Bimetallic Nanoparticles - Nanotechnologies for Water Environment Applications - page 163

Fe

0

+ 2H O → Fe

2 +

+ H + 2OH

−

(Eq. 7.1)

2 2

Because of the amphoteric nature, aluminum is capable of maintaining the
solution at near neutral pH. On the other hand, as a reactive metal with a standard
reduction potential of -1.66 V (Table 7.1), aluminum, however, readily forms a layer of
aluminum oxide under neutral conditions limiting its application as a reactive medium.
Therefore, the aluminum-based bimetallic system has been developed to overcome this
drawback. Bimetallic Fe/Al particles consisting of a core metal (aluminum) and a second
metal (iron) is designed to use aluminum as an electron source that can prevent the
precipitation of iron corrosion products at the iron surface and thus maintain the surface
reactivity of iron (Lee et al., 2006). Bimetallic Cu/Al particles is designed by taking
advantage of the low electron-donating tendency of copper (+0.34 V) that can better
protect Al against undesirable side reactions (e.g., oxidation with water) when the
bimetallic structure of Cu/Al is created (Lien and Zhang, 2002).

Preparation of bimetallic Fe/Al particles is conducted under acidic conditions
under which iron is readily deposited onto the aluminum surface while bimetallic Cu/Al
particles are synthesized at alkaline pH. Detailed procedures on the synthesis of
aluminum-based bimetallic particles have been previously published (Lien and Zhang,
2002; Lee et al., 2006).

7.2.1 Bimetallic Fe/Al Particles

7.2.1.1 Characterization

Scanning electron microscopy (SEM) images of bimetallic Fe/Al particles
showing in Figure 7.1(d) illustrate bimetallic Fe/Al particles exhibited knobs deposited
at the aluminum surface. Energy dispersive x-ray spectroscopy (EDX) mapping analysis
indicated the knobs were composed of iron. It is clear that an island-like structure was
formed for bimetallic Fe/Al particles (measured Fe:Al molar ratio of 60:40). The EDX
analysis performed on the bimetallic Fe/Al particles resulted in a spectrum with peaks
for Al and Fe indicating both elements are present (Data not shown).

The X-ray diffraction (XRD) analytic results for fresh bimetallic Fe/Al particles
are shown in Figure 7.2. For comparison, the XRD pattern for CCl 4 -reacted Fe/Al
particles taken after reacting with 31.7 mg/L CCl 4 for 24 h is also included in Figure 7.2.
The characteristic peaks of aluminum appear at 38.6°, 44.9°, 65.2°, and 78.5° where the
main diffraction peak is near at the diffraction angle (2θ) of 38.6°. The peaks assigned to
iron are at the positions at 44.7° (main peak) and 65.0°. The diffraction angle of peaks
shown in Figure 7.2(a) is 38.6°, 44.8°, 65.2°, and 78.4° indicating that bimetallic Fe/Al
particles consisted of zero-valent forms of aluminum and iron.