photoemission spectroscopy Royce K. Lam, Jacob …...Reversed interfacial fractionation of carbonate...

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Reversed interfacial fractionation of carbonate and bicarbonate evidenced by X-ray photoemission spectroscopy Royce K. Lam, Jacob W. Smith, Anthony M. Rizzuto, Osman Karslıoğlu, Hendrik Bluhm, and Richard J. Saykally Citation: The Journal of Chemical Physics 146, 094703 (2017); doi: 10.1063/1.4977046 View online: http://dx.doi.org/10.1063/1.4977046 View Table of Contents: http://aip.scitation.org/toc/jcp/146/9 Published by the American Institute of Physics

Transcript of photoemission spectroscopy Royce K. Lam, Jacob …...Reversed interfacial fractionation of carbonate...

Page 1: photoemission spectroscopy Royce K. Lam, Jacob …...Reversed interfacial fractionation of carbonate and bicarbonate evidenced by X-ray photoemission spectroscopy Royce K. Lam, Jacob

Reversed interfacial fractionation of carbonate and bicarbonate evidenced by X-rayphotoemission spectroscopyRoyce K. Lam, Jacob W. Smith, Anthony M. Rizzuto, Osman Karslıoğlu, Hendrik Bluhm, and Richard J.Saykally

Citation: The Journal of Chemical Physics 146, 094703 (2017); doi: 10.1063/1.4977046View online: http://dx.doi.org/10.1063/1.4977046View Table of Contents: http://aip.scitation.org/toc/jcp/146/9Published by the American Institute of Physics

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THE JOURNAL OF CHEMICAL PHYSICS 146, 094703 (2017)

Reversed interfacial fractionation of carbonate and bicarbonateevidenced by X-ray photoemission spectroscopy

Royce K. Lam,1,2 Jacob W. Smith,1,2 Anthony M. Rizzuto,1,2 Osman Karslıoglu,2

Hendrik Bluhm,2,3 and Richard J. Saykally 1,2,a)1Department of Chemistry, University of California, Berkeley, California 94720, USA2Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA3Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

(Received 19 December 2016; accepted 8 February 2017; published online 7 March 2017)

The fractionation of ions at liquid interfaces and its effects on the interfacial structure are of vitalimportance in many scientific fields. Of particular interest is the aqueous carbonate system, whichgoverns both the terrestrial carbon cycle and physiological respiration systems. We have investigatedthe relative fractionation of carbonate, bicarbonate, and carbonic acid at the liquid/vapor interfacefinding that both carbonate (CO2

3 ) and carbonic acid (H2CO3) are present in higher concentrationsthan bicarbonate (HCO

3) in the interfacial region. While the interfacial enhancement of a neutral acidrelative to a charged ion is expected, the enhancement of doubly charged, strongly hydrated carbonateanion over the singly charged, less strongly hydrated bicarbonate ion is surprising. As vibrational sumfrequency generation experiments have concluded that both carbonate and bicarbonate anions arelargely excluded from the air/water interface, the present results suggest that there exists a significantaccumulation of carbonate below the depletion region outside of the area probed by sum frequencygeneration. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4977046]

I. INTRODUCTION

The aqueous carbonate system is of central importancein nature and has been studied extensively. The speciesinvolved—carbon dioxide, carbonic acid, bicarbonate, andcarbonate—are involved in both the global carbon cycle andin physiological buffer and respiration systems. Oceanic car-bonate chemistry governs CO2 uptake by surface waters, andthe subsequent saturation of carbonate has a significant role inbiomineralization in marine organisms and in ecosystems.1,2

In mammalian systems, the carbonate buffer system regulatesblood pH and is responsible for CO2 transport across mem-branes.3 Clearly, understanding aqueous carbonate chemistryis also central to efforts involved in mitigating the effects ofclimate change (e.g., carbon capture, sequestration, etc.).4–6

Consequently, much effort has addressed the aqueous carbon-ate system, dating back over a century7 with past experimentalstudies characterizing the kinetic, thermodynamic, and struc-tural properties of these species in aqueous solution and in icematrixes.8–19 Recent theoretical studies, employing molecu-lar dynamics and ab initio quantum calculations, have soughtto characterize the thermodynamic and mechanistic details oftheir hydration and chemical reactions.20–27

