Sol-gel hot injection synthesis of ZnO nanoparticles into a porous silica matrix and reaction mechanism

Materials and Design 119 (2017) 270–276

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Sol-gel hot injection synthesis of ZnO nanoparticles into a porous silica matrix and reaction mechanism Ahmed Barhoum a,b,c,⁎, Guy Van Assche a,⁎, Hubert Rahier a, Manuel Fleisch b, Sara Bals d, Marie-Paule Delplancked e, Frederic Leroux d, Detlef Bahnemann b,f a

Department of Materials and Chemistry, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium Institute of Technical Chemistry, Photocatalysis and Nanotechnology Research Unit, Leibniz Universität Hannover, Callinstr.3, 30167 Hannover, Germany Chemistry Department, Faculty of Science, Helwan University, Helwan, Cairo 11795, Egypt d EMAT, Universiteit Antwerpen, Groenenborgerlaan 171, 2020 Antwerpen, Belgium e Department 4MAT, Universite Libre de Bruxelles, 50 avenue F.D. Roosevelt, 1050 Bruxelles, Belgium f Laboratory for Nanocomposite Materials, Department of Photonics, Faculty of Physics, Saint-Petersburg State University, Ulianovskaia Str.3, Peterhof, Saint-Petersburg 198504, Russia b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Sol-gel hot injection is an elegant onepot synthesis method to disperse ZnO NPs into a porous SiO2 matrix. • ZnO/SiO2 nanocomposites are thermally stable at 500 °C. • Extensive annealing (˃700 °C) of ZnO/ SiO2 nanocomposites forms Zn2SiO4 structures. • Zn:Si molar ratio affects significantly on morphological changes of ZnO/SiO2 nanocomposites.

a r t i c l e

i n f o

Article history: Received 14 September 2016 Received in revised form 18 January 2017 Accepted 19 January 2017 Available online 23 January 2017 Keywords: Sol-gel hot injection Zinc oxide nanoparticles Porous silica matrix Nanocomposite Zinc silicate Architectured nanostructures

a b s t r a c t Despite the enormous interest in the properties and applications of porous silica matrix, only a few attempts have been reported to deposit metal and metal oxide nanoparticles (NPs) inside the porous silica matrix. We report a simple approach (i.e. sol-gel hot injection) for insitu synthesis of ZnO NPs inside a porous silica matrix. Control of the Zn:Si molar ratio, reaction temperature, pH value, and annealing temperature permits formation of ZnO NPs (≤10 nm) inside a porous silica particles, without additives or organic solvents. Results revealed that a solid state reaction inside the ZnO/SiO2 nanocomposites occurs with increasing the annealing temperature. The reaction of ZnO NPs with SiO2 matrix was insignificant up to approximately 500 °C. However, ZnO NPs react strongly with the silica matrix when the nanocomposites are annealed at temperatures above 700 °C. Extensive annealing of the ZnO/SiO2 nanocomposite at 900 °C yields 3D structures made of 500 nm rod-like, 5–7 μm tube-like and 3– 5 μm needle-like Zn2SiO4 crystals. A possible mechanism for forming ZnO NPs inside porous silica matrix and phase transformation of the ZnO/SiO2 nanocomposites into 3D architectures of Zn2SiO4 are carefully discussed. © 2017 Elsevier Ltd. All rights reserved.

⁎ Corresponding authors at: Department of Materials and Chemistry, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium. E-mail addresses: [email protected] (A. Barhoum), [email protected] (G. Van Assche).

http://dx.doi.org/10.1016/j.matdes.2017.01.059 0264-1275/© 2017 Elsevier Ltd. All rights reserved.

