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Low Temperature Synthesis of Metal Oxides by a Supercritical Seed Enhanced Crystallization (SSEC) Process Henrik Jensen,*,† Karsten D. Joensen,‡ Steen B. Iversen,§ and Erik G. Søgaard Department of Chemistry, Aalborg UniVersity Esbjerg, Niels BohrsVej 8, 6700 Esbjerg, Denmark
A method for producing crystalline nanosized metal oxides by a supercritical seed enhanced crystallization (SSEC) process has been developed. The process is a modified sol-gel process taking place at temperatures as low as 100 °C with supercritical CO2 as the solvent and polypropylene as the seeding material. The nanocrystalline product is obtained without having to resort to costly postreaction processing, and the product is obtained directly after the SSEC process. TiO2 powders produced by the SSEC process were shown to have a crystallinity of 60% and a crystal size of 7.3 ( 2.5 nm. The crystallinity can be controlled by changing the heating rate of the initial formation of the nanoparticles, and the morphology can be altered by changing the process time. 1. Introduction The production of nanosized materials is of great interest in both industry and academic research. Despite progress in scaling up and reducing costs, nanoparticles remain relatively expensive materials and the price depends on the production volume, material type, powder characteristics, manufacturing method, and postsynthesis processing treatment.1 The applications for nanoparticles span a wide range of different areas depending on the properties of the nanoparticles. The use of nanosized metal oxides as photocatalysts has been studied intensively during the last decade.2 Several metal oxides cover the criteria for an optimal photocatalyst, but TiO2 is the most investigated semiconductor due to its promising catalytic performance and stability in aqueous media.3 Especially the anatase phase of TiO2 has shown great photocatalytic activity because of its larger band gap than the brookite and the rutile phases. Commercially available TiO2 and other metal oxides are normally produced by either a flame synthesis as Degussa P25 or by the sulfate process as Hombikat UV100. In academic research, metal oxides are often synthesized by a traditional solgel process followed by calcination at elevated temperatures. The reason for producing metal oxides by a sol-gel process and not by the flame synthesis or the sulfate process is that the sol-gel process is a simple low-cost technology, taking place at low temperatures. Furthermore, it is possible to design the final particle properties by changing the process parameters.4-7 Producing TiO2 in a traditional sol-gel process has some disadvantages such as an amorphous product. In photocatalysis and similar processes an amorphous product is unwanted due to the lack of activity of amorphous powders.8 Furthermore, the obtained powder has a finite particle size of 1-10 µm and, to get a crystalline product, a postheat treatment at high temperatures for up to 24 h is necessary. In addition to a higher energy usage, the postheat treatment has the unfortunate effect within, * To whom correspondence should be addressed. Phone: +45 22102520. E-mail:
[email protected]. † Present address: SCF Technologies A/S, Gl. Køge Landevej 22, Building H, 2500 Valby, Denmark. ‡ JJ X-ray Systems ApS, Gl. Skovlundevej 54, 2740 Skovlunde, Denmark. § SCF Technologies A/S, Gl. Køge Landevej 22, Building H, 2500 Valby, Denmark.
for example, catalysis applications that the specific surface area is decreased by up to 80% due to sintering and particle growth.9 Sol-gel derived powders often also contain impurities from the precursor such as nonreacted alkoxy groups.10 Tremendous advances have been made in supercritical fluid techniques for the synthesis of nanosized materials during the last decade.11-15 The possibility to fine-tune the solvent properties of supercritical fluids make them especially suitable as solvents in sol-gel processes. The use of supercritical fluids as solvents gives a significantly lower finite particle size in the nanometer range, believed to be due to the higher reaction rate obtained in supercritical fluids. The synthesis of ceramics and metal oxides in supercritical fluids has been further developed to improve upon the particle characteristics, for example, in terms of chemical homogeneity, structure, and morphology.15,16 Tadros et al. showed in 1996 the production of crystalline TiO2 by a supercritical sol-gel process.17 Later, Sarrade and co-workers reported a method for the production of metal oxides and silicon oxides in a supercritical process. This process yields amorphous oxides, which in order to become crystalline need further calcinations. These oxides have a particle size of 0.1-1 µm.18,19 A continuous supercritical process for the production of TiO2 based on the supercritical antisolvent precipitation (SAS) process has also been proposed.20 This process results in amorphous nanosized titanium hydroxide particles. The present method is a modified supercritical sol-gel process. The basic principle is a sol-gel reaction taking place in a supercritical environment. The sol-gel process starts with the hydrolysis of the precursor, normally a metal alkoxide, when it comes into contact with water. The hydrolysis continues simultaneously with the condensation of the hydrolyzed monomers, leading to the formation of a three-dimensional metal oxide network. The overall process for producing TiO2 can be expressed as21
Ti(OR)4 + 2H2O f TiO2 + 4ROH
(1)
The most used precursors for the production of titanium oxides are titanium-n-butoxide, Ti(OBun)4, titaniumisopropoxide, Ti(OPri)4, and titaniumethoxide Ti(OEt)4.22 In this paper, a method for producing crystalline nanosized metal oxides by a supercritical seed enhanced crystallization (SSEC) process will be described. The main focus is the synthesis of the anatase phase of TiO2 at low temperatures, and
10.1021/ie050694q CCC: $33.50 © 2006 American Chemical Society Published on Web 03/21/2006
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Figure 2. Schematic presentation of the SSEC process.
