Advance ternary surface-fluorinated TiO2 nanosheet/Ag3PO4/Ag composite photocatalyst with planar heterojunction and island Ag electron capture center

Article abstract of DOI:10.1039/c4ra08899a

Morphology, phase structure and optical performance of ternary surface-fluorinated TiO₂ nanosheet/Ag₃PO₄/Ag composite photocatalysts were characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-Vis diffuse reflectance spectroscopy (DRS). Wide band-gap TiO₂ crystal nanosheets were synthesized while stabilizing the {001} facet, which is highly active for oxidation reactions, with fluorination. Narrow band-gap Ag₃PO₄ crystal was then uniformly grown on a fluorinated-TiO₂ nanosheet {001} facet, forming a planar heterojunction between two nanosheet semiconductors. Ag particles were in situ reduced by visible light irradiation from Ag₃PO₄, forming island-distributed Ag on the Ag₃PO₄ surface. TiO₂/Ag₃PO₄/Ag exhibited excellent photocatalytic activity and high stability for methylene blue degradation under 410 nm LED light irradiation. The enhanced activities of TiO₂/Ag₃PO₄/Ag could be attributed to effective electron-hole separation, fast hole transportation and the Schottky barrier. This novel ternary TiO₂/Ag₃PO₄/Ag structure material is an ideal candidate in environmental treatment and cleaning applications.

Full text of DOI:10.1039/c4ra08899a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Advance ternary surface-fluorinated TiO2
nanosheet/Ag3PO4/Ag composite photocatalyst
with planar heterojunction and island Ag electron
capture center
Cite this: RSC Adv., 2014, 4, 62751
a
b
c
Kai Dai, Luhua Lu, Changhao Liang, Guangping Zhua and Lei Genga
*
*
Morphology, phase structure and optical performance of ternary surface-fluorinated TiO2 nanosheet/
Ag3PO4/Ag composite photocatalysts were characterized by field emission scanning electron
microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), energy dispersive
spectrometry (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-Vis diffuse
reflectance spectroscopy (DRS). Wide band-gap TiO2 crystal nanosheets were synthesized while
stabilizing the {001} facet, which is highly active for oxidation reactions, with fluorination. Narrow band-
gap Ag3PO4 crystal was then uniformly grown on a fluorinated-TiO2 nanosheet {001} facet, forming a
planar heterojunction between two nanosheet semiconductors. Ag particles were in situ reduced by
visible light irradiation from Ag3PO4, forming island-distributed Ag on the Ag3PO4 surface. TiO2/Ag3PO4/
Ag exhibited excellent photocatalytic activity and high stability for methylene blue degradation under 410
nm LED light irradiation. The enhanced activities of TiO2/Ag3PO4/Ag could be attributed to effective
electron–hole separation, fast hole transportation and the Schottky barrier. This novel ternary TiO2/
Ag3PO4/Ag structure material is an ideal candidate in environmental treatment and cleaning applications.
Received 19th August 2014
Accepted 5th November 2014
DOI: 10.1039/c4ra08899a
www.rsc.org/advances
have shown that the (001) facet is more favorable for oxidation
reactions, and the (101) facet is more suitable for reduction
reactions.16 Li and co-workers have recently proved that the
1. Introduction
Semiconducting nanomaterials have shown signicant value as
photocatalysts for environmental engineering and solar energy
storage applications.1–5 In addition to the selection of semi-
conducting materials with a desired energy band-gap, their
structure design for highly efficient photon-electron conversion
has been found to be very important.6–8 Three basic principles
for structure design of these materials are: high efficiency of
electron–hole pair separation under light irradiation, low
recombination ratio of free electrons and holes and highly
effective light absorption.9–12 Based on these principles, several
typical structures for a high photon–electron conversion rate
have been sequentially proposed and intensively studied in
recent decades.13–15
(010) and (110) facets of single crystal BiVO4 have an electron–
hole separation effect.17 The (010) facet of single crystal BiVO4 is
more favorable for electron location while the (110) facet is
more advantageous for hole location. Then, they have also
experimentally investigated the crystal facets of TiO2 single
crystal and have found a similar electron–hole separation
behavior on the facets of (001) and (101).18 These characters are
in well accordance with their photocatalytic activity. Further
modication of the exposed facet has also been found to be
effective for improving the photocatalytic activity of semicon-
ducting nanomaterials. In our previous work, we have proved
that anatase TiO2 nanosheets with a largely exposed (001) facet
that was stabilized by surface uorination can effectively
enhance the photoelectrical current intensity and photo-
catalytic activity, which indicated the role of surface uorina-
tion on reducing the recombination of electrons and holes.19
In the meantime, the concept of heterojunction was intro-
duced to the design of photocatalysts. The formation of het-
erojunctions within the interface of semiconductors with a
different energy band structure can largely improve the pho-
