Theoretical Investigation of Adsorption, Dynamics, Self-Aggregation

Article pubs.acs.org/JPCC

Theoretical Investigation of Adsorption, Dynamics, Self-Aggregation, and Spectroscopic Properties of the D102 Indoline Dye on an Anatase (101) Substrate Susanna Monti,*,†,‡ Mariachiara Pastore,*,§,∥ Cui Li,‡,⊥ Filippo De Angelis,§,# and Vincenzo Carravetta⊥ †

CNR-ICCOM, Institute of Chemistry of Organometallic Compounds, via G. Moruzzi 1, I-56124 Pisa, Italy Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden § Computational Laboratory for Hybrid Organic Photovoltaics (CLHYO), Institute of Molecular Science and Technologies (ISTM-CNR), Via Elce di Sotto, 8, I-06123, Perugia, Italy ∥ CNRS, Théorie−Modélisation−Simulation, SRSMC, , Boulevard des Aiguillettes, 54506 Vandoeuvre- lés-Nancy, France ⊥ CNR-IPCF, Institute of Chemical and Physical Processes, via G. Moruzzi 1, I-56124 Pisa, Italy # CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy ‡

S Supporting Information *

ABSTRACT: A coherent account of adsorption modes, dynamics, selfaggregation, and spectroscopic properties of an indoline organic dye adsorbed on TiO2 anatase (101) substrates is reported. The study is performed by combining reactive molecular dynamics (reaxFF) simulations with time-dependent density functional theory calculations, and the reliability of the results is assessed through comparison with theoretical and experimental data available in the literature. The use of a theoretical multilevel approach has proven to be crucial to gain a deep understanding, at an atomistic level, of the morphology and electronic properties of dye-sensitized heterogeneous interfaces. A realistic description of the functionalized anatase (101) interface, where a variety of binding modes are present, has been achieved by means of extensive molecular dynamics simulations of the adsorption of dye clusters made of different molecular units on medium/large size TiO2 anatase slabs. Our results disclose that the main driving forces toward formation of ordered surface aggregates are π stacking and T-shaped interactions between the aromatic rings of the donor moiety of the molecules, as well as the tendency to maximize the anchoring points with the surface. The dye aggregates were found to be organized in domains, characterized by a different orientation of the packing units, and, in the high coverage limit, presenting a certain degree of short-to-medium range order.

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determining the overall DSCs conversion efficiency.8−11 A recent work12 also reports on the use of dye SAM as hole transporting material, by exploiting intermolecular hole migration. The simplest picture, mainly grounded on the Langmuir-type adsorption kinetic,13,14 is that dye molecules organize themselves on the TiO2 surface into an highly ordered SAM,15,16 even if the existence of dye multilayers17 and highly inhomogeneous films18 has been reported. Although dye sensitizers generally give close packing when adsorbed on TiO2 substrates,19−25 the surface coverage is the result of the interplay of different factors, such as the molecular structure of the dye and the general dyeing conditions (e.g., solvent, dipping time, use of coadsorbents, etc.).26−28 Typical D-π-A organic

INTRODUCTION Self-assembly of organic molecules on semiconductor surfaces is of pivotal importance in a variety of technological applications, ranging from molecular electronics and medical devices to photocatalysis and photovoltaics.1−5 In particular, surface functionalization is at the heart of a Dye-Sensitized Solar Cell (DSC),5−7 which, in fact, in the simplest picture, consists of a mesoporous wide band gap oxide (generally TiO2) substrate sensitized by a self-assembled monolayer (SAM) of dye molecules. The dye layer absorbs the sun light radiation and injects the photoexcited electrons into the manifold of semiconductor unoccupied states. The oxidized dye is then regenerated by the redox shuttle, which is in turn reduced by a catalyst at the counter electrode, closing the circuit. As the charge separation processes taking place at the multiple metal oxide/dye/electrolyte heterointerfaces are crucial to the device functioning, the morphology and electronic and optical properties of the dye layer are of outstanding relevance in © 2016 American Chemical Society

Received: November 19, 2015 Revised: January 15, 2016 Published: January 19, 2016 2787

