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Visible-light-driven photocatalytic properties and electronic structures of nickel sulfide nanoflowers
Solid State Sciences 43 (2015) 59e62
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Visible-light-driven photocatalytic properties and electronic structures of nickel sulfide nanoflowers Junfeng Chao a, *, Degong Duan a, Shumin Xing b, Yuliang Zhao b, Xiutai Zhang a, Suling Gao a, Xiaohong Li a, Qiufeng Fan a, Junping Yang a a b
College of Electronic Information and Electric Engineering, Anyang Institute of Technology, Anyang 455000, PR China College of Mathematics and Physics, Anyang Institute of Technology, Anyang 455000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2015 Received in revised form 25 March 2015 Accepted 27 March 2015 Available online 28 March 2015
Nickel sulfide (NiS) nanoflowers with the thickness of ca. 5e10 nm and size up to several hundreds of nanometers were synthesized via a facile polyol refluxing process under the open-air condition. The photocatalytic properties of NiS nanoflowers were evaluated by the decomposition ratio of MB was up to nearly 98% after 3 h visible light irradiation, indicating the NiS nanoflowers were good candidates for high performance photocatalysts. Meanwhile, the influencing factor of the photocatalytic reaction had also been studied by calculating the electronic structure of NiS nanoflowers. The band structure indicates that charge transfer upon photoexcitation occurs from the S 3p orbital to the empty Ni orbital. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: Polyol refluxing NiS nanoflowers Photocataly Electronic structures
1. Introduction The photocatalytic technique for the degradation of organic contaminants in air or in solution has been widely studied to remedy the effects of environmental pollution because of its simple and exhaustive decomposition process [1e5]. Among all kinds of semiconductors, TiO2 is one of the most important oxides photocatalyst, which has been widely used [6e9]. However, with a wide band gap of 3.2 eV, TiO2 is only sensitive under ultraviolet light region and is not active to visible light. It is well known that only 4% of the solar spectra falls in the ultraviolet region, thus much work has been done to synthesize visible light photocatalyst in view of the better utilization of solar energy. Through modification of TiO2, including doping with metal, such as Fe, Au, Ag [10e12], and nonmetal atoms, such as C, N, and S [13e15], a lot of visible-lightdriven photocatalysts have been synthesized. On the other hand, many traditional non-titanium-based visible light photocatalysts have been synthesized, for example, some sulfides have been found to have visible-light-driven catalytic activity, such as SnS [16], Sb2S3 [17], CdS [18], and so on [19,20]. All of these studies may provide new insights for the design of visible-light-driven photocatalysts.
* Corresponding author. E-mail address: [email protected] (J. Chao). http://dx.doi.org/10.1016/j.solidstatesciences.2015.03.022 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
Flower-like semiconductor nanomaterials with a large surface area and high photocatalytic activity have been widely investigated due to their important applications in purifying water and air fields. For instance, 3D flower-like Ni(OH)2 products prepared by Shen's group showed high selective adsorption for cationic/anionic dyes in aqueous solutions under visible light irradiation [21]. From a hydrothermal method, Jing and his coauthors have synthesized flower-like Ni2þ doped ZnIn2S4 microsphere with greatly enhancing the activity of the photocatalyst [22]. However, few report concerns the yield of nanomaterials via a facile route, thus remains a considerable challenge. Thus, developing simple and effective methods to fabricate semiconductors with flower-like structures is important for future visible-light-driven photocatalysts applications. The facile polyol refluxing process has several advantages over the conventional hydrothermal method, such as facile, short heating time to the reaction temperature, high output and open-air condition [16,23]. Up to now, to the best of our knowledge, little work on the preparation of nickel sulfide nanoflowers in liquid systems has been reported. Here, the simple synthesis of flower-like NiS from onestep, facile solution chemical method under the open-air condition was realized for the first time. The optical properties, photocatalytic activity and the electronic structures of the NiS nanoflowers were investigated. It was found that the obtained NiS nanoflowers have high photocatalytic activity in degradating
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J. Chao et al. / Solid State Sciences 43 (2015) 59e62
Fig. 1. (a) SEM image, (b) TEM image, (c) XRD pattern and (d) EDS of the as-synthesized flowers-like NiS nanostructure.
organic dyes MB under visible light irradiation.
