1/28 S.C. Nues et al. SPECTROSCOPIC AND STRUCTURAL STUDIES OF DIUREASILS DOPED WITH LITHIUM PERCHLORATE S. C. Nues a, V. de Zea Bermudez a*, D. Ostrovskii b, P. B. Tavares a, P. C. Barbosa c, M. M. Silva c, M. J. Smith c a Departameto de Química ad CQVR, Uiversidade de TrásosMotes e Alto Douro 5001801 Vila Real, Portugal b Departmet of Applied Physics, Chalmers Uiversity of Techology, 41296 Göteborg, Swede. c Departameto de Química, Uiversidade do Miho, Gualtar, 4710057 Braga, Portugal Abstract Diurea crossliked POE/siloxae hybrid ormolytes (diureasils) doped with a wide cocetratio rage of lithium perchlorate trihydrate (LiClO 4.3H 2 O) (200 0.5, where expresses the salt cotet i terms of the umber of ether oxyge atoms per Li + io) have bee aalysed by Fourier Trasform ifrared ad Rama (FTIR ad FTRama, respectively) spectroscopies ad Xray diffractio (XRD). The results obtaied lead us to coclude that the xerogels with 5 are totally amorphous. At 1 free salt is observed. Free ClO 4 ios appear to be the mai charge carriers at the coductivity maximum located withi the 25 8 compositio rage of this family of ormolytes. At = 15 ClO 4 ios coordiated i moo/tridetate (C 3v symmetry) ad bidetate (C 2v symmetry) cofiguratios were detected. I saltrich samples with < 15 there is a marked tedecy for ioic associatio. The resultig decrease that occurs i the cocetratio of free ios is cosistet with the observed sigificat decrease of the ioic coductivity. The aalysis of the amide I ad * Tel. +351 259 350 253, Fax: +351 259 350 480, Email adress: vbermude@utad.pt
2/28 S.C. Nues et al. amide II regios provided solid proof that the Li + ios bod to the urea carboyl oxyge atoms over the etire rage of salt cocetratio studied. 1. Itroductio Sice Wright et al. first reported the ioic coductio of complexes formed betwee liear high molecular weight poly(ethylee oxide) (POE) ad alkali metal salts 1, great progress has bee achieved i the domai of solid polymer electrolytes (SPEs). These materials have potetial applicatio i allsolidstate electrochemical devices, such as higheergy desity rechargeable batteries, electrochromic displays (ECDs) ad fuel cells 2. The practical use of the POEbased electrolytes has bee somewhat delayed, however, by their poor processability ad a marked tedecy to crystallise. I recet years modified SPEs that represet a valuable alterative to covetioal polymer electrolytes have bee developed usig the solgel process 3. This chemical sythetic route provides access to POE/siloxae hybrid etworks 4] that offer a umber of beefits: (1) a major reductio or eve total suppressio of crystalliity; (2) good thermal, mechaical ad chemical stability; (3) accommodatio of high guest salt cocetratios without saltig out ; (4) easy processig ito thi films. The Li + doped ormolyte (orgaically modified silicate electrolyte) cocept has attracted particular iterest 5,6,7,8,9,10,11,12,13,14,15]. I this cotext our group has devoted cosiderable effort to the ivestigatio of ormolyte systems based o diurea crossliked POE/siloxae frameworks (diureasils) [16] doped with Li + [13,14], Mg 2+ [17] ad Z 2+ 18] ios, itroduced as triflate salts. Very recetly ecouragig results were obtaied with a ECD prototype icorporatig a ormolyte of the diureasil family doped with LiClO 4.3H 2 O (200 8) [19]. The diureasil host matrix employed, abbreviated as du(900) (where d idicates di, U deotes the urea
3/28 S.C. Nues et al. (NHC(=O)NH) group ad 900 correspods to the average molecular weight of the startig orgaic precursor), comprises POE chais with about 15.5 repeat uits. The doped ormolytes have bee represeted by the otatio du(900) LiClO 4. Samples with 200 8 are completely amorphous. Thermal degradatio starts at 209 ºC i xerogels with < 25. The highest decompositio temperature (283ºC) was registered at = 200. The coductivity maximum of this system is located at 15 8 (approximately 10 6 ad 10 4 Scm 1 at 30 ad 95 ºC, respectively) [19]. I the preset work, du(900) LiClO 4 samples with a wider rage of salt cocetratio (200 0.5) have bee examied by FTIR ad FTRama spectroscopy with the primary objective of elucidatig the Li + /ether oxyge (POE), Li + /carboyl oxyge (urea crosslik) ad Li + /ClO 4 iteractios. The mai motivatio of this spectroscopic aalysis has bee to gai isight ito the ature/cocetratio of the species resposible for charge trasport. The morphology ad structure of the ormolytes studied have bee determied by XRD. 2. Experimetal Sectio 2.1. Materials Lithium perchlorate trihydrate (LiClO 4 3H 2 O, Aldrich, 99,99%) ad O,O'Bis(2 amiopropyl) polypropylee glycolblockpolyethylee glycolblockpolypropylee glycol (commercially desigated as Jeffamie ED900, Fluka, average molecular weight 900 gmol 1 ) ad 3isocyaatepropyltriethoxysilae (Aldrich 95 %) were used as received. Ethaol (Merck, 99.8%) ad tetrahydrofura (Merck, 99,9%) were dried over molecular sieves prior to use. High purity distilled ad deioised water was used i all experimets.
