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Improved hydrogen evolution activities under visible light irradiation over NaTaO3 codoped with lanthanum and chromium
Materials Chemistry and Physics 121 (2010) 506–510
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Improved hydrogen evolution activities under visible light irradiation over NaTaO3 codoped with lanthanum and chromium Ming Yang a , Xianli Huang a , Shicheng Yan a,b , Zhaosheng Li a,b , Tao Yu a,c,∗ , Zhigang Zou a,b,c a b c
Eco-materials and Renewable Energy Research Center (ERERC), Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 25 December 2009 Accepted 9 February 2010 Keywords: Inorganic compound Sintering XPS Photocatalysis
a b s t r a c t Na1−x Lax Ta1−x Crx O3 and NaTa1−x Crx O3 (x = 0.01, 0.03, 0.05 and 0.10) have been synthesized by a solid state reaction method. These photocatalysts can produce H2 in the presence of methanol under visible light irradiation ( > 420 nm). The photocatalytic activities of Na1−x Lax Ta1−x Crx O3 are much higher than those of NaTa1−x Crx O3 , respectively. Especially, the H2 evolution rate of Na0.9 La0.1 Ta0.9 Cr0.1 O3 is 2.2 mol h−1 , which is nearly 4 times higher than that of NaTa0.9 Cr0.1 O3 (0.6 mol h−1 ). The improved activities of Na1−x Lax Ta1−x Crx O3 compared with NaTa1−x Crx O3 can be ascribed to two factors: one is smaller particle size and higher specific surface area which is caused by the doping of lanthanum; the other is that Na1−x Lax Ta1−x Crx O3 has less Cr6+ , which is induced by codoping of lanthanum and chromium. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Much attention has been paid to hydrogen production since photo-induced decomposition of water was discovered by Fujishima and Honda in 1972 [1]. Remarkable progress has been made in developing photocatalysts, such as NaTaO3 [2–6], K2 La2 Ti3 O10 [7], and SrTiO3 [8]. Among these photocatalysts, NaTaO3 is studied intensively for its high photocatalytic activity for water splitting into H2 and O2 in a stoichiometric amount [2]. However, these photocatalysts can only work under ultraviolet light, which accounts for only 4% of solar energy. By contrast, visible light accounts for more than 40%, so visible-light-driven photocatalysts have attracted more and more attention in recent years [9–13]. So far, doping has been proved to be a feasible method to get visible-light-driven photocatalysts. Generally, doping can adjust the band structure of semiconductors [9,10,14], and it can also have an impact on the particle size and surface structure of materials, which usually results in a better photocatalytic activity [2]. In the past, much effort has been devoted to photocatalysts doped by only one element with different valence from the hosts [9,10,14], which usually show poor photocatalytic activity. A possible reason is that defects, for example oxygen vacancy, usually form and function as recombination center of photo-generated electrons and holes [14]. Codoping may be a promising method to improve
∗ Corresponding author at: 22 Hankou Road, Nanjing, People’s Republic of China. E-mail address: [email protected] (T. Yu). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.02.015
the activity of photocatalysts, and there have been a few literatures on this method. For instance, Kato and Kudo found that SrTiO3 codoped with antimony and chromium showed a quite high H2 production activity, which was attributed to the charge balance kept by codoping of Sb5+ and Cr3+ [15]. However, there is lack of systemic researches on the effect of doping manners on photocatalytic activities. In the present study, codoped-NaTaO3 (NaTaO3 doped with lanthanum and chromium: Na1−x Lax Ta1−x Crx O3 ) and monodopedNaTaO3 (NaTaO3 doped with chromium: NaTa1−x Crx O3 ) were synthesized to investigate the effect of doping manners on photocatalytic activities. These photocatalysts were found for the first time to have the ability to produce H2 in the presence of methanol under visible light irradiation ( > 420 nm); furthermore, the photocatalytic activities of Na1−x Lax Ta1−x Crx O3 are much higher than those of NaTa1−x Crx O3 when x varies from 0.01 to 0.1. A possible mechanism for the improved activity of codoped samples is proposed. 2. Experimental 2.1. Synthesis of samples Solid state reaction was used to synthesize the powder samples of Na1−x Lax Ta1−x Crx O3 and NaTa1−x Crx O3 . The starting materials, Na2 CO3 , La2 O3 , Ta2 O5 and Cr2 O3 , in the corresponding atom ratios were mixed carefully by milling in a mortar with addition of ethanol, and 5 mol% excess of sodium was added to compensate for volatilization. The mixed powders were first calcined at 900 ◦ C for 1 h and then calcined at 1150 ◦ C for 10 h with intermediate grinding using an alumina crucible and a muffle furnace in air.
