Article pubs.acs.org/JPCC
Charge Transfer at the Hybrid Interfaces in the Presence of Water: A Theoretical Study Olga A. Syzgantseva,*,†,‡ Martti Puska,‡ and Kari Laasonen† †
COMP, Department of Chemistry, and ‡COMP, Department of Applied Physics, Aalto University, FI-00076 Aalto, Finland S Supporting Information *
ABSTRACT: The presence of water molecules at the interfaces of dye-sensitized solar cells can hinder the excited-state charge transfer (CT), which constitutes a crucial step in solar energy harvesting by photovoltaic devices. To rationalize the impact of water adsorption on interfacial CT, this process is simulated within the time-dependent density functional theory in a model system formed by perylene-3-carboxylic acid and the TiO2 (101) anatase surface. The adsorption of molecular water results in a moderate decrease of the CT efficiency, while dissociative adsorption of H2O is shown to substantially reduce the electron accepting capacity of TiO2. The amplitude of the effect depends smoothly on the amount of adsorbed water molecules, though distinct adsorption configurations contribute to it in different ways. The dissociation of the COOH anchor under the action of water species, simultaneous with the CT, results in an increased CT efficiency from the dye molecule to the TiO2 surface.
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INTRODUCTION The design of novel hybrid materials for photovoltaics requires understanding of the processes affecting the efficiency of solar energy harvesting at the atomistic level. Excited-state charge transfer at the organic−inorganic interface is one of the key processes in the photovoltaic and photocatalytic applications of hybrid systems.1,2 One of the factors susceptible to deteriorate the CT efficiency at the interface is the presence of water molecules.3 The presence of water can affect CT in different ways; for instance, it may provoke the desorption of the sensitizer from the surface.3 It can also directly change the CT capacity of the interface, modifying its structure by sticking of water molecules to the semiconductor surface or by formation of hydrogen bonds with the sensitizer. The current study aims to find out how the direct chemical interaction of water with the components of the interface affects its CT characteristics. For this purpose, we resort to CT simulations within the timedependent density-functional theory (TDDFT). The chosen model system is constructed from the TiO2 (101) anatase surface, which is known to be the most abundant facet of anatase crystals used in photovoltaic devices, and from the perylene-3-carboxylic acid (PeCOOH), which is a representative prototype of aromatic molecules applied for sensitization of TiO2 surfaces. To explore possible interactions of water with the interface components, both nondissociative (PeCOOH) and dissociative (PeCOO−) adsorption modes of PeCOOH were considered. They are referred to as the monodentate (MA) and bidentate (BA) modes,4 respectively, to avoid any confusion with the water dissociation. Concerning the interaction of water with the (101) anatase surface, the major question, currently debated in the literature, is whether water adsorbs molecularly or dissociatively. A particular interest to this interaction stemming from its © 2015 American Chemical Society
potential applicability to photocatalytic water splitting has stimulated an intensive research in the field.5,6 The pioneering experimental and theoretical studies7−10 on the clean (101) anatase surface have suggested the molecular adsorption of water, although a more recent experimental work11 reveals the possibility of comparable amounts of molecular and dissociated species in the first monolayer. This experimental finding is supported, in particular, by ab initio reverse engineering of experimental O1s XPS spectra12 and by molecular dynamics simulations using the ReaxFF force field,13 the last one revealing the possibility of water dissociation assisted by other water molecules. In addition to this fact, the presence of subsurface defects,14−18 namely oxygen vacancies, is shown to facilitate the dissociative water adsorption on the (101) TiO2 surface. Another issue is the influence of the solvent on the water adsorption state. For instance, it was recently shown that acetonitrile, a solvent frequently used in the dye-sensitized solar cells, can compete with the water for the adsorption on the (101) anatase TiO2 surface.3 Though the presence of solvent can affect the behavior of water molecules in a real system, to retrieve the pure effect of water interaction with the components of the interface, in the present study we exclude solvent molecules from the consideration. Though the controversy about the water adsorption state is not completely resolved, it should be borne in mind that most of the available studies address water adsorption on (quasi-)perfect surfaces under specific external conditions. However, the local environment at a dye-sensitized interface and the impact of working conditions are much less controllable. Thus, the presence of Received: October 18, 2015 Revised: November 27, 2015 Published: November 30, 2015 28347
DOI: 10.1021/acs.jpcc.5b10182 J. Phys. Chem. C 2015, 119, 28347−28352
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The Journal of Physical Chemistry C dissociated water species at the hybrid interfaces of dyesensitized solar cells cannot be excluded, as it can be promoted by such external factors as temperature, electric fields, presence of ions in the electrolyte solution (K+, Li+, I−, I3−1), local presence of hole carriers in TiO2, or the existence of the abovementioned subsurface defects in TiO2 facilitating the water splitting. Taking these arguments into account, we consider in the present work both the molecular and the dissociative adsorption of water, the last one providing potentially stronger changes to the CT properties of the interface.
