Boosting the Photocatalytic Activity of P25 for Carbon Dioxide Reduction by using a Surface-Alkalinized Titanium Carbide MXene as Cocatalyst

Article abstract of DOI:10.1002/cssc.201800083

Although they are widely used as cocatalysts in promoting photocatalysis, practical application of noble metals is limited by their high cost and rarity. Development of noble-metal-free cocatalysts is thus highly demanded. Herein titanium carbide (Ti

Full text of DOI:10.1002/cssc.201800083

Accepted Article
Title: Boosting the Photocatalytic Activity of P25 for Carbon Dioxide
Reduction using a Surface-Alkalinized Titanium Carbide MXene
as Co-catalyst
Authors: Minheng Ye, Xin Wang, Enzuo Liu, Jinhua Ye, and Defa
Wang
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Boosting the Photocatalytic Activity of P25 for Carbon Dioxide
Reduction using a Surface-Alkalinized Titanium Carbide MXene
as Co-catalyst
Minheng Ye,[a] Xin Wang,[a] Enzuo Liu,[a, b] Jinhua Ye,[a, b, c] and Defa Wang*[a, b]
Abstract: While used widely as co-catalysts in promoting
a
On the other hand, sufficient adsorption sites of CO2 molecules
are of great importance for enhancing the photocatalytic activity of
CO2 reduction, since the intimate interaction and stable binding
facilitates electron transfer from the catalytically active sites to
CO2.[4b,6] However, such adsorption sites rarely exist on noble metals.
In this context, the layered double hydroxides (LDHs) and metal-
organic frameworks (MOFs) are often chosen as the alternatives for
CO2 adsorption.[7] The question is that the unfavourable Fermi level
or limited electrical conductivity of LDHs and MOFs hinder their
application as co-catalyst. Actually, noble-metal-free co-catalysts
with high CO2 adsorption capacity have rarely been reported to date
for photocatalytic CO2 reduction.[8]
photocatalytic reaction, noble metals are suffered from the high cost
and rarity for practical applications. Development of noble-metal-free
co-catalysts is thus highly demanded. Here we report for the first
time on utilizing the titanium carbide (Ti3C2) MXene as a highly
efficient noble-metal-free co-catalyst with commercial titania (P25)
for photocatalytic CO2 reduction. In particular, surface alkalinization
of Ti3C2 dramatically enhances the activity, i.e., the evolution rates of
CO (11.74 μmol⋅g−1⋅h−1) and CH4 (16.61 μmol⋅g−1⋅h−1) are 3- and
277-fold those of bare P25, respectively. The significantly enhanced
activity is attributed to the superior electrical conductivity and
charge-carrier separation ability, as well as the abundant CO2
adsorption and activation sites of surface-alkalinized Ti3C2
MXene.Our findings provide a promising alternative to new highly-
active noble-metal-free co-catalysts for photocatalytic CO2 reduction.
MXenes, a novel type of two-dimensional (2D) materials, have
been attracting intensive attention since their discovery in 2011.[9] As
one of the most studied MXenes, Ti3C2 (TC) is usually produced
from Ti3AlC2 via a hydrofluoric acid (HF) etching method to remove
the aluminium atom layers while preserving the 2D nature of Ti3C2,
the planar surfaces of which are terminated by −O−, −OH and −F
groups.[9b] The computational work by Zhou et al[10a] studied the
effect of surface functional groups on the property of MXene.
Several recent works have demonstrated its potential in hydrogen
evolution reaction,[10b,10c] indicating that the excellent electrical
conductivity, superior structural stability and controllable surface
anchoring of functional groups might render Ti3C2 to be highly
Photocatalytic reduction of carbon dioxide into hydrocarbon
solar fuels with only solar light as the input energy source is an ideal
green technology for solar-to-chemical conversion.[1] In recent years,
a large number of research works have been reported in this field.[2]
However, photocatalytic CO2 reduction still remains fairly low
efficiency, due mainly to insufficient charge-carrier separation and
transfer, as well as poor adsorption and activation of CO2 molecules
on catalyst surface.[3] One solution to these problems is to introduce
co-catalysts, almost all of which developed so far are noble metals
such as Pt, Au, and Ag etc. The large work function of these noble
metals renders facile trapping of photo-excited electrons from the
semiconductor photocatalysts, and thus, favouring for the separation
of photo-generated electrons and holes.[2b,4] Recently, noble-metal-
contained alloys or core-shell structured bimetals are reported as the
co-catalysts, showing superior photocatalytic activity over pure noble
metal co-catalysts.[2c,5] Nevertheless, development of efficient and
noble-metal-free co-catalysts is highly necessary because the high
cost and rarity of noble metals restrict their extensive application.
