Enhancement of the photocatalytic activity of a TiO2/carbon aerogel based on a hydrophilic secondary pore structure

Article abstract of DOI:10.1039/c6ra08074j

Improving the separation and utilization of electrons and holes in a photocatalytic process is a guarantee for high photocatalytic efficiency. We report a strategy to enhance the photocatalytic performance based on fabrication of a hydrophilic secondary pore structure by incorporating TiO₂ into a porous carbon aerogel (CA) with a 9.3 nm pore diameter, where TiO₂ resides on both the inner and outer surfaces of CA as evidenced by N₂ sorption isotherms and transmission electron microscopy. In such a structure, the spatial separation efficiency of photoelectrons and photoholes is supposed to get enhanced with interface electrons transferring into the inner surface of the pores via conductive CA. As a result, HO formation can be promoted in the confined inter surface of the hydrophilic secondary channel through O₂ reduction with the participation of photoelectrons and H₂O. And the remaining photoholes on the outer surface can oxidize water to generate HO as well. In contrast, TiO₂ is mainly dispersed on the outer surface of CA as small pore diameters of 3.4 and 4.3 nm; as a result, only uncombined photoholes on the outer surface contribute to HO generation via the water oxidation route. In line with this understanding, TiO₂/CA (9.3 nm) shows the largest amount of HO and thereby the highest efficiency of dimethyl phthalate degradation, as respectively evidenced by the electron paramagnetic resonance spectroscopy and the photocatalytic degradation test. These findings unveil the contribution of the surface/interface synergy effect on the separation and utilization of electrons and holes in photocatalytic process, and provide a potential strategy to enhance the photocatalytic performance.

Full text of DOI:10.1039/c6ra08074j

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Enhancement of the photocatalytic activity of
a TiO2/carbon aerogel based on a hydrophilic
secondary pore structure†
Cite this: RSC Adv., 2016, 6, 68416
a
b
c
a
a
*
Hua'nan Cui, Zhenxing Liang, JinZhong Zhang, Hong Liu and Jianying Shi
Improving the separation and utilization of electrons and holes in a photocatalytic process is a guarantee for
high photocatalytic efficiency. We report a strategy to enhance the photocatalytic performance based on
fabrication of a hydrophilic secondary pore structure by incorporating TiO2 into a porous carbon aerogel
(CA) with a 9.3 nm pore diameter, where TiO2 resides on both the inner and outer surfaces of CA as
evidenced by N2 sorption isotherms and transmission electron microscopy. In such a structure, the
spatial separation efficiency of photoelectrons and photoholes is supposed to get enhanced with
interface electrons transferring into the inner surface of the pores via conductive CA. As a result, HOc
formation can be promoted in the confined inter surface of the hydrophilic secondary channel through
O2 reduction with the participation of photoelectrons and H2O. And the remaining photoholes on the
outer surface can oxidize water to generate HOc as well. In contrast, TiO2 is mainly dispersed on the
outer surface of CA as small pore diameters of 3.4 and 4.3 nm; as a result, only uncombined photoholes
on the outer surface contribute to HOc generation via the water oxidation route. In line with this
understanding, TiO2/CA (9.3 nm) shows the largest amount of HOc and thereby the highest efficiency of
dimethyl phthalate degradation, as respectively evidenced by the electron paramagnetic resonance
spectroscopy and the photocatalytic degradation test. These findings unveil the contribution of the
surface/interface synergy effect on the separation and utilization of electrons and holes in photocatalytic
process, and provide a potential strategy to enhance the photocatalytic performance.
