Artificial photosynthesis of methanol from carbon dioxide and water via a Nile red-embedded TiO2 photocathode

Article abstract of DOI:10.1039/c6ta10231j

The conversion of carbon dioxide into useful chemicals is a prospective strategy for alleviating the greenhouse effect and the depletion of energy. Herein, we report an artificial photosynthetic system composed of a photoanode and a photocathode comprised of NR_x@TiO₂ functionalized with Nile red via covalent linkage or Pd/NR_x@TiO₂ with additional palladium nanoparticles. The new Nile red derivatives and organic-inorganic composite electrodes were steadily prepared and well characterized using NMR, HRMS, UV-vis, FTIR, TEM, XPS, XRD and SEM. Methanol and oxygen were the products that could be detected in the liquid and gas phase. The main active species in this artificial photosynthesis system were proven using EPR spectroscopy to be hydroxy radicals releasing O₂ gas via H₂O₂. Moreover, the carbon source of methanol was validated using a ¹³CO₂ labeling experiment; ¹⁸O₂ was determined to come from H₂O using GC-MS. The optimal photoelectrocatalytic CO₂ reduction was carried out using Pd/NR₂@TiO₂ as the working electrode yielding methanol at a rate of 106 μM h^?1 cm^?2 with high light quantum efficiency (Φ_cell = 0.95).

Full text of DOI:10.1039/c6ta10231j

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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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ARTICLE
Artificial Photosynthesis of Methanol from Carbon Dioxide with
Water via Nile Reds-embeded TiO2 Photocathode
Received 00th January 20xx,
Accepted 00th January 20xx
Yongjian Jia,a Yanjie Xua, Rong Niea, Fenjuan Chena, Zhenping Zhub Jianguo Wangb and Huanwang
Jinga,b
DOI: 10.1039/x0xx00000x
Conversion of carbon dioxide into useful chemicals is a prospective strategy for alleviating the greenhouse effect and the
depletion of energy. Here, we report an artificial photosynthetic systems composed of a photoanode and a photocathode
of NRx@TiO2 functionalized by Nile reds via covalent linkage or Pd/NRx@TiO2 with additional palladium nanoparticles. The
new Nile red derivatives and organic-inorganic composite electrodes are steadily prepared and well characterized by NMR,
HRMS, UV-vis, FTIR, TEM, XPS, XRD and SEM. Methanol and oxygen are the products that can be detected in liquid and gas
phase. The main active species in this artificial photosynthesis system are proved by EPR to be hydroxy radicals releasing
O2 gas via H2O2. Moreover, the carbon source of methanol is validated by 13CO2 labeling experiment; the 18O2 is determined
by GC-MS coming from H2O18. The optimal photoelectrocatalytic CO2 reduction is carried out using Pd/NR2@TiO2 as
www.rsc.org/
1
2
working electrode yielding methanol in a rate of 106 μM h cm with high light quantum efficeincy (cell = 0.95).
Unfortunately, many promising semiconductors like TiO2,
especially oxides, have a wide band gap, which results in an
onset of the absorption below 400 nm.[2] A convenient method
1. Introduction
Solar energy regarded as the most prospective energy
resource on account of inexhaustibility and zero-pollution
character is capitalized on three distinct approaches:
photoelectric, photothermal, and photochemical conversion.[1]
The solar fuels derived from solar energy by photochemical
conversion would act an alternative of the traditional energy.[2]
The artificial photosynthesis is one of the promising
photochemical processes mimicking the photosynthesis in
nature,[3] except for the water splitting that was first reported
by Honda and Fujishima.[4] Although various catalyst systems
have achieved the water splitting to release oxygen and
hydrogen,[5,6] these only fulfil partial artificial photosynthesis
without CO2 participation. Therefore, light-driven CO2
conversion in water to carbon-based fuels allowing for better
integration into the existing energy infrastructure is the best
example of artificial photosynthesis in light of the coupling of
water splitting and CO2 reduction.[7-37]
to overcome this issue is using a molecular sensitizer adsorbed
on a semiconducting electrode, which would harvest visible
light and triggered the photocatalytic CO2 reactions.[38-39]
N-containing ligands have also demonstrated good
properties to absorb and activate CO2 in this reaction.[40-42]
Herein, we design a new approach to modify semiconductor by
embedding sensitizers of Nile reds via N-containing ligands
onto the surface of TiO2 films producing a series of organic-
inorganic composite photocathodes (NRx@TiO2). The new
artificial photosynthetic cells of NRx@TiO2Co-Pi/W:BiVO4
and Pd/NRx@TiO2Co-Pi/W:BiVO4 can realize efficiently both
water-splitting and CO2 reduction yielding solely methanol in
liquid phase and oxygen in gas phase (Scheme 1).
