Rutile Surface Properties Beyond the Single Crystal Approach: New

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Rutile Surface Properties Beyond the Single Crystal Approach: New Insights from the Experimental Investigation of Different Polycrystalline Samples and Periodic DFT Calculations Lorenzo Mino, Giuseppe Spoto, Silvia Bordiga, and Adriano Zecchina J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp401916q • Publication Date (Web): 18 Apr 2013 Downloaded from http://pubs.acs.org on April 19, 2013

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The Journal of Physical Chemistry

Rutile Surface Properties Beyond the Single Crystal Approach: New Insights from the Experimental Investigation of Different Polycrystalline Samples and Periodic DFT Calculations Lorenzo Mino*, Giuseppe Spoto, Silvia Bordiga, Adriano Zecchina Department of Chemistry, NIS Centre of Excellence, and INSTM Reference Center, University of Turin, via P. Giuria 7, 10125 Torino, Italy *[email protected] Abstract The relations between particles morphology and surface properties for different kind of rutile polycrystalline samples have been deeply investigated combining FTIR spectroscopy of CO adsorbed at 60 K through the carbon end on surface Ti4+ centers, electron microscopy and periodic DFT calculations. We provide a complete assignment of the FTIR spectra of CO adsorbed on micro and nano-rutile crystals with well defined morphology, on nano-rutile from Aldrich and on the P25 rutile component. From these results the spectrum of CO on native P251 is revisited. Of special interest is the fact that the (110) rutile main surface undergoes a thermally induced relaxation process leading to the shielding of exposed Ti4+ sites and consequently to the reduction of the polarizing power. This process can be reversed by inducing an outwards relaxation of the shielded Ti atoms by treating the sample in water at 573 K. A red-shifted band ascribed to CO adsorbed through the oxygen end on the low polarizing sites of relaxed surfaces is providing the signature of surface relaxation. CO species interacting through the oxygen end were already studied for CO on zeolites exchanged with low polarizing alkaline cations, but not yet properly discussed for CO on metal oxides. Keywords: FTIR of adsorbed CO, nanoparticles morphology, electron microscopy, rutile surfaces, ab initio modeling.

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Introduction Since the demonstration in 1972 by Fujishima and Honda2 of the photoelectrolysis of water by a TiO2 electrode under an anodic bias potential, strong efforts have been devoted to the investigation of the two main TiO2 polymorphs, anatase and rutile. As in photocatalytic applications the relevant phenomena are occurring at the oxide surface, many papers are focused on the structure, the relaxations and the reconstructions of the most common TiO2 surfaces. Although anatase is generally reported to be more active than rutile, the great majority of the singlecrystal studies on TiO2 surfaces deal with rutile, which is the thermodynamically stable phase for macroscopic crystals.3 In particular the rutille (110) surface has been mainly investigated, both theoretically and experimentally.4 On the contrary, most of the investigations performed on finely divided samples are devoted to anatase, which is the most stable polymorph in nanocrystalline form3 and indeed is the most used TiO2 phase in technological applications. The aim of this work is to contribute to fill the gap between the single-crystal and the powder and microcrystalline worlds by comparing the surface properties of a variety of polycrystalline rutile samples, ranging from well shaped micrometric rutile particles (assumed as models for mimicking single-crystal world) down to highly dispersed systems containing particles with mean size in the 10-50 nm interval. Concerning the experimental approach, Fourier transform infrared (FTIR) spectroscopy of adsorbed probe molecules, which is widely recognized as the leading method to investigate the surface properties,5-7 has been utilized. In particular carbon monoxide, a weak Lewis base, has been chosen because it is an efficient probe of the Lewis and Brønsted surface acidity.1,5,8-12

The ν(CO) stretching frequency of the molecule adsorbed through the carbon end is in fact related to the electrophilic character and the polarizing power of the surface Lewis acid sites. More in detail, the higher is the electrophilicity of the adsorbing cations, the larger is the blue-shift of the


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ν(CO) with respect to the gas phase value (2143 cm−1). Actually the shift in the adsorbed state originates from the combination of different effects: (i) the interaction between the CO dipole moment and the surface electric field (Stark effect);13 (ii) the repulsive potential due to the vibration of the CO molecule against a rigid surface (wall effect);14 (iii)

the dipole-dipole interactions between the adsorbed molecules (lateral interactions in the adsorbed monolayer).1

Effects (i) and (ii) give valuable information about the local structure of the adsorbing metal centres, effect (iii) on their two-dimensional arrangement. As both the structure and the spatial organization of the surface sites are specific properties of each crystal face, it is evident that the IR spectra of adsorbed CO in turn contain information about the faces distribution and, ultimately, on the morphology of the micro/nano crystals. The results obtained by spectroscopy of adsorbed CO are compared here with the parallel and more direct morphological information obtained by High Resolution Electron Microscopy (HRTEM) and Field Emission Scanning Electron Microscopy (FESEM). The spectroscopic results can be also directly compared with the vibrational frequencies obtained by first-principles calculations for CO adsorbed on different faces,1,11,12,15 thus validating the conclusions based on the experimental spectra and on the HRTEM and FESEM images. The reliability of this approach has been carefully proven on anatase surfaces as reported in our recent publication.1 It has also been shown for zeolites exchanged with low polarizing alkaline cations16-18 that the CO species adsorbed through the oxygen end (associated with an anomalous red-shift with respect to the gas phase) on low polarizing sites can become competitive from the energetic point of view and be simultaneously present in the adsorbed CO phase. This capability to selectively reveal also the presence of very low polarizing sites increases the utility and versatility of CO as surface probe.



