- Email: [email protected]
Solvothermal synthesis and characterization of ultrathin SnO nanosheets
Materials Letters 118 (2014) 69–71
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Solvothermal synthesis and characterization of ultrathin SnO nanosheets Guang Sun, Fengxiao Qi, Yanwei Li n, Naiteng Wu, Jianliang Cao, Saisai Zhang, Xiaodong Wang, Guiyun Yi, Hari Bala, Zhanying Zhang Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, School of Materials Science and Engineering, School of Chemistry and Physics, School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2013 Accepted 11 December 2013 Available online 18 December 2013
Two-dimensional (2-D) nanomaterials have attracted remarkable interest in recent years. Herein, we report the synthesis of 2-D ultrathin nanosheets of SnO via a simple solvothermal method by using SnCl2 2H2O and NH3 H2O as reactants and ethanol as solvent. The as-prepared sample was characterized by using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high resolution TEM (HRTM), respectively. Results indicate that that the as-synthesized ultrathin SnO nanosheets are well crystalline in nature and about 4 nm in thickness. N2 absorption–desorption measurement showed that the specific surface area of the ultrathin SnO nanosheets is 35.7 m2/g. The band gap energy of the ultrathin SnO nanosheets is estimated to be 3.62 eV, which is blue shifted to its bulk counterpart due to the quantum effect. & 2014 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Nanocrystalline materials SnO Ultrathin nanosheets Solvothermal synthesis
1. Introduction In the past decades, metal oxide nanomaterials with special morphology have attracted significant attention because of their unique morphology-dependent properties that are different from their bulk counterparts. Intensive efforts have been devoted to rational design and precise control over the morphology of metal oxide nanocrystals. Among various nanostructures, dimensionality is one of the most defining parameters that are closely relative to the property of metal oxide nanomaterials. By now, different dimensional nanostructures of metal oxide semiconductor have been reported. However, in contrast with the abundant researches on 0-D quantum dots, 1-D nanowires, nanotubes, and nanorods, and 3-D hierarchical structures that assembled from low dimensional building blocks, 2-D nanostructures, such as ultrathin nanosheets, are far less reported. Ever since the finding of grapheme [1–3], a typical 2-D nanomaterial owning excellent physical and chemical properties, 2-D ultrathin nanosheet materials have been paid more and more attentions in recent years [4–6]. Tin oxides, in particular divalent tin monoxide (SnO) and tetravalent tin dioxide (SnO2), have caused widespread concern due to their excellent optical and electrical properties. SnO2 is a well- known n-type semiconductor with a wide band gap of 3.6 eV. The synthesis and morphological control of SnO2 have
n
Corresponding author Tel.: þ 86 3913986952. E-mail address: [email protected] (Y. Li).
0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.048
been widely studied for different applications [7–10]. Different to SnO2, SnO is found to be a p-type semiconductor with band gap of 2.7–3.2 eV [11,12], and has potential applications in various fields, such as coating substance [13], catalyst [14], and anode materials for lithium-ion batteries with high theoretical specific capacity (875 mA/h/g) [15–17]. In general, the selective synthesis of SnO is more difficult than that of SnO2 because divalent tin ions are easily oxidized to the tetravalent state. Up to now, various morphologies of SnO have been synthesized, such as diskettes [18], nanobranches [19], nanosheets [20,21], nanoflowers [16,22], honeycomb-like hierarchical structures [12], and so on. However, the synthesis of SnO ultrathin nanosheets under mild condition is still a challenging work. In this paper, 2-D ultrathin nanosheets of SnO with the thickness about 4 nm have been successfully prepared by a simple solvothermal method. N2 adsorption–desorption analysis indicated that the specific surface area of the ultrathin SnO nanosheets is about 35.7 m2/g. Based on the result given by UV–vis absorption spectrum, the energy band gap is estimated to be 3.62 eV, which is blue shift to bulk SnO.
