Langmuir 1996,11, 4773-4778
4773
Reactions of Complex Ions of Platinum and Palladium in Langmuir-Blodgett Films of Metal Arachidates David J. Elliot,? D. Neil Furlong,*>?Thomas R. Gengenbach,? Franz Grieser,* Robert S. Urquhart,? Catherine L. Hoffman,§ and John F. Rabolts CSIRO, Division of Chemicals and Polymers, Private Bag 10, Rosebank MDC, Clayton, Victoria 3169, Australia, Advanced Mineral Products Research Centre, School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia, and IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120 Received May 3, 1995. In Final Form: September 1, 1995@ Surface pressurdarea per molecule (n-A)isotherms of arachidic acid monolayers on subphases containing various PtlI a n d Pd*I amine complexes were measured. Metal arachidate LB films derived from these monolayers were deposited on gold-coated glass and mica substrates. The reactions of these films with H2S were investigated using W-visible, grazing angle Fourier transform infrared, and X-ray photoelectron spectroscopic techniques a n d atomic force microscopy. The absorbance onsets of the H2S exposed films are dramatically blue shifted relative to the bulk bandgaps of PtS and PdS consistent with the formation of particles in the “Q state” size regime. Atomic force microscopy images of palladium arachidate films on mica exposed to H2S revealed particles with an average diameter of 4.8 nm.
Introduction
A diverse range of metal ions have been incorporated i n t o LB films. Some of these include Ca2+and Mg2+ from the g r o u p IIa metals,’ Ti4+,2Fe3f,3M2+(M = Mn,4 R u , Ni, ~ Pd, Pt,6C U , Cd,8 ~ Hg9), and Agf from the transition metal series,I0 and Pb2+from the main block elements.’l The reactions of s o m e of these metal ions with (X = 0, S, Se, Te), HX (X = halide)12and reducing agents’O t o give metal chalcogenides, metal halides, and the metallic element, respectively, have been studied. A common observation has been that the resulting species are in the form of particles 2-10 nm in size. This size range has been termed the “Q-state”regime in which the materials exhibit optical and redox properties intermediate between individual molecules and the bulk mate1-ia1.I~ The strategy employed for producing Q-state semiconductor particles in a q u e o u s media has b e e n t o r e a c t metal ions with an appropriate agent t o produce the semiconductor in the presence of stabilizing m e d i a , s u c h as thiol solutions,14phospholipid membranes,15 micelles,I6porous
* Author to whom correspondence should be addressed: fax, 613-9542-2515; telephone, 61-3-9542-2618; e-mail, nfurlong@ csiro.chem.au. + CSIRO. University of Melbourne. 8 IBM Almaden Research Center. Abstract published in Advance ACS Abstracts, November 1,
*
@
1995.
(1)Goddard, E. D.; Ackilli, J. A. J . Colloid Sci. 1963,18,585. (2)Ganguly, P.; Paranjape, D. V.; Sastry, M. Langmuir 1993,9,577. (3)Prakash, M.; Peng, J. B.; Ketterson, J. B.; Dutta, P. Thin Solid Films 1987,146,L15. (4)Bettarini, S.;Bonsoi, F.; Gabrielli,G.; Martini, G. Langmuir 1991, 7, 1082. (5) Samha, H.; DeArmond, M. K. Langmuir 1994,10,4157. (6) Burghard, M.; Schmelzer, M.; Roth, S.; Haisch, P.; Hanack, M. Langmuir 1994,10, 4265. (7)Chen, H.; Chai, X.; Wei, Q.; Jiang, Y.; Li, T. Thin Solid Films 1989,178,535. ( 8 )Furlong, D. N.; Urquhart, R. S.;Grieser, F.; Tanaka, K.; Okahata, Y.J . Chem. Soc., Faraday Trans. 1993,89,0. (9)Elliot, D. J.; Furlong, D. N.; Gengenbach, T.; Grieser, F. Colloid Surf. 1995,102,45. (10)Leloup, J.; Maire, P.; Ruadel-Teixier, A,; Barraud, A. J . Chim. Phys. 1985,82,695. (11)Vogel, C.; Corset, J.; Dupeyrat, M. J . Chim. Phys. 1979,76,903. (12) Ruaudel-Teixier, A.; Leloup, J.; Barraud, A. Mol. Liq. Cryst. 1986,134,347. (13)Weller, H. Adu. Mater. 1993,5,88.
