Langmuir 1986, 2, 165-169 in Figure 6. Then, we plotted y / y o against X and x / x o against X-lI2, where X is the overall extension ratio equal to Y/2b (see Figure 1)and xo and yo are the initial coordinates. The results are shown in Figure 9. We see that
165
the y coordinates of the markers are proportional to X, while the x coordinates are proportional X-lI2, as we have anticipated for the flow inside the drop being a simple elongation under a constant tensile force.
Synthesis and Characterization of Aqueous Tris(2,2'-bipyridine)ruthenium(11)-Zirconium Phosphate Suspensions Dominique P. Vliers,? Dirk Collin,t Robert A. Schoonheydt,*t and Frans C. De Schryved Laboratorium uoor Opperulaktechemie, K . U. Leuuen, Kard. Mercierlaan 92, B-3030 Leuuen (Heverlee),Belgium, and Departement Scheikunde, K . U.Leuuen, Celestijnenlaan 200F, B-3030 Leuuen (Heuerlee), Belgium Receiued August 14, 1985. I n Final Form: October 30, 1985 Tris(2,2'-bipyridine)ruthenium(II) (R~(bpy),~+) is ion-exchanged on stable suspensions of hydrogen zirconium phosphate (HZrP) and hexylammonium zirconium phosphate (HexA-ZrP) to a maximum extent of 1.20 mmol/g of solid. In the former case, the particles are almost completely disordered and there is no intercalate with R ~ ( b p y ) , ~ In + . the latter case, both interlamellar adsorption and adsorption on the external surface occur, because the material is only partially disordered. The adsorption and emission bands of adsorbed R ~ ( b p y )are ~ ~red-shifted + with respect to their aqueous solution values by 5 nm, and the extinction coefficient of the 458-nm band strongly decreases with loading. By quenching with Fe(CN):it is possible to distinguish between R~(bpy)~'+ on the external surface and in the interlamellar space: in the former case the Stern-Volmer constant is 11500 M-', in the latter case 7200 M-'.
Introduction The adsorption of the photosensitizer tris(2,2'-bipyridine)ruthenium(II), R ~ ( b p y ) ~on ~ +inorganic , colloids has currently obtained a large interest in the field of heterogeneous photochemistry, especially the water splitting.'r2 Among the colloidal systems, clay suspensions, especially smectite clays, are attractive supports?-* These are layered aluminosilicates with swelling behavior, a large surface area (700-800 m2/g), and a cation-exchange capacity of 1 mequivfg. The luminescence of R~(bpy),~+-clay suspensions is sensitive to the chemical composition and loading of the clays.g The wavelength of the emission maximum increases linearly with the negative charge of the lattice. The quantum yield is independent of the loading up to 40% and the quenching by structural Fe(II1) follows Perrin's law of quenching in the absence of diffusion. Quenching studies using neutral organic molecules or potassium ferricyanide differentiate Ru(bpy)? adsorbed on the external surface of kaolin clay and Ru(bpy)gl+intercalated between montmorillonite clay layers.6 The structure of zirconium phosphate is similar to that of smectite clays. Layers of zirconium octahedra are separated by phosphate tetrahedra like the aluminum octahedra and silicium tetrahedra in clays. The attractive features of zirconium phosphates are (i) the lack of iron and (ii) the very high cation exchange capacity (CEC) (6.64 mequiv/g). Three types of Ru(bpy):+ adducts of zirconium phosphates (ZrP) can be synthesized.ll (i) A crystalline intercalate is formed when Ru(bpy)z+is incorported during the synthesis of zirconium phosphate in the presence of HF. Its emission maximum is at 615 nm. (ii) By t Laboratorium
voor Oppervlaktechemie.
* Department Scheikunde.
