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

doi:10.4236/anp.2013.21002

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Advances in Nanoparticles, 2013, 2, 6-10

http://dx.doi.org/10.4236/anp.2013.21002 Published Online February 2013 (http://www.scirp.org/journal/anp)

Synthesis of La/N Co-Doped SrTiO3 Using Polymerized

Complex Method for Visible Light Photocatalysis

Uyi Sulaeman1, Shu Yin2, Tsugio Sato2

1Department of Chemistry, Jenderal Soedirman University, Purwokerto, Indonesia

2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan

Email: [email protected]

Received December 16, 2012; revised January 15, 2013; accepted January 25, 2013

ABSTRACT

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 La3+ and N3−. The high visible light photocatalytic activity of this substance was caused by a narrow band gap energy

that enables to absorb visible light.

Keywords: Photocatalysis; Visible Light; SrTiO3; Polymerized Complex; La-Doping; N-Doping

1. Introduction

Recently, synthesis of strontium titanate based photo-

catalyst for converting visible light energy to photoreac-

tion has been a great attention [1-6]. Among the modify-

ing of SrTiO3, 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. How-

ever, substituting O2− by N3− will generate residual ani-

onic vacancies which suppress the photocatalytic activity

of SrTiO3. To solve this problem, co-doping of lantha-

num and nitrogen into SrTiO3, can decrease the band gap

energy without forming lattice defect and lattice strain,

and consequently lead to generate high visible light

photocatalytic activity. The La3+ can substitute Sr2+

without large lattice strain because of the similar ionic

radius.

Many researchers had developed the synthesis of La/N

co-doped SrTiO3 catalysts. Miyauchi et al. [7] reported

that the synthesis of La/N co-doped SrTiO3 catalysts us-

ing 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 co-

doped SrTiO3 catalysts using mechanochemical reaction

enhanced the photocatalytic activity in a visible light.

However, they had a large particle and low specific sur-

face 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 synthe-

sis 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 seg-

regation [12]. Based on this consideration, the fine parti-

cles of La-doped SrTiO3 could be synthesized by the

polymerized complex process and then followed by cal-

cinations in ammonia to obtain the La/N co-doped SrTiO3

nanoparticles.

In the present paper, we report the synthesis of La/N

co-doped SrTiO3 using polymerized complex method.

The lanthanum and nitrogen co-doping effectively nar-

rowed the band-gap energy of SrTiO3. The photocatalytic

activity of SrTiO3 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.

2. Experiment

2.1. Preparation of Catalyst

The La/N co-doped SrTiO3 with variation of lanthanum

doping was prepared by the polymerized complex method

U. SULAEMAN ET AL. 7

[9]. The Ti(OC3H7)4 and SrCl2·6H2O and La(NO3)3·6H2O

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 (Sr1−xLaxTiO3)

was grinded. The products of Sr1−xLaxTiO3 were then

nitrogenized by heating at 700˚C for 5 h under NH3 (flow

rate 400 ml/L). The as-prepared sample of Sr1-xLax-

TiO3-yNy 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, respec-

tively.

2.2. Characterization

The powder product was characterized by XRD (Shima-

dzu XD-D1) using graphite-monochromized CuKα ra-

diation. The mean crystallite size of the powders was

determined by the XRD-Scherrer equation [13]. Micro-

structure 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 spe-

cific surface area was determined by the nitrogen adsorp-

tion at 77 K (BET, Quantachrome NOVA 4200e). Bind-

ing energies of element were analyzed at room tempera-

ture by XPS (Perkin-Elmer PHI5600).

2.3. Photocatalytic Activity

The photocatalytic activities were evaluated using NOx

analyzer (Yanaco, ECL-88) [5]. A 450 W high-pressure

mercury arc was used as the light source. The wave-

length 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.

3. Result and Discussion

3.1. XRD Analysis

The XRD profiles of La/N co-doped SrTiO3 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 in-

tensity of diffraction decreased with increasing La dop-

ing, indicating that the crystalline properties was strongly

affected by La doping. The particle sizes of La/N co-

doped SrTiO3 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.

3.2. Morphology

Figure 2 shows the morphology of STN and STN-0.5.

The particle size of 40 - 60 nm in diameter could be ob-

served 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.

3.3. Uv-Vis Diffusion Reflectance Spectroscopy

Figure 3 shows the absorbance spectra of La/N co-doped

SrTiO3. The absorption edge shifted to higher of wave-

length, indicating the narrow band gap was generated by

La/N co-doping. The band-gap energies of La/N co-doped

10 20 3040 5060 70 80

2θ/degree

Intensity/a.u.

STN-0.25

STN-0.5

STN-0.7

LTN

STN

Figure 1. XRD patterns of La/N co-doped SrTiO3 samples

synthesized by the polyme rized co mple x me thod.

Table 1. The crystallite sizes, specific surface areas, Sr/Ti

and La/Ti atomic ratios from EDX of La/N co-doped

SrTiO3 synthesized by the polymer i zed complex me thod.

