UNIVERSITY OF PORTO
MAP-FIS PHD RESEARCH CONFERENCE
H E M AT I T E N A N O W I R E S F O R S O L A R WAT E R S P L I T T I N G :
DEVELOPMENT AND STRUCTURE
O P T I M I Z AT I O N
J. Azevedo1,2, C.T. Sousa1, M.P. Fernandez-García1, A. Apolinário1, J. M. Teixeira1, A.M. Mendes2
and J.P. Araújo1
1IN-IFIMUP and
Dep. Física, Rua do Campo Alegre 687, 4169-007 Porto, Portugal
LEPAE – Dep. de Engenharia Quıímica, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto,
Portugal.
2
Porto, January 20, 2011
Outline
• Introduction
• Fabrication Methods
• Results
• Conclusions and Future work
2
Hydrogen Economy
powered by
Solar cells only
photoelectrochemical
generate
cells
Covering 0.16% of electricity
the surface of the during daytime;
earth with 10%
Solar energy is efficient solar cells
the most
would satisfy our
available
present energy
renewable
requirements;
energy
source;
Fossil fuels are
rapidly being
3
depleted;
2) Electrons (majority carriers) are conducted
to a metal electrode (typically Pt) where they
combine with H+ ions in the electrolyte solution
to make H2 :
1) Absorption of light near the surface of the
semiconductor creates electron-hole pairs.
h  e   h 
1.23 eV
4OH   4h   2 H 2O  O2
http:/newenergyandfuel/com/2011/05/11/
3) Holes (minority carriers) drift to the
surface of the semiconductor (the photo
anode) where they react with water to
produce oxygen:
4 H 2O  4e   4OH   2 H 2
4) Transport of H+ from the anode to the
cathode through the electrolyte completes the
electrochemical circuit.
1
2h  H 2O(l )  H 2 ( g )  O2 ( g )
2
H2
O2
4 H 2O  4e   4OH   2 H 2
4OH   4h   2 H 2O  O2
h

+
+
+
+
+
Photoanode
h  e  h

OHOH-
Counterelectrode
H 2O
OH-
Photoelectrochemical Cell
4
Hematite (α-Fe2O3) as photoanode
Advantages
High chemical
stability
Low cost
Adequate
band-gap of
2.2 eV
Disadvantages
Nanowires
Low absorption
coefficient
Red shifted
band-gap
Rapid electronhole
recombination
Nano
structuring
• Larger diffusion coefficients
• More efficient charge
collection
• Bang gap blue shift
• Independence on light
absorption coefficient
High
theoretical
efficiencies
5
Objectives
Create a highly oriented
alumina template
Fill the template with
Iron nanowires
Anneal the iron into
hematite
6
ELECTRODEPOSITION
7
Pulsed Electrodeposition
Pulsed Mode
Alumina
Pulsed
deposition
Barrier layer
thinning
Aluminium
8
I, II
a)
III
IV
V
VI
VII
I, II
III
IV
V
VI
b)
VII
VI
V
IV
III
I, II
c)
Perfis de Deposição
9
Filled Percentage (%)
100
80
60
40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
Concentration (M)
120
I) Low j(t) regime
II) High j(t) regime
Filled percentage (%)
100
80
60
40
20
10
0
0
100
200
300
400
-2
Current Density (mAcm )
500
Pore modulation
11
10 µm
12
1 µm
OXIDATION
13
Atmosphere dependence on oxidation
Comparison of oxidation state between different atmospheres: a) left in ambient conditions for 2 months,
b) and c) annealing for 6 h at 600oC in air and oxygen, respectively.
The α-Fe2O3 Bragg reflections are shown with their respective Miller indices.
14
Temperature dependence on oxidation
Annealing temperature study on samples with 60 μm thickness. Between 400oC - 600oC the
samples were annealed together with the Al substrate. For annealing’s above 600oC, the substrate
was removed prior to oxidation, due to the Al melting point
15
1.6
FeO(OH)
-Fe2O3
1.4
-Fe2O3
1.2
Fe3O4
norm 
1.0
0.8
0.6
XANES
EXAFS
chemical
composition
cristaline
struture
0.4
0.2
0.0
7000
7100
7200
7300
7400
7500
E (eV)
Reference samples
X-ray absorption spectroscopy measurements at the Fe k-edge in transmission
16
1.6
1.4
1.2
norm 
1.0
0.8
Fe
FeO(OH)
-Fe2O3
Higher Oxidation
-Fe2O3
> Temperature
Fe3O4
> Time
o
650 C for 6h in O2
O2 atmosphere
o
600 C for 19h in O2
0.6
< Temperature
0.4
< Time
0.2
Air atmosphere
0.0
-0.2
7105
Lower Oxidation
7110
7115
7120
7125
7130
7135
7140
E (eV)
Comparison with prepared samples
17
18
(a), (c) and (d) SEM images of annealed NWs; (b) EDS profile of annealed NWs.
