Ultrafast Spectroscopy Reveals Electron-Transfer Cascade That Improves Hydrogen Evolution with Carbon Nitride Photocatalysts

Abstract

Solar hydrogen generation from water represents a compelling component of a future sustainable energy portfolio. Recently, chemically robust heptazine-based polymers known as graphitic carbon nitrides (g-CN) have emerged as promising photocatalysts for hydrogen evolution using visible light while withstanding harsh chemical environments. However, since g-CN electron-transfer dynamics are poorly understood, rational design rules for improving activity remain unclear. Here, we use visible and near-infrared femtosecond transient absorption (TA) spectroscopy to reveal an electron-transfer cascade that correlates with a near-doubling in photocatalytic activity from 2050 to 3810 μmol h g when we infuse a suspension of bulk g-CN with 10% mass loading of chemically exfoliated carbon nitride. TA spectroscopy indicates that exfoliated carbon nitride quenches photogenerated electrons on g-CN at rates approaching the molecular diffusion limit. The TA signal for photogenerated electrons on g-CN decays with a time constant of 1/k' = 660 ps in the mixture versus 1/k = 4.1 ns in g-CN alone. Our TA measurements suggest that the charge generation efficiency in g-CN is greater than 65%. Exfoliated carbon nitride, which liberates only trace hydrogen levels when photoexcited directly, does not appear to independently sustain appreciable long-lived charge generation. Thus, the activity enhancement in the two-component infusion evidently results from a cooperative effect in which charge is generated on g-CN, followed by electron transfer to exfoliated carbon nitride containing photocatalytic chain terminations. This correlation between electron transfer and photocatalytic activity provides new insight into structural modifications for controlling charge separation dynamics and activity of carbon-based photocatalysts.

Ultrahigh‐Content Nitrogen‐doped Carbon Encapsulating Cobalt NPs as Catalyst for Oxidative Esterification of Furfural

“…The XRD patterns of g-C 3 N 4 showed ap eak at 27.78,w hich indicated ad ecreased galleryd istance between the basic sheets in the g-C 3 N 4 nanosheets ( Figure S2). [14,15,36,38] These agree well with the significant increase in volume during the exfoliation of bulk g-C 3 N 4 to g-C 3 N 4 nanosheets ( Figure S3). Then, acobalt(II) phenanthroline complex was impregnatedo ng -C 3 N 4 nanosheets or bulk g-C 3 N 4 ,f ollowed by pyrolysis under an itrogen atmosphere.…”

“…Compared with bulk g‐C 3 N 4 , the FT‐IR peaks of g‐C 3 N 4 in the region from 900 to 1800 cm −1 that were attributed to either trigonal C‐N(‐C)‐C or bridging C‐NH‐C units became sharper, probably due to the more ordered packing of hydrogen‐bond cohered long strands of polymeric melon units survived after thermal oxidation etching of bulk g‐C 3 N 4 (Figure S1). The XRD patterns of g‐C 3 N 4 showed a peak at 27.7°, which indicated a decreased gallery distance between the basic sheets in the g‐C 3 N 4 nanosheets (Figure S2) . These agree well with the significant increase in volume during the exfoliation of bulk g‐C 3 N 4 to g‐C 3 N 4 nanosheets (Figure S3).…”

Sulfone-containing covalent organic frameworks for photocatalytic hydrogen evolution from water

“…The partial exfoliation of FS-COF may also contribute. 10,44 The higher BET surface area of 1288 m 2 g -1 for FS-COF versus 919 m 2 g -1 for TP-COF might also enhance the availabliity of photogenerated charges for water reduction.…”

Light‐Controlled Ferromagnetism in Porphyrin Functionalized Ultrathin FeS Nanosheets

“…In a considerable range of laser power, we found that the decay dynamics of the induced TA signal keeps unchanged, which is obviously different from the previously reported no-geminate charge recombination. [25][26][27] Figure 3b shows the time-resolved kinetic curves, in which the temporal evolution of the 600 nm signal amplitude decreases with probe time. The carrier kinetic process can be divided into fast component (τ2 = 34.6 ps) and long lifetime channel (τ3 = 1.8 ns).…”