Zn phthalocyanine/carbon nitride heterojunction for visible light photoelectrocatalytic conversion of CO2 to methanol

Article abstract of DOI:10.1016/j.jcat.2019.01.022

Zinc phthalocyanine/carbon nitride nanosheets photoelectrocatalysts were obtained by evaporation and calcination of the mixture of ZnPc and carbon nitride. It exhibits highly efficient synergistic function between PC-CO₂RR and EC-CO₂RR. The energy band matching between the ZnPc and carbon nitride results in the faster electrons transfer and less electron-hole pairs recombination. The optimized ZnPc/carbon nitride at a ratio of 0.1 wt% demonstrates high efficiency in photoelectrocalytic reduction of CO₂ under visible light, generating a major product of methanol at a yield of 13 μmol·cm^?2 after 8 h. The efficiency of photoelectrocalytic reduction of CO₂ to methanol is 2.6 times as high as that of electrocatalytic CO₂ reduction reaction, and 5.9 times as high as that of photocatalytic CO₂ reduction reaction. The mechanism of the photoelectrocatalytic CO₂ reduction reaction to methanol was discussed in detail.

Full text of DOI:10.1016/j.jcat.2019.01.022

Journal of Catalysis 371 (2019) 214–223
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Zn phthalocyanine/carbon nitride heterojunction for visible light
photoelectrocatalytic conversion of CO to methanol
2
Jingui Zheng a,1, Xiaojie Li b,1, Yuhu Qin , Shuqin Zhang , Mingsheng Sun , Xiaoguang Duan , Hongqi Sun ,
Peiqiang Li , Shaobin Wang
a
a
a
a
b
c
a,
⇑
b,
⇑
College of Chemistry and Material Science, Shandong Agricultural University, Shandong, China
Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Zinc phthalocyanine/carbon nitride nanosheets photoelectrocatalysts were obtained by evaporation and
calcination of the mixture of ZnPc and carbon nitride. It exhibits highly efficient synergistic function
between PC-CO RR and EC-CO RR. The energy band matching between the ZnPc and carbon nitride
2 2
Received 9 October 2018
Revised 15 January 2019
Accepted 17 January 2019
results in the faster electrons transfer and less electron-hole pairs recombination. The optimized
ZnPc/carbon nitride at a ratio of 0.1 wt% demonstrates high efficiency in photoelectrocalytic reduction
ꢁ
2
of CO
h. The efficiency of photoelectrocalytic reduction of CO
trocatalytic CO reduction reaction, and 5.9 times as high as that of photocatalytic CO
The mechanism of the photoelectrocatalytic CO
2
under visible light, generating a major product of methanol at a yield of 13
to methanol is 2.6 times as high as that of elec-
reduction reaction.
l
molꢀcm after
Keywords:
Photoelectrocatalysis
CO reduction
2
Zinc phthalocyanine
Carbon nitride
Methanol
8
2
2
2
2
reduction reaction to methanol was discussed in detail.
Ó 2019 Elsevier Inc. All rights reserved.
1
. Introduction
hand, an external electric field significantly reduces electron-hole
recombination and improves the selectivity of products [19,23].
Nowadays, CO
2
emissions [1–5] and energy crisis [6–8] have
2
Catalyst materials are most important for PEC-CO RR [24]. Car-
become a serious threat to human society and the environment,
and therefore the conversion of CO to fuels [9,10] is believed to
provide an effective means for energy supply and reduction of
CO emission, solving both the problems simultaneously. Photocat-
alytic CO reduction reaction (CO RR), utilizing solar energy and
bon nitride, a non-metallic, inexpensive semiconductor with an
appropriate band gap [25], is an ideal choice. Yang et al. [26] devel-
oped a photochemical polymerization approach for the textural
engineering of triazine monomers to carbon nitride-polymer pos-
sessing extraordinary light-harvesting properties. Zhang et al.
[27] synthesized a highly active copolymer with donor-acceptor
(D-A) heterostructure by post-calcination of PCN in suitable eutec-
tic molten salts, promoting interface charge carrier transfer. Ou
et al. [28] found that biomimetic donor-acceptor motifs in conju-
gated polymers (DA-carbon nitride) can promote exciton splitting
and charge separation. However, the recombination of photo-
induced electrons and holes and poor electrochemical performance
limit its application. Menny Shalom et al. [29] found that the
charge separation of carbon nitride was improved by utilization
of cyanuric acid and melamine hydrogen-bonded complexes. Teru-
hisa et al. [30] prepared B-doped carbon nitride, by transforming n-
2
2
2
2
photocatalysts to mimic plant photosynthesis [11,12], is an envi-
ronmentally friendly method. However, inefficient photocatalysts,
suffering from low CO
excited electron-hole pairs, low conversion and selectivity to fuels,
severely limit the efficiency of PC-CO RR [13–15]. Although elec-
trocatalytic (EC) CO RR has some advantages as compared to pho-
tocatalysis, high-energy consumption is the major obstacle [16].
Thus, photoelectrocatalytic (PEC) CO RR by combining the advan-
tages of photocatalytic and electrocatalytic CO reduction has been
suggested and received more and more attention in recent years
17–20]. On one hand, the light irradiation reduces external energy
input for inducing photogenerated electrons [21,22]. On the other
2
adsorption, high recombination of photo-
2
2
2
2
[
2
type carbon nitride to p-type semiconductor for favoring CO RR.
Meanwhile, Au, Ag and Rh were used as a co-catalyst to suppress
electron-hole recombination. Yao et al. [31] fabricated a Z-
⇑
Corresponding authors.
