CRYSTAL GROWTH & DESIGN
Titanium Oxide Nanorods Extracted From Ilmenite Sands
2009 VOL. 9, NO. 2 1240–1244
Jun Yu, Ying Chen,*,† and Alexey M. Glushenkov† Department of Electronic Materials and Engineering, Research School of Physical Sciences and Engineering, the Australian National UniVersity, Canberra, ACT 0200, Australia ReceiVed October 9, 2008; ReVised Manuscript ReceiVed NoVember 24, 2008
ABSTRACT: One dimensional titanium oxides (TiO2) nanorods and nanowires have substantial applications in photocatalytic, nanoelectronic, and photoelectrochemical areas. These applications require large quantities of materials and a production technique suitable for future industry fabrication. We demonstrate here a new method for mass production of TiO2 nanorods from mineral ilmenite sands (FeTiO3). In this process, powder mixtures of ilmenite and activated carbon were first ball milled; the milled samples were then heated twice at two different temperatures. First high-temperature annealing produced metastable titanium oxide phases, and subsequent second low-temperature annealing in N2-5%H2 activates the growth of rutile nanorods. This solid-state growth process allows large-quantity production of rutile nanorods. Introduction Titanium oxides (TiO2) nanorods and nanowires have substantial applications in photocatalytic, nanoelectronic, and photoelectrochemical areas.1 These applications require a mass quantity of materials, but most current production methods can only produce a small amount of samples, suitable for laboratory research purposes only. Titanium oxide nanorods and nanowires have been produced using wet chemistry methods including electrodeposition,2 sol-gel electrophoresis,3,4 hydrothermal,5 and solvothermal methods.6 Physical thermal deposition7-9 and metal-organic chemical vapor deposition10 methods produce a thin layer of samples on a substrate surface. For example, Wu et al. reported catalyst-assisted growth of TiO2 nanowires on Ti-coated silicon substrates.7,11 Amin et al. described the growth of TiO2 nanowires and nanoribbons by using thermal annealing of nickel coated TiO powders at 760 Torr and 850-920 °C in argon (Ar).12 There is no report of a mass production technique, which could be scaled up in industries. Here we demonstrate a new method for mass production of TiO2 nanorods from mineral ilmenite sands. Ilmenite is a naturally occurring iron titanate (nominally FeTiO3) and thus significantly reduces the production cost. In addition, FeTiO3 contains both titanium oxide and catalytic iron (Fe) required for nanorod growth. In the new process, high-energy ball milling was used to assist the reduction of ilmenite into an intermediate structure containing metastable titanium oxide phases by a high-temperature annealing and the subsequent low-temperature annealing activates the growth of rutile nanorods. This solid-state growth process allows large-quantity production of rutile nanorods. Experimental Section The starting ilmenite used in this study is a naturally occurring iron titanate (FeTiO3) and abundant in nature. High-purity ilmenite (99%) was provided by Consolidated Rutile Limited located in Australia. Chemical composition (wt.%) of the ilmenite are TiO2 (dry basis) 49.6, iron (total) 35.1, FeO 32.8, Fe2O3 13.7, Al2O3 0.47, Cr2O3 0.25, SiO2 0.45. Several grams of the mixture of ilmenite (FeTiO3) and active carbon (weight ratio of 4:1) were milled in a Fritsch planetary ball mill with 10 steel balls (diameter 1 cm) for 50 h in vacuum atmosphere at room temperature. Isothermal annealing was conducted in a horizontal
Figure 1. (a) SEM image of ilmenite sands; (b) SEM image of ball milled ilmenite and activated carbon powder mixture; (c) XRD pattern of the milled sample. tube furnace at different temperatures (700-1200 °C) and in different atmospheres (Ar or N2-5%H2) at a flow rate of 100 mL/min. Milled and annealed samples were characterized by using an X-ray diffraction (XRD) spectrometer with a cobalt KR radiation (λ ) 0.1789 nm). Scanning electron microscopy (SEM) was conducted using Hitachi S4500 and S4300 instruments. X-ray energy dispersive spectroscopy (EDS) system was used for chemical composition analysis. Transmission electron microscopy (TEM) analysis was carried out using a Philips CM300 microscope operating at 300 kV. For TEM study, a powder sample was ultrasonicated in ethanol for a few minutes to form a diluted suspension solution, and a few drops of the suspension was dripped on a copper grid with carbon film. Backscattered electron imaging (BSE) using in-lens Energy Selective Backscatter (EsB) detector and X-ray intensity distribution maps were conducted with a Zeiss ULTRA plus SEM microscope. Powder sample was coated with carbon to increase conductivity. Shimadzu TA50 instrument was employed to conduct thermogravimetric analysis (TGA).
