Resonance Raman spectra of adsorbed species at solid-gas

CVD Germania on Pyrogenic Silica. V. M. Gun'ko, E. F. Voronin, V. I. Zarko, and E. M. Pakhlov. Langmuir 1997 13 (2), 250-258. Abstract | Full Text HTM...
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J. Phys. Chern. 1901, 85, 1005-1014

1005

Resonance Raman Spectra of Adsorbed Species at Solid-Gas Interfaces. 2. p-Nitrosodimethylaniline and p-Dimethylaminoazobenzene Adsorbed on Semiconductor Oxide Surfaces James F. Brazdllt and Ernest B. Yeager' Depaltment of Chemistry, Case Western Reserve Univers@, Cleveland, Ohio 44 106 (Received: July 17, 1980; In Final Form: December 2, 1980)

The resonance Raman spectra of two probe molecules adsorbed on the surface of n-type semiconductor oxides (ZnO, TiOz, SnOJ have been studied at the solid-gas interface. The probe molecules used were p-nitrosodimethylaniline (p-NDMA) and p-dimethylaminoazobenzene (p-DMAAl3) since they gave an intrinsic resonance enhancement when excited by argon ion laser radiation. These molecules were found to be sensitive probes for the acid-base nature of the adsorption sites on the surface of these oxides. The oxides were found to possess both Lewis-acid and -base sites as determined from the Raman spectra of adsorbed p-NDMA. Hydrogen bonding to surface hydroxyl groups was also observed on hydroxylated SnOz and TiOz. The existence of Bronsted-acid centers on hydroxylated ZnO and TiOz was confirmed from the Raman spectra of absorbed p-DMAAB. No Bronsted acidity was observed for SnOz. The physical nature and the acid strength of these adsorption sites are discussd in the context of the crystal structure of the adsorbents and the calculated surface charges of these sites.

Introduction Resonance Raman spectroscopy is a powerful tool to obtain vibrational information about molecules which are present in low concentrations. In a previous paper' the resonance Raman spectra of p-nitrosodimethylaniline (pNDMA) adsorbed on high-area silica and alumina surfaces were used to characterize the absorption properties of these insulators. The sensitivity afforded by resonance enhancement permits the extension of this technique to low-area surfaces where conventional infrared and Raman spectroscopic techniques have been experimentaly difficult and not always successful. This paper describes the resonance Raman spectroscopic investigation of p-NDMA and p-dimethylaminoazobenzene (p-DMAAB) adsorbed on the surface of ZnO, SnOz, and TiOP The objective of the work was to obtain information about the spectral properties of these molecules in various chemical environments and to use this information to gain an overall picture of the nature of the adsorption sites on these oxides. The study has been carried out over a wide range of adsorbate coverages as well as under conditions where competitive adsorption with other molecules was allowed to occur. In addition, thermal-desorption studies and adsorption-isotherm measurements were made to more fully characterize the adsorption properties of these materials.

water at 60 "C with constant stirring. It was further dried at 120 "C for 24 h in an oven. The residue was calcined in air at 300 OC for 24 h to decompose the nitrates. The resultant white powder was ground and sieved, and only particle sizes between 63 and 37 km were used. After thermal treatment in air a t 500 "C, the zinc oxide had a BET surface area of 0.3 m2/g. X-ray diffraction showed that the ZnO had a wurzite structure. The only major impurities detected by atomic-absorption and emissionspectroscopic analyses were Si and Pb, which were present in 530 ppm concentration. Tin oxide was prepared by oxidation of tin metal in an excess of nitric acid. After filtering, washing, and drying, the white precipitate was denitrified at 300 "C. This produced a hard, yellow, granular material. The material was ground and screened, and only particle sizes between 841 and 210 pm were used. Heat treatment of the tin oxide at 500 "C in oxygen resulted in a pronounced darkening of the material. The tin oxide returned to its original light yellow color when cooled to room temperature. The 500 "C treated tin oxide had a surface area of 27.7 m2/g, with the only major impurity being Ca, which was present in 130 ppm. X-ray diffraction showed the material to be amorphous. Titania powder was obtained from Degussa. After treatment in oxygen at 500 "C, it had a surface area of 55.8 m2/g and was present in the form of anatase.

Experimental Section The Raman spectrometer, the sample preparation, and the data collection procedures have been described in a previous paper.l The p-DMAAB was purchased from Aldrich and purified in the same manner as that described for p-nitrosodimethylaniline (p-NDMA) in the previous paper.' Zinc oxide powder was prepared by precipitation from an aqueous solution of reagent zinc nitrate. Concentrated ammonium hydroxide was added dropwise to the wellstirred solution until precipitation ceased at a pH of -6.5. The white precipitate was isolated by evaporation of the

Results and Discussion Absorption and Resonance Raman Spectra of p DMAAB in Solution. As was found for p-NDMA,l the electronic structure of p-DMAAB is also sensitive to changes in its chemical environment as revealed from its UV-visible absorption spectrum shown in Figure 1. As discussed for p-NDMA, changes in the position of the absorption maximum are attributable to the ability of the solvent to stabilize the excited state of the molecule. As can be seen from Figure 1, a dramatic shift in the absorption maximum occurs when the pH of the solvent is

t Sohio Research

Center, Cleveland OH 44128.

(1)J. F. Brazdil and E. Yeager, J.Phys. Chern., previous paper in this issue.

0 1981 American Chemical Society

1006

The Journal of Physical Chemistty, Vol. 85, No. 8, 1981

Brazdil and Yeager

TABLE I: Observed Vibrational Frequencies ( A v ), Relative Intensities (I),Depolarization Ratios (p ), and Vibrational Assignments of the Raman Bands of p-DMAABC

in solid state Av

I

in chloroform Av

I

p

in equimolar CH,OH/H,O, pH 6.0

I

Av

in equimolar CH,OH/H,O, pH 3.0a Av

I

1624 s 1604 w w 1593 m m 1461 w m 1440 w m-sh 1417 m-sh s s s 1405 s m m m 1373 m m m m 1316 w 1283 m 1190 m 1196 m 0.4 1196 m 1199 m 1183 m 1136 m 1143 m 0.34 1146 m 1150 m a Laser excitation at 5145 A . P. J. Trotter, Appl. Spectrosc., 31, are labeled as follows: s, strong; m, medium; w, weak; sh, shoulder, 1595 1588 1456 1435 1415 1403 1362 1309

