Anatase by Amorphous Al - American Chemical Society

Article pubs.acs.org/JPCC

Coating TiO2 Anatase by Amorphous Al2O3: Effects on Dyes Anchoring Through Carboxyl Groups U. Terranova†,‡ and D. R. Bowler*,†,‡ †

Department of Physics & Astronomy, University College London, London, WC1E 6BT, U.K. International Center for Materials Nanoarchitectonics (MANA), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

‡

ABSTRACT: We have studied by density functional theory the amorphous Al2O3 (a-Al2O3)/TiO2 anatase (101) interface. The a-Al2O3 sample was generated by following a “melt and quench” technique, in which the corundum phase of Al2O3 was first melted at the temperature of 5000 K and then gradually cooled to 0 K. Once placed on TiO2 anatase, the overlayer has been employed for the adsorption of formic acid (HCOOH). Compared to the bare anatase (101), the adsorption of HCCOH is enormously stabilized in the presence of the coating, regardless of its thickness. Additional calculations confirm the trend also when the benchmark N3 dye, binding through two carboxyl groups (−COOH), is used. These results help to understand the improvement in dye-sensitized solar cell efficiencies after the a-Al2O3 coating of the TiO2 electrode.

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INTRODUCTION While the demand for renewable sources of energy is increasing day by day, dye-sensitized solar cells (DSSCs) are gaining more and more attention as a promising alternative to the traditional silicon devices.1,2 In a DSSC, a layer of an organic or metalbased dye is bound to the surface of a nanoporous TiO2 film (usually exposing the (101) face of the anatase polymorph), forming a chromophore/semiconductor interface which has been extensively investigated (see refs 3−10 and ref 11 for a recent review). The carboxyl group −COOH constitutes the binding anchor of the most widely used Ru-based dyes. In the attempt to shed light on their adsorption, the interaction of formic acid (HCOOH) with anatase has been the subject of many works in the last years.12−17 However, the results in the literature are not always in agreement with each other, indicating that further investigations are necessary on this system. For example, some density functional theory (DFT) studies have suggested that the most stable structure is a molecular monodentate.12,15 Others have predicted that the latter is in equilibrium with a dissociative bidentate bridging,13 whereas an experimental work has shown that, besides to hydrogen bonded HCOOH, a formate (HCOO−) is also formed upon adsorption (in both bidentate chelating and monodentate mode).16 To the end of increasing the power conversion efficiency (PCE) of DSSCs, in the past few years, many groups have successfully coated the TiO2 film with an amorphous overlayer of a second higher conduction band (CB) oxide, such as Al2O3.18−21 Recently, Lin et al.22 have shown that it is also possible to employ atomic layer deposition (ALD)23 to overcoat the anatase nanoparticles. Due to its high thickness resolution (around 0.1 nm) and infiltrating capability, ALD is ideal to deposit monolayer films, and some steps toward the © 2012 American Chemical Society

understanding of the growth mechanism of the amorphous Al2O3 (a-Al2O3) on TiO2 have been taken by us and other groups.24−26 With this paper, we intend to shed light on the increased PCE resulting from the a-Al2O3 coating of the TiO2 anatase (101) substrate. At least four mechanisms are currently debated, and the question of which of them plays the predominant role still remains unsolved. In the first mechanism, the insulating layer works as a physical barrier that retards all of the electron transfer reactions at the interface.19 According to this scheme, while the injection yield into the semiconductor is insensitive to the small lengthening of the injection time caused by the coating, a retardation of the recombination reactions at the TiO2/dye and TiO2/electrolyte interfaces reduces the losses, bringing a larger electron concentration in the TiO2 film and consequently a higher Fermi level and open circuit voltage Voc. In the second mechanism, the overlayer creates a dipole moment at the oxide/oxide interface, the so-called surface dipole effect, which induces a positive shift of the CB and results in higher Voc. A dipole moment has been found in action when various oxides are used to coat the nanoporous TiO2 electrode, and a good correlation has been found between the isoelectric point of numerous overlayers and the induced Voc change.27 In the third mechanism, the second oxide is thought to passivate the surface states of TiO2, which are responsible for the recombination of the injected electron with the dye or the redox couple.28 In the fourth mechanism, the improved performance of DSSCs arises from an enhanced adsorption of the dye onto the overlayer, which, being more basic than Received: October 13, 2011 Revised: January 21, 2012 Published: January 21, 2012 4408

