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Sol–gel combustion synthesis and visible-light-driven photocatalytic property of perovskite LaNiO3
Journal of Alloys and Compounds 491 (2010) 560–564
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Sol–gel combustion synthesis and visible-light-driven photocatalytic property of perovskite LaNiO3 Yuanyuan Li a , Shanshan Yao a , Wei Wen b , Lihong Xue a,∗ , Youwei Yan a a b
State Key Laboratory of Materials Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 9 September 2009 Received in revised form 26 October 2009 Accepted 29 October 2009 Available online 6 November 2009 Keywords: Calcination LaNiO3 Visible-light-driven photocatalysis Methyl orange
a b s t r a c t A photocatalyst LaNiO3 with perovskite structure was synthesized via a sol–gel combustion technique. The effect of calcination process on the crystallinity, structure and visible-light-driven photocatalytic property of LaNiO3 has been studied. The photocatalysts were characterized by XRD, SEM, BET and UV–vis diffuse reflectance spectrum. The results showed that a single perovskite phase of LaNiO3 was obtained by suitable calcination process. The powder had high surface area. The band-gap of LaNiO3 was 2.26 eV, possessing potential visible-light-induced photocatalytic activity. The photocatalytic activity of the LaNiO3 powders was evaluated by degradation of methyl orange in water under visible light irradiation. The photo-degradation of MO over LaNiO3 was a zero-order reaction and the rate constant k of the best sample in our experiments is 0.1520 (10−2 g L−1 h−1 ). The degradation percentage after 5 h on the sample was about 74.9%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the photoinduced decomposition of water on a TiO2 electrode was discovered by Fujishima and Honda [1], semiconductor photocatalysts have attracted great interest for their environmental applications [2–6]. To date, most investigations have focused on the photocatalyst TiO2 because of its high photocatalytic activity, good photostability, non-toxicity and low price. However, the large band-gap (3.2 eV) of TiO2 limits the usage efficiency of solar energy. Therefore, finding a novel visible-light-driven semiconductor is of great interest. As a native p-type semiconductor with pervoskite structure [7], LaNiO3 has wide applications such as ferroelectrics [8], catalytic combustion [9], conductive thin films [10] and electrode materials [11,12] due to its excellent electric and magnetic properties. Zhao et al. [13] first studied the photocatalytic activity of LaNiO3 for organic contaminants in water. They prepared the powder by chemical coprecipitation method and the results revealed that 90% azo blueuse could be degraded under UV light irradiation for 80 min with 50 mg LaNiO3 . Zhu et al. [14] synthesized LaNiO3 powders by solid-state reaction and found the photodegradation percentage of methyl orange after 160 min on the best sample under UV light irradiation reached to 80%. However, the photocatalytic activity of LaNiO3 under visible light has not been reported.
It is well known that the properties of materials are related with the preparation method. LaNiO3 powder is conventionally synthesized using solid-state reaction [15]. However, conventional solid-state reaction requires high calcining temperature, which easily induces sintering and aggregation in the particles. In the present study, we synthesized a single phase of LaNiO3 using the sol–gel combustion technique [16–18] and subsequent calcination process. The as-synthesized powders were used as catalysts for the photo-degradation of organic pollutants under visible light irradiation. The effect of the calcination process on the LaNiO3 powders and their photocatalytic property were investigated. 2. Experimental procedures 2.1. Preparation Analytical-grade lanthanum nitrate (La(NO3 )3 ·6H2 O), nickel nitrate (Ni(NO3 )2 ·6H2 O) and citric acid (C6 H8 O7 ·H2 O) were used as raw materials. Firstly, 0.02 mol Ni(NO3 )2 ·6H2 O, 0.02 mol La(NO3 )3 ·6H2 O and 0.1 mol citric acid were dissolved in 200 mL distilled water. The pH value of the mixed solution was adjusted to 7 by adding a small amount of liquor ammonia to obtain a transparent solution. Then, the solution was heated at 130 ◦ C for 2 h on a hot plate with continuous stirring for dehydration. During dehydration process, polycondensation reaction happened between citric acid and nitrates. A transparent gel formed and was transferred to an oven of 300 ◦ C. After a few minutes, the gel was ignited and burnt in a self-propagating combustion manner until the gel was completely burnt out to form a loose ash. Finally, the ash was calcined under different calcination processes. 2.2. Characterization
