J. Phys. Chem. B 1999, 103, 10425-10432
10425
Collision Dynamics and Decomposition of NaCl Nanometer Particles on Hot Platinum Surfaces John G. Korsgren and Jan B. C. Pettersson*,† Department of Chemistry, Physical Chemistry, Go¨ teborg UniVersity, SE-412 96 Go¨ teborg, Sweden ReceiVed: May 14, 1999; In Final Form: September 16, 1999
The collision dynamics of nanometer size NaCl particles on hot (1200-1500 K) Pt surfaces are investigated with molecular beam techniques. A beam of particles with diameters of 15-80 nm is directed toward a polycrystalline Pt surface with a velocity of 260 m/s. The scattered flux is measured with a rotatable surface ionization detector that allows for counting and size determination of individual particles. The particlesurface interaction is highly inelastic, and angular distributions indicate that the scattered particles lose more than 90% of their kinetic energy in the surface normal direction. Particles also stick to the surface, and the sticking probability is strongly influenced by the incident particle velocity in the range 200-400 m/s. Particles deposited on the surface subsequently melt and decompose by evaporation of NaCl molecules. A small fraction of each deposited particle also gives rise to a pulse of desorbing Na+ ions, produced by surface ionization, which is used to monitor the decomposition process. The dynamics of the scattering, sticking, and decomposition processes are discussed, and the experimental findings are compared with results from earlier studies of clusters and micrometer-sized particles.
1. Introduction Interactions between particles and surfaces are of significant importance in several areas of research, engineering, and manufacturing. The present study is focused on surface collisions of particles with diameters of 10-100 nm. These particles contain 104-107 atoms and bridge the gap between particles of atomic/molecular dimensions and micrometer particles that may be considered as macroscopic objects with a behavior known from everyday experiences. Very few investigations of the collision dynamics of nanometer particles have been performed, mainly due to experimental difficulties. In general, the particles are too large to be studied by cluster techniques. At the same time, they are too small to be followed by methods employed for micrometer particles. Large clusters can be formed by expansion of condensable high-pressure gas into vacuum. Beams with high cluster density are produced, which makes the method suitable for surface scattering experiments with mass spectrometric detection of scattered particles. Several studies of cluster-surface collisions have been reported, see refs 1-6 and references therein for an overview. The surface collisions are in general found to be highly inelastic. Energy is rapidly transferred from cluster translation to internal degrees of freedom of the cluster, and the system may also couple effectively to surface modes. A variety of processes can take place depending on the available energy, including fragmentation, intracluster and surface reactions, and the emission of charged particles. For relatively low incident velocities, weakly bound atomic and molecular clusters are also found to partially survive scattering from solid surfaces.1-6 In general, the cluster size range is not easily extended above 104-105 molecules, mainly due to limitations in cluster source pressures and pumping capacity of vacuum systems. † Phone: +46 31 772 2828. Fax: +46 31 772 3107. E-mail: janp@phc. chalmers.se. Also at the School of Environmental Sciences, Go¨teborg University.
