Introduction

Since the first reports from Fojtik and Henglein on nanoparticle synthesis and Patil et al. on reactive target modification , pulsed laser synthesis and processing of colloids (LSPC) has been shown to be a scalable and versatile technique for nanoparticle synthesis, comprehensively reviewed regarding fundamentals and applications . The colloids reported in the literature contain mainly inorganic particles (hence, they are the focus of this review), although the literature on organic particle synthesis has been reported as well, ranging from dyes to natural substances and drugs . LSPC generation of inorganic particles is a physicochemical approach that claims to synthesize particles without surfactants or molecular additives. In contrast to other approaches of nanoparticle synthesis, LSPC only requires the neat (elemental) target materials while no other precursors or ligand exchange reactions are needed. Furthermore, properties such as surface structure or crystallinity can be tailored by adjusting experimental conditions while retaining the initial chemical composition of the educt material . LSPC can be classified into the method variants of laser ablation in liquid (LAL), laser reduction in liquid (LRL), laser fragmentation in liquid (LFL), and laser melting in liquid (LML), which are schematically shown in Figure 1. Molecular precursors are only required in LRL, whereas the other variants employ a solid as starting material, which is ablated/fragmented/molten in the dispersing liquid. Hereby, in ideal cases that fulfill the “purity” claim, the liquid shall not be degraded into reaction products that may adsorb to the nanoparticle surface as they are difficult to remove afterward. Here, water is less critical than organic solvents, where liquid hydrocarbons and other species may be unintendedly created as by-products, potentially found as surface adsorbates on the produced colloidal particles, compromising the nominal purity. However, molecular surface adsorbates may contribute to enhanced functionality in application scenarios, for example, through hindering kinetic accessibility and, thereby, slowing down oxidation, providing conductivity via carbon shells, or providing steric stability against aggregation.

Figure 1: Variants of pulsed laser synthesis and processing in organic liquids as well as classification of the by-products including reported formation pathways. The LSPC variants can be classified by the laser fluence regime (typically, LAL > LRL > LFL > LML), the characteristic size of starting material (LAL > LML > LFL > LRL), or obtained particle sizes (typically, LML >> LAL > LFL ≈ LRL). The synthesis products and by-products are schemed on the right side and may be categorized into three classes: (i) gaseous by-products (i.e., H2, CO, CO2, and C2Hx) emerging as persistent gas bubbles, (ii) liquid hydrocarbons resulting from chemical reactions (i.e., pyrolysis, polymerization, and dimerization), and (iii) carbonaceous solids consisting of amorphous carbon shells, graphitic onion-like carbon shells, elemental carbon, and carbon composites.

Regardless of the liquid medium, the LSPC variants have process-specific characteristics, mainly classified by the product but also by the starting material. LAL, LFL, and LRL yield nanoparticles as products, whereas LML creates submicrometer spheres. LAL, LFL, and LML process solids, whereas LRL employs solvates as precursors (Figure 1). In detail, LAL describes the laser irradiation of a macroscopic target and the subsequent removal of surface matter, which leads to the formation of nanoparticles and can be performed in either aqueous media or organic solvents . Further, it is possible to synthesize metastable phase nanomaterials (NMs) that are hardly obtainable by conventional, chemical methods . LFL utilizes commercial-grade powders or nanoparticles to downsize the particles by laser irradiation with high fluences ; LML, in contrast, is used to isochorically alter the shape or increase the size of nanoparticles by low-fluence irradiation of nanoparticles . A variant of LSPC is reactive laser ablation, fragmentation, or melting in liquids (RLAL, RLFL, or RLML), which refers to the synthesis of nanoparticles wherein molecular or galvanic replacement precursors, such as metal salts, are added to react in situ . The added precursors take part in chemical reactions leading to the generation of products that differ from the initial target’s composition . Besides LAL, LFL, and LML, where a solid bulk or particulate material is laser-excited, another method to synthesize colloidal nanoparticles is the laser reduction in liquid (LRL), where the liquid contains solvated molecular precursors and is excited itself. LRL was first published by Shafeev et al. in 1986, who reduced triphenylphosphine Au(I) complexes to form Au nanoparticles on different materials such as GaAs and was recently extended to synchronous LRL of multiple elements into high-entropy material nanoparticles formed in-place on various substrates . In general, LRL utilizes metal salt or metal-organic complex solutions to form nanoparticles by photochemical reduction of the respective metal ions . Besides the formation of metal nanoparticles, LRL can also induce nuclear reactions such as the (alpha) decay of uranium in the proximity of Au nanoparticles . This LSPC process variant has also been called pulsed laser photoreduction/-oxidation in liquids (LPL) , and LRL has recently been reviewed by the Tibbetts group emphasizing the involved redox reactions and radical species . Besides the reactions involving metal salts during LRL, gas formation and solvent decomposition have also been reported, highlighting the importance of chemical reactions during the processes, although LSPC is often considered to be a physical synthesis method. Moreover, the irradiation of pure organic solvents with femtosecond or picosecond radiation led to the formation of numerous products induced by the intense conditions, which can be attributed to laser-induced optical breakdown and/or shockwaves. The optical breakdown of the solvent and shockwaves can initiate bimolecular reactions that primarily lead to dimerization but also allow for fragmentation, polymerization, and other reactions to occur . Figure 1 schematically summarizes the classification of LSPC methods and the generated particles (left side), and by-products obtained during the processes by solvent decomposition (right side) in organic liquids. For the particle formation mechanisms of these process variants, we refer to a recent LSPC review article . While the nanoparticles obtained in aqueous liquids are at least partially oxidized , the conditions during LSPC in organic liquids are quite different. The solvent molecules themselves as well as the created hydrocarbons can adsorb on the nanoparticle surface and act as ligands. If a carbon shell is formed, it can be amorphous or onion-like graphitic. In addition, composites such as carbides, metallic glasses, or intermetallics can be synthesized. The origin of the formed carbon can be traced back to chemical reactions during the LSPC, which have been reported to be pyrolysis, di-/trimerization, and polymerization. The by-products of these reactions can be either liquid hydrocarbons, for example, polyynes or alkanes, or permanent gases forming persistent gas microbubbles.

