Spontaneous Shape Alteration and Size Separation of Surfactant-Free Silver Particles Synthesized by Laser Ablation in Acetone during Long-Period Storage

Laser synthesis of nanoparticles in organic solvents - products, reactions, and perspectives

Laser synthesis and processing of colloids (LSPC) is an established method for producing functional and durable nanomaterials and catalysts in virtually any liquid of choice. While the redox reactions during laser synthesis in water are fairly well understood, the corresponding reactions in organic liquids remain elusive, particularly because of the much greater complexity of carbon chemistry. To this end, this article first reviews the knowledge base of chemical reactions during LSPC and then deduces identifiable reaction pathways and mechanisms. This review also includes findings that are specific to the LSPC method variants laser ablation (LAL), fragmentation (LFL), melting (LML), and reduction (LRL) in organic liquids. A particular focus will be set on permanent gases, liquid hydrocarbons, and solid, carbonaceous species generated, including the formation of doped, compounded, and encapsulated nanoparticles. It will be shown how the choice of solvent, synthesis method, and laser parameters influence the nanostructure formation as well as the amount and chain length of the generated polyyne by-products. Finally, theoretical approaches to address the mechanisms of organic liquid decomposition and carbon shell formation are highlighted and discussed regarding current challenges and future perspectives of LSPC using organic liquids instead of water.

Solvent Influence on the Magnetization and Phase of Fe-Ni Alloy Nanoparticles Generated by Laser Ablation in Liquids

The synthesis of bimetallic iron-nickel nanoparticles with control over the synthesized phases, particle size, surface chemistry, and oxidation level remains a challenge that limits the application of these nanoparticles. Pulsed laser ablation in liquid allows the properties tuning of the generated nanoparticles by changing the ablation solvent. Organic solvents such as acetone can minimize nanoparticle oxidation. Yet, economical laboratory and technical grade solvents that allow cost-effective production of FeNi nanoparticles contain water impurities, which are a potential source of oxidation. Here, we investigated the influence of water impurities in acetone on the properties of FeNi nanoparticles generated by pulsed laser ablation in liquids. To remove water impurities and produce "dried acetone", cost-effective and reusable molecular sieves (3 Å) are employed. The results show that the Fe₅₀Ni₅₀ nanoparticles' properties are influenced by the water content of the solvent. The metastable HCP FeNi phase is found in NPs prepared in acetone, while only the FCC phase is observed in NPs formed in water. Mössbauer spectroscopy revealed that the FeNi nanoparticles oxidation in dried acetone is reduced by 8% compared to acetone. The high-field magnetization of Fe₅₀Ni₅₀ nanoparticles in water is the highest, 68 Am²/kg, followed by the nanoparticles obtained after ablation in acetone without water impurities, 59 Am²/kg, and acetone, 52 Am²/kg. The core-shell structures formed in these three liquids are also distinctive, demonstrating that a core-shell structure with an outer oxide layer is formed in water, while carbon external layers are obtained in acetone without water impurity. The results confirm that the size, structure, phase, and oxidation of FeNi nanoparticles produced by pulsed laser ablation in liquids can be modified by changing the solvent or just reducing the water impurities in the organic solvent.

Au-Ag Alloy Nanocorals with Optimal Broadband Absorption for Sunlight-Driven Thermoplasmonic Applications

Noble metal nanoparticles are efficient converters of light into heat but typically cover a limited spectral range or have intense light scattering, resulting in unsuited for broadband thermoplasmonic applications and sunlight-driven heat generation. Here, Au-Ag alloy nanoparticles were deliberately molded with an irregular nanocoral (NC) shape to obtain broadband plasmon absorption from the visible to the near-infrared yet at a lower cost compared to pure Au nanostructures. The Au-Ag NCs are produced through a green and scalable methodology that relies on pulsed laser fragmentation in a liquid, without chemicals or capping molecules, leaving the particles surface free for conjugation with thiolated molecules and enabling full processability and easy inclusion in various matrixes. Numerical calculations showed that panchromism, i.e., the occurrence of a broadband absorption from the visible to the near-infrared region, is due to the special morphology of Au-Ag alloy NCs and consists of a purely absorptive behavior superior to monometallic Au or Ag NCs. The thermoplasmonic properties were assessed by multiwavelength light-to-heat conversion experiments and exploited for the realization of a cellulose-based solar-steam generation device with low-cost, simple design but competitive performances. Overall, here it is shown how laser light can be used to harvest solar light. Besides, the optimized broadband plasmon absorption, the green synthetic procedure, and the other set of positive features for thermoplasmonic applications of Au-Ag NCs will contribute to the development of environmentally friendly devices of practical utility in a sustainable world.