2.3. Photocatalysis
2. Experimental
Photocatalytic decomposition of methylene blue (MB) was carried out in a cylindrical reaction vessel containing a suspension of 0.1 g NiS nanoflowers sample in a 100 mL MB or RhB solution (the dye concentration was 8 mg L1 in both cases). To get the adsorptionedesorption equilibrium, the suspension sample was kept in dark for 2 h with continuous stirring. Then the system was irradiated by a 500 W Xe-lamp equipped with a cutoff filter (l > 420 nm) and a water filter was used as the light source. At given time intervals, 3 mL of the reaction suspension was collected and filtrated to remove the NiS particles. The absorption spectra of a series of MB solutions were measured on a Shimadzu UV-2550 UVevis spectrometer at room temperature.
2.1. Synthesis of the NiS nanoflowers In this study, all of the reactants were of analytical grade and were used directly without further purification. NiS nanoflowers were prepared via a facile polyol refluxing process. 1 mmol of four hydrated nickel acetate (Ni(CH3COO)2$4$H2O, 98.5%, AR) and 3 mmol thiourea (CN2H4S, 99.0%, AR) were put into a three-neck flask (250 mL capacity) to which 50 mL ethylene glycol was added. After stirring for 10 min, all the reagents were dissolved, the reaction system was heated to 135 C and kept at that temperature for 1 h under vigorous stirring in atmosphere. Finally, black precipitates were collected, washed with distilled water and absolute ethanol and then dried in a vacuum at 70 C for 3 h. 2.2. Material characterization The as-prepared sample was characterized by X-ray powder diffractometer (X'Pert PRO, PANalytical B.V., the Netherlands) with radiation of a Cu target (Ka, l ¼ 0.15406 nm) at 25 C. The energy dispersive spectroscopy was achieved on a micro X-ray fluorescence system (EDX Inc.). The nanoelectrode morphologies were investigated by a JEOL JSM-6700F field emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) images (JEOL, JEM-2010) were prepared to analyze the morphologies of the samples. The UVevis absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer. Room-temperature photoluminescence (PL) spectra were recorded with an HORIBA Jobin Yvon LabRAM Spectrometer HR 800 UV with the excitation wavelength of 325 nm.
3. Results and discussion Fig. 1a displays the panoramic SEM image, indicating the formation of a large number of 3D flower-like architectures with the diameter of about 150e200 nm. Fig. 1b clearly exhibits that NiS nanoflowers are built from dozens of nanoflakes with the thickness of 5e10 nm. The formation of porous flowers is caused by the connections and overlaps of the adjacent nanoflakes. The little nanoflowers which were attracted by Van der Waals' force made up some nanoflowers with larger size. X-ray powder diffraction (XRD) was used to study the phase purity of the as-synthesized NiS nanoflowers as shown in Fig. 1c. The main reflection peaks can be indexed to the hexagonal phase of nickel monosulfide with lattice constants of a ¼ 3.425 and c ¼ 5.340 (JCPDS card No. 65e3419). Four main peaks ((100), (101), (102) and (110) planes) in the XRD pattern show wider full-widths at half-maxima than a bulk NiS [24]. In order to further indicate the lattice formation of the flowerlike product, an energy dispersive spectroscopy (EDS) was detected
J. Chao et al. / Solid State Sciences 43 (2015) 59e62
61
Fig. 4. The calculated (a) energy bands and (b) density of states (DOS) of assynthesized NiS nanoflowers samples.
Fig. 2. (a) UVevis absorption spectrum (the inset is a (ahn)1/2 vs hn plot) and (b) a room temperature photoluminescence (PL) spectrum of the NiS nanoflowers.
in Fig. 1d. Only S and Ni peaks can be detected, other element peaks such as N are not be found, which indicates the product is composed of the elements S and Ni. The result also reveals that the quality ratio is 35.56: 64.44 and the molar ratio is 50.25: 49.75, which is consistent with the composition of NiS.
The optical properties of the products were measured and the corresponding UVevis absorption spectrum is depicted in Fig. 2a. The absorption spectra were calculated according to the KubelkaeMunk Theory:
FðR∞ Þ ¼ K=S ¼
ð1 R∞ Þ2 2R∞
Were R∞ is the extreme limit of the reflectance coefficient, K is the absorption coefficient, S is the scattering coefficient. Black-colored NiS samples exhibit broad and strong absorption in the range from
Fig. 3. (a) UVevis absorption spectral of MB solution in the presence of NiS nanoflowers under visible light irradiation, inset is the photographs of photodegradation of MB over NiS samples against time. (b) UVevis spectral changes of RhB aqueous solution in the presence of NiS architectures. (c) Concentration changes of MB over the NiS nanoflowers as well as photolysis and P25 TiO2 with irradiation time, respectively.
