ISSN 0012�5008, Doklady Chemistry, 2012, Vol. 442, Part 1, pp. 4–6. © Pleiades Publishing, Ltd., 2012.Original Russian Text © I.V. Melikhov, N.B. Mikheev, S.A. Kulyukhin, V.A. Lavrikov, A.N. Kamenskaya, 2012, published in Doklady Akademii Nauk, 2012, Vol. 442, No. 1,pp. 60–62.

4

There are a lot of technologies consisting in modi�fying a solid by its evaporation with subsequent con�densation under conditions under which it acquiresnew properties. In this context, it is reasonable to askthe question of how the properties of nano� andmicrocrystals forming in a supersaturated vapor can becontrolled. Answering this question implies studyingelementary phenomena during collisions of moleculeswith solid particles. There is ample evidence that thesephenomena include elastic reflection of particles,their thermal accommodation, chemisorption, andtopochemical interactions without charging of parti�cles [1, 2]. However, these facts do not rule out that, atthe moment of the addition of a molecule to a solid,there can be electron transitions leading to charging ofthe solid. Under ordinary conditions, such chargingwas not detected [3, 4]. However, in a supersaturatedvapor over a boiling melt of a substance, growing crys�tals of the substance can be charged, as we showed inthis work.

EXPERIMENTAL

We investigated the behavior of nano� and micro�crystals of some substances (table) in a CsI aerosolplume in a reactor schematized in Fig. 1. The reactorwas filled with nitrogen, argon, or air, and a flat cruci�ble with a boiling melt of one of the above substanceswas placed in the reactor. The shape of the crucible wassuch that the vapor near the melt surface was saturatedat the boiling point of the melt. With distance from themelt, the vapor was cooled by the carrier gas andbecame supersaturated. Nano� and microcrystalsformed in the vapor and were entrained by an upwardflow of the vapor and the gas carrier. In the flow, these

particles grew and combined to form aggregates con�stituting a smoke plume of a stable shape. If a cold col�lector was introduced in the plume, the particlesdeposited on its surface. They were studied with anelectron microscope, and the size, shape parameters,and distance of each particle from the melt surfacewere determined. In special experiments, the averagevelocity Uу of vertical motion of smoke particles wasfound. A horizontal electric field was applied to theplume, and the velocity Uх of motion with the field forparticles at the periphery of the plume.

RESULTS AND DISCUSSION

On the collector introduced into the smoke plumein zone А1 (Fig. 1), accumulations of nano� andmicroparticles, partially gathered into aggregates,were found. The size distribution of these particles(Fig. 2) corresponded to the solution of the Fokker–Planck equation for a collection of particles growing ata rate independent of their size [5]. The particles hadthe shape of polyhedra with complex faceting. In theplume, the particles and aggregates combined intofloccules. The floccules broke up while contacting thecollector, on which they were observed as accumula�tions.

CHEMISTRY

Spontaneous Charging of Nanocrystals during Their Formationin a Supersaturated Vapor

Corresponding Member of the RAS I. V. Melikhov, N. B. Mikheev, S. A. Kulyukhin, V. A. Lavrikov, and A. N. Kamenskaya

Received June 22, 2011

DOI: 10.1134/S0012500812010028

Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia

Empirical characteristics of the motion of particles

Compound B ε, eV

FeCl3 0.36 ± 0.02 0.12

CdCl2 0.85 ± 0.03 0.27

MoO3 0.28 ± 0.02 0.16

CsI 0.20 ± 0.01 0.12

CuI 0.32 ± 0.02 0.19

KBr 0.30 ± 0.02 0.18

NaCl 0.09 ± 0.01 0.12

DOKLADY CHEMISTRY Vol. 442 Part 1 2012

SPONTANEOUS CHARGING OF NANOCRYSTALS 5

As judged from the stable shape of the plume, theparticles at its periphery moved directionally and wereonly slightly influenced by the Brownian motion andthe plume turbulence. In the absence of a field, theparticles hardly moved in the horizontal direction.Application of a field imparted to them the velocity

(1)

where В and Е0 are empirical parameters, ( is the potential difference across the electrodes,Н is the interelectrode distance, is the air viscosity,and η is the argon or nitrogen viscosity at the boilingpoint of the melt). The parameter В was constant inzone А1 and reduced to zero in zone А2. The parameterВ in zone А1 can be expressed as

, (2)

where Тb is the boiling point of the mel; is theВ value for the substance the melt of which boils at thetemperature ; is an empirical parameter, the val�ues of which at = 1763 K and = 0.09 are pre�sented in the table; and kB is the Boltzmann constant.

The velocity varied within one order of magnitude(Fig. 3).

