J. Phys. Chem. C 2009, 113, 8009–8015
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Cobalt-Doped TiO2 Nanocrystallites: Radio-Frequency Thermal Plasma Processing, Phase Structure, and Magnetic Properties Ji-Guang Li,† Robert Bu¨chel,†,‡ Masaaki Isobe,§ Takao Mori,| and Takamasa Ishigaki*,†,⊥ Nano Ceramics Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan, Particle Technology Laboratory, Department of Mechanical Engineering, ETH Zu¨rich, CH-8092 Zu¨rich, Switzerland, Superconducting Materials Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan, International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan, and Department of Chemical Science and Technology, Hosei UniVersity, Koganei, Tokyo 184-8584, Japan ReceiVed: September 9, 2008; ReVised Manuscript ReceiVed: January 22, 2009
Co2+-doped TiO2nanopowders have been synthesized via one-step Ar/O2 RF thermal plasma oxidizing mists of liquid precursors containing titanium tetra-n-butoxide and cobalt (II) nitrate. Co2+ doping was found to steadily promote rutile crystallization. The solubility of Co2+ in the TiO2 lattice was determined to be around 2.0 atom %, above which CoTiO3 and Co2TiO4 may be formed as impurity phases. The existence of metallic Co or any other ferromagnetic impurities were excluded by a variety of characterization techniques, including high-power XRD, HR-TEM, XPS, whereas the nanopowders exhibit room-temperature ferromagnetism, which was indicated to be of intrinsic nature. The ferromagnetic performances of the TiO2:Co samples tend to decrease with increasing the Co2+ content up to ∼7 atom %, which has been ascribed to the enhanced rutile crystallization by co-doping and the formation of paramagnetic impurities. 1. Introduction TiO2 exhibits interesting physicochemical properties, such as high transparency in the visible wavelength region (bandgap: 3.0-3.2 eV depending upon phase structure), high refractive index (∼2.4-2.7, depending upon phase), and remarkable chemical and thermal stabilities, thus allowing its direct applications in photocatalysis, solar cells, semiconducting gas sensors, and as building blocks for photonic crystals. TiO2 is known to have two common polymorphs of anatase (tetragonal, 19 14 space group: D4h ) and rutile (tetragonal, space group: D4h ), between which the arrangements of the structure-building blocks (Ti-O octahedrons) differ.1 Specifically, in rutile, two opposing edges of each octahedron are shared to form linear chains along the [001] direction, and the TiO6 chains are then linked to each other via corner connection. Anatase has no corner sharing but has four edges shared per octahedron, and thus the crystal structure of anatase can be viewed as zigzag chains of the octahedrons linked together through edge sharing. Owing to these different structural features, the anatase and rutile polymorphs exhibit varied properties, in terms of theoretical density, refractive index, bandgap, and carrier mobility,2 as shown in Table 1. Because of its technical importance, nanocrystalline TiO2 has long been a subject of focused studies and a handful of methodologies have been developed to date for its synthesis. We demonstrated previously the synthesis of pure TiO2 nanopowders via one-step Ar/O2 radio frequency (RF) thermal * To whom correspondence should be addressed. Tel/Fax: +81-42-3876134. E-mail:
[email protected]. † Nano Ceramics Center, National Institute for Materials Science. ‡ Department of Mechanical Engineering, ETH Zu¨rich. § Superconducting Materials Center, National Institute for Materials Science. | International Center for Materials Nanoarchitectonics, National Institute for Materials Science. ⊥ Department of Chemical Science and Technology, Hosei University, Koganei.
