A highly efficient covalent organic framework film photocatalyst for selective solar fuel production from CO2

Article abstract of DOI:10.1039/c6ta01625a

Two-dimensional covalent organic frameworks (2D COFs) are a class of crystalline polymers with a design controllable platform that may be developed into a new type of metal-free photocatalyst. The exploration of new frameworks is, however, critical for further progress in this emerging field. To realize their full potential in practical light harvesting applications, the fabrication of a film type photocatalyst is equally essential. Here, we report the successful development of a triazine based covalent organic framework (2D CTF) as an inexpensive and highly efficient visible light active flexible film photocatalyst for solar fuel production from CO₂. For this research work, the condensation polymerization between cyanuric chloride and perylene diimide has been exploited for the first time as a new synthetic approach to the construction of 2D CTFs. The visible light-harvesting capacity, suitable band gap and highly ordered π electron channels contribute to the excellent performance of the film photocatalyst. The current study is a benchmark example of COF based photocatalysts for solar fuel production from CO₂ and is expected to trigger further interest in potential solar energy conversion applications such as wearable devices.

Full text of DOI:10.1039/c6ta01625a

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Highly Efficient Covalent Organic Framework Film Photocatalyst for
Selective Solar Fuel Production from CO2
Rajesh K. Yadav, Abhishek Kumar, NoꢀJoong Park, Kiꢀjeong Kong and JinꢀOok Baeg*
Artificial Photosynthesis Research Group, Korea Research Institute of Chemical Technology
(KRICT), 100 Jangꢀdong, Yuseong, Daejeon 305 600, Republic of Korea
*Eꢀmail: jobaeg@krict.re.kr
Abstract: Twoꢀdimensional covalent organic frameworks (2D COFs) are a class of crystalline
polymers with design controllable platform that may be developed into new type of metalꢀfree
photocatalysts. The exploration of new frameworks is however critical for further progress in this
emerging field. To realize their full potential in practical light harvesting applications, fabrication
of film type photocatalyst is equally essential. Here we report the successful development of
triazine based covalent organic framework (2D CTF) as an inexpensive and highly efficient
visible light active flexible film photocatalyst for solar fuel production from CO2. For this
research work, the condensation polymerization between cyanuric chloride and perylene diimide
has been exploited for the first time as a new synthetic approach for the construction of 2D CTFs.
The visible lightꢀharvesting capacity, suitable band gap and highly ordered π electron channels
contribute to the excellent performance of film photocatalyst. The current study is a benchmark
example of COF based photocatalyst for solar fuel production from CO2 and is expected to
trigger further interest in potential solar energy conversion applications such as wearable devices.

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Introduction
Using sunlight to produce solar fuels/chemicals is one of the most promising strategies for
controlling our carbon habit.1–2 It has also spurred demand for wearable devices that are powered
by solar energy. Therefore substantial efforts are being devoted towards the practical
development of visible light harvesting systems. Among them metalꢀfree organic photocatalysts
are highly attractive as their properties can be tailored by suitable chemical modifications.
Moreover, they are inexpensive and easy to fabricate. They however suffer from low efficiencies
and stability which stems primarily from structural defects.3ꢀ4 Therefore identifying and
integrating suitable building blocks under optimal conditions holds the key to improving their
performance.
Covalent organic frameworks (COFs) are a class of polymer networks which possess
long range order due to structural regularity. Since their properties can be tuned by selecting
suitable organic building blocks, twoꢀdimensional COFs (2D COFs) with interesting
optoelectronic properties have emerged recently.5ꢀ7 Taking a cue from these results, photoactive
COFs with charge separation capabilities have also been prepared.8ꢀ11 These findings suggest that
2D COFs are ideal metalꢀfree organic scaffolds which can be developed into new flexible film
type photocatalysts. This is primarily due to columnar stacking of the 2D sheets which provides
highly ordered π electron channel systems thereby facilitating charge carrier transport in the
stacking direction. However it is highly surprisingly that despite the enormous potential, COFs
have been hardly explored as visible light active photocatalysts for the production of solar
fuels.12ꢀ14 Therefore intensive efforts need to be devoted towards the development of COF
photocatalysts for solar fuel production.

