Post on 13-May-2023
Enhanced photocatalytic activity of TiO2 nano-structured
thin film with a silver hierarchical configuration
Jinyu Zheng a,b, Hua Yu c, Xinjun Li a,*, Shanqing Zhang c,**a Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China
b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR Chinac Australian Rivers Institute and Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australia
Received 14 June 2007; received in revised form 10 July 2007; accepted 10 July 2007
Available online 28 July 2007
Abstract
TiO2 sol–gels with various Ag/TiO2 molar ratios from 0 to 0.9% were used to fabricate silver-modified nano-structured TiO2 thin films using a
layer-by-layer dip-coating (LLDC) technique. This technique allows obtaining TiO2 nano-structured thin films with a silver hierarchical
configuration. The coating of pure TiO2 sol–gel and Ag-modified sol–gel was marked as T and A, respectively. According to the coating order and
the nature of the TiO2 sol–gel, four types of the TiO2 thin films were constructed, and marked as AT (bottom layer was Ag modified, surface layer
was pure TiO2), TA (bottom layer was pure TiO2, surface layer was Ag modified), TT (pure TiO2 thin film) and AA (TiO2 thin film was uniformly
Ag modified). These thin films were characterized by means of linear sweep voltammetry (LSV), X-ray diffraction (XRD), scanning electron
microscopy (SEM), electrochemical impedance spectroscopy and transient photocurrent (Iph). LSV confirmed the existence of Ag0 state in the
TiO2 thin film. SEM and XRD experiments indicated that the sizes of the TiO2 nanoparticles of the resulting films were in the order of
TT >AT > TA > AA, suggesting the gradient Ag distribution in the films. The SEM and XRD results also confirmed that Ag had an inhibition
effect on the size growth of anatase nanoparticles. Photocatalytic activities of the resulting thin films were also evaluated in the photocatalytic
degradation process of methyl orange. The preliminary results demonstrated the sequence of the photocatalytic activity of the resulting films was
AT > TA > AA >TT. This suggested that the silver hierarchical configuration can be used to improve the photocatalytic activity of TiO2 thin film.
# 2007 Elsevier B.V. All rights reserved.
www.elsevier.com/locate/apsusc
Applied Surface Science 254 (2008) 1630–1635
Keywords: Photocatalysis; TiO2 thin film; Ag loading
1. Introduction
Substantial effort has been devoted to study the photo-
catalytic degradation of organic pollutants using nano-
structured TiO2 catalyst due to its high oxidation efficiency,
outstanding chemical stability and environmentally friendly
nature [1]. However, the efficiency of the photocatalytic
degradation reaction is limited by the high recombination rate
of photoinduced electrons and holes. Much effort has been
contributed to improve the photocatalytic efficiency of TiO2 by
ion dispersion and metal loading [2–9].
* Corresponding author. Tel.: +86 20 87057781; fax: +86 20 87057677.
** Corresponding author. Tel.: +61 7 5552 8155; fax: +61 7 5552 8067.
E-mail addresses: lixj@ms.giec.ac.cn (X. Li), s.zhang@griffith.edu.au
(S. Zhang).
0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2007.07.165
The sol–gel method is one of the popular techniques used for
the fabrication of TiO2 thin film modified by metal ions, which
were uniformly modified in the thin film in most of studies. In
recent years, we have developed a novel method using layer-by-
layer dip-coating (LLDC) processes for the fabrication of TiO2
thin film. In our previous works, metal ions, such as Mo6+ [10]
and Mn4+ [4] that can replace Ti4+ in the lattice of TiO2 due to
their similar ion radius, have been selected to modify TiO2 thin
film. The preliminary result suggested that the layer-by-layer
process (previously referred to as ‘‘uneven doping’’) was able to
improve the photocatalytic activity significantly compared with
the traditional process (previously referred to as ‘‘even doping’’).
Ag is one of the most promising transition metals for the
improvement of photocatalytic activity of TiO2 thin film [11]
and has been investigated extensively in the literature [12–16].
