Photoelectrocatalytic CO2 reduction to ethanol via graphite-supported and functionalized TiO2 nanowires photocathode

Article abstract of DOI:10.1016/j.jphotochem.2020.112368

The photoelectrocatalytic CO₂ reduction into value-added carbon-based energetic molecules is a promising strategy to store solar energy into chemicals, which lessen the concentration of atmospheric contamination. In this study, the graphitic supported and multiple functionalized nanowire TiO₂ semiconductor was firstly used to CO₂ reduction with water under photoelectrocatalytic conditions. These photocathodes of R-TiO₂?GS were designed to be functionalized by organic ligands that catch CO₂ and control the C-C coupling as the Calvin cycle in natural plants. These new organic–inorganic composite electrodes were facile to be prepared and well characterized by using NMR, UV–vis, FTIR, PL, TRPL, EIS and XPS spectra; XRD patterns; SEM, TEM images. The transient absorption spectra of photocathodes demonstrate the efficiency of electron transfer between the Eosin Y and semiconductor, ensuring CO₂ reduction. Their Mott–Schottky plots show that the flat band potentials are improved by organic ligands, favoring the ethanol production. The light quantum efficiency of the best photoelectrocatalytic cell of S-TiO₂?GS│SCE│Pt reaches to 1.0 % that is 2 times better than natural plant. To better understand the working process of the photoelectrocatalytic cell, a plausible mechanism of CO₂ reduction in water was carefully proposed as well.

Full text of DOI:10.1016/j.jphotochem.2020.112368

JournalofPhotochemistry&PhotobiologyA:Chemistry391(2020)112368
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photoelectrocatalytic CO2 reduction to ethanol via graphite-supported and
functionalized TiO2 nanowires photocathode
Liwen Wanga,1, Yan Weia,1, Ran Fanga,*, Jinyuan Wanga, Xiaogang Yua, Jiazang Chenb,
Huanwang Jinga,b,
*
a State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, 222 South Tianshui Road, Lanzhou, 730000, PR
China
b State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan, 030001, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The photoelectrocatalytic CO2 reduction into value-added carbon-based energetic molecules is a promising
strategy to store solar energy into chemicals, which lessen the concentration of atmospheric contamination. In
this study, the graphitic supported and multiple functionalized nanowire TiO2 semiconductor was firstly used to
CO2 reduction with water under photoelectrocatalytic conditions. These photocathodes of R-TiO2@GS were
designed to be functionalized by organic ligands that catch CO2 and control the C-C coupling as the Calvin cycle
in natural plants. These new organic–inorganic composite electrodes were facile to be prepared and well
characterized by using NMR, UV–vis, FTIR, PL, TRPL, EIS and XPS spectra; XRD patterns; SEM, TEM images. The
transient absorption spectra of photocathodes demonstrate the efficiency of electron transfer between the Eosin
Y and semiconductor, ensuring CO2 reduction. Their Mott–Schottky plots show that the flat band potentials are
improved by organic ligands, favoring the ethanol production. The light quantum efficiency of the best pho-
toelectrocatalytic cell of S-TiO2@GS│SCE│Pt reaches to 1.0 % that is 2 times better than natural plant. To better
understand the working process of the photoelectrocatalytic cell, a plausible mechanism of CO2 reduction in
water was carefully proposed as well.
