Revealing the Structural Complex of Adsorption and Assembly of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Revealing the Structural Complex of Adsorption and Assembly of Benzoic Acids at Electrode-Electrolyte Interfaces Using Electrochemical Scanning Tunneling Microscopy Cody Leasor, Kuo-Hao Chen, Trent Closson, and Zhihai Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01705 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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The Journal of Physical Chemistry

Revealing the Structural Complex of Adsorption and Assembly of Benzoic Acids at Electrode-Electrolyte Interfaces Using Electrochemical Scanning Tunneling Microscopy Cody Leasor, Kuo-Hao Chen, Trent Closson, and Zhihai Li* Department of Chemistry, Ball State University, Muncie, IN 47304 *Corresponding authors: [email protected]

ABSTRACT Self-assembly of benzenecarboxylic acids on well-defined noble metal has been intensively investigated using surface sensitive techniques. However, most studies were focused on the formation of nanostructures via benzene-tricarboxylic and benzene-dicarboxylic acids, which are composed of multiple carboxylic acid functional groups in either the meta or para positions of the benzene ring, allowing the formation of long-range ordered molecular arrays through -COOH mediated intermolecular hydrogen bonds. Two dimensional nanostructures of benzoic acid molecules that are comprised of a single carboxylic acid functional group on the phenyl ring at metal-electrolyte interfaces were rarely reported using scanning tunneling microscopy (STM) because there is only one carboxylic acid functional group for each benzoic acid available to form intermolecular hydrogen bonds, making it difficult to construct long-range ordered nanoarchitectures. In the present work, we employed electrochemical scanning tunneling microscopy (EC-STM) in combination with electrochemical cyclic voltammetry (CV) techniques to explore the adsorption and phase formation of benzoic acids (BZAs) at Au(111)/electrolyte interfaces. Our experiments show how electrolyte, molecular concentration, electrochemical potential, co-adsorption of aqueous ions affect the adsorption and self-assembly of BZA 1 ACS Paragon Plus Environment

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molecules. It is found that the BZA molecules are not assembled into long range ordered structures in the presence of sulfuric acid electrolyte due to the strong competing co-adsorption of sulfate ions on a gold electrode. BZA molecules can form flat-oriented ordered adlayers in perchloric acid electrolyte (containing weakly adsorbed ClO4- ion) at a negatively charged surface only when the concentration of the molecular solution reaches above 12 mM. Below 12 mM, the CVs of BZA on Au(111) in 0.1 M HClO4 shows only one pair of adsorption/desorption peaks. When the BZA concentration increases to 12 mM, the voltammogram exhibits 3 pairs of peaks, corresponding to the structural transformation of a disordered phase (phase I, ESample (ES): -0.600 V to -0.190 V), linear stripe pattern (phase II, ES: -0.190V to 0.108 V), zigzag pattern (phase III, ES: -0.108 V to -0.066 V) and upright packing pattern (phase IV, ES: -0.066 to 0.300V) adsorption revealed by STM within 4 electrochemical potential regions. Effect of parameters (electrolyte ions, concentration, and electrochemical potential) explored in this study will provide valuable information in the formation of molecular adlayers, adsorption and selfassembly, materials, corrosion inhibition, and molecular devices.

INTRODUCTION Adsorption and self-assembly of organic molecules on metal surfaces or at the metal-electrolyte interfaces is a fundamentally important topic because the study of the interactions between adsorbates (molecules) and metal substrates can provide valuable information to other chemical reactions and processes such as surface corrosion,1 asymmetric heterogeneous catalysis,2 electrochemical sensors,3 degradation of organic pollutants,4 surface-catalyzed reactions,5,6 and charge transfer in molecular devices.7 Self-assembly of molecules on substrates also provides a unique route to create supramolecular nanostructures.8 The self-assembly of supramolecular 2 ACS Paragon Plus Environment

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nanostructures is based on noncovalent interactions such as hydrogen-bonding, van der Waals forces, and metal ion-ligand coordination, etc.8 Both the molecule-molecule interaction (intermolecular) and molecule-substrate (often metal working electrode) interaction play a crucial role in determining the structural motif of supramolecular architectures that are formed. Recently, adsorption and molecular self-assembly of aromatic hydrocarbon (AHC) derivatives on different well-defined surfaces have been studied using surface sensitive techniques such as scanning tunneling microscopy (STM),9-11 and low energy electron diffraction (LEED)12 under ultra-high vacuum (UHV)9 and/or at low temperature conditions.12 As one category of AHC molecules, benzenecarboxylic acids, which are composed of a single benzene ring and a different number of carboxylic acid functional groups, have been intensively studied because benzenecarboxylic acids represent the smallest and a model-type of AHCs, i.e., studying the smallest AHC model can help to understand the expanded forms of AHC such as pyrenes,10,13 naphthalenes,12 and coronenes.14 Among the study of benzenecarboxylic acids, most reports are focused on 1,3,5-benzenetricarboxylic acid (trimesic acid, TMA)8,15-23 because each TMA molecule has 3 carboxylic acid functional groups. The multiple –COOH functional groups allow TMA molecules to form intermolecular hydrogen bonds stabilising the molecular adlayer on a metal substrate or at liquid-metal interfaces. There are also quite a few reports on the selfassembly of benzenedicarboxylic acids such as isophthalic acid (IA)24-26 and terephthalic acid (TA).24,27 However, fewer TA studies have been reported because the solubility of TA in water is extremely low (almost insoluble), thus hindering its application and making the study of TA inconvenient. Yet, there are even less STM studies found on the nanostructures of BZAs comprising of a single carboxylic acid functional group on the phenyl ring at metal-electrolyte interfaces. One possible reason is the challenge of using BZAs as molecular building blocks to 3 ACS Paragon Plus Environment