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J. Chao et al. / Solid State Sciences 43 (2015) 59e62
240 nm to 800 nm. The optical band gaps of the samples could be evaluated from the following equation: (ahn) ¼ A(hn-Eg)n/2, the a and n values at the steep edges of the absorption spectra were used to construct the plots of (ahn)2/n against photon energy. As seen in Fig. 2a inset, the curves for the sample show a linear region with n ¼ 4 for the indirect band gap. The intercepts of the tangents to the plots indicated that the optical bandgap energy of the nanomaterial was 1.22 eV, which is good consistent with the previously reported data [25]. Fig. 2b represents the room temperature photoluminescence (PL) spectrum of the as-prepared NiS nanoflowers with an excitation wavelength of 325 nm. One strong broad emission peak with a maximum at 565 nm can be clearly observed, which may be due to electronic transitions caused by defects in the interfacial region [26]. Taking into account the novel 3D architecture with nanoscale building block and nanometer size, also with a visible light band gap, the as-prepared NiS nanoflowers should have an excellent optical activity. To demonstrate the applications, we investigated the materials' photocatalytic activity by employing the photocatalytic degradation some organic dyes such as methylene blue (MB) and rhodamine B (RhB) under visible light irradiation at room temperature. Fig. 3a shows the UVevis absorption spectra of the MB and RhB solution in the presence of NiS nanoflowers. After adsorption/desorption equilibrium, the light was turned on. It clearly shows that when the MB solution system was irradiated, the main absorption peaks corresponding to MB (at 664 nm, 611 nm and 291 nm) gradually decreased continuously with increased irradiation time, and almost completely disappeared after 3 h. Inset of Fig. 3a shows the corresponding photographs of the initial MB solution and those after photocatalytic reactions for different times. The colour of MB solution system changes from blue to almost colorless, indicating that MB species were absorpted and degraded under visible light irradiation. Fig. 3b shows the great adsorption activities of the NiS architectures when catalyst was dispersed into the RhB solution. Furthermore, the plot for the concentration changes of MB determined from its characteristic absorption peak at 664 nm is shown in Fig. 3b. For comparison, the photolysis, the MB degradation over commercially P25eTiO2, the RhB degradation over NiS nanoflowers under visible light irradiation were also tested at the same condition, the corresponding results are also shown in Fig. 3c, respectively. NiS nanoflowers have a significantly stronger adsorption and photocatalytic degradation ability than P25eTiO2, and MB dye is considerably stable under visible light irradiation. It can be clearly seen that after 2 h of adsorption equilibrium and 3 h of visible light irradiation, the degradation ratio of MB for NiS nanoflowers is about 98%, while that of P25eTiO2 is about 10%. According to the adsorptionedesorption and photodegradation processes, we concluded that NiS nanoflowers show enhanced photocatalytic performance for the degradation of MB solution. The photocatalytic performance of sulfide photocatalysts depend on their electronic structures. In the present work, the electronic structures of NiS nanoflowers were investigated by plane-wave DFT calculation. Fig. 4a shows the conduction band minimum (CBM) of NiS crystal is located at the K point while the valence band maximum (VBM) is located in the region between the G point and the M point of Brillouin zone, indicating that the NiS is indeed an indirect semiconductor. The density of states of the NiS nanoflowers are shown in Fig. 4b. The conduction band in the range of 3e15 eV is mainly composed of Ni 5p and S 3p orbitals. The valence band at about 17 to 2 eV primarily consists of S 3p and Ni 5p orbitals. The bottom of the
