Langmuir 2008, 24, 14035-14041
14035
Formate Adsorption onto Thin Films of Rutile TiO2 Nanorods and Nanowires Thomas Berger, Jose´ M. Delgado, Teresa Lana-Villarreal, Antonio Rodes, and Roberto Go´mez* Departament de Quı´mica Fı´sica i Institut UniVersitari d’Electroquı´mica, UniVersitat d’Alacant, Apartat 99, E-03080 Alacant, Spain ReceiVed July 7, 2008. ReVised Manuscript ReceiVed October 1, 2008 We evaluate the applicability of silicon prisms to infrared (IR) spectroscopic investigations of the oxide/electrolyte interface in the attenuated total reflection (ATR) configuration. Using formic acid as a probe adsorbate, a comparison is done between a rutile nanowire film supported on Si and a rutile nanorod film supported on ZnSe. The nanowires were 2 nm in diameter and were deposited directly on Si by chemical bath deposition, whereas the nanorods were deposited on ZnSe by solution casting of an aqueous dispersion prepared from a commercial powder. The lower penetration depth of the evanescent wave for silicon was compensated by a higher internal surface area of the corresponding nanowire film. Advantageously, much higher solution concentrations can be used in the case of the Si prism without a significant contribution of solution species to the IR spectrum. Furthermore, the high chemical stability of Si opens up the possibility of performing experiments in highly acidic aqueous solutions. Upon formate adsorption at pH 3.5, a pair of intense IR bands was observed in the wavenumber range where antisymmetric νas(COO) vibrations are expected, namely at 1537 and 1592 cm-1 on nanorod films and at 1544 and 1586 cm-1 on nanowire films. The relative band intensities are different for nanorod and nanowire films. While the bands at 1537/1544 cm-1 are assigned to formate adsorbed on the (110) face, forming the bridging bidentate (µ) structure, those at 1592/1586 cm-1 are tentatively attributed to formate adsorbed at low coordination adsorption sites at the nanocrystal edges. Bands corresponding to the carboxylate symmetric stretching and the HCO deformation were also observed.
I. Introduction A plethora of synthetic routes for the tailoring of size, shape, and crystal structure of nanocrystalline metal oxides has become available recently and provides the basis for a systematic investigation of the structure-activity relationship of these materials.1,2 The question of how electronic and optical properties depend on the structure of the crystallite units has a high relevance for an ample body of applications ranging from sensors to optical devices and photocatalysts.2,3 Regarding the analysis of processes taking place at the metal oxide surface, morphologically uniform and highly dispersed systems provide, in addition to conveniently high concentrations of active sites, also the advantage of a high geometrical definition of the underlying structures. This is a basic requirement for the application of molecular spectroscopy to a systematic investigation aiming at an understanding of “real world” systems such as oxide powders. Attenuated total reflection (ATR) IR spectroscopy has turned out to be the method of choice to gain a molecular view of surface processes on metal oxides in contact with an aqueous phase in spite of the high absorptivity of water in the IR range.4,5 The adsorption and reaction of model organic molecules on porous metal oxide films have been studied by taking advantage of the high sensitivity of Fourier transform IR spectroscopy and the possibility to detect even small structural changes in the adsorbate geometries.6-10 The fact that the evanescent IR wave penetrates * Corresponding author. Tel: +34 965903536. Fax: +34 965903537. E-mail:
[email protected]. (1) Jun, Y.; Choi, J.; Cheon, J. Angew. Chem. Int. Ed. 2006, 45, 3414–3439. (2) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891–2959. (3) Hashimoto, K.; Irie, H.; Fujishima, A. Jpn. J. Appl. Phys 2005, 44, 8269– 8285. (4) McQuillan, A. J. AdV. Mater. 2001, 13, 1034–1038. (5) Hug, S.; Sulzberger, B. Langmuir 1994, 10, 3587–3597. (6) Christensen, P. A.; Eameaim, J.; Hamnett, A. Phys. Chem. Chem. Phys. 1999, 1, 5315–5321. (7) Araujo, P. Z.; Mendive, C. B.; Garcı´a Rodenas, L. A.; Morando, P. J.; Regazzoni, A. E.; Blesa, M. A.; Bahnemann, D. Colloids Surf. A 2005, 265, 73.