2. Experimental section All chemical reagents were of analytical grade, purchased from Shanghai Chemical Reagents Co. and used without further purification. In a typical procedure for ultrathin SnO nanosheets, 1.128 g SnCl2 2H2O powder was dissolved in 10 mL absolute
70
G. Sun et al. / Materials Letters 118 (2014) 69–71
30
40
50
60
70
2 theta (degree) Fig. 1. XRD pattern of the prepared SnO sample.
80
(114)
(213)
(301)
(310)
(220)
(103)
(211) (202)
(200)
(002)
(001)
20
(112)
(110)
Intensity (a.u.)
(101)
ethanol to form a homogenous solution. Then, 30 mL NH3 H2O (25–28 wt%) were added into above solution under stirring. The obtained mixture solution was sealed and then heated at 130 1C in a 50 mL Telfon-lined stainless autoclave for 15 h. After the autoclave was cooled down to room temperature naturally, the precipitations were collected by centrifugation and washed for several times with de-ionized water and absolute alcohol to remove impurities. Finally, blue black product was obtained after it was dried in a vacuum oven at 60 1C for 15 h. The phase structure and purity of the as-prepared product were characterized by powder X-ray diffraction (XRD) on Bruker D8 diffractometer with Cu Kα radiation (λ ¼1.54056 nm). The transmission electron microscopy (TEM) images, selected area electron diffraction (SAED) pattern, and high resolution TEM (HRTEM) images were collected on a JEM-2100 TEM. N2 adsorption–desorption isotherms were collected at liquid nitrogen temperature using a Quantachrome AsiQM0000-3 sorption analyzer.
90
The UV–vis absorption spectrum was recorded with a TU-1810 UV–vis spectrophotometer.
3. Results and discussion The phase structure and purity of the as-prepared sample was characterized by XRD. As show in Fig. 1a, all the diffraction peaks in the XRD spectrum can be readily indexed to tetragonal SnO (JCPDS card no. 06-0395, lattice parameters a¼ b¼3.802 Å, and c¼4.836 Å), and no diffraction peaks from other crystalline impurities such as Sn2O3, Sn3O4 and SnO2 are detected, indicating the formation of tetragonal SnO with high purity. The calculated lattice parameters are a ¼b¼ 3.77 Å, c¼4.83 Å, being consistent with the standard values. Fig. 2a and b shows the typical TEM images of the prepared SnO with different magnification. One can see that a large amount of sheet-like nanocrystals are formed in the product. Meanwhile, the light region suggested planar or blended thin sheets lying on the substrate and the relatively dark regions indicated that some sheets may either lie obliquely, perpendicularly to the substrate or spontaneously convolute to minimize their surface energy. The edge length of the nanosheets is most likely less than 50 nm in lateral dimension, and the average thickness of the nanosheets determined from Fig. 2b is about 4 nm. Fig. 2c shows the SAED pattern of the product. The bright and clear diffraction rings reveal the wellcrystalline nature of the as-prepared SnO ultrathin nanosheets. The diffraction rings from inner to outer can be indexed to (1 0 1), (1 1 0), (0 0 2) and (1 1 2) planes of the tetragonal SnO, respectively, being good agreement with XRD analysis. Fig. 2d shows a typical HRTEM image recorded from a single SnO nanosheet. The clear lattice fringes further confirmed the well-crystalline nature of the as-prepared SnO nanosheets. The spacing between the adjacent lattice fringes was measured to be 0.179 and 0.160 nm, corresponding to (1 1 2) and (2 1 1) planes of tetragonal SnO, respectively. The as-prepared SnO sample was investigated by the nitrogen adsorption and desorption isotherms. The specific surface area is measured by the Brunauer–Emmett–Teller (BET) method, and the
Fig. 2. ((a) and (b)) TEM images, (c) SAED pattern, and (d) HRTEM image of the as-prepared ultrathin SnO nanosheets.
G. Sun et al. / Materials Letters 118 (2014) 69–71
the band gap energy for as-prepared SnO ultrathin nanosheets is estimated to be 3.62 eV, which is larger than that of bulk SnO material (2.7–3.2 eV). The blue shift of the band gap energy can be ascribed to the quantum effect based on the ultrathin nature of the SnO nanosheets.
