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Optical and fluorescence properties of MgO nanoparticles in micellar solution of hydroxyethyl laurdimonium chloride
Chemical Physics Letters 636 (2015) 26–30
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Optical and fluorescence properties of MgO nanoparticles in micellar solution of hydroxyethyl laurdimonium chloride Jamil K. Salem a,∗ , Issa M. El-Nahhal a , Talaat M. Hammad a , Sylvia Kuhn b , Somaya Abu Sharekh a , Mohamad El-Askalani a , Rolf Hempelmann b a b
Department of Chemistry, Al-Azhar University, PO Box 1277, Gaza, Palestine Physical Chemistry, Saarland University, D-66123 Saarbrucken, Germany
a r t i c l e
i n f o
Article history: Received 14 May 2015 In final form 9 July 2015 Available online 20 July 2015
a b s t r a c t The critical micelle concentration of hydroxyethyl laurdimonium chloride (HY) is estimated in this Letter. MgO exhibited substantial absorbance and fluorescence in presence of HY micelles. The optical properties of MgO were utilized to estimate the critical micelle concentration of a surfactant. The signals of spectra were assigned and explained in terms of electron transitions produced by magnesium and oxygen vacancies of different coordinations. The CMC values of HY evaluated by absorbance, fluorescence and specific conductivity were found to be 9.8, 8.0 and 2.7 mM, respectively. The synthesized MgO nanoparticles were characterized by XRD, TEM, UV and fluorescence spectroscopy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Metal oxides are well known because of their industrial applications as adsorbents, catalysts, and catalyst supports [1,2]. Magnesium oxide (MgO) is an important functional metal oxide that has been widely used in various fields such as catalysis, refractory materials, paints, and superconductors [3,4]. Nanostructured MgO has high surface area and therefore acts as coating agent which helps in increasing the energy conversion efficiency [5]. More interestingly, MgO nanostructures not only exhibit the blue fluorescence emission at room temperature but they are also used as indicators and photon sources. Origin of the blue emission characteristics is still a subject of discussion [6,7]. It has been believed that the blue fluorescence in MgO is essential because of the presence of surface defects, such as F centers (F, F+ and F2+ , corresponding to the oxygen ion vacancies with two, one and zero electrons, respectively) [8–11]. Indeed, a defected surface usually plays a decisive role in catalysis, reactivity, optical and transport properties at oxide surfaces [9,12]. Generally defects, such as point defects, F-centers and intrinsic/extrinsic defects which are created during various synthetic methods may form luminescent centers on the insulating surface of materials [13]. The application of a surfactant to solubilize MgO nanoparticles provides a means to achieve control over the MgO nanoparticle size and size distribution in solution, which
∗ Corresponding author. E-mail address: [email protected] (J.K. Salem). http://dx.doi.org/10.1016/j.cplett.2015.07.014 0009-2614/© 2015 Elsevier B.V. All rights reserved.
is essential for tailoring optical, electrical, chemical, and magnetic properties of nanoparticles for specific applications. Micelles have attracted significant attention because of their ability to function as encapsulating systems to provide selective microenvironments [14–17]. The surfactant selected in this study hydroxyethyl laurdimonium chloride (HY) classified as a typical cationic surfactant whose hydrophilic characteristics are improved by the presence of a hydroxyl group in its structure (see Figure 1). The HY can affect local aggregation which in turn can substantially enhance the stability, the optical and fluorescence properties [18,19]. Importantly, we found that the enhancement in the optical and fluorescence were dependent on the concentration of HY. The results reported herein show the fluorescence response of MgO is sensitive to the microenvironment provided by the micelles. The microenvironment provided by organized micellar system offer an attractive medium to modulate absorbance and fluorescence detections of critical micelle concentration of HY in aqueous media.
2. Experimental All reagents used in the present work were analytical grade and directly used without further treatment. Magnesium acetate tetrahydrate, Mg(CH3 COO)2 ·4H2 O (Merck, 99.5% purity); oxalic acid, H2 C2 O4 ·2H2 O (Merck, 99.5% purity), hydroxyethyl laurdimonium chloride, C16 H36 NOCl (Praepagen HY, Clariant, 40%); and deionized water were used in the synthesis and preparation of all solutions.
J.K. Salem et al. / Chemical Physics Letters 636 (2015) 26–30
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Figure 1. Molecular structure of hydroxyethyl laurdimonium chloride.