Photoelectrocatalysis
CO2 reduction
TiO2
C-C coupling control
Nanowire structure
1. Introduction
reported by using semiconductors as the heterogeneous catalysts, in
which the major issue is the recombination of excitons. [26] The
Global warming caused by the massive consumption of fossil fuels
has become the most serious environmental issue. [1] One of the ef-
fective methodology to suppress this problem is carbon dioxide (CO2)
reduction that has received more and more attention for its resourceful
property. [2–10] In 1972, Honda et al. discovered that the TiO2 semi-
conductor could split water to H2 and O2 under photoelectrocatalytic
conditions that might be a promise methodology to store solar energy to
chemical fuels. [11] In 1978, p-type GaP was developed to decompose
water and reduce CO2 under photoelectrocatalytic conditions by Hal-
mann. [12] Therefrom, the semiconductors have been widely in-
vestigated and exhibit excellent thermal, mechanical, photosensitive,
and electrical properties and have been applied to water splitting and
CO2 reduction via electrocatalysis, [13–19] photocatalysis [20–25] and
photoelectrocatalysis. Among these strategies, there are various ad-
vantages and drawbacks. The photocatalytic CO2 reduction has been
electrocatalytic CO2 reduction is also widely investigated by different
catalysts, normally yielding C1 compounds like formate, CO and CH4
[8,27]. The photoelectrocatalysis is of the most promising strategy to
synthesize hydrocarbons via CO2 reduction since it can be regarded as
an artificial model for mimicking photosynthesis under lower mem-
brane voltage. [28,29] However, the selectivity of CO2 reduction to-
wards CeC coupling products over semiconductor electrodes is still low
owing to preferential H2 production. [30,31]
Semiconductors-catalyzed CO2 reduction has been considered pri-
marily a photocatalytic process, in which, the surface properties of
semiconductors are very important. Modifying the surface by various
methods such as doping, [2] metal deposition [28], and functional
groups anchoring [29] were commonly used to enhance the perfor-
mance of substrate. Apart from the number of reactive groups, the
precise molecular structure of the silane also influences the stability of
⁎ Corresponding authors at: State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, 222 South
Tianshui Road, Lanzhou, 730000, PR China.
E-mail addresses: fangr@lzu.edu.cn (R. Fang), hwjing@lzu.edu.cn (H. Jing).
1 These authors given equally contribution to this work.
https://doi.org/10.1016/j.jphotochem.2020.112368
Received 24 August 2019; Received in revised form 21 November 2019; Accepted 9 January 2020
Availableonline10January2020
1010-6030/©2020ElsevierB.V.Allrightsreserved.

L. Wang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry391(2020)112368
50 mL) under reflux for 12 h, respectively. After drying in vacuum at
80 °C, the organic ligand-modified and GS-supported electrodes were
successfully generated and nominated as S-TiO2@GS and B-TiO2@GS,
respectively.
2.2. Characterizations of photocathodes
The field-emission scanning electron microscope (FESEM, JSM-
6701 F) and transmission electron microscopy (TEM, Tecnai-G20-F30)
was used to observe the morphology of TiO2@GS, S-TiO2@GS and B-
TiO2@GS. X-ray diffraction (XRD, D/max-2400 Rigaku) patterns were
collected using Cu-Kα radiation (λ =0.1541 nm) with 2θ in the range
of 10° and 80°. Fourier transform infrared (FTIR) spectra were recorded
on a Nicolet NEXUS 670 instrument. The X-ray photoelectron spectro-
scopy (XPS) was recorded by the XPS spectrometer (Kratos AXIS Ultra
DLD) with an Mg Kα X-ray resource. The solid-state UV–vis absorption
spectra were conducted on UV–vis spectrophotometer (UV-2600,
Shimadzu). The steady state/transient PL spectra and the corresponding
time-resolved photoluminescence spectra were measured by a steady
state/transient photoluminescence spectrophotometer (FLS920,
Edinburgh Instruments).
The linear sweep voltammetry (LSV) curves of the photoelec-
trocatalytic cells were measured in a three-electrode system on an
electrochemical workstation (CHI660E). In these cells, R-TiO2@GS
electrodes served as a working electrode; the Pt sheet was employed as
a counter electrode and a saturated calomel electrode (SCE) was used as
a reference electrode. The LSV experiments were conducted in the same
electrolyte of CO2 reduction experiment under simulated sunlight ir-
radiation or dark condition. The electrochemical impedance spectra
(EIS) and the Mott–Schottky plots were both illustrated by the elec-
trochemical workstation under EC conditions. On the other hand, the
transient photocurrent responses were detected at -0.9 V with a two-
electrode system.
Scheme 1. Schematic diagram for the PEC cell of CO2 reduction.
its monolayer, as shown by the Linford’s group [32], when the Si-OR
groups of ligands react with the hydroxyl groups of TiO2 surfaces, the
anchored groups are quite stable and improve the catalytic properties of
TiO2. [33–35]
According to our previous investigations, [29,36–40] the nano-
sheets of graphite were used to support and activate TiO2 semi-
conductor in CO2 reduction due to their advantages, such as good
conductivity and mesoporous structure. [41–45] Herein, we report a
new photoelectrochemical cell (Scheme 1) composed of the graphite-
supported TiO2 nanowires modified by 3-aminopropyl-triethoxysilane
(APTES) and aldehydes (salicylaldehyde and benzaldehyde) as the
working electrode, a platinum plate as the counter electrode and Hg
(l)|Hg2Cl2 (s)|KCl (SCE) as reference electrode.
