Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

Abstract

CuWO₄ is a ternary semiconductor oxide with excellent visible light harvesting properties up to 550 nm and stability at high pH values, which make it a suitable material to build photoanodes for solar light conversion to hydrogen via water splitting. In this work, we studied the photoelectrochemical (PEC) performance of transparent CuWO₄ electrodes with tunable light absorption and thickness, aiming at identifying the intrinsic bottlenecks of photogenerated charge carriers in this semiconductor. We found that electrodes with optimal CuWO₄ thickness exhibit visible light activity due to the absorption of long-wavelength photons and a balanced electron and hole extraction from the oxide. The PEC performance of CuWO₄ is light-intensity-dependent, with charge recombination increasing with light intensity and most photogenerated charge carriers recombining in bulk sites, as demonstrated by PEC tests performed in the presence of sacrificial agents or cocatalysts. The best-performing 580 nm thick CuWO₄ electrode delivers a photocurrent of 0.37 mA cm^-2 at 1.23 V_SHE, with a 7% absorbed photon to current efficiency over the CuWO₄ absorption spectrum.

Figure 1

Field emission scanning electron microscopy (FESEM) top views of (A) 2 layers and (B) 5 layers of CuWO₄. Inset in (B): cross section of the CuWO₄ film obtained by depositing 5 layers. (C) XRD patterns of the CuWO₄ electrodes. The diffraction signals of WO₃ (JCPDF 89–4476), CuWO₄, and underlying FTO are identified with red bars, black arrows, and asterisks, respectively. (D) Absorption spectra of the CuWO₄ photoanodes with increasing semiconductor thickness. Inset: scheme of the optical transitions in CuWO₄. (E) Thickness of the electrodes with 1–6 CuWO₄ layers, from cross-sectional FESEM images (left column) and calculated from the absorption coefficient at 420 nm (right column). (F) Extinction coefficient vs wavelength of a 5-layered CuWO₄ electrode.

Figure 2

Linear sweep voltammetry (LSV) curves recorded with CuWO₄ multilayer photoanodes under (A) back-side or (B) front-side simulated solar light irradiation (100 mW cm^–2) in 0.1 M KBi buffer solution at pH 9. The labels within the figures refer to the thickness of the CuWO₄ layers, in nm. (C) Current density at 1.23 V_SHE as a function of the CuWO₄ thickness, under back- and front-side irradiation (empty and full symbols, respectively). (D) Overall photon flux in the AM 1.5 solar spectrum (black trace) and number of photons absorbed per second by CuWO₄:580 (blue area). The dashed vertical line marks the CuWO₄ absorption edge.

Figure 3

(A, C) IPCE and (B, D) IQE analyses of CuWO₄ multilayer photoanodes under (A, B) back-side and (C, D) front-side monochromatic irradiation, at 1.23 V_SHE.

Figure 4

(A) LSV curves recorded with the CuWO₄:580 photoelectrode under different irradiation intensities in contact with 0.1 M (0–1 sun) or 0.5 M (1.5–2 sun) (continuous lines) KBi solutions. The two dashed lines are the LSV curves recorded under 1.5 and 2 sun irradiation in contact with 0.1 M KBi solutions. (B) Photocurrent efficiency at 1.23 V_SHE vs absorbed photons per unit time at different irradiation intensities.

Figure 5

(A) Chopped chronoamperometry under 1 sun irradiation of an 80 nm thick CuWO₄ photoanode (black line) and a 75 nm thick BiVO₄ photoanode (red line), under back-side irradiation at 1.23 V_SHE. The green (light on) and blue (light off) boxes display expanded portions of the photocurrent curve. (B) Chronoamperometric stability test of the CuWO₄:580 electrode under simulated solar light irradiation in 0.1 M KBi. (C) Chopped chronoamperometry under 1 sun irradiation of the CuWO₄:580 photoanode in the presence (red line) or in the absence (black line) of 0.5 M H₂O₂. (D) Chopped LSV curves recorded with a bare (black line) and NiFeO_x-modified (red line) CuWO₄:80 photoanode under monochromatic irradiation at 420 nm; the light intensity is 9 mW cm^–2.