Sorption of Uranyl Cations on a Rutile - American Chemical Society

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Langmuir 2006, 22, 140-147

Sorption of Uranyl Cations on a Rutile (001) Single Crystal Monitored by Surface Second-Harmonic Generation Manuel Dossot,*,† Se´bastien Cremel,† Johan Vandenborre,‡ Je´roˆme Grausem,† Bernard Humbert,† Romuald Drot,‡ and Eric Simoni‡ Laboratoire de Chimie Physique et Microbiologie pour l’EnVironnement LCPME, UMR CNRS-UHP 7564, 405 rue de VandoeuVre, F-54600 Villers-Les-Nancy, France, and Institut de Physique Nucle´ aire IPN, UniVersite´ Paris XI, 15 rue Georges Clemenceau, F-91406 Orsay, France ReceiVed July 22, 2005. In Final Form: October 27, 2005 The rotational anisotropy of second-harmonic generation at the surface of a (001) single-crystal rutile is obtained in the presence of uranyl cations sorbed at the surface from acidic solutions at various concentrations. Surface secondharmonic generation appears to be sensitive to the presence of uranyl cations on the rutile samples. Evolution of the anisotropy pattern with initial uranyl concentration is analyzed through a phenomenological model. The elements obtained for the nonlinear susceptibility tensor χ(2) for each sample significantly constrain the geometry of the possible sorption complexes between uranyl cations and rutile and lead to the proposition of two sorption sites involving different oxygen atoms of the rutile surface.

Introduction Evaluation of the mobility of radionuclides in sediments and soils is crucial for long-term risk assessment analysis of nuclear waste disposal and for remediation of contaminated areas. Migration is a dynamic process that involves radionuclide partitioning between the mobile aqueous phase and solid phase through, for example, adsorption/absorption, solubility, or percolation steps.1 Retention time is influenced by the physicochemical properties and the chemical speciation of radionuclides in the aqueous phase as well as the mineral and organic composition of soils and sediments.2-7 By adopting reductionism, a part of this complex problem can be addressed by choosing a representative radionuclide and studying its physicochemical behavior at the interface between the aqueous phase and a reference mineral surface. Information should be collected at both the macroscopic and the molecular levels to understand the factors that impact interfacial reaction efficiency, for instance, pH, redox potential and ionic strength of the aqueous phase, speciation of the radionuclide, and acidity of the sorption sites and their distribution on exposed crystallographic faces. In the present study, uranium was selected as a radionuclide of environmental interest: it has a long half-life and remains stable in the ecosphere at various oxidation states, including uranyl species UO22+.2,3 To understand uranyl adsorption behavior toward a mineral surface, rutile TiO2 was selected as a reference oxide with surface chemistry that has been extensively studied.8 * Corresponding author. E-mail: [email protected]. Phone: (+33) 3 83 68 52 49 Fax: (+33) 3 83 27 54 44. † UMR CNRS-UHP 7564. ‡ Universite ´ Paris XI. (1) Wang, P.; Anderko, A.; Turner, D. R. Ind. Eng. Chem. Res. 2001, 40, 4428. (2) Sylwester, E. R.; Hudson, E. A.; Allen, P. G. Geochim. Cosmochim. Acta 2000, 64, 2431. (3) Casas, I.; De Pablo, J.; Pe´rez, I.; Gime´nez, J.; Duro, L.; Bruno, J. EnViron. Sci. Technol. 2004, 38, 3310. (4) Kowal-Fouchard, A.; Drot, R.; Simoni, E.; Ehrhardt, J.-J. EnViron. Sci. Technol. 2004, 38, 1399. (5) Lomenech, C.; Simoni, E.; Drot, R.; Ehrhardt, J.-J.; Mielczarski, J. J. Colloid Interface Sci. 2003, 261, 221. (6) Ordon˜ez-Regil, E.; Drot, R.; Simoni, E.; Ehrhardt, J.-J. Langmuir 2002, 18, 7977. (7) Drot, R.; Simoni, E. Langmuir 1999, 15, 4820. (8) Diebold, U. Surf. Sci. Rep. 2003, 48, 53.

