Rapid continuous flow synthesis of high-quality silver nanocubes and nanospheres† Hakim Mehenni, Lutfan Sinatra, Remi Mahfouz, Khabiboulakh Katsiev and Osman M. Bakr * We report a biphasic-liquid segmented continuous flow method for the synthesis of high-quality plasmonic single crystal silver nanocubes and nanospheres. The nanocubes were synthesized with controllable edge lengths from 20 to 48 nm. Single crystal nanospheres with a mean size of 29 nm were obtained by in-line continuous-flow etching of as-produced 39 nm nanocubes with an aqueous solution of FeNO 3 . In comparison to batch synthesis, the demonstrated processes represent highly scalable reactions, in terms of both production rate and endurance. The reactions were conducted in a commercially available flow-reactor system that is easily adaptable to industrial-scale production, facilitating widespread utilization of the procedure and the resulting nanoparticles. 1. Introduction The optical, magnetic and catalytic properties of metal nano- particles are highly dependent on their sizes and shapes. 1–4 As the number of compositions, shapes, and sizes of useful nanoparticles increases at a growing rate, it is becoming imperative to develop reaction methodologies that enable their scalable production without compromising their quality. For this reason, tubular and narrow channel continuous ow reactors, as alternatives to traditional batch synthesis, have drawn interest because they combine precise control over experimental conditions, such as temperature, pressure, resi- dence time (i.e. the time for which the reagents reside inside the reactor, equivalent to the reaction time for a traditional batch synthesis), and ow rate, with the advantage of minimal thermal and chemical gradients. 5–8 Traditionally, continuous ow reactors were conned to the realm of organic synthesis, in which the reagents and products were in liquid or gaseous states. More recently, they have been utilised in the synthesis of metal nanoparticles, 9–14 such as silver, 15,16 gold, 17,18 palladium, 9 platinum 19 and cobalt. 20 In this paper, we describe a novel method for the synthesis of high-quality single crystal silver nanocubes (NCbs) and nano- spheres (NSs) using a two-phase (liquid–liquid) segmented ow system in a Teon-PFA microtube coil reactor. Silver nano- crystals have garnered the attention of researchers as their surface plasmon resonance (LSPR) is stronger and more efficient than that of gold nanoparticles in the visible wave- length range. Silver's enhanced surface plasmon resonance makes silver nanocrystals useful for chemical and biological sensing (e.g., substrates for surface-enhanced Raman scat- tering), 21–24 plasmonic enhanced photovoltaics, 25 and plas- monic waveguides. 26 Several methods have been developed for the batch synthesis of silver nanocrystals with varying shapes, including spheres and cubes, 27–29 plates, 30 bars and wires. 31–33 Among these, silver NCbs have received considerable research interest due to their great potential as catalysts with well-dened facets 34,35 as building blocks for self-assembly, 36 and as sacricial templates for the synthesis of gold, platinum, and palladium nano- structures. 37,38 Moreover, etching silver NCbs is the only known way to obtain high-quality single crystal silver NSs larger than 40 nm. 29 Unlike multiply twinned and irregularly shaped silver nanoparticles (>15 nm), sometimes confusingly referred to as ‘nanospheres’, silver NSs have highly spherical shapes akin to polystyrene and silica spheres that are used to assemble highly ordered photonic crystals. 39 The combination of their spherical shape and monocrystalline nature is shown to reduce ohmic losses, which makes silver NSs a sought-aer material for self- assembled plasmonic nanostructures. 40 Unfortunately, due to the difficulty of producing them in signicant quantities (vide infra) their use has been seldom reported. There are several notable batch preparation methods for silver NCbs, 29,38,41 the most well known of which are (i) polyol reduction and (ii) hydrochloric-acid-assisted polyol reduction. 42 Polyol reduction can be rapidly carried out within 15 minutes using a trace amount of sodium hydrosulde (NaHS) as the reducing agent, yet this method is limited to small quantities and reaction volumes of a few millilitres. Using the hydro- chloric-acid-assisted polyol method, production of silver Division of Physical Sciences and Engineering, Solar and Photovoltaics Engineering Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. E-mail: osman.bakr@kaust.edu.sa † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra43295e Cite this: RSC Adv., 2013, 3, 22397 Received 28th June 2013 Accepted 19th September 2013 DOI: 10.1039/c3ra43295e www.rsc.org/advances This journal is ª The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 22397–22403 | 22397 RSC Advances PAPER Published on 20 September 2013. Downloaded by King Abdullah Univ of Science and Technology on 19/04/2014 13:40:16. View Article Online View Journal | View Issue