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In Situ Coating of Flame-Made TiO2 Particles with Nanothin SiO2 Films Alexandra Teleki,† Martin C. Heine,† Frank Krumeich,‡ M. Kamal Akhtar,§ and Sotiris E. Pratsinis*,† Particle Technology Laboratory, Department of Mechanical and Process Engineering, and Electron Microscopy Center (EMEZ), ETH Zurich, Sonneggstrasse 3, CH-8092 Zurich, Switzerland, and Research Center, Millennium Inorganic Chemicals (a Cristal Company), 6752 Baymeadow DriVe, Glen Burnie, Maryland 21060 ReceiVed May 27, 2008. ReVised Manuscript ReceiVed September 11, 2008 Rutile TiO2 particles made by flame spray pyrolysis (FSP) were coated in a single step with SiO2 layers in an enclosed flame reactor. This in situ particle coating was accomplished by a hollow ring delivering hexamethyldisiloxane (HMDSO) vapor (precursor to SiO2) through multiple jets in swirl cross-flow to Al-doped nanostructured rutile TiO2 aerosol freshly made by FSP of a solution of titanium tetraisopropoxide and aluminum sec-butoxide in xylene. The as-prepared powders were characterized by (scanning) transmission electron microscopy (STEM and TEM), energy dispersive X-ray analysis, X-ray diffraction, nitrogen adsorption, electrophoretic mobility, DC plasma optical emission (DCP-OES), and Fourier transform infrared (FT-IR) spectroscopy. The coating quality was assessed further by the photocatalytic oxidation of isopropyl alcohol to acetone. The effect of HMDSO injection point and vapor concentration on product particle morphology was investigated. The titania particles were uniformly SiO2-coated with controlled and uniform thickness at a production rate of about 30 g h-1 and exhibited limited, if any, photoactivity. In contrast, spraying and combusting equivalent mixtures of the above Si/Al/Ti precursors in the above reactor (without delivering HMDSO through the hollow ring) resulted in particles segregated in amorphous (SiO2) and crystalline (TiO2) domains which exhibited high photocatalytic activity.
1. Introduction Flame technology is an attractive route for materials synthesis for its proven capacity to scalably produce highly pure products (e.g., optical fibers) with unique morphology and composition.1 Recent advances in aerosol and combustion science have contributed to the synthesis of a wide spectrum of sophisticated nanostructured particles such as catalysts, gas sensors, phosphors, fuel cells, batteries, dental and orthopedic materials, and even nutritional supplements by this technology.2 These particles are often coated to condition their surfaces so they can be easily dispersed in liquid suspensions (inks, paints, wafer polishing slurries) or polymer nanocomposites (e.g., dental prosthetics, tires, toothpaste, and summer polymer furniture).3 For example, in the manufacture of pigments, the photocatalytic activity of TiO2 is inhibited by applying SiO2, Al2O3, and other oxide coatings in a postsynthesis, wet-phase treatment.4 The synthesis and coating of such particles are two distinct processes and, practically, two manufacturing plants. As a result, the cost and complexity of this functionalization or coating step is quite high. Therefore, there is a strong interest to simplify and improve this process by combining both flame synthesis and particle coating in a single gas phase process to facilitate the manufacturing of newly developed, nanostructured materials.2 In fact, one could say that the “holy grail” of this field has been the synthesis and coating of these particles in a single process. * To whom correspondence should be addressed. Telephone: +41 44 632 31 80. Fax: +41 44 632 15 95. E-mail:
[email protected]. † Department of Mechanical and Process Engineering, ETH Zurich. ‡ Electron Microscopy Center (EMEZ), ETH Zurich. § Millennium Inorganic Chemicals.
(1) Ulrich, G. D. Chem. Eng. News 1984, 62, 22–29. (2) Strobel, R.; Pratsinis, S. E. J. Mater. Chem. 2007, 17, 4743–4756. (3) Egerton, T. A. KONA 1998, 16, 46–59. (4) Braun, J. H.; Baidins, A.; Marganski, R. E. Prog. Org. Coat. 1992, 20, 105–138.
