master

1. Applied Surface Science 347 (2015) 454–460
Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
A study on photocatalytic activity of micro-arc oxidation TiO2 films
and Ag+
/MAO-TiO2 composite films
N. Xianga,b,c
, R.G. Songa,b,c,∗
, B. Xianga,b,c
, H. Lia,b,c
, Z.X. Wanga,b,c
, C. Wanga,b,c
a
School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China
b
Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou 213164, China
c
Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, Jiangsu, China
a r t i c l e i n f o
Article history:
Received 8 November 2014
Received in revised form 9 January 2015
Accepted 17 April 2015
Available online 28 April 2015
Keywords:
Micro-arc oxidation
MAO-TiO2 film
Ag+
/MAO-TiO2 composite film
Photocatalytic activity
Impregnation
Methylene blue
a b s t r a c t
First, micro-arc oxidation (MAO) TiO2 films have been prepared on pure titanium in a phosphate-based
electrolyte, and then the Ag+
/MAO-TiO2 composite films have been fabricated by Ag+
impregnation
in this paper. The microstructure and composition of MAO-TiO2 films and Ag+
/MAO-TiO2 composite
films have been studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and
energy-dispersive X-ray spectroscopy (EDS). The photocatalytic activity of both films was evaluated by
photocatalytic decolorization of methylene blue (MB) in aqueous solution as a model pollutant under
sunlight irradiation simulation with homemade ultraviolet–visible spectroscopy (UV–vis). The results
showed that the photocatalytic activity of MAO-TiO2 films increased with increasing the applied volt-
age and concentration in a certain scope. The morphology of Ag+
/MAO-TiO2 composite films were of
significantly difference and superior photocatalytic activity compared to the MAO-TiO2 film. Also, Ag+
impregnation was able to enhance the photocatalytic efficiency of MAO-TiO2 film.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Nowadays, with the development of dye industry, the water pol-
lution becomes more and more serious. Among the various types of
dyes, methylene blue (MB) is one of the most commonly used sub-
stances for coloring cotton, wood, silk and paper stock among the
various types of dyes. Severe exposure to MB will cause increase
heart rate, vomiting, shock, cyanosis, jaundice, quadriplegia and so
on in humans [1–3]. So it can be seen that MB should be wiped out
from the human environment concerning about the above negative
effects.
It is well known that metal oxide photocatalysis is a very promis-
ing approach to remedy the problem of chemical waste. Among
various metal oxide, titanium dioxide (TiO2) has been found to be
able to decompose different kinds of organic and inorganic waste
in gas and liquid phases, and it becomes one of the most popu-
lar photocatalysis because of its chemically and biologically inert,
photocatalytically stable, commercially available, inexpensive and
from environmental viewpoint is friendly [4–7]. A number of
commonly surface techniques have been developed to synthesize
∗ Corresponding author at: School of Materials Science and Engineering,
Changzhou University, Changzhou 213164, Jiangsu, China. Tel.: +86 519 86330069.
E-mail address: songrg@hotmail.com (R.G. Song).
titanium dioxide, such as sol–gel [8–13], physical vapor deposition
[14–17], chemical vapor deposition [18,19], laser nitriding [20–22],
hydrothermal process [23], spray pyrolysis [24,25], liquid phase
deposition [26] and anodizing. Among them, micro-arc oxidation
(MAO) is one of the most promising and environmentally friendly
technology by spark micro-discharges which move rapidly on the
vicinity of the anode surface [27–30]. The MAO process is carried
out at a voltage higher than the breakdown voltage of the gas layer
enshrouding the anode. Since the substrate is linked to positive
pole of the rectifier as anode, the gas layer is composed of oxygen.
When the dielectric gas layer completely covers the anode surface,
electrical resistance of the electrochemical circuit surges and the
process continues providing that the applied voltage defeats the
breakdown voltage of the gas layer. Applying such voltages leads to
formation of electrical discharges via which electrical current could
pass the gas layer, and the MAO process is characterized by these
electrical sparks [31,32]. Other applications of MAO processed on
titanium can be seen elsewhere [33,34].
