Synthesis of La / N Co-Doped SrTiO 3 Using Polymerized Complex Method for Visible Light Photocatalysis

Lanthanum and nitrogen co-doped SrTiO3 was synthesized using polymerized complex method with Ti(OC3H7)4, SrCl2·6H2O and La(NO3)3·6H2O as starting materials followed by calcinations in NH3. Ethylene glycol and anhydrous citric acid were used as the precursors of synthesis. The samples were characterized using XRD, TEM, DRS, BET, EDX and XPS. The cubic-perovskite type of La/N co-doped SrTiO3 nanoparticle could be successfully synthesized. The photocatalytic activity of SrTiO3 for DeNOx ability in visible light region (λ > 510 nm) could be improved by co-doping of La and N. The high visible light photocatalytic activity of this substance was caused by a narrow band gap energy that enables to absorb visible light.


Introduction
Recently, synthesis of strontium titanate based photocatalyst for converting visible light energy to photoreaction has been a great attention [1][2][3][4][5][6].Among the modifying of SrTiO 3 , doping with nitrogen is the most effective to enhance photoreaction in visible light.The mixing of N 2p with O 2p states narrows the band gap energy and enhances the photocatalytic ability in visible light.However, substituting O 2− by N 3− will generate residual anionic vacancies which suppress the photocatalytic activity of SrTiO 3 .To solve this problem, co-doping of lanthanum and nitrogen into SrTiO 3 , can decrease the band gap energy without forming lattice defect and lattice strain, and consequently lead to generate high visible light photocatalytic activity.The La 3+ can substitute Sr 2+ without large lattice strain because of the similar ionic radius.
Many researchers had developed the synthesis of La/N co-doped SrTiO 3 catalysts.Miyauchi et al. [7] reported that the synthesis of La/N co-doped SrTiO 3 catalysts using sol-gel method decreased the ionic vacancy and then increased the photocatalytic activity in a visible light.Wang et al. [8] found that the synthesis of La/N codoped SrTiO 3 catalysts using mechanochemical reaction enhanced the photocatalytic activity in a visible light.However, they had a large particle and low specific surface area which limited the catalytic ability.To improve the potocatalytic ability, synthesis of fine particle which has a large specific surface area should be realized.
Polymerized complex method using citric acid and ethylene glycol as polymeric precursors has been widely used for metal oxide synthesis [9,10].The polymerized complex process has great advantages over other synthesis techniques due to mixing of several components in atomic scale, good stoichiometry control, high purity, low cost and relatively low processing temperature [11].The metallic ions are dispersed in the polymeric network at the atomic scale without precipitation and phase segregation [12].Based on this consideration, the fine particles of La-doped SrTiO 3 could be synthesized by the polymerized complex process and then followed by calcinations in ammonia to obtain the La/N co-doped SrTiO 3 nanoparticles.
In the present paper, we report the synthesis of La/N co-doped SrTiO 3 using polymerized complex method.The lanthanum and nitrogen co-doping effectively narrowed the band-gap energy of SrTiO 3 .The photocatalytic activity of SrTiO 3 for NO decomposition in visible light region (λ > 510 nm) could be enhanced.The high visible light photocatalytic activity of this substance might be caused by the low band gap energy and high specific surface area.

Preparation of Catalyst
The La/N co-doped SrTiO 3 with variation of lanthanum doping was prepared by the polymerized complex method [9].The Ti(OC 3 H 7 ) 4 and SrCl 2 •6H 2 O and La(NO 3 ) 3 •6H 2 O were dissolved in ethylene glycol.As amount of 38.2 gram of anhydrous citric acid and 100 mL of methanol were added to the solution and the mixture was stirred at 130˚C until a transparent gel was formed.The polymer was carbonized at 350˚C and calcined in air at 620˚C for 2 h to remove carbon, and the product (Sr 1−x La x TiO 3 ) was grinded.The products of Sr 1−x La x TiO 3 were then nitrogenized by heating at 700˚C for 5 h under NH 3 (flow rate 400 ml/L).The as-prepared sample of Sr1-xLa x- TiO 3 -yN y with x = 0, 0.25, 0.5, 0.7 and 1 are named as STN, STN-0.25,STN-0.5, STN-0.7 and LTN, respectively.

Characterization
The powder product was characterized by XRD (Shimadzu XD-D1) using graphite-monochromized CuKα radiation.The mean crystallite size of the powders was determined by the XRD-Scherrer equation [13].Microstructure examinations were obtained by transmission electron microscopy (TEM, JEOL JEM-2010).The band gap energies of the products were determined using DRS (Shimadzu UV-2000).The chemical compositions were analyzed by EDX (Shimadzu, EDX-800HS).The specific surface area was determined by the nitrogen adsorption at 77 K (BET, Quantachrome NOVA 4200e).Binding energies of element were analyzed at room temperature by XPS (Perkin-Elmer PHI5600).

