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Controllable growth of SnO2 nanoparticles by citric acid assisted hydrothermal process
Colloids and Surfaces A: Physicochem. Eng. Aspects 327 (2008) 17–20
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Controllable growth of SnO2 nanoparticles by citric acid assisted hydrothermal process Zhijie Li a , Wenzhong Shen b , Xue Zhang c , Limei Fang a , Xiaotao Zu a,∗ a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610041, PR China State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying 257061, PR China c Equipment Manufactory, Well Tech-China Offshore Service Limited, 232 box, Beijing 101149, PR China b
a r t i c l e
i n f o
Article history: Received 15 June 2007 Received in revised form 27 May 2008 Accepted 28 May 2008 Available online 3 June 2008 Keywords: SnO2 Nanoparticles Citric acid Hydrothermal
a b s t r a c t SnO2 nanoparticles were prepared by citric acid assisted hydrothermal process and its microstructure and optoelectronic properties had been investigated. Based on XRD, TEM and FT-IR analyses, the SnO2 crystallites with the tetragonal rutile structure were directly formed. The crystalline grain size of SnO2 was 6.0 nm and its specific surface area reached 206 m2 /g. The room temperature photoluminescence proved that it had a blue emission, which increased remarkably after SnO2 calcined at 800 ◦ C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction SnO2 nanoparticles, as an n-type semiconductor with a wide band gap (Eg = 3.6 eV), have been attracting much attention due to wide range of applications in gas sensors [1–3], optoelectronic devices [4–5], dye-based solar cells [6], secondary lithium batteries electrode materials [7] and catalysts [8]. SnO2 has been used as the predominant sensing materials in the field of solid state gas sensors for environmental monitoring [1–3], such as CO, NO, and C2 H5 OH, etc. Furthermore, nanosized-SnO2 could enhance the sensor performance because of their characteristic microstructural and electronic properties. It is generally accepted that increasing the surface/bulk ratio and decreasing grain size for rutile SnO2 nanoparticles is crucial to achieving high-sensitivity gas sensors. One of the most common methods to modify the properties of SnO2 is introducing dopants. Many results have showed that several additives (Fe, Cu, Co, Cr, Al, Mn and Mg) could lead to an increase of the surface area of SnO2 -based powders [3,9–14]. The added active elements could stabilize the SnO2 surface, and decrease its grain size. However, a problem arising from the use of such dopants is that their migration and segregation can possibly occur during the heat treatment or the operation, leading to irreproducibility and aging of
∗ Corresponding author. Tel.: +86 28 83201939; fax: +86 28 83201939. E-mail address: [email protected] (X. Zu). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.05.043
the sensors. Moreover, for practical applications in sensor devices, SnO2 particles need to be printed on substrates and annealed at 300–600 ◦ C. The operation temperature of the sensors should also be high enough (typically 400 ◦ C) to obtain good response to gases. So, it is still of prime importance to develop a novel and simple synthetic route for pure SnO2 nanoparticles that are stable against heat. A variety of techniques have been proposed to prepare SnO2 nanoparticles including sol–gel [3,11,14], chemical vapor deposition [15], annealing precursor powders [16,17], thermal evaporation [18] and microwave heating [19]. Generally, these preparation methods are carried out at high-temperature synthetic process. In the process, the surface areas decrease and particle sizes increase due to the particle growth and sintering. The hydrothermal processing is an alternative to calcination for the crystallization of SnO2 under mild temperatures [20]. But it was still difficult to control the size of nanoparticles. Recently, it has been noted that citric acid can be used to assist the formation of nanoscaled materials. Fragi et al. [21] had prepared SnO2 nanopowders by a novel synthesis method called nitrate-citrate gel-combustion process. Bhagwat et al. [22] had prepared nanocrystalline SnO2 powder by morphous citrate route. But the key calcanation step made the surface area less than 100 m2 /g and particle size larger than 10 nm. So, here, a citric acid assisted hydrothermal process at 150 ◦ C was used to prepare SnO2 nanoparticles with a fairly narrow particle size distribution, high specific surface areas of 206 m2 /g. The resultant particles exhibited high specific surface areas even at
18
Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 327 (2008) 17–20
400 ◦ C. The room temperature photoluminescence showed remarkably increased blue luminescence after calcined at 800 ◦ C. 2. Experimental 2.1. Synthesis of samples Firstly, at 50 ◦ C, citric acid was added to 80 ml of H2 O until the pH reached 1.5 with magnetic stirring, then, 16 ml H2 O and 17.529 g SnCl4 ·5H2 O were added and dissolved in above solution. Secondly, 10 ml polyglycol were then added into the above solution and stirred for 10 min to form a sol. Thirdly, 30 ml NH3 ·H2 O (17 M) was added dropwise to the above sol within 30 min under magnetic stirring. Then the hydrolysis product was stirred for 30 min to form a gel. Finally, the gel was transferred into a Teflon-lined autoclave for hydrothermal reaction at 150 ◦ C for 12 h. After that, the hydrothermal product was filtered and dried at 120 ◦ C for 10 h. Then SnO2 nanoparticles were obtained. To compare the effect of temperature on the nanoparticle properties, the sample