CAREER: Advanced Photochemical Paradigms for Enhanced Photovoltaics and Photocatalysis

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Xu1150617 Intellectual MeritsSolar energy conversion and utilization can be implemented through photoelecrochemical (PEC) schemes that convert light energy to electricity, such as PEC-based photovoltaics (PV), or chemical energy such as solar fuels and solar environmental purification using photocatalysts. Therefore, understanding the basic science that defines and regulates the elementary light harvesting and charge transport processes in PEC systems is crucial for the advancement of PEC solar cells and photocatalysis. The overall goal of this proposal is to fundamentally transform and enhance the elementary light harvesting and charge transport efficiencies in current state-of-the-art PEC-based photovoltaic and photocatalytic systems. Specifically, the proposed research plan will aim to the following subtasks.1. Converting the photoanodes of conventional PEC solar cells, typically in the format of a thick PV layer on a planar transparent conducting oxide (TCO), to a set of 3-D nanoarchitectured conformal TCO/PV photoanodes. This innovation will drastically shorten the transport length in the PV layer, leaving the majority of the transport in the nearly metallic TCO layer for enhanced photocurrent collection, while still providing sufficient PV material to accommodate the solar photon flux. The PI will study the fundamental transport mechanisms resulting from these transformative structures. In particular, reducing the thickness of the PV layer to approximately the width of the space charge layer (typically 30 nm) at the interface of TCO/PV layer can subject the overall light-induced charge separation and transport processes to the strong influence of the built-in potential at this interface. As a result, the slow diffusive transport mechanism can be replaced by the more efficient field-driven drift transport mechanism to suppress charge recombination in the PV layer. In turn, alternative faster redox mediators with less overpotential can be applied for enhanced photovoltaic performance. 2. Synergistically, the transformation from 2-D to 3-D TCO enables the incorporation of a variety of 3-D photonic crystal (PC) configurations into the 3-D conformal TCO/PV architectures, which can trap light through multiple scattering of photons, resulting in synergy rather than conflict between the light harvest and charge transport processes. As such, PEC solar cells can be more affordable since less PV materials are needed for accommodating the solar flux. 3. Ti3+ and/or oxygen vacancy self-doped TiO2-x can be an effective visible-light active photocatalyst that using no additional dopants, thus to minimize the environmental concerns. In order to achieve efficient visible-light activity, the concentration of Ti3+ must be high enough to induce a continuous vacancy band of electronic states just below the conduction band edge of TiO2. Distinguished from the conventional reduction of TiO2 (Ti4+-¨Ti3+) under harsh conditions, we propose an oxidative conversion (Ti2+-¨Ti3+) scheme that uses TiH2 and H2O2 as precursors for Ti3+ self-doped TiO2-x photocatalysts. This process will provide a facile, low-cost and high-concentration doping of Ti3+ throughout bulk of the TiO2-x matrix. The reaction of TiH2 and H2O2 produces a hydrolyzed sol-gel-like precursory TiO2-x, allowing convenient incorporation of plasmonic nanoparticles and formation of versatile nanoarchitectured TiO2-x for enhanced solar photocatalytic activity. Broader ImpactsAs nanoscience provides more opportunities for more efficient PEC processes, the primary educational goal is to integrate introductory nanoscience experiments relevant to both this project and undergraduate chemistry into our existing curriculum with laboratory assignments, so as to provide students hands-on understanding in course work and basic nanoscience knowledge. Secondly, as an effective way to train the next generation scientists, the PI plans to broaden the research collaboration with Argonne scientists by developing a multifunctional program: Northern Student Scientists at Argonne. This program will incorporate PI's research collaborations with Argonne into student recruitment, education and training, research, employment and community outreach. The students will be exposed to world-class research environment, involved in frontline research projects at their young age, and trained with cutting-edge research facilities, cross-disciplinary knowledge, critical thinking and problem-solving methodologies, and team-work skills. Due to the growing concerns on environmental disasters related to petroleum and nuclear energy, PEC energy conversion becomes a highly attractive aspect of energy science. As energy science is Argonne's core research area, the PI will leverage Argonne'fs research and NIU's network to establish an outreach program Energy and Environment Workshop for students and teachers in local high schools via interactive presentations given by scientists at Argonne and Northern Illinois University.