Time-Resolved Spectroscopic Investigation of Charge Trapping in Carbon Nitrides Photocatalysts for Hydrogen Generation
Time-Resolved Spectroscopic Investigation of Charge Trapping in Carbon Nitrides Photocatalysts for Hydrogen Generation
复制标题
DOI：
10.1021/jacs.7b01547
复制
发表时间：
2017-04-12
影响因子：
15
通讯作者：
Durrant, James R.
中科院分区：
文献类型：
--
作者：
Godin, Robert;Wang, Yiou;Durrant, James R.
氮化碳(g-C3N4)作为一种基准聚合物光催化剂，由于其可见光催化性能、良好的稳定性和易于合成等优点，引起了人们极大的研究兴趣。然而，人们对g-C3N4的基本光物理过程知之甚少，这是解释和促进光活性的关键。利用时间分辨吸收光谱和光致发光光谱，我们在飞秒到秒的时间尺度上研究了一系列氮化碳的光物理。自由载流子在200fs的激发脉冲内形成，在皮秒时间尺度上陷阱，陷阱态在一定能量范围内，然后与指示电荷捕获-脱陷过程的幂函数衰变重新结合。延迟光致发光归因于俘获载流子回到导带/价带的热激发。我们建立了一个简单的、定量的模型来描述这些光催化剂中的载流子动力学，该模型将载流子弛豫包含在陷阱态的指数尾部，并延伸到禁带中，最高可达1.5 eV。这种捕获降低了表面光催化反应的效率。在微米到毫秒的时间尺度上观察到的深陷电子不能还原表面或溶液中的电子受体。在一系列g-C3N4中，这些非反应俘获电子的产额与H-2的析出速率成反比。我们最后认为，这些氮化碳材料的光物理与无机半导体比共轭聚合物更接近，优化这些材料的光催化活性的关键挑战是防止电子陷阱进入深度和光催化不活跃的电子陷阱状态。
Carbon nitride (g-C3N4) as a benchmark polymer photocatalyst is attracting significant research interest because of its visible light photocatalytic performance combined with good stability and facile synthesis. However, little is known about the fundamental photophysical processes of g-C3N4, which are key to explain and promote photoactivity. Using time-resolved absorption and photoluminescence spectroscopies, we have investigated the photophysics of a series of carbon nitrides on time scales ranging from femtoseconds to seconds. Free charge carriers form within a 200 fs excitation pulse, trap on the picosecond time scale with trap states in a range of energies, and then recombine with power law decays that are indicative of charge trapping-detrapping processes. Delayed photoluminescence is assigned to thermal excitation of trapped carriers back up to the conduction/valence bands. We develop a simple, quantitative model for the charge carrier dynamics in these photocatalysts, which includes carrier relaxation into an exponential tail of trap states extending up to 1.5 eV into the bandgap. This trapping reduces the efficiency of surface photocatalytic reactions. Deep trapped electrons observed on micro- to millisecond time scales are unable to reduce electron acceptors on the surface or in solution. Within a series of g-C3N4, the yield of these unreactive trapped electrons correlates inversely with H-2 evolution rates. We conclude by arguing that the photophysics of these carbon nitride materials show closer parallels with inorganic semiconductors than conjugated polymers, and that the key challenge to optimize photocatalytic activity of these materials is to prevent electron trapping into deep, and photocatalytically inactive, electron trap states.