Polymorphism Driven by Concentration at the Solid–Liquid Interface

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Polymorphism Driven by Concentration at the Solid Liquid Interface Nguyen Thi Ngoc Ha,*,† Thiruvancheril G. Gopakumar,‡ and Michael Hietschold† † ‡

Solid Surfaces Analysis Group, Institute of Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany Institut f€ur Experimentelle und Angewandte Physik der Universit€at Kiel Olshausenstr. 40, D-24098 Kiel, Germany

bS Supporting Information ABSTRACT: Self-assembly at the liquid solid interface is controlled by concentration of solute in solvent. Trimesic acid (TMA) dissolved in fatty acids shows a polymorphism on HOPG surface tuned by its concentration at the solid liquid interface via sonication of the solution. According to literature, TMA at long chain length fatty acids (Cn 1H2n 1COOH) HOPG interface, such as octanoic acid (n = 8) and nonanoic acid (n = 9) can only form low-packing density chicken-wire structure, whereas in shortchain length solutions (ranging from butyric acid (n = 4) to heptanoic acid (n = 7)), high-packing density flower structure is formed. Here we show that by adjusting the molecule concentration by sonication (using an ultrasonic bath), the different polymorphs can be deposited in a controlled manner, and it turns out that the observations previously made1 are valid only within a small window of low concentrations.

’ INTRODUCTION To tune the structures and properties of self-assembled architectures of organic molecules different building strategies like functional groups;2 12 substrate;14 19 temperature;20 22 chemical nature of solvent (aromatic interaction,13,23 odd even or parity effect,24 26 saturated and unsaturated solvent,27 alkyl chain length1,2,10); properties of solvent (solubility, hydrophilic, and hydrophobic properties,28 polarity,29 chirality, viscosity30); and so on have been used. Out of all of these, functional groups have been widely exploited because they give an excellent tunability over the strength and symmetry of the structures that one would like to design.31,32 Trimesic acid (TMA) with threefold symmetric carboxylic acid functionality is a suitable candidate for the formation of hydrogen-bond networks because it can act as both hydrogen-bond donor and acceptor at the same time; therefore, it forms stable supra-molecular structures via hydrogen-bonding with other TMA or with different molecules. Typically, TMA assembles on crystalline substrate into two porous networks from fatty acid solutions, the low-packing density “chicken-wire” (0.8 molecules/nm2) and higher-packing density “flower” structures (1.1 molecules/nm2)1 as depicted in Figure 1. Both structures are governed by intermolecular hydrogen bonding and exhibit periodic cavities of ∼1.0 nm diameter. These structures are found to form at both solid liquid interface and at vacuum solid interface (deposited in UHV) on different substrates.1,33,34 The building blocks for chicken-wire and flower structures are dimers or dimers and trimers, respectively. TMA also coassembles with other molecules (1, 3, 5-tris(4-pyridyl)2,4,6-triazine (TPT), terephthalic acid (TPA), coronene), which eventually form different patterns (honeycomb packing motif; bone-shaped packing motif, etc.).35,36 Lackinger et al. showed a solvent-induced polymorphism of TMA in different alkanoic acids (Cn 1H2n 1COOH) from butyric (n = 4) to nonanoic acid r 2011 American Chemical Society

(n = 9) on graphite (0001).1 These STM experiments revealed that in long-chain fatty acids (n = 7, 8, 9) TMA forms a lowpacking density chicken-wire structure, and in short-chain length fatty acids (n = 4, 5, 6, 7), it forms the high-density flower structure. Under electrochemical control depending on an applied electrode potential, TMA may also form high-packing density structures with its molecular plane perpendicular to the substrate plane at solid liquid interface.37 Under UHV condition, different polymorphs of TMA are obtained by varying the coverage of TMA molecules on Au(111) surface.19 The authors have demonstrated a variety of structures including the flower and chicken-wire structures and also shown at high deposition rate the formation of a rather densely packed phase (1.34 molecules/nm2).19 Tahara et al. showed that network formation is affected by the concentration-dependent surface coverage of DBA (hexadehydrotribenzo[12]annulene) derivatives at the TCB (1,2,4-trichlorobenzene)/graphite interface. DBA structures transform from higher density nonporous structures to low density porous structures by dilution of solution.9 We have recently shown the formation of a high density structure of TMA in phenyloctane, a structure which has never been previously observed, which is achieved by increasing the concentration of solute in phenyloctane by sonication.38 Therefore, the question appears whether one could fabricate different structures in a controlled manner by varying the concentration of TMA in the solvent systematically to form structures with different densities, as has been shown in UHV,1 at the solid liquid interface. Moreover, it is not yet clear from the previous investigations of TMA in different fatty acids what is Received: December 7, 2010 Revised: October 4, 2011 Published: October 04, 2011 21743

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a Burleigh ambient STM operating in constant height mode using mechanically cut Pt/Ir (80/20) tips. All experiments were carried out at room temperature. STM images were collected under various tunneling conditions (IT, UT). Experiments have been repeated with different tips and samples to ensure that the images were not influenced by occasional tip and sample artifacts.

