(Invited) CdTe Based Photocathodes and Photoanodes for Photoelectrochemical Water Splitting Under Sunlight

Abstract

CdTe based photocathodes and photoanodes for photoelectrochemical water splitting under sunlight Tsutomu Minegishi, Jin Su, Kazunari Domen Photoelectrochemical (PEC) and photocatalytic water splitting reactions are attractive means to obtain energy carriers such as hydrogen and Methylcyclohexane from only water and sunlight. For efficient solar hydrogen production, development of photoelectrodes with long absorption edge wavelength are an essential issue. Cadmium telluride (CdTe) has attractive properties for sunlight driven PEC water splitting such as narrow band gap of 1.5 eV and high optical absorption coefficients of ~104 cm-1 at wavelength shorter than 800 nm.1,2 Owing to these advantageous properties, polycrystalline thin film of CdTe has been employed as an absorber in thin film solar cells nowadays.1,2 Considering the features of CdTe, as mentioned above, the material should be suitable also as photoelectrodes for water splitting working under sunlight. In the present study, PEC properties of surface modified CdTe based photoanodes and photocathodes have been investigated, and overall water splitting by the PEC cell composed of CdTe based photoelectrodes was demonstrated. A CdTe photoelectrode directly prepared on FTO substrate by closed space sublimation (CSS) method showed clear cathodic photo-response in a phosphate buffer aqueous solution because of the p-type semiconducting property of prepared CdTe thin film. The photocurrent density and onset potential of CdTe photocathode were enhanced significantly by a surface modification with 80 nm-thick CdS layer and Pt. A post-deposition calcination of CdTe thin film with existence of CdCl2 as a sintering reagent and introduction of Au and Cu layer between the CdTe layer and FTO substrate resulted in extremely high incident photon-to-current conversion efficiencies (IPCEs) of >95%.3, 4 A Faradic efficiency of the photocathode during hydrogen evolution under simulated sunlight from water was confirmed to be almost unity.3 On the other hand, a CdTe based photoelectrode with CdS layer introduced at between the FTO substrate and CdTe worked as photoanode. It should be noted that a p-n junction formed at CdTe/CdS interface provides photovoltage in the photoanode. The photoanode showed photocurrent at above 0.2 VRHE, indicating that the photoanode possess a sufficient driving force for overall water splitting in bi-photoelectrode PEC cell with the CdTe photocathode. The PEC cell composed of the combination of the CdTe based photocathode and photoanode successfully produced hydrogen and oxygen from water under simulated sunlight without an external bias voltage. 5 1) J. S. Kaper and J. S. Prener, Acta Crystallogr., 7, 246-248 (1954). 2) K. Hashimoto and N.Kishimoto, J. Am. Chem. Soc., 1, 5521-5529 (2016). 3) J. Su, T. Minegishi, M. Katayama and K. Domen, J. Mater. Chem. A, 5, 4486 (2017). 4) J. Su, T. Minegishi and K. Domen, J. Mater. Chem. A, 5, 13154 (2017). 5) J. Su, T. Minegishi, Y. Kageshima, H. Kobayashi, T. Hisatomi, T. Higashi, M. Katayama, and K. Domen, J. Phys. Chem. Lett., 8, 5712–5717 (2017).