Photocatalytic reduction of CO2 over facet engineered TiO2 nanocrystals supported by carbon nanofibers under simulated sunlight irradiation

Article abstract of DOI:10.1016/j.catcom.2018.01.025

Carbon nanofibers (CNFs) supported facet engineered TiO₂ nanocrystals (NCs) were one-step synthesized by a solvothermal method and their photocatalytic CO₂ reduction performance were investigated under simulated sunlight irradiation. The results show that anatase TiO₂ NCs with {001} and {101} facets coexposure were homogeneously dispersed on CNFs and combined firmly with CNFs. The enhanced photocatalytic CO₂ reduction activity of the composites can be attributed to the formation of TiO₂?carbon heterojunction and the surface heterojunction between the {001}/{101} facets of TiO₂ crystals, which accelerate the separation of photogenerated electron-hole pairs and extend the light response of TiO₂ to the visible light region simultaneously.

Full text of DOI:10.1016/j.catcom.2018.01.025

CatalysisCommunications108(2018)27–32
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Photocatalytic reduction of CO2 over facet engineered TiO2 nanocrystals
supported by carbon nanofibers under simulated sunlight irradiation
Ze Lei, Zhuo Xiong, Yicheng Wang, Yuxiao Chen, Dingshan Cao, Yongchun Zhao⁎,
Junying Zhang, Chuguang Zheng
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science & Technology, 1037 Luoyu Road, Wuhan 430074,
China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Carbon nanofibers (CNFs) supported facet engineered TiO2 nanocrystals (NCs) were one-step synthesized by a
solvothermal method and their photocatalytic CO2 reduction performance were investigated under simulated
sunlight irradiation. The results show that anatase TiO2 NCs with {001} and {101} facets coexposure were
homogeneously dispersed on CNFs and combined firmly with CNFs. The enhanced photocatalytic CO2 reduction
activity of the composites can be attributed to the formation of TiO2‑carbon heterojunction and the surface
heterojunction between the {001}/{101} facets of TiO2 crystals, which accelerate the separation of photo-
generated electron-hole pairs and extend the light response of TiO2 to the visible light region simultaneously.
Photocatalysis
CO2 reduction
CNFs
TiO2
Facet engineering
1. Introduction
activity of TiO2 [10,11]. For example, Zhang et al. [12] reported the
enhanced photocatalytic activity on degradation and H2 evolution over
Photocatalytic reduction of CO2 with water into renewable fuels is a
promising technology to reduce CO2 emission and produce organic
fuels. Among a variety of photocatalysts, TiO2 is most frequently
evaluated for the CO2 photocatalytic reduction process because it is
inexpensive, photo-stable and low toxic [1,2]. However, TiO2 still has
limitations that include low light absorption in the visible range due to
its large band gap (3.2 eV for anatase) and low photocatalytic conver-
sion efficiency by reason of the rapid recombination rate of photo-
excited electrons and holes.
Recently, it has been reported that photocatalytic activity of TiO2
crystals could be enhanced by engineering coexposed multiple facets
[3]. Yu et al. [4] reported enhanced CO2 photocatalytic reduction over
anatase TiO2 crystals with coexposed {001} and {101} facets and found
that it could be attributed to the effect of surface heterojunction in a
single TiO2 particle with {001} and {101} facets coexposure, which is
conducive to the photogenerated electrons and holes separation.
Another effective approach to enhance photocatalytic activity of
TiO2 is to combine it with other materials to form composite materials,
for instance noble metals [5], oxide semiconductor [6], and carbon
materials [7]. Among them, carbon material is more abundant and
cheaper, meanwhile, some of them have several extraordinary proper-
ties, such as large specific area, rapid mobility of charge carriers, and
tunable surface properties [8,9]. Therefore, combining TiO2 with
carbon materials has been widely used to enhance the photocatalytic
mesoporous TiO2 layer coated on multiwall carbon nanotubes. Our
group previously reported the synthesis and characterization of gra-
phene supported TiO2 with coexposed {101} and {001} facets and their
enhancement of photocatalytic CO2 reduction performance [13].
