Article pubs.acs.org/crystal
Rapid Hydrothermal Preparation of Rutile TiO2 Nanoparticles by Simultaneous Transformation of Primary Brookite and Anatase: An in Situ Synchrotron PXRD Study Jian-Li Mi, Casper Clausen, Martin Bremholm, Nina Lock, Kirsten M. Ø. Jensen, Mogens Christensen, and Bo B. Iversen* Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, DK-8000 Aarhus C, Denmark ABSTRACT: The formation mechanism and crystal growth of TiO2 in hightemperature high-pressure fluids were studied using HCl or H2SO4 as additives. In situ synchrotron radiation powder X-ray diffraction reveals that phase-pure rutile TiO2 nanoparticles can be formed using HCl as additive, whereas phase-pure anatase TiO2 is obtained when H2SO4 is used as additive. The supercritical (or near-critical) conditions provide a fast, one-step synthesis of rutile TiO2 nanoparticles and when using a 1:1 volume ratio of isopropanol−water as solvent at a temperature of 300 °C and a pressure of 25 MPa particles with an average particle size of about 22 nm are obtained in 20 min. A detailed analysis by sequential Rietveld refinements shows that the formation of rutile TiO2 occurs by a combined transformation of anatase and brookite TiO2. Analysis of the unit cell dimensions of the nanoparticles shows a lattice expansion with decreasing particle size for anatase prepared with H2SO4 medium and this may explain the stability of anatase particles that are significantly larger than their critical size.
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INTRODUCTION TiO2 is an intensively studied material of growing interest in a variety of applications such as photoelectrochemical solar energy conversion and environmental photocatalysis of water splitting to generate hydrogen and treatment of polluted water.1 The properties of TiO2 depend strongly on particle size, crystal structure, morphology, and crystallinity.2,3 Controlled and tailored preparation of TiO2 nanoparticles is of great importance for the effective utilizations of such nanoparticles. TiO2 has three polymorphs in nature, namely rutile, anatase, and brookite.4 The anatase and rutile phases both have tetragonal structures, whereas brookite has an orthorhombic structure. All three crystal structures consist of (TiO6) octahedra, which share either edges or corners. At ambient conditions rutile is the thermodynamically stable phase, whereas anatase and brookite are metastable. On the nanoscale anatase has been found to be more stable due to surface effects. The critical size for anatase-to-rutile phase stability has been predicted to be 32 nm.3 Complete control over the synthesis of anatase TiO2 has been achieved by pulsed supercritical synthesis where particles having any desired size between 7 and 35 nm can be prepared.6 Nanostructured materials with small crystallite sizes and high surface areas are of great interest for improving optical, electrical, and catalytic properties. Rutile TiO2 can be obtained via calcination of anatase TiO2 or amorphous TiO2 at high © 2012 American Chemical Society
temperatures. However, the thermal treatments induce significant grain growth and the resulting rutile crystals are always large. Therefore, low temperature wet chemistry such as hydrothermal and sol−gel methods have been used for preparation of nanostructured rutile TiO2.7 Previous studies show that different TiO2 phases can be formed by using different acid media. It has been found that H2SO4 or acetic acid can be used to form anatase, whereas HCl or HNO3 acid media result in phase transformation from anatase nanoparticles to rutile nanoparticles.8 However, under conventional hydrothermal conditions long aging time is necessary to obtain phase pure rutile TiO2. Therefore, it is desirable to find new fast and convenient synthesis methods to produce rutile TiO2 nanoparticles. A supercritical fluid is a fluid that is heated and pressurized above its critical point.9 Supercritical fluids exhibit unique properties with gaslike transport properties in diffusivity, viscosity, and surface tension while also maintaining liquidlike properties such as high-solvation capability and density. Synthesis in supercritical or near-critical solvent is an efficient approach to produce highly crystalline nanoparticles with a narrow size distribution.9 In our previous studies, we have applied supercritical synthesis to produce anatase TiO2 and shown that the method allows quick synthesis with wellcontrolled size and crystallinity of the nanoparticles.10 Here, we report that rutile TiO2 nanoparticles also can be synthesized under supercritical or near-critical conditions. Considering the Received: August 25, 2012 Revised: October 23, 2012 Published: October 24, 2012 6092
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synthesis of rutile TiO2, the phase transformation of anatase to rutile in the solid state has been well studied.5,11 However, understanding of the formation and growth mechanism of rutile TiO2 in near-critical hydro/solvothermal conditions is still lacking. It is clearly of interest to develop a fast synthesis route of rutile TiO2 under supercritical/near-critical condition and to investigate the reaction processes. In recent years, in situ X-ray studies have been used to investigate a wide range of chemical reactions,12 and in our group we have focused on studying nanocrystal growth processes under near-critical or supercritical conditions.13 The present work is focused on fast synthesis of rutile TiO2 nanoparticles in near-critical condition and the reaction is followed by in situ synchrotron radiation powder Xray diffraction (SR-PXRD). The data provide clear insight into the phase formation and particle growth processes of TiO2 in different phases.
