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Preparation of CdIn2S4 microspheres and application for photocatalytic reduction of carbon dioxide
Applied Surface Science 288 (2014) 138–142
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of CdIn2 S4 microspheres and application for photocatalytic reduction of carbon dioxide Wanlin Jiang a , Xiaohong Yin a,∗ , Feng Xin b , Yadong Bi a,∗ , Yong Liu a , Xia Li a a b
School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 5 August 2013 Received in revised form 26 September 2013 Accepted 28 September 2013 Available online 9 October 2013 Keywords: CdIn2 S4 Photocatalytic reduction CO2 Methanol Dimethoxymethane Methyl formate
a b s t r a c t A series of novel microspheres of CdIn2 S4 were prepared by hydrothermal process, among them the CdIn2 S4 synthesized from l-cysteine exhibited higher photocatalytic activity for CO2 reduction and has potential application for using visible light. Characterization of X-ray diffractometer (XRD), UV–vis absorption spectrometry (UV–vis), field emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM) and N2 sorption analysis resulted in the crystal morphology, light absorption band and porous geometry. The mechanism of photocatalytic reduction of CO2 in methanol over CdIn2 S4 was also proposed. The narrow band gap of the as prepared catalyst promoted reducing CO2 to dimethoxymethane and methyl formate in methanol. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Since a variety of human activities, such as electric power generation and transportation, CO2 emission has increased greatly and caused environmental problems seriously, the greenhouse effect has changed the global climate. In order to abate total amount of CO2 emitted, various technologies have been exploring. Photocatalytically reducing CO2 came into notice due to utilizing solar energy and conversing CO2 to valuable chemicals or fuel. By now TiO2 acted as an inexpensive and easily obtained material has been employed in many photocatalytic reactions, but its wider band gap hinders in utilizing solar energy. Many researchers, therefore, modified the TiO2 by doping and compositing Ag2 S Au–Ag in TiO2 , etc. [1–4] or synthesizing novel materials, including ZnIn2 S4 , Zn2 SnO4 , etc. [5,6]. Up to date, many efforts have been focused on the photocatalytic reduction of CO2 . Zhou et al. reported that square nanoplates of Bi2 WO6 benefited photocatalytic reduction of CO2 into CH4 [7]. Liang et al. used grapheme-TiO2 nanocomposite for photocatalytically reducing CO2 [8]. But the conversion rate of CO2 to
hydrocarbons is still low. Developing novel photocatalysts to efficiently conversing CO2 will be severe challenge. CdIn2 S4 , belonging to ternary semiconductors of chalcogenide AB2 X4 and being considered to have potential applications in solar cells and optoelectronic devices [9], has been used for H2 evolution, bacterial inactivation and degradation of methyl orange [10–13]. To the best of our knowledge, the CdIn2 S4 nanocrystals for CO2 photocatalytic reduction have not been reported. In our previous work, methyl formate (MF) was produced through photocatalytic reduction of CO2 on Ag loaded SrTiO3 in the presence of methanol [14]. In our present work, a chemically stable CdIn2 S4 was synthesized using l-cysteine as sulfur resource via a facile hydrothermal process. The obtained microspheric CdIn2 S4 that was composed of many nano sheets had a considerable photocatalytic activity for photocatalytically reducing CO2 to quantificationally modulated dimethoxymethane (DMM) and methyl formate (MF), as a new finding, we exposed their reaction mechanism. 2. Experimental 2.1. Synthesis of CdIn2 S4
∗ Corresponding authors. Tel.: +86 13820315937. E-mail addresses: [email protected] (X. Yin), easecloud [email protected] (Y. Bi). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.165
Three kinds of CdIn2 S4 powders were prepared by hydrothermally synthetic method. All reagents were purchased from Tianjin Benchmark Chemical Reagent Company and used without further
W. Jiang et al. / Applied Surface Science 288 (2014) 138–142
purification. Firstly, InCl3 ·4H2 O, CdCl2 ·2.5H2 O, l-cysteine or thioacetamide or thiourea with a molar ratio of 2:1:4 were dissolved in 55 ml deionized water to obtain three transparent liquids via magnetically stirring for 15 min. Then, each of them was poured into a 75 ml of Teflon-lined stainless steel autoclave. Subsequently, the three autoclaves were kept at 180 ◦ C for 48 h and cooled down to the room temperature. The obtained brown precipitates were collected after centrifugal separation. Finally, three catalysts were obtained by washing the solid precipitates with absolute ethyl alcohol and deionized water for three times and drying at 80 ◦ C over night.
