Synthesis & characterization of silica coated and

Powder Technology 287 (2016) 439–446

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Synthesis & characterization of silica coated and functionalized silica coated zinc oxide nanomaterials Issa M. El-Nahhal a,⁎, Jamil K. Salem a, Sylvia Kuhn b, Talaat Hammad a, Rolf Hempelmann b, Sara Al Bhaisi a a b

Department of Chemistry, Al Azhar University of Gaza, PO box 1277, Palestine Physical Chemistry, Saarland University, D-66123 Saarbrucken, Germany

a r t i c l e

i n f o

Article history: Received 5 May 2015 Received in revised form 26 September 2015 Accepted 29 September 2015 Available online xxxx Keywords: ZnO nanostructure Synthesis of zinc oxide nanoparticles Silica coated ZnO X-ray diffraction of coated ZnO Microstructure of silica coated ZnO Optical properties zinc oxide nanoparticles

a b s t r a c t Silica or meso silica or silica-meso silica coated zinc oxide microspheres were prepared based on base hydrolysis of tetraethylorthosilicate (TEOS) in the presence of CTAB. Functionalization with amine or thiol organofunctional groups were conducted onto pure zinc oxide, silica, meso silica or silica-meso silica coated zinc oxide microsphere. The silica coated ZnO composites and their amine or thiol functionalized materials have been characterized by modern techniques, TEM, XRD, TGA, 13C NMR, FTIR, and UV/VIS spectroscopy. TEM analysis showed that the ZnO nanoparticles were encapsulated and dispersed into the silica or meso silica microspheres. XRD analysis indicated that the size of ZnO nanoparticles before and after coating with silica has been maintained almost unchanged. CP/MAS 13C NMR and FTIR spectra indicated that the coated meso silica zinc oxide materials have been successfully grafted by amine and thiol organofunctional groups. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Zinc oxide is an inorganic multifunctional material with very useful properties, it is used in various applications in catalysis, in cosmetics as UV absorber, in biomedical applications, in chemical sensors, electroluminescent and photochemical properties [1–12]. Zinc oxide nano materials can be prepared by several methods: hydrothermal methods [13], electrochemical depositions [14], sol–gel method [15], chemical vapor deposition [16], thermal decomposition [17], and combustion method [18,19]. Recently, ZnO nanoparticles were prepared by ultrasound [20] and co-precipitation [21]. Silica coated structured ZnO nanomaterials have recently been used to improve their stability and their dispersibility in suspensions [22] and therefore they can be used for wide range of applications [22–25]. Silica-based coatings are of particular interest because they have good environmental stability with different materials [26], ease of surface modification [27] and reduce potential for photocatalysis and formation of free radicals [28]. Silica coating of metal oxide nanoparticles has been widely studied such as Fe2O3, TiO2 and others [26,29,30]. However, the introduction of silica coating on ZnO nanomaterial is substantially rather difficult, due to its high surface energy activity and its large surface area and therefore

⁎ Corresponding author. E-mail address: [email protected] (I.M. El-Nahhal).

http://dx.doi.org/10.1016/j.powtec.2015.09.042 0032-5910/© 2015 Elsevier B.V. All rights reserved.

zinc oxide nanoparticles could be easily aggregated. There were some reported articles in which dispersant agents were used as coupling agents for the fabrication of silica coated ZnO nanoparticles [23,31]. Two main strategies were recently applied for the fabrication of silica coating of zinc oxide nanoparticles using modified Stöber method. The first strategy is to incorporate silane precursors during the growth process of zinc oxide as coating agent [32,33], the second strategy is to use sol–gel process for coating of previously prepared zinc oxide nanoparticles [33]. In our research article, we used the second strategy, where ZnO nanoparticles were firstly ultrasonicated in an ethanolic solution containing base in order to decrease aggregation and activate ZnO nanoparticles, followed by sol–gel coating process. CTAB was used as a coupling agent to incorporate with zinc oxide nanoparticles and also in formation of meso silica microspheres. Nine different silica or meso silica and their functionalized silica coated ZnO microspheres are prepared. Several techniques are used for structural determination. 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), CP/MAS 13C nuclear magnetic resonance (NMR) spectra, Fourier transform infrared spectroscopy (FTIR), Ultra violet-visible spectra(UV/VIS) and thermal analysis (TGA). TEM, XRD and UV/VIS are used to provide information about morphology and optical properties of pure and silica coated zinc oxide nanomaterials. 13C NMR, FTIR and TGA analyses are used to examine the surface ligand containing groups.