E-mail addresses: chem_carbon@outlook.com (P. Li), shaobin.wang@curtin.edu.
2
scheme BiOI/carbon nitride composite for PC-CO RR, and the indi-
rect Z-scheme electron transfer mode efficiently facilitated the
separation of photogenerated electron and hole. However, not
au (S. Wang).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.jcat.2019.01.022
021-9517/Ó 2019 Elsevier Inc. All rights reserved.
0

J. Zheng et al. / Journal of Catalysis 371 (2019) 214–223
215
many investigations on PEC-CO
reported.
2
RR based on carbon nitride are
2.2. Characterizations
Zinc phthalocyanine (ZnPc) has a strong absorption of visible
light [32,33]. Compared with FePc, NiPc, MnPc, and CuPc, ZnPc
shows more sensitivity for light irradiation, since the metal d-
orbit does not participate in the composition of the frontier molec-
ular orbital [34,35]. Xu et al. prepared self-floating graphitic carbon
nitride/zinc phthalocyanine nanofibers for photocatalytic degrada-
tion of contaminants [4,36]. Zhang et al. put forward a new route to
The surface morphologies of the as-prepared samples were
characterized by scanning electron microscopy (SEM, Philips
XL30 FEG) with an accelerated voltage of 20 kV. The crystalline
structures were characterized by X-ray diffraction (XRD, Rigaku
D/MAX-rA, Japan) using Cu K
a radiation (k = 15.4184 nm) in a
ꢁ1
range of 2h = 10–60° with a scan rate of 4° min . UV–visible dif-
fuse reflectance spectroscopy (UV–visible DRS) was obtained for
optical properties (Beijing Purkinje General Instrument Co., Ltd).
The electrochemical properties were measured by a CHI660D elec-
trochemical workstation (Shanghai Chenhua Instrument Co., Ltd).
achieve broad spectral responsive photocatalytic H
over carbon nitride co-sensitized by an indole-based D-
2
production
-A organic
p
dye (LI-4) and an asymmetric zinc phthalocyanine derivative with
complementary absorption spectra [37]. The superior photocat-
alytic properties of ZnPc/carbon nitride have been well known,
but the excellent photoelectric synergistic properties have not
been explored. It is hopeful to obtain more excellent properties
for the energy band matching between the ZnPc and carbon nitride
2
2.3. PEC-CO RR evaluation
The experiments were conducted in a double-cell reactor with
circulating water at the constant temperature of 25 °C in 0.1 mol
[
35].
In this work, the fabrication of ZnPc/carbon nitride composites
was presented and utilized this target catalyst in PEC-CO RR. The
ꢁ1
L
3
KHCO solution. ZnPc/carbon nitride nanosheets, platinum
wire and the SCE were used as the working electrode, counter elec-
2
trode and reference electrode, respectively. The electrode area was
energy band matching between the ZnPc and carbon nitride not
only efficiently improve the EC performance but also significantly
restrain the recombination of photo-generated electrons and hole.
2
2
CO
ꢂ 2 cm , and the electrode interval was 1.0 cm. The flow rate of
ꢁ1
2
was 40 mL min during the whole experiment. The initial
pH of the electrolyte (0.1 mol L KHCO
ꢁ1
3
solution) was 8.3, As
Moreover, it exhibited highly synergic effect between PC-CO
and EC-CO RR. The mechanisms for the three different CO RR pro-
cesses including PC, EC and PEC-CO RR are detailedly discussed.
The research will supply a certain theoretical support for the
PEC-CO RR.
2
RR
2
CO was introduced continuously, the pH was finally maintained
2
2
at 6.8. The samples were illuminated under visible light by a Xenon
2
ꢁ2
lamp (420 ꢃ k ꢃ 800 nm, 100 mWꢀcm ). The volume of the reac-
tion cell is 40 mL. The products were detected by gas chromatogra-
phy (6890-N, Agilent) with a column (2 m, inner diameter 3 mm,
Parapok Q, 80–100) and a flame ionization detector. The column
temperature was kept at 100 °C while the detector temperature
2
was at 150 °C. High purity N
rate of 30 mL min . The diagram of the PEC cell for CO
2
worked as the carrier gas with a flow
2
. Experimental section
ꢁ1
2
reduction
was shown in Scheme 1.
2.1. Preparation of catalyst samples
Preparation of carbon nitride was carried out by heat polymer-
3. Results and discussion
ization of urea [37]. About 3 g of urea was placed in an autoclave
and the autoclave was taken into a muffle furnace with a heating
Carbon nitride nanosheets were fabricated by heat polymeriza-
tion of urea. Small lamellar structure of carbon nitride was showed
in Fig. S1. XRD and FT-IR further proved the successful preparation
of carbon nitride (Figs. S2 and S3). ZnPc was synthesized via a
phthalic anhydride - urea route. From the spectrum of UV–vis
DRS (Fig. 1a), two strong absorptions at 250–350 nm and 600–
700 nm can be seen, corresponding to B band and Q band of Pc
complex [40], respectively. The Q band is a typical absorption band
belonging to the Pc compound and can be used as the characteris-
tic peak for the Pc identification. Usually, the Q band of non-
metallic Pc appears between 700 nm and 800 nm, and the devia-
tion of the Q band in ZnPc may attribute to the influence of the cen-
ꢁ1
rate of 5 °C min to 550 °C and kept the temperature for 4 h.
