Pt nanoparticles on titania nanotubes prepared by vapor-phase impregnation-decomposition method

Article abstract of DOI:10.1016/j.jallcom.2009.10.232

Platinum (Pt) nanoparticles were prepared on titania nanotubes (TNTs) by vapor-phase impregnation-decomposition method using platinum acetylacetonate as precursor. TNTs and Pt precursor were mixed in 3:1 weight ratio before vapor-phase impregnation. The mixed powders were heated at 453 K for 10 min to evaporate the precursor in a horizontal tube quartz reactor at a total pressure of 66.6 kPa. Then, the impregnated TNTs were moved to a higher temperature zone (673 K) inside the tube reactor to achieve the precursor decomposition. HAADF-STEM observations, AAS and XPS results revealed that this method allows the formation of uniformly distributed Pt nanoparticles on the surface of TNTs with a narrow distribution of particle size (2.1 nm mean size). Pt nanoparticles remain mainly as Pt⁰ oxidation state with a Pt⁰/Pt²⁺ atomic ratio of 3.9.

Full text of DOI:10.1016/j.jallcom.2009.10.232

Journal of Alloys and Compounds 495 (2010) 458–461
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Pt nanoparticles on titania nanotubes prepared by vapor-phase
impregnation–decomposition method
C. Encarnación Gómeza, J.R. Vargas Garcíaa,∗, J.A. Toledo Antoniob, M.A. Cortes Jacomeb,
C. Ángeles Chávezb
a National Polytechnic Institute, Dept. of Metallurgical Eng., México 07300 DF, AP 75-874, Mexico
b Petroleum Mexican Institute, Eje Central Lazaro Cardenas No. 152, México 07730 DF, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2008
Received in revised form 27 October 2009
Accepted 27 October 2009
Available online 4 November 2009
Platinum (Pt) nanoparticles were prepared on titania nanotubes (TNTs) by vapor-phase
impregnation–decomposition method using platinum acetylacetonate as precursor. TNTs and Pt
precursor were mixed in 3:1 weight ratio before vapor-phase impregnation. The mixed powders were
heated at 453 K for 10 min to evaporate the precursor in a horizontal tube quartz reactor at a total
pressure of 66.6 kPa. Then, the impregnated TNTs were moved to a higher temperature zone (673 K)
inside the tube reactor to achieve the precursor decomposition. HAADF-STEM observations, AAS and
XPS results revealed that this method allows the formation of uniformly distributed Pt nanoparticles
on the surface of TNTs with a narrow distribution of particle size (2.1 nm mean size). Pt nanoparticles
remain mainly as Pt0 oxidation state with a Pt0/Pt2+ atomic ratio of 3.9.
Keywords:
Titania
Nanotubes
Platinum nanoparticles
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
have proved to be successful in obtaining metallic nanoparticles on
powdered supports [15–18]. The former method allows the depo-
The interest in titania nanotubes (TNTs) has grown explosively
in the last decade because of their high technological potential
in diverse areas such as in solar energy conversion [1], hydrogen
sensing [2], lithium storage [3], photocatalysis [4], biocompatibility
[5,6] and catalysis [7,8]. Titania nanotubes or nanofibers obtained
by a hydrothermal treatment in NaOH solution exhibit specific
surface areas as large as 400 m2/g as a result of their internal
and external porosities. As a consequence, over the past 5 years,
these types of titania nanotubes have been investigated as large
surface area supports of a variety of catalytic species including
noble metals and metallic oxides [3,7,9]. For example, Pt nanopar-
ticles have been incorporated on titania nanotubes to improve
the selective oxidation of different compounds [9–12] and hydro-
genation processes [13,14]. Well-dispersed nanoparticles ranging
from 1 to 20 nm depending on their chemical nature and the
deposition conditions have been obtained on titania nanotubes.
Conventional wet impregnation has been the most frequently
employed method to incorporate the catalytic species on the tita-
nia nanotubes. However, other methods such as metal organic
chemical vapor deposition (MOCVD) or alternatively the vapor-
phase impregnation–decomposition method can be considered to
deposit the catalytic species on the titania nanotubes. Both methods
sition of the catalytic nanoparticles in only one-single step, but
a fluidized bed reactor is required. In contrast, the latter method
which includes two-step process that consist of (i) the vapor-phase
impregnation of the precursor on the support followed by (ii) the
thermal activation to decompose the precursor, does not require a
fluidized bed reactor. To the best of our knowledge, we have used
for the first time the vapor-phase impregnation–decomposition
method to incorporate Pt nanoparticles on TNTs leading to much
better characteristics than those obtained by conventional wet
impregnation.
2. Experimental
TNTs were prepared by the hydrothermal treatment in a NaOH solution previ-
ously reported [19]. TiO2 powder in anatase phase was used as the raw material for
the nanotube preparation. Platinum (Pt) nanoparticles were prepared on TNTs by
vapor-phase impregnation–decomposition method using platinum acetylacetonate
[(CH3–COCHCO–CH3)2Pt] as precursor. Before the vapor impregnation step, TNTs
and precursor powders were mechanically mixed in 3:1 weight ratio. To impreg-
nate the TNTs with the precursor vapors, the mixed powders were heated at 453 K
for 600 s inside a horizontal tube quartz reactor at a total pressure of 66.6 kPa. Then,
the impregnated TNTs were moved to a higher temperature zone (673 K) inside
the tube reactor to achieve the precursor decomposition. The crystal structure of
the Pt/TNTs was investigated by X-ray diffraction (XRD) using Cu K␣ radiation in a
Bruker D8 apparatus.
Scanning transmission electron microscopy (STEM) analysis was performed in
a JEM-2200FS microscope with an accelerating voltage of 200 kV. The microscope is
equipped with a Schottky-type field emission gun and an ultra high resolution (UHR)
configuration (Cs = 0.5 mm; Cc = 1.1 mm; point to point resolution = 0.19 nm) and in-
∗
Corresponding author.
E-mail address: rvargasga@ipn.mx (J.R. Vargas Garcia).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.10.232

