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Morphology and surface defects of zinc oxide
Morphology and Surface Defects of Zinc Oxide G. SENGUPTA, I N. K. M A N D A L , M. L. K U N D U , R. M. SANYAL, AND S. D U T T A Projects and Development India Ltd., Sindri, Bihar, India 828122
Received August 25, 1983; accepted April 8, 1986 Samples of zinc oxide prepared from different sources do not show the same type of selectivity for hydrogenation of carbon monoxide due to creation of various types of active sites on the surface. It is reasonable to expect that there may be some correlation between the nature of surface defects or active sites and morphologyof zinc oxide crystallites. In the present investigation,the surfacedefectshave been characterized by electrical conductivitymeasurement and the morphologyhas been studied by electron microscopy, electron diffraction, X-ray diffraction, and NIR spectroscopy. It has been concluded that the surface of zinc oxide prepared from carbonate consists of all types of planes, whereas basal planes predominate in zinc oxide prepared from oxalate and pigment. Becauseof this, the surfaceof zinc oxide from carbonate is less reduced than the other two varieties of zinc oxide in the presence of hydrogen under conditionssimilarto carbonmonoxidehydrogenationreaction. This leadsto variationsin selectivity. © 1987 Academic Press, Inc.
INTRODUCTION Zinc oxide is well known as the catalyst for the synthesis of methanol from carbon monoxide and hydrogen. Although it was shown by Natta (1) that not all types of zinc oxide have the same synthesis activity and that prepared from smithsonite gives a better yield of methanol, no satisfactory explanation could be given for the observed behavior at that time. Curnisio et at. (2) have shown that selectivity for methanol formation from carbon monoxide and hydrogen depends on the adsorption mode of carbon monoxide. On zinc oxide surface, the C-O bond is stabilized through participation of several surface oxygen atoms in the adsorption process. This results in hydrogenation of carbon monoxide to methanol on an "oxidized" surface. On a metallic or "reduced" surface, hydrogenation to methane is more favorable due to weakening of the C-O bond. Similar conclusions have been drawn by Voroshilov et al. (3) from quantum chemi To whom correspondenceshould be addressed.
ical calculations, IR spectroscopy, and electronic work function study. Since methanol synthesis takes place under highly reducing conditions, it is worthwhile to study the nature of defect sites on the surface of different types of zinc oxide subjected to the reducing pretreatment. This may help to understand the reasons for the variations in selectivity exhibited by zinc oxide obtained from different sources. In the present investigation, the surface of zinc oxide has been characterized by water vapor adsorption and electrical conductivity studies to identify the type of defects. At the same time the structure and morphology of zinc oxide have been studied by electron microscopy, X-ray diffraction, and reflectance spectroscopy to find out to what extent these parameters influence the nature of oxide surface. During methanol synthesis a small a m o u n t of carbon dioxide or water vapor mixed with the synthesis gas helps to produce a thermal quenching effect and also acts as a mild oxidizing agent (7). Moreover, synthesis of methane which always occurs in varying degrees
301 0021-9797/87 $3.00 Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
Copyright © 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.
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parallel to methanol synthesis produces water vapor. Hence, in the present investigation moist hydrogen was used instead of dry hydrogen to simulate the conditions of synthesis reaction as much as possible. EXPERIMENTAL
Four types of zinc oxide were used for the experiment. ZnO(P) was a pigment variety zinc oxide supplied by M/s. Hindusthan Pigment Limited and had a purity better than 99.9%. It was cured in air for 10 h at 773 K. ZnO(H), ZnO(C), and ZnO(X) were prepared by dissolving ZnO(P) in nitric acid, precipitating with ammonium hydroxide, carbonate, or oxalate, respectively, followed by curing in air at 773 K for 10 h. Hydrogen was purified by passing through reduced copper catalyst at 573 K. Oxygen was dried by passing through silica gel. Water was deaerated by repeated freezing and thawing in vacuum. The cell and the experimental setup for studying the effect of chemisorption of water vapor on electrical conductivity of zinc oxide have already been described in previous communications (4-6). Zinc oxide samples were taken in the form of tablets of 10 × 10 mm size and subjected to the following pretreatment: (a) evacuation of 673 K for 2 h; (b) heating in a flow of dry oxygen at 673 K for 2 h; (c) second evacuation at 673 K for 2 h; (d) reduction with moist hydrogen at 573 K for 12 h; (e) final evacuation at 673 K for 2 h. The oxide tablet was then cooled in vacuum to 523 K and the conductivity of the solid was measured. Water vapor was introduced into the chamber at a pressure of 5.33 × 103 Pa. The conductivity of the solid changed instantaneously due to adsorption of water vapor. The conductivity was measured at different intervals of time to follow the adsorption process. The solid was then cleaned by the pretreatment (a) to (e), cooled to 348 K, and then Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
the water vapor adsorption was studied as before. The X-ray diffractograms were taken with a Philips diffractometer using a scanning speed of 1/2 ° per min. CuK, radiation was used. Electron micrographs and diffractograms were taken with the help of Siemen Elmiskop 1A. Electron optical magnifications varied from 40,000 to 60,000× at 80 kV. The beam diameter used for electron diffraction studies was 10 it. A Cary 17D spectrophotometer with diffuse reflectance attachment was used to record the spectra of the zinc oxide samples in the region of 1200-1800 nm. The samples were mounted on an infrasil plate that was attached to the window of the integrating sphere. Spectra were recorded with samples stored in air for a long time. To identify the adsorption bands due to physically adsorbed water vapor, the sample ZnO(P) was treated at 773 K for 2 h with dry N2 + 02 mixture in a setup shown in Fig. 1. After treatment, the constrictions A
!