For ZnPc synthesis, 10 g phthalic anhydride, 25 g urea, 2 g anhy-
drous zinc chloride and 0.25 g ammonium molybdate were mixed
and placed in a three-necked flask. The mixture was heated and
stirred until the urea was completely melted. Then anhydrous
2 3 4
Na CO and NH Cl were added. After all materials were dissolved,
the temperature was kept for 1 h. Then, the temperature was
raised to 280 °C and kept for 4 h. The resulting crude product
was soaked in diluted HCl for 12 h, filtered and washed repeatedly
with double distilled water. After drying, the crude product was
2 4
dissolved in concentrated H SO , and the impurities were filtered
2
+
out with a sand core funnel. The filtrate was diluted with ice water.
After ZnPc was separated out, water was used to wash the sample
to neutral. And then the precipitate was further washed with N,
N-dimethylformamide, acetone and absolute ethanol to remove
the remaining organic impurities, dried and green ZnPc powder
was obtained [38,39].
tral metal ions (Zn ).
2
Generally, in FT-IR spectrum, the strongest peak of H Pc is at
ꢁ1
about 1022 cm , which refers to C-N stretching vibration and N-
H in-plane bending vibration [41]. It was also reported that the
ꢁ
1
strongest peak of ZnPc appears at about 1331 cm , which is
related to the N-H in-plane bending vibration, and benzene ring
In synthesis of composite catalysts, the loadings of ZnPc on car-
bon nitride nanosheets were controlled at 0.05%, 0.1%, and 0.15%. A
mixture of ZnPc and carbon nitride nanosheets were ground, and
then placed in 50 mL of tetrahydrofuran, heated and stirred. The
mixture was calcined in the muffle furnace at 300 °C for 2 h when
the solvent was completely volatilized. For the preparation of cat-
alyst electrode, a FTO glass was ultrasonically cleaned in anhy-
drous ethanol for 5 min to remove impurities from the surface.
The catalyst composite was dispersed in double distilled water,
stretching vibration [42]. From Fig. 1b,
a
strong peak at
ꢁ1
1331 cm was observed, suggesting the presence of ZnPc struc-
ꢁ1
ꢁ1
ꢁ1
ture. In addition, the peaks at 1331 cm , 1117 cm , 1098 cm
,
ꢁ
1
ꢁ1
ꢁ
1070 cm and 1028 cm are related to the Pc ring skeleton vibra-
1
tion. The peak at 1485 cm is the stretching vibration of the aro-
ꢁ1
matic ring, 1284 cm
is the vibration of the CAN bond, and
ꢁ1
899 cm is in plane bending vibration of the CAH bond. Thus,
the successful preparation of ZnPc is proved by Fig. 1a and b.
Several ZnPc/carbon nitride nanosheets composites at different
ratios were prepared by mixing ZnPc and carbon nitride in tetrahy-
drofuran (THF), evaporation and calcination. Fig. 2a illustrates their
2
and then pasted onto FTO glass for an area of 2 ꢂ 2 cm . After dry-
ing, fluorocarbon resin was dipped onto the surface for adhesion.

2
16
J. Zheng et al. / Journal of Catalysis 371 (2019) 214–223
(Fig. 4a) reveals that the predominant elements are Zn, N and C.
The Zn 2p XPS spectrum shows peaks at binding energies of
1
021.1 and 1044.1 eV in Fig. 4(b). Furthermore, N 1s peak is
detected at 398.2 eV in Fig. 4(c), and Fig. 4(d) shows N 1s peak at
2
84.6 eV. These Zn, N and C peaks confirmed the formation of ZnPc.
Fig. 5a and b shows I-t curves for carbon nitride nanosheets,
ZnPc and ZnPc/carbon nitride nanosheets under visible light condi-
tions, with every 50 s shading internal. With illumination, PC-
CO RR and EC-CO RR synergistically proceed, and therefore, the
2 2
current intensity is much larger. When it is shaded, PC reaction
begins to weaken until disappearance, and only EC reduction cur-
rent exists, which is much lower than that under light conditions.
From Fig. 5a–c, carbon nitride nanosheets, ZnPc and ZnPc/carbon
nitride nanosheets have a response to light, but the increase of
ZnPc/carbon nitride nanosheets currents are more obvious than
that of pure ZnPc and carbon nitride. The increased current of
2
Scheme 1. The diagram of PEC cell for CO reduction.
ꢁ3
ꢁ2
ZnPc/carbon nitride nanosheets (0.11 ꢂ 10 mA cm ) is 5.5 times
linear sweep voltammetry (LSV) performance. The current densi-
ties indicate that the ZnPc/carbon nitride nanosheets of a ratio of
as high as that of carbon nitride or about 2 times as high as that of
ZnPc, indicating more photoelectrons are stimulated for CO reduc-
2
0
.1% produces the highest current. From Fig. 2b, the impedance
tion in irradiation. Besides, the I-t curves of carbon nitride and ZnPc
size of the composites also suggests that the ratio of 0.1% is the
optimum, giving the smallest semi-circle. With the ratio of ZnPc
increasing from 0.05% to 0.1%, electron transfer capacity was
enhanced, resulting in the improved EC activity. If ZnPc loading
further increasing from 0.1% to 0.25%, excess ZnPc with hydropho-
bicity will accumulate on the surface of carbon nitride, leading to a
decrease in conductivity and electron transfer, and thus weaker the
EC activity.