DOI: 10.1021/acs.jpcc.5b11332 J. Phys. Chem. C 2016, 120, 2787−2796

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The Journal of Physical Chemistry C

arrangement, topography and interfacial energy level alignment of dye-sensitized TiO2. Here we focus on the computational modeling of the TiO2 sensitization process by the indoline D102 dye in vacuo, increasing the complexity of the static aggregation models proposed in our previous investigation.56 By mixing TDDFT excited state calculations with extensive reactive force fieldbased simulations, we develop a theoretical framework capable to provide an atomistic description of the morphology (aggregation, short/long-range order, different anchoring configurations) and optical properties of the D102 sensitized TiO2 substrates. Our MD simulations, at different surface coverages, reveal that the collective arrangement of the dye molecules on the substrate to form close packed aggregate “islands” is mainly governed by two forces: (i) maximization of the anchoring points with the surface via a quasi tridentate coordination involving both carboxylic oxygens and the rhodaninic sulfur atom; (ii) maximization of intermolecular π stacked and T-shaped interactions. High dye densities seem also to favor a more ordered organization of the molecules in separate domains and a larger probability of establishing a three-points coordination; similar density-dependent changes in the monolayer morphology were also reported for Z907sensitized TiO2 surfaces.25

dyes bearing the cyanoacrylic anchoring group show high surface coverage (ca. 2 molecules per nm2),20,21 whereas dyes characterized by a rhodanine-3−acetic acid anchoring group present lower coverages, in the range of 0.3−0.6 molecules per nm2.19 This is basically due to the different dye binding mode induced by the rhodanine anchoring group,29−32 which leads to the dye lying bent with an angle of ∼45° with respect to the plane of the TiO2 surface. This adsorption configuration, being peculiar of indoline dyes,33 such as D10234 and D149,35 is characterized by strong π-stacking interactions, which are manifested by both a broadened absorption spectrum and a fast hole diffusion when the dyes are adsorbed on the TiO2 substrate.13,36 Since dye-aggregation is considered responsible for reduced device efficiencies (mainly due to intermolecular excited state quenching phenomena34,35,37−40) it is generally controlled,41 or even prevented, by the use of antiaggregation coadsorbents.42,43 Beneficial effects (higher photocurrents) coming from dye aggregation have, however, been reported in the literature, mainly related to the extension of the spectral portion of absorbed light.38,44−46 This tight connection between the macroscopic DSCs efficiency and the molecular and supra-molecular properties of the dye layer justifies the extensive experimental and computational research conducted, in the past few years, on the conformational, self-assembling, optical, and charge transfer properties of differently sensitized TiO2 surfaces.20,24,25,36,47−54 By virtue of its high-efficiency in solid state devices55 and strong tendency to form molecular aggregates on the TiO2 surface, a number of works were focused on the indoline D102 dye33 both in solution and in dye-sensitized TiO2 substrates.33,36,56−61 To the best of our knowledge, the first computational study on the aggregation of organic dyes adsorbed on extended TiO2 models was reported by some of us in 2009.56 The proposed approach, based on quantum mechanical second order Möller− Plesset (MP2), density functional theory (DFT), and timedependent DFT (TDDFT) calculations, was able to fully account for the experimental evidence of strong aggregation of D102 when adsorbed on the TiO2 surface and to identify, in the case of dimeric molecular aggregates, the most stable mutual arrangements. Excited state calculations carried out on the preferred dimeric structures were also able to satisfactorily reproduce the measured red-shifts, delivering values within 0.1 eV from experiments and, thus, confirming the overall picture extracted from the static adsorption modeling framework. The zero temperature static picture is however grounded on the general view that, on an ideal substrate, all the dyes adopt the most stable molecule−surface configuration, which is usually realized through the effective coordination of both of the carboxylic oxygens of the molecules to the titanium sites of the interface. However, this is just a special case which could be hardly realized from a statistical point of view, and a realistic description of the dye/semiconductor interface should, in principle, adequately consider the effect of thermal fluctuations on the structural and electronic properties of the dye-sensitized TiO2 surface.32,36,62−66 This issue has been recently tackled by a number of works reporting the characterization of the multiconformational binding modes,24,36,49 self-assembling properties25,48 and competitive adsorption50 of both organic and inorganic sensitizers on TiO2 surfaces. By combining highresolution scanning tunnelling microscopy (STM) and spectroscopy (STS), atomic force microscopy (AFM), DFT, and molecular dynamic (MD) simulations, these studies got atomiclevel insights into the heterogeneous binding modes, molecular