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2. Materials and methods

Finally the product was dried at 100 °C for 12 h and calcinated at 500 °C for 3 h.

2.1. Materials Tetraethylorthosilicate (TEOS), cetyltrimethylammonium bromide (CTAB) alkyl hydroxyethyl dimethyl ammonium chloride (HY, R = 12–14) were purchased from Merck and used as received. The organoalkoxysilanes selected for the functionalization process were 3-aminopropyltrimethoxysilane (APTS, 99%) and 3thiolpropyltrimethoxysilane (TPTS, 99%), these reagents were purchased from Aldrich company and used without further purification. Toluene and ethanol (spectroscopic grade) were purchased from Aldrich company and were used as received. Zinc sulfate pentahydrate and ammonium hydroxide solutions (28%) were obtained from Merck. 2.2. Methodology Infrared spectra for the materials were recorded on a Perkin–Elmer FTIR spectrometer using KBr disk in the range of 4000 to 400 cm− 1. Thermogravimetric analysis (TGA) was carried out using Mettler Toledo TGA/SDTA851e analyzer in the range of 25–600 °C of heat rate 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 as-prepared 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). UV/VIS spectra were obtained on a Shimadzu1601 UV/VIS spectrophotometer in a quartz cell with optical length 1 cm in the range of 700–200 nm. 2.3. Synthesis 2.3.1. Synthesis of ZnO nanoparticles In typical synthesis of ZnO nanopowders [21], (20 mmol) of zinc sulfate pentahydrate ZnSO4.7H2O 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 zinc sulfate solution under magnetic stirring for 60 min, white precipitate of zinc 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, zinc oxalate was annealed in muffle furnace under air at 500 °C for 4 h to form ZnO nanostructure. 2.3.2. Synthesis of ZnO@SiO2 microspheres The coated silica zinc oxide microspheres labeled as ZnO@SiO2 were prepared in similar reported method [26] through a simple sol–gel process. Briefly, 0.10 g of ZnO particles were 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 solution, 0.43 mL of tetraethylorthosilicate (TEOS) was added dropwise. After stirring for 6 h, the product was collected and washed with ethanol and deionized water. The product was dried under vacuum at 60 °C for 8 h. 2.3.3. Synthesis of ZnO@mSiO2 microspheres Coated meso silica zinc oxide microspheres labeled as ZnO@mSiO2 were prepared by dispersed 0.10 g of ZnO nanoparticles in 60 mL ethanol and 1.2 mL concentrated ammonia solution (28 wt.%), then ultrasonicated for 1 h. An ethanolic solution of 0.30 g CTAB was added to the zinc 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 centrifuge of 4000 rpm and washed with deionized water.