only a few micrometers into the optically less dense (aqueous) medium in contact with the ATR crystal allows for an accurate subtraction of the water background. On the other hand, there is only a short optical path length available to achieve reasonable signal-to-noise ratios of bands related to molecules adsorbed on the crystallite surface. Therefore, highly dispersed samples, characterized by a high surface area and thus a high concentration of adsorption sites, should be used for ATR-IR spectroscopic experiments. Metal oxide crystallites are usually immobilized in the form of thin porous films on the surface of the ATR crystal. The penetration depth (dp) of the evanescent IR wave (of wavelength λ) at the interface of the ATR crystal (with the refractive index n1) and an optically less dense medium (n2) is determined in the case of total reflection (angle of incidence θ > critical angle for total reflection θc) by11
dp )
λ
2πn1
()
n2 sin θ n1 2
(1) 2
Therefore, ZnSe prisms (n ) 2.4) are frequently used for ATRIR spectroscopic experiments, as they provide a high penetration depth of the evanescent IR wave, thus assuring a high sensitivity. However, high penetration depths lead, on the other hand, to a significant contribution of absorbing species in solution to the overall spectrum. This may, in some cases, considerably complicate the interpretation of the ATR-IR spectroscopic experiment. In addition, the low stability of ZnSe in acidic solutions, especially when exposed to UV light,9 limits its range (8) Lana-Villareal, T.; Rodes, A.; Pe´rez, J. M.; Go´mez, R. J. Am. Chem. Soc. 2005, 127, 12601–12611. (9) Dolamic, I.; Bu¨rgi, T. J. Phys. Chem. B 2006, 110, 14898–14904. (10) Lana-Villareal, T.; Monllor-Satoca, D.; Rodes, A.; Go´mez, R. Catal. Today 2007, 129, 89–95. (11) Harrick, N. J. Internal Reflection Spectroscopy; Harrick Scientific Corp.: New York, 1987.
10.1021/la8021326 CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
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Figure 1. Transmission electron micrographs of rutile nanorods (a) and nanowires (b, c). The samples were detached from the ATR crystals.
of application. Silicon crystals (n ) 3.4), on the other hand, are characterized by a high stability in acidic solutions, but they have the drawback of a low penetration depth of the evanescent IR wave, resulting in a lower sensitivity. This, however, can be compensated by increasing the internal surface area of the porous layer. The deposition of nanostructured films consisting of ultrafine crystallite units would in this context allow for the study of surface processes excluding the interfering contributions from species in solution. It should be mentioned that porous films deposited onto the ATR crystal by application of aqueous particle suspensions and subsequent water evaporation are characterized by a very low mechanical stability. This makes the reproducibility of the experiments a crucial issue, and the film thickness often has to be optimized with regard to a reasonable mechanical stability. Direct chemical deposition of a porous layer from a precursor solution onto the ATR crystal, on the other hand, would result in very stable films and would, in addition, allow for a precise tuning of the film thickness with respect to the penetration depth. Ideally, the film thickness should be equal to or slightly larger than the penetration depth of the IR wave. Thus an optimum signal-to-noise ratio together with a rapid mass transport between the internal surface and the solution can be achieved. Due to their impact on a wide field of application, surfacerelated properties of TiO2-based materials, both in the form of single crystals and powders, have been studied extensively.2,12,13 Only recently we reported on the activity of morphologically well-defined rutile thin films for the photooxidation of model organic molecules in acidic solution.14,15 Improved photocatalytic properties were observed for thin films of ultrafine rutile nanowires (d e 2 nm) when compared to thin films consisting of larger (20 nm × 40 nm) ellipsoidal rutile nanoparticles. The higher reactivity of the nanowire films was referred to altered electronic and optical properties. The results pointed to an increased reactivity of both electrons and holes in the nanowire film as a consequence of an increased band gap observed on these quantum-sized nanocrystals. However, whereas photoelectrochemical measurements face exclusively integral properties of the investigated electrodes, in situ spectroscopic techniques need to be used to study their microscopic, chemical properties. We, therefore, address in the present study the adsorption of HCOOH on ultrafine rutile nanowires (deposited on a silicon prism) and on larger nanorods (immobilized on a ZnSe prism) in acidic solution by ATR-IR (12) Thompson, T. L.; Yates, J. T. Chem. ReV. 2006, 106, 4428–4453. (13) Diebold, U. Surf. Sci. Rep. 2003, 48, 53–229. (14) Berger, T.; Lana-Villarreal, T.; Monllor-Satoca, D.; Go´mez, R. J. Phys. Chem. C 2008, 112, 15920–15928. (15) Berger, T.; Lana-Villarreal, T.; Monllor-Satoca, D.; Go´mez, R. Chem. Phys. Lett. 2007, 447, 91–95.
spectroscopy, aiming at a detailed view of the adsorption sites present on the surface of these films. The nanowire films were directly grown onto the Si crystal by chemical bath deposition and turned out to be mechanically very stable. Because of the high internal surface area of the nanowire films, high signalto-noise ratios were obtained in spite of the much lower penetration depth of the evanescent IR wave in the case of the Si crystal. The advantage of the low penetration depth, however, is that experiments could be performed at much higher HCOOH concentrations without interference from species in solution.