construct two-dimensional nano-architectures. As Scheme 1 shows, for benzenecarboxylic acids with three or two –COOH functional groups, each molecule can readily form hydrogen bonds with three or two neighbouring molecules via the dimerization of –COOH hydrogen bonds. For example, each TMA molecule can form hydrogen bonds with three adjacent TMA molecules to form hexagonal adlayers (Scheme 1A). Isophthalic acids, which have two carboxylic acid functional groups, can form hydrogen bonds with other IA molecules and form zigzag structures (Scheme 1B). However, the major challenge for BZA is that there is only one carboxylic acid functional group for each benzoic acid molecule available to form intermolecular hydrogen bonds. If one BZA forms hydrogen bonds with one BZA neighbour, one dimer will be produced and there is no other carboxylic acid functional group available on the other side of the phenyl ring to form a hydrogen bond with another BZA molecule by intermolecular hydrogen bonding (Scheme 1C). This makes BZA difficult to self-assemble into long range ordered adlayers at metal-electrolyte interfaces. Once the BZA dimers are formed, the interactions between dimers are weak due to no hydrogen bonding occurring among dimers. As a result, the substratemolecule interaction will become dominant in terms of the self-assembly of BZA. Thus, the molecule-substrate interaction plays a critical role in the formation of a BZA adlayer on a substrate. Itaya et al. successfully imaged BZA at the solid-electrolyte interfaces by choosing Pt as a substrate due to the strong BZA-Pt molecule-substrate interactions.

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A

B

C

Scheme 1. Molecular structures and formation of intermolecular hydrogen bonds for (A) trimesic acid (TMA), (B) isophthalic acid (IA), and (C) benzoic acid (BZA), respectively. Benzenecarboxylic acids containing multiple –COOH (TMA and IA) tend to generate long-range ordered two-dimensional nanostructures. For BZA, there is only one –COOH available for the formation of dimerization of hydrogen bond, difficult to form long range ordered adlayers.

As a powerful tool in nanoscience, scanning probe microscopy (SPM), STM especially, plays a major role in studying two dimensional nanostructures and molecular interactions on surfaces or at interfaces.28 With these techniques, structures of assembled molecules on conductive substrates can be characterized with molecular or atomic resolution. Recently, this STM technique has been employed to study the adsorption of benzoic acid on surfaces such as Ge(100),29 TiO2,30 Cu(100),31 Au(100),32 Au(111),33 or benzoic acid molecules co-adsorbed with single-walled carbon nanotubes on Au(111) surface.34 Most of these studies have been performed in ultrahigh vacuum (UHV) conditions at room or low temperatures, and the molecules are deposited on the surface by evaporating molecules from a vacuum chamber. Since many chemical reactions take place in a solution medium or at the solid-liquid interfaces, adsorption and self-assembly studies of BZA molecules in UHV conditions provide less “chemical” information in terms of solution reactions. There are some pioneering works on the 5 ACS Paragon Plus Environment


assembly of benzoic acid at platinum35 or gold33 electrode-liquid interfaces using EC-STM, but these studies are focused on the static imaging of benzoic acid molecules. In this study, we choose the smallest aromatic carboxylic acids, BZA, which has only one carboxylic functional group at the phenyl ring to explore the effect of the electrolyte (strongly or weakly adsorbed ions), solution concentration, and electrochemical potential on the formation of two dimensional nanostructures at the Au(111)/electrolyte interfaces using EC-STM in combination with CV techniques. We not only spotted the static images of the smallest aromatic carboxylic acid with the minimum number of –COOH groups, but also recorded structural transition and involution of surface adlayers when the electrode potential was simultaneously varied during STM imaging, i.e., the STM images and voltammograms are simultaneously captured in real time and real space. High-quality EC-STM images and simultaneously recorded electrochemical voltammograms reveal adsorption structures, and other factors such as co-adsorption of electrolyte and the effect of solution concentration are also discussed.

EXPERIMENT In general, it is highly challenging to image molecular adlayers at interfaces using EC-STM, especially for BZA, in which each molecule has only one –COOH available to form intermolecular hydrogen bonds. To obtain a high-quality image, all these conditions have to be met, such as a sharp STM tip, the small vibration of the whole instrument or drift of the STM tip, insulation of STM tip to reduce the “leaking” (faradaic) current, and an extremely clean surface. To prepare a super clean surface, only three things are trusted and regarded as “clean” in our experiments – the first is piranha solution (a mixture of H2O2 (35%) and H2SO4 (96%) by volume in a ratio of H2O2:H2SO4 = 1:3), the second and third is a hydrogen flame and Milli-Q 6 ACS Paragon Plus Environment

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water (resistance > 18 MΩ, total organic carbon (TOC) < 3 ppb), respectively. In other words, it is suggested to clean things as thoroughly as possible, such as cleaning STM cells and O-rings with the piranha solution followed by rinsing with Milli-Q water, cleaning the gold crystal and associated tools with a hydrogen flame to burn off all organic contaminations, to prepare all solutions while preventing contamination, and rinse coated STM tips with the super clean MilliQ water, see more details below.

Solution and electrode preparation All solutions were prepared using the super clean Milli-Q water mentioned above with extreme care to avoid any contaminations. Pipette tips were also cleaned with concentrated sulfuric acid and then rinsed with milli-Q water. Glassware for preparing and containing sample and control solutions were cleaned with Piranha solution and rinsed thoroughly with Milli-Q water. Chemicals used in this study need to be in high-purity and they are: BZA (Sigma-Aldrich, standard for elemental analysis), H2SO4 (Alfa-Aesar, 99.9999%), HClO4 (Alfa-Aesar, 99.9999%). 0.05 M H2SO4 and 0.1 M HClO4 control solutions, as well as sample solutions containing different concentrations of BZA were used for EC-STM experiments.

