1. 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

2. 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

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

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

5. 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

6. 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

7. 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

8. 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

9. 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

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

11. 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

12. 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

13. 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,

14. 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:
Nano Lett., 2015, 15 (9), 6155–6161

15. 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.

16. 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

17. 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

18. “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

19. Thanks You!
Questions/Comments?