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Electrodeposition and characterisation of copper deposited from cyanide-free

alkaline glycerol complex bath

Article in Transactions of the Institute of Metal Finishing · January 2015

DOI: 10.1179/0020296714Z.000000000196

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 Electrodeposition and characterisation of
copper deposited from cyanide-free alkaline
glycerol complex bath
P. Sivasakthi, R. Sekar and G. N. K. Ramesh Bapu*
Electrodeposition of copper on mild steel substrate using a non-cyanide alkaline bath containing
glycerol as complexing agent has been developed. To improve the quality of the copper deposit,
current efficiency and throwing power of the plating electrolytes, the additives imidazole and
benzotriazole were employed and their effect on the surface morphology, grain size and hardness
were determined. The complexation of copper ions with glycerol was analysed using UV/Visible
absorption spectrophotometric techniques. XRD data obtained for electro- deposited copper
showed a polycrystalline nature with body centered cubic structure. SEM and AFM analysis of the
deposited copper revealed a uniform and smooth surface morphology with grain refinement in the
presence of additives.
Keywords: Copper electrodeposition, Alkaline non-cyanide, Glycerol, Imidazole, Benzotriazole

Introduction impurities and difficulty in control of the bath. In recent

years, research has been proposed on different alter-
Electrodeposition of copper and its alloys from cyanide native baths such as pyrophosphate, EDTA, citrate,
solutions has been widely used industrially for the ammonia, fluoborate, ethylenediamine, methylene dis-
production of high quality coatings. In decorative ulphonic acid, glycine, and tartrate, etc.4–6 However,
copper–nickel–chromium plating, copper deposition each of these electrolytes had limitations and hence the
helps in improving the appearance and under certain majority of these proposals have not been applied to
conditions also improves corrosion resistance. Because industrial level.
of high electrical conductivity, copper deposits have Studies on the electrodeposition of copper using
acquired importance for electrical contacts, production alkaline non-cyanide solutions containing tartrate and
of printed circuit boards and in selective case hardening glycine as complexing agents show that the deposits
of steel.1–3 Cyanide copper plating is used in the metal obtained from these solutions are of high quality.7–11
finishing industry for many applications, but not as Ballesteros et al. observed the effect of Cu (II)
extensively today as in the 1970s because of environ- concentration, glycine concentration and deposition
mental issues.4 Ground water contamination, high waste potential on the electrocrystallisation of copper onto
treatment, worker safety considerations, and high nickel electrode at pH 10 and reported that glycine acts
effluent treatment costs are encountered during the not only as an effective ligand in producing stable
usage of cyanide solutions for plating. During direct copper complexes but as a good levelling agent resulting
acid copper plating on mild steel metallic foil a galvanic in homogeneous copper coatings.9 According to Drissi-
displacement reaction (immersion deposition) occurs Daoudi et al.12 cuprous complex is an intermediate in
between the less noble iron and more noble copper the cupric complex reduction but is not detected during
which leads to poor adhesion, a problem which would the oxidation of electrodeposited copper in the solution
not happen with carcinogenic cyanide electrolytes. Thus, containing Cu (II)–glycinate complex at pH 10. The
to obviate these disadvantages and to replace the toxic effect of organic additives in copper plating is very
cyanide based solutions, an extensive search has been important in achieving good quality copper deposits and
made to develop a suitable non-cyanide based electrolyte therefore has been reported in numerous works.13–15
for depositing copper on mild steel. Glycerol is frequently used in the metal finishing
Alkaline non-cyanide copper plating solutions have industry as an addition agent in the deposition of metals
found increasing popularity since the mid-1980s in spite and electropolishing procedures.5,16 However, only a
of their higher operating costs, difficulty in using the few reports have discussed the role of glycerol as a
process on zinc die castings, greater sensitivity to complexing agent in the electrodeposition process and
its behaviour in alkaline media.17,18 Glycerol behaves as
a weak monoprotic acid with a dissociation constant of
Electroplating and Metal Finishing Technology Division, CSIR-Central 1?7610215. Moreover, the glycerolate anion exists as a
Electrochemical Research Institute, Karaikudi, Tamil Nadu, India predominant species only in strong alkaline media
*Corresponding author, email rameshbapugnk@cecri.res.in (pH.14), and the complexation of metallic cations can

