Direct Formation of Crystalline Phase Pure Rutile TiO2 Nanostructures

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Direct formation of crystalline phase pure rutile TiO2 nanostructures by a facile low temperature hydrothermal method Aref Mamakhel, Christoffer Tyrstedt, Espen D. Bojesen, Peter Hald, and Bo Brummerstedt Iversen Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg400858p • Publication Date (Web): 01 Oct 2013 Downloaded from http://pubs.acs.org on October 11, 2013

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Crystal Growth & Design

Direct formation of crystalline phase pure rutile TiO2 nanostructures by a facile hydrothermal method Aref Mamakhel, Christoffer Tyrsted, Espen Drath Bøjesen Peter Hald and Bo Brummerstedt Iversen*

Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Langelandsgade 140, DK-8000 Aarhus, Denmark

* Email: [email protected]

Abstract Small titania, TiO2, nanoparticles (< ~10 nm) are normally restricted to the anatase polymorph, and synthesis of stable rutile nanoparticles below the critical size is challenging. Here, we report on a fast, low temperature and environmentally benign hydrothermal method to prepare phase pure rutile TiO2 nanorods with an average diameter of 42 nm covered by crystalline spherical nanoparticles around 10 nm in size. The synthesis approach utilises titanium tetraisopropoxide and glycolic acid at 200 0C for 3 - 12h, and the samples were characterized by powder X-ray diffraction (PXRD), UV-VIS spectroscopy, FT-IR spectroscopy, and Transmission Electron Microscopy (TEM). In addition, in situ synchrotron PXRD measurements were carried out to follow the formation and growth of the rutile nanoparticles under the present mild hydrothermal conditions, and it is observed that rutile TiO2 is formed directly from solution without intermediate brookite or anatase phases.

Keywords In situ X-ray diffraction, nanoparticles, rutile, hydrothermal synthesis

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1. Introduction The natural polymorphs of titania (TiO2) include rutile (P42/mnm), anatase (I42/amd), and brookite (Pcab). Rutile is the preferred phase for different applications such as a dielectric ceramics, capacitors, power circuits, and potentially in future Li-ion battery materials.1-4 The optical properties of rutile are especially attractive, exhibiting UV absorption up to the proximity of the visible range and, simultaneously, transparency in the visible wavelength range.5 Rutile is therefore of interest e.g. as UV absorber in polymers, either to preserve the polymers themselves from degradation by UV light, or in the form of polymer-rutile composites which protect UV sensitive materials.6,

7

However, since polymers typically do not have high temperature stability, it is desirable in fabrication of polymer-rutile composites that the inorganic nanoparticle is synthesised at low temperature. The stability of the rutile phase in nanocrystalline TiO2 has been shown to be highly dependent on reaction-conditions (surface chemistry) making it difficult to obtain phase purity without post-calcination.8 Different proven routes for the synthesis of phase-pure rutile nanoparticles include hydrolysis, hydrothermal processing, laser ablation and more.9-18 However, a simple benign route is lacking to prepare phase pure rutile TiO2, which can be transferred to large scale production. Hydrothermal methods have proven themselves to be a viable low temperature approach. Recently, the formation of phase pure rutile nanocrystallites about 22 nm in diameter were investigated as synthesized from titanium tetraisopropoxide and 2 M HCl in a sapphire capillary at 300 oC (25 MPa) with a residence time of 20 min.19 There are, however, limitations in the upscaling of this approach. A conventional Teflon-lined stainless steel autoclave cannot withstand temperatures above 250 oC. More complex autoclaves may be used at the conditions of 300 oC and 25 MPa, but steel reactors will suffer from corrosion from hydrochloric acid at these extreme conditions. Reyes-Coronado et al. published a similar synthesis approach using titanium tetraisopropoxide and a high acid concentration of 4M HCl at 200 oC.16 This reaction was suggested to follow a dissolution-precipitation formation mechanism. However, both these approaches preclude the use of steel reactors due to the use of hydrochloric acid. Another disadvantage is that the long heating time required for full phase transformations excludes the use of flow reactors where maximum residence times typically are a few minutes.20-23


