UV-visible and near-infrared active NaGdF4:Yb:Er/Ag/TiO2 nanocomposite for enhanced photocatalytic applications

Article abstract of DOI:10.1039/c6ra10208e

A near infra-red (NIR) active NaGdF₄:Yb:Er/Ag/TiO₂ nanocomposite photocatalyst was successfully synthesized by a one-pot thermal decomposition method. The composite structure, morphology and elemental mapping of the synthesized NaGdF₄:Yb:Er/Ag/TiO₂ nanocomposite were characterized by X-ray diffraction and transmission electron microscopy analysis. The energy transfer among NaGdF₄:Yb:Er, Ag, and TiO₂ was revealed by upconversion photoluminescence measurements at 980 nm. The Ag and NaGdF₄ nanoparticles enhanced the visible and NIR light absorption property of the NaGdF₄:Yb:Er/Ag/TiO₂ nanocomposite. The NIR and UV-visible light induced photocatalytic study of the NaGdF₄:Yb:Er/Ag/TiO₂ composite was examined by rhodamine B degradation. The energy transfer among NaGdF₄:Yb:Er, Ag, and TiO₂ significantly influenced the photocatalytic activity under NIR irradiation. The catalysts produced oxidative species during NIR irradiation, which are responsible for the photocatalytic degradation of rhodamine B. NaGdF₄:Yb:Er/Ag/TiO₂ showed photocatalytic activity under NIR and UV-visible radiation (full solar irradiation), which is superior to a UV or visible light active photocatalyst. The study provided a UV-visible and NIR-responsive photocatalyst and its energy transfer mechanism.

Full text of DOI:10.1039/c6ra10208e

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UV-visible and near-infrared active NaGdF : Yb: Er/Ag/TiO2
4
nanocomposite for enhanced photocatalytic application
†
‡
†
Natarajan Prakash, Dheivasigamani Thangaraju, Rajan Karthikeyan,
§
‡
†,‡,
Mukannan Arivanandhan, Yosuke Shimura and Yasuhiro Hayakawa *
†
Graduate School of Science and Technology, Shizuoka University, 3ꢀ5ꢀ1 Johoku,
Nakaꢀku, Hamamatsu, Shizuoka 432ꢀ8011, Japan
‡
Research Institute of Electronics, Shizuoka University, 3ꢀ5ꢀ1 Johoku, Nakaꢀku,
Hamamatsu, Shizuoka 432ꢀ8011, Japan
§
Center for Nanoscience and Technology, Anna University, Chennai 600 025, Tamil
Nadu, India
*Corresponding author. Tel./Fax: +81 53 478ꢀ1310
Eꢀmail: royhaya@ipc.shizuoka.ac.jp (Yasuhiro Hayakawa)
1

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Abstract
A
near infraꢀred (NIR) active NaGdF
4
:Yb:Er/Ag/TiO
2
nanocomposite
photocatalyst was successfully synthesized by oneꢀpot thermal decomposition method.
A composite structure, morphology and elemental mapping of synthesized
NaGdF
transmission electron microscopy analysis. The energy transfer among NaGdF
Ag, and TiO were revealed by upconversion photoluminescence measurement under
80 nm. The Ag and NaGdF nanoparticles enhanced the visible and NIR light
4
:Yb:Er/Ag/TiO
2
nanocomposite was characterized by Xꢀray diffraction and
4
:Yb:Er,
2
9
4
absorption property of NaGdF :Yb:Er/Ag/TiO nanocomposite. NIR and UVꢀvisible
4
2
light induced photocatalytic study of NaGdF
4
:Yb:Er/Ag/TiO
2
composite was examined
:Yb:Er, Ag, and TiO
2
by Rhodamine B degradation. The energy transfer among NaGdF
4
influenced the photocatalytic activity significantly in NIR irradiation. The catalysts
produced oxidative species during NIR irradiation, which are responsible for
photocatalytic degradation of Rhodamine B. NaGdF :Yb:Er/Ag/TiO showed
4
2
photocatalytic activity under NIR and UVꢀvisible radiation (full solar irradiation),
which is superior compared to UV or visible light active photocatalyst. The study
provided a UVꢀvisible and NIRꢀresponsive photocatalyst with its energy transfer
mechanism.
Keywords: Photocatalysis, upconversion, NaGdF4, TiO2, Ag, nanocomposite
2