Since the band gap energy (Eg) of titania is relatively wide,
considerable efforts have been extended to broaden the absorp-
tion edge of TiO2 toward the visible part of the spectrum in the
last three decades. One of the effective way for acquiring a visible
response is to introduce defects into the titania lattice by doping
with metallic [35–37] and non-metallic species [38–40]. Among
these techniques, ions modification of TiO2 is a powerful way to
http://dx.doi.org/10.1016/j.apsusc.2015.04.136
0169-4332/© 2015 Elsevier B.V. All rights reserved.

2. N. Xiang et al. / Applied Surface Science 347 (2015) 454–460 455
Table 1
Chemical composition of the different applied electrolytes.
Electrolyte no. Na3PO4·12H2O (g/l) NaF (g/l)
A 4 1
B 6 1
C 8 1
D 10 1
E 12 1
extend the adsorption light from UV to visible region and to reduce
the recombination rate of photo-generated electrons and holes of
TiO2. Till now, non-metal doping [41] and noble metal loading
[42] with TiO2 has been reported. However, studies on metal ion
impregnation is extremely exiguous.
In the present work, the main objective was to study the pho-
tocatalytic activity of Ag+/MAO-TiO2 composite films under the
optimum MAO treatment condition which was found out by photo-
catalytic tests. The photocatalytic efficiency of MAO-TiO2 films and
Ag+/MAO-TiO2 composite films have also been compared under
the same experimental condition. Also, the most suitable impreg-
nation time and concentration have been discussed in detail. The
surface morphology and composition of films were characterized
by scanning electron microscopy (SEM), X-ray diffraction (XRD) and
energy-dispersive X-ray spectroscopy (EDS), respectively.
2. Experimental
2.1. Materials and MAO process
The substrate material in the present study was commer-
cially pure grade II titanium for MAO experiment. The specimens
(25 mm × 15 mm × 2 mm) were ground with 500, 800, 1200 and
1800 grit abrasive paper and ultrasonically cleaned in ethanol. Then
specimens were taken out for cleaning by distilled water and dried
in ambient air before MAO process. The electrolytes consisted of
8 g/l sodium phosphate (Na3PO4·12H2O) and 1 g/l sodium fluoride
(NaF) was applied under DC-voltages of (Va) from 360 to 440 V
with +20 V intervals. The synthesis time for each specimen was
30 min under 303 K by a recyclable water cooling system during
the treatment. Then according to the photocatalytic activity study
of MAO coatings under the different conditions above, electrolytes
consisted of sodium phosphate (Na3PO4·12H2O) and sodium fluo-
ride (NaF) in aqueous solution were carried out under the optimum
voltage 420 V for comparison of concentration. The exactly chem-
ical compositions of electrolytes are shown in Table 1 under the
constant-voltage mode.
2.2. Ag+ impregnation
In order to obtain the Ag+/MAO-TiO2 composite films, the MAO-
TiO2 films obtained in 10 g/l Na3PO4 and 1 g/l NaF aqueous solution
under 420 V were impregnated with 0.1 mol/l silver nitrate for 3, 6,
12, 24, 48, 72 h. To further investigate the influence of AgNO3 con-
centration on photocatalytic activity of Ag+/MAO-TiO2 composite
films, different concentrations (0.025, 0.05, 0.075, 0.1, 0.2 mol/l) of
Ag+-containing AgNO3 solution were applied as the steeping fluid
to impregnated the MAO-TiO2 films for 24 h.
2.3. Characterization
Surface morphology of MAO-TiO2 films and Ag+/MAO-TiO2
composite films were examined by scanning electron microscopy
(SEM, Hitachi S-4700). Phase composition of both films were ana-
lyzed by X-ray diffraction (XRD, Thermo ARL X’TRA) using Cu Kɑ
radiation between 2Â values of 10◦ and 80◦ with a step length of
ultraviolet lamp
solution
cooling
water
Fig. 1. The schematic diagram of photocatalytic device.
