Synthesis and characterization of silica-, meso-silica- and their functionalized silicacoated copper oxide nanomaterials Issa M. El-Nahhal, Jamil K. Salem, Sylvia Kuhn, Talaat Hammad, Rolf Hempelmann & Sara Al Bhaisi Journal of Sol-Gel Science and Technology ISSN 0928-0707 J Sol-Gel Sci Technol DOI 10.1007/s10971-016-4034-z
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Author's personal copy J Sol-Gel Sci Technol DOI 10.1007/s10971-016-4034-z
ORIGINAL PAPER: NANO-STRUCTURED MATERIALS (PARTICLES, FIBERS, COLLOIDS, COMPOSITES, ETC.)
Synthesis and characterization of silica-, meso-silica- and their functionalized silica-coated copper oxide nanomaterials Issa M. El-Nahhal1 • Jamil K. Salem1 • Sylvia Kuhn2 • Talaat Hammad1 Rolf Hempelmann2 • Sara Al Bhaisi1
•
Received: 28 December 2015 / Accepted: 29 March 2016 Ó Springer Science+Business Media New York 2016
Abstract Silica- or meso-silica- or silica-meso-silicacoated copper oxide microspheres were prepared based on base hydrolysis of tetraethyl orthosilicate in the presence of CuO and CTAB. Functionalization with amine or thiol organofunctional groups was conducted onto the surface of silica-meso-silica-coated copper oxide microspheres (Scheme 1). The silica-coated CuO composites and their amine- or thiol-functionalized materials have been characterized by TEM, XRD, TGA, FTIR and UV/Vis. TEM analysis showed that the CuO nanoparticles were encapsulated and dispersed into the silica or meso-silica microspheres. XRD analysis indicated that the size of CuO nanoparticles has decreased after coating with silica precursors. TGA and FTIR results indicated that the mesosilica-coated copper oxide materials have been successfully grafted by amine and thiol organofunctional groups.
Graphical Abstract
Keywords CuO nanostructure Synthesis of copper oxide nanoparticles Silica-coated CuO & Issa M. El-Nahhal
[email protected] 1
2
Department of Chemistry, Al-Azhar University, P O Box 1277, Gaza, Palestine Physical Chemistry, Saarland University, 66123 Saarbrucken, Germany
1 Introduction In recent years, nanoscale metal oxides have attracted a great deal of research interest because of both fundamental and technological point of view. Among all these metal
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Scheme 1 Description of formation of silica-coated or encapsulation of metal oxide nanomaterials
oxides, cupric oxide (CuO) has attracted considerable attention because of its peculiar properties. CuO has been used as a basic material in cuprate high-TC superconductors, and the superconductivity in these classes of systems is associated with Cu–O bonding [1, 2]. Apart from this, CuO has investigated as potential material for nanofluid in heat transfer applications [3, 4], catalysts for the water–gas shift reaction [5], steam reforming [6], CO oxidation of automobile exhaust gases [7] and photocathodes for photoelectrochemical water splitting application [8]. For technological applications, the detailed understanding of size and morphology controlled emergence of different properties is important. The synthesis procedure plays crucial role in controlling size, shape of the nanostructure and hence detecting different properties of the material. CuO nanoparticles have been prepared by wet-chemistry route [9], sonochemical preparation [10], alkoxide-based preparation [11], hydrothermal process [12] and solid-state reaction in the presence of a surfactant [13]. Silica-coated structured metal oxide nanomaterials have recently been used to improve their stability and their dispersibility in suspensions, and therefore, they can be used for a wide range of applications [14–17]. Silica-based coating precursors are of particular interest because it has good environmental stability with different materials, ease of surface modification and reduce potential for photocatalysis and formation of free radicals [18]. On the other hand,
