Germanium-substituted Zn2TiO4 solid solution photocatalyst for conversion of CO2 into fuels

Article abstract of DOI:10.1016/j.jcat.2019.01.017

Photocatalytic CO₂ reduction conjugated with H₂O oxidation is regarded as a promising artificial photosynthesis system because it can simultaneously solve energy and environment problems. Here, a novel solid solution, Zn₂T

Full text of DOI:10.1016/j.jcat.2019.01.017

Journal of Catalysis 371 (2019) 144–152
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Germanium-substituted Zn2TiO4 solid solution photocatalyst for
conversion of CO2 into fuels
b
a,
b
Yao Chai a, Li Li a, Jiaxue Lu a, Deli Li a, Jinni Shen b, , Yongfan Zhang , Jun Liang , Xuxu Wang
⇑⇑
⇑
a State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan
750021, China
b College of Materials Science and Engineering, State Key Laboratory of Photocatalysis on Energy and Environment, Department of Chemistry, Fuzhou University, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Photocatalytic CO2 reduction conjugated with H2O oxidation is regarded as a promising artificial photo-
synthesis system because it can simultaneously solve energy and environment problems. Here, a novel
solid solution, Zn2Ti1ÀxGexO4 (0 ꢀ x ꢀ 0.15), with high photocatalytic activity for the reaction was suc-
cessfully synthesized via a facile molten salts route. The Zn-based solid solutions with size about
200 nm have a homogeneous inverted cubic spinel structure (Fd3m) and a continuously modulated band
gap with the Ge content. For the CO2 reduction reaction with H2O under simulated solar irradiation, the
Zn2Ti1ÀxGexO4 solid solutions display not only high activity for the conversion of CO2 into CH4 and CO
fuels, but also long-term stability (>60 h of catalytic reaction). Experimental results and theoretical cal-
culations indicated that the conduction and the valence bands of the cubic spinel Zn2TiO4 are positively
shifted by introducing Zn2GeO4 with a pseudocubic inverse spinel structure, but the band gaps of solid
solutions are simultaneously modulated with the introduction of germanium. Nevertheless, the
Zn2TiO4 affords a light-carrier effective mass and strong electron delocalization by forming a solid solu-
tion with Zn2GeO4, which is beneficial to improving migration of photogenerated electrons and holes. As
a synergistic result of band gap narrowing and high carrier diffusion, good conversion efficiencies for pro-
duction of solar fuels through the reactions of CO2 reduction with H2O are achieved.
Received 20 November 2018
Revised 10 January 2019
Accepted 11 January 2019
Keywords:
Germanium-substituted Zn2TiO4
Solid solution
Molten salts
CO2 conversion
Photosynthesis
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
In the design and development strategies for various photocat-
alytic materials, constructing solid solutions has been experimen-
The conversion of CO2 and water into fuels and chemicals over
semiconductor-based photocatalysts has been attracting extensive
research interest as one of the best prospective routes to transform
solar energy into chemical energy [1,2]. Various photocatalysts for
the reaction, including single-metal oxides (e.g., TiO2, WO3) [3–7],
metal sulfides or phosphides (e.g., ZnS, CdS, p-GaP) [8–11], binary-
metal oxides (e.g., Bi2WO6, ZnGa2O4, Zn2SnO4, Zn2GeO4) [12–17],
tally demonstrated to be quite suitable for CO2 reduction with
H2O [13]. On one hand, the photoabsorption range of a solid solu-
tion can be optimized by controlling foreign atoms to simultane-
ously meet the requirements of the half-reaction of CO2
reduction and the half-reaction of water oxidation [5]. On the other
hand, the additional energy states associated with optimized for-
eign atoms can be highly delocalized, which is favorable for
improving charge mobility and enhancing photocatalytic perfor-
mance [23]. Many studies have demonstrated that the mixing of
orbitals in d10 oxides’ hybridized s and p orbitals confers a large
dispersion in k-space on the band and thereby a high mobility of
photogenerated electrons [24–31]. High mobility of photoexcited
electrons enhances the photocatalytic activity of a given semicon-
ductor photocatalyst. For instance, ternary metal oxides (e.g., zinc
germanate/zinc stannate) in which the Zn2+ ion is in a d10 elec-
tronic configuration have exhibited a competitive advantage in
the photoreduction of CO2 into solar fuels [15,16]. Among these
metal oxides with d10 electronic configuration, zinc titanate gener-
ally exists in three different crystalline phases, including the
and semiconductor-based composites (e.g., CdS/graphene, Zn1.7
-
GeN1.8O/graphenes, Fe2V4O13/rGO/CdS) [18–22], have been devel-
oped. However, most of them can be used only under UV light
and suffer from low conversion efficiency and poor photostability
during the photocatalytic reaction, which has greatly hindered
commercial applications in this field. So highly efficient and stable
photocatalysts still are a huge challenge facing research on CO2
photoreduction to fuel.
