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Processing of iron-doped titania powders in flame aerosol reactors
Powder Technology 114 Ž2001. 197–204 www.elsevier.comrlocaterpowtec
Processing of iron-doped titania powders in flame aerosol reactors Zhong-Min Wang a , Guixiang Yang a,1, Pratim Biswas a,) , Wayne Bresser b, Punit Boolchand b a
Aerosol and Air Quality Research Laboratory, EnÕironmental Engineering and Science DiÕision, UniÕersity of Cincinnati, Cincinnati, OH 45221-0071, USA b Electrical, Computer Engineering and Computer Science, UniÕersity of Cincinnati, Cincinnati, OH 45221-0071, USA Received 30 August 1999; received in revised form 17 May 2000; accepted 17 May 2000
Abstract A flame aerosol reactor was used to synthesize FeŽIII.-doped titania powders. The processing conditions were controlled to obtain varying ratios of Fe:Ti in the as processed powders. The iron was incorporated into the titania lattice and promoted the conversion of the anatase to the rutile phase. With an increase in the iron dopant concentration, a decrease in the crystal size of the resultant titania particles was observed, along with a conversion to the amorphous state. The defect structure was further explored by Raman spectroscopy, revealing an increased shift and broadening of the anatase peaks with an increasing iron dopant concentration, and was attributed to shrinkage in the grain size. Absorption spectra revealed a shift of the absorption band toward the visible frequencies. Powders with Fe:Ti ratio exceeding 0.8 resulted in a binary mixture that had superparamagnetic characteristics. q 2001 Elsevier Science S.A. All rights reserved. Keywords: Aerosol; Combustion; Titania; Iron-doped; Nanostructured; Photocatalyst
1. Introduction Combustion processes are used in industry for the large scale production of powders such as silica, titania and carbon black w1x. These processes typically operate at atmospheric pressure, are low cost and are readily scalable for production of large volumes w2x. Flame aerosol reactors have been extensively used in laboratory scale systems to produce ceramic powders such as magnetic oxides, high temperature superconductors and photocatalysts w3–7x. The gas phase route used in a flame reactor provides for a well-mixed system of precursors at the atomic level, thus making it feasible to process chemically homogenous powders. A mechanistic understanding of flame aerosol processes w8x allows the choice of processing conditions to also produce non-homogeneous composite powders and particulate coatings w9x. Titania is a biologically and chemically inert compound that finds applicability as a pigment and as a photocatalyst. Titania has been widely studied for use in air and water )
Corresponding author. Tel.: q1-513-556-3697; fax: q1-513-5562599. E-mail address: [email protected] ŽP. Biswas.. 1 Current address: Dupont, Wilmington, DE, USA.
purification w10x, photosplitting of water to produce hydrogen, for odor control, and as a disinfectant for destroying microorganisms w11x. Photocatalytic or photoactivated reactions are applicable to a wide range of valuable industrial processes such as organic synthesis w12x, photodestruction of toxic compounds, and purification of drinking water w13–16x. The anatase form of TiO 2 has been the most extensively employed in photocatalytic reactions because of its high activity and chemical stability w10,13–16x. For example, the anatase phase of titania has applicability as a photocatalyst for several problems of environmental interest w10,17x, as a catalyst for sulfur removal w18x, for toxic metals capture w19-21x, and as an additive in cosmetics due to its efficient sun screen properties w22x. Recently, Yang et al. w7,23x have used it for coating of steel substrates to provide its stainless and corrosion resistant characteristics. The electronic structure of titania is characterized by a filled valence band and an empty conduction band. When a photon with energy equal to or exceeding the optical band gap energy is incident, an electron is readily excited, creating an electron-hole pair which may then participate in oxidation–reduction reactions w24x. Two factors appear to be important in establishing the overall efficiency of the photoreaction: grain size and role of dopants w25x. It has been reported that as the grain size
0032-5910r01r$ - see front matter q 2001 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 0 . 0 0 3 2 1 - 1
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approaches the quantum domain, the photoreactivity increases due to higher efficiencies of interfacial charge transfer Žbefore possible recombination of electron and hole.. An increase in photo reactivity has been observed for doped titania powders, possibly due to the trapping of the charge carriers in defect centers w26,27x. In addition, the use of dopants has resulted in the shifting of the absorption spectra towards the visible spectrum w28x providing a potential for using solar light. Fe 3q has been used by many researchers as a dopant for titania. Bockelmann et al. w28x used a wet method of solution–evaporation–drying to obtain quantum-sized titania doped with 5% to 50% iron. The absorption band-gap edge was observed to shift toward the visible solar spectrum. Choi et al. w25x used a similar wet process to dope several transition metals Žincluding iron. in titania, and the effect of dopants on its quantum yield and bandgap shift was studied. Borgalleo et al. w29x used a solid state surface doping technique ŽCr. to extend the absorption band-gap of anatase titania in the visible region. Ohshima et al. w30x used a spray pyrolysis method to prepare a mixture of titania and ZnO composites, and a wider range of UV shielding was observed from 200 to 370 nm. The reported quantum efficiency and the band gap shift varied depending on the defect structures, grain sizes and attendant surface morphology that resulted from the different preparation techniques. Optical characterization of titania by absorption spectroscopy and Raman scattering has been carried out and reported as a function of stoichiometry w31x and grain size in the submicrometer and nanometer ranges w7,38x. It was found that the 141- Ž Eg ., 394- and 516.2cmy1 modes blue shifted whereas the 638-cmy1 mode red
shifted as the grain size of anatase titania decreased. This was attributed to the dispersion of optical phonos with k near the zone center Ž k s 0. w7,32x. There is a need to develop processes to produce powders with well-tailored properties for a variety of applications. For example, iron-doped titanium dioxide has applicability as a photocatalyst that is activated using solar light. At higher ratios of Fe:Ti, superparamagnetic materials with applications in magnetic refrigeration can be produced. In this paper, a flame aerosol process for synthesizing nanostructured, iron-doped titania powders is described. By controlling the processing conditions, it is demonstrated that powders with varying ratios of Fe:Ti can be readily produced. A detailed solid state characterization ŽXRD, Raman spectroscopy, UV–Vis absorption spectroscopy, Mossbauer spectroscopy. is carried out to estab¨ lish the properties of these powders.
