Ti-suboxides structures for water splitting

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duction→. Magnéli phases. Functionally graded fibres. Flat band potential ... were grown on a Ti mesh that were then heat-threated in a reducing atmosphere ...
Ti-suboxides structures for water splitting V. Adamaki1, P. Mougoyannis1, A. Sergejevs2, C. Clarke2, C. R. Bowen1, R. Stevens1 1 Mechanical Engineering Department, University of Bath, BA2 7AY, UK 2Electrical Engineering Department, University of Bath, BA2 7AY, UK

Introduction The photo-electro-chemical (PEC) splitting of water requires semiconductors with minimum energy gap of 1.23 eV and with conduction and valence bands overlapping the oxidation of H2O and the reduction of H+ respectively [1]. Titania (TiO2) has been extensively investigated as photo-anode for PEC water splitting due to its suitable band-edge position, chemical resistance, environmental suitability and low cost [2-5]. The main limitations of stoichiometric TiO2 are its wide band gap (3.2 eV) and electron-hole recombination. It has been shown that the oxygen vacancies in the structure of Ti-suboxides increase the carriers’ density and thus the photocurrent efficiency [6]. Other parameters to increase the efficiency is to increase the surface area and use composites structures (i.e. thin films on ferroelectrics) to enhance electron-hole separation. The approach of this work is investigating both parameters and measuring the photocurrent efficiency. TiO2 flowerlike structures were grown on a Ti mesh that were then heat-threated in a reducing atmosphere introducing oxygen vacancies in the structure. The structures on a Ti mesh offer very high surface area. The water-splitting efficiency of sub-stoichiometric functionally graded titania fibres was also investigated. Photoelectrochemical (PEC) measurements • •

3-electrode electrochemical system Saturated calomel electrode (SCE) reference electrode and a platinum wire as a counter electrode in 1M KOH electrolyte • 6 wavelengths UV to visible light • Homogeneous illumination • Overheating protection Sub-stoichiometric functionally graded titania fibres Electrochemical Impedance Spectroscopy (EIS)

Through carbo-thermal re duction

TiO2-x

TiO2

Functionally graded fibres

dark

Rutile TiO2

Magnéli phases

Ti8 O15/Ti9 O17/Ti10 O19

Magnéli phases

Photocurrent – 760W/m2 – 368nm

Ti3 O5/Ti4 O7/ Ti5 O9/Ti6 O11

Functionally graded fibres

Flat band potential

1.92 V vs SCE

-2.33 V vs SCE

Carriers density

4.16x1014 cm-3

5.72x1017 cm-3

under UV light (368nm)

under UV light (368nm)

Magnéli phases under UV light (368nm)

dark

dark

Material

IPCE (%)

TiO2

1.42

Magnéli phases

2.88

Graded (TiO2-x- Magnéli phases)

34.8

High surface area TiO2 flower structure Titanium dioxide flowerlike structure prepared by anodisation of Ti-mesh electrode • Potentiostatic conditions (30 volts) at room temperature for 9.16h. • Ethylene Glycol with 0.45 wt % Ammonium Fluoride • Heat treatment in a reducing atmosphere and PEC measurements will be conducted

Schematic diagram of the flower structure formation mechanism

Initial surface oxide formed



(a) Pores on the oxide layer

(b)

Further etching of the nanotubes walls

Due to surface stress the oxide structure separates

Flower-like structure due to stress relaxation by viscous flow Cracks run along the length of the wire

Tubes

(c)

(d)

(e)

[1]: A. Fujishima, K. Honda, Nature, 238 (1972) 37 [3]: J. Ba, Nature Nanotechnology, 10 (2015) 19 [5]: J. Yu, L. Qi, M. Jaroniec, J. of Physical Chemistry C, 114 (2010) 13118 [2]: C. Cheng, Y. Sun, Applied Surface Science, 263 (2012) 273 [4]: D. Regonini, A. K. Alves, F. A. Berutti, F. Clemens, Int. J. of Photoenergy, 2014 (2014) 1 [6]: G. Wang, H. Wang, Y. ling, Y. Tang, X. Yang, R. C. Fitzmorris, Y. Li, Nano Letters, 11 (2011) 3026

The research leading to the materials development and characterisation was from the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement no. 320963 on Novel Energy Materials, Engineering, Science and Integrated Systems (NEMESIS).