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Introduction
Increasing population around the world leads to increased environmental pollution
and energy deficient. Photocatalysis is one of the promising technologies, which solve
1
–3
the environmental problems since it utilizes vast abundant and cheap solar energy.
TiO is one of the most gifted photocatalysts because of its outstanding physical and
chemical properties such as high oxidative power, chemical and photostability,
2
4–7
biocompatible nature and commercially cheap. The UV light active TiO (3.2 eV)
2
only utilizes 5 % of sunlight. Therefore, there is still a huge energy (visible light 49 %,
near infraꢀred (NIR) 46 %) available in the sunlight, which isn’t harvested by TiO
2
8
,9
itself. Numerous strategies have been tried to extend the utilization of sunlight, such
1
0–12
13
14
as doping,
noble metal deposition, dye sensitizing, and mixed semiconductor
However, complete absorption of solar spectrum has not been achieved
1
5–17
composites.
2
yet. The modifications for shifting the absorption edge of TiO up to the visible light
18,19
(up to 725 nm) have been done, however the NIR light is still not used.
Researchers
are facing numerous challenges to develop an ideal catalyst, which is active in the entire
solar light region without losing their photocatalytic efficiency.
The semiconductor catalysts having upconversion luminescence are paving a path
20,21
to utilize NIR light for the photoꢀcatalytic application.
Upconversion phosphor emits
the high energy light (visible and UV light) by NIR light (980 nm) excitation via a
2
2
nonlinear process. The direct doping of upconversion ions (Tm, Er, Yb/Tm, Yb/Er) on
23–25
semiconductor catalyst did not show efficient conversion.
Qin et.al initially reported
TiO coated YF :Yb:Tm host for photodegradation of methylene blue under NIR
2
3
3

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26
irradiation. There are limited papers reporting the upconversion based NIR
27–29
3+
3+
photocatalysis.
Tang et.al investigated NaYF
4
:Yb ,Tm @TiO
2
coreꢀshell
structures for methylene blue degradation and discussed the energy transfer mechanism
30
under NIR irradiation. The structure utilizes only the UV and NIR light, the visible
3
+
3+
light is unused. The visible light photocatalytic activity of NaYF
4
:Yb ,Tm /TiO
Selection of host lattice, upconversion
doping ions and photocatalyst are important to design full solar light spectrum active
2
was
31
improved by linking CdS nanostructures.
32
33
26
34
photocatalyst. Fluorideꢀbased phosphors such as NaGdF4, NaYF4, YF3, and CaF2
were reported as efficient hosts for upconversion ion doping because of their low
phonon energy, high luminous intensity, and excellent chemical stability. NaGdF
4
ꢀ
based upconversion phosphors have significantly drawn attention due to their bright
upconversion luminescence emission along with magnetic property, which allows for
35,36
separation of the particles magnetically.
The doped Yb absorbs the NIR excitation and transfers the energy nonꢀ
3+
3+
3+ 36,37
radioactively to coꢀdopants such as Er , Tm and Ho .
The dopants were excited
during the twoꢀphoton or multiꢀphoton process and deꢀexcited to the ground state with
high energy light emission. The photocatalytic reaction could be initiated by absorbing
the emitted light from upconversion particles through appropriate semiconductor
3
1
3+
3+
composite. Yb ꢀTm upconversion ions are mainly used as a dopant in the NIR
driven rare earth fluorides/TiO composite photocatalyst since they emit UV light under
2
30
NIR excitation. The energy transfer process activates the photocatalytic reaction on
the catalyst surface (TiO ). However, visible light conversion from NIR light involves
2
fewer antiꢀstoke shifts, which is more profitable than UV light conversion from NIR
4

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29
3+
3+
light. Yb ꢀEr ions would be a promising dopant for visible light emission, which
38,39
emits visible light (650, 540, 520 nm) upon NIR excitation.
On the other hand, nano Ag shows size dependable surface plasmon resonance
SPR) absorption property in the visible region which contributes a new prospect to
(
40
overcome the limitation of visible light absorption of the photocatalysts. A collective
oscillation of free electrons in the plasmonic Ag structure produces a strong light
41
absorption and it can generate a strong electric fields near the surface. These
properties can be useful for photocatalytic reaction. Surface plasmon sensitized Ag
42–46
photocatalysts showed promising photocatalytic activity in the visible light region.
The absorption range of Ag lies on the emission position of upconversion phosphor, and
3+
it enhances local field which results in an enhancement of the energy transfer from Yb
3
+ 47,48
to Er .
Therefore, Ag can be used to absorb the emitted visible light from the
upconversion phosphor and transfer electrons to semiconductor catalyst. Xu et.al
3+
3+
3+
studied plasmonic Au based NaYF
4
:Yb ,Tm
, 2
Er /TiO NIR photocatalyst for
4
9
complete solar utilization.
NaYF :Yb,Tm@TiO catalyst to enhance electronꢀhole pair separation. However,
plasmonic silver utilization with upconversion phosphor has not been demonstrated yet.
The fabrication of lanthanide doped upconverting phosphor, Ag and TiO in a single
Ma et.al utilized Ag in NIR responsive
5
0
4
2
2
system may allow the effective consumption of UV, visible and NIR light for
photocatalytic purpose via plasmonic and upconversion property.
In this work, NIR active NaGdF
synthesized by a oneꢀpot thermal decomposition method. Structural, morphological and
energy transfer process of NaGdF :Yb:Er/Ag/TiO were investigated and compared
with the NaGdF :Yb:Er/Ag, NaGdF :Yb:Er/TiO composites, which were synthesized
4 2
:Yb:Er/Ag/TiO nanocomposite catalyst was
4
2
4
4
2
5