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due to the high surface energy and large surface area, CuO NPs tend to aggregate easily. Therefore, the surface of CuO needs to be modified for a better dispersion. More recently, metal oxides nanoparticles are incorporated into sol–gel matrices. These novel materials exhibit important optical, catalyst and other properties. There are two strategies: First strategy is concerned with insertion of metal oxides into silica precursors, which have been recently developed, by either impregnation or co-condensation methods [19–21]; and the second strategy is concerned with coating the metal oxide nanoparticles using the sol–gel method [18, 22–24]. The introduction of silica coating on CuO nanomaterial is substantially rather difficult, due to its high surface energy activity and its large surface area, and therefore, copper oxide nanoparticles could be easily agglomerated. However, only few articles have been devoted to preparation of silica-coated copper oxide composites in which CuO nanoparticles of different concentrations are imbedded into the host matrices [25, 26]. In our present research, we had adopted the second strategy, where CuO nanoparticles were firstly prepared by the co-precipitation method [27] followed by sol–gel coating process. The mesoporous layer can be deposited either directly onto the copper oxide cores or the copper oxide crystals can be precoated with a dense silica layer via a modified Sto¨ber method [18] followed by coating with mesoporous layer. The thus formed solid silica layer is used to protect the underlying copper oxide from acidic environments. The formation of mesoporous silica layer is useful for modification by amine and thiol silane functional groups. Six silica-, meso-silica- and their functionalized silica-coated copper oxide nanocomposites are prepared in a similar method [18, 28]. Free silica-coated materials are obtained by etching the CuO nanoparticles using HCl [29]. Several methods and techniques were used for structural characterization of these new materials. These methods include X-ray diffraction (XRD), transition electron microscopy with energy-dispersive X-ray spectrometer (TEM–EDX), Fourier transform spectroscopy (FTIR) and thermal analysis (TGA). TEM and XRD are used to provide information about morphology and optical properties of pure and silica-coated copper oxide nanomaterials. FTIR and TGA analysis are used to examine the surface ligand containing groups.
2 Materials and methods 2.1 Materials All chemicals given below were purchased and used as received. Tetraethyl orthosilicate (TEOS), cetyltrimethylammonium bromide (CTAB) and alkyl hydroxyethyl dimethyl ammonium chloride (HY, R = 12–14) were
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purchased from Merck. The organoalkoxysilanes selected for the functionalization process were 3-aminopropyltrimethoxysilane (APTS, 99 %) and 3-thiolpropyltrimethoxysilane (TPTS, 99 %), and these reagents were purchased from Aldrich company. Toluene and ethanol (spectroscopic grade) were purchased from Aldrich. Copper sulfate pentahydrate and ammonium hydroxide solutions (28 %) were obtained from Merck. 2.2 Synthesis 2.2.1 Synthesis of CuO nanoparticles In typical synthesis of CuO nanopowders [27, 28], 20 mmol of copper sulfate pentahydrate (CuSO45H2O) was dissolved into 25 mL of deionized water. Oxalic acid (20 mmol) was dissolved in an equal volume of deionized water and added dropwise to copper sulfate solution under magnetic stirring for 60 min. Copper oxalate was isolated, washed with water several times and dried at 100 °C for 24 h. The dried material was ground using mortar and
CuSO4.5H2O + C2O4H2
pestle to produce fine powder precursor. Subsequently, the precursor, copper oxalate was annealed in muffle furnace under air at 500 °C for 4 h to form CuO nanostructure (Scheme 2). Agglomeration of CuO nanoparticles is confirmed by TEM results (Table 1). 2.2.2 Synthesis of CuO@SiO2 microspheres The silica-coated copper oxide microspheres labeled as CuO@SiO2 were prepared in similar reported method [18, 28] through a simple sol–gel process. Briefly, 0.10 g of CuO particles was dispersed in a mixture of ethanol (40 mL), deionized water (10 mL) and concentrated ammonia solution (28 wt%, 1.2 mL) by ultrasonication for 1 h. To the above mixture, 0.43 mL of TEOS was added dropwise. After stirring for 6 h, the product was collected and washed with ethanol and deionized water. The material was dried under vacuum at 60 °C for 8 h (Scheme 1). Silica microspheres are flower-like of diameter 270 nm in which copper oxide nanoparticles of an average diameter 34 nm are dispersed (Tables 1, 2).