⇑
Corresponding author.
Co-corresponding author.
E-mail address: junliang@nxu.edu.cn (J. Liang).
⇑⇑
https://doi.org/10.1016/j.jcat.2019.01.017
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

Y. Chai et al. / Journal of Catalysis 371 (2019) 144–152
145
ZnTiO3 (hexagonal), Zn2Ti3O8 (cubic), and Zn2TiO4 (cubic). As com-
pared with ZnTiO3 and Zn2Ti3O8, cubic spinel Zn2TiO4 with d10
electronic configuration has been found to have not only important
electrical and photochemical properties (e.g., catalytic properties)
[32–34], but also high chemical stability and a band gap compara-
ble to that of TiO2 [35,36]. Meanwhile, the refractory spinel oxide
typically forms extended series of solid solutions, which is extre-
morphology and energy-dispersive spectroscopy (EDS) of the sam-
ples were observed by transmission electron microscope (TEM;
(FEI Tecnai G2 F30 S-Twin, USA). The surface chemical composi-
tions of samples were examined by X-ray photoelectron spec-
troscopy with an ESCALAB 250 XPS electron spectrometer with a
mono-achromatized AlK
a X-ray source. The binding energy was
referenced to the C1s peak at 284.8 eV. Nitrogen adsorption–des-
orption studies were performed at liquid nitrogen temperature
(77 K) using a Model ASAP 2020 Micromeritics apparatus for the
Brunauer–Emmett–Teller (BET) method. CO2 adsorption–desorp-
tion curves were measured at 273 K. The optical absorption prop-
erties of samples were recorded by ultraviolet–visible diffuse
reflectance spectra (UV–vis DRS) on a Varian Cary 500 Scan UV/
Vis system, in which BaSO4 was used as the background. The ele-
mental composition of the sample was measured by inductively
coupled plasma optical emission spectrometry (ICP-OES, Agilent
725).
mely tolerant of different cationic replacements (e.g., Sn4+, Fe3+
,
and Zr4+) [37–39]. Thus, to design and construct Zn2TiO4-based
solid solutions by introducing foreign atoms in a particular way
can be a feasible and effective route to achieving series of novel
and high-performance photocatalytic materials for the photocat-
alytic reduction of CO2.
In this study, a series of solid-solution photocatalysts consisting
of cubic spinel Zn2TiO4 and pseudocubic inverse spinel Zn2GeO4
were prepared by a facile molten salt route (Na2SO4 and K2SO4)
at 1273 K for 12 h using stoichiometric mixtures of ZnO, TiO2,
and GeO2 as precursors. The as-obtained nanocrystalline Zn2Ti1Àx
-
GexO4 (0 ꢀ x ꢀ 0.15) solid solution was shown to exhibit both sig-
nificantly improved photocatalytic activity and unprecedented
photocatalytic stability for conversion of CO2 to CO and CH4. The
enhanced performance of Zn2Ti1ÀxGexO4 is attributed to two main
causes: (i) band-gap narrowing, which is caused by upper shift of
the valence band edge from the enhanced p-d repulsion, and (ii)
enhanced mobility and diffusion of photogenerated carriers, which
is attributed to the light hole effective mass and strong electron
delocalization. These factors enhance the ability of the photocata-
lyst to convert CO2 and water into renewable fuels.
