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Microemulsion-Mediated Room-Temperature Synthesis of High-Surface-Area Rutile and Its Photocatalytic Performance M. Andersson,*,† A. Kiselev,‡ L. O 2 sterlund,‡ and A. E. C. Palmqvist§,*,† Applied Surface Chemistry, Department of Chemical and Biological Engineering, and Competence Centre for Catalysis, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden, and Department of EnVironment and Protection, FOI NBC-Defence, SE-901 82 Umeå, Sweden ReceiVed: January 12, 2007; In Final Form: February 25, 2007
Nanosized titania having the rutile crystalline structure was synthesized at room temperature using a microemulsion-mediated system. The formed rutile particles had a diameter of 3 nm, which corresponds well with the droplet size of the water-in-oil microemulsion used for their preparation. The crystallinity was monitored by both X-ray diffraction (XRD) and electron diffraction, together with dark-field electron microscopy (TEM) and high-resolution TEM. The rutile had a high specific surface area (∼300 m2/g) according to N2 adsorption and the BET equation. To our knowledge, this is the highest specific surface area ever reported for rutile. The rutile crystals aligned in a specific crystallographic direction forming elongated aggregates 200-1000 nm in size, as observed by TEM and high-resolution TEM. The titania formation was followed in situ using dynamic light scattering and UV-vis spectroscopy, and together with TEM and XRD performed on samples collected throughout the duration of the titania synthesis, the results gave support for a formation scheme involving the initial formation of amorphous titania followed by crystallization of rutile. The photocatalytic performance of the formed material was evaluated by in situ Fourier transform infrared spectroscopy and compared to that of a rutile sample having a lower specific surface area (∼40 m2/g). The TEM and formate adsorption experiments revealed that the high-surface-area rutile had a much higher fraction of (101) facets than the low-surface-area sample, which predominantly exposed (110) facets. In particular, a new bidentate formate (µ-formate) species bridge-bonded to the (101) facet could be identified with characteristic bands at 1547 and 1387 cm-1. The photodegradation rate of this species was found to be similar to the µ-formate species on the (110) facet. However, the overall formate degradation rate was larger on the high-surface-area rutile sample because of a high concentration of the more readily photodegradable monodentate formate (η1-formate) on that sample.
Introduction Titania (TiO2) has been extensively studied for various applications such as white pigments, gas sensors,1,2 solar cells,3 electrode materials in batteries,4,5 and photocatalysts.6,7 It has three common polymorphs, namely, anatase, brookite, and rutile, of which rutile is the thermodynamically stable form at all temperatures at atmospheric pressure, whereas anatase and brookite are metastable.8 However, the crystal stability appears to be size-dependent, and it has been reported that, when the particle diameter is below 14 nm, anatase is the most stable form.9,10 Above 35 nm, rutile is preferred, whereas brookite is predicted to be stable at intermediate particle sizes.10 Furthermore, the materials properties of the various polymorphs differ, making them suitable for different applications. For example, in the electronics industry, rutile is the most frequently used polymorph because it exhibits a higher dielectric constant, a desirable property in applications such as capacitors and temperature-compensating condensers. * Corresponding authors. Tel.: +46 31 7725611 (M.A.), +46 31 7722961 (A.E.C.P.). Fax: +46 31 160062 (M.A.), +46 31 160062 (A.E.C.P.). E-mail:
[email protected] (M.A.),
[email protected] (A.E.C.P.). † Applied Surface Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology. ‡ FOI NBC-Defence. § Competence Centre for Catalysis, Chalmers University of Technology.
