Hybrid carbon@TiO2 hollow spheres with enhanced photocatalytic CO2 reduction activity

Article abstract of DOI:10.1039/c6ta11121a

Photocatalytic conversion of carbon dioxide (CO₂) into solar fuels is an attractive strategy for solving the increasing energy crisis and greenhouse effect. This work reports the synthesis of hybrid carbon@TiO₂ hollow spheres by a facile and green method using a carbon nanosphere template. The carbon content of the carbon@TiO₂ composites was adjusted by changing the duration of the final calcination step, and was shown to significantly affect the physicochemical properties and photocatalytic activity of the composites. The optimized carbon@TiO₂ composites exhibited enhanced photocatalytic activity for CO₂ reduction compared with commercial TiO₂ (P25): the photocatalytic CH₄ production rate (4.2 μmol g^?1 h^?1) was twice that of TiO₂; moreover, a large amount of CH₃OH was produced (at a rate of 9.1 μmol g^?1 h^?1). The significantly improved photocatalytic activity was not only due to the increased specific surface area (110 m² g^?1) and CO₂ uptake (0.64 mmol g^?1), but also due to a local photothermal effect around the photocatalyst caused by the carbon. More importantly, UV-vis diffuse reflectance spectra (DRS) showed a remarkable enhancement of light absorption owing to the incorporation of the visible-light-active carbon core with the UV light-responsive TiO₂ shell for increased solar energy utilization. Furthermore, electrochemical impedance spectra (EIS) revealed that the carbon content can influence the charge transfer efficiency of the carbon@TiO₂ composites. This study can bring new insights into designing carbon@semiconductor nanostructures for applications such as solar energy conversion and storage.

Full text of DOI:10.1039/c6ta11121a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C6TA11121A
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Hybrid carbon@TiO2 hollow spheres with enhanced
photocatalytic CO2 reduction activity
Weikang Wang,a Difa Xu,ab Bei Cheng,a Jiaguo Yu*ac and Chuanjia Jiang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Photocatalytic conversion of carbon dioxide (CO2) into solar fuels is an attractive strategy for solving the increasing energy
crisis and greenhouse effect. This work reports the synthesis of hybrid carbon@TiO2 hollow spheres by a facile and green
method using carbon nanosphere template. The carbon content of the carbon@TiO2 composites was adjusted by changing
the duration of the final calcination step, and was shown to significantly affect the physicochemical properties and
photocatalytic activity of the composites. The optimized carbon@TiO2 composites exhibited enhanced photocatalytic
activity for CO2 reduction compared with commercial TiO2 (P25): the photocatalytic CH4 production rate (4.2 μmol g–1 h–1)
was twice that of TiO2; moreover, a large amount of CH3OH was produced (at a rate of 9.1 μmol g–1 h–1). The significantly
improved photocatalytic activity was not only due to increased specific surface area (110 m2 g–1) and CO2 uptake (0.64
mmol g–1), but also to a local photothermal effect around the photocatalyst caused by the carbon. More importantly, UV–vis
diffuse reflectance spectra (DRS) showed remarkable enhancement of light absorption owing to the incorporation of visible-
light-active carbon core with UV light-responsive TiO2 shell for increased solar energy utilization. Furthermore,
electrochemical impedance spectra (EIS) revealed that the carbon content can influence the charge transfer efficiency of the
carbon@TiO2 composites. This study can bring new insights in designing seminconductor@carbon nanostructures for
applications such as solar energy conversion and storage.
www.rsc.org/
utilize ultraviolet (UV) light. Therefore, tremendous efforts
have been devoted to solving these problems. For example,
photocatalysts with hierarchical, hollow, yolk-shell and other
Introduction
Solar energy conversion toward chemical energy by
photocatalysis is an attractive candidate solution for global
environmental and energy problems because solar energy is the
most abundant and sustainable natural energy source.1–3
Although significant accomplishments in semiconductor
photocatalytic materials have been achieved in the past years,
the efficiency of photocatalytic carbon dioxide (CO2)
conversion toward solar fuels is still rather low and far from
commercial applications.4–9 As an abundant, nontoxic,
photostable and highly efficient photocatalyst, titania (TiO2)
and Ti-based composites with anatase, rutile, and brookite
crystalline structures were exploited and investigated by
various methods to enhance photocatalytic activity.10–15
However, there are two disadvantages of TiO2 in part limiting
its applications in photocatalysis: one is the low quantum
efficiency due to the high recombination rate of electron-hole
pairs generated in photocatalytic reactions. Moreover, TiO2
with a large band gap energy of approximately 3.2 eV can only
novel structures have been fabricated in recent years.16–19
Hollow structure can increase light-harvesting efficiency due to
multi-scattering of light. More exposed TiO2 nanoparticles in
hollow structure lead to higher specific surface area. Thus, Ao
et al. reported the synthesis of TiO2 hollow microspheres based
on carbon-sphere templates for degradation of methylene blue
(MB).17 Afterward, Chen et al. used carbonaceous materials to
improve transfer efficiency of charge carriers and enhance light
absorption.19 Integration of semiconductors with carbon
materials to form hybrid nanostructure has recently attracted
much attention.10,18,20,21 In summary, carbon-based materials
can act as photo-sensitizer, which greatly improve visible-light
absorption. More importantly, both the high electron mobility
and increased separation efficiency of electron-hole pairs
endow carbon materials with superior electron acceptor and
transport channel.22,23
Herein, the carbon@TiO2 composite hollow spheres were
systematically synthesized based on colloidal carbon spheres.
