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CeO2 heterojunction photocatalyst for the degradation of Acid Orange 7 under visible light irradiation
Catalysis Communications 12 (2011) 794–797
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Preparation of Cu2O/CeO2 heterojunction photocatalyst for the degradation of Acid Orange 7 under visible light irradiation Shichao Hu, Feng Zhou, Lingzhi Wang ⁎, Jinlong Zhang ⁎ Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, People's Republic of China
a r t i c l e
i n f o
Article history: Received 11 August 2010 Received in revised form 23 January 2011 Accepted 25 January 2011 Available online 4 February 2011 Keywords: Cu2O CeO2 Heterojunction Photocatalyst
a b s t r a c t Cu2O/CeO2 heterojunction photocatalysts with different concentrations of Cu2O were prepared by a soakreduction method using glucose as the reductant, and characterized by XRD, EDS, XPS and UV-vis DRS spectroscopy. The photocatalytic efficiency of the Cu2O/CeO2 composite was evaluated by degradation of Acid Orange 7 in water under visible light (λ N 420 nm) irradiation. The Cu2O/CeO2 composite showed stronger visible light absorption capacity and higher photocatalytic activity than pure CeO2. The sample with a Cu2O / CeO2 molar ratio of 0.029 presented the best photocatalytic activity, which was 20% higher than that of pure CeO2. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, cerium oxide has been extensively studied and employed for various applications including fast ion conductors, oxygen storage capacitors and catalysts. In particular, CeO2 has some properties like titania such as wide band gap, nontoxicity and high stability. As for pure CeO2, it has been investigated under UV irradiation concerning water-splitting for the generation of hydrogen gas [1] and photodegradation of toluene in the gas phase [2]. Our group has recently reported the photocatalytic behavior of CeO2 for the degradation of Acid Orange 7 (AO7) under visible light irradiation [3]. However, CeO2 showed low degradation rate and good adsorption ability to AO7, which is attributed to the low light absorption efficiency of CeO2. Many strategies are commonly used to extend the spectral photoresponse of wide band gap semiconductors like titania to visible light region such as doping with transition metals [4], dye sensitization [5] and anionic doping [6]. Besides, another promising idea is to couple them with narrow band gap semiconductors. In this configuration, several advantages can be obtained: (1) an improvement of charge separation; (2) an increase in the lifetime of the charge carrier and (3) an enhancement of the interfacial charge transfer efficiency to adsorbed substrate [7]. Cuprous oxide (Cu2O) is a p-type semiconductor having a direct band gap of 2.0 eV, which has been previously studied for application in solar energy converting devices [8]. As reported by Li et al. [9], the Ce4f band potential of CeO2 is similar to TiO2, which is lower than that
⁎ Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (J. Zhang). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.01.027
of Cu2O, and the electron can be theoretically injected from Cu2O to the conduction band of CeO2. Therefore, we proposed that Cu2O is suitable for CeO2 as sensitizer due to the energy band match. In this study, nanocrystal Cu2O loaded CeO2 was fabricated via a soakreduction method to form a novel Cu2O/CeO2 heterojunction composite as a visible light-driven photocatalyst. This new coupled system, to our knowledge, was reported for the first time. Compared with pure CeO2 sample, the coupled photocatalyst presented better visible light absorption capacity and photocatalytic activity to AO7. 2. Experimentals 2.1. Preparation of Catalysts Cu2O/CeO2 heterojunction photocatalysts were prepared by a soak-reduction method using glucose as the reducing agent. Typically, a certain amount CuCl2 was dissolved in 50 mL distilled water to form different concentration solutions, which is 0.01, 0.04, 0.06 and 0.08 M, respectively. Then 0.6 g CeO2 prepared by direct calcination of cerium nitrate at 723 K for 6 h was dispersed in above solutions and treated with ulrasound to obtain uniform a suspension. The suspension was vigorously stirred at room temperature for 1 h to reach the adsorption equilibrium. After stirring, the mixtures were centrifuged and washed by deionized water for several times to dispose of free Cu2+ ions in solutions. The mixture was added into 40 mL 1 M glucose solution to form a suspension and then heated to 60 °C in water bath under stirring. Then 20 mL 2 M NaOH was slowly added into the suspension until the color turned from green to pale yellow. After 15 min stirring, the products were collected by filtration, washed with deionized water for several times and dried under vacuum for 4 h at 60 °C.
