically at higher potentials where Au oxides are formed (see the CVs in Figure S5 collected simultaneously with the spectra). After 3098

DOI: 10.1021/acs.jpclett.6b01342 J. Phys. Chem. Lett. 2016, 7, 3097−3102

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Figure 2. (a) Vibrational frequencies of the band at various pH values plotted against applied potential on the SHE scale during the forward sweep. The data at the potentials where Au surfaces are oxidized were omitted for clarity. (b) Speciation of phosphate anions in the bulk solution against pH. (c) Effect of pH on the band intensity for phosphate I (1120 cm−1) and for phosphate II (1080 cm−1) at 1.0 V vs RHE.

The bands observed between 900 and 1200 cm−1 correspond to the P−O stretching modes, but the effect of H/D exchange is still observable for H2PO4− and HPO42− as was evaluated in Figure S6. The SEIRA spectra of an Au thin-film electrode collected at 1.1 V (RHE) in 0.1 M phosphate buffer solutions prepared with H2O or D2O at pH 10 and pH 5 are shown in Figure 3a and b, respectively. The background spectrum was

surface oxides are reduced, spectral features in the reverse sweep trace those in the forward sweep, indicating the reversible adsorption/desorption kinetics of phosphate anions at Au surfaces. Such reversible kinetics is also observed at pH 10.4 where HPO42− is dominant in the bulk solution, but the spectral features are significantly different. At 0.5 V, two upward bands appear at 1007 and 1050 cm−1. As the potential is made more positive up to 1.2 V, the former disappears and the latter grows with a blue-shift at a rate of 78 cm−1 V−1 (Figure 1d). The spectral data obtained at different pH values are compiled in Figure 2. Figure 2a plots the peak frequency against the applied potential on the standard hydrogen electrode (SHE) scale. The linear relationships are found with different slopes of ∼20 and ∼80 cm−1 V−1, indicating that the state and/or geometry of adsorbed phosphate species are different at pH < 7 and pH > 10. We tentatively term these adsorbed phosphate species as phosphate I and II, respectively. The band intensities of phosphate I (1000−1012 cm−1) and phosphate II (1040−1100 cm−1) at 1.0 V (RHE) in the reverse sweep are plotted as a function of pH in Figure 2c. The band intensities at 6 < pH < 10, where the spectra of phosphate I and II are overlapped, were obtained by the spectral deconvolution using a Gaussian-Lorenzian mixed function. Phosphate I increases at 3 < pH < 6 and diminishes at pH ∼ 8, whereas phosphate II augments at pH > 5 and decreases at pH > 10, most likely due to the competitive adsorption of OH−. The intensity profiles can be linked with the acid−base equilibrium of phosphate anions in the bulk solution (H2PO4− ⇌ HPO42− + H+, Figure 2b) and suggest the acid−base equilibrium of the adsorbed phosphate species with a pKa ∼ 7 (the pH at which the intensity is one-half of the maximum). However, phosphate I and II are not necessarily to be H2PO4− and HPO42−, respectively, as for sulfuric acid anion adsorption on Au and Pt electrodes: Sulfate is preferentially adsorbed on the electrodes from the bisulfate dominant acidic solution.40,48−50 Similarly, adsorption-induced deprotonation or preferential adsorption of deprotonated ions has been suggested for phosphate anions in earlier electrochemical studies.20−23 To identify the adsorbed phosphate species, the effect of H2O/D2O exchange on the spectral features was examined.

