Defect‐Cluster‐Boosted Solar Photoelectrochemical Water Splitting by n‐Cu₂O Thin Films Prepared Through Anisotropic Crystal Growth

Abstract

Anisotropic growth of Cu2O crystals deposited on an indium‐doped tin oxide‐coated glass substrate through facile electrodeposition and low‐temperature calcination results in favorable solar photoelectrochemical water splitting. XRD, TEM, and SEM reveal that appreciable oxygen vacancies are populated in the Cu2O crystals with a highly branched dendritic thin film morphology, which are further substituted by Cu atoms to form Cu antisite defects exclusively along the [111] direction. The post‐thermal treatment presumably accelerates such migration of the lattice imperfections, favoring the exposure of the catalytically active (111) facets. The Cu2O thin film derived in this way shows n‐type conduction with a donor concentration in the order of 1017 cm−3 and a flat‐band potential of −1.19 V vs. Ag/AgCl, which is also confirmed by Mott–Schottky analysis. The material is employed as a photoanode and delivers a photocurrent density of 2.2 mA cm−2 at a potential of 0.3 V vs. Ag/AgCl, surpassing reported values more than twofold. Such superiority mostly originates from the synergism of the selective facet exposure within the Cu2O crystals, which have decent crystallinity, as shown by Raman and photoluminescence spectroscopy, and a favorable bandgap of 2.1 eV, as confirmed by UV/Vis spectroscopy. The n‐type Cu2O thin film reported herein holds excellent promise for solar‐related applications.

Defective Indium Tin Oxide Forms an Ohmic Back Contact to an n-Type Cu2O Photoanode to Accelerate Charge-Transfer Kinetics for Enhanced Low-Bias Photoelectrochemical Water Splitting

“…A leaflike morphology has been reported as a result of the dendritic growth of Cu 2 O crystals, which was, however, interfered by the oxygen vacancy and the indium metal manifested as pinholes and NPs, respectively, distributed over the ITO film subjected to the electrochemically forced reduction (Figures and and Supporting Information Note 1). − , The lateral branches of the crystalline Cu 2 O dendrites electrodeposited on ITO subjected to the electrochemically forced reduction are relatively shorter with respect to that of the Cu 2 O counterpart on pristine ITO (Figure ). Moreover, the average grain sizes D = 26.4, 24.8, and 25.4 nm of the Cu 2 O crystals precipitated on ITO subjected to the electrochemically forced reduction at potentials of −1.4, −1.5, and −1.6 V Ag/AgCl , respectively, are likewise in this regard comparatively smaller with respect to that ( D = 26.8 nm) of the Cu 2 O counterpart on pristine ITO.…”

“…Besides these aspects, wherein the close resemblance between Cu 2 O precipitated on pristine ITO and ITO subjected to the electrochemically forced reduction is insofar well attested, another common feature between them is the n-type conductivity, as first evidenced by the anodic photocurrent response seen in all of the corresponding linear sweep voltammetry (LSV) curves collected under chopped solar illumination (Figure a). This is further reinforced by the positive slope of the linear regression of the M–S plots measured for Cu 2 O deposited on pristine ITO and ITO subjected to the electrochemically forced reduction, for which the majority carrier is assigned in this case to be electrons (Figure b). − , Moreover, the electron density ( N d ) is additionally derived from the slope via the formula shown below where C is the space charge capacitance; ε and ε 0 stand for the permittivity of Cu 2 O (ε = 8) and the free space, respectively; V represents the applied potential; V FB refers to the flat-band potential; T corresponds to the temperature; and k B and e denote the Boltzmann constant and the elementary charge, respectively. − , N d = 1.5 × 10 18 , 4.9 × 10 18 , and 8.1 × 10 17 cm –3 for Cu 2 O precipitated on ITO subjected to the electrochemically forced reduction at potentials of −1.4, −1.5, and −1.6 V Ag/AgCl , respectively, highly close to that ( N d = 1.3 × 10 18 cm –3 ) of the Cu 2 O counterpart on pristine ITO. This good agreement is further seen in V FB of Cu 2 O deposited on pristine ITO and ITO subjected to the electrochemically forced reduction, which is −1.2 V Ag/AgCl determined likewise from the M–S plots via eq .…”

“…The broad band alongside at B.E. of ∼945 eV is attributed to the characteristic shake-up satellite of CuO. , CuO has been reported to originate most likely from the oxidation of Cu 2 O upon the exposure to air, which was nevertheless limited at the surface to in turn account for the relative abundance of CuO far below that of Cu 2 O. , In addition, Cu 2 O crystals electrodeposited on pristine ITO and ITO subjected to the electrochemically forced reduction are all featured with an optical band gap E g ∼ 2.0 eV determined from the Tauc plots in Figure c.…”

“…To meet the targeted 10% solar-to-hydrogen (STH) efficiency for practical implementation of the hydrogen economy, the ideal band gap E g ∼ 2.3 and ∼1.8/1.2 eV for the single and paired light absorbers in the Schottky- and tandem-type photoelectrochemical (PEC) cells, respectively, has been theoretically derived. − Cuprous oxide (Cu 2 O) has in this regard long been spotlighted as a promising candidate in relevant research fields in view of the characteristic direct band gap E g = 1.9–2.2 eV being close to those estimations and more importantly allowing the generation of the maximum photocurrent density amounting to 14.7 mA cm –2 under one sun illumination. − In a majority of the Cu 2 O-based PEC devices reported to date, Cu 2 O was mostly prepared via an electrochemical approach and directly deposited on a transparent conductive oxide (TCO)-coated glass substrate, between which the interface has, however, been frequently identified as a Schottky in lieu of an Ohmic junction. − ,− This is attributed to the large mismatch in the work function (Φ) of TCO, which has been reported to be Φ = 4.8–5.0 eV for fluorine-doped tin oxide (FTO) and Φ = 4.1–5.2 eV for tin-doped indium oxide (ITO), with respect to that of either p- or n-type Cu 2 O. ,,− In consequence, the severe charge accumulation at the Cu 2 O/TCO interface has been frequently stressed by previous junction studies to significantly undermine the charge collection efficiency and thereby the overall device performance. ,,− ,− To address this issue, nickel oxide (NiO)-based materials including copper–nickel mixed oxide (CuO/NiO), copper-doped nickel oxide (Cu/NiO), and so forth have been put forward by Grätzel, Gong, and others very recently as alternative contacts to p-Cu...…”

“…A leaflike morphology has been reported as a result of the dendritic growth of Cu 2 O crystals, which was, however, interfered by the oxygen vacancy and the indium metal manifested as pinholes and NPs, respectively, distributed over the ITO film subjected to the electrochemically forced reduction (Figures 1 and 3 and Supporting Information Note 1). [12][13][14]51 The lateral branches of the crystalline Cu 2 O dendrites electrodeposited on ITO subjected to the electrochemically forced reduction are relatively shorter with respect to that of the Cu 2 O counterpart on pristine ITO (Figure 3).…”