I. R.
Nizameev
ab,
M. K.
Kadirov
*ac,
V. A.
Semyonov
c,
L. Ya.
Zakharova
a,
T. I.
Ismaev
c,
R. A.
Safiullin
c,
I. Kh.
Rizvanov
a and
V. M.
Babaev
a
aA.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, 8, Akad. Arbuzova str., Kazan 420088, Russia. E-mail: kadirovmarsil@gmail.com
bKazan National Research Technical University, 10, K. Marx str., Kazan 420111, Russia
cKazan National Research Technological University, 68, K. Marx str., Kazan 420015, Russia
First published on 17th June 2016
Nanoscale palladium clusters in the form of parallel strips have been formed on the surface of graphite with the help of a surface micellar template of cetyltrimethylammonium bromide using a chemical deposition method. The repeat period of the palladium strips deposited at 25 °C is 65 nm, with a width of 40 nm and height of 2 nm. The elemental composition of the metal clusters was confirmed using X-ray fluorescence analysis and TEM-EDX. The fact that the strips are composed of metallic palladium was also confirmed by testing the membrane electrode assembly with the strips in a commercial fuel cell. Using the obtained micellar template, the radius of the curvature of the AFM probe tip was estimated with the help of a unique method. The radius is equal to 10 nm and matches the value provided by the manufacturer.
MNWs can also be used as memory elements with a high storage density,9 as highly sensitive nanoscale sensors,10 magnetoresistive multi-layer wires11 and so on. Ordered arrays of MNWs are of primary concern. Recently,12 experts in the R&D department at Hewlett-Packard (HP) proved the existence of a memristor13 – the fourth passive element of electronics, using a system of platinum parallel strips. However, there is no efficient, low cost and quick commercial method to produce such structures.
At the present stage of science development, ordered arrays of metal clusters are mainly produced with the help of nanolithography. This method requires extreme precision and expensive probe technologies, which allow manipulation of certain metal nanoclusters. Besides the fact that such technologies require expensive equipment, they are in situ methods and it is necessary to spend a lot of money and effort to create a small system of several nanometers.
Recently, one-dimensional (1D) self-assemblies of Pt nanoparticles such as wires or structures with a necklace-like morphology on a graphite surface have been synthesized. A template-directed sintering process of individual nanoparticles, using non-ionic/cationic mixed hemi-cylindrical micelle templates of dodecyldimethylamine oxide surfactant at the graphite/solution interface, was used.14,15 We improved the method of platinum lattice production on the graphite surface.16 This method allows control of the morphological characteristics of the synthesized platinum nanolattice. These characteristics directly depend on the morphological characteristics of the original hemi- and pre-cylindrical micellar templates, which can be controlled with the temperature of deposition. AFM proved to be very informative and was widely used to study the morphology of both the micellar self-associated templates and Pt clusters. In addition, it seems very promising for developing nanolattices from parallel strips of noble metals using non-conducting clear surfaces. Electron spin resonance (ESR) proved to be another very productive method that can shed light on the process of self-organization of the surfactants on non-conductive surfaces. The wide modern capabilities of ESR are known through its applications in the direct registration of decomposition products in perfluorosulphonated membranes,17–19 investigation of micelles of classical surfactants in bulk solutions,20 amphiphilic calix[4]arenes21 and simultaneous studies of paramagnetic intermediates using electrochemistry and ESR.22–30 The ESR spin probe method31 and a combination of AFM and ESR methods allow the closer study of surfactant surface aggregates.32 Direct use of ESR to study surfactant self-organization on the conductive surface is not possible, but the results of its application on non-conducting surfaces using AFM help answer difficult questions regarding surface micelle formation.
An urgent task is to replace platinum with palladium – the cheaper analogue from the viewpoint of catalytic and nanoelectronic applications. This paper discusses the results of the synthesis of nanolattices from this other noble metal, palladium, on the atomically smooth surface of pyrolytic graphite.
The 1 cm2 MEA was placed in the body of a commercial fuel cell and studied on a test station with an electrochemical unit ECL 450 (ElectroChem Inc.) and gas-distributive unit HAS (ElectroChem Inc.). The flow rate of both hydrogen and oxygen was 20 ml min−1.
