Nanocrystalline Copper by Pulsed Electrodeposition: The Effects of

Tamas Ungár , Jeno Gubicza. Zeitschrift f r Kristallographie 2007 222 .... Ch. Beck , W. Härtl , R. Hempelmann. Journal of Materials Research 1998 13,...
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J. Phys. Chem. 1996, 100, 19525-19532

19525

Nanocrystalline Copper by Pulsed Electrodeposition: The Effects of Organic Additives, Bath Temperature, and pH H. Natter and R. Hempelmann* Physikalische Chemie, UniVersita¨ t Saarbru¨ cken, D-66123 Saarbru¨ cken, Germany ReceiVed: June 17, 1996; In Final Form: September 17, 1996X

The nanostructure of nano-metals prepared by pulsed electrodeposition is to a large extent adjustable by the appropriate choice of physical and chemical parameters of the electrolysis. For nano-Pd we have demonstrated recently how the shape of the current pulses influences the grain size. Here we focus on the effects of organic additives, bath temperature, and pH on the nanostructure of nano-copper.

1. Introduction Since the introduction of nanostructured materials by H. Gleiter,1 a permanent search for new preparation methods employs scientists all over the world. In particular, methods for the fast production of large quantities are of economic and scientific interest. In the case of ceramics wet chemical procedures2,3 are in use. For the preparation of large quantities of pure metals and alloys, ball-milling4 (impure products), inert gas condensation5 (expensive), and precipitation methods6 are not suitable. The preparation of nanocrystalline copper in a flow system7 and by constant current electrolysis8 has recently been reported. The pulsed electrodeposition technique (PED) is a versatile method for the preparation of nanostructured metals and alloys by pulsed current electrolyis. In the last two decades PED has found much attention worldwide9,10 because it is a technique that allows the preparation of large bulk samples with high purity, low porosity,11 and enhanced thermal stability. Furthermore, this electrochemical procedure enables one to intentionally adjust the nanostructure (grain size, grain size distribution, microstress, crystallite shape), which determines the physical and chemical properties, e.g., hardness,12 conductivity, or chemical stability.13 The relevant experimental parameters are subdivided into physical parameters (pulse characteristics ton, toff, and Ipulse) and chemical parameters (addition of complex formers or inhibitors).14 In addition pH value, bath temperature, and hydrodynamic conditions also have some effect. For nanocrystalline palladium deposits we have recently demonstrated11 the influence of the plating parameters on the nanostructure of the samples. Due to the strong correlation of the parameters, a systematic variation of the deposition parameters is necessary; a careful analysis (XRD, chemical analysis) of the deposits should allow conclusions on the molecular kinetics of the deposition. In the present study we demonstrate that the main reasons for the nanocrystal formation are overpotential phenomena (high nucleation rate) and adsorption/desorption processes of inhibiting molcules (slow grain growth). 2. Analysis of the Nanostructure The nanostructure of our nanocrystalline materials has been characterized by XRD and by several kinds of electron microscopy. 2.1. Particle Size by X-ray Diffraction. Grain size and microstress can be evaluated from XRD data by different

techniques based on the integral width or the shape15-17 of a harmonic set of Bragg reflections. A single-line analysis was also suggested by different authors.18,19 We analyzed our copper deposits by a modified method of Warren and Averbach,20 which allows the simultaneous determination of grain size, microstress, and grain size distribution. 2.1.1. Resolution Correction. The measured XRD peak profile, h(2Θ), is always a convolution of the sample profile, f(2Θ), and the instrumental resolution, g(2Θ):

h(2Θ) ) g(2Θ) X f(2Θ)

The instrumental line width is due to the optical arrangement of the diffractometer (slits, monochromators) and the superposition of KR1/R2 radiation, whereas the sample profile originates from particle size and microstresses. The resolution correction was done by the numerical deconvolution method of Stokes,21 which requires a diffractrogram of the sample and a diffractogram of a standard sample with no extra broadening. We use a LaB6 standard sample, because of the large number (cubic primitive structure) of very narrow reflections. The method of Ricci22 uses the 2Θ dependence of the shape factors (m) and the half-widths at half-maximum (hwhm) of the Bragg reflections, fitted by a split Pearson VII function,

[

PVII(2Θ) ) I0

1

(

S0022-3654(96)01783-2 CCC: $12.00

)]

mL

+ 2Θ - P0 1 + (2 - 1) hwhmL 1 2Θ - P0 1 + (21/mR - 1) hwhmR 1/mL

[

2

(

)] ] mR

2

(2)

in order to model the instrumental line width and line shape at any position P0:

hwhm2(2Θ)R,L )

AR,Ltan2(P0) + BR,L tan(P0) + CR,L (3) 4

m(2Θ)R,L ) DR,L(P0)2 + ER,L(P0) + FR,L

(4)

(A, B, C, D, E, and F are empirical fitting parameters). The split form of the Pearson VII function enables the reproduction of the asymmetry of the reflections. The Stokes method performs the deconvolution in Fourier space:

F(n) ) H(n)/G(n) * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, November 1, 1996.

