Adsorption and Premicellar Aggregation of CTAB Molecules and

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Adsorption and Premicellar Aggregation of CTAB Molecules and Fabrication of Nanosized Platinum Lattice on the Glass Surface Marsil K. Kadirov,*,†,‡ Alexey I. Litvinov,†,‡ 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 ‡ Kazan National Research Technological University, 68, ul. K. Marx, Kazan 420015, Russia S Supporting Information *

ABSTRACT: Premicellar aggregation processes were investigated in a wide range of concentrations and temperatures of cetyltrimethylammonium bromide (CTAB) aqueous solutions. Two independent techniques were involved to study adsorption and aggregation of CTAB molecules at the glass/solution interface. Electronic spin resonance (ESR) was used to estimate microviscosity properties through the reorientation correlation time τc of (2,2,6,6tetramethylpiperidin-1-oxyl), while atomic force microscopy (AFM) was involved to evaluate the CTAB molecule morphology at the glass/solution interface. In the dependence of τc vs the CTAB concentration three discontinuities were revealed within 0.2−0.5, 0.5−1.02, and 1.02−1.1 mM narrow concentration ranges, which are probably connected with the formation of bilayer and hemispherical, hemicylindrical, cylindrical, and spherical admicelles. The images of some of them at the glass surface have been independently obtained by AFM. One-dimensional thin layer (2 nm) of Pt parallel strips on a glass surface have been synthesized by chemical vapor deposition of the Pt on the surface micellar CTAB linear templates followed by washing of the latter.

M

purpose of this study is to develop the way of platinum deposition on the glass support and to characterize clusters obtained. The most simple and advanced strategy for obtaining such clusters both as colloid nanoparticles in the bulk phase and as adsorbed structures at the interfaces is the template way with the help of surfactant aggregates.31−34 Physical and chemical properties of clusters are mainly associated with significantly higher effects of surficial atoms. For small enough clusters almost all atoms can be considered surficial, which explains their high chemical activity. In other words, the smaller the sizes of MC and the closer they are to each other, the higher is their functional efficiency. In order to minimize aggregation of metallic particles various stabilizers are applied. In the case of chemical deposition of MC, amphiphilic compounds are often used as stabilizers. Therefore, another purpose of this work is to elucidate the aggregation of surfactants at the glass surface. This part of the work is additionally stimulated by our recent investigations35 of amphiphilic solutions by ESR spectroscopy. They evidenced some premicellar changes in the correlation times of a spin probe, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). These results are in agreement with ref 36, revealing premicellar association in sodium dodecyl sulfate (SDS) solutions. Electron spin resonance (ESR) is widely used in research of both homogeneous and microheterogeneous systems. Wide

etal clusters (MC) of different dimensions and morphologies including colloid nanoparticles, nanowires, nanodots are in the focus of modern nanotechnologies addressing the development of catalysts, optoelectronics, solar cells, biomedical imaging, sensors, data storage devices, etc. Among conducting clusters, nanowires are of special interest. These systems basically represent a one-dimensional array of atoms of a metal or one-dimensional cluster. Nanoconductors are known to be used in resistive switches of programmable electromechanical memory,1 in single mode laser,2 transparent thin-film transistors,3 logic matrices,4 and memristors. Nanostructured platinum is of special interest due to its vast potential of applications, including catalysis,5−8 electrocatalysis in fuel cells with polymer electrolytes,8,9 sensors,10 and other devices.7,11−13 Great efforts were applied in the past years in the area of synthesis of nanosized Pt-structures, such as nanoparticles, 6, 14 nanowires, 15−17 nanosheets, 18 and others.19−21 There are a lot of protocols for the fabrication of nanoclusters and nanoparticles.22−29 In our previous work,30 an approach has been proposed for the fabrication of platinum clusters on the pyrolytic graphite substrate and their application as a fuel cell. However, pyrolytic graphite is a conductive and optically opaque material. For a more complete implementation of practically useful properties of the resulting one-dimensional nanoscale metal strips it is advisible to use an optically transparent dielectric as a substrate. Herein, we evolve this idea to fabricate nanostructured platinum at the glass support, since the latter is a transparent and nonconductive substrate, thereby answering the modern technological criteria. Therefore, one © 2014 American Chemical Society