2. Experimental section
2.3. Photoelectrocatalytic CO2 reduction experiments
2.1. Preparation of multi-functionalized photocathodes
All photoelectrocatalytic (PEC) CO2 reduction experiments were
carried out in a closed quartz PEC cells of 120 mL, equipped with three
electrodes and charges with electrolyte of aqueous solution (60 mL,
0.1 M potassium bicarbonate and 0.01 M Eosin Y) that was saturated
with pure CO2 or 13CO2. In these new PEC cells, there are no membrane
between the photocathode and photoanode; and the R-TiO2@GS elec-
trodes were served as the working electrode. The PEC reduction of CO2
was performed on the PEC cell under simulated sunlight irradiation and
an appropriate external voltage. Generally, the irradiation time was 4 h,
and the CO2 gas was refilled again after each 2 h. For each photo-
cathode, the formation rate of organic compounds was average of two
to four hours measurement according to the integrated area of 1H NMR
spectra based on the standard of 700 μL sample of electrolyte with 35 μL
deuterium oxide (D2O) containing dimethyl sulfoxide (10 mM) and
phloroglucinol (50 mM) using a water decoupling technique. The
Xenon lamp (solar simulator, AM 1.5 G, PLS-SXE300/300UV) with a
light intensity of 200 mW/cm2 was used as light source. 1H NMR
spectra were recorded on a NMR spectrometer (JNM-ECS 400 M, JOEL).
The production rates of hydrogen and oxygen were quantified by gas
chromatography (Varian CP-3800).
2.1.1. General
All chemicals were purchased from chemical reagent companies and
used without further purification. Typically, Eosin Y, potassium bi-
carbonate (KHCO3), salicylaldehyde, benzaldehyde, sodium hydroxide,
3-aminopropyl triethoxy silane, tetrabutyl titanate, and 3-aminopropyl-
triethoxysilane were ordered from J&K Chemical Company.
2.1.2. Preparation of the TiO2@GS photocathodes
Tetrabutoxytitanium (20 mL) and diethanolamine (5 mL) were
mixed in ethanol (50 mL) that was allowed to stir for 1 h at room
temperature. To the obtained mixture, a solution of ethanol (2 mL)-
deionized water (1 mL) was added, followed by 2 h additional stirring
and then stood for at least 24 h at room temperature to form TiO2 sol-
gel. Graphite sheets (GS, 1.4 × 1.4 cm2) were immersed into the sol-gel
for 15 min, taken out for 5 min and then immersed again in sol-gel for
3–5 times to form a layer with proper thickness. After annealing these
graphite sheets soaked the sol-gel at 550 °C for 45 min in the muffle
furnace at a heating rate of 5 °C/min, a TiO2 film with nanowire
structure on the graphitic sheet was successfully grown.
2.1.3. Fabrication of the R-TiO2@GS photocathodes
3. Results and discussion
The as-prepared sheets of nanowire TiO2@GS were firstly immersed
into 0.1 M NaOH for 30 min. This procedure was to open the hydroxyl
groups on the nanowires TiO2. After drying in vacuum at 80 °C, these
basic treated GS sheets were then modified by APTES in ethanol (5 % v/
v, 50 mL) under reflux for 1 h. The amine groups were covalently
bonded onto the surfaces of TiO2. To enlarge the conjugated organic
system, the salicylaldehyde or benzaldehyde (2.5 mL) was condensed
with amine groups of TiO2@GS sheets in an ethanol solution (5 % v/v,
3.1. Fabrication and physicochemical properties
Primarily, the SEM images of substrate showed that the graphitic
sheets have a porous structure (Fig. 1a). After growing TiO2 onto the
surface of GS, the morphology of TiO2@GS electrode in SEM image
(Fig. 1b) demonstrates nanowires. On the other hand, these nanowires
could be observed in the cross-section of GS image (Fig. 1c). The
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JournalofPhotochemistry&PhotobiologyA:Chemistry391(2020)112368
Fig. 1. SEM images: (a) GS; (b, c, d) TiO2@GS electrodes.