2.3. Photocatalytic activity measurement
The photocatalytic reduction of CO2 with H2O vapor was carried
out in a quartz glass reactor under atmospheric pressure at room
temperature. In a typical experiment, 50 mg of the as-prepared
photocatalyst was loaded into a 25-mL quartz glass reactor whose
top was sealed with a silicone rubber septum. Subsequently, the
whole system was subjected to vacuum degassing and then back-
filling with high-purity CO2 gas (99.995%, Changzhou Jinghua
Industrial Gas Co.). This evacuation-filling process was repeated
several times, and after the last cycle the reactor was backfilled
2. Experimental
with CO2 (1 bar). Finally, 20 lL of liquid deionized water was intro-
duced into the reactor from the silicone rubber septum with a
microsyringe, and liquid deionized water was gasified by heating
with a hair dryer. Prior to illumination irradiation, magnetic stir-
ring (600 rpm) in the dark for 1 h established adsorption/desorp-
tion equilibrium. A 300-W Xe arc lamp was used as a light
source. The photocatalytic reaction was carried out while the reac-
tor was evacuated after each 10 h and refilled with CO2 and deion-
ized water. Finally, 0.5 mL of gas was taken from the glass reactor
for subsequent gas concentration analysis using a gas chro-
matograph (GC-7890A, Shimadzu) equipped with a flame ioniza-
2.1. Material preparation
For synthesis of nanocrystalline Zn2Ti1ÀxGexO4 (0 ꢀ x ꢀ 0.15)
solid solutions, all regents used were from Sinopharm Chemical
Reagent Co. (SCRC) without further purification. In this experi-
ment, Na2SO4 and K2SO4 served as molten salts media because of
their low melting temperature (1104 K). In the typical synthesis
of Zn2Ti1ÀxGexO4 solid solutions, dry mixtures of ZnO, GeO2, TiO2,
Na2SO4, and K2SO4 with molar ratios of 2:0.05:0.95:15:15,
2:0.10:0.90:15:15, and 2:0.15:0.85:15:15 were thoroughly ground
in an agate mortar. After 1 h, the starting-mixture compositions
were pressed into disks by a mini tablet press machine and put into
an alumina crucible, and then the disk-shaped samples were cal-
cined in static air at 1273 K (ramp of 3 K/min) for 12 h, followed
by quenching. After cooling to room temperature, the as-
obtained solid products were collected and washed several times
with deionized water by centrifugation to remove Na2SO4 and
K2SO4. Finally, these precipitates were dried in an oven at 343 K
for 12 h. The bulk Zn2TiO4 particles used for comparison were pre-
pared by heating a stoichiometric mixture of TiO2 and ZnO at
1473 K (ramp of 5 K/min) for 24 h; the product was denoted as
Zn2TiO4-SSR.
tion detector (FID) and
a capillary column (GC-GASPRO,
30 m  0.320 mm). The generated O2 was detected by HP 7890
gas chromatography (Ar carrier) equipped with a TCD detector
and 5A molecular sieve packed column (10 m  mm). Product
gases were calibrated with a standard gas mixture and their iden-
tity was determined using the retention time and analyzed quanti-
tatively by an external standard method (Fig. S1 in the
Supplementary Material).
2.4. Photoelectrochemical measurements
Photoelectrochemical measurements were carried out with a
BAS Epsilon workstation using a standard three-electrode electro-
chemical cell with a working electrode, a platinum foil as the coun-
ter electrode, and a saturated Ag/AgCl electrode as the reference. A
sodium sulfate solution (0.2 M) was used as the electrolyte, and a
300-W LightningCure Model LC8 spotlight was introduced as the
2.2. Characterization
The crystalline phases of the prepared samples were
characterized by powder X-ray diffraction (XRD) by a Bruker D8
Advance
X-ray
diffractometer
with
CuKa
radiation
light source. At 365 nm, the output power was 4500 mW cmÀ2
.
(k = 0.15406 nm) operating at 40 kV and 30 mA with a scanning
rate of 8°/min from 3° to 85°. The morphologies and sizes of the
samples were analyzed by field-emission scanning electron micro-
scopy (FE-SEM, JEOL JSM–6701F). The chemical structure and mor-
phology of the sample were characterized using a JEOL Model JEM
2010 EX microscope at an accelerating voltage of 200 kV. The
The working electrode was prepared by SnO2 doped with F (FTO)
glass pieces, and was cleaned by sonication in cleanout fluid, ace-
tone, and ethanol in sequence. The photocatalyst was dispersed
in ethanol under sonication to form a suspension. A photocatalyst
film was fabricated by spreading the suspension onto the conduc-
tive surface of the FTO glass.

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Y. Chai et al. / Journal of Catalysis 371 (2019) 144–152
Fig. 1. (a) XRD patterns of Zn2Ti1-xGexO4 (0 ꢀ x ꢀ 0.15) solid solutions. (b) The enlarged part of the corresponding Zn2Ti1-xGexO4 solid solutions’ XRD patterns ranging from
2h = 34.0° to 36.0°. (c) Lattice constants and (d) lattice parameters of Zn2Ti1-xGexO4 (0 ꢀ x ꢀ 0.15) solid solutions.
