environmental applications of carbon-based nano materials

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  • 8/6/2019 Environmental Applications of Carbon-based Nano Materials

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    Critical Review

    Environmental Applications of Carbon-BasedNanomaterialsM E A G A N S . M A U T E R A N D M E N A C H E M E L I M E L E C H *

    Department of Chemical Engineering, Environmental Engineering Program,Yale University, P.O. Box 208286, New Haven, Connecticut 06520-8286

    Received March 8, 2008. Revised manuscript received May 21, 2008. AcceptedMay 23, 2008.

    Theunique andtunable propertiesofcarbon-basednanomaterials

    enable new technologies for identifying and addressing

    environmental challenges. This review critically assesses the

    contributions of carbon-based nanomaterials to a broad range

    of environmental applications: sorbents, high-flux membranes,

    depth filters, antimicrobial agents, environmental sensors,renewable energy technologies, and pollution prevention

    strategies. In linking technological advance back to the physical,

    chemical, and electronic properties of carbonaceous

    nanomaterials, this article also outlines future opportunities for

    nanomaterial application in environmental systems.

    Introduction

    Environmental researchers are actively exploring nanoma-terials as contaminants of emerging concern. Articulatingthe environmental consequences of engineered nanoma-terials will inform risk analysis and enable benign nanoma-terial design. An exclusive emphasis on implications, how-ever,mayovershadowthebroadapplicationsofnanotechnology

    toward improved environmental outcomes. The tunablephysical, chemical, and electrical properties of carbon-basednanomaterials inspire innovative solutions to persistentenvironmental challenges.

    Applied nanotechnology research in the biomedical fieldhascharted a path forparallelinnovationsin environmentalscience and engineering. Carbonaceous nanomaterials andtheir functional derivatives are exploited to optimize the fateandtransport of drugs through dense tissues (1), specificallytarget cancerous cells (2), and employ functionalized nano-tubes as synthetic transmembrane pores (3). Analogousenvironmental applications include targeted delivery ofremediation agents, engineered removal of hazardous con-taminants, and novel membrane structures for water filtration.

    The environmental applications of carbonaceous nano-materials outlined in the review are both proactive (prevent-ing environmental degradation, improving public health,optimizing energy efficiency) and retroactive (remediation,

    wastewater reuse, pollutant transformation). We begin witha brief outline of carbonaceous nanomaterials and theircorresponding properties. We then review leading applica-tions of carbon-based nanotechnologies in the fields ofsorption, high-flux membrane separation, depth filtration,pathogencontrol, environmental sensing, renewableenergyproduction, and pollution prevention.

    A suite of new technologies is made possible by recentprogress in the rational design and manipulation of nano-

    materials. Whendeveloped and applied within a sustainableframework, the unique properties of carbonaceous nano-materials enable environmental applications with limitedimplications.

    Carbon-Based Nanomaterials

    Molecular manipulation draws from a broad subset ofelements to exploit specific properties on the nanoscale.Structuredfullerenesand nanotubes of carbon, boron, MoS2,

    WS2, NbS2, TiS2, chrysotile (asbestos), kaolinite, and otherprecursors demonstrate exceptional physical, mechanical,and electronic properties compared to their bulk forms (4).Carbonsunique hybridization properties, and the sensitivityof carbons structureto perturbations in synthesisconditions,allow for tailored manipulation to a degree not yet matchedby inorganicnanostructures. Whileinorganic nanomaterialsare a promising area of future research, the science andapplication of carbonaceous nanostructures is more fullydeveloped. The remainder of this paper critically reviews arange of carbonaceous nanomaterials for their applicationin environmental systems.

    The physical, chemical, and electronic properties of car-bonaceous nanomaterials are strongly coupled to carbonsstructural conformation and, thus, its hybridizationstate (5).The ground-state orbital configuration of carbons sixelectrons is 1s2, 2s2, 2p2. The narrow energy gap between the2s and 2p electron shells facilitates the promotion of one sorbital electron to the higher energy p orbital that is emptyin the ground state. Depending on bonding relationships

    with the neighboring atoms, this promotion allows carbonto hybridize into a sp, sp2, or sp3 configuration. The energygained from covalent bonding with adjacent atoms com-pensates for the higher energy state of the electronic

    configuration. This compensation is nearly equal for the sp2

    and sp3 hybridization states after the out-of-plane bondingdue tobonds among unhybridized p orbitals is considered(6).