0.02◦ at a scanning rate of 1◦/min. The Ag+/MAO-TiO2 composite
film was examined by energy-dispersive X-ray spectroscopy (EDS)
incorporated into scanning electron microscopy after Au deposition
by sputtering additionally.
2.4. Photocatalytic activity of MB under simulated sunlight
irradiation
The photocatalytic activity of MAO-TiO2 films and Ag+/MAO-
TiO2 composite films were evaluated by photocatalytic decoloriza-
tion of MB aqueous solution under sunlight irradiation simulation.
A 300 W high-voltage mercury lamp was used as the light source of
simulated sunlight. The distance between the specimens and lamp
was 10 cm. The photocatalytic tests were carried out in a cylindri-
cal bottle (250 ml) with the aqueous solution of MB dyes (15 mg/l,
100 ml) and photocatalysts MAO-TiO2 films as well as Ag+/MAO-
TiO2 composite films. Sketch-map for photocatalytic device is
shown in Fig. 1. The aqueous suspensions containing MB with pho-
tocatalyst were stirred for 40 min continuously in dark to reach an
adsorption–desorption equilibrium before irradiation. After that,
the mixture was exposed to visible irradiation using high-voltage
mercury lamp. The decolorization rates of MB were monitored by
measuring the absorbance of MB solution at = 665 nm using a
UV 720 spectrophotometer. The decolorization efficiency (%) was
calculated as:
Á% =
c0 − ct
c0
× 100% (1)
where c0 is the initial concentration of MB, ct is a instantaneous
concentration of MB after photoirradiation by simulated sunlight.
Three samples were tested for the average value.
3. Results and discussion
Based on the Ag+ impregnation of MAO-TiO2 film, the MAO-TiO2
film and Ag+/MAO-TiO2 composite film are addressed as TiO2 film
and Ag+-TiO2 film for convenient.
3.1. Effects of applied voltage
Fig. 2 demonstrates the evolution of surface morphologies of
MAO-TiO2 films. It can be seen that all TiO2 films were of porosity
on their surface (Fig. 2b–f). At 360 V, it can be found that the sur-
face morphologies of substrate (Fig. 2a) and the film (Fig. 2b) were
rough, and the sharpening vestige of titanium substrate can still
be clearly observed over the surface, which indicated that the film
was still thin under such a low voltage. At 380 V, the abrasive traces
disappeared by the intensely growth of the film and the thickness
was also increased. On the morphologies at the higher voltages (up
to 440 V), some of the micropores linked up to a single stretch after
expansive growth, which led to a decrease in quantity and increase

3. 456 N. Xiang et al. / Applied Surface Science 347 (2015) 454–460
Fig. 2. Surface morphologies of (a) substrate and MAO-TiO2 films obtained at (b) 360 V, (c) 380 V, (d) 400 V, (e) 420 V, and (f) 440 V.
in size for micropores, especially for the film obtained under the
voltage of 440 V (Fig. 2f).
Fig. 3 shows the XRD patterns of TiO2 films fabricated under dif-
ferent voltage. It should be pointed out that the strong Ti peaks (see
Fig. 3) correspond to titanium substrate owing to the thin TiO2 film
(about 10 ␮m of thickness). The TiO2 film was mainly composed of
anatase which was generated during MAO process. With the volt-
age increased, some peaks corresponding to anatase increased in
intensity. Besides, the width of diffraction peaks advanced toward
Fig. 3. XRD patterns of TiO2 MAO films obtained under different voltage.
the narrow side which indicated that the granularity of TiO2
increased with increasing the applied voltage.
The effects of TiO2 films prepared under mutative voltages on
photocatalytic activity were studied and the curves are shown in
Fig. 4. It can be found that the photocatalytic activity of TiO2 film
increased with the increment of voltage up to the optimum value
Fig. 4. The photocatalytic activity of MAO-TiO2 films obtained under different volt-
age in photodegradation reaction of MB solution.