In this paper, we prepared the TiO2 thin films loaded with Ag
using the LLDC sol–gel process. The effects of various loading
modes and contents on the separation efficiency of photo-
J. Zheng et al. / Applied Surface Science 254 (2008) 1630–1635 1631
generated carriers as well as the mechanism of photocatalytic
activity enhancement were investigated.
2. Experimental
2.1. Preparation of TiO2 sol and Ag–TiO2 sol
Precursor solutions for TiO2 thin films were prepared at
room temperature by the method illustrated in our previous
work [10]. Briefly, 68 mL of tetrabutyl titanate and 16.5 mL of
diethanolamine were dissolved in 210 mL absolute ethanol,
and then the mixture was stirred vigorously for 1 h at room
temperature (solution A). A mixture of 3.6 mL H2O and
100 mL ethanol (solution B) was added dropwise into the
solution A under stirring. The resulting solution, an alkoxide
solution, aged for 24 h at room temperature will result in a
TiO2 sol.
The preparation of Ag–TiO2 sol was similar to that of TiO2
sol. The only difference was that various content of AgNO3
(AR grade) were added into solution B before it was added into
solution A. The Ag–TiO2 sols containing Ag loading content of
0.1, 0.3, 0.5 and 0.9% were prepared. The percentage of the Ag
concentration indicated, in this paper, refers to the molar
percentage to TiO2.
2.2. Preparation of thin films
Firstly, soda lime glass slices (200 mm � 35 mm � 2 mm)
were pretreated using acid solution and distilled water in an
ultrasonic bath before being coated with a SiO2 layer. Secondly,
the glass slices were dipped into TiO2 sol or Ag–TiO2 sol and
withdrawn at the speed of 2 mm s�1. They were allowed to dry
and then baked at 100 8C for 10 min. The second process was
repeated according to Table 1 in order to obtain a hierarchical
TiO2 structure. In this paper, this technique is defined as layer-
by-layer dip-coating (LLDC). Finally, the glass slices with
various TiO2 hierarchical structures were annealed at 500 8Cfor 2 h in an electric muffle furnace to assure the mechanical
strength of the TiO2 film. The coatings of pure TiO2 sol–gel and
Ag-modified sol–gel were marked as T and A, respectively.
According to the coating order and the nature of the TiO2 sol–
gel, four types of the TiO2 thin films were constructed and
marked as AT (bottom layer was Ag modified, surface layer was
pure TiO2), TA (bottom layer was pure TiO2, surface layer was
Ag modified), TT (pure TiO2 thin film) and AA (uniformly Ag
modified TiO2 thin film).
Table 1
Preparation of TiO2 thin films using the layer-by-layer dip-coating technique
Coating mode Thin film composition
Bottom Surface
TT Four coats of TiO2 sol Four coats of TiO2 sol
TA Four coats of TiO2 sol Four coats of Ag–TiO2 sol
AA Four coats of Ag–TiO2 sol Four coats of Ag–TiO2 sol
AT Four coats of Ag–TiO2 sol Four coats of TiO2 sol
2.3. Characterization
The thickness of TiO2 thin films was measured by scanning
electron microscopy (SEM) at 240 nm which was consistent
with our previous work [10]. The crystalline phase of the TiO2
thin films was determined by XRD using a diffractometer (D/
MAX-IIIA) with Cu Ka radiation. The transmittance spectra
of thin films were obtained using a UV–vis spectrophotometer
(U-3010).
2.4. Electrochemical experiments
2.4.1. Preparation of thin film electrode
The four types of the TiO2 thin films (i.e., AT, TA, AA and TT)
were coated on conducting substrates (ITO glasses) using the
same protocol described in Section 2.2, except without the
coating of SiO2 layer. ITO glasses were used in LSV, photo-
electrochemical current (Iph) and ESI Nyquist measurements.
2.4.2. Electrochemical measurement
Electrochemical characterization of LSV plots, and Iph and
ESI Nyquist plots were performed in a three-electrode system
made of quartz cells linked with CHI660 electrochemical
station. A TiO2 thin film electrode, a platinum sheet
(20 mm � 30 mm) and an Ag/AgCl electrode served as the
working electrode, counter electrode and reference electrode,
respectively. A 0.5 mol L�1 Na2SO4 aqueous solution was
employed as a supporting electrolyte. All the photoelectro-
chemical tests were carried out under the illumination of a
mercury lamp (5 W, lp = 365 nm) at room temperature.