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by the similar procedure. Photocatalytic activities of synthesized nanocomposite
catalysts were evaluated by Rhodamine B dye degradation experiments under NIR light
(
980 nm) and UVꢀvisible light (300 – 600 nm). The possible photocatalytic reaction
pathways were discussed. In this method, Nanoꢀscaled (less than 25 nm) upconversion
host (NaGdF ) and Ag particles are synthesized which increase the surface contact
among the NaGdF , Ag, and TiO particles. The up converting host and Ag
nanoparticles were closely contact with each other as well as TiO which is the major
advantages in this structure. These structure allows the efficient charge and/or energy
transfer among NaGdF :Yb:Er, Ag and TiO particles and enhance the plasmonic and
4
4
2
2
4
2
upconversion phenomenon in the photocatalytic reaction. The synthesized catalyst
would effectively use all the spectrum regions of the solar light.
2. Experimental section:
2.1 Materials
3 2 4
Titanium (IV) isopropoxide (Ti(OCH(CH ) ) , 95 %, Wako), silver nitrate
(AgNO
3
, Wako), oleylamine ( C18
H
35NH2, 70 %, Wako), gadolinium oxide (Gd
2 3,
O
99.9 %, Wako), ytterbium oxide (Yb
2
O
3, 99.9 %, Wako), erbium oxide (Er 3, 99.9 %,
2
O
Wako), sodium trifluoroacetate (CF COONa, 98 %, Wako), trifluoro acetic acid
3
(
2 3
C HF O2, 99 %, Wako) were used as precursors. All the oxides were dissolved in
trifluoro acetic acid to form trifluoroacetate, which was dried at 80 °C and preserved
in an airtight glass bottle. Chemicals such as ammonia (28 %, Wako), acetone
(99.5 %, Wako), cyclohexane (99.5 %, Wako) and ethanol (99.5 %, Wako) were used
as purchased. Photocatalytic activity of the catalyst was studied using Rhodamine B
(C
28
H31ClN
2
3
O , Wako) as a model pollutant.
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4 2
2.2 Synthesis of NaGdF :Yb:Er/Ag/TiO composite nanomaterials
NaGdF
thermal decomposition method. The schematic diagram of the synthesis procedure is
shown in Fig. 1. The precursors for NaGdF such as CF COONa (6 mmol),
Gd(CF COO) (2.34 mmol), Yb(CF COO) (0.6 mmol) and Er(CF COO) (0.06 mmol)
4 2
:Yb:Er/Ag/TiO nanocomposite was synthesized by the highꢀtemperature
4
3
3
3
3
3
3
3
were dissolved in a 15 mL of oleylamine solvent in a round bottom (RB) flask (100
mL) attached with a cooling condenser. The RB flask was evacuated with a vacuum
pump and the reaction temperature raised to 160 °C. The reaction was kept at 160 °C for
3
0 min to remove moisture and O
30 °C to initiate the decomposition of precursors. The reaction was carried out for 1 h
flow. The resultant product was cooled to 50 °C and silver nitrate (2
mmol) was added to the NaGdF :Yb:Er–oleylamine solution. The reaction temperature
was increased to 150 °C and kept for 1 h under N2 atmosphere to obtain
NaGdF :Yb:Er/Ag composite. The reaction solution colour was turned to red,
2
in oleylamine. The reaction solution was raised to
3
at 330 °C under N
2
4
4
specifying the formation of Ag particles. To the resultant solution, 3 mL of Titanium
isopropoxide was added and the temperature of the solution again raised to 280 °C and
kept for 1 h under air atmosphere. The resulting NaGdF :Yb:Er/Ag/TiO composite
4
2
solution was cooled to room temperature. The sample was precipitated, centrifuged and
thoroughly washed several times with hexane, acetone and ethanol solvents.
4 4 2
NaGdF :Yb:Er/Ag and NaGdF :Yb:Er /TiO composites were synthesized by the same
procedure as mentioned for the comparison of luminescence and catalytic activity
studies. The synthesized NaGdF :Yb:Er/Ag/TiO catalyst was taken in a 20 mL of
4
2
ammonia and ethanol mixed solution (50 mL/50 mL) and stirred for 30 min to remove
the surface oleylamine. The reaction mixture was centrifuged at 6000 rpm and the
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sample redispersed again in a 20 mL of ammonia/ethanol solution. The procedure was
repeated four times, the obtained product was washed well with water until pH 7. The
sample was dried and used for photocatalytic studies.
2.3 Photocatalytic degradation of Rhodamine B
Photocatalytic performance of NaGdF :Yb:Er/Ag/TiO
4
2
nanocomposite was
evaluated by monitoring degradation of Rhodamine B dye molecules under NIR
irradiation. The photocatalytic experiment was as follows: 8 mg of synthesized catalyst
was suspended in a 5 mL of Rhodamine B (5 mg/L) solution and stirred for 1 h in the
dark to reach adsorption–desorption equilibrium of the dye with the catalyst before
irradiation. The Rhodamine B solution was irradiated with a 980 nm NIR source (1.5
W). 1.5 mL of solution was taken out from the reaction mixture at constant time
intervals (3 h). The absorbance of Rhodamine B was monitored at 554 nm using a UVꢀ
Vis spectrophotometer and then the solution was put back into the cell. To evaluate UVꢀ
visible light driven photocatalytic activity, 40 mg of catalyst was added into a labꢀmade
borosilicate reactor containing 40 mL of Rhodamine B solution (5 mg/L). The solution
was illuminated with a UVꢀvisible light source (500 W, 300ꢀ600 nm). 3 mL of
suspensions were withdrawn from the reaction mixture at regular irradiation time
intervals. The suspension was centrifuged to remove catalysts and absorbance was
measured.
2.4 Characterization
The crystal structures of the synthesized composite catalysts were identified by
powder Xꢀray diffraction (XRD) using a Rigaku Xꢀray diffractometer RINTꢀ2200 (Cu
Kα radiation, λ=1.54178 Å)). JEOL JEM 2100F HRTEM (200 kV acceleration voltage)
8