There have been various reviews regarding nanoparticle synthesis (mainly addressing findings reported for aqueous media) , fundamental physical processes during LSPC , and the potential use of the laser-generated nanoparticles in nanomedicine and catalysis , including electro- and photocatalysis . The liquid’s influence on the nanoparticle properties as well as its decomposition products, in contrast, have received significantly less attention. Some past reviews addressed at least in part organic solvents as LSPC media. Batista et al. recently reviewed the possibilities of reactive laser synthesis . While the chemical reactions occurring during reactive laser synthesis in aqueous solutions and organic solvents are discussed, the review highlights RLAL, RLFL, RLML, and LRL of various (base, noble, and alloy) metals and their possible use in the future. Marabotti et al., in contrast, focused on polyyne formation only, highlighting the different formation mechanisms, the effect of aqueous or organic solvents, the influence of the irradiated target, and nanocomposites based on sp-hybridized carbon chains . Liang et al. specified the conditions for inhibition of phase crystallization and, hence, the formation of metallic glass nanoparticles in organic solvents, which was attributed to the carbon doping of the amorphous phase as well as carbon shell formation stabilizing the nanoparticles’ glass structure . While these reviews consider at least in part the chemical reactions occurring in organic liquids, they focus on specific topics. Thus, this perspective article aims to survey the current state of knowledge on the laser-based synthesis of colloidal nanoparticles in organic solvents as well as the underlying chemical reactions and their influence on particulate properties. Additionally, the knowledge base on the role of volatile and non-volatile molecular products forming during LSPC will be addressed.

Review LSPC in aqueous liquids

Laser-based nanoparticle synthesis in water is always accompanied by the production of gases . Although gas formation has often been attributed to the vaporization of water, the formation of hydrogen and oxygen also occurs. Additionally, the formation of hydrogen peroxide was observed during LAL and LRL . Depending on the process, gas formation can be attributed to different redox reactions that contribute to nanoparticle formation. For the laser ablation, fragmentation, and melting processes, the nanoparticles are found to be at least partially oxidized, ranging from a surface oxidation of 5–10% for gold to a completely oxidized bulk volume, for example, for nickel . In contrast, laser reduction in water leads to the reduction of metal salts and, thus, to the nucleation of metallic nanoparticles . Hence, water acts as an oxidizing agent in the context of LAL, LFL, and LML, while it is a reducing agent during LRL, indicating an ambivalent behavior during the chemical processes. Recently, this ambivalent character has been demonstrated for iridium-based microparticle LFL, where elemental iridium as starting material was oxidized, and the oxidized iridium reactant was (partially) reduced, under identical processing conditions with identical product nanoparticle diameters of 3 nm . Moreover, the degree of reduction or oxidation was gradually tuned to reach a thermodynamically driven equilibrium with the cumulative laser pulse energy input . It indicates a certain controllability of the redox reactions occurring during reductive or oxidative laser processing in aqueous media at constant particle size. Furthermore, focusing laser pulses in CO2-saturated water leads to the reduction of CO2, which selectively yields CO or oxocarbon-encapsulated nanoparticles . Beyond the formation of carbon monoxide, the direct reduction of permanent solutes in liquids via LRL, sometimes termed laser bubbling in liquid (LBL), has been demonstrated recently for alternative reactants, such as hydrogen extraction from ammonia or methanol , and direct synthesis of HCN .