Among a variety of carbon materials, carbon nanofiber has been re-
ported in several researches for photocatalysis degradation and H2
evolution because it has strong visible light absorption and can effec-
tively capture and transport photogenerated electrons [14,15]. Pant
et al. [16] reported the fabrication of CNFs decorated with CdS/TiO2
heteroarchitecture by electrosping process and found that the nano-
composites exhibited high photocatalytic hydrolysis of ammonia
borane due to their favorable electrons-transfer properties, good dis-
persion, and strong adsorption property. Kim et al. [17] synthesized
trilayer CdS/CNF/Pt-TiO2 heterostructures for photocatalytic H2 pro-
duction from water splitting and demonstrated that the CNF acted as an
efficient electron-transfer mediator which promotes the separation of
excited electron-hole pairs. Moreover, comparing to other carbon ma-
terials such as graphene reported in many researches for CO2 photo-
catalytic reduction, CNF is cheaper and easier to prepare [13]. There-
fore, it is possible to load facet engineered TiO2 NCs onto CNFs to
achieve enhanced CO2 photocatalytic reduction. However, as far as we
know, very few papers have reported the synthesis and application of
this kind of catalyst for CO2 photocatalytic reduction.
In this paper, a one-step solvothermal method was employed to
⁎
Corresponding author.
E-mail address: yczhao@hust.edu.cn (Y. Zhao).
https://doi.org/10.1016/j.catcom.2018.01.025
Received 31 August 2017; Received in revised form 28 December 2017; Accepted 21 January 2018
1566-7367/©2018ElsevierB.V.Allrightsreserved.

Z. Lei et al.
CatalysisCommunications108(2018)27–32
Fig. 1. TEM and HR-TEM images of 001/101TiO2 (A),
5CNF-001/101TiO2 (B) catalysts.
synthesize CNFs supported TiO2 NCs with {001} and {101} facets co-
exposure (CNF-001/101TiO2). The photocatalytic CO2 reduction ac-
tivity of as-prepared CNF-TiO2 composites was evaluated under simu-
lated sunlight. The effect of CNFs introduction on CO2 reduction
photocatalytic performance of CNF-001/101TiO2 was investigated. A
possible photocatalytic CO2 reduction mechanism of CNF-001/101TiO2
was analyzed.
transferred to a 200 mL Teflon-lined autoclave and heated at 180 °C for
24 h. After the autoclave was cooled down, the obtained precipitates
were collected by centrifugation, washed with ethanol and deionized
water for several times, respectively, and then dried at 60 °C for over-
night. Finally the fluoride ions on the surface of the as-prepared sample
were removed by calcination at 500 °C for 2 h in the nitrogen atmo-
sphere. The mass ratio of CNF to TiO2 is 1%, 5%, and 10% in the mixed
suspension, the samples prepared by solvothermal from these suspen-
sions were labeled as 1CNF-001/101TiO2, 5CNF-001/101TiO2, and
10CNF-001/101TiO2, respectively. The preparation of CNFs supported
TiO2 NCs with {101} facet exposure (CNF-101TiO2) was carried out in a
similar manner without the addition of HF solution. Pure TiO2 NCs with
{101} and {001} facets coexposure (denoted as 001/101TiO2) was
prepared using the same procedure without CNF. Pure TiO2 NCs with
{101} facets exposure (denoted as 101TiO2) was prepared using the
same procedure without CNF and HF.
2. Experimental
2.1. Catalyst preparation
CNFs supported TiO2 NCs with {001} and {101} facets coexposure
were prepared by a one-step solvothermal method. Firstly, different
amounts of CNFs were dispersed into 90 mL ethanol by ultrasonic
treatment for 30 min, and then 1.2 mL hydrofluoric acid solution and
3 mL titanium butoxide dropped into the homogeneous suspension.
