TeraWatts, TeraGrams, TeraLiters Workshop on Challenges and Opportunities for
Sustainable Production of Chemicals and Fuels beyond the Shale Gale
Solar Energy-driven Photoelectrochemical Conversion
Using Earth-Abundant Materials
Gengfeng Zheng
Fudan University
2015-02-02, UCSB
Acknowledgements
Collaborators:
Dongyuan Zhao (Fudan, Chemistry)
Wenbin Cai (Fudan, Chemistry)
Xingao Gong (Fudan, Physics)
Min Jiang (Fudan, Life Sciences)
Zhongqin Yang (Fudan, Physics)
Song Jin (U. Wisconsin, Chemistry)
Rong Fan (Yale, BioMedEng)
Ahmed Elzatahry (King Saud Univ.)
Students:
Jing Tang (PhD)
Biao Kong (PhD)
Yongcheng Wang (MS)
Peimei Da (PhD)
Yuhang Wang (PhD)
2
TeraWatt Scale Global Electricity Consumption
Substantial global energy demand, cost and environment footprints.
3
Research Goal
Material
Structure
Electronic
Structure
Interfaces
Research in synthesis of new nanomaterials/structures and developing novel
physical measurement methods for a variety of opportunities for catalysis,
photoconversion, energy storage and biointerface.
4
Solar Energy-driven Artificial Photosynthesis
• Semiconductor particles as photo harvester
• Needs co-catalysts (e.g., Pt, MoS2, and Co3O4)
• Return reaction of H2O need to be prevented
• e- or h+ scavengers can be used (electrolytes)
to generate only O2 or H2.
Osterloh and Parkinson, MRS Bulletin, 2011, 36, 17
A. Kudo and Y. Miseki. Chem. Soc. Rev. 2009, 38, 253-278
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Semiconductor Photoelectrochemical (PEC) Conversion
Water reduction
Water oxidation
Grätzel et al. Science 2014, 345, 1593
6
Earth-Abundant Materials in the Earth Crust
7
Sustainable Solar Energy Conversion
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Design of Semiconductor Heterostructure & Interface
ηtotal = ηabsorption × ηseparation × ηtransfer
Charge
Transfer
Energy
e−
ECB
EF
e− e− e−
Band Gap
Tuning
φ
e-
ħω
h+ h+ h+
DOx
Electrochem
& Kinetics
EF,Redox
e- D
Red
EVB
Charge
Carrier
Density
Surface
Chemistry
(catalyst,
sensitizer,
receptor…)
Electrolyte
Semiconductor Heterostructure & Interface
Goals:

Rational design/synthesis of material structures & interface for enhanced photocatalysis.

Utilizing charge transport behavior for energy conversion and probing molecule interfaces.
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Research Design
ηtotal = ηabsorption × ηseparation × ηtransfer
Bandgap
Tuning
Charge
Carrier
Density
Charge
Transfer
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Semiconductor Materials for PEC
Walukiewicz, Physica B 302-303, 123 (2001)
Van de Walle, Nature 423, 626 (2003)
Peidong Yang, Chem. Mater. 26, 415-422 (2014)
band gap, band alignment, conductivity, stability, cost, ...
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Mesoporous Fe2O3 Nanopyramid-Au NP Heterostructure
Mesoporous Fe2O3 nanopyramid-Au nanoparticle heterostructure:

Mesoporous Fe2O3 nanopyramids are formed by an interfacial oriented growth of Prussian
blue nanocubes w/o template, and subsequent calcination. AuNPs (5 nm) are then sputtered.
Kong B, Zheng GF*, Zhao DY*, et al. J. Am. Chem. Soc., 2014, 136, 6822.
Kong B, Zheng GF*, Zhao DY*, et al. Angew. Chem. Int. Ed., 2014, 53, 2888.
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Mesoporous Fe2O3 Nanopyramid-Au NP Heterostructure
Features:

Mesoporous nanopyramid
structure with Au NPs

High surface area (~ 175 m2/g)

Large mesopore size (~ 20 nm)

Excellent flexibility

Scalability (inch to meter)
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Fe2O3 Nanopyramids-Au NPs as LSP-Enhanced PEC

The integration of plasmonic
Au NPs with Fe2O3
nanopyramids enable
localized surface plasmon
(LSP) for PEC conversion,
leading to ~6- and ~83-fold
increase of photocurrent
under solar light and visible
light illumination, respectively.
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Simultaneous PEC Conversion & Energy Storage
TiO2 – Ni(OH)2
Si – Pt
Wang YC, Zheng GF*, et al. Nano Lett., 2014, 14, 3668.
Wang YH, Xia YY*, Zheng GF*, et al. Nano Lett., 2014, 14, 1080.
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Simultaneous PEC Conversion & Energy Storage
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Research Design
ηtotal = ηabsorption × ηseparation × ηtransfer
Bandgap
Tuning
Charge
Carrier
Density
Charge
Transfer
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Simultaneous Etching & W-Doping of TiO2 NWs
Post-doping of TiO2 NWs:


Conventional post-doping methods
require high processing temperatures.
Simultaneous etching (NH2OH∙HCl) and
W-doping (Na2WO4) can dope W atoms
into deeper layer of NWs.
Wang YC, Zheng GF*, et al. ACS Nano, 2013, 7, 9375.
Xu M, Da PM, Zheng GF*, et al., Nano Lett. 2012, 12, 1503.
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Enhanced Carrier Density and Charge Transfer
Mott-Schottky Plot
Flatband potential (EFB), Charge carrier density (Nd)
Samples
EFB (V) vs. Ag/AgCl
Nd / 1018 cm-3
pristine TiO2 NW
−0.89
3.86
doped TiO2 NW
−0.87
2.06
etched TiO2 NW
−0.58
1.36
dual etched/doped TiO2 NW
−0.60
5.04
DFT Simulation

DFT simulation shows W 5d states exist in the bandgap and is close to the conduction
band edge (Ti 3d), leading to the enhanced charge excitation and carrier density.
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Dual Etched & W-doped TiO2 NWs for Enhanced PEC

Dual etched/W-doped NWs: Substantial PEC activity enhancement compared to the
pristine, the etch-only, and the doped-only TiO2 NWs.
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Simultaneous Etching and Reducing of WO3 Nanoplates

Substantial etching and reducing of WO3 nanoplates allows for forming more reduced
W5+ ions via a facile solution process, leading to enhanced charge carrier densities.
Li WJ, Da PM, Zheng GF*, et al. ACS Nano, 2014, 8, 11770.
Peng Z, Jia DS, Zheng GF*, et al. Adv. Energy Mater., 2015, 5, 1402031.
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Dual Etched & Reduced WO3 for Enhanced PEC
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Research Design
ηtotal = ηabsorption × ηseparation × ηtransfer
Bandgap
Tuning
Charge
Carrier
Density
Charge
Transfer
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Reduced Mesoporous Co3O4 NWs for Water Oxidation
NaBH4 reduction of mesoporous Co3O4 NWs:

The enhanced surface area of mesoporous Co3O4 NWs allows for more efficient
NaBH4 solution reduction of Co3O4 NWs, leading to higher oxygen vacancy density.
Wang YC, Yang ZQ*, Zheng GF*, et al. Adv. Energy Mater., 2014, 4, 1400696.
Wang YC, Yang ZQ*, Zheng GF*, et al. Small, 2014, 10, 4967.
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Reduced Mesoporous Co3O4 NWs for Water Oxidation
Chemically reduced mesoporous Co3O4 NWs:

Current: 13.1 mA/cm2 at 1.65 V vs RHE, (7-fold of pristine Co3O4, also higher than IrO2)

Onset V: 1.52 V vs RHE, (50 mV ahead of pristine Co3O4, but 100 mV higher than IrO2)
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Tuning of Charge Carriers by Reduction/Oxidation
NaBH4 reduction of TiO2 NWs:

Oxygen vacancies in the reduced TiO2 cause defect states in the band structure
and result in enhanced carrier density and conductivity.
Wang YC, Yang ZQ*, Zheng GF*, et al. Adv. Energy Mater., 2014, 4, 1400696.
Wang YC, Yang ZQ*, Zheng GF*, et al. Small, 2014, 10, 4967.
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Co3O4 Nanosheet/Nanotube as OER Catalyst


Ultrathin CoOx nanosheets that are further assembled into a nanotube structure.
Highly active Co2+ electronic structure for efficient OER at the atomic scale, ultrahigh surface
area (371 m2·g-1) for interfacial electrochemical reaction at the nanoscale, and enhanced
transport of charge and electrolyte over CoOx nanotube building blocks at the microscale.
Peng Z, Jia DS, Zheng GF*, et al. Adv. Energy Mater., 2015, 5, 1402031.
Wang YC, Cai WB*, Zheng GF*, et al. Adv. Science, 2015, in press.
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Co3O4 Nanosheet/Nanotube as OER Catalyst


Low onset potential of ~1.46 V
vs. RHE, high current density of
51.2 mA·cm-2 at 1.65 V vs. RHE,
and a Tafel slope of 75 mV·dec-1.
1.5 V for full water splitting.
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Mass Production and Wide Applicability
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Properties of Fe2O3 Frameworks
1.2 kg
production
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3.8 m
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Research Goal
Research in synthesis of new nanomaterials/structures and developing novel
physical measurement methods can open a variety of opportunities for
catalysis, photoconversion, energy storage and biointerface.
Review: Wang YL, Zheng GF*, et al. Adv. Mater. 2013, 25, 5177.
Review: Tang J, Li J, Zheng GF*, et al. Chem. Eur. J. 2015, in press.
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Thank You !
Shanghai – Seen
from a Clear Sky
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