Received: April 23, 2014 Revised: August 6, 2014 Published: August 18, 2014 19785

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Preparation and Reduction of Pt Salts. Pt nanostructures on the glass surface were synthesized basically in two steps involving preparation of a surfactant self-assembly at the glass/surfactant solution interface and subsequent reduction of Pt salts. A drop of 1 mM CTAB and 0.1 mM H2PtCl6·6H2O water solution was applied on the surface of the glass and then kept for 30 min for the formation of surface micelles at the interface at desired temperature. The PtCl62− counterions bound electrostatically to the trimethylammonium groups of the surface micelles were then reduced with the excess of hydrazine (5 mM) during 1 h. Glass substrates were cleaned with water and ethanol to remove the adsorbed surfactant micelles on the glass and then were dried again at 40 °C. 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 has 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. ESR Spectroscopy. ESR measurements were carried out on a Bruker Elexsys E-500 spectrometer utilizing a 100 kHz field modulation and X-band microwaves. The concentration of TEMPO used was 5 × 10−4 M. The studied solutions were placed in a cylindrical Pyrex glass tube with an internal diameter of 1 mm. The relative error of the experiment and consequent calculation of the correlation time of the spin probe was ∼10% or 5 ps. The temperature of the sample was maintained in the course of experiment with an accuracy ±1 K. In order to investigate aggregation in solutions of amphiphilic compounds, both statically and dynamically, nitroxide radicals are added to these systems. Hydrophobic nature of TEMPO spin probe as well as many of its analogs determines its prevailing solubilization inside of micelles or other types of aggregates of amphiphilic compounds. ESR spectrum of free nitroxide radical contains information about its surroundings. If the molecule of surfactant, to which the nitroxide is attached, can rotate freely and isotropically, the ESR spectrum will contain three spectral lines of equal intensity and width. If rotation of the molecule is obscured, for example, by interaction with surrounding molecules, line width of spectral lines and their intensity will change. Broadening of the three lines will not be equal and will depend on orientation of nitrogen atom in magnetic field. Width of high-field line mN(−l) will be stronger, than of the low-field line mN(+l). The main feature of spin probe method is that the investigated sample does not induce additional splitting of observed spectra and does not create complications for analysis of results. For aggregation processes with correlation times in range between 0.01 and 10 ns the rate of nitroxide molecule rotation can be characterized by rotation correlation time τc, which can be calculated as follows:

capabilities of ESR method are depicted by its application in direct registration of decomposition products in perfluorosulfonated membranes,37−39 investigation of micelles of classical surfactants,40 amphiphilic calix[4]arenes41 and polymerization with spin probe method,42 ESR imaging of paramagnetic centers in polymers,43 and simultaneous studies of paramagnetic intermediates by electrochemistry and ESR.44−55 Surfactant molecules usually do not have paramagnetic properties and as a result do not give rise to ESR spectrum. In order to investigate their aggregation behavior with ESR method spin probes were added into the solution. Among the stable nitroxyl radicals, TEMPO is a most widely used spin probe.56−58 Importantly, in experimental procedures involving thin capillary tubes, processes of aggregation/adsorption at the walls may play a crucial role. As shown in the work of ref 30, the morphology and size of micellar template and deposited platinum structures can be controlled by the temperature. Based on these reasons we study the association of CTAB by ESR methods at different temperatures. Although some limitations of the use of TEMPO occur due to its partition between two phases in micellar systems, we used it, since many classical surfactants were explored by the spine probe technique with TEMPO.57 To visualize the micellar surface aggregates and platinum structures, atomic force microscopy (AFM) technique was used. The first direct image of morphology of ionic surfactant molecules adsorbed on a hydrophobic substrate from aqueous solution was achieved with AFM as a series of parallel strips of hemicylindrical premicelles that were epitaxially oriented on a graphite substrate, spaced apart by about two surfactant molecule lengths and located in an interphase layer between graphite and an aqueous solution of surfactant. Recently, the CTAB molecule morphology on graphite/solution interface is determined by AFM in an aqueous solution at a concentration range of 0.5−100 cmc (critical micelle concentration (cmc) = 0.9 mM) and temperature range of 22−32 °C.30 The mentioned paper provides the first systematic description and explanation of concentration and temperature-induced changes of the period of surface micellar strips of ionic surfactants on a hydrophobic substrate and also describes the amplitude of topographic modulation and its connection to the type of strips and their period. Herein we evolve this advanced approach, addressing the fabrication of platinum particles at more technological platform.