promising and
distinctive noble-metal-free
co-catalyst in
photocatalytic CO2 reduction.[11] Particularly, the hydroxyl groups can
supply abundant basic sites, which may benefit for adsorption and
activation of the acidic CO2 molecules. Thus, significant
enhancement in photocatalytic CO2 reduction could be expected.
Herein, we report for the first time on coupling the Ti3C2 MXene
as co-catalyst with commercial P25 via a quite facile mechanical-
mixing method for significant enhancement of photocatalytic CO2
reduction. More interestingly, surface-alkalinization of Ti3C2 to
replace − F with − OH further improves the photocatalytic activity
dramatically. When using surface-alkalinized Ti3C2 as the co-catalyst,
the evolution rates of CO (11.74 μ mol ⋅ g − 1 ⋅ h − 1) and CH4 (16.61
μ mol ⋅ g − 1 ⋅ h − 1) increase 3 times and 277 times, respectively, in
[a] M. Ye, X. Wang, Prof. J. Ye, Prof. D. Wang
TJU−NIMS International Collaboration Laboratory, Key Laboratory of
Advanced Ceramics and Machining Technology (Ministry of Education,
China), Tianjin Key Laboratory of Composite and Functional Materials,
School of Materials Science and Engineering,
Tianjin University
comparison with the bare P25. Essentially, such
a significant
enhancement could be ascribed to the intrinsic excellent electrical
conductivity and suitable thermodynamic energy level of TC, which
facilitate the photo-excited charge-carrier separation and transfer.
Moreover, the surface − OH groups serve as the active sites for
strongly adsorbing and activating CO2 molecules.
92 Weijin Road, Tianjin 300072, China
E-mail: defawang@tju.edu.cn
Detailed synthesis processes of TC and TC-OH are described in
the Supporting Information. Typically, Ti3AlC2 particles were etched
[b] Dr. E. Liu, Prof. J. Ye, Prof. D. Wang
Collaborative InnovationCenter of Chemical Science and Engineering
(Tianjin),
92 Weijin Road, Tianjin 300072, China
[c] Prof. J. Ye
by hydrofluoric acid to obtain the TC powders. As shown in X-ray
diffraction (XRD) patterns (Figure S1), the most intense peak at 2θ ൌ
39° disappeared and the (002) and (004) peaks shifted towards a
lower angle, confirming the successful conversion of Ti3AlC2 to TC.[9a]
The scanning electron microscopic (SEM) image (Figure S2a)
showed that, as the Al atoms between layers were removed after
HF-etching, the stacking of two-dimensional TC units became loose,
International Center for Materials Nanoarchitectonics (WPI-MANA),
National Institute for Materials Science (NIMS)
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
E-mail: jinhua.ye@nims.go.jp
Supporting information for this article is given via a link at the end of the
document.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201800083
ChemSusChem
COMMUNICATION
being consistent with the decreased intensities of (002) and (004)
peaks in the XRD patterns.
Ti in TC might be overlapped. Therefore, we chose to analyze the
chemical environment of the element carbon other than titanium,
since carbon was solely contained in TC or TC-OH. The XPS
spectrum of C1s from TC (Figure S15a) could be deconvoluted into
four peaks: sp2 bonded carbon (C − C) at 284.91 eV, hydrocarbon
fragments (C − O) at 286.67 eV, carbonate species (O − C = O) at
288.91 eV, and Ti−C bond at 281.7 eV.[11d,12] Compared to those for
TC, the C1s peaks for 5TC/P25 showed an apparent shift to lower
binding energy (0.15 − 0.43 eV, see Figure S15b), confirming the
electron transfer from P25 to TC after their coupling. Similar binding
energy shift of TC-OH was also observed in the TC-OH/P25 hybrid
(Figures S16a, b). The above results clearly evidenced the effective
electronic coupling between the TC-based co-catalyst and P25.
