- Nano Express
- Open Access
Nanostructured Silica/Gold-Cellulose-Bonded Amino-POSS Hybrid Composite via Sol-Gel Process and Its Properties
© The Author(s). 2017
- Received: 2 March 2017
- Accepted: 2 May 2017
- Published: 2 June 2017
The Erratum to this article has been published in Nanoscale Research Letters 2017 12:458
It is demonstrated in this paper that silica nanoparticles coated with core/shell gold provide efficient thermal, optical, and morphological properties with respect to the cellulose-polyhedral oligomeric silsesquioxanes (POSS) hybrid system. The one-step synthesis of a silica/gold nanocomposite is achieved with a simultaneous hydrolysis and reduction of gold chloride in the presence of formic acid, and the trimethoxysilane group acts as a silica precursor. The focus here comprises the synthesis of cellulose-POSS and silica/gold hybrid nanocomposites using the following two methods: (1) an in situ sol-gel process and (2) a polyvinyl alcohol/tetrakis (hydroxymethyl)phosphonium chloride process. Accordingly, the silica/gold core/shell nanoparticles are synthesized. The growth and attachment of the gold nanoparticles onto the functionalized surface of the silica at the nanometer scale is achieved via both the sol-gel and the tetrakis (hydroxymethyl) phosphonium chloride processes. The cellulose-POSS-silica/gold nanocomposites are characterized according to Fourier transformed infrared spectroscopy, Raman, X-ray diffraction, UV, photoluminescence, SEM, energy-dispersive X-ray spectroscopy, TEM, thermogravimetric, and Brunauer-Emmett-Teller analyses.
- Core/shell nanoparticles
- Optical transparency
The field of nanotechnology is one of the most popular current-research areas, and it is being developed in chemistry, physics, biology, and materials science; here, polymer science and technology are obviously included, as well as a broad range of topics. This area of research was utilized for microelectronics and nanoelectronics, as the critical dimension scale for modern devices is now less than 100 nm [1, 2]. Therefore, the synthesis protocols of metal-oxide-hybrid composites are already well established in the literature [2, 3] and most of them are multistep metal-nanoparticle processes. The synthesis of the silica/gold hybrid composite materials is achieved using the in situ sol-gel process via the hydrolysis of gold and silica precursors into a cellulose-polyhedral oligomeric silsesquioxanes (POSS) matrix [3–5].
The gold nanoparticles have recently been synthesized by the reduction of chloroaurate (HAuCl4) ions for which different methods such as those involving sodium borohydrate, citrate, and other reducing agents are used [6, 7]. Based on this synthesis process, the stabilizing agents such as thiols, amines, phosphines, phosphine oxides, and carboxylates have been used to control the morphology of the nanoparticles. In addition, cellulose constitutes the most abundant, currently available renewable-polymer resource material, and it has received great attention due to its renewability, availability, non-toxicity, low cost, biodegradability, thermal stability, and chemical stability [8, 9]. Moreover, the polyhedral oligomeric silsesquioxanes (POSS) comprise nanostructures that contain the empirical formula RSiO1.5, where R may be a hydrogen atom or an organic functional group, e.g., alkyl, alkylene, acrylate, and hydroxyl functional groups [10, 11]. The focus of the cellulose-metal-oxide hybrid is the synthesis of the uniform dispersion of nanoparticles in the composite that is utilized for flexible electronic devices, chemical sensors, disposable sensors, and biosensors [12–14]. The sol-gel chemistry to synthesize the cellulose-binary mixed oxides has been widely reported in the literature. The focus of the present study is the synthesis of cellulose-POSS silica/gold that is covalently bonded by an in situ sol-gel process that includes the involvement of surface-modified PVA and tetrakis (hydroxymethyl) phosphonium chloride (THPC) in the hybrid composites. Based on the two chemical processes in the presence of tetra ethoxysilane (TEOS), chlorauric acid (HAuCl4) and γ-aminopropyl triethoxysilane (γ-APTES) are bonded to the cellulose-POSS hybrid nanocomposites. The cellulose-POSS-silica/gold hybrid nanocomposites are characterized by Fourier transformed infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Ultraviolet-visible spectral (UV-VIS), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX), SEM, Brunauer-Emmett-Teller (BET), and transmission electron microscopy (TEM) analyses.
