Co-Adsorbent Effect on the Sensitization of TiO2 and ZnO Surfaces: A

Nov 18, 2015 - Co-adsorbents are often employed in DSSC to avoid electron/hole recombination at the hybrid organic/inorganic interface. Here we invest...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Co-Adsorbent Effect on the Sensitization of TiO2 and ZnO Surfaces: A Theoretical Study Francesca Risplendi*,† and Giancarlo Cicero†,‡ †

Department of Applied Science and Technology, Politecnico of Torino, C.so Duca degli Abruzzi 24, 10129, Turin, Italy Center for Space Human Robotics at Polito, Istituto Italiano di Tecnologia, C.so Trento 21, 10129, 10129, Turin, Italy

‡

S Supporting Information *

ABSTRACT: Co-adsorbents are often employed in DSSC to avoid electron/ hole recombination at the hybrid organic/inorganic interface. Here we investigate, by means of density functional theory simulations, the changes induced in the structural and electronic properties of a hybrid heterostructure by the presence of a co-adsorbent. In particular, we focus on a prototype interface obtained by co-grafting a dye molecule (the hemi-squaraine dye) and imidazole at the oxide surfaces mostly employed in DSSC (anatase-TiO2(101) and ZnO(1100)). Our results show that beside avoiding dye aggregation, the coadsorbent plays an active role in stabilizing the binding between the chromophore and oxide substrate and in tuning the optoelectronic properties of the system. We found that imidazole shifts downward the dye energy levels (HOMO and LUMO) while raising the oxide valence and conduction bands, strongly influencing the energy alignment of the interface. As such, a co-adsorbent can either modify the type of heterostructure (straddling, staggered, or broken gap) or change the open circuit voltage (Voc), with potentially both beneficial or detrimental effect of the final device performance.

■

INTRODUCTION Devised originally by O’Regan and Grätzel,1,2 nanocrystalline dye-sensitized solar cells (DSSC) have attracted a considerable amount of attention due to their low fabrication costs and relative high efficiency as a valid alternative to silicon-based photovoltaic devices. Up to now the best performances have been obtained for anatase-TiO2 nanoparticle-based DSSCs sensitized with ruthenium complexes (e.g., N719,3 which gives efficiency higher than 11%) or employing porphyrin sensitizers (efficiency values larger than 13%4,5). DSSC performances strongly depend on the interaction between its constituent elements: the dye molecule, the wide gap nanostructured metal oxide, and the electrolyte. To allow efficient harvesting of solar energy, it is essential that charge separation occurs at the metal oxide/dye interface through injection of the light-excited electron in the conduction band (CB) of the metal oxide. The efficiency of this process strongly depends on the electronic properties of the dye/semiconductor interface: the alignment of the electronic energy levels has to correspond to a type II (staggered) heterostructure;6 the electron in the excited state has to be delocalized both on the dye and on the oxide to favor adiabatic electron transfer. To this aim, it is required that the chromophore, besides absorbing the largest fraction of the solar spectrum, chemically binds to the metal oxide surface through anchoring groups that ensure a strong electron coupling with the substrate. In these types of solar cells, after the initial charge separation, the close proximity of the photoexcited electrons and the holes localized either in electrolyte or on the oxidized sensitizer and the lack of a substantial potential barrier at the interface may © XXXX American Chemical Society

entail interfacial charge recombination that degrades cell performances. To prevent this back-reaction, the semiconductor surface can be modified introducing organic molecules, other than the dyes, on the solvent-exposed part of metal oxide.7,8 Besides effectively protecting from the back electron transfer from the metal oxide CB to the electrolyte, cografting of dyes with organic molecules such as the chenodeoxycholic acid8 has been demonstrated to considerably improve the stability and the performance of the DSSCs.9 Herein, the co-adsorbent effects on the photovoltaic performances of DSSCs are discussed by means of a theoretical study based on density functional theory for a prototype organic/inorganic system. In particular, we investigate the attachment of a recently proposed organic dye, named hemisquaraine (CT1),10,11 to anatase-TiO2 and ZnO surfaces in the presence of the simplest nitrogen-containing heterocyclic molecule, imidazole, proposed as co-adsorbent.12 Our results show that the co-adsorbent strengthens the binding between the dye and the metal oxide surface and correspondingly shifts the dye molecular orbitals to lower energies and the metal oxide valence band (VB) and CB to higher energies. The observed shifts change the relative energy alignments of the heterostructure either giving rise to a different type of energy alignment (straddling, staggered, or broken gap) or to modifications of the Voc (larger or smaller). This demonstrates that a co-adsorbent, if not properly selected, may have Received: November 12, 2015 Revised: November 18, 2015