In the photoreduction reaction, water was used as the sole
hydrogen source. As shown in Figure S19, the activity of pure P25
To further increase the surface-anchoring of −OH groups on TC,
we performed the KOH-treatment to replace −F with −OH radicals.
Considering that the Ti-F bonds become unstable in basic aqueous
solution,[11b,e] potassium hydroxide (KOH) aqueous solution was
employed for the −OH anchoring. To confirm that − F groups were
replaced by hydroxyls, we first measured the energy dispersive
spectrometer (EDS) mapping of TC-OH and found a significantly
increased oxygen signal while the fluorine element signal decreased
in comparison with the untreated TC (Figures S4, S5). The Fourier
transform infrared spectra (FTIR) of TC-OH (Figure S8) showed a
stronger stretching vibration of − OH groups (3000 to 3500 cm − 1).
Moreover, replacement of such functional groups was further
confirmed by the X-ray photoelectron spectroscopy (XPS), as the
intensity of Ti-F peak decreased significantly while the intensity of Ti-
OH peak increased after the KOH-treatment (Figures S6, S7a, b).
was fairly low (3.9 μmol⋅g−1⋅h for CO; 0.06 μmol⋅g−1⋅h for CH4).
−
−
1
1
After loading 1 wt% TC on P25, the as-formed hybrid 1TC/P25
−
exhibited an obviously increased activity (5.4 μmol⋅g− 1 ⋅h for CO;
1
−
0.47 μmol⋅g−1⋅h for CH4) under the same reaction conditions. With
1
an optimal loading amount of TC (5 wt%), the 5TC/P25 hybrid
showed the highest activities (12.6 μ mol ⋅ g − 1 ⋅ h − for CO; 1.6
1
μmol⋅g−1⋅h−1 for CH4), which were 3-fold (for CO) and 27-fold (for CH4)
higher than pure P25, respectively. The activity of 5TC/P25 was
even better than that of 5 wt% Pt-loaded P25 (5Pt/P25). It should be
noted that while pure TC could absorb a wide wavelength range of
light (Figure S12a), TC itself showed almost no activity for CO2
reduction. Further increasing the loading amount of TC (e.g.,
10TC/P25) decreased the activity because the excess TC would
shell P25 from light irradiation. The above results demonstrated
clearly the significant role of TC as an effective co-catalyst in
improving the photocatalytic activity of TC/P25 for CO2 reduction.
Figure 1.(a) XRD pattern of the Ti3AlC2, TC, TC-OH particles and TC-
OH/P25 hybrid;(b) UV − Vis spectra of the P25, 5TC/P25 and 5TC-
OH/P25 samples; (c) Typical SEM image of the 5TC-OH/P25 hybrid; and
(d) HRTEM image of the interface structure of TC-OH and P25 NPs.
TC/P25 and TC-OH/P25 hybrids were obtained simply through
a facile mechanical mixing method. From the XRD pattern of TC-
OH/P25 as shown in Figure 1a, no characteristic diffraction peaks of
TC-OH was found due to its relatively low mass content and weak
crystallization. The diffraction peaks were indexed as the anatase
(PDF#21-1272) and rutile titania (PDF#21-1276), being consistent
with the dual-phase nature of P25. Mixing with TC or TC-OH (Figure
S11b) did not change the phase composition and the crystal
structure of P25. Transmission electron microscopic (TEM) image
(Figure 1c) showed that P25 nanoparticles were loaded on the 2D
TC-OH. From the magnified HRTEM image as shown in Figure 1d,
the distances of neighbouring lattice fringes were estimated to be ca.
0.352 nm and 1.0 nm, corresponding to the (101) planar space of
anatase TiO2 and (002) planar space of hexagonal TC-OH,
respectively. Such an intimate interaction undoubtedly favours the
formation of junction between P25 and TC-OH.
Figure 2. (a) Gas evolution (CO and CH4) over the 5TC-OH/P25 as a
function of irradiation time under a 300 W Xe lamp; (b) Evolution rates of
CO and CH4 over P25, 5Pt/P25, 5TC/P25, 5TC-OH/P25 under irradiation
of a 300 W Xe lamp; (c) Transient photocurrent responses; and (d) EIS
Nyquist plots of P25, 5TC/P25 and 5TC-OH/P25.