Materials and Methods
The cotton cellulose with a specific degree of polymerization (DP = 4500) was purchased from Buckeye Technologies Co., (USA). Lithium chloride was purchased from Junsei Chemical Japan. The cotton pulp (buckeye) is purified in the presence of the LiCl, and sulfuric acid is used in the synthesis of the cellulose solution. The molecular sieves (containing 4A°, four meshes to eight meshes) that are used for the additional purification were received from Acros Organics Ltd, New Jersey, USA. Dimethylacetamide (DMAc) (anhydrous, 99.8%) was received from Sigma-Aldrich, USA. The cotton pulp was mixed with LiCl/anhydrous DMAc according to a proportion of the cotton-cellulose pulp/LiCl/DMAc that is 2/8/90 by mass. The cotton pulp and LiCl in the presence of sulfuric acid was used to purify the cellulose solution from the bulk-cotton fibers. The cellulose solution, PSS [3-(2- amino ethyl) amino] propyl-Hepta isobutyl substituted (POSS-amine), tetra ethoxysilane (TEOS), chlorauric acid (HAuCl4), γ-aminopropyl triethoxysilane (γ-APTES), hydrocholoric acid (HCl), poly (vinyl alcohol) (PVA), and tetrakis (hydroxymethyl) phosphonium chloride (THPC) were purchased from Aldrich (South Korea).
Synthesis of Cellulose-POSS-Amine-Silica/Gold Hybrid Nanocomposites
Method 2. The stoichiometric amount of the cellulose solution (0.5 g) and the POSS-amine (0.35 g) are dissolved in (50 ml) of DMAc and then stirred for 1 h in the presence of terphthalic acid (0.5 g). The mixture is then continuously stirred (300 rpm) for another 2 h at 95 °C until the homogeneous solution is achieved. The reaction is followed by an application of the same temperature, and the calculated amount of 0.2 g of PVA in the presence of a hot-water solution is transferred into the reaction mixture to obtain the homogenous solution. The calculated amount (2 ml) of γ-APTES is added into the homogenous-sol-reaction mixture and then dispersed in the same temperature, followed by stirring for 2 h. The required 2 ml of TEOS and 5 ml THPC solution is added along with 2 ml of HAuCl4 (2 ml, 0.004 mM), and this is followed by the reduction of the formic acid 5 ml and stirring for 12 h. Further, the reaction mixture is transferred into a beaker, purified in ethanol, and kept in the oven at 95 °C for 12 h. Lastly, the resultant cellulose-POSS-silica/gold hybrid nanocomposites are collected in a sol-gel bottle to avoid the moisture content before the characterization process.
Measurements and Characterization
Fourier transformed infrared spectroscopy (FT-IR) analysis
FT-IR spectra of the cellulose-POSS-silica/gold hybrid composite were recorded by using the Brucker, IF5-859 spectrometer from Digilab (Cambridge, USA) with KBr beam splitter and detector at 8 cm−1 resolution.
Raman spectral analysis (Raman)
Raman spectral analysis was performed by using the RM200 confocal Raman spectromicroscope scanning from 100 to 400 cm−1 at room temperature in open air, and an He-Ne laser beam with a wavelength of 580–600 nm.
X-ray diffraction (XRD) analysis
Wide angle XRD pattern of the hybrid composite were recorded with the Riguku co D/max X-ray diffractometer for which Cu Kα radiation. The tube current and the voltage 300 mA and 40 kV, respectively, and data from the 2θ angular regions between 5 and 80 °C.
Ultraviolet-visible spectral (UV-VIS) and photoluminesence spectral analyses
A UV-VIS spectrophotometer UV6000 was used to analyze absorption spectra of hybrid composite samples. Photoluminescence (PL) spectral results were conducted at room temperature by using a SPEC-1403 PL spectrometer (HORIBA Ltd., Tokyo, Japan) with a He-Cd laser (325 nm) as the excitation source. The power of the He-Cd laser was used 55 mW, and the diameter of the focal spot was 1 mm. The power density at the surface of the sample was approximately 7 W/cm2 .
Scanning electron microscopy (SEM and EDX)
The collected hybrid composites were characterized by SEM (Hitachi S-4200, Hitachi Ltd., Tokyo, Japan), and the EDX analysis is performed using the AN-ISIS 310.
Transmission electron microscopy (TEM)
The transmission electron microscopy of hybrid composite results were obtained using the 100CX electron microscope (JEOL, Ltd., Japan).
Thermal properties (thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC))
TGA were carried out using the TA Instruments 2050 Universal V4.1D. Ceramic hybrid samples weighing 9.7 mg are heated up to 1000 °C at 10 °C/min. The DSC analysis of hybrid composite was also carried out using the TA Instruments 2050 Universal V4.1D.
