J . Chem. SOC., Faraday Trans. I, 1984,80, 225-235 Spectroscopic Studies of Hydrogen Adsorbed on Zinc Oxide (Kadox 25) BY JOSEPH HOWARD* AND IAN J. BRAID Chemistry Department, University of Durham, South Road, Durham DH 1 3LE AND JOHN TOMKINSON Neutron Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQX Received 13th June, 1983 The vibrational spectra (320-2230 cm-l) of ZnO and of hydrogen adsorbed on ZnO have been obtained by incoherent inelastic neutron-scattering spectroscopy (INS). The v, and v6 modes of type I Zn-H surface species are observed at 829 and 1708 cm-l, in accord with published i.r. data. The recent assignments by Boccuzzi et a f . of the v6 mode of type I 0-H species aIld the v,, mode of type I1 species to weak i.r. bands at ca.850 and 1475 cm-l, respectively, are not corroborated by the INS results. No resolved bands assignable to type I1 species are observed. The INS spectrum of ZnO contained a previously unreported, intense feature at 1346 cm-l which was unchanged on hydrogen adsorption. This feature is assigned to impurities within the bulk. Raman and i.r. data of ZnO are also discussed. H / \ Zn Zn There have been numerous studies of zinc oxide and of hydrogen adsorbed on it.l Although these systems are complex, four main types of hydrogen adsorption processes have been identified.l Type I adsorption is rapid and reversible at room temperature and gives two i.r.-active species, ascribedl to surface Zn-H and 0-H species. The stretching modes of these species are generally accepted to occur at ca.1710 and ca. 3490 cm-l, respective1y.l Recently, the bending modes were assigned by Boccuzzi et aL2 in the i.r. spectrum of hydrogen adsorbed by ZnO to a band of moderate intensity at 817 cm-l [vg(Zn-H)] and to a broad, weak feature at ca. 845-850 cm-l [va(O-H)]. Type I1 adsorption is irreversible at room temperature and was first reported by Boccuzzi et aL2 to give weak, broad i.r. bands at 1475 and ca. 3400 cm-l, assigned to / \ and 0-I+---0 bridged species, respectively. H Zn Zn Type 111 adsorption occurs at low temperatures (77 K), giving rise to i.r.-active species, and is reversible and non-dissociative. The internal modes and vibrations relative to the ZnO surface of type I11 hydrogen occurs outside the frequency range of our incoherent inelastic neutron-scattering (INS) experiment^.^ Type I V adsorption occurs principally at high temperatures but also, to a lesser extent, at room temperature.The purpose of the present work, which was motivated by statistical thermodynamic studies of the same system, was to observe and assign the normal modes of the types I and I1 species using a combination of i.r., Raman and I N S spectroscopies. 225226 SPECTROSCOPIC STUDY OF H, ON ZnO EXPERIMENTAL The INS measurements (320-2230 cm-l) were made at 77 K using the INlB spectrometer at the Institut Laue Langevin in Grenoble in its beryllium filter detector mode.4 Data were normalised to constant monitor counts and it has been shown that the resultant spectrum is proportional to the amplitude-weighted density of states multiplied by the Debye-Waller f a ~ t o r .~ The transition frequencies have been calculated from the positions of the band maxima using standard correction factors.6 For all of our measurements the ZnO used was Kadox 25 (obtained from the New Jersey Zinc Co.), which is produced by burning zinc in air. For the INS experiment the ZnO was calcined, in a silica tube, by heating to 723 K in a static, but periodically refreshed, dry-oxygen atmosphere. The sample was then cooled to room temperature under oxygen and subsequently evacuated to 7 x Torr (1 Torr = 133.3 N m-,). The ZnO was then transferred, under vacuum, to a thin-walled cylindrical aluminium cell (3.8 cm diameter) via a glass-metal seal and the cell sealed off from the vacuum line. After the INS spectrum of the ZnO had been obtained hydrogen was admitted to the cell via a glass break-seal and the INS spectrum of the ZnO + H, obtained.Hydrogen (99.9995%, Masonlite, Chatham) was adsorbed by exposing the sample to 390 Torr of H, for 10 min followed by cooling of the sample to 77 K. This method is expected to produce7-'0 types I, I1 and 111 surface hydrogen with concentrations in the order type 111 > I1 > I. However, type 111 is thought to be molecular3 and so will