In typical synthesis of MgO nanopowders 20 mmol of magnesium acetate tetrahydrate Mg(CH3 COO)2 ·4H2 O was dissolved into 25 mL of deionized water. 20 mmol of oxalic acid was dissolved in an equal volume of deionized water and dropwise added to magnesium acetate solution under magnetic stirring for 60 min, white precipitate of magnesium oxalate was isolated, washed with water several times and dried at 100 ◦ C for 24 h. The dried material was grounded using mortar and pestle to produce fine powder precursor. Subsequently, the precursor, Magnesium oxalate was annealed in muffle furnace under air at 500 ◦ C for 4 h to form MgO nanostructure. UV–vis absorption spectra were collected using a UV–vis spectrophotometer (Shimadzu, UV-2400) in the wavelength range from 200 to 800 nm. Fluorescence spectra were recorded with a spectrofluorometer (JASCO, FP-6500); the extinction wavelength was selected to be 310 nm. The scan rate was set at 600 nm/min with the entrance and exit slit width of 5 nm. The X-ray diffraction (XRD) patterns of the dried as-prepared and classified samples were obtained using an X-ray diffractometer with Cu Ka radiation (0.154 nm wavelength) under 40 kV and 200 mA. The TEM analysis was done with JEM2010 (JEOL) transmission electron microscope with energy dispersive X-ray spectrometer INCA (Oxford Instruments). Critical micelle concentration of HY was determined as following: 1.5 mg of dried MgO was mixed with varying concentrations of surfactant from 0.0038 M to 0.075 M and make up to 5 mL as the total volume. These solutions were sonicated for 5 min in order to equilibrate the solutions. After equilibrating the MgO surfactant solution, the absorbance as well as the emission spectra were recorded. Prior to each measurement a base correction was recorded for each surfactant concentration. From the graphical representation of the absorbance or fluorescence enhancement versus concentration, CMC values of the surfactant were evaluated. 3. Results and discussion The XRD pattern of the synthesized MgO is shown in Figure 2a. The diffraction pattern matched with the face centered cubic structure of periclase MgO (JCPDS No. 87-0653). The major peaks at 2 values of 36.8◦ , 42.9◦ , 62.2◦ , 74.6◦ , and 78.6◦ can be indexed to the lattice planes of (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) respectively. From XRD pattern the small peaks at 2 = 56◦ and 50.5◦ (designated by o) correspond to the peaks of Mg(OH)2 . It means only a negligible fraction of smaller Mg(OH)2 particles present on the surface of MgO nanocrystallite, which does not significantly contribute to the solubility of MgO particles in water [20]. The crystallite size is determined using Scherrer’s formula, d = 0.9/ˇ cos , where d is the crystallite size, is wavelength of X-ray radiation, ˇ is full width at half maximum and is the diffraction angle. The calculated crystallite size of MgO is 8.2 nm. The morphology of MgO nanoparticles suspended in ethanol was studied by TEM analysis. It is clear that the MgO nanocrystals are clustered as shown in Figure 2b. Here, nanocrystals are closely seen as grouping together and forming a rod-shape morphology. The EDAX spectrum (Figure 2c) indicated that the nanoparticles were composed of elemental Mg and O and it is found that the nanoparticles contain 54.6% magnesium and 45.4% oxygen. It is clear that the Mg/O atomic ratio, approximated from these data
Figure 2. Structural characterization of MgO nanoparticles: (a) XRD pattern, (b) TEM image and (c) EDX spectra.