Several experimental studies of collisions between micrometer particles and surfaces have also been performed. Dahneke7-9 developed a particle beam apparatus for determination of sticking probabilities of uncharged particles striking a target at normal incidence and studied the normal impact of polystyrene latex microspheres (0.5-2 µm) with incident velocities ranging from 2 to 35 m/s. Dahneke found that particles with an initial velocity less than a critical value were captured, and particles traveling faster than this threshold velocity rebounded from the substrate. Broom10 used high-speed photography to measure the velocities of microspheres impacting various target surfaces. Recent studies have been performed by Wall et al.11 and by Dunn et al.,12 who measured incident and rebound velocities during individual impact events. The available experimental information shows that the collision dynamics of micrometer particles are complex.13 Similar to the previously mentioned cluster studies, several parameters have been found to influence the results, including particle size, particle and surface material, particle and surface temperature, incident velocity, and incident angle. The experimental methods used in studies of micrometer particles are not easily employed in the nanometer range. The light scattering technique cannot be extended much below a diameter of 100 nm since the Rayleigh scattering efficiency scales as r6 with particle radius. As an alternative, surface impact with high collision energies may result in emission of charged particles and surface deformation, which can give information about the surface interaction of submicron particles.14 In the present study, we investigate the scattering and decomposition of NaCl particles with diameters of 15-80 nm on a polycrystalline Pt surface using molecular beam techniques. Some of the experimental obstacles mentioned above are overcome by employing the surface ionization (SI) technique that provides sensitive detection of alkali-containing compounds. Scattered particles are measured with a rotatable SI detector that allows for single particle counting and size determination of individual particles. We also investigate the behavior of
10.1021/jp991585q CCC: $18.00 © 1999 American Chemical Society Published on Web 11/03/1999
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particles that stick to the surface. These particles give rise to a pulse of desorbing Na+ ions produced by surface ionization on the hot Pt surface, and the ion pulse is used to monitor the sticking event. The results give detailed insight into the collision dynamics, and the scattering, sticking, and decomposition processes are discussed. The study is to our knowledge the first investigation of detailed surface collision dynamics with 15-80 nm particles, and the system and conditions employed have been chosen to make the best use of the sensitive SI technique. However, in addition to the fundamental interest in the collision dynamics, the alkali salt particle-surface interactions are also of interest from an applied point of view. NaCl and other alkali compounds play an important role in high-temperature corrosion and may cause damage to protective coatings on industrial devices.15 One important example is the high-temperature corrosion of gas turbines employed in combustion applications, where the longevity of the turbine blades may be significantly influenced by interactions with submicron alkali-containing salt particles. Closely related to the present study are also the surface ionization mass spectrometry techniques that have been developed for the monitoring and chemical analysis of aerosols.16-19 Particles are sampled into a vacuum chamber and directed toward a rhenium or tungsten filament for volatilization, where elements with sufficiently low ionization potentials are ionized and the emitted ions analyzed by mass spectrometry. Problems with inaccurate particle sizing with the method have been related to incomplete ionization and particle bounce from the surface.18 An alternative method was employed by Ja¨glid et al.,20 where surface ionization was used for detection of alkali salt particles under atmospheric conditions. In related cluster work, collisions of high-velocity alkali halide crystals such as Na23F22+ with solid surfaces have been investigated by Whetten and coworkers.21,22 The surface impact of (NaCl)32 clusters has also been studied by Cheng and Landman using molecular dynamics simulations.23 This paper is organized as follows. The surface ionization method and the experimental setup are described in the next section. The main results are presented in section 3, followed by a discussion of the collision and decomposition dynamics in section 4. The main conclusions from this study are given in section 5. 2. Experimental Method 2.1. Surface Ionization Detection of Alkali Salt Particles. An atom that thermally desorbs from a hot metal surface has a certain probability of being emitted in ionic form. This process is called surface ionization (SI) and is the basis for the detection method employed in the present study. The ionization probability β is predicted by eq 1, which has been derived from the Saha-Langmuir equation:24,25
β)
1 g0 e(IP - φ) 1+ exp g+ kBT
[
]
(1)
In eq 1, g0/g+ denotes the statistical sum ratio of neutrals and ions (g0/g+ ) 2 for alkali metals), and e, IP, φ, kB, and T denote elementary charge, ionization potential, work function of the surface, Boltzmann’s constant, and surface temperature, respectively. For most elements IP > φ and β approaches zero, and the emission of neutral species from the surface is strongly favored over ion desorption. However, when IP < φ, as for alkali metals on Pt, β approaches unity and ion emission is
Figure 1. Particle beam experimental setup. The inset Figure 1b shows a detailed view of the rotatable detector: (1) V-shaped Pt filament; (2) ion collector; (3) tube-shaped shield, which is also used as a collector for ion pulses from the Pt surface.
