ISSN 1027-4510, Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques, 2017, Vol. 11, No. 1, pp. 270–275. © Pleiades Publishing, Ltd., 2017. Original Russian Text © E.Yu. Kaniukov, A.L. Kozlovsky, D.I. Shlimas, M.V. Zdorovets, D.V. Yakimchuk, E.E. Shumskaya, K.K. Kadyrzhanov, 2017, published in Poverkhnost’, 2017, No. 2, pp. 99–105.

Electrochemically Deposited Copper Nanotubes E. Yu. Kaniukova, *, A. L. Kozlovskyb, c, D. I. Shlimasb, c, M. V. Zdorovetsb, d, D. V. Yakimchuka, E. E. Shumskayaa, and K. K. Kadyrzhanovb aScientific

and Practical Materials Research Centre of the National Academy of Sciences of Belarus, Minsk, 220072 Belarus bInstitute of Nuclear Physics, Almaty, 050032 Republic of Kazakhstan c Gumilyov Eurasian National University, Astana, 010008 Republic of Kazakhstan dUral Federal University, Yekaterinburg, 620002 Russia *e-mail: [email protected] Received July 6, 2016

Abstract⎯Copper nanotubes are electrochemically synthesized using ion-track polyethylene terephthalate templates. The structure and morphology of the nanotubes constructed under different synthesis conditions are studied with the assistance of scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction analysis, and the manometric gas-permeability method. The dependence of structural features on the electrodeposition potential is established. Keywords: template synthesis, ion-track technology, electrochemical deposition, nanotubes, growth mechanisms DOI: 10.1134/S1027451017010281

INTRODUCTION At present, nanoscale objects exhibiting a number of unusual properties are causing great interest among researchers [1‒3]. Nanostructures have already found application in biomedicine, chemistry, physics, electronics, and materials science while research directions are being continuously developed [4‒6]. However, nanostructures are not widespread because reliable techniques of mass production thereof are lacking. From this viewpoint, template synthesis is a very promising method in which porous matrices provide the basis for the mass creation of nanostructures with specified shapes and sizes [7‒10]. In this case, nanopores create the natural conditions for selforganization of nanoscale objects. One of the methods intended for creating nanopores is that dielectric layers are bombarded with swift heavy ions so as to generate extended regions of radiation damage (latent tracks) [11, 12]. Selective etching of the latent tracks leads to the formation of stochastically distributed pores, the shape and sizes of which are specified by choosing the irradiation and etching parameters [13‒15]. After the nanopores are filled with certain materials or their alloys, structures with the required characteristics are formed, ensuring a wide range of their possible use [16‒20]. Owing to low cost, simple synthesis, and high performance characteristics, copper structures are of special interest among nanoobjects. At present, there are a large number of techniques [21‒25] that make it

possible to synthesize different morphologies of copper nanostructures: cubes [26], nanorods [27], nanodisks [28], nanowires [29], and others. On account of their specific structure and unique properties, hollow nanoobjects are more attractive in terms of practical applications. Since their density and surface area are, respectively, lower and larger than other nanoobjects, they are very promising for use as components of nanoscale devices, catalysts, and carriers for the targeted delivery of drugs and biomedical and chemical agents [30‒36]. Unfortunately, the methods described in [21‒29] make it impossible to reliably obtain micro- and nanostructures with the assigned parameters. Such a state of affairs is the main constraint of practical applications. To solve the given problem, we propose the template synthesis of copper nanotubes with predetermined parameters and perform the complex characterization of their structural and morphological features. EXPERIMENTAL Copper nanotubes were prepared via template synthesis whose important feature is that the properties of synthesized nanostructures can be controlled. This is due to the possibility of selecting the template material, the pore geometry, and porosity thereof [7, 11, 37, 38]. In the template synthesis method, pores can be filled using, e.g., electrochemical deposition [39], electron-beam lithography [40], chemical vapor deposition [41], and several other procedures [42‒44].

