BaAu2S2: A Au-Based Intrinsic Photocatalyst for High-Performance Visible-Light Photocatalysis

Article abstract of DOI:10.1021/acs.inorgchem.7b00319

A new Au-based sulfide BaAu₂S₂ was obtained through solid-state reaction. It crystallizes in the tetragonal space group I4₁/amd with unit cell parameters of a = 6.389 72(2) ?, b = 6.389 72(2) ?, c = 12.7872(1) ?, and Z = 4. Its structure features [AuS_2/2]_∞ zigzag chains composed of corner-sharing AuS₂ linear units. With a direct band gap of 2.49 eV, BaAu₂S₂ is suitable for the visible-light harvesting. Moreover, it exhibits excellent visible-light photocatalytic activity, which is 1.3 times that of graphitic carbon nitride (g-C₃N₄) and also demonstrates excellent circulating stability. On the basis of the crystal and electronic structure analysis, the electrons are highly delocalized along the [AuS_2/2] chains, and the electron effective mass of BaAu₂S₂ is only approximately one-fifth of that of g-C₃N₄, which may help the separation of the electron/hole pairs during the photocatalytic process. Additionally, the absorption coefficient of BaAu₂S₂ is extremely high, exceeding 1 × 10⁴ cm^-1 over the entire absorbable visible spectrum (hν > E_g), which is significantly higher than that of g-C₃N₄. Such factors may contribute to its outstanding photocatalytic performances. According to our best knowledge, BaAu₂S₂ is the first noble metal-based chalcogenide photocatalyst reported as intrinsic light-harvesting and electron/hole-generating centers. This study may provide valuable insights for further research on photocatalytic materials.

Full text of DOI:10.1021/acs.inorgchem.7b00319

Article
pubs.acs.org/IC
BaAu2S2: A Au-Based Intrinsic Photocatalyst for High-Performance
Visible-Light Photocatalysis
Molin Zhou,†,§,∥ Xin Jiang,‡,§,∥ Xingxing Jiang,† Ke Xiao,¶ Yangwu Guo,†,§ Hongwei Huang,¶
Zheshuai Lin,† Jiyong Yao,*,† Chen-Ho Tung,‡ Li-Zhu Wu,*,‡ and Yicheng Wu†
†Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical
‡
Institute of Physics and Chemistry and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
§University of Chinese Academy of Sciences, Beijing 100049, P. R. China
¶Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials,
School of Materials Science and Technology, China University of Geosciences, Beijing 100083, P. R. China
S
* Supporting Information
ABSTRACT: A new Au-based sulfide BaAu2S2 was obtained through
solid-state reaction. It crystallizes in the tetragonal space group I41/amd
with unit cell parameters of a = 6.389 72(2) Å, b = 6.389 72(2) Å, c =
12.7872(1) Å, and Z = 4. Its structure features [AuS2/2]∞ zigzag chains
composed of corner-sharing AuS2 linear units. With a direct band gap of
2.49 eV, BaAu2S2 is suitable for the visible-light harvesting. Moreover, it
exhibits excellent visible-light photocatalytic activity, which is 1.3 times
that of graphitic carbon nitride (g-C3N4) and also demonstrates excellent
circulating stability. On the basis of the crystal and electronic structure
analysis, the electrons are highly delocalized along the [AuS2/2] chains,
and the electron effective mass of BaAu2S2 is only approximately one-fifth
of that of g-C3N4, which may help the separation of the electron/hole
pairs during the photocatalytic process. Additionally, the absorption
coefficient of BaAu2S2 is extremely high, exceeding 1 × 104 cm−1 over the
entire absorbable visible spectrum (hν > Eg), which is significantly higher than that of g-C3N4. Such factors may contribute to its
outstanding photocatalytic performances. According to our best knowledge, BaAu2S2 is the first noble metal-based chalcogenide
photocatalyst reported as intrinsic light-harvesting and electron/hole-generating centers. This study may provide valuable insights
for further research on photocatalytic materials.
