Platinum Nanoscale Lattice on a Graphite Surface Using

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Platinum Nanoscale Lattice on a Graphite Surface Using Cetyltrimethylammonium Bromide Hemi- and Precylindrical Micelle Templates Marsil K. Kadirov,* Irek R. Nizameev, and Lucia Ya. Zakharova A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, 8, ul. Akad. Arbuzova, Kazan 420088, Russia S Supporting Information *

ABSTRACT: One-dimentional (1-D) thin-layer (2−5 nm) parallel strips of Pt on a graphite surface have been synthesized via a template-directed chemical deposition of Pt. The templates are a surface micellar strip of cetyltrimethylammonium bromide (CTAB) at highly ordered pyrolytic graphite (HOPG). The concentration- and temperature-dependent morphology of surface micellar strips of CTAB at the graphite/aqueous solution is elucidated by using the atomic force microscopy (AFM) softcontacting techniques. The dimentions and repeat period of the Pt strips can be widely controlled by the temperature: the width is from 47 to 169 nm and the period from 134 to 233 nm in the temperature range 25−33 °C. The morphological characteristics of the Pt strips depend on those of the original surface micellar strips. The fact that the strips are composed of metallic platinum was confirmed by testing the membrane electrode assembly with the strips in a special fuel cell. This approach could be extended to fabricate a wide range of 1-D self-assembling metallic nanostructures on surfaces using micelle-like self-assemblies carrying metal ions at interfaces.

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The first stage in these strategies consists of self-organization of amphiphilic molecules at the solid substrate. Our sphere of interest is also in the precipitation of small but closely situated catalytic particles for membrane electrode assemblies of fuel cells and design of supramolecular systems based on amphiphilic compounds with various functionality, i.e., nanoscale reactors and containers as well as the examination of their structural behavior by a complex of methods such as tensiometry, dynamic light scattering, AFM, EPR, etc.21−28 The self-organization of surfactants in bulk solution and at the interface is the key feature determining the function of a majority of nanotechnological soft products. Therefore, understanding the factors controlling the surfactant interfacial behavior still remains a challenging task. Manne et al.29 have demonstrated that an atomic force microscope (AFM) can be used to image surfactant periodic structures at surfaces in a form of hemicylinders. The nature of substrate, surfactant geometry, counterion type and concen-

INTRODUCTION Nanostructured platinum is of particular interest because of its potential use for many applications, including catalysis,1−4 electrocatalysis in polymer electrolyte fuel cells,4,5 sensors,6 and other devices.3,7−9 Great efforts have been made to synthesize nanostructured Pt structures such as nanoparticles,2,10 nanowires,11−13 nanosheets,14 and others.15−18 The synthesis of additional types of Pt nanostructures is highly desirable and potentially technologically important. New methods for the synthesis of metal nanostructures are important for providing the reproducibility and control over properties required for advanced technological applications. One-dimensional (1-D) self-assemblies of Pt nanoparticles such as wire or necklace-like morphology on a graphite surface have been synthesized via a template-directed sintering process of individual nanoparticles, using nonionic/cationic mixed hemicylindrical micelle templates of dodecyldimethylamine oxide surfactant at graphite/solution interfaces.19,20 However, the factors that determine 1-D self-assembly morphology are not clear yet. In addition, the catalytic reactivity of Pt nanoparticles is also influenced by such interfacial or environmental factors. © 2012 American Chemical Society