was calcined at 400, 600 and 800 ◦ C in air for 1 h. 2.2. Characterization of samples The crystalline phase SnO2 nanoparticles were determined by X-ray diffraction (XRD, Cu K␣, 40 kV, 60 mA, Rigaku D/max-2400). The morphologies of the samples were observed with JEOL 2010 F field emission gun electron microscopes were used with accelerating voltage at 200 keV. The specific surface area was measured by Nitrogen adsorption isotherm at 77 K (Tristar3000, Micromeritics). The chemical structure information of the particles was collected by FT-IR spectra (Nicolet 560 Spectrometer). The element composition and the chemical state of particle surface were determined by a KRATOS XSAM 800 X-ray photoelectron spectrometer (XPS) with monochromatic Al K␣ (h = 1486 eV) radiation for elemental analysis. Diffuse reflectance spectra (DRS) recorded by a Shimadzu UV-2101 apparatus, equipped with an integrating sphere, using BaSO4 as reference. The Photoluminescence (PL) spectrum was recorded by RF-5301PC. 3. Results and discussion 3.1. XRD analyses The XRD patterns of SnO2 nanoparticles are displayed in Fig. 1 (120 ◦ C). It showed that rutile phases SnO2 crystallites with tetragonal structure could be directly obtained without calcination.
Fig. 2. TEM (a) and HRTEM (b) micrographs of SnO2 nanoparticles.
Furthermore, the crystallite grain size of SnO2 could be estimated as 5.98 nm according to Scherrer formula L = K/(ˇ cos ), where is the wavelength of the X-ray radiation (Cu K␣ = 0.15406 nm), K is a constant taken as 0.89, ˇ is the line width at half maximum height, and is the diffracting angle. But for the SnO2 nanoparticles synthesized by poly(ethylene glycol) (PEG)-mediated hydrothermal method, the average diameters were up to about 50 nm [23]. It is well known that calcination would result in the growth of SnO2 crystallite. In order to investigate the effect of temperature on the crystallization process of SnO2 , XRD results of SnO2 that was calcined at 400, 600 and 800 ◦ C were compared, respectively. The crystalline grain size SnO2 was 9.73, 19.69 and 33.26 nm when it was calcined at 400, 600 and 800 ◦ C, respectively. It could be seen that the growth of crystallite size was slow before 400 ◦ C. But at 600 and 800 ◦ C, there is significant grain growth with sintering. 3.2. TEM and BET surface area analyses
Fig. 1. XRD patterns of SnO2 nanoparticles treated at different temperature.
The TEM and HRTEM micrographs of SnO2 nanoparticles are shown in Fig. 2. It illustrated that the SnO2 nanoparticles were about 6.0 nm in diameter with a fairly narrow particle size distribution and were well dispersed. The crystallinity of SnO2 nanoparticles was well developed (see Fig. 2(b)). This result was similar to that
Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 327 (2008) 17–20
19
Fig. 3. FT-IR transmission spectra of SnO2 nanoparticles.
obtained from XRD analysis. Selected-area diffraction exhibited the polycrystalline rings that were all from the rutile SnO2 structure as indexed in Fig. 2(b). These rings were in agreement with the peaks in the power XRD spectra. By the BET surface area analyses, the specific surface area of SnO2 nanoparticles were 206 m2 g−1 . It was 172 m2 g−1 after calcined at 400 ◦ C for 1 h. The calcinations process took place slight effect on the specific surface area. The reason should be that the samples prepared by citric acid assisted hydrothermal route were well dispersed and with less uniform size. So, it was an effective method to synthesize the SnO2 nanoparticles with large surface areas by the citric acid assisted hydrothermal route. Because the prepared SnO2 nanoparticles owned much larger specific surface areas than that of bulk SnO2 , a high fraction of the atoms should present at the surface of SnO2 nanoparticles. The large surface areas should be in favor of gas sensors. The SnO2 nanoparticles prepared by citric acid assisted hydrothermal process could be as high-sensitivity gas sensors material for environmental monitoring. 3.3. Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectrometer (XPS) analyses The FT-IR transmission spectra of SnO2 nanoparticle are drawn in Fig. 3. The absorption peaks at 3431 and 1640 cm−1 were attributed to the vibration of hydroxyl. The absorption peaks of 3400 cm−1 were high, suggesting that there were more O–H bonds on the SnO2 nanoparticle. Additionally, those bands at 2927, 2856 and 1400 cm−1 were assigned to C–H vibrations. The C–H was attributed to the organic traces residuals. The band at approximately 2400 cm−1 resulted from the adsorption and interaction of atmospheric carbon dioxide with water according to the literature [24]. The bands at 1055 cm−1 were assigned to the vibration of different types of surface hydroxyl groups. The Sn–O–Sn vibration appeared in the range of 400–700 cm−1 as the result of condensation reaction. The peaks at 660 and 550 cm−1 are attributed to the Sn–O–Sn antisymmetric vibrations. To obtain information relative to the chemical states of the SnO2 nanoparticles, XPS analysis was performed in Fig. 4 and Table 1. The binding energy at 486.84 and 495.37 eV proved that the chemical state in the prepared SnO2 nanoparticles was Sn4+ in Fig. 4(a) [25,26]. Two obvious peaks were observed in the O (1s) spectrum at 530.62 and 531.79 eV, the former was Sn–O bonds, and the latter was O–H bonds in Fig. 4(b). Furthermore, the ratio of the nominal surface atomic O/Sn concentration was 4:1 calculated from XPS integral proportion. This implied that there was a rather high concentration of oxygen on the surface. The oxygen content of OH was
Fig. 4. The Sn 3d (a) and O 1s (b) XPS spectra of SnO2 nanoparticles.
