Synthesis and characterization of CuO nanofibers, and investigation for its suitability as blocking layer in ZnO NPs based dye sensitized solar cell and as photocatalyst in organic dye degradation

Article abstract of DOI:10.1016/j.jssc.2011.12.013

Electrospun copper based composite nanofibers were synthesized using the copper acetate/polyvinyl alcohol/water solution as starting material. Synthesized composite nanofibers were sintered at 500°C to obtain CuO nanofibers. XRD, FTIR and XPS techniques were used to confirm the presence of pure CuO nanostructures. The effect of annealing cycle on the crystalline structure of the CuO nanofibers was analyzed and observed that the decrease in crystallite size with an increase in the dwelling time improved the orientation of the CuO crystallite. The blue-shift in the band-gap energies of CuO nanofibers was observed as a result of quantum confinement from bulk CuO (1.2 eV) to one dimensional (1D) nanostructures (～1.746 eV). The catalytic activity of the CuO fibers for the degradation of methyl orange was carried out and as a blocking layer in ZnO based DSSC was fabricated and observed a ～25% increase in the current density.

Full text of DOI:10.1016/j.jssc.2011.12.013

Journal of Solid State Chemistry 186 (2012) 261–267
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Synthesis and characterization of CuO nanofibers, and investigation for its
suitability as blocking layer in ZnO NPs based dye sensitized solar cell and as
photocatalyst in organic dye degradation
n
R. Sahay a, J. Sundaramurthy a, P. Suresh Kumar a, V. Thavasi b,n, S.G. Mhaisalkar a, S. Ramakrishna b,
a School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
b NUS Nanoscience and Nanotechnology Initiative, National University of Singapore, Singapore 117576, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Electrospun copper based composite nanofibers were synthesized using the copper acetate/polyvinyl
alcohol/water solution as starting material. Synthesized composite nanofibers were sintered at 500 1C
to obtain CuO nanofibers. XRD, FTIR and XPS techniques were used to confirm the presence of pure CuO
nanostructures. The effect of annealing cycle on the crystalline structure of the CuO nanofibers was
analyzed and observed that the decrease in crystallite size with an increase in the dwelling time
improved the orientation of the CuO crystallite. The blue-shift in the band-gap energies of CuO
Received 17 August 2011
Received in revised form
6 December 2011
Accepted 9 December 2011
Available online 23 December 2011
Keywords:
Copper oxide
Nanofibers
Electrospinning
DSSC
nanofibers was observed as
a result of quantum confinement from bulk CuO (1.2 eV) to one
dimensional (1D) nanostructures (ꢀ1.746 eV). The catalytic activity of the CuO fibers for the
degradation of methyl orange was carried out and as a blocking layer in ZnO based DSSC was
fabricated and observed a ꢀ25% increase in the current density.
& 2011 Elsevier Inc. All rights reserved.
Catalysis
Band-gap
1. Introduction
from microspheres to nanowires with the increase in the process
temperature of the hydrothermal method. Only few works have
Transition metal oxides nanostructures have been widely used
due to their unique features such as high specific surface area,
chemical stability and electrochemical activity at nanoscale with
promising applications in applied science and technology. Among
various transition metal oxides, copper based oxides such as
cuprous oxide (Cu2O) and cupric oxide (CuO) has lot of significant
properties, which need to be explored. Cu2O is widely employed
in energy applications due to its high charge mobilities [1];
whereas, research is on-going to employ CuO p-type semicon-
ductor with a narrow band-gap of Eg¼1.2 eV, in energy applica-
tions due to its high carrier concentration [2]. It has been widely
noted that the chemical and physical properties of copper oxides
can be changed either by changing the fabricating process or the
process parameters [3]. One of the most analyzed process para-
meter is the post annealing temperature, which can alter the
oxidation states of copper resulting in the variation of the
physical and chemical properties. Figueiredo et al. [4] observed
an increase in the optical band-gap with the increase in the post
annealing temperature of the as-deposited Cu films. On the other
hand, Xi et al. [5] observed a change in the morphology of CuO
been performed to monitor the changes in the physical and
chemical properties with the changes in the dwelling time of
the annealing process. Therefore, an attempt was made to analyze
the growth mechanism of the CuO crystallite with the dwelling
time such as 4, 6, 8 and 12 h, respectively.