70
Volume absorbed (cm3g-1)
60
50
40
4. Conclusions 30
20
10
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 3. N2 adsorption–desorption isotherm and the corresponding pore size distribution of the as-prepared ultrathin SnO nanosheets (upper right inset).
In summary, SnO ultrathin nanosheets with high purity have been successfully synthesized via a simple solvothermal route. The obtained SnO nanosheets are about 4 nm in thickness, and their surface area is about 35.7 m2/g. The band gap energy of the ultrathin SnO nanosheets is estimated to be 3.62 eV, which is blue shift to bulk SnO due to the quantum effect. Such ultrathin nanosheets of SnO not only have potential applications in anode material for lithium rechargeable batteries and catalyst, but also can be used as sacrificial templates for fabricating SnO2 nanostructures.
Acknowledgments
3.5
This work was supported by the National Natural Science Foundation of China (51172065), Program for Innovative Research Team in the University of Henan Province (2012IRTSTHN007), Foundation of He’nan Scientific and Technology key project (132102210251, 112102310425, 112102310029), the Education Department Natural Science Foundation of Henan Province (13A430315, 2010B420012, 2011B150009), China Postdoctoral Science Foundation funded project (2012M521394), and Specialized Research Fund for the Doctoral Program of Higher Education (20124116120002).
3.0 2.5
Absorbance
71
2.0 1.5 1.0 0.5
References
0.0 300
400
500
600
700
800
Wavelength (nm) Fig. 4. UV–vis adsorption spectrum of the as-prepared ultrathin SnO nanosheets and corresponding plot of (αhv)2 versus photo energy (upper right inset).
pore-size distribution (PSD) is calculated using Barret–Joyner– Halenda (BJH) method. As show in Fig. 3, the isotherm curve of the SnO sample exhibited a type of IV-like behavior including a type H3 hysteresis loop (IUPC classification), suggesting the presence of mesopores in the material. The formation of mesopores in the material is attributed to the aggregation and convolution of SnO ultrathin nanosheets. The BET specific surface area of the product was found to be 35.7 m2/g. The inset in Fig. 4b reveals that the pore-size distribution of the sample is 2–15 nm, indicating that the aggregation between the SnO nanosheets is nonuniform. Fig. 4 shows the UV–vis absorption spectrum of the asprepared ultrathin SnO nanosheets. An obvious absorption band centered at 310 nm is observed. For direct band gap semiconductor, such as SnO [22], the absorption coefficient α can be expressed as α (hv Eg)1/2/hv [23], where α is the absorption coefficient, h is Planck's constant, ν is the radiation frequency, and Eg is the band gap energy. Thus, the plot of (αhv)2 versus hv can be derived from the absorption data in Fig. 4. The intercept of the tangent to the plot gives a good approximation of the band gap energy of the direct band gap material [24]. As shown in the inset of Fig. 4,
[1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Science 2004;306:666–9. [2] Geim AK, Novoselov KS. Nat Mater 2007;6:183–91. [3] Yoo E, Kim J, Hosono E, Zhou HS, Kudo T, Honma I. Nano Lett 2008;8:2277–82. [4] Hosono E, Fujihara S, Honma I, Zhou H. Adv Mater 2005;17:2091–4. [5] Han XG, Kuang Q, Jin MS, Xie ZX, Zheng LS. J Am Chem Soc 2009;131:3152–3. [6] Golberg D, Bando Y, Huang Y, Terao T, Mitome M, Tang C, et al. ACS Nano 2010;4:2979–93. [7] Zhu JJ, Lu ZH, Aruna ST, Aurbach D, Gedanken A. Chem Mater 2000;12: 2557–66. [8] Qin LP, Xu JQ, Dong XW, Pan QY, Cheng ZX, Xiang Q, et al. Nanotechnology 2008;19:185705. [9] Li KM, Li YJ, Lu MY, Kuo CY, Chen LJ. Adv Funct Mater 2009;19:2453–6. [10] Jia TK, Wang WM, Long F, Fu ZY, Wang H, Zhang QJ. J Phys Chem C 2009;113:9071–7. [11] Liang LY, Liu ZM, Cao HT, Pan XQ. ACS Appl Mater Interfaces 2010;2:1060–5. [12] Sun G, Wu NT, Li YW, Cao JL, Qi FX, Bala Hari, et al. Mater