2.3.4. Synthesis of ZnO@SiO2@mSiO2 microspheres A two layer microspheres labeled hereafter as ZnO@SiO2@mSiO2 composite was prepared in a similar method described [26] by dispersed 0.10 g of ZnO@SiO2 in 60 mL ethanol and 1.2 M concentrated ammonia solution (28 wt.%), then the mixture was ultrasonicated for 1 h. An ethanolic solution containing 0.3 g CTAB was added to the mixture under constant stirring for 6 h. Afterwards, 0.43 mL of TEOS was added dropwise under mechanical agitation for further 6 h, the obtained particles were separated by centrifuge at 4000 rpm and washed with deionized water. Finally the product was dried at 100 °C, then calcinated at 500 °C for 3 h. Silica spheres free of zinc oxide were obtained by treating ZnO@SiO2@mSiO2 materials with 2 M HCl, washed with excess of water and dried in vacuum at 80 °C overnight. 2.3.5. Synthesis of amine-functionalization of ZnO–NH2, ZnO@mSiO2–NH2, ZnO@SiO2@mSiO2–NH2 Amine-functionalized of ZnO or ZnO@mSiO2 microspheres were prepared as previously described [26] by disperse 1.0 g of pure ZnO, ZnO@mSiO2, ZnO@mSiO2 or ZnO@SiO2@mSiO2 with the appropriate amount (0.37 g, 0.002 mol) of 3-aminepropyltrimethoxy silane coupling agent in 20 ml of dry toluene. The mixture was refluxed for 24 h at 110 °C. The material was filtered off washed with ethanol and dried in vacuum at 80 °C. 2.3.6. Synthesis of thiol-functionalization ZnO@mSiO2–SH and ZnO@SiO2@mSiO2–SH Thiol-functionalized mesoporous materials were prepared by dispersed 0.10 g of the ZnO@mSiO2 or ZnO@SiO2@mSiO2 and the appropriate amount (0.37 g, 0.0019 mol) of 3-thiolpropyltrimethoxy silane coupling agent in 20 ml of dry toluene. The mixture was refluxed for 24 h at 110 °C. The material was filtered off washed with ethanol and dried in vacuum at 80 °C. 2.3.7. Synthesis of zinc oxide free silica microspheres Zinc oxide free silica spheres were obtained by treating 0.5 g ZnO@ mSiO2, ZnO@SiO2@mSiO2, ZnO@SiO2@mSiO2–NH2 with 20 mL 2 M hydrochloric acid with continuous stirring. Zinc oxide free silica materials were separated, washed with distilled water and dried in vacuum at 60 ° C for 8 h. 3. Results and discussion 3.1. Synthesis The following Schemes 1 & 2 represent the schematic views of the synthesis of zinc oxide nanoparticles and its coating with silica, meso silica coating and finally their functionalization. Zinc oxide nanoparticles were prepared by the co-precipitation method [21] by the reaction of zinc sulfate pentahydrate with oxalic acid, then the metal oxides were obtained by calcinations at 500 °C (Scheme 1). ZnO NPs were then dispersed in water/ethanol by ultrasonication for 1 h prior to TEOS coating through hydrolysis and subsequent polycondensation in the presence of NH4OH [26]. The ultrasonication in basic media is to secure dispersion of ZnO particles into solution to be stabilized by silica coating. The use of CTAB as cationic surfactant has two functions, it acts as a coupling agent to incorporate with zinc oxide nanoparticles to obtain well homogeneous dispersion and prevent

Scheme 1. Formation of ZnO nanoparticles.


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Scheme 2. Descriptive of formation silica coated or encapsulation of metal oxide nanomaterials.

aggregations of zinc oxide nanoparticles, and the second function is to form meso-wormlike silica [22,33]. In the sol–gel method, TEOS acts as a silica coating precursor and NH4OH acts as the catalyst. In this basic medium, the surface of the ZnO NPs was probably activated [33], the TEOS silane precursor undergoes hydrolysis and polycondensation process to establish ZnO–Si–O-linkages onto the nanoparticle surface [26,33]. This leads to formation of ZnO@SiO2, and ZnO@mSiO2 microspheres of diameter 260 nm and 300 nm, respectively. This depends on the path of the synthesis. The ZnO nanoparticles were probably embedded or encapsulated into the silica microspheres or introduced into the mesopores of meso silica microspheres (Scheme 2) as confirmed from TEM results. Synthesis of ZnO@SiO2@mSiO2 two-layer microspheres of diameter over 300 nm were obtained by treating ZnO@SiO2 microspheres with concentrated ammonia solution (28 wt.%) and CTAB and TEOS (Scheme 2) as confirmed from TEM results. The nanoparticles of ZnO were only embedded into the silica shell without appearance of zinc oxide nanoparticles in the meso silica shell as confirmed by TEM discussed later. Functionalization of ZnO or Zn@SiO2@m–SiO2 or Zn@m–SiO2 or Zn@ SiO2 nanomaterials was prepared by treating ZnO nanoparticles or their silica coated materials with 3-aminopropyltrimethoxysilane (APTMS) or 3-thiolpropyltrimethoxysilane (TPTMS) in dry toluene at 110 °C (Scheme 2) [26]. It is found clearly that two shells, (meso and functional silica) are formed around zinc oxide particles. Zinc oxide free spheres of coated silica and zinc oxide free spheres of functionalized silica coated materials are obtained easily by treating silica coated zinc oxide or

functionalized silica coated zinc oxide with hydrochloric acid (Scheme 2). This was confirmed by TEM-EDX which confirms the absence of zinc contents after treatment with HCl acid. The experimental data are summarized in Table 1.

