Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Fachgebiet Angewandte Nanophysik, Institut für Physik
Contact: [email protected]; [email protected]: Unterpoerlitzer Straße 38 (Heisenbergbau) (tel: 3748)
www.tu-ilmenau.de/nanostruk
Vorlesung: Thursday 13:00 – 14:30, F 3001Übung: Friday (G), 11:00 – 12:30, C 110
Prof. Yong Lei & Dr. Huaping Zhao
(a) (b2)(b1)
UTAM-prepared free-standing one-dimensional surface nanostructures on Sisubstrates: Ni nanowire arrays (a) and carbon nanotube arrays (b).
Nanostrukturphysik (Nanostructure Physics)
• Class 1: A general introduction of fundamentals of nano‐structured materials
• Class 2: Structures and properties of nanocrystalline materials• Class 3: Graphene• Class 4: 2D atomic‐thin nanosheets• Class 5: Optical properties of 1D nanostructures and nano‐generator• Class 6: Carbon nanotubes• Class 7: Solar water splitting I: fundamentals• Class 8: Solar water splitting II: nanostructures for water splitting• Class 9: Lithium‐ion batteries: Si nanostructures• Class 10: Sodium‐ion batteries and other ion batteries, and
Supercapacitors• Class 11: Solar cells• Class 12: Other nanostructures
Photosynthesis and nanostructures for solar water splitting II
• Brief review of Class 7• Advantages and disadvantages• Material designs and nanostructural architecture
N‐type semiconductor: top of valance band must be morepositive than water oxidization potential. Electric field generatedby band bending directs holes towards solution.
H2O + 2h+ ½ O2 + 2H+ + 4e‐
Ef H+ / H2
O2 / H2O
Thermodynamic Potential of Water: h
e-
h+
e-
Band Bending
Overpotential for O2 evolution
Photoanode for oxygen evolution
Requirement: proper bandgap; top of valance band must be morepositive than water oxidization potential; aganist corrosion.
TiO2: band gap is too large.
WO3: scarcity and high demand makes limited availability.
Fe2O3: short carrier diffusion length relative to absorption depth of light.
The semiconductor band‐edge position are plotted vesus their maximun theoretical photocurrent under Air Mass 1.5 illumination.
Photoanode materials
Photocathode (p‐type semiconductors) for hydrogen evolution
Requirement: proper bandgap; bottom of conduction band must bemore negative than water reduction potential; aganist corrosion.
P‐GaP: short carrier diffusion length relative to absorption depth of light
P‐InP: scarcity and high demand makes limited availability
P‐Si: stable in acidic environment
2H2O + 2e‐ H2 + 2OH‐ (high pH)
Typical photoelectrochemical (PEC) water splitting process
AnodeCathodeElectrolyte
One‐step photoexcitation system
Sufficiently narrow bandGap to harvest visiblephotons> 1.23 eV theoritically> 1.6 eV practically
Suitable thermodynamicpotential to achieveoverall water splitting
Stability against corrosion
Limited material meets all the requirements!
Nanostructures for solar water splitting II
• Brief review• Advantages and disadvantages• Material designs and nanostructural architecture
Advantages of nanostructured Photoelectrodes and photocatalysts
• Shorten carrier collection pathways• Improve light distribution• Quantum size confinement• Potential determining ions• Surface area‐enhanced charge transfer
1. Shorten carrier collection pathway
Mean free diffusion length LD: diffusion constant𝜏: lifetimeq: dimension factor
(q=2, 4, 6 for one‐, two‐, or three‐dimensional diffusion)
• Intrinsic semiconductors: Le > Lh• Doping: concentration of the majority carriers ↗
lifetime and diffusion length of the minority carriers ↘
d ~ Le and Lh
L2 = qDτ
2. Improve light distributionThe ability of a material to absorb light is determined by the Lambert BeerLaw and the wavelength‐dependent absorption coefficient α.
The light penetration depth l = α‐1 refers to the distance after which the lightintensity is reduced to 1/e of the original value.
Light loss by reflection Light scattering to improve distribution
Fe2O3 α‐1 = 118 nm @ λ = 550 nmCdTe α‐1 = 106 nm @ λ = 550 nmSi α‐1 = 680 nm @ λ = 510 nm
T = I/I0 = e‐αl
3. Quantum size confinement
The dependence of semiconductor energetics on particle size
Rates for charge transfer and water electrolysis ↗
4. Potential determine ions (PDIs)
nanostructures + Potential determine ions → external electric fields on surface → nanomaterial interior electric fields → local energe c structure
The band edge potential of nanomaterials, and resulting functions, including interfacial charge transfer can be controlled with PDIs.