tocatalytic performance.20,21 A variety of heterostructure pho-
tocatalysts have thus been developed and have shown superior
photocatalytic performance. Changjian Lin and co-workers
The very early structure design of semiconducting nano-
materials has focused on their crystal facet engineering. Ohno
and co-workers have investigated the role of different exposed
anatase TiO2 crystal facets in the process of photocatalysis and
aCollege of Physics and Electronic Information, Huaibei Normal University, Huaibei,
235000, P. R. China
bFaculty of Material Science and Chemical Engineering, China University of
Geosciences, Wuhan, 430074, P. R. China. E-mail: lhlu@cug.edu.cn
cKey Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and
Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science,
Chinese Academy of Sciences, Hefei, 230031, P. R. China. E-mail: chliang@issp.ac.cn
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 62751–62758 | 62751

View Article Online
RSC Advances
Paper
reported a TiO2/Cu2O hybrid with tubular TiO2 core and Cu2O
Based on these research progresses, we have designed a new
particles on its surface. The large amount of heterojunctions semiconducting composited photocatalyst, which is composed
distributed on the tubular walls of TiO2 effectively increased of TiO2, Ag3PO4 and Ag. Wide band-gap TiO2 crystal nanosheets
the electron–hole pair separation and reduced the recombi- with high oxidation reaction activity (001) facet that was stabi-
nation of free electrons and holes.22 Though nanomaterials lized by uorination were initially synthesized. Narrow band-
have a large specic surface area, the heterojunctions formed gap Ag3PO4 crystals were then uniformly grown on uori-
on the point contact regions between nanostructure semi- nated-TiO2 nanosheet {001} facets, forming a planar hetero-
conductors are limited. The heterojunction area of compos- junction between two nanosheet semiconductors, which favors
ited photocatalysts can be further increased for performance effective electron–hole separation and fast holes transportation
enhancement. In fact, increasing heterojunction area within to the oxidation surface. Finally, Ag particles were in situ
semiconducting composites has already been widely studied reduced by visible light irradiation from Ag3PO4. These
and known to be important for the enhancement of electron– distributed Ag particles on the Ag3PO4 surface, forming
hole pair separation in the eld of solar cells.23–25 Since the Schottky barrier, can prevent the electrons captured on Ag from
nanostructure of semiconducting materials can be well diffusing and recombining with holes. For the synergetic effect
controlled by intensively developed synthetic technologies, through a ne structure design, composited photocatalyst has
tuning the nanostructure of heterojunctions has become shown
a
largely improved visible light photocatalytic
possible. For this reason, we have recently designed a facet performance.
coupled nanosheet hybrid of graphitic-C3N4 nanosheet and
surface-uorinated TiO2 nanosheet and studied the role of
heterojunctions on improving the electron–hole pair separa-
2. Experimental section
2.1 Materials
tion and reducing the recombination of free electrons and
holes in detail, which were proved by photoluminescence
quenching and photoelectrical current enhancement.26 By
taking advantage of the short pathway of holes diffusion to the
surface for oxidation reactions, sufficient facet coupled het-
erojunction area can effectively separate electron–hole pairs
and form a strong built-in electrical eld. Thus, the quasi-two
dimensional nanosheet hybrid has shown largely improved
photocatalytic oxidation of organic molecules for the degra-
dation of pollutants.
Commercial TiO2 (P25, 20% rutile and 80% anatase) was
purchased from Degussa. Methylene blue (MB), dipotassium
hydrogen phosphate (K2HPO4), titanium sulfate (Ti(SO4)2) and
hydrogen uoride (HF) were provided from Shanghai Chemical
Reagent Co. Ltd., China. Silver nitrate (AgNO3) was purchased
from Sinopharm Chemical Reagent Co. Ltd., China. Double
distilled water was used for solution preparation.
In addition to the role of heterojunctions, these semi-
conductor composites also have shown a broadened light
absorption wavelength range in comparison with each constit-
uent semiconducting material,27–30 which can provide more
separated electrons and holes for photocatalysis. However, the
enhancement of light absorption is very limited. A more effec-
tive alternative approach of enhancing light absorption is to
synthesize semiconductors with graphene or noble metals.
Graphene and its derivatives are zero-gap semiconductors, and
they have a very wide light absorption wavelength range from
UV to IR.31,32
2.2 Preparation of TiO2 nanosheet
15 mL of Ti(OC4H9)4 and 3.2 mL HF were initially vigorously
sonicated for 0.5 h to form a mixture. Then, the mixture was
transferred into a 50 mL Teon-lined autoclave and heated at
ꢀ
180 C for 12 h. Aer hydrothermal reaction, the white precip-
itates were harvested by centrifugation and thoroughly washing
with water and ethanol, and then dried in an oven at 80 C for
4 h to obtain TiO2.