Electrochemical scanning tunnelling microscopy (EC-STM) EC-STM experiments were performed with Molecular Imaging microscope (Keysight). Electrodes for electrochemical STM experiments were Au(111) disk from Mateck (2 mm thick, 10 mm diameter). STM tips were electrochemically etched tungsten wires (0.25 mm diameter) coated with polyethylene or wax (Apiezon) to reduce leaking current between tip and electrolyte solution. Platinum wire (0.5 mm diameter) served as quasi-reference and counter electrodes. It is



noted that since Pt wire in 0.10 M HClO4 or 0.05 M H2SO4 was used as quasi-reference electrode, which has slight drift during the experiment or from one experiment to another. Thus, we compiled the voltammogram current peak values from 14 experiments (Table S1) and averaged them to indicate how these values are shifted from one experiment to another, and how the values are correlated to the potentials of a saturated calomel electrode (SCE) – a standard reference electrode. On average, the Pt wire in 0.10 M HClO4 or 0.05 M H2SO4 is -0.550 V vs. SCE, i.e., a potential of 0.000 V vs. SCE will be reported to be -0.550 V vs. a reference of Pt wire in 0.10 M HClO4. In this work, all the electrochemical potentials are reported vs. the quasireference electrode of Pt in 0.10 M HClO4 or 0.05 M H2SO4.

Before each experiment, the single crystal STM working electrode was flame annealed in hydrogen flame for 5 minutes, and then cooled down under argon atmosphere to avoid surface contamination. STM cells and O-rings were pre-cleaned with Piranha, rinsed with Milli-Q water and dried with argon. Then the three electrodes were assembled into the STM cell and a small amount (0.1 mL) of 0.1 M HClO4 or 0.05 M H2SO4 electrolyte solution was added to the STM cell under potential control at -0.60 V. Then the potential was scanned between -0.60 V to 0.30 V for 10 minutes to lift the surface reconstruction and create an island-free Au(111)-(1x1) surface to allow molecular assembly (Figure S1A). Figure S1B shows a gold crystal surface in which surface reconstruction lines can be clearly discerned. After the process of lifting the reconstruction lines – potential scan for 10 minutes, a Au(111)-(1x1) surface was created (Figure S1C), and molecule-containing solutions were added to the EC-STM cell at either a negatively charged or positively charged potential for imaging. All STM experiments were carried out at


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room temperature in constant current mode. All STM images are original data used as taken without any filtering such as Fourier transform.

RESULTS AND DISCUSSION Though understanding the adsorption of benzoic acids on noble metal electrodes is important, experiments of obtaining an ordered self-assembled adlayer is difficult compared with other benzene(poly)carboxylic acids as we discussed in the introduction. Due to the nature of single (unbalanced) carboxylic acid functional group, i.e., asymmetrical molecular structure, the adsorption process is complicated as it will be discussed in the following sections. Compared with isophthalic and trimesic acids, benzoic acid requires much more effort to explore the assembly conditions to create an ordered adlayer at the electrode/electrolyte interfaces. The reason of pursuing an “ordered” adlayer is that , it is almost impossible to image a small “single” molecule such as BZA using STM under ambient conditions due to the thermal movement of molecules on a surface at room temperature. In other words, molecules need to be “fixed” (without thermal lateral movement or rotating) on the surface in order to be imaged by STM at a room temperature. Therefore, to spot a BZA molecule, it is a pre-requisite to create an ordered adlayer so that molecular positions are fixed on the surface during STM tip scanning. That is often realized by using a substrate that has a strong molecule-substrate interaction, such as strong Au-thiol bonds, so that molecules can be strongly adsorbed (fixed) on surface during imaging.36 In this study, we have explored the effect of key parameters on creating an ordered adlayer: electrolyte, solution concentration, and electrochemical potential. We found that it is possible to construct ordered adlayers using BZA without thiol groups at both a positively charged surface and a negatively charged surface. However, at a negatively charged surface, BZA only can form



ordered adlayers in HClO4 electrolyte based on our experiments, where BZA molecules are adsorbed on the surface with a flat-oriented manner. In a H2SO4 electrolyte, where sulfate ions are strongly adsorbed on a gold electrode, our experiments show that no ordered BZA adlayer was detected at the negatively charged surface (Figure S2B), though the ordered TMA adlayers were observed for TMA in a H2SO4 electrolyte.8 We assume that this is because each TMA has 3 carboxylic acid functional groups to form hydrogen bonds with three neighbouring TMA molecules, and these hydrogen-bonded networks stabilize the long-range ordered two dimensional adlayers of TMA. However, for BZA, each molecule has only one carboxylic acid functional group forming hydrogen-bonded dimers with another BZA molecule. After these hydrogen-bonded dimers formed, there is no any additional –COOH group available to form hydrogen bonds with other BZA molecules (Scheme 1), so it is difficult for BZA molecules to form a long-range ordered adlayer lacking the hydrogen-bonded network. In this case, the molecule-substrate interaction of BZA is critical, and the intermolecular interactions among BZA molecules are weaker than the interaction among hydrogen-bonded TMA molecules. Therefore, the flat-adsorbed BZA adlayers are more fragile than TMA adlayers, and the formation of ordered BZA adlayers is influenced by the strongly adsorbed sulfate ions (SO42-) more than the formation of ordered TMA adlayers. This explains the observation of a clear electrolyte ion effect on the adsorption and assembly of BZA at Au(111)/electrolyte interfaces in the negative potentials, which was not observed for TMA.