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Finally, in this contribution we carefully investigate the FTIR spectra of CO adsorbed on the rutile component (about 15%) usually present in TiO2 P25 samples in order to more precisely assign its spectral features, thus better clarifying the still open issues concerning the contribution of this phase to the IR spectrum of CO adsorbed on native P25 presented in our previous paper.1

Experimental and computational details Computational parameters. Rutile surfaces and CO adsorption properties were modeled by employing the periodic CRYSTAL09 code,19 using the hybrid PBE020 DFT functional, which has been reported to give accurate structural and electronic properties for TiO2 crystals.21 The level of accuracy in evaluating the Coulomb and the exchange series is controlled by five parameters19 for which 10-7, 10-7, 10-7, 10-7, 10-18 values have been used for all calculations. The convergence threshold for SCF energy was set to 10-10 Ha. The reciprocal space was sampled according to a regular sublattice determined by the shrinking factor19 which was set to 6, resulting in 10 to 16 independent k-points in the irreducible part of the Brillouin zone depending on the considered surface. The surfaces of crystals were modeled with two-dimensional slabs of infinite dimensions along the x and y directions and a finite thickness. To determine the relative stability of the rutile faces an all electron basis set (B0) was employed: the O and Ti atoms were described with a [8-411-1]/(1s/3sp/1d) and a [8-6411-31]/(1s/4sp/2d) basis sets respectively. For the CO adsorption, in order to find an appropriate compromise between accuracy and computational cost, the CO molecule and the surface O and Ti atoms were described with a TZ-P basis set, while a [8-411-1]/(1s/3sp/1d) and a HW/411-31, exploiting the Hay-Wadt small-core pseudopotential,22 were employed for the inner O and Ti atoms respectively (B1 basis set). The reliability of these basis set combinations was carefully checked in our previous publication.11 The basis set superposition error (BSSE) was corrected using the counterpoise approach.23


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CO frequencies have been computed at the Г point, within the harmonic approximation, by diagonalizing the mass-weighted Hessian matrix only for the CO fragment, once checked that its stretching vibration is not coupled with other crystal phonons. Samples. Rutile nano- and micro-powders (hereafter nano-rutile and micro-rutile) were purchased from Sigma-Aldrich. The ICP analysis provided by Sigma-Aldrich highlights that nano-rutile contains 4,5% Si, while the other trace elements are below 100 ppm for both nano-rutile and microrutile. A thermal treatment in a furnace at 973 K for seven hours was performed on nano-rutile to monitor the variations in sample morphology. Very pure rutile samples (SiO2 wt % < 0.2) were alternatively obtained by thermal treatment in air at 1073 K of P25 from Evonik Industries (formerly Degussa). This treatment leads to sintering of the TiO2 particles and to the complete transformation of the P25 polymorphs mixture (~85% anatase and ~15% rutile) into pure rutile particles characterized by low surface area (see Table 1) and well defined morphology. The rutile component of P25 was also isolated by etching the native material with hydrofluoric acid (HF), which is known to dissolve the anatase phase only. To this purpose, a 10% HF solution (400 mL) was prepared by diluting 40% hydrogen fluoride (Merck) with deionized water, then 10 g of P25 were added and the suspension was stirred for 12 hours. After the treatment, the etched TiO2 particles (hereafter P25HF) were separated by centrifugation, washed with deionized water and heated in air at 773 K for 1 hour to remove the residual impurities. The phase composition, the particle size and the BET surface area of the investigated samples are reported in Table 1.

Table 1. Phase composition in weight percentage of anatase (anat) and rutile (rut), particle dimension (d) calculated from XRD and BET surface area (see Subsection “Adsorption of N2 at 77 K”) of the investigated samples.



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danat Sample

drut

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BET surface area

%wtanat %wtrut (nm) (nm)

(m2/g)

Micro-rutile

5

95

94

97

2

P25

83

17

26

42

61

P25 1073 K

0

100

116

9

P25HF

3

97

47

22

Nano-rutile

0

100

11

122

Nano-rutile 973 K

0

100

13

77

-

FTIR spectroscopy. All the TiO2 samples used for the CO adsorption IR experiments were in form of self-supporting pellets suitable for transmission measurements. Before CO adsorption, the pellets were outgassed under high vacuum (residual pressure < 10-4 mbar) at 773 K for two hours in the same cryogenic cell (a closed circuit liquid helium Oxford CCC 1204 cryostat properly modified) allowing infrared investigation of species adsorbed under controlled temperature (between 300 and 14 K) and pressure conditions. The activation at 773 K ensures that the main rutile faces are free from adsorbed water and hydroxyls. The residual OH groups are only located on edges, corners, steps and other defects, as already studied in detail.1 The thermal treatment was followed by an oxidation step at with 15 mbar of O2 in order to obtain fully stoichiometric TiO2. After outgassing O2 at room temperature, 40 mbar of CO were dosed and the temperature was gradually lowered down to 60 K in presence of the gas phase. The infrared spectra were recorded on a Bruker Equinox 55 FTIR spectrometer, equipped with an MCT cryogenic detector, with the sample compartment modified to accommodate the cryogenic head; 128 interferograms (recorded at 2 cm-1 resolution) were typically averaged for each spectrum. ACS Paragon Plus Environment