these strategies, noble metal deposition, which is capable of
producing high Schottky barriers to facilitate the electron
capture, has been considered as the optimal option to achieve
better charge carrier mobility and more efficient suppression of
e−/h+ pairs recombination. To date, numerous reports have
disclosed that various noble metals deposited on crystalline
TiO2 matrix can significantly enhance its photocatalytic activity
due to the ameliorative charge transfer ability across the grain
boundaries.8,12,13 In essence, it is worth noting that the role of
the noble metals in the above strategy is more like an auxiliary
accelerator rather than a catalyst. More specifically, they only
act as electron pitfalls to boost the fast separation of e−/h+ pairs
induced in photosensitive substrate. Nevertheless, noble metals
may also be used as light-sensitive component in virtue of its
noticeable surface plasmon resonance (SPR) effect.14−16 To
our best knowledge, very few preceding studies have designed
noble metal-based compounds as intrinsic light-harvesting and
INTRODUCTION
■
Semiconductor photocatalysts, as critical environmental
purification materials, have played increasingly important
roles in photophysics/chemistry domain during the past
decades.1−3 Until now, a series of compounds has been
discovered and reported to demonstrate photocatalytic activity;
among these, TiO2 is perhaps the most intensively investigated
material owing to its advantaged intrinsic physical properties,
high stability, and potential practicability.4 Nevertheless, TiO2
still exists with some self-defects, such as high-rate recombina-
tion between photoinduced electrons (e−) and holes (h+) and
nonresponsive to visible light, severely hindering its practical
application.5,6 For that reason, extensive investigations have
concentrated upon searching an efficient method to depress the
recombination of photoinduced e−/h+ pairs, as well as an
avenue to expand the photosensitive range of a photocatalyst.
In the past few years, several strategies, such as noble metal
deposition,7,8 selective ion doping,9,10 heterostructure con-
struction,11 and so forth, have been developed to settle the
concern of high recombination rate in pristine TiO2. Among all
Received: February 5, 2017
© XXXX American Chemical Society
A
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e−/h+-generating centers. It is, therefore, of significance to
explore noble metal-based compounds as intrinsic photo-
catalytic alternatives.
Table 1. Crystal Data and Structure Refinement for BaAu2S2
chemial content
Fw
BaAu2S2
595.39
To address the limited photosensitive spectral range of TiO2,
a logical choice is to find other substances with smaller band
gaps. As a large class of compounds, chalcogenides, have
complex and interesting structures and physical/chemical
properties,17−22 and they have already made significant impact
in energy applications as intriguing photovoltaic materials, such
as CuInGaSe2 (CIGS)23 and Cu2ZnSnS4 (CZTS).24 Compared
with oxides, they possess smaller band gaps and may provide an
enormous playground for searching new broad-light-sensitive
photocatalysts.25,26
Guided by above strategies, we focused our main attention
on the noble metal-based chalcogenides system and successfully
discovered a novel Au-based ternary sulfide BaAu2S2. Our
experimental measurements indicate that this intrinsic photo-
catalyst possesses excellent photocatalytic capability under
visible-light irradiation, even superior to the benchmark
visible-light-responsive photocatalyst graphitic carbon nitride
(g-C3N4). We believe that the present study may revive
explorations on new functionality of noble metals, which have
long been overlooked, and it may eventually lead to discovery
of more fascinating photocatalysts with outstanding properties.
Herein, we represent the synthesis, crystal growth, crystal and
electronic structures, and optical and visible-light photocatalytic
properties of the title compound.
a (Å)
6.389 72(2)
6.389 72(2)
12.7872(1)
90.00
b (Å)
c (Å)
β(deg)
space group
V (Å3)
Z
I41/amd
522.08(5)
4
T (K)
293(2)
0.7107
λ (Å)
ρc(g/cm3)
7.575
μ (mm−1
)
64.139
a
R(F)
0.0313
b
2
RW(Fo )
0.0789
a
b
2
2
2
R(F) = ∑∥F0| − |Fc∥/∑|F0| for F0 > 2σ(F0 ). RW(Fo ) =
{∑[w(F0 −Fc2)2]/∑wFo }1/2 for all data. w−1 = σ2(F0 ) + (zP)2,
2
4
2
where P = (max(F0 , 0) + 2Fc2)/3.
2
distances can be observed in Table 3. Further information may be
found in Supporting Information.