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Fig. 1 Schematic illustration of CTF film photocatalystꢀenzyme coupled system involved in
exclusive production of formic acid from CO2. Rhox = [Cp*Rh(bipy)H2O]2+, Rhred1
=
Cp*Rh(bipy), Rhred2 = [Cp*Rh(bipy)H]+ ; Cp*= pentamethylcyclopentadienyl, bpy = 2,2ʹꢀ
bipyridine.
As a part of our ongoing research work,15ꢀ16 we were interested in preparing perylene
containing triazine based COFs (or covalent triazine framework; hereinafter CTF) that may
provide an excellent platform to unify light harvesting capabilities of perylene with the desirable
properties of CTFs for artificial photosynthesis. The synthesis of CTFs is usually carried out by
cyclotrimerisation of appropriate nitrile building units.6,17 However obtaining a suitable perylene
building unit for cyclotrimerisation presented a major synthetic challenge in this research work.
To overcome this bottleneck, we sought to develop a new route for synthesizing the desired CTF.
We now herein report a direct condensation polymerization between cyanuric chloride and
perylene diimide for the first time that constitutes the development of a new facile synthetic
approach for the construction of 2D CTFs. As discussed later, this new framework constructed
from Cyanuric chloride (CC) and 3,4,9,10ꢀPerylenetetracarboxylic diimide (PDI) building units
proved to be an excellent visible light active flexible film photocatalyst for artificial

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photosynthesis (Fig. 1). To the best of our knowledge, this is the first report on solar fuel
production from CO2 by a visible light driven COF film photocatalystꢀbiocatalyst integrated
system.
Experimental
Synthesis of CTF
Cyanuric chloride (0.66 g, 3.6 mmol) was added to an Arꢀfilled 500ꢀmL, twoꢀneck, round bottom
flask equipped with a magnetic stirring bar and a condenser. Perylene diimide (2.17g, 5.16 mmol)
was then added. A mixture of N, Nꢀdiisopropylethylamine (DIPEA) (3.185g, 21.66 mmol) and
THF (tetrahydrofuran, 150 mL, anhydrous), previously deaerated by Ar, was then transferred
into the roundꢀbottom flask. The flask was placed in an oilꢀbath at 90 °C for 18 hours. Within a
few hours, precipitate started to form. The reaction mixture was further refluxed under Ar
atmosphere for 2 days. The solid product thus obtained was separated by suction filtration,
washed several times with DI water, methanol, dimethyl formamide and CH2Cl2, and dried in
oven at 100ºC to afford brown solid product (2.21g, 80.55 % based on CC).
Synthesis of the Monomer
Similar steps as for polymer mentioned above, except that excess PDI was used in order to
minimize polymerization. In this case CC (0.1418g, 0.76 mmol), PDI (1.1289g, 12.11 mmol),
and N,Nꢀdiisopropylethylamine (DIPEA) (0.746g, 5.78 mmol) in THF (50 mL, anhydrous) were
used. Thereafter following identical procedure the solvent was removed on a rotary evaporator to
yield a brown solid. The solid was washed several times with DI water, methanol, dimethyl
formamide and CH2Cl2, and then further dried in oven at 100ºC to afford brown solid product
(0.33g, yield 46.77 % based on CC).

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Preparation of monomer and CTF films and their optimized coatings
The monomer and CTF films were prepared by dropcasting method. For a single coating on a 1 x
1 cm2 polyimide sheet, 31ꢁl of 1mM concentration of the photocatalyst solution (in DMF) was
dropcasted and allowed to dry for 24 hours at 80ºC. The procedure was repeated to obtain films
with higher coatings. Optimum results were obtained for the films with three coatings. Therefore
all research work reported herein is with the triply coated film.
Photocatalytic NADH Regeneration
The photochemical regeneration of NADH was performed within a quartz reactor under an inert
atmosphere at room temperature, using a 450W Xenon lamp (Newport 66921) with a 420 nm
cutꢀoffꢀfilter as light source (100 mW/cm2). The photocatalytic regeneration of NADH was
carried out as follows. The reaction was performed in a quartz reactor. The reaction consisted of
β–NAD+ (1.24 ꢁmol), rhodium complex Rh (0.62 ꢁmol), Ascorbic acid (0.1 mmol) and 1 x 1
cm2 photocatalyst film in 3.1 mL of sodium phosphate buffer (100 mM, pH 7.0). The
regeneration of NADH was monitored by UVꢀvis spectrophotometer (UVꢀ1800, Shimadzu).
The artificial photosynthesis of formic acid from CO2
The artificial photosynthesis of formic acid from CO2 was also performed within a quartz reactor
at room temperature, using a 450W Xenon lamp with a 420 nm cutꢀoffꢀfilter as light source (100
mW/cm2). The reaction consisted of 1 x 1 cm2 photocatalyst film, β–NAD+ (1.24 µmol), rhodium
complex Rh (0.62 ꢁmol) and formate dehydrogenase enzyme (3 units) in 3.1 mL of sodium
phosphate buffer (100 mM, pH 7.0) with Ascorbic acid (0.1 mmol) in the presence of CO2 (flow
rate: 0.5 mL/min). The amount of formic acid was estimated by GC (7890A, Agilent
Technologies).

