Green radicals of potassium poly(heptazine imide) using light and benzylamine

Article abstract of DOI:10.1039/c9ta09500d

Tinted long-lived ionic carbon nitride radicals were recently introduced and applied in photocatalysis and energy storage. However, the reason for their higher activity in the photocatalytic reaction and optimal conditions for generating such radicals remain vague. Herein, we study the conditions for carbon nitride photocharging to achieve a higher charge density and validate a convenient method to quantify the number of electrons accumulated in carbon nitride semiconductors by quenching its radicals with methylviologen in the dark. In the presence of CO₂, potassium poly(heptazine imide) (K-PHI) can be charged by up to 1000 μmol of electrons per gram of the material using benzylamine as an electron donor. Under the same conditions, mesoporous graphitic carbon nitride can accumulate only 50 μmol of electrons per gram. The products of the benzylamine oxidative coupling are imine and ammonia.

Full text of DOI:10.1039/c9ta09500d

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Green radicals of potassium poly(heptazine imide)
using light and benzylamine†
Cite this: J. Mater. Chem. A, 2019, 7,
24771
Yevheniia Markushyna,‡a Paolo Lamagni,‡abc Christian Teutloff,d
c
bc
a
a
*
Jacopo Catalano,
Nina Lock,
Guigang Zhang,
Markus Antonietti
Received 28th August 2019
Accepted 7th October 2019
a
*
and Aleksandr Savateev
DOI: 10.1039/c9ta09500d
rsc.li/materials-a
Tinted long-lived ionic carbon nitride radicals were recently intro- poly(heptazine imides) are capable of photocharging.8 Fig. 1
duced and applied in photocatalysis and energy storage. However, the sketches the mechanism of IDEAS (Illumination-Driven
reason for their higher activity in the photocatalytic reaction and Electron Accumulation in Semiconductors) – a capacious
optimal conditions for generating such radicals remain vague. Herein, acronym that we propose for this process in the present
we study the conditions for carbon nitride photocharging to achieve communication.
a higher charge density and validate a convenient method to quantify
On the energy scale, it can be schematically explained as
the number of electrons accumulated in carbon nitride semi- follows. By absorbing a photon, potassium poly(heptazine
conductors by quenching its radicals with methylviologen in the dark. imide) (K-PHI) is converted to the excited state (K-PHI*). Elec-
In the presence of CO2, potassium poly(heptazine imide) (K-PHI) can tron transfer from the HOMO of the electron donor (ED) lying
be charged by up to 1000 mmol of electrons per gram of the material above the valence band (VB) of K-PHI reduces K-PHI* and gives
using benzylamine as an electron donor. Under the same conditions, a negatively charged radical anion (K-PHIcꢀ). IDEAS is accom-
mesoporous graphitic carbon nitride can accumulate only 50 mmol of panied by a distinct color change of K-PHI and related materials
electrons per gram. The products of the benzylamine oxidative from yellow to green or blue.8–11 The energy stored in K-PHI is
coupling are imine and ammonia.
then released by exposing K-PHIcꢀ to the electron acceptors
(EAs). This step leads to the recovery of K-PHI and formation of
Introduction
Photocatalysis has become a topical eld of research in
organic chemistry starting from the beginning of 21st century
due to its simple reaction conditions and omnitude.1 Indeed,
the same photocatalyst can promote different chemical reac-
tions.2,3 Due to their chemical stability, ease of preparation,
structural tunability and availability, carbon nitrides became
convenient catalysts in organic synthesis.4–7 Moreover,
aMax-Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry,
Research Campus Golm, 14476 Potsdam, Germany. E-mail: oleksandr.savatieiev@
mpikg.mpg.de
bCarbon Dioxide Activation Center (CADIAC), Aarhus University, Interdisciplinary
Nanoscience Center (iNANO), Department of Chemistry, DK-8000 Aarhus C,
Denmark. E-mail: nlock@eng.au.dk
cSection of Biological and Chemical Engineering, Dept of Engineering, Aarhus
˚
University, Abogade 40, DK-8200 Aarhus N, Denmark
Fig. 1 Schematic representation of IDEAS as exemplified for K-PHI.
