Thermodynamic Control of 2D Bicomponent Porous Networks of

Article pubs.acs.org/JPCC

Thermodynamic Control of 2D Bicomponent Porous Networks of Melamine and Melem: Diverse Hydrogen-Bonded Networks Shinobu Uemura,†,§ Masashi Aono,† Kenki Sakata,† Tamikuni Komatsu,‡ and Masashi Kunitake*,† †

Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan

‡

S Supporting Information *

ABSTRACT: Different bicomponent self-assembled structures comprising melamine and melem were formed at aqueous solution−Au(111) interfaces by varying the concentrations of melamine and melem and the electrochemical potential. The structures were observed by in situ scanning tunneling microscopy. The structures of the bicomponent networks were controlled thermodynamically through the surface concentration and the molecular ratio. Interestingly, the solution concentration did not directly reflect the surface density of the self-assembled structures because of the flocculation-like behavior around the interface. Furthermore, structural phase transitions between the melem monocomponent honeycomb network, the bicomponent honeycomb network, and the melamine monocomponent honeycomb network with aggregates were directly and reversibly observed by controlling the electrochemical potential.

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INTRODUCTION Two-dimensional (2D) self-assembled porous networks of organic molecules that have structural features and functional properties that can be controlled by molecular arrangement are eagerly anticipated in nanoscience, nanomaterials, and nanotechnology.1−6 Such “open” porous networks are composed of building blocks with multiple bridging sites for van der Waals interactions,7−11 hydrogen bonding,12−26 metal coordination,27−29 and covalent bonding.30−35 Among the bridging bonds, hydrogen bonds are frequently found in 2D porous networks formed under ultrahigh vacuum (UHV),12−16 and in organic17−24 and aqueous solutions25,26 because of their reversibility25,36 and diversity. Melamine (Figure 1, left) is a planar molecule with 3-fold symmetry and primary amino groups on each vertex that is

Melem (2,5,8-triamino-tri-s-triazine, Figure 1, right) is a structurally similar compound to melamine and possesses a heptazine ring with 3-fold symmetry and primary amino groups on its vertices. Melem and melon (a carbon nitride) were first observed by Berzelius in the 1830s and named by Liebig.48 Various derivatives with the heptazine ring have been synthesized to improve the solubility, because their inertness and insolubility lead to difficulty in characterizing their molecular structures.49−52 Recently, a visible light responsive metal-free catalyst of graphitic carbon nitrides (GCNs) was reported by Antonietti and Domen,53 and GCNs have attracted considerable attention as functional materials. The structure of GCN was fully characterized by Komatsu,54 and this led to the theoretical estimation that the catalysis reaction occurred around the area surrounded by heptazine rings. The GCN unit molecule melem is a promising candidate as a building block in crystal engineering and supramolecular chemistry to give novel nanomaterials.55−57 Previously, we reported an in situ scanning tunneling microscopy (STM) study of 2D monocomponent selfassembled structures of melamine and melem at aqueous solution−Au(111) interfaces.58 Lackinger et al. reported different self-assembled structures of melem on Ag(111) under UHV.59 Increasing the number of hydrogen bonding sites in the building blocks enhances the structural diversity. In this article, we report the construction of bicomponent selfassembled structures of melamine and melem at aqueous solution−Au(111) interfaces. Different highly ordered bicom-

Figure 1. Chemical structures of melamine (left) and melem (right).

frequently used to construct mono-37−40 and multicomponent 2D13,39,41−45 and three-dimensional (3D)46 supramolecular structures. As a typical 2D self-assembly, open honeycomb networks consisting of melamine and perylene bisimide on Ag/ Si(111)-√3 × √3R30° were constructed through the formation of multiple complementary hydrogen bonds under UHV.13 Guest molecules such as C60 have been encapsulated in the cavities of honeycomb networks.5,13,47 © 2013 American Chemical Society

Received: July 10, 2013 Revised: October 29, 2013 Published: November 1, 2013 24815

dx.doi.org/10.1021/jp406810c | J. Phys. Chem. C 2013, 117, 24815−24821

The Journal of Physical Chemistry C

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ponent structures were obtained by systematically changing the solution concentrations of melamine and melem and the electrochemical potential.