∗ Corresponding author. Tel.: +86 27 87543876; fax: +86 27 87541922. E-mail address: [email protected] (L. Xue). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.269
The thermal decomposition behavior of the gel was characterized by thermogravimetric and differential thermal analysis (DTA/TG, STA 409) at a heating
Y. Li et al. / Journal of Alloys and Compounds 491 (2010) 560–564 rate of 20 ◦ C/min in static air. The phase identification of the as-synthesized powders was performed using X-ray diffractometer (XRD, Philips PW 1710) with Cu K␣ radiation ( = 1.5405 Å). The surface area of the catalyst was measured by the Brunaue–Emmett–Teller (BET) method with nitrogen gas adsorption at 77 K (Micromeritics ASAP2100). The UV–vis diffuse reflectance spectra (DRS) of the samples were measured by a spectrophotometer (Hitachi U-3010). The obtained diffuse reflectance spectra were converted to absorption spectra on the basis of the Kubelka–Munk theory. The morphology of the powders was observed by scanning electron microscopy (SEM, SIRION 200). 2.3. Visible-light-induced photocatalytic activity evaluation The photocatalytic activities of LaNiO3 powders were evaluated by the decomposition of methyl orange (MO) in water. The photocatalytic degradation was performed in a glass beaker in which 0.4 g of LaNiO3 powders were suspended in 200 mL of MO solution (10 mg L−1 ). A 400 W xenon lamp with a maximum emission at 468 nm was used as a light source. A cutoff filter was used to remove wavelength shorter than 400 nm and to ensure that irradiation was achieved by the visible light wavelengths only. Prior to irradiation, the suspension was magnetically stirred for 30 min to establish adsorption/degradation equilibrium. The distance between the liquid surface and the light source was fixed 8 cm. During irradiation, 5 mL suspension was continually taken from the reaction cell at given irradiation time intervals and separated by centrifugation (4200 rpm, 15 min). The absorption spectrum of the centrifuged solution was measured on a UV–vis spectrophotometer (Hitachi U3010). The concentration of MO was determined by monitoring the change in the absorbance at 464 nm.
3. Results and discussion It is well known that the photocatalytic activity is concerned with the crystallinity and the particle size of the photocatalyst. XRD, SEM and BET were used to investigate the crystal structure, morphology and surface area of the as-prepared samples, respectively. The DTA and TG spectra were used to detect the combustion process of the precursor first. The simultaneous DTA/TG traces of the xerogel precursor in static air are shown in Fig. 1. It can be seen that there is one slight endothermic peak and two exothermic peaks in the DTA curve. The endothermic appeared at around 100 ◦ C accompanied by about 15% weight loss in the TG trace are due to the loss of residual water in the gel. The sharp exothermic peak at about 200 ◦ C accompanied by a drastic weight loss of 45% may be caused by the autocatalytic anionic oxidation–reduction reaction between the nitrates and citric acid. The slight exothermic peak appeared at 370 ◦ C, accompanied by a slight continued weight loss may be caused by the decomposition of the residual organic carbide. The XRD results show good agreement with the DTA and TG results. Fig. 2 shows the XRD patterns of powders calcined at different temperatures for 2 h. The precursor ash contains La2 O2 CO3 and NiO. These two phases are stable up to 500 ◦ C. When the calcination temperature is increased to 600 ◦ C, LaNiO3 is formed and
Fig. 1. DTA and TG traces of xerogel precursor.