Image of K-PHI suspension in acetonitrile in the presence of benzyl-
amine under a protective Ar atmosphere before light irradiation
(yellow) and after light irradiation (green). Abbreviations: VB – valence
band; CB – conduction band; ED – electron donor; EA – electron
acceptor; and EAred – reduced electron acceptor.
d
¨
Freie Universitat Berlin, Department of Physics, 14195 Berlin, Germany
† Electronic supplementary information (ESI) available: Catalyst preparation
procedure and characterization data; 1H, 13C and 15N NMR spectra;
supplementary gures. See DOI: 10.1039/c9ta09500d
‡ These authors contributed equally to this work.
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a reduced electron acceptor (EAred). K-PHI can accumulate up to (primary amine N–H bend), 2184 cmꢀ1 (nitrile stretch), and
several hundred mmol of electrons per gram of the material.9,12 2500–3600 cmꢀ1 (N–H stretch) (Fig. S2i†).22 In water, K-PHI
Due to high reactivity of K-PHIcꢀ, characterization of this breaks into particles of ca. 50 to 200 nm in diameter with an
material is a challenging task. Nevertheless, transient absorp- average size of 94 nm (Fig. S2j†). The ordered structure of the
tion spectroscopy has been used to characterize the green catalyst is readily observed in the high resolution transmission
radicals of cyanamide-functionalized poly(heptazine imide).13,14 electron microscopy (HR-TEM) image of K-PHI (Fig. S2l†).
Earlier, we have shown superior activity of K-PHI in Relaxation of the excited state via a radiative path occurs within
comparison with covalent carbon nitrides in reductive cyclo- 0.66 ns (Fig. S3†).
condensation of chalcones to cyclopentanoles,10 synthesis of N-
In order to quantify the number of electrons accumulated in
fused pyrroles15 and oxidative thiolation of toluene.9 On the K-PHI, we performed a series of experiments. K-PHIcꢀ was
other hand, tinted long-lived carbon nitride radicals have been generated in situ using benzylamine as an electron donor fol-
applied in H2 production8,16 and O2 reduction to H2O2.11,17 Based lowed by the addition of methylviologen dichloride (MV2+). The
on cyanamide-functionalized poly(heptazine imide), Lotsch latter was reduced via a one-electron process to a radical cation
et al. designed a “solar battery” that combines in one device (MVc+) that has a pronounced blue colour. The amount of MVc+
a light harvester and a charge storage reservoir.18
produced upon K-PHIcꢀ quenching with MV2+ was determined
While already plentiful chemistry and unusual properties measuring the solution absorbance at 600 nm (Fig. S4†). The
make K-PHI a promising material to address other synthetic amount of MVc+ is therefore equal to the amount of electrons
challenges, a deeper understanding of the IDEAS mechanism is accumulated in the catalyst. Fig. 2a summarizes this procedure.
required to use carbon nitrides more efficiently in light-
conversion applications.
Using the abovementioned methodology, the electron
capacity of mpg-CN was determined to be 43 ꢂ 5 mmol gꢀ1
,
It should be mentioned that the electron capacity of a rele- however no apparent colour change was observed (Fig. 2b). The
vant carbon nitride material, cyanamide-functionalized poly(- higher surface area of mpg-CN compared to K-PHI, i.e. 180 m2
heptazine imide), has been investigated electrochemically.18 gꢀ1 versus 89 m2 gꢀ1, apparently is not a decisive factor for
However, related to the application in organic photocatalysis by efficient IDEAS. Instead, the crystalline structure and the ability
suspending a semiconductor particulate in an organic solvent, of the material to stabilize negative charges observed for K-PHI
a more convenient method for measuring the electron capacity are important. It has also been reported that cyanamide groups
of carbon nitride would comprise a redox indicator. Such in poly(heptazine imide) are responsible for electron accumu-
a method would not require an electrolyte and a specialized lation.8,21 In the case of K-PHI, a deeper reaction mixture colour
electrochemical setup, while quenching of the reaction mixture was observed in the presence of CO2 (the interaction between
with a suitable redox indicator would allow for quasi in situ CO2 and benzylamine is investigated below), which correlates
quantication of IDEAS.
with a higher electron capacity of 957 ꢂ 82 mmol gꢀ1 compared
Herein, we develop and validate a convenient method to to 701 ꢂ 46 mmol gꢀ1 under Ar. Nevertheless, the electron
quantify IDEAS using benzylamine as a model electron donor capacity of K-PHI under Ar using benzylamine as an electron
and methylviologen as a redox indicator. We compare the donor is still 5.8 times higher compared to that using benzyl
capacity of K-PHI and covalent mesoporous graphitic carbon alcohol as an electron donor.9 This illustrates the much better
nitride (mpg-CN) under Ar and CO2.
ability of amines to quench reductively the excited state of the
carbon nitride photocatalyst. In the control experiment without
the carbon nitride photocatalyst, MV2+ is not reduced by ben-
zylamine alone.