It has been recently confirmed34 that the structure adsorbed by the support surface represents the hemi-cylindrical admicelles shown in Fig. 2. Hemi-cylinders stick to the surface of a hydrophobic substrate by means of the hydrophobic tails of surfactant molecules in the near-contact zone. The diameter of the hemi-cylinders is approximately twice the length of CTAB molecules. The length of CTAB molecules is 4.54 nm at 25 °C.34
Fig. 3 shows the topographic modulation dZm obtained during scanning of the graphite/solution interface with hemi-cylindrical micelles of radius rm using an AFM tip of radius rt. The adsorbed structure shown in Fig. 1A represents strips with repeat period Pm (Fig. 3) equal to 7.5 nm. The modulation dZm depends on the period Pm, and on the curvature radius of the probe rt.
The first direct image of the morphology of CTAB aggregates adsorbed from the aqueous solution on the hydrophobic substrate have been recently obtained using an AFM method.33 The aggregates exist in the form of “hemi-cylindrical half-micelles”, epitaxially oriented along the symmetry axes of the graphite surface. The distance between half-micelles is twice the length of the surfactant molecules. Such aggregates were observed in the concentration range 0.9–5.5 of the critical micelle concentration (CMC). According to modern concepts, at a surfactant concentration below the volume of CMC, amphiphilic molecules are adsorbed substantially, orienting parallel to the surface according to a “head-to-head”/“tail-to-tail” scheme. At higher concentrations (>CMC), semi-cylinders arrange at the phase interface, forming periodic extended structures (strips), separated by a distance approximately equal to twice the length of the surfactant molecules. This structuring of surfactant molecules on the graphite substrates was observed for both types of surfactant – ionic and non-ionic. A key role in controlling the morphology of the aggregates is played by the crystal lattice structure of graphite, which orients the surfactant molecules along the graphite crystallographic directions. The determining criterion in this case was supposed to be the correspondence between the graphite lattice period (0.246 nm) and the length of the methylene link (0.254 nm). An increase in the packing density of hemi-cylinders with an increase in surfactant concentration has also been confirmed.16 So the amplitude of the topographic modulation depends on the surfactant concentration of hemi-cylindrical CTAB micelles at the phase interface of graphite/solution; dZm decreases from 0.7 nm at C = 0.5 CMC to 0.4 nm at C = 10 CMC.
A dependence of the same nature for the surfactant aggregates’ period at the graphite/solution interface on the concentration of sodium dodecyl sulfate (SDS) has also been observed previously.35 The adsorbate period decreased from 7 to 5.2 nm with the increase in SDS concentration from 3 to 100 mM. It has been shown that an increase in concentration of Mg2+ ions leads to a denser packing of hemi-cylindrical micelles36 due to neutralization of the electrostatic repulsion between SDS head groups.
The temperature dependence of the repeat period has a critical nature: in the narrow temperature range of 1–1.5 °C, a steep 2-fold increase in the period is observed.16 Analysis of the interactions contributing to the temperature-induced structural changes gave the basis for introducing a new concept of a transition of the half-cylinders to pre-cylinders with a quantitative ratio of 2:1.
In ref. 32, similar hemi-cylindrical micelles from CTAB molecules have already been observed at the glass/solution phase interface; in a narrow temperature range of 28–28.6 °C, an approximate doubling of the repeat period of the strips (from 5 to 11 nm) was observed, i.e., hemi-cylindrical admicelles have been changed to cylindrical ones. It should be noted that the graphite surface is hydrophobic and the glass surface is hydrophilic (Table 1). In the case of the glass substrate, there are also no specific crystallographic axes with a distance between them that could be comparable to the length of the methylene link. However, it should be noted that the value of the glass surface dispersity is 5 nm (ref. 32) – approximately twice the length of the methylene link.
