Electrospraying route to nanotechnology: An overview

ARTICLE IN PRESS

Journal of Electrostatics 66 (2008) 197–219 www.elsevier.com/locate/elstat

Electrospraying route to nanotechnology: An overview A. Jaworek, A.T. Sobczyk Institute of Fluid Flow Machinery, Polish Academy of Sciences, Fiszera 14, 80-952 Gdansk, Poland Received 8 June 2007; accepted 12 October 2007 Available online 28 January 2008

Abstract Electrospraying (electrohydrodynamic spraying) is a method of liquid atomization by means of electrical forces. In electrospraying, the liquid at the outlet of a nozzle is subjected to an electrical shear stress by maintaining the nozzle at high electric potential. The advantage of electrospraying is that droplets can be extremely small, in special cases down to nanometers, and the charge and size of the droplets can be controlled to some extent by electrical means, i.e., by adjusting the flow rate and voltage applied to the nozzle. Due to its properties, electrospraying is considered as an effective route to nanotechnology. The paper considers the latest achievements in microand nano-thin-film production, including self-assembled nanostructures, in solid nano-particle generation, and in the formation of micro- and nanocapsules. r 2008 Elsevier B.V. All rights reserved. Keywords: Electrospraying; EHD spraying; Nanotechnology; Thin-film deposition; Fine powder production; Electro-encapsulation; Direct writing; Spray forming; Biotechnology

1. Introduction Electrospraying (electrohydrodynamic spraying) is a method of liquid atomization by means of electrical forces. In electrospraying, the liquid flowing out of a capillary nozzle, which is maintained at high electric potential, is forced by the electric field to be dispersed into fine droplets. Electrospray systems have several advantages over mechanical atomisers. The size of electrospray droplets can range from hundreds micrometers down to several tens of nanometer. The size distribution of the droplets can be nearly monodisperse. Droplet generation and droplet size can be controlled to some extent via the flow rate of the liquid and the voltage at the capillary nozzle. The fact that the droplets are electrically charged facilitates control of their motion (including their deflection and focusing) by means of an electric field. Charged droplets are selfdispersing in space, resulting also in the absence of droplet coagulation. The deposition efficiency of a charged spray on an object is higher than for an un-charged spray. This

Corresponding author.

E-mail address: [email protected] (A. Jaworek). 0304-3886/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2007.10.001

feature can be advantageous, for example, in surface coating, thin-film production, or electroscrubbing. Electrospraying can be widely applied to both industrial processes and scientific instrumentations. The interest in industrial or laboratory applications has recently prompted the search for new, more effective techniques which allow control of the processes in which the droplets are involved. Electrospraying has opened new routes to nanotechnology. Electrospray is used for micro- and nano-thin-film deposition, micro- or nano-particle production, and micro- or nano-capsule formation. Thin films and fine powders are (or potentially could be) used in modern material technologies, microelectronics, and medical technology. Research in electro-microencapsulation and electro-emulsification is aimed at developing new drug delivery systems, medicine production, and ingredients dosage in the cosmetic and food industries. Electrohydrodynamic spinning (electrospinning) of viscous liquids facilitates the production of nanofibers for masks, filters, scaffolds for biological tissue, and intelligent garment manufacturing. Recent advancements in nanoelectrospray technologies have been briefly reviewed by Salata [1]. This paper reviews many aspects of these emerging applications of electrospray in nanotechnology. In this

ARTICLE IN PRESS A. Jaworek, A.T. Sobczyk / Journal of Electrostatics 66 (2008) 197–219

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Nomenclature d ~ D ~ E ~e F ~g F ~r F ~Z F ~ g Ll p ~ q ~ vl Q gl

droplet diameter electric flux density electric field electrodynamic force gravitational force inertial force drag force gravity acceleration volume density of electromagnetic and polarization forces on the liquid phase pressure surface charge density liquid jet velocity liquid volume flow rate liquid bulk conductivity

paper, the term nanotechnology refers to the processing, production, or application of materials and structures having size scale less than 100 nm. Thus nanotechnology includes the investigation of physical phenomena at supraatomic levels, the creation of atomic clusters/compounds (e.g. polymers, quantum dots, molecular switches, carbon nanotubes, fullerenes of the size o10 nm), generation of nanoparticles (nanograins, nanocapsules of diameter o100 nm), deposition of nano-thin films (nanocoatings, self-assembling monolayers, nanowires of thickness o100 nm), spinning of nanofibers (carbon fibers, polymer fibers, textiles, nanoyarns of diameter o1000 nm), and production of nanostructured materials (nanocomposites which are conjunction all of above). There are two main approaches to the nanotechnology: ‘‘top-down’’ and ‘‘bottom-up’’. The ‘‘top-down’’ approach refers to physical or chemical machining of a bulk material down to nanometer scale by grinding, milling, etching, or lithographing. The ‘‘bottom-up’’ approach involves building a nanostructure from elementary components via moleculeby-molecule or grain-by-grain deposition onto a substrate, epitaxial growth, plating, and intercalation or implantation. Among these techniques, electrospraying is placed among the ‘‘bottom-up’’ techniques because the building blocks are produced from fine droplets after solvent evaporation.

e0 rl rq Zl sl ~ ~ Dp P ~ ~Z P ~ ~l L ~ ~ Xi ~ 1n ~ ~ 1

permittivity of the free space mass density of the liquid volume charge density liquid viscosity surface tension of the liquid stress tensor acting on the liquid surface due to hydrostatic pressure difference stress tensor acting on the liquid surface due to liquid viscosity tensor of electromagnetic and polarization stresses on inter-phase surface tensor of the surface tension unit vector (perpendicular to the inter-phase surface) unit tensor (perpendicular to the inter-phase surface)

strong electric filed is build-up at the capillary outlet. Liquid flowing out from the nozzle forms a meniscus, which becomes elongated in this electric field, and disintegrated into droplets due to electrical forces. There are two groups of forces, which cause deformation and disruption of the liquid jet (cf. Fig. 2): bulk forces on the jet, and normal and tangential stresses at the liquid surface. The bulk forces on the jet may be described as follows: 1. The electrodynamic force (Fe), proportional to the electric field. For a continuous medium, this force can be represented by the volume density of the electrodynamic forces (cf. Nomenclature): h i ~l ¼ rq E ~ þ 1 Dr ~ TE ~  Er ~ TD ~ . L (1) 2 The electrodynamic force occurs due to the electric field caused by the voltage imposed on the capillary nozzle, and also from the space charge of any previously emitted droplets (FQ).

2. Fundamentals A photograph of a typical electrohydrodynamic atomizer (electroatomizer) is shown in Fig. 1. The device consists of a capillary nozzle, usually made from a fine, hypodermic needle, and a ring extractor electrode. Usually, the capillary nozzle is connected to a high-voltage supply, while the ring electrode and a substrate are grounded. In another configuration, the nozzle is grounded, while the extractor electrode is at high voltage. By this means, a

Fig. 1. A photograph of an electroatomizer.

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2. Stress tensor due to pressure difference on both sides of the inter-phase surface: ~ ~ ~ Dp ¼ ~ 1ðpl  pg Þ. P

(5)

3. Stress tensor due to liquid dynamic viscosity, which is important for liquid in motion, and is proportional to the gradient of liquid velocity perpendicular to the inter-phase surface: ~ ~ Z ¼ Zl r~ P vl .

(6)

4. Stress tensor due to liquid inertia, which is proportional to the dyadic product of local liquid velocity at the inter-phase surface: ~ ~ r ¼ rl~ P vl . vl  ~

(7)

These forces and stresses cause the meniscus to elongate into a jet, which then disintegrates into droplets. The symbol  denotes the dyadic product of two vectors. The balance of bulk forces per unit volume on the jet is given by the following equation:   qrl~ vl ~ ~ ~ ~St  r P ~ Dp þ P ~Z þ P ~r ~l  F ¼ rl ~ gþL (8) qt and the stress balance by the equation ~ ~ ~ ~ ~ ~l . ~Z þ P ~r þ L ~ Dp þ P r~ X¼P

Fig. 2. Schematic illustration of stresses and forces on the electrohydrodynamic liquid jet.