Lett 2012;98:224–7. [13] Jia ZJ, Zhu LP, Liao GH, Yu Y, Tang YW. Solid State Commun 2004;132:79–82. [14] Banerjee Moloy, Roy Sujit. Org. Lett 2004;6:2137–40. [15] Yoshio I, Tadahiko K, Akihiro M, Yukio M, Tsutomu M. Science 1997;276:1395–7. [16] Ning JJ, Dai QQ, Jiang T, Men KK, Liu DH, Xiao NR, et al. Langmuir 2009;25:1818–21. [17] Ning JJ, Jiang T, Men KK, Dai QQ, Li DM, Wei YJ, et al. J Phys Chem C 2009;113:14140–4. [18] Dai ZR, Pan ZW, Wang ZL. J Am Chem Soc 2002;124:8673–80. [19] Shin JH, Song JY, Kim YH, Park HM. Mater Lett 2010;64:1120–2. [20] Sakaushi K, Oaki Y, Uchiyama H, Hosono E, Zhou HS, Imai H. Small 2010;6:776–81. [21] Wang HK, Wang Y, Xu, Jun, Yang, HH, et al. Langmuir 2012;28:10597–601. [22] Iqbal MZ, Wang FP, Javed QJ, Rafique MY, Qiu HM, Nabi G. Mater Lett 2012;75:236–9. [23] Mills G, Li ZG, Meisel D. J Phys Chem B 1988;92:822–8. [24] Xi HC, Ye JH. Inorg Chem 2010;49:2302–9.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Solvothermal synthesis and characterization of ultrathin SnO nanosheets Guang Sun, Fengxiao Qi, Yanwei Li n, Naiteng Wu, Jianliang Cao, Saisai Zhang, Xiaodong Wang, Guiyun Yi, Hari Bala, Zhanying Zhang Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in University of Henan Province, School of Materials Science and Engineering, School of Chemistry and Physics, School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2013 Accepted 11 December 2013 Available online 18 December 2013
Two-dimensional (2-D) nanomaterials have attracted remarkable interest in recent years. Herein, we report the synthesis of 2-D ultrathin nanosheets of SnO via a simple solvothermal method by using SnCl2 2H2O and NH3 H2O as reactants and ethanol as solvent. The as-prepared sample was characterized by using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high resolution TEM (HRTM), respectively. Results indicate that that the as-synthesized ultrathin SnO nanosheets are well crystalline in nature and about 4 nm in thickness. N2 absorption–desorption measurement showed that the specific surface area of the ultrathin SnO nanosheets is 35.7 m2/g. The band gap energy of the ultrathin SnO nanosheets is estimated to be 3.62 eV, which is blue shifted to its bulk counterpart due to the quantum effect. & 2014 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Nanocrystalline materials SnO Ultrathin nanosheets Solvothermal synthesis
1. Introduction In the past decades, metal oxide nanomaterials with special morphology have attracted significant attention because of their unique morphology-dependent properties that are different from their bulk counterparts. Intensive efforts have been devoted to rational design and precise control over the morphology of metal oxide nanocrystals. Among various nanostructures, dimensionality is one of the most defining parameters that are closely relative to the property of metal oxide nanomaterials. By now, different dimensional nanostructures of metal oxide semiconductor have been reported. However, in contrast with the abundant researches on 0-D quantum dots, 1-D nanowires, nanotubes, and nanorods, and 3-D hierarchical structures that assembled from low dimensional building blocks, 2-D nanostructures, such as ultrathin nanosheets, are far less reported. Ever since the finding of grapheme [1–3], a typical 2-D nanomaterial owning excellent physical and chemical properties, 2-D ultrathin nanosheet materials have been paid more and more attentions in recent years [4–6]. Tin oxides, in particular divalent tin monoxide (SnO) and tetravalent tin dioxide (SnO2), have caused widespread concern due to their excellent optical and electrical properties. SnO2 is a well- known n-type semiconductor with a wide band gap of 3.6 eV. The synthesis and morphological control of SnO2 have
n
Corresponding author Tel.: þ 86 3913986952. E-mail address: [email protected] (Y. Li).