thickness of TiO2 layer grown on working electrode is about 40 μm
(Fig. 1d). This 3D-dimention structure can construct a convenient en-
vironment as thylakoid in plant that is beneficial for the CeC coupling
reaction. The BET surfaces were measured and showed smaller values
after modification of organic ligands. It was reduced from 11.81 m2/g
of TiO2 nanowires to 6.03 m2/g of B-TiO2 and 4.61 m2/g S-TiO2 pho-
tocathode. After reaction, the morphology of TiO2 nanowires is not
changed (Fig. S16).
The XRD pattern of TiO2@GS photocathode was depicted in Fig. 2a,
in which, most of diffraction peaks can be assigned to anatase titanium
dioxide (PDF No. 21-1272). [18] The diffraction peaks at 25.2°, 48.3°
and 62.7° are assigned to the (101), (200) and (213) lattice planes,
respectively. The GS phase was attributed to the graphite-2H (PDF No.
41-1487). The diffraction peaks at 26.3°, 44.3° are assigned to the dif-
fraction of (002), (101) lattice planes. The XRD pattern of S-TiO2@GS
photocathodes is similar to TiO2@GS, as shown in the support in-
formation (Fig. S1). After reaction, all diffraction peaks of TiO2 were
the same compared with the original electrodes (Fig. S17, S18). It can
be seen that the diffraction signals of B-TiO2 electrodes are weakened
due to the partial loss of TiO2. However, the diffraction peaks of S-
TiO2@GS electrode are still stronger, showing better stability. In order
to further confirm the successful modification of organic ligands, FTIR
spectra of electrodes were also collected and illustrated in Fig. 2b. The
absorption peaks are assigned to νO-H
= 3418, νC-H = 2923,
νC=N = 1635, νAr-ring = 1460, νC-C = 1385, νSi-O = 1120 and νTi-
O = 562 cm−1, respectively. Therefore, the covalent bands are formed
between APTES and the hydroxyl groups of TiO2 surface and the imine
bands are built up between amine groups and aldehydes.
To further prove the modification of organic ligand, XPS spectra of C
1s, O 1s, Ti 2p, and Si 2p were detected and shown in Fig. 3, S6 and S7.
The binding energies of each sample were calibrated by the C 1s peak at
284.6 eV. The peaks of 287.8 and 285.9 eV can be assigned to the or-
bitals of C]C and CeO (Fig. 3a), respectively. Two peaks of O 1s were
clearly observed at 529.8 and 532.9 eV that can be assigned to OH
groups on the surfaces of TiO2 (Fig. 3b). Two distinctive peaks observed
at 458.5 and 464.2 eV were attributed to the 2p3/2 and 2p1/2 orbitals
(atomic terms) of Ti(IV) (Fig. 3c).The binding energy peak of Si 2p
(Fig. 3d) is appeared at 102.6 eV. [36]
Fig. 2. (a) XRD pattern of TiO2@GS; (b) FTIR spectra of different electrodes.
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JournalofPhotochemistry&PhotobiologyA:Chemistry391(2020)112368
Fig. 3. XPS spectra of S-TiO2/GS electrode.
3.2. Photoelectrocatalytic activity toward CO2 reduction
The linear sweep voltammetry (LSV) and cyclic voltammetry (CV) of
S-TiO2@GS and B-TiO2@GS electrodes were recorded on a CHI660E
electrochemical workstation with a Pt plate as counter electrode and
saturated calomel electrode (SCE) as reference electrode. The Xenon
lamp (solar simulator, AM 1.5 G, PLS-SXE300/300UV) with a light in-
tensity of 200 mW/cm2 was used as light source. Fig. 5a showed that
the reduction peak is obviously apparent in the range of −0.9 ∼ −
1.3 V for S-TiO2@GS electrode under CO2 atmosphere compared with
an unapparent peak for B-TiO2@GS electrode, indicating the more ac-
tivity of S-TiO2@GS electrode in CO2 reduction. In contrast, there are
no reduction peak in the LSV curves under Ar atmosphere. Their CV
spectra were recorded under the bias potential in the range of 0.0 ∼ −
1.8 V and shown in Fig. 5c, d. The current densities of the photoelec-
trocatalytic (PEC) cells in the range of − 0.2 ∼ − 1.8 V were larger
under light irradiation than under dark conditions because of the
photoelectrons generated by dye molecules of Eosin Y. On the other
hand, when the bias potential between the working and reference
electrode was set between 0.0 to − 0.2 V, the current densities were
positive, reflecting that the potential of working electrode is still higher
than that of anode Pt.