A. Barhoum et al. / Materials and Design 119 (2017) 270–276

1. Introduction The past two decades have seen exciting advances in synthesis and application of metal oxide nanoparticles (MOx NPs) and mixed-metal oxides in sensors, batteries, optical devices, and biomedical applications. MOx NPs, especially below 20 nm, show unusual structural disorder and interesting shape- and size- dependent physical, chemical, biological properties that cannot be achieved by their bulk counterparts [1]. For example, MgO, CaO, Al2O3, and ZnO NPs of 4–7 nm size exhibit unique destructive properties for acid gasses, polar organic compounds, and even chemical/biological warfare agents [2,3]. ɣ-Al2O3 NPs become energetically stable at size ≤10 nm [4]. Despite these key properties and applications, these NPs usually suffer from a high tendency toward aggregation [5]. ZnO is probably the richest material among all MOx, both in structures and properties [6,7]. The unique piezoelectric, semiconducting, and pyroelectric properties of ZnO NPs have shown novel applications from sensing, water treatment to energy production [8,9]. However, they have also shown some drawbacks: (i) without capping agents ZnO NPs can easily aggregate and lose their unique properties, i.e., photocatalytic efficiency; (ii) they undergo photo-corrosion upon UV irradiation, which results in a decrease in photocatalytic activity [10]; (iii) the optical and electrical properties of ZnO NPs are not very stable at high temperature [11]; (iv) a low tolerance toward acidic and alkaline solutions; and (v) a high cytotoxicity and potentially leading to damage of cellular DNA, thereby limiting its applications in environmental and biological systems [12]. Several efforts have been made in order to overcome the low dispersibility, the high toxicity and low durability of some MOx NPs, including ZnO NPs, by either insitu synthesis or introducing these NPs into solid matrices, such as carbon, polymers, or metal oxides [13,14]. Characteristics such as optical and catalytic activity may be lost if the dispersion of these particles is not adequately modulated. A way of avoiding such unwanted effects is an encapsulation of the MOx NPs in an adequate host matrix. Among the different solid supports “porous silica matrix” has been considered as an excellent transparent solid support because of its high surface area, thermal stability, chemical inertia, transmittance to radiation, and high absorption capacity, facilitating the interface reaction with organic compounds for photocatalytic degradation [15]. The porous silica framework facilitates the use of MOx NPs for the desired purposes while avoiding the drawbacks mentioned before. ZnO/SiO2 nanocomposites have been recently used in heterogeneous catalysis and in ceramics technology for applications as sensors, varistors, and photoluminescent materials [16,17]. ZnO/SiO2 nanocomposites can emit specific colors from blue to yellow-green, and, as a result, it is a potential material for use in white light sources. The doped ZnO NPs are stable upon aging since the ZnO NPs are trapped in and protected by the silica matrix due to the interfacial effect. We should emphasize that the chemical interaction between hosting matrix and ZnO NPs have often been deemed responsible for new properties and studies of these interactions are rather rare in the literature. Various synthetic methods, like sol–gel [16], molecular capping [18], impregnation [19], double-jet precipitation [20], reverse micelle [21], sputtering [22], and colloidal methods [21] have been developed to disperse ZnO NPs in a silica matrix. The obtained ZnO are amorphous and crystalline NPs dispersed in SiO2 matrix and the formation of Zn2SiO4 due to unwanted reactions between ZnO NPs and SiO2 matrix can not be avoided. Therefore, new synthesis methods need to be sought. This work reports a novel, one-step synthesis of ZnO NPs (≤10 nm) in porous silica matrix through injection of an aqueous ZnCl2 solution into a hot Na2SiO3 solution. The composition and structural properties are controlled by altering the Zn:Si molar ratio at a controlled reaction temperature and pH, and by the subsequent annealing of the produced materials. The chemical interactions between ZnO NPs and silica matrix were studied by XRD, UV–vis, Raman, FTIR and XPS spectroscopy. The sol-gel hot-injection synthesis exhibits significant advantages, such as

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high yield, low production cost, safety and relatively mild conditions of synthesis, precise control of the process, and it enables the modification/protection of ZnO particles within porous silica matrix. An aqueous sol-gel route and cheap precursors (ZnCl2 and Na2SiO3) were chosen for this study, as they are more appropriate for large-scale applications. Moreover, the organo-metal precursors such as Zn and Si alkoxides are not suitable because they are highly reactive, very sensitive to moisture, and remain rather expensive. The extensive annealing of the ZnO NPs into the porous silica matrix results in Zn-rich silicates, which is a material of great importance due to its large bandgap of 5.5 eV and exciton binding energies (75 meV) [23], transparency in the UV–visible range, and thermal and chemical stability. These properties make the Zn-rich silicates suitable for advanced applications in cathode ray tubes, laser crystals, plasma display panels, electroluminescent devices [24]. 2. Experimental section Three ZnO/SiO2 nanocomposites (ZS1, ZS2, and ZS3), with Zn:Si molar ratios of 3:1, 1:1 and 1:3, were synthesized by gradually injecting an aqueous ZnCl2 solution into a hot Na2SiO3 solution at 90 °C. Analytical grade zinc chloride (ZnCl2, + 98%, Sigma), sodium metasilicate (Na2SiO3, 44–47% SiO2, Sigma), hydrochloric acid (HCl, 37%, Sigma), sodium hydroxide (NaOH, +97%, Sigma), ethanol (C2H5OH, 96%, Sigma) and monodistilled water were used to synthesize the targeted materials. The pH of the pure Na2SiO3 solution was about 14, while that of the pure ZnCl2 solution was about 5.5. Aqueous solutions of HCl (5 M) and NaOH (5 M) and monodistilled water were used to control the pH of the mixture at pH 9. The produced gels were kept to cool under vigorous stirring for 30 min and were then aged for 18 h. The gels were broken, filtered and washed with 500 mL monodistilled water, followed by washing with 100 mL ethanol. The broken gels were thermally treated in two steps. The gels were allowed to dry at 150 °C for 24 h, ground, washed again with monodistilled water (100 mL) to remove any NaCl left and then annealed at either 500 °C or 900 °C for 2 h in air atmosphere. Pure porous ZnO (Z) and SiO2 (S) particles were synthesized via the same method using 5 M NaOH added to a hot ZnCl2 solution and 5 M HCl added to a hot Na2SiO3 solution, respectively, and used as reference samples. The detailed experimental conditions were listed in Table 1. Phase identification and the average crystallite size of the samples were determined using X–ray diffraction (XRD, Diffractometer Bragg Bantano, Bruker D500, Germany) with Cu-Kα (λ = 1.5406 Å) radiation and a secondary graphite monochromator. Scanning step size was 0.02°. The following slits were applied: divergence slit 0.3°, receiving slit 0.15° and Soller slit 2.3°. The average crystallite size was determined according to the Debye-Scherrer equation using TOPAS software (Bruker). Thermogravimetric analysis (TGA, TA Instruments TGA Q5000, USA) and differential scanning calorimetry (DSC, Netzsch STA 409 PC Luxx®) were used to determine the thermal transitions of the synthesized materials. TGA and DSC measurements were performed from 30 to 1000 °C at a heating rate of 10 K·min−1 under air atmosphere. The nanostructural investigation was carried out by transmission electron microscopy (Tecnai Osiris, FEI, The Netherlands) and scanning

