Novel high-efficiency visible-light responsive Ag4(GeO4) photocatalyst

Article abstract of DOI:10.1039/c7cy00393e

A novel high-efficiency visible-light responsive Ag₄(GeO₄) photocatalyst was prepared by a facile hydrothermal method. The photocatalytic activity of as-prepared Ag₄(GeO₄) was evaluated by photodegradation of methylene blue (MB) dye and water splitting experiments. The photodegradation efficiency and oxygen production efficiency of Ag₄(GeO₄) were detected to be 2.9 and 1.9 times higher than those of Ag₂O. UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence experiment and photoelectric effect experiments prove that the good light response and high carrier separation efficiency facilitated by the internal electric field are the main reasons for Ag₄(GeO₄)'s excellent catalytic activity. Radical-trapping experiments reveal that the photogenerated holes are the main active species. First-principles theoretical calculations provide more insight into understanding the photocatalytic mechanism of the Ag₄(GeO₄) catalyst.

Full text of DOI:10.1039/c7cy00393e

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M. Li, Q. Zhang, E. A. Rozhkova, X. Qin, X. Zhang, Y. Dai, Z. Wang and B. Huang, Catal. Sci. Technol., 2017,
DOI: 10.1039/C7CY00393E.
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Novel high-efficiency visible-light response Ag (GeO ) photocatalyst
4
4
a
a,
b
a
Xianglin Zhu , Peng Wang *, Mengmeng Li Qianqian Zhang , Elena A.
c,
a
a
b
a
,
Rozhkova * , Xiaoyan Qin , Xiaoyang Zhang , Ying Dai Zeyan Wang , Baibiao
a,
Huang *
a
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100,
China
b
School of Physics, Shandong University, Jinan 250100, China
c
Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois
6
0439, United States
Keywords: Photocatalytic; Ag (GeO ); Water splitting; Internal electric field
4
4
Abstract
4 4
A novel highꢀefficiency visibleꢀlight Ag (GeO ) photocatalyst was prepared by a
facile hydrothermal method. The photocatalytic activity of asꢀprepared Ag
4
4
(GeO )
was evaluated by photodegrading of methylene blue (MB) dye and water splitting
experiments. The photodegradation efficiency and oxygen production efficiency of
4 4 2
Ag (GeO ) were detected to be 2.9 and 1.9 times higher than that of Ag O. The UV–
vis diffuse reflectance spectra (DRS), photoluminescence experiment and
photoelectric effect experiments prove that the good light response and high carrier
separation efficiency facilitated by the internal electric field is the main reason for
4 4
Ag (GeO )’s excellent catalytic activity. The radicalꢀtrapping experiments reveal that
photo generated holes are the main active species. Firstꢀprinciples theoretical
calculations provide more insight into understanding of the photocatalytic mechanism
4 4
of Ag (GeO ) catalyst.
1

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Introduction
In the past decades, large massive of fossil fuels have been consumed to address
needs in energy and chemical products manufacturing for the society development. As
a result, global energy crisis and environment pollution issues have become two major
thorny issues which must be solved as soon as possible. One efficient approach to
resolve these global problems is to use photocatalysis technique, an inherently clean
way which can transform inexhaustible energy of solar irradiation into
environmentally benign fuels. Photocatalysis has been proved to have promising
1
ꢀ4
application in hydrogen generating from water splitting by using of solar energy
5
ꢀ7
and organic pollutants’ degradation . After years of extensive research efforts
toward materials design, synthesis and bandꢀengineering techniques challenges still
exist, and industrialization and commercialization of photocatalysts have not yet been
8
ꢀ14
realized
. Hence, the exploring and designing of novel photocatalysts with
enhanced performance is required for both the fundamental research and industrial
applications.
In order to get photocatalysts with high activity, light absorption ability and
efficient carriers’ separation efficiency the two main challenges must be solved in the
progress of designing and preparing materials. Up to now, visibleꢀlight sensitive
photocatalysts has been a hot spot in photocatalysis research. Agꢀbased structures
15ꢀ22
such as Ag@AgX (X=Cl, Br, I), Ag
3
PO
4
, Ag
2
O, Ag
2
Mo
3
O
11 and Ag
2
CO
3
have
drawn remarkable attention and been considered as one of the most attractive
photocatalysts owing to their excellent visible light absorption and high photocatalytic
activities in decomposing organic compounds. Besides, the influence of morphology,
structure and heterojunction for Ag based photocatalysts were also deeply discussed.
3
5ꢀ40
+
Due to the special d10 electronic configurations of Ag ions in AgOꢀbased
compounds, the top of valence band (VB) containing the unique hybridized Ag 4d and
O2p orbitals can lift the top position of the VB and narrow down the band gap. As a
result, these catalysts usually show outstanding visible light absorption properties and
excellent photocatalytic activities. Promoting the separation of photogenerated
2

