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Reports: ND1053140-ND10: Nanofabrication of Porous Membranes for Separations of Gaseous Energy Carriers Under Conditions of Single-File Diffusion

Kirk J. Ziegler, University of Florida

Sergey Vasenkov, University of Florida

Effective separation of light gases is important because of an increasing use of methane as a primary energy carrier. This work focuses on the development of a new separation strategy based on the single-file diffusion (SFD) of select components in the gas mixture inside a porous membrane. Under these conditions, the effective diffusivity of a molecule is expected to decrease by many orders of magnitude in comparison to normal diffusion thus allowing for highly selective separations. The aim of this study is to fabricate model membranes capable of inducing SFD of molecules and on investigating diffusion in these membranes using pulsed field gradient (PFG) NMR at high field and high gradients. Fabricate sacrificial nanowires for pore formation. Membranes for separations under the conditions of SFD require parallel cylindrical channels with diameters in the range of one nanometer and relatively smooth walls. Our work during this year has focused on the fabrication of silicon nanowire (SiNW) arrays that can be used as sacrificial materials for a porous membrane. Metal-assisted chemical etching is an attractive method to fabricate SiNWs because of its simplicity, controllability, and versatility. In a typical etching process, a silicon substrate is decorated with a noble metal (e.g., Ag) and immersed into an etching solution. The redox reactions between the Si and the etching solution cause the Si underneath the metal to dissolve. Nanosphere lithography (NSL) is an effective technique to generate well-patterned metal thin-films for fabricating SiNWs with controlled diameter and spacing. SiNW arrays were fabricated by a combined approach of NSL and chemical etching, as illustrated in Figure 1. First, monodisperse SiO2 nanospheres were assembled into a close-packed hexagonal pattern on a Si substrate. The diameter of nanospheres was then reduced by RIE to form a non-close-packed hexagonal pattern. Au was deposited (15 nm) on the substrate to form a nanoporous Au thin film. Finally, chemical etching was used to obtain SiNW arrays. Although the diameter of SiNWs can be controlled by using nanospheres with different initial diameters, the smallest SiNW diameter that can be achieved by NSL and chemical etching is ~100 nm. The diameter of the fabricated SiNW arrays was reduced by thermal oxidization at 950 °C and subsequent removal of the SiO2 shell. The SEM images of the resultant SiNWs are shown in Figure 2. It is seen that the NWs kept their vertical alignment without bundling after thermal oxidization. The oxidization occurred much faster at the initial oxidization stage. For example, the diameter of 100 nm NWs decreased by 39 nm in the initial 15 min but decreased only by another 12 nm in the next 45 min of oxidization. As the oxidization proceeds, the resultant SiO2 shell forms an O2 barrier that retarded the reaction and ceased oxidation beyond 60 min. The final NW diameters were 137 and 44 nm for initial NW diameters of 200 and 95 nm, respectively.

A two-step oxidization technique was used to fabricate sub-20 nm SiNW arrays, where a second oxidization was performed following the removal of the SiO2 shell. SiNWs with an initial diameter of 95 nm were used. The results after the second oxidization are shown in Figure 3. The NWs still have uniform spacing and good vertical alignment. The average diameter of the NWs was 16 nm, as shown in the histogram. NWs with average diameters smaller than 16 nm can be obtained by using a longer duration for the 2nd oxidization (>20 min).



 


First observation of single-file diffusion for molecular mixtures. In membrane-based separations of gas mixtures, more than one gas component is expected to be present in membrane channels. Hence, studies of the feasibility of gas separations under SFD conditions requires an ability to observe and investigate single-file diffusion of molecular mixtures. During the last year of the project, C-13 PFG NMR at a high magnetic field of 17.6 T and large magnetic field gradients up to 30 T/m was applied to study diffusion of C-13 labeled CO and CH4 in L-Alanyl-L-Valine (AV) dipeptide nanotubes. Both mixed and pure gases were used in diffusion studies. AV nanotubes were selected as a model nanotube system capable of inducing SFD conditions for small gas molecules. The PFG NMR diffusion studies resulted in the first direct observation of SFD of molecular mixtures by any type of experimental technique (Figure 4). While both CO and CH4 individually exhibit SFD with different effective rates of diffusion (viz. SFD mobilities) in AV nanochannels, it is shown that both species in the mixture start exhibiting SFD with the same SFD mobility. The observation of the coincident mobility is expected for the mixture because under the SFD conditions molecules cannot pass one another in the channels. In contrast to the relationship commonly observed for normal diffusion, this mixture mobility is only slightly smaller than that of pure CO which diffuses much faster than pure CH4. This relationship between the SFD mobilities in the mixture and single-component CH4 sample can be rationalized by noticing that, under the conditions of SFD, the displacement of any molecule strongly depends on the displacements of its neighbors, resulting in highly correlated transport. In the mixture sample, high local mobility of CO molecules trapped in-between CH4 molecules affords a high mobility for these CH4 molecules in comparison to the case when CO molecules in the channels are replaced by CH4 molecules (i.e., the pure CH4 sample with the same number density). Over the next year, PFG NMR studies of SFD will be expanded to other gas mixtures including CO/CO2 mixtures to obtain a fundamental understanding of the relationship between the SFD mobility of a single-component gas and that of the same gas in a gas mixture. Such knowledge will enable design of transport-optimized gas separations under the SFD conditions.   

Ms.  Akshita Dutta, a University of Florida PhD student working on the project, has developed an expertise in the area of diffusion studies of nanoporous materials as well as in the application of high field NMR in these studies.                    Chemical Communications cover art.