Figure 1. Ball-and-stick model of chicken-wire structure (a) and flower structure (b). Chicken-wire structure consists of TMA dimers, whereas the flower structure has intermediate trimers and dimers as building blocks.

the influence of the concentration and the formation mechanism on these high and low density structures. Here we have investigated the self-assembly of TMA molecules dissolved in octanoic acid at different concentrations on a HOPG substrate. Different concentrations are achieved by extended sonication of TMA in octanoic acid. With increasing sonication time, as a consequence of increasing concentration, high-density structures like filled chicken-wire, flower, filled-flower, dodeca-rim (a structure that was never observed before), and so on are formed, although only the low-density chicken-wire structures is to be expected in this solvent.1 A direct relation between the packing density of different structures and concentration of TMA in the solution is established. Analysis of the structures of TMA self-assembled from heptanoic acid (one carbon atom less than in octanoic acid) shows that TMA forms only flower and filled-flower structures and from nonanic acid (one carbon more than octanoic acid) TMA forms only chicken-wire and filled chicken-wire structures. This indicates that the polymorphic boundary between the chicken wire and flower structure of TMA in fatty acid is not at heptanoic acid as previously observed;1 instead it is at octanoic acid. Our studies add an important element of extension to the previous report by Lackinger et al.1 concerning the limiting chain length of fatty acid on the formation of molecular porous network structures of TMA. Moreover, this opens the possibility to tune the polymorphism by only one additional control parameter, the concentration.

’ EXPERIMENTAL SECTION TMA and fatty acids were purchased from Aldrich with a purity of 98% and used as they are for the experiments. TMA (0.05 g) dissolved in 7.5 mL of solvents (heptanoic acid, octanoic acid, and nonanoic acid) was used for preparing the solutions. After having the solutions with TMA sediment, the solutions were sonicated in an ultrasonic bath. The sonication times range from 1 to 10 h. With increasing sonication, the concentration of TMA increases and eventually reaches saturation at extended sonication time. The obtained solutions were kept for decanting (∼2 weeks). The solutions obtained this way were stable for several months because the polymorphs of TMA obtained from them were revealed in a reproducible manner. From this clear solution, 2 μL was applied on a graphite (0001) surface and in situ investigated using STM. Graphite (HOPG) substrates were freshly cleaved along the (0001) plane using adhesive tape. The liquid solid interface was scanned in the range of several hundred nanometers. STM measurements were performed with

’ RESULTS AND DISCUSSION Figure 2 shows STM constant-height images of different polymorphs obtained after different sonication times of TMA in octanoic acid deposited on the graphite interface. At low sonication time (0 to 2 h), clearly a typical chicken-wire structure is formed as depicted in Figure 2a. Three domains are also visible, which are rotated with respect to each other by (2π/3. The distance dc c, (double-headed arrow in Figure 2) is defined as the lattice parameter (distance between adjacent cavities) of the porous structure. dc c remains the same (1.8 nm) in both molecular packing directions. As the sonication time increases, between 2 and 4 h, all cavities of the chicken-wire structure are occupied. This structure is called filled chicken-wire and is depicted in Figure 2b. Although there is one additional molecule filled within the cavity of filled chicken-wire structure, dc c remains the same as that for the chicken wire structure. Including molecules in the cavities yields a packing density of 1.07 molecules/nm2, which is much higher than that for the chickenwire structure (0.71 molecules/nm2). The packing density obtained for the chicken-wire structure is in agreement with the literature.1 It was shown in a previous study that chicken-wire structure may act as a template for TMA itself because the size of one molecule fairly fits the cavity size. The guest TMA molecule in the cavity possesses six possible binding sites, as revealed by the symmetry of the structure. Therefore, the guest TMA has been observed in rather metastable conformations within the cavity.33 However, the formation of host guest complexes of TMA in chicken wire was showed occasionally and not at every pores.33 The filled chicken-wire structure observed in our experiment is rather reproducible, and the guest molecules are present in all cavities. However, the molecules within the cavity are appearing rather faint compared with the TMA molecules in the hexagonal network (marked with hexagon in Figure 2b). This is due to the fact that the guest molecules possess six adsorption sites and the STM contrast of the guest TMA is time averaged. Flower structure with higher packing density is observed from solutions sonicated for longer time window (4 to 5 h, Figure 2c). This is surprising because Lackinger et al.1 predicted that the formation of flower structure is unfavorable in octanoic acid and it should be expected only for lower-chain length fatty acids. The packing density of the flower structure is 1.1 molecules/nm2, which is roughly the same as filled chicken-wire structures but possesses a larger dc c (2.5 nm). Further sonication of TMA in octanoic acid for 5 to 7 h shows that single TMA molecules are filled within the cavities of the chicken-wire structure. This structure is formed again through the host guest mechanism as previously discussed for the filled flower structure. The corresponding STM image is shown in Figure 2d, and the inset is a structural model. TMA molecules within the cavities appear with intensities different from the surrounding molecules; in addition, certain defects (indicated by blue circles) and empty cavities (indicated by green circles) can be observed. This clearly indicates that the features seen within 21744