Results and Discussion * Corresponding author. E-mail:
[email protected]. † Present address: Institute for Technology Research and Innovation, Deakin University, Victoria 3217, Australia.
The SEM image in Figure 1a shows large grains of starting ilmenite sands in the range of 100 µm. Mixture of the ilmenite
10.1021/cg801125w CCC: $40.75 2009 American Chemical Society Published on Web 01/05/2009
TiO2 Nanorods Extracted From Ilmenite Sands
Crystal Growth & Design, Vol. 9, No. 2, 2009 1241
Figure 2. (a) XRD pattern of the sample after first annealing at 1200 °C; (b) BSE image; (c) Ti mapping; (d) Fe mapping.
powder and activated carbon at weight ratio of 4:1 was first ball milled in a high-energy planetary ball mill for 50 h; both FeTiO3 and C have been reduced down to small particles of about 100 nm as revealed by the SEM image in Figure 1b. The X-ray diffraction (XRD) pattern of the milled sample in Figure 1c displays a full set of the diffraction peaks of FeTiO3 phase (PDF 29-733). The relatively broadened peak shapes are caused by the crystal size reduction induced by intensive ball milling treatment.13 The broad, weak peak around 21 degrees is associated with the amorphous structure of activated carbon. The SEM image in Figure 1b also shows that fine particles of FeTiO3 and C were homogenously mixed and form large aggregates indicating close contacts between these two materials, which are beneficial to reduction reactions in the subsequent annealing.14 The ball milled powder sample was then annealed at 1200 °C for 8 h in Ar-5%H2 gases flowing at 100 mL/min to induce carborthermic reductions. The XRD diffraction pattern in Figure 2a was recorded from the heated sample, and it reveals the presence of monoclinic Ti3O5, Ti2O3 and R-Fe phases, indicating the partial reduction of FeTiO3 by C and H2. The BSE image of the heated sample in Figure 2b shows a mixture of large gray Ti containing particles and small bright Fe dominant particles. The different contrasts are due to different atomic masses of Ti and Fe. EDS mapping was used to examine chemical compositions of these particles. The Ti and Fe elemental mapping images in Figure 2, panels c and d, respectively, confirm the chemical nature of the Ti and Fe particles in the BSE image. Oxygen mapping shows most O in Ti particles. These results indicate that FeTiO3 has been reduced
into large titanium oxide particles and small iron particles. A small amount of Fe could exist in Ti particles. Because the weight ratio of ilmenite to carbon was 4:1, the available C is only sufficient for reduction of FeTiO3 to TiO2 and Fe.14 The further reduction of TiO2 to Ti3O5 and Ti2O3 is believed to happen due to the presence of hydrogen gas. The heated sample was annealed again at a lower temperature of 700 °C for 4 h in N2-5% H2 to activate the one-dimensional growth of rutile structure. The SEM image in Figure 3a shows the early stage of the nanorod growth, and we can see many nanorods appearing from the surfaces of large titanium oxide particles. After heating for 8 h, all titanium oxide surfaces were covered by a layer of nanorods of around 100 nm in diameter and a few micrometers in length (Figure 3b). Typical rectangular cross sections and sizes of the nanorods can be seen clearly in Figure 3c, which will be discussed later. The XRD pattern of the final sample in Figure 3d shows only TiO2 rutile and Fe phases. Ti3O5 and Ti2O3 phases are no longer detected. A selective chemical leaching treatment using 3 M HCl solution was found to effectively remove most Fe. Figure 4a shows a bright-field TEM image of a TiO2 nanorod, and its corresponding SAED pattern is displayed in Figure 4b. Electron diffraction pattern consists of a regular periodic array of dots and indicates that the nanorode has a single crystalline structure. The [002] direction of elongation was concluded from the SAED pattern (its orientation was carefully corrected for the rotation induced by the lenses of the microscope). The same direction of elongation is detected from other nanorods in the sample. The same growth direction has been observed by Yoo et al. in TiO2 nanowires.15 The nanorod has a nonuniform
1242 Crystal Growth & Design, Vol. 9, No. 2, 2009
Yu et al.