Received 29th March 2016
Accepted 14th July 2016
DOI: 10.1039/c6ra08074j
www.rsc.org/advances
addition, assembling TiO2 with carbon materials is another
effective way,12 as porous carbon support has been found to
selectively accumulate reactants into its abundant pores13–15 and
Introduction
As an advanced oxidation process, photocatalytic degradation
mediated via TiO2 is an efficient method to eliminate organic
pollutants from water. It has drawn much attention in the eld
of removing phthalate esters (PAEs) from an aqueous environ-
ment, because the end products aer photocatalysis on PAEs
are carbon dioxide, water, and inorganic mineral ions.1–7 For
TiO2 photocatalysts, improving the separation and utilization of
electrons and holes in photocatalytic processes is highly desired
for high photocatalytic efficiency.8 Surface modication,9
sensitization10 and transition-metal doping11 are widely used
strategies to optimize TiO2 photocatalytic performance. In
its excellent conductivity serves to accelerate the separation of
photogenerated carriers.16–18 Some studies have suggested that
such porous carbon powder is also capable of absorbing light,
and the generated photoelectrons can be transferred to the
conduction band of TiO2 for further use.19 Despite a large
number of assemblies of TiO2 with carbon were reported to
data,17,20–23 few systematic studies are undertaken to reveal the
function of pore structure on photocatalytic performance,
especially secondary pores with unique spatial features that are
formed by encapsulating TiO2 into carbon pore cavities. It has
been reported that the unique pore-conned space can poten-
tially play a vital role in enhancing catalytic activity and selec-
tivity in heterogeneous catalysis.24–26 Particularly, the CNT-
conned TiO2 exhibited improved visible-light activity as the
modication of the electronic structure of TiO2 induced by the
unique connement inside CNTs.27
aKey Laboratory of Environment and Energy Chemistry of Guangdong Higher
Education Institutes, School of Chemistry and Chemical Engineering, Sun Yat-sen
University, Guangzhou 510275, China. E-mail: shijying@mail.sysu.edu.cn
bGuangdong Provincial Key Laboratory of Fuel Cell Technology, School of Chemistry
and Chemical Engineering, South China University of Technology, Guangzhou
510641, China
Carbon aerogel (CA), possessing unique three-dimensional
ultra-porous network with high porosity (80–98%), low electrical
resistivity (#40 mU cm), and large surface area (400–1100 cm2
gꢀ1),28,29 is a promising material in the application of processing
contaminants.28,30 In this work, three kinds of mesoporous CA
cDepartment of Chemistry and Biochemistry, University of California, Santa Cruz, CA
95064, USA
† Electronic supplementary information (ESI) available: The thermogravimetry
(TG), differential thermal gravity (DTG), X-ray diffraction and UV-vis absorption
spectra for all samples. See DOI: 10.1039/c6ra08074j
68416 | RSC Adv., 2016, 6, 68416–68423
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with several nanometer pore diameters31 were employed as
support to assemble TiO2/CA-X composite catalysts, where X
denotes as the pore diameter of CA. Dimethyl phthalate (DMP),
a member of phthalate acid esters (PAEs) detrimental to human
being as endocrine disruptors, mutagens, reproductive and
developmental toxicants,7,32,33 was chosen as the target pollutant
for evaluating the photocatalytic properties of the TiO2/CA cata-
lysts. Only TiO2/CA-9.3 shows the enhanced photocatalytic
activity, and TiO2/CA-3.4 and TiO2/CA-4.3 even gives the lower
activity than bare TiO2. It was found that the HOc formation is
correlated with the secondary pore structure of TiO2/CA-X, and the
contribution of surface/interface synergy effect on the separation
and utilization of electrons and holes in the photocatalytic
performance is discussed.
Results and discussion
Pore structure of TiO2/CA-X
The pore structure of CA-X before and aer assembling TiO2 are
investigated by N2 adsorption–desorption isotherms, and the
results are shown in Fig. 1a and b, respectively. The isotherms of
CA-X and TiO2/CA-X featured IV-type character with specic
hysteresis loop at relatively high pressure, implying that both CA-X
and TiO2/CA-X are typical mesoporous materials. Fig. 2 shows the
pore distribution of CA-X and TiO2/CA-X samples calculated by
a DFT model on ASiQwin soware.34,35 It can be seen that meso-
pores dominate the pore structure in both CA-X and TiO2/CA-X
samples with a small proportion of micropores and macropores.
The detailed information of the pore structure is summarized in
Table 1, e.g. pore volume, pore diameter and BET surface area.