w w m m m-sh

1602 1592 1460 1441 1419 1409 1368 1314

w w m m s-sh

0.36 0.36 0.34 0.33 0.34 0.33 0.33 0.33

1600 1591 1460 1441 1417 1406 1370 1315

w

vibrational assignments

P

0.36 0.33 0.35 0.43 0.33 0.33 0.33

ref

C=C quinoline stretch (8a) ring stretch (8b) ring stretch (19a)ring deformation assymN-CH,deformation N=Nstretch symm N-CH, deformation phenyl-aminostretch ( 3 ) ring deformation

b, 2 2, 3

2, 3 b , 2, 3

2, 3 3 3

0.36 0.37 symmphenyl-azo bend (tentative) b 0.33 0.33 symmphenyl-azostretch b , 2, 3 30 (1977). Relative intensities at 4 8 8 0 4 excitation

c

6 3

T

i

2

2

W

I

3000

4000

xtX1

5000

Flgure 1. Absorption spectra of p-DMAAB in (1) diethyl ether, (2) chloroform, and equimolar methanol-water solution with (3) pH 6.0 and (4) pH N 2.7.

below -3. The maximum shifts from 4400 A in CH,OH/H,O to 5130 A when the solution is acidified with HC1. The absorption band for the acidified solution had a broad tail on its short-wavelength side which may contain some unresolved fine structure. The resonance Raman spectrum also shows a significant change when the p-DMAAB solution is acidified. The Raman spectra of p-DMAAB in various solvents and in the solid state are shown in Figure 2. Table I gives the observed vibrational frequencies and their vibrational assignments. Corresponding data for p-NDMA were given in the prior paper. All of the observed bands are symmetric, based on the measured depolarization ratios. These spectra agree well with that reported p r e v i ~ u s l y . ~ ~ ~ When the pH of the methanol/water solution was lower than the pK, of p-DMAAB (-3.3), the spectrum of the protonated form of the molecule dominated. The protonation transforms the molecule from an azo type to a quinoid type, as shown in Figure 3. Protonation occurs at the azo nitrogen farthest from the amino groups4 The quinoid structure is characterized in the resonance Raman spectrum by the intense C=C stretching band at 1624 cm-lS2 (2) K. Machida, B. Kim, Y. Saito, K. Igarashi, and T. Uno, Bull. Chem. SOC.Jpn., 47, 78 (1974). (3) J. L. Lorriaux, J. C. Merlin, A. Dupair, and E. W. Thomas, J. Raman Spectrosc. 8, 81 (1979). (4) I. M. Kolthoff, “Acid-Base Indicators”, Macmillan, New York, 1937.

A

‘b

1600

1500

1400

1200

1300

1100

nJ (crn-1) Flgure 2. Resonance Raman spectra of p-DMAAB In varlous solvents and in the solid state: (A) solid with ,A = 4880 A, power = 40 mW, resolution N 6 cm-’; (B) -lo-* M in chloroform with ,A = 4880 A, power = 100 mW, resolutlon N 5 cm-’; (C) IO3 M in equimolar methanol-water solution with pH N 6.0,,,A = 4880 A, power = 100 mW, resolution 5 cm-‘; (D) lo3 M In equimolar methanol-water solution with pH N 3.0, A, = 5145 A, power = 100 mW, resolution N 5 cm-’.

-

-

Flgure 3. Azo and quinoid forms of p-DMAAB.

The Journal of Physical Chemistry, Vol. 85, No. 8, 1981 1007

Adsorbed Species at Solid-Gas Interfaces

I

1375

-z5 0

1370

Z"

'E

r w

G 1365

I

1135

I

I

I

I150

I140 I I45 AV ( P H E N Y L - A Z O )

(cm-li

Figure 4. Correlation diagram for the observed changes in the phenyl-azo and phenyl-amino stretching frequencies of p-DMAAB in various chemical environments.

Figure 6. Adsorption isotherm for p-NDMA on ZnO at -25

-6

-5 -4 -3 LOG C (C in rnoles/liter)

-2

Figure 5. Adsorption isotherm for p-NDMAISnO, at -25 OC. C is the equilibrium concentration of p-NDMA in diethyl ether solution and N is the micromoles adsorbed per gram of SnO,.

The phenyl-azo stretching frequency can be seen to increase from 1136 cm-l in the solid state to 1150 cm-l in the acidified solution. This is as expected since the quinoid form will possess a higher phenyl-azo bond order than will the azo form. Further confirmation for this assignment is provided by the correlation diagram in Figure 4. Increased double-bond character is observed for both the phenyl-amino and phenyl-azo bonds as the polarity of the environment increases. The p-DMAAB was chosen as a surface probe because, like p-NDMA, its Raman spectrum is expected to be sensitive to the acidity of the adsorption sites on a solid surface. The molecule is expected to be especially sensitive to Bronsted-acid centers which can protonate the molecule and result in a structural change to its quinoid form. Adsorption Isotherms for the Adsorption of p-NDMA. The adsorption isotherm for p-NDMA adsorbed on SnOz a t -25 "C is shown in Figure 5. The isotherm was measured for the adsorption of p-NDMA onto tin oxide from a diethyl ether solution as described in the previous

OC.

paper.l The isotherm was observed to follow Langmuirian behavior for monolayer ads0rption.j From the flat portion of Figure 5 , a value of 3.6 pmol/m2 (99 pmollg) was obtained for a monolayer coverage of p-NDMA on Sn02. This shows that, at monolayer coverage, one p-NDMA molecule occupies -46 A2 of surface area on tin oxide. The adsorption isotherm for p-NDMA on ZnO shown in Figure 6 can be classified as Type I1 according to Brunauer et ale6 This type of isotherm has been treated theoretically by Brunauer, Emmet, and Teller.7 When the experimental points around the first inflection of the isotherm were fitted to theory, the monolayer value for p-NDMA on ZnO was found to be 3.9 pmol/m2 (1.2 kmol/g). The upper portion of the isotherm is a result of the multilayer coverage which occurs at high equilibrium adsorbate concentrations over the surface. The similarity in the monolayer values on both S n 0 2and ZnO suggests that the adsorption site density on these two oxides is similar and/or that the p-NDMA molecule has the same adsorption geometry on both surfaces a t monolayer coverage. This will be discussed in more detail shortly. It was not possible to obtain a similar adsorption isotherm for p-NDMA on TiOBsince extended exposure to the dilute diethyl ether solution of p-NDMA resulted in a discoloration of the titania surface and complete depletion of the p-NDMA from solution. Although no detailed analysis was made of the system, it is felt that the phenomenon is the result of a decomposition of the pNDMA on the titania which may be photocatalytic. Resonance Raman Spectra of p-NDMA and p-DMAAB on ZnO. The resonance Raman spectra of p-NDMA adsorbed on ZnO was studied for adsorbate coverages ranging from 0.14 to 13.6 pmol/g (0.47-45 pmol/m2). This amounts to between ca. one-tenth of a monolayer to a multilayer coverage equivalent to 10 times the monolayer amount (i.e,, % N 0.1-10). The Raman spectra for submonolayer and monolayer coverages are shown in Figure 7. The band positions and relative intensities are given in Table 11. At