4. N. Xiang et al. / Applied Surface Science 347 (2015) 454–460 457
Fig. 5. Surface morphologies of MAO-TiO2 films obtained at concentrations of (a) 4 g/l, (b) 6 g/l, (c) 8 g/l, (d) 10 g/l, and (e) 12 g/l phosphate-main electrolyte.
420 V, especially for the one obtained at 400 V with an obviously
leap compared to the one obtained at 380 V on photocatalytic
activity. Howbeit, there appeared a marginally decline when the
voltage increased to 440 V. The main reason for the change was
the anatase content. At 360 V, the peaks corresponding to anatase
in Fig. 3 was quite weak which suggested that the quantity of
anatase was relatively small, whereas the photocatalytic efficiency
was unfavorable. When the voltage applied up to 440 V, the pho-
tocatalytic efficiency of TiO2 film increased to 40% approximately
after 180 min irradiation which was slightly negative than that
of the one obtained under 420 V after the same period of irra-
diation. Considering about this phenomenon, another influencing
factor specific area is introduced. In Fig. 2, it can be found that
the TiO2 film fabricated under the voltage of 440 V (Fig. 2f) was of
less porosity and bigger size compared with that of 420 V (Fig. 2e)
in unit area. Due to the increasing voltage, the growth ability of
grain was enhanced which caused an increase in anatase content.
Therefore, the increment of photocatalytic ability was partially
neutralized owing to the decrease in corresponding specific area.
As a result, it can be concluded that the optimum voltage is 420 V
for obtaining TiO2 film with the most favorable photocatalytic
activity.
3.2. Effects of electrolytes concentration
Fig. 5 presents the surface morphologies of MAO-TiO2 films
fabricated under different concentrations of phosphate. The micro-
pores not only increased in size but also decreased in quantity
with increasing the phosphate concentration. At 4 g/l, the sparking
power was definitely less violent than that of the higher concentra-
tions. Therefore, the limited power led to the inhibition in the film
growth process, which caused the cellular structure full of indis-
tinction and crushing. Especially, when the concentration was up
to 12 g/l, the film surface existed some quenched trace of melt-
down substance owing to the high sparking energy during the MAO
process.
Fig. 6 shows the MAO-TiO2 film obtained in various concentra-
tions of electrolytes. The XRD patterns indicated that the TiO2 films
mainly consisted of anatase up to 10 g/l sodium phosphate, and the
anatase content increased with the increment of concentration in
overall trend. However, at 12 g/l, there appeared a pivotal point
which led to a component-change in the film, the rutile phase and
titanium oxide (Ti6O) are major phases of the film at that concen-
tration.
The photocatalytic activity of MAO-TiO2 films obtained under
different concentrations of electrolytes in photodegradation reac-
tion of MB solution is shown in Fig. 7. It can be found that the
photocatalytic activity increased with increasing the phosphate
concentration in all. Especially, when the concentration of sodium
phosphate increased to 10 g/l, the photocatalytic activity increased
to 45% after 180 min irradiation and showed the most positive pho-
tocatalytic efficiency compared to the rest. The anatase content was
the main factor for the difference in photocatalytic activity. At 6 g/l
phosphate-based electrolytes, the obtained anatase content was
relatively small. Therefore, after 180 min irradiation, the photocat-
alytic efficiency of TiO2 film was about 32% which was inferior to

5. 458 N. Xiang et al. / Applied Surface Science 347 (2015) 454–460
Fig. 6. XRD patterns of MAO-TiO2 films obtained under different concentration.
others. So, it can be concluded that the optimum concentration is
10 g/l under the applied voltage 420 V.
3.3. Effects of Ag+ impregnation
According to the investigation on the optimum applied voltage
and concentration for MAO-TiO2 film, the following study on the
effects of Ag+ impregnation is based on the optimum voltage and
concentration of 420 V and 10 g/l, respectively.