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Results and Discussion
CTF was synthesized by the condensation of the building blocks in THF at 90°C under an argon
atmosphere. The product was obtained as a brown solid. To remove any starting material or
entrapped solvent, CTF was filtered, washed several times with DI water, methanol, DMF and
DCM, and subsequently dried in oven at 100ºC. A triply coated film photocatalyst on polyimide
support was found to show optimum photocatalytic results which demonstrate the practical
feasibility of the newly prepared 2D CTF (details in experimental section). The photograph of
the flexible film photocatalyst is shown in figure 2 along with the chemical structure of CTF.
Fig. 2 Photograph of the flexible film photocatalyst alongwith detailed chemical structure of
CTF (Gray = C; Blue = N; Red = O).
(d)
(b)
(c)
(a)
(g)
(h)
(e)
(f)

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Fig. 3 (a) AFM roughness image of monomer film. (b) AFM roughness image of CTF film. (c)
AFM 3D image of monomer film. (d) AFM 3D image of CTF film. (e) SEM image of monomer.
(f) SEM image of CTF. (g) HRꢀTEM image of monomer. (h) HRꢀTEM image of CTF.
The atomic force microscopy (AFM) images (3D and roughness) of monomer unit (one
triazine unit bonded to perylene units from all the three sides) and CTF are shown in Figure 3.
The analysis of the films revealed that CTF is significantly thicker (15.1 nm) than the monomer
(3.2 nm).18 The AFM of polyimide substrate is provided in electronic supplementary information
ESI (Fig. S2). It indicates that the observed topography is due to ordering of the monomer and
CTF crystallites on the surface of polyimide substrate. In addition to this, the scanning electron
microscope (SEM) and highꢀresolution transmission electron microscopy (HRꢀTEM) images are
also provided in figure 3. These images show that the CTF consists of parallelly aligned 2D
sheet, which indicates a highly ordered arrangement. The photodeposition of platinum (Pt)
particles was carried out on CTF to visualize the electron channel structure. The HRTEM
analysis of the sample revealed presence of Pt nanoparticles with diameter in the range of 3ꢀ5 nm
(Fig. S3).
CTF is a crystalline material, as indicated by powder Xꢀray diffraction (PXRD)
measurements (Fig. S4). The 001 diffraction peak at 2θ = 26.9º corresponds to an interlayer
distance of 3.3 Å, suggesting a typical van der Waals contact between the aromatic layers of
CTF.13 The average CTF crystal domain size was calculated to be 12 nm from the line
broadening of reflection by using the Scherrer equation and it correlates well with the AFM.
In case of XPS analysis, the C1s spectrum of CTF was deconvoluted into three peaks
(Fig.S5) at 284.9, 283.2 and 281.6 eV. The peak at 284.9 eV was assigned to C=O while the peak
at 283.2 eV was assigned to CꢀN. The highest intensity peak at 281.6 was assigned to aromatic

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CꢀC. The intensity ratio between aromatic CꢀC: C=O: CꢀN was found to be 10:1.9:1 which
matches well the expected value. In a similar manner, the N1s spectrum of CTF was
deconvoluted into two peaks (Fig.S6) at 396.9 and 395.5 eV. The peak at 396.9 eV was assigned
to NꢀC of triazine ring while the higher intensity peak at 395.5 eV was assigned to imide NꢀC of
perylene diimide. The intensity ratio between NꢀC of triazine ring: imide NꢀC of perylene
diimide was found to be 1:1.2 which matches well the expected value.19
The FTꢀIR spectra of CTF and monomer share a number of distinct features that are
absent in the PDI reactant, which includes peaks at 812, 1094, and 1151 cm−1 (Fig. 4a). Such
similarities point to the common structural features between the monomer molecule and the CTF
and are associated with the triazine units. For instance, the medium FTIR peaks at near about
1094 and 1151 cm−1 can be ascribed to the stretch of the C−N bonds of the triazine ring.20ꢀ21
Similarly the peak at 812 cmꢀ1 can be ascribed to triazine semicircle stretching vibration.13
The thermal behavior of monomer and CTF was studied by thermogravimetric analysis
(TGA) (Fig. 4b). TGA analysis showed high thermal stability for monomer and CTF with no
decomposition up to 400 and 600 °C, respectively. These results are on expected lines as CTFs
are widely reported for their high thermal stability.

