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Low temperature synthesis of ZnIn2S4 microspheres as a visible light photocatalyst for selective oxidation
Catalysis Communications 41 (2013) 83–86
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Low temperature synthesis of ZnIn2S4 microspheres as a visible light photocatalyst for selective oxidation Zhixin Chen a,b,⁎, Jingjing Xu a, Zhuyun Ren a, Yunhui He a,b, Guangcan Xiao a,b a b
State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, PR China Instrumental Measurement & Analysis Center, Fuzhou University, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 23 April 2013 Received in revised form 4 July 2013 Accepted 8 July 2013 Available online 16 July 2013 Keywords: ZnIn2S4 Selective oxidation Visible light Photocatalyst
a b s t r a c t ZnIn2S4 microspheres have been synthesized by a facile hydrothermal method at 80 °C. The characterization results show that the as-synthesized sample is hexagonal phase ZnIn2S4 microspheres. The results of elemental mapping and thermogravimetric confirm that the sample is the pure ZnIn2S4. The ZnIn2S4 sample has been first used as visible-light-driven photocatalyst for selective oxidation of benzyl alcohol to benzaldehyde under ambient conditions, which shows the conversion is ca. 69% along with the high selectivity, ca. 94%, after 3 h irradiation. XRD and XPS investigations suggest that ZnIn2S4 is relatively stable in the photocatalytic reaction. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, photocatalysis is widely used as a “green” technology in the field of environmental remediation and energy conversion [1–3]. Recently, semiconductor photocatalysts have been reported to be applied in the partial oxidation process, for example, oxidation of alcohols to aldehydes [4,5]. Photocatalytic selective oxidation has become a vital embranchment in photocatalytic area [6]. The reaction of photocatalytic selective oxidation attracts burgeoning interest because it can be carried out under mild conditions, and has several advantages such as nontoxic, economical, environmental friendliness and promising potential for utilizing solar energy [6,7]. Selective oxidation of aromatic alcohols to corresponding aldehydes is a fundamental and significant reaction for the fine chemicals synthesis, because carbonyl compounds, e.g., aldehydes, are an important raw material for many useful chemicals, such as dyes, resins, fragrances and drugs [8]. For instance, Zhang et al. have synthesized the graphene-TiO2 nanocomposites, which exhibit a highly visible light photocatalytic activity toward selective oxidation of alcohols to aldehydes [7]. Marotta et al. have studied selective oxidation of benzyl alcohol to benzaldehyde in aqueous solution through the TiO2/Cu(II)/solar UV photocatalytic system [9]. However, as a potential type of photocatalyst, ternary chalcogenides have not been applied to the area of photocatalytic selective oxidation [10–14].
⁎ Corresponding author at: State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, PR China. Tel./fax: + 86 591 87892447. E-mail address: [email protected] (Z. Chen). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.07.016
ZnIn2S4, as one of ternary chalcogenides, has been reported as photocatalyst for hydrogen evolution or dye degradation under visible light irradiation [10–14]. However, it still remains unclear whether ZnIn2S4 can be applied to photocatalytic selective organic transformations. Herein, we have synthesized the ZnIn2S4 sample by a hydrothermal method at low temperature, 80 °C. The as-obtained ZnIn2S4 sample with microsphere morphology exhibits the good photocatalytic performance toward selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation and ambient conditions, thus pointing to the promising potential of ternary chalcogenides as photocatalysts for selective transformation. 2. Experimental section 2.1. Synthesis The procedures for synthesis of ZnIn2S4 sample were reported via a facile hydrothermal approach from our group [12]. ZnCl2 (1 mmol), InCl3·4H2O (2 mmol) and thioacetamide (6 mmol) were dissolved in a Teflon liner with 100 mL capacity containing 80 mL of deionized water. The pH was adjusted to 2.5 by the addition of hydrochloric acid. Then, it was maintained at 80 °C for 6 h. After the reaction, the sample was collected by centrifugation and washed with deionized water and absolute ethanol several times. The final samples were dried at 60 °C in vacuum. 2.2. Characterizations The powder X-ray diffraction (XRD) pattern was obtained on a Bruker D8 Advance X-ray diffractometer using Cu Kα irradiation.