In 1978, Halmann reported a pioneering research on the
photocatalytic CO2 reduction employing a p-type GaP cathode
to obtain small amount of formic acid, formaldehyde and
methanol.[20] Subsequently, the CO2 reduction with water in
the presence of TiO2 was reported by Honda et al.[21]
Scheme 1 The diagram of artificial photosynthetic cell and photocathode.
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electrolyte containing deuterium water, dimethyl sulfoxide
and phloroglucinol as internal standards.
2. Experimental
2.1 Syntheses of Nile red dyes
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DOI: 10.1039/C6TA10231J
The derivatives of Nile reds (NRx, x = 1, 3, 4), were fabricated
referring to a literature procedure[43] using 5-diethylamino-2-
nitrosophenol hydrochloride and various 2-naphthol
derivatives as starting materials. The detailed procedures for
preparation of them were described in supporting information.
The synthetic route of NR2 was depicted in Figure 1. 6-Bromo-
3. Results and discussion
Considering CO2 absorption of N-containing ligands and light-
harvesting ability of dyes, Nile reds were immobilized onto the
surface of FTO/TiO2. The designed organic-inorganic
photocathodes were successfully fabricated and characterized
using various techniques. The performance of their artificial
photosynthetic cell was documented as well.
2-naphthol (
precursor that was then treated with TBSCl for protecting
the free phenolic hydroxyl group yielding intermediate . The
protection strategy aimed to adjust the solubility of the
intermediate , which was beneficial to purification of
subsequent reactions. Suzuki cross-coupling between 2,3-
dibromonaphthalene-1,4-dione and was conducted to build
C-C bond getting the intermediate in modest yield. This key
intermediate could react with o-aminophenol via
intramolecular Michael addition to yield Nile red derivative
that was then converted to corresponding intermediate by
1) reacted with boron ester to give boron
2
3
3.1 UV-vis, IR spectra and TG of organic-inorganic composites
The UV-vis absorption spectra of NR series dyes in DMF
solution and organic-inorganic composites of dye-APS-TiO2 are
depicted in Figure 2 & Table 1. The results show that the
absorption bands of dyes in organic-inorganic composites have
evidently a bathochromic-shift comparing to that of free NR
series dyes in DMF solution. According to their FTIR spectra
2
3
4
4
5
1
(Fig. S5), the peaks at 1068 and 1680 cm are assigned to the
6
stretching vibration of Si-O and C=N bond respectively,
indicating the formation of linkage of Si-O-Ti and Schiff base.
Thermogravimetric analysis (Fig. S6) reveals the dyes are
embellished onto the surface of TiO2 film via APTES as well.
deprotection reaction using butyl ammonium fluoride. The
target molecule NR2 could be efficiently obtained by carrying
out a Duff reaction.
Figure 1 The schematic diagram for preparation of photocathodes
2.2 Preparation of photocathodes
Figure 2. UV-vis absorption spectra of NRx series dyes in DMF (a) and solid
UV-vis absorption spectra of photocathodes NRx@TiO2 (b).
The preparation procedures for photocathodes are illustrated
in Figure 1. The FTO/TiO2 films were firstly fabricated following
our previous reported procedure.[44] These electrodes were
then modified by 3-aminopropyl-triethoxysilane (APTES, J&K
Chemical) in ethanol under reflux condition. The organic-
inorganic composite photocathodes (dye-APS-TiO2) were in-
situ produced by merging the electrodes into a THF solution of
new Nile red dyes under reflux condition. The obtained
photocathodes were coloured from red to blue that were
nominated as NRx@TiO2. After depositing Pd nano-particles
onto the surfaces of electrodes NRx@TiO2, the more efficient
photocathodes named as Pd/NRx@TiO2 were successfully
obtained.