(Fig. 5c) slowly reached a steady state after about 19 s and 15 s in
irradiation, respectively, attributable to the slow separation of pho-
togenerated electron-hole pairs on carbon nitride and ZnPc. By
contrast, the photocurrent of ZnPc/carbon nitride nanosheets just
spends 8 s reaching a steady state from dark to light, further con-
firming that the combination of ZnPc and carbon nitride
nanosheets expedites the electrons transfer for enhanced CO2
reduction performance [43]. Fig. 5d displays the LSV curves of
0.1% ZnPc/carbon nitride nanosheets with and without illumina-
tion under N2 or CO2 atmosphere, carbon nitride nanosheets and
ZnPc under N2 atmosphere. It can be seen that the currents of
curves 2 and 4 with CO2 are higher than those of the curves 1
Fig. 3 shows the optical properties of carbon nitride and 0.1%
ZnPc/carbon nitride nanosheets. From UV–vis DRS spectra
(Fig. 3a), the absorbance of the composite is evidently enhanced
after ZnPc is added in carbon nitride. And, there is an apparent
absorption peak at approximately 700 nm. Its visible light absorp-
tion is also significantly better than pure carbon nitride. The band
gap of the composite was calculated by the Tauc formula. Com-
pared with the band gap of carbon nitride nanosheets (2.80 eV),
the band gap of 0.1% ZnPc/carbon nitride nanosheets (2.81 eV)
hasn’t changed a lot. Moreover, the fluorescence spectra (Fig. 3c)
shows that the intensity of ZnPc/carbon nitride nanosheets is
lower than that of carbon nitride, it means that the recombination
between electrons and holes is greatly inhibited on the target cat-
alyst comparing with the carbon nitride. It is attributed to the
energy band matching between the ZnPc and carbon nitride.
XPS was further used to study the chemical composition and
electronic structure of ZnPc materials. The wide scan XPS spectrum
and 3 with N . The LSV curves with light are obviously higher than
2
that without light, demonstrating ZnPc/carbon nitride nanosheets
have generated a good response to light with a good PC capacity.
In addition, the current of curve 3 is higher than that of curve 1,
and the current of curve 4 is higher than that of curve 2, indicating
that the current increase comes from the CO2 catalytic reduction.
The current density is ascribed to hydrogen evolution reaction
(HER) and CO RR [44]. The LSV curves 5 and 6 with N are shown
2
2
in Fig. 5d. The curve is relatively smooth, suggesting that the mate-
rial itself is not decomposed during CO RR. It shows that the cata-
2
lyst has excellent stability. Furthermore, photoelectrical
conversion efficiency (Fig. 5e) was calculated based on the fol-
lowed formula:
Fig. 1. Characterization of ZnPc by (a) UV–vis DRS; (b) FT-IR.

J. Zheng et al. / Journal of Catalysis 371 (2019) 214–223
217
2
Fig. 2. (a) Linear sweep voltammetry (LSV) for EC-CO RR; (b) electrochemical Impedance Spectroscopy (EIS) of ZnPc/carbon nitride nanosheets composites with different
ratios.
Fig. 3. Characterization of optimal properties of carbon nitride nanosheets and ZnPc/carbon nitride nanosheets (a) UV–vis DRS, (b) plot analysis of optical band gap; (c)
photoluminescence spectra.
Photoelectrical Conversion Efficiency
can be concluded that ZnPc/carbon nitride nanosheets facilitated
the CO RR comparing with HER, and thus demonstrated a stronger
CO reduction capacity. And the photoelectric conversion efficiency
of ZnPc/carbon nitride nanosheets was 38.7% higher than photo-
catalytic conversion efficiency in Fig. 5e. It was suggested that
¼ I
CO2ꢁwith light ꢁ IN2ꢁwith light
ꢂ 100%
ð1Þ
2
2
I
CO2ꢁwith light
where ICO2 is the current density of CO
2
reduction, while I 2
N
is the
current density of hydrogen evolution in saturated N
2
solution [43].
2
the light played a role in the reduction of CO , but the applied volt-
ZnPc/carbon nitride nanosheets has the best photoelectric con-
version efficiency of 47.9% at ꢁ1.0 V in Fig. 5e, which was 26.7%
higher than that carbon nitride nanosheets alone (in Fig. S4). It
age played a leading role. Meanwhile, the maximum conversion
efficiency at ꢁ1.0 V provides a basis for further exploring the opti-
2
mal potential of ZnPc/carbon nitride nanosheets for PEC-CO RR.

2
18
J. Zheng et al. / Journal of Catalysis 371 (2019) 214–223
Fig. 4. XPS spectra of the ZnPc. (a) The wide scan XPS spectrum of the ZnPc; (b–d) Zn 2p, N 1s and C 1s XPS spectra.
