STM Characterization of Self-Assembled Monolayers of Cysteine

Jul 19, 2017 - Improving the Spatial Resolution in Direct Laser Writing Lithography by Using a Reversible Cationic Photoinitiator. The Journal of Phys...
0 downloads 11 Views 8MB Size
Download PDF
Article pubs.acs.org/JPCC

STM Characterization of Self-Assembled Monolayers of Cysteine Betaine on Au(111) Electrode in Perchloric and Sulfuric Acids Shuehlin Yau,*,† Chun-Jen Huang,*,‡ and Weicheng Liao† †

Department of Chemistry, ‡Department of Biomedical Sciences and Engineering, National Central University, Jhongli, Taoyuan 320 Taiwan, ROC S Supporting Information *

ABSTRACT: A bioderived zwitterionic molecule, cysteine betaine (Cys-b), can be used as a biomaterial coating to evade fouling and damage by light radiation. In situ scanning tunneling microscopy (STM) has been used to study the structures of the cysteine betaine (Cys-b) molecule adsorbed on an Au(111) electrode in 0.1 M HClO4 and H2SO4. A number of Cys-b structures have been identified in 0.1 M HClO4 before adsorbed Cys-b is irreversibly oxidized, including (4 × 8), (6 × 6), and (√19 × 3√3). By contrast, very different Cys-b structures, including (√7 × 4), which is an incommensurate structure, and disordered structures, are seen in the same potential region in H2SO4. These results are reconciled by a coadsorption scheme involving the Cys-b cation and ClO4− (or HSO4−). The coverages of Cys-b are 1.31 × 1014 and 2.32 × 1014 molecules/cm2 at the same potential in HClO4 and H2SO4. Although Cys-b molecules are tethered to the Au(111) substrate via their S-ends, their spatial structures are influenced greatly by the interactions with the coadsorbed anions. As ClO4− and HSO4− anions are hydrated in the aqueous electrolyte, their hydrated shells can affect their interactions with the Cys-b cation, leading to different ordered structures as seen by STM.

■

INTRODUCTION L-Cysteine (Cys), a natural amino acid, has found applications as a coating for medical devices and biosensors to improve their biocompatibility and chemical functionality.1−3 The adsorption of Cys on ordered surfaces of copper,4,5 silver,6,7 and gold8,9 as a function of pH and ionic strength has been studied with and without electrochemical potential control. The Cys molecule is adsorbed on Au(111) in a well-ordered (3√3 × 6) in pH 4.2 ammonia acetate buffer,10 a (√7 × 4) in 0.1 M HClO4,11 and a local (√3 × √3)R30° structure in pH 3 perchloric acid.12 It is fair to state that the interactions between the adsorbed Cys molecules and their spatial structures are affected by sample preparation and the environment, including pH, ions in the solution, and electrochemical potential. However, to our knowledge, the role of an anion in structuring an electrified interface that uses an amino acid adsorbed on a well-ordered electrode, such as Au(111), has not been addressed thus far. A bioderived zwitterionic molecule, cysteine betaine (Cys-b), is prepared by quaternization of the amino group in the Cys to afford permanent positively charged ammonium.13,14 The Cysb coating can impart a greater resistance to fouling and a greater stability against light radiation than cysteine. The gold nanoparticles coated with Cys-b exhibit high biocompatibility and colloidal stability in complex media and at high temperature. The molecular structure of Cys-b as a function of pH is shown in Scheme 1. Cys-b has an isoelectric point at pH = 4.36, where it is in its zwitterion form . The deprotonations of the −COOH and −SH groups result in © XXXX American Chemical Society

pKa1 and pKa2 of 1.63 and 12.64, respectively. Therefore, Cys-b is predominantly cationic in acidic media. Specficially, 0.1 M HClO4 and H2SO4 were used in this study. Scanning tunneling microscopy (STM) has been used extensively to study the spatial structures of alkanethiol molecules adsorbed on an Au(111) substrate in vacuum and in solution.15−19 While the terminal group of thiol molecules can affect the spatial structures of adsorbed thiol molecules, the effect of an electrolyte on the charged thiol molecules has not been reported. Herein, we employ molecular-resolution STM to show that the electrified interface of the Cys-b-modified Au(111) is structured differently in HClO4 and H2SO4, indicating the important role of the anion to compensate for the positive charge on the Cys-b admolecules. The electrostatic interactions involved in these systems vary with the chemical identities of the anions, leading to different spatial structures.

2. EXPERIMENTAL SECTION 2.1. Preparation of Cys-b. The process for the preparation of Cys-b was described in detail in our previous works.13,14 Briefly, 1 g of cysteine was dissolved in 3 mL of deionized water in an ice bath and in nitrogen protection. KOH solution (8.46 mL, 6.5 M) was first introduced dropwise until the cysteine was dissolved. The residue KOH and dimethyl sulfate (5.15 mL) Received: May 18, 2017 Revised: July 9, 2017 Published: July 19, 2017 A

DOI: 10.1021/acs.jpcc.7b04786 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Scheme 1. Acid−Base Properties of Cys-b Measured by Potentiometric Titrationa

a The pKa values of the −COOH group and the −SH group of Cys-b are pKa1 = 1.63 (A) and pKa2 = 12.64 (C), respectively. The pI value of Cys-b is 4.36 (B).