A great number of studies has been devoted to the sorption of organic or inorganic pollutants on TiO2 single-crystalline surfaces or powders using an aqueous or gaseous mobile phase and various methods of investigation such as temperature-programmed desorption (TPD)9 and high-resolution electron energy loss spectroscopy (HREELS),10 X-ray photoelectron spectroscopy (XPS),11 X-ray standing wave (XSW) spectroscopy,12 scanning tunneling microscopy (STM),13 and atomic force microscopy (AFM)14. Concerning the (001) crystallographic plane of rutile, STM studies have brought interesting knowledge at the microscopic level by studying the reconstruction of the surface under various experimental conditions.13,15 The thermodynamic stability of some crystallographic faces was calculated, and theoretical work managed to obtain a quite satisfying description of rutile electronic states and optical properties for (110) and (001) faces.16-21 An extensive review of the literature8 indicates that the present knowledge on rutile single crystals mainly concerns the (110) face, certainly because (i) it is the more stable face from thermodynamic data and less subject to important reconstruction phenomena,8,18-20 (ii) it constitutes the major face exposed to the aqueous phase in rutile powders,8 and (iii) its photochemical properties are promising in a wide field of technical (9) Linsebigler, A.; Guangquan, L.; Yates, J. T. J. Chem. Phys. 1995, 103, 9438. (10) Henderson, M. A. Surf. Sci. 1996, 355, 151. (11) Bullock, E. L.; Patthey, L.; Steinemann, S. G. Surf. Sci. 1996, 352-354, 504. (12) Zhang, Z.; Fenter, P.; Cheng, L.; Sturchio, N. C.; Bedzyk, M. J.; Pøedota, M.; Bandura, A.; Kubicki, J. D.; Lvov, S. N.; Cummings, P. T.; Chialvo, A. A.; Ridley, M. K.; Be´ne´zeth, P.; Anovitz, L.; Palmer, D. A.; Machesky, M. L.; Wesolowski, D. J. Langmuir 2004, 20, 4954. (13) Tero, R.; Fukui, K.; Iwasawa, Y. J. Phys. Chem. B 2003, 107, 3207. (14) Sasahara, A.; Kitamura, S.; Uetsuka, H.; Onishi, H. J. Phys. Chem. B 2004, 108, 15735. (15) No¨renberg, H.; Dinelli, F.; Briggs, G. A. D. Surf. Sci. 1999, 436, L635. (16) Sano, H.; Mizutani, G.; Wolf, W.; Podloucky, R. Phys. ReV. B 2004, 70, 125411. (17) Pøedota, M.; Bandura, A. V.; Cummings, P. T.; Kubicki, J. D.; Wesolowski, D. J.; Chialvo, A. A.; Machesky, M. L. J. Phys. Chem. B 2004, 108, 12049. (18) Muscat, J.; Harrison, N. M. Surf. Sci. 2000, 446, 119. (19) Stashans, A.; Lunell, S.; Grimes, R. W. J. Phys. Chem. Solids 1996, 57, 1293. (20) Purton, J.; Bullett, D. W.; Oliver, P. M.; Parker, S. C. Surf. Sci. 1995, 336, 166. (21) Glassford, K. M.; Chelikowski, J. R. Phys. ReV. B 1992, 45, 3874.

10.1021/la0519913 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/03/2005

Sorption of Uranyl Cations on a Rutile Crystal

applications.8 However, the (001) face is also interesting to study because its reactivity in electrochemical or photochemical processes is sometimes higher than that of the (110) face8 and the precise knowledge of its physicochemical features is still demanding. Except for a few papers that report the interaction of uranyl cations with hydrous titanium dioxide,22,23 uranyl adsorption on rutile is a recent subject of investigation.24,25a A previous study dealt with the adsorption of uranyl on polycrystalline or (001) and (110) crystallographic planes of rutile studied by grazing X-ray absorption spectroscopy (G-XAS).25a G-XAS measurements have indicated that the uranyl rod is parallel to the surface for both planes. For the (110) face, uranyl cations formed an inner-sphere bidentate complex with the surface oxygen atoms of rutile. For the (001) face, the situation was not clear because of a low signal-to-noise ratio, and definitive conclusion could not be achieved. The reactivity difference between the two crystallographic planes was thus not fully explained by G-XAS measurements.25a A new experimental method is required to investigate the sorption behavior of UO22+ on the rutile (001) crystallographic plane. Surface second-harmonic generation (SSHG) is a nonlinear optical method of growing interest in surface chemistry.26-32 SSHG is very sensitive to surface symmetry change due to molecule or ion sorption on oriented crystallographic surfaces.33-36 The SSHG is null in centrosymmetric media such as rutile crystal that belongs to the centrosymmetric P42/mnm space group.8 In the electric dipole approximation, the SSHG signal would come only from the rutile surface where centrosymmetry is broken and uranyl cations are sorbed. Consequently, SSHG should be inherently surface-specific and should interestingly supplement G-XAS. SSHG was applied to the rutile/water interface for (110) and (001) crystallographic planes.37-40 The second-order nonlinear response of the clean surface was interpreted by a theoretical model coupled to ab initio calculations,16,37 and the results obtained convince us to use rutile single-crystals as reference oxide surfaces for uranyl sorption. The present article thus reports the first study of uranyl sorption on the (001) crystallographic plane of a rutile single crystal by SSHG with the aim of obtaining a deeper understanding of the binding modes of uranyl that were (22) Lieser, K. H.; Thybusch, B. Fresenius’ Z. Anal. Chem. 1988, 332, 351. (23) Gupta, A. R.; Venkataramani, B. Bull. Chem. Soc. Jpn. 1988, 61, 1357. (24) Zhijun, G.; Zhaoyun, Y.; Zuyi, T. J. Radioanal. Nucl. Chem. 2004, 261, 157. (25) (a) Den Auwer, C.; Drot, R.; Simoni, E.; Conradson, S. D.; Gailhanou, M.; Mustre de Leon, J. New J. Chem. 2003, 27, 648. (b) Vandenborre, J. unpublished results. (26) Corn, R. M.; Higgins, D. A. Chem. ReV. 1994, 94, 107. (27) Williams, C. T.; Beattie, D. A. Surf. Sci. 2002, 500, 545. (28) Fitts, J. P.; Shang, X.; Flynn, G. W.; Heinz, T. F.; Eisenthal, K. B. J. Phys. Chem. B 2005, 109, 7981. (29) Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. J. Phys. Chem. B 1998, 102, 6331. (30) Al-Abadleh, H. A.; Mifflin, A. L.; Bertin, P. A.; Nguyen, S. T.; Geiger, F. M. J. Phys. Chem. B 2005, 109, 9691. (31) Konek, C. T.; Musorrafiti, M. J.; Al-Abadleh, H. A.; Bertin, P. A.; Nguyen, S. T.; Geiger, F. M J. Am. Chem. Soc. 2004, 126, 11754. (32) Stack, A. G.; Higgins, S. R.; Eggleston, C. M. Geochim. Cosmochim. Acta 2001, 65, 3055. (33) Andersson, S. K.; Schanne-Klein, M. C.; Hache, F. Phys. ReV. B 1999, 59, 3210. (34) Schwab, C.; Meister, G.; Woll, J.; Gerlach, A.; Goldmann, A. Surf. Sci. 2000, 457, 273. (35) Yamada, C.; Kimura, T. Phys. ReV. B 1994, 49, 14372. (36) Yamada, C.; Kimura, T. Phys. ReV. Lett. 1993, 70, 2344. (37) Omote, M.; Kitaoka, H.; Kobayashi, E.; Suzuki, O.; Aratake, K.; Sano, H.: Mizutani, G.; Wolf, W.; Podloucky R. J. Phys.: Condens. Matter 2005, 17, S175. (38) Kobayashi, E.; Matsuda, K.; Mizutani, G.; Ushioda, S. Surf. Sci. 1999, 427-428, 294. (39) Kobayashi, E.; Wakasugi, T.; Mizutani, G., Ushioda, S. Surf. Sci. 1998, 402-404, 537. (40) Kobayashi, E.; Mizutani, G.; Ushioda, S. Jpn. J. Appl. Phys. 1997, 36, 7250.