Table 1 Crystal sizes and lattice spacings of CdIn2 S4 . Samples
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
Crystal size (nm) Lattice spacing (Å)
15.92 3.2593
18.33 3.2574
25.03 3.2574
Table 2 Surface areas of CdIn2 S4 samples. Photocatalyst
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
BET surface area (m2 /g)
24.03
23.56
13.20
2.2. Characterization The as prepared CdIn2 S4 powders were characterized by a Rigaku D X-ray powder diffractometer with Cu K␣ radiation at a scan rate of 2 min−1 . FE-SEM of JEOL-JSM6700F and HR-TEM of Tecnai G2 F20 were employed out to observe the morphology. Spectra of UV–vis by Shimadzu UV-2550 were obtained using Ba2 SO4 as a reflectance standard. The surface areas were measured by BET of APP V-Sorb 2800P using nitrogen sorption at 77 K. 2.3. Photocatalytic reaction Photocatalytic reactions were taken place in a slurry reactor with quartz window and cooling water jacket. Before irritation 0.02 g CdIn2 S4 and 20 ml methanol were put into the reactor. Under stirring, the CO2 was bubbled through the suspension at a rate of 200 ml/min for 30 min to sweep the air in the reactor and absorbed oxygen in methanol. Then the reactor was sealed and irradiated by a 250 W high pressure mercury lamp over the quartz window at an intensity of 2500 W/cm2 reaching the slurry surface. After 10 h irradiation, the concentration of methyl formate and dimethoxymethane in the solution was measured by a gas chromatography (GC) of Agilent 7890A equipped with a flame ionization detector (FID). Comparative tests were implemented under Ar instead of CO2 . 3. Results and discussion 3.1. Characterization of photocatalysts In Fig. 1 the XRD patterns of CdIn2 S4 that prepared with sulfur sources of l-cysteine, thioacetamide and thiourea respectively could refer to the cubic spinel structure of CdIn2 S4 (JCPDS-2760). The broad diffraction peaks in Fig. 1(a) indicated smaller crystals
Fig. 1. XRD pattern of CdIn2 S4 prepared with different sulfur source: (a) l-cysteine, (b) thioacetamide and (c) thiourea.
139
were formed when l-cysteine was sulfur source. Contrasting three patterns in Fig. 1, we could find that (b) and (c) had stronger and sharper peaks than (a), which meant the crystallinities of the CdIn2 S4 from thioacetamide and thiourea were better than that from l-cysteine. The facet [3 1 1] was chosen to calculate the crystallite size by using the Debyee–Scherrer equation and lattice spacing, the results are listed in Table 1. The morphologies and microstructures of the samples were further characterized by SEM and TEM. As shown in Fig. 2(a) and (b), marigold-like CdIn2 S4 spheres with an average diameter of 5 m were assembled by numerous flakes. Fig. 2(g) and (h) showed the HRTEM images of the CdIn2 S4 synthesized from l-cysteine. The lattice spacings between (3 1 1) and (2 2 0) facets were 3.295 A˚ and 3.846 A˚ respectively, which were in accordance with the XRD analysis. In Fig. 2(c) and (d) the morphologies of CdIn2 S4 synthesized from thioacetamide and thiourea were different from that synthesized from l-cysteine. Although synthesis condition was the same, the CdIn2 S4 from thioacetamide had an average diameter of 4 m and was composed of dozen of flakes and particles, which were accumulated to form irregular and looser spheres than that from l-cysteine. When sulfur source changed to thiourea, the CdIn2 S4 exhibited much larger diameter of 9 m, and some bipyramid-like particles aggregated on its spheric surface. Undoubtedly, coordination ability of sulfur sources plays a key role in morphologies of CdIn2 S4 . l-cysteine can coordinate with In3+ and CdIn2 S4 strongly to form precursor complexes, but lower coordination abilities are given by thioacetamide and thiourea [15]. The BET specific surface areas of the as-prepared CdIn2 S4 samples were calculated and summarized in