J. Phys. Chem. 1991, 95, 5924-5930
5924
hypothesize the presence of a broad distribution of surface states that straddles the Fermi leveL7 The electronic occupancy of these surface states can be reduced by the presence of interface states at the abrupt n-n heterojunction, Figure 9b, leading to a larger depletion width, as inferred from our intensity studies. Similar band profiles to that shown in Figure 9b have been proposed for the CuS/CdS2& and GaAs/InSb*I heterojunctions. It is noteworthy that charge neutrality in CdSe/Ag+ is no longer maintained on an axial basis but is, of course, for the overall volume. The interaction of the dipolar aniline species with the surface Cd sites can be likened to the formation of a weak donor-acceptor complex, Figure As shown in Figure loa, the donor HOMO and acceptor LUMO are slightly stabilized and destabilized, respectively, by the formation of the complex, but there is little transfer of charge density between them; accordingly, the complex is readily dissociated. The analogous picture for aniline adsorption onto CdSe can be represented in Figure lob. In this case, the surface states may interact with the aniline HOMO to produce a new distribution that can lead to a reduction in depletion width
by raising the energies of the surface states closer to the conduction band and reducing their overall electronic occupancy. The analysis of our experimental results, based on the dead layer model, suggests the following possible scenario for the case of CdSe/Ag+, shown in Figure 1Oc. The figure shows a larger initial depletion layer and lower electronic occupancy in the surface states of the bare CdSe region, presumably due to the trapping of electrons by the neighboring Ag,Se. It is thus likely that a similar shift of the Fermi energy, resulting from aniline adsorption, spans a region of larger density of surface states as compared with untreated CdSe. Consequently, this new equilibrium state results in a greater amount of charge having been transferred from localized surface states into the bulk semiconductor and leads to a larger change in the depletion width. Acknowledgment. We thank the Office of Naval Research for generous support of this work. We are also grateful to Dr.Ngoc Tran for helpful discussions. Registry No. CdSe, 1306-24-7; Ag2Se, 1302-09-6; aniline, 62-53-3; aniline p-OCH,, 104-94-9; aniline p-CH,, 106-49-0.
Infrared Spectra of Solid Films Formed from Vapors Containing Water and Nitric Acid Roland H. Smith: Ming-Taun Leu,* and Leon F. Keyser Earth and Space Sciences Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91 109 (Received: December 3, 1990; In Final Form: February 26, 1991)
Infrared spectra have been recorded at 188 K for crystalline mono- and trihydrates of nitric acid formed by vapor deposition. In addition, spectra of fully deuterated forms of these same compounds have been obtained. These spectra have been interpreted in terms of the known ionic structures of the hydrates and the known spectra of oxonium and nitrate ions. Two other less stable solids were formed, a molecular hydrogen-bonded HN03.H20complex, stable only at temperatures below 120 or 150 K, and a substance thought to be a crystalline mixture of trihydrate and ice which sometimes formed from water-rich vapors and which upon pumping andfor warming could be converted into crystalline trihydrate. While these four substances appear to be the four species recently reported by Ritzhaupt and Devlin, we disagree with their allocation of structures to two of them. In particular, we disagree with their claims that a stable dihydrate exists. The relevance of the results to the stratospheric "ozone hole" problem is discussed.
Introduction There is now considerable evidence'" that so-called type 1 polar stratospheric clouds consist of solid particles of nitric acid trihydrate (NAT), and it is widely believed7-'0 that these particles catalyze the conversion of the photolytically inactive compounds HCI and C10N02into active ones, C12and HOCI, and in so doing remove NO, as HN03 from the gas phase. These processes contribute significantly to the observed springtime decrease in ozone in the Antarctic stratosphere. Laboratory studies of these catalytic processes are being ~ n d e r t a k e n . ~ * ~To * " assist in the characterization of the solid deposits used in such studies and to improve understanding of the formation of NAT in the stratosphere, we have recorded the infrared spectra of the mono- and trihydrates of nitric acid and for comparative purposes that of pure nitric acid as well. In addition, we have made some measurements on the solids formed when nonstoichiometric nitric acid and water vapors are condensed. There have been two very recent reports12J3 of the infrared spectra of these hydrates: similarities and differences between those reports and our results will be discussed. Experimental Section
The low-temperature cell consisted of a silver chloride window (on which the vapors were condensed) mounted firmly on an 'On study leave from Maquarie University, N.S.W. 2109, Australia.
aluminum bracket suspended from the bottom of a stainless steel dewar vessel with the whole assembly mounted in an anodized aluminum jacket fitted with two KRS-5 windows (Figure 1). When placed in the sample compartment of a Bomem Model DA3.002 Fourier transform infrared spectrometer, the infrared beam passed through the center of the silver chloride window. Spectra were recorded in the range 500-4000 cm-I by using a globar source, a potassium chloride beam splitter, and a liquid (1) Toon, 0. B.; Hamill, P.; Turco, R. P.; Pinto, J. Geophys. Res. Lett.