Such a continuous, gas phase particle synthesis and coating process would not involve hard-to-clean liquid byproducts; particle collection from gases would be easier than that from liquid streams using fewer processes that can result in high purity products.5 Co-oxidation of precursors in the flame can lead to particles coated nicely by a second oxide in certain products. For example, in the flame synthesis of V2O5/TiO2 catalysts, the difference in physicochemical properties between the two oxides assures condensation of V2O5 films on top of earlier formed TiO2 particles in the flame even though the precursors of both oxides are simultaneously fed to the burner.6 Similar effects have been observed in the flame synthesis of superparamagnetic Fe2O3/ SiO27 and quantum dot ZnO/SiO28 where Fe2O3 and ZnO, respectively, are embedded in silica. In vapor-fed flames, the formation of SiO2-coated TiO2 has been demonstrated by controlling precursor concentration, flame temperature, and cooling rate.9,10 The operation window for the synthesis of such SiO2-coated TiO2 particles, however, is narrow and quite challenging, as co-oxidation at high temperature of their precursors results in typically particles with segregated silica and titania domains.10 Separate introduction of gaseous Ti and Si precursors to a flame burner is beneficial to promote surface enrichment of SiO2 on TiO2.11 (5) Pratsinis, S. E.; Mastrangelo, S. V. R. Chem. Eng. Prog. 1989, 85, 62–66. (6) Stark, W. J.; Wegner, K.; Pratsinis, S. E.; Baiker, A. J. Catal. 2001, 197, 182–191. (7) Zachariah, M. R.; Aquino, M. I.; Shull, R. D.; Steel, E. B. Nanostruct. Mater. 1995, 5, 383–392. (8) Madler, L.; Stark, W. J.; Pratsinis, S. E. J. Appl. Phys. 2002, 92, 6537– 6540. (9) Hung, C. H.; Katz, J. L. J. Mater. Res. 1992, 7, 1861–1869. (10) Teleki, A.; Pratsinis, S. E.; Wegner, K.; Jossen, R.; Krumeich, F. J. Mater. Res. 2005, 20, 1336–1347. (11) Moerters, M.; Hemme, I.; Hasenzahl, S.; Diener, U.; Habermann, H. U.S. Patent 2003129153, 2003.
10.1021/la801630z CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
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Figure 1. (a) Experimental setup for in situ SiO2 coating of FSP-made TiO2 nanoparticles in an enclosed reactor by injecting HMDSO-laden N2 through a torus pipe ring with 16 gas outlets. At (b) 5 and (c) 30 cm burner-ring-distance (BRD), this results in segregated SiO2/Al2O3/TiO2 or SiO2-coated Al/TiO2 particles, respectively, containing 4 wt % Al2O3 and 20 wt % SiO2.
Here, emphasis is placed on hermetically coating particles made in scalable liquid-fed flame reactors12 for their capacity to make an array of functional materials.2 This is demonstrated by coating with SiO2 predominantly rutile TiO2 nanoparticles made by flame spray pyrolysis (FSP).8 The TiO2 particles were doped with aluminum during their formation to promote rutile crystal composition13 consistent with TiO2 synthesis in hot-wall reactors,14 vapor-fed diffusion flames,15 and FSP reactors.16 The coating minimizes the TiO2 photocatalytic activity in liquid or polymer matrices.4,17 The coating quality can be assessed by microscopy, electrophoretic mobility (ζ-potential), and Fourier transform infrared (FT-IR) spectra.18 The present particle formation and coating process, however, can be used readily for other core or coating oxides (e.g., SiO2-coated FexOy19 or ZnO20). (12) Mueller, R.; Madler, L.; Pratsinis, S. E. Chem. Eng. Sci. 2003, 58, 1969– 1976. (13) Mezey, E. J. Pigments and reinforcing agents. In Vapor deposition; Powell, C. F., Oxley, J. H., Blocher, J. M., Eds.; John Wiley & Sons, Inc.: New York, 1966; pp 423-451. (14) Akhtar, M. K.; Pratsinis, S. E.; Mastrangelo, S. V. R. J. Mater. Res. 1994, 9, 1241–1249. (15) Vemury, S.; Pratsinis, S. E. J. Am. Ceram. Soc. 1995, 78, 2984–2992. (16) Kim, S.; Gislason, J. J.; Morton, R. W.; Pan, X. Q.; Sun, H. P.; Laine, R. M. Chem. Mater. 2004, 16, 2336–2343. (17) Werner, A. J. U.S. Patent 3437502, 1969. (18) Teleki, A.; Akhtar, M. K.; Pratsinis, S. E. J. Mater. Chem. 2008, 18, 3547–3555. (19) Ma, D. L.; Veres, T.; Clim, L.; Normandin, F.; Guan, J. W.; Kingston, D.; Simard, B. J. Phys. Chem. C 2007, 111, 1999–2007. (20) Wang, H. Z.; Nakamura, H.; Yao, K.; Uehara, M.; Nishimura, S.; Maeda, H.; Abe, E. J. Am. Ceram. Soc. 2002, 85, 1937–1940.
