H. DONG et al.: PREPARATION AND PHOTO-ELECTROCHEMICAL PERFORMANCE OF TiO2 NANOTUBE ...
397–404
PREPARATION AND PHOTO-ELECTROCHEMICAL
PERFORMANCE OF TiO
2
NANOTUBE ARRAYS FOR WATER
SPLITTING MODIFIED WITH CuO VIA A SIMPLE METHOD
ENOSTAVNA METODA PRIPRAVE MNO@ICE S CuO
MODIFICIRANIH TIO
2
NANOCEVK ZA POSTOPEK
RAZCEPLJANJA VODE TER NJIHOVE FOTOELEKTROKEMI^NE
LASTNOSTI
Hongxing Dong
1
, Qiuping Liu
2
, Yuehui He
3
1
College of Electromechanical Engineering, Hangzhou Polytechnic, Hangzhou, Zhejiang, China
2
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang, China
3
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
liuqiuping@zjut.edu.cn
Prejem rokopisa – received: 2017-08-22; sprejem za objavo – accepted for publication: 2018-01-09
doi:10.17222/mit.2017.138
A CuO/TiO2/Ti heterojunction photo-electrode was fabricated based on TiO2 nanotube arrays, prepared with anodic oxidation. A
facile and economical wet impregnation followed by annealing was employed to fabricate CuO-modified TiO2 nanotube arrays.
The structural and optical properties were characterized with SEM, EDX, XRD and UV-Vis spectrometry. The photo-electro-
chemical properties were measured with CHI 660E under a 150WXe lamp and in the dark. Moderate amounts of the
CuO-modified TiO2 nanotube arrays (TiO2-CuO (I)) exhibited photocurrents of 4.633 × 10
–5
A/cm
2
and 2.29 × 10
–5
A/cm
2
at
–0.35 V (vs. SCE) in a 0.2 M Na2SO4 electrolyte under the light and in the dark, respectively. The current density of TiO2-CuO
(I) was higher than that of the pure-TiO2 electrode in the light, but the current density of TiO2-CuO (I) was lower than that of
pure TiO2 in the dark. Therefore, the CuO on the TiO2 surface increased the electrical resistance. Significantly, the CuO films of
TiO 2-CuO (II) were too thick to decrease the photo-electrochemical activity, which was confirmed with electrochemical
impedance spectroscopy.
Keywords: TiO
2 nanotube arrays, CuO, photo-electrochemistry, water splitting
Avtorji prispevka so ve~sti~no CuO/TiO2/Ti fotoelektrodo izdelali na mno`ici TiO2 nanocevk, ki so bile izdelane z anodno
oksidacijo. Sledila je enostavna in ekonomi~na izdelava mno`ice s CuO modificiranih TiO2 nanocevk z mokro impregnacijo,
kateri je sledil postopek `arjenja. Avtorji so strukturne in opti~ne lastnosti dolo~ili s SEM, EDX, XRD in UV-Vis
spektrometrijo. Fotoelektrokemi~ne lastnosti so dolo~ili s CHI 660E pod 150WXe `arnico in v temi. Zmerna koli~ina mno`ice s
CuO modificiranih TiO2 nanocevk (TiO2-CuO (I)) je imela gostoto fotoelektri~nega toka 4,633 × 10
–5
A/cm
2
in
2,29 × 10
–5
A/cm
2
pri –0.35 V (vs. SCE) v 0,2 M elektrolitu Na2SO4 pod lu~jo oz. v temi. Gostota toka TiO2-CuO (I) je vi{ja kot
jo ima ~ista TiO2 elektroda pod lu~jo, toda v temi je gostota TiO2-CuO (I) ni`ja kot jo ima ~ista TiO2 elektroda v temi. Zato CuO
na povr{ini TiO2 pove~uje elektri~no upornost. Pomembna je ugotovitev, da so bili CuO filmi TiO2-CuO (II) predebeli, da bi se
zmanj{ala fotoelektrokemi~na aktivnost, ki so jo avtorji potrdili z elektrokemi~no impedan~no spektroskopijo.
Klju~ne besede: mno`ica TiO2 nanocevk, CuO, fotoelektrokemija, razcepljanje vode (lo~itev na vodik in kisik)
1 INTRODUCTION
Renewable energy is vital to the development of the
world. The application of solar energy for the production
of clean oxygen and hydrogen fuels, using photo-electro-
chemical splitting of water, is a promising way.