high as 21.7% in total oxygen calculated from XPS integral proportion. 3.4. UV–vis diffuse reflectance spectra (DRS) and photoluminescence (PL) spectra The diffuse reflectance spectra of samples treated at different temperatures are depicted in Fig. 5. Below 600 ◦ C, the spectra SnO2 nanoparticles displayed a red shift in the band gap transition with increasing of the treated temperature. It could be attributed to the well-known quantum size effect of semiconductors [27]. The results of XRD had proved that the crystalline grain sizes of SnO2 increased with treated temperature. According to size quantization, a decrease in the band gap happened with the increase in nanoparticle dimensions [28]. But for the SnO2 nanoparticles treated at 800 ◦ C, a remarkably blue shift in the band gap transition was found. The blue shift should due to the so-called Moss–Burstein effect caused by electrons generated by oxygen vacancies [29]. It meant that the lifting of the Fermi level into the conduction band of the degenerate semiconductor due to the increase in the carrier density leaded to the energy band broadening effect. In the case, the Table 1 XPS analysis of O 1s and Sn 3d of samples Peak
Binding energy (eV)
Area
FWHM
O 1s (Sn–O) O 1s (OH) Sn 3d5/2 Sn 3d3/2
530.619 531.790 486.836 495.365
1526.195 423.596 5372.303 3247.313
2.062 2.010 2.215 2.042
20
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in the SnO2 lattice. It should be noted that the position of the blue peak seems to blue shift from 466 to 464 nm at 800 ◦ C. The similar results are found in ZnO nanoparticles. A possible explanation was due to the changes in the local environments of the defect centers. 4. Conclusion
Fig. 5. The diffuse reflectance spectra of samples treated at different temperature.
SnO2 nanoparticles were prepared by citric acid assisted hydrothermal route. The rutile SnO2 crystallites with tetragonal structure could form directly during citric acid assisted hydrothermal process. It led to well-dispersed, spherical SnO2 nanoparticles with narrow size distribution and large specific surface area. The specific surface area of SnO2 nanoparticles was high as 206 m2 /g and it had well thermal stability against particle growth. The room temperature photoluminescence showed a near-UV emission and a blue emission. And the blue emission increased remarkably after calcined at 800 ◦ C. Narrow size distribution nanoparticles, excellent dispersibility and large surface areas of SnO2 nanoparticle prepared by citric acid assisted hydrothermal route make it particularly appealing in applications of gas sensors and optoelectronic devices. References
Fig. 6. The room temperature photoluminescence (PL) of SnO2 nanoparticles using an excitation wavelength of 350 nm.