ion exchange or impregnation of R ~ ( b p y ) , ~the + complex is adsorbed only on the external surface. These materials have their emission maximum at 640-645 nm. (iii) When Ru(bpy),Cl,, ZrOC12,and are refluxed, less crystalline materials are formed. Two Ru(bpy),2+species are distinguished. One which emits at 615 nm and another which emits at 590 nm. The latter is predominantly visible with excitation at 420 nm. The chemical species is believed to be ( R u ( b p ~ ) ~ ( b p y H ) ) ~ + . ~ ~ Although inorganic cations larger than K+ cannot be intercalated at room temperature into Zr(03POH)2.H20, a-HZrP,'O amines,12-14pyridine,', and amino acids15 are easily taken up in the interlamellar space. With vigorous stirring a-HZrP crystals, exchanged with alkylammonium (1)Duonghong, D.; Borgarello, E.; Gratzel, M. J. Am. Chem. Soc 1981, 103,4685. (2)Nijs, H.; Fripiat, J. J.; Van Damme, H. J . Phys. Chem. 1983,87, 1279. (3)Krenske, D.; Abdo, S.; Van Damme, H.; Cruz, M.; Fripiat, J. J. J . Phys. Chem. 1980,84,2447. (4)Abdo, S.;Canesson, P.; Cruz, M.; Fripiat, J. J.;Van Damme, H. J. Phvs. Chem. 1981.85. 797. 15) Nijs, H.; Cruz, M.;Fripiat, J. J.; Van Damme, H. J . Chem. SOC., Chem. Commun. 1981,1026. (6)Della Guardia, R.A.; Thomas, J. K. J. Phys. Chem. 1983,87,990. (7)Schoonhevdt, R. A.; Pelarims, J.; Heroes, Y.; Uytterhoeven, J. B. Clay Miner. 1978,13,435. (8) Ghosh, P. K.; Bard, A. J. J . Phys. Chem. 1984,88, 5519. (9)Schoonheydt, R.A,; De Pauw, P.; Vliers, D.; De Schryver, F. C. J . Phys. Chem. 1984,88,5113. (10)Clearfield, A,; Hagiwara, M. J . Inorg. Nucl. Chem. 1978,40,907. (11)Vliers, D. P.;Schoonheydt, R. A.; De Schryver, F. C. J. Chem. SOC.,Faraday Trans I , 1985,81,2009. (12)Clearfield, A.; Tindwa, R. H. J . Inorg. Nucl. Chem. 1979,41,871. (13)Yamanaka, S.;Haribe, Y.; Tanaka, M. J . Inorg. Nucl. Chem. 1976, 38,323. (14)Tindwa, R.W.; Ellis, D. K.; Peng, G.; Clearfield,A. J. Chem. SOC., Faraday Trans. 1, 1985,81, 545. (15)Kijima, T.;Ueno, S.; Gab, M. J. Chem. Soc., Dalton Trans. 1982, 2499.
0743-7463/86/2402-Ol65$01.50/0 0 1986 American Chemical Society
166 Langmuir, Vol. 2, No. 2, 1986
cations, exfoliate, and stable zirconium phosphate suspensions are obtained.I6 In this paper we report on the preparation of stable ZrP suspensions with the inorganic complex Ru(bpy)32+.The spectroscopy of this complex allows the characterization of the suspensions and a comparison of the spectroscopic properties of Ru(bpy)g2+on ZrP and on clays.
Vliers et al.
nL L N
m '&
1.50
1
I
Experimental Section Samples. a. P r e p a r a t i o n of S u s p e n s i o n s of Zirconium Phosphates. The crystalline Zr(HP04)z.Hz0, a-HZrP, was prepared by direct precipitation as described by Alberti and Torracca." The suspensions used in this work were prepared on the basis of the patent of Alberti and Constantino.16 n -Hexylammonium Zirconium P h o s p h a t e Suspension. a-HZrP, 1 g, was suspended in 60 cm3 of HzO and 40 cm3 of 0.1 M NaOH. This dispersion was refluxed under vigorous stirring and 40 cm3 0.1 M n-propylammonium chloride was dropwise added (0.03 cm3/s). At the end of the addition, the mixture was cooled, washed with water, and diluted to a volume of 100 cm3 with water. Under reflux and vigorous stirring 40 cm3 of 0.1 M n-hexylammonium chloride were dropwise added. With this amount the zirconium phosphate is half-exchanged.12p'6 The dispersion was washed with water, the p H was adjusted to p H 4 and the fraction with an average spherical particle diameter less than 0.3 pm was separated by centrifugation a t 4000 rpm for 15 min. This suspension is called HexA-ZrP. The amount of nhexylammonium adsorbed on ZrP, determined by the Kjeldahl method, was 3.64 mmol/g. The yield of HexA-ZrP in suspension was only 4-7% of the initial amount of a-HZrP. Hydrogen Zirconium P h o s p h a t e Suspension. A HZrP suspension was prepared in the same way as the HexA-ZrP suspension. Instead of adding 40 cm3 of 0.1 M n-hexylmmonium chloride, 50 cm3 of 0.1 M HCl were added. The yield of particles in suspension was 4-7 % . b. E x c h a n g e of