Figure 1. Experimental setup for the SSEC process.
it will be shown that by the invented SSEC process it is possible to produce metal oxide nanoparticles where the particle size, crystal phase, and degree of crystallinity can be controlled by external process parameters. 2. Experimental Section 2.1. SSEC Process. The process that will be presented in this paper makes use of a normal sol-gel reaction occurring in a supercritical environment with the reaction taking place in the proximity of a seeding material. The experimental setup is shown in Figure 1. The reactor is the central part of the setup, and a 24 mL nonstirred high pressure reactor is used. For a normal experiment, the reactor is loaded with the chosen seeding material. The precursor is injected in the seeding material in the upper third of the reactor, and the water is injected in the seeding material in the lower third of the reactor. The seeding material keeps the reactants separate until the supercritical CO2 is added. Furthermore, the seeding material is introduced to enable the production and collection of nanosized particles. The seeding materials can act both as seeds or catalyst as well as a reservoir for the collecting of the nanoparticles. Polymers, ceramics, metal fibers, and natural materials can be used as seeding material. The surface of the seeding material can be coated, resulting in different surface properties such as hydrophilic or hydrophobic surfaces. The procedure for the SSEC process is shown in Figure 2. At the beginning, tstart, the reactor is loaded as described above. Then, the reactor is inserted in the supercritical reaction chamber. Immediately after inserting the reactor in a preheated oven, the CO2 is added to the reactor and the pressure is raised to the desired start pressure, Pstart, which is higher than the critical pressure for CO2 to ensure supercritical conditions after the reactor temperature has reached the critical temperature, Tc. When CO2 enters the reactor, water and the alkoxide are mixed causing the sol-gel reaction to occur, eq 1. The initial hydrolysis and condensation are proceeding while the reactor is heated at a chosen heating rate, Tgradient ) (Tfinal - Troom)/theating, to the final temperature, Tfinal. After the final temperature is reached, it is assumed that the sol-gel reaction is completed, but the process is maintained at the final temperature for a given time,
tx, to produce stable nanocrystalline particles. The reactor is depressurized and cooled to room temperature before the seeding material is removed from the reactor. The powder is easily removed from the seeding material by, for example, mechanically shaking the powder or with ultrasonic sound. 2.2. Characterization. The produced powders are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), wide-angle X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) as described in Jensen et. al.23,24 In brief, the absolute crystallinity of the produced powders is defined from XRD with respect to a 100% reference sample, CaF2. The XRD spectra are obtained using a CuKR1 radiation (λ ) 1.54 Å) from a STOE Stadi P transmission diffractometer. The crystallinity is defined as being the background subtracted area of the 100% peak of the sample with unknown crystallinity divided by the background subtracted area of the 100% peak of the 100% crystalline CaF2. The crystallinity ratio is compared to table values of the ratio between the respective peaks for a 100% crystalline sample and CaF2. The sample with unknown crystallinity and CaF2 are mixed with a weight ratio of 50%. The method allows for determination of the absolute crystallinity, and thereby, the amorphous part of the sample is taken into account. Often the amorphous part is not considered, but it has a great impact in, for example, photocatalysis. The XRD spectra are also used for