Macromolecular Rapid Communications
Communication
Spatial Control over Brush Growth through Sunlight-Induced Atom Transfer Radical Polymerization Using Dye-Sensitized TiO2 as a Photocatalyst Bin Li, Bo Yu,* Feng Zhou*
Simulated-sunlight induced atom transfer radical polymerization is used for spatial control over polymer brush growth by in situ photo-generation of the CuI/L activator complex from its higher oxidation state CuII/L deactivator complex using dye sensitized titanium dioxide nanoparticles. The polymerization is well controlled under sunlight irradiation. Another attractive feature of this method is the possibility of creating various patterned surfaces of brushes using photomasks. When a nanoporous alumina oxide membrane is used as the template for confinement diffusion of photogenerated CuI/L catalyst, patterns with sub-50 nm resolution are obtained.
1. Introduction Surface-initiated controlled/living radical polymerization (SI-C/LRP) techniques have been widely used as a versatile and effective method for the fabrication of polymer brushes with predetermined molecular weight, architecture, and functionality.[1] Among several controlled radical polymerization strategies, atom-transfer radical polymerization has received significant attention for preparing polymer brushes from an initiator-decorated substrate. These polymers are chemically diverse and are well known to impart interesting physical behaviors. They have
B. Li, Dr. B. Yu, Prof. F. Zhou State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences 730000, China E-mail: [email protected]; [email protected] B. Li University of Chinese Academy of Sciences, Beijing 100049, China Macromol. Rapid Commun. 2014, 35, 1287−1292 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
a wide range of applications, including in stimuli-responsive materials,[2] antifouling,[3] actuators,[4] and lubrication.[5] Aqueous ATRP has several advantages; for example, it allows for the preparation of complex (co)polymers or biomolecule–polymer hybrids under biologically relevant conditions.[6] In recent years, the ATRP process has been improved by temporal control of polymerizations through an external stimulus, such as electrochemistry, light, and chemicals, where CuI activator is generated in situ by reducing a CuII/L deactivator complex. This has many advantages over conventional ATRP, such as controllability and better tolerance of the reaction conditions.[7] Photoinduced reactions are widely used in polymer synthesis.[8] Photoinitiated free radical polymerizations have attached great interest from both scientific and industrial viewpoints. Recently, semiconductors such as TiO2 or ZnO have been reported to initiate free radical polymerizations under UV light irradiation.[9] However, most simple semiconductors absorb only ultraviolet (UV) wavelengths and cannot be activated by the visible light that comprises most solar energy. Consequently, UV light sources are generally required for such photocatalysis
wileyonlinelibrary.com
DOI: 10.1002/marc.201400121
1287
Macromolecular Rapid Communications
B. Li et al.
www.mrc-journal.de
reactions. Long term UV irradiation may induce side reactions during the polymerization, leading to, for example, damage of polymers. It is therefore of great importance to explore the possibility of photo-induced ATRP under mild or realistic conditions, such as under sunlight irradiation with a broad range of wavelengths, because sunlight is effectively a “green” and inexhaustible energy source. Yagci and co-workers used an ecofriendly heterogeneous mesoporous graphitic carbon nitride (mpg-C3N4) to proceed ATRP under sunlight and UV light irradiation.[10] The excited electrons from the conduct band (CB) of mpg-C3N4 could reduce the CuII/L to CuI/L activators, which react with alkyl halide (R-X) and initiate ATRP. Herein, we report the successful use of dye sensitized TiO2 nanoparticles (d-TiO2) as a photosensitizer to initiate ATRP in aqueous media with sunlight without adding photoinitiator. Because of the dye doped TiO2, the light absorption wavelength can be extended to the visible range. Furthermore, pattered brushes can be grafted when a photomask is placed above the initiator coated substrate, and the size and the shape of the pattern can be tuned by varying the transparent area of the mask. In particular, when nanoporous alumina oxide membrane was used as the template for confinement diffusion of CuI catalyst, patterns with sub-50 nm resolution were obtained. Thus, this method could widen the range of polymerizable monomers using photo-induced ATRP techniques to proceed surface modifications, and open up new applications in photolithography.
2. Experimental Section 2.1. Materials 3-Sulfopropyl methacrylate potassium salt (SPMA, 95%, TCI), 2-hydroxyethyl methacrylate (HEMA, 97%, Aldrich), 2-(methacryloyloxy)ethyl trimethylammonium chloride (METAC, 80% in water, TCI), 2,2′-bipyridyl (bipy, AR, Shanghai, China), cupric bromide (CuBr2, AR, Shanghai, China), and Ruthenizer 620–1H3TBA (also known as “N749” or “Black Dye”, Solaronix) were purchased and used as received. The ultrapure water was prepared from a NANOpure Infinity system from Barnstead/Thermolyne Corporation. Commercial TiO2 granules (P25, 7:3 anatase to rutile crystalline phases, with a size range of 10∼40 nm) were purchased from Degussa (Germany). Photomasks containing transparent circles of 50 μm diameter and squares measuring 50 × 50 μm were used. Anodic aluminum oxide (AAO) membranes were purchased from Whatman International Ltd, England. Gold film was prepared via thermo evaporation of ∼100 nm gold on silicon wafer (100 orientation, one side polished) with 3∼5 nm Cr as the adhesive layer. Dye sensitized TiO2 (d-TiO2) nanoparticles were prepared by immersing P25 power in N749 ethanol solution (0.3 × 10–3M) for 24 h for the dye absorption (see the Supporting Information for details).
