An efficient near infrared photocatalyst of Er3+/Tm3+/Yb3+tridoped (CaWO4at(TiO2/CaF2)) with multi-stage CaF2nanocrystal formation

Article abstract of DOI:10.1039/c4ta03309d

The formation process of CaF₂is critical for the improvement of upconversion properties of the CaF₂based upconversion photocatalysts, and for this purpose a near-infrared (NIR) photocatalyst of Er³⁺/Tm³⁺/Yb³⁺tridoped (CaWO₄at(TiO₂/CaF₂)) (ETY-CTC) was synthesized. CaF₂nanocrystals are converted from CaWO₄precursors in a multi-stage process, and the remaining CaWO₄microspheres are wrapped in CaF₂and TiO₂nanocrystals to form the heterostructure of the photocatalyst. CaF₂is found to connect with TiO₂nanocrystals, instead of being coated by TiO₂, resulting in a higher upconversion luminescence efficiency of ETY-CTC than that of pure Er³⁺/Tm³⁺/Yb³⁺tridoped (CaWO₄@CaF₂). ETY-CTC possesses higher photocatalytic activities compared to Er³⁺/Tm³⁺/Yb³⁺tridoped (CaWO₄@TiO₂) under NIR and UV-vis-NIR light irradiations, since more OH and O₂^-radicals, and higher electron-hole separation efficiency are obtained in the ETY-CTC system. The multi-stage formation of luminescence agents can be an attractive method for the synthesis of NIR photocatalysts with enhanced upconversion properties and photocatalytic activities. This journal is

Full text of DOI:10.1039/c4ta03309d

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Materials Chemistry A
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An efficient near infrared photocatalyst of
Er3+/Tm3+/Yb3+ tridoped (CaWO4@(TiO2/CaF2))
Cite this: J. Mater. Chem. A, 2014, 2,
with multi-stage CaF2 nanocrystal formation†
16165
*
Shouqiang Huang, Ziyang Lou, Aidang Shan, Nanwen Zhu, Kaili Feng
and Haiping Yuan
The formation process of CaF2 is critical for the improvement of upconversion properties of the CaF2 based
upconversion photocatalysts, and for this purpose a near-infrared (NIR) photocatalyst of Er3+/Tm3+/Yb3+
tridoped (CaWO4@(TiO2/CaF2)) (ETY-CTC) was synthesized. CaF2 nanocrystals are converted from
CaWO4 precursors in a multi-stage process, and the remaining CaWO4 microspheres are wrapped in
CaF2 and TiO2 nanocrystals to form the heterostructure of the photocatalyst. CaF2 is found to connect
with TiO2 nanocrystals, instead of being coated by TiO2, resulting in
a higher upconversion
luminescence efficiency of ETY-CTC than that of pure Er3+/Tm3+/Yb3+ tridoped (CaWO4@CaF2). ETY-
CTC possesses higher photocatalytic activities compared to Er3+/Tm3+/Yb3+ tridoped (CaWO4@TiO2)
under NIR and UV-vis-NIR light irradiations, since more cOH and O2cꢀ radicals, and higher electron–hole
separation efficiency are obtained in the ETY-CTC system. The multi-stage formation of luminescence
agents can be an attractive method for the synthesis of NIR photocatalysts with enhanced upconversion
properties and photocatalytic activities.
Received 29th June 2014
Accepted 6th August 2014
DOI: 10.1039/c4ta03309d
www.rsc.org/MaterialsA
shells. The lower upconversion efficiencies in NaYF4:Yb,
Er@SiO2@TiO2 (ref. 9 and 10) were also observed compared to
Introduction
their bare upconversion phosphors, which limited the devel-
opment of solar cells with high conversion efficiencies. To solve
the issues above, slow formation of upconversion luminescence
agents might be one of the credible alternatives to control the
particle size, and the light penetration efficiency of the NIR
photocatalyst system can also be improved with a multi-stage
formation process, such as self-formed upconversion lumines-
cence agents before and aer the semiconductor coatings were
obtained. To identify a new Ca-containing precursor is the pre-
requirement for the highly efficient CaF2 based NIR photo-
catalyst formation.
