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Synthesis and characterization of mercaptoacetic acid-modified ZnO nanoparticles
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Solid State Sciences 10 (2008) 1563e1567 www.elsevier.com/locate/ssscie
Synthesis and characterization of mercaptoacetic acid-modified ZnO nanoparticles Rui Song*, Ying Liu, Linghao He Henan Provincial Key Laboratory of Surface and Interface Science, Zhengzhou University of Light Industry, A19 Yuquan Road, Henan Zhengzhou 450002, China Received 9 November 2007; received in revised form 4 February 2008; accepted 8 February 2008 Available online 15 February 2008
Abstract Mercaptoacetic acid (MAA)-modified ZnO nanoparticles have been prepared at low temperature by homogeneous precipitation method, using zinc nitrate hexahydrate and hexamethylenetetramine as initial agents. The modified zinc oxide nanorods were characterized using Xray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric (TG) analysis and photoluminescence (PL) spectroscopy. The XRD revealed the wurtzite structure of zinc oxide. The growth of ZnO crystals into rod shape was found to be closely related to its hexagonal nature. As indicated from SEM, the morphology of the modified ZnO nanorods changes with various MAA addition times. In addition, the results of TG and FT-IR confirmed the conjugation of MAA with ZnO nanorods, and the amount of carboxyl group in the samples’ surface was found to be 0.1943e0.3491 mmol/g through the titration experiment. The PL spectra indicated that the optical properties of ZnO nanorods were changed with the insertion of MAA, and showed a significant improvement in intensity. On the basis of these results, one might expect that the conjugate specific biomolecules on the functional ZnO nanorods are very potential to detect the complementary biomolecules by PL detecting. Ó 2008 Published by Elsevier Masson SAS. Keywords: ZnO nanoparticles; Surface modification; Characterization
1. Introduction In recent years, semiconductor nanocrystals show unique properties and are of great interest for applications in optoelectronic and photovoltaic devices [1,2] and biological labels [3,4]. The achievement of desired particle sizes over the largest possible range, narrow size distribution, good crystallinity, high luminescence and desired surface properties are the parameters that are considered to be the characteristics of a ‘‘high quality’’ of the chemically prepared semiconductor nanocrystals. Up to now, ‘‘surface modification’’ has been widely used on the nanocrystals for two reasons [5,6]. Firstly, surface coverage leads to electronic passivation, which enhances
* Corresponding author. Tel.: þ86 10 88256337; fax: þ86 10 88256092. E-mail address: [email protected] (R. Song). 1293-2558/$ - see front matter Ó 2008 Published by Elsevier Masson SAS. doi:10.1016/j.solidstatesciences.2008.02.006
room temperature photoluminescence and these nanocrystals can be used as fluorescent probes for the study of biological materials [7e9]. Secondly, via high quality surface modification one can synthesize monodispersed nanocrystals and incorporate some functional groups on the surface of nanocrystals [10]. As one major aim, the question of how to increase the PL efficiency has attracted a lot of efforts. Normally surface-modified semiconductor nanocrystals with high PL efficiency can be obtained by two different ways. The first is to replace the surface-capping molecules on particles prepared by trioctylphosphine oxide (TOPO) method with water-soluble thiols or silica shell [11,12]. However, TOPO is extremely toxic, expensive and the synthesis is relatively complicated. The second is to directly synthesize semiconductor nanoparticles in aqueous solutions using water-soluble thiols as stabilizing agent [13]. So some studies have reported the synthesis of semiconductor nanocrystals in aqueous solution using thiols
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as modifying agent directly, such as thioglycolic acid [14], mercaptopropionic acid [15,16], mercaptoethanol [17], etc. All the results indicated that water-soluble thiols play an important role and contribute greatly to the stability and the functionality of the resulting nanocrystals [18,19]. In addition, the choices of water-soluble thiols also critically determine the PL efficiency due to the difference in the resulting particle surface structures. Among various kinds of semiconductor nanocrystals, CdSe [20,21], CdTe [22] and ZnS [23,24] nanocrystals are widely investigated, but the researches on ZnO nanocrystals are few [25,26]. In fact, as one of the multifunctional inorganic nanoparticles, nano-sized ZnO has been attractive due to its many significant physical and chemical properties, such as chemical stability, high luminous transmittance, effective antibacterial and bactericidal function. In this paper, we use mercaptoacetic acid (MAA) as modifier to prepare ZnO nanoparticles with functional group (carboxyl group) in aqueous solution and their optical and thermal properties are systematically characterized. 2. Experimental All of the reagents used in these experiments including zinc nitrate hexahydrate (Zn(NO3)2$6H2O), hexamethylenetetramine ((CH2)6N4), mercaptoacetic acid (SHCH2COOH) and ethanol (CH3CH2OH) are of analytical grade. To prepare the ZnO nanorods, Zn(NO3)2$6H2O and (CH2)6N4 (HMT) were used as the starting materials. First, Zn(NO3)2$6H2O (0.595 g, 0.01 mol) was dissolved in 100 ml of distilled water, then the equimolar aqueous solution of HMT was slowly added into Zn(NO3)2 solution and reacted at 75 C for 1, 3, and 5 h (herein with marked samples b, c and d, respectively). Subsequently, mercaptoacetic acid (MAA) was added into the above three aqueous solutions, respectively and continued to react at 75 C until 10 h. For comparison, ZnO without MAA-modified sample (labeled as a) was prepared at the same temperature for 10 h. Later, the white powders were collected after the mixture was centrifuged, washed thoroughly with absolute alcohol and distilled water several times. The resultants were dried in vacuum at 80 C for 24 h prior to being used for further characterization. The details on the preparation were given in Ref. [27]:
1:40. Thermogravimetric (TG) analyses of the samples were carried out with Netzsch STA209PC in a flowing nitrogen atmosphere through the temperature range from room temperature to 500 C with heating rate of 10 C/min. The morphological features of the product were imaged by scanning electron microscope (SEM, JEOL JSM 5600F, Japan). The room temperature photoluminescence (PL) spectra were performed on Hitachi F-7000 spectrometer with the HeeCd laser. During the measurement the samples were dispersed in ethanol and the excitation wavelength used in PL measurement was 325 nm. The carboxyl group content of the samples (0.05 g of modified samples in 12 ml of methanol) was determined by titration with 0.0512 mol/L standard aqueous sodium hydroxide, using phenolphthalein as an indicator. The content of carboxyl group (ACOOH) is defined by the following equation [28]: ACOOH ¼
ðV V0 Þ C m
where V and V0 represent NaOH consumption volumes of the samples and the blank sample, respectively. C is the concentration of NaOH solution (0.0512 mol/L), m is the mass of the samples. 3. Results and discussion Fig. 1 shows XRD patterns of as-synthesized samples, which indicates that ZnO phase is of wurtzite structure. The diffraction peaks can be indexed to a hexagonal structure with cell constants of a ¼ 0.324 nm and c ¼ 0.519 nm (space group P63mc; JCPDS card no. 36-1451). Compared with the standard diffraction patterns, no characteristic peaks from impurities, such as Zn, are detected, indicating the high purity of the product. In addition, the sharp and strong peaks also confirm that the products are well crystallized. It can be observed that only slight changes of intensity occur in the patterns of XRD with the insertion of MAA, and the samples keep the characteristic peaks of ZnO intact (curve a in
ðCH2 Þ6 N4 þ 10H2 O . 6HCHO þ 4NH3 $H2 O NH3 þ H2 O . OH þ NHþ 4 OH þ Zn2þ . ZnOðsÞ þ H2 O X-ray powder diffraction (XRD) analysis was conducted on a Bruker D8 X-ray diffractometer with Ni-filtered Cu Ka radiation (l ¼ 0.15406 nm). The accelerating voltage was set at 40 kV, with 30 mA flux at a scanning rate of 0.5 /min from 20 to 70 (2q) angles. FT-IR spectra were recorded on a Bruker TENSOR 27 FTIR spectrometer in transmission mode with 32 scans at a resolution of 4 cm1. The samples were mixed with potassium bromide (KBr) in the ratio of
Fig. 1. XRD spectra of (a) ZnO, (b) modified ZnO.
R. Song et al. / Solid State Sciences 10 (2008) 1563e1567
Fig. 1). We also investigated the samples of different MAA adding times and found that their XRD patterns are very well consistent with the wurtzite ZnO. One typical XRD curve for the MAA-modified sample is given in curve b in Fig. 1. Thus we can conclude that the addition of MAA as modifier does not significantly affect the perfection of the ZnO crystals. FT-IR spectra of MAA (curve a), ZnO (curve b), modified ZnO (curve c). For MAA, the modifier (Fig. 2), the characteristic IR peaks appear at 3094, 2568, 1715 and 1293 cm1. There is a strong absorption peak at 3094 cm1, which is arising from the hydroxyl bond. The absorption peak at 2568 cm1 is the stretching vibration of SeH bond. The absorption peaks at 1715 and 1293 cm1 should be attributed to the stretching vibration of C]O and CeO bonds, respectively. Comparatively, IR spectrum of pure ZnO has only two strong characteristic peaks, one is the hydroxyl bond at 3398 cm1 and the other is the ZneO bond at 447 cm1. Comparing with the bulk ZnO, the peak at 447 cm1 presents a red shift considering the fact that the FT-IR spectrum of ZnO particles is generally influenced by particle size and morphology [29,30]. Compared with the pure ZnO, the spectra of modified ZnO (curve c in Fig. 2) display other characteristic peaks. The absorption peak at 1578 cm1 is ascribed to the stretching vibration of carbonyl group, which may be presented in the form of COO because of the constant pH value of the whole reacting system (w7). Both CeOH stretching (1387 cm1) and CeO stretching (1229 cm1) are also detected from the FT-IR spectrum. In addition, no SeH bond stretching appears at 2568 cm1, which implies that the mercapto group (eSH) has bounded to Zn2þ to form a complex and the polar carboxylic acid group retains the surface of ZnO nanoparticles. This result indicates that ZnO nanoparticles are modified by MAA successfully. Following TG measurement will be used further to verify this aspect. Thermogravimetric analyses are carried out to determine whether MAA exists on the ZnO nanoparticles (Fig. 3). Weight loss of 1.5 wt.% from room temperature to w150 C is mainly due to the removal of physically absorbed water.