dx.doi.org/10.1021/jp209862s | J. Phys. Chem. C 2012, 116, 4408−4415

The Journal of Physical Chemistry C

Article

of the same supercell with, respectively, only the substrate or a single molecule. Together with the density of states (DOS)

TiO2, favors the attachment through the carboxyl groups of the dye.29 Regardless of the mechanism, it is generally recognized that a coating which is too thick causes a decrease of the photocurrent Iph and a corresponding loss of PCE. To maximize the PCE, it is then necessary to employ an overlayer whose thickness has been optimized, and thicknesses ranging from 1 to 120 Å have been reported in the literature.30 Contrary to the information on the adsorption of HCOOH on TiO2 anatase, the information regarding the adsorption of HCOOH on a-Al2O3 is scarce. There is experimental evidence that formic acid is bonded to the surface by the two oxygens,31 supported by the finding that the spectrum bridging mode simulated by DFT best matches the experimental counterpart.32 It would be of vital importance, in order to understand the effect of the coating, to gain more insights into this system. Here, we present a DFT study on the adsorption of formic acid on TiO2 anatase (101), addressing the effect of an a-Al2O3 overlayer on the substrate. First, we describe the generation of the a-Al2O3 through the “melt and quench” technique with ab initio MD33 and test the resulting sample against others reported in the literature. Then, we characterize the a-Al2O3/ TiO2 interface in its coordination pattern and electronic structure and proceed to the adsorption of formic acid, for which we calculate adsorption energies and vibrational frequencies. Finally, by adsorbing the benchmark Ru-based N3 dye,34 anchoring through −COOH groups, we show that the results obtained for formic acid can be generalized and extended to larger dyes commonly employed in DSSCs.

ρ(E) = ΣjG(E − εj)

where εj is the eigenvalue of the KS orbital ϕj and G a Gaussian function, it is possible to define a partial DOS (PDOS) projected onto spherical harmonics ρlI (E) = Σj|⟨χlI |ϕj⟩|2 G(E − εj)

(3)

χlI

where is the spherical harmonic with angular momentum l centered on ion I. By summing over l and the ions I belonging to the same oxide (either Al2O3 or TiO2), we obtain an expression for the PDOS ρOX (E) = ΣlI εOX ρlI (E)

(4)

To calculate the vibrational frequencies of adsorbed formic acid, we employed the diagonalization of the dynamical matrix. Only the formic acid degrees of freedom were included in the calculation of the matrix components. Test calculations with the addition of some underlying surface ions gave negligible differences in the resulting vibrational frequencies (∼3 cm−1). All of the figures were made with VMD.40

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RESULTS AND DISCUSSION Al2O3/TiO2 Anatase (101) Interface. The a-Al2O3 sample was generated by the melt and quench technique on the corundum α-Al2O3. Since our final goal is the reproduction of the a-Al2O3/TiO2 anatase (101) interface, we have chosen the unit cells of the two oxides in a way to minimize the lattice strain, although in the limit of our computational capabilites. The calculated lattice vectors for TiO2 anatase and α-Al2O3 are reported in Table 1. From these values we construct a