Brunauer-Emmett-Teller (BET) analysis
The specific surface area and average pore volume of the hybrid nanocomposites were calculated according to the Brunauer-Emmett-Teller (BET) analysis (BELmax 00131 equipment, BELSORP, Tokyo, Japan).
Formation of Cellulose-POSS-Silica/Gold Nanoparticle Composite
The macromolecular structure of cellulose is presented with a number of hydroxyl groups, and the POSS-amine can be grafted into the cellulose macromolecular structure in the presence of terphthalic acid. As a crosslinking agent, the acid compound is able to form bonds between the cellulose and the POSS-NH2 hybrid compounds. The schematic representations of the cellulose and the POSS-NH2 are shown in Fig. 1a, b, respectively. In the graft reaction, the POSS particles are dispersed in the cellulose-host matrix and bond to the cellulose molecule, thereby forming cellulose-POSS hybrids. In addition, the bonding of the silica/gold nanoparticles via the sol-gel process is as follows. The original synthesis of the silica/gold nanoparticles is a four-step process in which the monodisperse-silica nanoparticles are first grown using the Stöber method to produce the spherical dielectric cores of the nanoparticles . The Stöber method produces spherical silica nanoparticles by means of the hydrolysis of alkyl silicates and the subsequent polycondensation of silicic acid in an acid or a base catalyst. In the second step, the surfaces of the silica nanoparticles are functionalized by the adsorption of γ-APTES with its amine tails protruding from the surface of nanoparticles. In the third step, the gold-colloid solution is added to the resultant silica solution. According to the phonthammachai and Jun-hyun Kim reports, the gold colloid is produced separately from the reduction of HAuCl4 by the formic acid and alkaline THPC [13–15]. The gold nanoparticles are bonded via the organo-aminosilane groups that produce the silica-hybrid nanoparticle composites. A final reduction process is used to produce silica nanoparticles with a uniform layer of gold nanoshell in the presence of formic acid. In the reduction process, the formed silica/gold particles that are covalently bonded to the silica core serve as nucleation sites for an aged mixture of the chloroauric acid and the reducing agents. Method 1. FT-IR spectroscopy is employed to study the chemical structure of the amino-POSS-bonded cellulose-hybrid nanocomposites in the presence of terphthalic acid during the sol-gel process.
X-ray Diffraction (XRD) Analysis
The XRD study for which the cellulose-POSS-silica/gold hybrid composites are synthesized by the sol-gel process is shown in Fig. 2e. The results indicate the values of 2θ = 22.56°, 25.14°, 27.90°, 30.08° (less intensity) for the silica/gold formation, and the board peaks of 8°, 17°, and 21° for the regeneration of the cellulose-POSS matrix. The XRD peak values indicate the planes of the face-centered cubic (fcc) gold structure (JCPDS 04- 0784), which indicate the crystalline behavior of the silica/gold nano-hybrid core/shell growth in the nanoparticle composite. The silica/gold core/shell hybrid nanocomposites show two phases, namely the fcc of Au and the tetragonal. This shows that the coated gold nanoparticles induce the silica crystallization at a lower temperature in situ during the sol-gel process and control the morphology [13–15]. Therefore, the metal disrupts the amorphous network, reducing the kinetic barrier regarding the crystallization. Method 2. Figure 2f shows the XRD study of the cellulose-POSS-silica/gold hybrid materials that are synthesized using PVA and THPC. The regeneration-peak value of the cellulose-POSS is 2θ = 7.96°. The other peak values of 2θ = 17.34°, 22.54°, 25.12°, and 27.88° (sharp peak) (silica/gold formation) represent the crystalline behaviors of the hybrid nanocomposites.
UV-Visible Spectral (UV-VIS) Analysis
Photoluminescence (PL) Properties
Figure 3c, d shows the PL spectra of the cellulose-POSS-silica/gold hybrid nanocomposites according to the sol-gel process. In this process, different amounts of the silica and gold nanoparticles (0.002 and 0.004 M, respectively) are present in the hybrid nanocomposites. The PL spectra results show the sharp peak in the red-band region of the fundamental absorption, and the peaks centered at 441.7, 451, 474, and 497 nm indicate the silica-based electrons. Another emission from the PL spectra shows the band gap between 2.3 and 2.80 eV (524 nm) wherein the broad and intense peaks of gold nanoparticles of different sizes are present. The smaller size indicates that the origin of these bands comes from the exciting laser and is penetrated through the porous layer of the gold nanoparticles, and the optical-coupling properties have been discussed in detail in previous studies [13–15]. The gold-crystallite size therefore becomes smaller, and the intensity in the PL properties becomes higher and stronger. It is also noted that the plasmon peak exhibits a blue shift with the decreasing of the particle size. The observed blue shift in the peak position of the plasmon absorption is due to the quantum-size effects from the gold nanoparticles.
