Pulsed Laser Ablation in Liquids for Fabrication of Noble Metal Nanostructures

Abstract

Pulsed laser ablation in liquids (PLAL) is a physical method that is applied for the fabrication of the noble metal nanostructures with different optical and morphological properties. The physical mechanism of laser ablation in a liquid environment, the subsequent growth of nanostructures, the essential laser technological parameters that determine the nanostructures’ properties, and the liquid medium’s influence are discussed. The main advantages and disadvantages of the PLAL method are noted. Post-ablation treatment at the optimal laser wavelength, fluence, and duration of exposure has been indicated as a means of mitigating and overcoming the latter. The aging effect of the colloids and some applications of them are also marked. The most commonly used methods for studying the nanostructures’ characteristics such as UV/Vis spectroscopy, high-resolution scanning transmission electron microscopy (HR-STEM), mass spectrometry (MS), and X-ray diffraction (XRD) are commented.

1. Introduction

The first ruby laser was developed by Maiman nearly 60 years ago in Hughes Research Laboratories and was employed to irradiate and to ablate materials [1, 2, 3]. After its appearance, the pulsed laser ablation (PLA) method was used to produce various types of nanostructures such as thin films (TFs), nanoparticles (NPs), nanowires (NWs), or nanonetworks (NNs), which may be further involved in distinguished applications [4, 5, 6, 7]. NPs (particles with dimensions in the interval from 1 nm to 100 nm) have attained significant interest in the past and even more nowadays because of their unique physicochemical properties (optical properties, electrical conductivity, melting point, density, chemical stability, etc.), which differ from that of the corresponding bulk materials [8, 9, 10, 11, 12]. Their specific characteristic is the large number of surface atoms compared to the number of such atoms in bulk, associated with high reactivity and determining their catalytic, optical, and magnetic properties. Another particularly significant feature is the strong dependence of these properties on the shape and size of nanomaterials. The latter provides an opportunity to create nanomaterials with independently and precisely controlled properties, such as size, composition, morphology, defect density, and atomic structure, that meet the specific requirements of a distinct application. Based on the material they are made of, nanostructures can be classified into four separate groups containing fullerenes, metallic NPs, ceramic NPs, and polymeric NPs, respectively [13, 14, 15, 16].

Due to their optical properties, the existence of surface plasmon resonance (SPR) resulting from an interaction between electromagnetic waves and electrons in the conduction band (plasmonic process) and the ability to control the optical field, metallic NPs are of great interest in nanotechnology, whose subject is the different fabrication strategies and morphological modifications of nanomaterials. Plasmonic materials are well-known for their harsh interaction with free electrons and incoming photons. Thus, metallic materials can function as a source in order to convert the light into a local electric field in metals, named surface-localized plasmonic [17, 18, 19, 20]. These interactions can also be tuned by modifying the morphological properties and the diameter size of the metal nanostructures. A special class of metallic NPs is represented by the noble metal nanoparticles (NMNPs), such as Ag, Au, Pd and Pt, characterized by tunable optical and remarkable photoelectric properties, high corrosion and oxidation stability, and exhibit a good biocompatibility related to their low biotoxicity. Based on these properties, an amazing variety of fields of application is established. We will mention some of them for illustration: in nano-enhanced catalysis and electrocatalysis, new energy materials, photoelectric information storage, for stable dyeing in the textile industry, colorimetric, and fluorometric sensors, diagnostic sensors in micro and nanoelectronics, for micro-joining of several electronic components in microelectronics, microelectronic systems for screen printing and inkjet printing, for drinking water purification decomposition of hazardous pollutants in the environment to minimize environmental pollution, bio diagnostics and biology, labels or probes in biosystems, as a biomaterial because of their anti-microbial, anti-inflammatory, anti-sterile and anti-allergic capability, in photothermal therapy, diagnostic and imaging applications, cancer treatment and diagnostic, for targeted delivery of active agents against tumor cells, etc. [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31].

NMNPs could enhance the response of organic or biomedical material having the plasmonic influence in the ultraviolet-visible spectrum (UV-Vis) because of the photon scattering and absorption. With their widespread use, various methods have been developed for the synthesis of metal, in particular, NMNPs. They can be classified into two main groups—top-down and bottom-up approaches. Applying physical and chemical techniques to the bulk materials to reduce them to nanosized particles is the essence of the first procedure. The essential disadvantages are large size variation, imperfect surface structure and expensive processing equipment to maintain high pressure and high-temperature conditions during synthesis. In the bottom-up procedure, nanoparticles are built by assembling atoms, molecules or clusters through chemical and biological processes. This method provides better control over the forming process, a more homogeneous size distribution, and chemical composition. It is generally less expensive but is limited by the use of toxic chemicals, organic solvents and reagents [32].

Another classification is also possible according to the processes involved in the manufacturing methods, specifically physical, chemical or biological [32, 33, 34, 35, 36, 37, 38, 39, 40, 41]. In general, regardless of the specifics of each of them, they present the disadvantages listed.

Synthesis of nanomaterial structures by pulsed laser ablation (PLA), which is a top-down physical method, can be performed either in a high vacuum deposition chamber at different pressure values with or without gases or in a liquid medium. The implementation of PLA in a liquid environment (PLAL), also known as laser ablation synthesis in solution (LASiS), is an approach that overcomes the main shortcomings of the other methods. It does not require expensive equipment; its application is under ambient pressure and temperature conditions and can produce chemically pure nanoparticles free of potentially toxic impurities. This approach was first reported in 1987 by Patil et al., and since then and especially in recent times, it has been frequently used [42]. In addition, the PLAL process can be split into various other methods to create complex nanomaterials including reactive PLAL, electric field-assisted PLAL, magnetic field-assisted PLAL, electrochemical PLAL and sequential PLAL [43, 44, 45, 46, 47]. Through this method, distinguish nanomaterials can be produced, such as pure metals, metal alloys, metal oxides, sulfides, polymer NPs, organic materials, semiconductors, ceramic nanostructures, nanocomposites with many different morphologies including core-shell, nanocubes (NCs), nanorods (NRs), NWs, NNs and other complex composites as a nano-resource for optical/photonics, catalytic, energy and biological applications [14, 15].