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EXPERIMENTAL SECTION All chemical reagents used in the experiments were analytical grade. A sapphire capillary was used as a high-pressure cell to study the formation and growth of TiO2 crystallites by timeresolved in situ SR-PXRD. A solution of 2.0 M titanium tetraisopropoxide (TTIP, Ti(OCH(CH3)2)4) was prepared by dissolving TTIP (>97%) in isopropanol. This solution was added dropwise into an equal volume of 2.0 M HCl (or 1.0 M H2SO4) aqueous solution under continuous stirring, and a sol was obtained. The sol precursor was injected into the sapphire capillary, and subsequently, the sapphire capillary was sealed, pressurized with a HPLC pump, and heated using a hot air flow.14 The experiments were performed at a temperature of 300 °C and a pressure of 25 MPa. The in situ SR-PXRD data were collected at beamline I711, MAX-II, MAX-lab, Sweden, using monochromatic X-rays with a wavelength of 1.0009 Å (E = 12.4 keV). The data was collected on a Mar165 CCD detector with a time resolution of 5 s between each frame, of which 1 s was detector dead time for readout. The SR-PXRD data were refined using the Rietveld method implemented in the FullProf program15 and corrected for instrumental broadening using data measured on a LaB6 standard. A ThompsonCox-Hastings pseudo-Voigt axial divergence asymmetry profile function and a background modeled with linear interpolation were used.
Figure 1. Time evolution of the in situ SR-PXRD patterns for the synthesis of TiO2 in HCl (a) and H2SO4 (b) media, respectively.
Parts a and b of Figure 2 show representative PXRD data with observed, calculated and difference patterns of the sample prepared in HCl medium at reaction times of 3.3 and 19.8 min, respectively. At a reaction time of 3.3 min, the diffraction pattern shows a mixture of rutile, anatase and brookite phases, whereas at a reaction time of 19.8 min, almost single phase rutile is obtained with only about 1 wt % anatase left. The high quality of the data allows us to refine all the three phases, and excellent agreement is observed between the calculated and observed patterns. The particle size was obtained from the peak broadening. The anisotropic particle shape can be modeled by a linear combination of spherical harmonic function.16
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RESULTS AND DISCUSSION Part a of Figure 1 shows the time evolution of the PXRD patterns for the sample, which is prepared in HCl medium. During the first 70 s, the PXRD patterns show the amorphous phase but then the anatase and brookite phases form rapidly. At the same time, the TiO2 rutile phase starts to form slowly. The intensities of the diffraction peaks of the anatase and brookite phases increase rapidly in about 20 s after being detected and subsequently decrease with prolonged reaction time. However, the intensities of the diffraction peaks of the rutile phase increase continuously and become almost constant after a reaction time of 8 min. When H2SO4 is used as medium, anatase TiO2 starts to form quickly after a reaction time of 30 s as can be seen in part b of Figure 1, and diffraction peaks from other phases are not observed. The results indicate that in the HCl medium, rutile TiO2 is formed at the expense of the anatase and brookite phases, rather than by formation directly from the amorphous phase or by phase transformation only from anatase, and this will be further discussed below.