(1)

(5)

(F(n), H(n), and G(n) are the respective Fourier transforms). © 1996 American Chemical Society

19526 J. Phys. Chem., Vol. 100, No. 50, 1996

Natter and Hempelmann

For that purpose the sample peak and the artificially created instrumental resolution (see above) have to be Fourier transformed. This procedure is extremely sensitive to noise in the data. An effective smoothing of the data is done by fitting also the sample peaks with a split Pearson VII function. Thus simultaneously overlapping peaks (e.g. 111 and 200) are separated. The corrected Fourier coefficients Ar and Ai in the Fourier series

f(2Θ) ) K{∑Ar(n) cos(n2π(2Θ - P0)) + n

∑n Ai(n) sin(n2π(2Θ - P0))}

(6)

result directly from this procedure. K is a normalization constant and n is a positive integer number. The real Fourier coefficients Ar(n) are required for the grain size analysis by Warren and Averbach. 2.1.2. The Evaluation by Warren and Averbach.17,23,24 First we have to transform the Arhkl(n) into Arhkl(L), where L ) na3. The quantity a3 is the length of a column perpendicular to the diffracting planes,17 and n is a harmonic number. Arhkl(L) are factorized in a size coefficient and a strain coefficient.

Arhkl(L) ) ALsize ALstrain

(7)

Using for the cubic system with the interplanar spacing d ) a/(h2 + k2 + l2)0.5 the equation

ln Arhkl(L) ) ln ALsize -

2π2〈L2〉L2 d2

(8)

the strain correction can be done by a logarithmic plot of Ar111(L), Ar222(L) versus m2 ) h2 + k2 + l2 and a subsequent linear regression. The intercepts at m ) 0 are the strainsize ) 1) corrected Fourier coefficients. After normalization (AL)0 the area-weighted grain size results from

( ) dALsize dL

)-

Lf0

1 〈D〉area

(9)

〈D〉volume 2

(10)

and the surface-weighted grain size distribution from

d2ALsize dL2

) psize(L)

3. Fundamental Aspects of Pulsed Electrodeposition (PED) The deposition of nanostructures by pulsed electrodeposition depends on two fundamental processes: (1) the nucleation rate; (2) the growth of existing grains. Factors that cause a high nucleation rate (current density) and a slow grain growth (inhibiting molecules) are favorable to the formation of a finegrained or, in special cases, nanostructured deposit. The parameters of PED are the pulse length (ton), the time between two pulses (toff), the peak height (Ipulse), and the average current density (Ia).

Ia )

Ipulseton ton + toff

(12)

The change of these parameters primarily modifies the cathodic overpotential, which influences the nucleation rate and activation energy of the nucleation (Ak). Ak is related to the cathodic overpotential (∆g) by a relation of Glasstone,25

Ak ∝

{

}

1 (∆g + [a′Me+/aMe+])2

(13)

where a′Me+ is the Cu2+ activity on the electrode film and aMe+ is the Cu2+ activity in the bulk solution. Equation 13 reveals that a high cathodic overpotential is responsible for a low Ak, resulting in an increased formation of nuclei. The use of the PED technique permits electrolysis with a high current density during a short time (ton: µs-ms). Current densities of 1-2 A/cm2 or more are not imaginable for a constant current electrolysis. Ipulse and the ton time determine the number of ions that are discharged during a pulse. A high deposition rate decreases strongly the ion concentration near the cathode. Toff controls the repetition rate. During this time Cu2+ ions migrate toward the electrode. Experimental results are given in section 6.2. 4. Inhibition and Structure of Inhibitors

the volume-weighted one from

∫0∞ALsize dL )

preparation. The main problem of microscopic techniques is the resolution of single grains because the crystallites are often overlapped.

(11)

The strain values are calculated from the slopes (see eq 8). 2.2. Grain Size Distributions by Electron Microscopy. The grain size distribution can be determined by transmission electron microscopy (TEM) or scanning tunneling microscopy (STM). A disadvantage of TEM is the difficult sample preparation (sample thickness < 100 nm) because the different preparation methods can modify the microstructure of the samples; textures can be initiated by cutting, and thinning by Ar ions can initiate crystal defects and grain growth. TEM measures the bulk grain size distribution whereas STM is a surface sensitive technique without any complicated sample

Colloids, solvent molecules, dyes, surfactants, and organic and inorganic ions interact with the surface of the electrode.14 If molecules are adsorbed by van der Waals attractions and the metal surface is incompletely covered with adsorbed molecules, we speak of inhibitation. A complete coating of molecules that are chemically bonded, e.g., oxygen, initiates a passivation. In this case only an electron transfer is possible but no ion transfer. During the negative polarization of the cathode, organic (or inorganic) cations interact electrostatically with the surface. Organic substances with hydrophilic groups like NH2, NH, COOH, CN, S, and OH interact with the water molecules in the Helmholtz double layer. The free electron pairs of the hydrophilic groups are able to form hydrogen bonds with the H atoms of the water molecules and block the surface. If the hydrogen bond is strong, the water molecules can be substituted by the added agents. Substances with this property are activators. Metal deposits resulting from such a bath are very coarse grained. The experimental verification of this effect is not easy because of a strong correlation of the experimental parameters. We examine the dependence of grain size of the deposits (section 6) on (1) different kinds of carboxylic acids and organic amines,