A

DOI: 10.1021/acs.jpcc.5b11113 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C detrimental effects on the cell performance; therefore, for each dye/metal oxide combination the co-adsorbent must be accurately chosen or chemically engineered in order to achieve optimized photovoltaic performances.

■

METHODS Our theoretical calculations are based on the density functional theory (DFT) as implemented in the Quantum Espresso package.13 The Kohn−Sham equations are solved using ultrasoft pseudopotential to describe the electron−ion interaction, employing the gradient corrected Perdew− Burke−Ernzerhof (PBE)14 functional to describe the exchange-correlation effects, and expanding the electronic wave functions in plane waves (PW). For all calculations we adopted a PW energy cutoff of 30 Ry for the wave functions and 300 Ry for the charge density and potentials. Surface calculations were performed in orthorhombic supercells containing either slabs of five O−Ti−O three layers or slabs of 12 ZnO atomic layers. A vacuum region of 10 Å thickness was added in the cells to avoid spurious interaction between periodic replicas along the surface orthogonal direction. The 3 × 2 and 2 × 3 surface supercells were adopted to investigate the attachment of the dye and the co-adsorbent to the ZnO(1100) and anatase-TiO2(101) surface, respectively. The Brillouin zone was sampled employing a 6 × 6 × 6 Monkhorst−Pack mesh for the bulk calculations and a 6 × 6 × 1 grid for the surface calculations.15 All structures were relaxed by minimizing the atomic forces; convergence was assumed when the maximum component of the residual forces on the ions was smaller than 10−4 Ry/bohr. To partially heal the PBE underestimation of the energy gap in transition metal oxides, such at TiO2 and ZnO, due to the overestimation of p− d coupling we applied to the relaxed structures a Hubbard U correction to the Ti-3d, Zn-3d, and O-2p orbital as reported in ref 16 (UZn = 12 eV, UO = 6.5 eV, and UTi = 6.68 eV).a Response charges to optical excitations were simulated through the Liouville−Lanczos approach to linearized TDDFT, as implemented in turboTDDFT code,17 and computed with 3000 iterations. To calculate the optoelectronic properties of CT1 attached to the anatase-TiO2 and ZnO surface, we employed two cluster models, representative of the sensitized surface. Our models, shown in Figure S13 of the Supporting Information, are intended to reproduce the chemical structure of the chromophore attached to the periodic oxide surface, and they have been obtained by extracting from a relaxed slab calculation the dye and the surface metal ions involved in the binding. To keep molecular neutrality of the cluster and to maintain the coordination (6 for Ti and 4 for Zn) and the oxidation state (+4 for Ti and +2 for Zn) of the metal ions at the surface, these have been saturated with hydroxyl groups and water molecules. During structural relaxation of the model cluster, the positions of the metal ions and of the oxygen atoms of the squaric acid group directly involved in the binding were kept fixed.