→ (C2O4)Cu calcinations → CuO nanoparticles
Scheme 2 Formation of CuO nanoparticles
Table 1 Experimental data Material
Synthesis description
Notes
CuO
CuSO4 ? H2C2O4 ? Cu (C2O4) calcinations ? CuO
Agglomeration of CuO nanoparticles in hexagona-like shape is formed
CuO@SiO2
CuO ? TEOS ? NH4OH ? sonication 1 h, annealed at 500 °C for 4 h
Overlapping of microspheres forming flower-like shape, 270 nm each, where CuO NPs (5–30 nm) are unevenly dispersed
CuO@mSiO2
CuO ? TEOS ? NH4OH ? CTAB ? sonication 1 h, annealed at 500 °C for 4 h
A composite of a wormlike m-silica is formed, where dark CuO NPs of different sizes are dispersed into its mesopores
CuO@SiO2@mSiO2
CuO@SiO2 ? TEOS ? NH4OH ? CTAB ? sonication for 1 h?
Microspheres of two layers (solid silica sphere, 270 nm ? mesosphere, 45 nm thickness). Dark diluted CuO NPs of size 5–30 nm are dispersed into silica spheres
CuO@mSiO2–SH
CuO@mSiO2 ? thiol silane, reflux in dry toluene at 110 °C?
CuO@SiO2@mSiO2– NH2
CuO@SiO2@mSiO2 ? amino silane, reflux in dry toluene at 110 °C?
Three layers (solid silica 260 nm ? m-silica 45–50 nm ? functionalizedsilica), CuO NPs of different sizes are dispersed into inner solid silica sphere
CuO@SiO2@mSiO2– SH
CuO@SiO2@mSiO2 ? thiol silane, reflux in dry toluene at 110 °C?
Silica ? m-silica ? functionalized silica micros layers of 144 nm diameter
CuO free @SiO2
CuO@SiO2 ? 6MHCl?
Overlapping of microspheres with the absence of CuO trapped inside the microsphere after treatment with HCl
CuO@SiO2–NH2
CuO ? amino silane, reflux in dry toluene at 110 °C?
Thin layer of coated functionalized silica (12 nm)
CuO–NH2
CuO nanoparticles ? amine silane ? reflux in dry toluene at 110 °C?
Dark hexagonal CuO NPs which are presumably coated by amine-functionalized silica thin layer
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Author's personal copy J Sol-Gel Sci Technol Table 2 Mean crystallite size of CuO and its silica-coated composites Material
Mean crystallite size (nm)
CuO pure
47
CuO@SiO2
34
CuO@SiO2@mSiO2 CuO@SiO2@mSiO2–SH
28 28
CuO@SiO2@mSiO2–NH2
23
CuO @mSiO2
28
CuO@mSiO2–SH
31
CuO–NH2
38
2.2.3 Synthesis of copper oxide-free silica microspheres Copper oxide-free silica microspheres were obtained by treating 0.5 g copper oxide-coated silica precursors with 20 mL 2M hydrochloric acid with continuous stirring (Scheme 1). The copper oxide-free silica materials were separated and washed with distilled water. The product was then dried in vacuum at 60 °C for 8 h. Flower-like silica microspheres of diameter 270 nm with no copper oxide nanoparticles are obtained (Table 1).
stirring for 6 h. Afterward, 0.43 mL of TEOS was added dropwise under mechanical agitation for further 6 h, and the obtained particles were separated by centrifugation at 4000 rpm and washed with deionized water. The product was dried at 100 °C for 6 h and then calcinated at 500 °C for 3 h to remove CTAB (Scheme 1). Two-layer microspheres of diameter 360 nm and solid silica sphere (270) nm covered by mesoporous sphere of 45 nm are formed (Tables 1, 2). 2.2.6 Synthesis of amine or thiol functionalization of CuO– SH(NH2), CuO@mSiO2–SH(NH2) and CuO@SiO2@mSiO2–SH(NH2) Amine or thiol functionalization of CuO or CuO@mSiO2 or CuO@SiO2@mSiO2 microspheres was prepared as previously described [18, 28] by dispersing 1.0 g of CuO, CuO@mSiO2, CuO@mSiO2 or CuO@SiO2@mSiO2 with the appropriate amount (0.37 g, 0.002 mol) of 3-thiolpropyltrimethoxy silane or 3-aminepropyltrimethoxy silane coupling agent in 20 mL of dry toluene. The mixture was refluxed for 24 h at 110 °C. The materials were filtered off, washed with ethanol and dried in vacuum at 80 °C (Scheme 1). Microsphere of diameter 357 nm is formed with three layers (solid silica sphere 271 nm, mesoporous sphere layer (43 nm thickness) and thin layer of functionalized sphere (Table 1).