5. Surface area‐enhanced charge transfer
• Larger specific surface area of nanomaterials promotes charge transfer across the interfaces (solid‐solid and solid‐liquid)
• Water redox reactions can occur at low overpotential
Disadvantages of nanostructured Photoelectrodes and photocatalysts
• Increased surface recombination• Lower absorbed photon flux• Slow inter‐particle charge transfer
1. Increased surface recombination
Major loss mechanism:
• Bulk recombination
• Depletion‐region recombination
• Surface state recombination
• Electron tunneling through the barrier
• Electron tunneling over the barrier
• Electron collection by back contact• Hole collection by redox coupling
2. Lower absorbed photon flux
Increased surface roughness a lower flux per unit area ofexposed semiconductor
3. Slow inter‐particle charge transport
• In nanocrystalline films, charge carriers move by diffusion instead of drift
• Charge transport is (much) slower than in bulk
Photosynthesis and nanostructures for solar water splitting II
• Brief review• Advantages and disadvantages• Material designs and nanostructured architecture
Material designs and nanostructured architecture to improve photoelectrochemical activity
Light absorption
Absorbance of single bandgap photo‐active materials
1. Branched (nano)structure
2. Band structure modification: doping• Homo‐ and inhomogeneous doping• Shift of the absorption edge toward longer wavelengths
1. Branched structure: 1D structural unit
Mean free diffusion length LEffective charge separation
and large surface areaL2 = qDτ
Branched 1D structure
• High surface area• Excellent light‐trapping• Efficient charge
separation
TiO2 branched nanorods
2. Band structure modification by inhomogeneous dopingDecrease bandgap and shift the absorption to visible region is a strategy to increase solar energy conversion efficiency (visible light has a large fraction of solar spectrum). Doping (N or C) into wide bandgap materials (TiO2 or ZnO).
𝐼𝑃𝐶𝐸 %1240𝐽λ 𝐼 100%
IPCE: incident‐photon‐to‐current‐conversion efficiency
𝐽: photocurrent density (mA cm‐2)λ: wavelength of incident light (nm)𝐼: intensity of incident light (mW cm‐2)
• Doped with heteroatoms• Decrease band gap• Visible light reactivity
2. Band structure modification by homogeneous doping
Homogeneous doping of nanomaterials: solid solution
• Nanomaterial dominating• Shifting conduction/valance band• Decreasing band gap
Absorbance of multi‐bandgap photo‐active materials
• Small increase of visible light absorption from branched structures
• Small wavelength shift towards visible light from band modification by doping
• Absorbance over a wide solar spectral region can be further increased using multi‐bandgap materials• Dye sensitization• Quantum dots sensitization
Dye:• Oxidize water• Transfer electrons to semiconductorSemiconductor:• Be sufficiently thick to maximize the adsorption of
the dye
A PEC cell with a RuP‐sensitized TiO2 photoanode coated with a Nafion film.
• Large light absorption• Visible light activity• Suitable oxidation potential
1. Dye sensitization
2. Quantum dots sensitizationReplace photosensitive dyes with semiconductor nanocrystals, know as quantum dots (QDs)
CdTe QDs decorated ZnO nanorods
• Tunable absorption spectrum: size‐dependent• Generation of multiple electron‐hole pairs• More suitable solar spectrum
Material designs and nanostructural architecture to improve photoelectrochemical activity
Charge separationand transportation
1. Hetero‐structures
TiSi2 nanonets:• Highly conductive framework• High surface areaFe2O3:• Thin film and cover the nanonets• Defect‐free by ALD
25 nm Fe2O3 photoanode @ TiSi2 nanonets
• Efficient charge collection• Excellent charge transportation
2. Co‐catalyst modification
• Promotes electron‐hole pair separation• Excellent charge collection
Metal nanoparticles can be used inphotocatalytic systems as co‐catalystsfor hydrogen and oxygen evolution,where they act as traps of photo‐generated electrons and holes tocollect electrons or holes from thesemiconductors.
ZnO nanorods decorated by Co‐based co‐catalyst
3. Plasmonic assistance
• Hot ejection: transfer of charge between metal/oxide (“hot” electron‐hole pairs)• Localized surface plasmon resonance: plasmonic induced electromagnetic field
Au nanoparticles decorated TiO2
When an incident light strikes the surface, localized surface plasmonsare excited. The field enhancement is greatest when the plasmon frequency ωp is in resonance with the radiation (ω = ωp/1.732 for spherical particles). The plasmon oscillations must be perpendicular to the surface.
Localized surface plasmon resonanceElectromagnetic field enhancement
More light scatteringand lower lightreflection
How does surface plasmonic effect affect the solar watersplitting performance?
A nanowire array‐based tandem cell to completely split water
Current artificial photosynthesis systems fabricated by National Renewable Energy Laboratory (NREL) show a solar hydrogen conversion efficiency of 12.4%. The theoretical limit of solar‐to‐hydrogen generation has been estimated to be ~30%.
• Class 1: A general introduction of fundamentals of nano‐structured materials
• Class 2: Structures and properties of nanocrystalline materials• Class 3: Graphene• Class 4: 2D atomic‐thin nanosheets• Class 5: Optical properties of 1D nanostructures and nano‐generator• Class 6: Carbon nanotubes• Class 7: Solar water splitting I: fundamentals• Class 8: Solar water splitting II: nanostructures for water splitting• Class 9: Lithium‐ion batteries: Si nanostructures• Class 10: Sodium‐ion batteries and other ion batteries, and
Supercapacitors• Class 11: Solar cells• Class 12: Other nanostructures
Thank you!