Building a Bridge from Papermaking to Solar Fuels

Article abstract of DOI:10.1002/anie.201909222

Black liquor, an industrial waste product of papermaking, is primarily used as a low-grade combustible energy source. Despite its high lignin content, the potential utility of black liquor as a feedstock in products manufacturing, remains to be exploited. Demonstrated here in is the use of black liquor as a primary feed-stock for synthesizing graphene quantum dots that exhibit both up-conversion and photoluminescence when excited using visible/near-infrared radiation, thereby enabling the photosensitization of ultraviolet-absorbing TiO₂ nanosheets. In addition, these graphene quantum dots can trap photo-generated electrons to realize the effective separation of electron-hole pairs. Together, these two processes facilitate the solar-powered generation of H₂ from H₂O, and CO from H₂O–CO₂, using broadband solar radiation.

Full text of DOI:10.1002/anie.201909222

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Building a Bridge from Papermaking to Solar Fuels
Authors: Zaiyong Jiang, Xinhan Zhang, Wei Sun, Deren Yang, Paul N.
Duchesne, Yugang Gao, Zeyan Wang, Tingjiang Yan, Zhimin
Yuan, Guihua Yang, Xingxiang Ji, Jiachuan Chen, Baibiao
Huang, and Geoffrey A. Ozin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201909222
Angew. Chem. 10.1002/ange.201909222
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http://dx.doi.org/10.1002/ange.201909222

