Direct growth of ultra-long platinum nanolawns on a semiconductor photocatalyst
© Shen et al; licensee Springer. 2011
Received: 18 February 2011
Accepted: 13 May 2011
Published: 13 May 2011
A template- and surfactant-free process, thermally assisted photoreduction, is developed to prepare vertically grown ultra-long Pt nanowires (NWs) (about 30-40 nm in diameter, 5-6 μm in length, and up to 80 NWs/100 μm2 in the wire density) on TiO2 coated substrates, including Si wafers and carbon fibers, with the assistance of the photocatalytic ability and semiconductor characteristics of TiO2. A remarkable aspect ratio of up to 200 can be achieved. TEM analytical results suggest that the Pt NWs are single-crystalline with a preferred 〈111〉 growth direction. The precursor adopted and the heat treatment conditions are crucial for the yield of NWs. The photoelectrons supplied by TiO2 gives rise to the formation of nano-sized Pt nuclei from salt melt or solution. The subsequent growth of NWs is supported by the thermal electrons which also generated from TiO2 during the post thermal treatment. The interactions between the ions and the electrons in the Pt/TiO2 junction are discussed in this study.
A summary of prior works on the template-less synthesis of Pt NWs
Surfactant (S) or reductant (R)
Song et al. 
Krishnaswamy et al. 
Vitamin B2 (R)
Several tens of nm
Pt metal salt
Nadagouda et al. 
Based on this, a recently developed process, thermally assisted photoreduction (TAP), has been applied to prepare metallic NWs via the photoreduction of metallic ions on the surface of thin-film TiO2 under certain irradiating and heating conditions [16, 17]. However, it has so far not been possible to produce Pt NWs with the commonly used precursor, H2PtCl6, which was ascribed to the high charge number of Pt ions.
By extending the selection of the precursors, a modified route for the synthesis of Pt NWs is proposed in this study. In addition to Si wafers, carbon cloths are also chosen as the substrate for investigation, since nanostructured Pt-TiO2 on carbon fibers or nanotubes has been suggested as excellent electrocatalysts for direct enthanol fuel cells [18, 19]. Considering the mechanism for forming the NWs was still elusive, this study also discusses the growth of Pt NWs from the view points of the transfer of ions and electrons, as well as their interactions.
Thin film preparation
As indicated by the step 2 in Figure 1, Na2Pt(OH)6 was selected as the precursor. Fifteen microliter of 0.05 M aqueous salt solution was dropped on the TiO2 coated substrates. Afterward the samples were isothermally heated at 300°C for 3 h in air by an infrared (IR) furnace (the same heating conditions as adopted in previous studies [16, 17]), followed by a furnace-cooling to the ambient temperature (namely the post thermal treatment, step 3 in Figure 1). For comparison, the commonly used precursor, H2PtCl6, was also adopted. To clarify how the state of the precursor affects the yield of NWs, the precursor was also applied in the form of powders.
The structure and phase of the NWs were characterized using a transmission electron microscope (FEI-TEM, Philips Technai G2) with an accelerating voltage of 200 kV and also an grazing incidence X-ray diffraction meter (GIXRD, Rigaku D/MAX2500) (incidence angle of 0.5°) with graphite monochromatic Cu Kα radiation (λ = 0.15418 nm) at a scanning rate of 2° per minute from 20° to 50°. The yield (i.e. the wire number per 100 μm2) and dimensions of NWs were measured using a scanning electron microscope (SEM, JEOL JSM-6700) and Scion Image 4.0.2 image analysis software. Each data was the average of 100 observations.
Results and discussion
The state of Pt salts indeed caused a great difference in the NW yield. With respect to the solution samples at 140°C, water soon evaporized upon heating and thus Na2Pt(OH)6 re-precipitated as massive nodules. The evidence that a very small number of Pt NWs could be found in the solution samples heated at 140°C implies that dissociated Na2Pt(OH)6 in water was still capable of forming NWs, however, the reaction time was quite limited because water vaporized and depleted fast. At 160°C, Na2Pt(OH)6 in both solution and powder samples melted and transformed into NWs effectively. Accordingly, it is reasonable to infer that free ions in an electrolyte, especially the molten salt, are important for the resultant yield of Pt NWs.
Electrical properties of TiO2 obtained from the Hall measurement
Carrier concentration (cm-3)
Mobility (cm2/V s)
Resistivity (Ω cm)
1.89 × 1017
6.23 × 10-1
where k is Boltzmann's constant, h is Planck's constant, m e and m h are the effective mass of electron and hole, T is calculated in Kelvin and E g is the band gap. It can be deduced that the great standard reduction potential of Pt4+ (1.15 V) can overcome the potential barrier, V o, and gave rise to a net electron flow for further reduction of the metallic ions. In order to maintain the charge neutrality at the Pt/TiO2 interface, hydroxide ions (OH-) in the salt melt or aqueous solution were able to balance the positive charge on the TiO2 side, as illustrated in Figure 8. As a result, the thermal electrons attracted by Pt4+ were allowed to cross the interface and diffuse to the growing front of Pt nuclei. Consequently, the reduced Pt atoms accumulated and stacked on the closest packed facet to form 〈111〉-oriented one-dimensional nanostructure.
By means of the TAP process, vertically grown ultra-long Pt NWs with the remarkable aspect ratio of up to 200 can be obtained on the TiO2 coated substrate in large quantities, without the additional assistance of surfactants, templates and seeding. The Pt NWs are single-crystalline with preferred growth direction along 〈111〉. The selection and the state of the precursor are crucial for the yield of NWs. The nucleation and the growth of NWs are, respectively, supported by the photoelectrons and thermal electrons subsequently excited from TiO2. A model describing the interactions between the ions (Pt4+ and OH-) and the electrons (photo- and thermal electrons) in the Pt/TiO2 junction was proposed and can be adopted to design the template-less fabrication process of ultra-long metallic NWs for further application.
fast Fourier transform
high-resolution transmission electron microscope
thermally assisted photoreduction
transmission electron microscope
We thank the National Science Council of Taiwan for funding NSC 95-2120-M-006-003 and NCKU Project of Promoting Academic Excellence and Developing World Class Research Center: D98-R048 for support of this work. The authors would like to thank Prof. T. K. Yeh (NTHU) and Prof. T. Y. Dong (NSYSU) for their valuable suggestions.
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