Fabrication of two-dimensionally ordered macroporous silica materials with controllable dimensions Mandakini Kanungo and Maryanne M. Collinson* Department of Chemistry, Kansas State University, 111 Willard Hall, Manhattan, KS 66506-3701 785-532-1468. E-mail: mmc@ksu.edu; Fax: 785-532-6666 Received (in West Lafayette, IN, USA) 26th September 2003, Accepted 12th December 2003 First published as an Advance Article on the web 4th February 2004 The formation of 2-D arrays of cavities of varying size and depth on an electrode surface via colloidal templating is described. Porous materials have many uses in chemistry and material science. They can be used as catalytic surfaces and supports, chromato- graphic stationary phases, adsorbents, chemical sensors, and nanosized reactors. 1–6 One promising approach to the formation of “spatially ordered” porous materials involves the use of templates. 7 For example, the M41S series of materials with channel diameters that range from ca. 20 to 100 Å and packed in a hexagonal or cubic array are prepared using surfactant liquid crystals as the template directing agents. 8–9 For larger diameter cavities, colloidal crystals can be prepared using latex spheres ranging in size from ca. 50–1000 nm as the templating agent. 10–12 Upon removal of the template, voids of a predefined size remain in the host material. Of utmost importance to many applications, particularly those in the areas of chemical sensing and catalysis, is the need to form thin films with an ordered array of cavities of controllable size that provides direct access to the underlying surface. Mesoporous silica films containing ordered channels can be prepared by spin casting or dip coating the surfactant doped sol on a suitable surface. 13–15 In most cases, however, the channels run parallel to the substrate thereby restricting its use in chemical sensor applications. 13–15 The use of colloidal particles that can be packed into a 2- or 3-D array via dip coating, spin coating, or Langmuir–Blodgett techniques appears to have the most promise in this regard. 16–22 In these investigations, multilayers have been created with the goal of developing “photonic band gap” materials. 1,18–22 A 2-D monolayer array of spheres has also been used in “nanoscale lithography” whereby metal is deposited between the spheres to create a regular array of triangular-shaped metallic structures. 1,16–17 In the present work, we describe how these colloidal crystal arrays can be used to create a closely spaced array of nanometer sized “channels” or “cavities” in a dense silica film cast on a conducting surface. Furthermore we show the size and depth of the cavities can easily be tuned by judiciously choosing the sol–gel processing conditions. The advantage of using a conducting surface to deposit these materials on is that these channels can be used as nanosized reaction vessels for electrochemical deposition, in electrochemical sensing, or catalysis applications since a small fraction of the underlying electrode is exposed. Polystyrene latex spheres (PS, 0.5 mm, 8%, sulfated, IDC) were added in a 1:1 volume ratio to a silica sol prepared by the sol–gel process. The sol was prepared by mixing tetramethoxysilane (TMOS), methanol, water, and HCl. Prior to the addition of the latex spheres, 5 mM sodium dodecylsulfate (SDS) was added to improve the wettability of the sol so that it can better coat the glassy carbon substrate used in this work. The PS-doped silica sol was then spin coated on a polished glassy carbon electrode surface at ca. 3000 rpm. After the film was dried, the latex spheres were removed from the silica film by soaking in chloroform for two-three hours. AFM images, unless otherwise noted, were acquired with a Digital Instruments Nanoscope IIIa in the contact mode at scan rate between 1 and 6 Hz. Fig. 1A shows an AFM image of PS doped silicate film on a glassy carbon substrate. The sol in this case was prepared using a mole ratio of 1:6:9:0.003 TMOS:MeOH:Water:HCl. As can be seen a two dimensional array of 500 nm diameter PS latex spheres is formed. The formation of a closely packed array of particles in the dense silica matrix is clearly much better than the randomly dispersing spheres across a surface as reported on in our lab 23–24 because it maximizes the porosity that can be obtained once the particles are removed. 25 Since glassy carbon is not atomically smooth, there are areas where there is an abrupt change in the continuality of the spheres as well as small gaps in the array. The number of defects in the arrays depends on the glassy carbon substrate due to the presence of pits and scratches on the surface. Defect-free domain sizes typically range from 25–100 mm 2 depending on smoothness of the substrate. The latex spheres can be removed from the film by soaking it in chloroform. Fig. 2A,B shows 2-D AFM images of the resultant cavities in the film. A line scan image of two cavities acquired in the tapping mode with a high aspect ratio tip is also shown in Fig. 2. The center-to-center distance between the cavities is ca. 500 nm, the top diameter is ca. 200–220 nm, and the depth of cavities are ca. 350–400 nm. It is apparent in this case that under these conditions, the latex spheres were embedded in the film so that only the top 1 4 of the surface was exposed. A large deep cavity results after template removal. The cavities are open at the top as well as the bottom as evident from the AFM images, which show a plateau, and from the fact that copper can be electrochemically deposited in the cavities via electrodeposition from a copper solution. 24 Both the depth of the cavity as well as its diameter can be easily tailored by judiciously choosing the sol–gel processing conditions. A “diluted” sol will give rise to a thinner film thus exposing a larger fraction of the embedded sphere. Fig. 3A,B shows AFM images of a 2-D arrangement of cavities embedded in a film prepared from a silica sol that has a Si:H 2 O ratio of ca. 1:100. At first glance, it may appear the cavities are further apart compared to those shown in Fig. 2. The center-to-center distance is still 500 nm, but now the Fig. 1 AFM image of 500 nm diameter latex spheres embedded in a silica film prepared by the sol–gel process (Si:H 2 O ratio of 1:9). The full gray scale image is 750 nm. This journal is © The Royal Society of Chemistry 2004 DOI: 10.1039/b311936j 548 Chem. Commun., 2004, 548–549