βh =
λ λ = D h ·cos θ cos θ
∑ almpYlmp(Θh , Φh) lmp
(1)
where βh is the size contribution to the integral breadth of reflection h, and Ylmp(Θh,Φh) are normalized real spherical harmonics. The refined coefficients almp were used to calculate the volume-weighted particle size along different crystallographic directions. An anisotropic refinement of the 19.8 min data shows that the rutile particle has sizes of 23(1) and 20(1) nm along the a and c directions respectively indicating isotropic growth. Therefore, a simplified model having only an isotropic size was used. The same method was used for the anatase and brookite particles. The refined parameters and crystallographic details from the Rietveld analysis of the SR-PXRD data for the sample prepared in HCl medium at reaction times of 3.3 and 19.8 min are listed in Table 1. Note that in our previous studies we have shown that the sizes of the nanoparticles calculated from PXRD data agree very well with the transmission electron 6093
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scale factor for each phase reflects the changing amount of the phase during the reaction. As can be seen, the amounts of anatase and brookite increase abruptly during the first frames after initial detection, and then decrease simultaneously at a reaction time of about 90 s. The brookite phase disappears after ∼10 min. The amount of rutile phase increases with reaction time and reaches a constant value after 10 min. Part b of Figure 3 shows the time dependence of the weight fractions for the three phases. The weight fractions are 1(1) %, 48(2) %, and 51(4) % for rutile, anatase, and brookite respectively at a reaction time of 70 s when the crystallites were initially observed. After 10 min, the weight fractions are 99(1) %, 1(1) %, and 0%, and they remain unchanged until the last frame at 19.8 min. To obtain the absolute normalized scale factor representing the total crystalline products within the probed volume, we divide any of the individual scale factors for the three phases by their respective weight fractions. Part c of Figure 3 shows the time evolution of the absolute scale factor of the total crystalline products, indicating a quick crystallization process from the amorphous phase. Part d of Figure 3 compares the normalized formation rates of the rutile phase and the total crystalline product obtained by differentiating the rutile curve in part a of Figure 3 and the curve in part c of Figure 3, respectively. The formation rate of the total crystalline product shows that the crystallization transformation from amorphous phase happens very fast in 20 s but that the formation of rutile TiO2 lasts until a reaction time of about 8 min. This indicates that both anatase and brookite are formed directly from the amorphous phase, whereas rutile is formed by a reaction between anatase and brookite. It is worth noting that, when brookite is consumed after 6 min, the formation of rutile slows down significantly. The above results show that the brookite phase plays an important role for the formation of rutile TiO2 with HCl medium. At the initial formation of rutile, the particle size of anatase is only 3 nm and even after ripening the size of anatase crystallites is only 12 nm as shown in part a of Figure 4. The transformation from anatase to rutile in such small particles appears to be due to the presence of another metastable phase, namely brookite. The initial nucleation of rutile possibly occurs at the interface between the anatase and brookite crystals due to a high interfacial energy. It has been reported that the presence of brookite phase will enhance the anatase-rutile transformation during heat treatment,17 which corroborates the present results. Brookite has also been proposed as an essential intermediate phase in mechanically induced anatase to rutile phase transformation.18 Our results show that brookite is not acting as an intermediate in the phase transformation from anatase to rutile since both phases nucleate in similar amounts and then decrease simultaneously. Part a of Figure 4 displays the time evolutions of particle sizes of rutile, anatase and brookite for the sample prepared with HCl medium. Because of the small and decreasing weight fraction of brookite, it is not possible to determine the particle size of brookite beyond reaction times of 4 min and similarly for anatase, where rather large error bars of particle size are obtained. The particle size of rutile TiO2 increases with reaction time until 5 min, and it is larger than those of anatase and brookite. It is known that the transition to rutile is accompanied by significant grain growth. The final particle sizes of rutile and anatase TiO2 of about 22 and 12 nm are obtained for the sample prepared in HCl medium. The fact that the particle size of rutile is much larger than for anatase and brookite could be
Figure 2. Observed, calculated, and difference patterns of SR-PXRD for the sample prepared in HCl medium at reaction times of 3.3 (a) and 19.8 min (b). The green markers indicate the Bragg reflections for the rutile, anatase, and brookite.
Table 1. Refined Parameters in the Rietveld Analysis of the SR-PXRD Data for the Samples Prepared in HCl Medium at Reaction Times of 3.3 and 19.8 Min reaction time (min) no. of data points no. of refined params no. of reflns RP (%) RWP (%) RF (%) (rutile) RF (%) (anatase) RF (%) (brookite) a (Å) (rutile) c (Å) (rutile) a (Å) (anatase)a c (Å) (anatase) rutile (wt%) anatase (wt%) brookite (wt%)
3.3 549 24 76 10.3 6.83 1.04 1.73 2.91 4.602(1) 2.971(1) 3.787(2) 9.484(4) 75(1) 10(1) 15(1)
19.8 549 21 76 12.1 7.00 1.94 12.7 4.613(2) 2.975(1) 3.794 9.491 99(1) 1(1) 0(0)
a
The cell parameters of anatase were refined for the data before 5 min and then fixed with the obtained values from the 5 min data for subsequent other frames.
