Formation of alloy nanoparticles by laser ablation of Au/Fe multilayer films in liquid environment

https://doi.org/10.1016/j.jcis.2016.10.023Get rights and content

Abstract

Laser ablation in liquids (LAL) emerged as a powerful technique for the synthesis of multielement nanoparticles (NPs) such as metal alloys with thermodynamically forbidden composition. Consequently, there is a great interest in expanding the current knowledge about NPs formation during LAL, in order to improve the control on product structure and to extend the range of compositions accessible by this technique. Here we performed a systematic investigation on alloy NPs formation by nanosecond LAL of Au/Fe/glass multilayers with different thickness and order of deposition. The experiments were carried out in ethanol and water, which have, respectively, favourable and unfavourable effects on alloy formation. Results were analyzed with optical absorption spectroscopy, transmission electron microscopy and Mie theory for simple and core-shell spheres. Since alloy NPs were obtained in all cases, our findings provide the evidence that the two metals are mixed during particles formation. Besides, our results suggest that the probability of interaction between ablated matter and solution species is higher for the topmost layer of the target, i.e. the one closer to the solid/liquid interface. This provides useful insight for the synthesis of nanoalloys with new compositions, that are of interest in several fields, from catalysis to photonics and nanomedicine.

Introduction

More than two decades after the pioneering report by Henglein and Fojtik [1], laser ablation in liquid (LAL) emerged as a versatile and effective approach for the synthesis of nanoparticles (NPs) with various compositions and functions [2], [3]. To date, for instance, LAL-generated NPs were used in catalytic substrates [4], [5], [6], [7], solar cells [8], linear [9] and nonlinear optics [10], [11], biomedical nanocomposites [12], immunolabeling [13], bioimaging [14], cancer therapy [15], nanostructured biomedical implants [16], sensing [17], [18], [19] and matrix assisted laser desorption ionization mass spectrometry [20]. The main advantages which motivated to the use of LAL for the synthesis of NPs are the simple experimental set-up, the environmentally friendly process, the high purity of products, and the formation of stable colloidal solutions with limited or no NPs agglomeration [2], [21], [22]. However, more recently, a growing interest was devoted also to the preparation of multielement NPs with unconventional composition, i.e. containing elements that are not thermodynamically miscible and, consequently, not accessible by traditional synthetic methods such as the wet chemistry approaches. The key advantage of LAL for the achievement of NPs with innovative composition is the fast kinetics of NP formation [2].

From this point of view, LAL provides a relevant contribution to nanoscience and nanotechnology by disclosing a whole library of new nanomaterials with unknown properties and appealing functions [2].

The synthesis by laser ablation of bimetallic and alloy NPs composed of immiscible elements is such an example. Our group previously described the synthesis with ns laser pulses of AuFe [23], [24] and AgFe [25] magneto-plasmonic alloy NPs, and showed that synthetic environment is key to control the composition and the structure of these nanoalloys, which is tuneable from iron-rich homogeneous metal spheres to core-shell metal-oxide structures [26], [27]. Interestingly, another group also reported the switch from AuFe alloy to core-shell Fe-Au NPs structure by changing the liquid environment during laser ablation with fs pulses of an equimolar AuFe alloy target [28]. Several authors also investigated a different synthetic approach based on laser irradiation of a mixture of pre-synthesized noble metal and transition metal colloids, to obtain nano or microspheres composed of the two immiscible elements [29], [30], [31]. However, for applications such as in catalysis, laser ablation of bulk targets in liquid was usually preferred, due to the possibility to obtain a larger amount of material in a one-step process and to control alloy composition by changing that of the bulk target [32], [33].

Given the interest attracted by laser ablation synthesis of bimetallic NPs, a more in deep understanding of the nanoalloy formation during LAL would be crucial to improve the control on NPs structure and to extend the range of accessible alloy compositions beyond those reported so far. Several investigation techniques have been applied to the study of LAL, with brilliant results, such as the rise, expansion and cooling of the plasma plume by emission and absorption spectroscopy [34], [35], the growth and collapse of the cavitation bubble by ultrafast shadography [36], the phase transition of bulk matter after laser heating by computer simulations [37], [38], and even NPs growth and agglomeration by small angle X-ray scattering and X-ray absorption [39], [40]. However, how alloy NPs form during LAL is still not well understood, in large part due to difficulties in the experimental study of a process that lasts few nanoseconds and involves a tiny amount of ablated material per single laser pulse [2].

In addition, the different stages of the synthesis are partially overlapping in time and space, due to the concentric geometry of LAL, where the inner hot core cannot be probed without the contribution from the colder external layer which surrounds the ablation site [2], [34], [35].

For this reason, we recently attempted a different approach where new information about the LAL-generated NPs is achieved by ex-situ investigation of products obtained by laser ablation of thin Au films [41]. These experiments provided new unexpected insights about the effect of LAL geometry on the structure of final Au NPs, showing that longitudinal and transversal confinement of the laser ablation process have direct effect on, respectively, NPs average size and polydispersity [41].

Here we extended the same approach to the synthesis of AuFe alloy NPs with LAL, in order to obtain more information about NPs formation mechanism by the comparison of final products and the pristine ablation target, which is constituted by a multilayer of alternating pure gold and pure metal iron films supported on an inert glass substrate. In this study, we investigated the effect of film thickness and deposition order, and we probed two different liquid environments (water and ethanol). The Au/Fe bimetallic system provides several advantages for the investigation of alloy formation mechanism, firstly because the two metals have different reactivity and store the trace of the interaction with surrounding chemical species in complementary ways [2], [26], [27]. In addition, the synthesis of AuFe alloy NPs by LAL and their thorough investigation by optical absorption spectroscopy and transmission electron microscopy (TEM) were already demonstrated [23], [26], [27]. On the other hand, AuFe alloy NPs are interesting for nanomedicine, photonics and catalysis, therefore the improved control on nanoalloys structure and composition is very important for the development of NPs with tailored properties for each specific application.

Section snippets

Synthesis

Au/Fe multilayers were prepared at room temperature by radiofrequency magnetron multitarget sputtering deposition of iron and gold, in pure Ar atmosphere at a working pressure of 40 · 10−4 mbar. Two different 13.56 MHz radiofrequency sources were used for iron and gold, respectively. Substrates were soda-lime glass slides cleaned according to a previously published procedure [42]. Before deposition, the soda-lime substrates were rf-biased at 20 W for 20 min, to remove possible surface contaminants

Results and discussion

Laser ablation experiments were performed on Au/Fe multilayers with different thickness and deposition order, and in different liquid environment (water or ethanol), however we adjusted in all cases the synthesis parameters (frequency of laser pulses and translation speed of the target below the fixed position of the laser beam), in order to ensure that each single pulse ablated a new region of the film not irradiated before.

In the first series of experiments, we considered Au/Fe/glass

Conclusions

In summary, we performed a systematic investigation on the synthesis of bimetallic NPs by LAL of Au/Fe multilayers with different thickness and order of deposition. The experiments were carried out in ethanol and water, which have, respectively, favourable and unfavourable effects on alloy formation. The LAL of Au/Fe multilayers evidenced that:

  • Bimetallic nanoparticles are formed, despite the targets were composed by alternated layers of the pure metals. This clearly means that mixing of matter

Acknowledgements

Financial support from University of Padova (PRAT no. CPDA114097/11 and Progetto Strategico STPD11RYPT_001) is gratefully acknowledged.

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