Synthesis and characterization of Zn2GeO4/Mg-MOF-74 composites with enhanced photocatalytic activity for CO2 reduction

Article abstract of DOI:10.1039/c7cy02286g

Strategies for effectively enhancing the photoreduction of CO₂ have been studied for several decades. In this work, Zn₂GeO₄/Mg-MOF-74 composites were prepared via a hydrothermal method. The X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images demonstrate the successful synthesis of Zn₂GeO₄/Mg-MOF-74 composites. The infrared spectra and TEM images indicate a close interaction between Zn₂GeO₄ and Mg-MOF-74, which is attributed to tetramethylammonium hydroxide (TMAOH) adsorbed on Zn₂GeO₄ and interacts with 2,5-dioxide-1,4-benzenedicarboxylate (H₄DOBDC) from Mg-MOF-74. The CO₂ adsorption isotherms illustrate that the incorporation of Mg-MOF-74 into Zn₂GeO₄ nanorods improved the adsorption of CO₂. Only with water as the sacrificial agent, the hydrothermally prepared Zn₂GeO₄/Mg-MOF-74 catalyst exhibited a superior performance for the conversion of CO₂ into carbon monoxide compared to that of single Zn₂GeO₄ or a physical mixture of Zn₂GeO₄/Mg-MOF-74, which is attributed to its higher adsorption capacity for CO₂ and reduced recombination of photogenerated electron-hole pairs. In situ photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) further demonstrated that the Zn₂GeO₄/Mg-MOF-74 catalyst exhibits superior charge-transfer properties. This work may provide a useful strategy to design novel and efficient photocatalysts to reduce the atmospheric concentration of CO₂.

Full text of DOI:10.1039/c7cy02286g

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Synthesis and characterization of Zn2GeO4/Mg-MOF-74
composites with enhanced photocatalytic activity for CO2
reduction
Received 00th January 20xx,
Accepted 00th January 20xx
Hui Zhao,ab Xusheng Wang,a Jifei Feng,a Yanning Chen,ab Xue Yang,a Shuiying Gao,a* and Rong
Caoab*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Strategies for effectively enhancing the photoreduction of CO2 have been studied for several decades. In this work,
Zn2GeO4/Mg-MOF-74 composites were prepared via a hydrothermal method. The X-ray diffraction (XRD) patterns,
scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) demonstrated the successful
synthesis of Zn2GeO4/Mg-MOF-74 composites. Infrared spectra and TEM images indicated a close interaction between
Zn2GeO4 and Mg-MOF-74, which is attributed to the tetramethylammonium hydroxide (TMAOH) adsorbed on Zn2GeO4 and
interacts with the 2,5-dioxide-1,4-benzenedicarboxylate (H4DOBDC) from Mg-MOF-74. The CO2 adsorption isotherms
illustrated that the incorporation of Mg-MOF-74 into Zn2GeO4 nanorods improved the adsorption of CO2. Only with water
as the sacrificial agent, the hydrothermally prepared Zn2GeO4/Mg-MOF-74 catalyst exhibited a superior performance for
the conversion of CO2 into carbon monoxide than that of single Zn2GeO4 or a physical mixture of Zn2GeO4/Mg-MOF-74,
which is attributed to its higher adsorption capacity for CO2 and reduced recombination of photogenerated electron-hole
pairs. In situ photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) further demonstrated that
Zn2GeO4/Mg-MOF-74 catalyst exhibits superior charge-transfer properties. This work may provide a useful strategy to
design novel and efficient photocatalysts to reduce the atmospheric concentration of CO2.
have the advantages of low toxicity, low cost and excellent stability.