TiO2 hollow spheres with different carbon contents were
fabricated by changing the duration of calcination. The
carbonaceous materials inside hollow structure not only
increased specific surface area and CO2 uptake capacities of the
composite samples, but also created a local photothermal effect
around the photocatalyst. As expected, the carbon@TiO2
composite nanostructure exhibited excellent photocatalytic CO2
a
State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070,
P. R. China. *E-mail: jiaguoyu@yahoo.com (J. Yu); jiangcj2016@yahoo.com (C.
Jiang)
b
Hunan Key Laboratory of Applied Environmental Photocatalysis, Changsha
University, Changsha, 410022, P. R. China
c
Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah
21589, Saudi Arabia
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reduction performance and selectivity under simulated solar spectrometer (Japan). X-ray photoelectron spectroscopy (XPS)
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light compared with P25 TiO2. Moreover, the fabrication was performed by using ultrahigh vacuumDVOGI: 10E.1X0C39A/CL6ATAB11122110A
process of the hollow structure and photocatalytic CO2 electron spectrometer using Al Kα radiation as X-ray source.
reduction mechanism of carbon@TiO2 composite products Thermal gravimetric analysis (TGA) was accomplished by a
were investigated. The results of this study provide a new idea Shimadzu DTG-60H analyzer (Japan) in air. Electrochemical
to finely control novel nanostructures in photocatalysis.
analysis was performed on a CHI660C workstation (Shanghai
Chenhua Instruments, China) using three-electrode
a
electrochemical cell with a working electrode, a platinum
counter electrode and a Ag/AgCl reference electrode in
electrolyte of 0.5 M Na2SO4 aqueous solution. The working
electrode was prepared by coating samples onto the fluorine-tin
oxide (FTO) glass.
Experimental
Sample preparation
All reagents were A.R. grade from Shanghai Chemical Reagent
Factory of China and used as received. Deionized (DI) water
was used in all experiments.
Photocatalytic CO2 reduction test
For the synthesis of carbon nanosphere (CNS) mold,24 12
g of glucose was dissolved in DI water (80 mL) to form a clear
solution, which was sealed in a 100 mL Teflon-lined autoclave
and maintained at 180 °C for 4 h. The obtained brown products
were isolated by centrifugation and ultrasonication, washed
with water and ethanol five times, and then dried at 55 °C for 8
h under vacuum.
The photocatalytic CO2 reduction tests were carried out in a 200
mL custom-made Pyrex glass reactor at atmospheric pressure
and ambient temperature, similar to our previously reported
procedures.25 The gaseous products were determined by the
retention time and quantified by calibration with a standard gas
mixture. Photocatalytic CO2 reduction studies of each
composite sample were repeated three times.
In a typical synthesis of hollow TiO2 spheres with CNS
cores, the initial suspension was prepared by mixing 0.3 g of
as-prepared CNS and 60 mL of ethanol. Then, 3 mL of
tetrabutyl titanate (TBOT) was added into the ethanolic
suspension, ultrasonically redispersed and then stirred
vigorously for 2 h in a sealed beaker. The products were
centrifuged, washed and ultrasonically redispersed in ethanol
for five cycles. After being aged in air at room temperature for
12 h and oven-dried at 55 °C under vacuum for 4 h, carbon
spheres with TiO2 precursor shell (CSTS) were formed.24 In
order to fabricate carbon@TiO2 hollow structure, the CSTS
were put into a crucible with a cover and then heated in static
air inside a furnace at 450 °C for 60, 90, 120 and 180 min, and
labeled as T60, T90, T120 and T180, respectively.
In situ diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS)
The in situ DRIFTS experiments were performed on a Nicolet
iS50 FTIR spectrometer (Thermo Fisher, US), similar to our
previously reported literature.25 A representative sample T120
was degassed at 100 °C prior to measurement. The specimen
chamber loaded with the sample was purged with flowing
nitrogen for 1 h to remove air, and then a mixed gas of CO2 and
water vapour was flowed into the chamber and irradiated with
monochromatic light (365 nm) for 2 h, during which the
infrared absorbance within the wavenumber range of 1900–
1200 cm–1 was recorded at certain time points.