CaWO4 is widely used in phosphors,11 microwave dielec-
trics,12 and scintillators13 owing to its high density and stable
physicochemical properties compared to other oxide materi-
als.11b The W6+ ions with a large electric charge and small radius
in CaWO4 can also decrease symmetries by their strong polar-
ization.14 Thus, CaWO4 is a good oxide host material for
lanthanide ions with high upconversion efficiency.15 Impor-
Full-spectrum utilization of solar energy in near-infrared (NIR)
photocatalysts with the aid of upconversion agents has attracted
enormous attention due to their promising applications in
water purication and solar cells.1 The NIR driven photo-
catalytic activities of upconversion photocatalysts are mainly
depending on the upconversion emission efficiencies of the
luminescence agents, in which uorides with low phonon
energies are commonly used as the host materials for lantha-
nide ions.2 CaF2 is a promising cost-effective upconversion
luminescence agent for the improvement of photocatalytic
activities of the NIR photocatalysts,3 while the unfavorably
large-sized particles of CaF2 are obtained immediately, when
Ca(NO3)2 is used as the calcium source in traditional methods
during the fabrication of lanthanide ions (Yb3+, Er3+ or Tm3+
)
doped CaF2.3,4 Furthermore, the upconversion emission inten-
sities of core–shell NIR photocatalysts, such as Er3+/Yb3+–
(CaF2@TiO2),3a NaYF4:Yb, Tm/TiO2,5 NaYF4:Yb, Tm/ZnO,6
NaYF4:Yb, Tm/CdS,7 and YF3:Yb, Tm/TiO2,8 were reduced
compared to those of pure uoride agents owing to the weak-
ened excitation light penetration caused by the semiconductor
tantly, the low solubility product of CaWO4 (Ksp ¼ 8.7 ꢁ 10ꢀ9
)
will control the release rate of Ca2+ ions,16 which provides an
opportunity to place CaF2 (Ksp ¼ 2.7 ꢁ 10ꢀ11) nanocrystals in
situ on the CaWO4 microsphere surfaces (CaWO4/CaF2)
continuously. The formation of lanthanide ions doped CaWO4/
CaF2 will fulll the requirements of high stability and excellent
upconversion properties. Moreover, CaWO4 is a wide-bandgap
semiconductor, and its coupling with TiO2 is benecial for the
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800
Dongchuan Road, Shanghai, 200240, P. R. China. E-mail: nwzhu@sjtu.edu.cn; Fax:
+86 21 54743710; Tel: +86 21 54743710
† Electronic supplementary information (ESI) available: Additional tables and
gures mentioned in the text. See DOI: 10.1039/c4ta03309d
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charge carrier separation due to the formed heterojunction,17
which will enhance photocatalytic performances for the
lanthanide ions doped CaWO4/CaF2/TiO2 systems.
In this work, a novel NIR photocatalyst of Er3+/Tm3+/Yb3+
tridoped (CaWO4@(TiO2/CaF2)) (ETY-CTC) was synthesized via
a hydrothermal method, and Er3+/Tm3+/Yb3+ tridoped CaWO4
(ETY-C), Er3+/Tm3+/Yb3+ tridoped (CaWO4@TiO2) (ETY-CT),
Er3+/Tm3+/Yb3+ tridoped (CaWO4@CaF2) (ETY-CC), and ETY-
CC@TiO2 (ETY-CC@T) were also fabricated simultaneously to
compare their upconversion properties. The structures and
morphologies of these samples were investigated, and their
photocatalytic activities on the degradation of methyl orange
(MO) under NIR and Ultraviolet-visible-Near Infrared (UV-vis-
NIR) irradiations were tested as well.
Experimental section
Chemicals and materials
Fig.
ETY-CTC.
