Benchmarked Photoelectrochemical Water Splitting by Nickel-Doped n-Type Cuprous Oxide

Abstract

Herein, Ni is for the first time put forward as a promising dopant to promote the photoelectrochemical property of cuprous oxide (Cu 2 O) with particularly n-type conduction. An electrochemical route is utilized to deposit Ni:Cu 2 O on the indium tin oxide (ITO) coated glass as a dense thin film with tree-like morphology, as revealed through scanning electron microscopy. Particularly, the incorporation of Ni that is verified via X-ray diffractometry leads to a compressive stress compensating well the tensile strain within Cu 2 O, which originates otherwise from large lattice mismatch between Cu 2 O and ITO. This in turn improves the crystallinity of the Ni:Cu 2 O thin film. More importantly, X-ray photoelectron spectroscopy indicates that the substitution of Ni(II) for Cu(I) boosts the carrier concentration to 8.4 × 10 18 cm −3 , as further evidenced by the Mott−Schottky analysis. Those enhancements along with an advantageous band structure including a flat-band potential of −1.1 V and a bandgap of ∼2.1 eV that is alternatively derived from UV−vis spectroscopy allow Ni:Cu 2 O to deliver an outstanding photocurrent of 2.5 mA cm −2 (at 0.3 V vs Ag/AgCl) for solar water splitting. Such performance well-surpasses those of other reported n-type Cu 2 O photoanodes more than 2fold, attesting the great promise of Ni:Cu 2 O in this work 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

“…Among them, the highest photocurrent density amounting to 2 mA cm –2 is seen in the LSV curve of Cu 2 O precipitated on ITO subjected to the electrochemically forced reduction at −1.5 V Ag/AgCl and, more importantly, at an applied potential of 0.05 V Ag/AgCl , which is far negative than that of the Cu 2 O counterpart on pristine ITO by 200 mV. Most significantly, this in turn renders ABPE of Cu 2 O deposited on ITO subjected to the electrochemically forced reduction at −1.5 V Ag/AgCl reach 1.1%, which is among the highest performance reported to date for a variety of state-of-the-art metal oxide-based photoanodes in the literature ,− (Figure a and Table S1). The best performance of this photoanode unambiguously highlights the important role of the external bias of −1.5 V Ag/AgCl applied to ITO, which decreases the work function of ITO via the oxygen vacancy introduced to ITO for compensating the charge caused by its electrochemically forced reduction into metallic indium (Figures c, and S1).…”

“…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.…”

“…A 150 W Xe lamp equipped with an AM 1.5 filter was employed in this experiment to provide the simulated sunlight with the calibrated intensity P light = 100 mW cm –2 , and 0.5 M sodium sulfate (Na 2 SO 4 ) aqueous solution (pH = 6.8) was used as the electrolyte. The photocurrent density ( J PEC ) delivered by Cu 2 O precipitated on either pristine ITO or ITO subjected to the electrochemically forced reduction under solar irradiation was then utilized to derive ABPE via the following equation , where V RHE is the applied potential reported against the reversible hydrogen electrode (RHE), and its relationship with the experimentally measured applied potential with respect to the Ag/AgCl reference electrode (V Ag/AgCl ) is expressed by Electrochemical impedance spectroscopy (EIS) was carried out at 0 V Ag/AgCl in dark conditions over a frequency range of 100 kHz–0.1 Hz with an AC potential amplitude of 10 mV. Mott–Schottky (M–S) analysis was performed over diverse applied potentials at a frequency of 10 kHz with an AC potential amplitude of 10 mV.…”

“…2 O precipitated on ITO subjected to the electrochemically forced reduction at −1.5 V Ag/AgCl and, more importantly, at an applied potential of 0.05 V Ag/AgCl , which is far negative than that of the Cu 2 O counterpart on pristine ITO by 200 mV. Most significantly, this in turn renders ABPE of Cu 2 O deposited on ITO subjected to the electrochemically forced reduction at −1.5 V Ag/AgCl reach 1.1%, which is among the highest performance reported to date for a variety of state-of-the-art metal oxide-based photoanodes in the literature14,[39][40][41][42][43][44][45][46] …”

“…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...…”