Fabrication of CuO/ZnO composite films with cathodic co-electrodeposition and their photocatalytic performance

Article abstract of DOI:10.1016/j.molcata.2010.08.011

CuO/ZnO composite films with different atomic Cu/Zn ratios were for the first time fabricated on indium tin oxide (ITO)-coated glass substrates via a cathodic co-electrodeposition route from baths containing Zn(NO ₃)₂ and Cu(CH₃COO)₂. The obtained composite films were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Hexavalent chromium (Cr(VI)) was used as a model pollutant to evaluate their photocatalytic activity. For a bath containing Zn(NO₃)₂ with a certain concentration, the concentration of Cu²⁺ ions added into this bath play an important role in fabrication of CuO/ZnO composite films. When the Cu²⁺ concentration in a bath is lower, the obtained composite is a Cu-doped ZnO film. The electrodeposited CuO/ZnO composite films show higher photocatalytic activity towards reduction of Cr(VI) compared to pure ZnO both under UV and under UV-vis light illumination. Furthermore, the photocatalytic activity of CuO/ZnO composite films is related to their atomic Cu/Zn ratios. The mechanism of CuO/ZnO composite films for improvement in photocatalytic activity was also discussed.

Full text of DOI:10.1016/j.molcata.2010.08.011

Journal of Molecular Catalysis A: Chemical 331 (2010) 112–116
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Fabrication of CuO/ZnO composite films with cathodic co-electrodeposition and
their photocatalytic performance
∗
Shouqiang Wei , Yuye Chen, Yuyan Ma, Zhongcai Shao
School of Environmental and Chemical Engineering, Shenyang Ligong University, Nanping Middle Road 6 #, Shenyang 110159, Liaoning Province, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 April 2010
Received in revised form 7 July 2010
Accepted 6 August 2010
Available online 14 August 2010
CuO/ZnO composite films with different atomic Cu/Zn ratios were for the first time fabricated on indium
tin oxide (ITO)-coated glass substrates via a cathodic co-electrodeposition route from baths containing
Zn(NO3)2 and Cu(CH3COO)2. The obtained composite films were characterized with X-ray diffraction
(XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Hexavalent
chromium (Cr(VI)) was used as a model pollutant to evaluate their photocatalytic activity. For a bath
2
+
containing Zn(NO3)2 with a certain concentration, the concentration of Cu ions added into this bath
Keywords:
2
+
play an important role in fabrication of CuO/ZnO composite films. When the Cu concentration in a bath
is lower, the obtained composite is a Cu-doped ZnO film. The electrodeposited CuO/ZnO composite films
show higher photocatalytic activity towards reduction of Cr(VI) compared to pure ZnO both under UV and
under UV–vis light illumination. Furthermore, the photocatalytic activity of CuO/ZnO composite films is
related to their atomic Cu/Zn ratios. The mechanism of CuO/ZnO composite films for improvement in
photocatalytic activity was also discussed.
CuO/ZnO composite films
Cathodic co-electrodeposition
Heterostructures
Photocatalytic reduction
Hexavalent chromium
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
[8–11]. It is worth noting that most of CuO/ZnO composite are used
for methanol synthesis, hydrogen production and gas sensor. More
recently, CuO/ZnO nano-composites were prepared via coordina-
tion oxidation homogeneous co-precipitation method and used for
photocatalytic degradation of methyl orange [12].