2. Experimental 2.1. System A multiport diffusion burner was used in this study to produce nanosized titania particles and films, and details of burner design are described elsewhere w7,33,34x. A schematic diagram of the system is shown in Fig. 1. Titanium isopropoxide Ž97%, Aldrich Chem.. and iron carbonyl Ž99%, Aldrich. vapors were entrained in particle free air and nitrogen streams, respectively, and then introduced into the flame through the center port of the burner. Methane was used as the fuel and passed through the
Fig. 1. Schematic diagram of the flame aerosol reactor system for producing composite particles.
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Table 1 List of typical flame experiments performed Test no. c
1 2 3 4 5 6 7 8
Flow rate Žlrmin.
Molar ratio
CH 4
O2
N2 ŽFe.
AirŽTi.
Fe:Ti
0.15 0.9 0.9 0.9 0.9 0.9 0.9 0.9
0.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.4 0.0 0.01 0.02 Žice. 0.02 0.03 0.08 0.15
0.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
Fe only Ti only 0.06 0.10 0.12 0.18 0.40 0.80
Flame temp. Ž8C.
Powder phase compositionb
810 1500 1500 1500 1500 1500 1500 1500
g-phase iron oxide Anatase 5% RutilerAnatase 30% RutilerAnataseq Amorphous 50% RutilerAnataseq Amorphous Amorphousq FeOrTiO a Amorphousq FeOrTiO a Amorphousq FeOrTiO a
a
FeOrTiO: binary mixtures of FeOrTiO nanocrystal phase. Phase compositions between anatase and rutile are calculated according to w33x, not accounting for the amount of amorphous state present. c In Test 1, 1 lrmin of air was supplied through the central port. b
second port. As this is a diffusion flame, air diffuses to the flame front to provide oxygen necessary for the combustion. To achieve a desired high temperature flame environment, oxygen was introduced through the outermost port Žthird port.. The feed rates of the two precursors were accurately controlled by controlling the flow rates of the carrier gases using mass flow controllers ŽMKS Instruments., and calibration experiments were performed to determine the molar ratios of titanium and iron. The as produced particles were collected on a water cooled rod by thermophoretic deposition in the high temperature zone, thus allowing the collection of particles before they could grow by coagulation. The iron and titanium concentrations in the collected powders were measured using inductively coupled plasma ŽICP. spectroscopy ŽModel 61E, Thermo Jarrell Ash.. 2.2. Characterization techniques Crystallinity of the as produced powders was examined by X-ray diffraction ŽRigaku D-2000.. Optical absorption was measured using a UV–Vis spectrophotometer ŽHewlett-Packard, 8452A Diode Array.. Raman scattering ŽModel T64000 Triple Monochromator System, Instruments, with a CCD multichannel detector and a Microprobe Attachment. in the powders was excited using 10 mW of the 514.5 nm line from an Arq laser. The superparamagnetic characteristics were examined by Mossbauer ¨ spectroscopy ŽConstant Acceleration Drive from Austin Science Assoc.; PCA-2 data acquisition; He Cryocooler based cold fingers; patented vibration free mounting for T dependent measurements in the range of 4 to 300 K. and vibration sample magnetometery Ž4500 EG & GPAR with 1 Tesla electromagnet and DMX-19 from APD, He cryocooler for measurements in the 12 to 350 K temperature range.. 2.3. Experimental plan Table 1 lists the experimental conditions used in this work. The gas flow rates Žmethane, oxygen, nitrogen, air.
were independently controlled to obtain different temperatures and precursor feed rates w33x. Tests 1 and 2 were carried out with an iron and titanium precursor feed only. The conditions for Test 2 were selected so as to obtain the pure anatase phase w33x, and this was used as the baseline condition Žflame temperature of 15008C. for Tests 3 to 8. The feed rate of nitrogen through the iron pentacarbonyl bubbler was varied to obtain different Fe:Ti molar ratios.