The oxidation and phase change of the target surface during LAL was initially published by Ogale et al. in 1987, and nanoparticle oxidation has been addressed in the literature frequently afterwards . During the plasma and cavitation bubble phase, reactive oxygen species (ROS), for example, hydrogen peroxide, hydroxyl radicals, or dissolved oxygen, react with the particles leading to their surface oxidation. During irradiation of water with intense laser pulses, a weakly ionized plasma forms because of optical breakdown, supercontinuum emission, or both. Optical breakdown occurs when the free-electron density surpasses a critical value, resulting in a high-density plasma, and the optical breakdown threshold is significantly reduced in the presence of metal nanoparticles . Supercontinuum emission can occur at low fluences, when pulses shorter than 100 fs are used, and will produce a low-density plasma . More recently, the Vogel group studied the energy spectrum of laser-induced conduction band electrons in water by introducing a simplified splitting scheme and corresponding rate equations, well suited also for the calculation of energy spectra at long pulse durations and high irradiance . This approach provides the essential understanding of the dependence of electron energy spectra on laser pulse duration, wavelength, and irradiance, which opens pathways for inducing energy-specific molecular modifications in dielectric media, such as water and even aqueous solutes. Thus, this model formed the basis that enabled the derivation of yield functions for a variety of direct electron-mediated DNA damage pathways and indirect damage by •OH radicals resulting from laser and electron interactions with water . In general, LSPC in aqueous solutions produces free electrons and ROS. The ROS lead to a certain degree of nanoparticle (surface) oxidation depending on the material’s standard electrochemical reduction potential for LAL and LFL. For LRL, both electrons and ROS contribute to the reduction reactions of the metal salts resulting in nanoparticles, depicting the ambivalent behavior of ROS.

Nanoparticle surface oxidation during LSPC can either be suppressed by degassing the water used or amplified by adding salts such as NaCl . Ziefuß et al. found an inverse linear trend during the LFL of gold when correlating the ionic strength of the added salts and the obtained gold particle size (Figure 2). The surface oxidation increased with the anion’s surface charge density until a plateau of around 60% was reached. A further increase in ionic strength showed no change in surface oxidation, which was attributed to an accumulation of anions in the Helmholtz layer indicated by zeta potential measurements. Scaramuzza et al., in contrast, investigated non-ionic additives and their influence on structure and composition during the ablation of metastable AuFe alloys . They used ethanol and water as solvents and added different additives. In the case of gaseous additives (nitrogen, carbon dioxide, and argon), they saturated the solution by bubbling. Further additives were 0.3 vol % hydrogen peroxide and 0.2 vol % water (for ethanol only). Depending on the chosen additive, they found a different degree of oxidation of the alloy nanoparticles. The highest oxidation was found for the mixture of water and hydrogen peroxide, yielding AuFe nanoparticles with an oxide shell; hence, the authors declared the mixture as highly oxidizing. In contrast, ethanol and hydrogen peroxide brought forth AuFe nanoparticles without an oxide shell. A moderate oxidation was found for the ethanol–water mixture, leading to oxide crescents on the nanoparticle surface; nitrogen-saturated ethanol led to scarcely oxidized AuFe nanoparticles. However, it has to be noted that the addition of such agents influences the ablation efficiency and gas formation. Scaramuzza et al. found varying ablation rates and cavitation bubble sizes depending on the used additive–solvent combination , and Zhang et al. found higher yields of gases when working in ethanol–water mixtures . This enhanced gas production can be used to alter the structure of the generated nanoparticles. Laser ablation in water–ethanol mixtures was reported to yield an increased amount of hollow nanoparticles compared to pure water, which was mainly attributed to an elongated lifetime of the cavitation bubble in the mixture . It is further possible to form porous nanoparticles when the liquid is saturated with hydrogen, depending on the specific metals and their properties (e.g., hydrogen permeability and diffusion coefficient of hydrogen) . To summarize, the generated nanoparticles can be oxidized or reduced depending on process parameters such as gaseous additives or atmospheric conditions, liquid additives, or salts, which determine the degree of oxidation, nanoparticle structure, productivity, and gas formation rates.