13. -1284. --. --. -(2) Hanson, D.; Mauersberger, K. Geophys. Res. Len. 1988, I S , 855.
-1986. -
(3) Smith, R. H. Geophys. Res. Leu. 1990, 17, 1291. (4) Fahey, D. W.; Kelly,.K. K.; Ferry, G.V.; Poole, L. R.; Wilson, J. C.; Murphy, D. M.; Loewenstein, M.; Chan, K. R. J . Geophys. Res. 1989, 94, 11299.. ( 5 ) Tum, R. P.; Toon, 0.B.; Hamill, P. J . Geophys. Res. 1989,94, 16493. ( 6 ) Toon. 0. B.; Browell, E. V.; Kinne, S.;Jordan, J. Geophys. Res. Lcrf. 1990, 17, 393. (7) Solomon, S.Reo. Geophys. 19%8, 26, 13 1. (8) Ouinlan. M. A.; Reihs, C. M.; Golden, D.M.; Tolbert, M. A. J . Phys. Chem. i990, 94, 3255. (9) Moore, S.B.; Keyser, L. F.; Leu. M.-T.; Turco, R. P.; Smith, R. H. Nature (London) 1990, 345, 333. (IO) Wofsy, S. C.; Salawitch, R. J.; Yatteau, J. H.; McElroy, M. 8. Geophys. Res. Lerr. 1990, 17, 449. ( 1 1 ) Leu, M.-T. Geophys. Res. Len. 1988, 15, 17, 851. (12) Ritzhaupt, G.;Dcvlin, J. P. J . Phys. Chem. 1991, 95, 90. (13) Tolbert. M. A.; Middlebrook, A. M. J . Geophys. Res. 1990, 95, 22423.
0022-3654 f 91 f 2095-5924$02.50 f 0 0 1991 American Chemical Society
IR Spectra of Crystalline Water-Nitric Acid VACUUM PUMP
I
4
The Journal of Physical Chemistry, Vol. 95, No. 15, 1991 5925
T
GASES INLET
L U
w"Do-
HEATER
.I
t
VECTOR PROCESSOR.
11
-
COMPUTER SYSTEM
1I u DEPOSITIONSUBSTRATE ONINFRAREDWINDOW
Figure 1. Schematic diagram of the low-temperature infrared cell.
nitrogen cooled MCT detector. A resolution of 1 .Ocm-' was used throughout; typically 30 scans were accumulated for each s p " taken. Because of the small energy throughput at the low end of the scanned range, no reliance was placed on measurements below 600 cm-I. The temperature of the sample was measured with a chromekonstantan thermocouple epoxied to the surface of the silver chloride window. Temperatures were maintained by intermittently adding small amounts of liquid nitrogen to the dewar; they could be held constant to f2 K. Vapors were admitted through a stainless steel tube which could be located opposite the center of the silver chloride window and then withdrawn out of the path of the infrared beam for measurements. A standard glass vacuum line with greaseless joints and s t o p k s was used for gas handling and cell evacuation. The design of the equipment did not allow routine visual inspection of the deposits formed. However, on two occasions immediately after deposition, the infrared cell was removed from the spectrometer, and it was observed that the deposits had a frostlike appearance. Anhydrous nitric acid was prepared by adding concentrated sulfuric acid to sodium nitrate crystals, collecting the volatile product in a liquid nitrogen trap, and purifying it by distillation. Deuterated nitric acid was prepared in the same way using D#04 (Aldrich, 99.5% D). Vapors having water and nitric acid in the desired ratios were prepared by vaporizing at 293 K aqueous solutions of nitric acid of suitable concentrations, using vapor pressurecomposition data from Clavelin and Mirabel" and some that had been measured in this laboratory. Such solutions were prepared either from pure nitric acid or from reagent grade concentrated nitric acid (70%) and were analyzed by acid-base titration. Deuterated solutions were prepared by diluting pure DNO, with D 2 0 (Baker, 99.8% D). Results and Discussion Pure Nitric Acid. Although the infrared spectrum of pure solid nitric acid has been previously reported,I5 we have measured it separately under our specific experimental conditions to facilitate comparison with the spectra of the hydrates. Figure 2 shows the spectra of ordinary and deuterated nitric acid at 188 f 3 K. Spectra were generally recorded by condensing HNO, at 188 K and quickly scanning the sample. Samples did not last longer than a few minutes on the AgCl window at 188 K,presumably due to migration to colder parts of the cell. Such spectra were quite reproducible. Occasionally samples were deposited at 168 or 1 13 K and then warmed to 188 K before recording the spectra. Some differences in the region 1250-1460 cm-l were observed as will be discussed below. Table I compares our band frequencies with those of previous measurements. McGraw et aht5measured the spectrum of gaseous nitric acid and of "annealed" solid at 75 K. Guillory and Bernsteint6and Ritzhaupt and Devlint7measured the spectrum of nitric (14) Clavelin, J. L.;Mirabcl, P. J . Chim. fhys. 1979, 76, 533. (15) McGraw, G.E.;Bernitt, D. L.; Hisatsune, 1. C. J . Chcm. fhys. 1965, 42, 237.