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(a)
(b)
(c)
Fig. 4 (a) FTIR spectra of CTF and monomer. (b) TGA thermograms of CTF and monomer. (c)
Absorption spectra of CTF and monomer in DMF.
The most prominent UV−vis absorption band of CTF (Fig. 4c) appears at 605 nm
corresponding to optical band gap energy of 2.05 eV. In principle, the observed optical band gap
of CTF appears to be sufficient to carry out visible light driven reduction of rhodium complex
Rh ([Cp*Rh(bpy)H2O]2+;Cp*= pentamethylcyclopentadienyl, bpy = 2,2ʹꢀbipyridine) which

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subsequently leads to NADH regeneration and formic acid production from CO2. This was
further verified by the cyclic voltammetric (CV) study which is discussed later in this work.
Fig. 5 Nitrogen adsorptionꢀdesorption isotherm of CTF. Inset shows the pore size distribution
curve of CTF.
Figure 5 shows the nitrogen adsorptionꢀdesorption isotherm and pore size distribution of
CTF. The nitrogen adsorptionꢀdesorption isotherm shows typical H3 hysteresis loop indicative of
mesoporous solid.22 This was further supported by size distribution which indicated that CTF has
a peak pore diameter of 3.9 nm.
The most significant part of the work was to evaluate the visible light harvesting ability
of the CTF film in comparison to the monomer film photocatalyst. Therefore, visible lightꢀdriven
1,4ꢀNADH photoꢀregeneration, and HCOOH formation ability of both these were examined. The
monomer afforded 20.57 % of NADH regeneration, and 60.76 µmol of HCOOH over a period of
120 minutes. In comparison, 75.88 % NADH regeneration, and 204.14 µmol of HCOOH was
obtained when CTF was evaluated for the photocatalytic activity (Fig. 6). In other words, nearly
3.7 times higher NADH regeneration, and 3.4 times higher HCOOH formation was carried out
by CTF in the same time period when compared to monomer. The high photocatalytic activity is
also evident from the formic acid formation rate which is 881.3 x 106 nmol/gcat/h for the CTF

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photocatalyst while it is 261.85 x 106 nmol/gcat/h for the monomer. In addition to systematic
arrangement of light harvesting units with spatial orientation towards visible light this
remarkably higher photocatalytic activity of CTF can be attributed to the columnar stacking
structure which provide highly ordered π electron channel systems and thus facilitate charge
carrier transport in the stacking direction.
(a)
(b)
Fig. 6 Photocatalytic activities of CTF and monomer for (a) NADH regeneration [β–NAD+
(1.24 ꢁmol), Rh (0.62 ꢁmol), AsA (0.1 mmol) and 1 x 1 cm2 photocatalyst film in 3.1 mL of
sodium phosphate buffer (100 mM, pH 7.0)] and (b) Selective enzymatic production of formic
acid from CO2 (flow rate: 0.5 mL/min) under visible light [β–NAD+ (1.24 µmol), Rh (0.62
ꢁmol), AsA (0.1 mmol), 1 x 1 cm2 photocatalyst film and formate dehydrogenase (3 units) in 3.1
mL of sodium phosphate buffer (100 mM, pH 7.0)].

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To rule out the presence or possible formation of carbon residues due to decomposition of
CTF during photocatalysis which may subsequently produce formic acid without the
consumption of CO2, a control experiment in the absence of CO2 was performed.23 However, no
formic acid formation was observed in this experiment. This clearly indicated the high stability
of CTF photocatalyst under visible light irradiation. In a related manner, one control experiment
without the CTF photocatalyst (but with formate dehydrogenase enzyme) and another one
without visible light irradiation were also performed. A lack of formic acid formation in these
cases confirmed photocatalytic NADH regeneration by CTF exclusively under visible light
irradiation. Additionally, no formic acid formation was detected in another set of control
experiments carried out in the absence of rhodium complex Rh and/or NAD+ which confirmed
their vital role in the functioning of the photocatalyst/biocatalyst integrated system. Alongwith
the high stability of CTF under visible light irradiation the control experiments clearly point
towards exclusive visible light driven photocatalytic NADH regeneration which subsequently
leads to formic acid formation from CO2 by the integrated biocatalyst.
The stability and reusability of the CTF film photocatalyst was examined by subjecting it
to 3 cycles of NADH regeneration. In comparison to the first cycle wherein the photocatalyst
carried out 75.88% (100%) of NADH regeneration, 71.3% (93.96%) was obtained in the third
cycle (Fig S7). This is remarkably high photocatalytic activity and indicates a stable and reusable
film photocatalyst for practical use.
Since cyclic voltammetry (CV) provides a useful insight into the mechanism of the
process, we studied the electrochemical properties of CTF with CV using glassy carbon
(working), silver–silver chloride (reference) and platinum (counter) electrodes at pH 7.0 in a
phosphate buffer (100 mM). The reduction potential of CTF was found to be ꢀ1.01 V (Fig. S8). A