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METHODS All of the DFT calculations have been performed with the VASP 4.6.34 package,35,36 which solves the Kohn−Sham (KS) equations of the system through a self-consistent procedure. We chose the generalized gradient approximation (GGA) for the exchange and correlation term, in the formalism of Perdew and Wang (PW91).37 A plane wave basis set with a cutoff of 400 eV was adopted, which, when employed with a (4 × 4 × 1) Monkhorst-Pack grid, is enough to converge the energy of the bulk anatase unit cell within 3 meV/atom. Ultrasoft pseudopotentials38 were used to represent the effect of the inner electrons. The 3s and 3p Ti electrons were treated as core electrons, after checking that their inclusion into valence electrons resulted in a difference in the anatase bulk parameters of less than 0.01 Å. Since VASP implements a periodic boundary conditions scheme, we introduced a vacuum of at least 10 Å between adjacent slabs to avoid spurious interactions between replica atoms. Relaxations of structures were performed with the conjugate gradient method and stopped when the force acting on each ion was less than 0.03 eV/Å. The surface calculations were performed with a (1 × 2 × 1) Monkhorst-Pack grid.39 The latter, obtained by rescaling the converged bulk mesh according to the new lattice vectors, was checked by calculating the anatase (101) surface energy for a four-layer slab. The resulting value of 0.035 eV/Å2 coincides with the one of a previous work employing the GGA.14 The adsorption energies were evaluated according to the formula ΔE = E(surf + mol) − Esurf − Emol

(2)

Table 1. Calculated Lattice Parameters of Bulk TiO2 Anatase and α-Al2O3 Compared with Experimental Results41,42,a lattice parameters (Å) α-Al2O3

TiO2 anatase this work experiment a

a1

c1

a2

c2

3.811 3.784 (+0.7%)

9.759 9.514 (+2.6%)

4.827 4.756 (+1.5%)

13.172 12.982 (+1.5%)

In parentheses are reported the percentage errors.

monoclinic (1 × 2) anatase (101) unit cell with dimensions of A1 = 7.622 and B1 = 10.477 Å and an orthorhombic α-Al2O3 unit cell with dimensions of A2 = 8.864, B2 = 10.233, and C2 = 13.962 Å. The latter values have already been multiplied by a factor of 1.06, necessary to bring the α-Al2O3 density to 3.2 g/cm3 (the density of amorphous alumina ranges from 2.95 to 3.3 g/cm343). With this choice of the unit cells, the alignment A1/A2 and B1/B2 gives lattice mismatches of, respectively, 14 and 2%, which will be compensated as reported below. The melt and quench procedure starts with 2 ps of ab initio MD of the α-Al2O3 at T = 5000 K (Figure 1, a), well above the experimental melting point of 2327 K, during which the oxide is brought to the liquid phase. Then, the system is rapidly quenched at a rate of 500 K/ps to the final temperature of 500 K (Figure 1, b). Now, while preserving the density, the system is equilibrated for 5 ps, during which the lattice vectors are

(1)

Here, E(surf+mol) is the energy of the supercell with an adsorbed molecule (HCOOH or N3), while Esurf and Emol are the energy 4409



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Table 2. Maximum Positions RM and Average Coordination Numbers n12 of the Partial PDF Compared with Other Results in the Literature RM (Å) this work Chagarov et al.44 Gutierrez et al.43

n12

Al−O

O−O

Al−Al

Al−O

O−O

Al−Al

1.85 1.77 1.76

2.75 2.82 2.75

3.25 3.07 3.12

4.63 4.23 4.25

13.96 10.66 9.47

8.96 6.98 8.26

states, and differ qualitatively only in the band gap width. The reduction of the gap following the transformation of phase (from 5.7 to 3.1 eV) has already been reported in the literature (from 6.0 to 3.8 eV44). After having been generated and tested, the a-Al2O3 sample was placed on the optimized anatase (101) substrate. From this configuration, by two stoichiometrical cleavages at a distance of 3 and 9 Å from the interface, a structure with a “thin” and another with a “thick” overlayer were obtained and subsequently relaxed (Figure 3, top).