4/28 S.C. Nues et al. 2.2. Sythesis The sythesis of the LiClO 4 doped du(900)based diureasils has bee described i detail elsewhere [19]. 2.3. Experimetal techiques XRD measuremets were performed at room temperature with a PW 3040/60 X Pert Pro Rötge diffractometer system, usig CuK radiatio ( = 1.54 Å) ad BraggBetao /2 geometry. The system icludes the ultrafast X Celerator detector PW3015/20 ad a secodary moochromator. The measuremets were performed over the 2 rage of 1080º. Asprepared ad dried xerogel samples were aalysed as films ( = 20 ad 5) ad powders ( = 1, 0.5 ad 0). The materials were dried by storage i a dessicator uder vacuum at 25 ºC for 20 days. A Setaram DSC131 Differetial Scaig Calorimeter was used to determie the thermal characteristics of the lithium salt. A mass of approximately 1020 mg was trasferred to a 40 µl alumiium ca ad stored i a dessicator over phosphorous petoxide uder vacuum for two weeks at room temperature. After this dryig treatmet, the ca was hermetically sealed ad the thermogram was recorded. The sample was heated from 25 to 300 ºC at 10 ºC mi 1. The purge gas used i both experimets was high purity itroge supplied at a costat 35 cm 3 mi 1 flow rate. The lithium salt sample for thermogravimetric studies was trasferred to a ope platium crucible ad aalysed usig a TA Q50 Thermogravimetric Aalyser at a heatig rate of 10º mi 1 usig dried itroge as purge gas (40 ml/mi ad 60 ml/mi i the balace ad sample chamber, respectively). Prior to measuremet, the sample was vacuumdried at room temperature for about 48 h. FTIR spectra were acquired at room temperature usig a Bruker 22 (Vektor) spectrometer placed iside a glovebox with a dry argo atmosphere. The spectra were
5/28 S.C. Nues et al. collected over the 4000400 cm 1 rage by averagig 150 scas at a spectral resolutio of 2 cm 1. Solid samples (2 mg) were fiely groud, mixed with approximately 175 mg of dried potassium bromide (Merck, spectroscopic grade) ad pressed ito pellets. Prior to recordig the spectra, the pellets were vacuum dried at 8090 ºC for about 60 h, i order to reduce the levels of adsorbed water ad solvet ad the trasferred ito a glovebox. The FTRama spectra were recorded at room temperature with a Bruker IFS66 spectrometer equipped with a FRA106 Rama module ad a earifrared YAG laser with wavelegth 1064 m. The spectra were collected over the 3200300 cm 1 rage at a resolutio of 2 cm 1. The accumulatio time for each spectrum was 4 hours. To evaluate complex FTRama bad evelopes ad to idetify uderlyig spectral compoets, the iterative leastsquares curvefittig procedure of the PeakFit [20] software package was used throughout this study. The best fit of the experimetal data was sought by varyig the frequecy, badwidth ad itesity of the bads ad by employig Voigt bad shapes. A liear baselie correctio with a tolerace of 0.2% was employed. The stadard errors of the curvefittig procedure were less tha 0.002. 3. Results ad Discussio 3.1 Structure The diffractograms of the du(900) LiClO 4 diureasils reproduced i Fig. 1(a) allow us to coclude that the samples with = 20 ad 5 are amorphous. The characteristic amorphous broad peak, Gaussia i shape ad cetred at approximately 21.4º i these XRD patters, is attributed to the coheret diffractig regios of the siliceous etwork [21]. Structural uit distaces of 4.2 ad 4.1 Å were obtaied for these hybrids, respectively, usig the Bragg law. The coheret legth L over which the structural uit survives i these two samples was estimated usig the modified Scherrer equatio L = I / (A cos ), where A, i radias, is the
Itesity (a.u.) 6/28 S.C. Nues et al. itegrated area of the peaks ad I their itesity. Coheret legths of 13 ad 12 Å were derived for this pair of xerogels, respectively. 0 0.5 0 1 20 21 22 23 24 25 2 (º) 0.5 20 10 20 30 40 50 60 70 80 2 (º) 5 10 20 30 40 50 60 70 80 2 (º) 1 Fig. 1. XRD curves of selected asprepared du(900) LiClO 4 diureasils: (a) 5 ; (b) 1 ; (c) magificatio of (b). I the XRD patters of the xerogels with = 1 ad 0.5 a series of sharp Bragg reflectios, the itesity of which icreases as more salt is added, are evidet (Fig. 1(b)). Fig. 1(c) reveals that the low agle peaks of these diffractograms ad of that of the pure salt are distorted ad asymmetrical, demostratig o the left side a broad shoulder that spreads. A possible explaatio for the presece of this asymmetric tail is the existece of itergrowth pheomea betwee salt with differet hydratio states (i.e., stackig faults) [22]. A Rietveld refiemet of these XRD data (Powder Cell 2.4) [23] idicated that the structure is cosistet with a hexagoal cell (P63mc) [24] (Table 1).