determining the crystallite size of the samples. If nothing else is referred to, the crystallite size is determined from the broadening of the (101) peak by Scherrer’s formula.25,26 Furthermore, Guinier derived a model for the line profile including a size distribution function in 1963. This formulation will in this study be used to extract information about the crystallite size distribution from XRD spectra assuming a log normal size distribution behavior.23,27 The primary particle size is defined as being the smallest size of the individual particles. The primary particles can be made up of several crystals or can consist of a crystalline core with an amorphous shell. The size of the primary particles was determined by SAXS, TEM, and SEM. The SAXS data were obtained using an adaptation of a Brukers AXS, Nanostar SAXS system, with a rotating anode X-ray generator, cross-coupled Goebel mirrors, three pinholes, and a Bruker AXS Hi-star area detector. The scattering intensity, I, was measured in terms of the scattering vector modulus q, where λ ) 1.54 Å. The data were corrected for background and azimuthally averaged to obtain a spectrum of average intensity vs q. The hard sphere model including a log normal
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Figure 3. (A) XRD spectrum of TiO2 from the hydrophilic PP fiber. (B) Closeup of the (101) reflection for anatase. Table 1. Measured Properties of TiO2 Produced with Different Seed Materials PP PP hydrophilic hydrophobic crystal phase anatase crystallinity [%] 60.6 ( 5.0 crystal size [nm] 6.2 ( 1.0
ceramic
metal fiber
natural fiber
anatase anatase anatase anatase 49.0 ( 4.0 42.5 ( 5.5 38.9 ( 5.5 24.3 ( 4.0 6.5 ( 1.0 6.7 ( 1.0 6.3 ( 2.0 9.3 ( 3.0
size distribution and a structure factor was used to model the SAXS curves. The structure factor describes the interference of scattering from different particles and contains information about the interaction between particles, and for very dilute systems, the structure factor is one and can be neglected.23,28,29 A transmission electron microscope was used to investigate the shape and morphology as well as the size of the samples. The TEM equipment used was a JEOL 100 CX, and the acceleration voltage was 100 kV. The samples were prepared by dispersion of the powder onto a formwar film. LEO1550 equipment was used for the SEM analysis, the powders were coated with a small gold layer, and a magnification of 70 500x was achieved. 3. Results 3.1. Seeding Material. Ten standard experiments following the procedure presented in section 2.1 were carried out. Titanium tetraisopropoxide (97%) was chosen as the precursor. The amounts of reactant were 2.10 mL of alkoxide and 1.00 mL of water. Five different seeding materials were used, and the amount of seeding material was adjusted separately ensuring that the reactants did not come into contact before the supercritical CO2 was added. The process time (see Figure 2) was for all experiments 4 h. A heating rate of 2.6 °C/min was chosen reaching a final temperature of 100 °C after 30-35 min. The starting pressure was 100 bar, and in Table 1, the results from these experiments are shown. The five different seeding materials all gave crystalline TiO2 on the anatase phase. The XRD spectrum for the hydrophilic polypropylene (PP) together with the anatase reference is shown in Figure 3A. In Figure 3B, a close up of the (101) reflection of the anatase phase is shown. From this spectrum, a crystallite size distribution of 7.3 ( 2.5 nm can be extracted using Guiniers model. The crystallinities of the five experiments were determined with respect to a 100% crystalline sample, as shown in Figure 4, and the highest crystallinity was achieved with hydrophilic PP as the seeding material, 60%. The natural material was not
Figure 4. XRD spectra for the seeding experiment (mixed with CaF2).
