中美自然科学基金双边会 议 2009.10.16 ~ 19 ,江苏 常州
Developments of Solar Energy Materials Based Nanotubular TiO2 Changjian Lin (林昌健)
[email protected]
State Key Laboratory of Physical Chemistry of Solid Surfaces, and College of Chemistry and Chemical Engineering , Xiamen University, Xiamen 361005, China
Highlights Photogenerated Cathodic Protection Hydrogen Production by
Photoelectrocatalytic Water Splitting Photoelectrochemical Solar Cell
( DSSC ) Orderly nano-micro structured biomaterials
A Photoelectrochemical Study of TiO2-based Nanotube Arrays as the Photoanodes for Cathodic Protection of Metals
Washington, DC, Oct. 7-12, 2007
Introduction e-
e-
CB
e- e-
Photoelectrochemistry
Photocatalysisphotodegredations
hν Metallic substrates
e- h+ h+ e-
VB
TiO2
“Green” Cathodic protection No
Wettability- Superhydrophilic Superhydrophobic
Photo-generated cathodic protection
Supply current
Consume light metals(Mg,Al,Zn)
TiO2 + hν→h+ + e-
Main critical technical problems e-h+ precursor in TiO2 is created only under an irradiation of UV light due to its large energy band-gap. Because of short excition diffusion lengths and high recombination of e-h+ pairs, the photoreactivity and quantum efficiency of TiO2 remains low. Pure TiO2 coatings cannot provide with a sustainable cathodic protection under dark conditions.
Aim and Motivation To enhance the photoabsorption to a visible light To increase the photoreactivity of TiO2 To last the cathodic protection in dark conditions.
Electrochemical Anodization & Doping Potentiostat
anode : 2H2O → O2 + 4e- + 4H+ Ti + O2 → TiO2 TiO2 + 6F− + 4H+ → TiF62− + 2H2O
Platinum Cathode Ti metal anode
Bath
two-electrode electrochemical cell
Highly ordered nanostructure of TiO2 N, Fe-doped TN
self-organized anodization in fluorinated electrolyte (H2SO4/ HF, Na2SO4/NaF containing different nitrogen, iron sources).
wet methods
Morphologies of TN Array a (a),
bb
(b) a
(a),(b) N-TN layers anodizeded in IMH2SO4+0.15wt%HF under 20V for 4h sintered at 4500C
100nm
100nm
dd
cc
diameter: ~ 90 nm length : ~ 500 nm (c),(d) N-TN layers anodizeded in 0.2MNa2SO4+0.15wt%NaF under 20V for 6h annealed at 5000C
100nm
200nm
Typical SEM top views(a,c) and cross-sectional (b,d) images for the N-doped TiO2 nanotubular layers anodized in different electrolyte.
diameter: ~ 100 nm length : ~1.5 μm
Chemical Compositions of the N-TN XPS total spectrum(a) and N1s high resolution spectrum (b) of the n-doped TN calcined at 4500C for 2h
400±0.2eV : chemisorbed γ- N2 on TiO2
surface 396±0.2eV: atomic β-N (interstitial state) concentration of N: ~2.7 atom%. Wet doping procedure : distribute
the N-dopant within the outer layer of the TN film Nitrogen: implanted in the nanotubular structure, presenting in a chemically bonded state.
Chemical Compositions of the Fe-TN XPS spectra of Fe doped TN arrays anodized in 0.5%(w)NaF+0.2 mol·L-1 Na2SO4 containing 0.1M Fe2+ annealed at 500 0C
459.8eV: Ti2p3/2 (anatase-type TiO2 ) 724.3eV: Fe(Ⅲ)2p1/2 710.7eV: Fe(Ⅲ)2p3/2 (γ–Fe2O3)
Anodization and anneal Fe(Ⅱ) was oxidized to Fe(Ⅲ) Fe ion or mixed titanium oxide-iron concentrated on the surface layer of the TN films.