3000
4000
1000 CM-'
2000
WAVENUMBER
Figure 2. Spectra at 188 K of normal and deuterated nitric acid. a and b denote bands that undergo isotope shifts and are assigned to OH stretch and NOH bend, respectively. TABLE I: B u d Frequeaekr (em-') for"V Form of F"re Nilrk Acid. Nz Or Ar "annealed" this study, gas phase. matrix, solid at solid at 298 Kb 10 Kc assignmentsd 75 Kb 188 K 737 (521)' 456 (342) 479 (361) v9 OH torsion v, ON@ bend 707' 579 597 647 660 v5 ON0 bend 722' 704 (702) 772 (772) 764 v, NO1 out-of-plane 773 762 v6 N U Stf 958 960 (954) 879 898 1324 1308 v4 NOz sym str 1256 1339 (1342) 1420 (1074) 1395 (1071) 1330 (1014) 1330 (1021) vt NOH bend 1460 uZ NO, antisym str 1708 1696 3550 (2622) 3498 (2580) vI OH str
1646 1650 (1609) 3106 (2274) 3060 (2210)
.Values in p a r c n t h arc for deuteratedcompounds; for this study values arc given for all bands; for other studia values are given only for bands that show significant isotope shifts. bMcGtsw et al.I5 'Average data from Guillory and Bernstein16and Ritzhaupt and Dev1in.I' 'From McGraw et a1.l' as modified by Ritzhaupt and D e v l i ~ ~'These ~' assignments are disputed in this study; scc text.
acid in dilute inert gas matrices at 10 K. With the exception of two regions, 1250-1460 and 700-740 cm-', our spectra are in reasonable agreement with those of McGraw et al. (though our bands are generally much sharper) and are reasonably consistent with the matrix isolation spectra which in turn agree well with the gas spectrum. At 188 K our nine separate samples of HNO, showed a strong band at 1339 cm-' with two unresolved peaks on each side (1 250, 1305,1395, 1460 cm-I). The spectrum of DNO, at 188 K (four separate samples) showed strong bands at 1071 cm-' (single peak) and at 1342 cm-I (with unresolved peaks at 1320 and 1300 cm-I). We assign the 1071-cm-'band in DNO, and the 1395-cm-' band (with 1460cm-l) in HNO, to the H-O-N bend and the 1342-cm-' (including 1320 and 1300 cm-I) in DNO, and the 1339-cm-'band (with 1250 and 1305 cm-I) in HNO, to the NO2 symmetric stretch. These NO2 band frequencies differ somewhat from those of McGraw et al., but the differences appear to be due to different temperatures, because when samples were deposited and scanned at 168 K, the strongest peak in HNO, occurred at 1270 cm-'(with unresolved peaks at 1250,1340 and 1410 cm-I), while in DNO, the expected two bands were at 1055 cm-l (single peak) and at 1342 cm-I (with unresolved peaks at 1330 and 13 15 cm-I), leading to the following assignments: H U N bend at 1055 cm-' in DNO, and 1410 cm-l in HNO,, NO2 symmetric stretch at 1342 cm-' (with 1330 and 1315 cm-l) in DNO, and 1270 cm-I (with 1250 and 1340 cm-I) in HNO,. McGraw et al. reported this NO2 band at 1300 cm-l in DNO, and 1256 cm-I in HNO,. The differences between their and our frequencies would appear to be due to the ~
~~
~~~
~
~
~
(16)Guillory, W.A.; Bernstein, hi. L. J . Chcm. fhys. 1975, 62, 1058. (17) Ritzhaupt. 0.; Devlin, J. P. J. fhys. Chcm. 1977, 81, 521.