probably not contribute bands to the spectral region measured in the INS experiments. The volume adsorbed was 85 cm3 and this quantity is calculated to scatter 1.4% of the incident neutron beam. The ratio of the incoherent cross-sections of the adsorbed H to that of the ZnO is 8.8 to 1.0 (at an incident neutron energy of 204 cm-l).Before pretreatment the catalyst had a surface area of 10 m2 g-' (N,, B.E.T.) and afterwards a surface area of 7.5 m2 g-l. This is in agreement with the results of Baranski and Cvetanovic." Using these values the uptake of hydrogen in our INS experiment corresponds to minimum surface coverage, 8, of 0.7 and maximum of 0.8 monolayers. We have obtained the i.r. spectra (4000-750 cm-l) of Kadox 25 during and after a calcination procedure similar to that used to prepare the INS samples. In the i.r. experiment, the sample was heated in 0, (10-20 Torr) to a maximum of 725 K. At various temperatures the sample was evacuated to ca. 5 x lop5 Torr and the i.r. spectrum then recorded, followed by re-admission of 0, to the ZnO and further heating.The i.r. spectrum was also recorded at 305 K after cooling in U ~ C U O at the end of the calcination procedure. 1.r. data were obtained using a Perkin-Elmer 580B spectrophotometer with data station. The sample was in the form of a self-supporting disc and the heating, adsorption and evacuation operations were carried out in situ with the sample in an all-metal infrared cell equipped with KRS5 windows. Because, for the self-supporting disc, the sample was totally absorbing below 700 cm-l, a spectrum of ZnO diluted with KBr was also obtained so that the region 700-300 cm-' could be studied. The Raman data were obtained using a Cary 82 spectrophotometer and the 514.5 mm line of an argon laser. No pretreatment of the sample was carried out. RESULTS AND DISCUSSION The space group of ZnO (wurtzite) is C,, and there are 4 atoms per unit cell.There are therefore 3 acoustic and 9 optic modes12 and the optic modes are classified as: 1 x A , mode 1 x E, mode 2 x E, mode 2 x B, mode inactive i.r. + R active i.r. + R active R active onlyJ. HOWARD, I. J. BRAID AND J. TOMKINSON 227 However, according to the Lyddane-Sachs-Teller (LST) theory,13 splitting of the formally i.r.-active optic phonons leads to transverse (t.0.) and longitudinal (1.0.) branches of which the t.0. branch generally occurs to lower wavenumbers. These branches retain the same i.r./R activity as the parent phonon mode with one exception: all the 1.0. branches are i.r. inactive regardless of their symmetry. RAMAN SPECTRUM OF ZnO The data and assignments from our Raman study of ZnO are summarised in table 1 together with the results of previously published work on similar l5 Our data on a polycrystalline sample are closer to those obtained from single-crystal studies by Damen et aI.l4 than to those obtained by Mitra and Bryant.15 In particular, our spectrum does contain a band at 100 cm-l and so provides some support for the assignment by Damen et al.of a band at 101 cm-l to an E, optic mode. Mitra and Bryant assigned a band at 180 cm-1 to this mode but we, like Damen et al., did not observe a transition at this value. Table 1. Frequencies (cm-l) and assignments of bands in the vibrational spectrum of ZnO this work uncalcined ZnO, room temperature calcined ZnO, Raman15 Raman14 77 K assignment assignment Raman INS assignment - _ 180 t.0.(E,) _ _ - - 373 t.0. (E,) 420 t.0. ( A , ) 438 t.0. (El) 538 1.0. 588 1.0. - - 101 E, 208 mp 334 mp 380 t.0. ( A , ) 407 t.0. ( E l ) 437 E, 574 1.0. ( A , ) 583 1.0. ( E l ) 986 mp 1084 b mp 1149 b mp - - 100 vs 204, 21 1 vw - - - - - 335 m 383 w 4 1 4 ~ , sh - 440 vs 437 vs 543 w, b 553 vs 588 w - 635 sh 758, 765 w, b 749 m 877 m 1007 m - - - - - - - - - 1072 sh, w 1105 sh - 1150 s, b - - 1346 vs, b t.0. phonon mP mP t.0. phonon t.0. phonon 1.0. phonon ( A , ) 1.0. phonon (El) mp (1 00 + 543 = 643 cm-l) mp(2 x 383 = 766 cm-l) mp (2 x 440 = 880 cm-') mp (440 + 588 = 1028 cm-l) mp (2 x 543 = 1086 cm-l) mp(543+588 = 1121 ern-') mp (2 x 588 = 1 176 cm-l) ? - - - - l.o., longitudinal optical phonon mode; t.o., transverse optical phonon mode; mp, multi- phonon mode; b, broad; m, medium; s, strong; sh, shoulder; v, very; w, weak.INFRARED SPECTRA 750-300 cm-l Our i.r. spectrum (750-300 cm-l) of