The first page of this article is displayed as the abstract.

The first page of this article is displayed as the abstract.

After storage under ambient conditions zinc oxide, prepared by the combustion of metallic zinc, carries both carbonate and hydroxylic species. The former are removed by heating the oxide in oxygen and water vapour at 673 K. Subsequent cooling to ambient temperatures in a moist oxygen atmosphere yields a surface carrying hydroxyls. The corresponding infra-red absorption spectrum exhibits four major absorption bands. Two narrow bands are present at 3670 and 3620 cm–1 and two broad absorptions are centred about 3555 and 3440 cm–1. Outgassing the sample at 625 K removes the surface species corresponding to the 3555 cm–1 band and reduces the concentration of that associated with the absorption at 3440 cm–1. The intensities of the 3670 and 3620 cm–1 absorptions are little affected by this outgassing procedure. Rehydroxylation of such outgassed samples by exposure to water vapour at ambient temperatures indicates that the thermal dehydroxylation process involves a “mobile” surface state. The absorption bands observed in the spectra are tentatively assigned to isolated and hydrogen-bonded surface hydroxyls sited on different surface crystal planes. Studies of the adsorption of CO2 show that this gas chemisorbs rapidly at ambient temperatures on a partially dehydroxylated sample to form a carbonate species but the chemisorption is much reduced if the surface is in the “fully” hydroxylated state. HCl gas also reacts with the surface layers of zinc oxide but in such a way as to promote the formation of the bulk hydroxide on exposure of the chlorine-containing sample to water vapour at ambient temperatures.

The first page of this article is displayed as the abstract.

A moderate surface reduction of TiO2(anatase or rutile) at 573–773 K under vacuum, or in the presence of carbon monoxide or hydrogen, or finally at room temperature under ultra-violet irradiation in vacuum or in the presence of hydrocarbons, creates two types of paramagnetic centres, which are identified by e.s.r. spectra as Ti3+ ions.Adsorption of oxygen (between 77 and 373 K) on TiO2 samples reduced in the ways described above leads to the formation of ionic species of the type O–2. The same species are also formed at room temperature on unreduced TiO2 samples submitted to an UV irradiation in the range of 2000–6000 Å.These results are paralleled with the occurrence of photocatalytic (in u.-v.) partial oxidation of isobutane on TiO2 at room temperature.

The first page of this article is displayed as the abstract.

The first page of this article is displayed as the abstract.

Sorption data of nitrogen at 77 K, water and ethanol at 298 K and n-pentane at 273 K have been obtained for pure rutile outgassed at 673 K and modified by presorption of water, ethanol, hexan-1-ol and hexan-1 : 6-diol. All adsorbates interact strongly with the unmodified surface. With presorbed water and organics the extent and energy of nitrogen sorption is reduced. Organic-pretreated surfaces are largely hydrophobic, but also show reduced n-pentane adsorption. Hexan-1 : 6-diol is oriented parallel to the rutile surface by strong interactions through the hydrocarbon chain and both hydroxyl groups. Although not initially adsorbed in perpendicular orientation, hexanol has this configuration at high relative pressures of n-pentane which then penetrates the presorbed layer. Ethanol exchanges with both presorbed water and hexan-1-ol but not with hexan-1 : 6-diol.

The chemisorption of water vapour on de-hydroxylated α-Fe2O3 has been investigated by heats of immersion into water at 25°C. The properties of physically adsorbed multilayers on the hydroxylated surface have been studied using dielectric relaxation techniques in conjunction with adsorption isotherms. The heat of immersion of the completely hydroxylated substrate is –367 ergs/cm2 and increases with increased outgassing temperature to –1073 ergs/cm2 for outgassing at 375°C. Differentiation of the integral heat curve gives the initial heat of chemisorption to be at least –50 kcal/mol of water vapour. The electrical capacitance at 70 c/s to 300 kc/s of physically adsorbed water vapour on the hydroxylated substrate is constant within the B.E.T. monolayer, but rises sharply with the onset of the second layer. This behaviour suggests that the first layer of physically adsorbed water is immobile, but that succeeding layers are mobile. Confirming evidence as to the immobile nature of the physisorbed monolayer is given on an entropy basis. The integral entropy of adsorption obtained from multi-temperature isotherms at constant spreading pressure is –33.2 cal/mol K for half a monolayer. This value agrees favourably with the theoretical value of –36.8 cal/mol K calculated from statistical thermodynamics for a localized monolayer, but not with the value –15.9 cal/mol K derived for mobile adsorption. Characteristic relaxation frequencies of adsorbed water vapour were obtained from Cole-Cole are plots of dielectric constant against dielectric loss. The characteristic frequencies increase smoothly with coverage from 10 c/s to a value of 10 kc/s at B.E.T. coverages of three or more, suggesting the development of a hydrogen-bonded ice-like structure for those coverages.