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Field emission scanning electron microscopy. Field emission scanning electron (FESEM) micrographs were acquired with a Zeiss Supra 40 scanning electron microscope based on Zeiss GEMINI field emission SEM column. Samples were deposited on a copper grid covered with a lacey carbon film. HRTEM. High-resolution transmission electron micrographs were obtained with a JEOL 3010UHR instrument operating at 300 kV, equipped with a 2k × 2k pixels Gatan US1000 CCD camera. Samples were deposited on a copper grid covered with a lacey carbon film. XRD. X-Ray Powder Diffraction patterns were collected in the 3-80 2θ range, with angular resolution 2θ = 0.016 and integration time of 90 seconds per step, with a PW3050/60 X'Pert PRO MPD diffractometer from PANalytical working in reflection mode in Bragg-Brentano geometry. The source used was a high power ceramic tube PW3373/10 LFF with Cu anode, equipped with Ni filter to attenuate Kβ. The scattered photons were collected by a RTMS (Real Time Multiple Strip) X'celerator detector and the samples were hosted in amorphous SiO2 holder. The quantitative analysis of the anatase/rutile ratio in the samples was performed using the following equation:24  I  X A = 1 + 1.26 R  IA  

−1

where XA is the weight fraction of anatase in the sample, IR the intensity of the (110) rutile diffraction peak and IA and intensity of the (101) anatase diffraction peak. The (101) anatase reflection and the (110) rutile reflection were also exploited to calculate the average crystallite size with the Scherrer equation. The instrumental broadening was corrected using a Si powder standard.

Adsorption of N2 at 77 K / BET. The surface area of the investigated samples was measured by adsorption of nitrogen at 77 K on samples pre-treated at 423 K under vacuum, applying the Brunauer–Emmett–Teller (BET) equation. Measurements were performed with a Micromeritics ASAP 2020 sorption analyzer. ACS Paragon Plus Environment


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Results and discussion 1) Periodic DFT calculations 1a) Surfaces analysis The first aim of the theoretical approach was to determine the relative stability of the rutile faces in order to pinpoint the most relevant surfaces for the study of the CO adsorption and to further validate the adopted computational approach, comparing our calculations with the results already reported in literature. To this end the surface energy (Es) of a series of low index surfaces has been computed, employing the PBE0 functional and the all electron basis set B0 (see the Computational parameters section). From Figure 1a we can see that the (110) is the most stable surface and the computed Es strongly depends on the number of Ti-layers considered in the slab, as already reported in previous papers.21,25-27 In particular, as more clearly noticeable in Figure 1b, slabs with an even number of Ti-layers always show a lower Es. This is due to the absence of a symmetry plane normal to the surface, which is associated with a lower flexibility of the odd Ti-layer slabs. However, the surface energies of both the odd and even Ti-layer slabs progressively converge to the surface energy expected for an infinite number of layers.

Figure 1. a) Comparison of the surface energies ES of the most relevant rutile surfaces


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as a function of the slab thickness obtained optimizing only the internal coordinates. The PBE0 functional and the all electron basis set B0 were employed. b) Surface energy as a function of a wider range of slab thickness for the (110) surface, showing the oscillating behaviour. c) Wulff construction, showing the equilibrium shape of the rutile crystals, obtained from the ES reported in part a).

Among the low-index rutile faces, the (110) surface has the lowest density of dangling bonds: this accounts for its low surface energy. As visible in Figure 2, it exposes two kinds of titanium and oxygen atoms. Along the [001] direction, rows of sixfold coordinated Ti atoms (as in the bulk) alternate with fivefold coordinated Ti atoms, Ti(5f), with one dandling bond perpendicular to the surface. The distances between Ti(5f) along the x and y directions are 6.49 Å and 2.96 Å respectively, resulting in a density of coordinatively unsaturated surface Ti atoms (Ti4+cus) of 5.2 nm-2. Concerning the oxygens, three-fold coordinated (as in the bulk), and two-fold coordinated bridging oxygens are present.



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Figure 2. Structure of the optimized 1x1 (110), (100), (011) and (001) rutile surfaces. The calculations were performed using basis set B0 and the PBE0 functional.

Although the second surface in order of stability is the (100) (see Figure 1a), it does not appear in the Wulff construction (Figure 1c) obtained from the computed surface energies. The (100) surface is however present to a small extent in equilibrium morphologies reported in literature.28. The (100) surface, shown in Figure 2, appears more corrugated than the (110). On the unrelaxed surface, the equatorial planes of the octahedra are rotated at 45° with respect to the surface normal: this gives rise to chains of 6-fold and 5-fold coordinated Ti and of twofold coordinated oxygens (bridging O atoms). The Ti(5f)-Ti(5f) distances are 4.59 Å (x direction) and 2.96 Å (y direction). The (011) surface exposes only fivefold coordinated Ti atoms (Ti4+cus density ~ 7.2 nm-2) which are bonded to twofold coordinated oxygens (see Figure 2). The Ti(5f)-Ti(5f) distances are 3.04 Å (x direction) and 4.59 Å (y direction). There are two different groups of bond lengths (1.81 Å and 1.86 Å) between the surface Ti and surface O atoms, giving rise to zig-zag chains of twofold coordinated oxygens at the highest level of the surface and threefold coordinated O atoms below the surface Ti atoms. The highest surface energy is shown by the (001) surface, where all surface Ti atoms are fourfold coordinated (Ti4+cus density ~ 4.8 nm-2) and all surface O atoms are twofold coordinated (Figure 2). The distances between Ti(4f) along the x and y directions are 4.59 Å and 4.59 Å respectively. Whether this high energy face is present or not in our finely divided systems will be discussed in the following. Table 2 reports the atomic relaxations occurring during the geometry optimization: the values are comparable to already published computational data.21,28 We can see that, as already observed for anatase surfaces,11 all coordinatively unsaturated surface Ti atoms relax downwards in a significant way in order to increase their coordination sphere: this behavior is particularly evident for the (110) and (001) surfaces. On the contrary the oxygen atoms generally relax outwards. ACS Paragon Plus Environment

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The atomic displacements away from the bulk-terminated structure of TiO2 (110) 1x1 surface have been also extensively investigated from experimental point of view.29-33 In particular a strong debate was present about the position of the bringing oxygen atom (O1 in Table 2) which was initially reported to undergo a relaxation of 0.27±0.08 Å toward the bulk in a surface x-ray diffraction (SXRD) study by Charlton et al.29 However more recent studies performed with different experimental techniques30-33 are in perfect agreement with the values reported in Table 2 and confirmed that the bridging oxygen shifts away from the bulk.

Table 2. Displacements along the z-axis of the Ti atoms belonging to the first three Ti-layers (Ti1, Ti2, Ti3) and of the oxygen atoms in the two outermost layers (O1, O2) with respect to the bulkterminated positions. Note that the Ti1 is the exposed coordinatively unsaturated Ti site for all surfaces except for the (110) surface in which Ti2 is the fivefold coordinated Ti atom. The slabs used to model surfaces were approximately 13 Å in thickness. The calculations have been performed at the PBE0/B0 level

(110)

(100)

(011)

(001)

∆z T1 (Å)

0.33

-0.02

-0.09

-0.28

∆z T2 (Å)

-0.17

0.03

0.05

0.37

∆z T3 (Å)

0.28

-0.01

-0.03

-0.19

∆z O1 (Å)

0.10

0.05

-0.03

0.10

∆z O2 (Å)

0.19

0.09

0.02

0.00

1b) Modeling of CO adsorption on the main rutile faces From the theoretical point of view, the CO adsorption on rutile has been investigated by many authors,14,15,27,34-37 but the calculations have been mainly focused on the (110) surface. To achieve a ACS Paragon Plus Environment


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more comprehensive description, in this paper we present a comparison of the adsorptive properties of CO on the (110), (100), (011) and (001) surfaces (full CO coverage conditions: θ = 1, see Figure 3).

Figure 3. Optimized geometry of CO adsorbed through the carbon end on rutile (110), (100), (011) and (001) surfaces at full coverage (θ = 1). The calculations were performed using basis set B1 and the PBE0 functional. Oxygen, titanium and carbon atoms are represented in red, cyan and dark yellow, respectively.

Table 3. Main computed CO adsorption properties for the (110), (100), (011) and (001) rutile surfaces. The slabs used to model surfaces were approximately 13 Å in thickness. For the (110) surface was employed also a thicker slab (110)-6L, containing 6 Ti-layers before geometry optimization, resulting in a thickness of 19 Å. The PBE0 functional with the B1 basis set was adopted for all calculations. BE and BEC are the binding energy and the BSSE corrected binding energy; d are the bond distances, ν is the CO stretching frequency scaled to match the experimental


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and computed frequencies in gas phase, ∆ are the absolute differences with respect to the free molecule. For free CO: d(C-O)=1.1249 Å and ν = 2249 cm−1 (TZ-P/PBE0).

d (C-Ti) ∆d (C-O)

BEC

BE

ν

∆ν

Surface (kJ/mol) (kJ/mol) (cm-1) (cm-1)

(Å)

(Å)

(110)-4L

2.5410

-0.0047

19.7

15.6

2194

54

(110)-6L

2.5266

-0.0048

20.8

16.6

2193

53

(100)

2.5210

-0.0042

20.8

16.1

2188

47

(011)

2.4671

-0.0016

22.1

18.1

2160

18

(001)

2.4363

-0.0038

36.0

31.5

2185

44

First of all, the effect of the slab thickness on the CO adsorption properties was checked for the (110) surface, which showed an oscillating behavior of the surface energy upon increasing the slab thickness. As visible in Table 3, the variation of the calculated properties is negligible when slabs containing 4 or 6 Ti-layers (which become 8 or 12 after geometry optimization) are compared, in agreement with the results reported in literature.27 For all surfaces the adsorption through the carbon end gives rise to a decrease of the CO bond length that linearly correlates with the corresponding blue-shift of ν(CO). This correlation can be explained according to the Stark effect and can be described by perturbation theory.13 The stretching frequencies of CO adsorbed at full coverage on (110), (100) and (001) are all comprised in a small interval (2194-2185 cm-1). Only the frequency of CO adsorbed on (011) face is occurring at a distinctly lower value. Notice that the frequency of CO on (110), (100) and (001) faces is only slightly higher than that of CO adsorbed on the most commonly exposed anatase surfaces, as fully documented in a previous paper.11 From the periodic calculations it is also



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emerging that the (110) surface, which should be predominant in rutile crystals, is slightly more electrophilic than the most abundant (101) anatase surface, as already highlighted also in a previous study which employed the B3LYP functional.15 In order to estimate the relative effect of CO coverage we can consider the CO - CO distances in x and y directions at full coverage on the different surfaces, which are 6.49 - 2.96 Å for the (110), 4.59 - 2.96 Å for the (100), 3.04 - 4.59 Å for the (011), 4.59 - 4.59 Å for the (001). As the dipoledipole interaction is depending upon the distance between parallel oscillators, it is inferred that the effect of lateral interactions on the stretching frequency should be similar for the (110) and (100) faces, because the shorter Ti-Ti distance along the rows of parallel oscillators is identical (2.96 Å), and lower for the (011). Moreover the dynamic coupling on the (011) face is expected to be negatively affected by the non parallel arrangement of oscillators on the two different rows1,38 (see Figure 3). This fact is likely explaining the low value of the calculated shift with respect to free CO. However, the coverage induced shifts will be discussed in detail on the basis of the experimental results. The high energy (001) face, characterized by Ti(4f) centers, shows the shortest Ti-CO distance and the highest adsorption energy, as expected.

