Materials Science and Engineering B 149 (2008) 99–104
Short communication
Fluorescence properties of Fe nanoparticles prepared by electro-explosion of wires Om Parkash Siwach, P. Sen ∗ School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India Received 6 August 2007; received in revised form 12 December 2007; accepted 15 December 2007
Abstract A novel electro-exploding wire technique has been employed to synthesize pure Fe nanoparticles. The Fe nanoparticles exist in a body-centered cubic (bcc) phase with an average size of the particles in the range ∼17 nm. A weak feature at ∼352 nm in the UV–vis absorption spectra has been established to be remnants of a surface plasmon peak, broadened due to the reduced particle size. Further, a fluorescence emission peak is observed at ∼303 nm for excitation wavelength in two different ranges, 220–250 nm and 255–280 nm. The position of the fluorescence peak remains fixed, irrespective of the excitation’s wavelength employed. The fluorescence is assigned to electronic transitions from excited states to d levels of the Fe nanoparticles. In concomitant with these, two resonant absorptions are observed at 224 nm and 270 nm, evident from the fluorescence excitation spectra which allow us to give a description of the electronic levels operating in this system. © 2008 Elsevier B.V. All rights reserved. Keywords: Fe nanoparticles; Synthesis; Optical properties
1. Introduction Metal nanoparticles [1,2] of size less than 100 nm have been a source of immense interest due to their novel properties. They differ from those of metal atoms and bulk metals [3–6]. A study of their fluorescence properties is expected to yield new insights into the energy band structure, besides their practical applications in various fields [5,6]. Fe nanoparticles are of special interest due to their possible use in magnetic recording [7], catalysis [8], as a magnetic resonance imaging (MRI) contrast agent [9], and in environment remediation [10]. This makes the synthesis and study of Fe nanoparticles an active field of research. As a result, various physical and chemical techniques have been employed to synthesize Fe nanoparticles, such as thermal and sonochemical decomposition of iron-containing complexes [11,12], reduction methods [13,14], chemical vapor condensation [15], reverse micelles [16], etc. Recently, a technique of electro-exploding wires (EEW) has been established by us [17–20] to synthesize metal nanoparticles. Fluorescence from noble metals is well known for a long time [21] and has been studied in detail, together with semiconductor
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nanoparticles [22–24]. However, studies involving Fe nanoparticles have mainly focused on aspects related to their magnetic and catalytic properties. There is hardly any literature, to the best of our knowledge, reporting fluorescence properties of Fe nanoparticles. Recently, Alqudami and Annapoorni [25] have reported fluorescence properties of Fe and polyvinylpyrrolidone (PVP) polymer-coated Fe nanoparticles dispersed in water and cyclohexane. According to these authors, for Fe and PVP-coated Fe nanoparticles dispersed in water, two fluorescence peaks are observed at ∼300 nm and ∼590 nm, respectively for excitation at 268 nm. While for the same excitation, when cyclohexane is the dispersive medium for the particles, the Fe and PVP-coated Fe nanoparticles exhibit new peaks at ∼310 nm, ∼360 nm and ∼590 nm, ∼695 nm. Amongst other reports, optical properties studied employing UV–vis absorption spectroscopy for very small (2–5 nm) particles, Basu and Chakravorty [26] and Guo et al. [2] both report absorption in the region of 300–350 nm which they assign to plasmons. In this work, we have studied in detail the excitation energy dependence of fluorescence from Fe nanoparticles. The synthesis technique involves a top-down approach producing pure metal nanoparticles, capped with an inorganic extension (like oxide, hydroxide, etc.) of the metallic lattice. X-ray diffraction (XRD) analysis of Fe nanoparticles show that the synthesized nanoparticles are in the body-centered cubic (bcc) phase.
