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90   Principles and Methods

        atmospheric pressure. Large quantities of nanotubes can be synthesized
        by these methods; advances in catalysis and continuous growth processes
        are making SWNTs more commercially viable.
          Nanotubes were first observed in 1991 in the carbon soot of graphite
        electrodes during an arc discharge that was intended to produce
        fullerenes. Because of the high temperatures caused by the discharge in
        this process, the carbon contained in the negative electrode sublimed. The
        fullerenes appear in the soot that is formed, while the CNTs are deposited
        on the opposing electrode. Tubes produced by this method were initially
        multiwalled tubes (MWNTs). However, in 1993 Bethune et al. reported
        that with the addition of cobalt to the vaporized carbon, it was possible
        to grow single-walled nanotubes [178]. This plasma-based process is
        analogous to the more familiar electroplating process in a liquid medium.
        This method produces a mixture of components and requires further
        purification to separate the CNTs from the soot and the residual catalytic
        metals. Producing CNTs in high yield depends on the uniformity of the
        plasma arc and the temperature of the deposit forming on the carbon
        electrode.
          Higher yield and purity of SWNTs may be prepared by the use of a
        dual-pulsed laser. In 1995, Guo et al. reported that SWNTs could be
        grown through direct vaporization of a Co/Ni doped graphite rod with
        a high-powered laser in a tube furnace operating at 1200 C [179, 180].
        By this method, it was possible to grow SWNTs in a 50 percent yield
        without the formation of an amorphous carbon overcoating. Samples are
        prepared by laser vaporization of graphite rods with a catalyst of cobalt
        and nickel (50:50) at 1200 C in flowing argon, followed by heat treatment
        in a vacuum at 1000 C to remove the C and other fullerenes. The ini-
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        tial laser vaporization pulse is followed by a second pulse, to vaporize
        the target more uniformly and minimize the amount of soot deposits. The
        second laser pulse breaks up the larger particle ablated by the first
        pulse (that would result in soot formation) and feeds the products to the
        growing SWNT structure. The material produced by this method
        appears as a mat of “ropes,” 10–20 nm in diameter and up to 100 
m or
        more in length. Each rope consists of a bundle of SWNTs, aligned along
        a common axis. By varying the growth temperature, the catalyst com-
        position, and other process parameters, the average nanotube diame-
        ter and size distribution can be varied.
          Although arc-discharge and laser vaporization are currently the prin-
        cipal methods for obtaining small quantities of high-quality SWNTs,
        both methods suffer from drawbacks. The first is that they involve evap-
        orating the carbon source, making scale-up on an industrial level diffi-
        cult and energetically expensive. The second issue relates to the fact that
        vaporization methods grow SWNTs in highly tangled forms, mixed with
        unwanted forms of carbon and/or metal species. The SWNTs thus produced
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