Weight (%) Heat flux (mw g 1 ) 7/28 S.C. Nues et al. Calculated cell parameters are preseted i Table 2 for asprepared ad dried samples (see Experimetal Sectio). Upo dryig, cell parameters decrease slightly for the pure salt ad for the ormolyte with = 0.5, but icrease at = 1. Crystallite sizes, calculated from Rietveld Powder Cell refiemet, rage from 120 to 150 m. Table 1 Hexagoal cell parameters for the asprepared ad dried samples asprepared dried a (m) c (m) a (m) c (m) 0 7.720 5.449 7.705 5.439 0.5 7.713 5.443 7.709 5.442 1 7.714 5.445 7.725 5.456 The recogitio that dryig of the ormolyte samples prior to recordig the XRD patters chaged the lattice parameters led us to perform DSC ad TGA measuremets with the guest salt. The DSC curve of the asreceived salt show i Fig. 2 exhibits two edotherms: a sharp, distict ad strog peak with oset at 93 ºC ad a illdefied, multicompoet, broad peak with oset at approximately 125 ºC. 105 50 100 150 200 250 300 100 95 exo 90 85 80 75 70 DSC TGA edo 65 1000 50 100 150 200 250 300 T(ºC) Fig. 2: TGA (left scale) ad DSC (right scale) curves of LiClO 4 3H 2 O
8/28 S.C. Nues et al. Quatitative aalysis of the correspodig TGA curve, also see i the plot of Fig. 2, allowed us to coclude that these evets correspod to the loss of two ad oe water molecules from the catio coordiatio sphere, respectively, idicatig that, after beig maipulated i the ope atmosphere, the perchlorate salt used i the preset work (acquired i the ahydrous state) cotais three hydratio water molecules. These fidigs cofirm that oly dryig at sufficietly elevated temperatures yields the ahydrous salt. For istace, we ote that the spectroscopic study of Chabael et al. [25] was carried out usig lithium perchlorate dried for 6 h uder vacuum ad the getly heated it to 300 ºC. The view of the pure salt structure, deduced from the JMOL software [26] usig the data of Table 1, clearly shows that i LiClO 4.3H 2 O the Li + ios, which lie up alog the caxis, are located iside regular hexagoalshaped chaels formed by water molecules (Fig. 3(a)). Fig. 3(a) also allows iferrig that the water chaels are i tur situated withi the voids of hexagoallike chaels composed of ClO 4 tetrahedra exhibitig a ABAB stackig sequece (plaes A solely composed of oxyge atoms ad plaes B also icludig Cl ios) (Fig. 3(b)). (a) (b) Fig. 3: Structural model of LiClO 4.3H 2 O: [001] view of the structure (a) ad 3D view of the uit cell (b). Violet spheres Li + ios; gree spheres Cl ios; red spheres perchlorate oxyge atoms ad blue spheres water molecules.
9/28 S.C. Nues et al. 3.2 Catio/aio, catio/poe ad catio/crossliks iteractios The charge carrier cocetratio ad ioic mobility are importat parameters that ifluece the coductivity of the electrolyte. Three mai types of ioic species ca be foud i salt/polymer electrolyte systems: (a) free or weakly boded ios with high mobility; (b) catios boded strogly to the host polymer ad with low mobility; (c) ioic aggregates (cotact io pairs ad ioic multiplets) with low to moderate mobility. Ioic associatio i POE/siloxae ormolytes ca be ivestigated usig ifrared ad Rama spectroscopies. Such studies typically ivolve the use of a io probe with vibratio modes that suffer characteristic chages (e.g., frequecy shifts, splittig ad/or itesity variatios) upo coordiatio. The ClO 4 io is oe of the most widely used aios i the framework of spectroscopic aalysis to idetify the ature of the ioic species i polymer electrolytes ad to determie their cocetratio [2]. The free ClO 4 io adopts a tetrahedral (T d ) symmetry ad its ie vibratioal degrees of freedom are distributed betwee four ormal Rama active modes: 1 (A 1 ), 2 (E), 3 (T 2 ) ad 4 (T 2 ). Of these, 1 (A 1 ) ad 2 (E) are iactive i ifrared. O the basis of the idividual ifrared values observed i differet solvets, the followig average waveumbers have bee deduced for free ClO 4 : 1 (A 1 ) = 931 cm 1, 3 (T 2 ) = 1100 cm 1 ad 4 (T 2 ) = 624 cm 1. A average waveumber of 458 cm 1 was derived for 2 (E) from Rama values [25,27]. The lowerig of the local symmetry aroud ClO 4 resultig from iteractio with a catio leads to bad splittig of degeerate vibratios ad to frequecy shifts of odegeerate vibratios. I additio, the ifraredforbidde 1 (A 1 ) ad 2 (E) modes are activated. The magitude of bad separatio ad frequecy shift depeds o the stregth of io associatio. A associatio of oe or three of the ClO 4 oxyge atoms with a catio (moo or tridetate coordiatio,
