Self-assembly of silica nanowires in a microemulsion system and their adsorption capacity
Denghui Jiang a, Yida Deng b, Guang Gao c, Liqiong Wu c, Huaming Yang*,a,d
a
Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China
b Tianjin
Key Lab of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
c
State Key Lab for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, China d
Hunan Key Lab of Mineral Materials and Application, Central South University, Changsha 410083, China
*
Corresponding author, Email:
[email protected], Fax: 86-731-88830549, Tel.: 86-731-88710804
1
ABSTRACT: The assembly process of nanostructures plays a crucial role in controlling the morphology of nanomaterial in a microemulsion system. However, controlling the assembly process of nanostructures becomes complex and difficult, due to the multiple factors of microemulsion system. Herein, we report a facile synthesis of amorphous caddice-like silica NWs by a self-assembly process in an oil-in-water (O/W) microemulsion system. In our method, the mass ratio of tetraethoxysilane (TEOS) to dimethylbenzene (DMB) would directly control the morphologies of silica nanostructures. Interestingly, the proper lowconcentration TEOS in oil phase favors the formation of silica NWs due to the self-assembly of nanosized oil droplets with silica porous shell, driving by the surface tension of oil droplets. The mean diameters of NWs could be slightly adjusted from 67 nm to 82 nm by prolonging reaction time. The as-obtained silica NWs exhibited an excellent adsorption capacity and rapid adsorption rate for methylene blue dye due to the negatively charged hydroxyl groups and high specific surface areas. Keywords: Silica; nanowires; microemulsion; assembly process; adsorption capacity
1. Introduction Silica nanomaterials have attracted great interest because of their remarkable properties and widespread potential applications in scientific and technological fields [1-9]. Among different morphologies of silica nanomaterials [10-15], silica nanowires (NW) have received considerable attention in the areas of photoluminescence (PL) [5], lowdimensional waveguides [16] and near-field optical microscopy [17]. Different synthesis 2
approaches have been reported for the preparation of silica NWs, including chemical vapor deposition [18], thermal evaporation [19], solid state reaction [20], oxide assisted growth mechanism [21], sol–gel [14, 22], hydrothermal method [23, 24], etc. Most of these methods are relatively complex and require harsh reaction conditions. Therefore, there is a need to explore simple and effective method for the controlled synthesis of silica NWs. Emulsion synthesis is a facile and mild solution-phase approach to prepare silica nanostructures [25]. The oil/water interface and the surfactant of emulsion and microemulsion system are used as a template and structure-directing agents to prepare various morphologies of silica nanostructures [10, 26], such as porous nanoparticles [27], hollow spheres or capsules [28-32], and hierarchically porous structures [33] and so on. In fact, the assembly and growth process of silica nanomaterials are affected by oil/water interface and surfactant of emulsion system. Different assembly process result in various morphologies of silica nanostructures. For example, Zhang et al. reported the synthesis of mesoporous silica nanoparticles with stellate, raspberry, or worm-like channel morphologies by different self-assembly process [11]. Theoretically speaking, the precise control of morphology can be achieved by controlling the assembly and growth process of nanomaterials. However, controlling the assembly process of nanostructures becomes complex and difficult due to the multiple factors of microemulsion system. This work reports a facile synthesis of amorphous caddice-like silica NWs by a selfassembly process in an oil-in-water (O/W) microemulsion system. In our method, the 3
mass ratio of tetraethoxysilane (TEOS) to dimethylbenzene (DMB) of oil phase would directly determine the morphologies of silica nanostructures. Interestingly, the oil phase with proper low-concentration TEOS favours formation of silica nanowires through the self-assembly of nanosized oil droplets with silica porous shell, driving by the surface tension of oil droplets. The possible formation mechanism of silica NWs were hypothesized. In addition, the adsorption capacity and adsorption behavior of MB on silica NWs has been investigated and analyzed by adsorption kinetic models and isotherms models, respectively.
