Growth mechanisms of CuSCN films electrodeposited on ITO in EDTA-chelated copper(II) and KSCN aqueous solution

Article abstract of DOI:10.1016/j.electacta.2004.10.048

Electrodeposition of β-CuSCN films was investigated on transparent conducting ITO substrates in an aqueous electrolyte containing EDTA-chelated Cu(II) and KSCN. It has been observed that the instability of CuSO₄ and KSCN aqueous solution without EDTA is due to the formation of Cu(SCN) ₂ precipitation, which can transform into CuSCN and (SCN)_x at room temperature. Research results illuminate that the deposited film at -0.5 V versus Ag/AgCl_sat at 298 K is uniform and dense and composed of nanocrystals. The film is p-type with stoichiometric excess of SCN and a direct transition gap of 3.7 eV. Deposition mechanisms of CuSCN films at varied temperatures are studied based on the proposed energetic model. At or below room temperature, the electron quantum tunnel through deposition layer is predominant at the very beginning. However, the growth is limited when the thickness of CuSCN film reaches the size comparable to the diffusion length of electrons. Above room temperature, the thermal activation of surface states plays an important role in the continuous growth of large crystals through holes transport in the valence band. The calculated activation energy for crystal growth is 0.5 eV.

Full text of DOI:10.1016/j.electacta.2004.10.048

Electrochimica Acta 50 (2005) 2343–2349
Growth mechanisms of CuSCN films electrodeposited on ITO in
EDTA-chelated copper(II) and KSCN aqueous solution
Weibing Wu, Zhengguo Jin∗, Zhen Hua, Yanan Fu, Jijun Qiu
School of Material Science and Engineering, Tianjin University, Tianjin 300072, China
Received 25 July 2004; received in revised form 29 September 2004; accepted 8 October 2004
Available online 21 November 2004
Abstract
Electrodeposition of ␤-CuSCN films was investigated on transparent conducting ITO substrates in an aqueous electrolyte containing
EDTA-chelated Cu(II) and KSCN. It has been observed that the instability of CuSO4 and KSCN aqueous solution without EDTA is due to
the formation of Cu(SCN)2 precipitation, which can transform into CuSCN and (SCN)x at room temperature. Research results illuminate
that the deposited film at −0.5 V versus Ag/AgClsat at 298 K is uniform and dense and composed of nanocrystals. The film is p-type with
stoichiometric excess of SCN and a direct transition gap of 3.7 eV. Deposition mechanisms of CuSCN films at varied temperatures are studied
based on the proposed energetic model. At or below room temperature, the electron quantum tunnel through deposition layer is predominant
at the very beginning. However, the growth is limited when the thickness of CuSCN film reaches the size comparable to the diffusion length of
electrons. Above room temperature, the thermal activation of surface states plays an important role in the continuous growth of large crystals
through holes transport in the valence band. The calculated activation energy for crystal growth is 0.5 eV.
© 2004 Elsevier Ltd. All rights reserved.
Keywords: CuSCN; Surface state; Activation energy; Deposition mechanism; Solar cell
1. Introduction
these, CuSCN is thought to be the most promising for its
stability and possibility in filling pores of porous film by dip-
ping [8], CBD [9], electrodeposition [10–12] and SILAR [9]
methods.
Nanocrystal photovoltaic cells (NPC), such as dye-
sensitized solar cells (DSSC) [1] and extremely thin absorber
solar cells (ETA) [2], etc., have recently been under intense
investigation because of their potentials in low cost and high
efficiency [3]. Generally, NPC cells mainly consist of porous
anode, sensitization layer and electrolyte layer as hole con-
ducting layer. In recent years, much attention is focused on
developing solid state or “quasi solid state” replacements for
liquid electrolyte. p-Type semiconductor is a good candi-
date for solid state electrolyte which has some advantages
in improving long-term durability and stability of NPC cells.
There are some wide band gap materials, such as CuI [4],
CuAlO2 [5], NiO [6], CuSCN [7], etc., which can meet the
requirements for p-type layer of NPC solar cells. Among
Electrodeposition is a standard method for coating the in-
sides of complex shapes [13]. It has been applied in NPC
solar cells to prepare CuSCN film. However, most deposi-
tions are carried out in organic solvents (ethanol) [10,11]
and seldom in aqueous solution because of the poor stability
of Cu(II) cations and SCN anions in water [10]. As known,
there are many advantages for electrodeposition in aqueous
solution, such as lower cost, strong dissolvability to solutes,
high deposition efficiency and especially preventing desorp-
tion of dyes [7]. In the present paper, electrochemical prepa-
ration of CuSCN films is realized in aqueous solution. To
improve the stability of aqueous solution, EDTA is added
as a chelating reagent for Cu(II). Electrodeposition mech-
anisms are discussed based on an energy model at varied
temperatures.
∗
Corresponding author. Tel.: +86 22 27890266; fax: +86 22 27404724.
E-mail address: zhgjin@tju.edu.cn (Z. Jin).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.10.048