Figure 1. Ball-and-stick representation of the co-absorption of the CT1 and imidazole molecules at the ZnO(1100) (top panels) and at the anatase-TiO2(101) surfaces (bottom panels). Ti atoms are represented in gray, Zn in light gray, O in red, C in black, N in blue, and H in white.

top of the outermost oxide layer at an initial distance of 3.5 Å and exposing the squaric acid group, which corresponds to the most reactive part of the molecule (see Supporting Information). The system was then fully relaxed. Results show that the squaric acid spontaneously deprotonates (nonactivated dissociative chemisorption) at both the analyzed oxide surfaces forming two partially covalent bonds between two oxygen atoms of the anchoring group and two surface metal ions in a bridge bond configuration as reported in Figure 2. The two O−M bonds formed at the surface saturate two initially undercoordinated metal ions, producing a localized outward relaxation of these ions, which restore their bulk-like coordination; the bond lengths (dO−M+) are 2.03 and 2.07 Å in the case of anatase and 2.01 and 1.97 Å in the case of ZnO. In the final configurations, the benzoindole (see Figure 2) part of CT1 is perpendicular to the surfaces while the anchoring group is slightly tilted with respect to the surface normal. The structure of the anchoring group is modified; in particular, the double CO bond elongates about 3.3%, partially losing its double-bond character due to the interaction with the surface metal ions, while the C−O shortens −3.8%, assuming partial double-bond character. Correspondingly the double bond C C of the squaric acid moiety elongates due to the delocalization of its π electrons toward the C−O group interacting with the surface. The hydrogen atom released by CT1 acid group attaches to an undercoordinated oxygen atom of the surface.

■

RESULTS AND DISCUSSION Before discussing the effect of the co-adsorbent on DSSC, we report on the structural and electronic properties of the metal oxide surfaces functionalized with CT1. (The structural and electronic properties of functionalization with imidazole molecule only are discussed in the Supporting Information.) Attachment of the hemi-squaraine dye to both the TiO2 and ZnO surfaces was addressed by placing one CT1 molecule on B

DOI: 10.1021/acs.jpcc.5b11113 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

anatase surface catechol adsorbs with a BE of 1.25 eV24 and the isonicotinic acid with BE of 1.26 eV.25 These higher BEs entail a more stable binding improving the stability of the systems. To understand if CT1 is a good sensitizer for ZnO and TiO2, the optoelectronic properties of the CT1/oxide systems are analyzed in terms of density of states (DOS), projected density of states (PDOS), charge density of the states involved in photoinjection process (the HOMO, LUMO, and LUMO+1 statesc), and the response charge computed for the first absorption peak (a more complete analysis is reported in the Supporting Information). In the anatase-based system, upon CT1 adsorption, four occupied molecular states appear in the substrate energy gap, while the LUMO state falls above the minimum of the anatase CB (CBM). The energy difference between the HOMO and LUMO state of the molecule (ΔELUMO−HOMO) adsorbed on the anatase surface is 1.93 eV similarly to what reported in a previous study.10 The HOMO state is located at 1.88 eV below the CBM, and the corresponding electronic density is localized on the atoms of CT1 anchoring group with contribution from the p orbitals perpendicular to the plane of the molecule (delocalized porbital) (see Figure 2A). Analyzing the density associated with the LUMO and LUMO+1 states, reported in Figure 2, it is apparent that the unoccupied states are localized both on the atoms of the anchoring group and on the titanium atoms of the substrate. In addition, Figure 3B illustrates the response charge computed for the first main absorption peak for a perturbation oriented along the long axis of the molecular cluster representing the CT1 adsorbed at TiO2 surface. Comparing the response charge of isolated CT1 and CT1−Ti cluster, it is evident that upon absorption a net electronic charge density moves toward the Ti atoms of the surfaces. In the hemi-squaraine/ZnO(1100) system, CT1 binds to ZnO, giving rise to four occupied molecular states appearing within the substrate energy gap. In this case the HOMO is 1.0 eV below the CBM while the LUMO is found at 1.07 eV above the ZnO CBM. The electron densities associated with the HOMO, LUMO, and LUMO+1 states are all localized on the atoms of the molecule (particularly on the anchoring group) as reported in Figure 2B, and the energy difference between the HOMO and the LUMO states is 2.07 eV, in agreement with ref 11. As reported in Figure 3C, the response charge computed for the first absorption peak for the cluster representing the CT1

Figure 2. Ball-and-stick representation of CT1 attached to the anataseTiO2(101) surface (top panel) and to the ZnO(1100) (bottom panel). The electron density isosurface of the interface states involving CT1’s HOMO (red isosurface) as well as LUMO and LUMO+1 (blue isosurface) are also reported.