10/28 S.C. Nues et al. respectively) lowers the symmetry of the aio to C 3v. O the other had, the associatio of a catio with two of the ClO 4 oxyge atoms (bidetate coordiatio) reduces the symmetry to C 2v. The bidetate cofiguratio of the ClO 4 io may be recogised by the splittig of the twofold degeerate 2 vibratio ito two bads (A 1 ad A 2 ) i the Rama spectrum ad by the splittig of the threefold degeerate 3 ad 4 vibratios ito three compoets (A 1, B 1 ad B 2 ) both i the ifrared ad Rama spectra. The moo or tridetate cofiguratios are maifested through the splittig of the threefold degeerate 3 ad 4 vibratios ito two compoets (A 1 ad E) both i the ifrared ad Rama spectra. It is accepted that the ClO 4 symmetry is lowered to C 3v whe this aio forms cotact io pairs with Li + (Li + ClO 4 ) ad to C 2v i the socalled dimerized state Li 2 (ClO 4 ) 2 [25,27,28]. 1 ClO 4 regio: The FTRama odegeerate symmetric stretchig vibratio mode of the ClO 4 io ( 1 ClO 4 ) usually shifts to higher waveumbers [29,30,31,32,33] upo coordiatio. The FTRama spectra of selected doped du(900)based diureasils i the 1 ClO 4 regio ad the results of the curvefittig performed i the 1 ClO 4 bad profiles are represeted i Figs. 4(a) ad 4(b), respectively. The variatio of the itegral area fractio of the isolated compoets with compositio is collected i Table 1 ad represeted i the graph of Fig. 5. The FTRama 1 ClO 4 bad of the du(900) LiClO 4 hybrids with 15 was decomposed ito three compoets (Fig. 4(b) ad Table 2): a bad at 930 cm 1 ad two shoulders at approximately 925 ad 910 cm 1. I the case of du(900) 15 LiClO 4 a ew evet emerges at 936 cm 1. I additio, samples with 5 give rise to a very weak feature at about 909 cm 1 (Fig. 4(b) ad Table 2). The FTRama 1 ClO 4 evelope of the saltrich sample with = 1 was resolved ito three compoets at 936, 930 ad 925 cm 1 (Fig. 4(b) ad Table 2). The 930 cm 1 bad, observed i all the FTRama spectra of the doped samples aalysed, is assiged to free ClO 4 ios [25,29,30,31,32,33,34]. The weak evet located at 909 cm 1 i
Itesity (u.a) 909 Itesity (a.u.) 907 925 930 930 936 934 11/28 S.C. Nues et al. the FTRama spectra of the Li + doped diureasils with 5 is associated with the first overtoe of the 2 (E) deformatio mode of the ClO 4 io (2 2 ClO 4 ) ehaced by Fermi preresoace with the 1 ClO 4 fudametal [25,29,30,31,32]. The 936 cm 1 feature probably arises as a result of io pair formatio ad is idicative of the presece of ClO 4 ios coordiated i a moodetate cofiguratio (C 3v symmetry) [25,29,30,31,32,33,34]. Accordig to Chabael et al. [25], the shoulder at 925 cm 1 may be correlated with the presece of C 2v species. No idicatio of the formatio of dimerised or aggregate species beyod the dimer state have bee foud i the FTRama spectra of the diureasils icluded i this study (at 948 ad 961 cm 1, respectively [25]). 1 1 5 10 10 970 960 950 940 930 920 910 900 890 Rama shift (cm 1 ) 15 60 15 60 960 950 940 930 920 910 900 Rama shift (cm 1 ) Fig. 4: FTRama spectra of selected du(900) LiClO 4 diureasils i the 1 ClO 4 regio (a) ad curvefittig results of represetative samples (b). The frequecies idicated represet the average value of the frequecies of all the samples cosidered. The graph icluded i Fig. 5 demostrates that the relative itesity of the three 1 ClO 4 compoets remais practically uchaged i xerogels with 60 5. Upo further additio
Itegral area fractio ( %) 12/28 S.C. Nues et al. of salt ( = 1) the cocetratio of moodetate ClO 4 ios icreases sigificatly at the expese of a reductio of the free aios (Fig. 5). 31.33 10.44 7.83 Si/Li (gg 1 ) 5.22 2.61 0.52 80 60 40 909 cm 1 924 cm 1 930 cm 1 935 cm 1 20 0 60 20 15 10 5 0 Fig. 5: Compositio depedece of the itegral area fractio of the isolated compoets of the FTRama 1 ClO 4 bad of the du(900) LiClO 4 diureasils. Table 2 Compositio depedece of the itegral area fractio (%) of the resolved compoets (cm 1 ) of du(900) LiClO 4 diureasils i the FTRama 1 ClO 4, 2 ClO 4 ad 4 ClO 4 regios. ClO 4 mode 1 4 2 60 15 10 5 1 Attributio Ref. Area Area Area Area Area cofiguratio ClO 4 935 13.3 936 6.2 935 14.7 935 43.1 moodetate (Li + ClO 4 ) 25,2934 930 87.9 930 72.1 930 82.7 930 73.3 931 47.5 free 25,2934 924 11.5 925 12.4 923 8.2 923 7.3 925 9.4 bidetate 25 912 0.66 909 2.2 909 2.9 909 1.7 2 2 ClO 4 25,2933 639 18.4 bidetate 25 630 2.8 630 6.6 628 21.0 632 40.9 bidetate 25 623 97.2 623 93.4 623 79.0 623 40.7 free 25 463 26.3 465 18.5 463 53.3 bidetate 25,38 457 60.8 458 54.7 460 37.7 free 3537 451 12.9 454 26.7 454 9.8 bidetate 25,38