2. Experimental 2.1 Synthesis of silica NWs All of chemical reagents were of analytical purity and purchased from the Sinopharm Chemical Reagent Company without further purification. Microemulsion system used in this experiment was reported in the published literature [34-37]. In a typical synthesis, 1 g sodium dodecyl sulfate (SDS) and 3.5 ml n-butanol were added to 14 mL of deionized water and stirred for several minutes at 50 °C. A mixture of tetraethoxysilane (TEOS) and dimethylbenzene (DMB) was employed as the oil phase, the mass ratio of TEOS to DMB was 1:3. Then, the oil phase (0.9 g) was added to the above aqueous solution with slow stirring for 40 s, forming the colorless and transparent microemulsion, and then 40 μL ammonia solution (25 wt%) was dropped into the microemulsion at once. This reaction solution was kept for 15 min at 50 °C under slow stirring. The white solid product was collected by centrifugation, washed several times with deionized water and ethanol. The final products were dried in a vacuum furnace at 4
80 °C for 2 h. In order to investigate the effects of reaction condition on the morphology control, the reaction parameters, including reaction time (3 min, 8 min, 15 min, 60 min), mass ratio of TEOS to DMB (pure TEOS, 1:1, 1:3, 1:6), and content of ammonia solution (20 μL, 40 μL, 60 μL, 80 μL) were changed to systematically control the morphology of the silica nanostructures.
2.2 Characterizations Transmission electron microscopy (TEM) images were recorded on a JEM-2100F microscope operating at 200 kV. For TEM, Samples after drying were sonicated in ethanol and subsequently deposited onto a carbon grid. Scanning electron microscopy (SEM) images were obtained using a FEI Sirion 200 field-emission SEM. The X-ray diffraction (XRD) patterns of the products were measured by using a D/max 2550VL/PC diffractometer with Cu-Kα radiation (λ = 0.15406 nm), and at a scanning rate of 0.02 deg/s in the 2θ range from 15 °to 80 °. Thermal gravimetric analysis (TGA) was carried out using a Mettler-Toledo TGA/DSC instrument at a heating rate of 6 °C/min with a gas feed (air) of 40 mL/min. The nitrogen adsorption measurement was conducted on a Micromeritics TriStar II 3020 analyzer. Before measurements, all of the samples were outgassed at 200 °C for 5 h under vacuum. FTIR spectra of the samples were taken on a Nicolet 6700 FTIR spectrometer. An ultraviolet–visible light (UV–vis) spectro-photometer (PE Lambda 650 s, Perkin-Elmer, USA) was used to perform the optical measurements of the sample.
2.3 Adsorption experiment The batch equilibrium experiment was carried out by 30 mg of silica NWs in 50 ml of methylene 5
blue solution at different initial concentrations. The mixture was stirred until the equilibrium was reached at room temperature. Aqueous samples were centrifuged to separate the powder. Then the concentration of MB was determined by the absorption spectra on a spectrophotometer (PE Lambda 650s). Kinetic and thermodynamic adsorption experiments were performed by 30 mg of silica NWs in 50 mL of methylene blue solution with an initial concentration of 50 mg/L. Aqueous samples were centrifuged to separate the powder at desired time intervals. The residual concentration of MB dye in the solution was determined by the absorption spectra on a spectrophotometer. The adsorption capacity (Qe) of sample for the MB dye can be calculated by:
Qe
C0 Ce V
(1)
m
where, C0 and Ce is the initial and equilibrium concentrations of MB (mg/L), respectively. V is the total volume of the solution (L) and m is the mass of adsorbent (g).
2.4 Kinetic models In order to investigate the mechanism of adsorption, Lagergren pseudo-first-order and pseudosecond-order equation models were analyzed. The linearized form of pseudo-first-order equation is expressed as follows [38]:
log(Qe Qt ) log Qe
K1 t 2.303
(2)
where, Qe and Qt represent the adsorption capacities at time t and at equilibrium, respectively. K1 is the rate constant of adsorption. A plot of log(Qe-Qt) vs. t is used to determine Qe and K1 from intercept and slope, respectively. The linearized form of pseudo-second-order kinetic can 6
be written as [38]: t 1 1 t 2 Qt K 2Qe Qe
(3)
where, K2 and Qe is the second-order rate constant of adsorption and the adsorption capacities at equilibrium. The values of K2 and Qe were calculated from the slope and intercept of plots of t/Qt vs t according to Eq. 2.