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a very small amount of La2 NiO4 phase is also observed, implying the LaNiO3 phase can be obtained at 600 ◦ C. With further increasing the calcination temperature, the intensity of diffraction of the LaNiO3 increases and that of La2 NiO4 phase disappears gradually. A pure LaNiO3 phase can be obtained above 700 ◦ C. The crystalline size of the as-synthesized powder estimated from the X-ray peak broadening of the (1 1 0) diffraction peak via the Scherrer formula. The calculated crystallite sizes of the powders calcined at 600, 700, 800 and 900 ◦ C are 23.1, 32.8, 34.3 and 47.2 nm, respectively. The morphology and the particle size of LaNiO3 were modified by changing the calcination temperature, which could be found from the SEM and BET results. Fig. 3 shows the SEM micrographs of the as-prepared powders at different calcination temperatures. The ash has bonding and aggregation structure, indicating large undecomposed organic components exist in the powder. After calcination at 500 ◦ C for 2 h, some aggregated particles appear. Combining with the DTA/TG and XRD results, it can be concluded that the aggregated particles consist of La2 O2 CO3 , NiO and a small amount of remaining organic matter. Increasing the calcination temperature to 600 ◦ C, fine particles are found (Fig. 3c). Further increasing the temperature, the particle size increases and agglomerates with the formation of different shapes, which is composed of fine primary particles. The BET measurement shows that the surface area of the as-prepared powders calcined at 600, 700, 800, and 900 ◦ C are 14.1, 12.7, 11.8 and 6.5 m2 /g, respectively. The influences of calcination time on the phase and the morphology of LaNiO3 powders are shown in Figs. 4 and 5. With increasing the calcination time, the diffraction intensity of the LaNiO3 phase increases and that of La2 NiO4 phase disappear, indicating a single phase LaNiO3 can be synthesized by prolonging the calcination time at 600 ◦ C. Meanwhile, from the SEM micrographs (Fig. 5), it can be found that when the calcination time increases, the remaining organic substance disappear gradually. Further increasing the calcination time, the LaNiO3 particles interconnect with each other and interfuse to the bigger particles. The BET surface area of the LaNiO3 powder calcined at 600 ◦ C for 4 and 6 h are 15.1 and 12.2 m2 /g, respectively. The above results indicate that LaNiO3 with single perovskite phase and high surface area can be obtained at a relatively low temperature and a suitable calcination time. Fig. 6 shows the Kubelka–Munk conversion spectra of the LaNiO3 powders synthesized at 600 and 900 ◦ C for 2 h. The result shows that the spectra have less difference between these two samples. The fine distinction between the spectra may be caused by
Fig. 2. XRD patterns of as-synthesized powders calcined at different temperatures for 2 h.
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Y. Li et al. / Journal of Alloys and Compounds 491 (2010) 560–564
Fig. 3. SEM micrographs of as-prepared powders calcined at different temperatures: (a) ash, (b) 500 ◦ C, (c) 600 ◦ C, (d) 700 ◦ C, (e) 800 ◦ C and (f) 900 ◦ C.
the impurity La2 NiO4 phase. The absorption edges extend to visible light region, indicating the LaNiO3 powder can markedly absorb the light in the range of visible light. The band-gap of LaNiO3 powders synthesized at 600 ◦ C for 2 h is evaluated by the extrapolation
Fig. 4. XRD patterns of as-prepared powders calcined at 600 ◦ C for different times.
method, which is shown in the inset of Fig. 6. The position of the absorption edge is determined by the interception of the straight line fitted through the low-energy side of the curve (F(R)·h)2 versus h, where F(R) is the Kubelka–Munk function and h is the energy of the incident photon. The sample shows a band-gapnarrowing energy at about 2.26 eV, which can effectively absorb visible light. The calculated value of band-gap has slight difference with Ref. [19] (2.42 eV). In order to further investigate the visible-light-induced photocatalytic activity of LaNiO3 powder, the photoreduction of MO solution in the presence of as-synthesized LaNiO3 powders under visible light irradiation were carried out. Figs. 7 and 8 show the temporal evolution of the concentration (C/C0 ) of MO, in which C0 and C represent the initial equilibrium concentration and reaction concentration of MO, respectively. As it can be seen, all the catalysts except for the sample calcined at 500 ◦ C exhibit activity in the degradation of MO solution under visible light. The concentration of MO linearly decreases with the increasing of radiation time, which means the photo-degradation of MO over LaNiO3 powders could be described as zero-order reaction. The values of the photo-degradation rate constant k (10−2 g L−1 h−1 ) are also shown in Figs. 7 and 8. The photocatalytic activity of LaNiO3 can be evaluated by using the value of k. The bigger the value of k, the higher the photocatalytic activity.
Y. Li et al. / Journal of Alloys and Compounds 491 (2010) 560–564
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Fig. 6. Kubelka–Munk conversion spectra of the samples calcined at 600 and 900 ◦ C for 2 h. Inset: band-gap energy of LaNiO3 calcined at 600 ◦ C for 2 h.