conduction band is mainly constituted by the Ni 5p orbital, while the conduction band top is composed mainly of the S 3p orbital. The total pDOS indicates that charges transfer upon visible light excitation occurs from the S 3p orbital to the empty Ni 5p orbital. The calculated band gap of NiS is 1.2 eV, in good agreement with the experimental result based on the UVevis absorption spectra. 4. Conclusions In summary, NiS nanoflowers were synthesized by using a simple polyol refluxing process. Under visible light irradiation, NiS nanoflowers showed much stronger adsorption and photodegradation activities for MB than commercial P25eTiO2 powders and the conversion ratio of MB was up to nearly 98%. The successful application of NiS in the photocatalytic field may also give useful insight for the development of other visible-light-driven semiconductors. The electronic structures of NiS were further investigated by plane-wave DFT calculation. Acknowledgments This work was supported by the National Natural Science Foundation (21001046, 51002059), the 973 Program of China (2011CB933300, 2011CBA00703), the Important Scientific Research Foundation of Henan Province Education Department (14A510002). The authors are grateful to professor Di Chen for help with the experiments and the samples measurements. References [1] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341e357. [2] D. Chen, J.H. Ye, Adv. Funct. Mater. 18 (2008) 1922e1928. [3] J.F. Chao, Z. Xie, X.B. Duan, Y. Dong, Z.R. Wang, J. Xu, B. Liang, B. Shan, J.H. Ye, D. Chen, G.Z. Shen, CrystEngComm 14 (2012) 3163e3168. [4] P. Hartmann, D.K. Lee, B.M. Smarsly, J. Janek, ACS Nano 4 (2010) 3147e3154. [5] D. Chen, Z. Liu, S.X. Ouyang, J.H. Ye, J. Phys. Chem. C 115 (2011) 15778e15784. [6] A. Fujishima, K. Honda, Nature 238 (1972) 37e38. [7] T.X. Wu, G.M. Liu, J.C. Zhao, H. Hidaka, N. Serpone, J. Phys. Chem. B 102 (1998) 5845e5851. [8] M. Liu, L.Y. Piao, W.M. Lu, S.T. Ju, L. Zhao, C.L. Zhou, H.L. Li, W.J. Wang, Nanoscale 2 (2010) 1115e1117. [9] Z.R. Wang, H. Wang, B. Liu, W.Z. Qiu, J. Zhang, S.H. Ran, H.T. Huang, J. Xu, H.W. Han, D. Chen, G.Z. Shen, ACS Nano 5 (2011) 8412e8419. [10] H. Tong, S.X. Ouyang, Y.P. Bi, N. Umezawa, M. Oshikiri, J.H. Ye, Adv. Mater. 24 (2012) 229e251. [11] S. Bingham, W.A. Daoud, J. Mater. Chem. 21 (2011) 2041e2050. [12] Y.R. Zhang, Q. Li, Solid State Sci. 16 (2013) 16e20. [13] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243e2245. [14] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269e271. [15] R. Bacsa, J. Kiwi, T. Ohno, P. Albers, V. Nadtochenko, J. Phys. Chem. B 109 (2005) 5994e6003. [16] J.F. Chao, Z.R. Wang, X. Xu, Q.Y. Xiang, W.F. Song, G. Chen, J.B. Hu, D. Chen, RSC Adv. 3 (2013) 2746e2753. [17] M. Sun, D.Z. Li, W.J. Li, Y.B. Chen, Z.X. Chen, Y.H. He, X.Z. Fu, J. Phys. Chem. C 112 (2008) 18076e18081. [18] N.Z. Bao, L.M. Shen, T. Takata, K. Domen, Chem. Mater. 20 (2008) 110e117. [19] B.B. Kale, J.-O. Baeg, S.M. Lee, H. Chang, S.-J. Moon, C.W. Lee, Adv. Func. Mater. 16 (2006) 1349e1354. [20] D. Chen, Z. Liu, X.F. Wang, B. Liang, J. Xu, H. Huang, Z. Xie, G.Z. Shen, CrystEngComm 13 (2011) 7305e7310. [21] S.H. Ran, Y.G. Zhu, H.T. Huang, B. Liang, J. Xu, B. Liu, J. Zhang, Z. Xie, Z.R. Wang, J.H. Ye, D. Chen, G.Z. Shen, CrystEngComm 14 (2012) 3063e3068. [22] D.W. Jing, M.C. Liu, L.J. Guo, Catal. Lett. 140 (2010) 167e171. [23] J.F. Chao, B. Liang, X.J. Hou, Z. Liu, Z. Xie, B. Liu, W.F. Song, G. Chen, D. Chen, G.Z. Shen, Opt. Express 21 (2013) 13639e13647. [24] S.H. Yu, M. Yoshimura, Adv. Funct. Mater. 12 (2002) 277e285. [25] P.S. Khiew, N.M. Huang, S. Radiman, S. Ahmad, Mater. Lett. 58 (2004) 762e767. [26] M. Liu, L.Y. Piao, W.M. Lu, S.T. Ju, L. Zhao, H.L. Li, W.J. Wang, Nanoscale 2 (2010) 1115e1117.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Visible-light-driven photocatalytic properties and electronic structures of nickel sulfide nanoflowers Junfeng Chao a, *, Degong Duan a, Shumin Xing b, Yuliang Zhao b, Xiutai Zhang a, Suling Gao a, Xiaohong Li a, Qiufeng Fan a, Junping Yang a a b
College of Electronic Information and Electric Engineering, Anyang Institute of Technology, Anyang 455000, PR China College of Mathematics and Physics, Anyang Institute of Technology, Anyang 455000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2015 Received in revised form 25 March 2015 Accepted 27 March 2015 Available online 28 March 2015