Fig. 8 shows the photocatalytic activity of Ag+/MAO-TiO2 com-
posite films prepared under the optimum condition above after
different period of Ag+ impregnation. It can be seen that there exists
an optimum impregnation time, i.e., 24 h. Furthermore, comparing
Fig. 8 with Figs. 4 and 7, it can be found that Ag+ impregnation was
of significantly ability to improve the photocatalytic activity of TiO2
film, i.e., the photocatalytic efficiency of Ag+-TiO2 films reaches a
maximum value 60% which was obviously higher than that of TiO2
films after 180 min exposure under simulated sunlight. And due to
the effects of Ag+ impregnation, the biggest increment of photocat-
alytic efficiency is nearly 20% after 180 min exposure. The results
proved that the photocatalytic activity of Ag+-TiO2 composite films
were significantly enhanced after Ag+ impregnation as compared
to TiO2 film. Once optical excitation takes place in Ag+-TiO2 films,
200180160140120100806040200-20
-5
0
5
10
15
20
25
30
35
40
45
degradation%
t/min
4g/L
6g/L
8g/L
10g/L
12g/L
Fig. 7. The photocatalytic activity of MAO-TiO2 films obtained under different con-
centration of electrolytes in photodegradation reaction of MB solution.
200180160140120100806040200-20
0
10
20
30
40
50
60
degradation%
t/min
3h
6h
12h
24h
48h
72h
Fig. 8. The photocatalytic activity of Ag+
/MAO-TiO2 composite films after different
time of Ag+
impregnation.
the photogenerated electrons can be transferred to lower-lying
conduction band of Ag+, while the positive holes moved toward
the valance band (VB) of TiO2 and accumulate there resulting in
reduction of the electron-hole recombination rate. Consequently,
the photocatalytic activity of Ag+-TiO2 films was improved. How-
ever, the photocatalytic activity and Ag+ content are not increasing
in linear because of the fluctuation in energy-level structure of Ag+.
While the Ag+ content is relatively small, the conduction band (CB)
of TiO2 is close to the energy-level of Ag+. At that time, the restricted
conducting energy owing to TiO2 CB led to an unapparent enhance-
ment on photocatalytic activity of Ag+-TiO2 film. Nevertheless, it
has also demonstrated that while the oxygen molecules adsorbed
on Ag+-TiO2 film surface with unpaired electrons affinity are on the
same level as compared to the energy-level of Ag+, the photocat-
alytic efficiency reaches to the maximum value with the increasing
n (quantum number). With the continually increment of n, the
energy-level of Ag+ is lower than O2 on film surface, while Ag+ is of
insufficient capability to conduct electrons to O2 on that condition,
the O2
•− decrease in quantity. On the contrary, the accepted abil-
ity of h+ enhances and leads to the formation of new electron-hole
recombination center, which eventually weaken the photocatalytic
200180160140120100806040200-20
0
10
20
30
40
50
60
degradation%
t/min
0.025mol/L
0.05mol/L
0.075mol/L
0.1mol/L
0.2mol/L
Fig. 9. The photocatalytic activity of Ag+
/MAO-TiO2 composite films with different
concentrations of Ag+
impregnation for 24 h.

6. N. Xiang et al. / Applied Surface Science 347 (2015) 454–460 459
Fig. 10. XRD analysis of Ag+
/MAO-TiO2 composite film under optimum condition.
Fig. 11. Surface morphology of Ag+
/MAO-TiO2 composite film under optimum con-
dition.
activity of Ag+-TiO2 composite film. Thus, the photocatalytic activ-
ity of Ag+-TiO2 film increased at first and then decreased with the
continuous time of Ag+ impregnation.
Fig. 9 shows the photocatalytic activity of Ag+/MAO-TiO2 com-
posite films prepared under the optimum condition above with
different concentrations of Ag+ impregnation for 24 h. It can
be found the optimum Ag+ concentration for impregnation is
0.075 mol/l. As a result, it can be concluded that the optimum time
and concentration for Ag+ impregnation are 24 h and 0.075 mol/l
under the optimum MAO condition, i.e., voltage and concentration
of 440 V and 10 g/l, respectively.
