Atomic Layer Deposition of Aluminum Oxide on TiO2 and Its Impact on

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Atomic Layer Deposition of Aluminum Oxide on TiO2 and Its Impact on N3 Dye Adsorption from First Principles Ville M€akinen,† Karoliina Honkala,*,‡ and Hannu H€akkinen†,‡ † ‡

NanoScience Center, Department of Physics, 40014 Jyv€askyl€a, University of Jyv€askyl€a, Finland NanoScience Center, Department of Chemistry, 40014 Jyv€askyl€a, University of Jyv€askyl€a, Finland ABSTRACT: The atomic layer deposition of aluminum oxide on an OH-terminated TiO2(101) anatase surface was studied employing density functional theory calculations. The formation of the Al2O3
TiO2 interface during the first atomic layer deposition cycle was modeled by studying the dissociative adsorption of an Al(CH3)3 precursor, followed with a H2Opulse reaction step that changes the surface termination. Calculations provide evidence for the formation of a discontinuous, atomically rough aluminum oxide layer after the first cycle. To explore the role of the aluminum oxide layer on adsorption of a ruthenium-based N3 dye molecule, various adsorption geometries were investigated. Calculations show that even one Al2O3 ALD cycle is enough to block N3 adsorption on the TiO2. Consequently, N3 anchors to the aluminum oxide layer, which increases the dye
TiO2 distance by ∼2 Å, changes the adsorption site, and weakens the coupling to TiO2. All these factors suppress forward electron injection from the dye molecule to the TiO2 conduction band edge. It is, therefore, questionable whether the aluminum oxide layer can offer improvements to the conversion efficiency in dye-sensitized solar cells.

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) comprise nanocrystalline porous metal oxide, typically anatase TiO2, covered with transition-metal complexes that absorb light.1
3 In an operating cell, a sensitized nanoporous TiO2 electrode is immersed under an electrolyte containing an I
/I
3 redox couple, and the cathode is coated with a thin layer of Pt. Owing to low-cost ingredients, DSSCs offer a cheap and sustainable method to convert light to electricity. To make these devices more competitive with commercial energy production technologies, a lot of research has been conducted to increase their conversion efficiency, and values exceeding 11% have been reported.4
6 The key issue to achieving an improved performance is to optimize the interfacial electron-transfer dynamics so that photogenerated electrons do not recombine with oxidized dyes or with I
3 molecules in the electrolyte but transport efficiently through the nanoporous TiO2 film. One way to do this is to cover TiO2 with a thin oxide layer.7
11 Among the promising coating oxides is Al2O3, which has a band gap ∼9 eV, and the valence (conduction band) band lies well below (above) that of TiO2.7,12,13 The optimization of the Al2O3
TiO2 interface can be performed using different techniques to deposit thin films.14
17 One of the methods, atomic layer deposition (ALD; see ref 18), offers an atomic level control to fabricate, on average, smooth and continuous thin films with known thicknesses. ALD processes are mostly based on self-limiting binary surface reactions with both gas-phase reactants proceeding in a sequential fashion.19 To deposit aluminum oxide, the precursors applied r 2011 American Chemical Society

are trimethyl aluminum (Al(CH3)3) and H2O, and the overall reaction is 2AlðCH3 Þ3 þ 3H2 O f Al2 O3 þ 6CH4

ð1Þ

including two half-reactions. Unfortunately, the coating decreases also the pivotal forward electron injection from a dye to a TiO2 electrode,20 and disadvantageous effects of the coating that is too thick overcome the benefits. Therefore, finding an optimal thickness for the Al2O3 film is of paramount importance. However, to measure the film thickness accurately for very thin films is difficult.19,20 In addition, the structure and the composition of the aluminum oxide at the TiO2 interface may differ substantially from the stoichiometric Al2O3, and therefore, in this work, it is referred to as an AlOx layer. Recently, it was estimated that the thickness increment per cycle is 0.1 nm calculated for thick AlOx coatings, whereas for ultrathin AlOx films (1
3 ALD cycles), the increment per cycle was estimated to be 0.2
0.3 nm.21 This indicates that the growth process is not well-understood. It is inaccurate to consider ultrathin AlOx films uniform and smooth, but atomic scale roughness is introduced during the first few AlOx ALD cycles. In ref 19, one ALD cycle was also found optimal for conversion efficiency and, already after the second cycle, the conversion efficiency was observed to be worse than on bare TiO2. On the contrary, ref 22 gives an order of magnitude larger Received: February 4, 2011 Revised: March 10, 2011 Published: April 13, 2011 9250