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EXPERIMENTAL SECTION Reagents and Materials. Hydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6·6H2O) and cetyltrimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich, and they were recrystallized two times from hot acetone. Hydrazine-hydrate and TEMPO spin probe were purchased from Sigma-Aldrich and used without further purification. Preparation of Solutions for Study of Surfactant SelfAssembly. Solutions were conditioned at room temperature before injection into the AFM fluid cell. Two mL 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.

⎛ h 0 τc = 6.5·10−10 ·W0⎜⎜ + h ⎝ −1

⎞ h0 − 2⎟⎟ h+1 ⎠

[s] (1)

where W is the line width of central spectral line, h0,h+1,h−1 are peak to peak intensity of central, low-field and high-field lines, 19786

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respectively.36 Correlation time τc is defined as a time it takes the molecule of probe to rotate to an angle of 1 rad. Investigations of platinum nanoscale lattice on pyrolytic graphite were reported in an already mentioned paper.30 Onedimensional thin-layered (2−5 nm) parallel strips of platinum (Pt) on the surface of pyrolytic graphite were synthesized by a method of template-guided chemical deposition. CTAB hemicyllindric and precyllindric micelles, mentioned earlier, acted as the template at graphite surface. The fact that these parallel strips consist of metallic platinum was proved by XRF method and tests in fuel cells with membrane-electrode blocks consisting of these strips. AFM of Surfactant Self-Assembles. 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, Veeco instruments Inc.) and sealed with a rubber Oring. Before each experiment, the fluid cell and the O-ring 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, 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 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 glass surfaces with the surface micelles in liquid were imaged using AFM operated in contact mode, while the substrate surfaces with the grown platinum strips were obtained in tapping mode in air. In the course of experiments on study of the morphology of the surface by the AFM method, a glass of Pyrex brand has been used as a solid substrate. The surface of the substrate was cleaned by the cleanser Piranha, which then has been washed off with distilled water. The adsorption of surfactant was always studied in areas that had no irregularities of surface topology. The lateral size of the objects is overestimated because of the tip convolution effect. Calibration of the effect yields their true dimensions. We calibrated the tip convolution effect by imaging the tip with special calibration grid (STR3-1800P, VLSI Standards Inc.) to determine its geometry. SEM-EDS. The SEM-EDS studies were carried out at five spots in the sample surface. The prepared sample was placed on a carbon conductive tape stuck on a metal stub. Each specimen is mounted on a separate stub in such way that the surface to be analyzed faces upward. Each specimen image was analyzed by SEM (model Carl Zeiss EVO LS 10) with an EDS attachment. Conditions: accelerating voltage = 20 kV, time of collection = 300 s, work distance = 9 mm, resolution =1024 × 768.

special precautions should be taken in order to obtain reliable and reproducible data and not to lose significant information, since changes of measured signals may reflect both the falsifying factors (instrumental and handling incorrectness, impurities, etc.) and true solution behavior, i.e., premicellar aggregation. We carried out careful investigation of the CTAB solution focusing on the premicellar range and taking into account data on CTAB adsorption on the glass surface that can markedly contribute to the experimental results. Figure 1 shows the dependence of the reorientation correlation time τc of TEMPO on the concentration of the

Figure 1. Dependence of the reorientation correlation time on the concentration of the aqueous solution of CTAB at temperatures 27 and 28 °C.