The nature of the ions and their effects on the water struc-ture at the air/water interface critically influence the uptakeof atmospheric gases such as carbon dioxide, which is subse-quently hydrolyzed to bicarbonate and carbonate. The aque-ous bicarbonate and carbonate ions present at the air/waterinterface have previously been examined by vibrational sum

a)Author to whom correspondence should be addressed. Electronic mail:[email protected]

frequency generation (VSFG), finding that the carbonate ionhas a significant effect on the orientation and structuring ofinterfacial water.28–30 These VSFG studies probe the interfa-cial water structure, focusing on the –OH stretching region ofthe vibrational spectrum, and are thereby an indirect probe ofthe ion. In contrast, X-ray photoemission spectroscopy (XPS)is an atom-specific probe of a system’s occupied states, whichenables both system composition and depth profiling. Depthprofiling is achieved through the exploitation of the effectiveattenuation length (EAL) of the emitted photoelectrons, viz.,photoelectrons with energies near 200 eV selectively probe theinterface to depths of∼20 Å, whereas those with higher kineticenergies typically have longer EALs and thereby probe deeperbelow the surface (e.g., ∼60 Å at 800 eV).31 Additionally, themeasured binding energies characterize the oxidation state ofthe probed atoms. The development of ambient pressure pho-toemission spectroscopy by Siegbahn et al. opened the fieldto the study of liquid interfaces32 either in the presence of theequilibrium vapor pressure33 or under vacuum conditions, asin the liquid microjet photoemission spectroscopy experimentsfirst performed by Faubel et al.,34 and has since been appliedto the study of a wide range of liquid systems.35–39 The spatialdistribution of K+ and CO2

3 in an aqueous solution was previ-ously investigated using XPS by Brown et al., who concludedthat the cation resides slightly closer to the interface than theanion.40

Here, we present a study of sodium carbonate (Na2CO3),sodium bicarbonate (NaHCO3), and carbonic acid (H2CO3)via liquid microjet XPS. We have previously explored theaqueous carbonate system using liquid microjet X-ray absorp-tion spectroscopy (XAS), a complementary technique whichprobes the unoccupied states of a system; this enabled the

0021-9606/2017/146(9)/094703/6/$30.00 146, 094703-1 Published by AIP Publishing.

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detailed characterization of the hydration structure of Na2CO3,NaHCO3, H2CO3, and dissolved CO2.41–43 Building on thoseresults, the measurement of XPS spectra at different incidentphoton energies, ranging from 490 eV to 1090 eV, has permit-ted the characterization of the surface fractionation of the indi-vidual ions. Investigating 50:50 mixtures of Na2CO3:NaHCO3

and H2CO3:NaHCO3 reveals that both Na2CO3 and H2CO3

are present in greater concentrations than that of NaHCO3 inthe probed interfacial region.

II. EXPERIMENTALA. Samples

Solutions were prepared using 18.2 MΩ·cm resis-tivity water obtained from a Millipore purification sys-tem. Concentrated HCl (12.1M) was obtained from Baker.Na2CO3 (>99.5% purity) was obtained from Fisher Chem-ical. NaHCO3 (>99.7% purity) was obtained from MacronFine Chemicals.

The solutions probed by XPS are detailed in Table I.The 0.5M H2CO3 solution (III) was generated in situ

using our fast-flow liquid microjet mixing system by mixingsolutions of 1M HCl and 1M NaHCO3. This mixing systemwas previously employed in XAS measurements of aqueousH2CO3 and dissolved CO2.42,43 Briefly, a dual syringe pumpsystem (Teledyne-ISCO 260D) drives two solutions througha Microvolume Y-connector. The mixed solution then trav-els through a 50 µm inner diameter fused silica capillary togenerate the liquid microjet. In this scheme, the interactiontime between the two solutions is ∼0.5 ms, facilitating theobservation of short-lived species in solution.

The 50:50 mixtures of Na2CO3:NaHCO3 (IV) were gen-erated by mixing 1M Na2CO3 with 1M NaHCO3, yielding a0.5M concentration of both carbonate and bicarbonate. 50:50H2CO3:NaHCO3 (V) was generated by mixing 1M NaHCO3

with 0.5M HCl yielding 0.25M concentrations of both species.To generate mixtures of 50:50 H2CO3:NaHCO3 with the sameconcentration Na2CO3:NaHCO3 as the Na2CO3:NaHCO3

solution (0.5M of each ion), a 2M solution of NaHCO3

would have been required. This exceeds the solubility limitof NaHCO3 in water (96 g/L, 1.14M). All mixing was donein situ within the fast-flow liquid microjet mixing system.

