Tunable Electric Field Enhancement and Redox Chemistry on TiO2

Article pubs.acs.org/JPCC

Tunable Electric Field Enhancement and Redox Chemistry on TiO2 Island Films via Covalent Attachment to Ag or Au Nanostructures Arumugam Sivanesan,†,‡,∥ Khoa H. Ly,†,∥ Witold Adamkiewicz,‡ Konstanze Stiba,§ Silke Leimkühler,§ and Inez M. Weidinger*,† †

Institut für Chemie, Technische Universität Berlin, Sekr. PC 14, Straße des 17. Juni 135, D-10623 Berlin, Germany Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland § Institut für Biochemie und Biologie, Universität Potsdam, Karl-Liebknecht Straße 24-25, H. 25, Golm D-14476, Germany ‡

ABSTRACT: Ag−TiO2 and Au−TiO2 hybrid electrodes were designed by covalent attachment of TiO2 nanoparticles to Ag or Au electrodes via an organic linker. The optical and electronic properties of these systems were investigated using the cytochrome b5 (Cyt b5) domain of sulfite oxidase, exclusively attached to the TiO2 surface, as a Raman marker and model redox enzyme. Very strong SERR signals of Cyt b5 were obtained for Agsupported systems due to plasmonic field enhancement of Ag. Timeresolved surface-enhanced resonance Raman spectroscopic measurements yielded a remarkably fast electron transfer kinetic (k = 60 s−1) of Cyt b5 to Ag. A much lower Raman intensity was observed for Au-supported systems with undefined and slow redox behavior. We explain this phenomenon on the basis of the different potential of zero charge of the two metals that largely influence the electronic properties of the TiO2 island film.

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measurements. Sufficient local field enhancement has been almost exclusively observed in the past for nanostructured noble metals like Ag, Au, and Cu that are able to create surface plasmon resonances upon light illumination. More recently, also semiconducting materials including TiO2 have shown to exhibit SER enhancement.13−16 It was proposed that for these materials plasmonic enhancement plays a very minor role and that the SER effect is mostly associated with the chemical enhancement mechanism caused by a charge transfer between a chemisorbed analyte and the substrate. However, the overall Raman enhancement for molecules adsorbed on TiO2 remains orders of magnitude lower than for classical plasmonic active materials such as Ag. To substantially increase the local electric field at the TiO2 surface, addition of plasmonic noble metal nanoparticles is needed. Such plasmonic TiO2−Ag or TiO2−Au hybrid systems have been widely studied because of their strong potential as plasmon-enhanced solar cells.17,18 The choice to use either Ag or Au as optical amplifiers is mainly based on the different plasmonic resonance conditions of Ag and Au. While Au is clearly superior in respect to stability and chemical inertness, its surface plasmon resonance can only be tuned to roughly 510 nm while Ag exhibits plasmonic activity up to the UV region. However, under the influence of an external potential Ag and Au exhibit very different charge properties, most prominently seen by their different potential of zero charge.19,20 Little is known how this will affect the electron

INTRODUCTION Metal oxide electrodes have proven to be a good and low-cost alternative to noble metals in a variety of applications. Especially TiO2 has been established itself as the material of choice for photovoltaic, photocatalytic, bioelectronic, and biomedical devices.1−4 For the latter applications high biocompatibility of TiO2 makes it superior to metal electrodes that are harmful to biomolecules when adsorbed directly on the metal surface. The main drawback of using TiO2 or metal oxide electrodes in general remains their low conductivity. To improve its electrical performance, either the material has to be treated with appropriate dopants3,5 or the device has to be operated at a potential that is more negative than the respective potential of zero charge.6 A promising possibility to tune the electronic properties of TiO2 is the formation of TiO2−noble metal nanocomposites. The presence of Au or Ag nanoparticles has shown to shift the Fermi level of TiO2, which in turn affected its photocatalytic performance.7,8 To better understand charge transfer reactions at the TiO2 interface in the presence of metals, direct spectroscopic monitoring of TiO2 bound molecules during the charge transfer process would be very helpful. Surface-enhanced Raman spectroscopy (SERS) is a technique that yields structural information on surface bound species on a molecular level. It is therefore suitable to analyze interfacial processes also in a time-resolved mode.9−12 However, as the amount of surface bound molecules is normally very low, high electric field enhancement at the interface is necessary to yield a sufficient signal-to-noise ratio, especially for time-resolved Raman © 2013 American Chemical Society