3.2 Electrochemical properties of dyes
To gain more insight into properties of dyes, energy levels of
molecular orbitals were calculated based on their cyclic
voltammetry (CV) curves and UV-vis spectra. The E0-0 values of
the dye molecules (Table 1 & Fig. S1) were estimated from the
intersection between the UV-vis absorption and emission
spectra as follows: 2.12 eV (NR1), 2.44 eV (NR2), 2.08 eV (NR3)
and 2.05 eV (NR4). In addition, the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) values of dyes were also calculated and listed in Table
1. We can see that the LUMOs of the dyes are located more
positively than the conduction band (CB) of TiO2, which
indicate the injection of photoelectrons from the LUMO of dye
into the conduction band of TiO2 is thermodynamically facile.
2.3 Artificial photosynthesis experiments
An two-electrode cell of Pd/NRx@TiO2)Co-Pi/W:BiVO4
equipped with a Si-solar cell was filled with 0.1M KHCO3
aqueous solution as electrolyte that was saturated with a
stream of high-purity CO2 for 30 min. Then the reactor was
irradiated by a Xenon lamp under 200 mW/cm2 when a tiny
voltage (~0.6V) was provided between the photoanode and
photocathode by the solar cell under 5W LED light. The liquid
products were quantified by 1H NMR spectroscopy using 70 μL
In particular, LUMO of NR2 sensitizer is positioned at 2.5 eV,
which shifts to more positive level than that of other
sensitizers implying lager driving force in the electronic
injection process. The HOMOs of the dyes are much higher
than
the
valence
band
(VB)
of
TiO2.
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Table 1 Photochemical and electrochemical properties of NRx dyes and dye-APS-TiO2
Dyes
NR1
NR2
NR3
NR4
λsolun /nma
545
λsolid /nm
550
474
571
602
λem/nm
614
570
612
615
HOMO/eVc
Eg/eVb
2.12
2.44
2.08
2.05
LUMO/eVd
5.30
5.31
5.21
5.24
3.18
457
539
579
2.5


3.13
3.19
a Absorption spectra were measured in DMF solutions. b Band gaps derived from the difference the UV-visible absorption and emission spectra. c HOMO＝−(Eox＋
4.8−EoxFe/Fe2+).
LUMO HOMO Eg
d
＝
＋
Figure 3. The Energy-level diagram of NRx dyes
3.3 XPS, SEM and TEM Characterization of Photocathodes
Pd/NRx@TiO2
The fabricated photocathodes Pd/NR2@TiO2 were
authenticated through some characterization techniques.
High-resolution transmission electron microscope (HRTEM)
image revealed clearly a lattice plane of Pd (111) with d111
=
0.2282 nm, and TiO2 (101) with d101 = 0.3492 nm respectively
(Figure 4). X-ray diffraction patterns of photacathodes (Fig. S8)
exhibited that the percentages of anatase, rutile phase and
SnO2 of FTO are not changed before and after investigations.
X-ray photoelectron spectroscopy (XPS) of Pd/NR2@TiO2
manifests the characteristic peaks of Pd are situated in 334.4
eV (3d5/2) and 339.7 eV (3d3/2). There are two chemical peaks
for nitrogen elements, in which, one peak of 397.2 eV derived
from C=N bond and another peak of 401.eV assigned to the N
elements of heterocycles (Fig. 4). These phenomena
validated the deposition of Pd and linkage of organic
molecules onto the surface of TiO2 films. Meanwhile, XRD
and XPS display photocathode has no palpable change before
and after the artificial photo-electrocatalytic experiments (Fig.
S7 and S8). Besides, the current density is slightly enhanced
after 100 cycles of cyclic voltammetry (Fig. S10). These results
confirm the high stability of our organic-inorgainc composite
photocathode.