S. Hu et al. / Catalysis Communications 12 (2011) 794–797
2.2. Characterizations of Photocatalysts The obtained samples were characterized by X-ray powder diffraction (XRD), energy dispersive X-ray spectrum (EDS), UV-visible absorption spectra and X-ray photoelectron spectroscopy (XPS). Details are given in the Supplementary Material. 2.3. Photocatalytic Activity Test The visible light photocatalytic activity test is similar to our previous report [3] and has been detailed in the supporting information. In this test, an aqueous suspension of AO7 (60 mL, 35 mg/L) and 0.06 g photocatalyst powder were applied. The change on the concentration of AO7 after irradiation was analyzed by UV-vis absorption spectra. The starting point of the concentration–time plot was collected after the suspension was stirred for 1 h in darkness to reach the adsorption equilibrium. C0 and Ct referred to the equilibrated and the original dye concentrations. 3. Results and Discussion 3.1. Characterization of the Cu2O/CeO2 Composites Fig. 1 displays XRD patterns of Cu2O/CeO2 composite photocatalysts prepared with different concentrations of CuCl2. From Fig. 1, however, we can find all the diffraction peaks only match the standard data for CeO2 and no additional phases such as Cu2O, CuO or Cu can be detected in all the samples. Since only very small amount of Cu2+ can be adsorbed on CeO2 after centrifuged from CuCl2 solution, which may be attributed to the relative low surface area of CeO2 particle prepared by direct calcination with Ce(NO3)3, the absence of detection can be explained as that cuprous oxide particles are so tiny (less than 3 nm) and beyond the detection limit of XRD [10,11]. To illustrate that the soak-reduction method can actually form Cu2O crystal, we prepared sample without the disposing of free Cu2+ in the solution. As seen from the XRD pattern shown in Fig. S1, the diffraction peak attributed to Cu2O can be obviously observed. However, this composite show much more severe dark adsorption than pure CeO2, which has negligible photocatalytic activity. As the cuprous oxide cannot be detected by XRD, EDS were employed to confirm the presence and content of cuprous oxide in the Cu2O/CeO2 composites. Table 1 shows the EDS data of Cu2O/CeO2 composites prepared with different concentrations of CuCl2. Only the elements of O, Ce and Cu can be observed and the molar ratios of Cu2O
Fig. 1. XRD patterns of the Cu2O/CeO2 composites prepared with different concentrations of CuCl2 solutions: (a) pure CeO2, (b) 0.01 M, (c) 0.04 M, (d) 0.06 M and (e) 0.08 M.
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Table 1 Elemental composition and BET surface area of Cu2O/CeO2 heterojunction photocatalyst prepared with different concentration of CuCl2. Samplea
Ce (Wt %)
Cu (Wt %)
O (Wt %)
Cu2O/CeO2b
BET surface area (m2/g)
1 2 3 4 5
– 88.89 88.27 87.34 86.52
– 0.99 1.26 2.31 2.50
– 10.12 10.47 10.35 10.98
– 0.012 0.016 0.029 0.032
63 47 54 53 56
a Samples are prepared with different concentrations of CuCl2: (1) 0, (2) 0.01, (3) 0.04, (4)0.06 and (5) 0.08 M. b Molar ratio.
to CeO2 in the composites prepared with different concentrations of CuCl2 (0.01, 0.04, 0.06 and 0.08 M) are calculated to be 0.012, 0.016, 0.029 and 0.032, which increases with the concentration of CuCl2. The result also reveals the content of cuprous oxide dispersed on CeO2 particle is so small that it is hard to highly crystallize at the reduction stage. It can be inferred that Cu2O probably forms nanocluster and attaches to the CeO2 surface. XPS was employed to confirm that Cu species on the surface of CeO2 particle exist as +1 valence state. Fig. 2 (A) shows the survey spectra of the sample prepared in 0.06 M CuCl2 solution. As shown in the survey spectra, element Cu, O, Ce, and C co-exist in the sample, where the carbon peaks are attributed to the residual carbon from the sample and adventitious hydrocarbon from the XPS instrument itself. This result is in good agreement with the EDS data. Fig. 2 (B) shows
Fig. 2. The survey (A) and Cu 2p3/2 (B) XPS spectra of the Cu2O/CeO2 composite prepared with 0.06 M CuCl2.