The calculated binding energiesb for CT1 attachment are 2.18 and 2.31 eV for TiO2 and ZnO, respectively. The adsorption energy is slightly higher in the ZnO case than in the anatase case due to a maximization of the chemical interaction between the surface metal ions and the oxygen atoms of the squaric acid group (the dO···O distance of the molecule is closer to the dZn3c−Zn3c distance than to the dTi5c−Ti5c one). It is interesting to notice that the squaric acid adsorption at both oxide surfaces is energetically stronger than that of other anchoring groups usually employed in dyes and characterized by a bond bridging binding configuration; for example, at the

Figure 3. Response charge computed for the first main absorption peak for a perturbation (electric field) oriented along the axis of molecular clusters (z direction) representing the isolated CT1 molecule (A), CT1 adsorbed at the TiO2 surface (B), and CT1 adsorbed at the ZnO surface (C). C

DOI: 10.1021/acs.jpcc.5b11113 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 4. Schematic energy level alignments between the metal oxide bands (shaded regions) and the molecular states of CT1 (red lines) and imidazole (blue lines) as obtained from an analysis of the PDOS after alignment of the vacuum level. H/L labels indicate the positions of the HOMO/LUMO states of the molecules. The position of the Fermi level (EF) is indicated by the dashed line.

adsorbed at the ZnO surface remains localized mainly on dye molecule. These results show that upon adsorption both interfaces realize a type II heterojunction; thus, one of the prerequisites for an efficient sensitization is satisfied: the dye’s HOMO falls in the substrate energy gap while the LUMO is above the CBM. The main difference between the CT1-functionalized ZnO and TiO2 surfaces is in the electronic coupling between the dye molecule and metal oxide CB. Stronger coupling occurs in the anatase case thanks to the partially occupied d states of the Ti ions, which are involved in the molecular binding. Indeed, the charge density associated with the molecular LUMO and LUMO+1 states (see Figures 2 and 3) and charge response in the CT1/anatase interface are localized both on the atoms of the anchoring group and on the titanium atoms of the substrate. Taken together, these results point at a higher efficiency of DSSC based on the TiO2/CT1 heterostructure with respect to those based on the ZnO/CT1 one, in agreement with experimental finding.11

We now analyze how the features of the hybrid interfaces are affected by the presence of imidazole as co-adsorbent. Figure 1 shows the fully relaxed structures for imidazole and CT1 molecules co-adsorbed at ZnO(1100) and TiO2(101) surfaces. Because of the presence of imidazole, the CT1 molecule is in a slightly distorted conformation, where the oxygen atom from the squaric acid group, which is not involved into bonding with the substrate, points toward the co-adsorbent. The binding energy for the co-adsorption of CT1 and imidazole at both oxide surfaces is higher than the sum of the adsorption energy obtained when the molecules are adsorbed separately (BECT1+imidazole > BECT1 + BEimidazole) of about 0.5 eV for ZnO and 0.35 eV for TiO2. Given that the intermolecular interaction between CT1 and imidazole at the surface was evaluated to be 0.13 eV, the co-adsorption energy gain indicates that the co-adsorbent strengthens the binding between dye molecule and metal oxide surfaces improving the stability of the sensitized systems. Co-grafting of CT1 and imidazole reduces the possibility of dye aggregations reducing the interaction D

DOI: 10.1021/acs.jpcc.5b11113 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Table 1. Binding Energy, Energy Difference between the HOMO State and CB Minimum, Work Function, and the Band Egdes Shift Calculated with Respect to the Clean Surface for Single Imidazole and Single CT1 Adsorptions and Co-Adsorption of Both Molecules at TiO2(101) and ZnO(1100) Surfaces anatase-TiO2(101) BE (eV) ΔECB‑HOMO (eV) Φ (eV) band shift (eV)

