Simple and efficient conversion of cellulose to γ-valerolactone through an integrated alcoholysis/transfer hydrogenation system using Ru and aluminium sulfate catalysts

Article abstract of DOI:10.1039/c8cy01971a

The direct conversion of cellulose to specific chemicals represents an important but challenging area that attracts much attention. In this study, we report, for the first time, a one-step conversion of cellulose to γ-valerolactone (GVL), a platform molecule with multiple applications, by integrating alcoholysis and transfer hydrogenation systems over mixed metal salt and Ru catalysts without external hydrogenation. A maximum GVL yield of 51.2% was obtained at 180 °C for 70 min reaction time with microwave heating. The metal salt effectively catalyzed cellulose alcoholysis to generate levulinate in isopropanol, which was also the hydrogen donor for the subsequent catalytic transfer hydrogenation of levulinate to GVL over the Ru/ZrO₂ catalyst. It was found that the types of metal center and support material had a significant influence on the reactivity of the catalyst for the catalytic transfer hydrogenation (CTH) reaction, i.e., concerning the existence of sulfuric acid species and water in the reaction system. Microwave heating was demonstrated to be an effective method for cellulose-to-GVL conversion as compared to conventional oil heating, through drastically reducing the reaction time and avoiding decomposition of the reagents. The catalysts were successfully recycled and reused with high reactivity. Finally, the system was also applied to the synthesis of GVL from real biomass, demonstrating the high applicability and potential of the catalytic system for industrial production.

Full text of DOI:10.1039/c8cy01971a

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PAPER
Group-IVA element-doped SrIn2O4 as potential
materials for hydrogen production from water
splitting with solar energy
Cite this: RSC Adv., 2018, 8, 32317
a
Hai-Cai Huang,a Chuan-Lu Yang,
Mei-Shan Wang,a Xiao-Guang Maa
*
and You-Gen Yib
Band gap engineering can efficiently improve the photocatalytic activity of semiconductors for hydrogen
generation from water splitting. Herein, we present a comprehensive investigation on the geometrical
structures, electronic, optical, and potential photocatalytic properties and charge carrier mobility of
pristine and group-IVA element-doped SrIn2O4 using first-principles density functional theory with the
meta-GGA+MBJ potential. The calculated formation energies are moderate, indicating that the synthesis
of the doped structures is experimentally feasible. In addition, the energy band gaps of the group-IVA
element-doped SrIn2O4 range from 1.67 to 3.07 eV, which satisfy the requirements for photocatalytic
water splitting, except for that of the Si mono-doped structure. Based on the deformation potential
theory, a high charge carrier mobility of 2093 cm2 Vꢀ1 sꢀ1 is obtained for the pristine SrIn2O4 and those
of the doped-structures are also large, although a decrease in the values of some are observed. The
optical absorption coefficient of the doped structures in the near ultraviolet (UV) and visible light range
significantly increases. Therefore, group-IVA element-doped SrIn2O4 are potential candidates as
photocatalysts for hydrogen generation from water splitting driven by visible light.