3. Results and discussion The resultant molar ratios of iron and titanium in the powders measured by ICP spectroscopy is listed in Table 1 and was varied from 0 Žpure titania. to 0.8 to ` Žpure iron oxide.. The mechanistic steps in the formation of the Fe–Ti nanocomposites are illustrated in Fig. 2. The organometallic precursors decompose very rapidly in the high temperature flame environment and result in the formation of TiO and FeO monomers. At low Fe-to-Ti ratios, the Fe is readily incorporated into the titania lattice by scavenging processes Žexcess of titania.. When Fe:Ti ratios are larger Žclose to 1., iron oxide nucleates as well and forms a composite with titania, very similar to the iron oxide-silica nanocomposites processed by McMillin et al. w5x. Unlike the titania–lead reaction w35x, chemical reaction is not promoted due to rapid quenching on the cooled collection rod ŽFig. 1.. 3.1. Crystal phase Fig. 3 shows the XRD patterns for the different powders produced. For the iron-only feed conditions ŽTest 1., the flame parameters were chosen to obtain the pure g-phase iron oxide. Though the local temperatures are high Žapproximately 8008C., and would favor the formation of the more stable a-phase w36x, the mode of particle collection on a cooled rod freezes the g-phase, similar to the premixed flame reactor system used by McMillin et al. w5x, Conditions in Test 2 ŽTi only. were selected so the pure
200
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Fig. 2. Cartoon illustrating the formation of iron-doped titania powders in a flame environment.
anatase phase of titania was obtained, and this is also a result of the metastable phase being formed due to high quench rates as was discussed in detail in an earlier paper w33x. As the focus of this study was on iron-doped anatase titania particles, the conditions of Test 2 were used in all the following experiments ŽTests 3 to 8.. The iron precursor feed rate was varied Žreadily done by varying the nitrogen flow rate through the iron carbonyl bubbler. to yield molar ratios of FerTi ranging from 0.06 to 0.8. The XRD patterns shown in Fig. 3 indicate that the anatase phase is gradually transformed to the rutile phase, and at higher Fe:Ti ratios, the particles become amorphous. The flame provides an environment where the particles are formed from an atomistic state, and iron substitutionally replaces Ti in the titania lattice. This is feasible because ˚ ., is close to that of the ionic radius of iron, Fe 3q Ž0.64 A 4q Ž ˚ . Ti 0.68 A . Due to its smaller size, iron compacts the titania lattice and, not surprisingly, the more compactly packed polymorph of TiO 2 , viz., rutile is formed. This is consistent with the findings of Shanon and Pask w37x. On increasing the Fe concentration, the grain size further decreases Žconsistent with our Raman scattering observations, described later. and the X-ray reflections broaden, and eventually give rise to the characteristic of an amorphous powder. In contrast to iron oxide-silica systems w5,8x even at the highest Fe concentration ŽTest 8., no crystalline phases of iron oxides or phase segregated iron oxide were
observed in the flame processed iron-doped titania powders by X-ray diffraction. 3.2. Raman spectroscopy Raman scattering signals of the iron-doped TiO 2 samples are shown in Fig. 4 for the different Fe:Ti ratios. A Gaussian profile was fitted to the observed line-shapes, to extract the mode-frequencies and mode-widths ŽTable 2.. At low Fe:Ti ratios ŽTests 3, 4, 5., the mode frequencies compared favorably to those of pure anatase w7,32x. However, at higher Fe:Ti ratios Ž) 0.4., significant modefrequency shifts occur, and we identify these as finite size effects. Parker and Siegel w38x have shown that the Raman mode frequency shifts could also result due to oxygen deficiency of samples. However, in the oxygen-rich ambient of flame synthesis, one does not expect oxygen deficiencies, as was confirmed by Auger spectroscopy measurements in our samples w7,43x. Furthermore, the as produced samples were annealed in an oxygen atmosphere Žflowing oxygen over sample in a furnace. at 3508C Žnot higher so as to prevent any crystalline phase transformations. for 24 h. The Raman mode frequencies remained unchanged, showing that the mode-frequency shift observed in the iron-doped titania sample was not due to oxygen deficiencies but most likely because of finite size effects. A similar observation was made by Gonzalez and Zallen w32x in sol–gel processed samples of TiO 2 . These
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gap energy Žshift in optical absorption towards the visible frequencies.. Fig. 5 is a plot of the optical absorption of flame generated iron-doped titania samples, and rather dramatic shifts of the edge toward the visible frequencies are observed, far greater than those observed for iron-doped titania samples prepared by other methods w25x. The estimated band gap energy and the broadening of the absorption band as a function of the Fe:Ti ratio is shown in Fig. 6. The pure anatase powders ŽTest 2. show an increased band gap energy compared to bulk anatase powders due to quantum effects Žbehaves like a particle in a box, separates the valence and conduction band energy states.. On adding iron, several dopant states’ electronic energy levels result in a reduction in the band-gap energies. Due to the incorporation of the Fe 3q into the Ti 4q lattice as the particles are formed from the atomistic states in the flame environment, the reduction in the band gap energies is greater than
Fig. 3. XRD patterns of the iron-doped titania powders for different ratios of Fe:Ti.
authors attributed the mode frequency shift to a relaxation of the k s 0 selection rule leading to a blue or red shift depending on the w Ž k . dispersion of phonon modes in the Brilloin Zone near k s 0. Using their results, we estimated grain sizes in our samples ŽTable 2.. The presence of iron is known to alter the size distributions of flame generated silica aerosols w8x due to alteration of viscosity and solid state diffusion characteristics. As the rutile phase is formed ŽTests 6, 7, 8., Raman modes of that polymorph are observed. The rutile phase nucleates around the iron and, thus, the Raman mode frequencies differ from those of pristine rutile w38x. 3.3. UV–Vis absorption spectroscopy Fe 3q-doped TiO 2 is of practical importance for use as a photocatalyst, and has been prepared by several researchers w25,28,39x using solid state and liquid precipitation methods. Transition metal doping is known to improve the quantum efficiencies of photocatalytic titania, and also to result in the reduction of the absorption band
Fig. 4. Raman scattering signals of the iron-doped titania powders for different ratios of Fe:Ti.