At the positively charged Au(111) surface, the carboxylic acid (-COOH) functional group will deprotonate by losing one proton to form a -COO- ion strongly adsorbed on the positively charged surface.8 The strong COO-Au adsorption at positive potentials should overcome the ion


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effect and allow the observation of ordered BZA on the positively charged electrode. Based on the observation of TMA adsorption at the Au(111)/H2SO4 interfaces, it should be possible to image BZA on Au(111) in 0.05 M H2SO4 electrolyte at a positive potential.8 However, our electrochemical STM experiments did not succeed to obtain an atomic resolution of BZA adlayers at a positively charged gold surface in 0.05 M H2SO4 electrolyte (Figure S2C). To avoid the possible effect of co-adsorption from sulfate ions, we did not continue testing the possibility of building ordered structures in sulfuric acid electrolyte. Instead, in this study we changed electrolyte from H2SO4 to HClO4, an electrolyte which dissociates into weakly adsorbed ClO4ions in water, and then we successfully obtained ordered BZA adlayers at different electrochemical potentials - both the positively and negatively charged surfaces.

We also discovered a solution concentration effect on the assembly of BZA molecules at Au(111)/electrolyte interfaces. To correlate the electrochemical responses to STM observations, most voltammograms presented in this paper were measured and recorded during STM experiments using STM electrochemical cell with a slow scan rate, typically 0.01V/s. Since the scan rate is small, and thin platinum wires (0.5 mm in diameter) with small surface areas were used as the counter and quasi-reference electrodes, so the current response in the voltammogram looks fussy, compared with a voltammogram measured by a large electrochemical cell using high scan rate such as 0.10 V/s or 0.20 V/s. It is found that the voltammogram of Au(111) in the presence of BZA exhibits three peaks in 0.10 M HClO4 electrolyte containing BZA molecules at a concentration of or above 12 mM. Figure 1 shows the cyclic voltammogram of Au(111) electrode in 0.1 M HClO4 in the presence of 20 mM BZA molecules. The voltammogram exhibits three anodic current peaks, which are labelled as Pa1, Pa2, and Pa3, respectively, and



three cathodic current peaks that are labelled as Pc1, Pc2, and Pc3, respectively (Figure 1). These three peaks separate the whole electrochemical potential into four regions labelled as region I, II, III, and IV, respectively (Figure 1). These four potential regions correspond to four different adsorption phases. The structural property at each phase as well as phase transitions will be discussed in the following chapters.

45

Pa2

30

Current (A)

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15

III

Pa1

I

Pa3

IV

II

0 -15

Pc1 Pc2

-30

Pc3

-45 -0.5 -0.4 -0.3 -0.2 -0.1

0.0

0.1

0.2

E(V) vs. Pt wire in 0.10 M HClO4 Figure 1. Cyclic voltammogram (CV) of Au(111) electrode in 0.1 M HClO4 in the presence of 20 mM BZA molecules. The voltammogram exhibits three anodic current peaks labelled as Pa1, Pa2, and Pa3, respectively. These peaks separate the whole potential into four regions: region I, II, III, and IV. Correspondingly, three cathodic current peaks are labelled as Pc1, Pc2, and Pc3, respectively.

However, there is only one single peak observed in the voltammogram of Au(111) at the molecular concentration below 12 mM. For example, Figure 2B shows the adsorption of BZA at the Au(111)/0.1 M HClO4 interfaces with a low concentration of 3 mM BZA. The in situ STM experiment of 3.0 mM BZA at Au(111)/HClO4 shows two type of adlayers: the disordered


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adlayer at the potential negative to (or the left of) the single current peak (Figure 2A) in the voltammogram, and partially ordered adlayer at the potential positive to (or the right of) the peak (Figure 2C), indicating the single current peak in the voltammogram of 3mM BZA is corresponding to the phase transition between the disordered and ordered BZA adlayer at Au(111)/ electrolyte interfaces. According to literature, the peak is also correlated to the phase transition of the surface adlayers between the flat-adsorbed adlayer and the vertical or tilted oriented adlayer.8 Also, Figure 2C shows that at this low concentration (3.0 mM), the surface was not completely covered by the ordered BZA adlayer at the positive potential, and it took hours to form the local ordered adlayer. At the concentration lower than 12 mM, no ordered BA adlayer was observed at negatively charged surface from our STM experiments (Figure 2A).

A

3.0

Current (A)

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B

C

1.5

0.0

-1.5

-3.0

-0.4

-0.2

0.0

0.2

E(V) vs. Pt wire in 0.10 M HClO4

Figure 2. (A) Electrochemical STM image of BZA on Au(111) in 0.1 M HClO4. Sample potential ES = 0.222 V, iT = 0.20 nA; (B) Cyclic voltammogram of Au(111) in 0.1 M HClO4 and 3 mM BZA, scanning rate 0.01 V/s; (C) Electrochemical STM image of BZA on Au(111) at positively charged potential ES = 0.500 V at a molecular concentration of 3 mM, iT = 0.15 nA.

When the concentration increases to 12 mM BZA, the single current peak in the voltammogram changes into three pairs of current peaks (Figure 3A), which separate the whole electrochemical



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potential into four regions: I, II, III, and IV (Figure 3A). At the positively charged potential region, region IV, the large-scale electrochemical STM image (Figure 3B) shows the structural phase IV featuring the highly ordered “oval” shaped bright spots aligned in parallel rows. Each spot represents a single BZA molecule according to the molecular size. The distance between the adjacent bright spot within the same diagonal row is 0.39 nm, which is much shorter than the distance between two flat-oriented benzene(poly)carboxylic acid molecules such as TMA or IA (0.75 nm).26 That indicates the BZA molecules may not be adsorbed on the surface in a horizontal (flat-oriented) way. The cross section measurement supports this assumption and shows that corrugation of these bright spots is averagely 0.33 nm, a height typically for upright adsorbed

benzene(poly)carboxylic

molecules.8

This

observation

is

consistent

with

benzene(poly)carboxylic acids with two or three –COOHs.8,26 Based on the conclusions from the TMA and IA studies, we can reasonably assume that, at the positively charged surface BZA acts similarly to trimesic and isophthalic acid, where the BZA molecules are also deprotonated, and each BZA adsorbs on the surface using the only –COO- group with the phenyl ring posing toward or into the solution. Based on the high-resolution STM image (Figure 3B), we tentatively build the adsorption model as Figure 3C shows. Each BZA molecule adsorbed at the bridge position of Au(111) surface with a unit cell indicated by a blue parallelogram (Figure 3C).8 The unit cells contain 2 molecules with a characteristic dimension of a = 0.58 nm, b = 0.76 nm, and an angle of = 79 degree. This model can interpret the experimentally observed characteristic well (experimental distance of a = 0.58 ± 0.05 nm, b = 0.80 ± 0.08 nm, and = 74 ± 5 degree). From this model, a unit cell area of 0.44 nm2 (containing two BZA molecules) can be calculated, giving a surface coverage  7.55 x 10-10 moles of BZA per cm2, which is consistent with the