2.2.4 Synthesis of CuO@mSiO2 composite 2.3 Characterization m-Silica-coated copper oxide composite labeled as CuO@mSiO2 was prepared in similar way as reported before [18, 24] by dispersing 0.10 g of CuO nanoparticles in 60 mL ethanol and 1.2 mL concentrated ammonia solution (28 wt%) and then ultrasonicated for 1 h. An ethanolic solution of 0.30 g CTAB was added to the copper oxide nanoparticles mixture under constant stirring at room temperature. To the solution, 0.43 mL of TEOS was then added dropwise under constant stirring for further 6 h. The product was separated by centrifugation at 4000 rpm and washed with deionized water. Finally, the product was dried at 100 °C for 12 h and calcinated at 500 °C for 3 h (Scheme 1). A wormlike m-silica-coated CuO composite with particle size 28 nm is dispersed into mesopores (Tables 1, 2).
Infrared spectra for the materials were recorded on a PerkinElmer FTIR spectrometer using KBr disk in the range 4000–400 cm-1. Thermogravimetric analysis (TGA) was carried out using Mettler Toledo TGA/SDTA 851e analyzer in the range of 25–600 °C of heat rate of 10 °C/min. The system was purged with nitrogen using a flow rate of 50 mL/min. The X-ray diffraction (XRD) patterns of the dried powder samples were obtained using an X-ray diffractometer with Cu K (0.154 nm wavelength) under 40 kV and 200 mA. The TEM analysis was performed with JEM2010 (JEOL) transmission electron microscope with energy-dispersive X-ray spectrometer INCA (Oxford Instruments).
3 Results and discussion 2.2.5 Synthesis of CuO@SiO2@mSiO2 microspheres 3.1 Synthesis Two-layer microspheres labeled hereafter as CuO@SiO2@mSiO2 composite were prepared in a similar method described [18, 28] by dispersing 0.10 g of CuO@SiO2 in 60 mL ethanol and 1.2M concentrated ammonia solution (28 wt%), and then the mixture was ultrasonicated for 1 h. An ethanolic solution containing 0.30 g CTAB was added to the mixture under constant
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Schemes 1 and 2 represent the schematic views of the synthesis of copper oxide nanoparticles and its silica-, m-silica-, silica/m-silica-coated composites and their amine or thiol functionalization. Copper oxide nanoparticles were prepared by the co-precipitation method [27, 28] by the reaction of copper sulfate pentahydrate with oxalic acid,
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3.2 FTIR spectra The FTIR spectra of copper oxide, silica-coated copper oxide, m-silica-coated copper oxide, silica/meso-silicacoated copper oxide microspheres and its amine-
functionalized materials are given in Fig. 1, respectively. Three regions of absorptions at 3300–3650, 1500–1700 and 500–1200 cm-1 are observed due to m(O–H), d(O–H) and m(Si–O–Si) or m(Cu–O–Cu) or m(Cu–O) vibrations, respectively [23, 25, 26, 28]. The FTIR spectrum of CuO (Fig. 1a) shows a strong absorption at 1114 cm-1 due to bridging (Cu–O–Cu) vibration due to the formation of nanocluster of CuO. The vibration band at 480 cm-1 for the sample can be attributed to the vibrations of m(Cu–O) bond, confirming the formation of CuO nanoparticles [26]. The three peaks at 3562 cm-1 (sharp), 3478 cm-1 (sharp) and 3253 cm-1 (broad) are associated with free m(O–H) and hydrogen bonding m(O–H) of CuO crystallizing water molecules, respectively. The absorption peak at 1624 cm-1 is assigned for the bending vibration of OH bond. The diminution of the m(O–H) vibrations at 3200–3550 cm-1 for CuO after coating with silica (Fig. 1b–e) provides evidence for the incorporation of silica into CuO particles and formation of Si–CuO [23]. The disappearance of peaks at 2980 and 1484 cm-1 upon calcinations gives clear evidence that CTAB was totally removed (Fig. 1d) [18, 28]. The presence of Si–O–Si stretching vibration bands at 960–1120 and 530 cm-1 is observed only for the silicacoated materials, which provides strong evidence for the silica-coated shells. The absorption band at 480 cm-1 for CuO and CuO-coated silica is for m(Cu–O), and small peak at 530 cm-1 is probably due to d(Si–O–Si) vibration. The FTIR spectrum of amine-functionalized copper oxide (Fig. 1e) shows the absorption bands at 2930 and 1560 cm-1 due to the m(C–H) of aliphatic hydrocarbons and d(N–H) of amine group which provide evidence for the introduction of the organofunctional ligand groups onto the meso-silica-coated layer. These assignments were based on IR spectral data of similar systems [18, 25, 26, 28].