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Figure 2. STM constant-height image of different polymorphs of TMA in octanoic acid observed in different sonication time windows between 0 and 7 h. (a) Chicken-wire structure (IT = 1.2 nA, UT = 1.4 V) and (b) filled chicken-wire structure (IT = 1.1 nA; UT = 1.5 V); hexagon marks six molecules in the network and inset is a tentative model. (c) Flower structure (IT = 1.1 nA; UT = 1.5 V) and (d) filled flower structure (IT = 1.3 nA; UT = 1.5 V); inset shows the tentative model. The double headed arrows (dc c) depict the lattice parameters of individual structures. The sonication time window at which different structures are obtained is also indicated in the Figure.

Figure 3. STM constant height image (IT = 1.5 nA, UT = 1.4 V) of rearrangement of filled-flower structure to dodeca-rim structure of TMA in octanoic acid above a sonication time of 7 h (a) and dodeca-rim structure (b). In panel a, the upper part (blue arrow on the right margin) depicts the filled-flower structure, whereas the lower one (red arrow on the right margin) is the dodeca-rim structure. The intermediate region (black arrow on the right margin) shows the rearrangement where a fuzzy transition structure is observed. The direction of arrows within the images indicates the scan direction. The inset of panel b is a tentative model for the dodeca-rim structure.

the pores are real molecules and not an imaging artifact. The packing density of this structure is higher than that of the flower structure and other observed structures: it reaches 1.29

molecules/nm2; however, the center-to-center distance between the cavities remains the same as for the flower structure (dc c = 2.5 nm). As previously reported, each hexagonal pore of TMA 21745

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Table 1. Different Adsorption Structures of TMA at Octanoic Acid/Graphite Interface

*

Above 7 h, sonication dc-c of filled flower structure is increased to 2.8 nm.

network structures fairly fits the size of one TMA molecule.33 Therefore, it is not surprising that dc c remains unchanged in filled flower and filled chicken-wire structures. TMA solutions prepared from higher sonication time (>7 h) show a new structure (Figure 3a), which has never been observed before. This structure is named dodeca-rim structure (based on 12 TMA molecules forming the ring). This structure has been observed at several occasions originating from a rearrangement of the filled flower structure, as shown in Figure 3b. The blue dashed lines indicate different domains of the rearrangement. The packing density of the dodeca-rim structure is 1.33 molecules/nm2, which is the largest of all observed polymorphs of TMA in our experiments. It is to be noted that the filled flower structure at higher sonication time (>7 h) from which dodecarim structure is formed (Figure 3a) the dc c is slightly larger (2.8 nm) compared with that of filler flower structure observed at lower sonication time (∼6 h). As previously shown, one TMA in the cavity induces no difference in dc c. This indicates that the observed slightly higher dc c (2.8 nm) is most likely due to more than one TMA molecules in the cavity of flower structure. However, the metastable molecules in the cavities are hard to discern within the STM images. The dodeca-rim structure consists of rings of 12 TMA molecules and a TMA trimer inside, as shown schematically in the inset of Figure 3. The distance between adjacent ring centers is 2.8 nm. This periodicity is large compared with that of both chicken-wire (dc c = 1.8 nm) and flower (dc c = 2.5 nm) structure. It is to be noted that compared with chicken-wire

