Modification of TiO2 semiconductor with molecules bearing functional phosphonic groups: a 31P solid state NMR study

Journal of Materials Processing Technology 161 (2005) 276–281

Modification of TiO2 semiconductor with molecules bearing functional phosphonic groups: a 31P solid state NMR study P. Falarasa,∗ , I.M. Arabatzisa , T. Stergiopoulosa , G. Papavassilioub , M. Karagiannib a b

Institute of Physical Chemistry, NCSR “Demokritos”, 153 10 Aghia Paraskevi Attikis, Athens, Greece Institute of Materials Science, NCSR “Demokritos”, 153 10 Aghia Paraskevi Attikis, Athens, Greece

Abstract In order to advance the work on the characterization of titanium oxide hetero-assemblies formed by surface modification with functional molecular materials, we are engaged in a solid state NMR study of the molecules chemisorption on the surface of the semiconductor. We present here the preliminary results of our study on two model organic compounds, methyl phosphonic acid and phenyl phosphonic acid, which are assembled to TiO2 via the phosphonic acid moiety. We are focusing on the 31 P magnetic behavior, considering the obtained data as a very interesting tool that may lead to the elucidation of the adsorbent/semiconductor interaction and the optimization of the corresponding dye-sensitized solar cells. The above results are further supported by FT-IR experiments on titania powder sensitized by a RuII dye bearing functional phosphonic acid groups. © 2004 Elsevier B.V. All rights reserved. Keywords: Titanium oxide; Surface modification; Functional anchoring groups; Phosphonic acids; 31 P solid state NMR; Dyes

1. Introduction Surface modification of nanostructured semiconducting materials by Ru-polypyridyl complexes is under extensive investigation for practical applications including molecular photovoltaic cells [1,2] and hetero-supramolecular photoelectrochromic devices [3]. Considerable interest has been recently shown in studying the photosensitization of large bandgap semiconductors, in order to achieve efficient solar to electrical energy conversion. The sensitized electrode usually consists of a thin layer of semiconducting oxide (i.e., TiO2 ), properly modified by chemical adsorption of a suitable dye monolayer. The cell performance both depends on the semiconductor and on the dye properties and good operation implies an intimate contact between them, especially characterized by the binding ability of the dye. Such a close interaction promotes electronic communication between the donor or∗

Corresponding author. Tel.: +30 210 6503644; fax: +30 210 6511766. E-mail address: [email protected] (P. Falaras).

0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.07.036

bitals of the dye and the acceptor orbitals of the semiconductor and controls the interfacial electron injection rates to the semiconductor conduction band and therefore the overall cell efficiency. Important effort has been spent to design and use of inorganic complexes with ligands that allow strong adsorption on the semiconductor. The key step in the synthesis of these ligands is the introduction of functional groups for effective attachment [4]. A number of Ru-polypyridyl sensitizers with different anchoring moieties have been prepared. Small energy conversion efficiencies have been obtained by using bpy complexes with sulphonic acid groups as anchor units [5], indicating weak adsorption properties of the sulphonic group on TiO2 . On the contrary, very efficient sensitization of titanium oxide thin films has been achieved by using Rupolypyridyl complexes bearing carboxylic acid groups [6]. However, new polypyridyl ruthenium sensitizers containing phosphonic acid as adsorbent groups [7–9] have been investigated in order to achieve better electron transfer efficiency and long-term stability of the cell. A lot of interest has been

