Anal. Chem. ISS3. 65, 2545-2548
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Multipurpose X-ray Absorption Cell Franqoise Villain:-t Valerie Briois,t Isabel Castro,tvs Chrystel Helary,t and Michel Verdaguer'StJ Laboratoire d'Utilisation d u Rayonnement Electromagn6tique (CNRS, CEA, MEN), Uniuersit6 Paris-Sud, 91405 Orsav. France. and Laboratoire de Chimie des M6tau.z de Transition, UA CNRS 419, UniuersitQ P. et M. Curie, 75252 Parii,'France '
INTRODUCTION X-ray absorption spectroscopy is a method for determination of the local structure of materials without long-range order andanelectronic probeof vacant statesof theabsorbing atoms. The availability of synchrotron radiation in the hard X-ray range and the development of fast acquisition techniques (dispersive EXAFS, fast EXAFS) have opened new perspectives in studies of solid state, surfaces, and solutions. In situ and time-resolved experiments are more and more appealing for chemists in the field of catalysis and reactivity. This trend increases the need for versatile ancillary devices allowing easy and good quality recording of spectra. We report herein the design of a cell for solution studies in transmission mode under various experimental conditions: variable-path length, controlled-temperature and -atmosphere, and static and semidynamic experiments.
EXPERIMENTAL SECTION The cell is adapted from the infrared (IR) variable-width cell built by the Specac Co.' The modifications take into account the constraints of transmission X-ray spectroscopy in solution studies and the particular chemical constraints arising from the systems studied. Prelimary versions of the cell were used in previous 8tudies.H The cell consists of a sample holder and tworemovahle windows (Figure 1). The sample holder is made of stainless steel coated with poly(tetrafluoroethy1ene) (PTFE). One part of the cell is fixed, the other is mobile, and the two parts are connected by a thread. The fixed part is equipped with a flatwindow,the mobile part with a flanged window. Thedesign featuremakes it possible for the internal faces of the two windows to be almost in contact with each other when the two parts of the cell are connected by the thread system. An adjustable stop can bloek the contact between the windows. The optical path length (i.e., the solution thickness) can be fixed continuously from 0 to 6 mm with a 5-rm accuracy because of the adjacent vernier scale. An x-em path length corresponds to a volume of x + 1 ems. The main modifications on the commercial cell are aa follows. The IR-transparent windows have been replaced by a series of easily removable windows transparent to X-rays and resistant to different chemical surroundings. Two window systems are available (Figure 2). The first one consists of a mechanical seal made of a plastic film of 10-pm width between two PTFE (or poly(monochlor0trifluoroethylene), PCTFE) rings. This kind of seal gives a safe, tight loek, without need for adhesives, and eliminates any risk of polluting the chemical system. The plastic films used are Kapton? Mylar: or poly(viny1 chloride) (PVC). Their small thicknesa allows the recording of spectra down to 4 keV. The
(A) (B) Figure 1. (A) Schematic cross section of the Celt 1, mobile part; 2, fixed part: 3, thread: 4, llanged window; 5, (Mng for flanged window; 6 , PTFE gaskets: 7, Neoprene washers: 8, flat window: 9. O-ring for flat wltniow: IO, adjustable stop: 11, knurled cap with vernier scale; 12. plastic films windows; 13, screw-on PTFEfIxturesforaccessports. (B) View of front park I, inlet port; 0.outlet port: C. inlet port for
circulation. Plaslic film
Flat
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Figure2. (A) Wlndows with PTFE (or PCTFE) rlngs and crlmpad plastic films. (B) Massive pyrolytic graphite windows (scale, 2).