Many semiconductors, such as ZnO,9 TiO2,17 CdS,18 and Zn2GeO4,19
have been used for the photocatalytic reduction of CO2. Specially,
Introduction
The atmospheric concentration of CO2 continues to rise due to the
increased burning of fossil fuels and has resulted in a series of
global environmental issues, such as global warming and disastrous
weather.1-4 Over the past several decades, numerous strategies to
convert CO2 into useful fuel, such as thermochemical conversion,5
electrocatalytic conversion,6,7 photoelectrochemical conversion,8,9
and photocatalytic conversion,9-11 have been developed to
chemically reduce CO2. Among these methods, the photocatalytic
reduction of CO2 into renewable fuels has been the most promising
activation method with many advantages,12 including the use of
relatively mild conditions, such as low temperatures and normal
pressure, and the direct use of solar energy without consumption of
auxiliary energy. Moreover, the raw photocatalyst materials can be
simply and easily obtained.13,14 Thus far, various photocatalysts,
including metal-incorporated zeolites,15 homogeneous transition-
metal compositeses,16 and semiconductors,17-19 have been studied
for the photocatalytic conversion of CO2. Semiconductor catalysts
the high crystallinity and more dispersed energy structure of
Zn2GeO4 nanorods cause a high mobility of the photoinduced
electrons in the conduction band, which inhibit most of the
photoinduced electron-hole pair recombination and increase the
life of the photogenerated electron-hole pairs.19-22 Moreover,
Zn2GeO4 has a unique light stability and thermal stability, and thus,
it can be used for the reduction of CO2. However, the conversion
efficiency of Zn2GeO4 catalyst is still low due to the weak adsorption
of CO2 and low solar energy utilization efficiency.23
Metal-organic frameworks (MOFs) with ultrahigh surface areas,
homogeneous active sites, large pore sizes, tunable and highly
ordered porous structures have been explored for many
applications including gas sorption and separation, sensing,
catalysis, and drug delivery.24-27 So far, several works had shown
that MOFs can be potential candidates for CO2 photoreduction
because it can integrate all key functions for solar drived CO2
reduction including light harvesting, charge separation, CO2
encapsulation, activation, and fixation.28-30 However, the sacrificial
agent TEOA must be utilized with pure MOFs as catalyst due to the
low catalytic efficiency. Mimicking the natural photoreduction of
CO2 with only water as sacrificial agent is a really important and
challenge work in the current situation. Incorporating MOFs into a
semiconductor photocatalyst is a promising strategy to increase the
31-33
conversion efficiency of the photocatalytic reduction of CO2,
however, the combination of semiconductor and MOF for CO2
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photoreduction are scarce, and most of them are just physically (UV-vis DRS) was performed using a spectrophotometer (Shimadzu
mixed and/or with TEOA as sacrificial agent. UV-2600), and BaSO4 was used as the background. The morphology
In this work, Mg-MOF-74 was combined with Zn2GeO4 for of the sample was characterized using field emission scanning
efficiently solar drived reduction of CO2 with only water as sacrificial electron microscopy (SEM, JSM6700) and transmission electron
agent.34 Mg-MOF-74 was chosen based on the following reasons: microscopy (TEM, JEM2010). The metal contents of the sample
(a) it can improves the electron transfer from the Zn2GeO4 to itself were measured by inductively coupled plasma mass spectrometry
and thus inhibits the photoelectron-hole recombination process; (b) (ICP-MS, ULTIMA 2). The photoluminescence (PL) spectra were
it has the highest CO2 adsorption ability at low pressure; (c) it investigated on a Horiba Jobin-Yvon Fluorolog-3 instrument. X-
possesses a high concentration of open alkaline metal sites (Mg2+) rayphotoelectron spectroscopy (XPS) was measured on a ESCALAB
that are beneficial for CO2 activation;35-37 (d) it can effectively 250Xi instrument.The photocurrent analysis was carried out in a
extends light sorption from the UV to visible region. Under the traditional three-electrode quartz cell. Pt foil plates were the
synergistic effect of Zn2GeO4 and Mg-MOF-74, photoelectron can counter electrode, and Ag/AgCl was the reference electrode.
effectively transfer from the Zn2GeO4 to open alkaline metal sites Electrochemical impedance spectra (EIS) was performed on an
(Mg2+) of Mg-MOF-74 for photocatalytic reduction of CO2. The IM6ex (Zahner, Germany). Brunauer-Emmett-Teller (BET) surface
unique Zn2GeO4/Mg-MOF-74 composites exhibit an ascendant area and N2 adsorption experiments were carried out on an
enhancement in photo-conversion of CO2 into CO efficiency by ASAP2020 instrument. The samples were degassed at 403 K for 12 h
more than 7 times, compared to the pure Zn2GeO4.
before the analysis. CO2 adsorption isotherms were measured at
room temperature using an ASAP2020 instrument.
Experimental
Photocatalytic Conversion of CO2
Before the photocatalytic reduction of CO2, the photocatalysts were
treated under vacuum at 383 K to remove any solvent. Then, 20 mg
of the photocatalyst was placed in a mixture of acetonitrile (1.6 ml)
and H2O (0.4 ml) that was exchanged with high purity CO2 (1 atm).