Nanocrystalline Copper by Pulsed Electrodeposition

J. Phys. Chem., Vol. 100, No. 50, 1996 19527

TABLE 1: Bath Composition and Experimental Parameters for the Deposition of Nanocrystalline Copper from Electrolytes Containing Different Complex Formers Pulse Parameters ton [ms]

toff [ms]

Ipulse [A/cm2]

Ia [A/cm2]

1

100

1.25

0.0125

Bath Parameters pH

T [°C]

hydrodynamic condition

1-2

40

mechanically stirred

Bath Composition bath

component

content [g/L]

A B C D E

CuSO4‚4H2O (NH4)2SO4 citric acid citric acid tartaric acid malonic acid DiNa-EDTA

28 50 21 0 17 12 42

complex

Kdis

[CuH2C6H5O7]+

1.0 × 10-6

Cu[Na2EDTA]

1.6 × 10-2 35

DV [nm]

29 50 polycrystalline polycrystalline 20

(2) concentration of inhibitors/complex formers, (3) temperature, and (4) variation of special factors, e.g., pH. 5. Experimental Section A heatable double-walled plating cell (250 mL volume) enables isothermal conditions and contains the anode and the cathode at a distance of 30 mm. The output current of the power supply is controlled with a pulse generator, which produces unipolar, rectangular pulses. A galvanostatic unit keeps the current (Ipulse) constant during the pulse duration (ton). The electrolyte is mechanically stirred. To reduce oxide formation nitrogen gas is bubbled through the electrolyte. The anode consists of a copper sheet (25 mm × 25 mm, thickness 2 mm). The use of a copper anode has the advantage of a constant copper ion concentration (anodic copper oxidation). For the production of copper films and coatings titanium cathodes (20 mm × 20 mm, thickness 1 mm) are used. The adhesion of the copper deposits is very strong, and for this reason the nanocrystalline copper samples cannot be removed. The preparation of bulk samples requires a special procedure. Iron foils (20 mm × 20 mm, thickness 100 µm) are covered on one side with a plastic foil (1 mm). To avoid the dissolution of the iron foil in acid electrolytes and the deposition of copper powder (resulting from hydrogen formation), we cover the other side with a thin film of arsenic. The arsenic film was prepared from a solution of 5 g of As2O3 in 50 mL of concentrated hydrochloric acid. After the deposition of the sample (up to 5 mm thick) the plastic foil was removed from the backside and the iron foil was dissolved in diluted hydrochloric acid. The X-ray measurements were performed on a SIEMENS diffractometer D 500 in reflection mode with secondarily monochromated copper radiation (Cu KR1/R2). The interstitial purity of the samples was determined by hot extraction. The metal sample was melted at 2000 °C, and the gaseous products were detected by mass spectra analysis. A calibration of the analytic system with a suitable substance allows the determination of the hydrogen, oxygen, and nitrogen content (in weight-ppm range). 6. Results 6.1. The Effect of Different Additives. To study the effect of organic additives on the structure of the deposit, a series of carboxylic acids were chosen with two (tartaric acid), one (citric acid), or without (malonic acid) any hydroxide groups. As an example of an organic amine ethylenediaminetetracetic acid

Figure 1. X-ray diffraction patterns of copper deposits prepared from electrolytes containing different kinds of complex formers.

disodium salt (DiNa-EDTA) was used. Table 1 shows the experimental conditions, the complexes that were formed under these conditions, and their dissociation constants. Even for copper solution without additives (Figure 1) we found broad Bragg reflections, indicating a reduced crystallite size (50 nm). This effect results from the high current used in the PED technique. During the cathodic polarization all Cu2+ ions in the vicinity of the electrode are discharged and deposited. This causes a high cathodic overpotential ∆g. According to eq 13 a small Ak value is the consequence, resulting in an increased formation of nuclei. The large surface energy of the small crystallites is reduced by grain growth (surface diffusion of the Cu atoms). To obtain smaller grains (30 nm or less), we have to increase the rate of nuclei formation and reduce the crystallite growth, which can be done by means of complex formers and inhibitors. In section 6.2 we explain the effect of citric acid as a complex former. Free citric acid has a tendency to adsorb at the surface of the copper electrode. Important for the adsorption are the number of free electron pairs and hydrophilic groups

19528 J. Phys. Chem., Vol. 100, No. 50, 1996

Natter and Hempelmann

Figure 2. Scheme of a deposit surface. Different sites for grain growth can be distinguish: 1, high-energy areas; 2, edges; 3, peaks; 4, hollow peaks; 5, hollow edges.