As for the TC-OH/P25 photocatalyst, in which TC was
decorated by hydroxyl groups, the activity was further enhanced. As
shown in Figure 2b, the activities of 5TC-OH/P25 were 3-fold (11.74
High-resolution X-ray photoelectron spectroscopy (HR-XPS)
was employed to further analyze the interfacial electronic interaction
between TC (TC-OH) and P25. Since the amount of TC or TC-OH in
the TC (TC-OH)/P25 hybrid was small (5 wt%) and the element
titanium is contained in both TC and P25, the characteristic peaks of
−
−
−
1
μmol⋅g−1⋅h 1 for CO) and 277-fold (16.61 μmol⋅g 1⋅h for CH4) higher
than those of bare P25, respectively. Using a band-pass filter (λ0
=
380 nm), the quantum yields (QY) over 5TC-OH/P25 were measured
to be 0.32% and 1.61%, for CO and CH4, respectively. In contrast to
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10.1002/cssc.201800083
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5TC/P25, over which the main catalytic product was CO, the
hydroxyl-decorated 5TC-OH/P25 showed an increased selectivity for
CH4 evolution (58.5%). The enhanced selectivity for CH4 is
particularly interesting as CH4 is the final C1 product of CO2
reduction. Kinetically, the process of CO2 reduction into CH4 involves
8e−/H+, which is much more difficult to realize than the CO evolution
involving only 2e− /H+ .The activity of 5TC-OH/P25 was even better
than that of P25 loaded with 5 wt% Pt (5Pt/P25) or some other
to cut off the light with wavelength shorter than 390 nm, the 5TC/P25
or 5TC-OH/P25 showed almost no activity. These results suggested
that the slightly enhanced light absorbance from TC or TC-OH was
unlikely to be used by P25 for the enhanced activity of 5TC/P25 or
5TC-OH/P25. To get a deep insight into the band structures of P25,
5TC/P25 and 5TC-OH/P25, the valence band (VB) XPS spectra
were measured and no difference in the valence band maximum
(VBM) of these three samples was found. Along with the UV − Vis
absorption analysis, we could reasonably judge that the conduction
band minimum (CBM) of P25 was not changed after coupling with
TC or TC-OH.
typical cocatalysts such as Au,[8a] reduced grapheme,[4a, b] and Pt-Cu
[5d]
bimetal
(Figure S29). We did not detect the molecule O2 but
another form of oxidized product O2−, which could be evidenced by a
dramatically enhanced signal of O2− in ESR (electron spin resonance,
Figure S30)[13a] and HR-XPS measurements (Table S3). Actually,
this phenomenon was often observed in photocatalytic CO2
reduction over TiO2.[13b] It needs to point out that, experimentally, we
did not detect the H2O2 that might be formed from O2 −. In the long-
term test for CO2 photoreduction over 5TC-OH/P25 (Figure S23),
only a slight decrease in catalytic activity and stability was found in
the initial stage, probably due to the newly formed O2−. We think that
We also conducted a series of characterizations including the
steady-state photoluminescence (PL) spectra, transient photocurrent
(TPC) response and electrochemical impedance spectra (EIS) to
unravel the charge separation and transfer process. The PL
intensities (Figure S25) were found to decrease after coupling with
TC or TC-OH, indicating an improvement of charge separation
efficiency. The TPC responses of 5TC/P25 and 5TC-OH/P25 were
much higher than that of pure P25 (Figure 2c), giving one more solid
evidence for the enhanced charge carrier separation. In addition, the
EIS results clearly showed that the semicircle diameters and
interfacial charge transfer resistances of the 5TC/P25 and 5TC-
OH/P25 were smaller than that of P25 (Figure 2d), which further
suggested an improvement of interfacial charge transfer after the
coupling with TC or TC-OH as co-catalyst. A previous calculation
work showed that the −OH groups on TC might lead to decrease of
work function,[16] which was not beneficial for capturing the photo-
electrons. However, such a work function fluctuation had only a
small impact on the charge separation capacity, as evidenced by the
quite close photoelectrochemical performances of TC/P25 and TC-
OH/P25 (Figure 2c). As will be discussed below, the increased CO2
adsorption ability by the −OH groups on TC played a more important
role in the significantly enhanced activity of TC-OH/P25.