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MODELS AND COMPUTATIONAL DETAILS The MD-ReaxFF code implemented in the Amsterdam Density Functional (ADF) package67 was employed in all the calculations together with a serial version of the program kindly provided by Adri C. T. van Duin. To define a reliable classical methodology for describing and characterizing the adsorption of the D102 dye on an anatase (101) substrate, a preliminary validation step of the chosen reactive force field (ReaxFF) was performed by direct comparison with the quantum mechanical data (DFT) obtained in our earlier studies.36,56,57 The benchmark results are reported and discussed in the Supporting Information (SI). Briefly, the classical calculations satisfactorily reproduced the quantum mechanical-optimized structures of the dyes, their conformational rearrangements during the dynamics runs and their most probable adsorption arrangements on the substrate. Moreover, considering the importance of dye−dye interactions in the description of adsorption, conformational organization and distribution of the dye on the surface, we also checked the ability of the chosen ReaxFF to properly take into account these interactions. The comparison between the dimeric D102 structures optimized at DFT level of theory in ref 56 and those optimized by means of the classical method is shortly discussed in the SI. In summary, all the molecules, adopting bent alignments in relation to the surface, were compactly organized and mutually interacting through their phenyl rings. Stable adsorption was obtained through the coordination of the carboxylic oxygens and the sulfur atom to the undercoordinated titanium sites of the surface. Modeling Single Dye Adsorption. The anatase periodic slab was cut from a crystal having the experimental lattice parameters, namely a = 3.782 Å and c = 9.502 Å, which was replicated 4 times in all directions. The resulting supercell, containing 768 atoms (256 Ti and 512 O), consisted of eight Ti32O64 layers and had a size of about 40.8397 and 15.1040 Å along x and y, which were identified with the [101¯] and [010] directions, respectively. In order to reduce spurious effects due to the presence of a z-image surface, the height of the 2788


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The Journal of Physical Chemistry C simulation box was fixed to 270 Å, which implies a vacuum space of about 250 Å where the dyes were free to move. The starting configuration consisted of one dye placed in the center of the slab with the two carboxylic oxygens at a distance of 1.9 Å from two Ti sites of the interface. After energy minimization, the system was equilibrated for 10 ps in the NVT ensemble at 298 K. The conformation of the dye was elongated and did not change during the initial phase of the reactive molecular dynamics run (equilibration). After the equilibration stage, the production phase was carried out for about 50 ps. The system was heated to 298 K very slowly in about 5 ps and then equilibrated at this temperature in 2 ps time intervals. The sampling was carried out in the NVT ensemble, without any constraints and snapshot were saved every 2.5 fs. Temperature was maintained by means of the Berendsen’s thermostat with a relaxation constant of 0.1 ps, and the equations of motion were solved through the Verlet leapfrog algorithm using a time step of 0.25 fs. Modeling Partially Covered Interfaces. In order to characterize the adsorption when more than one molecule was present on the surface and to investigate the possibility of selfaggregation and multisite-binding modes, 8 and 12 molecules were adsorbed on the slab model described above, by placing the carboxylic groups directly on top of selected titanium binding sites according to the schemes shown in Figure 1(a,b). The chosen coverages correspond to densities of about 1.30 and 1.95 molecules/nm2, respectively, which well compare with typical dye densities (1−2 molecules/nm2) reported in the literature.68 In the initial arrangement all of the dye conformations were elongated, and their carboxylic groups were almost perpendicular to the surface (Figure 1c). To explore the adsorption dynamics from a less biased pattern, a larger slab was prepared by replicating twice the small supercell in the y direction. The final substrate model had x, y dimensions of about 40.8397 and 30.2080 Å. This slab was used to simulate a random fall of 12 molecules (density close to 1 molecule/nm2), which were vertically aligned far from the surface and from each other (Figure 2), on top of the substrate. Several models, with the dyes at different distances from the interface, were prepared and tested to investigate their dynamics. After energy minimization of the whole system, a series of interconnected molecular dynamics runs at various temperatures (in the range 10−298 K) were performed. Temperature was increased very slowly in such a way that the dyes could reach the surface without being affected by abrupt perturbations to their conformation. However, when the molecules were positioned too far from the interface (at about 7 Å) it was difficult to obtain complete adsorption because of the strong competition between self-interaction and surface attraction. This suggests that at long distance the driving force toward the surface was overwhelmed by the ability of the dye to self-assemble. Indeed, the consequential formation of molecular clusters, where the molecules lost their effective alignments for the best adsorption, slowed down considerably their surface attachment. Adsorption took place by means of the progressive migration of bundles of molecules toward the interface. However, the release of each unit was very slow and strictly connected to the neighboring species. A complete detachment of the dyes from the clusters was not observed during the whole simulation time, which was extended beyond ten nanoseconds. Considering these findings, it can be speculated that, due to molecular size, number of molecules, and their attitude to self-