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was used for transmission electron microscope (TEM), energy dispersive Xꢀray
spectroscopy (EDX) and mapping images. The oxidation states of the catalyst were
analyzed with Xꢀray photoelectron spectroscopy (XPS) (ESCA 3100, Shimadzu).
Diffuse reflectance spectra (DRS) and ultravioletꢀvisible absorption spectra were
analyzed by UVꢀVISꢀNIR spectrophotometer (JascoVꢀ670). Photoluminescence (PL)
spectra were recorded in the range of 350–850 nm using a fluorescence
spectrophotometer (Jasco FP ꢀ 8600). Photocatalytic measurement was performed using
UVꢀvisible light source (MAX 303, Asahi, Japan) and 980 nm continuous NIR laser of
Changchun New Industries Optoelectronics Technology Co., Ltd.
4. Result and discussion
4
.1. Structural, morphological and elemental studies
The crystal phases of synthesized NaGdF :Yb:Er, NaGdF :Yb:Er/Ag and
4
4
NaGdF
4
:Yb:Er/Ag/TiO
:Yb:Er, NaGdF
compared in Fig. 2. Recorded reflections of NaGdF
2
samples were identified by XRD analysis. The acquired
:Yb:Er/Ag and NaGdF :Yb:Er/Ag/TiO were
:Yb:Er (Fig. 2a) were well matched
patterns of NaGdF
4
4
4
2
4
with a hexagonal crystal structure of NaGdF (JCPDS Card No. 27ꢀ0699). A low
4
intense peak appeared at 28.2° (●) corresponded to the cubic crystalline nature of the
NaGdF
present in the sample. When Ag is added, the characteristic Ag peaks appeared at 38.2°,
4.3°, and 64.4°, which corresponded (111), (200), and (220) planes of face centredꢀ
4
(JCPDS Card No. 27ꢀ0697), which indicated a small amount of cubic phase
4
cubic structure of silver particle (JCPDS Card No. 01ꢀ087ꢀ0720), which indicated the
presence of metallic silver in NaGdF :Yb:Er/Ag composite (Fig. 2b). XRD pattern of
4
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NaGdF
which are marked by the lattice planes in (Fig. 2c) along with NaGdF
XRD peaks of NaGdF , Ag and TiO were observed for NaGdF :Yb:Er/Ag/TiO as
4
:Yb:Er/TiO
2
shows anatase phase TiO
2
peaks (JCPDS Card No. 21ꢀ1272),
4
reflections. The
4
2
4
2
shown in Fig. 2d, which confirmed the formation of composites.
The morphology and size of the nanocomposite were studied by TEM images
and elemental mapping analysis. Fig.3.1 shows the NaGdF :Yb:Er particles which were
about 10 nm in diameter. NaGdF :Yb:Er/Ag (Fig. 3.2) composites exhibited distinct Ag
and NaGdF particles. TEM images of NaGdF :Yb:Er/Ag/TiO composite consisted of a
4
4
4
4
2
foamꢀlike structure with sphereꢀlike particles dispersed everywhere (Fig.3.3). The
elemental mapping of individual elements Na, Gd, F, Yb, Ag, Ti, and O (Fig (3.5 b) ꢀ
(
3.5 h)) evidently showed the existence of NaGdF
foamꢀlike structure. The elements were confirmed with EDS spectrum as shown in Fig.
.5 i. The elemental mapping of Gd (Fig. 3.5 c) and Ag (Fig. 3.5 f) displayed the
spherical shaped NaGdF :Yb:Er and Ag particles with diameter ranged between 8ꢀ20
nm. They were dispersed on the fine TiO grains. Over all size of
NaGdF :Yb:Er/Ag/TiO nanocomposite particles were in the range of 100 – 200 nm.
The spacing of lattice fringes 0.52 , 0.23 and 0.35 nm (Fig. 3.4) indicated the (100),
4 2
:Yb:Er and Ag particles in the TiO
3
4
2
4
2
(
111) and (101) planes of hexagonal NaGdF cubic Ag, and anatase TiO , respectively.
4, 2
4
TEM and mapping suggested that NaGdF and Ag nano particles formed a composite
2
structure with TiO particles.
The chemical environment of the elements present in the
:Yb:Er/Ag/TiO composite were analyzed using XPS analysis. The XPS
spectrum of Ti 2p, O 1s, and Ag 3d orbitals are measured as shown in Fig. 4 (aꢀc). The
NaGdF
4
2
1
0