were dropped in simultaneously for 1 h. The flask was allowed to stand for an additional 20 min at 25 °C, followed by addition of 1.15 mL of glacial acetic acid. White byproduct potassium methyl sulfate was precipitated and filtrated by adding 40 mL of ethanol. The filtrate was concentrated using an evaporator and precipitated by adding 50 mL of acetone. The white product was washed with acetone to produce cysteine betaine. The disulfide bond of cystine betaine was reduced in 0.1 M dithiothreitol (DTT) and stirred at 65 °C for 2 h. Afterward, acetone was added to precipitate the white product of Cys-b. The product was then dried in a vacuum. 2.2. Preparation of Cys-b-Coated Au(111). The Au(111) electrodes used to conduct STM and voltammetric experiments were single crystal beads made out of a polycrystalline Au wire.20,21,21 The conventional annealingand-quenching method was used to pretreat the Au(111) electrode, followed by soaking in a dosing solution made of 3 or 30 μM Cys-b + Millipore water (∼18.2 MΩ). Dosing time and temperature also influenced the extent of Cys-b adsorption. We examined dosing temperatures of 0 and 25 °C, and the results suggested that a lower dosing temperature yielded a longer range of molecular adlayers. The dosing time varied from 0.5 to 10 min, which affected the coverage of Cys-b adsorbed on the Au(111) electrode. STM results show that a 3 min dosing time gave the best ordered adlayer. It is thought that the coverage of Cys-b, which is crucial to the degree of ordering, reached the optimal value. The electrode was mounted onto the STM cell or placed in an electrochemical cell equipped with two Pt wires as a quasi-reference electrode and counter electrode. The potentiostat was a CHI 627 (Austin, TX, U.S.A.). 2.3. In Situ STM. The STM was produced by Veeco (Santa Barbara, CA) and equipped with a high-resolution scanner with a maximal scan size of 500 nm. The tip was a tungsten tip etched by alternating current in 1 M KOH and coated with Apeazon wax for insulating purposes. In order to minimize the interference from the faradaic process to the tunneling current measured at the tip electrode, it is necessary to set the tip potential at ∼0.3 V. As the tip might affect the structure of the adsorbed molecules in close proximity, we refrained from setting the tunneling current higher than 1 nA. The effect of the imaging condition on the stability and resolution of STM imaging can be realized by observing the molecular structures over a prolonged imaging period of 10 min. All STM images presented in this report were acquired with the constantcurrent mode. All potentials reported here are converted to a Ag/AgCl potential scale. Suprapur sulfuric acid (H2SO4) was purchased from Merck (Darmstadt, Germany) and used without further purification.

Triple-distilled water (Lotun Technology Co., Taipei, Taiwan, ROC) was used to prepare all electrolytes.

3. RESULTS 3.1. CVs Obtained with Cys-b-Modified Au(111) in 0.1 M HClO4 and H2SO4. Figure 1a shows the CV recorded at 10

Figure 1. CVs obtained with a Cys-b-modified Au(111) electrode in 0.1 M HClO4 (a) and 0.1 M H2SO4 (b). In panel (a), after a 5 min potential holding at 0.52 V, the potential is scanned at 10 mV/s to 0.2 V. The same potential program is applied in 0.1 M H2SO4, yielding the red trace shown in panel (b). The black trace shows the profile recorded after five potential cycles between 0.2 and 0.8 V.

mV/s with the Cys-b-modified Au(111) electrode in 0.1 M HClO4. The morphology of the CV profile varies notably with the potential-sweeping program. The negative-going scan recorded at 10 mV/s after a 5 min potential holding at 0.52 V results in a sharp peak at 0.46 V (C1) and a small hump at 0.38 V. The immediate positive-going scan from 0.2 to 1.0 V results in two sharp peaks at 0.49 and 0.84 V (A1 and A2), which are countered by weak peaks at 0.7 (C2) and 0.5 V (C1′) in the following negative scan. It is likely that C1′ and C1 peaks stem from the same electrode process, but their peak shapes vary greatly with the potential program. These results suggest that the electrode processes associated with A1/C1 and A2/C2 are slow in the time frame of this CV experiment. These electrode processes are examined by in situ STM imaging, and the results are described below. The rapid increase of the anodic current noted at 0.9 V is attributed to an irreversible oxidation of Cys-b admolecules. In order to elucidate the effect of the electrolyte on the spatial arrangement of Cys-b on Au(111), we follow the same procedure to prepare a Cys-b-modified Au(111) electrode, with CV experiments being conducted in 0.1 M H2SO4. Similar to B

DOI: 10.1021/acs.jpcc.7b04786 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C the potential-sweeping program used in perchloric acid, the potential is first held at 0.52 V for 5 min before it is scanned at 10 mV/s to 0 V, where the direction of potential scan is reversed. The resultant CV, shown as the red trace in Figure 1b, reveals a gradual decrease of current from 0.52 to 0 V. The following positive-going scan results in a broad peak between 0.4 and 0.6 V, which is a mirror image of the negative scan. This result contrasts markedly with the sharp peaks seen in the CV profile recorded in perchloric acid (Figure 1a). This lack of a well-defined feature in the CV profile recorded in H2SO4 suggests a weak interaction between Cys-b and the bisulfate anion. By contrast, the change in the Cys-b structure is more pronounced in perchloric acid, suggesting a greater influence of the perchlorate anion on the organization of the adsorbed Cysb on the Au(111) electrode. These views are made clear by STM results described below. When the potential excursion exceeds 0.7 V in H2SO4, an increase of current is noted, which is associated with irreversible changes in the Cys-b adlayer. This causes notable changes in the morphology of the CV profile, where two broad peaks at 0.30 and 0.45 V are noted in the fifth potential cycles between 0.2 and 1.0 V, as revealed by the black trace in Figure 1b. Ramping the potential negatively at 10 mV/s from 0.45 to −0.45 V, the onset potential for hydrogen evolution, does not yield any well-defined feature that can be associated with the desorption of Cys-b admolecule. This result shown in the Supporting Information (Figure S1) contrasts with that obtained with a typical thiol molecule, such as mercaptoacetic acid (MAA), adsorbed on Au(111),15 where a cathodicstripping peak is seen at −0.25 V. 3.2. In Situ STM Imaging. In situ STM imaging of the Cysb-modified Au(111) electrode yields its surface morphology and the spatial structures of Cys-b molecular adlayer as a function of potential in 0.1 M HClO4 and H2SO4. The CV profile recorded between 0.2 and 1.0 V in perchloric acid is defined as S1−S3 by the peaks seen in Figure 1a. All structures shown below have been seen more than three times. 3.2.1. STM Results Obtained in 0.1 M HClO4. Shown in Figure 2a is a topographic STM image acquired at 0.6 V by using −100 mV bias voltage and 1 nA set point current. Patched ordered arrays mixed with disordered domains are seen. A few pinholes that are 5−12 nm wide and 0.24 nm deep are noted, occupying ∼3% of the scan area. These vacancy defects in the gold substrate mean that a small portion of gold atoms at the electrode are relocated by the deposition of Cys-b molecules. This kind of pinhole has been observed with thiolmodified Au(111) surfaces in vacuum and in solution.15,19,22−24 The Cys-b adlayer is organized as patched arrays on Au(111), whose domain size varies with the potential. Meanwhile, Cys-b molecules are also adsorbed in disarrays, which could be related to coadsorption of gold adatoms released from the vacancy defects. 3.2.1.1. Ordered Cys-b Adlayer in S2. The ordered patches are further examined by a high-resolution STM scan, yielding a molecular resolution STM image shown in Figure 2b, which is filtered by a 2D Fourier transform technique. The unit cell as marked by the white lines is a rhombus, defined by edges aligned in the directions. (The orientation of the Au(111) substrate is determined by atomic-resolution STM imaging.) All edges are measured to be 1.73 nm long, which is 6 times the atomic spacing of gold atoms in the Au(111) plane. These results lead to a symmetric (6 × 6) structure, which has only one rotational domain in the 100 × 100 nm scan, as seen