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The Journal of Physical Chemistry C value for the optimal film thickness and ref 21 reports that even one AlOx ALD cycle slows down the forward electron injection. The reaction steps in the Al2O3 ALD deposition cycle are suggested to follow a mechanism where (1) water vapor in the atmosphere dissociates and OH groups occupy Ti sites, (2) dosed Al(CH3)3 in the first half-reaction binds to Ti
OH sites, forming Ti
O
Al(CH3)2 via a ligand-exchange reaction, and (3) the surface termination changes when CH3 groups are replaced with OH groups upon a water dosing step.18 It is also possible that two ligand-exchange reaction steps take place upon Al(CH3)3 adsorption. However, the verification of the reaction mechanism experimentally is difficult, and the process may depend on temperature.23 There exist various density functional theory (DFT) studies on different ALD processes, including TiO2, Al2O3, HfO2, and ZrO2,24
33 but none of them deals with Al2O3 formation on a TiO2(101) anatase substrate surface. As mentioned above, the barrier layer has an impact on the electron injection from a dye to a TiO2 electrode. This is suggested to be due to a longer dye
TiO2 distance and weaker coupling between the dye and the TiO2 electrode.20 Experimentally and theoretically, the binding of a dye on bare TiO2 is identified to take place via one or more carboxylate groups of the dye. The binding of different dyes on TiO2(101) was studied computationally by using the density functional theory (DFT).34
36 Recently, the adsorption properties of the common dye, N3 (Ru(dcbpy)2(NCS)2, dcbpy = 4,40 -dicarboxy-2,20 -bipyridine), were thoroughly computed via DFT.36 Numerous adsorption configurations on a defect-free TiO2 anatase (101) surface, including single, double, and triple bidentate, monodentate, and mixed monodentate/bidentate structures, were analyzed. The results show that the most stable structures are bound to the surface via two carboxylate groups along the [101] vector and the double bidentate bridging configurations are lower in energy than the mixed structures. Also, the effects of solvent/electrolyte on the binding energies were recently investigated.36,37 Here, we employ DFT to study the thermochemistry of two half-reactions in the first ALD cycle for AlOx formation on a hydroxylated TiO2 surface. The adsorption of a variable number of Al(CH3)3 molecules on different adsorption sites is investigated. With the help of differential adsorption energies, we determine the upper limit to Al(CH3)2 and AlCH3 coverages. The possible adsorption geometries and thermochemistry of the first Al(CH3)2 molecule in the second ALD step are also briefly addressed. Finally, we give comparison of the geometries and corresponding energetics of the N3 dye on AlOx/TiO2 and bare TiO2 substrates.

2. COMPUTATIONAL METHODS AND SYSTEMS The DFT calculations were performed with a GPAW code,38
40 which implements a projector-augmented wave method,41 with a grid basis and Perdew
Burke
Ernzerhof (PBE)42 exchange-correlation functional. The periodic boundary conditions were applied in all directions. In the Al(CH3)3 adsorption calculations, the grid parameter was set to 0.2 Å, the sampling of the 2  2  1 k-point was used, and the periodic images of slabs were separated by 13 Å of vacuum. The atoms in the bottom layer of the slab were fixed to their bulk positions, whereas the rest of the atoms in the slab and all the adsorbate atoms were allowed to relax with a convergence criterion of 0.05 eV/Å for the residual forces. The response of the total energy of a bare TiO2 slab to variation of the parameters was checked. The total energy change divided by the total energy was less than

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Figure 1. (a) The unit cell of the bare anatase (101) slab used in the calculations. Oxygen atoms are colored red and titanium atoms white. Different surface atoms are named according to their coordination number. (b) The completely hydroxylated anatase (101) surface. Oxygen atoms from the dissociated water are colored darker red.

0.002 with respect to the k-points and grid spacing, and the density of states did not change notably after three layers. In the calculations, including the dye molecule, N3, we employed a slightly larger grid parameter, 0.25 Å, and the k-point sampling included only the Γ-point. The vacuum between the periodic images was over 19 Å, the two lowest TiO2 layers were fixed, and the convergence criterion was 0.15 eV/Å. The lower accuracy for these calculation was chosen to ease the computational demands, as the number of electronic states was an order of magnitude larger. In TiO2 nanoparticles, the most abundant facet is anatase (101).43 Bulk anatase can be described with two cell parameters, a and c, which were determined to be, with our computational setup, 3.82 and 9.57 Å, respectively. They agree with experimental values 3.78 and 9.50 Å (ref 44) and previously calculated PBE values of 3.786 and 9.737 Å (ref 45). We modeled the surface of the nanoparticle with a three-layer-thick stoichiometric anatase (101) slab, which is also found sufficient in other studies.36,46 It is well known that TiO2 nanoparticles have numerous defects. Although we omitted the undercoordinated sites from the study, the role of oxygen vacancies is expected to be small as they have been predicted to diffuse into a nanoparticle core.47 The chosen unit cell is depicted in Figure 1a and has dimensions of 10.3 Å  7.6 Å  22.5 Å. The (101) surface consists of 5- and 6-fold coordinated titanium atoms and 2- and 3-fold coordinated oxygen atoms (labeled Ti5c, Ti6c, O2c, and O3c). In the N3 dye adsorption calculations, the unit cell was multiplied by 2 in the [101] direction and by 3 in the [010] direction to better accommodate the adsorbate. In computational studies of strongly correlated materials, the description of the on-site Coulomb interaction of localized d and f electrons is sometimes modified by adding a special Hubbard term to the total energy expression,39,48 usually to “correct” the band gap. This method is often termed as GGA þ U (or LDA þ U). It has been shown that, with the PBE xcfunctional, PBE þ U improves, for example, the description of defect states of anatase compared to plain PBE, but for stoichiometric anatase, they have been shown to give identical structural parameters.49 We calculated the total energies of a few systems with the Hubbard correction employing the Hubbard parameter U = 5.5 eV. It was chosen such that the calculations give 3.15 eV for a band gap with PBE lattice constants. Only minor changes in relative energies of the test systems were observed. Therefore, we conclude that the correction does not affect the quantities of our interest, and all the calculations in this work were carried out with the plain PBE exchange-correlation functional. 9251