Nickel sulfide (NiS) nanoflowers with the thickness of ca. 5e10 nm and size up to several hundreds of nanometers were synthesized via a facile polyol refluxing process under the open-air condition. The photocatalytic properties of NiS nanoflowers were evaluated by the decomposition ratio of MB was up to nearly 98% after 3 h visible light irradiation, indicating the NiS nanoflowers were good candidates for high performance photocatalysts. Meanwhile, the influencing factor of the photocatalytic reaction had also been studied by calculating the electronic structure of NiS nanoflowers. The band structure indicates that charge transfer upon photoexcitation occurs from the S 3p orbital to the empty Ni orbital. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: Polyol refluxing NiS nanoflowers Photocataly Electronic structures
1. Introduction The photocatalytic technique for the degradation of organic contaminants in air or in solution has been widely studied to remedy the effects of environmental pollution because of its simple and exhaustive decomposition process [1e5]. Among all kinds of semiconductors, TiO2 is one of the most important oxides photocatalyst, which has been widely used [6e9]. However, with a wide band gap of 3.2 eV, TiO2 is only sensitive under ultraviolet light region and is not active to visible light. It is well known that only 4% of the solar spectra falls in the ultraviolet region, thus much work has been done to synthesize visible light photocatalyst in view of the better utilization of solar energy. Through modification of TiO2, including doping with metal, such as Fe, Au, Ag [10e12], and nonmetal atoms, such as C, N, and S [13e15], a lot of visible-lightdriven photocatalysts have been synthesized. On the other hand, many traditional non-titanium-based visible light photocatalysts have been synthesized, for example, some sulfides have been found to have visible-light-driven catalytic activity, such as SnS [16], Sb2S3 [17], CdS [18], and so on [19,20]. All of these studies may provide new insights for the design of visible-light-driven photocatalysts.
* Corresponding author. E-mail address: [email protected] (J. Chao). http://dx.doi.org/10.1016/j.solidstatesciences.2015.03.022 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
Flower-like semiconductor nanomaterials with a large surface area and high photocatalytic activity have been widely investigated due to their important applications in purifying water and air fields. For instance, 3D flower-like Ni(OH)2 products prepared by Shen's group showed high selective adsorption for cationic/anionic dyes in aqueous solutions under visible light irradiation [21]. From a hydrothermal method, Jing and his coauthors have synthesized flower-like Ni2þ doped ZnIn2S4 microsphere with greatly enhancing the activity of the photocatalyst [22]. However, few report concerns the yield of nanomaterials via a facile route, thus remains a considerable challenge. Thus, developing simple and effective methods to fabricate semiconductors with flower-like structures is important for future visible-light-driven photocatalysts applications. The facile polyol refluxing process has several advantages over the conventional hydrothermal method, such as facile, short heating time to the reaction temperature, high output and open-air condition [16,23]. Up to now, to the best of our knowledge, little work on the preparation of nickel sulfide nanoflowers in liquid systems has been reported. Here, the simple synthesis of flower-like NiS from onestep, facile solution chemical method under the open-air condition was realized for the first time. The optical properties, photocatalytic activity and the electronic structures of the NiS nanoflowers were investigated. It was found that the obtained NiS nanoflowers have high photocatalytic activity in degradating
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J. Chao et al. / Solid State Sciences 43 (2015) 59e62
Fig. 1. (a) SEM image, (b) TEM image, (c) XRD pattern and (d) EDS of the as-synthesized flowers-like NiS nanostructure.
organic dyes MB under visible light irradiation.
