Nanomaterials for Solar Energy Harvesting and Conversion into Chemical Energy The last decade has seen tremendous progress in synthesizing and fabricating nanomaterials with precise control over their sizes, shapes, morphologies and chemical compositions. While these nanomaterials has exhibited promising applications in sensing, catalysis and biomedicine, to name a few, I will highlight how to use these emerging nanomaterials for solar energy harvesting and conversion into chemical energy. Zhaoxia Qian, Ph.D. University of Washington Solar Washington, May 4th, 2016

Solar Energy Harvesting and Conversion Electricity Thermal energy Chemical energy Solar cells 22% max efficiency so far Solar water heater Solar drying Solar cooking Solar distillation Over 30% efficiency Artificial photosynthesis Solar water splitting

Solar Energy to Chemical Energy: Photosynthesis Source: Royal Society of Chemistry

Solar Energy to Chemical Energy: Artificial Photosynthesis Light harvesting Charge separation Water splitting Fuel production Light harvesting Charge separation Water splitting Fuel production

Solar Water Splitting 2 H2O  2 H2 + O2 ∆G = + 237.2 KJ/mol Photoelectrolysis Cell Source: Empa, the Swiss Federal Laboratories for Materials Science and Technology

Semiconductors for Solar Water Splitting HER: hydrogen evolving center (photocathode) p-type semiconductor OER: oxygen evolving center (photoanode) n-type semiconductor A simple photoelectrolysis cell

Semiconductors for Solar Water Splitting Tungsten trioxide (WO3) Titanium dioxide (TiO2) Gallium arsenide (GaAs) Gallium phosphide (GaP) STH: solar to hydrogen conversion efficiency PEC: photoelectrolysis cell Although several semiconductors have band-edge positions that are appropriate for the photoelectrochemical reduction of water, the kinetics of HER on the bare semiconductor surface generally limit the efficiency of this reaction.41 Overcoming this kinetic limitation requires a stronger driving force, i.e. an overpotential, to drive the desired chemical reaction. In turn, the overpotential lowers the usable voltage output, and hence lowers the efficiency of the photocathode.5 Indium phosphide (InP) Silicon (Si) Chem. Rev. 2010, 110, 6446–6473

Semiconductors for Solar Water Splitting photocathode/photoanode Current research efforts: Photoanodes for Water Oxidation Silicon Photocathode Arrays Water-Splitting Membrane Hydrogen Evolution Catalysis Silicon Surface Chemistry Light-Material Interactions Non-conventional Solar Absorbers Chem. Rev. 2010, 110, 6446–6473

Replacing platinum (Pt) Semiconductors for Solar Water Splitting Silicon photocathode arrays on flexible substrate  nanostructured tungsten oxide non-conventional solar absorbers Replacing platinum (Pt) catalyst Synthesis of Cu2O substrates by high temperature thermal oxidation

Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting Nano Lett., 2013, 13 (6), 2989–2992 Peidong Yang Group, UC Berkeley

Nanowire–Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation genetically engineered  Escherichia coli  high-surface-area silicon nanowire array + anaerobic bacterium (Sporomusa ovata) Nano Lett., 2015, 15 (5), 3634–3639 Peidong Yang Group, UC Berkeley

Localized Surface Plasmon Resonance of Metal Nanoparticles Introducing Metal Nanoparticles into Solar Water Splitting Localized Surface Plasmon Resonance of Metal Nanoparticles Resonance induces strong absorption and scattering of light. Resonance frequency is highly tunable via varying nanoparticle size, shape and chemical compositions. Enhanced local electromagnetic field. Hot electron/hole generation near field map of a gold nanoparticle

Introducing Metal Nanoparticles into Solar Water Splitting Martin Moskovits Group, UC Santa Barbara 1st example proving that hot electrons generated on a metal surface can be used to drive water splitting. Solar-to-hydrogen efficiency (0.1%) (high for metal nanosparticles) all charge carriers involved in the oxidation and reduction steps arise from the hot electrons resulting from the excitation of surface plasmons in the nanostructured gold “The system shows long-term operational stability and could lead to new types of clean solar-energy harvesting device, fully exploiting the benefits of nanostructured electrodes.” -- Prof. Mark L. Brongersma, Stanford University Nature Nanotechnology, 2013, 8, 247-251

Metal Nanoparticles for Solar Water Splitting Direct Plasmon-Driven Photoelectrocatalysis ay’s best photovoltaic solar panels are the result of hot electrons that cool within a few trillionths of a second and release their energy as wasted heat. plasmons, waves of electrons that flow like a fluid across the metal surface of the nanoparticles. Plasmons are high-energy states that are short-lived, but researchers at Rice and elsewhere have found ways to capture plasmonic energy and convert it into useful heat or light. - See more at: http://news.rice.edu/2015/09/04/rice-researchers-demo-solar-water-splitting-technology-2/#sthash.Nemxzvgb.dpuf Nano Lett., 2015, 15 (9), 6155–6161

Remaining Challenges Improve photoconversion efficiency by designing novel nanomaterials/nanostructures with high photon absorption efficiency, high hot carrier generation efficiency, low hot carrier cooling rate, etc. Improve long-term operation stability of nanoelectrodes. Decrease solar energy conversion cost by simplifying the fabrication process and using cheaper/less nanomaterials. Understand the fundamental science and provide guidelines to facilitate the research and technological progress.

Global Research on Solar Water Splitting The Netherlands Includes 10 research institutions and 45 private industries; budget € 42 million (funded from public and private sources) Includes 13 European countries Swedish Consortium for Artificial Photosynthesis US’s largest research program dedicated to the advancement of solar-fuels generation science and technology; Led by California Institute of Technology and joint by Lawrence Berkeley National Laboratory, California campuses at Irvine (UCI) and San Diego (UCSD), and the SLAC National Accelerator Laboratory dedicated to the advancement of solar-fuels generation science and technology. Led by California Institute of Technology (Caltech) and joint by Lawrence Berkeley National Laboratory, California campuses at Irvine (UCI) and San Diego (UCSD), and the SLAC National Accelerator Laboratory. Northwestern University

Global Research on Solar Water Splitting Korean Centre for Artificial Photosynthesis (KCAP) (2009) The ICARE Institute Funded by the European Union, located on the premises of Huazhong University of Science and Technology (HUST) in Wuhan, China Launched in 2007 by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in a drive to build within Japan

“We are powerfully inspired that we have a photovoltaic industry that is worldwide, multi-scaled with integrated manufacturing,” he said. “We’re laying the foundations that could produce an artificial photosynthesis industry in the future.” - Harry Atwater, director of Caltech’s Joint Center for Artificial Photosynthesis (JCAP), 2015

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