The average pore diameters of three pristine CA samples are
3.1, 4.3 and 9.3 nm, as shown in Table 1. Aer assembling TiO2,
the pore diameters of CA-3.1 and CA-4.3 remain almost
unchanged, but it decreases from 9.3 to 6.1 nm for CA-9.3. As
seen in Table 1, both the volume of micropore (<2 nm) and
mesopore (2–50 nm) and the BET surface of three TiO2/CA-X
samples decrease sharply when compared with those of the
pristine CA-X samples. The decrease can be attributed to either
the blockage of pore channels by TiO2 dispersed on the carbon
surface or the shrinkage of pore channels as TiO2 was
Fig. 2 Pore distribution of (a) CA-X and (b) TiO2/CA-X and pure TiO2.
encapsulated into pore channels. The unchanged pore diameter
of CA-3.1 and CA-4.3 indicates that TiO2 was prominently
dispersed on the outer surface of CA-3.1 and CA-4.3. While for
CA-9.3, the dwindled 6.1 nm pore size suggests that, in addition
to being dispersed on the outer surface, a considerable amount
of TiO2 was dispersed into the pore channels of CA-9.3. It is
acknowledged that the capillary force is the driving force to
inltrate the precursor solution into the support channels
during the preparation of supported catalysts,36 which could
account for the formation of secondary pore in TiO2/CA-9.3. The
3.1 and 4.3 nm pore diameters of CA may be too small to allow
the inltrate of Ti precursor into the pore channel.
Composition and structure of TiO2/CA-X
The carbon content of TiO2/CA-X samples was analyzed by
thermogravimetry (TG) and differential thermal gravity (DTG)
ꢁ
(Fig. S1†). The same loss steps at around 500 C related to the
carbon combustion indicate three samples possess the
comparable carbon contents of about 15 wt% (Table 2), that is,
the same amount of TiO2 is dispersed on them. X-ray diffraction
was used to determine the crystal structure of pristine TiO2,
TiO2/CA-X and CA-X, and diffraction patterns are shown in
Fig. S2.† A set of diffraction peaks related to anatase (PDF#21-
1272) are observed in pristine TiO2 and TiO2/CA-X, in compar-
ison, CA-X presents amorphous structure with broad diffraction
patterns shown in insert of Fig. S2.† The results indicate that
Fig. 1 N2 adsorption–desorption isotherms of (a) CA-X and (b) TiO2/
CA-X and pure TiO2.
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and from ca. 8 to 6 mg gꢀ1 upon assembling TiO2 on CA-3.1 or
CA-4.3. It was reported the volume of mesopores and micropores
determined the adsorption of organic micro pollutants, such as
microcystins by activated carbons.37,38 The total volume of
micropores and mesopores for CA-3.1, CA-4.3 and CA-9.3 (listed
in Table 1) is about 0.88, 0.96 and 1.2 cm3 gꢀ1, respectively. The
volume data well accounts for the similar DMP adsorption
capacities of CA-3.1 and CA-4.3 and the highest adsorption
amount of CA-9.3. Aer assembling TiO2 on CA-X, the total
volume of all three samples decreases signicantly to the same
level of ꢂ0.17 cm3 gꢀ1, as presented in Table 1. The volume
decrease of all three samples are consistent with their adsorption
amount decrease, but the comparable 0.17 cm3 gꢀ1 total volume
cannot explain their different adsorption capacity, e.g. 4 mg gꢀ1
in TiO2/CA-9.3 and the 6 mg gꢀ1 in TiO2/CA-3.1 and TiO2/CA-4.3.
In addition to pore volume, the wettability of adsorbent also
plays a vital role on its absorption performance, for example,
the hydrophilic TiO2 possesses the negligible DMP adsorption
in Fig. 4. Fig. 5 presents the contact angle results and the
droplet images of all samples as well. It can be seen that all CA-X
samples are hydrophobic with apparent contact angles over
120ꢁ. Aer loading hydrophilic TiO2 with a contact angle of less
than 10ꢁ, all samples change to be hydrophilic accompanying
with the decrease of contact angles. Notably, TiO2/CA-9.3 shows
the smallest contact angle ca. 70ꢁ in three TiO2/CA-X samples,
which is consistent with its unique pore structure that hydro-
philic TiO2 was dispersed on both inner and outer surface of CA-
9.3. The hydrophilic nature of TiO2/CA-9.3 enables it to be more
inclined to adsorb H2O but not DMP. Thus, TiO2/CA-9.3 gives
the lowest DMP adsorption amount, as shown in Table 1.