Characterization
Results and Discussion
X-ray diffraction (XRD) patterns were recorded on a Rigaku
D/Max-RB X-ray diffractometer (Japan) with Cu Kα radiation Phase structures and morphology
source. Raman analysis was carried out on a Renishaw inVia
The phase structure and relative crystallinity of as-synthesized
micro-Raman spectrometer in the back-scattering geometry of
633 nm Ar+ laser as an excitation source. The morphology
observation of the as-prepared products was conducted on a
field emission scanning electron microscope (FESEM) (JSM
7500F). Transmission electron microscopy (TEM) images were
observed by a JEM-2100F electron microscope (JEOL, Japan).
The elemental mapping images were performed on Tecnai G2
F30 scanning transmission electron microscopy (STEM). The
nitrogen adsorption and desorption isotherms were obtained by
an ASAP 2020 (Micromeritics Instruments, USA) nitrogen
adsorption apparatus. The CO2 adsorption measurement was
carried out at ambient temperature by an ASAP 3020 carbon
dioxide adsorption apparatus (Micromeritics, USA). The UV–
vis diffuse reflectance spectra (DRS) of the composites were
measured on a Shimadzu UV–vis spectrophotometer (UV2550,
Japan). Fourier transform infrared (FTIR) spectra of all the
samples were recorded via a Shimadzu IR Affinity-1 FTIR
products were confirmed by XRD analysis. The XRD patterns
of the carbon@TiO2 composites (Fig. 1) show that anatase
phase titania formed after calcination at 450 °C for 60–180 min.
The characteristic diffraction peaks appeared at 2θ = 25.3°
(101), 36.9° (103), 37.9° (004), 38.6° (112), 48.0° (200), 53.9°
(105), 55.1° (211), 62.1° (213), which are easily assigned to
anatase TiO2 (JCPDS No. 71−1166). Notably, with increasing
calcination time, the intensity of the XRD diffraction peaks
increased, which arose from reduced shielding effect of
carbonaceous species in TiO2 hollow structure as well as grain
growth of TiO2. This indicates that the change of calcination
duration has evident positive effect on the crystallinity of TiO2.
Furthermore, FESEM and TEM images were used to
investigate the morphology and nanostructure of the samples.
Fig. 2a shows the FESEM images of the carbon nanospheres
(CNS) with an average diameter of approximately 350 nm.
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After TBOT was added into the ethanolic suspension of CNS
and aged for 24 h, the average diameter of the spheres increased
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DOI: 10.1039/C6TA11121A
Fig. 1 XRD patterns of as-prepared composite samples.
visibly to 420 nm (Fig. 2b). This change in diameter illustrates
that a TiO2 shell was coated on CNS uniformly by hydrolysis
and condensation with the surface functional groups of CNS.
Besides, from Fig. 2c, e and g, the carbon@TiO2 composite
samples exhibited regular spherical shape with coarse and
rough surface, which is beneficial for light-harvesting. The
surface morphology arises from the shrink of TiO2 spherical
shell with the CNS mold inside collapsed during calcination
treatment. Therefore, the diameter of carbon@TiO2 composite
spheres decreased obviously from 300 nm (T60) to 100 nm
(T180). The broken spheres (inset of Fig. 2c) imply a hollow
spherical structure with carbon core inside for the sample T60.
Moreover, the corresponding TEM image (Fig. 2d) further
confirms the hollow structure with carbon core inside, which
can be defined as yolk-shell spheres.
It can be seen in Fig. 2g and 2h that carbon cores
disappeared completely when the calcination duration increased
to 180 min. Furthermore, Fig. 2e and 2f reveal a stable and
uniform hollow spherical structure for sample T120, with a
certain content of carbon remaining inside. The carbon@TiO2
structure of T120 can not only enhance the visible-light
absorption by the carbon core, but also leave some space for
multi-scattering of light by the TiO2 shell. In addition,
elemental mapping images of an individual spherical yolk-shell
particle of T60 are shown in Fig. 2i. The colorful images reveal
that the C, O, and Ti elements are uniformly distributed over
the whole nanostructure with C inside the sphere and O and Ti
in the shell, demonstrating that uniform carbon@TiO2 hybrid
nanoarchitecture was formed successfully.
Fig. 2 FESEM images of typical samples CNS (a), CSTS (b),
T60 (c), T120 (e) and T180 (g); TEM images of T60 (d), T120
(f) and T180 (h). (i) STEM image of T60 and corresponding
elemental mapping images of C, O and Ti.
