Fe-modified TiO2 nanocatalyst under blue-light irradiation
Y. L. Weia兲 and K. W. Chen
Department of Environmental Science and Engineering, Tunghai University, Taichung City 407, Taiwan
H. P. Wang
Department of Environmental Engineering, National Cheng Kung University, Tainan City 701, Taiwan
and Sustainable Environment Research Center, National Cheng Kung University, Tainan City 701, Taiwan
共Received 8 September 2008; accepted 8 January 2009; published 27 May 2009兲
Ultraviolet 共UV兲 and visible light active photocatalysts, 13–20 nm in size, were prepared by doping
iron into anatase TiO2 共Fe/Ti at. ratio: 0.11% and 0.49%兲. Greater Fe dosage led to stronger light
absorbance in both UV and visible regions, and also to generally better photocatalytic activity. Fe2+
and Fe3+ coexisted in 0.11% Fe– TiO2, while only Fe3+ existed in 0.49% Fe– TiO2. A dramatic
change in the molecular environment of Fe was observed after the doping process; from cubic
共Fe3O4兲 and trigonal 共Fe2O3兲 structures into tetragonal TiO2. Different Fe speciation in catalysts led
to different kinetics for methylene blue photocatalytic degradation. © 2009 American Vacuum
Society. 关DOI: 10.1116/1.3079700兴
I. INTRODUCTION
II. EXPERIMENTAL METHODS
Anatase TiO2 can degrade many pollutants under ultraviolet 共UV兲 irradiation; electron/hole pairs are generated and
reacted with H2O and O2 to produce OH free radicals, which
are a strong oxidant. Modification of TiO2 with iron exhibits
enhanced photocatalytic activity.1–5 Zhu et al.1 prepared Fedoped anatase TiO2 with a hydrothermal approach that was
effective in degrading an aqueous yellow dye. Fe3+ was suggested to act as a trap for both electrons and holes; whereas
residual Fe2+ scavenges holes only. The hole trapping with
Fe3+ generates Fe4+.1 These roles played by iron dopants
promote photocatalytic activity. However, from a detrimental
viewpoint, Fe3+ and Fe2+ can also act as electron-hole recombination centers, which reduce the photocatalytic activity.1
Hung et al.2 used Fe-doped TiO2 共termed Fe– TiO2兲 to photocatalytically degrade gaseous chlorinated compounds and
proposed that the decrease in Cl adsorption on the catalyst
resulted from the doped Fe ion. Phenol degradation in an
aqueous solution with Fe– TiO2 under UV irradiation was
previously investigated, and Fe3+ cations were proposed to
lock into the TiO2 anatase structure during the catalyst modification process.3 Amorphous phases containing iron oxide in
the excessively doped Fe– TiO2 were suggested to be responsible for less photocatalytic activity.3 Methanol was chemically reduced to dimethyl ether with Fe– TiO2, resulting from
the Lewis acidic surface of Fe–Ti oxide.4 Their results indicated the existence of Fe2+ in small fraction in the Fe– TiO2.4
Xin et al.5 suggested that iron dopant in titania played both
roles of separation and recombination of electrons and holes;
depending on Fe dosage.
The photocatalytic mechanism associated with Fe– TiO2
is still controversial to date. Therefore, our present work investigates the Fe molecular environment in Fe– TiO2 with
x-ray absorption spectroscopy 共XAS兲. Fe– TiO2 characteristics are also studied with other instrumental techniques.
To synthesize Fe– TiO2 catalysts 共calculated Fe/Ti atomic
ratio: 0.10 and 0.50 mol %兲, a precalculated amount of
Fe共NO3兲3 · 9 H2O salt was dissolved in a mixture of 75 ml of
1-propanol and 15 ml of Ti共OC3H7兲4, followed by the dropby-drop addition of 5 ml of 1.0N NaOH 共pH 13.6兲. This
solution was thoroughly mixed for 24 h, placed at room temperature without mixing for 5 days, and heated at 70 ° C for
24 h to form a dry gel. The dry gel was calcined from 30 to
500 ° C at a temperature ramping rate of 5 ° C / min, plus two
temperature plateaus at 250 and 500 ° C each for 30 min.
The resulting material was washed with de-ionized water,
dried at 70 ° C, and ground to produce Fe– TiO2 catalyst.