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EXPERIMENTAL SECTION Melamine was purchased from TCI Co., Ltd. (Tokyo, Japan), and used without further purification. Melem was synthesized as described in a previous report.54 Weakly basic solutions (ca. pH 9−10) of melem and melamine containing electrolyte (NaClO4 and NaOH)60 were prepared in the same manner as a previous report.58 Because of the different solubilities of melamine and melem, concentrations of melamine nearly a thousand times higher than those of melon were deliberately used for construction of the bicomponent structures. Au(111) facets formed on single-crystal beads were directly used for electrochemical STM. The Au(111) electrodes were annealed with a hydrogen flame and cooled by immersion in ultrapure water immediately after annealing. The annealed Au(111) was transferred into the electrochemical STM cell containing the prepared solutions. Electrochemical STM investigations were carried out in aqueous solutions at room temperature using a scanning tunneling microscope (Nanoscope E, Digital Instruments (Present: Bruker AXS), Santa Barbara, CA, USA). A tungsten tip was prepared by electrochemical etching in 0.1 M KOH, and then covered with clear nail polish to minimize residual Faradaic currents. Platinum wires were used as reference and counter electrodes. All reported potentials are referenced to the saturated calomel electrode (SCE). STM images were obtained in constant current mode.

Figure 2. Phase diagram with the observed structure models for different concentrations of melamine and melem in aqueous solutions under the OCP. The saturated solution concentrations of melamine and melem were 2.5 × 10−2 and 1.8 × 10−5 M58 in ultrapure water.

MLr-T structure, the outer melem rings of the MLr-D structure are surrounded by rings of 12 melamine molecules (six dimers of melamine) to form a honeycomb network (Figure 3e). The unit cell (black diamond in Figure 3e) parameters of a, b = 3.4 ± 0.2 nm and α = 60 ± 5° are larger than those of the MLr-D structure. Interestingly, the MLr-T structure consistently contained black contrast features, which might be defect regions indicating the absence of melamine. The locations of the black contrast features were different at each sequential image captured at the same position, as shown in Figure S2 (Supporting Information). These defects were only observed for the MLr-T structure. This indicates a dynamic process between incorporation and desorption of melamine molecules in the lattice. Note that the MLr-T structure requires the highest concentration of melamine to shift the partition equilibrium from solution to surface, indicating that melamine easily desorbs from the lattice. A solution with high concentrations of both melamine (5000 μM) and melem (9 μM, ca. 50% of the saturated melem solution) produced AR networks (Figure 3f and g). In the AR structure, equal numbers of melamine and melem molecules are alternately aligned to form a six-membered honeycomb structure consisting of three melamine and three melem molecules. The AR network exhibited the lowest melamine/ melem ratio of the observed bicomponent honeycomb networks. The unit cell (black diamond in Figure 3h) parameters of the AR structure are a, b = 1.2 ± 0.1 nm and α = 60 ± 5°. The size of the unit cell is between the unit cell sizes of a monocomponent melamine honeycomb (ML-HC) network and a melem honeycomb (MM-HC) network, and is the smallest of the three bicomponent honeycomb networks. In all the mono- and bicomponent honeycomb networks, each molecule is bound to three others via side-by-side hydrogen bonds (Figure 4). In the observed bicomponent honeycomb networks, both homo- and hetero-intermolecular hydrogen bonds were found. The melem molecules in all the bicomponent networks were bound to three melamine

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RESULTS AND DISCUSSION Three individual bicomponent honeycomb networks with different unit cells and cavity sizes, melamine-rich double-ring (MLr-D), melamine-rich triple-ring (MLr-T), and alternating ring (AR) were constructed by changing the solution concentrations of melamine and melem at the open circuit potential (OCP), as shown in the phase diagram (Figure 2). Typical STM images and the corresponding models of the three networks on Au(111) surfaces are shown in Figure 3. All of the honeycomb networks were uniform over atomically flat whole terraces of Au(111) around the OCP (between −0.35 and −0.15 V). In the high resolution images (especially Figure 3b and g), two different-sized triangular features can be clearly distinguished. Such high submolecular resolution allows us to elucidate the arrangement of the molecules, and thus the hydrogen bond motifs of the respective structures. The MLr-D structure was observed at low concentrations of both melamine and melem, as shown in Figure 3a and b. Typically, the concentrations of melamine and melem were 100 μM (ca. 0.4% of the saturated solution) and 0.45 μM (ca. 2.5% of the saturated solution), respectively, which are about the lowest concentration of each component required to construct the monocomponent network.58 In the MLr-D structure, each isolated small ring consisting of six melamine molecules is surrounded by a larger ring of six melem molecules. The unit cell (black diamond in Figure 3c) parameters were a, b = 2.0 ± 0.1 nm and α = 60 ± 3°, respectively. In solutions with a high concentration of melamine (e.g., 5000 μM, ca. 20% of the saturated solution) and a low concentration of melem (e.g., 0.45 μM), the MLr-T structure with a high proportion of melem appeared (Figure 3d). In the 24816



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Figure 4. Hydrogen bonding systems of melem−melem (upper), melamine−melamine (bottom right), and melamine−melem (bottom left).