M. Yang et al. / Materials Chemistry and Physics 121 (2010) 506–510
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Fig. 1. X-ray diffraction patterns of doped NaTaO3 (x = 0.03). The inset shows a magnified X-ray diffraction peak of (1 0 1).
2.2. Characterization Powder X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (Ultima III; Rigaku Corp., Japan) using Cu K␣ radiation. The UV–vis diffuse reflectance spectra were measured on a UV–vis spectrophotometer (UV-2550; Shimadzu Corp., Japan) and were converted to absorbance spectrum by the KubelkaMunk method. The morphologies of the samples were determined by a field emission scanning electron microscope (JSM-7000F; JEOL, Japan). Specific surface area was measured using the Brunauer–Emmett–Teller (BET) method (TriStar-3000; Micromeritics, U.S.A.) by nitrogen absorption at 77 K. Concentration of chromium in the supernatant was analyzed on an ICP-AES (Optima-5300DV, PE, U.S.A.). The valence state of Cr was analyzed on an X-ray photoelectron spectroscope (ESCALAB 250; Thermo, U.S.A.). 2.3. Photocatalytic activities Photocatalytic H2 evolutions were conducted with CH3 OH as sacrificial agent. Cocatalyst Pt (0.5 wt%) was loaded on the photocatalyst surface by an in situ photodeposition method. 0.5 g of photocatalyst was suspended in 220 mL ultra pure water and 50 mL methanol by magnetic stirring in an outer irradiation Pyrex glass cell. A 300 W xenon arc lamp was focused on the side window of the cell through a long-pass cutoff filter (420 nm, L42, HOYA). The reaction cell was connected to a closed gas circulation system. The evolved H2 was analyzed by a gas chromatography with a thermal conductivity detector and molecular sieve 5 Å columns (GC-8A, Shimadzu Corp., Japan).
3. Results and discussion 3.1. Powder X-ray diffraction analysis Fig. 1 shows XRD patterns of pure and doped (x = 0.03) NaTaO3 powder samples. The samples synthesized by solid state reaction are assigned to be orthorhombic [4] (JCPDS 73-0878). It can be seen that both Na1−x Lax Ta1−x Crx O3 and NaTa1−x Crx O3 have nearly the identical crystal structure as pure NaTaO3 , no crystalline phase involving chromium oxides and lanthanum oxides can be observed. The reason is probably as follows: ionic radii of 12-coordinated Na+ (1.39 Å) and La3+ (1.36 Å) are almost the same [2], while the ionic radii of six-coordinated Ta5+ (0.64 Å) [2] and Cr3+ (0.61 Å) [16,17] are similar, so substitution of Na+ by La3+ and Ta5+ by Cr3+ would not cause much strain in the crystalline lattice. XRD patterns for other doped samples have also been studied (Fig. S1, see Supporting Information). From these results, it can be found that single phase samples can be synthesized in a wide range of doping concentration. The inset of Fig. 1 shows a magnified image of the XRD patterns for (1 0 1) diffraction peak of the samples. It is noticed that the peaks of the doped NaTaO3 shift to lower degree comparing with that of pure NaTaO3 , confirming that at least a part of La and Cr are doped
Fig. 2. UV–vis diffuse reflectance spectra of: (a) pure and doped NaTaO3 (x = 0.03); (b) Na1−x Lax Ta1−x Crx O3 (x = 0.01, 0.03, 0.05 and 0.10), the inset shows a magnified image of spectra from 550 to 750 nm.
into the crystal lattice of NaTaO3 . The shift of the peak to a lower angle may be like this: although the majority of La takes part of Na, a part of La which is six-coordinated will substitute Ta, but the ionic radius of the six-coordinated La3+ ion (1.032 Å) [2] is remarkably larger than that of Ta5+ ion (0.64 Å) [2], thus the unit cell will expand distinctly, and thus the small shift to a lower angle is observed.
3.2. UV–vis diffuse reflectance spectra The absorption spectra of investigated samples are shown in Fig. 2. It can be seen from Fig. 2(a) that two types of absorption are generated in the visible region when Cr is doped, and the band gap absorption of NaTaO3 with an absorption edge around 310 nm (4.00 eV) is not affected. Comparing spectra of NaTaO3 , NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 , it can be concluded that visible light response of NaTaO3 is derived from the doping of Cr, but not from the doping of La. In Fig. 2(b), the absorption edge of 420 nm is ascribed to the charge transfer from Cr3+ 3d orbital to Ta5+ 5d orbital [14], while broad absorption ranging from 550 to 750 nm is ascribed to a d-d transition of 4 A2 → 4 T2 in Cr3+ ions in octahedral systems [18]. The samples also show an intermediate absorption peak near 370 nm, and as the Cr content increases, the absorption peak became flat, which is induced by the larger absorption of Cr-containing species for example LaCrO3 [14].