ß 2015 Institute of Materials Finishing

Published by Maney on behalf of the Institute
Received 21 August 2013; accepted 17 April 2014
32 DOI 10.1179/0020296714Z.000000000196 Transactions of the IMF 2015 VOL 93 NO 1
 Sivasakthi et al. Electrodeposition of copper deposited from non-cyanide alkaline bath

be predicted in electroplating baths containing high and at different current densities ranging from 1 to
amounts of alkalies.17 Almeida et al. presented their 4 A dm22. The cathodes were weighed before and after
results on electrodeposition of copper onto steel in deposition and the cathode current efficiency and rate of
glycerol solutions at various NaOH concentrations and deposition were calculated. Throwing power was mea-
reported that the plating bath was stable with no sured using a rectangular Haring and Blum cell,
immersion deposition on steel for NaOH concentrations consisting of two mild steel sheet cathodes of 565 cm
(0?6M.18 In another investigation, the dissolution of filling the entire cross section at both ends of the walls,
copper in monoethanolamine (MEA)-complexed cupric and one perforated copper anode of the same size. The
ion solution containing halides, thiocyanates, and latter was placed between the cathodes so that its
different oxidisers as additives was reported.19 It was distance from one of the cathodes was one-fifth of its
proposed that copper dissolution proceeds through an distance from the other. Values of throwing power for
‘inner-sphere’ pathway in solution containing bridging different solutions used were calculated using the Field’s
ligands and the electron was transferred from the copper formula,
surface into the cupric species through the ligands which
L{M
greatly influenced the copper dissolution rate.19 Ben- Throwing powerð%Þ~ |100 (1)
zotriazole was shown to be the most effective inhibitor LzM{2
for copper and copper alloys in various aggressive where M is the metal distribution ratio between the near
environments.20 and far cathode and L is the ratio of the respective
Despite these many important contributions to the distances of the far and near cathodes from the anode.
electrochemical reduction of Cu (II) from alkaline non- To evaluate the adhesion of the copper deposits on
cyanide complexes, a detailed systematic study on the mild steel, a bend test as per ASTM Test method B 571-
development of Cu (II)–glycerol complex plating bath 84 was employed. The microhardness of the 30 mm thick
and operating conditions has not been explored. Hence, electrodeposited copper coatings was measured using
in the present investigation, the deposition parameters of an MH6 Everone micro hardness tester by an indenta-
copper deposition that affect the quality of the deposit, tion technique at a load of 10 g for 10 s with a dia-
and current efficiency and throwing power of the mond pyramid indenter. Measurements were conducted
process, in the presence and absence of imidazole, and six times on different areas on each sample, and the
benzotriazole additives have been determined. The role results were averaged. Hardness was expressed as
of additives in modifying the hardness of the deposit, Vickers microhardness (kg mm22). The plating solu-
surface morphology and grain size of the deposited tions were analysed by UV-visible molecular absorption
copper has also been reported. spectrophotometry, using a Varian, Cary 500 Scan
Spectrophotometer.
Experimental X-ray diffraction patterns were obtained using the X-
pert pro powder diffraction system PE 3040/60 for
All solutions were prepared using Analar grade chemi- copper deposits obtained from various copper baths in
cals with deionised water. Copper electrodeposition was the absence and presence of additives. The samples were
carried out using a freshly prepared non-cyanide bath, scanned at 20–100u (2h) at a rate of 1 degree per minute
containing 32 g L21 CuSO4, 40 g L21 NaOH, and using Cu Ka (l51?5405 Å) radiation. The peaks due to
40 mL L21 glycerol, in the absence and presence of the different phases were identified and the correspond-
additives (see Table 1). The experiments were carried ing lattice parameters calculated.23,24 The crystal size of
out in triplicate. Surface preparation prior to deposition the copper deposits was calculated using the Scherrer
is an important factor and can be achieved by formula for the predominant peak25
mechanical and electrochemical methods.21,22 The pro-
cedure adopted was the removal of surface scales using 0:9l
t~ (2)
acid dipping, mechanical polishing to get a smooth b cos h
surface, degreasing with trichloroethylene and final
where t is the average size of the crystallites, 0?9 is the
electrocleaning at 6 A dm22 in a solution of Na2CO3
Scherrer constant, l is the wavelength of the radiation
and NaOH (30 g L21 each). Mild steel metallic foil of
employed, b is the peak width at half maximum and h
2?565 cm size was used as cathodes in an electroplating
corresponds to the diffraction angle.
assembly consisting of two 99?99% pure copper anodes
The surface morphology of deposits obtained from
on either side of the cathode. Mechanical agitation was
different electrolytes was observed using scanning elec-
used throughout the experiments that reduced the
tron microscopy. The surface morphology of the depo-
polarisation of the electrodes and improved the quality
sited copper was analysed with an SEM (Hitachi, Model
of the deposits. The plating bath was operated at 30uC
S-3000H) at 15 kV. Molecular imaging atomic force
microscopy (AFM PicoScan 2100, Molecular Imaging,
Table 1 Bath composition and operating parameter
USA) was employed in a contact mode with a silicon
Constituents Bath (A) Bath (B) Bath (C) nitride tip to reveal the 3D surface topography of the
deposits.
21
Copper sulphate/g L 32 32 32
Sodium hydroxide/g L21 40 40 40
Glycerol/mL L21 40 40 40 Results and discussion
Imidazole/g L21 … 0.2 …
Benzotriazole/g L21 … … 0.2 Deposition bath stability and immersion
Current density/A dm22 1–4 1–4 1–4 deposition
pH 11.0–11.5 11.0 –11.5 11.0–11.5 Table 1 shows the operating parameters and composi-
Temperature/uC 30 30 30
tion of baths A, B and C used in the present study. In