Figure 1. Adsorption schemes on anatase (101)−small supercell (40.8793 × 15.1040 Å2). Only the top layer of the slab is shown for clarity. (a,b) Carboxyl oxygens of the dyes, Ti and O atoms of the interface are represented by means of light blue, white and red balls, respectively. (a) Low density coverage (8 molecules) corresponding to a density of 1.30 molecules/nm2. (b) High density coverage (12 molecules) corresponding to a density of 1.95 molecules/nm2. (c) Initial configuration: the carboxylic groups are placed on top of the selected titanium atoms according to the schemes shown in (a) and (b).

assemble, it was impossible to simulate their full spontaneous deposition on the interface in a reasonable amount of computational time. Thus, to speed up their adsorption on the support the starting configuration was redefined by placing all of the dyes closer to the interface (at about 3.5 Å) and the whole equilibration procedure was repeated again. As in the previous dynamics runs the sampling phase was carried out in the NVT ensemble, without any constraints for about 100 ps. Snapshots were saved every 2.5 fs. In the case of 8 molecules on the small supercell two simulations, namely fast and slow heating (1 and 5 ps long, respectively), were performed. In sum, the results of the following simulations: • 8 dyes on a small supercell (40.8397 × 15.1040 Å2)- slow heating (8DsS); • 8 dyes on a small supercell (40.8397 × 15.1040 Å2)- fast heating (8DsF); • 12 dyes on a small supercell (40.8397 × 15.1040 Å2)slow heating (12DsS); 2789


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induced by the reorientation of the sulfur atom which had the tendency to point toward the top layer of the slab to establish direct interactions with titanium. However, during the whole dynamics, sulfur adsorption was not observed and the shortest STi distance found was about 5.5 Å. By analyzing the evolution of the OTi separations, it was noted that both the connection lengths remained substantially at about 2.04 Å (with a standard deviation, SD, of about 0.1 Å) implying that the molecule was well balanced on top of the layer. Similarly, the inclination of the carboxylic carbon - aliphatic carbon bond with respect to to the interface plane remained around 67° (SD = 5°), whereas the C(carboxylic)CN angle of the dye explored a wider range of values oscillating between 113° and 128°, which suggests that the molecule had the propensity to rotate the five member ring close to the carboxylic group and bend slightly toward the surface to maximize dye-surface interactions. The classical data are in perfect agreement with the DFT description reported in ref 56 and give a reliable picture of the behavior of the isolated molecule adsorbed on the slab. Dye Layer Dynamics on the TiO2 Surface. Initial and final configurations of all the simulations are displayed in Figure 3; periodic boundary conditions are visualized to show how the

Figure 2. Adsorption schemes on anatase (101)−large supercell (40.8793 × 30.2080 Å2). Only the top layer of the slab is shown for clarity. (a) Side view. Configuration obtained after adsorption of the molecules on the substrate from a distance of 3.5 Å (vertically aligned molecules). Carboxyl oxygens and sulfur atoms of the dyes, Ti, and O atoms of the interface are represented by means of light blue and yellow, light gray and red balls, respectively. (b) Top view. The distances of the sulfur atoms from the closest Ti sites are reported. The dye is depicted by means of a gray wire. The density is around 1 molecule/nm2.

