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Facile synthesis of Ni(OH)1.4(SO4)0.3 nanoribbons and their photocatalytic properties
Journal of Alloys and Compounds 540 (2012) 127–132
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Facile synthesis of Ni(OH)1.4(SO4)0.3 nanoribbons and their photocatalytic properties Hai Zhou a,c, Baoliang Lv a,⇑, Dong Wu a, Yuhan Sun a,b a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China Low Carbon Conversion Center, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, PR China c Graduate University of the Chinese Academy of Science, Beijing 100049, PR China b
a r t i c l e
i n f o
Article history: Received 2 November 2011 Accepted 14 June 2012 Available online 22 June 2012 Keywords: Hydrothermal Ni(OH)1.4(SO4)0.3 Nanoribbons Photocatalytic
a b s t r a c t With the presence of SCN anions, Ni(OH)1.4(SO4)0.3 nanoribbons have been successfully synthesized by one-step hydrothermal method. The synthesized nanoribbons are with width of several tens of nanometers and length of several tens of micrometers. X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analysis revealed that the products were mainly dominated by (0 0 2) lattice plane. On the basis of condition-dependent experiments, a recrystallization–oxidation-split growth mechanism was proposed to explain the formation process of the Ni(OH)1.4(SO4)0.3 nanoribbons. As-synthesized products showed a photocatalytic capability under the irradiation of ultraviolet light for the decomposition of methyl orange as a probe reaction. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the shape control and modification of nanostructures has been a major topic for functional materials, since the morphology, size and chemical composition of materials had a great influence on their physical and chemical properties [1,2]. Among those materials, Nickel hydroxide has attracted great interest due to its importance as cathode material in a number of rechargeable alkaline batteries (e.g., Ni/Cd, Ni/H2, Ni/MH and Ni/Zn) [3], and the control of micro-structured nickel hydroxides led to the products with different morphologies such as nanowires [4], nanobelts [5], nanoboards [6], and nanoflowers [7], nanosheets and hierarchical microsphere [8]. And a-Ni(OH)2 is a promising one which raised great attention. It is well-known that a-Ni(OH)2 transforms from b-Ni(OH)2 by insert various anions (SO42 , CO32 , Cl and so on) or water molecules between interlayer [9], being a family of layered compounds with many new properties [10]. Thus, they had potential applications in the fields including catalysts [11], catalyst precursors [12], anionic exchange and electrode materials [13]. Ni(OH)1.4 (SO4)0.3, as a typical compound with such a structure, have been reported by several groups. But all of these works introduced the SO42 before the synthesis [5,14,15]. Herein, we report a new route to synthesize Ni(OH)1.4(SO4)0.3 with the nanoribbon structure in the presence of SCN anions, and their photocatalytic properties as well.
⇑ Corresponding author. Tel.: +86 0351 4049859; fax: +86 0351 4041153. E-mail address: [email protected] (B. Lv). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.059
2. Experimental All the reagents used without further purification: Ni(NO3)26H2O (98.5 wt.%, analytical reagent (A. R.), Tianjin Beichen Fangzheng Chemicals Co. Ltd.), NaSCN (98 wt.%, A. R., Tianjin Tianda Chemicals Co. Ltd.), NaOH (98 wt.%, A. R., Beijing Beida Chemicals Co. Ltd.). Double-distilled water was used throughout the experiment.
2.1. Synthesis of nickel basic sulfate nanoribbons For the preparation of Ni(OH)1.4(SO4)0.3 nanoribbons, a typical synthesis process was as follows: 1.8 mM of NaSCN and 1.7 mM of Ni(NO3)26H2O were dissolved in 80 mL of distilled water under ultrasonic radiation, and then 1 mL of 0.15 M NaOH aqueous solution was dropped into the above mixture under ultrasonic radiation. The as-formed bright green solution was sealed in a Teflon-lined autoclave of 150 mL capacity, and maintained at 220 °C for 24 h. After cooling to room temperature, the products were collected by naturally sedimentation, washed with distilled water three times and absolute ethanol twice to remove impurities. The final products were dried at 80 °C.
2.2. Characterizations The products were characterized by transmission electron microscopy (TEM, JEOL JEM-1011) and high-resolution TEM (HRTEM, JEM2010), scanning electron microscopy (SEM, XL30 S-FEG), powder X-ray diffraction (XRD, Rigaku D/max-rB diffractometer using Cu Ka radiation, k = 0.15408 nm), Ultraviolet–Visible (UV–Vis Shimadazu 3150).
2.3. Photocatalytic reaction Photocatalytic experiments were carried out in a SGY-1 photoreactor (with a 300 w medium-pressure mercury lamp, Nanjing Sidongke electrical equipment Co. Ltd.) [16]. During the experiment 0.2 g as-obtained samples were dissolved in 400 mL of methyl orange (20 mg/L) aqueous solution, and then the suspension was magnetic stirred in the dark for 30 min to ensure an adsorption/desorption
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H. Zhou et al. / Journal of Alloys and Compounds 540 (2012) 127–132 equilibrium before ultraviolet light irradiation. The concentration of methyl orange aqueous solution was detected using the Shimadazu 3150 UV–Vis spectrophotometer by collecting the absorbance of methyl orange aqueous solution at 465 nm.