1
Schematic illustration for the multi-stage fabrication of
Ca(NO3)2$4H2O, Na2WO4$2H2O, Er(NO3)3$5H2O, Tm(NO3)3$5H2O,
Yb(NO3)3$5H2O, and titanium isopropoxide (TTIP, 98%) were
all purchased from Sinopharm chemical Reagent Co. Ltd
(China) with analytical grade and they were used as received.
eld emission scanning electron microscope (FESEM) equipped
with the AZTec X-Max80 energy-dispersive spectroscopy (EDS)
instrument, and a JEM-2100F transmission electron microscope
(TEM) instrument. The Brunauer–Emmett–Teller (BET) specic
surface areas of the samples were determined on a nitrogen
adsorption apparatus (Micromeritics, ASAP 2010M + C) by N2
adsorption at 77 K. The surface analysis was studied by X-ray
photoelectron spectroscopy (XPS, Kratos Axis UltraDLD), and all
the binding energies were calibrated with the C 1s peak at 284.8
eV. The absorption spectra were performed on the Lambda 750
UV-vis-NIR spectrophotometer, and the band gap energy was
calculated referring to Tauc's formula.3 Room temperature
upconversion uorescence spectra and the photoluminescence
(PL) spectra were measured by a Hitachi F-7000 uorescence
spectrophotometer equipped with a 980 nm semiconductor
solid laser with tunable power (Table S1†). The electron spin
resonance (ESR) spectra were recorded at room temperature
with an EMX-8 ESR spectrometer (Bruker BioSpin Corp.).
Preparation of CaWO4 microspheres
5 mmol of Na2WO4$2H2O was dissolved into 25 mL distilled
water. Then, 25 mL Ca(NO3)2 aqueous solution (0.2 M) was
added drop by drop into the Na2WO4 solution with continuous
stirring for 30 min. The resulting mixture was maintained at
room temperature for 1 h. The products were obtained by
centrifugation, washed withꢂ distilled water and absolute
ethanol, and then dried at 60 C for 12 h.
Preparation of ETY-CTC samples
In a typical synthesis, 2 mmol of TTIP was dissolved in 20 mL
absolute ethanol. CaWO4, Er(NO3)3$5H2O, Tm(NO3)3$5H2O, and
Yb(NO3)3$5H2O (molar ratio of Ti4+ : Ca2+ : Yb3+ : Er3+ : Tm3+ is
1 : 0.3 : 0.2 : 0.02 : 0.02) were added into 40 mL distilled water
and completely dispersed by ultrasonication. The pH of the
mixture was adjusted to 1 by adding nitric acid, followed by the
slow addition of TTIP solution. The obtained mixture was stirred
for 30 min, and 0.1 mL of HF (40%) was introduced under
continuous stirring for 30 min. Then, the mixture was trans-
ferred into a 150 mL Teon-lined stainless steel autoclave, and
heated at 180 ꢂC for 12 h (Fig. 1). The precipitates collected were
dried at 60 ꢂC for 12 h aer separation and washing. The
obtained powders were heated at 500 ꢂC for 4 h to form ETY-CTC
samples. For comparison purposes, ETY-CC, ETY-CT, ETY-C,
ETY-CC@T, and ETYN@T were also synthesized following the
same procedure mentioned above.
Photocatalytic activities
The photocatalytic activities of the samples were evaluated by
the decomposition of MO under 980 nm light and the NIR light
(l $ 780 nm) irradiations (provided by 1000 W high pressure
mercury lamp with a lter). 20 mg of the samples were added in
the MO aqueous solutions (20 mg Lꢀ1, 20 mL). Before irradia-
tion, the mixtures were kept in the dark with magnetic stirring
for 2 h to establish an adsorption/desorption equilibrium. Every
30 min intervals, 2.0 mL of the mixtures were collected and
centrifuged. The changed absorption peak intensities at 464 nm
were detected by a UV-vis spectrophotometer (Hitachi U-3900).
Three dimension excitation-emission matrix (EEM) spectra
measurements were conducted using a Hitachi F-7000 uores-
Characterization
The crystal structures of the samples were identied by X-ray cence spectrophotometer. Emission scans were preformed from
diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer 250 to 500 nm with 10 nm steps, with excitation wavelengths
˚
at 40 kV and 40 mA using Cu Ka radiation (l ¼ 1.5406 A), and the from 200 to 350 nm at 10 nm widths. Moreover, the photo-
crystal sizes were determined through the Scherrer formula.3a activity tests of cOH and O2cꢀ radicals were similar to the
The morphologies were investigated by a NoVa NanoSEM 230 method mentioned for the photodegradation of MO.
