LETTER pubs.acs.org/JPCL
Visualizing Interfacial Charge Transfer in Ru-Dye-Sensitized TiO2 Nanoparticles Using X-ray Transient Absorption Spectroscopy Xiaoyi Zhang,*,† Grigory Smolentsev,§,|| Jianchang Guo,‡,#,z Klaus Attenkofer,† Chuck Kurtz,† Guy Jennings,† Jenny V. Lockard,‡ Andrew B. Stickrath,‡ and Lin X. Chen*,‡,^ X-ray Science Division and ‡Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § Research Center for Nanoscale Structure of Matter, Southern Federal University, Sorge 5, Rostov-na-Donu, 344090 Russia Department of Chemical Physics, Lund University, P.O. Box 124, Lund SE-22100, Sweden z Department of Chemistry, the University of Chicago, Chicago, Illinois, 60637, United States ^ Department of Chemistry, Northwestern University, Evanston, Illinois, 60208, United States
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bS Supporting Information ABSTRACT: A molecular level understanding of the structural reorganization accompanying interfacial electron transfer is important for rational design of solar cells. Here we have applied XTA (X-ray transient absorption) spectroscopy to study transient structures in a heterogeneous interfacial system mimicking the charge separation process in dye-sensitized solar cell (DSSC) with Ru(dcbpy)2(NCS)2 (RuN3) dye adsorbed to TiO2 nanoparticle surfaces. The results show that the average Ru-NCS bond length reduces by 0.06 Å, whereas the average Ru-N(dcbpy) bond length remains nearly unchanged after the electron injection. The differences in bond-order change and steric hindrance between two types of ligands are attributed to their structural response in the charge separation. This study extends the application of XTA into optically opaque hybrid interfacial systems relevant to the solar energy conversion. SECTION: Kinetics, Spectroscopy time resolution of our XTA measurements is limited by the ∼80 ps X-ray pulse duration from the synchrotron source, only the RuN3 transient structure of the (TiO2)n-/RuN3þ state after photoinduced interfacial electron injection is captured, rather than the MLCT state of RuN3. The XTA measurements reported here used a laser “pump” pulse to initiate the electron injection from the RuN3 dye molecules to TiO2 anatase nanoparticles and an X-ray “probe” pulse at a specified delay time with respect to the pump pulse to monitor the transient structural changes by X-ray absorption spectroscopy (XAS).12-15,21 Figure 1 displays the XANES (X-ray absorption near edge structure) spectra measured at the Ru K-edge with and without the laser excitation. The laser-on spectrum was taken at nominally 50 ps delay after the laser pump pulse. The photoexcited RuN3 molecules are mostly in the oxidized form, RuIIIN3þ, at this delay time based on previous studies and our optical transient absorption (OTA) measurements under the same conditions. Detailed description of OTA measurements is in the Supporting Information. The absorption edge energy of the laser-on XANES spectrum is slightly up-shifted by ∼0.23 eV, as expected for RuIIIN3þ due to an effectively higher oxidation state of the Ru center after the photoinduced electron injection to the TiO2 lattice.
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hotodriven interfacial charge transfer from transition-metal complex dye molecules to semiconductor nanoparticles is a core reaction for dye-sensitized solar cells (DSSCs).1-8 Extensive studies have been carried out to obtain the energetics, kinetics, and structural dynamics of the interfacial charge transfer processes.1-8 However, both the structural evolution of the adsorbed dye sensitizer and the rearrangement of the nanocrystal surface associated with the electron density shift during and after the interfacial charge injection are mainly obtained through theoretical calculations9-11 and have not been characterized experimentally with atomic scale resolution. Using X-ray transient absorption (XTA),12-18 we report here transient electronic and geometric structures of a ruthenium complex dye sensitizer undergoing interfacial photoinduced charge separation, mimicking the electron injection process in DSSCs. One of the most successful DSSCs uses RuII(dcbpy)2(NCS)2 [aka RuN3 or N3, dcbpy = bis(4,40 -dicarboxy-2,20 -bipyridine] as the sensitizer attached onto the surface of TiO2 nanocrystals in a film as a working electrode. The MLCT (metal-to-ligand-chargetransfer) excited state of RuN3 injects one electron into the conduction band of TiO2, leading to an interfacial chargeseparated state, (TiO2)n-/RuN3þ (Figure 1 inset). The electron injection dynamics from the MLCT state of RuN3 to TiO2 have been described as a dual-exponential function with a fast time constant of