Powder X-ray Diffraction (PXRD). The PXRD pattern of the
ground powder was performed at room temperature on a Bruker D8
Focus diffractometer with Cu Kα (λ = 1.5418 Å) radiation. The
scanning step width of 0.05° and a fixed counting time of 0.2 s/step
were applied to record the patterns in the 2θ range of 10−70°. The
measured XRD powder pattern was found to perfectly match with the
simulated pattern generated using the CIF of the refined structure (see
Figure 1).
Diffuse Reflectance Spectroscopy. Appropriate amount of
BaAu2S2 polycrystalline sample was thoroughly ground. A Cary 5000
UV−vis−NIR (NIR = near-infrared) spectro-photometer with a
diffuse reflectance accessory (the spectral resolution in the UV−vis
and NIR range is 0.05 and 0.2 nm, respectively) was used to measure
the spectrum of the title compound and BaSO4 (as a reference) in the
range from 400 nm (3.1 eV) to 2000 nm (0.62 eV).
First-Principles Calculation. The electron structure of BaAu2S2
was calculated using CASTEP,30 a total energy package based on
plane-wave pseudopotential density functional theory (DFT).31 The
functionals developed by Perdew, Burke, and Enzerhof in generalized
gradient approxiamation (GGA) form32 were employed to describe
the electron exchange-correlation energy. The effective interaction
between atom cores and valence electrons (Ba 5s25p66s2, Au 6s15d10,
and S 3s23p6) was modeled by ultrasoft potential33 in Kleinman−
Bylander form,34 which allows us to adopt a relatively small basis set
without compromising the computational accuracy. The plane-wave
energy cutoff of 360 eV and Monkhorst−Pack35 k-point mesh
spanning less than 0.04/Å in the Brillouin zone were chosen.
Convergence test shows that the above computational parameters are
sufficiently accurate for present purpose.36
Photocatalytic Activity Experiment. The photocatalytic activity
of BaAu2S2 was evaluated by degradation of organic dye molecule
Rhodamine B (RhB) under visible-light irradiation from a 1000 W Xe
lamp coupled with 420 filters (λ > 420 nm). In a typical
photodegradation process, the powder photocatalyst (50 mg) was
dispersed into 100 mL of RhB solution (1 × 10−5 mol/L). Before
illumination, the photocatalyst powder and dye solution suspension
was vigorously stirred in dark for 1 h to obtain the adsorption−
desorption equilibrium. Subsequently, the light was turned on, and 5
mL of the reaction suspension was taken off at appropriate intervals
and centrifuged to remove the solid. Then, the concentration of RhB
was determined by UV−vis spectra on the basis of the intensity of the
characteristic absorption band at 554 nm. Herein, g-C3N4, which is
one of the conventional commercialized benchmark visible-light
photocatalysts, was used as a reference under the same conditions.
In our experiments, g-C3N4 was obtained by sintering melamine.
EXPERIMENTAL SECTION
■
Synthesis. The following reagents were used as obtained: Ba
(Sinopharm Chemical Reagent Co., Ltd., 99.9%), Au (Aladdin Co.,
Ltd., 99.999%), and S (Sinopharm Chemical Reagent Co., Ltd., 99%).
The binary starting material BaS was prepared by the stoichiometric
reaction of the elements at high temperatures in sealed silica tubes
evacuated to 1 × 10−3 Pa. Polycrystalline sample of BaAu2S2 was
obtained by traditional solid-state reaction27 in a stoichiometric
mixture of BaS, Au, and S. The raw materials were loaded into fused-
silica tubes under the Ar atmosphere in a glovebox, and, then, the
ampules were flame-sealed under a high vacuum of 1 × 10−3 Pa. The
tubes were then placed in a computer-controlled furnace and heated to
1173 K within 15 h, left for 240 h, and, finally, the furnace was turned
off.
Crystal Growth. Single crystals of BaAu2S2 were prepared through
spontaneous crystallization. The reaction charge of BaS, Au, and S in a
molar ratio of 1:2:1was mixed and loaded into a fused-silica tube under
the Ar atmosphere in a glovebox. Then, the tubes was flame-sealed
under a high vacuum of 1 × 10−3 Pa. The tube was gradually heated to
1373 K in a computer-controlled horizontal furnace, kept for at least
120 h, and then, slowly cooled at a rate of 3 K/h, and finally cooled to
room temperature by switching off the furnace. The product consisted
of hundreds of yellow block crystals, which were manually selected for
structure characterization.