Results and discussion
K-PHI was characterized using different techniques and the
Fig. 2c illustrates the effect of the benzylamine concentra-
summary is given in Fig. S2 and S3.†19 The number and position tion, i.e. the electron donor concentration, on the number of
of diffraction peaks in the PXRD pattern are identical to the electrons accumulated in K-PHI. K-PHI reaches saturation at
ones reported earlier (Fig. S2a†).20,21 The valence band a benzylamine concentration of 60 mmol mLꢀ1, at which the
maximum (VBM) in K-PHI determined by ultraviolet photo- electron capacity approaches 957 ꢂ 82 mmol gꢀ1. This number
electron spectroscopy (UPS) is located at ꢁ2.5 V (Fig. S2e†). K- corresponds to 92 ꢂ 8 C gꢀ1 and is higher than that reported
PHI has the main onset of absorption at 460 nm related to p–p* for cyanamide-functionalized poly(heptazine imide) deter-
transitions and a band of lower intensity with the edge at 650 mined from electrochemical discharge proles (45 C gꢀ1).18
nm (n–p* transitions) (Fig. S2f†). By using the p–p* band, an Given that the molecular weight of a K-PHI monomer unit is
optical band gap of 2.72 eV was determined. K-PHI shows 223 g molꢀ1 (Fig. S5†),23 it can be concluded that every 4th
maximum uorescence at ꢁ500 nm upon sample excitation heptazine unit in K-PHI bears an uncoupled electron, while for
with 350 nm (Fig. S2g†). K-PHI does not possess distinct cyanamide-functionalized poly(heptazine imide) one electron
porosity as evidenced by the N2 sorption isotherm (Fig. S2h†). is being stored on every 9th and 10th cyanamide-heptazine
The specic surface area is 89 m2 gꢀ1. FT-IR shows peaks typical motif.8,18 Taking into account that K-PHI is represented by
for K-PHI and carbon nitrides in general: 800 cmꢀ1 (heptazine particles with an average diameter of 100 nm, the volume-to-
out-of-plane bending), 916 cmꢀ1 and 985 cmꢀ1 (symmetric surface ratio of heptazine units is 26. In other words, in order
vibrations of the metal–NC2 moiety), 1116–1390 cmꢀ1 (C–O to achieve such a high density of trapped electrons, they must
stretch), 1574 cmꢀ1 (secondary amine N–H bend), 1653 cmꢀ1 be stored not only on the surface of the material, but to be
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Fig. 2 Insights into carbon nitride charge storage capacity. (a) Visual appearance of the reaction mixture – K-PHI, benzylamine, acetonitrile,
under Ar (1 bar). From left to right: reaction mixture (under Ar) before light irradiation (yellow), after light irradiation (green), and methyl viologen
(MV2+) addition (blue). Schematic representation of the procedure for measuring the absorption of a MVc+ solution; (b) electron capacity of
carbon nitrides and appearance of the reaction tubes after light was switched off under Ar (1 bar) and CO2 (1 bar). In the blank experiment, no
carbon nitride catalyst was added. Conditions: benzylamine 50 mmol, MeCN 1 mL, time 24 h, T ¼ 35 ꢃC, gas pressure 1 bar; (c) capacity of K-PHI
depending on the benzylamine concentration. Conditions for K-PHIcꢀ generation: MeCN 3 mL, CO2 1 bar, time 24 h, T ¼ 35 ꢃC. Conditions for K-
PHIcꢀ quenching: MV2+ 0.1 mmol; (d) Number of electrons (y-axis) transferred from K-PHIcꢀ to MV2+ versus K-PHI amount (x-axis) equal to the
number of electrons accumulated in K-PHIcꢀ. Conditions for K-PHIcꢀ preparation: initial benzylamine concentration 63 mmol mLꢀ1, reaction
temperature 35 ꢃC, time 24 h, MeCN 3 mL, CO2 1 bar. K-PHIcꢀ quenching conditions: MV2+ 0.1 mmol. See the ESI† for calculation details; (e) EPR
spectra. EPR spectra of K-PHI suspension in MeCN in the dark (grey curves) and under 461 nm light (brown curves) irradiation (BA – benzyl-
amine); (f) quenching of K-PHIcꢀ with AgOTf. Reaction was performed under CO2. Appearance of the reaction mixture and schematic repre-
sentation of the quenching reaction.
distributed in the bulk of the K-PHI particles. These results signal increased drastically supporting that K-PHI* was
underline the similarity between the cathode of an electric quenched by benzylamine and the K-PHIcꢀ radical was formed.
battery and the K-PHI nanoparticle.