and the size of the inhibitor molecules. In this experiment the ratio of citric acid to copper ions is 1:1. The reduced grain size (29 nm) is mainly caused by complex formation (Kdis ) 1.0 × 10-6 L2/mol2) and less by inhibition because the free acid concentration is low in this bath. As shown in section 6.2, acid surplus causes a marked inhibition. DiNa-EDTA has two free electron pairs (amino groups) and four hydrophilic carboxylic groups. For this reason we expect a strong inhibitor effect, but DiNa-EDTA is a strong complex former. The copper complex formation has a strong dependence on the pH value, and for this reason we can drastically decrease the complex stability (Kdis ) 1.6 × 10-2 L2/mol2) at pH 1-2. The smaller grain size (20 nm) can be explained with the inhibitor effect of DiNa-EDTA. In contrast to the deposition from a bath without any agents we obtain very coarse grains by the addition of malonic acid. Malonic acid activates the metal deposition by the displacement of adsorbed water molecules, because the acid molecules have a strong affinity to the copper surface. In this way the copper ions can be discharged easily. The addition of tartaric acid shows a further effect of inhibitors. This carboxylic acid has four hydrophilic functional groups (two carboxylic and two hydroxyl groups). At pH 1-2 tartaric acid forms no complexes with copper ions,32 and for this reason the electrolyte consits of free Cu2+ ions and free tartaric acid [C4H6O6]. Because of the four functional groups, tartaric acid should be a good inhibitor. In the experiment we found a very strong textured and coarse-grained copper deposit. An explanation for this result is the strong adsorption (irreversible adsorption or chemisorption might be possible) of tartaric acid. The inhibiting agents occupy primarily the active sites; that is the edges of a growth step and the copper ions have to form a new layer on the less active areas. A continuous repetition leads to a textured structure. 6.2. Variation of the Citric Acid Concentration. Citric acid is a carboxylic acid with three carboxyl groups and one hydroxide group. This multifunctional organic substance has several possibilities for complex formation and inhibitation. The free acid is soluble in water and dissociates in protons (pK1 3.2) and acid anions. The hydrophilic acid groups are polar, and the hydroxide group is able to form hydrogen bonds. Citric acid forms various complexes with copper that exist in definite pH ranges. If the citric acid occupies the acitive sites (Figure 2, numbers 2 and 3) and hamper, in this way, the crystal growth, we should find a dependence of the crystallite size on the citric acid concentration. At low concentrations the inhibitor cannot occupy all active sites, and the growth of the copper deposit advances unlimited at active zones, resulting in large particle sizes. For higher concentrations the inhibitor occupies statistically the surface (active sites and less active sites) and the copper ions are forced to occupy the areas (Figure 2, number 1) with a lower current density. The diffusion of the copper ions to the active sites is hindered by the inhibitor molecules, and the

Figure 3. X-ray diffraction patterns (111-reflections) of nanocrystalline copper deposits prepared from electrolytes with variable citric acid contents.

grain growth is disturbed. For this reason the crystallite size should decrease. We verify this consideration in the following experiment: Pulse parameters (Ipulse 1.25 A/cm2, ton 1 ms, toff 100 ms), temperature (40 °C), pH (1-2), and hydrodynamic conditions are kept constant, and the citric acid concentration is varied (up to 100 g/L). The experimental conditions are given in Table 1 with citric acid concentrations of 0, 2.5, 5.0, 25.0, 50.0, and 100.0 g/L. The X-ray diffraction pattern (Figure 3) shows an increasing line width with increasing citric acid concentration. The resulting particle sizes decrease from >50 to 11 nm (Figure 3). As demonstrated for nanaocrystalline palladium,11 the grain size depends also on the pulse parameters, which therefore have to be optimized, too. In the present study we briefly investigated also the influence of the ton and toff time on the formation of nanocrystalline copper. The first experiment was carried out with variable toff time (ton is constant). To also keep the average current density constant, the peak current density (Ipulse) was changed appropriately. The pH value (1-2) and the temperature (40 °C) are kept constant for all experiments as well. The bath consists of copper sulfate (28 g/L), citric acid (30 g/L), and ammonium sulfate (50 g/L). Table 2 shows the volumeweighted grain sizes for each deposit. We observe a continuous increase in grain size from 10 nm up to 83 nm with decreasing toff. This result is reasonable in view of the nucleation processes: a long toff time means a high Ipulse, which causes a high cathodic overpotential and a high nucleation rate. We use a slight citric acid surplus to measure the inhibitor effect’s dependence on the pulse parameters. A further indication of inhibition could be the increased grain size at small toff times. Metal ions migrate faster than the big organic acid molecules. The metal ions are deposited first, and then the inhibitor molecules occupy the active sites. In a short toff time a decreased number of inhibitor molecules reach the active steps, before the growth continues at the next pulse. In this experiment we changed two parameters (toff and Ipulse). For this reason a further experiment is necessary to verify the results. In this mode, toff is nearly constant (see Table 3) because of a small variation of ton. A short ton time means a high current density. In the case of a high current density we obtain very small crystallites. The grain size increases slightly for longer ton times. We determine at the same current density (e.g. 12