Fig. 2. FT-IR spectra of (a) mercaptoacetic acid, (b) ZnO and (c) modified ZnO.
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Fig. 3. TGeDTG curves of modified ZnO nanoparticles.
Weight loss of 2 wt.% from w150 to w350 C is due to the removal of chemically adsorbed water and the decomposition of the hydroxide group on the surface of the nanoparticles [31]. Weight loss of 1.5 wt.% from w350 to w460 C is very likely due to the decomposition of the MAA and other impurities. Therefore, a total weight loss of w5 wt.% occurred when the sample is heated from room temperature to w460 C. Therefore, the results of TGeDTG indicate the presence of MAA on the sample surface, which is in accordance with the above result of FT-IR. Further morphology characterization of ZnO and modified ZnO with different MAA addition times are performed using SEM and the results are shown in Fig. 4. As indicated, the morphology of the modified ZnO nanorods changes profoundly with MAA addition time. When MAA was added at 1 h, the morphology of the samples seems to be disordered. Most samples present helical structures while only a small part is rod-shaped (Fig. 4b). When MAA was added at 3 h, the disordered ZnO nanorods were observed (Fig. 4c). However, the slenderness ratio was small, so the morphology seems nubby-shaped. When MAA was added at 5 h, the morphology as shown in Fig. 4d was similar to that of ZnO nanorods as indicated in Fig. 4a. Obviously, the structure change is closely related to the addition of MAA, which promotes the dissolution and re-growth of ZnO during the growth process at different MAA adding times [7]. As one interesting phenomenon, we found that when MAA was added at 5 h, two kinds of morphology appeared in the SEM images (Fig. 4d). One is rodshaped, as mentioned above; another is the column structure that formed by arrays of ZnO nanoplates, but the yield is very low. Similar structure has been reported previously [32e36]. In Ref. [33], the authors refer to the ‘‘dissolutione reprecipitation’’ mechanism to explain the change of morphology. They suggest that along with the increase in reaction time (about 2e3 d), the hydrolysis of HMT, i.e. (CH2)6N4, proceeds slowly to form ammonia, and then forms tetrahedral structure, eventually the morphology of ZnO is changed. However, similar morphology has been found in the current case, although the whole reaction process proceeds for 10 h. So we speculate that MAA would play an important role during the formation
R. Song et al. / Solid State Sciences 10 (2008) 1563e1567
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Fig. 4. SEM images of (a) ZnO, (b) modified ZnO (addition time: 1 h), (c) modified ZnO (addition time: 3 h), (d) modified ZnO (addition time: 5 h).
of ZnO. The interaction mechanism between ZnO and MAA is complicated and further works on this line are currently underway to make further progress in this respect. Titration experiment was used to determine the content of carboxyl group (ACOOH) on the sample surfaces and the results are list in Table 1. It can be found that the value of ACOOH increased with the increment of MAA addition time. The reason is that when MAA was added at 1 h, the ZnO nanorods are not formed and therefore a large number of Zn2þ exists in aqueous medium. Zn2þ can react with MAA, so the carboxyl group was consumed and only 0.1943 mmol/g carboxyl groups can be detected on the surface of ZnO. However, when MAA was added at 5 h, perfect nanorods exist in the system. At this point, MAA would couple to Zn2þ, which is on the surface of ZnO nanorods. And so the carboxyl group is left. In this case, the content of carboxyl groups increased from 0.1943 to 0.3491 mmol/g. Fluorescence semiconductor nanocrystals and fluorescent organic nanoparticles could dramatically improve the use of
fluorescent marks in biological imaging [3,4]. The major drawbacks encountered in the use of semiconductor nanocrystals as markers are the stability of nanoparticles and their solubility in water. Hence the preparation of MAA-modified ZnO nanorods will be of immense use in biological imaging. The photoluminescence spectrum of ZnO (curve a) and modified ZnO (curve b) was investigated using a PL spectrum and the results are shown in Fig. 5. Nano-ZnO displays an intensive
Table 1 Carboxyl group content of the samples Samples
m (g)
V
V0
C (mol/L)
ACOOH (mmol/g)
b (Addition time: 1 h) c (Addition time: 3 h) d (Addition time: 5 h)
0.0527 0.0475 0.0440
0.4 0.4 0.5
0.2 0.2 0.2
0.0512 0.0512 0.0512
0.1943 0.2156 0.3491
Fig. 5. Photoluminescence spectra of (a) ZnO, (b) modified ZnO.