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surface coverage of 7.5 ± 0.5 (x 10-10) mol cm-2 that is calculated by the measured experimental distance a, b, and angle values above.

8

Pa3

Pa2

III

C

Pa1

4 Current (A)

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I

II

IV

0

A Pc1 Pc2

-4

-8

B

Pc3 -0.4

-0.2 0.0 0.2 0.4 E (V) vs. Pt wire in 0.1 M HClO4

Figure 3. (A) Electrochemical cyclic voltammogram of Au(111) in 0.1 M HClO4 and 12 mM BZA. (B) The large scale EC-STM image of BZA on Au(111) at ES = 0.400 V (potential region IV), iT = 0.09 nA; (C) tentative adsorption model.

The observation of three sets of current peaks in the voltammogram suggests multiple structural phase transitions exist since there is no redox reaction within this potential range. Compared with benzenedi- or tri-carboxylic acid, adsorption and assembly of BZA on an Au(111) electrode is the most complex processes and revealing the self-assembled structures is challenging. To explore the structural properties of BZA as a function of electrochemical potential, i.e., to discover the effect of potential on the formation of two-dimensional nanostructures, we took advantage of the in situ imaging ability of EC-STM by recording STM images while simultaneously running electrochemical CVs. This in situ technique allows the matching of electrochemical potential and the structural change of surface species, so that both the static and structural transition can be monitored in a real space and in real time.



A Current (A)

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B C

0

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D

-2

-4

IV

III

-6

-0.2

Pc3 0.0

0.2

0.4

E(V) vs. Pt in 0.10 M HClO4

Figure 4. (A) Partial electrochemical voltammogram (similar to the CV found in Figure 3A, but ES was swept only in a cathodic direction (from right to left) as black arrow shows) recorded when the EC-STM image (B) was captured; (B) STM image captured while the sample potential was swept from 0.400 V to 0.136 V (A). Image size 30 nm x 30 nm. The ordered BZA adlayer starts showing up, indicated by a purple arrow on STM image B, while the electrochemical potential passes the voltammogram peak at 0.106 V; (C) Zoom-in static STM image of BZA on Au(111) in 12 mM BZA at ES = -0.136 V after (B), size: 20 nm x 20 nm; (D) STM image recorded on the same surface area with the same imaging parameters shows different contrast due to a tip scanning effect. The blue and red arrows in (A) and (B) show where the ES scan starts and ends.

Figure 4 demonstrates how we can change the sample potential by running a voltammogram while simultaneously recording STM images to discover the formation and structural transformation of the BZA adlayer at the electrified electrode-electrolyte interfaces. The STM image (Figure 4B) was recorded from up to down when the electrochemical potential was swept from 0.400 V to -0.136 V as a part of a cyclic voltammogram (Figure 4A). The thin blue arrows in Figure 4A and 4B indicates the time when the STM image was initially recorded and simultaneously the potential sweep also started at ES = 0.400 V, so that STM image Figure 4B was recorded from the thin blue arrow to the thin red arrow while the voltammogram was swept from 0.400 V to -0.136 V. After the potential scan was stopped at -0.136 V, the STM imaging continued from the thin red arrow to the bottom end of the image. In other words, the upper part 16 ACS Paragon Plus Environment

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of the STM image between the thin blue to the thin red arrows (Figure 4B) matches the potential sweep from 0.400V to -0.136 V in the voltammogram (Figure 4A). Also, a purple thick arrow on the STM image Figure 4B shows the appearance of the ordered molecular structure, corresponding to the current peak of the voltammogram indicated by the thick purple arrow in Figure 4A. Therefore, this in situ STM experiment with the simultaneously recorded potential sweep provides direct evidence that the peak Pc3 in Figure 4A is attributed to the formation of the adlayer in the potential region III (Figure 3). From the STM image in Figure 4B, it is difficult to judge what type of pattern was formed. It seems there are some zigzag patterns indicated by the zigzag blue lines in Figure 4B. Figure 4C shows a zoom-in static STM image of BZA at a fixed sample potential of ES = -0.136V after the potential sweep and imaging of Figure 4B. Interestingly, in the middle of the STM image in Figure 4D, a highly ordered pattern composed of round spots are clearly discerned for a moment, indicated by a thick blue arrow, and then the image again became a zigzag-like feature, though all the parameters were the same as Figure 4C. This “show-up” and disappearing phenomena piqued our interest to carry out more experiments to discover the adsorption patterns of BZA at this potential and the whole potential region.



end

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4 0 -4

start

-8 -12 -0.4

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0.4


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D 8

start

4

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0 -4

end

-8 -0.4

-0.2

0.0

0.2

0.4


Figure 5. (A) An electrochemical voltammogram of Au(111) in 0.10 M HClO4 and 12 mM BZA. Scan rate = 0.010 V/s. The potential was swept from 0.400 V to -0.400 V, and then ended at -0.125 V as the purple arrow shows; (B) EC-STM image of BZA on Au(111) at ES = -0.125 V after running the voltammogram (A); (C) An electrochemical voltammogram of Au(111) in 0.10 M HClO4 and 12 mM BZA. Scan rate = 0.010 V/s. The potential was swept from -0.400 V to 0.400 V, and then ended at -0.156 V as the purple arrow shows; (D) EC-STM image of BZA on Au(111) at ES = -0.156V after running the voltammogram (C).

