ARTICLE pubs.acs.org/JPCC
DFT Investigations of Formic Acid Adsorption on Single-Wall TiO2 Nanotubes: Effect of the Surface Curvature Francesca Nunzi*,†,‡ and Filippo De Angelis*,‡ †
Dipartimento di Chimica and ‡Istituto CNR di Scienze e Tecnologie Molecolari (ISTM-CNR), c/o Dipartimento di Chimica, Universita degli Studi di Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy
bS Supporting Information ABSTRACT: We carried out a theoretical study based on DFT calculations to provide a detailed characterization of the structural, electronic, and adsorption properties of single-walled TiO2 anatase nanotubes. We investigated nanotube models of increasing diameter, formally obtained by rolling a TiO2 anatase monolayer around the [101] and [010] directions, giving rise to (n,0) and (0,m) nanotubes, respectively. We considered finite cluster models for both (n,0) and (m,0) TiO2 nanotubes, with diameters ranging from 5 to 30 Å, thus approaching realistic nanotube dimensions. Our results show that (n,0) tubes are lower in energy with respect to (0,m) tubes. For (n,0) tubes with diameters greater than 23 Å, the electronic energy and the band gap are almost converged with respect to the diameter length. We then investigated the adsorption of formic acid on the TiO2 nanotube sidewalls, as the simplest model of photosensitizers binding to the TiO2 surface, relevant to dye-sensitized solar cells. Adsorption of formic acid was investigated on (12,0) and (0,4) TiO2 nanotubes, optimizing two monodentate modes and one bidentate adsorption mode, and comparing the results to those obtained for a planar TiO2 surface. We find that while for a planar surface a bridged bidentate configuration is the more stable, the effect of the curvature in TiO2 nanotubes leads a monodentate configuration to be the more stable structure. These results are interpreted in terms of the peculiar electronic properties of TiO2 nanotubes and their implications for use of nanotubes in dye-sensitized solar cells are discussed.
’ INTRODUCTION Nanocrystalline TiO2 is an interesting material because of its unique optical and electrical properties, which are suitable for solar energy conversion applications, such as photocatalysts, photochromics, and photovoltaics.1 In particular, dye-sensitized solar cells (DSSCs)2,3 for the conversion of sunlight into electricity have attracted increasing interest because of their low cost and high efficiency. DSSCs typically consist of a nanocrystalline TiO2 film covered by a monolayer of dye molecule (photosensitizer), acting as the photoanode. Several ruthenium complexes containing anchoring groups such as carboxylic acid, dihydroxy, and phosphonic acid on pyridine ligands have been used as dyes.3-6 Upon photoexcitation, the sensitizer injects an electron in the conduction band of the oxide and is regenerated by hole injection into an electrolyte or hole transporting material. The injected electrons flow through the semiconductor network to the front and back contacts of the cell, where they are collected as electric current. The nanocrystalline morphology of the oxide semiconductor film is essential for the highpower conversion efficiency of DSSCs.7 A whole range of nanostructures have been tested thus far, ranging from simple assemblies of nanoparticles to one-dimensional (1D) nanostructures (nanorods, nanowires, and nanotubes).8-15 These studies are motivated by the expectation that the transport of charge carriers along 1D-nanostructures could be more efficient than within a 3D random network r 2010 American Chemical Society
of nanoparticles, where the electrons have to cross many particle boundaries. Therefore one-dimensional nanostructured materials should improve the collection of the photogenerated charge carriers. Three general approaches16,17 have been developed in the last 15 years for the synthesis of nanostructured TiO2 materials, consisting of chemical template,18 electrochemical,19 and alkaline hydrothermal synthesis.20-22 Among these methods, the alkaline hydrothermal synthesis initially proposed by Kasuga and coworkers20-22 has revealed a simple and effective method to prepare thin-wall nanotubes without the requirement of templates. This pioneering method, consisting in the treatment of the metal oxide precursor with a concentrated solution of NaOH followed by acid washing with HCl, allows one to obtain nanotubes with finite lengths of ∼100 nm with inner and outer diameters of ∼5 and 8 nm, respectively. The main variables in this soft-chemistry process are the reaction temperature, the caustic concentration and the type of TiO2 raw-material. All polymorphs of TiO2 (anatase, rutile, brookite, or amorphous forms) have been transformed into TiO2 nanotubular or nanofibrous forms.23-27 Four different morphologies of nanostructures Received: October 22, 2010 Revised: December 3, 2010 Published: December 21, 2010 2179