Figure 1. Scheme for the generation of the amorphous Al2O3 phase. (a) High-temperature melting; (b) linear quenching; (c) equilibration at rescaled lattice vectors; and (d) relaxation at 0 K.

adjusted to match the A1 and A2 values of anatase (Figure 1, c). The last 3 ps of this run represent the production run, in which data are extracted. Finally, the sample is relaxed at the temperature of 0 K (Figure 1, d). Figure 2 shows the partial pair-distribution functions (PDFs) g12, defined as the probability of finding one atom of type 2 in a

Figure 2. Partial PDFs for the a-Al2O3 sample generated by the “melt and quench” technique.

spherical shell between r and r + Δr from one atom of type 1.44 From the curves, we can extract the pair nearest-neighbor distances RM, corresponding to the maximum positions, and the average coordination number n12, obtained by integrating the PDFs until the position of their first minimum Rm (5)

Figure 3. Top: front view of the thin and thick overlayers relaxed onto the (101) anatase substrate. Bottom: PDOS of the two oxides for the three systems (coating 0, 3, and 9 Å). The highest occupied levels are indicated by the vertical dashed lines.

Maximum positions and average coordination numbers are reported in Table 2, where they are compared with previous results in the literature. While the RM values are similar, the results show poorer agreement for n12, which tends to be overestimated compared to other theoretical works. As a final check, we have compared the electronic structure of our a-Al2O3 sample to that of the α-Al2O3 phase (data not shown). The two densities of states are very similar, without any defect

In general, the Al 2O 3 overlayer could differ from the amorphous phase (for example, the Al2O3 grown on NiAl presents a well-ordered structure45). However, it has been shown that when the Al2O3 is deposited on the TiO2 electrode of DSSCs by ALD it has an amorphous form.46 Although our approach does not simulate the atom-by-atom deposition typical of ALD, our a-Al2O3 sample reproduces the main

n12 = 4πρ2

∫0

Rm

g12(r )r 2dr

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respectively, of 1.1 and 0.1 eV, though still underestimated by DFT, are reduced compared to the bare TiO2. The thickness of the deposited film strongly affects the electronic structure, in line with a previous work on the Al2O3/TiO2 interface, where, for very similar thicknesses, the resulting gaps have been found to be, respectively, of 0.75 and less than 0.1 eV.49 Formic Acid Adsorption. In this section, we make use of the two Al2O3/TiO2 samples to study the adsorption of formic acid. This small ligand constitutes the binding group of larger Ru-based dyes, such as the N3, the paradigm of sensitizers in DSSCs. On anatase (101), many different bindings are possible.12,13,15,17 We have exploited the results present in the literature to focus only on the three most stable, i.e., two monodentates (one molecular and one dissociative) and a bridging adsorption. On the amorphous overlayer, the number of different configurations for the same type of adsorption is even larger than on bare anatase. Our approach was to test the same adsorptions of the bare surface on different binding sites, reporting only the most stable for each. Structures and energetics of formic acid on anatase (101), with and without the a-Al2O3 coating, are presented in Figure 4 and Table 4. On anatase (101), a molecular monodentate (MON(H)-0) is more stable then a dissociative monodentate mode (MON-0) and presents adsorption energies similar to the bridging (BRI-0), in agreement with a recent work by Nunzi et al.13 The adsorption of HCOOH on the coated system presents a remarkably different scenario. The emerging picture does not seem to depend on the a-Al2O3 thickness, with the only exception being that all of the molecular adsorptions were found to relax toward dissociative ones (MON-9) on the thick system. This result, together with the lower adsorption energies for the dissociative (MON-3) over the molecular monodentate modes (MON(H)-3) on the thin layer, allows us to conclude that the coating reverses the results of the bare anatase, where the molecular monodentate adsorption was favored over the dissociative. Regardless of the overlayer thickness, however, the preferential adsorption mode is bridging (BRI-3 and BRI-9), consistently with experimental results predicting that the molecule is bonded to the surface by the two oxygen atoms.31 Interestingly, when the coating is present, every adsorption mode becomes more stable, a fundamental characteristic for the stability of DSSCs based on carboxylate dyes. This finding agrees with a recent work employing the N3 dye on Al2O3 overcoated TiO2, in which the enhanced dye adsorption, together with a reduction in carrier recombination, was identified as the reason for the increase in Iph.30 The resulting enhanced adsorption is also in line with a different study on the adsorption of a carboxylate-containing dye (Rhodamine B) on the same system, where the interaction between −COO and Al was found to be stronger than that between −COO and Ti sites.50 In the same work, the authors have proved that the binding mode of Rhodamine B is dominantly the monodentate linkage. We find instead that bridging configurations, with energies larger than 2 eV, stabilize mostof the interaction of HCOOH. To validate our results, we have calculated the vibrational frequencies of adsorbed formate with and without the thick overlayer. The six intrinsic normal modes are reported in Table 5, where they are compared to some data from experiments. It has been shown that the difference Δ between the antisymmetric νas(OCO) and symmetric strectching of vibration νs (OCO) depends on the nature of the bond between the