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Table 2 Characteristics of the Raman modes for the various iron-doped titania particles Sample Žtest no..
FerTi mole ratio
Major mode frequency Žcmy1 . whalf width Žcmy1 .x
Grain size a Žnm.
Grain size b Žnm.
2 3 4 5 6 7 8
0 0.06 0.1 0.12 0.18 0.40 0.80
142.18 w11.361x, 196.05, 396.26, 518.45, 638.11 142.30 w12.98x, 197.43, 397.03, 519.95, 636.34 142.30 w11.61x, 196.28, 395.08, 516.68, 635.96 142.19 w11.61x, 196.61, 395.91, 517.86, 637.74 144.03 w16.90x 146.505 w23.51x 151.379 w25.1x
25 24 24 25 17 13 9
16 14 12 – – – –
a b
Estimated based on 142.18 nm Raman line shifts w32x. Calculated according to the Scherrer equation w44x.
that observed for iron-doped titania powders prepared by other methods. 3.4. Magnetic properties The as produced particles in Test 8 were examined by Mossbauer spectroscopy and vibration sample magnetome¨ tery for their magnetic properties. Fig. 7 displays the observed line shape at room temperature and 30 K. The results at room temperature reveal paramagnetic relaxation leading to a collapse of the magnetic splitting into a doublet Ž d s 0.37, D s 0.74., The shift d is characteristic of Fe 3q in the high spin state. The reverse is the case at 30 K because of a slowdown of the Fe 3q spins upon cooling.
The temperature at which magnetic hyperfine splitting is first observed upon cooling is called the blocking temperature w41x ŽT B f K eVrk B , where K e is anisotropy energy constant ŽJrm3 .; V is volume of particle Žm3 .; k B is Boltzmann constant ŽJrK.. and it depends on the particle size directly. It has been shown that the T-dependence of the spin relaxation time is usually given by
t s to exp Ž K eVrk B T .
Ž 1.
where K e represents the magnetic anisotropy energy constant ŽJrm3 . and to is a constant typically on the order of 10y1 0 to 10y1 2 s. When the spin relaxation time, t , exceeds the characteristic time window Žt L . for the nu-
Fig. 5. Absorption spectra in the UV–Vis range of the different iron-doped titania powders.
Z.-M. Wang et al.r Powder Technology 114 (2001) 197–204
Fig. 6. Band gap energy of powders as a function of the FerTi molar ratio.
cleus to sense the local field, a static magnetic hyperfine structure is observed. In the case of the 57 Fe resonance, the time window t L is typically 10y8 to 10y9 s and is the Larmor precession time. Eq. Ž1. shows that for particles of smaller size Ž V . one has to go to lower temperatures T for t to exceed t L . The characteristic temperature T at which t ) t L is known as the blocking temperature. In the room temperature spectrum ŽFig. 7. more than 90% of the Fe nuclei contribute to the paramagnetic doublet Žsince K eV
203
Fig. 8. Magnetic moment of M of the iron-doped titania powder prepared in Test 8 as a function of temperature, T.
- k B T ., a result in harmony with the small size wŽ V 1r3 s Žp d 3r6.1r3 x of the grains. Magnetization as a function of magnetic field and temperature was measured for the powders produced in Test 8 and is shown in Fig. 8. It further confirms that the as obtained pseudo-binary mixture of Fe 2 O 3 :TiO 2 has superparamagnetic properties, as also reported by others w40,42x.
4. Conclusions A flame aerosol reactor has been successfully used to produce iron-doped titania powders. Due to the assembly of structures from the atomistic state in the flame, homogeneous iron-doped titania powders were obtained. By controlling the processing conditions, powders with varying ratios of Fe: Ti could be readily produced. At low Fe:Ti ratios, the process resulted in substitutional incorporation of Fe 3q in the titania lattice. This resulted in the gradual transformation of the anatase phase to the rutile phase, a shrinkage in grain size and corresponding broadening of the Raman modes. Absorption spectroscopy indicated a shift towards visible frequencies associated with a reduction in the band-gap energies. At the higher ratios of Fe:Ti Ž) 0.8., a binary mixture of Fe 2 O 3 –TiO 2 was obtained and these powders exhibited superparamagnetic characteristics.
Fig. 7. Mossbauer spectrum of iron-doped titania powder prepared in test ¨ 8 ŽFe:Tis 0.8. at two different temperatures.
List of symbols d particle diameter Žm. kB Boltzmann constant ŽJrK. anisotropy energy constant ŽJrm3 . Ke T temperature ŽK. volume of particle Žm3 . V t spin relaxation time Žs. to characteristic time constant ŽEq. 1. characteristic time tL d isomer shift in Mossbauer spectra Žmmrs. ¨ D quadrupole splitting in Mossbauer spectra Žmmrs. ¨
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Acknowledgements Partial support provided by the USEPA Contract 8CR313-NAEX and NSF Grant DMR-97-02189 is gratefully acknowledged.