84
Z. Chen et al. / Catalysis Communications 41 (2013) 83–86
112
102
a 006
1012 202
108
104
Intensity /a.u.
c
b
JCPDS No. 65-2023 10
20
30 40 50 2 Theta / degree
60
Fig. 1. XRD pattern (a), FESEM (b) and TEM (c) images of the ZnIn2S4 sample.
The morphology of the sample was examined by field emission scanning electron microscopy (FESEM, FEI NanoSEM 230) and transmission electron microscopy (TEM, JEOL JEM 2010F). Thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses were carried out on a NETZSCH STA449C instrument in an air atmosphere from room temperature to 700 °C at a heating rate of 10 °C min−1.
2.3. Photocatalytic activity The photocatalytic selective oxidation of benzyl alcohol was performed as the following [7]. A mixture of benzyl alcohol (0.1 mmol) and 8 mg of catalyst was dissolved in the solvent of benzotrifluoride (1.5 mL). The above mixture was transferred into a 10 mL Pyrex glass bottle filled with molecular oxygen at a pressure of 0.1 MPa and stirred for half an hour to make the catalyst blend evenly in the solution. The suspensions were irradiated by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd.) with a UV-Cut filter to cut off light of wavelength b420 nm. After the reaction, the mixture was centrifuged at 12,000 rmp for 20 min to completely remove the catalyst particles. The remaining solution was analyzed with an Agilent Gas Chromatograph (GC-7820 fitted with a capillary FFAP analysis column and helium as gas carrier). Conversion of benzyl alcohol, yield of benzaldehyde, and selectivity for benzaldehyde were defined as the follows: Conversionð%Þ ¼ ½ðC0 −Cbenzylalcohol Þ=C0 100 Yieldð%Þ ¼ Cbenzaldehyde =C0 100 Selectivityð%Þ ¼ ½Cbenzaldehyde =ðC0 −Cbenzylalcohol Þ 100
where C0 is the initial concentration of benzyl alcohol and Cbenzyl alcohol and Cbenzaldehyde are the concentration of the substrate benzyl alcohol
a
b
SK
and the corresponding benzaldehyde, respectively, at a certain time after the photocatalytic reaction.
3. Results and discussions Our previous report has shown that the ZnIn2S4 sample prepared by hydrothermal method at 80 °C has a steep absorption edge in the visible range and exhibits good photocatalytic property toward degradation of several dyes [10,14]. Therefore, it is selected here for the further study. Fig. 1a shows the XRD pattern of the ZnIn2S4 sample. The diffraction peaks can be indexed to a hexagonal phase ZnIn2S4 (JCPDS No. 65-2023). Fig. 1b–c shows that the ZnIn2S4 sample is composed of a large quantity of microspheres with an average diameter, ca. 2 μm, and from the Figures, it is seen that the microsphere is made up of numerous nanosheets. The above characterizations of XRD, FESEM and TEM suggest that the as-synthesized sample is hexagonal phase. The elemental mapping is employed to investigate the element spatial distribution at the sample interface, which collects by scanning transmission electron microscope cooperate with energy dispersive X-ray spectroscopy (STEM-EDX). Fig. 2 shows the high angle annular dark field (HAADF, Fig. 2a) and STEM-EDX elemental mapping images of the ZnIn2S4 sample. As displayed in Fig. 2b–d, the red, yellow and blue colors of the elemental mapping images are acquired at the K-line spectra of S, Zn and In elements respectively. It can be seen that the elements of S, Zn and In have a homogenous distribution in the ZnIn2S4 sample. To further confirm the purity of as-synthesized ZnIn2S4 sample, the thermogravimetric (TG) and differential scanning calorimetry (DSC) characterizations are carried out, and commercial ZnS and In2S3 are selected as references. Fig. 3a shows that the TG profiles of ZnIn2S4, ZnS and In2S3. ZnS have no any weight loss when the temperature is lower than 600 °C, whereas In2S3 and ZnIn2S4 undergo oxygenolysis in the temperature range of 200–700 °C and 500–700 °C, respectively. Furthermore, ZnIn2S4 occupies about 9.1% weight loss, which is consistent with the literatures [11,15]. When temperature is
c
Zn K
d
In K
Fig. 2. HAADF image (a) of ZnIn2S4 and corresponding STEM-EDX elemental mapping images of S (b), Zn (c) and In (d).