XRD analysis of Ag+/MAO-TiO2 composite film under the opti-
mum condition is shown in Fig. 10. It can be seen the Ag+-TiO2
composite film was mainly composed of AgNO3 and a small
amount of anatase, and the peak corresponding to AgNO3 was
extremely strong. This indicated that AgNO3 layer was rela-
tively thick and nearly covered the whole TiO2 film after 24 h
impregnation.
To testify the XRD analysis above, surface morphology and
EDS spectra are shown in Figs. 11 and 12, respectively. From
Fig. 11, it can be found that the surface was mostly covered
by AgNO3 with bulky grain and only some scattered micropo-
res can be seen. The EDS spectra shows there exists a large
quantity of Ag on surface of Ag+-TiO2 composite film, however,
only a small amount of Ag+ penetrate into the pores. The results
are matched well with the XRD results above. All elements in
electrolytes and substrate were detected. On the whole, Ag+
impregnation can enhance the photocatalytic activity of MAO-TiO2
films.
4. Conclusions
The MAO-TiO2 films and Ag+/MAO-TiO2 composite films were
prepared on pure titanium in a phosphate-based electrolyte with-
out and with Ag+ impregnation after MAO treatment, respectively.
The microstructure and composition of the MAO-TiO2 films were
of significantly change after Ag+ impregnation under the optimum
condition.
The optimum voltage and concentration of phosphate were
420 V and 10 g/l for photocatalytic activity, and under such supe-
rior condition, the most superior time and concentration for Ag+
impregnation were 24 h and 0.075 mol/l. After such impregnation
treatment, the maximum value of photocatalytic activity for the
Ag+/MAO-TiO2 composite films was enhanced to 60% around which
was about 20% higher than that of the MAO-TiO2 films after 180 min
exposure to simulated sunlight.
Fig. 12. EDS spectra of Ag+
/MAO-TiO2 composite film under optimum condition.

7. 460 N. Xiang et al. / Applied Surface Science 347 (2015) 454–460
Acknowledgments
The financial aids of the National Natural Science Foundation
of China under grant No. 51371039 and the Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD), Jiangsu Province, China are gratefully acknowledged.
References
[1] K.V. Kumar, V. Ramamurthi, S. Sivanesan, J. Colloid Interface Sci. 284 (2005)
14–21.
[2] V. Vadivelan, K.V. Kumar, J. Colloid Interface Sci. 286 (2005) 90–100.
[3] K. Ravikumar, B. Deebika, K. Balu, J. Hazard. Mater. B 122 (2005) 75–83.
[4] Y. Shin, W. Chae, Y. Song, Electrochem. Commun. 8 (2006) 465–470.
[5] M.R. Bayati, A.Z. Moshfegh, F. Golestani-Fard, Electrochim. Acta 55 (2010)
2760–2766.
[6] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C: Photochem. Rev.
1 (2000) 1–21.
[7] U. Diebold, Surf. Sci. Rep. 48 (2003) 53–229.
[8] M.F. Hsieh, L.H. Perng, T.S. Chin, Mater. Chem. Phys. 74 (2002) 245–250.
[9] E. Milella, F. Cosentino, A. Licciulli, C. Massaro, Biomaterials 22 (2001)
1425–1431.
[10] U.G. Akpan, B.H. Hameed, Appl. Catal. A: Gen. 375 (2010) 1–11.
[11] R. Akbarzadeh, S.B. Umbarkar, R.S. Sonawane, S. Takle, Appl. Catal. A: Gen. 374
(2010) 103–109.
[12] K. Prasad, D.V. Pinjari, A.B. Pandit, Ultrason. Sonochem. 17 (2010) 409–415.
[13] H. Zhao, S. Bennici, J. Shen, Appl. Catal. A: Gen. 356 (2009) 121–128.
[14] V. Rico, P. Romero, J.L. Hueso, J.P. Espinós, A.R. González-Elipe, Catal. Today 143
(2009) 347–354.
[15] S.H. Lee, E. Yamasue, H. Okumura, K.N. Ishihara, Appl. Catal. A: Gen. 371 (2009)
179–190.
[16] S.G. Seong, E.J. Kim, Y.S. Kim, K.E. Lee, Appl. Surf. Sci. 256 (2009) 1–5.