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Tomita et al. were able to synthesize phase pure rutile by a different approach using (NH4)6[Ti4(C2H2O3)4(C2H3O3)2(O2)4O2]·4H2O.23 The complex was prepared by dissolving metallic titanium powder in a solution of concentrated H2O2 (30%) and NH3 (28%) and later adding glycolic acid as a complexing agent. However, a major drawback of this approach is that it requires the use of two rather aggressive chemicals and takes considerable time. Nevertheless, inspired by the approach envisioned by Tomita et al.,24 we have developed a scalable, faster, and more benign route to the synthesis of phase pure rutile nanoparticles. In addition, in order to understand the formation mechanism we have also carried out in situ PXRD studies of the hydrothermal reaction, and it is observed that rutile forms directly from solution without intermediate crystalline phases.

2. Experimental section 2.1 Materials All chemicals were purchased from a commercial source (Sigma-Aldrich) and used as received. Titanium tetraisopropoxide Ti[OCH(CH3)2]4 (97%, CAS: 546-68-9) was used as the precursor for the TiO2 nanoparticles. Isopropanol (99.8%, CAS: 67-63-0) was used as solvent to dilute titanium isopropoxide and glycolic acid HOCH2CO2H (99%, CAS: 79-14-1) was used to induce the formation of the rutile polymorph of titanium dioxide.

2.2 Synthesis A mixture of 5.0 mL Ti[OCH(CH3)2]4 and 5.0 mL isopropanol was poured into 50 mL (1.6 M) aqueous solution of glycolic acid HOCH2CO2H. Upon mixing, a white suspension was obtained as seen in Figure 1a. When heated, the suspended material dissolved to give a transparent solution (Figure 1b) with a [Ti4+] concentration of 0.28 M and pH around 1.6. The transparent solution can be condensed to a transparent gel like material which can be re-dissolved to obtain a desired concentration. The water miscible precursor solution makes it possible to use flow reactors avoiding sedimentation and clogging. For autoclave syntheses, the transparent solution was sealed in a 175 mL Teflon-lined stainless steel autoclave and heated to 200 oC for reaction times from 3 to 12 hours and then cooled to room temperature. The resulting titanium dioxide powders (Figure 1c) were separated in a centrifuge and washed three times with ethanol before being dried at ambient temperature.



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Figure 1. a) Suspension obtained upon mixing TTIP solution with glycolic acid solution. b) Upon heating, the suspended material is dissolved to from a clear transparent solution. c) Hydrothermal processing yields a white phase pure nanopowder of rutile TiO2.

2.3 Powder X-ray diffraction Powders X-ray diffraction was measured on a SmartLab Rigaku diffractometer equipped with a Cu Kα source, parallel beam optics and a D/tex Ultra 1D detector. Powder patterns were analyzed using a Le Bail refinement in the Fullprof software suite.25 The average volume weighted crystallite size was determined using the Scherer equation with the peak width corrected for instrumental broadening.26

2.4 Fourier Transform Infrared Spectroscopy FT-IR spectra of Rutile nanoparticles were recorded on a NICOLET 380 with smart orbit spectrophotometer in 4000-400 cm-1 range.

2.5 UV-VIS Spectroscopy Optical diffuse reflectance measurements were performed at room temperature using a Shimadzu UV-3101 PC spectrometer operating in the 200-2500 nm region. Reflectance versus wavelength data were used to estimate the band gap of the material by converting reflectance to absorption data according to the Kubelka-Munk equation: α/S = (1-R)2(2R)-1, where R is the reflectance and α and S are the absorption and scattering coefficients, respectively.

2.6 Transmission Electron Microscopy


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TEM images were recorded on a Philips CM20 electron microscope equipped with a LaB6 filament at 200 kV. Particle sizes were estimated from the TEM images using the software ImageJ.