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change in reduction peak was observed when CTF and rhodium complex Rh
([Cp*Rh(bpy)H2O]2+;Cp*= pentamethylcyclopentadienyl, bpy = 2,2ʹꢀbipyridine) were present
together in solution (Fig. 7a). It can be attributed to the interactions between Rh and CTF
photocatalyst. The reduction was observed at _0.85 V which is a cathodic shift of Rh reduction.16
It can also be interpreted as an anodic shift of CTF reduction. The CTF–Rh system showed an
increase in the reduction peak current at ꢀ0.95 V with NAD+, implying that the system consisting
of CTF and Rh was able to catalyze the reduction of NAD+ (Fig. 7a). The absence of any
additional oxidation peak further indicated that CTF–Rh complex followed the electrochemistry
of Rh (twoꢀelectron reduction followed by chemical protonation). The photoꢀelectrochemical
activity of photocatalyst originates from the excitation of electrons from highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), followed by
transfer of the excited electrons to Rh. In the present case, following the photoexcitation of the
CTF electron from HOMO (E = ꢀ5.69 eV) to LUMO (E = ꢀ3.44 eV), it cascades into Rh.16 The
vicinity and potential gradient between the light harvesting CTF photocatalyst and
electrocatalytic Rh center enables this efficient electron transfer from the former to the latter.
The resultant electrically reduced Rh, [Cp*Rh(bpy)] is chemically protonated in aqueous media,
followed by its catalytic reaction with NAD+ leading to 75.5% NADH regeneration.

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(a)
(b)
Fig. 7 (a) Cyclic voltammograms of Rh and CTF in the absence and presence of NAD+. The
potential was scanned at 100 mVsꢀ1 using glassy carbon (working), silverꢀsilver chloride
(reference) and platinum (counter) electrodes in sodium phosphate buffer (100 mM, pH 7.0). (b)
Photocurrentꢀtime (IꢀT) profiles of FTO/CTF and FTO/monomer electrodes under simulated
solar light (1 sun) illumination (three electrodes; scan rate 50 mV/s; input power: 100 mW/cm2;
and electrolyte: 0.1 M NaCl in water; bias potential: 0 to 0.1 V (vs Ag/AgCl)).
To gain further insight, the transient photocurrent was measured on a photo
electrochemical test device fabricated by drop casting CTF in dimethylformamide (DMF) onto
fluorine doped tin oxide (FTO) glass. Figure 7b shows the transient photocurrent response of
CTF under simulated solar light (one sun) illumination. The photocurrent of CTF photocatalyst
was found to be the much higher (~17.5 µA/ cm2) as compared to the monomer (~3.5 µA/ cm2).

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The result indicates that the COF generates charge carriers (electron–hole pairs) upon light
irradiation which thus proves the possibility of using CTF as a visible light active photocatalyst
for the targeted application.
Conclusions
In summary we have successfully developed a new approach for the construction of twoꢀ
dimensional triazine based covalent organic framework (2D CTF) by carrying out the
condensation polymerization between cyanuric chloride and perylene diimide for the first time.
Following the development of new approach, we have successfully demonstrated 2D CTF as a
highly efficient film photocatalyst in the photocatalystꢀbiocatalyst couple for artificial
photosynthesis. The improved lightꢀharvesting capacity resulting from the polymeric structure,
suitable band gap and highly ordered π electron channel systems that greatly facilitates charge
carrier transport are useful features of the current system that may be attributed to the excellent
performance of film photocatalyst. Undoubtedly this research work sets a new benchmark as it
expands the functional and synthetic scope of COFs and in particular triazine based COFs
(CTFs) in the area of visible light active photocatalysts. It constitutes an important step toward
obtaining flexible film type metal free organic systems for artificial photosynthesis and other
potential solar energy conversion applications such as wearable devices.
Acknowledgements
This work was supported by the KRICT Flagship Research Program.
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Journal of Materials Chemistry A
Page 18 of 18
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DOI: 10.1039/C6TA01625A
Graphical Abstract
The successful development of triazine based covalent organic framework as an inexpensive and
highly efficient visible light active flexible film photocatalyst for solar fuel production from CO2
is described.