Figure 2. In situ STM images obtained with a Cys-b-modified Au(111) at 0.6 V in 0.1 M HClO4 with a scan size of 100 × 100 (a) and 5 × 5 nm2. Panels (c−e) show STM images (1 × 1 nm2) of the Cys-b molecules adsorbed at the corners and edges of the (6 × 6) unit cell marked in panel (b).

in Figure 2a. This (6 × 6) structure has never been observed for studies on cysteine, homocysteine, and methionine adsorbed on Au(111) in pH 1−4 media.8,10,11,25,26 All protrusions seen in Figure 2b appear as trios with any two apexes within a trio are separated by 0.28 ± 0.01 nm. Trios at the corners and on the edges of the unit cell are highlighted in Figure 2c−e, showing unlike corrugation patterns. These trio features coincide with the fact that the Cys-b molecule has three different functional groups of −S, −N(CH3)3+, and −COOH, which are arranged in a scalene triangle with two apexes separated by ∼0.28 nm. These structural features match with those seen in the molecular STM images. Meanwhile, similar trio STM patterns are also reported with Cys adsorbed on Au(111) and Au(110) surfaces.11,27 However, it is difficult to associate a particular spot with a functional group, because the intensity of an adsorbed entity in the STM image is determined by not only its chemical nature but also its adsorption site on the substrate. If this trio STM appearance of Cys-b is considered as a footprint on the Au(111) electrode, Cys-b is adsorbed horizontally with the molecular plane that is defined by three functional groups (−S, N(CH3)3+, and − COOH,) all aligned parallel to Au(111) substrate. The functional groups in Cys-b molecules interact with one another, as judged from the regular orientations of trios in this structure, In addition to the prominent Cys-b features, three weaker spots are noted inside the (6 × 6) unit cell. They are lower than those of Cys-b by 0.09 nm, which is too large to be associated with Cys-b adsorbed on surface sites such as 3-fold hollow sites. The different corrugated spots can arise from differences in adsorption sites, for example, iodine atoms adsorbed atop the site and 3-fold sites on Pt(111) are corrugated by 0.06 nm.28For the present Cys-b case, the 0.09 nm difference in corrugation height seems to be too much to stem from differences in adsorption sites. Rather, they can be coadsorbed perchlorate anions. This view is substantiated below. 3.2.1.2. Cys-b Adlayer Formed in S1 and S3. The STM results obtained in the S1 region are described. In particular, we C

DOI: 10.1021/acs.jpcc.7b04786 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

irreversible, as the ordered (6 × 6) Cys-b structure is not restored by changing the potential from 0.85 to 0.6 V. 3.2.2. STM Imaging of Cys-b Adsorbed on Au(111) in 0.1 M H2SO4. Because of the poorly defined morphology of the CV profile recorded with Cys-b-modified Au(111) in H2SO4 (red trace in Figure 1b), the potential window between 0.2 and 1.0 V is not divided into sections. 3.2.2.1. Surface Morphology of the Electrode. We gather four STM snapshots in Figure 4a−d to show the surface

show an STM snapshot recorded at 0.4 V in Figure 3a, revealing patched Cys-b arrays, pits, and disordered domains.

Figure 3. In situ STM images obtained with a Cys-b-modified Au(111) electrode at 0.4 (a,b) and 0.85 V (c,d) in 0.1 M HClO4. The ordered domains marked I, II, and III are noted in panel (a). The Cysb adlayer is mostly disordered with some local structures highlighted in panel (d). Figure 4. In situ STM images collected with a Cys-b-modified Au(111) electrode at 0.2 (a), 0.5 (b), 0.6 → 0.5 (c), and 0.8 V (d) in 0.1 M H2SO4. All images are 100 × 100 nm2 and are collected over the roughly same area in a single STM imaging experiment. Panel (c) is acquired at 0.5 V after the potential is held at 0.6 V for 1 min. New structures are produced, as highlighted by the dotted squares. The Cys-b adlayer appears to be disordered at 0.8 V (d).

Close examination of Figure 3a discloses three ordered domains (I−III). They can be superimposed on one another by an in-plane rotational operation of 60 or 120°, suggesting that they are rotational domains of the same structure. The better ordered domain, II, is examined by the high-resolution STM mode, resulting in the internal molecular arrangement of this adlayer. The unit mesh of this structure is outlined by the dotted rhombus in Figure 3b. The edges of this rhombus are measured to be 1.15 and 2.3 nm long and aligned in the directions of the Au(111) substrate, leading to a (4 × 8) structure. On the positive end of the potential window (the S3 region), in situ STM reveals drastic changes in the Cys-b adlayer, as illustrated by a STM snapshot shown in Figure 3c recorded after the potential is held at 0.85 V for 10 min, a mostly rough surface with some local ordered arrays, whose internal structure is revealed by the high-resolution STM image shown in Figure 3d. The geometry of this structure indicates the (√19 × 3√3) structure. However, this structure is temporary at 0.85 V, as it is transformed into disarray after 1 h of STM imaging. This STM image reveals spots with different corrugation heights, suggesting coadsorption of Cys-b and anions. These STM results suggest consecutive phase transition from (6 × 6) to (√19 × 3√3), and further to a disordered state as the potential is switched from 0.5 to 0.85 V. This view is consistent with the doublet peak shape of the A2 feature in Figure 1a. These structural transitions appeared to be overlapped in potential, making it difficult to be resolved by STM. The oxidation of adsorbed Cys-b commences at 0.9 V, which is