2.3. Photocatalysis
2. Experimental
Photocatalytic decomposition of methylene blue (MB) was carried out in a cylindrical reaction vessel containing a suspension of 0.1 g NiS nanoflowers sample in a 100 mL MB or RhB solution (the dye concentration was 8 mg L1 in both cases). To get the adsorptionedesorption equilibrium, the suspension sample was kept in dark for 2 h with continuous stirring. Then the system was irradiated by a 500 W Xe-lamp equipped with a cutoff filter (l > 420 nm) and a water filter was used as the light source. At given time intervals, 3 mL of the reaction suspension was collected and filtrated to remove the NiS particles. The absorption spectra of a series of MB solutions were measured on a Shimadzu UV-2550 UVevis spectrometer at room temperature.
2.1. Synthesis of the NiS nanoflowers In this study, all of the reactants were of analytical grade and were used directly without further purification. NiS nanoflowers were prepared via a facile polyol refluxing process. 1 mmol of four hydrated nickel acetate (Ni(CH3COO)2$4$H2O, 98.5%, AR) and 3 mmol thiourea (CN2H4S, 99.0%, AR) were put into a three-neck flask (250 mL capacity) to which 50 mL ethylene glycol was added. After stirring for 10 min, all the reagents were dissolved, the reaction system was heated to 135 C and kept at that temperature for 1 h under vigorous stirring in atmosphere. Finally, black precipitates were collected, washed with distilled water and absolute ethanol and then dried in a vacuum at 70 C for 3 h. 2.2. Material characterization The as-prepared sample was characterized by X-ray powder diffractometer (X'Pert PRO, PANalytical B.V., the Netherlands) with radiation of a Cu target (Ka, l ¼ 0.15406 nm) at 25 C. The energy dispersive spectroscopy was achieved on a micro X-ray fluorescence system (EDX Inc.). The nanoelectrode morphologies were investigated by a JEOL JSM-6700F field emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) images (JEOL, JEM-2010) were prepared to analyze the morphologies of the samples. The UVevis absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer. Room-temperature photoluminescence (PL) spectra were recorded with an HORIBA Jobin Yvon LabRAM Spectrometer HR 800 UV with the excitation wavelength of 325 nm.
3. Results and discussion Fig. 1a displays the panoramic SEM image, indicating the formation of a large number of 3D flower-like architectures with the diameter of about 150e200 nm. Fig. 1b clearly exhibits that NiS nanoflowers are built from dozens of nanoflakes with the thickness of 5e10 nm. The formation of porous flowers is caused by the connections and overlaps of the adjacent nanoflakes. The little nanoflowers which were attracted by Van der Waals' force made up some nanoflowers with larger size. X-ray powder diffraction (XRD) was used to study the phase purity of the as-synthesized NiS nanoflowers as shown in Fig. 1c. The main reflection peaks can be indexed to the hexagonal phase of nickel monosulfide with lattice constants of a ¼ 3.425 and c ¼ 5.340 (JCPDS card No. 65e3419). Four main peaks ((100), (101), (102) and (110) planes) in the XRD pattern show wider full-widths at half-maxima than a bulk NiS [24]. In order to further indicate the lattice formation of the flowerlike product, an energy dispersive spectroscopy (EDS) was detected
J. Chao et al. / Solid State Sciences 43 (2015) 59e62
61
Fig. 4. The calculated (a) energy bands and (b) density of states (DOS) of assynthesized NiS nanoflowers samples.
Fig. 2. (a) UVevis absorption spectrum (the inset is a (ahn)1/2 vs hn plot) and (b) a room temperature photoluminescence (PL) spectrum of the NiS nanoflowers.
in Fig. 1d. Only S and Ni peaks can be detected, other element peaks such as N are not be found, which indicates the product is composed of the elements S and Ni. The result also reveals that the quality ratio is 35.56: 64.44 and the molar ratio is 50.25: 49.75, which is consistent with the composition of NiS.
The optical properties of the products were measured and the corresponding UVevis absorption spectrum is depicted in Fig. 2a. The absorption spectra were calculated according to the KubelkaeMunk Theory:
FðR∞ Þ ¼ K=S ¼
ð1 R∞ Þ2 2R∞
Were R∞ is the extreme limit of the reflectance coefficient, K is the absorption coefficient, S is the scattering coefficient. Black-colored NiS samples exhibit broad and strong absorption in the range from
Fig. 3. (a) UVevis absorption spectral of MB solution in the presence of NiS nanoflowers under visible light irradiation, inset is the photographs of photodegradation of MB over NiS samples against time. (b) UVevis spectral changes of RhB aqueous solution in the presence of NiS architectures. (c) Concentration changes of MB over the NiS nanoflowers as well as photolysis and P25 TiO2 with irradiation time, respectively.