IH C
S
li FIG. 1. Setup for treatment of ZnO in dry N2 + 02 at 773 K.
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MORPHOLOGY OF ZINC OXIDE
and B were sealed off and the sample was transferred to the holder H fitted with an infrasil window. The constriction at C was sealed off and the holder with the sample was attached to the spectrophotometer. All the spectra were recorded at r o o m temperature.
1.0~~_____..~
O.
~
RESULTS AND DISCUSSION
Water Vapor Chemisorption Results of conductivity measurements are presented as plots of a/tro vs time (Figs. 2 and 3) where tr0 is the conductivity of the oxide sample just before the adsorption of water vapor at the temperature of measurement. F r o m Fig. 2 it can be seen that upon chemisorption of water vapor at 523 K, ZnO(P) and ZnO(X) show a slight increase in conductivity. But adsorption of water vapor on ZnO(C) and ZnO(H) causes a continuous decrease in conductivity and in case of ZnO(C) it is preceded by a small increase in conductivity. In a previous communication (6) it has been shown that if the adsorption of water vapor on zinc oxide at 523 K occurs on Zn+O - defect pairs, then the water molecules act as electron acceptors. But in the case of Zn+O -2 defect sites acting as adsorption sites, the water molecules act as electron donors:
OI 0
.
.
20 40 TIME IN MINUTES
FIG. 3. Changes in electrical conductivity upon chemisorption of water vapor at 348 K on different types of zinc oxide. (~, ZnO(P); O, ZnO(X); (9, ZnO(H); ~, ZnO(C).
Zn+O - + H 2 0 ~
OH I Zn
OH Zn+O -2 -b H20--~ I Zn ÷
0.5-
o¸ 0
2'0
4'0
6'0
TIME IN MINUTES
FIG. 2. Changes in electrical conductivity upon chemisorption of water vapor at 523 K on different types of zinc oxide. (~, ZnO(P); e, ZnO(X); ®, ZnO(H); ~k ZnO(C).
H I OH I + e. O-
At 348 K a decrease in conductivity was observed for all the samples due to adsorption of water vapor (Fig. 3), with that in the case of ZnO(C) being least. It has been shown in a previous communication (5) that Zn+Zn ÷ ion pairs can also act as adsorption sites for water molecules and the adsorption is acceptor type OH Zn+Zn - + H 2 0 -'~ [ Zn +
1.o.
6"0
H ]
Z n +"
The unpaired electron of the O H radical combines with the free electron of the Zn ÷ site forming a strong covalent bond, and the trapping of the free electron will cause a decrease in conductivity. The chemisorption of the H radical takes place on the other Zn ÷ site of the ion pair as Zn+-H. Such adsorption without involving electron transfer is a weak adsorption and occurs below 373 K. But at these temperatures adsorption on Zn+O - is not possible since formation of O-H bond requires higher temperature (6). Therefore the observed decrease in conductivity due to water vapor adsorption at 348 and 523 K can be attributed Journal of Colloid and Interface Science. Vol.117,No. 2, June 1987
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to two different types of adsorption sites viz. Zn+Zn+ and Zn+O -, respectively. Hence, it can be concluded that after reduction with hydrogen at 573 K, the surfaces of ZnO(P) and ZnO(X) contain mostly Zn+O-2 sites, whereas those of ZnO(H) and ZnO(C) contain both Zn+O - and Zn+O -2 sites. Zn+Zn+ sites are present in all oxides but in varying degrees. From Figs. 2 and 3 it appears that ZnO(C) contains more Zn+O sites and less Zn+Zn+ sites than the other three oxides. Both ZnO(P) and ZnO(X), as well as ZnO(H), contain appreciable Zn+Zn + sites. The relative abundance of these sites can be taken as a measure of relative concentration of surface oxygen. For Zn+O- sites, the oxygen zinc ratio is 1, but for Zn+Zn+ and Zn+O -2 sites this ratio is O and 1/2, respectively. The Zn+O-2 sites can be depicted as Zn+O-ZZn+ to maintain charge neutrality. Therefore, it can be concluded that the surface oxygen of ZnO(P), ZnO(X), and ZnO(H) is more easily removed (i.e., more reducible) by hydrogen than that of ZnO(C).
Morphology of Zinc Oxide Crystallites The surface of zinc oxide corresponds to the (0001), (0001), (101l), (1011), and (1010) cleavage planes of the wurtzite crystal structure. It is possible that the predominant planes constituting the surface of different varieties of zinc oxide may not be the same, which would lead to observed differences in chemisorptive properties of these oxides. Curnisio et al. (2) have drawn similar conclusions from auger electron spectroscopic studies. In the present investigation electron microscopy, Xray, and electron diffraction techniques have been used to find the types of surface planes predominating in different varieties of zinc oxide. Figs. 4a-f depict the morphology of ZnO particles obtained by various routes and reduction treatments. In Fig. 4a corresponding to ZnO(C) the particles are the smallest, the average particle measuring about 0.05/z across. The photograph shows random orientation with all types of faces equally formed. The high Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
E T AL.
