Carboxylic Anchoring Dye p-Ethyl Red Does Not Adsorb Directly onto TiO2 Particles in Protic Solvents (original) (raw)
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Physical Chemistry Chemical Physics, 2012
First-principles computer simulations can contribute to a deeper understanding of the dye/ semiconductor interface lying at the heart of Dye-sensitized Solar Cells (DSCs). Here, we present the results of simulation of dye adsorption onto TiO 2 surfaces, and of their implications for the functioning of the corresponding solar cells. We propose an integrated strategy which combines FT-IR measurements with DFT calculations to individuate the energetically favorable TiO 2 adsorption mode of acetic acid, as a meaningful model for realistic organic dyes. Although we found a sizable variability in the relative stability of the considered adsorption modes with the model system and the method, a bridged bidentate structure was found to closely match the FT-IR frequency pattern, also being calculated as the most stable adsorption mode by calculations in solution. This adsorption mode was found to be the most stable binding also for realistic organic dyes bearing cyanoacrylic anchoring groups, while for a rhodanine-3-acetic acid anchoring group, an undissociated monodentate adsorption mode was found to be of comparable stability. The structural differences induced by the different anchoring groups were related to the different electron injection/recombination with oxidized dye properties which were experimentally assessed for the two classes of dyes. A stronger coupling and a possibly faster electron injection were also calculated for the bridged bidentate mode. We then investigated the adsorption mode and I 2 binding of prototype organic dyes. Car-Parrinello molecular dynamics and geometry optimizations were performed for two coumarin dyes differing by the length of the p-bridge separating the donor and acceptor moieties. We related the decreasing distance of the carbonylic oxygen from the titania to an increased I 2 concentration in proximity of the oxide surface, which might account for the different observed photovoltaic performances. The interplay between theory/simulation and experiments appears to be the key to further DSCs progress, both concerning the design of new dye sensitizers and their interaction with the semiconductor and with the solution environment and/or an electrolyte upon adsorption onto the semiconductor.
ChemPhysChem, 2003
Dye sensitisation of wide bandgap semiconductors has been extensively studied in the fields of photography and photovoltaics. Strong electronic coupling between dyes featuring suitable anchoring groups and metal oxides can lead to electron injection from the excited state into the solid in the subpicosecond domain. [3±9] Time-resolved transient absorption studies of TiO 2 and ZrO 2 nanoparticles sensitised by alizarin have demonstrated that ultrafast injection may occur not only into the conduction band but also into empty surface states located in the bandgap. These studies also showed that the lifetime of the charge separated state in the TiO 2 ± alizarin system exhibits a multiexponential relaxation with a fast component of the order of 430 fs. However, a substantial fraction of the charge separated state feature lifetimes beyond the nanosecond time scale. Previous studies by Moser and Gr‰tzel have shown that the slower component is of the order of 500 ms. This slow back electron transfer has been rationalised in terms of the inverted Marcus region for electron transfer. On the other hand, Haque et al. have indicated that population/depopulation dynamics in traps levels can be the determining factor in the rate of back electron transfer in dye-sensitised mesoporous TiO 2 nanocrystalline photoelectrodes. The lifetime of the charge separated state is one of the key points behind the high quantum efficiencies exhibited by dye sensitised nanocrystalline (DSNC) solar cells. In these systems, the fast injection process is followed by the regeneration of the dye by a redox couple present in the electrolyte phase. This process is equivalent to the cosensitisation reaction employed in photography. Assuming that all injected electrons arriving to the back contact are collected in the external circuit (short-circuit conditions), the photocurrent efficiency is determined by the diffusion of electrons across the mesoporous film and the rate of electron capture by species in solution. Independent measurements of both parameters revealed opposite dependencies on the light intensity, which is consistent with the weak relationship between the incident photon-to-current conversion efficiency (IPCE) and the illumination intensity. [13±16] In our previous publication, we explored a fundamentally different approach in which TiO 2 colloids and redox couples are separated by a polarisable liquid j liquid junction. Under potentiostatic (short-circuit) conditions, the origin of the photoresponses is connected to charge transfer processes across the boundary between the two immiscible electrolyte solutions. For instance, the transfer of valence band holes from the particles in the aqueous electrolyte to an electron donor in the organic phase manifests itself by a photocurrent signal. In the present communication, we demonstrate that photocurrent responses originating from the photooxidation of ferrocene can be extended into the visible region by dye sensitisation of TiO 2 particles assembled at the water j 1,2-dichloroethane (DCE) interface.
Heterogeneous ruthenium dye adsorption on nano-structured TiO 2 films for dye-sensitized solar cells
Current Applied Physics, 2010
TiO2 particles of single-phase anatase nanocrystallites were prepared by the hydrolysis of titanium-tetraisopropoxide under acidic condition and characterized by XRD, FE-SEM, and BET analysis. The adsorption isotherms of dye molecule on TiO2 particles were obtained at three different temperatures (298.15, 313.15, 333.15 K) and the experimental data were correlated with Sips isotherm model. Also the isosteric enthalpies of dye adsorption were calculated by the Clausius–Clapeyron equation. The influence of heterogeneous adsorption of cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabutylammonium dye (N719) on the energy conversion efficiency of solar cell was investigated on the basis of photocurrent–potential curves. The results showed that the conversion efficiency of dye-sensitized solar cell was highly dependent on the heterogeneous adsorption properties of N719 dye on TiO2 films.
