Synthesis of Nanostructured Titania Powders via Hydrolysis of

William E. Stallings† and H. Henry Lamb*. Department of Chemical Engineering, North Carolina State University,. Raleigh, North Carolina 27695-7905...
4 downloads 0 Views 324KB Size
Download PDF
Langmuir 2003, 19, 2989-2994

2989

Synthesis of Nanostructured Titania Powders via Hydrolysis of Titanium Isopropoxide in Supercritical Carbon Dioxide William E. Stallings† and H. Henry Lamb* Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905 Received September 5, 2002. In Final Form: December 30, 2002 Titania powders were synthesized via hydrolysis of titanium(IV) isopropoxide (TIP) in supercritical carbon dioxide (SCCD). Injection of TIP into water-in-CO2 (w/c) dispersions resulted in precipitation of spherical titania particles, and free-flowing white titania powders were isolated in 65-70% yield by slow isothermal depressurization. Qualitatively similar results were obtained with and without the addition of an anionic phosphate fluorosurfactant (DuPont Zonyl FSP) to stabilize the w/c dispersions. The titania powders had broad particle size distributions (20-800 nm) and specific surface areas in the 100-500 m2/g range. Addition of Zonyl FSP resulted in a decrease in specific surface area at a given water-to-alkoxide molar ratio (hydrolysis level). The specific surface area increased as the hydrolysis level was increased, irrespective of the presence of surfactant. The surface area is associated primarily with internal porosity of the spherical titania particles, as evidenced by scanning transmission electron microscopy and N2 porosimetry. Calcination of a surfactant-free titania powder at 300 °C in air decreased the specific surface area from ∼300 to 65 m2/g and increased the mean cylindrical pore diameter from 2.6 to 4.9 nm, consistent with collapse of micropores. Titania nanoparticle synthesis via TIP hydrolysis in SCCD was attempted using w/c microemulsions formed with ammonium carboxylate perfluoropolyether (PFPE-NH4); however, injection of TIP into PFPE-NH4-stabilized microemulsions resulted in precipitation of 0.3-2 µm titania particles. The use of other CO2-soluble titanium(IV) alkoxides gave qualitatively similar results.

Introduction Titania is used widely as a white pigment and opacifier in paper, plastics, paints, inks, and cosmetics.1 Titania also has many important catalytic applications, e.g., as a support for noble metals and other transition metal oxides and as a photocatalyst for water and air purification. For catalytic applications, titania powders with high specific surface areas and tailored pore-size distributions are desirable.2 Commercial processes for TiO2 production, e.g., flame hydrolysis of titanium(IV) chloride, result in powders with low-to-moderate specific surface areas (50-100 m2/ g). Solution-sol-gel (SSG) processing of titanium alkoxides and supercritical drying techniques have been used to prepare titania aerogels with much higher specific surface areas and tailored pore-size distributions.2 The SSG processing of titanium(IV) alkoxides involves two principal reactions:

Ti(OR)4 + 4H2O f Ti(OH)4 + 4ROH hydrolysis Ti(OH)4 f TiO2 + 2H2O condensation Both reactions proceed via associative nucleophilic substitution, and the (poly)condensation step may comprise both water and alcohol elimination reactions:

Ti-OH + HO-Ti f Ti-O-Ti + H2O Ti-OR + HO-Ti f Ti-O-Ti + ROH * To whom correspondence should be addressed. Telephone: (919) 515-6395. Fax: (919) 515-3465. E-mail: [email protected]. † Present address: Shell Chemical Company, Geismar, LA. (1) Balfour, J. G. Surf. Sci. Ser. 1994, 52, 69-104. (2) Schneider, M.; Baiker, A. Catal. Today 1997, 35, 339-365.