    These mutable hybridization states account for thediversity of organic compounds as well as the considerabledifferences among carbons bulk configurations (Figure 1).

    At high temperatures or pressures, carbon assumes thethermodynamicallyfavorable trigonometricsp3 configurationof diamond. At lower heats of formation, however, carbonadopts the planar sp2 conformation and forms monolayersheets bound by three sigma covalent bonds and a single bond. Weak out-of-plane interactions are a sum of van der

    Waals forces and the interaction between overlapping orbitals of parallel sheets. Mild shear forces, physical separa-* Corresponding author e-mail: [email protected]

    10.1021/es8006904 CCC: $40.75 2008 American Chemical Society VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5843

    Published on Web 07/12/2008

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    tion, and chemical modification disrupt these weak inter-planar forces and cause graphites planes to slip past eachother.

    On the nanoscale, graphite sheets are more thermody-namically stable as three-dimensional structures. The strainenergygenerated by curvature of theplanar graphite is com-pensated by the reduction of unfavorable dangling bonds(7). Theresult arefullerenesand nanotubesthat share manyof graphites attributes,but alsoexhibit a distinct and tunableset of properties due to quantum effects at the nanoscale,enhancedsp3 character of the bonds, quantum confinement

    of wave functions in oneor more dimensions (8), andclosedtopology.Thus, carbonaceousnanomaterials share the samebonding configurations as macroscopic carbon structures,but their properties and morphology are dominated by thestability of select resonance structures rather than the bulkaverages of their crystalline forms.

    Fullerenes. The electronic structure of fullerenes deter-mines their unique chemical, optical, and structural proper-ties. In defect-free form, fullerenes are enclosed cage-likestructures comprised of twelve 5-member rings and anunspecifiednumber of 6-member rings.Structureswithfewerhexagonsexhibitgreater sp3 bonding character, higher strainenergies, and more reactive carbon sites. Isomers withadjacent pentagons also display lower stability and relative

    abundance than isomers with isolated pentagons in whichresonance structures delocalize bonds over the fullerenestructure (9, 10). While the extent of charge delocalizationis debated, the chemical behavior of C-60 lies between anaromatic molecule and a straight chained alkene (11).

    Thisbalancebetween stability and reactivity distinguishesBuckminsterfullerene, C-60, from the degenerate C-20 ful-lerenes and inert, planar graphite. C-60 is an icosahedrallysymmetric fullerene (12) stabilized by resonance structuressupporting an equivalent electronic state and bonding geo-metry for each carbon atom (13). The relative stabilityimparted by this symmetry has redefined C-60 as a startingmaterial for chemical reactions in its own right. Covalent,supramolecular, and endohedral transformations enablemolecular manipulation and polymeric material develop-

    ment for specific environmental applications.Though C-60 is highly resistant to oxidation, up to six

    electrons can be accommodated in the lowest unoccupiedmolecular orbital (LUMO). This opens the route to covalentchemistry and the exploitation of fullerenes as structuralscaffolding for reactive adducts. Diederich and Thilgen (14)provide an excellent reviewof the extensivecovalent fullerenechemistry that has developed in the past two decades.

    Development of supramolecular pathways for fullerenemodification has also been critical for exploiting propertiesrelevantto environmental applications. Supramolecular tec-hniques, including molecular self-assembly, are formulatedupon noncovalent van der Waals, electrostatic, and hydro-phobic interactions intrinsic to fullerenes or their comple-

    mentaryreactants.Of particular relevance to biomedical and

    environmental applications are supramolecular techniquesto enhance the solubility and reduce the aggregation ofhydrophobic fullerenemolecules (15). Suchtechniqueshaverefined control over secondary architectures ranging frommonolayered films to complex three-dimensional macro-structures (16). Combinations of covalentand supramoleculartechniques have further expanded the range of tailoredstructures available (17).

    The electric and conductive properties of fullerenes andother carbonaceous nanomaterials form the basis for manyof their unique characteristics on the nanoscale. Thus,

    modification of these electric properties via single substitu-tionor endohedral doping hasgenerated significant researchattention. Single atom substitutions in a fullerene structurehave ramifications for the photosensitivity and bindingenergies of the fullerene molecule (18). With an i