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Terephthalic acid (TA, 4 ꢁ 10ꢀ4 M in a 2 ꢁ 10ꢀ3 M NaOH CaWO4 shows the perfect solid microsphere morphology
solution) and Nitroblue tetrazolium (NBT, 1 ꢁ 10ꢀ3 M) solu- (Fig. 3a and b), which is built by many CaWO4 nanocrystals
tions were used to determine the amounts of cOH and O2cꢀ (Fig. S3†). In the case of ETY-CT (Fig. 3c), the CaWO4 micro-
radicals generated from the samples, respectively.3b,18
sphere structure is not being destroyed, and the three-dimen-
sional sheet-like surfaces are coated with TiO2 nanoparticles
(Fig. 3d). In Fig. 4a, it can be observed the CaWO4@TiO2 core–
shell structure, since the lattice fringe spacings of 0.35 and 0.48
nm meet the (101) planes of TiO2 shell and the (101) planes of
Results and discussion
Characterization of the samples
Fig. 2 shows the XRD patterns of ETY-CT and ETY-CTC. For ETY- CaWO4 core (Fig. 4b), respectively.
CT, in addition to the detected broad diffraction peak of anatase
With the addition of HF, the surface layers of the CaWO4
TiO2 (PDF-#84-1285) at 25.26ꢂ, the main diffraction peaks are microspheres are dissolved gradually, and substituted simul-
assigned to CaWO4 (PDF-#72-1624). The crystal phases of ETY- taneously by the in situ formation of CaF2 layers, where ETY-CC
CTC consist of CaWO4, anatase TiO2, (Ca0.8Yb0.2)F2.2 (PDF-#87- is nally obtained (Fig. 3e and S4†). In the ETY-CTC system,
0976), and YbF3 (PDF-#74-2178). (Ca0.8Yb0.2)F2.2 is a complex most of the CaWO4 microspheres have been etched seriously,
system containing two components, CaF2 and YbF3, since the resulting in various near-spherical aggregates coated by many
Yb3+ ions take the Ca2+ lattice positions in CaF2.3 YbF3 is bound nanoparticles making them porous (Fig. 3f and g and Table
to form immediately once the HF is added into the precursor S1†). These aggregates consist of CaWO4 cores and TiO2/
solution (Fig. 1), owing to the fact that large amount of disso- (Ca0.8Yb0.2)F2.2 coatings (Fig. 4c), and present a polycrystalline
ciated Yb3+ ions are presented, while the Ca2+ ions have to be nature based on the selected-area electron diffraction pattern
released slowly from CaWO4. Although there are sufficient Fꢀ (top inset of Fig. 4c). A tooth-like nanoparticle (ꢃ28 nm) is
ions in the solution, the complete transformation of CaF2 from presented in Fig. 4d with a lattice fringe spacing of 0.32, which
CaWO4 needs a long time, i.e., ve days at room temperature.16 is consistent with the (111) planes of (Ca0.8Yb0.2)F2.2, respec-
Thus, parts of CaWO4 are maintained in the nal products, tively, and the Fourier transform electron diffraction pattern
which contribute to the charge carrier separation due to the (inset of Fig. 4d) further conrms the particle is a single crystal.
formed CaWO4/TiO2 heterostructure.
Moreover, a thin sheet-like crystal of (Ca0.8Yb0.2)F2.2 is also
According to the Scherrer formula, the average crystal sizes found in Fig. 4e, where the lattice fringe spacing of 0.27 nm is
of TiO2, CaWO4, YbF3, and (Ca0.8Yb0.2)F2.2 in ETY-CTC are close to its (200) planes.