After sonication for other 1 h, the mixed suspension was then
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CatalysisCommunications108(2018)27–32
2.2. Catalyst characterization
X-ray diffraction (XRD) patterns were obtained on an Empyrean
diffractometer. Specific surface area (BET) measurement was per-
formed on a Micromeritics ASAP2010 analyzer. An FEI Tecnai G2 F30
instrument was used to obtain transmission electron microscopy (TEM)
images. UV–Vis absorbance spectra were measured by a Hitachi U3900
spectrophotometer. Photoluminescence (PL) spectra were collected
using a confocal laser Raman microscope (Horiba Jobin Yvon, LabRAM
HR800). Electrochemical impedance spectra (EIS) were obtained by an
electrochemical system (CHI-660D). Fourier transform infrared (FTIR)
spectra were recorded by a Bruker VERTEX 70 FTIR spectrometer. X-ray
photoelectron spectroscopy (XPS) measurement was performed on a
Shimadzu/KRATOS AXIS-ULTRA DLS-600W instrument.
2.3. Photocatalytic reduction of CO2
CO2 photocatalytic reduction was performed under simulated sun-
light
irradiation
(18.7 mW cm−2
,
300 nm < λ < 400 nm;
45.0 mW cm−2, 400 nm < λ < 800 nm) in an internal circulated
reaction system reported in our previous work [18]. First, 10 mL of
deionized water was added to the reactor, and then 20 mg of catalyst
dispersed uniformly on a Petri plate was placed on above the water.
Prior to illumination, the reactor was first vacuumed through a vacuum
pump and then was performed a purification process with ultra-pure
CO2 (99.999%). The amounts of H2, CO, and CH4 in the reactor were
analyzed every 30 min by the GC during 6 h of light irradiation.
First, background tests were conducted using CO2 and water as re-
actants for the cases of (1) empty reactor and (2) blank Petri plate in the
reactor. No hydrocarbons were detected under simulated sunlight ir-
radiation in these two cases. This indicated that the CO2 reduction re-
action could not occur in the absence of catalyst. Second, test was
performed using carbon nanofibers under simulated sunlight irradia-
tion, no hydrocarbons were detected with or without CO2 and water,
This demonstrated that the carbon nanofibers did not produce hydro-
carbons under simulated sunlight irradiation, Third, instead of CO2, N2
was used to be purged gas, no hydrocarbons were detected under si-
mulated irradiation in the presence of water and catalyst. This confirms
that any produced hydrocarbons must be derived from the purged CO2.
Fig. 2. UV–Vis absorbance spectra of the as-prepared catalysts.
[21]. The absorption edges of pure TiO2 is higher than the CNF-TiO2
composites (Fig. S2), and the band gaps for the composites are further
narrowed by the increase of CNF content (Table S1), which is attributed
to the formation of TieC bonds, as indicated by XPS analysis (Fig. S4C).
The formation of TieC bonds results in the upward shift of the valence
band top, which is beneficial for band-gap narrowing [22].
The PL spectra of the samples are presented in Fig. 3. The CNF-TiO2
composites show the diminished PL intensities compared with that of
pure TiO2. This can be explained by the carbonaceous species bound to
TiO2 in the TiO2‑carbon heterojunction that can receive the photo-
generated electrons easily to efficiently suppress the recombination of
the photogenerated electron-hole pairs [12,23], which is beneficial to
the separation of photogenerated charges on TiO2.
The EIS spectra are presented in Fig. 4, it is observed that the arc
radius of the EIS Nyquist plot of the CNF-TiO2 composites are smaller
than that of pure TiO2, indicating that the interface layer resistance and
the charge transfer resistance on the surface are diminished [24], which
reveals that charge migration is facilitated due to the interfacial inter-
action between the TiO2 and CNF caused by the TiO2‑carbon hetero-
junction. The FTIR spectra are presented in Fig. S3. The broad peaks
located at 3422 and 1625 cm−1 are assigned to the physically adsorbed
H2O and the stretching mode of hydroxyl groups [25]. The weak peak
3. Results and discussion
3.1. Characterization of catalysts
The XRD patterns are presented in Fig. S1, only diffraction peaks
indexed to anatase appeared and all samples exhibited similar diffrac-
tion peaks, indicating that the obtained samples were anatase TiO2 and
CNFs would not affect the crystal structure of the TiO2 nanocrystals.
The BET surface of composites decreased with the increase of content of
CNF because the BET surface of CNF is smaller than that of TiO2 (Table
S1).