CTAB aqueous solution at the temperatures 27 and 28 °C in the ESR tube. There are three discontinuities in this curve against the plateau-like character with a subsequent increase beginning at the first critical micelle concentration (cmc = 0.9 mM). The discontinuities take place at concentrations of 0.2− 0.5, 0.5−1.02, and 1.02−1.1 mM. The phenomenon of premicellar association in water was the subject of much controversy in the literature. McBain suggested59,60 that at concentrations below the first cmc small aggregates of surfactant ions can be formed. These aggregates are considered to be responsible for deviation of the behavior of ionic surfactants from the behavior of 1−1 electrolytes. Mukerjee et al. have made61 an attempt to explain the deviation of the conductivity versus concentration curve of SDS solution from Onsager’s theory with assumption on extensive dimerization of SDS ions below the first cmc. Parfitt and Smith have measured conductivity of SDS solutions in concentration range from the SDS cmc value to 0.4 mmol/L at 25 °C.62 They have related the conversion degree of the anions with the formation of dimers or small premicellar aggregates or formation of ionic pairs. ESR spin probe experiments in SDS aqueous solutions at concentrations below and near cmc were originally conducted by Ernandes et al.63 From the shape of EPR spectra, they have reached the conclusion on the existence of premicellar aggregates at concentrations just below cmc (6− 8 mmol/L SDS). Using four different types of spin probes in their experiments and after extensive analysis of the data, they have suggested that the observed aggregation processes cannot be attributed to any probe-induced effect. A rather detailed study of the probe correlation time in a wide premicellar concentration range was reported in a paper.36 The authors have suggested the formation of micellar type of aggregates. However, no one could propose an idea that would clarify the nature of such aggregates.

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RESULTS AND DISCUSSION The ESR Study and AFM Visualization of the CTAB Premicellar Adsorption and Aggregation. There are many difficulties in the investigation of low concentration range of solutions including the surfactant systems. In the latter case 19787

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determined by the interplay of electrostatic and hydrophobic interactions. At the lowest concentrations the surfactant adsorbs via electrostatic interactions to oppositely charged surface sites. At the highest concentrations (around the cmc), hydrophobic interactions lead to the formation of aggregates or admicelles at the solid/water interface. Where the two models differ is in the relative importance of electrostatic and hydrophobic interactions at the intermediate concentrations. In the two-step model there is only a low coverage of (isolated) molecules bound electrostatically to the substrate, which then nucleate the formation of admicelles in the steeply rising part of the isotherm. In the four-region model, there is stronger adsorption at low concentrations leading to the formation of hemimicelles before the attachment of a second layer. As could be seen from the above description, the nature of the interaction of CTAB molecules with the surface of the glass corresponds to a four-region model of adsorption. However, we will see below that the ESR spin probe and AFM methods reveal amazing possibilities for study of surface structure properties by varying the surfactant concentration from bilayer formation to the first cmc. Further increase in the surfactant concentration (part 3−4 of Figure 2) leads to formation of hemispherical surface micelles on the surfactant layer attached to the glass surface (image 4). The driving forces of this process are hydrophobic interactions and solvation effects. This process is a first restructurization of aggregation (RA), at which the probe environment becomes more hydrophilic due to the increasing in the curvature of aggregate surface around the probe. Probe becomes closer to the water molecules surrounding the hydrophilic surfactant head groups from different sides, and τc was decreasing. Figure 3 shows AFM images of the glass substrate (A) and adsorbed structure (B−D) for different CTAB concentrations in water solutions (B) 0.35 mM; (C) 0.80 mM; (D) 1.02 mM and corresponding histogram plots of the surface height at the interface glass/water at 27 °C. AFM image in Figure 3B shows an atomically smooth surface with roughness of 0.16 nm, corresponding to the surfactant bilayer film covering the rougher glass substrate and leveling roughness (0.50 nm) of the original surface. The increase of the surfactant concentration leads to a fusion (part 4−5) of hemispherical micelles into hemicylindrical surface micelles71 on top of surfactant layer attached to the glass surface (image 5). This is a second restructurization of aggregation. It increases the aggregate surface curvature around the probe in admicelles, and τc grows accordingly. Visual confirmation of the hemicylindrical surface micelles formation by the AFM results is shown in Figure 3C. Distance between the highest peaks of histogram of height distribution of parts of the surface of 1.1 Å indicates the height modulation depth of the cantilever needle scanning the onedimensional hemicylindrical surfactant admicelles lattice. With further concentration increase in the concentration range of 0.7−1.02 mM (part 5−6, Figure 2) the third RA occurs, and hemicylinders are rearranged into cylindrical admicelles71 (image 6, Figure 2). This process is accompanied by increasing in the curvature of aggregate surface in the lower half of the cylinders, as a result τc decreases again due to the more moist and polar microenvironment of the probe. This rearrangement process is confirmed by AFM results shown in Figure 3D, where images of cylindrical admicelles could be seen. Their repetition period is approximately equal to the doubled period of repetition of hemicylindrical micelles (Figure 4). Distance between peaks of surface height histogram