Photocurrent of the N-TN nanotubular
(A) Photocurrent spectra of the pure TN film and the nanoparticles film a: anodized in 0.2MNa2SO4+0.15wt%NaF,4h b: anodized in IMH2SO4+0.15wt%HF, 2h c: nanoparticle films prepared by sol-gel d: blank Ti substrates e: n-doped TiO2 nanoparticle films.
N-doped
(B) A comparison of photocurrent spectra of the samples a,b,c: n-doped TN films recorded under the different bias voltages:a-1V,b-3V,c-300mV d: no-n-doped TN films.
N intercalation : absorption edge extends into the visible region. Nanotubular films: harvest UV or visible light much more effectively than the irregular structure.
Photocurrent Fe-TN Sharply increasing absorption in 625-650 nm. Band gap energy: 1.9~ 2.0 eV
A comparison of photocurrent spectra of the Fe-doped TN films anodized in 0.2MFe2+ /0.15wt %HF +1MH2SO4 and the nanoparticles film sintered at 500 0C, for different times, a-7h,b5h,c-4h,d-2h,e-sol-gel
Photocathodic protection-N-TN Time dependence of OCP for 316 SS coupled with the TiO2 nanotube layers and nanoparticle films under dark and illumination conditions.
1: TiO2 nanotubular electrodes; 2: TiO2 nanoparticle films.
n-doped TN
exhibiting a higher photoeffect on UV, the OCP drops immediately to -300mV the recovery is very slight (~0.039mV/s) the negative value can keep for several hours remain a complete cathodic protection for the SS in the dark condition.
Photocathodic protection N-TN A comparison of the OCP shifts for 316L SS coupled with the Ndoped TiO2 nanotube electrodes under irradiation and dark condition 1: visible light , λ=540nm 2: UV illumination, λ=350nm
Visible light
OCP reaches a more negative value (- 400mV) compared to the sample under UV.
N-TN electrode
exhibits a much higher photoresponse to visible light illumination.
Dark condition
the negative OCP shift maintains for a period and the photocathodic protection can last for a reasonable time.
Solar-hydrogen production by photoelectrocatalytic water splitting using highly ordered nanotubular TiO2 arrays
TiO2 photocatalysts for hydrogen production Potentiostat
B
A e-
Pt
SCE
e-
+0.0 V
h
eH+ H2
+
CB
E0(H3O+/H2)
D+
O2
membrane
Anodized nanotubular TiO2 arrays
Schematic illustrating the process of hydrogen generation using annealed nanotubular TiO2 arrays.
H2+OH-
H2+OH-
H2O
ligh t
e- H2O
e- H2O
+1.23 V E (H2O/O2) 0
O2+H3O+
D
e-
eH2O
VB
Potential energy diagrams for photochemical water splitting at pH = 0;(A) single semiconductor system (B) with an electron donor.
Electrochemical anodization
• The highly ordered TiO2 arrays constructed by Potentiostat
electrochemical anodization are using as photoanode for hydrogen production in photoelectrochemical cell.
Platinum cathode
Ti anode electrolyte
Schematic illustration of two-electrode electrochemical cell for the electrochemical anodization of Ti .
Ti - 4e
Ti4+
Ti4+ + 2H2O → TiO2 + 4H+ (or: Ti4+ + 4H2O → Ti(OH)4↓+ 4H+ Ti(OH)4 → TiO2 + 2H2O) TiO2 + 6F- + 4H+
TiF62- + 2H2O
Nanotubular TiO2 (TNT) arrays • Dimethylsulfoxide electrolyte a
b
c
d
e
f
Highly ordered TiO2 array: tubelength of 18 µ m, pore diameter of 100 nm, wall thickness of 20 nm
Nanotubular TiO2 (TNT) arrays notable highly ordered TiO2 arrays by multi-anodization in ethylene glycol electrolyte
Normal TNT arrays
smooth nanotubes
Notable highly ordered TNT arrays
nanoporous morphology with 55nm wall thickness
Multi-step anodization
Structure and photoelectrochemical activities
Photocurren Density (mA/cm2)
25
o
350 C o 450 C o 550 C o 650 C
20
15
10
5
0
-0.9
-0.6
-0.3
0.0
0.3
0.6
0.9
Potential (V vs SCE)
Photocurrent densities generated from the nanotubular TNT arrays by third anodization in 0.5 M KOH with a 300 W Xenon lamp.