-

(5) I. Langmuir, J . Am. Chem. SOC.,40,1361 (1918). (6)S.Brunauer, L. S. Deming, W. E. Deming, and E. Teller, J. Am. Chem. SOC.,62,1723 (1940). (7) S. Brunauer, P. H. Emmett, and E. Teller, J.Am. Chem. Soc., 60, 309 (1938).

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The Journal of Physical Chemistty, Vol. 85,No. 8, 1981

Brazdll and Yeager

TABLE 11: Observed Vibrational Frequencies ( A u )and Relative Intensities ( I ) of the Raman Bands of p-NDMA Adsorbed on ZnO at Various Coveragesa 0.14b (e = 0.12) A u , cm-'

I

1611

s

0.43 (e = 0 . 3 7 ) Au,

I

cm-'

1610

s

1445

m

1447

m

1398 1368 1321 1305

m m m w

1399 1370 1327 1308

s m w w

1161

s

1121

m

1160 1140 1127

m m m-sh

0.87b (e = 0.74) Au,

1.05b (e = 0.90) A u , cm-' I

I

cm-'

1.3b (e = 1.1) A u , cm-' I

w

1602 1592 1445

m w m

1602 1593 1443

w m

1593 1443

w m

1398

s

1398 1363

s m

1398 1363

s

w

1310 1193 1140

w m

1311 1197 1140

1127

m-sh

1127

w

4.76b (e = 4 . 1 ) A u , cm-' I

w

m

1593 1445 1420 1398 1365

m

1310 1196 1140

w w m

1197 1142

w m

m-sh

1127

m-sh

1127

m-sh

w

Relative intensities at 4880-8 excitation are labeled as follows: s, strong; m, medium; w, weak; sh, shoulder. NDMA coverage in p mol/g. a

m m-sh s

m

p-

TABLE 111: Raman Intensity Ratio for p-NDMA Adsorbed on ZnOa

coverage, pmol/g 0.14 (e = 0.12) 0.43 (e = 0.37) 0.87 (e = 0.74) 1.05 (e = 0.90) 1 . 3 (e = 1.1) 4.76 (e Y 4 . 1 ) 13.58 (e Y 11.6) 0.14 (e = 0.12)

a

description of adsorbent 500 " C dehydroxylated ZnO 500 " C dehydroxylated ZnO 500 "C dehydroxylated ZnO 500 " C dehydroxylated ZnO 500 "C dehydroxylated ZnO 500 "C dehydroxylated ZnO 500 "C dehydroxylated ZnO 500 "C dehydroxylated ZnO t 100 torr of NH, 500 "C dehydroxylated ZnO + 100 torr of NH, (evacuated) 500 "C dehydroxylated ZnO + 20 torr of pyridine 500 "C dehydroxylated ZnO t 20 torr of pyridine (evacuated)

I(1611I ( 161 11593 1593 cm-')/ cm-')/ I(1398I ( 13981399 1399 cm-' ) coverage, pmol/g description of adsorbent cm-' ) 1.08 0.34 0.20 0.17 0.15 0.15 0.15 0.13

0.14 (e = 0.12) 0.35 (e 0.30) 0.69 ( 0 = 0.59)

500 'C evacuated ZnO 500 "C evacuated ZnO 500 "C evacuated ZnO

3.04 2.44 1.31

0.55 0.13 0.26

Laser excitation at 4880 A.

low coverage, the spectrum is characterized by intense 1610- and 1160-cm-l bands which have been assigned to the 8a benzene-ring vibration and the phenyl-nitroso stretching vibration, respectively. These bands decrease in intensity as the surface coverage approaches a monolayer, while the band at 1398 cm-l, which has been assigned to the symmetrical N-CH, deformation, progressively dominates the spectrum. More importantly, the intensity ratio between the 8a benzene-ring mode and the symmetric dimethylamino stretching mode decreases from 1.08 to 0.15 with increasing coverage, as shown in Table 111. This intensity ratio has been shown to be proportional to the acid strength of the adsorption site for p-NDMA.' Thus at low coverage the p-NDMA is occupying an acidic surface center which, as in the case of 7-Al2O3,lcan be assigned as a Lewis-acid center. Hydrogen bonding to surface hydroxyl groups on ZnO, however, does not appear to be a preferred mode of adsorption for p-NDMA and thus can be ruled out as the site responsible for adsorption of pNDMA at low coverage. An intense Raman band between 1260 and 1280 cm-l was observed when this molecule was adsorbed on silica. This band is apparently characteristic of the molecule hydrogen bonding to surface hydroxyl groups.' This band is, however, very weak in the Raman spectra of p-NDMA adsorbed on ZnO. Even in the spectrum obtained from a ZnO surface which was exposed to water vapor for 24 h, this band is barely visible both at high and low p-NDMA coverages.