Received: December 8, 2011 Revised: April 17, 2012 Published: May 15, 2012 11326

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at the HOPG/surfactant solution interface and subsequent reduction of Pt salts. A drop of 1 mM CTAB and 0.1 mM H2PtCl6·6H2O water solution has been applied on the surface of freshly cleaved HOPG and then kept for 30 min for the formation of surface micelles at the interface at the desired temperature. The PtCl62− counterions bound electrostatically to the trimethylammonium groups of the surface micelles were then reduced with excess hydrazine (5 mM) during one hour. HOPG substrates were cleaned with water and ethanol to remove the adsorbed surfactant micelles on the HOPG and then were dried again at 40 °C. 4. Control and Measurement of Cell Temperature. The cell temperature was varied using a Peltier device under the solid sample. The temperature was measured using a small thermocouple, placed directly into the fluid cell through a special channel. The temperature was measured using a DigiSense Thermocouple Thermometer. After setting a specific temperature, we waited for some time until the system had reached equilibrium. Achievement of equilibrium was determined from the equalization of temperature on the Peltier element and a thermocouple located in the fluid cell, i.e., when the temperature at the bottom of the fluid cell reaches the temperature in the liquid. 5. Atomic Force Microscopy (AFM) of Surfactant SelfAssemblies. Microscopic images were captured in situ at the fluid cell of the scanning probe microscopy MultiMode V (Veeco instruments Inc., USA) using silicon cantilevers RTESP (Veeco instruments Inc., USA) with nominal spring constants of 40 N/m (tip curvature radius is of 10−13 nm). The solution was held in a fluid cell (MMTMEC model) and sealed with a rubber O-ring. Before each experiment, the fluid cell and the Oring were cleaned first with water and then acetone. Images were captured with the following feedback settings: integral gain was in the range of 0.5−1, and the proportional gain was 5−10. The scan rate was maintained in the range of 1−2 Hz. Distances in lateral dimensions were calibrated by an imaging special calibration grid (STR3-1800P, VLSI Standards Inc.) in the temperature range 20−60 °C. Distances normal to the surface were calibrated by measuring the depth of the bars of the same grid. The nonlinearity of the piezoelectric crystal has not been observed in this range. The antivibrational system (SG0508) was used to eliminate external distortions. Images were obtained using contact AFM techniques. While maintaining a stable image, the AFM tip was as close to the surfactant film as possible. The HOPG surfaces with the surface micelles in liquid were imaged using AFM operated in contact mode and the substrate surfaces with the grown platinum strips in tapping mode in air. 6. X-ray Fluorescence Spectra. Spectra were obtained using an energy-dispersive X-ray spectrometer EDX 800HS2 (Shimadzu). It has an active diameter of 10 mm. All measurements were made using a 50 W low-power Rh tube (50 kV, 625 μA) with an acquisition time of 150 s (live time). 7. Procedure for Preparing and Testing the Membrane Electrode Assembly (MEA). Production of MEA with a platinum catalyst was carried out in three phases: the first two of them coincide with subparagraphs 2 and 3 of this section, but instead of HOPG a carbon paper (gas diffusion layer Sigracet 25CC) with a size 17 × 10 mm2 has been taken. In the the third stage, the final carbon paper with platinum aggregates is cut into two equal parts with dimensions 17 × 5 mm2 (anodic and cathodic sides of the MEA) and covered with a layer of a liquid nafion (the liquid nafion has been prepared in accordance with

tration, pH, and ionic force can influence the shape of the surface aggregates.30−36 The temperature was a parameter which did not attract much attention in the course of studying the surface surfactant aggregation. Meanwhile, it is the key factor determining the solution behavior of amphiphilic compounds. Several vectors of the temperature effects upon the interfacial behavior of surfactants can be identified, e.g., temperature effects on adsorption/desorption behavior, aggregation, crystal lattice, etc. Thus, the aggregation stage assumes a complex type of the temperature dependence of the interfacial dynamics (exchange of monomers between aggregates and solution, reversible disintegration of surface aggregates, etc.), the negative hydration of alkyl chains (the source of the hydrophobic effect) and head groups, cmc values, and size of aggregates, with the listed processes dependent on the thermal motion of molecules. On the whole, the analysis of literature showed that since the first observation by Manne et al.29 of the micellar strips in the cetyltrimethylammonium bromide (CTAB) solution at the graphite/solution interface in 1994, many papers dealing with the morphology of cationic, anionic, and nonionic surfactants on substrates such as graphite, mica, quartz, and gold were published. However, there was no clear understanding of the period dependence on temperature and concentration, of the surface micellar phase transition to the bulk one, and of the balance between these two phases. To elucidate the tendencies that govern the temperature effect upon the surface aggregation of the cationic surfactant CTAB on the graphite/aqueous solution interface, the temperature and concentration dependences of the metric characteristics of surfactant aggregates on the graphite/CTAB aqueous solution interface have been studied in this paper. The second problem that has been solved in the present study is a regulation of the morphological characteristics of the synthesized platinum nanoscale lattice by a changing temperature of deposition, which is a direct consequence of the temperature dependence of the original hemi- and precylindrical micelle template morphological characteristics.