Itesity (u.a) Itesity (a.u.) 457 453 463 459 465 13/28 S.C. Nues et al. 2 ClO 4 regio: The FTRama spectra of the doped diureasil materials i the regio characteristic of the doubly degeerate deformatio mode of the ClO 4 io ( 2 ClO 4 ) are reproduced i Fig. 6(a). The results of the curvefittig performed o the 2 ClO 4 bad of selected samples are depicted i Fig. 6(b). The variatio of the itegral area fractio of the isolated compoets with compositio is collected i Table 2. The FTRama 2 ClO 4 bad of the doped du(900)based hybrids with = 10 ad 5 was decomposed ito three compoets (Fig. 6(b) ad Table 2): a bad at 459 cm 1 ad two weak shoulders at 465 ad 453 cm 1. The FTRama ClO 4 evelope of the diureasil with = 1 was also resolved ito the same three compoets (Fig. 6(b). ad Table 2). However, i the latter case the 465 cm 1 evet domiates this spectral regio, followed by the bad at 460 cm 1 ad fially by the feature at 453 cm 1 (Fig. 6(b) ad Table 2). 1 1 5 5 10 15 10 500 480 460 440 420 400 Rama shift (cm 1 ) 490 480 470 460 450 440 430 420 Rama shift (cm 1 ) Fig. 6: FTRama spectra of selected du(900) LiClO 4 diureasils i the 2 ClO 4 regio (a) ad curvefittig results of represetative samples (b). The frequecies idicated represet the average value of the frequecies of all the samples cosidered.
14/28 S.C. Nues et al. The 459 cm 1 bad is assiged to free ClO 4 ios [35,36,37]. The existece of the two additioal bads strogly supports the occurrece of C 2v symmetry ad thus bidetate cotact associatio betwee Li + ad ClO 4 [25,38]. Withi the C 2v symmetry the 465 ad 453 cm 1 compoets are attributed to 2 (A 1 ) ad 2 (A 2 ) modes, respectively [25,38]. 3 ClO 4 ad COC regios: I the FTIR spectra of the du(900) LiClO 4 samples the triply degeerate asymmetric stretchig mode of the ClO 4 io ( 3 ClO 4 ) is superimposed o the polymer skeleto COC stretchig ( COC) mode (Fig. 7). The FTIR spectrum of du(900) exhibits a strog bad aroud 1110 cm 1 ad a shoulder at about 1145 cm 1 (Fig. 7) [39]. These evets are attributed to the COC vibratio mode ad to the coupled vibratio of the COC ad rch 2 modes, respectively, ad are characteristic of ocomplexed, amorphous polyether chais [40,41,42]. As the itesity ad frequecy of both features persists essetially uchaged i samples with 20, we are led to coclude that i this salt compositio iterval the ether oxyge atoms of the polymer segmets do ot participate i the coordiatio of the Li + ios ad that the POE chais remai amorphous. With the icrease of salt cocetratio ( = 5) four bads emerge (Fig. 7): two itese, sharp features at 1122 cm 1 (very strog) ad 1108 cm 1 (strog) ad two illdefied, broad bads at 1144 cm 1 ad 1090 cm 1 (uresolved multiplet). Fig. 7 demostrates that the further additio of salt ( = 1) leads to a marked growth of the 1144, 1108 ad 1092 cm 1 evets. The liftig of the triply degeeracy of the 3 ClO 4 mode ito three compoets provides evidece that the local eviromet of ClO 4 must be at least C 2v [27]. The spacig betwee the most separated compoets of the 3 ClO 4 regio (1144 1092 = 52 cm 1 ) is idetical to that foud i solid alkalie metal perchlorates [25], but cosiderably lower tha that reported for argo matrices (226 cm 1 ) [43]. The 1122 cm 1 feature may be tetatively ascribed to the
1104 Absorbace (a.u.) 1144 1090 1108 1122 15/28 S.C. Nues et al. presece of coordiated ClO 4 ios i a C 3v cofiguratio [25]. The 1144, 1122 ad 1108 cm 1 evelopes are characteristic of the 3 (T 2 ) vibratio of the pure crystallie salt [25]. 1 5 10 20 1200 1150 1100 1050 1000 Waveumbers (cm 1 ) Fig. 7: FTIR spectra of selected du(900) LiClO4 di 3 ClO 4 ad COC regios. The frequecies idicated represet the average value of the frequecies of all the samples cosidered. 4 ClO 4 regio: The FTIR ad FTRama spectra of selected du(900) LiClO 4 diureasils i the regio characteristic of the triplydegeerate deformatio vibratio mode of the ClO 4 io ( 4 ClO 4 ) are reproduced i Figs. 8(a) ad 9(a), respectively. The results of the curvefittig carried out i the FTIR ad FTRama 4 ClO 4 bads for represetative xerogels are