2. Gravitational force (Fg), expressed also as a volume density: ~g ¼ rl~ L g.

(2)

3. Inertial force (Fr) volume density: vl ~r ¼ rl d~ . L dt

Many modes of spraying are distinguished in the literature depending on the form of the meniscus, the pattern of motion of the jet, and a way it disintegrates into droplets. Various forms of the modes of electrospraying are schematically shown in Fig. 3. These modes can be grouped into two principal categories:

 (3)

4. The drag force (FZ). The drag force is well known for a moving droplet as the Stokes drag. However, to our best knowledge, for a moving free jet, the force on the jet-head has not been presented in general form. Lateral motion of the jet, observed for higher electric field, imposes additional complications on the force description. It can be only stated that the drag force is proportional to the surrounding gas viscosity and jet velocity. The stresses on the jet surface, which deform the jet ~ ~i of shape, are opposed to the tensor of the surface tension X the liquid. The following stress tensors can be distinguished: 1. Electrodynamic stress tensor, resulting from the surface charge density (q) and the local electric field E: h i ~ ~ ~l ¼ ~ ~E ~l  E ~þ~ ~g Þ  Eð ~D ~l  D ~g Þ . L qE 1  12 Dð

(4)

(9)



Dripping modes: These modes are characteristic in that only fragments of liquid are ejected directly from the capillary nozzle; these fragments can be in the form of regular large drops (dripping mode), fine droplets (microdripping mode), elongated spindles (spindle or multispindle modes), or sometimes irregular fragments of liquid. At some distance from the nozzle outlet, however, these fragments contract into spherical droplets. Jet modes: In this case, the liquid is elongated into a long, fine jet, which can be smooth and stable (cone-jet mode) or can move in any regular way. For example, it may rotate around the capillary axis (precession mode) or oscillate in its plane (oscillating mode). Sometimes, a few fine jets on the circumference of the capillary can also be observed. This specific mode is known as the ‘‘multi-jet’’ mode. In each case, the jet disintegrates into droplets due to electrostatic forces.

The most important mode of spraying is the cone-jet mode. In this mode, the liquid meniscus assumes the form of regular, axisymmetric cone with a thin jet (o100 mm in

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Fig. 3. Various modes of electrospraying.

diameter) at its apex, stretching along the capillary axis. The end of the jet undergoes instabilities of one of two types: varicose and kink. These modes of spraying are systematically presented by Hayati et al. [2,3], Cloupeau and Prunet-Foch [4,5], Shiryaeva and Grigor’ev [6], and Jaworek and Krupa [7,8]. In recent years a novel approach to the classification of the spraying modes has been based on Fourier analysis of current oscillations correlated with fast jet imaging [9–13]. This analysis opens a new way to quantitative analysis of electrospraying, however, an effect of electrical discharge (corona discharge), which was observed many times during electrospraying [2,3,14–19], on the spray current, and on the spraying modes has not been considered. Eqs. (8) and (9) have never, to our knowledge, been solved in the general case. There are, however, considerations regarding specific modes of spraying, mainly the cone-jet mode. For the cone-jet mode, the size of the droplets is usually given by the following equation [20–24]: d¼a

ar QaQ a 0 rl ag . sas l gl

(10)

The constant a in Eq. (10) depends on the liquid permittivity. The given exponents for the spraying parameters vary, depending on author (cf. Table 1). This scaling law has been confirmed by many experiments for single and coaxial jet, and recently, for a jet generated within an insulating liquid. Nowadays this law is commonly accepted and widely used in the literature [21,25,26]. Recently, the scaling laws were modified for strong ionic solutions and

Table 1 Constants proposed for Eq. (8) to determine droplet size Authors

aQ

ae

ar

as

ag

Fernandez de la Mora and Loscertales [20] Gan˜an-Calvo [21] and Gan˜an-Calvo et al. [22] Hartman et al. [23]

1 3 1 2 1 2

1 3 1 6 1 6

0

0

1 6 1 6

1 6 1 6

1 3 1 6 1 6

dry particles by Basak et al. [27], and for pulsed drop generation by Chen et al. [28]. The main conclusion from Eq. (10) is that the droplet size can be decreased via decreasing the liquid flow rate and increasing liquid conductivity or surface tension. This equation does not take liquid viscosity into consideration because the droplet size only weakly depends on this parameter [29]. This phenomenon was explained by Paine et al. [30]. For liquids of higher viscosity, more energy is required to drive the liquid from the meniscus, and, therefore, the droplets are generated with lower frequency than for liquids of low viscosity. The size of the capillary also has an effect on the droplet diameter. In general, changing liquid physical properties independently of each other is not an easy task, hence droplet size control by this method is limited in practice. The minimum flow rate at which the cone-jet mode can operate at steady state was determined by Barrero and Loscertales [31]: Qmin 

sl 0 r . rl gl

(11)

ARTICLE IN PRESS A. Jaworek, A.T. Sobczyk / Journal of Electrostatics 66 (2008) 197–219

In this case, the size of droplets can be on the order of 1 mm, when the liquid’s electrical conductivity is 103 S/m. The droplet’s size decreases to about 10 nm when the conductivity assumes a value of 1 S/m. The electrospraying has the following advantages over conventional mechanical atomizers: 1. Droplet size is smaller than that available from conventional mechanical atomisers, and can be smaller than 1 mm. 2. The size distribution of the droplets is usually narrow, with small standard deviation that allows production of particles of nearly uniform size. 3. Charged droplets are self-dispersing in space (due to their mutual repulsion), resulting also in the absence of droplet coagulation. 4. The motion of charged droplets can be easily controlled (including deflection or focusing) by electric fields. 5. The deposition efficiency of a charged spray on an object is order of magnitudes higher than for un-charged droplets. These characteristics characterize electrospraying as a versatile tool for micro- and nano-thin-film deposition, or micro- and nano-particle production. 3. Applications 3.1. Micro- and nano-thin-film deposition Thin solid films, thinner than 10 mm, are used in manufacturing micro- and nano- electromechanical systems (MEMS or NEMS), in microelectronic devices as semiconducting, insulating or conducting layers, or for improving surface properties of mechanical elements. There are several conventional methods available for thin-film deposition on a substrate. These include: 1. Casting of a solution or colloidal suspension on a substrate, followed by solvent evaporation;

201

2. 3. 4. 5.

Cathode spraying, applicable to metal layer preparation; Condensation of vapors of a material on the substrate; Laser ablation for material evaporation; Chemical vapor deposition and plasma assisted/enhanced chemical vapor deposition; 6. Physical vapor deposition; 7. Electroplating, applicable only to metal film formation. Recently, many researchers have tested the electrospray deposition technique of liquid-phase materials on various substrates. Electrospray deposition (Fig. 4a), is a process in which droplets produced by electrospraying from a solution or suspension of a material to be deposited are targeted to a substrate to form a tight surface layer. A solid layer is obtained after solvent evaporation. Evaporation can be sped-up by heating the substrate. To improve mechanical properties, the layer may be sintered at higher temperatures, if applicable. Usually, the material to be deposited is sprayed directly onto the substrate, but the layer can also be formed from a precursor. The precursor is a compound which is decomposed at high temperature or converted to another substance in chemical reactions with other compound sprayed simultaneously or delivered in the gaseous phase. The reactions usually take place on the substrate, and a new product is obtained (Fig. 4b). Initially, electrospray was used to produce thin layers of radioactive materials, such as a- or b-particle sources or neutron emitters (e.g. obtained from U233, Pu238, Am241 or Cm242 nitrates) [32–35] or targets prepared for activation in particle accelerators or nuclear reactors. Nowadays, electrospray is involved in nanotechnology and nanoelectronics for thin-film deposition. The following applications have been reported in the literature:

 

Solar cells, thin films produced from CdS, CdSe, SnO2, or TiO2 [36–43]; Fuel cells, thin films made of La(Sr)MnO3 on Zr substrate, YOx on Ni–ZrO2, and ZrO2:Y2O3, GdxCe1xO2, La1xSrxCo1yFeyO3, or ZrO2:Y2O3 deposited on Ni72Cr16Fe8 substrate [44–58];

Fig. 4. Scheme of electrospray deposition of micro- and nano-thin film: (a) from a solution or suspension of particles to be deposited and (b) from a precursor thermally decomposed on the substrate.

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A. Jaworek, A.T. Sobczyk / Journal of Electrostatics 66 (2008) 197–219

Lithium batteries, thin cathode layers made of LiMn2O4, LiNiO2, LiCoO2, LiCoxMn2xO4, LiCo0.5Ni0.5O2, LiAl0.25 Ni0.75O2, and V2O5; anodes made of SnO2 [37,59–82]; Micro- and nanoelectronic devices, metal-oxide layers used as dielectric: Al2O3 [83–89], CeO2 [90–92], CoO [83], MgO [93–95], Mn3O4 [73], RuO [96], SnO2 [36,40,65,97–101], Ta2O5 [102], TiO2 [38,103–107], ZnO [83,108–110], ZrO2 [51,83,103,111–117], WO3 [118], alumina–zirconia composite [119], or semiconductor films: CdSe, CdS or ZnS [39,41,42,107,120–122], SiC [111–115,123,124], SnO2+LaOCl [125] or ZrO2:Y2O3 [51,126]; MEMS, piezoelectric microactuators made of zirconium n-propoxide, Zr(C3H7O)4, titanium tetraisopropoxide, Ti((CH3)2CHO)4, lead acetate, Pb(CH3COO)2, or lead– zinc–niobate (PZN) [127,128], piezoelectric transducers of PbTiO3 [129,130], or BaZrO3 [131], or polymer ferroelectric films on an Si wafer [132]; Other applications, electrocatalysts made from NiCo2O4 [133], catalyst made of platinum [134], ITO glasscovered with In2O3:Sn [135], optochemical sensors with SnO2 layer [136,137], plasma displays with (Zn,Mn)2SiO2 dielectric film [138].