The TEM images in Fig. 1A clearly shows the facet engineered TiO2
NC is a truncated tetragonal bipyramidal with coexposed {001} and
{101} facets. The HR-TEM images exhibited that the fringe spacing of
0.23 nm parallel to the top and bottom facets corresponds to the {001}
plane of anatase TiO2 [19]. Fig. 1A also showed another set of the
lattice fringes with spacing of 0.35 nm corresponding to the {101}
plane of anatase TiO2 [20]. For 5CNF-001/101TiO2 samples (Fig. 1B),
the TiO2 NCs with {001} and {101} facets coexposure were homo-
genously dispersed on CNFs. And the formation of TiO2‑carbon het-
erojunction can be clearly observed in the HR-TEM images of 5CNF-
001/101TiO2 composites.
The UV–Vis absorbance spectra are shown in Fig. 2A. The absorp-
tion intensities of the composites were higher than of pure TiO2 at
visible wavelengths. This can be attributed the presence of CNFs in the
composites, reducing reflection of light due to the black color of CNFs
Fig. 3. PL spectra of the as-prepared catalysts.
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CatalysisCommunications108(2018)27–32
Fig. 4. Electrochemical impedance spectroscopy Nyquist plots of the as-prepared cata-
lysts.
Fig. 5. Photocatalytic CO yield of as-prepared catalysts after 6 h irradiation.
at 1056 cm−1 of all samples corresponds to the vibration of CeO
stretching [26]. The peak at 1383 cm−1 of the spent 5CNF-001/
101TiO2 composite is attributed to the C]O stretching mode of COOH
groups [27]. The presence of wide transmittance band at
500–700 cm−1 in the composites is ascribed to TieO bonding [28].
The XPS spectra of Ti 2p in Fig. S4A exhibit that the binding energy
(BE) are located around 458.6 eV (Ti 2p3/2) and 464.3 eV (Ti 2p1/2),
indicating a chemical state of Ti4+ in the TiO2 [29]. The spectra of O
1 s shown in Fig. S4B reveal that the peak at around 529.7 and 531.0 eV
are assigned to lattice oxygen in TiO2 and the surface hydroxyl of CNF
and TiO2, respectively [30]. There are four peaks in the spectra (Fig.
S4C) of CNF-001/101TiO2 composites with the binding energies cen-
tered around 290.0 eV, 287.8 eV, 286.4 eV, and 285.0 eV, respectively.
The peaks at 285.0 eV and 286.4 eV is ascribed to the delocalized al-
ternant hydrocarbon (CeC) and CeO bond, while the other two peaks
at 287.8 and 290.0 eV are attributed to the C]O bond and carboxyl or
ester carbon (O]CeO), respectively [8,25]. The peak at 283.4 eV in
1CNF-001/101TiO2 can be attributed to the TieC bond [31]. It is no-
table that the chemical states of C on the surface of pristine CNF do not
change obviously after the solvothermal process (Fig. S4C), demon-
strating that CNF were stable during the solvothermal process. How-
ever, in comparison with the XPS spectra of CNF, a positive shift of the
CeO and C]O bond peaks of CNF-001/101TiO2 catalysts can be ob-
served, demonstrating the oxidation of the CNFs in CNF-001/101TiO2
catalysts during the solvothermal process, which could be caused by
introduction of TiO2. A similar phenomenon in the hydrothermal
synthesis of carbon nanotubes-TiO2 nanocomposites was also found by
Jitianu et al. [32]. This also indicated that the good combination of
TiO2 and CNF, which might be beneficial for the formation of
TiO2‑carbon heterojunction.
content in the composites. In addition, the 101TiO2 and CNF-101TiO2
has lower photocatalytic than 001/101TiO2 and CNF-001/101TiO2
respectively, which is attributed to the “surface heterojunction” within
the facet engineered TiO2 NCs that can facilitate the spatial migration of
photogenerated electrons and holes [13]. The CO yield of CNF-001/
101TiO2 is much higher than that of RGO-TiO2 composite reported by
Tomoaki Takayama [33]. Moreover, the quantum yield of CNF-001/
101TiO2 is higher than that of graphene oxide-supported TiO2 catalysts
reported by Lling-Lling Tan [34]. Quantum yield of the CO2 photo-
catalytic reduction over the composites were calculated by the fol-
lowing Eqs. [35].