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Processing of iron-doped titania powders in flame aerosol reactors Zhong-Min Wang a , Guixiang Yang a,1, Pratim Biswas a,) , Wayne Bresser b, Punit Boolchand b a
Aerosol and Air Quality Research Laboratory, EnÕironmental Engineering and Science DiÕision, UniÕersity of Cincinnati, Cincinnati, OH 45221-0071, USA b Electrical, Computer Engineering and Computer Science, UniÕersity of Cincinnati, Cincinnati, OH 45221-0071, USA Received 30 August 1999; received in revised form 17 May 2000; accepted 17 May 2000
Abstract A flame aerosol reactor was used to synthesize FeŽIII.-doped titania powders. The processing conditions were controlled to obtain varying ratios of Fe:Ti in the as processed powders. The iron was incorporated into the titania lattice and promoted the conversion of the anatase to the rutile phase. With an increase in the iron dopant concentration, a decrease in the crystal size of the resultant titania particles was observed, along with a conversion to the amorphous state. The defect structure was further explored by Raman spectroscopy, revealing an increased shift and broadening of the anatase peaks with an increasing iron dopant concentration, and was attributed to shrinkage in the grain size. Absorption spectra revealed a shift of the absorption band toward the visible frequencies. Powders with Fe:Ti ratio exceeding 0.8 resulted in a binary mixture that had superparamagnetic characteristics. q 2001 Elsevier Science S.A. All rights reserved. Keywords: Aerosol; Combustion; Titania; Iron-doped; Nanostructured; Photocatalyst
1. Introduction Combustion processes are used in industry for the large scale production of powders such as silica, titania and carbon black w1x. These processes typically operate at atmospheric pressure, are low cost and are readily scalable for production of large volumes w2x. Flame aerosol reactors have been extensively used in laboratory scale systems to produce ceramic powders such as magnetic oxides, high temperature superconductors and photocatalysts w3–7x. The gas phase route used in a flame reactor provides for a well-mixed system of precursors at the atomic level, thus making it feasible to process chemically homogenous powders. A mechanistic understanding of flame aerosol processes w8x allows the choice of processing conditions to also produce non-homogeneous composite powders and particulate coatings w9x. Titania is a biologically and chemically inert compound that finds applicability as a pigment and as a photocatalyst. Titania has been widely studied for use in air and water )
Corresponding author. Tel.: q1-513-556-3697; fax: q1-513-5562599. E-mail address: [email protected] ŽP. Biswas.. 1 Current address: Dupont, Wilmington, DE, USA.
purification w10x, photosplitting of water to produce hydrogen, for odor control, and as a disinfectant for destroying microorganisms w11x. Photocatalytic or photoactivated reactions are applicable to a wide range of valuable industrial processes such as organic synthesis w12x, photodestruction of toxic compounds, and purification of drinking water w13–16x. The anatase form of TiO 2 has been the most extensively employed in photocatalytic reactions because of its high activity and chemical stability w10,13–16x. For example, the anatase phase of titania has applicability as a photocatalyst for several problems of environmental interest w10,17x, as a catalyst for sulfur removal w18x, for toxic metals capture w19-21x, and as an additive in cosmetics due to its efficient sun screen properties w22x. Recently, Yang et al. w7,23x have used it for coating of steel substrates to provide its stainless and corrosion resistant characteristics. The electronic structure of titania is characterized by a filled valence band and an empty conduction band. When a photon with energy equal to or exceeding the optical band gap energy is incident, an electron is readily excited, creating an electron-hole pair which may then participate in oxidation–reduction reactions w24x. Two factors appear to be important in establishing the overall efficiency of the photoreaction: grain size and role of dopants w25x. It has been reported that as the grain size
0032-5910r01r$ - see front matter q 2001 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 0 . 0 0 3 2 1 - 1
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approaches the quantum domain, the photoreactivity increases due to higher efficiencies of interfacial charge transfer Žbefore possible recombination of electron and hole.. An increase in photo reactivity has been observed for doped titania powders, possibly due to the trapping of the charge carriers in defect centers w26,27x. In addition, the use of dopants has resulted in the shifting of the absorption spectra towards the visible spectrum w28x providing a potential for using solar light. Fe 3q has been used by many researchers as a dopant for titania. Bockelmann et al. w28x used a wet method of solution–evaporation–drying to obtain quantum-sized titania doped with 5% to 50% iron. The absorption band-gap edge was observed to shift toward the visible solar spectrum. Choi et al. w25x used a similar wet process to dope several transition metals Žincluding iron. in titania, and the effect of dopants on its quantum yield and bandgap shift was studied. Borgalleo et al. w29x used a solid state surface doping technique ŽCr. to extend the absorption band-gap of anatase titania in the visible region. Ohshima et al. w30x used a spray pyrolysis method to prepare a mixture of titania and ZnO composites, and a wider range of UV shielding was observed from 200 to 370 nm. The reported quantum efficiency and the band gap shift varied depending on the defect structures, grain sizes and attendant surface morphology that resulted from the different preparation techniques. Optical characterization of titania by absorption spectroscopy and Raman scattering has been carried out and reported as a function of stoichiometry w31x and grain size in the submicrometer and nanometer ranges w7,38x. It was found that the 141- Ž Eg ., 394- and 516.2cmy1 modes blue shifted whereas the 638-cmy1 mode red
shifted as the grain size of anatase titania decreased. This was attributed to the dispersion of optical phonos with k near the zone center Ž k s 0. w7,32x. There is a need to develop processes to produce powders with well-tailored properties for a variety of applications. For example, iron-doped titanium dioxide has applicability as a photocatalyst that is activated using solar light. At higher ratios of Fe:Ti, superparamagnetic materials with applications in magnetic refrigeration can be produced. In this paper, a flame aerosol process for synthesizing nanostructured, iron-doped titania powders is described. By controlling the processing conditions, it is demonstrated that powders with varying ratios of Fe:Ti can be readily produced. A detailed solid state characterization ŽXRD, Raman spectroscopy, UV–Vis absorption spectroscopy, Mossbauer spectroscopy. is carried out to estab¨ lish the properties of these powders.