characteristic of amorphous alumina. We are confident then that the so-generated a-Al2O3/TiO2 interface provides a realistic model to study the adsorption of carboxylic groups, relevant to DSSCs. The type of bonds across the interface are reported in Table 3. While the total number of bonds is equal to 6 in both cases, the Table 3. Pattern of Bonds Across the Interfacea occurrances

average length

bonds

thin

thick

thin

thick

Ti{1}−O O{1}−Al Ti{3}−O O{2}−Al

3 3 0 0

1 3 1 1

2.04 1.88 -

1.94 1.86 1.99 2.04

a

The ions of the substrate are classified according to the numbers of overlayer ions to which they bond, as indicated by the superscript.

coordination pattern is rather different. For example, in the thicker coating, a Ti ion of the top layer is dragged out by about 1 Å and can thus accommodate three bonds with the O of the coating. We estimate the induced corrugation of the TiO2 substrate by the root-mean-square deviation (rmsd) of its top layer atoms with respect to the bare surface. The values for the thin and thick structures are, respectively, 0.18 and 0.35 Å. In the thick coating, a large contribution to the rmsd is given by the upward relaxation of the Ti ion which is dragged out. We have tested the effects of the temperature on the interfacial bonding by carrying out some MD on the thick system at the typical ALD temperature of 500 K. However, no significant differences were found with the coordination pattern previously described, in which the anatase substrate mantains its bare structure. In Figure 3 (bottom), we show the PDOS of the two oxides for the three systems (bare, thin, and thick coating). The experimental band gap value of anatase is 3.2 eV.47 Our underestimated value of 2.2 eV is due to the well-known selfinteraction error of DFT.48 We note that the Fermi level can lie anywhere in the gap without changing the DFT total energy, and its position within that energy range therefore should not be considered significant. As expected, given the larger band gap of Al2O3 compared to TiO2, the bottom of the CB has contributions only from Ti ions. A surface coating can in principle increase the PCE of a DSSC through the surface dipole effect: the overlayer can induce a shift of the CB edge of TiO2 toward positive energies (resulting in higher Voc) as a consequence of a dipole moment formed at the oxide/TiO2 interface. In the thick coating, a net dipole moment (0.67 eÅ) is present along the direction perpendicular to the interface. Rather than to a charge transfer across the interface, we attribute this dipole to the charge distribution within the Al2O3 slab, as suggested by the similar value of the dipole moment (0.80 eÅ) in the overlayer when the TiO2 substrate is removed. However, the dipole in the overlayer is too small to affect the energy levels: in the thick system the CB is at the same energy as in bare anatase, while in the thin system it is slightly shifted negatively. The results seem thus to exclude any kind of dipole effect in the mechanism behind the PCE increase following the Al2O3 coating. While no difference can be detected in the CBs, when the layer of the Al2O3 film increases, its valence band extends beyond (toward higher energies) the TiO2 one. As a consequence, the band gaps of both the thin and thick systems, 4411



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Figure 4. Most stable adsorption modes of formic acid on TiO2 anatase (101) for different thicknesses of the a-Al2O3 overlayer. MON(H) and MON stand, respectively, for molecular and dissociative monodentate, and BRI stands for bridging. The number associated with each figure corresponds to the thickness of the overlayer in Å.