1
The
usage of metal-oxide semiconductors in the conversion
of solar energy to electrical energy has recently attracted
more attention due to their chemical stability and envi-
ronmentally friendly and cost-efficient production
methods.
2,3
Titanium dioxide (TiO
2
) is considered to be a favor-
able photocatalyst due to its relatively cheap, chemically
stable high photocatalytic performance and nontoxi-
city.
4,5
ATiO
2
nanotube exhibits a unique combination of
morphological and physicochemical properties, in-
cluding a high aspect ratio, mesoporous structure and
efficient electron conductivity.
3
It was reported that TiO
2
nanotubes have a great potential for the application in the
areas including gas sensing, environmental purification
and photocatalytic hydrogen production.
3,6,7
The hydro-
gen-generation efficiency over bare TiO
2
is relatively low
because of the fast recombination of electro-hole pairs.
8,9
To overcome the shortcoming, many modification me-
thods have been reported, involving semiconductor com-
position, metal-ion doping and noble-metal loading.
10–12
Recently, copper oxide (Cu
2
O, Cu
2+
, CuO, etc.) based
TiO
2
composites have been reported to be efficient
photocatalysts for water splitting and many other
photo-oxidation reactions.
13,14
Copper compounds are
good for the charge separation and provide reduction
sites for hydrogen formation. Moreover, a surface modi-
fication with CuO can widen the absorption of the wave
range from 200 nm to 500 nm.
15
The wet-impregnation
Materiali in tehnologije / Materials and technology 52 (2018) 4, 397–404 397
UDK 544.6:544.52:620.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(4)397(2018)

(WI) method proved to be an effective way to incorpo-
rate semiconductor compositions.
16,17
However, few
results showed detailed influence effects of the wet-im-
pregnation methods on the photocatalytic properties. In
addition, there are few works focusing on TiO
2
nanotube
arrays modified with wet impregnation.
In this paper, TiO
2
nanotube arrays were produced
using the anodic oxidation method, and then CuO was
incorporated with WI followed by sintering. The photo-
electrochemical performance of the resulting CuO-TiO
2
was obtained in the same impregnation liquid with a
different treatment. In order to elucidate the influence
effects, electrochemical impedance spectroscopy (EIS)
was carried out. The influence factors that contribute to
the high photo-electrochemical performance were dis-
cussed. And the factors influencing the charge transfer in
the TiO
2
nanotube arrays modified by CuO were also
evaluated. The result will help us to design a new kind of
efficient photo-electrode material for water splitting
through wet impregnation.
2 EXPERIMENTAL PART
2.1 Synthesis of TiO
2
nanotube arrays
Commercially pure titanium plates (0.5 mm thick,
purity > 99.5 %) were first degreased in acetone, then
mechanically polished, and finally chemically polished
at 25 °C in a solution consisting of H
2
O:HNO
3
:HF =
6:3:1 (V %) for 30 s. The pretreated titanium plates were
anodic oxidized in 1 % mass fraction hydrofluoric-acid
solution at 20 V for 30 min to produce TiO
2
nanotube
arrays. Then the TiO
2
nanotube arrays were washed with
de-ionized water three times and dried for the next step.
The prepared TiO
2
nanotube arrays were finally sintered
inairat500°Cfor2htoprepare sintered TiO
2
nanotube
arrays.
2.2 Preparation of TiO
2
-CuO photo-electrodes
The CuO-modified TiO
2
nanotube arrays were ob-
tained with WI, followed by sintering. Cu (NO
3
)
2
·3H
2
O
was used as the Cu
2+
precursor. 30g Cu (NO
3
)
2
·3H
2
Owas
dispersed into 100 mL of de-ionized water. The
as-prepared TiO
2
nanotube arrays were immersed into
the Cu (NO
3
)
2
solution treated with ultrasonic waves for
30 min and then dried in a furnace at 80 °C for 30 min.
The obtained sample was denoted as TiO
2
-Cu (NO
3
)
2
.
TiO
2
-CuO (I) was obtained by carefully drip washing
TiO
2
-Cu (NO
3
)
2
with de-ionized water to remove the
redundant Cu (NO
3
)
2
followed by sintering at 500 °C for
2h. TiO
2
-CuO (II) was obtained by sintering TiO
2
-Cu
(NO
3
)
2
in air at 500 °C for 2 h. The fabrication process of
the TiO
2
-CuO (I) and TiO
2
-CuO (II) photo-electrodes is
shown in Figure 1.