The actual content of Fe in these Fe– TiO2 catalysts was
analyzed to be 0.11 and 0.49 mol %, termed “0.11Fe–Ti”
and “0.49Fe–Ti,” respectively. In addition, nondoped TiO2
共termed “nondoped TiO2”兲 was prepared in a manner identical to the procedure described above, without adding the Fe
precursor. We used a N2-based Brunauer-Emmett-Teller
共BET兲 surface-area analyzer, x-ray diffractometer 共XRD兲,
scanning electron microscopy 共SEM兲, transmission electron
microscopy 共TEM兲, diffuse reflectance ultraviolet/visible
共DR-UV/VIS兲 spectrometer, and XAS technique to investigate the characteristics of the Fe– TiO2 catalysts. All
Fe K-edge XAS spectra were recorded on the wiggler C 共BL17C兲 beamline at the National Synchrotron Radiation Research Center 共NSRRC兲 of Taiwan. XAS data were reduced
with WINXAS 3.1 software.6
Experiments of photocatalytic degradation of methylene
blue 共MB兲 共15 mg l−1兲 in the presence of 0.100 g of
Fe– TiO2 catalyst in 1 l of water were carried out in a cylindrical quartz reactor, surrounded by eight blue-light tubes
共each being 10 W and having a peak flux at a wavelength of
450 nm, but also having a very small light flux near 367 nm兲,
and seated on a magnetic stirrer. MB concentration in the
sampled aliquots was determined by measuring its light absorption at 664 nm with an UV/VIS spectrophotometer.
a兲
Electronic mail: yulin@thu.edu.tw
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FIG. 1. TGA/DSC results from Ti-containing dry gel.
Prior to the experiments of MB photocatalytic degradation, some pretests were performed for quality assurance and
quality control. First, vaporization loss of MB in 1.0 l of
water over 36 h was determined to be negligible in the absence of catalysts and blue-light irradiation. Second, MB
concentration in 1.0 l water, in the absence of catalyst, under
blue-light irradiation for 36 h was determined to be nearly
invariant. Finally, to rule out the error caused by MB adsorption on catalysts, 1.0 l 15.0 mg l−1 MB adsorptiondesorption isotherms on each catalyst without blue-light irradiation were determined. The amount of the MB adsorbed on
each catalyst under equilibrium condition was deducted from
15.0 mg l−1 for properly expressing the level of MB degradation in the photocatalytic experiments in the presence of
both catalyst and blue-light irradiation.
III. RESULTS AND DISCUSSION
Figure 1 shows the result from measuring dry gel with a
thermogravimetric analyzer 共TGA兲/differential scanning
calorimeter 共DSC兲. It shows considerable weight loss in the
25– 200 ° C range with a concomitant heat-absorption peak
at 116 ° C, indicating the vaporization of moisture and
1-propanol.7 There is a heat-release peak at about 250 ° C,
which is attributed to oxidation of organic components because air was used as the gaseous stream during the TGA/
DSC experiment. The exothermic DSC peak at 474 ° C indicates the transformation of amorphous TiO2 to anatase form.
In addition, the DSC peak at 779 ° C indicates the transformation from anatase TiO2 to rutile form.
Table I shows that the surface areas are 78 共for the nondoped TiO2兲, 84 共0.11Fe–Ti兲, and 79 m2 g−1 共0.49Fe–Ti兲,
corresponding to diameters of 20, 19, and 20 nm, respectively, assuming spherical particles. XRD patterns in Fig. 2
only show the presence of anatase TiO2. Note that our
FIG. 2. XRD results from nondoped TiO2, 0.11Fe–Ti, and 0.49Fe–Ti.
Fe– TiO2 and nondoped TiO2 catalysts consist of about 5%
rutile phase and 95% anatase according to Ti K-edge x-ray
absorption near-edge structure 共XANES兲 simulation 共the
spectra are not shown in our present study兲. The characteristic peak of anatase TiO2 is located between 25.36° and
25.48°. Based on the calculation with the Scherrer formula
by using the characteristic peak 共2␪ = 25.36– 25.48°兲, the particle diameters are 15 共for nondoped TiO2兲, 13 共0.11Fe–Ti兲,
and 16 nm 共0.49Fe–Ti兲, respectively. SEM morphologies
共not shown here兲 from 0.11Fe–Ti and 0.49Fe–Ti indicate that
the catalyst particles agglomerate. TEM results 共not shown
here兲 reveal that the particles are quite uniform in size, at
about 20 nm.
Figure 3 shows the level of MB degradation with nondoped TiO2, 0.11Fe–Ti, and 0.49Fe–Ti under blue-light irradiation. Photodegradation of MB can be promoted by doping
Fe in anatase TiO2, and heavier Fe dosage leads to greater
photodegradation of MB under 24 and 36 h irradiation; the
activity is identical for 0.11Fe–Ti and 0.49Fe–Ti after 12 h
irradiation. The observation is explained when discussing the
results from Fig. 4.
Figure 4 shows that the 0.49Fe–Ti has the greatest UV
and visible absorption level. The observation, that more light
absorption results from more Fe dopant, is consistent with
the trend of increasing level of MB degradation with in-
TABLE I. BET results from nondoped TiO2, 0.11Fe–Ti, and 0.49Fe–Ti.
