6284 Chem. Commun., 2011, 47, 6284–6286 This journal is c The Royal Society of Chemistry 2011

Cite this: Chem. Commun., 2011, 47, 6284–6286

On-substrate, self-standing hollow-wall Pt and PtRu-nanotubes and their

electrocatalytic behaviorw

Robert Minch and Mohammed Es-Souni*

Received 10th March 2011, Accepted 31st March 2011

DOI: 10.1039/c1cc11398d

We report on a versatile approach for preparing on-substrate,

self-standing and hollow-wall nanotubes (NTs) of Pt and PtRu-

alloys. The method takes advantage of wet chemically processed

ZnO-NTs that are used as sacrificial templates for electrodepo-

sition. The electrocatalytic activity of these Pt-based NTs is

exemplary demonstrated on the electro-oxidation of methanol.

One-dimensional (1D) nanostructures (NSs) of noble metals

such as nanotubes and nanowires (NWs) are in the focus of

intense research and development activities owing to advantageous

high surface area and unique optical and catalytic properties.1–3

Pt and Pt-based nanomaterials occupy a place of choice

among catalysts, and, particularly, are critical in all types of

proton exchange fuel cells.4 Based on the topological features

specific to 1D Pt-NSs one should expect outstanding electro-

catalysis properties of methanol and other fuels in comparison

to usual carbon supported Pt-nanoparticles (Pt-NPs), and

indeed a better electrocatalytic activity and a higher stability

to dissolution and aggregation were reported.3,5

Basically 1D Pt-NTs were processed using two methods,

both relying on fragile alumina templates (AAO). The first one

relies on the direct electroplating of Pt into the AAO pores and

subsequent dissolving of the template.2,6–8 The second type

takes advantage of galvanic exchange reactions of primarily

prepared NWs of less noble elements, e.g. Ag, Ni or Te.5,9,10

In the present communication we propose a versatile

electrochemical preparation method for the fabrication of

large area on-substrate, self-standing Pt- and PtRu-NTs. PtRu

alloy was chosen because of its interesting electrocatalytic

properties in methanol fuel cells.11 The approach is based on

electroplating judiciously prepared ZnO-NTs, and has the

advantage that ZnO is dissolved while nucleation and growth

of Pt/PtRu proceeded. The delicate handling of AAO

templates that also involves a number of critical steps, and

the eventuality of losing the character of the nanostructure

after dissolving the AAO-template (this should ensue via the

collapsing of high aspect ratio 1DNSs), should thence be obsolete.

Fig. 1a schematically summarizes the procedure steps for

the synthesis of Pt- and/or PtRu-NTs that are fast, straight-

forward and reproducible. Scanning electron microscopy

(SEM) images of the NS arrays obtained after each step are

shown in Fig. 1b–d. The first step consists of pre-coating the

stainless steel substrate with a thin TiO2 layer which, in our

preliminary studies, had proven to be crucial for growing a

homogeneous and very dense ZnO-NR coating.12 A ZnO layer

that is generally used as a seed layer wouldn’t work in our

process, see below. Steps 2 and 3 depict wet chemical growth

of ZnO-NRs with subsequent etching to form ZnO-NTs

(Fig. 1b and c; see ESIw for details); these steps were

performed according to the process described by Chu et al.13

By varying the growth conditions it is possible to tune area

coverage and aspect ratio of ZnO-NRs; consequently Pt-based

NTs can be easily ‘‘designed’’ for specific applications.

Fig. 1 (a) Schematic representation of the different process steps

(marked as numbers in boxes) for the synthesis of Pt-based NTs using

sacrificial templates of ZnO-NTs. (b) SEM images of ZnO-NR array,

showing the high density of ZnO-NRs obtained on TiO2-terminated

steel substrate and their hexagonal character. (c) ZnO-NT array

obtained after ageing the ZnO-NRs in an aqueous NaOH solution.

(d) Pt-NT array obtained after 200 s deposition time; in the higher

magnification micrograph one sees that the Pt-NTs are composed of

self-organized Pt particles, approximately 20 nm big. The SEM images

are dual magnification micrographs. Notice that SEM images are

shown in the same sequence as that of the schematic representation

shown in (a).

Institute for Materials & Surface Technology (IMST), 24149,UAP Kiel, Germany. E-mail: me@fh-kiel.de;Fax: +49 0431-210-2660; Tel: +49 0431-210-2660w Electronic supplementary information (ESI) available: Detailedsynthesis of ZnO-NTs, electrochemical plating and tests procedures,EDX spectra and SEM image of PtRu-NTs. See DOI: 10.1039/c1cc11398d

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 6284–6286 6285

Electrodeposition of Pt-based NTs is conducted on ZnO-NTs

that were pre-coated with an ultra thin Au-film. Pt and PtRu

nucleation and growth onto Au-coated ZnO-NTs takes place

via cathodic reduction of the Pt(IV) complex, or simultaneous

reduction of Pt(IV) and Ru(III) ions, according to Reaction (1)

and (2), respectively:

[PtCl6]2� + 4e - Pt0 + 6Cl� (1)

RuCl3 + 3e - Ru0 + 3Cl� (2)

Energy dispersive spectroscopy (EDS) analyses (Fig. S1, ESIw)of Pt- and PtRu-NTs obtained via this process do not show

any of the Zn peaks, at least within the sensitivity of the

method. This indicates that ZnO was completely leached out

during electrodeposition, primarily as results of chemical

dissolution by the acidic electrolyte solution.