Figure 3. SEM images and XRD patterns showing sample morphology changes during the second annealing at 700 °C in N2-5%H2. (a) 4 h, (b) 8 h; (c) cross-sections of nanorods; (d) XRD pattern of the sample after second annealing.
Figure 4. TEM image of a nanorod: (a) low magnification image; (b) SAED pattern; (c) high magnification image taken from the edge of the rod.
contrast in a bright-field image, which is related to the complex shape of the cross section of the nanorods. The darker area in the center of the nanorod corresponds, most likely, to a larger thickness of its crystal in this area. Figure 4c shows a high resolution image taken from the edge of the same nanorod. Two types of lattice planes are resolved with the distances of 0.23 and 0.25 nm corresponding to (200) and (101) crystal planes of rutile TiO2, respectively. TEM analysis reveals that the nanorod growth direction is parallel to the c axis of tetragonal cell of rutile structure, and the nanorod side-walls of (002) and (101) planes actually construct unit cells of tetragonal rutile. This can explain the rectangular cross sections observed in Figure 3c.
Figure 5. (a) TGA curve up to 1200 °C in Ar-5%H2 at a flow rate of 50 mL/min; (b) TGA curve of sample (a) up to 700 °C in N2-5%H2 at a flow rate of 50 mL/min.
To examine possible reduction reactions during the first annealing, TGA analysis of the milled sample was conducted in Ar-5%H2 gases (50 mL/min) up to 1200 °C with a heating rate of 10 °C/min. Slow weight loss is observed in the TGA curve (Figure 5a) under 800 °C with a total weight loss of only 6.30%. Previous investigation has found the slow weight loss is due to the slow reduction reactions by solid C:14,16
TiO2 Nanorods Extracted From Ilmenite Sands
Crystal Growth & Design, Vol. 9, No. 2, 2009 1243
FeTiO3+ C f TiO2 + Fe + CO Fast weight loss is found when the temperature is above 800 °C, which is due to the following fast reduction reactions induced by CO gas:14
FeTiO3+ CO f TiO2+ Fe + CO2 CO2 + C f 2CO 3TiO2+ CO f Ti3O5+ CO2 Because a small amount of hydrogen was introduced into the annealing atmosphere, the following reactions also possibly occur: 17
2TiO2+ H2 f Ti2O3+ H2O 2FeTiO3+ 3H2 f 3H2O + 2Fe + Ti2O3 3TiO2+ H2 f Ti3O5+ H2O Several reactions could occur at the same time, and thus the weight loss has different speeds as the TGA curve indicates. Figure 2a XRD pattern proved these possible reactions because Ti3O5, Ti2O3, and Fe phases are present in the powder sample after first step annealing. The above reactions normally take place at a temperature at least above 1200 °C.18,19 Preball-milling treatments have an activation role and reduces reaction starting temperatures. High energy ball milling greatly reduces the particle size of ilmenite and active carbon (Figure 1a,b) and increases of the total external surface area, which presumably increases the contact area between them. The enhanced reduction process in premilled samples is attributed mainly to intimate mixing of carbon with disordered ilmenite nanocrystallites. This substantially reduces the diffusion length of oxygen to carbon in the solid-state reaction, thus increasing the rate of CO production for initiating the main gaseous reduction. The efficiency of the carbothermic reaction is initially greatly increased by ball milling treatment.14 No further reduction appears to be detected during the second annealing at 700 °C as the TGA curve in Figure 5b suggests, which was conducted in N2 and 5% H2 mixture gas up to 700 °C with heating rate 20 °C/min and flow rate 50 mL/min. The sample weight does not reduce but increases slightly 2.36% from room temperature to 700 °C, which might be due to the following possible reactions:
2Ti2O3+ O2 f 4TiO2 2Ti3O5+ O2 f 6TiO2 XRD pattern in Figure 3d has agreement with above two reactions. The oxygen needed for rutile formation comes from the furnace environment. On the other hand, both Ti2O3 and Ti3O5 phases are thermally metastable phases and readily transformed into more stable rutile phase, which may or may not require a very small amount of oxygen.20,21 Hydrogen reduction may still exist, but the oxidation speed is probably faster than the reduction speed, leading to a slight weight increase in the TGA curve. Amin et al. also found that Nicoated TiO particles become TiO2 nanowires in a low vacuum system due to the absorption of a small amount of oxygen.12 The limited concentration of oxygen played a very important role, and to confirm, the sample after first step annealing was heated in oxygen at 700 °C for 6 h. All Ti2O3 and Ti3O5 have become TiO2 rutile, but the nanorods could not be found because Fe becomes Fe2O3 which cannot catalyze nanorod formation. The limited concentration of the oxygen level needs further study.
Figure 6. (a) SEM image indicating an iron particle at the tip of a nanorod; (b) and (c) EDS spectra recorded at point A and B of the nanorod in panel a, respectively; (d) SEM image showing possible hydrogen etched areas, indicated by the arrows.
The formation of special nanorod structure is because of onedirectional growth of the rutile crystals in the second annealing treament. We observed that Fe particles produced by first annealing act as catalysts to help nanorod growth. The SEM image in Figure 6a shows a small particle at the tip of one nanorod. The EDS analysis revealed that the particle is enriched with Fe and the nanorod is titanium oxide. The corresponding
1244 Crystal Growth & Design, Vol. 9, No. 2, 2009
EDS spectra are displayed in Figure 6, panels b and c, respectively. Although most iron forms large particles and they are not found to help nanorod growth, some fine iron particles (diameter less than 100 nm) might be left among the TiO particles and act as catalysts. A similar catalytic role of Fe to the growth of TiO2 nanorod and other one-dimensional structures has been observed.15,22 But SEM images show that many nanorods do not have Fe particles at the tips, and therefore they are produced in different mechanisms. Figure 6d shows irregular cross sections and etched traces on the rod external surfaces. SEM analysis of a number of nanorods formed at different stages found that nanorods with large cross sections seem to be etched during extended annealing in hydrogen containing gas. One large nanorod became several nanorods with smaller diameters. This phenomena might be caused by hydrogen gas reduction along [002] directions. It has been reported that open atomic channels exist between TiO6 octahedra along the [002] direction of rutile crystals and hydrogen gas can have an etching effect along the channels.15,23 Tsujiko et al. found inverse-pyramid holes on (001) surface after heating in hydrogen gas at 700 °C.24 Therefore, hydrogen gas has played important roles in the two annealing processes. Without hydrogen gas, rutile particles can be produced from ilmenite but not in nanorod form. Nitrogen gas cannot be used in the first annealing as nitrides (TiN and FeN) can be formed at 1200 °C. N2-5% H2 gases were preferred the atmosphere compared to Ar-5%H2 in the second annealing because nitrogen gas helps the transformation from Ti2O3 to TiO2 as reported by Berger25 and Lyubimov et al.26 Conclusions We have demonstrated a practical, cost-effective method for producing rutile nanorods from ilmenite sands, and this method can be scaled up for large quantity production. In this method, ilmenite and active carbon powders were first ball milled in a planetary milling device, followed by two-step annealing treatment. The first high-temperature annealing at 1200 °C in Ar-5%H2 results in reduction of ilmenite into metastable titanium oxides and catalytic iron; the second low-temperature annealing at 700 °C in N2-5%H2 atmosphere leads to the growth of nanosized rutile rods. Fe can be removed using selective acid leaching. These results show a promising future for large-quantity production of rutile nanorods required by various environmental, biomedical, transportation, and chemical manufacturing applications.