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W. Wu et al. / Electrochimica Acta 50 (2005) 2343–2349
absorption band at about 1220 cm−1 is attributed to (SCN)x
2. Experimental
according to the reported structure of parathiocyanogen [11],
which may be self-polymerizated by SCN produced by de-
composition of Cu(SCN)2 in water at 353 K. The related pro-
cesses are shown as following:
CuSO4·5H2O, KSCN and ethylenediamine tetraacetic
acid disodium salt (EDTA, C10H14N2O8Na2·2H2O) were
used as received. A black precipitate was firstly prepared by
mixing cupric sulphate solution with the solution of potas-
siumthiocyanateatthemolarratioofCu2+/SCN− = 1:2. Then
it was rinsed with deioned water to remove impurity ions. The
as-prepared black precipitate turned into white through its re-
action in deioned water under stirring for about 30 min. After
filtering, a yellow powder is obtained by drying the filtrate at
353 K in vacuum.
The stable aqueous electrolyte, was prepared by mix-
ing 0.06 mol L−1 cupric sulphate, 0.06 mol L−1 EDTA and
0.03 mol L−1 SCN anions, in which EDTA was used
as a chelating reagent for Cu(II). The resultant solu-
tion could be stored at room temperature for several
months without apparent changes. The prepared electrolyte
was dried at 383 K to characterize its stability by FT-IR
method.
CuSO4 + 2KSCN → Cu(SCN)2 ↓ + K2SO4
Cu(SCN)2 → CuSCN + SCN
xSCN → (SCN)x
(1)
(2)
(3)
The stability of aqueous solution can be improved by chelat-
ing between EDTA and Cu(II) anion. From FT-IR spectra
of EDTA and the dried electrolyte in Fig. 3, it is found that
the asymmetric stretching of carboxylate at 1672 cm−1 in
Fig. 3b shifts to 1718 cm−1 due to the replacement of Na site
by Cu, which indicates that EDTA plays an important role in
improving the stability of electrolyte.
3.2. CuSCN film characterization
CuSCN films were deposited in the above obtained elec-
trolyte. A normal three electrodes cell was used. The elec-
trochemical measurement was performed on a potentio-
stat/galvanostat (TD3691, Tianjin, China). Indium tin oxide
coated glass (ITO, 10 ꢀ/cm) was used as a working electrode,
a large area Pt foil as a counter electrode and an Ag/AgClsat
as the reference electrode. Before deposition, ITO is cleaned
ultrasonicallyinacetone, 0.1 mol L−1 NaOHanddeionedwa-
ter, respectively. CuSCN film was electrodeposited potentio-
statically ranging from −0.1 to −0.6 V versus Ag/AgClsat
and galvanostatically at 0.05 mA cm−2 at varied tempera-
tures.
Fig. 4 shows a SEM micrograph of CuSCN film deposited
potentiostatically on ITO at −0.5 V versus Ag/AgClsat for
5 min at room temperature (298 K). Morphology of the film is
uniform, smooth and dense, and consists of faceted nanocrys-
tals. This shape of crystals suggests that the growth of films
is kinetic limit. The XRD patterns of the electrodeposited
CuSCN film, ITO substrate and CuSCN powder are shown in
Fig. 5. All characteristic peaks of the electrodeposited mate-
rial are consistent with ␤-CuSCN (JCPDS card number 29-
581) except those of the conducting layer on substrate. By
XPS method, the molar ratio of SCN/Cu = 1.2 indicates that
the deposited CuSCN film is p-type [14].
The optical transmittance spectrum of CuSCN film is
shown in Fig. 6. The relationship between absorption co-
efficient (α) and photon energy (hν) is given in the plot of
(αhν)2 versus hν. The straight line is due to the direct band
gap of CuSCN film. Band gap value of 3.7 eV is determined
by extrapolating the straight line portion to the energy basis
at α = 0. The apparent absorption at photon energy less than
2.5 eV is due to interference in the layers. The wider band
gap compared with other reported results may be caused by
the smaller crystal size and the larger stoichiometric excess
of SCN in deposited film [14].
Phase composition of as-prepared powders and films were
analyzed by X-ray diffraction (XRD) on a Japan D/Max-2500
˚
X-ray diffractometer with Cu K␣ radiation (λ = 1.541 A).
FT-IR spectra of the yellow powder and EDTA-chelated
cupric(II) complex were recorded on a BIO-RAD FTS3000
spectrophotometer. Microstructures of the films were ob-
served by scanning electron microscopy (SEM) with a
Philips XL-30 scanning electron microscope. Absorption
spectra of the films were recorded on Beckman DU-8B
ultraviolet–visible (UV–vis) absorption spectrometer. X-ray
photoelectron spectra (XPS) of powders and films were car-
ried out on an Esca Multiplex Philips-1600 X-ray photoelec-
tron spectrometer.
3.3. Electrodeposition mechanisms
3. Results and discussion
Fig. 7 depicts the potential transients for galvanostatic
deposition at the current density of 0.05 A cm−2 at varied
temperatures. Shapes of the curves illuminate the process of
nucleation at early stage and subsequent crystal growth. The
sharp drop in potential at the very beginning is charging of
double layer [15], which followed by a nonlinear increase of
the potential corresponding to the coinstantaneous nucleation
and growth of nuclei. It is known that the potential is more
negative for nucleation on a foreign substrate than growth
3.1. Powder characterization
Fig. 1 shows the XRD patterns of the black and white pow-
ders. The characteristic peaks are in good agreement with
the normal JCPDS (card numbers 22-253 and 29-581) corre-
sponding to Cu(SCN)2 and CuSCN, respectively. Fig. 2 is the
FT-IR spectrum of the yellow powder. The strong and broad