calculated to be 12.35, 45.56, 38.05, and 28.02 nm (Table S2†),
The crystal lattices at the bottom of the tooth-like
respectively, exhibiting no signicant difference in comparison (Ca0.8Yb0.2)F2.2 crystal belong to TiO2 with (101) planes, as well
with those of ETY-CC (Fig. S1†) and ETY-CT. It was found that as the TiO2 nanosheet overlapped with the sheet-like
the obtained crystal size of (Ca0.8Yb0.2)F2.2 was much smaller (Ca0.8Yb0.2)F2.2 crystal, indicating that the (Ca0.8Yb0.2)F2.2
than those (90–150 nm) fabricated by the direct addition of nanocrystals are generated continuously aer the TiO2 coatings
Ca(NO3)2.3,4 Thus, the (Ca0.8Yb0.2)F2.2 nanocrystals are able to be formed and are, thus, present in the outermost surfaces. It is
produced in situ on CaWO4 surfaces and the remaining CaWO4 reasonable to suppose that the Ca2+ ions in CaWO4 cores can
will be wrapped by CaF2 to self-assemble the core–shell struc- diffuse through the porous TiO2 coatings (Fig. 4c and Table
ture, and their synergistic effect might provide efficient S1†), and thus (Ca0.8Yb0.2)F2.2 generated can embed around the
upconversion emissions.
TiO2 nanocrystals. These bared (Ca0.8Yb0.2)F2.2 nanocrystals will
The morphologies and microstructures of the samples are contribute to the high upconversion emission efficiency for
examined by SEM (Fig. 3) and TEM (Fig. 4). The initial pure ETY-CTC, since there are no TiO2 layers weakening the
light penetration to (Ca0.8Yb0.2)F2.2. Furthermore, the combined
TiO2 lattice and CaWO4 lattice (Fig. 4f) can also facilitate
the charge carrier transmission because of the well-dened
heterostructure.
The element components in ETY-CT and ETY-CTC are eval-
uated by EDS spectra and shown in Fig. S2a and b,† respectively.
The Ti, Ca, Yb, Er, Tm, W, F, and O elements are found in ETY-
CTC, and the atomic ratio of Ti : Ca : Yb : Er : Tm is
16.08 : 3.88 : 2.27 : 0.17 : 0.20 (Table S3†), which is close to its
nominal atomic ratio of 1 : 0.3 : 0.2 : 0.02 : 0.02. As shown in
Fig. S5a,† the survey-scan XPS spectra further demonstrate that
the Ti, Ca, Yb, W, F, and O elements are present in ETY-CTC.
Fig. 5a shows two peaks of Ti 2p at 458 and 463.6 eV, which
match the Ti 2p3/2 and Ti 2p1/2, respectively. The Ca 2p peaks at
346.8 and 350.3 eV correspond to Ca 2p2/3 and Ca 2p1/2 (Fig. 5b),
respectively. The F 1s peak is detected at 683.9 eV (Fig. 5c), and
the Yb 4d peaks at 185.7 and 199.5 eV are assigned to Yb 4d5/2
Fig. 2 XRD patterns of (a) ETY-CT and (b) ETY-CTC.
and Yb 4d3/2 (Fig. S5b†), respectively. The spin–orbit
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Fig. 3 SEM images of (a and b) pure CaWO4, (c and d) ETY-CT, (e) ETY-CC, (f and g) ETY-CTC.
components of W 4f7/2 at 34.9 eV and W 4f5/2 at 36.9 eV are luminescence peaks mainly include the red (659 nm), green
attributed to W atoms with the +6 oxidation state (Fig. 5d).17b (554 and 525 nm), violet (410 nm), and UV (382 nm) emissions,
4
4
2
4
4
The atomic concentration ratio of Ti : Ca : F : W : Yb is calcu- which are attributed to the F9/2 / I15/2, H11/2 / I15/2, S3/2
lated to be 33.41 : 4.06 : 5.43 : 9.84 : 2.57, according to the XPS / I15/2, H9/2 / I15/2, and G11/2 / I15/2 transitions of Er3+
4
2
4
4
4
results. Thus, TiO2 is the predominant component in the ions, respectively (Fig. 6c). Moreover, the blue (477 nm) and UV
1
3
1
outermost layers of ETY-CTC, combined with the present (365 nm) emissions correspond to the G4 / H6 and D2 /
(Ca0.8Yb0.2)F2.2 and CaWO4 nanocrystals, which will be helpful 3H6 transitions of Tm3+ ions, respectively.
for the NIR light harvesting and the electron–hole pair
separation.