2.3 Characterization
The surface morphology of the sintered TiO
2
nano-
tube arrays, TiO
2
-CuO (I) and TiO
2
-CuO (II) was ob-
served with a Hitachi S-4700 field-emission scanning
electron microscope (FESEM) after spraying the con-
ducting layer of platinum. The bulk composition was
investigated with energy dispersive X-ray spectroscopy
(EDS). The phases present in the coatings were charac-
terized with a small-angle diffractometric study carried
out on a Riga KUD/max 2550PC X-ray automatic
diffractometer. The optical performance was evaluated
using an Agilent Technologies Cary 500 spectrophoto-
meter with its wavelength ranging from 200 nm to 800
nm.
The photo-electrochemical performance was eva-
luated in a three-electrode electrochemical cell with a
quartz window to allow illumination. The working
electrodes were the sintered TiO
2
,T i O
2
-CuO (I) and
TiO
2
-CuO (II). A saturated calomel electrode (SCE) and
Pt silk served as the reference electrode and counter
electrode, respectively. All the working electrodes were
tested in 0.2 M Na
2
SO
4
and analyzed with a CHI660E
electrochemical analyzer. Linear sweep voltammetry
(LSV) was measured at a scan rate of 0.01 VS
–1
.
Electrochemical impedance spectroscopy (EIS) was
carried out under an open-circuit voltage with frequen-
cies ranging from 10
5
to 10
–2
Hz with an AC voltage
amplitude of 5 mV. The potentials in the I–V and I–t
curves, and in the PEC degradation experiments, were
controlled by CHI660E. A 150WXe lamp was used to
H. DONG et al.: PREPARATION AND PHOTO-ELECTROCHEMICAL PERFORMANCE OF TiO2 NANOTUBE ...
398 Materiali in tehnologije / Materials and technology 52 (2018) 4, 397–404
Figure 1: Fabrication process of TiO
2
-CuO photo-electrodes: a) TiO
2
nanotube arrays produced with anodic oxidation, b) wet impregnation of
the Cu (NO
3
)
2
solution, c) air drying of sample b at 80 °C for 30 min, d)TiO
2
-CuO (II) produced by sintering in air at 500 °C for 2 h, e) drip
washing of sample c to remove the redundant Cu(NO
3
)
2
on the surface, f) TiO
2
-CuO (I) produced by sintering sample e in air at 500 °C for 2 h

provide visible light. Electrochemical impedance spec-
troscopy was used to explore the conductivity of the
electrodes under the dark and illumination environment.
The solution was the same as that used for the photo-
electrochemical-performance evaluation.
3 RESULTS AND DISCUSSION
3.1 SEM analysis
Surface morphology was observed with FESEM. It is
found that TiO
2
nanotube arrays are formed having a
regular and orderly structure (Figure 2a). The average
diameter of these nanotubes is around 100 nm and the
thickness of a wall is around 20 nm. The length of TiO
2
nanotube arrays is around 200 nm. There is an interstice
between TiO
2
nanotubes. H. Zao et al.
18
also reported
that there is an interstice between TiO
2
nanotube arrays.
The length of the TiO
2
nanotubes is around 200 nm.
Porous clusters were found on the TiO
2
-CuO (II) surface
(Figure 2b), and the thickness of a nanosheet is around
100 nm. The top and cross-section images of TiO
2
-CuO
(I) are shown in Figures 2c and 2d, respectively. The
thickness of TiO
2
-CuO (I) is around 200 nm. The inset
image in Figure 2d is the cross-section image of
TiO
2
-CuO (II), and the thickness of the film is around 3
μm. The energy-dispersive-spectrometer analysis shows
that there are Cu (x = 54.5 %), O (x = 36.75 %) and Ti
(x = 8.75 %) on the TiO
2
-CuO (II) surface, and the Cu
amount is much larger than that of Ti, indicating the Cu
oxide on the TiO
2
surface is thick. Nanoparticle and
porous films were found on the TiO
2
-CuO (I) surface.
The Cu (x = 28.82 %) amount is almost as big as that of
Ti (x = 20.01 %), indicating the Cu oxide on the surface
is thinner than that of TiO
2
-CuO (II).