(e) (d) 200
(c)
(b) 100 580 530 480
Transmittance (%T)
and then the metal oxides were obtained by calcinations at 500 °C (Scheme 2). CuO NPs were then dispersed in water/ ethanol by ultrasonication for 1 h prior to sol–gel coating through hydrolysis and subsequent polycondensation in the presence of NH4OH [29]. The ultrasonication in basic media is to secure dispersion of CuO particles into solution to be stabilized by silica coating. The use of CTAB as cationic surfactant has two functions: It acts as coupling agent to incorporate with copper oxide nanoparticles to obtain well homogeneous dispersion and prevent aggregations of copper oxide nanoparticles; and the second function is to form wormlike m-silica [14, 23]. In the sol–gel method, TEOS acts as a silica coating precursor and NH4OH acts as catalyst. In this basic medium, the surface of the CuO NPs was probably activated [23], and the TEOS silane precursor undergoes hydrolysis and polycondensation process to establish CuO–Si–O– linkages onto the nanoparticle surface [23]. This leads to formation of CuO@SiO2 or CuO@mSiO2 or CuO@SiO2@mSiO2 composites (Scheme 1) in which the copper oxide nanoparticles of (5–30 nm) are dispersed into the silica or m-silica. The CuO nanoparticles were probably encapsulated into the silica microspheres or inserted into the mesopores of m-silica as confirmed by TEM results discussed later. Synthesis of CuO@SiO2@mSiO2 two-layer microspheres of diameter of 360 nm was obtained by treating CuO@SiO2 microspheres with concentrated ammonia solution (28 wt%) and CTAB and TEOS (Scheme 1) as confirmed from TEM results. The nanoparticles of CuO were only embedded into the silica shell without appearance of copper oxide nanoparticles in the m-silica shell as confirmed by TEM discussed later. Functionalization of CuO or CuO@SiO2@mSiO2 or CuO@mSiO2 nanomaterials was prepared by treating CuO nanoparticles or their silica-coated materials with 3-aminopropyltrimethoxysilane (APTMS) or 3-thiolpropyltrimethoxysilane (TPTMS) in dry toluene at 110 °C (Scheme 1) [24]. It is found that CuO NPs can be functionalized forming very thin shell of functionalized silica (5 nm). Copper oxide-free spheres of silica-coated material and copper oxide-free spheres of functionalized silica-coated material are obtained easily by treating silica-coated copper oxide or functionalized silica-coated copper oxide with hydrochloric acid (Scheme 1). This was confirmed by TEM–EDX, which confirms the absence of copper contents after treatment with HCl acid. All experimental data are summarized in Table 1.
(a)
4000
3500
3000
2500
2000
1500
1000
500
wavenumber (cm-1)
Fig. 1 FTIR spectra of a CuO, b CuO@SiO2, c CuO@CTAB/mSiO2, d CuO@mSiO2, e CuO@S@mSiO2–NH2
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3.3 XRD results
(d)
140000
Intensity (a.u.)
120000 (c) (111) (111)
100000 80000
(202) (020)
(b)(110)
(202)
(113)
(311) (022) (113)
60000 40000 20000
(a) 0 -20000 30
40
50
60
70
2θ (Degrees)
Fig. 2 XRD patterns of a pure CuO, b CuO@SiO2, c CuO@mSiO2, d CuO@SiO2@mSiO2
(c)
Intensity (a.u.)