2.5. Evaluation of photocatalytic activity
The photocatalytic degradation of methyl orange was carried
out in a home-built reactor. A high-pressure mercury lamp
(125 W, lp = 365 nm) was used as the light source. In each run
five glass slices of TiO2 thin film were placed into a 10 mg L�1
methyl orange solution in the reactor. After bubbling for 30 min
in order to obtain absorption equilibrium on the surface of the
thin films, the illumination was applied to initiate the reaction.
The UV–vis spectrophotometer was used to determine the
concentration of methyl orange before and after photocatalytic
degradation. The degradation percentage of methyl orange in the
reaction process could be calculated by the following formula:
Degradation percentage ¼ C0 � Ct
C0
� 100%
where Ct is the concentration of methyl orange after 80 min
reaction and C0 is the initial concentration.
3. Results and discussion
3.1. Characterizations of the thin film structure
Metallic Ag nanoparticles, formed on the surface of the Ag-
modified TiO2 films from Ag+-containing TiO2 sol–gel, can be
observed under SEM [17,18]. Fig. 1 shows the SEM images of
Fig. 1. SEM images of the TiO2 thin films modified with 0.5% Ag content (a, b
and c correspond to AA, TA and AT, respectively) and TT (d) thin films.
J. Zheng et al. / Applied Surface Science 254 (2008) 1630–16351632
the various TiO2 thin films annealed at 500 8C for 2 h. The Ag/
TiO2 ratio of the Ag-loaded TiO2 sol was 0.5%. The SEM
indicated that the average particle size of the thin films is
approximately 20–30 nm. More importantly, it can be observed
that the average particle size decreased according to the
sequence of AA <TA < AT < TT. The XRD patterns of these
TiO2 films (as shown in Fig. 2) demonstrated that the films
consist of pure anatase with the average crystallite size of the
TiO2 films being ca. 17, 18, 21 and 28 nm for AA, TA, AT and
TT, respectively, calculated using the Scherrer equation [19]
which was consistent with the SEM results. On the basis of the
SEM observation and XRD calculation results, we can
conclude that Ag loading in TiO2 thin film is deemed to
inhibit the growth of the average particle size [18] and the
extend of the inhibition is dependent on the amount of Ag
present in the TiO2 thin films. This is in line with the reported
Fig. 2. The XRD patterns of the Ag-modified TiO2 thin films with 0.5% Ag
content and TT thin film under UV illumination.
XRD investigation [17]. Ag+ ions introduced by the sol–gel
process cannot get into the lattice of the TiO2 because the radius
of Ag+ ions (ca. 126 pm) is much larger than that of Ti4+ (ca.
68 pm) [11]. The Ag+ ions will, therefore, spread evenly on
TiO2 nanoparticles during the dip-coating process. In other
words, the TiO2 nanoparticles will be ‘‘wrapped’’ and separated
by the Ag+ ions. During the sintering process, the Ag element
here became a physical barrier to confine the TiO2
nanoparticles. Hence the energy necessary for the movement
of the grain boundary for the merging between TiO2
nanoparticles increased and this consequently led to the
inhibition of the TiO2 particle size growth. Apparently, the
extent of the inhibition is dependent on how well the TiO2
particles are separated, which relies on the amount of Ag+
present in the films. This size inhibition effect can be used to
explain the sequence of the average TiO2 particle size shown in
the SEM. The reason the AA film had the smallest size, is that it
had eight coats of Ag+ loaded on the TiO2 sol. As the TT film
does not have any Ag+ ions at all, it’s free from the size
inhibition effect and subsequently has the largest TiO2
particles. The AT film and TA film contain the same amount
of Ag and TiO2, which should lead to the same size of TiO2.