ꢁ1
2
The PEC-CO RR experiments were carried out in 0.1 mol L
2 2
of PC, EC, PC + EC and PEC-CO RR. EC is better for CO RR to metha-
KHCO solution. The liquid products were focused. The main prod-
3
nol than PC, and PEC is much better than PC plus EC, suggesting an
excellent synergic effect generated between PC and EC. Further
processing the methanol yield with reaction time, we found that
a linear relationship between lnc and time could be obtained
before 1 h or after 2 h (Fig. 6e). The correlation fits to the first-
order reaction kinetic equation. The apparent rate constants (K)
ucts on carbon nitride nanosheets, ZnPc and ZnPc/carbon nitride
nanosheets were found to be methanol, and a small amount of for-
mic acid was found. At different voltages (ꢁ0.8 V, ꢁ0.9 V, ꢁ1.0 V,
ꢁ
1.1 V, ꢁ1.2 V), the methanol product yields (PYs) with reaction
time are presented in Fig. 6a. The methanol yield increased first
and then decreased with the change of voltage. Specifically, from
1 2
were 3.4 and 0.28, respectively. By comparing K and K , the men-
ꢁ
0.8 V to ꢁ1.0 V, the product yield was gradually increased, but
the product yield was slowly reduced at high potential from
1.0 V to ꢁ1.2 V. This is due to the increase of the competitive
HER process. Therefore, the best product yield can be obtained at
thol generation rate during the first 1–2 h was higher than that of
2–8 h. After 2 h, the curve almost became a straight line, which
indicated that the catalyst has an excellent stability. In addition,
the reason of the different reaction rate at different time region
was further analyzed. Firstly, ZnPc/carbon nitride had an excellent
ꢁ
ꢁ2
the voltage of ꢁ1.0 V, reaching 13 mmol cm in 8 h. And, com-
ꢁ1
pared with Pd nanoparticles (TOF = 600 h ) for CO
2
RR in recent
selective adsorption for CO
(0.77 nmol) was 3.5 times that of adsorption N
(Fig. S5). Secondly, the adsorption amount of CO
at 1 h and 3 h. The adsorption amount of CO (0.77 nmol) at 1 h
was 1.8 times that of CO at 3 h (0.43 nmol) (Fig. S6). We specu-
lated that the different concentration of CO on the surface of
material may cause the different reaction rate.
2
. The adsorption amount of CO
(0.22 nmol)
was analyzed
2
ꢁ
1
research [45], ZnPc/carbon nitride nanosheets (TOF = 626 h
)
2
exhibited a higher catalytic activity (in the supporting informa-
tion). During the experiment, a product of formic acid at a very
low yield was found. Fig. 6d displays the formic acid can be
obtained at the voltage of ꢁ1.0 V, reaching 0.37 mmol cm in 8 h.
2
2
2
ꢁ2
2
At the optimal voltage, a comparison of the PEC-CO RR effi-
2
ciency on carbon nitride nanosheets, ZnPc and ZnPc/carbon nitride
nanosheets was further made (Fig. 6b). Compared with ZnPc/car-
bon nitride nanosheets, the methanol PYs on carbon nitride
nanosheets and ZnPc are quite low, demonstrating that the energy
band matching between the ZnPc and carbon nitride is the key for
According to the structural and optical characterisations and
catalytic performances, a mechanism is proposed for the three dif-
ferent CO
Scheme 2. The electrons coming from the external voltage on
ZnPc/carbon nitride nanosheets can react with the CO directly in
2 2
RR processes including PC, EC and PEC-CO RR in
2
the PEC-CO
2
RR to methanol. Fig. 6c shows the methanol genera-
Scheme 2a. When the light is applied as shown in Scheme 2b, the
electrons of ZnPc can be excited from the HOMO to the LUMO,
tions on ZnPc/carbon nitride nanosheets in the different processes

J. Zheng et al. / Journal of Catalysis 371 (2019) 214–223
219
Fig. 5. I-t curves of 0.1% ZnPc/carbon nitride nanosheets, carbon nitride (a) and ZnPc (b), (c) partial enlarged details of circle parts in a and b, (d) LSV results of 0.1%
ZnPc/carbon nitride nanosheets with and without light under N or CO atmosphere, carbon nitride and ZnPc with N , and (e) photoelectrical conversion efficiency and
photocatalytic conversion efficiency of ZnPc/carbon nitride nanosheets.
2
2
2
and the electrons of carbon nitride are also excited. But the excited
electrons of the carbon nitride will transfer from the CB of the car-
bon nitride to the LUMO of ZnPc. Because the LUMO of ZnPc
come from light exciting, but also come from the external voltage.
Secondly, the external voltage enhances the transfer rate of the
photo-generated electrons from the CB of carbon nitride (ꢁ1.4 eV)
to LUMO of ZnPc (ꢁ0.68 eV). The above two reasons jointly result
(
ꢁ0.68 eV) is more positive than that of the carbon nitride
(ꢁ1.4 eV), then the electrons also attend the CO reduction reaction.
2
2
in the synergistic reduction of CO between PC and EC.
Which significantly inhibits the recombination of photo-generated
electron-hole pairs. Meanwhile, absorbed CO on ZnPc is reduced to
From the literature [46] and experimental results, the mecha-
nism of six electrons reaction for the PEC reduction of CO to
2
2
be methanol by electrons coming from ZnPc and carbon nitride.
When light and the external voltage are simultaneously applied
as shown in Scheme 2c, firstly the reductive electrons not only
methanol was inferenced (Scheme 2d). The active center is the
Co ion in the mentioned literature (reference [51]), but it is the
Zn(II) metal ion in our research. Compared with Co ion, the Zn(II)

2
20
J. Zheng et al. / Journal of Catalysis 371 (2019) 214–223
Fig. 6. (a) Time-dependent curves of methanol yield at different potentials on ZnPc/carbon nitride nanosheets for PEC-CO
1.0 V on carbon nitride nanosheets, ZnPc, ZnPc/carbon nitride nanosheets for PEC-CO RR; (c) time-dependent curves of methanol yield for PC, EC, PC + EC and PEC-CO
ZnPc/carbon nitride nanosheets; (d) time-dependent curve of formic acid yield at ꢁ1.0 V on ZnPc/carbon nitride nanosheets for PEC-CO RR; (e) time-dependent curve of
lnCMethanol for PEC-CO RR on ZnPc/carbon nitride nanosheets.