Finally, K-PHIcꢀ can be quenched not only with MV2+, but
The plot of the number of trapped electrons versus K-PHI also with AgOTf, necessarily under oxygen-free conditions. In
mass shows a linear trend, supporting the reproducibility of the this case, Ag+ is instantly reduced to Ag0 (Fig. 2f). Formation of
method of IDEAS quantication in the range of K-PHI masses Ag0 was conrmed by the presence of (111) and (200) diffrac-
up to 10 mg per experiment, which is 787 ꢂ 35 mmol gꢀ1 tion peaks in the powder X-ray diffraction pattern of the
(Fig. 2d).
resulting solid (Fig. S6†). Upon the reaction mixture workup,
Furthermore, we studied the suspension of K-PHI in aceto- benzylammonium triate was isolated (Fig. S7†). It suggests
nitrile by EPR (Fig. 2e). In the dark, a broad signal of low that the negative charge of K-PHIcꢀ is compensated by
intensity at 3350 G (g ¼ 2.016, aer magnetic eld calibration) benzylammonium cations, similar to K+ when phosphate
was detected. The intensity of the signal remained almost the buffer is used.18 Taking into account the successful quenching
same when the EPR spectrum was acquired while the sample of K-PHIcꢀ with AgOTf and MV2+ and their standard redox
was irradiated with 461 nm (6 mW cmꢀ2). Upon addition of potentials, E(Ag/Ag+)0 ¼ +0.80 V and E(MV2+/MVc+)0 ¼ ꢀ0.5 V vs.
benzyl amine to the sample in the dark, the broad signal at 3350 SHE, energetically the SOMO of K-PHIcꢀ lies at a potential more
G disappeared and a narrow signal of low intensity appeared at positive than ꢀ0.5 V vs. SHE.
3370 G (g ¼ 2.0048). Under light irradiation, the intensity of the
In order to elucidate the path of benzylamine oxidation, we
performed several experiments in CD3CN using (15N)-
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dichloride. This method is generally applicable for the charac-
terization of semiconducting materials regarding their ability to
accumulate electrons. Carbon nitride materials can be charged
up to at least 957 ꢂ 82 mmol gꢀ1 (92 ꢂ 8 C gꢀ1) using benzyl-
amine as a convenient electron donor in the presence of CO2.
Every 4th heptazine unit in K-PHI thus holds an uncoupled
electron. Optimization of the poly(heptazine imide) structure
toward a higher capacity by IDEAS is one of the strategies to
enhance further the photocatalytic performance of the material.
Selection of stronger electron donors can also potentially
increase the density of accumulated electrons in
semiconductors.
Fig. 3 (a) 15N NMR spectrum of a (15N)benzylamine solution in
CD3CN : DMSO-d6 (4 : 1) before (bottom) and after (top) saturation
with CO2; (b) 15N NMR spectrum of the reaction mixture in CD3CN
after the photocatalytic experiment. The up-field area was magnified
and expanded for clarity.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
substituted benzylamine. In excess of CO2, (15N)benzylamine is
quantitatively converted into benzylcarbamic acid as we
deduced from the 15N, 1H and 13C NMR spectra (Fig. 3a).
Therefore, benzylcarbamic acid is a more plausible electron
donor than benzylamine itself, if the reaction is performed in
the presence of CO2. In the 15N NMR spectrum of the reaction
mixture obtained using (15N)benzylamine as a substrate, we
registered a signal of (15N)imine at 329.3 ppm and a broad peak
The authors are grateful to the Deutsche For-
schungsgemeinscha for the nancial support (DFG-An 156 13–
1). PL and NL acknowledge the Danish National Research
Foundation (DNRF 118, CADIAC). PL, NL and JC thank the AU
Synergic grant for partially supporting their work.
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