Interfacial Donor-Acceptor Engineering of Nanofiber Materials To Achieve Photoconductivity and Applications

Self-assembly of π-conjugate molecules often leads to formation of well-defined nanofibril structures dominated by the columnar π-π stacking between the molecular planes. These nanofibril materials have drawn increasing interest in the research frontiers of nanomaterials and nanotechnology, as the nanofibers demonstrate one-dimensionally enhanced exciton and charge diffusion along the long axis, and present great potential for varying optoelectronic applications, such as sensors, optics, photovoltaics, and photocatalysis. However, poor electrical conductivity remains a technical drawback for these nanomaterials. To address this problem, we have developed a series of nanofiber structures modified with different donor-acceptor (D-A) interfaces that are tunable for maximizing the photoinduced charge separation, thus leading to increase in the electrical conductivity. The D-A interface can be constructed with covalent linker or noncovalent interaction (e.g., hydrophobic interdigitation between alkyl chains). The noncovalent method is generally more flexible for molecular design and solution processing, making it more adaptable to be applied to other fibril nanomaterials such as carbon nanotubes. In this Account, we will discuss our recent discoveries in these research fields, aiming to provide deep insight into the enabling photoconductivity of nanofibril materials, and the dependence on interface structure. The photoconductivity generated with the nanofibril material is proportional to the charge carriers density, which in turn is determined by the kinetics balance of the three competitive charge transfer processes: (1) the photoinduced electron transfer from D to A (also referred to as exciton dissociation), generating majority charge carrier located in the nanofiber; (2) the back electron transfer; and (3) the charge delocalization along the nanofiber mediated by the π-π stacking interaction. The relative rates of these charge transfer processes can be tuned by the molecular structure and nanoscale interface engineering. As a result, maximal photoconductivity can be achieved for different D-A nanofibril composites. The photoconductive nanomaterials thus obtained demonstrate unique features and functions when employed in photochemiresistor sensors, photovoltaics and photocatalysis, all taking advantages of the large, open interface of nanofibril structure. Upon deposition onto a substrate, the intertwined nanofibers form networks with porosity in nanometer scale. The porous structure enables three-dimensional diffusion of molecules (analytes in sensor or reactants in catalysis), facilitating the interfacial chemical interactions. For carbon nanotubes, the completely exposed π-conjugation facilitates the surface modification through π-π stacking in conjunction with D-A interaction. Depending on the electronic energy levels of D and A parts, appropriate band alignment can be achieved, thus producing an electric field across the interface. Presence of such an electric field enhances the charge separation, which may lead to design of new type of photovoltaic system using carbon nanotube composite.