2
RR; (b) time-dependent curves of methanol yield at
ꢁ
2
2
RR on
2
2
+
metal ion can make carbon easier combine with H (Fig. S7). Due to
the difference in the electronic structure of the outer layer of the
metal, the number of N combining with Zn (two number) is less
than that of N combining with Co (three number). Comparing with
Zn (2+), Co (3+) results in the reduction of the electron cloud den-
sity around contacted O. It further results in the reduction of the
electron cloud density around C. So the C in the ACAOAZn
(3+), which makes C in the ACAOAZn (2+) easier combine with the
proton. The mechanism of reduction of CO to methanol was infer-
enced in Scheme 2d in our research. At first, CO is directly reduced
radical combines with
the Zn kation of ZnPc to form ZnAO [47], the product P1 is gener-
ated. Then, the [ZnPc(CO )] anion (P1) with negative charge is in
2
2
–
–
2 2
to CO radical by electron, and then the CO
2
combination with the dissociative hydrogen proton, produces the
product P2. The P2 is also reduced under the action of electron,
(
2+) has higher electron cloud density than the C in the ACAOACo

J. Zheng et al. / Journal of Catalysis 371 (2019) 214–223
221
2 2 2
Scheme 2. Proposed mechanisms for (a) EC-CO RR, (b) PC-CO RR and (c) PEC-CO RR on ZnPc/carbon nitride nanosheets.
2
Scheme 2d. Mechanism of six electrons reaction for the PEC reduction of CO to methanol on ZnPc/carbon nitride nanosheets.
and further forms Zn-O getting the product P3. And then, The P3
combines with the hydrogen proton to form the ZnPc-CO (coor-
dination compound P4). The P4 is unstable resulting in fracture of
C-O, and combines with hydrogen proton to produce the product
P5 and P6. Finally, the Zn-O separates and combines with hydrogen
proton to receive the ZnPc, H O and CH OH.
2 3
2
H
2

2
22
J. Zheng et al. / Journal of Catalysis 371 (2019) 214–223
[
16] M. Zhang, J. Cheng, X. Xuan, J. Zhou, K. Cen, CO
2
synergistic reduction in a
Pt-modified TiO
4
. Conclusions
photoanode-driven photoelectrochemical cell with
a
2
nanotube photoanode and a Pt reduced graphene oxide electrocathode, ACS
Sustain. Chem. Eng. 4 (2016) 6344–6354.
In this investigation, ZnPc/carbon nitride nanosheets catalysts
were assembled by mingling, evaporation and calcination pro-
cesses. The hybrid catalysts could be effectively excited by visible
[17] W. Lu, B. Jia, B. Cui, Y. Zhang, K. Yao, Y. Zhao, J.J. Wang, Efficient
photoelectrochemical reduction of CO to formic acid with functionalized
2
ionic liquid as absorbent and electrolyte, Angewandte Chemie Int. Ed. (2017).
18] J.T. Song, H. Ryoo, M. Cho, J. Kim, J.G. Kim, S.Y. Chung, J. Oh, Nanoporous Au
thin films on Si photoelectrodes for selective and efficient
photoelectrochemical CO reduction, Adv. Energy Mater. 7 (2017).
2
19] Q. Kong, D. Kim, C. Liu, Y. Yu, Y. Su, Y. Li, P. Yang, Directed assembly of
nanoparticle catalysts on nanowire photoelectrodes for photoelectrochemical
2 2
light for PEC-CO RR. The major product of the PEC-CO RR was
[
[
[
[
methanol and the highest methanol generation efficiency was
ꢁ1
achieved. at ꢁ1.0 V, the methanol yield 13
mechanisms for the three different CO RR processes including PC,
EC and PEC-CO RR are detailedly proposed. In PEC-CO RR, ZnPc/-
carbon nitride nanosheets exhibited a synergic effect and methanol
l
molꢀL ꢀafter 8 h. The
2
2
CO reduction, Nano Lett. 16 (2016) 5675–5680.
2
2
20] E. Han, F. Hu, S. Zhang, B. Luan, P. Li, H. Sun, S. Wang, Worm-like FeS
2
/TiO
2
to methanol under
2
nanotubes for photoelectrocatalytic reduction of CO
visible light, Energy Fuels 32 (2018) 4357–4363.
2
production efficiency is much better PC- and EC-CO RR.
21] Y.J. Jang, J.-W. Jang, J. Lee, J.H. Kim, H. Kumagai, J. Lee, T. Minegishi, J. Kubota, K.
Domen, J.S. Lee, Correction: selective CO production by Au coupled ZnTe/ZnO
Acknowledgement
in the photoelectrochemical CO
2016), 1114–1114.
22] G. Sahara, H. Kumagai, K. Maeda, N. Kaeffer, V. Artero, M. Higashi, R. Abe, O.
Ishitani, Photoelectrochemical reduction of CO coupled to water oxidation
2
reduction system, Energy Environ. Sci. 9
(
[
This research was supported by the National Natural Science
Foundation of China (Grant No. 21203114), Natural Science Foun-
dation of Shandong Province (Grant No. ZR2017MB018), Planning
Project of Science and Technology in Colleges of Shandong Province
Grant No. J14LC16) and partially supported by the Australian
Research Council (DP170104264). We are also grateful to the sup-
port from Shandong Jingbo Holdings Corporation.
2
using a photocathode with a Ru (II)–Re (I) complex photocatalyst and a CoO
TaON photoanode, J. Am. Chem. Soc. 138 (2016) 14152–14158.
x
/
[
23] X. Chang, T. Wang, P. Zhang, Y. Wei, J. Zhao, J. Gong, Frontispiz: stable aqueous
photoelectrochemical CO reduction by a Cu O dark cathode with improved
selectivity for carbonaceous products, Angewandte Chemie 128 (2016).