W. Wu et al. / Electrochimica Acta 50 (2005) 2343–2349
2345
Fig. 1. X-ray diffraction spectra of (a) the black powder Cu(SCN)2 and (b) the white powder CuSCN.
[16]. A similar phenomenon was also found in the present
system, as shown in Fig. 7, potential for CuSCN nucleation
on bare ITO substrate is more negative than growth. Once
a certain number of nuclei are formed, further growth of
CuSCN is facilitated and the potential for growth is less neg-
ative. This is also seen in the linear sweep voltammograms
between 0.6 and −0.6 V at the rate of 50 mV s−1 (Fig. 8).
The critical potential for deposition is about −0.05 V as
seen in the first scan curve. While in the next four scans, the
potential where the current density is considerable is 0.3 V or
more positive, which is less negative about 0.4 V than the first
scan.
At low temperature, there is a steady decrease in poten-
tial during the growth of the layer, especially at temperature
lower than 288 K (Fig. 7). When potential is too negative
(around −0.63 V), Cu is co-deposited, causing the oscilla-
tions of potential. At the same potential, the third reduction
peak for Cu deposition in the fifth scan appears, seen in Fig. 8.
When temperature elevated, potentials for grain growth get
less negative and comparatively stable (Fig. 7). This means
that higher current density is permissible and thicker layers
are possible to be deposited at temperature above 298 K. For
example, a film with the thickness up to several microm-
eters is formed at 318 K. From the above discussion, it is

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W. Wu et al. / Electrochimica Acta 50 (2005) 2343–2349
Fig. 5. XRD patterns of (a) ITO, (b) CuSCN powder and (c) CuSCN thin
film deposited at −500 mV vs. Ag/AgClsat
Fig. 2. FT-IR spectrum of the yellow powder (SCN)x.
.
Fig. 6. Plot of (αhν)2 vs. hν and the optical transmittance spectra of CuSCN
film obtained at −500 mV vs. Ag/AgClsat
.
Fig. 3. FT-IR spectra of (a) EDTA powder and (b) dried electrolyte.
Fig. 4. SEM micrograph of the CuSCN thin film prepared at −500 mV vs.
Fig. 7. Potential transients for galvanostatic deposition at the current density
of 0.05 mA cm−2
Ag/AgClsat at 298 K.
.