2.5. Models and computational methods
3. Results and discussion
Density functional theory (DFT) calculations were carried out
with the CASTEP package from Accelrys [40]. The generalized gra-
dient approximation (GGA) plus the Perdew–Burke–Ernzerhof
functional (GGA-PBE) [41,42] was set as the exchange-correlation
functional. Brillouin zone sampling was chosen according to the
The XRD patterns of the Zn2Ti1-xGexO4 (0 ꢀ x ꢀ 0.15) samples
are shown in Fig. 1a. All samples have the same crystal structure
as Zn2TiO4 (JCPDS 25-1164), and the enlarged image indicates a
gradual shift of the (3 1 1) diffraction peaks toward higher angles
with increasing Ge content (Fig. 1b), suggesting formation of a
cubic-spinel-structured Zn2Ti1-xGexO4 (0 ꢀ x ꢀ 0.15) solid solution
by incorporation of Ge4+ into the lattice structure of Zn2TiO4. Addi-
tionally, the lattice parameters of the Zn2Ti1-xGexO4 solid solution
Monkhorst–Pack [43] scheme with a grid spacing of 0.04 ÅÀ1
,
and the k-point mesh was 5 Â 5 Â 5, 5 Â 5 Â 1 for Zn2TiO4 and Zn2-
Ti0.9Ge0.1O4, respectively.
Fig. 2. (a) Wide scan XPS survey spectra of the Zn2Ti0.9Ge0.1O4 solid solution, Zn2TiO4, and Zn2GeO4. High-resolution XPS spectra of the Zn2Ti0.9Ge0.1O4 solid solution, Zn2TiO4,
and Zn2GeO4: (b) Ti2p, (c) Zn2p, (d) Ge2p, and (e) O1s.

Y. Chai et al. / Journal of Catalysis 371 (2019) 144–152
147
Fig. 3. (a) Large-area and (b) partially enlarged FESEM images of the as-obtained Zn2Ti0.9Ge0.1O4 solid solution. (c and d) TEM and (e) HRTEM images of the Zn2Ti0.9Ge0.1O4
solid solution. (f) Energy-dispersive X-ray spectroscopy (EDX) spectrum of the Zn2Ti0.9Ge0.1O4 solid solution. (g) Dark-field TEM and corresponding EDX elemental mappings
of Zn2Ti0.9Ge0.1O4 solid solution obtained by TEM. Insert in panel (e) is the corresponding SAED pattern Zn2Ti0.9Ge0.1O4 solid solution.
are calculated by fitting the XRD data, as shown in Fig. 1c. It is
important to note that the lattice constant is increased linearly
with decreasing Ge content, obeying the generally known Vegard’s
law [39]. This may be attributed to isomorphous substitution of
lattice Ge4+ with a smaller radius (0.053 nm) by Ti4+ with a larger
radius (0.0605 nm). Nevertheless, it is further demonstrated in
Fig. 1d that the lattice of the solid solution Zn2Ti1-xGexO4
for the Zn2Ti0.9Ge0.1O4 sample, which are assigned to Ti2p3/2 and
Ge2p3/2 (Fig. 2b and d), respectively [44]. The two peaks centered
at 1022.41 and 530.28 eV are attributed to Zn2p3/2 and O1s
(Fig. 2c and e), respectively. However, in contrast to Zn2TiO4
(Ti2p3/2 at 458.85 eV, O1s at 530.33 eV, and Zn2p3/2 at
1022.15 eV) [45] and Zn2GeO4 (Ge2p3/2 at 1220.63 eV, O1s at
530.99 eV, and Zn2p3/2 at 1022.44 eV) [46], the Ti2p (458.66 eV)
and Ge2p3/2 (1220.5 eV) peak positions in the Zn2Ti0.9Ge0.1O4 sam-
ples shift slightly toward lower binding energy, indicating the
presence of a new bond (Fig. 2b and d). Compared to the Zn2TiO4
and Zn2GeO4 samples, the O1s binding energy is slightly decreased,
(0 ꢀ x ꢀ 0.15) evolves with composition from Zn2TiO4 to Zn2Ti0.85
-
Ge0.15O4. However, it should be noticed that when the x value is
higher than 0.15, the product is a two-phase mixture of the cubic
Zn2Ti1-xGexO4 solid solution and rhombohedral Zn2GeO4 (Fig. S2).