ꢀ
Noble metals, such as Ag and Au, act as electron traps to
reduce the recombination of photogenerated electron–hole
2.3 Preparation of TiO2/Ag3PO4/Ag
pairs.33,34 In recent published works, we have shown that both In a typical synthesis, a certain part of TiO2 nanosheet, 0.544 g
graphene and noble metals are very effective in enhancing light K2HPO4 and 2.045 g AgNO3 were added to 40 mL water, and
absorption in the visible light wavelength range, which have then ultrasonicated at 20 ꢀC for 30 min to obtain a uniform
highly improved the catalysis efficiency of composited photo- mixture. Then, the mixture were transferred into a Teon-lined
catalysts. Moreover, the work function of graphene and noble autoclave and subsequently heated at 180 ꢀC for 24 h. Aer
metals are larger than that of semiconductors, and thus the ltering with double distilled water and drying at 60 ꢀC for 6 h,
Schottky barrier formed on their interface further reduces the the TiO2/Ag3PO4 composite photocatalyst was obtained. To
recombination of free electrons and holes.35 However, different investigate the effect of Ag3PO4 content on the photocatalytic
from the increasing area of the heterojunction formed between performance rates of TiO2/Ag3PO4 composites, the as-
two semiconductor materials with different band-gaps for synthesized photocatalysts were labelled as X% TiO2/Ag3PO4,
performance enhancement, too much area covered by graphene where X refers to the weight ratio of TiO2 and Ag3PO4. The
or noble metals reduces the photocatalytic reaction area of the resulting X% TiO2/Ag3PO4 was irradiated by sunlight for 4 h,
composite, and thus the amount of graphene or noble metals is and X% TiO2/Ag3PO4/Ag was obtained. Ag3PO4 was also
normally lower than 5%.36,37
prepared using the above mentioned method without TiO2.
62752 | RSC Adv., 2014, 4, 62751–62758
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
2.4 Analytical and testing instruments
The structure of TiO2/Ag3PO4/Ag heterostructures was observed
by Q FEI SIRION 200 eld emission scanning electron micros-
copy (FESEM) with an accelerating voltage of 10 kV and JEOL
JEM-2010F high resolution transmission electron microscopy
(HRTEM) with an INCA x-act energy dispersive spectrometer
(EDS) at an accelerating voltage of 200 kV. The X-ray photo-
electron spectra (XPS) of the catalyst were measured using a
Thermo ESCALAB 250 with an Al Ka X-ray photoelectron spec-
trometer at 150 W. X-ray diffraction (XRD) data for the photo-
Fig. 1 Schematic diagram of the formation of the TiO2/Ag3PO4/Ag
composite.
adsorption of Ag+ cations. When TiO2 nanosheets are heated in
the mixture solution, Ag3PO4 grown on the TiO2 substrates
under high pressure may be simplied as follows:
catalysts were acquired using
a Rigaku D/MAX 24000
diffractometer at room temperature with Cu Ka radiation (l ¼
ꢀ
ꢀ
˚
1.5406 A) in the 2q range from 10 to 70 , operated at 40 kV and
30 mA, and a scanning speed of 0.02ꢀ sꢁ1. The Brunauer–
Emmett–Teller (BET) specic surface area values were deter-
mined using nitrogen adsorption data at 77 K obtained by a
Micromeritics ASAP 2010 system with the multipoint BET
method. UV-Vis diffuse reectance spectroscopy (DRS)
measurements were carried out using a Hitachi UV-3600 UV-Vis
spectrophotometer equipped with an integrating sphere
attachment. The analysis range was from 200 to 800 nm, and
BaSO4 was used as a reectance standard. Photoluminescence
(PL) spectra of Ag3PO4-based photocatalysts were measured by
FLS920 combined uorescence lifetime and steady state spec-
trometry using a 450 W Xe lamp as an irradiating light source.
3AgNO3 + K2HPO4 / Ag3PO4 + HNO3 + 2KNO3
(3)
4Ag3PO4 + 6H2O + 12h+ + 12eꢁ / 12Ag + 4H3PO4 + 3O2 (4)
As indicated in stage (II), initial Ag3PO4 nuclei were formed
by the reaction of AgNO3 and K2HPO4, and then Ag3PO4 nano-
sheets grew directly on Ag3PO4 nuclei. At stage (III), the Ag+ ions
on the outer surface of the Ag3PO4 nanosheets are reduced by
sunlight irradiation when Ag3PO4 sheet oxidizes water auto-
matically,39 and then a certain amount of Ag nanoparticles are
in situ produced naturally on the surface of Ag3PO4
nanoparticles.