Ru(bpy),'+ on Zirconium P h o s p h a t e Suspensions. HZrP or HexA-ZrP suspensions, 10 cm3, were + a known initial conexchanged in a solution of R ~ ( b p y ) , ~with centration, a t pH 4 by end-over-end shaking in polyethylene tubes a t 295 K for 1 week. The extent of adsorption was determined by measuring the Ru(bpy)? concentration in the supernatant solution spectrophotometrically a t 452 nm after exchange. We call these exchanged suspensions Ru-HZrP and Ru-HexA-ZrP. The adsorption of C1- was measured using 36C1-labeled Ru(bpy),Cl,. Countings were performed on 2-cm3 aliquots of the supernatant solution by using a Tricarb 460C liquid-scintillation counter. The quenching by R ~ ( b p y ) ~ was ' + taken into account by the standard addition method. Na%C1solution, 0.5 cm3, with known activity was added to the same samples, which were measured again. Procedures a n d Techniques. X-ray powder (XRD) patterns (Ni-filtered CuKa radiation) were taken with a Seiffert Pad11 diffractometer connected to a HPlOOO computer and equipped with a position-sensitive detector (M. Braun) and a multichannel analyzer (Mikras, Frieseke and Hoepfner). The zirconium phosphates were precipitated by centrifugation at 14000 rpm for 10 min. The wet solid was put into the sample holder and analyzed as such. The drying of the sample is minimized as a consequence of the rapid scanning (50" 20 in 15 min). IR spectra of HZrP particles, separated from the HZrP suspension and air-dried, were recorded with a Perkin-Elmer 580B spectrophotometer from 4000 to 1500 cm-' on KBr pellets. The absorbance at 452 nm of concentrated aqueous solutions of Ru( b ~ y ) ~was ? + measured with a Cary 17 instrument using the reference attenuation method. A M aqueous solution of Ru(bpy)?+ was put in the reference beam. The absorption spectra of the Ru-HZrP and Ru-HexA-ZrP suspensions were measured in the range 700-230 nm with the same instrument. Water was put in the reference beam. The Ru-HZrP and Ru-HexA-ZrP particles were precipitated by centrifugation and resuspended in water to make sure that only adsorbed Ru(bpy):+ complexes (16) Alberti. G.; Constantino, U. European Patent EP 0 094 919 A,
1983.
(17)
Alberti, G.; Torracca, E. J . Inorg. Nucl. Chem. 1968, 30, 3127.
0.00
10
.20
.30
.40
m m o l Ru ( b p y ) 3 2 ' /I Figure 1. Adsorption isotherms of Ru(bpy),'+ on HZrP (A)and on HexA-ZrP (0). were measured. Both the Ru-HZrP and the Ru-HexA-ZrP suspensions are stable for a t least 3 days. Some sedimentation is observed after standing for 1 week. For luminescence measurements, the suspensions were diluted with water to obtain an absorbance of about 0.1 at the 455-460-nm band. On these diluted suspensions luminescence spectra were recorded on a Spec Fluorolog instrument in the range 500-800 nm with excitation a t 452 nm. The emission spectra were corrected for the sensitivity of the photomultiplier. The suspensions were measured in equilibrium with air and after deaeration by bubbling argon through them. The quatum yield, ,,$, was calculated as
I,, and Zref are respectively the areas under the luminescence spectra of the Ru-ZrP suspension and a R ~ ( b p y ) , ~7+X loa M aqueous solution with quantum yield $ref = 0.046. They are calculated by integration of the digitalized spectrum on a P D P l l computer. A,, and Arefare the absorptions of respectively the suspensions and the reference solution. n is the index of refraction. nref= 1.333 and nau8= 1.67. The latter is obtained from the experimental data (see Discussion). The quenching of R ~ ( b p y ) , ~on + a Ru-HZrP suspension with a Ru(bpy),*+loading of 0.781 mmol/g of ZrP and a Ru-HexA-ZrP suspension with a loading of 0.715 mmol/g of ZrP by potassium ferricyanide was studied at different ferricyanide concentrations in the range of to 3 X low3M. The deaerated suspensions were excited at 470 nm to minimize the absorption of Fe(CN)63-. Quenching was expressed as Zo/I wherein Zo and Z are the areas under the luminescence spectra of the suspension in the absence and presence of the quencher, respectively.