plasma oxidizing mists of titanium organometallic compounds,3 phase structure, and particle size control of the plasma-generated TiO2 nanopowders via quench-gas injection,4 and the processing of doped TiO2 nanocrystals for photocatalytic (Fe3+ doping),5 magnetic (Fe3+ doping),6 and photoluminescent (Eu3+/Er3+ doping)7,8 applications. One salient advantage of the plasma processing technique is that its direct products are well crystallized, thus saving the postannealing stage frequently needed by soft-chemical synthetic procedures. Since the discovery of room temperature ferromagnetism (RTF) in Co-doped TiO2 thin films by Matsumoto et al.,9 the physical properties of dilute magnetic semiconductors (DMS) have been drawing increasing attention due to the potential applications of these materials in optoelectronics, magnetoelectronics, spintronics, and microwave devices. Doping semiconducting oxides with transition-metal (TM) ions to induce RTF has been one of the major challenges for many research groups, and to date a number of studies have reported on hightemperature ferromagnetism in oxide semiconductors such as TM-doped TiO2, ZnO, and SnO2 (TM ) Co, Ni, Cr, Mn, V, and Fe).2,9-25 For the Co-doped TiO2 (TiO2:Co) system, there is a report26 showing that RTF can only be achieved in the anatase form, whereas others showed RTF in rutile.11,12,19 These contradictions suggest that the magnetic properties of TiO2:Co heavily depend upon the methods and the conditions of sample preparation, as can be seen from Table 2, which summarizes some typical observations reported in recent years.9-22 For DMS materials, one issue that remains under debate is the origin of RTF. Whereas a number of reports showed its intrinsic nature, others claimed that it might be extrinsic, that is, due to the existence of isolated metallic clusters of the doping element.13,14 Though both thin film and powdered forms have been prepared for ferromagnetic TiO2:Co, it was noticed that in the majority of previous studies the samples were synthesized under high vacuum or in reducing atmospheres (Table 2), adding
10.1021/jp8080047 CCC: $40.75 2009 American Chemical Society Published on Web 04/20/2009
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J. Phys. Chem. C, Vol. 113, No. 19, 2009
Li et al.
TABLE 1: Comparison of Structural Features and Some Properties between the Anatase and Rutile Polymorphs of TiO2
phase
corner sharing
number of edge sharing
Eg (eV)
refractive index
density (g/cm3)
carrier mobility (cm2/V · s)
polarity
anatase rutile
no yes
4 2
3.23 3.02
2.52 2.72
3.84 4.26
6-10 0.05-0.2
n n
TABLE 2: Some Typical Reports on the TiO2:Co Systema
a
ref
preparation technique
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
MBE MBE MBE sputtering MBE MBE PLD sol-gel SP PLD SSR PLD sputtering PLD Exfoliation
form
crystal structure
film anatase film anatase film rutile film rutile film anatase film anatase film anatase powder anatase film anatase film anatase powder rutile film rutile film anatase film anatase 2D nanosheet
Co content (atom %)
Ms (µB/Co)
remarks
7 3 5 4 4 7 7 10 10 5 1 10 2 10 20
0.32 1.25 1.0 0.94 1.7 1.55 1.44 4.1 no RTF 0.16 1.75 1.5 1.1 0.23 1.4
as made O2-plasma assisted as made as made Co cluster found Co cluster found Co cluster @ >2 atom % of Co after H2-treatment @ 573 K ferromagnetic @ e5 K as made CoTiO3 found @ >2 atom % of Co as made UHV annealed as made as made
MBE, molecular beam epitaxy; PLD, pulsed laser deposition; SP, spray pyrolysis; SSR, solid state reaction; UHV, ultrahigh vacuum.
