Properties/Material aspects of photoelectrodes
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Electrochemical and stability properties
including the size of the band gap (Eg)
Position of the band edges against H20 redox
reaction
Electrical resistance
Corrosion resistance etc..
In search for suitable material
TiO2 ideally suites and has been widely
used for the past two decades
Disadvantages:
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Poor light absorption
Wide band gap (3 eV)
Remedies:
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Band gap engineering
Optimal band gap for high efficiency
Figure: Conversion efficiency of solar energy vs. band gap according to
Gerischer (1) hypothetical efficiency without any losses, (2) efficiency including
losses in semiconducting electrode,(3) efficiency of PEC with two photoelectrodes, (4) efficiency of PEC with one photo-electrode.
Critical issues – semiconductor
electrodes for PEC
3.3 eV 2 eV
The band gap of a
semiconductor should satisfy
two contradicting criteria:
(i) be as wide as possible (≥1.8
eV) for providing enough
voltage for the
redox reaction to take place and
(ii) as narrow as possible to
absorb more of the sun light.
Promising scenario – multi-layered
structure
Figure: Schematic of a PEC cell having multilayered semiconductor
electrode consisting of band gap engineered light absorber and TiO2
covering layer.
MiNalab approaches - multi-layered
structure
Catalyst assisted nanowire growthVLS mechanism
Fig. Schematic illustration: Growth of a silicon crystal by VLS
a) Initial condition with liquid droplet on substrate b) Growing
crystal with liquid droplet at the tip
Fig. Sketch of the nanowire and droplet. The nanowire of radius r0 grows at a
rate V, and the solid-liquid interface is located at z=h0 when V=0.Growth is
caused by a constant flux Fr0 at the curved liquid-vapor interface, causing a
deflection of the solid-liquid interface to z=hr, where r is the radial coordinate.
Total contact angle between the droplet and solid is c. Undeformed liquid-vapor
and liquid-solid interfaces are shown; they make angles qLV and qLS with the
horizontal
Models for VLS growth
Fig. 1. Schematic of the three VLS growth models of the nanowires:
•
(1) mass transport of the
gaseous species, (2)
diffusion of the species
from the substrate to the
liquid–solid interface, (3)
incorporation into the solid
by diffusion along the
liquid–solid interface.
J. Chem. Phys. 37 (1962) 428.
(b) (1) mass transport of the
gaseous species, (2)
intermediate reaction at the
vapor–liquid interface, (3)
diffusion of the reaction product
along the vapor–liquid
interface, (4) incorporation into
the solid by diffusion along the
liquid–solid interface.
J. Appl. Phys. 76 (1994) 1557.
(c) (1) mass transport of
gaseous
species,
(2)
intermediate reaction at the
vapor–liquid interface, (3)
penetration of the product to
the melt and subsequent
diffusion to the liquid–solid
interface, (4) incorporation
into the solid at the liquid–
solid interface and growth.
J. Cryst. Growth 31 (1975) 20.
The VLS mechanism depends on factors:
• Formation of liquid alloy at the deposition temperature
• Vapor-liquid-solid interface energies
• Distribution coefficient
• Inertness to the reaction products
The growth rate depends on:
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The equilibrium composition at which solid precipitates from the vapor
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Surface kinetics of the solid-liquid interface – governed by temperature
and the radius of the nanowire
Phase diagram of Au-Ge system
How nanowires grow
Choice of catalyst for ZnO growth
• Zn forms solid solutions with Ag, Al, As, Au, Ba, Bi, Ca, Cd, Ce, Co, Cu, Fe,
Ga, Ge, Hg, In, La, Li, Mg, Mn, Nd, Ni, P, Pb, Pd, Pr, Pu, Sb, Si, Sm, Sn, Sr,
Te, Th, Tl, Y, Yb
• Some of the alloys which has the same phase group (P63/mmm) is Ag, Au,
Ba, Bi, Ca, Cd, Ce, Co,Cu, Fe, La, Li, Mg, Mn, Nd, Pr, Pu, Sm, Sr, Te, Th, Tl, Y,
Yb
• Of which the rare earth alkali elements are highly reactive and limits the no
of catalyst to Ag, Au, Cu, Fe, Pd.
• Lattice mismatch for Au is 0.48% along Au(111) with ZnO(0001)
Lattice mismatch between ZnO and Sn
catalyst
Lattice mismatch a) 0.7 % and 3.6% , b) 12 % and 2.1 %, c) 3.6 % and 12 %
Tetragonal Sn – lattice constant (a = 0.5831 nm , c = 0.3182 nm)
Hexagonal ZnO (a = 0.3249 nm , c = 0.5206 nm)
Growth of ZnO nanorods by thermal
evaporation method
What is MOCVD?
MOCVD
(Metal-organic Chemical Vapor deposition) or
MOVPE (Metal-organic Vapor Phase Epitaxy)
MO Precursors decompose/react at the heated substrate surface
Ga(CH 3 )3  AsH 3  GaAs  3CH 4 
Ga(CH 3 )3  NH 3  GaN  3CH 4 
History:
1953, Scott et al used TEIn (triethylindium) and stibine to form InSb
layers (UK Patent)
1965, Miederer et al OMVPE growth of GaAs (US Patent)
1968, Manasevit and Simpson termed MOCVD.
Basic Principle of the MOCVD Process
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A gas mixture containing the precursors needed for growth,
and if necessary for doping, is passed over a heated substrate.
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The precursor molecules (DEZn – Zn, t-BuOH – O in this
case) pyrolyze leaving the atoms, e.g., Zn and O atoms on the
substrate surface.
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The atoms bond to the substrate surface and a new crystalline
ZnO layer is grown.
Decomposition Kinetics of Precursors
for ZnO growth
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Pyrolysis of tertiarybutanol (t-BuOH) leads to
water (H2O) and isobutene (C4H8). Complete
decomposition at 480oC
Co-pyrolysis with DEZn in addition ethylene
(C2H4) and (C2H6) appears. Complete
decomposition of DEZn is at 360oC
C. Thiandoume, O. Ka, and O. Gorochov – Poster presentation
The factors affect the MOCVD
process
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Mass flow of precursors
Molecule Ratio of reactants
Flow rate of precursors
Growth temperature
Chamber working pressure
Geometry of the chamber
Others
Why MOCVD?
 Very
high quality of grown layers (high growth rate and
doping uniformity/reproducibility)
 High throughput and no ultra high vacuum needed
(compared to MBE), therefore economically
advantageous, high system up-time
 Different materials can be grown in the same system,
therefore highest flexibility
 Growth of sharp interfaces possible - therefore very
suitable for heterostructures, e.g., multi quantum wells
(MQW)
ZnO Nanorods
With 3nm Au catalyst
With ZnO buffer layer
Tg < 425 oC
Tg > 425oC
MOCVD in MiNa Lab
Thanks for your attention !!!