2. Experimental 2.1. System A multiport diffusion burner was used in this study to produce nanosized titania particles and films, and details of burner design are described elsewhere w7,33,34x. A schematic diagram of the system is shown in Fig. 1. Titanium isopropoxide Ž97%, Aldrich Chem.. and iron carbonyl Ž99%, Aldrich. vapors were entrained in particle free air and nitrogen streams, respectively, and then introduced into the flame through the center port of the burner. Methane was used as the fuel and passed through the
Fig. 1. Schematic diagram of the flame aerosol reactor system for producing composite particles.
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Table 1 List of typical flame experiments performed Test no. c
1 2 3 4 5 6 7 8
Flow rate Žlrmin.
Molar ratio
CH 4
O2
N2 ŽFe.
AirŽTi.
Fe:Ti
0.15 0.9 0.9 0.9 0.9 0.9 0.9 0.9
0.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.4 0.0 0.01 0.02 Žice. 0.02 0.03 0.08 0.15
0.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
Fe only Ti only 0.06 0.10 0.12 0.18 0.40 0.80
Flame temp. Ž8C.
Powder phase compositionb
810 1500 1500 1500 1500 1500 1500 1500
g-phase iron oxide Anatase 5% RutilerAnatase 30% RutilerAnataseq Amorphous 50% RutilerAnataseq Amorphous Amorphousq FeOrTiO a Amorphousq FeOrTiO a Amorphousq FeOrTiO a
a
FeOrTiO: binary mixtures of FeOrTiO nanocrystal phase. Phase compositions between anatase and rutile are calculated according to w33x, not accounting for the amount of amorphous state present. c In Test 1, 1 lrmin of air was supplied through the central port. b
second port. As this is a diffusion flame, air diffuses to the flame front to provide oxygen necessary for the combustion. To achieve a desired high temperature flame environment, oxygen was introduced through the outermost port Žthird port.. The feed rates of the two precursors were accurately controlled by controlling the flow rates of the carrier gases using mass flow controllers ŽMKS Instruments., and calibration experiments were performed to determine the molar ratios of titanium and iron. The as produced particles were collected on a water cooled rod by thermophoretic deposition in the high temperature zone, thus allowing the collection of particles before they could grow by coagulation. The iron and titanium concentrations in the collected powders were measured using inductively coupled plasma ŽICP. spectroscopy ŽModel 61E, Thermo Jarrell Ash.. 2.2. Characterization techniques Crystallinity of the as produced powders was examined by X-ray diffraction ŽRigaku D-2000.. Optical absorption was measured using a UV–Vis spectrophotometer ŽHewlett-Packard, 8452A Diode Array.. Raman scattering ŽModel T64000 Triple Monochromator System, Instruments, with a CCD multichannel detector and a Microprobe Attachment. in the powders was excited using 10 mW of the 514.5 nm line from an Arq laser. The superparamagnetic characteristics were examined by Mossbauer ¨ spectroscopy ŽConstant Acceleration Drive from Austin Science Assoc.; PCA-2 data acquisition; He Cryocooler based cold fingers; patented vibration free mounting for T dependent measurements in the range of 4 to 300 K. and vibration sample magnetometery Ž4500 EG & GPAR with 1 Tesla electromagnet and DMX-19 from APD, He cryocooler for measurements in the 12 to 350 K temperature range.. 2.3. Experimental plan Table 1 lists the experimental conditions used in this work. The gas flow rates Žmethane, oxygen, nitrogen, air.
were independently controlled to obtain different temperatures and precursor feed rates w33x. Tests 1 and 2 were carried out with an iron and titanium precursor feed only. The conditions for Test 2 were selected so as to obtain the pure anatase phase w33x, and this was used as the baseline condition Žflame temperature of 15008C. for Tests 3 to 8. The feed rate of nitrogen through the iron pentacarbonyl bubbler was varied to obtain different Fe:Ti molar ratios.
3. Results and discussion The resultant molar ratios of iron and titanium in the powders measured by ICP spectroscopy is listed in Table 1 and was varied from 0 Žpure titania. to 0.8 to ` Žpure iron oxide.. The mechanistic steps in the formation of the Fe–Ti nanocomposites are illustrated in Fig. 2. The organometallic precursors decompose very rapidly in the high temperature flame environment and result in the formation of TiO and FeO monomers. At low Fe-to-Ti ratios, the Fe is readily incorporated into the titania lattice by scavenging processes Žexcess of titania.. When Fe:Ti ratios are larger Žclose to 1., iron oxide nucleates as well and forms a composite with titania, very similar to the iron oxide-silica nanocomposites processed by McMillin et al. w5x. Unlike the titania–lead reaction w35x, chemical reaction is not promoted due to rapid quenching on the cooled collection rod ŽFig. 1.. 3.1. Crystal phase Fig. 3 shows the XRD patterns for the different powders produced. For the iron-only feed conditions ŽTest 1., the flame parameters were chosen to obtain the pure g-phase iron oxide. Though the local temperatures are high Žapproximately 8008C., and would favor the formation of the more stable a-phase w36x, the mode of particle collection on a cooled rod freezes the g-phase, similar to the premixed flame reactor system used by McMillin et al. w5x, Conditions in Test 2 ŽTi only. were selected so the pure
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Fig. 2. Cartoon illustrating the formation of iron-doped titania powders in a flame environment.
anatase phase of titania was obtained, and this is also a result of the metastable phase being formed due to high quench rates as was discussed in detail in an earlier paper w33x. As the focus of this study was on iron-doped anatase titania particles, the conditions of Test 2 were used in all the following experiments ŽTests 3 to 8.. The iron precursor feed rate was varied Žreadily done by varying the nitrogen flow rate through the iron carbonyl bubbler. to yield molar ratios of FerTi ranging from 0.06 to 0.8. The XRD patterns shown in Fig. 3 indicate that the anatase phase is gradually transformed to the rutile phase, and at higher Fe:Ti ratios, the particles become amorphous. The flame provides an environment where the particles are formed from an atomistic state, and iron substitutionally replaces Ti in the titania lattice. This is feasible because ˚ ., is close to that of the ionic radius of iron, Fe 3q Ž0.64 A 4q Ž ˚ . Ti 0.68 A . Due to its smaller size, iron compacts the titania lattice and, not surprisingly, the more compactly packed polymorph of TiO 2 , viz., rutile is formed. This is consistent with the findings of Shanon and Pask w37x. On increasing the Fe concentration, the grain size further decreases Žconsistent with our Raman scattering observations, described later. and the X-ray reflections broaden, and eventually give rise to the characteristic of an amorphous powder. In contrast to iron oxide-silica systems w5,8x even at the highest Fe concentration ŽTest 8., no crystalline phases of iron oxides or phase segregated iron oxide were
observed in the flame processed iron-doped titania powders by X-ray diffraction. 3.2. Raman spectroscopy Raman scattering signals of the iron-doped TiO 2 samples are shown in Fig. 4 for the different Fe:Ti ratios. A Gaussian profile was fitted to the observed line-shapes, to extract the mode-frequencies and mode-widths ŽTable 2.. At low Fe:Ti ratios ŽTests 3, 4, 5., the mode frequencies compared favorably to those of pure anatase w7,32x. However, at higher Fe:Ti ratios Ž) 0.4., significant modefrequency shifts occur, and we identify these as finite size effects. Parker and Siegel w38x have shown that the Raman mode frequency shifts could also result due to oxygen deficiency of samples. However, in the oxygen-rich ambient of flame synthesis, one does not expect oxygen deficiencies, as was confirmed by Auger spectroscopy measurements in our samples w7,43x. Furthermore, the as produced samples were annealed in an oxygen atmosphere Žflowing oxygen over sample in a furnace. at 3508C Žnot higher so as to prevent any crystalline phase transformations. for 24 h. The Raman mode frequencies remained unchanged, showing that the mode-frequency shift observed in the iron-doped titania sample was not due to oxygen deficiencies but most likely because of finite size effects. A similar observation was made by Gonzalez and Zallen w32x in sol–gel processed samples of TiO 2 . These
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201
gap energy Žshift in optical absorption towards the visible frequencies.. Fig. 5 is a plot of the optical absorption of flame generated iron-doped titania samples, and rather dramatic shifts of the edge toward the visible frequencies are observed, far greater than those observed for iron-doped titania samples prepared by other methods w25x. The estimated band gap energy and the broadening of the absorption band as a function of the Fe:Ti ratio is shown in Fig. 6. The pure anatase powders ŽTest 2. show an increased band gap energy compared to bulk anatase powders due to quantum effects Žbehaves like a particle in a box, separates the valence and conduction band energy states.. On adding iron, several dopant states’ electronic energy levels result in a reduction in the band-gap energies. Due to the incorporation of the Fe 3q into the Ti 4q lattice as the particles are formed from the atomistic states in the flame environment, the reduction in the band gap energies is greater than
Fig. 3. XRD patterns of the iron-doped titania powders for different ratios of Fe:Ti.
authors attributed the mode frequency shift to a relaxation of the k s 0 selection rule leading to a blue or red shift depending on the w Ž k . dispersion of phonon modes in the Brilloin Zone near k s 0. Using their results, we estimated grain sizes in our samples ŽTable 2.. The presence of iron is known to alter the size distributions of flame generated silica aerosols w8x due to alteration of viscosity and solid state diffusion characteristics. As the rutile phase is formed ŽTests 6, 7, 8., Raman modes of that polymorph are observed. The rutile phase nucleates around the iron and, thus, the Raman mode frequencies differ from those of pristine rutile w38x. 3.3. UV–Vis absorption spectroscopy Fe 3q-doped TiO 2 is of practical importance for use as a photocatalyst, and has been prepared by several researchers w25,28,39x using solid state and liquid precipitation methods. Transition metal doping is known to improve the quantum efficiencies of photocatalytic titania, and also to result in the reduction of the absorption band
Fig. 4. Raman scattering signals of the iron-doped titania powders for different ratios of Fe:Ti.