Transactions of the IMF 2015 VOL 93 NO 1 33

Sivasakthi et al. Electrodeposition of copper deposited from non-cyanide alkaline bath

order to test the immersion deposition of copper on mild

steel substrates and the bath stability, mild steel plates
were immersed in the solution containing copper
sulphate, glycerol and various concentration of sodium
hydroxide (NaOH) from 10 to 50 g L21 and observed at
periodic intervals. When the concentration of NaOH in
the bath is 10, 20 and 30 g L21, an immersion deposit
was formed on mild steel by the displacement of copper
ions in the solution resulting in a thin poorly adherent
layer hindering subsequent deposition. The dissolution
of the steel substrate during immersion deposition led to
bath contamination. When the NaOH concentration is
maintained to 40 and 50 g L21, a clear deep blue colour
solution was obtained which gave no immersion
deposition. The solution was stable without precipita-
tion and no discolouration occurred, showing the
stability of the bath. The NaOH concentration was
optimised at 40 g L21 and used in further studies. 1 Effect of current density on throwing power for differ-
ent baths at 30uC
Current efficiency of deposition
The results of the current efficiency and rate of act as hydrogen suppressors leading to the observed
deposition measurements carried out at various current increase in cathode current efficiency.
densities are given in Table 2. For bath A, the current
efficiency was found to increase from 87 to 92% up Throwing power
to 2 A dm22 and thereafter decreased with increasing Figure 1 illustrates the variation of throwing power of
current densities, whereas the rate of deposition steadily baths A, B and C with different current densities.
increased at current densities up to 3 A dm22 and then Throwing power of bath A containing no additives
decreased to 24?9 mm h21 at 4 A dm22. This may be due increased with current density. Similarly, in bath B and
to hydrogen evolution reaction occurring at high current C containing imidazole and benzotriazole as additives,
densities which in turn explains the observed decreasing respectively, the throwing power increased with increas-
trend of current efficiency. It was concluded that ing current densities. This may be attributed to the
2 A dm22 is optimum for producing a smooth uniform increase in cathodic polarisation with increasing current
dull deposit with cathode current efficiency of 92% at density.
30uC. The results of studies from bath B containing
0?2 g L21 of imidazole as additive shows that the Adhesion and microhardness
current efficiency gradually decreased from 92 to 62% Adhesion of the copper deposits was tested by subjecting
with increasing current densities from 1 to 4 A dm22. the plated samples to standard bend tests as per ASTM
However, with increasing current density, the rate of Test method B 571-84. The deposits were found to
deposition was found to increase steadily from 14?9 to withstand the bend test, showing that the keying and
32?9 mm h21. Moreover, the quality of the deposits was adhesion of the deposit to the base metal were very good
changed from semi bright to a dull and powdery nature. in all cases.
For bath C containing 0?2 g L21 of benzotriazole as The microhardness of the deposited copper was
additive, the current efficiency steadily decreased from determined by the Vickers method at a load of 10 g.
94 to 62% with increasing current densities whereas the Table 3 shows the microhardness of the copper deposits
rate of deposition increased with increasing current obtained from baths A, B and C at 2 A dm22 at 30uC.
densities. In general, additives containing electrolytes Additive-free bath A exhibited a microhardness of
(baths B and C) displayed higher cathode current 196 VHN10 whereas the deposits obtained from imida-
efficiency compared to the additives-free electrolyte zole and benzotriazole containing bath B and bath C
(bath A). It was concluded that the additives studied show hardness values of 218 VHN and 239 VHN