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given in the literature at the semiconductor interface and especially on the study and elucidation of the chemical nature and strength of molecular dye-oxide bonding. Although the formation of an ester-like linkage between carboxylic acid groups of the corresponding bipyridyl ligands and TiO2 microcrystallites is now well established [4,10,11], there is a lack of experimental data on the kind of chemisorption of dyes bearing phosphonic acid functional groups. Much stronger (ca. 80 times) surface adhesion than that obtained for Ru(dcbpy)2 (NCS)2 complex as well as a more efficientcharge transfer sensitization of nanocrystalline TiO2 were observed with a ruthenium terpyridine complex involving the phosphonic acid bridging group [7]. The phosphonic acid terpy complex, in contrast to the carboxylate analogues, does not desorb from the surface of the nanocrystalline TiO2 film when it is exposed to water. Furthermore, it has been demonstrated that the use of phosphonic acid groups to link the ruthenium complex component to the surface of the semiconductor prevents their desorption at any applied potential [3]. A wide range of techniques has been used to study the fundamental properties of the semiconductor interface [12]. Resonance Raman spectroscopy has shown the role of PO3 terpy as the bringing ligand [13] but this surface modification needs to be further investigated. In order to advance the work on the characterization of photosensitized solar cells, we are engaged in a solid state NMR study of the dye chemisorption on the surface of the semiconductor. As the phosphorus (31 P) nucleus gives very strong peaks in NMR, we believe that it would be very interesting to study the 31 P NMR spectrum of molecules adsorbed on TiO2 , expecting to find significant shifts compared to the spectrum of the free ligand. Simple phosphonic acids from methyl to cetyl react with the TiO2 surface [14]. In fact, the dye itself is a very big and robust molecule with a special structure and particular space orientation, which may influence its real concentration on the semiconductor surface. As we work with powders, it is understandable that only the Ti nuclei situated on the outer surface of the grains “react” with the complex. Therefore, simple anchoring moieties must be first studied in order to obtain significant and unambiguous results, which in turn will be used to compare the corresponding behavior of the real dyes. In this work, we studied the surface modification of titanium oxide by using two simple phosphonic acid derivatives (methyl- and phenyl-phosphonic acids) as model compounds. Chemical attachment of these organic molecules on titanium oxide is generally expected to produce significant changes in the molecule and especially in the environment of the 31 P nucleus, which can be detected by solid state NMR. For that reason the work was focused on the 31 P NMR lineshape study. We present here the preliminary results of our study, considering these data as a very interesting tool, which may lead to the elucidation of the dye/semiconductor interaction and therefore to the optimization of the sensitization process via efficient chemical binding by functional anchoring groups.

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2. Experimental 2.1. Reagents (chemicals) Methylphosphonic acid [CH3 P(O)(OH)2 ] and phenylphosphonic acid [C6 H5 P(O)(OH)2 ] were of reagent grade (Aldrich Chem. Co.) and were used without further purification. The dye, [(2,2 :6 ,2 -terpyridine-4 -iodophenyl)(2,2 : 6 ,2 -terpyridine-4 -phenylphosphonic-acid)-ruthenium(II)] dichloride [[RuII -(terpy-C6 H5 I) (terpy-C6 H5 PO3 H2 ](Cl)2 ] (1) was available from a previous work. TiO2 powder (Degussa P25, a mixture of ca. 75% anatase and 25% rutile) was used as received. 2.2. TiO2 modification Coating of the TiO2 with phosphonic acids as well as with the dye was carried out by soaking 0.5 g of titanium oxide powder into a saturated ethanolic solution of the adsorbents and the resulting colloidal mixture was stirred for 24 h. The TiO2 powder was heat-treated for 30 min at 120 ◦ C before immersion, in order to eliminate physisorbed water. After completion of the acid adsorption the modified powder was filtered, thoroughly rinsed with ethanol and dried in air. 2.3. NMR characterisation 31 P

NMR measurements were performed on powder samples of methylphosphonic acid [CH3 P(O)(OH)2 ] and phenylphosphonic acid [C6 H5 P(O)(OH)2 ] as well as on titanium dioxide powders chemically modified by the above two phosphonic acids, using a Bruker MSL-200 spectrometer with a superconducting magnet of 4.7 T. An Oxford CF1200 continuous flow cryostat was used for measurements below room temperature. The time domain signal was acquired with the Hahn spin-echo pulse sequence ((π/2) − τ − π). This technique was applied for both lineshape and spin-spin relaxation time (T2 ) measurements, whereas for the spin-lattice relaxation time (T1 ) measurements the “saturation recovery” pulse sequence ((π/2) − t − (π/2) − τ − π) was employed. In order to eliminate unwanted ghost signals due to amplitude and phase errors of the quadrature detection we used the “cyclops” phase cycling technique [15]. The frequency spectrum was obtained after a Fourier transformation of the time domain signal performed on the Aspect-3000 computer of the MSL spectrometer. Fourier-transform IR measurements were carried out using a Perkin-Elmer 1600 spectrometer, the sample being in the form of KBr pellets.