2.5 Photocatalytic experiments
Fig. 2 shows the XRD patterns of the Ag3PO4, TiO2 and TiO2/
Ag3PO4/Ag hybrids. The very strong diffraction angles at 2q ¼
25.34ꢀ, 37.82ꢀ, 48.08ꢀ, 53.94ꢀ, 55.10ꢀ, 62.78ꢀ and 68.80ꢀ can be
assigned to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4) and (1 1 6)
crystal planes, respectively, of pure TiO2 with the anatase phase,
and the lines match well with the reported values (JCPDS no. 21-
1272). The weak peaks at 38.16ꢀ can be assigned to (111)
reections of the cubic metal Ag (JCPDS no. 65-2871). For
Ag3PO4, the very strong diffraction angles at 2q ¼ 20.90ꢀ, 29.72ꢀ,
33.34ꢀ, 36.62ꢀ, 42.46ꢀ, 47.84ꢀ, 52.74ꢀ, 55.42ꢀ, 57.66ꢀ and 61.88ꢀ
can be assigned to (1 1 0), (2 0 0), (2 1 0), (2 1 1), (2 2 0), (3 1 0),
(2 2 2), (3 2 0), (3 2 1) and (4 0 0), respectively, bodycentered
cubic phase of Ag3PO4 (JCPDS no. 84-0510). These results
In all the photocatalytic experiments, 0.10 g of as-prepared
catalysts were dispersed in a glass reactor system containing
100 mL 10 mg Lꢁ1 methylene blue (MB) solution under
magnetic stirring. 50 W 410 nm LED arrays light was used as a
light source. The distance between irradiation light was 5 cm.
The concentration of MB was tested by the absorbance, and the
degradation efficiency was calculated as follows:38
C0 ꢁ C
h ¼
ꢂ 100%
(1)
C0
where, C0 is the absorbance of original MB solution, and C is the
absorbance of the dye solution aer LED light irradiation.
According to the Langmuir–Hinshelwood kinetics model,
the photocatalytic process of dyes can be expressed as the
following apparent pseudo-rst-order kinetics equation:
C0
ln
¼ kappt
(2)
C
where, kapp is the apparent pseudo-rst-order rate constant, C0
is the original MB concentration and C is MB concentration in
aqueous solution at time t.
3. Results and discussion
As illustrated in Fig. 1, the formation of as-prepared TiO2/
Ag3PO4/Ag composited samples can be divided into three
stages. At stage (I), dispersed TiO2 nanosheets are synthesized
via a solvothermal reaction in the presence of HF, and then Fꢁ is
Fig. 2 XRD patterns of (a) Ag, (b) Ag3PO4, (c) 20% TiO2/Ag3PO4/Ag, (d)
coated on the surface of TiO2 nanosheets, which favors the 40% TiO2/Ag3PO4/Ag, (e) 60% TiO2/Ag3PO4/Ag and (f) TiO2.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 62751–62758 | 62753

View Article Online
RSC Advances
Paper
indicate that TiO2/Ag3PO4/Ag composites exhibit the same
Ag3PO4 crystalline integrity based on the present synthetic
technology.
Fig. 3a and b show the SEM and TEM images of 40% TiO2/
Ag3PO4/Ag. Moreover, it is clear that TiO2 nanosheets are
modied by Ag3PO4 nanoparticles, which display a good
combination between TiO2 and Ag3PO4. EDS indicates the
existence of Ti, O, Ag, P and F in Fig. 3c. Fig. 3d–h show the
elemental mapping images of a relatively homogeneous distri-
bution of Ag3PO4 sheets on the outer surface of F–TiO2 nano-
sheets, and Ag nanoparticles in situ deposited on the TiO2/
Ag3PO4 nanosheets.
Fig. 4a shows the XPS spectra, including the signals for Ag3d,
P2p, C1s, Ti2p, O1s and F1s of the 40% TiO2/Ag3PO4/Ag nano-
composites to probe the chemical environment of the elements
in the near surface range. Fig. 4b shows the typical high
Fig.
4
Overview (a) and the corresponding high-resolution XPS
spectra (b) Ag3d, (c) P2p, (d) O1s, (e) Ti2p and (f) F1s of the as-prepared
40% TiO2/Ag3PO4/Ag nanocomposites.