X-ray photoelectron spectrum (XPS) measurements were car-
ried out to obtain the chemical state and composition of the Zn2-
Ti1-xGexO4 solid solution. For comparison, the XPS spectra of
Zn2TiO4 and Zn2GeO4 samples obtained by a similar molten salt
route were also determined. As shown in Fig. 2a, in contrast to
the chemical compositions of both Zn2TiO4 and Zn2GeO4 samples,
the survey spectrum of the Zn2Ti0.9Ge0.1O4 sample confirms the
simultaneous existence of Zn, Ti, Ge, and O elements in the
Zn2Ti1-xGexO4 solid solution. To further identify the chemical states
of elements in various samples, binding energies of core-level elec-
trons from constituent elements for all samples are illustrated in
Fig. 2b–e. Two peaks at 458.66 and 1220.5 eV are clearly observed
while Zn2p binding energy is slightly increased for the Zn2Ti0.9
-
Ge0.1O4 sample (Fig. 2c and e). The change in binding energies
can result from introduction of the Ge atom into the Zn2TiO4. In
addition, the energetic separation (DE) between the Zn2p3/2 and
Ti2p3/2 peaks for Zn2TiO4 and Zn2Ti0.9Ge0.1O4 samples was deter-
mined to be ca. 563.3 eV and 563.75 eV, respectively (Fig. 2b and
c), much lower than that for the mixture of ZnO and TiO2
(565.80 eV) [47], confirming the formation of spinel Zn2TiO4. The
result indicates that a more complete phase transformation to
cubic spinel structure can occur, as demonstrated by XRD
(Fig. 1). On the other hand, we also observed that the
DE value of
the Zn2Ti0.9Ge0.1O4 sample exhibits certain increase in
a

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Y. Chai et al. / Journal of Catalysis 371 (2019) 144–152
Fig. 4. (a) UV–visible diffuse reflection spectra of the various samples. (b) The optical band gap energy (Eg) of the various corresponding samples. (c) Valence-band XPS
spectra of Zn2TiO4, Zn2GeO4, and Zn2Ti0.9Ge0.1O4 solid solution prepared by a molten-salts route. (d) Energy band structure schematic of these materials, calculated from the
data in (a), (b), and (c).
comparison with that of Zn2TiO4 particles, which further demon-
strated the formation of Ge-substituted Zn2TiO4 solid solution with
a cubic inverse spinel structure.
Fig. 3a and b present typical SEM images of the Zn2Ti0.9Ge0.1O4
solid solution. It can be seen clearly that the solid solution sample
is composed of a large number of polyhedral nanoparticles, with
sizes ranging from 100 to 300 nm (Fig. S3). The morphological
characteristics of the sample are also supported by TEM observa-
in agreement with the theoretical value. In addition, as shown in
Fig. 3g, the elemental mapping images reveal that the Zn2Ti0.9
-
Ge0.1O4 are indeed composed of the elements Zn, Ti, Ge, and O,
and all the elements are distributed uniformly over the whole area
of the solid solution, suggesting uniformity of Zn2Ti0.9Ge0.1O4 with-
out any phase separation. This indicates that the Zn2Ti0.9Ge0.1O4
solid solution can be successfully obtained via the simple molten
salt route.
tion (Fig. 3c and d), further disclosing that the as-prepared Zn2Ti0.9
-
Fig. 4a and b show UV–visible diffuse reflectance spectra (DRS)
of samples. The band gap (Eg) of bare Zn2TiO4 prepared by the mol-
ten salt route is estimated to be 3.73 eV, corresponding to an opti-
cal absorption edge of ca. 332 nm. Zn2Ti0.95Ge0.05O4,
Zn2Ti0.9Ge0.1O4, and Zn2Ti0.85Ge0.15O4 have band gaps of ca. 3.72,
3.70, and 3.71 eV, respectively, corresponding to optical adsorption
edges of about 333, 335, and 334 nm. This means that the Eg of the
solid solution is less than the that of Zn2TiO4 and of Zn2GeO4
(4.5 eV) [31]. According to Fig. S4, the light-absorption edge of
the nanostructured Zn2TiO4 sample prepared by the molten salt
route exhibit a certain extent of blue shift from those of Zn2TiO4-
SSR prepared by the high-temperature solid state method, which
can be explained by size effects [48]. Simultaneously, the band
edge position of the conduction band of the cubic spinel Zn2TiO4
can be positively shifted by introducing Zn2GeO4 with a pseudocu-
bic inverse spinel structure. To understand the electronic structure
of the Ge-substituted Zn2TiO4 solid solution, valence band XPS (VB
XPS) was used for the observation of the valence band electronic
Ge0.1O4 solid solution is highly crystalline and the sizes are below
300 nm. Moreover, to make sure of the detailed crystal structure of
the polyhedral Zn2Ti0.9Ge0.1O4 solid solution, HRTEM characteriza-
tion was also done, as shown in Fig. 3e. Both a uniform lattice
fringe and a spot pattern of the selected-area electron diffraction
(SAED) (the inset in Fig. 3e) verify that the Zn2Ti0.9Ge0.1O4 solid
solution shows apparently single-crystalline characteristics. The
distance between the adjacent lattice fringes is 0.485 nm
(Fig. 3e), which can be assigned to the interplanar distance of the
cubic Zn2Ti0.9Ge0.1O4 (1 1 1) plane. The data are almost consistent
with the (1 1 1) crystal planes of cubic Zn2TiO4, according to JCPDS
card 25-1164. Furthermore, EDS and TEM mapping are used to
establish the composition and distribution of all elements in the
solid solution, as shown in Fig. 3f. The Zn, Ti, Ge, and O belonging
to the sample are detected, and its composition is very close to the
feed ratios. Meanwhile, on the basis of the ICP-OES analysis, the
atomic ratio of Zn, Ti, and Ge is ca. 2.05:0.89:0.1, which is nearly

Y. Chai et al. / Journal of Catalysis 371 (2019) 144–152
149
Fig. 5. (a) Gas evolution rates under different conditions. (b) Mass spectra of the reaction product from 12CO2 to 13CO2 over Zn2Ti0.9Ge0.1O4 solid solution in the presence of
H2O vapor. (c) The photocatalytic performance of P25 TiO2, Zn2TiO4-SSR, and various Zn2Ti1-xGexO4 samples. (d) The photocatalytic stability of the Zn2Ti0.9Ge0.1O4 solid
solution. (e) The CH4 and (f) CO production rates of various samples.