and flower structure, doceca-rim structure is rather unstable as it is revealed by the fuzzy contrast of individual bright spots within the layer. Each bright spot is assigned to one TMA molecule and thus arrives at a tentative model as proposed in the inset of Figure 3a. The model suggests that the outer 12-membered ring consists of six pairs of TMA molecules, which are linked by closed hydrogen bonds (red oval) and by weaker open hydrogen bonds (white ovals) between adjacent pairs. Open hydrogen-bonded dimers are defined as dimers that possess free hydrogen bond valencies. Consequently, this structure consists of more open hydrogen bonds than the chickenwire (no “open” bonds) and flower structures. Therefore, the stability of dodeca-rim structure is expected to be lower compared with other structures. It might also be possible that the open dimer (white oval) in the 12-membered ring and the trimer in the center are stabilized by invisible solvent molecules. The details of different structures observed at different sonication time together with geometric models are listed in Table 1. To understand different types of structures deposited out of solution at different sonication times, we have performed UV vis adsorption spectroscopy of the solution. By characterizing TMA solved in octanoic acid with known concentration using UV vis spectroscopy, the concentration after different sonication time has been evaluated. Figure 4 shows a plot of concentration as a function of sonication time as well as the packing density of different structures obtained at corresponding sonication time. It is obviously seen that the concentration increases by a factor of ∼1.4 for 8 h sonicated compared to nonsonicated 21746

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Figure 4. Concentration as a function of sonication time. A reference solution with known concentration is compared with the optical density of solutions (UV vis absorption at λ = 286 nm) at different sonication time to obtain their concentration. The UV vis spectra used for the evaluation of the concentration of solutions with different sonication time is provided in the Supporting Information S3. The bar diagram shows different structures obtained at different sonication time windows and their packing density.

solution. The concentration saturates as the time of sonication exceeds 6 h. This demonstrates that by sonication one can increase the concentration of molecules within the solution until to some saturation value. To explain the scenario, one can easily correlate the concentration of the TMA molecules and the physical process associated with sonication. The chemical effects of ultrasound on alkane solutions have been studied since the 1980s.39 44 The ultrasonic energy is used to break H-bonded molecular aggregates that are prevalent in short-range ordered structures of solvents. This effect has been studied in aqueous ethanol mixture to increase the rate constant of solvolysis of 2-chloro-2methyl propane in ethanol39,41 and increase the rate of alkaline hydrolysis.40 Similarly, the rate of reactive dissolution of p-chloranil increased in the presence of ultrasound.42 A solute dissolves in a solvent when it forms favorable bonding interactions like ionic, hydrophobic, hydrophilic, and so on with the solvent. That is, concentration of a solvent is directly related to the number of available interactions for an individual solute molecule within the solvent. At the same time, the solute molecules themselves must be separated from each other. A liquid is stabilized by cohesive molecular forces. Ultrasound waves consist of cycles of compression and expansion. During compression cycles a positive and during expansion a negative pressure is exerted on the local volume of the liquid, which pushes and pulls the molecules together and apart from each other, respectively. During the expansion cycles, the magnitude of the negative pressure may become high enough to overcome the cohesion between neighboring molecules of the solvent.39 41 Consequently, because of sonication, the internal interactions within the solvent weaken and favorable configurations emerge for solute molecules to interact with those of the solvent. As a consequence the concentration of solute may increase with sonication time. In addition, the mechanical energy of the sound is converted into heat during prolonged sonication.40 However, the final temperature achieved for a given time of sonication also depends on the room temperature in the laboratory.43 We therefore determine initially the maximum