To visualize the changes in aggregation behavior and rationalize the EPR and AFM data, we present the schematic images in Figure 2. The dependence of the reorientation

Figure 2. Simplified representation of the adsorbed structures for low and intermediate concentrations of CTAB in water solutions at the glass/solution interface with the increase of surfactant concentration: (1) sparse monomers of surfactant at the surface of glass; (2) the formation of hemimicelles; (3) bilayer film of adsorbed surfactant; (4) hemispherical surface micelles at surfactant monolayer film; (5) hemicylindrical surface micelles on the surfactant layer; and (6) cylindrical and (7) spherical admicelles.

correlation τc on aqueous solution concentration of CTAB shown in conjunction with surface structures formed in the glass/solution interface border reveals the existence of adsorption and premicellar aggregation. Before the concentration of 0.2 mM (image 1) only single surfactant molecules are adsorbed on the glass, resulting in the charge neutralization. No changes in τc value occur in this concentration range due to the absence of aggregates. First critical point of 0.2 mM indicates the formation of hemimicelle with surfactant head groups electrostatically interacting with dissociated silanol groups and hydrophobic tails facing the bulk solution. It has also been reported previously64 that formation of SDS hemimicelles on the alumina surface does not affect the rotational mobility of the spin probe TEMPO. Initial growth phase of the first discontinuity is the result of formation of bilayer admicelles with increasing of surfactant concentration. It is assumed65 that the transition from hemimicelles to bilayer admicelles has been accompanied by compensation of the surface charge. A marked increase in τc was also observed. This increase indicates66 that TEMPO molecules are likely to reside in the second layer and the rotational movement of TEMPO is inhibited.67 Further increase in the concentration leads eventually to the formation of the bilayer film (image 3) of CTAB adsorbed molecules. Adsorption of surfactants at hydrophilic substrates is widely studied, including the interaction of CTAB with mica, glass, or silica. Two principal models have been proposed in the literature to describe the adsorption of ionic surfactants to hydrophilic surfaces: the so-called two-step model and the fourregion model.68−71 In both models, the adsorption isotherm is 19788

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Figure 3. AFM images of glass substrate (A) and adsorbed structures (B−D) for different concentrations of CTAB solutions in water (B, 0.35 mM; C, 0.80 mM; D, 1.02 mM); corresponding histogram plots of the surface height at the glass/solution interface at 27 °C and AFM cross-section profile along the black solid line.

The cmc for CTAB at normal conditions is 0.9 mM, i.e., it falls on part 5−6 in Figure 2. At this range, unfortunately, the conditions for obtaining clear images of the surface structures by AFM begin to deteriorate, as a transparency of the solution was dramatically reduced and an implementation of the AFM experiments became impossible. However, we can continue to monitor the system in the concentration range above the cmc. In the range 6−7 and further (Figure 2), τ c increases due to increasing of concentration of the bulky micelles. However, at concentration 1.07 mM a marked decrease in τc has been observed. It is associated with the fourth RA, which is a transition of cylindrical admicells into spherical71 ones (image 7, Figure 2), leading to a corresponding increase in the surface curvature and τc, respectively. The Temperature Effect. The CTAB adsorption at the glass substrate was investigated by AFM technique at different temperatures. Figure 4A exemplifies the AFM images of adsorbed structure of 1 mM CTAB in water at the glass/ solution interface at 28 °C. The images reveal periodic strips of aggregates with constant period in the cross-section, which correlates with temperature. As shown in Figure 5, the period of strips increases from 4.9 nm at solution temperature 28.0 °C to 10.6 nm at temperature 28.6 °C (Figure 4B). Summarizing these results, a 2-fold increase30 of surfactant strips repetition period is observed in narrow range of temperatures 28−29 °C. Further, AFM investigations of morphology of CTAB cationic molecules have been conducted in aqueous solution at glass/solution interface at concentrations from 0.5 to 1.1 cmc (cmc = 0.9 mM) and at temperature range of 25−31 °C

