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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.29 No.3 pp.183-189
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2019.29.3.183

Preparation of Spherical TiO2 Nanoparticles Using Amphiphilic PCZ-r-PEG Random Copolymer Template Membrane

Jae Hun Lee*, Rajkumar Patel**†
*Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea
**Energy and Environmental Science and Engineering, Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, 85, Songdogwahak‐ro, Yeonsu-gu, Incheon 21983, Korea
Corresponding author(e-mail: rajkumar@yonsei.ac.kr)
June 24, 2019 ; June 27, 2019 ; June 27, 2019

Abstract


Amphiphilic PCZ-r-PEG random copolymer assisted solvothermal process is used to prepare mesoporous TiO2 microspheres generated from nanoparticles by self-assembly method. Synthesized PCZ-r-PEG is characterized by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), gel permeation chromatography (GPC) and transmission electron microscopy (TEM). The mesoporous TiO2 are prepared by PCZ-r-PEG, glucose, water in tertrahydrofuran solution at 150°C for 12 h and the TiO2 microspheres are calcined at 550°C for 30 min to further crystallize and organic residue are removed. Morphology and crystallization phase is characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) respectively. The mesoporous TiO2 crystallized in pure anatase phase with diameter of 300 ± 20 nm.


TiO2 , solvothermal , polymer , self-assembly

양친성 PCZ-r-PEG 랜덤 공중합체 분리막을 이용한 구형 이산화티타늄 나노입자의 제조

이 재 훈*, 라 즈쿠마 파텔**†
*연세대학교 화공생명공학과
**연세대학교 융합과학공학부

초록


양친성 PCZ-r-PEG 랜덤 공중합체를 기반으로 한 수열합성법을 통해 자가조립된 메조기공 이산화티타늄 마이크로 스피어를 합성하였다. 중합된 PCZ-r-PEG는 푸리에 변환 적외분광법(fourier transform infrared spectroscopy, FT-IR), 핵자기 공명(nuclear magnetic resonance, NMR), 젤 투과 크로마토그래피(gel permeation chromatography, GPC) 그리고 투과전자 현 미경(transmission electron microscopy, TEM)을 통해 그 특성이 분석되었다. 다공성 이산화티타늄 입자는 PCZ-r-PEG, 글루 코스(glucose), 물을 테트라히드로푸란(tetrahydrofuran, THF) 용액에 분산시킨 뒤 150°C, 12시간 동안 반응시켰다. 다공성 이 산화티타늄 입자의 구조와 결정성 분석을 위해 주사전자현미경(scanning electron microscopy, SEM)과 엑스선 회절(X-ray diffraction, XRD)이 사용되었다.


    1. Introduction

    Mixed matrix membranes (MMMs) composed of inorganic fillers with organic polymer matrix is a promising candidate for gas separation process. Incorporation of porous inorganic filler and high permselective polymer matrix can overcome intrinsic limitation of polymer membrane. There were various inorganic fillers to achieve high gas separation performance such as carbon-based materials[1,2], zeolite[3] and metal oxides[ 4,5].

    Nanostructured materials synthesized are one of the most important area of research due to the correlation between the specific morphology and unique properties[ 6,7]. Mesoporous TiO2 has large surface area and unique pore structure distribution which resulted in several applications in electrochemical devices, air protection and membranes[8-12]. TiO2 with specific morphology can be prepared by hydrothermal, sol-gel, hydrolysis and precipitation[13-20].

    Solvothermal process is one of the simplest method for preparation of well-designed morphology with porosity and high crystallinity. Structure directing agent induced solvothermal process leads to in situ growth of crystallized metal oxide. Yu et al. prepared hollow inorganic microsphere by chemically induced self-transformation method. In this solvothermal process two different approach for self-transformation is followed. Firstly, polymer (poly(4-styrenesulfonate)) mediated method to prepare calcium carbonate hollow microsphere. Secondly fluoride mediated approach with NH4F, SnF2 as source to prepare TiO2 and SnO2 hollow sphere respectively[21]. Ti (SO4)2 and NH4F is used to prepare TiO2 hollow sphere by hydrothermal method[22]. Xi et al. prepared hybrid TiO2 in French fries like morphology by P123 as the structure directing directing agent[23]. Hydrated salt assisted solvothermal (HAS) strategy is utilized to prepare TiO2 hollow sphere with hybrid composition[24]. Roh et al. prepared TiO2 hollow nanosphere by solvothermal process with polyethylene glycol (PEG) as the structure directing agent. It is used to prepare mixed matrix membrane for carbon dioxide (CO2) separation process[25]. Chen et al. reported the synthesis of monodisperse mesoporous anatase TiO2 by sol-gel and solvothermal process with hexadecylamine as the structuring agent[26]. In a similar process Cheng et al. reported TiO2 microsphere with nonanoic acid as the structure directing agent[27].