B. Experimental design

Carbon 1s and the corresponding valence band X-ray pho-toemission spectra were measured at Beamline 11.0.244 at theAdvanced Light Source (ALS), Lawrence Berkeley NationalLaboratory (LBNL) using the Ambient Pressure Photoemmi-sion Spectrometer (APPES-II) endstation, which is based ona NAP Phoibos 150 hemispherical analyzer (Specs Surface

TABLE I. Aqueous carbonate solutions probed by XPS.

I 0.5M Na2CO3

II 0.5M NaHCO3

III 0.5M H2CO3 (1M NaHCO3 + 1M HCl)IV 50:50 Na2CO3:NaHCO3 (1M Na2CO3 + 1M NaHCO3)V 50:50 H2CO3:NaHCO3 (1M NaHCO3 + 0.5M HCl)

Nano Analysis GmbH, Berlin). The sample was introducedvia a liquid microjet, into the vacuum chamber, operating at apressure of ∼10 mTorr. The liquid jet is orientated normal toboth the electron optical axis of the input lens of the electronspectrometer and the incident X-ray beam. The carbon 1s pho-toemission spectra of aqueous Na2CO3 (I), NaHCO3 (II), andH2CO3 (III) and 50:50 mixtures of Na2CO3:NaHCO3 (IV)and H2CO3:NaHCO3 (V) were measured using incident pho-ton energies of 490 eV, 690 eV, 890 eV, and 1090 eV, resultingin photoelectrons with ∼200, 400, 600, and 800 eV kineticenergy, respectively. To account for shifts induced by charg-ing of the liquid jet and the detector work function, measuredbinding energies were aligned to the liquid water 1b1 bindingenergy at 6.5 eV.45

III. RESULTS AND DISCUSSION

The C(1s) and their respective valence band photoemis-sion spectra, measured with an incident photon energy of490 eV, are shown in Figure 1 for 0.5M solutions of sodium car-bonate, sodium bicarbonate, and carbonic acid (I-III). A largesystematic shift to higher binding energies is observed betweenthe various carbonate species, from carbonate to carbonicacid. Measured binding energies are Na2CO3 (289.1 eV),NaHCO3 (290.1 eV), and H2CO3 (291.0 eV) with correspond-ing FWHMs of 1.1 eV, 1.1 eV, and 1.3 eV, respectively. Thespectra shown in Figure 1 are background-corrected using alinear baseline subtraction but are otherwise unnormalized.As the photoemission cross sections are not expected to differsignificantly between the various carbonate species, the drastic

FIG. 1. X-ray photoemission spectra with an incident photon energy of490 eV. (a) Measured C(1s) binding energies for 0.5M solutions of Na2CO3(solution I, blue), NaHCO3 (solution II, red), and H2CO3 (solution III, black).The low intensity peak centered at 292.8 eV corresponds to gas phase CO2.(b) Measured valence band photoemission spectra for the respective solu-tions. The energy axes, for all measurements, were aligned relative to water1b1 feature, located at 6.5 eV (dashed purple line).

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difference observed in the intensity between species is strik-ing. The spectra of both sodium carbonate and carbonic acidexhibit higher intensity than that of the sodium bicarbonate. Toaccount for differences in alignment between the liquid jet, theincident X-ray beam, and the detector, the spectra were scaled,relative to the valence band intensity of sodium carbonate. Themeasured signal intensity in the valence band spectra primarilycorresponds to that of the water valence band photoemission,which is largely unchanged upon the addition of 0.5M solute.The scaled spectra, which also exhibit this anomalous differ-ence in intensity, are shown in Figure 2. Relative to that ofsodium bicarbonate, the signals originating from carbonateand carbonic acid solutions are ∼1.5 and ∼10.2 times larger,respectively.

The measured carbonate and bicarbonate spectra were fitto single Gaussians while the carbonic acid spectrum was fitto two Gaussian peaks due to the presence of a small peakat ∼292.8 eV corresponding to a gas phase CO2 backgroundpresent in the chamber resulting from the decomposition ofH2CO3 to form CO2 and H2O. The fitted peaks, shown inFigure 3, reproduce the measured spectra well and were usedto deconvolute the spectra of Na2CO3:NaHCO3 (IV) andH2CO3:NaHCO3 (V) mixtures.