Photocatalytic treatment of organic or inorganic pollutants
from water and air using semiconductors as photocatalysts has
been a promising process among advanced oxidation technolo-
gies [1,2]. Semiconductor ZnO with broad-band-gap (3.2 eV) has
been extensively investigated as semiconductor photocatalysts
for the contaminant remediation [3–5]. However, ZnO can only
absorb a small portion of solar spectrum in the UV region, which
means that they does not allow the efficient utilization of visible
light. Compared with those broad-band-gap semiconductors, the
photoresponse of semiconductors with narrow-band-gap extends
much more into the visible wavelength range. Nevertheless, the
narrow-band-gap semiconductors exhibit low photocatalytic effi-
ciency due to fast recombination rate for electron–hole pairs
photogenerated on the semiconductors [6]. The reports in the liter-
ature indicated that coupling of a narrow-band-gap semiconductor
with another having broad-band-gap results in a more efficient
separation and consequently, a higher visible light induced pho-
tocatalytic activity compared with the single narrow-band-gap
semiconductor [7]. In the past several years, composite semicon-
ductors of ZnO coupled with narrow-band-gap semiconductor,
including Fe O , WO , CdS, Cu O and CuO, have been reported
Electrodeposition has been received more attention as a method
for the synthesis of oxide thin films due to its interesting charac-
teristics for large area, low cost, generally low temperature and
easily controlling of the film thickness [13]. A great number of
unary oxides such as TiO , ZnO, WO3 and Cu O have been pre-
2
2
pared with electrodeposition [6,13–17]. On the other hand, very
little information is available on the cathodic co-electrodeposition
of multinary oxides. Pauporte et al. reported co-precipitation of zinc
and europium oxides and co-precipitation of TiO2 and WO3 with
cathodic electrodeposition [18].
Hexavalent chromium (Cr(VI)) is a carcinogenic contaminant in
waste waters arising from industrial processes such as electroplat-
ing, leather tanning, or paint manufacture. The preferred treatment
is reduction of Cr(VI) to the less harmful Cr(III), which can be pre-
cipitated in neutral or alkaline solutions as Cr(OH) . Compared
3
with conventional reduction processes, photocatalytic reduction
of Cr(VI) over semiconducting materials has some obvious advan-
tages, such as no addition of other chemicals, operation at mild
conditions and low cost [19]. The utilization of photocatalysis in
reducing Cr(VI) has been reported in the literature since 1979 [20].
TiO , ZnO, WO , CdS and so on were used as photocatalysts for
2
3
3
2
∗ Corresponding author. Tel.: +86 24 24681936; fax: +86 24 86259041.
E-mail address: weisq1961@126.com (S. Wei).
2
3
reduction of Cr(VI) under UV or visible light irradiation [21–24]. To
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.08.011

S. Wei et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 112–116
113
the best of our knowledge, there is no report on the photocataytic
reduction of Cr(VI) over CuO/ZnO composite films.
In the present work, ZnO/CuO composite films have been suc-
cessfully prepared with cathodic electrodeposition on indium tin
oxide (ITO)-coated glass substrates from Zn(NO ) + Cu(CH COO)
3
2
3
2
aqueous solutions. The obtained composite films were character-
ized with X-ray diffraction (XRD), scanning electron microscopy
(
SEM) and energy dispersive X-ray spectroscopy (EDS). Hexavalent
chromium (Cr(VI)) was used as a model pollutant to evaluate the
photocatalytic activity of the ZnO/CuO composite films both under
UV and UV–vis light irradiation.
2
. Experimental
2.1. Materials
All chemicals were of analytic reagent grade and used as
received. Except Potassium dichromate (from Tianjin Damao
Chemical Reagent Factory, China), the other reagents were pur-
chased from Sinopharm Chemical Reagent Co. Ltd., China. All
solutions were prepared with distilled water. Cr(VI) stock solu-
tion (0.02 mol/L) was prepared by dissolving K Cr O7 into distilled
2
2
water.
Fig. 1. XRD patterns of the composite films obtained from electrodeposition baths
2
+
2.2. Preparation of films
containing 0.5 mol/L Zn(NO3)2 and different Cu concentrations: 0 mmol/L (a),
mmol/L (b), 15 mmol/L (c) and 30 mmol/L (d), respectively.
1
Before use, an indium tin oxide (ITO)-coated glass slide
2 cm × 5 cm) was rinsed successively in acetone, absolute ethanol
and distilled water. Following the pre-treatment, the substrate was
immediately transferred to a bath containing 0.5 mol/L Zn(NO ) , in
(
where C0 and Ct are the concentrations of Cr(VI) before and after
photocatalytic reactions. All the photocatalytic experiments were
carried out without stirring.