Kinetically Stable Nonequilibrium Gold-Cobalt Alloy Nanoparticles with Magnetic and Plasmonic Properties Obtained by Laser Ablation in Liquid

Nonequilibrium nanoalloys are metastable solids obtained at the nanoscale under nonequilibrium conditions that allow the study of kinetically frozen atoms and the discovery of new physical and chemical properties. However, the stabilization of metastable phases in the nanometric size regime is challenging and the synthetic route should be easy and sustainable, for the nonequilibrium nanoalloys to be practically available. Here we report on the one-step laser ablation synthesis in solution (LASiS) of nonequilibrium Au-Co alloy nanoparticles (NPs) and their characterization on ensembles and at the single nanoparticle level. The NPs are obtained as a polycrystalline solid solution stable in air and water, although surface cobalt atoms undergo oxidation to Co(II). Since gold is a renowned plasmonic material and metallic cobalt is ferromagnetic at room temperature, these properties are both found in the NPs. Besides, surface conjugation with thiolated molecules is possible and it was exploited to obtain colloidally stable solutions in water. Taking advantage of these features, an array of magnetic-plasmonic dots was obtained and used for surface-enhanced Raman scattering experiments. Overall, this study confirms that LASiS is an effective method for the formation of kinetically stable nonequilibrium nanoalloys and shows that Au-Co alloy NPs are appealing magnetically responsive plasmonic building blocks for several nanotechnological applications.

Effect of Pd Ions on the Generation of Ag and Au Heterogeneous Nanoparticles Using Laser Ablation in Liquid

Heterogeneous Ag/Au nanoparticles combined with Pd ions were generated by irradiating Ag/Au metal targets in a Pd solution with nanosecond and femtosecond lasers. AgPd and AuPd nanoparticles were generated by laser fragmentation and bonded. We numerically analyzed the hot spots with electromagnetic field enhancement of nanoparticles of different sizes separated by various distances. AgPd and AuPd nanoparticles differing in diameter were generated and showed different characteristics compared to typical core-shell heterogeneous nanoparticles. Pd ions played an important role in the generation of nanoparticles in liquid via laser ablation. The femtosecond laser produced both pure and heterogeneous nanoparticles of uniform size. The nanosecond laser produced pure nanoparticles with a relatively non-uniform size, which developed into spherical heterogeneous nanoparticles with a uniform (small) size in the presence of Pd ions. These nanoparticles could optimize applications such as photothermal therapy and catalysis.

Multiscale Hierarchical Micro/Nanostructures Created by Femtosecond Laser Ablation in Liquids for Polarization-Dependent Broadband Antireflection

In this work, we present the possibility of producing multiscale hierarchical micro/nanostructures by the femtosecond laser ablation of transition metals (i.e., Ta and W) in water and investigate their polarization-dependent reflectance. The hierarchical micro/nanostructures are composed of microscale-grooved, mountain-like and pit-rich structures decorated with hybrid laser-induced periodic surface structures (LIPSSs). The hybrid LIPSSs consist of low/high and ultrahigh spatial frequency LIPSSs (LSFLs/HSFLs and UHSFLs). LSFLs/HSFLs of 400–600 nm in a period are typically oriented perpendicular to the direction of the laser polarization, while UHSFLs (widths: 10–20 nm and periods: 30–50 nm) are oriented perpendicular to the curvatures of LSFLs/HSFLs. On the microstructures with height gradients, the orientations of LSFLs/HSFLs are misaligned by 18°. On the ablated W metasurface, two kinds of UHSFLs are observed. UHSFLs become parallel nanowires in the deep troughs of LSFLs/HSFLs but result in being very chaotic in shallow LSFLs, turning into polygonal nanonetworks. In contrast, chaotic USFLs are not found on the ablated Ta metasurfaces. With the help of Fourier transform infrared spectroscopy, it is found that microgrooves show an obvious polarization-dependent reflectance at wavelengths of 15 and 17.5 μm associated with the direction of the groove, and the integration of microstructures with LSFs/HSFLs/UHSFLs is thus beneficial for enhancing the light absorbance and light trapping in the near-to-mid-infrared (NIR-MIR) range.