as applicable for producing crystalline TiO2, reaching only an absolute crystallinity of 24%. Using the hydrophobic PP, the ceramic, and the metal fibers resulted in crystallinities between the case of the hydrophilic PP and the natural fiber. These three reactor filling materials gave 39-49% crystalline TiO2, and due to uncertainties, it is not possible to distinguish between these three reactor filling materials regarding crystallinity. With these three materials and the hydrophilic PP, a crystal size of 6.26.7 nm was obtained. The natural material gave a larger crystal size of 9.3 nm, which could be caused by larger uncertainties in determining the full width at half-maximum (fwhm) from a much smaller and less intense peak. The question of whether the fiber has anything to do with the nucleation and the particle growth can be seen in Figure 5. From Figure 5A, it is observed that the TiO2 nanoparticles are growing on the PP fiber, and Figure 5B shows that the final powder after removal from the fiber has a structure defined by the shape of the fiber acting as the seeding material. Finally, in Figure 5C, the powder after removal from the fiber is shown. A close up of the nanoparticles is shown in Figure 5D. The SEM images show that the powder consists of small nanoparticles, 10-20 nm, which are agglomerated into larger clusters, 1000 nm, which again are arranged in a larger macrocluster, 10 000-20 000 nm. This form of no symmetrical crystal clusters or conglomerates is frequently seen in industrial crystallization processes and is probably caused by poor agitation, the presence of impurities, or the presence of too many seeding crystals.30
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Figure 5. SEM images of TiO2 powder. (A) Particle growth on PP fiber. (B) Micrometer shape of the final TiO2 powder. (C) Submicrometer shape of the produced TiO2 powder. (D) Closeup of the primary particles. Table 3. Experiments with Different Process Times
Figure 6. TEM image of TiO2 powder synthesized with the hydrophilic PP fiber. Table 2. Experiments with Different Heating Rates and Final Temperatures exp
Tgradient (°C/min)
Tfinal (°C)
seeding material
phase/cr (%)
crystallite size (nm)
SC 1 SC 2 SC 3 SC 4
2.6 2.6 2.6 4.1
43 96 96 175
PP PP metal metal
amorphous 62.6 38.9 69.0
5.6 6.3 6.7
The size of the primary particles extracted from XRD and SEM for the hydrophilic PP fiber is confirmed by TEM, Figure 6. 3.2. Heating Rate and Final Temperature. To investigate the influence of the heating rate and the final temperature on the crystallinity, the experimental procedure presented in sections 2.1 and 3.1 was followed, and only the heating rate and the final temperature were changed. Furthermore, experiment SC 1 and SC 2 were carried out with PP fibers as seeding material, and in experiment SC 3 and SC 4, metal fibers were used. The metal fibers were used due to the fact that PP fibers cannot withstand temperatures above 100 °C. The results in Table 2 show that an amorphous powder is obtained at 43 °C with PP fiber, and by increasing the final temperature to 96 °C, a 63% crystalline titanium dioxide is obtained. For the metal fiber, a further temperature increase from
exp
tprocess (min)
crystallinity (%)
crystal size (nm)
primary particle size (nm)
SC 5 SC 6 SC 7
120 240 480
50.0 54.1 48.9
4.2 5.5 5.3
3.9 ( 2.1 5.1 ( 3.3
96 to 175 °C increased the crystallinity by more than 75% without changing the crystallite size. 3.3. Process Time. The process time was investigated following the procedure presented in sections 2.1 and 3.1, and only the process time was changed. The results are presented in Table 3. The XRD data did not show changes when the process time was increased from 2 to 8 h. The crystallinities and the crystallite sizes were similar; however, a change in the morphologies was observed going from 4 to 8 h, Figure 7. Extracting the size of the primary particles from the SAXS spectra in Figure 7 shows that by increasing the process time from 4 to 8 h particle growth increases the primary particle size from 3.9 ( 2.1 to 5.1 ( 3.3 nm. For both the 4 h and the 8 h experiment, an interparticle interference factor has been introduced in the SAXS model to explain the secondary growth as shown in Figure 5. 4. Discussion In the SSEC process, the crystallization temperature for producing nanocrystalline TiO2 on the anatase phase is lowered by 100-250 °C compared to traditional sol-gel processes. This can be attributed to the fact that the rate of nucleation in supercritical fluids is significantly higher than the crystal growth rate, which is well-known to decrease the crystallization temperature.31 It is also believed that the seeding material is especially helpful in facilitating the formation of crystalline phases at these low temperatures. Furthermore, the seeding material is homogeneously dispersing the reactants on/in the seeding material, contributing to the very narrow size distribution, and primary particles with a diameter of 6 nm and a narrow size distribution are obtained. The mechanism for the SSEC process is that the alkoxide is homogeneously dispersed in the seeding material. The seeding material hinders the sol-gel reaction from starting before supercritical CO2 is added to the system. The supercritical CO2
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Figure 7. SAXS analysis of the process time experiments.
mechanically transports the water to the alkoxide, but because water is only soluble in very low concentrations in supercritical CO2,