R. Song et al. / Solid State Sciences 10 (2008) 1563e1567
ultraviolet emission peak at w380 nm. It is well known that the UV emission peak usually originates from a near-bandedge (NBE) transition of the wide band gap [37,38]. And the impurities and structural defects, such as oxygen vacancies, may induce the visible emission [39]. The PL spectrum of MAA-modified ZnO samples (curve b) reveals two emitting bands. One is a strong ultraviolet emission at around 392 nm, which has a slight red shift compared with the PL spectrum of ZnO. It might be attributed to the defect passivation on the surface region of ZnO nanorods due to the conjugation of MAA [40]. Another is a weak blue emission at 440e470 nm, which may be in correlation with ionized oxygen defects or others on the surface of the nanoparticles. 4. Conclusion In summary, a simple method has been presented using MAA as the modifier agent to prepare the chemically modified ZnO nanorods with high dispersion. This method employs an aqueous solution, and the reaction process does not require high temperature or complicated procedures. The properties of ZnO nanoparticles are studied by SEM, XRD, FT-IR, TG and PL measurements. The ZnO nanoparticles obtained are rod-shaped by SEM observation. FT-IR and TG results confirm that MAA can modify the nanoparticles successfully and the carboxylic groups do exist on the surface of ZnO nanoparticles. All these findings demonstrate a novel protocol for the preparation of the modified ZnO nanoparticles with functional groups, which foresee a potential application in the preparation of composites and the detection of biomolecules. Acknowledgment This work was partially subsidized by Henan Innovation Project for University Prominent Research Talents (‘‘HAIPURT’’) program. References [1] D.L. Klein, R. Roth, A.K.L. Lim, A.P. Alivisatos, P.L. McEuen, Nature 389 (1997) 699. [2] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [3] W.C.W. Chan, S. Nie, Science 281 (1998) 2016. [4] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [5] X.G. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, J. Am. Chem. Soc. 119 (1997) 7019.
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[6] H.M. Chen, X.F. Huang, L. Xu, K.J. Chen, D. Feng, Superlattices Microstruct. 27 (2000) 1. [7] T.Y. Liu, H.C. Liao, C.C. Lin, S.H. Hu, S.Y. Chen, Langmuir 22 (2006) 5804. [8] L. Wang, L.Y. Wang, C.Q. Zhu, X.W. Wei, X.W. Kan, Anal. Chim. Acta 468 (2002) 35. [9] X.F. Hua, T.C. Liu, Y.C. Cao, B. Liu, H.Q. Wang, J.H. Wang, Z.L. Huang, Y.D. Zhao, Anal Bioanal. Chem. 386 (2006) 1665. [10] H. Zhang, Z. Zhou, B. Yang, M.Y. Gao, J. Phys. Chem. B 107 (2003) 8. [11] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 123 (2001) 183. [12] M.Q. Chu, Y. Sun, X.Y. Shen, G.J. Liu, Physica E 35 (2006) 75. [13] N. Gaponik, D.V. Talapin, A.L. Rogach, K. Hoppe, E.V. Shevchenko, A. Kornowski, A. Eychmuller, H. Weller, J. Phys. Chem. B 106 (2002) 7177. [14] A.G. Young, D.P. Green, A.J. McQuillan, Langmuir 22 (2006) 11106. [15] D.J. Zhou, J.D. Piper, C. Abell, D. Klenerman, D.J. Kang, L.M. Ying, Chem. Commun. 38 (2005) 4807. [16] P. Bergese, E. Bontempi, C. Dragonetti, D. Roberto, R. Ugo, I. Colombo, L.E. Depero, J. Phys. Chem. B 109 (2005) 711. [17] S.L. Shishido, T.M. Watanabe, H. Tada, H. Higuchi, N. Ohuchi, Biochem. Biophys. Res. Commun. 351 (2006) 7. [18] E. Hao, H. Zhang, B. Yang, H. Ren, J.C. Shen, J. Colloid Interface Sci. 238 (2001) 285. [19] D. Lawless, S. Kapoor, D. Meisel, J. Phys. Chem. 99 (1995) 10329. [20] J.A. Kloepfer, S.E. Bradforth, J.L. Nadeau, J. Phys. Chem. B 109 (2005) 9996. [21] Y.H. Gao, C.J. Liang, A.W. Tang, F. Teng, D. Li, Z.B. Deng, S.H. Huang, J. Lumin. 122 (2007) 646. [22] S.P. Wang, N. Mamedova, N.A. Kotov, W. Chen, J. Studer, Nano Lett. 2 (2002) 817. [23] S. Wageh, S.L. Zhao, X.R. Xu, J. Cryst. Growth 255 (2003) 332. [24] S. Wageh, S.M. Liu, T.Y. Fang, X.R. Xu, J. Lumin. 102 (2003) 768. [25] J. Dvorak, T. Jirsak, J.A. Rodriguez, Surf. Sci. 479 (2001) 155. [26] D. Palms, C. Priest, R. Sedev, J. Ralston, G. Wegner, J. Colloid Interface Sci. 303 (2006) 333. [27] Q.C. Li, V. Kumar, Y. Li, H.T. Zhang, T.J. Marks, R.P.H. Chang, Chem. Mater. 17 (2005) 1001. [28] W.D. He, The Experiments of Polymer Chemistry, University of Science and Technology of China Press, Hefei, 2001, p. 26. [29] M. Ristic, S. Music, M. Ivanda, S. Popovic, J. Alloys Compd. 397 (2005) L1. [30] S. Music, D. Dragcevic, S. Popovic, J. Alloys Compd. 429 (2007) 242. [31] M.G. Li, H. Bala, X.T. Lv, X.K. Ma, F. Sun, L.Q. Tang, Z.C. Wang, Mater. Lett. 61 (2007) 690. [32] Z.