Before the photocatalytic reaction, the reactor was tightly closed
and allowed to rest for 30 minutes in the dark. A 300 W xenon arc
lamp was used as the light source for the photocatalytic reaction.
After the reactions, the gases produced were analyzed and
quantified by an Agilent 7820A gas chromatography (GC) and
Synthesis of Zn2GeO4 nanorods
All solvents were analytical grade and used without further
purification. In a typical synthesis of Zn2GeO4 nanorods,19 5 mmol
germanium dioxide (GeO2) and 10 mmol Zn(CH3COO)2·2H2O were
added to
a 25% tetramethylammonium hydroxide (TMAOH)
aqueous solution (30 ml). Then, the mixture was transferred to a
100 ml Teflon-lined stainless-steel autoclave and heated in an oven
at 453 K for 24 h under autogenous pressure. After naturally cooling
to room temperature, the product was obtained, washed with
deionized water and alcohol several times, and then dried at 343 K
for 24 h.
quantified with
a flame ionization detector and thermal
conductivity detector. The amount of HCOOH was detected by an
ion chromatograph (Metrohm 883) and compact autosampler
(Metrohm 863).
Synthesis of the Mg-MOF-74
Mg-MOF-74 was synthesized according to the previously reported
method.32 Mg(NO3)2·6H2O (1.67 mmol) and 0.56 mmol 2,5-dioxide-
1,4-benzenedicarboxylate (H4DOBDC) were dissolved in a 15:1:1
(v/v/v) mixture of DMF-ethanol-water (50 ml). Then, the mixture
was poured into three 20 ml Teflon-lined stainless-steel autoclaves
and heated at 398 K for 24 h. After naturally cooling, the as-
obtained product was collected and washed with DMF, ethanol and
deionized water.
Results and discussion
The X-ray diffraction (XRD) patterns of the as-synthesized samples
are shown in Fig. 1. The XRD pattern of the Zn2GeO4 nanorods (Fig.
1a) and Mg-MOF-74 (Fig. 1b) both match well with the published
pattern.23,38 The hydrothermally prepared Zn2GeO4/Mg-MOF-74
composites show diffraction peaks for Zn2GeO4 and Mg-MOF-74
(Fig. 1c). In addition, no other peaks appeared in the patterns,
which indicated that the composite material only contains Zn2GeO4
and Mg-MOF-74.
Preparation of the Zn2GeO4/Mg-MOF-74 composites
The similar hydrothermal method was used to prepare
Zn2GeO4/Mg-MOF-74. Mg(NO3)2·6H2O (1.67 mmol) and H4DOBDC
(0.56 mmol) were added to a 15:1:1 (v/v/v) mixture of DMF-
ethanol-water (50 ml) and uniformly ultrasonicated. Then, 0.75
mmol Zn2GeO4 was added to the above solution. The autoclave was
sealed and heated at 398 K for 24 h. After cooling to room
temperature, the obtained Zn2GeO4/Mg-MOF-74 composites were
washed with DMF, water and alcohol several times.
Characterizations
The as-prepared samples were characterized by powder X-ray
diffraction (PXRD) on
a
Miniflex 600 under CuKα radiation (λ=
VERTEX70
1.5406). Infrared spectra were collected on
a
spectrometer by Bruker. UV-vis diffuse reflectance spectroscopy
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Fig. 1. PXRD patterns, (a) Zn2GeO4 nanorods, (b) pure Mg-MOF-74,
functional alkylammonium group plays a vital role in the formation
and (c) Zn2GeO4/Mg-MOF-74 composites.
of Zn2GeO4/Mg-MOF-74.
Fig. 3. (a and b) TEM images, and (c) HRTEM of the Zn2GeO4/Mg-
MOF-74 composites.
The FT-IR spectra of the as-prepared samples further illustrate
the formation of the Zn2GeO4/Mg-MOF-74 composites (Fig. 4). The
spectral band at approximately 3300-3500 cm-1 was ascribed to the
amine stretching from TMAOH on the surface of the Zn2GeO4
sample (Fig. 4a). The bands at 700-850 cm-1 and 532 cm-1 were
ascribed to the vibrations of the GeO4 and ZnO4 tetrahedra,
respectively. After the successful deposition of Zn2GeO4 on Mg-
MOF-74, several obvious absorbance peaks appeared in Fig. 4d. The
absorption bands at 1578 and 1418 cm-1 were attributed to the C=O
carboxyl absorption peak and the stretching vibration of the
aromatic ring C=C bond, respectively. In addition, a new, obvious
peak at 1664 cm-1 appeared in the Zn2GeO4/Mg-MOF-74 composites
(Fig. 4d), which was assigned to the carboxyl absorption peak.