state of these as-prepared samples (Fig. 4c). We find that the
valence band (EVB) potentials of Zn2TiO4, Zn2GeO4, and Zn2Ti0.9
Ge0.1O4 solid solution are ca. 2.45, 3.2, and 2.5 V. Accordingly, the
conduction band potential (ECB) of Zn2TiO4, Zn2GeO4, and Zn2Ti0.9
The electron structures of Zn2TiO4 and Zn2Ti1ÀxGexO4
(0 ꢀ x ꢀ 0.15) solid solution samples can meet requirements for
the reduction of CO2 by H2O under simulated sunlight. As dis-
played in Fig. 5a, a blank experiment reveals that no products are
detected in the absence of a photocatalyst or light irradiation. In
the absence of H2O, it is found that only small amounts of CH4
and CO were produced over Zn2Ti0.9Ge0.1O4. Once water was
added, as expected, the amounts of the main products (CH4 and
CO) produced by CO2 photoreduction were significantly increased
under light irradiation. This result indicated that CO2 reduction
and H2O oxidation might occur simultaneously in the present pho-
tosynthesis system to generate CH4 and CO from direct photocat-
alytic reaction of CO2 and H2O, while H2O can act as an electron
donor for photocatalytic CO2 reduction [49]. To further confirm
the origin of the CO and CH4 products, isotope experiments using
13CO2 and 12CO2 as substrates were conducted on a GC-MS system
(Fig. 5b). As revealed by the peaks at m/z = 17 and 29, 13CH4 and
13CO were the main products of the reaction; this further confirms
that the carbon source of the formed reduction products derived
-
-
Ge0.1O4 solid solutions is estimated at about À1.28, À1.20, and
À1.20 V, respectively, according to Eg = EVB À ECB [49]. (Eg: semi-
conductor band gap; EVB: valence band edge potential; ECB: con-
duction band edge potential.) This means that the band edge
positions of the conduction and valence bands of the cubic spinel
Zn2TiO4 can be positively shifted simultaneously by introducing
Zn2GeO4 with a pseudocubic inverse spinel structure. However, it
is noteworthy that the upshift of the valence band edge potential
is much greater than that of the conduction band edge potential.
Thus, introducing Zn2GeO4 into Zn2TiO4 can effectively narrow
the band gap, as shown in Fig. 4d. The band gap narrowing may
promote the photoinduced reaction due to more light absorption.
The results of our present work show that Zn2GeO4 plays a domi-
nant role in tuning the energy band structure of Zn2TiO4 with Ge
substituting at the Ti site [50].

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Y. Chai et al. / Journal of Catalysis 371 (2019) 144–152
Fig. 6. (a) CO2 adsorption isotherms of Zn2TiO4, Zn2Ti0.9Ge0.1O4 solid solution, and Zn2GeO4 at 273 K. (b) Specific surface areas of the as-prepared Zn2Ti1-xGexO4 samples. (c)
Transient photocurrent responses and (d) Nyquist plots for the samples obtained by a molten salts route.