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, M.J. Mcdermott, J. Am. Chem. Soc. 124 (2002) 12954. [33] Y. Shu, T. Sato, J. Mater. Chem. 15 (2005) 4584. [34] A. Taubert, G. Glasser, D. Palms, Langmuir 18 (2002) 4488. [35] A. Taubert, D. Palms, O. Weiss, M.T. Piccini, D.N. Batchelder, Chem. Mater. 14 (2002) 2594. [36] A. Taubert, C. Kubel, D.C. Martin, J. Phys. Chem. B 107 (2003) 2660. [37] J.M. Cao, J. Wang, B.Q. Fang, X. Chang, M.B. Zheng, H.Y. Wang, Chem. Lett. 33 (2004) 1332. [38] S.J. Chen, Y.C. Liu, C.L. Shao, R. Mu, Y.M. Lu, J.Y. Zhang, D.Z. Shen, X.W. Fan, Adv. Mater. 17 (2005) 586. [39] Z.Q. Li, Y.J. Xiong, Y. Xie, Inorg. Chem. 42 (2003) 8105. [40] M. Schvartzman, V. Sidorov, D. Ritter, Y. Paz, Semicond. Sci. Technol. 16 (2001) L68.
Solid State Sciences 10 (2008) 1563e1567 www.elsevier.com/locate/ssscie
Synthesis and characterization of mercaptoacetic acid-modified ZnO nanoparticles Rui Song*, Ying Liu, Linghao He Henan Provincial Key Laboratory of Surface and Interface Science, Zhengzhou University of Light Industry, A19 Yuquan Road, Henan Zhengzhou 450002, China Received 9 November 2007; received in revised form 4 February 2008; accepted 8 February 2008 Available online 15 February 2008
Abstract Mercaptoacetic acid (MAA)-modified ZnO nanoparticles have been prepared at low temperature by homogeneous precipitation method, using zinc nitrate hexahydrate and hexamethylenetetramine as initial agents. The modified zinc oxide nanorods were characterized using Xray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric (TG) analysis and photoluminescence (PL) spectroscopy. The XRD revealed the wurtzite structure of zinc oxide. The growth of ZnO crystals into rod shape was found to be closely related to its hexagonal nature. As indicated from SEM, the morphology of the modified ZnO nanorods changes with various MAA addition times. In addition, the results of TG and FT-IR confirmed the conjugation of MAA with ZnO nanorods, and the amount of carboxyl group in the samples’ surface was found to be 0.1943e0.3491 mmol/g through the titration experiment. The PL spectra indicated that the optical properties of ZnO nanorods were changed with the insertion of MAA, and showed a significant improvement in intensity. On the basis of these results, one might expect that the conjugate specific biomolecules on the functional ZnO nanorods are very potential to detect the complementary biomolecules by PL detecting. Ó 2008 Published by Elsevier Masson SAS. Keywords: ZnO nanoparticles; Surface modification; Characterization
1. Introduction In recent years, semiconductor nanocrystals show unique properties and are of great interest for applications in optoelectronic and photovoltaic devices [1,2] and biological labels [3,4]. The achievement of desired particle sizes over the largest possible range, narrow size distribution, good crystallinity, high luminescence and desired surface properties are the parameters that are considered to be the characteristics of a ‘‘high quality’’ of the chemically prepared semiconductor nanocrystals. Up to now, ‘‘surface modification’’ has been widely used on the nanocrystals for two reasons [5,6]. Firstly, surface coverage leads to electronic passivation, which enhances
* Corresponding author. Tel.: þ86 10 88256337; fax: þ86 10 88256092. E-mail address: [email protected] (R. Song). 1293-2558/$ - see front matter Ó 2008 Published by Elsevier Masson SAS. doi:10.1016/j.solidstatesciences.2008.02.006
room temperature photoluminescence and these nanocrystals can be used as fluorescent probes for the study of biological materials [7e9]. Secondly, via high quality surface modification one can synthesize monodispersed nanocrystals and incorporate some functional groups on the surface of nanocrystals [10]. As one major aim, the question of how to increase the PL efficiency has attracted a lot of efforts. Normally surface-modified semiconductor nanocrystals with high PL efficiency can be obtained by two different ways. The first is to replace the surface-capping molecules on particles prepared by trioctylphosphine oxide (TOPO) method with water-soluble thiols or silica shell [11,12]. However, TOPO is extremely toxic, expensive and the synthesis is relatively complicated. The second is to directly synthesize semiconductor nanoparticles in aqueous solutions using water-soluble thiols as stabilizing agent [13]. So some studies have reported the synthesis of semiconductor nanocrystals in aqueous solution using thiols
1564
R. Song et al. / Solid State Sciences 10 (2008) 1563e1567
