magnetite

Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 159–166

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Preparation and characterization of superparamagnetic nanocomposites of aluminosilicate/silica/magnetite Chiung-Fen Chang a,∗ , Yi-Ling Wu a , Sheng-Shu Hou b a b

Department of Environmental Science and Engineering, Tunghai University, Taichung 407, Taiwan Department of Chemical Engineering, National Cheng-Kung University, Tainan 701, Taiwan

a r t i c l e

i n f o

Article history: Received 6 October 2008 Received in revised form 21 November 2008 Accepted 22 November 2008 Available online 30 November 2008 Keywords: Superparamagnetic Magnetite Silica Aluminosilicate Sol–gel route Substitution method

a b s t r a c t Three types of SPASMs (i.e., aluminosilicate materials incorporated with superparamagnetic particles) comprised of magnetite, silica and aluminosilicate have been successfully synthesized via three sequential steps: chemical precipitation of nano-sized Fe3 O4 , coating of SiO2 on Fe3 O4 using an acidifying method or sol–gel route, and further surface functionalization to form aluminosilicates adopting sol–gel or aluminum substitution methods. The characteristics of the resulting composites were well identified using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX), Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), superconducting quantum interference device (SQUID), solid nuclear magnetic resonance (solid NMR), nitrogen adsorption and electrophoresis. The IR and XRD spectra can well explain the bonding interaction and crystal structures of various composites respectively. The poor crystallinity of silica in the magnetic carrier can promote the substitution of Al in the silica framework. The SPASM obtained from surface functionalization via the sol–gel route possesses a greater ratio of Al(IV)/Al(VI) confirmed by the solid NMR spectra, larger surface acidity verified by SEM/EDX, and higher sensitivity to the pH value of solution near pHPZC of composites. Furthermore, such superparamagnetic materials can easily overcome the difficulty in the solid–liquid separation process when applied in the heterogeneous system. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Aluminosilicate (AS) materials are widely used as catalysts and adsorbents in the fabrication and purification processes. The main framework of AS materials is built from the corner-sharing tetrahedral structure of SiO4 and A1O4 [1,2]. The substitution of aluminum for silicon in the tetrahedral site results in a net negative charge due to the charge difference of SiO4 4− and AlO4 5− . Therefore, the surface acidity or electrostatic properties of these materials significantly depends on the number of aluminum atoms in their framework. The nature of electrostatic interactions gives AS materials the ability of ion exchange or adsorption of cations from streams. Applications of ASs and related modified materials as adsorbents have been reported in the literature, such as dye [3], aflatoxin B-1 and fumonisin B-1 [4], naphthalene [5], humic materials [6], and ions [7–9]. The synthesis of ASs by various methods has been investigated, such as substitution, coprecipitation, the hydrothermal method and sol–gel routes. Among the methods reported, the sol–gel route is regarded as the best method to obtain the homogeneous compos-

∗ Corresponding author at: Tunghai University, P.O. Box 818, Taichung 407, Taiwan. Tel.: +886 4 23590121x33622; fax: +886 4 23594276. E-mail address: [email protected] (C.-F. Chang). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.11.042

ite under a considerably lower temperature [10], and the resulting ASs possess various properties when different precursors are used in the admixture of the sol–gel route [11]. Magnetic materials are anticipated to have great applicability in heterogeneous systems due to the advantages of easy control and simple separation. The previous study showed that a nano-sized magnetic adsorbent can possess comparable or even higher adsorption capacity as compared with commercial ones [12]. Aluminosilicate materials incorporated with superparamagnetic particles (denoted as SPASMs) have been synthesized using the self-assembly (SA) approach [13], in which the superparamagnetic ␥-Fe2 O3 particles are embedded in the wall of the mesoporous AS matrix. Although research has been performed on a surfactant-based SA approach for mesoporous SPASMs embedded with magnetic particles, research on the nano-scale composites of SPASMs synthesized via sol–gel is still scarce. The objectives of this study were to synthesize composites comprised of magnetite, silica and AS as SPASMs via sol–gel and substitution methods. The structure of SPASMs is like the core–shell nanoparticle, in which the structure from inner to outer is sequentially comprised of a magnetic core, passive film of silica, and functional groups of aluminosilicates. In order to well protect the magnetic core of the SPASMs, the magnetic carrier (SiO2 /Fe3 O4 ) was first formed and then further functionalized to become a SPASM.

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Fig. 1. SEM pictures of various SPASMs.