In the photocatalytic degradation of organic compounds, wide-band-gap semiconductors, such as TiO2, WO3, and SnO2, have shown great potential,11 where the most promising is titania, having a high photocatalytic activity and a low cost in addition to being environmentally benign.7,12-14 The advantages of utilizing photocatalytic degradation are that the organic contaminants are completely decomposed into CO2, water, and mineral acids and the reactions can proceed at low temperatures. The technique has gained much attention in the purification of wastewater from households and industry.15-17 In the field of photocatalysis, the most efficient catalysts have often been regarded to be single-phase anatase or mixtures of anatase and rutile, although some studies have shown that pure rutile sometimes has a higher photocatalytic activity.18 Commercially available P-25, which is a titania photocatalyst manufactured by Degussa, has turned out to be particularly effective. Its high activity is believed to be due to the facts that P-25 is a mixture of anatase (80%) and rutile (20%) and has a relatively high specific surface area of approximately 50 m2/g.19 One explanation for the high activity of this mixture has been suggested to be an extended lifetime of the UV-excited state, which gives the photocatalytic reaction a higher probability of occurrence.20,21 Other experimental reports have shown that there is an optimum ratio between the two titania polymorphs and that the optimum depends on the type of degradation reaction studied.22-24
10.1021/jp070284a CCC: $37.00 © 2007 American Chemical Society Published on Web 04/14/2007
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Considering the many studies presented in which titania has been photocatalytically evaluated, it is clear that the results are far from consistent. Most likely, this is due to differences in evaluation conditions and sample preparation, which are both of utmost importance for the photocatalytic activity of titania. Several formation routes have been exploited to synthesize titania in the form of nanoparticles, e.g., sol-gel methods,25 Langmuir-Blodgett films,26-28 and reverse microemulsions,29-36 and many of these studies have also included photocatalytic evaluations.29,30,34-36 Most often, titania is hydrothermally treated in order to form a crystalline sample at relatively low temperature22,37-40 or even at room temperature.8,41-43 Furthermore, the type of reaction (reduction or oxidation) as well as the reaction conditions seem to play a major role in the outcome of the photoinduced reaction. In a previous study, we reported that both anatase and rutile are photocatalytically active for the decomposition of phenol, although they follow different reaction mechanisms as evidenced by the formation of different intermediates upon phenol degradation.30 In that study, the two titania polymorphs were formed under the same mild hydrothermal conditions (120 °C) using a reverse microemulsion system in order to evaluate the role of the crystalline structure while keeping all possible parameters constant. The two catalyst materials were either pure anatase or pure rutile having a specific surface area of 250 or 40 m2/g, respectively. In the present work, a method utilizing a similar microemulsion system was used. However, the formation procedure was performed at room temperature, resulting in nanocrystalline rutile with an extremely high specific surface area (∼300 m2/g), which, to our knowledge, is the highest specific surface area ever reported for rutile. The rutile particles were thoroughly characterized using transmission electron microscopy (TEM), nitrogen adsorption, and Fourier transform infrared (FTIR) spectroscopy, and the synthesis procedure was monitored in situ using dynamic light scattering (DLS) and UVvis spectroscopy. The influence of the microemulsion on the synthesis is discussed, and a detailed description of the synthesis mechanism is provided. The photocatalytic performance of the formed material was evaluated by degradation of formic acid using mass spectrometry (MS) and in situ FTIR spectroscopy. The results are compared to those obtained using rutile having a lower specific surface area. Experimental Section Materials. TritonX-100 [(tert-octylphenoxy)polyethoxyethanol], 1-hexanol (98% GC), cyclohexane (99%), titanium(IV) butoxide (97%), nitric acid (concentrated), and formic acid (>98%) were purchased from Aldrich and used as received. All water used was doubly distilled. Formation of Titania Nanocrystals. The titania nanoparticles were synthesized using a reverse microemulsion system according to the procedure described by Andersson et al.,30 except that in this case, the synthesis was performed at room temperature. The composition of the microemulsion used for the material preparation is reported in Table 1. In short, one mixture containing the surfactant (TritonX-100), cosurfactant TABLE 1: Compositions Used for Titania Synthesis titania precursor aqueous component surfactant cosurfactant oil component
component
volume (mL)
tetrabutyl titanate HNO3 (5 M) TritonX-100 n-hexanol cyclohexane
0.85 2 2.5 1.5 4