Sample Crystallite

Size (nm)

Specific Surface

Area (m2/g)

Atomic

Ratio Sr/Ti

Atomic

Ratio La/Ti

STN 52 17.30 1.076 -

STN-0.2539 25.75 0.757 0.218

STN-0.517 65.43 0.467 0.443

STN-0.723 30.36 0.280 0.653

LTN 18 58.25 - -

U. SULAEMAN ET AL.

Figure 2. TEM images of STN (a) and STN-0.5 (b) synthe-

sized by polymeri zed comple x method.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

200 300 400 500600 700 800

Wavelength/nm

Absorbance

STN-0.5

LTN

STN-0.7

STN-0.25

STN

Figure 3. DRS of La/N co-doped SrTiO3 synthesized using

polymer complex method.

SrTiO3 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 color 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, respec-

tively. 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 con-

duction 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.

3.4. XPS Analys is

Figure 4 shows XPS profiles of the STN-0.5 after sput-

tering at 3 kV for 3 minutes. The peak N1s could be ob-

served at 396.0 eV shown in Figure 4(a), indicating the

formation of nitrogen doped SrTiO3 [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 ex-

hibits 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 Ti4+ [17-19]. The peak of O1s was observed at

529.1 eV which is the characteristic of metal oxides [20].

3.5. Photocatalytic Activity

Figure 5 shows the photocatalytic activity of La/N

co-doped SrTiO3 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 sig-

nificant activity in visible light (λ > 510) for STN, pre-

sumably 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 va-

cancy 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 TiO2 (P-25) in the ultraviolet

light, i.e., 42.3% of NO could be destructed. The excel-

lent 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 attrib-

uted to low crystallinity of the sample.

4. Conclusion

Cubic perovskite of La/N co-doped SrTiO3 nanoparticles,

Sr1−xLaxTiO3−yNy (x = 0, 0.25, 0.5, 0.7 and 1), could be

U. SULAEMAN ET AL.

(a) (b)

Figure 4. XPS profiles of La/N co-doped SrTiO3 (STN-0.5) synthesized using the polymerized complex method.

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0510 15 20 2530

Time / min.

NO Concentration / ppm

LTN

STN-0.25

STN-0.7

P-25

STN

STN-0.5

λ>510nm λ>400n λ>290nm

Figure 5. The photocatalytic NO destruction activities of

La/N co-doped SrTiO3.

synthesized by the polymer complex (PC) method using

Ti(OC3H7)4, SrCl2·6H2O, and La(NO3)2·6H2O as starting

materials followed by calcinations in ammonia. Ethylene

glycol and anhydrous citric acid could be used as the

precursors of synthesis. The catalytic degradation of NO

over La/N co-doped SrTiO3 is significantly improved in

the presence of visible-light irradiation. The sample with

x = 0.5 is the highest photocatalytic activity for NO deg-

radation under visible light irradiation (λ > 510 nm).

5. Acknowledgements

This research was partly supported by the Management

Expenses Grants for National Universities Corporations

from the Ministry of Education, Culture, Sports, Science

for Technology of Japan (MEXT).

REFERENCES

[1] Y. Sun, J. Liu and Z. Li, “Design of Highly Ordered

Ag-SrTiO3 Nanotube Arrays for Photocatalytic Degrada-

tion of Methyl Orange,” Journal Solid State Chemistry,

Vol. 184, No. 8, 2011, pp. 1924-1930.

doi:10.1016/j.jssc.2011.05.037

[2] G. Wang, Y. Qin, J. Cheng and Y. Wang, “Influence of

Zn Doping on the Photocatalytic Property of SrTiO3,”

Journal of Fuel Chemistry and Technology, Vol. 38, No.

4, 2010, pp. 502-507.

doi:10.1016/S1872-5813(10)60042-4

[3] J. Wang, S. Yin, M. Komatsu, Q. Zhang, F. Saito and T.

U. SULAEMAN ET AL.

Sato, “Preparation and Characterization of Nitrogen Doped

SrTiO3 Photocatalyst,” Journal of Photochemistry and

Photobiology A: Chemistry, Vol. 165, No. 1-3, 2004, pp.

149-156. doi:10.1016/j.jphotochem.2004.02.022

[4] A. Jia, Z. Su, L. Lou and S. Liu, “Synthesis and Charac-

terization of Highly-Active Nickel and Lanthanum Co-

Doped SrTiO3,” Solid State Science, Vol. 12, No. 7, 2010,

pp. 1140-1145.

doi:10.1016/j.solidstatesciences.2010.04.005

[5] J. Wang, H. Li, H. Li, S. Yin and T. Sato, “Preparation

and Photocatalytic Activity of Visible Light-Active Sul-

fur and Nitrogen Co-Doped SrTiO3,” Solid State Science,

Vol. 11, No. 1, 2009, pp. 182-188.

doi:10.1016/j.solidstatesciences.2008.04.010

[6] T. Ohno, T. Tsubota, Y. Nakamura and K. Sayama, “Pre-

paration of S, C Cation-Codoped SrTiO3 and Its Photo-

catalytic Activity under Visible Light,” Applied Catalysis

A: General, Vol. 288, No. 1-2, 2005, pp. 74-79.