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EXPERIMENTAL SECTION 1. Reagents and Materials. Hydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6·6H2O) was purchased from SigmaAldrich. CTAB was purchased from Sigma-Aldrich and was recrystallized two times from hot acetone. Hydrazine-hydrate was purchased from Sigma-Aldrich. The highly ordered pyrolytic graphite (HOPG, Veeco instruments Inc., USA) was used in all measurements as a solid substrate. Before each experiment, HOPG was dried at 40 °C for 2 h. The experiments showed that this phase of preliminary preparation of the substrate is an important role. 2. Preparation of Solutions for Study of Surfactant Self-Assemblies. Solutions were conditioned at room temperature before injection into the AFM fluid cell. Two milliliters of the initial solution was placed into the fluid cell AFM. When the liquid is poured into the cell, special attention should be paid to the absence of air bubbles in the liquid. Otherwise investigating with AFM methods is impossible because of the inhomogeneity of the medium in which the cantilever is located. After each experiment, the cell was cleaned with distilled water and acetone to remove residual solution. 3. Preparation and Reduction of Pt Salts. Pt nanostructures on the HOPG surface were synthesized basically in two steps involving preparation of a surfactant self-assembly 11327

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the described procedure).37 Then the two parts by the sides covered with liquid nafion are pressed onto a membrane (Nafion 117) from two sides under the influence of short-term pressure of 4.1 MPa at room temperature. The MEA was placed in the body of a special Fuel Cell38 and tested on a test station with electrochemical unit ECL 450 (ElectroChem Inc.) and gas-distributive unit HAS (ElectroChem Inc.). The flow rate of hydrogen was 14 cm3 min−1, and that of oxygen was 7 cm3 min−1.

h=

(rt + rm)2 − (rm + 0.5g )2

we obtain dZm = rt + rm −

rt2 + rm(2rt − g ) − 0.25g 2

for the range of g in accordance with the inequality r2t + rm(2rt − g) − 0.25g ≥ 0. In the limits of the inequality, the amplitude of topographic modulation dZm increases with increasing distance g between the walls of hemicylinders. Figure 4 shows the line of the section of AFM images of adsorbed structures in the 1.1 cmc CTAB solution in water at the graphite/solution interface at the temperature of 25.2 °C and the corresponding topographic modulation of the needle in the vertical direction. Probably, the choice of the experimental temperature (Figures 4 and 5) of 25.2 °C requires some comments because it is close to the bulk Krafft temperature for CTAB. This choice is due to the fact that above 25 °C the concentration-dependent critical temperature behavior occurs, which can affect the results. In addition, there are essential doubts regarding the occurrence of the Krafft boundary for the surface aggregation and its identity to the bulk parameters. The possibility of studying the vertical section of the interphase boundary on height variation of the AFM tip was discussed previously.39,40 However, due to the low (less than 0.2 nm) sensitivity of the needle on the height, the authors failed to obtain satisfactory information on the section of the interphase boundary. Using a sharp silicon tip, the authors39,40 managed to increase the sensitivity of the needle height, i.e., the amplitude of topographic modulation, up to ≈1.3 nm. In our studies, measuring the amplitude of topographic modulation yielded important information on trends in the distance between the aggregates and the packing density. For small periods of parallel strips up to the Pj, an interesting dependence of the period of hemicylindrical CTAB micelles (Phc) at the graphite/solution interface on the concentration of a surfactant in the range of 0.5−11.1 cmc has been revealed (Figure 5A). The period decreases from about 8.7 nm and tends to less than 5 nm during the increase of concentration. Considering that twice the length of the CTAB molecule is 4.54 nm, we can assume that the radius of hemicylinders approximately equals twice the length of the surfactant molecule, and the distance between the hemicylinders decreases with the increase of concentration, approaching the close packing of hemicylinders at c = 11.1 cmc. The growth of the packing density of the hemicylinders is confirmed by the dependence (Figure 5B) of the amplitude of topographic modulation on the concentration of surfactant hemicylindrical micelles at the graphite/solution interface, when dZm decreases from 0.7 nm for c = 0.5 cmc to 0.4 for c = 11.1 cmc. The same character dependence of the period of aggregates of surfactants on the boundary of the graphite/solution on the concentration of SDS was observed previously.40 With the concentration of SDS increasing from 3 to 100 mM, the adsorbate period was reduced from 7 to 5.2 nm. The seal packing of hemicylindrical micelles occurred39 under increasing concentrations of Mg2+ due to the neutralization of electrostatic repulsion of the headgroups of SDS. For intermediate concentrations of 1.1−11.1 cmc at temperatures of period jump, a gradual increase in the period is observed (Figure 2), and then at the temperatures after the jump, the stabilization of the period occurs at values equal to about twice the period before the jump.

