Valentina Shymanovska

ABSTRACT The fundamental absorption edge of polydisperse rutile doped with transition metal cations (Cr, Fe, Cu, Co) was investigated by UV spectroscopy at room temperature. It was shown that the fundamental absorption edge of TiO2 is determined by two electron transition mechanisms: one is the direct forbidden transition, another is the indirect allowed transition. The fundamental band gap DE=3.013eV for pure rutile was determined. The Fe-doped TiO2 absorption shows spectral redistribution near the fundamental edge while Co-doped TiO2 absorption is strictly similar to that of pure rutile.

Abstract The effect of surface Fe-doping on the structural, photocatalytic, optical properties of polydisperse anatase and rutile TiO2 has been comprehensively studied. The TiO2 surface has been modified by Fe3+ ions through the adsorption from the diluted FeCl3 aqueous solutions. The prepared powders were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) surface area analysis, temperature programmed desorption techniques (TPD), Raman and UV–vis spectroscopy. With the adsorption isotherms of Fe3+ ions from FeCl3 aqueous solutions, it was found that adsorption of Fe3+ ions on the anatase surface is twice more efficient compared to that one on the rutile. The XRD and Raman spectroscopy measurements confirmed the monophase structure of the Fe-doped TiO2 with a subtle change in the lattice parameters and O/Ti ratio compared to the undoped counterparts. The red shift of the UV absorption edge has been registered for the Fe-doped anatase resulting in a significant decrease of its bandgap from 3.2 to 2.7 eV. The photocatalytic destruction of organic dye Safranin T over the prepared Fe-doped TiO2 under UV irradiation has been investigated. The influence of various energetically nonequivalent active sites on the surfaces of anatase and rutile on their photoactivity is considered.

Abstract We studied the effect of the phenothiazine (PTZ) surface doping on the structural and optical properties of nanocrystalline titanium dioxide powder with a single anatase phase (A-TiO2). We found that the mixing and interaction of PTZ and A-TiO2 leads to the intense dark-blue color of PTZ-doped A-TiO2 (A/PTZ). The appearance of new bands in the Fourier transform IR spectrum indicated the formation of PTZ•+ radical cations and reduced Ti3+species as well as extra oxygen vacancies (VO) in the TiO2 matrix. The UV-vis absorption spectrum of A/PTZ exhibited an increase in absorption in the visible region (>400 nm). The doping of A-TiO2 with PTZ causes a noticeable redshift of the absorption edge and a significant narrowing of the band gap by 0.24 and 0.49 eV for direct and indirect electronic transitions, respectively. The photoluminescence spectra show that the photoluminescence originating from Ti3+ and [VO-Ti3+] states for A/PTZ is stronger than for A-TiO2, but the excitonic photoluminescence of A/PTZ is quenched. An increased number of surface defects in A/PTZ can essentially increase the nonradiative charge recombination, and therefore may considerably enhance the photoactivity of the dark-blue TiO2 in the visible range of the spectrum.

ABSTRACT Nanocrystalline titanium-manganese mixed oxides (TMMO) with Mn content of 13–16 at% were first synthesized by manganese hydroxide precipitation on anatase or rutile particles. UV-vis absorption spectra were investigated in 2.4–6.0 eV region and revealed that compared with pure TiO2 the presence of TMMO results in an increase of the absorption intensity and red shift of the absorption edge. Photocatalytic activity of pure TiO2, Mn2+-doped TiO2 and TMMO during the photocatalytic decomposition of the safranine dye under UV irradiation was studied. A great improvement of photocatalytic activity is registered for Mn2+-doped TiO2 anatase and rutile and rutile-based TMMO.