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temperature the bath reaches and maintain this temperature by filling the bath with water heated to this final sonication temperature (41 °C in winter time). Therefore, the sonication can be performed under the well-defined temperature conditions (steady state). The heat-generated during sonication within the liquid enables the breaking of intermolecular cohesive interactions (endothermic) in both TMA and octanoic acid, which further enhances the concentration of TMA. The solutions obtained after different sonication time have been stable over several months (proved by the same polymorphs always deposited from them indicating the unchanged concentration of the solute). Although TMA dissolves in all fatty acids, its solubility (concentration) decreases with increasing aliphatic chain length.1 TMA has three carboxylic acid groups ( COOH); each of them can act as both hydrogen-bond donor and acceptor at the same time. Fatty acid consists of nonpolar aliphatic tail group (CH3(CH2)n ) and polar headgroup ( COOH). The molecular dipole moments align fatty acids with either head-to-head (COOH groups interact with each other) or tail-to-tail orientation in the solvent.45 For increasing length of the tail group of fatty acid, the solubility of TMA decreases due to the fact that the polar nature of molecule effectively decreases. Alhough it is expected that octanoic acid and TMA interact via COOH groups, the long tail groups of octanoic acid hinder the interaction; therefore, the solubility of TMA is only moderate. As discussed above, during sonication, the head-to-head interactions of solvent get weakened, and the solvent molecules become more and more individual, which considerably increases the concentration of TMA in octanoic acid. As shown above, the molecular concentration increases with sonication time and the packing density of structures deposited after longer sonication time increases as well. The packing density of different structures observed after different sonication time windows is depicted using a bar diagram in Figure 4. The packing density observed clearly correlates with the sonication time. When a freshly prepared surface is exposed to the solution, the solute molecules (TMA) are forced toward an equilibrium configuration at the surface. In other words, the structure formation in this system is possible by entropy export to the environment because lowering of molecular concentration in the solvent due to adsorption of molecules at the surface enables the solution to reach its equilibrium via an exothermic process. The energy intruded into the solution during sonication is used to “break the short-range order” in it. As the sonication time increases, more and more solute molecules are available in the solution and may become involved in the formation of the adsorbate structure on surface. Therefore, the structures formed at higher sonication time consist of more densely packed structures compared with that formed at lower sonication time, in accordance with the experiments. The nonequilibrium between TMA solved in the octanoic acid and TMA adsorbed on the surface might be the origin of rearrangement of filled flower structures to the dodeca-rim structure during the scanning with the STM tip. The relatively unstable adsorbate structure together with the quite large amount of molecules in the solution may force a rearrangement of the adsorbate structure to a much higher packing density, which finally accommodates more molecules on the surface. The energy transfer from the scanning tip to the surface region might help to overcome a possible energy barrier hindering the spontaneous formation of the new structures. 21747

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The Journal of Physical Chemistry C The scenario of sonication time-controlled polymorphism is compared also for TMA solved in heptanoic acid (aliphatic chain contains one carbon atom less than octanoic acid) and for nonanoic acid (one carbon atom more than octanoic acid). As known from the literature,1 TMA forms both chicken-wire and flower structures in heptanoic acid and only chicken-wire in nonanoic acid. These results are clearly reproduced with corresponding solutions at low sonication time (Supporting Information S1 and S2). However, when sonication time increases, TMA forms filled flower and dodeca-rim structure in heptanoic acid and filled-chicken-wire structure in nonanoic acid. This clearly indicates the formation of structures with high packing density as a consequence of the increasing concentration of TMA in heptanoic and nonanoic acid after increasing the sonication time. As shown in ref 1, the solubility of TMA decreases in the order heptanoic acid > octanoic acid > nonanoic acid, and it has been shown that the high packing density structures also follow the same order. The limit for TMA molecules self-assembled to both chicken-wire and flower structures was found for heptanoic acid.1 Here we have shown that this boundary of coexistence can be shifted to octanoic acid by tuning the concentration of TMA, which reveals the consequence of concentration toward the equilibrium structure. Moreover, it has also been shown that the same type of polymorphism can be seen within one solvent depending on the concentration of the solution.

’ CONCLUSIONS By tuning the concentration of TMA in fatty acid solutions, 2D polymorphic structures of TMA segregated on HOPG substrates have been observed. Their packing density increases as a function of concentration of TMA within the solvent. The previously found adsorbate structures in literature correspond to a window of relatively low concentration of TMA in the solutions. A more complete set of adsorbate structures has been shown in the experiments presented here for a larger window of concentrations. This has become accessible by increasing the concentration via a sonication treatment of the solutions. The formation of high-packing density structures is correlated to the increasing amount of TMA molecules in the solution. ’ ASSOCIATED CONTENT

bS

Supporting Information. Different structures formed at heptanoic acid-graphite (S1) and nonanoic acid-graphite (S2) interfaces. The UV vis spectra used for calculating concentration of solutions with different sonication time (S3). This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +49 37153121639.

’ ACKNOWLEDGMENT Nguyen Thi Ngoc Ha acknowledges financial support from Deutscher Akademischer Austauschdienst (DAAD).

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dx.doi.org/10.1021/jp111640t |J. Phys. Chem. C 2011, 115, 21743–21749