The physicochemical properties of the SPASM were characterized with scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, superconducting quantum interference device (SQUID), specific surface area measurement with nitrogen gas, and electrophoresis for the zeta potential. The relationship between preparation methods and the physicochemical properties of resulting materials identified via the above methods were also discussed in this study.

in the filtrate was determined by the inductively coupled plasma atomic emission spectrometry, ICP-AES analysis (JY-24, Jobin Yvon, Lonjumeau, Paris, France). The well protection of silica film was ensured as the value of TOT[Fe] of filtrate was lower than the value of method determination limits.

2. Materials and methods 2.1. Synthesis of nanocomposites comprised of magnetite, silica and aluminosilicate The synthesis of SPASMs was designed in three sequential steps. The step initially began with the formation of the magnetite (Fe3 O4 ) as a magnetic core by means of chemical precipitation. Second, a silica film was further coated on the magnetic core to compose a magnetic carrier (SiO2 /Fe3 O4 ). The obtained magnetic carriers were finally treated to become SPASMs by substitution and sol–gel methods. Five kinds of composites were obtained via different routes as listed below: C1: C2: C11: C21: C22:

Magnetic carrier. Silica film on magnetic core adopting the acidification method. Magnetic carrier. Silica film on magnetic core adopting the sol–gel method. SPASM. The magnetic carrier is C1. The further functionlization adopted the substitution method. SPASM. The magnetic carrier is C2. The further functionalization adopted the substitution method. SPASM. The magnetic carrier is C2. The further functionalization adopted the sol–gel method.

The preparations of magnetic core and C1 in details are seen in Reference 12. With respect to C2, the admixture of the sol–gel process was composed of ammonium hydroxide, isopropanol, ultra-pure water, tetraethylorthosilicates and the magnetic core (in the amount of 10 g). The ratio of isopropanol:water:NH4 OH (25 vol.%):TEOS was 30:2:7.8:1 with a total volume of 2 dm3 . After drying with an infrared-ray lamp for overnight, the particles were then calcined under 673 K for 8 h. Before the step of further functionalization, the magnetic core was ensured to be well protected by the silica film. The protection of the silica film on Fe3 O4 particles was judged by the leaching concentrations of total iron ions (TOT[Fe]) from the particles of C1 and C2 in the acidic solution. The particle was placed in the hydrochloric acid solution of 0.01N at a ratio of mass to volume equal to 1 g L−1 . After shaking 20 h at 150 rpm and 298 K, the particles were separated by magnet and then filtrated through a 0.22-␮m pore size membrane. TOT[Fe]

Fig. 2. EDS of various SPASMs: (a) composite of C11; (b) composite of C21; (c) composite of C22.

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Table 1 Weight ratios of the main elements on the surface of various superparamagnetic composites obtained from SEM/EDX analysis. Element O Al Si Fe

68.68 0.35 25.78 5.18

Total a

C11

100

C21 –a 0.72 44.47 54.81 100

C22 61.32 16.57 20.06 2.05 100

The weight ratio of oxygen was excluded.

Fig. 3. Adsorption and desorption curves of N2 at 77 K on various SPASMs. ,  and ♦: C22, C21 and C11.

Fig. 5. XRD spectra of various composites. (a) C1 series and (b) C2 series.

Table 2 Nitrogen gas adsorption characteristics at 77 K of various superparamagnetic composites.

Fig. 4. Magnetization curves of various superparamagnetic composites. , , ,  and ♦: C2, C21, C22, C1 and C11.

Properties

C11

C21

C22

BET surface area (m2 g−1 )

29.7

96.7

158.5

Total pore volume (cm3 g−1 ) Micropore volume (cm3 g−1 ) Mesopore volume (cm3 g−1 ) Macropore volume (cm3 g−1 ) Ratio of macropore volume to total pore volume (%)

0.0727 0.0084 0.0208 0.0435 60

0.1113 0.0230 0.0222 0.0660 59

0.2147 0.0307 0.0508 0.1332 62

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Fig. 6. FTIR spectra of various composites in the 400–4000 cm−1 range.

