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Chemical Physics Letters 445 (2007) 271–275

Capturing electrified nanodroplets under Rayleigh instabilityby coupling electrospray with a sol–gel reaction

Dan Li a,b, Manuel Marquez c, Younan Xia a,*

a Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USAb INEST Group Postgraduate Program, Philip Morris USA, Richmond, VA 23234, USA

c Research Center, Philip Morris USA, 4201 Commerce Road, Richmond, VA 23234, USA

Received 20 June 2007; in final form 31 July 2007Available online 7 August 2007

Abstract

We demonstrate that electrospray could be combined with a sol–gel process to produce non-spherical nanostructures. The electro-sprayed nanodroplets solidified under Rayleigh instability via a controlled sol–gel reaction, enabling us to capture elongated intermedi-ates in the solid form. This work demonstrates the potential of electrospray, a remarkably simple and well-established technique, forfabricating one-dimensional nanostructures by taking advantage of the Rayleigh instability phenomenon. It also provides direct exper-imental evidence to confirm the existence of Rayleigh instability for electrified nanodroplets.� 2007 Elsevier B.V. All rights reserved.

1. Introduction

The behavior of electrified droplets has been of greatinterest both fundamentally and technologically for overa century. As an effective technique for generating bothmicro- and nanoscale charged droplets, electrospray hasfound many technological applications such as ink-jetprinting, spray painting, production and patterning ofnanoparticles, and particularly ionization of moleculesfor mass spectrometry [1–8]. In an electrospray process, acharged liquid jet is produced by applying a high-voltageto a solution that flows through a nozzle. If the solutionis not sufficiently viscous, the jet will break up into manysmall droplets due to varicose instability. As confined bysurface tension, the resultant particles usually take a spher-ical shape [1]. Here we demonstrate that nanostructureswith non-spherical shapes could also be produced usingelectrospray by taking advantage of the Rayleigh instabil-ity of electrified droplets.

0009-2614/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2007.07.090

* Corresponding author. Fax: +1 206 685 8665.E-mail address: xia@chem.washington.edu (Y. Xia).

In 1882, Rayleigh predicted that an electrified dropletwould become unstable in terms of shape once the densityof surface charges had exceeded a threshold (now known asthe Rayleigh limit), beyond which the electrostatic repul-sion would overcome the surface tension, leading to elon-gation of the droplet and emission of fine charged jets [9].As demonstrated in a recent experiment [10], the densityof charges on an electrified droplet could increase to passthe Rayleigh limit as the droplet shrank in volume due tosolvent evaporation. The Rayleigh instability forced thedroplet into a series of elongated intermediates. After los-ing a significant amount of charges via the emission ofhighly charged jets, the stretched droplet would relax backto spheres as driven by surface tension while the jets imme-diately broke up into smaller droplets as a result of varicoseinstability. We suspect that if the droplets could be rapidlysolidified under Rayleigh instability via a chemical reac-tion, the elongated intermediates could be captured andlocked in the solid form. As a result, non-spherical particlescould be produced.

This concept can be demonstrated by coupling the elec-trospray process with a sol–gel reaction. As schematicallyshown in Fig. 1, the density of charges on an electrified

Fig. 1. Schematic illustrating the shape evolution of a drying electrifieddroplet that contains a sol–gel precursor. Differing from neutral droplets,the drying of electrified droplets could lead to the formation of solidparticles with various shapes through three different routes (I–III). See thetext for more details.

272 D. Li et al. / Chemical Physics Letters 445 (2007) 271–275

droplet will gradually increase during its flight to the col-lecting electrode as the solvent evaporates [9]. If the dropletcontains a sol–gel precursor, the moisture in air will diffuseinto the droplet to hydrolyze the precursor while the sol-vent is being evaporated. As a result, the viscosity of thesolution in the droplet is expected to increase as well.Depending on the experimental conditions, the dryingdroplet may take three different routes: (I) Rayleigh insta-bility occurs when the charge density exceeds the Rayleighlimit, but the sol–gel reaction is too slow to allow the gela-tion to take place before the deformed intermediates relaxback into spheres or break up into smaller spheres. Theproduct will contain spherical particles with a bimodal dis-tribution in size. (II) Rayleigh instability occurs togetherwith quick gelation in the droplet. The gelation will preventbreakup of the jets formed via Rayleigh instability andrelaxation of these jets to spheres, resulting in elongatednanostructures. (III) Rayleigh instability is prevented anda spherical particle is formed because gelation and solidifi-cation occur before the charge density reaches the Rayleighlimit. In this case, if gelation and solidification only occuron the surface of the droplet, the final product will becomea hollow particle.

2. Experimental

We demonstrated the concept by electrospraying alco-holic solutions of titanium butoxide (Aldrich), a commonlyused sol–gel precursor for preparing titania. It is well doc-umented that the hydrolysis rate of a metal alkoxide can becontrolled by choosing appropriate solvents and additives(e.g., acetic acid in our experiments), and by adjustingthe concentration of water in the solution [11]. Differingfrom a conventional sol–gel process in which water is oftenadded to the bulk solution directly, no water was added tothe stock solution in our electrospray experiments. Instead, water was introduced into the droplets through thediffusion of moisture from the atmosphere during electro-spray. The hydrolysis rate was controlled by adjustingthe humidity in the atmosphere and/or the concentrationof the precursor in the solution. Since no polymers were

added to the solutions, the viscosities of the solutionsbefore electrospray are close to the values of the pure sol-vents and not sufficiently high for electrospinning [12,13].The electrospray setup is similar to the one we previouslyused for electrospinning [12,13]. In a typical procedure,the solution containing the sol–gel precursor was loadedinto a plastic syringe equipped with a stainless steel needle.The needle was connected to a high-voltage supply (ES30P-5W, Gamma High Voltage Research Inc., Ormond Beach,FL). The feed rate was controlled using a syringe pump(KDS-200, Stoelting Co., Wood Dale, IL). In all experi-ments, the feed rate was fixed at 0.03 mL/h. A piece ofgrounded aluminum foil was placed �5 cm below the tipof the needle and used as the collector. The electrosprayprocess was conducted at room temperature in a chamberin which the relative humidity was controlled by purginga mixture of saturated moisture/dry nitrogen with con-trolled ratios through the chamber. The voltage wasadjusted to a level at which only one steady jet was formed(cone-jet mode). It was observed that the most uniformparticles could be obtained at this jet mode. Dependingon the composition of the solution and humidity, the volt-age could vary from 3.0 kV to 5.1 kV. Higher voltagesmight lead to the formation of smaller particles withbroader size distributions, but this parameter seemed tohave a minor effect on the shape of the resulting particles.Samples for SEM studies were directly prepared by placingsilicon wafers on the aluminum foil during electrospraying.SEM images were taken using a field-emission scanningelectron microscope (Sirion, FEI, Hillsboro, OR) operatedat an accelerating voltage of 5 kV.

3. Results and discussion

Fig. 2 shows how the shape of collected particles wasaffected by the relative humidity or the hydrolysis rate ofthe sol–gel precursor. When the electrospray atmospherewas very dry, only small spherical nanoparticles wereobtained (Fig. 2A). In this case, the low humidity in theenvironment limited the hydrolysis of the precursor. WhenRayleigh instability occurred, the viscosity of the solutionin the deformed droplets did not reach a sufficient levelto fix the shape of the elongated intermediates, leading tothe formation of spherical particles (i.e., the Route-I shownin Fig. 1). When the humidity was increased (Fig. 2B andC), Rayleigh instability occurred together with rapid gela-tion and solidification of the deformed droplets; the elon-gated intermediates were captured and locked in a solidform (Route-II). At higher levels of humidity, the dropletsbecame less stretchable, resulting in the formation of elon-gated particles with shorter tails (Fig. 2C). When thehumidity became too high (Fig. 2D), the surface of thedroplets were rapidly hydrolyzed and gelated, leading tothe formation of hollow particles (Route-III). In this exper-iment, the hollow particles collapsed when the solventinside them evaporated. It is worth noting that very smallparticles also existed in the samples shown in Fig. 2B and

Fig. 2. SEM images of samples that were prepared by electrospraying a solution of 3% (v/v) titanium butoxide in a mixture of ethanol and 2-methoxyethanol (9:1 by volume) at four different levels of humidity: (A) 14%, (B) 28%, (C) 38%, and (D) 55%, respectively. Note that the final products aremade of amorphous titania.

D. Li et al. / Chemical Physics Letters 445 (2007) 271–275 273

C, which can be attributed to the breakup of some emittedjets. The number of such small particles decreased as thehumidity was increased.

We have tested a number of solvents, including ethanol,2-methoxyethanol, 1-propanol, 2-propanol, 1-butanol andtheir mixtures. It was found that elongated particles couldbe obtained from all these solutions as long as the humidityand concentration of the precursor were adjusted to theappropriate range although their sizes might vary withthe composition of the solution. In particular, non-spheri-cal nanostructures were also observed when acetic acid wasadded into the spraying solution (Fig. 3). The introductionof acetic acid could increase the conductivity of the solu-tion, facilitating the production of smaller droplets. How-ever, acetic acid decreased the hydrolysis and gelationrates of the precursor [11]. As a result, a more humid envi-ronment was needed in order to generate elongated parti-cles (Fig. 3). As shown in Fig. 3, the shape of resultingparticles could be controlled by humidity and the concen-tration of sol–gel precursor.

The shape of resulting particles also sheds some light onthe deformation mechanism of electrified nanodroplets.The behavior of charged droplets under Rayleigh instabil-ity has been experimentally examined for macroscopicdroplets and recently for microdroplets with the aid of

high-speed optical microscopy [10,14]. However, whenthe droplet size is further reduced to the nanometer regime,it becomes extremely difficult to follow the deformationprocess by conventional means, especially when rapidevaporation of solvent is involved [15]. As a result, thereis essentially no experimental evidence to confirm theexistence of Rayleigh instability for electrified nanoscaledroplets. Rapid fixing of electrified nanodroplets underRayleigh instability via a sol–gel reaction has allowed usto take snapshots of the droplets as their shape evolves.We have clearly observed three types of shapes that corre-spond to different deformation modes: Some particles exhi-bit a spindle-like shape with two thin tails, similar to that ofthe an electrified microdroplet of ethylene glycol fromwhich two symmetrical jets were ejected [10]. Some parti-cles show a tadpole-like shape, indicating a deformationbehavior similar to that of an electrosprayed microdroplet[14]. Interestingly, nanorods and nanowires with slightlythicker ends were also observed in many samples, suggest-ing that their deformation might be caused by unidirec-tional stretching of the droplets. These results maypresent much-needed experimental observations for furthertheoretical modeling of this complex process [15].

It is worth pointing out that the production of particlesby electrospray is a very complex process. Besides Rayleigh

Fig. 3. SEM images of samples that were prepared by electrospraying 1-propanol/acetic acid solutions containing titanium butoxide at differentconcentrations (v/v%): (A, D) 10%, (B, E) 3%, and (C, F) 0.6%. Samples A–C were prepared at a relative humidity level of 20% while samples D–F wereobtained at a humidity of 60%. The content of acetic acid was 10% for all these experiments. Based on the volume of elongated particles, the diameter ofdroplets that underwent Rayleigh instability was estimated to be in the range of 10–300 nm.

274 D. Li et al. / Chemical Physics Letters 445 (2007) 271–275

instability, it involves many other complicated phenomenaincluding rapid evaporation of solvent, chemical reactions,friction with air, as well as internal droplet hydrodynamics.For example, due to the variation in spatial distribution forthe droplets in the electrospray process, different dropletscould experience different local physical conditions (e.g.,temperature and humidity). Such variation could lead tosome difference in their evaporation and sol–gel reactionrates and eventually lead to polydispersity in size and shapefor the resultant particles (see Figs. 2 and 3). Despite thiscomplexity, our experiments clearly demonstrated thatthe shape of electrosprayed particles is controllable andfixable when coupled with a sol–gel reaction.

4. Summary

This work suggests a new route to the fabrication ofnon-spherical and hollow nanostructures from low-viscos-ity solutions by electrospray. This work, together withprevious demonstrations of electrospray [1–7] and electro-spinning [12,13,16], indicates that judicious use of charges

on the surface of a liquid droplet as well as chemical reac-tions to manipulate solidification of the liquid could lead tosimple and unusual approaches to the production of vari-ous nanostructures with controllable shapes. This workalso provides direct experimental evidence to confirm theexistence of Rayleigh instability for electrified nanodro-plets. The hollow nanostructures shown in Fig. 2D couldbe applied to encapsulation – a process that has foundwidespread use in a variety of applications, including con-trolled release of drugs, contrast agents, cosmetics, inks,pigments, or chemical reagents; protection of biologicallyactive species; and removal of wastes. The one-dimensionalnanostructures could serve as inorganic fillers for makingnanocomposites, or as active components for fabricatingsolar cells.

Acknowledgements

This work was supported in part by an AFOSR-MURIgrant on smart skin materials awarded to the UW, and aresearch fellowship from the David and Lucile Packard

D. Li et al. / Chemical Physics Letters 445 (2007) 271–275 275

Foundation. Y.X. is a Camille Dreyfus Teacher Scholar.Part of the work was performed in the Nanotech UserFacility at the UW, a member of the National Nanotech-nology Infrastructure Network (NNIN) funded by theNSF.

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