Structure Determination. Single-crystal X-ray diffraction experi-
ment was performed on an Xcalibur Ecos diffractometer equipped with
a graphite-monochromated Mo Kα (λ = 0.710 73 Å) radiation at 293
K. The collection of the intensity data and cell refinement was
performed with CrysAlisPro software (Agilent Technologies, Version
1.171.35.11).28 Multiscan absorption corrections were performed
numerically with the use of the program XPREP.29
The structure was solved with the direct methods using SHELXTLS
program and refined with the least-squares program SHELXL of the
SHELXTL.PC program suite.29 The main structural data and
refinement parameters for BaAu2S2 are given in Table 1. The atom
positional coordinates and equivalent isotropic displacement param-
eters for the title compound are given in Table 2. The selected bond
B
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Table 2. Positional Coordinates and Equivalent Isotropic Displacement Parameters for BaAu2S2
atom
Wyckoff
x/a
y/b
z/c
0.1250
Ueq [Å2]
occupancy
Ba
Au
S
4b
8c
8e
0.5000
0.7500
0.5000
0.2500
0.2500
0.2500
0.0052(5)
0.0058(5)
0.0075(11)
1
1
1
−0.2500
−0.1197(4)
Table 3. Selected Bond Lengths (Å) for BaAu2S2
BaAu2S2
Au―S
Ba―S
Ba―S
2.3082(4)×2
3.1290(5)×2
3.1956(1)×4
Figure 2. Crystal structure of BaAu2S2 with the unit cell marked.
Figure 1. Experimental (red) and simulated (black) powder X-ray
diffraction data of BaAu2S2. The differences in peak intensity for the
same crystallographic index between the two patterns are believed to
be caused by the preferential orientation of the powder samples.
Photo-Electrochemical Test. A three-electrode electrolytic cell
with 0.1 M NaOH as electrolyte solution was employed to perform the
photo-electrochemical test. The platinum wire and Ag/AgCl electrode
served as the counter electrode and reference electrode, respectively.
BaAu2S2 film deposited on indium tin oxide (ITO) glass was used as
the working electrode. Before testing, the ITO glass was thoroughly
cleaned by deionized water and ethanol and then dried in air. After
this, the 1 mM stock suspension of BaAu2S2 powder was prepared by
using dichloromethane as solvent. The as-prepared suspension was
ultrasonicated to achieve homogeneous dispersion. Subsequently, to
form the BaAu2S2 layer, the catalyst suspension was deposited onto an
ITO glass by drop-casting. Then, the glass was baked in an oven (the
temperature was set as 100 °C) for 10 min. For the following
photocurrent measurement, the 300 W Xe lamp was chosen as a light
source, and the light intensity at the electrode surface was ∼100 mW
cm−2. The photoresponse of the photocatalyst as light on and off was
measured at 1.0 V versus Ag/AgCl. The results were recorded using a
Zennium electrochemical workstation (Germany, Zahner Company).
4−
Figure 3. A single one-dimensional infinite [AuS2/2
]
anionic chain
along the a axis (a), coordination environments of Ba1 cations (b).
RESULTS AND DISCUSSION
■
Structure. BaAu2S2 crystallizes in the tetragonal centrosym-
metric space group I41/amd with unit cell parameters of a =
6.389 72(2) Å, b = 6.389 72(2) Å, c = 12.7872(1) Å, and Z = 4.
Its asymmetric unit is comprised of one crystallographically
independent Ba atom, one Au atom, and one independent S
atom, which occupy Wyckoff sites 4b, 8c, and 8e, respectively.
Note that no metal−metal/S−S bonds existing in its structure,
the valence of 2+, 1+, and 2− could be allotted to Ba, Au, and S
atoms, respectively. The bond valence sums (BVS)37 values for
Ba, Au, and S atoms were then calculated as 2.019, 0.870, and
1.879, respectively, which is close to the expected values.
Figure 4. Diffuse reflectance spectrum of BaAu2S2.