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RESULTS Surfactant Self-Assemblies. Figure 1 shows AFM images of adsorbate structure of 1.1 cmc (here cmc is critical micelle

Figure 1. AFM images of adsorbate structure of 1.1 cmc CTAB on the graphite surface at (A) 25.2 °C and (B) 29.0 °C.

concentration) CTAB on the graphite surface at 25.2 and 29.0 °C. This structure is a system of strips with a characteristic distance that depends on the temperature, in the direction perpendicular to the direction of the strips. The period of strips denoted as P in the case shown in Figure 1 increases from 7 nm at the temperature of the solution 25.2 °C to 14 nm at 29.0 °C. The results exemplified by the AFM images in Figure 1 encouraged us to conduct a systematic study of the morphology of the CTAB cationic molecules in aqueous solution at the graphite/solution interface in the wide concentration range of 0.5−111 cmc (cmc = 0.9 mmol/L) and at temperatures 22−33 °C. Figure 2 shows the temperature dependence of the period of parallel strip repetition of the CTAB surface micelles at different concentrations at the graphite/solution interface. As can be seen, at low (0.5 cmc) and high (111 cmc) surfactant concentrations the period of strip is weakly dependent on temperature, remaining within the range of 8−8.5 nm at low concentrations and 12−14 nm at high. However, in the intermediate concentrations of ≈0.7−11.1 cmc, a sharp increase of the period (let us call it a jump of period or a period jump Pj) in the narrow temperature range of 1−1.5 °C is observed. The temperature of the Pj is in the range from 26.5 to 28.5 °C. We should like to draw your attention to another detail, which is important in explaining these patterns. This is the temperature dependence of the value of the so-called topographic modulation dZ for intermediate concentrations. Figure 3 shows the topographic modulation dZ during scanning with the AFM tip in the form of a hemisphere of radius rt of the graphite/solution interface with the boundary micelles in the form of hemicylinders of radius rm. The amplitude of topographic modulation is dZm = rt + rm − h

Considering that the height 11328

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Figure 2. Repeat period of parallel strip temperature dependences of the CTAB surface micelles at different concentrations in a perpendicular direction to the bands on the AFM images at the graphite/solution interface.

interval of 22−32 °C. At concentrations below bulk critical micelle concentration, parallel strips of hemicylindrical surface micelles with a repeat period 5−8.5 nm in the perpendicular direction to the strips are observed. At the concentrations of ≈0.7−11 cmc, the hemicylinders are observed at low Figure 3. Image of the topographic modulation dZ during scanning by the AFM tip in the form of a hemisphere of radius rt of the graphite/ solution interface with the boundary micelles in the form of hemicylinders of radius rm. Here g is the distance between neighboring strips.

The dependence (Figure 5C) of the amplitude of the topographic modulation of the surface adsorbed structure is similar to the corresponding period of structure dependence in Figure 2. It demonstrates a critical behavior and looks like a plateau at the temperatures below the period jump and rises sharply in the transition temperature range. So, cetyltrimethylammonium bromide morphology at the interface graphite/aqueous solution has been studied by AFM within the concentration range of 0.5−111 bulk critical micelle concentration (cmc = 0.9 mmol/L) and the temperature

Figure 4. Line of section of AFM images of adsorbed structures in 1.1 cmc CTAB solution in water at the graphite/solution interface at a temperature of 25.2 °C (A) and the topographic modulation of the needle in the vertical direction (B) (here X is a direction along the section line). 11329

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Figure 5. Dependences of the period Phc (A) and amplitude of topographic modulation (B) on the concentration of surfactant hemicylindrical micelles at the graphite/solution interface, 25.2 °C; the amplitude of the topographic modulation temperature dependence (C) of the surface adsorbed structure of 1.1 cmc CTAB solution in water at the graphite/solution interface.