Moss–Burstein effect should cause by electrons generated by the bridging and in-plane oxygen vacancies of SnO2 nanoparticles. The surface of SnO2 nanoparticles was covered with chemisorbed OH. When the SnO2 nanoparticles was heated at 800 ◦ C, their associated Sn–OH groups were decreased remarkably, and in the process, the bridging and in-plane oxygen vacancies formed. The room temperature photoluminescence of SnO2 nanoparticles at excitation wavelength of 350 nm was shown in Fig. 6. A narrow near-UV emission peak at 365 nm was observed. It should be interference from the source in the equipment used. A visible blue wideband peak was observed when the SnO2 was treated above 400 ◦ C. The blue emission was related to the singly ionized oxygen vacancy, and this emission resulted from the recombination of a photogenerated hole with a singly ionized charge state of the specific defect. The green emission increased above 400 ◦ C. When the temperature reached 800 ◦ C, the blue emission was dramatically increased. And the blue emission was eventually dominated in the PL spectrum. It suggested that there was a high concentration of defects (oxygen vacancies) in the SnO2 nanoparticles likely due to the evaporation of O. This behavior should be due to the competition between the O atoms getting into the lattice and those evaporating out of the SnO2 lattice in O2 atmosphere. The kinetic energy of the surface atoms was large, resulting in a larger escaping rate of O atoms than the adsorption rate to make more O vacancies
[1] A. Teeramongkonrasmee, M. Sriyudthsak, Sens. Actuators B 66 (2000) 256. [2] A. Forleo, L. Francioso, M. Epifani, S. Capone, A.M. Taurino, P. Siciliano, Thin Solid Films 490 (2005) 68. [3] D. Kotsikau, M. Ivanovskaya, D. Orlik, M. Falasconi, Sens. Actuators B 101 (2004) 199. [4] H. Cachet, V. Vivier, T. Toupance, J. Electroanal. Chem. 572 (2004) 249. [5] Tomokatsu Hayakawa, Masayuki Nogami, Sci. Technol. Adv. Mater. 6 (2005) 66. [6] S. Ferrere, A. Zaban, B.A. Gsegg, J. Phys. Chem. B 101 (1997) 4490. [7] V. Subramanian, K.I. Gnanasekar, B. Rambabu, Solid State Ionics 175 (2004) 181. [8] S.R. Stampfl, Y. Chen, J.A. Dumesis, Ch. Niu, C.G. Hill, J. Catal. 105 (1987) 445. [9] O.K. Tana, W. Caoa, W. Zhua, J.W. Chaib, J.S. Pan, Sens. Actuators B 93 (2003) 396. [10] Shunji Abe, U-Sung Choi, Kengo Shimanoe, Noboru Yamazoe, Sens. Actuators B 107 (2005) 516. [11] Feng Gu, Shu Fen Wang, Meng Kai Lu, Yong Xin Qi, Guang Jun Zhou, Dong Xu, Duo Rong Yuan, Inorg. Chem. Commun. 6 (2003) 882. [12] Haiying Jin, Yaohua Xu, Guangsheng Pang, Wenjun Dong, Qiang Wan, Yan Sun, Shouhua Feng, Mater. Chem. Phys. 85 (2004) 58. [13] Yude Wang, Xinghu Wu, Qun Sun, Yanfeng Li, Zhenlai Zhou, Solid State Electron. 45 (2001) 347. [14] Lincoln A. Kurihara, Sergio T. Fujiwara, Ren´ı V.S. Alfaya, Yoshitaka Gushikem, Antonio A.S. Alfaya, Sandra C. de Castro, J. Colloid Interface Sci. 274 (2004) 579. [15] Y. Liu, J. Dong, M. Liu, Adv. Mater. 16 (2004) 353. [16] C. Nayral, E. Viala, P. Fau, F. Senocq, J.C. Jumas, A. Maisonnat, B. Chaudret, Chem. Eur. J. 6 (2000) 4082. [17] Y. Liu, C. Zheng, W. Wang, C. Yin, G. Wang, Adv. Mater. 13 (2001) 1883. [18] Z.R. Dai, Z.W. Pan, Z.L. Wang, Adv. Funct. Mater. 13 (2003) 9. [19] A. Chandra Bose, P. Thangadurai, S. Ramasamy, Mater. Chem. Phys. 95 (2006) 72. [20] Shinobu Fujihara, Takahiro Maeda, Hirotoshi Ohgi, Eiji Hosono, Hiroaki Imai, Sae-Hoon Kim, Langmuir 20 (2004) 6476. [21] L. Fragi, D.G. Lamas, N.E. Walsoe de Reca, Nanostruct. Mater. 11 (1999) 311. [22] Mahesh Bhagwat, Pallavi Shah, Veda Ramaswamy, Mater. Lett. 57 (2003) 1604. [23] Enhong Shen, Chunlei Wang, Enbo Wang, Zhenkui Kang, Lei Gao, Changwen Hu, Lin Xu, Mater. Lett. 58 (2004) 3761. [24] S. Emiroglu, N. Barsan, U. Weimar, V. Hoffmann, Thin Solid Films 391 (2001) 176. [25] Changhao Liang, Yoshiki Shimizu, Takeshi Sasaki, Naoto Koshizaki, J. Phys. Chem. B 107 (2003) 9220. [26] Hyo-Jin Ahn, Hyun-Chul Choi, Kyung-Won Park, Seung-Bin Kim, Yung-Eun Sung, J. Phys. Chem. B 108 (2004) 9815. [27] M. Anpo, T. Shima, S. Kodama, Y. Kobokawa, J. Phys. Chem. 91 (1987) 4305. [28] K.M. Reddy, C.V.G. Reddy, S.V. Manorama, J. Solid State Chem. 158 (2001) 180. [29] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1988) 10244.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Controllable growth of SnO2 nanoparticles by citric acid assisted hydrothermal process Zhijie Li a , Wenzhong Shen b , Xue Zhang c , Limei Fang a , Xiaotao Zu a,∗ a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610041, PR China State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying 257061, PR China c Equipment Manufactory, Well Tech-China Offshore Service Limited, 232 box, Beijing 101149, PR China b
a r t i c l e
i n f o
Article history: Received 15 June 2007 Received in revised form 27 May 2008 Accepted 28 May 2008 Available online 3 June 2008 Keywords: SnO2 Nanoparticles Citric acid Hydrothermal