Figure 4. HRTEM, and N element-XPS of Pd/NR2@TiO2 electrode
three-electrode system with Ag/AgCl reference electrode and
Pt wire as counter electrode. The experiments were achieved
in KHCO3 aqueous solution saturated by CO2 under a
simulated sunlight irradiation and the results are depicted in
Figure 5a. Their current densities increase evidently when the
bias potentials are over
photoelectrocatalytic reduction of CO2 and electrochemical
water splitting. Taking notice that considerable
photocurrent is detected around bias potential of 0.4 V, a
0.6 V reflecting a good coupling of
a

photocatalytic water splitting and photoelectrocatalytic CO2
reduction would happen in these systems. Therefore, we
decide to set up a new artificial photosynthesis system
containing two electrodes under a voltage below 1.23 V of
theoretical water splitting. The LSV diagrams of two-
electrode cell for Pd/NR2@TiO2Co-Pi/W:BiVO4 under argon
were also detected and presented in Figure 5b. The voltage of
water splitting was removed from 1.43 V under dark
condition to 1.38V under light irradiation condition. The
current density of cell was larger under light irradiation than
that of under dark condition owing to the photoelectrons
generating by Nile reds.
3.4 Linear Sweep Voltammetry
To reveal the photoelectrocatalytic activities of new organic-
inorganic composite photocathodes of NRx@TiO2 and
Pd/NRx@TiO2, linear sweep voltammetry (LSV) curves of
them were recorded by electrochemical workstation in the
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The formation rates of products and quantum efficiency
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DOI: 10.1039/C6TA10231J
versus various conditions were illustrated in Fig. 6 as
histogram. Firstly, when the organic-inorganic composite
photocathodes of NRx@TiO2 was applied in this artificial
photosynthetic system, the methanol was unique product in
electrolyte. The formation rates of methanol for various
electrodes were 2.32 (NR1@TiO2), 6.94 (NR2@TiO2), 3.47
1
2
(NR3@TiO2) and 5.79 μM h cm (NR4@TiO2), respectively.
In contrast, a bare TiO2/FTO photocathode worked in artificial
photosynthesis could not generate target product. These
results imply that the dye molecules of NRx embedded on the
surfaces of TiO2 nanoparticles would be not only a good
sensitizer harvesting sun-light, but also a good ligand catching
CO2 molecules. When the palladium nanoparticles were
deposited on the basis of NRx@TiO2 films generating new
photocathodes of Pd/NRx@TiO2, the production rate of
artificial photosynthetic systems augmented sharply to 21.76
(Pd/NR1@TiO2), 106.4 (Pd/NR2@TiO2), 54.88 (Pd/NR3@TiO2)
1
2
and 29.09 μM h cm (Pd/NR4@TiO2), respectively (Fig. 5).
These manifest that the Pd loading has a great impact on CO2
conversion performance. The reasons would be that Pd nano-
particles embedded on the surface of TiO2 have good ability
to adsorb protons from electrolyte. The protons combine in-
situ with photoelectrons of LUMOs in TiO2 films generating
active hydrogen atoms that reduce CO2 to target products.
The highest light quantum efficiency (cell) is up to 0.95 for
photocathode of Pd/NR2@TiO2 that is more than two times
of plants in nature.
Figure 5. LSV diagrams for Pd/NRx@TiO2 at a scan rate of 50 mV s-1
3.5 Artificial photosynthetic experiments
The artificial photosynthetic cells were assembled with novel
photocathode of NRx@TiO2 or Pd/NRx@TiO2, photoanode of
Co-Pi/W:BiVO4, and electrolyte of 0.1 M KHCO3 aqueous
solution, mimicking a plant environment. This equipment
shown in Scheme 1 were irradiated by a simulated sunlight
and also powered by a Si-solar cell on the electrodes with
As comparison, this artificial photoelectrochemical system
bearing Pd/NR2@TiO2 as a photocanthode was also working
without external power supply of solar cell. The formation
rate of methanol was dropped obviously from 106.4 to 5.92

0.6 V. After termination of experiment, products in the
electrolyte and gas phase were then detected by proton NMR
and GC-MS, respectively.