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In this work, the choice was made in favor of electrochemical deposition because such an approach provides simplicity, a low cost, the possibility to control the physical and chemical properties of synthesized nanostructures, and a high degree of control of the process [45‒47]. In the synthesis of copper nanotubes, track membranes based on polyethylene terephthalate (PET) served as templates. A Mitsubishi Polyester Film (Germany) Hostaphan® PET film 12 μm thick, which was irradiated by swift heavy 132Xe22+ ions with an energy of 1.75 MeV/nucleon and a fluence of 4 × 107 cm–2 with the help of a DC-60 accelerator (Astana, Republic of Kazakhstan), was employed to create the templates [48, 49]. The UV sensitization of each side of the irradiated films was carried out with the help of a UV-C lamp with a wavelength of 253.7 nm for 30 min. Irradiation-induced highly defective regions (latent tracks) were transformed into pores by means of electrochemical etching in a 2.2-M NaOH solution at a temperature of 85 ± 1°С within 4.5 min. Afterward, the prepared templates were treated in a neutralizing solution (aqueous 1% solution of acetic acid) and washed in deionized water. The pores had a cylindrical shape, and their diameter was ~380 nm. The pores were filled with a metal via electrochemical deposition. For this purpose, individual pieces with sizes of 1.0 × 1.5 cm were cut from the prepared track membranes and coated with a gold layer 10 nm thick, which served as the working electrode (cathode) during deposition, via magnetron deposition in vacuum. The templates with deposited gold films were tightly pressed to the holder so that the electrolyte could reach the cathode only through the pores. Potentiostatic deposition was performed in an electrolyte containing CuSO4 · 5H2O (238 g/L) and H2SO4 (21 g/L) at the voltages U = 1.0, 1.25, and 1.5 V. During deposition, a constant acidity level was maintained by adding ascorbic acid. The degree of pore filling with the metal was controlled by means of the chronoamperometric method in which the current strength was measured using an Agilent 34410A multimeter. The composition and morphological and structural features of the electrodeposited copper nanostructures were studied with the help of an Hitachi TM3030 scanning electron microscope (SEM) equipped with a Bruker XFlash MIN SVE energy-dispersive analysis system at an accelerating voltage of 15 kV. The internal diameters of the copper nanotubes were measured using the manometric gas-permeability method [50, 51] in which a Sartocheck® 3 Plus 16290 device was employed. Measurements were carried out at pressures of 8‒20 kPa. The crystal structure of the metal phase was investigated via the X-ray diffraction method in which a D8 Advance diffractometer with CuKα radiation and a graphite monochromator were employed. The diffrac-

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tion patterns of each point were recorded at angles 2θ of 20°–120° with a step of 0.03° for 9 s. RESULTS AND DISCUSSION The golden cathodic layer has a thickness 10 nm and deposites not olny on the template surface but on the pores walls. (Fig. 1a). Thus, copper nuclei generation at the cathode was confirmed (the inset in Fig. 1a) and the nanotube-growth mechanism is specified (Figs. 1b and 1c). In the polymer template, the pore filling process was controlled via chronoamperometry. The experimental dependences between the current strength I and the time t, which were obtained at the chosen deposition potentials, are presented in Fig. 1d. It is seen that all of the potentiostatic-copperdeposition chronoamperograms are quantitatively identical. At small deposition times, a sharp decrease in the deposition current I is replaced by its flattening. Afterward, its value increases gradually and attains saturation. Such a type of the dependence is caused by the fact that metal electrodeposition into the pores includes four stages [52, 53]. At the first stage, template pores begin to fill with metal and the current strength falls greatly. The latter is associated with the fact that the 3D formation of nuclei occurs at the cathode (Fig. 1a). Deposition current flattening corresponds to the second stage, at which nanotubes grow inside the pores (Figs. 1b, 1c). The process continues until the nanotube length becomes equal to the template thickness. In this case, t = 130, 110, and 40 s at U = 1.0, 1.25, and 1.5, respectively. At the third stage, upon achievement of the template surface, the metal begins to grow in three directions (i.e., exhibits volume growth) above the polymer film surface and generates “cups” growing from the nanotube walls. Since this process is accompanied by an increase in the effective area of the deposition surface, the recorded current increases as well. The third stage continues until metal islands arranged within the places of nanotube localization overlap each other. The fourth stage is related to the fact that a continuous metal film is formed on the template surface. As a result, the current attains saturation. It should be emphasized that, in the chronoamperograms, the I(t) behavior depends on the deposition parameters [53] and pore shape characteristics [54]. It was found from the chronoamperograms that the deposition times of copper nanotubes are 120 (1.0 V), 100 (1.25 V), and 30 s (1.5 V). To eliminate plugging and the formation of covers on the nanotube surface intended for the study of structural and electrical characteristics, pores were partially filled with metal, as is diagrammatically shown in Fig. 1c. Typical SEM images of the copper nanotubes, which were obtained after chemical dissolution of the polymer matrix, are depicted in Fig. 2.

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(а) PET

Cu

Au (b)

(c)

(d) 150 3

I, mA

125

100 2

75

50

0

50

100

150

200 t, s

1

250

300

350

Fig. 1. (a‒c) Schematic representation of the main stages of nanotube growth in the PET pores under electrochemical-synthesis conditions. (d) Potentiostatic-copper-deposition chronoamperograms observed at the voltages (1) U = 1.0, (2) 1.25, and (3) 1.5 V.