Here, the FSP reactor was enclosed to enable judicious injection of hexamethlydisiloxane (HMDSO) vapor, the SiO2 precursor, downstream of the TiO2 formation zone. The influence of injection location and HMDSO vapor concentration, which are most important when applying the present process to other core-shell particle systems, on product particle characteristics was investigated. Conditions for turbulent mixing between the coating precursor and TiO2 aerosol were selected to minimize the formation of separate SiO2 and uncoated TiO2 particles.21 The coating quality was assessed also by suspending such particles in isopropyl alcohol (IPA) and monitoring its photocatalytic conversion to acetone.
2. Experimental Section 2.1. In Situ Coating of FSP-Made Nanoparticles. Figure 1a shows the experimental setup. A FSP reactor, described in detail elsewhere,22 was enclosed by a 5-30 cm long (burner-ring-distance: BRD) quartz glass tube (ID ) 4.5 cm).23 At the top of that tube, a stainless steel metal torus (pipe diameter ) 0.38 cm) ring (ID ) 4.5 cm) was placed. That ring had 16 radially equispaced openings (ID ) 0.06 cm each) directed 10° away from the center line of the ring and pointing downstream by 20° to ensure no stagnation flow (21) Teleki, A.; Buesser, B.; Heine, M. C.; Krumeich, F.; Akhtar, M. K.; Pratsinis, S. E. Ind. Eng. Chem. Res., published online Aug 27, http://dx.doi.org/ 10.1021/ie800226d. (22) Madler, L.; Stark, W. J.; Pratsinis, S. E. J. Mater. Res. 2002, 17, 1356– 1362. (23) Ernst, F. O.; Buechel, R.; Strobel, R.; Pratsinis, S. E. Chem. Mater. 2008, 20, 2117–2123.
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upstream.21,24 Through these openings, 0.8 L min-1 N2 gas carrying hexamethyldisiloxane (HMDSO, Aldrich, purity >98%) vapor (the SiO2 coating precursor) was injected along with an additional 15 L min-1 N2 at room temperature.21 Above the ring, another 30 cm long quartz tube (ID ) 4.5 cm) was placed. Al-doped TiO2 particles were produced by FSP of a 1 M solution in total metal concentration of titanium tetraisopropoxide (TTIP, Aldrich, purity >97%) and aluminum sec-butoxide (Al(s-BuO)3, Alfa Aesar, purity >95%) in xylene (Riedel-de Hae¨n, puriss). Through the innermost FSP nozzle capillary, 5 mL min-1 of that solution was fed and dispersed by 5 L min-1 O2 (Pan Gas, purity >99%) from the surrounding annulus and sheathed by an additional 40 L min-1 O2 flowing through the surrounding outermost sinter metal plate. The pressure drop at the nozzle tip was 1.5 bar. The solution spray was ignited by a ringshaped, methane/oxygen (1.5/3.2, total inlet gas flow 4.7 L min-1) premixed flame at the nozzle base.22 The HMDSO vapor was supplied by bubbling 0.8 L min-1 N2 through about 500 cm3 of liquid HMDSO in a 1 L glass flask (Schott) immersed in silicon oil at 10.5 °C. At saturated conditions, this corresponds to 5.9 g h-1 of SiO2 and 20 wt % SiO2 in the product powder (standard coating conditions). Lower silica contents (2.5, 5, 7.5, 10, and 15 wt %) in the product powder were attained by reducing the bubbler temperature to 4.5 °C and selecting the N2 flow rate accordingly (0.1-0.8 L min-1) assuming its complete saturation with HMDSO vapor.25 Similarly, higher silica contents (25 and 30 wt % SiO2) were achieved by increasing the silicon oil temperature to 20 °C and adjusting the N2 flow rate (0.6-0.8 L min-1). Unless otherwise stated, standard coating conditions refer to a spray flame producing 23.5 g h-1 of titania particles containing 4 wt % Al2O3 (4Al/TiO2) coated with 20 wt % SiO2 from HMDSO vapor at 20 cm BRD resulting in 29.4 g h-1 of product. For comparison, co-oxidized SiO2/Al2O3/TiO2 particles were made in the above FSP unit from a mixture of HMDSO, Al(s-BuO)3, and TTIP in xylene (1 M total metal concentration). Nitrogen gas was supplied from the above torus ring (but without HMDSO) to preserve the flow and temperature characteristics26 of the in situ coating process at above standard conditions. These particles are denoted ySi/zAl/TiO2, where y and z refer to the weight fractions of SiO2 and Al2O3, respectively, with the balance TiO2. For example, 10Si/4Al/ TiO2 is composed nominally of 10 wt % SiO2, 4 wt % Al2O3, and 86 wt % TiO2 made from HMDSO/Al(s-BuO)3/TTIP at a volume ratio 5/6/100. The y range was 2.5-20 wt %, while z was 4 wt %. 