and repeat periods PPt from 134 to 233 by changing the temperature in the range of 25−33 °C (Table 1), while their thickness was around 3.5 ± 1 nm, according to the AFM crosssectional profiles (Figures 6A, 6B, and 6D). The most solid bands are obtained at 25 °C, while at 27 °C the breaks appear in the strips and at 33 °C the structure resembling a necklace. The heights of the strips on their axis are 3.5−4.5 nm at 25 °C, 2.0−3.0 nm at 27 °C, and 2.5−5.0 nm at 33 °C.

temperatures of the investigated temperature range, while at higher temperatures there occur so-called precylinders, which are almost cylinders, but coupled with the hydrophobic substrate by the tails of their constituent surfactant molecules. The rapid growth of the period called the period jump occurs in a narrow temperature range of about 1−1.5 °C. The temperature of the period jump is in the range from 26.5− 28.5 °C. A systematic description and explanation of the temperature and concentration dependences of the period of surface micellar strips of ionic surfactants at the hydrophobic substrate are presented for the first time. Platinum Nanoscale Self-Assemblies. Figure 6 shows tapping-mode AFM images in air for Pt self-assemblies on graphite from the CTAB surface micelle templates at various temperatures with cross-sectional profiles along the black lines on the AFM images and cross-sectional profile of one of the strips at 25 °C. The images indicate the parallel arrays of linear chains of Pt nanoparticles over relatively large length scales of micrometer sizes. With increasing temperature, the structure of the platinum band similar to the necklace begins to be formed. Perhaps the formation of nanoparticles rather than the bands is preferable at higher temperatures. This is in line with the discussion, where the temperature effect on different stages of the formation of surface micelle is treated. Regardless of what stage (micellization or adsorption) controls the formation of surface aggregates, an increase in temperature would result in less effective micellization or weakening the adherence between alkyl radicals and the substrate. In both cases surface aggregates become shrunken and move away from each other, and therefore changes in their morphology (necklace-like structure versus continuous band) can occur. Probably, random inclusion of hemispherical micelles is observed, which is responsible for the formation of the “bump” in Figures 6B and 6D. Morphological parameters of surface micelles and Pt surface strips at temperatures 25, 27, and 33 °C are represented in the Table 1. The widths of Pt bands dPt change from 47 to 113 nm

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DISCUSSION Surfactant Self-Assemblies. According to data,41 73% of the surface (graphitized carbon black) is covered with cationic surfactant molecules with interfacial aggregation. The repeat unit, or period, is the sum of the aggregate diameter and the separation.42 In general, they both may undergo changes in concentration43 and temperature. Therefore, before explaining the observed effects, two possible models for the interpretation of the AFM images should be discussed; i.e., apparent changes in the periodicity may reflect changes in (i) the width (the diameter or small axis of hemimicelles) of the aggregates as such or (ii) the distance between them. The first model assumes that the observed increase in periodicity is due to the increase in the aggregation number or size of the surface hemicylindrical micelles, followed by the decrease in their curvature. Such an increase of periodicity from 6.4 to 12 nm was documented in ref 41 in the case of swollen SDS micelles with the addition of alkanol. Another case may be observed, when the aggregates become more distant from each other. This case can probably be due to the (i) decrease in the aggregation number (“shrinkage” of aggregates), (ii) strengthening the “head−head” repulsive interactions, and (iii) the rearrangement or the partial changes in the morphology of aggregates. Accordingly, opposite trends occur, when a decrease in the period is observed. In a similar way, increasing the value of dZ can be the result of an increase in the surface curvature of hemicylinders or/and interaggregate spacing and vice versa. 11330

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Figure 6. Tapping-mode AFM images in air for Pt self-assemblies on graphite from the CTAB (c = 1.1 cmc) surface micelle templates at 25 °C (A), 27 °C (B), and 33 °C (D) with cross-sectional profiles along the black lines on the AFM images; cross-sectional profile of one of the strips at 25 °C (C).