Ti02 membranes,17 zeolites,18 and inverse microemulsions. l9 Size quantization of semiconductor particles provides a means of controlling optical band gaps and generating large third-order optical effects.13 Thus the fabrication of semiconductor particles in LB films m a y be of practical use in the development of optoelectronic devices s u c h as optical switches, infrared detectors, o r photovoltaics. The reactions of PtI1 and PdI1 complex ions in LB films of arachidic acid with H2S were investigated t o further o u r understanding of the role of L B films in Q-state particle formation.
Experimental Section Materials. Arachidic acid (eicosanoic acid) (99%)was obtained from Fluka. The ethylenediamine (99%) and MCl2 complexes (M = Pt, Pd 98%)were purchased from Aldrich. Triethylenetetraamine (trien) was purchased from Tokyo Kasei. The 25% ( w h ) ammonia solution was an analytical grade reagent from BDH. All other materials were as described in another publicationz0 Preparation of Subphase Solutions. (i) [M(NH&]C12 Subphases, M = Pt, Pd. The [M(NH3)41Clz complexes were prepared using a modified version of Reinhardt et aLZ1 Approximately 2 x low4mol ofMClz complexes was stirred in excess ammonia (5 mL of 25% (w/v)) giving a green precipitate of the complex [M(NH&] [MCld]in the case of platinum (Magnus’salt)22 and a pink precipitate in the case of palladium (Vauquelin’s salt).23 With further stirring the solid redissolved. The solution was filtered through a sintered glass frit and was diluted to 1 L for use as a subphase for monolayer compression and LB film deposition. The absorbance spectrum of the [Pd(NH3)41Clz (14)Herron, N.;Wang, Y.; Eckert, H. J.Am. Chem. SOC.1990,112, 1322. (15)Zhao, X.K.; Barai, S.; Rolandi, R.; Fendler, J. H. J . A m . Chem. SOC.1988,110,1012. (16) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigenvald, M. L.; Carroll, P. J.; Brus, L. E. J . Am. Chem. Soc. 1990, 112, 1327. (17)Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990,174,241. (18) Wang, Y.; Herron, N. J. Phys. Chem. 1987,91,257. (19)Motte, L.; Petit, C.; Boulanger, L.; Nixon, P.; Pileni, M. P. Langmuir 1992,8,1049. (20) Elliot, D. J.; Furlong, D. N.; Gengenbach, T. R.; Grieser, F.; Urquhart, R. S.; Hoffman, C. L.; Rabolt, J. F. Colloid Surf., in press. (21)Reinhardt, R. A.; Brenner, N. L.; Sparkes, R. K. Inorg. Chem. 1967,6,254. (22)Keller, R. N. Inorg. Synth. 1946,2,250. (23)Rasmussen, L.;Jorgensen, C. K. Acta. Chem. Scand. 1968,22, 2313.
0743-746319512411-4773$09.00/0 0 1995 American Chemical Society
4774 Langmuir, Vol. 11, No. 12, 1995 subphase (A, = 297 nm, molar extinction coefficient E = 203 M-I cm-l) is in good agreement with that found in a previous The previouslyreported aqueous spectrum of Pt(NH3)JClz is as follows: 284 n m (broad) E = 44 M-l cm-l; 238 nm (shoulder) E = 155 M-l cm-l; 220 nm (shoulder) E = 484 M-l cm-l, and 196 E = 10 600 M-l cm-1.26 The subphase solution prepared by dissolving PtClz in excess ammonia exhibits a shoulder at about 285 nm ( E = 44 M-l cm-l) confirming the presence ofthe [Pt(NH3)4lZ+ion. The subphase spectrum absorbs strongly below 220 nm (at 196 nm, E = 18 000 M-' cm-l) and is likely that some other species is present. The pH values of the [Pd(NH&]Clz and [Pt(NH3)41C12subphases were 10.8 and 11.1, respectively. (ii) [Pt(en)z](ClO& Subphases. Ethylenediamine (en) (0.53g, 8.7 mmol) in 2 mL of water was added to 0.255 g (0.96 mmol) of PtClZ, and the mixture was stirred. The initial pink precipitate of [Pt(en)z][PtC14Iz6redissolved to give a pale yellow solution. This solution was added to 40 mL of ethanol and kept below 0 "C for an hour. The resulting white precipitate was filtered and washed with ethanol, dried under vacuum, and dried to constant weight at 80 "C to give 0.316 g (0.817 mmol) of [Pt(en)zlClz (85% yield). The [Pt(en)z][Clh (0.0850,0.22 mmol) was dissolved in 5 mL of water, and 0.0863 g (0.42 mmol) of AgC104 in 1 mL of water was added. A white solid immediately precipitated and the mixture was stirred for 30 min in the dark. The mixture was filtered through a sintered glass frit and then aMillipore 45pm pore filter paper fitted to a glass syringe. Water (150 mL, 3 x 50 mL) was passed through the filter paper prior to filtering the [Pt(en)2](C104)2solution. The filtered solution mol of HC104 for use was then diluted to 1 L with 1.02 x as a subphase. The pH of the subphase was 4.1. The UVvisible spectrum of this solution (Am, = 203 nm, E = 7800 M-' cm-I; A,, = 224 nm, E = 471 M-1 cm-l; Am= = 283 nm, E = 39.5 M-' cm-l) is in good agreement with a previous study of the [Pt(en)zlZ+ion.25 Stable monolayers of arachidic acid were formed on subphases of [Pt(en)z](C104)2and regular 'Y type deposition was observed for hydrophobic quartz dipped through these monolayers. XPS results indicated that very little Pt(I1) was incorporated into the film and studies of this system were discontinued. (iii) [Pd(trien)]X2Subphases. Typically, subphases of [Pd(trien)](X)z(X = C104or NO3)were prepared as follows. To 0.0541 g (0.305 mmol) of PdClz was added 5 mL of 25% ( w h ) ammonia solution. The mixture was stirred until all ofthe solid dissolved, at which point 0.04476 g (0.3061 mmol) oftrien in 1mL ofwater was added. The ammonia was boiled off over 15 min during which time the solution changed from colorless to pale yellow. At this point 0.61 mmol ofAgX (X = NO3, C104) in 1mL ofwater was added resulting in the immediate precipitation of a white solid. The solution was stirred for 30 min in the dark and then filtered through a sintered glass frit and then Millipore filter paper as described above. The filtrate was diluted to 1L for use in subphase solutions. The pH ofthe [Pd(trien)](ClO4)zsubphase was 8.1. The absorbance spectrum (Am, = 298 nm, E = 754 M-' cm-l) was in good agreement with a previous study of the [Pd(trien)12+ ion.27 For all the subphases prepared above, the absorbance spectrum was measured before and after LB film deposition. There was no significant change in the spectra during the course of each experiment. Experimental Techniques. A 3.8 x M solution of arachidic acid (ArH) in CHC13 was used for all LB films and R-A isotherms. Further details on monolayer compression and LB film deposition, substrate hydrophobation, preparation of metal sulfide particles in film, and other experimental techniques can be found in another publication.20
Results and Discussion LB Film Preparation and Properties. The surface pressure/area per molecule (n-A) isotherms for arachidic (24) Chatt, J.;Gamlen, G. A,; Orgel, L. E. J . Chem. SOC.1968,486. (25)Mason, W.R.Inorg. Chem. 1986,25,2925. (26)Basolo, F.;Bailar, J. C.; Tarr, B. R. J . Am. Chem. SOC.1960,72, 2433. (27)Hori, F.;Matsumoto, K.; Ooi, S.; Kuroya, H. Bull. Chem. SOC. Jpn. 1977,50,138.
Elliot et al.
Figure 1. Surface pressure versus area per molecule (n-A) isotherms for air-water monolayers of arachidic acid spread on subphases of [M(NH&]C12 (M = Pt,Pd) and [Pd(trien)I(NO& complexes.