The proposed surface structure for p-NDMA adsorbed on a Lewis-acid center is the same as that described previously for alumina.l The adsorbed molecule is viewed as forming an adsorptive bond between the basic nitroso oxygen and a coordinately unsaturated zinc cation on the surface. Exposure of the low-coverage (0.14 pmol/g) sample of p-NDMA on ZnO to 100 torr of anhydrous ammonia resulted in a Raman spectrum which was very similar in both the band positions and their relative intensities to that which was observed at monolayer coverage (see Figure 8). Under these conditions, the 1398-cm-l band is the most intense in the spectrum and the 1445-cm-' band is now quite prominent. These bands, which are characteristic of the unpolarized form of the p-NDMA molecule, indicate that the molecule is displaced from the polarizing acidic sites on ZnO in the presence of ammonia. The phenylnitroso stretching region was unfortunately masked in these spectra by the intense ZnO band at 1149 cm-'. This shift of the adsorbed p-NDMA molecule from acidic to basic surface sites is best evidenced by the decrease in the intensity ratio from 1.08 to 0.13 when ammonia was admitted to the cell (see Table 111). Evacuation of the gaseous ammonia from the cell, which leaves only the strongly adsorbed ammonia molecules on the ZnO surface, resulted in the increase in the intensity ratio to 0.55. In the presence of 20 torr of pyridine, the p-NDMA is also displaced from acidic surface sites on ZnO (see Figure

The Journal of Physical Chemistty, Vol. 85, No. 8, 1981 1009

Adsorbed Species at Solid-Gas Interfaces

*t !rVI

z W t-

z z

a 2

a a

I

1600

I

I

1500

1400

I

1300

I

1200

I

1100

A+ (crn-1)

h

I

1600

I

1500

1400 1300 AX' (cm-')

I200

1100

1398

t

>

cIn z

W

k

I

z a I a a

AV (ern-1)

Flgure 7. Resonance Raman spectra of p-NDMA adsorbed on ZnO; power = 40 mW,,,A, = 4880 A; (A) coverage = 1.05 pmol/g (0 = 0.9),resolution = 4.5cm-l; (B) coverage = 0.4 pmol/g (0 = 0.3), resolution = 5.5 cm-'; (c)coverage = 0.1 pmol/g (0 = o.I), resolution = 5.5 cm-'.

8C). However, the benzene-ring vibration is shifted downward only to 1598 cm-l, which is not as low as was observed in the presence of gaseous ammonia. In fact, two bands at 1598 and 1593 cm-* were apparent in this region of the spectrum. This indicates that p-NDMA molecules are still occupying some acidic surface sites. This was further apparent when the pyridine was evacuated from the cell, as shown in Figure 8D. In this case these two bands appeared at 1600 and 1592 cm-', indicating that pyridine, in contrast to ammonia, is not able to occupy all of the acidic sites on ZnO. The nature of these acidic and basic surface sites can be understood by examining the dehydroxylated ZnO s u r f a ~ e .For ~ ~ example, ~ dehydroxylation of the (0001) and (8) A. A. Tsyganenko,D. V. Pozdnyakov, and V. N. Filimonov, J.Mol. Struct., 29, 299 (1975).

n\r

(crn-l)

Figure 8. Resonance Raman spectra of p-NDMA adsorbed on ZnO (A) in the presence of 100 torr of NH,, (B) after evacuation of NH3from the cell, (C) in the presence of -20 torr of pyridine, and (D) after evacuation of the pyridine from the cell. Coverage = 0.1 pmol/g (0 = 0.11, A,, = 4880 A, power = 100 mW, resolution = 5 cm-I.

(0005) planes of wurtzite ZnO may produce isolated zinc cations located above the plane of the underlying oxygen anions. These cation centers are normally tetrahedrally coordinated in the bulk, but they are coordinately unsaturated on the dehydroxylated surface. These surface cations are expected to be the most acidic adsorption centers on ZnO based on the calculated net charges shown in Table IV. These Lewis-acid centers were also observed in infrared studies of adsorbed ammonia on ZnO surfaces which were previously treated a t 450-500 OC.* These reported spectra only showed evidence of ammonia coordi(9) K. Atherton, G. Neubold, and J. A. Hockey, Discuss. Faraday Soc., 52, 33 (1971).

1010

The Journal of Physical Chemistry, Vol. 85, No. 8, 198 1

Brazdil and Yeager

TABLE IV: Calculated Net Charges of Surface Groups on ZnO, SnO,, and TiOZa net charge for surface site

coordination no. of lattice site for M

M = Zn

4 6

- 0.50

surface hydroxyl group Y

M,

M,

4 6

4 6

M = Sn

M = Ti

-0.33

-0.33

+0.33

+0.33

+ 0.67

+ 0.67

-0.67

- 0.67

0.0

surface cation 4 6

M

surface oxygen anion

Y1 6

a

+0.50

M* 4 6

- 1.00

See ref 1 for method of calculation.

nated to surface zinc cations. The competitive adsorption experiments with pyridine and ammonia suggest that there is a heterogeneity in the acid strength of these Lewis-acid centers on ZnO. After evacuation of the basic molecules from the cell, the Raman intensity ratio for p-NDMA does not return to the value it had in the absence of these molecules, but it does increase somewhat (see Table 111). As mentioned before, adsorption of p-NDMA onto a hydroxyl group on ZnO does not appear to occur. Instead, these results can be viewed as indicating the presence of at least two different Lewis-acid sites. The most acidic and most abundant Lewis-acid sites will result from the dehydroxylation of tetrahedrally coordinated zinc cations on the surface. Analysis of the cleavage planes of the wurtzite structure, however, shows that all surface zinc cations do not occupy normal tetrahedral lattice position^.^ Zinc cations on a dehydroxylated surface can also occupy a position between four surface oxygen anions in the (1011) and (lO1T) planes. Coordination to this additional oxygen will reduce the net positive charge on the cation and thus make it a less acidic adsorption center. Ammonia and pyridine can be desorbed from this site by evacuation at room temperature, allowing p-NDMA to reoccupy this site. Ammonia and pyridine strongly adsorb to the more acidic Lewis-acid centers and prevent the readsorption of p-NDMA on these centers. The basic surface centers on ZnO, on which p-NDMA adsorbs at monolayer coverage and in the presence of a more basic adsorbate, can be assigned to exposed surface oxygen anions. As in the case of alumina,l these oxygen anions result from the dehydroxylation of ZnO surface which removes a proton from an acidic hydroxyl group. The p-NDMA is viewed as adsorbing onto these centers in a nearly perpendicular configuration with a weak adsorptive interaction occurring between the nitroso nitrogen and the lattice oxygen on the surface. In this way the molecule is not strongly polarized by the surface, although partial delocalization of the a electrons of the N-0 double bond is expected to occur. This was observed previously' on the surface of y-A1203by the decrease in the frequency of the N-0 stretch. The adsorbate thus takes on some of the spectral characteristics of its corresponding nitro compound. Thermal desorption experiments show that the pNDMA is weakly adsorbed on these basic centers. When a monolayer coverage of p-NDMA on ZnO was subjected to evacuation at elevated temperatures, no Raman spec-