Figure 2: Anion solute effects on pulsed laser synthesis in water. (a) Effect of ionic strength on the gained particle diameter during the LFL of Au. (b) Relation between surface oxidation of the first atomic layer and surface charge density delivered by adsorbed anions. Figure 2 was reprinted with permission from , Copyright 2020 American Chemical Society. This content is not subject to CC BY 4.0.

Further differences have been found depending on the ablated metal. Gold, for instance, shows surface oxidation degrees of 5–10% , while platinum surfaces are partially oxidized with 20–73% , and nickel particles are completely oxidized . This was further investigated by Kalus et al., who ablated seven different metals (Au, Pt, Ag, Cu, Fe, Ti, and Al) in water while quantifying the formed hydrogen and oxygen via the liquid displacement method and gas chromatography (Figure 3) . The amount of formed hydrogen was found to be inversely correlated with the metal’s standard electrochemical reduction potential, resulting in higher hydrogen formation rates during the ablation of less noble metals such as Ti, Fe, and Al. As the H2/O2 ratio follows the same trend, redox reactions occurring between the formed nanoparticles and the formed oxygen species lead to the particles’ surface oxidation. This correlation shows that metals with a lower standard electrochemical reduction potential than hydrogen (E0 < 0 V) undergo bulk oxidation during the ablation process, while metals with a higher standard electrochemical reduction potential are less oxidized. In addition to hydrogen and oxygen, hydrogen peroxide was also formed during the ablation process. The formation rate of hydrogen peroxide during LAL of Pt (and Cu) was superior compared to that of the other metals (Figure 3d,e) because of surface-catalytic reactions of the metal . In summary, the total water decomposition products formed during ablation increase for less noble metals, which in turn form higher fractions of surface oxide because of the redox reactions of the released oxygen with the generated nanomaterials, resulting in a depletion of oxygen and a higher fraction of the generated hydrogen.

Figure 3: Water splitting and gas formation during LAL of seven metals with increased standard electrochemical reduction potential. (a) Molar gas volume formation rate obtained by LAL of the respective metals in water. (b) Molar hydrogen and oxygen gas volume formation rates depend on the standard electrochemical reduction potential of the used metals. (c) H2/O2 ratio gained during the LAL of metals compared to values known from literature. (d) H2O2 concentrations as well as the molar H2O2 formation rate (insert) found after laser ablation. (e) Molar ratio of formed by-products related to the molar nanoparticle productivity during the LAL in water. (f) Oxidation degree of ablated metal nanoparticles as measured by XPS. Figure 3 was republished with permission of PCCP Owner Societies, from (“Determining the role of redox-active materials during laser-induced water decomposition” by M.-R. Kalus et al., Phys. Chem. Chem. Phys., vol. 21, issue 34, © 2019 and the related correction); permission conveyed through Copyright Clearance Center, Inc. This content is not subject to CC BY 4.0.

The process of LRL stands in direct contrast to oxidation during LAL and LFL, but the (effective) reducing character of the water can also be explained by the previously mentioned ROS, solvated electrons, hydrogen radicals, and hydroxyl radicals. Whereas solvated electrons are generally accepted as the dominant reducing agents in LRL, the hydrogen peroxide produced by the recombination of hydroxyl radicals significantly contributes to the reduction of Au salts during LRL. Moreover, formed molecular hydrogen can contribute to reduction. Hydrogen formation was first reported by Maatz et al. in 2000 by irradiating water, saline solutions, and gelatine phantoms . Meader et al. correlated the formation of free electrons and H2O2 with the [AuCl4]− reduction rate through a two-step autocatalytic nanoparticle growth mechanism . The autocatalytic growth step is driven by H2O2 and can be slowed down by adding hydroxyl radical scavengers resulting in smaller nanoparticles (Figure 4) . Hence, the water decomposition leads to the formation of reducing agents for metal nanoparticle formation during irradiation of metal salt solutions with ultrashort pulses. Several LRL syntheses of nanoparticles, which provide either noble metals, base metals, or oxides, are available and, given the recent review of Frias Batista et al., will not be listed here .

Figure 4: (a) Influence of hydroxyl radical scavengers on the size distribution obtained during the LRL of AuCl4− in water. (b) Quantified yields of H2O2 generated in aqueous scavenger solutions containing varying amounts of 2-propanol and acetate. Inset: absorption spectra of pertitanic acid (TiO2H2O2) in water and radical scavenger solutions. Figure 4 was reprinted with permission from , Copyright 2019 American Chemical Society. This content is not subject to CC BY 4.0.

