Ultrafast Charge Carrier Dynamics in CuWO4 Photoanodes

Abstract

CuWO₄ is a ternary metal oxide semiconductor with promising properties for photoelectrochemical (PEC) water splitting and solar light conversion, due to its quite low band gap (2.3 eV) and high stability in an alkaline environment. Aiming at understanding the origin of the relatively low PEC efficiency attained with CuWO₄ photoanodes, we here investigate transparent CuWO₄ electrodes prepared by a simple solution-based method through the combination of femtosecond transient absorption spectroscopy with electrochemical, PEC, and photochromic characterizations. The very fast recombination dynamics of the charge carriers photogenerated in CuWO₄, which is the reason for its low efficiency, is discussed in relation with its PEC performance and with the recently calculated band structure of this material, also in comparison with the behavior of other semiconductor oxides employed in PEC applications, in particular Fe₂O₃.

Figure 1

(A) Absorption spectrum of a CuWO₄ photoelectrode. (B) X-ray diffraction pattern of the CuWO₄ thin film coated on FTO glass. The gray bars are the reference Bragg reflections of CuWO₄ from JCPDF 72-0616. The FTO reflections are marked separately.

Figure 2

(A) SEM top view and (B) cross section image of the CuWO₄/FTO electrode employed in the PEC measurements. The scale bars are 500 (A) and 200 nm (B).

Figure 3

(A) Linear sweep voltammetry under AM 1.5 G simulated solar light for the CuWO₄ film in contact with 0.1 M potassium borate (KBi) aqueous solutions at pH 9, (1) in the presence of 0.5 M Na₂SO₃ (dash dot line) and (2) in the absence of Na₂SO₃ (continuous line) and (3) in the absence of irradiation and of Na₂SO₃. (B) Incident photon to current efficiency (IPCE, black circles) and internal quantum efficiency (IQE, red squares) of the CuWO₄ photoanode at 1.23 V_RHE in KBi at pH 9.

Figure 4

(A) Cyclovoltammetry recorded in 0.1 M tetrabutylammonium hexafluorophosphate, with a scan rate of 100 mV s^–1. (B) Mott–Schottky plot of the CuWO₄ photoanode in 0.3 M Na₂SO₄ solution at pH 7.

Figure 5

Photochromic measurements of a CuWO₄ electrode recorded in deaerated ethanol at increasing time under irradiation with a 300 W Xe arc lamp.

Figure 6

(A) Femtosecond TA spectra recorded at different times after photoexcitation of CuWO₄ at 400 nm under vacuum and ΔA decay traces (B) at 500 nm upon excitation with different pump fluences and (C) at different wavelengths upon excitation with a 420 μJ cm^–2 fluence. (D) Evolution associated spectra (EAS) obtained from Global Analysis of the ΔA(λ,Δt) data recorded upon photoexcitation at 400 nm with a 170 μJ cm^–2 fluence.

Scheme 1. Ultrafast Relaxation and Recombination Pathways of Electrons Photopromoted in CuWO₄ upon 400 nm Excitation