The relative rate of hydrolysis to condensation dictates the structure of the wet gel and has a direct bearing on the textural properties of the final solid. Water-to-alkoxide molar ratio (hydrolysis level), acid or base catalysis, alkoxide reactivity modification, and solvent (typically an alcohol) are SSG parameters used to manipulate the relative hydrolysis rate.3 The stoichiometric hydrolysis level for TiO2 synthesis from titanium(IV) alkoxides is 2, and higher hydrolysis levels typically favor colloidal titania precipitates rather than polymeric gels.4,5 Supercritical drying (with or without exchange of the original solvent for carbon dioxide) after SSG processing yields aerogels (in which the network structure of the wet gel is largely retained) by avoiding surface-tension-driven shrinkage and pore collapse.6 Supercritical carbon dioxide (SCCD) is an attractive environmentally benign substitute for conventional organic solvents (typically alcohols) in SSG processing due to its readily accessible critical point, low toxicity, and low cost.7 Titanium alkoxides are reasonably soluble in SCCD, and titanium(IV) isopropoxide (TIP) exhibits the highest solubility of the commercially available compounds.8 Thermal decomposition of TIP in supercritical CO2-2-propanol mixtures has been used to produce titania films.9 Water is only slightly soluble in SCCD;10 however, stable water-in-CO2 (w/c) dispersions can be formed with (3) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: New York, 1990. (4) Barringer, E. A.; Bowen, H. K. Langmuir 1985, 1, 414-420, and references therein. (5) Campbell, L. K.; Na, B. K.; Ko, E. I. Chem. Mater. 1992, 4, 13291333. (6) Hu¨sing, N.; Schubert, U. Angew. Chem., Int. Ed. 1998, 37, 2245. (7) Loy, D. A.; Russick, E. M.; Yamanaka, S. A.; Baugher, B. M.; Shea, K. J. Chem. Mater. 1997, 9, 2264. (8) Tadros, E. M.; Adkins, C. L. A.; Russick, E. M.; Youngman, M. P. J. Supercrit. Fluids 1996, 9, 172-176.

10.1021/la020760i CCC: $25.00 © 2003 American Chemical Society Published on Web 02/20/2003

2990

Langmuir, Vol. 19, No. 7, 2003

Stallings and Lamb

Figure 1. Schematic diagram of the experimental apparatus.

the aid of fluorinated anionic surfactants.11 Earlier work by Tadros et al. indicated that a surfactant-stabilized w/c dispersion was necessary for the formation of spherical titania particles via TIP hydrolysis in SCCD.8 The formation of reverse micellar solutions or microemulsions, which contain stable aggregates of water surrounded by a monolayer of surfactant molecules at the CO2/water interface, has been established for D2O/CO2/ammonium carboxylate perfluoropolyether (PFPE-NH4) mixtures by small angle neutron scattering.12 Microemulsion-templated synthesis of CdS nanoparticles in SCCD has been reported.13 The water-to-surfactant molar ratio has been shown to correlate directly with the size of the reverse micelle core for PFPE-NH4-stablized w/c microemulsions,12 and the CdS particle size closely corresponded to the expected diameter of the w/c reverse micelles. Titania nanoparticles have been synthesized via hydrolysis of Ti(IV) n-butoxide14 and titanium(IV) chloride15 contained within surfactant reverse micelles in conventional organic solvents. In this paper, we report on the synthesis of titania powders via TIP hydrolysis in SCCD. Our experiments were performed using a high-pressure view cell so that w/c/surfactant phase behavior and titania particle formation could be monitored visually. w/c dispersions were (9) Bocquet, J. F.; Chhor, K.; Pommier, C. Surf. Coat. Technol. 1994, 70, 73-78. (10) King, M. B.; Mubarak, A.; Kim, J. D.; Bott, T. R. J. Supercrit. Fluids 1992, 5, 296-302. (11) Johnson, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle; S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624-626. (12) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. Langmuir 1997, 13, 3934-3937. (13) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langmuir 1999, 15, 6613-6615. (14) Romano, S. D.; Acosta, E. O.; Du¨rrschmidt, T.; Kurlat, D. H. Colloids Surf. 2001, 183-185, 595-605. (15) Chhabra, V.; Pillai, V.; Mishra, B. K.; Morrone, A.; Shah, D. O. Langmuir 1995, 11, 3307-3311.

