Reported laser focusing conditions and resulting AuNP sizes.
Abstract
Laser-assisted metallic nanoparticle synthesis is a versatile “green” method that has become a topic of active research. This chapter discusses the photochemical reaction mechanisms driving AuCl4− reduction using femtosecond-laser irradiation, and reviews recent advances in Au nanoparticle size-control. We begin by describing the physical processes underlying the interactions between laser pulses and the condensed media, including optical breakdown and supercontinuum emission. These processes produce a highly reactive plasma containing free electrons, which reduce AuCl4−, and radical species producing H2O2 that cause autocatalytic growth of Au nanoparticles. Then, we discuss the reduction kinetics of AuCl4−, which follow an autocatalytic rate law in which the first- and second-order rate constants depend on free electrons and H2O2 availability. Finally, we explain strategies to control the size of gold nanoparticles as they are synthesized; including modifications of laser parameters and solution compositions.
Keywords
- femtosecond laser pulses
- nanocolloids
- optical breakdown
- gold nanoparticles
- in-situ spectroscopy
- photochemical reduction mechanisms
1. Introduction
The unique chemical and physical qualities of metallic nanoparticles have attracted the attention of researchers. Their size- and shape-dependent optical properties make them especially appealing due to the potential technological applications [1, 2, 3, 4]. In particular, gold nanoparticles (AuNPs) have strong absorptions in the visible spectrum that come from the collective oscillations of surface conduction-band electrons as they interact with light, which is called the surface plasmon resonance (SPR). The dependence of the SPR absorption on particle size and shape opens a range of application possibilities for AuNPs, including surface enhanced Raman spectroscopy [5]; non-invasive diagnostic imaging [2]; photothermal cancer therapy [3, 6]; plasmon-enabled photochemistry; and catalytic reactions such as water-splitting [7, 8]. It is necessary to these ends that the NP sizes and shapes are controllable during synthesis [9, 10]. Control can be achieved chemically, by modifying experimental conditions like temperature, reaction time, metal-ion concentration, and the absence or presence of reducing agents and surfactants [2]. Laser-assisted approaches to AuNP fabrication allow the manufacture of “pure” NPs which lack chemical reducing agents or surfactants, making this synthesis method ideal for NPs intended to be used in catalysis, and other electronic, biological or medical applications [11, 12].
There are two common approaches to colloidal AuNP synthesis using laser-assisted methods. The first is bulk-metal ablation, in which metal atoms are ejected from the target material and form nanoparticles in solution [13]. The second is by irradiating a metal-salt solution to produce reducing agents via solvent-molecule photolysis [14, 15, 16]. Controlling nucleation and growth of the nanoparticles during metal-salt reduction by changing laser parameters (focusing conditions, pulse duration, pulse energy, irradiation time), and chemical parameters (metal-ion concentration, solvent composition, presence of capping agents), determines the size, shape, and stability of the colloidal products [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. Laser-assisted AuNP fabrication requires a simple setup, which facilitates experimentation [12].
Section 2 of this chapter examines
2. Background: interactions of ultrashort laser pulses with condensed media
In a dielectric medium with a band gap that exceeds the laser photon-energy, ultrashort laser pulses can produce quasi-free electrons in the conduction band by two processes: (1) nonlinear multiphoton ionization and tunneling photoionization [31], and (2) high-kinetic-energy free electron collisions with neutral molecules, causing cascade ionization, also called avalanche ionization [32]. The formation of free electrons initially generates a localized, weakly-ionized plasma [32, 33, 34], which can initiate optical breakdown (OB), supercontinuum emission (SCE), or both [35, 36, 37]. This section provides an overview of the theory behind both processes, and some experimental measurements.
2.1. Optical breakdown
Optical breakdown (OB) of a transparent dielectric medium occurs when the free-electron density
Conventionally, the laser pulse propagates in the
where
The time evolution of the free-electron density
Free electrons are produced according to the photoionization rate
At a given laser wavelength, the peak-intensity needed to reach critical electron density for OB is highly dependent on pulse duration, due to the interplay between the photoionization and cascade ionization rates [33, 34]. To illustrate this effect over a wide range of pulse durations used in recent
The high pulse-energies of up to 5 mJ and tight-focusing conditions often used in
2.2. Supercontinuum emission and filamentation
The filamentation process leading to SCE arises from self-focusing of the laser pulse in a nonlinear Kerr medium. A full discussion of the details of nonlinear light propagation leading self-focusing is beyond the scope of this work and may be found in Refs. [35, 36, 37]. Briefly, filamentation depends on the laser power
where
2.3. Experimental measurement of OB and SCE
The presence of OB and SCE may be measured with a spectrometer arranged as shown in Figure 2(a) [35, 36]. To detect OB from light that has scattered off of the OB plasma, the fiber mount is placed at a 90