The substitution method for preparation of SPASMs was executed by mixing magnetic carriers in a solution composed of NaNO3 of 0.1N and Al(NO3 )3 of 8.5 × 10−3 N under an inert atmosphere and a constant pH of 5. The obtained particles were freeze-dried after a reaction time of 24 h. For C22, the admixture was comprised of aluminum isopropoxide, TEOS, isopropanol, ultra-pure water, and C2. The ratio of aluminum isopropoxide:TEOS:water:isopropanol was 0.6 g:21 cm3 :140 cm3 :55 cm3 . After a reaction time of 20 h, the particles were dried at 373 K in the oven overnight. During the synthesizing and washing processes, the separation of particles from solutions adopted a permanent magnet to make sure that all particles possess magnetism. 2.2. Characterization of physicochemical properties Specific surface areas of the SPASMs were measured by nitrogen gas adsorption at 77 K with an automated adsorption instrument (ASAP 2010, Micromeritics). The crystal structures of the SPASMs were determined by a powder XRD analyzer (MXP diffraction collector, MAC Science, Japan) with Cu K␣ (wavelength  = 1.5418 Å) and confirmed by the JCPDS (Joint Committee on Powder Diffraction Standards) database. Scanning electron microscopy (SEM, Hitachi S-2400, Tokyo, Japan) with an X-ray energy dispersive spectrometer (EDX, S3000-N, Hitachi, Tokyo, Japan) was used to obtain the surface information. The saturation magnetization of SPASMs was performed by a superconducting quantum interference device (SQUID) magnetometer (Model: MPMS7, Quantum Design Company, San Diego, USA). The zeta potentials of SPASMs at various pH values under ionic strength of 0.1N NaNO3 were determined by a Zetasizer 3000 (Malvern Instruments, Worcestershire, United Kingdom). A Fourier-transform infrared spectrometer (FT/IR-460 Plus, Jasco, Tokyo, Japan) was used to determine the structure of specific functional groups of SPASMs. A solid-state nuclear magnetic resonance spectrometer (solid NMR, InfinityPlus-500, Varian, United Kingdom) was used to obtain the chemical environment of elements. A single pulse (90◦ ) of 4.5 ␮s was used with a repetition time of 5 s between pulses at a magnetic filed of 7.04 T for samples. The resonance frequencies were 59.6 and 78.18 MHz for 29 Si and 27 Al, respectively. Furthermore, the spin frequencies were 4 and 6 kHz for 29 Si and 27 Al, respectively. The experiments were executed at room temperature with primary standards of Al(H2O)6 3+ (prepared from aluminum sulfate) of 1 M and tetramethylsilane for aluminum and silicon, respectively.

3. Results and discussion 3.1. Morphology and specific surface areas of SPASMs The SEM photographs of various SPASMs are shown in Fig. 1. The appearances of C11, C21 and C22 are nearly similar and considerably uniform. The exact size of the specific particles is difficult to define due to the tendency to form aggregates. However, the size of the aggregates is approximately below 100 nm. Fig. 2 and Table 1 represent the SEM/EDX analyses of SPASMs, interpreting the weight ratio of the main elements on the surface. Comparing Al content with C11, C21 and C22, one can note that the Al is much more abundant on the surface of the SPASMs synthesized via the sol–gel route rather than the substitution method. The adsorption and desorption isotherms of N2 at 77 K on SPASMs are illustrated in Table 2 and Fig. 3. All SPASMs possess both Type I and Type II characteristics according to the BDDT classification, which is interpreted as indicating that they are both microand mesoporous structures. Furthermore, it is also observed that the macroporous volume dominates the porous structure of the SPASMs at a high ratio of 60–62%. The BET specific surface areas of C11, C21 and C22 are 29.7, 96.7, 158.5 m2 g−1 , respectively. The combination of the results of pore structures and SEM pictures indicates that the main contribution to the specific surface areas of SPASMs is the surface areas associated with voids between the particles rather than with the pores between the walls. 3.2. Magnetization of magnetic particles The magnetization curves of the resulting magnetic materials determined from the SQUID magnetometer are shown in Table 3 Table 3 Saturation magnetizations (MS ) of various superparamagnetic composites. Material

Saturation magnetization (emu g−1 )

Weight ratio of Fe3 O4 in particles (%)

Magnetitea C1 C11 C2 C21 C22

59 8.4 9.4 20.4 19.1 16.3

100 14 16 35 32 26

a

Quoted from Ref. [12].

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Fig. 7. FTIR spectra of various composites in the 400–1200 cm−1 range.

and Fig. 4. Due to the zero coercive force and freedom from hysteresis, the SPASMs obtained in this study are all superparamagnetic, indicating the negligible magnetic interactions between particles. The saturation magnetizations (MS ) of magnetic carriers C1 and C2 are 8.4 and 20.4 emu g−1 , respectively. The magnetism of the magnetic particle is assumed to be provided by magnetite, therefore the weight ratio of silica to magnetite via the sol–gel method is smaller than that via the acidification method. Comparing the MS of C2 and C22, we see that further modification (or coating) of C2 with the sol–gel method reduces the MS value to 74% of the original

value. On the contrary, MS of C11 is slightly higher than that of C1, which may be due to the loss of silica during the process of substitution. According to the MS value, the magnitude of magnetism of the resulting SPASMs in order is C21 > C22 > C11. 3.3. Crystal structures of magnetic particles The crystal structures of superparamagnetic composites were characterized by the XRD spectra, as illustrated in Fig. 5. The magnetic core and magnetic carriers of C1 and C2 were magnetite and