nature of the film is user-defined and adapted to the chemical constraints. The absorption percentage is approximately 3.5% for Kapton and Mylar and 15% for PVC at 5 keV (vanadium K-edge). These numben become 0.5 % and 3%, respectively,at 10keV (zincK-edge). Inthisconfwation, theavailablediameter of the cell allows work with a 13-mm X-ray beam width. The second systemconsistsofmassivepyrolyticgraphite(PG) windows: to he used at higher energy. The central portion of the windows has a reduced thickness of 0.2 mm leading to 9% absorption at 10 keV. The useful width of the cell in this configuration is then 18 mm. With both systems, the entrance and flow of liquid has been t Universith Paris-Sud. 1 Universit4 P. et M. Curie. designed to allow the renewal of the solution without removing t Permanent address: Departament de Qnimica Inorganica,Fadtat the cell from the X-ray beam. Two working modes are then de Quimica, Burjsasot, Valencia, Spain. allowed. (1) Instruction manual for variable-path length IR cell. Swac, In static mode, the cell is a simple sample holder for liquid Analytical Accessories Limited, Unit 3, Lagoon Rd. St. Mary Cray, medium with a continuous variable path length. The filling and Orpington, Kent, BR5 30X. emptying is achieved through the upper openings in (I)and out (2) Brioia, V. Thbse, Universite P. et M. Curie, Paris. 1991. (3) Prieto.C.;Lagarde.P.;Derpe~H.;Briois.V.;ViUain,F.;Ve~dagu~, ( 0 )in Figure 1,through PTFE access ports. The access ports M.J. Phys. Chem. Solids 1992,53,233. can be closed by PTFE plugs or valves to isolate the solution (4) Prieto,C.;Lagarde.P.;Derpert,H.;BrioigV.;Villain.F.;Verdsguer, from the atmosphere in the caae of systems sensitive to oxygen M.Meas. Sei. Teehnol. 1992.3.325. (5) Kapton (palyimidefilm) and Mylar (poly(ethyleneterephthalate) fdm), registered trademarks of Du Pant de Nemours, were purchased (6) PyrolyticgraphiteisapmduetofUnionCarhideandwaspurehased from Du Pont Eleetronirs. from Union Carbide Coatings Service Corp., Advanced Ceramics. 0003-2700/93/0365-255$04.00/0
0 1993 American ChrrmCal Scclety
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ANALYTlCAL CHEMISTRY, VOL. 65. NO. 18, SEPTEMBER 15, 1993
cell
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Schemeofthewholesetup: 1, cell: 2,thermostatedreactor; 3. peristaitic pump. Flgure3.
orlandatmosphericmoisture. The filling of the cellin such cases is realized in a glovebag.
In semidynamic mode, the cell can be used for in situ studies of evolving chemical systems and for kinetic studies. In this case, the contents of the cell are renewed (openings C and 0 in Figure 1) by circulation of solution from the reactor to the cell by meansofaperistalticpump (Figure 3). During theacquisition of the EXAFS data circulation must be interrupted, to ensure perfect control over volume of the cell, by isolating it from the rest of the circulation device using valves up- and downstream (whence the semidynamic appellation). For in situ controlled-temperature studies, a thermostated liquid circulation support is available, connected to either a thermostat or a cryothermostat. The Duralumin support has been hollowed in the center to receive the fixed part of the cell. The mobile part remains free and the optical path length may he adjusted in the X-ray beam. The cell may be heated or cooled by thermal conduction. When the temperature is less than 15 O C , water vapor condensation on the cell windows can he avoided by helium gas flow through a hermetically sealed chamber where the cell and its support are fixed.
RESULTS AND DISCUSSION The originality of the present cell is its facility of adaptation tovarious chemicalsystems becauseof its capacityforflexible adjustment, possibilities offered by the materials employed and the accessories available (accessible energy range of 4-30 keV, solution concentrationof 1W-1 molL-', studiesin highly reactivemedia, insitu studies, temperature- and atmospherecontrolled studies). The flexibility of the cell makes a difference with other multipurpose devices described succinctly in the literature?-'9 The only cells widely described are the ones dedicated to a specific use: a cell for the study of gaseous water at the oxygen K-edgeFO cells for EXAFS spectroele~trochemistry,2~-~~ and a cell based on photocon(7) Sandstrom. D. R.:. Dodeen. - . H. W.:. Lvtle. _ .F. W. J. Chem. Phvs. 19i7;67,473. ' (8) Cramer, S. P.: Gray, H. 9.: Dori, Z.: Bino, A. J . Am. Chem. Soe. 1979 m i ._._ ~
2770~
(9) Yamapehi,T.; Lingqvist, 0.; Claeson,T.; Boyce,J. B. Chem.Phys. Lett. 1982,93,528. (10) Beagley, B.: Gahan, B.; Greaves, G. N.: McAuliffe, C. A. J. Chem. Soc., Chem. Commun. 1983,1265. (11)Cramer, S.:Eidem, P. K.; Paffet, M. T.: Wikler, J. R.; Dori, Z.: Gray, H. B. J. Am. Chem. Soc. 1983,105,799. (12) Sano, M.;Maruo, T.; Masuda, Y.: Yamatera, H. Znorg. Chem. 19PA 9.7 Adfifi .-_. .,_._.