5926
Smith et al.
The Journal of Physical Chemistry, Vol. 95, No. 15. 19'91 7
TABLE II: Band Frequencies (em-') for Nitric Acid Monohydrate (NAM)' Bethel and 160 Kb
Giguere,19 93 K
670
2.0-
d
WAVENUMBER
Figure 3. Spectra of crystalline HN03-Hz0and DN03.D20 at 188 K. a 4 are oxonium ion bands while arrows indicate nitrate bands.
fine structure at different temperatures altering the position of the apparent band maxima. We attribute the fine structure in these bands to effects of the crystal lattice. The second region of difference between our spectrum and McGraw et al.'sI5 is around 700 cm-I; we observe only one band at 704 f 2 cm-' in H N 0 3 and at 702 f 2 cm-'in D N 0 3 whereas McGraw et al. claim that after annealing they have three bands, at 707,722, and 737 cm-I. None of our €€NO3and D N 0 3 spectra showed this feature, though we consistently obtained such a triplet structure in our spectra of crystalline monohydrate (Figure 3 and Table 11). Perhaps in their annealing process McGraw et al. inadvertently converted some of their nitric acid to monohydrate. We find the shift in band frequencies that they propose in going from gas to solid to explain this trio of peaks unconvincingly high (Le., for u7, 0 - N 4 ' bend, and u9, OH torsion). We are inclined to assign the 704-cm-I band to the O N 0 bend with u7 and ug at frequencies below 550 cm-I where we could not see them. The band assignments of Guillory and BernsteinI6 based upon those of McGraw et are shown in Table I along with our assignments for bands we observed (i.e., above about 550 cm-'). Spectra were also recorded at 173 and 113 K,but apart from the apparent changes discussed above the band frequencies did not change by more than experimental error from the 188 K values. Condensation of 1:l Vapor. The vapor from a 70 f 1 wt 7% aqueous nitric acid solution a t 293 K has the composition [H,O]/ [HN03]= 0.9 k 0.1. Figure 3 shows a typical spectrum of the solid formed when this vapor was condensed at 188 K. Eleven such spectra were recorded Over a Emonth period. Table I1 lists the band frequencies along with those of previous rep o r t ~ ' ~ of J ~the J ~spectrum of nitric acid monohydrate (NAM). Our spectrum agrees well with the corresponding one from Ritzhaupt and Devlin.lz While these spectra are of superior quality to the earlier ones, there are no major discrepancies to cast doubt upon the identity of the species being examined. To help with band assignments, the spectrum of fully deuterated NAM has been recorded; it is also shown in Figure 3, and the band frequencies are given (in parentheses) in Table 11. A band assignment is given in Table 111. It is essentially that of Ritzhaupt and DevlinIz and is completely consistent with the observed isotope effects. The nitrate antisymmetric stretch, u3, occurs at a lower frequency than in inorganic nitrates,," but this is consistent with there being significant hydrogen bonding between the H30+ and NO3- ions. The absence of the symmetric stretch, uI (expected at about 1050 cm-I), confirms that the nitrate ion is virtually symmetrical in this substance. On the basis of the frequencies of the deuterated compound, we are inclined to agree (18) Bethel, D.E.;Sheppard, N.J . Chcm. fhys. 1953,21, 1421. (19) Savoic, R.;Gigure, P.A. J. Chem. fhys. 1964, 41, 2698. (20) Miller, F.A.; Wilkins. C. H.Anal. Chem. 1952, 24, 1253.
Ritzhaupt and Devlin,12
Savoie and
Sheppard,'*
723? 738
675 700 723 737
816
815
1134
1135
1361 1670
1386 1680 2260 2780
2800
this study:
50 Kd
188
K
680?' 702' 722 735 (7341 775? 813 (813)
702 721 735 780? 814 1 102? 1135 1279 (1300)
1115 (851) 1260h 1316 (1320)
1674 2246 2635
1671 (1197) 2230 (1410) 2644 (1988)
'Values for deuterated compound are given in parentheses. ? means weak band or one not due to the stated species. bTemperature not stated; rough estimate only. cAverage of 11 spectra. dSample deposited at 80 K, warmed to 175 K for annealing, and then recooled to 50 K before scanning the spectrum. cShifted to outside our measuring range. /One peak only; not 722 and 735 cm-l. #Their tabulated value, but in their Figure 3 the peak maximum occurs at 1300 cm-I with a shoulder at 1279 cm-I; they argue that 1300 cm-l is due to molecular complex. "ee text.