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J. Chao et al. / Solid State Sciences 43 (2015) 59e62
240 nm to 800 nm. The optical band gaps of the samples could be evaluated from the following equation: (ahn) ¼ A(hn-Eg)n/2, the a and n values at the steep edges of the absorption spectra were used to construct the plots of (ahn)2/n against photon energy. As seen in Fig. 2a inset, the curves for the sample show a linear region with n ¼ 4 for the indirect band gap. The intercepts of the tangents to the plots indicated that the optical bandgap energy of the nanomaterial was 1.22 eV, which is good consistent with the previously reported data [25]. Fig. 2b represents the room temperature photoluminescence (PL) spectrum of the as-prepared NiS nanoflowers with an excitation wavelength of 325 nm. One strong broad emission peak with a maximum at 565 nm can be clearly observed, which may be due to electronic transitions caused by defects in the interfacial region [26]. Taking into account the novel 3D architecture with nanoscale building block and nanometer size, also with a visible light band gap, the as-prepared NiS nanoflowers should have an excellent optical activity. To demonstrate the applications, we investigated the materials' photocatalytic activity by employing the photocatalytic degradation some organic dyes such as methylene blue (MB) and rhodamine B (RhB) under visible light irradiation at room temperature. Fig. 3a shows the UVevis absorption spectra of the MB and RhB solution in the presence of NiS nanoflowers. After adsorption/desorption equilibrium, the light was turned on. It clearly shows that when the MB solution system was irradiated, the main absorption peaks corresponding to MB (at 664 nm, 611 nm and 291 nm) gradually decreased continuously with increased irradiation time, and almost completely disappeared after 3 h. Inset of Fig. 3a shows the corresponding photographs of the initial MB solution and those after photocatalytic reactions for different times. The colour of MB solution system changes from blue to almost colorless, indicating that MB species were absorpted and degraded under visible light irradiation. Fig. 3b shows the great adsorption activities of the NiS architectures when catalyst was dispersed into the RhB solution. Furthermore, the plot for the concentration changes of MB determined from its characteristic absorption peak at 664 nm is shown in Fig. 3b. For comparison, the photolysis, the MB degradation over commercially P25eTiO2, the RhB degradation over NiS nanoflowers under visible light irradiation were also tested at the same condition, the corresponding results are also shown in Fig. 3c, respectively. NiS nanoflowers have a significantly stronger adsorption and photocatalytic degradation ability than P25eTiO2, and MB dye is considerably stable under visible light irradiation. It can be clearly seen that after 2 h of adsorption equilibrium and 3 h of visible light irradiation, the degradation ratio of MB for NiS nanoflowers is about 98%, while that of P25eTiO2 is about 10%. According to the adsorptionedesorption and photodegradation processes, we concluded that NiS nanoflowers show enhanced photocatalytic performance for the degradation of MB solution. The photocatalytic performance of sulfide photocatalysts depend on their electronic structures. In the present work, the electronic structures of NiS nanoflowers were investigated by plane-wave DFT calculation. Fig. 4a shows the conduction band minimum (CBM) of NiS crystal is located at the K point while the valence band maximum (VBM) is located in the region between the G point and the M point of Brillouin zone, indicating that the NiS is indeed an indirect semiconductor. The density of states of the NiS nanoflowers are shown in Fig. 4b. The conduction band in the range of 3e15 eV is mainly composed of Ni 5p and S 3p orbitals. The valence band at about 17 to 2 eV primarily consists of S 3p and Ni 5p orbitals. The bottom of the