Conclusions
 Fabrication
of
highly
organized
alumina
templates;
120
nanowires deposition method;
II) High j(t) regime
100
Filled percentage (%)
 Optimization of an industrially viable Fe
I) Low j(t) regime
80
60
40
20
0
0
100
200
300
400
500
-2
Current Density (mAcm )
 Fabrication of Fe nanowires with high degree of
organization with lengths from 1 μm to 10 μm
up to 99 % of pore filling;
19
Conclusions
 Enlarged nanowire surface area through
pore modulation;
1 µm
 Oxidation studies indicate the presence
of hematite after an annealing.
20
Future Work
 Expose only a fraction of the
nanowires by a partial removal of
the alumina template;
 Test
solar
water
splitting
efficiencies;
 Reproduce results in TiO2 templates.
21
Acknowledgments
22
Thank you for your attention
João Carlos Azevedo
[email protected]
https://sites.google.com/site/azevedojcam/
23
INTRODUCTION SUPPORT
24
O2
H2
Potentiostat
1
2h   H 2O(l )  O2 ( g )  2 H 
2
+
+
+
+
H+
H+
H+
Photoelectrochemical Scheme
Counterelectrode
h
+
Photoanode
h  e   h 
2e  2H   H 2 ( g )
25
1)
Absorption of light near the surface of the
semiconductor creates electron-hole pairs.
h  e   h 
2)
Holes (minority carriers) drift to the surface
of the semiconductor (the photo anode)
where they react with water to produce
oxygen:
1
2h   H 2O(l )  O2 ( g )  2 H 
2
3)
Electrons (majority carriers) are conducted
to a metal electrode (typically Pt) where
they combine with H+ ions in the electrolyte
solution to make H2 :
2e  2H   H 2 ( g )
4)
Transport of H+ from the anode to the
cathode through the electrolyte completes
the electrochemical circuit.
The overall reaction :
1
2h  H 2O(l )  H 2 ( g )  O2 ( g )
2
26
Photoelectrolysis
Electrolysis
Electric
Current
Photoelectrolysis
Solar Cell
Electric
Current
27
Nernst Equation
For an oxidation/reduction reaction we have:
Where F is the Faraday constant and n is the number of necessary electrons (in this case two).
Energy losses
28
Theoretical efficiencies
The overall solar energy conversion efficiency can be written as the product of the efficiencies of
the cell in performing these processes:
29
Quais as dificuldades?
 Óxidos
 Quimicamente
estáveis
mas baixa eficiência (baixa
condutividade)
 Não óxidos
 Boa
condutividade mas
fraca estabilidade química
Adapted from M. Grätzel, Nature 414, 388 (2001)
30
Maximum efficiency possible
Depending upon semiconductor bandgap, under xenon arc
lamp and AM1.5 solar illuminations.
31
Armazenamento de Hidrogénio
http://en.wikipedia.org/wiki/File:XASEdges.svg
• Compressed hydrogen
• Liquid hydrogen
• Chemical storage
• Physical storage
• Carbon nanotubes
32
PAA SUPPORT
33
First Anodization
1) 2)
3)
4)
Al
The four major stages of nanoporous
alumina template formation:
1)
oxide barrier formation;
2)
pore initial nucleation;
3)
pore initial growth;
4)
pore continuous growth;
34
Two Step Anodization
Dissolution of
Oxide Layer
2nd Anodization
Alumina
1st Anodization
Aluminium
No organization
1 µm
SEM surface
Pattern formed
1 µm
Better organization!