0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.048
been widely studied for different applications [7–10]. Different to SnO2, SnO is found to be a p-type semiconductor with band gap of 2.7–3.2 eV [11,12], and has potential applications in various fields, such as coating substance [13], catalyst [14], and anode materials for lithium-ion batteries with high theoretical specific capacity (875 mA/h/g) [15–17]. In general, the selective synthesis of SnO is more difficult than that of SnO2 because divalent tin ions are easily oxidized to the tetravalent state. Up to now, various morphologies of SnO have been synthesized, such as diskettes [18], nanobranches [19], nanosheets [20,21], nanoflowers [16,22], honeycomb-like hierarchical structures [12], and so on. However, the synthesis of SnO ultrathin nanosheets under mild condition is still a challenging work. In this paper, 2-D ultrathin nanosheets of SnO with the thickness about 4 nm have been successfully prepared by a simple solvothermal method. N2 adsorption–desorption analysis indicated that the specific surface area of the ultrathin SnO nanosheets is about 35.7 m2/g. Based on the result given by UV–vis absorption spectrum, the energy band gap is estimated to be 3.62 eV, which is blue shift to bulk SnO.
2. Experimental section All chemical reagents were of analytical grade, purchased from Shanghai Chemical Reagents Co. and used without further purification. In a typical procedure for ultrathin SnO nanosheets, 1.128 g SnCl2 2H2O powder was dissolved in 10 mL absolute
70
G. Sun et al. / Materials Letters 118 (2014) 69–71
30
40
50
60
70
2 theta (degree) Fig. 1. XRD pattern of the prepared SnO sample.
80
(114)
(213)
(301)
(310)
(220)
(103)
(211) (202)
(200)
(002)
(001)
20
(112)
(110)
Intensity (a.u.)
(101)
ethanol to form a homogenous solution. Then, 30 mL NH3 H2O (25–28 wt%) were added into above solution under stirring. The obtained mixture solution was sealed and then heated at 130 1C in a 50 mL Telfon-lined stainless autoclave for 15 h. After the autoclave was cooled down to room temperature naturally, the precipitations were collected by centrifugation and washed for several times with de-ionized water and absolute alcohol to remove impurities. Finally, blue black product was obtained after it was dried in a vacuum oven at 60 1C for 15 h. The phase structure and purity of the as-prepared product were characterized by powder X-ray diffraction (XRD) on Bruker D8 diffractometer with Cu Kα radiation (λ ¼1.54056 nm). The transmission electron microscopy (TEM) images, selected area electron diffraction (SAED) pattern, and high resolution TEM (HRTEM) images were collected on a JEM-2100 TEM. N2 adsorption–desorption isotherms were collected at liquid nitrogen temperature using a Quantachrome AsiQM0000-3 sorption analyzer.
90
The UV–vis absorption spectrum was recorded with a TU-1810 UV–vis spectrophotometer.