The structure of BaAu2S2 along the a axis is demonstrated in
Figure 2. As it is shown, the title compound features typical
[AuS2/2] zigzag chains. Every Au atom is linearly bound to two
S atoms and connects with the other two Au atoms by sharing
vertical S atoms, then propagating themselves along the b axis
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commutative at lengths of c/2 with Au atoms sited at z = 0 and
1/2, and parallel to the a axis metagenic at lengths of b/2 with
Au atoms situated at z = 1/4 and 3/4. These zigzag chains are
further partitioned to each other with Ba2+ cations locating in
the interspaces, which act as the counterions to achieve
electrical neutrality. It is worth noting that, although the d10
element Au has variable coordination numbers (2−4) with
chalcogen atoms, which has already led to the discoveries of a
variety of complex and interesting chalcogenide structures,38−43
BaAu2S2 is the first Au-based chalcogenide member that adopts
such a specific structure type. Besides, from the structural
viewpoint, the similarity (e.g., primary covalent bonding) and
differences (e.g., structural dimensionality) between BaAu2S2
and its binary prototype Au2S can be regarded as a
representative example in metal chalcogenides that can
perfectly corroborate the dimensional reduction formalism,44
which describes how an ionic agent AaX reacts with a parent
compound MXx to form a child compound AnaMXx+n. Note
that the dimensional reduction formalism has already been
successfully exploited to describe the structure conversion of
borates, in Li2O−B2O3 and BaO−B2O3 systems,45,46 and
phosphates in Li2O−Pb3P4O13 system.47
Figure 5. Electron band structure along highly symmetrical path in
Brillouin zone of BaAu2S2.
The detailed bonding characteristics are depicted in Figure 3.
All Au atoms are bound with two S atoms with Au−S bond
length of 2.3082(4) Å, comparable to those of 2.3063(8)−
2.3221(1) Å in K2Au4CdS4,48 2.2824(2)−2.323(15) Å in
49
Rb2Au6Sb4S10
,
and 2.3052(6)−2.3144(5) Å in
K2Au2Ge2S6,40 but slightly larger than that of 2.1737(8) Å in
Au2S.50 The S atoms act as bridges to connect two adjacent Au
atoms with Au−S−Au angle of 87.6(1)°, a little deviating from
the perpendicular geometry. Ba atoms are surrounded by six S
atoms with Ba−S bond lengths ranging from 3.129(5) to
3.1956(1) Å, similar to those of 3.171(4)−3.305(4) Å in
BaHgS2 and 3.120(4)−3.307(4) Å in BaCu4S3,52 and the
51
values are slightly smaller than that of 3.246(1)−3.626(4) Å in
Ba2Fe4S553 and 3.40(1)−3.678(7) Å in BaGa4S7.54 The detailed
coordination configuration of Ba atom is demonstrated in
Figure 3b.
Optical Properties. The UV−vis−NIR diffuse reflectance
spectroscopy was applied to study the band gap of BaAu2S2.
The absorption F(R) data were directly obtained from the
Kubelka−Munk function as listed below:
Figure 6. PDOS projected onto the constituted atoms of BaAu2S2.
Figure 7. Photocatalytic degradation curve of RhB in the presence of BaAu2S2 (a), cyclability test curve for BaAu2S2 (b).
D
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Figure 8. SEM images of BaAu2S2 (a) and g-C3N4 (b) samples.
Figure 9. Transient photocurrent response of BaAu2S2 under light
irradiation.
(1 − R)2
K
S
F(R) =
=
(1)
2R
In the above equation, R, K, and S represent the reflectance,
absorption, and scattering, respectively.55,56 The band gap of
BaAu2S2 can be deduced from the curve of F(R) versus hν. As
displayed in Figure 4, the band gap (Eg) value of BaAu2S2 was
estimated to be 2.49 eV by using the straightforward
extrapolation method, which is consistent with its yellow
color. Note that this value is significantly smaller than that of
TiO2 (Eg = 3.2 eV), indicating that the title compound may
demonstrate meliorative visible-light absorption capacity
compared with TiO2. The results will be further discussed in
next section, accounting the electronic structure calculations.