It should be pointed out that ETY-CTC exhibits a better
upconversion property compared to those of ETY-CT, ETY-C,
ETY-CC@T, and even the pure ETY-CC (Fig. 6c and S8†), except
for the 365 and 382 nm UV light absorbed by TiO2. Based on the
previously reported NIR core–shell photocatalysts,3a,5–10 all of
Upconversion luminescence properties
The upconversion emission spectra of ETY-C, ETY-CC, ETY- their upconversion emission intensities are lower than those of
CC@T, ETY-CT, and ETY-CTC upon 980 nm light excitation are pure luminescence agents, due to the barriers of the semi-
recorded in Fig. 6. As the output currents increased from 1.0 to conductor shells and the weakened excitation light penetration.
3.0 A, the emission spectra proles of ETY-CC (Fig. 6a and S6a†) There are also some different cases, where the upconversion
and ETY-CTC (Fig. 6b and S6b†) are almost unchanged, while properties of NIR photocatalysts are not decreased in the
ETY-CTC possesses higher emission intensities in the visible emission wavelength of 250–700 nm, in addition to the absor-
light wavelength range (400–700 nm) compared to those of ETY- bed parts,19 while in the longer wavelength, such as the 800 nm,
CC (Fig. 6c), and their UV light emissions (365 and 382 nm) the emission intensity of Tm3+ is still unknown. The main
remain almost at the same levels. Considering the band gap reason might be related to the surface morphologies and the
energy of 2.92 eV for ETY-CTC (Table S2 and Fig. S7†), it can be thickness of the semiconductor shells. Nevertheless, the semi-
inferred that the UV light is stronger absorbed by TiO2 conductor shells, such as TiO2, ZnO, and CdS, are different from
compared to the violet light (410 nm). The upconversion the CaF2 or NaYF4 passivation shells with excellent optical
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Fig. 4 (a) TEM and (b) HRTEM images of ETY-CT. (c) TEM (inset: selected-area electron diffraction pattern) and (d, e and f) HRTEM images of ETY-
CTC. Inset of (d) is the Fourier transform electron diffraction pattern from the image.
transparency to greatly improve the upconversion emission sufficient exciting light harvesting to produce strong upcon-
intensities.20 With increasing the light scattering by the used version emission intensity. Taking the fabrication process of
semiconductor shells, the NIR light penetration to the upcon- ETY-CC and ETY-CTC into account, the distribution of Er3+
/
version luminescence agent cores will be oen more or less Tm3+/Yb3+ ions has been changed by the additional TiO2 coat-
weakened. To reduce the loss of the excitation light penetration ings. According to the surface compositions estimated from the
rate, a special structure is formed in the ETY-CTC system, where XPS analysis (Table S3†), the atomic ratios of Ca : F : W : Yb are
the (Ca0.8Yb0.2)F2.2 nanocrystals located at the outermost 1 : 1.34 : 2.42 : 0.63 for ETY-CTC and 1 : 1.65 : 0.27 : 0.25 for
surfaces are not fully coated by TiO2 (Fig. 4), resulting in ETY-CC. The increase of Yb3+ content in the surface layer of
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Fig. 5 XPS spectra of ETY-CTC: (a) Ti 2p, (b) Ca 1s, (c) F 1s, and (d) W 4f.
ETY-CTC can absorb more 980 nm light energy,20b,21 which will be seen from Fig. S10† that there is no MO degradation in the
transfer more energy to Er3+ and Tm3+ ions to generate higher absence of photocatalysts, and neither the pure TiO2 nor
emission intensity compared to ETY-CC. The Er3+ and Tm3+ CaWO4 has photocatalytic activity under NIR light irradiation.