3. Results and discussion The phase structure and purity of the as-prepared sample was characterized by XRD. As show in Fig. 1a, all the diffraction peaks in the XRD spectrum can be readily indexed to tetragonal SnO (JCPDS card no. 06-0395, lattice parameters a¼ b¼3.802 Å, and c¼4.836 Å), and no diffraction peaks from other crystalline impurities such as Sn2O3, Sn3O4 and SnO2 are detected, indicating the formation of tetragonal SnO with high purity. The calculated lattice parameters are a ¼b¼ 3.77 Å, c¼4.83 Å, being consistent with the standard values. Fig. 2a and b shows the typical TEM images of the prepared SnO with different magnification. One can see that a large amount of sheet-like nanocrystals are formed in the product. Meanwhile, the light region suggested planar or blended thin sheets lying on the substrate and the relatively dark regions indicated that some sheets may either lie obliquely, perpendicularly to the substrate or spontaneously convolute to minimize their surface energy. The edge length of the nanosheets is most likely less than 50 nm in lateral dimension, and the average thickness of the nanosheets determined from Fig. 2b is about 4 nm. Fig. 2c shows the SAED pattern of the product. The bright and clear diffraction rings reveal the wellcrystalline nature of the as-prepared SnO ultrathin nanosheets. The diffraction rings from inner to outer can be indexed to (1 0 1), (1 1 0), (0 0 2) and (1 1 2) planes of the tetragonal SnO, respectively, being good agreement with XRD analysis. Fig. 2d shows a typical HRTEM image recorded from a single SnO nanosheet. The clear lattice fringes further confirmed the well-crystalline nature of the as-prepared SnO nanosheets. The spacing between the adjacent lattice fringes was measured to be 0.179 and 0.160 nm, corresponding to (1 1 2) and (2 1 1) planes of tetragonal SnO, respectively. The as-prepared SnO sample was investigated by the nitrogen adsorption and desorption isotherms. The specific surface area is measured by the Brunauer–Emmett–Teller (BET) method, and the
Fig. 2. ((a) and (b)) TEM images, (c) SAED pattern, and (d) HRTEM image of the as-prepared ultrathin SnO nanosheets.
G. Sun et al. / Materials Letters 118 (2014) 69–71
the band gap energy for as-prepared SnO ultrathin nanosheets is estimated to be 3.62 eV, which is larger than that of bulk SnO material (2.7–3.2 eV). The blue shift of the band gap energy can be ascribed to the quantum effect based on the ultrathin nature of the SnO nanosheets.
70
Volume absorbed (cm3g-1)
60
50
40
4. Conclusions 30
20
10
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 3. N2 adsorption–desorption isotherm and the corresponding pore size distribution of the as-prepared ultrathin SnO nanosheets (upper right inset).
In summary, SnO ultrathin nanosheets with high purity have been successfully synthesized via a simple solvothermal route. The obtained SnO nanosheets are about 4 nm in thickness, and their surface area is about 35.7 m2/g. The band gap energy of the ultrathin SnO nanosheets is estimated to be 3.62 eV, which is blue shift to bulk SnO due to the quantum effect. Such ultrathin nanosheets of SnO not only have potential applications in anode material for lithium rechargeable batteries and catalyst, but also can be used as sacrificial templates for fabricating SnO2 nanostructures.
Acknowledgments
3.5
This work was supported by the National Natural Science Foundation of China (51172065), Program for Innovative Research Team in the University of Henan Province (2012IRTSTHN007), Foundation of He’nan Scientific and Technology key project (132102210251, 112102310425, 112102310029), the Education Department Natural Science Foundation of Henan Province (13A430315, 2010B420012, 2011B150009), China Postdoctoral Science Foundation funded project (2012M521394), and Specialized Research Fund for the Doctoral Program of Higher Education (20124116120002).