troscopic evidence for the p-NDMA adsorbed on basic surface site was apparent once the evacuation temperature reached 100 "C. The room-temperature Raman spectrum showed the p-NDMA to be adsorbed primarily on the Lewis-acid sites on the ZnO surface. Evidence for the proposed configuration for the adsorbed p-NDMA molecule at monolayer coverage is provided by the measured adsorption isotherm which showed that the p-NDMA molecule occupies -43 A2 on the ZnO surface. If one uses standard bond lengths, bond angles, van der Waals radii, and an N-0 bond length of 1.21 A and a C-N-0 bond angle of 116O,'O ap-NDMA molecule, which adsorbs nonspecifically with a parallel orientation on the surface, would occupy -70 A2 of surface area. The fact that this is greater than the actual value for monolayer coverage on ZnO requires that the interaction with the surface be more specific and suggests that the orientation on the surface be other than parallel. If the same calculation is made by assuming a perpendicular orientation through the nitroso group, a freely rotating p-NDMA molecule would sweep out an area of -35 A2 on the surface. This is much closer to the experimentally determined value at monolayer coverage and supports the end-on adsorption model for the molecule. The difference between these calculated and experimental values is probably due to the fact that the molecule is not truly perpendicular with respect to the surface. Site geometry may also account for this difference. The resonance Raman spectrum of p-DMAAB adsorbed on ZnO was also studied for surface coverages between 0.14 pmol/g (0.47 pmol/m2) and 10 pmollg (33 pmol/m2). No significant changes in the spectra were observed within this range of adsorbate coverage. The spectrum for the 0.62 pmol/g (2.1 pmol/m2) coverage is shown in Figure 9A and is representative of that observed at the other coverages studied. The spectrum is dominated by the 1142-cm-l band due to the phenyl-azo stretching vibration as well as the intense 1405-cm-' band which can be assigned to the symmetric dimethyl-amino stretching mode. These results indicate that the p-DMAAB molecule is not strongly affected by the ZnO surface at these coverages. The only indication of a perturbation of the adsorbed p-DMAAB by the dehydroxylated ZnO surface is a change in the intensity of the 1593-cm-' band relative to the 1405-cm-' band. This intensity ratio decreased from 0.20 at 0.62 pmol/g to 0.09 at 10 pmol/g. The 0.20 ratio also (10)Y. Hanyu and J. E. Boggs, J. Chem. Phys., 43, 3454 (1965).

The Journal of Physical Chemistry, Vol. 85,No. 8, 198 1

Adsorbed Species at Solid-Gas Interfaces

1011

TABLE V: Observed Vibration Frequencies ( A u ) and Relative Intensities ( I ) of the Raman Bands of p-NDMA Adsorbed on SnO, at Various Coveragesa 13.7b (e = 0.14) Av,

cm-'

1612

2 0 . 0 ~( e = 0.20)

I

A v , cm-'

s-br

1612 1453 1399 1372

1370

m-br

1263 1187 1175

W

m m

1263 1187 1176

I s-br m W

40.2b (0 = 0.40) 1611 1453 1399

A v , cm-'

m m-w m m-w

1610 1445 1399 1362 1322 1263 1182 1176 1142

S

m S

m-br W

m m

1322 1263 1182 1175 1145

80.3b (0 = 0.80)

I

A v , cm-'

W

I

m m S W

W

m W

m m

l O O b (0

= 1.0)

I

A v , cm-'

1609 1444 1397 1363 1322 1262 1187 1176 1142 1127

m m S W W

m W W

m m-sh

a Relative intensities at 4880-A excitation are labeled as follows: s, strong; m, medium; w, weak; sh, shoulder; and br, broad. Coverage in pmol/g.

~d (crn-1) Figure 9. Resonance Raman spectra of p-DMAAB adsorbed on ZnO; coverage = 0.6 prnol/g, A, = 4880 A, power = 40 rnW, resolution N 5 crn-'; (A) on partially dehydroxylated ZnO and (B) on rehydroxylated ZnO.

decreased to 0.09 when the 0.62 pmol/g sample was exposed to 100 torr of anhydrous ammonia. A significant change in the Raman spectrum was observed only when 0.62 pmol/g of p-DMAAB was adsorbed on the hydroxylated ZnO surface, as shown in Figure 9B. A strong band at 1620 cm-l became apparent in the spectrum, as well as bands at 1278 and 1176 cm-l. All of these bands have been previously assigned to the quinoid form of p-DMAAB. The spectrum thus reveals the presence of ionizable hydrogens on the hydroxylated ZnO surface. These bands are, however, shifted to lower energy by -4 cm-l compared to the solution spectrum because of the adsorptive interaction of the molecule with the surface. A duality in the surface sites is also apparent since the 1142-cm-l band, characteristic of the azo form of the molecule, is also seen in the Raman spectrum. When exposed to 100 torr of ammonia, the spectrum returned to that observed on the dehydroxylated ZnO surface since the more basic ammonia molecule competes more favorably for the acidic surface hydrogens. The existence of the quinoid form of p-DMAAB when adsorbed on the hydroxylated ZnO surface clearly reveals the presence of Bronsted-acid sites on this surface. The