3.2. FTIR spectra Fig. 1a–d presents, the FT-IR spectra of ZnO, ZnO@SiO2, ZnO@SiO2@ mSiO2 and their amine functionalized ZnO@SiO2@mSiO2–NH2, respectively. The peaks at 1550–1600 cm− 1 and 3100–3600 cm− 1 can be attributed to the hydroxyl groups (O–H) and amine (N–H) bending and stretching vibrations, respectively. The Si–O–Si asymmetric stretching vibration band of 980–1190 cm − 1 was found only in the ZnO@SiO2 NPs spectrum compared to the ZnO NPs [33– 35]. The Si–ZnO bond formation was confirmed by the reduction in peak intensity at 450–500 cm− 1 of Si–O–Si. This manifests the existence of a silica layer and successfully grafted over the ZnO NPs. The absorption bands at 2980 and 1560 cm− 1 due to the ν(C–H) of aliphatic hydrocarbons and δ(N–H) (Fig. 1d) respectively as well as the disappearance of the shoulder at 960 cm− 1 due to free silanol hydroxide groups (Si–OH) provide evidence for the introduction of the organofunctional ligand groups onto the coated silica layers. The removal of CTAB was confirmed from the FTIR spectra where the peaks assigned due to CTAB at 2984 and 1475 cm− 1 due to ν(C–H) vibrations are removed.

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Table 1 Experimental data. Material

Synthesis description

ZnO@SiO2

ZnO + TEOS + NH4OH + Microspheres are formed, 260 nm diameter. sonication 1 h, annealed

ZnO@mSiO2

Notes

at 500 °C for 4 h. ZnO + TEOS + NH4OH + Microspheres, wormlike meso silica, 300 nm CTAB + sonication 1 h, diameter, ZnO NPs are annealed at 500 °C for 4 dispersed into mesopores. h.

Microspheres of two layers, N300 nm diameter, ZnO NPs dispersed into silica sonication for 1 h → shell. ZnO@mSiO2 + amine Amine-functionalized and ZnO@mSiO2–NH2 silane, reflux in toluene at meso silica two layers are 110 °C → formed around ZnO. ZnO@mSiO2 + thiol Thiol-functionalized and ZnO@mSiO2–SH silane, reflux in dry meso silica two layers of 13 toluene at 110 °C → and 5 nm thickness ZnO@SiO2@mSiO2–NH2 ZnO@SiO2@mSiO2 + ZnO@SiO2@mSiO2

ZnO@SiO2 + TEOS +

NH4OH + CTAB +

ZnO–NH2

ZnO free @SiO2@mSiO2

ZnO free@mSiO2

amino silane,reflux in dry toluene at 110 °C → ZnO nanoparticles + amine silane → reflux in dry toluene at 110 °C → ZnO@SiO2@mSiO2 + 2 M HCl →

Low density ZnO free silica and meso layers, no Zn content is observed ZnO@mSiO2 + 2 M HCl → Low density ZnO free meso silica layer, no ZnO is present

3.3. 13C CP/MAS NMR results The CP/MAS 13C NMR spectra for amine-functionalized coated silicameso silica zinc oxide and thiol-functionalized coated silica-meso silica zinc oxide are depicted in Fig. 2a & b respectively. The spectrum of amine-functionalized composite shows three methylene carbons at 11, 23 and 43 ppm corresponding to the Si–CH2, C–CH2 and CH2–N. the signal at 165 ppm is probably due to absorbed CO2 [32]. The spectrum of thiol-functionalized composite shows three methylene carbons at 11 and 28 ppm corresponding to the Si–CH2 and the two carbons C– CH2 and CH2–S, respectively. The signal at 129 ppm is probably due to

Fig. 1. FTIR spectra of a) pure ZnO b) ZnO@mSiO2 c) ZnO@SiO2@mSiO2 and d) ZnO@SiO2@ mSiO2–(CH2)3–NH2.