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two peaks Ti 2p3/2 and Ti 2p1/2 are positioned at 458.8 eV and 464.5 eV, which
4
+
3+
attributed to the Ti state of anatase TiO
which appeared at 457.3 eV. The asymmetric O 1s peak was deconvoluted into two
peaks centered at 530.3 and 532.1 eV, which are related to the lattice oxygen of TiO
2
. Surface disorders led to Ti formation,
2
and hydroxyl oxygen present on the surface, respectively. The two peaks, Ag 3d 5/2 and
Ag 3d 3/2 separated with ~6 eV were located at 367.9 and 373.9 eV, which designated
the metal state of Ag particle. A small quantity of silver exists as an oxide form,
observed around ~367eV.
F
1s peak of NaGdF4, NaGdF :Yb:Er/Ag and
4
NaGdF
4
:Yb:Er/Ag/TiO
:Yb:Er/Ag (686.6 eV) and NaGdF
:Yb:Er at 686.0 eV. This suggested the chemisorption of
particle with Ag and TiO2. The Yb 4d and Er 4d peaks (Fig. 4 (e)) were
observed at 172.6 eV and 188.4 eV respectively, well matched with reported values,
2
was studied and compared in Fig 4. (d). The F 1s peak of
NaGdF
4
4
:Yb:Er/Ag/TiO (685.1 eV) were shifted
2
from the F 1s peak of NaGdF
NaGdF
4
4
51
confirming the doping on NaGdF
.2 Optical studies
Optical properties of synthesized composite catalysts were analyzed by UVꢀ
4
.
4
visible absorption measurement and upconversion luminescence analysis. Recorded
absorption spectra of NaGdF :Yb:Er, NaGdF :Yb:Er/Ag and NaGdF :Yb:Er/Ag/TiO
4
4
4
2
are shown in Fig. 5 (a). Absorption spectrum of NaGdF
4
:Yb:Er (black curve) showed
3
+
small absorption peaks of Er (488, 520, 540 and 653 nm). For NaGdF :Yb:Er/Ag (red
4
curve), a wide absorption peak (350ꢀ750 nm) was observed in the visible region, which
corresponded to the localized surface plasmon resonance peak delivered by Ag NPs.
1
1

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The absorption spectrum of NaGdF
absorption peaks of Ag combined with TiO
Upconversion luminescence spectra of asꢀsynthesised NaGdF :Yb:Er,
4
2
:Yb:Er/Ag/TiO (blue curve) showed the wide
2
absorption in the UV region.
4
4 4 2 4 2
NaGdF :Yb:Er/Ag, NaGdF :Yb:Er/TiO and NaGdF :Yb:Er/Ag/TiO nanoparticles
were studied under an excitation wavelength of 980 nm as shown in Fig 5 (b). The
luminescence spectra showed four significant peaks at 523, 542, 656 and ~ 800 nm,
2
4
4
4
4
which corresponded to the transitions of H11/2, S3/2,
F9/2 and I9/2 levels to the I15/2
3
+
51–53
level of Er , respectively.
emission intensities were decreased slightly because the excitation light was shielded by
TiO particles on the surface of the NaGdF :Yb:Er. The peak intensities of
NaGdF :Yb:Er/Ag and NaGdF :Yb:Er/Ag/ TiO were drastically decreased because of
silver and TiO addition to the NaGdF :Yb:Er particles. Fig. 5 (c) shows the visual
emitting images of NaGdF :Yb:Er, NaGdF :Yb:Er/Ag and NaGdF :Yb:Er/Ag/TiO
After addition of TiO to NaGdF , the upconversion
2 4
2
4
4
4
2
2
4
4
4
4
2,
excited by NIR 980 nm laser. The brightness of green color light of NaGdF
4
:Yb:Er was
4
4
reduced and turned to weak red ( I9/2 ꢀ I15/2) by the addition of Ag and TiO2,
respectively. Ag particles induced larger reduction of green light emission than red light
since the absorption maximum exactly matched with the green light region. These
observations can be explained by the schematic diagram as shown in Fig. 6, which
shows the photoluminescence and NIR light influenced photocatalytic reaction
mechanism of the NaGdF
4
:Yb:Er/Ag/TiO
2
composite. It is well known that
upconversion phosphors can convert longer wavelength NIR radiation into shorter
wavelength light (UV or visible light) through a two or multiphoton absorption
3
9,51
mechanism
. During the excitation of NaGdF
4
:Yb:Er under 980 nm by NIR laser,
3
+
Yb ion acts as a NIR sensitizer, which absorbs the photons (980 nm), and results in an
1
2