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Figure 1. Rutile (001) hydroxylated crystallographic plane. Ti(t) labels pentacoordinated titanium atoms, and Ti(o) labels titanium atoms in octahedral sites. O(t) stands for oxygen atoms linked to Ti(t), whereas O(b) denotes bridging oxygen atoms that are linked to one Ti(t) and one Ti(o).

not attained by G-XAS measurements.25a This work performed on the (001) plane is also envisaged as a test of the validity of SSHG spectroscopy to investigate the sorption of uranyl onto rutile surfaces before turning to the other crystallographic planes that are more exposed to solution in the rutile powder. Experimental Section Protocol for Uranyl Sorption on the Rutile (001) Single Crystal and Labeling of Atoms on the Surface. Rutile single crystals were purchased from CERAC. The sample dimensions were 10 mm × 10 mm × 1 mm thick. The uranium (VI) solutions were prepared by dissolving solid uranyl nitrate in a 0.1 mole L-1 NaClO4 solution previously acidified with HClO4 to avoid cation hydrolysis. Initial uranyl concentrations in solution were measured by R-liquid scintillation using a Tri-Carb 2700TR Packard spectrometer. The sorption protocol has been previously described.25a Briefly, the rutile single crystal was placed in contact with 2 mL of uranyl solution at pH 3 for 24 h. Three initial uranyl concentrations were studied: 10-7, 10-4, and 10-2 mole L-1. The samples were washed with distilled water and air dried. One crystal was hydrated by 24 h contact time with a pH 3 solution and no uranyl to constitute a blank sample. As previously explained,25a uranyl adsorption did not lead to site saturation for 10-7 and 10-4 mole L-1 uranyl solutions, and it is suspected that site saturation is not reached even at 10-2 mole L-1.25b Figure 1 illustrates the atomic arrangement on the (001) crystallographic plane of rutile after hydration of the surface but without reconstruction. Some titanium atoms are in octaedric sites, and they have been labeled as Ti(o). The truncation of the bulk and further hydroxylation also lead to pentacoordinated titanium atoms labeled as Ti(t). Figure 1 indicates only the oxygen atoms of the hydroxyl groups and labels them as O(t). Finally, the oxygen atoms that are linked to Ti(o) and Ti(t) are noted as O(b) for “bridging oxygens”. Surface Second-Harmonic Generation (SSHG). The experimental setup for performing SSHG measurements is indicated in Figure 2. The 1064 nm radiation of a pulsed Nd:YAG laser (pulse duration 5 ns, energy 600 mJ/pulse/cm2, pulse repetition 20 Hz) is reflected on a BK7 blade M set at the Brewter angle to purify the beam polarization. Next, it is sent through lenses using an inverse galilean setup to reduce the beam diameter from 9 to 3 mm. This beam passes through a Glan polarizer to control the polarization of the beam and through a hot filter that rejects the possible 532 nm harmonic wave generated by the optics but lets the IR beam be sent to the sample by a gold-coated mirror. The sample is irradiated with an energy of around 50 mJ/pulse/cm2, and no sample degradation due to any laser damage was observed. The 532 nm harmonic wave generated by the sample is sent by a mirror through a cold filter that rejects the residual 1064 nm radiation and then is analyzed through a second Glan polarizer and collected by a photomultiplier tube (Hamamatsu R928). The signal is visualized with a transient digitizer