Fig. 2. CP/MAS 13C NMR spectra of ZnO@mSiO2–(CH2)3–NH2 and ZnO@mSiO2–(CH2)3–SH.

the presence of some impurities. These assignments are based on spectral data reported for similar systems [36].

3.4. XRD results The XRD patterns for pure ZnO powder prepared by the co-precipitation method [21], silica, meso silica coated zinc oxide and their amine or thiol functionalized materials are shown in Fig. 3a–i. All the diffraction peaks are well indexed to the hexagonal ZnO wurtzite structure (JCPDS no. 36–1451) [37]. Diffraction peaks corresponding to the impurity were not found in the XRD patterns, confirming high purity of the synthesized products. The mean

Fig. 3. XRD results of: a) ZnO free@SiO2@mSiO2, b) ZnO, c) ZnO@SiO2, d) ZnO@mSiO2, e) ZnO@SiO2@mSiO2, f) ZnO@SiO2@mSiO2–(CH2)3–NH, g) ZnO@mSiO2–(CH2)3–SH, h) ZnO@mSiO2–(CH2)3–NH and i) ZnO free@mSiO2.


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Fig. 4. TEM of a) ZnO, b) TEM-EDX of ZnO NPs, c) low resolution TEM of ZnO@mSiO2, d) high resolution TEM of ZnO@mSiO2, e) TEM of ZnO@SiO2, f) TEM of ZnO@SiO2@mSiO2, g) TEM of ZnO@mSiO2–(CH2)3–SH, h) TEM-EDX of ZnO@mSiO2–(CH2)3–SH, i) TEM of ZnO@mSiO2–(CH2)3–NH2 and j) TEM-EDX of ZnO@mSiO2–(CH2)3–NH2.

crystallite size of ZnO particles was determined by Sherrer's equation (where D = 0.89λ/β cosθ where D is the crystallite size (nm), λ is the wavelength of incident X-ray (nm), β is the full width at half maximum, and θ is the diffraction angle). The obtained particle size was 13 nm for pure ZnO. There was a small shift to lower angle width and change in the line width revealing that silica coating was successfully conducted (Fig. 3b–h). This leads in slight change of the particle size of the coated zinc oxide. The zinc oxide free silica (Fig. 3a & i) showed no XRD peaks, this may confirm that complete removal of zinc oxide nanoparticles form silica coated zinc oxide microspheres, when treated with hydrochloric acid.

3.5. TEM results TEM images along with EDX of pure ZnO, ZnO@SiO2, (ZnO@mSiO2 ZnO@SiO2@mSiO2 are given in Fig. 4a–g. TEM image of pure zinc oxide shows different bar sizes of hexagonal shape of 20–70 nm length and 15–30 nm width (Fig. 4a). The TEM-EDX of ZnO nanomaterial shows only zinc and oxygen peaks with no silicon is present (Fig. 4b). Fig. 4c & d illustrates low and high resolution TEM images of ZnO@mSiO2 microspheres of 300 nm diameter. The meso silica microsphere was evident from the gray wormlike structure with zinc oxide appearing as dark nanoparticles (dark color) which were capsulated into silica

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Fig. 4 (continued).

mesopores as shown in Fig. 4d. The small particles of zinc oxides are probably formed through the ultrasonication process of colloidal solution in the presence of CTAB, so these small particles are fitted into the mesopores of the meso silica [22]. For ZnO@SiO2, it can be seen from

TEM-image Fig. 4e, that roughly microspheres of size 260 nm of silica, in which ZnO nanoparticles are dispersed. EDX confirms the presence of peaks due to components of silicon and oxygen and zinc. In the case of ZnO@SiO2@mSiO2 microspheres, it can be seen from TEM

Fig. 5. Thermograms for (a) ZnO@SiO2@mSiO2 and (b) ZnO@SiO2@mSiO2-NH2.