W. Wu et al. / Electrochimica Acta 50 (2005) 2343–2349
2347
Fig. 10. Current transients for potentiostatic stepping from 0.6 to −0.6 mV
Fig. 8. Five linear sweep voltammograms in the range of 0.6 to −0.6 V
consecutively on the same ITO electrode at the rate of 50 mV s−1
at varied temperatures.
.
As CuSCN is a p-type semiconductor, holes are the major-
ity charge carriers. This means that charge transfer through
holes in the valence band is possible.
concluded that the growth of CuSCN film is kinetics limit
and the thermal activation is necessary.
In addition, the morphology of the films is strongly af-
fected by temperature. Crystals grown at 328 K are about
1 ␮m (Fig. 9a), while those formed at 288 K are very small,
e.g. less than 50 nm (Fig. 9b). From Fig. 10, it can be seen
that the growth of CuSCN film is strongly influenced by tem-
perature. The deposition current density is increased from 0.6
to 2.6 mA cm−2 when temperature rose from 288 to 338 K in
the steady deposition region II.
As known, chemical reorganization energy of reaction
complex is about 1 eV and Franck–Condon’s split energy
is 1–3 eV [17,18]. The thermal fluctuation of the solvation
structure causes a corresponding thermal distribution of the
energy levels of both the highest occupied energy level Eoc
(reduction potential Ered) and the lowest unoccupied energy
level Eunoc (oxidation potential Eox) [18]. Band edges of the
semiconductor are pinned at its surface energy level. Inter-
facial electron transfer generates a space charge layer. The
thickness of the space charge layer typically ranges from
10 nm to several micrometers dependent on the conductivity
of the semiconductor and external potential, which is much
larger than the lengthof electronquantum tunnel. The valence
band edge at the surface of CuSCN (Evs) is about 0.5 eV ver-
sus Ag/AgClsat and the conduction band edge (Ecs) −3.1 eV
[13]. The redox potential of this electrolyte solution (Eredox
)
is about −0.05 eV as seen in Fig. 8. The upper limit, where
the lowest density of possible receiving states is near zero, is
about −3.05 eV which is lower than Ecs.
According to Frank–Condon principle [19], electrons
must move from an initial state (on an electrode or a reduc-
tant) to a receiving state (on another solved species or on an
electrode) of the same energy. There are three possible paths
for electron transfer across the semiconductor–electrolyte in-
terface: conduction band, valence band and thermal activa-
tion. In present deposition system, minority carries (elec-
trons) are accumulated at conduction band of the surface
when deposition potentials are negative than −0.1 V while
the energy of the available electrons in conduction band is
higher than the upper limit energy where the density of pos-
sible receiving states in the electrolyte is near zero. In other
words, there are nearly no available receiving states match-
ing the energy of electron in accumulation layer at the surface
Fig. 9. Morphologies of CuSCN film deposited at galvanostatic
0.05 mA cm−2 for 300 s (a) at 328 K and (b) at 288 K.