2. Experimental setup

The PLAL method used for the synthesis of nanostructures has a simple and basic setup in which three essential elements such as a laser, a target and a liquid medium are required. In other words, it consists of a laser system (ms, ns, ps or fs) [48, 49, 50, 51, 52], an optical system and a container where the target is placed. Figure 1 shows the most used experimental setup for the PLAL method. Among the systems mentioned above, at least one of them is more expensive, and that is the laser system, but if you have a laser in the lab that can be used, then the other elements are less expensive. Almost all literature has reported the use of an identical diagram, except for systems using a rotating table or magnetic stirring of a liquid. Therefore, the target is placed on the bottom of the container, the latter is located on a movable XY stage, and the optical system is adjusted so that the laser beam should reach the target. Generally, the target is located prior to the focal plane of the optical system, specifically, the focusing lense, to avoid optical breakdown in the liquid medium during the ablation process, while the laser spots on the target surface have diameter sizes ranging from millimeters to microns. Also, suitable modifications to the experimental setup can be made in order to increase the efficiency of the ablation rate by positioning the target at different angles (0°, 45°, 90°) during PLAL processes [53, 54, 55, 56, 57].

Figure 1.

Experimental setup of PLAL process.

As mentioned above, the technological parameters of the PLAL such as pulse duration, laser wavelength, laser fluence, laser energy, repetition rate (RR), number of shots, ablation time, the nature of target, solvent and solutes have an important role in obtaining NPs with different optical and morphological properties, but the laser parameters are fundamental and required when PLAL occurs. Typically, laser systems with different pulse durations from nanoseconds up to femtoseconds operating from ultraviolet to infrared wavelengths were utilized in the synthesis of NPs applying a broad range of laser fluence values [52]. It should be noted that the usages of PLAL in industrial applications have some limitations regarding the difficulties to control the mean size (MS), standard deviation (SD) and size distribution, respectively, for NPs and their production rate if mass concentration is lower in a shorter ablation time. Among the advantages is that the laser ablation process only needs a minimal working procedure in which you will learn how easily the NPs can be created, the bulk material to be ablated instead of metal salts or chemical reagents, and minimal waste fabrication during PLAL instead of getting toxic waste by using other synthesis methods.

In the following sections, the technological parameters and the synthesis mechanism of growing NPs, as mentioned above, will be described and commented in detail for a better understanding of when the PLAL process is used.

3. Basic technological parameters

Fundamentally, the main technological parameters such as laser wavelength, laser beam width, laser fluence, laser RR, laser ablation duration, and desired medium are involved during PLAL process in the formation of the NPs.

The laser wavelength (λ) is an important parameter when MNPs with the desired morphology are produced through the laser ablation process. Using different types of laser sources, from Nd:YAG lasers to excimer lasers (KrF, ArF, XeCl), NPs with different shapes or sizes can be obtained [58]. As known from the literature, Nd:YAG laser is mainly used in PLAL process to create NMNPs as a laser source with a fundamental wavelength of 1064 nm because of their simplicity and capability to double, triple, or quadruple the frequency by optical systems [59]. Excimer lasers such as ArF (λ = 193 nm), KrF (λ = 248 nm) or XeCl (λ = 308 nm) could deliver short pulses of tens of nanoseconds and, due to their versatility of ultraviolet light, may be a choice in the laser ablation procedure to ablates different and numerous materials [60]. Mafune et al. utilized the fundamental wavelength (λ = 1064 nm) and second harmonic (λ = 532 nm) in pure water to prepare Pt-NPs with an average diameter of 6.2 nm [61]. For instance, Mortazavi et al. fabricated Pd-NPs using two different laser sources, Nd:YAG (1064 nm) and ArF (193 nm), and their influence on the production and structure of the obtained NPs was studied [62]. Censabella et al. produced Pt-NPs with a mean diameter size of around 10 nm also by an Nd:YAG laser in deionized water [63]. Nikolov et al. utilized the fundamental (λ = 1064 nm), second (λ_SHG = 532 nm), third (λ_THG = 355 nm), and fourth harmonic (λ_FHG = 266 nm) of a Nd:YAG laser system to ablate Ag-NPs. The best result was achieved with the fundamental wavelength, specifically, an average size of 12 nm and a standard deviation of 4 nm [64].

The laser beam width (τ). The working principle of the PLAL process depends on the pulse duration, which represents the time at which the laser energy is driven to the target surface and as a result, different effects such as light absorption, heat generation and target ionization can occur. Various scientific papers have reported that the pulse width probably significantly influences the structure and size of NMNPs. The effect of pulse width on the growth mechanism for the fabrication of NMNPs in the initial phase of PLAL was investigated by Shih et al. [65]. The influence of the femtosecond pulses on Au-NPs synthesized in water was studied by Kabashin et al. [66].

