Artificial photosynthesis is a very promising method to turn solar energy into fuels.¹ In a photoelectrochemical (PEC) cell, the generation of H₂ by water splitting takes place on two different electrodes, which facilitates the separation of the products by the addition of a proton exchange membrane.  PEC cells have led to some of the highest solar-to-hydrogen efficiencies achieved to date.² The setup of a successful PEC cell requires the preparation of efficient and stable photoelectrodes.

Subestequiometric copper telluride, Cu_2-xTe, is a very interesting material that, as far as we know, has not been used in a PEC cell yet. It has already been used in photovoltaic cells due to its small band gap of around 1.5 eV and its high conductivity.³ In this work, we used Cu_2-xTe nanocrystals synthesized by a colloidal method⁴ to prepare thin films and use them as photoelectrodes. After their organic capping was successfully removed by a thermal treatment in Ar, two different crystallographic phases were obtained depending on the heating temperature. Then, the photoelectrochemical and optical properties of both phases were measured. Electrochemical impedance spectroscopy was used to build the Mott-Schottky diagrams and determine the flat band potential, charrier density and conduction type, which is p-type for both phases. This conductivity type was ratified by photopotential measurements. Both electrochemical impedance spectroscopy and UV-Vis-nIR spectroscopy allowed us to build the bands energy diagram, which confirms the ability of this material to reduce water to hydrogen. This means that Cu_2-xTe thin films are good candidates to be used as photocathodes, which was also confirmed by the measurement of their photocurrents. As they show some stability problems, two strategies were proved to solve this: the addition of an amorphous TiO₂ layer and the change of the electrolyte pH. Moreover, H₂ generation by water splitting in a two-electrode PEC cell was observed and quantified.

References

1.           Sivula, K. & van de Krol, R. Semiconducting materials for photoelectrochemical energy conversion. Nat. Rev. Mater. 1, 1–16 (2016).

2.           Young, J. L. et al. Direct solar-to-hydrogen conversion via inverted metamorphic multijunction semiconductor architectures. Nat. Energy 17028, 1–8 (2017).

3.           Rungtaweechai, N. & Tubtimtae, A. Cu_2-xTe/MnTe co-sensitized near-infrared absorbing liquid-junction solar cells. Mater. Lett. 158, 70–74 (2015).

4.           Arciniegas, M. P. et al. Self-Assembled Dense Colloidal Cu₂Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater. 26, 4535–4542 (2016).