The Potential of Silver Nanoparticles for Antiviral and Antibacterial Applications: A Mechanism of Action

Rapid development of nanotechnology has been in high demand, especially for silver nanoparticles (AgNPs) since they have been proven to be useful in various fields such as medicine, textiles, and household appliances. AgNPs are very important because of their unique physicochemical and antimicrobial properties, with a myriad of activities that are applicable in various fields, including wound care management. This review aimed to elucidate the underlying mechanisms of AgNPs that are responsible for their antiviral properties and their antibacterial activity towards the microorganisms. AgNPs can be synthesized through three different methods-physical, chemical, and biological synthesis-as indicated in this review. The applications and limitations of the AgNPs such as their cytotoxicity towards humans and the environment, will be discussed. Based on the literature search obtained, the properties of AgNPs scrutinizing the antibacterial or antiviral effect shown different interaction towards bacteria which dependent on the synthesis processes followed by the morphological structure of AgNPs.

Two Birds with One Stone: Spontaneous Size Separation and Growth Inhibition of Femtosecond Laser-Generated Surfactant-Free Metallic Nanoparticles via ex Situ SU-8 Functionalization

Laser ablation in liquids (LAL) offers a facile technique to develop a large variety of surfactant-free nanomaterials with high purity. However, due to the difficulty in the control of the particle synthesis process, the as-prepared nanomaterials always have a broad size distribution with a large polydispersity (σ). Surfactant-free properties can also cause problems with particle growth, which further increases the difficulty in size control of the colloids. Therefore, searching for strategies to simultaneously unify the sizes of colloids and inhibit particle growth has become significantly important for LAL-synthesized nanomaterials to be extensively used for biological, catalytic, and optical applications, in which fields particle size plays an important role. In this work, we present a facile way to simultaneously realize these two goals by ex situ SU-8 photoresist functionalization. Ag nanoparticles (NPs) synthesized by femtosecond laser ablation of silver in acetone at laser powers of 300 and 600 mW were used as starting materials. The synthesized Ag NPs have a broad size distribution between 1 and 200 nm with an average size of ca. 5.9 nm and σ of 127-207%. After ex situ SU-8 functionalization and 6 months storage, most particles larger than 10 nm become aggregates and precipitate, which makes the size distribution narrow with an average diameter of 4-5 nm and σ of 48-78%. The precipitation process is accompanied by the decrease in colloid mass from the initial ∼0.2 to 0.10-0.11 mg after ex situ SU-8 functionalization and 6 months colloid storage. Morphology analysis indicates that ex situ SU-8 functionalization inhibits the particle growth into polygonal nanocrystals. Radical polymerization of SU-8 on Ag NPs is considered to be the reason for both spontaneous size separation and growth inhibition phenomena. Benefiting from Ag NPs embedment and acetone dissolution, the glass-transition temperature of SU-8 photoresist increased from 314 to 331 °C according to thermogravimetric analysis. The universality of ex situ SU-8 functionalization-induced growth inhibition and size separation behaviors is further proved using the Au colloids generated by LAL in acetone. This work is expected to provide a new route for better size control of LAL-synthesized colloids via ex situ photoresist functionalization, although a half of colloidal mass is wasted due to radical polymerization-induced colloidal precipitation.

Magnetic Fe@FeOx, Fe@C and α-Fe₂O₃ Single-Crystal Nanoblends Synthesized by Femtosecond Laser Ablation of Fe in Acetone

There are few reports on zero-field-cooled (ZFC) magnetization measurements for Fe@FeO_x or FeO_x particles synthesized by laser ablation in liquids (LAL) of Fe, and the minimum blocking temperature (T_B) of 120 K reported so far is still much higher than those of their counterparts synthesized by chemical methods. In this work, the minimum blocking temperature was lowered to 52 K for 4⁻5 nm α-Fe₂O₃ particles synthesized by femtosecond laser ablation of Fe in acetone. The effective magnetic anisotropy energy density (K_eff) is calculated to be 2.7⁻5.4 × 10⁵ J/m³, further extending the K_eff values for smaller hematite particles synthesized by different methods. Large amorphous-Fe@α-Fe₂O₃ and amorphous-Fe@C particles of 10⁻100 nm in diameter display a soft magnetic behavior with saturation magnetization (M_s) and coercivities (H_c) values of 72.5 emu/g and 160 Oe at 5 K and 61.9 emu/g and 70 Oe at 300 K, respectively, which mainly stem from the magnetism of amorphous Fe cores. Generally, the nanoparticles obtained by LAL are either amorphous or polycrystalline, seldom in a single-crystalline state. This work also demonstrates the possibility of synthesizing single-crystalline α-Fe₂O₃ hematite crystals of several nanometers with (104), (113), (116) or (214) crystallographic orientations, which were produced simultaneously with other products including carbon encapsulated amorphous Fe (a-Fe@C) and Fe@FeO_x core-shell particles by LAL in one step. Finally, the formation mechanisms for these nanomaterials are proposed and the key factors in series events of LAL are discussed.