Received: April 2, 2013 Revised: May 16, 2013 Published: May 17, 2013 11866

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Figure 1. SEM pictures of (A) pAu−TiO2 (B) pAg−TiO2, and (D) rAg−TiO2 electrodes. (C) Schematic sketch of the setup including attachment of Cyt b5.

laser beam was focused onto the sample by a Nikon 20× objective with a working distance of 20.5 mm and a numeric aperture of 0.35. Accumulation times of the (SE)RR spectra were between 1 and 10 s for Ag- and Au-supported systems. Time-resolved (TR) SERR experiments were carried out as described previously.21 UV vis spectra were recorded with a Cary 4000 (Agilent). Cyclic voltammetric experiments were performed with a CH Instruments 660C (Austin, TX). Scanning Electron Microscopy. SEM measurements were performed in FEI Nova NanoSEM 450 with an accelerating voltage of 10 kV under high vacuum. Electrode Preparation. Ag or Au electrodes were mechanically polished with 0.5 and 0.3 μm alumina slurry and sonicated in Millipore water for 15 min. The electrochemical roughening of Ag electrodes was performed in 0.1 M KCl by continuous oxidation−reduction cycle. Au electrodes were electrochemically cleaned prior to roughening by performing CVs in 0.1 M H2SO4. Roughening of Au was then achieved by running oxidation−reduction cycles in 0.1 M KCl solution.22 A self-assembled monolayer (SAM) of MUA was formed by immersing Ag or Au electrodes in 2 mM ethanolic solution of MUA for 12 h. Subsequently, the MUAmodified electrodes were soaked in TiO2 dispersed aqueous solution (0.1 g/10 mL) at pH 4.6 for 6 h, followed by the electrodes soaked in 10 mM phosphate buffer solution at either pH 4.6 or pH 7 containing 2 μM cytochrome b5 (Cyt b5). For simplification the polished and roughened Ag−MUA−TiO2

transfer properties of redox probes attached to the TiO2 interface. In this paper we have created nanostructured Ag−TiO2 and Au−TiO2 electrodes via covalent linkage of an organic spacer. To understand how the choice of metal support influences the optical and chemical properties at the TiO2/analyte interface, we investigate the redox chemistry of the protein cytochrome b5 exclusively adsorbed on the TiO2 surface using a combined approach of electrochemical methods and time-resolved surface-enhanced resonance Raman (SERR) spectroscopy.

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MATERIALS AND METHODS Chemicals. 11-Mercaptoundecanoic acid (MUA) and TiO2 nanoparticles were purchased from Sigma-Aldrich. All solutions were prepared with Millipore water with a resistance >18 MΩ. Spectroelectrochemistry. SERR and electrochemical measurements were performed using custom-made spectroelectrochemical cells with a volume of about 10 mL, a rotating Ag or Au ring as the working electrode, an Ag/AgCl (3 M KCl) reference electrode (+0.21 V vs SHE, World Precision Instruments), and a platinum wire as counter electrode. The SERR spectra were measured using a confocal Raman spectrometer (LabRam HR 800, Jobin Yvon) coupled to a liquid nitrogen cooled CCD detector. The 413 nm laser line of a Coherent Innova 400 krypton CW laser was used for excitation. The laser power on the sample was 1.0 mW. The 11867

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Figure 2. (A) Structure of the Cyt b5 domain showing the carboxyl residues close to the heme. (B) Detail of the Cyt b5 UV−vis spectrum before electrode incubation (trace a) and after incubation in a pAu−TiO2 (b), rAu−TiO2 (c), pAg−TiO2 (d), and rAg−TiO2 (e) electrode. (C) SERR spectra of Cyt b5 attached to rAg−TiO2 at −0.3 V (a) and +0.1 V (b). (D) SERR intensity of the ν4 band of Cyt b5 on Ag- and Au-supported electrodes at different ionic strength and pH.