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2
2
3+
3+
electron excitation from F7/2 to F5/2 level. The excited Yb transports energy to Er
4
4
3+
ions during relaxation. The process induced excitation from I15/2 to I11/2 in Er ions.
3
+
Two or more similarly excited photons were populated at higher energy levels of Er
4
4
2
29
3+
(
F
9/2, S3/2 and H11/2). The excited electrons of Er relaxed to the ground state with
the emission of visible light. The absorption range of the Ag overlaps the emitted visible
light of NaGdF :Yb:Er, which indicated that the emitted light from NaGdF :Yb:Er
4
4
could be recaptured by Ag. This reabsorption led to the strong decrement of
luminescent intensity. TiO encapsulation over the NaGdF :Yb:Er/Ag led to a further
2
4
2
decrease of intensity, indicating the energy transfer from Ag to TiO particles. Similar
kind of luminescent intensity reduction was observed and reported for
29,48
4 4 2
NaGdF :Yb:Er/Au and NaYF :Yb:Er/Cꢀdoped TiO composites.
4.3 Photocatalytic studies
Rhodamine B was utilized as a model pollutant to demonstrate the performance of
the prepared catalysts under NIR irradiation (980 nm). Dye degradation (C/C ) was
calculated from the initial (C ) and absorbance values (C) of Rhodamine B at different
0
0
interval of time. The absorbance of Rhodamine B decreased gradually because of the
photocatalytic degradation reaction catalyzed by NaGdF :Yb:Er/Ag/TiO
4
2
nanocomposites as shown in Fig. 7. The timeꢀdependent relative concentration ratios
C/C ) of Rhodamine B was studied for NaGdF :Yb:Er/Ag and NaGdF :Yb:Er/Ag/TiO2,
as shown in Fig. 8 (a). There is no degradation of Rhodamine B in the absence of a
catalyst at NIR irradiation. The C/C value against time revealed that
(
0
4
4
0
NaGdF :Yb:Er/Ag/TiO degraded 40 % of Rhodamine B under 18 h NIR irradiation.
4
2
However, NaGdF
4
. 2
:Yb:Er/Ag were able to degrade only 21 % Generally, TiO and Ag
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were inactive in NIR light. In the case of NaGdF
light of NaGdF :Yb:Er was recaptured by Ag particles and generated hot electrons and
holes. In NaGdF :Yb:Er/Ag/TiO , during surface plasmon excitation the electrons of Ag
4
:Yb:Er/Ag, the emitted upconversion
4
4
2
2
obtained sufficient energy to overcome schottky barrier and transferred to TiO surface
5
4
by charge transfer mechanism. Hence, the recombination of photogenerated electron
and hole pair minimized significantly and improved the electron – hole pair separation
in Ag nano particles. Photoꢀgenerated holes in Ag particles plays a vital role to oxidize
the Rhodamine B into oxidized products (CO and H O). The moved electrons to the
2
2
•
ꢀ
•
2 2
conduction band of TiO generated O radicals which in turn converted to OH radicals.
•
•
ꢀ
The photo generated OH radicals (and
2
O ) actively oxidized the conjugated
•
chromophore of the dye molecule. The OH formation reaction are shown in equation (1.1) ꢀ
(
1.5).
-
•O ꢀ
O
2
(TiO
2
) + e
2
(TiO
2
)
(1.1)
(1.2)
(1.3)
(1.4)
•
ꢀ
+
•
O (TiO ) + H
HO (TiO )
2
2
2
2
•
•
HO
2
+ HO
2
H
2
O
2 2
+ O
−
•
•
-
H
2
O
2
+ O
2
OH + OH + O
2
+
h (Ag) + RB
degraded or mineralized product (1.5)
degraded or mineralized product (1.6)
•
−
•
OH (or O
2
) + RB
From the Fig. 8a, NaGdF :Yb:Er/Ag/TiO showed higher photocatalytic activity
4
2
than the NaGdF
keen energy transfer between NaGdF
Terephthalic acid fluorescence probe test was carried to identify the formation of
4
:Yb:Er/Ag. It confirmed that Ag acts as an intermediate carrier for the
4
:Yb:Er and TiO . For more confirmation,
2
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4