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S. Hu et al. / Catalysis Communications 12 (2011) 794–797
the pattern of the X-ray photoelectron scan for the Cu 2p3/2 of composite photocatalyst prepared in 0.06 M CuCl2 solution. The only peak located at 931.8 eV can be attributed to Cu (I) in Cu2O, although it is lower than the reported binding energy of Cu2O at 932.4 eV. The similar result was also observed in Huang's experiment and the shift can be explained as the small content of Cu2O on the surface of CeO2 particles [12] and the different chemical environment around Cu2O compared to bulk Cu2O. It is known Cu2O can be easily oxidized to CuO during the washing and drying treatment process. However, from the XPS data, there is no peak attributed to Cu (II), where the range of binding energy is from 933.4 to 933.9 eV. It is probably because that nanocrystal Cu2O is more stable than bulk Cu2O under ambient atmosphere because Cu2O has a high-symmetry (cubic) structure and a lower anion/cation ratio, which is more stable at small particle size [12]. 3.2. Photocatalytic Activity Fig. 3 shows the photocatalytic degradation curves of AO7 using CeO2 and Cu2O/CeO2 composites prepared with different CuCl2 concentrations as photocatalyst. From the curves, it is found that under visible light irradiation, all the composites exhibit higher photocatalytic activity for the degradation of AO7 than pure CeO2. It can also be found that the photocatalytic activity of Cu2O/CeO2 composites increases with the increasing amount of CuCl2 and reaches to the best for the sample prepared in 0.06 M CuCl2 solution. About 96.2% of AO7 is degraded after 4 h irradiation, while only 77.6% of AO7 can be decomposed using pure CeO2 as photocatalyst. The degradation percentage is not further increased when the concentration of CuCl2 solution increases from 0.06 to 0.08 M. That is to say, the amount of Cu2O in the composites plays an important role in the photocatalytic performance. The GC-MAS was also applied to identify the intermediates during the photodegradation of AO7 for the sample prepared in 0.06 M CuCl2 solution. The main products included substituted benzene, such as benzoic acid, phenol, phthalic anhydride and low molecular weight organic 4-oxo-2-butenoic acid, which suggests that AO7 is almost completely photodegradated by Cu2O/CeO2. Because adsorption also plays an important role in most photocatalytic reactions, the adsorption capacity of all catalysts during the whole decolorization process was investigated (Fig. 4). As we studied before [3], CeO2 has the superior adsorption capacity to AO7 and from this pattern, we can also see that in the whole decolorization process
Fig. 3. The photocatalytic degradation curves of AO7 using (a) pure CeO2 and Cu2O/CeO2 composite prepared with different CuCl2 concentrations: (b) 0.01 M, (c) 0.04 M, (d) 0.06 M and (e) 0.08 M.
Fig. 4. Decolorization rate of (a) pure CeO2, and Cu2O/CeO2 composites prepared with different CuCl2 concentrations: (b) 0.01 M, (c) 0.04 M, (d) 0.06 M and (e) 0.08 M.
of pure CeO2, almost 60% of AO7 molecules adsorbs on CeO2 surface on the dark adsorption stage and only 30% of AO7 are decomposed by photocatalysis (Fig. 4a). Hereby, the high decolorization efficiency of pure CeO2 can be mainly attributed to its high adsorption capability to AO7 molecules. After loaded with Cu2O, however, the dark adsorption capacity becomes much lower even with the least loading amount of Cu2O (Fig. 4b), which may be attributed to the reduced specific surface area as seen from Table 1. Actually, the photocatalytic activity usually improves with adsorption capacity during the photocatalysis process. But it is observed in Fig. 4d that sample prepared in 0.06 M CuCl2 solution with best photocatalytic activity only adsorbs about 30% of AO7 during dark adsorption process. From these data, it is apparent that the high decolorization percentage of Cu2O/CeO2 composite is not mainly attributed to the adsorption capability any more. UV-vis diffuse reflectance spectra (DRS) of pure CeO2 and Cu2O loaded CeO2 (prepared in 0.06 M CuCl2 solution) are shown in Fig. 5. It shows that the Cu2O/CeO2 composite has a red shift and increased absorption in the visible range. The optical band gap energy (Eg) was calculated based on the absorbance spectrum of the powders according to equation of Eg = 1240/λAbsorp.Edge. Pure CeO2 and Cu2O/ CeO2 composite show the band gap absorption onset at 425 and 437 nm, which correspond to band gap energies of 2.92 and 2.84 eV, respectively. Consequently, the optical absorption edge of the Cu2O/
Fig. 5. UV-vis diffuse reflectance spectra of (a) pure CeO2 and (b) Cu2O/CeO2 composite prepared with 0.06 M CuCl2.