ABSTRACT The optical absorption and photoluminescence of anatase and rutile TiO2 were studied at room temperature. TiO2 nanocrystalline powders were synthesized in the form of pure anatase or rutile. The samples were characterized by X-ray diffraction, X-ray fluorescence, Raman spectroscopy, optical absorption and photoluminescence (PL) methods. The PL spectra were studied under the intensive UV (3.68eV) laser excitation. Some interesting features in the PL spectra including the well-resolved peaks of excitonic and band–band transitions in TiO2 anatase and rutile were observed, to our knowledge, for the first time. It is shown that PL bands including peaks at 2.71–2.81 eV and its phonon replicas in anatase and rutile TiO2 arise from the excitonic e--h+ recombination via oxygen vacancies. The excitonic peak at 2.91 eV is attributed to the recombination of self-trapped excitons in anatase or free excitons in rutile TiO2. The PL peaks within 3.0–3.3 eV in anatase TiO2 are ascribed to indirect allowed transitions due to the band-band e--h+ recombination. The peaks at 3.03 eV and 3.26 eV are attributed to the free exciton emission near the fundamental band edge of rutile and anatase TiO2, respectively. The influence of TiO2 crystal structure and calcinations temperature on the PL spectra is discussed.

We studied the effect of the phenothiazine (PTZ) surface doping on the structural and optical properties of nanocrystalline titanium dioxide powder with a single anatase phase (A-TiO2). We found that the mixing and interaction of PTZ and A-TiO2 leads to the intense dark-blue color of PTZ-doped A-TiO2 (A/PTZ). The appearance of new bands in the Fourier transform IR spectrum indicated the formation of PTZ •+ radical cations and reduced Ti3+ species as well as extra oxygen vacancies (VO) in the TiO2 matrix. The UV-vis absorption spectrum of A/PTZ exhibited an increase in absorption in the visible region (> 400 nm). The doping of A-TiO2 with PTZ causes a noticeable redshift of the absorption edge and a significant narrowing of the band gap by 0.24 and 0.49 eV for direct and indirect electronic transitions, respectively. The photoluminescence spectra show that the photo-luminescence originating from Ti3+ and [VO-Ti3+] states for A/PTZ is stronger than for A-TiO2 , but the excitonic ph...

Titanium-manganese mixed oxides TiO2 /MnOx (TMO) were prepared by chemical precipitation of manganese hydroxide on pure nanocrystalline rutile TiO2 particles with subsequent thermal treatment at different temperatures. The morphology, structural and optical properties of TMO samples were investigated using standard XRD, SEM, EDS, XRF, TGA, FT-Raman and UV–vis absorption spectroscopic techniques. The photo-luminescence (PL) in TMO was excited by the UV laser pulses (337.1 nm). All measurements were carried out at room temperature. Examination of PL spectra revealed that they are strongly dependent on the phase composition and Mn concentration in TMO compounds. The quenching of the PL is connected with the oxygen vacancies in the TMO structure, in particular, the intensity lowering for the near band edge emission at 3.06 eV (405 nm) and the free exciton emission at 2.92 eV (424 nm) is interpreted by the non-radiative Auger recombination. The relative increase and decrease in the ratio...

The optical absorption and photoluminescence of nanocrystalline TiO2 samples of anatase and rutile were investigated at room temperature. Nanocrystalline TiO2 samples were synthesized in the form of pure anatase or rutile and studied by X-ray diffraction, X-ray fluorescence, Raman spectroscopy, optical absorption and photoluminescence (PL). PL was studied at room temperature when excited by intense UV (3.68 eV) by a nitrogen laser. For the first time for nanocrystalline TiO2 a features in the high-resolution PL spectra, including the exciton band and interband transitions were registered. It is concluded that the processes of absorption and emission of light near the edge of the forbidden zone occur with the participation of the same electronic transitions. PL bands, including the peaks at 2.71-2.81 eV in the anatase and rutile arise due to exciton recombination in the TiO2 lattice oxygen vacancies. The exciton peak at 2.91 eV is attributed to the recombination of self-trapped excit...

