Observation of pH-Dependent Back-Electron-Transfer Dynamics in Alizarin/TiO₂ Adsorbates:  Importance of Trap States

Abstract

The dependence of the interfacial electron transfer in alizarin-sensitized TiO2 nanoparticles on the sample pH has been examined via transient absorbance spectroscopy in the visible spectral region (443-763 nm). Excitation of the alizarin/TiO2 system with visible pump pulses (lambdaexc = 500 nm) leads to a very fast electron injection (tauinj < 100 fs) over a wide pH range. Back electron transfer shows complicated multiphasic kinetics and strongly depends on the acidity of the solution. The strong dependence of back-electron-transfer dynamics on the ambient pH value is explained by a Nernstian-type change in the semiconductor band energy. Indeed, a variation of pH values over 7 units leads to a approximately 0.42 eV change of the conduction band edge position (i.e., the nominal free energy of the electron in the electrode). Assuming a pH-independent redox potential of the dye, this change was sufficient to push the system to a condition where direct photoinitiated electron injection to intraband gap surface states could be investigated. The existence of an electron-transfer pathway via surface trap states is supported by the similarity of the observed back-electron-transfer kinetics of alizarin/TiO2 at pH 9 and alizarin/ZrO2 reported in earlier work (J. Phys. Chem. B 2000, 104, 8995), where the conduction band edge is approximately 1 eV above the excited state of the dye. The influence of surface trap states on interfacial electron transfer has been studied, and a detailed analysis of their population, depopulation, and relaxation kinetics is performed. Therefore, alizarin adsorbed on the surface of TiO2 nanoparticles is an ideally suited system, where pH-dependent investigations allow a detailed study of the electron dynamics in trap states of TiO2 nanoparticles.

Ultrafast Photoinduced Processes in Alizarin‐Sensitized Metal Oxide Mesoporous Films

“…[12,19,[22][23][24] The system consisting of the organic dye alizarin and TiO 2 is characterized by a strong electronic coupling. Theoretical [30] and experimental [21,25,27] approaches showed that the excited state of alizarin is close to the conduction band edge of TiO 2 , a position with a reduced density of acceptor states, which makes this system an interesting candidate for investigations on interfacial ET. Despite the relative position of the alizarin excited state and the conduction band edge of TiO 2 , the FET is not less efficient than that from dyes with an excited state well above the conduction band edge.…”

“…[35][36][37] The importance of these trap states for the BET dynamics in the dyesensitized TiO 2 nanoparticles was shown earlier by our group. [21,27] The great surface-to-volume ratio of semiconductor nanoparticles ensures a highly efficient coupling of dye molecules. However, this high surface-to-volume ratio also leads to a high density of trap states, which inevitably influences the BET kinetics in these dye-sensitized semiconductors.…”

Electronic Structure of the Perylene–Zinc Oxide Interface: Computational Study of Photoinduced Electron Transfer and Impact of Surface Defects

“…In such devices, the photoinduced electron-transfer (ET) reactions at the dye-semiconductor interface, in particular the processes of electron injection from an electronically excited state of a chemisorbed dye molecule into the semiconductor substrate, represent a key step for photonic energy conversion [33][34][35][36][37][38] . In recent years, photoinduced ET processes in dye-semiconductor systems have been studied in great detail experimentally with femtosecond spectroscopy techniques 34,35,37,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] . It has been shown that electron-injection processes at the dye-semiconductor interfaces often take place on an ultrafast sub-picosecond timescale 37,43,45,46,57,58 .…”

Donor/Acceptor Adsorbates on the Surface of Metal Oxide Nanoporous Films: A Spectroscopic Probe for Different Electron Transfer Pathways

“…[12,22,23] The fast electron injection (< 100 fs) from photoexcited alizarin directly to the surface state of the TiO 2 colloid has also been observed in alkaline (pH 9) aqueous solution. [24] The injected electrons in the dye-sensitized solar cell then travel through TiO 2 nanoporous film to the collecting contact or may recombine with the dye cation [back electron transfer (BET)]. Widespread BET dynamics on a timescale of femtoseconds up to microseconds has been observed [18,[25][26][27] and the variability of BET is normally explained by strong contributions of semiconductor effects such as charge trapping and spatial diffusion, which is strongly defined by the presence of the trapping states.…”

“…The ultrafast dynamics in such a system was investigated by femtosecond time-resolved spectroscopy in the UV/Vis and mid-IR spectral regions. We used the dye alizarin, which is well known as a suitable electron donor, [12,13,24] as a sensitizer (see Scheme 1).…”