Table 1 Pore parameters of carbon, TiO2, and TiO2/CA
Da
(nm)
SBET
Vmicro + Vmeso
DMP adsorption
b
c
d
Sample
(m2 gꢀ1
)
(cm3 gꢀ1
)
amount (mg gꢀ1
)
CA-3.1
CA-4.3
CA-9.3
TiO2
TiO2/CA-3.1
TiO2/CA-4.3
TiO2/CA-9.3
3.1
4.3
9.3
4.4
3.8
4.1
6.1
1173
954
709
84
237
215
124
0.88
0.96
1.2
0.07
0.17
0.18
0.17
8.6 ꢃ 0.4
7.7 ꢃ 0.5
10.6 ꢃ 0.6
0 ꢃ 0.2
6.4 ꢃ 0.5
6.2 ꢃ 0.5
4.4 ꢃ 0.5
a
b
Average mesopore size calculated form DFT method. Brunauer–
Emmett–Teller (BET) surface area calculated from adsorption branch
c
of the N2-sorption isotherm. Micropore (<2 nm) volume calculated
from cumulative pore volume of DFT method. Mesopore (2–50 nm)
d
volume calculated from cumulative pore volume of DFT method.
Table 2 Carbon content of TiO2/CA-X samples
Sample
C content (%)
TiO2 content (%)
C/TiO2 ratio
TiO2/CA-3.1
TiO2/CA-4.3
TiO2/CA-9.3
15.0
14.9
15.1
85.0
85.1
84.9
0.18 : 1
0.18 : 1
0.18 : 1
three TiO2/CA-X samples possess the same TiO2 crystal struc-
ture, irrespective of the pore diameter of CA.
The structure of CA-X before and aer assembling TiO2 was
further studied by TEM and HRTEM, and the representative
images of these samples are shown in Fig. 3. It can be seen that
both CA-4.3 (Fig. 3a) and CA-9.3 (Fig. 3c) exhibit disordered
wormhole-like mesopore structures, which are consistent with
the previous report.31 Aer assembling TiO2 on CA-4.3 (Fig. 3b),
abundant and spherical domains (marked in the dash line
frame) on several nanometer scale with clear lattice fringes of
TiO2 are disorderly dispersed on the surface of CA-4.3. When
comparing CA-9.3 (Fig. 3c) vs. TiO2/CA-9.3 (Fig. 3d), the pore
channels of CA-9.3 are partially lled by the assembled TiO2,
deduced from the variation on contrast of the two images. In
addition, TiO2 particles (marked in the dash line frame) with
identiable lattice fringes are observed on the edge of CA-9.3
without serious aggregation. The TEM results further conrm
that TiO2 is dispersed on outer surface of CA-4.3, while on both
inner and outer surfaces of CA-9.3, which are consistent with
the pore structure analysis (vide supra).
Photocatalytic degradation of DMP
DMP was chosen as the target pollutant to study the inuence of
pore structure on the photocatalytic activities of TiO2/CA-X
samples. It should be noted that no DMP degradation occurred
when irradiating TiO2/CA-X photocatalysts with visible light (l >
420 nm). As previous reports,39,40 the photodegradation process
in our case occurs with UV-light irradiation of photocatalysts
and carbon does not sensitize TiO2, though the visible absorp-
tion related to carbon is observed in UV-vis absorption spectra
(Fig. S3†).
Fig. 6 shows the dynamic curves of DMP photocatalytic
degradation during 3 hours. It can be seen that TiO2/CA-9.3
gives the highest degradation efficiency of 83%, and TiO2
shows the secondary degradation efficiency ca. 70%. TiO2/CA-
3.1 and TiO2/CA-4.3 respectively exhibit the third and fourth
degradation efficiency of 55% and 44%, which are followed by
Effect of porous structure and wettability on DMP adsorption
The DMP adsorption on TiO2/CA-X is evaluated by kinetic the mechanical mixture of TiO2 and CA-9.3 with 25% efficiency.
