Manipulation of mesoporous structure and crystallinity in spontaneously self-assembled hierarchical metal oxides Sikander H. Hakim, Brent H. Shanks * Dept. of Chemical and Biological Engineering, 2114 Sweeney Hall, Iowa State University, Ames, IA 50011, United States article info Article history: Received 8 October 2009 Received in revised form 25 June 2010 Accepted 29 June 2010 Available online 3 July 2010 Keywords: Hierarchical metal oxides Mesopore Macropore Hydrothermal Self-assembly abstract Spontaneous self-assembly starting from liquid metal alkoxide precursors offers a convenient template- free route to hierarchical meso/macroporous metal oxide materials with unique macroporous structure. Tailoring of the macropore morphology in these materials has been the focus of recent studies. However, the freshly prepared solids contained amorphous walls and, in many cases, poor structure at the micro/ meso scale. While being closely associated to one another, the formation of pores at different scales is governed by independent mechanisms. Examined is the extent of manipulation and control of the struc- ture at meso scale by means of hydrothermal aging in alumina, titania and zirconia materials that possess macroporous structure. The influence of this aging on the textural properties and crystalline phase com- position as well as on the previously formed macropore structure is demonstrated. The results indicate that hierarchical materials with well-defined mesopores and macropores with crystalline walls can be successfully synthesized for all three metal oxides by a selective combination of spontaneous self-assem- bly, hydrothermal aging and thermal treatment. Aging results in the formation of crystalline phases for aluminum hydroxides/oxyhydroxides and titania, while the aged zirconia materials were amorphous. Hydrothermal treatment of the titania and zirconia materials produced higher surface area and mesop- ores with larger apertures while preserving the macropore structure. However, for aluminum hydrox- ides/oxyhydroxides, there was a relatively small increase in surface area, which was also accompanied by deterioration of the macropore structure. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Metal oxides with hierarchical nanoporous structures have at- tracted attention due to their capability to offer multiple benefits from having pores at different length scales. Mesopores provide high surface area to materials, while macropores allow enhanced transport of the reactant species to the mesopore network. Such materials hold potential in a variety of applications such as heter- ogeneous catalysis, separations and storage materials. Significant effort has gone into synthesizing materials that integrate pores at different length scales in the same particle [1], with dual templat- ing commonly employed [2]. However, these methods require post-synthesis removal of the sacrificial templates by heating or extraction, leaving final oxides that are contaminated with residual organic matter. Efforts have also been directed towards template- free pathways for obtaining hierarchical materials. Examples of these include redox cycling producing mesopores in a regenerative manner [3], selective leaching of one phase from a two phase composite [4], and coupling of polymer induced phase separation with gelation [5]. Our group has reported hierarchically structured alumina mate- rials via spontaneous self-assembly in solution [6]. In this tech- nique, starting from very reactive liquid alkoxides, solid metal oxides/oxyhydroxides are instantaneously produced in excess water as a result of very rapid hydrolysis-condensation reactions. Unique structures containing a parallel array of macropores (0.5– 2 lm) with mesoporous (4–5 nm) walls were obtained. In this work, a single surfactant was used [7]. The synthesis of analogous zirconia [8] and titania [9] materials using a similar technique have also been shown. The formation of patterns in the absence of any surfactant in the latter study suggested that there is no direct role of the surfactant in the formation of the macropores. Thus, this technique offers a very convenient approach for obtaining hierar- chical materials such as alumina, titania and zirconia. To exploit these architectures in many potential applications, a better under- standing and control of the structure at the individual pore lengths is desirable. It has been demonstrated that independent mechanisms under- lie the formation of macropores and mesopores in hierarchical nanoporous materials obtained by spontaneous self-assembly [6]. 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.06.017 * Corresponding author. Tel.: +1 515 294 1895; fax: +1 515 294 2628. E-mail address: bshanks@iastate.edu (B.H. Shanks). Microporous and Mesoporous Materials 135 (2010) 105–115 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso