Langmuir 1992,8, 25962600
2598
Fourier Transform Infrared Spectroscopy of H20 on the Hydroxylated ZnO Surface S. Kittaka,' T. Sasaki, and N. Fukuhara Department of Chemistry, Faculty of Science, Okayama University of Science, 1 - 1 Ridaicho, Okayama 700, Japan Received May 4,1992. In Final Form: July 13, 1992 Phase properties of physisorbed HzO molecules on the hydroxylated ZnO were studied through IR spectroscopy at low temperatures. It was confirmed that a two-dimensionally condensed phase of HzO cannot be completed even below the two-dimensionalcritical temperature which has been determined by adsorptionmeasurements. Below 233 K interaction between physisorbedHzO and surface OH is weakened, by which physisorbed HzO molecules are detached from OH sites and form some kind of condensed HzO phase.
Introduction The stepwise adsorption isotherm of gases on the solid surface indicates the presence of lateral attractive interactions between adsorbed molecules. It makes us expect the Occurrence of two-dimensional phase changes of adsorbed gases. There are a number of examples for this phenomenon such as inert gases on the graphite and layered material surfaces.lg Since such a type of isotherm was found in the system of H20 on the hydroxylated ZnO surface, it has been studied by various techniques such as adsorption$ calorimetry,6 and dielectric measurement^.^-^ But so far there has not been evidence for the occurrence of a condensed phase except the fact that the twodimensional critical temperature has been determined to Zwicker and be 236 K from adsorption Jacobi proposed the three-dimensional and two-dimensional ices a t 152 and 168 K, respectively,on the singlecrystal clean ZnO surfaces of (Ool), (Ool), and (loo)? However,their experimente did not afford direct evidence for such phases. Recently, researchers have observed clearly the phase change in the system of HzO molecules on the hydroxylated Cr209 surface through Fourier transform IR spectroscopy under varying temperatures.1° Present work has been conducted on the same line to study the phase property of adsorbed H20 on the hydroxylated ZnO surface by using IR spectroscopy a t varying temperatures.
Experimental Section Materials. The ZnO was a purchased sample, Kadox 15 which was produced by burning Zn in air. Before every sequence of measurements the sample was evacuated in each apparatus at 688 K to remove volatile surface impurities such as H20, COz, and organics, and was hydrated with saturated HzO vapor for a night. The surface area thus treated was determined to be 7.83 m2 g l by applying the Brunauer-Emmett-Teller (BET)equation on the Nz adsorption isotherm measured at 77 K. (1) Steele, W.A. Interactionof Gaseswithsolidsurfacea. Inhoperties of Interfaces; Everett, D. H., Ed.; Pergamon Press: Oxford, 1974; Topic 4. (2) Enault, A.; Larher, Y. Surf. Sci. 1977, 62, 233. (3) Dash, J. G.;R u d d s , J. Phase Transitions in Surface F i l m ; Plenum Press: New York, 1980. (4) Nagao, M. J. Phys. Chem. 1971, 75, 3822. (6)Nagao, M.;Yunoki,K.;Muraishi, H.; Morimoto,T. J.Phys. Chem. 1978,82, 1032. (6) Iwaki, T.; Morimoto, T. Langmuir 1987,3, 282. (7) Iwaki, T.;Morimoto, T. Langmuir 1987, 3, 287. (8) Morishige, K.; Kittaka, S.;Morimoto, T. Surf. Sci. 1981,109, 291. (9) Zwicker, G.; Jacobi, J. Surf. Sci. 1983, 131, 179. (10) Kittaka, S.;Sasaki, T.; Fukuhara, N. To be published in Surf. Sci.
Adsorption of Hs0. Adsorption of HzO was performed gravimetrically on the 564-mg sample at 259.1-298.4 K by using a Cahn 2000 electrobalance which was connected with a H20 vapor dosing system constructed with greasafree lines. Before HzO adsorption, the sample was evacuated at 298.4 K until a constant weight was attained by use of a turbomolecular pump (- 10-3 Pa). The equilibrium HzO vapor pressure of the system was determined by a capacitance manometer, Baratron 390 B. The temperatureof the sample was controlled by using methanol and HzO bathe at 259.2-298.4K. The adsorption was equilibrated within 30 min. Fourier Transform Infrared Spectroscopy. IR measurements were carried out by use of a Fourier transform IR spectrometer,JEOL JIR-100,which was equipped with an MCT detector. A disk of 100 mg of sample (diameter 13 mm) was mounted on a copper sample holder in a cell which permitted the heating of the sample in a vacuum of lo-' Pa, measurements at temperatures from 93 to 298.4 K, and varying HzO vapor pressures. The cell consists of two chambers: the inner one in which the sample disk was set has a couple of quartz windows and the outer one CaFz windows. The outer chamber was continuously evacuated for thermal insulation of the inner one. Pretreatment of the sample was done as in the HzO adsorption.
