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C. Huang et al. / Electrochimica Acta 56 (2011) 8319–8324
strates have been used in this and previous studies, including
hydrophilic porous anodic alumina and plasma treated glass and
hydrophobic Ag substrate and carbon branches in TEM grids. How-
ever, the experimental results show that the morphology of the
nanoparticles is mainly controlled by the addition of Ni2+ ions in
the electrolyte. With Ni2+, spherical nanoparticles appear on all
these surfaces, and without Ni2+ either no particles or just hemi-
sphere nanoparticles form on the surface. The possible explanation
is that with Ni, which increases the hydrogen evolution efficiency,
the supersaturation of hydrogen molecules is close to the level at
which the homogenous nucleation can take place. Therefore, the
bubbles formed on the substrate (heterogeneous nucleation) are
close to spherical shape (homogenous nucleus).
current density of Pt or Pd is much higher than Ni, we expect the
addition of Pt or Pd ions in the electrolyte would reduce the bubble
size.
The synthesis process can be summarized as doing electrode-
position using an electrolyte for electroless deposition. In the last
three decades, many electrolytes for metal electroless deposition,
such as for Cu, Ni, Co, Ag, Pt, and Pd, have been developed. Currently,
we are applying this synthesis method to different electroless-
depositable metals, which not only provide us different kinds of
nanoparticles, but also help us to better understand this electro-
chemical reaction mechanism.
Acknowledgements
This work was supported by the National Science Foundation
(ECCS-0901849 and CMMI-1000831) and the Texas Higher Educa-
tion Coordinating Board Norman Hackerman Advanced Research
Program. We thank the Characterization Center for Materials and
Biology (CCMB) at University of Texas at Arlington for providing
financial and technical support for the electron microscopic char-
acterization.
4. Conclusions
We have confirmed a new role for electrochemically evolved
hydrogen nanobubbles, serving as both templates and reducing
agents for synthesizing hollow Au nanoparticles via a electro-
less deposition from Na3Au(SO3)2 electrolyte. The process involves
three steps: (1) electrochemical evolution of hydrogen bubbles; (2)
reduction of Au+ complex around hydrogen bubbles into Au clus-
ters; and (3) further electroless deposition of Au triggered by Au
clusters through an autocatalytic disproportionation reaction.
We have also found that the addition of Ni2+ ions in the elec-
trolyte affect the formation of the hollow Au nanoparticles. This
can be explained by the increase of the hydrogen evolution effi-
ciency from electrodeposited Ni metal. Such observation gives rise
to another way to control the bubble size. The hollow core diam-
eter in nanoparticles is an indication of the hydrogen bubble size.
Formation of hydrogen bubbles is a typical nucleation and growth
process of the second phase inside the parent phase. The nucle-
ation and growth rate, which control the bubble size, is determined
by the supersaturation of hydrogen molecules in the solution.
Monodispersed bubbles with a critical nucleus size forms at the
nucleation stage. The higher the supersaturation, the smaller the
critical nucleus size. Besides applied potential, the supersaturation
can also be controlled by the additives which can increase hydro-
gen evolution efficiency such as Ni, Pt, and Pd. Since the exchange
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