The first page of this article is displayed as the abstract.

The conditions under which an oxide surface in aqueous solution will obey the Nernst equation with respect to H+/OH– as potential-determining ions are investigated and a modified form of the Nernst equation is derived. This is combined with a model of the inner part of the double layer involving adsorption of both anions and cations of a supporting uni-univalent electrolyte and a discreteness-of-charge correction in their adsorption isotherms. Theoretical total differential capacities at the interface are compared with experimental data for TiO2 and SiO2.

Ferric oxide hydrosols are composed of a haematite core surrounded by an hydrated oxide shell. Interactions have been studied, on both sides of the p.z.c., between ferric oxide surface and a series of anions and cations having characteristics ranging from structure promoters to structure breakers. Although ferric oxide has a moderate crystalline field, the observed adsorption sequences show that the interface acts as a structure promoter for water molecules. The large spreading of the measured adsorption sequences, especially at extreme pH values, proved the coagulation technique to be a good tool for the study of ion-surface interactions. A new type of behaviour has been observed; at low surface potentials the ions behave as indifferent but at higher surface potentials they are specifically adsorbed.

Experimental results are given for the temperature dependency of the zeta potential and adsorption density in the system of quartz + dodecylammonium acetate solutions. Utilizing the experimental adsorption densities and the Stern-Grahame equation, calculations indicate large entropic effects which are attributed to the phenomenon of hydrophobic bonding. A change in the adsorption mechanism in the temperature range greater than 25°C may be due to a major breakdown of the ordered water structure at the solid surface. The electrokinetic results appear contrary to the prediction of both the simple Gouy-Chapman and the modified Stern theory allowing for a distance of closest approach. It is concluded that the measured effects must be due to changes occurring at, or associated with, both the plane of closest approach and the shear plane.

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A review is given of some advances since 1967 in this field as it applies to homomolecular crystals of the anthracene type. The topics discussed include carrier generation and recombination, carrier motion and trapping, and the relationship between these processes and the observed current-voltage dependence in the crystal.

Phonon modes in aromatic hydrocarbon crystals are classified to guide discussion of the electron-phonon interaction. The principal qualitative electron-phonon effects on charge carrier transport arise from the exchange integral variations due to lattice modes and from the binding energy due to frequency changes in out-of-plane molecular modes; other lattice and molecular modes have quantitative effects. The use of linear-response theory to calculate the conductivity for the two principal interactions is described. Each calculation embodies results applicable in the region intermediate between hopping and band transport, but as yet neither is completely analyzed in this region. Nevertheless, the fundamental processes governing charge carrier transport in these crystals now appear to be understood.

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The photo-induced electrical conductivity of a number of arylidene-1,3-indandiones was investigated in order to determine the effect of charge carrier generation on the intramolecular electron shifts. The variation in the polarity of the compounds was achieved by placing electron donor and electron acceptor substituents on the arylidene group. The comparison of dark and photoconductivity of 15 arylidene-1,3-indandiones was obtained on sandwich cells of polycrystalline layers. More detailed experiments with the better photoconductors were carried out with sublimed films and pressed pellets.The photoconductivity of benzylidene derivatives indicated that photoconductivity increases with polarity of the molecule and achieves a maximum value in p-dimethylaminophenyl compound (DMAPID). Photovoltaic cells, giving 1.17–1.24 V open circuit photovoltage, were prepared from DMAPID sublimed film sandwich cells. The spectral response, voltage dependence and temperature dependence of several photoconductors were investigated. Although the generation of light-induced charge carriers is facilitated by increasing the dipole moment, the dark conductivities remained unaffected by such structural modifications.

Conductivity measurements are reported on both isotactic and atactic polyvinyl pyridinium complex TCNQ salts. The isotactic materials are significantly better conductors than those derived from atactic materials, but not as good as those derived from the monomeric base. Seebeck and microwave Hall mobility measurements indicate that the isotactic materials behave as polycrystalline complex radical salts, whereas the atactic materials exhibit behaviour consistent with hopping conduction.

Field-induced switching between two impedance states has been observed in both pure and doped organic thin films. Aromatic hydrocarbon films display this phenomenon reproducibly when a mobile electrode material such as gallium-indium alloy is employed. Controlled doping of the hydrocarbons with electron acceptors leads to reproducible switching characteristics which are not electrode dependent. For the doped films, activation energies of conduction typical of bulk charge-transfer complexes characterize the low impedance states whereas the activation energies of the high impedance states are typical of organic insulating films. It is suggested that the switching mechanism involves the formation of conducting filaments.