The group’s research has focused on two main areas (namely, nanotube chemistry and nanostructure synthesis) that will broaden the potential impact and practical applicability of nanostructures.
A. Carbon Nanotube (CNT) Functionalization: In this work, we have reacted nanotubes as if there were chemical ligands. Indeed, the protocols that we have created have significantly enhanced the ability to purify, exfoliate, process, solubilize, and even render biocompatible CNTs, thereby permitting more facile photophysical and biomedical applications of these systems. This work is summarized in a number of invited contributions including Chem. Eur. J., v.9, 1898 (2003), Adv. Mater., v.17, 17 (2005), Chem. Soc. Rev., v.21, 682 (2009), as well as Adv. Mater., v.21, 625 (2009). By providing new ways to isolate, purify, and functionalize CNTs, our research is bringing their potential for novel technological uses closer to fruition. Thus, our research accomplishments address key challenges facing the incorporation of CNTs into practical, functional devices.
Successful Demonstrations of Carbon Nanotube Functionalization:
(i). Inorganic Systems
(ii). Organic Systems
(iii). Nanotube-Nanocrystal Heterostructures
(iv). Biological Systems
B. Green Nanostructure Synthesis: We have also embarked on developing innovative syntheses of nanotubular, nanorod, and nanocube formulations of oxide (particularly, technologically significant perovskites) and fluoride materials. This work is summarized in invited contributions including Chem Commun., v. 46, 8093 (2010), Chem. Commun., 4598 (2005), and Small, v.3, 1122 (2007). Specifically, we have implemented viable environmentally friendly synthetic methodologies in the fabrication of a range of ternary/binary metal oxides, titanates, fluorides, tungstates, zirconates, and ferrites. In fact, most of our processes run under either ambient conditions or low temperatures, and can be efficiently scaled up. Moreover, our simple protocols are generally cost-effective; use mainly nontoxic precursors; limit the numbers of reagents and reaction steps; minimize waste, reagent use, and power consumption; and involve the development of high-yield processes with an absence of volatile and toxic byproducts.
In particular, we have made important advances in the use of molten-salt synthetic methods, hydrothermal protocols, and ambient template-directed techniques as green, cost-effective methodologies to generate monodisperse nanostructures with precise size and shape control without sacrificing on sample quality, purity, and crystallinity. Our as-prepared nanomaterials maintain fundamentally interesting size-dependent electronic, optical, and magnetic properties. In terms of applications, these nanostructures have wide-ranging utility in areas as diverse as catalysis, energy storage, fuel cells, biomedicine, computation, power generation, photonics, remediation, and sensing.
Successful Demonstrations of Nanoscale Metal & Metal Oxide Synthesis:
(iii). Magnetic Nanostructures
(iv). Perovskite Nanostructures
(v). Titanate Nanostructures
(vi). Binary Systems
(vii). Nanostructures for Biological Labeling
(viii). Nanomaterials for Fuel Cells
C. Near-edge X-ray absorption fine structure (NEXAFS) Studies of Nanomaterials:
We have recently used synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as a particularly useful and effective technique for simultaneously probing the surface chemistry, surface molecular orientation, degree of order, and electronic structure of carbon nanotubes and related nanomaterials. This work is summarized in an invited contribution, namely Small (Concepts Article), v.2, 26 (2006). That is, we employed NEXAFS as an exciting, complementary tool to microscopy and spectroscopy for providing localized information about single-walled carbon nanotube and multi-walled carbon nanotube (MWNT) powders, films, and arrays as well as of boron nitride nanotubes. Specifically, for SWNTs, we analyzed their structure as a function of oxygenation/oxidation (e.g. comparison of wet-air oxidized, ozonized, and pristine tubes).
In additional experiments, we compared the degree of order and alignment in nanotube powder, film, and aligned samples with those of graphite. Specifically, we analyzed the surface order of vertically-aligned single-walled and multi-walled carbon nanotube arrays of varying length and composition by means of NEXAFS. Both NEXAFS and scanning electron microscopy (SEM) studies concluded that the nanotubes in these samples were oriented vertically to the plane of the surface. However, NEXAFS polarization analysis provided for a more quantitative and nuanced description of the surface structure, indicative of far less localized surface order, an observation partially attributed to misalignment and bending of the tubes. Moreover, it was demonstrated by NEXAFS that the surface order of the arrays was imperfect and relatively independent of the height of the nanotube arrays. Furthermore, we have shown that NEXAFS can be used to correlate the extent of chemical functionalization and oxygenation with disruption of the electronic and physical structure of nanotubes embedded in array motifs.
(i). Carbon Nanotubes
(ii). Additional Systems
D. Site-Selective Chemistry Induced by Probe Microscopy Techniques.
Working on the nanometer scale requires the ability to synthesize, manipulate, and organize matter in a controllable manner as well as to predict and understand the properties of the resulting structure. One means of synthesizing material at the nanoscale is to actually create it molecule by molecule. In this regard, novel approaches are needed to understand surface reactions at a molecular scale. Scanning probe techniques offer the prospect of manipulating atoms and molecules on surfaces. In this regard, the project herein described involves using an atomic force microscopy (AFM) tip, derivatized at the end with a catalytic nanocrystal/powder, to initiate and catalyze localized nanometer-scale chemical reactions on a surface, one molecule at a time. In effect, this work takes advantage of the ultralow dimension of the functionalized tip to catalyze reactions on a single molecule level.
(a). Selective Borohydride Reduction Using Functionalized Atomic Force Microscopy Tips
A powder of a selective reducing agent, sodium triacetoxyborohydride (Na(OAc)3BH), was attached to an AFM tip and used to selectively reduce a monolayer of imines to their corresponding secondary amines within a well-defined region. Reaction completion was confirmed using surface mid-IR results and the chloranil test. Ref.: Langmuir, v.18, 5055 (2002).
(b). Current-less Photoreactivity Catalyzed by Functionalized AFM Tips
Spatially confined photocatalytic oxidation of a synthetic textile azo dye (Procion Red MX-5B) was carried out using TiO2-functionalized AFM probes on a homogeneous surface of a synthetic textile dye deposited on a glass substrate. Reaction process was confirmed through optical microscopy and AFM images and analyses as well as through surface FT-mid-IR spectroscopy studies. Ref.: Chem. Commun., (36), 4598 (2005).