Hollow porous nanoshells can be tailored to have unique physical and chemical properties for potential use in catalysis and biosensor applications. Plasma treatment can be applied to facilitate nanoshell fabrication and alter physical properties and morphology.
applications of porous nanomaterials
With recent advances in the ability to fabricate periodic nanostructures with well controlled size, shape, and composition, researchers are able to develop nanostructures with unique physical, chemical, and structural properties to serve multiple functions [1,2].
In particular, porous noble metal nanostructures have been investigated as favorable materials for multifunctional use in catalysis, plasmonics, and chemical sensors.
For example, noble metal nanoparticles (NP) assembled into a hollow shell configuration provide a large surface area and increases the number of active sites for catalytic reactions.
In addition, NPs assembled with numerous nanogaps can yield highly localized surface plasmon resonance (LSPR) response and Surface Enhanced Raman Scattering (SERS) enhancement. LSPR depends on the size, shape, and composition of nanoparticles as well as the interparticle distance and junction between NPs. Nanogaps and sharp metal protrusions concentrate LSPR response locally and give rise to more intense (stronger) electromagnetic fields (local hot spots), greatly enhancing SERS response signals for chemical sensing .
Thus, the combination of large surface area (increased number of active sites) and local electromagnetic field enhancement (hot spots) would be beneficial for both catalysis and sensors.
In this note, we explore recent studies that use plasma treatment as part of a multi-step process to fabricate hollow porous nanoshells with hybrid composition and controlled morphology to serve dual functions in catalysis and chemical sensing.
fabricating hollow porous nanoshells
In Jeong et al.’s (2021) study, the researchers developed a unique technique to synthesize hollow porous nanoshells of single noble metal (Au, Pt) or hybrid composition (Au/Pt) by employing electrostatic interactions and O2 plasma treatment to sinter (coalesce) adjoining NPs. This fabrication method was considered a simpler alternative to common dealloying techniques which use corrosive wet chemistry to selectively isolate one metal in an alloy.
To fabricate the hollow porous nanoshells, Au, Pt, or a combination of Au/Pt NPs were adsorbed by electrostatic attraction onto oppositely charged polystyrene (PS) spheres in a colloidal solution, The PS colloid served as a sacrificial template for the NP assembly. A monolayer of the NP-PS colloidal solution was then coated onto a target substrate. Finally, O2 plasma treatment was applied to etch and remove the PS colloid and promote NP sintering, with final formation of the hollow porous metal nanoshell (Figure 1).
The PDC-32G Basic Plasma Cleaner was employed for O2 plasma treatment (HIGH RF power, 10 sccm O2 gas flow rate for 25 min). Initial plasma exposure etched and shrank the PS sphere, bringing the NPs closer in contact. With further plasma treatment, the PS sphere was completely removed followed by removal of the NP’s citrate capping layer, thus exposing the metal NP surface to facilitate sintering.
Previous work by these researchers indicated that varying O2 plasma etch conditions can vary the degree of NP sintering. Transmission Electron Microscopy (TEM) imaging of fabricated nanoshells showed ligament connection between NPs, and coalescence and joining (“necking”) of the metal NPs at their contact points. It is suggested that plasma treatment facilitates sintering through local heating and atomic diffusion of metal across nanoparticle contact points to reduce total surface energy (Lee et al., 2018, Jeong et al., 2019).
The final hollow porous nanoshell comprised of an interconnected metal nanostructure with a large surface area of random porous networks. Scanning Electron Microscopy (SEM) imaging indicated nanoshells with overall size of 500-620 nm diameter, and shell thickness and ligament connections that were both nanoscale in size.
The nanoshell structure formed a stiff framework that was mechanically stable, retaining its shape and structure even when detached from the substrate, and was able to withstand mechanical scrubbing and ultrasonic agitation (Lee et al., 2018).
In addition, the surface of the hybrid metal nanoshells preserved their elemental metal composition and were not alloyed.
Figure 1.Illustration of procedure to fabricate hollow porous nanoshells with different metal composition using O2 plasma treatment. The PDC-32G Basic Plasma Cleaner was utilized at HIGH RF power and 10 sccm O2 gas flow rate for 25 min. Recreated from Jeong S, Lee S, Kim M and Kim J. Appl. Surf. Sci (2021) 543: 148831. DOI: 10.1016/j.apsusc.2020.148831
catalytic activity and sers response
To further explore the effect of the nanoshell metal composition on the catalytic activity and SERS signal response, the researchers studied the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by sodium borohydride (NaBH4) and in the presence of the nanoshells as a model catalytic reaction.
Through UV-vis absorption spectroscopy and monitoring peak intensity with reaction time, the relative 4-NTP concentration (ln C/C0) was used to quantify the catalytic reaction for the various metal nanoshells. The catalytic activity (ln C/C0) was linear with time and rated as Pt > Au/Pt > Au. The catalytic reduction occurred quickly for Pt and Au/Pt nanoshells (within 1 min) while Au nanoshells showed negligible catalytic activity even after 60 min.
In contrast, the resulting SERS spectra signal was rated as Au > Au/Pt > Pt. Hollow porous Au exhibited the strongest SERS signal but no change in the 4-NTP peak intensity or position, suggesting no compositional change (no catalytic activity).For Au/Pt, the SERS signal was weaker than that of Au, however a signal change occurred after 1 min of adding NaBH4 and a new peak emerged at 1588 cm-1 (associated with 4-ATP). Pt nanoshells showed poor SERS signal, with no peaks associated with 4-NTP.
Only the Au/Pt nanoshells displayed both strong (detectable) SERS signal as well as high catalytic activity. This was attributed to the hybrid Au/Pt composition, where Au elicited electromagnetic field enhancement (SERS enhancement) while Pt enhanced catalytic activity.
Jeong et al.’s work demonstrated the fabrication of porous metal nanostructures with both catalytic and SERS functionality. They suggest the ability to synthesize nanoshells to serve multiple functions by adjusting nanostructure size, shape, morphology, and metal composition.
Relevant Articles from Harrick Plasma Users
- Jeong S, Lee S, Kim M and Kim J. “Multifunctional hollow porous Au/Pt nanoshells for simultaneous surface-enhanced Raman scattering and catalysis”. Appl. Surf. Sci (2021) 543: 148831. DOI: 10.1016/j.apsusc.2020.148831
- Jeong S, Kim M, Jo Y, Kim N, Kang D, Lee S, Yim S, Kim B and Kim J. “Hollow Porous Gold Nanoshells with Controlled Nanojunctions for Highly Tunable Plasmon Resonances and Intense Field Enhancements for Surface-Enhanced Raman Scattering”. ACS Appl. Mater. Interfaces (2019) 11: 44458-44465. DOI: 10.1021/acsami.9b16983
- Lee DH, Park JS, Hwang JH, Kang D, Yim S-Y and Kim JH. “Fabrication of hollow nanoporous gold nanoshells with high structural tunability based on the plasma etching of polymer colloid templates”, J. Mater. Chem. C (2018) 6: 6194-6199. DOI: 10.1039/C8TC01658E
Supplemental References (Do not report using Harrick Plasma Instruments)
 Baig N, Kammakakam I, and Falathabe W. “Nanomaterials: a review of synthesis methods, properties, recent progress, and challenges. Mater. Adv. (2021) 2: 1821.
 Polarz S and Smarsly B. “Nanoporous Materials”. J. Nanosci. Nanotechnology 2 (2002) 6: 581-612.
 Willets KA and Van Duyne RP. “Localized Surface Plasmon Resonance Spectroscopy and Sensing.” Annu. Rev. Phys. Chem. (2007) 58: 267-297.