Raman analysis
The broad peak in the 2θ region of 19° to 28° indexed to carbon
materials were too weak to find in the XRD spectra of the
composites, so Raman spectroscopy was used to verify the
existence of carbon in the carbon@TiO2 composites. As shown
in Fig. 3, there was no obvious difference in the four main
Raman vibrational peaks (A1g + B1g + two Eg) of carbon@TiO2
composite samples (T60, T120, T180), which are characteristic
of the standard anatase spectrum.26 More importantly, the
Raman pattern of CNS and composite samples T60 and T120
Fig. 3 Fourier Transform Raman spectra of typical samples
(using 633 nm Ar+ laser excitation source).
(b)
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showed two characteristic Raman bands corresponding to ascribed to the surface hydrolysis and condensation reactions.
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–1
carbon species: the one at 1363 cm–1 corresponds to the carbon
Furthermore, two major bands centered atD1O5I:9130.a10n3d9/1C369T3A11c1m21A
defect-induced Raman band (D band), and the other at 1582 are attributed to asymmetric and symmetric stretching
cm–1 corresponds to the ordered graphitic structure (G vibrations of COO– groups,30 respectively. Therefore, it can be
band).27,28 The relative intensity of the D and G bands changed inferred that when TiO2 precursor was coated on CNS surface,
very little in different samples, indicating that the form of the composite products were carboxylates before calcination.
carbon in the carbon@TiO2 composite samples was similar to
Herein, the fabrication mechanism of the carbon@TiO2
that in CNS template. Moreover, the Raman spectra of sample composite hollow structure is illustrated in Scheme 1. Glucose
T180 exhibited no D or G band, suggesting the absence of undergoes a series of reactions under hydrothermal conditions,
carbon in this sample, which is consistent with the TEM which results in a complex mixture of organic compounds. The
observations in Fig. 2h. Consequently, the carbon content growth of CNS conforms to the LaMer model in the sealed
remaining in the TiO2 hollow spheres can be adjusted by vessel.31 Some aromatic compounds and oligosaccharides are
varying the duration of the calcination treatment.
formed initially, which can be called the “polymerization”
process. Then cross-linking occurs in the carbonization process,
induced by intermolecular dehydration of linear or branched
oligosaccharides, or other macromolecules. The resulting nuclei
grow uniformly and isotropically by diffusion of solutes toward
the particle surfaces until the final size is attained eventually.24
Blue wavy lines on sphere surface in Scheme 1 represent
hydrophilic CNS surface and a distribution of oxygenous
functional groups (–OH and C=O). The functional groups on
CNS can combine with TBOT during stirring in the ethanolic
CNS suspension. Thus, large amounts of TiO2 are formed on
the CNS surface by hydrolysis and condensation with
functional groups, as shown in Scheme 1.32 After calcination
for different time, hybrid carbon@TiO2 hollow nanospheres
with various carbon contents are formed.
Thermal gravimetric analysis (TGA)
In order to investigate the content of carbon in the
carbon@TiO2 composites, we studied the TG curves of
composite samples prepared with different calcination durations
(Fig. 4). All of the composite products show initial weight loss
from approximately 60 to 300 °C due to the evaporation of
surface adsorbed water, gases and disintegration of amorphous
carbon impurities. Between 400 and 580 °C, the T180 sample
exhibited no weight loss, while significant weight loss occurred
for the carbon@TiO2 samples T60, T90 and T120, which
corresponded to the decomposition of remaining carbon core.
The weight loss listed in Table 1 was used to estimate the
weight percentage of the carbon core in the carbon@TiO2
composites: 60.3% for sample T60 to 0% for sample T180.
These results further confirm that carbon@TiO2 composite
samples with different carbon contents can be obtained as
designed by controlling the calcination duration.
FTIR spectra and formation mechanism
FTIR spectroscopy study was used to analyze the formation
mechanism of TiO2 hollow spheres with different contents of
CNS cores. The FTIR spectra (Fig. 5) show the typical
vibrations associated with Ti–O–Ti bonding (500–800 cm–1)
and surface adsorbed water molecules (1630 cm–1, 3000–3700
cm–1) by anatase TiO2 in T60, T120 and T180 samples. The
adsorbed water is beneficial for photocatalytic CO2 reduction.
More importantly, there is a moderate intensity band near 1050
cm–1 observed for the carbon@TiO2 composite nanospheres,
which is characteristic of Ti–O–C bonds.29 However, with
decreasing carbon content, the band was absent in T120 and
T180. The Ti–O–C bond derived from carbonaceous species
formed between TiO2 and CNS core in the as-prepared
carbon@TiO2 nanocomposites.
Fig. 4 TG curves of thermal decomposition of carbon@TiO2
composite samples. (Preheated at 50 °C for 1 h and heated at a
constant rate of 5 °C min–1 in air.)