existence of Bronsted-acid centers on mixed oxides, such as silica-alumina, whose surfaces are partially hydroxylated is well k n 0 ~ n . l l - l ~However, infrared spectroscopic evidence for Bronsted acidity on pure oxides using ammonia or pyridine as probes is rare,14 and such oxides usually exhibit only Lewis acidity.15J6 This is due in part to the fact that these Bronsted-acid centers are present in low concentration on the surface of pure oxides and can escape detection by infrared absorption spectroscopy. The low concentration is probably due to the fact that Bronsted acidity on pure oxides results from defects or nonstoichiometry on the surface. These lower the normal coordination of a cation and therefore raise the acidity of the hydrogen of the hydroxyl group attached to this cation. As the resonance Raman results of p-DMAAB on hydroxylated ZnO show, spectroscopic detection of Bronsted acidity requires a probe molecule whose spectral properties are uniquely sensitive to this type of acid center. High sensitivity is also required to observe the molecules adsorbed on these low density sites. Resonance Raman Spectra of p-NDMA and p-DMAAB on Sn02 A surface heterogeneity in the adsorption sites on SnOz was also observed from the resonance Raman spectra of adsorbed p-NDMA. The resonance Raman spectra for low, intermediate, and high coverages are shown in Figure 10 and summarized in Table V. The frequency shifts in the structure-sensitive bands for p-NDMA adsorbed a t low coverage indicate that much stornger acid centers are present on the dehydroxylated tin oxide surface than were observed for zinc oxide. This is particularly apparent from the doublet a t 1187 and 1170 cm-l a t a surface coverage of -0.5 ymol/m2 (0 N 0.1). These bands reveal a large double bond character in the phenyl-nitroso bond due to strong polarization of the molecule by an acidic surface site. The doublet further indicates the existence of two types of acid sites, each of which is more acidic than that observed on ZnO. The high-frequency band can be assigned to p-NDMA adsorbed on a Lewisacid center since this band decreases in intensity relative to the low-frequency band as the adsorbate coverage is increased. These Lewis-acid sites are probably coordinately unsaturated tin cations which are in a normally 6-fold coordination site in the lattice. As can be seen from Table IV, these sites possess a higher net positive charge (11) J. E. Mapes and R. P. Eischens, J. Phys. Chem., 58,1059 (1954). (12) F. P. Paray, J. Catal., 2, 371 (1963). (13) J. A. Schwartz, J. VUC.Sci. Technol., 12, 321 (1975). (14) T. Morimoto, H. Yanai, and M. Nagao, J. Phys. Chem., 80,471 (1976). (15) J. B. Peri, J.Phys. Chem., 69, 231 (1965); 70, 2937 (1966); Discuss.

Trans. Far-

The Journal of Physical Chemistry, Vol. 85,No. 8, 1981

1012

TABLE VI:

Brazdil and Yeager

Raman Bands ( A v ) and Relative Intensities of p-NDNIA Adsorbed on SnOZa

in the presence of 100 torr of NH,

in the presence of adsorbed NH,

of -in20thetorrpresence of pyridine

A u , cm-’

I

A u , cm-’

I

A u , cm-’

I

1602 1453 1422 1399 1322 1310 1259 1196

m m m-sh

1602 1450 1422 1398 1320 1305 1260 1190

m m m-sh

1605 1445 1422 1398 1321

m m m-sh

S W W

W

w-br

S W W

1260 1190 1181 1140

W

W

A u , cm-’

I m m m m-sh

m

1608 1600 1445 1421 1398 1320

m

1260

m

1182 1140

m-br m

S

S

m

W

m-sh m

on rehydroxylated surface

I

A u , cm-’

-

1610 1449 1420 1399 1322

S

1259 1190 1180 1141

m

m m-sh m m-br W

m-br

1140 m W 11 28 m-sh a Exciting wavelength = 4880 A. Relative intensities are labeled as follows: s, strong; m, medium; w, weak; sh, shoulder, and br, broad. Surface coverage was 13.7 kmol/g (e = 0.14). 1140

m

in the presence of adsorbed pyridine

t

>-

k v) z W c z -

z a I a K

1600

1500

1400

1300

1100

1200

AU (ern-')

Flgure 10. Resonance Raman spectra of p-NDMA adsorbed on SnO,; A, = 4880 A, power = 100 mW, resolution = 5.5 cm-‘; (A) coverage = 13.7 pmol/g (6 = 0.1); (B) coverage = 40 pmol/g (6 0.4); (C)coverage = 100 lmot/g (8 1.0).

compared to a Lewis-acid center on dehydroxylated zinc oxide. Additionally, the larger covalent character in the metal-oxygen bonds in the SnO, lattice makes the surface tin cations more acidic than a similar zinc cation. In addition, the Raman spectra show evidence of p NDMA hydrogen bonded to surface hydroxyl groups on tin oxide. Specifically the 1263-cm-’ band, which was previously observed for p-NDMA adsorbed on a surface silanol group on SiOZ,lincreases in intensity as the adsorbate coverage is increased to 0 = 0.4. The 1175-cm-’ band is also observed to increase in intensity and can thus be assigned to the phenyl-nitroso stretching mode of p NDMA adsorbed on a surface hydroxyl group on tin oxide. On the rehydroxylated surface with 8 N 0.1, these bands are also observed to increase in intensity while the 1187cm-I band, characteristic of adsorption on Lewis-acid sites, disappears. The Lewis-acid centers are thus blocked after

-

the surface is exposed to water vapor. These results are in contrast to the results for zinc oxide, where no evidence of hydrogen bondiqg to surface hydroxyl groups was observed. The greater acidity of hydroxyl groups attached to normal cation sites on SnO,, compared to that on ZnO, can be seen from Table IV. The table shows that hydroxyl groups on SnO, are expected to possess a greater net positive charge, thus making them stronger adsorption sites for p-NDMA than similar sites on ZnO. The large covalent character of the Sn-0-H moiety would also be expected to contribute to the higher acidity of this surface site. Further evidence for the existence of two acid sites is provided from the Raman spectra of p-NDMA adsorbed on SnOz in the presence of ammonia or pyridine. The Raman spectra are summarized in Table VI. In the table, the headings “adsorbed ammonia” or “adsorbed pyridine” refer to the Raman spectra after the evacuation of the gaseous ammonia or pyridine from the cell at room temperature. In the presence of gaseous ammonia or pyridine, the p-NDMA is displaced from both of the acidic sites on SnO,. The 1187- and 1176-cmT1doublet is replaced by a single band at 1140 cm-I which is characteristic of the molecule adsorbed on a basic site. In the presence of adsorbed ammonia, however, the 1260-cm-’ band is barely visible in the spectrum, while, in the presence of adsorbed pyridine, this band is moderately intense. Therefore, since pyridine is a strong Lewis base, the molecule preferentially occupies the Lewis-acid centers on SnOz and allows the p-NDMA to remain hydrogen bonded to the surface hydroxyl groups. Ammonia, being a stronger base than pyridine, displaces the adsorbed p-NDMA from both acid centers. The difference in the acidity of these two sites can be seen from the measured Raman intensity ratio of the 8a benzene-ring vibration to the symmetric vibration of the dimethylamino group shown in Table VII. Some of the p-NDMA molecules occupy acidic sites in the presence of pyridine where the intensity ratio was observed to be 0.47 compared to a value of 0.19 in the presence of ammonia. As the p-NDMA coverage on SnO, is increased, the spectral characteristics for the adsorption on acidic sites are reduced in intensity and are replaced by those characteristics of adsorption on basic surface sites. This heterogeneity in surface acidity is clearly evident from the change in the intensity ratio with coverage listed in Table VII. At 0 = 0.1, the 1399-cm-l band is not clearly visible and the spectrum is dominated by the intense 1612-cm-’ band. The intensity ratio drops rapidly as the acidic sites become occupied and the p-NDMA adsorbs on basic sites. A t monolayer coverage, the intensity ratio is 0.25. This