However, the FESEM and EDS analyses do not allow
us to distinguish between TiO
2
and Cu oxide. Therefore,
other method, i.e., X-ray diffraction was applied.
3.2 XRD analysis
Figure 3 indicates the XRD patterns for the sintered
TiO
2
nanotube arrays and CuO-modified TiO
2
nanotube
arrays. The major peaks corresponding to the CuO
crystallites at 2 = 35.5° and 38.4° are clearly shown in
Figures 3b and 3c. The peak intensity at 2 = 35.5° and
of 38.4° in Figure 3b is much stronger than that in
Figure 3c, indicating that the amount of CuO in
TiO
2
-CuO (II) is larger than that in TiO
2
-CuO (I). The
XRD pattern for the CuO on TiO
2
nanotube arrays can
be indexed to the monoclinic phase of CuO without Cu
and Cu
2
O, showing that the CuO nanostructure is well
crystallized and pure after being sintered in air at 500 °C.
L. Valladares et al.
19
showed that Cu was totally trans-
formed into CuO when the sintering temperature was
above 300 °C. All the results demonstrated that CuO was
well crystallized on the surface of the TiO
2
nanotube
arrays.
The average crystal sizes for CuO with a cell volume
of 20.63 nm in TiO
2
-CuO (II) are calculated, after an
appropriate background correction, from the X-ray line,
broadening the diffraction peaks at 2 = 35.5° and 38.4°
using Debye-Scherrer’s formula:
20
D=0.89 / Cos  (1)
where is 0.15406 nm (the wave length of the X-ray),  is the angular peak width at half maximum in radians,
and  is Bragg’s diffraction angle.
3.3 UV-Vis diffuse-reflectance-spectra analysis
Figure 4 displays the diffuse-reflectance spectra of
sintered TiO
2
,T iO
2
-CuO (I) and TiO
2
-CuO (II). The bad
H. DONG et al.: PREPARATION AND PHOTO-ELECTROCHEMICAL PERFORMANCE OF TiO2 NANOTUBE ...
Materiali in tehnologije / Materials and technology 52 (2018) 4, 397–404 399
Figure 3: XRD patterns for sintered TiO
2
,T i O
2
-CuO (I) and
TiO
2
-CuO (II)
Figure 2: SEM images of the as-prepared photochemical electrodes:
a) SEM image of the sintered TiO
2
surface, the inset is the
cross-section image, b) SEM images of the TiO
2
-CuO (II) surface, c)
SEM images of the TiO
2
-CuO (I) surface, d) cross-section image of
TiO
2
-CuO (I), the inset is the cross-section image of TiO
2
-CuO (II)

gap was changed dramatically with the increasing CuO
amount on the surface of the TiO
2
nanotube arrays. The
broad band between 390 nm and 800 nm implies a signi-
ficant improvement of the absorption in the visible-light
region. It can be seen that the adsorption edge of the
samples did not shift, which indicates that CuO was
deposited onto the TiO
2
nanotube arrays rather than
doped into the crystalline TiO
2
.
21
The enhanced light
adsorption increased the possibilities to separate holes
and electrons. Therefore, it can be primarily inferred that
the TiO
2
-CuO (I) and TiO
2
-CuO (II) composites might
show a higher photocatalytic performance than the
sintered TiO
2
nanotube arrays.
The band-gap energy values (Eg) for different sam-
ples were calculated using the following equation:
22
Eg = 1239.89 m / (-b) (2)
The Eg values were 3.25 eV, 3.17 eV and 3.14 eV for
the sintered TiO
2
,T iO
2
-CuO (I) and TiO
2
-CuO (II), res-
pectively.
3.4 Photo-electrochemical (PEC) properties
3.4.1 I-V and I-t analysis
The PEC properties of the electrodes were measured
by examining their linear-sweep voltammetric profiles in
0.2M Na
2
SO
4
in the dark and under illumination. The
PEC behavior of the sintered TiO
2
,T i O
2
-CuO (I) and
TiO
2
-CuO (II) electrodes exposed to illumination is
presented in Figure 5a. The current densities in all the
photo-electrodes increased with the increasing potential
applied to the electrodes, indicating the p-type con-
ductivity. As shown, the maximum current densities of
about –4.269 × 10
–5
A/cm
2
, –4.633 × 10
–5
A/cm
2
and
–1.1 × 10
–5
mA/cm
2
at –0.35 V (vs. SCE) were obtained
for the sintered TiO
2
,T i O
2
-CuO (I) and TiO
2
-CuO (II),
respectively. TiO
2
-CuO (I) exhibits an improved photo-
activity compared to the sintered TiO
2
nanotube arrays.