Theoretical Calculation. The electron band structure
along highly symmetrical path of BaAu2S2 is plotted in Figure
5. Obviously, BaAu2S2 belongs to a direct-transition semi-
conductor with a calculated band gap of 1.66 eV, a bit smaller
than the experimental value 2.49 eV, that resulted from the
discontinuity of the exchange-correlation in standard DFT.57,58
The partial density of states (PDOS) projected onto the
constituent atoms is displayed in Figure 6, and thus some
typical electron distribution characteristics could be inferred:
(1) the 5s orbitals of Ba atoms are strongly localized at ca. −27
eV, indicating that it is difficult to excite these orbitals by
perturbation of external photoelectric field. Therefore, it can be
further deduced that these orbitals nearly have nothing to do
with the optical property involved in the electrons transition
near the forbidden band edge. (2) The 5p orbitals of Ba atoms
Figure 10. Schematic diagram of the photodegradation of RhB over
BaAu2S2 under visible-light illumination (a), electron density map of
BaAu2S2 in (100) plane (b).
and 3s orbitals of S atoms comprise most of the electron energy
levels between −15 and −10 eV. The hybridization between
these orbitals implies certain covalency in Ba−S bonds.
However, the excitement of the electrons at these states
require the energy larger than 10 eV, so these orbitals also
contribute little to the optical property of BaAu2S2, especially
for visible-light wave range. (3) The energy levels at the valence
bands’ (VB) top (−5 to 0 eV) are constituted dominantly by
Au 5d and S 3p, and the conduction bands’ (CB) bottom is
mainly composed of Ba 5d, Au 6p, and S 3p orbitals, indicating
that the optical property of BaAu2S2 is mainly decided by
[AuS2/2 ∞
]
infinite chains, despite some contribution of BaS6
polyhedra.
Photocatalytic Activity. The photocatalytic behavior of
the obtained BaAu2S2 sample in decomposition of RhB under
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Table 4. Locations of the Brillouin Zone Paths in Reciprocal
Space for BaAu2S2 and g-C3N4
BaAu2S2
from
g-C3N4
from
to
to
Z (0.000, 0.000, A (0.500, 0.500, G (0.000, 0.000,
0.500) 0.500) 0.000)
A (0.000, 0.000,
0.500)
A (0.500, 0.500, M (0.500, 0.500, A (0.000, 0.000,
0.500) 0.000) 0.500)
H (−0.333, 0.667,
0.500)
M (0.500, 0.500, G (0.000, 0.000, H (−0.333, 0.667, K (−0.333, 0.667,
0.000) 0.000) 0.500) 0.000)
G (0.000, 0.000, Z (0.000, 0.000, K (−0.333, 0.667, G (0.000, 0.000,
0.000)
Z (0.000, 0.000, R (0.000, 0.500, G (0.000, 0.000,
0.500) 0.500) 0.000)
0.500)
0.000)
0.000)
M (0.000, 0.500,
0.000)
R (0.000, 0.500, X (0.000, 0.500, M (0.000, 0.500, L (0.000, 0.500,
Figure 11. Curves of absorption coefficients (α) vs incident photon
energy (hν) for BaAu2S2 and g-C3N4.
0.500)
X (0.000, 0.500, G (0.000, 0.000, L (0.000, 0.500,
0.000) 0.000) 0.500)
0.000)
0.000)
0.500)
H (−0.333, 0.667,
0.500)
visible light irradiation (λ > 420 nm) is presented in Figure 7a.
For convenience of comparison, the RhB photodegraded by g-
C3N4, and the blank reference was also performed under the
same condition. After 240 min of irradiation, the g-C3N4
sample can degrade ∼36% of RhB, while the RhB photolysisis
is very low. Interestingly, BaAu2S2 exhibits excellent photo-
catalytic performance, and as much as 45% of RhB can be
removed in the same period, which is even superior to the well-
known benchmark photocatalyst g-C3N4.
The degradation kinetics of the photocatalytic processes can
be further compared by Langmuir−Hinshelwood (L−H)
kinetics model as listed below59 (assuming the reaction is a
pseudo-first-order model).
microscopy (SEM) images in Figure 8a,b, respectively). This
means that the surface area of BaAu2S2 sample in this
experiment is drastically lower than that of the photocatalysts
reported before. Therefore, if the size effect is to be taken into
account, there is still plenty of room for the improvement of the
photocatalytic efficiency of BaAu2S2 owing to the fact that
smaller particles usually produce larger specific surface area,
further leading to faster separation rate of the e−/h+ pairs and
much better degradation performances. Meanwhile, our efforts
to synthesize nanoscale BaAu2S2 polycrystalline sample by
other methods (hydrothermal and solvothermal methods) are
ongoing. Furthermore, it should be stressed that the band gap
of BaAu2S2 is still a little larger than the optimal band gaps for
solar harvesting despite its distinguished visible-light-responsive
photocatalytic activity. Hence, it can be inferred that, if some
well-known band gap modulation techniques60,61 will be
applied to extend the photosensitive range of the title
compound, its photocatalytic efficiency will be further
enhanced.
