Enhancement of visible light photocatalytic activity of Ag2O/F-TiO2 composites

Article abstract of DOI:10.1016/j.molcata.2015.06.015

We synthesized the Ag₂O/F-TiO₂ composites via an aqueous precipitation method and characterized their morphology, structure and photocatalytic activity in the degradation of methylene blue (MB) using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis absorption spectroscopy, and electrochemical impedance spectra, respectively. The results show that the Ag₂O addition could enhance the visible light photocatalytic activity of Ag₂O/F-TiO₂ composites with a maximum MB degradation rate of 93% for 60 min, much higher than that of the F-TiO₂ (30%). The excellent photocatalytic activity is due to the increased visible light absorption, and promoted separation of photo-generated electrons and holes in F-TiO₂ with the introduction of Ag₂O.

Full text of DOI:10.1016/j.molcata.2015.06.015

Journal of Molecular Catalysis A: Chemical 407 (2015) 25–31
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Enhancement of visible light photocatalytic activity of Ag O/F-TiO
2
2
composites
a
b,∗
b
c
a
a
Wei Yu , Xinjuan Liu , Haipeng Chu , Guang Zhu , Jinliang Li , Junying Liu ,
b
a
a,∗∗
Lengyuan Niu , Zhuo Sun , Likun Pan
Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Department of Physics, East China Normal University,
Shanghai 200062, China
Institute of Coordination Bond Metrology and Engineering, College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China
Anhui Key Laboratory of Spin Electron and Nanomaterials, Suzhou University, Suzhou 234000, China
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
We synthesized the Ag O/F-TiO composites via an aqueous precipitation method and characterized
2
2
Received 17 March 2015
Received in revised form 5 June 2015
Accepted 10 June 2015
their morphology, structure and photocatalytic activity in the degradation of methylene blue (MB) using
scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron
spectroscopy, UV–vis absorption spectroscopy, and electrochemical impedance spectra, respectively. The
results show that the Ag2O addition could enhance the visible light photocatalytic activity of Ag2O/F-TiO2
composites with a maximum MB degradation rate of 93% for 60 min, much higher than that of the F-TiO2
Available online 16 June 2015
Keywords:
Ag2O/F-TiO2
Coprecipitation method
Photocatalytic
(30%). The excellent photocatalytic activity is due to the increased visible light absorption, and promoted
separation of photo-generated electrons and holes in F-TiO2 with the introduction of Ag2O.
©
2015 Elsevier B.V. All rights reserved.
Methylene blue (MB)
1
. Introduction
mentioned problems. TiN/, Bi2O3/ and CuO/F-TiO composites have
2
been found to exhibit excellent visible light photocatalytic activity
Photocatalysis has already attracted significant interest world-
compared with F-TiO2 [29–31]. Ag O with a narrow energy band
2
wide, due to its potential applications in many areas including clean
energy production [1–4], environment remediation and chemi-
cal reaction technologies [5–19]. Exploring visible light-responsive
photocatalysts with good photocatalytic performance is one of the
core issues for photocatalysis. Currently, doping with nonmetal ele-
ments such as N, C, F and B was demonstrated to be an effective
approach to enhance the visible light photocatalytic activity of wide
semiconductor [20,21]. In particular, F doped TiO (F-TiO ) has been
gap of ∼1.2 eV has been found to be an efficient visible light photo-
catalyst [32–35]. Ag O-based composites have been demonstrated
2
with the excellent visible light photocatalytic activity owing to the
synergistic effects on promoting the generation and separation of
the photo-generated electron–hole pairs [34–38]. Therefore, devel-
opment of composite material based on Ag O and F-TiO2 should
2
be a promising method to enhance the visible light photocatalytic
activity of F-TiO2 because of the stepwise structure of band-edge
2
2
proven to be a promising visible light photocatalyst [22–26]. The F
doping in TiO2 can increase the visible light absorption and reduce
electron–hole pair recombination by creating of oxygen vacancies
levels constructed in the Ag O/F-TiO2 (AT) composites (Scheme 1).
2
The conduction band (CB) of TiO2 is −0.34 V and valence band
(VB) 2.86 V (vs. SHE) [39,40], while the corresponding values of
3
+
and Ti surface states in TiO2 [27,28], leading to the enhancement
Ag O are 0.20 and 1.40 V (vs. SHE), respectively [35,36,41]. Such
2
of photocatalytic activity of TiO . However, the quick recombina-
energy levels are beneficial to the transfer of photo-induced elec-
2
tion of photo-generated charge carriers still exists in F-TiO2 and
significantly decreases its photocatalytic activity.
Currently, a particularly attractive strategy is to design and
develop hybrid materials based on F-TiO2 to overcome the afore-
trons from F-TiO2 CB to Ag O CB, which could efficiently separate
2
the photo-induced electrons and hinder the charge recombination
in the electron-transfer processes, thus enhance the photocatalytic
performance. Unfortunately, there has been no report on the syn-
thesis and application of AT composites in photocatalysis to date.
In this work, AT composites were synthesized via an aqueous
precipitation method. The as-synthesized AT composites exhibit an
enhanced photocatalytic activity in the degradation of methylene
blue (MB) under visible light irradiation compared with the F-TiO2.
∗
Corresponding author. Fax: +86 57186872475.
Corresponding author. Fax: +86 21 62234132.
∗
∗
E-mail addresses: lxj669635@126.com (X. Liu), lkpan@phy.ecnu.edu.cn (L. Pan).
http://dx.doi.org/10.1016/j.molcata.2015.06.015
381-1169/© 2015 Elsevier B.V. All rights reserved.
1