Table 4. Adsorption Energies for HCOOH on TiO2 Anatase (101) for Different Thicknesses of the Coatinga

Table 5. Vibrational Frequencies (In the Harmonic Approximation) of HCOOH Adsorbed on TiO 2 Anatase (101) with and without the Thick a-Al 2O 3 Overlayer

adsorption energies (eV)

a

mode

0Å

3Å

9Å

MON(H) MON BRI

−0.94 −0.36 −0.94

−1.23 −1.37 −2.04

unstable −1.91 −2.78

frequencies (cm−1) 0Å type

MON(H) BRI

ν(CH) νas(OCO) δ(CH) νs (OCO) π(CH) δ(OCO) Δ ≡ νas(OCO) − ν(OCO) s

See Figure 4 for the abbreviations.

carboxylate and the oxide. In particular, separations greater than the free carboxylate ion (250−270 cm−1 for sodium formate, see ref 51 and references within) or significantly smaller are indicative, respectively, of monodentate or bidentate (chelating or bridging) adsorptions.52 When no overlayer is present, the bridging Δ value of 184 cm−1 agrees with the experimental one of 180 cm−1. We note also that the very different values of νas(OCO) for MON(H) and BRI characterize the two adsorption modes. The result reproduces the equilibrium between the

3017 1631 1379 1325 1017 785 306

2978 1515 1369 1331 1007 705 184

9Å expa 2883 1665/1550b 1380 1370c 180c

MON BRI

expd

2956 1604 1381 1348 1020 718 256

2894 1629 1392 1380 1061 788 249

3020 1632 1400 1382 1010 705 250

a

Data from ref 16. bAssigned to MON/BRI. cOnly the BRI value was detected. dThe assignments of frequencies and experimental peaks for the coated system have been taken from ref 31, where the HCOOH was adsorbed on amorphous Al2O3. 4412



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Figure 5. Relaxed structures of the N3 dye in the BRIMON (top) and in the 2MON (bottom) mode with and without the thin a-Al2O3 coating on anatase (101).

experimental bands,16 confirming that monodentate and bridging structures are likely to coexist. The DFT vibrational frequencies for HCOOH on the coated substrate give further evidence to the bridging adsorption. A very good match is obtained when comparing the BRI frequencies to the experimental peak positions.31 Our Δ values for MON and BRI are, respectively, 256 and 250 cm−1, the latter agreeing nicely with the experimental value of 249 cm−1 for formic acid on a-Al2O3. Finally, we tackle the question of whether the results obtained for formic acid can be generalized and extended to larger dyes commonly employed in DSSCs. To this end, we test the effect of the overlayer on the entire N3 dye. As these calculations are rather expensive, we have limited our study to two adsorptions. The N3 dye has been adsorbed in a double monodentate/bridging (BRIMON) and monodentate (2MON) interaction (between the most favored on bare anatase53), and the placement of the proton in the monodentate linkage has been derived from the formic acid behavior discussed above (on the dye for the bare anatase, on the surface for the overlayer). The relaxed structures, with and without the overlayer, are illustrated in Figure 5. From the adsorption energies and bond distances in Table 6, it is evident that not onlyfor HCOOH but also for the entire N3 dye the interaction is much stronger when the TiO2 is coated by a layer of a-Al2O3. To accommodate both the carboxyl groups to the crystalline substrate, the monodentate bonds are forced to mantain larger

Table 6. Bond Distances and Adsorption Energies for the N3 Dye in the BRIMON and 2MON Modes with and without the Thin Coating of a-Al2O3 on Anatase (101)a bond distances (Å) mode BRIMON(TiO2) BRIMON(Al2O3/ TiO2) 2MON(TiO2) 2MON(Al2O3/ TiO2)

adsorption energies (eV)