However, the adsorption peak did not exist in the physically mixed
Zn2GeO4/Mg-MOF-74 (Fig. 4c), which suggested that alkylammo-
nium group that adsorbed on the Zn2GeO4 nanorods favorably
interacted with the H4DOBDC from Mg-MOF-74. Therefore, the
functional alkylammonium group plays a vital role in the formation
of Zn2GeO4/Mg-MOF-74.
Fig. 2. SEM images of the (a) and (b) Zn2GeO4/Mg-MOF-74
composites, (c) Zn2GeO4 nanorods and (d) Mg-MOF-74.
The SEM images show that the as-prepared Zn2GeO4 sample was
composed of nanorods. The length of the individual nanorods was
300-500 nm with uniform diameters of 20-40 nm (Fig. 2c). The Mg-
MOF-74 morphology was column-like with a length of 1-3 µm and
smooth surfaces (Fig. 2d). A low-magnification SEM image shows
that after the Zn2GeO4 nanorods covered the Mg-MOF-74, the
column-like Mg-MOF-74 particles tended to form polyhedral
aggregates with lengths of 30-50 µm with grainy surfaces (Fig. 2a).
A
high-magnification SEM image of the Zn2GeO4/Mg-MOF-74
sample reveals that the Mg-MOF-74 surface was covered in
nanorods and indicated the successful growth of the Zn2GeO4
nanorods on the Mg-MOF-74 (Fig. 2b). TEM images of Zn2GeO4
/ZIF-8 nanorods are shown in Fig. 3. Fig. 3a and 3b illustrates that
Mg-MOF-74 was covered with discrete Zn2GeO4 nanorods.
Moreover, the HRTEM image shows that the inter-planar d-spacing
of ca. 0.706 nm corresponds to the (110) lattice plane of an
orthorhombic Zn2GeO4 (Fig. 3c). The amounts of Ge and Mg in the
composites were quantified by ICP. The results show that the
percentages of Ge and Mg in Zn2GeO4/Mg-MOF-74 are 10.89% and
5.88%, respectively. Thus, the mole ratio of Zn2GeO4 and Mg-MOF-
74 is 1.2:1.
Based on the above characteristics, the formation process of the
Zn2GeO4/Mg-MOF-74 composites are illustrated in Scheme 1. First,
the Zn2GeO4 nanorods coated with TMAOH were obtained via a
hydrothermal method (Scheme 1a). Then, the Zn2GeO4 nanorods
were added to the precursor solution of Mg-MOF-74 (Scheme 1b).
Under hydrothermal conditions, the Zn2GeO4/Mg-MOF-74 compo-
sites were prepared (Scheme 1c). Due to the strong interaction
between Mg2+ and H4DOBDC, Mg-MOF-74 was preferentially synth-
esized. Meanwhile, the alkylammonium groups (from TMAOH)
adsorbed on the Zn2GeO4 nanorods and interact with the H4DOBDC
from Mg-MOF-74. Therefore, the Zn2GeO4 nanorods well covered
the Mg-MOF-74.
The FT-IR spectra of the as-prepared samples further illustrate
the formation of the Zn2GeO4/Mg-MOF-74 composites (Fig. 4). The
spectral band at approximately 3300-3500 cm-1 was ascribed to the
amine stretching from TMAOH on the surface of the Zn2GeO4
sample (Fig. 4a). The bands at 700-850 cm-1 and 532 cm-1 were
ascribed to the vibrations of the GeO4 and ZnO4 tetrahedra,
respectively. After the successful deposition of Zn2GeO4 on Mg-
MOF-74, several obvious absorbance peaks appeared in Fig. 4d. The
absorption bands at 1578 and 1418 cm-1 were attributed to the C=O
carboxyl absorption peak and the stretching vibration of the
aromatic ring C=C bond, respectively. In addition, a new, obvious
peak at 1664 cm-1 appeared in the Zn2GeO4/Mg-MOF-74 composites
(Fig. 4d), which was assigned to the carboxyl absorption peak.