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Figure 2. SSHG experimental setup. oscilloscope (Tektronix TDS2024) and treated by a boxcar to increase the signal-to-noise ratio. The resulting signal is acquired by a personal computer, and homemade software allows the determination of the SSHG intensity. Each signal corresponds to an average of at least 256 pulses that smooth the slight variations in laser intensity from one pulse to another. The sample is fixed on a horizontal (x, y) translational stage that allows us to select the laser-irradiated area and on a vertical z-axis rotational stage that allows us to perform the SSHG rotational anisotropy measurement. The polarization of the fundamental 1064 nm beam and the polarization of the harmonic 532 nm beam were set parallel (p) to the plane of incidence. Indeed, the SSHG signal was too low to be adequately measured in the other possible combinations (namely, p/s, s/p, and s/s polarization for fundamental/ harmonic beams). The intensity of the SSHG signal was plotted versus the z-axis rotational angle in polar coordinates to unravel the symmetry of the sample surface. Phenomenological Three-Layer Model for C1W Surface Symmetry. The generation of the surface second-harmonic wave at the uranyl/rutile interface can be theoretically analyzed using the phenomenological three-layer model.41,42 The physical origin of second-harmonic generation at an interface can be separated into two distinct contributions. The first one is an electrical dipolar contribution that comes from the centrosymmetry break that occurs at the boundary between the crystal bulk and the upper medium, which is the atmosphere in our case. This SH contribution is purely surface and local. Second, a nonlocal contribution can arise from the bulk because of either electrical quadrupole terms or gradients observed at the boundary for the electromagnetic field and the nonlinear electrical susceptibility.41-46 Depending on the working conditions and the samples, the nonlocal contribution can be as important as the local one, and it is sometimes very difficult to separate them.41,44 Concerning rutile single-crystalline surfaces, it has been shown that the electric dipole contribution is mainly responsible for the SH generation when the incident fundamental wavelength is lower than 800 nm.37 In that case, electronic resonant SHG was observed, exalting the dipolar contribution. However, to (41) Brevet, P. F. J. Chem. Soc., Faraday Trans. 1996, 92, 4547. (42) Brevet, P. F. "Surface Second Harmonic Generation", Presses Polytechniques et Universitaires Romandes, 1997, Lausanne. (43) Sipe, J. E.; Moss, D. J.; van Driel, H. M. Phys. ReV. B 1987, 35, 1129. (44) Guyot-Sionnest, P.; Shen, Y. R. Phys. ReV. B 1988, 38, 7985. (45) Guyot-Sionnest, P.; Chen, W.; Shen, Y. R. Phys. ReV. B 1986, 33, 8254. (46) Lu¨pke, G. Surf. Sci. Rep. 1999, 35, 75.

the best of our knowledge, no data are available at 1064 nm for the (001) face. Nevertheless, the phenomenological model can be treated in the electric dipole approximation assuming that the second-order electric susceptibility tensor χ(2) that describes the formation of the second-harmonic wave at the rutile sample is an effective surface tensor.41,42 Because our experimental results will show a marked change in the SSHG rotational anisotropy pattern with uranyl sorption and because we are interested in obtaining information on the evolution of the effective tensor elements with uranyl sorption, this assumption is fully justified. Rigorously, the second-order suscep(2) tibility tensor should be written as χs,eff under this framework, but

for the sake of clarity, we will use the simplified notation χ(2). Figure 3 indicates the axis frames used in the model. The frame (x1, y1, z) is the laboratory frame and corresponds to the incident plane of the fundamental electromagnetic wave at pulsation ω. The incident angle θω1 corresponds to the angle between the fundamental beam and the z axis. The frame (x2, y2, z) is attached to the sample and thus is a rotating frame that defines the angle Φ of rotation along the z axis. It is important to note that because the crystallographic

Figure 3. Definition of axis frames, angle of incidence θ1ω, and angle of rotation Φ.


Langmuir, Vol. 22, No. 1, 2006 143 m ) R1 + (1 - R)2

(1)

1 (2) ω ω P(2) Ω ) 0χ :Em Em 2

(2)

Re(xΩ 1) ω2 × 3 ω Ω 2 20c Re (x )|x cos(θΩ)|2

ISHG )

1

Figure 4. Notation for the phenomenological three-layer model of SSHG. (x, z) here is the incident plane and corresponds to the (x1, z) plane of Figure 3.

directions were not determined the x2 axis is defined here as the axis of maximal SHG intensity and is not directly related to any crystallographic axes of the sample. However, the samples were always positioned on the rotating stage at the beginning of the experiment with the same orientation toward the laboratory frame, and they are cut identically during manufacturing. One can thus confidently postulate that the x2 axis is the same for all of the samples studied in this article but cannot be safely related to any crystallographic direction. To simplify the notation, subscripts 1 and 2 will be ignored in the rest of the text if the context is clear enough. The second-order polarization is created at the rutile/air interface by the fundamental electric field Eω of pulsation ω and is represented in the three-layer model by a sheet of nonlinear polarization in the horizontal (x, y) plane corresponding to the interface of two centrosymmetric media, namely, the air (medium 1) and the rutile crystal (medium 2) (Figure 4). The three-layer model embeds the (x, y) plane of polarization inside an intermediary layer between media 1 and 2, which is a linear slab of vanishing thickness.41,42 The existence of this slab permits us to assign to this boundary area a relative permittivity constant m using the Maxwell-Garnett relationship (eq 1) and to retrieve the wave vectors easily by solving boundary conditions for the propagation equations. In eq 1, R is the volumic fraction that can be taken as an adjustable parameter or be arbitrarily fixed to a given value. In the present article, R ) 0.5. The optical relative permittivity constants were taken from the literature ω 2ω as follows: ω1 ) 2ω 1 ) 1 for medium 1 (air); 2 ) 6.7 and 2 ) 47 8 for rutile, neglecting the very small absorption at 2ω. The fact that rutile is optically birefringent is neglected because the slab is supposed to be thin enough to neglect the walk-off and the difference in the propagation direction between ordinary and extraordinary waves.33 The optophysical parameters of the slab are all designated by the subscript m. Figure 4 also indicates the other notations used in the phenomenological model41,42 and notes the laboratory frame (x, y, z) instead of (x1, y1, z) for simplification. Bold characters are used for vectors, and the electrical fields are decomposed as usual on the s and p unit vectors that define the polarization state of the fields. The fundamental beam has a pulsation ω, and the harmonic beam, a pulsation Ω ) 2ω. Within the electric dipole approximation, the second-order nonlinear polarization P(2) Ω is related to the incident electrical field Eωm propagating in the inner slab (Figure 4) by the second-order electric susceptibility tensor χ(2) (eq 2). Within this framework, the general expression for SHG intensity in the reflection geometry adopted in our experimental setup is given by eq 3.41 (47) Mo, S. D.; Ching, W. Y. Phys. ReV. B 1995, 51, 13023.