However, a very minor difference between the AT film and TA
film can be observed if we compare the two images carefully. In
particular, the particle size of AT film was slightly bigger than
that of the TA film. This occurred because the SEM image
shows the surface structure rather than the size of the whole
film, and hence SEM image does not reflect the composition of
the whole film. In this case, although the same amount of Ag
was distributed unevenly in AT and TA films, Ag was
distributed with a concentration gradient in these two films.
Compared with TA film, AT film has a lower Ag concentration
at the surface because of the last four coatings of pure TiO2 sol.
Therefore, AT has a larger particle size than TA film. Compared
with TT film, AT film has a higher Ag concentration at the
surface because of the Ag diffusion to the surface from the
bottom layer. Consequently, AT has a smaller size than TT film.
In general, the average particle size sequence of the TiO2 films
(AA <TA < AT < TT) is opposite to the order of Ag amount
on the surface of the film (AA >TA > AT > TT). This
suggests that the Ag distribution in the thin film was in a
fashion of vertically gradient rather than in a way of perfect
layer form.
3.2. Electrochemical characterization
Electrochemical tools are powerful and effective tools for
the characterization of TiO2 thin film [20,21]. Immobilization
of TiO2 onto conducting substrates, such as Ti sheet and ITO
glass, makes the application of the electrochemical techniques
(such as LSVand chronoamperometry) possible to the resulting
thin films.
LSV was firstly conducted to study the redox properties of
the TiO2 films. Fig. 3a shows the LSV voltammograms of the
various TiO2 thin films described in Fig. 1. The experiments
were carried out at a scanning rate of 2 mV s�1 without UV
illumination. Distinctive oxidation peaks in the range of 0.175–
Fig. 3. LSV voltammograms of the TiO2 thin films in (a) 0.5 M Na2SO4
aqueous solution in the dark; (b) 0.5 M Na2SO4 aqueous solution under UV
illumination and (c) 0.5 M Na2SO4 aqueous solution under illumination with
0.03 mM ethanol.
Fig. 4. The photocurrent of AT thin films with varied Ag content and the TT
thin film under zero bias.
J. Zheng et al. / Applied Surface Science 254 (2008) 1630–1635 1633
0.25 V of the Ag-loaded thin films (AT, TA, AA) suggests the
existence of metal Ag with its valence state being Ag0.
Electrochemical behavior of Ag/TiO2 composite surfaces
suggested that Ag can be oxidized electrochemically [22].
Fig. 3b shows LSV voltammogram of the TiO2 thin films under
UV illumination. Under UV illumination, the current remark-
ably increased compared to the LSV voltammograms without
UV illumination (see Fig. 3a). Distinctive oxidation peaks were
observed in the range of 0.175–0.25 V, within the same
potential range as in Fig. 3a. This again indicated that the
oxidation state of the Ag element in the films was 0, i.e., Ag0.
This conclusion is in line with the result that Ag+ in the TiO2
thin film was converted into Ag0 atom or cluster after annealing
at 500 8C in the literature [18], which was confirmed by XPS.
In Fig. 3b, the current for all the TiO2 films was enhanced by
the UV illumination. The photocurrents can be used to represent
the photocatalytic activity of the TiO2 films over the oxidation
of water. The profiles of TA and AT thin films are similar, and
their photocurrents were much higher than that of AA and TT
thin films, under the same experimental conditions. The
photocatalytic activity sequence for the TiO2 films over the
oxidation of water were AT > TA > AA >TT.
The current was further enhanced with the addition of
0.03 mM ethanol into the supporting electrolyte (see Fig. 3c).
Organic compounds are generally more effective photohole
scavengers than water, i.e., easier to be oxidized than water at
the TiO2 electrodes [20]. The enhanced current was due to more
electrons being captured from ethanol besides water. The
photocatalytic activity sequence for the TiO2 films over the
oxidation of ethanol were AT > TA > AA >TT, which is
consistent with the results in Fig. 3b.
Fig. 4 shows the photocurrent profiles of the AT with
different Ag content under the same potential bias (0 mV versus
Ag/AgCl). The photocurrent increased gradually with the
increase of Ag loading from 0.1 to 0.5%, but then descended
sharply at 0.9% content. The optimal content of AT thin films
was 0.5%. The photocurrent response reflects the amount of
free carriers in the catalyst. The results indicate that the Ag
loading content has an effect on the amount of free carriers in
the catalyst, which in turn affect the photocatalytic activities of
TiO2 thin films.
