Preparation and characterization of azobenzene-smectite photoactive mineral nanomaterials

Abstract

Smectites are 2:1 layered minerals built of one octahedral sheet located between two tetrahedral sheets. The layer charge, derived from the isomorphic substitutions in the mineral structure, is compensated by the interlayer cations. The capability to exchange the interlayer cations is an important property of smectites as it enables to design and produce new nanomaterials based on natural minerals through their organic modification. Such hybrid materials are highly desirable in industry and environmental protection due to their specific properties that may be controlled in nanoscale. Preparation of photoactive materials through intercalation of layered minerals, including mainly synthetic micas, with azobenzene and other azo-compounds was proposed previously (Fujita et al., 2003; Ogawa et al., 2003; Heinz et al., 2008). Azobenzene molecules show a change in their shape and dimensions upon the UV irradiation, which may affect the structure of host mineral. The photoactivate materials may find application in nanotechnology as molecular nanoswitches and nanosensors controlled by UV radiation. The objective of the study was to prepare azobenzene-smectite intercalation compounds. The obtained materials were subjected to structural and chemical characterization, which is crucial for further advancement of their photoresponsive properties.

Na-montmorillonite (SWy), Ca-montmorillonite (STx), beidellite (BId) and synthetic laponite (SynL) were used for the experiments. The modification procedure involved at first the intercalation of smectites with hexadecyltrimethylammonium bromide (abbreviated C16) and after that the insertion of azobenzene into the interlayer space. The reaction with C16, in amount equal to 1.0 CEC of the smectite, was performed in an aqueous suspension (20g/L) for 2 h in 60°C. The obtained organo-smectites were prepared as thin films on glass plates and reacted with azobenzene in a teflon vessel at ~100°C for 24 h.In such conditions the azobenzene vaporizes and penetrates the interlayer space of the organo-mineral. The azobenzene/smectite weight ratio was equal to 0.2. The chemical and structural analysis of all obtained samples was carried out with use of X-ray diffraction  (XRD), infrared spectroscopy (FTIR) and CHN elemental analysis.

The increased amount of nitrogen and carbon in modified samples confirmed the occurrence of intercalation process of both the ammonium salt and the azobenzene. Moreover, new bands appeared in the infrared spectra of the C16-smectites at ~2924 cm^-1 and ~2851 cm^-1 due to the C-H stretching vibrations in the C16 molecules. The spectra of azobenzene intercalation compounds showed additionally a series of bands corresponding to the vibrations characteristic for the azobenzene molecule at: ~3061 cm^-1, ~1581 cm^-1, ~1455 cm^-1, and ~1302 cm^-1. . The basal spacing of tested minerals increased after the C16 intercalation, as confirmed by the XRD analysis. The increase was equal to 6.1 Å, 3.3 Å, 4.1 Å and 3.5 Å for SWy, STx, BId and SynL samples, respectively. This suggests a nearly horizontal arrangement of C16 molecules and formation of a monolayer in the interlayer space of smectite. Introduction of azobenzene lead to a further increase of d₀₀₁ value. The increase is visibly different for all the samples and is equal to 7.0 Å, 15.0 Å, 21.7 Å and 23.5 Å for SWy, STx, BId and SynL samples, respectively. The arrangement of organic molecules in the interlayer space is influenced by a number of factors including: the type of the mineral, the layer charge and its location in the layer, or the amount and arrangement of the cationic surfactant. A correlation between the azobenzene location in the interlayer space and the photoresponse behavior of tested materials will be the subject of further studies.