morphology of the Cys-b-modified Au(111) electrode between 0.2 and 0.8 V in 0.1 M H2SO4. Patches of ordered arrays, attributable to adsorbed Cys-b, can be seen between 0.2 and 0.6 V (Figure 4a−c). The average size of an ordered patch is ∼30 nm. Pinholes ∼3 nm wide and 0.24 nm deep are clearly seen at all potentials. They occupy only 1.7% of the entire scan area and are mostly found in the vicinity of disordered domains. While the morphology of the electrode hardly changes with the modulation of potential, the Cys-b adlayer can rearrange with potential, as revealed by the high-resolution STM images shown in Figure 5 and reflected by the broad peak seen between 0.3 and 0.6 V (Figure 1b). Switching the potential from 0.5 to 0.6 V gradually transforms the ordered Cys-b adlayer into a disordered state. The reversibility of this transition is tested by stepping back to 0.5 V after a brief stay (∼60 s) at 0.6 V. An STM image acquired afterward is shown in Figure 4c, where new structures emerge, as highlighted by the dotted rectangles, amid ordered structures that are the same as those present on the electrode. This potential induced-phase transition becomes irreversible upon a prolonged potential holding at E > 0.8 V (Figure 4d). D

DOI: 10.1021/acs.jpcc.7b04786 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 5. In situ STM images collected with a Cys-b-modified Au(111) electrode at 0.2 (a) and 0.5 V (b) in 0.1 M H2SO4. Panel (c) shows the corrugation profiles along the dotted lines marked in panel (b).

Figure 6. In situ molecular-resolution STM images (a−c) acquired at 0.5 V with a Cys-b-modified Au(111) electrode after the potential is stepped to 0.6 V for 1 min in 0.1 M H2SO4. These structures correspond to those highlighted in Figure 4c.

Table 1. Summary of the Spatial Structures of Cys-b Adsorbed on Au(111) in 0.1 M H2SO4 and HClO4 potential

0.2−0.51 V

0.52−0.80 V

0.81−0.83 V

0.1 M HClO4

(4 × 8) 0.2−0.35 V (√7 × 4)

(6 × 6) 0.36−0.55 V incommensurate

disordered + (√19 × 3√3) >0.56 V disordered + local (7 × 9), (√21 × 6), (6 × √37)

0.1 M H2SO4

3.2.2.2. Molecular Structures of Cys-b. The ordered Cys-b structure seen by high-resolution STM imaging at 0.2 V is shown in Figure 5a. Protrusions are aligned in the y-axis of the image, which is rotated from the axis of the Au(111) substrate by 19°, indicating a preferential molecular alignment in the √7 direction. Protrusions aligned along the √7 column are equally bright and separated by 0.763 nm. Spots in two neighboring columns have different STM heights (Δz = 0.07 nm), suggesting that there are two types of Cys-b orientations. Two neighboring columns (center-to-center) are separated by 0.65 nm, which roughly equals the dimension of Cys-b molecule. This structure is characterized as (√7 × 4) with a coverage of 2/12 or 2.32 × 1014 molecules/cm2. Three rotational domains are found in the image shown in Figure 4a. Meanwhile, Cys is also adsorbed in the same structure on Au(111) in 0.1 M perchloric acid.11 The dimension of the adsorbed entity, judging from the different physical sizes of Cys and Cys-b molecules, is not the prime factor in guiding how admolecules are arranged on Au(111). The Cys-b structure seen at 0.5 V is revealed by the molecular-resolution STM image shown in Figure 5b. This adlattice appears to be hexagonal, with molecules aligned in the √7 direction of the Au(111) substrate. (Two close-packed molecular rows enclose a 60 ± 2° angle and nearest neighbor spacing of 0.73 ± 0.02 nm.) This array consists of spots with an irregular pattern of corrugation heights, as revealed by the

cross-section profiles shown in Figure 5c. The relative corrugation heights of protrusions modulate randomly between 0.01 and 0.1 nm, which are reconciled with one kind of admolecules with different orientations. This feature suggests an incommensurate structure, where adsorbates do not have a fixed registry pattern on a substrate. This structure has not been reported for organosulfur molecules adsorbed on Au(111). The intermolecular spacing of 0.73 nm is larger than that (0.65 nm) of (√7 × 4) structure seen at 0.2 V. The structures produced by the brief potential stay at 0.6 V (highlighted in Figure 4c) are substantiated by using the highresolution STM images shown in Figure 6a−c. These structures have distinct pairwise features, surrounded by poorly resolved weaker features (Δz = 0.06 nm). Two spots within a pair are separated by 0.23 ± 0.01 nm (center-to-center), which is too small to be associated with two Cys-b entities adsorbed side-byside. This paired feature resembles those reported with the selfassembled monolayers of octanethiol and bis(4-pyridyl) disulfide adsorbed on Au(111).29,30 It is likely that Cys-b admolecules could dimerize to give di(Cys-b) at 0.6 V. This molecule is tethered to the Au(111) substrate via the −S−S− headgroup. This anchoring group is clearly imaged by the STM, but the remaining portion of the di(Cys-b) entity is obscure under the present imaging conditions. The surrounding weaker features could be coadsorbed HSO4− anions. E

DOI: 10.1021/acs.jpcc.7b04786 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 7. Ball models proposed for the structures of Au(111): (6 × 6), (a); (4 × 8), (b); (√19 × 3√3), (c). Cys-b + ClO4− observed in HClO4.