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Figure 2. One Al(CH3)3 adsorbed on the hydroxylated surface. In (a)
(c), Al(CH3)3 is adsorbed via a single ligand-exchange reaction and, in (d)
(g), via two ligand-exchange reactions. The numbers refer to the bond lengths (in Å) between the aluminum and oxygen atoms.

3. RESULTS AND DISCUSSION 3.1. Hydration of the Anatase (101) Surface. TiO2 electrodes are exposed to water vapor in the air prior to the ALD process. Therefore, the electrodes are hydrated, but the exact coverage of the OH groups is not known.50 Here, we assume that the anatase (101) surface has 1 ML (monolayer) coverage of dissociated water (Figure 1b). This choice gives us an upper limit for the OH coverage. It also affects the results as coverage of OH groups has an influence on Al(CH3)3 adsorption due to different surface sites present on a hydrated surface.51 From earlier computational studies on TiO2 anatase surfaces, it is known that, on a stoichiometric (101) surface, nondissociative H2O adsorption is favored and that the adsorption energy is independent of the coverage, being only 0.02 eV smaller at 1 ML than at 0.25 ML coverage.52 At all coverages, water dissociation is found as an endothermic process, but the energy difference between molecular and dissociated moieties decreases with increasing coverage.52 Different vacancies have been found to play a crucial role in the adsorption of water molecules, and recent DFT calculations predict facile water dissociation on subsurface oxygen vacancies, titanium interstitial defects, and on undercoordinated titanium atoms on step edges.46,53 On TiO2 nanoparticles, various defects are always present, providing active sites for water dissociation from which dissociated fragments are assumed to diffuse to the facets of the nanoparticle. We find the adsorption energy of a single water molecule to be
0.64 eV. At the best adsorption site, an oxygen binds to a Ti5c atom, and the hydrogens point toward O2c atoms. The adsorption energy agrees well with the previously reported values of
0.74 and
0.72 eV for a single molecule and a monolayer, respectively.46,52
54 For the dissociated water, there are, in principle, four possible adsorption sites available for the H and OH radicals. However, the radicals do not compete for the same sites. The OH can adsorb on either a 5- or a 6-fold coordinated titanium atom (Ti5c and Ti6c in Figure 1) and the hydrogen on a 2- or 3-fold coordinated oxygen (O2c and O3c in Figure 1). Energetically, the best adsorption is onto Ti5c and O2c atoms, and it is exothermic by
0.28,
0.40,
0.34, and
0.53 eV for the first, second, third, and fourth dissociated water, respectively. A similar trend in adsorption energies has been verified in ref 52.

Table 1. Binding Energies for Different Adsorption Geometries and the Height of the Adsorbed Aluminum from the TiO2 Surface adsorption (Figure 2)

energy [eV]

Al height [Å]

a


2.63

2.93

b


2.36

2.34

c


2.03

2.49

d


3.68

2.16

e


4.06

2.02

f g


4.08
3.67

1.66 1.69

3.2. The First ALD Cycle. The surface chemistry during AlOx ALD on TiO2 can be described with two surface reactions 



AlðCH3 Þ3 -pulse : TiOH þ AlðCH3 Þ3 f TiOAlðCH3 Þ2 þ CH4

ð2Þ 



H2 O-pulse : AlðCH3 Þ2 þ 2H2 O f AlðOHÞ2 þ 2CH4 ð3Þ where “/” refers to adsorbed species. At the present time, the details of the reaction steps on the TiO2 are not accurately understood, and also, the surface coverage after the first ALD cycle is not clearly determined. In the first trimethyl aluminum pulse, Al(CH3)3 adsorbs dissociatively on the surface via the ligand-exchange reaction, where either one or more methyl groups react with surface OH groups, forming and releasing CH4, and making an O
Al bond. Molecular adsorption of Al(CH3)3 on TiO2 is unlikely as we find adsorption thermoneutral. An adsorption process releasing one methane is called single ligand exchange, and if two methane molecules are formed, the process is called double ligand exchange. The various Al(CH3)x (x = 1,2) adsorption geometries are summarized in Figure 2, and the corresponding energies are given in Table 1. Figure 2a
c shows a single ligand-exchange adsorption, and Figure 2d
g shows a double ligand-exchange adsorption. We employ the following naming convention for adsorption sites, where oxygen atoms involved are identified by supercripts, as shown in Figure 1. The first letter (in the case of single ligand exchange) or the first two letters (double ligand exchange) express from which surface oxygens the hydrogens are removed. 9252