3. Results and discussion The 31 P spectra of the investigated samples at room temperature are shown in Fig. 1. The 31 P NMR resonance frequency was 81.022 MHz for the methylphosphonic acid and

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Fig. 1. 31 P NMR lineshapes at room temperature (293 K) and at T = 140 K of (a) CH3 P(O)(OH) (· · ·) and C6 H5 P(O)(OH)2 (–) acids, (b) CH3 P(O)(OH) acid before (· · ·) and after (c) adsorption on TiO2 , and (c) C6 H5 P(O)(OH)2 acid before (· · ·) and after (–) adsorption on TiO2 .

81.019 MHz for the phenylphosphonic acid. The 31 P nucleus has spin I = 1/2 and in a NMR experiment the lineshape is mainly determined by the chemical shift which probes the electronic distribution around the nucleus (The P–P dipolar interaction is considered to be negligible due to large P–P intermolecular distances). The interaction of the nucleus with the surrounding electrons has the effect of shifting the resonance relative to the “bare” nucleus by an amount of −γσH0 , where H0 is the external magnetic field, γ is the gyromagnetic ratio of the nucleus and σ is a second rank tensor known as the chemical shift tensor. This relative frequency shift depends on the electronic distribution around the nucleus and generally σ is anisotropic. The screening tensor includes also contributions from the induced magnetic moments in nearby atoms. Any change in the electronic environment of the nucleus when the molecule undergoes some chemical transformation is thus manifested as a change in the NMR parameters. The 31 P NMR lines of the phosphonic acids methylphosphonic [CH3 P(O)(OH)2 , abbreviated as MPA] and phenylphosphonic [C6 H5 P(O)(OH)2 , abbreviated as PPA] at room temperature and at 140 K are shown in Fig. 1a. It is observed that both lines are Lorentzian at room temperature with a

full width at half intensity (FWHH) of ≈0.27 kHz for MPA and ≈1.1 kHz for PPA. These lineshapes may be attributed to rapid molecular reorientation, which leads to a motionally averaged chemical shift anisotropy [16]. As a result the observed spectra exhibit the motionally averaged Lorentzian shape. The significantly narrower width of the methyl-31 P NMR line indicates that the rate of reorientation at room temperature is much higher for the methyl than for the phenyl group. We may also observe a small positive (downfield) shift (≈2.26 ppm) of the 31 P peak in PPA relative to the corresponding peak in MPA, which may be attributed to the delocalized ␲ electrons in the phenyl ring. The 31 P NMR spectra of CH3 P(O)(OH)2 and C6 H5 P(O)(OH)2 molecules adsorbed on TiO2 at room temperature and at 140 K are shown in Fig. 1b and c respectively. For comparison each spectrum is shown together with the spectrum of the corresponding free acid. At room temperature, the 31 P peak of chemisorbed PPA shows a significant negative (upfield) shift of ≈18 ppm relative to the corresponding peak of free PPA. A similar behavior is also observed in MPA, but the corresponding shift is much smaller (of the order of ≈3 ppm). In the literature an upfield shift of

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the 31 P NMR line is also reported upon coordination of the terpy-P(O)(OH)2 ligand to ruthenium (Ru) in the case of the Ru[terpy-P(O)(OH)2 ](Me2 bpy)(NCS) sensitizer [8]. In addition, at room temperature the 31 P lines of the chemisorbed molecules are significantly broader than the corresponding lines of the free acid molecules as can be easily seen in Fig. 1b and c. Particularly, the FWHH for the CH3 P(O)(OH)2 /TiO2 is about twice the FWHH for the free ligand (i.e., ≈0.492 kHz), whereas for C6 H5 P(O)(OH)2 /TiO2 the FWHH amounts to ≈4.6 kHz, i.e., approximately four times larger than the width of the resonance line of the corresponding free ligand. This may indicate that at room temperature the rotation of the P(O)(OH)2 groups is restricted upon chemisorption on TiO2 . Since the rotation of molecular groups close to 31 P affects its chemical shift anisotropy the effect is less pronounced in the CH3 P(O)(OH)2 /TiO2 molecule where the CH3 - groups rotate at very high rates. The presence of rapid molecular reorientations in the phosphonic acids at intermediate temperatures (300–200 K) is fur-