density of dislocation lines suggest highly strained crystals. Fig. 5a, which is the electron diffraction pattern of a selected area of ZnO(C), confirms the random orientation of the crystallites since the rings are nearly continuous. Fig. 4b, representing the ZnO(X) sample, demonstrates that the crystallites have their basal plane mostly parallel to the substrate. The projected shape of most of the crystallites is hexagonal. Figs. 5b and 5c are electron diffraction patterns taken from two different regions of the sample ZnO(X). In Fig. 5b the spots like (1120) and (1013) are distributed in hexagonal array which indicates that the c axis was parallel to the beam. In Fig. 5c, a large single crystal diffraction pattern is illustrated, which corroborates that the major surfaces of crystallites exposed in this sample consists of basal planes. The micrograph of ZnO(H) is presented in Fig. 4c and the corresponding diffraction pattern in Fig. 5d. Apart from one lath-shaped particle, most of the crystallites are near hexagonal in outline, although they have angular terminations. The diffraction pattern suggests a larger degree of orientation variation since the rings are more densely populated than in ZnO(X). The prominent spots like (1120), (1013), and (1122) all can be traced to be comers of regular hexagons which means that the most predominant orientation is with c axis parallel to the beam. Among other orientations possible [011 ] zone can be distinguished. Fig. 4d is the micrograph of the ZnO(P) sample and Figs. 5e-h show the diffraction patterns of this sample taken from various regions of this sample. The electron micrograph shows clearly both the basal planes and the prismatic planes. Fig. 5e shows all prominent reflections like (10i2) arranged in hexagonal array indicating the [hko] zones. Fig. 5f shows a thick crystallite in [111] orientation. Fig. 5g shows a texture pattern with fiber axis parallel to (010) direction. Fig. 5h shows a superposition of diffraction patterns from two crystallites, both with the [001] parallel to the beam. Thus, in general the crystallites in ZnO(P) are found to lie with their (0001) planes parallel to the substrate.
MORPHOLOGY OF ZINC OXIDE
C
305
0.5#
FIG. 4. Electron micrographs of different types of zinc oxide. (a) ZnO(C), (b) ZnO(X), (c) ZnO(H), (d) ZnO(P), (e) reduced ZnO(H), (f) reduced ZnO(P).
Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
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FIG.5. Electrondiffractionpatterns of differenttypesof zinc oxide.(a) ZnO(C), (b) ZnO(H), (c) ZnO(X), (d-h) ZnO(P). Figs. 4e and 4f show the micrographs of ZnO(H) and ZnO(P) after reduction at 673 K by H2. The micrographs reveal that the defects have been annealed out after reduction in sample ZnO(H) while in ZnO(P) prismatic surfaces have disappeared leaving crystallites bounded mostly by basal planes. X-ray diffraction data provides further information about differences in surface structure of the various zinc oxide samples. The crystallite sizes of the oxides were determined in directions perpendicular to different crystallographic planes using Scherrer's formula. The Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
data are presented in Table I, from which it can be seen that all oxides exhibit anisotropy in crystallite size but in varying degrees. In the case of ZnO(C), the anisotropy is small which means that all the planes are more or less equally developed.
NIR Spectra of Adsorbed Water The diffuse reflectance spectra of the oxide samples are presented in Figs. 6a-d. The common feature of all the spectra is the broad adsorption band at about 1620 nm which is
MORPHOLOGY OF ZINC OXIDE
307
FIG. 5 - - C o n t i n u e d .
TABLE I Crystallite Size (t) of Zinc Oxide from Different Sources Measured in Direction Perpendicular to DifferentCrystal Planes
10i0 0002 1011 10i2 1120 1013 1122 2021
ZnO(C) (t in A)
ZnO(H) (t in/~)
ZnO(P) (t in A)
ZnO(X) (t in A)
510 480 460 500 660 470 640 770
1320 950 1120 1740 1390 3730 1920 1930
1320 1330 1340 3480 2410 3730 3840 1930
940 950 840 1740 1200 1490 1280 1290
probably due to electron transition from valence band to zinc vacancy. According to Kroger (8), the position of the singly charged zinc vacancy is 0.8 eV above the valence band of zinc oxide. Such an assumption is further supported by the observation that this absorption band is practically absent in zinc oxide cured at 623 K (Fig. 6a) as curing at lower temperature does not favor the formation of zinc vacancies. In the case of samples ZnO(P) and ZnO(X), one more adsorption band is observed at 1480-1470 nm, which disappears upon heating in dry N2 + 02 mixture at 773 K (Fig. 6f) indicating that this band arises due Journal of Colloid and Interface Science, Vol. 117, No. 2,.June 1987
308
SENGUPTA
Lid
z ,<
e
re
m <
i
i
1400
i
i
1600
i
t
1800
ET
AL.
stretching vibration and no deformation vibration (11) since the latter is a characteristic of molecularly adsorbed water. Consequently, the observed combination band (Fig. 6) of 1470 to 1480-nm must be due to molecularly adsorbed water. To account for the surface sensitivity of this adsorbed water, it is assumed that molecular adsorption of water is related to the chemisorbed OH group. According to Von Mattmann et al. (12), below a relative humidity of 0.2% at 298 K, two hydrogen bridges connect one physisorbed water molecule with two chemisorbed surface OH groups in the manner shown below.