    In this work PCZ-r-PEG random copolymer is synthesized by solution condensation process and characterized by FTIR, 1H-NMR, TEM etc. Sol-gel solution of TTIP is prepared and poured into THF solution containing random copolymer which act as a structure directing agent in the solvothermal process. The prepared spherical TiO2 is calcined and characterized by SEM and XRD.

    2. Experiments

    2.1. Materials

    Poly(ethylene glycol) (2,000 g/mol), pyridine (99%), bisphenol Z (98%), triphosgene (98%), dicholoromethane (99.9%), titanium (iv) isopropoxide (TTIP, 97%), hydrogen chloride solution (HCl, 37%) and D-glucose are procured from Aldrich. Diethyl ether (99%), and tetrahydrofuran (99.5%) are bought form J. T. Baker. All the chemicals are used without further purification.

    2.2. Synthesis of PCZ-r-PEG random copolymer

    Amphiphilic random PCZ-r-PEG copolymer is synthesized by solution condensation polymerization. Fixed amount of poly(ethylene glycol) and bisphenol Z are taken in an round bottom flask and 75 mL of dichloromethane is added into it. The solution is stirred on a magnetic stirrer. Pyridine (50 mmol) is added into the round bottom flask and mixed well to prepare completely homogenous solution. Fixed amount of triphosgene is dissolved in 20 mL of dichloromethane. Round bottom flask is kept on an ice bath to reduce the temperature of the reaction mixture. Triphosgene solution is added drop by drop for a long period of time for completion of reaction. Further the reaction is carried out overnight and then precipitated in diethyl ether. The product is filtered and dissolved further in dichloromethane for several times. Finally, the product is dried completely in a vacuum oven. The molar ratio of the bisphenol Z : triphosgene : poly(ethylene glycol) is 9 : 4 : 50 respectively. 0.18 g of PCZ-r-PEG amphiphilic random copolymer is dissolved in 35 mL of tetrahydofuran. 0.6 g of glucose is dissolved in 3 mL of deionized water. Sol-gel solution of titanium isopropoxide is prepared by adding hydrochloric acid with a volume ratio of 2 : 1 and stirred for some time. Glucose solution is added slowly to the polymer solution with continuous stirring. Then the sol-gel solution is added to the above solution gradually and stirred for about 2 h. The solution is transferred to a teflon autoclave and packed in a stainless jacket which is transferred to an oven and kept for 12 h at 150°C. The autoclave is cooled to room temperature and the resultant solution is centrifuged at 5,000 rpm to separate the product. The brown color product is washed with tetrahydrofuran and ethanol to remove the side product. The solution is kept in oven at 100°C for some time to dry the solvent. The product is then calcined in a furnace at 550°C for 30 min with a ramping rate of 4 °C/min. Then the furnace is cooled to room at a control rate of 0.5 °C/min. The obtained spherical TiO2 product is checked by scanning electron microscope and X-ray diffraction.

    2.3. Characterization

    FT-IR spectra of the samples are measured in Excalibur series FT-IR (DIGLAB Co., Hannover, Germany) in the frequency range of 4,000~600 cm-1 with an attenuated total reflection facility. 1H-NMR characterization is performed in 600 MHz high resolution NMR spectrometer (Avance 600 MHz FT-NMR spectrometer, Bruker, Ettlingen, Germany). XRD of the samples are measured in Rigaku RINT2000 (Japan) wide-angle goniometer with a Cu cathode operated at 40 kV and 300 mA. The wavelength of the radiated wave is 1.54 Å, operate in a 2θ range of 10~80° with a scan rate of 4 min-1. Field emission-scanning electron microscope (FE-SEM) model no S-4700, Hitachi, is used to measure the morphological properties of the samples. Energy filtered transmission electron microscopy (EF-TEM) is measured by Philips CM30 microscope operating at 300 kV after casting the solution on a copper grid.

    3. Results and Discussion

    Aromatic polycarbonate is prepared by different methods like interfacial condensation reaction of phosgene and aromatic diol, base catalyzed transesterification of bisphenol and diphenyl carbonate, reaction of phosgene with a bisphenol solution containing tertiary amine [28-31]. Thermotropic liquid crystalline polycarbonate is prepared by using triphosgene[32-34]. Randomly branched polycarbonate is prepared by bisphenol A, triphosgene and tris(4-hydroxy-phenyl) ethane[35]. In this work amphiphilic PCZ-r-PEG random copolymer is prepared by reaction between bisphenol Z, triphosgene and PEG which is presented in Fig. 1.