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O.P. Siwach, P. Sen / Materials Science and Engineering B 149 (2008) 99–104
Transmission electron microscopic (TEM) analysis shows an average size of the particles ∼17 nm, which however, is an overestimate as we find these nanoparticles to be superparamagnetic from Mossbauer spectroscopy [17]. Superparamagnetism in Fe nanoparticles is reported for size less than 10 nm. A broad feature at ∼352 nm in UV–vis absorption spectroscopy is assigned to a plasmon excitation as no fluorescence emission is seen at this excitation. Fluorescence emission spectra from the Fe nanoparticles dispersed in de-ionized water (water, henceforth) at different excitation wavelengths (λex ) in the range 220–280 nm provides fluorescence emission at 303 nm for two absorption ranges (a) 220–250 nm and (b) 255–280 nm. Variation in the intensity of the fluorescence emission peak is experimentally observed as λex is tuned in the range 220–280 nm. Simultaneously, resonant absorption is observed for Fe nanoparticles at ∼224 nm and ∼270 nm, evident from the fluorescence excitation spectra. 2. Experimental details Fe nanoparticles, reported here, have been synthesized by employing a novel, physical, top-down approach of EEW. A schematic diagram of the EEW technique has been published by us earlier [20]. In the EEW technique, a wire is exploded on a plate of the same material by passing a current density ∼1010 A/m2 ; in a time ∼10−6 s. Flow of current through the wire-plate leads to a series of processes [20] culminating in explosion of the parent material. Fe nanoparticles are thus synthesized by fragmentation of the parent material, in water medium, which simultaneously caps and collects the particles. Synthesized Fe nanoparticles are free from extraneous impurities, as no chemicals have been used in the nanoparticles synthesis. Parameters like current density, material-type, wire dimension and the medium in which explosion is carried out are monitored to have control over the entire process of exploding the wire [17–20]. The advantage of this technique lies in the purity of the nanoparticles produced, small particle size and a capping layer which is an extension of the metallic lattice. A comment on Fe nanoparticle sizes, reported by authors who employed other methods of capping such as PVP coatings are in order. Nanoparticles of Fe3 O4 reported by Liu et al. [27], coated with polyvinylpyrrolidone (PVP), exist as sub5 nm particles and exhibit a tight size distribution, obtained by employing TEM. Similarly, controlled sizes down to 10 nm (as seen by TEM) have been reported by Wang et al. [28] for ␣Fe particles prepared by employing a technique which involved capping with PVP. These authors however do not report any measurement of electronic properties. We too observe a welldefined superparamagnetic behaviour as observed by Mossbauer spectroscopy [17] and measurement of the magnetization curve with zero coercivity, comparable to Liu et al. [27]. However, our TEM data shows considerably larger size for the Fe particles. But, as the electronic properties are similar, we believe that our particles, observed by TEM does not define the actual size where electron confinement takes place, but are aggregates of considerably smaller particles (where electronic confinement is active).
Fig. 1. X-ray diffraction patterns of the Fe nanoparticles.
For preparing XRD samples, the powder was extracted by centrifugation of the Fe nanoparticles suspension in water. XRDpatterns were recorded on a Bruker D8 advance diffractometer ˚ For TEM investigations using Cu K␣ radiation (λ = 1.5418 A). a small drop of the diluted suspension was put on a carboncoated copper grid. After drying the grid, TEM characterization was carried out employing a JEOL JEM-2000EX transmission electron microscope. After ultrasonication for 10 min of the Fe nanoparticles dispersed in water, UV–vis absorption spectrum was recorded, employing a Hitachi 3300 UV–vis double beam spectrophotometer. With similar samples of the Fe nanoparticles dispersed in water, fluorescence measurements were done by using a Carey Eclipse fluorescence spectrophotometer from Varian, equipped with a Xenon light source and two Czerny–Turner monochromators for excitation and emission, respectively. 3. Results and discussion In the XRD spectrum reported in Fig. 1, there are peaks at 2θ = 44.6, 65.0, and 82.4. The XRD peaks position for Fe nanoparticles matches with those from bulk Fe in bcc phase and correspond to (h k l) planes (1 1 0), (2 0 0), and (2 1 1), respectively. XRD peaks at 2θ = 65.0, 82.4 are very weak, i.e., they are suppressed. Due to the nonequilibrium nature of the synthesis process the planes of the Fe nanoparticles gets reoriented [19]. The crystallite size (d) is measured using the Scherer’s equation, employing the peak corresponding to the (1 1 0) reflection at 2θ = 44.6. The Scherer’s formula to calcu˚ B (in late d is given as d = 0.9λ/B cos θ B ; where λ is 1.5418 A, radians) is full width of the XRD peak at half maximum or B = 1/2(2θ 2 − 2θ 1 ), θ B = 1/2(θ 1 + θ 2 ); 2θ 1 and 2θ 2 are the limiting angles (in degrees) at which the diffracted X-rays intensity drop to zero, 2θ 1 = 44.45 and 2θ 2 = 44.97. The particle size as calculated using the Scherer’s equation turns out to be ∼35 nm (instrument broadening has not been taken into account). Fig. 2 shows the TEM image of the Fe nanoparticles. Most of the particles have size ≤20 nm, only few particles have size >20 nm and also, there are particles whose size is ∼10 nm. Furthermore, all the particles are spherical in shape possibly due to van der Waals clustering of smaller particles. Mossbauer spectroscopy [17] shows superparamagnetism which is known to arise from
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Fig. 4. Fluorescence emission spectra of the Fe nanoparticles dispersed in water at λex = 320 nm (squares), 350 nm (circles), 370 nm (stars), curves have been electronically shifted to enhance clarity.
Fig. 2. TEM image of the Fe nanoparticles.
particles of size