Table 2 Current efficiency, rate of deposition, and quality and nature of deposit from different baths at 30uC

Bath Current density/A dm22 Current efficiency/% Rate of deposition/mm h21 Quality and nature of the deposit

A 1 87 11.4 Smooth uniform dull deposit

2 92 24.4 Smooth uniform dull deposit
3 77 34.3 Dull with powdery deposit
4 57 24.9 Dull with powdery deposit
B 1 92 14.9 Smooth uniform semibright deposit
2 88 22.6 Smooth uniform semi bright deposit
3 71 28.3 Dull with powdery deposit
4 62 32.9 Dull with powdery deposit
C 1 94 11.2 Smooth, uniform bright deposit
2 91 21.9 Smooth uniform bright deposit
3 67 26.6 Semi-bright deposit
4 62 33.1 Semi-bright deposit

34 Transactions of the IMF 2015 VOL 93 NO 1

 Sivasakthi et al. Electrodeposition of copper deposited from non-cyanide alkaline bath

respectively. The deposits obtained from baths B and C

are more compact and of finer grained structure. Hence
the azole derivatives, benzotriazole and imidazole, tested
increase the hardness of the electrodeposits, acting as
grain refiners. In general, fine grained deposits have
higher hardness values than coarse grained ones.
Absorption spectra
Figure 2 shows the electronic absorption characteristics
of the aqua copper and copper (II) complexes in the
wavelength region between 400 and 1200 nm. Figure 2a
shows the spectrum of 32 g L21 CuSO4 in aqueous
solution, exhibiting the absorption band at 810 nm.
Figure 2b shows the absorption spectra of bath A
containing 32 g L21 CuSO4 solution in the presence of
40 mL L21 glycerol, and 40 g L21 NaOH at 600 nm.
Figure 2a clearly shows that the change of ligand has a
a 32 g L21 CuSO4, b 32 g L21 CuSO4, 40 mL L21 glycerol, strong influence on the positions of the absorption
and 40 g L21 NaOH; c 32 g L21 CuSO4, 40 mL L21 gly- maxima of the copper (II) complex. As is seen from
cerol, 40 g L21 NaOH, and 0?2 g L21 imidazole; d 32 g L21
Fig. 2c and d, imidazole-containing bath B and benzo-
CuSO4, 40 mL L21 glycerol, 40 g L21 NaOH, and 0?2 g L21
benzotriazole
triazole-containing bath C exhibited the absorption
2 Absorption spectra of the copper electrolyte solutions band at 600 nm and there was no significant change in
the absorption spectra by the addition of imidazole
and benzotriazole additives. The absorption maximum
observed in Fig. 2b is characteristic of the absorption
spectra of copper (II) coordination complexes where
copper (II) is attached to four oxygen atoms. The
appearance of a dark blue clear solution in presence of
glycerol indicates the formation of copper–glycerolate
coordination compounds.
X-ray diffraction studies
Figure 3a–d shows the X-ray diffraction patterns of the
copper electrodeposits obtained from baths A to C at
30uC. The XRD patterns show that all the deposits are
polycrystalline and body centred cubic structure (Table 4).
The observed ‘d’ values are in good agreement with
standard ‘d’ values of copper.26 Figure 3a shows the XRD
pattern of copper, deposited from bath A at 1 A dm22 in
which the reflection from the (111) plane was more
predominant (2h543?4579u) compared to other peaks.
a bath A at 1 A dm22; b bath A at 3 A dm22; c bath B The crystal size was calculated using the Scherrer formula
at 1 A dm22; d bath C at 1 A dm22 for the predominant peak and the average crystal size was
3 Pattern (XRD) of copper deposit obtained at 30uC from about 51 nm. The XRD pattern of copper deposited using
different baths bath A at 3 A dm22 (Fig. 3b) shows the same preferred
crystal orientation of the (111) plane that was more
predominant, and the crystal size was marginally reduced
to 43 nm. Generally grain size decreased with increasing
Table 3 Microhardness (load 10 gf) of copper deposits current densities. The observed reduction in grain size can
obtained from different copper plating baths at
2 A dm22 and at 30uC
be attributed to the higher current density employed
giving rise to a high degree of adatoms saturation at the
Bath Hardness, VHN10/kg mm22 electrode surface.
The reflection from the (111) plane was predomi-
A 196 nant for the copper deposit obtained from bath B
B 218
containing imidazole (Fig. 3c) and the grain size was
C 239
about 42 nm. The same reflection from the (111) plane