    1H-NMR spectra of the amphiphilic PCZ-r-PEG is presented in Fig. 2. The aromatic proton peak appears in the region of 7~8 ppm. Phenyl proton “b” from bisphenol Z adjacent to the electron withdrawing ether group appears in the downfield region of 7.4 ppm. The proton “c” from the same phenyl group adjacent to the cyclohexane group appear at the upfield region of 7.2 ppm. Aliphatic methylene protons “c” of polyethyleglycol appears at higher field of 3.4 ppm. The percentage of PCZ and PEG in the amphiphilic PCZ-r-PEG random copolymer is calculated from representative peak at 7.4 and 3.4 respectively. The actual mass ratio of PCZ : PEG are around 47 : 53. Synthesis of PCZ-r-PEG is checked by FTIR spectroscopy as presented in Fig. 3. The carbonate group of the polycarbonate copolymer appears at 1,769 cm-1. The aromatic benzene ring of bisphenol Z appears at 1,503 cm-1. The ether group present in the polyethylene glycol is represented by peak at 1,103 cm-1. The FTIR character showed that the amphiphilic PCZ-r-PEG random copolymer is successfully synthesized.

    Transmission electron microscope of the amphiphilic PCZ-r-PEG random copolymer is presented in Fig. 4 which showed phase separated morphology. The darker region represented by the hydrophobic polycarbonate Z region with higher electron density. The hydrophilic poly(ethylene glycol) group is represented by brighter region. The induced repulsion between the hydrophilic and hydrophobic region leads to the self-assembly in to nanophase region of the amphiphilic random copolymer.

    Amphiphilic PCZ-r-PEG copolymer with micro-phase separated morphology due to the hydrophilic and hydrophobic domain played a crucial role of structure directing agent for crystalline growth. Titanium precursor TTIP interact specifically with the hydrophilic poly(ethylene glycol) of the copolymer and crystallization is initiated during the hydrothermal process. Hydrophobic PCZ remain ideal during the process due to the inertness towards the TTIP precursor. Hydrophilic sugar molecule plays an important role of growth directing agent of TiO2 crystal. The steric hindrance between the amphiphilic macromolecule and TTIP precursor is reduced by the presence of small hydrophilic glucose molecule. It induces better interaction between the TiO2 precursor and the amphiphilic macromolecules. During calcination the crystal growth is enhanced and the hydrophobic PCZ group present in the amphiphilic PCZ-r-PEG random copolymer generate mesoporous structure in the TiO2 sphere.

    TiO2 mesoporous sphere is characterized by SEM as presented in Fig. 5. The size of the mesosphere is around 300 nm. The enlarged size of the sphere is presented in Fig. 5(d) that shows the interconnected porous structure. The individual crystals are segregated to form the mesoporous large TiO2 sphere. The crystallinity phase of the TiO2 spheres are characterized by XRD analysis as presented in Fig. 6. Mesoporous TiO2 beads are crystallized in pure anatase phase which is indicated in the XRD. The peaks at 25.3, 37.86, 48.02, 54.06, 54.9, 62.56, 68.52, 70.08, and 75.07° corresponds to the reflection from 101, 004, 200, 105, 211, 204, 116, 220 and 215 crystal plane of the anatase phase, respectively[36].

    4. Conclusions

    Micro-phase separation behavior of an amphiphilic random PCZ-r-PEG copolymer is used as a structure directing agent for preparation of mesoporus TiO2 by solvothermal process. Smaller hydrophilic glucose molecules induced better interaction between the random macromolecules and the TiO2 precursor leading to self-assembly of nanoparticles and formation of mesoporous TiO2 structure. TiO2 mesospheres prepared after calcination crystallize in pure anatase phase.

    Figures

    MEMBRANE_JOURNAL-29-3-183_F1.gif

    Synthesis scheme of PCZ-r-PEG copolymers.

    MEMBRANE_JOURNAL-29-3-183_F2.gif

    1H-NMR spectrum of PCZ-r-PEG copolymer.

    MEMBRANE_JOURNAL-29-3-183_F3.gif

    FTIR spectra of PCZ homopolymer and PCZ-r-PEG copolymers.

    MEMBRANE_JOURNAL-29-3-183_F4.gif

    TEM images of PCZ-r-PEG copolymers.

    MEMBRANE_JOURNAL-29-3-183_F5.gif

    SEM images of (a) TiO2 microspheres (b~d) Enlarged images of TiO2 microspheres.

    MEMBRANE_JOURNAL-29-3-183_F6.gif

    XRD patterns of TiO2 microspheres.

    Tables

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