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Ultrasonic spray pyrolysis of TiO2 nanoparticles
NanoStructured Materials, Vol. 9.pp. 125-128, 1997 Elsevier Science Lid O 1997 Acts Metallurgica Inc. Printed in the USA. All rights reserved 0965-9773/97 $17.00 + .00
Pergamon
PII S0965-9773(97)00034-2
ULTRASONIC SPRAY PYROLYSIS OF TiO 2 NANOPARTICLES J.M. Nedeljkovi~1, Z.V. ~aponji~ 1, Z. Rako~evi~1, V. Jokanovi~ 2, D.P. Uskokovi~3
llnstitute of Nuclear Sciences "Vin~a", Belgrade 21nstitute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade 3Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Belgrade, Yugoslavia A b s t r a c t - Synthesis of ultrafine spherical TiO2 particles was achieved by using ultrasonic spray pyrolysis and nanoparticles of TiO2 (=2.5nm) as a precursor. The size distribution and morphology of TiO2 particles obtained after ultrasonic spray pyrolysis were characterized by SEM and STM. Agreement between experimentally obtained results and theoretical calculation of particle size was found. © 1 9 9 7 A c t a Metallurgica Inc.
INTRODUCTION Photoredox chemistry of quanlized semiconductor nanoparticles is enhanced (I). Since Henglein's pioneering work in the preparation of nanoparticle powders that can be redissolved in water (2), various techniques have been employed to obtain nanoparticles in the condensed phase (3). The process of ullrasonic spray pyrolysis provides the possibility to produce particles with desired characteristics (4,5). The purpose of this work was to estimate, for the first time, the capability of ultrasonic spray pyrolysis for the preparation of submicronie spheres by using 2.5 nm TiO2 particles as building units. 2.5 nm TiO2 particles were purposefully chosen as a building block because: (a) they exhibit size quantization effects; (b) their highly concentrated colloidal aqueous solution is very stable and easy to manipulate, and (e) TiO2 is a widely used photocatalyst.
EXPERIMENTAL PROCEDURE A colloidal solution of 10-2M TiO2 (d=2.5nm) was synthesized by using an already developed procedure (6). An aerosol was generated by an ultrasonic atomizer (Gapnsol 9001, RBI), with three transducers operating at a frequency of 2.5MHz. The laboratory setup for powder synthesis by ultrasonic spray pyrolysis consists of the ultrasonic atomizer and a reaction chamber - furnace (Heraues ROF 7/50) with a quartz tube and a vessel for particle collection. The flow rate of the carrier gas (N2)was 0.66 dm3/min, the temperature was 1073K, and the residence time of the droplets in the furnace was about 20s. The flow rate of the droplets was assumed to be equal to the flow rate of the carrier gas (0.05m/s), while the mean heating rate of the aerosol was 60K/s. 125
126
JM NEDELJKOVlCET AL
Thermogravimetric analysis of TiO2 particles was carried out using a Perkin Elmer instrument, model TGS-2. TiO 2 particles before and after ultrasonic spray pyrolysis were analyzed using X-ray diffraction on a Philips PW 1710 diffiractometer. The size distribution of TiO 2 particles before the pyrolysis was determined by a Philips EM 420 electron microscope in the transmission mode. Particle size of TiO 2 particles after the pyrolysis was determined by surface analysis with a semiautomatic picture analyzer (Videoplan, Kontron) connected to a scanning electron microscope (SEM JEOL 5300). Morphology of TiO 2 particles after ~trasonic spray pyrolysis was analyzed by STM - scanning tunneling microscopy (NanoScope III).
RESULTS AND DISCUSSION
Characterization of TiO2 particles In order to make a clear difference between the TiO 2 particles before and after the pyrolysis, we will further on use terms primary and secondary particles, respectively. TGA analysis of the primary TiO 2 particles revealed relative weight loss (26%) corresponding to the presence of two OH groups per one TiO2 molecule (TiO2(OH)2 , m.w. 113.9). Weight loss, under the same experimental conditions, was negligible in the case of secondary TiO 2 particles indicating formation of true TiO2 particles after the pyrolysis. Based on the X-ray analysis, it was concluded that the primary and the secondary TiO 2 particles are amorphous. Experimentally determining value of the mean diameter was 286 nm (feret-x mean value 300 nm and feret-y mean value 272 nm) (Fig. 1). Diameter of the particle in the x and y directions (feret-x, feret-y) varied between 100-500 nm, where the values between 100-450 nm (feret-x) and 100-500 nm (feret-y), predominantly occur. 70% of the particles have a quite close distribution (between 140 and 350 nm, feret-y; and 140 and 420 nm, feret-x). The standard deviation (feret-x) was 0.130, and 0.113 (feret-y). The perimeter form factor had a value of 0.951 and the area form factor 0.987, which led to the conclusion that the obtained particles are of high sphericity. A small part of the surface image of secondary TiO2 particles, obtained by STM, is shown in Fig. 2. It can be seen that the surface of secondary TiO2 particles consists of grains whose size corresponds to the size of primary TiO2 particles determined by TEM (6). Although primary TiO2 particles coalesced forming secondary Fig. 1. Scanning electron micrograph of TiO2 submicronic particles, it seems that an particles prepared by ultrasonic spray pyrolysis intensive mass transport did not occur.