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Fig. 3. (a) Comparison of hydrogen evolving activity between NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 ; (b) Time dependence in hydrogen evolving of (䊉) Na0.97 La0.03 Ta0.97 Cr0.03 O3 and () NaTa0.97 Cr0.03 O3 under visible light irradiation ( > 420 nm).
3.3. Photocatalytic performance Fig. 3(a) shows the comparison of hydrogen evolving activity (average rate) between NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 . Under the irradiation of visible light ( > 420 nm), pure NaTaO3 and La-doped NaTaO3 show no activity for water splitting. However, when Cr is doped, photocatalytic activities driven by visible light are obtained. When La and Cr are doped simultaneously, improved activity is obtained in all the cases when x takes the values from 0.01 to 0.10. Especially, when x takes the value of 0.10, the activity of Na1−x Lax Ta1−x Crx O3 (2.2 mol h−1 ) is nearly 4 times higher than that of NaTa1−x Crx O3 (0.6 mol h−1 ). Time dependence of hydrogen evolving of Na0.97 La0.03 Ta0.97 Cr0.03 O3 and NaTa0.97 Cr0.03 O3 is shown in Fig. 3(b). Three cycles of reaction were carried out. It can be found that hydrogen evolves steadily during the reaction; no decline of activity has been detected during the span of time (about 40 h). The H2 evolution rate of Na0.97 La0.03 Ta0.97 Cr0.03 O3 is about 4.0 mol h−1 , which is comparable with some photocatalysts reported before [12,14,16]. Time dependences of hydrogen evolving of the other samples are shown in Fig. S2 (see Supporting Information). All these results reflect the stability of the photocatalysts. It can be found that with the increase of doping concentration, the photocatalytic activities will firstly increase and then decrease: when x takes the value of 0.03, both NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 show the highest activity (2.8 and
Fig. 4. SEM images of doped NaTaO3 (b) NaTa1−x Crx O3 .
(x = 0.03). (a) Na1−x Lax Ta1−x Crx O3 ;
4.0 mol h−1 , respectively). The reason is probably like this: when the doping concentration is low, the samples will absorb more visible light with the increase of Cr (Fig. 3), and the particles’ size will become smaller with the increase of La and hence samples will have higher specific surface areas (see Section 3.4); when the doping concentration is high, defects for example oxygen vacancy usually form and function as recombination center of photo-generated electron and holes [14], which results in poor photocatalytic activity. 3.4. Particle size and specific surface area analysis The SEM images of doped NaTaO3 (x = 0.03) are shown in Fig. 4. Although particles with large size for about 300–400 nm have been found in Fig. 4(a), the majority of particles have smaller size of about 100 nm; while in Fig. 4(b), the majority of particles have the size of about 400–600 nm. The doping of La results in smaller particle size, which can be ascribed to that lanthanum localizing near the surface of crystal grain prevents the growth of crystal [2]. As is well known that when particle size becomes smaller, samples usually have larger specific surface area, which is also confirmed in Table 1. A higher specific surface area can supply more active sites, thus usually shows better photocatalytic activity [19]. So the improved activity of Na1−x Lax Ta1−x Crx O3 can be partly attributed to the larger specific surface area. 3.5. XPS analysis The oxidation state of chromium in NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 (x = 0.03) was investigated by XPS. As indi-
M. Yang et al. / Materials Chemistry and Physics 121 (2010) 506–510 Table 1 BET specific surface area of doped samples. Value of x
0.01 0.03 0.05 0.10
BET specific surface area (m2 g−1 ) NaTa1−x Crx O3
Na1−x Lax Ta1−x Crx O3
1.1 1.5 2.8 2.6
3.7 4.0 3.5 3.8
cated in Fig. 5, the binding energy for the major peak of Na1−x Lax Ta1−x Crx O3 is 579.1 eV, while the binding energy for the major peak of NaTa1−x Crx O3 is 579.6 eV (the binding energy was corrected by taking a C1s level as 284.6 eV). Furthermore, the shape of XPS spectra of the two samples is different, so a curve fitting was performed to analyze the quantitative composition of the spectra. As depicted in Fig. 5(a), the raw spectrum of Na1−x Lax Ta1−x Crx O3 can be decomposed into two spectra. One locates at 577.1 eV, and the other locates at 579.2 eV. These two spectra correspond to Cr3+ and Cr6+ [16,20,21], respectively, and the area of Cr6+ to Cr3+ is 2:1. While in Fig. 5(b), the raw spectrum of NaTa1−x Crx O3 can also be separated into two spectra. One locates at 576.3 eV, which corresponds to Cr3+ ; and the other locating at 579.4 eV is the binding energy of Cr6+ . Being different from the case of Na1−x Lax Ta1−x Crx O3 , the ratio of Cr6+ to Cr3+ in NaTa1−x Crx O3 is about 13:1, which is much larger than that of Na1−x Lax Ta1−x Crx O3 , indicating much