20000 (111) (111) (202)
(b)
(113) (020)
(110)
(202)
(311) (113) (022)
0
(a)
30
40
50
60
70
2θ (Degrees)
Fig. 3 XRD patterns of a CuO@SiO2@mSiO2, b CuO@SiO2@ mSiO2–NH2, c CuO@SiO2@mSiO2–SH
Fig. 4 TEM image of CuO NPs and their TEM/EDAX spectra
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The XRD patterns for CuO, CuO@SiO2, CuO@mSiO2 and CuO@SiO2@mSiO2 are presented in Fig. 2a–d, respectively. The crystal structure parameters obtained show that all the diffraction peaks with lattice planes (110, -111, 111, -202, 020, 202, -113, 022, -311, 113) can be indexed as monoclinic phase of CuO (JCPDS 05-0661) [22, 30]. Diffraction peaks corresponding to the impurity were not found in the XRD patterns, confirming high purity of the synthesized products. The mean crystallite size of CuO particles was determined by Sherrer’s equation [where D = 0.89k/b cosh where D is the crystallite size (nm), k is the wavelength of incident X-ray (nm), b is the full width at half maximum and h is the diffraction angle]. The mean crystallite size results are given in Table 2. There was a decrease in the crystallite size from 47 nm for pure CuO to an average of 27 nm after coating with silica precursors (Table 2) [28]. The decrease in the particle size could be ascribed to partial dissolution or breaking nanoparticles during the silica covering in the sol–gel process. Comparing the XRD pattern of CuO (Fig. 2a) with those of silica-coated CuO materials (Fig. 2b–d), it showed that there was no shift of diffraction angle of all peaks, but there are a decrease in intensity and a slight line broadening which is probably associated with slight change in particle size. There is no change in either the diffraction angle or line broadening upon functionalization of CuO@SiO2@mSiO2 with amine or thiol silane precursors CuO@SiO2@mSiO2– (SH)NH2 as shown in Fig. 3. This means that coating with thiol and amine silane precursors does not alter the morphology of copper oxide nanostructure.
Author's personal copy J Sol-Gel Sci Technol Fig. 5 TEM image of CuO@SiO2 microspheres and their TEM/EDAX spectra
Fig. 6 TEM image of CuO@SiO2 microspheres and their TEM/EDAX spectra (treated with HCl)
Fig. 7 TEM image of CuO@mSiO2 composite and its TEM/EDAX spectra
3.4 TEM results TEM image along with EDAX of CuO is shown in Fig. 4. TEM image of CuO shows agglomeration of CuO nanoparticles due to their high surface energy activity. An agglomeration of nanoscale particles is clearly observed,
showing a uniform distribution of particle size and a homogeneous morphology [31]. TEM–EDAX of CuO shows the presence of Cu and O compound and the absence of Si peak (Fig. 4). Figure 5 shows the TEM image of CuO@SiO2. It shows overlapping of silica microspheres forming a flower-like shape (diameter of 270 nm), in which
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Author's personal copy J Sol-Gel Sci Technol Fig. 8 TEM image of CuO@SiO2@mSiO2 microspheres and their TEM/ EDAX spectra
Fig. 9 TEM image of CuO@SiO2mSiO2–NH2 microsphere and its TEM/ EDAX spectra
Fig. 10 TEM images of CuO– NH2 composite and its TEM/ EDAX image spectra
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3.5 TGA analysis Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA) for CuO@SiO2@mSiO2 and
(a) 0.000 -0.005 10
Weight(mg)
-0.010 -0.015 -0.020
-0.025 -0.030 8 0
100
200
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Temperature(°C)
(b)
0.002 0.000
-0.002 10
Weight(mg)
-0.004 -0.006 -0.008 -0.010 8
-0.012 0
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Temperature(°C)
(c) 18
0.005 0.000 -0.005
16
Weight(mg)
CuO NPs (dark color) of size range 5–30 nm are dispersed unevenly into silica spheres (gray color) [14]. The TEM– EDAX analysis indicates the presence of a silicate amorphous material (SiO2), and it showed peaks correspond to Si, O and Cu components. Figure 6 shows the TEM image of free CuO@SiO2 microspheres with the absence of CuO NPs trapped inside the microsphere after treatment with HCl, indicating complete removal of CuO particles. The TEM/EDAX analysis shows only two main peaks consisting of Si and O components with no peaks of copper. Figure 7 exhibits TEM image of CuO@mSiO2 composite, and it shows a wormlike shape mesoporous silica in gray color, which dark color CuO NPs are inserted into its mesopores. EDAX showed peaks corresponding to Si, O and Cu. This confirmed that copper oxide particles are encapsulated into the m-silica