acid spread on subphases containing [M(N4)12+(N4 = 4(NH3),M = Pd, Pt; N4 = trien, M = Pd} ions are given in Figure 1. For all the n-A isotherms the area per molecule reached a t the surface pressure of 20 mN m-l maintained for dipping is between 0.20 and 0.19 nm2.This is consistent with the area of the carboxylate head group of the fatty acid in a condensed mono1ayer.l~~ LB deposition experiments were conducted 5 min after a surface pressure of 20 mN had been obtained. Typically a there was also a l-min delay a t the top of each dip cycle. For all films in Figure 1, there was virtually no change in the areas ofthe films during the delay periods indicating that the monolayers of arachidic spread on these subphases are stable. LB film deposition of metal arachidate films onto hydrophobed quartz substrates was found to exhibit typical "Y-type" behavior with transfer ratios of 1 on both up and down dips. Transfer of surfactant monolayers onto freshly cleaved mica was attempted from [Pd(trien)l(C104)~ and [Pd(NH&I Clz subphases. For the [Pd(trien)l(C104)~subphase, monolayers were deposited as the substrate was moved upward through the air-water interface. The monolayers were subsequently removed during each downward passage through the interface. Thus a maximum of one monolayer could be deposited. This phenomenon was observed for transfer of films onto mica from dimethyldioctadecylammonium bromide (DDAB) monolayers on a KzPtCls subphase.20 For the [Pd(NH3)41(C104)~ subphase the average amount of film deposited on the up dip exceeded the amount lost on the down dip and more than a monolayer could be deposited. Absorbance Data. The absorbances of the [M(N4)I2+ subphases were measured before and after LB film deposition and no changes were detected in the subphase over the period of time required for a typical dipping experiment (3-4 h). The previously reported absorbance spectra of the [M(N4)I2+ions are in good agreement with those measured in this study with the exception of the [Pt(NH&I2+ion as discussed in the Experimental Section. The only feature in the absorbance spectrum of the ArW [Pt(NH&]C12film is a shoulder a t 215 nm. After exposure to HzS a distinct band develops a t about 330 nm (Figure 2). The absorbance spectrum changes with continued exposure to H2S. The difference between subsequent spectra, however, becomes progressively featureless and resembles that of a scattering curve. Bandgaps can be estimated by plotting (uhw)2versus energy (in electronvolts, eV), where u is the absorption coefficient and h w is the photon energy, and interpolating the linear region of
Pt and Pd Reactions in LB Films
Langmuir, Vol. 11, No. 12, 1995 4775
-
0.2 I -...- 20 layer ArW[Pt(NH,),lCI, film
20 layer ArH/[Pd(NH,),lC12film
1.25 hours H2S
0.25
;
e 2
%
I
30 min. H2S
0.15
13.3 hours H2S
........- remeasured after 16 hours
0.2 0.15 0.1
0.05 0
200
300
400
500 600 wavelength (nm)
700
Figure 2. Absorbance curves (corrected for absorbance of quartz) of a A I - W [ P ~ ( N H ~film )~]C at~0,~ 1.3, 13, 100,240,and 290 h of exposure to H2S. lo00 h
7g
800
8
1
[Pd(t~ien)](ClO,)~ subphase
...___.._ ArH/[Pd(trien)](C104)2film
0
250
350 wavelength (nm)
300
400
0 200
800
450
Figure 3. Absorbance of a [Pd(trien)l(ClOJzsubphase and an LB film of arachidicacid (correctedfor the absorbanceof quartz) prepared from this subphase.
the curve down to the energy axis.28 A (uhw)z/eVplot for the gassed ArW[Pt(NH&]Clz film provides a bandgap of 3.0 eV after 13.3h of H2S exposure, whereas the bandgap ofbulk PtS is 0.8 eV (1550 nm).29 The bandgap in the film decreases slightly as the gassing time is increased. The large blue-shifted bandgaps observed for semiconductors in LB films have been attributed to restriction of particle growth, by the film, to the Q state regime.9 The absorbance spectrum of the LB film deposited from monolayers of ArH on [Pd(trien)l(C104)~ containing subphases34(Figure 3) exhibits a peak at 345 nm. The large red shift of the peak from the aqueous spectrum (48 nm) could be rationalized as being due to ion pair interaction^.^ ArW[Pd(NH&IClz films absorb strongly below 250 nm and contain a shoulder at about 215 nm. After exposure to HzS for 30 min the films become orange-brown with a n absorbance maximum at 355 nm (Figure 4). When remeasured after 16 h in air, the maxima have not shifted but rather the absorbance tails off more gradually toward the longer wavelengths. The bandgap of bulk PdS is 0.5 eV.29 The bandgaps estimated from (aho)2/eVplots for a film gassed for 30 min and then remeasured after 16 h are 2.3 and 2.0 eV, respectively. One explanation for this is (28)Zhao, X. K.;Yuan, Y.; Fendler, J. H. J . Chem. SOC.,Chem. Commun. 1990, 1249. (29)Goodenough, J. B.;Hamnett, A.; Huber, G.;Hulliger, F.; Leib, M.; Ramasesha, S. K.; Werheit, H. In Landolt-Bornstein, Numerical Data and Functional Relationships in Science and Technology, New series Group III, Volume 17,Semiconductors Subvolume g, Physics of Non-Tetrahedrally Bonded Binary Compounds III; Madelung, O., Ed.; Springer-Verlag: Heidelberg, 1984;pp 617,620.