Table 1 Weight loss of carbon@TiO2 composite samples
Sample
T60 T90 T120 T180
~0
The characteristic absorption bands of CNS at 1709 and
1622 cm–1 are attributed to C=O and C=C vibrations,
respectively. The two characteristic vibrations support the
concept of aromatization glucose during hydrothermal
treatment. Moreover, the bands in the range of 1000–1300 cm–1
involve the C–OH stretching and O–H bending vibrations,
which imply the existence of hydroxyl groups. After TBOT was
added in the ethanolic CNS suspension, the amount of surface
reducing organic functional groups decreased, which was
Weight loss (%) 60.7 35.3
7
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DOI: 10.1039/C6TA11121A
Fig. 5 FTIR spectra of typical samples.
Fig. 6 XPS survey spectrum (a) and corresponding high-
resolution XPS spectra of Ti 2p (b), C 1s (c) and O 1s (d) of the
T120 sample.
can be attributed to the sp2 hybridized carbon (C–C bonds) and
carbon contaminants from ambience. The C 1s peaks at 286.1 and
288.7 eV can be assigned to the hydroxyl carbon (C–OH) and
carboxyl carbon (C=O) functional groups, respectively. These
groups are ascribed to the surface functional groups of exposed
carbon in some broken spheres. More importantly, the peak at 288.7
eV also corresponds to carbonaceous species that form Ti–O–C
structure between TiO2 and carbon in the composites as also shown
in the FTIR spectra (Fig. 5).34 This structure results from the
condensation reaction between the CNS surface groups and the
Ti−OH groups during TBOT adsorption on CNS surface. With
respect to the XPS spectra of O 1s in Fig. 6d, two peaks of 529.8 and
531.3 eV were fitted, which can be assigned to lattice oxygen (Ti–
O–Ti) and surface hydroxyls (Ti–OH), respectively.35,36 The O 1s
binding energy for Ti–OH was slightly lower than that for pure TiO2
(531.5 eV),35 indicating the presence of C–O–Ti bonds through
which the carbonaceous materials was grafted onto the TiO2 shell.
The structure is favorable for the desired charge transfer upon light
excitation.37,38
Scheme
1 Illustration of formation mechanism for the
carbon@TiO2 composite hollow structure.
XPS analysis
Pore structures
XPS measurements were carried out to further analyze the
interaction between TiO2 and carbon, and identify the chemical
status of Ti and O on the surface of the typical sample T120. Fig. 6a
demonstrates that Ti, O, and C elements existed in the carbon@TiO2
composite sample. Fig. 6b shows two bands locating at binding
energies of 464.3 and 458.6 eV, which were assigned to the Ti 2p1/2
and Ti 2p3/2 spin-orbital splitting photoelectrons in Ti4+ chemical
state. The splitting between these bands is 5.7 eV and the binding
energy of O 1s is 529.8 eV, demonstrating the normal state Ti4+ in
the prepared carbon@TiO2 nanocomposites. In Fig. 6c, the C 1s
spectrum of T120 can be fitted as three peaks at binding energies of
284.8, 286.1 and 288.7 eV. Notably, no carbonaceous species with
binding energy around 282 eV was observed, which can rule out the
lattice doping of carbon element for C–Ti.33 The peak at 284.8 eV
The BET specific surface area (SBET) and pore structure are two
important factors of the carbon@TiO2 nanocomposites for
photocatalytic CO2 reduction, which were investigated via
nitrogen adsorption–desorption measurements. Fig. 7 shows the
nitrogen
adsorption–desorption
isotherms
and
the
corresponding pore size distribution (inset) of the typical
samples T60, T120 and T180. According to Brunauer–
Deming–Deming–Teller (BDDT) classification, the isotherms
for the three samples are type IV, revealing the presence of
mesopores.39 Meanwhile, the nonlimiting adsorption at a high
P/P0 is characteristic of type H3 hysteresis loop, which is often
associated with slit-shape pores due to the aggregation of plate-
like particles. The pore size distribution (inset of Fig. 7) of T60
exhibits bimodal distribution including small mesopores (3–4
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nm) and macropores; in addition, micropores (<2 nm) also Photocatalytic activity of semiconductors isViepwaArrttiiccleuOlanrlilnye
DOI: 10.1039/C6TA11121A
existed in T60, as indicated by the sharp increase in N2 relevant to their optical absorption properties. In Fig. 8, the
adsorption amount at extremely low relative pressure (P/P0 <
Table 2 Textural properties and CO2 adsorption capacities of
the as-prepared samples.