Nanocrystalline Copper by Pulsed Electrodeposition

J. Phys. Chem., Vol. 100, No. 50, 1996 19529

TABLE 2: Experimental Data for an Electrolysis with Different toff Timesa Bath Parameters pH

T [°C]

hydrodynamic condition

1-2

40

mechanically stirred

Bath Composition CuSO4 [g/L]

(NH4)2SO4 [g/L]

citric acid [g/L]

28

50

30

Pulse Parameters toff [ms] ton [ms] Ipulse [mA/cm2] Ia [mA/cm2] grain size 〈D〉V [nm] 100 50 25 18 12 6

1 1 1 1 1 1

1250 625 312 225 156 78

12 12 12 12 12 12

10 14 25 32 51 83

a A long t off time means a high current density and an increased nucleation rate. The inhibiting molecules migrate to the active growth sites and block the grain growth process.

TABLE 3: Experimental Data for an Electrolysis with Different ton Timesa Bath Parameters pH

T [°C]

hydrodynamic condition

1-2

40

mechanically stirred

Figure 4. Transmission electron micrograph of nanocrystalline copper. The most frequent grain size is 15 nm.

Bath Composition CuSO4 [g/L]

(NH4)2SO4 [g/L]

citric acid [g/L]

28

50

30

Pulse Parameters ton [ms] toff [ms] Ipulse [mA/cm2] Ia [mA/cm2] grain size 〈D〉V [nm] 1 2 4 8

100 100 100 100

1250 625 312 156

12 12 12 12

10 12 19 25

a An increase in ton time causes a decrease in crystallite size because of a reduced pulse current density.

mA/cm2, see Tables 2 and 3) smaller grains than in the toff mode. This fact supports the theory of inhibitation. The grain size distributions were measured by TEM (Figure 4) and STM (Figure 5). The sample was prepared from a bath containing 75 g/L citric acid (Table 1). We determined three distributions from electron microscopy: two from STM (computer analyzed and counted by “hand”) and one from TEM (counted by “hand”). Further we determined a distribution by XRD (see section 2.1). A good fit could be obtained by a log normal particle size distribution in the form of

n(D) )

1

x2πD ln σ

(

exp -

)

(ln D - ln µ)2 2(ln σ)2

(14)

where µ is the median and σ is a parameter related to the width of the distribution. The results are represented in Figure 6. Compared to the optical methods, the median of the grain size distribution determined by XRD shifts to a smaller grain size. 6.3. Temperature Dependence of the Grain Size. The temperature influences the following. a. The Electrochemical Kinetics. Τhe Velocity (diffusion and migration) of the metal ions and inhibitor molecules are functions of the temperature. A high temperature causes an increased ion supply toward the cathode and the cathodic overpotential decreases. An increased energy for the nucleation

Figure 5. Scanning tunneling microscopy measurement of a nanocrystalline copper deposit.

process (eq 13) means a decreased rate of nuclei formation and a preferred growth of existing nuclei. The consequence is the formation of coarse grains. Τhe Viscosity of the electrolyte decreases at high temperature. The diffusion rate and the velocity of copper ions and inhibitor molecules were influenced in this way. The adsorption rate of inhibitor molecules decreases at high temperature and increases the surface energy of the crystallites. Volmer has shown that Ak is proportional to the third power of the surface energy (Ak ∝ σ3).27,28 Due to a high desorption of the inhibitor at high temperature, σ was increased, resulting in an increased grain growth.

19530 J. Phys. Chem., Vol. 100, No. 50, 1996

Natter and Hempelmann

Figure 7. 111 and 200 reflections from copper prepared at different temperatures. The grain size decreases with decreasing bath temperature.

TABLE 5: Citric Acid Complexes That Were Formed at Different pH Values and Their Dissociation Constants pH 2-4 4-5 5-7 7-11 Figure 6. Grain size distributions of nanocrystalline copper deposits obtained from different kinds of electron microscopy and by X-ray diffraction.

TABLE 4: Experimental Data for the Preparation of Copper Deposits at Different Temperatures Pulse Parameters ton [ms]

toff [ms]

Ipulse [A/cm2]

Ia [A/cm2]

1

100

1.25

0.0125

Bath Parameters pH

T [°C]

hydrodynamic condition

1-2

20, 30, 40, 50, 60, 80

mechanically stirred

Bath Composition CuSO4 [g/L]

(NH4)2SO4 [g/L]

citric acid [g/L]

28

50

50

b. The Electrochemical Thermodynamics. The addition of a complex former generates more or less stable complexes. The increased dissociation of a complex at high temperatures is the reason for an increased Cu2+ concentration (reduced cathodic overpotential). The result is the formation of large grains at high temperatures. For experimental verification we use a bath with a citric acid concentration of 50 g/L. The experimental details are given in Table 4. The line width (Figure 7) decreases in the temperature range from 20 to 80 °C, which indicates a strong temperature dependence of the crystallite size on the temperature. Small metal grains have a high interface energy and a high tendency for the reduction of the interface energy by grain growth. Migration of atoms in the interface is a function of temperature. To exclude this cause for grain growth during the deposition, we carry out a further experiment. A copper foil was deposited at 20 °C (bath, see Table 4). We detect grain sizes below 8 nm (by X-ray analysis). After heat treatment of the foil at 90 °C for 24 h, we found the same grain size. This result proves that the different grain sizes in this study do not result from grain growth based on interface diffusion. It is very