as modifying agent directly, such as thioglycolic acid [14], mercaptopropionic acid [15,16], mercaptoethanol [17], etc. All the results indicated that water-soluble thiols play an important role and contribute greatly to the stability and the functionality of the resulting nanocrystals [18,19]. In addition, the choices of water-soluble thiols also critically determine the PL efficiency due to the difference in the resulting particle surface structures. Among various kinds of semiconductor nanocrystals, CdSe [20,21], CdTe [22] and ZnS [23,24] nanocrystals are widely investigated, but the researches on ZnO nanocrystals are few [25,26]. In fact, as one of the multifunctional inorganic nanoparticles, nano-sized ZnO has been attractive due to its many significant physical and chemical properties, such as chemical stability, high luminous transmittance, effective antibacterial and bactericidal function. In this paper, we use mercaptoacetic acid (MAA) as modifier to prepare ZnO nanoparticles with functional group (carboxyl group) in aqueous solution and their optical and thermal properties are systematically characterized. 2. Experimental All of the reagents used in these experiments including zinc nitrate hexahydrate (Zn(NO3)2$6H2O), hexamethylenetetramine ((CH2)6N4), mercaptoacetic acid (SHCH2COOH) and ethanol (CH3CH2OH) are of analytical grade. To prepare the ZnO nanorods, Zn(NO3)2$6H2O and (CH2)6N4 (HMT) were used as the starting materials. First, Zn(NO3)2$6H2O (0.595 g, 0.01 mol) was dissolved in 100 ml of distilled water, then the equimolar aqueous solution of HMT was slowly added into Zn(NO3)2 solution and reacted at 75 C for 1, 3, and 5 h (herein with marked samples b, c and d, respectively). Subsequently, mercaptoacetic acid (MAA) was added into the above three aqueous solutions, respectively and continued to react at 75 C until 10 h. For comparison, ZnO without MAA-modified sample (labeled as a) was prepared at the same temperature for 10 h. Later, the white powders were collected after the mixture was centrifuged, washed thoroughly with absolute alcohol and distilled water several times. The resultants were dried in vacuum at 80 C for 24 h prior to being used for further characterization. The details on the preparation were given in Ref. [27]:
1:40. Thermogravimetric (TG) analyses of the samples were carried out with Netzsch STA209PC in a flowing nitrogen atmosphere through the temperature range from room temperature to 500 C with heating rate of 10 C/min. The morphological features of the product were imaged by scanning electron microscope (SEM, JEOL JSM 5600F, Japan). The room temperature photoluminescence (PL) spectra were performed on Hitachi F-7000 spectrometer with the HeeCd laser. During the measurement the samples were dispersed in ethanol and the excitation wavelength used in PL measurement was 325 nm. The carboxyl group content of the samples (0.05 g of modified samples in 12 ml of methanol) was determined by titration with 0.0512 mol/L standard aqueous sodium hydroxide, using phenolphthalein as an indicator. The content of carboxyl group (ACOOH) is defined by the following equation [28]: ACOOH ¼
ðV V0 Þ C m
where V and V0 represent NaOH consumption volumes of the samples and the blank sample, respectively. C is the concentration of NaOH solution (0.0512 mol/L), m is the mass of the samples. 3. Results and discussion Fig. 1 shows XRD patterns of as-synthesized samples, which indicates that ZnO phase is of wurtzite structure. The diffraction peaks can be indexed to a hexagonal structure with cell constants of a ¼ 0.324 nm and c ¼ 0.519 nm (space group P63mc; JCPDS card no. 36-1451). Compared with the standard diffraction patterns, no characteristic peaks from impurities, such as Zn, are detected, indicating the high purity of the product. In addition, the sharp and strong peaks also confirm that the products are well crystallized. It can be observed that only slight changes of intensity occur in the patterns of XRD with the insertion of MAA, and the samples keep the characteristic peaks of ZnO intact (curve a in
ðCH2 Þ6 N4 þ 10H2 O . 6HCHO þ 4NH3 $H2 O NH3 þ H2 O . OH þ NHþ 4 OH þ Zn2þ . ZnOðsÞ þ H2 O X-ray powder diffraction (XRD) analysis was conducted on a Bruker D8 X-ray diffractometer with Ni-filtered Cu Ka radiation (l ¼ 0.15406 nm). The accelerating voltage was set at 40 kV, with 30 mA flux at a scanning rate of 0.5 /min from 20 to 70 (2q) angles. FT-IR spectra were recorded on a Bruker TENSOR 27 FTIR spectrometer in transmission mode with 32 scans at a resolution of 4 cm1. The samples were mixed with potassium bromide (KBr) in the ratio of
Fig. 1. XRD spectra of (a) ZnO, (b) modified ZnO.