Figure 3. SEIRA spectra of adsorbed phosphate species on an Au thinfilm electrode at 1.1 V (RHE) in 0.1 M phosphate buffer electrolyte at pH (a) 10 and (b) 5 prepared with H2O (black) or D2O (red). The background was recorded at 0.2 V. The bleach at 1192 cm−1 is due to the removal of interfacial heavy water molecules induced by phosphate adsorption.

taken at 0.2 V for both conditions. The slight red-shift from 1091 to 1088 cm−1 observed at pH 10 is smaller than the spectral resolution used (4 cm−1), indicating that virtually no hydrogen is involved. On the other hand, at pH 5, a noticeable blue shift from 1117 to 1124 cm−1 is found for the higher peak, in line with the shift observed for HPO42− in the bulk solution (Figure S6b). Consequently, phosphate I and II are attributed to the adsorbed HPO4 and PO4, respectively, where the charges of the adsorbed ions are unknown and omitted for the sake of simplicity. 3099

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Figure 4. Computed IR spectra of phosphate species adsorbed on Au(111) surfaces in the absence of electric field (F = 0) for (a) PO4 and (b) HPO4, and the effect of the electric field on the vibrational frequency of each band in the corresponding spectrum for (c) PO4 and (d) HPO4.

S1), the observed weak band is speculated to be the asymmetric P−O* vibration of the PO4 activated by a tilted orientation. The frequency of the ν(P−O) mode is highly sensitive to the applied electric field as shown in Figure 4c (the Stark effect53,54). The calculated Stark tuning rate of 34.9 cm−1 (V nm−1)−1, which is converted to 116 cm−1 V−1 assuming the double-layer thickness of 0.3 nm,55,56 is in line with the experimental value of 78 cm−1 V−1 under alkaline conditions. For the adsorbed HPO4, three spectral features at 913, 1006, and 1077 cm−1 are attributed to the symmetric P−O*3 stretching, νs(P−O*3), the OH bending mode, δ(OH), and δ(OH) coupled with the asymmetric P−O*3 stretching, δ(OH) + νas(P−O*3), respectively. The δ(OH) band is hardly detected in experiments because it is broadened by inhomogeneous hydrogen bonding with water molecules surrounding (Table S1).57 Including the effect of hydrogen bonding appropriately in DFT simulations is still challenging even for phosphate anions in the bulk solution.57,58 Despite that, the DFT calculations reasonably reproduce the SEIRA spectra obtained at pH 5 including the weak potential dependence of peak positions (Figure 4d) and the weaker band intensity than that at pH 10 (Figures 4a and b). The blue-shift of the ν(P−O) vibration observed upon H/D exchange (Figure 3b) was also simulated by adding a water molecule hydrogen bonded to the OH (OD) moiety (Figure S9). Combining the insights from the above, the adsorbed phosphate at Au surfaces is rationally identified as (1) HPO4 in H2PO4− dominant acidic solutions and (2) PO4 in HPO42− dominant alkaline solutions, implying that deprotonation is vital upon adsorption. This finding is different from those obtained in earlier IRAS studies.34,36 The present SEIRAS study observes a band between 1073 and 1094 cm−1 that exhibits a Stark shift of ∼80 cm−1 V−1 at 7 < pH < 12.5, in good agreement with a SERS study.35 IRAS studies on single- and poly crystalline Au electrodes also observed a band at similar frequencies with a Stark shift of 85 cm−1 V−1 at pH 12.6. However, at pH 9−10, the band was observed at much higher frequency (1100−1120 cm−1).34,36 Because the dominant

To support the band assignments and determine the geometric preference of adsorbed phosphate species, harmonic vibrational frequencies of PO4 and HPO4 adsorbed on Au19 clusters were calculated by DFT implemented in Gaussian 09, Revision E.0151 in the presence of electric field (F) applied perpendicular to the surface (full calculation details in SI). The (111) surface was selected for the calculation because the SEIRAS-active Au thin-film electrode used for the experiments was preferably (111) oriented and phosphate adsorption on Au surfaces is insensitive to the surface crystallography.34,36 Geometry optimization revealed that the binding via three O atoms at fcc sites (Figure S7) is the most favorable for both PO4 and HPO4. Calculated IR spectra of the PO4 and HPO4 tridentately coordinated to Au(111) surfaces without electric field (F = 0) are shown in Figure 4a and b, respectively, where the Lorenzian function with fwhm of 10 cm−1 were assumed for all the bands. The surface selection rule was incorporated into the calculation by its mathematical expression of I ∝ I°cos2θ (where I° is the calculated intensity and θ is the tilt angle of the transition dipole moment from the surface normal).52 The enhancement of the band intensity by the SEIRA effect was not taken into account in the calculation because it does not affect the relative band intensities. For the adsorbed PO4, two spectral features at 843 and 1038 cm−1 are attributed to the symmetric P−O(coordinated) stretching, νs(P−O*3), and the P−O(uncoordinated) stretching, ν(P−O), respectively, where O* denotes the oxygen atom coordinated to the Au surfaces. However, the νs(P−O*3) mode at 843 cm−1 cannot be experimentally detected because it is below the cutoff of the Si window. Other P−O stretching modes (Figure S8) have transition dipole moments parallel to the surface and thus are not observable due to the surface selection rule. The calculated IR spectrum for the adsorbed PO4 is in good agreement with the SEIRA spectra obtained under alkaline conditions, except for the weak band at 1007 cm−1 which was only observed at low potentials. Taking into account that the calculated harmonic frequencies of PO4 are lower by 50−100 cm−1 than the experimental values (Table 3100