pores. TEM image of CuO@SiO2@mSiO2 microspheres showed two shells, a solid silica shell of 270 nm diameter and a m-silica shell of 45 nm thickness which covered the silica microsphere (Fig. 8), and the whole microsphere diameter is 360 nm (0.36 lm), which is composed of a hard silica sphere (270 nm) covered by mesoporous layer of 45 nm thickness. The CuO particles with size range (5–30 nm) are well dispersed into silica microspheres. TEM–EDAX showed three main peaks of Si, O and small peak of Cu. This may explain that CuO is diluted and dispersed into silica microsphere. TEM image of CuO@SiO2@mSiO2–NH2 showed microspheres of the diameter of 360 nm (Fig. 9), and it is composed of multi-shells, a hard silica sphere of 260 nm diameter, covered by m-silica shell of 45–50 nm thickness and very thin amine-functionalized shell not seen properly (Fig. 9). CuO particles are presumably dispersed into the inner silica shell which covered by m-silica and very thin amine-functionalized silica layers (Fig. 9). TEM–EDAX showed several peaks of high-intensity peaks of Si and O and low-intensity peaks of Cu. Figure 10 shows two TEM images of CuO–NH2. It shows a dark hexagonal CuO NPs which are presumably covered by amine-functionalized silica thin layer of 4–5 nm thickness as shown in white strips. The aminefunctionalized silica layer is about 23 nm thickness as indicated by arrows. This is due to silica layer covering the agglomerated copper oxide. A hexagonal CuO particle size of 30 nm was covered by a thin layer of 4–5 nm thickness of functionalized silica around hexagonal shape of CuO (Fig. 10). Four components, Si, O, Cu, and N and C, were detected in TEM–EDAX analysis.
-0.010 -0.015
14
-0.020 -0.025 12 -0.030 0
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Fig. 11 TG/DTA analysis of a CuO@SiO2@mSiO2, b CuO@SiO2@ mSiO2–NH2, c CuO @SiO2@mSiO2–SH
their amine (CuO@SiO2@mSiO2–NH2 and thiol CuO@SiO2@mSiO2–SH)-functionalized nanoparticles were examined under nitrogen atmosphere at 20–600 °C (Fig. 11). The thermogram of CuO@SiO2@mSiO2 (Fig. 11a) showed
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two stages of total weight loss ca 15.6 % of its initial weight: The first stage at 75 °C is attributed to the evaporation of physically absorbed water or alcohol from silica-coated CuO porous materials and a second stage (broad peak) is at [250 °C due to dehydroxylation of silanols and formation of siloxane bonds [32]. The thermogram of CuO@SiO2@ mSiO2–NH2 (Fig. 11b) showed four stages of total weight loss ca 21.6 % of its initial weight. The extra two peaks are observed: One is observed at 120 °C, which is related to adsorption of CO2 onto the amine groups [33], and the second is observed at 380 °C, which is related to the decomposition of and the removal of functional propylamine precursor [33]. The thermogram of CuO@SiO2@mSiO2–SH (Fig. 11c) showed two stages of total weight loss ca 27.5 % of its initial weight at 70 and 331 °C. The extra peak at 331 °C is attributed to the decomposition of the thiol functional groups [33].
4 Conclusion Silica-coated CuO microspheres were synthesized by modified Sto¨ber method. A wormlike mesoporous silicacoated copper oxide composite is directly obtained in the presence of CTAB as cationic surfactant. TEM analysis has confirmed that CuO nanoparticles are encapsulated into silica microspheres or inserted into the mesopores of the mesoporous silica shell. XRD results showed a decrease in CuO particle size due to its encapsulation into silica precursors and due to the dissolution of agglomerated particles during the incapsulation process. TGA and FTIR spectra revealed that the amine and thiol organofunctional groups are covalently attached to the m-silica layer. Functionalization with amine or thiol silane coupling agent is successfully grafted directly onto the surface of copper oxide or onto the surface of meso-silica shells. TEM analysis has confirmed that three silica-coated shells (silica shell of 270 nm diameter, m-silica shell of 45 nm thickness and a functionalized silica shell of about 4–5 nm thickness) covered copper oxide nanoparticles are formed. Copper oxidefree silica materials have low-density silica spheres which showed no XRD peaks due to complete etching of core CuO. These materials could be tested for removal of toxic heavy metals and dyes which is proposed for further research.
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