278559 Amorphous-to-Crystalline Transformation of Pt Nanoparticles: Dependency On Size, Support and Adsorbates

Wednesday, October 31, 2012
Hall B (Convention Center )
Long Li1, Lin-Lin Wang2, Duane D. Johnson2, Zhongfan Zhang3, Sergio Sanchez4, Joo H. Kang4, Ralph G. Nuzzo5, Qi Wang6, Anatoly I. Frenkel6, James Ciston7,8, Eric A. Stach9, Jie Li10 and Judith C. Yang11,12, (1)Department of Chemical and Petroleum Engineering, Uinversity of Pittsburgh, Pittsburgh, PA, (2)Materials Science, Iowa State University, Ames, IA, (3)University of Pittsburgh, Pittsburgh, PA, (4)University of Illinois at Urbana-Champaign, Urbana, IL, (5)Chemistry, university of Illinois at Urbana-Champaign, Urbana, IL, (6)Physics Department, Yeshiva University, New York, NY, (7)Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, (8)Lawrence Berkeley National Laboratory, Berkeley, CA, (9)School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN, (10)Center for Research on Health Care Data, University of Pittsburgh, Pittsburgh, PA, (11)Chemical & Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA, (12)Physics & Astronomy, University of Pittsburgh, Pittsburgh, PA

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,5 Jie 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 (Ih) 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/-Al2O3 or Pt/C clusters by using the incipient wetness method to impregnate Pt(NH3)4(OH)2H2O (Strem Chemicals, Inc.) onto the -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.

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