400 500 600 wavelength (nm)
300
700
800
Figure 4. Absorbance of a film from a [Pd(NH3)&12/NH3 subphase after exposure to HzS, and then remeasured after 16 h in air.
that “Qstate”partic1es in the films continue to grow after the initial absorbance measurement. Gassed ArW[Pd(trien)l(C104)2and D D A B K Z P ~ films C ~ ~also ~ ~ produce a n absorbance maximum a t 355 nm and it appears that the same material forms in these films as in gassed ArW [Pd(NH3l41 Clz films. Absorbance data have been used to gauge particle size in the Q state regime for materials such as CdS, ZnS,30-32 and PbS.33 No such studies have been conducted with platinum or palladium sulfides. Thus, while quantum size effects are apparent in this study, a particle size cannot be extracted from absorbance spectra. The rate of reaction of Pt(I1) and Pd(I1) ions in LB films of arachidic ccid with HzS is very different from that for PtC14’-, PtC1&, and PdC1d2- ions in LB films made from DDAB.20 For example, major changes of the absorbance spectra of the A r m ” films associated with metal sulfide formation are observed within minutes of HzS exposure. For the same exposure period there are only minute changes observed in (DDA)Z(MCl,) films. Surrounding the metal ions in the (DDA)(MCl,) films are chlorides which are very weak bases and the quarternary alkylammonium head group of the DDA cation. In the films made from arachidic and metal(I1) amine complexesthere are basic carboxylates and in some cases amine ligands surrounding the metal. This strongly suggests that the rate-determining step of the reaction of metal complexes in LB films with HzS is deprotonation of the HzS. For MCln2- (M = Pt, n = 4 or 6; M = Pd, n = 4) ions incorporated into DDAB films there was a good correlation between aqueous spectra of MCln2-ions in the subphase and LB films prepared from their respective subphases.20 In contrast, with the films made from arachidic acid there is no conclusive evidence from absorbance data about the nature of the MI1 ion in the film. XPS Data. (i) Films before Exposure to HzS. The XPS results for LB films prepared from ArH monolayers on subphases containing [M(N4)I2+complexes are presented in Table 1. Hypothetically, the exchange of ~~
~
~
(30)Lippins, P.E.;Lannoo, M. Phys. Rev. B 1989,39, 10935. (31)Wang, Y.;Herron, N. J . Phys. Chem. 1991,95,525. (32)Wang, Y.;Herron, N. Phys. Rev. B 1990,42,7253. (33)Wang, Y.;Suna, A.; Mahler, W.; Kasowski, R. J . Chem. Phys. 1987,87,7315. (34)The extinction coefficient E (units of M-l cm-l) in the films was defined as E = l W n 0 whereA is the absorbance, 0 is the concentration of complex metal ion in the film expressed as moles per m2, and n is the number of layers. For example, from the n-A isotherm of ArH on the [Pd(trien)](ClO& subphase the area occupied by each arachidate m2. Assuming 100% in the deposited film is approximately 19 x ion exchange (eq 1)this corresponds to 5.26 x 1018 mol of Pt(II)/mz for each bilayer or 8.74x mol/m2. There are 30 bilayers for the film in Figure 2 and thus the expression becomes E = 3814M.
Elliot et al.
4776 Langmuir, Vol. 11, No. 12, 1995 Table 1. X P S Data of LJ3 Films Containing Comdex Ions of Pt" and PdlIa
1 h with
His
a surf = surfactant, M = metal, N = nitrogen, C1= chlorine, S = sulfur, 0 = oxygen. The values in parentheses are calculated based on the formula given in the left-hand column. The surfactant to metal and chlorine to surfactant ratio are based on the total carbon in the sample assuming that the surfactant is the primary source of carbon in the case of [M(NH3)41Ar2films and is corrected for -N(CH2)2Ncarbons in films containing (trien) and (en) ligands. A silicon peak from the Si02 substrate was detected which contributes to the oxygen signal.
