Department of Chemistry, University of Washington , Seattle, Washington 98195, United States.
J Am Chem Soc. 2017 Jun 14;139(23):7904-7912. doi: 10.1021/jacs.7b02869. Epub 2017 Jun 2.
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.
从水中制取太阳能氢气是未来可持续能源组合的一个重要组成部分。最近，被称为石墨相氮化碳（g-CN）的化学稳定性强的七嗪基聚合物作为可见光下析氢的光催化剂而崭露头角，同时还能在苛刻的化学环境中使用。然而，由于 g-CN 的电子转移动力学还没有被很好地理解，因此，提高其活性的合理设计规则仍不清楚。在这里，我们使用可见和近红外飞秒瞬态吸收（TA）光谱来揭示一个电子转移级联过程，它与光催化活性的近两倍增加相关联，从 2050μmol h g 增加到 3810μmol h g ，当我们将块状 g-CN 的悬浮液注入 10%质量负载的化学剥离氮化碳时。TA 光谱表明，剥离的氮化碳以接近分子扩散极限的速率在 g-CN 上猝灭光生电子。在混合物中，g-CN 上光生电子的 TA 信号衰减时间常数为 1/k'=660 ps，而在 g-CN 中为 1/k=4.1 ns。我们的 TA 测量表明，g-CN 的电荷产生效率大于 65%。剥离的氮化碳在直接光激发时只释放痕量的氢气，似乎不能独立地维持可观的长寿命电荷产生。因此，在两相注入中活性的增强显然是由于一种协同效应，即电荷在 g-CN 上产生，然后电子转移到含有光催化链终止的剥离氮化碳上。这种电子转移与光催化活性之间的相关性为控制基于碳的光催化剂的电荷分离动力学和活性的结构修饰提供了新的见解。