To illustrate the conditions in which OB, SCE, or both are present, tightly focused pulses [16] and unfocused, collimated pulses [28] were measured using the setup in Figure 2(a). Figure 2(b) shows spectra obtained at detector (i) for tightly focused 30 and 1500 fs pulses at 0.3 and 0.03 mJ pulse energy. The broadened spectrum for 30 fs pulses indicates the presence of SCE along with OB, while the narrow spectrum with 1500 fs pulses indicates that no SCE occurs. Figure 2(c) and (d) show SCE spectra obtained for 30 fs pulses at a series of pulse energies for tightly focused and collimated pulses. Under both conditions, asymmetric broadening towards the visible region of the spectrum grows with increasing pulse energy. The spectral broadening saturates for tightly focused pulses at energies above 1.2 mJ (Figure 2(c)), while greater pulse-energies would be needed to saturate the spectral width for unfocused pulses (Figure 2(d)). No OB is observed when the beam is collimated, indicating that a low-density plasma (LDP) with
3. Mechanisms of [AuCl4]− reduction
3.1. Reactions of water
The key role that water photolysis plays, in the photochemical reduction of
Although both hydrated-electrons and hydrogen radicals are capable of reducing
Another product of water photolysis,
where the existing AuNPs act as a catalyst for
3.2. Kinetics
Controlling the sizes and shapes of AuNPs starts with kinetic control of their nucleation and growth. LaMer’s nucleation theory, developed in 1950 [60], was used to describe AuNP formation first [4, 9, 61], but Turkevich’s studies [62] on reduction of
where [A] is the precursor (metal salt) concentration, [B] is the metal nanoparticle concentration,
where [A(0)] is the initial precursor concentration. The rate law in Eq. (13) has been used to describe AuNP formation from reducing ionic precursors via wet chemical routes [67, 68, 69] and for femtosecond laser-induced
The time-dependent concentrations of
Under certain experimental conditions where the
The third rate constant,
Extracting the rate constants at a series of experimental conditions (e.g., solution pH [15], pulse energy [16, 28]) provides significant insight into the roles that the reactions in Eqs. (4)–(9) play in the conversion of
The power law dependence of
Under both tight-focusing and LDP conditions, the power law dependence of
The kinetics results quantifying the dependence of both nucleation and autocatalytic growth rate constants demonstrate the importance of both short and long-lived reducing species to control the formation of AuNPs via photochemical reduction of aqueous
4. Controlling Au nanoparticle sizes
Several recent articles have reported some degree of control over the size of AuNPs synthesized by photochemical reduction of
4.1. Laser parameters
4.1.1. Focusing-geometry
Focusing-geometries influence the nature of the nonlinear interactions between the laser and solution (c.f., Section 2). Without strong-focusing, SCE yields a LDP environment containing electron-densities on the order of
Another phenomenon to consider when experimenting with focusing-conditions is cavitation bubble formation, which happens when the OB electron-density threshold is exceeded [38]. The generated cavitation bubbles are sensitive to the shape of the laser-plasma [20, 72]. Under low-NA focusing-conditions with filamentation and SCE, the bubbles are ejected from the focus with low kinetic energy, seen as a small stream of bubbles rising from the center of the cuvette in Figure 5(b). This condition results in inefficient and asymmetrical mixing-dynamics of the reactive species throughout the solution, but can be improved with the addition of a magnetic stir-bar [20]. Similarly, a stir bar is needed when operating under LDP conditions to ensure that the solution is being mixed [26, 28]. Both high-NA focusing and SSTF produce more spherical plasmas, which eject high kinetic energy bubbles into solution radially [72], causing turbulent mixing of the reactive species into the solution (evident in Figure 5(c)) and removing the need for stirring [16, 20].
The complex interactions between high-intensity fs laser pulses and aqueous solution result in the particle size being sensitive to different focusing-conditions. Table 1 summarizes the results of AuNP syntheses prepared in aqueous solutions without capping agents across focusing conditions. Of the reported sizes, the largest AuNPs resulted from the LDP conditions [26, 28], which limits production of the reducing species, driving AuNP formation through aggregative growth and agglomeration. Smaller AuNPs were formed with the high-NA focusing conditions [16, 18], which produces high electron-densities because of tight laser-focus; filamentation is suppressed, and the reducing species (electrons and
4.1.2. Pulse energy and duration
Several studies on focusing-conditions have demonstrated that increasing the pulse energy reduces AuNP size [16, 20, 28]. When tight-focusing geometry was used, increasing the energy of a 30 fs pulse from 0.15 mJ to 2.4 mJ decreased AuNP size from
While the pulse energy strongly influences the size of the AuNPs from photochemical reduction of
4.2. Chemical composition
4.2.1. Scavengers
Water photolysis produces reactive species, which govern
The hydrated-electron scavenger
Belmouaddine et al. [26] investigated the effect of adding
In another study, Uwada et al. [21] investigated the effects of alcohols (1-propanol, 2-propanol, ethanol) on aqueous
act as the primary reducing agents of
4.2.2. pH
Changing the pH of irradiated aqueous
For comparison with the results in Ref. [15], experiments performed in our laboratory using the tight-focusing conditions in Ref. [16] also showed that the AuNP size depends on solution pH. Aqueous solutions (0.1 mM
4.2.3. Capping agents
One of the most widely used strategies for controlling AuNP size during chemical synthesis is the addition of capping agents, including polymers and surfactants, to stop particle growth [2]. Increasing the molar ratio of a capping agent to Au(III) salt typically results in smaller AuNPs in chemical syntheses [79]. A similar trend is observed for femtosecond laser-based syntheses of AuNPs, as shown in Table 2. Nakamura et al. [14] found that addition of polyvinylpyrrolidone (PVP) to aqueous
In addition to controlling AuNP size in femtosecond laser-based syntheses, capping agents like PEG
5. Conclusion
Photochemical reduction of
Acknowledgments
This work was supported by the American Chemical Society Petroleum Research Fund under Grant no 57799-DNI10 and Virginia Commonwealth University.
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