On close examination of the SEM-micrographs it can be

seen that Pt-NTs (Fig. 1d) consist of an assemblage of Pt-NPs

with a mean size o20 nm that had grown on top of the thin

gold film. Basically the PtRu-NTs are rather similar to

PT-NTs with the only difference that they contain some

porosity, probably arising from the different electroplating

voltage used for this alloy (Fig. S2, ESIw). The XRD patterns

recorded after the 3rd and 4th procedure steps are shown in

Fig. 2; tick marks denote reflex positions of all phases appearing:

TiO2, ZnO-NTs, Au, Pt or PtRu. Characteristic reflexes

corresponding to the wurtzite hexagonal structure with

preferred orientation along the pseudo-cubic direction [002]

at 34.41 (2y) obtained for ZnO-NTs disappeared after Pt/PtRu

electrodeposition. This supports the complete removing of the

ZnO template first obtained from EDS analysis. Using

Debye-Sherrer equation an average particle size from peak

position and full width at half maximum of 111-Pt reflex for Pt

and PtRu alloy was estimated to be ca. 10 and 8 nm,

respectively.14 This also confirms the crystalline size evaluated

from SEM images. In the case of PtRu, there are no other

reflections present than those of the face-centered cubic Pt

phase which indicates the formation of a PtRu solid solution,

i.e. incorporation of Ru atoms into the Pt lattice. This explains

the positive shift of the 111-Pt reflection in PtRu-NTs,

and indicates Pt lattice volume decrease. From the 111-Pt

reflection we have obtained values of the cubic lattice para-

meter a of 3.92 A and 3.90 A for Pt and PtRu, respectively. We

estimated the Ru content from the calculated lattice para-

meters by comparing our results to those of Bock et al., who

did a calibration of the Pt lattice parameter versus the nominal

atomic concentration of Ru in a series of PtRu alloys.15 A

good agreement for the lattice parameter of pure Pt can be

asserted, while the lattice parameter of our PtRu-alloy rather

corresponds to that of an alloy containing 17 at.% Ru.

Among the many known catalytic application possibilities

of Pt-based NSs, we explored the electrocatalytic activity of

ours toward methanol oxidation in acidic media using cyclic

voltammetry (CV). Fig. 3a shows CV curves of Pt- and

PtRu-NTs in N2 saturated 0.5 M H2SO4 aqueous solution in

the absence of methanol. The anodic limit for PtRu nano-

structured electrode was set to 0.95 V in electrochemical

Fig. 2 XRD patterns obtained after the 3rd and 4th process steps

depicted in Fig. 1a. The patterns are shifted vertically for clarity. Tick

marks denote reflection positions for (from down to the top) ZnO,

TiO2, Au and Pt. Stars represent reflections of the steel substrate.

Notice that the patterns corresponding to step 4 contain neither

reflections corresponding to ZnO nor to Ru (hexagonal), in the case

of PtRu-plating. With respect to the latter, notice the positive 2Y shift

of the Pt-reflections, denoting a lattice volume decrease.

Fig. 3 (a) Cyclic voltammograms of Pt- (solid line) and PtRu-NTs (dashed line) after 200 s deposition time at a scan rate of 50 mV s�1 in 0.5 M

H2SO4. The Pt–O reduction peak, H adsorption and desorption areas are indicated. (b) Cyclic voltammograms of Pt- (solid line) and PtRu-NTs

(dashed line) in 0.5 M H2SO4 + 0.5 M CH3OH at the same scan rate. Peak current densities for forward, f, and backward, b, scans during

methanol electro-oxidation are shown. Lines at 0.52 and 0.62 V represent onset potentials for Pt- (solid) and PtRu-NTs (dashed), respectively.