The data presented show that the nanocrystalsforming from aerosols in the vapor near the surface of

/00

1 ,x yEU BUE

⎛ ⎞= − η η⎜ ⎟

⎝ ⎠= Δ /E V H

ΔVη0

b

B b

maxmax

max max

exp 1T T

B BT k T T

⎡ ⎛ ⎞⎤ε= −⎜ ⎟⎢ ⎥⎣ ⎝ ⎠⎦

maxB

maxT ε

maxT maxB

xU

melts of the studied substances grew at a rate indepen�dent of their size. In the course of their growth, theyacquired a charge, which manifested itself in theirmotion in an electric field. After completion of theirgrowth, the particle charge vanished. In the CsI aero�sol, this took the time t = (y1/Uy) ∼ 1 s. The chargecould not emerge because of the dissociation of vapormolecules into ions or a nonequivalent emission ofcations and anions from the melt since the vaportemperature was too low and the equilibrium vaporformed on the melt surface. This is confirmed byrelation (2), which shows the tendency that thehigher the boiling point Tb of a substance, the lesscharged the particles of the substance (Fig. 3).Therefore, it can be assumed that the charging ofthe particles was caused by electron emission or theionization of adsorbed vapor molecules with subse�quent nonequivalent emission of cations and anionsfrom the particle surface to the vapor. Such an adsorp�tion–ionization–desorption process can be consid�ered to be characterized by relation (1). This relationdescribes the steady motion of particles in a field ofstrength (Е – Е0), which is typical of the periphery of

the plume, at and

(3)

where Z is the drag coefficient of a particle in themedium, Q is the effective particle charge, and Е0 isthe field distortion within the plume. Condition (3)was applicable to particles the sizes of which differby an order of magnitude (Fig. 2). This indicatesthat ZQ was independent of the particle size l.

= − 0( )xU ZQ E E

−

=

10( ) ,yQ BU ZE

1

2

34 4

33

2

XX

YY

H A2

A1

y1y1

(а) (b)

A0

Fig. 1. CsI aerosol plumes in an air�filled reactor at E =(a) 0 and (b) 715 V/cm: (1) platinum crucible with heatedwalls, (2) aerosol plume, (3) electrodes, (4) thermocou�ples. A0 is the vapor layer saturated at the boiling point,A1 is the zone of the determination of the velocity Ux atconstant parameter B, and A2 is the zone of change of theparameter B. H is the interelectrode distance, and y1 is thedistance between the heater and the electrode along the 0Yaxis.

2.5

2.0

1.5

1.0

0.5

0

F(l

), µ

m−

1

0.4 0.8 1.2 1.6l, µm

Fig. 2. Differential CsI particle size (l) distribution func�tion on a glass collector introduced in the smoke plume inzone A1 at E = 0.

6

DOKLADY CHEMISTRY Vol. 442 Part 1 2012

MELIKHOV et al.

If we take, as in electrophoresis studies [6, 7], thatthe drag coefficient is described by the Stokes formula

, then we have to assume that

, (4)

where Q0 is the charge of the particle of the minimalsize l0 among the particles in the plume.

Relations (3) and (4) make it possible to estimatethe fractions of atoms carrying a nonequivalent chargein particles of size l0 (Fig. 2). The fraction of such par�ticles in CsI (l0 = 0.2 µm) was 7 × 10–6 at the numberof charge carriers per particle of 2 × 103. According torelation (4), each nanocrystal, growing to a micronsize, acquired a charge on special regions of its surface,with the number and area of these regions remainingunchanged during the growth. On each of theseregions, adsorbed molecules were ionized and the

= πη/( 1 3 )Z l

= /0 0Q Q l l

forming anions or electrons passed to the vapor withmuch higher probability than on other parts of the sur�face. The forming cations were surrounded by mole�cules coming from the vapor and turned out to beimmured in the bulk of the crystal, because of whichthe charge vanished only in ~1 s. Under such condi�tions, integration of the equation for the rate of chargeaccumulation by a crystal leads to relation (4). Specialregions can be areas at vertices, accumulations ofpoint defects on faces, etc. The spontaneous chargingso strongly affected the motion of crystals that theycan be controlled by a field.

Thus, our experiments showed that, on coolingof the vapor of a number of substances that is satu�rated at the boiling point of the melt, charged solidparticles form, the motion of which can be con�trolled by applying an electric field. The charging ofeach particle occurs by an adsorption–ionization–desorption process, in which adsorbed vapor mole�cules emit electrons to the vapor or undergo disso�ciation into ions with subsequent nonequivalentdesorption of like ions and passage of counterions tothe bulk of the particle.

REFERENCES

1. Gooman, F. O. and Wachman, H.Y., Dynamics of Gas�Surface Scattering, New York: Academic, 1976. Trans�lated under the title Dinamika rasseyaniya gaza pov�erkhnost’yu, Moscow: Mir, 1980.

2. Masel, R., Principles of Adsorption on Solid Surface,New York: Wiley, 1995.

3. Banic, C.M. and Iribarne, J.V., J. Aerosol Sci., 1986,vol. 17, no. 1, pp. 95–105.

4. Friedlander, S.K., Fundamentals of Aerosol Science,New York: Wiley, 2000.

5. Melikhov, I.V. and Berliner, L.B., Teor. Osn. Khim.Tekhnol., 1985, vol. 19, no. 2, pp. 158–165.

6. Dukhin, S.S. and Deryagin, B.V., Elektroforez (Elec�trophoresis), Moscow: Nauka, 1972.

7. Hunder, R.J., Foundations of Colloid Science, Oxford:Oxford Univ. Press, 2001.

5

0

Ux/Uy

600 1000 1400 1800Tb, K

4

3

2

1

T0

Fig. 3. Ratio of the velocities of particles in a field at astrength ratio of E/E0 = 7 at Uy = 20 cm/s in an airmedium.