However, the adsorption peak did not exist in the physically mixed
Zn2GeO4/Mg-MOF-74 (Fig. 4c), which suggested that alkylamm-
onium group that adsorbed on the Zn2GeO4 nanorods favorably
interacted with the H4DOBDC from Mg-MOF-74. Therefore, the
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Fig. 4. FT-IR spectra of the (a) Zn2GeO4 nanorods, (b) Mg-MOF-74,
(c) Zn2GeO4/Mg-MOF-74 physical mixture and (d) Zn2GeO4/Mg-
MOF-74 composites.
MOF-74 could effectively enhance the separation of photo-
generated electron-hole pairs, which is consistent with the result of
the in situ PL measurements. Therefore, a possible reduction
mechanism for CO2 molecules was considered and proposed. Upon
light irradiation, electron-hole pairs were excited and generated on
the Zn2GeO4 nanorods. Photoinduced electrons were separated and
migrated to the conduction band of the Zn2GeO4 semiconductor.
Then, the electrons on the conduction band of the semiconductor
readily transferred to the surface of Mg-MOF-74 and subsequently
interacted with the CO2 molecule that adsorbed on Mg-MOF-74.
Finally, CO and HCOOH were produced in this system.38 In this
reduction reaction, the addition of the MOF enhances the
adsorption and activation of CO2, promotes the separation effici-
ency of the excited electron-hole pairs, which results in more long
lifetime electrons that can reduce the CO2 adsorbed on Mg-MOF-
74. Thus, the photocatalytic reduction of CO2 in the system substa-
ntially improved.
The recommended reduction mechanism was also verified by the
transient photocurrent responses of Zn2GeO4/Mg-MOF-74 and bare
Zn2GeO4 (Fig. 5(c)). When the photogenerated electrons diffuse to
the back contact and the photoinduced holes are effectively
separated and taken up by the hole acceptor in the electrolyte, i.e.,
when the photogenerated electron-hole pairs are effectively sep-
arated, a photocurrent was eventually obtained.38 Under light irra-
diation, the Zn2GeO4/Mg-MOF-74 composites coated on the FTO
glass showed a stronger photocurrent response than that of the
pure Zn2GeO4 nanorods, which suggested that the photoexcited
electrons can efficiently transfer to the active sites. In addition, the
photocurrent rapidly decreased to zero with the light was switched
off. These results agree well with the existing characterizations and
conclusions.
Scheme 1. Schematic illustration of the synthesis of the
Zn2GeO4/Mg-MOF-74 composites.
The UV/visible DRS spectrum of the as-obtained Zn2GeO4/Mg-
MOF-74 is shown in Fig. S1, and the reflectance data were
converted into absorbance using the Kubelk-Munk function (K-M).39
Fig. S1a shows that the Zn2GeO4 nanorods had a photocatalytic
reduction capability under ultraviolet light. Although Mg-MOF-74
(Fig. S1c) had superior absorption in the visible region, the pure Mg-
MOF-74 did not show any photocatalytic activity. Moreover, a red
shift occurred in the absorption spectra of the as-prepared
Zn2GeO4/Mg-MOF-74 sample (Fig. S1b) and the shoulder peak
extended to 580 nm. Additionally, the absorption intensity in the
range of 300-500 nm increased, which suggested that the hybrid
structure was responsive to light in the visible range. Therefore, the
composites had a shorter band gap (2.71 eV) than the pure Zn2GeO4
nanorods (4.7 eV).40
The adsorption capacity for CO2 and the surface area play
important roles in the photocatalytic reduction of CO2. The as-
prepared samples (Zn2GeO4/Mg-MOF-74, Zn2GeO4 and Mg-MOF-
74) were analyzed via N2 and CO2 adsorption isotherm techniques.
After the incorporation of Mg-MOF-74 into Zn2GeO4, a significantly
improvement in the N2 adsorption was observed at a very low
relative pressure (Fig. S2). The BET surface area of Zn2GeO4/Mg-
MOF-74 (406.7 m2·g-1) was higher than that of Zn2GeO4 (56.4 m2·g-
1),
The in situ PL measurements of the samples are shown in Fig.
5(a). The generation of emission spectra in semiconductors is
known to be related to the recombination of electron-hole pairs.