DOI: 10.1021/acs.jpclett.6b01342 J. Phys. Chem. Lett. 2016, 7, 3097−3102

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Society for the Promotion of Science (JSPS) KAKENHI Grant Number 24550143, MEXT Project of Integrated Research on Chemical Synthesis, and the New Energy and Industrial Technology Development Organization (NEDO).

phosphate anions in the bulk solution at pH 12 and pH 9−10 are PO43− and HPO42−, the bands observed were ascribed to the adsorbed PO4 and HPO4, respectively. The discrepancy in the spectral features between SEIRAS and IRAS at pH 9−10 can be explained in terms of the overlay of solution bands. At pH > pKa3 (= 12.32), the vibrational frequency of PO43− in the bulk solution (1004 cm−1) is located far below of the adsorbed PO4 (1073−1094 cm−1) and hence the misleading superposition does not occur, whereas, at pH 9−10, the band of adsorbed PO4 is strongly perturbed by the negative absorption band of HPO42− in the bulk solution (1077 cm−1), resulting in the appearance of the ghost peak at 1100−1120 cm−1. The same issues account for the controversial IRAS results at other metal surfaces30,32,33,37 and re-examinations are highly desirable. In conclusion, we have developed a simple chemical deposition method to obtain a very stable and moderately (111) oriented SEIRAS-active Au thin-film on a Si prism. With this Au thin-film, ATR-SEIRAS was applied to study phosphate anion adsorption at Au surfaces over a wide range of pH. Spectral features including their potential dependences at pH < 7 and pH > 8 are largely different and only the spectrum at pH < 7 displays the isotopic effect upon H2O/D2O exchange, suggesting that there are two different adsorbates depending on the dominant phosphate anion in the bulk solution. The DFTcomputed vibrational frequencies for the HPO4 and PO4 adsorbed on Au(111) surfaces via three oxygen atoms are consistent with the SEIRAS results obtained at pH < 7 and pH > 8, respectively. These results demonstrate that the acid−base equilibrium of adsorbed phosphate species, (HPO4)ads ⇌ (PO4)ads + H+, is significantly shifted to lower pH compared to that of phosphate anions in the bulk solution. The capability of ATR-SEIRAS is explicitly emphasized in providing detailed insights at electrochemical interfaces.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01342. Full experimental and calculation details, AFM images, additional SEIRA spectra and CVs, standard IR spectra, optimized geometry for adsorbed PO4 on Au19 clusters, series of the vibrational modes of adsorbed HPO4 and PO4 on Au(111) surfaces, DFT-simulated IR spectrum of the HPO4 in D2O, and comparison between simulation and experimental data for vibrational frequencies of phosphate species. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

(K.M.) Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Professor Wen-Bin Cai for helpful advice in preparing more stable SEIRAS-active Au thin-film electrodes. We thank Can Liu for helping the AFM measurements. This research was supported by Japanese 3101

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DOI: 10.1021/acs.jpclett.6b01342 J. Phys. Chem. Lett. 2016, 7, 3097−3102