Z. Chen et al. / Catalysis Communications 41 (2013) 83–86
a
a
100 96 92
9.1%
ZnS In2S3
88
Intensity / a.u.
Weight / %
85
ZnIn2S4
After Before
84 0
200
400
600
Temperature / C
10
20
30
40
50
60
250
0
2 Theta / degree
b 0
b
-10
ZnS In2S3
-15
ZnIn2S4
Counts / a.u.
mW / mg
-5
After Before
-20 0
200
400
600
Temperature / C 1250 Fig. 3. TG (a) and DSC (b) profiles of commercial ZnS, In2S3 and as-synthesized ZnIn2S4 sample.
1000
750
500
Binding Energy / eV Fig. 5. Comparison of XRD (a) and XPS (b) patterns of ZnIn2S4 before and after reaction.
a
Percentage/ %
100 Conversion Yield Selectivity
80 60 40 20 0
0
1
2
3
the conversion of benzyl alcohol and the yield of benzaldehyde reach ca. 69% and 66% respectively. As shown in Fig. 4b, there is almost no concentration change as the solution without catalyst under illumination 3 h. The concentration of benzyl alcohol decreases slightly but no benzaldehyde is detected when the ZnIn2S4 is added in the dark for 3 h, it can be indicated that the decrease of the benzyl alcohol is because of adsorption rather than photocatalytic reaction. All of these results strongly suggest that the catalyst and the light are the essential for the photocatalytic reaction. This is the first report that using the ZnIn2S4 sample to the photocatalytic selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation and ambient conditions. Furthermore, it is encouraging that the selectivity to benzaldehyde of the ZnIn2S4 sample at different irradiation time stages are all between 94 and 95%.
Remaining fraction of benzyl alcohol
lower than 500 °C, the weight loss may come from the absorbed water and some small organic molecules absorbed by the ZnIn2S4 sample. In Fig. 3b, the DSC curves of ZnIn2S4 and In2S3 show that the oxidation peaks appear at 522 and 480, respectively. It indicates that the oxidation temperature of In2S3 is lower than ZnIn2S4 or ZnS. Moreover, the DSC curve of ZnIn2S4 shows that no obvious peak appears when the temperature is lower than 500 °C. It indirectly demonstrates that no In2S3 as impurity is contained in the ZnIn2S4 sample [15]. The results of STEM-EDX elemental mapping, TG–DSC and XRD strongly confirm that the as-synthesized sample is the pure phase ZnIn2S4. The photocatalytic activity of the ZnIn2S4 sample and some comparative experiments are evaluated by selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation and ambient conditions. As shown in Fig. 4a, after 0.5 h irradiation, the conversion of benzyl alcohol and the yield of benzaldehyde are ca. 47% and 45% respectively. With increasing of the irradiation time, the conversion of benzyl alcohol enhances gradually. When the irradiation time is 3 h,
0.10
b
0.08 0.06 0.04 0.02 0.00 initial
no catalyst no irradiation
Irradiation Time / h Fig. 4. Photocatalytic performance toward selective oxidation of benzyl alcohol to benzaldehyde over ZnIn2S4 under visible light irradiation: a) time-dependent photocatalytic performance and b) under visible light with no catalyst and in the dark with catalyst for 3 h.