conduction band is mainly constituted by the Ni 5p orbital, while the conduction band top is composed mainly of the S 3p orbital. The total pDOS indicates that charges transfer upon visible light excitation occurs from the S 3p orbital to the empty Ni 5p orbital. The calculated band gap of NiS is 1.2 eV, in good agreement with the experimental result based on the UVevis absorption spectra. 4. Conclusions In summary, NiS nanoflowers were synthesized by using a simple polyol refluxing process. Under visible light irradiation, NiS nanoflowers showed much stronger adsorption and photodegradation activities for MB than commercial P25eTiO2 powders and the conversion ratio of MB was up to nearly 98%. The successful application of NiS in the photocatalytic field may also give useful insight for the development of other visible-light-driven semiconductors. The electronic structures of NiS were further investigated by plane-wave DFT calculation. Acknowledgments This work was supported by the National Natural Science Foundation (21001046, 51002059), the 973 Program of China (2011CB933300, 2011CBA00703), the Important Scientific Research Foundation of Henan Province Education Department (14A510002). The authors are grateful to professor Di Chen for help with the experiments and the samples measurements. References [1] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341e357. [2] D. Chen, J.H. Ye, Adv. Funct. Mater. 18 (2008) 1922e1928. [3] J.F. Chao, Z. Xie, X.B. Duan, Y. Dong, Z.R. Wang, J. Xu, B. Liang, B. Shan, J.H. Ye, D. Chen, G.Z. Shen, CrystEngComm 14 (2012) 3163e3168. [4] P. Hartmann, D.K. Lee, B.M. Smarsly, J. Janek, ACS Nano 4 (2010) 3147e3154. [5] D. Chen, Z. Liu, S.X. Ouyang, J.H. Ye, J. Phys. Chem. C 115 (2011) 15778e15784. [6] A. Fujishima, K. Honda, Nature 238 (1972) 37e38. [7] T.X. Wu, G.M. Liu, J.C. Zhao, H. Hidaka, N. Serpone, J. Phys. Chem. B 102 (1998) 5845e5851. [8] M. Liu, L.Y. Piao, W.M. Lu, S.T. Ju, L. Zhao, C.L. Zhou, H.L. Li, W.J. Wang, Nanoscale 2 (2010) 1115e1117. [9] Z.R. Wang, H. Wang, B. Liu, W.Z. Qiu, J. Zhang, S.H. Ran, H.T. Huang, J. Xu, H.W. Han, D. Chen, G.Z. Shen, ACS Nano 5 (2011) 8412e8419. [10] H. Tong, S.X. Ouyang, Y.P. Bi, N. Umezawa, M. Oshikiri, J.H. Ye, Adv. Mater. 24 (2012) 229e251. [11] S. Bingham, W.A. Daoud, J. Mater. Chem. 21 (2011) 2041e2050. [12] Y.R. Zhang, Q. Li, Solid State Sci. 16 (2013) 16e20. [13] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243e2245. [14] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269e271. [15] R. Bacsa, J. Kiwi, T. Ohno, P. Albers, V. Nadtochenko, J. Phys. Chem. B 109 (2005) 5994e6003. [16] J.F. Chao, Z.R. Wang, X. Xu, Q.Y. Xiang, W.F. Song, G. Chen, J.B. Hu, D. Chen, RSC Adv. 3 (2013) 2746e2753. [17] M. Sun, D.Z. Li, W.J. Li, Y.B. Chen, Z.X. Chen, Y.H. He, X.Z. Fu, J. Phys. Chem. C 112 (2008) 18076e18081. [18] N.Z. Bao, L.M. Shen, T. Takata, K. Domen, Chem. Mater. 20 (2008) 110e117. [19] B.B. Kale, J.-O. Baeg, S.M. Lee, H. Chang, S.-J. Moon, C.W. Lee, Adv. Func. Mater. 16 (2006) 1349e1354. [20] D. Chen, Z. Liu, X.F. Wang, B. Liang, J. Xu, H. Huang, Z. Xie, G.Z. Shen, CrystEngComm 13 (2011) 7305e7310. [21] S.H. Ran, Y.G. Zhu, H.T. Huang, B. Liang, J. Xu, B. Liu, J. Zhang, Z. Xie, Z.R. Wang, J.H. Ye, D. Chen, G.Z. Shen, CrystEngComm 14 (2012) 3063e3068. [22] D.W. Jing, M.C. Liu, L.J. Guo, Catal. Lett. 140 (2010) 167e171. [23] J.F. Chao, B. Liang, X.J. Hou, Z. Liu, Z. Xie, B. Liu, W.F. Song, G. Chen, D. Chen, G.Z. Shen, Opt. Express 21 (2013) 13639e13647. [24] S.H. Yu, M. Yoshimura, Adv. Funct. Mater. 12 (2002) 277e285. [25] P.S. Khiew, N.M. Huang, S. Radiman, S. Ahmad, Mater. Lett. 58 (2004) 762e767. [26] M. Liu, L.Y. Piao, W.M. Lu, S.T. Ju, L. Zhao, H.L. Li, W.J. Wang, Nanoscale 2 (2010) 1115e1117.