[17] M. Kitano, M. Matsuoka, M. Ueshima, M. Anpo, Appl. Catal. A: Gen. 325 (2007)
1–14.
[18] P. Evans, T. English, D. Hammond, M.E. Pemble, D.W. Sheel, Appl. Catal. A: Gen.
321 (2007) 140–146.
[19] J. Mungkalasiri, L. Bedel, F. Emieux, J. Doré, F.N.R. Renaud, Surf. Coat. Technol.
204 (2009) 887–892.
[20] C. Hu, H. Xin, L.M. Watson, T.N. Baker, Acta Mater. 45 (1997) 4311–4322.
[21] I. Garcia, J.J. Dedamborenea, Corros. Sci. 40 (1998) 1411–1419.
[22] R.S. Razavi, M. Salehi, M. Ramazani, Corros. Sci. 51 (2009) 2324–2329.
[23] T. An, J. Liu, G. Li, Appl. Catal. A: Gen. 350 (2008) 237–243.
[24] I.O. Acik, A. Junolainen, V. Mikli, Appl. Surf. Sci. 256 (2009) 1391–1394.
[25] M.O. Abou-Helal, W.T. Seeber, Appl. Surf. Sci. 195 (2002) 53–62.
[26] M. Pourmand, N. Taghavinia, Mater. Chem. Phys. 107 (2008) 449–455.
[27] L. Wan, J.F. Li, J.Y. Feng, W. Sun, Z.Q. Mao, Mater. Sci. Eng. B 139 (2007)
216–220.
[28] A.L. Yerokhin, A. Leyland, A. Matthews, Appl. Surf. Sci. 200 (2002) 172–184.
[29] X. Sun, Z. Jiang, S. Xin, Z. Yao, Thin Solid Films 471 (2005) 194–199.
[30] P. Gupta, G. Tenhundfeld, E.O. Daigle, D. Ryabkov, Surf. Coat. Technol. 201 (2007)
8746–8760.
[31] A.L. Yerokhin, A. Leyland, A. Matthews, Surf. Coat. Technol. 200 (2002) 172–184.
[32] A.L. Yerokhin, V.V. Lyubimov, R.V. Ashitkof, Ceram. Int. 24 (1998) 1–6.
[33] J.H. Wang, J. Wang, Y. Lu, M.H. Du, Z. Han, Appl. Surf. Sci. 324 (2015)
405–413.
[34] S. Aliasghari, P. Skeldon, G.E. Thompson, Appl. Surf. Sci. 316 (2014) 463–476.
[35] B. Gao, T.M. Lim, D.P. Subagio, T.T. Lim, Appl. Catal. A: Gen. 375 (2010) 107–115.
[36] E.A. Kozlova, T.P. Korobkina, A.V. Vorontsov, V.N. Parmon, Appl. Catal. A: Gen.
367 (2009) 130–137.
[37] R. Sasikala, A.R. Shirole, V. Sudarsan, V. Jagannath, C. Sudakar, R. Naik, R. Rao,
S.R. Bharadwaj, Appl. Catal. A: Gen. 377 (2010) 47–54.
[38] M.M. Joshi, N.K. Labhsetwar, P.A. Mangrulkar, S.N. Tijare, S.P. Kamble, S.S. Ray-
alu, Appl. Catal. A: Gen. 357 (2009) 26–33.
[39] S. Higashimoto, W. Tanihata, Y. Nakagawa, M. Azuma, H. Ohue, Y. Sakata, Appl.
Catal. A: Gen. 340 (2008) 98–104.
[40] S. Song, J. Tu, Z. He, F. Hong, W. Liu, J. Chen, Appl. Catal. A: Gen. 378 (2010)
169–174.
[41] J. Niu, B. Yao, Y. Chen, C. Peng, X. Yu, J. Zhang, G. Bai, Appl. Surf. Sci. 271 (2013)
39–44.
[42] C. Quinones, W. Vallejo, J. Ayala, Appl. Surf. Sci. 257 (2010) 367–371.