resolution of Ag3d. The peaks at ꢃ374 and ꢃ368 eV are assigned
to Ag3d3/2 and Ag3d5/2, respectively. The Ag3d3/2 is further
divided into two different peaks at 373.72 and 374.06 eV, and
the Ag3d5/2 peak is also divided into two different peaks at
367.48 and 368.04 eV. The peaks at 374.06 and 368.04 eV can be
attributed to metal Ag0,40,41 whereas the peaks at 373.72 and
367.48 eV can be attributed to the Ag+ of Ag3PO4.42,43 Fig. 4c
shows the P2p peak at the binding energy of 132.74 eV.44 Fig. 4d
shows the high-resolution spectra of O1s. The O1s spectra were
tted into two components centered at 529.22 and 531.28 eV
using Gaussian–Lorentzian peak shapes, which are attributed
to the lattice oxygen and surface hydroxyl groups,45,46 implying
that oxygen vacancies and surface hydroxyls are formed in 40%
TiO2/Ag3PO4/Ag nanocomposites, which are active species in
composited photocatalysis. As indicated in Fig. 4e, the binding
energies of Ti2p3/2 and 2p1/2 are centered at 459.02 and 464.96
eV, in agreement with those of pure TiO2.47 Fig. 4f shows the F1s
centered at 683.41 eV, F species play an important role in the
formation of oxygen vacancies, and the uorination on the
surface of F–TiO2 can accelerate the photocatalytic degradation
of a wide range of organic pollutants, since the cOH radicals
generated on the surface of F–TiO2 are more mobile than those
generated on pure TiO2.48
Fig. 5 shows the UV-Vis absorption spectra of the Ag3PO4,
TiO2 and TiO2/Ag3PO4/Ag composite. The loading of Ag3PO4
nanoparticles onto the surface of the TiO2 nanosheet affects the
optical property of TiO2 nanoparticles signicantly. The UV-Vis
spectrum of the Ag3PO4 sample indicates that it absorbs
Fig. 3 (a) SEM image, (b) TEM image, (c) EDS spectrum and (d)–(h)
corresponding elemental mapping images of TiO2/Ag3PO4/Ag.
62754 | RSC Adv., 2014, 4, 62751–62758
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Fig. 7 Comparison of the PL spectra of pure Ag3PO4 and TiO2/
Ag3PO4/Ag samples.
Fig. 5 (a) UV-Vis DRS spectra for pure TiO2, Ag3PO4 and TiO2/
Ag3PO4/Ag composites.
TiO2, Degussa P25 and no catalyst was also carried out under
identical conditions. As shown in Fig. 8a, the degradation of MB
in Degussa P25, TiO2, 40% TiO2/Ag3PO4, 20% TiO2/Ag3PO4/Ag,
40% TiO2/Ag3PO4/Ag, 60% TiO2/Ag3PO4/Ag, Ag3PO4 and Ag3PO4/
Ag was 12%, 31%, 82%, 81%, 95%, 86%, 66% and 72%,
respectively. Fig. 8b shows that there is a linear relationship
sunlight with a wavelength less than 517 nm, corresponding to
2.40 eV of band gap energy. Pure TiO2 nanosheets show an
absorption edge at about 403 nm, corresponding to the band
gap of 3.07 eV. Moreover, the heterostructured TiO2/Ag3PO4/Ag
photocatalyst could be used for visible light photocatalytic
reactions.
Fig. 6 summarizes the BET surface area of the TiO2/Ag3PO4/
Ag heterostructure with different loading amounts of TiO2. It is
found that the surface area of pure TiO2 nanosheets is much
larger than that of Ag3PO4 (6.2 m2 gꢁ1). When Ag3PO4 combines
with TiO2 nanosheets, the surface area of TiO2/Ag3PO4/Ag het-
erostructure enlarges with the added content of TiO2.
PL analysis was applied to investigate the separation effi-
ciency of photogenerated electrons and holes in TiO2/Ag3PO4/Ag
samples (Fig. 7). The pure Ag3PO4 sample has a strong and wide
peak between 370 and 600 nm, which is consistent with the
result from the literature.49 For the TiO2/Ag3PO4/Ag hybrid
materials, the emission intensity signicantly decreased, which
indicated that the TiO2/Ag3PO4/Ag composites had a much
lower recombination rate of photogenerated electrons and
holes.
The photocatalytic activity of TiO2/Ag3PO4/Ag photocatalysts
was studied by the degradation of MB under 410 nm LED light
irradiation sources. As a comparison, MB degradation with pure
Fig. 8 (a) Photocatalytic degradation of MB under 410 nm LED light
irradiation, (b) linear transform ln(C0/C) of the kinetic curves of MB
degradation, (c) the apparent pseudo-first-order rate constant kapp
with different catalysts.
Fig. 6 BET surface area versus different TiO2 contents sample.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 62751–62758 | 62755

View Article Online
RSC Advances
Paper
between ln C0/C and t, conrming that the photodegradation
reaction is indeed pseudo-rst-order. According to eqn (2) and
Fig. 8b and c shows the apparent pseudo-rst-order rate
constant kapp with different catalysts. kapp of the photo-
degradation of MB are 0.0032 minꢁ1, 0.0096 minꢁ1, 0.0422
minꢁ1, 0.0405 minꢁ1, 0.0716 minꢁ1, 0.0479 minꢁ1, 0.0255
minꢁ1 and 0.0288 minꢁ1 for Degussa P25, TiO2, 40% TiO2/
Ag3PO4, 20% TiO2/Ag3PO4/Ag, 40% TiO2/Ag3PO4/Ag, 60% TiO2/
Ag3PO4/Ag, Ag3PO4 and Ag3PO4/Ag, respectively. An optimal
degradation performance of 95% MB was found for 40% TiO2/
Ag3PO4/Ag, where 40% TiO2/Ag3PO4/Ag shows the highest
photocatalytic activity among commercial Degussa P25, pure
TiO2 and other Ag3PO4-based composites. This result is in
accordance with the PL test.