m

m

|

ω ω (2) eΩ 1 ‚ χ : em e m

|

2

× (Iω1 )2 (3)

In eq 3, subscript 1 stands for medium 1 (the air), subscript m ω stands for the inner slab, eΩ 1 and em are the polarization vectors (Figure 4) including the Fresnel factors of the interface for EΩ 1 propagating in medium 1 and Eωm propagating in the inner slab. Iω1 is the intensity (J m-2) of the incident laser beam in medium 1. The (001) crystallographic plane of rutile has a C2V symmetry (Figure 1). However, our experimental results will show that this symmetry will be broken when uranyl cations are sorbed on rutile surface. Only one mirror plane will be conserved, reducing the symmetry to C1V (Figure 5). To account for this change in surface symmetry with uranyl sorption, the theoretical model will use a second-order susceptibility tensor χ(2) that respects the C1V symmetry. In that case, (2) (2) (2) (2) the tensor has 10 independent components: χ(2) xxx, χxyy, χxzz, χxxz, χyxy, (2) (2) (2) (2) (2) χyyz, χzxx, χzyy, χzxz, and χzzz. One can represent the components of this tensor using the following notation in which each matrix refers to axes x, y, and z of the sample:

[

χ(2) xxx 0

(2)

χ

≡ 0 χ(2) xxz

χ(2) xxz

χ(2) xyy

0

0

χ(2) xzz

][

0

χ(2) yxy 0

χ(2) yxy

0

0

χ(2) yyz

][

χ(2) zxx 0

χ(2) yyz 0

0 χ(2) zxz

χ(2) zxz

χ(2) zyy

0

0

χ(2) zzz

]

(4)

If one performs a rotation of angle Φ around the vertical z axis, then the components of χ(2) transform themselves and the product χ(2): eωm eωm used in eq 3 can be calculated. The components of the polarization vectors depend on the Fresnel factors and the polarization state of electrical fields and are given by eq 5-8. The Fresnel factors are compiled in Appendix A. In eq 6, γ is the polarization angle of the fundamental beam (γ ) 0 for p polarization, γ ) π/2 for s polarization), and similarly in eq 8, Γ is the polarization angle of the SSHG beam (Figure 4). The respective phases δ and ∆ will be arbitrarily fixed to zero because we did not perform experiments to measure them, for example, in relation to a reference SHG wave obtained with a quartz plate. eωm ) eωx x + eωy y + eωz z

|

(5)

p p eωx ) t1m (1 - rm2 )cos(γ) eiδ cos(θωm) s s eωy ) - t1m (1 + rm2 )sin(γ)

eωz

)

p t1m (1

+

p rm2 )cos(γ)

iδ

e

(6) sin(θωm)

Ω Ω Ω eΩ 1 ) ex x + ey y + ez z

|

(7)

p p i∆ Ω eΩ x ) Tm1(Rm2 - 1)cos(Γ) e cos(θm ) s s eΩ y ) Tm1(1 + Rm2)sin(Γ)

(8)

p p Ω i∆ eΩ z ) Tm1(1 + Rm2)cos(Γ) e sin(θm )

Using the commutation of the dyadic products in the case of SHG, the dyad eωm eωm is given by eωmeωm ) eωx eωx xˆ xˆ + eωy eωy yˆ yˆ + eωz eωz zˆ zˆ + 2eωx eωy xˆ yˆ + 2eωx eωz xˆ zˆ + 2eωy eωz yˆ zˆ (9) Then the vector χ(2): eωm eωm has three components for each axis and

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Figure 5. Rotational anisotropy of SSHG for the rutile (001) crystallographic plane exposed to uranyl solutions. Circles are experimental data, and blue lines are theoretical curves obtained by the phenomenological SSHG model loaded with the indicated components of tensor. All other components are null. can be written as (2) (2) χ(2): eωm eωm ) χ(2) x (Φ)x + χy (Φ)y + χz (Φ)z

(10)

(2) Appendix B gives the mathematical expressions of χ(2) x (Φ), χy (Φ), (2) and χz (Φ). Finally, reporting this vector in eq 3 and performing the dot product with eΩ 1 leads to the expression of the SSHG intensity as a function of the rotational angle Φ for a surface with C1V symmetry:

ISHG(Φ) )