Fig. 7. Photocatalytic degradation of MO solution (10 mg L−1 ) under visible light ( > 400 nm) for 5 h on LaNiO3 powders prepared at different calcination temperatures for 2 h. The inset shows the value of the photo-degradation rate constant k (10−2 mg L−1 h−1 ).
Fig. 8 shows the photocatalytic activities of the LaNiO3 particles synthesized at 600 ◦ C for different times. With the increase of the calcination time, the photocatalytic activity increases firstly and then decreases. The highest photocatalytic activity is obtained at
Fig. 5. SEM micrographs of as-prepared powders calcined at 600 ◦ C for different times: (a) 2 h, (b) 4 h and (c) 6 h.
The photocatalytic activities of the photocatalysts are highly corresponding to the surface structure and crystallinity. It can be found in Fig. 7 that with the increase of calcination temperature, the photocatalytic activities of LaNiO3 first increase and then decrease. From the XRD results (Fig. 2), with the increase of the calcination temperature, the impurity phase disappears and the crystallinity of LaNiO3 increases, which is benefit to the photocatalytic properties. However, from the SEM (Fig. 3) and BET results, the particle size and the surface area of the photocatalyst decrease with the calcination temperature increases, which leads to the lower absorptive capacity of MO on the surface of LaNiO3 particles.
Fig. 8. Photocatalytic degradation of MO (10 mg L−1 ) under visible light ( > 400 nm) for 5 h on LaNiO3 powders prepared at 600 ◦ C for different times. The inset shows the value of the photo-degradation rate constant k (10−2 mg L−1 h−1 ).
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4 h. The mechanism is the same with the sample calcined at different temperatures. Therefore, to enhance the photocatalytic activity, LaNiO3 had better calcined at a low temperature for a suitable time. The significant temporal changes in the concentration of the MO clearly indicate the degradation of MO in the presence of LaNiO3 powders under visible light irradiation. Photocatalytic processes are based on electron/hole pairs generated by means of bandgap radiation, which can give rise to redox reactions with species absorbed on the surface of the catalysts. The LaNiO3 powders with a narrow band-gap (2.26 eV) can absorb visible light to generate charge carriers and lead to oxidization of the MO molecules. The rate constant k of the best sample in our experiments is 0.1520. The degradation percentage after 5 h on the sample was about 74.9%. 4. Conclusion Nanocrystalline LaNiO3 powders were synthesized by the sol–gel combustion method and subsequent calcination process. The crystallinity, structure and photocatalytic property of the LaNiO3 powders can be modified easily by changing the calcination temperature and time. The LaNiO3 powders exhibit a good activity in the degradation of MO under visible light irradiation, indicating that the material is not only responsive but also activated in the visible light range. The photo-degradation of MO over LaNiO3 is a zero-order reaction. The rate constant k of the best sample in our experiments is 0.1520. The degradation percentage after 5 h on the sample is about 74.9%.