II. Experimental Section TiOSO4 (Aldrich, 99.99%, 15 wt % solution in sulfuric acid), HCl (Prolabo, Rectapur 35%), NaOH (Merck, p.a.), HClO4 (Merck, 70%, Suprapur), NaClO4 · H2O (Scharlau, extrapure), Na2SO4 (Merck, p.a.), and HCOOH (Merck, 98-100%) were used as received. All solutions were prepared in ultrapure water (Millipore Elix 3). ATR-IR spectroscopic measurements were performed on a Nicolet Magna 850 spectrometer equipped with a MCT detector and a variable angle Veemax reflectance accessory (Pike Technologies). The ATR cell consists of a glass body pressed mechanically against a semicylindrical ZnSe prism or a triangular Si prism, respectively. Prism and glass cell are separated by a Teflon joint to avoid leakage. Spectra were obtained by averaging 50 scans at a resolution of 8 cm-1, the angle of incidence being 45° in the case of the ZnSe prism and 60° in the case of the Si prism. Background spectra were taken using HCOOH-free blank solutions. Nanorod films were prepared by applying 1.2 µL/mm2 of a 0.16 M aqueous suspension of rutile TiO2 nanorods onto the ZnSe prism and subsequent drying in air at 60 °C overnight. Assuming a porosity of 0.5, the resulting film thickness can be estimated to be ∼8 µm. The rutile powder (Sachtleben Nano-Rutile) was a gift from Sachtleben Chemie GmbH (Duisburg, Germany). Rutile nanowire films were grown by direct deposition from aqueous TiOSO4 solutions.14-17 The precursor (3 mM Ti(IV)) was prepared by adding TiOSO4 to aqueous solutions containing HCl. The pH after 1 h of stirring at room temperature was adjusted to 1. The ATR cell was then filled with the precursor solution and maintained at 60 °C for 14 h. The film was washed extensively with 0.1 M HClO4 aqueous solution and finally conditioned with the blank solution. Due to adsorbed sulfate anions, nanowire films contain around 2.5% S as determined by energy dispersive X-ray analysis (EDX). The crystal phase of the TiO2 films was determined by both Raman spectroscopy (LabRam spectrometer, Jobin-Yvon Horiba) and X-ray diffraction (Seifert JSO-Debyeflex 2002) using the Cu KR line. The film thickness was measured by scanning electron microscopy (SEM, Hitachi S-3000N). After removing the oxide films from the substrates, (16) Yamabi, S.; Imai, H. Chem. Mater. 2002, 14, 609–614. (17) Berger, T.; Lana-Villarreal, T.; Monllor-Satoca, D.; Go´mez, R. J. Phys. Chem. C 2007, 111, 9936–9942.
Formate Adsorption onto Thin Films of Rutile TiO2
Langmuir, Vol. 24, No. 24, 2008 14037
Figure 2. IR spectra of a 1 M HCOOH/0.1 M ClO4- aqueous solution at pH 1 (a, b) and pH 3.5 (c, d). Background spectra were taken at the respective pH using HCOOH-free blank solutions. ATR crystals: Si, 60° (a, c); ZnSe, 45° (b, d). Spectral resolution: 8 cm-1; 50 scans.
transmission electron microscopic (TEM) measurements were carried out with a JEM-2010 (JEOL) microscope equipped with an INCA Energy TEM100 (Oxford instruments) for EDX. Images were recorded with a MegaView II camera (SIS).
III. Results and Discussion A. Structural Characterization. TEM was used to analyze the morphology of the TiO2 samples after detachment of the respective films from the ATR prism. Nanorods (Figure 1a) have a width of 6-9 nm and a length of ∼40 nm and are of pure rutile phase (Figure S1a of the Supporting Information). A previous study has shown that the majority of exposed facets on nanorods are (110) faces.18 Nanowires grown by chemical bath deposition crystallize in the rutile phase (Figure S1b,c, Supporting Information) and form on the Si prism an inhomogeneous film with a thickness of ∼2 µm (Figure S2, Supporting Information). The film is formed by bundles (d ∼ 1 µm) of rutile nanowires (d e 2 nm) (Figure 1b,c). As shown in previous studies, growth of the nanowires proceeds in the [001] direction.14-17 The crystallographic orientation of the surface planes cannot be deduced unambiguously from the TEM images due to the limited resolution of the microscope and the fact that considerable damage to the surface was observed at high magnifications in the electron beam. However, taking into account the high thermodynamic stability of the rutile (110) surface and the elongation of the nanowires in the [001] direction, we assume that the majority of surface planes is also