The precursors used for the formation of these thin films by electrospray pyrolysis and solvents applied by this process are summarized in Table 2. For example, metal nitrates or acetates dissolved in water, methanol, ethanol, or their mixtures are electrosprayed as precursors for metal–oxide layer production. The suspensions tested as source of nanoparticles for thin-film deposition via electrospraying are listed in Table 3. All the authors reported that the films produced by electrospraying were homogeneous and composed of small agglomerates built of particles smaller than 1 mm, which were the particles of dry powder used for preparing a suspension, or crystallites smaller than 1 mm. The crystallites are formed as an effect of solvent evaporation from the droplet during its flight towards the substrate. In specific cases the layer exhibit better properties than those obtained by other methods. For example, in the case of lithium batteries, the cathode or anode deposited by the electrospraying method showed very stable charging/ discharging characteristics. The spray systems used usually operate in the cone-jet mode, but sometimes the multi-jet mode also is used [93,94]. The multi-jet mode made it possible to obtain simultaneously a large number of emission cones and droplets smaller than those obtainable from a single cone. Changing the physical properties of the liquid to be sprayed allows tailoring, to some extent, of the film morphology. The quality of thin film formed on a substrate strongly depends on the size of particles or droplets forming the layer, and their monodispersity. Even layers of uniform thickness are obtained when the droplets are uniformly dispersed over the substrate. Smaller particles of narrow size distribution are required in order

to reduce the number and size of voids, flaws, and cracks in the film. Thus electrospray is a promising tool for the production of high quality layers, because it fulfills all of these requirements. This technique allows generating fine droplets in micro- and submicron size ranges, with narrow size distribution. Electrostatic forces disperse the droplets homogeneously in the space between the nozzle and the substrate. The film thickness, its crystallinity, texture, and deposition rate can be easily controlled by varying the voltage, flow rate, concentration of the material to be deposited, and the substrate temperature. Many authors have optimized the electrospray process with regard to its application to thin-film deposition in nanotechnology (cf., for example [139–141]). The electrospray methods used for thin-film deposition have been reviewed by Jaworek [142]. Fig. 5 illustrates the progress in the thinnest films produced by electrospraying over the last decade. By this method, also organic films thinner than 100 nm have been produced [143]. At the present time, the thinnest film reported is 15 nm [132]. From analysis of the experimental data from over 20 papers, a general law can be formulated that the deposition rate of a layer is proportional to the final thickness of the layer. This trend is illustrated in Fig. 6. The power in the trend-line equation is close to unity. Large scattering of the points around the trend line is a result of different experimental conditions and various properties of the sprayed liquids. This law means that thinner layers are deposited with lower rates than those of larger final thickness. In other words, the reciprocal of the deposition rate (or the time required for deposition of a unit-thick film) is inversely proportional to the final film thickness. This means that the time of deposition of, for example, 1 mm layer takes about half an hour while a 100 mm film deposition lasts only 10–20 s. This unobvious effect results from the fact that lower particle suspension or precursor concentrations have to be used for thinner films to obtain coatings that are more uniform and have a lower roughness factor. From the reviewed literature it can be concluded that electrospray facilitates the production of extremely thin layers, which can be crack-free and more homogeneous than those obtained by other methods. The process is simple, cheap, flexible, and easy to control. Compared to other methods such as chemical vapor deposition (CVD) or physical vapor deposition (PVD), its main advantage is that the growth rate of the layer is relatively high. For example, the deposition rate of a 1-mm thin-film layer produced by electrospray is about 0.1 mm/min [144], while for PVD it is in the range of 0.006–0.06 mm/min [145], and for CVD from 0.02 to 0.05 mm/min [144]. The electrospraying process can be carried out in an ambient atmosphere, in air or other gas, and at low temperature, without the need for complex reactors and high-vacuum systems. Using the electrospray deposition technique, highly pure materials can be produced, with structural

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Table 2 Precursors and solvents used for thin film deposition or microstructure formation by electrospraying Layer

Precursor

Solvent

Substrate temperature (1C)

References

CaP

Ca(NO3)2  4H2O+H3PO4

300 325–400

CdS CdSe CeO2 CeO2:NiO:Gd CoO

Ca(OH)2+H3PO4 CdCl2+(NH2)2CS CdCl2+(NH2)2CSe Ce(NO3)2 Ce(NO3)3  6H2O, Gd(NO3)3  6H2O, Ni(NO3)2  6H2O Co(NO3)2  6H2O

Ethanol+butyl carbitol Butyl carbitol (C8H18O3) Ethanol Ethanol+water Ethanol+water Ethanol+water Ethanol Ethanol or ethanol+butyl carbitol (+acetic acid) Ethanol+butyl carbitol Ethanol Methanol Ethanol+methyl glycol Ethanol+butyl carbitol

200–450 100–450 400–600 200–450 100–250

[146,147] [148] [149] [39–43,120,122] [42,120,150] [91,92] [47] [83]

230–350 200–550 350–600 180–400 230–350

[57] [135] [36] [48] [56]

225–500 450 250

[53,54] [79] [78]

230–450 300

[74–77] [80] [61,62] [79] [82] [59,72,73,104]

GdxCe1xO2 In2O3:Sn La0.7Ca0.3CrO3d La1xSrxCo1yFeyO3

LiAl0.25Ni0.75O2 LiBPO4 LiCoO2 LiCoxMn2xO4 LiCo0.5Ni0.5O2 LiMn2O4

Gd(NO3)3  6H2O+Ce(NO3)3  6H2O SnCl4+InCl3 Sn(CH3COO)+HF La(NO3)3+Ca(NO3)2+Cr(NO3)3 La(NO3)3  6H2O+Co(NO3)2  5H2O+SrCl2  6H2O+Fe(NO3)2  9H2O (La0.85Sr0.15)0.95MnO3d La(NO3)3  6H2O+Sr(NO3)2+Mn(NO3)2  6H2O+SrCl2  6H2O Li(CH3COO)  2H2O+Ni(CH3COO)2  4H2O+Al(NO3)3  9H2O Li(CH3COO)  2H2O+H3BO4+P2O5 Li(CH3COO)+Co(NO3)2  6H2O LiNO3+Co(NO3)2  6H2O LiNO3+Co(NO3)2+Mn(NO3)2 Li(CH3COO)  2H2O+Ni(CH3COO)2  4H2O+Co(NO3)2  6H2O LiCl+Mn(CH3COO)2  4H2O Li(CH3COO)2  2H2O+Mn(CH3COO)2  4H2O

LiNiO2 MgO

LiNO3+Mn(NO3)2  6H2O Li(CH3COO)2  2H2O+Mn(NO3)2  4H2O Li(CH3COO)+Ni(CH3COO)2 Mg(C11H19O2)2

Mn3O4 NiCo2O4

Mg(CH3COO)2 Mn(CH3COO)2 Ni(NO3)2  6H2O+Co(NO3)2  6H2O

PbTiO3 Pb(Zn1/3Nb2/3)O3 Pb(Zr, Ti)O3 Pt

Pb(CH3COO)2  3H2O+[CH3(CH2)3O]4Ti Pb(Zn1/3N2/3)O3+PbTiO3+BaTiO3 Zr(C3H7O)4+Ti((CH3)2CHO)4+Pb(CH3COO)2 Pt(NH3)4(OH)2H2O, or PtCl4, or platinum acetylacetonate

RuO SiC SnO2 SnO2:Cu SnO2:F SnO2+LaOCl SnO2:Mn2O3

RuCl3  xH2O Ter-polysilane SnCl4  5H2O SnCl4  5H2O+Cu(NO3)2  2.5H2O SnCl4  5H2O+HF SnCl4  5H2O+LaCl3  6H2O SnCl4  5H2O+Mn(CH3COO)2  4H2O

Ta2O5 TiO2

Ta(OC2H5)5 Ti(i-C3H7O)4

V2O5 WO3 (Zn,Mn)2SiO2

Ti((CH3)2CHO)4 Titanium diisopropoxide bis(2,4-pentamedionate) [(CH3)2CHO]3VO W(C2H5O)6 Mn(CH3COO)2+Zn(CH3COO)2+tetraethyl orthosilicate

ZnO

SnCl4  5H2O+Zn(CH3COO)2  2H2O Zn(CH3COO)2 Zn(C2F3COO)2 Zn(NO3)2  2H2O

ZnS ZrO2

ZnCl2+(NH2)2CS Zr(OCH2 CH2CH3)4 ZrO(NO3)2  xH2O

ZrO2:Y2O3

Zr(C5H7O2)4+Y(C5H7O2)3 Zr(C6H7O2)4+YCl3  6H2O Zr(C6H7O2)4+YCl3  6H2O Zr(C5H7O2)4+YCl3  6H2O or Y(NO3)3  6H2O Zr(C3H7O)4+YCl3  6H2O or Y(NO3)3  6H2O