To interpret the concentration dependences (Figures 5A and 5B) it should be noted that the increase in the surfactant concentration usually results in the micellar growth, i.e., the enlargement of the size and the aggregation number of micelles. This, in turn, gives rise to a decrease in the curvature of aggregates. All these transformations could contribute to the increase of the period; therefore, they contradict the data in

Figure 5A. Probably, another interpretation is valid, which involves decreasing the distance between aggregates when the solution is concentrated. This interpretation is in line with diminishing the parameter dZm (Figure 5B). The source of such behavior is probably due to an increase in the counterion binding when the surfactant is concentrated in analogy with 11331

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be an exothermic process,47,48 and it is highly probable that this holds true for the surface aggregation of the cationic surfactant too. Indeed, ref 40 has confirmed that the surface-induced hemimicellization is always reversible, with the process being moderately exothermic for ionic surfactants. Therefore, a rise in temperature would suppress the micellization capacity, i.e., the model treating the increase in periodicity as micellar growth is invalid. Another boundary situation assumes that epitaxial adsorption (the first step) mainly controls the temperature dependence of the surface aggregation. In this case, the variation in the temperature will change the balance between the van der Waals’ interactions and thermal motion. Usually, the former dominate over the latter; however, an increase in the temperature may result in the intensification of the thermal motion and in the shift of the above balance. Consequently, weakening the adherence between alkyl radicals and the substrate can occur, followed by a decrease in the length of contact of a surfactant molecule along the graphite symmetry axes. Hemicylinders become shrunken in width, which may be imaged as an increase in periodicity within the framework of the model assuming an increase in the distance between aggregates rather than an increase in micellar size (≡ decrease in the curvature). This mechanism is in line with the exothermic character of the adsorption, as it is evident from ref 45. It may additionally be contributed by the decrease in the degree of the counterion binding because of the more intense thermal motion of bromide ions, which would increase the repulsion between head groups and move them away. The explanation offered is plausible, but it assumes that the temperature-induced changes are gradual, which is not consistent with the critical character of the observed temperature effect. Therefore, a certain intermediate mechanism involving both adsorption/aggregation stages should be assumed. Taking into account the above considerations, we propose the following mechanism of the observed critical temperature behavior, which introduces the pioneer concept of the surface hemicylinder-to-precylinder transition. An increase in the temperature results in an increase in the thermal motion of the surfactants and in a decrease in their adherence to the surface, so that hemimicelles become shrunken in width and elevated under the surface. This indeed may look as moving them away. To the point of this moment, temperature-induced structural changes of the first stage of adsorption are dominant and pave the way for further transformations. At a definite critical temperature, the epitaxial adherence is practically broken down except a very weak binding due to residuary surfactant molecules (Figure 7). The intermediate structures formed are in dynamic equilibrium with the bulk micelles and adhere to the surface of the hydrophobic substrate with the hydrophobic tails of surfactant molecules in the contact area. At this stage the so-called precylinders are probably formed (Figure 7). Under the assumption of the constant concentration of surfactant molecules at the surface, one precylinder may be formed from two hemicylinders in a narrow range (≈ 1°) of the temperature of period jump, and thus the period of surface micelles doubles. The structural transition through the scheme “two hemicylinders−one precylinder” satisfactorily explains approximately the period doubling of the surface aggregates under conditions of preservation of the material balance and an increase in the amplitude of topographic modulation with temperature (Figure 5C). However, to substantiate the energetic validity of this

Table 1. Morphological Parameters of Surface Micelles and Pt Surface Strips at Various Temperatures parameters of surface micelles

temperature, °C 25 27 33

distance between walls of surface repeat period micelles gm = Pm − dm, nm Pm, nm 7 7 14