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COMPUTATIONAL METHODS The considered systems are investigated within the densityfunctional theory (DFT) applying the Perdew−Burke− Ernzerhof (PBE) exchange-correlation functional.19 Wave functions are represented on a real space grid with a spacing parameter h of 0.20 Å. Atomic core−shells are described within the projector augmented wave (PAW) formalism;20 the number of electrons in the valence shell is equal to 12, 6, 4, and 1 for Ti, O, C, and H atoms, respectively. Electron injection calculations are conducted applying the TDDFT realtime propagation of electronic wave functions with fixed atomic positions21 and Ehrenfest dynamics.22 In both approaches we use the GPAW program23 and employ the timesteps of 25 and 20 as, respectively. The initial excitation from the dye molecule HOMO orbital situated in the energy range of the TiO2 band gap to the localized molecular-shaped LUMO orbital, overlapping with the TiO2 conduction band, was produced by the ΔSCF method.24,25 A four-layer TiO2 structural model of the anatase (101) surface containing 12 TiO2 units in each layer was considered within the orthorhombic unit cell with the following lattice parameters: 35.0, 11.3526, and 10.2395 Å. Periodic boundary conditions were applied in all three directions. The Monkhorst−Pack (1 × 2 × 2) k-point mesh was used. To produce the CT profiles, the partitioning of the total electron density was accomplished using the Bader program.26 Images of structures were produced with the VESTA program.27 A more detailed description of the computational procedure can be found in ref 28.
Figure 1. Dissociative and molecular water adsorption on anatase surface (101) containing dye molecule in PeCOOH (MA, top) or PeCOO− (BA, bottom) form. Colors: Ti = red, O = green, C = beige, H = blue. Atoms of water molecules are shown in white.
Because of the presence of the dye molecule on the TiO2 surface, the Ti5c adsorption sites in the unit cell become inequivalent with respect to the PeCOOH position (Figure 1). In the ab initio simulations, presumably, water adsorption at various surface sites can have distinguishable effects on the CT process. Besides, the amount of adsorbed water molecules can have a cumulative effect on CT. Exploring all the possible configurations of dissociated and molecular water emplacements for a variable amount of adsorbed water and different adsorption states of PeCOOH produces several hundreds of structures for our unit cell, making the consideration of the complete structure set impossible. Therefore, to overcome this difficulty, the following strategy of water molecule distribution on the surface was adapted. At the first step, all the Ti5c sites were filled in with OH groups (dissociative adsorption) or H2O (molecular adsorption) molecules. The corresponding structures cumulate the effect of water presence at each particular site and the water−water interactions between different sites (Figure 1). The analysis of CT for those allows to observe if there is any effect due to the water presence considering only one structure per adsorption mode. Thus, we initially consider four structures, two per each adsorption type of PeCOOH (MA and BA) with water adsorbed molecularly or dissociatively on each available adsorption site, forming the 1 monolayer (ML) coverage. Hence, the slabs with PeCOOH in the MA form contain five water molecules (MA5), while those with the BA form have four water molecules (BA4). The density-of-states (DOS) curves with projections on PeCOOH and H2O (Figure 2) evidence only a tiny contribution of H2O states within the conduction band region, while the H2O presence mostly affects the upper edge of the TiO2 valence band and overlaps with some occupied molecular states at deeper energy levels. The adsorption of molecular water has a minor effect on the CT profile (Figure 3), which is more prominent in the case of PeCOO− compared to PeCOOH. The difference can be associated with the more important role of the surface states in the accommodation of injected charge in the former case (see ref 28). Besides, for PeCOO− the adsorption geometry of water molecules is substantially affected by the presence of the OH group due to the dissociation of PeCOOH: the neighboring H2O molecules form H-bonds with the hydroxyl group. This can also be a reason for the more significant impact
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RESULTS AND DISCUSSION To assess the impact of water present in the real dye−TiO2 systems, we concentrate our attention on the possible alteration of CT parameters due to the chemical interaction of the interface with water species, considering both dissociative and molecular adsorption forms. According to available experimental and theoretical data,7−9 water molecules on the anatase (101) are adsorbed through their O atoms on the Ti5c sites, and their H atoms form hydrogen bonds with the O2c atoms of the adjacent rows. Consequently, this adsorption geometry was adopted in our simulations. In the case of dissociative adsorption, OH groups are positioned on the Ti5c adsorption sites, while H atoms are put on top of the neighboring O2c atoms. The surface adsorbing an undissociated PeCOOH molecule has five Ti5c sites out of the total number of six Ti5c sites in the calculation unit cell (see Figure 1), while that holding a dissociated PeCOO− molecule has four Ti5c positions in the unit cell (two Ti5c sites from the total of six per unit cell are taken for adsorption of PeCOO− through the Ti···O bonds). 28348
DOI: 10.1021/acs.jpcc.5b10182 J. Phys. Chem. C 2015, 119, 28347−28352
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Figure 2. DOS structure for MA5 (top) and BA4 (bottom) systems, containing dissociated (left) or molecular (right) water.