In the next experiment, we ran the voltammogram from 0.400 V to -0.400V, and then, swept the potential in an anodic direction to -0.125 V. The potential was stopped at the top of the second peak in an anodic scan. The STM image (Figure 5B) at this sample potential ES = - 0.125 V, i.e., the top of the second peak, shows clearly a zigzag pattern, which means, the second peak corresponds to the transformation into or the formation of zigzag molecular structures. This observation together with the STM images and the voltammogram in Figure 4 indicate that at the potential range between the second and third peaks, the region III, BZA molecules form an adlayer with a zigzag pattern. As we observed in Figure 4A that when the potential was swept 18 ACS Paragon Plus Environment

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negatively and stopped immediately after passing the cathodic current peak 3 (Pc3 in Figure 3) in a cathodic potential sweep, a zigzag adlayer was formed (Figure 4A). Herein, we were wondering what would happen if we sweep the potential negatively slightly further until the cathodic current peak Pc2 starts showing up. Following this idea, the potential was stopped at the top of peak Pc2 (Figure 5C), and then, the electrode was immediately imaged with STM. What surprised us is that we observed a totally different pattern, and the STM image Figure 5D shows the highly ordered “striped” structures. It seems there are some bright stripes packed with molecules (blue arrow in Figure 5D) where in between each bright stripe are relatively dark rows (green arrow in Figure 5D).

By correlating the voltammogram (Figure 5C) and STM image (Figure 5D), one may conclude that within the potential range between the cathodic current peak 1 (Pc1 in Figure 3) and the cathodic current peak 2 (Pc2 in Figure 3), BZA molecules form the striped features –structural Phase II. This hypothesis was further tested, and seemed to be correct, by another type of potential sweep. In Figure 6A, the electrochemical potential was swept first from 0.400 V to 0.400 V, and then swept from -0.400 V to -0.234 V, before the appearance of the second current peak. The simultaneously recorded STM image during the potential sweep is shown in Figure 6B. When the potential was scanned in the anodic direction passing the peak Pa1, the striped molecular adlayer started appearing in Figure 6B. The absence of ordered structures at the upper part of the image indicates that potential Region I (more negative than anodic current peak Pa1) corresponds to a disordered adlayer – structural Phase I. It also further suggests that within potential Region II (between the anodic current peak Pa1 and Pa2), a striped molecular adlayer forms structural Phase II. Figure 6C is an STM image recorded immediately after the STM



image of Figure 6B and the potential sweep (Figure 6A). It shows that the whole surface (Figure 6C) was covered with a striped molecular adlayer, in comparison with Figure 6B, where no ordered striped pattern was observed at the upper part of the image. In order to resolve the structural details of the striped molecular adlayer, a high-resolution STM image was recorded by zooming in on the surface without adlayer defects. It was still difficult to resolve the individual molecules within adlayers even when we rotated the scanning direction of the STM tip by an angle of 50 degrees (Figure 6D). More high-resolution STM images and images that were rotated by different angles can be found in supporting information Figure S5. Though the structural details of this striped adlayer can’t be resolved at this stage, this experiment does reveal the fact that the anodic current peak 1 (Pa1) in the voltammogram (Figure 6A) is related to the formation of the striped adlayer from the disordered phase I that is the potential negative to (or left of) Pa1. Combined with the knowledge in Figure 5, it also suggests that anodic current peak 2 (Pa2) represents a structural transformation between the striped structure and the zigzag structure.


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A

4

B

end

0 Current (A)

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start

-4

-8 -0.3

0.0

0.3

E (V) vs. Pt wire

D

C

Figure 6. (A) Partial electrochemical voltammogram of Au(111) in 0.10 M HClO4 and 12 mM BZA. Scan rate = 0.010 V/s. The potential was swept from 0.400 V to -0.400V and then to -0.234 V; (B) The ordered striped type of BZA adlayer starts showing up, indicated by a green arrow on STM image B, when the electrochemical potential passes the peak Pa1; (C) Static STM image at ES = -0.234 V after the sweeping of potential (A) and STM imaging (B). Size of images B and C: 60 nm x 60 nm; (D) High resolution STM image of (B) after rotating the tip scanning direction at 50 degrees. Size: 12 nm x 12 nm.

Many experiments have been carried out with extreme care aiming to obtain high-quality STM images to reveal the mystery of BZA adsorption on Au(111). The extraordinary effort was spent in the aspects of improving surface cleanness, increasing tip quality (sharper tips, smaller leakage), and also in reducing the vibration of instrument and tip drifting. The STM image (Figure 7A) shows the zigzag patterns (structural Phase III) in the potential Region III, and we made substantial effort to optimize the STM imaging parameters in order to discern how the molecules are arranged within the surface adlayer. Although most of time the STM images only