Figure 4. AFM images of adsorbed surficial aggregates in 1 mM CTAB solution at the glass/solution interface at temperatures 28.0 °C (A) and 28.6 °C (B).

increased to 1.6 Å, indicating a higher modulation depth of cantilever needle while scanning one-dimensional lattice of cylindrical surfactant admicelles. 19789

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Figure 5. Temperature dependences of repetition period of surficial strips of CTAB interfacial micelles at glass/solution interface at concentrations 0.5 and 1.1 cmc according to AFM images.

(Figure 5). At CTAB concentration 0.5 cmc a repetition period of strips is independent from temperature and lies at range of 4.3−4.5 nm; while at a concentration 1.1 cmc, it is in the range of 4.5−5 nm at 25−27.5 °C, then the period jump30 occurs in the temperature range of 28−28.6 °C. The curve of reorientation correlation time τc versus CTAB concentration at 37 °C is shown in Figure 6. Here we do not

Figure 6. TEMPO rotation correlation time τc versus concentration of CTAB in water solution at 37 °C.

see the increase and following decrease of τc that is typical for curves in Figure 4 at 27 and 28 °C. Reorientation correlation time changes in concentration range from 0.5 to 1.0 mM are insufficient for one to speak about the adsorption and aggregation processes in said concentration range. Temperature dependence of repetition period of surface micellar parallel strips was reported in paper,30 in which AFM method was used to investigate CTAB surface aggregates at graphite/solution interface. For temperatures above period jump temperature, when hemicylinders turn into precylinders, at concentrations above 0.5 mM surface structures from the phase 4 convert to the phase 6 directly. The second discontinuity at the reorientation correlation time versus surfactant concentration curve is not observed. The Platinum Deposition on the Glass Surface Using Admicellar Templates. AFM contact mode images of parallel platinum bands was registered at the surface of glass in normal air atmosphere after the process of assisted synthesis (Figure 7A,B). The process of synthesis was based on chemical deposition of platinum in liquid medium at 25 °C with the aid of surficial micellar template, which was formed by CTAB aggregates and was later rinsed off after the synthesis was completed. These images depict parallel matrices of linear chains of platinum nanoparticles of relatively high lateral

Figure 7. AFM images (A−C) of parallel platinum bands at glass surface in normal air atmosphere, synthesized by template aided chemical deposition at 25 °C. AFM cross-section profile of platinum structure along the black solid line (D) in AFM image C and along the black dotted line (E).

dimensions. At some points these bands are combined into relatively thick bars, as can be seen in Figure 7A. As glass substrate is quite rough, the baseline, which can be plotted along the lower points of periodic structure, is not flat but resembles cross-section of irregular glass substrate. Period of repetition of CTAB surface micelles at temperature 25 °C was at a level of 4.7 nm, while the period of platinum strips at the same temperature is roughly 50 nm, width of strips is approximately 24−45 nm. According to these results, one band of platinum matches with roughly 7−10 strips of CTAB surface micelles. Process of template-mediated synthesis of Pt nanograting on glass surface can be divided into four stages (Figure 8). It occurs similarly to the same processes on the surface of pyrolytic graphite.30 Initially, separate Pt nanoparticles are formed and localized along the axis of hemicylindrical micelles (Figure 8B), and then they combine into linear chains with one 19790

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Figure 9. Representative SEM-EDS spectra of parallel platinum bands at glass surface synthesized by template-aided chemical deposition at 25 °C (A) and glass surface without platinum (B).

