Electrochemical engineering of hollow nanoarchitectures: pulse/step anodization (Si, Al, Ti) and their applications

Hollow nanoarchitectured materials with straight channels play a crucial role in the fields of renewable energy, environment and biotechnology due to their one-dimensional morphology and extraordinary properties. The current challenge is the difficulty on tailoring hollow nanoarchitectures with well-controlled morphology at a relatively low cost. As a conventional technique, electrochemistry exhibits its unique advantage on machining nanostructures. In this review, we present the progress of electrochemistry as a valuable tool in construction of novel hollow nanoarchitectures through pulse/step anodization, such as surface pre-texturing, modulated, branched and multilayered pore architectures, and free-standing membranes. Basic principles for electrochemical engineering of mono- or multi-ordered nanostructures as well as free-standing membranes are extracted from specific examples (i.e. porous silicon, aluminum and titanium oxide). The potential of such nanoarchitectures are further demonstrated for the applications of photovoltaics, water splitting, organic degradation, nanostructure templates, biosensors and drug release. The electrochemical techniques provide a powerful approach to produce nanostructures with morphological complexity, which could have far-reaching implications in the design of future nanoscale systems.