S. Hu et al. / Catalysis Communications 12 (2011) 794–797
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C sensitization on the photocatalytic activity, we studied the photodegradation for AO7 using sample prepared without the addition of CuCl2. The result shown in Fig. S3 indicate CeO2 treated only with glucose and NaOH shows reduced activity toward AO7, which well excludes the C sensitization effect. 4. Conclusions
Scheme 1. The photocatalytic mechanism over Cu2O/CeO2 composite under irradiation of visible light.
CeO2 composite shifts to the lower energy region compared to the pure CeO2, and its absorption is stronger in the wavelength range of 400–600 nm. That is to say, Cu2O/CeO2 composite photocatalyst has high optical absorption capability than pure CeO2 and more photointroduced electrons/holes pairs can be generated under visible light irradiation. 3.3. Mechanism According to the DRS data, one of the factors for enhanced activity of Cu2O/CeO2 composite can be probably attributed to the improved optical absorption. Furthermore, following the model described by Chen et al. [13], n-type CeO2 and p-type Cu2O semiconductors can form p–n junction photocatalysts when Cu2O is loaded on CeO2 surface, which is favorable for the separation of photo-generated charges. As shown in Scheme 1, the generated electrons in Cu2O and holes in CeO2 immigrate to the conduction band of CeO2 and the valence band of Cu2O, respectively. At the equilibrium, the inner electric field forms, where p-Cu2O region has the negative charge while n-CeO2 region has the positive charge. Under visible light irradiation, electron–hole pairs may be generated. With the effect of the inner electric field, the holes flow into the negative field while the electrons move to the positive field. Hereby, the photo-generated electron–hole pairs will be separated more effectively in the Cu2O/ CeO2 composite. Moreover, the Cu2O nanocrystal dispersed on CeO2 surface broadens the band gap of Cu2O due to the quantum size effect. Consequently, the oxidative power of photo-generated holes on valence band should be stronger than those in bulk Cu2O, which may accelerate the degradation process. Moreover, to exclude the effect of
Cu2O/CeO2 composite was prepared by reduction of Cu2+ adsorbed on CeO2 surface using glucose as the reductant. The adsorption capability of Cu2O/CeO2 composites to AO7 was much lower than pure CeO2, but the photocatalytic activity for the degradation of AO7 under visible light irradiation was significantly enhanced. The enhancement of photocatalytic degradation could be explained by the improved visible light absorption and the formation of p–n junction. Acknowledgements This work has been supported by National Nature Science Foundation of China (21007016, 20773039 and 20977030), National Basic Research Program of China (973 Program, 2007CB613301 and 2010CB732306), Science and Technology Commission of Shanghai Municipality (10520709900, 10JC1403900) and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/ j.catcom.2011.01.027. References [1] K.-H. Chung, D.-C. Park, Catal. Today 30 (1996) 157–162. [2] M.D. Hernández-Alonso, A.B. Hungría, A. Martínez-Arias, M. Fernández-Garcí´a, J.M. Coronado, J.C. Conesa, J. Soria, Appl. Catal. B: Environ. 50 (2004) 167–175. [3] P. Ji, J. Zhang, F. Chen, M. Anpo, Appl. Catal. B 85 (2009) 148–154. [4] J.L. Zhang, L. Xiao, Y. Cong, M. Anpo, Top. Catal. 47 (2008) 122–130. [5] V. Iliev, J. Photochem. Photobiol. A: Chem 151 (2002) 195–199. [6] Y. Cong, J.L. Zhang, F. Chen, M. Anpo, J. Phys. Chem. C 111 (2007) 6976–6982. [7] Y. Bessekhouad, D. Robert, J.-V. Weber, J. Photochem. Photobiol. A: Chem. 163 (2004) 569–580. [8] M.K.I. Senevirathna, P.K.D.D.P. Pitigala, K. Tennakone, J. Photochem. Photobiol. A: Chem. 171 (2005) 257–259. [9] F.B. Li, X.Z. Li, M.F. Huo, K.W. Cheah, W.C.H. Choy, Appl. Catal. A 285 (2005) 181–189. [10] Y.-N. Chan, R.-S. Hsu, J.-J. Lin, J. Phys. Chem. C 114 (23) (2010) 10373–10378. [11] W. Wang, M. Song, Microporous Mesoporous Mater. 96 (2006) 255–261. [12] L. Huang, F. Peng, H. Yu, H. Wang, Solid State Sci. 11 (2009) 129–138. [13] S.F. Chen, W. Zhao, W. Liu, S.J. Zhang, Appl. Surf. Sci. 255 (2008) 2478–2484.