>11 a

complex

dissociation constant (Kdis)a [L2/mol2]

[CuH2C6H5O7]+ [CuHC6H5O7] [CuC6H5O7][Cu(OH)C6H5O7]2[CuC6H4O7]2[Cu(OH)C6H4O7]3-

1.0 × 10-6 1.0 × 10-7 7.8 × 10-6 1.8 × 10-18 1.5 × 10-16 1.8 × 10-18

Zolotukhim, V. K. Ukr. Khim. Zh. 1965, 31 (5), 525.

likely that the molecular processes, described above, are responsible for these results. 6.4. Dependence of the Grain Size on pH. The structure of the copper citrate complex depends on the pH value.29-31 Zolotukhim32 has found (Table 5) in an acid medium a positive complex ion, in a nearly neutral medium a complex with one negative charge, and in an alkaline medium a very stable, dark blue complex with two negative charges. The stability of these complexes rises rapidly with a progressive increase of pH. While deposition of free Cu2+ ions (reactions a, b) from an unstable complex is possible (Kdis 10-2-10-6 L2/mol2), direct deposition from a very stable complex is hardly conceivable (Kdis 10-16-1020 L2/mol2). For the deposition from a stable complex we propose the following deposition mechanism. In the vicinity of the cathode the complex loses its ligands stepwise (reactions c, d). The final step is the discharge of a simple positve charged complex with one negative ligand (reaction e).

a.

[Cu(Ligand)2](0 f Cu2+ + 2 Ligands(dissociation before the charge transfer)

b.

Cu2+ + 2e- f Cu(0

c.

[Cu(Ligand)3]- f [Cu(Ligand)2] (0 + Ligand-

d.

[Cu(Ligand)2] (0 f [Cu(Ligand)]+ + Ligand-

e.

[Cu(Ligand)]+ + e- f Cu(0 + Ligand-

Experimental details are given in Table 6. The molar ratio of copper to citric acid is 1:1. The pH was adjusted with decreasing quantities of NaOH. A change of the electrolyte color from light blue (pH 2) to dark blue (pH 11.5) indicates a complex formation. The results are shown in Figure 8. We

Nanocrystalline Copper by Pulsed Electrodeposition

J. Phys. Chem., Vol. 100, No. 50, 1996 19531

TABLE 6: Bath Composition and Experimental Parameters for the Deposition from a Bath with a Variable pH Value Pulse Parameters ton [ms]

toff [ms]

Ipulse [A/cm2]

Ia [A/cm2]

1

100

1.25

0.0125

Bath Parameters a

pH

T [°C]

hydrodynamic condition

1.5, 5.0, 8.0, 11.5

40

mechanically stirred

Bath Composition CuSO4 [g/L]

(NH4)2SO4 [g/L]

citric acid [g/L]

28

50

21

a

pH adjusted by addition of NaOH.

TABLE 7: Chemical Analysis of a Copper Prepared by Constant Current Electrolysis and by PEDa impurities [at.-ppm] sample

hydrogen

nitrogen

oxygen

carbon

nanocrystalline sample polycrystalline sample

1000 700

4000 340

2300 419

1500

a The analysis was performed by hot extraction at 2000 °C combined with mass spectra detection.

Figure 8. X-ray diffraction patterns of deposits prepared from citric acid electrolytes with different pH values.

obtain as our smallest grain size 8 nm at pH 1.5-2.0, and a continuous increase of the grain size up to 100 nm at pH 11.5 is observed. 6.5. Chemical Analysis. The nanocrystalline sample was prepared from a citric acid bath containing 28 g/L CuSO4, 50 g/L (NH4)2SO4, and 50 g/L citric acid (Ipuls 1.25 A/cm2, ton 1 ms, toff 100 ms). The results (Table 7) of the PED sample were compared with a coarse-grained sample prepared by constant current electrolysis (250 g/L CuSO4, 50 g/L ethyl alcohol, 1 × 10-2 A/cm2). The low carbon content of the deposit shows that no inhibitor molecules were occluded. 7. Discussion In the present systematic study of the pulsed electrochemical deposition of nanocrystalline copper we have clearly demostrated how and how extensively the nanostructure of the resulting material can deliberately be tuned by taking appropriate physical and chemical measures. The former include the parameters of the current pulses and the temperature, whereas the latter