9.36 keV, Lα1 = 9.44 keV and averaged line of Mβ = 2.134, Mα1 = 2.065, Mα2 = 2.059) (Figure 9 A). The rest of the lines correspond to oxides: SiO2, Na2O, Al2O3, K2O, and MgO with average wt % of 73.1, 13.3, 1.2, 0.3, and 3.5, respectively. These oxides belong to the glass itself (SEM-EDS spectrum of clear glass surface without platinum is shown in Figure 9B).

Figure 8. Illustration of the platinum linear bands formation mechanism with the aid of micellar template: surface micelles with Pt-based counterions (A), Pt nanoparticles nucleation (B) along the long axes of the of micellar template, and the forming of Pt linear chains (C) and Pt linear bands (D).

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CONCLUSIONS Summarizing the results, for the first time nanoscale clusters of metallic platinum shaped as periodic linear bands were achieved at the surface of glass by chemical deposition aided by CTAB surficial micellar template. Period of repetition of CTAB surface micelles at temperature 25 °C was at a level of 4.7 nm, while the period of thin-layer (1.6 nm) parallel platinum strips at the same temperature is roughly 33 nm, width of strips is approximately 24 nm. The fact that these strips are composed of metallic platinum was confirmed by testing the chemically deposited one-dimensional bands with the SEM-EDS method. Saw-shaped discontinuities at ESR spin probe correlation times plot were explained by AFM method and attributed to surfactant aggregate formation on the inner surface of ESR glass tube. These aggregates were imaged by AFM on the surface of the same glass plate and were shown to be shaped as film, hemicylinders, and admicellar cylinders with transitions between them with the increase of surfactant concentrations. This approach could be extended to fabricate a wide range of one-dimensional self-assembling metallic nanostructures on transparent and nonconductive surfaces using micelle-like selfassembled carrying metal ions at interfaces.

predominant orientation, as shown in Figure 8C. Particle growth takes place between hemicylindrical surface micelles. After rinsing the micellar template off, merging or coalescence of roughly 7−10 neighboring metallic chains occurs (Figure 8 D). Periodicity and repetition rate of these metallic bands on glass substrate are slightly worse than in experiments with pyrolytic graphite.30 It is probably related to the worse smoothness of the surface and higher instability of morphological properties of glass substrate. Verification that the observed bands are formed by metallic platinum and do not contain CTAB molecules was originally performed through elemental analysis of these bands on pyrolytic graphite by X-ray fluorescence spectroscopy.30 The obtained platinum bands were also tested in H2/O2 fuel cell in order to investigate their catalytic activity in reaction of hydrogen oxidation. The elemental composition of the material deposited on glass was analyzed by SEM-EDS. The SEM-EDS studies were carried out at five spots in a surface of the sample. Results are shown in Figure 9. Despite the presence of many elements, it was possible to identify lines of platinum in the spectrum. Analyzed surface layer thickness is about 1 μm when SEM-EDS is used. Therefore, the signal of platinum is very small compared with silicon. It was found that the sample contains metallic Pt because of the markedly observed characteristic lines of M and L series of the platinum (LI = 8.27 keV, Lα2 =

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ASSOCIATED CONTENT

S Supporting Information *

Temperature-dependent AFM images of CTAB surface micelles, AFM images of Pt structures on glass surface, and temperature-dependent curves of TEMPO correlation time in 19791

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CTAB solutions. 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]. Fax: +7-843-2732253. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from State contract no. 02.552.11.7070 on a Theme 2009-07-5.2-00-08-003 and Russian Foundation for Basic Researches (grant nos. 14-03-00258A and 12-03-97066) are greatly appreciated.

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ABBREVIATIONS AFM, atomic force microscope; ESR, electron spin resonance; CTAB, cetyltrimethylammonium bromide; MC, metal cluster; SDS, sodium dodecyl sulfate; SEM-EDS, scanning electron microscopy energy-dispersive X-ray spectroscopy

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REFERENCES

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