constituted by (110) faces. B. ATR-IR Spectroscopic Measurements. IR spectra of 1 M HCOOH aqueous solutions at pH 1 (Figure 2a,b) and pH 3.5 (Figure 2c,d) were measured using the uncoated ZnSe and Si prism, respectively. As a consequence of both the lower refractive index of ZnSe (nZnSe ) 2.4 versus nSi ) 3.4) and the smaller angle of incidence (45° versus 60°), around a 10 times higher band intensity was observed in the former case.19 At pH 1, formic acid (pKa ) 3.75) is present exclusively in the protonated form. The spectra (Figure 2a,b) contain bands at 1716 cm-1 [ν(CdO)] and 1211 cm-1 [ν(C-OH)], respectively. At pH 3.5, the deprotonated form gives rise to additional bands at 1579 cm-1 [antisymmetric νas(COO) vibration] and at 1351 and 1383 cm-1 in the region of the symmetric νs(COO) and δ(HCO) vibrations. All bands are in agreement with previous studies.18 (18) Rotzinger, F. P.; Kesselman-Truttmann, J. M.; Hug, S. J.; Shklover, V.; Gra¨tzel, M. J. Phys. Chem. B 2004, 108, 5004–5017. (19) The 10-fold higher intensity results from a 10-fold higher value of the so-called effective path length (de) in the case of ZnSe. The value of de represents the path length in a (hypothetic) transmission experiment resulting in the same absorption as in the ATR experiment. For bulk aqueous solutions, de can be r 2, where n ) n /n , calculated according to ref 11 as de ) (n21dp)/(2 cos θ)E0,2 21 2 1 r is the relative electric field amplitude in medium 2 at a distance z ) 0 from E0,2 the interface, dp is the penetration depth (eq 1), and θ is the angle of incidence.
Figure 3. ATR-IR spectra of a 25 mM HCOOH/0.1 M ClO4- aqueous solution at pH 3.5 in contact with a layer of rutile nanorods on ZnSe (a) and nanowires on Si (b). The solution spectra in the absence of an oxide layer are shown in both cases for comparison. Background spectra were taken using HCOOH-free blank solutions. ATR crystals: ZnSe, 45° (a); Si, 60° (b). Spectral resolution: 8 cm-1; 50 scans.
Figure 3a shows the IR spectrum of a 25 mM HCOOH aqueous solution at pH 3.5 in contact with a layer of rutile nanorods deposited on ZnSe.20 The spectrum measured in the absence of the oxide layer is also shown for comparison. Only traces of bands resulting from formate in solution are observed at this concentration, making minimal the contribution of solution species to the overall spectrum in the presence of the oxide film. Five intense IR bands at 1586 and 1537 cm-1 [νas(COO)] and at 1388, 1351, and 1315 cm-1 [δ(HCO) and νs(COO)] are observed. Furthermore, a spectrum fit (see below) reveals a minor contribution at 1530 cm-1. The spectra in Figure 3a agree well with recent results published by Rotzinger et al.,18 who have investigated the adsorption of formate and acetate onto Sachtleben rutile nanorods. On the basis of experimental results as well as on ab initio calculations, the authors assigned a band at 1540 cm-1 to the νas(COO) vibration of formate adsorbed onto the (110) face of the nanorods and forming the bridging bidentate (µ) structure. A band at 1581 cm-1, on the other hand, was assigned to dissolved formate, as it was found to scale linearly with HCOOH concentration in solution. In contrast, we assign the corresponding band in Figure 3a (at 1586 cm-1) to a surfacerelated species. In fact, a significant contribution of dissolved species to the overall spectrum seems in our case unlikely, as all solution bands exhibit a very low intensity at this concentration when using the uncoated prism, whereas a high intensity is observed for the band at 1586 cm-1 in the presence of the TiO2 film. Figure 3b shows the respective IR spectrum (25 mM HCOOH aqueous solution at pH 3.5) for a rutile nanowire film deposited on Si. As seen from the spectrum measured with the uncoated prism, contributions from dissolved HCOOH are completely absent in the case of the Si prism. This is a result of both a higher refractive index and a higher angle of incidence than in the case (20) Measurements using the Si prism instead of the ZnSe prism were also performed but resulted in too low signal-to-noise ratios.
14038 Langmuir, Vol. 24, No. 24, 2008
Figure 4. Results of a fit of the experimental ATR-IR spectra from Figure 3. The solution spectra were subtracted prior to performing the fit.