Re(xeΩ 1) ω2 × 3 20c Re2(xω)|xΩcos(θΩ)|2

|

1

m

m

|

2

Ω (2) Ω (2) Ω χ(2) × (Iω1 )2 (11) x (Φ)ex + χy (Φ)ey + χz (Φ)ez

Results Figure 5 gives the experimental SSHG intensity plotted versus the angle Φ and normalized to its maximum value in polar coordinates for the four samples studied. The rotational anisotropic patterns are observed for samples without uranyl (a) or with uranyl adsorbed during 24 h from an aqueous phase at the following initial concentrations: (b) 10-7, (c) 10-4, and (d) 10-2 mole L-1. It can be seen that without uranyl, the rutile (001)

single crystal gives an SSHG pattern with two lobes. The general C2V symmetry expected from the (001) crystallographic plane is then rather well observed, and Figure 5a reports the two planes of symmetry σV for this sample. The fact that the two lobes are not perfectly symmetric could come from a slight miscut of the crystal or from a slight angle between the true horizontal (x, y) plane and the surface of the crystal. To discriminate between these two possibilities, future measurements in X-ray scattering of the crystal mounted on a goniometer head will be performed. When uranyl cations are sorbed on the rutile surface, the SSHG pattern changes. For an aqueous phase with [UO22+] ) 10-7 mole L-1, the C2V symmetry is maintained, but the two lobes merge and the SSHG signal is more intense in the direction of the σV2 plane perpendicular to the lobes’ axis. With initial [UO22+] g10-4 mole L-1, the C2V symmetry is lost, only one mirror plane of symmetry is conserved, and surface symmetry thus becomes C1V with a lack of SHG signal in the right-half quadrant of the polar plot. To understand the evolution of the SSHG signal with uranyl concentration, a simulation of the interface was made using the model described in the previous section. The number of independent components for χ(2) is 10 in C1V symmetry, and it is not meaningful to fit experimental data by optimizing such a


Langmuir, Vol. 22, No. 1, 2006 145

Figure 6. SSHG calculated by the phenomenological model for various combinations of the polarization state of input/output beams in the case of two different sets of components corresponding to the sample with [UO22+] ) 10-7 mole L-1. Experimental observations are also reported for comparison.

large number of parameters. Consequently, the strategy to account for the essential features of the experimental data of Figure 5 was the following: (i) For a clean rutile surface and rutile submitted to an initial concentration of [UO22+] ) 10-7 mole L-1, the data are consistent with a C2V description of the surface symmetry, which reduces (2) (2) (2) (2) the tensor components to only five: χ(2) xxz, χyyz, χzxx, χzyy, and χzzz. The x axis of the sample was defined as the axis of maximum (2) SHG intensity, thus the contribution of χ(2) xxz or χzxx should be (2) (2) higher than the contributions χyyz and χzyy. Two sets of tensor components can be used to represent the experimental results: (2) (2) (2) (χ(2) xxz, χyyz) or (χzxx,χzyy). To choose the appropriate set, we tried to collect experimental data in polarization combinations other than pin/pout (where in ) fundamental beam and out ) harmonic beam). The result is that no significant SHG signal could be separated from noise for all of the other possible combinations (i.e., sin/pout, pin/sout, and sin/sout). If one calculates the theoretical SHG intensity versus angle Φ for these polarization combinations and for the two sets of tensor components, then one obtains the results gathered in Figure 6. The magnitudes of the components were fixed at the values found for the sample submitted to an initial concentration of [UO22+] ) 10-7 mole L-1 as an example, but the same conclusions can be obtained for a clean rutile surface. (2) Examining Figure 6 reveals that if the set (χ(2) zxx, χzyy) is chosen then the SHG intensity should be of the same order of magnitude for pin/pout and sin/pout polarization combinations, whereas for (2) the set (χ(2) xxz, χyyz), the nonzero combination pin/sout is 1 order of magnitude lower than the combination pin/pout. In that case, experimental detection was not possible under our working conditions. Indeed, the signal-to-noise ratio was not high enough in the pin/pout combination to allow any correct measurement

with 10 times lower SHG intensity. In conclusion, Figure 6 (2) indicates that only the set (χ(2) xxz, χyyz) is compatible with our experimental data. We then chose to fit the experimental data for the clean rutile surface and rutile submitted to an initial concentration of [UO22+] ) 10-7 mole L-1. The resulting values (2) of χ(2) xxz and χyyz are reported in Figure 5a and b. (2) (ii) Once two components (χ(2) xxz, χyyz) were determined to fit the experimental data of Figure 5a and b properly, it was decided to fit the remaining data by adding only one supplementary tensor component to avoid too many fitting parameters and a loss of physical meaning. The main aim was indeed to unravel the secondorder susceptibility components that were predominantly affected by uranyl sorption. It was found that χ(2) zxz was the appropriate component to account reasonably for the data of Figure 5c and d, which give the corresponding values obtained for the three (2) (2) components χ(2) xxz, χyyz, and χzxz. Figure 5 indicates that at a low initial concentration of uranyl (10-7 mole L-1) the sorption of cations slightly increases χ(2) yyz from 0.3 to 0.5 au. When the [UO22+] initial concentration reaches 10-4 mole L-1, χ(2) yyz (2) becomes equal to χ(2) xxz, and the component χzxz must be introduced to account for the strong decrease in SHG intensity in the right-half quadrant of the polar plot in Figure 5c. Finally for [UO22+] ) 10-2 mole L-1, χ(2) zxz reaches 1.7 au, but neither (2) χ(2) xxz nor χyyz has to be changed.

Discussion The rotational anisotropy of the bare rutile (001) sample in Figure 5a indicates that the surface symmetry is C2V, as expected from crystallographic considerations (Figure 1). A detailed study of the (110) crystallographic face has been previously performed

146 Langmuir, Vol. 22, No. 1, 2006

Dossot et al.

