Template Synthesis of Nanostructured Metals Using Cellulose Nanocrystal - The Nanoscience and Technology of Renewable Biomaterials - page 317

We here focus on the preparation of homogeneously crystalline metals using CNXL
as a template and reducing agent. We also describe the preparation of metal oxides and
metal carbides using CNXL as a template and carbon precursor in an early discussion.

12.3 Metal Oxide and Metal Carbides

12.3.1 Porous Anatase

Synthesis of porous metal oxides has attracted much attention in the fields of adsorp-
tion (14), separation (15), and catalysis (16) because of their high surface area and
tunable pore sizes. Since Mobil researchers reported the synthesis of M41S family in
1992 (17), significant amount of research work in fundamental and applied areas has
been conducted. As one of the most important metal oxides, titania exhibits important
potential applications in photocatalysis (18, 19) and energy conversion (20). The titania
materials, particularly of the nanometer sizes are especially promising for their different
nonlinear and photo-induced properties (21). Considerable effort has been devoted to
the development of porous and high surface area titania materials even though there are
some difficulties in controlling the hydrolysis rate and crystallization process of titania
precursors during particle growth.
The previous approach to synthesize metal oxides with multidimensional pores to
mineralize wood cellular structures using a surfactant templated sol-gel solution suc-
cessfully copied wood cellular networks (22). The surfactant-templated sol-gel solution
was replaced to insure complete reaction and avoid precipitation. The removal of residual
organics by thermal treatment at 550 ◦ C in air produced white silica monoliths with wood
cellular structures. Microscopic images of the calcined samples mineralized revealed that
they retained their original cellular structures, which contained intact cells, cell walls and
pit structures along the cell walls, ray pitting of poplar, rectangular-type cells with fine
fibrous arrays, and bordered pits.
Water-stable Ti precursor solution is important due to CNXL being in an aque-
ous suspension. An aqueous Tyzor-LA solution (Titanium (IV) bis(ammonium lac-
tate)dihydroxide, 50%) is a cheap and stable titania precursor. Other titanium alcoxide
precursors cannot be controlled to prevent their precipitation in water. CNXL colloids
are stabilized by hydrogen bond between them. The hydrogen bonds between CNXL
colloids are broken upon adding Tyzor-LA to form Tyzor-LA-CNXL complexes, which
precipitate very slowly (23). The dried thick free-standing, transparent film is very sta-
ble after centrifugation because hydrogen bonding is stronger upon drying in air. After
calcination at 500 ◦ C, all organic contents including CNXL were burned off, leaving the
white titania material (24). Figure 12.1 shows pictures of as-prepared (Figure 12.1a)
and calcined (Figure 12.1b) Tyzor-LA-CNXL composite samples. Figure 12.1b shows
shrinked pure titania after calcination due to collapse of CNXL-templated pore struc-
tures. Figure 12.2 displays XRD patterns of as-prepared and calcined Tyzor-LA-CNXL
composites. As-prepared Tyzor-LA-CNXL composite contains four peaks at 2 θ = 14 . 50,

16.65, 22.80, and 34 . 33 ◦ , which are indicative of crystalline cellulose I (25). No other
crystalline phases corresponding to titania were found in the complexes. The titania
after calcination at 500 ◦ C in air has a typical anatase structure. There are nine crystal