Although many techniques such as chemical vapor deposition
(CVD) [6], hydrothermal [7], sol–gel [8] and electrochemical
deposition [9] have already been used for the fabrication of CuO
with various morphologies, research is on-going on a reliable and
economic process for the production of large-scale high-quality
CuO nanomaterials due to their applicability in various technolo-
gical areas such as as-catalysis [10], batteries [11], solar energy
conversion [12], gas sensing [13] and field emission applica-
tions [7]. Recently, it has been observed that the re-exploration
of 1D-nanostructure production technique known as electrospin-
ning, due to its simplicity and uniqueness of producing high
aspect ratio nanofibers. This technique involves preparation of a
sol–gel with the compatible inorganic precursor and polymer
dissolved in a particular solvent. The technical details about the
effect of processing as well as system parameters on electrospin-
ning technique can be seen elsewhere [14,15]. These nanofibers
were found to be of great significance because of their inherent
property such as large surface area and the engineered property
such as porosity.
n Corresponding authors. Fax: þ65 6872 5563.
E-mail addresses: velnanotech@gmail.com (V. Thavasi),
seeram@nus.edu.sg (S. Ramakrishna).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.12.013

262
R. Sahay et al. / Journal of Solid State Chemistry 186 (2012) 261–267
Majority of transition metal oxides are being employed for the
zinc acetate and sodium hydroxide (NaOH). The mixed aqueous
solution was stirred using a magnetic stirrer at room temperature
until a clear white solution was obtained. The clear white solution
thus obtained was refluxed at 80 1C for 4 h and then allowed to cool
down to room temperature. The as-prepared ZnO powder samples
were subjected to calcination in a muffle furnace at 250 1C for 1 h.
Finally, the calcinated white ZnO powder was obtained after gradual
cooling down to room temperature.
photo-catalytic activities such as TiO2, SnO2, ZnO, Fe2O3, CuO, etc.
Nevertheless, CuO has been scarcely employed as a photocatalyst to
degrade organic pollutants such as textile dyes, due to its inability to
produce good amount of OHd radicals. This phenomenon may be the
result of fast electron/hole recombination and the position of the
valence band edge of CuO with respect to the redox potential
necessary for the generation of OHd [16]. Yu et al. [3] observed that
a CuO thin film depicts no photo-catalytic activity under visible-light
irradiation, unless H2O2 is added as one of the sources of OHd. They
found that the presence of CuO complimented H2O2 for an improved
degradation of methyl orange dye. The other approach includes
fabrication of composite nanomaterials employing CuO as one of the
ingredients to improve the overall catalytic activity. These config-
urations include CuO/TiO2 [17], CuO/ZnO [18], CuO/X zeolite [19],
etc. In this report, photo-degradation of methyl orange was carried
out under UV irradiation employing CuO nanofibers as the catalyst,
thus completely eliminating H2O2 or composite counterpart, thus
paving the way for stable and affordable photocatalyst. It was noted
that 60% of the dye degraded within 1 h of exposure to the UV
radiation in the presence of CuO nanofibers. Kim et al. [20] observed
that the high conduction band edge of the blocking layer can retard
the back transfer of electrons from the photoanode to the electrolytes
or the dye. The usage of CuO nanofibers as the blocking layer in ZnO
based dye sensitized solar cells (DSSCs) is also analyzed as it has high
conduction band edge as compared to ZnO.
2.2. DSSC fabrication
Fluorine doped indium oxide (FTO) (2.0 cmꢃ 1.5 cm; 15
O
/cm2,
Asahi Glass Co. Ltd., Japan) substrates were first cleaned in a
detergent solution using an ultrasonic bath for 20 min and then
rinsed with water and ethanol successively and dried at 80 1C in an
oven. Typically, 0.25 g of prepared ZnO nanoparticles was ultra-
sonically dispersed in 0.6 mL of acetic acid (99%) for 20 min.
Further, 0.1 g of polyethylene glycol (MW¼100,000) was added as
the binder and allowed to agitate for 2 h. As-prepared ZnO paste
was coated on to the cleaned FTO substrates using doctor blade
technique to an average thickness of 20 mm. The dispersion of CuO
nanofibers in deionised water (5 mg/ml) is then spin-coated on top
of the ZnO layer to form a blocking layer having an average
thickness of 1 mm. The substrate was then heated at 500 1C for
1 h under the normal atmosphere to remove the binder. Two
different dyes were employed to visualize their effect on the
performance of the solar cell. These dyes include Eosin-Y and Ru
based N719 dye. The as-prepared ZnO photo-electrode with area of
0.28 cm2 was soaked in a given dye for 24 h at the ambient
temperature. The dye-anchored electrode was then washed in
ethanol to remove unbound dye and dried under vacuum for
2. Materials and methods
Copper acetate (99%), zinc acetate (99%), polyvinyl alcohol PVA
(MW¼80,000–124,000) and 100% pure acetic acid were purchased
from Aldrich and used as received without any further purifica-
tion. 0.5 g Of the copper acetate (Cu(CH3COO)2 ꢁ H2O) was dis-
solved in 4.5 ml of 10 wt% aqueous PVA solution. This solution was
then stirred for 24 h at room temperature for the formation of pale
blue colored copper hydroxide. Eq. (1) represents the chemical
reaction depicting the hydrolysis of the employed precursor to the
formation of CuðOHÞ2. Later, as-prepared solution was introduced
in 10 ml syringe with a hypodermic needle (dia. 27 G) in a
controlled electrospinning setup (ELECTROSPUNRA, Singapore).
15 min, and was sealed using a 30 mm thick spacer. These photo-
electrodes were then sandwiched with Pt coated on the conducting
substrate (FTO) as the counter electrode to complete the device
as shown in Fig. 1. Electrolyte containing 0.1 M lithium iodide,
0.03 M iodine, 0.5 M 4-tert-butylpyridine and 0.6 M 1-propyl-2,
3-dimethyl imidazolium iodide in acetonitrile solvent was filled in
the device.
2.3. Characterization
CuðCH3COOÞ UH2OþH2Oyꢂie!ldsCuðOHÞ þ2CH3COOH:
ð1Þ
A
PANanalytical (XPERT-PRO) X-ray diffractometer using
˚
2
2
CuKa1 radiation (l¼1.5406 A) was employed to characterize the
The flow rate and applied electric filed was varied to obtain the
optimal conditions for the electrospun fibers. The obtained optimal
condition was as follows: flow rate of 0.2 ml/h and the applied
electric field of 1.25 kV/cm. High electric field strength (1.4 kV/cm)
was employed to enable for the high stretch rates of the electrospun
jet. The distance between the needle tip and the collector against the
applied electric field was set as 18 cm. The longer distance between
the needle tip and the collector aided the stretching of the jet due to
the increase in the distance covered by the spiraling electrospun jet
before being deposited on the collector. The fiber mesh obtained was
then annealed to obtain CuO nanofibers. Annealing of the electrospun
fibers at 500 1C for 4, 6, 8 and 12 h caused the elimination of water
molecules from the copper hydroxide and resulted in the formation of
cupric oxide (CuO) as given in Eq. (2). The heating and cooling rates
employed were kept constant at 2 and 5 1C/min, respectively,
crystallographic properties of the post-annealed CuO nanofibers
samples. The surface morphologies of the samples were charac-
terized by a scanning electron microscopy (SEM, JEOL JSM).
Photocurrent measurements were carried out using a solar
simulator (XES-151S, San Ei, Japan) under AM 1.5 G condition.
UV–vis spectral analysis was recorded on Shimadzu UV-3150. XPS
measurements were made on a Kratos Analytical AXIS HSi
spectrometer with
a monochromatized AlKa X-ray source
D
CuðOHÞ2-H2OþCuO:
ð2Þ
2.1. Synthesis of ZnO nanoparticles for photoanode
DSSC was constructed using ZnO as the photo-electrode. ZnO
nanoparticles were prepared by sol–gel technique by using equimolar
Fig. 1. Schematic diagram of ZnO based DSSC with CuO as the blocking layer.

R. Sahay et al. / Journal of Solid State Chemistry 186 (2012) 261–267
263
(1486.71-eV photons) at a constant dwell time of 100 ms and pass
energy of 40 eV. The core-level signals were obtained at a
photoelectron takeoff angle of 901 (with respect to the sample
surface). The X-ray source was run at a reduced power of 150 W.