2.5 Adsorption isotherm models The equilibrium adsorption isotherm plays an important role in understanding the mechanism and process of adsorption. The Langmuir model assumes that adsorption process takes place at specific homogeneous sites, resulting in a monolayer adsorption processes. The Freundlich model is employed to describe heterogeneous system with multilayer surface phase. The linearized form of Langmuir model is presented by the following equation [38]： Ce C 1 e Qe Qmax K LQmax
(4)
where, Qe and Ce represent the amount and equilibrium concentration of dye adsorbed at equilibrium time, Qmax is the maximum adsorption capacity and KL is the energy constant for Langmuir. The values of Qmax and KL are determined from the slope and intercept of the linear plot of Ce/Qe versus Ce, respectively. The linearized form of Freundlich equation can be expressed as [38]:
1 ln Qe ln K F (ln Ce ) n
(5)
where, KF and n is Freundlich constants related to capacity of the adsorbent and adsorption intensity, respectively. A plot of lnQe versus lnCe is used to determine KF and n from intercept 7
and slope, respectively.
3. Results and discussion 3.1 Morphology and structure of silica NWs Fig. 1 shows the SEM and TEM images of silica NWs. Tortuous nanowires with mean diameters of 72 nm were formed (Fig. 1C), and looked like a pile of intertwined caddice, while the surfaces were smooth. The XRD pattern of the as-obtained samples indicated the characteristic silica glass hump centred on 2θ=22° [39, 40] (Fig. S1A). As shown in TGA curve of the as-prepared silica NWs (Fig. S1B), only one main weight losses corresponding to the surface stage happens in the temperature range 20-150 °C [41]. Almost no obvious weight loss above 150 °C of the samples was observed, indicating no residual surfactant and impurities. The typical FTIR spectra of the as-prepared silica NWs are demonstrated in Fig. S1C. Two bands at 3458 and 1631 cm−1 are attributed to adsorbed water or the surface −OH group. The strong band at 1097, 798, and 470 cm−1 is assigned to the asymmetric stretching vibration, symmetric stretching, and bending modes of Si−O−Si groups [13, 42, 43], respectively. Compared with the FTIR spectra of SDS, it showed no residual SDS in the obtained samples. Fig. 1D is the typical N2 adsorption/desorption isotherms of silica NWs. The N2 adsorption–desorption isotherms of silica NWs exhibited typical type-IV isotherms, and the BET surface areas of silica NWs reached 282.1m2/g.
3.2 Growth process of silica NWs 8
To well understand the formation process of silica NWs, samples formed at different reaction times were examined by TEM technology (Fig. 2). In the initial stage of reaction (3 min), only conterminous hollow spheres with thin shells formed (Fig. 2A). The mean diameters of the hollow structures was about 67 nm (Fig. S2A). When the reaction time was extended (8 min), hypogenetic nanowires with uneven and rough surfaces formed clearly. The mean diameters of the NWs was about 70 nm (Fig. S2B). Some parts of the nanowires were very thin, as indicated by the black arrows (Fig. 2B). As the reaction proceeded (15min), the nanowires became uniform and full-grown (Fig. 1A). When prolonged to 60 min (Fig. 2C), the full-grown nanowires with small burrs formed and the mean diameter of NWs was increased to 82 nm (Fig. S2C). Many hollow particles with diameter of about 41 nm appeared accompanying the NWs. More importantly, it was clearly observed that these small hollow particles connected together and fused to form a relatively big particles and then gradually self-assembled to form onedimensional structure, as shown in a yellow open rectangular area (Fig. 2C&2D). Therefore, the obtained silica NWs were self-assembled by the silica nanoparticles. In order to further investigate the formation process of silica NWs, the effect of mass ratio of TEOS to DMB on the morphology control was analyzed (Fig. 3). When pure TEOS was used as oil phase, many micron-sized irregular cysts and aggregates were formed, no uniform product was obtained (Fig. 3A). However, when the mass ratio was decreased to be 1:1, well-maintained hollow spheres with an average diameter of about 600 nm were prepared (Fig. 3B). When the mass ratio was decreased to be 1:3, the uniform and full-grown nanowires formed (Fig. 1A). The ratio was further decreased to 9
be 1:6. Some hypogenetic nanowires formed, as shown in Fig. 3C. From the above experiments, it was found that the mass ratio of TEOS to DMB directly determined the morphologies of silica nanostructures. Proper low concentration of TEOS in oil phase favors the formation of nanowires. The optimized mass ratio of TEOS to DMB is 1:3 for the formation of nanowires in our experiment. Aiming to clarify effects of the content of ammonia solution on the formation of silica NWs, different content of ammonia solution was used in synthesis of silica NWs (Fig. S3). When the content of ammonia solution was 20 μL, net-cross nanostructures was obtained, no single well-structured nanowires was observed. When the content of ammonia solution was increased to 60 μL, some separate nanoparticles seemed to adhere to the NWs (Fig. S3B). The content of ammonia solution was further increased to 80 μL, many nanoparticles with mean size of 80 nm formed. This result suggests that only the appropriate content of ammonia solution (40 μL) can form single well-structured nanowires.