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Z. Chen et al. / Catalysis Communications 41 (2013) 83–86
It is known that the photocorrosion or photodissolution of catalysts might occur on the photocatalyst surface in the photocatalytic reaction. Therefore, stability of catalysts is a very important issue in the photocatalysis area. Our previous works [10,12] suggest that the ZnIn2S4 is stable in the photocatalytic degradation of MO aqueous solution. For the purpose of knowing the ZnIn2S4 situation in photocatalytic reactions toward the selective oxidation of benzyl alcohol to benzaldehyde. The photocatalysts before and after reaction are compared by XRD and XPS investigation. Fig. 5a shows the XRD patterns of ZnIn2S4 before and after reaction. The position, intensity, and ratio of peaks are nearly the same, and no new peak is created. Furthermore, the XPS is applied to test the photocatalysts' surface chemical states. Fig. 5b indicates that the main peaks are owned by elements Zn, In, and S. The binding energies of Zn2p, In3d, and S2p before and after reaction are nearly the same. These results suggest that the photocatalyst ZnIn2S4 is relatively stable in the photocatalytic reaction. Consequently, as-synthesized ZnIn2S4 sample should be exploited to other photocatalytic selective transformation, and more other ternary chalcogenides should be exploited to the area of photocatalytic selective transformation. 4. Conclusion ZnIn2S4 microspheres are synthesized by a facile hydrothermal method at 80 °C. The results of XRD, FESEM, TG–DSC and STEM-EDX elemental mapping demonstrate that as-synthesized sample is pure hexagonal phase ZnIn2S4 microspheres. The ZnIn2S4 sample is first used in photocatalytic selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation and ambient conditions. The ZnIn2S4 sample shows the conversion of benzyl alcohol is ca. 69% along with the high selectivity, ca. 94%, after 3 h visible light irradiation.
XRD and XPS investigations suggest that ZnIn2S4 is relatively stable in the photocatalytic reaction. Consequently, the ZnIn2S4 sample should be exploited to other photocatalytic selective transformation, and more other ternary chalcogenides should be exploited to the area of photocatalytic selective transformation. Acknowledgment This work is financially supported by the Natural Science Foundation of Fujian, China (2011J05024). References [1] M. Sun, D.Z. Li, W.J. Zhang, Z.X. Chen, H.J. Huang, W.J. Li, Y.H. He, X.Z. Fu, J. Solid State Chem. 190 (2012) 135–142. [2] T.Y. Peng, D.N. Ke, J.R. Xiao, L. Wang, J. Hu, L. Zan, J. Solid State Chem. 194 (2012) 250–256. [3] G. Patrinoiu, M. Tudose, J.M. Calderon-Moreno, R. Birjega, P. Budrugeac, R. Ene, O. Carp, J. Solid State Chem. 186 (2012) 17–22. [4] M.D. Tzirakis, I.N. Lykakis, G.D. Panagiotou, K. Bourikas, A. Lycourghiotis, C. Kordulis, M. Orfanopoulos, J. Catal. 252 (2007) 178–189. [5] K. Ohkubo, K. Suga, S. Fukuzumi, Chem. Commun. (2006) 2018–2020. [6] S. Higashimoto, N. Kitao, N. Yoshida, T. Sakura, M. Azuma, H. Ohue, Y. Sakata, J. Catal. 266 (2009) 279–285. [7] Y.H. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 5 (2011) 7426–7435. [8] C.J. Li, G.R. Xu, B.H. Zhang, J.R. Gong, Appl. Catal. B Environ. 115 (2012) 201–208. [9] R. Marotta, I. Di Somma, D. Spasiano, R. Andreozzi, V. Caprio, Chem. Eng. J. 172 (2011) 243–249. [10] Z.X. Chen, D.Z. Li, G.C. Xiao, Y.H. He, Y.J. Xu, J. Solid State Chem. 186 (2012) 247–254. [11] B. Chai, T.Y. Peng, P. Zeng, X.H. Zhang, Dalton Trans. 41 (2012) 1179–1186. [12] Z.X. Chen, D.Z. Li, W.J. Zhang, Y. Shao, T.W. Chen, M. Sun, X.Z. Fu, J. Phys. Chem. C 113 (2009) 4433–4440. [13] S.H. Shen, L. Zhao, L.J. Guo, J. Phys. Chem. Solids 69 (2008) 2426–2432. [14] Z.X. Chen, D.Z. Li, W.J. Zhang, C. Chen, W.J. Li, M. Sun, Y.H. He, X.Z. Fu, Inorg. Chem. 47 (2008) 9766–9772. [15] Z.D. Xu, Y.X. Li, S.Q. Peng, G.X. Lu, S.B. Li, RSC Adv. 2 (2012) 3458–3466.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Low temperature synthesis of ZnIn2S4 microspheres as a visible light photocatalyst for selective oxidation Zhixin Chen a,b,⁎, Jingjing Xu a, Zhuyun Ren a, Yunhui He a,b, Guangcan Xiao a,b a b