10.1002/anie.201909222
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Building a Bridge from Papermaking to Solar Fuels
Zaiyong Jiang,# Xinhan Zhang,# Wei Sun,* Deren Yang, Paul N. Duchesne, Yugang Gao, Zeyan Wang,
Tingjiang Yan, Zhimin Yuan, Guihua Yang, Xingxiang Ji, Jiachuan Chen,* Baibiao Huang, Geoffrey A.
Ozin*
[
#] These authors contribute equally to this work.
Abstract: Black liquor, an industrial waste product graphene quantum dots can trap photo-generated
of papermaking, is primarily used as a low-grade electrons to realize the effective separation of
combustible energy source. Despite its high lignin electron-hole pairs. Together, these two processes
content, the potential utility of black liquor as a facilitate the solar-powered generation of H2 from
feedstock in products manufacturing, remains to be H2O, and CO from H2O-CO2, using broadband
exploited. Demonstrated here in is the use of black solar radiation.
liquor as a primary feed-stock for synthesizing
Black liquor is a major waste product of industrial
papermaking.[1] It is composed primarily of lignin,
graphene quantum dots that exhibit both up-
conversion and photoluminescence when excited
together with small amounts of hemicellulose and
sodium salts (i.e., hydroxides and sulfides).[2]
using visible/near-infrared radiation, thereby
enabling the photosensitization of ultraviolet-
Today, black liquor is utilized as a low-grade
absorbing TiO2 nanosheets. In addition, these
combustible energy source, despite being is
considered environmentally unsound and resource-
inefficient.[3] In the Kraft papermaking process,
Dr. Z. Jiang, H. Zhang, Dr. Z. Yuan, Prof. G. Yang, Prof. X. Ji,
Prof. J. Chen
State Key Laboratory of Biobased Material and Green
Papermaking, Qilu University of Technology, Shandong
Academy of Sciences, Jinan 250353, P.R. China
E-mail address: chenjc@qlu.edu.cn
every ton of pulp produced results in 10 tons of
black liquor.[4] Such levels of waste generation via
papermaking is the worldwide norm.[5,6] The vision
Dr. Z. Jiang, Prof. W. Sun, Dr. P. N. Duchesne, Prof. T. Yan,
Prof. G. A. Ozin
of a sustainable papermaking process is imagined
Department of Chemistry, University of Toronto, 80 St. George
St., Toronto, Ontario, M5S 3H6, Canada.
in this work: the transformation of black liquor
E-mail: gozin@chem.utoronto.ca
from a liability to a resource for producing value-
Prof. W. Sun Prof. D. Yang
added products.[7,8]
State Key Laboratory of Silicon Materials, School of Materials
Science and Engineering, Zhejiang University, Hangzhou,
In recent years, polycyclic aromatic
Zhejiang 310027, PR China
E-mail: sunnyway423@zju.edu.cn
hydrocarbons (PAHs) have been shown to be useful
Dr. G. Gao, Prof. Z. Wang, Prof. B. Huang
precursors for synthesizing graphene quantum dots
State Key Lab of Crystal Materials, Shandong University
Jinan 250100 (China)
Supporting information for this article is available on the WWW
(GQDs) with appealing optical properties.[9-11]
under http://dx.doi.org
Biomass from agriculture and forestry, is available
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in large quantities and is rich in PAHs, making it a lattice spacing of approximately 0.205 nm, which
useful precursor for producing GQDs.[12-15] Thus, was close to that of graphene (100) planes.[16] The
in the work described herein it is proposed that height of the GQDs was further measured by
lignin-rich black liquor, could serve as a useful atomic force microscopy (AFM), see Figure 1d-e,
feed-stock for the large-scale production of GQDs, yielding values of 0.3 to 2.1 nm. As far as we know,
thereby significantly increasing the value of this these GQDs are the first ever prepared via using the
waste product.
This proposition is reduced to practice by
black liquor from papermaking.
employing black liquor sourced GQDs as a
visible/near infrared up-conversion sensitizer and
electron collector, thus enabling ultraviolet-
absorbing TiO2 nanosheets to function as a
broadband photocatalyst, generating H2 from H2O,
and CO from H2O and H2O-CO2, under solar
irradiation.
The GQDs were prepared using poplar sulfate
black liquor lignin, as shown in Figure 1a. Inspired
by the method for processing biomass, we isolated
the lignin-rich portion and then applied oxidative
cleavage and aromatic refusion procedures. The
synthesized GQDs could be uniformly dispersed in
water without signs of sedimentation for several
months, indicating their high colloidal stability. As
discussed below, a combination of various
spectroscopic tools demonstrated the characteristic
features of GQDs, indicating the success of the
aforementioned transformation.
Figure 1. (a) Synthetic steps for producing GQDs
from black liquor generated via papermaking; (b-f)
Morphology and size distribution of GQDs, (b)
TEM and (c) HR-TEM images of the GQDs; (d)
AFM image and (e) height distributions of GQDs
by AFM.
The morphology of the GQDs was
characterized via transmission electron microscopy
(TEM), see representative image Figure 1b and
Figure S1, revealing that they were uniformly
dispersed with sizes in the range of 2 to 6 nm. High-
resolution TEM imaging, see Figure 1c, revealed a
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The power X-ray diffraction (PXRD) pattern of to 520 nm (Figure 2b). The GQDs also displayed
GQDs, shown in Figure S2a, exhibited a graphene up-conversion PL, Figure 2c, which has been
oxide (001) peak at 2θ = 12.9° and a graphite peak ascribed to multiphoton excitation.[20] The UV-Vis
at 2θ = 28.9°.[17] The interlayer spacings are 6.86 Å absorption spectrum of GQDs, Figure 2d, has an
and 3.09 Å, respectively, corresponding well with optical adsorption edge around 450 nm. Together,
the PXRD peaks of graphene oxide and bulk both the optical properties and well-known
graphite. Raman spectroscopy, Figure S2b, reveals electrons-capturing properties of the GQDs allow
two diagnostic peaks of graphene at 1375 cm−1 and them to function in a broadband composite
1588 cm−1, corresponding to the D peak (sp3 C) and photocatalyst for generating H2 and CO from H2O
the G peak (sp2 C), respectively.
and H2O-CO2, as described below.
X-ray photoelectron spectroscopy (XPS) of the
GQDs (Figure S2c-f) revealed three types of
carbon: C=C/C-C, C-N and C-O/C=O. Four
distinct nitrogen environments were also observed
in these samples: C-N-C (399.9 eV), N-(C)3 (400.9
eV), N-H (402.0 eV) and N-O (406.3 eV).[18] The
C-N bond in the C 1s spectrum is indicative of
nitrogen doping of the GQDs by nitric acid. The O
1s peaks contributions, observed at 531.6 eV
(C=O), 532.1 eV (-NO2) and 533.1 eV (C-O),
indicated some oxidative cleavage of lignin bonds
during the synthesis of the GQDs from the black