Moreover, laser fluence is an essential laser parameter that can influence the PLAL process during the NMNPs formation and evaluates the quantity of laser energy per area on the ablated target (J cm⁻²). The minimum laser energy required to ablate the target and achieve vaporization during the PLAL procedure is also called threshold laser fluence. The latter depends on the type of pulses used during the ablation process (i.e., ns, ps, or fs pulses) and can have different behaviors such as being independent of the pulse length (for ns pulses) or being dependent on the pulse length (for ps or fs pulses). Several scientific papers have been reported in the study of the influence of laser fluence during the formation of NMNPs. Amendola et al. produced Au-NPs in water and dimethyl sulfoxide solution using laser fluence values between 12 and 442 mJ cm⁻² [67]. By using these values, a trend of increasing and decreasing NMNPs diameter sizes can occur. A theoretical explanation of the dependence Au-NPs diameter size on the laser fluence used to ablate Au target was made by Pyatenko et al., while this correlation was studied experimentally by Tsuji et al. [68, 69]. The impact of the laser fluence on the Ag-NP characteristics ablated in distilled water was investigated by Xu et al. [70]. They observed that the MS values of Ag-NP diameter decrease when the applied laser fluence is less than 6.4 J cm⁻² and increases when its value is higher. They established a decrease in size distribution when the fluence is less than 4.2 J cm⁻² and higher than 6.4 J cm⁻² and increases between these two values. This result matches the full width at the half maximum (FWHM) of the corresponding plasmonic band in the UV/Vis absorption spectra. Regarding the average NPs diameter size, it initially decreases from about 22 nm to about 17 nm when the laser fluence changes from 3.4 to 4.2 J cm⁻². With a subsequent increase of the laser fluence to 8.4 J cm⁻², the average value of the diameter increases, reaching about 39 nm at the indicated fluence. Moniri et al. fabricated Pt-NPs with a mean size value of 18 nm by using the fundamental wavelength at a RR of 10 Hz and laser fluence of 2 J cm⁻² [71]. Lazar et al. fabricated Pt-NPs in double-distilled water (DDW) using KrF excimer laser by varying the laser fluence (2.3, 4.0 and 5.8 J cm⁻²) and RR (10, 20, 30 ,40 and 50 Hz) while the time ablation was fixed at 10 min. They observed that by increasing the laser fluence and keeping the RRs constant, the mean size enhanced from 2.2 nm up to 4.0 nm, while the smallest mean size value of 2.2 nm for Pt-NPs created with the smallest laser fluence was achieved [72].

Pulse repetition rate (RR) is a fundamental laser parameter and depends on the applied laser energy used in the PLAL process. As it is known from other scientific works, RR is defined as the number of pulses released per second. Moreover, RR optimization and adjustment are necessary because of the shielding effect that can be generated by the plasma over laser pulses and can influence the yield of the created NPs. Therefore, to minimize this effect, the RR variation, the laser spot size and a suitable movement between the laser beam and target are needed. The impact of the RR varying from 1 Hz to 10 Hz in the productivity of ablated Ag-NPs in ethanol was studied by Valverde-Alva et al. [73]. They observed that the laser ablation efficiency of NPs grew with the enhancement of the laser RR. Pt-NPs ablated in aqueous ethanol solution using KrF excimer laser were obtained by Lazar et al. [74]. By varying the laser RRs from 10 Hz to 50 Hz using 2.3 J cm⁻² laser fluence in 40% ethanol solution, an enhancement in the mean size values from 2.0 nm up to 2.45 nm was observed, while the standard deviation values remained similar at ±0.8 nm. Altowyan et al. produced bimetallic NPs from Ag and Au, and they studied the effect of the laser fluence on the created nanostructures [75]. They also showed that the formation of Au (core)-Ag (shell) depends on the applied laser energy, and the shell thickness enhances with increasing laser energy. The influence of the laser fluence on the properties of the fabricated Pd-NPs was investigated by Mendivil et al. [76]. The formation of cubic nanostructures was observed, the MS and SD values simultaneously decreased from 27 ± 9 nm to 17 ± 6 nm with increasing laser fluence varying from 8 to 40.5 J cm⁻².

In general, the target material and the liquid environment used in the pulsed laser ablation process are also among the most important technological parameters. The optical and morphological properties of the obtained colloids depend on the material type used as the target and on the liquid medium employed to ablate the target.

Usually, the target that is utilized in PLAL method to obtain different nanostructures could be made from various materials such as pure metals (i.e., Ag, Au, Pt, Pd and Cu), metal alloys (i.e., AgAu, PtPd, FePt, AuFe, AgFe, PtCu and PtAu), carbon-based targets (graphite and graphene oxide), oxides (GaO and SiO), selenides (ZnSe and CdSe), nitrides (GaN and TiN), sulfides (PbS and CdS), tellurides (CdTe), antimonides (InSb), and pressed targets (Cu/Zn and WO₃/GO) [63, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96].

Normally, the properties of the liquid environment, such as height level over the target surface, the relative movement and the physical/chemical nature, can affect the nanostructures properties and the efficiency of the laser ablation process [97, 98, 99]. The height level of the liquid on the top of the target surface could influence the efficiency of the laser ablation process. Two groups, Jiang et al. and Al-Mamun et al., showed that when a lower height level of liquid of 1.2 mm and 2.0 mm was used, a higher ablation rate was achieved [97, 100]. They affirmed that a confinement effect could occur at a low height level of the liquid above the target surface, and a high efficiency can be obtained as a result. Cristoforetti et al. ablated Pd-NPs in different liquids media such as water, acetone, ethanol, 2-propanol, toluene and n-hexane. They observed a lower efficiency in ablation process when water, acetone, toluene and n-hexane were used, while in ethanol and 2-propanol a comparable efficiency was achieved [101]. Moura et al. synthesized Ag-NPs in DDW, acetone and ethanol. The smallest NPs were achieved in DDW and the biggest in acetone and ethanol [102]. Lee et al. fabricate Au-NPs in water, methanol, hexane and acetonitrile, obtaining spherical, polycrystalline nanostructures and agglomerated chains. They also observed an enhancement in the catalytic activity of Au-NPs [103].