4. DISCUSSION 4.1. Anionic Effect on the Structures of Cys-b Adsorbed on Au(111). The structures of Cys-b adsorbed on Au(111) in HClO4 and H2SO4 are summarized in Table 1. The very different structures seen in these two media clearly reflect the marked influence of anions (ClO4−, HSO4−, and SO42−) on the spatial arrangement of Cys-b cations on Au(111). The coadsorption scheme involving a cation and anion is well-established in the modern study of interfacial electrochemistry, as exemplified by the underpotential deposition systems, where deposited metal cations are accompanied by anions on Au and Pt electrodes.31,32 Meanwhile, molecular cations such as aniline and pyrrole are coadsorbed with anions of bisulfate, perchlorate, and chloride on the Au(111) electrode.33−37 Similarly, this ionic interaction can hold for organosulfur compounds with terminal groups of −SO3−, −COOH, or cysteine on a gold electrode, where an adsorbate can arrange in different spatial structures in different electrolytes.11,15,16,25 Although the interactions between Cys-b and perchlorate or bisulfate anions are electrostatic, the hydration of these charged particles can affect these interactions and their final spatial arrangements. The hydration phenomenon of charged species is traditionally described by the Hoffmeister’s series,38 which ranks the ability of an anion to organize its water shell. Within the context of this study, we compare bisulfate and perchlorate anions. The ranking of (bi)sulfate is much higher than perchlorate in the Hoffmeister’s series, which may explain the fact that bisulfate anions and water molecules form the (√3 × √7) structure on Au(111), Pt(111), Rh(111), Pd(111), and a distorted (√3 × √7) structure on Cu(111) electrode,39−42 whereas no ordered structure has been observed with perchlorate on metal electrodes thus far. It is known that this hydration difference affects the interaction between organic entities such as proteins, macromolecules, and anions.43 With a more-organized water shell, the (bi)sulfate anion is shielded from the Cys-b cation to a greater extent than perchlorate. This view can hold for Cys in acids, which should yield different structures in different acidic media. However, this issue is overlooked by previous studies.11,25 The extent of this anion effect on the structure of the adsorbed molecule varies with the nature of admolecule. For example, aniline is arranged in the same Au(111)(3 × 2√3) structure in HClO4 and H2SO4.35 4.2. Spatial Structures of the Adsorbed Cys-b Molecules on Au(111). The molecular configurations of thiol molecules adsorbed on Au(111) have been extensively studied. It is shown that the adsorption of thiol molecules can induce restructuring of the Au(111) substrate by relocating gold atoms from the surface to the top of the substrate, yielding

vacancy defects that are one gold atom in depth. These defects can coalesce into pinholes, which have been found consistently on the Au(111) surface.18,19,44−46 The gold adatom-mediated RS-Au-SR motif, initially observed with methanethiol adsorption on Au(111) in vacuum, has gained wide attention and support.47 However, this model is complicated by the chemical structure of the molecule, temperature,48 and surface defects on the gold substrate.49,50 This adatom-mediated motif can occur when thiol molecules are adsorbed on Au(111) from a solution phase. For example, it is applied to account for the adsorption of 1-propanethiol (C3SH) and 1-butanethiol (C4SH) on Au(111).24 The rather low density of the pinhole (∼3%), as compared with the typical 12−20% found for SAMs of methanethiol, leads to a “mining” model, where the vacancy defects remain segregated in the Au(111) substrate. For the present Cys-b/Au(111) system, the pinhole density at ∼3% is comparable to those reported by C3SH and C4SH. Alternatively, the ordered Cys-b structures summarized in Table 1 are very different from the (3 × 2√3) and (7 × √3) structures reported for C3SH and C4SH. Meanwhile, disordered Cys-b domains and pinholes always coexist simultaneously. Thus, rather than adopting the RS-Au-SR motif to account for the adlayer of Cys-b, we propose that gold adatoms and Cys-b are coadsorbed in the disordered domains, which coexist with ordered Cys-b + ClO4− structures. Because of the lack of an ordered pattern, it is difficult to substantiate the gold adatommediated motif in the present Cys-b/Au(111) system. 4.3. Ball Models for the Cys-b Structures Formed in 0.1 M HClO4. Aided by the STM results described above, we propose a coadsorption scheme involving Cys-b and an anion to account for the fact that there are two kinds of spots with a 0.09 nm difference in corrugation height in the (6 × 6) structure (Figure 2b). Although the perchlorate anion interacts weakly with the bare gold electrode, the adsorbed Cys-b molecules enhance its adsorption on the gold electrode. The separation of ∼0.5 nm between two weaker spots in the (6 × 6) structure greatly exceeds the 0.237 nm spacing between the two oxygen atoms in the ClO4− structure,51 implying three perchlorate anions are adsorbed in a unit cell. These STM results lead to coverages of 3/36 or 0.0833 for Cys-b and ClO4−. This is only one-half of that (0.167) reported with Cys adsorbed on Au(111)(√7 × 4) and (3√3 × 6) in perchloric acid and pH 4.6 ammonia acetate buffer.11,27 This low coverage of Cys-b mostlty derives from the coadsorption of ClO4− anions, rather than their different physical sizes. The ball model of the (6 × 6) structure is depicted in Figure 7a. The three Cys-b admolecules (one at the corners and two on the edges) are adsorbed in different orientations to account for their different STM appearances shown in Figure 2c−e. In F

DOI: 10.1021/acs.jpcc.7b04786 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C this model, Cys-b cations and ClO4− are spatially placed in a way to allow electrostatic interactions between their neighboring partners. The in-plane spatial distance between ClO4− and the −N+(CH3)3 entity of a neighboring Cys-b (center-tocenter) is ∼0.47 nm, which is comparable to the distance between N and Cl atoms in the crystal structure of NH4+ClO4−.51 Similarly, a ball model for the (4 × 8) structure is shown in Figure 7b, which shows adsorbed Cys-b cations and perchlorate anions in a ratio of 3:2, resulting in Cys-b and ClO4− coverages of 0.09375 and 0.0625. As the potential is lowered from 0.5 to 0.2 V, the coverage of Cys-b increases by about 13%, and that of ClO4− decreases by 25%, implying the loss of some adsorbed ClO4− anions to give a more-compact Cys-b adlayer. This restructuring event corresponds to the A1/C1 peaks seen in the CV (Figure 1a). The local structure of (√19 × 3√3) seen at 0.85 V (Figure 3d) is reconciled with a ball model depicted in Figure 7c. Each unit cell contains 2 Cys-b and 4 ClO4−, translating to coverages of 0.0833 and 0.166. Thus, shifting the potential from 0.6 to 0.8 V populates the interface with ClO4−, triggering reorientation and rearrangement of the Cys-b adlayer. The formation of multiple adlattices of Cys-b on Au(111) in perchloric acid is unique for thiol molecules adsorbed at the electrified interface of Au(111). For example, the spatial arrangement of cysteine and homocysteine adsorbed on Au(111), before they are desorbed and oxidized, is unchanged with respect to the electrode potential.25,52 This contrast is also manifested in the CV profiles, where multiple peaks are seen with Cys-b, and essentially featureless CV profiles are reported for cysteine and homocysteine. This comparison reflects the complicated intermolecular interactions and charged species at electrified interfaces, which both contribute to guide the spatial structures. 4.4. Ball Models for the Cys-b Structures Formed in 0.1 M H2SO4. Figure 8a shows a ball model proposed for the