[24] B. Kumar, M. Llorente, J. Froehlich, T. Dang, A. Sathrum, C.P. Kubiak,
2
2
(
2
Photochemical and photoelectrochemical reduction of CO , Annu. Rev. Phys.
Chem. 63 (2012) 541–569.
[
25] J. Liu, H. Wang, M. Antonietti, Graphitic carbon nitride ‘‘reloaded”: emerging
applications beyond (photo) catalysis, Chem. Soc. Rev. 45 (2016) 2308–2326.
26] P. Yang, R. Wang, M. Zhou, X. Wang, Photochemical construction of
carbonitride structures for red-light redox catalysis, Angew. Chem. Int. Ed.
57 (2018) 8674–8677.
Appendix A. Supplementary material
[
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.01.022.
[
27] G. Zhang, L. Lin, G. Li, Y. Zhang, A. Savateev, S. Zafeiratos, X. Wang, M.
Antonietti, Ionothermal synthesis of triazine–heptazine-based copolymers
with apparent quantum yields of 60% at 420 nm for solar hydrogen production
from ‘‘Sea Water”, Angew. Chem. Int. Ed. 57 (2018) 9372–9376.
28] H. Ou, X. Chen, L. Lin, Y. Fang, X. Wang, Biomimetic donor-acceptor motifs in
conjugated polymers for promoting exciton splitting and charge separation,
Angew. Chem. Int. Ed. 57 (2018) 8729–8733.
29] M. Shalom, S. Inal, C. Fettkenhauer, D. Neher, M. Antonietti, Improving carbon
nitride photocatalysis by supramolecular preorganization of monomers, J. Am.
Chem. Soc. 135 (2013) 7118–7121.
References
[
[
[
[
[
2
1] S.J. Davis, K. Caldeira, H.D. Matthews, Future CO emissions and climate change
from existing energy infrastructure, Science 329 (2010) 1330–1333.
[
2] P. Smith, S.J. Davis, F. Creutzig, S. Fuss, J. Minx, B. Gabrielle, E. Kato, R.B. Jackson,
A. Cowie, E. Kriegler, Biophysical and economic limits to negative CO
emissions, Nat. Clim. Change 6 (2016) 42–50.
2
30] N. Sagara, S. Kamimura, T. Tsubota, T. Ohno, Photoelectrochemical CO
2
[
[
[
[
3] C.B. Dutta, D.K. Das, Does disaggregated CO
Evidence from thirty countries, Renew. Sustain. Energy Rev. 66 (2016) 825–
33.
4] T. Xu, D. Ni, X. Chen, F. Wu, P. Ge, W. Lu, H. Hu, Z. Zhu, W. Chen, Self-floating
graphitic carbon nitride/zinc phthalocyanine nanofibers for photocatalytic
degradation of contaminants, J. Hazard. Mater. 317 (2016) 17–26.
5] Q. Wang, Y.-E. Zeng, B.-W. Wu, Exploring the relationship between
2
urbanization, energy consumption, and CO emissions in different provinces
of China, Renew. Sustain. Energy Rev. 54 (2016) 1563–1579.
6] Y. Zheng, Z. Li, S. Feng, M. Lucas, G. Wu, Y. Li, C. Li, G. Jiang, Biomass energy
utilization in rural areas may contribute to alleviating energy crisis and global
warming: A case study in a typical agro-village of Shandong, China, Renew.
Sustain. Energy Rev. 14 (2010) 3132–3139.
2
emission matter for growth?
reduction by a p-type boron-doped g-C
Appl. Catal. B 192 (2016) 193–198.
3 4
N electrode under visible light,
8
31] J.-C. Wang, H.-C. Yao, Z.-Y. Fan, L. Zhang, J.-S. Wang, S.-Q. Zang, Z.-J. Li, Indirect
Z-scheme BiOI/g-C photocatalysts with enhanced photoreduction CO
activity under visible light irradiation, ACS Appl. Mater. Interfaces 8 (2016)
765–3775.
N
3 4
2
3
[
[
32] J. Mack, M.J. Stillman, Photochemical formation of the anion radical of zinc
phthalocyanine and analysis of the absorption and magnetic circular
dichroism spectral data. Assignment of the optical spectrum of [ZnPc (-3)], J.
Am. Chem. Soc. 116 (1994) 1292–1304.
33] H. Dai, S. Zhang, G. Xu, Y. Peng, L. Gong, X. Li, Y. Li, Y. Lin, G. Chen, Highly
3 4
photoactive heterojunction based on g-C N nanosheets decorated with
dendritic zinc(ii) phthalocyanine through axial coordination and its
ultrasensitive enzyme-free sensing of choline, RSC Adv. 4 (2014) 58226–
[
[
[
7] D. Pimentel, L. Hurd, A. Bellotti, M. Forster, I. Oka, O. Sholes, R. Whitman, Food
production and the energy crisis, Science 182 (1973) 443–449.
8] R.L. Hirsch, Impending United States energy crisis, Science 235 (1987) 1467–
58230.
[34] J.O. Morley, M.H. Charlton, Theoretical investigation of the structure and
spectra of zinc phthalocyanines, J. Phys. Chem. 99 (1995) 1928–1934.
[35] H.J.-D.Z. Yong-Fan, L. Jun-Qian, Ab Initio study on some metal
phthalocyanines, Chinese J. Struct. Chem. 2 (2002) 021.
[
1
474.
9] M. Aresta, A. Dibenedetto, A. Angelini, Catalysis for the valorization of exhaust
carbon: from CO to chemicals, materials, and fuels. Technological use of CO
Chem. Rev. 114 (2013) 1709–1742.