Additionally, the composition of the liquid and their additives can be classified into different batches like water and hydrogen-peroxide, acids and alkalis, organic solvents, monomers and polymers, salts, surfactants, superfluid and supercritical fluids, and combined liquids [15].

4. Ablation mechanism and nanostructures formation

The working mechanism of the PLAL remains misunderstood due to the physicochemical process including laser-matter interaction (electron excitation, lattice heating and lattice disintegration), plasma plume, cavitation bubble, and liquid phase, which are involved during the NMNPs formation [104].

In essence, the PLAL process is based on the removal of the matter from the target through a laser beam that irradiates the target surface submerged in a liquid environment. The interaction between the laser beam and the target material and resulting ablation depends on the pulse duration, the liquid medium, the sample properties, and the focusing conditions used during the PLAL procedure. Usually, when a pulsed laser beam hits a metal plate, various materials can be carried out from the target surface in the form of vapors, fragments or clusters, which also interact with the liquid environment in order to form NMNPs [105]. Frequently, when short-pulse lasers with high laser fluence are used, the mostly vapors are produced, while using long-pulse lasers with lower laser fluence, mainly fragments or nanoclusters are created. Two processes have been identified in the literature, such as thermal vaporization of species from the liquid-solid interface and thermally induced emission of clusters/fragments from the surface target [1].

Supplementary, several steps during the ablation process were suggested like the easiest formation of atoms in the metal plate was realized by laser fluence from the laser beam. In the meantime, a plasma plume with high temperature was produced at the interface between the target and the liquid. Thus, the concentrated species within the plasma plume can be fast thermally scattered, resulting from their numerous aggregations, collisions and condensations. Lastly, as the plasma plume was extinguished by the surrounding liquid, the temperature of the plume began to decrease, and the obtaining nuclei started to spread and agglomerated into a larger nanostructure [105]. Nevertheless, the expansion of the plasma plume is confined by the surrounding liquid in order to form bubbles consisting of ablated species in gaseous form, which can lead the laser-induced plasma into thermodynamic conditions with high temperatures varying from 4000 K to 6000 K, high- density and high- pressure [106, 107, 108, 109]. This thermodynamic condition of the plasma is interconnected with the laser parameters and the lifetime of the bubbles [107, 110, 111]. After that, the plasma plume will extend into the ambient liquid. During this expansion, the rapid pressure gradient caused by the shock wave emission and the energy transfer from plasma to liquid creates the cavitation bubble. Thus, during the laser ablation process, since the cavitation bubble can extend and reduce with time ablation, the high-pressure condition when the cavitation bubble drops down can impact the formation of the NMNPs [112]. The representative schematic of the laser-induced cavitation bubble can also be seen in Figure 2.

Figure 2.

Scheme for the cavitation bubble formation.

Due to the falling of bubbles, the nanostructures are created during the cooling stage of the plasma plume, and afterward, the NMNPs will be scattered in the surrounding liquid to compose the colloidal solution. In addition, during this procedure, the bubbles release huge energy into the liquid with a considerably minimized temperature and pressure, which determines the rapid extinguishment of the plasma and the emission of the ablated material [113, 114, 115]. Furthermore, a representative illustration can be drawn that consists of the fundamental steps described above about the ablation mechanism and this can be observed in Figure 3.

Figure 3.

Experimental steps that occur when the ablation mechanism begins.

As it is already well-known, the synthesis of the nanostructures in the laser ablation field can be carried out either in the non-aqueous medium or in the liquid environment. The adjustable laser synthesis method of nanomaterials in a liquid environment has been widely used to create diverse colloidal nanostructures. Three laser synthesis techniques—laser ablation in liquids (LAL), laser fragmentation in liquids (LFL), and laser melting in liquids (LML)—can be distinguished based on the processing mode and on the formation mechanism of NMNPs [23]. Then, in this section, we reviewed the most current research in the nanomaterials field that utilized new LAL and LFL liquid phases. Figure 4 shows the representative illustration of the experimental setup for an LFL technique type.

Figure 4.

Representative scheme of the laser fragmentation in liquid.

Furthermore, the liquid environment used in the PLAL can be non-reactive or reactive. By using a non-reactive liquid medium, the material ejected from the target will not interact with the liquid compounds, and the final colloidal solution will contain only the material from the target. When the nanostructure synthesis takes place with a reactive liquid medium, the atomic species from the target will interact with the molecules from the surrounding liquid leading to the new nanostructures [116].

5. Methods to study the PLAL process

PLAL is a complicated method whose implementation is related to the occurrence of physical and chemical processes under non-equilibrium conditions in order of nano-, pico-, and femtoseconds time scale. Their study must be carried out by ensuring sufficient temporal and spatial resolution. Such processes are the laser-matter interaction when the laser pulses are short or ultrashort, plasma formation, cavitation bubbles appearance and their dynamics, and the nanoparticles creation. This investigation, in addition to purely theoretical interest, also aims to manage them in order to control the characteristics of NMNPs as a final product. Several extensive review articles have been devoted to this topic [109, 110, 111, 112].