hydrogen bonding combinations increased as the symmetry of the structure decreased and the size of the unit cell increased. The hetero-intermolecular hydrogen bonds between melamine and melem were of similar strength to the homo-intermolecular hydrogen bonds. No phase separation of melamine and melem was observed under the same conditions. Schematic models of the mono- and bicomponent honeycomb networks with unit cell parameters are summarized according to the surface density in Figure 5. The surface densities of the bicomponent honeycomb networks were between those of the monocomponent ML-HC and MM-HC networks. The molecular ratios of melamine:melem in the unit cells were 3:1, 2:1, and 1:1 for the MLr-D, MLr-T, and AR structures, respectively. As the melamine/melem ratio decreased, the average surface density of melamine and melem decreased and the cavity increased because of differences in molecular size. Thermodynamically self-assembled structures formed at solution−solid interfaces are governed by the interface concentrations of the components, which are essentially controlled by the adsorption and partition equilibriums as a function of temperature36 and solution concentration.25,58 For solutions with high concentrations of both melamine and melem, the AR structure with the highest proportion of melem (melamine:melem = 1:1 in Figure 5) was observed. The MLr-T network was observed with an intermediate proportion of melem (melamine:melem = 2:1), that is, for solutions with a high concentration of melamine and a low concentration of melem. The correlation between the AR and MLr-T structures can be explained as a change of the thermodynamically controlled partition equilibrium, which is controlled by the concentration of melem (Figure 2). Curiously, the MLr-D network, which has the highest proportion of melamine (melamine:melem = 3:1) and surface density in the observed bicomponent networks, formed for solutions with low concentrations of both melamine and melem. This inconsistency between the MLr-D and MLr-T networks can be explained by the shift of the partition

Figure 3. Typical STM images (a, b, d, f, and g) and models (c, e, and h) of bicomponent self-assembled structures of melamine and melem at the aqueous solution−Au(111) interfaces under OCPs of between −0.35 and −0.15 V vs SCE. (a−c) Melamine-rich double-ring (MLrD) network. The concentration ratio of melamine:melem was 100 μM:0.45 μM. Es = −190 mV vs SCE, Ebias = −200 mV, and It = 20 nA. (d and e) Melamine-rich triple-ring (MLr-T) network. The concentration ratio of melamine:melem was 5000 μM:0.45 μM. Es = −295 mV vs SCE, Ebias = −200 mV, and It = 5 nA. (f−h) Alternating ring (AR) network. The concentration ratio of melamine:melem was 5000 μM:9 μM. Es = −355 mV vs SCE, Ebias = −200 mV, and It = 20 nA.

molecules, but the combinations of the three molecules bound to each melamine systematically changed for the different honeycomb networks. Each melamine molecule was bound to three melem molecules in the AR structure and two melamine and one melem molecules in the MLr-D structure. In the MLrT structure, there were two hydrogen bonding combinations: one with the melamine molecules in the center melamine rings bound to two melamine and one melem molecules and the other with melamine molecules in the outer melamine rings bound to one melamine and two melem molecules. Thus, the 24817



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Figure 5. Schematic models, unit cell parameters, surface densities, and hydrogen-bonded neighbor molecules of the observed mono- and bicomponent honeycomb networks. “All” surface density is the sum of surface densities of melamine and melem. Neighbor molecules indicate the number of molecules hydrogen bonded to melamine or melem.

Figure 6. In situ STM images of self-assembled structures of melamine and melem at aqueous solution−Au(111) interfaces. (a) Es = −555 mV, It = 2 nA, and Ebias = −200 mV. (b) Es = −355 mV, It = 20 nA, and Ebias = −200 mV. (c) Es = −55 mV, It = 20 nA, and Ebias = −400 mV.

melem-rich honeycomb networks such as melem-rich double rings (MMr-Ds) (Figure S1, Supporting Information). The observed MM-HC networks occasionally contained an unclear feature, probably a guest melamine38 or gold atom40 in their cavities (Figure S3, Supporting Information). The cavities of melem rings might be too large to immobilize a guest melamine by hydrogen bonds or trap a single gold atom, while they might be too small to trap a melem molecule. Considering the adsorption and partition equilibrium conditions of the solution, this may suggest that a host−guest-like structure with (nonhydrogen-bonded) melamine incorporated in the cavities is thermodynamically favored rather than melem-rich networks (see Figure S1, Supporting Information). In situ phase transitions of the self-assembled networks were achieved by controlling the surface potential. Figure 6 shows

equilibrium of melem from the solution phase to the Au surface with melamine. The UV−vis spectra (Figure S4, Supporting Information) revealed that the coexistence of melamine decreased the solubility of melem by flocculation, indicating that the surface concentration of melem increased in the mixed solution. Eventually, the surface density of melem in the MLr-T structure would become higher than that in the MLr-D structure, even though the solution ratio of melem for the MLrT structure was lower than that for the MLr-D structure. Consequently, all of the results were essentially governed by thermodynamics. In a solution with a relatively low concentration of melamine (ca. 0.4% of the saturated concentration) and high concentration of melem (50−90% of the saturated concentration), MM-HC networks were observed instead of the presumed 24818