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•
27,30
oxidative OH radicals on the surface of the nanocomposite during NIR irradiation.
Fig. 8b shows the fluorescence spectra of hydroxy terephthalic acid monitored at 424
nm under 320 nm excitation. Terephthalic acid didn’t show any fluorescent emission,
but the nonꢀfluorescent terephthalic acid produced fluorescent emission at 424 nm when
4 2
it became hydroxyl terephthalic acid. NaGdF :Yb:Er/Ag/TiO catalyzed terephthalic
acid photocatalytic reaction driven by NIR irradiation produced hydroxy terephthalic
acid intermediate when exposed by 980 nm light. The gradual increment in the intensity
with time confirmed the increasing concentration of hydroxy terephthalic acid. The
•
intensity change means OH radical generated during the reaction.
For further photocatalytic investigation, the photocatalytic degradation was
performed under UVꢀvisible light irradiation and the results are shown in Fig.9 (aꢀd).
The absorption spectra of Rhodamine B decreased gradually with NaGdF
4 2
:Yb:Er/TiO
(
a) NaGdF :Yb:Er/Ag (b), and NaGdF :Yb:Er/Ag/TiO (c) catalysts under UVꢀvisible
,
4
4
2
light irradiation. Two different degradation patterns were observed for the composite
catalysts. NaGdF :Yb:Er/TiO catalysed photodegradation curves of Rhodamine B
4
2
hypsochromic shifted to shorter wavelength (~498 nm) during 30 min irradiation; after
that the absorption position did not change, but the absorption intensity decreased to
4 4 2
nearly zero. NaGdF :Yb:Er/Ag and NaGdF :Yb:Er/Ag/TiO catalysed reaction showed
55
the traditional degradation pattern. The hypsochromic shift can be explained through
two different cleavage pathway of Rhodamine B molecule under UVꢀvisible irradiation.
Generally, Rhodamine B adsorbs on the surface of TiO
2
via carboxylic group
5
6
(
COOH).
chromophore ring, which is identified from the decrease of Rhodamine B absorption at
54 nm. On the other hand, few reports suggest that surface fluorination or anionic
The oxidative species of TiO are actively attacked the conjugated
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56
surfactant modification on TiO
2
convert the adsorption modes of TiO
2
.
Surface
fluorination on TiO
2
increases more polarity (negative charge) on the surface. Therefore,
Rhodamine B tend to adsorb through ꢀNEt (cationic group). This type of adsorption led
2
to the cleavage of amine group initially and produce intermediate rhodamine (498
5
7,58
nm).
catalyst surface. The adsorption (%) of NaGdF
NaGdF :Yb:Er/Ag/TiO are 48, 36, 15 % respectively, calculated from the absorbance
difference between the fresh dye and before irradiation (0 min). The Rhodamine B
adsorbed on the NaGdF :Yb:Er/TiO surface was much higher than that of
NaGdF :Yb:Er/Ag and NaGdF :Yb:Er/Ag/TiO composites. The adsorption behaviour
attributed to the increased surface area and polarity of the composites. In
NaGdF :Yb:Er/TiO composite NaGdF has changed the surface characteristic of
This interpretation is supported by the adsorption of Rhodamine B on the
4
:Yb:Er/TiO2, NaGdF :Yb:Er/Ag and
4
4
2
4
2
4
4
2
4
2
,
4
NaGdF :Yb:Er/TiO and orient the Rhodamine B adsorption through amine group. The
4
2,
adsorption on TiO
2
facilitated the Nꢀdeꢀalkylation of Rhodamine B (Fig. 9 e) and
produce Rhodamine, which was observed at 498 nm (Fig.9 a). The resultant
intermediate degraded gradually with the irradiation of light. It is understood that for
semiconductor based photocatalysts adsorption of reactants are very important for
enhanced photocatalytic activity. Since it introduces reactants rich environment and
59
reinforce the interaction between reactant and catalyst surface. On the other hand, for
silver
nanoparticle
containing
composites
NaGdF
4
:Yb:Er/Ag
and
NaGdF
4
:Yb:Er/Ag/TiO
2
(Fig. 9 b and c) the absorption intensity of Rhodamine B
,
decreases with slight shift but did not approach to 498 nm. The observation suggested
that silver addition to the NaGdF reduces the surface charge of the catalyst by making
4
chemisorption. Therefore, the rhodamine B was predominantly decomposed by typical
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conjugated chromophore cleavage pathway rather than Nꢀdeꢀalkylation. Even though,
nanocomposite catalysts decomposed the rhodamine B by two different pathways,
NaGdF :Yb:Er/Ag/TiO composite took only 30 min for almost complete degradation.
4
2
Fig.9 d shows that NaGdF
NaGdF :Yb:Er/Ag and NaGdF
efficient generation of photogenerated species
4
:Yb:Er/Ag/TiO
2
had a higher photocatalytic activity than
4
4
:Yb:Er/TiO
2
because of enhanced light absorption and
.
The results conclude that NaGdF
4
introduced not only upꢀconversion luminescence emission but also increase of the
adsorption of dyes (cationic groups) on the surface of the catalysts. The results
exhibited the excellent UVꢀvisible light driven photocatalytic ability of the catalyst.
The reusability of the catalyst was verified under UVꢀvisible irradiation, as shown
in Fig. 10. The catalyst showed 97, 95, 92, 94, 80 and 77 % of Rhodamine B
degradation for 30 min. The catalyst exhibited nearly equal value for first four cycles.
After that, it started to decrease. This might be due to the aggregation of particles during
prolonged usage.
5
. Conclusions
Oneꢀpot thermal decomposition method was developed to synthesize
NaGdF :Yb:Er/Ag/TiO nanocomposite XRD, TEM, EDS and elemental mapping
4
2
.
results confirmed the composite structure of the NaGdF :Yb:Er, Ag particles, and TiO
4
2
nanostructrures. The materials showed excellent luminescent quenching behavior upon
80 nm irradiation by adding Ag and TiO with NaGdF :Yb:Er. The emission region of
NaGdF :Yb:Er overlapped with the absorption range of Ag/TiO2, which was identified
9
2
4
4
by UVꢀVis absorption spectra. The emitted upconversion light was recaptured by
Ag/TiO which created a photocatalytic reaction under NIR irradiation. Interestingly,
2
,
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two different fragmentation pathways were identified for Rhodamine B with the
composites under UVꢀvisible light because of the two different adsorption modes. The
promising photocatalytic results of NaGdF :Yb:Er/Ag/TiO under the UVꢀvisible and
4
2
NIR lights revealed a new pathway to harvest UVꢀvisible and infraꢀred light from solar
light
Acknowledgements
This research was financially aided by a GrantꢀinꢀAid for JSPS Fellows related
to a JSPS Postdoctoral Fellowship for Foreign Researchers (No. 26ꢀ04050) from the
Ministry of Education, Culture, Sports, Science and Technology of Japan and the
cooperative research project of the Research Institute of Electronics, Shizuoka
University. We thank Centre for Nanoꢀdevice Fabrication and Analysis for the use of
the Instruments. We sincerely thank Mr. Koyama and Mr. Tomada for their help to
measure various instruments.
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Figure captions:
4 2
Fig. 1 schematic diagram of synthesis of βꢀNaGdF :Yb:Er /Ag/TiO nanocomposites
Fig. 2. XRD patterns of (a) NaGdF :Yb:Er, (b) NaGdF :Yb:Er/Ag, (c)
4
4
NaGdF :Yb:Er/TiO and (d) NaGdF :Yb:Er/Ag/TiO
4
4
2
2
Fig 3. TEM images of (1) NaGdF
4
:Yb:Er, (2) NaGdF
4
:Yb:Er/Ag, (3) and (4)
NaGdF :Yb:Er/Ag/TiO ; (5a) STEM image, EDS mappings of (b) Na, (c) Gd, (d) F, (e)
4
2
Yb (f) Ag, (g) Ti, and (h) O and (i) EDS spectrum of NaGdF :Yb:Er/Ag/TiO2
4
composite
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3

