Summary. The investigation of different zinc oxide samples by means of thermogravimetry and infrared spectroscopy has shown that the surface of the samples is covered by an approximately monoatomic layer of hydroxide groups. Furthermore, varying amounts of carbonate groups are found which are due to the presence of zinc hydroxide carbonate I1 [Zn,(OH),(CO,),]. Below relative water vapour pressures of p/po = 0.2 (25"C), two hydrogen bridges connect one physisorbed water molecule with two chemisorbed surface hydroxide groups. In addition, about the same amount of water is physically adsorbed between vapour pressures of p/po 0.2 and 0.8. ,4t still higher relative humidity, a multiinolccular layer is built up which reaches a thickness of about 200 water molecules at p/po = 1.0.All samples show in the v-OH region of the IR. spectrum a broad absorption with four bands, A , B, C, and D. The position of the bands and the change of their intcnsities when rising the temperature of the samples up to 600°C indicate that both bands of longer wave lengths, C and D, arise from physically adsorbed water molecules, while the bands A and B are due to hydroxide groups located on the crystallographic faces (0001) and (OOOX), respectively.

o,o′‐Dihydroxyazo compounds are spectral sensitizers for the photosemiconductor properties of zinc oxide. It is shown that 1‐(2‐hydroxyphenylazo‐)‐2‐naphthol (II) forms a 1:1 Zn complex on the surface of ZnO particles if adsorbed from toluene solution. With most samples of ZnO a Langmuir type adsorption isotherm is obtained. The concentration of the dye at saturation relative to the BET surface of the ZnO particles correponds to a slightly oblique edge‐on arrangement of the dye molecules on the surfaces. In the case of ZnO pretreated with propionic acid and having taken up dye in excess of a monolayer, it can be shown by electron microscopy that tail‐like complexes (zinc: dye ratio = 1:1) are formed, and that these complexes contain zinc which was originally on the particle surfaces.

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Zur Klärung des Mechanismus der Sensibilisierung der Photohalbleitereigenschaften von ZnO durch die im Titel genannten Farbstoffe wird an ZnO verschiedenen Typs das Sorptionsverhalten der Farbstoffe (I)‐(III) aus Toluol bei Zimmertemperatur untersucht.