2.2. Particle Characterization. The particles were deposited onto a holey carbon foil supported on a copper grid for analysis by (scanning) transmission electron microscopy (TEM: CM30ST, LaB6 cathode; STEM: Tecnai F30, field emission cathode). Both microscopes were operated at 300 kV and had a SuperTwin lens with a point resolution of ∼2 Å (Fa. FEI, Eindhoven), while STEM images were recorded with a high angle annular dark field detector (HAADF). The presence of Ti, Al, and Si at selected spots in STEM images was determined by energy-dispersive X-ray (EDX) analysis (Fa. EDAX detector). The product powder composition was determined by DC plasma optical emission spectroscopy (DCP-OES). Samples prepared by sodium peroxide fusion were dissolved in 25% sulfuric acid, diluted, and analyzed by using a Thermo Jarrel Ash Corporation (IRIS) instrument. X-ray diffraction (XRD) patterns were obtained with a Bruker AXS D8 Advance diffractometer (40 kV, 40 mA, Karlsruhe, Germany) operating with Cu KR radiation. The anatase and rutile sizes, xa and xr, respectively, and phase composition were determined with TOPAS 3 software. The Brunauer-Emmett-Teller (BET) powder-specific surface area (SSA), was measured by nitrogen adsorption at 77 K (Micromeritics TriStar) after degassing the samples, at least, for 1 h at 150 °C in nitrogen. Fourier transform infrared (FT-IR; Bruker, Vector 22) spectra were measured by the KBr pellet method: 0.25 g of KBr (Fluka, g99.5%) was ground with
3.1. Effect of Burner-Ring-Distance (BRD) on Coating Quality. Early injection of HMDSO (BRD ) 5 and 10 cm) resulted in some 20Si-coated 4Al/TiO2 particles and separate SiO2 and TiO2 regions or particles (Figure 1b). Injection of HMDSO vapor at low BRD where high temperatures prevail and TiO2 formation still takes place leads to fast oxidation of HMDSO resulting in separate SiO2 and TiO2 particles or domains reminiscent of SiO2/TiO2 formation by precursor vapor cooxidation.9,10,15 In contrast, all particles appeared homogeneously coated with 2-3 nm thick SiO2 films and no separate domains or particles when HMDSO was injected at BRD g 20 cm (Figure 1c). This is similar to such particles coated by introducing Siprecursor vapor downstream of the TiO2 formation zone in hotwall reactors.28 As SiO2 can drastically alter the physical properties of flamemade TiO2 by promoting anatase formation13 and high specific surface area (SSA),29 the effect of BRD on titania particle size, crystallinity, and SiO2 coating quality of pure and 20Si-coated 4Al/TiO2 was investigated. Figure 2a shows the rutile weight fraction (triangles) and SSA (circles) of uncoated (dashed line) and 20Si-coated 4Al/TiO2 (solid line) at standard conditions as a function of BRD. The corresponding crystallite sizes are depicted in Figure 2b. For both particle compositions, the SSA is reduced with increasing BRD that delays the cooling of the FSP-made aerosol by mixing with the injected HMDSO-laden N2 at room temperature.26 That way, the high temperature particle residence time is prolonged and aerosol dilution by N2 is delayed. This facilitates particle coagulation and sintering, resulting in larger grains or primary particles (with lower SSA) without affecting crystal composition. The final anatase size, xa, is reached at 10 cm BRD, while the rutile size, xr, increases with increasing BRD that prolongs the high temperature particle residence time as aerosol cooling is delayed (Figure 2b, dashed line). The rutile content of SiO2-coated titania, however, increases steadily from about 40 to 70 wt % with increasing BRD. At 30 cm BRD, the rutile fraction (Figure 2a) and the corresponding crystallite sizes of anatase, xa, and rutile, xr (Figure 2b), reach the values of uncoated TiO2 (at BRD ) 20 cm). Injecting HMDSO at low BRD brings Si into contact with newly formed TiO2 during its formation by nucleation and growth, promoting anatase formation13 and slowing down TiO2 sintering29 that results in
(24) Hansen, J. P.; Jensen, J. R.; Livbjerg, H.; Johannessen, T. AIChE J. 2001, 47, 2413–2418. (25) Yaws, C. L.; Narasimhan, P. K.; Gabbula, C. Yaws’ Handbook of Antoine Coefficients for Vapor Pressure (Electronic Edition); Knovel: New York, 2005. (26) Schulz, H.; Madler, L.; Strobel, R.; Jossen, R.; Pratsinis, S. E.; Johannessen, T. J. Mater. Res. 2005, 20, 2568–2577.