For photocatalysts, a high electron-hole pair recombination efficie-
ncy can result in an increase in the fluorescence intensity.41 When
the filter cut was less than 380 nm at room temperature, the PL
spectral intensity of Zn2GeO4/Mg-MOF-74 was less than that of the
Zn2GeO4 nanorods or Mg-MOF-74, which indicated that the
electron-hole recombination of the composites was inhibited and
the separation efficiency of the excited electron-hole pairs incre-
ased. In order to further prove the improvement of separation
efficiency, electrochemical impedance spectroscopy (EIS) of
photocatalysts was performed. In general, the radius of the arc of
Nynquist plots reflects the reaction rate occurring on the surface of
electrode.42-44 The smaller the radius of the arc, the faster
interfacial charge transfer.45 As shown in Fig. 5b, the arc radius on
EIS Nynquist plot of the Zn2GeO4/Mg-MOF-74 is smaller than
Zn2GeO4. It shows that a faster interfacial charge transfer occurred
on Zn2GeO4/Mg-MOF-74 photocatalyst, which results in a more
effective separation of photogenerated electron-hole pairs. These
results clearly illustrated that the combination of Zn2GeO4 and Mg-
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structure. Fig. 6 shows that the Zn2GeO4/MOF-74 sample had a
higher CO2 adsorption capability than that of the pure Zn2GeO4
nanorods due to the high adsorption ability of Mg-MOF-74. At 1
atm and 298 K, the maximum CO2 adsorptions of Zn2GeO4, Mg-
MOF-74 and Zn2GeO4/Mg-MOF-74 were 3.15 m3·g-1, 136.3 m3·g-1
and 64.88 m3·g-1, respectively. The increase in the CO2 adsorption
ability of a material may increase the photocatalytic performance of
the composites.
To understand the influence of Mg-MOF-74 on the photocatalytic
performance, the photocatalytic activity was explored under mild
environmental conditions and no other special sacrificial agent
appears in the system. The main product of CO2 photoreduction in a
complicated aqueous system was CO and trace HCOOH and O2.
When Zn2GeO4/Mg-MOF-74 was irradiated under 200-1100 nm
light, 12.94 µmol/g CO was detected after a 9 h irradiation (Fig.
7(I)). However, only 1.8 µmol/g CO was produced by the Zn2GeO4
nanorods, which is approximately 7 times more than that produced
by pure Zn2GeO4 under the same ambient conditions. As expected,
the photocatalytic conversion of CO2 was enhanced due to the
introduction of Mg-MOF-74. The photocatalytic performance of the
Zn2GeO4/Mg-MOF-74 composites is listed with those of other ph-
otocatalysts reported in the literature (Table S1). Based on the
literature reports,s1-s7 Zn2GeO4 catalysts with a higher activity were
modified with the noble metal Pt and Ru and the reduction
products is methane. It is difficult to make a direct comparison due
to different experimental conditions. In our work, the main product
of CO2 photoreduction was CO and the higher activities were finally
achieved .
For comparison, a mechanical mixture of Zn2GeO4 and Mg-MOF-
74 (with a mole ratio of Zn2GeO4 and Mg-MOF-74) was investigated.
As the mixture ratio of Zn2GeO4 and Mg-MOF-74 changed from
3.6:1 to 1:4.3 (Fig. 7(II)), an obvious change in the photocatalytic
activity for the CO2 reduction was not observed. In addition, the
reduction activities of the hydrothermal Zn2GeO4/Mg-MOF-74
samples were better than those of the mixture samples. This
suggested a dynamic interaction between Zn2GeO4 and Mg-MOF-74
after the hydrothermal synthesis. The interaction can act as a
transportation path to accelerate the electron transfer from
Zn2GeO4 to Mg-MOF-74 and reduce the CO2 adsorbed on Mg-MOF-
74 to generate CO gas.
Fig. 5. (a) Photoluminescence (PL) spectra, (b) EIS Nynquist plots
and (c) Transient photocurrent responses of photocatalysts.
For the CO2 photocatalysis reduction reaction, water is not an
efficient electron donor due to its weak oxidation capacity.46
However, water in a solution system can be oxidized by the
photogenerated holes in the valence band via the half reaction
H2O→ 1/2O2 + 2H+ + 2e−.19,47 Fig. 7(III) shows that no gas was
produced in the water-free system, which was ascribed to the lack
of reducing agents. When water was added into the system, a
certain amount of CO appeared. Moreover, the gas evolution
increased as the amount of water increased. When 0.4 ml of water
was added to the system, the CO production substantially
increased. Therefore, water has a vital effect on the reduction
activity. In addition, water can be used as a hydrocarbon hydrogen
source for the photocatalytic reduction of CO2. In this reduction
system, the photogenerated electrons from the conduction band
reduce CO2 to CO and HCOOH via the reactions CO2 + 2e- + 2H+ →
HCOOH and CO2 + 2e- + 2H+ → CO + H2O.14 Therefore, the
production of CO and a trace amount of HCOOH was obtained.