weight to the speculation that in these cases the observed RTF might be arising from metallic Co. In contrast to these previous synthetic routes, we have synthesized Co-doped TiO2 nanocrystallites via Ar/O2 RF thermal plasma oxidation of atomized liquid precursors containing titanium tetra-n-butoxide and cobalt (II) nitrate. Such an oxidative atmosphere was expected to prevent the possible formation of any metal elements, as also evidenced by our detailed analysis given herein. The plasmagenerated nanopowders were found to be room-temperature ferromagnetic, and detailed characterizations were made on the nanocrystallites by high-power XRD, HR-TEM, Raman spectroscopy, BET, XPS, and magnetic measurements. In the following sections, we report the synthesis via RF thermal plasma processing, characterization, and magnetic properties of the TiO2:Co nanopowders. 2. Experimental Section 2.1. RF Thermal Plasma Processing of TiO2:Co Nanocrystallites. The experimental apparatus for powder synthesis mainly consists of a water-cooled induction plasma torch (Model PL-50, TEKNA Plasma System Inc., Sherbrooke, QC, Canada), a 2-MHz radio frequency power supply system (Nihon Koshuha Co. Ltd., Yokohama, Japan), a water-cooled stainless steel reactor, and a stainless steel filter connecting the reactor and a vacuum pump for reactor-pressure control. Details of the experimental setup may be found elsewhere.3-8 All of the chemicals used in this work are reagent grade supplied by Wako Pure Chemical Industries Ltd., Tokyo, Japan, and were used as received. The liquid precursor solution for powder synthesis was made according to our previously established procedures.3-8 Briefly, 0.1 mol (34 g) of titanium tetra-n-butoxide (Ti(OC4H9)4, TTBO) was added to 0.4 mol (42 g) of diethanolamine (HN(OC2H5)2, DEA, a chelate for Ti4+) at room temperature under magnetic stirring to obtain a clear solution (solution I). Separately, proper amounts of cobalt (II) nitrate and citric acid (C6H8O7, CA, a chelate for Co2+, CA/ Co2+ ) 2:1 in molar ratio) were dissolved in 20 mL of distilled water to make solution II, whose pH was then adjusted to >8.0
with 25% ammonia solution. Mixing solutions I and II yields a stable clear solution to be used as the liquid precursor for powder synthesis of TiO2:Co (aimed Co2+ contents: Co/(Ti + Co) ) 0, 0.5, 1, 2, 3, 7, and 50 atom %, respectively). The fundamental conditions to generate RF thermal plasma for powder synthesis are as follows: central gas, 15 L/min of Ar; sheath gas, 60 L/min of Ar plus 10 L/min of O2; carrier gas, 5 L/min of Ar; plate power, 37.8 kW; chamber pressure, 200 Torr (22.3 kPa). For powder synthesis, the liquid precursor is delivered at ∼4 mL/min by a peristaltic pump into the center of the plasma plume through an atomization probe (Model SA792-260-100, TEKNA Plasma System Inc., Sherbrooke, QC, Canada). The probe was water-cooled to resist the high plasma temperature, and the precursor was atomized into mists at the tip of it by the exiting Ar carrier gas flowing through the probe at 5 L/min. The TiO2:Co nanoparticles formed via instantaneous oxidation of the liquid precursor mists by the O2 in the plasma sheath were recovered from the filter and the inner wall of the reactor. 2.2. Characterization Techniques. High-power XRD (50kV/ 300mA, 15 kW) analysis was performed on a RINT-TTRIII diffractometer (Rigaku, Tokyo, Japan), using nickel filtered Cu KR irradiation. For a mixture of TiO2 polymorphs, a phase constituent of the powder was analyzed by the method of Spurr and Myres.27 Lattice constant was measured for both rutile and atatase phases using pure Si as an internal standard material. HR-TEM observation was made on a JEM-4000EX transmission electron microscope (JEOL, Tokyo) operating at 400 kV; Raman spectroscopy was made using Ar+ laser excitation (514.5 nm) with an incident power of 50 mW and a resolution of 1 cm-1 (Model NR-1800, JASCO, Tokyo); specific surface area of the powder was assayed by the Brunauer-Emmett-Teller (BET) method via N2 adsorption at 77 K (Belsorp 18, Bell Japan Inc., Tokyo); X-ray photoelectron spectroscopy (XPS) was performed with monochromatized Al KR irradiation (1486.6 eV) with an incident power of 200 W (XPS5700, PHI, Chanhassen, MN, USA); magnetic properties of the samples were measured on a
Cobalt-Doped TiO2 Nanocrystallites
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TABLE 3: Cobalt Contents and Some Physical Properties of the Plasma-Generated TiO2:Co Nanopowders Co in precursor (Co/(Co+Ti), atom %) 0 0.5 1.0 2.0 3.0 7.0 50
Co in final powder (Co/(Co+Ti), atom %)