Titanium dioxide (TiO2) nanocrystalline powders were prepared by the thermal hydrolysis method in the form of pure anatase or rutile and were investigated by X-ray diffraction, X-ray fluorescence, FT-Raman spectroscopy, optical absorption, and photoluminescence (PL) methods. PL spectra were studied under the intense UV laser excitation at 337.1 nm (3.68 eV) at room temperature. Some interesting features in the PL spectra including the well-resolved peaks of excitonic and band-band e-h transitions in TiO2 were observed for the first time. It is shown that PL bands with peaks at 2.71–2.81 eV and its phonon replicas in anatase and rutile TiO2 arise from the excitonic e-h recombination via oxygen vacancies. The excitonic peak at 2.91 eV is attributed to the recombination of self-trapped excitons in anatase or free excitons in rutile TiO2. The PL peaks within 3.0–3.3 eV in anatase TiO2 are ascribed to indirect allowed transitions due to the band-band e-h recombination. The peaks at 3.03 ...

ABSTRACT The optical absorption and photoluminescence of anatase and rutile TiO2 were studied at room temperature. TiO2 nanocrystalline powders were synthesized in the form of pure anatase or rutile. The samples were characterized by X-ray diffraction, X-ray fluorescence, Raman spectroscopy, optical absorption and photoluminescence (PL) methods. The PL spectra were studied under the intensive UV (3.68eV) laser excitation. Some interesting features in the PL spectra including the well-resolved peaks of excitonic and band–band transitions in TiO2 anatase and rutile were observed, to our knowledge, for the first time. It is shown that PL bands including peaks at 2.71–2.81 eV and its phonon replicas in anatase and rutile TiO2 arise from the excitonic e--h+ recombination via oxygen vacancies. The excitonic peak at 2.91 eV is attributed to the recombination of self-trapped excitons in anatase or free excitons in rutile TiO2. The PL peaks within 3.0–3.3 eV in anatase TiO2 are ascribed to indirect allowed transitions due to the band-band e--h+ recombination. The peaks at 3.03 eV and 3.26 eV are attributed to the free exciton emission near the fundamental band edge of rutile and anatase TiO2, respectively. The influence of TiO2 crystal structure and calcinations temperature on the PL spectra is discussed.

ABSTRACT The effect of nanocrystalline TiO2 doping with transition metal cations (Cu2+, Fe3+, Co2+, Cr3+) on their optical absorption and photocatalytic properties was investigated. The obtained metal-doped TiO2 samples were characterized by X-ray diffraction, scanning electron microscopy, and UV-vis absorption spectroscopy. It is shown that doping effect on anatase (A) and rutile (R) properties is quite different, being much stronger and complicated on A than on R. Contrary to doped R, doped A revealed a significant red shift of the absorption edge along with the band gap narrowing. Photocatalytic activity of anatase increases upon doping in the order: A<A/Co<A/Cu<A/Fe. On the contrary, photocatalytic activity of rutile samples decreases upon doping in the series R>R/Co>R/Cu>R/Fe>R/Cr, indicating the inhibitory effect of impurity cations. This fact correlates with the decrease in the UV absorption of the doped rutile in the region of the Hg-lamp irradiation at 4.88 eV.

ABSTRACT Nanocrystalline anatase (A) and rutile (R) TiO2 were synthesized by the thermal hydrolysis and then were surface doped with Cr ions by adsorption. The samples were characterized by X-ray diffraction, X-ray fluorescence, Raman spectroscopy, optical absorption and photoluminescence (PL) methods. It is found that the PL emission and optical absorption near the band gap edge occur with the same electronic transitions in TiO2. The Cr3+ doping leads to the blue and red shifts of the PL peaks in A/Cr TiO2 due to the Burstein-Moss and band tailing effects, respectively. The Auger type non-radiative recombination in R/Cr TiO2 quenches the PL emission intensity. For anatase samples at low concentrations of Cr ions (0.5 at%) the PL emission increases compared to the undoped A TiO2 due to the formation of additional radiative recombination centers. At higher contents of Cr (1.0 at%), the PL intensity decreases due to the concentration quenching effect