Ethanol+butyl carbitol Ethanol Ethanol or ethanol+butyl carbitol (+acetic acid) Ethanol or ethanol+butyl carbitol Ethanol Ethanol Ethanol Ethanol Ethanol, or ethanol+butyl carbitol (+acetic acid), or ethanol+glycerol Ethanol Ethanol Ethanol Ethanol+acetic acid; or tetrahydrofuran+1-butyl alcohol Ethanol Ethylene glycol or butyl carbitol Ethanol)+di(ethylene glycol) butyl ether 2-Ethoxy ethanol water+ethanol+acetic acid Water+ethanol, or water+butyl carbitol Ethanol+butyl carbitol Toluene Ethanol Ethanol Ethanol Ethanol Ethanol or ethanol+butyl carbitol (+acetic acid) Ethanol or ethanol+butyl carbitol Ethanol or ethanol+butyl carbitol (+acetic acid) Ethanol 2-Propanol Ethanol Ethanol Ethanol or ethanol+butyl carbitol (+acetic acid) Ethanol or ethanol+butyl carbitol (+acetic acid) Ethanol+hydrochloric acid Methanol Water+izopropyl alcohol+acetic acid Water Ethanol Ethanol+butyl carbitol+water+(0–0.5%) polyvinyl alcohol (87%) or ethanol Ethanol+butyl carbitol+acetic acid Ethanol+butyl carbitol Ethanol+diethylene glycol Ethanol+diethylene glycol

450 235–300 200–400 700 400–500

[64,66–69,81] [59,70,71] [79] [93,94]

265–400 170–220 350–400

[95] [73] [133]

150 200 25–100 343–575

[129,130] [128] [127] [134]

200 1300 300–500 150–400 500–550 400 400

[96] [123] [65,97,98,136,137] [40] [100,101] [125] [97,98]

100–210 250

[102] [104]

100–220 350–550 200 200–400

[105] [38,107] [63] [118] [138]

100–250

[83]

350 340–400 350

[108] [109] [110]

450–550 100–220 200–500

[106] [83] [139,140]

300–400 285–340 480–575 300–335 325

[51] [50] [141] [52] [52]

ARTICLE IN PRESS A. Jaworek, A.T. Sobczyk / Journal of Electrostatics 66 (2008) 197–219

204 Table 2 (continued ) Layer

Precursor

Solvent

Ethanol+butyl carbitol or ethanol+1-methoxy-2-propanol Ethanol+butyl carbitol or ethanol+1-methoxy-2-propanol Ethanol+butyl carbitol or ethanol+1-methoxy-2-propanol Ethanol+butyl carbitol or ethanol+1-methoxy-2-propanol

ZrO(NO3)2  xH2O+YCl3  6H2O or Y(NO3)3  6H2O Zr(C5H7O2)4+ZrO(NO2)3+YCl3 ZrCl4

Substrate temperature (1C)

References

265–325

[52]

196–365

[58]

325

[52]

Table 3 Submicron layers formed from suspensions deposited by electrospraying Layer

Particles size (sintering temperature)

Solvent

Flow rate (spray time)

References

Alumina (Al2O3)

500 nm 100 nm (100–250 1C)

Ethanol Ethanol or ethanol+butyl carbitol

[84–89] [83]

Molybdenite (MoS2) Nickel (Ni)

120  1000 nm (platelets) 2–3 nm (600 1C)

Platinum on carbon (Pt//C) Silica (SiO2)

5 nm 20 nm 5 nm 3 nm 410 nm (1450 1C) 410 nm (1500 1C)

Isopropanol, acetone, alcohol, or toluene Ethylene glycol monoethyl ether acetate+alkylnaphthalene+polyamine Isopropanol or isopropanol+Nafion Ethylene glycol Ethylene glycol 1-Octanol Ethanol+0.5% dispersant Ethanol+0.5% dispersant

0.0036–6 ml/h 0.8 ml/h or 1.5 ml/h (60 min) 2.4 ml/h (25 min)

[154] [155,156] [157] [158] [159] [160]

200 nm 500 nm (Al2O3), 400 nm (ZrO2) (1200 1C)

Butyl acetate+ethanol Glycerol (for alumina), olive oil (for zirconia)+1 wt% dispersant

0.2–1 ml/h 36 ml/h 22 ml/h 0.1 ml/h 3.3 ml/h (2 h) 0.36 ml/h (153 s for 100 layers) 0.6–45 ml/h (0.2 g/h) 0.25–250 ml/h

Silicon (Si) Zirconia (ZrO2)

Zirconia+alumina composite (Al2O3+ZrO2)

[117] [119]

100

100

y = 0.0023x0.9959 10

1

0.1

0.01 1990

1

2

Deposition rate, µm/h

Film thickness in micrometers

[151,152] [153]

3 4 5

6

7

8 9

1995

2000 Year

10

10

1

11

2005

2010

Fig. 5. Thinnest films produced by electrospraying over the last decade: 1. Hall and Hemming (1992) [161] (photosensitive resist); 2. Chen et al. (1996) [45] (ZrO2); 3. Denisyuk (1996) [162] (plastic); 4. Hoyer et al. (1996) [163] (celulose acetate); 5. Sobota and Sorensen (1997) [151] (MoS2); 6. Heine et al. (1998) [150] (CdSe); 7. Rhee et al. (2001) [95] (MgO); 8. Matsushima et al. (2004) [99] (SnO2); 9. Saf et al. (2004) [143] (organic); 10. Dam et al. (2005) [164] (MEH-PPV); 11. Rietveld et al. (2006) [132] (PVDF).

control at the nanometer scale. The electrospray method is also a very efficient process because at least 80–90% of base material can be deposited onto the substrate.

0.1

10

100

1000 10000 Film thickness, nm

100000

Fig. 6. Deposition rate vs. final film thickness.

3.2. Micro- and nano-particle production Fine particles of size smaller than 10 mm are, for example, applied for ceramic coatings, paints, or emulsion production, as powder in the cosmetic or pharmaceutical industries, or as toner in electro-reprographic systems. Nanoparticles may also be deposited to create thin solid films. Conventional methods of synthesis of nanoparticles in the gas phase for electronic, optical and magnetic

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An electrospraying system using ac/dc excitation was first proposed by Vonnegut and Neubauer [196]. The process of drop formation was controlled by an ac voltage at 60 Hz applied to the capillary. Later, ac/dc electrospray systems were developed by Sample and Bollini [188], Bollini et al. [189], Sato et al. [190–194], and Balachandran et al. [195]. The synchronous mode of droplet generation operates within a limited range of flow rate and ac frequency. Low viscosity of the liquid to be sprayed is favorable to synchronous generation over wider frequency and flow-rate ranges [29,30,197]. An increase in the flow rate allows droplet generation with higher frequency. From the reviewed literature it can be concluded that electrospraying allows production of fine solid particles over broad size range, from about 100 mm down to a few nanometers, and of small standard deviation. The production of such particles via electrospraying is easier than by other methods. This technique is particularly useful for the production of metal oxides or ceramic powders from the liquid phase (cf. Table 4). The size of the droplets and particles can be controlled by flow rate of the liquid, the voltage, or by the frequency of ac excitation. It was reported by many authors that electrospraying allows fine powder production without significant change in chemical 100 Particle size in micrometers

applications were reviewed by Kruis et al. [165] with only minor attention to electrospraying. Electrospraying was also successfully tested as a tool for fine particle production [16,166–185]. A diagram of the steps involved in micro- and nano-particle production is shown in Fig. 7. The solvent from the electrosprayed droplets evaporates, and the remaining solid material forms a fine powder. The particles are produced from a solution or suspension of a solid material. For the solutionbased droplets, the remaining substance crystallizes forming solid particles. When a suspension is used for powder production, the nanosized particles suspended in the solvent form a tight cluster after the droplets dry. The size of such particles can be controlled by changing the concentration of the dissolved or suspended substance. Electrospraying allows the generation of particles of small size, down to 10 nm, and of high monodispersity [27,106,182,184,185]. Fig. 8 illustrates the size of the smallest particles produced by electrospraying over the past decade. At the present time, the size of smallest particle reported is 4 nm [182]. The materials from which the micro- and nanoparticles were produced via electrospraying, the size of the obtained particles, and the production rate are summarized in Table 4. From this table we can see that the production rate of the particles is usually in the range of 105–104 particles/s for particles of about 10 mm, increasing to 1010–1011 particles/s for particles smaller than 10 nm. The cone-jet mode, most frequently used for the production of powders, operates only over limited voltage and flow-rate ranges. Any change in physical properties of the liquid—due, for example, to temperature variation— may also change this mode to another, or shift the operating range to other values of voltage and flow rate. The size and monodispersity of the droplets and particles can also undergo unacceptable change. The problem was solved by using pulsed or ac voltage superimposed on dc-bias voltage (ac/dc excitation) [188–195]. When the frequency of the ac voltage is properly tuned, the jet disintegrates into a stream of droplets of uniform size. The size of the droplets can be controlled by the dc-bias and ac-voltage magnitudes, ac frequency, and flow rate of the liquid. The production rate of particles can be controlled by the flow rate and excitation frequency.