Figure 3. Charge transfer from PeCOOH (left) and PeCOO− (right) on clean and hydrated TiO2 surfaces. RT-TDDFT calculations with fixed nuclei positions.
known30 to be more favorable for the accommodation of reduction electrons. The introduction of OH on the surface thus reduces its capacity to accept injected charge. Remarkably, for the MA case the CT profile flattens out substantially, and the amplitude of the CT peaks decreases. For the BA case, the position of the first maximum is well reproduced, while the overall CT amplitude is decreased. Interestingly, the water species present on the surface also accommodate quite a lot of the injected charge. The corresponding layer-by-layer analysis is presented in Figure S1. The next step is to understand how much the amount of the adsorbed water impacts CT. As the molecular water has a much weaker effect on CT than the dissociated water, in the following discussion only the latter will be considered. To explore the effect of water concentration, water molecules are gradually
of molecular water adsorption, as the local environment becomes more disturbed by the hydration, than in the case of nondissociated PeCOOH. The position of the maximal CT displaces to higher times in the case of PeCOOH and stays almost unchanged in the case of PeCOO−. The dissociative adsorption of water leads to a drastic decrease in the amount of transferred charge. According to the DOS structure (Figure 2), the position of the first excited state gets closer to the conduction band edge compared to the clean TiO2 surface;28 hence, fewer TiO2 states are available for injection. The CT decrease is also in line with the previously observed important role of the surface layer in sheltering of injected electrons.29 Indeed, the hydroxylation of the surface Ti5c sites restores the bulk coordination number of 6 for these Ti atoms. At the same time, lower coordination states of Ti are 28349
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excited-electron injection, observed previously for the PeCOOH−TiO2 interface.28 The interaction of the interface with water, considered up to now, concerned mainly the TiO2 surface. No formation of hydrogen bonds with the polar anchoring COO(H) group was observed in the simulations, where the water species were positioned on the most favorable adsorption sites (Ti5c and O2c), suggesting that the topology of the COOH anchor binding to the (101) anatase surface does not allow the formation of static surface hydrogen bonds. Nevertheless, it does not exclude a dynamical H-bond formation due to the thermal fluctuations. Besides, species forming H-bonds do not necessarily have to be sticked to the TiO2 surface. Water molecules can form10,31,32 a network of predissociated species with elongated H−O···H bonds. Some of them can interact with the COOH anchor. In addition, OH− species can penetrate from the solution to the TiO2 surface and provoke, in particular, the dissociation of the COOH anchor. The penetration of OH− from the H2O solution to the TiO2 (101) anatase surface was previously investigated in a first-principles molecular dynamics study.33 It was shown that OH− anion prefers to stick at the (101) anatase TiO2 surface, rather than to stay in the solution, so that its presence near the TiO2 surface is probable. To explore the excited-state CT process, taking place simultaneously with the dissociation of the COOH group, an OH− anion was introduced to the system in the neighborhood of the H anchor atom in a configuration, which presumably would be favorable for trapping H from the COOH group and resulting in water formation. Essentially, the OH− group is chosen here for the sake of simplicity as a model case of a base, allowing for a fast proton transfer. Moreover, COOH dissociation can in principle be initiated by any other species, such as a H-bound water molecule, though a priori a dynamical simulation of this process would require longer simulation times. The HO−···HOOC distance is set to 1.053 Å, while the H+ of the COOH group is rotated to form a C−O−H angle of 138°, and the corresponding O−H distance is set equal to 1.026 Å. To compensate for the negative charge of OH−, a proton is positioned on one of the O2c surface atoms. This manipulation of the initial geometry allows to reproduce this quite a rare event. This is justified as, on the one hand, the presence of water can lead to the COOH-anchor dissociation under the working conditions and, on the other hand, our main objective here is to observe the CT under the dissociation process. Using the obtained initial geometry, the system was excited and relaxed within an ED simulation of 50 fs. Interestingly the obtained CT profile is substantially different from those observed previously. It has inflection points and continues to grow up to 0.61 e−; i.e., the increase in the CT is 100% with respect to nondissociated case (Figure 6). The animation illustrating the corresponding CT process is provided as Supporting Information. This particular case demonstrates the importance of the anchor chemical state for the CT characteristics.