Figures 4 and 5 show the measured photoemissionspectra (black and green markers) for 50:50 mixtures ofNa2CO3:NaHCO3 (IV) and H2CO3:NaHCO3 (V) at incidentphoton energies of 490 eV, 690 eV, 890 eV, and 1090 eV.The differences in intensity that were observed in the singlecomponent solutions are clearly maintained in these mixtures,although at slightly different ratios. The measured photoe-mission spectra of the mixtures were fit by fixing the widthand center obtained from the Gaussian fits of the single com-ponent solutions shown in Figure 3 and varying only theamplitudes of the Gaussians. For the spectra measured at ener-gies higher than 490 eV, the relative centers of the Gaussiansbetween species were maintained. The fitted spectra are shownin the solid black and solid green lines in Figures 4 and 5,respectively, exhibit excellent agreement with the measuredspectra. The individual components of the fit are shown inblue (Na2CO3), red (NaHCO3), black (H2CO3), and purple(CO2 gas).

FIG. 2. Scaled C(1s) X-ray photoemission spectra for 0.5M solutions ofNa2CO3 (solution I, blue), NaHCO3 (solution II, red), and H2CO3 (solu-tion III, black). Spectra were scaled relative to the valence band intensityof Na2CO3 to account for intensity differences originating from the liquidjet alignment. Applied scaling factors are Na2CO3 (1.0), NaHCO3 (1.1), andH2CO3 (1.25).

FIG. 3. Gaussian fits for the measured C(1s) X-ray photoemission spectra forNa2CO3 (solution I, top), NaHCO3 (solution II, middle), and H2CO3 (solu-tion III, bottom) with an incident photon energy of 490 eV. The experimentalmeasurement is represented by the markers (+), and the solid blue, red, andblack lines correspond to the respective fit curves for Na2CO3, NaHCO3, andH2CO3. The individual Gaussian peaks are represented by the offset solidpurple lines. Measured binding energies are Na2CO3 (289.1 eV), NaHCO3(290.1 eV), and H2CO3 (291.0 eV) with corresponding FWHMs of 1.1 eV,1.1 eV, and 1.3 eV, respectively.

The peak area ratios for 50:50 mixtures ofNa2CO3:NaHCO3 (IV) and H2CO3:NaHCO3 (V) are plot-ted as a function of electron kinetic energy (eKE) inFigure 6. As the electron kinetic energy increases, the peak arearatio between the probed species approaches a limit of unity,as would be expected for an equimolar mixture. However,our results indicate that concentrations of both Na2CO3 andH2CO3 are significantly enhanced relative to that of NaHCO3

throughout the probed region. While the enhancement of a neu-tral acid (H2CO3) over a charged ion (HCO

3) is not surprising,and has previously been observed for protonated acetic acid,46

the significant enhancement of CO23 over HCO

3 appearsto conflict with recent models for interfacial ion adsorption.

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FIG. 4. Measured X-ray photoemission spectra of 50:50Na2CO3:NaHCO3 mixtures (solution IV) at 490 eV,690 eV, 890 eV, and 1090 eV incident photon ener-gies, corresponding to electron kinetic energies (eKE) of200 eV, 400 eV, 600 eV, and 800 eV, respectively. Themeasured spectra were fit with two Gaussian peaks withthe same parameters (width, center) as those measuredfor the pure components (Fig. 3). Peak areas representthe absolute area in the measured spectrum. The decreasein absolute signal as the photon energy is increased is aresult of the reduction in photoemission cross section.