Overall, LSPC in aqueous liquids is characterized by in situ water splitting. More precisely, highly reactive radicals and free electrons are formed, which act as reducing agents for metal precursor salts during LRL, as well as molecular hydrogen, oxygen, and hydrogen peroxide. However, when LAL or LFL of metals is performed, the targets are oxidized by redox reactions with these ROS. The degree of target oxidation and hydrogen formation rate are inversely correlated and determined by the metal’s standard electrochemical reduction potential. The relation of such material-related effects during LAL would require ideally more single-pulse studies as, during prolonged LAL, the formed nanoparticles will cause in-process LFL and LML, making it difficult to distinguish between reaction in the liquid caused by the target ablation and nanoparticle excitation. With increasing nanoparticle mass concentration, the fraction of permanent gas caused by nanoparticle excitation during LAL reaches already 50% of the total gas at 200 mg/L and can be responsible for as much as 80% of the gaseous laser synthesis reaction products .

LSPC in organic solvents with and without solutes

While LSPC in aqueous solutions leads to (partial) oxidation of the generated nanoparticles in the colloid , the laser-based synthesis of nanoparticles in organic solvents is more likely to retain the initial composition of the material or to form metal carbides while modifying the surface with an amorphous or graphitic carbon shell . As previously mentioned, gas formation is apparent during LSPC in organic solvents, too, but to a larger degree . Kalus et al. found that LAL yielded around 20 times more gas in acetone and ethylene glycol than in water, reaching absolute values of 60 cm3/h (at only 5 W laser power), and specific values of 0.2 cm3/(mg·W) normalized to the ablated target mass and laser power . This indicates the relevance of processes and by-products formed during LSPC in organic solvents as the gas formation rates are proportional to the applied laser power , resulting in the generation of large gas volumes when working with high laser powers, such as state-of-the-art pulsed lasers with several hundred of watts of output power. While the decomposition species of nanoparticle syntheses in water are limited, the possible products formed during irradiation in organic solvents are plenty. First, the ionization of the organic solvent molecules produces free electrons and radical cations with longer lifetimes than in water . Because of their longevity, the radical cations can participate in various reactions such as dehydration, dimerization, and hydrogen transfer before recombination . This was recently shown for fs-laser irradiation of C5 to C11 alkanes by Ishikawa et al., who reported C–C bond formation. They analyzed the formed products via gas chromatography and found dimers with different constitutions to be the main products. Besides dimer formation, the production of shorter carbon chains (down to C4 for the irradiation of undecane), longer carbon chains (Cn+1 to C2n−1), and, although in very small quantities, trimers were observed. They concluded that laser-driven mechanical shockwaves induced these bond-formation reactions . Irradiation of benzene was shown to produce biphenyl as a dimerization product and higher molecular aromatics . However, additional products including hydrogen, methane, acetylene, ethylene , and polyynes were observed from ablation of benzene and toluene. Overall, a large quantity of different products was found for the irradiation of solvents with pulsed lasers. Although the reaction products resulting from the photolysis of solvents have been characterized reasonably well, the influence of nanoparticle production on photolysis reactions and by-product formation is largely unknown. Thus, this chapter will discuss the known by-products obtained during laser-based synthesis in organic solvents. Because of the manifold reaction products, the formation of gases will be discussed first, followed by hydrocarbons and carbonaceous products.

