ductionâ. 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).