a b s t r a c t SnO2 nanoparticles were prepared by citric acid assisted hydrothermal process and its microstructure and optoelectronic properties had been investigated. Based on XRD, TEM and FT-IR analyses, the SnO2 crystallites with the tetragonal rutile structure were directly formed. The crystalline grain size of SnO2 was 6.0 nm and its specific surface area reached 206 m2 /g. The room temperature photoluminescence proved that it had a blue emission, which increased remarkably after SnO2 calcined at 800 ◦ C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction SnO2 nanoparticles, as an n-type semiconductor with a wide band gap (Eg = 3.6 eV), have been attracting much attention due to wide range of applications in gas sensors [1–3], optoelectronic devices [4–5], dye-based solar cells [6], secondary lithium batteries electrode materials [7] and catalysts [8]. SnO2 has been used as the predominant sensing materials in the field of solid state gas sensors for environmental monitoring [1–3], such as CO, NO, and C2 H5 OH, etc. Furthermore, nanosized-SnO2 could enhance the sensor performance because of their characteristic microstructural and electronic properties. It is generally accepted that increasing the surface/bulk ratio and decreasing grain size for rutile SnO2 nanoparticles is crucial to achieving high-sensitivity gas sensors. One of the most common methods to modify the properties of SnO2 is introducing dopants. Many results have showed that several additives (Fe, Cu, Co, Cr, Al, Mn and Mg) could lead to an increase of the surface area of SnO2 -based powders [3,9–14]. The added active elements could stabilize the SnO2 surface, and decrease its grain size. However, a problem arising from the use of such dopants is that their migration and segregation can possibly occur during the heat treatment or the operation, leading to irreproducibility and aging of
∗ Corresponding author. Tel.: +86 28 83201939; fax: +86 28 83201939. E-mail address: [email protected] (X. Zu). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.05.043
the sensors. Moreover, for practical applications in sensor devices, SnO2 particles need to be printed on substrates and annealed at 300–600 ◦ C. The operation temperature of the sensors should also be high enough (typically 400 ◦ C) to obtain good response to gases. So, it is still of prime importance to develop a novel and simple synthetic route for pure SnO2 nanoparticles that are stable against heat. A variety of techniques have been proposed to prepare SnO2 nanoparticles including sol–gel [3,11,14], chemical vapor deposition [15], annealing precursor powders [16,17], thermal evaporation [18] and microwave heating [19]. Generally, these preparation methods are carried out at high-temperature synthetic process. In the process, the surface areas decrease and particle sizes increase due to the particle growth and sintering. The hydrothermal processing is an alternative to calcination for the crystallization of SnO2 under mild temperatures [20]. But it was still difficult to control the size of nanoparticles. Recently, it has been noted that citric acid can be used to assist the formation of nanoscaled materials. Fragi et al. [21] had prepared SnO2 nanopowders by a novel synthesis method called nitrate-citrate gel-combustion process. Bhagwat et al. [22] had prepared nanocrystalline SnO2 powder by morphous citrate route. But the key calcanation step made the surface area less than 100 m2 /g and particle size larger than 10 nm. So, here, a citric acid assisted hydrothermal process at 150 ◦ C was used to prepare SnO2 nanoparticles with a fairly narrow particle size distribution, high specific surface areas of 206 m2 /g. The resultant particles exhibited high specific surface areas even at
18
Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 327 (2008) 17–20
400 ◦ C. The room temperature photoluminescence showed remarkably increased blue luminescence after calcined at 800 ◦ C. 2. Experimental 2.1. Synthesis of samples Firstly, at 50 ◦ C, citric acid was added to 80 ml of H2 O until the pH reached 1.5 with magnetic stirring, then, 16 ml H2 O and 17.529 g SnCl4 ·5H2 O were added and dissolved in above solution. Secondly, 10 ml polyglycol were then added into the above solution and stirred for 10 min to form a sol. Thirdly, 30 ml NH3 ·H2 O (17 M) was added dropwise to the above sol within 30 min under magnetic stirring. Then the hydrolysis product was stirred for 30 min to form a gel. Finally, the gel was transferred into a Teflon-lined autoclave for hydrothermal reaction at 150 ◦ C for 12 h. After that, the hydrothermal product was filtered and dried at 120 ◦ C for 10 h. Then SnO2 nanoparticles were obtained. To compare the effect of temperature on the nanoparticle properties, the sample was calcined at 400, 600 and 800 ◦ C in air for 1 h. 2.2. Characterization of samples The crystalline phase SnO2 nanoparticles were determined by X-ray diffraction (XRD, Cu K␣, 40 kV, 60 mA, Rigaku D/max-2400). The morphologies of the samples were observed with JEOL 2010 F field emission gun electron microscopes were used with accelerating voltage at 200 keV. The specific surface area was measured by Nitrogen adsorption isotherm at 77 K (Tristar3000, Micromeritics). The chemical structure information of the particles was collected by FT-IR spectra (Nicolet 560 Spectrometer). The element composition and the chemical state of particle surface were determined by a KRATOS XSAM 800 X-ray photoelectron spectrometer (XPS) with monochromatic Al K␣ (h = 1486 eV) radiation for elemental analysis. Diffuse reflectance spectra (DRS) recorded by a Shimadzu UV-2101 apparatus, equipped with an integrating sphere, using BaSO4 as reference. The Photoluminescence (PL) spectrum was recorded by RF-5301PC. 3. Results and discussion 3.1. XRD analyses The XRD patterns of SnO2 nanoparticles are displayed in Fig. 1 (120 ◦ C). It showed that rutile phases SnO2 crystallites with tetragonal structure could be directly obtained without calcination.