strongly favored. For Na (IP ) 496 kJ/mol) on a Pt surface (φ = 531 kJ/mol26), β ) 0.94 at a temperature of 1200 K and decreases to 0.89 at 1500 K. As will be discussed below, the decomposition of melted alkali salt particles on a hot surface is more complex and also includes the possibility of alkali evaporation in molecular form. 2.2. Experimental Setup. A schematic view of the experimental setup is shown in Figure 1. A polydisperse aerosol was generated by nebulization of 0.100 mol/L NaCl solution with filtered compressed air (p ) 6 bar). The aerosol could be diluted with filtered air to a suitable particle concentration in order to avoid coincidence measurements of successive particles in the beam. After dilution, the aerosol was led through a diffusion dryer (length 0.55 m) to remove water vapor and then passed a ball valve. The pressure downstream of the valve was around 10 mbar and the aerosol reached a capillary (20 mm long, 0.5 mm i.d.), through which a fraction of the aerosol expanded into the first vacuum chamber (p ) 5 × 10-4 mbar during the experiments). The central part of the formed particle beam passed through a skimmer placed 3 mm downstream of the capillary exit and entered the second chamber (p ) 1.2 × 10-5 mbar), which contained a mechanical beam chopper used for velocity distribution measurements. The beam then passed through a 1 mm aperture into the third chamber and impacted upon a Pt surface with an incident angle, θi, of 45°. The polycrystalline Pt surface (99.9% nominal purity) was in the form of a thin foil (thickness 0.025 mm) with dimensions 20 × 5 mm, which was resistively heated and the temperature was controlled by pyrometric measurements. The third chamber was pumped by liquid N2 cooled shields and a diffusion pump, and had a base pressure of 1 × 10-8 mbar. In the present experiments the partial pressure of O2 was raised to 1 × 10-7 mbar, which together with the high surface temperatures employed (12001500 K) ensured that the surface was maintained free of carbon impurities. A SI detector that can be rotated in the scattering plane around the Pt surface was used for measurement of scattered particles and for characterization of the incident beam (with the Pt surface moved out of the beam). A closer view of the rotatable detector is shown in Figure 1b. The detector consisted of a flat ion collector and a V-shaped Pt filament with an opening angle of 20°, surrounded by a tube-shaped shield with a thin slit aperture. The V-filament has been designed to ensure complete melting and ionization of the incoming particles, and it was kept at 1500
Dynamics and Decomposition of NaCl Particles
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Figure 2. Examples of signal peaks recorded for NaCl particles when placing the rotatable detector in the beam path. The two peaks correspond to particles with diameters of 33 and 31 nm, respectively.
K during all particle measurements. The dimensions of the apertures in the shield and ion collector were 1.5 × 5 mm and 2 × 6 mm, respectively. The distance from the particle beam impact site on the flat Pt surface to the outer shield of the detector was 42 mm, and the total distance to the far end of the V-filament was 56 mm. The configuration resulted in an inplane resolution of (2°, and an out-of-plane resolution of (7°. During particle measurements, the Pt surface and the ion collector were connected to ground while potentials of +200 and +150 V were applied to the V-filament and shield, respectively. The ion collector was connected to a fast current amplifier with a gain of 108 V/A and a rise time of 0.01 ms. The output signal from the amplifier was recorded by a PC at a sampling rate of 100 kHz. We estimate that particle sizes down to 10-15 nm (corresponding to ion pulses of about 2 × 104 ions in the detector) can be detected with the present system under typical experimental conditions. A salt particle that sticks and decomposes on the hot Pt surface may produce Na+ ions by surface ionization, and a characteristic ion pulse is produced for the sticking event. When measuring these ion pulses from the Pt surface, the rotatable detector was placed in the surface normal direction (θ ) 0°), and the outer shield on the SI detector was used as ion collector. A voltage of +100 V was applied to the Pt surface to accelerate the produced Na+ ions toward the ion collector, while all parts of the detector were grounded and the V-filament turned off. 2.3. Data Analysis. Particle count frequencies and size distributions have been determined in the following three ways: Particle beam: the rotatable detector was placed directly in the particle beam when the Pt surface was temporarily removed. Scattering: the scattering of particles from the surface was measured at different angles with the rotatable detector. Ion pulses: the emission of Na+ ion pulses was measured upon particle impact at the flat Pt surface. Examples of typical signal peaks for scattered particles are shown in Figure 2, where the time scale is arbitrary and is not related to the time-of-flight. Each NaCl particle entering the detector gives rise to a current peak that is integrated over time to yield the particle size. We assume complete ionization of the Na content of each particle and have not corrected the data for small losses of neutral Na species through the narrow opening of the V-filament. In Figure 2, the total ion charges of the two peaks are 6.6 × 10-14 and 5.7 × 10-14 C, respectively. Assuming that the particles have a spherical shape and the density of bulk salt, diameters of 33 and 31 nm can be calculated for the two particles. We have decided to use the mass
Figure 3. Velocity distribution of the aerosol beam. The upper graph shows the signal data recorded upon the opening of the chopper and the best fit to a Maxwell distribution. The velocity distribution is shown in the lower graph. The most probable particle velocity is 260 m/s, and the distribution width at half-maximum is 110 m/s.