UV–vis DRS of the composite samples demonstrate a significant
fundamental optical absorption threshold near 400 nm, which can be
assigned to the intrinsic band gap absorption of anatase TiO2 in the
composites.40–43 Notably, the curves of all the composite samples
show significant enhancement of light absorption in visible light
range of 400–800 nm as shown in Fig. 8. Moreover, the visible light
absorbance increased for composites with higher carbon contents,
i.e., T60 > T120 > T90 > T180. It is obvious that remnant carbon in
the core of hollow spheres affects the light absorption characteristics
of TiO2. The enhanced visible light absorption is mainly attributed to
carbon inside the spheres which can effectively absorb visible
light.44
SBET
PV
APS
CA
Sample
(m2 g–1) (cm3 g–1) (nm) (mmol g–1)
T60
T120
T180
P25
275
110
74
0.16
0.17
0.21
0.16
2.4
6.2
1.19
0.64
0.24
0.11
11.9
13.4
45
Photocatalytic activity and CO2 adsorption capacity
Notes: PV: pore volume; APS: average pore size; CA: CO2
adsorption quantity corresponding to P/P0 of 1.0.
Photocatalytic CO2 reduction activity was evaluated using the
prepared samples under simulated solar light (λ > 200 nm).
Control experiments show that no reduction products were
detected in the absence of photocatalyst, gaseous CO2 or light
irradiation. This suggests that CO2 reduction indeed proceeded
via photocatalytic reaction on the prepared samples, and
possibilities of contamination from other carbon sources can be
excluded.45,46
Fig. 9 shows the photocatalytic methane (CH4) and
methanol (CH3OH) production rates of the carbon@TiO2
composite samples. It is obvious that carbon inside the hollow
spheres has significant influences on photocatalytic activity of
the composites. P25 without carbon exhibited a relative low
photocatalytic CH4-production rate (2.02 μmol h–1 g–1) due to
low specific surface area and high recombination rate of
photogenerated electrons and holes in TiO2. By contrast, all the
carbon@TiO2 composite samples can yield two kinds of
products (i.e., CH4 and CH3OH) in the photocatalytic reaction.
Sample T60 exhibited a relative low performance compared
with other carbon@TiO2 samples. This can be ascribed to the
fact that CNS of large size inside the T60 sample absorbed
most of incident light due to the black body effect. Meanwhile,
the large volume of carbon in
Fig. 7 N2 adsorption–desorption isotherms and pore size
distribution curves (inset) of T60, T120 and T180.
0.02) of the adsorption isotherm (Fig. 7). The micropores are
attributed to the amorphous carbonaceous materials of the CNS
core, while the small mesopores and macropores were primarily
derived from the aggregation of primary TiO2 nanoparticles and
that of the carbon@TiO2 spheres, respectively. In the inset of
Fig. 7, with increasing calcination duration, the amount of
mesopores and macropores increased due to the decrease of
carbonaceous content. Meanwhile, the SBET decreased from 275
(T60) to 74 m2 g–1 (T180), while both pore volume and average
pore size increased with longer calcination duration (Table 2).
These changes indicate that CNS core released gases during
calcination, which can change the pore structure of the
composite samples. Moreover, the results can also verify that
the primary particles are packed more closely with increasing
calcination time, consistent with the TEM results. In summary,
the presence of carbon materials resulted in increased specific
surface area and improved pore structure of TiO2 hollow
spheres, which can lead to enhancement of photocatalytic
performance.
Fig. 8 UV–vis diffuse reflectance spectra of CNS and
carbon@TiO2 composites.
UV–vis diffuse reflectance spectra (DRS)
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further investigated the CO2 adsorption capacity of typical
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composite samples, P25 and CNS. AsDsOhI:o1w0.n103i9n/Ct6hTeA11C12O1A2
adsorption isotherms (Fig. 10), T60, T120 and CNS showed a
rapid rise of CO2 adsorption with increasing CO2 partial
pressure in the low pressure region (P/P0 < 0.2), which is
attributed to adsorption by the small mesopores of carbon. The
enhanced CO2 adsorption capacity of the composite samples
cannot be explained by the increased specific surface area alone
(Table 2), but is also associated with carbon content. Thus, it is
reasonable to assume that the delocalized π-conjugated binding
π66 in the core of carbon nanospheres (as shown in Scheme 1)
conduces to the adsorption of CO2. CO2 molecules also contain
delocalized π-conjugated binding π43 . Therefore, the unique π–π
conjugation interaction between CO2 and CNS can significantly
enhance the adsorption of CO2 molecules onto the
Fig. 9 Comparison of the photocatalytic CH4 or CH3OH
evolution rate of carbon@TiO2 composite samples and P25 carbon@TiO2 nanocomposite samples.51 As a consequence,
(under simulated solar light).
T60 and T120 with certain content of carbon and high specific
surface area exhibited much higher CO2 uptake capability
compared with T180 and P25. Moreover, CO2 adsorption of
CNS was not as much as those of T60 and T120, which results
from TiO2 hollow structure and pore structure of carbon
changed during calcination treatment. More CO2 can be
adsorbed due to the plentiful porous structure (inset of Fig. 7)
and high specific surface area (Table 2). The results show that
carbon inside TiO2 hollow spheres plays a very important role
in enhancing the CO2 adsorption capability besides the specific
surface area. Such enhanced CO2 concentration by
carbonaceous materials inside hollow spheres is beneficial for
improving photocatalytic CO2 reduction performance.