It is well known that the enhancement of photocatalytic
performance of composite photocatalysts is mainly attributed
to electrons and holes transfer at the interfaces of photo-
catalysts. When TiO2 is coupled with Ag3PO4 and Ag, the band
edge potential position of the TiO2 and TiO2/Ag3PO4/Ag mate-
rials plays an important role in studying the efficient generation
and separation process of the electrons and holes pairs. The
valence band (VB) potentials of a semiconductor at the point of
zero charge can be theoretically predicted by the following
empirical equation:50
Fig. 9 A proposed visible LED light photodegradation mechanism of
the TiO2/Ag3PO4/Ag hybrid photocatalyst.
When the as-prepared TiO2/Ag3PO4/Ag samples were irradi-
ated by 410 nm LED light, the energy of LED light exceeds the
band-gap of Ag3PO4, and the electrons of Ag3PO4 VB are excited
to the CB, creating holes in the VB. As far as Ag3PO4 is con-
cerned, the electron–hole pairs quickly recombine and only a
few electrons and holes can participate in the photocatalytic
process. However, as for the TiO2/Ag3PO4/Ag composite, the
electrons of Ag3PO4 can immediately transfer to metallic Ag0
nanoparticles through the Schottky barrier, because the CB
potential of Ag3PO4 is more negative than the Fermi level of the
loaded metallic Ag0 nanoparticles. Then, electrons can also
photogenerate from plasmon-excited Ag nanoparticles.54 Pho-
togenerated holes in the VB of Ag3PO4 can easily transfer to the
VB of TiO2 and get trapped by TiO2. Therefore, electron–hole
pairs can be easily and effectively separated. At almost the same
time, the enrichment of electron on the Ag nanoparticles can
react with water or oxygen molecular in the MB solution and
cO2ꢁ radicals will form. As a result, a more stable TiO2/Ag3PO4/
Ag composite is obtained, which displays better photocatalytic
performance on the organic pollutants treatment than pure
TiO2 or Ag3PO4. However, when the content of Ag3PO4 is further
increased above its optimum value, the photocatalytic
EVB ¼ X ꢁ Ec + 0.5Eg
(5)
where EVB is the VB edge potential, X is the electronegativity of
the semiconductor, which is the geometric mean of the elec-
tronegativity of the constituent atoms, Ec is the energy of free
electrons on the hydrogen scale (about 4.5 eV), Eg is the band
gap energy of the semiconductor. Moreover, the conduction
band (CB) edge potential (ECB) can be obtained by ECB ¼ EVB
ꢁ
Eg. The Eg of Ag3PO4 and F–TiO2 were estimated as 2.40 eV and
3.12 eV, respectively, and the X values for the Ag3PO4 and TiO2
materials are 6.15 eV and 5.70 eV, respectively. Herein, the CB
and VB edge potentials of Ag3PO4 were determined at +0.45 eV
and +2.85 eV, respectively. The CB and VB edge potentials of
TiO2 were calculated at ꢁ0.31 eV and +2.76 eV, respectively. The
potentials of both CB and VB of TiO2 are negative compared
with those of Ag3PO4.
Based on the abovementioned information, the band struc-
ture of TiO2/Ag3PO4/Ag composites and the possible charge
separation processes is shown in Fig. 9. The major photo-
catalytic reaction steps in this system can be described as the
following equations:51–53
TiO2/Ag3PO4/Ag + hn / Ag3PO4(h+ + eꢁ) + TiO2(h+) (6)
ꢁ
eꢁ + O2 / cO2
(7)
(8)
2eꢁ + 2H+ + O2 + 2H+ / H2O2
ꢁ
H2O2 + cO2 / cOH + OHꢁ + O2
(9)
h+ + H2O / cOH + Hꢁ
(10)
(11)
Fig. 10 Photodegradation performance within five cycles for 40%
TiO2/Ag3PO4/Ag.
cOH + MB / degradation product
62756 | RSC Adv., 2014, 4, 62751–62758
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
performance deteriorates. Some Ag3PO4 and Ag will act as a 15 M. R. Hoffmann, S. T. Martin, W. Y. Choi and
kind of recombination center, which will hinder the transfer of D. W. Bahnemann, Chem. Rev., 1995, 95, 69.
electrons; thus, 40% TiO2/Ag3PO4/Ag is the most suitable pho- 16 T. Ohno, K. Sarukawa and M. Matsumura, J. Phys. Chem. B,
tocatalyst in this study. 2001, 105, 2417.