205

1

10 1 0.1

2 4

0.01 0.001 1990

3 1995

5

6

2000 Year

7

9 8

10 2005

11 2010

Fig. 8. Finest particles produced by electrospraying over the last decade. 1. Verma et al. [186] (sodium silicate); 2. Meesters et al. [170] (DOP); 3. Chen et al. [182] (sucrose); 4. Lenggoro and Okuyama [185] (ZnS); 5. Dudout et al. [184] (CaCl2); 6. Choy and Su [39] (CdS); 7. Lenggoro et al. [173] (PSL); 8. Nakaso et al. [177] (SiO2, TiO2, ZrO2); 9. Erven et al. [174] (Pt); 10. Suh et al. [187] (HAuCl4); 11. Basak et al. [27] (FeO).

Fig. 7. Steps of micro- and nanoparticle production via electrospraying.

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Table 4 Electrosprayed materials used for micro- and nanoparticle production Material (precursor)

Solvent

Particle size (droplet size)

Production rate (flow rate, capillary i.d.)

References

C12H22O11 (sucrose)

Water Water (10 ppm-10%)+nitric acid

4  105 particles/s 2.5  1010 particles/sa (3–18 ml/h, 81 mm)

[166] [182]

CaCl2 CdS (CdCl+CH4N2S) CeO2 (Ce(NO3)3  6H2O) Dextrin DOP

Ethanol Water Ethanol+diethylene glycol butyl ether Water Ethanol ethylene glycol Ethanol or water

1–20 mm (20–600 mm) 4 nm–0.18 mm (0.04–1.8 mm) 8 nm 10 nm 10–100 nm (1.45 mm)

[184] [106] [90]

1–20 mm (20–600 mm) 21 nm (o0.1 mm)

(5 ml/h, 100 mm) (10–30 ml/h) 3.5–8.7  107 particles/sa (0.2–0.5 ml/h, 460 mm) (14.5 l/h for 100 nozzles, 53 mm) 108–1010 particles/s (0.1–30 ml/h, 200 mm) 108–1010 particles/s (0.1–30 ml/h, 200 mm) (0.6–5.4 ml/h—water, 12–144 ml/h—ethanol, 160 mm) 4  105 particles/s 5.3  1010 particles/sa (0.1 ml/h, 100 mm)

8-12 nm (0.42 mm)

6  107 particles/s (8 ml/h, 60 mm)

[174]

Water+methanol+surfactants, methanol+surfactants+ammonium acetate (0.2–20 mM) Ethanol+water mixture Ethylene glycol 1-Butanol Ethanol 1-Butanol Ethanol 1-Butanol Water Ethanol (0.0025–0.2 mol/l)

109 nm (o0.1 mm)

5.3  1010 particles/sa (0.1 ml/h, 100 mm)

[173]

50 nm (1 mm) 20 nm 10–40 nm 2.4–5 mm (5–20 mm) 10–40 nm 2.4–5 mm (5–20 mm) 100 nm-4 mm (5.7 mm) 65 mm 20 nm (0.125–0.64 mm)

[175] [176] [177] [179] [177] [179] [178] [168] [172,185]

1-Butanol 1-Butanol

100 nm–4 mm (5.7 mm) 10–40 nm

3.2  108 particles/sa (0.6 ml/h) (0.11 ml/h, 200 mm) (0.02 ml/h, 300 mm) (5–20 ml/h) (0.02 ml/h, 300 mm) (5–20 ml/h) 2.9  107 particles/sa (1–9 ml/h) 5  102 particles/s (3.6–5.16 ml/h, 170 mm) 3.2  108–1.3  1010 particles/sa (0.05–0.16 ml/h, 400 mm or 900 mm) 2.9  107 particles/sa (1–9 ml/h) (0.02 ml/h, 300 mm)

FeO [Fe(NO3)3] NaCl PSL

Pt (0.2 wt% H2PtCl6  6H2O SiO2

SiO2 (Si(OC2H5)4) SnO2 (Sn(OC4H9)4) TiO2 (Ti(OC3H7)4) TiO2 (Ti(OC4H9)4) Water glass ZnS (Zn(NO3)2+SC(NH2)2) ZrO2 (Zr(OC4H9)4) a

Water Water+methanol+surfactants, or methanol+surfactants+ammonium acetate (0.2–20 mM) Ethanol

100–150 mm 80–300 nm (1.3–1.5 mm) 80–300 nm (1.3–1.5 mm) 6–40 nm

[167,169] [16,170,171] [16,170,171] [27] [166] [173]

[178] [177]

Production rate estimated from the flow rate and mean diameter of the droplet.

composition and physical properties of the material to be sprayed. This process seems to be useful for industries requiring fine powders of material that must remain unaffected during the production process. 3.3. Electroencapsulation Encapsulation is a process for capturing solid particles, liquid droplets, or a gaseous bubble as a core material in a solid or liquid envelope (shell) made of another material. Encapsulation can be regarded as the conversion of liquid phase to powder particles having the physical and chemical properties of the shell for easier transport and processing of the core material. Under certain conditions (heating, diffusion, or shell dissolution), the core material can be released from the shell. Conventional techniques of micro- and nano-encapsulation include: sol–gel encapsulation [198], polymerization or atomization of an emulsion of core in shell material for shell hardening [199], coextrusion, a process of simultaneous spraying of two liquids using two coaxial nozzles [200], fluidized-bed mixing of core material (larger particles) with dispersed particles of a size of about 1 mm

which cover the core and are subsequently stabilized by spraying a coating formulation [201], coacervation, which is a process of two immiscible soles separation after addition of an electrolyte [202], and rapid expansion of a solution of a core and shell materials in a supercritical fluid (CO2) [203–205]. Solid envelopes are obtained by subsequent solidification of the shell material, for example, by solvent evaporation or chemical reactions. Regardless of the method of encapsulation, the process can be effective when the two materials forming a capsule are immiscible but are mutually wettable, the core material has higher surface tension, and the two droplets are about of the same size [206]. Employing electrical forces can increase the effectiveness of micro- and nano- encapsulation. This process is called electro-encapsulation. The following electro-encapsulation techniques may be found in the literature:



Impacting of two oppositely charged droplets (Fig. 9a) [206]: Both of the droplet’s streams are emitted from two separated capillary nozzles maintained at opposite potentials, one of the capillaries at positive, and the other at negative. The droplets collide due to Coulomb

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207

Fig. 9. Schematics of various electro-encapsulation processes: (a) impacting of two oppositely charged droplets; (b) electrospraying/evaporation of colloidal suspension; (c) electrospraying/gelatinization of colloidal suspension; (d) electro-coextrusion; and (e) structure of microcapsule.







attraction, forming a capsule via submerging the droplet of higher surface tension within that of smaller surface tension. Electrospraying/evaporation of colloidal suspension (Fig. 9b) [207,208]: The suspension is electrosprayed and next the shell is solidified by solvent evaporation. The surplus solvent of the solution evaporates, forming a hardened envelope. Electro-encapsulation via spraying of colloidal suspension requires low concentration of the particles in order to generate droplets with only one particle inside. Usually a surfactant is added to the colloidal suspension prepared for electrospraying to prevent particle coagulation, aggregation, or flocculation. Electrospraying/gelatinization of colloidal suspension (Fig. 9c) [191,209]: A suspension of core material is electrosprayed into a bath with gelatinizing or polymerizing agent. The agent forms a hard envelope on the core material. Ultraviolet light has also been tested for surface polymerization in order to form hardened envelope. Electro-coextrusion (Fig. 9d) [25,210–216]: The process is a simultaneous spraying of two different liquids from two coaxial capillaries. In this case, the capillaries are at the same potential. The core liquid flows from the central capillary, and the envelope liquid flows through the annular nozzle between the capillaries. Electrocoextrusion allows spraying of core liquid of high

resistivity, but only if the envelope has sufficiently high conductivity. The opposite arrangement, i.e., a conducting core liquid and dielectric envelope, is also feasible [210]. Electro-encapsulation was also used for micro- and nano-composite materials production. For example, the core material (acrylic powder) was charged with an electrostatic gun, while envelope powder was suspended in an ethanol+water mixture, and electrosprayed [201]. The droplets were deposited onto acrylic particles due to Coulomb attraction and, after drying, composite particles built from core acrylic powder and dry envelope were formed. Encapsulation permits the protection of environmentsensitive core materials, such as cultures, vitamins, flavours, dyes, enzymes, salts, sweeteners, acidulates, nutrients, or preservatives. Oil-in-water or water-in-oil emulsions are most frequently used as a carrier for such ingredients [210,217,218]. The shell may consist of starch, gums, fats, waxes, oils, dextran, polysaccharides, proteins, or glucoses. For their properties, micro- and nano-capsules are applied in pharmaceutical, cosmetic, or food industries. For example, by encapsulation, a drug used as a core material can be targeted and released in a controlled manner. The electro-encapsulation process was used in biotechnology, for example, for the capturing of living cells