Acknowledgment This work is financially supported by the National Natural Science Foundation of China, NNSFC 50574042. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [2] S. Kohtani, M. Tomohiro, K. Tokumura, R. Nakagaki, Appl. Catal. B: Environ. 58 (2005) 265–272. [3] S. Kohtani, M. Koshiko, A. Kudo, Appl. Catal. B: Environ. 46 (2003) 573–586. [4] S.C. Zhang, C. Zhang, Y. Man, Y.F. Zhu, J. Solid State Chem. 179 (2006) 62–69. [5] Z.G. Zou, J.H. Ye, H. Arakawa, J. Mater. Res. 17 (2002) 1419–1424. [6] M. Machida, J. Yabunaka, T. Kijima, S. Matsushima, M. Arai, Int. J. Inorg. Mater. 3 (2001) 545–550. [7] T.R. Ling, Z.B. Chen, M.D. Lee, Appl. Catal. A: Gen. 136 (1996) 191–203. [8] M. Detalle, D. Remiens, J. Cryst. Growth 310 (2008) 3596–3603. [9] G. Valderrama, A. Kiennemann, M.R. Goldwasser, Catal. Today 133 (2008) 142–148. [10] D.H. Bao, X. Yao, N. Wakiya, K. Shinozaki, N. Mizutani, J. Phys. D: Appl. Phys. 36 (2003) 1217–1221. [11] P. Delobelle, G.S. Wang, E. Fribourg-Blanc, D. Remiens, J. Eur. Ceram. Soc. 27 (2007) 223–230. [12] H. Miyazaki, H. Suzuki, T. Naoe, Y. Suyama, T. Ota, M. Fuji, M. Takahashi, Ferroelectrics 335 (2006) 51–59. [13] X.H. Zhao, D.P. Chen, X.D. Lou, Q.T. Cheng, Y. Lu, Ziran Kexueban. 33 (2005) 69–72. [14] J. Zhu, N. Zhang, J.L. Zhong, S.S. Tong, Gongye Cuihua 14 (2006) 48–51. [15] J. Echigoya, S. Hiratsuka, H. Suto, Mater. Trans. 30 (1989) 789–799. [16] Y.Y. Li, S.S. Yao, L.H. Xue, Y.W. Yan, J. Mater. Sci. 44 (2009) 4455–4459. [17] M.K. Shobana, S. Sankar, V. Rajendran, J. Alloy. Compd. 472 (2009) 421–424. [18] A. Mali, A. Ataie, Scripta Mater. 53 (2005) 1065–1070. [19] L.S. Jia, J.J. Li, W.P. Fang, J. Alloy Compd. (2009), doi:10.1016/ j.jallcom.2009.09.104.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Sol–gel combustion synthesis and visible-light-driven photocatalytic property of perovskite LaNiO3 Yuanyuan Li a , Shanshan Yao a , Wei Wen b , Lihong Xue a,∗ , Youwei Yan a a b
State Key Laboratory of Materials Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 9 September 2009 Received in revised form 26 October 2009 Accepted 29 October 2009 Available online 6 November 2009 Keywords: Calcination LaNiO3 Visible-light-driven photocatalysis Methyl orange
a b s t r a c t A photocatalyst LaNiO3 with perovskite structure was synthesized via a sol–gel combustion technique. The effect of calcination process on the crystallinity, structure and visible-light-driven photocatalytic property of LaNiO3 has been studied. The photocatalysts were characterized by XRD, SEM, BET and UV–vis diffuse reflectance spectrum. The results showed that a single perovskite phase of LaNiO3 was obtained by suitable calcination process. The powder had high surface area. The band-gap of LaNiO3 was 2.26 eV, possessing potential visible-light-induced photocatalytic activity. The photocatalytic activity of the LaNiO3 powders was evaluated by degradation of methyl orange in water under visible light irradiation. The photo-degradation of MO over LaNiO3 was a zero-order reaction and the rate constant k of the best sample in our experiments is 0.1520 (10−2 g L−1 h−1 ). The degradation percentage after 5 h on the sample was about 74.9%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the photoinduced decomposition of water on a TiO2 electrode was discovered by Fujishima and Honda [1], semiconductor photocatalysts have attracted great interest for their environmental applications [2–6]. To date, most investigations have focused on the photocatalyst TiO2 because of its high photocatalytic activity, good photostability, non-toxicity and low price. However, the large band-gap (3.2 eV) of TiO2 limits the usage efficiency of solar energy. Therefore, finding a novel visible-light-driven semiconductor is of great interest. As a native p-type semiconductor with pervoskite structure [7], LaNiO3 has wide applications such as ferroelectrics [8], catalytic combustion [9], conductive thin films [10] and electrode materials [11,12] due to its excellent electric and magnetic properties. Zhao et al. [13] first studied the photocatalytic activity of LaNiO3 for organic contaminants in water. They prepared the powder by chemical coprecipitation method and the results revealed that 90% azo blueuse could be degraded under UV light irradiation for 80 min with 50 mg LaNiO3 . Zhu et al. [14] synthesized LaNiO3 powders by solid-state reaction and found the photodegradation percentage of methyl orange after 160 min on the best sample under UV light irradiation reached to 80%. However, the photocatalytic activity of LaNiO3 under visible light has not been reported.