Permanent gas evolution during LSPC

Compared to water, the laser irradiation of organic solvents (in the absence of nanoparticles) leads to decomposition reactions that form permanent gases. Baymler et al. irradiated water, ethanol, isopropyl alcohol, diethyl ether, and isobutanol with nanosecond laser pulses while quantifying the formed hydrogen pressure with amperometric sensors. The organic alcohols, while having a lower number of hydrogen atoms per molecule, showed a ten times higher hydrogen evolution rate than water . Additionally, the molecular structure of the solvent affects the hydrogen evolution rate. Ethanol and isopropyl alcohol produced greater hydrogen yields than isobutyl alcohol and diethyl ether, which was attributed to the higher ratio of hydrogen to carbon and, thus, more C–H bonds relative to C–C bonds. In addition to hydrogen, other gases including methane, ethylene, acetylene, and ethane have been reported by multiple groups during irradiation of neat alkanes, alcohols, and aromatics with nanosecond, picosecond, and femtosecond laser pulses . Gas formation also occurs during laser-based nanoparticle synthesis. Kalus et al. used the liquid displacement method to quantify the formed gases during the ablation of gold and observed gas evolution rates an order of magnitude (around 20 times) higher than in water . Although hydrogen formation has been observed in a variety of ways , the influence of the solvent on the rate of hydrogen formation has not been discussed widely. Fromme et al. recently quantified the overall gas generation as well as hydrogen formation rate during the ablation of gold in several C6 solvents as well as n-pentane and n-heptane (Figure 5). While the overall gas formation was ruled by the solvent’s molar enthalpy of vaporization, the correlation of hydrogen formation with the physical properties of the solvents was not possible. Instead, the hydrogen formation depends on the liquid’s pyrolysis behavior. As such, 1-hexene and benzene, both prone to polymerization reactions and coke formation, showed the largest hydrogen production, while cyclohexane and cyclohexene released the lowest amount of hydrogen compared to other C6 solvents. Interestingly, ablation in n-pentane produced the least amount of hydrogen of all solvents. This may be attributed to differences in bond dissociation energies, which increase with decreasing solvent chain length as well as chemical constitution. Hence, the solvent decomposition and (permanent) gas evolution are influenced by both physical and chemical properties of the solvents. Interestingly, solvent decomposition could be connected to the chemical behavior of the solvent during pyrolysis, although laser ablation is not a thermodynamically driven process .

Figure 5: (a) Gas formation rates per laser pulse versus ablated gold per laser pulse. (b) Hydrogen formation rate per laser pulse versus ablated gold per laser pulse for linear hydrocarbons. (c) Influence of the molar enthalpy of vaporization of organic solvents on the gas formation rate during LAL of Au. (d) Impact of the molar activation energy during the first step in pyrolysis on the hydrogen formation rate during LAL. Figure 5 was reproduced from (© 2023 T. Fromme et al., published by Wiley, distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License, https://creativecommons.org/licenses/by-nc/4.0/). This content is not subject to CC BY 4.0.

However, hydrogen is not the only gas observed during nanoparticle formation processes. Kalus et al. reported the formation of CO and CO2, along with CH4 and C2 hydrocarbons . Further, almost no O2 is formed during the ablation of gold in ethylene glycol, resulting in a conversion of chemically bound oxygen to possible molecular non-volatile products and/or CO and CO2. Frias Batista et al. recently investigated the chemical reactions occurring during the LRL of AuCl4 and AgClO4 in water–isopropyl alcohol solution using femtosecond and nanosecond lasers (Figure 6).

Figure 6: a) Laser mass spectra obtained after femtosecond (red) and nanosecond (green) LRL of AuCl4− in a water–isopropyl alcohol mixture with major products indicated. The • denotes that multiple species can contribute to this peak and the * indicates a peak due to fragmentation. The ns-LRL spectrum is magnified by a factor of ten. Insets magnify the regions m/z 49–57 and 63–91 in the fs-LRL spectrum to show low-yield products. Schematic of different reaction pathways for the LRL of (b) Ag+ and (c) AuCl4− and detected reaction products and reaction intermediates indicated for each reaction condition: femtosecond (pink) or nanosecond (green) laser excitation; water (top) or water–isopropyl alcohol (bottom). Figure 6 was reproduced from (“Understanding photochemical pathways of laser-induced metal ion reduction through byproduct analysis”, © 2023 L. M. F. Batista et al., published by The Royal Society of Chemistry, distributed under the terms of the Creative Commons Attribution-NonCommercial 3.0 Unported License, https://creativecommons.org/licenses/by-nc/3.0/). This content is not subject to CC BY 4.0.

Isopropyl alcohol was found to decompose to numerous products, including methane, acetylene, propene, and C5 alkyne hydrocarbons. However, the product yields found during laser mass spectrometry measurements were higher for fs-LRL than for ns-LRL, indicating that femtosecond irradiation enhances chemical reactions of the solvent, possibly caused by laser-induced shockwaves or optical breakdown of the liquid. The decomposition of isopropyl alcohol was enhanced by the Au nanoparticles resulting from LRL, producing the previously mentioned C1–C5 products. In addition to this, the authors found a difference regarding the nucleation of Ag nanoparticles depending on the used solvent. For the formation of Ag nanoparticles during LRL in isopropyl alcohol, they proposed a mechanism of electron transfer from the solvent, which produced acetone as a by-product. Since this transfer is not possible in water and only plasma reactions are available, Ag could not nucleate during LRL in water because of the oxidizing activity of hydroxyl radicals . Overall, LSPC in organic solvents leads to higher gas volumes and a more complex gas mixture, consisting of hydrogen and highly volatile hydrocarbons, than in water. The gas and hydrogen formation rates were also shown to be connected to the pyrolysis processes and chemical properties of the used solvent. Furthermore, LSPC in organic solvents enables different reaction pathways compared to water and, hence, allows the formation of different nanoparticle products.