Yu et al.
Acknowledgment. This work is supported in part by the Australian Research Council under the Centre of Excellence and Discovery programs. The authors thank Mr. David Llewellyn, Dr. Frank Brink, and Dr. Cheng Huang for their assistance in microscopic analysis.
References (1) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (2) Lei, Y.; Zhang, L. D.; Fan, J. C. Chem. Phys. Lett. 2001, 338, 231. (3) Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.; Tanemura, M. J. Cryst. Growth. 2004, 264, 246. (4) Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.; Tanemura, M. Appl. Surf. Sci. 2004, 238, 175. (5) Zhang, Q.; Gao, L. Langmuir 2003, 19, 967. (6) Wen, B.; Liu, C.; Liu, Y.; New, J. Chem. 2005, 969. (7) Wu, J. M.; Shih, H. C.; Wu, W. T. Chem. Phys. Lett. 2005, 413, 490. (8) Wu, J. M.; Shih, H. C.; Wu, W. T.; Tseng, Y. K.; Chen, I. C. J. Cryst. Growth 2005, 281, 384. (9) Xiang, B.; Zhang, Y.; Wang, Z.; Wang, X. H.; Luo, X. H.; Zhang, H. Z.; Yu, D. P. J. Phys. D 2005, 38, 1152. (10) Wu, J. J.; Yu, C. C. J. Phys. Chem. B 2004, 108, 3377. (11) Wu, J. M.; Wu, W. T.; Shih, H. C. J. Electrochem. Soc. 2005, 152, G613. (12) Amin, S. S.; Nicholls, A. W.; Xu, T. T. Nanotechnology 2007, 18, 445609. (13) Chen, Y.; Bibole, M.; Le Hazif, R. And Martin, G.; Phys. Rev. B. 1993, 48, 14. (14) Chen, Y.; Hwang, T.; Marsh, M.; Williams, J. S. Metall. Mater. Trans. A 1997, 28A, 1115. (15) Yoo, S.; Akbar, S. A.; Sandhage, K. H. AdV. Mater. 2004, 16, 260. (16) Chen, Y.; Hwang, T.; Williams, J. S. Mater. Lett. 1996, 28, 55. (17) Kellerman, D. G.; Zainulin, Y. G.; Perelyaev, V. A. Inorg. Mater. 1983, 19, 305. (18) Becher, R. G. Aust. Pat. 1963, 241. (19) Becher, R. G.; Canning, R. G.; Goodheat, B. A.; Uusna, S. Proc. Aust. Inst. Min. Metall. 1965, 214, 21. (20) Zheng, L. Y.; Li, G. R.; Xu, T. X.; Yin, Q. R. J. Inorg. Mater. 2002, 17, 1253. (21) Duyar, O.; Placido, F.; Durusoy, H. Z. J. Phys. D-Appl. Phys. 2008, 41, 9. (22) Chen, Y; Chadderton, L. T.; Williams, J. S.; Fitz Gerald, J. Appl. Phys. Lett. 1999, 74, 2782. (23) Sasaki, J.; Peterson, N. L.; Hoshino, K. J. Phys. Chem. Solids. 1985, 46, 1267. (24) Tsujiko, A.; Kisumi, T.; Magari, Y.; Murakoshi, K.; Nakato, Y. J. Phys. Chem. B 2000, 104, 4873. (25) Berger, L. M. J. Mater. Sci. Lett. 2001, 20, 1845. (26) Lyubimov, V. D.; Shveikin, G. P.; Shestakova, T. V.; Alyamovskii, S. I. Inorg. Mater. 1978, 14, 372.
CG801125W