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ther supported by the observed broadening of the NMR lines as the temperature is lowered. This behavior is clearly seen in Fig. 2. Fig. 2a shows the 31 P NMR lines in MPA as a function of temperature. A similar broadening of the P line shape is observed in all of the four investigated samples. The temperature dependence of the FWHH of the absorption lines is shown in Fig. 2b. It is observed that in the free acids, the linewidth exhibits a step-like increase as the temperature is lowered. This behavior is generally attributed to internal motions (usually rotations), characterized by a correlation time τ c , which has a temperature dependence similar to that observed for the linewidth [17]. In particular, the high temperature plateau corresponds to the so-called “motional narrowing” regime where the correlation time describing the rate of molecular rotations is small and consequently the resulting NMR line is narrow. As the temperature decreases, internal motions slow down and the line broadens. The linewidth reaches its rigid lattice value at temperatures below 230 K for MPA and 200 K for PPA. In this low temperature regime, rotational motions

Fig. 2. (a) 31 P NMR line shapes of the methylphosphonic acid (MPA) as a function of temperature. (b) Temperature dependence of the full width at half intensity (FWHH) of the 31 P NMR lines.

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are described by a large correlation time so that the lattice may be considered as practically rigid. In the chemisorbed acids on the other hand, a continuous broadening of the 31 P lines is observed as the temperature is lowered. Since, as stated above, rotational motions become restricted upon adsorption, the measured values of the linewidth in the temperature range between room temperature and 200 K correspond to the intermediate region between the two plateaux, where τ c is neither large nor small. Below 200 K the linewidths reach their rigid lattice values, which however are lower than the rigid lattice values in the free acids. This is also shown in Fig. 1b and c where it is observed that at 140 K the 31 P lines of the chemisorbed acids become narrower than the corresponding lines of the free acids. This may be considered as an indication that chemical attachment to TiO2 induces some kind of partial averaging of the P chemical shielding tensor. Adsorption of the phosphonic acid moieties on TiO2 is also manifested in the asymmetry of the corresponding 31 P NMR lines. Indeed, in the adsorbed compounds the observed lines (which are the unresolved powder patterns of P) are asymmetric as can be seen in Fig. 1, whereas in the free acids the corresponding lines are symmetric. At room temperature the 31 P absorption line of C6 H5 P(O)(OH)2 /TiO2 exhibits the characteristic features of a powder lineshape in case of an axially symmetric chemical shift tensor. Since the above results indicate that rotational motions are restricted in the adsorbed compounds, it seems that the axial symmetry observed in the C6 H5 P(O)(OH)2 /TiO2 31 P powder spectrum reflects an inherent axial symmetry of the 31 P screening tensor. It is worth to mention that the asymmetry of the 31 P NMR line depends on the ligand CH3 or C6 H5 , which is attached to P(O)(OH)2 . In particular, in C6 H5 P(O)(OH)2 /TiO2 the 31 P line has a high frequency asymmetric tail, while in CH3 P(O)(OH)2 /TiO2 a low frequency asymmetric tail is observed. The experimental results presented above demonstrate that adsorption of the phosphonic acids on TiO2 is manifested through changes in the NMR parameters of P nucleus. In particular, the lineshape studies have shown that chemical bonding of the phosphonic moieties to TiO2 restricts the reorientational motions of PPA and MPA units. In addition, preliminary studies of the 31 P spin-lattice relaxation time T1 in PPA have shown that it increases by three orders of magnitude upon chemisorption. The measured values of T1 at room temperature were ≈8 ms for the free ligand and ≈6 s for the adsorbed molecule (it is worth to mention that in the free PPA the 31 P spin-lattice relaxation time T1 increases from 8 ms at room temperature to 91 ms at T = 200 K and reaches a value of 2.5 s at T = 140 K). The bonding of the phosphonic acid moieties to TiO2 is also reflected in the chemical shift anisotropy of P nucleus, which moreover, was observed to be sensitive to the specific ligand attached to P(O)(OH)2 . In order to relate the above solid-state NMR study to the real sensitizer/semiconductor interaction, the vibrational properties of a dye bearing phosphonic acid functional groups before and after adsorption on titanium oxide were investi-