2
6
W. Yu et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 25–31
Table 1
Atomic ratios of Ag and F in T1, T2 and T3.
Samples
Ag (at.%)
F (at.%)
T1
T2
T3
7.23
11.42
14.59
2.33
2.19
2.12
UV–vis spectrophotometer equipped with an integrated sphere
attachment. Photoluminescence (PL) spectra at room tempera-
ture were examined by fluorescence spectrophotometer (HORIBA
Jobin Yvon fluoromax-4). X-ray photoelectron spectroscopy (XPS)
measurement was performed on an Imaging Photoelectron Spec-
trometer (Axis Ultra, Kratos Analytical Ltd.) with a monochromatic
Al K␣ X-ray source. EIS measurements were carried out on an
electrochemical workstation (AUTOLAB PGSTAT302N) under dark
conditions using a three electrode configuration with the as-
prepared films as working electrode, a Pt foil as counter electrode
and a standard calomel electrode as reference electrode. The elec-
Scheme 1. Schematic diagram of energy levels of F-TiO2 and Ag2O.
The possible photocatalytic mechanism for the photo-degradation
process over AT composites was also studied in terms of a series of
characterization.
−
1
trolyte was 10 mg l MB aqueous solution. EIS were recorded in
the frequency range of 0.1 Hz–1 MHz, and the applied bias volt-
age and ac amplitude were set at open-circuit voltage and 10 mV,
respectively.
2
. Experimental
2
.1. Synthesis of F-TiO2
F-TiO was prepared by a sol–gel method [26]. 8 ml of tetrabutyl
2.4. Photocatalytic experiments
2
titanate was first dissolved in 15 ml of ethanol by stirring for 30 min
at room temperature to obtain solution A. 390 mg NH4F was dis-
solved in 14 ml ethanol, and then 5 ml acetic acid and 2 ml deionized
water were successively added into the solution with stirring for
The photocatalytic performance of the as-prepared samples was
evaluated by photocatalytic degradation of MB under visible light
irradiation. The samples (120 mg) were dispersed in 80 ml MB solu-
tions (10 mg/l). The mixed suspensions were first magnetically
stirred in the dark for 30 min to reach the adsorption–desorption
equilibrium. Under ambient conditions and stirring, the mixed
suspensions were exposed to visible light irradiation (ꢀ > 400 nm)
produced by a 400 W metal halogen lamp with cut-off filter. At cer-
tain time intervals, 2 ml of the mixed suspensions were extracted
and centrifuged to remove the photocatalyst. The filtrates were
analyzed by recording UV–vis spectra of MB using a Hitachi U-3900
UV–vis spectrophotometer.
3
0 min at room temperature to obtain solution B. Solution B was
then added dropwise into solution A under vigorous stirring. Sub-
sequently, the mixture solution was continuously stirred at room
temperature for the hydrolysis of tetrabutyl titanate until a trans-
◦
parent sol was formed. Finally, the sol was dried in air at 100 C for
◦
2
4 h, ground and heated at 500 C for 1 h.
2
.2. Synthesis of Ag O/F-TiO composite
2
2
A certain amount of AgNO solution, 0.4 g F-TiO powder and 2 g
3
2
polyethylene glycol (average molecular weight 200) were dissolved
in 100 ml water by sonication for 15 min to produce a uniform
dispersion. Then a dilute NaOH solution was dropped in above
solution to adjust the pH value to be 13, and the mixture was
stirred for 30 min. The AT samples synthesized using 0.02, 0.03,
3
. Results and discussion
The FESEM images of F-TiO and Ag O are shown in Fig. 1(a) and
2
2
(b). Both of the F-TiO and Ag O display the particle nanostructure.
2 2
The morphology of Ag O in AT composites (T2) is similar to that
of pure Ag O, and the F-TiO2 nanoparticles are well distributed in
Ag O, as shown in Fig. 1(c). The morphologies of T1 and T3 (not
shown here) are similar to that of T2. The existence of Ag O and
F-TiO2 in the composites is proved from elemental mapping image
by EDS measurement, as shown in Fig. 1(d). Fig. 2 shows the ele-
ment distribution, Ag, Ti and F elemental mapping images of T2.
The distribution of Ag, Ti and F elements indicates that they are
highly dispersed in the composites. Furthermore, the atomic ratios
of Ag and F in T1, T2 and T3 were also characterized by the EDS, as
2
0
.04 mol AgNO3, named as T1, T2 and T3, were isolated by filtration,
2
◦
washed three times with distilled water, and finally dried at 80 C
2
for 4 h. Pure Ag O using 0.02 mol AgNO were also synthesized by
2
3
2
the similar method in the absence of F-TiO . For the electrochemi-
2
cal impedance spectra (EIS) testing, the as-synthesized composites
with 5 wt% cellulose binder were homogenously mixed in terpi-
neol to form a slurry. Then, the resultant slurries were coated on
the FTO using a screen-printing approach. Finally, these prepared
◦
electrodes were dried at 100 C for 30 min.
shown in Table 1. It can be observed that the Ag O content in the
2
2
.3. Characterizations
composites increases with the increase of AgNO3 in the precursor
solution, while the F doping amount remains almost unchanged.
Fig. 3(a) and (b) shows the low-magnification and high-
magnification HRTEM images of T2. It confirms the presence of
The surface morphology, structure and composition of the
samples were characterized by field-emission scanning electron
microscopy (FESEM, Hitachi S-4800), high-resolution transmis-
sion electron microscopy (HRTEM, JEOL-2010), energy dispersive
X-ray spectroscopy (EDS, JEM-2100, X-ray diffraction (XRD, Hol-
land Panalytical PRO PW3040/60) with Cu K␣ radiation (V = 30 kV,
I = 25 mA), and Fourier transform infrared spectroscopy (FTIR,
NICOLET NEXUS 670), respectively. The UV–vis diffuse absorp-
tion spectra of the samples were recorded using a Hitachi U-3900
F-TiO nanoparticles with diameters in the range of 20–50 nm con-
2
tacting with Ag O particles. The lattice fringes with an interplanar
2
distance of 0.24 nm can be assigned to the (2 0 0) plane of Ag O
2
(JCPDS 41-1104). Around the Ag O crystallite edge, fine crystallites
2
are observed. The crystallites connected to the Ag O have lattice
2
fringes of 0.35 nm, which is ascribed to the (1 0 1) plane of anatase
F-TiO2 (JCPDS 21-1272) [26].