2.46 2.46 9.46

parameters of Pt surface strips repeat period PPt, nm

width dPt, nm

calculated width d′Pt = (PPt:Pm)gm, nm

134 169 233

47 59 113

47 62 161

bulk aggregation, which can diminish the screening effect of the adjacent head groups. To interpret the temperature effects observed, the following should be emphasized. The geometry factor is assumed to control the behavior of surfactants at the graphite/aqueous solution interface; i.e., the size of the two surfactant methylene units matches the unit cell of the exposed graphite surface. However, to treat the temperature effect one should be aware that the energetic rather than geometric factors are sensitive to temperature changes, although a minor temperature dependence of the crystal lattice should also be taken into account. Keeping this in mind, the temperature effects on the interactions of components involved in the adsorption/ aggregation processes at the interface should be considered. Among these, a substrate, surfactant molecules (and probably counterions as separate species), interfacial water, and air bubbles can be listed. According to the two-step adsorption mechanism, the interfacial ordering of organized structures is assigned to the earliest adsorption process of isolated molecules. Further, they act as nuclei for two-dimensional growth at the expense of surface-bound water. At this stage, a good match in size allows the alkyl chains to be very close to the graphite lattice and form strong van der Waals forces. Besides, in ref 44 a negative zeta potential of −21.7 mV is revealed at the planar graphite surface. Therefore, the contribution of attractive electrostatic forces between the support and positively charged head groups may aid adsorption. The electrostatic attraction between the surfactant head groups and the mobile electrons in the conducting graphite surface may also serve to stabilize the closely spaced head groups. It had been documented45 that the head groups of cationic surfactants with even numbers of carbon atoms ≥12 are in contact with the hydrophobic surface, which makes adsorption irreversible. At the second stage, the ordered packing of the first layer of surfactant templates the formation of a hemicylindrical aggregate. The typical hydrophobic effect resulting from a decrease in the entropy of the system prior to aggregation is responsible for the assembly similar to the bulk surfactant aggregation. Water−headgroup interactions contribute to the stabilization of the system, while repulsive electrostatic forces between head groups, both intraand intermicellar, destabilize aggregates, thereby limiting the number of aggregation (Nagg) and inneraggregative spacing. Values Nagg which equal 5 for C12TA41 and 7 for SDS42 per cross-section of hemicylindrical surface micelles are available in the literature against 17 for the CTAB rodlike bulk micelles.46 To begin with, one of the two stages is mainly assumed to control to the observed temperature effect. It is unlikely that the temperature dependence of the surface micellization (the second stage) would govern the observed critical behavior. Solution aggregation of the ionic surfactant is reliably proved to 11332

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Figure 7. Simplified picture of the adsorbed structures for intermediate concentrations of C16TAB solutions in water at the graphite/solution interface with increasing temperature: A, hemicylinders; B, intermediate aggregates; C, precylinders.

scheme, the following should be stressed once again. An increase in the period value occurs in a short temperature range, i.e., shows a critical behavior. Therefore, it results from the cooperative contributions of interactions, which originate from the growth of temperature and, in turn, provoke structural changes providing a marked gain in the free energy, i.e., the stabilization of the system. Among the interactions contributing to the aggregative behavior of surfactants, both in the bulk solution and at the interface, the hydrophobic effect complies with this criterion in full measure. As mentioned above, owing to the destruction of water clusters formed around alkyl radicals of a surfactant molecule, the hydrophobic effect as such declines with the temperatures. However, accumulation of the clusters can occur due to an increase in the water contacts with alkyl chains or/and with the hydrophobic surface of substrate resulting from the temperature-induced structural changes. It is these factors that can stimulate the critical mechanism of further structural reorganizations capable of stabilizing the system. Platinum Nanoscale Self-Assemblies. The templatedirected sintering process of Pt nanoparticles mediated by the hemi- or precylindrical micelles (Figure 8A) could be divided into several stages. Initially, individual Pt nanoparticles are formed through nucleation along the long axis of hemi- or precylindrical micelles (Figure 8B) and then linked together into a linear chain oriented in the same direction, as schematically shown in Figure 8C. The Pt particles would grow between the hemi- or precylindrical micelles. After washing off the surface template micelles, coalescence of several (from 17 to 24, depending on temperature) neighboring Pt linear chains has been observed, and the last have pulled together into a broader band of Pt (Figure 8D) under the action of surface tension forces. These thin-film metal strips have a regular form and certain period and width as a function of temperature. There is an interesting relationship between the morphological parameters of the surface micelle templates and the parameters of the final platinum bands. It turns out that the width of the final band of platinum at a temperature of 25 °C is exactly equal to the algebraic sum of the widths of platinum strips between surface micelles in the sintered platinum linear structures. It can be assumed that the width of the latter is the distance between the outer walls of the half-cylinders of surfactant gm = Pm − dm, where Pm is repeat period and dm the diameter of the surface micelles. After washing off the surface micelles, the number of fused platinum strips is equal to the

Figure 8. Schematic illustration of platinum strip formation using surface micelle templates.