Samples
Nondoped TiO2
0.11Fe–Ti
0.49Fe–Ti
BET surface
area
共m2 g−1兲
Particle size
共nm兲
78
84
79
20
19
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J. Vac. Sci. Technol. B, Vol. 27, No. 3, May/Jun 2009
FIG. 3. Photocatalytic degradation of methylene blue with nondoped TiO2,
0.11Fe–Ti, and 0.49F–Ti.
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Wei, Chen, and Wang: Fe-modified TiO2 nanocatalyst
FIG. 4. DR-UV/VIS results from nondoped TiO2, 0.11Fe–Ti, and 0.49Fe–Ti
prior to MB degradation.
creased Fe dosage 共see Fig. 3兲. To explain the identical result
obtained with 0.11Fe–Ti and 0.49Fe–Ti after 12 h irradiation,
three factors are considered: MB adsorption rate, MB degradation kinetics on the catalyst surface, and desorption rate of
degraded organic product. Because the reactants were well
stirred during the degradation experiment, the adsorption and
desorption rates are assumed to be rapid and negligible, suggesting different MB degradation kinetics on the surface of
the catalysts. This inference is supported by the difference in
their Fe speciation as shown below.
Figure 5 compares Fe K-edge XANES spectra from
0.11Fe–Ti and 0.49Fe–Ti with Fe references 关Fe共0兲, FeO,
FeOOH, Fe2O3, and Fe3O4兴. XANES simulation was attempted for both 0.11Fe–Ti and 0.49Fe–Ti with these Fe
references, yet no satisfactory result could be obtained; this
suggests that the fraction of the iron dopant has been incorporated into the anatase TiO2 crystalline structure. Such dopant incorporation might have enhanced the level of photodegradation of MB. The intensity of the white band 共i.e., the
hump at 7132 eV兲 in the XANES spectra from both Fe– TiO2
catalysts is considerably greater than all Fe references, with
an unusually great intensity for the 0.11Fe–Ti. The enhanced
intensity is usually associated with greater Debye-Waller factor and/or greater coordination number in the first coordination shell.
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As shown in Fig. 5, the oxidation state of Fe in 0.11Fe–Ti
is almost equivalent to that in the Fe3O4 reference, which has
an average Fe oxidation state of 2.67, as suggested by a
comparison between the photon energy of the main edge of
the XANES from 0.11Fe–Ti with the Fe3O4 reference spectrum. TiO2 exists in a tetragonal structure, whereas both FeO
and Fe3O4 are cubic, and Fe2O3 is trigonal. Therefore, introducing Fe into crystalline TiO2 would result in a large
change in Fe environment, thereby leading to the great enhancement in intensity of the white band. Because more fraction of Fe was introduced into the TiO2 structure for the
0.11Fe–Ti 共as compared with the 0.49Fe–Ti兲, greater enhancement in the white band is observed in 0.11Fe–Ti
XANES than in 0.49Fe–Ti. The fact that 0.11Fe–Ti shows
more photocatalytic activity than nondoped TiO2 is consistent with the proposal made by Zhu et al. that Fe2+ in
Fe– TiO2 can trap holes and Fe3+ acts as a scavenger for both
electrons and holes if Fe is not overdoped.1 For 0.49Fe–Ti,
its average Fe valence is about 3.0, as compared to the Fe2O3
reference XANES. Because the Fe3+ acts as a scavenger for
both electrons and holes,1 it enhanced MB degradation as
compared to the nondoped TiO2. The 0.49Fe–Ti was not
overdoped with Fe3+; thus, it has better photocatalytic activity in MB degradation after 24 and 36 h reactions than the
0.11Fe–Ti. It is suggested that due to the difference in Fe
speciation between the catalysts, the MB degradation kinetics on the catalyst surfaces is somewhat different.
IV. CONCLUSIONS
Photodegradation of aqueous MB with TiO2 under bluelight irradiation was affected by Fe dosage and speciation in
TiO2. Heavier Fe dosage leads to more UV and visible light
absorption, compared to nondoped TiO2. Introducing Fe into
the structure of TiO2 results in a large change in the Fe
environment, from cubic and/or trigonal into tetragonal. The
observation that 0.11Fe–Ti shows more photocatalytic activity than nondoped TiO2 is consistent with a previous study
that Fe2+ traps holes whereas Fe3+ scavenges both electrons
and holes if Fe is not overdoped. For the 0.49Fe–Ti, a negligible fraction of Fe2+ is observed.
ACKNOWLEDGMENT
The authors thank NSRRC of Taiwan for the XAS
experiment.
1
FIG. 5. 共Color online兲 Comparison among Fe K-edge XANES results from
0.11Fe–Ti, 0.49Fe–Ti, and various Fe references: Fe共0兲, FeO, FeOOH,
Fe2O3, and Fe3O4.
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