protons of the arachidic acid (ArH) {Ar = CH3(CH2)&00-} monolayer for [M(N4)I2+ions in the films should occur according to eq 1.The values in parentheses
[M(N,)IX,
+ 2ArH
-
[M(N,)IAr,
+ 2HX
(1)
in Table 1are calculated based on this stoichiometry. The binding energies of the Pt 4 f 7 ~and Pd 3d5D peaks are consistent with PtI' and Pd" being present in the film, respectively. For ArW[M(NH3)4lCl2(M = Pt or Pd) films, the surfactantlmetal ratio and oxygedmetal ( O M )ratios are consistent with one metal ion being associated with two surfactant molecules. The nitrogedmetal (NM)ratios are 2.3 and 1.2 f0rArW[Pt(NH3)~lCl2 andArW[Pd(NH3)41Clz films, respectively, which are smaller than the expected value of 4 for the [M(NH&IAr2 formulation. Loss of amine ligand by the action of the X-ray beam or vacuum used for XPS may account for the low nitrogedmetal ratio. IR evidence (see IR data section) indictates, however, that the nitrogen is present in the film as a n ammonium ion. This suggests that the loss of amine occurs during complexation of the metal ion by the surfactant rather than during the XPS analysis. There is also approximately one chloride per surfactant for both ArW[M(NH3)41C1z (M = Pt or Pd) films. Triethylenetetraamine ( t r i e d was used to facilitate incorporation of a [M(N4)12+species into LB films by suppressing ligand dissociation. The N M metal ratio for films prepared from [Pd(trien)12+containing subphases is only 62%of the expected value. Assuming that no cleavage of the (trien) ligand has occurred, there must be some Pd(I1) in the film without associated nitrogen. In fact there are two types of palladium in the film, distinguished by the positions of their peaks in the XPS analysis. One of the peaks constitutes 76% of the palladium in the film which correlates reasonably well with the 62% of the expected N/M ratio. The surfactantlmetal ratio for the ArH/[Pd(trien)lZffilm is roughly two times higher than that expected, meaning that arachidic acid proton exchange for the M(I1) complex ions has occurred to only about 50% of its potential. No chlorine was detected in ArW[Pd(trien)](C104)2film. (ii) Films after Exposure to H2S. The reactions of the films, assumed to have the formula [M(N)41Arz,with H2S could have the stoichiometry given in eq 2 and the values in parentheses for gassed films in Table 1 were calculated on this basis. [M(N),IAr,
+ H,S
-
MS
+ 2ArH + 4NH,
(2)
For the gassed films the sulfurlmetal (S/M) ratios of 3.7 {ArWl'Pt(NH3)41Cl~ film}and2.2 (ArWPd(NH3)41C12 film] and 1.8 {ArW[Pd(trien)l(C104)2film} indicate a n excess of sulfur in the films if MS (M = Pt or Pd) is considered to be the final product. Possible explanations for the excess
of sulfur in the film are polysulfide (Sn2-) formation, elemental sulfur deposition, both of which require oxidation of sulfide (S2-)and bisulfide (SH-) complex formation. The presence of only one type of sulfur peak in the XPS data makes it difficult to reach any conclusions about the nature of the excess sulfur. The excess of sulfur in the films after gassing has also been noted for LB films made from DDAB containing PtCle2-,PtCL2-, and PdC1dZ-ions.20 After the arachidic acid films were gassed, the N/M ratios for the ArW[Pt(NH&l Cl2 and ArW[Pd(NH3)41Clz films drop to 0.78 and 0, respectively. Also in both cases, the CYM ratio almost dropped to zero after gassing. Apossible explanation is that the metal ion, once exposed to H2S, releases the complexed chloride and the NH&l produced in the film is removed under the vacuum used for the XPS analysis (NH4C1- NH3 HC1). There is a shiR in the Pd 3d 512 and Pt 4f 712 peak positions in the films after gassing to values consistent with PdS and PtS, r e ~ p e c t i v e l y .Also, ~ ~ the position of the sulfur 1s peaks arising from gassing the ArW[Pd(NH3)41Cl2 and ArW[Pt(NH&IClz films is eonsistent with PdS and PtS, respectively. IR Data. LB films {ArW[M(NH3)4lC12(M = Pd, Pt)}, deposited onto gold-coated glass slides, were examined by grazing angle FTIR spectroscopy. The results are given in Figures 5 and 6. The different regions of the spectra are assigned according the various vibrational modes of the aliphatic and carboxyl portions of the fatty acid (Table 3) and of the complexed ammonia in the film as detected by XPS (Table 2). Complexed amine species exhibit a strong band in the region 1240-1430 cm-l attributed to the 6,(NH3) vibration (6,= symmetric bending mode).40 For the range of complexes in Table 2 the 6JNH3) band of metal-complexed ammonia (1245-1325 cm-') clearly falls a t a lower wavenumber than in an ammonium ion (1403- 1410cm-l). There are signals a t 1420 and 1406 cm-l in the spectra of the ArW[Pt(NH3)41Clz and ArW[Pd(NH3)41C1zfilms, respectively, which are probably broadened by a contri) = symmetrical stretchingmode) bution from the v , ( C 0 ~(v, of the surfactant. Also the absence of a strong signal in the 1245-1325 cm-l band in the LB films suggests that a n ammonium ion is present in the film. The spectra ofthe ArW[M(NH&IC12 films in this study before gassing (spectra a) are clearly similar to the other
+
(35) Vissers, J. P. R.; Groot, C. K.; Van Oers, E. M.; de Beers, V. H. J.: Prins. R. Bull. Chim. Belg. 1984. 93. 813. ~-~ '(36) Nyquist, R. A.; fagel, R. 0. Infrared Spectra of Inorganic Compounds; Academic Press: New York, 1971. (37) Shimanouchi, T.; Nakagawa, I. Inorg. Chem. 1964,3,1805. (38) %bolt, J. F.; Bums, F. C.; Schlotter, N. E.; Swalen, J. D. J . Chem. Phys. 1983,78, 946. (39) Marshbanks, T. L.; Jugduth, H. K.; Delgass, W. N.; Franses, E. I. Thin Solid Films 1993,232,126. (40) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds, 2nd ed.; Wiley-Interscience: New York, 1970.