The recycling stability of the photocatalyst sample is 17 R. Li, F. X. Zhang, D. Wang, J. X. Yang, M. R. Li, J. Zhu,
crucially important for practical applications. To evaluate the X. Zhou, H. X. Han and C. Li, Nat. Commun., 2013, 4, 1432.
stability of 40% TiO2/Ag3PO4/Ag composites, we carried out the 18 R. F. Chong, J. Li, X. Zhou, Y. Ma, J. X. Yang, L. Huang,
recycled experiment with a reaction time of 40 min. As shown in
Fig. 10, the photocatalytic activity of the as-prepared 40% TiO2/
H. X. Han, F. X. Zhang and C. Li, Chem. Commun., 2014,
50, 165.
Ag3PO4/Ag hybrids photocatalyst exhibited almost no decrease 19 K. Dai, L. Lu, Q. Liu, G. Zhu, Q. Liu and Z. Liu, Dalton Trans.,
even aer 5 times cycling.
2014, 2202.
20 H. H. Yang, W. G. Fan, A. Vaneski, A. S. Susha, W. Y. Teoh
and A. L. Rogach, Adv. Funct. Mater., 2012, 10, 2821.
21 A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253.
4. Conclusions
In summary, a TiO2/Ag3PO4/Ag composite with a well-designed 22 M. Y. Wang, L. Sun, Z. Q. Lin, J. H. Cai, K. P. Xie and C. J. Lin,
structure has been synthesized. Beneting from the planar Energy Environ. Sci., 2013, 6, 1211.
Ag3PO4/TiO2 heterojunction and the Schottky barrier between 23 S. G. Kumar and K. S. R. Koteswara Rao, Energy Environ. Sci.,
Ag and Ag3PO4, which favor the enhancement of light absorp-
2014, 7, 45.
tion, effective electron–hole separation and reduction recom- 24 S. V. Kershaw, A. S. Susha and A. L. Rogach, Chem. Soc. Rev.,
bination of free electrons and holes, the composite has shown a 2013, 42, 3033.
wide spectral response and exhibited an obviously increased 25 Y. S. Xu, Z. J. Zhang and W. D. Zhang, Dalton Trans., 2014,
photocatalytic activity in the degradation of organic molecules
for pollutant treatment.
3660.
26 K. Dai, L. Lu, C. Liang, Q. Liu and G. Zhu, Appl. Catal., B,
2014, 156–157, 331.
27 M. Z. Xie, X. D. Fu, L. Q. Jing, P. Luan, Y. J. Feng and H. G. Fu,
Adv. Energy Mater., 2014, 4, 1300995.
Acknowledgements
This work was supported by the National Natural Science 28 W. Z. Wang, J. Wang, Z. Z. Wang, X. Z. Wei, L. Liu, Q. S. Ren,
Foundation of China (51302101, 21303129, 11004071 and W. L. Gao, Y. J. Liang and H. L. Shi, Dalton Trans., 2014, 6735.
51302100), the Natural Science Foundation of Anhui Province 29 C. X. Kronawitter, L. Vayssieres, S. H. Shen, L. J. Guo,
(1408085QE78).
D. A. Wheeler, J. Z. Zhang, B. R. Antoun and S. S. Mao,
Energy Environ. Sci., 2011, 4, 3889.
30 L. Zhang, W. Z. Wang, S. M. Sun, Y. Y. Sun, E. P. Gao and
Z. J. Zhang, Appl. Catal., B, 2014, 27, 164.
Notes and references
1 X. Lang, X. Chen and J. Zhao, Chem. Soc. Rev., 2014, 43, 473. 31 W. J. Jie and J. H. Hao, Nanoscale, 2014, 6, 6346.
2 M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, 32 K. Dai, L. Lu, Q. Liu, G. Zhu, X. Wei, J. Bai, L. Xuan and
B. J. Marinas and A. M. Mayes, Nature, 2008, 452, 301.
3 A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253.
4 C. K. Prier, D. A. Rankic and D. W. C. MacMillan, Chem. Rev.,
2013, 113, 5322.
H. Wang, Dalton Trans., 2014, 6295.
33 Q. Hua, F. C. Shi, K. Chen, S. J. Chang, Y. S. Ma, Z. Q. Jiang,
G. Q. Pan and W. X. Huang, Nano Res., 2011, 4, 948.
34 Z. H. Wang, S. P. Zhao, S. Y. Zhu, Y. L. Sun and M. Fang,
CrystEngComm, 2011, 13, 2262.
¨
5 X. B. Chen, C. Li, M. Gratzel, R. Kostecki and S. S. Mao, Chem.
Soc. Rev., 2012, 41, 7909.