of experiments with the ZnSe prism (Figure 3a). The IR bands in Figure 3b can thus be attributed exclusively to adsorbed species. Bands at 1586 and 1544 cm-1 [νas(COO)] and at 1390 and 1352 cm-1 as well as a shoulder at 1328 cm-1 [δ(HCO) and νs(COO)] are observed. A spectrum fit (see below) reveals additional contributions at 1626 cm-1 and 1528 cm-1. Apart from major differences in the relative band intensities, spectra in Figure 3a,b are similar. For both, the nanorod film and the nanowire film, HCOOH adsorbs exclusively in the deprotonated form at pH 3.5. It is remarkable that the intensity of the IR bands in the case of the nanowire film on Si (Figure 3b) is higher than in the case of nanorods on ZnSe (Figure 3a) in spite of the fact that a 10-fold higher intensity was found for the spectra of HCOOH in solution when using the ZnSe prism (Figure 2).21 This points on the one hand to a huge internal surface area of the nanowire film and, on the other hand, to the accessibility of its pores for the HCOOH molecules.22 In order to determine the relative contributions of single IR bands to the overall signal, IR spectra were fitted by nonlinear least-squares fit using Gaussian-shaped peak functions.23 The results of the spectrum fit, both for nanorods (Figure 4a) and for (21) Assuming a volume fraction of particles of 0.5 for both TiO2 layers, the refractive index n2 of the films can be determined from the volume-weighed average of the refractive index of the particle material and the aqueous solution (ref 5). With n(H2O) )1.34 and n(TiO2) ) 2.0, n2 is calculated to be 1.67. Thus, the penetration depth (dp) of the evanescent wave at 1500 cm-1 amounts to 3.52 µm in the case of a nanorod film on ZnSe and 0.44 µm in the case of a nanowire film on Si. It has to be mentioned that the determination of the penetration depth for the system Si(ZnSe)/TiO2/solution by using a two-layer model is only valid for a film thickness d . dp. With d ) 8 µm in the case of the nanorod film and d ) 2 µm in the case of the nanowire film, this requirement is fulfilled for both samples. (22) In a recent study (ref 17) we found that, for a given thickness, the real surface area of a nanowire film is around 24 times larger than the surface area of a film prepared from nanorods by thermal annealing. Assuming that the decrease of the specific surface area during thermal annealing is compensated by the decrease of the film porosity and estimating the difference of the effective path lengths from the measurements using the uncoated prisms (de[ZnSe] ∼ 10de[Si], Figure 2), the surface area of the nanowire film on Si being probed in the ATR-IR experiment can be estimated to be around 2.4 times that of the nanorod film on ZnSe. This estimate approximately matches the relative intensities of the IR bands in Figure 3. (23) Origin 7, OriginLab Corp., 1991-2002.
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Figure 5. ATR-IR spectra of aqueous solutions (pH 3.5, 0.1 M ClO4-) of HCOOH in contact with a layer of rutile nanorods on ZnSe (a) and nanowires on Si (b). Background spectra were taken using HCOOHfree blank solutions. Solution spectra were subtracted prior to performing the fit. The residuals from spectrum fit are shown at the top. ATR crystals: ZnSe, 45° (a); Si, 60° (b). Spectral resolution: 8 cm-1; 50 scans. Table 1. Band Parameters Resulting from Spectrum Fit nanorods -1
ν˜ /cm fwhm/cm-1
1315 70
1351 16
1388 19
1530 51
1537 23
1592 52
nanowires ν˜ /cm-1 fwhm/cm-1
1328 70
1352 16
1390 18
1528 63
1544 29
1586 48
1626 62
nanowires (Figure 4b), are shown and the respective fit parameters are listed in Table 1. The band at 1626 cm-1 can be attributed to surface coordinated water species;24 all other bands are assigned to adsorbed formate. In a next step, HCOOH was adsorbed from solutions of different concentration onto each of the TiO2 films to investigate the saturation behavior of the resulting IR bands (Figure 5). All bands are saturated at c(HCOOH) ) 25 mM. The experimental spectra were then fitted using the band parameters from Table 1, and the respective residuals are shown at the top of Figure 5. The saturation behavior of the bands at 1537 and 1592 cm-1 as observed for the nanorod film (Figure 5a) is shown in Figure 6a together with the respective Langmuirian fits. The saturation characteristics of these bands differ significantly from each other (Table 2). A very similar behavior is observed in the case of the nanowire film for the bands at 1544 and 1586 cm-1 (Figure 6b). Thus, on both samples a change of the relative band intensities is observed in this wavenumber range by increasing the HCOOH concentration (Figure 5). In conclusion, a pair of two clearly distinguishable IR bands was observed for both samples in the wavenumber range where antisymmetric νas(COO) vibrations are expected, namely at 1537 and 1592 cm-1 on nanorod films and at 1544 and 1586 cm-1 on (24) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1999, 15, 2402–2408.