Table 4 XRD and crystal size data of copper deposits obtained from various copper plating baths at 30uC

Bath Current density/A dm22 2h value FWHM Plane Rel. Int/% Crystal size/nm

A 1 43.3908 0.1673 (111) 100 51

A 3 43.4239 0.2007 (111) 100 43
B 1 43.4136 0.2007 (111) 100 42
C 1 43.5431 0.2676 (111) 100 31

Transactions of the IMF 2015 VOL 93 NO 1 35

Sivasakthi et al. Electrodeposition of copper deposited from non-cyanide alkaline bath

a bath A at 1 A dm22; b bath A at 3 A dm22; c bath B at 1 A dm22; d bath C at 1 A dm22

4 SEM images of copper deposit obtained at 30uC from different baths

was predominant for the copper deposit obtained from indicates that the deposit obtained from bath A without
the bath C containing benzotriazole as additive (Fig. 3d) additive shows the presence of flat mount like structures
and the crystal size is still reduced to 35 nm. These with no well defined grain boundaries and average grain
results confirm that both the additives employed in the size of 51 nm. Figure 5b is the AFM image for deposits
present study acted as grain refiner. The observed higher from bath B containing imidazole as additive and the
hardness values for deposits from bath B and bath C crystals are well defined grains of about 42 nm size with
compared to those from bath A (Table 3) is due to the smooth surface morphology.
reduction of grain size in the presence of additives which
is clearly evident from the XRD data. Conclusion
Scanning electron microscopy Smooth and adherent deposits of copper from alkaline
The surface morphology of the copper deposits obtained glycerol based electrolytes with high current efficiency
from baths A to C and different current densities are shown and good throwing power have been obtained. The
in Fig. 4a–d. Figure 4a represents the copper deposits deposits obtained at 1 A dm22 from additive-containing
obtained from bath A without additive at 1 A dm22. It is (imidazole and benzotriazole) baths (B and C) are
observed that in the absence of an additive, clusters of big compact and of fine grained structure compared to
crystals and a non-uniform surface were evident, and copper deposits obtained from additive-free bath A. The
agglomerated crystal grains are seen. Similar behaviour was hardness of the copper deposit obtained at 2 A dm22
observed from deposits obtained at 3 A dm22 (Fig. 4b), but was found to increase with the additives studied. It is
the grains were slightly reduced. Figure 4c shows the believed both benzotriazole and imidazole act as grain
electrodeposited copper obtained from bath B containing refiners. UV absorption spectra revealed that complex-
0?2 g L21 imidazole as an additive, revealing a regular and free copper exhibited the absorption band at 810 nm
smooth surface morphology with fine grained structure. whereas the presence of glycerol, NaOH and imidazole/
Figure 4d shows that incorporation of benzotriazole benzotriazole additives exhibited the absorption band at
(0?2 g L21) as additive, produced smooth surface mor- 600 nm. X-ray diffraction studies revealed that the
phology and fine grained structure. Figure 4c and d copper deposits obtained from all three baths exhibited
clearly indicates that both imidazole and benzotriazole the (111) plane reflection being more predominant. SEM
act as grain refiners. The observed hardness values and images show that the deposits obtained in the absence of
the calculated average grain sizes fully agree with the additives have a cluster of coarse grains and the crystals
morphology of the deposited copper. are non-uniformly arranged, whereas deposits obtained
from the additive-containing baths have smooth and
AFM measurements uniform surface morphology. AFM analysis revealed
AFM measurements give a perspective of the ‘Z’ that smoothening of three dimensional surface images
direction with three dimensional images. Figure 5a, the and grain refinement was brought about by the additives
representative AFM scanned over an area of 565 mm, studied.

36 Transactions of the IMF 2015 VOL 93 NO 1

 Sivasakthi et al. Electrodeposition of copper deposited from non-cyanide alkaline bath

a bath A; b bath B
5 AFM images of copper deposit obtained at 30uC and 1 A dm22 from different baths

14. C. N. Tharamani, B. N. Maruthi and S. M. Mayanna: Trans. IMF,

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Transactions of the IMF 2015 VOL 93 NO 1 37

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