removed (one by one) from the reference of the heavily hydroxylated structure (MA5), starting from H−OH pairs which are the most distant from the dye molecule (structures MA4−MA1, Figure 4). The amount of CT smoothly decreases
Figure 4. CT dependence on the H2O molecule concentration on the TiO2 surface. RT-TDDFT calculations with fixed nuclei positions.
with the increasing number of H−OH pairs on the surface. From the point of view of electrostatics, the presence of polar hydroxyl groups on the TiO2 surface with a δ+ charge on hydrogen atoms creates a layer of positive charges above the surface, which sums up with the δ′+ charge of the PeCOO(H) molecule upon the loss of the injected electron, creating a higher effective positive charge above the TiO2 surface. Thus, the smooth decrease of CT is in line with this electrostatic description. To find out to what extent this decrease is related to the amount of water molecules on the surface and to what degree it is due to the water adsorption on qualitatively different surface sites, three different MA1 configurations were explored (MA1a−MA1c, Figure S2). The corresponding CT profiles differ substantially depending on the respective positions of the H and OH groups (Figure 5). Interestingly, the CT profile for the MA1c configuration is very close to that for the MA2 structure, suggesting that not only the relative occupancy but also the nature of the particular adsorption site affects CT. This is not surprising, taking into account the anisotropy of the
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CONCLUSIONS In this study we have investigated the impact of the chemical interaction of water species with the TiO2−perylene interface on the excited state charge transfer. It was shown that both the molecular and dissociative adsorption of water on TiO2 decrease the CT capacity; the stronger effect was observed in the case of the dissociative adsorption. The CT decrease
Figure 5. CT dependence on the position of the H2O molecule adsorption site. RT-TDDFT calculations with fixed nuclei positions. 28350
DOI: 10.1021/acs.jpcc.5b10182 J. Phys. Chem. C 2015, 119, 28347−28352
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge PRACE for awarding us access to resource Curie based in France at CEA. We thank CSC-IT Center for Science Ltd. and Aalto Science-IT project for providing computational resources. The work is a part of the QUASIMODO project (project number 258547) and the Centers of Excellence Program (project number 284621) funded by the Academy of Finland. K.L. thanks Prof. Jouko Korppi-Tommola for useful discussions.
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Figure 6. Enhancement of CT upon the COOH anchor dissociation. Ehrenfest dynamics calculation is presented in the case of anchor dissociation. For the cases of the clean surface (no water) and water monolayer in the PeCOOH−TiO 2 system, the RT-TDDFT calculations with fixed nuclei positions are performed. For the COOH dissociation process the initial CT value is set to q(0) instead of qideal (equal to stoichiometric number of electrons in PeCOOH, used as a reference in all other cases; this choice is the most appropriate for the current situation, as the system’s reference state is changing upon dissociation from PeCOOH to PeCOO−).
smoothly depends on the amount of water species adsorbed in the first monolayer, though their various distributions over adsorption sites result in different CT profiles, indicating that the interaction with different sites gives different contributions. For both PeCOOH adsorption modes, the drop of the CT efficiency upon the dissociative water adsorption is approximately 50%. The interaction with OH− species is shown to induce the COOH anchor dissociation, resulting in a monodentate dissociative adsorption associated with a CT enhancement by 100%. Essentially, these findings demonstrate that the presence of water at the perylene−TiO2 surface will impede the CT through a direct chemical modification of the interface components. We also suggest that any chemical modification of the surface is capable of altering the CT. In the case of TiO2 usually the CT is reduced. Hence, the possible coadsorption of the substances present in the system under real conditions should be taken into account during the setup of interfacial CT experiments or simulation procedures.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10182. Decomposition of the injected charge into the contributions of layers; different adsorption configurations of dissociated water molecule (structures M1a− M1c) (PDF) Animation of the CT process during the anchor dissociation (AVI)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Phone +358-50-5972111; e-mail olga.syzgantseva@aalto.fi (O.A.S.). 28351
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