W. Yu et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 25–31
27
Fig. 1. FESEM images of (a) F-TiO2, (b) Ag2O and (c) T2; (d) EDS elemental mapping image of T2.
Fig. 2. (a) O, (b) Ag, (c) Ti and (d) F elemental mapping image by EDS measurement of T2.
Fig. 3. (a) Low-magnification and (b) high-magnification HRTEM images of T2.

2
8
W. Yu et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 25–31
Fig. 4. XRD patterns of Ag2O, TiO2, F-TiO2 and T2.
The XRD patterns of Ag O, TiO , F-TiO and T2 are shown in
2
2
2
◦
◦
◦
◦
Fig. 4. The peaks in the Ag O pattern at 26.6 , 32.8 , 38.3 , 54.9 ,
2
◦ ◦
6
5.4 and 68.7 are indexed to (1 1 0), (1 1 1), (2 0 0), (2 2 0), (3 1 1)
and (2 2 2) planes of cubic Ag O (JCPDS 41-1104), respectively [42].
2
The main diffraction peaks of F-TiO2 are similar to those of pure
TiO , which can be indexed to the anatase TiO2 (JCPDS 21-1272).
2
Furthermore, as shown in Fig. 4(b), the F doping causes the shift
of the TiO2 peak to a larger angle. Compared with F-TiO , the new
2
peaks corresponding to cubic phase of Ag O appear in XRD pattern
2
of T2, which further confirms the existence of Ag O in the compos-
2
ite. No other impurity peaks for T2 are observed, indicating the high
purity of the products.
Fig. 5 shows the FTIR spectra of F-TiO , Ag O, T1, T2 and T3. The
2
2
−1
−1
broad absorption bands at 3425 cm and 1661 cm are assigned
2 2
Fig. 5. FTIR spectra of F-TiO , Ag O, T1, T2 and T3.
to the hydroxyl groups of absorbed H O molecules [42,43]. The
2
−
1
absorption peak of Ag O around 500 cm is related to Ag O bond
2
−
1
vibration [44]. The absorption band of AT composites at 650 cm is
spectrum of Ag 3d, displaying the spin-orbit split lines of Ag 3d3/2
and Ag 3d5/2 at 368.2 and 374.2 eV, respectively, which indicates
that the Ag exists in one valence state (Ag ) and further confirms
similar to that of F-TiO , which is owing to the stretching modes of
2
+
Ti O [43]. Compared with the F-TiO , new absorption peak around
2
−
1
5
00 cm
appears in FTIR spectra of AT composites, which also
the existence of Ag O in the composite [45,46]. Fig. 6(b) shows the
2
proves the existence of Ag O in the composite.
high-resolution XPS spectrum of Ti 2p, displaying the spin–orbit
split lines of Ti 2p3/2 and Ti 2p1/2 at 458.7 and 464.6 eV, respectively,
2
The surface chemical composition of T2 was characterized by
XPS measurements. Fig. 6(a) demonstrates the high-resolution XPS
which is in good agreement with that in TiO . As shown in Fig. 6(c),
2
Fig. 6. XPS spectra of (a) Ag 3d, (b) Ti 2p, (c) O 1s, and (d) F 1s for T2.