The pressure in the analysis chamber was maintained at
7.5 ꢃ 10ꢂ9 Torr or lower during each measurement. FT-IR analysis
was performed by AVATAR 360, Fourier Transforms Infrared
spectroscope.
peaks with the increase in the dwelling time resulted in the
reduction of the crystallite size of CuO nanofibers as observed in
Fig. 3(b). The average crystallite sizes of these post-annealed
samples were calculated from the peak widths using the follow-
ing Scherrer equation
0:94l
D ¼
ð3Þ
b
cos
y
whereas the relationship for strain is
b
cot
4
y
e
¼
ð4Þ
3. Results and discussion
where, D is the crystallite size,
Cu Ka),
intensity peak after subtraction of the equipment broadening and
is the diffraction angle. The lattice strain of the (111) orientated
l
is the wavelength (1.546 A1 for
3.1. Surface and structural analysis
b
is the full-width at half-maximum (FWHM) of main
SEM image of the composite CuO/ PVA nanofibers and
annealed CuO nanofibers for 12 h is shown in Fig. 2(a) and (b).
It can be seen from the SEM images that a random mesh of
nanofibers was obtained. This randomness of the fiber mesh
obtained was the direct consequence of the bending instability
associated with the electrospun jet [21]. The distribution of
annealed CuO fibers soaked at 12 h was found to be around
141754 nm whereas 269776 nm represent the distribution of
composite CuO/PVA fibers. This reduction of CuO fiber diameter
upon annealing was due to the removal of residual polymer as
carbon dioxide and water vapor. The composite fiber mesh
obtained was then annealed at 500 1C in the air for 4, 6, 8 and
12 h. The heating and cooling rates employed were kept constant
at 2 and 5 1C/min, respectively. Finally, annealing of the compo-
site fibers resulted in the formation of poly-crystalline CuO
structure with predominate (111) phase orientation. During the
analysis, no appreciable change in the morphology of the CuO
nanofibers was observed with the dwelling time from 4 to 12 h.
XRD analysis performed at different dwelling times with the
other parameters kept constant is shown in the Fig. 3(a). All the
y
CuO nanofibers prepared at the different dwelling time are shown
in Fig. 3(b) was calculated using the Eq. (4). As expected, the
lattice strain increased with the decrease in the crystallite size
leading to the improved orientation of CuO crystallites. XRD
pattern of the synthesized ZnO powder samples showed the
well-crystalline wurtzite ZnO phases. All the diffraction peaks
orientated along (100), (002), (101), (102), (110), (103), (200),
(112), (201), (004), (202) correspond to the pure ZnO phase
(JCPDS Card No. 36–1451) as observed from Fig. 4(a) and corre-
sponding SEM image typically shows the formation of spherical
ZnO nanoparticles with the diameter ranging from 100 nm to
150 nm confirmed from the inset in Fig. 4(b).
3.2. Chemical composition analysis
XPS is a powerful technique for analyzing the chemical
composition of elements on the surface of compounds and
studying the transition metal compounds having the localized
valence d orbitals. The detailed XPS analysis confirmed the
formation of CuO nanofibers, which are shown in Fig. 5(a)–(c).
The peaks at 932.8 eV and 953.1 eV confirmed the presence of Cu
2p3/2 and Cu 2p1/2 peaks, respectively (Fig. 5(a)). Along with the
main peak at 932.8 eV, the shake-up satellite peaks were
observed at the higher binding energy side, 934.3 eV, 940.6 eV
diffraction peaks at 2y¼32.541, 35.541, 38.701, 48.771, 53.481,
58.291, 61.651, 66.281 and 68.141 were purely assigned to the
(110), (002), (111), (ꢂ202), (020), (202), (ꢂ113), (ꢂ311), (220)
crystal planes, respectively. It can be seen that all the diffraction
peaks belong to the CuO monoclinic phase (JCPDS 48-1548) with
lattice constant of a¼0.4598 nm, b¼0.346 nm and c¼0.523 nm.
Compared to the standard diffraction patterns, there were no
characteristic peaks from impurities, such as Cu(OH)2 or Cu2O,
indicating that Cu(OH)2 was completely transformed into CuO
after annealing. It was also noted that the reflections of (111)
orientation was the strongest, suggesting as the preferential
crystal planes of the CuO nanofibers. Broadening of the diffraction
and 943.1 eV (Fig. 5(b)). The main peak was characteristic of Cuþ
,
whereas the shake-up satellite peaks were characteristics of
partially filled d-block (3d9) of Cu2þ. The peak positions and
relative intensities of the satellites against the core levels were
indicative of the formation of CuO particles. The detailed XPS
analysis of O 1s is shown in Fig. 5(c). The main peak at 529.5 eV
Fig. 2. CuO fibers fabricated by electrospinning. (a) Composite fibers: The chemical reagents (copper acetate and water) were dissolved in 10 wt% aqueous poly
(vinyl) alcohol solution. (b) Annealed fibers: The composite fibers were annealed at 500 1C for 12 h to remove residual polymer as water vapor and CO2 leading to yield pure
CuO fibers.