The photoelectrochemical performance of the photocathodes was
displayed in supporting information. The transient photocurrent re-
sponse of the photocathode was tested at −0.9 V (vs. SCE) in a two-
electrode system and illustrated in Fig S19. Concretely, when the light
was turned on, the photocurrent of the photocathodes increased
sharply. It can be found that the modified TiO2 electrodes have better
photocatalytic performance in comparison with the bare TiO2. The S-
TiO2@GS electrode exhibits the highest photocurrent value, which
could be attributed to the high separation efficiency of electron-hole
pairs.
Fig. 4. The solid UV–Vis spectra: (a) S-TiO2@GS electrode (red), (b) B-
TiO2@GS electrode (black), (c) TiO2@GS electrode (blue).
The solid UV–vis absorption spectra of photocathodes are shown in
Fig. 4. The absorbance of S-TiO2@GS is stronger than that of TiO2@GS
and B-TiO2@GS. These results prove that the organic covalent ligands
increase the absorption intensity and cause a slightly red shift com-
pared with TiO2@GS electrode. The absorption edge of S-TiO2@GS is
enlarged to 500 nm owing to aromatic groups of organic ligand. The
band gaps of semiconductors of electrodes were acquired by intersec-
tions between UV–vis spectra of electrodes and the X axis, being 2.9 and
3.15 eV for S-TiO2@GS and B-TiO2@GS respectively. The narrow band-
gap of photocathodes tuned by organic ligands is benefit of the visible
light absorption.
The interfacial charge transfers between the photocathodes and
electrolyte were measured by the electrochemical impedance
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L. Wang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry391(2020)112368
Fig. 5. Linear sweep voltammetry curves: (a) S-TiO2@GS, (b) B-TiO2@GS; Cyclic voltammetry without CO2: (c) S-TiO2@GS, (d) B-TiO2@GS.
spectroscopy (EIS) in a CO2 saturated KHCO3 aqueous solution under
dark conditions. As shown in Fig S20, the semicircle of intermediate
frequency of S-TiO2 electrode is the smallest, indicating the weakest
resistance between this modified electrode and the electrolyte and the
highest electron transfer. [39]
To investigate the separation efficiency of photogenerated electron-
hole pairs in the prepared catalysts, their photoluminescence (PL)
spectra were measured under an excitation wavelength of 320 nm. As
shown in Fig. S21, the synthesized catalyst exhibited photo-
luminescence at about 460 and 520 nm, which could be contributed
from the surface defect emission of anatase TiO2 [46]. Compared with
pure TiO2, the emission process of TiO2 electrodes modified by the li-
gand was obviously inhibited, which indicated a good transmit of
photogenerated electrons and reduced the recombination rate of pho-
togenerated carriers.
The above results were further confirmed by the corresponding
time-resolved photoluminescence spectra. As shown in Fig. S22 and
Table S3, the PL lifetime of TiO2 after modification was significantly
reduced from 1.28 ns to 0.92 ns. The short PL lifetime indicated that the
modification of ligands accelerated the electron transfer from TiO2 to
GS [47]. Thus, the probability of rapid recombination of electron-hole
pairs would be small, which leads to weak photoluminescence and is
consistent with PL results. These photoelectrons transferred from TiO2
to GS substrate can be captured by the protons on GS to produce H
atoms that reduce immediately CO2.