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In single ligand-exchange adsorption, the Al(CH3)2 prefers binding to two oxygen atoms, and in total, four adsorption sites can be found. A site between two Oh atoms is named an h-h site, and it is exothermic by
2.63 eV. Two sites are found between the Oh and O2c atoms because one can remove a hydrogen from either oxygens. According to our naming convention, if a hydrogen is taken from an Oh (O2c) atom, the site is called h-2c(2c-h). Al(CH3)2 adsorption on these sites is exothermic by
2.36 and
2.03 eV, respectively. The fourth site is 2c-2c, but Al(CH3)2 adsorption is endothermic. As mentioned above, Al(CH3)3 can lose two methyl groups, in which case, two surface hydrogens are needed to produce two methane molecules. We find AlCH3 binding to three oxygen atoms and forming tetrahedral structures. There are, in principle, two different adsorption sites available. AlCH3 binds either to two Oh atoms and to one O2c atom or to two O2c atoms and to one Oh atom. In the first case, both hydrogens can be removed both from Oh atoms or one from an Oh and one from an O2c atom; thus, it is possible to identify two sites, namely, h-h-2c and h-2c-h. AlCH3 adsorption on these sites is exothermic with adsorption energies of
3.68 and
4.06 eV, respectively. In the second case, AlCH3 binds to one Oh atom and to two O2c atoms. Now both hydrogens can be removed from O2c atoms (2c-2c-h) or again one from each kind of oxygen (h-2c-2c). The computed adsorption energies are
4.08 and
3.67 eV for 2c-2c-h and h-2c-2c sites, respectively. Looking at the structures of single ligand-exchange adsorption geometries (Figure 2a
c), the average Al
O bond length is found to be 1.88 Å and the bond is, on average, 0.07 Å longer if a hydrogen is attached on an oxygen. For a double ligand-exchange reaction (Figure 2d
g), the average Al
O bond length is 1.84 Å and a hydrogen on an oxygen elongates the bond ∼0.06 Å on average. To further analyze the geometric properties, we define the AlOx thickness, which is calculated by subtracting the average height of Al atoms from the average height of the TiO2 surface, defined by the average height of the Ti5c atoms in the (101) direction. Note that, for one adsorbate, the average height of Al atoms is just a vertical Al
Ti5c distance. In a case of a single ligand-exchange process, vertical distances are 2.93, 2.34, and 2.49 Å for structures given in Figure 2a
c, respectively. For a double ligand-exchange process, the aluminum is closer to the TiO2, and structures from Figure 2d
g have distances of 2.16, 2.02, 1.66, and 1.69 Å, respectively. Table 1 shows that structures given in Figure 2e,f (also Figure 2d,g) are isoenergetic, despite different adsorption sites and vertical distances, indicating that the aluminum oxide coating is amorphous close to the titanium dioxide interface. A similar conclusion was also made in ref 55 where the roughness of ALD grown aluminum oxide on Si(100) was studied with atomic force microscopy. 3.3. Surface Coverage of Al(CH3)2. Our aim is to determine the Al(CH3)2 coverage after the first ALD cycle, which, for one, determines the AlOx coverage. Adsorption geometries given in Figure 2a,b,e were selected as starting points. The hydroxylated anatase (101) surface was attempted to fill in, occupying only the same type of sites at a time. However, each adsorbate induces surface relaxations, and therefore, for the following Al(CH3)3 molecules, there might not be identical adsorption sites available. The differential adsorption energy for the Nth Al(CH3)3 was calculated employing equation ENdiff :ads: ¼ ENsurf : þ x 3 ECH4
ENsurf
: 1
EAlðCH3 Þ3

ð4Þ

Table 2. Differential Adsorption Energies of Al(CH3)3 Molecules Binding via Different Adsorption Configurations Shown in Figure 2a adsorption geometry h-h

1st [eV]
2.63

2nd [eV]
2.38

3rd [eV] †


1.15

4th [eV]
0.20†

2c-h


2.36


2.35


1.50


0.28

h-2c-h


4.06


3.67


3.37†


3.64†

The “†” indicates that the adsorption site is different from the previous ones. a

