hematite nanowires for solar water splitting: development and structure optimization j. azevedo 1,2,...
TRANSCRIPT
Hematite nanowires for solar water splitting: development and structure
optimization
J. Azevedo1,2, C.T. Sousa1, M.P. Fernandez-García1, A. Apolinário1, J. M. Teixeira1, A.M. Mendes2 and J.P. Araújo1
University of porto
MAP-Fis PhD Research Conference
Porto, January 20, 2011
1IN-IFIMUP and Dep. Física, Rua do Campo Alegre 687, 4169-007 Porto, Portugal2 LEPAE – Dep. de Engenharia Quıímica, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal.
Outline
• Introduction
• Fabrication Methods
• Results
• Conclusions and Future work
2
3
Phot
oano
de
Coun
tere
lect
rode
4
heh
h
22244 OOHhOH 22 2444 HOHeOH
+++
++
OH-
OH-
OH-
H2O2
Photoelectrochemical Cell
1) Absorption of light near the surface of the semiconductor creates electron-hole pairs.
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 :
heh
22244 OOHhOH
22 2444 HOHeOH
)(2
1)()(2 222 gOgHlOHh
3) Holes (minority carriers) drift to the surface of the semiconductor (the photo anode) where they react with water to produce oxygen:
4) Transport of H+ from the anode to the cathode through the electrolyte completes the electrochemical circuit.
1.23 eV
http:
/new
ener
gyan
dfue
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/201
1/05
/11/
Hematite ( -Feα 2O3) as photoanode
5
Nano structuring
Objectives
6
Electrodeposition 7
Pulsed Electrodeposition
Pulsed ModePulsed Mode
Alumina
Aluminium
Barrier layer thinning
Pulseddeposition
8
Perfis de Deposição 9
I, II III IV V VII, II III IV V VI VII
VII
VI
V
IVIII
I, II
a) b)
c)
10
0.0 0.2 0.4 0.6 0.8 1.0
0
20
40
60
80
100
Fill
ed P
erc
enta
ge (
%)
Concentration (M)
0 100 200 300 400 500
0
20
40
60
80
100
120II) High j(t) regime
Fill
ed p
erce
ntag
e (%
)
Current Density (mAcm-2)
I) Low j(t) regime
Pore modulation
11
12
10 µm
1 µm
oxidation 13
Atmosphere dependence on oxidationComparison 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 oxidationAnnealing 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 substratewas removed prior to oxidation, due to the Al melting point
15
Reference samplesX-ray absorption spectroscopy measurements at the Fe k-edge in transmission
16
7000 7100 7200 7300 7400 7500
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
cristaline struture
FeO(OH) -Fe
2O
3
-Fe2O
3
Fe3O
4
norm
E (eV)
XANES EXAFS chemical composition
Comparison with prepared samples 17
7105 7110 7115 7120 7125 7130 7135 7140-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6 Fe FeO(OH) -Fe
2O
3
-Fe2O
3
Fe3O
4
650oC for 6h in O2
600oC for 19h in O2
norm
E (eV)
Higher Oxidation
Lower Oxidation
(a), (c) and (d) SEM images of annealed NWs; (b) EDS profile of annealed NWs.
18
Conclusions Fabrication of highly organized alumina
templates;
Optimization of an industrially viable Fe
nanowires deposition method;
Fabrication of Fe nanowires with high degree of
organization with lengths from 1 μm to 10 μm
up to 99 % of pore filling; 19
0 100 200 300 400 500
0
20
40
60
80
100
120II) High j(t) regime
Fill
ed p
erce
ntag
e (%
)
Current Density (mAcm-2)
I) Low j(t) regime
Conclusions Enlarged nanowire surface area through
pore modulation;
Oxidation studies indicate the presence
of hematite after an annealing.
20
1 µm
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
Introduction support 24
Photoelectrochemical Scheme 25
Potentiostat
Phot
oano
de
Coun
tere
lect
rode+
++
++
H2O2
H+
H+
H+
heh
HgOlOHh 2)(2
1)(2 22 )(22 2 gHHe
h
26
1) Absorption of light near the surface of the semiconductor creates electron-hole pairs.
2) Holes (minority carriers) drift to the surface of the semiconductor (the photo anode) where they react with water to produce oxygen:
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 :
4) Transport of H+ from the anode to the cathode through the electrolyte completes the electrochemical circuit.
The overall reaction :
heh
HgOlOHh 2)(2
1)(2 22
)(22 2 gHHe
)(2
1)()(2 222 gOgHlOHh
Photoelectrolysis
27
Electric Current
Solar Cell
Electric Current
Electrolysis
Photoelectrolysis
Nernst Equation
28
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
Theoretical efficiencies
29
The overall solar energy conversion efficiency can be written as the product of the efficiencies of
the cell in performing these processes:
Quais as dificuldades?
Adapted from M. Grätzel, Nature 414, 388 (2001)30
Óxidos Quimicamente estáveis
mas baixa eficiência (baixa condutividade)
Não óxidos Boa condutividade mas
fraca estabilidade química
Maximum efficiency possibleDepending upon semiconductor bandgap, under xenon arc
lamp and AM1.5 solar illuminations.
31
Armazenamento de Hidrogénio
32
http://en.wikipedia.org/wiki/File:XASEdges.svg
• Compressed hydrogen
• Liquid hydrogen
• Chemical storage
• Physical storage
• Carbon nanotubes
PAA support 33
First Anodization
The four major stages of nanoporous alumina template formation:
1) 2) 3) 4)
1) oxide barrier formation;
2) pore initial nucleation;
3) pore initial growth;
4) pore continuous growth;
34
Al
Two Step Anodization
1 µm
Aluminium
Pattern formed Better organization!
1 µm
No organization
1 µm1 µm
Alu
min
a
1st Anodization Dissolution of Oxide Layer
2nd Anodization
SEM surface 35
Ordered triangular lattices
36
Ordered triangular lattices
37
Electrodepositon support 38
Experimental parameters
39
General Concepts
40
Different methods
41Electrodeposition different methods
Simulação numérica da influência do pulso de repouso na deposição 42
Influência do tamanho de poro na qualidade da deposiçãoAmostras de 10μm de espessura, preparadas a 20oC, 0.43M e 14mA/cm2
43
Characterization support 44
Estrutura Cristalina
45
• 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.
Estrutura Cristalina
46
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:
Taxamento da deposição
47
Magnetic Characterization48
//
//
Oxidation Support 49
FC and ZFC measurementsZFC 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
Spectra of loose Fe oxide NWs, annealed at 800oC. The α-Fe2O3 Bragg reflections are identified.
51
Synchrotron radiation
52
Synchrotron radiation is produced from the electromagnetic
radiation emitted when charged particles are accelerated radially.
Synchrotron radiation
53
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).
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.
57
7105 7110 7115 7120 7125 7130 7135 7140-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6 Fe FeO(OH) -Fe
2O
3
-Fe2O
3
Fe3O
4
650oC for 6h in O2
600oC for 19h in O2
norm
E (eV)
• NEXAFS can also
determine the chemical
state of elements which
are present in bulk in
minute quantities