Nowadays,
the photochemical conversion of CO2 to high-value
products is attracting tremendous research interest. Developing artificial
photocatalysts with excellent catalytic activity and long-term stability
is still a challenge. This work demonstrates that solution-processable
naphthalenedimide (NDI)-based conjugated polymers, PNDI-BT, PNDI-DTBT,
and PNDI-BP, which are copolymerization products of NDI with bithiophene
(BT), 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT), and
biphenyl (BP), respectively, were capable of catalyzing the photochemical
reduction of CO2 to produce CO in the presence of water
without the need for metal-containing co-catalysts or sacrificial
agents. In particular, the PNDI-BP-catalyzed reaction generated CH4 as well as CO. Results from time-resolved photoluminescence,
photovoltage decay, electrochemical impedance spectroscopy, and transient
photocurrent response experiments indicate that PNDI-BP with the largest
dihedral angles along the conjugated backbone possessed the longest
electron lifetime, the lowest charge-carrier recombination rate, and
the smallest interfacial charge transfer resistance. Consequently,
it had the best catalytic performance. Notably, PNDI-BP exhibited
excellent recyclability, robust structural stability, and extremely
steady catalytic activity for more than 330 h during a photocatalytic
CO2 reaction. Furthermore, the solution-processability
of the linear polymer allows the incorporation of porous substrates,
which improve the reaction interface. The catalyst system of PNDI-BP@molecular
sieves with H2O/triethylamine doubled the CO yield to 214.8
μmol·gcat
–1 and enhanced the
CH4 yield by ∼36 times to 61.4 μmol·gcat
–1 in an 18 h reaction.
The application of organic small molecules as metal-free
photocatalysts
for light-driven photoreduction of carbon dioxide (CO2)
has seldom been explored. This work developed four naphthalene diimide
(NDI)-derived donor–acceptor–donor small molecules with
different numbers of thiophene units, namely, NDI-2T, NDI-TT, NDI-4T,
and NDI-6T, as metal-free photocatalysts to catalyze the reduction
of CO2 under irradiation with an air mass 1.5G solar simulator
at one-sun intensity. The structure–property relationship was
investigated by exploring the effects of the electron-donating ability
of the donor units on the optical properties, redox potential, electron–hole
distribution, and exciton lifetime. NDI-6T exhibited the most red-shifted
absorption, longest exciton lifetime, and strongest electron–hole
separation. However, the large upshift in oxidation potential because
of the elevated electron-donating ability of the hexathiophene unit
significantly reduced the driving force for catalyst regeneration,
leading to poor catalytic performance. Alternatively, NDI-4T possessed
proper redox potentials, reduced charge-transfer resistance, and excellent
photocurrent intensity; therefore, it effectively converted CO2 to a single product of CO in the presence of water as an
electron donor without a sacrificial reagent or cocatalyst with a
product yield of 168.6 μmol gcat
–1 24 h–1, which was considerably higher than those
of NDI-TT (111.9 μmol gcat
–1 24
h–1), NDI-2T (88.4 μmol gcat
–1 24 h–1), and NDI-6T (40.5 μmol
gcat
–1 24 h–1). This
study provides a practical guideline for the molecular design of conjugated
organic molecules as promising photocatalysts for CO2 photoreduction.
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