Hydrogen production potentiostat
O2
W E
to RE
PV = n R T
C E
vacuomete r
P,V,T known
Thermoelectric couple
gas collection
H2 H2 Reaction cell
TNT arrays photoanode
Remove water vapor
Cooling flask
SCE
Pump
Pt electrode
Illustrative drawing of a threeelectrode photoelectrochemical cell for hydrogen production by water splitting.
A gas-closed circulation system equipped with a three electrodes reaction cell and a volumetric device with a vacuum line.
Photoelectrochemical activities Enhanced photocatalytic activities for hydrogen production by multi-anodization. 400 µ mol·h-1 ·cm-2 T5(1.2 µm) S5(1.2 µm) F90 (1.2 µm) F300 (10 µm)
1500
25
Photocurrent Density (m A/cm2)
-1
-2
H2 generation amount(µmol⋅h ⋅cm )
2000
1000
500
20
15
10
5
0
0
T5 (1.2 µm ) S5 (1.2µm ) F90 (1.2 µm) F300 (10 µm)
-0.9
-0.6
-0.3
0.0
0.3
0.6
0.9
Potential (V vs SCE) 0
1
2
3
4
5
Time (h)
The amount of hydrogen generated as a function of time using the sample T5, S5, F90 and F300 in 2 M KOH with 0.5 M ethylene glycol.
UV-Vis photocurrent densities generated from the annealed nanotubular TNT arrays sample T5, S5, F90, F300 in 0.5 M KOH. T, S, F represent third, second and first anodization respectively.
Composite nanostructures of TiO2 Ti foils
Degreased Ti foils
(a)
(b)
(c)
(d)
Treatment in an oxidizing agent Potentiostatic anodization nanocomposite TiO2 film
Bamboo leaf-like TiO2
Schematic presentation of the two-step fabrication process used to synthesize nanocomposite TiO2 film.
The bamboo leaf-like TiO2 are uniform and dense with film thickness of 400 nm.
Composite TiO2 nanostructures (b)
(c)
TNT arrays
bamboo leaf-like TiO2
Enhanced photocurrent densities of 150% higher than those of the two components alone.
(d)
Structure of composite TiO2 consists of highly ordered TNT arrays of 1.5 µ m and bamboo leaflikeTiO2 films of 0.1 µ m on top surface.
Photocurrent Density (mA/cm2)
(a)
composite TiO2
18
nanotubular TiO2
15 12 9 6 3 0
-1.0
-0.5
0.0
Potential (V vs SCE)
0.5
1.0
Hydrogen production The combination of TNT arrays and bamboo leaf-like TiO2 enhance the hydrogen production rate significantly up to 150 µ mol·h-1 ·cm-2 . The O2 evolution was lower than the stoichiometric (H2 : O2=2:1) (b)
800
composite TiO2
700
700
normal TNT arrays bamboo leaf-like TiO2
-2
500 400 300 200 100 0
H2 O2
600
-1
600
Generation amount(µmol⋅h ⋅cm )
-1
-2
Generation amount(µmol⋅h ⋅cm )
(a)
0
1
2
3
4
5
Time (h)
500 400 300 200 100 0
0
1
2
3
4
5
Time (h)
The separate evolution of H2 and O2 by photoelectrocatalytic water splitting using composite TiO2 films under UV-Vis irradiation in 2 M Na2CO3+0.5 MCH3OH.
The evolution of H2 using three morphologies TiO2 materials under UV-Vis irradiation in 2 M Na2CO3+0.5 M CH3OH.
