Gold particle formation via photoenhanced deposition on lithium niobate
Introduction
Metallic nanostructures have a variety of unique optical [1], electronic [2], and catalytic properties [3]. Therefore, metal nanoparticles have been studied for a range of applications including: solar energy conversion [4], [5]; information processing [6]; transparent conducting electrodes [7], [8]; medicine [9]; and sensing [10].
Researchers have developed several techniques for controlling metal nanoparticle formation. For instance, thin films of metal evaporated onto lithographically defined masks provides the ability to pattern nanoparticles, but is a relatively expensive and time consuming approach [11]. On the other hand, if patterning is not a concern, simply heating thin metal films can result in their dewetting from the substrate surface to form islands [7], [12], resulting in polydispersed particles.
Metal nanoparticles can also be formed in solution with in-situ reduction of metal salts into colloidal nanoparticles. Gold chloride reduction mediated by chemical reagents such as citrate is an example of this approach, which dates back to 1953 [13] and is still widely used and studied [14]. This technique requires relatively high temperatures, due to the weak reduction potential of citrate [15], and often involves the incorporation of stabilizing agents and surfactants [15]. Gold-citrate aggregates may form as a by-product of the citrate method [16]. Contemporary work on this reaction involves fine-tuning the pH [17], concentration [18], and order of reactions [19] in order to finely-control nanoparticle sizes. Such fine control over size is essential for tuning the optical properties of gold nanoparticles. By tuning the size of a gold nanoparticle, the wavelength of light which is resonant with the plasmonic excitation of the particle is correspondingly shifted [1].
In this work, we present an alternate technique for reducing gold chloride into nanoparticles: the photoexcitation of a polar insulating substrate acting as a driver for the reaction. In contrast with other in situ techniques, nanoparticle formation occurs at room temperature without additional reagents or stabilizing agents. In principle, this technique is similar to gold particle formation on surfaces such as silicate [20] or graphene [21]. However, in this case, the energy for the reaction comes from photoexcited electrons as opposed to oxidation of the surface.
The substrate we choose is lithium niobate, a ferroelectric semi-conducting material. Ferroelectrics have the property of holding permanent polarizations and surface charge. By locally controlling the polarization of domains, the local reactivity of ferroelectrics can be controlled [22]. When illuminated with above-band gap light, internal electric fields drive photogenerated electrons to the surface, preferentially to the positively-terminated domains and the boundaries between domains. These charges, in turn, enable surface reactions [23]. Thus, lithium niobate illuminated with above band gap light (>3.9 eV) has been used for the reduction of silver nitrate into silver nanoparticles [24]. Lithium niobate patterned to have alternating polarized domains, periodically polarized lithium niobate (PPLN), can be used to produce silver nanowires, as a result of the tendency of silver nanoparticles to preferentially form on the boundaries, consistent with the availability of electrons [25]. While lithium niobate has been the most ubiquitous substrate for these experiments, other ferroelectrics, such as lead zirconate titanate [26] and barium titanate [27] have also been studied for this technique.
Nanoparticles templated in such a manner may in principal be lifted from the ferroelectric and placed on a target substrate, and the ferroelectric template may be reused. Alternately, the nanoparticles can be left in place; this technique has been exploited for the growth of silver nanoparticles on PPLN for surface-enhanced Raman spectroscopy [28], controlled emission from Nd+ [29] and enhanced emission from nanoscale lasers [30], [31], [32]. Lithium niobate also exhibits evanescent light-induced photovoltaic fields, which may be used as optical tweezers [33]; metallic nanoparticles could aid in this effort. Thus, beyond nanoparticle formation, this system holds promise for a range of nanophotonics applications.
While the deposition of silver on illuminated PPLN has been well established, there are fewer studies on the deposition of other materials with this technique. In this work, we explore the reduction of gold chloride into nanoparticles via illuminated PPLN. Park et al studied this process for a single gold chloride solution concentration, varying exposure times for the deposition [34]. This work contributes a concentration-dependent study and uses this concentration dependence to further understand nanoparticle growth on PPLN. We find that there is a substantial dependence on the concentration for the nucleation rate, size, and deposition pattern for gold nanoparticles. We also directly compare the gold nanoparticle deposition to that of silver. We find the silver and gold depositions to be quite dissimilar, even when accounting for optical absorption and molecule motility.
Section snippets
Materials and methods
The lithium niobate samples used have dimensions of 5 mm × 5 mm × 0.5 mm and come cut perpendicular to the polar axis, maximizing surface charge. The samples have a periodic polarization, and are obtained from Crystal Technologies. The polarizations have 180° domain walls. Prior to use, the PPLN substrates are cleaned via sonication for 10 min in acetone and methanol, and dried with nitrogen. The samples are then placed in a shallow petri dish, and a 25 μL drop of the gold (III) chloride or silver
Theory
The reaction required for gold chloride to form particles can proceed via several pathways. In particular, dissolving HAuCl4 in water results in molecules of the form AuClx where x may range from 2 to 4, with the percent of molecules of each species dependent upon the pH, with a lower x for more acidic solutions [35]. In our solutions the only source of acid is the HAuCl4; thus the higher concentrations are more acidic. While a number of pathways for reduction are possible [36], [37], here we
Results and discussion
Typical AFM results for select concentrations are shown in Fig. 1. The particle size and number density analysis is summarized in Table 1. For both high and low levels of gold chloride concentration, nanoparticle deposition is sparse, and particles tend to be large, >100 nm. However, for concentrations of 1E−7 to 2E−6, nanoparticles deposit with a high density and smaller average size (<100 nm). When discussing the size of the particles, we refer to the diameter of the base looking onto the
Conclusions
In conclusion, we find that UV illuminated PPLN can be used as a reducing surface for gold chloride, resulting in gold particles, and the size of the particles is highly sensitive to the concentration of the gold chloride. No surfactants or reagents are used: deposition occurs as a result of interaction with the photo-excited PPLN surface. The mechanism for gold reduction is explored, and the chemistry of this reaction plays a critical role in the resulting deposition patterns. Gold
Acknowledgements
This work is supported through the National Science Foundation under Grant # DMR-1206935. We gratefully acknowledge the use of facilities with the LeRoy Eyring Center for Solid State Science at Arizona State University. We thank Yang Sun for UV–Vis data, and Brandon Palafox for material preparation. VM acknowledges support from the Arizona NASA Space Grant Internship program.
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