Fig. 6. CO2 adsorption isotherms (298 K) of the samples.
which demonstrated that more Zn2GeO4 nanorods were dispersed
on Mg-MOF-74 and more reactive sites existed on the hybrid
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Further experiments were performed. If the volume of water was (III) The production of CO as a function of water added to the
further increased and the amount of acetonitrile was decreased, reaction system (The total solvent volume was unchanged).
the amount of CO generated was markedly decreased because the Reaction conditions: solvent - 2 ml, MeCN : H2O = 4:1; CO2-1 atm;
reaction solution had a poor affinity for CO2 molecules with a 1100 nm ≥ n ≥ 200 nm; room temperature; 9 h.
decrease in the acetonitrile content.
To validate the involvement of CO2 in the system, we performed
a control experiment using a high purity N2 atmosphere instead of
CO2 (Fig. S3). After 7 h, no CO was generated under identical
conditions. This illustrated that the photocatalytic production of CO
is from pure CO2 and not from Mg-MOF-74 or other organic
compounds.
X-ray photoelectron spectroscopy (XPS) analysis confirms
thepresence of Zn, Ge, Mg and N in the Zn2GeO4 /Mg-MOF-
74composites (Fig. S4). The results show that Zn 2p, Ge 3d and Mg
1s of the fresh and used Zn2GeO4/Mg-MOF-74 catalysts have similar
peak of binding energy, which illustrates the valence change of
metals has no obvious change before and after the reaction. the
The peak of N 1s is from TMAOH. Therefore, the photocatalysts are
stable during the photocatalytic reduction reaction.
In order to prove the durability of the catalysts, three cycles of
CO2 reduction were examined, as shown in Fig. S5. After each cycle
for 3h, the reaction was stopped, the composite were redispersed
in a fresh MeCN/H2O medium and CO2 atmosphere. It was found
that the reaction activity was nearly unchanged as in Fig S5. Besides,
the stability of the photocatalysts during the photocatalytic reduc-
tion reaction is also showed by the PXRD patterns and durability.
The results show that the fresh and used Zn2GeO4/Mg-MOF-74
catalysts have similar XRD patterns (Fig. S6), This further demons-
trates that the Zn2GeO4/Mg-MOF-74 catalyst has good stability.
Conclusion
In summary, Zn2GeO4/Mg-MOF-74 composites were successfully
prepared via a hydrothermal method. The Zn2GeO4/Mg-MOF-74
sample showed a higher photocatalytic activity for the CO2 reduc-
tion reaction than that of the bare Zn2GeO4 nanorods in solution
with only water as sacrificial agent, and this is mainly attributed to
the strong CO2 adsorption property of Mg-MOF-74, the decreased
recombination of photogenerated electron-hole pairs and the exis-
tence of more alkaline metal sites on Mg-MOF-74. In addition, the
effect of H2O was also investigated in this work. The results
demonstrated that H2O was a reducing agent and a hydrogen
source for the hydrocarbons that participated in the reaction. This
work provides a strategy for incorporating MOFs as co-catalysts to
artificially imitate the phototransformation of CO2 and will provide
new opportunities to enhance the CO2 photocatalytic reduction
reaction.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Fig. 7. (I) Time-production plot of CO over irradiated
The authors acknowledge the financial support of 973 Program
Zn2GeO4/Mg-MOF-74. (II) the physical mixture of Zn2GeO4 and Mg-
(2014CB845605), NSFC(21331006, 21520102001, 51572260 and
MOF-74 with different mole ratios (Zn2GeO4:Mg-MOF-74) (a) 3.6:1,
(b) 2:1, (c) 1.2:1, (d) 1:1.92, (e) 1:4.3, and (f) Zn2GeO4/Mg-MOF-74.
6 | J. Name., 2012, 00, 1-3
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Catalysis Science & Technology
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Zn2GeO4/Mg-MOF-74 composites are explored for artificial CO2
phototransformation under mild condition.