adsorption curves in Fig. 4, and the one-hour absorption amount The intermediates in DMP degradation process are identi-
is listed in Table 1. CA-3.1 and CA-4.3 present the similar ed from positive modes of MS (Fig. S4†). The signal at m/z ¼
adsorption behaviors (dashed line) with ca. 8 mg gꢀ1 DMP 195 (+H) which belongs to DMP has been strongly weakened
amount upon one-hour absorption, which is less than the and two major intermediates were formed according to the
amount of 10 mg gꢀ1 on CA-9.3. Aer assembling TiO2, DMP signals in the mass spectra. The signals at m/z ¼ 149 (+H) and
adsorption on three TiO2/CA-X samples (solid lines) decreases in 171 (+Na) might belong to o-phthalic anhydride. The signal at
different levels compared with each pristine CA-X. It decreases m/z ¼ 167 (+H) might be phthalic acid. The similar intermedi-
from 10 to 4 mg gꢀ1 when comparing CA-9.3 with TiO2/CA-9.3, ates were observed by Wang et al.3
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Fig. 3 HRTEM images of (a) CA-4.3, (b) TiO2/CA-4.3, (c) CA-9.3, and (d) TiO2/CA-9.3.
Photocatalytic activity is acknowledged to rely on the
In consideration of the similar absorption features of TiO2/
adsorption capacity of organic molecules upon photocatalyst, CA-X in UV-vis absorption spectra (Fig. S3†), the amounts of
and high adsorption capacity is favorable for the photocatalytic photogenerated carriers in these samples are comparable,
degradation due to the accumulation of organic molecules and which cannot account for such a big difference in photo-
the intermediates, if any.41 However, in our case, the high degradation efficiency (see Fig. 6) as well. The HOc was further
adsorption amount of DMP in TiO2/CA-3.1 and TiO2/CA-4.3 did studied to address the difference in DMP degradation effi-
not result in high degradation efficiency. TiO2/CA-9.3, which ciency. Fig. 7 gives the EPR spectra of DMPO-OH adduct
has lowest adsorption amount of DMP, presents the highest produced by in situ illuminating aqueous solutions respectively
degradation efficiency. That is, the DMP accumulation in TiO2/ containing TiO2, TiO2/CA-9.3 and TiO2/CA-3.1. The control
CA-X relying on the adsorption capacity is unessential for its experiment result of DMPO aqueous solution is also shown in
photodegradation in this case.
Fig. 7. It can be seen that TiO2/CA-9.3 exhibits the largest
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of above MS results, the major intermediates should be formed
by the attack of hydroxyl radicals on the alkyl chains of DMP.
Therefore, we focus on the formation of HOc to correlate the
structures of TiO2/CA-X with their photocatalytic properties
hereinaer.
The mechanism of photocatalytic degradation
A typical photocatalytic process begins with the light absorption
to produce electrons and holes (eqn (1)). The photogenerated
holes are consumed by the oxidization of H2O to produce active
HOc (eqn (5));42 meanwhile, the photoelectrons are consumed
by O2 reduction to cO2ꢀ (eqn (2)), followed by the generation
(eqn (3)) and decomposition (eqn (4)) of H2O2, in which process
active HOc is also generated. It is noted that active HOc with
strong oxidation ability leads to the fast degradation of organic
molecules (eqn (6)).43
Fig. 4 The adsorption curves for DMP onto CA-X, TiO2/CA-X, and
pristine TiO2.
ꢀ
+
TiO2(surface) + hn / eCB + hVB
(1)
(2)
(3)
(4)
(5)
(6)
ꢀ
ꢀ
eCB + O2 / cO2
ꢀ
cO2 + H2O / H2O2
H2O2 / HOc + OHꢀ
H2O + hVB / HOc + H+
+
HOc + C10H10O4 / H2O + CO2
In above processes, there are two ways for HOc formation: H2O
oxidation in eqn (5) and O2 reduction with the participation of
H2O in eqn (2)–(4). Most important of all, in both ways, H2O
molecule is indispensable for HOc formation. Thus, the more
hydrophilic surface in TiO2/CA-9.3 can account for its higher HOc
generation ability than that of TiO2/CA-3.1 with hydrophobic
surface. However, it is still puzzling that the HOc amount
produced in TiO2/CA-9.3 is even larger than that in pristine TiO2,
concerning the super-hydrophilic TiO2 vs. hydrophilic TiO2/CA-
9.3. We propose that a structure-related effect exists in TiO2/CA-
Fig. 5 Contact angle and droplet images of CA-X and TiO2/CA-X.
amount of HOc, TiO2 gives the secondary HOc amount, and
TiO2/CA-3.1 shows the lowest HOc amount. This order of HOc
amount is consistent with the DMP degradation sequence in
Fig. 6. It was reported that water treatment processes was highly
dependent on hydroxyl radical and it is critical to maximize
hydroxyl radical generation during TiO2 photocatalysis.4 In light
Fig.
