Structural and Textural Studies of ZnO Modified with Dodecyltrimethylammonium Chloride

Abstract

The effect of dodecyltrimethylammonium chloride — a cationic surfactant — on the texture and structure of ZnO, which is commonly used in the photocatalytic degradation of surfactants, has been investigated together with the effect of thermal treatment of the resulting materials at temperatures ≤350°C. Structural changes were followed by combining diffraction and infrared (IR) spectroscopic measurements while textural variations were monitored via low-temperature nitrogen adsorption studies. In addition, the effect of soaking the original sample of ZnO in water for 5 d has been monitored together with the effect of heat treatment on the resulting products.

Bands corresponding to a carbonate complex were observed in the IR spectrum of all samples subjected to thermal treatment below 275°C. This complex gave rise to new bands in the XRD patterns of such samples situated at d-distances of 2.339, 2.023 and 1.431 Å, respectively, which disappeared when the samples were subjected to heat treatment above 275°C. The intensities of such bands decreased when increasing surfactant concentrations were employed in the initial treatment of the ZnO solid.

Acid OH groups on the surface of the solid appear to provide sites for the formation of the carbonate complex. Such sites are attacked either by the aqua complex [Zn(H₂O)₆]²⁺ formed from dissolved ZnO in the presence of water alone or by the cationic surfactant when the latter is present in the system. Attack of the surfactant on the surface probably occurs through exchange with an H⁺ ion. The cationic surfactant was also adsorbed on to basic sites present on the surface of the solid since the concentration of such sites diminished as the surfactant concentration was increased.

Measurements of the specific surface areas and pore structural analyses for all the samples studied indicated the presence of micro- and meso-pores in most cases, the former being absent from the parent ZnO employed.

A comparison between the application of the V _l versus t plot and the α _s -plot for such pore structural analyses is presented together with a discussion of the criteria necessary for the application of the α_s-method.