Fig. 2. TEM (a) and HRTEM (b) micrographs of SnO2 nanoparticles.
Furthermore, the crystallite grain size of SnO2 could be estimated as 5.98 nm according to Scherrer formula L = K/(ˇ cos ), where is the wavelength of the X-ray radiation (Cu K␣ = 0.15406 nm), K is a constant taken as 0.89, ˇ is the line width at half maximum height, and is the diffracting angle. But for the SnO2 nanoparticles synthesized by poly(ethylene glycol) (PEG)-mediated hydrothermal method, the average diameters were up to about 50 nm [23]. It is well known that calcination would result in the growth of SnO2 crystallite. In order to investigate the effect of temperature on the crystallization process of SnO2 , XRD results of SnO2 that was calcined at 400, 600 and 800 ◦ C were compared, respectively. The crystalline grain size SnO2 was 9.73, 19.69 and 33.26 nm when it was calcined at 400, 600 and 800 ◦ C, respectively. It could be seen that the growth of crystallite size was slow before 400 ◦ C. But at 600 and 800 ◦ C, there is significant grain growth with sintering. 3.2. TEM and BET surface area analyses
Fig. 1. XRD patterns of SnO2 nanoparticles treated at different temperature.
The TEM and HRTEM micrographs of SnO2 nanoparticles are shown in Fig. 2. It illustrated that the SnO2 nanoparticles were about 6.0 nm in diameter with a fairly narrow particle size distribution and were well dispersed. The crystallinity of SnO2 nanoparticles was well developed (see Fig. 2(b)). This result was similar to that
Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 327 (2008) 17–20
19
Fig. 3. FT-IR transmission spectra of SnO2 nanoparticles.
obtained from XRD analysis. Selected-area diffraction exhibited the polycrystalline rings that were all from the rutile SnO2 structure as indexed in Fig. 2(b). These rings were in agreement with the peaks in the power XRD spectra. By the BET surface area analyses, the specific surface area of SnO2 nanoparticles were 206 m2 g−1 . It was 172 m2 g−1 after calcined at 400 ◦ C for 1 h. The calcinations process took place slight effect on the specific surface area. The reason should be that the samples prepared by citric acid assisted hydrothermal route were well dispersed and with less uniform size. So, it was an effective method to synthesize the SnO2 nanoparticles with large surface areas by the citric acid assisted hydrothermal route. Because the prepared SnO2 nanoparticles owned much larger specific surface areas than that of bulk SnO2 , a high fraction of the atoms should present at the surface of SnO2 nanoparticles. The large surface areas should be in favor of gas sensors. The SnO2 nanoparticles prepared by citric acid assisted hydrothermal process could be as high-sensitivity gas sensors material for environmental monitoring. 3.3. Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectrometer (XPS) analyses The FT-IR transmission spectra of SnO2 nanoparticle are drawn in Fig. 3. The absorption peaks at 3431 and 1640 cm−1 were attributed to the vibration of hydroxyl. The absorption peaks of 3400 cm−1 were high, suggesting that there were more O–H bonds on the SnO2 nanoparticle. Additionally, those bands at 2927, 2856 and 1400 cm−1 were assigned to C–H vibrations. The C–H was attributed to the organic traces residuals. The band at approximately 2400 cm−1 resulted from the adsorption and interaction of atmospheric carbon dioxide with water according to the literature [24]. The bands at 1055 cm−1 were assigned to the vibration of different types of surface hydroxyl groups. The Sn–O–Sn vibration appeared in the range of 400–700 cm−1 as the result of condensation reaction. The peaks at 660 and 550 cm−1 are attributed to the Sn–O–Sn antisymmetric vibrations. To obtain information relative to the chemical states of the SnO2 nanoparticles, XPS analysis was performed in Fig. 4 and Table 1. The binding energy at 486.84 and 495.37 eV proved that the chemical state in the prepared SnO2 nanoparticles was Sn4+ in Fig. 4(a) [25,26]. Two obvious peaks were observed in the O (1s) spectrum at 530.62 and 531.79 eV, the former was Sn–O bonds, and the latter was O–H bonds in Fig. 4(b). Furthermore, the ratio of the nominal surface atomic O/Sn concentration was 4:1 calculated from XPS integral proportion. This implied that there was a rather high concentration of oxygen on the surface. The oxygen content of OH was
Fig. 4. The Sn 3d (a) and O 1s (b) XPS spectra of SnO2 nanoparticles.