and Au−MUA−TiO2 electrodes are denoted as pAg−TiO2, rAg−TiO2, pAu−TiO2, and rAu−TiO2, respectively. Expression and Purification of the Cytochrome b5 Domain of Human Sulfite Oxidase. The cytochrome b5 domain of human sulfite oxidase was expressed from vector pKS1 in a heterologous expression system in Escherichia coli and purified by Ni-NTA chromatography as described previously.23

surface. Cytochrome b5 (Cyt b5) is a small heme protein that functions as an electron transfer subunit in human sulfite oxidase. High quality surface-enhanced resonance Raman (SERR) spectra of Cyt b5 have been obtained in the past under violet light excitation that matches the molecular absorbance maximum of the heme cofactor.11,23 As under these conditions solely the vibrational specra of the redox-active heme center are observed, electron transfer reactions can be followed very precisely by potential and time-resolved SERR spectroscopy.10,26 Additionally, Cyt b5 is an ideal candidate to study the redox chemistry with TiO2 surfaces as it exhibits several carboxylic amino acids in close environment of the solvent exposed heme cofactor (see Figure 2A) that can bind to the TiO2 surface in the same way as the carboxyl groups of the MUA layer. Finally, it has been shown that Cyt b5 does not adsorb on Ag-MUA functionalized surfaces23 such that exclusive attachment to the TiO2 overlayer can be guaranteed. Therefore, in a last step the metal−TiO2 electrodes were incubated in a Cyt b5 solution for 2 h under pH 4.6 and 7. The schematic setup of the so-created metal−TiO2/protein complex is shown in Figure 1C. The relative amount of Cyt b5 attached to TiO2 surfaces for polished and roughened Ag(Au)−TiO2 supports was monitored by measuring the UV−vis spectrum of a Cyt b5 solution before and after the solution has been incubated with the respective electrodes (Figure 2B). From the loss in Cyt b5 Soret band intensity, the relative amount of adsorbed Cyt b5 was estimated to increase in the order 1:3:4.5 for pAu−TiO2, rAu− TiO2, and pAg−TiO2 electrodes. Interestingly, no difference in

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RESULTS Polished or roughened Ag or Au electrodes (pAg, rAg, pAu, and rAu) were incubated in 2 mM MUA/ethanol solution overnight, resulting in the formation of a compact MUA SAM with the carboxylic functional groups facing the solution. These metal−MUA electrodes were further incubated in a solution containing TiO2 nanoparticles at pH 4.6. From experimental and theoretical investigations it is known that under acidic conditions preferentially the carboxylate groups will bind to TiO2 via bridging bidentate structures.24,25 Figures 1A,B,D show SEM pictures of the resulting metal−TiO2 nanocomposites using respectively pAu, pAg, and rAg as support. In all cases a randomly nanostructured TiO2 overlayer island film was created on the electrode. The TiO 2 nanostructure looked similar for all supports; however, the TiO2 surface coverage on Au was lower than on Ag under the same preparation conditions. To study the SER intensity and electron transfer properties of redox probes attached to these hybrid electrodes, it is required that analyte adsorption exclusively occurs to the TiO2 11868

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Figure 3. (A) Molar ratio of oxidized (squares) and reduced (circles) Cyt b5 on rAg−TiO2 electrodes for pH 7 (solid) and pH 4 (hollow). The solid lines correspond to a Nernstian fit. (B) Molar ratio of oxidized (squares) and reduced (circles) Cyt b5 on pAu−TiO2 electrodes at pH 4.6.

Figure 4. (A) Cyclic voltammograms of Cyt b5 on rAg−TiO2 at pH 4.6; scan rates: 100, 300, 600, 800, and 1000 mV/s. (B) Cyclic voltammograms of rAg−TiO2 (a) and rAu−TiO2 (b) electrodes in a broader potential range; scan rate: 100 mV/s.