Formate Adsorption onto Thin Films of Rutile TiO2
Figure 6. Concentration dependence of the normalized absorbance of antisymmetric νas(COO) vibrations as observed on rutile nanorods (a) and rutile nanowires (b). The absorbance was determined by fitting the IR spectra from Figure 5. Table 2. Equilibrium Adsorption Constants Kads for Nanorods and Nanowires As Deduced from the Saturation Behavior of the Respective νas(COO) Vibrations nanorods
nanowires
ν˜ /cm-1 Kads/M-1 ν˜ /cm-1 Kads/M-1 1537
230
1544
290
1592
130
1586
190
assignment formate at rutile TiO2 (110) in a bridging bidentate coordination (species I) formate at low coordination adsorption sites (e.g., at the nanocrystal edge) (species II)
nanowire films.25 The fact that wavenumber, fwhm, and saturation behavior of the bands change only slightly when using different TiO2 films (Table 1) indicates that similar adsorbate structures should be expected in both cases. We assign the bands at 1537/ 1544 cm-1 to a formate structure, which we call species I. Accordingly, bands at 1592/1586 cm-1 are associated with formate species II. The observation of a close to Langmuirian saturation behavior is a clear indication for the location of the corresponding species at the surface of the nanocrystals. It should be mentioned that in ref 18 the band associated with species II is reported to scale linearly with the HCOOH concentration. This is in contrast with our result and could be a consequence of the different porosities of the nanorod films. The contribution of the solution spectrum to the overall spectrum increases with increasing porosity. As a result, the saturation behavior will be influenced more and more by solution species shifting from a Langmuirian characteristic to a linear dependence. This points to the relevance of finding optimal parameters for the experimental setup (angle of incidence), the ATR prism (refractive index), the TiO2 film (internal surface area, thickness and porosity), and the aqueous solution (concentration range). (25) The broad peak at 1530/1528 cm-1 may result from a number of additional adsorbate geometries due to the presence of various, less abundant adsorption sites (crystal faces) on the nanocrystals.
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As shown by Rotzinger et al.,18 species I can be assigned to formate adsorbed onto the (110) face forming the bridging bidentate (µ) structure. This is in line with our former assumption that, also in the case of the nanowire crystals, (110) faces constitute the majority of surface planes. Species II differs in two ways from species I. The wavenumber of the νas(COO) bands (1592/ 1586 cm-1) strongly resembles that of solution species (1579 cm-1), but it is much broader (fwhm ∼ 50 cm-1 versus 25 cm-1). Furthermore, a smaller equilibrium adsorption constant is found for species II (Table 2). These observations point to a weaker adsorption of species II onto TiO2. On the basis of our experimental results, very similar adsorbate geometries are expected on both samples, in spite of major morphological differences (nanorods vs nanowires). However, the knowledge of the nanocrystal morphology together with the observation of changed relative band intensitiessthat is, the higher contribution of species II to the overall spectrum in the case of the nanowire film when compared to the nanorod film (Figure 5)smay give an indication of the location of the adsorption complex. As the relative concentration of edge atoms increases as the diameter of the elongated nanocrystal decreases (from nanorods to nanowires), so does the number of possible adsorption sites at the edges. In contrast to (110) terraces, 4-fold-coordinated Ti surface atoms should be taken into account at edge and corner sites. These tetra-coordinated atoms are characterized by two dangling bonds, which may render the bidentate mononuclear adsorption possible. Indeed, additional adsorbate structures [including bidentate mononuclear (η2) and monodentate (η1) structures] have been predicted at low coordinated adsorption sites (as e.g. edges and corners) by quantum chemical calculations.18 In addition, a particle-size-dependent distribution of carboxylate adsorption sites on TiO2 nanoparticles was observed by Zhang et al.26 only recently and was rationalized by a change of the relative concentrations of 6-, 5-, and 4-fold-coordinated surface Ti atoms. Putting all these pieces of information together, we assign species II, therefore, to formate adsorbed at low coordinated adsorption sites at the edge of the nanocrystals. Interestingly, two formate species were also identified on TiO2(110) in ultrahigh vacuum by Hayden et al.27 A νas(COO) band at 1536 cm-1 was attributed to bridging bidentate species adsorbed on 5-fold-coordinated Ti(IV) sites along the [001] direction, whereas a second νas(COO) band at 1566 cm-1 was assigned to bridging bidentate species adsorbed at oxygen vacancies. In addition, bridging bidentate and monodentate structures have been observed on the (111) surface of rutile TiO2 by Uetsuka et al.28 As mentioned above, nanowire films contain around 2.5% S. This is a result of the adsorption of SO42- onto the nanowire surface in the course of film deposition.29 We have shown in a previous study that the S content can be reduced to ∼0.5% by treatment of native films for 20 h in 0.1 M NaOH (in the case of films deposited on transparent conductive glass).15 However, due to the limited stability of Si in alkaline solution, this treatment was avoided in the present case. We investigated, therefore, the influence of SO42- on the adsorption of formate onto sulfate-free TiO2 nanorods. Figure 7 shows IR spectra of a nanorod film in contact with a 25 mM HCOOH aqueous solution on the one hand (Figure 7a) and with a 25 mM HCOOH/25 mM Na2SO4 aqueous solution on the other hand (Figure 7b). The solution (26) Zhang, Q. L.; Du, L. C.; Weng, Y. X.; Wang, L.; Chen, H. Y.; Li, J. Q. J. Phys. Chem. B 2004, 108, 15077–15083. (27) Hayden, B. E.; King, A.; Newton, M. A. J. Phys. Chem. B 1999, 103, 203–208. (28) Uetsuka, H.; Henderson, M. A.; Sasahara, A.; Onishi, H. J. Phys. Chem B 2004, 108, 13706–13710. (29) Yamabi, S.; Imai, H. Thin Solid Films 2003, 434, 86–93.
