Preparation of Ultrafine Fe–Pt Alloy and Au Nanoparticle Colloids by KrF Excimer Laser Solution Photolysis
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We prepared ultrafine Fe–Pt alloy nanoparticle colloids by UV laser solution photolysis (KrF excimer laser of 248 nm wavelength) using precursors of methanol solutions into which iron and platinum complexes were dissolved together with PVP dispersant to prevent aggregations. From TEM observations, the Fe–Pt nanoparticles were found to be composed of disordered FCC A1 phase with average diameters of 0.5–3 nm regardless of the preparation conditions. Higher iron compositions of nanoparticles require irradiations of higher laser pulse energies typically more than 350 mJ, which is considered to be due to the difficulty in dissociation of Fe(III) acetylacetonate compared with Pt(II) acetylacetonate. Au colloid preparation by the same method was also attempted, resulting in Au nanoparticle colloids with over 10 times larger diameters than the Fe–Pt nanoparticles and UV–visible absorption peaks around 530 nm that originate from the surface plasmon resonance. Differences between the Fe–Pt and Au nanoparticles prepared by the KrF excimer laser solution photolysis are also discussed.
KeywordsNanoparticle Excimer laser Laser solution photolysis Precursor Fe–Pt alloy Au
Recently, nanomaterials have been researched due to their diverse application potentials. Particular attention has focused on nanoparticles of Fe–Pt alloys because, they demonstrate potentials for ultra-high density recording media  of which materials require a high magnetocrystalline anisotropy for thermal stability of magnetization, biomedical applications [2, 3] of which materials require chemical stability and biocompatibility, catalysts for fuel cells with high poisoning resistance , and other magnetic application potentials . Besides the preparation method for well-defined self assembly of Fe–Pt nanoparticles , precise deposition control of nanoparticles employing Langmuir–Blodgett method has also been reported [7, 8].
Processes for nanoparticle production by light irradiations, which are clean, one-step processes different from conventional physical or chemical ones, have been proposed [9–17]. Syntheses of gold nanoparticles by UV light irradiation to gold chloride solutions, referred to as “photolysis”, have long been known  and “laser photolysis” using UV laser light has also been applied for syntheses of gold nanoparticles [10, 11] and iron based nanoparticles from ferrocene and iron(II) acetylacetonate that are UV-absorbing complexes [12, 13]. In addition to photolysis, laser using processes under other generation principles such as “laser pyrolysis” based on thermal decomposition of gas phase sources by far-infrared laser irradiation [14, 15] and “laser ablation in liquid phase” based on laser ablation phenomena in solutions resulted in monodisperse nanoparticles of target materials submerged in solutions [16, 17], has also been reported.
In the present study, we prepared ultrafine Fe–Pt alloy nanoparticles of 0.5–3 nm diameters dispersed in methanol solvent by KrF excimer laser solution photolysis for the first time, employing precursors of methanol solutions into which iron and platinum complexes were dissolved together with polymer dispersant of polyvinylpyrrolidone, PVP. Au nanoparticles with diverse application potentials  were also prepared using the same preparation technique. The differences between the results of Au and Fe–Pt nanoparticle colloids by this method are discussed in this article.
Precursor solutions for Fe–Pt were methanol (CH3OH, Wako 99.8+% dehydrated) solutions in which iron(III) acetylacetonate (Fe(III)(C5H7O2)3, Aldrich 99.9+%), denoted by Fe(III)(acac)3, and platinum(II) acetylacetonate (Pt(II)(C5H7O2)2, Aldrich 97%), denoted by Pt(II)(acac)2were completely dissolved. Polyvinylpyrrolidone ((C6H9NO)n, Aldrich average molecular weight ~10,000), denoted by PVP, was dissolved in all cases to prevent aggregation. Concentrations for Fe(III)(acac)3, Pt(II)(acac)2, and PVP were varied while keeping constant the sum of both metal complex concentrations and PVP ones to 3 mM and 6 mM, respectively. The combination of ferrocene (Fe(II)(C5H5)2, Aldrich 98%), denoted by Fe(II)Cp2, and Pt(II)(acac)2has also been investigated while keeping constant the sum of both metal complex concentrations and PVP ones to 10 mM and 50 mM, respectively. For Au nanoparticle preparation, water solutions into which chloroauric acid (HAu(III)Cl4 · H2O, Aldrich 99.999%) and PVP were completely dissolved with concentrations of 0.5 mM and 1.0 mM, respectively. After laser irradiation, the resulting solutions were centrifuged at 3,000 rpm for 10 min for both the Fe–Pt and Au cases. In the case of Fe–Pt, irradiated solutions were dissolved into hexane for removal of decomposed and undecomposed matter.