high as 21.7% in total oxygen calculated from XPS integral proportion. 3.4. UV–vis diffuse reflectance spectra (DRS) and photoluminescence (PL) spectra The diffuse reflectance spectra of samples treated at different temperatures are depicted in Fig. 5. Below 600 ◦ C, the spectra SnO2 nanoparticles displayed a red shift in the band gap transition with increasing of the treated temperature. It could be attributed to the well-known quantum size effect of semiconductors [27]. The results of XRD had proved that the crystalline grain sizes of SnO2 increased with treated temperature. According to size quantization, a decrease in the band gap happened with the increase in nanoparticle dimensions [28]. But for the SnO2 nanoparticles treated at 800 ◦ C, a remarkably blue shift in the band gap transition was found. The blue shift should due to the so-called Moss–Burstein effect caused by electrons generated by oxygen vacancies [29]. It meant that the lifting of the Fermi level into the conduction band of the degenerate semiconductor due to the increase in the carrier density leaded to the energy band broadening effect. In the case, the Table 1 XPS analysis of O 1s and Sn 3d of samples Peak
Binding energy (eV)
Area
FWHM
O 1s (Sn–O) O 1s (OH) Sn 3d5/2 Sn 3d3/2
530.619 531.790 486.836 495.365
1526.195 423.596 5372.303 3247.313
2.062 2.010 2.215 2.042
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in the SnO2 lattice. It should be noted that the position of the blue peak seems to blue shift from 466 to 464 nm at 800 ◦ C. The similar results are found in ZnO nanoparticles. A possible explanation was due to the changes in the local environments of the defect centers. 4. Conclusion
Fig. 5. The diffuse reflectance spectra of samples treated at different temperature.
SnO2 nanoparticles were prepared by citric acid assisted hydrothermal route. The rutile SnO2 crystallites with tetragonal structure could form directly during citric acid assisted hydrothermal process. It led to well-dispersed, spherical SnO2 nanoparticles with narrow size distribution and large specific surface area. The specific surface area of SnO2 nanoparticles was high as 206 m2 /g and it had well thermal stability against particle growth. The room temperature photoluminescence showed a near-UV emission and a blue emission. And the blue emission increased remarkably after calcined at 800 ◦ C. Narrow size distribution nanoparticles, excellent dispersibility and large surface areas of SnO2 nanoparticle prepared by citric acid assisted hydrothermal route make it particularly appealing in applications of gas sensors and optoelectronic devices. References
Fig. 6. The room temperature photoluminescence (PL) of SnO2 nanoparticles using an excitation wavelength of 350 nm.