State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, PR China Instrumental Measurement & Analysis Center, Fuzhou University, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 23 April 2013 Received in revised form 4 July 2013 Accepted 8 July 2013 Available online 16 July 2013 Keywords: ZnIn2S4 Selective oxidation Visible light Photocatalyst
a b s t r a c t ZnIn2S4 microspheres have been synthesized by a facile hydrothermal method at 80 °C. The characterization results show that the as-synthesized sample is hexagonal phase ZnIn2S4 microspheres. The results of elemental mapping and thermogravimetric confirm that the sample is the pure ZnIn2S4. The ZnIn2S4 sample has been first used as visible-light-driven photocatalyst for selective oxidation of benzyl alcohol to benzaldehyde under ambient conditions, which shows the conversion is ca. 69% along with the high selectivity, ca. 94%, after 3 h irradiation. XRD and XPS investigations suggest that ZnIn2S4 is relatively stable in the photocatalytic reaction. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, photocatalysis is widely used as a “green” technology in the field of environmental remediation and energy conversion [1–3]. Recently, semiconductor photocatalysts have been reported to be applied in the partial oxidation process, for example, oxidation of alcohols to aldehydes [4,5]. Photocatalytic selective oxidation has become a vital embranchment in photocatalytic area [6]. The reaction of photocatalytic selective oxidation attracts burgeoning interest because it can be carried out under mild conditions, and has several advantages such as nontoxic, economical, environmental friendliness and promising potential for utilizing solar energy [6,7]. Selective oxidation of aromatic alcohols to corresponding aldehydes is a fundamental and significant reaction for the fine chemicals synthesis, because carbonyl compounds, e.g., aldehydes, are an important raw material for many useful chemicals, such as dyes, resins, fragrances and drugs [8]. For instance, Zhang et al. have synthesized the graphene-TiO2 nanocomposites, which exhibit a highly visible light photocatalytic activity toward selective oxidation of alcohols to aldehydes [7]. Marotta et al. have studied selective oxidation of benzyl alcohol to benzaldehyde in aqueous solution through the TiO2/Cu(II)/solar UV photocatalytic system [9]. However, as a potential type of photocatalyst, ternary chalcogenides have not been applied to the area of photocatalytic selective oxidation [10–14].
⁎ Corresponding author at: State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, PR China. Tel./fax: + 86 591 87892447. E-mail address: [email protected] (Z. Chen). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.07.016
ZnIn2S4, as one of ternary chalcogenides, has been reported as photocatalyst for hydrogen evolution or dye degradation under visible light irradiation [10–14]. However, it still remains unclear whether ZnIn2S4 can be applied to photocatalytic selective organic transformations. Herein, we have synthesized the ZnIn2S4 sample by a hydrothermal method at low temperature, 80 °C. The as-obtained ZnIn2S4 sample with microsphere morphology exhibits the good photocatalytic performance toward selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation and ambient conditions, thus pointing to the promising potential of ternary chalcogenides as photocatalysts for selective transformation. 2. Experimental section 2.1. Synthesis The procedures for synthesis of ZnIn2S4 sample were reported via a facile hydrothermal approach from our group [12]. ZnCl2 (1 mmol), InCl3·4H2O (2 mmol) and thioacetamide (6 mmol) were dissolved in a Teflon liner with 100 mL capacity containing 80 mL of deionized water. The pH was adjusted to 2.5 by the addition of hydrochloric acid. Then, it was maintained at 80 °C for 6 h. After the reaction, the sample was collected by centrifugation and washed with deionized water and absolute ethanol several times. The final samples were dried at 60 °C in vacuum. 2.2. Characterizations The powder X-ray diffraction (XRD) pattern was obtained on a Bruker D8 Advance X-ray diffractometer using Cu Kα irradiation.