Fig. 6. UV/VIS spectra for a) ZnO, b) ZnO@SiO2@mSiO2, and c) ZnO@SiO2.


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3.7. UV/VIS spectra The UV/VIS spectra for the solution contain ZnO@SiO2 and ZnO@ SiO2@mSiO2 NPs in HY surfactant Fig. 6(a–c). HY surfactant was used to obtain a homogenous solution of un-coated and coated silica zinc oxide particles. Two peaks at 359, 290 nm are shown in Fig. 6(a–c), that characteristic for the presence of ZnO nanoparticles [22,33]. It is found that there is a decreasing of the absorption band intensities as the number of shells of coated silica is build up on zinc oxide (ZnO@ SiO2 to ZnO@SiO2@mSiO2). There was a shift of the absorption band at 307 nm for the free uncoated ZnO to shorter wavelength at 290 nm for ZnO@SiO2 and at 283 nm for ZnO@SiO2@mSiO2 . The blue shift of the absorption band 307 nm to shorter wavelength is probably due to decreasing of the particle size. No peaks are observed for the ZnO free coated silica-mesosilica indicating the absence of the shell core of zinc oxide (Fig. 7b) when treated with HCl acid in comparison with that of ZnO@SiO2(Fig. 7b).

Fig. 7. UV/VIS spectra for a) ZnO@SiO2 and b) ZnOfree@SiO2.

image Fig. 4f that zinc oxide nanoparticles of different sizes are dispersed only into the silica microspheres and none are present in the meso silica shell. This is in consistence with experimental work, where zinc oxide nanoparticles were firstly accommodated into the silica microsphere, and later coated with a meso silica shell. The size distribution histogram of ZnO nanoparticles coated silica showed that ca. 85% of ZnO nanoparticles has mean particle size of 16.1 nm, whereas ca. 8% of ZnO nanoparticles has particle size of 5 nm and 7% have particle size of 40 nm. The TEM-EDX of ZnOfree@mSiO2 and ZnOfree@SiO2@mSiO2 microspheres showed only the presence of silicon and oxygen and absence of zinc content which is evident for complete removal of ZnO core upon treatment with HCl. This also was confirmed by UV/VIS spectra discussed below. The TEM and TEM-EDX images Fig. 4(g & i) and (h & j) of ZnO@mSiO2–SH and ZnO@mSiO2–NH2 NPs, respectively showed ZnO core bare covered with distinguishable meso silica layer, 13 nm in thickness covered by thin layer of functionalized thiol-silane precursor layer, 5 nm in thickness resulting from the condensation of thiol-silane over the meso silica layer [33,37,38] as marked in Fig. 4g. This was also evident from TEM-EDX (Figs. i & j) in which the silicon content has increased from 1.3% for ZnO@mSiO2 to 9.13% after coating with thiol-silane and the zinc content has decreased from 96% to 68% after.

3.6. TGA analysis Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA) for ZnO@SiO2@mSiO2 and its amine functionalized ZnO@SiO2@mSiO2–NH2 nanoparticles were examined under nitrogen atmosphere at 20–600 °C at rate 10 °C/min. The thermogram of the ZnO@SiO2@mSiO2 nanomaterial (Fig. 5a) shows two peaks, the main peak occurs at ~75 °C due to loss of 9.9% of its initial weight. This attributed to loss of physisorbed water and alcohol from the system pores [37]. The second peak is at ~390 °C due to loss of 3.6%, which is probably due to dehydroxylation and loss of water or alcohol from silica [37]. The total loss of weight was 13.3%. Fig. 5b shows the thermogram of the ZnO@SiO2@mSiO2–NH2 nanomaterial, three peaks were observed, the first peak occurs at ~75 °C due to loss of 7.2% of its initial weight. This attributed to loss of physisorbed water and alcohol from the system pores [37]. The second peak and third peak at 350 °C and at 450 °C, respectively are due to lost of the amine functional groups from the system loss 17.6% and dehydroxylation and loss of water or alcohol from silica [3]. The total loss of weight was 23.8%. The difference between the total loss of the two materials (10.5%) is probably due to the amine organofunctional group.

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