Figure 7. Proposition of two sorption sites for uranyl rods on the TiO2 rutile (001) crystallographic plane. Uranyl atoms are in yellow, and oxygen atoms of the rods are in green. The three oxygen atoms of the uranyl first hydration sphere are not represented for better clarity.

by SSHG, but only partial results were available for the (001) plane.37 Data of ref 37 were not reported for the incident wavelength at 1064 nm but were reported at 532 nm. At this last wavelength, a part of the SHG signal was suspected to come from an electronic resonant contribution.40 The SSHG signal was almost isotropic, and no C2V symmetry was observed.37 The signal-to-noise ratio appeared to be low under the working conditions of ref 37. Because the rutile (001) face is less stable than the (110) face, the preparation of the sample seems crucial and may explain that under different working conditions the sample surface changes (for instance, by different reconstruction). The surface electronic states of the sample may change dramatically with the preparation, resulting in a variation of the SSHG signal. Besides, the results of Figure 5a are obtained far from significant electronic resonance, which required us to use a higher energy density for the fundamental laser beam than in ref 37. This important difference in working conditions can be invoked to explain the variation of our results with those reported in this article. What is important to note is that our four samples were prepared under the same conditions and can thus be compared to each other. Scanning tunneling microscopy (STM) experiments that studied a polished rutile (001) wafer cleaned by cycles of Ar+ ion sputtering and annealing under UHV at 900 K have shown that the surface has a lattice-work structure that holds the C2V symmetry of the bulk-terminated TiO2 (001) structure.13 The anisotropy pattern in Figure 5c is perfectly consistent with such surface symmetry, indicating that the hydroxylation phenomenon due to contact with aqueous solutions maintains the C2V symmetry of our rutile sample. The main result of Figure 5 is that the rotational anisotropy pattern markedly changes with uranyl sorption if the initial uranyl concentration in the contact solution is increased. Figure 5 clearly shows that SSHG spectroscopy is very sensitive to uranyl sorption even at a concentration as low as 10-7 mole L-1. Considering

that the specific area of the samples is very low because they are single crystals, this confirms the potential of SHG as a surface spectroscopy technique that complements other methods such as XPS and G-XAS very well. The phenomenological model is able to account for this interesting change by using only three tensor elements. However, the difficulty of interpreting the values (2) (2) found for components χ(2) xxz, χyyz, and χzxz in Figure 5 relies on the fact that the x axis (x2 in Figure 3) does not necessarily correspond to a crystallographic axis. It was found on the (110) face irradiated at 532 nm that SSHG is mainly produced along the Ti-O-Ti chains that are present on this surface plane. These chains, along which electronic delocalization can occur, do not exist on the (001) plane, which explains that SSHG intensity was lowered for this face compared to that of the (110) plane.37 However, if one examines the atomic arrangement along the [110] direction, one can discover the following chain: Ti(t)-O(b)-Ti(b)-O(b)Ti(t). If the electrical field of the incident beam is parallel to these chains, then electron movements can be induced along the chains and can primarily contribute to the second-order polarization. We consequently make the hypothesis that the [110] direction can be identified with the x2 axis. This assumption is fruitful for (2) (2) interpreting the change in tensor components χ(2) xxz,χyyz, and χzxz with uranyl sorption. In Figure 5, the sorption of uranyl appears to change the tensor components in two different manners. An increase in χ(2) yyz can be observed for initial concentrations of uranyl from 10-7 to 10-4 mole L-1. Then a new component χ(2) zxz appears, and its magnitude also increases until the initial concentration of uranyl reaches 10-2 mole L-1, whereas χ(2) yyz remains constant for this highest examined concentration. This peculiar behavior could be explained by invoking two different sorption sites. The first one would be occupied by uranyl cations even at a low covering rate and would lead to an increase in χ(2) yyz, suggesting that a second-order polarization term should appear


perpendicular to the x direction. The second sorption site would be occupied at a higher covering rate and lead to an increase in (2) χ(2) zxz but maybe also to an increase in χyyz to a certain extent. Making the assumption that the x2 axis is the [110] direction and considering the atomic arrangement on the (001) crystallographic plane of rutile, geometrical constrains bring about the proposition of two sorption complexes that would be consistent with the previous observations. Figure 7 shows two uranyl rods placed on the rutile sample in the two sorption sites that have been identified and that are consistent with G-XAS measurements for the U-O bond lengths and the fact that uranyl rods are parallel to the surface plane.25a For the sake of clarity, only oxygen atoms of uranyl rods have been represented, and the three oxygen atoms of the first hydration sphere have been omitted. Figure 7 indicates that the first sorption site corresponds to a link between the uranium atom and two O(b) atoms; this site is thus noted as the bb site. Uranyl rods are perpendicular to the [110] direction, and this may contribute to an increase in χ(2) yyz if the x axis is identified as the [110] axis. Indeed, in solution uranyl rods are centrosymmetric and would not give rise to any SHG contribution, but the sorption on the rutile (001) surface may change the surface electronic state of rutile and induce a small electronic delocalization along the y axis. The second sorption site proposed in Figure 7 corresponds to the linking of the uranium atom to one O(b) and one O(t) and has been labeled the bt site. One can clearly see in Figure 7a and b that for this second site for the uranium atom makes a bridge between O(t) and O(b). This could 2+ explain the introduction of component χ(2) zxz for [UO2 ] g10 4 mole L-1 because the electronic delocalization in the chain Ti(t)-O(b)-Ti(b)-O(b)-Ti(t) should be affected by such a bridge. This is also consistent with a plausible increase in χ(2) yyz by the second sorption site because the rod remains perpendicular to the [110] direction. Figure 7 thus proposes two sorption sites that explain rather well the change in the tensor components with uranyl sorption within the assumption that the x axis can be identified as the [110] direction. The atomic arrangements reported in Figures 1 and 7 pertain to a clean hydroxylated (001) face with no reconstruction. Concerning the bb sorption site, only 1.60 Å separates the oxygen atoms of the uranyl rod and the neighboring O(t) atoms. This is a rather short distance, but it should be mentioned that Figure 7 is only a sketch that places O(t) directly perpendicular to the crystallographic plane. Turning to the real rutile/uranyl interface, both the Ti(t)-O(t) and UdO bonds can be slightly distorted to permit the complexation of uranyl cations in the bb sorption site. Furthermore, the (001) face can undergo reconstruction, rendering the situation more complex than the ideal schemes of Figure 7, but the overall C2V symmetry of the bare surface can be conserved.13 Consequently, the propositions of Figure 7 are quite reasonable even for nonideal rutile surfaces.