3.3 Possible formation mechanism of silica NWs Based on above results, a model for the formation process of silica NWs was proposed (Fig. 4). The formation process can be roughly divided into three stages. Firstly, the mixed oil phase with low concentration of TEOS was dispersed into nanosized oil droplets when oil phase was added into water phase. Hydrolysis and condensation of TEOS occurred continuously at the interface, and then more TEOS molecules migrated to the oil/water interface. In this stage, the surface of oil droplets formed solid porous 10
shell of silica. Therefore, hollow spheres with thin porous shells were observed clearly when reaction time was very short (Fig. 2A). Secondly, after collision of the interdroplets with porous shell, the oil droplets were connected together due to the fusion of oil phase induced by surface tension. The linked oil droplets will not be easily separated due to continuous formation of solid silica. It should be noted that multiple oil droplets with smaller sizes may connect together and rapidly fuse to form a relatively big oil droplets. The oil droplets with porous shell continuously linked and gradually selfassembled to form a caddice-like structure. So some hollow nanowires with porous shell also were observed (Fig. S4). In growth process of silica NWs, many oil droplets with relatively small sizes continued to integrate into the hollow porous nanowires. These oil droplets may come from small mixed oil droplets formed at the initial stage and partially hydrolyzed TEOS droplets fled from mixed oil droplets. The liquid TEOS in the interior of the nanowires continuously reacted and generated solid silica until the interior of nanowires was completely filled. Finally, the complete nanowires were formed. It is noteworthy that the self-assembly and growth process of nanowires were carried out simultaneously. Based on the proposed formation mechanism, it was inferred that the key points for formation of silica NWs were the formation and self-assembly of nanosized oil droplets with porous shell. An assembly process of building blocks is actually a packing of building blocks. The driving force and shape of building blocks for the self-assembly process are two key factors that govern the self-assembly of building blocks substantially [44]. Lenz et al. demonstrated that irregular building blocks with short-range interactions can self11
assemble into one-dimensional structures by using a minimal model of ill-fitting and sticky particles [45]. In our work, the oil droplet with solid porous shell is a representative building block with short-range interactions. Because the oil droplets with solid porous shell are uneven and irregular, and surface tension of oil phase is a typical short-range interaction [45]. Hence, the assemble process of silica NWs is good consistent with Lenz’s theory. However, in our previous works [35, 36, 46], Ni or CuS nanoparticles obtained by similar method in the same microemulsion system only grow at the interface and are difficult to assemble into one-dimensional structures, due to lacking of a driving force. Therefore, it was concluded that the driving force for the selfassembly of NWs is the surface tension of oil droplets. The formation of silica porous shell of oil droplets is another important factor for selfassembly process. In our experiments, the oil phase with high concentration of TEOS cannot be sufficiently dispersed and form micron-sized aggregates due to the instantly rapid formation of solid silica (Fig.3A&3B). High content of ammonia solution will also increase reaction rate of TEOS, resulting in the formation of solid silica particles in a short time (Fig. S3B). Hence, in the case of high concentrations of reactants, the oil droplets have no chance to assemble into nanowires due to high reaction rate. The proper low-concentration TEOS in oil phase and moderate content of ammonia solution facilitate formation of the nanosized oil droplets with silica porous shell. These results indicate that optimized reaction rate controlled by appropriate reaction conditions favours formation of nanosized oil droplets with silica porous shell.