anion, the coverage of bisulfate anion in this structure cannot be determined. Tentatively, bisulfate anions can be placed in the vacant sites in this Cys-b adlayer, which yields a coverage of 0.0833. Switching the potential from 0.2 to 0.5 V results in an incommensurate structure (Figure 5b), which is slightly different from the (√7 × 4)Cys-b structure. If judged from the increase of intermolecular spacing from 0.65 to 0.73 nm, the Cys-b adlayer becomes more loosely packed with more positive potential. This restructuring event can result from either a lateral shift of anchoring sites or reorientations of Cys-b admolecules to accommodate the increase of positive charges at the interface. In Figure 8b, we show a ball model for one of the disulfide structures of Cys-b produced by a brief polarization at 0.6 V. We arbitrarily choose the one highlighted in Figure 6c, (6 × √37)di(Cys-b) + HSO4− structures. It is likely that this molecule is adsorbed on Au(111) via its −S−S− ends, as shown in the inset of Figure 8b. The weak spots seen in between admolecules are associated with bisulfate anions, which could interact with the −COOH groups of their neighboring admolecules.

5. CONCLUSION In situ STM coupled with voltammetry has yielded a molecular view of the electrified interface of Cys-b adsorbed on Au(111) in 0.1 M HClO4 and 0.1 M H2SO4. Although adsorbed Cys-b molecules are tethered at the Au(111) substrate via their Sterminals, their functional groups can rearrange to accommodate the perchlorate anions as the potential is modulated, leading to several distinctively different spatial structures, as revealed by molecular-resolution STM imaging. These microscopic events are manifested by yielding sharp peaks in the CV profile recorded in 0.1 M HClO4. By contrast, the spatial structure of Cys-b adsorbed on Au(111) does not change appreciably in 0.1 M H2SO4, as reflected by the nearly featureless CV profile. Although the interactions between the Cys-b cation and ClO4− and HSO4− anions are largely electrostatic, the strength of interactions are varied by the different hydration extents of these anions. The bisulfate anion could have a more-organized hydration shell, which weakens its interaction with the Cys-b cation, as compared with ClO4−. The strong interaction between Cys-b and ClO4− leads to notable restructuring of the Cys-b ionic adlayer on Au(111) as the potential is changed in 0.1 M HClO4.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04786. Cathodic-stripping voltammogram recorded with the Cys-b-modified Au(111) in 0.1 M H2SO4 (PDF)

Figure 8. Ball models proposed for the structures of Au(111): (√7 × 4), Cys-b + HSO4−, (a) and (6 × √37), di(Cys-b) + HSO4−, (b) structures observed in H2SO4. The inset in panel (b) shows the side view of the di(Cys-b) molecule.

■

Au(111)(√7 × 4) − Cys-b structure, observed at 0.2 V in 0.1 M H2SO4. The parallelogram unit cell contains two Cys-b cations with different orientations, which accounts for the fact that there are two types of spots with different corrugation heights in the STM image (Figure 5a). It is possible that Cys-b molecules belonging to two neighboring columns can pair up to enable intermolecular H-bonds via their −COOH groups. The coverage of Cys-b is 2/24 or 0.167. Because the obtained STM results do not unambiguously discern the adsorbed bisulfate

AUTHOR INFORMATION

Corresponding Authors

*S.Y.: E-mail: [email protected] *C.-J.H.: E-mail: [email protected] ORCID

Shuehlin Yau: 0000-0001-8268-9574 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.jpcc.7b04786 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

