Black Fiona A, Clark Charlotte A, Summers Gareth H, Clark Ian P, Towrie Michael, Penfold Thomas, George Michael W, Gibson Elizabeth A
School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK.
School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK.
Phys Chem Chem Phys. 2017 Mar 15;19(11):7877-7885. doi: 10.1039/c6cp05712h.
Understanding what influences the formation and lifetime of charge-separated states is key to developing photoelectrochemical devices. This paper describes the use of time-resolved infrared absorption spectroscopy (TRIR) to determine the structure and lifetime of the intermediates formed on photoexcitation of two organic donor-π-acceptor dyes adsorbed to the surface of NiO. The donor and π-linker of both dyes is triphenylamine and thiophene but the acceptors differ, maleonitrile (1) and bodipy (2). Despite their structural similarities, dye 1 outperforms 2 significantly in devices. Strong transient bands in the fingerprint region (1 and 2) and nitrile region (2300-2000 cm) for 1 enabled us to monitor the structure of the excited states in solution or adsorbed on NiO (in the absence and presence of electrolyte) and the corresponding kinetics, which are on a ps-ns timescale. The results are consistent with rapid (<1 ps) charge-transfer from NiO to the excited dye (1) to give exclusively the charge-separated state on the timescale of our measurements. Conversely, the TRIR experiments revealed that multiple species are present shortly after excitation of the bodipy chromophore in 2, which is electronically decoupled from the thiophene linker. In solution, excitation first populates the bodipy singlet excited state, followed by charge transfer from the triphenylamine to the bodipy. The presence and short lifetime (τ ≈ 30 ps) of the charge-transfer excited state when 2 is adsorbed on NiO (2|NiO) suggests that charge separation is slower and/or less efficient in 2|NiO than in 1|NiO. This is consistent with the difference in performance between the two dyes in dye-sensitized solar cells and photoelectrochemical water splitting devices. Compared to n-type materials such as TiO, less is understood regarding electron transfer between dyes and p-type metal oxides such as NiO, but it is evident that fast charge-recombination presents a limit to the performance of photocathodes. This is also a major challenge to photocatalytic systems based on a "Z-scheme", where the catalysis takes place on a µs-s timescale.
了解哪些因素会影响电荷分离态的形成和寿命是开发光电化学器件的关键。本文描述了如何使用时间分辨红外吸收光谱（TRIR）来确定吸附在NiO表面的两种有机供体-π-受体染料光激发后形成的中间体的结构和寿命。两种染料的供体和π-连接体均为三苯胺和噻吩，但受体不同，分别是马来腈（1）和硼二吡咯（2）。尽管它们结构相似，但在器件中染料1的性能明显优于染料2。染料1在指纹区（1和2）和腈区（2300 - 2000 cm）有很强的瞬态吸收带，这使我们能够监测溶液中或吸附在NiO上（有无电解质存在）的激发态结构以及相应的动力学，其时间尺度在皮秒到纳秒之间。结果表明，在我们的测量时间尺度上，电荷从NiO快速（<1 ps）转移到激发态染料（1），仅产生电荷分离态。相反，TRIR实验表明，染料2中的硼二吡咯发色团激发后不久会出现多种物种，它与噻吩连接体发生电子解耦。在溶液中，激发首先使硼二吡咯单重激发态布居，随后电荷从三苯胺转移到硼二吡咯。当染料2吸附在NiO上（2|NiO）时，电荷转移激发态的存在及其短寿命（τ≈30 ps）表明，2|NiO中的电荷分离比1|NiO中的电荷分离更慢和/或效率更低。这与两种染料在染料敏化太阳能电池和光电化学水分解器件中的性能差异一致。与TiO等n型材料相比，关于染料与NiO等p型金属氧化物之间的电子转移了解较少，但显然快速的电荷复合限制了光阴极的性能。这也是基于“Z-方案”的光催化系统面临的一个重大挑战，在该系统中催化作用发生在微秒到秒的时间尺度上。
