Nanoscale PAPER Cite this: DOI: 10.1039/c4nr06469k Received 3rd November 2014, Accepted 22nd December 2014 DOI: 10.1039/c4nr06469k www.rsc.org/nanoscale Adsorption in alumina pores open at one and at both ends† Lorenzo Bruschi, a Giampaolo Mistura,* b Phuong T. M. Nguyen, c Duong D. Do,* c David Nicholson, c Sang-Joon Park d and Woo Lee* d,e We have studied adsorption in regular, self-ordered alumina pores open at both ends or only at one end. The straight, non-connected pores have diameters ranging from 22 to 83 nm, with a relative dispersion below 1% in the pore size. Adsorption isotherms measured in open pores with a torsional microbalance show pronounced hysteresis loops characterized by nearly vertical and parallel adsorption and desorption branches. Blocking one end of the pores with glue has a strong influence on adsorption, as expected from classical macroscopic arguments. However, the experimental measurements show an unexpectedly rich phenomenology dependent on the pore size. For large pores (D p ≥ 67 nm), the isotherms for closed end pores present much narrower hysteresis loops whose adsorption and desorption boundaries envelop the desorption branches of the isotherms for the corresponding open pores of the same size. The loop for small closed end pores (D p = 22 nm) is slightly wider than that for open pores while the adsorption branches coincide. For large pores, in contrast, the desorption branches of pores with the same D p overlap regardless of the pore opening. These observations are in agreement with our grand canonical Monte Carlo (GCMC) simulations for a cylindrical pore model with constrictions, suggesting that the alumina pores could be modeled using a constricted pore model whose adsorption isotherm depends on the ratio of the constriction size to the pore size (D c /D p ). Introduction Advances in nanotechnology have allowed the fabrication of porous matrices formed from straight, unconnected pores with the characteristic size ranging from a few to a couple of hundred nanometers. 1 Examples of such materials include porous silicon, 2,3 silica SBA-15 4 and MCM-41, 5 and porous alumina. 6–8 Because of their regularity, these matrices have been exploited as templates for the realization of new func- tional materials. 8–11 They have also been widely employed to study the behavior of fluids under confinement. 12,13 On a planar open surface, an adsorbed fluid exhibits the coexistence between the adsorbate and the vapor-like phases, but the first-order boundary curve and the critical point are shifted when the fluid is confined in a pore. At a temperature T below a critical temperature which depends on the pore size, vapor condenses at a pressure P less than the saturation vapor pressure P 0 . If the liquid completely wets the pore inner walls, the condensation pressure is related to the curvature C of the meniscus formed in the pore using the macroscopic Kelvin equation: 14 ln P P 0 ¼À γ n l K B T C ð1Þ where γ is the adsorbate surface tension and n l is the liquid argon number density both evaluated at T, and K B = 1.38 × 10 -16 erg K -1 being the Boltzmann constant. The order of the transition is determined by the confining geometry. In rectangular grooves, the capillary condensation is continuous if the liquid completely wets the cap, and first- order otherwise. 15 In open cylindrical pores, Cohan 16 origi- nally proposed that the transition is first-order because of the different shape of the meniscus during adsorption (the adsor- bate is added to the pore) and desorption (the adsorbate is removed from the pore). It follows that in a closed bottom cylindrical pore, a continuous transition is expected because the meniscus nucleates at the bottom corners and will be the same in both adsorption and desorption. Classical theories, 17–20 mean field density functional theory, 21–23 simu- lations of adsorption and desorption studied by mean field † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4nr06469k a CNISM Unità di Padova, via Marzolo 8, 35131 Padova, Italy b Dipartimento di Fisica e Astronomia G. Galilei, Università di Padova, via Marzolo 8, 35131 Padova, Italy. E-mail: giampaolo.mistura@unipd.it c School of Chemical Engineering, University of Queensland, St. Lucia, QLD 4072, Australia. E-mail: d.d.do@uq.edu.au d Korea Research Institute of Standards and Science (KRISS), Yuseong, 305-340 Daejeon, Korea. E-mail: woolee@kriss.re.kr e University of Science and Technology (UST), Yuseong, 305-333 Daejeon, Korea This journal is © The Royal Society of Chemistry 2015 Nanoscale Published on 23 December 2014. Downloaded by University of Queensland on 24/01/2015 06:23:46. View Article Online View Journal