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Fig. 4. XPS binding energy curves of NaGdF
Ag 3d, (d) F 1s, and (e) Yb 4d and Er 4d
Fig. 5. (a) UVꢀVisꢀNIR absorption spectra, (b) upconversion luminescence spectra, (c)
visual fluorescence emission images under 980 nm excitations of NaGdF :Yb:Er,
NaGdF :Yb:Er/Ag and NaGdF :Yb:Er/Ag/TiO
4
:Yb:Er/Ag/TiO2, (a) Ti 2p, (b) O 1s, (c)
4
4
4
2
Fig. 6 Schematic diagram of energy transfer and photocatalytic mechanism among Yb,
Er, Ag, TiO in NaGdF :Yb:Er/Ag/TiO nanocomposite catalyst
2
4
2
Fig. 7. NaGdF :Yb:Er/Ag/TiO catalyzed degradation of Rhodamine B at constant
4
2
irradiation time (3 h) under NIR irradiation (980 nm)
Fig. 8. (a) Relative ratio (C/C ) of Rhodamine B at different time catalyzed by
NaGdF :Yb:Er/Ag/TiO and NaGdF :Yb:Er/Ag composites under NIR (980 nm) (b)
0
4
2
4
Terephthalic acid fluorescence spectra at different time catalyzed by
NaGdF :Yb:Er/Ag/TiO under NIR irradiation
4
2
Fig.9. Rhodamine B degradation spectra with different catalysts (a) NaGdF
4 2
:Yb:Er/TiO
(
b) NaGdF :Yb:Er/Ag, (c) NaGdF :Yb:Er/Ag/TiO2 ; (d) relative absorbance of
4
4
Rhodamine B with three different catalyst under UVꢀvisible light irradiation; (e)
Rhodamine B structural change with NaGdF :Yb:Er/ TiO under UVꢀvisible light
4
2
irradiation.
Fig. 10. Rhodamine B degradation with NaGdF
4
2
:Yb:Er/Ag/TiO under UVꢀvisible light
irradiation from the first to sixth cycles.
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4

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4 2
Fig. 1 schematic diagram of synthesis of βꢀNaGdF :Yb:Er /Ag/TiO nanocomposites
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5

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Fig. 2. XRD patterns of (a) NaGdF :Yb:Er, (b) NaGdF :Yb:Er/Ag, (c)
4
4
NaGdF
4
:Yb:Er/TiO and (d) NaGdF
4
:Yb:Er/Ag/TiO2
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6

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7

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Fig 3. TEM images of (1) NaGdF
4
4
:Yb:Er, (2) NaGdF :Yb:Er/Ag, (3) and (4)
NaGdF :Yb:Er/Ag/TiO ; (5a) STEM image, EDS mappings of (b) Na, (c) Gd, (d) F, (e)
4
2
4 2
Yb (f) Ag, (g) Ti, and (h) O and (i) EDS spectrum of NaGdF :Yb:Er/Ag/TiO
composite
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Fig. 4. XPS binding energy curves of NaGdF :Yb:Er/Ag/TiO (a) Ti 2p, (b) O 1s, (c)
4
2,
Ag 3d, (d) F 1s, and (e) Yb 4d and Er 4d
2
9