As can be seen, nanotubes have different orientations after the polymer matrix dissolves. This is a consequence of the action of mechanical forces observed upon PET film removal and drying. It is obvious that nanotubes are arranged in parallel inside the polymer matrix. This follows from the corresponding arrangements of pores in the PET template. The analysis of the SEM images is evidence that copper nanostructures are hollow nanotubes whose length correspond to the initial-PET template thickness (11.8 ± 0.2 μm). The external diameters D correspond to pore diameters of 380 ± 20 nm. Since the SEM resolution is insufficient, the internal diameters of the nanotubes grown in the PET template were investigated using the manometric gas-permeability method based on gas-

pressure measurements in a closed chamber. In this case, the pressure was varied with a step of 4 kPa in the range of 8−20 kPa. The internal diameters were calculated from the formula [50]

Q3l (1) , 2π Δ p4n RTM where r is the pore radius, Q is the efficiency in air, l is the film thickness, ∆p is the applied pressure, R is the universal gas constant, M is the molar air mass, n is the surface density of pores, and T is the temperature. The measurement results are presented in Fig. 3. From the analysis of the internal diameters of copper nanotubes obtained at different deposition potenr = 3

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1 μm

(a)

273

200 nm

(b)

Fig. 2. SEM images: (a) the set of copper nanotubes after removal of the PET template and (b) the sheared single nanotube.

tials, it is possible to reveal an unusual regularity: when the deposition potential increases from 1.0 to 1.5 V, the internal diameter of the copper nanotubes grows from 170 to 200 nm. This implies that the wall thickness decreases from 100 to 85 nm. The elemental compositions determined via energy-dispersive analysis indicate that the nanotube samples under study are composed of 100% copper without any impurities. Diffraction patterns enabling us to analyze the crystal structure of the copper nanotubes are depicted in Fig. 4. In accordance with the X-ray diffraction data, all nanotube samples have the fcc (face-centered cubic) structure with lattice parameter a differs from the standard one (3.6130 Å). The peak with the angular position 2θ = 20°–33° corresponds to the PET template. The average sizes τ of crystallites composing the

copper nanotubes were determined using the Scherrer equation [56]:

kλ , (2) β cos θ where k = 0.9 is the particle’s dimensionless form factor (Scherrer constant), λ = 1.54 Å is the X-ray wavelength, β is the reflection half width at half maximum (HWHM), and θ is the diffraction angle (Bragg angle). For different deposition potentials, lattice parameters and the average crystallite sizes are presented in Table 1. The analysis of the tabular data is evidence that lattice parameter a tends to grow with increasing cathode potential of copper deposition. There is no avidience dependence between the average sizes of crystallites constituting the nanotube wall and the deposition potential. The crystallite shape dynamics was investigated by analyzing the nanotube texture under changes in the electrodeposition potential. The texture τ=

1.00 V 1.25 V 1.50 V

210

PET 1000

190

I, arb. units

d, nm

111

1200

200

180 170

200

800 600

220

311 222

400

400 200

160 10

15 Р, kPа

20

Fig. 3. Dependences between the internal diameter of copper nanotubes and the applied pressure obtained by the manometric gas-permeability method.

1 2

3 0 20 30 40 50 60 70 80 90 100 110 120 2θ, deg Fig. 4. Diffraction patterns of copper nanotubes obtained at the deposition voltages (1) U = 1.0, (2) 1.25, and (3) 1.5 V.

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Table 1. Copper-nanotube parameters (lattice parameter a and average crystallite size τ) obtained at different deposition potentials U U, V

a, Å

τ, nm

1.00 1.25 1.50

3.6124(9) 3.6132(9) 3.6135(7)

32.23 28.82 43.85

Table 2. Texture coefficients TC(hkl) 2θ, deg

(hkl) U = 1.0 V U = 1.25 V U = 1.5 V

43.39 50.53 74.18 90.00 95.20 116.99

(111) (200) (220) (311) (222) (400)

1.584974 0.826349 1.145673 0.687315 0.755689 −

1.775872 1.030679 0.825306 0.661072 0.707071 −

1.780566 1.121368 0.902701 0.848477 0.897925 0.448962

coefficients TC(hkl) were calculated according to the Harris formula [57]

TC (hkl ) =

I (hkl ) 1 I 0(hkl ) n

) , ∑ II ((hkl hkl )

(3)

0

where I(hkl) is the experimentally found relative intensity, I0(hkl) is the relative intensity corresponding to the given plane in accordance with the JCPDS base, and n is the number of planes. The calculated data are summarized in Table 2. The texture coefficients exceeding unity signify the predominant orientation of the set of nanotubes in the corresponding planes, implying an increase in crystallite size along the chosen direction. The determined values of TC(hkl) confirm the assumption that the copper nanotube structure is polycrystalline in the dominant [111] direction. Certain deviations from the general pattern are observed at a small deposition potential (U = 1.0 V) when the texture coefficients of the (111) and (220) planes have comparable values (greater than unity). In the case of multidirectional planes, the low ratio of coefficients TC(hkl) indicates the rounded shape of crystallites forming the nanotube wall structure. An increase in the electrodeposition potential is accompanied by the fact that the ratio of texture coefficients grows and the [111] direction becomes predominant. This is evidence that the crystallite length increases along the nanotube axis. In particular, as is apparent from Table 2, the texture coefficients of the (111) plane vary by 10% if the cathode potential increases by 0.25 V. A comparison between the X-ray diffraction data and those obtained via the gas-permeability method (Fig. 3) indicates that the nanotube-wall thickness

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Translated by S. Rodikov

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