concern the concentration and chemical composition of organic additives, the concentration of the metal ions, and the pH value. According to their effects on the nanostructure, the agents can be classified into (i) grain refinement agents, (ii) graincoarsening agents, and (iii) texture-forming agents. Based on a systematic series of experiments, we propose atomistic deposition mechanisms that allow the interpretation of the above phenomena as subtle interplays between nucleation, physisorption, and grain growth processes. The cathodic overpotential and the exchange current are the central electrochemical quantities. A large cathodic oVerpotential favors a high nucleation rate, resulting in the formation of crystallite nuclei in large quantities but small size. The necessary huge current density cannot be achieved for direct current electrolysis because of ionic transport limitations in the electrolyte. But huge current densities are feasible as peak current densities in short current pulses because in this way sufficient ionic transport takes place in the off-times between two pulses provided these times are long enough and there is sufficient stirring in the electrolyte. With the maximum possible current of our apparatus, 5 A, a current density of 1.25 A/cm2 (i.e a sample size of 4 cm2) in combination with a pulse length of 1 ms turned out as an acceptable compromise to give on one hand sufficient small crystallite nuclei and on the other hand a reasonable overall deposition rate of several grams per day. A further possibility to increase the cathodic overpotential is the decrease of the concentration of free Cu2+ ions. Usually, a decreased ion concentration can be achieved by dilution or by complex formation. For this reason a small grain size can be expected for the deposition from a bath with a strong complex former. Because of the complex instability in acid medium, we observe in section 6.1 the adsorption effects of the different substances. The interaction of citric acid and DiNa-EDTA with the metal surface is a reversible physisorption, whereas tartaric acid blocks irreversibly the low-energy growth sites and an oriented grain growth takes place. Malonic acid is an activator substance because it removes the water molecules from the inner Helmholtz plane and the copper ions can discharge easily. The exchange current is the main reason for the grain growth, which takes place during the off-times between the pulses in such a way that larger grains grow at the expense of smaller ones, with the interface energy as the driving force. Grain growth is hardly avoidable, but it should occur only for a short toff-time and only at a low rate. To limit the time of grain growth, the off-time between two current pulses has to be optimized: as low as possible and as large as necessary for the recovery of the bulk ion concentration in the depleted layer in the vicinity of the cathode. In the case of an inhibitor-free bath we found for nanocrystalline palladium11 a large crystallite size for long off-times. In this way, however, just by optimizing the pulse parameters, nanocrystalline Pd was prepared with a grain size down to 12 nm. From Table 2 of the present study it is evident that for nanocrystalline copper deposits prepared from a citrate bath the contrary effect can be observed. For long off-times we found a small crystallite size (10 nm). This is achieved by the addition of organic inhibitor molecules, which adsorb during the off-time on the freshly deposited layer and thus, if the adsorption enthalpy is sufficiently large, severely inhibit the ion exchange processes. For short off-times only a small fraction of the inhibitor molecules reach the growth sites because of the slow diffusion rate from the large organic molecules. Simultaneously these adsorbed large molecules inhibit the surface diffusion of adatoms, which represents another mechanism of grain growth.

19532 J. Phys. Chem., Vol. 100, No. 50, 1996 Of course the adsorbed organic molecules also have a certain inhibiting effect on the ion deposition during the pulse; but since the strong cathodic polarization and thus the driving force for the electrodeposition are very large, organic molecules adsorbed with not too large adsorption enthalpy are easily replaced, and the inhibitor molecules are not deposited on the electrode by metal atoms. Therefore the free enthalpy of the adsorption must on one hand be large enough to disturb the ion exchange between the pulses (inhibition) but not too large to prevent the desorption during the pulses (passivation). Citric acid and even better DiNa-EDTA fulfill these requirements; for these inhibitors the temperature and concentration dependence of inhibition, as experimentally verified, is straightforward: both an increase in temperature and a decrease in concentration decrease the coverage of the deposit surface with adsorbed molecules, decrease the inhibition, and increase the exchange current, the grain growth, and, eventually, the resulting particle size. Also the surface diffusion is increased, another reason for larger particles. In section 6.2 we determine the resulting particle size distributions by STM and TEM. From the STM image we get one distribution (D1) by computer-assited image analysis and a second distribution (D2) by counting by “hand”. The median of the first distribution is smaller than the median of distribution D2. The contrast between two grains is often very weak, and the computer system is not able to distinguish between two overlapping grains. The count by “hand” describes the surface distribution, but it has to be compared with the distribution from the bulk (TEM micrograph). Compared with distribution D2 we found a decreased median. Compared to the optical procedures the XRD measurement delivers a distribution of the coherently diffracting crystallites. The distribution median we get from the Warren/Averbach method is even smaller than the TEM distribution median. These observations show that grain size distributions have to be critically interpreted. 8. Conclusion and Outlook Pulsed electrodeposition has turned out to be a very valuable method for the preparation not only of nancrystalline copper but also of several other nanocrystalline metals. The apparatus is simple and cheap; large sample quantities can be prepared with low impurity content and, as shown in ref 11, with low porosity. The particle size can intentionally be tuned to a large extent by appropriate physical and chemical measures. Apart from possible practical applications, the thus prepared materials enable fundamental research with methods that require larger amounts of sample. Investigations of the diffusion of positive muons in nanocrystalline copper33 and the annihilation of positrons in nanocrystalline palladium34 have meanwhile been submitted for publication and many other studies are in progress. Acknowledgment. This study has arisen from the generous support by the Deutsche Forschungsgemeinschaft (DFG) in the framework of Sonderforschungsbereich 277 Grenzfla¨chen-