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Preparation of Cu2O/CeO2 heterojunction photocatalyst for the degradation of Acid Orange 7 under visible light irradiation Shichao Hu, Feng Zhou, Lingzhi Wang ⁎, Jinlong Zhang ⁎ Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, People's Republic of China
a r t i c l e
i n f o
Article history: Received 11 August 2010 Received in revised form 23 January 2011 Accepted 25 January 2011 Available online 4 February 2011 Keywords: Cu2O CeO2 Heterojunction Photocatalyst
a b s t r a c t Cu2O/CeO2 heterojunction photocatalysts with different concentrations of Cu2O were prepared by a soakreduction method using glucose as the reductant, and characterized by XRD, EDS, XPS and UV-vis DRS spectroscopy. The photocatalytic efficiency of the Cu2O/CeO2 composite was evaluated by degradation of Acid Orange 7 in water under visible light (λ N 420 nm) irradiation. The Cu2O/CeO2 composite showed stronger visible light absorption capacity and higher photocatalytic activity than pure CeO2. The sample with a Cu2O / CeO2 molar ratio of 0.029 presented the best photocatalytic activity, which was 20% higher than that of pure CeO2. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, cerium oxide has been extensively studied and employed for various applications including fast ion conductors, oxygen storage capacitors and catalysts. In particular, CeO2 has some properties like titania such as wide band gap, nontoxicity and high stability. As for pure CeO2, it has been investigated under UV irradiation concerning water-splitting for the generation of hydrogen gas [1] and photodegradation of toluene in the gas phase [2]. Our group has recently reported the photocatalytic behavior of CeO2 for the degradation of Acid Orange 7 (AO7) under visible light irradiation [3]. However, CeO2 showed low degradation rate and good adsorption ability to AO7, which is attributed to the low light absorption efficiency of CeO2. Many strategies are commonly used to extend the spectral photoresponse of wide band gap semiconductors like titania to visible light region such as doping with transition metals [4], dye sensitization [5] and anionic doping [6]. Besides, another promising idea is to couple them with narrow band gap semiconductors. In this configuration, several advantages can be obtained: (1) an improvement of charge separation; (2) an increase in the lifetime of the charge carrier and (3) an enhancement of the interfacial charge transfer efficiency to adsorbed substrate [7]. Cuprous oxide (Cu2O) is a p-type semiconductor having a direct band gap of 2.0 eV, which has been previously studied for application in solar energy converting devices [8]. As reported by Li et al. [9], the Ce4f band potential of CeO2 is similar to TiO2, which is lower than that
⁎ Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (J. Zhang). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.01.027
of Cu2O, and the electron can be theoretically injected from Cu2O to the conduction band of CeO2. Therefore, we proposed that Cu2O is suitable for CeO2 as sensitizer due to the energy band match. In this study, nanocrystal Cu2O loaded CeO2 was fabricated via a soakreduction method to form a novel Cu2O/CeO2 heterojunction composite as a visible light-driven photocatalyst. This new coupled system, to our knowledge, was reported for the first time. Compared with pure CeO2 sample, the coupled photocatalyst presented better visible light absorption capacity and photocatalytic activity to AO7. 2. Experimentals 2.1. Preparation of Catalysts Cu2O/CeO2 heterojunction photocatalysts were prepared by a soak-reduction method using glucose as the reducing agent. Typically, a certain amount CuCl2 was dissolved in 50 mL distilled water to form different concentration solutions, which is 0.01, 0.04, 0.06 and 0.08 M, respectively. Then 0.6 g CeO2 prepared by direct calcination of cerium nitrate at 723 K for 6 h was dispersed in above solutions and treated with ulrasound to obtain uniform a suspension. The suspension was vigorously stirred at room temperature for 1 h to reach the adsorption equilibrium. After stirring, the mixtures were centrifuged and washed by deionized water for several times to dispose of free Cu2+ ions in solutions. The mixture was added into 40 mL 1 M glucose solution to form a suspension and then heated to 60 °C in water bath under stirring. Then 20 mL 2 M NaOH was slowly added into the suspension until the color turned from green to pale yellow. After 15 min stirring, the products were collected by filtration, washed with deionized water for several times and dried under vacuum for 4 h at 60 °C.