quench effect can also be reduced in the outermost layers of In addition, ETY-CC@T also shows low degradation rates of
ETY-CTC, since they can be well-separated by the TiO2 nano- 9.17% and 33.69% under 980 nm and NIR light driven condi-
crystals and the Yb3+ ions in high concentration.4 Moreover, the tions (Fig. S12†), respectively, due to its large decreased
surface quenching effect may be suppressed in ETY-CTC, due to upconversion emission intensity and the inert TiO2 shells
the modulation through TiO2 coatings.22 Other decisive factors without active centers (Er3+/Tm3+/Yb3+) compared to those of
may be related to the suitably increased coating thickness and ETY-CTC.3a
the changed morphologies.23 On the other hand, owing to the
In the ETY-CTC system, Yb3+ ions absorb the NIR light and
multi-stage formed CaF2 agents, the obtained NIR photocatalyst transfer energy to Er3+ and Tm3+ ions to emit UV (365 and 382
ETY-CTC still possesses an excellent upconversion property nm) and violet (410 nm) light (Fig. 8), which enables the exci-
compared to ETY-CC, and it is the guarantee of high NIR light tation of TiO2 to produce oxidative valance band (VB) holes (h+)
driven photocatalytic activities.
and reductive conduction band (CB) electrons (eꢀ). To elucidate
the heterostructure between CaWO4 and TiO2, their relative
band edge positions are investigated through the following
equation: ECB ¼ X ꢀ E0 ꢀ 0.5Eg.24 X is the absolute electroneg-
Photocatalytic activities
The photocatalytic activities of ETY-CT and ETY-CTC are tested ativity of the semiconductors (for TiO2, X ¼ 5.81 eV;25 for
and compared in MO decomposition experiments under 980 CaWO4, X ¼ 7.045 eV)17a, E0 is the energy of free electrons on the
nm and NIR light (l $ 780 nm) irradiations. As shown in hydrogen scale (ꢃ4.5 eV), Eg is the band gap of the semi-
Fig. S9,† the maximum absorbance of MO at 464 nm decreased conductor (for TiO2, Eg ¼ 2.83 eV;3a for CaWO4, Eg ¼ 4.09 eV).17a
steadily over ETY-CTC as the irradiation time increased to 180 The ECB values of TiO2 and CaWO4 are calculated to be ꢀ0.105
min, and higher degradation rates of 20.29% and 46.57% were and 0.5 eV, respectively, and the corresponding EVB values are
obtained using 980 nm and NIR light irradiations (Fig. 7), fol- 2.725 and 4.59 eV for TiO2 and CaWO4, respectively. As shown in
lowed by ETY-CT with 12.69% and 38.23%, respectively. It can Fig. 8, the generated electron can be transferred from the CB of
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Fig. 7 C/C0 conversion plots of MO for ETY-CT and ETY-CTC under
(a) 980 nm (output current ¼ 2.0 A) and (b) NIR light ($780 nm)
irradiations.
Fig. 6 Upconversion luminescence spectra of (a) ETY-CC and (b) ETY-
CTC under 980 nm NIR excitation with different output currents (1.0–
3.0 A). (c) Upconversion luminescence spectra (inset: the UV and violet
light emission spectra) of ETY-CC, ETY-CTC, ETY-CT, ETY-C, and
ETY-CC@T under 980 nm NIR excitation (output current ¼ 3.0 A).
TiO2 to the lower CB edge of CaWO4, and thus the electron–hole
pair separation is promoted. Then, the cOH radicals can be
generated through the reaction between holes and the surface
adsorbed H2O, and electrons can be scavenged by the surface
adsorbed O2 to yield O2cꢀ radicals.26 It was found that the
production of both cOH and O2cꢀ radicals increased linearly as
the irradiation time extended (Fig. 10a and b, S16 and 17†). ETY-
CTC has a stronger ability to produce both cOH and O2cꢀ radi-
cals, and a better decomposition performance is displayed
compared to ETY-CT.
Fig. 8 Schematic electronic band structure and electron–hole pair
separation of ETY-CTC under NIR light irradiation.
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Fig. 9 EEM fluorescence spectra of MO for ETY-CTC under NIR light irradiation: (a) 0 min, (a) 30 min, (a) 120 min, and (a) 180 min.