Received 29th May 2018
Accepted 4th September 2018
DOI: 10.1039/c8ra04569k
rsc.li/rsc-advances
of hydrogen from water splitting by TiO2 was reported in 1972,9
much effort has been dedicated to improving the hydrogen
production efficiency.10 In recent years, the integration of
1. Introduction
Hydrogen is considered as one of the future fuels, and thus has
attracted increasing attention as renewable and clean energy.1–4
The conversion of solar energy into hydrogen by a photocatalyst
via water splitting is an attractive technology, which can effi-
ciently reduce the severe environment and energy issues.5–7
Photocatalytic water splitting is regarded as articial photo-
synthesis, which involves three main steps: (i) absorption of
photons with equivalent or greater energy than the band gap of
the semiconductor photocatalyst, where electrons are excited
from the valence band to the conduction band, leading to the
generation of electron–hole pairs. (ii) The photoexcited carriers
migrate to the surface of the photocatalyst. (iii) The separated
photoexcited carriers reduce H+ to produce H2 and oxidize H2O
to produce O2.8 Generally, only water, solar energy, and photo-
catalyst are needed for photocatalytic water splitting, where
solar energy is inexhaustible and water is one of the most
abundant resources on earth. Therefore, the key factor to ach-
ieve efficient photocatalytic water splitting is the development
of suitable photocatalysts. Since the photocatalytic production
atomic dopants has been conrmed to be an important
approach for improving the efficiency of photocatalytic water
splitting.11 Doping can efficiently enhance the photocatalytic
performance by extending the absorption edge of semi-
conductors to a lower energy range. For example, Mg-doped
anatase TiO2 had been synthesized successfully by Gao et al.12
They found that the absorption coefficient of TiO2 increased
with an increase in the Mg doping concentration. Highly effi-
cient photocatalytic water splitting was achieved on Pt-
embellished 0.5% Mg–TiO2, and the hydrogen evolution
reached as high as 850 mmol gꢀ1 hꢀ1. Also, metal ion (Nb, Cr,
Cu, and Ni) and nitrogen (N) co-doped TiO2 photocatalysts were
synthesized by Lin et al.,13 and they found that the signicant
photocatalytic enhancement of the Cu, N co-doped TiO2 was
attributed to the enhanced visible light absorption. Similarly,
Cu-doped ZnS microspheres were prepared via microwave
hydrothermal method by Lee et al.14 They reported that the
absorption coefficient of ZnS in the visible light range was
signicantly enhanced with the introduction of Cu dopant, and
the hydrogen evolution rate reached as high as 973.1 mmol gꢀ1
hꢀ1 under ZnS with 2 mol% Cu dopant.
aSchool of Physics and Optoelectronics Engineering, Ludong University, Yantai 26425,
People's Republic of China. E-mail: ycl@ldu.edu.cn; Tel: +86 535 6672870
bHunan Key Laboratory for High-Microstructure and Ultrafast Process, College of
Physics and Electronics, Central South University, Changsha 410083, People's
Republic of China
Two groups of elements can act as active components in
photocatalytic water splitting.15 Photocatalysts with a d0 or d10
electronic conguration generally present a better photo-
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response than that with partially lled d orbitals. As one of the IVA atoms, which correspond to a doping molar concentration
p-block metal oxides with a d10 electronic conguration, of 1.5%. Hereaer, the doped structures are denoted as C@Sr,
SrIn2O4 has been reported to exhibit stable and high photo- Si@Sr, Ge@Sr, Sn@Sr, and Pb@Sr. In the present study, all the
catalytic activity for water splitting.16 However, its energy band density functional theory (DFT) simulations were carried out
gap is 3.6 eV,17 indicating that it only responds to ultraviolet using the Vienna Ab initio simulation package (VASP).27 The
(UV) light, which is only about 7% of solar energy.18 Therefore, projector augmented wave (PAW) potential28 was used to
SrIn2O4 photocatalytic water splitting cannot efficiently utilize describe the ion–electron interactions, which means that 2s22p4
solar energy. Also, its rate of hydrogen formation is only about 7 for O, 4d105s2 for Sr, 5s25p1 for In, 2s22p2 for C, 3s23p2 for Si,
mmol gꢀ1 hꢀ1, which is much smaller than that of TiO2 (106 4s24p2 for Ge, 5s25p2 for Sn and 6s26p2 for Pb were treated as
mmol gꢀ1 hꢀ1).19,20 Considering that band gap engineering is an valence electrons. The electron exchange-correlation functional
effective method to extend the absorption edge of photo- was treated as the generalized gradient approximation (GGA)
catalysts to the visible light range, there are several reports on with the Perdew–Burke–Ernzerhof (PBE) parametrization for
cation/anion doped SrIn2O4 to efficiently use solar energy.21–24 structural optimization.29 All the atomic positions were fully
For example, Dy3+/Pr3+/Tb3+-doped SrIn2O4 was successfully relaxed until the residual forces and energy convergence were
prepared by Liu et al.25 They found that SrIn2O4:Dy3+/Pr3+/Tb3+ less than 0.01 eV Aꢀ1 and 10ꢀ6 eV, respectively. The plane-wave
˚
exhibited stronger near white, red and green luminescence, cutoff kinetic energy was set as 450 eV throughout the calcula-
which results from the efficient energy transfer from the host tion. For the geometry optimization and electronic properties
lattice to the dopants. Additionally, Eu3+, Sm3+-doped SrIn2O4 calculation, Monkhorst–Pack k-point grids of 3 ꢁ 3 ꢁ 5 and 5 ꢁ
was successfully synthesized by Li et al.26 through a combustion 5 ꢁ 7 were used. The meta-GGA+MBJ potential30 was used to
method. They found that Sm3+-doped SrIn2O4 effectively obtain credible calculations for the energy gap and optical
extended and strengthened the absorption range from 400 to absorption coefficient.