Only the diagnostic techniques used to study the individual processes will be indicated in the present work. The processes that occur during PLAL are plasma formation and expansion (t ≅ 0.1–1 μs), cavitation bubble formation and spread time (t ≅ 1–100 μs), nanoparticle growth (t ≅ 300 μs), and shockwave emission (t ≅ 1 μs) [5, 108, 117]. Only the research techniques with appropriate temporal resolution will be listed in this paper, and their more detailed description can be found in the suggested literature. The interferometry (∼10⁻⁹–10⁻⁶ s) and optical emission spectroscopy (∼10⁻¹²–10⁻⁹ s) methods to investigate the plasma generation, expansion and quenching mechanism are applied [118, 119]. The shockwave and the cavitation bubble dynamics are studied by fast shadowgraph (∼10⁻⁹–10⁻⁶ s), X-ray radiography (∼10⁻⁶ s), photoacoustic (∼10⁻⁶ s), and photoelastic imaging (∼10⁻⁹ s). Optical beam deflection (∼10⁻⁹ s), small angle x-ray scattering (SAXS) (∼10⁻⁶ s), laser light scattering (∼10⁻⁹–10⁻⁶ s), and x-ray radiography (∼10⁻⁶ s) are used to examine the nanoparticle growth. Using techniques such as high-speed photography (∼10⁻¹⁰–10⁻⁶ s) and optical beam deflection (∼10⁻⁹ s), the entire dynamics of PLAL procedure can be researched [118, 119, 120, 121, 122, 123, 124, 125].

6. Manufacture of NMNPs through laser ablation process

The fabrication of several NMNPs using the PLAL method is covered in this section. The characteristics of the created NMNPs and the corresponding characterization methods will be discussed in detail. Several authors reported the production of NMNPs as Pt, Ag or Au applying the PLAL method in different liquid media – acetone, ethanol and methanol, aqueous solution of sodium dodecyl sulphate (SDS), solution of TSC (trisodium citrate, non-toxic and biocompatible solution) or ethanol in water with different concentrations [71, 72, 74, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136].

6.1 Platinum nanoparticles (Pt-NPs)

The Pt-NPs were synthesized in either DDW or different ethanol concentration using an excimer laser, KrF. First, the synthesis of Pt-NPs in DDW will be discussed. To obtain these NPs, the technological parameters were optimized. Three technological parameters were kept constant, and another one was varied to adjust the fabrication of Pt-NPs. The laser fluence was fixed at 2.3 J cm⁻², the time ablation was only 15 min, the initial volume of DDW was 6.7 mL every time and the repetition rate was varied from 10 Hz up to 40 Hz. In Figure 5 are represented the optical properties of the as-created Pt-NPs acquired in the wavelength interval from 200 nm to 800 nm by using the already mentioned technological parameters during the PLAL process.

Figure 5.

UV/-Vis measurements of Pt-NPs with 2.3 J cm⁻², 15 min ablation time and varied RR: 10 Hz—blue curve, 20 Hz—purple curve, 30 Hz—green curve, and 40 Hz—red curve.

As can be seen from Figure 5, the optical spectrum of the Pt-NPs obtained with 10 Hz RR has the highest transmission (T) value, while the lowest T values were achieved for the Pt-NPs created with 40 Hz RR. In our opinion, these differences between the spectra under consideration it can be due to the difference in the mass concentration of the Pt-NPs in the final solution. Two bands with altered profiles relative to those in the nearby areas may be distinguished in all spectra with reproducible features. This is because of the transmission’s lower levels (enhanced extinction values). A distinct local minimum at about 220 nm clearly defines the first band which is located in the 200–230 nm range. This band’s existence is most likely caused by interband transitions [137, 138, 139, 140, 141]. Conversely, a very weakly expressed band with reduced transmission and ambiguous borders can be seen in the wavelength range of 230–330 nm. The local minimum of this band is situated at roughly 270 nm and exhibits relatively modest local minima in all spectra under consideration.

Furthermore, the obtained Pt-NPs were investigated through HR-STEM technique to achieve their morphological properties. Figure 6 shows the morphological features of the as-prepared Pt-NPs using the above-mentioned technological properties. In addition, the ZC-phase contrast images, Figure 6a, and transmission electron (TE) micrographs, Figure 6b, which are co-localized images, were simultaneously acquired for each sample at the same magnification (x250K) and position on the sample as a result of a unique characteristic of the HR-STEM used for these measurements.

Figure 6.

Co-localized STEM images of obtained Pt-NPs using 2.3 J cm⁻²: (a) ZC micrographs, (b) TE micrographs and (c) histograms of laser RR: (I) 10 Hz, (II) 20 Hz, (III) 30 Hz, and (IV) 40 Hz.

The below images showed that the Pt-NPs are mostly spherical, and, especially, agglomerated small Pt-NPs exist on the Cu grid. The Pt-NPs obtained at 10 Hz RR have the lowest MS of 2.2 nm, as shown in Figure 6(Ic). When RR is increased up to 20 Hz the MS value rises to 2.8 nm, as is shown in Figure 2(IIc). Further, raising the laser RR to 30 Hz, the MS value remains similar, 2.8 nm, as in the case when 20 Hz RR was used, see Figure 6(IIIc). Nevertheless, the Pt-NPs produced with 40 Hz RR have the largest MS values of 3.8 nm. Regarding the SD values, a decrease ranging from ±1.2 nm (10 Hz RR) to ±0.9 nm (40 Hz RR) can be observed when laser RR is enhanced.

A few research groups produced Pt-NPs in pure water via PLAL method, and they showed that using the lowest values of laser fluence and RR, the smallest mean size value can be obtained. For instance, as mentioned above, Moniri et al. obtained the smallest MS value for Pt-NPs using the lowest laser parameters of the Nd:YAG laser [71]. Similarly, Cueto et al. produced Pt-NPs in pure water with the fourth harmonic ($\lambda$ = 266 nm) at RR of 10 Hz and laser fluence of 1.5 J cm⁻². They observed a bimodal distribution of Pt-NPs with two MS values of 1–4 nm and 6–8 nm [137]. Nichols et al. fabricated Pt-NPs in pure water, by the same laser type with the third harmonic ($\lambda$ = 355 nm) at laser fluences of 1–110 J cm⁻², and the determined diameter size was between 1 and 30 nm [130].