2) The FTIR spectra of CO adsorbed on micro-rutile In order to compare the periodic DFT calculations with the FTIR results, we decided to start the experimental investigation with a rutile polycrystalline sample characterized by the presence of crystals with micrometric size (micro-rutile) and with well defined morphology. As visible in the FESEM images reported in Figure 4b, the particles show a well defined crystalline habit and many crystals exhibit a shape very similar to the Wulff construction (see Figure 1c). Despite the presence of a small amount of anatase (see Table 1), this sample can therefore be considered as an ideal model system to compare experimental and computational results. Only at a later stage we will move to more complex nanocrystalline samples characterized by a less defined morphology. ACS Paragon Plus Environment

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The FTIR spectral sequence reported in Figure 4a corresponds to decreasing CO coverages. Several components are present: 1) A small shoulder at 2137 cm-1 which will be analyzed later when the spectroscopy of CO species adsorbed through the oxygen end will be discussed 2) An intense and easily reversible signal at 2141 cm−1 due to physically adsorbed CO forming a surface multilayer.7 3) A sharp peak centered at 2149 cm−1, shifting to 2152 cm−1 at low CO coverage. Considering the small shift with respect to the gas phase value, this peak is assigned to CO adsorbed on Ti sites possessing a very low polarizing power (vide infra). 4) A weak and broad band in the 2160-2155 cm-1 interval. This absorption is mainly originated by the interaction between CO and the residual OH groups located on edges, steps and corners. This assignment is in agreement with: a) the intensity which is observed to decrease in samples containing less hydroxyl groups obtained via prolonged outgassing times at 773 K (data not shown for brevity); b) the parallel perturbation of the complex and structured OH absorption in the 3750-3600 cm-1 range, as visible in the inset of Figure 4a. The detailed assignment of the single ν(OH) components is outside the scope of this study. The interested readers are referred to our previous paper.1 5) A main component centered at 2181 cm−1 (with a shoulder at 2178 cm-1) which shifts to 2189 cm−1 upon lowering the coverage θ. At low θ also a weak peak at 2197 cm-1 is clearly observable, which is very resistant upon outgassing. The main band is located in a spectral range similar to that observed on anatase surfaces.1 It can be assigned to CO interacting through the carbon end with Ti centers on the predominant (110) rutile face. Considering the asymmetric shape, the broad character of the absorption and the presence of a shoulder at 2178 cm-1 the contribution of other less abundant faces is quite probable (see Table 3 for the computed CO blue-shifts). By analogy with the results obtained on anatase1 the component at 2197 cm-1, which is the most resistant to outgassing, can be ascribed to CO on low ACS Paragon Plus Environment


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coordinated and hence highly polarizing sites located on edges and steps. A contribution of CO adsorbed on Ti(4f) sites located on the (001) surface cannot be fully excluded. 6) The very weak and broad tail, present over 2210 cm-1, can be ascribed to a combination mode involving the ν(CO) mode responsible for the main peak and low frequency modes of the CO molecules adsorbed on facets, as already observed on anatase.1,10. On this point we shall return in the following. 7) The small components in the 2145-2080 cm-1 range, clearly observable only at low coverages, are due to 13C16O (~1%): from these signals it is possible to obtain information about the static interactions occurring in the CO monolayer (see Section 4).

Figure 4. a) FTIR spectra, recorded at 60 K, of CO adsorbed at increasing coverages on microrutile activated for 2 hour at 773 K. In the inset the OH stretching region is reported. The black curve refers to maximum coverage (40 mbar of CO), the red curve to complete CO outgassing. b) FESEM images of the micro-rutile sample.

Comparing the FTIR spectra with the results of the periodic DFT calculations (Table 3), the assignment of the band centered at 2181 cm-1 to Ti centers located on the most stable and hence ACS Paragon Plus Environment

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more abundant (110) rutile surface is confirmed. Notice that, as already observed for anatase surfaces,11 the PBE0 functional tends in general to overestimate the ∆ν(CO) blue-shifts. A band in a similar frequency range has already been observed on rutile by other authors39-41 and generically assigned to CO molecules adsorbed at Ti4+cus sites: small differences in the peak position can be explained by the fact that our measurements are performed at 60 K, while the experimental data already present in literature are recorded at a real temperature of 100 K or higher.9,12,39-41 The assignment of the main component to the (110) surface is further supported by recent ReflectionAbsorption Infrared Spectroscopy (RAIRS) results for CO adsorbed on single-crystalline (110) surface.42 However, considering the broad character of the 2181 cm-1 band, it could be more properly considered as originated by the superposition of one major (110) component and others minor components associated with less abundant exposed faces. In particular the shoulder at 2178 cm-1 could be possibly originated by the (100) surface, which shows a slightly smaller computed ∆ν(CO) (see Table 3). The assignment of the sharp peak centered at 2149 cm−1, which is due to easily reversible species, is more tricky. The narrow width at half maximum (FWHM) of this band seems to suggest that: a) the responsible species are adsorbed on well defined and homogeneous sites characterized by a very low polarizing power; b) owing to the very weak perturbation, the induced dipole moment of these CO oscillators is so small that the adsorbate-adsorbate interaction caused by the surrounding parallel oscillators is necessarily very limited (indeed the peak shifts only of about 3 cm-1 with coverage). For the same reason the ν(CO) is not seriously influenced by the inhomogeneous broadening effects associated with surface disorder. In our opinion this second observation is able to explain why the FWHM of the bands centered in the 2200-2175 cm-1 interval is distinctly higher even on materials treated at high temperature and hence characterized by highly ordered faces (vide infra). On the basis of the periodic DFT calculations, the only unreconstructed rutile surface present showing a low blue-shift is the (011) (see Table 3), which is also present in the equilibrium shape of the rutile crystals (see Wulff construction in Figure 1c). However the experimental ∆ν(CO) = 6 cm-1 ACS Paragon Plus Environment