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amorphous silica confirmed by the JCPDS database. Comparison of characteristic peaks at 2 between 15◦ and 30◦ with C1 and C2, greater amount of silica coatings exited in the sample of C1due to the higher intensity. With respect to the peaks at the same angular ranges (i.e., 2 between 15◦ and 30◦ ) of C11, C21 and C22, the characteristic peaks of 2 between 15◦ and 30◦ mainly represent short-range order silica and the AS in SPASMs. For the sake of removing the disturbance of peaks of magnetite and directly determining the crystal structures of ASs, the syntheses of pure ASs via the same procedure without magnetic core were executed. The XRD patterns are defined as AS1 (referred to C11) and AS2 (referred to C22), as shown in Fig. 5. However, the characteristic peaks of special crystallites related to aluminosilicates are not distinguishable in the broad peak between 15◦ and 30◦ of 2. 3.4. FTIR characterizations of various composites The FTIR spectra of resulting materials are recorded between 4000 and 400 cm−1 and illustrated in Fig. 6. There are three obvious absorption bands at wave numbers of 3400, 1620 and 576 cm−1 in the spectrum of synthesized magnetite. The first two bands basically represent the vibrations of O H stretching and H O H band bending [14], while the band at wavenumber 576 cm−1 is mainly attributed to Fe O stretch vibrations [14,15]. Being similar to the magnetite, the broad peaks at about 3420 and 1620–1650 cm−1 of composites are also related to the O H stretching and H O H band bending, respectively [16]. Most peaks of IR spectra occur in the range of 400–1200 cm−1 , which needs further discussion as illustrated in Fig. 7. The absorption bands at around 1092, 800 and 472 cm−1 of C1 and C2 reflect the Si O Si asymmetric, Si O Si symmetric stretching vibrations, and deformation mode of Si O Si, respectively [15]. The bands at 561, and 954 cm−1 , they are possibly due to the Fe O Si, and Si O Si stretching vibrations caused by the perturbation of the metallic ion in SiO4 tetrahedra [12], respectively. The spectra of C11, C21 and C22 possess the additional bands at 636 and 726 cm−1 as compared with C1 and C2. The former mainly represents the vibration of AlO6 while the latter indicates the vibration of AlO4 [10]. Furthermore, the characteristic peak related to the bonding of Al O Si, which ought to be located at wavenumber 1000–1100 cm−1 is not definable due to the overlap with the peak of Si O Si. 3.5. NMR characterizations of various composites In order to understand the elemental environment of the resulting materials, the solid NMR technique was used to obtain the 29 Si and 27 Al MAS NMR spectra. Since magnetite has a significant effect on NMR spectra, the samples used in this analysis were synthesized using the same procedures without a magnetic core, denoted as CA (referred to C1), AS1, CS (referred to C2), AS3 (referred to C21) and AS2. 29 Si MAS NMR spectra of various materials are shown in Fig. 8. A broad signal between 90 and 130 ppm, of which the main peak is around 111 ppm, can be observed in all the samples. This broad line in the spectra of all samples represents the Q4 structure of the silica, manifesting that the crystal structure of SiO2 is amorphous or short-range order [16]. Furthermore, the signal at around 103 ppm, which represents the Q3 structure of SiOH, is obviously seen in the sample of CA. With respect to the weak lowfield shoulder of the broad signal, the Q3 structure of SiOH possibly exists in CS as well. The characteristic signal at 89 ppm related to Q2 (i.e., with two OH groups attached to each silicon) is not seen in both CA and CS [17,18], indicating that both magnetic carriers CA and CS are comprised of amorphous SiO2 and SiOH at a minor level. The peaks of 29 Si MAS NMR spectra for Si(4Al), Si(3Al,1Si), Si(2Al,2Si), Si(1Al,3Si), and Si(4Si) substitutions occurred at −84, −90, −96, −101 and −106 ppm, respectively [16]. It is notewor-

Fig. 8.