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Table 2 Characteristics of the Raman modes for the various iron-doped titania particles Sample Žtest no..
FerTi mole ratio
Major mode frequency Žcmy1 . whalf width Žcmy1 .x
Grain size a Žnm.
Grain size b Žnm.
2 3 4 5 6 7 8
0 0.06 0.1 0.12 0.18 0.40 0.80
142.18 w11.361x, 196.05, 396.26, 518.45, 638.11 142.30 w12.98x, 197.43, 397.03, 519.95, 636.34 142.30 w11.61x, 196.28, 395.08, 516.68, 635.96 142.19 w11.61x, 196.61, 395.91, 517.86, 637.74 144.03 w16.90x 146.505 w23.51x 151.379 w25.1x
25 24 24 25 17 13 9
16 14 12 – – – –
a b
Estimated based on 142.18 nm Raman line shifts w32x. Calculated according to the Scherrer equation w44x.
that observed for iron-doped titania powders prepared by other methods. 3.4. Magnetic properties The as produced particles in Test 8 were examined by Mossbauer spectroscopy and vibration sample magnetome¨ tery for their magnetic properties. Fig. 7 displays the observed line shape at room temperature and 30 K. The results at room temperature reveal paramagnetic relaxation leading to a collapse of the magnetic splitting into a doublet Ž d s 0.37, D s 0.74., The shift d is characteristic of Fe 3q in the high spin state. The reverse is the case at 30 K because of a slowdown of the Fe 3q spins upon cooling.
The temperature at which magnetic hyperfine splitting is first observed upon cooling is called the blocking temperature w41x ŽT B f K eVrk B , where K e is anisotropy energy constant ŽJrm3 .; V is volume of particle Žm3 .; k B is Boltzmann constant ŽJrK.. and it depends on the particle size directly. It has been shown that the T-dependence of the spin relaxation time is usually given by
t s to exp Ž K eVrk B T .
Ž 1.
where K e represents the magnetic anisotropy energy constant ŽJrm3 . and to is a constant typically on the order of 10y1 0 to 10y1 2 s. When the spin relaxation time, t , exceeds the characteristic time window Žt L . for the nu-
Fig. 5. Absorption spectra in the UV–Vis range of the different iron-doped titania powders.
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Fig. 6. Band gap energy of powders as a function of the FerTi molar ratio.
cleus to sense the local field, a static magnetic hyperfine structure is observed. In the case of the 57 Fe resonance, the time window t L is typically 10y8 to 10y9 s and is the Larmor precession time. Eq. Ž1. shows that for particles of smaller size Ž V . one has to go to lower temperatures T for t to exceed t L . The characteristic temperature T at which t ) t L is known as the blocking temperature. In the room temperature spectrum ŽFig. 7. more than 90% of the Fe nuclei contribute to the paramagnetic doublet Žsince K eV
203
Fig. 8. Magnetic moment of M of the iron-doped titania powder prepared in Test 8 as a function of temperature, T.
- k B T ., a result in harmony with the small size wŽ V 1r3 s Žp d 3r6.1r3 x of the grains. Magnetization as a function of magnetic field and temperature was measured for the powders produced in Test 8 and is shown in Fig. 8. It further confirms that the as obtained pseudo-binary mixture of Fe 2 O 3 :TiO 2 has superparamagnetic properties, as also reported by others w40,42x.
4. Conclusions A flame aerosol reactor has been successfully used to produce iron-doped titania powders. Due to the assembly of structures from the atomistic state in the flame, homogeneous iron-doped titania powders were obtained. By controlling the processing conditions, powders with varying ratios of Fe: Ti could be readily produced. At low Fe:Ti ratios, the process resulted in substitutional incorporation of Fe 3q in the titania lattice. This resulted in the gradual transformation of the anatase phase to the rutile phase, a shrinkage in grain size and corresponding broadening of the Raman modes. Absorption spectroscopy indicated a shift towards visible frequencies associated with a reduction in the band-gap energies. At the higher ratios of Fe:Ti Ž) 0.8., a binary mixture of Fe 2 O 3 –TiO 2 was obtained and these powders exhibited superparamagnetic characteristics.
Fig. 7. Mossbauer spectrum of iron-doped titania powder prepared in test ¨ 8 ŽFe:Tis 0.8. at two different temperatures.
List of symbols d particle diameter Žm. kB Boltzmann constant ŽJrK. anisotropy energy constant ŽJrm3 . Ke T temperature ŽK. volume of particle Žm3 . V t spin relaxation time Žs. to characteristic time constant ŽEq. 1. characteristic time tL d isomer shift in Mossbauer spectra Žmmrs. ¨ D quadrupole splitting in Mossbauer spectra Žmmrs. ¨
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Acknowledgements Partial support provided by the USEPA Contract 8CR313-NAEX and NSF Grant DMR-97-02189 is gratefully acknowledged.
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