RSC Adv. 4 (2014)

UV light-induced surface-initiated atom-transfer radical polymerization (ATRP) was reported. This method uses TiO 2 nanoparticles as photoactive materials to reduce Cu(II)/L to a Cu(I)/L complex under UV irradiation by a one-electron transfer process for ATRP with multiple usage of monomer solutions. The growth of polymer brushes can be manipulated by either varying the content of photoactive materials or regulating the irradiation intensity, thereby yielding polymer brushes with controllable thickness, composition, and architecture. P olymer brushes produced by controlled or living surfaceinitiated radical polymerization 1,2 provide a superior route for surface functionalization, such as creation of smart surfaces, 3−5 antibiofouling materials, 6,7 and lubrication. 8 Compared with various controlled radical polymerization techniques 9 and organic polymer synthesis strategies, 10 atomtransfer radical polymerization (ATRP) 11−14 facilitates the construction of diverse polymer assemblies. 15−18 Generally, a Cu(I)−ligand complex and high monomer concentration are necessary to maximize polymer growth, 19−21 which highly relies on the reversible redox activation/deactivation of Cu(I)/ Cu(II). Control over the Cu(I)/Cu(II) ratio is important to obtain fast a polymerization rate while prolonging the lifetime of propagation chains. 22,23 The scope of ATRP has been expanded to external stimulusinduced polymerizations, 24,25 such as in electrochemically mediated ATRP (eATRP) for controlling polymerization by the electrically one-electron reduction of air-stable Cu(II). 26,27 We extended the eATRP technique to surface-initiated eATRP for controllable fabrication of surface-attached polymer brushes. 28 External visible light-stimulated living radical polymerization has been proposed through the excited Ir(III) species to reduce an alkyl bromide initiator for alkyl radical formation to initiate polymerization, 29,30 click reaction, 31,32 light-mediated atom-transfer radical addition, 33 and photoiniferter-mediated surface attached polymers formation. 34 The key to external stimulus-induced polymerization is continuous in situ generation of the activator catalyst. In this report, we propose a novel approach to achieve surface-initiated ATRP with multiple usage of monomer solutions, wherein the polymerization activators, Cu(I)−ligand, can be continuously generated from a photochemical reduction process by the excited electrons under ultraviolet (UV) illumination that uses the TiO 2 nanoparticle as the photosensitive material. Scheme 1(b) displays the mechanism of UV light-induced ATRP. A commercially available TiO 2 nanoparticle, P25, was

This review starts with a brief introduction to TiO 2 nanotubes (NTs), and then discusses in more detail how to optimize the structure of TiO 2 NTs for the fabrication of highly efficient solar cells, including the controllable fabrication of perfectly aligned TiO 2 NTs, optimizing the tube parameters, strategies of sensitization, improvement of the interface adhesion in polymer/TiO 2 solar cells and how to make devices flexible. Some key challenges and perspectives for future research are also tentatively discussed.