2.7 In situ PXRD In situ PXRD measurements were performed during synthesis of rutile at 200 oC, 300 °C and 400 °C and at a solvent pressure of 250 bar. The data were collected at beamline I711 at MAX-lab (Lund, Sweden), using a capillary setup described in detail by Becker et al.27 The solution prepared for conventional autoclave synthesis was loaded into a 1.5 mm diameter sapphire capillary which was pressurized by a HPLC pump. Synthesis was initiated by directing a heated air jet onto the capillary thereby reaching set point temperatures within 20 seconds. Simultaneously, the capillary was penetrated by a monochromatic 1.009 Å X-ray beam. 2D X-ray scattering images were recorded with 5 s resolution on an Oxford Diffraction Titan CCD detector at a detector distance of 88.1 mm. Integrated PXRD patterns were analysed by sequential Rietveld refinement using the Fullprof program suite to obtain unit cell dimensions and crystallite sizes for each obtained PXRD pattern.28-30 Here, crystallite size refers to the diameter of individual scattering domains.

3. Results and discussions 3.1 Structure The purity and crystallinity of the as synthesized samples were investigated by PXRD (Figure 2). A gel-like material is obtained when vacuum-drying the transparent precursor solution (Figure 1b) at room temperature. The PXRD pattern obtained from the gel material (Figure 2a) is dominated by the scattering pattern of glycolic acid. No crystalline forms of TiO2 are observed at this stage. The hydrothermal treatment at 200 oC yields phase pure rutile, and PXRD estimates of the crystallite size are 5(1) nm, 12(1) nm and 13(2) nm for the reactions carried out for 3 h, 6h and 12 h, respectively (Figure 2a). Le Bail refinement (Figure 2b) reveals that all reflections in the PXRD patterns can be assigned to the rutile phase (ICDD#00-021-1276) exhibiting tetragonal symmetry (P42/mnm, a = b = 4.5993(6) Å, c = 2.9559(3) Å, α = β = ϒ = 90o). No reflections are observed from anatase or brookite phases of TiO2. This indicates that rutile is the first and only crystallographic phase of TiO2 formed, and this is further investigated by the in situ measurements discussed below.



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Figure 2. a) PXRD diagrams of the precursor gel and rutile nanoparticles synthesized for 3h, 6h and 12h at 200 oC. b) Le Bail refinement of the powder pattern (12 h sample) showing phase pure rutile. c) Simulated powder pattern for rutile TiO2 measured with Cu Kα1 radiation. As expected, the glycolic acid affects the surface chemistry of the TiO2 nanopowder as observed through the FT-IR spectrum of the rutile sample obtained with a reaction time of 12 h (Figure 3a). The bands at 1430 and 1556 cm-1 are attributed to the symmetric COO- stretching indicating a covering of the particle surface with carboxyl groups. A broad band at 3200-3600 cm-1 is attributed to the O-H stretching while the band at 500 cm-1 can be attributed to the Ti-O stretching vibrations


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of crystalline TiO2.31 The reported direct band gap of bulk rutile is around 3.0 eV,32 compared with the slightly blue-shifted band gap of around 3.1 eV measured for the nanoparticles synthesized for 12h (Figure 3b). No significant difference in band gap was observed between samples prepared with varying synthesis durations.

Figure 3. a) FT-IR spectrum of Rutile nanoparticles synthesized for 12 h. b) Band gap measurement of Rutile nanoparticles synthesized for 12 h.

3.2 Morphology The morphological visualization was carried out only for the rutile nanoparticles synthesized for 12 h, and Figure 4 shows TEM images. The sample contains clusters of pointed crystalline nanorods with a diameter of around 42 nm and lengths of several hundred nm. The surface of the rods is covered by crystalline spherical particles with a mean diameter below 10 nm. The crystallite size estimated by the Bragg peak broadening in PXRD data (Figure 2), therefore, originates as an average of the size domains observed in the TEM images. Both nanostructures must be phase-pure



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rutile TiO2 since no other crystalline phases were observed by PXRD. An Ostwald ripening mechanism is suspected to be the origin of the nanorods obtained from for the prolonged autoclave synthesis, where the smaller particles may be dissolved and re-deposited onto the growing rod structures. TEM investigations of the nanopowders obtained at different synthesis times support this hypothesis; showing nanoparticle assemblies gradually forming well-defined nanorods with prolonged synthesis time (see Figure S3-S6).


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Figure 4: TEM images of the sample prepared for 12 h at 200 °C showing coexisting rutile nanorods with sharp ends decorated by smaller nanoparticles.