(13)Ludwig, K.F.. Jr.; Warburton. W. K.;Wilson. L.; Bienenatock. A. 1. J Chcm. Ph>s. 1987.87,604. (14,Yamaaurhi, T.: Numura. M.;Wekita. H.Ohtaki, H. J. Chem.
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115)Rabonneau, F.; Doeuff. S.:Leaurtie. A,; Sanchez. C.: Cartier, C.: Verdaguer. M. Inory. C h m . 1988.27.3166. (16, Sanchez.C.:Bsbonneau. 4,;Doeuff.S.:Lesus1ir.A. Ultrastructure Proeessmg of Adunneed Ceramics; Maekenzie. J. D., Ulrieh, D. R., Eds.; Wiley: New York, 1988;p 71. (17) Leaustic,A.:Babonneau,F.;Livage,J.Chem.Mater. 1989,1,240. (18)Garcia. J.: Benfstto. M.: Natoli. C. R.: Bianconi.. A,:. Fontaine.. A,:. Toieniino, ~.'chem.Phyi KISS,132,'zgs. (19)Pandya. I. I.: O'Grady, W. E.: Corrigan, D. A,: MeBreen, J.; Hoffman. ~.~ R. W.. ,J. Phv.q. 94. 21. ~ ~ Chcm. , 1990. ..., ~ ~~,~~ ~~~
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(20) Yang. B. X.; Kim. J. Phys. Rev. B 1987,36,1361. (21) Dewald, H. D.; Watkins, J. W.; Elder, R. C.; Heineman, W. R. Anal. Chem. 1986,58,2968.
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Flgure 4. Absorption spectra of copper(I1)sulfate aqueous solutlons,
and path lengths (1 [Cu"(H20h]*+,at two different concentrations(0 recorded in fast-scanning mode: (A, top) experimental spectra; (E. bonom)EXAFSsignal. ThespectrumatC= 0.4molL-'wasobtained from an accumulation of five scans. The spectrum at C = 0.1 mol L-' was obtained from an accumulation of 10 scans. ductivity detection for organometallics in hydrocarbon solvents.24 Very recently a paper was devoted to a detailed description of a solution cell built for multipurpose uses, followingadesigncompletelydifferent from the one described herein.% Our cellmay be used in fluorescence detectionmode but was not conceived for such a n use. Experiments with solution cells optimized for fluorescence detection are in progress.% Some characteristic aspects of the cell and/or its operation are illustrated in the results given herein, obtained at the K-edge of transition elements of the first period27 or at the Lm-edge~flanthanides.~.~ These results show both theextent of studies which may be carried out and the performance achieved in the various fields. (1)ControlandReliabilityoftheOptiealPathLength. The optical path length is an essential factor for the quality of the measurements. Its control is illustrated in the following examples. Continuous adjustment, with a filled cell, makes possible the optimization of the measurements. Figure 4 shows that whenever the chemical system so allows, the use of concentrated solutions and a low path length is preferable, so as to reduce solvent absorption and enhance the s i g n a noise ratio. Theconstancyoftheopticalpathlength,i.e.,theconstancy of the thickness of the sample, is particularly important in EXAF'S analysis to determine the number of neighbors. It is illustrated in Figure 5 through a series of five spectra in the fast EXAFS mode developed at LURE on the EXAFS I11 station of the DCI ring.2" The absorbance difference A A ~~
(22) Sharpe, L. R,Heineman, W.
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R.; Elder, R. C. Chem. Re". 1990,
90,705. (23)lm. D. H.: Elder. R. C.: Heineman. W. R.: Dewald. H. D. Anal. Chem. l%l, 63,2535. (24)Sham, T. K. Top. Cum. Chem. 1988,145,81. (25) Ertel, T. S.; Bertagnoli, H. Nuel. Instrum. Methods 1993,B73,
199.
(26) Villain, F.; Verdaguer, M., work in progress. (27) Castro, I.; HBlary,C.; Villain. F.; Verdaguer, M., work in progress.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
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