SEM, SEM-EDX, and TEM Analyses
Thermogravimetric Analysis (TGA)
Figure 10b shows the thermal properties of the hybrid nanocomposites in the presence of PVA and THPC, whereby the silica/gold hybrid shows a 34.5% char residue at 999 °C. The TGA regarding both methods for the silica/gold hybrid shows a greater thermal stability compared with those of a previous report [13–16]. Differential scanning calorimetry (DSC) is one of the important thermal-characteristic properties regarding the cellulose-POSS-silica/gold hybrid composites. The DSC results are indicative of the pure cellulose and the am-POSS-grafted cellulose hybrids [15, 16]. The DSC plots for the am-POSS-grafted cellulose hybrids respectively show the first endothermic peaks at the temperatures of 80.47 and 78.29 °C. These endothermic peaks (Fig. 10c, d) are probably associated with the removal of the water from the am-POSS-grafted cellulose materials that is due to the decrease of the amino-POSS amount. The cellulose shows the second endothermic peak at 358.92 °C. The endothermic change that is shown in the DSC plot for the cellulose is associated with the decomposition processes that may occur within the chemical-heating process. The cellulose-POSS hybrid nanocomposites respectively show the second endothermic peaks at 366 and 364 °C. The thermal properties of the am-POSS-grafted cellulose are different from those of the pure-cellulose polymer because of the difference between the organic/inorganic hybrids. The endothermic peaks are estimated according to the interaction between the organic components and the inorganic components. The DSC plots of the am-POSS-bonded cellulose hybrids also indicate that the smallness of the two endothermic peaks is due to the interaction of the organic/inorganic hybrids. In addition, the cellulose-POSS-grafted silica-gold hybrid results improve the Tg, and the melting temperature increases due to an interfacial bonding between the two components [17–28].
BET Analysis of Cellulose-POSS-Silica/Gold Hybrid Nanocomposites
In this paper, cellulose-POSS-silica/gold hybrid nanocomposites are synthesized using an in situ sol-gel process in the presence of γ-APTES, PVA, and THPC. Both of the samples show the homogeneous formation of cellulose-POSS-silica/gold hybrid nanocomposites in the stable colloids. From the colloid nanoparticles, the uniform core/shell consisting of gold nanoparticles is formed on the surface of γ-aminopropyl-modified silica/gold hybrid composites. The first method uses the colloidal gold solution to form the shell on the modified silica core in the hybrid composites. The second method involves the formation and simultaneous deposition of silica/gold nanoparticles in the presence of PVA and THPC, whereby the HAuCl4 is reduced with formaldehyde. A comparison of both deposition methods indicates that the direct deposition of colloidal gold on the modified silica particles affords a more-uniform and homogeneous distribution of the Au nanoparticles; therefore, the deposition can be easily controlled to achieve the desired size and concentration of the gold nanoparticles on the silica surface in the presence of a coupling agent and surface modifiers. The homogeneity of the hybrid nanocomposites is influenced by the hydrolysis rate and the condensation reaction of the alkoxysilanes, which plays an important role in the sol-gel process; this might be due to the amounts of hydrochloric acid and the tetraethoxysilane/gold precursors in the presence of γ-APTES. The hybrid nanocomposites indicate that an optical transparency and a thermal stability are achieved compared to the pristine cellulose-POSS materials. The XRD results show crystalline behavior in the low-temperature PVA/THPC via silica/gold hybrid nanocomposites. The hybrid nanocomposites represent the achievement of thermal stability, PL behavior, surface morphology, and a controlled particle size via a coupling agent or surface modifiers.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2014R1A1A2A10054019), funded by the ministry of Education. It was also supported by the “Technology Innovation Industrial Program” (10051977, 30W/mK Super Thermal Nanoceramic Adhesive Material for Fuel Effiencency of EV/HEV), funded by the ministry of Trade, Industry & Energy (MI,Korea) and partial funding was supported by a grant from Endowment Project of “Infrastuctures Establishment of Thermal Energy Conversion and Desalination using Sea water Thermal Energy (2/3)” funded by the Korea Research Institute of Ships and Ocean Engineering (PES9060).
Equal contributions for all authors and discussed the results. All authors read and approved the final manuscript.
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