Langmuir 1992,8,645-649
645
Adsorption of Water Vapor by Iron Oxides. 3. Inelastic Incoherent Neutron Scattering from Water Adsorbed on Magnetite: Evidence for an Icelike Structure N.S.Clarke+and P.G.Hall'J Department of Chemistry, University of Exeter, Exeter EX4 4QD,U.K., and Atomic Weapons Research Establishment, Aldermaston, U.K. Received April 29, 1991. In Final Form: November 13, 1991 Inelastic incoherent neutron scattering spectra from water adsorbed on magnetite, measured using a beryllium filter spectrometer at 77 K, are compared with the neutron spectrum from bulk ice Ih. The adsorbate is shown to have a remarkably icelike structure which is independent of coverage in the range examined (up to 0.028 g of waterlg of solid); the central feature in each case is the strong librational peak at ca. 80 meV.
Introduction There is widespread interest in the structure of water at interfaces, and in particular to what extent i t may be icelike. Examples of specific areas are colloid science and heterogeneous nucleation. This part 3 concerns the inelastic neutron scattering spectra from water adsorbed on magnetite. The spectra obtained are compared with that from bulk ice. Neutron scattering measurements with water adsorbed on silica have been reported.'-4 In part 1: the preparation and interconversion of iron(III) oxides (includingmagnetite) and hydroxides and their characterization by X-ray diffraction and small angle neutron scattering (SANS)were described. In part 2: water vapor isotherms on a- and y-FezOa suggested that the structure of the water adlayer is closepacked and nonlocalized. Wide energy (survey) X-ray photoelectron spectroscopy (XPS) scans of haematite and magnetite surfaces showed the presence of adsorbed water down to pressures as low as mbar after pumping overnight. The isotherm and XPS methods mentioned above are just two examples of a variety of experimental techniques which have been used to investigate water vapor adsorption on iron oxide surfaces. Thus one may quote, as examples, infrared s ~ o s c o p y , 7heats ~ * of immersion! and dielectric relaxati~n.~I n this context XPS being an ultrahigh vacuum technique is necessarily confined to low coverages of adsorbate, whereas the present neutron studies are able to extend the range to higher coverages. Also, neutron scattering has particular advantages in that it is possible to use large samples (high surface areas) in controlled (water vapor pressure) environments and the fact that it is not subject to the same selection rules as infrared spectroscopy. + Atomic Weapons Research Establishment.
* University of Exeter.
(1) Boutin, H.; Prask, H. Surf.Sci. 1964,2, 261. (2) Anderson, R. G. W.; White, J. W. Spec. Discuss. Faraday SOC. 1970, 205. (3) Tempelhoff, K.; Feldman, K. Z . Phys. Chem. (Leiprig)1975,256, 369. (4) Hall, P. G.; Pidduck, A.; Wright, C. J. J. Colloid andznterfaceSci. 1981, 79, 339. ( 5 ) Clarke, N. S.; Hall,P. G.Langmuir 1991, 7, 672. (6) Clarke, N. S.; Hall,P. G.Langmuir 1991, 7, 678. (7) Blyholder, G.; Richardson, E. A. J. Phys. Chem. 1962, 66, 2597. (8) Rochester, C. H.; Topham,S. A. J. Chem. Soc., Faraday Trans. I 1979, 75, 1073. (9) McCafferty,E.A.; Zettlemoyer, A. C. Discuss. Faraday Soc. 1971, 52, 258.
Experimental Section We measure an amplitude weighted density of states, C(Q,o), as a function of energy transfer. This function has been defined previ~usly.~ The experimental details, using a beryllium filter spectrometer (BeF), are largely as described in the silica work4 and have been reported in detail by Clarke.Io The instrument used in the present work was the former BeF spectrometer mounted on the 10H hole of the DIDO reactor at A.E.R.E. Harwell. The 10H Be filter instrument uses a monochromated beam of neutrons, selected from the Maxwellian distribution of reactor neutron velocities by Bragg scattering from an alumium crystal. To extend the incident energy range up to 150meV, it is necessary to use both the (111)and (200) monochromator planes. Only those neutrons losing sufficient energy to the sample to be scattered with final energies Ef< 5.2 meV are available for detection by the two banks of 1°BF3counters, all those of higher energies being Bragg scattered out of the beam by the beryllium filter (cooled to 77 K to minimize Debye-Waller attenuation of Bragg reflection). The counters are arranged at angles between 48and 90°to the incident beam, one bank collectingtransmitted radiation and the other collecting reflected radiation. Scans were performed by stepping the incident energybetween 10 and 150 meV and counting until a fixed number of monitor counts had been recorded at each point. The samples were contained in aluminum cans, either cylindrical, in the case of samples of low percentage scattering, or of circular slab geometry with samplesof high percentagescattering. The slab containers were aligned at angles of 3 5 O to 50' to the beam. Percentage scattering was kept within the range 5 to 20% ; above 20% ,multiple scattering problems can arise. The sample cans were clamped centrally and vertically in position in a liquid-helium-filled cryostat and cooled to 77 K. With temperatures as low as 77 K the possibility arises of the cooling leading to desorption of the water from the iron oxide surface onto the walls of the container. However, any such effect would seem to be insignificant. Thus, the similarity between our previous results with silica' at low temperature and the results of the time of flight measurements at room temperature by Tempelhoff and Feldman3 discount that. The same conclusion is supported by our time of flight measurement' at room temperature. Samples were outgassed at about 200 OC for 12 h to a pressure