6 H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri and J. Ye,
Adv. Mater., 2012, 24, 229.
35 G. C. Xie, K. Zhang, B. D. Guo, Q. Liu, F. Liang and J. R. Gong,
Sci. Adv. Mater., 2013, 25, 3820.
36 Y. H. Zheng, L. R. Zheng, Y. Y. Zhan, X. Y. Lin, Q. Zheng and
K. M. Wei, Inorg. Chem., 2007, 46, 6980.
37 K. Dai, L. Lu, C. Liang, J. Dai, G. Zhu, Z. Liu, Q. Liu and
Y. Zhang, Mater. Chem. Phys., 2014, 143, 1410.
38 K. Dai, G. Dawson, S. Yang, Z. Chen and L. Lu, Chem. Eng. J,
2012, 191, 571.
39 Z. G. Yi, J. H. Ye, N. K. Kikugawa, T. Kako, S. X. Ouyang,
H. S. Williams, H. Yang, J. Y. Cao, W. J. Luo, Z. S. Li, Y. Liu
and R. L. Withers, Nat. Mater., 2010, 9, 559.
7 Y. Q. Qu and X. F. Duan, Chem. Soc. Rev., 2013, 42, 2568.
8 W. J. Foo, C. Zhang and G. W. Ho, Nanoscale, 2013, 5, 759.
9 J. C. Colmenares and R. Luque, Chem. Soc. Rev., 2014, 43,
765.
10 K. Dai, L. Lu, J. Dong, Z. Ji, G. Zhu, Q. Liu, Z. Liu, Y. Zhang,
D. Li and C. Liang, Dalton Trans., 2013, 42, 4657.
11 P. D. Tran, L. H. Wong, J. Barber and J. S. C. Loo, Energy
Environ. Sci., 2012, 5, 5902.
12 B. M. Rajbongshi and S. K. Samdarshi, Appl. Catal., B, 2014, 40 W. S. Wang, H. Du, R. X. Wang, T. Wen and A. W. Xu,
144, 435. Nanoscale, 2013, 5, 3315.
13 L. L. Yang, H. Zhou, T. X. Fan and D. Zhang, Chem. Phys., 41 W. Q. Zhang, J. Z. Yang and X. M. Lu, ACS Nano, 2012, 6,
2014, 16, 6810. 7397.
14 H. L. Zhou, Y. Q. Qu, T. H. Zeid and X. F. Duan, Energy 42 L. Q. Ye, J. Y. Liu, C. Q. Gong, L. H. Tian, T. Y. Peng and
Environ. Sci., 2012, 5, 6732.
L. Zan, ACS Catal., 2012, 2, 1677.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 62751–62758 | 62757

View Article Online
RSC Advances
Paper
43 B. Z. Tian, T. T. Wang, R. F. Dong, S. Y. Bao, F. Yang and 49 Y. S. Xu and W. D. Zhang, Dalton Trans., 2013, 42, 1094.
J. L. Zhang, Appl. Catal., B, 2014, 147, 22.
44 S. Zhang, S. Zhang and L. Song, Appl. Catal., B, 2014, 152–
153, 129.
45 A. L. Linsebigler, G. Lu and J. T. Yates, Chem. Rev., 1995, 95,
735.
50 X. Zhang, L. Z. Zhang, T. F. Xie and D. J. Wang, J. Phys.
Chem., 2009, 113, 7371.
51 Z. Chen, W. Wang, Z. Zhang and X. Fang, J. Phys. Chem. C,
2013, 117, 19346.
52 M. A. Asi, C. He, M. Su, D. H. Xia, L. Lin, H. Q. Deng,
Y. Xiong, R. L. Qiu and X. Z. Li, Catal. Today, 2011, 175, 256.
46 Y. Zhao, C. Z. Li and F. Gu, Appl. Catal., B, 2008, 79, 208.
`
47 E. Barborini, A. M. Conti, I. Kholmanov, P. Piseri, A. Podesta, 53 W. F. Yao, B. Zhang, C. P. Huang, C. Ma, X. L. Song and
P. Milani, C. Cepek, O. Sakho, R. Macovez and M. Sancrotti,
Adv. Mater., 2005, 17, 1839.
48 T. R. Gordon, M. Cargnello, T. J. Paik, F. Mangolini,
R. T. Weber, P. Fornasiero and C. B. Murray, J. Am. Chem.
Soc., 2012, 134, 6751.
Q. J. Xu, J. Mater. Chem., 2012, 22, 4050.
54 C. Cui, Y. P. Wang, D. Y. Liang, W. Cui, H. H. Hu, B. Q. Lu,
S. Xu, X. Y. Li, C. Wang and Y. Yang, Appl. Catal., B, 2014,
158–159, 150.
62758 | RSC Adv., 2014, 4, 62751–62758
This journal is © The Royal Society of Chemistry 2014