However, the TiO
2
-CuO (II) sample exhibits a poor
photo-activity compared to the sintered TiO
2
nanotube
arrays. In the dark area, the maximum photocurrent
densities of about –3.367 × 10
–5
A/cm
2
, –2.29 × 10
–5
A/cm
2
, –7.908 × 10
–6
A/cm
2
at –0.35 V (vs. SCE) were
obtained for the sintered TiO
2
,T i O
2
-CuO (I) and
TiO
2
-CuO (II), respectively. The sintered TiO
2
exhibits
the best electrical behavior of the three samples. And the
TiO
2
-CuO (II) sample shows the poorest electrical
behavior. The results indicate that the bare TiO
2
nanotube arrays modified with CuO through wet
impregnation cannot enhance the electron mobility.
A current-time evaluation performed at a fixing bias
of –0.2 V vs. SCE for 600 s is shown in Figure 5b. The
light was continuously chopped with 20-s intervals
during the current-time evaluation. Figure 5c shows
enlarged views of the current-time relationship from the
boxed area in Figure 5b, corresponding to the whole
stages of the current-time measurement of TiO
2
-CuO
(II). The result indicates that the current density of the
sintered TiO
2
and TiO
2
-CuO (II) starts to decline
immediately after several seconds and remains stable
from the 200–300 s measurement to the end of the
evaluation. On the other hand, the TiO
2
-CuO (I) elec-
trode shows a superior stability under the same evalu-
ation conditions. The current stability of the electrode
was quantified as the ratio of the current at the end of the
measurement to the current at the beginning of the
measurement.
23
The stability of the TiO
2
-CuO (I)
electrode shows an excellent stability of 95 %. The LSV
and current-time results indicate that the growth of the
moderate CuO porous film on the TiO
2
nanotube arrays
greatly improved the stability of the electrode, although
it causes a minor negative effect on the LSV measure-
H. DONG et al.: PREPARATION AND PHOTO-ELECTROCHEMICAL PERFORMANCE OF TiO2 NANOTUBE ...
400 Materiali in tehnologije / Materials and technology 52 (2018) 4, 397–404
Figure 5: PEC results for the sintered TiO
2
,T iO
2
-CuO (I) and TiO
2
-
CuO (II) electrodes: a) electrical-current measurements of the electro-
des in the dark and under illumination, b) current-time-measurement
results of the sintered TiO
2
,T i O
2
-CuO (I) and TiO
2
-CuO (II) photo-
electrodes in the dark and under illumination, c) magnified view of the
boxed region from (b); (d) long-time-stability-measurement result for
the TiO
2
-CuO (I) electrode under illumination
Figure 4: UV-Vis diffuse-reflectance-spectra analysis of sintered
TiO
2
,T iO
2
-CuO (I) and TiO
2
-CuO (II); m and b are obtained with the
linear fit (y=mx+ b) of the flat section of the UV-Vis spectrum

ment by reducing the electrical-current generation. An
abundant CuO amount on the TiO
2
nanotube arrays
causes a large negative effect on the LSV measurements
by reducing the photocurrent generation and electronic
transportation. Therefore, it is important to control the
CuO content on the surface of TiO
2
nanotube arrays.
During the 6-h reaction process (Figure 5d), the
photocurrent density over TiO
2
-CuO (I) remained vigo-
rous without any obvious deactivation. High efficiency
and an excellent current-time stability indicate a great
potential of TiO
2
-CuO (I) for a wide usage in the
hydrogen production. Therefore, drip washing is very
important during the wet impregnation followed by
sintering.
3.4.2 Electrochemical-impedance-spectroscopy (EIS)
analysis
In order to further investigate the effects of the sin-
tered condition and the CuO amount on the PEC
performance of TiO
2
nanotube arrays, EIS was per-
formed in a frequency range of 0.01 Hz–100 kHz in
0.2 M of the Na
2
SO
4
electrolyte and the results are
represented as Nyquist and Bode plots. The EIS
measurements were performed on sintered TiO
2
,
TiO
2
-CuO (I) and TiO
2
-CuO (II). Figure 6a shows the
Nyquist plot of the sintered TiO
2
, which demonstrates
the beginning of a large semicircle usually associated
with resistive and capacitive processes. The relationship
between Z’ (the real part) and Z” (the imaginary part) of
a semicircle with a radius of R/2 can be expressed as
follows:
(Z’-R/2)
2
+Z’’=(R/2)
2
(3)
The relative size of a circular arc radius corresponds
to the charge transfer resistance and electron-hole sepa-
ration efficiency from the Nyquist plot.