The performance of S-TiO2@GS and B-TiO2@GS electrodes for PEC
CO2 reduction was further examined by maintaining constant-potential
at −0.5, −0.7, and −0.9 V (vs. SCE), respectively (Fig. 6, Table S1,
S2). The main products were detected as acetic acid, formic acid, me-
thanol and ethanol for both working electrodes. The control experiment
was also carried out by using GS as working electrode generating
nothing in CO2 reduction. Interestingly, S-TiO2@GS and B-TiO2@GS
electrodes can convert CO2 to chemicals under low potential of −0.5 V
(vs. SCE). The advantages of these PEC cells of S-TiO2@GS|SCE|Pt are of
good selectivity for CeC coupling, especially for ethanol generation
with a production rate of 20.7 μM h−1 at −0.9 V (vs. SCE). The light
quantum efficiency of this cell reaches up to 1.0 % that is 2 times better
than plants. As show in Fig. 6b, the production rate of formic acid for S-
TiO2@GS is larger than that of B-TiO2@GS in the range of −0.5 ∼
−0.9 V (vs. SCE). These results suggest the performance of S-TiO2@GS
electrode for photoelectrocatalytic CO2 reduction is superior to B-
TiO2@GS electrode due to the assistance of hydroxyl groups of ligands.
3.3. The stability of PEC cell
To test the stability of photocathode of S-TiO2@GS, the concentra-
tions of products were detected each 2 h during 6 h reaction time
(Fig. 7). It can be seen that the TER of products grown linearly, im-
plying no decay of photocathodes in catalytic reaction. The photo-
current density of PEC cell of S-TiO2@GS││Pt seldom decreased during
8 h irradiation (Fig. S14). It revealed that the multi-functionalized
TiO2@GS photocathodes were quite stable.
3.4. The Energy levels of PEC cell
To estimate the CB band potential of these organic-inorganic com-
posites of electrodes, Mott–Schottky (MS) plots were gained under
working conditions of 1.0 kHz in the dark. The CB positions of S-
TiO2@GS and B-TiO2@GS (Fig. 8a,b) show positive linear slopes, in-
dicating that the electrodes behave as n-type semiconductors. Ac-
cording to the principle of thermodynamics, the ability of semi-
conductors to transfer photogenerated charge carriers to adsorbed
molecules is governed by the alignment of the Fermi levels that as-
sumed to locate very near the conduction band edges in n-type semi-
conductors such as TiO2. Therefore, the flat band potential (Efb) can be
determined as an x-axis point of intersection. With this understanding,
organic ligands as electron donor could inject electrons to Fermi levels
and were linked to the surfaces of semiconductor of TiO2. The Efb are
calculated according to the following Eq. (1)
E (vs. NHE) = Efb (vs. SCE) + 0.24 V
(1)
As expectation, the derived Efb potentials of S-TiO2@GS and B-
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JournalofPhotochemistry&PhotobiologyA:Chemistry391(2020)112368
Fig. 6. Production rates: (a) ethanol and methanol, (b) formic and acetic acid.
Fig. 8. Mott–Schottky plots: (a) S-TiO2@GS, (b) B-TiO2@GS; (c) The band gaps:
TiO2, S-TiO2@GS, and B-TiO2@GS.
B-imine group and Eosin Y were fully computerized using a hybrid DFT
method at the level of B3LYP/6−31 G(d)/LANL2DZ (Fig. S9). Their
molecular orbital diagrams of HOMOs and LUMOs are exhibited in Fig.
S10. The conduction band of TiO2 is located between the LUMO and
HOMO of dyes and organic molecules. These results confirm that the
propulsion of electron injection from LUMOs to the CB band of TiO2 is
quite sufficient.
Fig. 7. The accumulation results of photoelectrocatalytic CO2 reduction using
S-TiO2@GS-Pt sheet as photocathode at -0.9 V vs SCE. TER (total electron
rate) = 6RMeOH+12REtOH+2RHCOOH+8RHOAc
.
TiO2@GS in the electrolyte of KHCO3 are approximately − 0.5 V and −
0.9 V (Fig. 8c), respectively. These results show that the imine mole-
cules make the conduction band of the electrode move to a more ne-
gative position, so that it has enough energy to reduce CO2, giving the
selectivity and distribution of products.