• 12 dyes on a large supercell (40.8397 × 30.2080 Å2)slow heating (12DlS); will be presented and discussed in the next sections. Excited State Calculations. In order to simulate the electronic absorption spectra of both isolated D102 and aggregate models, a number of snapshots were randomly extracted from the production phase of the D102-TiO2 and 12DsS simulations respectively, and modified to comply with the requirements of the quantum mechanical calculations. In particular, the TiO2 slab was removed and only selected clusters, made of five dyes, were considered for the excited states computations. The absorption spectra of the deprotonated form of D102 and of its five-member aggregates were calculated by TDDFT using the hybrid B3LYP exchange and correlation functional69 and a 6-31G* basis set. The solvation effects (water) were included by means of the conductor-like polarizable continuum model (C-PCM)70,71 as implemented in the Gaussian 09 package.72

Figure 3. (a,b) Initial configurations of the molecular dynamics runs involving the adsorption of 8 (a) and 12 (b) molecules on the small supercell (40.8397 × 15.1040 Å2). Configurations obtained at the end of the simulations (i.e., after 100 ps) are in (c) 8 dyes (small supercell fast heating); (d) 12 dyes (small supercell−slow heating); (e) 8 dyes (small supercell - slow heating). In (f) final configuration of the 12 dyes randomly adsorbed on the large supercell (40.8397 × 30.2080 Å2). The slab is rendered by means of a gray solid surface, whereas the dyes are rendered through the CPK model (color codes: O = red, C = cyan, S = yellow, N = blue, H = white). Cell borders are displayed as dark gray lines and a white rectangle evidencing the simulation box is also shown.

surface was decorated by the molecular assemblies at the end of the simulation in each case. As can be evinced from the inspection of the final patterns, the initial vertical order was not maintained but it was disrupted by the intermolecular interactions between the dyes. These forces could act concertedly by inducing cooperative bending of the molecular chains in specific directions, which were determined essentially by the orientation of the surface connected moieties. Indeed, even though the initial contact points with the interface were preserved, the redistribution of the positions of both the surface and dye atoms, due to their reciprocal attraction, generated new links which drove the free portion of the molecules (donor region), to a mutual reorganization through stacking and T-

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RESULTS AND DISCUSSION Single Dye Dynamics on the TiO2 Surface. During the trajectory, the molecule maintained an extended conformation and a close contact of the carboxylic oxygens with the Ti sites of the substrate although the orientation of the carboxylic group was no longer perfectly perpendicular to the layer but slightly inclined (at most 20°). This molecular arrangement was 2790