It is well known that the properties of materials are related with the preparation method. LaNiO3 powder is conventionally synthesized using solid-state reaction [15]. However, conventional solid-state reaction requires high calcining temperature, which easily induces sintering and aggregation in the particles. In the present study, we synthesized a single phase of LaNiO3 using the sol–gel combustion technique [16–18] and subsequent calcination process. The as-synthesized powders were used as catalysts for the photo-degradation of organic pollutants under visible light irradiation. The effect of the calcination process on the LaNiO3 powders and their photocatalytic property were investigated. 2. Experimental procedures 2.1. Preparation Analytical-grade lanthanum nitrate (La(NO3 )3 ·6H2 O), nickel nitrate (Ni(NO3 )2 ·6H2 O) and citric acid (C6 H8 O7 ·H2 O) were used as raw materials. Firstly, 0.02 mol Ni(NO3 )2 ·6H2 O, 0.02 mol La(NO3 )3 ·6H2 O and 0.1 mol citric acid were dissolved in 200 mL distilled water. The pH value of the mixed solution was adjusted to 7 by adding a small amount of liquor ammonia to obtain a transparent solution. Then, the solution was heated at 130 ◦ C for 2 h on a hot plate with continuous stirring for dehydration. During dehydration process, polycondensation reaction happened between citric acid and nitrates. A transparent gel formed and was transferred to an oven of 300 ◦ C. After a few minutes, the gel was ignited and burnt in a self-propagating combustion manner until the gel was completely burnt out to form a loose ash. Finally, the ash was calcined under different calcination processes. 2.2. Characterization
∗ Corresponding author. Tel.: +86 27 87543876; fax: +86 27 87541922. E-mail address: [email protected] (L. Xue). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.269
The thermal decomposition behavior of the gel was characterized by thermogravimetric and differential thermal analysis (DTA/TG, STA 409) at a heating
Y. Li et al. / Journal of Alloys and Compounds 491 (2010) 560–564 rate of 20 ◦ C/min in static air. The phase identification of the as-synthesized powders was performed using X-ray diffractometer (XRD, Philips PW 1710) with Cu K␣ radiation ( = 1.5405 Å). The surface area of the catalyst was measured by the Brunaue–Emmett–Teller (BET) method with nitrogen gas adsorption at 77 K (Micromeritics ASAP2100). The UV–vis diffuse reflectance spectra (DRS) of the samples were measured by a spectrophotometer (Hitachi U-3010). The obtained diffuse reflectance spectra were converted to absorption spectra on the basis of the Kubelka–Munk theory. The morphology of the powders was observed by scanning electron microscopy (SEM, SIRION 200). 2.3. Visible-light-induced photocatalytic activity evaluation The photocatalytic activities of LaNiO3 powders were evaluated by the decomposition of methyl orange (MO) in water. The photocatalytic degradation was performed in a glass beaker in which 0.4 g of LaNiO3 powders were suspended in 200 mL of MO solution (10 mg L−1 ). A 400 W xenon lamp with a maximum emission at 468 nm was used as a light source. A cutoff filter was used to remove wavelength shorter than 400 nm and to ensure that irradiation was achieved by the visible light wavelengths only. Prior to irradiation, the suspension was magnetically stirred for 30 min to establish adsorption/degradation equilibrium. The distance between the liquid surface and the light source was fixed 8 cm. During irradiation, 5 mL suspension was continually taken from the reaction cell at given irradiation time intervals and separated by centrifugation (4200 rpm, 15 min). The absorption spectrum of the centrifuged solution was measured on a UV–vis spectrophotometer (Hitachi U3010). The concentration of MO was determined by monitoring the change in the absorbance at 464 nm.