Cyt b5 coverage was found between polished and roughened Ag supported electrodes. In Figure 2C, the resulting SERR spectra of Cyt b5 on a rAg−TiO2 electrode at an external potential of +0.1 and −0.3 V are shown. The spectrum shows the characteristic porphyrine ring vibrations (ν4, ν3, ν2) of the reduced and oxidized heme b cofactor, respectively.11,23 No contributions from non-native species are observed, indicating that the native environment of the heme cofactor has been preserved upon immobilization. This further supports the proposed high biocompatibility of TiO2. The spectra show an extraordinary high signal-to-noise ratio, even higher than observed for Cyt b5 directly adsorbed on Ag electrodes functionalized with aminoundecanethiol (AUT) reported in a previous publication by our group.11 This is surprising as direct adsorption of Cyt b5 on the AUT interface is expected to result in a much smaller distance between the protein and the plasmonic Ag surface. A comparison of the SERR intensity of the most prominent ν4 vibrational band using different electrode architectures and buffer conditions is shown in Figure 2D. The highest intensity was obtained for rAg−TiO2 electrodes where Cyt b5 adsorption was done under acidic conditions (pH 4.6). Less than 50% intensity was achieved under neutral conditions (pH 7). The second highest intensity was observed upon electrostatic adsorption of the protein on rAg−AUT electrodes under very low ionic strength conditions.11 However, in this case a drastic loss in signal intensity was observed when the ionic strength was increased up to 50 mM due to desorption of the protein. In the case of pAg−TiO2

electrodes the Raman signal decreased by a factor of 6 compared to the same measurements done with roughened Ag. Using pAu−TiO2 electrodes, the Raman signal decreased by more than a factor of 60 compared to rAg−TiO2. The difference in Raman signal intensity between pAu−TiO2 and rAu−TiO2 was roughly 1:2. Nevertheless, also for Ausupported systems the signal-to-noise ratio was still good enough to determine the contributions of reduced and oxidized species during a redox titration. The vibrational spectra of the protein when attached to Ag− TiO2 corresponded to a fully reduced and oxidized state at −0.3 and +0.1 V, respectively, demonstrating that an electrical contact between Cyt b5 and the Ag electrode had been established. For both pH conditions SERR spectra of Cyt b5 were acquired as a function of electrode potential. In both cases the protein could be almost completely oxidized and reduced. Relative surface concentrations of reduced and oxidized protein were calculated using component fit analysis.27 As seen in Figure 3A, a sharp redox transition was observed. The data points could be fitted very well by the Nernst equation: xox =

1

(

1 + exp −

nF(E − E 0) RT

)

(1) 0

Here E stands for the applied potential, E for the redox potential, and n for the apparent number of transferred electrons. A value for n = 0.7 was obtained independent of pH, indicating homogeneous adsorption of the proteins on the 11869

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Table 1. Electron Transfer Rates of Cyt b5 on rAg−TiO2 Electrodes

TiO2 surface. However, a pH-dependent shift of the redox potential E0 was observed in the redox titration as E0 was determined to be −0.19 V at pH 7 and −0.14 V at pH 4.6. In both cases the redox potential was distinctly more negative than measured in solution (−0.13 V) and on AUT-functionalized Ag electrodes (−0.12 V). Interestingly, complete different redox behavior was observed when the Ag electrode was replaced by Au. As shown in Figure 3B, a very broad redox transition was observed in this case. The molar fractions of reduced and oxidized species followed an almost linear dependence that could not be fitted reasonably by a Nernst function. Such broad redox transitions are usually observed if the electrical communication between the redox probe and the electrode is not properly established.28 One has to note that using roughened Au electrodes instead of polished ones did not change the overall redox behavior. Electron transfer rates of Cyt b5 on Ag and Au systems were determined individually by cyclic voltammetry and timeresolved SERR spectroscopy following a previously described protocol.29,30 For Ag-supported systems high quality and wellresolved CVs could be obtained (Figure 4A). From the scan rate dependent peak-to-peak positions,31 the electron transfer rate was determined to be 13 s−1. On Au-supported systems no peaks in the potential region close to the protein’s redox potential could be detected at any applied scan rates. However, at more positive potentials two very intense redox peaks at 0.1 and 0.3 V were observed (Figure 4B, trace b). Also, on Agsupported systems redox peaks that are not connected to the protein were observed; however, these peaks occurred at much more negative potentials, namely at −0.4 and −0.5 V. On rAg−TiO2 electrodes the Cyt b5 Raman intensity was good enough to perform time-resolved SERR spectroscopy. In Figure 5, the time dependent molar ratio of the oxidized Cyt b5

kET/s−1 pH 4 pH 7 a

CV

SERRSa

13 n.d.

34 (0.8) 44 (0.5)

For the SERR measurements the second rate is given in parentheses.