Adsorbed Species at Solid-Gas Interfaces

The Journal of Physical Chemistry, Vol. 85,No. 8, 198 1

TABLE VII: Raman Intensity Ratio for p-NDMA Adsorbed on SnOZa

TABLE VIII: Raman Intensity Ratio for p-NDMA Adsorbed on TiO,'"

I(16121602 cm-' )/ I(1398coverage, pmol/g (fractional coverage) 13.67 (e = 0.14) 20.0 (e = 0.20) 40.2 (e = 0.40) 80.35 ( 0 = 0.81) 100.0 (e = 1.0) 13.67 (e = 0.14)

20.0 (e

= 2.0)

1400

sample description 500 "C treated SnO, 500 " C treated SnO, 500 "C treated SnO, 500 " C treated SnO, 500 "C treated SnO, 500 "C treated SnO, + 100 torr of NH, 500 "C treated SnO, + 20 torr of pyridine 500 "C treated SnO, + 20 torr of pyridine (evacuated) rehydroxylated SnO,

cm- ' )

1593 cm-')/ I( 1398coverage, molk

0.51 a

is similar to the value observed in the presence of adsorbed ammonia, indicating that the p-NDMA is primarily occupying basic sites at high coverages. As in the case of ZnO and A1203,the adsorption of p-NDMA on basic sites on Sn02 is viewed as a weak bonding of the nitroso nitrogen to surface oxygen anions. As was shown for ZnO, this specific interaction with the surface is supported by the fact that, at monolayer coverage on Sn02, each p-NDMA occupies 46 A2 of surface area. A nonspecific parallel orientation on the surface is ruled out, since calculations made in the previous section show that in such a configuration each molecule would occupy -70 A2 of surface area while a perpendicular configuration requires only -35 A2 of surface area. When p-DMAAB was adsorbed on Sn02, the resonance Raman spectra showed no evidence of Bronsted acidity. No bands associated with the quinoid form of the adsorbed molecule were observed in the Raman spectra from either the hydroxylated or partially dehydroxylated surfaces. This result is in accord with infrared studies of adsorbed pyridine and ammonia on tin oxide gel." This fact attests to the high degree of covalent character in the bonding in the tin-hydroxyl moiety.18 The hydroxyl groups on tin oxide, as on other covalent metal oxides such as Si02,19 show a strong propensity to hydrogen bond to bases but do not usually act as proton donors. Resonance Raman Spectra of p-NDMA and p-DMAAB Adsorbed on TiOz.The resonance Raman spectrum of p-NDMA adsorbed on anatase was obtained for surface coverages between 50 pmol/g (0.9 pmol/m2) and 300 pmol/g (5.4 pmol/m2). When one assumes the same monolayer coverage which was observed for ZnO, this range amounts to a fractional surface coverage on Ti02 of 8 N 0.2 to 6 N 1.4. As in the case of ZnO and SnOz,these spectra show that both acidic and basic adsorption sites are present on the TiOp surface. This is clearly evident from the observed change in the Raman intensity ratio as a function of coverage which is listed in Table VIII. The intensity ratio decreased from a high of 3.82 at 50 pmol/g coverage to 0.19 at 300 pmol/g coverage. The large value (17) P. G. Harrison and E. W. Thornton, J. Chern. Soc., Faraday Trans. 1, 71, 1013 (1975). (18) E. W . Thornton and P. G. Harrison, J. Chern. Soc., Faraday Trans. 1, 71, 461 (1975). (19) N. W. Cant and L. H. Little, Can. J. Chern., 43, 1252 (1965).

1400 sample description

500 "C dehydroxylated TiO, 500 "C dehydroxylated TiO, 500 "C dehydroxylated TiO, 500 "C dehydroxylated TiO, 500 "C dehydroxylated TiO, 500 "C dehydroxylated TiO, + 100 torr of NH, 50 500 "C dehydroxylated TiO, + 100 torr of NH, (evacuated) 50 rehydroxylated TiO, Exciting wavelength = 4880 A. 50 75 100 200 300 50

0.47

The a Intensities measured at 4880-8excitation. 1398 cm-' band was not clearly observed in the Raman spectrum at this coverage.

-

I( 1602-

3.79 0.80 0.52 0.25 0.19

1.61

1013

cm-') 3.82 3.07

1.04 0.57 0.19 0.13 0.29 3.97

for the intensity ratio at low coverage as well as the absence of a strong Raman band in the 1260-1280-~m-~ region indicate that the molecule is primarily adsorbed on Lewis-acid sites on the surface. These coordinately unsaturated titanium atoms are expected to have acid strengths similar to the Lewis-acid sites on Sn02 based on their calculated net charges (see Table IV). The similarity in the ionic radii of the Sn4+and Ti4+ cations will also be responsible for producing a similar charge density on the Lewis-acid centers of these oxides. This accounts for the similar Raman intensity ratios for p-NDMA adsorbed on TiOa and S n 0 2 at comparable coverages. These Raman spectra also show evidence of hydrogen bonding to surface hydroxyl groups by the presence of the weak band at -1265 cm-l. When the surface was rehydroxylated by exposure to water vapor, this band increased in intensity and shifted to 1270 cm-l. The appearance of the band at 1140 cm-l and the decrease in intensity ratio to 0.19 when the p-NDMA coverage was increased indicate that the molecule adsorbs on basic adsorption sites once the acidic sites have been occupied. In the presence of ammonia, the Raman intensity ratio for the low-coverage sample decreased to 0.13 when the p-NDMA was displaced from the Lewis-acid centers. This basic adsorption site is probably the oxygen anion of the Ti-0-Ti bridge which is formed during dehydroxylation of the surface.m This is identical with the situation discussed earlier for ZnO and SnOz. The greater adsorption strength of Lewis-acid sites on Ti02 compared to those on ZnO was demonstrated in thermal desorption experiments wherein a 300 pmol/g sample was subjected to evacuation temperatures between 25 and 400 "C. After evacuation the cells were cooled to room temperature, and the Raman spectra recorded. From these experiments it was observed that the p-NDMA molecule begins to desorb from the basic sites only at 200 "C. These molecules are completely desorbed by -300 O C , leaving only the p-NDMA molecules which are more strongly adsorbed on Lewis-acid sites, as indicated by the increase in the intensity ratio from 0.19 to -3.7. The p-NDMA molecules were completely desorbed from the acid sites only when the evacuation temperature exceeded 350 "C. The resonance Raman spectra of p-DMAAB on TiOz were studied for coverages between 100 pmol/g (1.8 pmol/m2) and 500 pmol/g (9.0 pmol/m2). In each case, the spectra showed a moderately intense band at 1622 cm-' (20) M. Primet, P. Pichat, and M. V. Mathieu, J. Phys. Chern., 75, 1221 (1971).

