Chmelik, Christian

Kärger, Jörg

Lauerer, Alexander

Lenzner, Jörg

Valiullin, Rustem

Zeigermann, Philipp

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The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and ApplicationIR Micro-imaging of mesoporous silicon as a model system for theinvestigation of hysteresis phenomenaAlexander Lauerer1∗, Philipp Zeigermann1, Jo¨rg Lenzner2, Christian Chmelik1,Rustem Valiullin1, Jo¨rg Ka¨rger11Department of Interface Physics, Universita¨t Leipzig, Leipzig, Germany2Department of Semiconductor Physics, Universita¨t of Leipzig, Leipzig, Germany∗lauerer@physik.uni-leipzig.deEnsembles of molecules confined to mesopore spaces reveal features which may notably deviate fromtheir behavior both in the bulk phase and under dominating host-guest interaction. A previous de-tailed investigation of molecular dynamics in Vycor porous glass with a pore diameter of 6 nm usingpulsed field gradient nuclear magnetic resonance (PFG NMR) indicated that under identical externalconditions (temperature, pressure), different “histories” of sample preparation may give rise to dra-matic differences in their microdynamics [1–3]. One approach to rationalize the occurrence of history-dependent states in mesoporous host-guest systems is the occurrence of a geometric disorder-inducedrugged landscape in the free energy. To get deeper insight into this phenomenon, novel options providedby the development of IR micro-imaging [4] have been used in the present work.IR micro-imaging is a powerful tool for the investigation of sorption phenomena because it allowsobtaining time- and space-resolved images of the concentration of guest molecules within the hostmaterial. The possibility of visualizing the variation of probe molecule arrangements at different pres-sures (see figure 1) is a promising experimental approach for a quantification of hysteresis phenomenaand could also be the key for a more detailed insight in their correlation with underlying pore fillingmechanisms.Electrochemically etched mesoporous silicon with a spatially ordered pore system was chosen as hostsystem for this study. The pore system has been designed in a way that 5 nm pores of 55 µm lengthare continued by channels of about 10 nm pore diameter with a length of 45 µm. The application offocused ion beam milling to a freshly etched porous silicon film enabled the preparation of a siliconsample with the channels open on both sides and aligned perpendicular to the incident IR beam. Beforethe measurements, the sample was oxidized under oxygen atmosphere to increase its optical transmis-sibility. Benzene was used as a guest molecule, the experiments were performed at room temperature.Figure 1: Selected FPA maps of benzene adsorption in a 175 µm × 100 µm × 52 µm piece of mesoporous siliconat a pressure of 75 mbar (left map) and 115 mbar (right map) with the 5 nm pores being arranged on the lefthalf and the 10 nm pores on the right half of the sample. Light color represents low, dark color high benzeneconcentration.From the maps recorded with the focal plane array (FPA) detector for pressures from 0 mbar up to sat-urated vapour pressure of benzene, concentration profiles were obtained (figure 2). Integration gave therespective isotherm with a well-observable hysteresis loop. The kinetics of the concentration equilibra-tion following each pressure step were studied separately using a single element detector (right graphof figure 2). In particular, at the onset of the hysteresis the uptake kinetics were found to be sloweddown.Figure 2: Concentration profiles for benzene adsorption (left graph). Sorption isotherms obtained upon integrationof the concentration profiles (middle). Uptake curves showing the slowing down of the system kinetics within thehysteresis region (right graph).By the development of suitable procedures of sample preparation and activation, we succeeded inusing the full potential of IR micro-imaging for the application to mesoporous silicon. By this, thebasis for further in-depth studies of a first, simply designed model system for both ad- and desorptionof benzene, including FPA maps, concentration profiles, the corresponding isotherm and the uptakecurves for each pressure step, was provided.References[1] R. Valiullin, S. Naumov, P. Galvosas, J. Ka¨rger, H.J. Woo, F. Porcheron, P.A. Monson: Explorationof molecular dynamics during transient sorption of fluids in mesoporous materials. Nature 443,965–968 (2006)[2] S. Naumov, R. Valiullin, P.A. Monson, J. Ka¨rger: Probing memory effects in confined fluids viadiffusion measurements. Langmuir 24, 6429–6432 (2008)[3] R. Valiullin, J. Ka¨rger, R. Gla¨ser: Correlating phase behaviour and diffusion in mesopores: Per-spectives revealed by pulsed field gradient NMR. Physical Chemistry Chemical Physics 11, 2833–2853 (2009)[4] J. Ka¨rger, D.M. Ruthven, D.N. Theodorou: Diffusion in Nanoporous Material. Wiley-VCH, Wein-heim (2012)

IR Micro-imaging of mesoporous silicon as a model system for the investigation of hysteresis phenomena

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IR Micro-imaging of mesoporous silicon as a model system for the investigation of hysteresis phenomena

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