N
1 where EN surf and Esurf are the total energies of the surface with N and N
1 adsorbed Al(CH3)3 molecules, ECH4 and EAl(CH3)3 are the gas-phase energies, and x is 1 for a single and 2 for a double ligand-exchange reaction. At maximum, four Al(CH3)3 molecules can be adsorbed to our unit cell selected for the present study. If all four sites are occupied, this corresponds to full monolayer coverage. At the h-h geometry, the adsorption energy of the first Al(CH3)2 is
2.63 eV, as shown in Table 2. The dissociative adsorption of the second Al(CH3)3 to the identical site gives
2.38 eV for the adsorption energy. Because of the surface relaxations, there are no longer similar adsorption sites available. Therefore, the third and fourth molecules were attached to Oh and O3c atoms; for structures, see Figure 3a,b. The adsorption energy of the third Al(CH3)2 is
1.15 eV, and for the fourth one, it is
0.20 eV, only marginally exothermic. When using the 2c-h site, the first and second Al(CH3)3 adsorb to identical sites with similar energies of
2.36 and
2.35 eV. In contrast to the previous situation, there are also identical adsorption sites available for the third and fourth Al(CH3)3. The third adsorption energy is
1.50 eV. During the relaxation of the surface, one Al(CH3)2 rotated about 45°, blocking the adsorption of the fourth Al(CH3)2 (Figure 3c). Sterically, it is possible to force the fourth Al(CH3)2 to the remaining 2c-h site (see Figure 3d), but this is exothermic only by
0.28 eV The results indicate that the main reason for the weaker adsorption with increasing coverage is the repulsion between the methyl groups. The methyl group surface densities for single ligand-exchange adsorption are 2.5, 5.1, 7.6, and 10.2 nm
2 for 0.25, 0.5, 0.75, and 1 ML coverages, respectively. A C
Al
C angle, R, is employed as a descriptor to analyze the compression of a Al(CH3)2 as a function of coverage. Table 3 displays the obtained results for two different adsorption geometries. For an isolated Al(CH3)2 on the h-h site, the angle is 119.5°, dropping monotonically to 101.9° at 1 ML coverage. The decrease of the angle indicates that molecules become more compressed at higher surface coverage. We assign the observed decrease of the adsorption energy of the second Al(CH3)2 at the h-h site to be due to the compression of the molecule: The Al(CH3)2 adsorbates are aligned in the [101] direction of the surface, and already with two adsorbates, methyl groups become too close to each other. At the 2c-h site, the orientation of the Al(CH3)2 adsorbates is different and the methyl groups can relax more freely. Therefore, even at 0.75 ML coverage, R angles differ only slightly from those at 0.25 ML coverage, and the repulsion between the methyl groups is seen as the rotation of adsorbates. Forcing the fourth Al(CH3)2 to the 2c-h site, the R angle clearly changes along with a substantial decrease in adsorption energy. The differential adsorption energies of a double ligandexchange reaction are
4.06,
3.67,
3.37, and
3.64 eV for

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Figure 3. Al(CH3)3 molecules adsorbed on the hydroxylated surface. In (a), there are three molecules and, in (b), there are four molecules via a single ligand exchange (h-h). In (c), there are three molecules and, in (d), four molecules via a single ligand-exchange reaction (2c-h). In (e), there are four molecules via a double ligand-exchange reaction (h-2c-h). 9254

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Table 3. Average C
Al
C Angles (r) of the Al(CH3)2 Adsorbates as a Function of the Surface Coverage adsorption geometry

coverage [ML]

R [°]

h-h

0.25

119.5

0.5 0.75

115.1 105.0

Figure 3a

1

101.9

Figure 3b

0.25

2c-h

.

Table 4. Aluminum Oxide Layer Thickness and Aluminum Surface Density after the First Half-Reaction of the ALD Cycle with Different Adsorption Configurations structure

thickness [Å]

Al coverage [nm
2]

Figure 3a

2.68

3.8

Figure 3b

2.56

5.1

Figure 3c

2.43

3.8

102.9

Figure 3d

2.46

5.1

0.5

104.5

Figure 3e

1.81

5.1

0.75

105.0

Figure 3c

98.6

Figure 3d

1

the first, second, third, and fourth AlCH3, respectively. The variation in energies is minor as a function of coverage, indicating a full AlCH3 coverage (Figure 3e). Also, in this case, we observe that the Oh atoms (from OH groups) relax toward each other due to the adsorption of the first and second Al(CH3)3 molecules. Therefore, the third and fourth AlCH3 have to bind to Oh, O2c, and O3c oxygens, but this has a small effect on adsorption energies. The methyl group surface densities are half of those for single ligand-exchange processes; thus, at 1 ML coverage, the density is only 5.1 nm
2 . A nearly coverage-independent adsorption energy indicates that, at a 5.1 nm
2 methyl group density, the repulsion between methyl groups is negligible. The thickness of the aluminum oxide is defined as previously, and it is the average vertical distance of the aluminum atoms from the titanium dioxide surface. The computed distances are shown in Table 4. The single ligand-exchange process provides an approximately 0.75 Å thicker AlOx layer than the double ligand-exchange process, for which the film thickness is ∼1.8 Å. The comparison of calculated and measured values is difficult because the determination of thickness increment for thin AlOx layers is demanding. The increment estimated from HRTEM figures is in the range of 2
3 Å (ref 21). Our calculations support this finding, which indicates that, at the interfacial region, the increment is higher than in the case of thick ALD AlOx films Note that, finally, kinetics determines whether a single or a double exchange process takes place. Even though there is a clear thermodynamical driving force to cover the entire anatase surface with adsorbates, it might be hindered by high reaction barriers for adsorption and ligand-exchange reactions. Also, in reality, probably both processes take place, and thus, Al(CH3)2 and AlCH3 adsorb simultaneously on the surface.12,23 For simplicity, in this study, we focus on one adsorbate type at a time. To model the second half of the ALD cycle (H2O-pulse), all the methyl groups were replaced with OH groups (eq 3) from the structure in Figure 3a. This is exothermic by
1.85 eV/ligand. The Al
O bonds shorten slightly, but otherwise, no structural changes or large relaxations are seen. 3.4. The Second ALD Cycle: Initial Stage. Next, the adsorption of the first Al(CH3)3 of the second ALD cycle is briefly addressed. From the calculations, it is clear that the TiO2 surface is not fully covered after the first ALD cycle. Interesting questions arising are how smooth is the interface after two or more cycles and what is the coverage. To perform a detailed study for the entire second ALD cycle is not possible due to substantial computational effort owing to various adsorption geometries. Therefore, we focus on the structure in Figure 3a after the methyl groups have been replaced with OH groups, shown in Figure 4.