3
2
which the concentration of Cu(CH COO) was 0 mmol/L, 1 mmol/L,
3
2
1
4
5 mmol/L and 30 mmol/L, respectively, and pH was in the range
–6. Cathodic electrodeposition of CuO/ZnO composite films onto
3. Results and discussion
ITO substrates (each sample efficient area: 2 cm × 2.5 cm) was
3
.1. Characterization of films
performed from the above baths at constant current density of
−
2
3
mA cm for 15 min in an ordinary cell with a zinc plate used as
Fig. 1 shows XRD patterns of the films prepared with differ-
the counter electrode. The deposition bath temperature was fixed
2+
ent concentrations of Cu (0 mmol/L, 1 mmol/L, 15 mmol/L, and
3
◦
at 65 C.
0 mmol/L). The peaks indicated by “D” come from the ITO sub-
strates. The peaks marked with “B” correspond to ZnO (JCPDS,
36-1451), indicating that ZnO has been deposited on ITO-coated
glass substrates at each concentration of Cu2+. When Cu concen-
trations in electrodeposition baths are 15 mmol/L and 30 mmol/L,
respectively, the diffraction peaks (marked with “A”) matching with
CuO (JCPDS, 49-1830) were observed, which clearly indicates that
co-precipitation of CuO and ZnO takes place and CuO/ZnO com-
posite films have been successfully fabricated on ITO-coated glass
2
.3. Characterization of films
2+
The morphology and elemental composition of the result-
ing films were examined with a scanning electron microscope
equipped with an EDS facility (S3400, HITACHI, Japan). The
crystalline structure of films was determined with an X-ray diffrac-
tometer (D/Max-RB, RIGAKU, Japan).
substrates. The peaks marked with “C” come from Zn(OH) , which
2
may mean that the dehydration of Zn(OH)2 into ZnO is incomplete
in our experiments.
2.4. Photocatalytic reactions
2
In the previous work [25], It was found that the copper oxide
potentiostatically electrodeposited from an acetate bath is cuprous
The obtained films (efficient area: 5 cm ) were placed inside
5 mL of Cr(VI) aqueous solutions (containing 0.2 mmol/L Cr(VI)
1
oxide (Cu O) rather than cupric oxide (CuO). And the following for-
and 0.4 mmol/L citric acid (CA), pH = 4.6) filled in a cylinder quartz
tube. A 20 W high-pressure mercury lamp with a maximum emis-
sion at 365 nm and a 300 W metal halide lamp (ꢀ > 365 nm) were
used as UV and UV–vis light sources, and the illumination density
available at the film surface is about 2 mW/cm2 and 14 mW/cm ,
respectively. The concentrations of Cr(VI) during the photocatalytic
reactions were determined spectrophotometrically by measuring
the absorbance at 349 nm with a spectrophotometer (721E, Shang-
hai Spectra Instrument Co. Ltd., China). Photocatalytic activity of
the CuO/ZnO composite films was evaluated with Cr(VI) residual %
which can be expressed as follows:
2
mation mechanism for cuprous oxide from acetate bath has been
proposed.
2+
−
+
2
Cu + H O + 2e → Cu O + 2H
(2)
2
2
2
However, in the case of present baths containing Zn(NO3)2 and
Cu(CH COO) , because the nitrate system has a more positive redox
3
2
0
potential (E = 0.93 V vs. SHE, pH = 0) than that of Cu(II)/Cu(I) redox
couple (E = 0.16 V vs. SHE, pH = 0) [26] and furthermore, the con-
0
3−
2+
centration of NO is much higher than that of Cu , the reduction
of NO3 at the surface of electrode occurs predominantly according
−
to Eq. (3)
Ct
Cr(VI) residual % =
%
(1)
NO3 + H O + 2e → NO2− + 2OH−
−
−
(3)
C0
2

1
14
S. Wei et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 112–116
This reduction reaction leads to a pH increase in the vicinity of the
electrode and a local supersaturation due to the release of hydrox-
ide ions. In the present baths, apart from the reaction of these
hydroxide ions with zinc ions to form ZnO on the electrode surface
(
Eq. (4)) [27]
Zn2 + 2OH− → Zn(OH) → ZnO + H O
+
(4)
2
2
a precipitation reaction for CuO similar to that for ZnO may simul-
taneously take place on the same electrode surface (Eq. (5)). This
process is the so-called cathodic co-electrodeposition [18].