ULTRASONICSPRAYPYROLY'81$OF"nO2 NANOPARTICLE$
127
20,0
20.0 nit
Fig. 2. STM line plot of a part of the secondary TiO2 particles on highly oriented pyrolytic graphite in air (It = 4.7 nA, Vt = 36.4 mV)
Particle size distribution, internal structure and mechanism of precipitation
Forced oscillating frequency of the ultrasonic atomizer creates equivalent oscillations of the liquid column in the sprayer dish causing development of transverse and longitudinal disturbances. Their superposition, dependent on the surface tension, liquid viscosity, liquid column height, dish shape, and particularly forced frequencies of the oscillator, produces complex area waves which in general have an elliptical shape. If damping is negligible, then the tension within the cross section of liquid through a number of lameUar layers is of minor importance and is comparable to the damping of liquid lamellar surfaces. Under these conditions a standing wave of spherical shape is created. Based on Bernoulli's equation and Laplace's law, and assuming boundary conditions where the liquid velocity at the sides of the dish is equal to zero, the corresponding differential equation can be used (7): =
[I1
128
JM NEOELJKOVICET AL.
where d is the aerosol droplet diameter, p is the colloidal dispersion density, T is the surface tension of the TiO2 colloidal dispersion, and f is the ultrasonic oscillator frequency. In order to determine the aerosol droplet diameter, besides the above equation, it is possible to use Lang's equation as well (8): d = 0.34
[21
For determination of the mean diameter of the particle (do) formed from the aerosol droplets, the following equation may be used: do
= d/cp~MTi% .) ~ \ ~
[31
The diameter of aerosol droplets was calculated using the following parameters: y=TZ9-103N/m (surface tension of pure water), p = 1030kg/m 3, and f = 2.5MHz. The obtained values of the mean diameters of the aerosol droplets are 3.32~tm and 2.26~trn, based on equation 1 and 2, respectively. Substituting these values in equation (3) and the corresponding constants for the pyrolysed system (precursor concentration %r = 0.01M, M~o~ = 79.9, Pvio2 -- 3900kg / m
3 ,
and estimated molecular mass of the precursor based on the burning
loss Mpr= 113.9), theoretically expected values of particle diameters were obtained (dr = 195nm and dp = 132nm). Comparison of the theoretically obtained results with the experimentally determined value (ds=286nra), regardless of the equations being used for determination of the aerosol droplets diameter (equation 1 or 2), undoubtedly shows that there is a substantial difference between the theory and the experiment, if theoretical density of primary TiO2 particles packing was assumed. Density of the packing of the particles is the highest on the surface of the sphere where the highest densification occurs during the shrinking and drying, whereas its internal sphere corresponds to the boundary conditions of precipitation from the saturated mixture of suspension appearing originates during the process of diffusion of the mixture towards the surface of the droplets and of dissolved substance towards the centre. The investigation of optical properties and photochemical behaviour of secondary TiO2 particles is on the way. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
J.M.Nedeljkovi6, M.T.Nenadovi6, O.I.Mi6i6, A.J.Nozik, J. Phys. Chem., 90, 12 (1986). A.Fojtik, H.Weller, U.Koch, A.Henglein, Ber. Bunsenges. Phys. Chem., 88, 969 (1984). H.Weller, Angew. Chem. Int. Ed. Engl., 32, 41 (1993). G.L.Messing, S.C. Zhang, G.V. Jayanthi, J. Amer. Ceram. Soc. 76, 2707 (1993). Dj. Jana~kovi6, V. Jokanovi6, Lj. Zivkovi6, D. Uskokovi~, J.Mat. Res. 11, 1706 (1996). O.I.Mi6i6, T. Rajh, J.M. Nedeljkovi6, M. Comor, Israel. J. Chem. 33, 59 (1993). V. Jokanovi6, Dj. Jana6kovi6, A. Spasi6, D. Uskokovid, Mat.Trans. JIM. 37, 627 (1996). R.J.Lang, J. Acoust. Soc. Am. 3, 6 (1962).