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The Journal of Physical Chemistry C shaped interactions. The maximization of number of contact points and ring−ring coupling seems to be the dominant cause of domains formation. In fact, after establishing (subps time scale) strong interactions with the interface (three contact points) and after packing the phenyl rings, the dyes flexibility was reduced and their configuration remained stable. Apparently, the established STi contacts favored the formation of quite strong and stable connections between the molecule and the top layer thus preventing major rearrangements. It is evident in all of the final configurations that the motion of the tethered chains produced islands of ordered molecules. A slightly more coherent organization of the dyes was observed in the case of slow heating (8DsS and 12DsS) due to the fact that the dyes could reorganize their structure more effectively in relation to the surrounding species and place their binding groups in more appropriate locations on the TiO2 top layer. As discussed more in detail in the SI, during all the dynamics runs, surface relaxation took place in agreement with experimental data and theoretical calculations.73−77 However, the effect is not prominent and affects mainly the interfaces, where the dyes and the top layer of the slab are mutually attracted. Dye Aggregates Optical Properties. The prominent tendency of D102 to form J-aggregates on the TiO2 electrode is manifested by a broadening and a significant red-shift of its absorption spectrum when passing from solution (absorption maximum at 2.53 eV) to sensitized-TiO2 film (absorption maximum at 2.30 eV).34 Although, the observed spectral shift on going from dye solution to TiO2 film is usually the result of a complex interplay of different effects (intermolecular interaction, protonation/deprotonation, solvation, etc.),78 such a large red-shift and band broadening are clear signature of strong dye−dye interactions on the TiO2 substrate. Our previously published static model of the D102 aggregation,56 even if limited to dimeric aggregates, reliably reproduced the optical response of the adsorbed dye, delivering for the preferred aggregation pattern a red-shift of 0.15 eV. In this work, from the dynamics of self-assembled and isolated D102 on the TiO2 surface, we can evaluate the thermal-averaged spectral shift due to dye aggregation on the TiO2 surface. In the bottom panel of Figure 4, the spectrum of five-molecules aggregates averaged over the randomly extracted MD snapshots (red line) is compared with the corresponding one obtained from the MD trajectory of the isolated D102 molecule adsorbed on the TiO2 substrate (blue line). In the top panel of Figure 4, the main contributions to the spectral broadening (isolated D102) ascribable to different conformations sampled during the MD simulation are displayed together the relative D102 structures. The lowest-energy transition, responsible of the UV−vis absorption band, is associated with the strongly dipole allowed HOMO → LUMO excitation (see isodensity surface plots in the bottom of Figure 4). The averaged spectrum of the isolated dye has the maximum absorption at 2.24 eV, almost coincident with the peak at 2.23 eV calculated for dye conformation 3, Figure 4. Red- and blue-shifted maxima (2.12 and 2.41 eV) are obtained from conformations 2 and 1 respectively, which, as is apparent from the molecular structures in Figure 4, are characterized by a different relative orientation between the triphenyl ethylene and the indoline-rhodanine moieties. Conformation 1 shows a large distortion around the indolinic nitrogen causing a significant reduction in the coplanarity

Figure 4. Top: Calculated averaged absorption spectrum of isolated D102 (blue full line) and absorption spectra of selected MD snapshots (light to dark blue dotted lines); the corresponding molecular structures are also reported. The spectra have been normalized and simulated with a Gaussian broadening with σ = 0.2 eV. Bottom: Comparison between the calculated averaged absorption spectrum of selected D102 aggregates composed of 5 molecules (red full line) and that of the isolated D102 monomer (blue full line); red (blue) vertical lines correspond to the unbroadened calculated excitation energies and oscillator strengths. The spectra have been normalized and simulated with a Gaussian broadening with σ = 0.3 eV. D102 HOMO and LUMO isodensity surface plots are also shown.

between the two subunits and thus weakening in the electronic conjugation along the donor−acceptor axis. This results in a sizable destabilization (ca. 0.2 eV with respect to conformation 3) of the LUMO level, that cannot effectively delocalize toward the triphenyl ethylene unit, and in the consequent blue-shift of the absorption maximum. Contrarily, the almost coplanarity between the two moieties in 2 favors a larger spatial delocalization of the LUMO, its stabilization (about 0.15 eV) and an absorption maximum shifted at lower energies. When clusters of five molecules are considered (bottom panel), the absorption spectrum is red-shifted by 0.16 eV (maximum absorption at 2.08 eV), and the low-energy tail, characteristic of J-aggregates, clearly appears. Notably, the calculated shift almost coincides with that reported in our previous quantum mechanical investigation for the most stable dimeric D102 aggregate.56 This confirms that the inclusion of thermal fluctuations by classical simulations and the use of extended aggregate models, deliver a reliable description of the dye layer optical properties, with a satisfactory agreement between the calculated and the measured spectral shifts. Dye Adsorption Mode and Dynamics. All the simulations showed that D102 could bind to the TiO2 anatase (101) interface through four different direct-contact modes (Figure 5): monodentate (with one carboxylic oxygen), homogeneous bidentate (with both carboxylic oxygens), heterogeneous 2791


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Figure 5. Possible binding modes identified during the MD simulations. Monodentate (with one carboxylic oxygena), homogeneous bidentate (with both carboxylic oxygensb), heterogeneous bidentate (one carboxylic oxygen and the sulfur atomc) and tridentate (both carboxylic oxygens and the sulfur atomd). Only a few atoms of the surface and the connected portion of the dye are displayed.