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carriers is also a key point in designing efficient photocatalysts. Most studies on
speeding up the rates of charge carriers are mainly focused on construction of
heterojunction catalysts, such as BiPO
4
/Bi
2
6
WO , BiOCl/BiOBr and BiPO
4
/BiOI etc.
2
3
24ꢀ26
27
Different from these work, our group
and some other researchers reported
that intrinsic internal electric field covering the whole system is formed within polar
materials. The buildꢀin internal electric field plays a critical role in decreasing the
recombination of carriers. As a result, Agꢀbased materials especially with polar
structures are considered as new prospective visibleꢀlightꢀdriven photocatalysts
because of their unique crystal and electronic structures.
4 4
Herein, Ag (GeO ) was first synthesized through a facile oneꢀstep hydrothermal
method. The photocatalytic activity of asꢀprepared sample was characterized by
photodegrading MB dye and water splitting in presence of a sacrificial reagent. Our
results demonstrate that wide absorption of the visible light and efficient separation of
carriers caused by intrinsic internal electric field determine high efficiency of the
Ag (GeO ) photocatalyst.
4
4
Experimental
Synthesis of Ag (GeO ) photocatalyst.
4
4
3
4
Firstly, Na was synthesized through solid state reaction method. Typically
2
GeO
3
Na CO and GeO were grinded together at an equimolar ratio and then pressed into a
2
3
2
disk with1.5 cmꢀdiameter, 5 mmꢀthick. The disk was heated in air at 900 °C (5°C/min)
for 10 hrs and then was grinded into powder for the further use.
Ag
details, 10.0 mmol AgNO
deionized water. Then the Na
4 4
(GeO ) was synthesized through a simple oneꢀstep hydrothermal method. In
3
and 2.5 mmol Na
2
GeO
3
were dissolved in 40 mL
solution slowly
2
GeO solution was dropped into AgNO
3
3
under vigorous stirring. After stirring for three hours, the mixed solution was
transferred into 100 mL Teflonꢀlined autoclaves, which were sealed at 120°C for 10 h
and then cooled down naturally. Precipitate was filtered through a filter membrane
and washed with deionized water and absolute ethanol for three times and then dried
3