Langmuir, Vol. 22, No. 1, 2006 147

surface symmetry change with uranyl sorption, the phenomenological model41 is derived in the case of the general C1V symmetry and accounts rather well for the experimental data. It brings about the corresponding evolution of the elements of the nonlinear susceptibility tensor χ(2) with uranyl concentration. This interesting information constraints the possible geometry of the sorption complexes. In agreement with present SSHG data and previous G-XAS results,25a two binding sites are proposed. Further ab initio calculations will be devoted to the study of these surface complexes to test their relative stability, whereas other SSHG measurements will be realized at different incident wavelengths and with rutile (110) single crystals.

Appendix A For the fundamental wave, the Fresnel factors are given by s rm2 )

p rm2 )

s t1m )

p t1m )

nωm cos(θωm) - nω2 cos(θω2 ) nωm cos(θωm) + nω2 cos(θω2 ) nω2 cos(θωm) - nωm cos(θω2 ) nω2 cos(θωm) + nωm cos(θω2 ) 2nω1 cos(θω1 )

ω nω1 cos(θω1 ) + nωm cos(θmm )

2nωm cos(θω1 ) nωm cos(θω1 ) + nω1 cos(θωm)

For the SSHG harmonic wave, the Fresnel factors have exactly the same form except that the refractive indexes and the angles are those for the pulsation Ω ) 2ω defined in Figure 2.

Appendix B Mathematical expressions of the three x, y, and z components of the vector χ(2): eωm eωm at an angle of rotation Φ are (2) (2) (2) 2 2 χ(2) x (Φ) ) {cos (Φ)χxxx + sin (Φ)(χxyy + 2χyxy)}cos 2 (2) (2) (Φ)eωx eωx + {cos2(Φ)χ(2) xyy + sin (Φ)(χxxx - 2χyxy)}cos ω ω (2) 2 (2) (Φ)eωy eωy + χ(2) xzzcos(Φ)ez ez + 2{cos (Φ)(χxyy + χyxy 2 2 2 (2) ω ω (2) χ(2) xxx) - sin (Φ)χyxy}sin(Φ)ex ey + 2{cos (Φ)χxxz + sin ω ω (2) (2) ω ω (Φ)χ(2) yyz}ex ez + 2 cos(Φ) sin (Φ)(χyyz - χxxz)ey ez (B1) (2) (2) 2 2 (2) χ(2) y (Φ) ) {cos (Φ)(2χyxy - χxxx) - sin (Φ)χxyy}sin 2 (2) (2) (Φ)eωx eωx - {cos2(Φ)(χ(2) xyy + 2χyxy) + sin (Φ)χxxx}sin

Conclusions

2 2 ω ω (2) (Φ)eωy eωy - χ(2) xzzsin (Φ)ez ez + 2{cos (Φ)χyxy + sin (Φ)

This article reports the change in the SSHG rotational anisotropy pattern of the rutile (001) crystallographic plane when uranyl cations are sorbed on the surface. Whereas G-XAS measurements were not sensitive enough for this crystallographic plane to show the nature of the uranyl/rutile complexes clearly, SSHG demonstrates its surface specificity and indicates a surface change even for the single crystal in contact with an initial uranyl concentration as low as 10-7 mol L-1. This is particularly interesting because titration methods are not sensitive enough to follow uranyl sorption onto surfaces of such low area. These results prompt us to develop in the near future a flow cell to study sorption kinetics by SSHG spectroscopy. Concerning the

(2) (2) ω ω (2) (χ(2) xxx - χxyy - χyxy)}cos(Φ)ex ey + 2 cos(Φ) sin (Φ)(χyyz 2 2 ω ω (2) (2) ω ω χ(2) xxz)ex ez + 2{cos (Φ)χyyz + sin (Φ)χxxz}ey ez (B2) 2 2 (2) (2) ω ω χ(2) z (Φ) ) {cos (Φ)χzxx + sin (Φ)χzyy} ex ex + 2 (2) ω ω (2) ω ω {cos2(Φ)χ(2) zyy + sin (Φ)χzxx} ey ey + χzzz ez ez + (2) ω ω (2) ω ω 2cos(Φ)sin (Φ)(χ(2) zyy - χzxx) ex ey + 2cos(Φ)χzxz ex ez ω ω 2 sin (Φ)χ(2) zxz ey ez (B3)

LA0519913

Sorption of Uranyl Cations on a Rutile - American Chemical Society

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