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In the solutions for other bicomponent honeycomb networks such as MLr-Ds, the phase transitions between the MM-HC, each bicomponent, and the ML-HC networks could be induced by controlling the potential. Other bicomponent networks were not observed in the overlapping potential range of monocomponent melamine and melem. This might be because the potential range was too narrow for several phase transitions and thermodynamic control dictated by the solution concentration dominates.

typical STM images and a schematic illustration of the change of the self-assembled network at the concentrations for AR networks (5000 μM melamine and 9 μM melem) by electrochemical control. An AR network was formed at the OCP (ca. −0.2 V) as mentioned above and consistently observed in the potential region near the OCP (−0.35 to −0.15 V), as shown in Figure 6b. Upon shifting the potential to the negative region, the MMHC network appeared between −0.35 and −0.6 V (Figure 6a), and no adsorbate was observed below −0.6 V. This adsorbatefree region was similar to that of the monocomponent solution of melem,58 and was caused by desorption or rapid surface diffusion of molecules. Conversely, aggregates of melem and short periodic honeycomb networks of ML-HC with a few disordered areas were observed at potentials between −0.15 and 0 V (Figure 6c). MM-HC, AR, and ML-HC networks reversibly and repeatedly formed by changing the surface potential. Generally, under electrochemical control, neutral molecules physically adsorb onto the electrode with the highest density at the potential of zero charge, and the adsorption strength and surface density of the molecules decrease as the surface polarization increases.61,62 For instance, absorption of TMA25 and melem58 on Au(111) exhibits a similar potential dependence, with phase transitions between desorption, honeycomb networks, and close-packed structures from negative (low density because of weak adsorption) to positive potential (high density caused by strong adsorption). At each potential, the surface density is thermodynamically driven by the balance between adsorption and desorption, and the most thermodynamically stable structures preferentially self-assemble. In the case of the melamine−melem bicomponent system, MM-HC, AR, and ML-HC networks were observed in order as the potential was shifted from negative to positive. The order that these structures appear is logical from a thermodynamic viewpoint, because it agrees with the order of the average surface density of the networks in Figure 4 (MM-HC < AR < ML-HC). The observed potential region of the AR network overlapped with those of each monocomponent self-assembled structure. Both order-to-order phase transition potentials of the bicomponent networks, ca. −0.35 and −0.15 V, roughly agreed with the desorption potential of melamine and the potential for an order−disorder phase transition of melem in each monocomponent solution, respectively (bottom of Figure 6). Considering the surface densities of the self-assembled structures, the melem monocomponent close-packed structures (MM-CPs) had a similar surface density to the AR networks (Figure S1, Supporting Information), so MM-CP may form in the same potential range. The reason why the AR network formed in this range is related to both the solution concentration and hydrogen bonding systems. The head-totail hydrogen bonds in the MM-CP structure are weaker than the side-by-side hydrogen bonds between melem molecules and between melem and melamine molecules, because structures consisting of only head-to-tail hydrogen bonds were not observed (Figure 4). Specific intermolecular interactions can overcome the decrease in thermodynamic energy from the low surface density.25,58 As a result, the AR network composed of hetero-intermolecular side-by-side hydrogen bonds formed rather than the MM-CP even though they have similar surface densities.

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CONCLUSIONS We have constructed tunable nanoarchitectures of melamine and melem by spontaneous adsorption from aqueous solutions. The close structural similarity and multiple hydrogen bonds between melamine and melem allow us to achieve controllable systematic diversity with long-range periodicity in the bicomponent nanoarchitectures. The bicomponent nanoarchitectures are thermodynamically controlled by the surface density and molecular ratio, which are functions of the solution concentration and the surface potential.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis spectra of melamine and melem, an additional STM image, and all respective self-assembled structures of melem and melamine with surface densities. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, 2217-20 Hayashi-cho, Takamatsu, Kagawa 761-0396, Japan. Author Contributions

All of the authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Young Scientists (A) (23681016) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and partially by Core Research for Evolutional Science and Technology, Japan Science and Technology Agency (CREST, JST), and Grant-in-Aid for Scientific Research on Innovative Areas Elements Block (24102006).

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