WAVE LENGTH (nm)
FiG.6. NIR reflectancespectraofzincoxide.(a) ZnO(C), (b) ZnO(H), (c) ZnO(X),(d) ZnO(P),(e) ZnO(C)curedat 623 K, (f) ZnO(P) heatedin dry N2 + 02.
to adsorbed species. According to Herzberg (9), this band can be assigned to the (2vl + v2) combination of molecular water, where Vl and v2 are the frequencies of stretching and deformation vibration, respectively. In the case of porous glass, the 1460-nm band has been assigned to capillary condensation of water vapor (10). But in the present investigation, simple physical adsorption of water cannot be taken as the cause of the 1470 to 1480-nm band because in that case it would be a common feature of the adsorption spectra of all four types of zinc oxide. The fact that it is observed only in the cases of ZnO(P) and ZnO(X), but is nearly absent in ZnO(C), indicates that the adsorption phenomenon related to the 1470 to 1480-nm band is dependent on the nature of the adsorbent surface, and hence it is related to a chemisorption process. Atherton et al. (11) and Von Mattmann et al. (12) have studied the adsorption of water vapor on different types of zinc oxide by IR spectroscopy and have shown that the surface of the samples is covered by approximately a monoatomic layer of hydroxide species. The chemisorbed OH species exhibits only Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
/ lOx-.
HI
"H
f
[
0
O
"/////////////////L
~1////////////////I
This physisorbed water can exhibit both stretching and deformation vibration and will give rise to the 1470 to 1480-nm band. It is obvious that this type of hydrogen-bonded adsorption of water molecules can take place only if there is no H-bond formation among the surface OH groups. According to Atherton et al. (11), both the hydroxylated (0001) and (0001) faces of zinc oxide have an outermost layer of cation, each of which carries a single OH group. The latter are coplanar and each OH group has six equidistant nearest neighbors at 3.25 A. The combination of this distance and the coplanarity suggests that the hydroxyl groups will not be perturbed by hydrogen-bonding interactions. In the case of a hydroxylated (1010) plane each cation is attached to two hydroxyls and since surface zinc atoms of a particular row are coplanar with the surface oxygen atoms of the adjacent row, hydrogen bond formation between adjacent OH pairs is favored; similar situations also arise in the cases of the (10il) and (1011) planes. The same type of conclusions have been drawn by Von Mattmann et al. (12).
309
MORPHOLOGY OF ZINC OXIDE CONCLUSION The electron microscopy investigations have shown that the exposed surfaces of ZnO(P) and ZnO(X) are made up of predominantly basal planes and that of ZnO(C) consists of all kinds of planes. The observed physisorption of hydrogen-bonded water molecules in ZnO(P) and ZnO(X) is most likely to occur on the basal planes, which constitute most of the surface in these samples as confirmed by electron microscopy. The weak shoulder of the 1480-nm band in the N I R spectrum of ZnO(H) is expected because only a part of the surface consists of basal planes in this case. F r o m a study of the effect of water vapor adsorption at 523 and 348 K on the electrical conductivity of H2-treated zinc oxide, it has been concluded that ZnO(P) and ZnO(X) are more reduced than ZnO(C). McVicker et aL (13) and Hirschwald and Stolze (14) have shown that the rate of the evaporation of oxygen due to the dissociative process ZnO(s) ~ Zn(g) + O(g) from the (0001) face is three times higher than that from the (0003) face, whereas the rate from the (1030) face is of the same magnitude as that from the (000i) face (15). Since the rate of evaporation is a measure of the ease of oxygen removal from the zinc oxide surface, it can be assumed that the greater reducibility of ZnO(P) and ZnO(X) can be due to predominance of the (0001) face in these samples. Since in reducing environment the surface of ZnO(C) will be less reduced than ZnO(P), ZnO(X), or ZnO(H), it will have better selectivity for methanol formation from carbon
monoxide and hydrogen as observed by Natta (1). ACKNOWLEDGMENTS The authors expresstheir gratitude to Mr. N. C. Ganguli and Dr. S. K. Ghosh for their constant encouragement and valuable suggestions. REFERENCES 1. Natta, G., "Catalysis" (D. H. Emett, Ed.), Vol. 3, p. 349, Reinhold, New York, 1955. 2. Curnisio,G., Garbassi, F., Petrini, G., and Parravano, G., J. Catalysis 54, 66 (1978). 3. Voroshilov,I. G., Reev, L. M., Kozub, G. M., Lunev, N. K., and Rusov, M. T., TezisyDokl. Vses. Semin "Primen Opt. Spektrosk Adsorbtsil Katal" 17 (1974) (CA 85, 52260 j). 4. Sengupta,G., Ahluwalia, H. S., and Sen, S. P., Proc. Indian NatL Sci. Acad. 41(1), 102 (1975). 5. Sengupta,G., Ahluwalia, H. S., and Sen, S. P., J. Catalysis36, 111 (1975). 6. Sengupta,G., Ahluwalia, H. S., Banerjee, S., and Sen, S. P., J. Colloid Interface Sci. 69, 217 (1979). 7. Herman, R. G., Klier, K., Simmons, G. W., Finn, B. P., Balko, J. B., and Kobylinski, T. P., J. Catalysis 56, 407 (1979). 8. Kroger,F. A., "The Chemistryof ImperfectCrystals," p. 691, North-Holland, Amsterdam, 1974. 9. Herzberg, G., "Molecular Spectra and Molecular Structure," p. 28, D. Van NorstrandCo., Inc., New Jersey, 1959. 10. Little, L. H., "Infrared Spectra of Adsorbed Species," p. 215, Academic Press, New York, 1966. 11. Atherton, K., Newbold, G., and Hockey, J. A., Disc. FaradSoc. 52, 33 (1971). 12. Von Mattmann, G., Oswald, H. R., and Schweizer, F., Helv. Chim. Acta 55(4), 1249 (1972). 13. McVicker,J. E., Rapp, R. A., and Hirth, J. P., J. Chem. Phys. 57, 1381 (1972). 14. Hirschwald,W., and Stolze, F., Z. Phys. Chem. N.F. 77, 21 (1972). 15. Leonard, R. B., and Searcy, A. W., J. Appl. Phys. 42(10), 4047 (1971).
Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
Received August 25, 1983; accepted April 8, 1986 Samples of zinc oxide prepared from different sources do not show the same type of selectivity for hydrogenation of carbon monoxide due to creation of various types of active sites on the surface. It is reasonable to expect that there may be some correlation between the nature of surface defects or active sites and morphologyof zinc oxide crystallites. In the present investigation,the surfacedefectshave been characterized by electrical conductivitymeasurement and the morphologyhas been studied by electron microscopy, electron diffraction, X-ray diffraction, and NIR spectroscopy. It has been concluded that the surface of zinc oxide prepared from carbonate consists of all types of planes, whereas basal planes predominate in zinc oxide prepared from oxalate and pigment. Becauseof this, the surfaceof zinc oxide from carbonate is less reduced than the other two varieties of zinc oxide in the presence of hydrogen under conditionssimilarto carbonmonoxidehydrogenationreaction. This leadsto variationsin selectivity. © 1987 Academic Press, Inc.
INTRODUCTION Zinc oxide is well known as the catalyst for the synthesis of methanol from carbon monoxide and hydrogen. Although it was shown by Natta (1) that not all types of zinc oxide have the same synthesis activity and that prepared from smithsonite gives a better yield of methanol, no satisfactory explanation could be given for the observed behavior at that time. Curnisio et at. (2) have shown that selectivity for methanol formation from carbon monoxide and hydrogen depends on the adsorption mode of carbon monoxide. On zinc oxide surface, the C-O bond is stabilized through participation of several surface oxygen atoms in the adsorption process. This results in hydrogenation of carbon monoxide to methanol on an "oxidized" surface. On a metallic or "reduced" surface, hydrogenation to methane is more favorable due to weakening of the C-O bond. Similar conclusions have been drawn by Voroshilov et al. (3) from quantum chemi To whom correspondenceshould be addressed.
ical calculations, IR spectroscopy, and electronic work function study. Since methanol synthesis takes place under highly reducing conditions, it is worthwhile to study the nature of defect sites on the surface of different types of zinc oxide subjected to the reducing pretreatment. This may help to understand the reasons for the variations in selectivity exhibited by zinc oxide obtained from different sources. In the present investigation, the surface of zinc oxide has been characterized by water vapor adsorption and electrical conductivity studies to identify the type of defects. At the same time the structure and morphology of zinc oxide have been studied by electron microscopy, X-ray diffraction, and reflectance spectroscopy to find out to what extent these parameters influence the nature of oxide surface. During methanol synthesis a small a m o u n t of carbon dioxide or water vapor mixed with the synthesis gas helps to produce a thermal quenching effect and also acts as a mild oxidizing agent (7). Moreover, synthesis of methane which always occurs in varying degrees
301 0021-9797/87 $3.00 Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
Copyright © 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.
302
SENGUPTA ET AL.
parallel to methanol synthesis produces water vapor. Hence, in the present investigation moist hydrogen was used instead of dry hydrogen to simulate the conditions of synthesis reaction as much as possible. EXPERIMENTAL
Four types of zinc oxide were used for the experiment. ZnO(P) was a pigment variety zinc oxide supplied by M/s. Hindusthan Pigment Limited and had a purity better than 99.9%. It was cured in air for 10 h at 773 K. ZnO(H), ZnO(C), and ZnO(X) were prepared by dissolving ZnO(P) in nitric acid, precipitating with ammonium hydroxide, carbonate, or oxalate, respectively, followed by curing in air at 773 K for 10 h. Hydrogen was purified by passing through reduced copper catalyst at 573 K. Oxygen was dried by passing through silica gel. Water was deaerated by repeated freezing and thawing in vacuum. The cell and the experimental setup for studying the effect of chemisorption of water vapor on electrical conductivity of zinc oxide have already been described in previous communications (4-6). Zinc oxide samples were taken in the form of tablets of 10 × 10 mm size and subjected to the following pretreatment: (a) evacuation of 673 K for 2 h; (b) heating in a flow of dry oxygen at 673 K for 2 h; (c) second evacuation at 673 K for 2 h; (d) reduction with moist hydrogen at 573 K for 12 h; (e) final evacuation at 673 K for 2 h. The oxide tablet was then cooled in vacuum to 523 K and the conductivity of the solid was measured. Water vapor was introduced into the chamber at a pressure of 5.33 × 103 Pa. The conductivity of the solid changed instantaneously due to adsorption of water vapor. The conductivity was measured at different intervals of time to follow the adsorption process. The solid was then cleaned by the pretreatment (a) to (e), cooled to 348 K, and then Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
the water vapor adsorption was studied as before. The X-ray diffractograms were taken with a Philips diffractometer using a scanning speed of 1/2 ° per min. CuK, radiation was used. Electron micrographs and diffractograms were taken with the help of Siemen Elmiskop 1A. Electron optical magnifications varied from 40,000 to 60,000× at 80 kV. The beam diameter used for electron diffraction studies was 10 it. A Cary 17D spectrophotometer with diffuse reflectance attachment was used to record the spectra of the zinc oxide samples in the region of 1200-1800 nm. The samples were mounted on an infrasil plate that was attached to the window of the integrating sphere. Spectra were recorded with samples stored in air for a long time. To identify the adsorption bands due to physically adsorbed water vapor, the sample ZnO(P) was treated at 773 K for 2 h with dry N2 + 02 mixture in a setup shown in Fig. 1. After treatment, the constrictions A
!
