Preparation of nanocrystalline Fe-doped TiO2 powders as a visible-light-responsive photocatalyst
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- Ma, J., Wei, Y., Liu, W. et al. Res Chem Intermed (2009) 35: 329. doi:10.1007/s11164-009-0027-7
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Nanocrystalline Fe-doped TiO2 powders were prepared using TiOSO4, urea, and Fe(NO3)3 · 9H2O as precursors through a hydrothermal method. The as-synthesized yellowish-colored powders are composed of anatase TiO2, identified by X-ray diffraction (XRD). The grain size ranged from 9.7 to 12.1 nm, calculated by Scherrer’s method. The specific surface area ranged from 141 to 170 m2/g, obtained by the Brunauer–Emmett–Teller (BET) method. The transmission electron microscopy (TEM) micrograph of the sample shows that the diameter of the grains is uniformly distributed at about 10 nm, which is consistent with that calculated by Scherrer’s method. Fe3+ and Fe2+ have been detected on the surface of TiO2 powders by X-ray photoelectron spectroscopy (XPS). The UV–Vis diffuse reflection spectra indicate that the light absorption thresholds of the Fe-doped TiO2 powders have been red-shifted into the visible light region. The photocatalytic activity of the Fe-doped TiO2 was evaluated through the degradation of methylene blue (MB) under visible light irradiation. The Fe-doped TiO2 powders have shown good visible-light photocatalytic activities and the maximum degradation ratio is achieved within 4.5 h.
KeywordsFe-doped TiO2Visible lightHydrothermal method
TiO2 has been extensively researched for the degradation of organic pollutants, for air purification, antibacterial products, as a deodorant, as a demister, etc. [1, 2]. However, the photocatalyst TiO2 can only be excited by ultraviolet light with a wavelength less than 387 nm to generate electron–hole pairs due to its wider bandgap of 3.2 eV (anatase). Therefore, it is necessary to improve its visible-light activities by extending its absorption threshold from the UV light region to the visible light region [3, 4]. Doping with Fe ion is one of the best choices to enhance the visible-light photocatalytic activity of TiO2 [5–8] because the recombination of photogenerated electrons and holes can be hindered by increasing the charge separation. So many works have been done to synthesize Fe-doped TiO2 powders by a hydrothermal method in recent years . However, the synthesized products need be post-calcinated for the crystallization of TiO2, which results in the quick grain growth and agglomeration .
In this paper, nanocrystalline Fe-doped TiO2 powders were prepared by a modified hydrothermal method without any post-calcination. The photocatalytic activities of the synthesized Fe-doped TiO2 powders have been demonstrated by the degradation of methylene blue (MB) aqueous solution under visible-light irradiation.
Fe-doped TiO2 powders have been prepared by a hydrothermal method using TiOSO4 · 2H2O, urea, distilled water, and Fe(NO3)3 · 9H2O as starting materials. The TiOSO4 · 2H2O (0.5 mol) and urea (1 mol) were firstly mixed into distilled water (1 L) and stirred at the speed of 1,000 rpm for 5 min. After that, the Fe(NO3)3 · 9H2O was solved into the suspension based on the desired Fe/Ti ratio: Fe/Ti = 1, 2, 3, 4, 5 at.% (the samples were denoted as FT1, FT2, FT3, FT4, and FT5, respectively). Then, the suspension was stirred for another 10 min. Finally, the obtained suspension was charged into the autoclave with an internal volume of 2 L. The initial pressure was set at 6 MPa and the stirring speed was fixed at about 300 rpm. The autoclave was heated to 150 °C and remained constant for 4 h. The synthesized products were washed by distilled water repeatedly until residual SO42− radical could not be detected by 0.5 M Ba(NO3)2 solution. Finally, the yellowish Fe-doped powders were obtained after vacuum drying at 60 °C for 24 h.