unpretreated ZnO (KBr disc) showed strong absorptions at 440,490 and 525 cm-l which, in accord with other authors,16 we assign to surface optic phonons. The i.r. band at 440 cm-l is not assigned to the E, bulk228 SPECTROSCOPIC STUDY OF H, ON ZnO phonon which was also observed at 440 cm-l in our Raman data. Our reasons are: (i) group-theory arguments predict that the E, mode is Raman active but i.r. inactive and (ii) particle-size arguments13- l6 predict that only surface phonons will be observed in the i.r. spectra of our sample. Measurements using a Coulter counter indicate that 90% of the particles had a diameter < 0.5 pm.The original assignment of the E, mode was made from a polarised Raman study of a single crystal14 in which the polarisation characteristics of a Raman band at 437 cm-l lead to its unambiguous assignment to the E, bulk phonon. Therefore, the Raman band at 440 cm-l and the i.r. maximum at 440 cm-l have different physical origins. 4000-750 cm-l Before starting the calcination treatment, but after mild evacuation to 4.5 x lop2 Torr, the ZnO sample at 299 K showed three sharp i.r. bands in the 3000-2800 cm-l region, at 2960,2930 and 2875 cm-l. Calcination completely removed these bands, which have not been repolded elsewhere for ZnO. The frequencies are typical of the C-H symmetric stretching mode of aliphatic hydrocarbons. We therefore assign the bands to the v,(C-H) mode of surface contaminants on the ZnO. Some support for our assignment is provided by the i.r.spectrum of SiO, (Cabosil) obtained by McDonald." An untreated SiO, sample showed bands at 2960, 2959 and 2875 cm-l which disappeared after heating for several minutes at 773 K in 0,. These bands were also assigned to the C-H stretch of surface impurities. With the exception of the 3000-2800 cm-l region, the i.r. spectra (4000-750 cm-I) of ZnO obtained during the calcination treatment to 725 K in 0, were in accord with the results of other w ~ r k e r s . l ~ - ~ ~ After cooling the calcined sample in vacuo to 305 K the i.r. spectrum showed the presence of only hydroxyl and carbonate groups on the calcined ZnO surface. Such species are therefore the only likely contaminants on the ZnO samples used in our INS experiments, these samples having been prepared using a similar calcination technique. INS SPECTRUM OF ZnO The INS spectrum obtained at 77 K on IN 1 B of Kadox 25 after calcination is shown in fig.1 and the results are summarised in table 1. Two very intense bands are observed at 437 and 553 cm-l. By comparison with our Raman data (table 1) these bands are assigned to t.0. and 1.0. phonon modes, respectively. Although, in the INS technique both bulk and surface phonons will be active, the measured spectra of our sample will be dominated by bulk modes because of the large bu1k:surface ratio. The bands at 635, 749, 877, 1007 and 1072 cm-l in the INS spectrum (fig. 1) are tentatively assigned to multiphonon processes (table 1).In addition to these features the INS spectrum of ZnO (fig. 1) shows an intense, broad band at 1346 cm-l. No i.r. or Raman bands have been reported for ZnO in this region. On adsorption of H, the scattered-neutron intensity due to this transition remains constant whereas the intensity increases at all other frequencies (see fig. 1). This suggests that the 1346 cm-l band is not due to phonon processes. Also, the highest value at which multiphonon bands have been observed in the Raman data is 1155 cm-l. It is unlikely that the 1346 cm-l band is due to surface carbonate because the cross-sections of carbon and oxygen and their likelyz3 concentration (ca. 0.05 0) are too low to explain a band of such high intensity. Moreover, surface hydroxyls are not reported to give rise to bands near 1346 cm-l nor are they likely to be present in high enough concentration to explain the observed intensity.In summary, we are unable to make a definite assignment of the band at 1346 cm-'.J. HOWARD, I. J. BRAID AND J. TOMKINSON 229 I I I I 500 1000 1500 2000 incident neutron energylcm-' Fig. 1. INS spectra at 77 K of (a) ZnO after calcination and (b) ZnO after adsorption of H,. + and x indicate data collected using the (200) plane and 0 and 0 those collected using the (220) plane of the copper monochromator. However, because its intensity is unchanged on H, adsorption, it