Table 1 Experimental conditions for synthesis ZnO, SiO2 and ZnO/SiO2 nanocomposite. Sample Zn:Si mole ratio

Z ZS1 ZS2 ZS3 S

1:0 3:1 1:1 1:3 0:1

ZnCl2 solution

Na2SiO3 solution

ZnCl2 (mole)

H2O (mL)

Na2SiO3 (mole)

H2O (mL)

0.4 0.3 0.2 0.1 –

600 450 300 150 –

– 0.1 0.2 0.3 0.4

– 150 300 450 600

pH controller

5 5 5 5 5

M NaOH M NaOH M HCL M HCL M HCL

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The bandgap of the samples was identified by UV–visible spectra (UV–vis) at the wavelength range of 200–800 nm using a UV-2450 spectrophotometer (Shimadzu, Japan). Estimation of the bandgap carried out by plotting (αhʋ)2 versus hʋ. The absorption coefficient α, excitation energy (hʋ = h/c) and bandgap energy (Eg) of is correlated by the following formula: (αhʋ)2 = const (hʋ-Eg) where hʋ is the excitation energy and c is the wavelength. The intersection of the linear part with hʋ will give us the value of band gap energy (Eg). The specific surface area was calculated from the adsorption isotherm of nitrogen (BET; model Nova 1200, Yuasa Ionics, Osaka, Japan). 3. Results and discussion Three ZnO/SiO2 nanocomposites (ZS1, ZS2, and ZS3), with Zn:Si molar ratio of 3:1, 1:1 and 1:3, were synthesized via injecting an aqueous ZnCl2 solution gradually in a Na2SiO3 solution at 90 °C. Pure microporous ZnO (Z) and SiO2 (S) were also synthesized via the same method and used as reference samples. The samples were annealed in air at different temperatures (150, 500 and 900 °C) to investigate the variation of grain size (crystallite size) with annealing temperature. 3.1. Composition, polymorph and phase transformation Fig. 1. XRD patterns of the synthesized samples annealed at (a) 150 °C, (b) 500 °C, and (c) 900 °C. (▲) zincite.

electron microscopy (Phenom ProX, Phenom-World, The Netherlands). The composition and elemental distribution were further mapped through energy dispersive X-Ray spectroscopy (EDX) by integrating the intensity of the peaks corresponding to Zn and Si as a function of the beam position when operating the TEM in scanning mode. Bonding structures were analyzed using Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet 6700, USA). KBr pellets (ratio 1:100) were made using 2 mg of dried sample powder per 200 mg of KBr. Spectra were taken in the range of 4000–500 cm− 1 with a resolution of 4 cm−1 and 64 scans were averaged per spectrum [25]. The IR peak positions were determined using OMNIC software. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI 5600 spectrophotometer (PerkinElmer, USA) with an Al-Kα monochromated source, which was used to identify the core levels of the elements of the synthesized samples. The powder samples were measured without further treatment. The survey scan spectra were recorded with a pass energy of 187.85 eV, an X-ray flux of 300 W, and a takeoff angle of 45°. The photo-electrons were analyzed using a hemispherical analyzer using a slit aperture of 0.8 mm. Sample charging was compensated by using the charge neutralizer. Therefore all spectra were mathematically shifted in such a way that the aliphatic part of the C 1 s peak is located at 285 eV [26]. The spectra acquisition parameters (channel exposition, number of scans, analyzer parameters, etc.) were selected in order to provide the best energy resolution and signal/noise ratio. Peak position, peak intensity, and full-width at half– maximum (FWHM) of the samples were calculated using Multipack software (provided by Physical Electronics).