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at 60°C for 2h.
Ag O was synthesized by the similar solid state method as Ag (GeO ), only
2
4
4
replacing Na
2
GeO
was obtained by heating melamine at 550 °C with a ramp rate of
°C/min for 4 h.
3
with 10 mmol NaOH.
3 4
Bulk gꢀC N
2
Characterization
The structure of Ag (GeO ) was characterized by Xꢀray diffraction (XRD) using
4
4
Bruker AXS D8 advance powder diffractometer (Cu Kα Xꢀray radiation, λ=0.154056
nm). The morphologies of asꢀprepared samples were characterized using scanning
electron microscopy (SEM, Hitachi S4800). UVꢀVis diffuse reflectance spectra were
recorded using a Shimadzu UV 2550 spectrophotometer equipped with an integrating
sphere; and BaSO
4
was used as a reference. PLSꢀSXE 300, Beijing Trusttech Co. Ltd
2
3
00 W Xe arc lamp equipped with an ultraviolet cutoff filter (λ > 420 nm, 85 mW/cm )
was used as visibleꢀlight source. The amount of produced O was determined with a
2
gas chromatograph (Varian GCꢀ3800) equipped with thermal conductivity detector
and 5 Å molecular sieve column.
Photocatalytic experiments
The photocatalytic activities of asꢀprepared samples were evaluated through the
degrading MB dye and photocatalytic oxygen generation under visible light
irradiation (λ>420 nm). Typically, 50 mg photocatalyst was dispersed in 50 mL MB
dye solution (20 mg/L in deionized water), and the suspension was stirred and kept
in dark for half an hour to attain adsorption saturation. Then, photocatalytic
experiment was carried out under irradiation of 300 W Xe arc lamp. 3 mL of the
mixed suspension was taken every 10 minutes and centrifuged to remove solid
photocatalyst. The residual organic dye solutions were analyzed using a UVꢀvis
spectrophotometer. Photocatalytic production of oxygen was carried out in presence
of AgNO . Typically, 100 mg photocatalyst was added into 100 mL of aqueous
3
AgNO
3
solution (0.015 M), which was kept at 20°C in a quartz reactor. The reactor
4

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was sealed and vacuumed in order to remove the residual oxygen and switched on the
00 W Xe lamp. The output of oxygen was detected every half an hour.
3
Results and discussion
2 3 4 4
Fig. 1. XRD patterns of the asꢀsynthesized Na GeO and Ag (GeO ).
The crystal structures of asꢀprepared Na
by XRD. As it can be seen from the Fig. 1, the peaks of synthesized raw material
Na GeO are in well accordance with the peaks of the standard card. Peaks of neither
2 3 4 4
GeO and Ag (GeO ) were characterized
2
3
side products nor initial materials were detected, revealing purity of the synthesized
Na GeO . The peaks of synthesized Ag (GeO ) also exactly correspond to the
2
3
4
4
standard card (JPCDS No.89ꢀ312).
5

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4 4
Fig. 2. SEM images of the asꢀsynthesized Ag (GeO ): (a), (b) low magnification
and (c),(d) high magnification SEM images.
The morphology of asꢀprepared Ag
scanning electron microscopy (SEM), as shown in Fig. 2. Lower magnification SEM
images (Fig. 2a and b) show that the prepared Ag (GeO ) samples represent
irregularly sized carpetꢀlooking microfilms. The higher magnification SEM images
Fig. 2c and d) demonstrate that these ~300 nmꢀthick films are composed of uniform
crissꢀcrossing nanorods.
4 4
(GeO ) photocatalyst was characterized by
4
4
(
6

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4 4 2 3 4
Fig. 3. UV–vis diffuse reflectance spectra (DRS) of Ag (GeO ), Ag O and C N .
The optical properties of asꢀprepared Ag
studied using UV–vis diffuse reflectance spectra (DRS). As shown in Fig. 3, both
Ag (GeO ) and Ag O demonstrate better performance in light absorbance compared
with C N . While Ag (GeO ) shows lower absorption in visible light region than
4 4 2 3 4
(GeO ), Ag O and C N samples were
4
4
2
3
4
4
4
2
Ag O, it still covers the 200ꢀ800nm UVꢀvisible light region. Efficient absorption in
visible light region is propitious for enhancing photoexcitation efficiency
consequently leading to higher photocatalytic activity.
7