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more Cr6+ exists in NaTa1−x Crx O3 , and this fact simply shows the superiority of codoping method. One thing has to be pointed out here is that Cr6+ is a toxic element; it will be a problem of affecting the environment when Cr6+ leaches from the photocatalyst. ICP-AES tests were conducted to detect the concentration of sodium and chromium in the supernatant of Na0.97 La0.03 Ta0.97 Cr0.03 O3 . Concentration of sodium is 12.7 g mL−1 (sodium ions have usually been found to elute from the surface of NaTaO3 [2]), which means that this photocatalyst is indissolvable. No chromium can be detected in the solution, which means that no Cr6+ leaches from the photocatalyst, and this result confirms the stability of the photocatalyst further. The reason why there is more Cr6+ in the NaTa1−x Crx O3 samples can be explained by the formula bellow: 3Cr2 O3 −→4Cr•Ta (VI) + 2CrTa (III) + 9O0
(1)
In formula (1), Cr•Ta (VI) represents that a defect shows one unit positive charge when a Cr6+ substitutes the site of Ta5+ , and CrTa (III) denotes that another defect shows two unit negative charge when a Cr3+ substitutes the site of Ta5+ . In order to keep the charge balance, Cr6+ and Cr3+ may form simultaneously and the ratio of Cr6+ to Cr3+ is 2: 1, which explains why more Cr6+ exists, as was depicted in the XPS spectrum. The whole point of codoping La3+ for Na+ and Cr3+ for Ta5+ will not induce any other defects, however, Cr6+ has been found in Na1−x Lax Ta1−x Crx O3 (Fig. 5a). Cr6+ at the surface layer was formed in the synthesizing process, because Cr3+ is easily oxidized into Cr6+ in oxygen atmosphere. It is known that Cr6+ cations can be reduced to Cr3+ by obtaining three electrons, and these parts of electrons will not reduce water, so when there is too much Cr6+ , the catalyst usually shows very poor activity [16]. So the differences in the valence of Cr can partly explain the differences in water splitting activity. 4. Conclusions In summary, codoped-NaTaO3 (NaTaO3 doped with lanthanum and chromium: Na1−x Lax Ta1−x Crx O3 ) and monodoped-NaTaO3 (NaTaO3 doped with chromium: NaTa1−x Crx O3 ) showed intense visible light absorption, and these photocatalysts can produce H2 in the presence of methanol under visible light irradiation ( > 420 nm). The photocatalytic activities of Na1−x Lax Ta1−x Crx O3 are much higher than those of NaTa1−x Crx O3 when x varies from 0.01 to 0.1. The present study proves that codoping is a possible method to improve the activity of photocatalyst and also provides some guideline to design visible-light-driven photocatalysts. Acknowledgments The authors would like to acknowledge financial support from the National Natural Science Foundation of China (nos. 50732004, 10874077, and 20773064), the Jiangsu Provincial Natural Science Foundation (no. BK2008252), as well as the National Basic Research Program of China (973 Program, grant nos. 2007CB613301 and 2007CB613305). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2010.02.015. References
Fig. 5. X-ray photoelectron spectra of Cr 2p of doped NaTaO3 (x = 0.03). (a) Na1−x Lax Ta1−x Crx O3 ; (b) NaTa1−x Crx O3 .
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[4] C.C. Hu, H. Teng, Appl. Catal. A 331 (2007) 44. [5] A. Iwase, H. Kato, A. Kudo, Chem. Sus. Chem. 2 (2009) 873. [6] M. Maruyama, A. Iwase, H. Kato, A. Kudo, H. Onishi, J. Phys. Chem. C 113 (2009) 13918. [7] T. Takata, K. Shinohara, A. Tanaka, M. Hara, J.N. Kondo, K. Domen, J. Photochem. Photobiol. A 106 (1997) 45. [8] K. Domen, A. Kudo, T. Onishi, J. Phys. Chem. 90 (1986) 292. [9] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [10] Z.G. Zou, J.H. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625. [11] K. Maeda, K. Teramura, D.L. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Nature 440 (2006) 295. [12] Z.G. Yi, J.H. Ye, Appl. Phys. Lett. 91 (2007) 254108.