The HR-STEM image of the separated Pt-NPs shows their spherical shape, crystallinity and fully metallic nature as can be seen in Figure 7a. This image was used in order to determine the corresponding interplanar profile of d-spacing value measurements for Pt material, see Figure 7b. Afterwards, through the white squares created in the transmitted electron micrograph, the interplanar distance of 0.22 nm for the crystalline plane of Pt (111) was established. The achieved value is nearly close to the ideal platinum material with FCC nanostructure value of 0.23 nm [142, 143, 144].

Figure 7.

(a) High resolution STEM micrograph of created Pt-NPs with 2.3 J cm⁻², 10 Hz RR and (b) corresponding profile.

In addition, direct analysis in real time-mass spectrometry (DART-MS) was applied to confirm the presence of Pt material in the final solution without supplementary sample preparation of the colloid as a direct investigation.

Figure 8 shows the DART-MS spectrum in which a visible line matching to the Pt-NPs atomic mass can be seen at 195.33 u. where the real atomic mass for Pt is 195.09 u.

Figure 8.

DART-MS spectrum of Pt-NPs.

In the following part, the synthesis of Pt-NPs in different ethanol concentrations and the manufacturing parameters such as laser fluence (2.3 J cm⁻²), laser RR (10 Hz), time ablation (10 min), will be discussed. The initial volume of solution (6.7 mL) was fixed while the aqueous ethanol solution was varied: 10, 20, 40, and 60% ethanol concentration. An UV/Vis spectrometer in the wavelength intervals from 200 nm up to 800 nm to obtain the optical spectra of the fabricated Pt-NPs was used. Figure 9 show the T spectra of the produced Pt-NPs in different ethanol concentrations.

Figure 9.

UV/Vis transmission measurements of Pt-NPs with 2.3 J cm⁻² laser fluence, 10 Hz RR in different ethanol ratios: 10%—blue curve, 20%—purple curve, 40%—green curve, and 60%—red curve.

As can be seen in Figure 9, the Pt-NPs colloids obtained in 10% ethanol concentration have the highest T values than the other Pt-NPs ablated in 20, 40, and 60% aqueous ethanol solution. The SPR in the Pt-NPs is attributed to the optical T band between 230 and 340 nm. As ethanol concentration rises, the minimum position oscillates and the T values of the corresponding spectra decrease. In our opinion, two aspects affect the magnitude of transmission values.

Furthermore, the Pt-NPs’ morphological characterization by HR-STEM are shown in Figure 10 from I to V (a, b), and in Figure 10c, the corresponding size distributions are represented.

Figure 10.

HR-STEM images of the created Pt-NPs using constant technological parameters but varying ethanol concentration: (I) 10%, (II) 20%, (III) 40%, and (IV) 60%.

All images as can be seen in Figure 10(I–III (a, b)) show a significant degree of agglomeration in the colloids that are being studied. From S1 to S3 samples, the distinct size of the aggregates gradually rises, the largest aggregate being seen at S3. The specific size of the aggregates rapidly decreases when the ethanol content is increased further, the S4 sample. As can be observed in Figure 10(I–IVc), when ethanol concentration is increased from 10% to 60%, the MS values of the Pt-NPs decrease ranging from 2.17 nm to 1.7 nm accompanied by a parallel decrease in SD from 0.92 nm up to 0.7 nm, respectively. Therefore, the smallest MS and the narrowest SD values of 1.7 nm ± 0.7 nm are established for the Pt-NPs ablated in 60% ethanol concentration with 2.3 J cm⁻² and 10 Hz RR.

A few scientific groups used either pure ethanol or aqueous ethanol solution as a liquid medium to achieve Pt-NPs with the smallest mean size values [71, 74, 127].

In addition, the X-ray diffraction technique was applied to investigate the structure of the platinum material, and the corresponding diffractogram can be seen in Figure 11. The Pt with indexed crystalline planes of (111), (200) and (311), are represented through the peaks at 2θ = 39.9, 46.4 and 81.7°, respectively, confirming the fabrication of Pt-NPs with nanostructure of FCC. The interplanar distance (d) for the Pt (111), (200) and (311) planes, respectively, were found to be 0.25 nm, 0.19 nm and 0.12 nm [145, 146].

Figure 11.

XRD analysis on fabricated Pt-NPs colloid.

6.2 Gold nanoparticles (Au-NPs)

The Au-NPs were ablated in DDW through of a third harmonic Nd:YAG laser, λ = 355 nm. The technological parameters such as laser RR (10 Hz), laser ablation duration (20 min) and initial volume of the liquid environment (3.7 mL) were constant, while the laser fluence were varied from 0.9 J cm⁻² up to 14.5 J cm⁻². The laser fluence (the laser spot) was varied in order to obtain different nanostructures from the Au target.

The optical transmission spectra to assess the optical properties of the fabricated nanostructure, as can be seen in Figure 12, were acquired in the wavelength interval from 300 nm to 900 nm.

Figure 12.

The UV/Vis extinction spectra on the created nanostructure: (a) 0.9 J cm⁻², 2.8 J cm⁻², 9.6 J cm⁻², and 14.5 J cm⁻². Image taken with the permission from [147] and license number 5426541006535.

As shown in Figure 12, the Au-NPs obtained with the lowest laser fluence has the highest transmission values, while Au colloids fabricated with the highest laser fluence have a significant decrease in transmission values. In our opinion, this effect can happen when the high-density energy of the electron beam is absorbed on the target surface and results in a high quantity of ablated material. Increasing the laser fluence from 0.9 J cm⁻² to 14.5 J cm⁻² a red shift of the SPR band maximum from 511 nm to 528 nm can be seen.