3. Results and discussion It is well known that the photocatalytic activity is concerned with the crystallinity and the particle size of the photocatalyst. XRD, SEM and BET were used to investigate the crystal structure, morphology and surface area of the as-prepared samples, respectively. The DTA and TG spectra were used to detect the combustion process of the precursor first. The simultaneous DTA/TG traces of the xerogel precursor in static air are shown in Fig. 1. It can be seen that there is one slight endothermic peak and two exothermic peaks in the DTA curve. The endothermic appeared at around 100 ◦ C accompanied by about 15% weight loss in the TG trace are due to the loss of residual water in the gel. The sharp exothermic peak at about 200 ◦ C accompanied by a drastic weight loss of 45% may be caused by the autocatalytic anionic oxidation–reduction reaction between the nitrates and citric acid. The slight exothermic peak appeared at 370 ◦ C, accompanied by a slight continued weight loss may be caused by the decomposition of the residual organic carbide. The XRD results show good agreement with the DTA and TG results. Fig. 2 shows the XRD patterns of powders calcined at different temperatures for 2 h. The precursor ash contains La2 O2 CO3 and NiO. These two phases are stable up to 500 ◦ C. When the calcination temperature is increased to 600 ◦ C, LaNiO3 is formed and
Fig. 1. DTA and TG traces of xerogel precursor.
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a very small amount of La2 NiO4 phase is also observed, implying the LaNiO3 phase can be obtained at 600 ◦ C. With further increasing the calcination temperature, the intensity of diffraction of the LaNiO3 increases and that of La2 NiO4 phase disappears gradually. A pure LaNiO3 phase can be obtained above 700 ◦ C. The crystalline size of the as-synthesized powder estimated from the X-ray peak broadening of the (1 1 0) diffraction peak via the Scherrer formula. The calculated crystallite sizes of the powders calcined at 600, 700, 800 and 900 ◦ C are 23.1, 32.8, 34.3 and 47.2 nm, respectively. The morphology and the particle size of LaNiO3 were modified by changing the calcination temperature, which could be found from the SEM and BET results. Fig. 3 shows the SEM micrographs of the as-prepared powders at different calcination temperatures. The ash has bonding and aggregation structure, indicating large undecomposed organic components exist in the powder. After calcination at 500 ◦ C for 2 h, some aggregated particles appear. Combining with the DTA/TG and XRD results, it can be concluded that the aggregated particles consist of La2 O2 CO3 , NiO and a small amount of remaining organic matter. Increasing the calcination temperature to 600 ◦ C, fine particles are found (Fig. 3c). Further increasing the temperature, the particle size increases and agglomerates with the formation of different shapes, which is composed of fine primary particles. The BET measurement shows that the surface area of the as-prepared powders calcined at 600, 700, 800, and 900 ◦ C are 14.1, 12.7, 11.8 and 6.5 m2 /g, respectively. The influences of calcination time on the phase and the morphology of LaNiO3 powders are shown in Figs. 4 and 5. With increasing the calcination time, the diffraction intensity of the LaNiO3 phase increases and that of La2 NiO4 phase disappear, indicating a single phase LaNiO3 can be synthesized by prolonging the calcination time at 600 ◦ C. Meanwhile, from the SEM micrographs (Fig. 5), it can be found that when the calcination time increases, the remaining organic substance disappear gradually. Further increasing the calcination time, the LaNiO3 particles interconnect with each other and interfuse to the bigger particles. The BET surface area of the LaNiO3 powder calcined at 600 ◦ C for 4 and 6 h are 15.1 and 12.2 m2 /g, respectively. The above results indicate that LaNiO3 with single perovskite phase and high surface area can be obtained at a relatively low temperature and a suitable calcination time. Fig. 6 shows the Kubelka–Munk conversion spectra of the LaNiO3 powders synthesized at 600 and 900 ◦ C for 2 h. The result shows that the spectra have less difference between these two samples. The fine distinction between the spectra may be caused by
Fig. 2. XRD patterns of as-synthesized powders calcined at different temperatures for 2 h.
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Fig. 3. SEM micrographs of as-prepared powders calcined at different temperatures: (a) ash, (b) 500 ◦ C, (c) 600 ◦ C, (d) 700 ◦ C, (e) 800 ◦ C and (f) 900 ◦ C.
the impurity La2 NiO4 phase. The absorption edges extend to visible light region, indicating the LaNiO3 powder can markedly absorb the light in the range of visible light. The band-gap of LaNiO3 powders synthesized at 600 ◦ C for 2 h is evaluated by the extrapolation
Fig. 4. XRD patterns of as-prepared powders calcined at 600 ◦ C for different times.