[13] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76. [14] D.W. Hwang, H.G. Kim, J.S. Lee, J. Kim, W. Li, S.H. Oh, J. Phys. Chem. B 109 (2005) 2093. [15] H. Kato, A. Kudo, J. Phys. Chem. B 106 (2002) 5029. [16] D.F. Wang, J.H. Ye, T. Kako, T. Kimura, J. Phys. Chem. B 110 (2006) 15824. [17] M. Ari, P.M. Jardim, B.A. Marinkovic, F. Rizzo, F.F. Ferreira, J. Solid State Chem. 181 (2008) 1472. [18] N. Serpone, D. Lawless, Langmuir 10 (1994) 643. [19] G.H. Tian, H.G. Fu, L.Q. Jing, B.F. Xin, K. Pan, J. Phys. Chem. C 112 (2008) 3083. [20] J.B. Yin, X.P. Zhao, Chem. Mater. 16 (2004) 321. [21] C.C. Pan, J.C.S. Wu, Mater. Chem. Phys. 100 (2006) 102.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Improved hydrogen evolution activities under visible light irradiation over NaTaO3 codoped with lanthanum and chromium Ming Yang a , Xianli Huang a , Shicheng Yan a,b , Zhaosheng Li a,b , Tao Yu a,c,∗ , Zhigang Zou a,b,c a b c
Eco-materials and Renewable Energy Research Center (ERERC), Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 25 December 2009 Accepted 9 February 2010 Keywords: Inorganic compound Sintering XPS Photocatalysis
a b s t r a c t Na1−x Lax Ta1−x Crx O3 and NaTa1−x Crx O3 (x = 0.01, 0.03, 0.05 and 0.10) have been synthesized by a solid state reaction method. These photocatalysts can produce H2 in the presence of methanol under visible light irradiation ( > 420 nm). The photocatalytic activities of Na1−x Lax Ta1−x Crx O3 are much higher than those of NaTa1−x Crx O3 , respectively. Especially, the H2 evolution rate of Na0.9 La0.1 Ta0.9 Cr0.1 O3 is 2.2 mol h−1 , which is nearly 4 times higher than that of NaTa0.9 Cr0.1 O3 (0.6 mol h−1 ). The improved activities of Na1−x Lax Ta1−x Crx O3 compared with NaTa1−x Crx O3 can be ascribed to two factors: one is smaller particle size and higher specific surface area which is caused by the doping of lanthanum; the other is that Na1−x Lax Ta1−x Crx O3 has less Cr6+ , which is induced by codoping of lanthanum and chromium. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Much attention has been paid to hydrogen production since photo-induced decomposition of water was discovered by Fujishima and Honda in 1972 [1]. Remarkable progress has been made in developing photocatalysts, such as NaTaO3 [2–6], K2 La2 Ti3 O10 [7], and SrTiO3 [8]. Among these photocatalysts, NaTaO3 is studied intensively for its high photocatalytic activity for water splitting into H2 and O2 in a stoichiometric amount [2]. However, these photocatalysts can only work under ultraviolet light, which accounts for only 4% of solar energy. By contrast, visible light accounts for more than 40%, so visible-light-driven photocatalysts have attracted more and more attention in recent years [9–13]. So far, doping has been proved to be a feasible method to get visible-light-driven photocatalysts. Generally, doping can adjust the band structure of semiconductors [9,10,14], and it can also have an impact on the particle size and surface structure of materials, which usually results in a better photocatalytic activity [2]. In the past, much effort has been devoted to photocatalysts doped by only one element with different valence from the hosts [9,10,14], which usually show poor photocatalytic activity. A possible reason is that defects, for example oxygen vacancy, usually form and function as recombination center of photo-generated electrons and holes [14]. Codoping may be a promising method to improve
∗ Corresponding author at: 22 Hankou Road, Nanjing, People’s Republic of China. E-mail address: [email protected] (T. Yu). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.02.015
the activity of photocatalysts, and there have been a few literatures on this method. For instance, Kato and Kudo found that SrTiO3 codoped with antimony and chromium showed a quite high H2 production activity, which was attributed to the charge balance kept by codoping of Sb5+ and Cr3+ [15]. However, there is lack of systemic researches on the effect of doping manners on photocatalytic activities. In the present study, codoped-NaTaO3 (NaTaO3 doped with lanthanum and chromium: Na1−x Lax Ta1−x Crx O3 ) and monodopedNaTaO3 (NaTaO3 doped with chromium: NaTa1−x Crx O3 ) were synthesized to investigate the effect of doping manners on photocatalytic activities. These photocatalysts were found for the first time to have the ability to produce H2 in the presence of methanol under visible light irradiation ( > 420 nm); furthermore, the photocatalytic activities of Na1−x Lax Ta1−x Crx O3 are much higher than those of NaTa1−x Crx O3 when x varies from 0.01 to 0.1. A possible mechanism for the improved activity of codoped samples is proposed. 2. Experimental 2.1. Synthesis of samples Solid state reaction was used to synthesize the powder samples of Na1−x Lax Ta1−x Crx O3 and NaTa1−x Crx O3 . The starting materials, Na2 CO3 , La2 O3 , Ta2 O5 and Cr2 O3 , in the corresponding atom ratios were mixed carefully by milling in a mortar with addition of ethanol, and 5 mol% excess of sodium was added to compensate for volatilization. The mixed powders were first calcined at 900 ◦ C for 1 h and then calcined at 1150 ◦ C for 10 h with intermediate grinding using an alumina crucible and a muffle furnace in air.