■

Propanesulfonic Acid and Bis(3-Sulfopropyl)-Disulfide on a Au(111) Surface. Langmuir 2010, 26 (16), 13263−13271. (18) Poirier, G. E.; Pylant, E. D. The Self-Assembly Mechanism of Alkanethiols on Au(111). Science 1996, 272 (5265), 1145−1148. (19) Poirier, G. E. Characterization of Organosulfur Molecular Monolayers on Au(111) Using Scanning Tunneling Microscopy. Chem. Rev. 1997, 97 (4), 1117−1128. (20) Itaya, K. In Situ Scanning Tunneling Microscopy in Electrolyte Solutions. Prog. Surf. Sci. 1998, 58 (3), 121−247. (21) Hamelin, A. The Crystallographic Orientation of Gold Surfaces at the Gold-Aqueous Solution Interphases. J. Electroanal. Chem. Interfacial Electrochem. 1982, 142 (1), 299−316. (22) Petri, M.; Kolb, D. M.; Memmert, U.; Meyer, H. Adsorption of Mercaptopropionic Acid onto Au(1 1 1): Part I. Adlayer Formation, Structure and Electrochemistry. Electrochim. Acta 2003, 49 (1), 175− 182. (23) Hagenström, H.; Schneeweiss, M. A.; Kolb, D. M. Modification of a Au(111) Electrode with Ethanethiol. 1. Adlayer Structure and Electrochemistry. Langmuir 1999, 15 (7), 2435−2443. (24) Wang, Y.; Chi, Q.; Hush, N. S.; Reimers, J. R.; Zhang, J.; Ulstrup, J. Gold Mining by Alkanethiol Radicals: Vacancies and Pits in the Self-Assembled Monolayers of 1-Propanethiol and 1-Butanethiol on Au(111). J. Phys. Chem. C 2011, 115 (21), 10630−10639. (25) Zhang, J.; Chi, Q.; Nielsen, J. U.; Friis, E. P.; Andersen, J. E. T.; Ulstrup, J. Two-Dimensional Cysteine and Cystine Cluster Networks on Au(111) Disclosed by Voltammetry and in Situ Scanning Tunneling Microscopy. Langmuir 2000, 16 (18), 7229−7237. (26) Humblot, V.; Tielens, F.; Luque, N. B.; Hampartsoumian, H.; Méthivier, C.; Pradier, C.-M. Characterization of Two-Dimensional Chiral Self-Assemblies L- and D-Methionine on Au(111). Langmuir 2014, 30 (1), 203−212. (27) Zhang, J.; Chi, Q.; Nazmutdinov, R. R.; Zinkicheva, T. T.; Bronshtein, M. D. Submolecular Electronic Mapping of Single Cysteine Molecules by in Situ Scanning Tunneling Imaging. Langmuir 2009, 25 (4), 2232−2240. (28) Sawaguchi, T.; Sato, Y.; Mizutani, F. In Situ Stm Imaging of Individual Molecules in Two-Component Self-Assembled Monolayers of 3-Mercaptopropionic Acid and 1-Decanethiol on Au(111). J. Electroanal. Chem. 2001, 496 (1−2), 50−60. (29) Fenter, P.; Eberhardt, A.; Eisenberger, P. Self-Assembly of NAlkyl Thiols as Disulfides on Au(111). Science 1994, 266 (5188), 1216−1218. (30) Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Direct Observation of 4-Mercaptopyridine and Bis(4-Pyridyl) Disulfide Monolayers on Au(111) in Perchloric Acid Solution Using in Situ Scanning Tunneling Microscopy. Langmuir 1998, 14 (13), 3565−3569. (31) Herrero, E.; Buller, L. J.; Abruña, H. D. Underpotential Deposition at Single Crystal Surfaces of Au, Pt, Ag and Other Materials. Chem. Rev. 2001, 101 (7), 1897−1930. (32) Magnussen, O. M. Ordered Anion Adlayers on Metal Electrode Surfaces. Chem. Rev. 2002, 102 (3), 679−726. (33) Yau, S.; Lee, Y.; Chang, C.; Fan, L.; Yang, Y.; Dow, W.-P. Structures of Aniline and Polyaniline Molecules Adsorbed on Au(111) Electrode: As Probed by in Situ Stm, Ex Situ Xps, and Nexafs. J. Phys. Chem. C 2009, 113 (31), 13758−13764. (34) Lee, Y.; Chang, C.; Yau, S.; Fan, L.; Yang, Y.; Yang, L. O.; Itaya, K. Conformations of Polyaniline Molecules Adsorbed on Au(111) Probed by in Situ Stm and Ex Situ Xps and Nexafs. J. Am. Chem. Soc. 2009, 131 (18), 6468−6474. (35) Lee, Y.; Chen, S.; Tu, H.; Yau, S.; Fan, L.; Yang, Y.; Dow, W.-P. In Situ Stm Revelation of the Adsorption and Polymerization of Aniline on Au(111) Electrode in Perchloric Acid and Benzenesulfonic Acid. Langmuir 2010, 26 (8), 5576−5582. (36) Chen, S.; Yau, S. Self-Organized Electropolymerization of Aniline on Au (111) Electrode in Hydrochloric Acid. J. Electrochem. Soc. 2014, 161 (10), H612−H618. (37) Khairunnisa, A.; Liao, W.; Yau, S. Adsorption and Electrochemical Polymerization of Pyrrole on Au(100) Electrodes as

ACKNOWLEDGMENTS This research was funded by the Ministry of Science and Technology of the ROC (contract number: MOST 106-2113M-008-005).

■

REFERENCES

(1) Liu, W. H.; Choi, H. S.; Zimmer, J. P.; Tanaka, E.; Frangioni, J. V.; Bawendi, M. Compact Cysteine-Coated CdSe (ZnCdS) Quantum Dots for in Vivo Applications. J. Am. Chem. Soc. 2007, 129 (47), 14530−14531. (2) Tengvall, P.; Lundstrom, I.; Liedberg, B. Protein Adsorption Studies on Model Organic Surfaces: An Ellipsometric and Infrared Spectroscopic Approach. Biomaterials 1998, 19 (4−5), 407−422. (3) Rosen, J. E.; Gu, F. X. Surface Functionalization of Silica Nanoparticles with Cysteine: A Low-Fouling Zwitterionic Surface. Langmuir 2011, 27 (17), 10507−10513. (4) Mateo Marti, E.; Methivier, C.; Pradier, C. M. (S)-Cysteine Chemisorption on Cu(110), from the Gas or Liquid Phase: An FtRairs and Xps Study. Langmuir 2004, 20 (23), 10223−10230. (5) Ismail, K. M. Evaluation of Cysteine as Environmentally Friendly Corrosion Inhibitor for Copper in Neutral and Acidic Chloride Solutions. Electrochim. Acta 2007, 52 (28), 7811−7819. (6) Brolo, A. G.; Germain, P.; Hager, G. Investigation of the Adsorption of L-Cysteine on a Polycrystalline Silver Electrode by Surface-Enhanced Raman Scattering (Sers) and Surface-Enhanced Second Harmonic Generation (Seshg). J. Phys. Chem. B 2002, 106 (23), 5982−5987. (7) Santos, E.; Avalle, L. B.; Scurtu, R.; Jones, H. L-Cysteine Films on Ag(1 1 1) Investigated by Electrochemical and Nonlinear Optical Methods. Chem. Phys. 2007, 342 (1−3), 236−244. (8) Nazmutdinov, R. R.; Zhang, J.; Zinkicheva, T. T.; Manyurov, I. R.; Ulstrup, J. Adsorption and in Situ Scanning Tunneling Microscopy of Cysteine on Au(111): Structure, Energy, and Tunneling Contrasts. Langmuir 2006, 22 (18), 7556−7567. (9) Kühnle, A.; Linderoth, T. R.; Schunack, M.; Besenbacher, F. LCysteine Adsorption Structures on Au(111) Investigated by Scanning Tunneling Microscopy under Ultrahigh Vacuum Conditions. Langmuir 2006, 22 (5), 2156−2160. (10) Zhang, J.; Demetriou, A.; Welinder, A. C.; Albrecht, T.; Nichols, R. J.; Ulstrup, J. Potential-Induced Structural Transitions of DlHomocysteine Monolayers on Au(1 1 1) Electrode Surfaces. Chem. Phys. 2005, 319 (1−3), 210−221. (11) Xu, Q.-M.; Wan, L.-J.; Wang, C.; Bai, C.-L.; Wang, Z.-Y.; Nozawa, T. New Structure of L-Cysteine Self-Assembled Monolayer on Au(111): Studies by in Situ Scanning Tunneling Microscopy. Langmuir 2001, 17 (20), 6203−6206. (12) Dakkouri, A. S.; Kolb, D. M.; Edelstein-Shima, R.; Mandler, D. Scanning Tunneling Microscopy Study of L-Cysteine on Au(111). Langmuir 1996, 12 (11), 2849−2852. (13) Huang, C. J.; Chu, S. H.; Li, C. H.; Lee, T. R. Surface Modification with Zwitterionic Cysteine Betaine for NanoshellAssisted near-Infrared Plasmonic Hyperthermia. Colloids Surf., B 2016, 145, 291−300. (14) Huang, C. J.; Chu, S. H.; Wang, L. C.; Li, C. H.; Lee, T. R. Bioinspired Zwitterionic Surface Coatings with Robust Photostability and Fouling Resistance. ACS Appl. Mater. Interfaces 2015, 7 (42), 23776−23786. (15) Lai, C.-S.; Chen, Y.-Y.; Yau, S.; Dow, W.-P.; Lee, Y.-L. In Situ Scanning Tunneling Microscopy Imaging Self-Assembled Monolayers of Mercaptoacetic Acid and Cupric Ion on Au(111) Electrode. J. Electrochem. Soc. 2014, 161 (14), D742−D748. (16) Jian, Z.-Y.; Chang, T.-Y.; Yang, Y.-C.; Dow, W.-P.; Yau, S.-L.; Lee, Y.-L. 3-Mercapto-1-Propanesulfonic Acid and Bis(3-Sulfopropyl) Disulfide Adsorbed on Au(111): In Situ Scanning Tunneling Microscopy and Electrochemical Studies. Langmuir 2009, 25 (1), 179−184. (17) Liu, Y.-F.; Lee, Y.-L.; Yang, Y.-C.; Jian, Z.-Y.; Dow, W.-P.; Yau, S.-L. Effect of Chloride Ions on the Adsorption of 3-Mercapto-1H