Cu2 + 2OH− → Cu(OH) → CuO + H O
+
(5)
2
2
Fig. 2 shows the EDS distribution of elements for the compos-
ite films prepared with different Cu2 concentrations in the baths
+
(
1 mmol/L, 15 mmol/L and 30 mmol/L). The atomic Cu/Zn ratios in
the composite films were calculated from the data to be 0.02, 0.73
and 1.10 corresponding to those Cu2 concentrations, respectively.
It is indicated that the atomic Cu/Zn ratio in a composite film can be
tuned over a large composition range just by varying the molar ion
ratio in the deposition bath. For the composite film (atomic Cu/Zn
ratio: 0.02) obtained from a bath containing 1 mmol/L Cu2 , no
diffraction peaks corresponding to CuO were observed (see Fig. 1),
which means that the co-deposition process for CuO and ZnO do
not occur. The reason may be attributed to the fact that the local
+
+
2+
supersaturation is not achieved for CuO due to lower Cu concen-
2+
2+
tration. Considering that Cu (0.072 nm) and Zn ions (0.074 nm)
have almost the same ionic radius [28], Cu2 ions may be incorpo-
rated into ZnO lattice in such a case. As a result, Cu-doped ZnO films
may be obtained.
+
It is worth noting that the atomic Cu/Zn ratio in a CuO/ZnO com-
posite film is much higher than that in a corresponding deposition
bath (0.03 and 0.06 corresponding to 15 mmol/L and 30 mmol/L
of Cu2+ concentrations, respectively). This phenomenon can be
attributed to the difference of solubility between Cu(OH)2 and
Zn(OH) . Solubility constants (pKs) for Cu(OH)2 and Zn(OH)2 at
2
room temperature are 18.59 and 16.92, respectively [29].
SEM micrographs of pure ZnO and composite films are shown in
Fig. 3. No significant morphological differences between pure ZnO
Fig. 2. EDS element distribution of the composite films obtained from electrode-
2
+
position baths containing 0.5 mol/L Zn(NO3)2 and different Cu concentrations:
mmol/L (a); 15 mmol/L (b) and 30 mmol/L (c), respectively.
1
(
corresponding to atomic Cu/Zn ratio being 0) and the composite
Fig. 3. SEM micrographs of the composite films with different atomic Cu/Zn ratios: 0 (a), 0.02 (b), 0.73 (c) and 1.10 (d), respectively.

S. Wei et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 112–116
115
For comparison, the results obtained from pure ZnO is included in
the same figures. It can be seen that the Cr(VI) residual (%) at given
time during the photocatalytic reduction period over the composite
films is lower that over pure ZnO both under UV and under UV–vis
light illumination. The results indicate that the composite films are
photocatalytically more active than the corresponding pure ZnO.
Moreover, the CuO/ZnO composite film exhibits a higher photo-
catalytic activity than that Cu-doped ZnO film. It should be noted
that the CuO/ZnO composite film (atomic Cu/Zn ratio: 1.10) is less
active than that (atomic Cu/Zn ratio: 0.73). The improvement in
photocatalytic activity of the composite films can be explained as
follows:
Under UV light illumination, Cu-doped ZnO films show a slightly
higher photocatalytic activity than that of pure ZnO films (see
Fig. 4). This result indicates that Cu(II) plays a minor role in UV
light-driven photocatalytic reduction of Cr(VI) over Cu-doped ZnO.
However, the photocatalytic activity of Cu-doped ZnO films is
enhanced obviously compared with pure ZnO films under UV–vis
light illumination. The photocatalytic activity enhancement of ZnO
when doped with small quantities of Cu(II) under UV–vis light illu-
mination may be resulted from the formation of Cu(I) induced by
visible light. As shown in Fig. 6, ZnO cannot be activated by vis-
ible light due to its broad-band-gap, but electrons in the valence
band (VB) of ZnO can be directly transferred to Cu(II) and conse-
quently, holes in the VB of ZnO and Cu(I) are formed [30]. The holes
produced in the VB of ZnO oxidize CA, and meanwhile the formed
Cu(I) reduces Cr(VI) to Cr(III) and itself will return to original Cu(II)
form.
Fig. 4. Photocatalytic reduction of Cr(VI) under UV light illumination over the com-
posite films with different atomic Cu/Zn ratios: 0 (a), 0.02 (b), 0.73 (c) and 1.10 (d),
respectively.
film with the atomic Cu/Zn ratio being 0.02 (i.e., the Cu-doped ZnO
film) can be noticed. Compared with pure ZnO, the morphologies of
CuO/ZnO composite films are changed significantly. Furthermore,
the atomic Cu/Zn ratios have great influence on the morpholo-
gies of the composite films. When Cu/Zn ratio is 0.73, flower-like
superstructures are obtained. As Cu/Zn ratio increases to 1.10, the
morphologies of the composite films evolve to assemblies of flakes
and grains.
As a result, the photocatalytic activity of Cu-doped ZnO films is
improved. On the other hand, as atomic Cu/Zn ratio is increased to
0
.73 or 1.10, the enhancement of activity for CuO/ZnO composite
films should be mainly explained in terms of efficient separation of
charges photogenerated in CuO/ZnO heterostructures in addition
to the above-mentioned electron transfer.
3
.2. Photocatalytic activity
Initially, direct photolysis of Cr(VI) in the absence of catalysts
CuO is a p-type semiconductor with narrow-band-gap (ca.
1.7 eV) [31], and it may have acted as a visible light-driven
photocatalyst; however, it was found that CuO shows very low
photocatalytic activity [32], which may be attributed to the inef-
ficient charge transport within CuO [6]. The enhanced activity for
CuO/ZnO both under UV and UV–vis light irradiation can be inter-
preted using a schematic diagram of the energy band structure of
CuO/ZnO heterojunction shown in Fig. 7.
and adsorption of Cr(VI) onto catalysts under dark conditions with-
out light illumination were performed, respectively. The negligible
change in Cr(VI) concentrations was observed for each experiment.
These results indicate that the reduction of Cr(VI) is caused by pho-
tocatalytic reaction on catalysts under light illumination.
The photocatalytic reduction of Cr(VI) was carried out under
UV and UV–vis light illumination over the composite films with
different atomic Cu/Zn ratios, respectively. The change in Cr(VI)
concentrations with illumination time is depicted in Figs. 4 and 5.
Fig. 5. Photocatalytic reduction of Cr(VI) under UV–vis light illumination over the
composite films with different atomic Cu/Zn ratios: 0 (a), 0.02 (b), 0.73 (c) and 1.10
Fig. 6. Energy diagram of ZnO and redox potential of Cu(II)/Cu(I) (energy levels are
referred to NHE).
(
d), respectively.

1
16
S. Wei et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 112–116
remarkablyfrom the former(see Fig. 3), whichmayindicate that the
interfaces between CuO and ZnO are also changed significantly. It
was found that the interfaces among the particles of a semiconduc-
tor composite play an important role in the photocatalytic reaction
[
34]. The details of the mechanism for the effect of atomic Cu/Zn
ratio on the photocatalytic activity of CuO/ZnO composite films are
under investigation.