12
Based on proposed formation mechanism of silica NWs, the mean diameter of NWs is mainly depend on the size of oil droplets. According to our previous report [35], the mean sizes of oil droplets using pure DMB as an oil phase was about 70 nm. In the synthesis of silica NWs, DMB is the main component of the oil phase with low concentration of TEOS. The mean size of the oil droplets should be compared with that of pure DMB oil droplets. Therefore, the silica NWs had similar diameter (70 nm) with the hollow aggregates obtained in the initial stage of reaction (Fig. 2A). The size of oil droplets in microemulsion system can be controlled by simply varying the content of oil phase[10]. Theoretically speaking, the diameters of NWs would be adjusted by varying the content of oil phase. Further work is in progress.
3.4 Adsorption kinetics and isotherms The adsorption ability of the as-prepared silica NWs herein was tested by adsorption of methylene blue (MB) dye. Fig. 6A showed the time dependence of silica NWs for the removal of MB from solution in the dark. The adsorption rate of the silica NWs was very high within the initial 10 min. 91.6% of the MB dye were removed for silica NWs. The equilibrium was established after about 30 min and the amount of MB adsorbed at equilibrium time (Qe) was 51 mg/g. Silica NWs have rapid adsorption rate and high adsorption ability due to its large external surface area. As a contrast, the adsorption ability of the silica mesoporous hollow spheres prepared with mass ratios of TEOS to DMB at 1:1 was tested (Fig. S5). The silica mesoporous hollow spheres (MHSs) had
13
higher adsorption ability (Qe = 64mg/g) than that of silica NWs, because of its larger external surface area (368.0 m2/g), as shown in Fig. S5C. The Lagergren pseudo-first-order and pseudo-second-order kinetic models were used to further investigate the kinetic behavior of the adsorption process [47]. Linear plots of log(Qe−Qt) vs. t, and t/Qt vs. t were shown in Fig. 6A&6B, respectively. All constants of the kinetic models were calculated (Table S1). The determination coefficient (R2) of the pseudo-first-order was very low, implying a poor fit in pseudo-first-order model. On the other hand, the determination coefficient of the pseudo-second-order rate model was higher than 0.9970 for silica NWs, and the calculated Qe value also agreed very well with the experimental value, indicating that the adsorption process of MB adsorbed by silica NWs followed the pseudo-second-order kinetic model. The equilibrium adsorption isotherm played an important role in understanding the mechanism and process of adsorption [38]. Adsorption isotherms of MB on silica NWs were analyzed by using the linearized form of Freundlich and Langmuir models. Linear plots of lnQe vs. lnCe, and Ce/Qe vs. Ce were shown in Fig. 5C&5D, respectively. The relative parameters of Langmuir and Freundlich model were calculated (Table S2), it was observed that the calculated Langmuir isotherm constant Qmax of silica NWs was 58.55 mg/g, its adsorption capacity could be comparable to that of mesoporous SBA-15 materials [7, 48]. The adsorption isotherm data of MB on silica NWs was well fitted through the Langmuir model with higher R2 value (0.9717), while was poorly fitted to the Freundlich model with relatively lower R2 value (0.7662). The results indicated that
14
the adsorption of MB by silica NWs took place at the specific homogeneous sites, following a monolayer adsorption processes. MB is a basic cationic dye and can dissociate a positive ammonium cation in aqueous solution [49]. The silica NWs exhibited excellent adsorption capacity for MB, which was attributed to the electrochemical interaction between the MB molecules and the functional hydroxyl groups on the surface of silica nanostructures [38, 49]. Therefore, the adsorption capacity for MB of our silica nanostructures is mainly related to its specific surface area. In addition, the values of the adsorption capacity were normalized by the BET surface area of the silica NWs and MHSs (Table S3). As expected, the normalized adsorption capacity of two silica nanostructures are almost the same. This results indicated that two different silica nanostructures have homogeneous surfaces. So the adsorption behavior of MB on the silica NWs and MHSs is monolayer adsorption, which is agreed with the results of the equilibrium adsorption isotherm fitted by Langmuir model.