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Figure 7. ATR-IR spectra for 25 mM HCOOH/0.1 M ClO4- (a) and 25 mM HCOOH/25 mM Na2SO4/0.1 M ClO4- (b) aqueous solutions at pH 3.5 in contact with a layer of rutile nanorods on ZnSe. The solution spectra in the absence of an oxide layer are shown in both cases for comparison. Background spectra were taken using blank solutions free of formic acid and sulfate. The band at 1108 cm-1 in spectrum (a) can be attributed to uncompensated ClO4-. The spectra in part b were multiplied by a factor of 2.5. ATR crystal: ZnSe, 45°. Spectral resolution: 8 cm-1; 50 scans.
spectra in the absence of an oxide layer are shown in both cases for comparison. Most importantly, coadsorption of SO42- and HCOO- does not influence either the wavenumber or the relative intensities of the bands assigned to the antisymmetric νas(COO) vibrations (Figure 7). However, the absolute intensity of these bands decreases by a factor of 2.5 (Figure 7b). At the same time, new bands appear at 1182, 1120, and 1035 cm-1, which can be assigned to the stretching modes of surface adsorbed sulfate anions.5,29 SO42- in solution, in contrast, gives rise to a band at 1101 cm-1 (Figure 7b, spectrum in the absence of the TiO2 layer). Obviously, SO42- competes with HCOO- for adsorption sites, resulting in a drastic reduction of the concentration of adsorbed formate, whereas it does not lead to a change either of the relative intensities of the νas(COO) bands or of their wavenumber. This points to a nonselective adsorption of SO42onto formate adsorption sites. Additional adsorption experiments have been carried out in pH 1 solutions in order to check the effect of the pH on the formate adsorption. Figure 8 shows the IR spectra of HCOOH aqueous solutions at pH 1 in contact with a layer of rutile nanorods (Figure 8a) and in contact with a nanowire film (Figure 8b). At pH 1, formic acid (pKa ) 3.75) is present in solution exclusively in the protonated form, as shown in Figure 2. All formate bands in Figure 8 may thus be attributed to surface adsorbed species. Obviously, even at pH 1, adsorption of formic acid occurs on TiO2 mainly in the deprotonated form. However, adsorption is much less favorable under these conditions, so that high HCOOH concentrations should be used in order to obtain a reasonable signal-to-noise ratio. This results in a considerable contribution of HCOOH in solution to the overall spectrum (compare with spectra in the absence of TiO2). Nevertheless, the fact that there are no intense bands from solution species in the spectral range between 1650 and 1250 cm-1 makes the detection of bands originating from adsorbed formate feasible. Apart from formate bands and the above-mentioned contribution from solution species, no other bands are observed in Figure 8. However, an additional adsorption of formic acid in the protonated form cannot completely be discarded due to the masking of the wavenumber range above 1650 cm-1 and below 1250 cm-1 by solution HCOOH. In any case, a significant wavenumber shift of ν(CdO) and/or ν(COH) vibrations would be expected in the case of specific adsorption of HCOOH molecules. Such a shift, however, cannot be observed in Figure 8. Furthermore, after replacing the HCOOH solution by the HCOOH-free blank, an immediate disappearance of the bands at 1716 and 1211 cm-1 was observed, whereas the
Figure 8. ATR-IR spectra of aqueous solutions (pH 1, 0.1 M ClO4-) of HCOOH in contact with a layer of rutile nanorods on ZnSe (a) and nanowires on Si (b). The solution spectra in the absence of an oxide layer are shown in both cases for comparison. Background spectra were taken using HCOOH-free blank solutions. ATR crystals: ZnSe, 45° (a); Si, 60° (b). Spectral resolution: 8 cm-1; 50 scans.