S. Hu et al. / Catalysis Communications 12 (2011) 794–797
2.2. Characterizations of Photocatalysts The obtained samples were characterized by X-ray powder diffraction (XRD), energy dispersive X-ray spectrum (EDS), UV-visible absorption spectra and X-ray photoelectron spectroscopy (XPS). Details are given in the Supplementary Material. 2.3. Photocatalytic Activity Test The visible light photocatalytic activity test is similar to our previous report [3] and has been detailed in the supporting information. In this test, an aqueous suspension of AO7 (60 mL, 35 mg/L) and 0.06 g photocatalyst powder were applied. The change on the concentration of AO7 after irradiation was analyzed by UV-vis absorption spectra. The starting point of the concentration–time plot was collected after the suspension was stirred for 1 h in darkness to reach the adsorption equilibrium. C0 and Ct referred to the equilibrated and the original dye concentrations. 3. Results and Discussion 3.1. Characterization of the Cu2O/CeO2 Composites Fig. 1 displays XRD patterns of Cu2O/CeO2 composite photocatalysts prepared with different concentrations of CuCl2. From Fig. 1, however, we can find all the diffraction peaks only match the standard data for CeO2 and no additional phases such as Cu2O, CuO or Cu can be detected in all the samples. Since only very small amount of Cu2+ can be adsorbed on CeO2 after centrifuged from CuCl2 solution, which may be attributed to the relative low surface area of CeO2 particle prepared by direct calcination with Ce(NO3)3, the absence of detection can be explained as that cuprous oxide particles are so tiny (less than 3 nm) and beyond the detection limit of XRD [10,11]. To illustrate that the soak-reduction method can actually form Cu2O crystal, we prepared sample without the disposing of free Cu2+ in the solution. As seen from the XRD pattern shown in Fig. S1, the diffraction peak attributed to Cu2O can be obviously observed. However, this composite show much more severe dark adsorption than pure CeO2, which has negligible photocatalytic activity. As the cuprous oxide cannot be detected by XRD, EDS were employed to confirm the presence and content of cuprous oxide in the Cu2O/CeO2 composites. Table 1 shows the EDS data of Cu2O/CeO2 composites prepared with different concentrations of CuCl2. Only the elements of O, Ce and Cu can be observed and the molar ratios of Cu2O
Fig. 1. XRD patterns of the Cu2O/CeO2 composites prepared with different concentrations of CuCl2 solutions: (a) pure CeO2, (b) 0.01 M, (c) 0.04 M, (d) 0.06 M and (e) 0.08 M.
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Table 1 Elemental composition and BET surface area of Cu2O/CeO2 heterojunction photocatalyst prepared with different concentration of CuCl2. Samplea
Ce (Wt %)
Cu (Wt %)
O (Wt %)
Cu2O/CeO2b
BET surface area (m2/g)
1 2 3 4 5
– 88.89 88.27 87.34 86.52
– 0.99 1.26 2.31 2.50
– 10.12 10.47 10.35 10.98
– 0.012 0.016 0.029 0.032
63 47 54 53 56
a Samples are prepared with different concentrations of CuCl2: (1) 0, (2) 0.01, (3) 0.04, (4)0.06 and (5) 0.08 M. b Molar ratio.
to CeO2 in the composites prepared with different concentrations of CuCl2 (0.01, 0.04, 0.06 and 0.08 M) are calculated to be 0.012, 0.016, 0.029 and 0.032, which increases with the concentration of CuCl2. The result also reveals the content of cuprous oxide dispersed on CeO2 particle is so small that it is hard to highly crystallize at the reduction stage. It can be inferred that Cu2O probably forms nanocluster and attaches to the CeO2 surface. XPS was employed to confirm that Cu species on the surface of CeO2 particle exist as +1 valence state. Fig. 2 (A) shows the survey spectra of the sample prepared in 0.06 M CuCl2 solution. As shown in the survey spectra, element Cu, O, Ce, and C co-exist in the sample, where the carbon peaks are attributed to the residual carbon from the sample and adventitious hydrocarbon from the XPS instrument itself. This result is in good agreement with the EDS data. Fig. 2 (B) shows
Fig. 2. The survey (A) and Cu 2p3/2 (B) XPS spectra of the Cu2O/CeO2 composite prepared with 0.06 M CuCl2.