4. Conclusions In summary, caddice-like silica NWs have been successfully synthesized through a selfassembly process in an oil-in-water microemulsion system. The mass ratio of TEOS to DMB of oil phase would directly control the morphologies of silica nanostructures. Interestingly, the oil phase with proper low-concentration TEOS favours formation of silica nanowires through the self-assembly of nanosized oil droplets with silica porous shell, driving by the surface tension of oil droplets. It is a key for forming oil droplets 15
with silica porous shell to optimize the reaction rate by controlling reaction conditions. The mean diameters of NWs can be slightly adjusted from 67 nm to 82 nm by prolonging reaction time, due to continuous growth process. The as-prepared silica NWs exhibited an excellent adsorption capacity and rapid adsorption rate for MB by electrostatic interactions because of negatively charged hydroxyl groups and high specific surface areas. Importantly, the self-assembly process in microemulsion system could be further expanded to prepare polymer and TiO2 nanowires through changing the reactant in oil phase. Moreover, nanowires with a tuneable diameter may be possible to obtain by adjusting content of oil phase.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51402346), the National Key R&D Program of China (2017YFB0310903), the Strategic Priority Research Program of Central South University (ZLXD2017005), the National "Ten Thousand Talents Program" in China, the Hunan Provincial Science and Technology Project (2016RS2004, 2015TP1006) and the Postdoctoral Science Foundation of Central South University (182043).
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20
(A)
(B)
100 nm
500 nm
500 nm 71.8
12.4 nm
(D) 400 Volume Adsorbed (cm /g)
(C)
Frequency
3
0.3
0.2
0.1
300
200
100
Adsorption Desorption
0
0.0
40
60
80
Diameter (nm)
100
0.0
120
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 1 (A) TEM (inset is high-magnification image of the surface of silica NWs), (B) SEM images, (C) diameter distributions and (D) N2 adsorption–desorption isotherms of silica NWs. The mean diameters and diameters distribution of silica NWs were determined by the statistical analysis of TEM images.
21
(A)
(B)
200 nm
500 nm
500 nm
(D)
(C)
500 nm Fig. 2
100 nm
TEM images of silica NWs prepared at different reaction times: (A) 3 min (inset is high-
magnification image); (B) 8 min (inset shows the surface of a nanowire at higher magnification) and (C) 60 min; (D) high-magnification TEM image of the hollow spheres in the sample C.
22
(B)
(A)
(C)
1μm
500 nm
200 nm
Fig. 3 TEM images of silica nanostructures prepared with different mass ratios of TEOS to DMB: (A) pure TEOS, (B) 1:1 and (C) 1:6.
23
Initial stage
Growth process
Self-assembly
Water
: SDS
: Oil droplet with porous silica shell
: SiO2
Fig. 4 Proposed mechanism for the formation of silica NWs
24
(B)
(A)
Fig. 5 (A) The adsorption kinetic behavior and (B) adsorption isotherms of MB adsorbed by silica NWs.
25
1.0
(A)
3
(B)
t
log(Qe-Q )
0.5 t
2
t/Q
0.0
1
-0.5
-1.0
0
0
20
40
60
80
100
t (min)
120
140
0
160
20
40
(C)
80
100
120
140
160
t (min)
4.4
1.2
60
(D) 4.0
lnQe
Ce/Qe
0.8
3.6
0.4
3.2
0.0 -10
0
10
20
30
40
50
60
70
1.0
Ce
1.5
2.0
2.5
3.0
3.5
4.0
4.5
lnC e
Fig. 6 Adsorption kinetics fitted by linearized form of (A) pseudo-first-order and (B) pseudo-second-order models. Adsorption isotherms fitted by linearized form of (C) Langmuir and (D) Freundlich model of MB adsorbed by silica NWs.
26