is in good agreement with that of the bulk ratio. A slightly lower oxygen atomic ratio compared to that of magnesium is probably high oxygen vacancies in the MgO nanoparticles. The optical transitions generally arise when a photon is absorbed or emitted by the defect center [9]. Therefore, optical absorption and fluorescence emission were studied to know the impact of HY micelles on surface defect centers created on the synthesized MgO nanoparticles. Figure 3a shows the UV–vis spectra of MgO dispersed in water and in 0.1 M HY micellar solution. The UV–vis spectrum (Figure 3a inset) of MgO in water displayed continuous absorbance increasing from 220 nm to 800 nm which is in agreement with relevant literature [21]. The UV–vis absorption spectrum of MgO dispersed in HY solution exhibit two well-defined absorption peaks at 230 nm and 300 nm as shown in Figure 3a. In the literature, bulk MgO is reported to exhibit absorptions in the UV region in between 160 and 200 nm [22]. In the present study the absorption peak at 230 nm may be due to F or F + center in agreement with the results obtained by Chen et al. [23]. The broad absorption peak at 300 nm is attributed to absorptions of surface and bulk F centers [24]. In nanoscale MgO a larger percentage of atoms and defects reside on the surface than in the bulk. These defects are located on the different coordination sites on the surface and thus, undergo excitation when exposed to UV photon energy. It has been shown that the cubic-MgO is a wide-band gap energy material and normally does not demonstrate the fluorescence characteristic [13,25,7]. In the MgO crystalline structure, some structural defects are present, such as magnesium vacancies (3 and 4-coordinated Mg2+ ) and oxygen vacancies (3 and 4-coordinated O2− ) [10,26]. There are three active sites or surface anions (oxygen, O2− ) present; (i) 3-coordinated at the corner (ii) 4-coordinated at edges, and (iii) 5-coordinated at the planar site in the MgO nanoparticles, which belong to surface defects at the stepped and kinked surfaces [27–30]. Figure 3b provides the fluorescence emission spectra (excitation wavelength: 300 nm) of
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MgO in aqueous and in 0.1 M HY micellar solutions. Fluorescence spectra of aqueous HY surfactant solution without MgO was also recorded and showed no emission at same excitation wavelength (300 nm). The spectrum reveal one broad emission peak and two accompanying shoulder/small peaks for aqueous solution (Figure 3b inset). The occurrence of the major fluorescence emission peak at about 407 nm (3.30 eV) is due to the presence of oxygen vacancy with low coordinate oxygen anions (3-coordinated O2− ) in the MgO [27,8]. The emission peak at 407 nm agrees well with the main feature detected in previous photo-luminescence experiments on cube-shaped MgO nanocrystals and MgO films [31,32]. The various structural defects in their crystalline structure and high coordinated oxygen anions (4 and 5-coordinated O2− ) are responsible for the presence of almost suppressed shoulder emissions in the fluorescence spectra of the MgO at about 446 nm (2.78 eV) and 475 nm (2.61 eV). It was interesting to see from Figure 3b that fluorescence spectrum of MgO in micellar solution increased drastically in intensity with a wide emission peak centered at 411 nm without appearance of any shoulders. The results show that the fluorescence intensity increased by 50-fold in the presence of 0.1 M HY. The possible optical transitions (absorbance and emission process) observed from the UV–vis absorption and fluorescence spectra are illustrated in Figure 3c. The changes observed in absorption and emission may be because of extrinsic point defects which are created from the hydrophilic interactions between the surface of MgO nanorods with HY micelles. These results are in agreement with previous observations revealing that the origin of fluorescence emission can be attributed to the existence of oxygen vacancies/intrinsic or extrinsic point defects on the metal oxide surface [33,34]. Since MgO has a large surface area and binds surfactants. This property, along with its interesting photophysical characteristics, led us to evaluate MgO as a probe to determine the CMC of HY. Due to the presence of hydroxyl group on the surface of head portion of the surfactant, strong hydrophilic interaction with MgO was exhibited leading to the change of the surface as well as the optical properties of the MgO. The impact of HY on the absorbance and the fluorescence properties of MgO is shown in Figures 4 and 5 which is the preliminary criterion of the determination of CMC. Figures 4a and 5a represents the absorbance and emission spectra of MgO in different concentrations of HY. On increasing the concentration of HY, the absorbance values increases with a marginal blue shift from 303 to 299 nm in the absorption maxima and about