-
-I
I
Pt and Pd Reactions in LB Films
Langmuir, Vol. 11, No. 12, 1995 4777
Table 2. Assignments of the IR Active Vibrational Modes of the (NHs) Moiety in Metal Complexes and in LB Films Containing Complex Ions of Pd” and Pt1IU complex
v(NH3)
6d(N H3
ds(NH3)
ArW[Pt(NH3)41Clzfilm ArWPt(NHd41Cl2
3255 m, br, 3125 sh
1650-1580 br 1580 w
in the region 1380-1440 sb
1630 m
not evident
not evident
fildgassed
ArW[Pd(NH&]Clz film
br, 3100 sh
not evident
not evident
in the region 1370-1430 sb not evident
3175,3045,2803 3195,3045,2810 3236,3156 3268,3142
1625 1650 1563 1601
1410 1407 1325 1285
3250 m,
a All peak positions are given in wavenumbers (cm-l). v = stretchingmodes, 6 d = bending modes, 6, = symmetric bending modes. Band assignments are qualified as follows: w = weak, m = medium, s = strong, br = broad, sh = shoulder. Overlapping with v(C02).
Table 3. Assignments of IR Active Vibrational Modes of Aliphatic and Carboxyl Portions of Metal-Fatty Acid Complexes in LB Films” film
hW[Pt(NHd41Clz gassed
hW[Pd(NHd41Clz gassed cadmium a r a ~ h i d a t e(reflectance) ~~ cadmium ~ t e r a t (transmission) e~~
v(CH3) + ~ ( C H Z ) Y(OH) m-s not evident m-s 3150 br, 2650 br not evident m-s m-s 3300 m, br, 2675 m, br m not evident not present
2961-2849 2961-2850 2960-2850 2962-2849 2918-2962 2956-2850
Va(C0z)
1538 m 1538 w 1577 m, 1540 sh 1575 w,1540 sh 1554 sh, 1544 sh 1544
&CHd 1470 sh 1470 sh 1468 m 1470 sh
not evident 1472,1461
Vs(CO2)
in region 1380-1440 sb 1422 s, sharper in the region 1370-1430 sb 1437 s 1450-1460 sh 1433 1422
All peak positions are given in wavenumbers (cm-l). v = stretching mode, va = asymmetric stretching mode, 6 = bending mode, Y , = symmetric stretchingmode. Assignments are qualified as follows: vw,very weak; w, weak; m, medium; s, strong; br, broad; sh, shoulder. Overlapping with v,(NH3). a
ArWIPdCNH,),lClz
0.015
2 hours
H,S(b)
3400 3200 3000 2800 2600 1900 1700 1500 1300 1100
wavenumbers (cm.’)
Figure 6. Grazing angle FTIR spectra of LB film deposited from ArH monolayer on [Pt(NH3)41C1~ subphase before (a)and after (b)exposure t o HzS.
metal-fatty acid films presented in Table 3. The spectra of the films in this study are complicated, however, due to the presence of complexed ammonia in the film. In particular the v,(CO2) falls in the same region as the 6,(NH3) of the complexed ammonia which broadens the peaks and makes determination of its exact position tenuous. Also the dd(NH3) ( 6 d = bending mode) falls in between the v(C=O) ( Y = stretchingmode)and v,(COz) (va = asymmetric stretching mode). It should be noted that the presence of the v(C=O) band in spectra a indicates that some of the surfactant in the film is still protonated while the absence of any v(OH) band suggests that the amount of protonated acid is small. For both ArH[M(NH3)2]Clz(M = Pt or Pd) films there are significant changes in the IR spectrum after gassing (spectra b). The bands associated with complexed ammonia disappear completely in the case of the ArW[Pd(NH3)&12 film while with the ArW[Pt(NH&] C12 film there is a small peak remaining a t 1630 cm-l associated with the dd(NH3)mode. This is consistent with XPS data which indicates that the 0% and 33% of the nitrogen remain after gassing for the ArW[Pd(NH3)4IClz and ArW[Pt(NH3)4]Clz films, respectively. The remaining ammonia in the ArW[Pt(NH3)$C12film is probably due to incomplete
3400 3200 3000 2800 2600 1900 1700 1500 1300 1100
wavenumber (cm.’) Figure 6. Grazing angle FTIR spectra of LB film deposited from ArH on [Pd(NH3)41Cl2subphase before (a) and after (b)
exposure to HzS.