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in polymer beads (for example, sodium alginate) [209,216,219–224]. It has been reported by the authors that the viability of the living cells was over 80–90%. In pharmaceuticals production, various drugs or antibodies have been encapsulated in polyethylenoxide, dextrose, starch, sucrose, lactose, poly-lactic-co-glycolic acid, or poly-lactic acid [31,225–227]. Poly(lactide) and polylactic-co-glycolic acid are used most frequently due to their biocompatibility and biodegradable properties, solubility in many organic solvents, and ease of manipulation of their physical properties. In the literature, there are also reports on applications of microcapsules in agrochemistry used to control insecticides or pesticides with higher efficiency and a smaller amount of an active agent, thereby reducing the environmental pollution [228]. In the textile industry, durable fragrances added to textile and skin softeners using microcapsules have been demonstrated by Nelson [229]. Capsules can also be generated synchronously by pulsed or ac-voltage excitation [191,209,230]. The capsules can be smaller than 1 mm and uniform. Application of alternating current [230] allowed the production of capsules of zero net charge with a frequency of 20 kHz. An advantage of ac excitation is that it does not require two capillaries of opposite potentials for capsule production. In contrast to dc electro-encapsulation, by which only polar liquids having free electric charge can be dispersed, the ac encapsulation technique also allows the use of dielectric liquids based on organic solvents. Various types of capsules (core and shell materials) produced via electro-encapsulation are summarized in Table 5. Fig. 10 illustrates progress in the electroencapsulation technology leading to a decrease in the size of microcapsules produced over the past decade. At the present time, the smallest microcapsules reported are 150 nm. Such capsules were produced by Loscertales et al. [211] using two coaxial nozzles. Many authors have shown experimentally that electroencapsulation offers several advantages over conventional methods used for capsule production. Most of the authors claim that this process does not disturb physical and chemical stability of the core material. Electro-encapsulation thus appears to be promising in the pharmaceutical, cosmetics, and food industries. In biotechnology, where low-temperature processes that preserve heat-sensitive materials are needed, electro-encapsulation also has been found to be very useful [215,230,231]. The shortcoming of this method is its low throughput, but for effective and rapid mass production of microcapsules, simultaneously operated capillary nozzles can be used. 3.4. Electrospray forming and direct writing Spray forming is a process by which fine, semi-solid droplets of a material to be deposited are placed layer-bylayer onto a substrate to form a bulk product or thick coating. During this process, the droplets impacting a

surface mix with previously deposited layer, penetrating its pores to form a tight product. The result is similar to a sintering process [233]. A comparison between spray forming technology and conventional casting was performed by Chaudhury et al. [234], Mesquita and Barbosa [235], Srivastava et al. [236], and Yu et al. [233]. The authors concluded that the spray-formed materials can be less porous, and their microstructure can be more uniform that those obtained via other methods, for example, casting. A bulk product is cooled faster without shrinkage, and strength within the material is relatively low. Spray forming from nanometer droplets additionally offers the possibility of production of alloys of controlled composition and microstructure down to molecular layers. Simultaneous spraying of metal droplets and ceramics can be used for micro- or nano-composite material production. It was noticed that the particles in the matrix are distributed uniformly, and the hardness of the spray-formed composite or alloy can be relatively great [234–236]. An advantage of spray-formed products is that the composition can be changed on demand during the course of production. Spray formed composites were, for example, produced from alumina and titania [234,237] components. Drop-bydrop, bulk materials were produced, for example, by Wang et al. [176] who applied non-electrified ink-jet printing using a suspension of micrometer ceramic particles. The height of the prism formed by this method was 10 mm. Electrospray forming is a process of layer-by-layer deposition of electrosprayed droplets of a solution or a colloidal suspension which are continuously dried or solidified to form a thick coating or bulk product. In conventional spray forming systems, 40–50% of the material is oversprayed [238], but in the electrospray forming process the efficiency can exceed 80%. Electrospray forming was proposed by Chen et al. [239]. The authors produced a SnPb coating by agitating an electrified jet with a piezoelectric transducer tuned to 10 kHz. The droplets were charged by induction at the instant they detached from the cone. The electrospray production of composite layers from crystallites of different materials was demonstrated by Diagne and Lumbreras [125] (SnO2+LaOCl for CO2 gas sensors), Nguyen and Djurado [51] (ZrO2:Y2O3 for fuel cells), and Choy [106,126] (Y2O3:Eu) (cf. Table 2). Multilayer films from different materials were deposited on a substrate by Choy et al. [46] and Choy [126] for fuel cell production, Wei and Choy [92] as a buffer layer for superconducting YBa2Cu3O7x film, Yoon et al. [81] for deposition a double-layer cathode of LiMn2O4-coated LiCoO2 for lithium batteries, and Miao et al. [114] (ZrO2+SiC) as a ceramic coating. Such films were deposited in consecutive processes by changing the precursor solution. Recently, the electrospray method was utilized by Balasubramanian et al. [119] using two coaxial capillaries for the production of composite ceramics from an alumina–zirconia mixture (cf. Table 3). Deotare and Kameoka [175] have fabricated nano-composite

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Table 5 Microcapsules produced by electrospraying Core material

Shell material

Capsule size

Method

References

Astrocytoma cells (1321N1) Bacteria in poly(vinyl alcohol)+water

Poly-dimethylsiloxane (PDMS)

Electro-coextrusion

[216]

Spraying a suspension

[223]

Bovine serum albumin

1–4% Poly(lactide) in 1,2-dichloroethane or dichloromethane+acetone Poly (D,L-glycolide-co-lactide) (PLGA) Sodium alginate Sodium alginate

10–40 mm (33 mm mean) 0.25–0.4 mm fibers, 1–2 mm capsules 4.77 mm

Spraying an emulsion

[227] [231] [219] [224]

Cells Enzymes Ethanol Ethylene glycol

Gas Glycerol Hepatocytes HepG2, C3A (2  106 or 4  106 cells/ml) Kerosene

Medicine Oil (66%)+water (33%)+emulsifier Paclitaxel (a drug) (up to 10%) Silicone oil Tributyl phosphate Triethylene glycol Water Water (50%)+sugar (49.5%)+colorant (0.5%)

Olive oil Photopolymer (Somos 6120-DuPont) Olive oil Somos oil Water Butyl phthalate

1 mm 0.15–10 mm 8 mm 25–60 mm 70 mm 2–20 mm

Sodium alginate (1.5%) +NaCl (0.9%)+water (solvent)

440–830 mm

Water Water

500 mm 100 mm

Polyurehane

100 mm

Bovine serum albumin (5, 20 and 50/ml) in 10vol% ethanol/water mixture Cocoa butter

10–20 mm

Spraying an emulsion Spraying into a bath Spraying into a bath, nozzle-toextractor system Electro-coextrusion Electro-coextrusion Electro-coextrusion Electro-coextrusion Electro-coextrusion Impacting of oppositely charged droplets Spraying into gelling bath CaCl2 (1.67%)+NaCl (0.9%)+HEPES (0.26%)+water Electro-coextrusion Spraying within a bath (nozzle at the bottom of vessel) Spraying within a bath (nozzle at the bottom of vessel) Surface polymerization

9–21 mm

Electro-coextrusion

[210]

Poly-lactic-co-glycolic acid (2–16 wt%)+surfactant (0–16%)+organic salts (0–2 mM)+acetonitrile Glycerol Olive oil Olive oil Olive oil Sunflower oil Cocoa butter

1–5 mm 0.2 mm (@0.1 ml/h) Microns 4 mm 1–8 mm 0.15–10 mm 25–60 mm 12.4–22.5 mm

Spraying an emulsion and drying

[208]

Electro-coextrusion Electro-coextrusion Electro-coextrusion Electro-coextrusion Electro-coextrusion Electro-coextrusion

[214] [215] [215] [211] [25] [210]

hemispherical cups from a blended polymer–sol–gel solution of SiO2 and polyvinylpyrrolidine (PVP) in ethanol/ water (0.5/0.5) mixture. The SiO2 nanocups were 50 nm in size. An example of the production of composite material via layer-by-layer electrospray deposition of two materials is shown in Fig. 11. In electrospray forming, the particles are of the same size, and have similar thermodynamic states. This feature offers significant advantages in reduction of the number and size of voids and cracks in the bulk product. Simultaneous, layer-by-layer or drop-by-drop deposition of solutions of materials of different properties allows composite bulk materials having novel and unique properties to be formed. Direct writing is a maskless process of drop-by-drop deposition of a material in a liquid or semi-solid phase to

