Modeling Ruthenium-Dye-Sensitized TiO₂ Surfaces Exposing the (001) or (101) Faces: A First-Principles Investigation

Abstract

We present a first-principles computational investigation on the adsorption mode and electronic structure of the highly efficient heteroleptic ruthenium dye C101, [NaRu(4,4′-bis(5-hexylthiophene-2-yl)-2,2′-bipyridine)(4carboxylic acid-4′-carboxylate-2,2′-bipyridine)(NCS) 2 ], on anatase TiO 2 models exposing the (001) and ( 101) surfaces. The electronic structure of the TiO 2 models shows a conduction band energy upshift for the (001)-surface ranging between ∼50 and ∼110 meV compared with the (101) surface, in agreement with previous interfacial impedance and recent spectro-electrochemical data. TDDFT excited-state calculations provided the same optical band gap, within 0.01 eV, for the (001)-and (101) models. Two dominant adsorption modes for C101 dye adsorption on the (001) and (101) surfaces were found, which differ by the binding of the dye carboxylic groups to the TiO 2 surfaces (bridged bidentate vs monodentate), leading to sizably different tilting of the anchoring bipyridine plane with respect to the TiO 2 surface. The different adsorption mode leads to a smaller dye coverage on the (001) surface, as experimentally found, due to partial contact of the thiophene and alkyl bipyridine substituents with the TiO 2 surface. For the energetically favored adsorption modes, we calculate a larger average spatial separation, by 1.3 Å, between the dye-based HOMO and the semiconductor surface in (001) and (101) TiO 2 models. In terms of simple nonadiabatic electron-transfer considerations, our model predicts a retardation of the charge recombination kinetics, in agreement with the experimental observations.

Selective photoredox activity on specific facet-dominated TiO2 mesocrystal superstructures incubated with directed nanocrystals

“…Laskova et al also reported that the surface concentration (0.4-0.5 molecules nm −2 ) of C101 dye [cis-bis(isothiocanate)(4,4 -bis(5-hexylthiophene-2-yl)-2,2 -bipyridi-ne)(4-carboxylic acid-4 -carboxylate-2,2 -bipyridine) ruthenium-(II) sodium] was lower on TiO 2 (0 0 1) nanosheets than that on TiO 2 (1 0 1) nanoparticles (0.7-0.8 molecules nm −2 ) [61]. Recent first-principle theoretical calculations suggested that the observed smaller dye coverage on the {0 0 1} facets than that on the {1 0 1} facets is a consequence of a partial contact of the thiophene and alkyl bipyridine substituents of C101 with the TiO 2 surface [59]. Compared with those systems, Ruthenizer 470 does not have bulky substituents and its surface concentrations on the TMC samples are quite low (e.g., 0.023 molecules nm −2 for TMC-4).…”

“…During this key step, electrons are preferentially injected into the {1 0 1} facet, which has a CB level lower by ca. 0.05-0.1 V than that of the {0 0 1} facet [58,59]. Finally, electrons are trapped by Pt and consumed in H + reduction.…”

First Principle Modelling of Materials and Processes in Dye-Sensitized Photoanodes for Solar Energy and Solar Fuels

“…Finally, the sensitizer's grafting group should establish a stable binding of the dye onto the metal oxide surface, thus ensuring long-term stability of the cell [163][164][165]. Investigation of the adsorption of organic dyes onto TiO2 cluster models [161,162,166], has largely shown that the bidentate bridging adsorption mechanism with proton transfer to a nearby surface oxygen is the energetically favored one (Figure 6), while the monodentate anchoring is usually predicted to be less stable, although some dependency on the employed methodology can be outlined [166], For Ru(II) sensitizers different adsorption modes onto the TiO2 surface can be found: while homoleptic dyes, such as N3 or N719, can adsorb on TiO2 using carboxylic anchoring groups residing on different bipyridine ligands and hence using up to three carboxylic groups (Figure 6c) [159,167], heteroleptic dyes, e.g., N621, C106, or Z907, necessarily adsorb via carboxylic groups residing on the same bipyridine (two carboxylic groups, Figure 6d) [168,169]. From an electronic structure point of view, when a dye is anchored to a semiconductor surface, two effects might interplay: (i) the electrostatic (EL) effect, due to the dye dipole moment; and (ii) the effect of the CT between the dye and the semiconductor, which may accompany the dye/semiconductor bond formation.…”

Facet engineering of TiO2 nanocrystals for solar energy conversion

“…The large surface area and high crystallinity are usually required for high dye adsorption capability and rapid electron transport. Faceted anatase TiO2 single crystals with unique surface atomic and electronic structure were also introduced into the electrodes of DSCs [134][135][136][137][138][139][140][141][142][143][144][145][146][147][148][149]. Many researches used faceted anatase TiO2 crystals to fabricate photoanodes.…”

“…From Mott-Schottky (MS) plots, the flatband potential value of {001} facet was negatively shifted by 0.06 V compared to that of {101} facet. Later, De Angelis et al presented a first-principles computational investigation on the adsorption mode and electronic structure of the dye C101 on anatase TiO2 {001} and {101} facet [142]. They theoretically confirmed the negative flatband shift of {001} facet, and explained that the slower charge recombination was attributed to the different dye-anchoring geometries of C101 on {001} and {101} facets ( Furthermore, the plane surface has superior reflectance ability, which can increase the light harvesting of photoanode.…”