Moss–Burstein effect should cause by electrons generated by the bridging and in-plane oxygen vacancies of SnO2 nanoparticles. The surface of SnO2 nanoparticles was covered with chemisorbed OH. When the SnO2 nanoparticles was heated at 800 ◦ C, their associated Sn–OH groups were decreased remarkably, and in the process, the bridging and in-plane oxygen vacancies formed. The room temperature photoluminescence of SnO2 nanoparticles at excitation wavelength of 350 nm was shown in Fig. 6. A narrow near-UV emission peak at 365 nm was observed. It should be interference from the source in the equipment used. A visible blue wideband peak was observed when the SnO2 was treated above 400 ◦ C. The blue emission was related to the singly ionized oxygen vacancy, and this emission resulted from the recombination of a photogenerated hole with a singly ionized charge state of the specific defect. The green emission increased above 400 ◦ C. When the temperature reached 800 ◦ C, the blue emission was dramatically increased. And the blue emission was eventually dominated in the PL spectrum. It suggested that there was a high concentration of defects (oxygen vacancies) in the SnO2 nanoparticles likely due to the evaporation of O. This behavior should be due to the competition between the O atoms getting into the lattice and those evaporating out of the SnO2 lattice in O2 atmosphere. The kinetic energy of the surface atoms was large, resulting in a larger escaping rate of O atoms than the adsorption rate to make more O vacancies
[1] A. Teeramongkonrasmee, M. Sriyudthsak, Sens. Actuators B 66 (2000) 256. [2] A. Forleo, L. Francioso, M. Epifani, S. Capone, A.M. Taurino, P. Siciliano, Thin Solid Films 490 (2005) 68. [3] D. Kotsikau, M. Ivanovskaya, D. Orlik, M. Falasconi, Sens. Actuators B 101 (2004) 199. [4] H. Cachet, V. Vivier, T. Toupance, J. Electroanal. Chem. 572 (2004) 249. [5] Tomokatsu Hayakawa, Masayuki Nogami, Sci. Technol. Adv. Mater. 6 (2005) 66. [6] S. Ferrere, A. Zaban, B.A. Gsegg, J. Phys. Chem. B 101 (1997) 4490. [7] V. Subramanian, K.I. Gnanasekar, B. Rambabu, Solid State Ionics 175 (2004) 181. [8] S.R. Stampfl, Y. Chen, J.A. Dumesis, Ch. Niu, C.G. Hill, J. Catal. 105 (1987) 445. [9] O.K. Tana, W. Caoa, W. Zhua, J.W. Chaib, J.S. Pan, Sens. Actuators B 93 (2003) 396. [10] Shunji Abe, U-Sung Choi, Kengo Shimanoe, Noboru Yamazoe, Sens. Actuators B 107 (2005) 516. [11] Feng Gu, Shu Fen Wang, Meng Kai Lu, Yong Xin Qi, Guang Jun Zhou, Dong Xu, Duo Rong Yuan, Inorg. Chem. Commun. 6 (2003) 882. [12] Haiying Jin, Yaohua Xu, Guangsheng Pang, Wenjun Dong, Qiang Wan, Yan Sun, Shouhua Feng, Mater. Chem. Phys. 85 (2004) 58. [13] Yude Wang, Xinghu Wu, Qun Sun, Yanfeng Li, Zhenlai Zhou, Solid State Electron. 45 (2001) 347. [14] Lincoln A. Kurihara, Sergio T. Fujiwara, Ren´ı V.S. Alfaya, Yoshitaka Gushikem, Antonio A.S. Alfaya, Sandra C. de Castro, J. Colloid Interface Sci. 274 (2004) 579. [15] Y. Liu, J. Dong, M. Liu, Adv. Mater. 16 (2004) 353. [16] C. Nayral, E. Viala, P. Fau, F. Senocq, J.C. Jumas, A. Maisonnat, B. Chaudret, Chem. Eur. J. 6 (2000) 4082. [17] Y. Liu, C. Zheng, W. Wang, C. Yin, G. Wang, Adv. Mater. 13 (2001) 1883. [18] Z.R. Dai, Z.W. Pan, Z.L. Wang, Adv. Funct. Mater. 13 (2003) 9. [19] A. Chandra Bose, P. Thangadurai, S. Ramasamy, Mater. Chem. Phys. 95 (2006) 72. [20] Shinobu Fujihara, Takahiro Maeda, Hirotoshi Ohgi, Eiji Hosono, Hiroaki Imai, Sae-Hoon Kim, Langmuir 20 (2004) 6476. [21] L. Fragi, D.G. Lamas, N.E. Walsoe de Reca, Nanostruct. Mater. 11 (1999) 311. [22] Mahesh Bhagwat, Pallavi Shah, Veda Ramaswamy, Mater. Lett. 57 (2003) 1604. [23] Enhong Shen, Chunlei Wang, Enbo Wang, Zhenkui Kang, Lei Gao, Changwen Hu, Lin Xu, Mater. Lett. 58 (2004) 3761. [24] S. Emiroglu, N. Barsan, U. Weimar, V. Hoffmann, Thin Solid Films 391 (2001) 176. [25] Changhao Liang, Yoshiki Shimizu, Takeshi Sasaki, Naoto Koshizaki, J. Phys. Chem. B 107 (2003) 9220. [26] Hyo-Jin Ahn, Hyun-Chul Choi, Kyung-Won Park, Seung-Bin Kim, Yung-Eun Sung, J. Phys. Chem. B 108 (2004) 9815. [27] M. Anpo, T. Shima, S. Kodama, Y. Kobokawa, J. Phys. Chem. 91 (1987) 4305. [28] K.M. Reddy, C.V.G. Reddy, S.V. Manorama, J. Solid State Chem. 158 (2001) 180. [29] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1988) 10244.