84
Z. Chen et al. / Catalysis Communications 41 (2013) 83–86
112
102
a 006
1012 202
108
104
Intensity /a.u.
c
b
JCPDS No. 65-2023 10
20
30 40 50 2 Theta / degree
60
Fig. 1. XRD pattern (a), FESEM (b) and TEM (c) images of the ZnIn2S4 sample.
The morphology of the sample was examined by field emission scanning electron microscopy (FESEM, FEI NanoSEM 230) and transmission electron microscopy (TEM, JEOL JEM 2010F). Thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses were carried out on a NETZSCH STA449C instrument in an air atmosphere from room temperature to 700 °C at a heating rate of 10 °C min−1.
2.3. Photocatalytic activity The photocatalytic selective oxidation of benzyl alcohol was performed as the following [7]. A mixture of benzyl alcohol (0.1 mmol) and 8 mg of catalyst was dissolved in the solvent of benzotrifluoride (1.5 mL). The above mixture was transferred into a 10 mL Pyrex glass bottle filled with molecular oxygen at a pressure of 0.1 MPa and stirred for half an hour to make the catalyst blend evenly in the solution. The suspensions were irradiated by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd.) with a UV-Cut filter to cut off light of wavelength b420 nm. After the reaction, the mixture was centrifuged at 12,000 rmp for 20 min to completely remove the catalyst particles. The remaining solution was analyzed with an Agilent Gas Chromatograph (GC-7820 fitted with a capillary FFAP analysis column and helium as gas carrier). Conversion of benzyl alcohol, yield of benzaldehyde, and selectivity for benzaldehyde were defined as the follows: Conversionð%Þ ¼ ½ðC0 −Cbenzylalcohol Þ=C0 100 Yieldð%Þ ¼ Cbenzaldehyde =C0 100 Selectivityð%Þ ¼ ½Cbenzaldehyde =ðC0 −Cbenzylalcohol Þ 100
where C0 is the initial concentration of benzyl alcohol and Cbenzyl alcohol and Cbenzaldehyde are the concentration of the substrate benzyl alcohol
a
b
SK
and the corresponding benzaldehyde, respectively, at a certain time after the photocatalytic reaction.
3. Results and discussions Our previous report has shown that the ZnIn2S4 sample prepared by hydrothermal method at 80 °C has a steep absorption edge in the visible range and exhibits good photocatalytic property toward degradation of several dyes [10,14]. Therefore, it is selected here for the further study. Fig. 1a shows the XRD pattern of the ZnIn2S4 sample. The diffraction peaks can be indexed to a hexagonal phase ZnIn2S4 (JCPDS No. 65-2023). Fig. 1b–c shows that the ZnIn2S4 sample is composed of a large quantity of microspheres with an average diameter, ca. 2 μm, and from the Figures, it is seen that the microsphere is made up of numerous nanosheets. The above characterizations of XRD, FESEM and TEM suggest that the as-synthesized sample is hexagonal phase. The elemental mapping is employed to investigate the element spatial distribution at the sample interface, which collects by scanning transmission electron microscope cooperate with energy dispersive X-ray spectroscopy (STEM-EDX). Fig. 2 shows the high angle annular dark field (HAADF, Fig. 2a) and STEM-EDX elemental mapping images of the ZnIn2S4 sample. As displayed in Fig. 2b–d, the red, yellow and blue colors of the elemental mapping images are acquired at the K-line spectra of S, Zn and In elements respectively. It can be seen that the elements of S, Zn and In have a homogenous distribution in the ZnIn2S4 sample. To further confirm the purity of as-synthesized ZnIn2S4 sample, the thermogravimetric (TG) and differential scanning calorimetry (DSC) characterizations are carried out, and commercial ZnS and In2S3 are selected as references. Fig. 3a shows that the TG profiles of ZnIn2S4, ZnS and In2S3. ZnS have no any weight loss when the temperature is lower than 600 °C, whereas In2S3 and ZnIn2S4 undergo oxygenolysis in the temperature range of 200–700 °C and 500–700 °C, respectively. Furthermore, ZnIn2S4 occupies about 9.1% weight loss, which is consistent with the literatures [11,15]. When temperature is
c
Zn K
d
In K
Fig. 2. HAADF image (a) of ZnIn2S4 and corresponding STEM-EDX elemental mapping images of S (b), Zn (c) and In (d).