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2.2. ATRP Initiator Formation 2.2.1. Silicon A plasma oxidized clean silicon substrate was placed in a vacuum desiccator with a vial containing 10 μL of 3-(trichlorosilyl) propyl 2-bromo-2-methylpropanoate initiator. The chamber was pumped down to
Communication
Spatial Control over Brush Growth through Sunlight-Induced Atom Transfer Radical Polymerization Using Dye-Sensitized TiO2 as a Photocatalyst Bin Li, Bo Yu,* Feng Zhou*
Simulated-sunlight induced atom transfer radical polymerization is used for spatial control over polymer brush growth by in situ photo-generation of the CuI/L activator complex from its higher oxidation state CuII/L deactivator complex using dye sensitized titanium dioxide nanoparticles. The polymerization is well controlled under sunlight irradiation. Another attractive feature of this method is the possibility of creating various patterned surfaces of brushes using photomasks. When a nanoporous alumina oxide membrane is used as the template for confinement diffusion of photogenerated CuI/L catalyst, patterns with sub-50 nm resolution are obtained.
1. Introduction Surface-initiated controlled/living radical polymerization (SI-C/LRP) techniques have been widely used as a versatile and effective method for the fabrication of polymer brushes with predetermined molecular weight, architecture, and functionality.[1] Among several controlled radical polymerization strategies, atom-transfer radical polymerization has received significant attention for preparing polymer brushes from an initiator-decorated substrate. These polymers are chemically diverse and are well known to impart interesting physical behaviors. They have
B. Li, Dr. B. Yu, Prof. F. Zhou State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences 730000, China E-mail: [email protected]; [email protected] B. Li University of Chinese Academy of Sciences, Beijing 100049, China Macromol. Rapid Commun. 2014, 35, 1287−1292 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
a wide range of applications, including in stimuli-responsive materials,[2] antifouling,[3] actuators,[4] and lubrication.[5] Aqueous ATRP has several advantages; for example, it allows for the preparation of complex (co)polymers or biomolecule–polymer hybrids under biologically relevant conditions.[6] In recent years, the ATRP process has been improved by temporal control of polymerizations through an external stimulus, such as electrochemistry, light, and chemicals, where CuI activator is generated in situ by reducing a CuII/L deactivator complex. This has many advantages over conventional ATRP, such as controllability and better tolerance of the reaction conditions.[7] Photoinduced reactions are widely used in polymer synthesis.[8] Photoinitiated free radical polymerizations have attached great interest from both scientific and industrial viewpoints. Recently, semiconductors such as TiO2 or ZnO have been reported to initiate free radical polymerizations under UV light irradiation.[9] However, most simple semiconductors absorb only ultraviolet (UV) wavelengths and cannot be activated by the visible light that comprises most solar energy. Consequently, UV light sources are generally required for such photocatalysis
wileyonlinelibrary.com
DOI: 10.1002/marc.201400121
1287
Macromolecular Rapid Communications
B. Li et al.
www.mrc-journal.de
reactions. Long term UV irradiation may induce side reactions during the polymerization, leading to, for example, damage of polymers. It is therefore of great importance to explore the possibility of photo-induced ATRP under mild or realistic conditions, such as under sunlight irradiation with a broad range of wavelengths, because sunlight is effectively a “green” and inexhaustible energy source. Yagci and co-workers used an ecofriendly heterogeneous mesoporous graphitic carbon nitride (mpg-C3N4) to proceed ATRP under sunlight and UV light irradiation.[10] The excited electrons from the conduct band (CB) of mpg-C3N4 could reduce the CuII/L to CuI/L activators, which react with alkyl halide (R-X) and initiate ATRP. Herein, we report the successful use of dye sensitized TiO2 nanoparticles (d-TiO2) as a photosensitizer to initiate ATRP in aqueous media with sunlight without adding photoinitiator. Because of the dye doped TiO2, the light absorption wavelength can be extended to the visible range. Furthermore, pattered brushes can be grafted when a photomask is placed above the initiator coated substrate, and the size and the shape of the pattern can be tuned by varying the transparent area of the mask. In particular, when nanoporous alumina oxide membrane was used as the template for confinement diffusion of CuI catalyst, patterns with sub-50 nm resolution were obtained. Thus, this method could widen the range of polymerizable monomers using photo-induced ATRP techniques to proceed surface modifications, and open up new applications in photolithography.
2. Experimental Section 2.1. Materials 3-Sulfopropyl methacrylate potassium salt (SPMA, 95%, TCI), 2-hydroxyethyl methacrylate (HEMA, 97%, Aldrich), 2-(methacryloyloxy)ethyl trimethylammonium chloride (METAC, 80% in water, TCI), 2,2′-bipyridyl (bipy, AR, Shanghai, China), cupric bromide (CuBr2, AR, Shanghai, China), and Ruthenizer 620–1H3TBA (also known as “N749” or “Black Dye”, Solaronix) were purchased and used as received. The ultrapure water was prepared from a NANOpure Infinity system from Barnstead/Thermolyne Corporation. Commercial TiO2 granules (P25, 7:3 anatase to rutile crystalline phases, with a size range of 10∼40 nm) were purchased from Degussa (Germany). Photomasks containing transparent circles of 50 μm diameter and squares measuring 50 × 50 μm were used. Anodic aluminum oxide (AAO) membranes were purchased from Whatman International Ltd, England. Gold film was prepared via thermo evaporation of ∼100 nm gold on silicon wafer (100 orientation, one side polished) with 3∼5 nm Cr as the adhesive layer. Dye sensitized TiO2 (d-TiO2) nanoparticles were prepared by immersing P25 power in N749 ethanol solution (0.3 × 10–3M) for 24 h for the dye absorption (see the Supporting Information for details).
1288
2.2. ATRP Initiator Formation 2.2.1. Silicon A plasma oxidized clean silicon substrate was placed in a vacuum desiccator with a vial containing 10 μL of 3-(trichlorosilyl) propyl 2-bromo-2-methylpropanoate initiator. The chamber was pumped down to