M. Yang et al. / Materials Chemistry and Physics 121 (2010) 506–510
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Fig. 1. X-ray diffraction patterns of doped NaTaO3 (x = 0.03). The inset shows a magnified X-ray diffraction peak of (1 0 1).
2.2. Characterization Powder X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (Ultima III; Rigaku Corp., Japan) using Cu K␣ radiation. The UV–vis diffuse reflectance spectra were measured on a UV–vis spectrophotometer (UV-2550; Shimadzu Corp., Japan) and were converted to absorbance spectrum by the KubelkaMunk method. The morphologies of the samples were determined by a field emission scanning electron microscope (JSM-7000F; JEOL, Japan). Specific surface area was measured using the Brunauer–Emmett–Teller (BET) method (TriStar-3000; Micromeritics, U.S.A.) by nitrogen absorption at 77 K. Concentration of chromium in the supernatant was analyzed on an ICP-AES (Optima-5300DV, PE, U.S.A.). The valence state of Cr was analyzed on an X-ray photoelectron spectroscope (ESCALAB 250; Thermo, U.S.A.). 2.3. Photocatalytic activities Photocatalytic H2 evolutions were conducted with CH3 OH as sacrificial agent. Cocatalyst Pt (0.5 wt%) was loaded on the photocatalyst surface by an in situ photodeposition method. 0.5 g of photocatalyst was suspended in 220 mL ultra pure water and 50 mL methanol by magnetic stirring in an outer irradiation Pyrex glass cell. A 300 W xenon arc lamp was focused on the side window of the cell through a long-pass cutoff filter (420 nm, L42, HOYA). The reaction cell was connected to a closed gas circulation system. The evolved H2 was analyzed by a gas chromatography with a thermal conductivity detector and molecular sieve 5 Å columns (GC-8A, Shimadzu Corp., Japan).
3. Results and discussion 3.1. Powder X-ray diffraction analysis Fig. 1 shows XRD patterns of pure and doped (x = 0.03) NaTaO3 powder samples. The samples synthesized by solid state reaction are assigned to be orthorhombic [4] (JCPDS 73-0878). It can be seen that both Na1−x Lax Ta1−x Crx O3 and NaTa1−x Crx O3 have nearly the identical crystal structure as pure NaTaO3 , no crystalline phase involving chromium oxides and lanthanum oxides can be observed. The reason is probably as follows: ionic radii of 12-coordinated Na+ (1.39 Å) and La3+ (1.36 Å) are almost the same [2], while the ionic radii of six-coordinated Ta5+ (0.64 Å) [2] and Cr3+ (0.61 Å) [16,17] are similar, so substitution of Na+ by La3+ and Ta5+ by Cr3+ would not cause much strain in the crystalline lattice. XRD patterns for other doped samples have also been studied (Fig. S1, see Supporting Information). From these results, it can be found that single phase samples can be synthesized in a wide range of doping concentration. The inset of Fig. 1 shows a magnified image of the XRD patterns for (1 0 1) diffraction peak of the samples. It is noticed that the peaks of the doped NaTaO3 shift to lower degree comparing with that of pure NaTaO3 , confirming that at least a part of La and Cr are doped
Fig. 2. UV–vis diffuse reflectance spectra of: (a) pure and doped NaTaO3 (x = 0.03); (b) Na1−x Lax Ta1−x Crx O3 (x = 0.01, 0.03, 0.05 and 0.10), the inset shows a magnified image of spectra from 550 to 750 nm.
into the crystal lattice of NaTaO3 . The shift of the peak to a lower angle may be like this: although the majority of La takes part of Na, a part of La which is six-coordinated will substitute Ta, but the ionic radius of the six-coordinated La3+ ion (1.032 Å) [2] is remarkably larger than that of Ta5+ ion (0.64 Å) [2], thus the unit cell will expand distinctly, and thus the small shift to a lower angle is observed.