(27) Cundall, R. B.; Rudham, R.; Salim, M. S. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1642–1651. (28) Powell, Q. H.; Fotou, G. P.; Kodas, T. T.; Anderson, B. M.; Guo, Y. X. J. Mater. Res. 1997, 12, 552–559. (29) Akhtar, M. K.; Pratsinis, S. E.; Mastrangelo, S. V. R. J. Am. Ceram. Soc. 1992, 75, 3408–3416.
1 mg of sample and pressed into a pellet at 10 tons for 5 min (Specac). Spectra were recorded at normal incidence to the pellet surface in the range 4000-500 cm-1 with a resolution of 2 cm-1 and collection of 128 scans. Electrophoretic mobility measurements (converted to ζ-potential) as a function of pH were performed with a Malvern Zetasizer (Nano series) instrument and a MPT-2 multipurpose autotitrator as described in detail elsewhere.18 The photocatalytic activity of all particles was evaluated by photooxidation of isopropyl alcohol (IPA).27 Prior to that test, moisture was removed by heating the particles in vacuum at 80 °C for at least 1 h. A stirred suspension of the particles (0.5 g) in 20 mL of IPA was oxygenated for 60 s and then placed in a bath at 30 °C beneath a UV lamp at 36 W cm-2 for 30 min. The irradiated suspensions were centrifuged for 15 min at 10 000 rpm, and 2 mL of the clear supernatant was analyzed by gas chromatography (GC) to determine the amount of acetone formed. A UV-irradiated blank IPA sample was also analyzed by GC for calibration.
3. Results and Discussion
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Figure 3. Nominal and measured (by DCP-OES) weight fractions of Al2O3 and SiO2 in coated and co-oxidized TiO2 particles made by FSP.
Figure 2. Effect of BRD for uncoated (- - -) and 20Si-coated (s) Al-doped TiO2 on (a) rutile weight fraction (4) and specific surface area (SSA; O), (b) anatase (O) and rutile (4) crystallite sizes, and (c) acetone concentration formed by photooxidation of IPA from slurries of the coated particles. The corresponding ordinates for rutile weight fraction (left axis) and SSA (right axis) are indicated by arrows (a).
smaller particles (higher SSA) and smaller anatase and rutile crystallites (Figure 2b). Mixing the two streams at progressively higher BRDs reduces the impact of Si on TiO2 characteristics, as the particles have grown larger so that Si can no longer interfere with TiO2 formation and growth. Figure 2c shows the acetone concentration released from photooxidized IPA slurries containing 20Si-coated titania particles made at various BRD. The released acetone decreases from 99 µg (high activity) to 7 µg (low activity) as the BRD is increased from 5 to 30 cm, respectively. This is a result of the formation of SiO2 coatings onto the TiO2 particles with increasing BRD (Figure 1c) and to a lesser extent of the increasing rutile content (Figure 2a). The released acetone by IPA photooxidation decreases as SiO2 coatings reduce the rate of generation of the catalytically active hydroxyl radicals.30
3.2. Effect of Silica Content on Coating Quality. Figure 3 shows the nominal composition of ySi-coated zAl/TiO2 (open symbols) made at standard conditions as well as co-oxidized Si/Al/TiO2 (filled symbols) particles with various SiO2 and Al2O3 contents compared to their measured elemental composition. Particles coated with SiO2 at 5 and 10 cm BRD are also included in Figure 3. The nominal Si and Al contents are in reasonable agreement with their measured values, indicating consistent precursor delivery and product composition. The influence of silica content on coating morphology was investigated by setting BRD ) 20 cm, as this yielded coated particles of low photocatalytic activity having crystallite sizes of