405 nm.
To conrm the validity of the present calculational results,
However, to the best of our knowledge, there are no experi- we performed additional calculations for CaIn2O4/GaN/GaAs/
mental or theoretical investigations on group-IVA element- ZnS and C/Pb-doped ZnS, which were synthesized experimen-
doped SrIn2O4. Therefore, we comprehensively investigate the tally. For the pristine structures, we investigated the geometrical
electronic, optical, and potential photocatalytic properties and structures and energy band gaps of CaIn2O4, GaN, and GaAs, as
carrier mobility (m) for pristine and IVA-doped SrIn2O4 to listed in Table 1. The results from Table 1 show that the lattice
˚
understand the inuence of the dopants on the photocatalytic parameters of CaIn2O4 are a ¼ 9.735, b ¼ 11.389 and c ¼ 3.249 A,
properties of SrIn2O4 for water splitting. A high m is observed for which are slightly larger than the experimental values.31 The
the pristine SrIn2O4, which implies that the carriers can transfer calculated energy band gap of CaIn2O4 is 3.67 eV, which is also
to its surface efficiently in the process of photocatalytic water close to the experimental value31 and that of GaN and GaAs,32–35
splitting. The absorption coefficient of SrIn2O4 in the visible indicating that the present theoretical level is rational for the
light range is signicantly enhanced with the introduction of pristine structure. For the doped structure, we investigated the
the dopants, and the mechanism is explored in accordance with energy band gap of C, Pb-doped ZnS. The results from Table 2
the total density of states (TDOS) and partial density of states show that the parameters and energy band gaps of the pristine
(PDOS). The results obtained are benecial for the design of ZnS are almost close to that obtained in the previous works,36,37
new photocatalytic materials driven by visible light.
whereas, the calculated energy band gaps of C, Pb-doped ZnS
are 0.3 eV smaller than the experimental value.38,39 However, it is
still acceptable for us. In addition, the carrier mobility of GaAs
was also investigated. The results in Table 3 demostrate that the
calculated carrier mobility of the pristine GaAs is 0.594 m2 Vꢀ1
sꢀ1, which is in good agreement with the experimental values.40
All of these results indicate that the present theoretical level is
rational and that the results for SrIn2O4 herein are credible.