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(full coverage) seems too small to fit the blue-shift value computed for the (011) surface (18 cm-1). The only possible explanation is that this band is originated by the interaction via carbon end of CO with Ti sites very effectively shielded by the surrounding oxygen atoms and located on a reconstructed rutile surface. This issue will be more deeply discussed in the following. About the presence of the (001) face, which should originate a peak located at about 2160 cm-1, it is difficult to draw a definite conclusion because in the same region also the CO species interacting with the residual OH groups are absorbing. Concerning the small shoulder at 2137 cm-1, redshifted with respect to CO in gas phase, we can note that, upon outgassing, it shows the same behavior of the band at 2149 cm-1 . Therefore we can ascribe it to CO molecules adsorbed on the same kind of sites which originate the 2149 cm-1 band. As this band is more clearly observable on other rutile systems the discussion about its assignment is postponed

3) CO adsorption on nano-rutile As second step of our FTIR study we investigated the CO adsorption on Sigma-Aldrich nanocrystalline rutile. The motivation was that a high surface area nanocrystalline material, potentially exposing a different proportion of the above mentioned faces together with a higher fraction of defects, could represent an ideal playground to complete and substantiate the assignments given before for the CO bands observed on micro-rutile.


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Figure 5. a) FTIR spectra, recorded at 60 K, of CO adsorbed at increasing coverages on nano-rutile activated for 2 hour at 773 K. The black curve refers to maximum coverage (40 mbar of CO), the red curve to complete CO outgassing. b) As part a) for the the OH stretching region. From Figure 5 it is clearly evident that the spectrum of adsorbed CO is completely different from that observed on micro-rutile because of the total absence of the main band at 2181 cm-1 and of the presence of a dominating broad band in the 2175-2160 cm-1 interval, typical of CO interacting with OH groups. The band centered at 2140 cm-1, due to multilayer CO growing at the highest equilibrium pressures, is also present. In the high frequency region associated with the stretching modes of surface hydroxyl groups, we can observe a very strong and asymmetric band centered at 3749 cm-1, which has no equivalent in the spectrum of micro rutile. Upon increasing the CO equilibrium pressure, this band is progressively eroded with the formation of a new broader component in the 3700-3600 cm-1 interval. At the same time the intensity of the ν(CO) band in the 2175-2160 cm-1 range proportionally increases. On this basis we can conclude that the 2175-2160 cm-1 absorption is associated with CO interacting via a weak hydrogen bond with anomalous OH groups abundantly present on the nano-rutile particles. It is most noticeable that a 3749 cm-1 band is currently observed on silica treated at the same temperature and that the nano-rutile from Sigma Aldrich is containing a fairly large amount of Si impurities (see Experimental Section). So the hypothesis can be advanced that the surface of rutile particles is fully covered by a layer of amorphous silica. In order to ascertain whether these anomalous OH groups are not a characteristic feature of high surface area nano-rutile, we have sintered the sample in air at 973 K. In this way the surface area can be decreased and the particle size and crystallinity of the rutile phase increased, thus approaching the situation of micro-rutile. Another expectation of the sintering procedure was the elimination of the hypothesized amorphous silica layer fully covering the surface of rutile with formation of separate silica and titania particles. However, even after the thermal treatment, the FTIR spectra of CO (see Figure S1 in the Supporting Information) show the same bands, only less ACS Paragon Plus Environment


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intense owing to the reduction of the surface area caused by sintering. Hence we conclude that the rutile particles from Sigma-Aldrich are fully covered by a silica layer that is not influenced by thermal treatments. This statement will be further confirmed on the basis of the spectra of CO adsorbed on very pure rutile samples derived from P25.

4) CO adsorption properties of rutile particles from P25 4a) Rutile particles obtained by chemical etching of P25 Degussa P25 is a mixture of about 85% anatase and 15% rutile with a surface area of 60 m2/g, which is considered a benchmark in photocatalysis owing to its outstanding photocatalytic activity. Chemical etching with an HF solution allows to fully eliminate the anatase polymorph and to isolate the pure rutile component. The resulting phase is highly crystalline and with moderate surface area (22 m2/g, see Table 1). The highly crystalline character and perfect chemical purity (minimal presence of trace elements) of P25 is associated with the preparation method (flame hydrolysis of TiCl4) where the formation of the particles in the flame occurs at very high temperatures (≥ 1300 K). The habit of the particles is shown in the inset of Figure 6. Also in this case, we studied the surface properties of this phase using the CO as spectroscopic probe. The motivation of this experiment was twofold: a) to investigate the properties of highly crystalline rutile particles with surface area larger than that of micro-rutile; b) to answer some of the unresolved questions concerning the contribution of the rutile phase (about 15%) to the IR spectrum of CO adsorbed on native P25 described in our previous contribution.1


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Figure 6. FTIR spectra, recorded at 60 K, of CO adsorbed at increasing coverages on the P25 rutile component. The particles were previously etched with hydrofluoric acid and then outgassed at 773 K for 2 hours. The black curve refers to maximum coverage (40 mbar of CO), the red curve to complete CO outgassing. In the insets the corresponding OH stretching region and FESEM image are reported