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Fig. 4. (a),(b) Photoꢀdegradation of MB dye solutions under visible light irradiation
2
(
λ>420 nm, 85 mW/cm ), (c) Photocatalytic O
2
production from water under visible
4 4 2
light irradiation (λ>420 nm), (d) Photocurrent results of Ag (GeO ) and Ag O (λ>420
nm).
Photocatalytic performance of asꢀprepared samples was evaluated by
photodegradation of methylene blue (MB) solution, photocatalytic O generation and
photocurrent response under visible light irradiation (Fig. 4). As presented in Fig. 4a
and b both Ag (GeO ) and Ag O can bleach MB solution more efficiently than C
Ag (GeO ) sample showed the best performance, and over 95% of the dye was
removed within 20 min. The ln(C /C)ꢀtime curves of the three catalysts excluding the
2
4
4
2
3 4
N .
4
4
0
effect of adsorption also prove that Ag
degradation rater of Ag (GeO ) is 2.9 times that of Ag
In photocatalytic water splitting experiments (Fig. 4c), similar tendencies were
observed, water splitting over the Ag (GeO ) catalysts resulted in higher oxygen yield
than over the Ag O under the same conditions. The oxygen generation rater of
Ag (GeO ) is 1.9 times that of Ag
4 4
(GeO ) has the highest reaction rate and the
4
4
2
O.
4
4
2
4
4
2
O. In order to further understand the reasons of
8

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better performance of Ag
and Ag O samples were compared. The photocurrent measurements results shown in
Fig. 4d demonstrate that Ag (GeO ) has higher photocurrent response than that of
4 4 4 4
(GeO ), the photoelectrochemical properties of Ag (GeO )
2
4
4
2
Ag O, indicating that more electrons and holes are generated and separated under the
visible light irradiation. BET surface area of Ag
4
(GeO
4
), Ag
2
O and C
3
N
4
was also
2
characterized. Compared with the BET surface area of Ag O and C N (5.4 m /g and
2
3
4
2
2
1
4.0 m /g), Ag
4
(GeO
4
) has the biggest BET surface area of 18.8 m /g and this is one
of the reasons that Ag
4
(GeO ) shows best activity.
4
Fig. 5. Photoluminescence experiment of Ag (GeO ) and Ag O.
4
4
2
From photoluminescence spectra, the recombination rates of electronꢀhole pairs in
photocatalysts can be evaluated. Typically,the higher PL intensity means the higher
recombination of electron hole pair. Fig. 5 is the PL spectra of Ag
4
(GeO
4
2
) and Ag O
excited at 330 nm. Ag (GeO ) and Ag O have weak PL intensity, while Ag
4
4
2
4
(GeO )
4
has lower intensity than Ag
2
O. This indicates the better separation efficiency of
carriers in Ag (GeO ).
4
4
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Fig. 6. Photocatalytic experiments in presence of ammonium oxalate, AgNO3,
tertꢀbutanol respectively under visible light irradiation (λ>420 nm).
Hydroxyl radicals •OH have been identified among few typical reactive species to
2
8
be involved into photocatalytic degradation of organic dyes . Therefore, to identify
the major reactive species involved into photodegradation of MB solution over the
4 4
Ag (GeO ) catalyst, radicalꢀtrapping experiments were carried out using ammonium
oxalate, AgNO and tertꢀbutanol as holeꢀ, electronꢀ and •OH radical ꢀ scavengers
3
(
respectively) under identical conditions. As shown in Fig. 6, adsorption changed after
adding of these radical scavengers. Degradation efficiencies didn’t change noticeably
in presence of AgNO and tertꢀbutanol compared to pure Ag (GeO ), proposing that
3
4
4
electrons and •OH radicals are not the main reactive species during the MB
degradation. However, when ammonium oxalate was added, the photocatalytic
4 4
activity of Ag (GeO ) was greatly suppressed, indicating that the photogenerated
holes were the major reactive species involved into the MB photodegradation.
1
0