Photocatalytic Activity
The photocatalytic activities were evaluated using NO x analyzer (Yanaco, ECL-88) [5].A 450 W high-pressure mercury arc was used as the light source.The wavelength of the irradiation light was controlled by selecting filters, i.e., Pyrex glass for λ > 290 nm, Kenko L41 Super Pro (W) filter λ > 400 nm and Fuji, tri-acetyl cellulose filter λ > 510 nm.The photocalyst sample was placed in a hollow of 20 mm length × 15 mm width × 0.5 mm depth on a glass holder plate and set in the bottom center of the reactor.The concentration of NO gas at the outlet of the reactor during the photoirradiation was monitored for 10 minutes for every filter.

XRD Analysis
The XRD profiles of La/N co-doped SrTiO 3 are shown in Figure 1.The single phase of cubic-perovskite could be observed at samples of STN, STN-0.25 and STN-0.5, while the sample of LTN contains an impurity.The intensity of diffraction decreased with increasing La doping, indicating that the crystalline properties was strongly affected by La doping.The particle sizes of La/N co-doped SrTiO 3 calculated by Scherrer equation are listed in Table 1.The particle of 52 nm could be obtained in the sample of STN, and decreased with increasing La doping to 39 nm, 17 nm, 23 nm and 18 nm for STN-0.25,STN-0.5, STN-0.7 and LTN, respectively.The smallest of particle size could be found on the sample of STN-0.5.

Morphology
Figure 2 shows the morphology of STN and STN-0.5.The particle size of 40 -60 nm in diameter could be observed in STN and 15 -20 nm in STN-0.5.The particle size observed by TEM agreed with that measured by Scherrer equation (see Table 1).The particle size of STN-0.5 is smaller than that of STN, indicating that the La doping affected the process of crystallization.

Uv-Vis Diffusion Reflectance Spectroscopy
Figure 3 shows the absorbance spectra of La/N co-doped SrTiO 3 .The absorption edge shifted to higher of wavelength, indicating the narrow band gap was generated by La/N co-doping.The band-gap energies of La/N co-doped   SrTiO 3 were calculated according to the equation of Eg = 1240/λ [14].The results were 2.95, 1.87, 2.13, 2.16 and 2.08 eV for STN, STN-0.25,STN-0.5, STN-0.7 and LTN, respectively.The variations of could be observed in the samples, they are grey, blue, greenish-yellow and yellow, for STN, STN-0.25,STN-0.5, STN-0.7,respectively.The highest band gap energy of 2.95 eV could be observed for STN, indicating that the doping of nitrogen without lanthanum did not effectively narrow the band gap energy.The broad absorption above 500 nm could be found in the spectra of STN and STN-0.25.The sample of STN-0.25 showed high broad absorption which was assigned to the oxygen vacancy states.They were located between 0.75 and 1.18 eV below the minimum level of the conduction band [15].The similar results were also found in the samples prepared with different methods [7].The strontium titanate with oxygen vacancies can absorb a broad range of visible light above 500 nm.The lower broad absorption above 500 nm could be found in the sample of STN-0.5, STN-0.7 and LTN, indicating that the samples have lower oxygen vacancy.

XPS Analysis
Figure 4 shows XPS profiles of the STN-0.5 after sputtering at 3 kV for 3 minutes.The peak N1s could be observed at 396.0 eV shown in Figure 4(a), indicating the formation of nitrogen doped SrTiO 3 [7].This result proves that nitrogen was incorporated in the lattice.The lanthanum ion could be identified at 833.7 eV and 850.5 eV (Figure 4(b)), which correspond to La 3d5/2 and La 3d3/2, respectively [16].The spectrum for titanium exhibits two different signals corresponding to the Ti 2p3/2 and 2p1/2 with binding energies of 457.5 and 463.2 eV, respectively.The peak position of Ti 2p3/2 agreed with that of the Ti 4+ [17][18][19].The peak of O1s was observed at 529.1 eV which is the characteristic of metal oxides [20].

Photocatalytic Activity
Figure 5 shows the photocatalytic activity of La/N co-doped SrTiO 3 for the NO elimination under visible light irradiation (λ > 510 nm, λ > 400 nm), and UV light irradiation (λ > 290 nm).It took about 10 min to reach the steady state after light irradiation.There is no significant activity in visible light (λ > 510) for STN, presumably due to higher band gap energy of 2.95 eV.The sample of STN-0.25 exhibits low photocatalytic activity both in visible light and UV light.It showed the highest broad absorption above 500 nm which is assigned to the oxygen vacancy states.The existences of oxygen vacancy increase recombination of hole-electron pairs and then decrease the photocatalytic ability [7].The sample of STN-0.5 showed the highest activity under the visible light irradiation (λ > 510 nm), i.e., 28.8% NO could be destructed.Moreover, the photocatalytic ability of STN-0.5 was also higher than TiO 2 (P-25) in the ultraviolet light, i.e., 42.3% of NO could be destructed.The excellent photocatalytic was attributed to both narrow band gap energy and high specific surface area (see Table 1).The photocatalytic activity decreased with increasing lanthanum doping (STN-0.7 and LTN).It may be attributed to low crystallinity of the sample.

Figure 1 .
Figure 1.XRD patterns of La/N co-doped SrTiO 3 samples synthesized by the polymerized complex method.