(600d) Amorphous-to-Crystalline Transformation of Pt Nanoparticles: Dependency On Size, Support and Adsorbates

Amorphous-to-Crystalline Transformation of Pt Nanoparticles:

Dependency on Size,
Support and Adsorbates

Long Li,¹ Lin-Lin
Wang,² Duane D. Johnson,² Zhongfan Zhang,³
Sergio I. Sanchez,⁴ Joo H. Kang,⁴ Ralph G. Nuzzo,⁴
Qi Wang,⁵ Anatoly I. Frenkel,⁵ James Ciston,^6,7
Eric A. Stach,⁵Jie Li,⁸ Judith
C. Yang¹

¹Department
of Chemical and Petroleum Engineering, Department of Physics, University of
Pittsburgh, Pittsburgh, PA 15261, USA.

²Division of Materials Science and Engineering, 311 TASF, Ames
Laboratory, Department of Materials Science and Engineering, Iowa State
University, Ames, IA 50011, USA.

³Department of Mechanical Engineering and Materials Science,
University of Pittsburgh, Pittsburgh, PA 15261, USA.

⁴Materials Research Laboratory, Department of Chemistry, University
of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.

⁵Department of Physics, Yeshiva University, New York, NY 10016,
USA.

⁶Center for Functional Nanomaterials, Brookhaven National
Laboratory, Upton, NY 11973, USA. ⁷National Center of Electron Microscopy, Lawrence
Berkeley National Laboratory, Berkeley, CA 94720, USA.

⁸ Center for Research
on Heath Care Data Center, University of Pittsburgh, Pittsburgh, PA 15261, USA.

Here we reveal an amorphous state of Pt NPs, where
its manifestation is due to the mesoscopic nature of clusters interacting with
their environment, since the amorphous state of an elemental metal is not
stable in bulk. We use complementary experimental methods, high-resolution
transmission electron microscopy (HRTEM),  aberration-corrected environmental TEM
(ETEM), and in situ X-ray absorption
spectroscopy (XAS), combined with first-principles simulations of relevant
model systems.

First-principles
calculations predicted that the energetically preferred structure of a 1.1 nm
NP in an inert atmosphere on C or g-Al₂O₃ lacks crystalline order while
adsorbates stabilize a truncated fcc structure, which
is more pronounced on C supports. In contrast to
previous theoretical reports on free-standing NPs, icasohedral
(I_h) particles are not stable on supports,
the support material stabilizes this amorphous phase, and hydrogen adsorbates
cause a crystalline fcc transition.

To confirm the theoretical
predications, we synthesized Pt/¦Ã-Al₂O₃ or Pt/C
clusters by using the incipient wetness method to impregnate Pt(NH₃)₄(OH)₂°¤H₂O
(Strem Chemicals, Inc.) onto the ¦Ã-Al₂O₃
support (Aldrich, surface area 220 m­­²/g) or carbon black  (Cabot, Vulcan XC72, surface area 250 m²/g.
The sizes of Pt particle were controlled by the
loading amount.  To ensure statistically significant results, we examined by focal-series
(FS) HRTEM over 3000 Pt NPs individually on both g-Al₂O₃ and C supports.  FS-HRTEM observations revealed that
disordered and ordered NPs co-exist in the small size range with a non-abrupt amorphous-to-ordered
transition; a narrower transition zone exists for Pt/g-Al₂O₃
(1.2 - 2.5 nm) than that of Pt/C (1.2 ¨C 5 nm). Statistical analysis was
performed to quantify the confidence of disordered measurement of the
nanoparticle.

Furthermore, to
verify experimentally the predictions of the adsorbate effect, we performed
careful structural characterization of the Pt NPs under different environmental
conditions using aberration-corrected ETEM and in situ EXAFS.  The Pt NPs on g-Al₂O₃
were exposed in the ETEM or EXAFS environmental chamber to 1 torr H₂(g) at a temperature of 385 °C and then
cooled down to room temperature. We measured the crystallinity
of tens of NPs through FS-HRTEM to determine the crystallinity fraction within
the transition zone which revealed that the H₂ treatments led to
more Pt NPs to be more ordered.  The
in situ EXAFS measurements on the
static disorder of the Pt NPs before and after H₂ anneal also showed
that the static disorder decreased with H₂ exposure.  Both the ETEM and in situ EXAFS are in agreement with the theoretical predictions of
the dramatic impact of adsorbates on nanoparticles' crystallinity,
with H stabilizing the truncated fcc structure of
ultra-small supported clusters. This data indicate the existence of several
metastable phases that depend intimately on the size, adsorbates, and support
material.

Hence, combined HRTEM
(including ETEM) and EXAFS are in excellent agreement with the theoretical
predictions, but also revealed that the NPs
structural behavior is more diverse than implied from theory using a limited
sampling of NP sizes. Complementary tools are
needed to examine the mesoscopic structural behaviors of supported catalysts,
one where a statistical, not deterministic, approach is necessary to describe
this regime. This work establishes a richer, albeit more complex, picture of
the local structures in supported metal nanoclusters, ones that well model
structural habits present in the real, technologically-relevant materials used
as heterogeneous catalysis

We
gratefully acknowledge DOE-BES funding: DE-FG02-03ER15476 and DE-AC02-98CH10886.  The structural characterizations were
performed at the CFN and NSLS at Brookhaven National Laboratory and the NFCF at
the University of Pittsburgh.