Electrochemical impedance analysis
Carbon-based materials play a very important role in acting as
an electron storage and transfer channel, which can hinder the
recombination of photogenerated electron-hole pairs,52,53 thus
EIS of the carbon@TiO2 composites were measured to
investigate their charge transfer ability. As shown in the
Nyquist plots (Fig. 11), the diameter of the semicircles for
carbon@TiO2 samples T120 and T180 (inset of Fig. 11) are
much smaller than those of T60 and T90, indicating that the
composite samples with lower carbon content had superior
charge transfer capacity. The result can be attributed to the fact
that carbon inside dispersed more evenly and contacted more
closely to the inner surface of TiO2 hollow spheres when
calcined for 120 min or longer. On the other hand, too much
amorphous carbon in the composites leads to relative low
conductivity of the samples, such as T60 and T90. However,
the conductivity of the samples did not increase monotonically
with lower carbon content, and sample T120 had the smallest
semicircle diameter (and thus the lowest charge transfer
resistance) among the composite samples. When the calcination
time was 180 min, the carbon content was close to 0% (Table
1), and interface states between carbon and TiO2 greatly
decreased, which resulted in the higher charge transfer
resistance of T180 than that of T120. Therefore, optimized
Fig. 10 CO2 adsorption isotherms of tested samples. (All the
samples were degassed at 100 °C prior to measurement.)
TiO2 hollow spheres shielded the active sites on the inner
surface of TiO2 hollow spheres, which is called the “shielding
effect”. In addition, due to the high carbon content (Table 1),
per unit mass of T60 contains much less TiO2 than the other
samples, which can negatively affect electron photoexcitation.
On the other hand, highest CH3OH evolution rate was observed
for sample T120, which was about 3 times that observed for
T60. Moreover, high CH4-production rate of 4.22 μmol h–1 g–1
was also obtained using T120. It can be observed that CNS in
the composite samples calcined longer than 60 min were
dispersed more evenly in the hollow spheres (Fig. 2). Thus, a
suitable content of carbon is crucial for optimizing
photocatalytic activity of the carbon@TiO2 composites. Similar
conclusions were made in previous studies on graphene-based
photocatalytic hydrogen production, reduction of organics and
metal ions.47–49 In addition, the optimized carbon@TiO2
composite photocatalyst exhibited
a higher activity for
photocatalytic CO2 reduction than that of the previously
reported TiO2-graphene and TiO2-multiwalled carbon nanotube
nanocomposites.50
Since the adsorption of CO2 onto the photocatalyst surface
is the first step essential for photocatalytic reduction of CO2, we
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DOI: 10.1039/C6TA11121A
Fig. 11 Electrochemical impedance spectra of carbon@TiO2 Fig. 12 In situ DRIFTS spectra of T120 in a flow of CO2/H2O
composite samples and magnified spectra of T120 and T180 vapour mixture as a function of irradiation time.
(inset).
carbon content was obtained by calcination for 120 min, which
led to the best charge transfer efficiency for sample T120. The
improved conductivity of nanocomposites has significant
positive impacts on the photocatalytic reaction, which is
consistent with the trend of photocatalytic CO2 reduction
activity (Fig. 9).
In situ DRIFTS
In situ DRIFTS of typical sample T120 was conducted to identify
adsorbed intermediates and understand the photocatalytic conversion
process. When T120 was irradiated with monochromatic light (365
nm) in a flow of CO2/H2O vapour mixture, multiple peaks were
observed in the DRIFTS spectra (Fig. 12), which could be assigned
Scheme
composite hollow structure.