−
the evolved O2 was just the minority oxidation product while O2
should be the majority one, which was adsorbed on the catalyst
surface and/or the interior surface of the reaction cell.
It should be noted that the pure TC-OH showed no activity for
CO2 photoreduction. Moreover, while keeping other conditions
identical, no CO and/or CH4 could be detected when the CO2 gas
was replaced by Ar gas (Table S1). These results undoubtedly
excluded the possibility that the high yields of CO and/or CH4 were
from the carbon contained in TC or the adsorbed impurities on
TiO2.[14] When the reaction was performed in dark (Figure S24) or in
the absence of 5TC-OH/P25, neither CO nor CH4 was detected,
indicating that the light and 5TC-OH/P25 were both necessary for
the photocatalytic reduction of CO2.
To investigate the mechanism of photocatalytic CO2 reduction
on TC-OH/P25 photocatalyst, it is necessary to make clear whether
or not the significantly enhanced activity was resulted from the
residual alkali after the KOH-treatment. According to some previous
reports, alkalinization treatment or direct mixing with alkali might
influence the photocatalytic CO2 reduction activity or product
selectivity.[15] In our experiment (Figure S21), the KOH-treated P25
(P25-OH), or the P25-OH further coupled with TC (5TC/P25-OH)
only showed a slight enhancement in the photoreduction activity with
a low selectivity for CH4 (~9%). In contrast, once coupled with 5 wt%
of TC-OH as the co-catalyst, the as-obtained hybrids (5TC-OH/P25,
5TC-OH/P25-OH) exhibited significant enhancement in both the
photoreduction activity and the selectivity for CH4 (Table S1).
Figure 3. (a) CO2 adsorption behaviours for TC and TC−OH; side views
of the adsorption models of CO2 on (b) 2 ×2 ×1 TC−F and (c) 2× 2 ×1
TC−OH supercells.
Moreover, KOH-treatment of the 5TC/P25 hybrid as
a whole
((5TC/P25)-OH), a high selectivity for CH4 could also be obtained.
However, while the mixture 5KOH/P25 showed an enhanced activity
to some extent, the selectivity for CH4 (3.3%) was very low (Figure
S22). The above experimental results indicated the important role of
TC-OH in achieving a high activity and selectivity for CH4, ruling out
the impacts from the residual KOH.
In terms of thermodynamics, the CO2 molecule is extremely
stable owing to the large dissociation energy (~750 kJ⋅mol−1) of C=O
bond. Nonetheless, once adsorbed on the surface of a photocatalyst,
δ−
the CO2 molecule often tends to be partially charged as CO2 upon
light irradiation, and this adsorbate can be easily reduced due to the
loss of linear symmetry.[1a] Thus, the barrier for CO2 to accept
electrons is reduced because the energy level of the lowest
unoccupied molecular orbital (LUMO) of CO2 decreases after losing
the linear symmetry of CO2 molecule. In addition, the basic sites are
generally the active sites for adsorbing the CO2 molecules with
acidic nature.[1c] Figure 3a shows the CO2 adsorption behaviours of
TC and TC-OH at room temperature. The maximum CO2 uptake for
It is well known that the band structure, charge mobility, CO2
adsorption and activation are generally crucial factors governing the
photocatalytic CO2 conversion efficiency. To verify which factor
played the most important role in the significant enhancement of TC-
OH/P25 for photocatalytic CO2 reduction, we carried out various
characterizations on P25, 5TC/P25 and 5TC-OH/P25. Shown in
Figure 1b is the UV−Vis diffuse reflectance spectra of P25, 5TC/P25
and 5TC-OH/P25. No obvious shift of the absorption edge (∼390 nm)
was observed among these three samples while the coupling of TC
or TC-OH slightly increased the absorbance. When a filter was used
−
−
1
1
the TC and TC-OH samples were 0.18 cm3 ⋅g and 7.01 cm3 ⋅g
,
indicating that the CO2 molecule could be adsorbed on TC-OH
surface much more easily. Based on the density functional theory
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10.1002/cssc.201800083
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(DFT), the adsorption energy (Ead) of CO2 on TC-F (TC-F … CO2)
was calculated to be −0.13 eV (Figure 3b), which was much higher
than that (−0.44 eV, Figure 3c) of CO2 on TC-OH (TC-OH…CO2).