ratio of the repeat period of the final platinum bands to the repeat period of surface micelles Pm − PPt:Pm. The calculated width of the platinum band d′Pt = (PPt/Pm) × gm is 47 nm, which coincides exactly with an average width of the final platinum bands. For a temperature of 27 °C, the calculated width of platinum bands of 62 nm is in good agreement with the experimentally found one of 59 nm. For the temperature 33 °C, the calculated width d′Pt = 161 nm is significantly different from the experimentally measured one dPt = 113 nm. The reason for this discrepancy may be a friability and a larger diameter of the surface micelles−precylinders and/or nonuniformity of the final structure of the platinum strips at 33 °C, where there are structural elements resembling a necklace. The elemental composition of the material deposited on graphite was analyzed by X-ray fluorescent spectroscopy. To increase the sensetivity of the XRF method, the top layer of pyrolytic graphite with deposited metal lattice of platinum was peeled off with the help of a scotch and then investigated as a sample. The results are shown in Figure 9A. It was found that the sample contains metallic Pt because of the markedly observed characteristic lines Lα = 9.44 keV and Lβ = 11.07 keV of the platinum. Characteristic lines of Br (BrKα 11.91 keV, BrKβ 13.29 keV) were not observed, indicating the absence of this element and, therefore, the CTAB molecules on the surface of the top layer of the graphite. The XRF spectrum of the next layer of the graphite substrate is presented in Figure 9B. This spectrum does not contain the characteristic lines of Pt. The presence of lines of Rh (RhLα 2.70 keV, RhLβ1 2.83 keV, RhKα 20.17 keV) is due to the material of the X-ray spectrometer anode tube, and lines S (SKα 2.31 keV), Ho (HoLα 6.72 keV, HoLβ1 7.53 keV), and Ti (TiKα 4.51 keV) are due to the presence of these elements in the system of graphite substrate− adhesive tape. The diagnostic curves of the MEA with the surface density of Pt of 0.05 mg cm−2 in H2/O2 fuel cell are shown in Figure 10. The curve of the dependence of the power density on the 11333

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surface self-organization of CTAB is shown to be consistent with the previously documented two-step adsorption model. AFM images look like oriented epitaxially parallel strips of hemicylindrical surface micelles with a period of repeat depending on the surfactant concentration and temperature. An increase in the concentration results in the decrease in the period and the amplitude of topographic modulation, which were assumed to originate from the decrease of interaggregative spacing. The temperature dependence of the period shows a critical character; i.e., a sharp 2-fold increase in the period occurs within a short range of 1−1.5 °C. The analysis of interactions contributing to the temperature-induced structural transformations makes it possible to introduce the novel concept of hemicylinder-to-precylinder transition following the 2:1 ratio. This approach could be extended to fabricate a wide range of 1-D self-assembling metallic nanostructures on surfaces using micelle-like self-assemblies carrying metal ions at interfaces.

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Figure 9. XRF spectrum of the top layer of pyrolytic graphite with the deposited metal lattice of platinum (A) and the next layer (B) at 25 °C.

ASSOCIATED CONTENT

S Supporting Information *

Temperature-dependent AFM images of CTAB surface micelles. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the help of Litvinov A.I. Financial support from State Contract No. 02.552.11.7070 on a Theme 2009-07-5.2-00-08-003 is greatly appreciated.

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Figure 10. Diagnostic curves of the MEA with the surface density of Pt 0.05 mg cm−2 in H2/O2 FC.

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current density shows that the maximum power density of 170 mW mg −1 is achieved at a current density of 600 mA mg −1. These values are more than an order of magnitude lower than the same characteristics of known fuel cell with the optimized parameters and MEA with the optimized sizes of Pt particles. However, these data confirm that the precipitated bands are composed of metallic platinum.

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CONCLUSIONS One-dimensional (1-D) thin-layer (2−5 nm) parallel strips of Pt on a graphite surface have been synthesized via a templatedirected chemical deposition of Pt. The templates are surface micellar strips of cetyltrimethylammonium bromide (CTAB) at HOPG. The dimensions and repeat period of the Pt strips can be widely controlled by the temperature: the width is from 47 to 169 nm and the period from 134 to 233 nm in the temperature range 25−33 °C. The morphological characteristics of Pt strips depend on those of the original surface micellar strips. The fact that these strips are composed of metallic platinum was confirmed by testing the MEA with the strips in a special fuel cell. The concentration- and temperature-dependent interfacial behavior of CTAB at the graphite/aqueous solution is elucidated by using the AFM soft-contacting techniques. The 11334

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