7 EPR spectra of DMPO-OH adduct in aqueous solution
Fig. 6 Dynamic curves of photocatalytic degradation DMP with TiO2/ respectively containing TiO2, TiO2/CA-9.3 and TiO2/CA-3.1 illumi-
CA-X, TiO2 and TiO2 + CA-9.3.
nating with Xe lamp.
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Fig. 8 Schematic illustration of DMP degradation in TiO2/CA-9.3 under light irradiation. The red ring describes the reactions occurring in the
secondary pores. The processes include: (I) on the external surface, HOc formation with H2O reduction and oxidation, (II) the photoelectrons
generated on external surface migrates to the internal surface through the carbon layer, (III) on the internal surface, the photoelectrons are
trapped by O2 to produce cO2ꢀ, (IV) HOc are generated with the participation of H2O and cO2ꢀ, (V) DMP is oxidized by active HOc on both external
and internal surface.
9.3 toward HOc generation and the mechanism of HOc formation
on the outer and inner surfaces of TiO2/CA-9.3 is illustrated in TiO2 on both outer and inner surface of CA-9.3 support,
Fig. 8 (vide infra). prepared by introducing CA-9.3 into titanium precursor solu-
Comparing with TiO2/CA-9.3 with uniform distribution of
It is understandable that the primary factor inhibiting HOc tion prior to sol–gel formation, TiO2 + CA-9.3 is a simply
formation via either the oxidation way (eqn (5)) or reduction way mixture of TiO2 and CA-9.3 and no hydrophilic porous struc-
(eqn (2)–(4)) is the recombination of photoholes and photo- ture exists, in addition, part of CA-9.3 covering on TiO2 could
electrons in the photodegradation process. With respect to the block the incident light by its self-absorption. Therefore, the
special structure of TiO2/CA-9.3 with TiO2 dispersed on both degradation activity of TiO2 + CA-9.3 is far lower than TiO2/CA-
inner and outer surfaces, it is possible that the photoelectrons 9.3, even lower than TiO2/CA-3.1 and TiO2/CA-4.3, where TiO2
generated on the outer surface transfer to its inner surface mainly dispersed on the outer surface of CA supports as their
through the conductive CA (process II in Fig. 8). This process small pore diameter.
potentially increases the separation efficiency of photoelectrons
and photoholes. As a result, HOc formation via H2O oxidation
with photoholes (eqn (5)) is enhanced on the outer surface TiO2
(process I in Fig. 8). Meanwhile, the formation of HOc via O2
Conclusions
reduction with participation of H2O and photoelectrons (eqn
(2)–(4)) is improved in the hydrophilic secondary channels of
TiO2/CA-9.3 (process IV in Fig. 8 amplifying red-circle region),
because in this secondary channels the three integral compo-
nents of photoelectrons, O2 and H2O needed for HOc formation
are simultaneously satised with the photoelectrons transfer
aforementioned, air bubbling in aqueous solution, and surface
TiO2/CA-X photocatalysts with variable pore structures were
fabricated and the correlation between the pore structure and
the photocatalytic property was systematically studied. Our date
indicates that dispersion of TiO2 on CA-X relies on the pore
diameter of CA support. The large diameter enables TiO2 to be
dispersed on both inner and outer surface of CA support,
accompanying with the formation of the secondary pore with
hydrophilic property, respectively. In addition, the spatial
narrower channel. This special porous structure of TiO2/CA-9.3
connement effect44,45 arising from narrower secondary channel
leads to the separation of electrons and holes in space in the
further facilitates the HOc formation by stabilizing the inter-
photocatalytic process. Correspondingly, the two routes of HOc
mediates of cO2ꢀ and H2O2. In summary, the interface electron
formation in H2O oxidation and O2 reduction with participation
of H2O are promoted. The largest amount HOc nally causes the
highest DMP degradation efficiency in TiO2/CA-9.3. It was
concluded that the surface/interface synergy effect plays a posi-
transfer between TiO2 and CA-9.3 promotes the spacial sepa-
ration of photoelectrons and photoholes, and hydrophilic
secondary-pore surface with conned space results in the
increase of HOc formation possibility in reduction way (eqn (2)–
(4)), and thereby leads to the highest DMP degradation activity
and holes in photocatalytic process, which suggests a potential
of TiO2/CA-9.3 (process V in Fig. 8).
tive role on the spatial separation and utilization of electrons
strategy for designing high efficiency photocatalysts.