3. Results and discussion 3.1. Phase and morphology of the Ni(OH)1.4(SO4)0.3 nanoribbons
Fig. 1. XRD patterns for: (a) Ni(OH)1.4(SO4)0.3 nanoribbons, (b) PDF#41-1424.
Fig. 1 shows the XRD patterns of as-prepared nanoribbons. Accordingly, the product consisted of Ni(OH)1.4(SO4)0.3 (JCPDS 411424). This indicated that crystalline Ni(OH)1.4(SO4)0.3 was obtained via the present hydrothermal process. Furthermore, the relative diffraction intensity of (1 0 0), (0 0 2) and (1 0 2) planes appeared higher than the corresponding standard data [17] (especially the (0 0 2) plane) with the lower intensity of (1 0 3), (0 1 0) and (21-2) planes, being illustrative of predominated (0 0 2) facet for the Ni(OH)1.4(SO4)0.3 nanoribbons. SEM, TEM and HRTEM were carried out for both morphology and microstructure of as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons (see Fig. 2). Clearly, so-produced samples were composed of
Fig. 2. SEM, TEM and HRTEM images of the as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons: (a) and (b) SEM images of the sample, (c) TEM images of the sample, (d) HRTEM images of the sample, (e) enlarged image of the area marked by white frame in (d), the inset show corresponding FFT pattern.
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Fig. 3. SEM images of samples obtained at the synthetic pH of (a) 4.0, (b) 6.9, (c) 7.5 and (d) 7.7.
Fig. 4. XRD pattern of samples obtained at the synthetic pH of (a) 4.0, (b) 6.9, (c) 7.3, (d) 7.5, and (e) 7.7.
uniform ribbon-like structures with the width of several ten nanometers and the length of several ten micrometers (see Fig. 2c and d). Fig. 2e shows an enlarged image of the area marked by white frame in Fig. 2d, and the inset of Fig. 2e was the corresponding fast Fourier transform (FFT) image. The lattice fringe was clearly observed with the distance of adjacent lattice fringes about 0.24 nm (2.4 Å), which was in good agreement with the d value of (0 0 6) plane of the monoclinic Ni(OH)1.4(SO4)0.3. This was well consistent with the XRD data, and identified the (0 0 1) planes as the predominate plane in the obtained nanoribbons. 3.2. Effect of conditions and plausible mechanism Nickel Basic Sulfate is very sensitive to the pH value of synthetic system [18], and then several experiments were carried out to
determine the role of pH value in the formation of Ni(OH)1.4(SO4)0.3 nanoribbons. Only uniform ribbon-like structure were produced at the pH of 7.3 (see Fig. 2a), and irregular shapes were observed at the pH of 4.0 (NaOH was not added into the synthetic system), 6.9, 7.5 and/or 7.7 (see Fig. 3). Simultaneously, the XRD patterns of samples produced at different pH values indicated that NiS and Ni3S4 appeared without adding NaOH into the system, and Ni(OH)1.4(SO4)0.3 was observed between the pH range of 6.9 and 7.3 (see Fig. 4). However, the patterns of b-Ni(OH)2 and NixSy (NiS: JCPDS 12-0041, Ni0.96S: JCPDS 50-1791, Ni3S4: JCPDS 431469) appeared at the pH values higher than 7.5. In addition, accompanying with the increase of the amount of OH , the peak intensity of b-Ni(OH)2 (JCPDS 14-0117) became more stronger, while the peaks of NixSy became weaker (see Fig. 4d and e). This indicated that the more OH existed in the system the less SO42 formed, and the peaks of Ni(OH)1.4(SO4)0.3 were hardly observed at the pH higher than 7.5. It was related to the hydrolytic process involved in the first step of the hydrothermal synthesis, and a trace alteration of the OH concentration would dramatically change the composition and structure of final product. As a result, there were not enough OH to formed uniform nanoribbon structure with the composition of Ni(OH)1.4(SO4)0.3 at pH of 4.0 and 6.9. However, when excessive OH existed in the system, b-Ni(OH)2 and NixSy appeared in the products and the morphology of the samples became irregular. Thus, appropriate pH value was required in order to get uniform Ni(OH)1.4(SO4)0.3 nanoribbons. In order to observe the growth process of as-prepared Ni(OH)1.4 (SO4)0.3 nanoribbons, the time-dependent experiments were carried out. The synthetic times were 4, 8, 16, and 24 h, respectively (see the corresponding SEM images in Fig. 5). Fluey irregular particles were formed for 4 h (see Fig. 5a), corresponding to the amorphous Ni(OH)2 (see Fig. 6a). Film-like structures and compact particles were observed for 8 h (see Fig. 5b), and the XRD patterns of hexagonal b-Ni(OH)2 appeared together with a few peaks of Ni3S53H2O (JCPDS 37-1082) and (H3O)2NiO2 (JCPDS 73-2408) (see Fig. 6b). With the time prolonged to 16 h (see Fig. 5c and 6c), irregular ribbon-like structures appeared with the main ingredient
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Fig. 5. SEM images of samples obtained at the synthetic time of (a) 4, (b) 8, (c) 16, and (d) 24 h.
