Nanocomposites and nanomaterials

Emission Bands of Adsorbed Water and (Н2О)n Clusters 
in IR Spectra of MoS2 Nano- and Microcrystallites
Kornienko N.E., Naumenko A.P.
Taras Shevchenko National University of Kyiv, 64/13, Volodymyrs’ka Str., 01601, Kyiv, Ukraine
E-mail: nikkorn@univ.kiev.ua

 The physical mechanisms of increased catalytic activity of layered MoS2 during water decomposition for alternative energy purposes are studied in the report. The vibrational spectra of MoS2 have been studied very widely, but the state of adsorbed water  in MoS2 has not been studied sufficiently. We studied the FTIR spectra of nanocrystallites of MoS2 (6 ‒ 8 monolayers and sizes 5x17 (40) nm) and natural microcrystals ~ 200 nm using the methods of ATR and diffuse scattering (DS). In the IR spectra of changes in the optical density ΔD for MoS2 nano- and microcrystallites, a strong broadening of the valence νOH and deformation νδ bands of adsorbed water  in MoS2 was revealed, as shown in Fig. 1. A feature of adsorbed water in nanocrystallites of MoS2 is an abnormal enhancement of its absorption capacity. The key role of collective vibrational modes of a system of hydrogen bonds of water [1] in the formation of wide bands of adsorbed water νОН, νδ has been established. The large bandwidths νδ, νOH are associated with the manifestation of composite tones with the participation of transverse and longitudinal acoustic (TA, LA) and optical (TO, LO) vibrational modes [1]. The most important for the catalytic activity of MoS2 structures is the detection of IR bands of negative polarity (emission) in the region of ν'ОН ~ 3700 cm-1 and sum tones νδ + 2ТО, νδ + 2LO, as well as the emission bands of (Н2О)n clusters (see inset in Fig. 1). For water clusters, the maximum emission intensities were observed for vibrations of (Н2О)2 dimers (3736 and 3743 cm–1) and (Н2О)3 trimmers at about 3727 cm–1 in MoS2 microcrystals. Emission in the regions of the νOH, νδ bands of adsorbed water and cluster vibrations clearly indicate excessive populations of these states, which are associated with the nonlinear-wave interaction of vibrational modes. This is due to the high catalytic activity of MoS2 with adsorbed water. 
1. Kornienko N.E. Low-frequency collective modes of the system  of Hydrogen bonds in Water. Ukr. J. Phys. 2002, Vol. 47, N 4, p.361-364