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The Journal of Physical Chemistry C have been experimentally observed, i.e., nanotubes, nanosheets, nanorods, or nanowires and nanofibers, nanobelts, or nanoribbons.16,28 Nanotubes can be produced by folding nanosheets, that are usually observed in the early stage of preparation or as a small impurity in the final product. Nanotubes are always multilayered, the number of layers varying from two to ten,16 while nanorods or nanowires are long, solid cylinders with a circular base-with nanowires being longer than nanorods. Finally, the long, solid parallelepiped titanates are labeled in the literature as nanofibers, nanobelts, or nanoribbons.28 Due to various experimental impediments, the detailed crystal structure of nanostructured titanate materials is still unclear. While in the early study of Kasuga20 the needle-shaped titanium oxide was characterized as anatase, presently it is acclaimed that the synthesized structures are more and more sophisticated. Different crystal structures and compositions have been proposed besides tetragonal TiO2 anatase with the general formula A2TinO2nþ1, where A = Naþ or Hþ and n = 2-4.29 It is generally believed that the nanotube formation involves breaking of the Ti-O-Ti bonds in the starting 3D TiO2 structure, that rearranges into single- or multiwall nanosheets (2D structure), which subsequently scroll or wrap into nanotubes (1D structure), the driving force being essentially the saturation of the undercoordinated sites or dangling bonds.16,29 Compared with the large amount of experimental studies on the preparation and applications of titania nanostructures, theoretical investigations on the properties of TiO2 nanotubes have been carried out consistently only in the past few years30-44 and, in particular, only one study has so far investigated the functionalization of the nanotube sidewalls with hydrogen.45 The structure and stability of titanate nanotubes H2TinO2nþ1 (n = 1-4) have been investigated by several research groups by means of Density Functional Theory (DFT)-based methods,30-35 while Hart et al. applied atomistic simulation techniques to compare the stability of different nanotube structures (tritanate, lepidocrocite, and anatase).36 The formation mechanism and structural and electronic properties of TiO2 nanotubes formed from rutile have also been investigated with plane wave DFT methods.37,38 A different structure with TiO2 nanotubes constituted by hexagonal ABC PtO2 pattern were also investigated with DFT calculations, together with the effect of B and N doping on the electronic properties.39 Starting from the early theoretical study of Ivanovskaya et al. at the semiempirical level,40 TiO2 nanotubes formed by anatase sheets were recently considered by means of DFT calculations.30,35,41-45 In particular, assuming a simplified hexagonal lattice model in analogy with carbon nanotubes, Seifert and Enyashin30 concluded that anatase nanotubes are more stable than nanorolls and nanostrips and are semiconductors with a wide direct band gap, while Wang et al.41 found that TiO2 nanotubes are semiconductors with indirect band gaps, regardless of the tube size and chirality. More realistic models were considered in References 44-48 where, assuming a centered rectangular 2D unit cell for the (101) sheet of the bulk anatase, TiO2 nanotubes were formed by rolling the anatase (101) sheet along either the [101] and [010] directions, giving rise to (n,0) and (0,m) tubes, respectively. TiO2 nanotubes were found semiconductors with a wide band gap, with the nature of the gap depending on the chirality of the tube, i.e., direct and indirect gap respectively for (n,0) and (0,m) tubes, and increasing with the tube diameter.42,43 A more extended investigation on the electronic and optical properties of anatase TiO2 nanotubes revealed that the band gap can also be affected by other parameters, such as the wall thickness and the atomic arrangement.44 Recently, nanotubes constituted by anatase (001) layers were computed more stable