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Fig. 7. XPS results of the asꢀprepared Ag (GeO ).
4
4
The chemical valence states and surface compositions of Ag
4
(GeO
4
) have been
(GeO
investigated, as shown in Fig.7. Fig. 7 (a) is the full XPS survey scan of Ag
4
4
)
which exhibits the characteristic peaks of Ag, Ge and O. The high resolution XPS of
Ag 3d, Ge 3d and O 1s are shown in Fig. 7b, c and d. Two peaks at 367.9 eV and
+
29
0
3
73.9 eV corresponded to characteristic peaks of Ag in Ag
4 4
(GeO ) . No Ag was
detected in XPS test. Signals around the 31.2eV are the Ge 3d characteristic peaks
30.Two peaks at about 530.0 eV and 531.4 eV corresponded to signals of O1s crystal
31
O element and adsorption O element.
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4 4
Fig. 8. Energy band diagram, DOS calculation results, crystal structure of Ag (GeO )
2 4 4
and Ag O and polarization direction of Ag (GeO ).
In order to get further insight into the mechanism of photo excitation and separation
of photogenerated electrons and holes, the energy band diagrams, DOS diagrams and
crystal structures of Ag
methods,(Fig. 8). As it can be seen from total DOS and local DOS, the top of valence
band of Ag (GeO ) is mainly consisted of hybridized Ag 4d and O2p states and the
4 4 2
(GeO ) and Ag O were produced using DFT calculation
4
4
bottom of conduction band is majorly formed with Ag 5s and Ag 4d states, which are
very similar to the results of Ag O. As it follows from the calculations, in the
2
Ag
4
(GeO
4
) crystal Ge element does little contribution to construction of Ag
4
4
(GeO )’s
VB and CB. Similarly to Ag
2
O the unique d10 and sp/p configurations are the main
1
2

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32, 33
reason for the visible light absorption of Ag
4
(GeO
4
).
In view of another key
factor influencing the photocatalytic properties, the separation efficiency of photo
generated carriers is the vital factor that determines the photocatalytic performance of
4 4 4 4
Ag (GeO ). As for Ag (GeO ) crystal, there is no symmetric center exists because
of the nonꢀsymmetric GeꢀO tetrahedron present in the structure. As a result, an
intrinsic internal electric field covering the whole material forms and promotes the
separation of photogenerated carriers. The latter results in more separated holes
available to join the oxidation reaction under irradiation of visible light.
Based on all the experimental and calculation results, we propose a possible
4 4
mechanism which can explain the high photocatalytic activity of Ag (GeO )
photocatalyst. Under the irradiation of visible light, electrons and holes are generated
and join the photocatalytic reaction. More carriers could be separated owing to the
4 4
existing intrinsic internal electric field in the Ag (GeO ) crystal. As a result, more
photo generated holes can be separated and, consequently, get involved into the
photocatalytic reaction, that leads to higher activities of Ag (GeO ) compared with
4
4
2
Ag O.
Conclusion
4 4
A novel photocatalytic material Ag (GeO ) was synthesized for the first time
through a hydrothermal method. The photocatalytic activities of the asꢀprepared
Ag (GeO ) were evaluated through photoꢀdegradation of MB dye solution and
photocatalytic O evolution. Ag (GeO ) photocatalyst showed excellent visible light
photocatalytic activities compared with Ag O photocatalyst. Both the experiments and
4
4
2
4
4
2
calculations results demonstrate that the wide range of light absorption and higher
separation of generated holes and electrons are the main reason that Ag (GeO ) has
4
4
advanced performance. Compared to Ag
electric field formed in the Ag (GeO ) arising from the polar structure that promotes
separation of photogenerated electrons and holes. As a result, the Ag (GeO ) shows
2
O photocatlyst, there is intrinsic internal
4
4
4
4
higher photoꢀoxidation abilities. Our new results provide further guidance for the
1
3

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designing of advanced high efficiency photocatalysts.
Acknowledgements
This work was financially supported by a research Grant from the National Basic
Research Program of China (No. 2013CB632401), the National Natural Science
Foundation of China (No. 21333006, 21573135, 11374190 and 51602179),
Recruitment Program for Young Professionals, China, Taishan Scholar Foundation of
Shandong Province, China. Dr. Rozhkova was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, contract no. DEꢀAC02ꢀ
06CH11357.
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Page 17 of 17
Catalysis Science & Technology
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DOI: 10.1039/C7CY00393E
The exiting of internal electric field promotes the separation of generated carriers, enhances the
photocatalytic activity in the polar materials
TOC Graphic
.