Historically, it had been assumed that all ions were repelledfrom the air/water interface. This repulsion was originallyexplained by classical electrostatic theory47 and supported bysurface tension measurements which showed that the surfacetension increased as a function of salt concentration.48,49 Morerecently, molecular dynamics simulations,50,51 surface specificsecond-order nonlinear optical experiments,52–54 and XPSmeasurements33,39 have predicted and observed the enhance-ment of certain simple ions at the air/water interface. Thesemodels have shown that weakly hydrated, charge-diffuse ionsare generally enhanced at the air/water interface.55–57 Our cur-rent experiments suggest that carbonate, a strongly hydrateddoubly charged anion, is present in higher concentrations thanis the singly charged, less strongly hydrated, bicarbonate anionin the probed region. This can be rationalized if carbonateadsorbs to the air/water interface as an ion pair with sodium(Na+:CO2

3 ). Adsorption to the air/water interface as a contaction pair has previously been observed, in both experiment andtheory, for aqueous solutions of strong acids.58–60 In previousVSFG measurements, carbonate was shown to exert a muchlarger effect on the interfacial water than does bicarbonate.28,29

More recently, Allen et al. employed phase sensitive VSFG

measurements, finding that bicarbonate is accommodated bythe interfacial region while carbonate is excluded.30 In theseexperiments, the presence of aqueous carbonate was found toreorient the surface waters so that the water hydrogens pointdownward into the bulk.

While these VSFG results appear to conflict with thepresent XPS measurements, they are not necessarily irreconcil-able. Second order nonlinear experiments probing the air/waterinterface are sensitive only to regions of broken inversionsymmetry. While the exact thickness of the probed interfa-cial region is not quantified, the penetration depth of the probeis approximately half the input wavelength. In aqueous sys-tems containing simple ions, the probe depth is likely less than1 nm. However, in systems with a more complex depth pro-file a greater depth may be accessible.61 Theoretical modelstypically define the interface in terms of the Gibbs divid-ing surface, wherein the solvent density reaches half of thebulk density. In either case, these measurements are certainlymore surface specific than our low-energy XPS measurements,wherein the effective attenuation length of an electron in wateris at a minimum (∼2 nm) when the electron has a kinetic en-ergy of ∼200 eV. As such, even with 200 eV photoelectrons, a

FIG. 5. Measured X-ray photoemission spectra of 50:50H2CO3:NaHCO3 mixtures (solution V) at 490 eV, 690 eV,890 eV, and 1090 eV incident photon energies, correspond-ing to electron kinetic energies (eKE) of 200 eV, 400 eV,600 eV, and 800 eV, respectively. The measured spectrawere fit with two Gaussian peaks with the same parameters(width, center) as those measured for the pure compo-nents (Fig. 3). Peak areas represent the absolute area inthe measured spectrum. The decrease in absolute signal asthe photon energy is increased is a result of the reductionin photoemission cross section.

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FIG. 6. Peak area ratios vs. eKE for 50:50 Na2CO3:NaHCO3 mixtures (solu-tion IV) and 50:50 H2CO3:NaHCO3 mixtures (solution V). Correspondingapproximate electron attenuation length values adapted from Ref. 31.

significant proportion of the signal arises from photoelectronsgenerated 2–5 nm below the surface. At 800 eV, the EAL is∼6 nm. Although the EAL is expected to vary slightly with sys-tem composition, we do not expect these variations to affectour interpretation as the EAL should always increase as a func-tion of eKE from 200 eV to 800 eV. Our results would thereforeindicate that there exists a significant accumulation of CO2

3below the depletion region outside of the region probed by theSFG measurements.

IV. CONCLUSIONS

We have presented the X-ray photoemission spectra ofaqueous solutions of Na2CO3, NaHCO3, and H2CO3 whichexhibit a systematic shift to higher measured C(1s) bindingenergies from carbonate to carbonic acid. The measured spec-tra of 50:50 mixtures indicate that both carbonate and carbonicacid are present at higher concentrations in the probed regionthan bicarbonate. Further theoretical modeling is required toaddress the apparent conflict with current models describingion adsorption to aqueous interfaces, which would suggest thatthe singly charged anion, bicarbonate, should be present inhigher concentrations relative to the doubly charged carbonateanion. This new result could reflect interesting and importantdifferences in the hydration and counterion interactions (i.e.,ion pairing) of the carbonate species.

ACKNOWLEDGMENTS

The authors thank the staff at the Advanced Light Sourcefor excellent experimental support. This work was supportedby the Director, Office of Science, Office of Basic EnergySciences, Division of Chemical Sciences, Geosciences, andBiosciences and Materials Sciences Division of the U.S.Department of Energy at the Lawrence Berkeley NationalLaboratory under Contract No. DE-AC02-05CH11231. X-rayphotoemission spectra were collected at Beamline 11.0.2 atthe Advanced Light Source. The data presented are availableupon request to [email protected].

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