The surface coverage for this structure is 0.75 ML after the first cycle, and there is clearly one site available on the bare TiO2 surface. Our aim is to explore the possibilities to cover the parts of the TiO2 surface left exposed after the first cycle. Four different adsorption geometries were studied for a single ligand-exchange reaction. The adsorption geometries are shown in Figure 4. In the first two cases (Figure 4b,c), the Al(CH3)2 binds to the bare TiO2, whereas in the other two structures (Figure 4d,e), Al(CH3)2 is on the aluminum oxide layer. The structures on the bare TiO2 surface differ slightly due to a different adsorption geometry. In Figure 4b, the Al(CH3)2 occupies a similar site as the third Al(CH3)2 of the first ALD cycle; that is, it binds to O3c and Oh atoms. In the second structure (Figure 4c), the Al(CH3)2 is attached to an Oh atom and to the hydroxyl group on the aluminum atom. The adsorption geometries on the aluminum oxide are rather similar, as can be seen from Figure 4d,e. In contrast to all previous geometries, in these two cases, Al(CH3)2 binds to only one oxygen atom. This also indicates that the thickness increment is likely to change during the first ALD cycles. Surprisingly, the adsorption energies of these four different geometries are virtually identical, being roughly
1.6 to
1.7 eV. We note that all the energies are considerably more exothermic than those of the fourth (or even third) Al(CH3)2 following a single ligand-exchange reaction during the first ALD cycle, which is due to decreased steric repulsion. We conclude that, from the energetic point of view, it is possible that at least some of the holes in the aluminum oxide layer after the first ALD cycle can be filled during the subsequent cycles, but a more sophisticated prediction would require the knowledge of the reaction barriers. 3.5. N3 Adsorption. Recently, the adsorption of the N3 dye on a clean defect-free TiO2 anatase (101) surface has been thoroughly studied by means of DFT calculations.34
36 Various coordination types and orientations on the most reactive adsorption site, Ti5c, were calculated together with their relative energies.36 The N3 binds via carboxylate groups, and the comparison of relative energies shows that single-bound dyes are less stable than double- and triple-bound dyes. This supports the spectroscopic measurements, which give evidence of two groups participating in the dye anchoring on the surface. However, one should keep in mind that several factors can affect the binding of the dye, including solvent, defects, surface protons, and surface hydroxyl groups. Usually, from each carboxylate group taking part in the adsorption, one hydrogen is removed. In ref 36, it was found that placing these hydrogens on the surface affects notably the relative adsorption energies. In our case, so many factors affect already the structure of the coated TiO2 surface that we simply remove the hydrogens to gas phase. Therefore, the binding energy 9255

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Figure 4. Adsorption of the first Al(CH3)3 in the second ALD cycle. (a) The surface (Figure 3a) after the water dose. (b) Al(CH3)2 adsorbed in the cavity as the other molecules in the first cycle. (c) Al(CH3)2 in the cavity, but adsorbed on aluminum rather than titanium. (d, e) Al(CH3)2 adsorbed on the surface of the aluminum oxide layer from the first ALD cycle. The structures (a)
(c) are drawn from two different directions.

for the dye anchoring via N carboxylate groups is defined as N EN3 ads ¼ Esurf þdye þ EH2
EN3
Esurf 2

ð5Þ

where Esurfþdye and Esurf are the total energies of the adsorption structure and the surface before adsorption and EH2 and EN3 are

the gas-phase energies. The binding configurations are categorized according to the coordination of the carboxylate groups to the oxide, including monodentate, bidentate bridging, and bidentate chelating, where the first two are found for the N3 on the TiO2 surface.36 The N3 distance from the TiO2 is computed as follows. The height of the TiO2 surface is the 9256

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Figure 5. Four different N3 adsorption geometries for different surfaces. Configuration (a) is the best configuration for the dye in a clean anatase (101) surface found in ref 36. In (b) and (c), the surface is OH-deficient. In (b), the dye is connected to Ti atoms, in (c), to Al atoms. In (d), the surface is OHrich and the dye is connected to the Al atoms.