W. Yu et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 25–31
29
Fig. 9. PL spectra of F-TiO2, T1, T2, and T3.
Fig. 7. UV–vis diffuse absorption spectra of F-TiO2, T1, T2 and T3.
Fig. 9 shows PL spectra of F-TiO , T1, T2, and T3 with the exci-
2
tation wavelength at 400 nm. It can be clearly observed that the
introduction of Ag O markedly decreases the PL intensity, which
2
further confirms that the recombination of photo-induced elec-
trons and holes in F-TiO2 can be effectively inhibited.
Photocatalytic degradation of MB by F-TiO , T1, T2 and T3 was
2
performed under visible light irradiation. Fig. 10(a) shows the
UV–vis absorption spectra of MB with the irradiation time under
visible light irradiation using T2. It is observed that the UV–vis
absorption peak of MB, related to its concentration in the solution,
becomes weak with the increase in the irradiation time. Fig. 10(b)
displays the time-dependent degradation rates of MB by F-TiO2,
Ag O, T1, T2, T3 and without catalyst under visible light irradia-
2
Fig. 8. Nyquist plots of F-TiO2, T1, T2, and T3. Inset is the corresponding equivalent
tion. The normalized temporal concentration changes (C/C0 ) of MB
during the photocatalytic process are proportional to the normal-
ized maximum absorbance (A/A0 ), which can be derived from the
change in the MB absorption profile at a given time interval. It is
observed that the concentration of MB is hardly reduced under vis-
ible light irradiation in the absence of the photocatalyst, and the AT
composites exhibit better photocatalytic activity than F-TiO2 and
circuit model.
the O 1s spectrum of T2 is fitted to two peaks. The peak of O 1s at
5
30 eV corresponds to lattice oxygen of TiO , and a higher binding
2
energy of 531.7 eV is assigned to mixed contributions from surface
hydroxides. Fig. 6(d) gives the high-resolution XPS spectrum of F
1
s. The F 1s peak can be deconvoluted into two peaks with Gaussian
Ag O. The photocatalytic activity of AT composites is dependent
2
distributions, which indicates that two forms of F are present. The
low binding energy of about 684.8 eV could be ascribed to the F
on the proportion of Ag O in the composites. The degradation rate
2
of MB for F-TiO2 and Ag O are 30% and 63% for 60 min, respec-
2
anions physically adsorbed on the surface of TiO , while the high
tively. When Ag O is introduced into F-TiO , the degradation rate
2
2
2
binding energy located at 689.3 eV could be ascribed to the F atoms
substituting for the O atoms, forming the Ti F bonds. The F content
in the T2 is 2.58 at.%, which is almost consistent with the result of
EDS measurement.
is increased to 81% for T1 and reaches a maximum value of 93%
for T2 for 60 min. It is known that during photocatalysis, the light
adsorption, and the charge transportation and separation are cru-
cial factors [50–52]. The enhancement of the photocatalytic activity
should be mainly ascribed to the increase in light absorption and
the reduction in electron–hole pair recombination in F-TiO2 with
The UV–vis diffuse absorption spectra of F-TiO , T1, T2 and T3
2
are shown in Fig. 7. It is observed that the AT composites exhibit
higher absorbance in the visible light region and the intensity
the introduction of Ag O due to the stepwise energy level in the
2
increases with the increase of Ag O content, which is similar to
composites, which have been confirmed by the UV–vis absorption,
EIS and PL measurements. Furthermore, the self-sensitized degra-
dation of the dyes should also play a critical role in the improvement
of photocatalytic performance of AT composites under visible light