1
2
μM h cm (Fig. 7). This implies that the external power
supplying is very important for our new artificial
photosynthesis systems to give electrons from photoanode to
cathode. Excitingly, these new artificial photosynthesis
systems could also work under weak irradiation conditions of
1
2
32 mW/cm2 giving methanol in a rate of 6.61 μM h cm .
Besides, the oxygen releasing from photoanode minicking
photosystm II could be detected by GC-MS (Fig. S18). In order
to certify the stability of the prepared photoelectrodes, the
repeated experiments were conducted with Pd/NR2@TiO2 in
our new photoelectrochemical cell. The results exhibit the
Figure 6. Conversion results of CO2 reduction with various photocathodes
Figure 7 Conversion results of CO2 reduction under various conditions
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photocathode shifted negatively from 0.58 V toView0A.r7tic2leVOn(livnes
activity of photocathode keep in the same level after four
cycles because that the reduction reactions are taken place in
this photocathode differing from the oxidation reaction in the
pollutant decomposition. Otherwise, the used photocathodes
were also demonstrated by XRD and XPS characterization (Fig.
S7, S8 & S19).
DOI: 10.1039/C6TA10231J
Ag/AgCl) compared with Pd/NR1@TiO2 photocathode, which
means an enhancement in the bending band edge and
smaller barrier for the transfer of photogenerated charge
carriers. As shown in this Figure S15, the CB potential of
Pd/NR2@TiO2 was high enough (ca. 0.14 eV vs NHE) for CO2
reduction to produce methanol. In contrast, the potential of
Pd/NR1@TiO2 could not thermodynamically afford adequate
driving-force for methanol formation. The more negative flat
band potential, higher carrier density and abundant driving
force promised more active for CO2 conversion.
3.6 Mechanism investigation
3.6.1 Electrochemical Impedance Spectroscopy (EIS).
The charge transfer resistance (Rct) can be illustrated by the
electrochemical impedance spectra. The impedance
experiments are conducted at 0.6 V (vs. Ag/AgCl) of the CO2
3.6.3 EPR experiments
saturated electrolyte solution (0.1 M KHCO3) using the
Pd/NRx@TiO2 as working electrode. An equivalent Randles
circuit model was fit the data to obtain Rct for each artificial
system,[19] inidicating the interface transfer resistance
between electrode and electrolyte. The Nyquist plots and Rct
values are shown in Figure 8 and Table S1. Photocathode of
Pd/NR2@TiO2 exhibits a smaller circular radius than other
photocathodes, which suggests that Pd/NR2@TiO2 has a
lower resistance in electron transfer process and acquire
enough electrons for the CO2 reduction.
The EPR experiment is an effective technique to detect the
formation of short-lived radicals. 5,5-Dimethyl-1-pyrroline-N-
oxide (DMPO) was widely used as a spin trapper for OH
radicals. The result was depicted in Figure 9. The signals with
an intensity ratio of 1:2:2:1 (blue hexagram, hfs : α αH =
14.73G) are assigned to DMPO-OH
agreement with the literature values.[24] The other 6 signals
marked with red star are assigned to DMPO-O2H radicals
hfs : α 16.1 G, αHβ = 24.5 G) that would exist in neutral
solution.[46] Hence,
OH and OOH radicals should be the
• radicals that are in
•
•
•
3.6.2 Mott-Schottky Ananlysis of Pd/NRx@TiO2 Electrode
main active species during carbon dioxide reduction under
visible light irradiation.
The Mott–Schottky plots of two photocathodes under dark
conditions is presented in Figure 6b, in which Pd/NR1@TiO2
and Pd/NR2@TiO2 behaved a greater difference on carbon
dioxide conversion rates. The positive slopes, indicating the
film behaves as an n-type semiconductor, obtained at 0.5 kHz
for each electrode but the slopes at a given frequency for the
two electrodes are slightly different. According to the
equation:
1
2
E Ef
)
s
0
r
Figure 9 EPR spectra of spin adducts (DMPO-OH• and DMPO-OOH•)
generated in artificial photocatalysis cell
3.7 Isotopic labelling experiments of 13CO2 and H2O18
To verify the carbon source of methanol derived from CO2
reduction for our artificial photosynthesis system, an isotopic
labelling experiment using 13CO2 for CO2 reduction was
carried out. After bubbled argon 30 min to eliminate oxygen
of the whole system, 13CO2 gas was introduced into the
system for 15 min. The related artificial photosynthesis was
performed for 8h under solar-simulated light irradiation.