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W. Wu et al. / Electrochimica Acta 50 (2005) 2343–2349
Scheme1. EnergymodeloftheinterfacebetweenCuSCNandtheelectrolyte
at external potential.
Fig. 11. Arrhenius plots for three potentials in EDTA-chelated copper(II)
and KSCN solution.
of CuSCN. Therefore, the rate constant for electron transfer
from conduction band of deposited CuSCN film to oxidant
is very small and not relative with temperature, although
electron (minority carrier) concentration is closely associ-
ated with temperature. For another possible path of holes
transferring from oxidant to valence band of CuSCN, the
pinning of valence band edge at surface energy level lower
than redox potential forms a wide hole depletion layer, which
preventing electron transfer from valence band to receiving
states according to Frank–Condon principle. So, only ther-
mal activation of the surface states (Ess) is possible for the
deposition reaction. In this case, the thermal activation is the
rate-determining step for the overall formation process of
CuSCN.
A valence band mechanism including surface states ac-
tivation for electrodeposition of CuSCN film can be drawn
from the above discussion. The mechanism model is shown
in Scheme 1.
The cathodal current density for semiconductor elec-
trodeposition is usually expressed by the following relation
[17]:
In this model, the surface states are formed on the sur-
face of CuSCN, holes are thermally activated from the sur-
face states into the valence band and subsequently transport
through the bulk to the back substrate. The activation energy
of 0.5 eV corresponds to the energy of activating a hole from
a surface state (Ess) into the valence band edge at the sur-
face (Evs). The surface state could be formed both on the
surface of p-CuSCN by stoichiometrical excess of SCN and
in Helmholtz layer by absorption [20,21]. The surface state
(Ess) is equal to the redox energy. A lower activation energy
could be expected if the reorganization energy of CuII com-
plex ions were decreased. This is demonstrated by the poor
stability of the aqueous solution containing cupric(II) ions
and SCN ions without any chelating reagent. At this condi-
tion, the redox energy of cupric(II) (the dark Cu(SCN)2) is
lower than the valence band edge of CuSCN, causing holes in
redox energy of cupric(II) ions transfer to valence band edge
[11]. The deposition process of CuSCN film is described as
following:
• Surface states formed by stoichiometrical excess of SCN
jc = qkcnscox exp (−Ucat/kT)
in p-type CuSCN or absorption in Hemholtz layer:
where kc is a rate constant, ns the surface electron density,
CuIIEDTA + SCN−
c
ox the oxidant concentration and Ucat is the activation energy
→ CuIISCN (surface state) + EDTA
of cathodal reaction. However, for the deposition of CuSCN
semiconductor, as discussed above, electrons in the conduc-
tion band at the surface do not take part in deposition process.
So, the relation should be moderately modified. In the modi-
fied relation, ns is no longer seemed as variable and combined
to kc:
or
CuIIEDTA + 2SCN−
→ CuII(SCN)2 (surface state) + EDTA
jc = qkccox exp (−Ucat/kT)
• Thermal activation of surface states:
This relation is very similar to Arrhenius equation [19]:
CuII(SCN)+(surface state) → CuSCN(solid) + h+(VB)
jc = Acox exp (−Ucat/RT)
Arrhenius plots for three applied potentials are shown in
Fig. 11. Activation energy Ucat obtained from the slope of
the plot is about 0.5 eV.
or
−
CuII(SCN)2(surface state) → CuI(SCN)2 + h+(VB)

W. Wu et al. / Electrochimica Acta 50 (2005) 2343–2349
2349
crystals is realized through holes transport in the valence
band, resultinginthethickfilmwithseveralmicrometers.
• Chemical reaction:
−
CuI(SCN)2 → CuISCN(solid) + SCN−(solution)
Acknowledgements
It is also shown from Fig. 11 that the activation energy is
almost independent of the applied cathodal potentials. This
may be caused by the pinning of valence band edge at the sur-
face of CuSCN [18]. This reasonably accords with previous
discussion and the energy model.
The financial support is provided by Key Basic Research
Program of TSTC (No. F103004). We thank Dr. GuoHusen,
He Fei, Du Haiyan and Jiang Dongqing, for their kind as-
sistance in the XRD, XPS SEM and UV–vis measurements,
respectively.
For deposition at low temperatures, the valence band
mechanism, involving large activation energy, is difficult to
match the deposition process for the deficiency of thermal
activation. Another charge transport mechanism may be suit-
able to explain low temperature process. The diffusion length
of the minority charge carriers is in the range of 10–100 nm
[18]. This means that if the distance between the conduct-
ing substrate and the CuSCN–electrolyte interface is within
the diffusion length range, it is possible that electrons tune
through the thin deposited layer. The charge tunnel through
the layer can explain the small rounded particles formed at
lower temperatures (Fig. 9b) and the strong increase in nega-
tive potential when the particles reach the size corresponding
to the diffusion length of electrons just as shown in the curve
at 288 K in Fig. 7.
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4. Conclusions
(1) The instability of aqueous electrolyte is caused by pre-
cipitation of Cu(SCN)2 and Cu(SCN)2 transformation to
CuSCN. The stability of aqueous electrolyte can be im-
proved by EDTA chelating with Cu(II) cations, restrain-
ing precipitation of Cu(SCN)2.
(2) A dense film with nanocrystals is deposited at poten-
tialstat −0.5 V at room temperature. The film prepared
through aqueous electrolyte is ␤-CuSCN and p-type
semiconductor with stoichiometric excess of SCN in
CuSCN. The CuSCN film has a wide direct band gap
of 3.7 eV.
(3) ThegrowthofCuSCNcrystalsisthermalactivationlimit.
The activation energy obtained from the logarithm of the
deposition current density versus the inverse temperature
is 0.5 eV and is independent of the applied potentials.
(4) The growth of CuSCN film follows two different mech-
anisms at varied temperatures. Below or at room temper-
ature, electron quantum tunnel through thin deposited
layer is predominant, and the thin dense film consisting
of nanocrystals is obtained; at higher temperature, a ther-
mal activation process is involved and the growth of large
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