362
T. A. Khalyavka et al.
recombination. It was shown that the lifetime of trapped electrons correlates with
the photocatalytic activity of pure and doped TiO 2 ; the higher photoactivity of the
samples, the lower recombination rate [8, 9]. The mutual recombination of electrons
and holes reduces the activity of the photocatalyst. A dopant can trap electrons and
transfer them to an electron acceptor which reduces the rate of recombination of an
electron-hole pair [10]. Oxygen adsorbed on the surface of the photocatalyst captures
electrons and forms a superoxide anion as well as the holes can oxidize adsorbed
hydroxide ions to hydroxyl radicals [11, 12]. Lanthanum and its compounds have
numerous applications as catalysts, various additives in glasses, in ignition elements,
electronic cathodes, in medicine, etc. [5, 13–15].
Lanthanum has an electronic configuration [Xe] 5d
1 6s
2 , very effective to increase
electron-hole carrier lifetime [16]. The size of the La atom radius is 1.87 Å and that
of La
3+ ion is 1.17 Å, which is almost two times bigger than the radius of Ti
4+ (0.64
Å) and Ti
3+ (0.67 Å). In the ground state, the La atom is monovalent. Transferring it
to the trivalent state requires an expense of 8 kcal/g-atom. The successive ionization
potentials for La are 5.61, 11.43, and 19.17 eV. In chemical reactions, lanthanum
usually gives away three valence electrons from 5d and 6s subshells in order to form
an oxidation state of +3, reaching a stable configuration. In an oxygen atmosphere
at 450 °C, lanthanum is oxidized to A-La 2 O 3 .
The aim of this study was to investigate the effect of La on the structural and optical
properties of titanium dioxide, to evaluate Safranine T (ST) destruction (decolorization) and H 2 evolution over pure and La-doped TiO 2 under UV irradiation. For this
purpose, we have synthesized TiO 2 samples with different lanthanum content by
the sol–gel method. The sol–gel process is a widespread method used to obtain
titanium dioxide-based nanocomposite materials. This process is a low-temperature
synthesis which allows one to obtain polycrystalline powders of different chemical
composition and various physicochemical properties [2, 3, 9].
2 Experimental Section
2.1 Synthesis of TiO 2 and La/TiO 2 Samples
All the samples were synthesized by sol–gel method. All the chemical reagents were
of analytical grade and used directly without further purification. The experimental
procedure was as follows.
Preparation of pure TiO 2 . Titanium (IV) tetrabutoxide (C 16 H 36 O 4 Ti ≥ 99.9%,
Aldrich, 3 g), citric acid (C 6 H 8 O 7 , 0.06 g), and glycerol (C 3 H 8 O 3 , 2 g) were mixed
at room temperature and calcinated at 500 °C for 2 h in the presence of air oxygen.
Preparation of La/TiO 2 samples. The mixture of titanium (IV) tetrabutoxide,
La(NO 3 ) 3 ·6H 2 O, citric acid, and glycerol was carefully stirred in order to obtain
uniform mass and further was calcined at 500 °C for 2 h in the presence of air.
Lanthanum was doped at various atomic percentages (1.44 and 6.03% at., Table 1).
T. A. Khalyavka et al.
recombination. It was shown that the lifetime of trapped electrons correlates with
the photocatalytic activity of pure and doped TiO 2 ; the higher photoactivity of the
samples, the lower recombination rate [8, 9]. The mutual recombination of electrons
and holes reduces the activity of the photocatalyst. A dopant can trap electrons and
transfer them to an electron acceptor which reduces the rate of recombination of an
electron-hole pair [10]. Oxygen adsorbed on the surface of the photocatalyst captures
electrons and forms a superoxide anion as well as the holes can oxidize adsorbed
hydroxide ions to hydroxyl radicals [11, 12]. Lanthanum and its compounds have
numerous applications as catalysts, various additives in glasses, in ignition elements,
electronic cathodes, in medicine, etc. [5, 13–15].
Lanthanum has an electronic configuration [Xe] 5d
1 6s
2 , very effective to increase
electron-hole carrier lifetime [16]. The size of the La atom radius is 1.87 Å and that
of La
3+ ion is 1.17 Å, which is almost two times bigger than the radius of Ti
4+ (0.64
Å) and Ti
3+ (0.67 Å). In the ground state, the La atom is monovalent. Transferring it
to the trivalent state requires an expense of 8 kcal/g-atom. The successive ionization
potentials for La are 5.61, 11.43, and 19.17 eV. In chemical reactions, lanthanum
usually gives away three valence electrons from 5d and 6s subshells in order to form
an oxidation state of +3, reaching a stable configuration. In an oxygen atmosphere
at 450 °C, lanthanum is oxidized to A-La 2 O 3 .
The aim of this study was to investigate the effect of La on the structural and optical
properties of titanium dioxide, to evaluate Safranine T (ST) destruction (decolorization) and H 2 evolution over pure and La-doped TiO 2 under UV irradiation. For this
purpose, we have synthesized TiO 2 samples with different lanthanum content by
the sol–gel method. The sol–gel process is a widespread method used to obtain
titanium dioxide-based nanocomposite materials. This process is a low-temperature
synthesis which allows one to obtain polycrystalline powders of different chemical
composition and various physicochemical properties [2, 3, 9].
2 Experimental Section
2.1 Synthesis of TiO 2 and La/TiO 2 Samples
All the samples were synthesized by sol–gel method. All the chemical reagents were
of analytical grade and used directly without further purification. The experimental
procedure was as follows.
Preparation of pure TiO 2 . Titanium (IV) tetrabutoxide (C 16 H 36 O 4 Ti ≥ 99.9%,
Aldrich, 3 g), citric acid (C 6 H 8 O 7 , 0.06 g), and glycerol (C 3 H 8 O 3 , 2 g) were mixed
at room temperature and calcinated at 500 °C for 2 h in the presence of air oxygen.
Preparation of La/TiO 2 samples. The mixture of titanium (IV) tetrabutoxide,
La(NO 3 ) 3 ·6H 2 O, citric acid, and glycerol was carefully stirred in order to obtain
uniform mass and further was calcined at 500 °C for 2 h in the presence of air.
Lanthanum was doped at various atomic percentages (1.44 and 6.03% at., Table 1).