method, which is shown in the inset of Fig. 6. The position of the absorption edge is determined by the interception of the straight line fitted through the low-energy side of the curve (F(R)·h)2 versus h, where F(R) is the Kubelka–Munk function and h is the energy of the incident photon. The sample shows a band-gapnarrowing energy at about 2.26 eV, which can effectively absorb visible light. The calculated value of band-gap has slight difference with Ref. [19] (2.42 eV). In order to further investigate the visible-light-induced photocatalytic activity of LaNiO3 powder, the photoreduction of MO solution in the presence of as-synthesized LaNiO3 powders under visible light irradiation were carried out. Figs. 7 and 8 show the temporal evolution of the concentration (C/C0 ) of MO, in which C0 and C represent the initial equilibrium concentration and reaction concentration of MO, respectively. As it can be seen, all the catalysts except for the sample calcined at 500 ◦ C exhibit activity in the degradation of MO solution under visible light. The concentration of MO linearly decreases with the increasing of radiation time, which means the photo-degradation of MO over LaNiO3 powders could be described as zero-order reaction. The values of the photo-degradation rate constant k (10−2 g L−1 h−1 ) are also shown in Figs. 7 and 8. The photocatalytic activity of LaNiO3 can be evaluated by using the value of k. The bigger the value of k, the higher the photocatalytic activity.
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Fig. 6. Kubelka–Munk conversion spectra of the samples calcined at 600 and 900 ◦ C for 2 h. Inset: band-gap energy of LaNiO3 calcined at 600 ◦ C for 2 h.
Fig. 7. Photocatalytic degradation of MO solution (10 mg L−1 ) under visible light ( > 400 nm) for 5 h on LaNiO3 powders prepared at different calcination temperatures for 2 h. The inset shows the value of the photo-degradation rate constant k (10−2 mg L−1 h−1 ).
Fig. 8 shows the photocatalytic activities of the LaNiO3 particles synthesized at 600 ◦ C for different times. With the increase of the calcination time, the photocatalytic activity increases firstly and then decreases. The highest photocatalytic activity is obtained at
Fig. 5. SEM micrographs of as-prepared powders calcined at 600 ◦ C for different times: (a) 2 h, (b) 4 h and (c) 6 h.
The photocatalytic activities of the photocatalysts are highly corresponding to the surface structure and crystallinity. It can be found in Fig. 7 that with the increase of calcination temperature, the photocatalytic activities of LaNiO3 first increase and then decrease. From the XRD results (Fig. 2), with the increase of the calcination temperature, the impurity phase disappears and the crystallinity of LaNiO3 increases, which is benefit to the photocatalytic properties. However, from the SEM (Fig. 3) and BET results, the particle size and the surface area of the photocatalyst decrease with the calcination temperature increases, which leads to the lower absorptive capacity of MO on the surface of LaNiO3 particles.
Fig. 8. Photocatalytic degradation of MO (10 mg L−1 ) under visible light ( > 400 nm) for 5 h on LaNiO3 powders prepared at 600 ◦ C for different times. The inset shows the value of the photo-degradation rate constant k (10−2 mg L−1 h−1 ).
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4 h. The mechanism is the same with the sample calcined at different temperatures. Therefore, to enhance the photocatalytic activity, LaNiO3 had better calcined at a low temperature for a suitable time. The significant temporal changes in the concentration of the MO clearly indicate the degradation of MO in the presence of LaNiO3 powders under visible light irradiation. Photocatalytic processes are based on electron/hole pairs generated by means of bandgap radiation, which can give rise to redox reactions with species absorbed on the surface of the catalysts. The LaNiO3 powders with a narrow band-gap (2.26 eV) can absorb visible light to generate charge carriers and lead to oxidization of the MO molecules. The rate constant k of the best sample in our experiments is 0.1520. The degradation percentage after 5 h on the sample was about 74.9%. 4. Conclusion Nanocrystalline LaNiO3 powders were synthesized by the sol–gel combustion method and subsequent calcination process. The crystallinity, structure and photocatalytic property of the LaNiO3 powders can be modified easily by changing the calcination temperature and time. The LaNiO3 powders exhibit a good activity in the degradation of MO under visible light irradiation, indicating that the material is not only responsive but also activated in the visible light range. The photo-degradation of MO over LaNiO3 is a zero-order reaction. The rate constant k of the best sample in our experiments is 0.1520. The degradation percentage after 5 h on the sample is about 74.9%.
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