2 Photoexcitation process of the carbon@TiO2
to carbonate, adsorbed formate (HCOO), methoxyl group (CH3O),
and molecularly adsorbed formaldehyde (HCHO). The peaks at 1681,
1618, 1520, 1507, 1437 and 1312 cm–1 are attributed to carbonate
species, those at 1868, 1844, 1825, 1700, 1649, 1592, 1558, 1540,
1455, 1395, 1372 and 1340 cm–1 are ascribed to adsorbed HCOO,
and those at 1770, 1749, 1715, 1418 and 1249 cm–1 can be assigned
to adsorbed HCHO.54,55 More importantly, the peaks at 1732 and
1472 cm–1 result from the vibration of CH3O groups.55 However,
CH4 cannot be detected because of its nonpolar property and low
affinity onto sample surface. In addition, the peak intensity of
formate, formaldehyde and methoxyl group was gradually increased,
while those of carbonate species did not change significantly. These
results indicate that HCOOH, HCHO and CH3OH are the products in
a multi-step photocatalytic conversion process of CO2 to CH4. 25,56
Photocatalytic CO2 reduction mechanism
As discuss above, photocatalytic reduction of CO2 into solar
fuels demands energy input to break C=O bonds and form C–
H/C−C bonds, involving the participation of multiple electrons
and a corresponding number of protons.45 Photocatalytic CO2
reduction with H2O into CH4 and CH3OH is an upward reaction
with a highly positive change in Gibbs free energy, and a multi-
step reduction process is presented for the conversion of CO2 to
CH4: CO2 → HCOOH → HCHO → CH3OH→ CH4.56,57
During the photocatalytic activity measurement, the
temperature of the whole custom-made Pyrex glass reactor,
especially the bottom of the flask with catalyst powder on it,
increased significantly after irradiation for 1 h. This observation
indicates that carbon materials like graphene absorb almost the
whole spectrum of solar light due to its black color and zero
band gap, which results in increasing temperature around the
photocatalyst to create a local photothermal effect.51,53,58 The
Near Infrared Ray (NIR) light absorption into carbon of
composite samples particularly allows photogenerated electrons
to obtain more energy and move faster in carbon@TiO2
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composite samples with rising temperature.59 Therefore, local
photothermal effect of carbon also has positive effects on the
activity of photocatalytic CO2 reduction over the
nanocomposites.
1
2
3
4
5
6
7
P. D. Tran, L. H. Wong, J. Barber and J. S. C.ViLewooAr,ticElenOenrlginye
Environ. Sci., 2012, , 5902.
S. N. Habisreutinger, L. Schmidt-Mende and J. K. Stolarczyk,
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5
DOI: 10.1039/C6TA11121A
S. Yan, S. Ouyang, H. Xu, M. Zhao, X. Zhang and J. Ye, J.
Based on the above results, a probable reaction mechanism
for photocatalytic over carbon@TiO2 composites is proposed
and illustrated in Scheme 2. Firstly, CO2 is adsorbed on the
surface of TiO2 hollow spheres. Meanwhile, the CO2 adsorption
on CNS core also plays a very important role via π–π
conjugation interaction. During light irradiation, charge
separation occurs in TiO2 generating a large amount of
electron-hole pairs. The electron-hole pairs were separated and
the electrons transferred through carbon materials. Then
electrons initiated photocatalytic reduction reaction by reacting
with CO2 and water. Water was oxidized by holes, generating
oxygen and H+ ions. And CO2 was reduced into methane and
methanol by reaction with H+ ions and electrons.60,61
Mater. Chem. A, 2016,
M. Marszewski, S. W. Cao, J. G. Yu and M. Jaroniec, Mater.
Horiz., 2015, , 261.
4, 15126.
2
J. G. Yu, K. Wang, W. Xiao and B. Cheng, Phys. Chem.
Chem. Phys., 2014, 16, 11492
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,19936.
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8 J. X. Low, B. Cheng and J. G. Yu, Appl. Surf. Sci., 2017, 392
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In this process, difference in the quasi Fermi energies
created nonequilibrium charging state between TiO2 and
carbon. Thus, these photoexcited electrons transfer from
conduction band (CB) of TiO2 to neighboring carbon. The
process improves electron transfer efficiency, which leads to
charge equilibrium between these two components in the
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reduces the amount of electrons in the lattice of TiO2, which
can hinder electron-hole recombination efficiently and allow
more holes to be generated on the TiO2 surface for the
oxidation of water. Therefore, overall photocatalytic activity of
the carbon@TiO2 nanocomposites can be enhanced during the
process.
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Conclusions
In summary, a hybrid carbon@TiO2 hollow spherical structure
was fabricated by a facile and green method. The as-designed
hybrid semiconductor@carbon products not only enhanced the
visible-light absorption and CO2 adsorption, but also improve
the photogenerated charge transfer efficiency. As a result, the
hybrid carbon@TiO2 photocatalyst exhibited a much higher
activity for photocatalytic CO2 reduction and produced more
variety of solar fuels compared with P25. The approach for
fabricating the hybrid nanostructure may also bring new
insights for a variety of semiconductor-based applications such
as supercapacitor, catalysis, sensors and energy conversion.
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Acknowledgements
This study was partially supported by the 973 program
(2013CB632402), NSFC (51572209, 51320105001, 51372190,
21433007 and 21573170). Also, this work was financially
supported by the Natural Science Foundation of Hubei Province
of China (2015CFA001), the Fundamental Research Funds for
the Central Universities (WUT: 2015-III-034) and Innovative
Research Funds of SKLWUT (2015-ZD-1)
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA11121A
Graphic abstract
Photocatalytic CO2 conversion toward solar fuels via hybrid
carbon@TiO2 hollow spheres.