show zigzag pattern, occasionally STM images did disclose some features allowing discernment of the arrangement of BZA molecules inside the zigzag patterns. Interestingly, one STM image disclosed that those zigzag patterns actually correspond to the arranged bright features (Figure 7A). For example, from the bottom part of Figure 7A, one can clearly see that the zigzag patterns are essentially composed of many round spots, and each bright round spot represents one individual BZA molecules according to their size because each bright feature is about 0.3 nm in length and distance between two adjacent bright features is 0.79 ± 0.05 nm. This dimension analysis suggests each bright feature corresponds to one benzoic acid molecule according to the crystallography data of benzoic acid in the solid state.37 Intensive STM experiments show that the majority of patterns that BZA formed in the potential region III is a zigzag shaped adlayer, however, these zigzag patterns occasionally can show the long-range ordered linear spots that comprise of quadratic tetramers as indicated by a blue rectangle at the left-lower corner of STM image in Figure 7A. As Figure 7B shows, the zigzag pattern was switched to the long-range ordered linear patterns and then switched back to zigzag patterns again, though during this process, no imaging parameter or experimental conditions were changed. This switching behaviour can’t be explained by the STM imaging technique alone. The hypothesis could be that each benzoic acid has only one –COOH group, so that it can’t form hydrogen bonds in two locations around the phenyl ring with two other neighbouring BZA molecules like IA or TA, or even form three H-bonds like TMA. This makes the BZA molecular adlayer fragile on the Au surface, i.e., BZA molecules are not fixed on the Au surface with an intermolecular hydrogenbonded network as other works with carboxylic acids that have multiple –COOH functional groups have shown. This makes it possible that BZA molecules can change or slightly shift their orientation that can show different appearances or arrangements under STM imaging. It is also


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possible that there are co-adsorbed ions on the top of BZA molecules to co-stabilize the adlayer, and these loosely adsorbed ions can be disturbed by STM tip scanning, making these “metastable” adlayers exhibit distinct visual patterns. Figure 7A and 7B are examples that zigzag pattern can switch to linear spot during STM imaging.

B

A

Figure 7. (A) An electrochemical STM image of BZA on Au(111) in 0.10 M HClO4 and 12 mM BZA at ES = -0.075V, after The potential was swept from 0.400 V to -0.048 V at a scan rate = 0.01 V/s. iT = 0.10 nA, size: 20 nm x 20 nm; (B) EC-STM image of BZA on Au(111) at ES = -0.088 V, iT = 0.10 nA, size: 22 nm x 22 nm.

Plenty of STM experiments were carried out with extreme care, fortunately some large-scale and high-resolution STM images were captured in potential region III (Figure 8), allowing to resolve the adsorption of the BZA features under the veil of the zigzag pattern. Figure 8A shows the large-scale EC-STM image at sample potential of ES = -0.075 V. This image was captured immediately after the surface pattern was flipped from the zigzag to the quadratic packing. It is our hypothesis that the zigzag pattern and quadratic pattern are essentially the same type of adlayer and the same molecular structure but with different visual patterns under STM observation. This hypothesis is supported by the observation that from the quadratic pattern in Figure 8A, one still can discern the zigzag pattern feature, guided by the zigzag blue line in



Figure 8A. Also, this image was recorded when the previous image was still at a zigzag pattern at the exact same potential and the same imaging conditions. After this image was captured, the surface pattern changed back to zigzag again. Therefore, compared to zigzag pattern, this quadratic pattern is just a “short-time event” and most images collected in this region show a zigzag pattern from different experiments. The hypothesis that zigzag and quadratic patterns are essentially the same molecular packing which is also supported by the observation that these two patterns can exist in the same image and flip between each other (Figure 7A and B). It is also observed that compared with the “clear” quadratic features, the zigzag patterns are often “blurry” or “cloudy”. For the zigzag feature, it seems there are species on the top of the adlayer and the BZA molecules are interacting with some species on the top, which could possibly be coadsorbed ions or molecules. For example, the zigzag pattern in Figure 5C shows that we do not observe the individual separated bright spots (the phenyl rings of BZA molecules) as we detected from the quadratic pattern. Instead, one always sees the bright spots that seem “glued” together and it seems that something is on the top of the zigzag-pattern of the BZA adlayer (Figure 5C). So, it is reasonable to hypothesize that there might be some co-adsorbed ions or BZA molecules interacting with the BZA adlayer, complexing the STM observation. When these clouds of coadsorbed ions or molecules were temporarily removed or disturbed by STM tip scanning, the quadratic patterns underneath were exposed and clearly discerned (the highly ordered bright spots at the bottom of Figure 7A). This hypothesis cannot be completely verified by the STM imaging technique alone. Hopefully, this research could inspire the curiosity and promote further comprehensive experimental approaches, and even theoretical investigation to unveil the mystery of switching behaviour between the zigzag and quadratic patterns.


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A

B

C

D a 

Figure 8. (A) An electrochemical STM image of BZA on Au(111) in 0.10 M HClO4 and 12 mM BZA at ES = -0.075V, iT = 0.10 nA. Size: 20 nm x 20 nm; (B) High-resolution STM image of BZA on Au(111) at ES = -0.075 V, iT = 0.10 nA. Size: 10 nm x 10 nm; (C) Tentative structural model for molecular adlayer of BZA in potential region III. BZA molecules form hydrogen bonded dimers which become the basic units of quadratic or zigzag adlayers; (D) Alternative adsorption model of BZA for quadratic or zigzag patterns with the four BZA molecules forming hydrogen-bonded tetramers, and these tetramers forms the quadratic or zigzag patterns.

Based on the crystallographic literature, benzoic acids tend to form hydrogen-bonded dimers in the solid state.37 So that it is reasonable to assume that these BZA molecules also form the hydrogen-bonded dimers, as their benzene-carboxylic acid members – benzenedicarboxylic acid and TMA. Therefore, the structural model was built as Figure 8C, in which each two BZA molecules take the form of hydrogen-bonded dimerization.



Considering the majority repeating patterns are quadratic-based units, this inspired us to pursue another way for possible intermolecular interactions. The Figure 8D shows an alternative structural model. In this model, every four BZA molecules form a quadratic hydrogen-bonded tetramer, and then a long range ordered two-dimensional nanostructure is formed using these tetramers as nanoscale building blocks. The high-resolution STM image (Figure 8B) also suggests that BZA may form the hydrogen-bonded tetramers as one sees that molecules seem to form groups where each group has 4 molecules as indicated by a blue rectangle in Figure 8B. On the basis of this observation, the alternative structural model is tentatively built as Figure 8D. Though the tetramer model is reflected better from the STM images, for example, one can discern the tetramers from Figure 8 as well as from the zigzag structure, our experiments cannot absolutely exclude the possibility of the model in Figure 8C. The unit cell of the tetramer model (Figure 8D) has a side length of a = 1.44 nm, b = 1.50 nm, and an angle of  = 90 degrees; which fits well with the experimentally observed characteristic data a = 1.55 ± 0.10 nm, b = 1.58 ± 0.10 nm, and an angle of  = 85 ±8 degrees.