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DISCUSSION From detailed investigations of dye-sensitized solar cells it is known that strong covalent binding of a dye’s carboxylic groups to TiO2 is one of the best ways to establish a fast electron transfer between dye and support.33 Cyt b5 exhibits several carboxylic residues close to the heme pocket; hence, we suggest that these residues are responsible for binding to TiO2 in the same way as the carboxyl groups of the MUA layer. This proposed binding mechanism is supported by our observation of a much higher Cyt b5 SERR intensity when adsorbed at pH 4.6 compared to pH 7. Furthermore, the Cyt b5 SERR intensity did not depend on ionic strength (data not shown), which also strongly points to a nonelectrostatic binding mechanism. More than 1 order of magnitude higher Raman signals of Cyt b5 are obtained for pAg−TiO2 electrodes than for pAu−TiO2 while the increase in Cyt b5 coverage on pAg−TiO2, determined from UV−vis measurements, only yields a factor of 4.5. The increase in Raman intensity can therefore only be partially attributed to the different amount of Cyt b5 molecules probed with SERRS in both systems. This effect becomes even more evident when roughened Ag and Au supports are used. While the increase of a factor 2 in Raman intensity from pAu− TiO2 to rAu−TiO2 is even lower than the observed increase in Cyt b5 surface coverage (factor 3), a more than 6 times higher Raman signal is observed in rAg−TiO2 systems compared to pAg−TiO2 at equal high Cyt b5 surface coverage. For Au systems plasmonic field enhancement under violet light excitation can be ruled out; therefore, the observed Raman signal is exclusively a result of the resonance Raman effect combined with a possibly small surface enhancement from the TiO2 support. In contrast, the strong effect of Raman intensity increase upon roughening in Ag-supported systems can only be rationalized if the plasmonic properties of the metal support are responsible for the observed Raman intensity. Polished Ag electrodes should in principle show no plasmonic field enhancement. However, due to the mechanical polishing procedure, some scratches might remain on the surface which results in a small plasmonic enhancement of pAg. In conclusion, the different Raman intensities observed for different metal support materials and structures strongly suggest that the plasmonic properties of Ag are the main contributor to the observed high Raman signals. However, even when Ag as plasmonic amplifier is considered, these intense signals are still surprising if one takes into account that the TiO2 island film is rather thick, and thus the majority of the Cyt b5 molecules are far away from the Ag surface. Comparably high SERR signals were also observed for similar Ag hybrid electrodes were the overlayer consisted of an Au or Pt island film.34,35 In these cases the high surface enhancement at the overlayer film was attributed to a long-range plasmon coupling between the resonant Ag and the nonresonant metal overlayer film.36 Such a plasmon coupling, however, requires first the existence of a plasmonic amplifier (i.e., Ag) and second the

Figure 5. Molar ratio of oxidized Cyt b5 as a function of delay (δ) time after a potential jump: solid squares, pH 7; hollow squares, pH 4.6.

species is shown subsequent to a potential jump from 0 (+50) mV for to the respective redox potentials at pH 4.6 and pH 7. For both pH measurements the data could be fitted assuming a second-order exponential decay. For pH 4.6 (pH 7) this resulted in a faster decay rate of 68 (87) s−1 and a slower decay rate of 1.6 (1.1) s−1. The decay rates were divided by a factor of 2 in order to get the electron transfer rates at zero driving force32 and are listed in Table 1. Interestingly, the fast decay rate measured with SERRS exceeded the rates obtained upon electrostatic adsorption of Cyt b5 measured in ref 11 by almost 1 order of magnitude. 11870

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the protein. However, the peaks appear at a potential close to EPZC of Au and thus may indicate charging and discharging of the TiO2 layer. If this hypothesis is true, similar redox peaks should also appear in Ag-supported systems close to EPZC of Ag. This is indeed observed in the CVs of Ag−TiO2 electrodes at −0.4 and −0.5 V.