R. Song et al. / Solid State Sciences 10 (2008) 1563e1567
Fig. 1). We also investigated the samples of different MAA adding times and found that their XRD patterns are very well consistent with the wurtzite ZnO. One typical XRD curve for the MAA-modified sample is given in curve b in Fig. 1. Thus we can conclude that the addition of MAA as modifier does not significantly affect the perfection of the ZnO crystals. FT-IR spectra of MAA (curve a), ZnO (curve b), modified ZnO (curve c). For MAA, the modifier (Fig. 2), the characteristic IR peaks appear at 3094, 2568, 1715 and 1293 cm1. There is a strong absorption peak at 3094 cm1, which is arising from the hydroxyl bond. The absorption peak at 2568 cm1 is the stretching vibration of SeH bond. The absorption peaks at 1715 and 1293 cm1 should be attributed to the stretching vibration of C]O and CeO bonds, respectively. Comparatively, IR spectrum of pure ZnO has only two strong characteristic peaks, one is the hydroxyl bond at 3398 cm1 and the other is the ZneO bond at 447 cm1. Comparing with the bulk ZnO, the peak at 447 cm1 presents a red shift considering the fact that the FT-IR spectrum of ZnO particles is generally influenced by particle size and morphology [29,30]. Compared with the pure ZnO, the spectra of modified ZnO (curve c in Fig. 2) display other characteristic peaks. The absorption peak at 1578 cm1 is ascribed to the stretching vibration of carbonyl group, which may be presented in the form of COO because of the constant pH value of the whole reacting system (w7). Both CeOH stretching (1387 cm1) and CeO stretching (1229 cm1) are also detected from the FT-IR spectrum. In addition, no SeH bond stretching appears at 2568 cm1, which implies that the mercapto group (eSH) has bounded to Zn2þ to form a complex and the polar carboxylic acid group retains the surface of ZnO nanoparticles. This result indicates that ZnO nanoparticles are modified by MAA successfully. Following TG measurement will be used further to verify this aspect. Thermogravimetric analyses are carried out to determine whether MAA exists on the ZnO nanoparticles (Fig. 3). Weight loss of 1.5 wt.% from room temperature to w150 C is mainly due to the removal of physically absorbed water.
Fig. 2. FT-IR spectra of (a) mercaptoacetic acid, (b) ZnO and (c) modified ZnO.
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Fig. 3. TGeDTG curves of modified ZnO nanoparticles.
Weight loss of 2 wt.% from w150 to w350 C is due to the removal of chemically adsorbed water and the decomposition of the hydroxide group on the surface of the nanoparticles [31]. Weight loss of 1.5 wt.% from w350 to w460 C is very likely due to the decomposition of the MAA and other impurities. Therefore, a total weight loss of w5 wt.% occurred when the sample is heated from room temperature to w460 C. Therefore, the results of TGeDTG indicate the presence of MAA on the sample surface, which is in accordance with the above result of FT-IR. Further morphology characterization of ZnO and modified ZnO with different MAA addition times are performed using SEM and the results are shown in Fig. 4. As indicated, the morphology of the modified ZnO nanorods changes profoundly with MAA addition time. When MAA was added at 1 h, the morphology of the samples seems to be disordered. Most samples present helical structures while only a small part is rod-shaped (Fig. 4b). When MAA was added at 3 h, the disordered ZnO nanorods were observed (Fig. 4c). However, the slenderness ratio was small, so the morphology seems nubby-shaped. When MAA was added at 5 h, the morphology as shown in Fig. 4d was similar to that of ZnO nanorods as indicated in Fig. 4a. Obviously, the structure change is closely related to the addition of MAA, which promotes the dissolution and re-growth of ZnO during the growth process at different MAA adding times [7]. As one interesting phenomenon, we found that when MAA was added at 5 h, two kinds of morphology appeared in the SEM images (Fig. 4d). One is rodshaped, as mentioned above; another is the column structure that formed by arrays of ZnO nanoplates, but the yield is very low. Similar structure has been reported previously [32e36]. In Ref. [33], the authors refer to the ‘‘dissolutione reprecipitation’’ mechanism to explain the change of morphology. They suggest that along with the increase in reaction time (about 2e3 d), the hydrolysis of HMT, i.e. (CH2)6N4, proceeds slowly to form ammonia, and then forms tetrahedral structure, eventually the morphology of ZnO is changed. However, similar morphology has been found in the current case, although the whole reaction process proceeds for 10 h. So we speculate that MAA would play an important role during the formation
R. Song et al. / Solid State Sciences 10 (2008) 1563e1567
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Fig. 4. SEM images of (a) ZnO, (b) modified ZnO (addition time: 1 h), (c) modified ZnO (addition time: 3 h), (d) modified ZnO (addition time: 5 h).