10] Y. Zhu, Z. Xu, W. Jiang, S. Zhong, L. Zhao, S. Bai, Engineering on the edge of Pd
2
2
,
36] W. Lu, T. Xu, Y. Wang, H. Hu, N. Li, X. Jiang, W. Chen, Synergistic photocatalytic
[
[
[
3 4
properties and mechanism of g-C N coupled with zinc phthalocyanine
catalyst under visible light irradiation, Appl. Catal. B 180 (2016) 20–28.
37] X. Zhang, T. Peng, L. Yu, R. Li, Q. Li, Z. Li, Visible/near-infrared-light-induced H
2
nanosheet cocatalysts for enhanced photocatalytic reduction of CO
Mater. Chem. A 5 (2017) 2619–2628.
2
to fuels, J.
[
[
[
[
[
[
11] A. Dhakshinamoorthy, S. Navalon, A. Corma, H. Garcia, Photocatalytic CO
reduction by TiO and related titanium containing solids, Energy Environ. Sci.
(2012) 9217–9233.
12] Z. Jiang, X. Liang, H. Zheng, Y. Liu, Z. Wang, P. Wang, X. Zhang, X. Qin, Y. Dai, M.-
H. Whangbo, Photocatalytic reduction of CO to methanol by three-
dimensional hollow structures of Bi WO quantum dots, Appl. Catal. B 219
2017) 209–215.
13] L. Liang, F. Lei, S. Gao, Y. Sun, X. Jiao, J. Wu, S. Qamar, Y. Xie, Single unit cell
bismuth tungstate layers realizing robust solar CO reduction to methanol,
Angewandte Chemie Int. Ed. 54 (2015) 13971–13974.
14] W. Tu, Y. Zhou, Q. Liu, S. Yan, S. Bao, X. Wang, M. Xiao, Z. Zou, An In Situ
simultaneous reduction-hydrolysis technique for fabrication of TiO -graphene
sandwich-like hybrid nanosheets: graphene-promoted selectivity of
2
3 4
production over g-C N Co-sensitized by organic dye and zinc phthalocyanine
2
derivative, ACS Catal. 5 (2015) 504–510.
5
38] A. Burczyk, A. Loupy, D. Bogdal, A. Petit, Improvement in the synthesis of
metallophthalocyanines using microwave irradiation, Tetrahedron 61 (2005)
2
179–188.
2
6
39] H. Uchida, P.Y. Reddy, S. Nakamura, T. Toru, Novel efficient preparative method
for phthalocyanines from phthalimides and phthalic anhydride with HMDS, J.
Org. Chem. 68 (2003) 8736–8738.
(
[
[
2
40] A. Henriksson, B. Roos, M. Sundbom, Semiempirical molecular orbital studies
of phthalocyanines, Theor. Chem. Accounts: Theory Comput. Model.
(
Theoretica Chimica Acta) 27 (1972) 303–313.
2
41] D. Verma, R. Dash, K.S. Katti, D.L. Schulz, A.N. Caruso, Role of coordinated metal
ions on the orientation of phthalocyanine based coatings, Spectrochimica Acta
Part A: Mol. Biomol. Spectrosc. 70 (2008) 1180–1186.
42] R. Seoudi, G. El-Bahy, Z. El Sayed, FTIR, TGA and DC electrical conductivity
studies of phthalocyanine and its complexes, J. Mol. Struct. 753 (2005) 119–
2
D
photocatalytic-driven hydrogenation and coupling of CO
2
into methane and
ethane, Adv. Funct. Mater. 23 (2013) 1743–1749.
[
15] W. Tu, Y. Zhou, Z. Zou, Photocatalytic conversion of CO into renewable
2
hydrocarbon fuels: state-of-the-art accomplishment, Challenges Prospects
Adv. Mater. 26 (2014) 4607–4626.
126.

J. Zheng et al. / Journal of Catalysis 371 (2019) 214–223
223
[
[
[
43] Q. Shen, Z. Chen, X. Huang, M. Liu, G. Zhao, High-yield and selective
photoelectrocatalytic reduction of CO to formate by metallic copper
decorated Co nanotube arrays, Environ. Sci. Technol. 49 (2015) 5828–5835.
44] P. Li, J. Xu, H. Jing, C. Wu, H. Peng, J. Lu, H. Yin, Wedged N-doped CuO with
more negative conductive band and lower overpotential for high efficiency
[46] Z. Yang, J. Xu, C. Wu, H. Jing, P. Li, H. Yin, New insight into photoelectric
converting CO to CH OH on the one-dimensional ribbon CoPc enhanced Fe
NTs, Appl. Catal. B 156–157 (2014) 249–256.
[47] K. Lv, C. Teng, M. Shi, Y. Yuan, Y. Zhu, J. Wang, Z. Kong, X. Lu, Y. Zhu,
Electrocatalysts: hydrophobic and electronic properties of the E-MoS
nanosheets induced by FAS for the CO electroreduction to syngas with a
ratios (Adv Funct. Mater. 49/2018), Adv. Funct. Mater. 28
2
2
3
2 3
O
3 4
O
2
photoelectric converting CO
2
to methanol, Appl. Catal. B 156 (2014) 134–140.
2
45] D. Gao, H. Zhou, J. Wang, S. Miao, F. Yang, G. Wang, J. Wang, X. Bao, Size-
wide range of CO/H
(2018) 1870351.
2
dependent electrocatalytic reduction of CO
Chem. Soc. 137 (2015) 4288–4291.
2
over Pd nanoparticles, J. Am.