XRD diffraction patterns in Fig. 1 show that the prepared samples exhibit different crystallization habit (see Table 2), from highly amorphous to crystalline, as a function of the Zn:Si molar ratio and annealing temperature. Z and ZS1 samples annealed at 150 °C have a hexagonal zincite structure (JCPDS036-1451). ZS2, ZS3 and S samples annealed at 150 °C do not show any crystalline diffraction peaks. Up on annealed at 500 °C, Z and ZS1 show a hexagonal zincite structure (JCPDS036-1451) while ZS2, ZS3 and S do not show crystalline diffraction peaks, suggesting that these samples are amorphous or contain ultrafine ordered clusters (Fig. 1b). The crystallinity of Z and ZS1 particles increases significantly by increasing the annealing temperature from 150 to 500 °C. The crystallite size for pure ZnO particles significantly increases from 64 to 98 nm while the crystallite size of ZS1shows a slight increase from 27 to 28 nm (Table 2). The XRD diffraction patterns (Fig. 1b) of the composite ZS1 shift toward higher 2θ compared with pure Z, i.e. the most intense peak of the composite ZS1 appears at ~36.5° while this for pure Z appears at ~36.2°. This slight shift is due to the formation of a zinc silicate (willemite) via a solid state reaction between ZnO and SiO2 at the interface [27]. The amount of zinc silicate (willemite) formed at 500 °C is a quite low to be accurately detected by XRD. A further increase in annealing temperature from 500 to 900 °C induces formation of willemite (Fig. 1b, c). Fig. 2 shows the TGA and DSC curves of the synthesized samples after drying at 150 °C. ZnO particles (Z) shows two mass losses at 155 °C (13.3 wt%) and 417 °C (18 wt%). The first one is probably due to decomposition of a hydrated ZnO phase. It is very likely that some amorphous (not visible in XRD) Zn(OH)2 is formed and that it further decomposes above 150 °C, as most of this compound should have dehydrated during the drying at 150 °C. The weight loss at higher temperatures might be due to the decomposition of other hydrates (see

Table 2 XRD data of the prepared samples: polymorph and crystallite size. Sample

Annealing at 150 °C Polymorph

Crystallite size

Annealing at 500 °C Polymorph

crystallite size

Annealing at 900 °C Polymorph

Z ZS1

Zincite Zincite

64 nm 27 nm

Zincite Zincite

98 nm 28 nm

Zincite Zincite & Willemite

ZS2 ZS3

Amorphous Amorphous

None None

Amorphous Amorphous

None None

S

Amorphous

None

Amorphous

None

Willemite Cristobalite & willemite Cristobalite & Tridymite

crystallite size 240 nm 126 nm 136 nm 151 41 nm 83 nm 45 nm 50 nm

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Fig. 2. Thermograms of the synthesized samples dried at 150 °C for 24 h: (a) mass loss (TGA) curves; (b) heat flow (DSC) curves. (▲) endothermic transition; (▼) exothermic transition.

Table 3 Data extracted from TGA curves: temperature at peak maximum and weight loss. Sample

Zn:Si mole ratio

Z

1:0

ZS1

3:1

ZS2

1:1

ZS3

1:3

S

0:1

Temp. range 60–500 °C

Temp. range 500–1000 °C

Temp. at peak max.

Wt. loss %

Temp. at peak max.

Wt. loss

155 °C 417 °C 290 °C

13 wt% 18 wt% 5 wt%

Non

Non

105 °C 219 °C 126 °C 263 °C 80 °C 460 °C

1.7 3.8 3.1 3.4 7.5 1.5

wt% wt% wt% wt% wt% wt%

684 681 660 738 non

°C °C °C °C

non

0.7 wt% 0.4 wt% 0.01 wt% 0.8 wt%

Table 3). According to the literature, a Zn(OH)2 shell on a nanoparticle decomposes in several steps up to 450 °C, which is also the temperature range of the highest endotherm [27]. No clear crystallization signal is visible and no other phenomena are observed till 1000 °C. This is in agreement with the XRD results. Pure SiO2 (S) shows mass loss steps at 80 °C to 200 °C (7.5 wt%) and a very broad one between 200 and 460 °C (1.5 wt%) due to endothermic evaporation of physically and chemically bonded water, respectively [28]. The mass loss from 500 to 1000 °C (b1 wt%) is due to the dehydroxylation of silanol groups from the SiO2 surface. The exothermic transition at 920 °C is due to the crystallization of amorphous SiO2 into cristobalite and tridymite. The mass loss curves of ZS1 to ZS3 are rather different from the ones of the pure compounds. Below 200 °C, sorbed water is released, causing an endothermic transition in DSC. The second step between 200 and 400 °C is probably again due to the decomposition of hydrated ZnO structures, but as ZnO is trapped in the SiO2 matrix, it is very well possible that another type of hydrate is formed, which releases its water at a lower temperature. The glass transition temperatures (Tg) of ZS2 and ZS3 are clearly visible in the range of 650 to 700°C and are followed

Fig. 3. BF-TEM images of the synthesized samples annealed at 500 °C for 2 h.

by the crystallization exotherms in the range of 700–800 °C (see Table 4). This crystallization involves the solid-state reaction of ZnO with SiO2 forming crystalline willemite. It is, however, remarkable that these exotherms do not occur at the same temperatures, nor is there a systematic trend in the series for the first exotherm. The second exotherm shifts to higher temperatures with increasing amount of SiO2. The presence of willemite after annealing the nanocomposites at 900 °C for 2 h was already confirmed with XRD. Our synthesis approach, however, leads to well dispersed and tiny ZnO NPs in a silica matrix, allowing the formation of crystalline willemite at 900 °C. According to the XRD, TGA and DSC results, almost complete crystallization has been obtained after only 2 h. 3.2. Morphological changes TEM images in Fig. 3 reveal that all samples annealed at 500 °C have a porous nanostructure (≤30 nm). The nanocomposite ZS1 has a structure in which amorphous SiO2 nanoparticles are dispersed within a ZnO matrix (TEM-EDS elemental mapping, Fig. 4). In contrast to ZS1, STEM-

Table 4 Data extracted from DSC curves: temperature at peak maximum and enthalpy change. Sample Zn:Si mole ratio

Temp. range 60–500 °C

Temp. range 500–1000 °C

Temp. at peak max.