Furthermore, the morphological properties of the synthesized nanostructures which are presented in Figure 13 were recorded by using the Transmission Electron Microscope (TEM) technique.

Figure 13.

TEM images of the obtained nanostructures and their corresponding size distributions: (a) 0.9 J cm⁻², 2.8 J cm⁻², 9.6 J cm⁻², and 14.5 J cm⁻². Image was taken with permission from [147] and license number 5426541006535.

In Figure 13(a–d) different morphologies obtained during PLAL process using different laser fluence values are shown. By using the lowest laser fluence of 0.9 J cm⁻², Figure 13a, Au-NPs of different sizes but having spherical shape with MS value of 4.35 nm can be obtained while increasing the laser fluence value at 2.8 J cm⁻², see Figure 13b, these Au-NPs begin to be interconnected to each other forming NWs or“nanochains”with a width value of nearly 7 nm. A further increase in the laser fluence at 9.6 J cm⁻², as can be observed in Figure 13c, the morphological properties of the NWs remain the same as in the previous case, and when the highest laser fluence of 14.5 J cm⁻² is finally used the NWs begin to disintegrate into Au-NPs with the MS value of 9.3 nm as is shown in Figure 13d. In conclusion, using lower and higher laser fluence values, the Au-NPs can be produced with a parallel increase in the MS and SD values from 4.35 ± 1.6 nm to 9.6 ± 3.7 nm.

Several results are listed in the following text regarding the fabrication Au-NPs in DDW. Nikolov et al. utilized Nd:YAG laser and achieved the best result with the second harmonic wavelength of 532 nm, specifically, MS of 7 nm and SD of 2 nm for Au-NPs [148]. Al-Azawi et al. obtained with the fundamental wavelength 13.9 nm, mean particle size with a SD of 6.78 nm. After post-ablation illumination with the second harmonic (532 nm) these results were improved to a mean particle size of 9.50 nm and a SD of 3.12 nm [149]. Jamaludin et al. fabricated Au-NPs by the same laser type with the RR of 1 Hz using the fundamental and second harmonic wavelengths i.e., 1064 nm and 532 nm, respectively. They employed 4 different laser energies for each fundamental and second harmonic wavelengths for 30 minutes at 3 Hz pulsed RR. The average MS of the produced NPs is in the range 60–100 nm [150].

6.3 Silver nanoparticles (Ag-NPs)

Another interesting study was done by using PLAL process to ablate and to irradiate the Ag-NPs. The Ag-NPs were ablated through Nd:YAG laser using the fundamental wavelength (λ = 1064 nm) with 10 Hz RR, 12.2 J cm⁻² laser fluence and 15 min laser ablation duration. The third harmonic (λ = 355 nm) was used to irradiate the as-prepared colloid to minimize the size distribution. In other words, the laser beam became unfocused using the PLAL setup without a focusing lens. In this case, different values of irradiation time (5–25 min) and pulse laser energy (6 mJ, 17 mJ and 12.5 mJ) were used to optimize the PLAL process to obtain the desired narrowest size distribution. The extinction spectra of the as-prepared and irradiated colloid acquired in the range between 300 and 800 nm are illustrated in Figure 14a–c.

Figure 14.

UV/Vis extinction spectra of the Ag-NPs fabricated with third harmonic, 355 nm, using three different pulse laser energy: (a) 12.5 mJ, (b) 6 mJ and (c) 17 mJ. Image was taken with permission from [151] and license number 5426540770582.

As can be seen from Figure 14, the transmission values acquired for the unirradiated Ag-NPs in all three cases are lower in the entire wavelength range while their corresponding size distribution is wider but still pronounced. Their size distributions are changed when the Ag-NPs colloids were irradiated at the same laser pulse energies but using different irradiation times. For instance, in Figure 14b and c, the NPs size distributions become wider but still noticeable when 6 mJ laser energy is used, while the highest laser energy is utilized the wider and less pronounced size distribution can be obtained. The best result was obtained when the middle value of 12.5 mJ laser energy was used. As can be observed in Figure 14a, with increasing the irradiation time the size distribution becomes narrower and more pronounced.

Additionally, the morphological properties were studied through TEM method and are presented in Figure 15a and b.

Figure 15.

TEM micrographs of fabricated Ag-NPs: (a) unirradiated and (b) irradiated colloids for 25 min performed using third harmonic and 12.5 mJ laser energy. Image taken with permission from [151] and license number 5426540770582.

Taking a look at the TEM micrographs from Figure 15, the unirradiated Ag-NPs are mainly spherical and agglomerated with MS value of 15 nm while those irradiated for 25 min are less agglomerated but the shape differs having the average size of 16 nm. Nevertheless, the TEM results regarding the SD values, ±9.5 nm and ±4.6 nm, respectively, are in good agreement with the UV/Vis results.

The application of PLAL to the synthesis of Ag-NPs is illustrated by the results of several research groups listed below. Tsuji et al. investigated the influence of the laser wavelength on particle size [152]. They fabricated colloidal solutions of Ag-NPs in water by ablation with 1064, 532 or 355 nm laser wavelength. The NPs mean diameter became smaller from 29 to 12 nm with decreasing laser wavelength under laser irradiation at the high fluence of 36 J cm⁻². Pyatenko et al. utilized the second harmonic wavelength (532 nm) [153]. They investigated the influence of the beam spot size, the laser power, and ablation time. Using high laser power and small spot sizes (high laser fluence values), they succeeded in producing very small spherical Ag-NPs with a mean size of 2–5 nm.