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S. Hu et al. / Catalysis Communications 12 (2011) 794–797
the pattern of the X-ray photoelectron scan for the Cu 2p3/2 of composite photocatalyst prepared in 0.06 M CuCl2 solution. The only peak located at 931.8 eV can be attributed to Cu (I) in Cu2O, although it is lower than the reported binding energy of Cu2O at 932.4 eV. The similar result was also observed in Huang's experiment and the shift can be explained as the small content of Cu2O on the surface of CeO2 particles [12] and the different chemical environment around Cu2O compared to bulk Cu2O. It is known Cu2O can be easily oxidized to CuO during the washing and drying treatment process. However, from the XPS data, there is no peak attributed to Cu (II), where the range of binding energy is from 933.4 to 933.9 eV. It is probably because that nanocrystal Cu2O is more stable than bulk Cu2O under ambient atmosphere because Cu2O has a high-symmetry (cubic) structure and a lower anion/cation ratio, which is more stable at small particle size [12]. 3.2. Photocatalytic Activity Fig. 3 shows the photocatalytic degradation curves of AO7 using CeO2 and Cu2O/CeO2 composites prepared with different CuCl2 concentrations as photocatalyst. From the curves, it is found that under visible light irradiation, all the composites exhibit higher photocatalytic activity for the degradation of AO7 than pure CeO2. It can also be found that the photocatalytic activity of Cu2O/CeO2 composites increases with the increasing amount of CuCl2 and reaches to the best for the sample prepared in 0.06 M CuCl2 solution. About 96.2% of AO7 is degraded after 4 h irradiation, while only 77.6% of AO7 can be decomposed using pure CeO2 as photocatalyst. The degradation percentage is not further increased when the concentration of CuCl2 solution increases from 0.06 to 0.08 M. That is to say, the amount of Cu2O in the composites plays an important role in the photocatalytic performance. The GC-MAS was also applied to identify the intermediates during the photodegradation of AO7 for the sample prepared in 0.06 M CuCl2 solution. The main products included substituted benzene, such as benzoic acid, phenol, phthalic anhydride and low molecular weight organic 4-oxo-2-butenoic acid, which suggests that AO7 is almost completely photodegradated by Cu2O/CeO2. Because adsorption also plays an important role in most photocatalytic reactions, the adsorption capacity of all catalysts during the whole decolorization process was investigated (Fig. 4). As we studied before [3], CeO2 has the superior adsorption capacity to AO7 and from this pattern, we can also see that in the whole decolorization process
Fig. 3. The photocatalytic degradation curves of AO7 using (a) pure CeO2 and Cu2O/CeO2 composite prepared with different CuCl2 concentrations: (b) 0.01 M, (c) 0.04 M, (d) 0.06 M and (e) 0.08 M.
Fig. 4. Decolorization rate of (a) pure CeO2, and Cu2O/CeO2 composites prepared with different CuCl2 concentrations: (b) 0.01 M, (c) 0.04 M, (d) 0.06 M and (e) 0.08 M.
of pure CeO2, almost 60% of AO7 molecules adsorbs on CeO2 surface on the dark adsorption stage and only 30% of AO7 are decomposed by photocatalysis (Fig. 4a). Hereby, the high decolorization efficiency of pure CeO2 can be mainly attributed to its high adsorption capability to AO7 molecules. After loaded with Cu2O, however, the dark adsorption capacity becomes much lower even with the least loading amount of Cu2O (Fig. 4b), which may be attributed to the reduced specific surface area as seen from Table 1. Actually, the photocatalytic activity usually improves with adsorption capacity during the photocatalysis process. But it is observed in Fig. 4d that sample prepared in 0.06 M CuCl2 solution with best photocatalytic activity only adsorbs about 30% of AO7 during dark adsorption process. From these data, it is apparent that the high decolorization percentage of Cu2O/CeO2 composite is not mainly attributed to the adsorption capability any more. UV-vis diffuse reflectance spectra (DRS) of pure CeO2 and Cu2O loaded CeO2 (prepared in 0.06 M CuCl2 solution) are shown in Fig. 5. It shows that the Cu2O/CeO2 composite has a red shift and increased absorption in the visible range. The optical band gap energy (Eg) was calculated based on the absorbance spectrum of the powders according to equation of Eg = 1240/λAbsorp.Edge. Pure CeO2 and Cu2O/ CeO2 composite show the band gap absorption onset at 425 and 437 nm, which correspond to band gap energies of 2.92 and 2.84 eV, respectively. Consequently, the optical absorption edge of the Cu2O/
Fig. 5. UV-vis diffuse reflectance spectra of (a) pure CeO2 and (b) Cu2O/CeO2 composite prepared with 0.06 M CuCl2.
