Photoluminescence of montmorillonite clay minerals modified by cetyltrimethylammonium bromide

Abstract

The structure and photoluminescent properties of natural montmorillonite (MMT) aluminosilicates from three mineral deposits that were modified by cations of sodium or the surfactant cetyltrimethylammonium bromide (CTAB) have been investigated. X-ray fluorescence analysis showed significant differences in the concentrations of the inorganic dopants in these materials. An x-ray diffraction analysis established that incorporation of the CTA + long-chain cations into the MMTs expands remarkably the mineral interplanar distances, the values of which are close to each other for all organoclay samples under study and equal to 1.8-2.0 nm. The photoluminescent properties of the Na-form of the MMTs are shown to depend on the amount of introduced dopant capable of either enhancing or quenching the emission. Deposition of CTAB on the mineral surface influences the luminescence pattern, the spectral contour and wavelength range being practically the same for all modified samples. The emission may intensify or weaken because of such surface modification.Introduction. Natural layered clay minerals (CM) with the 2:1 structure type are highly disperse and have a large specific surface area. They are known to be selective sorbents and effective ion-exchangers [1, 2], active catalysts for various processes (isomerization, polymerization, addition, etc.[3]), fillers of organic media, plastifiers of polymeric materials [4-8], etc. The particles of these CM are stratified in a liquid medium. Their thickness is of the order of nanometers; the width/length, 25-1000 nm. Their surface is organophobic in the unaltered state. Owing to the presence in the CM interplanar space of metal cations that are susceptible to ion exchange, it is rather easy to convert the surface into an organophilic one via exchange of the inorganic cations by organic ones using ion exchange. As a result, the affinity of the resulting organoclays for an organic medium [polymers, liquid crystals (LC), aromatic compounds, etc.] increases sharply. Heterocomposites with a homogeneous structure (coagulation network) consisting of nanoparticles of the organoclay and the organic matrix are formed at a certain concentration of them. For example, LC filled with CM particles modified by various surfactants (SA) exhibit a variety of electro-optical properties [9][10][11].The starting CM in the present work was montmorillonite (MMT) from three deposits: Pyzhev (Ukraine), Cherkassy (Ukraine), and Askan (Georgia). MMT is a layered natural aluminosilicate of the 2:1 structure type that consists of two layers of SiO 2 tetrahedra between which a layer of Al 2 O 3 octahedra is located. These samples differ in the impurity profile, dispersion, cation-exchange capacity (CEC), and magnitude of surface charge. This should affect their properties (in particular, optical ones). We studied two series of these CM. One of them was the Na-form of the samples; the other, MMT modified with an organic molecule. The organic surface modifier was cetyltrimethylammonium bromide (CTAB) because this SA...

Structure and spectroscopic properties of organoclays doped by multiwall carbon nanotubes

“…According to elemental analysis, the contents of Fe, Ti, Cr, and Cl in Cherkassy MMT were substantially greater than in Pyzhevsk MMT. This was reported previously [25] and agrees with the literature [17,24]. A band with principal maximum at 1040 cm -1 corresponded to Si-O stretching vibrations of the MMT crystal lattice and also of aerosil, which was used in the synthesis of the carbon nanotubes [23].…”

Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanoclay Materials

“…It can thus be assumed that a titanium-related center is responsible for the blue emission in Br-Hec and I-Hec. According to previous literature, this blue emission can be attributed to TiO 6 entities as reported for Benitoite (BaTiSi 3 O 9 ) [43] or to Ti 3+ -V O (oxygen vacancy) pairs [23,24,47,48]. Moreover, the doping of the X-Hec materials with Ti 3+ (x Ti = 0.1 mol %) increases the intensity of the main emission ( Figure A6), without significant changes to its band position confirming titanium as the luminescent center.…”

“…The multisite origin for emission is also witnessed in the decrease of emission band width [42] with increasing excitation wavelength. Similar blue-green emission has previously been observed for other silicates such as fluorohectorite [23], chlorohectorite [24], montmorillonite [43], kaolinite [44], pyrophyllite [45], topaz (Al2SiO4(F,OH)2) [46], synthetic hackmanite (Na8Al6Si6O24(Cl,S)2) [47,48], benitoite (BaTiSi3O9) [49], and SiO2 [50,51]. The literature presents mainly two different explanations for the origin of this blue-green emission.…”

“…The literature presents mainly two different explanations for the origin of this blue-green emission. One proposition categorizes this emission as photoelectric response as well as inner center and recombination type of luminescence [52], attributing the blue emission to Similar blue-green emission has previously been observed for other silicates such as fluorohectorite [23], chlorohectorite [24], montmorillonite [43], kaolinite [44], pyrophyllite [45] [50,51]. The literature presents mainly two different explanations for the origin of this blue-green emission.…”