equivalent “spherical” diameter as a convenient measure of the particle size in the present study, even though large salt particles have earlier been found to have cubical or irregular shapes.27 The measured particle size distributions have in most cases been found to agree reasonably well with a log-normal distribution, which is defined as28
[
]
(ln dp - ln dg)2 df 1 exp ) d(dp) x2πdp ln σg 2(ln σg)2
(2)
where f denotes the number density, dp the particle diameter, dg the geometric mean diameter, and σg the geometric standard deviation. The size distribution for ion pulses generated by the Pt surface has been treated in the same way as the data from the rotatable detector. In the case of incomplete ionization of particles at the flat Pt surface, the ion pulse measurements do not actually correspond to whole particles but have been assigned an apparent diameter for comparison with beam and scattering measurements. 3. Results We have studied the impact of NaCl particles on polycrystalline Pt with an incident angle θi ) 45°. The main results consist of particle size distributions measured for several scattering angles. The size distributions are also integrated to obtain angular distributions for the total scattered flux. In addition, ion pulses produced by particles sticking and decomposing on the Pt surface are presented as apparent particle size distributions. A beam chopper with a 50% duty cycle was used for velocity measurements in the incident particle beam. Figure 3(top) shows
10428 J. Phys. Chem. B, Vol. 103, No. 47, 1999 the increase in signal after the opening of the chopper, together with a best fit based on a velocity-shifted Maxwell-Boltzmann distribution. Without dilution of the aerosol in the beam source, 102 particles per second were typically detected in the beam, and the experimental curve in Figure 3 (top) is the integrated signal from a large number of detected particles. The obtained velocity distribution, shown in Figure 3 (bottom), has a most probable particle velocity of 260 m/s, and a full width at halfmaximum of 110 m/s. The main parameters governing the velocity distribution are the source pressure and the capillary dimensions, and these parameters were kept constant in this study if not stated otherwise. It is well-known that particles accelerating into a vacuum achieve velocities that depend on size.7 In the present study, small particles are likely to have a higher average velocity compared to larger particles. However, the width of the size-independent velocity distribution shown in Figure 3 is considered sufficiently small for the purpose of this study, and the influence of particle size has therefore been neglected. The size distribution for the incident NaCl beam is shown in Figure 4, together with scattering results and results for ion pulses from the Pt surface for a surface temperature Ts ) 1500 K. In each panel, the ordinate is given as the measured number density for each diameter interval in integer nanometers. The values of dg and σg obtained from fits to log-normal distributions are indicated in the panels. The incident beam distribution for NaCl particles, shown in Figure 4a, is quite broad and covers the diameter range 15-80 nm. The distribution is well fitted by a log-normal distribution, with a mean diameter dg ) 33 nm and a standard deviation σg ) 1.36. Compressed air nebulization generally produces polydisperse aerosols with values for σg in the range 1.5-2.2.28 The narrow distribution observed here is possibly due to deposition of larger particles during transport to the capillary in the beam source, and a wider angular spread of smaller particles from the particle beam centerline. Size distributions for NaCl particles scattered in three different angles are shown in Figure 4b-d. The distributions are similar to the incident beam, with comparable mean diameters and standard deviations, and the NaCl particles thus scatter essentially intact from the surface. No clear trend with scattering angle, θf, is observed, except for the change in intensity that is found to be largest for θf ) 75°. As described above, ion pulses produced by particles on the Pt surface itself are transformed into apparent particle sizes, and the resulting distributions are shown in Figure 4e. While the shape of the beam and scattered distributions are similar, the distributions for ion pulses from the Pt surface are substantially different. The ion pulses correspond to very small diameters, and a mean diameter of only 17 nm is obtained from the log-normal fit, which is close to the detection limit in the present study. As will be discussed in the next section, the data show that NaCl particles that stick to the surface are only partially transformed into Na+ ions during the decomposition. Size distributions of the type shown in Figure 4b-d have been integrated to obtain the flux as a function of scattering angle. The particle number density per unit time is thus used as a measure of the scattering intensity. The scattered flux was measured at θf angles ranging from 0 to 90°, and angular distributions for three different surface temperatures are shown in Figure 5. The distributions are normalized to facilitate the comparison between them. The distributions are shifted from the specular direction (θf ) 45°) with the highest intensity at θf ) 75°. The angular distribution is slightly shifted toward the
Korsgren and Pettersson
Figure 4. Size distributions showing the relative intensities of NaCl particles: (a) particle beam; (b) surface scattering at θf ) 85°; (c) θf ) 75°; (d) θf ) 65°; (e) ion pulses from the Pt surface. The panel ordinates denote particle number density. The surface temperature was 1500 K, and the incident angle, 45°.