from CO2. Moreover, we can observed distinctly from Fig. 5c that
each of the prepared Zn2Ti1ÀxGexO4 (0 ꢀ x ꢀ 0.15) samples can
photocatalytically reduce CO2 to CO and CH4 in the presence of
H2O, and their activities for evolution of gases (CH4 and CO) were
remarkably better than those of the standard photocatalyst P25
TiO2. Meanwhile, upon full arc irradiation, CO and CH4 are
generated as the main reduction products of CO2, while O2 is
evolved simultaneously as the oxidation product of H2O. To the
best of our knowledge, this is the first examples of CO2 photore-
duction over Zn2TiO4 and Zn2Ti1ÀxGexO4 (0 ꢀ x ꢀ 0.15) solid solu-
tions. Furthermore, according to the results in Fig. 5c, for Zn2TiO4
samples, the nanostructured Zn2TiO4 sample obtained by the
molten-salt route is superior in activity to the bulk Zn2TiO4-SSR
sample prepared by the solid-state reaction route. In view of
Figs. S5 and S7a, it is evident that the difference in photocatalytic
activity is related to their particle sizes. Of the Zn2Ti1ÀxGexO4
(0 ꢀ x ꢀ 0.15) samples, the Ge-substituted Zn2TiO4 solid solutions
displayed better activity than the nanostructured Zn2TiO4 sample.
It is therefore noteworthy that the amounts of these products
increased almost linearly with increasing irradiation time (Fig. 5e
and f); especially, the Zn2Ti0.9Ge0.1O4 sample shows the highest
activity, and the maximal evolution rates of CO and CH4 over the
Zn2Ti0.9Ge0.1O4 solid solution photocatalyst are as high as 11.9
SSR > P25-TiO2 (Fig. 5c). At the same time, Zn2TiO4 and Zn2Ti0.9-
Ge0.1O4 solid solution also show higher photocatalytic activity than
common semiconductors (Tables S1 and S2 in the Supplementary
Material). In addition, Fig. 5d shows that the photocatalytic activity
of the Zn2Ti0.9Ge0.1O4 solid solution remains unchanged after 60 h
of reaction, and the amount of these products increases almost lin-
early with increasing irradiation time, indicating good activity and
stability of the photocatalyst. From the results of the XRD (Fig. S6),
the phase composition of the solid solution photocatalyst is still
maintained after 60 h photoreaction. On the basis of analysis of
the above results, the germanium-substituted Zn2TiO4 solid solu-
tions have the competitive advantages of tunable bandgap
(Fig. 4), good photocatalytic activity, and powerful stability. These
make it a promising candidate as a new photocatalyst to convert
CO2 into solar fuel in the presence of H2O.
To gain insight into the photocatalytic performance of Zn2Ti1-
xGexO4 solid solutions, the basic physicochemical properties of
these Zn2Ti1-xGexO4 solid solutions were characterized. As seen in
Fig. S7 and Fig. 6b, the as-prepared Zn2Ti1-xGexO4 samples almost
have comparable specific surface area and particle size, implying
that the texture is not the reason for the difference in the photocat-
alytic activity (Fig. 5d) of those Zn2Ti1-xGexO4 photocatalysts. The
CO2 adsorption on various photocatalysts was evaluated by the
BET method at 273 K, as shown in Fig. 6a. The adsorption amount
of CO2 decreased in the order Zn2TiO4 > Zn2Ti0.9Ge0.1O4 > Zn2GeO4,
and 0.38 l
mol hÀ1 gÀ1, respectively. The photocatalytic activity
decreased in the order Zn2Ti0.9Ge0.1O4 > Zn2Ti0.95Ge0.05O4 > Zn2-
Ti0.85Ge0.15O4 > Zn2TiO4 + Zn2GeO4 (prepared by physical mixing
at a molar ratio of Zn2TiO4/Zn2GeO4 = 9:1) > Zn2TiO4 > Zn2TiO4-
while the photocatalytic activity decreased in the order Zn2Ti0.9
-
Ge0.1O4 > Zn2GeO4 > Zn2TiO4. Obviously, the photocatalytic activity

Y. Chai et al. / Journal of Catalysis 371 (2019) 144–152
151
Fig. 7. (a) Structure of Zn2TiO4, viewed from the [1 1 0] direction with the unit cell outlined. (b) Constructed model of Zn2TiO4 with the primitive cell, viewed from the [1 0 1]
direction. Pink: ZnO4 tetrahedron, light blue: Zn/TiO8 octahedron, dark blue: TiO8 octahedron, green: ZnO8 octahedron. Calculated band structures and density of states (DOS)
of (c) Zn2TiO4 and (d) Zn2Ti0.9Ge0.1O4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
difference cannot be related to the CO2 adsorption. Furthermore,
the photoelectrochemical characterization of the photocatalyst
was performed and is shown in Fig. 6c. As compared with Zn2TiO4
and Zn2GeO4, the Zn2Ti0.9Ge0.1O4 solid solution electrode gives a
significant improvement in transient photocurrents under light