of ZnO. The interaction mechanism between ZnO and MAA is complicated and further works on this line are currently underway to make further progress in this respect. Titration experiment was used to determine the content of carboxyl group (ACOOH) on the sample surfaces and the results are list in Table 1. It can be found that the value of ACOOH increased with the increment of MAA addition time. The reason is that when MAA was added at 1 h, the ZnO nanorods are not formed and therefore a large number of Zn2þ exists in aqueous medium. Zn2þ can react with MAA, so the carboxyl group was consumed and only 0.1943 mmol/g carboxyl groups can be detected on the surface of ZnO. However, when MAA was added at 5 h, perfect nanorods exist in the system. At this point, MAA would couple to Zn2þ, which is on the surface of ZnO nanorods. And so the carboxyl group is left. In this case, the content of carboxyl groups increased from 0.1943 to 0.3491 mmol/g. Fluorescence semiconductor nanocrystals and fluorescent organic nanoparticles could dramatically improve the use of
fluorescent marks in biological imaging [3,4]. The major drawbacks encountered in the use of semiconductor nanocrystals as markers are the stability of nanoparticles and their solubility in water. Hence the preparation of MAA-modified ZnO nanorods will be of immense use in biological imaging. The photoluminescence spectrum of ZnO (curve a) and modified ZnO (curve b) was investigated using a PL spectrum and the results are shown in Fig. 5. Nano-ZnO displays an intensive
Table 1 Carboxyl group content of the samples Samples
m (g)
V
V0
C (mol/L)
ACOOH (mmol/g)
b (Addition time: 1 h) c (Addition time: 3 h) d (Addition time: 5 h)
0.0527 0.0475 0.0440
0.4 0.4 0.5
0.2 0.2 0.2
0.0512 0.0512 0.0512
0.1943 0.2156 0.3491
Fig. 5. Photoluminescence spectra of (a) ZnO, (b) modified ZnO.
R. Song et al. / Solid State Sciences 10 (2008) 1563e1567
ultraviolet emission peak at w380 nm. It is well known that the UV emission peak usually originates from a near-bandedge (NBE) transition of the wide band gap [37,38]. And the impurities and structural defects, such as oxygen vacancies, may induce the visible emission [39]. The PL spectrum of MAA-modified ZnO samples (curve b) reveals two emitting bands. One is a strong ultraviolet emission at around 392 nm, which has a slight red shift compared with the PL spectrum of ZnO. It might be attributed to the defect passivation on the surface region of ZnO nanorods due to the conjugation of MAA [40]. Another is a weak blue emission at 440e470 nm, which may be in correlation with ionized oxygen defects or others on the surface of the nanoparticles. 4. Conclusion In summary, a simple method has been presented using MAA as the modifier agent to prepare the chemically modified ZnO nanorods with high dispersion. This method employs an aqueous solution, and the reaction process does not require high temperature or complicated procedures. The properties of ZnO nanoparticles are studied by SEM, XRD, FT-IR, TG and PL measurements. The ZnO nanoparticles obtained are rod-shaped by SEM observation. FT-IR and TG results confirm that MAA can modify the nanoparticles successfully and the carboxylic groups do exist on the surface of ZnO nanoparticles. All these findings demonstrate a novel protocol for the preparation of the modified ZnO nanoparticles with functional groups, which foresee a potential application in the preparation of composites and the detection of biomolecules. Acknowledgment This work was partially subsidized by Henan Innovation Project for University Prominent Research Talents (‘‘HAIPURT’’) program. References [1] D.L. Klein, R. Roth, A.K.L. Lim, A.P. Alivisatos, P.L. McEuen, Nature 389 (1997) 699. [2] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [3] W.C.W. Chan, S. Nie, Science 281 (1998) 2016. [4] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [5] X.G. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, J. Am. Chem. Soc. 119 (1997) 7019.
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