Table 2. It is obvious that the CdIn2 S4 synthesized from l-cysteine and thioacetamide had much larger surface area than that from thiourea, and the largest one was CdIn2 S4 synthesized from l-cysteine. The reason could be found by observing the puffy appearance of two samples with larger surface area and the dense appearance of the sample from thiourea in Fig. 2(b), (d) and (f). The smaller crystallite size of sample from l-cysteine also leads to a larger specific surface area. Compared images in Fig. 2, the CdIn2 S4 prepared from l-cysteine has more perfect morphology, which will provide larger irritated area and result in a higher absorbance. Large surface area of microsphere not only provides more active sites, but also enhances the ability of light absorption [16,17]. Fig. 3 showed the UV–vis diffuse reflection spectra (DRS) of the catalysts prepared from three different sulfur sources. The absorption edges of the CdIn2 S4 samples synthesized from lcysteine, thiourea and thioacetamide were 737, 592 and 605 nm, the corresponding band gaps were 1.68, 2.09 and 2.05 eV, respectively. The smaller value of absorption edge of the sample from l-cysteine could contribute to its smaller crystal size and regular morphology, which reflected an opened architecture constituted
140
W. Jiang et al. / Applied Surface Science 288 (2014) 138–142
Fig. 2. SEM images of CdIn2 S4 at different magnifications: (a–b) l-cysteine, (c–d) thioacetamide, (e–f) thiourea and (g–h) TEM and HRTEM image of l-cysteine synthesized sample.
of many nanosheets. Such kind of 3D nanocomposite could prohibited recombination of electron–hole pairs [18]. The band gap energy of CdIn2 S4 synthesized from l-cysteine is suitable to visible light photocatalysis even more. The conduction band edge (ECB ) and valence band edge (EVB ) were also calculated by the empirical equation [21] and displayed in Table 3.
Table 3 Band gap energies of CdIn2 S4 samples. Photocatalyst
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
Band gap (eV) ECB vs. NHE (V) EVB vs. NHE (V)
1.68 −0.50 1.18
2.09 −0.705 1.385
2.05 −0.685 1.365
W. Jiang et al. / Applied Surface Science 288 (2014) 138–142
141
Fig. 4. Schematic mechanism of photocatalysis. Fig. 3. UV–vis DRS spectra of CdIn2 S4 synthesized by: (a) l-cysteine, (b) thiourea and (c) thioacetamide. Table 4 Photocatalytic activities of CdIn2 S4 for CO2 reduction. Photocatalyst
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
DMM (mol h−1 g−1 ) MF (mol h−1 g−1 )
2968 2857
0 3604
0 5258
Table 5 Photocatalytic activities of CdIn2 S4 under Ar condition. Photocatalyst
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
DMM (mol h−1 g−1 ) MF (mol h−1 g−1 )
0 2884
0 2030
0 2163
3.2. Photocatalytic activity Table 4 shows a comparison of the photocatalytic activity of the CdIn2 S4 catalysts prepared from three different sulphur sources. Obviously, the catalyst prepared from l-cysteine had a unique photocatalytic property in both products of dimethoxymethane (DMM) and methyl formate (MF). While the other two catalysts only promoted the formation of MF, and the total formation rates were lower than CdIn2 S4 synthesized from l-cysteine. In order to deeply understand the reaction, blank experiments were carried out. Firstly, we substituted Ar for CO2 and the results were displayed in Table 5. The formation rate of MF over the catalyst synthesized from l-cysteine was almost the same as that in CO2 passing through the reactor, but DMM was not detected. Meanwhile, the bubbling of Ar gas results in MF evolution rate decreased for the catalysts synthesized from thioacetamide and thiourea. We summarized the photocatalytic mechanism in Fig. 4. Due to the narrow band gap of 1.68 eV and the potential energies of conduction and valance bands of CdIn2 S4 (l-cysteine) were more negative than that of reducing CO2 to formaldehyde and more positive than that to oxidize methanol to formic acid [19,20]. So the formaldehyde from reducing CO2 on the conduction band (CB) would react with methanol to produce DMM through aldol condensation. Meanwhile, the formic acid coming from methanol oxidation could react with methanol to produce MF through esterification. While at the presence of the Ar, no formaldehyde was generated and led to an undetected DMM. But for the rest two kinds of CdIn2 S4 catalysts synthesized from thioacetamide and thiourea, their potential energies of CB were much more negative than that of CdIn2 S4 catalysts synthesized
from l-cysteine, so that CO2 could be reduced to formic acid, and on the VB methanol would be oxidized to formic acid too, which would react with methanol in bulk to form MF. That is the reason of the unique product to be MF. While under the Ar condition the MF generated from CO2 reduction was impossible, the only way was methanol oxidation and etherification. In summary, the catalyst from l-cysteine has a tendency to transform CO2 to DMM, while the other two catalysts enhance to form MF only. The large surface area and an extensive light absorption resulted in the most positive photocatalytic activity. Here methanol was taken as both sacrificial reagent and solvent to effectively enhance the separation the electron–hole pairs in VB or CB and the solubility of CO2 in liquid phase. 4. Conclusion Microspheric CdIn2 S4 photocatalysts were prepared by hydrothermal synthesis and using three different sulfur sources, which resulted in different band gaps and potential energies on CB and VB. Among them CdIn2 S4 prepared from l-cysteine possessed the most perfect spheric morphology, larger surface area and stronger absorbility to visible light, which collaborated to the high evolution rates of DMM and MF. However, the MF was the only product of photocatalytically conversing CO2 in methanol over the CdIn2 S4 microspheres synthesized from thioacetamide and thiourea. The photocatalytic mechanisms were proposed for deeply understanding the reaction and designing the photocatalysts. Furthermore, the novel CdIn2 S4 specially synthesized from l-cysteine has a potentially prospective application in utilizing the solar energy. Acknowledgments We gratefully acknowledge financial support by the National Natural Science Foundation of China (NSFC) (No. 21003095), (No. 21176192) and the Tianjin natural science foundation (No. 12JCZDJC29400). References [1] Z. Lei, M. Zeda, T. Ghosh, O. Won-Chun, Chin. J. Catal. 33 (2012) 254–260. [2] D. Tsukamoto, A. Shiro, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka, T. Hirai, ACS Catal. 2 (2012) 599–603. [3] J. Zhang, S. Yan, S.L. Zhao, Q. Xu, C. Li, Appl. Surf. Sci. 280 (2013) 304–311. [4] Y.T. Zhu, W. Wei, Y. Dai, B.B. Huang, Appl. Surf. Sci. 258 (2012) 4806–4812. [5] Z.D. Li, Y. Zhou, J.Y. Zhang, W.G. Tu, Q. Liu, T. Yu, Z.G. Zou, Cryst. Growth Des. 12 (2012) 1476–1481. [6] Z.X. Chen, D.Z. Li, G.C. Xiao, Y.H. He, Y.-J.J. Xu, Solid State Chem. 186 (2012) 247–254.
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[7] Y. Zhou, Z.P. Tian, Z.Y. Zhao, Q. Liu, J.H. Kou, X.Y. Chen, J. Gao, S.C. Yan, Z.G. Zou, ACS Appl. Mater. Interfaces 3 (2011) 3594–3601. [8] Y.T. Liang, B.K. Vijayan, K.A. Gray, M.C. Hersam, Nano Lett. 11 (2011) 2865–2870. [9] L. Fan, R. Guo, J. Phys. Chem. 112 (2008) 10700–10706. [10] B.B. Kale, J.-O. Baeg, S.M. Le, H. Chang, S.-J. Moon, C.W. Lee, Adv. Funct. Mater. 16 (2006) 1349–1354. [11] W.J. Wang, T.W. Ng, W.K. Ho, J.H. Huang, S.J. Liang, T.C. An, G.Y. Li, J.C. Yu, P.K. Wong, Appl. Catal. B: Environ. 129 (2013) 482–490. [12] J. Mu, Q.L. Wei, P.P. Yao, X.L. Zhao, S.-Z. Kang, X.Q. Li, J. Alloys Compd. 513 (2012) 506–509. [13] A. Bhirud, N. Chaudhari, L. Nikam, R. Sonawane, K. Patil, J.-O. Baeg, B. Kale, Int. J. Hydrogen Energy 36 (2011) 11628–11639.
[14] D.D. Sui, X.H. Yin, H.Z. Dong, S.Y. Qing, J.S. Chen, W.L. Jiang, Catal. Lett. 142 (2012) 1202–1210. [15] L.-Y. Chen, Z.-D. Zhang, W.-Z. Wang, J. Phys. Chem. 112 (2008) 4117–4123. [16] J.C. Yu, X.C. Wang, X.Z. Fu, Chem. Mater. 16 (2004) 1523–1530. [17] H.X. Li, Z.F. Bian, J. Zhu, D.Q. Zhang, G.S. Li, Y.N. Huo, H. Li, Y.F. Lu, J. Am. Chem. Soc. 129 (2007) 8406–8407. [18] M. Nanu, J. Schoonman, A. Goossens, Nano Lett. 5 (2005) 1716–1719. [19] X.B. Chen, S.H. Shen, L.G. Guo, S.S. Mao, Chem. Rev. 110 (2010) 6503–6570. [20] S.C. Roy, O.K. Varghese, M. Paulose, C.A. Grimes, ACS Nano 4 (2010) 1259–1278. [21] Y. Xu, M.A.A. Schoonen, Am. Mineral. 85 (2000) 543–556.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of CdIn2 S4 microspheres and application for photocatalytic reduction of carbon dioxide Wanlin Jiang a , Xiaohong Yin a,∗ , Feng Xin b , Yadong Bi a,∗ , Yong Liu a , Xia Li a a b
School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 5 August 2013 Received in revised form 26 September 2013 Accepted 28 September 2013 Available online 9 October 2013 Keywords: CdIn2 S4 Photocatalytic reduction CO2 Methanol Dimethoxymethane Methyl formate
a b s t r a c t A series of novel microspheres of CdIn2 S4 were prepared by hydrothermal process, among them the CdIn2 S4 synthesized from l-cysteine exhibited higher photocatalytic activity for CO2 reduction and has potential application for using visible light. Characterization of X-ray diffractometer (XRD), UV–vis absorption spectrometry (UV–vis), field emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM) and N2 sorption analysis resulted in the crystal morphology, light absorption band and porous geometry. The mechanism of photocatalytic reduction of CO2 in methanol over CdIn2 S4 was also proposed. The narrow band gap of the as prepared catalyst promoted reducing CO2 to dimethoxymethane and methyl formate in methanol. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Since a variety of human activities, such as electric power generation and transportation, CO2 emission has increased greatly and caused environmental problems seriously, the greenhouse effect has changed the global climate. In order to abate total amount of CO2 emitted, various technologies have been exploring. Photocatalytically reducing CO2 came into notice due to utilizing solar energy and conversing CO2 to valuable chemicals or fuel. By now TiO2 acted as an inexpensive and easily obtained material has been employed in many photocatalytic reactions, but its wider band gap hinders in utilizing solar energy. Many researchers, therefore, modified the TiO2 by doping and compositing Ag2 S Au–Ag in TiO2 , etc. [1–4] or synthesizing novel materials, including ZnIn2 S4 , Zn2 SnO4 , etc. [5,6]. Up to date, many efforts have been focused on the photocatalytic reduction of CO2 . Zhou et al. reported that square nanoplates of Bi2 WO6 benefited photocatalytic reduction of CO2 into CH4 [7]. Liang et al. used grapheme-TiO2 nanocomposite for photocatalytically reducing CO2 [8]. But the conversion rate of CO2 to
hydrocarbons is still low. Developing novel photocatalysts to efficiently conversing CO2 will be severe challenge. CdIn2 S4 , belonging to ternary semiconductors of chalcogenide AB2 X4 and being considered to have potential applications in solar cells and optoelectronic devices [9], has been used for H2 evolution, bacterial inactivation and degradation of methyl orange [10–13]. To the best of our knowledge, the CdIn2 S4 nanocrystals for CO2 photocatalytic reduction have not been reported. In our previous work, methyl formate (MF) was produced through photocatalytic reduction of CO2 on Ag loaded SrTiO3 in the presence of methanol [14]. In our present work, a chemically stable CdIn2 S4 was synthesized using l-cysteine as sulfur resource via a facile hydrothermal process. The obtained microspheric CdIn2 S4 that was composed of many nano sheets had a considerable photocatalytic activity for photocatalytically reducing CO2 to quantificationally modulated dimethoxymethane (DMM) and methyl formate (MF), as a new finding, we exposed their reaction mechanism. 2. Experimental 2.1. Synthesis of CdIn2 S4
∗ Corresponding authors. Tel.: +86 13820315937. E-mail addresses: [email protected] (X. Yin), easecloud [email protected] (Y. Bi). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.165
Three kinds of CdIn2 S4 powders were prepared by hydrothermally synthetic method. All reagents were purchased from Tianjin Benchmark Chemical Reagent Company and used without further
W. Jiang et al. / Applied Surface Science 288 (2014) 138–142
purification. Firstly, InCl3 ·4H2 O, CdCl2 ·2.5H2 O, l-cysteine or thioacetamide or thiourea with a molar ratio of 2:1:4 were dissolved in 55 ml deionized water to obtain three transparent liquids via magnetically stirring for 15 min. Then, each of them was poured into a 75 ml of Teflon-lined stainless steel autoclave. Subsequently, the three autoclaves were kept at 180 ◦ C for 48 h and cooled down to the room temperature. The obtained brown precipitates were collected after centrifugal separation. Finally, three catalysts were obtained by washing the solid precipitates with absolute ethyl alcohol and deionized water for three times and drying at 80 ◦ C over night.
Table 1 Crystal sizes and lattice spacings of CdIn2 S4 . Samples
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
Crystal size (nm) Lattice spacing (Å)
15.92 3.2593
18.33 3.2574
25.03 3.2574
Table 2 Surface areas of CdIn2 S4 samples. Photocatalyst
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
BET surface area (m2 /g)
24.03
23.56
13.20
2.2. Characterization The as prepared CdIn2 S4 powders were characterized by a Rigaku D X-ray powder diffractometer with Cu K␣ radiation at a scan rate of 2 min−1 . FE-SEM of JEOL-JSM6700F and HR-TEM of Tecnai G2 F20 were employed out to observe the morphology. Spectra of UV–vis by Shimadzu UV-2550 were obtained using Ba2 SO4 as a reflectance standard. The surface areas were measured by BET of APP V-Sorb 2800P using nitrogen sorption at 77 K. 2.3. Photocatalytic reaction Photocatalytic reactions were taken place in a slurry reactor with quartz window and cooling water jacket. Before irritation 0.02 g CdIn2 S4 and 20 ml methanol were put into the reactor. Under stirring, the CO2 was bubbled through the suspension at a rate of 200 ml/min for 30 min to sweep the air in the reactor and absorbed oxygen in methanol. Then the reactor was sealed and irradiated by a 250 W high pressure mercury lamp over the quartz window at an intensity of 2500 W/cm2 reaching the slurry surface. After 10 h irradiation, the concentration of methyl formate and dimethoxymethane in the solution was measured by a gas chromatography (GC) of Agilent 7890A equipped with a flame ionization detector (FID). Comparative tests were implemented under Ar instead of CO2 . 3. Results and discussion 3.1. Characterization of photocatalysts In Fig. 1 the XRD patterns of CdIn2 S4 that prepared with sulfur sources of l-cysteine, thioacetamide and thiourea respectively could refer to the cubic spinel structure of CdIn2 S4 (JCPDS-2760). The broad diffraction peaks in Fig. 1(a) indicated smaller crystals
Fig. 1. XRD pattern of CdIn2 S4 prepared with different sulfur source: (a) l-cysteine, (b) thioacetamide and (c) thiourea.
139
were formed when l-cysteine was sulfur source. Contrasting three patterns in Fig. 1, we could find that (b) and (c) had stronger and sharper peaks than (a), which meant the crystallinities of the CdIn2 S4 from thioacetamide and thiourea were better than that from l-cysteine. The facet [3 1 1] was chosen to calculate the crystallite size by using the Debyee–Scherrer equation and lattice spacing, the results are listed in Table 1. The morphologies and microstructures of the samples were further characterized by SEM and TEM. As shown in Fig. 2(a) and (b), marigold-like CdIn2 S4 spheres with an average diameter of 5 m were assembled by numerous flakes. Fig. 2(g) and (h) showed the HRTEM images of the CdIn2 S4 synthesized from l-cysteine. The lattice spacings between (3 1 1) and (2 2 0) facets were 3.295 A˚ and 3.846 A˚ respectively, which were in accordance with the XRD analysis. In Fig. 2(c) and (d) the morphologies of CdIn2 S4 synthesized from thioacetamide and thiourea were different from that synthesized from l-cysteine. Although synthesis condition was the same, the CdIn2 S4 from thioacetamide had an average diameter of 4 m and was composed of dozen of flakes and particles, which were accumulated to form irregular and looser spheres than that from l-cysteine. When sulfur source changed to thiourea, the CdIn2 S4 exhibited much larger diameter of 9 m, and some bipyramid-like particles aggregated on its spheric surface. Undoubtedly, coordination ability of sulfur sources plays a key role in morphologies of CdIn2 S4 . l-cysteine can coordinate with In3+ and CdIn2 S4 strongly to form precursor complexes, but lower coordination abilities are given by thioacetamide and thiourea [15]. The BET specific surface areas of the as-prepared CdIn2 S4 samples were calculated and summarized in Table 2. It is obvious that the CdIn2 S4 synthesized from l-cysteine and thioacetamide had much larger surface area than that from thiourea, and the largest one was CdIn2 S4 synthesized from l-cysteine. The reason could be found by observing the puffy appearance of two samples with larger surface area and the dense appearance of the sample from thiourea in Fig. 2(b), (d) and (f). The smaller crystallite size of sample from l-cysteine also leads to a larger specific surface area. Compared images in Fig. 2, the CdIn2 S4 prepared from l-cysteine has more perfect morphology, which will provide larger irritated area and result in a higher absorbance. Large surface area of microsphere not only provides more active sites, but also enhances the ability of light absorption [16,17]. Fig. 3 showed the UV–vis diffuse reflection spectra (DRS) of the catalysts prepared from three different sulfur sources. The absorption edges of the CdIn2 S4 samples synthesized from lcysteine, thiourea and thioacetamide were 737, 592 and 605 nm, the corresponding band gaps were 1.68, 2.09 and 2.05 eV, respectively. The smaller value of absorption edge of the sample from l-cysteine could contribute to its smaller crystal size and regular morphology, which reflected an opened architecture constituted
140
W. Jiang et al. / Applied Surface Science 288 (2014) 138–142
Fig. 2. SEM images of CdIn2 S4 at different magnifications: (a–b) l-cysteine, (c–d) thioacetamide, (e–f) thiourea and (g–h) TEM and HRTEM image of l-cysteine synthesized sample.
of many nanosheets. Such kind of 3D nanocomposite could prohibited recombination of electron–hole pairs [18]. The band gap energy of CdIn2 S4 synthesized from l-cysteine is suitable to visible light photocatalysis even more. The conduction band edge (ECB ) and valence band edge (EVB ) were also calculated by the empirical equation [21] and displayed in Table 3.
Table 3 Band gap energies of CdIn2 S4 samples. Photocatalyst
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
Band gap (eV) ECB vs. NHE (V) EVB vs. NHE (V)
1.68 −0.50 1.18
2.09 −0.705 1.385
2.05 −0.685 1.365
W. Jiang et al. / Applied Surface Science 288 (2014) 138–142
141
Fig. 4. Schematic mechanism of photocatalysis. Fig. 3. UV–vis DRS spectra of CdIn2 S4 synthesized by: (a) l-cysteine, (b) thiourea and (c) thioacetamide. Table 4 Photocatalytic activities of CdIn2 S4 for CO2 reduction. Photocatalyst
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
DMM (mol h−1 g−1 ) MF (mol h−1 g−1 )
2968 2857
0 3604
0 5258
Table 5 Photocatalytic activities of CdIn2 S4 under Ar condition. Photocatalyst
CdIn2 S4 (l-cysteine)
CdIn2 S4 (thioacetamide)
CdIn2 S4 (thiourea)
DMM (mol h−1 g−1 ) MF (mol h−1 g−1 )
0 2884
0 2030
0 2163
3.2. Photocatalytic activity Table 4 shows a comparison of the photocatalytic activity of the CdIn2 S4 catalysts prepared from three different sulphur sources. Obviously, the catalyst prepared from l-cysteine had a unique photocatalytic property in both products of dimethoxymethane (DMM) and methyl formate (MF). While the other two catalysts only promoted the formation of MF, and the total formation rates were lower than CdIn2 S4 synthesized from l-cysteine. In order to deeply understand the reaction, blank experiments were carried out. Firstly, we substituted Ar for CO2 and the results were displayed in Table 5. The formation rate of MF over the catalyst synthesized from l-cysteine was almost the same as that in CO2 passing through the reactor, but DMM was not detected. Meanwhile, the bubbling of Ar gas results in MF evolution rate decreased for the catalysts synthesized from thioacetamide and thiourea. We summarized the photocatalytic mechanism in Fig. 4. Due to the narrow band gap of 1.68 eV and the potential energies of conduction and valance bands of CdIn2 S4 (l-cysteine) were more negative than that of reducing CO2 to formaldehyde and more positive than that to oxidize methanol to formic acid [19,20]. So the formaldehyde from reducing CO2 on the conduction band (CB) would react with methanol to produce DMM through aldol condensation. Meanwhile, the formic acid coming from methanol oxidation could react with methanol to produce MF through esterification. While at the presence of the Ar, no formaldehyde was generated and led to an undetected DMM. But for the rest two kinds of CdIn2 S4 catalysts synthesized from thioacetamide and thiourea, their potential energies of CB were much more negative than that of CdIn2 S4 catalysts synthesized
from l-cysteine, so that CO2 could be reduced to formic acid, and on the VB methanol would be oxidized to formic acid too, which would react with methanol in bulk to form MF. That is the reason of the unique product to be MF. While under the Ar condition the MF generated from CO2 reduction was impossible, the only way was methanol oxidation and etherification. In summary, the catalyst from l-cysteine has a tendency to transform CO2 to DMM, while the other two catalysts enhance to form MF only. The large surface area and an extensive light absorption resulted in the most positive photocatalytic activity. Here methanol was taken as both sacrificial reagent and solvent to effectively enhance the separation the electron–hole pairs in VB or CB and the solubility of CO2 in liquid phase. 4. Conclusion Microspheric CdIn2 S4 photocatalysts were prepared by hydrothermal synthesis and using three different sulfur sources, which resulted in different band gaps and potential energies on CB and VB. Among them CdIn2 S4 prepared from l-cysteine possessed the most perfect spheric morphology, larger surface area and stronger absorbility to visible light, which collaborated to the high evolution rates of DMM and MF. However, the MF was the only product of photocatalytically conversing CO2 in methanol over the CdIn2 S4 microspheres synthesized from thioacetamide and thiourea. The photocatalytic mechanisms were proposed for deeply understanding the reaction and designing the photocatalysts. Furthermore, the novel CdIn2 S4 specially synthesized from l-cysteine has a potentially prospective application in utilizing the solar energy. Acknowledgments We gratefully acknowledge financial support by the National Natural Science Foundation of China (NSFC) (No. 21003095), (No. 21176192) and the Tianjin natural science foundation (No. 12JCZDJC29400). References [1] Z. Lei, M. Zeda, T. Ghosh, O. Won-Chun, Chin. J. Catal. 33 (2012) 254–260. [2] D. Tsukamoto, A. Shiro, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka, T. Hirai, ACS Catal. 2 (2012) 599–603. [3] J. Zhang, S. Yan, S.L. Zhao, Q. Xu, C. Li, Appl. Surf. Sci. 280 (2013) 304–311. [4] Y.T. Zhu, W. Wei, Y. Dai, B.B. Huang, Appl. Surf. Sci. 258 (2012) 4806–4812. [5] Z.D. Li, Y. Zhou, J.Y. Zhang, W.G. Tu, Q. Liu, T. Yu, Z.G. Zou, Cryst. Growth Des. 12 (2012) 1476–1481. [6] Z.X. Chen, D.Z. Li, G.C. Xiao, Y.H. He, Y.-J.J. Xu, Solid State Chem. 186 (2012) 247–254.
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