Chemical doping with foreign atoms is an effective approach to significantly enhance the electrochemical performance of the carbon materials. Herein, sulfur-doped three-dimensional (3D) porous reduced graphene oxide (RGO) hollow nanosphere frameworks (S-PGHS) are fabricated by directly annealing graphene oxide (GO)-encapsulated amino-modified SiO2 nanoparticles with dibenzyl disulfide (DBDS), followed by hydrofluoric acid etching. The XPS and Raman spectra confirmed that sulfur atoms were successfully introduced into the PGHS framework via covalent bonds. The as-prepared S-PGHS has been demonstrated to be an efficient metal-free electrocatalyst for oxygen reduction reaction (ORR) with the activity comparable to that of commercial Pt/C (40%) and much better methanol tolerance and durability, and to be a supercapacitor electrode material with a high specific capacitance of 343 F g(-1), good rate capability and excellent cycling stability in aqueous electrolytes. The impressive performance for ORR and supercapacitors is believed to be due to the synergistic effect caused by sulfur-doping enhancing the electrochemical activity and 3D porous hollow nanosphere framework structures facilitating ion diffusion and electronic transfer.
Over the past decade or so, polymerization-induced self-assembly (PISA) has become a versatile method for rational preparation of concentrated block copolymer nanoparticles with a diverse set of morphologies. Much of the PISA literature has focused on the preparation of well-defined linear block copolymers by using linear macromolecular chain transfer agents (macro-CTAs) with high chain transfer constants. In this review, a recent process is highlighted from an unusual angle that has expanded the scope of PISA including i) synthesis of block copolymers with nonlinear architectures (e.g., star block copolymer, branched block copolymer) by PISA, ii) in situ synthesis of blends of polymers by PISA, and iii) utilization of macro-CTAs with low chain transfer constants in PISA. By highlighting these important examples, new insights into the research of PISA and future impact these methods will have on polymer and colloid synthesis are provided.
Block
copolymer polymersomes offer considerable access for applications
in a variety of fields; however, the traditional cosolvent self-assembly
method can only produce polymersomes at a low solids content (typically
<1%). Recently, an in situ growth method, termed
polymerization-induced self-assembly (PISA), has been developed to
allow the preparation of polymersomes at high solids (10–50%).
Synthesis and self-assembly of block copolymers occur simultaneously
in PISA, and therefore, morphological evolution occurs throughout
the polymerization. It is highly desirable to provide mechanistic
insights into morphological evolution that enables one to rationally
synthesize a variety of morphologies. Herein, we demonstrate that
the further growth of polymersomes in aqueous PISA can be conveniently
driven by temperature via two different pathways: (i) at high temperatures
from polymersomes with a thin membrane to polymersomes with a thick
membrane and (ii) at low temperatures from spherical polymersomes
to tubular and donut-like polymersomes. We show that both the hydrodynamic
diameter and membrane thickness of polymersomes increase during PISA
at high temperatures, indicating that the membrane of polymersomes
grows inward and outward simultaneously as the polymerization proceeds.
Furthermore, careful characterization of samples withdrawn during
the kinetic study at low temperatures by transmission electron microscopy
and dynamic light scattering reveals various intermediate morphologies
that provide important insights into the formation of tubular and
donut-like polymersomes. We expect that this study not only provides
mechanistic insights into the morphological evolution of PISA but
also expands the scope of PISA for the preparation of a variety of
structures.
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