C. Encarnación Gomez et al. / Journal of Alloys and Compounds 495 (2010) 458–461
459
Fig. 1. Typical XRD patterns of (a) the TNTs obtained by the hydrothermal treatment
and (b) the Pt/TNTs material.
column omega-type energy filter. The morphological images were obtained using
the high angle annular dark field (HAADF) detector, which collects electrons that
undergo Rutherford scattering. High resolution transmission electron microscopy
(HRTEM) was performed in a Titan 80-300 microscope which operates at 300 kV
of acceleration voltage and has a spherical aberration corrector in the TEM mode
which gives a point to point resolution near a 0.1 nm. The samples were ground,
suspended in isopropanol at room temperature, and dispersed with ultrasonic agi-
tation. An aliquot of the solution was then dropped onto a 3 mm in diameter lacey
carbon copper grid. Pt content in TNTs was determined by atomic absorption spec-
troscopy (AAS) with a Perkin-Elmer 2380 apparatus. Pt oxidation state and surface
chemical composition was determined by X-ray photoelectron spectroscopy (XPS).
The spectra were recorded on a THERMO-VG SCALAB 250 spectrometer equipped
with Al K␣ X-ray source (1486.6 eV) and a hemispherical analyzer.
Fig. 2. (a) TEM image of Pt/TNTs prepared by vapor-phase impregnation–
decomposition method, (b) HAADF image of Pt nanoparticles distributed on the
TNTs, and (c) particle size distribution of Pt/TNTs.
HRTEM analysis revealed that Pt nanoparticles were grown
as small crystallites with a faceted morphology such as cubic or
truncated octahedron (Fig. 3). A higher magnification of some Pt
nanoparticles is illustrated in Fig. 4. The faceting of the small Pt par-
ticles is clearly appreciated. The d-spacing of 0.191 nm, 0.191 nm
and 0.136 nm obtained by the Fourier transform (FT) well corre-
sponded to the (2 0 0), (0 2 0) and (2 2 0) planes of the fcc crystal
structure of Pt indicating a viewing direction parallel to the [0 0 1]
direction of the Pt cubic structure. Then, the Pt particles show a sur-
face face corresponding to the {2 0 0} planes with truncated edges
associated to the {2 2 0} planes, marked by black arrows in Fig. 4a.
The HRTEM image of the Pt nanoparticle shown in Fig. 4b displayed
3. Results and discussion
The Pt content determined by AAS was 14.5 wt%. Fig. 1 shows
the typical XRD patterns of (a) the TNTs support obtained by the
hydrothermal treatment and (b) the Pt/TNTs material. The angu-
lar Bragg positions of Pt fcc (JCPDS 04-0802) and TiO2 tetragonal
anatase phase (JCPDS 21-1272) are indicated as a reference. The
XRD pattern of Fig. 1a exhibits low-intensity and broad reflec-
tions clearly located at different angular positions (2ꢀ = 11.96◦,
24.72◦ and 48.28◦) from those of anatase phase, implying a dif-
ferent crystal structure which has been associated to the formation
of TNTs by various research groups [20,21]. The small angle reflec-
tion at 2ꢀ = 11.96◦, in particular, has been assigned to the distance
between the walls of the nanotubes [22]. Various reflections from
the Pt/TNTs material (Fig. 1b) correspond to those of anatase phase
suggesting a collapse of the TNTs structure resulting from the incor-
poration of Pt at 673 K. This has been previously observed during the
heat treatments of TNTs which cause a return to the anatase phase
of the raw material used for the nanotube preparation [21]. How-
ever, it should be noted that the characteristic small angle reflection
of TNTs remains even after the Pt deposition at 673 K, implying the
coexistence of the anatase phase and TNTs structures. The asym-
metric nature of the broad reflection at 2ꢀ = 39.63◦ in Fig. 1b could
be the result of the presence of Pt (1 1 1) at 2ꢀ = 39.76◦.
Transmission electron microscopy analysis performed under
HAADF-STEM mode revealed the presence of small white dots uni-
formly distributed on the titania nanotube. High density of white
dots can be observed in HAADF image of Fig. 2. In this case, the
white contrast was generated by the high intensity of the scattered
electrons collected of the heavy atoms by the high angle annular
detector where the intensity of the scattering electrons is directly
proportional to the square of atomic number of the sample. Then,
the small white dots were produced by Pt atom presence while
the gray contrast was generated for the Ti and O atoms of the nan-
otube. Statistical analysis indicated that Pt nanoparticles have a size
of about 2 nm.
Fig. 3. HRTEM image showing the crystalline arrangement of the small Pt nanopar-
ticles.