2. Computational details
In this study, a 2 ꢁ 2 ꢁ 2 supercell of SrIn2O4 with the space
group Pnam belonging to orthorhombic symmetry, containing 8
Sr atoms, 16 In atoms and 32 O atoms, was constructed. The
IVA-doped structures were built by substituting the Sr atom with
Table 1 Calculated lattice parameters a, b, and c (A˚), and energy band gaps, Eg (eV), for the experimentally synthesized semiconductors
Lattice parameter
a
b
c
Eg
CaIn2O4
GaN
9.735
9.720 (ref. 31)
3.219
11.389
11.300 (ref. 31)
3.219
3.249
3.240 (ref. 31)
5.230
3.67 eV
3.90 eV (ref. 31)
3.32 eV
3.189 (ref. 32)
5.726
3.189 (ref. 32)
5.726
5.185 (ref. 32)
5.726
3.40 eV (ref. 33)
1.45 eV
GaAs
5.653 (ref. 34)
5.653 (ref. 34)
5.653 (ref. 34)
1.44 eV (ref. 35)
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Table 2 Calculated lattice parameters a, b, and c (A˚), and energy band
gaps, Eg (eV), for the pristine and C/Pb doped ZnS
To evaluate the feasibility of the synthesis of the doped
SrIn2O4, the formation energy (Eform) was calculated as follows:42
a ¼ b ¼ c
Eg
Eform ¼ E(doped) ꢀ E(pristine) ꢀ nmX + nmSr
(1)
Pristine
C@ZnS
Pb@ZnS
5.436
5.412 (ref. 36)
5.422
—
5.480
—
3.57 eV
3.70 eV (ref. 37)
2.25 eV
1.95 eV (ref. 38)
2.59 eV
2.30 eV (ref. 39)
where, E(doped) and E(pristine) are the total energy of the
doped and pristine SrIn2O4, respectively. mX and mSr represent
the chemical potential of the dopants and Sr atom, respec-
tively. The calculated Eform of the doped structures are
collected in Table 4. The results demonstrate that the Eform of
the doped structures are in the range of 4.40 to 6.74 eV, which
is acceptable for their general synthesis.
Table 3 Calculated carrier mobility, m (m2 ꢀ1 sꢀ1), for GaAs
V
Type
Cb
Eb
m*dos
m
Pristine
Electron
0.65
—
16.68
—
0.11
—
0.594
0.700 (ref. 40)
3.2 Photocatalytic water splitting and band engineering
The calculated energy band gaps (Eg) of all the structures are
listed in Table 4. The Eg of 3.61 eV for the pristine SrIn2O4 is
close to the experimental value,17 which signies that the
present theoretical level is credible. However, it is too large to
respond to visible light. Thus, band gap engineering of the
electronic properties of SrIn2O4 is necessary. Fortunately, the
results in Table 4 demonstrate that the Eg of SrIn2O4 signi-
cantly decreased with the introduction of IVA elements, espe-
cially for C@Sr and Si@Sr. However, one can nd from Table 4
that the change tendency of the energy band gaps for the doped
structures is not singlet, which is similar to the case of IVA-
doped ZnS.43 The change tendency of the ionization energy of
IVA elements is responsible for the characteristic of the energy
band gaps. For examples, the ionization energy of Si is smallest
and that of Pb is the largest, which results in the smallest Eg of
Si@Sr and the largest Eg of Pb@Sr among the doped structures,
respectively. The Egs of the doped structures are in the range
from 1.32 eV to 3.07 eV, implied that all the doped structures
can respond to the visible light. The Eg for photocatalytic water
splitting should be higher than 1.23 eV considering the
minimum potential for water splitting is 1.23 eV, while an Eg
smaller than 3.0 eV means that the photocatalytic water split-
ting reaction can be efficiently driven by visible light.44 More-
over, the suitable band edge position of a semiconductor is also
indispensable for photocatalytic water splitting. The valence
band maximum (VBM) of a photocatalyst must be lower than
the oxidation potential (VO2/H2O ¼ ꢀ5.67 eV) so that it can be
used for oxygen production, while its conduction band
minimum (CBM) must be higher than the reduction potential
3. Results and discussion
3.1 Geometrical structures and energy stability of SrIn2O4
The optimized structure of Sn@Sr in Fig. 1 is used to represent
the IVA-doped SrIn2O4 structures and the others are omitted for
brevity. The optimized lattice parameters of the pristine and
doped SrIn2O4 are presented in Table 4. The calculated lattice
parameters of the pristine SrIn2O4 are a ¼ 9.906, b ¼ 11.602 and
˚
c ¼ 3.305 A, which are in great agreement with the previous
works.26,41 For the doped structures, all the changes in the lattice
˚
parameters are less than 0.1 A compared to that of the pristine
structure, which means that the dopants did not lead to
signicant deformation of the lattice.