microscopy (TEM) studies exemplified e.g. with anatase TiO2 nanoparticles10 and Zirconia nanoparticles.13h Part a of Figure 3 shows the time evolutions of normalized scale factors of rutile, anatase, and brookite phases for the sample prepared with HCl medium. The time evolution of the 6094
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Figure 3. (a) Time evolution of normalized scale factors of rutile, anatase and brookite phases for the sample prepared in HCl medium, (b) time evolution of weight fractions for each phase, (c) absolute scale factor for the total crystalline products, (d) normalized formation rates of the rutile phase and the total crystalline product obtained by differentiation of the curves in (a) and (c).
the a and c directions, respectively indicating that the particles are almost isotropic. A particle size of 18 nm is obtained after 10 min reaction. The anatase phase prepared with H2SO4 medium is much more stable than that prepared with HCl medium, even though the anatase crystallites are larger. In addition, the particle growth of anatase is much faster in H2SO4 medium. As seen in part a of Figure 5, the particle growth almost stops once the crystallization is finished, and the ripening growth is not obvious in this case. When comparing part c of Figure 4 and part b of 5, it is seen that the microstrain characteristics are fundamentally different for the anatase nanoparticles prepared with H2SO4 medium compared with those prepared with HCl medium. Thus, in H2SO4 medium the cell parameters increase at small particle sizes, whereas they decrease in HCl medium. The microstrain characteristics of anatase previously have been found to vary considerably depending on the synthesis route. For example, Zhang et al.19 have determined the cell parameters as function of particle size of nanocrystalline anatase prepared by hydrolysis of titanium ethoxide or titanium isopropoxide, and the results show lattice contractions along both the a and c axes. In contrast, Swamy et al.20 have shown a small lattice expansion at reduced crystallite size (expansion of cell parameter a and the unit cell volume, and contraction for cell parameter c). This has been explained as a Ti deficiency, which leads to lattice expansion, whereas the contraction of the anatase lattice is due to surface stress. The microstrain was also found to vary with the shape anisotropy of nanocrystalline anatase.21 In our experiments, the anatase nanocrystallites prepared with HCl medium show lattice contraction with reduced crystallite size, whereas the anatase nanocrystallites prepared with H2SO4 medium have lattice expansion. Cell parameters of a = 3.806(1) Å and c = 9.565(2) Å are observed at 10 min for
taken as an indication that the anatase and brookite particles fuse to form the rutile nanoparticles. It has been suggested that HNO3 or HCl might contribute to the reorientation of anatase to produce rutile crystals which is possibly via a dissolution− precipitation mechanism.7a However, our results clearly show that under these conditions it is more likely a solid state reaction mechanism with combination of anatase and brookite nanoparticles to form the rutile phase. Even though the formation process of anatase and brookite crystallites is rapid, it is found that the particle growth of anatase and brookite continues for a much longer time which is attributed to Ostwald ripening. For rutile, the particle growth ceases almost at the same time as the formation stops. This further indicates that the formation of rutile is a solid state reaction mechanism involving a reconstructive process. Parts b and c of Figure 4 show the time dependences of cell parameters of rutile and anatase prepared with HCl medium. The cell parameters are fixed in the refinements for anatase after 5 min and for brookite for all the data. Both the rutile and the anatase phase show contractions of the cell parameters along both a and c direction with decreasing particle size. It is known that cell parameters deviate from bulk values when the particle size decreases to nanoscale due to the microstrain. The cell parameters of the rutile phase are a = 4.572(1) Å, c = 2.960(1) Å and a = 4.613(2) Å, c = 2.975(1) Å for the reaction times of 2 and 19.8 min corresponding to the particle sizes of 16(1) nm and 22(1) nm, respectively. This is equivalent to a lattice contraction of 0.9% along a direction and 0.5% along c direction. Part a of Figure 5 shows the time dependences of the normalized scale factor and particle size of anatase TiO2 prepared using H2SO4 as medium. An anisotropic refinement shows that the 10.4 min particles are 17(1) and 18(1) nm along 6095
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anatase nanoparticles are formed due to the lower surface energy as compared to other phases when the particles are sufficiently small. On the other hand, a lower surface energy significantly accelerates the nucleation kinetics of the formation of anatase.22 Zhang et al. has suggested that the surface free energy of nanocrystalline anatase has a strong size dependence with a maximum value at around 14 nm, where the particles have lattice contractions along both the a and c axes.19 Even though the detailed mechanism for the variation of the microstrain with particle size is still unknown, the abnormal microstrain behaviors of anatase nanoparticles prepared in H2SO4 medium may explain the stabilization of the anatase particles at a size much larger than the expected critical size.