formed with and without the aid of fluorinated anionic surfactants. Our original intent was to produce titania nanoparticles via TIP hydrolysis in w/c microemulsions, but rapid precipitation of titania particles was observed in the presence and absence of added surfactant. We examined the textural properties of the resultant powders by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and nitrogen porosimetry. Experimental Section Reagent-grade TIP, titanium(IV) n-butoxide, titanium(IV) tertbutoxide, titanium(IV) 2-ethylhexoxide, and titanium bisacetylacetonate diisopropoxide were obtained from Strem Chemicals and used as received. Deionized water (18 MΩ‚cm) was obtained from a Barnstead Nanopure unit. Liquid carbon dioxide was Coleman grade (99.99%) from National Welders. Zonyl FSP was obtained from DuPont and used as received. The anionic phosphate fluorosurfactant has an average formula (RfCH2CH2O)xP(O)(ONH4)y, where Rf ) F(CF2CF2)z and x + y ) 3 and a molecular weight of ∼600 g/mol. The perfluoropolyether carboxylic acid, Fluorolink 7004, was obtained from Ausimont and converted to its ammonium (NH4+) salt by reaction with excess ammonium hydroxide followed by drying under vacuum at 45 °C for 8 h. PFPE-NH4 has an average formula of [CF3O(CF2CF(CF3)O)∼3CF2COO]-[NH4]+ and an average molecular weight of 672 g/mol. PFPE-NH4 was stored in a desiccator prior to use. The experimental apparatus is shown in Figure 1. Water and surfactant were loaded into a 5-mL high-pressure stainless steel view cell with opposing sapphire windows. The view cell was purged of air and filled with liquid CO2. The fluid in the cell was agitated continuously using a Teflon-coated magnetic stir bar. The pressure was increased to 2600 psig by using a manual pressure generator (HIP). The view cell was heated to 45 °C by applying power to an external heating tape, and the pressure increased to 3800 psig. Preliminary experiments established that TIP was soluble in SCCD under these conditions. In a typical experiment, 250 µL of TIP (5.2 wt %) was injected into the view cell using a six-port injection valve (Valco) with a calibrated

Titania Powders via Hydrolysis of TIP in SSCD

Langmuir, Vol. 19, No. 7, 2003 2991

Table 1. BET Surface Areas of Titania Powders Synthesized via TIP Hydrolysis in SCCD Ti(i-OPr)4 (µL)

Zonyl FSP (µL)

250 100 250 250 250 250 250 250

40 16 40 40

water (µL)

46 92 30 60 67 90

water/surfactant molar ratio

water/alkoxide molar ratio

SBET, 125 °C degas (m2/g)

SBET, 235°C degas (m2/g)

43 43 140 233

1.4 1.4 4.5 7.5 2.0 4.0 4.5 6.0

154 109 276 393 274 323 366 454

175 138 230 225 261 293 418 474

Figure 2. SEM images of titania particles synthesized in SCCD (a) with Zonyl FSP surfactant and (b) without added surfactant. sample loop. An HPLC pump (Beckman) was used to inject the reactant; the valve was switched to inject and 2-propanol was pumped through the sample loop and into the cell at 0.1 mL/min for approximately 3 min. Stirring was continued for 5-10 min after injection. The cell was depressurized isothermally by slowly opening a two-way valve. Depressurization took between 1 and 1.5 h. Similar experiments were performed using the PFPE-NH4 surfactant. Microemulsions were prepared by loading 130 mg (3 wt %) of PFPE-NH4 in the view cell with varying amounts of water. The experimental procedure was the same one used with the Zonyl FSP surfactant. w/c microemulsions were formed at 45 °C and 3800 psig with water-to-surfactant ratios of 5, 10, 15, and 20. A Micromeritics Flowsorb 2100 instrument was used for singlepoint Brunauer-Emmett-Teller (BET) surface area16 and pore volume measurements. Samples were degassed at 125 and 235 °C (typically for 2 h) in a stream of flowing 30% He in N2 prior to BET measurements. Samples for pore volume analysis were degassed at 125 °C for 2 h. Titania powders were also characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), SEM, STEM, X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) analysis. FTIR spectra (16) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309.

Figure 3. Influence of hydrolysis level on the specific surface area of the resultant titania powder. were measured in reflectance mode using a Bio-Rad Win-IR FTS175 instrument equipped with a UMA-250 microscope. TGA was performed using a Perkin-Elmer TGA7 instrument with Pyris software. SEM images were obtained using a JEOL JSM6400F cold field emission microscope. A Hitachi HD-2000 cold field emission STEM was used to obtain high-resolution images of the titania particles. STEM samples were prepared by ultrasonic dispersion in 2-propanol and dip coating of holey carbon grids. XRD measurements were made using an INEL XRG 3000 instrument using Cu KR radiation. EDX measurements were

2992

Langmuir, Vol. 19, No. 7, 2003

Stallings and Lamb

Figure 4. FTIR spectrum of a titania powder synthesized using 250 µL of TIP and 60 µL of H2O in SCCD. Table 2. Influence of Postsynthetic Treatments on the Textural Properties of Titania Powders Synthesized in SCCD treatment

Ti(i-OPr)4 (µL)

water (µL)

SBET, 125 °C degas (m2/g)

SBET, 235 °C degas (m2/g)

Vp, 125 °C degas (cm3/g)

mean pore diam (nm)

fast depressurization slow depressurization calcined in air 5 °C/min to 235 °C calcined in air 10 °C/min to 300 °C

250 250 250 250

60 67 60 60

476 366

408 418 141 65

0.33 0.24 0.15 0.08

2.7 2.6 4.3 4.9

made using a Princeton Gamma Tech (PGT) Spirit with a lightelement detector.