+
(VH2/H ¼ ꢀ4.44 eV) for hydrogen production.45 The VBMs and
CBMs of the present structures can be calculated from the
following equations:46
1
EVBM ¼ ꢀc ꢀ Eg
(2)
(3)
2
1
ECBM ¼ ꢀc þ Eg
2
where, c is the Mulliken electronegativity value of the compound
and Eg is the energy band gap of the structure. The electroneg-
ativity can be determined using the following equation:47
Fig. 1 Optimized structure of Sn@Sr. The green, purple, red and gray
spheres represent the Sr, In, O, and Sn elements, respectively.
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Table 4 Calculated lattice parameters a, b, and c (A˚), energy band gaps Eg (eV), formation energies Eform (eV) and electronegativity c (eV) for
SrIn2O4
Lattice parameters
a
b
c
Eg
Eform
c
Pristine
9.906
11.602
3.305
3.61 eV
—
4.84
9.830 (ref. 24)
9.809 (ref. 29)
10.006
9.994
9.954
11.496 (ref. 24)
11.449 (ref. 29)
11.578
11.586
11.586
3.269 (ref. 24)
3.265 (ref. 29)
3.306
3.303
3.297
3.60 eV (ref. 17)
C@Sr
1.67 eV
1.32 eV
2.64 eV
2.31 eV
3.07 eV
4.92
4.40
6.14
6.62
6.74
4.94
4.91
4.91
4.91
4.90
Si@Sr
Ge@Sr
Sn@Sr
Pb@Sr
9.926
9.920
11.616
11.620
3.302
3.305
ꢀ
ꢁ
CBM of the pristine SrIn2O4 is ꢀ3.04 eV, which is more positive
than the reduction potential VH2/H+, and the VBM is more
negative than the oxidation potential VO2/H2O, indicating that it
can be used for hydrogen production from water splitting. It can
also be observed that the CBMs and VBMs of the IVA-doped
structures shi toward the potential of the water oxidation
and reduction reactions in Fig. 2. Fortunately, all the CBMs and
VBMs except that of Si@Sr satisfy the requirements for the water
splitting reaction. Therefore, the pristine SrIn2O4 and the C@Sr,
Ge@Sr, Sn@Sr and Pb@Sr structures are favorable for photo-
catalytic water splitting.
vE
vN
c ¼ ꢀ
n
¼ ½EðN þ 1Þ ꢀ EðNÞ þ EðNÞ ꢀ EðN ꢀ 1Þꢂ=2 z ðI þ AÞ=2 (4)
where, I is the ionization energy and A is the electron affinity of
the atom. Using eqn (4), the electronegativities of Sr, In, O, C, Si,
Ge, Sn, and Pb were calculated. The Mulliken electronegativity
of a compound can be considered as the electronegativity of its
constituent atoms, which corresponds to the following
equation:48
c(compound) ¼ ca1c2b.cnc
(5)
where, c1, c2 and cn represent the electronegativities of the
atoms in the compound, and a, b, and c are the molar fractions
of the atoms. The electronegativities of the pristine and doped
SrIn2O4 were calculated based on eqn (5), and the results are
listed in Table 4. The results reveal that the electronegativities
of the doped structures are larger than that of the pristine
structure, which can be understood from the fact that the
electronegativities of IVA elements are larger than that of the Sr
atom. As a result, the reductive ability of the doped structures is
stronger than that of the pristine structure. Base on eqn (2)–(5),
the calculated absolute CBMs and VBMs of the pristine and
doped SrIn2O4 are shown in Fig. 2. This gure shows that the
3.3 Enhancement of absorption coefficient of IVA-doped
SrIn2O4
Enhancement of the absorption coefficient of SrIn2O4 in the
visible light range can promote the full use of solar energy to
produce hydrogen from photocatalytic water splitting. Thus, to
evaluate the absorption coefficient of SrIn2O4, its complex
dielectric function was rst calculated. The details of the
calculation of the complex dielectric function can be found in
the previous literature49 and is thus omitted here for brevity.
