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Title: Templating Mesoporous Hierarchies in Silica Thin Films Using the Thermal Degradation of Cellulose Nitrate

Abstract

Materials containing a hierarchical pore structure (i.e. large pores leading to small pores) are highly desirable because they combine the advantages of high surface area with the rapid mass transport. Mesoporous SiO2 with hierarchical pore structure was prepared by a novel dual templating approach using a combination of cellulose nitrate and surfactants as the templates. Both ionic and nonionic surfactants, or mixtures of surfactants, in conjunction with cellulose nitrate were used as the pore templates. Low angle XRD patterns show well-defined pore structures and BET shows surface areas from 500 to over 800m2/g, with tunable bimodal or trimodal pore-size distributions from 18Å to 0.3µm. The hierarchical pore structure can be controlled by manipulating the template composition.

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
902671
Report Number(s):
PNNL-SA-54344
20291; 20291a; TRN: US200718%%51
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Microporous and Mesoporous Materials, 99(3):308-318; Journal Volume: 99; Journal Issue: 3
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; CELLULOSE; NITRATES; PORE STRUCTURE; SILICA; SURFACE AREA; THERMAL DEGRADATION; THIN FILMS; reverse micelles; combustion synthesis; porous silica; nanoparticles; powders; particles; composite; mechanism; Environmental Molecular Sciences Laboratory

Citation Formats

Li, Xiaohong S., Fryxell, Glen E., Wang, Chong M., and Young, James A.. Templating Mesoporous Hierarchies in Silica Thin Films Using the Thermal Degradation of Cellulose Nitrate. United States: N. p., 2007. Web. doi:10.1016/j.micromeso.2006.09.037.
Li, Xiaohong S., Fryxell, Glen E., Wang, Chong M., & Young, James A.. Templating Mesoporous Hierarchies in Silica Thin Films Using the Thermal Degradation of Cellulose Nitrate. United States. doi:10.1016/j.micromeso.2006.09.037.
Li, Xiaohong S., Fryxell, Glen E., Wang, Chong M., and Young, James A.. Sat . "Templating Mesoporous Hierarchies in Silica Thin Films Using the Thermal Degradation of Cellulose Nitrate". United States. doi:10.1016/j.micromeso.2006.09.037.
@article{osti_902671,
title = {Templating Mesoporous Hierarchies in Silica Thin Films Using the Thermal Degradation of Cellulose Nitrate},
author = {Li, Xiaohong S. and Fryxell, Glen E. and Wang, Chong M. and Young, James A.},
abstractNote = {Materials containing a hierarchical pore structure (i.e. large pores leading to small pores) are highly desirable because they combine the advantages of high surface area with the rapid mass transport. Mesoporous SiO2 with hierarchical pore structure was prepared by a novel dual templating approach using a combination of cellulose nitrate and surfactants as the templates. Both ionic and nonionic surfactants, or mixtures of surfactants, in conjunction with cellulose nitrate were used as the pore templates. Low angle XRD patterns show well-defined pore structures and BET shows surface areas from 500 to over 800m2/g, with tunable bimodal or trimodal pore-size distributions from 18Å to 0.3µm. The hierarchical pore structure can be controlled by manipulating the template composition.},
doi = {10.1016/j.micromeso.2006.09.037},
journal = {Microporous and Mesoporous Materials, 99(3):308-318},
number = 3,
volume = 99,
place = {United States},
year = {Sat Feb 10 00:00:00 EST 2007},
month = {Sat Feb 10 00:00:00 EST 2007}
}
  • Ordered mesoporous silica thin films have potential applications as, e.g., sorbents, catalysts, and analytical instruments. In addition to high internal surface areas, these applications also require high permeability, which can be obtained by engineering hierarchical porosity into the film. A novel approach uses cellulose nitrate to generate a system of larger pores (15-30 nm dia.) connected by the smaller mesopores (2-10 nm dia.) generated using traditional organic surfactants. The unimolecular reaction for cellulose nitrate deflagration avoids the usual diffusion limitations of traditional combustion synthesis. A family of hierarchically porous films was produced using a range of surfactant mixtures, and exhibitedmore » a range of surface areas along with relatively impermeable ?skins? on the film free boundaries. These skins were formed by an extrusion mechanism due to pore expansion coupled with the lateral (in-plane) symmetry constraints unique to thin films. The skin thickness depends primarily on the ratio of the large pore spacing (l0) over its diameter (d). The specific surface area (SA, m2/g) of the film increases monotonically as l0/d decreases, reaching a maximum at about l0/d{approx}0.67. Precipitous reductions in SA for smaller values of l0/d are caused by pore intersections and the associated excluded surface areas. At high pore intersection (low l0/d), the film structure evolves from ''a solid with pores'' to ''pores surrounded by a cage-like silica matrix''. The cage-like film structure is less susceptible to thin film cracking behavior, but is probably in a more metastable state susceptible to thermal or hydrothermal exposure. This metastability might be mitigated by controlling the structure of the skin through several avenues, including processing conditions (e.g., drying rates or thermal schedules) and the sol-gel chemistry.« less
  • The synthetically tailored morphologies of mesoporous silica thin films are important for applications to sensing, separations, and catalysis. Sol-gel methods using surfactant templates can produce a variety of mesoporous phases, including one composed of long cylindrical micelles aligned parallel to the substrate. This hexagonal phase is particularly useful for analysis of the thin film stresses that produce the circular-elliptical-rectangular micelle shape evolution as the film dries. A simple linear elastic model employing an effective medium concept is proposed to describe this shape evolution using a minimal set of parameters: mesopore aspect ratios are predicted from the film’s known Poisson ratiomore » and its measured thickness strain. The model may be of general utility for guiding the structural design of thin film mesoporous materials. Additional analysis reveals that the material fails when the internal stress system departs significantly from the plane stress condition. This occurs for severe gel shrinkage when the micelle shape exceeds its maximum aspect ratio, defined by conservation of volume and the geometric constraints of adjacent micelles. Localized stresses associated with micelle pressurization cause failure of the small silica ligaments between micelle ends, thus limiting the range of mesopore aspect ratios attainable for the structural tailoring of these materials.« less