4. Conclusions
CuO/ZnO composite films have been successfully fabricated on
ITO-coated glass substrates via a cathodic co-electrodeposition
route from baths containing Zn(NO ) and Cu(CH COO) . For a bath
3
2
3
2
containing Zn(NO ) with a certain concentration, the concentra-
3
2
tion of Cu2+ ions added into this bath play an important role in
cathodic co-electrodeposition of CuO/ZnO composite films. When
the Cu2+ concentration in a bath is lower, the obtained composite
is Cu-doped ZnO film. The atomic Cu/Zn ratio in the obtained com-
posite films can be tuned over a large composition range just by
varying the molar ion ratio in the deposition baths. And the atomic
Cu/Zn ratio in a composite film is much higher than that in a corre-
sponding deposition bath. CuO/ZnO composite films show higher
photocatalytic activity towards reduction of Cr(VI) compared to
pure ZnO both under UV and under UV–vis light illumination. The
photocatalytic activity of CuO/ZnO composite films is related to
their atomic Cu/Zn ratios. The enhanced activity of CuO/ZnO com-
posite films may be mainly attributed to the efficient separation of
charges photogenerated in CuO/ZnO heterostructures.
Fig. 7. Energy band diagram for ZnO/CuO heterostructure materials in contact,
showing corresponding valence and conduction band positions and photogenerated
hole and electron transfer.
Under UV light illumination, both CuO and ZnO can be excited
according to process (1) and (3), respectively. The conduction band
(
CB) edge of CuO (−4.96 eV vs. absolute vacuum scale (AVS)) is
higher than that of ZnO (−4.19 eV vs. AVS); the valence band (VB)
edges of CuO and ZnO are situated at −3.26 eV and −0.99 eV vs
AVS, respectively [33]. From thermodynamic view points, the pho-
togenerated electrons transfer from CB of CuO to that of ZnO, while
the photogenerated holes immigrate in the opposite direction from
VB of ZnO to that of CuO. Consequently, more electrons are accu-
mulated in CB of ZnO and consumed for reduction of Cr(VI). Thus
CuO/ZnO composite films show higher photocatalytic activity than
that of pure ZnO films. The photocatalytic reduction process for
Cr(VI) over CuO/ZnO composite films under UV light illumination
can be proposed as follows:
References
[
[
[
1] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1.
2] J. Yu, L. Qi, J. Hazard. Mater. 169 (2009) 221.
3] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V.
Murugesan, Sol. Energy Mater. Sol. Cells 77 (2003) 65.
[4] E. Evgenidou, K. Fytianos, I. Poulios, Appl. Catal. B 59 (2005) 81.
[
[
[
5] A. Shafaei, M. Nikazar, M. Arami, Desalination 252 (2010) 8.
6] C.C. Hu, J.N. Nian, H. Teng, Sol. Energy Mater. Sol. Cells 92 (2008) 1071.
7] J. Georgieva, S. Armyanov, E. Valova, I. Poulios, S. Sotiropoulos, Electrochem.
Commun. 9 (2007) 365.
[
8] S. Sakthivel, S.U. Geissen, D.W. Bahnemann, V. Murugesan, A. Vogelpohl, J.
Photochem. Photobiol. A 148 (2002) 283.
CuO/ZnO + hv → CuO(e− + h )/ZnO(e− + h+)
+
[9] C. Xu, L. Cao, G. Su, W. Liu, H. Liu, Y. Yu, X. Qu, J. Hazard. Mater. 176 (2010) 807.
10] I. Eswaramoorthi, V. Sundaramurthy, A.K. Dalai, Appl. Catal. A 313 (2006) 22.
[11] H.C. Yang, F.W. Chang, L.S. Roselin, J. Mol. Catal. A 276 (2007) 184.
12] Z.L. Liu, J.C. Deng, J.J. Deng, F.F. Li, Mater. Sci. Eng. B 150 (2008) 99.
[13] Th. Pauporté, D. Lincot, Sol. Energy Mater. Sol. Cells 45 (2000) 3345.
14] I. Zhitomirsky, L. Gal-Or, A. Kohn, H.W. Hennicke, J. Mater. Sci. 30 (1995) 5307.
15] I. Shiyanovskaya, M. Hepel, J. Electrochem. Soc. 146 (1999) 243.
16] J. Georgieva, S. Armyanov, E. Valova, Ts. Tsacheva, I. Poulios, S. Sotiropoulos, J.
Electroanal. Chem. 585 (2005) 35.
17] J. Georgieva, S. Armyanov, E. Valova, I. Poulios, S. Sotiropoulos, Electrochim.