grazing angle when the temperature is lowered from 1500 to 1200 K. The distribution at 300 K is very similar to the hightemperature data, although contamination of the surface by background gas may have changed the properties of the surface in this case. Ion pulses from the Pt surface were also measured at surface temperatures of 1200-1600 K, and the derived size distributions are shown in Figure 6. The values for dg, σg, and the integrated number of counts over the entire size range, I, for each distribution are given in Table 1. The integrated signal increases when the temperature is lowered from 1600 K and reaches its
Dynamics and Decomposition of NaCl Particles
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Figure 5. Polar plot showing the angular distribution of scattered NaCl particles from the Pt surface at 300, 1200, and 1500 K. The distributions are normalized to the same peak value, and the normalization factors are indicated.
Figure 6. Size distributions for the ion pulses produced by NaCl particles on the Pt surface in the temperature range 1200-1600 K. The surface temperature for each distribution is indicated.
TABLE 1: Parameter Values dg and σg Obtained from Lognormal Fits to the Size Distributions Shown in Figure 6a T (K)
1600
1500
1400
1300
1200
dg (nm) σg I
20 1.25 37
17 1.13 54
19 1.15 58
21 1.15 124
24 1.18 52
a I is the integrated number of counts over the entire size range for each distribution.
highest value at 1300 K. With the exception of the value at 1600 K, the dg values for the NaCl distributions increase with decreasing temperatures. 4. Discussion The experimental data show that NaCl particles, which collide with the Pt surface, may either stick or scatter from surface. The particles that stick to the surface will in subsequent steps melt and decompose by evaporation. In the following, we will treat these different aspects separately and first discuss the dynamics of the scattering process, followed by discussions of the sticking and decomposition processes. 4.1. Scattering Dynamics. The angular distributions show that the particle-surface interactions are highly inelastic, with the peak of the distributions being strongly shifted toward the surface tangential direction. Similar angular distributions peaked at large scattering angles have previously been observed for superthermal atoms scattering from surfaces,29-31 and also for clusters scattering from surfaces.1-6 In these earlier cases, kinetic energy in the surface normal direction is efficiently transferred
to surface modes or to internal degrees of freedom, while the momentum parallel to the surface plane is much less affected. In the present study, we have not measured the final particle velocity of the scattered particles due to the short flight distance from the surface to the detector, which limits the time resolution. However, the angular distributions can be used to estimate the loss in kinetic energy. The NaCl particles in the incident beam have kinetic energies of 2.5-375 keV depending on particle size, and it is highly unlikely that the fast incoming particles will gain kinetic energy in surface contact. If we assume that the particle velocity parallel to the surface plane is conserved in the collision, we can calculate the expected loss of velocity in the surface normal direction. For an incident velocity of 260 m/s and θi ) 45°, the velocity parallel and perpendicular to the surface is 184 m/s. Using the final scattering angle of highest intensity θf ) 75°, the final velocity in the surface normal direction is calculated to be 49 m/s. We expect that the particles actually lose some of their velocity parallel to the surface during the collisions, although not to the same degree as for the normal component, and the calculated value for the velocity loss can therefore be considered an upper limit. On the basis of these considerations, we conclude that NaCl particles lose g93% of their kinetic energy in the surface normal direction in the collisions. 