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C. Encarnación Gomez et al. / Journal of Alloys and Compounds 495 (2010) 458–461
d-spacing of 0.222 nm, 0.221 nm and 0.191 nm which well corre-
sponded to the (1 1 1), (1 −1 −1) and (0 0 2) planes, respectively,
with a viewing direction parallel to the [1 −1 0] direction. Accord-
ing with these data the Pt particle could be described as a truncated
octahedron with {1 1 1} and {0 0 2} planes as the truncated sur-
faces, marked by white and black arrows, respectively.
Fig. 5a shows XPS details of the Pt 4f signal. After subtract-
ing inelastic background, two doublets were necessary to fit the
Pt 4f signal. The binding energy (BE) values were assigned to dif-
ferent oxidation states of Pt atoms according to literature data
[23,24]. As shown in Fig. 5a, the peak at 70.5 eV was associated
to the 4f 7/2 signal of metallic Pt (Pt0), with its corresponding
4f 5/2 peak at 73.9 eV, whereas the doublet with signals 4f 7/2
at 72.3 eV and 4f 5/2 at 75.7 eV was associated to oxidized Pt2+
in PtO particles. The Pt0/Pt2+ atomic ratio of 3.9 obtained in this
study indicates that significant large amounts of Pt0 were produced
by vapor-phase impregnation–decomposition method on the sur-
face of TNTs. Comparing with a wet impregnation method recently
reported for Pt/TNTs [11] only 10% of Pt is reduced to Pt0, and about
90% of Pt remains in high oxidation Pt4+ and Pt2+ states. Then, the
preparation method explored in this study represents a simple and
high efficient growing procedure to obtain uniformly distributed Pt
nanoparticles in a reduced oxidation state on the surface of TNTs.
Two doublets were also necessary to fit the Ti 2p signal in the XPS
spectrum shown in Fig. 5b. The BE value reported for Ti4+ 2p3/2 is
458.5 eV. This peak shifts to lower energies when the valence state
of Ti4+ is reduced to Ti3+ [25]. In this case, one of the peaks 2p3/2 is
about 458.5 eV and the other is about 457.5 corresponding to Ti4+
and Ti3+, respectively. The surface atomic ratio of Ti3+/Ti4+ was 0.4
suggesting that the noble metal impregnation induce the partial
reduction of a considerable amount of Ti atoms.
Fig. 4. HRTEM image of single Pt nanoparticles. (a) Viewing direction parallel to
[0 0 1] direction and its corresponding FT and (b) viewing direction parallel to[1 −1 0]
direction and its corresponding FT. Scale bar represent 1 nm.
4. Conclusions
Pt nanoparticles were prepared on TNTs by the vapor-phase
impregnation–decomposition method. This method proved to be
successful to incorporate homogeneously distributed Pt nanopar-
ticles on the surface of TNTs with a narrow distribution of particle
size (2.1 nm mean particle size). Pt nanoparticles remain mainly as
Pt0 oxidation state with a Pt0/Pt2+ atomic ratio of 3.9.
Acknowledgments
The authors would like to acknowledge the IPN and IMP for the
financial support through the projects: IPN-SIP 20080927 and IMP-
D.00446. One of the authors (C. Encarnacion) is grateful to CONACYT
for the scholarship for graduate studies.
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