Protein‐Mediated Synthesis of Nanostructured Titania with Different Polymorphs at Room Temperature

2010 WILEY-VCH Verlag Gmb Nature possesses an extraordinary and unique ability to assemble inorganic ions into intricate mineral structures (biominerals) with features on the microto nanoscale through the use of specific biomolecules. However, the majority of natural biomineralized structures are composed of calcium carbonate or silica, which are not always well suited for technological applications. Nonetheless, the molecules and mechanisms of biomineralization may provide new paradigms for the environmentally benign syntheses of non-natural inorganic materials. Recently, methods that mimic biomineralization have received a great deal of attention because of nature’s ability to produce interesting nanostructures under inherently benign conditions. Morse and coworkers demonstrated that biomolecules implicated in biosilicification could be utilized to induce the formation of TiO2. [2] Following this initial study, the use of biomolecules to direct the in-vitro syntheses of non-biological oxides is a rapidly developing field in which the synthesis of nanoscale materials is often facilitated by the presence of biomolecules. In particular, titania has become one of the most extensively studied materials because of its myriad of technological applications. The majority of the biomolecules investigated in the production of TiO2 have been derived from, or are analogues of, biomolecules believed to be associated with biosilicification. The investigated molecules include recombinant silafins, peptide R5, spermidine, and spermine. Recently, lysozymes, viral capsids, protamines, digestive enzymes, and several peptides identified through the screening of a phage-displayed peptide library were also explored for the biomimetic production of TiO2. Despite these achievements, two problems have thus far hindered the practical applications of TiO2 mediated by biomolecules; first, the tailoring of the size, shape, and polymorphs of TiO2 is less sophisticated than that of traditional methods. For example, amorphous titania as well as the partially crystallized anatase phase have been produced to date for the majority of the in-vitro mineralizations; only Kröger reported fully crystallized rutile TiO2 microspheres using recombinant silafin, however, the obtained TiO2 was a mixed phase of highly crystalline rutile microspheres and weakly crystallized anatase nanocrystals. The control of the titania polymorph is, thus, highly desirable for technological applications and remains a challenging task for protein-mediated synthetic pathways. Secondly, the resulting TiO2 often remains in intimate contact with the biomolecules, rendering them largely unsuitable for device applications that require high-purity materials. Herein, we report three enzyme-induced (glucose oxidase, catalase, and lysozyme) syntheses of titania nanocrystallites with different polymorphs; rutile, anatase, and mixed phases of monoclinic b-TiO2 and anatase are obtained by the unique combination of enzyme and titanium precursor and subsequent aging for three to four weeks. Specifically, we obtained nanostructured rutile TiO2 by adding glucose oxidase to a solution of titanium sulfate; anatase nanocrystallites were obtained through adding catalase to a solution of titanium sulfate; and mixed phases of monoclinic b-TiO2 and anatase were obtained through adding lysozyme to a solution of titanium(IV) bis-(ammonium lactato)-dihydroxide (Ti-BALDH). After adding catalase or lysozyme, the precursor solutions instantly became turbid, and precipitates were formed after only a few minutes; however, when glucose oxidase was added to the solution of titanium sulfate, the solution became yellowish but still remained clear and homogenous for several days, after which white precipitates were formed gradually and the color of the solution became lighter. In the absence of enzymes, the titanium precursor solutions remained clear and homogenous for several weeks, as did the titanium sulfate solutions containing lysozyme and the Ti-BALDH solution containing glucose oxidase or catalase. The composition and structure of the resulting TiO2 forms were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and thermogravimetric analysis (TG). The results revealed that the obtained rutile was free of most enzymes, whereas the anatase andmixed phases remained in intimate contact with a large amount of the enzymes. Considering the high purity of the resulting rutile and the increased interest in the lithium-insertion performance of nanostructured rutiles, electrochemical experiments were carried out on the resulting rutile phase. The rutile structure exhibited a high lithium storage and high rate performance when used as the anode material for lithium ion batteries. Figure 1a shows the XRD pattern of the titania precipitate mediated by glucose oxidase from a titanium sulfate solution (sample 1). It was highly surprising to find that the pattern of sample 1 matched well with the standard pattern of rutile, because it is well known that the anatase phase is usually formed first in all solution synthesis pathways; and the formation of nanocrystalline rutile has so far only been achieved at extremely acidic pH values (pH< 1) and elevated temperatures. Moreover, there are no observable diffraction peaks at 258 or 318, indicating