Results and Discussion A commercial anionic fluorosurfactant, DuPont Zonyl FSP, was employed in our initial titania synthesis experiments. An optically transparent fluid that appeared to be a single-phase w/c microemulsion was obtained at 45 °C and 3800 psig when 40 µL of Zonyl FSP (containing 35 wt % fluorosurfactant, 20 wt % 2-propanol, and 45 wt % water) was charged to the view cell; however, two phases were observed when any additional water was added. Injection of TIP resulted in rapid precipitation of titania particles irrespective of the amount of added water. The fluid in the cell became turbid immediately, and the resultant suspension was completely opaque after 20-30 s. When the magnetic stirrer was turned off, a fine white powder settled to the bottom of the view cell occupying approximately 50% of the cell volume (for a typical injection of 250 µL of TIP). An SEM image of the titania powder is shown in Figure 2a. Spherical particles are observed, but there is a very broad particle size distribution ranging from ∼20 to 800 nm. Single-point N2 BET measurements (Table 1) evidence that the titania powders had specific surface areas (SBET) in the 100-500 m2/g range. BET measurements were made after degassing the samples for 2 h at 125 °C and subsequently for 2 h at 235 °C. Equivalent results (within the experimental uncertainty of (10%) were obtained in most cases; however, sample discoloration (which we attribute to fluorosurfactant decomposition) was observed after degassing at 235 °C. TIP hydrolysis in w/c dispersions without added surfactant was also investigated. Rapid precipitation of titania particles was observed, although a cloudy w/c dispersion was present at the time of injection.17 A typical yield of 65-70 mg of free-flowing white powder was obtained after slow isothermal depressurization. The white precipitate was characterized by SEM and found to consist of spherical particles ∼20-500 nm in diameter, as illustrated in Figure 2b. This result demonstrates that a stable w/c micro(17) Rapid titania precipitation also was observed when a water/2propanol mixture was injected into a solution of TIP in SCCD.

emulsion is not required in order to form nanometer-sized primary titania particles via TIP hydrolysis in SCCD. The titania powders synthesized without added surfactant exhibited consistently higher specific surface areas than those synthesized using Zonyl FSP, as illustrated in Figure 3a. Specific surface area was found to increase with the water-to-alkoxide ratio (hydrolysis level) irrespective of the presence of surfactant. A universal trend of increasing titania surface area with the quantity of water charged to the view cell was observed, as illustrated in Figure 3b. We infer from this correlation that the primary titania particles form via a common nucleation and growth mechanism proceeding from TIP hydrolysis.4 We discovered that the rate of depressurization had a marked effect on the bulk density of the titania powder. When the view cell was depressurized slowly over a period of 1-2 h (as was typical), the volume occupied by the titania powder did not change significantly. However, when the view cell was depressurized rapidly (within 30-60 s), the powder collapsed to a volume less than 10-20% of the original. The reduction in bulk volume on rapid depressurization results from collapse of the macropore structure. Specific surface area and pore volume (Vp) measurements (Table 2) demonstrate that the rate of depressurization has little effect on the micro- and mesopores. The SBET and Vp values measured after degassing at 125 °C are actually higher for the sample subjected to rapid depressurization; however, the average cylindrical pore diameters (dp ) 4Vp/SBET) are equivalent. The average cylindrical pore diameter is in the mesoporous range (2-50 nm). Table 2 also shows the effect of calcining the assynthesized titania powders in air at 235 and 300 °C. Both SBET and Vp decreased markedly after calcination, and the average pore diameter increased. We infer that calcination in air decreases the N2-accessible surface area via collapse of micropores. Titania powders synthesized without added surfactant were characterized by TGA, FTIR spectroscopy, EDX, and XRD. TGA experiments evidenced a 30% weight loss on heating to 900 °C in N2 and a 35% weight loss on heating to 900 °C in air. Most of the weight loss was accomplished below 200 °C, and we infer that it was associated with

Titania Powders via Hydrolysis of TIP in SSCD

Langmuir, Vol. 19, No. 7, 2003 2993

Figure 5. STEM images of titania particles synthesized from 250 µL of TIP and 60 of µL water in SCCD.

