A remarkable aspect ratio of up to can be achieved. The precursor adopted and the heat treatment conditions are crucial for the yield of NWs. The photoelectrons supplied by TiO 2 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 TiO 2 during the post thermal treatment.
Platinum has become a crucial material due to its outstanding catalystic characters in fuel-cell technology, hydrogenation reaction, three-way automobile catalytic conversion and gas sensing [ 1 — 4 ]. A great deal of effort has been devoted to the synthesis of one-dimensional Pt nanostructures, however, it still remains a grand challenge to synthesize long and oriented single-crystalline Pt nanowires NWs.
Table 1 summarizes the relevant reports concerning template-free chemical means for preparing Pt NWs. Xia et al. Moreover, using neither templates nor surfactants, HCOOH [ 10 — 13 ] and vitamin B 2 [ 14 ] have been suggested, respectively, to act as the reductant agents in the chemical routes for the synthesis of Pt NWs. It is well known that TiO 2 is an excellent photocatalyst under exposure to ultraviolet UV light. According to the Honda-Fujishima effect [ 15 ], electrons and holes on the surface of TiO 2 films can be activated by UV light, which enables the reduction of metallic ions from the solution.
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 TiO 2 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, H 2 PtCl 6 , which was ascribed to the high charge number of Pt ions.
Single Crystal Growth of Semiconductors from Metallic Solutions
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-TiO 2 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. To make a gel coating film, TiO 2 was deposited by dipping Si wafers and carbon cloths into the sol and then only the wafers were spun at rpm for 30 s. Fifteen microliter of 0. For comparison, the commonly used precursor, H 2 PtCl 6 , 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 yield i. Each data was the average of observations. No byproducts were found and thus no purificatory procedures are needed. Figure 3c displays Pt NWs grown radially on carbon fibers. The above results manifest that the selection of the precursor was crucial. The inserted electron diffraction pattern constructed by fast Fourier transform FFT also verifies this preferred growth direction.
To clarify how the state of the precursor affects the yield of NWs, Na 2 Pt OH 6 powders and aqueous solution were placed or dropped on the TiO 2 coated Si wafers, respectively, and isothermally treated at chosen temperatures for 3 h in air by an IR furnace. Similar results were observed if the aqueous solution was replaced by powders.
Histograms of the quantitative data of Pt NWs under different growth conditions : a diameter, b wire length and c wire density the yield of NWs. The XRD patterns shown in Figure 7 support the above observations. The state of Pt salts indeed caused a great difference in the NW yield.
Growth of Binary III–V Semiconductors from Metallic Solutions
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. It has been demonstrated that metallic ions can be reduced to form nano-sized nuclei on the surface of photocatalytic TiO 2 [ 22 ].
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As sketched in Figure 8 , in order to align Fermi levels for the metal E Fm and semiconductor E Fs at equilibrium, electrons move to the metal and then the positive charge due to ionized donor ions within W , the depletion region adjacent to the junction, matches the negative charge on the metal [ 25 ]. The equilibrium contact potential V o so-called built-in potential barrier, the difference in work functions between Pt and TiO 2 prevents further net electron diffusion from the semiconductor conduction band into the metal.
It is suggested that during the subsequent thermal treatment, thermal electrons generated in TiO 2 can supply the follow-up reduction of Pt ions to grow NWs from the nano-sized nuclei. The formula for calculating the number of thermal electrons n in a semiconductor as a function of temperature is shown below [ 26 ]:. By means of the TAP process, vertically grown ultra-long Pt NWs with the remarkable aspect ratio of up to can be obtained on the TiO 2 coated substrate in large quantities, without the additional assistance of surfactants, templates and seeding.
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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 TiO 2. Phys Chem Chem Phys , 8: Nano Lett , 5: Chem Rev , J Am Chem Soc , Adv Mater , Chen J, Herricks T, Xia Y: Polyol synthesis of platinum nanostructures: control of morphology through manipulation of reduction kinetics. Angew Chem Int Ed , Physik 32 , Google Scholar.
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