The results are plotted in Fig. 3, which shows that the imaginary
Fig. 2 Absolute VBM and CBM energy levels relative to the NHE Fig. 3 Imaginary part of the dielectric function of the pristine and
potential for the pristine and doped SrIn2O4.
doped SrIn2O4.
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part 32(u) of the dielectric function of the pristine SrIn2O4 is mainly comprising of p states from the dopant is observed in
close to zero in the visible and infrared light range. Neverthe- the band gap of SrIn2O4 aer the IVA doping. Specically, the
less, a signicant red-shi of 32(u) was observed when SrIn2O4 is electron transfer from the p states of the dopants to the 5s states
doped with IVA elements, especially for C@Sr and Si@Sr.
of In is responsible for the signicant enhancement in the
The optical absorption properties can be deduced from the absorption of the doped SrIn2O4 in the visible light range.
complex dielectric function according to the following Moreover, the energy of the CBM for the doped structures
expression:50
remained unchanged although the components are changed
compared to that of the pristine structure.
1
2
!
1
2
ꢂ
ꢃ
2
2
31 ðuÞ þ 32 ðuÞ ꢀ 31ðuÞ
aðuÞ ¼ 2u
(6)
2
3.5 Effective mass and carrier mobility
where, a is the optical absorption coefficient. Fig. 4 shows the
absorption coefficient of the pristine and doped SrIn2O4. The
results reveal that the pristine SrIn2O4 shows no signicant
absorption in the visible light range, which is in agreement with
the previous works.51 Therefore, the pristine SrIn2O4 does not
respond to visible light, and as a result, its photocatalytic effi-
ciency using solar energy is low. Nevertheless, signicant red-
shis due to absorption are identied for the IVA-doped
structures, especially for C@Sr, Si@Sr, and Ge@Sr, where
a signicant enhancement in absorption in the visible light
range is observed. This indicates that these structures are
preferable to the pristine structure to capture solar energy in the
photocatalytic water splitting reaction.
The efficient separation of charge carriers and high mobility m
signicantly impact the efficiency of photocatalytic water split-
ting. Thus, m and relative ratio (D) between the effective mass of
electrons ðm*eÞ and holes ðm*hÞ for the pristine and doped
SrIn2O4 were calculated considering the relaxation time (s) as
follows:52
qs
m*
m ¼
(7)
where, q is the carrier charge and m* is the effective mass of the
charge carriers. s can be calculated depending on the detailed
scattering mechanisms. For a three-dimensional system, s
along the b-direction at temperature T can be expressed as
follows:53
pffiffiffiffiffiffi
2pCbħ4
3.4 Density of states and partial density of states
2
sb ¼
(8)
À
Á
3 kBTm*dos 2=3Eb
2
To understand the origin of the visible light absorption of the
IVA-doped structures, their TDOS and PDOS were calculated, as
shown in Fig. 5 and 6, respectively. Fig. 5 shows new energy
levels resulting from the doped IVA elements in the band gap of
the pristine structure, which are responsible for the reduction
in the energy band gap of the doped structures. The effect of the
dopants on PDOS of SrIn2O4 is demonstrated in Fig. 6. Fig. 6a
shows that VBM of the pristine SrIn2O4 is mainly dominated by
O 2p states with a small number of 4p, 4d and 5s states of Sr as
well as 4d states of In, while its CBM is mainly dominated by 5s
states of In with a small number of 5s states of Sr, as well as 2s
and 2p states of O. However, a new additional donor energy level
where, ħ represents the reduced Planck constant and kB stands
for the Boltzmann constant. Cb is the elastic constant, which
1
V0
