Synthesis and Characterization of Rutile TiO2Nanopowders Doped with Iron Ions
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Titanium dioxide nanopowders doped with different amounts of Fe ions were prepared by coprecipitation method. Obtained materials were characterized by structural (XRD), morphological (TEM and SEM), optical (UV/vis reflection and photoluminescence, and Raman), and analytical techniques (XPS and ICP-OES). XRD analysis revealed rutile crystalline phase for doped and undoped titanium dioxide obtained in the same manner. Diameter of the particles was 5–7 nm. The presence of iron ions was confirmed by XPS and ICP-OES. Doping process moved absorption threshold of TiO2into visible spectrum range. Photocatalytic activity was also checked. Doped nanopowders showed normal and up-converted photoluminescence.
KeywordsTitanium dioxide Rutile Doping Characterization
Since its commercial production in the early 20th century, titanium dioxide (TiO2) has been widely used as a pigment  and in sunscreens [2, 3], paints , ointments, toothpaste , etc. In 1972, Fujishima and Honda [6, 7, 8] discovered the phenomenon of photocatalytic splitting of water on a TiO2 electrode under ultraviolet (UV) light. Since then, enormous efforts have been devoted to the research of TiO2 material, which has led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo/electrochromics and sensors [9, 10, 11, 12]. These applications can be roughly divided into “energy” and “environmental” categories, many of which depend not only on the properties of the TiO2 material itself but also on the modifications of the TiO2 material host (e.g., with inorganic and organic dyes) and on the interactions of TiO2 materials with the environment .
An exponential growth of research activities has been seen in nanoscience and nanotechnology in the past decades [14, 15, 16, 17, 18]. New physical and chemical properties emerge when the size of the material becomes smaller and smaller, and down to the nanometer scale. Among the unique properties of nanomaterials, the movement of electrons and holes in semiconductor nanomaterials is primarily governed by the well-known quantum confinement and the transport properties related to phonons and photons are largely affected by the size and geometry of the materials [14, 15, 16, 17]. The specific surface area and surface-to-volume ratio increases dramatically as the size of a material decreases [14, 19]. The high surface area brought about by the small particle size is beneficial to many TiO2-based devices, as it facilitates reaction/interaction between the devices and the interacting media, which mainly occurs on the surface or at the interface and strongly depends on the surface area of the material. Thus, the performance of TiO2-based devices is largely influenced by the sizes of the TiO2 building units, apparently at the nanometer scale.
Titanium dioxide can be obtained in three crystalline phases: anatase, rutile, and brookite. The most stable phase is rutile and it is usually obtained after annealing at temperature above 500 °C . TiO2 is transparent normally in the visible light region; its band gap is 3.0 eV for rutile and 3.2 eV for anatase crystalline phase. By doping or sensitization, it is possible to improve the optical activity of TiO2 and to move its absorption threshold into the visible light region.
The subject of this work is the synthesis by low temperature coprecipitation method of Fe-doped TiO2nanopowders and their characterization. Several concentrations of Fe ions were implied. Detailed characterization was conducted and photodegradation of mecoprop was chosen as a probe reaction for evaluation of photocatalytic activity of prepared samples. The relationship between optical properties (PL) and photoactivity of samples is discussed.
All chemicals used were of p.a. purity and were used without further purification. Triply distilled water was used for aqueous solutions.
Fe-doped TiO2 powders were prepared by a modified synthetic procedure of Abazović et al. . An appropriate amount of FeCl3 (Aldrich) was dissolved in 200 mL of triply distilled water. Then, 5 mL of TiCl4 (Fluka) prechilled to −20 °C was added dropwise into solution containing FeCl3 under stirring. After 2 h of stirring at room temperature, the obtained dispersions were heated and kept at 50 °C for 16 h with constant stirring. The resulting precipitates were dialyzed against water until test reaction for Cl− ions was negative and subsequently dried in vacuum at 40 °C. Pure TiO2 powder was synthesized in the same manner, without FeCl3 in the reaction solution.
The obtained powders were characterized by several techniques. For UV/vis spectrometry a Perkin–Elmer λ-35 spectrophotometer, equipped with reflectance accessory and referenced with BaSO4, was used. Photoluminescence was measured using a Perkin–Elmer LS-3b instrument. Raman spectra were obtained using a Raman system R-2001TM.
The X-ray diffraction measurements of the powders were performed on a Philips PW1710 diffractometer.