Enthalpy change

Temp. at peak max.

Enthalpy change

Z

1:0

Exo

3:1

ZS2

1:1

ZS3

1:3

S

0:1

Endo Endo Exo Endo Exo Endo Exo Endo Exo Endo

585 °C

ZS1

178 °C 225 °C 410 °C 70 °C 350 °C 80 °C 350 °C 93 °C 350 °C 84 °C

754 853 740 882 845 920 936

Exo Exo Exo Exo Exo Exo Exo

°C °C °C °C °C °C °C

Fig. 4. STEM-EDX elemental mapping of the synthesized nanocomposites annealed at 500 °C. Elemental distributions of Zn and Si are represented in red and blue, respectively.

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Fig. 5. SEM images of the synthesized samples annealed at 900 °C.

EDX of ZS2 and ZS3 shows an individual ZnO particles (≤10 nm) dispersed in a silica matrix. By decreasing the Zn:Si mole ratio to 1:1 or 1:3, the number of SiO2 NPs significantly increases in the reaction medium. Therefore the SiO2 NPs aggregate with each other and on the ZnO NPs, preventing the extensive aggregation of the ZnO NPs. Upon annealing these nanocomposites at 500 °C, tiny ZnO NPs dispersed in silica matrix are obtained, instead of forming SiO2 NPS dispersed in ZnO matrix, as opposed to ZS1 (see Figs. 3 and 4). The extensive heating of pure ZnO and pure SiO2 particles at 900 °C results in 250–400 nm ZnO NPs (Fig. 5) and architectured SiO2 structures, respectively. XRD along with SEMEDS analysis indicate that the nanocomposites ZS1, ZS2 and ZS3 are made of 3D architectured structures made of 500 nm rod-like (inset ZS1), 5-ä7 μm tube-like (inset ZS2) and 3–5 μm and needle-like (inset ZS3) of Zn2SiO4 crystals, respectively (Fig. 5). The specific surface area is about 90 m2/g for pure SiO2. The specific surface area of the nanocomposites decreases to 52 m2/g (ZS1), 66 m2/g (ZS2) and 56 m2/g (ZS3) on loading the ZnO inside the SiO2 channels. 3.3. FTIR, XPS and UV–visible spectroscopy The bonding structure at the molecular level, after annealing at 500 °C, is further investigated using FTIR, XPS and UV–visible spectroscopy (Fig. 6). The nanocomposites exhibit several IR absorption peaks, which are the combination of the stretching/bending vibrations of Zn\\O and Si\\O\\Si (Fig. 6a). ZS1 and ZS2 display the main SiO2 absorption (1104 cm−1 Si\\O\\Si stretching for pure SiO2, S) shifted to 942 and 957 cm− 1, respectively. In the literature [29], the peaks at 942–957 cm−1 are also considered as symmetric vibrational Si\\O\\Ti or Si\\O\\Zn stretching, and the intensity is related to the dispersion of TiO2 or ZnO NPs over the SiO2 matrix, respectively. Therefore, we suggest that the increase in peak intensity at 942–957 cm−1 was brought on by the chemical reaction between \\Si\\OH and ZnO NPs [29]. According to XRD (Fig. 1), ZS1 contains crystalline domains of ZnO. It is clear from TEM-EDS (Fig. 4) and FTIR that the remaining SiO2 rich phase contains ZnO or Zn2+ ions. The ZnO crystals in ZS1 cause the absorption at 428 cm− 1, located next to a bending modes of SiO2 at 494 cm−1. The stretching Si\\O\\Si absorption of pure SiO2 appears at about 1104 cm− 1. The stretching Si\\O\\Si absorption for ZS1, ZS2 and ZS3 shifted to lower wavenumbers by increasing the ZnO NPs content. A second absorption, the so-called ‘Si-O stretching’ at 792 cm−1 is visible for ZS2 and ZS3 (809 cm−1) has not been observed in case of ZS1 and ZS2 [30]. This reflects high ZnO and Zn2+ content in ZS2 and ZS3 samples. The XPS spectra (Fig. 6b, c) of the nanocomposites also show a peak shift compared to pure ZnO (red shift) and pure SiO2. However, sample charging was compensated by using the charge neutralizer. At a Zn:Si ratio of 1:1 (ZS2) tiny ZnO domains are formed, and the interaction

Fig. 6. Spectra of the synthesized samples annealed at 500 °C: (a) FT-IR spectra; (b and c) XPS spectra; (d) UV–visible spectra.