is probably related to transitions within the bulk rather than at the surface of the ZnO particles. Since incorporation of impurities within the lattice during manufacture is conceivable there is a chance that the 1346 cm-l INS band is due to contaminants, e.g.carbonate or water, trapped within the bulk of the sample. INS SPECTRUM OF ZnO + H, The spectrum of ZnO+H,, 68 pmol g-l coverage, obtained on IN1B at 77 K is shown without any background subtraction in fig. 1 . The result of subtracting the ZnO background is shown in fig. 2. Table 2 lists the frequencies of bands observed after the subtraction and includes the i.r. data of Boccuzzi et aL2 for comparison. The two optical phonon modes which arose at 437 and 553cm-l in the ZnO (fig. 1) now appear at 458 and 538 cm-l on adsorption of H, (fig. 2) and the lower- frequency, t.0. band apparently gains in intensity relative to the higher-frequency, 1.0.band. However, a change in the sloping background is the most likely cause of the change in relative intensities of these two phonon modes upon hydrogen adsorption. Modification of the bonding at the ZnO surface on hydrogen adsorption may be the cause of the shift in frequency of the two phonon modes. The frequency shift is notSPECTROSCOPIC STUDY OF H, ON ZnO 400 600 800 1000 1200 1400 1600 1800 2000 2200 incident neutron energy/cm-' Fig. 2. Difference spectrum : INS spectrum of ZnO + H, minus INS spectrum of ZnO. Symbols as for fig. 1 . due to plasmon effects since the adsorbed hydrogen does not form an accumulation layer2, at temperatures as low as 77 K. A shoulder at 584 cm-l in the ZnO + H, INS spectrum (fig. 2) may arise from either the bulk phonon mode at 588 cm-l (table I ) or from the bending mode of surface H /' \ species.The assignment to the phonon mode is preferred for the reasons Zn Zn given later. vg(Zn-H) The intense band at 829 cm-I in the subtracted spectrum (fig. 2 ) is not present in the ZnO data. This frequency is typical of metal-hydrogen bending modes, e.g. vg for terminally bound H in transition-metal hydridocarbonyls lies in the region 600-800cm-l, and the bending modes in GeH,, SnH, and GeH, (matrix isolated) occur at 931, 758 and 928 cm-l, re~pectively.~~ We assign the 829 cm-l band to v6 of surface Zn-H at a type I site, in agreement with previous work.2 INS SPECTRUM OF TYPE I SPECIES v, (Zn-H) The subtracted spectrum (fig. 2) shows a broad, strong band at ca. 1665 cm-l which is also absent from the ZnO spectrum (fig. 1).This band may be assigned immediately to the symmetric stretch of Zn-H at type I sites. It has been reported, at 1708 cm-' in the i.r. spectrum (table 2). The discrepancy between the frequencies observed by INS and i.r. spectra is explained by the temperature and coverage dependence of the frequency of this mode and the difficulty of locating the exact centre of the broad INS transition. The INS spectrum of ZnO+H, with background subtracted (fig. 2) shows unresolved bands in the region between the 584 cm-l shoulder and the 829 cm-l band. This intensity we assign to scattering from multiphonon bands (observed at 635 and 749 cm-l in the INS spectrum of ZnO).Table 2. Frequencies and assignments of bands in the vibrational spectrum of ZnO plus adsorbed hydrogen (cm-l) assignment (Boccuzzi et ~ 1 .~ ) INS i.r. Boccuzzi2 type of assignment (this work) mode type of H adsorption INS ZnO ZnO + H,-background ZnO + H, mode adsorption t.0. phonon 1.0. phonon 1.0. phonon mP mP v,(Zn-H) mP mP mP - - vs(0-H) ? - v, (Zn-H) - __ 437 vs 553 vs - 635 sh I - - - - 749 - - - - - 877 m - 1007 m - 1027 sh, w - - - I 1346 vs, b - 458 vs 538 vs 584 sh, m - - 829 s - - - - - 1125 sh (1346 vs, b)" - - - - - 817 m 845 to 850 b, w 870 s 990 s - ca. 1100 sh, m - 1475 vb 1708 s ca. 3400 b 3498 a The band at 1346 cm-l occurs with equal intensity in the background spectrum of ZnO and in the spectrum of ZnO + H, before subtraction of the background. A decrease in intensity is therefore observed at 1346 cm-l in the spectrum of ZnO + H, -background.232 SPECTROSCOPIC STUDY OF H, ON ZnO The INS spectrum in the region 829 to 1125 cm-l (fig.2) shows the high-frequency edge of a band with a maximum at ca. 1125 cm-l and some unresolved intensity in the region down to ca. 850 cm-l. There are three possible (not necessarily mutually