average hydrogen generation rate was 275 and 159 mL minÀ1gÀ1
on Rh nanotubes and Rh nanospheres (NaBH4 reduc-
tion), respectively, demonstrating the superior catalytic
properties of the Rh nanotubes to spherical nanoparticles.
The improved activity of the Rh nanotubes might be due to
the higher surface area and mesoporous structure and poten-
tially more active surface in comparison to smooth, crystalline
structures.
In summary, we have demonstrated an effective approach to
produce hollow Rh nanotubes with uniform and perfect
structures through the galvanic replacement reaction between
Ag nanowires and aqueous RhCl3 in the saturated NaI solu-
tion at room temperature. In this replacement process, the
presence of saturated IÀ ions greatly reduces the redox poten-
tial of the Ag species from +0.80 V (Ag+/Ag pair) to À0.15 V
(AgI/Ag pair), hence Rh3+ ions can be rapidly displaced by
Ag nanowires and hollow Rh nanotubes formed. Further-
more, we believe that this rational room-temperature synthetic
route should be adapted for the preparation of other metal
nanocrystals with hollow structures and applied in many fields
such as electronics, sensing, photonics and catalysis.
This work was supported by the National Basic Research
Program of China (no. 2003CB214500, 2007CB613305 and
2009CB220003).
Fig. 4 UV-visible absorption spectra of silver nanowires and hollow
rhodium nanotubes.
Notes and references
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Fig. 5 Catalytic activities of colloidal rhodium nanocatalysts for the
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as a result of the very small imaginary part of the dielectric
constant. Recently, Xia and co-workers15 reported that
branched Rh nanocrystals displayed an attractive surface
plasmon resonance (SPR) feature and exhibited a weak and
broad peak around 380 nm. In this case, the plasmon absorp-
tion peak appearance should be related to the formation of
hollow structures and uniform walls for the Rh nanotubes.
This demonstration of optical tuning indicates that such
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Finally, the catalytic behaviours of the hollow Rh nano-
structures were explored for hydrogen generation from HCHO
solution at room temperature.16 To the best of our knowledge,
these colloid rhodium nanotubes were used, for the first time,
as catalysts for this reaction. For comparison, catalytic prop-
erties of Rh nanoparticles reduced by NaBH4 reduction have
also been investigated. The comparison in the reactivity for the
various shaped Rh nanocrystals has been shown in Fig. 5. It
can be seen that the catalytic activities were greatly dependent
on the structural evolutions of Rh nanocatalysts. Further-
more, the Rh nanotubes showed higher activity than that of
spherical nanoparticles. During 30 min reaction, the calculated
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ꢀc
This journal is The Royal Society of Chemistry 2008
6404 | Chem. Commun., 2008, 6402–6404