In the photocatalytic degradation process, the chromophoric CTC is lower than that of ETY-CT (Fig. 10c), indicating its better
azo group (–N]N–) of MO molecules (Fig. S13†) can be attacked electron–hole separation ability, which is responsible for the
and destroyed by the cOH and O2cꢀ radicals, producing higher photocatalytic activities compared to ETY-CT. The
aromatic intermediates and further forming various low UV-vis-NIR driven EEM spectra of MO over ETY-CTC are shown
molecular weight byproducts before mineralizing.27 Fig. 9 in Fig. S14,† and they are very different from those obtained
shows the EEM spectra of MO over ETY-CTC with different NIR under NIR light irradiation. The complete decolorization and
light irradiation time. Before irradiation, the MO solution partial mineralization reactions have occurred in the initial
mainly contained one uorescence peak at excitation/emission irradiation of 30 min (Fig. S14b†), and the purity of the solution
wavelengths Ex/Em ¼ 220–250 nm/330–350 nm (Fig. 9a), which obtained aer 180 min (Fig. S14d†) is close to the distilled water
represented aromatic-like compounds.28 As the NIR light irra- used in this study (Fig. S15†).
diation time extended, the peak intensity of aromatic-like
The broad PL spectra of the samples include several
compounds increased gradually (Fig. 9b–d), and two other emission peaks. Accordingly, the 396 nm peak is emitted by
uorescence peaks were displayed clearly at Ex/Em ¼ 230–250/ CaWO4,11a and the 412 nm peak is attributed to defect levels
390–440 nm and 290–320/400–430 nm. These new peaks may be near the valence band of TiO2.29 The emission peaks at 452,
attributed to phototransformation intermediates, such as 470, 484, and 494 nm are associated with oxygen defects,
hydroxy derivatives.28a Based on the enhanced uorescence representing the different types of charged (F, or F+ centers)
peaks, it can be inferred that decolorization is the predominant oxygen vacancy states.30 Furthermore, the oxygen vacancies
reaction, and the mineralization of MO has not yet occurred of the samples are characterized by ESR spectroscopy
during the use of NIR light irradiation time (ꢃ180 min).
(Fig. 10d), and the symmetry ESR signal located at g ¼ 1.9965
The photocatalytic activities are further evaluated under UV- is assigned to oxygen vacancy, and no Ti3+ (g ¼ 1.965, 1.938,
vis-NIR irradiation (Fig. S11 and S12†). In the rst irradiation and 1.916) or O2ꢀ (g ¼ 2.02) signals can be measured.31 With
for 30 min, the degradation rates of ETY-CTC, ETY-CT, and ETY- the addition of CaF2, the ESR signals of ETY-CTC are stronger
CC@T are 91.69%, 84.77%, and 58.71%, respectively, consistent than those of ETY-CT, and the increased oxygen vacancies
with the NIR-driven photocatalytic results. A benet from the can be considered electron traps to enhance the electron–
incorporation of CaF2 is that the PL emission intensity of ETY- hole pair separations.32 The oxygen vacancies can also react
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Journal of Materials Chemistry A
Fig. 10 Photocatalytic activities for the (a) cOH and (b) O2cꢀ production under 980 nm light irradiation (output current ¼ 2.0 A). (c) PL emission
spectra excited at 300 nm at room temperature and (d) ESR spectra for ETY-CT and ETY-CTC.
with the absorbed O2 to generate bridging O2 dimers,33 which Program of China (2014BAL02B03), and the “Chenguang”
contribute to the photocatalytic activity for the NIR project by Shanghai Municipal Education Commission and
photocatalysts.
Shanghai Education Development Foundation (no. Z1126862).
Conclusions
Notes and references
In conclusion, a heterostructured NIR photocatalyst, ETY-CTC,
was synthesized, in which CaF2 nanocrystals were slowly fabri-
cated from CaWO4 precursors and embedded in CaWO4
surfaces, while the remaining CaWO4 microspheres were
wrapped in CaF2 and TiO2 nanocrystals to form the hetero-
structure. Benetting from this special structure, ETY-CTC
exhibits a better upconversion property compared to those of
ETY-C, ETY-CT, ETY-CC@T, and even to the pure ETY-CC
luminescence agent. Under NIR and UV-vis-NIR light irradia-
tions, ETY-CTC has higher MO degradation rates compared to
ETY-CT, owing to the enhanced upconversion property, the
increased electron–hole separation efficiency, and the
enhanced oxygen vacancy signals in the ETY-CTC system.
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