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desorption isotherm at 77 K (Quantachrome, AutosorbiQ,
American). Before measurement, the samples were out-gassed
at 200 ꢁC for 12 h. The specic surface area was determined by
Brunauer–Emmett–Teller (BET) model, while the pore size
and volume were calculated by density functional theory
(DFT) method on Quantachrome ASiQwin soware. Diffuse
reection spectra (DSR) were recorded on UV-vis spectro-
photometer (Shimadzu, UV-3150, Japan, equipped with an
integrating sphere) to study the optical properties of the
samples.
Experimental
Materials
Titanium(IV) isopropoxide (95%) and tetraethyl orthosilicate
(TEOS, 98%) were purchased from Aladdin (Shanghai, China).
Other reagents used were all analytical reagents and purchased
from Guangzhou Chemical Reagent Plant (Guangzhou, China).
All reagents were used as received without further purication,
and the double distilled water was used in all experiments.
Electron Paramagnetic Resonancse spectrometer (EPR,
Bruker, A 300, Germany) were used to detect the hydroxyl
radical with 5,5-dimethy-1-pyrroline-N-oxide (DMPO, Sigma-
Aldrich) as the radical trap. The samples were irradiated with
xenon arc source system (LOT Oriel 75-150, UK). Data acquisi-
tion parameters were as follows: magnetic eld, 3514 G;
microwave power, 5.95 mW; modulation frequency, 100 kHz;
modulation amplitude, 1.00 G; microwave frequency, 9.86 GHz;
sweep time, 20.48 s. The hydrophilic properties of CA-X and
TiO2/CA-X were measured by OCA contact angle system (Data-
physics, OCA20, Germany). The intermediates in the degrada-
tion process were identied from positive modes of MS (AB
Sciex, triple QuadTM 4500).
Preparation
The carbon aerogels were synthesized following a previously
published procedure.31 Briey, the raw materials of glucose (2
g), tetraethyl orthosilicate (TEOS, 3 mL) and sulfuric acid
(H2SO4) solution (4 mL, pH ¼ 2) were mixed and stirred until
a homogenate was formed. The mixture was then treated by
hydrouoric acid (HF, 4 wt%) under stirring, gelated, and aged
in a plastic bottle as follows: 40 ꢁC for 2 days, followed by 100 ꢁC
for 6 h, and nally 160 ꢁC for 6 h. The xerogel was subsequently
carbonized in nitrogen atmosphere at 900 ꢁC for 3 h, and
washed by 40 wt% HF to remove silica. The HF/TEOS molar
ratio used was 0, 1/30 and 1/7, and as a result, the pore diam-
eters were controlled by changing the amount of HF during the
sol–gel process of TEOS. The notation of “CA-X” is used to
denote the samples, with X being the pore diameter of CA
calculated by the density function theory (DFT) model. The
TiO2/CA-X sample was synthesized with CA-X as the support
material.
TiO2/CA-X samples were prepared by a sol–gel method with
titanium isopropoxide as Ti source. A typical procedure is as
follows. Titanium(IV) isopropoxide (3 mL) was introduced into
ethanol (20 mL) with vigorous stirring for 30 min, and then
concentrated nitric acid (0.16 mL) was added into this solu-
tion. Aer stirring for 5 minutes, CA (0.16 g) was distributed
into this solution. The mixture solution was constantly stirred
until a homogenous CA-contained gel was formed. The gel was
aged in air to form a ergoes, which was then ground into ne
powder. The powder was calcined at 400 ꢁC for 5 h in the
nitrogen atmosphere, and TiO2/CA-X samples were thus ob-
tained. For comparison, pure TiO2 was also prepared
following the same procedure in the absence of CA. Mechan-
ically mixed TiO2 and CA-X sample with the same mass ratio
as TiO2/CA-X samples, marked as TiO2 + CA-9.3 was also
prepared.
