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J. Phys. Chem. B 2006, 110, 702-710
Synthesis and Growth Mechanism of Titanate and Titania One-Dimensional Nanostructures Self-Assembled into Hollow Micrometer-Scale Spherical Aggregates Yuanbing Mao,† Mandakini Kanungo,‡ Tirandai Hemraj-Benny,† and Stanislaus S. Wong*,†,‡ Department of Chemistry, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-3400, and Materials and Chemical Sciences Department, BrookhaVen National Laboratory, Building 480, Upton, New York 11973 ReceiVed: August 9, 2005; In Final Form: NoVember 7, 2005
Three-dimensional, dendritic micrometer-scale spheres of alkali metal hydrogen titanate 1D nanostructures (i.e., nanowires and nanotubes) have been generated using a modified hydrothermal technique in the presence of hydrogen peroxide and an alkali metal hydroxide solution. Sea-urchin-like assemblies of these 1D nanostructures have been transformed into their hydrogen titanate analogues (lepidocrocite HxTi2-x/40x/4O4 (x ∼ 0.7, 0: vacancy)) by neutralization as well as into their corresponding anatase TiO2 nanostructured counterparts through a moderate high-temperature annealing dehydration process without destroying the 3D hierarchical structural motif. The as-prepared hollow spheres of titanate and titania 1D nanostructures have overall diameters, ranging from 0.8 to 1.2 µm, while the interior of these aggregates are vacuous with a diameter range of 100 to 200 nm. The constituent, component titanate and TiO2 1D nanostructures have a diameter range of 7 ( 2 nm and lengths of up to several hundred nanometers. A proposed two-stage growth mechanism of these hollow micrometer-scale spheres was supported by time-dependent scanning electron microscopy, atomic force microscopy, and inductively coupled plasma atomic emission spectrometry data. We have also demonstrated that these assemblies are active photocatalysts for the degradation of synthetic Procion Red dye under UV light illumination.
Introduction There have been innumerable reports on the synthesis and characterization of nanotubes, nanorods, nanowires, and nanoparticles, due to their interesting size- and shape-dependent properties.1 What is now a relevant area of focus in nanoscience involves the preparation of higher-order assemblies, arrays, and superlattices of these various, individual nanostructures.2-7 In particular, in the preparation of organized assemblies of inorganic materials, bioinspired strategies, for example, have tended to rely on the use of organic ligands, additives, or templates.6,7 Generally, nonbiological synthetic strategies aimed at the synthesis of these ordered structures involve a number of different approaches. Specifically, curved architectures of CuO nanoribbons can spontaneously attach into rhombic crystal strips that can subsequently self-assemble into dandelion-like architectures with hollow interiors, while the Kirkendall effect is responsible for the formation of hollow ZnO nanorod dandelions.8,9 More generally, one assembly approach is associated with the use of various template precursors, such as droplets, spherical silica, polystyrene spheres, and block copolymer micelles. Upon synthesis, these templates are removed by separation techniques such as etching and calcination.5 In another generalized scheme, two- and three-component rodlike building blocks consisting of gold and polymer block domains can be organized into a series of single-layer superstructures * To whom correspondence should be addressed. Tel: 631-632-1703. E-mail:
[email protected]. Tel: 631-344-3178. E-mail:
[email protected]. † State University of New York at Stony Brook. ‡ Brookhaven National Laboratory.
comprised of bundles, tubes, and sheets.4 Very recently, a few reports have shown the ability to synthesize and simultaneously generate highly ordered structures in situ without the need for templates, under hydrothermal conditions.8-13 Hence, our strong inclination to generalize this idea to other important classes of oxide materials has been a major motivation for this work. One such class of materials, consisting of alkali metal titanates, are analogous to charged polyelectrolytes.14,15 In general, alkali metal titanates can be delaminated to form exfoliated nanosheets, which are useful (a) as building blocks for new materials such as nanocomposites containing bulky guest molecules such as organic polymers, polyoxocations, and biocatalytic hemoglobin16 and (b) as ion exchangers and photocatalysts.15 Another exciting class of metal oxide materials is associated with titanium oxide, a wide band gap semiconductor. These materials have been utilized as components of batteries, photocatalytic materials, pigments, cosmetics, optoelectronic devices, and gas sensors.17-20 Titanate and titania nanotubes, nanowires, and nanoparticles are particularly interesting, because their catalytic activity and their sensitivity to hydrogen, as examples of highly desirable properties, are dependent upon their inherent crystal structure, particle size, surface area, and porosity.19,21 As a result of the wide range of valuable functional properties associated with these materials, it is important to rationally control the size, morphology, and assembly of titanate and titania nanostructures and to develop strategies for their large-scale syntheses.22-24 The current work, inspired by previous studies aimed at creating continuous films of TiO2-based nanotubes,25 describes an in situ organization of either sodium or potassium hydrogen titanate one-dimensional (1D) nanostructures, measuring several hundreds of nanometers in length and up to several nanometers
10.1021/jp0544538 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/18/2005
Titanate and Titania 1D Nanostructures in diameter, into hollow micrometer-scale spherical aggregates or sea-urchin-like structures, under a variety of reaction conditions (including temperature variations).22 These hierarchical structures have been produced using a general redox strategy combined with a hydrothermal reaction involving a titanium source (e.g., either Ti foil or Ti powder), a basic NaOH or KOH solution, and an oxidizing H2O2 solution. Whereas a similar study was limited exclusively to the formation of titania nanorods on a Ti plate surface,26 by contrast, we have been able to readily generate large quantities of discrete sea-urchinlike structures not only of both titanate and titania 1D nanostructures but also in different reaction media, including in solution and on the surfaces of our Ti reagent foils and powders. In other words, we have for the first time generated threedimensional (3D) dendritic assemblies of (a) potassium hydrogen titanate, (b) sodium hydrogen titanate, (c) hydrogen titanate, and (d) anatase titania 1D nanostructures in solution and on surfaces. Furthermore, we have been able to conveniently section these 3D structures and have developed insights into their formation mechanism. Our one-pot assembly process does not involve the use of sacrificial templates to render spatial confinement which tend to yield amorphous or semicrystalline products.5 Our initially formed assemblies of alkali metal hydrogen titanate 1D nanostructures can be subsequently transformed into their analogous anatase TiO2 1D nanostructures by annealing intermediate hydrogen titanate 1D nanostructures in air. Moreover, our reported protocol for the controlled synthesis of assemblies of 1D nanostructures of titanates and anatase can be easily scaled up to achieve gram quantities of product in a simplistic manner, without loss of structure. The detailed time-dependent investigation by scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectrometry (ICP-AES), and atomic force microscopy (AFM) of the growth of these nanoscale materials demonstrates that the evolution of the initial hollow alkali metal hydrogen titanate micrometer-scale spheres involves a two-stage process. Experimental Section Preparation of Materials. The synthesis and assembly of alkali metal hydrogen titanate 1D nanostructures were performed through a one-step approach. In a typical protocol, 16.5 mL of either a 1-10 M NaOH (to synthesize sodium hydrogen titanate) or KOH (to generate potassium hydrogen titanate) solution and 1.5 mL of 50% H2O2 solution were initially mixed into a 23 mL autoclave. Thereafter, either a 1 × 1 cm2 titanium foil or a 0.5-2 mL aqueous suspension of metallic titanium powder (