Microstructural characterization of the iron ions doped TiO2nanopowders was carried out on a Philips EM-400 transmission electron microscope operated at 100 kV and on a Cambridge 250MKIII scanning electron microscope. Samples for TEM analysis were dispersed in methanol, ultrasonicated for 1 h and deposited on C-coated Cu grids. Samples for SEM were deposited on carbon tapes.
The X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG Scientific ESCALAB 210 spectrometer using non-monochromatic MgKα radiation. The calibration of peak position was made using the Ag 3d5/2 line at 368.26 eV of a standard silver foil, used as a reference sample. It was in situ sputtered with argon ions in order to remove the surface oxide and acquire a clean reference spectrum. We used C1s to calibrate the peak positions after experimental acquisitions, because on the surface region carbon was well detected and unequivocally associated to adventitious carbon (for air exposure of the samples) expected at 285 eV of binding energy .
Chemical quantitative analysis was performed by inductively coupled plasma optical emission spectroscopy (Spectroflame ICP, 2.5 kW, 27 MHz). ICP-OES analysis was performed by measuring the intensity of radiation of the specific wavelengths emitted by each element. The samples dispersed in liquid were introduced into the plasma as aerosol, where they were vaporized, atomized, and excited.
Photocatalytic degradation was carried out in a cell made of Pyrex glass (total volume of ca. 40 mL, liquid layer thickness 35 mm), with a plain window on which the light beam was focused, equipped with a magnetic stirring bar and a water circulating jacket. A 125 W medium-pressure mercury lamp (Philips, HPL-N, emission bands in the UV region at 304, 314, 335, and 366 nm, with maximum emission at 366 nm), together with an appropriate concave mirror, was used as the radiation source.
Experiments were carried out using 20 mL of the solution of mecoprop (2.7 mmol dm−3) and 40 mg of catalyst. Herbicide mecoprop (RS-2-(4-chloro-otolyloxy) propionic acid, C10H11ClO3) was chosen as a model compound of a photodegradable organic waste substance in water because of its worldwide use for the selective control of many annual and some perennial weeds and because it is the herbicide most often found in drinking water [23, 24].The aqueous suspensions were sonicated in the dark for 15 min before illumination, to make the photocatalyst particles uniform and attain adsorption equilibrium. The suspensions thus obtained were thermostated at 40 ± 0.5 °C, in a stream of O2 and then irradiated. During the irradiation, the mixtures were stirred at a constant speed.
Photocatalytic activity was checked on a spectrophotometer (Secomam anthelie Advanced 2). Namely, 0.25 cm3aliquots of the samples were taken at different illumination times and diluted to 10.00 cm3with double distilled water. The suspensions containing photocatalyst were filtered through Millipore (Milex-GV, 0.22 μm) membrane filters and spectra were recorded on a spectrophotometer. Kinetics of the aromatic ring degradation was monitored at 228 nm.
Results and Discussion
Diameters are in good agreement with the diameters obtained from XRD measurements using the Scherrer diffraction formula. Similar TiO2morphology was observed for all dopant concentrations.
Concentration of Fe in doped TiO2powders, determined by ICP-OES
Sample (% Fe–TiO2)
Effect of at.% Fe in catalysts on mecoprop photocatalytic degradation rate with UV light
Sample (% Fe–TiO2)
High energy peaks can be assigned to band edge luminescence of the TiO2 particles, while lower energy peaks/shoulders are induced by the presence of the oxygen vacancies [21, 30]. Among the other facts, it seems that radiative recombination of photogenerated charge carriers through oxygen vacancy–cascade can also be considered as the process which can decrease the photocatalytic activity of iron doped TiO2.
TiO2nanopowders doped with different concentration of Fe ions were synthesized by coprecipitation method. Applied synthetic procedure induced formation of pure rutile crystalline structure. TEM measurements revealed formation of flower-like agglomerates with diameters in the size range from 100 to 150 nm. XPS measurements showed that Fe ions are mainly in Fe3+oxidation state and that concentrations of incorporated iron ions are much lower than stoichiometric. Doping with Fe ions has great influence on optical characteristics of the host material. Reflection measurements showed that doping of TiO2with Fe3+causes shift of the absorption threshold toward visible spectral region. No increase of TiO2photocatalytic activity after doping, was observed. The induced photoluminescence as well as the decrease of photocatalytic activity is probably the consequence of the introduction of oxygen vacancies through doping procedure. For higher dopant concentrations (>5%, stoichometric concentration) also recombination of photogenerated charge carriers occurs with higher probability.
The authors are grateful to Dr Amelia Montone for help in performing SEM measurements. Financial support for this study was granted by the Ministry of Science and Technological Development of the Republic of Serbia (Project No. 142066 and 142029).
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