ΦCO(%) = 100% × 2 × CO yield/moles of photon reached the catalyst
As shown in Table S1, For 001/101TiO2, the quantum efficiency is
calculated to be 0.09%, which is much higher than that of the rutile
TiO2 nanocrystals with exposed high-index facets reported by Quang
Duc Truong [36]. Fig. S5 shows the cycle performance of 5CNF-001/
101TiO2 catalyst. After the first cycle, the reactor was vacuumed and
the same experimental process was repeated. The CO yield in the
second and the third cycle only slightly decreased, indicating good
stability of 5CNF-001/101TiO2 catalyst. As shown in Fig. S6 and Table
S2, the chemical states of Ti, O and C on the surface of 5CNF-001/
101TiO2 catalyst do not show clearly change after photocatalytic re-
action, which can well explain the stability of the catalyst.
On the basis of the above experimental results, a simple mechanism
to explain the observed enhancement in the photocatalytic activity of
CNF-001/101TiO2 catalysts was proposed and shown in Fig. 6. Firstly,
the introduction of CNF into composites can extend the spectral re-
sponse of TiO2 to visible light region and enhance the visible absorption
capability. Secondly, the photogenerated electron and hole pairs can
efficiently migrate to {101} and {001} facets, respectively, due to the
formation of surface heterojunction [4]. Then the electrons continued
to accumulate on {101} facets and transfer to CNFs owing to its high
exhibit electron storage capacity, electron conductivity, and the for-
mation of TiO2‑carbon heterojunction [37]. Thus, the combination of
photogenerated electron-hole pairs is more effectively suppressed. Fi-
nally, the hydrocarbons are produced by the reaction of CO2 with the
electrons on CNFs and the generation of O2 is due to the oxidation of
H2O by the hole on the TiO2 NCs.
3.2. CO2 photocatalytic reduction
Fig. 5 shows the photocatalytic CO yield of as-prepared catalysts
after 6 h. CO was found to be the main product and the generation of
CH4 and H2 was not detected. It can be seen that CNF-TiO2 composites
exhibit higher photocatalytic activity than pure 001/101TiO2 and 101
TiO2, suggesting that the support of CNF promotes the migration of
electron from TiO2 to CNF leading to the restrained recombination of
electrons and holes, as evident in PL spectra and EIS analysis. However,
the photocatalytic activity of 10CNF-001/101TiO2 is lower than that of
5CNF-001/101TiO2 even though the PL intensity of 10CNF-001/
101TiO2 is lower than that of 5CNF-001/101TiO2. The reason for this
phenomenon may be due to the decreased active component TiO2
4. Conclusions
In this paper, CNFs supported TiO2 NCs with {001} and {101} facets
coexposure were synthesized by one-step solvothermal method. The
30

Z. Lei et al.
CatalysisCommunications108(2018)27–32
Fig. 6. Proposed mechanism for the photocatalytic CO2
reduction of CNF-TiO2 composites under simulated irra-
diation.
TiO2 NCs with {001} and {101} facets coexposure were uniformly
loaded on the CNFs after the solvothermal reaction. The photocatalytic
activity of facet engineered TiO2 catalyst was effectively improved after
the supporting of CNF. The CO yield can effectively enhanced by adding
an appropriate amount of CNFs in the catalyst. The optimization CO
yield of 2.4 μmol g−1 was obtained by 5CNF-001/101TiO2 after simu-
lated sunlight irradiation for 6 h. The enhanced photocatalytic activity
of CNF-001/101TiO2 catalysts can be attributed to the formation of
{001}/{101} surface heterojunction within TiO2 crystals and
TiO2‑carbon heterojunction between TiO2 and CNF, which effectively
promote the separation of photogenerated charges and extend the light
response of TiO2 from UV to visible light region.
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Acknowledgments
This project was supported by the National Natural Science
Foundation of China (U1610110, 51706082), the Foreign Science and
Technology Cooperation of Hubei Province (2016AHB021), and China
Postdoctoral Science Foundation (2017M620319). The Analytical and
Testing Center of Huazhong University of Science & Technology were
acknowledged by the authors.
Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://
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