Pergamon
PII S0965-9773(97)00034-2
ULTRASONIC SPRAY PYROLYSIS OF TiO 2 NANOPARTICLES J.M. Nedeljkovi~1, Z.V. ~aponji~ 1, Z. Rako~evi~1, V. Jokanovi~ 2, D.P. Uskokovi~3
llnstitute of Nuclear Sciences "Vin~a", Belgrade 21nstitute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade 3Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Belgrade, Yugoslavia A b s t r a c t - Synthesis of ultrafine spherical TiO2 particles was achieved by using ultrasonic spray pyrolysis and nanoparticles of TiO2 (=2.5nm) as a precursor. The size distribution and morphology of TiO2 particles obtained after ultrasonic spray pyrolysis were characterized by SEM and STM. Agreement between experimentally obtained results and theoretical calculation of particle size was found. © 1 9 9 7 A c t a Metallurgica Inc.
INTRODUCTION Photoredox chemistry of quanlized semiconductor nanoparticles is enhanced (I). Since Henglein's pioneering work in the preparation of nanoparticle powders that can be redissolved in water (2), various techniques have been employed to obtain nanoparticles in the condensed phase (3). The process of ullrasonic spray pyrolysis provides the possibility to produce particles with desired characteristics (4,5). The purpose of this work was to estimate, for the first time, the capability of ultrasonic spray pyrolysis for the preparation of submicronie spheres by using 2.5 nm TiO2 particles as building units. 2.5 nm TiO2 particles were purposefully chosen as a building block because: (a) they exhibit size quantization effects; (b) their highly concentrated colloidal aqueous solution is very stable and easy to manipulate, and (e) TiO2 is a widely used photocatalyst.
EXPERIMENTAL PROCEDURE A colloidal solution of 10-2M TiO2 (d=2.5nm) was synthesized by using an already developed procedure (6). An aerosol was generated by an ultrasonic atomizer (Gapnsol 9001, RBI), with three transducers operating at a frequency of 2.5MHz. The laboratory setup for powder synthesis by ultrasonic spray pyrolysis consists of the ultrasonic atomizer and a reaction chamber - furnace (Heraues ROF 7/50) with a quartz tube and a vessel for particle collection. The flow rate of the carrier gas (N2)was 0.66 dm3/min, the temperature was 1073K, and the residence time of the droplets in the furnace was about 20s. The flow rate of the droplets was assumed to be equal to the flow rate of the carrier gas (0.05m/s), while the mean heating rate of the aerosol was 60K/s. 125
126
JM NEDELJKOVlCET AL
Thermogravimetric analysis of TiO2 particles was carried out using a Perkin Elmer instrument, model TGS-2. TiO 2 particles before and after ultrasonic spray pyrolysis were analyzed using X-ray diffraction on a Philips PW 1710 diffiractometer. The size distribution of TiO 2 particles before the pyrolysis was determined by a Philips EM 420 electron microscope in the transmission mode. Particle size of TiO 2 particles after the pyrolysis was determined by surface analysis with a semiautomatic picture analyzer (Videoplan, Kontron) connected to a scanning electron microscope (SEM JEOL 5300). Morphology of TiO 2 particles after ~trasonic spray pyrolysis was analyzed by STM - scanning tunneling microscopy (NanoScope III).
RESULTS AND DISCUSSION
Characterization of TiO2 particles In order to make a clear difference between the TiO 2 particles before and after the pyrolysis, we will further on use terms primary and secondary particles, respectively. TGA analysis of the primary TiO 2 particles revealed relative weight loss (26%) corresponding to the presence of two OH groups per one TiO2 molecule (TiO2(OH)2 , m.w. 113.9). Weight loss, under the same experimental conditions, was negligible in the case of secondary TiO 2 particles indicating formation of true TiO2 particles after the pyrolysis. Based on the X-ray analysis, it was concluded that the primary and the secondary TiO 2 particles are amorphous. Experimentally determining value of the mean diameter was 286 nm (feret-x mean value 300 nm and feret-y mean value 272 nm) (Fig. 1). Diameter of the particle in the x and y directions (feret-x, feret-y) varied between 100-500 nm, where the values between 100-450 nm (feret-x) and 100-500 nm (feret-y), predominantly occur. 70% of the particles have a quite close distribution (between 140 and 350 nm, feret-y; and 140 and 420 nm, feret-x). The standard deviation (feret-x) was 0.130, and 0.113 (feret-y). The perimeter form factor had a value of 0.951 and the area form factor 0.987, which led to the conclusion that the obtained particles are of high sphericity. A small part of the surface image of secondary TiO2 particles, obtained by STM, is shown in Fig. 2. It can be seen that the surface of secondary TiO2 particles consists of grains whose size corresponds to the size of primary TiO2 particles determined by TEM (6). Although primary TiO2 particles coalesced forming secondary Fig. 1. Scanning electron micrograph of TiO2 submicronic particles, it seems that an particles prepared by ultrasonic spray pyrolysis intensive mass transport did not occur.