Results The adsorption isotherms of Ru(bpy)32+on HZrP and HexA-ZrP suspensions are shown in Figure 1. The Ru(bpy)32+adsorption, expressed in millimoles per gram of unexchanged ZrP, as a function of the Ru(bpy)3z+concentration of the equilibrium solution is, within the experimental error, equal for HZrP and HexA-ZrP suspensions. The adsorption is less selective than in the case of a hectorite clay,' where all the added Ru(bpy),2+ is adsorbed till the maximum R ~ ( b p y ) ~ loading ~ + of 0.775 mmol/g is reached. For the HZrP and HexA-ZrP suspensions the maximum amount adsorbed is 1.2 mmol of R ~ ( b p y ) , ~per + g. This is significantly above the 0.79 mmol/g that can be intercalated during the synthesis into crystalline ZrP.ll To determine if R ~ ( b p y )is~ adsorbed ~+ as a cation or precipitated as Ru(bpy),Clz the C1- adsorption of the Ru-HZrP sample with a Ru(bpy)F loading of 0.913 mmol/g was measured. No C1- adsorption was observed.
Ru( bpy):+-Zirconium
Langmuir, Vol. 2, No. 2, 1986 167
Phosphate Suspensions
6.104
1.00
.50
I
0.00
.50
00
1.00
1.50
m m o l Ru (bpy)32* / g
2.00
ZrP
F i g u r e 3. Lambert-Beer's extinction coefficient expressed as a function of the Ru(bpy)?+ content of Ru-HZrP (A)and RuHexA-ZrP (0). . 2 0
4 0
6 0
6.0
10 0
12 0
14.0
2e Figure 2. XRD patterns of a HZrP suspension (a), Ru-HZrP with 0.824 mmol R u ( b p y ) p / g of ZrP (b), HexA-ZrP suspension (c), and Ru-HexA-ZrP with 0.632 mmol/g of ZrP (d). The scale of spectrum c is 5 times reduced with respect to the others.
The XRD spectra of HZrP-, HexA-ZrP-, and Ru(bpy)?+-exchanged suspensions are shown in Figure 2. Only a few, weak and broadened peaks are visible. The first peak can be interpreted a t the (002) reflection by analogy with the spectra of a-HZrP, alkali exchanged ZrP, Zr(03POM)2-nH20(M = cation), and the organic derivatives ZI-(O~POR)~. The basal distances, d,, of HZrP and HexA-ZrP are 1.21 nm (20 = 7.3) and 2.10 nm (20 = 4,2), respectively. The adsorption of Ru(bpy),2+on HZrP, even at high loadings, doesn't change the XRD spectra of HZrP. On the contrary the interbasal distance of HexA-ZrP decreases to 1.63 nm (20 = 5.4) after adsorption of Ru(bpyI3'+. The IR spectrum of the HZrP particles, precipitated from the HZrP suspension, is nearly identical with that of y-HZrP. Only very weak shoulders a t 3600 cm-' and at 3515 cm-', typical for a-HZrP, are reminders of its origin.18 The UV-visible absorption spectra of Ru-HZrP and Ru-HexA-ZrP suspensions are almost identical with that of Ru(bpy)?+ solutions with three minor differences. (i) The maximum of the MLCT band at 452 nm in aqueous solution is red-shifted to 458 nm for both Ru-HZrP and Ru-HexA-ZrP. (ii) The maximum of the X-X* band is at 283 and 285 nm for Ru-HexA-ZrP and Ru-HZrP, respectively, and a shoulder at 276 nm can, especially at high loadings, hardly be observed. In solution the maximum is a t 280 nm with a weak shoulder a t 273 nm. The full bandwidth a t half-maximum (fwhm) of the ir--a* band is 30 f 1 nm for Ru-HZrP and Ru-HexA-ZrP and 20 f 1 nm for the solution. (iii) The intensity ratio of the 280-nm band to the 456-nm band is 5 in solution and at low loadings for Ru-HZrP but decreases to 3.3 for the Ru-HZrP suspensions at higher loadings and for Ru-HexA-ZrP. The ratio of the integrated intensities of the X-T* band and the MLCT band decreases from 2.2 in aqueous solution and a t low loadings to 1.7 a t high loadings. The extinction coefficient of the MLCT band system at 458 nm, calculated from the Lambert-Beer's law, decreases (18) Horsely, S. E.; Nowell, D. V.; Stewart, D. T. Spectrochim. Acta, Part A 1974, 30A, 535.