1 µm
35
Ordered triangular lattices
36
Ordered triangular lattices
37
ELECTRODEPOSITON SUPPORT
38
Experimental parameters
2º Anodization
240min at 40V
Dendrite Formation
8V
Electrodeposition
P.andodization (8V; 2ms)
P.deposition (70mA/cm2; 8ms)
P.rest (700ms)
39
General Concepts
40
Different methods
Electrodeposition different methods
41
Simulação numérica da influência do pulso de repouso na
deposição
42
Influência do tamanho de poro na qualidade da deposição
Amostras de 10μm de espessura, preparadas a 20oC, 0.43M e 14mA/cm2
43
CHARACTERIZATION SUPPORT
44
Estrutura Cristalina
•
Os eletrões emitidos pelo cátodo de uma ampola onde foi previamente realizado vácuo são
acelerados por um potencial elevado aplicado ao longo dela, dirigindo-se a alta velocidade em
direção a uma placa metálica (alvo) utilizada como ânodo. Quando os eletrões chocam com o alvo
dá-se a emissão de raios-X.
•
O espectro emitido é composto por radiação-X cujo comprimento de onda varia continuamente, ao
qual se sobrepõe uma série de riscas muito estreitas e em posições discretas.
45
Estrutura Cristalina
Fatores que contribuem para o alargamento dos picos medidos experimentalmente:
•
tensões mecânicas não homogéneas
•
variações de composição ao longo da amostra
•
a sua espessura
•
as larguras e alturas das fendas de colimação do feixe (instrumento)
•
falta de monocromatismo do feixe incidente (instrumento)
o
o tamanho médio das cristalites que compõem a amostra (policristalina)
A relação entre o tamanho L e o alargamento é dada pela fórmula de Scherrer, que se escreve do
seguinte modo:
46
Taxamento da deposição
47
//

//
Magnetic Characterization
Coercive field:
Saturation field:
• HC// (~ 1550 Oe) >> HC (~ 385 Oe)
• HS// (~ 4 kOe) << HS (~ 15 kOe)
48
OXIDATION SUPPORT
49
FC and ZFC measurements
ZFC and FC measurements in a 100 Oe field.
The annealing temperature was 800oC.
(C. H. Kim et al, “Magnetic anisotropy of vertically aligned alpha-fe2o3 nanowire array”, Ap.
Phys. Let., vol. 89.)
50
51
Spectra of loose Fe oxide NWs, annealed at 800oC. The
α-Fe2O3 Bragg reflections are identified.
Synchrotron radiation
Synchrotron radiation is produced from the electromagnetic radiation emitted
when charged particles are accelerated radially.
52
Synchrotron radiation
Properties of synchrotron radiation:
• Broad Spectrum (which covers from microwaves to hard X-rays);
• High Flux of energy;
• High Brilliance (highly collimated photon beam);
• High Stability (submicron source stability);
• Polarization (both linear and circular);
• Pulsed Time Structure (pulsed length down to tens of picoseconds allows
the resolution of process on the same time scale).
53
X-ray absorption spectroscopy
• X-ray absorption spectroscopy (XAS)
is a widely-used technique for
determining the local geometric
and/or
electronic
structure
of
matter.
• XAS data are obtained by tuning
the
photon
energy
using
a
crystalline monochromator to a
range where core electrons can be
excited.
54
http://en.wikipedia.org/wiki/File:XASEdges.svg
X-ray absorption spectroscopy
• There are two main regions found on a spectrum generated by XAS data
55
http://en.wikipedia.org/wiki/File:XASEdges.svg
XANES
• X-ray Absorption Near Edge Structure (XANES), also known as Near edge X-ray
absorption fine structure (NEXAFS) is the absorption of an x-ray photon by a core
level of an atom in a solid and the consequent emission of a photoelectron.
• The resulting core hole is filled either via an Auger process or by capture of an
electron from another shell followed by emission of a fluorescent photon.
56
http://en.wikipedia.org/wiki/File:XASEdges.svg
XANES
• The great power of XANES derives from its elemental specificity. Because the
various elements have different core level energies, XANES permits extraction of
the signal from a surface monolayer or even a single buried layer in the presence of
a huge background signal.
1.6
• NEXAFS
also
1.4
determine the chemical
1.2
state of elements which
1.0
are present in bulk in
minute quantities
norm 
can
0.8
Fe
FeO(OH)
-Fe2O3
-Fe2O3
Fe3O4
o
650 C for 6h in O2
o
600 C for 19h in O2
0.6
0.4
0.2
0.0
-0.2
7105
57
7110
7115
7120
7125
E (eV)
7130
7135
7140