between ZnO and the SiO2 matrix reaches its maximum and consequently the material exhibits the highest peak shift. The above-mentioned XPS analysis results indicate that Si–O–Zn exists in ZnO–SiO2 composites. Our results are in agreement with the literature [31,32]. For pure ZnO, there exist two different oxygen species: lattice oxygen, termed as ‘O2 −’, and oxygen ions termed as ‘O−’. The oxygen ions exist at sites where the coordination number of oxygen ions ‘O−’ is smaller than that on regular sites ‘O2−’. In addition to these oxygen species, there exists another oxygen ‘O2−’ in the ZnO/SiO2 nanocomposite, which is attributed to the loosely bound oxygen in the amorphous SiO2 or partially weakly adsorbed oxygen species such as OH−. The shift of the O 1 s peak position reflects the variation of the relative amount of ‘O2−’ (Zn\\O\\Zn, Zn\\O\\Si, and Si\\O\\Si) when the ZnO content vary in the nanocomposite samples [32]. UV–vis spectra show that Z and S annealed at 500 °C absorb in the UV region at approximately 378 and 271 nm, respectively (Fig. 5d). The nanocomposites ZS1, ZS2, and ZS3 have the maximum absorption at about 369 and 277 nm, respectively. ZS1 presents two bands at 279 and 369 nm due to the presence of SiO2 and ZnO, respectively. ZS2 and ZS3 show one peak at 277 nm while the peak at 369 nm disappeared due to the decrease of ZnO content comparing to SiO2. The decrease of absorbance with increasing the SiO2 content is acquiring to the low UV–Vis absorption characteristic of SiO2. The bandgap of the semiconductor determines its optical property and application. The bandgap of the nanocomposites is about 4.4 eV, which is higher than for pure ZnO (3.2 eV) and lower than for SiO2 (4.7 eV). The size of ZnO NPs is smaller than that of pure ZnO, as confirmed by XRD and TEM results. The bandgap of the semiconductor material increases with decreasing size and discrete energy levels arise at the bandedges. This phenomenon is known as “quantum confinement effects” [29]. It is also possible, the bandgap shift is attributed to the formation of Si\\O\\Zn bonds, and can probably be attributed to a zinc silicate structure, formed along with separated phases ZnO and silica. 3.4. Growth and crystallization mechanism of ZnO NPs in porous silica matrix The Zn:Si molar ratio, temperature and pH of the reaction medium and the subsequent annealing and heating/cooling rate are the main parameters controlling the formation, growth, crystallization, and phase transformation of ZnO and SiO2. Generally, the formation and growth

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Fig. 7. Schematic representation shows the solid state reaction inside the ZnO/SiO2 nanocomposites occurs with increasing the annealing temperature: (a) ZnO NPs formed in the porous SiO2 matrix after annealing at 500 °C; (b) ZnO NPs react strongly with the silica matrix when the nanocomposites are annealed at above 700 °C; and (c) Extensive annealing of the ZnO/SiO2 nanocomposite at 900 °C yields rod-like, tube-like and needle-like Zn2SiO4 (willemite) crystals.

of ZnO via injection of HCl (5 M) into a hot aqueous ZnCl2 solution (pH ~5.5) involves: (i) hydrolysis of ZnCl2 forming Zn(OH)x. Hydrolysis and complexation of Zn2+ ions with increasing pH from 5 to 9. ZnCl2 exists 2− in aqueous solution as Zn2 +, Zn(OH)2, Zn(OH)− and 3 and Zn(OH)4 their concentration ratios change with increasing the pH of the reaction medium. Generally, Zn2+ ions hydroxylate in the presence of OH– ions forming a white precipitate (Zn2+ + 2 OH– → Zn(OH)2). If the concentration of OH– exceeds a certain limit, Zn(OH)2 will dissolve forming water-soluble zincate ion (Zn(OH)2 + 2 OH– → Zn(OH)2− 4 ). Other complexes such as [Zn(OH)(H2O)x]+ and [Zn2(OH)(H2O)x]3+ can also be formed; (ii) condensation of Zn(OH)x forming stable molecular clusters with oxo (Zn\\O\\Zn) or hydroxo (Zn\\OH\\Zn) bridges and generation of H2O as a by-product [33]; (iii) growth of the nuclei by addition of reactive species (Zn(OH)x and clusters) available in the solution on the surface of the nuclei, forming colloidal sol NPs [34]; (iv) coagulation of the ZnO sol NPs by surface condensation reaction forming a rigid and porous inorganic network enclosing a continuous water phase, the “gel” [35]; (v) drying of the produced gel at 150 °C results in a significant amount of shrinkage, densification and formation of dry hard aggregates (xerogel); (vi) controlled calcination of the xerogel at 500 °C forming a porous nanostructure (Fig. 3-Z), while the extensive heating at 900 °C forms uniform ZnO NPs (Fig. 5-Z). The sol-gel chemistry of silica is similar to this of pure ZnO, but Si is more strongly bonded to oxygen than Zn [36]. The high binding energy of Si\\O bonds makes the produced porous silica particles more thermally stable than porous ZnO particles. When the pure silica (S) is annealed at 900 °C, the SiO2 particles sinter and the pores collapse and architecture silica particles (Fig. 5-S) are formed upon cooling. The sol-gel injection of an aqueous ZnCl2 solution into a hot Na2SiO3 solution results in nanocomposite particles having different morphologies, characteristics and compositions depending on the Zn:Si molar ratio. The combination of high precursor concentrations, the polycondensation at the reaction temperature of 90 °C, the final pH of 9, the Zn:Si mole ratio that is controlled at either 1:1 or 1:3, the aging of the hybrid gel for 18 h, and the drying at 150 °C for 24 h followed by annealing the nanocomposites at 500 °C for 2 h, bring to completion the polycondensation and crystallization, giving rise to tiny ZnO NPs welldispersed inside the SiO2 matrix. The pH of the pure Na2SiO3 solution is about 14, while that of the pure ZnCl2 solution is about 5.5. Therefore, the injection of the ZnCl2 solution into the Na2SiO3 solution allows forming individual ZnO sol NPs, before the condensation reaction of the silicate is completed. When the ZnCl2 solution is injected into the hot Na2SiO3 solution, firstly ZnCl2 hydrolyses (step i) and then condenses (step ii) forming ZnO sol NPs (step iii–iv). The gradual consumption of OH– ions in the solution results in a gradual decrease in the solution pH from 14 to 9. Within the pH range of 14–11, the Zn2+ ions transform to Zn(OH)x and a polycondensate, forming monodispersed