3.3. Evaluation of photocatalytic activity
Fig. 5 shows the effect of the LLDC modes and Ag loading
percentage of the thin films on the photocatalytic degradation of
methyl orange aqueous solutions under UV illumination. For
the AT and TA thin films, the photocatalytic activities were
Fig. 5. The dependence of photocatalytic degradation for aqueous methyl
orange on loading modes and loading content of the thin films.
J. Zheng et al. / Applied Surface Science 254 (2008) 1630–16351634
improved with the initial increase in Ag loading percentage.
The maximum degradation percentage was acquired at 0.5%.
Afterward, the photocatalytic activities decreased with the
increasing Ag loading percentage. For the AA thin film, the
photocatalytic activity was marginally better than that for TT,
with the optimal loading content being 0.3%. Overall, at the
optimal Ag loading percentage, the photocatalytic activities for
the investigated films are in the order of AT > TA > AA >TT.
This can be explained by the experimental results from the
characterization of the investigated TiO2 films.
In TiO2 photocatalysis, the surface area plays an important
role on the photocatalytic activity. A larger crystalline size will
lead to a smaller specific photoactive area, and therefore, lower
photocatalytic activity, vice versa. The fact that TT had the
lowest photocatalytic activity among all the thin films is
consistent with the observation from SEM, where TT had the
largest TiO2 particle size. If the particle size effect is the
dominant factor for the photocatalytic activity, the size
sequence of AT > TA > AA resulted form the Ag size
inhibition effect should have resulted in the photocatalytic
activity order of AT < TA < AA. This is opposite to the
experimental observation as the photocatalytic activity order
was reversed being AT > TA > AA. This conflict suggested
that the size variable was not the determining factor.
The dominant factor responsible for the photocatalytic
activity order of AT > TA > AA >TT may be the formation
of Schottky barriers between TiO2 and Ag0. As aforementioned,
the radius of the Ag+ and Ag atom is much larger than the Ti4+, so
very unlikely for Ag+ and Ag atom to enter the TiO2 lattice. In
fact, the Ag atoms were in direct contact with the TiO2
nanoparticles only. Because the work function of the metal Ag is
higher than that of TiO2, electrons are removed from the TiO2
particles to the vicinity of the Ag particle. This results in the
formation of Schottky barriers at the Ag–TiO2 contact region and
results in charge separation [23]. More specifically, Schottky
barriers facilitate the electron transfer from TiO2 nanoparticles
(with high Fermi level) to Ag (with low Fermi level), resulting in
higher transferring efficiency of electrons [24]. The effect of
Schottky barriers in our particular case was evidenced by the fact
that all the Ag-loaded films (AT, TA and AA) had higher
photocatalytic efficiencies than the pure TiO2 film (TT).
The photocatalytic activity order of AT > TA > AA needs
to be explained from the features brought by the proposed
LLDC technique combined with the Schottky barriers theory.
The use of the proposed LLDC technique will result in the
vertically gradient distribution of Ag in the TiO2 thin film with
the Ag element concentration at the surface being in the order
of AT < TA < AA. This will have double effect on the
photocatalytic activity. Firstly, the Schottky barriers facilitate
the photoelectron movement to a certain direction, from the
semiconductor TiO2 nanoparticles to adjacent Ag atom or
cluster. AT has the highest photocatalytic activity and can be
attributed to the fact that Ag was distributed at the back of the
TiO2 surface layer for the AT film. For the AT thin films, the
Schottky barriers attract the electrons from the surface layer to
the bottom layer, and the holes will be left on the surface layer
of the thin film. This attraction greatly inhibits the recombina-
tion of electron–hole pairs. The enriched holes in the surface
layer can directly degrade methyl orange, at the same time, it
can oxidize H2O adsorbed on the surface and produce hydroxyl
radicals (OH�) that can degrade organic matters effectively.