2 nm red shift in the emission maxima. As the HY concentration increased the absorbance or emission intensity increase was very small initially and sudden increase occurred as the concentration approached CMC. It means that more MgO are solubilized in micellar pseudo-phase than aqueous monomer pseudo-phase. Constant increase in absorbance or emission intensity was observed above CMC as the concentration increased implying that MgO surface is still unsaturated with HY micelles. Figures 4b and 5b shows the plot of the absorbance or fluorescence intensity as a function of the surfactant concentration. The CMC value was determined from the intersection of two lines drawn through the experimental points for surfactant concentrations below and above the critical surfactant concentration. The obtained values are 9.8 × 10−3 and 8.0 × 10−3 M by absorbance and fluorescence, respectively. For further investigation, electrical conductivity method has been used to study the aggregation behavior of surfactant in the presence of MgO nanoparticles. The changes in specific conductivity were measured during titration of surfactant into aqueous solution at 25 ◦ C, and the CMC value of the surfactant has been estimated at the break point of nearly two straight line portions of the specific conductivity vs concentration plot as shown in Figure 6. The obtained value is 2.7 × 10−3 M. However, the uncertainity in the CMC values are usually due to the limitations of the experimental methodologies. In electrical conductivity method the overall increase in conductivity of surfactant in the studied range is due to conducting nature of charged nanoparticles dispersed in surfactant solution. Whereas, in absorbance and fluorescence methods the surfactant micelles are responsible for the solubilization of the MgO which may intern enhance the absorbance or fluorescence intensity and the onset of the intensity is at the CMC of the surfactant. Comparing the CMC value of HY with that of similar cationic surfactant cetyltrimethyl ammonium bromide (CTAB). It is clear that HY having higher CMC than CTAB. Since both surfactant have similar structures except HY has additional hydroxyl group attached to its ammonium head group. The hydroxyl functional group possesses hydrogen bonding with water molecules. The formation of hydrogen bonding between HY monomers and water molecules makes the micellization process more hard and needs excluding water molecules exist between head groups of monomers. This may be due to the fact that the head group structure of the surfactant is one of the factors that decides the aggregation of the surfactant monomers into a micelle; if so, we would expect the CMC value of HY is higher than that of CTAB
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J.K. Salem et al. / Chemical Physics Letters 636 (2015) 26–30
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(7 × 10−4 M) [35]. It is important to note that HY forms transparent colloid with MgO but CTAB does not form. For this reason CTAB is not examined in this study. 4. Conclusions Spectrophotometric approach for estimation of critical micelle concentration (CMC) of HY surfactant with the assistance of MgO nanostructure has been developed. MgO exhibited enhanced absorbance and fluorescence in presence of HY micelles. The spectra were assigned and explained in terms of electron transitions produced by magnesium and oxygen vacancies of different coordinations. CMC values of HY estimated by absorbance, fluorescence and specific conductivity were found to be 9.8, 8.0 and 2.7 mM, respectively. The obtained CMC values of HY is higher than that of CTAB. References [1] H. Thakuria, B.M. Borah, G. Das, J. Mol. Catal. A: Chem. 2741 (2007) 1. [2] Y.H. Ren, B. Yue, M. Gu, H.Y. He, Materials 3 (2010) 764.
[3] U. Zavyalova, M. Geske, R. Horn, G. Weinberg, W. Frandsen, M. Schuster, Chem. Catal. Chem. 3 (2011) 949. [4] H. Heidari, H. Abedini, A. Nemati, M.M. Amini, Catal. Lett. 130 (2009) 266. [5] H.S. Jung, J.K. Lee, M. Nastasi, S.W. Lee, J.Y. Kim, J.S. Park, K.S. Hong, H. Shin, Langmuir 21 (2005) 10332. [6] A. Anjiki, T. Uchino, J. Phys. Chem. C 116 (2012) 15747. ˘ [7] A. Chandran, J. Prakash, K.K. Naik, A.K. Srivastava, R. Danbrowski, M. Czerwinski, A.M. Biradar, J. Mater. Chem. C 2 (2014) 1844. [8] D. Vingurt, D. Fuks, M.V. Landau, R. Vidrukb, M. Herskowitz, Phys. Chem. Chem. Phys. 15 (2013) 14783. [9] E. Ertekin, L.K. Wagner, J.C. Grossman, Phys. Rev. B 87 (2013) 155210. [10] A.R. Gerson, T. Bredow, Phys. Chem. Chem. Phys. 1 (1999) 4889. [11] H. Yang, S.H. Yang, S. Takahashi, S. Maekawa, S.S.P. Parkin, Nat. Mater. 9 (2010) 586. [12] S.R. Lu, R. Yu, J. Zhu, Phys. Rev. B 87 (2013) 165436. [13] B.M. Maoz, E. Tirosh, M.B. Sadan, I. Popov, Y. Rosenbergd, G. Markovich, J. Mater. Chem. 21 (2011) 9532. [14] D.K. Rana, A. Sarkar, S. Dhar, T.K. Mandal, S.C. Bhattacharya, J. Lumin. 130 (2010) 1106. [15] N.A. Begum, N. Roy, S. Mandal, S. Basu, D. Mandal, J. Lumin. 