ZnO(1100)

imidazole

CT1

imidazole and CT1

imidazole

CT1

imidazole and CT1

−0.89 3.82 1.51 0.12

−2.18 1.85 1.53 0.10

−3.42 2.33 1.48 0.15

−0.70 2.72 1.28 0.20

−2.31 1.01 1.37 0.11

−3.55 1.54 1.27 0.21

achieving a stronger binding between dye and metal oxide and optimizing the energy alignment between occupied and uncoccupied states to maximize the photovoltaic performances.

between CT1 molecules; moreover, saturating the surface undercoordinated metal atoms and increasing the distance between the electrolyte (such as triiodate) and the surface, it prevents back electron/hole recombination. Co-adsorbent has strong effects also on the electronic properties of the CT1/oxide interfaces as demonstrated by a detailed analysis of the DOS and the PDOS reported in Figures S9 and S10. When the co-adsorbent is present, an additional molecular peak appears in the energy range close to the valence band maximum (VBM) that can be identified as the HOMO state of imidazole; this molecular state is located immediately above (+0.51 eV) or below (−0.58 eV) the metal oxide VBM for TiO2 and ZnO, respectively, while the LUMO is well above the oxide CBM (ΔELUMO−HOMO 5.18 eV). The most important effect corresponds to a change in the alignment between the dye molecular states and oxide band edges (CBM and the VBM) as summarized in the scheme of Figure 4. Indeed, the metal oxide bands are shifted to higher energies while CT1 states are pushed downward by the presence of imidazole. In particular, in the ZnO-based interface the HOMO (LUMO) of CT1 is located at 1.14 (0.69) eV above the VBM (CBM), while in the anatase-based system the HOMO of the dye is 0.79 eV above the VBM and the LUMO is 0.31 eV below the CBM. The observed VB and CB shifts of the functionalized surfaces with respect to the band structure of the clean surface (see Table 1) are induced by the dipole of the adsorbed molecules: The larger is the surface dipole, the lower is the work function of the functionalized surfaces (Table 1) and the distance of the VBM from the vacuum level (see Figure 4). In the case of our hybrid systems, the component of the calculated molecular dipole along the surface normal is 3.68 and 3.24 D for imidazole and CT1 molecules, respectively. The lowering of the CT1 molecular orbital is instead due to the interaction of CT1 with imidazole molecules, which stabilize the hybrid interface, as discussed above. In summary, our calculations demonstrate that the coadsorbent changes the relative energy alignments between CT1 molecular states and CB and VB of metal oxides and specifically to CT1 and imidazole at the anatase surfaces; both CT1 HOMO and LUMO states fall in the substrate energy gap transforming the interface in a type I heterojunction, which limits the photovoltaic performance of this hypothetical photoanodes. The effect is opposite for the CT1/ZnO system for which the final type II heterostructure is characterized by a larger staggered energy gap and thus a larger open circuit voltage. We stress the fact that the imidazole/CT1 coadsorption has been chosen only as a model system to understand how the electronic properties of a hybrid organic/ inorganic interface benefit or not by the presence of an otherwise inert co-adsorbent. Changing the dye or the coadsorbent or both molecules, it is possible to tune accurately the structural and electronic properties of the interface both

■

CONCLUSION In conclusion, the adsorption of the CT1 dye molecule at the anatase-TiO2(101) and ZnO(1100) has been analyzed in terms of geometry, stability, and electronic structure within DFT framework. Our calculations demonstrate that CT1 spontaneously deprotonates at both oxide surfaces binding in a bridging configuration. The adsorption energy is slightly higher for the ZnO surface since the oxygen−oxygen distance of the anchoring group is closer to the Zn ion distance at the ZnO surface. Our study shows that both ZnO/CT1 and TiO2/CT1 interface form a type II heterojunction, and as such, they are potentially good candidates to be employed in DSSC. Yet, due to the reduced hybridization occurring between the CT1 unoccupied energy states and the ZnO CB and in view of the strong electronic coupling between the anatase and hemisquaraine orbitals, we predict that DSSC based on anatase-TiO2 would be characterized by larger quantum yield and higher solar cell efficiency values. Finally, we studied the effects of a co-adsorbent (imidazole) on the properties of the hybrid CT1/ oxide interface and found that co-grafting considerably enlarges the stability of the interface strengthening the binding between dye and metal oxide. More importantly, the co-adsorbent changes the energy level alignment of the heterostructure, strongly affecting the final efficiency of the device.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11113. Discussion of the structural and electronic properties of the isolated hemi-squaraine molecule, of the clean anatase TiO2(101) and ZnO(1100) surfaces, and of imidazole adsorption at anatase TiO2(101) and ZnO(1100) surfaces; the equilibrium structure (Figure S6), the band structure, DOS, and PDOS for CT1 chemisorbed at the anatase (Figure S7) and ZnO (Figure S8) surfaces and for the co-adsorption of CT1 and imidazole at the anatase-TiO2(101) (Figure S9) and ZnO(1100) surfaces (Figure S10); comparison between PBE+U and hybrid functionals, the cluster models used for TDDFT calculations (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.R.). Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.jpcc.5b11113 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