A post-ablation treatment is illustrated in Figure 16, which aims to maximum reduce the NP’s MS and, in parallel, the maximum narrowing of the size distribution. Sequential irradiation of Ag-NPs solution with 355 nm and 266 nm, respectively, was used. As a result of the irradiation and absorption of the light energy, photo-fragmentation takes place. The peculiarity is that the energy at both wavelengths is absorbed by two different mechanisms—plasmonic at 355 nm and interband at 266 nm. The photo-fragmentation process is also different for the two types of absorption. In interband absorption, fragmentation leads to a more significant decrease in the MS of Ag-NPs with a relatively wider size distribution. In plasmonic absorption, the MS is less affected, but a stronger narrowing of the size distribution occurs. The synergistic effect when applying the considered procedure is expressed in achieving a strong reduction of the average size combined with a strong narrowing of the size distribution. This effect is illustrated by changing the corresponding optical transmission spectra in Figure 16 and the TEM images in Figure 17(I–III).

Figure 16.

UV/Vis transmission spectra of Ag-NPs: (a) initial solution immediately after PLAL and laser irradiation with third harmonic after: (b) 85 min, (c) 90 min, and by 266 nm for: (d) 5 min, (e) 10 min, (f) 15 min, (g) 20 min. Image taken with permission from [154] and license number 5426541168853.

Figure 17.

(a) TEM electron images, (b) histogram graphs, c) SAED patterns of Ag-NPs created after the first phase (I)—immediately after preparation, the second phase (II) with third harmonics—after 85 min, third phase (III) with fourth harmonics (III)—after 40 min, and (IV)—applied third harmonics (after 85 min) and fourth harmonics (after 15 min), respectively. Image was taken with permission from [154] and license number 54265411.

The aging process is expressed in the formation of aggregates and their deposition at the bottom of the cuvette in which they are stored. The change in the optical properties of the aqueous colloid of Ag-NPs within 30 days after the preparation process was followed by the respective optical transmission spectra and is illustrated in Figure 18a while in the morphology—by the corresponding TEM images is shown in Figure 19(I–III). The restoration of the colloid’s properties, by manual shaking and using ultrasonic treatment, is demonstrated by the optical transmission spectrum, see Figure 18a and its TEM image in Figure 19d.

Figure 18.

UV/Vis extinction measurements of Ag-NPs: (a) at different time after preparation ((1) on the preparation day; (2) after 24 h; (3) after 72 h; (4) after 144 h; (5) after 720 h); (b) on the 30th day ((1) initial solution; (2) after manual agitating; (3) after manual agitating followed by a ultrasonic treatment. Image was taken with permission from [155] and license number 5426540458906.

Figure 19.

TEM micrographs of Ag-NPs: (a) on the preparation day; (b) after 720 h; (c) on the 30th day from the upper surface; (d) on the 30th day after manual agitating. Image was taken with permission from [125] and license number 5426540458906.

7. Conclusions

Nanostructures of noble metals like Ag, Au, Pt, Pd, due to their unique anti-microbial, anti-allergic, anti-inflammatory and catalytic properties, find applications in a variety of areas – from nanoelectronics, minimization of environmental pollution, in the industry to biomedicine with an emphasis on cancer treatment. The PLAL method is a versatile top-down physical method to produce them. The synthesized nanoparticles are chemically pure, not containing potentially toxic impurities for the human body, and are therefore biocompatible. Their optical and morphological properties can be largely controlled with an appropriate choice of process parameters such as laser performance and different fluid media. By suitable wavelength, fluence, repetition rate, laser pulse length, type of liquid, and ablation time, nanoparticles on the order of 2–3 nm with a narrow size distribution of the same order can be created. Double-distilled or deionized water is most often used as a liquid medium, but other liquids such as ethanol and its aqueous solution with different concentrations are also used. Controlling the ongoing processes allows more precise handling of the nanostructure’s properties. Post-ablation irradiation with the optimal wavelength, fluence and duration of the resulting colloid essentially reduce the average size and narrows the size distribution due to photo-fragmentation. Sequential irradiation of the colloid with different wavelengths enables maximum reduction of the mean size and narrowing of the nanoparticles’ size distribution. Such a result is possible using two different absorption mechanisms, plasmonic and interband, which affect the photo-fragmentation process differently. The aging process of silver and gold colloids was studied for 1 month after their creation. Through manual shaking and ultrasonic treatment, the formed aggregates can be destroyed, and the properties of the as-prepared colloids can be restored.

Conflict of interest

The authors declare no conflict of interest.

Author contributions

O.A.L., A.S.N. and M.E.—conceptualization; O.A.L., A.S.N., C.C.M. and M.E.—methodology; O.A.L. and A.S.N.—analysis; O.A.L. A.S.N. and C.C.M.—measurements; M.E.—resources; O.A.L., A.S.N. and C.C.M.—data curation; O.A.L. and A.S.N.—writing original draft; O.A.L., A.S.N. and M.E.—writing review and editing; M.E.—supervision; M.E.—project administration; M.E.—funding acquisition.

Funding statement

This research was funded by ECSEL JU under the following grant agreements: No. 826422 (PIn3S), No. 876124 (BEYOND5) and No. 875999 (IT2). The JU receives support from the European Union’s Horizon 2020 research and innovation program and Italy, Switzerland, Germany, Belgium, Sweden, Austria, Romania, Slovakia, France, Poland, Spain, Ireland, Israel, Portugal, Greece, Netherlands, Hungary, United Kingdom. This work is financially supported by the Romanian Ministry of Research, Innovation and Digitalization, under the following ECSEL-H2020 Projects: PIn3S-Contract no. 10/1.1.3H/03.04.2020, POC-SMIS code 135127, BEYOND5-Contract no. 12/1.1.3/31.07.2020, POC-SMIS code 136877 and IT2-Contract. no. 11/1.1.3H/06.07.2020, POC-SMIS code 136697.