However, good stability may be the prerequisite for a
photocatalyst to become practically usable. To test the potential
applicability of BaAu2S2, cyclic runs in the photodegradation of
RhB were performed under identical conditions. As demon-
strated in the Figure 7b, after three recycles, BaAu2S2 still
exhibited high photocatalytic performance without any evident
loss of activity, indicating that the title compound is highly
stable and does not corrode in the entire photocatalytic process.
Photoelectrochemical Property. Photocurrent measure-
ment can be exploited to intuitively interpret the charge carrier
mobility of a specific photocatalyst. In general, photocurrent is
ln(C0/C) = kapp
t
(2)
where C0, C, and kapp represent the initial concentration of RhB
(mol/L), the instantaneous concentration of RhB (mol/L) at
time t, and the apparent rate constant (h−1) based on the
pseudo-first-order model, respectively. The corresponding
apparent rate constants of g-C3N4 and BaAu2S2 for photo-
degradation of RhB are 0.112 and 0.149 h−1, respectively. In
other words, BaAu2S2 exhibits higher photocatalytic activity,
which is ∼1.3 times that of g-C3N4. Note that g-C3N4 is one of
the benchmark materials for visible-light photocatalysis; the
above value for BaAu2S2 is particularly noteworthy. Moreover,
according to our best knowledge, the majority of the
photocatalysts reported before are nanomaterials, while our
preparation method in this study (traditional solid-state
reaction and manually grinding) can only generate particles at
most of micrometer level (the morphologies of BaAu2S2
together with C3N4 are recorded by scanning electron
Figure 12. Brillouin zone paths in reciprocal space for BaAu2S2 (a) and g-C3N4 (b).
F
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Figure 13. Second-order differential curves of E(ka) vs ka for BaAu2S2 (a) and g-C3N4 (b).
propotional to the photocatalytic capability. Large photocurrent
customarily implies efficient separation and migration of the
e−/h+ pairs.62,63 Figure 9 displays the generated photocurrent
of BaAu2S2 sample in electrolyte solution under light
illumination. The blank ITO electrode without catalyst was
used as the background reference. Obviously, the photocurrent
generated in the blank ITO electrode was negligible. In
comparison, when the light was on, the photocurrent of
BaAu2S2 electrode was generated immediately, and it can
eventually reach ∼30 μA/cm2, indicating BaAu2S2 may have
good photocatalytic capability and agrees well with the previous
measurements.
BaAu2S2 is even comparable with some leading photovoltaic
65 66
materials, such as CuInS2 and CuSbS2 (>1 × 104 cm−1).
The electron effective mass of BaAu2S2 was calculated by
applying the following standard equation
∂2E(ka)
1
1
=
ℏ2 ∂ka2
*
m
(3)
where m*, ℏ, ka and E(ka) are the electron effective mass,
reduced Planck constant, Brillouin zone path in reciprocal
space, and the energy state, respectively. Here, for the sake of
simplicity, the electron effective mass at Γ-point of the bottom
of the CB was chosen as the representative. For intuitionistic
comparison, the corresponding value of the benchmark material
g-C3N4 was also calculated. Figure 12 depicts the Brillouin zone
paths in reciprocal space for BaAu2S2 and g-C3N4; the detailed
locations are tabulated in Table 4. As is shown in Figure 13, the
second-order differential curves of E(ka) versus ka yield the
values of 177.85 and 99.80 eV·m2 for BaAu2S2 and g-C3N4,
respectively. After normalization processing, their respective
electron effective masses were calculated as 3.1 × 10−31 kg (0.34
m0, in units of the electron mass) and 15.8 × 10−31 kg (1.74
m0). In other words, the electron effective mass of BaAu2S2 is
only one-fifth of that of g-C3N4, demonstrating that the carrier
Photocatalytic Mechanism. Figure 10a depicts the
possible photocatalytic mechanism over BaAu2S2 under
visible-light irradiation. As depicted, under the illumination by
photons with enough energy (hν > Eg), the photoinduced e−/
h+ pairs will promptly appear. The generated e− can be injected
to the CB, while the h+ are left in the VB. Then the superoxide
−
radicals (·O2 ) can be generated by the reaction of the
electrons in the CB and adsorbed O2 molecules on the surface
of BaAu2S2 sample. Simultaneously, the reactive hydroxyl
radicals (·OH) can also be produced via the redox of holes and
OH− ions in solution. All of the active species (·O2 , h+, and ·
−
OH) can participate in the next photooxidation reaction. Figure
10b demonstrates the electron density map of BaAu2S2 in (100)
plane, where obviously the electron cloud is mainly located in
mobility of BaAu2S2 (mainly in the [AuS2/2]∞ chains) is much
higher than that of g-C3N4. Therefore, it can be inferred that
the combination of high absorption efficiency and low carrier
effective mass is responsible for the excellent charge
generating/transport abilities of BaAu2S2.