liquor, likely by nitric acid.[13] Oxygen- and
nitrogen-containing functional groups in the GQDs
exist as mid-gap states, which can act as capture
centers for excitons and the source of PL described
below.[19]
Figure 2. (a) Photographs taken of a GQDs
suspension while illuminated by sunlight or a 365
nm UV light, (b) PL spectra of GQDs excited from
280 nm to 450 nm, (c) Up-conversion PL spectra of
GQDs excited from 600 nm to 900 nm, (d)
Ultraviolet-visible (UV-Vis) absorption spectrum
of GQDs.
The GQDs also exhibited some anticipated
diagnostic optical properties. Under 365 nm UV
excitation, a notable blue-green down-conversion
photoluminescence (PL) was observed in Figure
2a. The PL was dependent on the excitation
wavelength and could be tuned in the range of 430
With this background information regarding
the synthesis-structure-property relations of black-
liquor-derived GQDs, their function and utility in
H2O and H2O-CO2 photoreduction was explored.
Different quantities of GQDs were first
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immobilized on the surface of TiO2 nanosheets and confirmed that the photogenerated 13CO originated
subsequently reduced using NaBH4 to improve the from the reduction of 13CO2, Figure S4. In
conductivity of graphene.[21] For comparison, the addition, the PXRD and Raman spectra of used
pristine TiO2 nanosheets (henceforth denoted TiO2) samples, shown in Figure S5, revealed no obvious
were also treated using the same amount of NaBH4. changes in the morphology or crystallinity of TiO2
In both cases, residual NaBH4 was removed via after immobilizing the GQDs.
washing with de-ionized water. The expected
splitting of H2O to produce H2 and O2 was not
initially observed; therefore, methanol (CH3OH)
was employed as a sacrificial reagent for H2
evolution. For this process, it is well-documented
that the H2 is evolved reductively via the reaction
of photoelectrons by H2O, while CH3OH is
oxidized via the reaction of photo-generated
holes.[22, 23]
The results shown in Figure 3a, 3c
demonstrate a significant enhancement of the CO2
to CO and H2O to H2 reduction activities of
GQDs/TiO2 over those of TiO2 under UV-Vis
irradiation, with 0.6 wt% GQDs/TiO2 exhibiting
the best overall rates. The pristine rectangular TiO2
nanosheets shown in the TEM images of Figure 3e
were observed to have average widths and lengths
of 50 nm and 100 nm, respectively. GQDs
immobilization on the TiO2 nanosheets is seen to
Figure 3. (a) Photocatalytic CO2 reduction
activities of GQDs/TiO2 samples under UV-Vis
light, (b) Cycling test results of CO2 reduction
photoactivity of 0.6wt% GQDs/TiO2, (c)
Photocatalytic H2 evolution activity of TiO2 and
GQDs/TiO2 samples, (d) Cycling test results of
photocatalytic H2 evolution activity of 0.6 wt%
GQDs/TiO2, (e) TEM image of pristine TiO2
be fairly uniform, see Figure 3f and Figure S3,
with essentially no change in nanosheets
morphology. The stability of the most active 0.6 wt%
GQDs/TiO2 sample can be seen from the
reproducibility of the H2 and CO evolution
reactions shown in Figure 3b and Figure 3d.
Isotopic 13CO2 tracing experiments analyzed using
gas chromatography-mass spectroscopy (GC-MS)
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nanosheets, (f) TEM image of the 0.6 wt% molecules to O2.[32-34] A similar process occurs for
GQDs/TiO2, (g) Transient photocurrent responses the reduction of adsorbed H2O molecules to H2 as
of the TiO2 and 6 wt% GQDs/TiO2, (h) shown in Figure 4b. Meanwhile, h+ are consumed
Electrochemical impedance spectroscopy (EIS) of by the sacrificial methanol reagent. For both cases,
TiO2 and 6 wt% GQDs/TiO2.
the GQDs/TiO2 faciliated separation of electrons
and holes. Furthermore, the up-conversion PL of
To explore the origin of the aforementioned
GQDs,
can
advantageously
generate
enhancement,
transient
photocurrent
and
photoelectrons in TiO2, thereby exploiting the
wider visible/near-infrared wavelength range of
sun-light and improving the photocatalytic
efficiency.
electrochemical impedance spectroscopy (EIS)
experiments were performed, see Figure 3g-h. The
photocurrent of GQDs/TiO2 increased relative to
pristine TiO2, which implied GQDs can efficiently
promote the separation of photo-generated
electron-hole pairs. Figure 3h shows the arc radius
of GQDs/TiO2 in the EIS Nyquist plot was smaller
compared with that of pristine TiO2, again
supporting the fact that the photogenerated
electron-hole pairs could be well separated.[24-30] In
addition, the UV-Vis spectra of GQDs/TiO2
differed from that of TiO2 showing a visible light
absorption band from 400 to 600 nm from HOMO-
LUMO excitations in GQDs, see Figure S6[31]
.
With the collected results of this study one can
propose a model for the photocatalytic CO and H2
evolution reactions from CO2-H2O and H2O
respectively, as delineated in Figure 4.
In brief, GQDs/TiO2 can be excited by UV-Vis
irradiation to produce photo-generated electrons (e-)
and holes (h+). As shown in Figure 4a, the photo-
generated e- in the conduction band of TiO2 can be
transferred to GQDs, where they reduce adsorbed
CO2 molecules to CO. Concurrently, the
photogenerated h+ can directly oxidize water
Figure 4. Schematic of the photocatalytic (a) CO2-
H2O and (b) H2O reduction pathways to CO and H2
over GQDs/TiO2 under UV-Vis light irradiation.
In summary, GQDs sourced from industrial
black liquor have been successfully immobilized
on the surface of TiO2 nanosheets. Relative to
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pristine TiO2, the resulting GQDs/TiO2 catalysts Carbon Innovation Fund (LCIF); Ontario Centre of
exhibited a remarkable improvement in CO2 and Excellence Solutions 2030 Challenge Fund;
H2O photoreduction activity by GQDs-TiO2 under Alexander von Humboldt Foundation; Imperial
simulated solar light irradiation. The up-conversion Oil; University of Toronto Connaught Innovation
and charge trapping abilities of the GQDs work Fund (CIF); Connaught Global Challenge (CGC)
synergistically to enhance the photocatalytic Fund; Natural Sciences and Engineering Research
performance of GQDs/TiO2 relative to TiO2 alone. Council of Canada (NSERC). Wei Sun and Deren
Thus, the envisioned bridge between the black Yang acknowledge the fund support by Zhejiang
liquor of papermaking and fuels from the sun has University.
been realized in this work, enabling the conversion
Keywords: Graphene quantum dots, Black liquor,
of two waste products (black liquor and CO2) into
Papermaking, Solar fuels, Photocatalytic CO2 Reduction
value-added products. As a result, this fundamental
scientific advance shows great promise for such
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