Figure 6. Atom−Atom Radial Distribution Functions of the oxygen and sulfur atoms of the dye, in relation to the Ti sites of the slab, in the different simulations (see legend).

the substrate and slowly relaxed toward their better interacting configurations (compare blue and magenta curves in the top panel of Figure 6), the majority of them tended to improve and reinforce the binding to the surface by moving the sulfur atom toward the close neighboring Ti sites. This was better achieved when there was enough space available around them, that is in the case of average density (compare blue and red curves in the top panel of Figure 6). On the contrary, kinetic effects, sparse arrangements, and lower density of the dye might reduce the probability of three-contact adsorptions. As far as the carboxylic group is concerned, the RDFs plots show that it was strongly adsorbed and remained in contact with the layer until the end of the simulations. This is confirmed by the presence of a flat region between the first two peaks and the third peak located at about 3.25 Å (corresponding to the carbonyl oxygen), which suggests no motion of the moiety toward other locations. Taking into consideration that in addition to the geometry and spacing of the molecules, their alignment also could be important, the inclination of the C(COO)C(CH2) bond relative to the z axis (perpendicular to the interface) was analyzed in detail, considering its evolution and average values. Four distributions were identified, and the dispersion in the alignment was estimated from the full width at half-maximum of these curves. The selected regions fell in the following narrow ranges: 20°−24°, 35°−38°, 42°−45°, and 53°−62°. The molecules were not aligned collectively, denoting a random rotation of the phenyl rings, but in all cases, the two intervals 42−45 and 53−62 were remarkably preferred, with the first one being twice more populated than the second one. Instead, almost vertical alignments were more rarely found and usually

bidentate (one carboxylic oxygen and the sulfur atom of the rhodanine moiety), and tridentate (both carboxylic oxygens and the sulfur atom). The monodentate anchoring mode was rarely observed, being energetically unfavored and short-lived due to the relatively high coverage and packed molecular arrangement. The other binding modes were all possible but the preferred one was that having three contact points, which could be realized through the readjustment of the Ti atom of the top layer of the slab and the simultaneous bend of the dye toward the neighboring adsorbed molecules (see the adsorption scheme chosen for these simulationsFigure 1), which induced a rotation of the CH2COO moiety of the adsorbate. However, to reach such a configuration all of the connected atoms needed to increase their optimal XTi distance, which became regulated by the interactions with the surface and the neighboring species. For example, it was observed that for an STi distance of about 2.8 Å, the carboxylic oxygen closer to the sulfur atom lengthened the separation from its Ti-coordinated site of around 0.3 Å (passing from 2.0 to 2.3 Å), whereas the other oxygen maintained a distance of about 2.1 Å. This configuration could be obtained through a combined rotation of the torsional angles defining the conformation of the carboxylic portion of the dye. The RDFs of the oxygen and sulfur atoms of D102 around the surface Ti sites are shown in Figure 6. The main features are the presence of marked peaks at about 2.7 Å in all the TiS RDFs and two sharp peaks at about 2.0 and 2.3 Å in all of the TiO RDFs, which confirm the adsorption picture described above. Moreover, the different heights of the TiS peaks suggest that when the molecules were carefully positioned on 2792


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populated region (first peak at about 16°) corresponds to the slab with the highest density of molecules. Even though the rings are located preferentially far from each other (highest peaks at 6 and 7 Å) comparison with the plane−plane angle plot suggests that the structures are almost always aligned due to mutual interactions which determine coordinated movements and a final order. A few examples of dye-surface adsorption geometries are displayed in the SI.