(1-hexanol), and oil (cyclohexane) and another mixture containing the acid (5 M HNO3) and titania precursor [titanium(IV) butoxide] were prepared separately. The acid and alkoxide mixture was then added to the organic solution under stirring, and a clear reverse microemulsion formed. All of the above steps were performed at room temperature. The microemulsion was then either left stirring at room temperature or put into a stainless steel autoclave and hydrothermally treated at 120 °C for selected time periods. After completion, powder samples were collected from the synthesis mixtures via centrifugation. The samples were washed in a procedure that was repeated five times and involved redispersion in ethanol and centrifuging, followed by drying at 100 °C for 10 h. Characterization. Dynamic light scattering (DLS) measurements were performed using a Malvern Instruments Series 7032 Multi-8 correlator and PCS 100 spectrometer at a wavelength of λ ) 632.8 nm and a scattering angle of 90°. The samples were filtered (0.2-µm syringe filter) before the measurements, and the temperature was kept at 25 °C. UV-vis absorption spectra of the titanium compound were measured using a GBC 920 UV-vis double-beam spectrometer. The formed material was collected by filtration and redispersed in ethanol prior to the measurements. The reference used was pure ethanol, and the measurements were performed in a 1-mm quartz cuvette at 22 °C. X-ray diffraction (XRD) patterns were obtained on a Siemens D5000 X-ray diffractometer equipped with a Cu KR radiation source (λ ) 1.54 Å). Scans were collected on washed and dried powders in the 2θ range of 20-60°. Transmission electron microscopy (TEM) was performed on a JEOL 1200 EX II microscope equipped with a Wolfram filament operated at 120 kV. High-resolution TEM (HRTEM) micrographs were obtained using a Philips 200 microscope equipped with a LaB6 filament operated at 200 kV. The TEM sample powders were ground in an agate mortar, dispersed in ethanol, placed on a holey carbon grid, and finally dried in open air for 2 h prior to TEM investigations. Nitrogen adsorption and desorption isotherms were collected at 77 K using a Micromeretic TriStar instrument. Before the measurements, the samples were exposed to vacuum treatment for 12 h at 120 °C to remove remaining moisture. The specific surface areas were obtained using the BET method.44 In situ diffuse reflectance FTIR (DRIFT) spectra were collected using a Bruker IFS-66v/S FTIR spectrometer equipped with a broad-band MCT detector and a modified reaction cell mounted inside a Praying Mantis (Harrick) diffuse reflectance optical house. The background was collected in a feed of 20% O2 in N2 (AGA research grade) at 50 mL/min on a fresh TiO2 sample, which was pretreated at 450 °C in synthetic air. All DRIFT measurements were performed at 20 °C. Photocatalytic Activity Experiments. Photocatalytic activity experiments were carried out with simulated solar light illumination from a 300 W Xe arc lamp source (Oriel) with a focusing lens assembly combined with AM1.5 filters. The total photon flux at the sample was measured to be 166 mW/cm2, which corresponds to 13 mW/cm2 at λ < 386 nm and 23 mW/ cm2 at λ < 411 nm (the band-gap energies of anatase and rutile, respectively). The powder samples (50 mg) were pretreated in synthetic air (50 mL/min) at 450 °C for 30 min in the reaction cell prior to experiments. This resulted in a white and contamination-free sample as determined by DRIFT spectroscopy. All DRIFT background spectra were acquired in synthetic air on samples pretreated in this manner. When present, formic acid was added to the synthetic air flow (50 mL/min) at a concentration of 7900 ppm through a home-built gas generator.29
Microemulsion-Mediated Synthesis of Rutile
Figure 1. DLS results showing the autocorrelation curves for microemulsions, 100 and 240 min after the addition of titanium alkoxide. The corresponding radius after 100 min is given in the figure, together with its standard deviation.
Results Titania Formation and Characterization. Several measuring techniques were utilized to follow the titania formation procedure, as well as to characterize the prepared product. The reaction carried out at room temperature was followed in situ using DLS and UV-vis spectrometry, and the formed titania was characterized using XRD, N2 adsorption, TEM, HRTEM, and FTIR spectroscopy. The results from the DLS measurements are presented in Figure 1. Here, the autocorrelation function, F(q,t), is shown for the microemulsion at times 100 and 240 min after the addition of titanium alkoxide. After 100 min, the average microemulsion droplet size was measured to be 1.6 nm in radius, which correlates well with the microemulsion droplet size reported elsewhere for a similar system.45 Measurements were performed prior to 100 min but without any reasonable results correlating to droplet size. This is because the titanium alkoxide is soluble in the oil phase, but following reaction with water, it forms hydrolyzed polar species. Hence, prior to 100 min, it was not possible to achieve a sufficient count rate because the refractive index of water is very similar to the refractive index of cyclohexane, which is the major constituent of the oil phase. After 100 min, the titanium species become water-soluble, and the condensation reaction starts. The autocorrelation function obtained 240 min after titanium addition indicates the formation of another population of aggregates. These new aggregates are much larger (200-1000 nm) and increase further in number with time and form a turbid mixture. It should be mentioned that measurements were conducted at 10-min intervals during the first 300 min after the reaction commenced, but no significant difference could be observed prior to approximately 240 min. UV-vis absorption spectra measured at different time intervals during the particle formation reaction are presented in Figure 2. Here, it is seen that the absorption edge of the synthesis mixture is strongly shifted toward shorter wavelengths between 24 and 48 h after the reaction started. There is a further shift in absorbed wavelength between 48 and 72 h, after which the absorption spectra remain approximately the same. The final absorption edge was estimated to be 370 ( 5 nm as derived from a plot of (Ahυ)1/2 vs hυ (not shown). These results indicate that the crystallization commences prior to 72 h after the addition of the titania precursor and continues for another 24-48 h. X-ray diffraction of the prepared products confirms that crystalline titania (rutile) formed within 4 days at room
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Figure 2. UV-vis absorption spectra monitoring the formation of titania in a water-in-oil microemulsion, collected after different reaction times following the addition of the titania alkoxide.