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than the corresponding flat slabs with periodic DFT calculations.35 More interestingly, a DFT study45 on the molecular and atomic adsorption of hydrogen on TiO2 nanotubes sidewalls investigated the dependence of the electronic properties on the hydrogen concentration in view of an understanding of the nature of the TiO2 nanotube-based hydrogen gas sensing. In this work, we report a fully first principles quantum mechanical investigation based on DFT on the adsorption of the simplest species containing a carboxylic group, i.e., formic acid (HCOOH), on TiO2 sidewalls nanotubes with the aim to model the interaction of the photosensitizers anchored onto the oxide semiconductor surface in DSSCs. Since anatase (101) is one of the most frequently exposed surface in TiO2 nanoparticles for solar cells,7 we considered TiO2 nanotubes constituted by anatase (101). The structural, energetic, and electronic properties of anatase TiO2 nanotubes have been initially investigated, followed by inspection of the binding modes of formic acid on the tube sidewalls and by analyzing the nature of the adsorbate/substrate interaction compared to planar TiO2 structures. We find formic acid to adsorb in different adsorption modes on planar and curved TiO2 nanostructures, i.e., dissociative bidentate or nondissociative monodentate, respectively, reflecting the competition between the basicity of surface oxygens and hydrogen bonding on the surface. This work is expected to promote the realization and applications of these novel structures in nanoscience and nanotechnology.
’ COMPUTATIONAL DETAILS All of the DFT calculations reported in this work have been performed by the Amsterdam density functional (ADF) program package.46 Full geometry optimizations were carried out using the local density approximation of Vosko, Wilk, and Nusair (LDA VWN),47 augmented with the gradient corrections of Becke48 and Perdew49 for exchange and correlation, respectively. The molecular orbitals were expanded in an uncontracted double-ξ (DZ) Slatertype orbital (STO) basis set for all atoms. The frozen core is constituted by 1s-2p for Ti, 1s for O and C. Solvation effects were modeled by the “Conductor-like Screening Model” (COSMO)50,51 of solvation, as implemented in the ADF code,46 using the structures optimized in vacuo. We considered finite cluster models for both (n,0) and (0,m) TiO2 nanotubes, with diameter ranging from 5 Å up to 30 Å, thus approaching a realistic nanotube dimension, while we considered a length of 13 Å along the axis direction, checking the reliability of this length size by doubling it and comparing the energy stability per TiO2 unit. The employed cluster models show Cm and Cnv symmetries, respectively, for (n,0) and (0,m) tubes. We considered the attachment of two HCOOH molecules on each nanotube fragment with the aim to preserve a higher symmetry, thus reducing the computational effort. In order to evaluate the effect of the curvature on the adsorption of formic acid, we also optimized at the same level of theory, a planar sheet of anatase (101) surface constituted by 82 TiO2 units and the main adsorption configurations of HCOOH molecule on this model.52 The adsorption energy of the formic acid on the TiO2 surface is defined as the energy difference between the reactants (formic acid in the trans isomeric form and TiO2 fragment) and the complex product (formic acid adsorbed on TiO2 fragment). As a preliminary test, we performed DFT calculations on the HCOOH molecule with different basis sets (DZ, DZP, and TZP). It is well-known that in the gas phase, the isolated HCOOH molecule adopts a trans rather than cis conformation with respect to the H 2180
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Figure 1. Centered rectangular 2D unit cell for anatase (101) TiO2, top view (top of the panel) and side view (bottom of the panel). Ti and O atoms are in gray and red, respectively, in this and the following.
atoms. The trans conformer can dimerize through the formation of two intermolecular hydrogen bonds. The optimized geometries for these three species have been reported in the Supporting Information (SI) section (see Figure S1), together with the computed value for the bond distances, bond angles and dimerization energy (see Table S1 of the SI). Our results show that the geometrical parameters for both the isolated HCOOH molecule and the dimeric form are almost insensitive to the quality of the basis set. Similarly, the stabilization energy due to the formation of two hydrogen bonds in the dimeric (HCOOH)2 form is equal to 0.85, 0.83, and 0.74 eV employing the DZ, DZP, TZP basis set, respectively, to be compared with the experimental value of 0.66 eV.53 Thus the dimerization energy is reproduced to a good degree of accuracy also with the smaller DZ basis set, with our results being in excellent agreement with the data by Vittadini et al.54 (0.84 eV).