Results and Discussion Figure 1 shows the adsorption isotherms of HzO determined a t various temperatures on the hydroxylated ZnO which was evacuated at 298.4 K for a night before each run. The result was not different from that found previously upon volumetric method? Le., adsorption of H20 on the active sites at low pressures and adsorption on the specific homogeneous surface which might bring about the two-dimensional condensation. The two-dimensional critical temperature, which was determined through the extrapolation method introduced by Enault and Larher? was also similar to that reported previously (236 K). A monolayer coverage can be estimated to be 0.25 cm3m-2 (STP) by the boiling point method which is fairly smaller than 0.35 cm3 m-2 (STP) calculated by molecular area (0.106 nm2). This indicates that 28% of the surface is inactive for HzO adsorption. Figure 2a shows the IR absorption spectra of the sample determined at 298.4 K (above the two-dimensional critical temperature) under increasing HzO coverages. The spectrum for the starting sample is very similar to those reported by Atherton et al." and Mattmann et al.12 All the peaks 3681,5656,3639,3617,3558, and 3448 cm-l were shifted by deuterium exchange to those expected on the (11) Atherton, K.; Newfold, G.; Hockey, J. A. Discuss. Faraday SOC. 33. -(12) Mattmann, G.; Oswald, H. R.; Schweizer, F. Helu. Chim. Acta 1972,55, 1249. 1971. ~. -,52. --r
0743-7463/92/2408-2598$03.00/00 1992 American Chemical Society
Langmuir, Vol. 8, No. 11, 1992 2699
Letters
0
03
IO
I5 20 P I kPa
25
3.0
Figure 1. Adsorption isotherms of HzO on the hydroxylated ZnO surface determined at varying temperatures (K): 1,259.1; 2, 263.8; 3, 274.2;4,283.5;5,293.4;6,298.4.
4000
3600 3200 WAVLWUYER / em-'
2000
4000
3600 3200 WAMNUYBER I cm-I
2IOO
Figure 3. Infrared absorption spectra observed on the sample cooled at lower temperatures in the closed system where initial coverage for the spectrum (2)is 0.68cm3 m-2 (STP)for P = 0.948 kPa. (a) Temperatures (K): 1,298.4under vacuum; 2,298.4;3, 273;4,253;5,243;6,233;7,193;8,153;9,93. (b) Temperatures (K)all under vacuum: 1, 298.4; 2,273;3, 253;4,223;5, 193;6, 153;7,93. T I K 313.2
273.2
233.2
103.2
I
l
4000
3800
3200
2000
4000
I
.
3600
t
I
3200
,
L
2800
Figure 2. (a) Infrared absorption spectra of hydroxylated ZnO determined at 298.4 K under increasing amounts of HzO adsorption (cm3m-2 (STP)): 1,O; 2,0.045;3,0.107;4,0.170;5, 0.209; 6, 0.264; 7, 0.317. (b) Infrared absorption intensity incrementa calculated by subtracting the spectrum in (a) from that just above: 1, 2-1; 2, 3-2; 3, 4-3; 4,5-4; 5,6-5;6,7-6.
