Application of Hierarchical Titanate and Titania nanotubes in Dye Sensitized Solar Cells Dmitry V. Bavykin and Frank C. Walsh

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1 Application of Hierarchical Titanate and Titania nanotubes in Dye Sensitized Solar Cells Dmitry V. Bavykin and Frank C. Walsh Energy Technology and Materials Engineering Research Groups School of Engineering Science University of Southampton 1

2 Publication growth related to TiO 2 nanotubes Data were taken from the ICI Web of Science database using TiO 2 and nanotube* as keywords. 2

3 Three types of TiO 2 nanotubes Titanate nanotubes via alkaline hydrothermal route TiO 2 nanotubes array via anodic oxidation in F - containing electrolyte TiO 2 nanotubes via sol gel synthesis using templates 3

4 Alkaline hydrothermal synthesis of titanate nanotubes 10 mol dm -3 NaOH, ºC (autoclave) or NaOH/KOH mixture at 106ºC (reflux) One pot synthesis. 99 % conversion after 24 hours. Easy scale up and recycling of NaOH. Low cost process. Feedstock (TiO 2 ) is readily available. 4

5 Sequence of events in alkaline solution TiO2 + NaOH nanosheets nanotubes nanofibre 5

6 Mechanism of titanate nanotube growth: closing the loop Onion Concentric Snail 6

7 Early studies of use titanate nanotubes for a DSSC load hν Pt SnO 2 e - 3I- I 3 - H2T3O7 TiO H 2 Ti 2 3 O 7 Solaronix N719 No particular improvements compare to DSSC with TiO 2 7

8 How titanate nanotubes can work better in DSSC? In aqueous suspension titanate nanotubes develop neganive zeta potential due to dissociation H 2 Ti 3 O 7 H + + HTi 3 O - 7 MB uptake / mol(mb) mol(tio 2 ) nanotubes P Concentration of MB / mmol dm -3 Approx molecules of dye per single nanotube D.V. Bavykin, K.A. Redmond, B.P. Nias, A.N. Kulak, F.C. Walsh Aust. J. Chem. 2010, 63,

9 Cationic dyes for DSSC UV absorption λ max 740 nm UV absorption λ max 655 nm UV absorption λ max 483 nm UV absorption λ max 466 nm 9

10 How titanate nanotubes can work better in DSSC? How thick should be layer of titanate nanotubes with dye in order to absorb 90 % light? Assuming extinction coefficient of dye ε = 10 5 dm 3 mol -1 cm -1 ; adsorption is 10-2 mol(dye)mol(tio 2 ) -1 ; density of TiNT is 3.2 g cm -3 ; The thickness of the film should be ca 250 nm. However Random orientation in situ films EPD dip coating etc. electron collection efficiency can be small even for thin films Horizontal orientation LB films 10

11 Anodic synthesis of TiO 2 nanotubes array Fluoride ions containing electrolytes Anodic oxidation of titanium V = V, Time: up to several hours Ambient temperature Feedstock (titanium) is available. 11

12 Titanate nanotubes vs. TiO2 nanotubes array Temperature of synthesis Crystal structure Inner diameter Length Wall thickness BET surface area Band gap Thermal stability Titanate C H 2 Ti 3 O nm nm nm ca. 250 m 2 g ev up to 400ºC Array 25 C amorph. or anatase µm µm ~ 5 40 nm 20 m 2 g -1 (estimated) 3.34 ev up to 500ºC 12

13 Application of TiO 2 nanotubes array in DSSC 30 µm J.M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer, P. Schmuki, Curr. Opin. Solid State Mater. Sci., 2007, 11, 3. 13

14 Hierarchical structure: Titanate nanotubes inside TiO2 nanotubes array 14

15 Incorporation of Titanate nanotubes into TiO 2 nanotubes array. First attempts anodic TiO 2 NT in glycerol : water (90:10) electrolyte, 40 V, 3 hours electrophoretic deposition of TiNT on anode from aqueous suspension, 140 V, 5 min long tubes lay on the top, small amount of TiNT Bridge 15

16 Control of the nanotubes length: ultrasound initial 0.5 hours 3 hours N um be r o f tu be s / a.u. N um be r o f tu be s / a.u. N u m b e r o f tu b e s / a.u Length of nanotubes / nm Length of nanotubes / nm Length of nanotubes / nm D.V. Bavykin, F.C. Walsh, J. Phys. Chem. C:, 2007, 111, (40),

17 Incorporation of short TiNT into TiO 2 nanotubes array anodic TiO 2 NT in glycerol : water (90:10) electrolyte, 40 V, 3 hours Ultrasonic treatment of TiNT 3 hours electrophoretic deposition of TiNT on anode from aqueous suspension, 140 V, 5 min short TiNT embedded into TiO 2 nanotubes array. However amount of TiNT is small, optimization is needed. 17

18 Adsorption of Methylene Blue on the surface of nanotubes Incorporation of TiNT into the pores of TiO 2 nanotube array increases the amount of cationic dye, which can be adsorbed without changing film thickness (electron diffusion path). 18

19 Other hierarchical structures 19

20 Other hierarchical structures 20

21 Principle of synthesis of hierarchical structures 21

22 Conclusions Titanate nanotubes have a great potential for photocatalytic and photovoltaic scientific community providing additional synthetic route for catalysts, facile approach for decoration of TiO 2, and mesoporous morphology for improved mass transport. TiO 2 nanotubes array amongst many applications can be promising candidate for DSSC electrodes, photocatalyst of water splitting and advanced oxidation. Due to improved current collection efficiency and better transport of iodine ions in the DSSC based on the hierarchical TiNT/TiO 2 NT structures, their higher performance is anticipated. Acknowledgment EPSRC (EP/F044445/1 ), NATO (Collaborative linkage grant) Prof. Tom Markvart, Dr.Lefteris Danos, Ben Nias. 22

23 Contact Dmitry Bavykin Energy Technology and Materials Engineering Research Groups School of Engineering Science University of Southampton Tel