Physical Chemistry Chemical Physics, 2010
The electronic and molecular properties of three organic dye molecules with the general structure donor-linker-anchor have been investigated using core level photoelectron spectroscopy (PES). The molecules contain a diphenylaniline donor unit, a thiophene linker unit, and cyanoacrylic acid or rhodanine-3-acetic acid anchor units. They have been investigated both in the form of a multilayer and adsorbed onto nanoporous TiO 2 and the experimental results were also compared with DFT calculations. The changes at the dye-sensitized TiO 2 surface due to the modification of either the donor unit or the anchor unit was investigated and the results showed important differences in coverage as well as in electronic and molecular surface properties. By measuring the core level binding energies, the sub-molecular properties were characterized and the result showed that the adsorption to the TiO 2 influences the energy levels of the sub-molecular units differently.
International Journal of Photoenergy, 2013
We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfophenylazo)salicylic acid disodium salt, known as Mordant Yellow 10 (MY-10), used as sensitizer for TiO 2 dye-sensitized solar cells (DSSCs). Given the need to better understand the behavior of the dyes adsorbed on the TiO 2 nanoparticle, we studied various single and double deprotonated forms of the dye bound to a TiO 2 cluster, taking advantage of the presence of the carboxyl, hydroxyl, and sulfonic groups as possible anchors. We discuss various binding configurations to the TiO 2 substrate and the charge transfer from the pigment to the oxide by means of DFT calculations. In agreement with other reports, we find that the carboxyl group tends to bind in bidentate bridging configurations. The salicylate uses both the carboxyl and hydroxyl substituent groups for either a tridentate binding to adjacent Ti(IV) ions or a bidentate Ti-O binding together with an O-H-O binding, due to the rotation of the carboxyl group out of the plane of the dye. The sulfonic group prefers a tridentate binding. We analyze the propensity for electron transfer of the various dyes and find that for MY-10, as a function of the anchor group, the DSSC performance decreases in the order hydroxyl + carboxyl > carboxyl > sulfonate. An efficient solar cell sensitizer should demonstrate (i) strong adsorption to the semiconductor surface through anchoring groups, (ii) intense absorption in the visible part of the spectrum, (iii) proper energy level alignment of the excited state of the dye and the conduction band edge of the semiconductor, as well as the redox level of the electrolyte and the ground state of the dye, (iv) fast charge transfer from the dye to the substrate, with low loss of photoelectrons, and (v) electrochemical and thermal stability [1, 4, 12].
Physical Review B, 2009
Using the first-principles plane-wave pseudopotential method within density functional theory, we systematically investigated the interaction of perylenediimide (PDI)-based dye compounds (BrPDI, BrGly, and BrAsp) with both unreconstructed (UR) and reconstructed (RC) anatase TiO2(001) surfaces. All dye molecules form strong chemical bonds with surface in the most favorable adsorption structures. In UR-BrGly, RC-BrGly and RC-BrAsp cases, we have observed that HOMO and LUMO levels of molecules appear within band gap and conduction band region, respectively. Moreover, we have obtained a gap narrowing upon adsorption of BrPDI on the RC surface. Because of the reduction in effective band gap of surface-dye system and possibly achieving the visible light activity, these results are valuable for photovoltaic and photocatalytic applications. We have also considered the effects of hydration of surface to the binding of BrPDI. It has been found that the binding energy drops significantly for the completely hydrated surfaces.
Rapid Dye Adsorption via Surface Modification of TiO2 Photoanodes for Dye-Sensitized Solar Cells
ACS Applied Materials & Interfaces, 2013
A facile method for increasing the reaction rate of dye adsorption, which is the most time-consuming step in the production of dyesensitized solar cells (DSSCs), was developed. Treatment of a TiO 2 photoanode with aqueous nitric acid solution (pH 1) remarkably reduced the reaction time required to anchor a carboxylate anion of the dye onto the TiO 2 nanoparticle surface. After optimization of the reaction conditions, the dye adsorption process became 18 times faster than that of the conventional adsorption method. We studied the influence of the nitric acid treatment on the properties of TiO 2 nanostructures, binding modes of the dye, and adsorption kinetics, and found that the reaction rate improved via the synergistic effects of the following: (1) electrostatic attraction between the positively charged TiO 2 surface and ruthenium anion increases the collision frequency between the adsorbent and the anchoring group of the dye; (2) the weak anchoring affinity of NO 3 − in nitric acid with metal oxides enables the rapid coordination of an anionic dye with the metal oxide; and (3) sufficient acidity of the nitric acid solution effectively increases the positive charge density on the TiO 2 surface without degrading or transforming the TiO 2 nanostructure. These results demonstrate the developed method is effective for reducing the overall fabrication time without sacrificing the performance and long-term stability of DSSCs.