129 (2009) 158. [16] A. Mallick, P. Purkayastha, N. Chattopadhyay, J. Photochem. Photobiol. C: Rev. 8 (2007) 109. [17] P. Goswami, D.K. Das, J. Lumin. 131 (2011) 760. [18] J.K. Salem, T.M. Hammad, S. Kuhn, M. Abu Draaz, N.K. Hejazy, R. Hempelmann, J. Mater. Sci.: Mater. Electron. 25 (2014) 2177. [19] J.K. Salem, T.M. Hammad, S. Kuhn, I. Nahal, M. Abu Draaz, N.K. Hejazy, R. Hempelmann, J. Mater. Sci.: Mater. Electron. 25 (2014) 5188. [20] S. Baumann, J. Schneider, A. Sternig, D. Thomele, S. Stankic, T. Berger, H. Grönbeck, O. Diwald, Langmuir 31 (2015) 2770. [21] B. Choudhury, A. Choudhury, Mater. Res. Express 1 (2014) 025026. [22] L. Shirmane, V. Pankratov, A. Lushchik, A. Kotlov, V.E. Serga, L.D. Kulikova, G. Chikvaidze, J. Zimmermann, Nucl. Instrum. Methods B325 (2013) 1023. [23] Y. Chen, T.R. Williams, W.A. Sibley, Phys. Rev. B 182 (1969) 960. [24] F. Illas, G. Pacchioni, J. Chem. Phys. 108 (1998) 7835. [25] A.K. Srivastava, M. Deepa, N. Bahadur, M.S. Goyat, Mater. Chem. Phys. 114 (2009) 194. [26] C. Chizallet, G. Costentin, H. Lauron-Pernot, J. Krafft, M. Che, F.O. Delbecq, P. Sautet, J. Phys. Chem. C (2008) 16629. [27] R. Hacquart, J.M. Krafft, G. Costentin, J. Jupille, Surf. Sci. 595 (2005) 172. [28] S. Benedetti, H.M. Benia, N. Nilius, S. Valeri, H.J. Freund, Chem. Phys. Lett. 430 (2006) 330. [29] M.L. Bailly, G. Costentin, H.L. Pernot, J.M. Krafft, M. Che, J. Phys. Chem. B 109 (2005) 2404. [30] K.K. Moorthy, J.Y. Moon, H.B. Hyun, S.K. Cho, S.J. Kim, J. Mater. Chem. 22 (2012) 24610. [31] M. Anpo, Y. Yamada, Y. Kubokawa, S. Coluccia, A. Zecchina, M. Che, J. Chem. Soc. Faraday Trans. I 84 (1988) 751. [32] H.R. Moon, J.J. Urban, D.J. Milliron, Angew. Chem. 121 (2009) 6396. [33] A.K. Srivastava, M. Deepa, A. Srivastava, H.J. Kleebe, D.P. Singh, in: A. Malik, R.J. Rawat (Eds.), New Nanotechniques, vol. 13, Nova Science Publications, USA, 2009. [34] A. Sternig, S. Stankic, M. Müller, N. Siedl, O. Diwald, Nanoscale 4 (2012) 7494. [35] S.P. Moulik, M.E. Haque, P.K. Jana, A.R. Das, J. Phys. Chem. 100 (1996) 701.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Optical and fluorescence properties of MgO nanoparticles in micellar solution of hydroxyethyl laurdimonium chloride Jamil K. Salem a,∗ , Issa M. El-Nahhal a , Talaat M. Hammad a , Sylvia Kuhn b , Somaya Abu Sharekh a , Mohamad El-Askalani a , Rolf Hempelmann b a b
Department of Chemistry, Al-Azhar University, PO Box 1277, Gaza, Palestine Physical Chemistry, Saarland University, D-66123 Saarbrucken, Germany
a r t i c l e
i n f o
Article history: Received 14 May 2015 In final form 9 July 2015 Available online 20 July 2015
a b s t r a c t The critical micelle concentration of hydroxyethyl laurdimonium chloride (HY) is estimated in this Letter. MgO exhibited substantial absorbance and fluorescence in presence of HY micelles. The optical properties of MgO were utilized to estimate the critical micelle concentration of a surfactant. The signals of spectra were assigned and explained in terms of electron transitions produced by magnesium and oxygen vacancies of different coordinations. The CMC values of HY evaluated by absorbance, fluorescence and specific conductivity were found to be 9.8, 8.0 and 2.7 mM, respectively. The synthesized MgO nanoparticles were characterized by XRD, TEM, UV and fluorescence spectroscopy. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Metal oxides are well known because of their industrial applications as adsorbents, catalysts, and catalyst supports [1,2]. Magnesium oxide (MgO) is an important functional metal oxide that has been widely used in various fields such as catalysis, refractory materials, paints, and superconductors [3,4]. Nanostructured MgO has high surface area and therefore acts as coating agent which helps in increasing the energy conversion efficiency [5]. More interestingly, MgO nanostructures not only exhibit the blue fluorescence emission at room temperature but they are also used as indicators and photon sources. Origin of the blue emission characteristics is still a subject of discussion [6,7]. It has been believed that the blue fluorescence in MgO is essential because of the presence of surface defects, such as F centers (F, F+ and F2+ , corresponding to the oxygen ion vacancies with two, one and zero electrons, respectively) [8–11]. Indeed, a defected surface usually plays a decisive role in catalysis, reactivity, optical and transport properties at oxide surfaces [9,12]. Generally defects, such as point defects, F-centers and intrinsic/extrinsic defects which are created during various synthetic methods may form luminescent centers on the insulating surface of materials [13]. The application of a surfactant to solubilize MgO nanoparticles provides a means to achieve control over the MgO nanoparticle size and size distribution in solution, which
∗ Corresponding author. E-mail address: [email protected] (J.K. Salem). http://dx.doi.org/10.1016/j.cplett.2015.07.014 0009-2614/© 2015 Elsevier B.V. All rights reserved.