Secondly, the most efficient UV illumination (in terms of
effective illumination geometric area) was achieved on the AT
film because there was the lowest amount of Ag on the AT film
surface and the Ag particles on the top of TiO2 surface block the
UV light. This was certainly beneficial to higher photocatalytic
activity.
For the TA and AA thin films, the Schottky barriers force the
photoelectrons being transferred from the TiO2 nanoparticles to
the Ag atom or cluster on the surface, which will physically
extend the distance between photoholes and photoelectrons.
Therefore, they have stronger photocatalytic activities than the
TT thin film. Among the AT, TA and AA film, the Ag loaded
amount on the surface was in the order of AT < TA < AA, and
the Ag atom or Ag cluster will block light illumination, with the
efficiency of UV illumination being in the order of
AT > TA > AA. In other words, under the same UV
illumination intensity, the amounts of the resulting photoholes
and photoelectrons were in the order of AT > TA > AA.
The Ag loading content has an important effect on the
photocatalytic activity. Too much Ag loading content may
result in a photohole trapping effect. Due to the fact that the Ag
particles and clusters on the TiO2 nanoparticles are relatively
negative charged due to the formation of the Schottky barriers,
the photoholes in the interfacial region of the TiO2 film may be
trapped by the negatively charged Ag particles and clusters
before they react with water and organics. This trapping effect
was neglectable when the Ag concentration is below 0.5%, and
the Schottky barrier predominantly promoted the TiO2 charge
separation efficiency. The TiO2 photocatalytic activities,
therefore, increased with the increasing Ag concentration.
With the Ag concentration increasing beyond 0.5%, however,
the trapping effect became prevailing and the photocatalytic
activity declined. The decline in the photocatalytic activity can
also be associated with the change of the reaction site density.
Fig. 6. EIS Nyquist plots of the Ag-modified TiO2 thin films with 0.5% Ag
content and TT thin film under UV illumination.
J. Zheng et al. / Applied Surface Science 254 (2008) 1630–1635 1635
As the Ag+ concentration increased beyond 0.5%, the coverage
of Ag on TiO2 surface will increase, in other words, the density
of reaction sites (for adsorption of UV light, produce photohole
and photoelectrons) at the surface of the resulting film would
decrease. Consequently, the TiO2 photocatalysis is depressed
when the Ag+ concentration was greater than 0.5%.
It is well established that of EIS Nyquist plots are associated
with the charge transfer resistance and the separation efficiency
of the photohole–electron pairs [25]. Fig. 6 shows the results of
EIS Nyquist plots of the TiO2 thin film investigated. A larger
circular radius usually has a larger charge transfer resistance,
and therefore, a lower separation efficiency of the photohole–
electron pairs [26,27]. The frequencies for EIS measurement
are scanned from 105 to 0.1 Hz. As shown in Fig. 6, the
curvature radiuses varied with the loading mode following the
below sequence: AT < TA < AA �TT. In other words, the
electron separation efficiency of the TiO2 films was in the order
of AT > TA > AA �TT. These experimental results supported
the conclusions from the photoelectrochemical characterization
and photocatalytic degradation of methyl orange.
4. Conclusions
Four types of the TiO2 thin films (i.e., AT, TA, AA and TT)
were prepared using the LLDC sol–gel technique, and were
characterized using SEM, XRD and electrochemical methods.
The experimental results demonstrated that the AT film has the
higher photocatalytic activity to water and organic compound
oxidation than conventional Ag modified TiO2 thin films (AA)
and pure TiO2 thin film (TT). The sequence of the photocatalytic
activity was AT > TA > AA >TT, which was explained
systematically using the Schottky barrier, the photohole trapping
effect and the reactive reaction site density. This investigation
demonstrated the LLDC coating technique can be used to
construct a tailored hierarchical configuration of TiO2 nano-
structured thin film, which has a significantly higher photo-
catalytic activity over traditional evenly-modified method.
Acknowledgements
The authors are grateful for Griffith University encourage-
ment grant and financial support from Guangzhou Science and
Technology Bureau (No. 2005J1-C0261), China, and Austra-
lian Research Council discovery grant.
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