TABLE ilk h a d As8igamnQ I&
(In cm-I) for Nitrate and Oxonium
NO3- bands NAM NAT
I :::
antisym bend u, sym bend u2
813
sym str v I antisym
{
Str "3
1260 13 16
743 804 1026 1262
1452
inorganic nitrates 7 30 830 (1040 1370
H30t bands
NAM torsional (librational) vL sym bend vZ antisym bend Y, sym str v I antisym str v3
{
;;;:
1115 (851) 1671 (1197) 2230 (1410) 2644 (1988)
NAT 669c 1262 (899) 1731 (1293) 2236 (1622) (1695)) 2702 (2030)
Raman) from HIOtCl-b
620 (450)
IO00 (780) 1670 (1250) 2100 (1550) 2550 (2000)
'Values for deuterated compounds are given in parentheses. ? means weak band or one not due to the stated species. "Average of values from Huston et aLZ6and references contained therein. CIsotope shifted out of our measuring range. with Ritzhaupt and Devlin'* that the 2230 (1410) cm-I band is a fundamental stretching frequency, ulr of the oxonium ion; an earlier interpretation'* had considered it an overtone band, 219. Two differences between our spectrum and that of Ritzhaupt and DevlinIz warrant comment. Firstly, in our spectra the intensity of the 1115-cm-' band (and to a lesser extent the other oxonium bands also) is greater, relative to the nitrate bands than in theirs; there is no obvious explanation for this. Secondly, we observe some variability in the 1300-cm-' nitrate band. On some occasions the band occurred solely at 1316 cm-' and on others, solely at 1260 cm-I, and on still other bands appeared at both frequencies (Figure 4). There is actually a hint of an unresolved band at about 1300 cm-I in Ritzhaupt and Devlin's Figure 2aI2 which they attribute to HN03. This variability-not unlike that observed with nitric acid-may be due to imperfections in the crystals formed, leading to loss of symmetry in the nitrate ion and a splitting of the degenerate u3 band. In the deuterated compound this nitrate band occurs near the oxonium ion symmetric stretch band (1410 cm-I), sometimes just as a shoulder and other times as a distinct but unresolved peak.
The Journal of Physical Chemistry, Vol. 95, No. 15, 1991 5921
IR Spectra of Crystalline Water-Nitric Acid TABLE I V Band Frequencies (em-') for the Unstable 1:l Species Ritzhaupt and Devlin molecular amorphous Tolbert and Middlebrook" this study glassy NAM, 1:l species, complex, film (l:l), IO K1' 80 KIL 183 K 173 and 141 K 770 779 776b 775 954 945 947 945 1036? 1303 I309 1307 1305 1432 I424 1425 1415 1673 1680 1665 1670 2680 2660 2650 "Major bands in the 1:l spectrum in their Figure I . bNot listed in ref 17 but included in ref 12.
On a few occasions depositions were made at significantly lower temperatures (173 and 140 K). A distinctly different spectrum was first observed, but when the sample was warmed to about 180 K,the spectrum changed completely to that of Figure 3 on each occasion. A recurring question throughout this study is whether or not the solid formed had the same composition as the vapor being used. This assumes that the deposition efficiencies of H N 0 3 and H 2 0 vapors are the same. To test this, we varied the deposition conditions and found (a) that the same spectrum is observed when the vapor is condensed at 140 K and warmed to 188 K as when it is condensed at 188 K directly and (b) that no large spectral variations are observed when a variety of vapor flow rates is used at 188 K. These observations indicate that the solid formed from 1:l vapor does have this same composition. The spectrum of the unstable species formed by condensation at 173 or 140 K shows a strong resemblance to that of the amorphous films reported by Ritzhaupt and Devlin12 for 1:l mixtures and to the "glassy NAM" spectrum reported by Tolbert and Middlebrook13 as shown by the listing of main absorption bands in Table IV. Ritzhaupt and DevlinI2 interpret their spectra of amorphous films at 80 K as being due to a hydrogen-bonded molecular complex, HN03.H20, which gradually changes to an ionic form, H30+NO