existence of free electrons in the conducting band of the overlayer film. The latter is the case for Au or Pt but not for TiO2. However, there have been several reports on the influence of Ag on the electronic properties of TiO2.8,37 It is believed that Ag, due to its larger work function, can inject electrons into the conducting band of TiO2.8 This injection leads to an occupation of the conducting band and as a consequence would allow plasmon resonance induction in the TiO2 by the supporting Ag. Similar to the reported metal hybrid systems, this could explain the high SE(R)R intensities of probes on the TiO2. Additionally a photon-induced charge transfer (PICT) between the heme cofactor and TiO2, as discussed for TiO2/ dye systems,14 might further increase the SERR signal of Cyt b5. Such an effect may also explain why on Au-supported systems, in the absence of a plasmonic amplifier, a decent Raman signal could be observed. It might furthermore explain the faster electron transfer rate obtained from SERRS compared to CV measurements on rAg−TiO2 electrodes. However, with SERRS a biphasic redox kinetic is observed, most likely due to slightly different orientations of the protein on the surface. The rate observed in the CV measurements lies in between the fast and slow rate obtained with SERRS and could very well represent an average value of the redox process. Most surprisingly, a very different redox chemistry of Cyt b5 is observed when the underlying Ag support is changed to Au. While the different Raman intensities of Cyt b5 on Ag- and Ausupported systems can easily be explained by their different plasmonic activity under violet light excitation, the different electronic properties are highly surprising as no obvious reason is given why a proper electrical communication is only established on Ag-supported systems and not on Au. As these differences in conductivity are also observed in CV measurements, light-induced effects can be ruled out as a possible source. Therefore, the main reason has to lie in the different response of the Ag−TiO2 and Au−TiO2 electrodes to the applied potential. It is known that the intrinsic low conductivity of pure TiO2 can be tuned by applying an external potential.38 The potential of zero charge (EPZC) of pure TiO2 lies at −0.3 V. Below this potential an exponential increase of conductivity in nanostructured TiO2 films is observed.6 EPZC of MUA coated Au was determined to be 0.04 V,20 which is more positive than the respective value of TiO2. Covalent linkage of TiO2 to Au NPs has shown to influence the Fermi level of TiO2;7 therefore, a shift in EPZC of TiO2 is expected for TiO2−Au hybrid systems. For MUA-coated Ag electrodes EPZC was determined to be at −0.45 V,30 which is, in contrast to Au, more negative than the respective value of TiO2. EPZC of TiO2 in Ag−TiO2 systems thus will most likely be shifted in the other direction. The redox potential of Cyt b5 was measured to be at −0.14 V (−0.19 V). In this potential range Ag will carry a positive net charge while Au is negatively charged. It seems reasonable that, in order to retain maximum charge neutrality, the electron density in the interfacial TiO2 layer will be increased in the case of Ag and decreased in the case of Au supported systems. As a consequence, a positive shift in EPZ of TiO2 will be induced in Ag-supported systems such that good conductivity is achieved in the investigated potential range around −0.14 V. Such a scenario might also be able to explain the strong anodic and cathodic peaks observed in the CVs of Au−TiO2 electrodes. The peaks occur in a region far away from the redox potential of Cyt b5 and are not related to

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CONCLUSIONS TiO2 island films bound to noble metal supports via a covalent organic linker exhibit very different optical and electronic properties depending on the nature of the metal support. Systems supported by nanostructured Ag created a high lightinduced field enhancement at the TiO2 interface, demonstrated by the extraordinary high SERR intensity of the attached redox enzyme Cyt b5 under violet light excitation. Using nonresonant Au supports instead, the Raman intensity fell off by a factor of 60, indicating that plasmonic coupling of Ag to TiO2 is a crucial reason for the observed high Raman signals. On Ag-supported systems a fast electron transfer between Cyt b5 and the electrode could be established, while Au-supported electrodes showed very slow and undefined redox behavior. We explain this observation by the different charge densities that exist in Ag and Au at the measured potential range. While Ag is positively charged at the proteins redox potential, Au exhibits a negative net charge at this potential. To maintain maximum charge neutrality, the potential of zero charge EPZC for TiO2 is shifted to negative potentials in Au systems and to positive potentials for Ag systems. Therefore, only in the case of Ag redox measurements are performed at potentials below EPZC of TiO2, where a significant increase of conductivity is expected.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (I.M.W.). Author Contributions ∥

A.S. and K.H.L. contributed equally to the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank P. Hildebrandt, A. Fischer, and A. Weidinger for helpful discussions. Financial support from the Fonds der Chemie (I.M.W.) and the DFG (UniCat) is gratefully acknowledged.

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REFERENCES

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dx.doi.org/10.1021/jp4032578 | J. Phys. Chem. C 2013, 117, 11866−11872