20 mm 160–180 mm 100 mm

[215] [211] [215] [25] [213] [206] [209]

[212] [212] [191] [232]

draw a pattern on a substrate. The pattern can be directly printed under computer control by moving the substrate on an x–y table. The electrospray technique seems to be ideal tool for this goal, because it is a low-cost, flexible, and high-speed process which, additionally, can trace patterns of higher resolution compared to ink-jet printing or conventional lithography. In contrast to traditional inkjet printing techniques, in which the droplet size is about twice of the nozzle diameter, the electrospray process can produce droplets at least an order of magnitude smaller than the nozzle size. Thus, using coarser nozzles prevents them from clogging, but at the same time, the droplets can be of micrometer size. A schematic of direct writing using electrospray droplets is shown in Fig. 12a. Experiments with electrospray direct writing on solid surfaces were carried out by Jayasinghe et al. [155,240]

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210

using silica suspension in ethylene glycol and alumina suspension in ethanol. The silica layer was deposited onto a quartz glass, and alumina on a polymer substrate. The mean diameter of the deposited relics was between 4 and 8 mm. Wang et al. [160], composed a 5 mm long and 100 mm thick wall by depositing 100 layers build from 0.41 mm zirconia particles. The smoothness of the surface was better than 10 mm. The lines produced by Wang et al. [156] from SiO2 suspension in ethylene glycol were 80 mm in width. Recently, Lee et al. [241] applied a direct writing technique for the production of miniature spiral-type inductors by depositing silver nanoparticles suspended in ethylene glycol onto a polyimide substrate. The line width was about 100 mm, and its thickness 100–300 nm. The printing velocity was 10 mm/s. Nanometer-thin trace was produced by Park et al. [158] using 3 nm Si particles suspended in 1-octanol. The line width traced with a speed of 10–20 mm/s on a SiO2 layer was about 700 nm. A lattice of individual Ni dots of the size of 1 mm was deposited by Ishida et al. [153]. These

Capsule size in micrometers

1000 1 100

2

3

4

11 5 7

10

8

12

9

1

10

13

6 0.1 1990

1995

2000 Year

2005

2010

Fig. 10. Smallest microcapsules produced by electro-encapsulation over the last decade: 1. Sakai et al. [212] (kerosene/water); 2. Sato et al. [191] (kerosene/polyurethane); 3. Ganan-Calvo [21] (gas/water); 4. Watanabe et al. [224] (enzymes/sodium alginate); 5. De et al. [201] (acrylic powder/ fluorescent particles); 6. Loscertales et al. [211] (ethylene glycol/oil); 7. Lopez-Herrera et al. [25] (ethylene glycol/oil); 8. Bocanegra et al. [210] (oil+water/cocoa butter); 9. Yeo et al. [230] (water/PLA polymer); 10. Xu et al. [227] (bovine serum albumin/poly-lactide); 11. Xie and Wang [231] (bovine serum albumin/PLGA); 12. Marin et al. [214] (silicone oil/ glycerol); and 13. Mei and Chen [215] (ethylene glycol/olive oil).

examples demonstrate feasibility of deposition of various structures on a substrate using electrospraying. The printing quality was improved via placing a particle concentrator in the form of a sharp needle under the dielectric substrate co-axially with the capillary nozzle. This method was developed by Jayasinghe and Edirisinghe [87,89]. The concentrator was grounded or maintained at high potential of polarity opposite that of the droplets. In such a configuration, the electrode attracts the droplets and helps to converge their trajectories onto a target point on the substrate. Another method of direct writing was developed by Krinke et al. [242]. The authors deposited charged particles on an oxidized silicon substrate directly from the gaseous phase. The pattern to be deposited was drawn via contact charging with a sharp needle maintained at high potential. The charge remaining on the non-conducting surface attracted oppositely charged ions from the surrounding gas. Single charged indium particles of diameter of 30 nm were deposited by this way to form lines of about 100 nm in width on the substrate. Lenggoro et al. [243] have modified this method by using electrospraying as a source of particles to be deposited. This process is illustrated in Fig. 12b. The authors deposited Au or SiO2 particles of about 10–30 nm in size on a silicon/silicon oxide substrate. The pattern was drawn with a stainless steel needle at highpotential gliding over the substrate with zero force imposed. The substrate was next subjected to electrosprayed droplets of polarity opposite that of the pattern. The line width was about 100 nm. An ‘‘inverse pattern’’ was also produced by the authors. When the polarity of the particles was the same as that of the pattern on the substrate, and additionally an opposite potential was applied to the substrate, the particles were preferentially deposited off the pattern, leaving a particle-free surface in the location where the pattern was drawn. Although the achievements of electrospraying using spray forming and direct writing are less spectacular than those in fine particle and thin-film production, these few reports have opened a new route of electrospraying to nanotechnology, because the process is cheap, and more environmentally friendly than conventional techniques which use toxic substances to draw a pattern by removing unwanted material from the surface.

Fig. 11. Schematic of the production of composite material using layer-by-layer electrospray deposition.

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Fig. 12. Schematic of direct writing: (a) one-step process using electrospray droplets and (b) two-step process: ion deposition with a sharp needle followed by nanoelectrospray.

3.5. Biotechnology Biotechnology refers to the application of biological processes on molecular level for the fabrication of new materials for medicine, the food industry, the cosmetic industry, or for the improvement of conventional technologies. These processes require new technical, physical, and chemical tools for their manipulation. New such tools based on electrospraying have been developed in recent years. The experimental work concerning electrospray applications in medicine and biotechnology are summarized in Table 6. In the 1990s, electrospraying was tested as a means for generating on-demand particles. Initially, electrospray nebulisers were developed for testing the efficiency of medicine inhalation [244–246]. The authors emphasised that the electrospraying technique is very simple to operate and can produce particles in the ‘‘respirable’’ size range without disturbing the physical and chemical stability of the drug. The ‘‘respirable size range’’ refers to particles of aerodynamic size between 2 and 5 mm. Such particles are effectively transported to lower airways in adults [247–251]. Smaller particles have been produced in recent years. Submicron drug particles (smaller than 400 nm) have the advantage that they are not detected by the human immune system and can be effectively transported via the blood

vessels to a target human organ [31]. Electrospraying has also been tested for the production of biodegradable drugcarrying agents, for example, as nanoparticles and microcapsules from poly-lactic-co-glycolic acid (PLGA) [208]. For investigation purposes or for medical diagnostics, biologically active substances, such as enzymes and antibodies have also been electrosprayed. For example, antibodies electrosprayed by Moerman et al. [252–254] formed spots of diameter of 130–350 mm and remained biologically active after this process. The authors noticed that electrospraying in the stable cone-jet mode allowed for an accurate and reproducible dispersion of liquid without splashing, contrary to other deposition techniques such as piezodispensing or contact-printing. Biologically active proteins (a-lactalbumina) produced by Uematsu et al. [255] for protein-based biomaterials, biosensors, and biochips were also unaffected by the high potential used for electrospraying. A fine porous (reticular) structure having pores in the size range 40–600 nm was obtained. Pareta et al. [232] and Xu et al. [227] also have confirmed that the high voltage used for electrospraying a suspension of bovine serum albumin molecules for the encapsulation of drugs does not degrade the molecules. Thundat et al. [256] applied electrospray to the deposition of DNA molecules onto a gold substrate for investigation under a scanning tunnelling microscope. The sample was uniformly

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Table 6 Applications of electrospraying in biotechnology Material to be sprayed

Application

Solvent

Flow rate (deposition time)

References

Antibodies

Biologically active micrometer spots Medicine production Analytical chemistry Implants Samples for scanning tunnelling microscopy Biologically active micrometer spots Implants for bone repair Medicine delivery

Water, or ethylene glycol+water mixture

0.36–4 ml/h

[253,254]

300 nm–20 mm 3 nm Ethanol or butyl carbitol

0.36 ml/h 3.6 ml/h 0.5–2 ml/h (60 min) (2–6 s)

[251] [180] [146–148] [256]

Water, or ethylene glycol+water mixture

0.36–4 ml/h

[253,254]

Ethanol or methanol Ethanol+water (4 vol%)

[149,259] [244]

Medicine delivery

Ethanol

6–10 ml/h 0.23 mg/h (150 mm thickness) 1–3 ml/h

[250]

Medicine delivery Drug-carrying agent

Ethanol (7.5 or 10 wt%) Acetonitrile+surfactant+drug+organic salts

1.2–4.2 ml 0.1 ml/h

[245] [208]

Tissue repair

Chloroform, ethyl acetate, methylene chloride, acetone, trifluoroethanol, or acetonitrile Ethanol+HCl

0.01–10 ml/h

[266]

3.6 ml/h

[258]

0.2–0.29 ml/h (2–4 h) 3.3 ml/h (2 h) (10 min)