irradiation. MB is excited under visible light irradiation to MB*, fol-
2
those reported in the literatures [29,47]. Such an enhancement of
light absorption is beneficial to the increase of photo-generated
electrons and holes [43,48,49].
The charge transfer and recombination behaviors of the samples
were studied by analyzing the EIS spectra under dark conditions.
lowed by photo-induced electrons transferring from MB* to Ag O or
2
Fig. 8 shows the typical Nyquist plots of F-TiO , T1, T2 and T3. The
F-TiO , which react with adsorbed oxidants, usually O , to produce
reactive oxygen radicals (O2 ) [53]. This leads to the enhancement
2
2
2
−
semicircle in the EIS spectra is due to the contribution from the
charge transfer resistance (Rct) and constant phase element (CPE) at
the photocatalyst/electrolyte interface [48,49]. The corresponding
equivalent circuit is shown in the inset of Fig. 8. It is clearly observed
of photocatalytic performance. However, when the Ag O content
2
is further increased, the degradation rate decreases to 87% for
T3 at 60 min, which may be due to its increased recombination
of photo-generated electron–hole pairs [29,47]. Furthermore, the
repeatability of as-prepared samples was also investigated under
visible light irradiation with three times of cycling uses, and the
experimental error bars are shown in Fig. 10(b). It can be observed
that the experimental error is small, showing that the experiment
results are repeatable.
that the Rct decreases with the increase of Ag O content, which
2
indicates that the introduction of Ag O in F-TiO favors the electron
2
2
transfer and suppresses the charge recombination in F-TiO2 due to
the stepwise energy level structure in the composites. However,
when the Ag O content is further increased (T3), the Rct increases,
2
which is possibly because excessive Ag O can act as recombination
2
centers instead of providing an electron pathway and promote the
recombination of electron–hole pairs [29].
The values of rate constants (Kapp), as shown in Fig. 11, can be
obtained directly from the fitted straight-line plots of ln (C /Ct)
0

3
0
W. Yu et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 25–31
Fig. 10. (a) UV–vis absorbance of MB with the variation of visible light irradiation time using T2, (b) photocatalytic degradation of MB by F-TiO2, Ag2O, T1, T2, T3 and without
catalyst under visible light irradiation.
4. Conclusion
AT composites were successfully synthesized via an aqueous
precipitation method and their photocatalytic activities under vis-
ible light irradiation were investigated. Results indicate that (1)
AT composites exhibit an enhanced photocatalytic activity than F-
TiO ; (2) the photocatalytic activity of AT composites is dependent
2
on the proportion of Ag O in the composites and the T2 achieves
2
a highest MB degradation rate of 93% for 60 min; (3) the enhanced
photocatalytic activity is mainly ascribed to the increase in light
adsorption and the reduction in photoelectron–hole pair recombi-
nation with the introduction of Ag O.
2
Fig. 11. Reaction constant Kapp for MB degradation by F-TiO2, T1, T2 and T3 using
pseudo-first order kinetic equation.
Acknowledgements
Financial support from the National Natural Science Foundation
of China (No. 21401180), Provincial Natural Science Foundation of
Anhui (1508085ME104), and Scientific Research Foundation for the
Anhui Provincial Returned Overseas Chinese Scholars (Electrospun
low dimensional TiO2 nano-structures and application in photo-
voltaic devices) is gratefully acknowledged.
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