DOI: 10.1021/acs.jpcc.7b04786 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Examined by in Situ Scanning Tunneling Microscopy. J. Phys. Chem. C 2016, 120 (46), 26425−26434. (38) Xie, W. J.; Gao, Y. Q. A Simple Theory for the Hofmeister Series. J. Phys. Chem. Lett. 2013, 4 (24), 4247−4252. (39) Wan, L.-J.; Suzuki, T.; Sashikata, K.; Okada, J.; Inukai, J.; Itaya, K. In Situ Scanning Tunneling Microscopy of Adsorbed Sulfate on Well-Defined Pd(111) in Sulfuric Acid Solution. J. Electroanal. Chem. 2000, 484 (2), 189−193. (40) Kim, Y.-G.; Soriaga, J. B.; Vigh, G.; Soriaga, M. P. AtomResolved Ec-Stm Studies of Anion Adsorption at Well-Defined Surfaces: Pd(111) in Sulfuric Acid Solution. J. Colloid Interface Sci. 2000, 227 (2), 505−509. (41) Wan, L.-J.; Yau, S.-L.; Itaya, K. Atomic Structure of Adsorbed Sulfate on Rh(111) in Sulfuric Acid Solution. J. Phys. Chem. 1995, 99 (23), 9507−9513. (42) Funtikov, A. M.; Stimming, U.; Vogel, R. Anion Adsorption from Sulfuric Acid Solutions on Pt(111) Single Crystal Electrodes. J. Electroanal. Chem. 1997, 428 (1), 147−153. (43) Zhang, Y.; Cremer, P. S. Interactions between Macromolecules and Ions: The Hofmeister Series. Curr. Opin. Chem. Biol. 2006, 10 (6), 658−663. (44) Zhang, J.; Chi; Ulstrup, J. Assembly Dynamics and Detailed Structure of 1-Propanethiol Monolayers on Au(111) Surfaces Observed Real Time by in Situ STM. Langmuir 2006, 22 (14), 6203−6213. (45) Maksymovych, P.; Voznyy, O.; Dougherty, D. B.; Sorescu, D. C.; Yates, J. T., Jr Gold Adatom as a Key Structural Component in Self-Assembled Monolayers of Organosulfur Molecules on Au(1 1 1). Prog. Surf. Sci. 2010, 85 (5−8), 206−240. (46) Kautz, N. A.; Kandel, S. A. Alkanethiol/Au(111) Self-Assembled Monolayers Contain Gold Adatoms: Scanning Tunneling Microscopy before and after Reaction with Atomic Hydrogen. J. Am. Chem. Soc. 2008, 130 (22), 6908−6909. (47) Maksymovych, P.; Sorescu, D. C.; Dougherty, D.; Yates, J. T. Surface Bonding and Dynamical Behavior of the CH3SH Molecule on Au(111). J. Phys. Chem. B 2005, 109 (47), 22463−22468. (48) Nenchev, G.; Diaconescu, B.; Hagelberg, F.; Pohl, K. SelfAssembly of Methanethiol on the Reconstructed Au(111) Surface. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80 (8), 081401. (49) Askerka, M.; Pichugina, D.; Kuz’menko, N.; Shestakov, A. Theoretical Prediction of S−H Bond Rupture in Methanethiol Upon Interaction with Gold. J. Phys. Chem. A 2012, 116 (29), 7686−7693. (50) Zhou, J.-G.; Hagelberg, F. Do Methanethiol Adsorbates on the Au(111) Surface Dissociate? Phys. Rev. Lett. 2006, 97 (4), 045505. (51) Venkatesan, K. The Crystal Structure of Ammonium PerchlorateNH4ClO4. Proceedings of the Indian Academy of Sciences - Section A 1957, 46, 134−142. (52) Zhang, H.-M.; Su, G.-J.; Wang, D.; Wan, L.-J.; Jin, G.; Bai, C.-L.; Zhou, X. Adlayer Structures of Dl-Homocysteine and L-Homocysteine Thiolactone on Au(1 1 1) Surface: An in Situ Stm Study. Electrochim. Acta 2004, 49 (9−10), 1629−1633.

I

DOI: 10.1021/acs.jpcc.7b04786 J. Phys. Chem. C XXXX, XXX, XXX−XXX