An HF2000 (Hitachi, 200 kV) was used for transmission electron microscopic (TEM) observations and Vantage (Noran, a minimum analytical beam size of ϕ1 nm) attached to the TEM apparatus enabled energy dispersive X-ray spectroscopy (EDXS) measurement. TEM and EDXS measurements were performed on the samples of C-supported Cu grids on which colloids were dropped and allowed to dry. UV3600 (Shimadzu) and Zetasizer Nano (Malvern) were used for measurements of absorbance spectra in the UV–visible light region and dynamic light scattering (DLS) intensities as a function of Zeta potentials, respectively. Quartz or polystyrene cells (a path length of 10 mm) were used for these optical measurements.
Results and Discussion
Increase of Fe(II)Cp2 concentration in precursors did not cause increase of iron concentration in generated nanoparticles, which may be considered to be attributable to a difficulty in photolysis of Fe(II)Cp2 compared with Fe(III)(acac)3. Ouchi et al. reported the investigation results of Fe-based nanoparticle formation by ArF laser solution photolysis of Fe(II)Cp2 in hexane, including its very low quantum yield <10−3. Thus, the low iron concentration in Fe–Pt nanoparticles with Fe(II)Cp2/Pt(II)(acac)2 complex combination might be related to the reported low quantum yield of Fe(II)Cp2 solution photolysis. The harder photolysis of Fe(II)Cp2 can be also explained from the mass difference between the ligands of Cp and acac as follows: frequency of vibration ω is known to be proportional to (k/m)1/2, where k is the elastic constant and m is the reduced mass on the iron–ligand bond. Thus, ω of Fe(II)Cp2 can be estimated to be higher than that of Fe(III)(acac)3 because Fe(II)Cp2 has a Cp ligand lighter than an acac of Fe(III)(acac)3 if the same value of k is assumed. We think that the higher ω of Fe(II)Cp2 would be one of the possible reason for its harder photolysis.
Adiabatic dissociation energies of metal–ligand bonds in iron complexes including Fe(CO)5, Fe(II)Cp2 and Fe(III)(acac)3 were reported to be nearly equal to 6.0 eV from their photodissociation and thermodynamic investigation . In particular, Fe(II)Cp2 has been investigated due to its unusual photochemical behavior . The dissociation energy of nearly 6.0 eV is not sufficient for a single 248 nm photon energy of 5.0 eV, and hence two photon dissociation can be considered for these dissociations. The dissociation energy of nearly 6.0 eV is only for the cleavage of metal–ligand bonds and solvent effect such as the scavenging effect in alcohols , energies for cleaved ion reduction to zero-valent iron and the formation scheme of alloy nanoparticles are not taken into consideration. Therefore, further investigations are required to elucidate nanoparticle formation by UV laser solution photolysis of Fe and Pt complex solutions.
As mentioned in the section “Fe–Pt nanoparticles”, Fe or Fe–Pt nanoparticle formation by UV laser photolysis of iron and platinum complexes is considered to be based on multiphoton dissociation of metal complexes, which might be the reason for relatively high-laser pulse energies for generation of Fe–Pt nanoparticles. Conversely, the laser powers for Au nanoparticle formation by UV solution photolysis are relatively low compared with the metal complex case. It has been known that UV incoherent light of relatively low intensity compared with laser light is sufficient for Au nanoparticle formation in chloride solutions, which may be considered to originate from the above-mentioned difference in decomposition mechanism between the multiphoton dissociation of metal complexes and the photoreduction of gold chloride ions.
Fe–Pt and Au nanoparticles were prepared by KrF excimer laser solution photolysis. TEM observations revealed that the Fe–Pt nanoparticles are composed of FCC A1 phase and are ultrafine with diameters of 0.5–3 nm. From EDXS analyses, compositions of Fe–Pt nanoparticles are found to be mainly influenced by irradiated laser powers, which implies that Fe acetylacetonate is harder to decompose compared with Pt acetylacetonate. Although the Zeta potentials are lower than those of the Au colloids, the Fe–Pt colloids are stable for longer time periods than the case of Au colloids due to the steric hinderance of PVP. The Au nanoparticles are over 10 times larger than those of Fe–Pt nanoparticles.
We are grateful for Mr. T. Miyazaki of Tohoku University for his skilled TEM observations and EDXS analyses and Mr. K. Tamura for his assistance with sample preparations.