24
The Nyquist
plots (shown in Figure 6a) indicate that the diameter of
the semicircle of the sintered TiO
2
under illumination in
the high-to-medium frequency region is much smaller
than that in the dark.
The relationship between the parallel circuit impe-
dance (Z), resister (R) and capacitor (C) can be expressed
as follows:
Z=R/(1+ CR)-j CR
2
/(1+ CR
2
) (4)
As we all know, a low resistance indicates a recom-
bination suppression via an improved charge transport to
the electrolyte.
25
The plot of the frequency against the
electrochemical impedance of the sintered TiO
2
is shown
in Figure 6b, indicating that the impedance of the
sintered TiO
2
in the dark is about two-fold higher than
that under illumination in the low-frequency region. The
Nyquist plots in Figure 6b show that the diameter of the
semicircle of TiO
2
-CuO (I) under illumination in the
high-to-medium frequency region is much smaller than
that in the dark. The plot of the frequency against the
electrochemical impedance of the TiO
2
-CuO (I) elec-
trode materials is shown in Figure 6d. The impedance of
the sintered TiO
2
-CuO (I) in the dark is about eight-fold
higher than that under illumination in the low-frequency
region. Moreover, the resistance of the sintered TiO
2
in
the dark is lower than that of TiO
2
-CuO (I) in the dark
according to the analysis from Figures 6b and 6d.I n
addition, the resistance of the sintered TiO
2
in the dark
increases after the CuO loading. Therefore, the electrical
activity of the sintered TiO
2
is higher than that of
TiO
2
-CuO (I), as shown in Figure 5a.
The reduction of the TiO
2
-CuO (I) resistance under
illumination also indicates that CuO might induce a
better charge separation and an efficient electron transfer
in the TiO
2
-CuO. From all the above results, it can be
concluded that a moderate amount of CuO provides a
way for the hole-electron separation and enhances the
photo-electrochemical activity of the electrode. Fig-
ure 6e shows the Nyquist plot for the TiO
2
-CuO (II)
electrode. As shown in Figure 6e, the diameter of the
semicircle of TiO
2
-CuO (II) under illumination in the
high-to-medium frequency region is smaller than that in
H. DONG et al.: PREPARATION AND PHOTO-ELECTROCHEMICAL PERFORMANCE OF TiO2 NANOTUBE ...
Materiali in tehnologije / Materials and technology 52 (2018) 4, 397–404 401
Figure 6: Electrochemical-impedance-spectroscopy results for the
sintered TiO
2
,T iO
2
-CuO (I) and TiO
2
-CuO (II) photo-electrodes: a)
Nyquist and b) Bode plots obtained for the sintered TiO
2
, c) Nyquist
and d) Bode plots obtained for TiO
2
-CuO (I), e) Nyquist and f) Bode
plots obtained for TiO
2
-CuO (II), g) Bode phase plot for the sintered
TiO
2
,TiO
2
-CuO (I) and TiO
2
-CuO(II) electrodes under illumination

the dark. The plot of frequency against the electro-
chemical impedance of the TiO
2
-CuO (II) photo-elec-
trodes is provided in Figure 6f. The impedance of
TiO
2
-CuO (II) in the dark is about 1.5 times higher than
that under illumination in the low-frequency region. The
resistance value for the sintered TiO
2
changed signifi-
cantly after loading a large amount of CuO. And that
may be the most important factor contributing to the
higher PEC properties of TiO
2
-CuO (I) and lower PEC
properties of TiO
2
-CuO (II).
Furthermore, the Bode plots for the three photo-elec-
trodes (Figure 6g) under illumination indicate the pre-
sence of time constants at the low- and middle-frequency
peaks, corresponding to the diffusion in the electrolyte
and the electron transport-recombination process, respec-
tively. The Bode plot for the TiO
2
-CuO (I) electrode
suggests that the electron transport-recombination
process benefited from loading a moderate amount of
CuO, thus leading to the highest PEC efficiency of
TiO
2
-CuO (I). However, the Bode phase plot for the
TiO
2
-CuO (II) electrode suggests that the electron
transport-recombination process did not benefit from an
abundant amount of CuO, resulting in the lowest PEC
efficiency of the TiO
2
-CuO (II) photo-electrode.