A sequence of IR spectra corresponding to variable coverages of CO at 60 K on the P25 rutile component is shown in Figure 6. Although the sequence looks quite similar to that obtained for micro-rutile, significant differences are present. In particular: a) the FWHM of all the IR bands is reduced, in agreement with the fact that the rutile phase, being highly crystalline, is exposing more regular faces; b) owing to the band narrowing, the composite character of the envelope centred at 2181 cm-1 is more clearly observable (in particular the 2178 cm-1 shoulder is more evident); c) weak components in the 2170-2160 cm-1 range become observable which were masked or absent on micro-rutile; d) because of the band narrowing effect associated with the increased crystallinity, the component at 2137 cm-1 is now more easily visible and its behaviour upon CO pressure changes looks clearly very similar to that of the 2149 cm-1 peak (i.e. the ratio of their intensities has a well ACS Paragon Plus Environment


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defined trend). This allows to definitely assign both peaks to species weakly adsorbed on the same sites. In particular we think that the band at 2137 cm-1 can be ascribed to CO interacting through the oxygen end. This assignment is supported by the experimental16,43 and computational17 data obtained for CO interacting with Lewis acid sites showing different polarization power (e.g. alkaline cations in zeolites). Indeed, although binding through the C end is invariably preferred, considering alkaline cations of increasing size (and therefore with lower polarization power), the interaction through the O end becomes progressively less unfavored.17,18

4b) Rutile particles obtained from P25 by thermal annealing A second way to obtain a very pure rutile sample is to induce the anatase-rutile transformation of P25 samples by thermal heating at 1073 K in air. The so obtained sample is constituted only by very pure rutile, has very low surface area (see Table 1) and is characterized by particles with regular polyhedral shape (see inset of Figure 7). However the particle size distribution is wider than that of micro-rutile.


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Figure 7. a) FTIR spectra, recorded at 60 K, of CO adsorbed at increasing coverages on the rutile particles obtained from P25 by thermal annealing at 1073 K. The sample was outgassed at 773 K for 2 hours. The black curve refers to maximum coverage (40 mbar of CO), the red curve to complete CO outgassing. In the insets the corresponding OH stretching region and HRTEM image are reported

The usual sequence of spectra of adsorbed CO at different coverages is shown in Figure 7. Significant differences can be evidenced when it is compared with the similar sequences of Figures 4a and 6. In particular: a) the intensity of the composite absorption centered at 2181 cm-1 (with an evident shoulder at 2178 cm-1) is greatly reduced; b) the peak at 2149 cm-1 is clearly enhanced and is now the dominant spectroscopic feature. The FWHM of this peak is larger than that observed in the HF treated sample (Figure 6). This is not unexpected since during the P25 thermal annealing the rutile phase is obtained from the anatase component at a temperature lower with respect to the temperature reached in the flame hydrolysis process, a fact which can affect the surface regularity and the surface defects population (both associated with band broadening). The increased intensity of the 2149 cm-1 peak suggests that in this sample the responsible face, characterized by Ti sites with very low polarizing power, is now dominant. This surface should derive from a conversion of the (110) face, which is dominant in the P25 native rutile component and is responsible of the composite main absorption centered at 2181 cm-1 (Figure 6). In our opinion this transformation is due to a thermally induced relaxation process occurring on the (110) face leading to the shielding of Ti sites and consequently to the reduction of the polarizing power. If this hypothesis is correct, the process could be reversed by inducing an outwards relaxation through the agency of an adsorbate able to “extract” the Ti4+cus cations from their shielded position. The water molecule, which is able to dissociate or become coordinated on the surface sites, appeared to us a possible candidate. Therefore the pure rutile sample obtained by thermal annealing of P25 was treated for 12 hours in water vapor at 573 K. Finally, before CO adsorption, the sample was outgassed under high vacuum ACS Paragon Plus Environment


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at 773 K, following the same procedure utilized for the other samples. In this way the residual hydroxyl groups concentration was very similar in all cases as confirmed by the comparable intensity of the OH stretching signals in the 3750-3600 cm-1 range.

Figure 8. a) FTIR spectra, recorded at 60 K, of CO adsorbed at increasing coverages on the rutile particles obtained from P25 by thermal annealing at 1073 K and then treated in water vapor at 573 K. The sample was outgassed at 773 K for 2 hours. In the inset a magnification of the weak signals in the 2280-2200 cm-1 range is reported. The black curve refers to maximum coverage (40 mbar of CO), the red curve to complete CO outgassing. b) Magnification of the 13C16O stretching region.

The result of this experiment (Figure 8a) supports the hypothesis that the 2149 cm-1 peak is due to CO adsorbed on a thermally relaxed (110) surface. In fact, the treatment in water at 573 K seemed able to induce the outwards relaxation of the Ti4+cus cations , so changing the relative intensity of the 2149 and 2181 cm-1 peaks in favor of the second one as expected. Till now we referred to the structural changes showed by the (110) surface in general terms as a thermally induced relaxation, however the observed behavior could be also described as a reconstruction of the rutile (110) 1x1 surface leading to a shielding of the Ti(5f) atoms. Indeed, in literature many reconstruction have been proposed for this surface under reducing conditions.44

4,45,46

Also few reconstructions under

oxidizing conditions have been also reported.4,47,48. However, as the thermal treatments of our samples are always terminating with an oxidizing step at 773 K, the most studied reconstructions ACS Paragon Plus Environment

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mentioned before, occurring in reducing conditions, look unlikely to be present on our samples. This is the reason why we prefer to speak about relaxation instead of reconstruction. Of course it is fear to conclude that a fully definite assignment of the structural changes which leads to the appearance of the IR band centered at 2149 cm-1 cannot be made on the basis of the sole IR results. The sequence of spectra reported in Figure 8b offers the opportunity to comment in greater detail the behavior upon coverage of the 13C16O components in the 2145-2080 cm-1 range. The component observed at 2127 cm-1 at full coverage and shifting to 2142 cm-1 upon decreasing the coverage is the 13 16