Figure 3. Powder X-ray diffractograms of the titania synthesized at both room temperature and 120 °C. The Bragg peaks are identified as rutile and anatase as seen from the peak positions noted in the figure. Details of the different samples are presented in Table 2.
temperature (Figure 3A). Samples collected prior to this did not show crystallinity by XRD. In contrast, when the synthesis was performed at 120 °C, anatase was instead formed initially (after 9 h), as shown in Figure 3B. However, after an additional 1 h, rutile also appeared (Figure 3C), and within the following 2 h, the anatase was converted to rutile (Figure 3D,E). This means that, at 120 °C, there is a window in the time frame where the anatase polymorph is favored. It also shows that rutile is formed by different paths depending on temperature, appearing via a metastable anatase phase at 120 °C and forming via an amorphous structure at 20 °C. As expected, the size of the formed titania crystals also depended on the formation conditions, as indicated by the difference in the width of the Bragg peaks in the diffractograms. The sizes of the crystals were calculated from the full width at half-maximum (fwhm) of the anatase (101) and rutile (110) diffraction peaks using the Scherrer equation and are reported in Table 2. The content of each polymorph, estimated from the relative integrated intensities of these XRD peaks, is also presented in Table 2. The size of the crystals increased with synthesis temperature and time as expected, and the rutile crystals were smaller in size than the anatase crystals. The TEM micrographs presented in Figure 4 were collected at different stages of the formation of the titania particles
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TABLE 2: Synthesis Temperatures and Times for the Formation of Titania Using the Compositions Presented in Table 1 sample
temp (°C)
time
rutile contenta (%)
BET surface areab (m2/g)
sizec (nm)
A B C D E
20 120 120 120 120
4 days 9h 10 h 11 h 12 h
100 0 46 52 100
300 308 274 246 39
5 (rutile) 16 (anatase) 18 (anatase) 10 (rutile) 8 (rutile)
a Rutile contents estimated from the individual XRD peaks. b Specific surface areas calculated from nitrogen adsorption measurements using the BET method. c Crystal size calculated using the Scherrer equation on the anatase (101) and rutile (110) Bragg peaks
Figure 5. Nitrogen adsorption/desorption isotherms (77 K) for titania produced under various conditions; see Table 2 for details. A pronounced difference is seen between the (B-D anatase and (A,E) rutile samples.
Figure 4. TEM micrographs of titania prepared at room temperature: (A) Image collected 24 h after the start of the reaction showing an amorphous aggregate. (B) Image collected 48 h after the start of the reaction showing agglomerated needle-shaped particles. These particles are crystalline with the rutile structure as indicated by the (C) electron diffraction pattern collected 48 h after the start of the reaction. The (110) and (002) reflections of the rutile structure are indicated. The needle-shaped particles consist of smaller crystals as seen from (D) dark-field TEM micrographs and (E) an HRTEM image. In the HRTEM image, the spacing between the lattice fringes is 0.32 nm, which corresponds well with the (110) crystal plane of rutile. (F) Image collected 7 days after the start of the reaction showing no difference in crystal structure compared to the image obtained after 48 h.
prepared at 20 °C. Figure 4A shows a micrograph of a sample obtained after 24 h of reaction. Here, agglomerates (between 200 and 1000 nm in size) of amorphous nanoparticles were observed, and no electron diffraction patterns were observed. After 48 h of reaction, Figure 4B shows the appearance of needle-shaped particles within the agglomerates. These needleshaped particles are crystalline, and Figure 4C shows an electron diffraction pattern that is consistent with rutile. When viewed using dark-field TEM (Figure 4D), it can be seen that the needleshaped particles consist of small crystals with a brick-shaped morphology having a size of approximately 3 × 5 nm that are stacked with their short axis perpendicular to the direction of the needles. These crystals are shown in the high-resolution TEM image in Figure 4E. Seemingly, the particles aggregate along a specific crystallographic direction, which results in the elongated needle-shaped particles. This type of crystal was previously observed by Gopal et al. using another reaction system.8 The distance between the crystal fringes is 0.32 nm, which is also the distance between the atoms forming the (110) crystal plane (0.325 nm according to the literature8). The crystal morphology of rutile prepared at 20 °C differs from that prepared at 120 °C. In the latter case, large crystal particles form that preferentially grow in the most stable (110) direction forming needle-like particles along the (001) direction.39,46 At 20 °C, the needle-shaped particles remain after additional reaction time, and Figure 4F shows a micrograph of a sample collected after 7 days of reaction time. In Figure 5, the nitrogen adsorption/desorption isotherms are presented for titania particles prepared both at room temperature and at 120 °C (see Table 2 for details). The anatase-containing samples all showed type IV hystereses and a baseline that increased with increasing relative pressure. The pure rutile samples showed a type I isotherm. The type IV behavior is characteristic of mesoporous materials having a pore size between 2 and 50 nm, and type I behavior is characteristic of microporous materials (