Natter and Hempelmann bestimmte Materialien at the Universita¨t des Saarlandes. We thank our colleagues in the SFB for many useful discussions. We thank L. Huang (Institut fu¨r Festko¨rperforschung of the Forschungszentrum Ju¨lich) for the STM measurement. We would also like to thank R. Herrig for the TEM measurement and A. Hartenberger for the hot extraction analysis. Financial support by the Fonds der Chemischen Industrie is gratefully acknowledged. References and Notes (1) Gleiter, H. Prog. Mater. Sci. 1989, 33, 223. (2) Osseo-Assare, K.; Arriagada, J. F. Ceramic Powder Science Vol. 3; Ceramic Transactions 12; American Ceramic Society: Westerville, OH, 1990; p 3. (3) Herrig, H.; Hempelmann, R. Mat. Lett. 1996, 27, 287. (4) Politis, C.; Johnson, W. L. J. Appl. Phys. 1986, 60, 1147. (5) Birringer, R.; Gleiter, H. In Encyclopedia of Material Science; Cahn, R. W., Ed.; Pergamon: Oxford, 1988; Suppl. Vol. 1, p 339. (6) Yang, H.; Nguyen, G.; McCormick, P. G. Scr. Metall. Mater. 1995, 32, 681. (7) Becker, A.; Langel, W.; Kno¨zinger, E. Z. Phys. Chem. 1995, 17, 188. (8) Czira´ko, A Ä .; Gero¨cs, I.; To´th, E.; Bakonyi, I. Nanostruct. Mater. 1995, 6, 547. (9) Osmola, D.; Renaud, E.; Erb, U.; Wong, L.; Palumbo, G.; Aust, K. T. Mat. Res. Soc. Symp. Proc. 1983, 286, 191. (10) Erb, U.; El-Sherik, A. M.; Palumbo, G.; Aust, K. T. Nanostruct. Mater. 1993, 2, 383. (11) Natter, H.; Krajewski, T.; Hempelmann, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 55. (12) Candlish, L. E.; Kear, B. H.; Kim, B. H. Nanostruct. Mater. 1992, 1, 119. (13) Rofagha, R.; Langer, R.; El-Sherik, A. M.; Palumbo, G.; Aust, K. T. Scr. Metall. Mater. 1991, 25, 2867. (14) Fischer, H. Elektrolytische Abscheidung und Elektrokristallisation Von Metallen; Springer Verlag: Berlin, 1954. (15) Enzo, S.; Benedetti, A. J. Appl. Crystallogr. 1988, 21, 536. (16) Benedetti, A.; Battagliarin, M. J. Appl. Crystallogr. 1988, 21, 543. (17) Warren, B. E. X-Ray Diffraction; Addison-Wesley: Reading, MA, 1968. (18) Scherrer, P. Go¨ ttinger Nachrichten 1918, 2, 98. (19) Klug, H. P.; Alexander, L. E. X-Ray Diffraction Procedures, 2nd ed.; John Wiley: New York,1974. (20) Balzar, D. J. Res. Natl. Inst. Stand. Technol. 1993, 98, 321. (21) Stokes, A. R.; Wilson, A. J. C. Proc. Phys. Soc. 1944, 56, 174. (22) Galiotti, G.; Paoletti, A.; Ricci, F. P. Nucl. Instrum. 1958, 3, 223. (23) Warren, B. E.; Averbach, L. E. J. Appl. Phys. 1950, 21, 536. (24) Warren, B. E.; Averbach, L. E. J. Appl. Phys. 1952, 23, 497. (25) Glasstone, S. Trans. Faraday Soc. 1935, The metallic coatings of films and surfaces, 1232. (26) Le Febvre, J. J. Chim. Phys. 1957, 54, 553. (27) Volmer, M. Die Kinetik der Phasenbildung; Verlag Steinkopff: Dresden and Leipzig, 1939. (28) Volmer, M.; Weber, A. Z. Phys. Chem. 1926, 119, 277. (29) Suzuki, S. J. Chem. Soc. Jpn. 1951, 72, 974. (30) Le Febvre, J. J. Chem. Phys. 1957, 54, 553. (31) Das, R.; Pattanaik, R. K.; Pani, S. J. Indian Chem. Soc. 1960, 37, 59. (32) Zolotukhim, V. K. Ukr. Khim. Zh. 1965, 31, 525. (33) Soetratmo, M.; Natter, H.; Hempelmann, R.; Hartmann, O.; Wa¨ppling, R.; Ekstro¨m, M. Hyperfine Interactions, in press. (34) Wu¨rschum, R.; Gruss, S.; Natter, H.; Hempelmann, R.; Scha¨fer, H.-E. Nanostruct. Mater., in press. (35) Jander, G.; Blasius, E. Einfu¨ hrung in das anorganisch-chemische Praktikum, 12th ed.; S. Hirzel Verlag: Stuttgart, 1987.

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