The X-ray diffraction (XRD) measurements were carried out with a diffractometer (type HZG41B-PC) using Cu Kα radiation at a scan rate (2θ) of 0.05°/s to identify the phases composition. The grain size was calculated from the half-height width of different diffraction peaks of the XRD patterns by Scherrer’s formula . The lattice constants were calculated by the software package Celerf. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Scientific ESCALAB MkII XPS system with a Mg Kα source to analyze the surface elemental composition and valent state of TiO2. The Brunauer–Emmett–Teller (BET) surface area of the powders was measured by the amount of nitrogen adsorption at 77 K (Quanta chrome, NOVA 1000-TS). A transmission electron microscope (TEM, Hitachi, JEOL-200CX) was used to observe the morphologies of the powders. The UV–Vis diffuse reflection spectra described in this paper were obtained by using a UV–Vis Spectrophotometer (Shimadzu, UV-2100S).
Results and discussion
Grain size, Brunauer–Emmett–Teller (BET) area, crystal constant, and absorption edge of Fe-doped TiO2 powders
Grain size (nm)
BET area (m2/g)
a = b (Å)
Absorption edge (nm)
According to Table 1, the tetragonal lattice parameters determined from the XRD data are slightly larger than those reported from JCPDF data (a = 3.785 Å, c = 9.514 Å, JCPDF #84-1285). Considering that the ionic radius of octahedrally coordinated Fe3+ (0.738 Å) is larger than that of Ti4+ (0.610 Å), the increase of the lattice parameter of the as-synthesized samples may suggest that the Fe ion has been incorporated into the anatase lattice and distorted the crystal structure of the host compound.
The UV–Vis diffuse reflection spectra of Fe-doped TiO2 powders are shown in Fig. 5. There are two absorption edges located at 415–450 nm and about 540 nm, respectively. The first absorption threshold located at 415–450 nm was larger than that of non-doped anatase TiO2. It was thought that some oxygen deficiency in the TiO2 lattice accounted for the slight red-shift of the absorption thresholds . The second threshold occurring at about 540 nm may be attributed to the doping of Fe atoms in the lattice of TiO2. Based on the theory of metal-induced gap states (MIGS), Fe ion doping in the lattice would form an impurity energy level below the conduction band of TiO2 and, thus, reduce the band-gap energy.
All of the synthesized Fe-doped TiO2 powders exhibited good visible-light photocatalytic activities when decomposing MB solution under visible-light irradiation. Zhu et al.  reported that the Fe atoms could be incorporated into a lattice of anatase and substitute the Ti atoms with lower Fe concentrations less than 2 wt%, which causes local distortion of the anatase lattice, while some of the Fe atoms will aggregate and form α-Fe2O3 when the concentration of Fe was increased to 5 wt%. As XPS data has detected the existence of Fe3+ on the surface of the synthesized TiO2 powders, Fe3+ can act as both hole and electron traps to enhance the lifetimes of electrons and holes . If the Fe3+ is inserted into the matrix interior of the TiO2 lattice, the Fe acts as recombination centers for the electron–hole pair , as a result of decreased photocatalytic activities. It is deduced that a certain amount of Fe doping, such as FT2 and FT5, may enhance the photocatalytic activities of TiO2 powders with different mechanisms.
Visible-light active Fe-doped TiO2 powders were synthesized through a hydrothermal method by using TiOSO4, H2O, urea, and Fe(NO3)3 · 9H2O as starting materials. All of the as-synthesized spherical powders are composed of anatase, the grain size ranged from 9.7 to 12.1 nm, and the specific surface area ranged from 141 to 170 m2/g. The X-ray photoelectron spectroscopy (XPS) data shows that the Fe atoms on the surface of TiO2 powder exist as both Fe3+ and Fe2+. The UV–Vis diffuse reflection spectra of each of the Fe-doped TiO2 powders have an additional absorption threshold occurring at about 540 nm. The prepared Fe-doped TiO2 powders have good visible-light response and show good visible-light photocatalytic activities in the degradation of methylene blue (MB). The maximum degradation rate of MB is achieved within 4.5 h of visible-light irradiation by FT2 and FT5.
The authors appreciate the financial support by the National Natural Science Foundation of China under Grant No. 50772014 and by the Program for New Century Excellent Talents in University under Grant No. NCET-07-0064.