Apparently, the higher CO2 adsorption capability of TC-OH could be
attributed to the lower adsorption energy of CO2 on TC-OH. The
modifying the methods for surface-functionalizing of Ti3C2 and their
effective coupling with photocatalysts. We believe that this work
could shed a light on developing highly efficient noble-metal-free co-
catalysts for artificial photosynthesis.
FTIR spectra (Figure S26a) showed that the absorption bands of Acknowledgements
5TC/P25 were a little different before and after reaction. However,
the strong absorption bands of various carbonate species such as
νs(CO3) (~1320 cm−1) and νas(CO3) (~1580 cm−1) were emerged on
5TC-OH/P25 after irradiation (Figure S26b), indicating that the up-
taken CO2 was effectively converted to the easily activated form of
The authors are grateful to Dr. Jian Ren for critical reading of the
manuscript and valuable discussion. We would also like to thank Dr.
Hong Pang and Dr. Yunxiang Li for their help with the CO2
adsorption measurement. This work was supported by the National
Natural Science Foundation of China (51572191, 21633004), and
the National Basic Research Project of China (973, 2014CB239300).
CO3 − . Moreover, HR-XPS analysis showed that the characteristic
2
peak of carbonates (O−C=O) was shifted from 288.71 eV to a lower
binding energy 288.26 eV (Figures S16b, c). In contrast, no obvious
shift of the binding energy (O − C = O) was found in the 5TC/P25
before and after reaction (Figure S15b, c). These results implied a
close interaction between the electron-rich TC-OH surface and
adsorbed carbonate species.
Conflict of interest
The authors declare no conflict of interest.
Based on above experimental results, a mechanism illustrating the Keywords:CO2 photoreduction •co-catalyst •heterogeneous
outstanding activity and high selectivity for CH4 of TC-OH/P25 is catalysis• MXenes• photocatalysis
proposed in Figure 4. Upon light irradiation, electrons are excited
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(Ef) of TC-OH is much lower than that of P25, the photo electrons
migrate from the CB of P25 to TC-OH, thereby electrons and holes
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are reduced by electrons into CO and CH4 (Figure S28). Here, the
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adsorption and activation of CO2 molecules. The enrichment of
electrons, strong interaction between the TC-OH co-catalyst and
CO2 molecules, all facilitate the multiple proton/electron transfer,
thereby the photocatalytic activity of CO2 reduction to the final C1
product of CH4 is boosted significantly.
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Figure 4. Illustration of the photocatalytic CO2 reduction mechanism of
TC-OH/P25 under irradiation, due to the mixed phase of P25, here we
utilize the anatase titania structure as a model for P25.
In summary, we succeeded in remarkably boosting the activity
of P25 for photocatalytic CO2 reduction by using Ti3C2 MXene as the
noble-metal-free cocatalyst. Particularly, the surface-alkalinization of
Ti3C2 further enhanced the photocatalytic activity and selectivity for
CH4 evolution dramatically. The excellent photocatalytic activity was
attributed to the synergistic effects of superior electrical conductivity
and CO2 adsorption/activation ability of Ti3C2−OH. A systematic work
is underway in coupling the Ti3C2 MXene-based co-catalyst with
other semiconductor photocatalysts. Further improvement in the
activity of photocatalytic CO2 reduction could be expected by
properly reducing the size and/or layer number of Ti3C2 together with
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Ti3C2 MXene is used as co-catalyst for
Minheng Ye, Xin Wang, Enzuo Liu,
Jinhua Ye, Defa Wang*
the
first
time
to
boost
the
photocatalytic activity of titania (P25)
for CO2 reduction into hydrocarbon
solar fuels. In particular, surface
alkalinization of Ti3C2 dramatically
enhances the activity, and especially
the selectivity for CH4 evolution.The
high intrinsic electrical conductivity,
Page No. – Page No.
Boosting the Photocatalytic Activity
of P25 for Carbon Dioxide Reduction
using a Surface-Alkalinized Titanium
Carbide MXene as Co-catalyst.
strong
CO2
adsorption/activation
capability of the hydroxyl-terminated
Ti3C2 MXene could account for the
significantly enhanced photocatalytic
activity.
This article is protected by copyright. All rights reserved.