626 Absorbace (a.u.) Absorbace (a.u.) 619 637 630 640 627 633 622 630 637 626 16/28 S.C. Nues et al. give i Figs 8(b) ad 9(b), respectively. The variatio of the itegral area fractio of the FT Rama/FTIR isolated compoets with compositio is collected i Table 2 ad Fig. 10/ Table 3. The FTIR 4 ClO 4 mode of the du(900) LiClO 4 hybrid with = 10 was decomposed ito five compoets at 637, 633, 630, 626 ad 622 cm 1 (Fig. 8(b) ad Table 2). I the samples with = 5 ad 1 two ew compoets develop at 640 ad 619 cm 1 respectively (Fig. 8(b) ad Table 2). 1 1 5 10 20 5 10 650 640 630 620 610 600 Waveumbers (cm 1 ) 650 640 630 620 610 Waveumber (cm 1 ) Fig. 8: FTIR spectra of selected du(900) LiClO 4 diureasils i the 4 ClO 4 regio (a) ad curvefittig results of represetative samples (b). The frequecies idicated represet the average value of the frequecies of all the samples cosidered. The FTRama 4 ClO 4 evelope of the Li + based du(900) doped hybrids with = 15, 10 ad 5 was resolved ito oly two compoets: oe bad at 623 cm 1 ad a shoulder at 630
Itesity (u.a) 648 Itesity (a.u.) 639 623 623 632 632 17/28 S.C. Nues et al. cm 1 (Fig. 9(b) ad Table 2). Upo further additio of salt ( = 1), this shoulder becomes the strogest 4 ClO 4 compoet ad a ew evet appears at 639 cm 1 (Fig. 9(b) ad Table 2). The presece of three compoets i the FTRama 4 ClO 4 evelope of the du(900) 1 LiClO 4 compoud supports the explaatio that the local symmetry aroud ClO 4 is C 2v [25]. These fidigs lead us to propose that the FTRama feature see at 623 cm 1 is simultaeously associated with free ad bidetate coordiated ClO 4 ios [25]. 1 1 5 10 5 15 60 650 640 630 620 610 600 Rama shift (cm 1 ) 650 640 630 620 610 600 Rama shift (cm 1 ) 15 Fig. 9: FTRama spectra of selected du(900) LiClO 4 diureasils i the 4 ClO 4 regio (a) ad curvefittig results of represetative samples (b). The frequecies idicated represet the average value of the frequecies of all the samples cosidered. The 633, 626 ad 622 cm 1 bads observed i the FTIR 4 ClO 4 regio (Fig. 8(b) ad Table 3) are attributed to ClO 4 ios i a bidetate cofiguratio, suggestig that the local symmetry aroud ClO 4 must be C 2v [25]. We ote that the FTIR feature see at 626 cm 1 appears to be commo to free ad bidetate coordiated ClO 4 ios. The FTIR evets at
Itegral area fractio ( %) 18/28 S.C. Nues et al. 630 ad 619 cm 1 (Fig. 8(b) ad Table 3) correspod to 37 ClO 4 isotopomers [25]. It is very likely that the feature at 637 cm 1 ad the shoulder at 640 cm 1 are associated with moodetate boded ClO 4 ios [25,32]. 7.83 5.22 Si/Li (gg 1 ) 2.61 0.52 100 80 623 cm 1 630 cm 1 639 cm 1 60 40 20 0 16 14 12 10 8 6 4 2 0 Fig. 10: Compositio depedece of the itegral area fractio of the isolated compoets of the FTRama 4 ClO 4 bad of the du(900) LiClO 4 diureasils. Fig. 10 reveals that the relative itesity of the three 4 ClO 4 compoets remais essetially ualtered i samples with = 15 ad 10. The icorporatio of more salt ( = 5) leads to a moderate icrease of the proportio of bidetate ClO 4 ios (Fig. 10). A reductio of the fractio of free ios occurs i parallel (Fig. 10). At = 1 the cocetratio of free ios markedly decreases ad the proportio of bidetate ClO 4 ios cotiues to grow (Fig. 10). As expected, the tred exhibited by the free 4 ClO 4 compoet as a fuctio of compositio is i perfect agreemet with that displayed by the correspodig 1 ClO 4 feature (cf. Fig. 5). The spectroscopic aalysis performed i the preset work has eabled us to