OD stretching modevibrations, i.e., 2711,2696,2686,2669, 2629, and 2551 cm-l, respectively. Characteristic changes in the IR spectrum with H20 shown hereafter have mostly been confirmed with D20. The spectral changes upon H2O dosing are not sharp ones (Figure 2a). Therefore, the growth of peaks was detailed by subtracting each spectrum from that just above (Figure2b). First dosing of H20 vapor gave negative peaks, a sharp one at 3617 cm-l and a broad one at 3448 cm-I, and rather sharp positive peaks at 3379 and 3695 cm-I which continued to increase up to 0.107 cm3 m-2 (STP). A chemical shift of peaks upon H20 adsorption is not clear. At the stage 0.170.209 cm3m-2 (STP),corresponding to the sudden step around0.9 kPa in the adsorption isotherm, the main increase was observed at 3471 cm-', accompanied by the diffusing of the peak at 3558 cm-l. Above 0.209 cm3m-2 (STP)the peak was broadened further and shifted to somewhat lower wavenumbers. The IR spectra observed in the lower relative pressures at temperatures lower than 250 K (i.e., smaller adsorption range) were similar to those at 298.4 K, but differed at a higher pressure range. That is, IR absorption spectra do not increase their intensity after repeated dosing of H20 vapor. This result was demonstrated more clearly by the observation of spectra at decreasingtemperatures in which the sample cell was closed after equilibrating with given H2O pressures at 298.4 K. One example is shown in Figure 3a for the system equilibrated initially at 0.948 kPa. The absorption intensity increases first due to adsorption of H2O in a chamber by an increase in the humidity of the system. After reaching the maximum around 253 K, the
30
40 IIT IXIO-~I
50
Figure 4. Equilibrium H20 vapor pressures at a constant amount of adsorption. Thick lines: saturated H2O vapor pressure which is equilibrated with liquid H20 and ice. Broken line: HzO vapor pressure over supercooled HzO. Adsorbed amount of H20: (cma m-2 (STP)): 0, 0.1;A, 0.2;0 , 0.3. spectrum begins to decrease: the sequential spectral change is very similar to the direction of the spectral change observed at decreasing H2O vapor pressures of Figure 2a. Below 193 K small but clear peaks appear and increase their intensity at 3650,3605-3574, and 3423 cm-l. This process was reversed with fairly good reproducibility. Figure 3b shows the IR spectra observed at decreasing temperatures under vacuum where every peak becomes sharper as the temperature goes down but integrated intensities ranging 3700-2800 cm-I are almost unchanged. New tiny peaks observed under vacuum exactly coincide with those observed under humid conditions at lower temperatures in Figure 3a. Therefore, we came to two important conclusions. (1)Some amount of H2O molecules are desorbed at lower temperatures. This was confirmed when the sampleweight in a closed system was traced with decreasing temperature from 298 K. The sample weight was increased expectedly at first and decreased after reaching the maximum. The maximum was displaced to lower temperatures and became less pronounced with lower starting equilibrium vapor pressures. The result was considered to be rational from the illustration in Figure 4, where the pressurea giving the adsorbed amounts of H20,0.1,0.2, and 0.3 cm3 m-2 (STP), were plotted as a function of the inverse of temperature. Equilibrium vapor pressures of pure H20 (333-273.2 K) and of ice (273.2-190 K)are drawn as thick solid linea. It is clearly shown that the extrapolation of the line for 0.3
2600 Langmuir, Vol. 8,No.11,1992 cm3m-2 (STP),which is, if it occurs, near the density for the two-dimensionallycondensed phases, crossesthe vapor pressure line of ice around 245 K and goes over it at lower temperatures. This signifies that below 245 K H20 vapor adsorption is depressed below 0.3 cm3m-2 (STP).Therefore, there will be quite a small amount of H2O molecules on the surface which is expected to be the stage of twodimensional condensation of H2O. (2)The signals due to surface hydroxyls, which have once disappeared upon adsorption of HzO, reappear at much lower temperatures. It is reasonable to consider that adsorbed HzO molecules on the OHs are detached at low temperatures to form some kind of condensed phase and surface OHs are exposed. However, it was not possible to find a characteristic peak for ice around 3220 cm-l but a broad peak around 3378cm-'. The reversibleappearance
Letters of threetiny new p& (3604,3594,and 3577cm-1) between 3617 and 3560 at lower temperatures suggests the occurrence of phase changes probably characteristic of surface OHs of ZnO. In the case of the Cr203 surface,13which also presents the stepwise adsorption isotherm of HzO, the vibration mode of OHs in either system which includes adsorbed H2O or not does not change at decreasing temperatures down to 93 K. Simply, observed are sharpening of peaks and resolution of overlappings. Therefore, detailed analysis is required in the future in the case of ZnO surface.
Acknowledgment. The authors express sincere thanks to Professor H. Katoh of the Department of Chemistry for his valuable discussions and encouragements during this work. (13) Unpublished data obtained during the work of ref 10.
