65-6790 4400fax. 65-6791 1859/6792 4062

email: msyzhang@ntu.edu.sg http://www.ntu.edu.sg/home/msyzhang

Prof. Sam ZhangSchool of Mechanical & Aerospace Engineering

Nanyang Technological University50 Nanyang Avenue

Singapore 639798

Editor-in-Chief, Nanoscience & Nanotechnology Letter (NNL)Editor-in-Chief, Nanoscience & Nanotechnology Letter (NNL)Principal EditorPrincipal Editor ，， Journal of Materials Research (JMR)Journal of Materials Research (JMR)Fellow, Institute of Materials, Minerals and Mining, UK (IOMMM)Fellow, Institute of Materials, Minerals and Mining, UK (IOMMM)President, Thin Films Society (TFS)President, Thin Films Society (TFS)

Hello from Singapore!Hello from Singapore!(currently on sabbatical (currently on sabbatical @Central Iron & Steel Research Institute, Beijing@Central Iron & Steel Research Institute, Beijing北京钢铁研究总院北京钢铁研究总院 ))

Anodic Titania Nanotube Arrays for Applicationin Dye-Sensitized Solar Cells

•Lidong Sun; Sam Zhang, Xiao Wei Sun, Xiaodong He, Effect of Electric Field Strength on the Length of Anodized Titania Nanotube Arrays, Journal of Electroanalytical Chemistry : in press (2009) •Lidong Sun, Sam Zhang, Xiaowei Sun, Xiaodong He, Effect of TiO2 Nanotube Geometries on the Performance of Dye-Sensitized Solar Cells, Journal of Nanoscience and Nanotechnolog, Journal of Nanoscience and Nanotechnology, 10 : 1-10 (2009)

Sam Zhang, Lidong SunSchool of Mechanical and Aerospace Engineering

Nangyang Technological University, Singapore

Outline

1. Dye-Sensitized Solar Cell (DSSC) typical structure and working principle why choose titania (TiO2) nanotube array current TiO2 nanotube array based DSSCs

2. Nanoparticle or Nanotube Array? efficiency: 11% vs. 7%

3. Nanoparticle + Nanotube Array composite structure

4. Anodic Titania Nanotube Array

5. Conclusions

M. Grätzel, J. Photochem. Photobiol. A: Chem. 164 (2004): 3

TCO

TiO2

Dye

Electrolyte Counter Electrode

• electrons scattered at connections• randomly walk to the collecting electrode• large surface area

• drastically reduced connections• vectorial transpoprt• comparable surface area with nanoparticle based photoanode

Particle vs Nanotube array in DSSC

Limit the photoanode thickness to approx. 10 µmEnable the photoanode thickness larger than 10 µm

Inhibit absorption of low-energy photonsAchieve light-harvesting

L. Sun, S. Zhang, X. W. Sun, X. He, Chapter 2, Anodized Titania Nanotube Array and its Application in Dye-Sensitized Solar Cells, in Vol. 3, CRC Handbook of Nanostructured Thin Films and Coatings Edited by Sam Zhang, Published by CRC Press Taylor & Francis Group, in press, 2009

(a) (b)

Problems in TiO2 Nanotube Array Based DSSCs

• short nanotube array (less than 5 µm, compared to optimal 20~30 µm)

• increased resistance of FTO during annealing

• reflected by platinized counter electrode

• absorbed by iodine in the electrolyte• Increased barrier layer thickness by approx. 1 µm during annealing

Another Issue: Effective Surface Area

3

5

r

Rcomparable surface area

3

5

r

RSNT < SNP

r = 9 nm

R = 15 nm D = 30 nm

typical nanoparticle size15 ~20 nm

L. Sun, S. Zhang, X. W. Sun, X. He, Chapter 2, Anodized Titania Nanotube Array and its Application in Dye-Sensitized Solar Cells, in Vol. 3, CRC Handbook of Nanostructured Thin Films and Coatings Edited by Sam Zhang, Published by CRC Press Taylor & Francis Group, in press, 2009

Nanoparticles (fcc packing) Nanotubes (hcp)

R radius of nanotuber radius of nanoparticle Too small to achieve!

Another Issue: Effective Surface Area

L. Sun, S. Zhang, X. W. Sun, X. He, J. Nanosci. Nanotechnol. 10 (2010): 1-10

Consequences:

Less effective area less efficiency!

That explains why most nanotube array DSSCs are less efficient than nanoparticle counterpart

Nanoparticle + Nanotube Array?

TiO2 nanotube

TiO2 nanoparticle

Composite Structure

In combination of nanotube array: superior electron transport and suppressed

electron recombination nanoparticle: high surface area

Anodic Titania Nanotube Array

Preparation & Characterization

Electrochemical Anodization: Experimental Set Up

Electrochemical Anodization: Principle

Ti foil

Pt

Ethylene Glycol +

2 vol% H2O +

0.3 wt% NH4F

Ti → Ti4+ + 4e

Ti4+ 2H2O / 2OH → TiO2 4H+/ 2H+

TiO2 6HF → [TiF6]2 2H2O 2H+

2H+ 2e → H2

anioncation

Surface morphology and cross-sectional view of the as-anodized titania nanotube arrays for differentanodizing durations:

2 h (a, d)14 h (b, e)24 h (c, f)

L. Sun, S. Zhang, X. W. Sun, X. He, J. Electroanal. Chem. (2009) Accepted.

Variation of Length

L. Sun, S. Zhang, X. W. Sun, X. He, J. Electroanal. Chem. (2009) Accepted.

Longer nanotube arrays are obtainable at higher potential for longer anodizing duration.

Variation of Pore Diameter

Pore diameter of the nanotubes increases with applied potential, whereas decreases with increased working distance.

L. Sun, S. Zhang, X. W. Sun, X. He, J. Electroanal. Chem. (2009) Accepted.

25 V 40 V 50 V 60 V

40 mm 30 mm 20 mm 13 mm

The as-anodized nanotubes are amorphous.

TEM Images of As-anodized Nanotubes

20 30 40 50 60 70 80

a a

Inte

nsity

(a.

u.)

2 (degree)

Ti substrate

as-prepared

annealeda

r

a

ra a

a

XRD Patterns

As anodized, the nanotubes are amorphous; after annealing, mainlyanatase phase (with a little rutile phase).

Conclusions

• Longer titania nanotube arrays are obtainable at higher applied potential for prolonged durations. Length of the nanotubes can be controlled from ~500 nm to ~120 m;

• Pore diameter of the nanotubes increases with applied potential, whereas decreases with increased working distance;

• The as-anodized titania nanotubes are amorphous. The nanotubes crystallize mainly into anatase phase upon annealing.• The composite structure of nanoparticle-nanotube array points to another direction for efficiency enhancement in DSSCs;

Thanks for your attention!

M. A. Green, K. Emery, Y. Hishikawa and W. Warta, Prog. Photovolt: Res. Appl. 17 (2009): 320-326