19/28 S.C. Nues et al. coclude that i the saltrich du(900) LiClO 4 samples with 5 there is clearly a marked tedecy for ioic associatio. Table 3 Compositio depedece of the itegral area fractio (%) of the resolved compoets (cm 1 ) of the more cocetrated du(900) LiClO 4 diureasils i the FTIR 4 ClO 4 regio. 10 5 1 Atributio Ref. Area Area Area 642 6.8 640 6.4 moodetate ClO 4 (Li + ClO 4 ) 25 637 8.5 637 23.7 637 25.3 moodetate ClO 4 (Li + ClO 4 ) 25,32 633 9.9 633 11.7 633 9.9 bidetate ClO 4 25 628 27.3 630 14.9 630 17.6 isotopic 37 ClO 4 25 625 29.1 626 27.4 626 24.0 free ad bidetate ClO 4 25 622 24.1 621 11.6 623 14.1 bidetate ClO 4 25 617 3.9 620 2.7 isotopic 37 ClO 4 25 This observatio is cosistet with the sigificat decrease of the ioic coductivity observed at < 8 [19]. However, we would like to emphasize that the iterpretatio proposed should be treated with cautio, sice although the frequecies of the perchlorate bads i the FTIR ad FTRama spectra of the hybrid materials studied here exactly match those reported for the same aio i solutios of LiClO 4 i several aprotic door solvets [27], the species resposible for the bads foud i the diureasils may be cosiderably differet. Oly by measurig the trasferece umbers, a study that is beyod the scope of the preset work, could we cofirm the claim that the free ClO 4 species are the mai charge carriers i samples with compositios i the viciity of the coductivity maximum. Amide I ad amide II regios: I this sectio the examiatio of the FTIR spectra of the du(900) LiClO 4 hybrids i the amide I (18001600 cm 1 ) ad amide II (16001500 cm 1 ) regios, reproduced i Fig. 11 was carried out with the objective of determiig the rages of salt cocetratio i which the carboyl oxyge atoms of the urea crossliks bod
20/28 S.C. Nues et al. to the Li + ios. This aalysis will eable us to obtai i parallel valuable iformatio regardig the extet of hydroge bodig i these xerogels. The amide I regio of the diureasils correspods to the amide I [44] regio of polyamides [45]. The amide I mode is a complex vibratio that receives a major cotributio from the C=O stretchig vibratio ad is sesitive to the specificity ad magitude of hydroge bodig. [45] Usually the amide I bad cosists of several compoets which correspod to differet C=O eviromets kow as aggregates, associatios or structures. As the absorptio coefficiets of C=O groups belogig to these aggregates ca be differet, it is ot possible to compare itesity values of differet compoets. Cosequetly, oly the chages suffered by each mode represet cocetratio variatios of each aggregate [45,46]. The amide II mode of the diureasils correspods to the amide II [44] mode of polyamides [45]. The amide II bad is a mixed mode that cotais a major cotributio from the NH iplae bedig vibratio [44]. As it is sesitive to both chai coformatio ad itermolecular hydroge bodig, this mode gives reliable iformatio about the distributio of hydroge bod stregths [44]. The aalysis of Fig. 11 demostrates that the additio of Li + ios to du(900) host matrix affects the amide I ad amide II regios of all the diureasils examied, a idicatio that the Li + ios iteract with the carboyl oxyge atoms of the urea crossliks over the etire cocetratio rage cosidered. It may be immediately iferred from Fig. 11 that the amide I regio of the diureasils is extremely complex. Curvefittig of the amide I mode of the du(900) matrix (ot show) permitted the idetificatio of the strog compoets aroud 1678 ad 1646 cm 1 ad the two shoulders located at about 1756 ad 1723 cm 1. The bad at 1756 cm 1 is associated with the absorptio of urea groups i which the NH or C=O groups are free from ay iteractios [47]. The bads cetred ear 1723 ad 1678 cm 1 are ascribed to the absorptio of hydrogeboded
1756 Absorbace (a.u.) 1723 1565 1572 1678 1660 1646 1624 1615 21/28 S.C. Nues et al. C=O groups of disordered hydrogeboded POE/urea associatios of icreasig stregth [47]. Fially, the 1646 cm 1 feature is assiged to the absorptio of C=O groups icluded i sigificatly more ordered hydrogeboded urea/urea associatios [47]. The bad at 1565 cm 1 is ascribed to the amide II mode (Fig. 11). 0.5 1 5 10 60 200 1800 1700 1600 1500 Waveumbers (cm 1 ) Fig. 11: FTIR spectra of selected du(900) LiClO 4 diureasils i the amide I ad amide II regios. The frequecies idicated represet the average value of the frequecies of all the samples cosidered. The aalysis of Fig. 11 demostrates that the additio of Li + catios to the du(900) host matrix perturbs the amide I regio, solid evidece that the alkali metal ios coordiate to the carboyl oxyge atoms of the urea crossliks over the whole rage of salt cocetratio examied. The two itese broad bads of the amide I evelope of du(900), situated