average height of the Ti5c atoms from which the average height of the oxygens in the carboxylate groups forming bonds with titanium atoms is subtracted. The best configuration found in previous studies is shown in Figure 5a. The N3 prefers to bind via two carboxylate groups so that both oxygens from both groups form a bond with different Ti5c atoms on the surface. This configuration is called double bidentate bridging.36The adsorption energy of this configuration is
3.46 eV and the dye
TiO2 distance is 2.0 Å. We employed two surface models to describe the TiO2 surface after the first ALD cycle: OH-deficient and OH-rich. The structure shown in Figure 4a is selected as a starting point. In experiments, the coated TiO2 films are heat-treated to remove excess water from the surface before sensitization. This is simulated by removing one of the two OH groups from each aluminum atom and an equal number of hydrogen atoms from the surface to produce water. The reason for removing only one OH group is that removing the second OH group is energetically more expensive. Removing the first three water molecules requires energy of 0.89 eV per molecule on average. For the second two water molecules, the average energy is already 1.55 eV per molecule. This surface is called OH-deficient, whereas the coated surface, including all OH groups, is called OH-rich. Next, the binding of the dye on both model surfaces is investigated. The main point is to find out if the N3 can bind directly to the Ti atoms after the first ALD cycle and compare the N3 adsorption structures on OH-deficient and OH-rich surfaces to shed light on the impact of OH groups. The starting point and the reference system is the best N3 configuration on clean TiO2; see Figure 5a. This adsorption geometry is not possible after the first cycle as 75% of the Ti5c sites are occupied by O
Al
(OH)2 moieties; therefore, only less-reactive Ti6c atoms are available. Even at 0.5 ML Al(CH3)2 coverage, only the Ti6c atom are available. The only possible configuration is along the [010] surface vector, as shown in

Figure 5b. Again, the dye attaches to the surface via two carboxylate groups. The configuration is a mixed monodentate/ bidentate geometry as one carboxylate group binds to both the titanium atom and aluminum atoms, whereas the other carboxylate group attaches to one titanium via one oxygen. The dye
TiO2 distance is only 1.2 Å, which is 0.8 Å smaller than on the ideal TiO2, and it is due to the fact that only Ti6c are available for anchoring the dye. The binding configuration induces notable relaxations to the aluminum atoms in the vicinity of the dye and renders the binding endothermic by 3.15 eV. Figure 5c presents the N3 binding geometry over the barrier coating layer on the OH-deficient surface. The binding configuration is double monodentate as two carboxylate groups bind to two aluminum atoms from one oxygen each, and the configuration is also along the [010] surface vector. This binding is exothermic by
1.56 eV, and it is also reasonable to assume that the attaching on the barrier coating layer is a nonactivated process. Note that one can probably find even better adsorption configurations than the one given in Figure 5c, but this was not further explored. The dye
TiO2 distance is 3.7 Å, which is 1.7 Å longer than on the clean TiO2 surface. Finally, the N3 binding on the OH-rich surface was addressed. The high number of OH groups inhibits the N3 from attaching directly on the Ti6c atoms; thus, only anchoring over the barrier coating layer was calculated. We selected the same double monodentate configuration as on the OH-deficient surface. The adsorption is exothermic by
1.65 eV, practically identical compared to the adsorption on the OH-deficient surface. The comparison the dye
TiO2 distances indicate that the higher number of the OH groups increases the dye
TiO2 distance, which is 4.2 Å on the OH-rich surface. 3.6. Discussion. The adsorption of Al(CH3)3 molecules via single and double ligand exchange-reactions was studied at various coverages. Calculations show that the molecular adsorption of Al(CH3)3 on the TiO2(101) is highly unlikely, and the 9257