Fig. 6. XRD pattern of samples obtained at the synthetic time of (a) 4, (b) 8, (c) 16, and (d) 24 h.
of Ni(OH)1.4(SO4)0.3. It could be found that some film-like structures splitted into ribbons. With the reaction proceeded, clearer and more uniform ribbon structures were formed (see Fig. 5d) with the composition of Ni(OH)1.4(SO4)0.3 (see Fig. 6d). The results revealed that the reaction time was a crucial factor for the formation of uniform Ni(OH)1.4(SO4) 0.3 nanoribbons. On the basis of characterization results and the conditionsdependent experiments, a possible recrystallization–oxidationsplit growth mechanism was proposed as shown in Fig. 7. The molar ratio of SCN to Ni2+ was about 1:1, and the solution was alkalescent because of the addition of NaOH. In consideration of the existence of [Ni(SCN)x]2 x (0 6 x 6 4), there should be uncombined Ni2+ in the alkaline solution. Therefore, amorphous b-Ni(OH)2 irregular particles formed firstly when the solution was heated. With the
reaction proceeded, the concentration of Ni2+ decreased quickly and then hardly met the requirement of the hydrolytic reaction, but [Ni(SCN)x]2 x could not release enough uncombined Ni2+ timely in order to sustain hydrolytic process of the system. As a result, the dissolution and recrystallization processes of the pre-formed amorphous b-Ni(OH)2 began, and finally formed the film-like structures because b-Ni(OH)2 could easily form layered structure [19]. The change of samples showed in Fig. 5a and b revealed the existed of dissolution and recrystallization processes. In Fig. 5a fluey irregular particles were observed, however, in Fig. 5b Film-like structures and compact particles appeared. With the continuous hydrolysis of Ni2+, the [Ni(SCN)x]2 x ions would decompose stepwisely to release Ni2+ and S2 in this process [20,21]. Ni3S53H2O and (H3O)2NiO2 were formed besides Ni(OH)2, due to nickel element existing in the main form of [Ni(SCN)x]2 x ions which led the rate of release S2 was faster than that of Ni2+, and the amount of OH decreased with the reaction proceeded. The percentage composition of Ni3S53H2O and (H3O)2NiO2 would be higher than the percentage of b-Ni(OH)2. This was why the peak intensity of Ni3S53H2O and (H3O)2NiO2 had the advantage over b-Ni(OH)2 at 8 h (see Fig. 6b). With the hydrolytic process and recrystallization process taken further, (H3O)2NiO2 and [Ni(SCN)x]2 x ions would disappear stepwisely and finally be exhausted. Meanwhile, S2 would be transformed into SO42 when the concentration of H+ ions was enough to form an oxidation atmosphere with NO3 ions. That is, with the continuous hydrolysis of Ni2+ ions, the concentration of H+ ions would increase continuously. The oxidation atmosphere would be formed unavoidably, and then S2 would be oxidized to bulky SO42 . Out of the view of reaction equilibrium, the adsorption of SO42 ions on the layered b-Ni(OH)2 would be inevitable. According to the crystal structure of b-Ni(OH)2, (001) planes possessed the highest concentration of Ni2+ and various anions (SO42 , CO32 , Cl and so on or water molecules) could be easily inserted between the interlayer [9]. Consequently, the SO42 anions would be inserted into the interlayer of (0 0 1) planes of layered b-Ni(OH)2 [22]. Finally, the film-like structures would split into ribbon-like structures along the (0 0 1) plane.
H. Zhou et al. / Journal of Alloys and Compounds 540 (2012) 127–132
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Fig. 7. The possible growth mechanism of as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons: (a) the growth process, (b) the oxidation and split process.
3.3. Photocatalytic properties of the Ni(OH)1.4(SO4)0.3 Nanoribbons
Fig. 8. The relationship of photodegradation ratio of methyl orange with time irradiated by UV–Vis light: (a) the adsorption curve of methyl orange (20 mg/L) aqueous solution containing the as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons in the dark, (b) the curve of MO in blank experiment, (c) the curve of as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons.
Although TiO2 and ZnO have been appreciated as esteemed photoactive catalysts [23,24] due to their excellent performance, some endeavors have been made to explore that possibility with the compounds of nickel until now. For instance, Song et al. [16] and Sarkar et al. [25] have reported their jobs about NiO and/or Ni(OH)2 and investigated the photocatalytic activity of the materials. In the case of Ni(OH)1.4(SO4)0.3, defects and oxygen vacancies would be formed simultaneously in order to keep the electric neutrality when the SO42 ions are inserted into the interlayer of b-Ni(OH)2 in the formation process [26]. This indicated that the as-obtained Ni(OH)1.4 (SO4)0.3 nanoribbons may possess a high photocatalytic activity, because the defects and oxygen vacancies were propitious to the fast transmission of electrons. Fig. 2c showed that the as-obtained samples have many pores along its growth direction as the white arrows showed, and that might result from the defects arisen in the crystal growth process. To investigate the photocatalytic activity of the as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons, the decomposition of methyl orange in aqueous solution was selected as a probe reaction [16]. Fig. 8a was the adsorption curve of methyl orange (20 mg/L) aqueous solution containing the as-prepared
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Ni(OH)1.4 (SO4)0.3 nanoribbons in the dark and the concentration change of methyl orange was about 2%. The change of the methyl orange concentration versus the irradiation time at 465 nm was presented in Fig. 8c, which demonstrated that the methyl orange was obviously degraded (about 87%) with the nanoribbon Ni(OH)1.4(SO4)0.3 as photocatalysts. Compared with the blank experiment, in which only 45% of methyl orange was degraded (see Fig. 8b), the photocatalytic activities of synthesized material could be clearly identified. So it can be concluded that the as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons could take as a new photocatalytic material on the basis of above research. However, why the asprepared Ni(OH)1.4(SO4)0.3 nanoribbons exhibited photocatalytic activities are still unclear and the research is underway.
4. Conclusions Ni(OH)1.4(SO4)0.3. nanoribbons were synthesized by the one-step hydrothermal method. They had several tens of nanometers in width and several tens of micrometers in length. The analysis revealed that the nanoribbons were mainly dominated by (0 0 2) facet. On the basis of condition-dependent experiments, a recrystallization–oxidation-split growth mechanism was proposed to explain the formation process of nanoribbons. The as-obtained samples showed significant photocatalytic activity to decompose methyl orange pollutant.
(2011SJCRC07) and the State Key Laboratory of Coal Conversion (SKLCC) in-house Project (No. 2011BWZ005). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Acknowledgements
[22] [23]
We are grateful for Grants from the National Nature Science Foundation of China (No. 21003147), Natural Science Foundation of Shanxi (2011011007-3), Distinguished Young Scholar Project of Institute of Coal Chemistry, Chinese Academy of Sciences
[24] [25] [26]
S.H. Im, Y.T. Lee, B. Wiley, Y. Xia, Angew. Chem. Int. Ed. 44 (2005) 2154–2157. E.R. Leite, C. Vila, J. Bettini, E. Longo, J. Phys. Chem. B 110 (2006) 18088–18090. S.R. Ovshinsky, M.A. Fetcenko, J. Ross, Science 260 (1993) 176–181. D.L. Chen, L. Gao, Chem. Phys. Lett. 405 (2005) 159–164. Y. Tang, Z. Jia, Y. Jiang, L. Li, J. Wang, Nanotechnology 17 (2006) 5686–5690. D. Yang, R. Wang, M. He, J. Zhang, Z. Liu, J. Phys. Chem. B 109 (2005) 7654– 7658. Y.Y. Luo, G.T. Duan, G.H. Li, J. Solid State Chem. 180 (2007) 2149–2153. D.B. Kuang, B.X. Lei, Y.P. Pan, X.Y. Yu, C.Y. Su, J. Phys. Chem. C 113 (2009) 5508– 5513. P.V. Kamath, G.H.A. Therese, J. Gopalakrishnan, J. Solid State Chem. 128 (1997) 38–41. M. Rajamathi, P.V. Kamath, J. Power Sources 70 (1998) 118–121. W.T. Reichle, J. Catal. 94 (1985) 547–557. J. Cernak, J. Chomic, C. Kappenstein, R. Brahmi, D. Duprez, Thermochim. Acta 276 (1996) 209–220. M. Meyn, K. Beneke, G. Lagaly, Inorg. Chem. 32 (1993) 1209–1215. K. Zhang, J.B. Wang, X.L. Lu, L.Y. Li, Y.W. Tang, Z.Y. Jia, J. Phys. Chem. C 113 (2009) 142–147. M. Ocaña, J. Colloid Interface Sci. 228 (2000) 259–262. X.F. Song, L. Gao, J. Phys. Chem. C 112 (2008) 15299–15305. JCPDS File No. 41-1424. L.H. Dong, Y. Chu, W.D. Sun, Chem. Eur. J. 14 (2008) 5064–5072. N. Huang, W.K. Zhang, H. Huang, Y.P. Gan, Zhejiang chemical industry 37 (2006) 9–14. W.Q. Zhang, L.Q. Xu, K.B. Tang, F.Q. Li, Y.T. Qian, Eur. J. Inorg. Chem. 4 (2005) 653–656. Z.C. Wu, C. Pan, T.W. Li, G.J. Yang, Y. Xie, Cryst. Growth Des. 7 (2007) 2454– 2459. E.H. Nickel, J. Graham, Can. Mineral 25 (1987) 409–411. J.C. Yu, W. Ho, J. Lin, H. Yip, P.K. Wong, Environ. Sci. Technol. 37 (2003) 2296– 2301. J.G. Yu, X.X. Yu, Environ. Sci. Technol. 42 (2008) 4902–4907. S. Sarkar, M. Pradhan, A.K. Sinha, M. Basu, Y. Negishi, T. Pal, Inorg. Chem. 49 (2010) 8813–8827. J. Liu, L.Y. Li, X.L. Lu, J.B. Wang, Z.Y. Jia, Y.W. Tang, J. Chin. Electr. Microsc. Soc. 25 (2006) 57–58.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Facile synthesis of Ni(OH)1.4(SO4)0.3 nanoribbons and their photocatalytic properties Hai Zhou a,c, Baoliang Lv a,⇑, Dong Wu a, Yuhan Sun a,b a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China Low Carbon Conversion Center, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, PR China c Graduate University of the Chinese Academy of Science, Beijing 100049, PR China b
a r t i c l e
i n f o
Article history: Received 2 November 2011 Accepted 14 June 2012 Available online 22 June 2012 Keywords: Hydrothermal Ni(OH)1.4(SO4)0.3 Nanoribbons Photocatalytic
a b s t r a c t With the presence of SCN anions, Ni(OH)1.4(SO4)0.3 nanoribbons have been successfully synthesized by one-step hydrothermal method. The synthesized nanoribbons are with width of several tens of nanometers and length of several tens of micrometers. X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analysis revealed that the products were mainly dominated by (0 0 2) lattice plane. On the basis of condition-dependent experiments, a recrystallization–oxidation-split growth mechanism was proposed to explain the formation process of the Ni(OH)1.4(SO4)0.3 nanoribbons. As-synthesized products showed a photocatalytic capability under the irradiation of ultraviolet light for the decomposition of methyl orange as a probe reaction. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the shape control and modification of nanostructures has been a major topic for functional materials, since the morphology, size and chemical composition of materials had a great influence on their physical and chemical properties [1,2]. Among those materials, Nickel hydroxide has attracted great interest due to its importance as cathode material in a number of rechargeable alkaline batteries (e.g., Ni/Cd, Ni/H2, Ni/MH and Ni/Zn) [3], and the control of micro-structured nickel hydroxides led to the products with different morphologies such as nanowires [4], nanobelts [5], nanoboards [6], and nanoflowers [7], nanosheets and hierarchical microsphere [8]. And a-Ni(OH)2 is a promising one which raised great attention. It is well-known that a-Ni(OH)2 transforms from b-Ni(OH)2 by insert various anions (SO42 , CO32 , Cl and so on) or water molecules between interlayer [9], being a family of layered compounds with many new properties [10]. Thus, they had potential applications in the fields including catalysts [11], catalyst precursors [12], anionic exchange and electrode materials [13]. Ni(OH)1.4 (SO4)0.3, as a typical compound with such a structure, have been reported by several groups. But all of these works introduced the SO42 before the synthesis [5,14,15]. Herein, we report a new route to synthesize Ni(OH)1.4(SO4)0.3 with the nanoribbon structure in the presence of SCN anions, and their photocatalytic properties as well.
⇑ Corresponding author. Tel.: +86 0351 4049859; fax: +86 0351 4041153. E-mail address: [email protected] (B. Lv). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.059
2. Experimental All the reagents used without further purification: Ni(NO3)26H2O (98.5 wt.%, analytical reagent (A. R.), Tianjin Beichen Fangzheng Chemicals Co. Ltd.), NaSCN (98 wt.%, A. R., Tianjin Tianda Chemicals Co. Ltd.), NaOH (98 wt.%, A. R., Beijing Beida Chemicals Co. Ltd.). Double-distilled water was used throughout the experiment.
2.1. Synthesis of nickel basic sulfate nanoribbons For the preparation of Ni(OH)1.4(SO4)0.3 nanoribbons, a typical synthesis process was as follows: 1.8 mM of NaSCN and 1.7 mM of Ni(NO3)26H2O were dissolved in 80 mL of distilled water under ultrasonic radiation, and then 1 mL of 0.15 M NaOH aqueous solution was dropped into the above mixture under ultrasonic radiation. The as-formed bright green solution was sealed in a Teflon-lined autoclave of 150 mL capacity, and maintained at 220 °C for 24 h. After cooling to room temperature, the products were collected by naturally sedimentation, washed with distilled water three times and absolute ethanol twice to remove impurities. The final products were dried at 80 °C.
2.2. Characterizations The products were characterized by transmission electron microscopy (TEM, JEOL JEM-1011) and high-resolution TEM (HRTEM, JEM2010), scanning electron microscopy (SEM, XL30 S-FEG), powder X-ray diffraction (XRD, Rigaku D/max-rB diffractometer using Cu Ka radiation, k = 0.15408 nm), Ultraviolet–Visible (UV–Vis Shimadazu 3150).
2.3. Photocatalytic reaction Photocatalytic experiments were carried out in a SGY-1 photoreactor (with a 300 w medium-pressure mercury lamp, Nanjing Sidongke electrical equipment Co. Ltd.) [16]. During the experiment 0.2 g as-obtained samples were dissolved in 400 mL of methyl orange (20 mg/L) aqueous solution, and then the suspension was magnetic stirred in the dark for 30 min to ensure an adsorption/desorption
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H. Zhou et al. / Journal of Alloys and Compounds 540 (2012) 127–132 equilibrium before ultraviolet light irradiation. The concentration of methyl orange aqueous solution was detected using the Shimadazu 3150 UV–Vis spectrophotometer by collecting the absorbance of methyl orange aqueous solution at 465 nm.
3. Results and discussion 3.1. Phase and morphology of the Ni(OH)1.4(SO4)0.3 nanoribbons
Fig. 1. XRD patterns for: (a) Ni(OH)1.4(SO4)0.3 nanoribbons, (b) PDF#41-1424.
Fig. 1 shows the XRD patterns of as-prepared nanoribbons. Accordingly, the product consisted of Ni(OH)1.4(SO4)0.3 (JCPDS 411424). This indicated that crystalline Ni(OH)1.4(SO4)0.3 was obtained via the present hydrothermal process. Furthermore, the relative diffraction intensity of (1 0 0), (0 0 2) and (1 0 2) planes appeared higher than the corresponding standard data [17] (especially the (0 0 2) plane) with the lower intensity of (1 0 3), (0 1 0) and (21-2) planes, being illustrative of predominated (0 0 2) facet for the Ni(OH)1.4(SO4)0.3 nanoribbons. SEM, TEM and HRTEM were carried out for both morphology and microstructure of as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons (see Fig. 2). Clearly, so-produced samples were composed of
Fig. 2. SEM, TEM and HRTEM images of the as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons: (a) and (b) SEM images of the sample, (c) TEM images of the sample, (d) HRTEM images of the sample, (e) enlarged image of the area marked by white frame in (d), the inset show corresponding FFT pattern.
H. Zhou et al. / Journal of Alloys and Compounds 540 (2012) 127–132
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Fig. 3. SEM images of samples obtained at the synthetic pH of (a) 4.0, (b) 6.9, (c) 7.5 and (d) 7.7.
Fig. 4. XRD pattern of samples obtained at the synthetic pH of (a) 4.0, (b) 6.9, (c) 7.3, (d) 7.5, and (e) 7.7.
uniform ribbon-like structures with the width of several ten nanometers and the length of several ten micrometers (see Fig. 2c and d). Fig. 2e shows an enlarged image of the area marked by white frame in Fig. 2d, and the inset of Fig. 2e was the corresponding fast Fourier transform (FFT) image. The lattice fringe was clearly observed with the distance of adjacent lattice fringes about 0.24 nm (2.4 Å), which was in good agreement with the d value of (0 0 6) plane of the monoclinic Ni(OH)1.4(SO4)0.3. This was well consistent with the XRD data, and identified the (0 0 1) planes as the predominate plane in the obtained nanoribbons. 3.2. Effect of conditions and plausible mechanism Nickel Basic Sulfate is very sensitive to the pH value of synthetic system [18], and then several experiments were carried out to
determine the role of pH value in the formation of Ni(OH)1.4(SO4)0.3 nanoribbons. Only uniform ribbon-like structure were produced at the pH of 7.3 (see Fig. 2a), and irregular shapes were observed at the pH of 4.0 (NaOH was not added into the synthetic system), 6.9, 7.5 and/or 7.7 (see Fig. 3). Simultaneously, the XRD patterns of samples produced at different pH values indicated that NiS and Ni3S4 appeared without adding NaOH into the system, and Ni(OH)1.4(SO4)0.3 was observed between the pH range of 6.9 and 7.3 (see Fig. 4). However, the patterns of b-Ni(OH)2 and NixSy (NiS: JCPDS 12-0041, Ni0.96S: JCPDS 50-1791, Ni3S4: JCPDS 431469) appeared at the pH values higher than 7.5. In addition, accompanying with the increase of the amount of OH , the peak intensity of b-Ni(OH)2 (JCPDS 14-0117) became more stronger, while the peaks of NixSy became weaker (see Fig. 4d and e). This indicated that the more OH existed in the system the less SO42 formed, and the peaks of Ni(OH)1.4(SO4)0.3 were hardly observed at the pH higher than 7.5. It was related to the hydrolytic process involved in the first step of the hydrothermal synthesis, and a trace alteration of the OH concentration would dramatically change the composition and structure of final product. As a result, there were not enough OH to formed uniform nanoribbon structure with the composition of Ni(OH)1.4(SO4)0.3 at pH of 4.0 and 6.9. However, when excessive OH existed in the system, b-Ni(OH)2 and NixSy appeared in the products and the morphology of the samples became irregular. Thus, appropriate pH value was required in order to get uniform Ni(OH)1.4(SO4)0.3 nanoribbons. In order to observe the growth process of as-prepared Ni(OH)1.4 (SO4)0.3 nanoribbons, the time-dependent experiments were carried out. The synthetic times were 4, 8, 16, and 24 h, respectively (see the corresponding SEM images in Fig. 5). Fluey irregular particles were formed for 4 h (see Fig. 5a), corresponding to the amorphous Ni(OH)2 (see Fig. 6a). Film-like structures and compact particles were observed for 8 h (see Fig. 5b), and the XRD patterns of hexagonal b-Ni(OH)2 appeared together with a few peaks of Ni3S53H2O (JCPDS 37-1082) and (H3O)2NiO2 (JCPDS 73-2408) (see Fig. 6b). With the time prolonged to 16 h (see Fig. 5c and 6c), irregular ribbon-like structures appeared with the main ingredient
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Fig. 5. SEM images of samples obtained at the synthetic time of (a) 4, (b) 8, (c) 16, and (d) 24 h.
Fig. 6. XRD pattern of samples obtained at the synthetic time of (a) 4, (b) 8, (c) 16, and (d) 24 h.
of Ni(OH)1.4(SO4)0.3. It could be found that some film-like structures splitted into ribbons. With the reaction proceeded, clearer and more uniform ribbon structures were formed (see Fig. 5d) with the composition of Ni(OH)1.4(SO4)0.3 (see Fig. 6d). The results revealed that the reaction time was a crucial factor for the formation of uniform Ni(OH)1.4(SO4) 0.3 nanoribbons. On the basis of characterization results and the conditionsdependent experiments, a possible recrystallization–oxidationsplit growth mechanism was proposed as shown in Fig. 7. The molar ratio of SCN to Ni2+ was about 1:1, and the solution was alkalescent because of the addition of NaOH. In consideration of the existence of [Ni(SCN)x]2 x (0 6 x 6 4), there should be uncombined Ni2+ in the alkaline solution. Therefore, amorphous b-Ni(OH)2 irregular particles formed firstly when the solution was heated. With the
reaction proceeded, the concentration of Ni2+ decreased quickly and then hardly met the requirement of the hydrolytic reaction, but [Ni(SCN)x]2 x could not release enough uncombined Ni2+ timely in order to sustain hydrolytic process of the system. As a result, the dissolution and recrystallization processes of the pre-formed amorphous b-Ni(OH)2 began, and finally formed the film-like structures because b-Ni(OH)2 could easily form layered structure [19]. The change of samples showed in Fig. 5a and b revealed the existed of dissolution and recrystallization processes. In Fig. 5a fluey irregular particles were observed, however, in Fig. 5b Film-like structures and compact particles appeared. With the continuous hydrolysis of Ni2+, the [Ni(SCN)x]2 x ions would decompose stepwisely to release Ni2+ and S2 in this process [20,21]. Ni3S53H2O and (H3O)2NiO2 were formed besides Ni(OH)2, due to nickel element existing in the main form of [Ni(SCN)x]2 x ions which led the rate of release S2 was faster than that of Ni2+, and the amount of OH decreased with the reaction proceeded. The percentage composition of Ni3S53H2O and (H3O)2NiO2 would be higher than the percentage of b-Ni(OH)2. This was why the peak intensity of Ni3S53H2O and (H3O)2NiO2 had the advantage over b-Ni(OH)2 at 8 h (see Fig. 6b). With the hydrolytic process and recrystallization process taken further, (H3O)2NiO2 and [Ni(SCN)x]2 x ions would disappear stepwisely and finally be exhausted. Meanwhile, S2 would be transformed into SO42 when the concentration of H+ ions was enough to form an oxidation atmosphere with NO3 ions. That is, with the continuous hydrolysis of Ni2+ ions, the concentration of H+ ions would increase continuously. The oxidation atmosphere would be formed unavoidably, and then S2 would be oxidized to bulky SO42 . Out of the view of reaction equilibrium, the adsorption of SO42 ions on the layered b-Ni(OH)2 would be inevitable. According to the crystal structure of b-Ni(OH)2, (001) planes possessed the highest concentration of Ni2+ and various anions (SO42 , CO32 , Cl and so on or water molecules) could be easily inserted between the interlayer [9]. Consequently, the SO42 anions would be inserted into the interlayer of (0 0 1) planes of layered b-Ni(OH)2 [22]. Finally, the film-like structures would split into ribbon-like structures along the (0 0 1) plane.
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Fig. 7. The possible growth mechanism of as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons: (a) the growth process, (b) the oxidation and split process.
3.3. Photocatalytic properties of the Ni(OH)1.4(SO4)0.3 Nanoribbons
Fig. 8. The relationship of photodegradation ratio of methyl orange with time irradiated by UV–Vis light: (a) the adsorption curve of methyl orange (20 mg/L) aqueous solution containing the as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons in the dark, (b) the curve of MO in blank experiment, (c) the curve of as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons.
Although TiO2 and ZnO have been appreciated as esteemed photoactive catalysts [23,24] due to their excellent performance, some endeavors have been made to explore that possibility with the compounds of nickel until now. For instance, Song et al. [16] and Sarkar et al. [25] have reported their jobs about NiO and/or Ni(OH)2 and investigated the photocatalytic activity of the materials. In the case of Ni(OH)1.4(SO4)0.3, defects and oxygen vacancies would be formed simultaneously in order to keep the electric neutrality when the SO42 ions are inserted into the interlayer of b-Ni(OH)2 in the formation process [26]. This indicated that the as-obtained Ni(OH)1.4 (SO4)0.3 nanoribbons may possess a high photocatalytic activity, because the defects and oxygen vacancies were propitious to the fast transmission of electrons. Fig. 2c showed that the as-obtained samples have many pores along its growth direction as the white arrows showed, and that might result from the defects arisen in the crystal growth process. To investigate the photocatalytic activity of the as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons, the decomposition of methyl orange in aqueous solution was selected as a probe reaction [16]. Fig. 8a was the adsorption curve of methyl orange (20 mg/L) aqueous solution containing the as-prepared
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Ni(OH)1.4 (SO4)0.3 nanoribbons in the dark and the concentration change of methyl orange was about 2%. The change of the methyl orange concentration versus the irradiation time at 465 nm was presented in Fig. 8c, which demonstrated that the methyl orange was obviously degraded (about 87%) with the nanoribbon Ni(OH)1.4(SO4)0.3 as photocatalysts. Compared with the blank experiment, in which only 45% of methyl orange was degraded (see Fig. 8b), the photocatalytic activities of synthesized material could be clearly identified. So it can be concluded that the as-prepared Ni(OH)1.4(SO4)0.3 nanoribbons could take as a new photocatalytic material on the basis of above research. However, why the asprepared Ni(OH)1.4(SO4)0.3 nanoribbons exhibited photocatalytic activities are still unclear and the research is underway.
4. Conclusions Ni(OH)1.4(SO4)0.3. nanoribbons were synthesized by the one-step hydrothermal method. They had several tens of nanometers in width and several tens of micrometers in length. The analysis revealed that the nanoribbons were mainly dominated by (0 0 2) facet. On the basis of condition-dependent experiments, a recrystallization–oxidation-split growth mechanism was proposed to explain the formation process of nanoribbons. The as-obtained samples showed significant photocatalytic activity to decompose methyl orange pollutant.
(2011SJCRC07) and the State Key Laboratory of Coal Conversion (SKLCC) in-house Project (No. 2011BWZ005). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Acknowledgements
[22] [23]
We are grateful for Grants from the National Nature Science Foundation of China (No. 21003147), Natural Science Foundation of Shanxi (2011011007-3), Distinguished Young Scholar Project of Institute of Coal Chemistry, Chinese Academy of Sciences
[24] [25] [26]
S.H. Im, Y.T. Lee, B. Wiley, Y. Xia, Angew. Chem. Int. Ed. 44 (2005) 2154–2157. E.R. Leite, C. Vila, J. Bettini, E. Longo, J. Phys. Chem. B 110 (2006) 18088–18090. S.R. Ovshinsky, M.A. Fetcenko, J. Ross, Science 260 (1993) 176–181. D.L. Chen, L. Gao, Chem. Phys. Lett. 405 (2005) 159–164. Y. Tang, Z. Jia, Y. Jiang, L. Li, J. Wang, Nanotechnology 17 (2006) 5686–5690. D. Yang, R. Wang, M. He, J. Zhang, Z. Liu, J. Phys. Chem. B 109 (2005) 7654– 7658. Y.Y. Luo, G.T. Duan, G.H. Li, J. Solid State Chem. 180 (2007) 2149–2153. D.B. Kuang, B.X. Lei, Y.P. Pan, X.Y. Yu, C.Y. Su, J. Phys. Chem. C 113 (2009) 5508– 5513. P.V. Kamath, G.H.A. Therese, J. Gopalakrishnan, J. Solid State Chem. 128 (1997) 38–41. M. Rajamathi, P.V. Kamath, J. Power Sources 70 (1998) 118–121. W.T. Reichle, J. Catal. 94 (1985) 547–557. J. Cernak, J. Chomic, C. Kappenstein, R. Brahmi, D. Duprez, Thermochim. Acta 276 (1996) 209–220. M. Meyn, K. Beneke, G. Lagaly, Inorg. Chem. 32 (1993) 1209–1215. K. Zhang, J.B. Wang, X.L. Lu, L.Y. Li, Y.W. Tang, Z.Y. Jia, J. Phys. Chem. C 113 (2009) 142–147. M. Ocaña, J. Colloid Interface Sci. 228 (2000) 259–262. X.F. Song, L. Gao, J. Phys. Chem. C 112 (2008) 15299–15305. JCPDS File No. 41-1424. L.H. Dong, Y. Chu, W.D. Sun, Chem. Eur. J. 14 (2008) 5064–5072. N. Huang, W.K. Zhang, H. Huang, Y.P. Gan, Zhejiang chemical industry 37 (2006) 9–14. W.Q. Zhang, L.Q. Xu, K.B. Tang, F.Q. Li, Y.T. Qian, Eur. J. Inorg. Chem. 4 (2005) 653–656. Z.C. Wu, C. Pan, T.W. Li, G.J. Yang, Y. Xie, Cryst. Growth Des. 7 (2007) 2454– 2459. E.H. Nickel, J. Graham, Can. Mineral 25 (1987) 409–411. J.C. Yu, W. Ho, J. Lin, H. Yip, P.K. Wong, Environ. Sci. Technol. 37 (2003) 2296– 2301. J.G. Yu, X.X. Yu, Environ. Sci. Technol. 42 (2008) 4902–4907. S. Sarkar, M. Pradhan, A.K. Sinha, M. Basu, Y. Negishi, T. Pal, Inorg. Chem. 49 (2010) 8813–8827. J. Liu, L.Y. Li, X.L. Lu, J.B. Wang, Z.Y. Jia, Y.W. Tang, J. Chin. Electr. Microsc. Soc. 25 (2006) 57–58.