22/28 S.C. Nues et al. aroud 1678 ad 1646 cm 1, due to ordered POE/urea structures, are replaced by a sigle broad bad cetred at 1660 cm 1 i samples with =10 ad 5 (Fig. 11). The 1756 ad 1723 cm 1 features practically disappear upo salt additio. At = 1 saturatio of the urea crossliks is attaied. I additio, at this compositio, the bad at 1660 cm 1 becomes a shoulder ad a bad at 1646 cm 1, attributed to urea/urea aggregates, is trasformed ito a more itese evet. At = 0.5 the itesity maximum is located at 1624 cm 1 ad a ew evet emerges at 1624 cm 1 (Fig. 11). These fidigs suggest that the hydrogeboded associatios formed at high salt cotet are more ordered (ad thus stroger) tha those foud i the less cocetrated hybrids. The itese amide II mode of the du(900) observed at 1565 cm 1 (Fig. 11) shifts to 1572 cm 1 upo additio of salt, a clear idicatio that the hydroge bods established are stroger tha those observed i the host matrix. I saltrich materials with = 1 ad 0.5 the amide I ad amide II bads are superimposed (Fig. 11). 4. Coclusios Diureasil ormolytes doped with a wide cocetratio of LiClO 4.3H 2 O (200 05) were prepared by the solgel method ad aalysed by FTIR ad FTRama spectroscopy ad XRD. Structural data revealed that free salt exists i samples with = 1 ad 0.5. The hydratio state of the guest salt is a critical parameter to cosider i the characterisatio of the samples. Xerogels with high to moderate ioic coductivity are amorphous. Free ClO 4 ios appear to be the mai charge carriers of the coductivity maximum of this family of ormolytes located withi the 25 8 compositio rage. At = 15 ClO 4 ios coordiated through moo/tri ad bidetate cofiguratios (C 3v ad C 2v symmetry, respectively) appear. I the saltrich samples with < 15 the marked tedecy for ioic associatio that occurs at
23/28 S.C. Nues et al. the expese of a drastic reductio i the cocetratio of free ios is cosistet with a sigificat decrease of the ioic coductivity. The aalysis of the amide I ad amide II regios provided solid proof that the Li + ios bod to the urea carboyl oxyge atoms over the etire rage of salt cocetratio studied. Ackowledgmets The authors are pleased to ackowledge the support provided by Uiversidade do Miho ad Uiversidade de TrásosMotes e Alto Douro ad Fudação para a Ciêcia e a Tecologia (cotracts POCI/QUI/59856/2004, POCTI/SFA/3/686 ad SFRH/BD/2207/2005). List of figure captios Fig. 1. XRD curves of selected du(900) LiClO 4 diureasils. Fig. 2: TGA (left) ad DSC (right) curves of LiClO 4.3H 2 O Fig. 3: Structural model of LiClO 4.3H 2 O: [001] view of the structure (a) ad 3D view of the uit cell (b). Violet spheres Li + ios; gree spheres Cl ios; red spheres perchlorate oxyge atoms ad blue spheres water molecules. Fig. 4. FTRama spectra of selected du(900) LiClO 4 diureasils i the 1 ClO 4 regio (a) ad curvefittig results of represetative samples (b). The frequecies idicated represet the average value of the frequecies of all the samples cosidered. Fig. 5: Compositio depedece of the itegral area fractio of the isolated compoets of the FTRama 1 ClO 4 bad of the du(900) LiClO 4 diureasils.
24/28 S.C. Nues et al. Fig. 6: FTRama spectra of selected du(900) LiClO 4 diureasils i the 2 ClO 4 regio (a) ad curvefittig results of represetative samples (b). The frequecies idicated represet the average value of the frequecies of all the samples cosidered. Fig. 7: FTIR spectra of selected du(900) LiClO 4 di 3 ClO 4 ad COC regios. Fig. 8: FTIR spectra of selected du(900) LiClO 4 diureasils i the 4 ClO 4 regio (a) ad curvefittig results of represetative samples (b). The frequecies idicated represet the average value of the frequecies of all the samples cosidered. Fig. 9: FTRama spectra of selected du(900) LiClO 4 diureasils i the 4 ClO 4 regio (a) ad curvefittig results of represetative samples (b). The frequecies idicated represet the average value of the frequecies of all the samples cosidered. Fig. 10: Compositio depedece of the itegral area fractio of the isolated compoets of the FTRama 4 ClO 4 bad of the du(900) LiClO 4 diureasils. Fig. 11: FTIR spectra of selected du(900) LiClO 4 diureasils i the amide I ad amide II regios. The frequecies idicated represet the average value of the frequecies of all the samples cosidered. Tables Table 1 Hexagoal cell parameters for the asprepared ad 20 days vacuum dried samples
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