is essential for tailoring optical, electrical, chemical, and magnetic properties of nanoparticles for specific applications. Micelles have attracted significant attention because of their ability to function as encapsulating systems to provide selective microenvironments [14–17]. The surfactant selected in this study hydroxyethyl laurdimonium chloride (HY) classified as a typical cationic surfactant whose hydrophilic characteristics are improved by the presence of a hydroxyl group in its structure (see Figure 1). The HY can affect local aggregation which in turn can substantially enhance the stability, the optical and fluorescence properties [18,19]. Importantly, we found that the enhancement in the optical and fluorescence were dependent on the concentration of HY. The results reported herein show the fluorescence response of MgO is sensitive to the microenvironment provided by the micelles. The microenvironment provided by organized micellar system offer an attractive medium to modulate absorbance and fluorescence detections of critical micelle concentration of HY in aqueous media.
2. Experimental All reagents used in the present work were analytical grade and directly used without further treatment. Magnesium acetate tetrahydrate, Mg(CH3 COO)2 ·4H2 O (Merck, 99.5% purity); oxalic acid, H2 C2 O4 ·2H2 O (Merck, 99.5% purity), hydroxyethyl laurdimonium chloride, C16 H36 NOCl (Praepagen HY, Clariant, 40%); and deionized water were used in the synthesis and preparation of all solutions.
J.K. Salem et al. / Chemical Physics Letters 636 (2015) 26–30
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Figure 1. Molecular structure of hydroxyethyl laurdimonium chloride.
In typical synthesis of MgO nanopowders 20 mmol of magnesium acetate tetrahydrate Mg(CH3 COO)2 ·4H2 O was dissolved into 25 mL of deionized water. 20 mmol of oxalic acid was dissolved in an equal volume of deionized water and dropwise added to magnesium acetate solution under magnetic stirring for 60 min, white precipitate of magnesium oxalate was isolated, washed with water several times and dried at 100 ◦ C for 24 h. The dried material was grounded using mortar and pestle to produce fine powder precursor. Subsequently, the precursor, Magnesium oxalate was annealed in muffle furnace under air at 500 ◦ C for 4 h to form MgO nanostructure. UV–vis absorption spectra were collected using a UV–vis spectrophotometer (Shimadzu, UV-2400) in the wavelength range from 200 to 800 nm. Fluorescence spectra were recorded with a spectrofluorometer (JASCO, FP-6500); the extinction wavelength was selected to be 310 nm. The scan rate was set at 600 nm/min with the entrance and exit slit width of 5 nm. The X-ray diffraction (XRD) patterns of the dried as-prepared and classified samples were obtained using an X-ray diffractometer with Cu Ka radiation (0.154 nm wavelength) under 40 kV and 200 mA. The TEM analysis was done with JEM2010 (JEOL) transmission electron microscope with energy dispersive X-ray spectrometer INCA (Oxford Instruments). Critical micelle concentration of HY was determined as following: 1.5 mg of dried MgO was mixed with varying concentrations of surfactant from 0.0038 M to 0.075 M and make up to 5 mL as the total volume. These solutions were sonicated for 5 min in order to equilibrate the solutions. After equilibrating the MgO surfactant solution, the absorbance as well as the emission spectra were recorded. Prior to each measurement a base correction was recorded for each surfactant concentration. From the graphical representation of the absorbance or fluorescence enhancement versus concentration, CMC values of the surfactant were evaluated. 3. Results and discussion The XRD pattern of the synthesized MgO is shown in Figure 2a. The diffraction pattern matched with the face centered cubic structure of periclase MgO (JCPDS No. 87-0653). The major peaks at 2 values of 36.8◦ , 42.9◦ , 62.2◦ , 74.6◦ , and 78.6◦ can be indexed to the lattice planes of (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) respectively. From XRD pattern the small peaks at 2 = 56◦ and 50.5◦ (designated by o) correspond to the peaks of Mg(OH)2 . It means only a negligible fraction of smaller Mg(OH)2 particles present on the surface of MgO nanocrystallite, which does not significantly contribute to the solubility of MgO particles in water [20]. The crystallite size is determined using Scherrer’s formula, d = 0.9/ˇ cos , where d is the crystallite size, is wavelength of X-ray radiation, ˇ is full width at half maximum and is the diffraction angle. The calculated crystallite size of MgO is 8.2 nm. The morphology of MgO nanoparticles suspended in ethanol was studied by TEM analysis. It is clear that the MgO nanocrystals are clustered as shown in Figure 2b. Here, nanocrystals are closely seen as grouping together and forming a rod-shape morphology. The EDAX spectrum (Figure 2c) indicated that the nanoparticles were composed of elemental Mg and O and it is found that the nanoparticles contain 54.6% magnesium and 45.4% oxygen. It is clear that the Mg/O atomic ratio, approximated from these data