264
R. Sahay et al. / Journal of Solid State Chemistry 186 (2012) 261–267
Fig. 3. (a) XRD patterns of CuO nanofibers samples prepared against the dwelling time (i) 4, (ii) 6, (iii) 8, (iv) 12 h, the other parameters were kept constant during the
annealing cycle. (b) Crystallite size and lattice strain as a function of the dwelling time.
corresponds to O 1s of CuO. This particular O 1s peak accounting
for 60% of the total spectral area confirming Cu–O bonds, whereas
the left-over portion of the total spectral area can be associated
with (OH)2 groups (530.5–531.6 eV) and oxygen defective sites
(531.6 eV) [22]. Ratio of the sum of the peak areas of Cu2þ and
Cuþ were performed to measure their relative amount on the
surface, and it suggested that some reduction of Cu2þ to Cuþ state
[23]. This reduction of Cu2þ to Cuþ can be due to a group of
reasons such as, (a) exposure to X-ray during the acquisition of
XPS data [23], (b) the presence of carbon obtained during the
treatment of these substrates in the air, (c) it was argued that the
acetic acid formed during the hydrolysis of copper acetate (Eq. (1))
can act as a reducing agent. This reducing agent may be the reason
for the phase transformation of CuO to Cu2O as shown in [3]
depicted by Eq. (5).
results in the increase of the amount of atoms available on the
surface for the reduction to Cuþ. It was also observed that the
binding energy of the Cu 2p3/2 line and the characteristic area
ratio decreased with the increase in the particle size. This
phenomenon may be due to an increase in the bond ionicity
associated with the reduction in the co-ordination number
induced by the particle size reduction down to the nanometer
scale and the resulting narrowing of the electron energy levels.
FTIR analysis was performed to confirm the presence of char-
acteristic vibrational peaks pertaining to CuO nanofibers as
shown in Fig. 6. The wavenumbers positioned at 531 cmꢂ1 (4 h)
and 533 cmꢂ1 (6, 8 and 12 h) correspond to Cu–O stretching
vibrations. The wavenumbers at 487 cmꢂ1 (4 h), 484 cmꢂ1 (6 h),
481 cmꢂ1 (8 h) and 483 cmꢂ1 (12 h) represented the other
characteristic peaks re-confirming the poly-crystalline nature of
CuO nanofibers [24]. Weak bands at 2926 cmꢂ1 (4 h), 2926 cmꢂ1
(6 h), 2917 cmꢂ1 (8 h) and 2919 cmꢂ1 (12 h) were assigned to
alkyl (sp3) (C–H) stretching. The corresponding CH2 bend occurred
at 1405ꢂcm1 ꢂ1 (4 h), 1404 cmꢂ1 (6 h), 1399 cmꢂ1 (8 h) and
8CuOþCH3COOHyꢂie!lds4Cu2Oþ2CO2 þ2H2O
ð5Þ
As the CuO crystallite size decreased with an increase in the
dwelling time, the relative amount of Cuþ/Cu2þ increased. This
increase in the relative amount of Cuþ with respect to Cu2þ can
be attributed to the increased reduction by X-ray exposure due to
the decrease in the particle size. The decrease in the particles size
results in the increase of the surface to the volume ratio, which
1400 cm
(12 h). Weak peaks were also observed at
1632 cmꢂ1 (4 h), 1633 cmꢂ1 (6 h), 1629 cmꢂ1 (8 h) and
1629 cmꢂ1 (12 h) correspond to frequency against stretching of
CQC bond. These week peaks may correspond to impurities
present on the surface of the nanofibers during measurement.

R. Sahay et al. / Journal of Solid State Chemistry 186 (2012) 261–267
265
10
20
30
40
50
60
70
80
2θ (degree)
b
Fig. 5. XPS analysis to confirm the formation of pure CuO nanofibers. (a) XPS
spectra of Cu 2p core levels of nanofibers. (b) Peak fit of XPS Cu 2p3/2 core level.