IH C
S
li FIG. 1. Setup for treatment of ZnO in dry N2 + 02 at 773 K.
303
MORPHOLOGY OF ZINC OXIDE
and B were sealed off and the sample was transferred to the holder H fitted with an infrasil window. The constriction at C was sealed off and the holder with the sample was attached to the spectrophotometer. All the spectra were recorded at r o o m temperature.
1.0~~_____..~
O.
~
RESULTS AND DISCUSSION
Water Vapor Chemisorption Results of conductivity measurements are presented as plots of a/tro vs time (Figs. 2 and 3) where tr0 is the conductivity of the oxide sample just before the adsorption of water vapor at the temperature of measurement. F r o m Fig. 2 it can be seen that upon chemisorption of water vapor at 523 K, ZnO(P) and ZnO(X) show a slight increase in conductivity. But adsorption of water vapor on ZnO(C) and ZnO(H) causes a continuous decrease in conductivity and in case of ZnO(C) it is preceded by a small increase in conductivity. In a previous communication (6) it has been shown that if the adsorption of water vapor on zinc oxide at 523 K occurs on Zn+O - defect pairs, then the water molecules act as electron acceptors. But in the case of Zn+O -2 defect sites acting as adsorption sites, the water molecules act as electron donors:
OI 0
.
.
20 40 TIME IN MINUTES
FIG. 3. Changes in electrical conductivity upon chemisorption of water vapor at 348 K on different types of zinc oxide. (~, ZnO(P); O, ZnO(X); (9, ZnO(H); ~, ZnO(C).
Zn+O - + H 2 0 ~
OH I Zn
OH Zn+O -2 -b H20--~ I Zn ÷
0.5-
o¸ 0
2'0
4'0
6'0
TIME IN MINUTES
FIG. 2. Changes in electrical conductivity upon chemisorption of water vapor at 523 K on different types of zinc oxide. (~, ZnO(P); e, ZnO(X); ®, ZnO(H); ~k ZnO(C).
H I OH I + e. O-
At 348 K a decrease in conductivity was observed for all the samples due to adsorption of water vapor (Fig. 3), with that in the case of ZnO(C) being least. It has been shown in a previous communication (5) that Zn+Zn ÷ ion pairs can also act as adsorption sites for water molecules and the adsorption is acceptor type OH Zn+Zn - + H 2 0 -'~ [ Zn +
1.o.
6"0
H ]
Z n +"
The unpaired electron of the O H radical combines with the free electron of the Zn ÷ site forming a strong covalent bond, and the trapping of the free electron will cause a decrease in conductivity. The chemisorption of the H radical takes place on the other Zn ÷ site of the ion pair as Zn+-H. Such adsorption without involving electron transfer is a weak adsorption and occurs below 373 K. But at these temperatures adsorption on Zn+O - is not possible since formation of O-H bond requires higher temperature (6). Therefore the observed decrease in conductivity due to water vapor adsorption at 348 and 523 K can be attributed Journal of Colloid and Interface Science. Vol.117,No. 2, June 1987
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to two different types of adsorption sites viz. Zn+Zn+ and Zn+O -, respectively. Hence, it can be concluded that after reduction with hydrogen at 573 K, the surfaces of ZnO(P) and ZnO(X) contain mostly Zn+O-2 sites, whereas those of ZnO(H) and ZnO(C) contain both Zn+O - and Zn+O -2 sites. Zn+Zn+ sites are present in all oxides but in varying degrees. From Figs. 2 and 3 it appears that ZnO(C) contains more Zn+O sites and less Zn+Zn+ sites than the other three oxides. Both ZnO(P) and ZnO(X), as well as ZnO(H), contain appreciable Zn+Zn + sites. The relative abundance of these sites can be taken as a measure of relative concentration of surface oxygen. For Zn+O- sites, the oxygen zinc ratio is 1, but for Zn+Zn+ and Zn+O -2 sites this ratio is O and 1/2, respectively. The Zn+O-2 sites can be depicted as Zn+O-ZZn+ to maintain charge neutrality. Therefore, it can be concluded that the surface oxygen of ZnO(P), ZnO(X), and ZnO(H) is more easily removed (i.e., more reducible) by hydrogen than that of ZnO(C).