3.0 2.5
Absorbance
71
2.0 1.5 1.0 0.5
References
0.0 300
400
500
600
700
800
Wavelength (nm) Fig. 4. UV–vis adsorption spectrum of the as-prepared ultrathin SnO nanosheets and corresponding plot of (αhv)2 versus photo energy (upper right inset).
pore-size distribution (PSD) is calculated using Barret–Joyner– Halenda (BJH) method. As show in Fig. 3, the isotherm curve of the SnO sample exhibited a type of IV-like behavior including a type H3 hysteresis loop (IUPC classification), suggesting the presence of mesopores in the material. The formation of mesopores in the material is attributed to the aggregation and convolution of SnO ultrathin nanosheets. The BET specific surface area of the product was found to be 35.7 m2/g. The inset in Fig. 4b reveals that the pore-size distribution of the sample is 2–15 nm, indicating that the aggregation between the SnO nanosheets is nonuniform. Fig. 4 shows the UV–vis absorption spectrum of the asprepared ultrathin SnO nanosheets. An obvious absorption band centered at 310 nm is observed. For direct band gap semiconductor, such as SnO [22], the absorption coefficient α can be expressed as α (hv Eg)1/2/hv [23], where α is the absorption coefficient, h is Planck's constant, ν is the radiation frequency, and Eg is the band gap energy. Thus, the plot of (αhv)2 versus hv can be derived from the absorption data in Fig. 4. The intercept of the tangent to the plot gives a good approximation of the band gap energy of the direct band gap material [24]. As shown in the inset of Fig. 4,
[1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Science 2004;306:666–9. [2] Geim AK, Novoselov KS. Nat Mater 2007;6:183–91. [3] Yoo E, Kim J, Hosono E, Zhou HS, Kudo T, Honma I. Nano Lett 2008;8:2277–82. [4] Hosono E, Fujihara S, Honma I, Zhou H. Adv Mater 2005;17:2091–4. [5] Han XG, Kuang Q, Jin MS, Xie ZX, Zheng LS. J Am Chem Soc 2009;131:3152–3. [6] Golberg D, Bando Y, Huang Y, Terao T, Mitome M, Tang C, et al. ACS Nano 2010;4:2979–93. [7] Zhu JJ, Lu ZH, Aruna ST, Aurbach D, Gedanken A. Chem Mater 2000;12: 2557–66. [8] Qin LP, Xu JQ, Dong XW, Pan QY, Cheng ZX, Xiang Q, et al. Nanotechnology 2008;19:185705. [9] Li KM, Li YJ, Lu MY, Kuo CY, Chen LJ. Adv Funct Mater 2009;19:2453–6. [10] Jia TK, Wang WM, Long F, Fu ZY, Wang H, Zhang QJ. J Phys Chem C 2009;113:9071–7. [11] Liang LY, Liu ZM, Cao HT, Pan XQ. ACS Appl Mater Interfaces 2010;2:1060–5. [12] Sun G, Wu NT, Li YW, Cao JL, Qi FX, Bala Hari, et al. Mater Lett 2012;98:224–7. [13] Jia ZJ, Zhu LP, Liao GH, Yu Y, Tang YW. Solid State Commun 2004;132:79–82. [14] Banerjee Moloy, Roy Sujit. Org. Lett 2004;6:2137–40. [15] Yoshio I, Tadahiko K, Akihiro M, Yukio M, Tsutomu M. Science 1997;276:1395–7. [16] Ning JJ, Dai QQ, Jiang T, Men KK, Liu DH, Xiao NR, et al. Langmuir 2009;25:1818–21. [17] Ning JJ, Jiang T, Men KK, Dai QQ, Li DM, Wei YJ, et al. J Phys Chem C 2009;113:14140–4. [18] Dai ZR, Pan ZW, Wang ZL. J Am Chem Soc 2002;124:8673–80. [19] Shin JH, Song JY, Kim YH, Park HM. Mater Lett 2010;64:1120–2. [20] Sakaushi K, Oaki Y, Uchiyama H, Hosono E, Zhou HS, Imai H. Small 2010;6:776–81. [21] Wang HK, Wang Y, Xu, Jun, Yang, HH, et al. Langmuir 2012;28:10597–601. [22] Iqbal MZ, Wang FP, Javed QJ, Rafique MY, Qiu HM, Nabi G. Mater Lett 2012;75:236–9. [23] Mills G, Li ZG, Meisel D. J Phys Chem B 1988;92:822–8. [24] Xi HC, Ye JH. Inorg Chem 2010;49:2302–9.