v2E
can be calculated as Cb ¼
jl¼l , where, E is the total
0
2
vðDl=l0Þ
energy of the structure, l0 is the lattice constant along the b-
direction, Dl ¼ l ꢀ l0 is the corresponding lattice distortion, and
V0 is the equilibrium volume of the unit cell. The deformation
vEedge
potential constant Eb is calculated by Eb ¼
, where,
vðDlÞ=l0
Eedge is the energy level of the electrons in the CBM and holes in
the VBM. The density-of-states effective mass m*dos is given by
ꢀ
ꢁꢀ1
d2Eedge
dk2
m*dos ¼ ħ2
. The calculated Cbs, Ebs, m*dos and ms of
the pristine and doped SrIn2O4 are summarized in Table 5. A
larger m represents a higher migration rate of photogenerated
charge carriers, which leads to charge carriers quickly reaching
the surface of the materials to drive the redox reaction for
hydrogen and oxygen production. The results in Table 5 reveal
that the calculated m of the electrons in the pristine SrIn2O4 is
2093.00 cm2 Vꢀ1 sꢀ1, while that of the holes is only about 0.97
cm2 Vꢀ1 sꢀ1, indicating that the separation of electrons and the
holes is signicant. Therefore, the pristine SrIn2O4 is benecial
for water splitting. Moreover, the m of the electrons in the doped
structures is higher than that of other effective photocatalytic
materials, such as MoS2 (200 cm2 Vꢀ1 sꢀ1),54 although it
signicantly decreases compared with that of the pristine
structure. As a result, IVA-doped SrIn2O4 also exhibits efficient
photocatalytic behavior theoretically.
Fig. 4 Absorption coefficient of the pristine and doped SrIn2O4.
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Fig. 5 Total density of states for the pristine and doped SrIn2O4.
Fig. 6 Partial density of states for pristine and doped SrIn2O4.
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Table 5 Calculated elastic constant Cb (N mꢀ1), deformation potential
constant Eb (eV), density-of-states effective mass m*dos, relaxation time
PDOS of these structures demonstrate that the new additional
donor energy levels mainly comprising of the p states of the
dopants are responsible for the reduction in energy band gap
and enhancement of absorption coefficient in the visible light
range. In addition, a high carrier mobility and efficient sepa-
ration of photogenerated charge carriers are observed for both
the pristine and doped structures, which lead to efficient pho-
tocatalytic performances. Therefore, we predict that C@Sr and
Ge@Sr are the most promising candidates as photocatalysts for
hydrogen production from water splitting driven by visible
light.
s (fs) and carrier mobility m (cm2 Vꢀ1
s
ꢀ1) for SrIn2O4
Type
Cb
Eb
m*dos
m
D
Pristine
C@Sr
Electron
Hole
Electron
Hole
Electron
Hole
Electron
Hole
Electron
Hole
Electron
Hole
1.55
1.53
1.51
1.54
1.52
1.53
16.15
11.61
14.80
13.08
15.56
13.82
16.12
13.19
16.49
14.11
16.35
13.17
0.24
6.68
0.59
11.41
0.90
7.34
0.29
5.01
0.47
2.67
0.37
2.82
2093.00
0.97
250.98
0.20
79.37
0.53
1214.70
1.54
353.02
6.43
674.00
6.49
27.83
19.34
8.16
17.28
5.68
7.62
Si@Sr
Ge@Sr
Sn@Sr
Pb@Sr
Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
The recombination of electrons and holes is one of the
factors that limits the photoactivity of a semiconductor. Thus,
to further analyze the recombination rate of electrons and
holes, the relative ratio (D) between the effective mass of elec-
trons ðm*eÞ and holes ðm*hÞ was calculated according to the
following expression:55
This study was supported by the National Natural Science
Foundation of China (NSFC) under Grant Nos. NSFC-11374132
and NSFC-11574125, as well as the Taishan Scholars Project of
Shandong Province (ts201511055).
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