C O analogue of the 2181 cm-1 band for 12C16O. The coverage induced shift (15 cm-1) is larger

than that observed for the

12 16

C O analogue (13 cm-1). This is due to the absence of the dynamic

coupling since the highly diluted

13 16

C O molecules (~1%) are basically isolated in a layer

constituted by parallel 12C16O oscillators.1,38 Therefore, since the global shift for the 12C16O bands is originated by the sum of static and dynamic effects (∆νst+∆νdyn), while the

13 16

C O signals are

affected only by the static effect (for a more detailed explanation of the physical origin of the static and dynamic effects see the Supporting Information), we can conclude that for the 2181 cm-1 band: ∆νdyn = +2 cm-1 and ∆νst = -15 cm-1. In a similar way, considering the peak at 2149 and its 13C16O analogue at 2099 cm-1, we can obtain the corresponding shifts associated with the dynamic and static contributions: ∆νdyn = +2 cm-1, ∆νst = -5 cm-1. Finally, observing the inset of Figure 8a, we can notice that the 2280-2210 cm-1 spectral region reveals an unexpected complexity. In fact, besides a very weak complex and hardly reversible band at 2220-2210 cm-1 probably due to CO on corner sites, a very broad component at 2260-2230 cm-1 is clearly visible. This band can be originated by the combination of the internal ν(CO) mode responsible for the main peak at 2181 cm-1 and of low frequency external modes of CO molecules, as already observed for anatase.1,10,12

Conclusions



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The close comparison of the results obtained by electron microscopy, FTIR spectroscopy of adsorbed CO and periodic DFT calculations allowed us to fully explore the relations between particles morphology and surface properties for different kind of rutile polycrystalline samples. The data obtained on crystals with micrometric size (micro-rutile) and with well defined morphology can represent a link between the single-crystal world of classical surface science and the nanocrystalline samples widely employed in technological applications. In particular it is demonstrated that the FTIR spectrum of CO on micro-rutile is dominated by a main component at 2181 cm-1 assigned, by comparison with the periodic DFT calculations, to molecules adsorbed on the (110) surface (which is the surface that shows the lowest computed surface energy). The other very significant spectroscopic feature is a sharp peak at 2149 cm-1, which is present also in the spectrum of native P25. By investigating the pure P25 rutile component obtained by HF etching we demonstrated that its FTIR spectrum of adsorbed CO is similar to the spectrum for micro-rutile, but the FWHM of all the IR bands is reduced because of the higher crystallinity of the material. Owing to the band narrowing, it was possible to highlight the composite character of the envelope centred at 2181 cm-1 which, besides the main contribution related to the (110) surface, shows components related to CO adsorbed on other minor surfaces, e.g. (100), and on edges and steps. The origin of the 2149 cm-1 band was clarified by the FTIR spectra of CO adsorbed on pure rutile obtained by thermal annealing of P25: in this case the intensity of the composite absorption centered at 2181 cm-1 is greatly reduced, while the peak at 2149 cm-1 is clearly enhanced. This experiment suggests that the surface related to the 2149 cm-1 peak derives from a thermal induced relaxation involving the (110) face leading to the shielding of Ti sites and consequently to the reduction of the polarizing power. This hypothesis is also supported by the fact that a successive treatment in water vapor at 573 K is able to induce the outwards relaxation of the Ti4+cus cations. This process is accompanied by the expected modification the relative intensity of the 2149 and 2181 cm-1 peaks in favor of the second one. This complete assignment of the FTIR spectra of CO adsorbed on the P25 rutile component helps also to clarify the open issues concerning the ACS Paragon Plus Environment

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contribution of the rutile phase to the FTIR spectrum of CO on native P25 discussed in our recent study.1. In particular it is emerging that the spectrum of CO on P25 is the sum of that observed on separate anatase and rutile phases and that no sign of anomalous features associated with anataserutile junctions are observable. During our investigation we also discussed the origin of the red-shifted band at 2137 cm-1 which has been ascribed to CO adsorbed through the oxygen end on the same structures associated with the 2149 cm-1 band. This kind of signal was already observed for alkaline cations in zeolites, but never clearly described and properly discussed for metal oxides.17 The unique power and extreme sensitivity of the FTIR spectroscopy of CO adsorbed at very low temperature (60 K) to the material surface properties are further proven by the spectra of CO adsorbed on Sigma-Aldrich nano-rutile. Although the sample is commercialized as almost pure rutile nanopowder containing only 4,5% Si and the XRD analysis shows that the crystalline component is 100% rutile, the FTIR measurements highlight that the surface properties are completely different from that of pure highly crystalline rutile. Indeed, the FTIR spectra are dominated by signals ascribed to a layer of amorphous silica and no bands related to CO interacting with exposed Ti atoms are observable. Finally, the study of the coverage dependence of the ν(CO) for the

12 16

C O and

13 16

C O isotopes

allowed us to determine the frequency shifts associated with the static and dynamic interactions between CO species adsorbed on the various faces .

Acknowledgements. Dr. G. Agostini is acknowledged for the acquisition of the HRTEM images and Dr. A. M. Ferrari for the helpful discussion on the DFT results. The work has been supported by the Italian MIUR through the FIRB Project RBAP115AYN “Oxides at the nanoscale: multifunctionality and applications” and by Ateneo-Compagnia di San Paolo-2011-1A line, ORTO11RRT5 project.



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Supporting Information Available. Additional details about the CO adsorption on nano-rutile and about the CO isotopic mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.


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Rutile Surface Properties Beyond the Single Crystal Approach: New

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