As discussed above and also in the introduction, self-assembly and adsorption of BZA is challenging and complicated compared with other benzenecarboxylic acids with two or three – COOH groups. For example, it is observed that within potential region II BZA molecules form highly ordered striped molecular rows. It seems that the bright molecular rows and the “empty” dark rows appear alternately (Figures 6 and 9B). However, some breakthrough was made after intensive STM experiments, and the high-resolution STM image shows that the previously observed “empty rows” are actually adsorbed molecules (Figure 9C). High-resolution STM image Figure 9C clearly demonstrates that there are molecules (indicated by a thick blue arrow)


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among each bright molecular row (indicated by thick green arrows). From this observation, when we look back at the previous STM images (Figure 6, and Figure SI5), it is found that there are molecules with dark contrast sitting between each two bright molecular rows. They seem dark or “empty” just because in contrast to the bright molecules, they are not as bright or not bright enough to be discerned when the size of the image is large, or the resolution is not high enough. Also, from the high resolution STM image Figure 9C, it seems there are co-adsorbed ions or molecules on the top of the underlining molecules, indicated by green circles in Figure 9C.

B Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C

0

-5

-10

A -0.4

-0.2 0.0 0.2 0.4 E (V) vs. Pt wire in 0.1 MHClO4

Figure 9. (A) Partial electrochemical voltammogram of an Au(111) in 0.1 M HClO4 and 12 mM BZA; sweeping rate: 10 mV/s. The potential was first swept in a cathodic direction as indicated by the thin blue arrows and then in an anodic direction and stopped at ES = -0.234 V, the potential region II; (B) Large scale EC-STM image of BZA on Au(111) in 0.1 M HClO4 that was directly recorded after the potential sweep (A). ES = -0.234 V, iT = 0.150 nA, size: 40 nm x 40 nm; (C) High-resolution STM image of (B). ES = -0.234 V, iT = 0.300 nA, size: 16 nm x 16 nm. The blue and green arrows indicate molecular rows.

Unlike other benzene(poly)carboxylic acid molecules with multiple -COOH functional groups, that are relatively easier to obtain very clear high-resolution images. For BZA, even though we obtain very clear high-resolution images, it is still challenging to assign the STM features to molecules as the patterns observed are complexed by the possible co-adsorption of molecules or 27 ACS Paragon Plus Environment


ions on the top of the BZA molecules. One of the examples is that in STM image Figure 9C, it is quite obvious that each spot highlighted by the red circles represent one BZA molecule based on the size measurement. However, for the features highlighted by the blue rectangle, it is difficult to assign the image features to molecules because it seems there are four molecules underneath the two bright features that are highlighted by two green circles. Herein, we tentatively assign each rectangle that includes two co-adsorbed BZA molecules (bright features) and four molecules underneath (dark blurry features underneath the two bright features inside the blue rectangle in Figure 9C). This assignment of molecules assumes that the two bright features on the top of the molecular rows are co-adsorbed while underneath, there are four BZA molecular building blocks formed by carboxylic acid mediated hydrogen bonds. This is the best observation we could achieve by using electrochemical STM techniques. We hope this study can stimulate more interest from surface scientists, and a more comprehensive approach can be employed to investigate the assembly of surface adsorption of BZA at electrolyte-solid interfaces.

CONCLUSIONS We have intensively investigated the self-assembly and molecular adsorption of BZA at Au(111)/electrolyte interfaces using in situ STM and CV techniques. Different parameters and experimental conditions have been explored including different electrolytes, electrochemical potentials, molecular concentrations, and the transformation of different adlayers under potential controls. It is found that BZA molecules are very sensitive to electrolyte solutions, and highly ordered BZA adlayers were only observed in weakly adsorbed electrolyte such as perchloric acid, not in the strongly adsorbed electrolyte such as sulfuric acid. It is also observed that adsorption of BZA is concentration-dependent. At the molecular concentration of 12 mM or lower than 12


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mM, voltammogram of BZA on a Au(111) in 0.1 M HClO4 shows only a single peak, and no ordered molecular structures were observed by electrochemical STM imaging. Increasing solution concentration to 12 mM gives rise to three peaks which separate potential range into potential regions I, II, III, and IV. The effect of electrochemical potential on the formation and transformation of adlayers were studied by recording STM images and simultaneously sweeping the surface potential and monitoring the current peaks in voltammograms. This in situ technique allows the discovery of the four correlating BZA molecular adlayers, named disorder phase I, striped phase II, zigzag phase III, and upright packing adlayer, corresponding to the four electrochemical potential regions I, II, III, and IV, respectively. Structural models were tentatively built based on the high-resolution STM images.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]. ASSOCIATED CONTENT Supporting Information. Cyclic voltammograms, Table of statistical data of current peaks in cyclic voltammograms, STM images of reconstructed Au(111) electrode and Au(111) - (1x1) surface after lifting reconstruction, EC-STM images of Au(111) in 0.05 M H2SO4 and 3 mM BZA, voltammogram and STM image of Au(111) oxidation and reduction, additional STM images of phase II, III and mixture of phase II and III adlayers. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS 29 ACS Paragon Plus Environment


Authors acknowledges the financial support from Indiana Academy of Science Senior Research Grant and Ball State University ASPiRE Junior Faculty Awards, and Ball State University CRISP program for supporting undergraduate research. REFERENCE (1)

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