Z. Chen et al. / Catalysis Communications 41 (2013) 83–86
a
a
100 96 92
9.1%
ZnS In2S3
88
Intensity / a.u.
Weight / %
85
ZnIn2S4
After Before
84 0
200
400
600
Temperature / C
10
20
30
40
50
60
250
0
2 Theta / degree
b 0
b
-10
ZnS In2S3
-15
ZnIn2S4
Counts / a.u.
mW / mg
-5
After Before
-20 0
200
400
600
Temperature / C 1250 Fig. 3. TG (a) and DSC (b) profiles of commercial ZnS, In2S3 and as-synthesized ZnIn2S4 sample.
1000
750
500
Binding Energy / eV Fig. 5. Comparison of XRD (a) and XPS (b) patterns of ZnIn2S4 before and after reaction.
a
Percentage/ %
100 Conversion Yield Selectivity
80 60 40 20 0
0
1
2
3
the conversion of benzyl alcohol and the yield of benzaldehyde reach ca. 69% and 66% respectively. As shown in Fig. 4b, there is almost no concentration change as the solution without catalyst under illumination 3 h. The concentration of benzyl alcohol decreases slightly but no benzaldehyde is detected when the ZnIn2S4 is added in the dark for 3 h, it can be indicated that the decrease of the benzyl alcohol is because of adsorption rather than photocatalytic reaction. All of these results strongly suggest that the catalyst and the light are the essential for the photocatalytic reaction. This is the first report that using the ZnIn2S4 sample to the photocatalytic selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation and ambient conditions. Furthermore, it is encouraging that the selectivity to benzaldehyde of the ZnIn2S4 sample at different irradiation time stages are all between 94 and 95%.
Remaining fraction of benzyl alcohol
lower than 500 °C, the weight loss may come from the absorbed water and some small organic molecules absorbed by the ZnIn2S4 sample. In Fig. 3b, the DSC curves of ZnIn2S4 and In2S3 show that the oxidation peaks appear at 522 and 480, respectively. It indicates that the oxidation temperature of In2S3 is lower than ZnIn2S4 or ZnS. Moreover, the DSC curve of ZnIn2S4 shows that no obvious peak appears when the temperature is lower than 500 °C. It indirectly demonstrates that no In2S3 as impurity is contained in the ZnIn2S4 sample [15]. The results of STEM-EDX elemental mapping, TG–DSC and XRD strongly confirm that the as-synthesized sample is the pure phase ZnIn2S4. The photocatalytic activity of the ZnIn2S4 sample and some comparative experiments are evaluated by selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation and ambient conditions. As shown in Fig. 4a, after 0.5 h irradiation, the conversion of benzyl alcohol and the yield of benzaldehyde are ca. 47% and 45% respectively. With increasing of the irradiation time, the conversion of benzyl alcohol enhances gradually. When the irradiation time is 3 h,
0.10
b
0.08 0.06 0.04 0.02 0.00 initial
no catalyst no irradiation
Irradiation Time / h Fig. 4. Photocatalytic performance toward selective oxidation of benzyl alcohol to benzaldehyde over ZnIn2S4 under visible light irradiation: a) time-dependent photocatalytic performance and b) under visible light with no catalyst and in the dark with catalyst for 3 h.