■

(11) Shahzad, N.; Risplendi, F.; Pugliese, D.; Bianco, S.; Sacco, A.; Lamberti, A.; Gazia, R.; Tresso, E.; Cicero, G. Comparison of HemiSquaraine Sensitized TiO2 and ZnO Photoanodes for DSSC Applications. J. Phys. Chem. C 2013, 117, 22778−22783. (12) Manzhos, S.; Giorgi, G. Bridging the Fields of Solar Cell and Battery Research to Develop High-Performance Anodes for Photoelectrochemical Cells and Metal Ion Batteries. Challenges 2013, 4, 116−135. (13) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; et al. QUANTUM ESPRESSO: A Modular and OpenSource Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21 (39), 395502. (14) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation for the Exchange-Correlation Hole of a Many-Electron System. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 16533− 16539. (15) H. J. Monkhorst, H. J.; Pack, J. D. Special Points for BrillouinZone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (16) Vogel, D.; Krüger, P.; Pollmann, J. Self-Interaction and Relaxation-Corrected Pseudopotentials for II-VI Semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 5495−5511. (17) Malcioğlu, O. B.; Gebaur, R.; Rocca, D.; Baroni, S. A Code for the Simulation of Molecular Spectra Using the Liouville-Lanczos Approach to Time-Dependent Density-Functional Perturbation Theory. Comput. Phys. Commun. 2011, 182, 1744−1754. (18) Hughbanks, J. K.; Miller, T.; Richardson, G.; Smith, J.W. J. Structural-Electronic Relationships in Inorganic Solids: Powder Neutron Diffraction Studies of the Rutile and Anatase Polymorphs of Titanium Dioxide at 15 and 295 K. J. Am. Chem. Soc. 1987, 109, 3639−3646. (19) Rossler, U. Landolt-Bornstein, New Series, Group III; Springer: Heidelberg, 1999; 17B, 22, 41B. (20) Shirley, R.; Kraft, M. Electronic and Optical Properties of Aluminium-Doped Anatase and Rutile TiO 2 from Ab initio Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 075111. (21) Pham, A.; Zhang, Y. B.; Assadi, M. H. N.; Yu, A. B.; Li, S. Ferromagnetism in ZnO:Co Originating from a Hydrogenated Co− O−Co Complex. J. Phys.: Condens. Matter 2013, 25, 116002−116011. (22) Welte, A.; Waldauf, C.; Brabec, C.; Wellmann, P. J. Application of Optical Absorbance for the Investigation of Electronic and Structural Properties of Sol−Gel Processed TiO2 Films. Thin Solid Films 2008, 516, 7256−7259. (23) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Cho, S.-J.; Morkoc, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98, 041301−041404. (24) Li, S. C.; Wang, J. G.; Jacobson, P.; Gong, X. Q.; Selloni, A.; Diebold, U. Correlation between Bonding Geometry and Band Gap States at Organic−Inorganic Interfaces: Catechol on Rutile TiO2(110). J. Am. Chem. Soc. 2009, 131, 980−984. (25) Nilsing, M.; Persson, P.; Ojamae, L. Anchor Group Influence on Molecule−Metal Oxide Interfaces: Periodic Hybrid DFT Study of Pyridine Bound to TiO2 via Carboxylic and Phosphonic Acid. Chem. Phys. Lett. 2005, 415, 375−380.