Table 1. Quenching Rate Constants of Ru(bpy),*+ with O2 in Heterogeneous Media system k,, M-' ref Ru-HZrP (1.20-1.26) X lo9 this work Ru-HexA-ZrP (1.20-1.26) X lo9 hectorite 8.7 x 107 3 Sephadex SP-C50 (1.2 f 0.1)x 109 21 PVS 2.7 x 109 22 NaLS" 2.1 x 109 23 Nalco SiOz 1115 23 1.3 x 109 porous colloidal SiOz inside 1.3 x 107 23 outside 4.9 x 108 23 aqueous soln 3.3 x 109 19, 20 Sodium lauryl sulfate.
drastically with the loading for Ru-HexA-ZrP and RuHZrP as visualized in Figure 3. The limiting value for infinitely small loadings approachesthe value of 14600 M-' cm-' for aqueous solutions with a concentration below lo4 M. These small extinction coefficients should be compared with the values 12380,8700, and 4450 M-l cm-I obtained for aqueous solutions of 0.3, 0.5, and 1 mM, respectively. On clays Ghosh and Bard* obtained 21 000 M-l cm-' while we found 16500 M-l cm-' a t small loading^.^ The emission spectra of the Ru-HZrP and Ru-HexAZrP suspensions are identical with the spectrum of Ru(bpy)32+in aqueous solution. The band maximum is at 620-625 nm. The quantum yields are independent of the loading. The average values are 0.073 and 0.061 for RuHZrP and Ru-HexA-ZrP suspensions, respectively. For nondeaerated suspensions the quantum yields are 0.068 and 0.057. This O2 quenching is much less effective than in aqueous solution. In the latter case, the quenching can be analyzed with the Stern-Volmer equation:
I o / I = 1 + kq~o[Ql= 1 + Ksv[QI
Io and I are the areas under the luminescence curves of the deaerated solution and the solution in equilibrium with air, respectively, k, and T~ a r e equal to 3.3 X lo9 M-l s-l and 600 ns, r e s p e c t i ~ e l y , 'and ~ , ~ ~[Q] is the quencher con(19) Demas, J. N.; Diemente, D.; Harris, E. W. J . Am. Chem. SOC. 1973, 95, 6864.
(20) Lin, C . T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N. J . Am. Chem. SOC.1976,98, 6536. (21)Slama-Schwok,A.; Feitelson, Y.; Rabani, J. J . Phys. Chem. 1981, 85, 2222. (22) Meisel, D.; Mastheson, M. S. J . Am. Chem. SOC.1977, 99, 6577. (23) Wheeler, J.; Thomas, J. K. J . Phys. Chem. 1982, 86, 4540.
Vliers et al.
168 Langmuir, Vol. 2, No. 2, 1986 30.0
r
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( b p ~ ) , ~is+adsorbed exclusively on the external surface. R ~ ( b p y ) , ~adsorbed + in the interlamellar space of montmorillonite or incorporated in silica and NaLS micelles is quenched less effectively.
Discussion The absence of C1- adsorption shows that R ~ ( b p y ) , ~is+ ion-exchanged on the surfaces of HZrP and HexA-ZrP in i / aqueous medium. The maximum amount adsorbed, 1.20 mmol/g, corresponds to 36% of the cation exchange capacity of the material. This number should be compared i /, to the 25% or 0.83 mmol/g which can be incorporated in the interlamallar space during the synthesis of the highly crystalline R~(bpy),~+-HzrP complexes.ll The increase 0 0 I / . . . . , . . , , , . . , . ' , 1 , . from . 0.83 to 1.20 mmol/g can be explained by delamina0 00 1 00 2 00 3 00 4 00 tion of ZrP layers and disordered stacking of these layers, [Fe (CNIC3-] mM making available large (external) surfaces for ion exchange and adsorption. This is explained for the two materials, Figure 4. Stern-Volmer plots for the quenching of Ru-HZrP HZrP and HexA-ZrP, in more detail below. with a loading of 0.781 mmol/g (A)and Ru-HexA-ZrP with a The high crystallinity of the a-HZrP particles is lost loading of 0.715 mmol/g (0) with potassium ferricyanide. during the synthesis of the HZrP suspension. Intercalation of n-propylammonium into a-HZrP a t loadings between Table 11. Stern-Volmer Constants of Ru(bpy)3z+ 3 and 6 mmol/g produces amorphous gels.14 Since hyQuenching by Fe(CN)63-in Heterogeneous Systems drolysis of the phosphate groups is improbable in the system Ksv, M-' ref synthesis conditions, the poor crystallinity should be exRu-HZrP 11500 this work plained by a disordered stacking of the ZrP layers caused Ru-HexA-ZrP 7200 this work by the swelling after intercalation of propylammonium kaolin 8820 6 cations. The broad and weak line observed in the XRD montmorillonite 600 6 60" 24 Nalco SiOz 1115 spectrum of a HZrP suspension (Figure 2) corresponds to Si02gel 660" 24 an interbasal distance of 1.21 nm, typical for y-HZrP.26
