Interfacial Electron Transfer between Fe(II)(CN)₆^4- and TiO₂ Nanoparticles:  Direct Electron Injection and Nonexponential Recombination

Abstract

Photoinduced electron-transfer (ET) dynamics in Fe(II)(CN) 6 4sensitized TiO 2 nanoparticles in D 2 O solution are studied by subpicosecond tunable laser spectroscopy in the mid-infrared and visible region. The dynamics of the injected electrons are monitored by the mid-IR absorption of electrons in the semiconductor, and the corresponding dynamics of the adsorbate are monitored by the vibrational spectra of the CN stretching mode region and electronic absorption in the visible. After 400 nm excitation, the forward electron injection time from Fe(II)(CN) 6 4to TiO 2 occurs in <50 fs, indicating a direct photoinduced charge-transfer process. The back ET from TiO 2 to Fe(III)(CN) 6 3in the <1 ns time scale is found to be a non-single-exponential process. The best three-exponential fit to the data yields back ET time constants of 3 ps (35%), 40 ps (30%), and >1 ns (35%). Combining with previous measurements in the nanosecond to microsecond time scale (Lu et al.

Quantum Dynamics of Photoinduced Electron-Transfer Reactions in Dye−Semiconductor Systems: First-Principles Description and Application to Coumarin 343−TiO2

“…84 The adsorption site of the clusters is characterized by two titanium atoms from the (101) surface of anatase (see below). The minimal model that can capture all of these features is the cluster (TiO 2 ) 2 (H 2 O) 5 ( Figure 1, left image). However, significantly larger clusters had to be considered to minimize boundary effects.…”

“…3,5,6,13,81 This discrepancy can have a variety of reasons. As discussed above, the electron-injection dynamics are not a single-exponential decay but also exhibit a slower injection component as well as oscillatory structures, which may result effectively in an overall longer injection time.…”

“…In particular, the process of electron injection from an electronically excited state of a dye molecule into a semiconductor substrate has been investigated in great detail experimentally in recent years. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] This process represents a key step for photonic energy conversion in nanocrystalline solar cells. 2,6,9,18,19 Employing femtosecond spectroscopy techniques, it has been demonstrated that electron-injection processes at dye-semiconductor interfaces often take place on an ultrafast (sub-picosecond) time scale.…”

The Role of Surface States in the Ultrafast Photoinduced Electron Transfer from Sensitizing Dye Molecules to Semiconductor Colloids

“…If this state lies energetically above the conduction band edge of the colloid, electron injection to the semiconductor can occur on a fast or ultrafast time scale. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] The resulting charge-separated system was reported to undergo relaxation and recombination processes, with typical time constants in the range from 10 fs up to 500 µs, 9,12,21,24,25 dependent on the dye/semiconductor system. According to Marcus theory one of the main parameters for the electron injection rate is the difference in free energy ∆G between the donor and acceptor states.…”

Transient Absorption Spectroscopy of Ruthenium and Osmium Polypyridyl Complexes Adsorbed onto Nanocrystalline TiO2 Photoelectrodes

“…Transient absorption experiments in the visible and infrared spectral regions have revealed that the electron injection process occurs on femtosecond to picosecond time scales. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Many of the studies reported to date have focused on the dynamics of electron injection for Ru(H 2 L′) 2 (NCS) 2 (the socalled "N3" sensitizer), where H 2 L′ is 4,4′-dicarboxylic acid-2,2′-bipyridine, adsorbed on nanocrystalline TiO 2 . [1][2][3][4][5][6][7][8][9] This system has been well-studied because Ru(H 2 L′) 2 (NCS) 2 adsorbed onto TiO 2 has been reported to produce 5-10% solar energy conversion efficiencies under Air Mass (AM) 1.5 conditions.…”