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The Journal of Physical Chemistry C previous calculations indicate that CH4 adsorption is thermoneutral.56 Thus, upon adsorption, Al(CH3)3 dissociates and Al(CH3)2 adsorbs on the TiO2 surface and CH4 desorbs to gas phase. At 0.25 ML, several adsorption sites and geometries were explored, and the energetically most favorable ones were selected for further studies. Our aim was to determine the maximum surface coverage of Al(CH3)2 by filling the surfaces using one adsorption site at a time. This was not always possible as, in some cases, relaxations of the surface atoms introduced by a previously adsorbed Al(CH3)2 molecule forced the next Al(CH3)3 molecules to different adsorption sites. Adsorption energies indicate that, in the first ALD cycle, the discontinuous AlOx layer is formed. For a single ligand-exchange process, the differential adsorption energies of the first two precursor molecules are about the same; however, the adsorption energies of the third and fourth Al(CH3)2 are one-half and nearly zero, respectively. Compared with previous DFT calculations on single Al(CH3)2 and AlCH3 adsorption on a Al2O3 model surface, the adsorption energies on OH
TiO2(101) are 0.73 and 0.38 eV more exothermic, respectively. In the case where a double ligand-exchange reaction mechanism is employed, the adsorption energy is nearly coverage-independent. We assign this to the lack of repulsion between the methyl groups present in single ligand-exchange adsorption. When the methyl group surface density reaches a certain limit, the repulsion appears and it is seen as a compression of a molecule, especially in the variation of an R angle, and decrease in adsorption energy. We estimate that the critical density is somewhere between 5.1 and 7.6 nm
2 . This is in good agreement with a maximum methyl group surface density of 7.2 nm
2, calculated using a van der Waals radius of 0.20 nm (ref 18). Despite that the OH coverage has an impact on available adsorption sites for Al(CH3)2, we note that it has at most minor effect on repulsion between methyl groups. Adsorbate distribution, that is, the overlayer structure, on the surface is more important, in particular, for a single ligandexchange process. DFT calculations provide evidence for the submonolayer coverage of AlOx on the TiO2 surface after the first ALD cycle; thus, the AlOx layer is discontinuous. The coverage is higher than the value of 0.25 ML obtained from XPS measurements applying a simple geometrical model57 and the value of 0.3
0.4 ML estimated with the phenomenological model.58 The high coverage indicated by DFT is supported by TOF-ERD measurements, which give a similar areal Al density as calculations.21 Also, the experimental observation that forward electron injection significantly slows down already after the first ALD cycle20 supports a high submonolayer AlOx coverage. The exact structure of the AlOx
TiO2 interface is not known. Our calculations provide evidence that it might be quite amorphous, because different adsorption geometries are energetically close to each other, pointing toward a flexible interface geometry. The adsorption properties of the first Al(CH3)2 of the second ALD cycle indicate that an AlOx layer is rough at the atomic level as adsorption on the bare part of TiO2 is isoenergetic with the adsorption on the barrier coating layer. The N3 adsorption on the anatase (101) has been extensively studied computationally.36 In the best configuration, the N3 is coordinating with double bidentate bridging to the surface. The presence of submonolayer AlOx introduces new adsorption sites for N3 but simultaneously removes sites present on the clean TiO2 surface. Our results indicate that it is highly unlikely that the N3 dye adsorbs at AlOx-free areas on the surface at 0.75 ML

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coverage as adsorption is highly endothermic due to the less reactive Ti6c sites and the surrounding AlOx coating. Even lower, 0.5 ML, AlOx coverage does not improve the situation as in the best configuration where Al(CH3)2 molecules occupy the surface unit cell in a zigzag fashion, blocking still all Ti5c sites from N3. The discontinuity and atomic level roughness of the AlOx layer may introduce variations in dye
TiO2 distances. Calculations show that, after the first ALD cycle, the average dye
TiO2 distance is 2 Å longer compared with the distances on the clean TiO2 surface. The increased distance leads to weaker electronic coupling between the dye and TiO2 and higher tunneling barrier. In addition, the dye binds to the AlOx coating via only two oxygens, whereas four oxygens take part in binding on the clean anatase (101) TiO2 surface. All these factors work in the same direction, and therefore, already one ALD cycle suppresses electron injection from the N3 dye, which is also seen in transient absorption measurements.21

4. CONCLUSIONS The first steps of ALD growth characteristics of Al2O3 deposited from Al(CH3)3 and H2O on TiO2 anatase (101) and the binding of the N3 molecule on the barrier layer coated TiO2 were studied by means of DFT calculations. Adsorption energies of Al(CH3)2 molecules give evidence for both submonolayer and full monolayer coverage depending on whether a single or double ligand-exchange process takes place. Calculations also show that the AlOx
TiO2 interface displays atomic level roughness owing to steric repulsion between methyl groups and the fact that the first Al(CH3)3 of the second ALD cycle adsorbs equally well on the bare TiO2 surface than on the AlOx layer. The calculations indicate that, already at the submonolayer AlOx coverage, the adsorption geometry of the N3 dye differs significantly from that on the clean TiO2 surface, as the most active Ti5c sites are blocked by the AlOx layer. On the basis of calculations, we conclude that even one ALD cycle is enough to inhibit direct N3 binding to TiO2. This leads to the increase in a N3
TiO2 distance by 2 Å and, therefore, less efficient forward electron injection from the dye to the TiO2 conduction band. Thus, the overall effect of the aluminum oxide coating for improving the conversion efficiency is minor. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: karoliina.honkala@jyu.fi.

’ ACKNOWLEDGMENT This work was financially supported by the Academy of Finland through project 118532 and through the FINNANO program. The authors thank Liisa Antila for fruitful discussions. The computational resources were provided by the Nanoscience Center University of Jyv€askyl€a and the Finnish IT Center for Science (CSC) Espoo. ’ REFERENCES (1) O’Regan, B. C.; Gr€atzel, M. Nature 1991, 353, 737–740. (2) Gr€atzel, M. J. Photochem. Photobiol., C 2003, 4, 145–153. (3) Ardo, S.; Meyer, J. Chem. Soc. Rev. 2009, 38, 115–164. 9258

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