3.5. The transient absorption spectra and proposed mechanism
The laser flash photolysis experiments were carried out to reveal the
photocatalytic dynamic processes, which certify
a rapid electron
Furthermore, the geometries and energy levels of the S-imine group,
transfer between the photosensitizer and imine-TiO2. The absorption
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L. Wang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry391(2020)112368
Fig. 9. Transient absorption spectra: (a) EY, (b) EY + S-TiO2 mixture, (c) EY + B-TiO2 mixture, (d) fitted decay profiles of 3EY* at 560 nm for EY and EY with S-TiO2
and B-TiO2.
peaks of 480 nm were the typical absorption of Eosin Y molecules
(Fig. 9). Along with the laser irradiation to the Eosin Y aqueous solu-
tion, the transient absorbance of 480 nm has a decrease accompanying
the diminution of 560 nm that is assigned to the formation of excited
triplet state of 3EY* (Fig. 9a). [48,49] When the sample of S-TiO2 was
added to the Eosin Y solution, the similar phenomenon can be observed
with smaller decreases at 480 nm and 560 nm (Fig. 9b), which indicates
a slowdown decay rate of Eosin Y reflecting a good electron charge
transfer from the triplet state of dye molecule to TiO2 semiconductor via
S-imine group. On the contrary, when the sample of B-TiO2 was added
to the Eosin Y solution, the dye molecule of Eosin Y was decomposed
evidently (Fig. 9c). Thereafter, a triplet state of 3EY* peak of 560 nm
decreased and a radical cation of EY+% peak of 460 nm enhanced,
which indicates a worse electron charge transfer from dye molecule to
TiO2 semiconductor via B-imine group. The time-resolved decay curves
of 3EY* in various samples at 560 nm are in consistent with the tran-
Fig. 10. The proposed mechanism of PEC process of CO2 reduction.
sient absorption spectra (Fig. 9d).
To demonstrate the transportation of electrons in the PEC cell
4. Conclusions
clearly, a proposed mechanism is drawn in Fig. 10. The electrons in the
ground states (HOMO) of dye molecules harvest photons to jump into
their excited states (LUMO); then photoelectrons inject into the con-
duction band (CB) of anatase-TiO2, transfer to protons adsorbed onto
the GS, producing hydrogen atoms that reduce directly CO2 molecules,
in which, the CeC coupling is ensured by the neighbouring imine
groups. When the electrons of OH― group transfer to the Pt electrode
accompanied by the oxygen release, and then flow in the external cir-
cuit. After arriving at the valence band of TiO2, the external electrons
fill into the HOMO of dye molecules under the action of external vol-
tage, completing the recovery of dye molecules.
For clarifying the carbon source of products, the labelling experi-
ment was taken placed as well using the 13CO2 gas as reactant, the
48 m/z datum of major product ethanol 13CH313CH2OH was detected by
GC–MS(Fig. S15), proving the carbon source of organic compound
come from CO2 gas.
In this paper, TiO2 nanowires successfully grown onto the surface of
graphite sheet by a sol-gel method. This uniform and 3D structure of
semiconductors mimicked the Calvin cycle of natural plant is in favour
of CeC coupling reaction, leading to C2+ chemicals. The CB band
energy of semiconductor could be adjusted by organic ligands anchored
by covalent bonds, which supplied enough potential to reduce CO2
toward carbon-based chemicals. Ethanol was the predominant product
of photoelectrocatalytic reduction, approaching to 124.2 μM after 6 h
irradiation. The transient absorption spectra show that the photoelec-
trons generated by dye molecules would transfer from the triplet state
of 3EY* to TiO2 via imine groups, which can provide enough electrons
for the CO2 reduction. On the other hand, the ligands with imine groups
can efficiently catch the CO2 molecules to improve the concentration
around the active sites of TiO2 semiconductor. This work would open
up
a
new avenue for the development of graphite-based
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L. Wang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry391(2020)112368
photoelectrocatalytic material for CO2 reduction.
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Material fabrication: Liwen Wang, Yan Wei
Characterization: Liwen Wang, Yan Wei, Xiaogang Yu, Jinyuan
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DFT calculation: Ran Fang
Designing and Writing: Jiazang Chen, Huanwang Jing, Liwen Wang,
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Declaration of Competing Interest
No conflict of interests are reported.
Acknowledgments
This work was funded by the Natural Science Foundation of Gansu
Province (17JR5RA212) and the Foundation of State Key Laboratory of
Coal Conversion (Grant No. J19-20-913-1).
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2020.
112368.
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