bands attributed to adsorbed formate species remained. Such formate bands were removed only after prolonged purging with blank solution. Obviously, HCOOH adsorbs on rutile TiO2 mainly in the deprotonated form. A similar behavior was found for the adsorption of HCOOH on metal surfaces.30,31 In contrast, protonated formic acid adsorption was recently evidenced on WO3 nanoparticles by ATR-IR.32 The formate bands at pH 1 differ slightly from those at pH 3.5. The most significant difference observed at c(HCOOH) ) 100 mM in the case of the nanorod film (Figure 8a) is the absence of the band at 1586 cm-1, whereas a band at 1530 cm-1 and a shoulder at 1537 cm-1 remain unchanged in their wavenumber, although with altered relative intensities. In contrast, for the nanowire film (Figure 8b), a shoulder at 1586 cm-1 is still observed at c(HCOOH) ) 100 mM. By increasing the concentration to 500 mM, the band at 1586 cm-1 can clearly be distinguished, although at a much lower relative intensity than at pH 3.5. The fact that in the case of the Si prism much higher HCOOH concentrations can be used without major contributions from solution species to the overall spectrum makes the detection of a low concentration of surface species, therefore, feasible and highlights one benefit of Si crystals for ATR-IR spectroscopic investigations of the oxide film/electrolyte interface. The pH dependence of the formate adsorption on rutile TiO2 nanorods has been investigated between pH 9 and 3 in a detailed study only recently.18 The formate IR band at 1541 cm-1 was found to shift only slightly over the pH range from 3 to 6. At pH 9, no formate adsorption was observed. In the present study, we have been able to extend the investigation of formate adsorption on TiO2 to highly acidic solutions by using a Si prism. (30) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc. 2003, 125, 3680–3681. (31) Endo, M.; Matsumoto, T.; Kubota, J.; Domen, K.; Hirose, C. J. Phys. Chem. B 2000, 104, 4916–4922. (32) Monllor-Satoca, D.; Borja, L.; Rodes, A.; Go´mez, R.; Salvador, P. ChemPhysChem 2006, 7, 2540–2551.
Formate Adsorption onto Thin Films of Rutile TiO2
The absolute surface concentration as well as the distribution of formate species is obviously governed by a site-dependent acid-base equilibrium of hydroxyl groups on the TiO2 surface. As a result, both the absolute as well as the relative intensities of νas(COO) bands change with pH (Figures 5 and 8). Finally, the presence of the band at 1586 cm-1 at pH 1, i.e. in the absence of solution formate, clearly supports the assignment of species II to adsorbed HCOO-. It is worth mentioning that Si prisms are routinely used in ATR experiments with nanostructured metal films to take advantage of the so-called surface-enhanced IR absorption or SEIRA effect,30 which compensates for the low penetration depth of the evanescent wave. In addition, Si prisms may also be applied to semiconductors, as demonstrated in the present study. ATR spectroscopy can thus be extended to all those films, providing a high inner surface area, which require a substrate of high chemical stability for either film deposition or actual ATR experiments.
IV. Conclusions Two types of nanoporous, pure rutile layers (i.e., nanorod and nanowire thin films) were deposited onto prisms of ZnSe and Si, respectively. The most significant morphological difference of the two films consists in the size of the nanocrystals as well as in their aspect ratio (nanorods, 7 nm × 40 nm; nanowires, 2 nm × >50 nm). The adsorption of formate onto these films was investigated by ATR-IR spectroscopy. At pH 3.5, two distinct adsorbed species (formate species I and II), with similar spectral response are observed on both films. However, the relative concentrations of these species change as a consequence of morphological differences. Species I features a νas(COO) vibration located at 1537-1544 cm-1 and is assigned to a bridging bidentate
Langmuir, Vol. 24, No. 24, 2008 14041
adsorbate, in agreement with previous studies.18 Species II is characterized by a νas(COO) band at 1586-1592 cm-1 and is attributed to formate species adsorbed on edge sites. Even at pH 1, that is, in the absence of solution formate, formic acid adsorbs onto TiO2 in the deprotonated form. However, major differences concerning the absolute and relative formate band intensities are found at this pH. In a more general vein, the potential advantages of using a Si prismatic ATR element instead of that of ZnSe are highlighted: (i) working in highly acidic media is possible due to the high chemical stability of silicon; (ii) such a stability allows for direct chemical bath deposition of TiO2 films from acidic solutions, as illustrated in the case of the nanowire films; (iii) the lower penetration depth of the evanescent wave minimizes the interference from solution species. Obviously, one can take advantage of this if working with films of high inner surface area. Acknowledgment. This work was financially supported by the Spanish Ministry of Education and Science (MEC) through projects CTQ2006-06286 (FONDOS FEDER) and HOPE CSD2007-00007 (Consolider Ingenio 2010). T.B. gratefully acknowledges the support of the Austrian Science Fund (Project J2608-N20). We thank C. Almansa, V. Lo´pez-Belmonte, and A. Bernal for performing the TEM, SEM, and XRD measurements, respectively. Supporting Information Available: Raman spectra of the rutile samples and X-ray diffraction and scanning electron microscopy cross section of the nanowire film. This material is available free of charge via the Internet at http://pubs.acs.org. LA8021326