ULTRASONICSPRAYPYROLY'81$OF"nO2 NANOPARTICLE$
127
20,0
20.0 nit
Fig. 2. STM line plot of a part of the secondary TiO2 particles on highly oriented pyrolytic graphite in air (It = 4.7 nA, Vt = 36.4 mV)
Particle size distribution, internal structure and mechanism of precipitation
Forced oscillating frequency of the ultrasonic atomizer creates equivalent oscillations of the liquid column in the sprayer dish causing development of transverse and longitudinal disturbances. Their superposition, dependent on the surface tension, liquid viscosity, liquid column height, dish shape, and particularly forced frequencies of the oscillator, produces complex area waves which in general have an elliptical shape. If damping is negligible, then the tension within the cross section of liquid through a number of lameUar layers is of minor importance and is comparable to the damping of liquid lamellar surfaces. Under these conditions a standing wave of spherical shape is created. Based on Bernoulli's equation and Laplace's law, and assuming boundary conditions where the liquid velocity at the sides of the dish is equal to zero, the corresponding differential equation can be used (7): =
[I1
128
JM NEOELJKOVICET AL.
where d is the aerosol droplet diameter, p is the colloidal dispersion density, T is the surface tension of the TiO2 colloidal dispersion, and f is the ultrasonic oscillator frequency. In order to determine the aerosol droplet diameter, besides the above equation, it is possible to use Lang's equation as well (8): d = 0.34
[21
For determination of the mean diameter of the particle (do) formed from the aerosol droplets, the following equation may be used: do
= d/cp~MTi% .) ~ \ ~
[31
The diameter of aerosol droplets was calculated using the following parameters: y=TZ9-103N/m (surface tension of pure water), p = 1030kg/m 3, and f = 2.5MHz. The obtained values of the mean diameters of the aerosol droplets are 3.32~tm and 2.26~trn, based on equation 1 and 2, respectively. Substituting these values in equation (3) and the corresponding constants for the pyrolysed system (precursor concentration %r = 0.01M, M~o~ = 79.9, Pvio2 -- 3900kg / m
3 ,
and estimated molecular mass of the precursor based on the burning
loss Mpr= 113.9), theoretically expected values of particle diameters were obtained (dr = 195nm and dp = 132nm). Comparison of the theoretically obtained results with the experimentally determined value (ds=286nra), regardless of the equations being used for determination of the aerosol droplets diameter (equation 1 or 2), undoubtedly shows that there is a substantial difference between the theory and the experiment, if theoretical density of primary TiO2 particles packing was assumed. Density of the packing of the particles is the highest on the surface of the sphere where the highest densification occurs during the shrinking and drying, whereas its internal sphere corresponds to the boundary conditions of precipitation from the saturated mixture of suspension appearing originates during the process of diffusion of the mixture towards the surface of the droplets and of dissolved substance towards the centre. The investigation of optical properties and photochemical behaviour of secondary TiO2 particles is on the way. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
J.M.Nedeljkovi6, M.T.Nenadovi6, O.I.Mi6i6, A.J.Nozik, J. Phys. Chem., 90, 12 (1986). A.Fojtik, H.Weller, U.Koch, A.Henglein, Ber. Bunsenges. Phys. Chem., 88, 969 (1984). H.Weller, Angew. Chem. Int. Ed. Engl., 32, 41 (1993). G.L.Messing, S.C. Zhang, G.V. Jayanthi, J. Amer. Ceram. Soc. 76, 2707 (1993). Dj. Jana~kovi6, V. Jokanovi6, Lj. Zivkovi6, D. Uskokovi~, J.Mat. Res. 11, 1706 (1996). O.I.Mi6i6, T. Rajh, J.M. Nedeljkovi6, M. Comor, Israel. J. Chem. 33, 59 (1993). V. Jokanovi6, Dj. Jana6kovi6, A. Spasi6, D. Uskokovid, Mat.Trans. JIM. 37, 627 (1996). R.J.Lang, J. Acoust. Soc. Am. 3, 6 (1962).