4.2. Sticking Dynamics. Considering the large loss of kinetic energy in the surface normal direction, it is not surprising that some particles also stick to the surface. To quantify the scattering and sticking probabilities, we have estimated the detection efficiencies of the beam, ion pulse, and scattering measurements. The spread of the particle beam in the final chamber, the cross sections of the flat Pt surface and the rotatable detector, as well as the angular distribution of the scattered flux have been taken into account. The size distributions are shown in Figure 7a, where the ordinate represents a number density scaled to account for the different detection efficiencies. For the ion pulse and scattering measurements, the surface temperature was 1500 K, and the scattered flux is represented by the distribution for θf ) 75°. The comparison shows that about 50% of the NaCl particles scatter from the Pt surface for a beam with a most probable velocity of 260 m/s. We have also performed a limited number of tests where the incident beam velocity was increased to 400 m/s (with a full width at half-maximum of 60 m/s) by increasing the source pressure to 40 mbar. The resulting size distributions are shown in Figure 7b, and the scattering probability is seen to increase considerably compared to the data for 260 m/s in Figure 7a. An angular distribution was also measured with the higher incident velocity, and it is compared with results for the low velocity beam in Figure 8. The faster beam is seen to scatter closer to the specular direction, which indicates that the scattered particles to a higher degree conserve their velocity in the surface normal direction. Several studies of interactions between micrometer-sized aerosol particles and surfaces have been reported.32 A threshold velocity is normally found to exist for particle rebound, and particles travelling slower than this critical Velocity are captured upon collision with the surface. As the critical velocity is exceeded, the ratio between the rebound and incident velocity (Vr/Vi) quickly rises to a value of 0.8 or higher. Several empirical models have been developed to account for the particle scattering probability. Johnson et al.33,34 have derived a theoretical treatment for estimation of particle adhesion based on purely elastic deformation. In the model, generally referred to as the
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Figure 7. Incident beam, ion pulse, and scattering size distributions corrected for the detection probabilities of the different types of measurement. The most probable velocity in the incident beam was (a) 260 m/s and (b) 400 m/s. The ion pulse and scattering measurements are for a surface temperature of 1500 K.
Figure 8. Effect of incident velocity on the angular distribution of NaCl particles scattered from Pt. The most probable velocity in the incident beam for each distribution is indicated. The distributions are normalized to the same peak value. The surface temperature was 1500 K, and the incident angle, 45°.
JKRS model, the critical velocity Vcr is given by
Vcr )
[
]
37π4(Ks + Kp)2σp,s5 Fp3dp5
(3)
where Kp and Ks are mechanical constants of the particle and surface and σp,s is the specific adhesion energy at the interfacial contact area. We use this model to estimate the expected critical velocity for NaCl particles on Pt. The particle diameter dp is set to 32 nm, and the bulk density of the salt is used (Fp ) 2165 kg m-3 at 298 K). The mechanical constant KNaCl is estimated to be 5.5 × 10-11 Pa1- from the work of Raj et al.35 and KPt ) 1.56 × 10-12 Pa1-.36 The specific adhesion energy σp,s is difficult to determine, and it is not available for most materials. A specific adhesion energy σp,s ) 0.7 J/m2 has been
obtained for a NaCl-KCl melt on Al.37 Using σp,s values of 0.5-1.0 J/m2, the critical velocity is 63-112 m/s for NaCl particles on Pt. Although the quantitative significance of these figures is limited, the estimated critical velocity agrees within a factor of 2-3 with the experimental data in the present study. The strong effect of changing the incident velocity in Figure 7 indicates that the critical velocity is in the range 200-400 m/s in the present case. Because of the width of the velocity distribution in the incident beam, particles probably impact upon the surface at velocities on both sides of the critical value. The shift of the angular distributions in Figure 8 with incident velocity is also consistent with the results for micron-sized particles where the ratio (Vr/Vi) quickly rises as the critical velocity is exceeded. The sticking probability should be strongly influenced by the contact area developed between a particle and the surface during impact. The angular distributions shown in Figure 5 reveal that the scattering probability does not sensitively depend on surface temperature in the range 1200-1500 K. This indicates that particle melting induced by the hot Pt surface during impact is not governing the sticking probability, since the melting process is expected to be accelerated by an increased surface temperature. 