Developments Of Solar Energy Materials Based Nanotubular Tio

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!