Considering the inconveniences of the traditional photo-
catalysts, and the limitations of previous studies on the role of
noble metals, we believe the title compound may trigger further
interest in exploring new functionality of noble metals, as well
as provide some insights in enriching the source of high-
efficiency photocatalysts.
the infinite [AuS2/2 ∞ chains, and the electrons within the
]
chains are highly delocalized. The superior photoactivity of
BaAu2S2 may originate from its admirable light-harvesting
efficiency and excellent cruise ability of the charge carriers in
the intrinsic [AuS2/2]∞ chains.
To quantitatively corroborate the above speculation, we
theoretically investigated the absorption coefficient and
electron effective mass of BaAu2S2, which are two inherent
parameters for a specific material, and they can be regarded as a
direct reflection of the light-absorption capacity and charge
carrier mobility, respectively.64,65 Figure 11 displays the curve
of absorption coefficient versus incident photon energy for
BaAu2S2. For intuitionistic comparison purposes of the visible-
light absorption capacity, the corresponding curve of g-C3N4
was also depicted. As is shown, the absorption coefficient of
BaAu2S2 is extremely high, exceeding 1 × 104 cm−1 over the
entire absorbable visible spectrum (hν > Eg). As a comparison,
the absorption coefficient of g-C3N4 is significantly smaller than
that of BaAu2S2, which is in accordance with the photocatalytic
activity experiment. Additionally, it is worth noting that high
absorption coefficient acts a key role in the choice of materials
for photovoltaic applications. The aforementioned value for
CONCLUSION
■
The first member in A−Au−Q system (A = alkali earth metal;
Q = S, Se), BaAu2S2, has been discovered and characterized. Its
structure features [AuS2/2 ∞ zigzag chains, which are further
]
separated from each other by Ba2+ cations residing in the
interspaces. BaAu2S2 has a direct band gap of 2.49 eV, which is
conducive to the visible-light harvesting. Most importantly,
despite the large particle sizes, BaAu2S2 still exhibits superior
photocatalytic activity, which is 1.3 times that of the visible-light
photocatalysis benchmark material g-C3N4, and also demon-
strates excellent circulating stability, which is favorable for its
potential application. Furthermore, to the best of our
G
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knowledge, BaAu2S2 is the first Au-based chalcogenide
photocatalyst as intrinsic light-harvesting and electron/hole-
generating centers that exhibits superior photocatalytic activity.
The outstanding photocatalytic performance may be explained
based on its high light-absorption capacity (the visible-light
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00319.
The crystallographic file in CIF format for BaAu2S2
(CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: jyao@mail.ipc.ac.cn. (J.Y.)
*E-mail: lzwu@mail.ipc.ac.cn. (L.-Z.W.)
ORCID
Xin Jiang: 0000-0002-7836-1784
Hongwei Huang: 0000-0003-0271-1079
Jiyong Yao: 0000-0002-4802-5093
Chen-Ho Tung: 0000-0001-9999-9755
Li-Zhu Wu: 0000-0002-5561-9922
■
Author Contributions
∥These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This research was supported by National Natural Science
Foundation of China (No. 51472251 and No. 91622123).
■
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