No | Substrate | CA(M) [°] | CA(L) [°] | CA(R) [°] | WoA [mN cm−1] | Hydrophilic (90−) or hydrophobic (90+) properties |
---|---|---|---|---|---|---|
CA(M) is average contact angle, CA(L) and CA(R) are left and right contact angles, respectively, WoA is work of adhesion. | ||||||
1 | HOPG | 96.05 | 96.47 | 95.62 | 65.87 | HPB |
2 | Glass | 45.31 | 46.98 | 43.65 | 122.78 | HPL |
h = rt + rm − dZm, | (1) |
(rt + rm)2 = h2 + (Pm/2)2. | (2) |
A solution of the system of eqn (1 and 2) in relation to rt is
rt = dZm/2 + Pm2/8 dZm − rm. |
By substituting dZm ∼ 0.7 nm and Pm ∼ 7.5 nm from Fig. 3, a value for the curvature radius of the probe tip rt ∼ 10 nm was obtained. This corresponds to a value of 10–13 nm declared by the manufacturer.
The cross-sectional shape and arrangement of the palladium strips are very different from those of the platinum strips.16 Palladium spreads better over the hydrophobic surface of HOPG and has a contact angle less than an order of magnitude than platinum has in the same conditions. The palladium strips lie close to each other and are almost in contact, while the distance between the platinum strips is nearly 100 nm. Despite these inherent differences, it is necessary to emphasize the universality of the developed method to obtain one-dimensional nanostructures of precious metals on a solid surface.
For the synthesis of parallel Pd strips on a graphite surface, a drop of 1 mM CTAB solution and 0.1 mM PdCl2 solution in water was applied to the clean surface of HOPG in the liquid cell.
Surface admicelles were formed over the next 30 minutes. Then, Pd2+ ions were reduced with 5 mM hydrazine solution over 1 h. Then, surface micelles were removed from the surface by washing with water and ethanol and the fluid cell was dried at 40 °C for 4 h.
The elemental composition of the material deposited on the surface of HOPG was analyzed using X-ray fluorescence spectroscopy. According to the characteristic lines at 21.12 (Kα) and 23.80 (Kβ) keV in Fig. 5, it can be concluded that the material contains Pd particles. There were no characteristic lines of bromine at 11.91 keV (Kα) and 13.29 keV (Kβ) indicating the absence of CTAB molecules containing Br ions.
To investigate Pd using TEM, the metal was deposited on the surface of copper grids (300 mesh) covered with a formvar membrane. The deposition occurred using the method mentioned above. During the deposition of Pd on formvar, separate metal particles were obtained instead of an arranged system of metal strips. The sizes of the particles are 2–5 nm (Fig. 6). The properties of the formvar surface significantly differ from the properties of the HOPG surface. This fact did not allow arranged metal strips to be formed. However, the elemental composition of the deposited material was confirmed. In Fig. 7 the EDX spectrum relating to the TEM image area (Fig. 6) is shown. Two lines of Pd (Mζ = 0.286 kV, Mγ = 0.532 kV) are clearly seen in the spectrum. Lines of carbon, silica and copper are also seen in the spectrum. These lines correspond to the substrate.37 In addition, the lack of bromine lines in the spectrum confirms that other components of the reaction are not included in the obtained product.
Fig. 7 Spectrum of TEM-EDX analysis of Pd particles deposited on the surface of formvar using CTAB micellar template. |
The diagnostic curves of MEA with a surface density of Pd of 0.5 mg cm−2 on both the anode and cathode sides in an H2/O2 fuel cell are shown in Fig. 8. The curve of the dependence of the power density on the current density shows that a maximum power density of 2.4 mW cm−2 is achieved at a current density of 11 mA cm2. These data confirm that the precipitated strips are composed of metallic palladium.
Fig. 8 Diagnostic curves of the MEA with a surface density of Pd of 0.5 mg cm−2 in an H2/O2 fuel cell. |
The template directed sintering process of Pd nanoparticles mediated by surface micelles (Fig. 9A) could be divided into several stages. During chemical reduction, individual Pd nanoparticles are formed through nucleation along the long axis of the surface micelles (Fig. 9B) and then linked together into linear chains oriented in the same direction, as schematically shown in Fig. 9C. Particle growth will occur between the admicelles. After washing off the template from the surface micelles, coalescence of several (about 9 in this case) neighboring Pd linear chains into a broader strip of Pd (Fig. 9D) was observed. The height of the strips is 2 nm, the width is 40 nm and the repeat period is 67 nm (according to Fig. 4).
Footnote |
† Electronic supplementary information (ESI) available: AFM images of palladium on HOPG surface, AFM images of the structure of CTAB at the graphite-water interface. See DOI: 10.1039/c6dt01234e |
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