Hydrocarbon formation

While the production of gases might initially appear unrelated to the widespread observations of carbon shells surrounding nanoparticles produced by LSPC in organic solvents, another category of carbon-based molecules is formed that might link the two species, namely pyrolysis products. Pyrolysis products were previously found during photolysis of organic solvents as well as laser ablation , laser fragmentation , laser melting , and laser reduction . While the formation of short hydrocarbons during laser-based synthesis of nanoparticles in organic solvents is known, the amount of literature in this regard is scarce. An overview of formed hydrocarbons is given in Table 1.

Table 1: Molecular hydrocarbons formed through laser ablation, laser fragmentation, laser reduction, and laser melting in organic solvents or irradiation of pure solvents.

Process and used material Used solvent Formed by-products Ref. AuLAL glycols CH4, C2H6, C2H4, C2H2, CO, CO2, H2 ZnLAL tetrahydrofuran olefinic and carbonyl species AlLAL acetone enolates and carboxylates Nd2Fe14BLFL cyclohexane C2–C7 hydrocarbon fragments FeLML ethanol C2H4O, C4H10 FeLML ethyl acetate C2H4O, C4H10 CuLML ethanol CO2, CO, CH4, C2H6, C2H4 Cu(OAc)2,LRL acetonitrile CH4, HCN, H2 Cu(OAc)2,LRL propionitrile CH4, HCN, H2 Cu(OAc)2,LRL benzonitrile CH4, H2 KAuCl4, AgClO4,LRL 2-propanol CH4, C2H2, C3H4, C3H6, C2H4O, C5H4, C5H6 SolventLRL C5–C11 alkanes dimers, trimers, C4 to C9 species SolventLRL octane polyynes (up to C14), C2H2, C2H4, C3H4, H2 SolventLRL benzene biphenyl, terphenyl, styrene, and many more SolventLRL benzene H2, CH4, C2H2, C2H4, toluene, (methyl-)biphenyl, phenanthrene, (methyl-)anthracene SolventLRL cyclohexane CH4, C2H2, C2H4, C2H6 SolventLRL toluene biphenyl, anthracene, pyrenes SolventLRL toluene C2H2, C2H4, C3H4, C4H2, C8H2, acetophenone, benzyl alcohol, dimers, phenylacetylene, naphthalenes, fluorene SolventLRL ethanol CH4, C2H2, C2H4, C3H4, C3H6, C4H4, dimers SolventLRL acetone CH4, C2H2, C2H4, C3H4, C3H6, acetylacetone, dimers SolventLRL n-hexane CH4, C2H2, C2H4, C3H4, C3H6, C4H2, C4H4, C4H6, C6 alcohols and ketones, C9–C11 alkanes, dimers, styrene, phenylacetylene, naphthalenes

Kalus et al. also reported the formation of CH4, C2H2, C2H4, and C2H6 during ns-LAL (Figure 7a). Similar results (but for a totally different LSPC process) were published by Tangeysh et al., who performed fs-LRL of copper salts in acetonitrile, propionitrile, and benzonitrile. Besides the formation of methane, mass spectrometry measurements of the obtained solvents showed the formation of HCN and propionitrile. HCN formation did not occur during LRL in benzonitrile, which was ascribed to the lack of α-hydrogen in the solvent molecules. The authors further proposed the formation of CuCN-polyacetonitrile chains during the reduction step . LML of iron oxide in ethyl acetate and ethanol was reported to yield ethyl aldehyde and butane. The aldehyde was proposed to form via dehydrogenation of ethanol, while butane forms via dimerization of formed C2H5 radicals . Van’t Zand et al. investigated the pyrolysis of tetrahydrofurane (THF) at a total energy input of 2250 J comparing fs-, ps-, and ns-pulsed lasers using FTIR spectroscopy. They found a significant decrease in the dominant C–O–C bond at 1070 cm−1 after ablation, which indicates cleavage of the C–O–C bonds in THF. C–H and C–C bonds were detected before and after ablation, while at wavenumbers between 1650 cm−1 and 1725 cm−1 signals could be detected after the ablation process. Bonds at those wavenumbers represent olefinic species or carbonyl groups possibly formed during the C–O–C bond cleavage. Apparently, fewer olefinic and more carbonyl products are formed when using nanosecond lasers compared to picosecond and femtosesond lasers . In general, the irradiation of organic solvents (during LSPC) leads to the formation of various by-products, also through pyrolysis reactions. While formed substances with short carbon chains are gaseous, the formation of liquid compounds, which include saturated, unsaturated, and aromatic hydrocarbons, is also possible. Furthermore, the pulse duration affects the quantity of the generated by-products; ultrashort pulses (picoseconds and femtoseconds) yield larger amounts than nanosecond pulses.