4.3. Decomposition Dynamics. The surface temperatures of 1200-1500 K employed here are above the melting point of NaCl (Tm ) 1074 K). Particles that stick to the surface will therefore subsequently melt and decompose on the hot Pt surface. In the present study, information about the decomposition process is obtained by monitoring the Na+ ions emitted from the Pt surface. For ion emission to occur, the particle decomposition must have produced atomic Na on the surface, since only Na atoms in direct contact with the Pt surface will desorb in ionic form. As illustrated by the ion pulse distribution shown in Figure 4e, complete ionization of the Na content of NaCl particles on the surface is unlikely to occur. The mean diameter values for the beam and ion pulse distributions suggest that only about 20% of the Na content in the particles becomes ionized. To account for the low ionization efficiency of the particles, three different mechanisms may be proposed: Particles stick to the surface, but a substantial fraction of the material evaporates in neutral (molecular) form from the particle itself or from the surface, thus lowering the ionization efficiency. Small particles (dp < 25 nm) stick and are efficiently ionized at the surface, while large particles are predominantly scattered. A small fraction of each particle melts and becomes ionized at the surface, while the main part of the particle is scattered. For the third mechanism to be valid, a NaCl particle should leave up to 20% of its mass on the surface to become ionized, while the remaining fraction should scatter from the surface. The mechanism would involve a large contact area between the particle and the surface, and as discussed above, this would make scattering highly improbable. For this reason we rule out the last mechanism as a likely cause for the observed results. To distinguish between the first and second mechanisms, additional measurements were made using an incident beam with the size distribution shifted to larger particles. The beam and ion pulse distributions of NaCl particles produced from 0.100 and 0.500 mol/L solutions are compared in Figure 9. For the beam formed with the more concentrated NaCl solution, the dg value is a factor of 1.2 higher, and the corresponding value for the ion pulse distribution is 1.3. The results do not favor the second mechanism since the ion pulse distribution shifts to larger diameters when the size of the incoming particles is increased. If, instead, a fraction of each particle becomes ionized at the
Dynamics and Decomposition of NaCl Particles
J. Phys. Chem. B, Vol. 103, No. 47, 1999 10431 the drop continues to expand on the surface during the evaporation process, until a monolayer thick island remains. The remaining island will mainly evaporate as Na+, and the total ion signal is therefore a measure of the drop extension on the surface. The approximate value of 20% ionization efficiency of each melted particle indicates that the melt effectively wets the hot surface and expands to cover a relatively large area. Further details about the decomposition process are given by the ion pulse distributions shown in Figure 6. The ion pulses become less pronounced with increasing surface temperature. This may be interpreted as a competition between the two decomposition channels, with high temperature favoring NaCl evaporation over Na+ emission. The matter is further discussed in a related study,41 where the surface interactions of submicron NaCl, NaI, NaNO3, and Na2SO4 particles are investigated. 5. Conclusions
Figure 9. Incident beam and ion pulse size distributions of particles produced from 0.100 and 0.500 mol/L NaCl solutions. The surface temperature was 1500 K.