B1

B2

M1

M2

2.11 1.94

2.11 1.86

2.24 1.73

-

−1.08 −5.35

-

-

2.25 1.78

2.19 1.76

−0.98 −4.58

a

B1 and B2 are the bridging bonds, M1 and M2 the monodentate ones.

distances than with formic acid (2.24, 2.25, and 2.19 Å vs 2.13 Å), and also the hydrogen bonds have larger distances (3.18, 3.13, and 2.27 Å vs 1.38 Å). On the contrary, the amorphous substrate allows us to accommodate both the carboxyl groups even at shorter monodentate distances for the N3 dye compared to formic acid (1.78, 1.76, and 1.73 Å vs 1.83 Å). Negligible differences have been found for the bridging bond lengths, in both substrates. This argument explains why for the bare TiO2 the energies are higher than what is expected on the basis of the previous results with formic acid, while for the a-Al2O3/TiO2 interface the N3 energetics is even more favorable. Despite this quantitative deviation, we stress however that the investigation of the N3 dye reproduces qualitatively the trend 4413



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the UKCP Consortium, which is funded by EPSRC grant EP/ F040105.

already seen for formic acid, both the adsorptions being enormously stabilized in presence of the a-Al2O3 overlayer.

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CONCLUSIONS We have presented a DFT study of the a-Al2O3/TiO2 anatase (101) interface, which has been employed to adsorb formic acid and the N3 dye. The a-Al2O3 was generated through the melt and quench ab initio technique, in which the corundum phase of Al2O3 was first melted at 5000 K, well above its fusion point of 2327 K, and then gradually cooled to 0 K. We have characterized the amorphous sample by its partial PDFs, whose maximum positions, average coordination numbers, and electronic structures are in qualitative agreement with the literature, and use it to construct two models of the oxide/oxide interface differing in the a-Al2O3 thickness. When no coating is present on anatase, both monodentate and bridging modes are in equilibrium. The effect of the overlayer, regardless of its thickness, is to considerably stabilize all of the HCOOH adsorptions investigated, while shifting the equilibrium toward the bridging mode, as also confirmed by the vibrational analysis. The stronger interaction with the substrate, beneficial for the stability of DSSCs, has been reported by a recent work employing the N3 dye on Al2O3 overcoated TiO2, in which the enhanced dye adsorption was identified as one reason for the higher Iph.30 The enhanced adsorption is also in line with a second experimental study employing the Rhodamine B dye, where the interaction between −COO and Al was found to be stronger than that between −COO and Ti sites.50 Additional evidence for the coating acting as an enhancer of the dye adsorption is provided by calculations on the N3 dye, which revealed much lower adsorption energies in the case of an overcoated TiO2. As evidenced by the CB position in the electronic structures, a surface dipole effect does not seem to be in action. While the passivation effect and the retardation of the recombination reactions can not be excluded or supported by this work, the role that we propose for the a-Al2O3 overlayer in increasing the efficiency of DSSCs is close to the fourth mechanism discussed in the Introduction. However, differently from it, we do not attribute the enhanced adsorption to the higher basicity of a-Al2O3 compared to TiO2, but to its higher affinity toward carboxyl anchoring groups.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +44 (0)20 76797229. Fax: +44 (0)20 7679 0595. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS UT is supported by the MANA-WPI project through a collaboration with Cambridge University, and DRB was funded by the Royal Society. We are grateful to Angelos Michaelides, Conn O’Rourke, and Yoshitaka Tateyama for stimulating discussions. This work made use of the facilities of HECToR, the UK’s national high-performance computing service, which is provided by UoE HPCx Ltd. at the University of Edinburgh, Cray Inc., and NAG Ltd., and funded by the Office of Science and Technology through EPSRC’s High End Computing Programme. Calculations were performed at HECToR through 4414



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