limited to the case of the isolated molecules. Remarkably, this result fully confirms the reliability of the static DFT picture, which predicts an inclination of about 45° with respect to the substrate plane for molecules anchored via the rhodanine-3acetic acid group.32,56,59 A better description of the arrangement of the adsorbed acceptor portion of the dye in relation to the surrounding units can be obtained by measuring ring−ring distances (considering the ring centers) and ring plane angles for all the neighboring adsorbates, that is those molecules where the rings were within 8 Å from each other (this choice was based on visual inspection of the trajectories). This was analyzed in all cases, but only the results obtained for the adsorption of 8 and 12 molecules on the smaller slab are reported. This is because they reflect the general behavior of the systems in the case of high and relatively low density when a finely tuned decoration of the interface is realized. Indeed, to develop an efficient hybrid device it could be important to create donor−acceptor layers with highly ordered nanoscale domains. Motifs formation has been observed in these simulations even in the case of random adsorption at lower dye density. In Figure 7, the most probable distance and orientation distributions of the five member rings of the selected dyes pairs during the final steps of the trajectory (last 2.5 ps) are depicted. From inspection of the data shown in the bottom panel of Figure 7, one can infer that the tendency of the acceptors to adopt parallel orientations is marked and that the most densely

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CONCLUSIONS Classical molecular dynamics simulations based on a reactive force field (ReaxFF), developed during the past few years to describe titanium dioxide substrates and their interactions with amino acids, peptides, and other types of molecules, were carried out to characterize the adsorption of the D102 dye on a anatase (101) surface in the gas phase. Different densities of the adsorbates at the interface, compatible with experimental observations, were analyzed to get insights into the preferential anchoring modes of the dyes and their self-assembling properties. The simulations reveal that the main forces driving the formation of ordered aggregate motifs are the maximization of the contact points with the TiO2 surface on the one hand, and the of the π stacking and Tshaped interactions between the rings of the donor moieties on the other hand. In line with other theoretical and experimental studies, we have found that the most stable adsorbates were bound to the substrate through a bidentate coordination of their carboxylic groups to the available titanium sites of the surface, and when possible, by means of a tridentate coordination where the carboxylic anchoring was reinforced by direct contacts between the sulfur atom protruding out of the acceptor ring and the atoms of the slab. These types of binding determined the inclination of the dye in relation to the surface (which was around 60° as already observed by other authors) and favored clustering of the adsorbed units. However, the observed clusters were organized in domains, namely aggregates separated by empty regions, which were characterized by a different orientation of the packing units. Essentially, the orientation was driven by the reorganization of the donor portion but also by the location of the contact points. Indeed, not all the potential binding sites of the surface were saturated during the adsorption process and some of the adsorbates had around them space enough to rearrange their groups relatively to the other species. We believe our combined ab initio and MD study, although limited to gas phase, will further motivate future studies on the formation of dye aggregates on TiO2 and their impact on the photoelectrochemical properties of the related solar cell devices. In particular, it would be of pivotal relevance exploring the self-assembling properties of dyes in the presence of the commonly employed solvents (acetonitrile, chloroform, ethanol etc..). In this perspective, we are currently parametrizing some of these solvents to perform reactive classical simulations with the methodologies we have recently developed.

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Figure 7. Statistically significant values distributions of centroidcentroid ring distances (top) and ring plane-ring plane angles (bottom) between the different molecules adsorbed on the small anatase (101) supercell (40.8397 × 15.1040 Å2). Data relative to low (8 molecules) and high (12 molecules) densities are depicted with cyano and red histograms. The chosen molecules are the ones located in close proximity to each other (interacting distance lower than 8.0 Å).

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11332. Validation of the reactive force field, preliminary checks through comparison with previously published quantum mechanical results, data relative to surface deformation 2793


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along the MD trajectories, and selected snapshots extracted from the dynamic runs (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.M.). *E-mail: [email protected], [email protected] (M.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.M. is grateful to Adri C. T. van Duin for the serial version of the reactive dynamics program (ReaxFF), for his support, useful comments, and suggestions.

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REFERENCES

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Theoretical Investigation of Adsorption, Dynamics, Self-Aggregation

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