1014

J. Phys. Chem. 1981, 85, 1014-1016

which had its greatest intensity a t the lowest surface coverage. The observed decrease in its intensity with increasing coverage may be caused by the attenuation of the laser beam in the sample due to absorption by the adsorbate. This band, which is characteristic of the protonated p-DMAAB molecule, reveals the presence of Bronsted acidity on the 500 "C treated surface. Bronsted acidity has been observed previously for anatase by using trimethylamine as a probe molecule in an infrared spectroscopic study, but only after the surface had been hydroxylated with water vapor.20 In this investigation, exposure of the heat-treated TiOz surface to water vapor at room temperature was found to increase the number of Bronsted-acid sites as shown by the increase in the relative intensity of the 1622-cm-l band in the Raman spectrum. This acidic proton can be associated with a hydroxyl group which bridges two titanium cations on the anatase surface. This 0-H bond is expected to have increased ionic character due to the polarizing effect of the two cations.21 Complete removal of these protons may not be possible a t 500 "C because of the lack of a more basic hydroxyl group in close enough proximity once the surface becomes depleted of OH groups during dehydroxylation. Although the net positive charge for the hydroxyl group bridging two cations on SnOz is the same as that calculated for Ti02, tin oxide was not observed to possess any Bronsted acidity. This is primarily because tin oxide does not show as strong a tendency to form hydroxyl bridges as does anatase. Rather, because of the high covalent character of the tin-hydroxyl moiety,18 a bridging hydroxyl group would instead dissociate into a singly coordinated ~

~~

(and therefore more basic) OH group and a coordinately unsaturated tin cation on the surface.

Concluding Remarks It should be remembered that the intensity of the observed resonance Raman lines for any surface species depends upon both the amount of the species present and the frequency of its absorption maximum. For this reason, it was not possible to make accurate estimates of the relative amounts of the individual surface species present at any given time. Instead, it was only possible to confirm the existence of these species from the Raman spectra under controlled adsorption conditions. The results of this investigation have demonstrated that the use of adsorbates which give an intrinsic resonance Raman enhancement is a powerful tool for probing the adsorption properties of surfaces a t low absorbate coverages. Detailed information about the acid-base nature of the adsorption sites of a solid can be obtained by appropriate. choice of probe molecules whose Raman spectra are sensitive to various chemical environments. When the resonance Raman phenomenon is used, additional enhancements which result from unique interactions between the adsorbate and the surface, as in the case of pyridine adsorbed on metallic silver,22are not needed to readily observe the vibrational spectrum of an adsorbed molecule.

Acknowledgment. One of us (J.F.B.) thanks The Standard Oil Company (Ohio) for a leave of absence to conduct this research and for the use of their equipment and facilities.

~~~

(21)H.P.Boehm, Adu. Catal., 16,179 (1966);Discuss,Faraday SOC., 62,1221 (1971).

~~~

(22)B. Pettinger, U.Wenning, and D. M. Kolb, Ber. Bunsenges. Phys. Chem., 82, 1326 (1978).

Low-Frequency Raman Studies of Metal-Ammonia Solutions B. De Bettignies Laboratoire de Spectrochimie Infrarouge et Raman, C.N.R.S., U.S. T.L., 59650 Villeneuve 0'A8C2, France

and J. P. Lelleur" Laboratoire d'Etude des Surfaces et Interfaces, L.A. 253, 59046 Lllle Cedex, France (Received: June 17, 1979; In Final Form: December 3, 1980)

Raman spectroscopy of several solvated and unsolvated lithium and sodium salts provides new information about the structure of concentrated metal-ammonia solutions. Spectra of the frozen salt solutions are similar to those of solvated salts and solid amides, whose structure are known. In the liquid state, the spectra of the salt solutions have bands below 700 cm-l that are similar to the low-frequency bands of the correspondingmetal solutions. These results suggest that the metal-ammonia solutions can be interpreted in the same manner as the salt-solution spectra in the liquid state. Although several methods have been employed to study metal-ammonia solutions, little definitive information has been obtained from laser Raman spectra.lP2 In such spectra, two ranges must be considered. The first concerns the internal vibrational modes of ammonia, while the second deals with the low-frequency modes of ammonia and the metal-nitrogen vibrations. In this paper, we (1)M.G.De Backer, P. F. Rusch, B. De Bettignies, and G. Lepoutre in "Electrons in Fluids", J. Jortner and N. R. Kestner, Eds., SpringerVerlag, Berlin, 1973,p 161. (2)Proceedings of Colloque Weyl IV, J.Phys. Chem., 79,2789-3114 (1975). 0022-365418112085-1014$0 1.2510

briefly report results for the ammonia vibrations, and then we focus attention on the low-frequency region of the spectra. We compare spectra of sodium-ammonia solutions with those of ammoniated sodium iodide, and spectra of lithium-ammonia solutions with lithium amide and lithium nitrate, and give evidence that the organization of ammonia molecules around the metal cations is similar for these systems.

Experimental Section Solutions were prepared in cylindrical 10-mm 0.d. Pyrex tubes with ammonia which was distilled twice before it was 0 1981 American Chemical Society