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6286 Chem. Commun., 2011, 47, 6284–6286 This journal is c The Royal Society of Chemistry 2011

investigations to avoid selective dissolution of Ru.11 In the

potential range 0 o E o 0.35 V the Pt-NTs curve in forward

and reverse scans shows large broad peaks corresponding

to H adsorption/desorption, accompanied with bisulfate

adsorption/desorption. The peak profiles observed point to

some possible structure perturbation from the Au substrate

and randomly oriented Pt crystal surfaces.16,17 In the case of

the PtRu electrode, the CV is additionally biased by adsorp-

tion of oxygen-containing species on Ru sites.18 Fig. 3b

demonstrates the electrocatalytic activity of our Pt-based

NTs towards electrooxidation of methanol. The current was

normalized by the real surface (SR) area that was determined

using Cu-UPD (copper under potential deposition) technique.19

The roughness factor, SR/Sg, where Sg is the geometrical

surface area, is approximately 40 and 25 for Pt- and

PtRu-NTs electrodes, respectively. For comparison, commercial

Pt/C 30 and PtRu/C 30 (E-TEK) catalysts have a roughness

factor of approximately 10 and 3, respectively.20,21 As can be

seen from Fig. 3a, the onset potentials of methanol oxidation

are approximately 0.62 and 0.52 V for Pt- and PtRu-NTs,

respectively. The negative shift in the case of PtRu is in

agreement with bifunctional or pseudo-bifunctional mecha-

nism of methanol oxidation on PtRu, and is associated

with adsorbed OH species, OHads, originating from water

dissociation, on Ru atoms.11,22 OHads oxidizes products of

methanol dehydrogenation (COad) to CO2. Water dissociation

and building of essential OHads required for COad oxidation

occur on pure Pt at more positive potentials than on Ru sites.

The offset potential observed for our Pt-NTs is smaller than

that of Pt/C 30 (B0.65 V), but the offset potential of

PtRu-NTs is rather similar to that of PtRu/C 30 (B0.52).20,23

The anodic peak maximum in the forward scan (jf) asso-

ciated with methanol oxidation is 0.65 mA cm�2 at 0.9 V for

Pt-NTs, and 0.50 mA cm�2 at 0.92 V for PtRu-NTs. The lower

activity of PtRu-NTs probably arises from the presence of Ru

atoms that are inactive at the surface for methanol adsorption/

dehydrogenation, consequently reducing the active surface

area of the electrode for methanol oxidation.24 The specific

electrochemical active surface area (SESA = SR/mPt) calcu-

lated for our Pt-NTs (25.6 m2/gPt) is lower than that of Pt/C30

(36–53.3 m2/gPt) catalyst, which is amenable solely to different

processing methods, but higher than that of Pt-nanobelts

(13.57) reported in our previous paper.12,20,21 Using the ratio

of the peak current density j of forward (jf) to reverse (jb)

anodic peaks, the resistance of Pt-based catalysts to poisoning

by accumulation of carbonaceous species from methanol

electrooxidation can be estimated.25 The jf/jb value obtained

for PtRu-NTs (1.4) is close to that of PtRu/C 30, but larger in

comparison to Pt-NTs (1.2) and Pt/C 30 (0.76), and indicates

that PtRu-NTs should have a better resistance to poisoning

due to the bifunctional mechanism of ethanol oxidation

mentioned above.

In summary, we have demonstrated a versatile and rather

efficient electrochemical approach to prepare large area of

on-substrate self-standing nanostructured Pt-based NTs, using

wet chemically processed ZnO-NTs as a sacrificial template.

Inherent advantages of this low cost method lies in avoiding

handling of fragile AAO templates, preserving the nano-

structured character of the electrode, the possibility to coat

large area of any substrate, including soft materials, and any

geometry. We have shown that ZnO was dissolved while

Pt-nucleation and growth proceeded, leaving Pt-base-NTs

with hollow walls. The electrocatalytic activity of these

Pt-based NTs was exemplary demonstrated on the electro-

oxidation of methanol. It is found that PtRu-NTs had better

performance in comparison to pure Pt-NTs, but lower activity

due to lesser number of Pt active sites. The method reported

should possess a high potential for the fabrication of nano-

tubes of noble metals, and their alloys, for a wide range of

catalytic applications.

Financial support of this work is provided by the European

council and the Land of Schleswig-Holstein Project #

TraFo 08139.

Notes and references

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2004, 151(8), A1269.16 P. Liu, X. Ge, R. Wang, H. Ma and Y. Ding, Langmuir, 2009, 25,

561.17 J. Solla-Gullon, F. Vidal-Iglesias, A. Lopez-Cudero, E. Garnier,

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18 F. Maillard, F. Gloaguen and J.-M. Leger, J. Appl. Electrochem.,2003, 33, 1.

19 C. L. Green and A. Kucernak, J. Phys. Chem. B, 2002, 106, 1036.20 L. Liu, E. Pippel, R. Scholz and U. Gosele,Nano Lett., 2009, 9(12),

4352.21 L. Liu, R. Scholz, E. Pippel and U. Gosele, J. Mater. Chem., 2010,

20, 5621.22 H. A. Gasteiger, N. Markovic, P. N. Ross and E. Cairns,

J. Electrochem. Soc., 1994, 141, 1795.23 C. Xu, L. Wang, X. Mu and Y. Ding, Langmuir, 2010, 26(10), 7437.24 J. M. Sieben, M. M. E. Duarte and C. E. Mayer, J. Appl.

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