exclusive) origins of this intensity. (i) There is the possibility of a symmetric stretching mode of a / \ surface species, formed by type I1 adsorption, occurring in this region. This is discussed below. (ii) Multiphonon bands were observed in this region in the INS spectrum of the ZnO background (fig. 1, table 2). Thus the first overtones of the intense t.0. (458 cm-l) and 1.0. (538 cm-l) bulk phonons in the INS spectrum of ZnO + H (fig.2) are therefore expected at ca. 916 and 1076 cm-l. (iii) v6 of type I (0-H) was assigned2 to a broad i.r. band at ca. 850 cm-l (table 2). Although no band was resolved in our INS spectrum of ZnO+H2 at 850 cm-l (fig. 2), this mode should be INS active. Thus we have calculated the predicted intensities of an INS band due to vs(O-H) in the region 850 to 1125 cm-l relative to v,(Zn-H). In general, the vg modes of metal-OH bonds in organometallic and inorganic compounds are found between 700 and 1200 cm-l in the Now since type I adsorption is said to occur at Zn-0 pair sites, the surface concentration of type I Zn-H and 0-H species will be equal. vg(Zn-H) was assigned above to an intense band at 829 cm-l (fig. 2) and as a first approximation we might expect to observe the vd(O-H) mode with similar intensity.There are two possible assignments of a band to vs(O-H) in fig. 2; the first at 1125 cm-l (shoulder) and the second at ca. 850 cm-l [this being unresolved from vg(Zn-H) at 829cm-lI. We recall that the vs(O-H) band in the i.r. spectrum was broad.2 Because type I Zn-H and 0-H have equal concentrations, the intensities ( I n ) in the INS spectra of the fundamental modes of these species can be estimated (in the harmonic approximation) from:26 H Zn Zn where fie is the momentum transfer during the scattering process, Z(C,) is the vibrational density of states, Fn is the normal mode frequency in cm-l and exp [ - 2 W, (PA)] is the mode-dependent anisotropic Debye-Waller factor. The relative INS intensities predicted for the v, and vg fundamentals of type I Zn-H species are compared in table 3 with those of the proposed vs(O-H) fundamentals at 850 and 1125 cm-l.In the calculations we have taken 3498 cm-l as the frequency of the v,(O-H) mode.27 We need also to consider the intensity in the region 1500-2300 cm-l Table 3. Relative INS intensities predicted for the normal modes of type I hydrogen on ZnO type I surface species Zn-H 0-H 0-H Zn-H frequency/cm-' 1665 1125* 850* 829 relative Ih'S intensity 1 .o 1 .5 3.0 3.0 a The frequencies 1125 and 850 cm-I are taken as the two most likely possibilities for vs(,O--H) (see text). The large difference between the two predicted relative INS intensities for v,(O-H) is a consequence of the use of a mode-dependent Debye-Waller factor.J. HOWARD, I.J. BRAID AND J. TOMKINSON 233 from the first overtones of vs(Zn-H) and vs(O-H). Using published formulae26 we estimate the intensity of each overtone to be ca. that of its fundamental. When the overtone contribution is included, we find that the intensities of the observed bands (fig. 2) cannot be measured with sufficient accuracy to distinguish between the alternative assignments of vb(0-H). This is largely due to the uncertainty in the baseline position of fig. 2. However, because we expect to observe a strong INS band due to vs(O-H), we tentatively assign this mode to the shoulder at 1125 cm-l rather than to the region at ca. 850 cm-l where our INS data show no resolved features (fig. 2). Our assignment of vs(O-H) disagrees with that of Boccuzzi2 who ascribed a broad i.r.band at 845-850 cm-l to this mode. The published i.r. spectra, of hydrogen adsorbed on ZnO in the region ca. 845-775 cm-l show a general increase in i.r. absorbance with increasing hydrogen coverage. We submit that the feature at 850-845 cm-l in the published spectra, is not sufficiently well differentiated from the general decrease in i.r. transmission to be definitely ascribed to a band arising from a vibrational mode of a particular surface species. The previous assignment, of the vs mode of type I (0-H) to the i.r. feature at 850-845 cm-l should therefore also be regarded as tentative. No bands appear in the i.r. spectra in the region of 1125 cm-l on adsorption of H,., (A band at ca. llOOcm-l in the ZnO spectrum of Boccuzzi et al.before H, adsorption was not