’ RESULTS AND DISCUSSION Geometric and Electronic Properties of Anatase (101) Nanotubes. Cylindrical surfaces of nanotubes are constructed
by rolling up a two-periodic lattice defined by translation vectors a1 and a2 and the angle between them γ. Among the five 2D Bravais lattices, a monolayer (or slab) of anatase (101) possesses the cen6 |a2|, γ = 90° (see Figure 1), tered rectangular 2D lattice, i.e., |a1| ¼ for which, unlike the well-known carbon nanotube structures,55 only (n,0) and (0,m) nanotubes can be constructed, due to symmetry restrictions.56 Accordingly, we constructed (n,0) and (0,m) TiO2 nanotubes by rolling up an anatase (101) sheet along the [101] and [010] directions, respectively, see Figure 1 and 2. In our preliminary studies, we assumed simplified models for the experimentally observed anatase TiO2 nanotubes by rolling up only one slab of the (101) surface, while the possibility to model TiO2 nanotubes with multiple slab thicknesses is currently under investigation. As shown in Figure 1, the anatase (101) surface is characterized by the presence of acidic-basic pairs of coordinative unsaturated ions, i.e., 5-fold coordinated Ti4þ (Ti5c) and 2-fold coordinated bridging oxygen O2- (O2c) ions, besides coordinated ions, i.e., 6-fold Ti4þ
Figure 2. (n,0) (top of the panel) and (0,m) (bottom of the panel) TiO2 nanotubes structures obtained by rolling up an anatase (101) sheet along the [101] and [010] directions, respectively. Side view and top view are shown for clarity on the left and right side of the panel, respectively.
(Ti6c) and 3-fold O2- (O3c,) ions. Due to the employed nanotube models, the coordinated ions pointing inside the surface in the anatase planar sheet (Ti6c and O3c in Figure 1) become undercoordinated 5-fold Ti4þ and 2-fold O2- ions in the nanotube models, labeled as Ti6c0 and O3c0 , respectively, in Figure 2, so that the employed TiO2 nanotube structures are lacking coordinated Ti6c ions. The optimized geometries of (n,0) and (0,m) TiO2 nanotubes show that the curvature strain leaves the essential geometrical features of the (101) anatase sheet almost unaltered, imposing 2181
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Figure 3. Total energy of TiO2 nanotubes vs tube diameter. The computed data are reported in detail in the SI.
only a slight rearrangement of the Ti-O bond distances and Ti-O-Ti angles, which gradually decreases going toward nanotubes with larger diameters, i.e., approaching a planar structure. In particular, on the largest computed (n,0) tube, i.e., (18,0), we found Ti5c-O2c bond distances of 1.84 ÷ 1.86 Å and Ti5c-O3c bond distances of 1.93 ÷ 2.11 Å, that differ only within 0.1 Å with respect to the correspondent bond distances computed for the planar cluster models (see SI for further details). We also compared the Ti-O-Ti angles, finding differences within 5° between the (18,0) tube and the planar surface. An analogous rational attains for the computed bond distances and angles on the largest computed (0,m) tube, i.e., (0,8), with differences within 0.1 Å and 4°, respectively, with respect to the planar structure (see Figure S2 in SI). In agreement with previous investigations,30,41-45 our calculations show that the nanotubes' total energy per TiO2 unit depends on both the tube diameter and the rolling direction. As shown in Figure 3, we found that the nanotubes' total energy per TiO2 unit is inversely proportional to the tube diameter, which can be considered to be converged with respect to the tube diameter for the largest computed tube (i.e., ca. 30 Å). In particular, for large tube diameters (∼10-30 Å), the changes on total energy per TiO2 unit values are very small (