3.4.3 Schematic illustration of TiO
2
-CuO photo-elec-
trodes
It is well known that combing the p-type photo-elec-
trodes with wide-band-gap n-type semiconductors could
not only improve the stability of photo-electrodes, but
also enhance the PEC properties due to the formation of
a p-n junction. There are many schematic illustrations of
p-CuO and n-TiO
2
composites contacting the electrolyte
in the thermal-equilibrium state.
Here, our results show that the positive effect of the
p–n junction might be suppressed if the thickness of the
secondary semiconductor is too high. This is remarkably
noticeable when we reckon the CuO with a thick porous
sheet layer as a protecting buffer (increasing the
resistance). Similar results were observed by A. Kargar
26
and Ulugbek Shaislamov
27
for their p-n PEC electrode,
where the thickness of the secondary semiconductor
exceeding a certain limit might result in decreasing the
PEC performance of photo-electrodes. Moreover, the
effect of graphene sheets with various thicknesses on
Cu
2
O NW electrodes was also reported.
28
High graphene
concentrations reduce PEC properties of Cu
2
O/graphene
electrodes.
The reduced PEC properties of our TiO
2
-CuO elec-
trode can be explained based on several influence factors
as shown in Figure 7: (i) the grain boundaries in the
thick porous CuO layer act as recombination sites for the
photo-induced electron-hole pairs;
29
(ii) the porous CuO
nanosheet layer (including the nanosheet layer and
porous CuO shown in Figure 2b) leads to additional
electrochemical charge-transfer resistance in the dark
(shown in Figure 5); (iii) the drip washing after WI can
prevent the formation of large clusters of CuO, acting as
the recombination centers resulting in a lower hydro-
gen-generation activity;
16
and (iv) the CuO layer (shown
in Figure 2c)o nt h eT i O
2
nanotubes increases the
resistance of the electrode in the dark. However, the CuO
layer improves the cohesion between CuO and TiO
2
,
facilitating the charge transfer from TiO
2
to CuO, and
resulting in enhancing the proton reduction. These
results indicate that the TiO
2
-CuO photo-electrode
produced through WI by drip washing is more favorable
than the TiO
2
electrodes (shown in Figures 5b and 5d)
with respect to the long-term stability and PEC per-
formance under photo-electrochemical conditions.
4 CONCLUSIONS
Wet impregnation followed by sintering in air was
used to prepare a CuO/TiO
2
/Ti heterojunction photo-
electrode. High-density porous-nanosheet CuO films on
the TiO
2
nanotube arrays (TiO
2
-CuO (II)) were obtained
without drip washing before sintering, while the CuO
films on the TiO
2
nanotube arrays (TiO
2
-CuO (I)) were
obtained after sintering the sample with drip washing
after air drying. Both of the heterojunction photo-elec-
trodes extend the absorption of visible light. It was found
that the CuO growth could significantly reduce the PEC
properties of a photo-electrode due to the additional
electrochemical charge-transfer resistance. The photo-
catalytic stability of TiO
2
was remarkably enhanced up
to 95 % because of the formation of CuO/TiO
2
interfaces
due to the drip washing during the preparation. The EIS
measurement indicates that an abundant amount of CuO
on the TiO
2
nanotube arrays could increase the electric
resistance, resulting in decreasing the photocurrent
density during the LSV test. The fabricated TiO
2
/CuO
photo-electrode exhibited an excellent electrochemical
stability, which is highly favorable, having a great
potential for the application of other unstable metal-
oxide semiconductor-based electrodes.
H. DONG et al.: PREPARATION AND PHOTO-ELECTROCHEMICAL PERFORMANCE OF TiO2 NANOTUBE ...
402 Materiali in tehnologije / Materials and technology 52 (2018) 4, 397–404
Figure 7: Schematic illustration of the TiO
2
-CuO electrode

Acknowledgments
This work was supported by the Zhejiang Province
Science Foundation for Youths (No. LQ16E020002).
The work of material characterization was supported by
the Zhejiang Metallurgical Research Institute Co., Ltd.
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