surface, as in the first mechanism, the ionized fraction should increase with the particle sizes. The results thus favor the first mechanism over the second, and we conclude that NaCl particles partially decompose by evaporation of Na in neutral form. The ionization probability for single NaCl molecules on the Pt surface does not differ from the ionization probability of Na atoms.38 We therefore believe the dominating process to be evaporation of NaCl in molecular form from the melted NaCl drop itself. This is consistent with the high vapor pressure of NaCl melts.39 We have not been able to directly detect desorbing NaCl molecules. The angular distribution of evaporating NaCl monomers should be broad and cosine-like, and the flux in the angular range seen by the rotatable SI detector is therefore too low to be detectable with the present experimental setup. In studies of the desorption of NaCl and Na+ from Pt, Kawano et al.40 deposited 102-103 monolayers of NaCl on the surface and the system was heated to high temperatures at a heating rate of 2-65 K s-1. The main fraction of the salt evaporated as NaCl molecules in the temperature range 800950 K, and a small fraction desorbed as Na+ in the range 11001300 K. They concluded that NaCl evaporation only occurred when the surface coverage was larger than half a monolayer, while Na+ only desorbed when the surface coverage was less than a monolayer. The total NaCl mass deposited on the Pt surface was very large in the work by Kawano et al. compared to the deposition of less than 107 NaCl molecules at a time in the present study. However, the main conclusions from the two studies are in agreement, with desorption of NaCl from on top of the salt melt, followed by molecular dissociation and Na+ desorption from the layer in direct contact with the surface. The NaCl layer in direct contact with the Pt surface develops strong bonds to the surface, as revealed by the high temperatures needed for evaporation of Na+.40 We may therefore assume that
The interactions of nanometer NaCl particles (15-80 nm) with a hot Pt surface (1200-1500 K) have been studied by measuring the scattering of particles from the surface. The emission of Na+ ions was also measured to obtain information about the decomposition of NaCl particles that stick to the surface. The main conclusions are that the particle-surface interactions are highly inelastic, and the angular distributions indicate that the scattered particles lose more than 90% of their kinetic energy in the surface normal direction. The particle sticking probability is strongly influenced by the incident velocity in the range 200-400 m/s. Particles that are captured on the hot surface subsequently melt and decompose by evaporation of NaCl in molecular form. About 20% of each deposited particle give rise to a pulse of desorbing Na+ ions, which indicates that the melted salt drop effectively wets the surface and expands to cover a relatively large surface area. The observations are in qualitative agreement with the behavior observed for micrometer-sized particles colliding with solid surfaces, and also with results for cluster-surface collisions, in particular concerning the high inelasticity of the surface collisions. Further experimental and modeling studies will be needed to make a closer, quantitative, coupling between macroscopic and microscopic dynamics of particles on solid surfaces, and work in this direction is currently being performed. Acknowledgment. This project was supported by the Swedish Research Council for Engineering Sciences. References and Notes (1) Gspann, J.; Krieg, G. J. Chem. Phys. 1974, 61, 4037. (2) Chaˆtelet, M.; De Martino, A.; Pettersson, J. B. C.; Prade`re, F.; Vach, H. Chem. Phys. Lett. 1992, 196, 563. (3) Vach, H.; De Martino, A.; Benslimane, M.; Chaˆtelet, M.; Prade`re, F. J. Chem. Phys. 1994, 100, 8526. (4) Mironov, S. G.; Rebrov, A. K.; Semyachkin, B. E.; Vostrikov, A. A. Surf. Sci. 1981, 106, 212. (5) Vostrikov, A. A.; Zadorozhny, A. M.; Dubov, D. Yu.; Witt, G.; Kazakova, I. V.; Bragin, O. A.; Kazakov, V. G. Z. Phys. D 1997, 40, 542. (6) Andersson, P. U.; Pettersson, J. B. C. J. Phys. Chem. B 1998, 102, 7428. (7) Dahneke, B. In Recent DeVelopments in Aerosol Science; Shaw, D. T., Ed.; John Wiley & Sons: New York, 1978; p 187. (8) Dahneke, B.; Hoover, J.; Cheng, Y. S. J. Colloid Interface Sci. 1982, 87, 167. (9) Dahneke, B. J. Colloid Interface Sci. 1973, 45, 584. (10) Broom, G. P. Filtration Separation 1979, 16, 661. (11) Wall, S.; John, W.; Wang, H.-C.; Goren, S. L. Aerosol Sci. Technol. 1990, 12, 926. (12) Dunn, P. F.; Brach, R. M.; Janson, G. G. Aerosol. Sci. Technol. 1996, 25, 445. (13) John, W. Aerosol. Sci. Technol. 1995, 23, 2. (14) Benner, W. H.; Otto, R. J. J. Aerosol Sci. 1990, 21, 277. (15) Stringer, J. Annu. ReV. Mater. Sci. 1977, 7, 477.
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