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CONCLUSIONS The formation and growth of TiO2 nanoparticles in near-critical solution were studied by in situ SR-PXRD. The synthesis method provides a fast route for preparation of rutile TiO2 nanoparticles. The formation of rutile TiO2 is a result of a solid reaction with phase transformation from anatase and brookite to rutile. The anatase nanocrystallites prepared with HCl medium show lattice contractions with reduced crystallite size, whereas lattice expansion is observed for the crystallites prepared with H2SO4 medium. The different microstrain characteristics of anatase nanoparticles prepared under different conditions may explain the different properties in phase formation, phase stability, and crystal growth.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
Figure 4. (a) Time evolutions of particle sizes of rutile, anatase, and brookite TiO2 for the sample prepared with HCl medium, (b) cell parameters for rutile TiO2, (c) cell parameters for anatase TiO2.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Danish National Research Foundation (Center for Materials Crystallography), the Danish Strategic Research Council (Center for Energy Materials), and the Danish Research Council for Nature and Universe (DanScatt). The authors are grateful for the beamtime obtained at the beamline I711, MAX-lab synchrotron radiation source,
the anatase particles prepared with H2SO4 medium. Compared with the cell parameters of a = 3.794(2) Å, c = 9.491(7) Å for the anatase particles observed after 5 min in HCl medium as well as most reported anatase cell parameters,20 the elongation of the cell parameter along the a axis is moderate, whereas the value is much larger along the c axis. It has been suggested that
Figure 5. (a) Time dependence of the normalized scale factor and particle size of anatase TiO2 prepared using H2SO4 as medium. (b) Anatase cell parameters as a function of time for synthesis with H2SO4 medium. 6096
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B. Chem.Eur. J. 2012, 18, 5759−5766. (i) Jensen, K. M. Ø.; Christensen, M.; Tyrsted, C.; Iversen, B. B. J. App. Cryst 2011, 44, 287−294. (j) Tyrsted, C.; Jensen, K. M. Ø.; Bøjesen, E. D.; Lock, N.; Christensen, M.; Billinge, S. J. L.; Iversen, B. B. Angew. Chem. 2012, DOI: 10.1002/anie.201204747. (k) Jensen, H.; Bremholm, M.; Nielsen, R. P.; Joensen, K. D; Pedersen, J. S.; Birkedal, H.; Chen, Y.-S.; Almer, J.; Søgaard, E. G.; Iversen, S. B.; Iversen, B. B. Angew. Chem. 2007, 46, 1113−1116. (l) Jensen, K. M.; Christensen, M.; Bremholm, M.; Iversen, B. B. Cryst. Growth Des. 2011, 11, 753−758. (14) Becker, J.; Bremholm, M.; Tyrsted, C.; Pauw, B.; Jensen, K. M. Ø.; Eltzholt, J.; Christensen, M.; Iversen, B. B. J. Appl. Crystallogr. 2010, 43, 729−736. (15) Rodriguez-Carvajal, J. Physica B 1993, 192, 55−69. (16) Jarvinen, M. J. Appl. Crystallogr. 1993, 26, 525−531. (17) Hu, Y.; Tsai, H. L.; Huang, C. L. J. Eur. Ceram. Soc. 2003, 23, 691−696. (18) Rezaee, M.; Khoie, S. M. M.; Liu, K. H. CrystEngComm 2011, 13, 5055−5061. (19) Zhang, H.; Chen, B.; Banfield, J. F. Phys. Chem. Chem. Phys. 2009, 11, 2553−258. (20) Swamy, V.; Menzies, D.; Muddle, B. C.; Kuznetsov, A.; Dubrovinsky, L. S. Appl. Phys. Lett. 2006, 88, 243103. (21) Jensen, G. V.; Bremholm, M.; Lock, N.; Deen, G. R.; Jensen, T. R.; Iversen, B. B.; Niederberger, M.; Pedersen, J. S.; Birkedal, H. Chem. Mater. 2010, 22, 6044−6055. (22) Deyes-Coronodo, D.; Rodríguez-Gattorno, G.; EspinosaPesqueira, M. E.; Cab, C.; de Coss, R.; Oskam, G. Nanotechnology 2008, 19, 145605.
Lund University, Sweden, and Jeppe Christensen is thanked for assistance during measurements.
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