ZnO sol NPs, while the condensation reaction of silica is slow. The silicate slowly assembles and forms polycondensates on the surface of ZnO sol NPs formed in the early stages of the reaction (heterogeneous nucleation) [37,25], or condensates and aggregates in the bulk solution, forming SiO2 sol NPs (homogeneous nucleation). The condensation of silicate on the surface of ZnO sol NPs controls the growth of these NPs. Within the pH range 11–9, the number of produced SiO2 sol NPs significantly increases, but there is still some Zn2+ or Zn(OH)x in solution, which will be incorporated into the silicate network. Aggregation and polycondensation of Zn(OH)x and SiO4− 4 results in hybrid sol NPs and finally in a gelatinous network made of ZnO, SiO2, and ZnO/SiO2 sol NPs are obtained (step iv). Annealing the produced nanocomposites at 500 °C results in a significant amount of shrinkage, and the formation of ZnO NPs (zincite) dispersed into amorphous silica matrix (Fig. 7a). The mechanism of thermal transformation of the nanocomposite samples upon heating from 500 to 900 °C seems to follow the coalescence and Ostwald ripening mechanism [39]. Upon heating, especially above 700 °C, the porous ZnO microparticles break into smaller ZnO nanoparticles. Then in the second step, the smaller and bigger ZnO nanoparticles combine to form particles of a more uniform shape and size, following “Ostwald ripening”. During this process, combining and densification of the particles reducing the porosity thereby making the particles denser. When SiO2 is mixed with ZnO the breaking-up of the obtained ZnO/SiO2 nanocomposite becomes not possible due to the high strength of SiO2 skeleton, which is much stronger than this of ZnO. Above the glass transition temperature (700 °C), Zn2+ ions diffuse through the silica matrix as a result willemite crystals are formed [38]. Extensive heating of the ZS1, ZS2, and ZS3 nanocomposite samples at 900 °C produces highly crystallized 3D structures made of cristobalite, tridymite, and willemite crystals (Fig. 7c). The nucleation and formation of willemite crystals starts at the surface of the ZnO NPs (heterogeneous nucleation) (Fig. 7b). Diffusion of Zn2+ ions from ZnO NPs into silica matrix replaces Si\\O\\Si bonds by weaker Si\\O−‖Zn2+‖− O\\Si electrostatic interactions facilitating SiO2 melting. Melting and collapse of ZnO/ SiO2 nanocomposite particles result in the partial loss of their porosity. 4. Conclusion Finding new methods for the synthesis of ZnO nanoparticles (NPs) with a good control on their dispersibility, size, and crystallinity is of great importance. Here, we demonstrate a low cost, one-step, green synthetic route “sol-gel hot injection” for incorporating ZnO NPs (≤10 nm) into a porous silica matrix. The sol-gel hot injection is an elegant one-pot synthesis technique to disperse ZnO NPs within a porous SiO2 matrix. The nanocomposites were successfully prepared through: (i) insitu synthesis of ZnO NPs inside a porous silica matrix through injection of ZnCl2 into hot Na2SiO3 solution at 90 °C, without additives or