reaction with HzS. For both A ~ H [ M ( N H ~ M(M C ~= ZPt or Pd) films the v(NH3) in the 3000-3500 cm-l region is replaced by a stronger and broader band assigned to the 4OH) mode of the reprotonated carboxylate function of arachidic acid. After gassing, the v(C=O) increases dramatically in intensity while the v,(COZ) decreases in intensity. This is expected as the carboxylate function is reprotonated during the reaction of the MI1ion to give the metal sulfide as given in eqs 2 and 3. The v,(C02) band sharpens and increases in intensity upon gassing. Theoretically if the reaction in eq 2 or 3 goes to completion, both the v,(COz) and the vs(C02)bands should completely disappear. Previous s t u d i e ~on ~ !the ~ ~reactions of Cd2+/ fatty acid LB films with H2S have indicated that even after compete conversion of CdAr to CdS there is a certain amount of deprotonated acid remaining. It has been hypothesized that the remaining acid salt is associated with the metal sulfide particle and that this surfactant “capping” restricts the particle growth to the Q state regime. (41) Urquhart, R. S.; Hoffman, C. L.; Furlong, D. N.; Geddes, N. J.; Rabolt, J. F.; Grieser, F. J. Phys. Chem., in press.
4770 Langmuir, Vol. 11, No. 12, 1995
Elliot et al.
X
a rJ
/ 600
I
“1 0
%
15
Ll
P
1
0
gc
10
5 0
0
Figure 7. AFM surface plot of PdS particles on a mica substrate. PdS was formed from the reaction of a ArMPd(NH3)4]C12film with HzS, andthe organic film was subsequently washed away with CHCl3.
AFM Data. LB films were deposited onto freshly cleaved mica from arachidic acid monolayers on a [Pd(NH3)41C12 subphase. The plate was exposed to H2S and then immersed in CHCl3 with gentle stimng to wash off the surfactant prior to AFM examination. A typical image observed for the ArWPt(NH3)4]C12film is given in Figure 7. Approximately spherical particles were found with vertical dimensions between about 1and 10 nm. Much larger, irregularly shaped particles were also imaged; however, a section analysis suggested that these were probably aggregates made up of several individual particles. A histogram (Figure 8) obtained from the section analysis of several spherical particles provided a mean particle diameter of 4.8 nm.
Conclusions Monolayers of arachidic acid, compressed to a surface pressure 20 mN m-l, are stable on all the [M(N4)I2+((N4) = (NH3)4,M = Pt,Pd; (N4) = trien, M = Pd} subphases. Multilayer LB films could be transferred to quartz and gold-coated glass substrates with an overall transfer of approximately 1. W-visible absorbance data do not
1
2
3
4 5 6 7 8 9 particle diameter (nm)
1011
Figure 8. Histogram of PdS particles obtained h m AFM images of a gassed Ar€UPd(NH3)4]C12 film rinsed with CHCla to remove the surfactant.
provide any conclusive evidence as to the nature of the P t I 1 and PdlI ions in the films. XPS and grazing angle FTIR data suggest that for films made from [M(NH&lC12 (M = Pt, Pd) subphases there are ammonium ions (NH4+) rather than amine ligands in addition to chloride ions in the film. For the [Pd(trien)](C104)2subphase, XPS data suggest that the exchange of arachidic acid protons for the [Pd(trien)12+complex ion is not stoichiometric. The reactions of metal ions in arachidic acid films proceed more quickly than those of platinum and palladium complex halides in DDAB films.20 Band gaps estimated from the absorbance spectra of the reaction products of the metal ions in the arachidic films with H2S indicate the formation of “Q state” particles. AFM measurements have confirmed that when films of ArH/[Pd(NH3)4lC12 deposited on mica plates are exposed to H2S, particles in the size range 1-10 nm are formed.
Acknowledgment. D.J.E. gratefully acknowledges the financial support of the Australian Research Council. The authors would also like to thank Frank Caruso and Elke Rodda for their assistance with this work. LA9503461