Figure 10 presents the result of labelling experiment, in which,
Figure 8 Electrochemical impedance spectra plots (Nyquist plots) in a CO2
saturated KHCO3 solution (0.1 M, ref. Ag/AgCl) and representative Mott-
Schottky plots (b) measured under dark condition at 0.5 kHz
Therein, 0 is the dielectric constant, r is the permittivity of
free space (F cm−1), q is the electronic charge (C), Efb is the
flat band potential (V), Csc is the capacitance of the space
charge layer. Charge carrier densities (ND) were calculated
and obtained assuming a relative permittivity of TiO2 = 80
r
according to the literature [45]. The carrier concentration of
Pd/NR2@TiO2 was estimated to be 1.098 × 1019 cm−3,
somewhat larger than that of Pd/NR1@TiO2 (0.94 × 1019
cm−3). This suggests that Pd/NR2@TiO2 should affect more
carrier density relative to Pd/NR1@TiO2. In addition, the flat
band potential (Efb) plays an important role on the
photoelectrocatalytic performance. Efb of Pd/NR2@TiO2
Figure 10 1H NMR spectra of 13CH3OH and 12CH3OH
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under the light density of 200 mW/cm2. Encouragingly, these
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cells can work in very weak light irraDdOiaI:ti1o0n.10a39s/Cw6eTAll1.02T3h1eJ
isotopic labelling experiments of 13CO2 and H2O18 show that
the carbon source of product methanol comes from gas
supplying instead of decomposition of organic moieties of
Figure 11 GC-MS of 18O2.
electrode. The EPR experiments demonstrate that the •OH
radicals are the key intermediated in this process of artificial
photosynthesis. The light quantum efficiency of new artificial
photosynthetic cells can reach to 0.95 that is higher than 0.4
of plants.
a double single peak, 3.03 and 3.42 ppm, is attributable to
the coupling of proton and 13C of methyl group caused by
the influence of nuclear magnetic moment of isotope carbon
on hydrogen nucleus. This result clearly indicates that the
carbon source of methanol comes from 13CO2.
The artificial photosynthesis with 98 atom% H2O18 was also
carried out under the same conditions. Isotopic labelled 18O2
was detected by gas chromatography-mass spectrometry
using Pd/NR2@TiO2 as photocathode (Fig. 11). Which prove
the oxygen releasing from photoanode minicking
photosystem II. Synchronously, the result reveals that no side
products can be detected.
Acknowledgements
This work was funded by the National Natural Science
Foundation of China (NSFC 21173106, 21401091), the
Foundation of State Key Laboratory of Coal Conversion (Grant
No. J16-17-913) and the Fundamental Research Funds for the
Central Universities (No. lzujbky-2016-K09).
In general, under supplying tiny voltage of cell, proton (H+)
and hydroxyl group (OH) move oppositely to negative or
positive electrode. The CO2 reduction is taken place in the
photocathode. In there, the CO2 molecules are caught by the
nitrogen atoms of organic moieties and the protons are
absorbed by Pd nano-particles (Fig. 12). When the dye
molecules harvest photons, the electrons in HOMO of dyes
jump to LUMO generating photoelectrons that would transfer
immediately to the protons on the surfaces of Pd nano-
particles making active hydrogen atoms that reduce in-situ
CO2 to methanol. Simultaneously, the hydroxyl radicals are
generated under light irradiation by OH group transferring to
BiVO4 electrode and releasing oxygen gas via H2O2. On the
other hand, when OH groups transfer directly electrons to
the HOMO of dyes rather than formation of hydroxyl radicals,
this electron transportation is a photocatalytic way of
sensitizer recovery.
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA10231J
A novel TiO2 based organic-inorganic composite photocathode was prepared for the efficient
artificial photosynthsis of methanol from carbon dioxide with Water.