3.2. UV–vis diffuse reflectance spectra The absorption spectra of investigated samples are shown in Fig. 2. It can be seen from Fig. 2(a) that two types of absorption are generated in the visible region when Cr is doped, and the band gap absorption of NaTaO3 with an absorption edge around 310 nm (4.00 eV) is not affected. Comparing spectra of NaTaO3 , NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 , it can be concluded that visible light response of NaTaO3 is derived from the doping of Cr, but not from the doping of La. In Fig. 2(b), the absorption edge of 420 nm is ascribed to the charge transfer from Cr3+ 3d orbital to Ta5+ 5d orbital [14], while broad absorption ranging from 550 to 750 nm is ascribed to a d-d transition of 4 A2 → 4 T2 in Cr3+ ions in octahedral systems [18]. The samples also show an intermediate absorption peak near 370 nm, and as the Cr content increases, the absorption peak became flat, which is induced by the larger absorption of Cr-containing species for example LaCrO3 [14].
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Fig. 3. (a) Comparison of hydrogen evolving activity between NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 ; (b) Time dependence in hydrogen evolving of (䊉) Na0.97 La0.03 Ta0.97 Cr0.03 O3 and () NaTa0.97 Cr0.03 O3 under visible light irradiation ( > 420 nm).
3.3. Photocatalytic performance Fig. 3(a) shows the comparison of hydrogen evolving activity (average rate) between NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 . Under the irradiation of visible light ( > 420 nm), pure NaTaO3 and La-doped NaTaO3 show no activity for water splitting. However, when Cr is doped, photocatalytic activities driven by visible light are obtained. When La and Cr are doped simultaneously, improved activity is obtained in all the cases when x takes the values from 0.01 to 0.10. Especially, when x takes the value of 0.10, the activity of Na1−x Lax Ta1−x Crx O3 (2.2 mol h−1 ) is nearly 4 times higher than that of NaTa1−x Crx O3 (0.6 mol h−1 ). Time dependence of hydrogen evolving of Na0.97 La0.03 Ta0.97 Cr0.03 O3 and NaTa0.97 Cr0.03 O3 is shown in Fig. 3(b). Three cycles of reaction were carried out. It can be found that hydrogen evolves steadily during the reaction; no decline of activity has been detected during the span of time (about 40 h). The H2 evolution rate of Na0.97 La0.03 Ta0.97 Cr0.03 O3 is about 4.0 mol h−1 , which is comparable with some photocatalysts reported before [12,14,16]. Time dependences of hydrogen evolving of the other samples are shown in Fig. S2 (see Supporting Information). All these results reflect the stability of the photocatalysts. It can be found that with the increase of doping concentration, the photocatalytic activities will firstly increase and then decrease: when x takes the value of 0.03, both NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 show the highest activity (2.8 and
Fig. 4. SEM images of doped NaTaO3 (b) NaTa1−x Crx O3 .
(x = 0.03). (a) Na1−x Lax Ta1−x Crx O3 ;
4.0 mol h−1 , respectively). The reason is probably like this: when the doping concentration is low, the samples will absorb more visible light with the increase of Cr (Fig. 3), and the particles’ size will become smaller with the increase of La and hence samples will have higher specific surface areas (see Section 3.4); when the doping concentration is high, defects for example oxygen vacancy usually form and function as recombination center of photo-generated electron and holes [14], which results in poor photocatalytic activity. 3.4. Particle size and specific surface area analysis The SEM images of doped NaTaO3 (x = 0.03) are shown in Fig. 4. Although particles with large size for about 300–400 nm have been found in Fig. 4(a), the majority of particles have smaller size of about 100 nm; while in Fig. 4(b), the majority of particles have the size of about 400–600 nm. The doping of La results in smaller particle size, which can be ascribed to that lanthanum localizing near the surface of crystal grain prevents the growth of crystal [2]. As is well known that when particle size becomes smaller, samples usually have larger specific surface area, which is also confirmed in Table 1. A higher specific surface area can supply more active sites, thus usually shows better photocatalytic activity [19]. So the improved activity of Na1−x Lax Ta1−x Crx O3 can be partly attributed to the larger specific surface area. 3.5. XPS analysis The oxidation state of chromium in NaTa1−x Crx O3 and Na1−x Lax Ta1−x Crx O3 (x = 0.03) was investigated by XPS. As indi-