[268] [159] [255]

10, 15, or 20 ml/h

[181]

Aspirin Biomolecules Calcium phosphate DNA Enzymes Hydroxyapatite Insulin Methylparahydroxybenzoate (0.5–3%) Paracetamol Poly-lactic-co-glycolic acid Poly(D,L-lactidecoglycolide Siloxane Starch Zirconia a-Lactalbumina (0.4–1.8 mg/ml) a-Lactose monohydrate

Bio-compatible self-supporting scaffold structure Food industry Scaffold for bone regeneration Protein-based biomaterials (a-lactalbumina//Al// poly(ethylene terephthalate)) Drug production

De-ionized water or ethanol Ethanol+0.5% dispersant Water

Polyvinylpyrrolidone+de-ionised water (6, 8 or 10 wt%)

distributed with isolated strands, and the number of molecular aggregates was lower than for standard electrodeposition or drop evaporation techniques. In recent years, electrospraying and electrospinning were tested as tools for the production of scaffolds for tissue regeneration and for bone repairing. For example, calcium phosphate coatings on endosseous titanium implants were produced by Siebers et al. [146], and Leeuwenburgh et al. [147,148,257]. The properties of the coating were similar to those produced by ESD and RF magnetron sputtering, although the two coatings differed in roughness, Ca/P ratio, and molecular composition. The coatings were amorphous, irrespective of the initial Ca/P solution ratio, but they crystallized into different crystalline phases after annealing at 650 1C. Self-supporting scaffold structures with high macroporosity free of micro- or mesopores produced by Jayasinghe and Sullivan [258] using electrospraying were similar to natural structures. From NMR investigations it was determined that electrospray-produced structures had a degree of network condensation of 79% that was higher than for those obtained by the sol–gel method (66–70%). Bioceramic scaffolds based on zirconia for host tissue regeneration with reduced number and size of microcracks and micropores in struts (as compared to the slurry-dip coating technique) were produced by Chen et al. [159]. The mechanical strength and Young’s modulus

of the scaffold were higher. Huang et al. [149] produced hydroxyapatite relics on glass from 50 to 80 nm nanoparticles suspended in ethanol for the formation of implants for bone repair. The relics produced by electrospraying were able to support the growth of primary human osteoblast cells. Hydroxyapatite film was also prepared by Kim et al. [259] using sol–gel assisted electrostatic spray deposition with subsequent annealing at a temperature of 500 1C for 30 min. The method is potentially applicable to orthopedic and dental implants manufacturing, production of scaffolds for bone growth, or the creation of micro- and nano-scale surface morphology for a favorable cell response. Electrospun fibers were tested also for nerve regeneration by Bini et al. [260], and Yang et al. [261], blood vessel reconstruction by Ma et al. [262], Xu et al. [263,264], and muscle cells proliferation by Mo et al. [265]. Electrospray experiments in medicine and biotechnology often use poly-lactic-co-glycolic acid (PLGA) or poly (D,L-lactide-coglycolide) (PLG) due to their high biocompatibility, low toxicity and immunogenicity, solubility in many organic solvents, and ease of manipulation of their physical properties such as permeability, porosity, and crystallinity and biodegradability [208,266]. PLG is also used for drug encapsulation, because the drug can be easily released due to diffusion and erosion of the polymer. Water, water–methanol, or water–ethanol mixtures are

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the most frequently used solvents for biotechnological applications. In tissue engineering the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is used because it is biodegradable, biocompatible, inexpensive, and its characteristics are similar to conventional thermoplastics [267]. In the food industry and in pharmaceutical applications, starch is frequently used because products based on it are tasteless, odorless, colorless, and are non-toxic. Traditionally, starch dissolution was deposited by casting or extrusion, but Pareta and Edirisinghe [268] applied an electrospraying technique for starch film deposition. 4. Conclusions This review provides a summary of current achievements in electrospraying in its route to nanotechnology. Electrospraying is a versatile tool for liquid atomization that has the advantage of uniform droplet generation from inexpensive equipment. Electrospray devices can operate under atmospheric conditions, and the rate of particle production is easy to control via voltage and flow rate. Electrospraying is a single-step, low-energy, and low-cost material processing technology, which can deliver products having unique properties. It has been shown in this paper that electrospraying can be effectively used for micro- and nano-thin-film deposition, micro- and nano-particle production, and micro-and nano-capsule fabrication. Our brief review shows that interest in industrial and laboratory applications of electrospraying, which allows droplets smaller than 1 mm to be generated, has grown over the last decade. It has been prompted by the need for new, more effective techniques which will allow automatic control of droplet size and charge, as well as generation on-demand particles, especially in nanotechnology. In recent years, many companies have sought versatile methods for direct writing of electrically conducting ink, containing nanosized particles of silver or carbon to pattern thin, electrically conducting circuits. This process should be cheap, and more environmentally friendly than conventional techniques [269]. It seems plausible that electrospraying will be an ideal tool for this goal. It is expected that this technique will find many new applications in various industries due to its low investment, low operating costs, and high versatility. Electrospray operation at atmospheric conditions has the advantage that uniform micro- and nano-thin-film deposition of large areas is easily controlled with respect to deposition rate and film thickness. In micro- and nanoparticle production, electrospray allows particles of uniform size over a wide size range to be produced. Electrostatic micro- and nano-encapsulation utilizing electrospray will provide capsules smaller than a micron from a wide spectrum of core and shell materials. It can be envisioned that this technique will increasingly be applied in materials technology for the production of nanocomposite materials or materials having on-demand properties, as well as in biotechnology as a new research

213

tool for the production of medicine. In recent years, electrospraying has been successfully tested as a low-cost tool for composing small and complex structures via spray forming and direct writing. Although there are still a number of challenges to be faced before commercialization will be possible, advances in electrospray applications in nanotechnology and biotechnology will certainly continue in near future, and new achievements in this field can be expected. We have attempted to consider a wide spectrum of relevant refereed papers, but the Authors feel that the literature cited in this paper is by no means complete. However, we believe that it should provide the reader with the development and application of electrospraying, particularly in nanotechnology. Acknowledgments The paper is supported by Polish Ministry of Science and Higher Education within the Project no. 4078/T02/2007/32. References [1] O.V. Salata, Tools of nanotechnology: electrospray, Curr. Nanosci. 1 (2005) 25–33. [2] I. Hayati, A.I. Bailey, Th.F. Tadros, Investigations into the mechanisms of electrohydrodynamic spraying of liquids. Pt. I. Effect of electric field and the environment on pendant drop and factors affecting the formation of stable jets and atomisation, J. Colloid Interface Sci. 117 (1) (1987) 205–221. [3] I. Hayati, A.I. Bailey, Th.F. Tadros, Investigations into the mechanisms of electrohydrodynamic spraying of liquids. Pt. II. Mechanism of stable jet formation and electrical forces acting on a liquid cone, J. Colloid Interface Sci. 117 (1) (1987) 222–230. [4] M. Cloupeau, B. Prunet-Foch, Electrostatic spraying of liquids. Main functioning modes, J. Electrostat. 25 (1990) 165–184. [5] M. Cloupeau, B. Prunet-Foch, Electrohydrodynamic spraying functioning modes. A critical review, J. Aerosol Sci. 25 (6) (1994) 1121–1136. [6] S.O. Shiryaeva, A.I. Grigor’ev, The semiphenomenological classification of the modes of electrostatic dispersion of liquids, J. Electrostat. 34 (1) (1995) 51–59. [7] A. Jaworek, A. Krupa, Classification of the modes of EHD spraying, J. Aerosol Sci. 30 (1999) 873–893. [8] A. Jaworek, A. Krupa, Jet and drops formation in electrohydrodynamic spraying of liquids A systematic approach, Exp. Fluids 27 (1999) 43–52. [9] I. Marginean, L. Parvin, L. Heffernan, A. Vertes, Flexing the electrified meniscus: the birth of a jet in electrosprays, Anal. Chem. 76 (2004) 4202–4207. [10] I. Marginean, P. Nemes, A. Vertes, Order–chaos–order transitions in electrosprays: the electrified dripping faucet, Phys. Rev. Lett. 97 (6) (2006) Paper no. 064502. [11] I. Marginean, R.T. Kelly, J.S. Page, K. Tang, R.D. Smith, Electrospray characteristic curves: in pursuit of improved performance in the nanoflow regime, Anal. Chem. 79 (21) (2007) 8030–8036. [12] I. Marginean, P. Nemes, A. Vertes, A stable regime in electrosprays, Phys. Rev. E 76 (2) (2007) Paper no. 026320. [13] L. Parvin, M.C. Galicia, J.M. Gauntt, L.M. Carney, A.B. Nguyen, Y. Park, L. Heffernan, A. Vertes, Electrospray diagnostics by Fourier analysis of current oscillations and fast imaging, Anal. Chem. 77 (2005) 3908–3915.

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