  Taking into account the electron-rich and visible light response of
thiophene, first-principles calculations have been carried out to explore the
photocatalytic activity of donor-acceptor polymers incorporating thiophene and
boron. The designed honeycomb-kagome boron thiophene (BTP) polymers with
varying numbers of thiophene units and fixed B center atoms are direct band gap
semiconductors with tunable band gaps ranging from 2.41 eV to 1.88 eV, and show
high absorption coefficients under the ultraviolet and visible regions of the
solar spectrum. Fine-tuning the band edges of the BTP polymer is efficiently
achieved by adjusting the pore size through the manipulation of thiophene units
between the B centers. This manipulation, achieved without excessive chemical
functionalization, facilitates the generation of an appropriate quantity of
photoexcited electrons and/or holes to straddle the redox potential of the
water. Our study demonstrates that two units between B centers of thiophene in
BTP polymers enable overall photocatalytic water splitting, whereas BTP
polymers with larger pores solely promote photocatalytic hydrogen reduction.
Moreover, the thermodynamics of hydrogen and oxygen reduction reactions proceed
either spontaneously or need small additional external biases. Our findings
provide the rationale for designing metal-free and single-material polymer
photocatalysts based on thiophene, specifically for achieving efficient overall
water splitting.