Morphology of Zinc Oxide Crystallites The surface of zinc oxide corresponds to the (0001), (0001), (101l), (1011), and (1010) cleavage planes of the wurtzite crystal structure. It is possible that the predominant planes constituting the surface of different varieties of zinc oxide may not be the same, which would lead to observed differences in chemisorptive properties of these oxides. Curnisio et al. (2) have drawn similar conclusions from auger electron spectroscopic studies. In the present investigation electron microscopy, Xray, and electron diffraction techniques have been used to find the types of surface planes predominating in different varieties of zinc oxide. Figs. 4a-f depict the morphology of ZnO particles obtained by various routes and reduction treatments. In Fig. 4a corresponding to ZnO(C) the particles are the smallest, the average particle measuring about 0.05/z across. The photograph shows random orientation with all types of faces equally formed. The high Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
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density of dislocation lines suggest highly strained crystals. Fig. 5a, which is the electron diffraction pattern of a selected area of ZnO(C), confirms the random orientation of the crystallites since the rings are nearly continuous. Fig. 4b, representing the ZnO(X) sample, demonstrates that the crystallites have their basal plane mostly parallel to the substrate. The projected shape of most of the crystallites is hexagonal. Figs. 5b and 5c are electron diffraction patterns taken from two different regions of the sample ZnO(X). In Fig. 5b the spots like (1120) and (1013) are distributed in hexagonal array which indicates that the c axis was parallel to the beam. In Fig. 5c, a large single crystal diffraction pattern is illustrated, which corroborates that the major surfaces of crystallites exposed in this sample consists of basal planes. The micrograph of ZnO(H) is presented in Fig. 4c and the corresponding diffraction pattern in Fig. 5d. Apart from one lath-shaped particle, most of the crystallites are near hexagonal in outline, although they have angular terminations. The diffraction pattern suggests a larger degree of orientation variation since the rings are more densely populated than in ZnO(X). The prominent spots like (1120), (1013), and (1122) all can be traced to be comers of regular hexagons which means that the most predominant orientation is with c axis parallel to the beam. Among other orientations possible [011 ] zone can be distinguished. Fig. 4d is the micrograph of the ZnO(P) sample and Figs. 5e-h show the diffraction patterns of this sample taken from various regions of this sample. The electron micrograph shows clearly both the basal planes and the prismatic planes. Fig. 5e shows all prominent reflections like (10i2) arranged in hexagonal array indicating the [hko] zones. Fig. 5f shows a thick crystallite in [111] orientation. Fig. 5g shows a texture pattern with fiber axis parallel to (010) direction. Fig. 5h shows a superposition of diffraction patterns from two crystallites, both with the [001] parallel to the beam. Thus, in general the crystallites in ZnO(P) are found to lie with their (0001) planes parallel to the substrate.
MORPHOLOGY OF ZINC OXIDE
C
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FIG. 4. Electron micrographs of different types of zinc oxide. (a) ZnO(C), (b) ZnO(X), (c) ZnO(H), (d) ZnO(P), (e) reduced ZnO(H), (f) reduced ZnO(P).
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FIG.5. Electrondiffractionpatterns of differenttypesof zinc oxide.(a) ZnO(C), (b) ZnO(H), (c) ZnO(X), (d-h) ZnO(P). Figs. 4e and 4f show the micrographs of ZnO(H) and ZnO(P) after reduction at 673 K by H2. The micrographs reveal that the defects have been annealed out after reduction in sample ZnO(H) while in ZnO(P) prismatic surfaces have disappeared leaving crystallites bounded mostly by basal planes. X-ray diffraction data provides further information about differences in surface structure of the various zinc oxide samples. The crystallite sizes of the oxides were determined in directions perpendicular to different crystallographic planes using Scherrer's formula. The Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
data are presented in Table I, from which it can be seen that all oxides exhibit anisotropy in crystallite size but in varying degrees. In the case of ZnO(C), the anisotropy is small which means that all the planes are more or less equally developed.
NIR Spectra of Adsorbed Water The diffuse reflectance spectra of the oxide samples are presented in Figs. 6a-d. The common feature of all the spectra is the broad adsorption band at about 1620 nm which is
MORPHOLOGY OF ZINC OXIDE
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FIG. 5 - - C o n t i n u e d .
TABLE I Crystallite Size (t) of Zinc Oxide from Different Sources Measured in Direction Perpendicular to DifferentCrystal Planes
10i0 0002 1011 10i2 1120 1013 1122 2021
ZnO(C) (t in A)
ZnO(H) (t in/~)
ZnO(P) (t in A)
ZnO(X) (t in A)
510 480 460 500 660 470 640 770
1320 950 1120 1740 1390 3730 1920 1930
1320 1330 1340 3480 2410 3730 3840 1930
940 950 840 1740 1200 1490 1280 1290
probably due to electron transition from valence band to zinc vacancy. According to Kroger (8), the position of the singly charged zinc vacancy is 0.8 eV above the valence band of zinc oxide. Such an assumption is further supported by the observation that this absorption band is practically absent in zinc oxide cured at 623 K (Fig. 6a) as curing at lower temperature does not favor the formation of zinc vacancies. In the case of samples ZnO(P) and ZnO(X), one more adsorption band is observed at 1480-1470 nm, which disappears upon heating in dry N2 + 02 mixture at 773 K (Fig. 6f) indicating that this band arises due Journal of Colloid and Interface Science, Vol. 117, No. 2,.June 1987
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i
i
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stretching vibration and no deformation vibration (11) since the latter is a characteristic of molecularly adsorbed water. Consequently, the observed combination band (Fig. 6) of 1470 to 1480-nm must be due to molecularly adsorbed water. To account for the surface sensitivity of this adsorbed water, it is assumed that molecular adsorption of water is related to the chemisorbed OH group. According to Von Mattmann et al. (12), below a relative humidity of 0.2% at 298 K, two hydrogen bridges connect one physisorbed water molecule with two chemisorbed surface OH groups in the manner shown below.
WAVE LENGTH (nm)
FiG.6. NIR reflectancespectraofzincoxide.(a) ZnO(C), (b) ZnO(H), (c) ZnO(X),(d) ZnO(P),(e) ZnO(C)curedat 623 K, (f) ZnO(P) heatedin dry N2 + 02.
to adsorbed species. According to Herzberg (9), this band can be assigned to the (2vl + v2) combination of molecular water, where Vl and v2 are the frequencies of stretching and deformation vibration, respectively. In the case of porous glass, the 1460-nm band has been assigned to capillary condensation of water vapor (10). But in the present investigation, simple physical adsorption of water cannot be taken as the cause of the 1470 to 1480-nm band because in that case it would be a common feature of the adsorption spectra of all four types of zinc oxide. The fact that it is observed only in the cases of ZnO(P) and ZnO(X), but is nearly absent in ZnO(C), indicates that the adsorption phenomenon related to the 1470 to 1480-nm band is dependent on the nature of the adsorbent surface, and hence it is related to a chemisorption process. Atherton et al. (11) and Von Mattmann et al. (12) have studied the adsorption of water vapor on different types of zinc oxide by IR spectroscopy and have shown that the surface of the samples is covered by approximately a monoatomic layer of hydroxide species. The chemisorbed OH species exhibits only Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
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This physisorbed water can exhibit both stretching and deformation vibration and will give rise to the 1470 to 1480-nm band. It is obvious that this type of hydrogen-bonded adsorption of water molecules can take place only if there is no H-bond formation among the surface OH groups. According to Atherton et al. (11), both the hydroxylated (0001) and (0001) faces of zinc oxide have an outermost layer of cation, each of which carries a single OH group. The latter are coplanar and each OH group has six equidistant nearest neighbors at 3.25 A. The combination of this distance and the coplanarity suggests that the hydroxyl groups will not be perturbed by hydrogen-bonding interactions. In the case of a hydroxylated (1010) plane each cation is attached to two hydroxyls and since surface zinc atoms of a particular row are coplanar with the surface oxygen atoms of the adjacent row, hydrogen bond formation between adjacent OH pairs is favored; similar situations also arise in the cases of the (10il) and (1011) planes. The same type of conclusions have been drawn by Von Mattmann et al. (12).
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MORPHOLOGY OF ZINC OXIDE CONCLUSION The electron microscopy investigations have shown that the exposed surfaces of ZnO(P) and ZnO(X) are made up of predominantly basal planes and that of ZnO(C) consists of all kinds of planes. The observed physisorption of hydrogen-bonded water molecules in ZnO(P) and ZnO(X) is most likely to occur on the basal planes, which constitute most of the surface in these samples as confirmed by electron microscopy. The weak shoulder of the 1480-nm band in the N I R spectrum of ZnO(H) is expected because only a part of the surface consists of basal planes in this case. F r o m a study of the effect of water vapor adsorption at 523 and 348 K on the electrical conductivity of H2-treated zinc oxide, it has been concluded that ZnO(P) and ZnO(X) are more reduced than ZnO(C). McVicker et aL (13) and Hirschwald and Stolze (14) have shown that the rate of the evaporation of oxygen due to the dissociative process ZnO(s) ~ Zn(g) + O(g) from the (0001) face is three times higher than that from the (0003) face, whereas the rate from the (1030) face is of the same magnitude as that from the (000i) face (15). Since the rate of evaporation is a measure of the ease of oxygen removal from the zinc oxide surface, it can be assumed that the greater reducibility of ZnO(P) and ZnO(X) can be due to predominance of the (0001) face in these samples. Since in reducing environment the surface of ZnO(C) will be less reduced than ZnO(P), ZnO(X), or ZnO(H), it will have better selectivity for methanol formation from carbon
monoxide and hydrogen as observed by Natta (1). ACKNOWLEDGMENTS The authors expresstheir gratitude to Mr. N. C. Ganguli and Dr. S. K. Ghosh for their constant encouragement and valuable suggestions. REFERENCES 1. Natta, G., "Catalysis" (D. H. Emett, Ed.), Vol. 3, p. 349, Reinhold, New York, 1955. 2. Curnisio,G., Garbassi, F., Petrini, G., and Parravano, G., J. Catalysis 54, 66 (1978). 3. Voroshilov,I. G., Reev, L. M., Kozub, G. M., Lunev, N. K., and Rusov, M. T., TezisyDokl. Vses. Semin "Primen Opt. Spektrosk Adsorbtsil Katal" 17 (1974) (CA 85, 52260 j). 4. Sengupta,G., Ahluwalia, H. S., and Sen, S. P., Proc. Indian NatL Sci. Acad. 41(1), 102 (1975). 5. Sengupta,G., Ahluwalia, H. S., and Sen, S. P., J. Catalysis36, 111 (1975). 6. Sengupta,G., Ahluwalia, H. S., Banerjee, S., and Sen, S. P., J. Colloid Interface Sci. 69, 217 (1979). 7. Herman, R. G., Klier, K., Simmons, G. W., Finn, B. P., Balko, J. B., and Kobylinski, T. P., J. Catalysis 56, 407 (1979). 8. Kroger,F. A., "The Chemistryof ImperfectCrystals," p. 691, North-Holland, Amsterdam, 1974. 9. Herzberg, G., "Molecular Spectra and Molecular Structure," p. 28, D. Van NorstrandCo., Inc., New Jersey, 1959. 10. Little, L. H., "Infrared Spectra of Adsorbed Species," p. 215, Academic Press, New York, 1966. 11. Atherton, K., Newbold, G., and Hockey, J. A., Disc. FaradSoc. 52, 33 (1971). 12. Von Mattmann, G., Oswald, H. R., and Schweizer, F., Helv. Chim. Acta 55(4), 1249 (1972). 13. McVicker,J. E., Rapp, R. A., and Hirth, J. P., J. Chem. Phys. 57, 1381 (1972). 14. Hirschwald,W., and Stolze, F., Z. Phys. Chem. N.F. 77, 21 (1972). 15. Leonard, R. B., and Searcy, A. W., J. Appl. Phys. 42(10), 4047 (1971).
Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987