Figure 2. Structural characterization of MgO nanoparticles: (a) XRD pattern, (b) TEM image and (c) EDX spectra.
is in good agreement with that of the bulk ratio. A slightly lower oxygen atomic ratio compared to that of magnesium is probably high oxygen vacancies in the MgO nanoparticles. The optical transitions generally arise when a photon is absorbed or emitted by the defect center [9]. Therefore, optical absorption and fluorescence emission were studied to know the impact of HY micelles on surface defect centers created on the synthesized MgO nanoparticles. Figure 3a shows the UV–vis spectra of MgO dispersed in water and in 0.1 M HY micellar solution. The UV–vis spectrum (Figure 3a inset) of MgO in water displayed continuous absorbance increasing from 220 nm to 800 nm which is in agreement with relevant literature [21]. The UV–vis absorption spectrum of MgO dispersed in HY solution exhibit two well-defined absorption peaks at 230 nm and 300 nm as shown in Figure 3a. In the literature, bulk MgO is reported to exhibit absorptions in the UV region in between 160 and 200 nm [22]. In the present study the absorption peak at 230 nm may be due to F or F + center in agreement with the results obtained by Chen et al. [23]. The broad absorption peak at 300 nm is attributed to absorptions of surface and bulk F centers [24]. In nanoscale MgO a larger percentage of atoms and defects reside on the surface than in the bulk. These defects are located on the different coordination sites on the surface and thus, undergo excitation when exposed to UV photon energy. It has been shown that the cubic-MgO is a wide-band gap energy material and normally does not demonstrate the fluorescence characteristic [13,25,7]. In the MgO crystalline structure, some structural defects are present, such as magnesium vacancies (3 and 4-coordinated Mg2+ ) and oxygen vacancies (3 and 4-coordinated O2− ) [10,26]. There are three active sites or surface anions (oxygen, O2− ) present; (i) 3-coordinated at the corner (ii) 4-coordinated at edges, and (iii) 5-coordinated at the planar site in the MgO nanoparticles, which belong to surface defects at the stepped and kinked surfaces [27–30]. Figure 3b provides the fluorescence emission spectra (excitation wavelength: 300 nm) of
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MgO in aqueous and in 0.1 M HY micellar solutions. Fluorescence spectra of aqueous HY surfactant solution without MgO was also recorded and showed no emission at same excitation wavelength (300 nm). The spectrum reveal one broad emission peak and two accompanying shoulder/small peaks for aqueous solution (Figure 3b inset). The occurrence of the major fluorescence emission peak at about 407 nm (3.30 eV) is due to the presence of oxygen vacancy with low coordinate oxygen anions (3-coordinated O2− ) in the MgO [27,8]. The emission peak at 407 nm agrees well with the main feature detected in previous photo-luminescence experiments on cube-shaped MgO nanocrystals and MgO films [31,32]. The various structural defects in their crystalline structure and high coordinated oxygen anions (4 and 5-coordinated O2− ) are responsible for the presence of almost suppressed shoulder emissions in the fluorescence spectra of the MgO at about 446 nm (2.78 eV) and 475 nm (2.61 eV). It was interesting to see from Figure 3b that fluorescence spectrum of MgO in micellar solution increased drastically in intensity with a wide emission peak centered at 411 nm without appearance of any shoulders. The results show that the fluorescence intensity increased by 50-fold in the presence of 0.1 M HY. The possible optical transitions (absorbance and emission process) observed from the UV–vis absorption and fluorescence spectra are illustrated in Figure 3c. The changes observed in absorption and emission may be because of extrinsic point defects which are created from the hydrophilic interactions between the surface of MgO nanorods with HY micelles. These results are in agreement with previous observations revealing that the origin of fluorescence emission can be attributed to the existence of oxygen vacancies/intrinsic or extrinsic