29

Si MAS NMR spectra of various composites.

thy that the series of characteristic signals may differ by several ppm from those mentioned above due to the nature of materials [19]. Comparing the spectrum with the magnetic carrier CA and its derivative AS (i.e., AS1), the signal is very similar in the investigated range. The main difference between these two spectra is that AS1 possesses a slightly broader peak, indicating the possible presence of Si(1Al,3Si). With respect to the comparison of the signals in the spectra of CS, and its derivative AS (i.e., AS2 and AS3), the spectra of CS and AS3 are similar while AS2 shows a significantly different pattern in the range between −90 and −111 ppm. Furthermore, the deterioration of the resolution and the presence of a strong lowfield shoulder are clearly observed in the AS2 spectrum, indicating that the substitution of Al for Si possibly occurs in the composite. Although it can be obviously seen that the signals representative of the specific environment are difficult to extract due to extensive overlap, the broad peak still can be additionally attributed to the signals of Si(1Al,3Si) of −101 ppm and Si(4Si) of −106 ppm [16]. The spectra of 27 Al NMR MAS of SPASMs are illustrated in Fig. 9. The spectra of AS3 contain a main peak at 0 ppm and two minor signals at 29 and 59 ppm, referred to a octahedral, pentahedral and tetrahedral aluminum environment, respectively [16]. The spectrum of AS2 is comprised of two main peaks at 49 and -1 ppm, which are attributed to the tetracoordinated and octacoordinated center of aluminum, respectively [20]. The Gaussian-like spectrum of AS2 indicates the symmetric environment for aluminum. In the spectrum of AS1, the peaks at 55 and 9 ppm are attributed to the chemical shift of tetrahedrally coordinated and octahedrally coordinated centers of aluminum. Generally speaking, the signals of tetracoordinated and pentacoordinated aluminum manifest the incorporation of Al into the silica framework. By contrast, the signal of octahedrally coordinated aluminum is normally attributed to the non-framework aluminum species [18]. Therefore, the substitution of Al for Si is obviously present in all ASs. For the case of AS1, the weak signals of tetrahedral and strong octahedral centers may imply the existence of alumina clusters. Furthermore, compared with AS1 and AS3, the poor crystallinity promotes the substitution of Al in the silica framework.

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(i.e., the point of zero charge) values of C1, C11, C2, C21 and C22 are 3.2, 5.5, 5.5, 5.7 and 5.4, respectively. The pHPZC of C1 is lower than that of C2, indicating that the crystallinity of C1 is better than that of C2 [21]. Comparison of the pHPZC value with C1 and C11, the substitution of aluminum for silicon can effectively increase the pHPZC for the sample of C11. The discrimination of the pHPZC value between C2, C21 and C22 is not obviously observed in the case of the C2 series, of which the zeta potentials are very sensitive to the pH value of the solution near the pHPZC , especially for the sample of C22. As shown in Table 3, the zeta potentials of C2, C21 and C22 at the pH value equal to 6 were −14.5, −16 and −24 mV, which indicates C22 possesses the much more basic Al OH groups on the surface then C21 and C2. 4. Conclusions

Fig. 9.

27

Al MAS NMR spectra of various composites.

Table 4 The zeta potentials of various superparamagnetic composites as a function of pH values under the ionic strength of 0.1N NaNO3 . Adsorbents

pHPZC (point of zero charge)

Potential (pH 6)

C1 C11 C2 C21 C22

3.2 5.49 5.5 5.7 5.38

−12.2 −20.5 −14.5 −16 −24

3.6. Zeta potential The surface potential of catalysts plays a critical role in the adsorption of ionic species from aqueous solutions due to the electrostatic force of attraction or repulsion. The zeta potentials of the various composites are illustrated in Table 4 and Fig. 10. The pHPZC

Three nano-scale composites comprised of magnetite, silica and aluminosilicate as SPASMs have been synthesized via three sequential steps: formation of magnetite with chemical precipitation, coating of silica on magnetite through the acidification method or sol–gel route, and development of aluminosilicates via the sol–gel route or substitution method. SEM, SEM/EDX, XRD, SQUID, IR, and solid NMR give an insight into the nature and structure of various composites and the interactions between various layers. The crystallinity of silica in the magnetic carrier prepared by the acidification method is better than that synthesized by the sol–gel method in this study, which is confirmed by the spectra of XRD and solid NMR, and the pHPZC . The SPASMs can be prepared by the substitution and sol–gel methods, in which the latter possesses a greater ratio of Al(IV)/Al(VI) and larger amount of aluminosilicate-phase of the composites. This study has indicated that the innovative SPASMs produced by sol–gel the route possess great potential for application due to the higher surface acidity and the simple solid–liquid separation. Acknowledgements The authors thank for the National Science Council of Taiwan for financial support under Grant NSC 95-2221-E-029-013-MY3, and the Sustainable Environment Research Center, National Cheng Kung University for assistance in particle characterization. References

Fig. 10. Zeta potentials of various superparamagnetic composites as function of pH values under 0.1N NaNO3 . , , ♦,  and : C21, C22, C11, C2 and C1.

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