(c) Peak fit of XPS O 1s.
Fig. 4. (a) XRD confirming the formation of pure ZnO nanoparticles depicting its
characteristic peaks. (b) FESEM of poly-crystalline ZnO nanoparticles.
3.3. Effect of crystallite size on the band-gap
UV–vis analysis was performed to investigate the optical
properties of the CuO nanofibers as a function of the dwelling
time. The thin film samples for UV–vis analysis were prepared by
using spin-coating techniques at 1000 rpm for 30 s on the glass
substrate. These thin films were prepared from the CuO nanofi-
bers dispersed in deionised water (5 mg/ml). The band-gap
energies of the CuO nanofibers were found to be of the order of
1.7 eV. The band-gap energy of bulk CuO is estimated to be in the
range of 1.2–1.5 eV [25]. The blue-shift observed in the band-gap
can be attributed to the result of quantum confinement usually
observed in 1D nanostructure [26]. It was noted that the band-
gap of the nanofibrous sample increased with an increase in the
dwelling time as seen in Fig. 7. The band-gap energy values as a
function of dwelling time are 1.746 (4 h), 1.769 (6 h), 1.771 (8 h)
and 1.771 (12 h). The obtained result complies with the effective
mass theory (EMA), which predicts rꢂ2 dependence of the band-
gap, with rꢂ1 correction term in the strong confinement regime,
where r is the crystallite size. As mentioned earlier, the crystallite
size decreases with the increase in the dwelling time resulting in
the increase in the band-gap energy according to the effective
mass theory (EMA).
Fig. 6. FTIR analysis of CuO nanofibers as a function of the dwelling time, (a) 4,
(b) 6, (c) 8 and (d) 12 h.
4. Applications of CuO nanofibers
4.1. Dye degradation with CuO as photocatalyst
Cu2þ in CuO has a partially empty d and s block, which
increases its affinity for the available free electrons and thus
making it a good p type material. Whereas, Cuþ has only empty s
block achieves better charge transfer density in comparison to
Cu2þ. This is due to the strong affinity for electron by the empty d
block in case of Cu2þ. The research has been on-going globally to

266
R. Sahay et al. / Journal of Solid State Chemistry 186 (2012) 261–267
Fig. 7. (a) Band-gap energy as a function of dwelling time. An increase in the band-gap energy was observed with the increase in the dwelling time, (b) close-up of (a) to
visualize the small variation of the band-gap energy for 6, 8 and 12 h. The dwelling time are represented by (i) 4, (ii) 6, (iii) 8 and (iv) 12 h in (a, b).
employ Cu2þ in applications such as catalysis due to its high
carrier concentration [1]. In our present study, highly crystalline
CuO nanofibers obtained at dwelling time of 12 h were used for
photocatalysis. Methyl orange (4-dimethylaminoazobenzene-40-
sulfonic acid sodium salt—C14H14N3NaO3S) was chosen for the
analysis as it is being widely employed in textile industry, making
it as a classic case-study material. 10ꢂ5 M of dye solution was
prepared in deionized water. UV–vis analysis was performed on
dye solution to obtain the maxima pertaining to the dye, which
was found to be 464 nm. Dye to the catalyst concentration ratio
was kept as 1:10 for the analysis. The methyl orange solution
with CuO nanofibers was stirred for 12 h and then degraded by
placing it under a 4 W UV lamp. In aqueous dye solutions the
holes (hþ) were scavenged by surface hydroxyl groups to gen-
erate the oxidizing hydroxyl radical (OHd) and electrons (eꢂ
)
were scavenged by oxygen groups to generate the Oꢂ2 radical to
promote the oxidation of dye. UV light has energy ranges between
1.24 and 3.1 eV, which is more than the energy required for the
primary reaction as methyl orange energy band-gap is 2.6 eV. CuO
nanofibers as photocatalyst accelerated the degradation of dye on
virtue of its small band-gap (1.77 eV), which enhanced the
creation of eꢂ/hþ pair. We observed 60% of the dye decomposi-
tion within 1 h of UV illumination as shown in Fig. 8. A similar
study was performed by employing CuO thin films as the catalyst
under UV irradiation for the degradation of methyl blue [16].