illumination. Those transient photocurrent results are in agree-
ment with the photocatalytic activity reported above (Fig. 5). How-
ever, we also observed that the photocurrent density for these film
electrodes decreased slowly with the on–off illumination cycle
increasing, which could be attributed to the establishment of
steady diffusion gradients in the system [15]. In order to further
reveal the charge transfer process, electrochemical impedance
spectroscopy (EIS) was also carried out. The Nyquist plots of the
photocatalyst electrodes of Zn2TiO4, Zn2Ti0.9Ge0.1O4 solid solution,
[51], in which the tetrahedral interstices are occupied by Zn
cations (ZnO4), while the octahedral interstices are occupied by
both Zn and Ti cations (MO8, Zn:Ti = 0.5:0.5, Fig. 7a). To further
explore the origin of the optical and photocatalytic properties of
the Zn2Ti0.9Ge0.1O4 solid solution, theoretical calculations were
performed for the electronic structure of Zn2TiO4 and Zn2Ti0.9Ge0.1
-
O4 solid solution. Considering the fractional occupancies of Ti and
Zn atoms, a possible structure replacing two octahedral sites with
Ti and the others with Zn was predicted (Fig. 7b). The solid solution
was simulated by replacing 1 Ti at the octahedral site with 1 Ge in
the 1 Â 1 Â 5 supercell of Zn2TiO4 (shown in Fig. S8). A primitive
cell (ainit ꢁ binit ꢁ cinit = 5.989 Å,
a = b = c = 60°), with space group
À
reduced from Fd 3 mto P1, was constructed. The electronic struc-
ture of Zn2TiO4 and Zn2Ti0.9Ge0.1O4 solid solution was researched
by DFT calculations. The electronic band structures and densities
and Zn2GeO4 are shown in Fig. 6d. It can be seen that the Zn2Ti0.9
-
Ge0.1O4 solid solution shows more decrease in charge-transfer
resistance than Zn2TiO4 and Zn2GeO4. The EIS analysis result indi-
cates higher conductivity of the Zn2Ti0.9Ge0.1O4 solid solution than
of the pseudocubic Zn2TiO4 and Zn2GeO4. Therefore, more effective
charge transfer was achieved, as well as higher photocatalytic
performance.
The band structure of a semiconductor photocatalyst is of vital
importance in determining the variation of photocatalytic activity
in the Zn2Ti1-xGexO4 solid solutions. To understand the electronic
structures of cubic Zn2TiO4 and Zn2Ti0.9Ge0.1O4 solid solution,
density functional theory (DFT) calculations and generalized gradi-
of states (DOS) near the Fermi levels of Zn2TiO4 and Zn2Ti0.9Ge0.1
-
O4 are shown in Fig. 7c and d. Zn2TiO4 is an indirect band gap semi-
conductor with a calculated band gap of about 1.51 eV. This value
is much lower than the experimental value of 3.52 eV determined
from UV/vis spectra. This can be attributed to the drawbacks of the
generalized gradient approximation (GGA) method, which under-
estimates band gaps [52]. Nevertheless, the calculation offers qual-
itative comparisons. The conduction band (CB) and valence band
(VB) are mainly composed of Ti3d and O2p orbitals. On the other
hand, Zn2Ti0.9Ge0.1O4 is an indirect band gap semiconductor with
a calculated gap of 1.0 eV. Seen from its PDOS, it reveals the pres-
ence of an additional VB at the top of the original VB, which is from
Ge4s4p orbitals. It has been reported that p-d repulsion in II–VI
ent approximation (GGA) functional calculation were performed.
À
Zn2TiO4 possesses an inverse spinel-type structure (Fd 3 m, Z = 8)

152
Y. Chai et al. / Journal of Catalysis 371 (2019) 144–152
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In summary, we reported a controllable synthesis strategy for
the Ge-substituted Zn2TiO4 solid solution as a highly efficient pho-
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By changing the molar ratio of Ti/Ge atoms in Zn2Ti1-xGexO4, a ser-
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This research was supported by the National Natural Science
Foundation of China (Grants 21862015, 21463019, 51702053,
and 21865022), the National First-Rate Discipline Project of Ningx-
ia (Chemical Engineering and Technology NXYLXK2017A04), the
Major Innovation Projects for Building First-Class Universities in
China’s Western Region, and the Natural Science Foundation of
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Appendix A. Supplementary material
Supplementary data to this article can be found online at
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