Adsorption and photocatalytic degradation of DMP
Dimethyl phthalate (DMP, 2 mg Lꢀ1) was chosen as the model
target to investigate the photocatalytic properties of porous
TiO2/CA-X samples. Prior to the photocatalytic reaction, the
adsorption experiment of DMP on TiO2/CA-X was carried out
in dark at room temperature by adding 0.01 g of TiO2/CA-X
sample into 100 mL 2 mg Lꢀ1 DMP aqueous solution. The
photodegradation experiments were carried out in a cylin-
drical glass reactor (F 9.0 cm ꢄ 10.6 cm) covered by a cylin-
drical quartz vessel (F 9.0 cm ꢄ 9.0 cm), which was full-lled
with double distilled water to remove the infrared light
radiated from 300 W Xe lamp (Changtuo, PLS-SXE 300/300
UV, Beijing). The light intensity was 85 mW cmꢀ2 deter-
mined by a radiometer (FZ-A, Photoelectric Instrument
factory of Beijing Normal University). The emission spectrum
of the Xe lamp has been reported in ref. 8. For a typical
photocatalytic experiment, 0.1 g Lꢀ1 of TiO2/CA-X catalysts
were rstly pre-adsorbed with 100 mL 2 mg Lꢀ1 DMP in dark
before irradiation to establish adsorption–desorption equi-
librium. Then photocatalytic experiments were initiated by
air bubbling and irradiation. During the experiments, 0.5 mL
of solution was sampled at preset time interval for the
Characterization
The crystalline structures of TiO2/CA-X samples were analyzed analysis of the concentration of DMP by high performance
by D8 Advance X-ray Diffractometer (XRD, Bruker, D8 liquid chromatography (HPLC, Techcomp, LC2130,
advance, Germany) with Cu-Ka radiation source at scanning Shanghai, China) equipped with a reverse phase column
rate of 10 minꢀ1. The morphologies of TiO2/CA-X samples (Waters, XT erra MS C-18, 5 mm) and a UV detector. The
were observed by transmission electron microscope (TEM, mobile phase was composed of 50% acetonitrile and 50%
JEOL, JEM-2100, Japan) on a JEM 2100F 200 kV eld emission water, and the detection wavelength was 276 nm. For
transmission electron microscope. The carbon content in comparison, pristine TiO2 and mechanical mixtures of TiO2
TiO2/CA-X samples was determined by thermogravimetry and CA-9.3 (TiO2 + CA) were also tested under the same
analysis (TG, Netzsch, TG209, Germany) in air atmosphere. condition. The amount of TiO2 was to the same as that in
The pore structures of samples were probed by N2 adsorption– TiO2/CA-X.
68422 | RSC Adv., 2016, 6, 68416–68423
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Paper
RSC Advances
21 X. H. Xia, Z. H. Jia, Y. Yu, Y. Liang, Z. Wang and L. L. Ma,
Carbon, 2007, 45, 717–721.
Acknowledgements
22 L. Gu, Z. X. Chen, C. Sun, B. Wei and X. Yu, Desalination,
2010, 263, 107–112.
23 M. F. Wu, Y. N. Jin, G. H. Zhao, M. F. Li and D. M. Li, Environ.
Sci. Technol., 2010, 44, 1780–1785.
24 D. Wang, G. H. Yang, Q. X. Ma, M. B. Wu, Y. S. Tan,
Y. Yoneyama and N. Tsubaki, ACS Catal., 2012, 2, 1958–1966.
25 H. Q. Yang, L. Zhang, L. Zhong, Q. H. Yang and C. Li, Angew.
Chem., Int. Ed., 2007, 46, 6861–6865.
The research was nancially supported by the 973 Program
(2014CB845600), NSFC (No. 21103235), NSF of Guangdong
Province (No. S2012010010775), the Key Laboratory of Fuel Cell
Technology of Guangdong Province, and the Key Laboratory of
Environmental Pollution Control and Remediation Technology
of Guangdong Province (No. 2011K0011). JZZ is grateful to the
BES Division of the US DOE for nancial support.
26 Z. J. Chen, X. H. Li and C. Li, Chin. J. Catal., 2011, 32, 155–
161.
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RSC Adv., 2016, 6, 68416–68423 | 68423