Nevertheless, low-level degradation of the methyl blue was
observed due to the scarcity of the OHd radicals. It is argued that
the large surface area and oriented crystallites of CuO nanofibers
resulted in the improved degradation of the dye in the present
analysis.
Fig. 8. Change of absorbance values of methyl orange in the presence of CuO
nanofibers under UV illumination. UV illumination was provided from a 4 W UV
lamp under ambient conditions, i.e. 295 K and relative humidity (RH), 60% air.
size. We observed the increase in photocurrent with the incor-
poration of CuO nanofibers as a blocking layer. The retardation of
the interfacial recombination by the CuO blocking layer was the
reason for the reduction in the reverse current thus improving the
current density. This phenomenon was possible as the conduction
band edge of CuO is situated above ZnO creating a potential
gradient, which allowed the forward movement of electron
leading to the low reverse current. The inverse of the slope at
the Voc to the (J–V) characteristic curve is proportional to the
series resistance of the cell. This series resistance was a function
of metal contact resistance and the resistance to the flow of
electron through the dye to the FTO attached to ZnO. It was noted
that the inverse of the slope at the Voc decreases with the
incorporation of CuO blocking layer, particularly for N719 dye
4.2. Device performance
The preliminary analysis of DSSC employing CuO as a blocking
layer was presented. The photocurrent densities versus photo-
voltage (J–V) characteristic are shown in Fig. 9. The resultant
photovoltaic parameters such as open-circuit voltage (Voc), short-
circuit current density (Jsc), fill factor (FF) and the conversion
efficiency are summarized in Table 1. CuO nanofibers obtained at
12 h dwelling time were employed as a blocking layer because of
its improved orientation that resulted from its small crystallite

R. Sahay et al. / Journal of Solid State Chemistry 186 (2012) 261–267
267
5. Conclusions
The effect of the dwelling time of the annealing cycle for the
formation of the crystallite CuO nanofibers was analyzed. It was
observed that the crystallite size of CuO nanofibers decreased
with an increase in the dwelling time. This decrease in the
crystallite size resulted in the improved orientation of the CuO
nanofibers. The highly ordered morphology obtained at 12 h was
employed for photo-catalytic study. It was noted that 60% of the
organic dye was decomposed within 1 h of UV illumination.
Preliminary results based on the study on the suitability of CuO
nanofibers as the blocking layer in ZnO based DSSC was also
presented. A 25% increase in the current density was observed
with the application of CuO as blocking layer. We have hereby
fabricated highly crystalline CuO nanofibers with possible energy
applications.
Acknowledgments
This work was supported by Singapore NRF-CRP grant on
‘‘Nanonets for Harnessing Solar Energy and Storage’’ and also
NUS and NTU for providing facilities to carry out the research.
Dr. V. Thavasi acknowledges NUSNNI for support.
Fig. 9. Photocurrent density–voltage characteristics of DSSCs.
Reference
[1] J.R. Hayes, G.W. Nyce, J.D. Kuntz, J.H. Satcher, A.V. Hamza, Nanotechnology 18
(2007) 275602.
[2] A.H. Jayatissa, K. Guo, A.C. Jayasuriya, Appl. Surf. Sci. 255 (2009) 9474–9479.
[3] H.G. Yu, J.G. Yu, S.W. Liu, S. Mann, Chem. Mater. 19 (2007) 4327–4334.
[4] V. Figueiredo, E. Elangovan, G. Gonc-alves, N. Franco, E. Alves, S.H.K. Park,
R. Martins, E. Fortunato, Phys. Status Solidi A 206 (2009) 2143–2148.
[5] Y. Xi, C. Hu, P. Gao, R. Yang, X. He, X. Wang, B. Wan, Mater. Sci. Eng. B 166
(2009) 113–117.
Table 1
Device characteristics of ZnO based DSSCs in the presence of CuO.
Configuration
Voc
Jsc
FF
Efficiency
(%)
(mV)
(mA/cmꢂ2
)
[6] T. Maruyama, Sol. Energy Mater. Sol. Cells 56 (1998) 85–92.
[7] M.A. Dar, Y.S. Kim, W.B. Kim, J.M. Sohn, H.S. Shin, Appl. Surf. Sci. 254 (2008)
7477–7481.
[8] S.C. Ray, Sol. Energy Mater. Sol. Cells 68 (2001) 307–312.