doi:10.1016/j.apcata.2005.04.035

[7] M. Miyauchi, M. Takashio and H. Tobimatsu, “Photo-

catalytic Activity of SrTiO3 Codoped with Nitrogen and

Lanthanum under Visible Light Illumination,” Langmuir,

Vol. 20, No. 1, 2004, pp. 232-236.

doi:10.1021/la0353125

[8] J. Wang, S. Yin, M. Komatsu and T. Sato, “Lanthanum

and Nitrogen Co-Doped SrTiO3 Powders as Visible Light

Sensitive Photocatalyst,” Journal of the European Ce-

ramic Society, Vol. 25, No. 13, 2005, pp. 3207-3212.

doi:10.1016/j.jeurceramsoc.2004.07.027

[9] A. Kasahara, K. Nukumizu, G. Hitoki, T. Takata, J.N.

Kondo, M. Hara, H. Kobayashi and K. Domen, “Photo-

reactions on LaTiO2N under Visible Light Irradiation,”

The Journal of Physical Chemistry A, Vol. 106, No. 29,

2002, pp. 6750-6753. doi:10.1021/jp025961+

[10] S. Farhikhteh, A. Maghsoudipour and B. Raissi, “Synthe-

sis of Nanocrystalline YSZ (ZrO2-8Y2O3) Powder by Po-

lymerized Complex Method,” Journal of Alloys and Com-

pounds, Vol. 491, No. 1-2, 2010, pp. 402-405.

doi:10.1016/j.jallcom.2009.10.196

[11] J. P. Zhao, M. H. Quan and L. Zhang, “Preparation of

Potassium Lithium Niobate Powders with Low Li Content

via the Pechini Method,” Ceramics International, Vol. 32,

No. 7, 2006, pp. 843-846.

doi:10.1016/j.ceramint.2005.05.010

[12] S. Qiu, H. Fan and X. Zheng, “Pb(Zr0.95Ti0.05)O3 Powders

Synthesized by Pechini Method: Effect of Molecular Weight

of Polyester on the Phase and Morphology,” Journal of

Sol-Gel Science and Technology, Vol. 42, No.1, 2007, pp.

21-26. doi:10.1007/s10971-006-1509-3

[13] B. Dcullity, “Elements of X-Ray Diffraction,” 3rd Edition,

Addison-Wesley Publishing Co., Boston, 1976.

[14] S. X. Liu, X. Y. Chen and X. Chen, “Preparation of

N-Doped Visible Light Response Nanosize TiO2 Photo-

catalyst Using the Acid-Catalyzed Hydrolysis Method,”

Chinese Journal of Catalysis, Vol. 27, No. 8, 2006, pp.

697-702. doi:10.1016/S1872-2067(06)60037-5

[15] D. C. Cronemeyer, “Infrared Absorption of Reduced Rutile

TiO2 Single Crystals,” Physical Review, Vol. 113, No. 5,

1959, pp. 1222-1226. doi:10.1103/PhysRev.113.1222

[16] A. J. Nelson, T. Buuren, T. M. Willey, C. Bostedt, J. J.

Adams, K. I. Schaffers, L. Terminello and T. A. Callcott,

“Electronic Structure of Lanthanum Calcium Oxoborate

LaCa4O(BO3)3,” Journal of Electron Spectroscopy and Re-

lated Phenomena, Vol. 137-140, No. 2004, pp. 541-546.

[17] S. Martınez-Mendez, Y. Henrıquez, O. Domınguez, L.

D’Ornelas and H. Krentzien, “Catalytic Properties of Sil-

ica Supported Titanium, Vanadium and Niobium Oxide

Nanoparticles towards the Oxidation of Saturated and

Unsaturated Hydrocarbons,” Journal of Molecular Ca-

talysis A: Chemical, Vol. 252, No. 1-2, 2006, pp. 226-

234. doi:10.1016/j.molcata.2006.02.041

[18] M. Ziolek and I. Nowak, “Characterization Techniques

Employed in the Study of Niobium and Tantalum-Con-

taining Materials,” Catalysis Today, Vol. 78, No. 1-4, 2003,

pp. 543-553. doi:10.1016/S0920-5861(02)00353-X

[19] R. E. Tanner, Y. Liang and E. I. Altman, “Structure and

Chemical Reactivity of Adsorbed Carboxylic Acids on

Anatase TiO2(0 0 1),” Surface Science, Vol. 506, No. 3,

2002, pp. 251-271. doi:10.1016/S0039-6028(02)01388-2

[20] M. Z. Atashbar, H. T. Sun, B. Gong, W. Wlodarski and R.

Lam, “XPS Study of Nb-Doped Oxygen Sensing TiO2

Thin Films Prepared by Sol-Gel Method,” Thin Solid Films,

Vol. 326, No. 1-2, 1998, pp. 238-244.

doi:10.1016/S0040-6090(98)00534-3