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organic solvents; (ii) thermal treatment of the obtained gels at 150 °C for 24 h and 500 °C for 2 h to form and crystallize the ZnO phase in the silica matrix, resulting in ZnO/SiO2 nanocomposites. Thermal treatment at high temperature 900 °C for 2 h (iii) facilitates the diffusion of Zn and Si atoms through the SiO2 and ZnO matrices, enabling the transformation of the nanocomposites into different 3D Zn2SiO4/silica or ZnO composite structures. Confining the ZnO NPs in an inert matrix “silica” offers a way to control their particle sizes, size distribution, and contrasting the tendency of nanoparticles to aggregate. Thus opening new possibilities to the fine control of its performance and applications in optical detection and heterogeneous catalysis. Characterization of the nanocomposites obtained at 500 °C shows that the formation of ZnO NPs inside porous SiO2 matrix is effective to keep the ZnO NPs highlydispersed, well-protected from acids/bases and make them safe to use in optical devices. The developed approach is highly versatile and may be applicable for many other metal oxide NPs, like Fe2O3 or TiO2. Acknowledgment A.B. would like to thank FWO - Research Foundation Flanders (grant no. V450315N) and the Strategic Initiative Materials in Flanders (SBOproject no. 130529 - INSITU) for financial support. TEM and TEM-EDX analyses were performed by Dr. F. Leroux (EMAT, Universiteit Antwerpen). XRD and DSC measurements were performed by T. Segato (4MAT, Université Libre de Bruxelles). Notes: the authors declare no competing for financial interest. References [1] C. Yuan, H. Bin Wu, Y. Xie, X.W.D. Lou, Mixed transition-metal oxides: design, synthesis, and energy-related applications, Angew. Chem. Int. Ed. 53 (2014) 1488–1504, http://dx.doi.org/10.1002/anie.201303971. [2] E. Lucas, S. Decker, A. Khaleel, A. Seitz, S. Fultz, A. Ponce, W. Li, C. Carnes, K.J. Klabunde, Nanocrystalline metal oxides as unique chemical reagents/sorbents, Chem. Eur. J. 7 (2001) 2505–2510, http://dx.doi.org/10.1002/15213765(20010618)7:12b2505::AID-CHEM25050N3.0.CO;2-R. [3] V.H. Grassian, P.T. O'Shaughnessy, A. Adamcakova-Dodd, J.M. Pettibone, P.S. Thorne, Titanium dioxide nanoparticles: Grassian et al. respond, Environ. Health Perspect. 116 (2008) A152–A153, http://dx.doi.org/10.1289/ehp.10915R. [4] J.M. McHale, Surface energies and thermodynamic phase stability in nanocrystalline aluminas, Science 277 (1997) 788–791, http://dx.doi.org/10.1126/science.277.5327.788. [5] A.B. Djurišić, Y.H. Leung, A.M.C. Ng, X.Y. Xu, P.K.H. Lee, N. Degger, R.S.S. Wu, Toxicity of metal oxide nanoparticles: mechanisms, characterization, and avoiding experimental artefacts, Small 11 (2015) 26–44, http://dx.doi.org/10.1002/smll.201303947. [6] K. Omri, I. Najeh, L. El Mir, Influence of annealing temperature on the microstructure and dielectric properties of ZnO nanoparticles, Ceram. Int. 42 (2016) 8940–8948, http://dx.doi.org/10.1016/j.ceramint.2016.02.151. [7] K. Omri, I. Najeh, R. Dhahri, J. El Ghoul, L. El Mir, Effects of temperature on the optical and electrical properties of ZnO nanoparticles synthesized by sol–gel method, Microelectron. Eng. 128 (2014) 53–58, http://dx.doi.org/10.1016/j.mee.2014.05.029. [8] Y.T. Chung, M.M. Ba-Abbad, A.W. Mohammad, N.H.H. Hairom, A. Benamor, Synthesis of minimal-size ZnO nanoparticles through sol–gel method: Taguchi design optimisation, Mater. Des. 87 (2015) 780–787, http://dx.doi.org/10.1016/j.matdes.2015.07.040. [9] A. Barhoum, J. Melcher, G. Van Assche, H. Rahier, M. Bechelany, M. Fleisch, D. Bahnemann, Synthesis, growth mechanism, and photocatalytic activity of zinc oxide nanostructures: porous microparticles versus nonporous nanoparticles, J. Mater. Sci. 1–17 (2016)http://dx.doi.org/10.1007/s10853-016-0567-3. [10] V.V. Shvalagin, A.L. Stroyuk, S.Y. Kuchmii, Role of quantum-sized effects on the cathodic photocorrosion of ZnO nanoparticles in ethanol, Theor. Exp. Chem. 40 (2004) 378–382, http://dx.doi.org/10.1007/s11237-005-0003-2. [11] Z.-Y. Ye, H.-L. Lu, Y. Geng, Y.-Z. Gu, Z.-Y. Xie, Y. Zhang, Q.-Q. Sun, S.-J. Ding, D.W. Zhang, Structural, electrical, and optical properties of Ti-doped ZnO films fabricated by atomic layer deposition, Nanoscale Res. Lett. 8 (2013) 1–6, http://dx.doi.org/10. 1186/1556-276X-8-108. [12] I.L. Hsiao, Y.J. Huang, Titanium oxide shell coatings decrease the cytotoxicity of ZnO nanoparticles, Chem. Res. Toxicol. 24 (2011) 303–313, http://dx.doi.org/10.1021/ tx1001892. [13] R. Kumar, R.K. Singh, D.P. Singh, R. Savu, S.A. Moshkalev, Microwave heating time dependent synthesis of various dimensional graphene oxide supported hierarchical ZnO nanostructures and its photoluminescence studies, Mater. Des. 111 (2016) 291–300, http://dx.doi.org/10.1016/j.matdes.2016.09.018. [14] Y. Wang, J. Ma, Q. Xu, J. Zhang, Fabrication of antibacterial casein-based ZnO nanocomposite for flexible coatings, Mater. Des. 113 (2017) 240–245, http://dx.doi.org/ 10.1016/j.matdes.2016.09.082.

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