point defects on the metal oxide surface [33,34]. Since MgO has a large surface area and binds surfactants. This property, along with its interesting photophysical characteristics, led us to evaluate MgO as a probe to determine the CMC of HY. Due to the presence of hydroxyl group on the surface of head portion of the surfactant, strong hydrophilic interaction with MgO was exhibited leading to the change of the surface as well as the optical properties of the MgO. The impact of HY on the absorbance and the fluorescence properties of MgO is shown in Figures 4 and 5 which is the preliminary criterion of the determination of CMC. Figures 4a and 5a represents the absorbance and emission spectra of MgO in different concentrations of HY. On increasing the concentration of HY, the absorbance values increases with a marginal blue shift from 303 to 299 nm in the absorption maxima and about
2 nm red shift in the emission maxima. As the HY concentration increased the absorbance or emission intensity increase was very small initially and sudden increase occurred as the concentration approached CMC. It means that more MgO are solubilized in micellar pseudo-phase than aqueous monomer pseudo-phase. Constant increase in absorbance or emission intensity was observed above CMC as the concentration increased implying that MgO surface is still unsaturated with HY micelles. Figures 4b and 5b shows the plot of the absorbance or fluorescence intensity as a function of the surfactant concentration. The CMC value was determined from the intersection of two lines drawn through the experimental points for surfactant concentrations below and above the critical surfactant concentration. The obtained values are 9.8 × 10−3 and 8.0 × 10−3 M by absorbance and fluorescence, respectively. For further investigation, electrical conductivity method has been used to study the aggregation behavior of surfactant in the presence of MgO nanoparticles. The changes in specific conductivity were measured during titration of surfactant into aqueous solution at 25 ◦ C, and the CMC value of the surfactant has been estimated at the break point of nearly two straight line portions of the specific conductivity vs concentration plot as shown in Figure 6. The obtained value is 2.7 × 10−3 M. However, the uncertainity in the CMC values are usually due to the limitations of the experimental methodologies. In electrical conductivity method the overall increase in conductivity of surfactant in the studied range is due to conducting nature of charged nanoparticles dispersed in surfactant solution. Whereas, in absorbance and fluorescence methods the surfactant micelles are responsible for the solubilization of the MgO which may intern enhance the absorbance or fluorescence intensity and the onset of the intensity is at the CMC of the surfactant. Comparing the CMC value of HY with that of similar cationic surfactant cetyltrimethyl ammonium bromide (CTAB). It is clear that HY having higher CMC than CTAB. Since both surfactant have similar structures except HY has additional hydroxyl group attached to its ammonium head group. The hydroxyl functional group possesses hydrogen bonding with water molecules. The formation of hydrogen bonding between HY monomers and water molecules makes the micellization process more hard and needs excluding water molecules exist between head groups of monomers. This may be due to the fact that the head group structure of the surfactant is one of the factors that decides the aggregation of the surfactant monomers into a micelle; if so, we would expect the CMC value of HY is higher than that of CTAB
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(7 × 10−4 M) [35]. It is important to note that HY forms transparent colloid with MgO but CTAB does not form. For this reason CTAB is not examined in this study. 4. Conclusions Spectrophotometric approach for estimation of critical micelle concentration (CMC) of HY surfactant with the assistance of MgO nanostructure has been developed. MgO exhibited enhanced absorbance and fluorescence in presence of HY micelles. The spectra were assigned and explained in terms of electron transitions produced by magnesium and oxygen vacancies of different coordinations. CMC values of HY estimated by absorbance, fluorescence and specific conductivity were found to be 9.8, 8.0 and 2.7 mM, respectively. The obtained CMC values of HY is higher than that of CTAB. References [1] H. Thakuria, B.M. Borah, G. Das, J. Mol. Catal. A: Chem. 2741 (2007) 1. [2] Y.H. Ren, B. Yue, M. Gu, H.Y. He, Materials 3 (2010) 764.
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