(Invited) Synthesis and Visible-Plasmonic Properties of Intermetallic Nanoparticles
Authors: Teranishi Toshiharu 
Published: August 2024

The past research on inorganic nanomaterials that exhibit intrinsic localized surface plasmon resonance (LSPR) in the visible region has focused on well-disperse nanoparticles (NPs) including group 11 elements (Au, Ag, and Cu) [1]. Recent theoretical studies on photo-excited electron dynamics in Au NPs have revealed that not only the interband transition but also the screening caused by the vibrations of bound electrons by visible light are important for visible LSPR [2]. However, the correlation between the electronic and crystal structure (as a means of controlling the screening effect) remains unclear because most alloys based on group 11 elements have a face-centered cubic (fcc)-based structure. Here, we show that well-disperse spherical intermetallic Pd-based alloy NPs and Pt-based alloy NPs with non-close packed structure intrinsically exhibit LSPR absorption in the visible region.
It is well known that chemical conversion of inorganic NPs via element substitution reactions, such as ion exchange reactions [3] and galvanic replacement reactions, can overcome the difficulties associated with controlling the size, shape, chemical composition, and crystal structure in conventional syntheses [4-6]. Especially, alloying enables us to drastically modulate the electronic properties of metallic NPs. The retained shape of the parent-NPs in element replacement reactions provides an opportunity to obtain non-equilibrium unique structures and even new structures of inorganic NPs. We demonstrate that various Pd-based alloy NPs with a B2 structure were synthesized by the element replacement reactions with Pd–P NPs and showed LSPR in the visible region [7]. On the other hand, we successfully synthesized pseudospherical PtIn2 intermetallic NPs with a C1 structure as uncommon types of visible plasmonic NPs [8].
Consequently, we can extract the design principles of intermetallic NPs that exhibit LSPR in the visible region in terms of the crystal and electronic structures as follows. (i) The electronic structure of intermetallic NPs should be similar to that of group 11 element NPs, that is, the sp-band crossing the Fermi level and d-band edge of noble metal like Pt around the visible region. The deeper d-band edge of another base metal like indium less affects the red-shift of LSPR wavelength. (ii) The intermetallic NPs should have non-close packed structure to cause strong screening effect by a small number of d-electrons of noble metal. The intermetallic NPs, which satisfy the above two requirements, are expected to show LSPR in the visible region and serve as novel plasmonic photocatalysts [9].
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[7] Sato, R.; Takekuma, H.; Teranishi T. et al., to be submitted.
[8] Takekuma, H.; Sato, R.; Iida, K.; Teranishi, T. et al., Adv. Sci., in revision.
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