Figure 7: (a) Gas chromatograms (GC-TCD) showing the production of hydrogen and oxygen during LAL in water (black line) and the formation of hydrogen, carbon monoxide, carbon dioxide, and other hydrocarbons during LAL in glycols (green and red lines). Figure 7a was republished with permission of PCCP Owner Societies, from (“How persistent microbubbles shield nanoparticle productivity in laser synthesis of colloids – quantification of their volume, dwell dynamics, and gas composition” by M.-R. Kalus et al., Phys. Chem. Chem. Phys., vol 19, issue 10, © 2017); permission conveyed through Copyright Clearance Center, Inc. This content is not subject to CC BY 4.0. (b) Mass spectra of headspace gas of unirradiated acetonitrile (blue) and irradiated acetonitrile without copper acetate (red, top) and with copper acetate (red, bottom). (c) Schematic depiction of the reaction between copper(II) acetate dimer with HCN leading to the formation of copper(I) acetate through reductive elimination of CN-CN. Figure 7b, c was reprinted with permission from , Copyright 2019 American Chemical Society. This content is not subject to CC BY 4.0.

Not only was the formation of sp2- or sp3-hybridized short-chain hydrocarbons observed but also the formation of sp-hybridized polyynes, which are linear hydrocarbons consisting of alternating single and triple bonds. They are often synthesized by laser ablation of carbon-based materials in water or organic solvents with carbon chain lengths of up to 26 . However, they are also generated when irradiating pure organic solvents or during the ablation of metals . Pan et al. synthesized polyynes with a carbon chain length of ten by ablation of gold in ethanol. They proposed that both the gold target and ethanol have a fundamental impact on the carbyne formation. While gold catalyzes the dehydrogenation of ethanol to form carbon–carbon triple bonds, the structure of ethanol utilizes the C2 carbon chain as a building block of the carbyne chain and the hydroxyl group forms the initial Au–H adduct needed for the catalytic reaction. Other alcohols, such as methanol or propanol, did not lead to carbyne formation during the experiments . Condorelli et al. were able to synthesize carbon-encapsulated Pt nanoparticles by RLAL of graphite in a colloidal Pt solution . The chemical processes leading to the formation of polyynes are still not fully understood, but a few models have been proposed over the years. Tsuji et al. proposed a model for ns-LAL. They suggested a stepwise growth of hydrogen-capped polyynes by the addition of carbon radical fragments, depicted in Figure 8. This radical propagation process competes with the hydrogenation reaction, which terminates the elongation reaction . The carbon source for this polymerization reaction is either the ablated material or decomposed solvent molecules . Besides the termination reaction with hydrogen, different end-capping groups such as CN or CH3 can be added .

Figure 8: Scheme of the formation of hydrogen-capped polyynes during ns-LAL. The carbon source can be either a carbon-based target or an organic solvent. Figure 8 was redrawn from .

While this mechanism for polyyne formation is widely accepted for ns-LAL, the synthesis of polyynes by fs-LAL is proposed to undergo different reactions. In contrast to ns-LAL, where elemental carbon is used as a source to form the initial polyyne fragments, fs-LAL reaches power densities that enable direct ionization and dissociation of the solvent, which may form ionized, short polyynes or cumulenes without intermediate steps. Short C4 polyynes have been observed by femtosecond laser mass spectrometry of irradiated organic solvents . Long-chained polyynes, however, cannot be formed this way and, hence, require follow-up reactions of the short cumulenes . The difference between the proposed mechanisms for ns-LAL and fs-LAL is that Tsuji et al. proposed a radical propagation mechanism, while the fs-LAL mechanism proposed by Zaidi et al. uses ionic propagation . In general, the necessary species for both formation mechanisms are either elemental carbon or solvent fragments, depending on the used pulse width. In this regard, laser fluence is crucial for the synthesis of polyynes. On the one hand, if the fluence is below a certain thresh