Vibrational Spectroscopy 69 (2013) 1–7 Contents lists available at ScienceDirect Vibrational Spectroscopy jou r n al hom ep age: www.elsevier.com/locate/vibspec Far-infrared investigation of kaolinite and halloysite intercalates using terahertz time-domain spectroscopy D. Zich a , T. Zacher a,b , J. Darmo c , V. Szöcs d , D. Lorenc e , M. Janek a,∗ a Comenius University, Faculty of Natural Sciences, Department of Physical and Theoretical Chemistry, Mlynská dolina CH1, SK-84215 Bratislava, Slovakia b Slovak Institute of Metrology, Department of Chemistry, Karloveská 63, SK-84255 Bratislava, Slovakia c Institute of Photonics, Vienna University of Technology, Gußhausstraße 27-29, A-1040 Vienna, Austria d Comenius University, Faculty of Natural Sciences, Institute of Chemistry, Mlynská dolina H2, SK-84215 Bratislava, Slovakia e International Laser Center, Ilkovicova 3, SK-81219 Bratislava, Slovakia a r t i c l e i n f o Article history: Received 7 March 2013 Received in revised form 4 September 2013 Accepted 4 September 2013 Available online 18 September 2013 Keywords: THz-TDS Far-infrared Layered clay minerals Kaolinite Halloysite Dielectric properties Refractive index Absorption index Absorption coefficient a b s t r a c t Two clay minerals from the kaolin group, namely well-ordered kaolinite and poorly ordered halloysite, were investigated by terahertz time-domain spectroscopy (THz-TDS). Both clay samples were used for preparation of their respective intercalates using dimethyl sulfoxide (DMSO) and potassium acetate (KAc) with water. The intercalates were also characterized by X-ray powder diffraction and Fourier transform infrared spectroscopy. The dielectric behaviour of clay samples was investigated in the far-infrared region of 0.2–2.7 THz corresponding to about 6.7–89.9 cm −1 . The frequency dependence of the power absorp- tion coefficient revealed clear absorption bands for DMSO intercalates but not for KAc with water. For kaolinite – DMSO intercalate a distinct doublet at 1.70 THz (56.6 cm −1 ) and 1.88 THz (62.6 cm −1 ), and for halloysite – DMSO intercalate a single broad band centred around 1.72 THz (57.3 cm −1 ) were found. These bands are reported for the first time in this type of intercalation substances and indicate the appli- cation potential of THz time-domain spectroscopy for use in the investigation and detection of chemical behaviour of molecular species introduced into the interlayer space of layered substances such as clays and clay minerals. Additionally, the qualitative characteristics of observed bands of DMSO intercalates in the THz region reasonably resembled the structural order/disorder of used kaolinite and halloysite samples. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The major use of THz spectroscopy by chemists is in the spec- tral characterization of the rotational and vibrational resonances and thermal-emission lines of simple molecular species in the far- infrared region of 3.3–333 cm −1 (0.1–10 THz). As the last years have revolutionized THz systems with new and higher-power sources for advanced physics and materials research, new significant research and application areas have been found simultaneously [1–8]. The main advance of THz-TDS in comparison to the conven- tional far-infrared technique is a much better signal to noise ratio mainly at frequencies below 3 THz (<100 cm −1 ) [9]. The fact that the transmitted THz electric field is measured coherently, enables measurement with high signal sensitivity and time-resolved phase information, allowing rich spectroscopic, dielectric and image anal- ysis. Nevertheless, the main strength of this technique is the ability to achieve time-resolved spectroscopic information, which enables insight into material charge carrier diffusion and trapping ∗ Corresponding author. Tel.: +421 2 60296418; fax: +421 2 60296231. E-mail address: marian.janek@fns.uniba.sk (M. Janek). processes at sub-picoseconds resolution of photogenerated con- ductivity carriers [10–14]. Despite the broad application field of THz-TDS, there are no papers yet to our knowledge dedicated to the inspection of con- fined systems or host–guest complexes based on, e.g., layered clay minerals. Clays including kaolinite and/or halloysite are generally considered as easily accessible natural low-cost materials suitable for very specific applications [15–17], which can be easily modi- fied by physical and/or chemical treatments [18–26]. As THz-TDS can provide information which falls in the far-infrared fingerprint region, it can be supposed that this method should be capable of following the changes connected to the modification of interlayer space and reactivity of confined intercalated molecules. Previous studies of micaceous clays with different chemical compositions of interlayer cations showed that THz-TDS is capable of distin- guishing the differences between samples with different chemical composition in the far-infrared region [27]. The structural units forming a single kaolinite layer are formed by two-dimensional tetrahedral sheets composed of tetrahedrons [SiO 4 ] x− , which are linked to octahedral sheets composed of octahe- drons [Al(O,OH) 6 ] y− (Fig. 1). In a single mineral layer the tetrahedral sheets are terminated with oxygen atoms forming siloxane 0924-2031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vibspec.2013.09.003