gated by FT-IR spectroscopy. It is reasonable to suppose that the surface derivatization proceeds by chemical adsorption of the dye on the semiconductor via the phosphonic acid group. FT-IR spectroscopy allows identifying the ␯ (P O) vibration of the complex from the tree bands at 1240, 1138 and 975 cm−1 , Fig. 3a. A comparison with the corresponding spectrum of the derivatized TiO2 powder, Fig. 3b, shows significant differences on the ␯ (P O) region. Especially, the second vibration peak shifts from 1138 in the free complex to 1143 cm−1 for the chemisorbed dye. The above results permit us to confirm that the surface modification of the titanium oxide by the RuII sensitizer occurs via P O Ti bonds. Such a surface bonding could probably result from a maximum overlapping and subsequent strong electron coupling between the Ti 3d orbitals of the semiconductor and the ␲* orbitals of the PO3 -phenyl-terpy ligand. These results are consistent with the model proposed by Gratzel and co-workers in Ref. [8] and indicate the creation of a surface entity involving P O Ti bonds. Such a hetero-molecular formation is a general approach and readily promotes the use of dyes bearing phosphonic acid functional groups on the azoaromatic ligands. This results in dye stability and strong and intimate contact between the dye and the semiconductor. The obtained results fully justify the continuation of the solid state NMR research with real sensitizers anchored on TiO2 via phosphonic acid functional groups. The general idea relies on the possibility to observe how the magnetic properties and especially the relaxation kinetics of both the semiconductor (47 Ti and 49 Ti) and the dye (31 P) nuclei are affected by the presence of the chemisorbed complex, excited or not. Works are currently in progress with the use of one of the most efficient dyes, the Ru[terpy-P(O)(HO)2 ](dmbpy)(NCS) complex and the introduction of a laser light fiber optic. The

Fig. 3. FT-IR spectra in the region of the ␯ (P O) vibration band for the dye 1 (a) bearing phosphonic acid functional groups and for the chemisorbed dye on TiO2 (b).

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first results are very encouraging and could help us in deeply characterizing the dye/oxide interaction and thus to better understand the photosensitization mechanism.

Acknowledgements Financial support from NCSR Demokritos (“Dimoerevna” project) is greatly acknowledged.

References [1] U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weisso, J. Salbeck, H. Spreitzer, M. Gratzel, Nature 395 (1998) 583. [2] R. Hoyle, J. Sotomayor, G. Will, D. Fitzmaurice, J. Phys. Chem. 101 (1997) 10791. [3] J. Sotomayor, G. Will, D. Fitzmaurice, J. Mater. Chem. 10 (2000) 685. [4] P. Falaras, Solar energy mater, Solar Cells 53 (1998) 163. [5] O. Schwartz, D. van Loyen, S. Jockush, N. Turro, H. Durr, Photochem. Photobiol. A: Chem. 132 (2000) 91. [6] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E.M. Uller, P. Liska, N. Vlachopoulos, M. Gratzel, J. Am. Chem. Soc. 115 (1993) 6382.

281

[7] P. Pechy, F.P. Rotzinger, M.K. Nazeeruddin, O. Kohle, S.M. Zakeeruddin, R. Humphry-Baker, M. Gratzel, J. Chem. Soc., Chem. Commun. (1995) 65. [8] S.M. Zakeeruddin, M.K. Nazeeruddin, P. Pechy, F.P. Rotzinger, R. Humphry-Baker, K. Kalyanasundaram, M. Gratzel, V. Shklover, T. Haibach, Inorg. Chem. 36 (1997) 5937. [9] B. Jing, H. Zhang, M. Zhang, Z. Lu, T. Shen, J. Mater. Chem. 8 (1998) 2055. [10] K. Murakoshi, G. Kano, Y. Wada, S. Yanagida, H. Miyazaki, M. Matsumoto, S. Murasawa, J. Electroanal. Chem. 396 (1995) 27. [11] T.J. Meyer, G.J. Meyer, B.W. Pfennig, J.R. Schoonover, C.J. Simpson, J.F. Wall, C. Kobusch, Y. Chen, B.M. Peek, C.G. Wald, W. Ou, B.W. Erickson, C.A. Bignozzi, Inorg. Chem. 33 (1994) 3952. [12] J. Sotomayor, R. Hoyle, G. Will, D. Fitzmaurice, J. Mater. Chem. 8 (1998) 105. [13] A. Hugot- Le Goff, S. Joiret, P. Falaras, J. Phys. Chem. B 103 (1999) 9569. [14] I. Lukes, M. Kalbac, 33rd Heyrovsky Discusions, Poster Presentation, Trest Castle, Czech Republic, 2000. [15] G. Bodenhausen, R. Freeman, D.L. Turner, J. Magn. Reson. 27 (1977) 511. [16] M. Mehring, NMR Basic Principles and Progress, Springer-Verlag, Berlin, 1976. [17] A. Abragam, The Principles of Nuclear Magnetism, Oxford University Press, London, 1961.