[9] L. Chen, S. Shet, H.W. Tang, H.L. Wang, T. Deutsch, Y.F. Yan, J. Turner, M. Al-
Jassim, J. Mater. Chem. 20 (2010) 6962–6967.
[10] P. Bera, A.L. Camara, A. Hornes, A. Martinez-Arias, J. Phys. Chem. C 113 (2009)
10689–10695.
[11] M.A. Dar, S.H. Nam, Y.S. Kim, W.B. Kim, J. Solid State Electrochem. 14 (2010)
1719–1726.
FTO/ZnO/N719/
electrolyte/Pt
0.399
0.439
0.283
0.332
0.643
0.787
0.089
0.092
0.381
0.439
0.512
0.609
0.098
0.152
0.013
0.019
FTO/ZnO/CuO/N719/
electrolyte/Pt
FTO/ZnO/Eosin-Y/
electrolyte/Pt
FTO/ZnO/CuO/Eosin-
Y/electrolyte/Pt
[12] K.P. Musselman, A. Wisnet, D.C. Iza, H.C. Hesse, C. Scheu, J.L. MacManus-
Driscoll, L. Schmidt-Mende, Adv. Mater. 22 (2010) E254–258.
[13] J.J. Chen, K. Wang, L. Hartman, W.L. Zhou, J. Phys. Chem. C 112 (2008)
16017–16021.
[14] D.H. Reneker, A.L. Yarin, Polymer 49 (2008) 2387–2425.
[15] C.J. Thompson, G.G. Chase, A.L. Yarin, D.H. Reneker, Polymer 48 (2007)
6913–6922.
[16] M. Miyauchi, A. Nakajima, T. Watanabe, K. Hashimoto, Chem. Mater. 14
(2002) 2812–2816.
[17] G.H. Li, N.M. Dimitrijevic, L. Chen, T. Rajh, K.A. Gray, J. Phys. Chem. C 112
(2008) 19040–19044.
[18] B. Rohe, R. Weiss, S. Vukojevic, C. Baltes, M. Muhler, M. Tausch, M. Epple, Eur.
J. Inorg. Chem. (2007) 1723–1727.
[19] A. Nezamzadeh-Ejhieh, S. Hushmandrad, Appl. Catal. A 388 (2010) 149–159.
[20] J.Y. Kim, S. Lee, J.H. Noh, H.S. Jung, K.S. Hong, J. Electroceram. 23 (2009) 422–425.
[21] D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhongse, J. Appl. Phys. 87 (2000)
4531–4547.
implying the reduction of series resistance. Similarly, Rsh repre-
sents the obstruction to an alternate path for the flow of current,
which can be deduced by inverse of the slope at the Isc. High value
of Rsh implied that the series resistance was the major reason for
lower current density. As expected, the highest current density
was obtained for the cells having lower series resistance. Lower
series resistance was realized by incorporating CuO as a blocking
layer which restricted the recombination electron–hole pairs
leading to relatively high current density. The difference in the
current density for N719 and Eosin-Y can be explained on the
basis of carboxylate anchoring group. The carboxylate anchoring
group was the cause for the grafting of the sensitizer on to the
ZnO surface [27]. Two carboxylate anchoring groups in N719
compared to one for Eosin-Y resulted in improved grafting of
sensitizer and thus resulted in the improved current density for
N719 dye. Work is still in the process to further improve the
efficiency of solar cells and analyze the scattering nature of the
poly-crystalline CuO nanofiber blocking layer.
[22] S.J. Stewart, M. Multigner, J.F. Marco, F.J. Berry, A. Hernando, J.M. Gonzalez,
Solid State Commun. 130 (2004) 247–251.
[23] Y. Iijima, N. Niimura, K. Hiraoka, Surf. Interface Anal. 24 (1996) 193–197.
[24] G. Kliche, Z.V. Popovic, Phys. Rev. B 42 (1990) 10060–10066.
[25] T. Ito, H. Yamaguchi, K. Okabe, T. Masumi, J. Mater. Sci. 33 (1998) 3555–3566.
[26] J. Tang, L. Brzozowski, D.A.R. Barkhouse, X.H. Wang, R. Debnath, R. Wolowiec,
E. Palmiano, L. Levina, A.G. Pattantyus-Abraham, D. Jamakosmanovic,
E.H. Sargent, ACS Nano 4 (2010) 869–878.
[27] M. Gratzel, MRS Bulletin 30 (2005) 23–27.