water desorption. The FTIR spectrum (Figure 4) of an as-synthesized titania sample confirmed the presence of adsorbed water, as evidenced by the characteristic OH stretching (∼3000-3500 cm-1) and HOH bending (1645 cm-1) bands. No bands associated with residual TIP are observed. Elemental analysis by EDX confirmed that the powder was titanium dioxide with a background level of carbon contamination. We infer from these results that the TIP hydrolysis in SCCD was essentially complete. The average synthetic yield of titania was 65-70% of theoretical after accounting for desorption of water and volatile byproducts, e.g., 2-propanol. XRD analysis of the as-synthesized titania powder indicated that it was amorphous. STEM images of particles from a typical sample synthesized without added surfactant are shown in Figure 5. Larger spherical particles (200-300 nm in diameter) and irregularly shaped agglomerates of primary nanoparticles (∼20 nm in diameter) are observed in the lowresolution image in Figure 5a. The aggregate structure of the larger particles is clearly visible in Figure 5b. Their remarkably regular spherical shape suggests that interfacial tension plays a role in their formation. Dark voids (pits) are observed on the surface that suggest internal porosity. The size of the voids is consistent with the average pore size determined by N2 porosimetry. Titania nanoparticle synthesis via TIP hydrolysis in SCCD was attempted using w/c microemulsions formed with PFPE-NH4. Visual inspection indicated that w/c microemulsions were formed at 45 °C and 3800 psig using

Figure 6. SEM images of titania particles synthesized using PFPE-NH4 with wo of (a) 20, (b) 15, and (c) 10.

3 wt % surfactant for water-to-surfactant molar ratios (wo) of 5 and 10. Higher wo values (15 and 20) resulted in the appearance of two phases. Injection of 250 µL of TIP into a PFPE-NH4-stabilized microemulsion with wo ) 5 resulted in precipitation of titania particles. When the magnetic stirrer was turned off, these particles formed a thin layer covering the bottom of the cell. A waxy solid (∼100 mg) was isolated by isothermal depressurization. Injection of TIP into w/c microemulsions with wo ) 10, 15, and 20 resulted in rapid precipitation of titania particles that settled to occupy approximately 50% of the cell volume; free-flowing white powders (145-165 mg) were isolated by slow isothermal depressurization. SEM images (Figure 6) of these powders are qualitatively similar to those of powders formed using Zonyl FSP (Figure 2). Larger (0.3-2 µm) spherical particles are prominent in

2994

Langmuir, Vol. 19, No. 7, 2003

Figure 6c; this SEM image is representative of the particles produced using a single-phase w/c microemulsion (wo ) 10). The spherical particles obviously are much larger than the expected size (2-3 nm) of PFPE-NH4-stabilized reverse micelles in SCCD. Particles with a more uniform size distribution were obtained with wo ) 15, as evidenced in Figure 6b, although the initial w/c microemulsion was at the cloud point. The powder produced via TIP hydrolysis using a two-phase w/c mixture (wo ) 20) had a broad particle size distribution that is closely similar to that of powders produced using Zonyl FSP. The specific surface areas of powders synthesized using PFPE-NH4-stabilized microemulsions with wo ) 10 and 15 were 6 and 20 m2/g, respectively, after degassing at 100 °C for 4 h. Degassing the wo ) 10 sample at 235 °C for 4 h resulted in 53% weight loss and increased SBET to 61 m2/g. Several other titanium alkoxides were investigated in an attempt to vary the hydrolysis rate of the sol-gel precursor in PFPE-NH4-stabilized microemulsions. Titanium (IV) n-butoxide, titanium (IV) tert-butoxide, and titanium-2-ethylhexoxide are all soluble in SCCD and were rapidly hydrolyzed to form titania precipitates. In contrast, titanium(IV) bisacetylacetonate diisopropoxide, which is also soluble in SCCD, did not react to form precipitates

Stallings and Lamb

(after 12 h) consistent with its lower reactivity in alcohol solutions. Conclusions Nanostructured titania powders containing ∼20-nm primary particles and larger spherical agglomerates are formed by TIP hydrolysis in SCCD. Qualitatively similar results are obtained with and without the addition of fluorinated anionic surfactants to stabilize the w/c dispersions. Powders produced via TIP hydrolysis without added surfactant have high specific surface areas (275-475 m2/ g) due primarily to internal porosity of the spherical agglomerates. Calcination of the titania powders in air at 300 °C reduces the specific surface area to