S. Hu et al. / Catalysis Communications 12 (2011) 794–797
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C sensitization on the photocatalytic activity, we studied the photodegradation for AO7 using sample prepared without the addition of CuCl2. The result shown in Fig. S3 indicate CeO2 treated only with glucose and NaOH shows reduced activity toward AO7, which well excludes the C sensitization effect. 4. Conclusions
Scheme 1. The photocatalytic mechanism over Cu2O/CeO2 composite under irradiation of visible light.
CeO2 composite shifts to the lower energy region compared to the pure CeO2, and its absorption is stronger in the wavelength range of 400–600 nm. That is to say, Cu2O/CeO2 composite photocatalyst has high optical absorption capability than pure CeO2 and more photointroduced electrons/holes pairs can be generated under visible light irradiation. 3.3. Mechanism According to the DRS data, one of the factors for enhanced activity of Cu2O/CeO2 composite can be probably attributed to the improved optical absorption. Furthermore, following the model described by Chen et al. [13], n-type CeO2 and p-type Cu2O semiconductors can form p–n junction photocatalysts when Cu2O is loaded on CeO2 surface, which is favorable for the separation of photo-generated charges. As shown in Scheme 1, the generated electrons in Cu2O and holes in CeO2 immigrate to the conduction band of CeO2 and the valence band of Cu2O, respectively. At the equilibrium, the inner electric field forms, where p-Cu2O region has the negative charge while n-CeO2 region has the positive charge. Under visible light irradiation, electron–hole pairs may be generated. With the effect of the inner electric field, the holes flow into the negative field while the electrons move to the positive field. Hereby, the photo-generated electron–hole pairs will be separated more effectively in the Cu2O/ CeO2 composite. Moreover, the Cu2O nanocrystal dispersed on CeO2 surface broadens the band gap of Cu2O due to the quantum size effect. Consequently, the oxidative power of photo-generated holes on valence band should be stronger than those in bulk Cu2O, which may accelerate the degradation process. Moreover, to exclude the effect of
Cu2O/CeO2 composite was prepared by reduction of Cu2+ adsorbed on CeO2 surface using glucose as the reductant. The adsorption capability of Cu2O/CeO2 composites to AO7 was much lower than pure CeO2, but the photocatalytic activity for the degradation of AO7 under visible light irradiation was significantly enhanced. The enhancement of photocatalytic degradation could be explained by the improved visible light absorption and the formation of p–n junction. Acknowledgements This work has been supported by National Nature Science Foundation of China (21007016, 20773039 and 20977030), National Basic Research Program of China (973 Program, 2007CB613301 and 2010CB732306), Science and Technology Commission of Shanghai Municipality (10520709900, 10JC1403900) and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/ j.catcom.2011.01.027. References [1] K.-H. Chung, D.-C. Park, Catal. Today 30 (1996) 157–162. [2] M.D. Hernández-Alonso, A.B. Hungría, A. Martínez-Arias, M. Fernández-Garcí´a, J.M. Coronado, J.C. Conesa, J. Soria, Appl. Catal. B: Environ. 50 (2004) 167–175. [3] P. Ji, J. Zhang, F. Chen, M. Anpo, Appl. Catal. B 85 (2009) 148–154. [4] J.L. Zhang, L. Xiao, Y. Cong, M. Anpo, Top. Catal. 47 (2008) 122–130. [5] V. Iliev, J. Photochem. Photobiol. A: Chem 151 (2002) 195–199. [6] Y. Cong, J.L. Zhang, F. Chen, M. Anpo, J. Phys. Chem. C 111 (2007) 6976–6982. [7] Y. Bessekhouad, D. Robert, J.-V. Weber, J. Photochem. Photobiol. A: Chem. 163 (2004) 569–580. [8] M.K.I. Senevirathna, P.K.D.D.P. Pitigala, K. Tennakone, J. Photochem. Photobiol. A: Chem. 171 (2005) 257–259. [9] F.B. Li, X.Z. Li, M.F. Huo, K.W. Cheah, W.C.H. Choy, Appl. Catal. A 285 (2005) 181–189. [10] Y.-N. Chan, R.-S. Hsu, J.-J. Lin, J. Phys. Chem. C 114 (23) (2010) 10373–10378. [11] W. Wang, M. Song, Microporous Mesoporous Mater. 96 (2006) 255–261. [12] L. Huang, F. Peng, H. Yu, H. Wang, Solid State Sci. 11 (2009) 129–138. [13] S.F. Chen, W. Zhao, W. Liu, S.J. Zhang, Appl. Surf. Sci. 255 (2008) 2478–2484.