M. Yang et al. / Materials Chemistry and Physics 121 (2010) 506–510 Table 1 BET specific surface area of doped samples. Value of x
0.01 0.03 0.05 0.10
BET specific surface area (m2 g−1 ) NaTa1−x Crx O3
Na1−x Lax Ta1−x Crx O3
1.1 1.5 2.8 2.6
3.7 4.0 3.5 3.8
cated in Fig. 5, the binding energy for the major peak of Na1−x Lax Ta1−x Crx O3 is 579.1 eV, while the binding energy for the major peak of NaTa1−x Crx O3 is 579.6 eV (the binding energy was corrected by taking a C1s level as 284.6 eV). Furthermore, the shape of XPS spectra of the two samples is different, so a curve fitting was performed to analyze the quantitative composition of the spectra. As depicted in Fig. 5(a), the raw spectrum of Na1−x Lax Ta1−x Crx O3 can be decomposed into two spectra. One locates at 577.1 eV, and the other locates at 579.2 eV. These two spectra correspond to Cr3+ and Cr6+ [16,20,21], respectively, and the area of Cr6+ to Cr3+ is 2:1. While in Fig. 5(b), the raw spectrum of NaTa1−x Crx O3 can also be separated into two spectra. One locates at 576.3 eV, which corresponds to Cr3+ ; and the other locating at 579.4 eV is the binding energy of Cr6+ . Being different from the case of Na1−x Lax Ta1−x Crx O3 , the ratio of Cr6+ to Cr3+ in NaTa1−x Crx O3 is about 13:1, which is much larger than that of Na1−x Lax Ta1−x Crx O3 , indicating much
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more Cr6+ exists in NaTa1−x Crx O3 , and this fact simply shows the superiority of codoping method. One thing has to be pointed out here is that Cr6+ is a toxic element; it will be a problem of affecting the environment when Cr6+ leaches from the photocatalyst. ICP-AES tests were conducted to detect the concentration of sodium and chromium in the supernatant of Na0.97 La0.03 Ta0.97 Cr0.03 O3 . Concentration of sodium is 12.7 g mL−1 (sodium ions have usually been found to elute from the surface of NaTaO3 [2]), which means that this photocatalyst is indissolvable. No chromium can be detected in the solution, which means that no Cr6+ leaches from the photocatalyst, and this result confirms the stability of the photocatalyst further. The reason why there is more Cr6+ in the NaTa1−x Crx O3 samples can be explained by the formula bellow: 3Cr2 O3 −→4Cr•Ta (VI) + 2CrTa (III) + 9O0
(1)
In formula (1), Cr•Ta (VI) represents that a defect shows one unit positive charge when a Cr6+ substitutes the site of Ta5+ , and CrTa (III) denotes that another defect shows two unit negative charge when a Cr3+ substitutes the site of Ta5+ . In order to keep the charge balance, Cr6+ and Cr3+ may form simultaneously and the ratio of Cr6+ to Cr3+ is 2: 1, which explains why more Cr6+ exists, as was depicted in the XPS spectrum. The whole point of codoping La3+ for Na+ and Cr3+ for Ta5+ will not induce any other defects, however, Cr6+ has been found in Na1−x Lax Ta1−x Crx O3 (Fig. 5a). Cr6+ at the surface layer was formed in the synthesizing process, because Cr3+ is easily oxidized into Cr6+ in oxygen atmosphere. It is known that Cr6+ cations can be reduced to Cr3+ by obtaining three electrons, and these parts of electrons will not reduce water, so when there is too much Cr6+ , the catalyst usually shows very poor activity [16]. So the differences in the valence of Cr can partly explain the differences in water splitting activity. 4. Conclusions In summary, codoped-NaTaO3 (NaTaO3 doped with lanthanum and chromium: Na1−x Lax Ta1−x Crx O3 ) and monodoped-NaTaO3 (NaTaO3 doped with chromium: NaTa1−x Crx O3 ) showed intense visible light absorption, and these photocatalysts can produce H2 in the presence of methanol under visible light irradiation ( > 420 nm). The photocatalytic activities of Na1−x Lax Ta1−x Crx O3 are much higher than those of NaTa1−x Crx O3 when x varies from 0.01 to 0.1. The present study proves that codoping is a possible method to improve the activity of photocatalyst and also provides some guideline to design visible-light-driven photocatalysts. Acknowledgments The authors would like to acknowledge financial support from the National Natural Science Foundation of China (nos. 50732004, 10874077, and 20773064), the Jiangsu Provincial Natural Science Foundation (no. BK2008252), as well as the National Basic Research Program of China (973 Program, grant nos. 2007CB613301 and 2007CB613305). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2010.02.015. References
Fig. 5. X-ray photoelectron spectra of Cr 2p of doped NaTaO3 (x = 0.03). (a) Na1−x Lax Ta1−x Crx O3 ; (b) NaTa1−x Crx O3 .
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