Acta 51 (2006) 2076.
(6)
(7)
(8)
(9)
[
−
+
−
+
+
−
CuO(e + h )/ZnO(e + h ) → CuO(h )/ZnO(e )
Cr O72− + 14H+ + 6e− → 2Cr3+ + 7H O
[
2
2
[
[
[
+
h
+ CA → oxidation products
In the case of UV–vis light illumination, ZnO can only be excited
[
[
[
by a small portion of UV light from the UV–vis light source, so
it shows lower photocatalytic activity compared to UV light illu-
mination (see Figs. 4 and 5). Under visible light illumination, the
interband transition of electrons within CuO (process (1)) still
occurs, while ZnO cannot be activated due to its broad-band-gap,
i.e., process (3) does not take place. In such a case, the mechanism
for photogenerated charges transfer between the two semiconduc-
tor materials is different from that under UV light illumination,
which can be expressed as follows:
18] Th. Pauporté, A. Goux, A. Kahn-Harari, N. de Tacconi, C.R. Chenthamarakshan,
K. Rajeshwar, D. Lincot, J. Phys. Chem. Solids 64 (2003) 1737.
19] B. Xie, H. Zhang, P. Cai, R. Qiu, Y. Xiong, Chemosphere 63 (2006) 956.
[20] H. Yoneyama, Y. Yamashlta, H. Tamura, Nature 282 (1979) 817.
[
[
21] X. Xu, H. Li, J. Gu, Chemosphere 63 (2006) 254.
22] S. Chakrabarti, B. Chaudhuri, S. Bhattacharjee, A.K. Ray, B.K. Dutta, Chem. Eng.
J. 153 (2009) 86.
[23] A. Watcharenwong, W. Chanmanee, N.R. de Tacconi, C.R. Chenthamarakshan,
P. Kajitvichyanukul, K. Rajeshwar, J. Electroanal. Chem. 612 (2008) 112.
24] S. Wang, Z. Wang, Q. Zhuang, Appl. Catal. B l (1992) 257.
25] R.P. Wijesundera, M. Hidaka, K. Koga, M. Sakai, W. Siripala, Thin Solid Films 500
(2006) 241.
26] A.J. Bard, R. Parsons, J. Jordan (Eds.), Standard Potentials in Aqueous Solution,
Marcel Dekker, New York, 1985.
[
[
CuO/ZnO + hv → CuO(e− + h )/ZnO
+
(10)
(11)
[
[
[
−
+
+
−
CuO(e + h )/ZnO → CuO(h )/ZnO(e )
27] T. Yoshida, D. Komatsu, N. Shimokawa, H. Minoura, Thin Solid Films 451–452
(
2004) 166.
Based on the above-mentioned reason, it is deduced that
CuO/ZnO composite films also exhibit higher photocatalytic activ-
ity for reduction of Cr(VI) in comparison with pure ZnO films even
under visible light irradiation. The exact reason why CuO/ZnO
composite film (atomic Cu/Zn ratio: 1.10) is less active than that
28] C. Yu, X. Zhang, J. Xu, D. Yu, J. Chin. Electr. Microsc. Soc. 27 (2008) 2.
[29] J.A. Dean, Lange’s Handbook of Chemistry, 15th ed., McGraw-Hill, 1999.
30] H. Irie, S. Miura, K. Kamiya, K. Hashimoto, Chem. Phys. Lett. 457 (2008) 202.
31] M.A. Butler, D.S. Ginley, J. Electrochem. Soc. 125 (1978) 228.
32] T. Arai, M. Yanagida, Y. Konishi, Y. Iwasaki, H. Sugihara, K. Sayama, Catal. Com-
mun. 9 (2008) 1254.
[
[
[
[
[
33] Y. Xu, M.A.A. Schoonen, Am. Miner. 85 (2000) 543.
34] Z. Liu, Z. Zhao, M. Miyauchi, J. Phys. Chem. C113 (2009) 17132.
(
atomic Cu/Zn ratio: 0.73) is not clear, but there are several possi-
ble explanations. For example, the morphologies of the latter differ