ACKNOWLEDGMENTS We acknowledge the CINECA award under the ISCRA initiative for the availability of high performance computing resources and support.

■

ADDITIONAL NOTES By employing this computational scheme to the bulk oxide systems, we found equilibrium lattice parameters of abulk = 3.794 Å (0.24%), cbulk = 9.656 Å (1.50%) for the anatase case and abulk = 3.288 Å(1.17%), cbulk = 5.308 Å (2.08%), and u = 1.614 (0.90%) for wurtzite ZnO. These values are in good agreement with the experimentally observed data18,19 (deviations are reported in parentheses) and previous PBE calculated results.20,21 The corresponding energy gaps, estimated after applying the Hubbard correction, are 3.19 eV for anatase (indirect) and 3.22 eV for ZnO (direct) to be compared with the experimental values of 3.21 and 3.3 eV, respectively.22,23 b The binding energy (BE) of the dye molecule to the metal oxide surfaces was evaluated by calculating BE = Etot − Esurf − Emol, where Etot is the total energy for the functionalized surfaces and Esurf and Emol correspond to total energy of the relaxed clean surface and isolated molecule, respectively. c According to TDDFT analysis performed on model clusters, the excitations that mostly contribute to visible light absorption involve HOMO−LUMO and HOMO−LUMO+1 transitions. a

■

REFERENCES

(1) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (2) Grätzel, M. Dye-Sensitized Solar Sells. J. Photochem. Photobiol., C 2003, 4, 145−153. (3) Chen, C.-Y.; Wang, M.; Li, J.-Y.; Pootrakulchote, N.; Alibabaei, L.; Ngoc-le, C.-H.; Decoppet, J.-D.; Tsai, J.-H.; Grätzel, C.; et al. Highly Efficient Light-Harvesting Ruthenium Sensitizer for Thin-Film Dye-Sensitized Solar Cells. ACS Nano 2009, 3, 3103−3109. (4) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, Md. K.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved Through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242−247. (5) Yella, A.; Lee, H.-W.; Nok Tsao, H.; Yi, C.; Chandiran, A. K.; Nazeenddin, M. K.; Wei-Guan Dian, E.; Yen, C.-Y.; Zakeeenddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)− Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (6) Calzolari, A.; Ruini, A.; Catellani, A. Anchor Group versus Conjugation: Toward the Gap-State Engineering of Functionalized ZnO(1010) Surface for Optoelectronic Applications. J. Am. Chem. Soc. 2011, 133, 5893−5899. (7) Han, L.; Ashraful, I.; Han, C.; Chandrasekharam, M.; Barreddi, C.; Shufang, Z.; Xudong, Y.; Masatoshi, Y. High-Efficiency DyeSensitized Solar Cell with a Novel Co-Adsorbent. Energy Environ. Sci. 2012, 5 (3), 6057−6060. (8) Magne, C.; Urien, M.; Ciofini, I.; Tugsuz, T.; Pauporte, T. Amphiphilic Acids as Co-Adsorbents of Metal-Free Organic Dyes for the Efficient Sensitization of Nanostructured Photoelectrode. RSC Adv. 2012, 2, 11836−11842. (9) Lim, J.; Kwon, Y. S.; Park, T. Effect of Coadsorbent Properties on the Photovoltaic Performance of Dye-Sensitized Solar Cells. Chem. Commun. 2011, 47, 4147−4149. (10) Cicero, G.; Musso, G.; Lamberti, A.; Camino, B.; Bianco, S.; Pugliese, D.; Risplendi, F.; Sacco, A.; Shahzad, N.; Ferrari, A. M.; et al. Combined Experimental and Theoretical Investigation of the HemiSquaraine/TiO2 Interface for Dye Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15 (19), 7198−7203. F

DOI: 10.1021/acs.jpcc.5b11113 J. Phys. Chem. C XXXX, XXX, XXX−XXX