Summary The notable highly ordered TNT arrays by multi-step anodization show excellent photocatalytic activities for H2 production by photoelectrocatalytic water splitting. The nanocomposite of TiO2 photoanode is capable of enhancing the photocatalytic activities for water splitting with H2 evolution rate of 150 µ mol•h-1•cm-2. The prepared highly ordered nanocomposite titania possess good stability, and it is expected to become an efficient photoanode materials for water splitting.
Electrochemical Solar Cell ( DSC ) based on Nanotubular TiO2
Structure and principle
Table 1 Performances of Main Solar Cells Type of cell
Efficiency (%)
Research and technology needs
Crystalline silicon
Cell 24
Multicrystalline silicon
18
9-12
Amorphous silicon
13
7
CuInSe2
19
12
Replace indium (too expensive and limited supply), replace CdS window layer, scale up production
Dye-sensitized solar cells
10-11
7
Improve efficiency and high-temperature stability, scale up production
Bipolar AlGaAs/Si 19-20 photoelectrochemical cell
-
Reduce materials cost, sale up
-
Improve stability and efficiency
Organic solar cell
2-3
Module 10-15 Higher production yields, lowering of cost and energy content Lower manufacturing cost and complexity Lower production costs, increase production volume and stability
M. Grätzel, Photoelectrochemical cells, Nature 2001(414), 338
Why- DSSC? High cost
Low cost
Rare materials
Cheap materials
Si—DSC
Complex fabrication
Easy fabrication
pollution
green
PEC or DSSC new generation of solar cells ?
Important Issues for DSC TiO2 , ZnO , SnO2 , Nb2O5 等
photoanodes
Organic/ inorganic dyes Organic or inorganic dyes L, S, q-S
Electrolytes
< 1% Pt, C
7.1%
10%
Counter Electrode
Grätzel team
11.18%
Preparation of TiO2 Photoanode TiO2 Nanocrystalline
Tio 2 光 阳 极
Sol-gel synthesized hydrolyzate of TiCl4 Screen Printing Electrochemical deposition ................. megnetron sputtering
1-D Nanostructures
from up to down from down to up free-standing
PVD Ion beam evaporation
templated assisted Hydrothermal Synthesis
1.1-1.2μm
5.6-5.9μm
3min
5min
20min
3min
5min
20min
0.8-0.9μm
SEM images of the nanoporous TiO2 films after third anodization at 50V in ethylene glycol system including NH4F and H2O for 3min,5min and 20min with different film thickness.
modified with Pd-QD nanoparticles
modified with Pt-QD nanoparticles
a
b
c
d
100nm
100nm
e
Photocurrent (a.u.)
100nm
100nm
TiO2 CdS dc - 2 min dc - 10 min sono - 2 min sono -10 min
250 f CdS
300
350
400
450
500
Wavelength (nm) (a)
100nm
100nm
SEM images of TiO2 nanotube array electrode (a, b) and CdS-TiO2 nanotube array electrodes after CdS sonoelectrochemical deposition at 0.5 mA/cm2 for 2 min (c), 5 min (d) and 10 min (e, f), respectively.
Photocurrent spectra of TiO2 nanotube array (black), CdS layer (blue) and six types of CdS modified TiO2 nanotube array electrodes, respectively(a). The main charge-transfer process between TiO2 and CdS after being excitated by the light (b)
A1
B1
10min
10min
A2
20min
A3
30min
B2
20min
B3
30min
TiO2/ZnO nanocomposite and heterojunction structures
Large-Scale, Noncurling, and Free-Standing Crystallized TiO2 Nanotube Arrays for Dye-Sensitized Solar Cells
0.25 wt % NH4F 50 V
0.25 wt % NH4F 12 V
Fig 13: SEM images of the membranes with different thicknesses of 9 μm (a,d), 25 μm (b,e), and 42 μm (c,f) Procedure for Preparing Free-Standing Crystallized TNT Arrays
J. Phys. Chem. C, 113, 15, 2009
Design of structures of DSSC based on TiO2 nanotube arrays
Many Thanks!