86
Z. Chen et al. / Catalysis Communications 41 (2013) 83–86
It is known that the photocorrosion or photodissolution of catalysts might occur on the photocatalyst surface in the photocatalytic reaction. Therefore, stability of catalysts is a very important issue in the photocatalysis area. Our previous works [10,12] suggest that the ZnIn2S4 is stable in the photocatalytic degradation of MO aqueous solution. For the purpose of knowing the ZnIn2S4 situation in photocatalytic reactions toward the selective oxidation of benzyl alcohol to benzaldehyde. The photocatalysts before and after reaction are compared by XRD and XPS investigation. Fig. 5a shows the XRD patterns of ZnIn2S4 before and after reaction. The position, intensity, and ratio of peaks are nearly the same, and no new peak is created. Furthermore, the XPS is applied to test the photocatalysts' surface chemical states. Fig. 5b indicates that the main peaks are owned by elements Zn, In, and S. The binding energies of Zn2p, In3d, and S2p before and after reaction are nearly the same. These results suggest that the photocatalyst ZnIn2S4 is relatively stable in the photocatalytic reaction. Consequently, as-synthesized ZnIn2S4 sample should be exploited to other photocatalytic selective transformation, and more other ternary chalcogenides should be exploited to the area of photocatalytic selective transformation. 4. Conclusion ZnIn2S4 microspheres are synthesized by a facile hydrothermal method at 80 °C. The results of XRD, FESEM, TG–DSC and STEM-EDX elemental mapping demonstrate that as-synthesized sample is pure hexagonal phase ZnIn2S4 microspheres. The ZnIn2S4 sample is first used in photocatalytic selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation and ambient conditions. The ZnIn2S4 sample shows the conversion of benzyl alcohol is ca. 69% along with the high selectivity, ca. 94%, after 3 h visible light irradiation.
XRD and XPS investigations suggest that ZnIn2S4 is relatively stable in the photocatalytic reaction. Consequently, the ZnIn2S4 sample should be exploited to other photocatalytic selective transformation, and more other ternary chalcogenides should be exploited to the area of photocatalytic selective transformation. Acknowledgment This work is financially supported by the Natural Science Foundation of Fujian, China (2011J05024). References [1] M. Sun, D.Z. Li, W.J. Zhang, Z.X. Chen, H.J. Huang, W.J. Li, Y.H. He, X.Z. Fu, J. Solid State Chem. 190 (2012) 135–142. [2] T.Y. Peng, D.N. Ke, J.R. Xiao, L. Wang, J. Hu, L. Zan, J. Solid State Chem. 194 (2012) 250–256. [3] G. Patrinoiu, M. Tudose, J.M. Calderon-Moreno, R. Birjega, P. Budrugeac, R. Ene, O. Carp, J. Solid State Chem. 186 (2012) 17–22. [4] M.D. Tzirakis, I.N. Lykakis, G.D. Panagiotou, K. Bourikas, A. Lycourghiotis, C. Kordulis, M. Orfanopoulos, J. Catal. 252 (2007) 178–189. [5] K. Ohkubo, K. Suga, S. Fukuzumi, Chem. Commun. (2006) 2018–2020. [6] S. Higashimoto, N. Kitao, N. Yoshida, T. Sakura, M. Azuma, H. Ohue, Y. Sakata, J. Catal. 266 (2009) 279–285. [7] Y.H. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 5 (2011) 7426–7435. [8] C.J. Li, G.R. Xu, B.H. Zhang, J.R. Gong, Appl. Catal. B Environ. 115 (2012) 201–208. [9] R. Marotta, I. Di Somma, D. Spasiano, R. Andreozzi, V. Caprio, Chem. Eng. J. 172 (2011) 243–249. [10] Z.X. Chen, D.Z. Li, G.C. Xiao, Y.H. He, Y.J. Xu, J. Solid State Chem. 186 (2012) 247–254. [11] B. Chai, T.Y. Peng, P. Zeng, X.H. Zhang, Dalton Trans. 41 (2012) 1179–1186. [12] Z.X. Chen, D.Z. Li, W.J. Zhang, Y. Shao, T.W. Chen, M. Sun, X.Z. Fu, J. Phys. Chem. C 113 (2009) 4433–4440. [13] S.H. Shen, L. Zhao, L.J. Guo, J. Phys. Chem. Solids 69 (2008) 2426–2432. [14] Z.X. Chen, D.Z. Li, W.J. Zhang, C. Chen, W.J. Li, M. Sun, Y.H. He, X.Z. Fu, Inorg. Chem. 47 (2008) 9766–9772. [15] Z.D. Xu, Y.X. Li, S.Q. Peng, G.X. Lu, S.B. Li, RSC Adv. 2 (2012) 3458–3466.