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3.3 In situ PXRD The direct formation of rutile nanocrystallites was investigated through in situ PXRD measurements, which is a way to directly observe crystallite formation and growth as well as possible phase transformations during hydrothermal syntheses.33-34 Figure 5a shows a time-resolved stacking of PXRD patterns with intensity visualized through brightness. The PXRD patterns were obtained during a heating of the transparent Ti-complex solution at 400 oC and 25 MPa for 15 minutes. No intermediate crystalline phase is observed between the existence of the precursor solution and the nucleation of phase pure rutile nanocrystallites. This directly reveals a difference in the formation of rutile under the present synthesis conditions and synthesis methods relying on transformation of either anatase or brookite phases.16, 19 A similar plot for the synthesis performed at 300 oC (25 MPa) is included in the supporting information. In addition an in situ experiment was carried out at 200 oC, but for this synthesis the large scattering signal from the solvent obscures the weak Bragg peaks and reliable data analysis is not possible. Figure 5b shows the change in crystallite diameter with time obtained from Rietveld refinement of the in situ PXRD patterns. At 400 oC the nanocrystallites reach a stable size of around 20 nm within the first 2 minutes, whereas at 300 oC it takes around 15 minutes to reach a crystallite size of around 15 nm. This suggests that above 300 oC flow synthesis is possible, since the required residence is below a few minutes. The short synthesis durations make spherical particles the most plausible morphology present for the in situ studies in contrast to the existence of nanorods observed for the 12 h autoclave synthesis. However, in the present in situ reactor it is not possible to recover a representative sample for ex situ TEM characterization. The dimensions of the tetragonal unit cell evolve during synthesis (Figure 5c). The (a,b)-axes contract ~0.2 Å over time whereas the c-axis expands ~0.2 Å. The change in unit cell dimensions is directly dependent on the average crystallite diameter as seen in Figure 5d. The unit cell volume expands drastically for crystallite diameter sizes below ~6 nm. This behaviour has previously been observed for nanocrystalline TiO2 and may be ascribed to an increased abundance of Ti4+ vacancies with decreasing crystallite size.35


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Figure 5. a) Time-resolved PXRD patterns for TiO2 synthesized at 400 oC and 250 bar. b) Crystallite growth curves for rutile TiO2 synthesized at 300 oC and 400 oC (both at 250 bar). c) Evolution in unit cell dimensions over time. d) Evolution in unit cell dimensions and volume as a function of crystallite size.

4. Conclusion Phase pure rutile TiO2 has been synthesized through a fast low temperature hydrothermal method using titanium isopropoxide and an aqueous solution of glycolic acid at pH≈1.6. Autoclave synthesis at 200 oC resulted in crystalline nanorods with a diameter of ~42 nm covered by smaller nanoparticles below 10 nm in size. The direct band gap for rutile nanoparticles was determined to be 3.1 eV. In situ PXRD reveals that rutile TiO2 is formed directly from solution without any crystalline intermediates within the time resolution of the experiment (5 s.). The unit cell of the rutile crystal structure is observed to be highly dependent on the crystallite size, and below 6 nm a drastic increase in the unit cell volume is observed. At 400 oC crystalline rutile is obtained within a few minutes, and this suggests that the reaction conditions may be transferred to conventional supercritical flow reactors. Overall, the present study has demonstrated a facile, scalable, low



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temperature method to obtain rutile nanoparticles below 10 nm, and this makes it highly promising e.g. for fabrication of polymer-nanoparticle composites.

Acknowledgments This work was supported by the Danish National Research Foundation (Center for Materials Crystallography, DNRF93), 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, Lund University, Sweden and Carsten Gundlach is thanked for assistance during measurements.

Supporting Information Additional TEM pictures and in situ scattering data. This information is available free of charge via the Internet at http://pubs.acs.org/.


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For Table of Contents Use Only

Synopsis: A simple hydrothermal route has been devised for the preparation of phase pure rutile (TiO2) nanostructures. Water miscibility of the prepared precursor allows for easy hydrothermal processing. in situ X-ray diffraction shows that crystalline rutile nanoparticles are formed directly from solution without intermediate crystalline phases.


Direct Formation of Crystalline Phase Pure Rutile TiO2 Nanostructures

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