Disorder−Order Transition in Mesoscopic Silica Thin Films

Abstract

Electron microscopy has been used to study the mesoscopic (nanometer-level) and microscopic (micrometer-level) structural evolution of mesoscopic silica thin films grown at the air−water interface under dilute, acidic (pH < 2) conditions. Transmission electron microscope observations reveal that the film begins with a disordered (amorphous) structure. Over time, mesoscopically ordered regions (hexagonally packed cylindrical channels) nucleate and grow within the film. Scanning electron microscopy reveals microscopic structural features such as ribbons, protrusions, domain boundaries, microindentations, and pits. Our work shows that mesoscopic order develops within the film through a “disorder to order transition.” Our observations also clarify the role of the air−water interface in confining film growth to two dimensions during the initial stages. We note that a two-dimensional (in-plane) to three-dimensional (unconstrained) growth transition occurs when the film exceeds a critical thickness. We extend the current understanding of the structural evolution of the film by providing a detailed mechanism for the development of mesoscopic order and microscopic features and consider the possibility of a universal growth mechanism for films and particles.

Order−Disorder in Self-Assembled Mesostructured Silica Films: A Concepts Review

“…28 The process of organization involves the formation of intermediate species, as it has been shown studying the structural evolution taking place during the processing of the silica self-assembled film; the final Pm3n cubic structure, for instance, is obtained via the formation of lamellar and hexagonal intermediate structures. 29 The formation of a surfactant concentration gradient, from the air-liquid interface to the film substrate, which is larger in thicker films, is responsible for the presence of the intermediate species, 30 and this the reason why several different phases can be found in the same film. In general, in fact, the chemicalphysical phenomena during evaporation are kinetically controlled; 31 a water-surfactant richer liquid-air interface is formed during EISA, and this produces different mesophases in the films.…”

Preparation and characterization of ordered mesoporous silica membrane

“…According to the LCT mechanism it needs a mass of surfactant (about 60 times of its critical micelle concentration (cmc) [20,22]) to form MCM-41 materials. However, in our experiments the products formed while the surfactant concentration was only 1.08 times of its cmc possessed a sharp peak at 2h = 2.580°(see Fig.…”

“…Mesoporous silica membranes can be synthesized at solid-liquid and air-liquid interfaces by means of an interfacial silica-surfactant self-assembly process [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. All of these membranes were formed using acidic conditions and a mass of surfactant.…”

From template-assisted mesoporous titania powders to thin films: Differences and similarities

“…This induces a concentration gradient between the air-liquid and liquid-substrate interface [16,17]. This concentration gradient, which is larger in thick films, can lead to several differently organized phases in the same film [15,18]. Since the powder synthesis can be regarded as a very thick film, the above mentioned concentration gradient could have a more pronounced effect on the powder synthesis.…”