assigned by the authors. From our i.r. resuits for calcined ZnO we suggest that this is a multiphonon band.) The discrepancy between the values for vs(O-H) (type I) as assigned by Boccuzzi et al., in their i.r. data and by us using INS cannot be resolved in the present work. We submit that both assignments must be regarded as tentative. An INS study of HD adsorbed on ZnO may help to clarify the situation, though it would be difficult with current neutron sources. INS SPECTRUM OF TYPE I1 SPECIES The INS data show no strong bands assignable to the symmetric or antisymmetric stretches of the species proposed by Boccuzzi et aL2 for adsorption at type I1 sites. A very broad band in the i.r. spectrum of ZnO+H, at 1475 cm-l was assigned23 to the asymmetric stretch. The symmetric stretch, although not observed in the i.r.data of Boccuzzi et al., is expected to occur in the region 895-1300 cm-l on the basis of hydridocarbonyl [@,-H) M,] data2* (M is a transition-metal atom). We cannot discount the possibility of a low concentration of a / \ species on our ZnO surface. It is possible that a weak asymmetric stretching band might be present at ca. 1475 cm-l in the INS spectrum of ZnO + H, (fig. 2), unresolved from the broad 1665 cm-l band. The INS spectrum also shows unresolved intensity in the /H\ Zn Zn H Zn Zn H range 829-1 125 cm-l which may include a contribution from a / \ symmetric Zn Zn stretch. There are two possible assignments for the shoulder at 584 cm-1 (fig. 2): (i) a bulk ). If we assume (a) that the 584 cm-l feature is / \ , expected in the range 490-630 cm-l from H / \ Zn Zn phonon mode and (ii) vg( H Zn Zn due to the bending mode of234 SPECTROSCOPIC STUDY OF H, ON ZnO hydridocarbonyl data,28 (6) that the symmetric stretch occurs within the range 829-1 125 cm-l and (c) that the antisymmetric stretch occurs at 1475 cm-l, then using the modified Katovic equation2g we calculate the / \ bond angle (a) to lie in the range 99-108'.Since type I1 adsorption is said to occur on the (1010) planes1 (Zn- - - -Zn distance30 of 3.25 A unaffected by surface reconstr~ction~l), this corresponds to a Zn-H distance of 2.G2.1 A. We consider this to be too long. Since the charge of the ions at the ZnO surface are less than those in the bulk,31 we expect the surface Zn-H bonds to be predominantly covalent. The predicted Zn-H bond length is longer than that usually found in covalent h y d r i d e ~ ~ ~ and is also greater than the sum of the covalent radii (1.57 A).34 This indicates that the assignment of the 584 cm-l band to the vs mode of &-H) Bn, is probably incorrect.The alternative assignment to a bulk phonon mode is therefore preferred. In view of the above discussion, and the fact that the 1475 cm-l i.r. band has not been reported by other workers, we submit that its assignment, and the bridge structure of type I1 hydrogen, proposed by Boccuzzi et aL2 should be regarded as tentative. H Zn Zn CONCLUSIONS The adsorption conditions used in the INS experiment are expected to give rise to type I, I1 and I11 hydrogen.The modes of type 111 hydrogen and the stretching modes of surface hydroxyls lie outside the wavenumber range (320-2230 cm-l) of our data. The bending and stretching modes of Zn-H at type I sites were assigned at 829 and 1665 cm-l in the INS data. These frequencies are in close agreement with the published i.r. results.'. From our INS spectra, we were unable to confirm the recent assignments by Boccuzzi et aL2 of the vs mode of type I (O-H) to a broad i.r. band at 845-850 cm-l and of the v,, mode of type I1 hydrogen at zinc sites to an i.r. band at 1475 cm-l. We suggest that these assignments2 of the i.r. bands are to be regarded as tentative. An intense broad band was observed at 1346 cm-l in the INS spectrum of ZnO itself. The intensity of this band was unchanged on H, adsorption and since it is not observed in the optical spectra it is suggested that this band arises from modes of carbonate impurities in the bulk.We thank the S.E.R.C. and A.E.R.E. Harwell for the award of a CASE studentship to one of us (I. J. B.) and for the provision of access to neutron-beam facilities. We also thank the New Jersey Zinc Co. for the gift of the Kadox 25. C. S. John, in CatafyAis, ed. A. Dowden and C . 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