Reconstructing Oxygen Vacancies in the Bulk and Nickel Oxyhydroxide Overlayer to Promote the Hematite Photoanode for Photoelectrochemical Water Oxidation

Abstract

Surface engineering, as an efficient strategy, can improve the photoelectrochemical water splitting (PEC-WS) performance for converting inexhaustible sunlight into clean hydrogen fuel. Oxyhydroxides and p–n heterojunctions have been demonstrated as efficient catalysts for the water oxidation reaction. In this work, to address the drawbacks of poor conductivity and sluggish oxidation kinetics of hematite, we introduce a p-type NiOOH overlayer as a surface catalyst onto n-type Sn-doping hematite (Sn@α-Fe2O3) photoanode. The oxygen vacancies (Ov) are reconstructed both in the bulk of Sn@α-Fe2O3 and the surface decoration layer of NiOOH via Ar plasma treatment, effectively reducing unavoidable defects introduced by the NiOOH overlayer. Compared with the original Sn@α-Fe2O3 photoanode, the Sn@α-Fe2O3/NiOOH–Ar photoanode exhibits a significant increase in photocurrent density (at 1.23 VRHE) of ∼3 times and a decrease in the onset potential of ∼200 mV. The performance improvement can be ascribed to the synergistic effect of the p–n junctions formed by NiOOH decoration and improved conductivity through oxygen vacancy reconstruction, which remarkably improves carrier separation in the bulk of α-Fe2O3 and suppresses carrier recombination on the photoanode surface. Moreover, the density functional theory (DFT) calculation proves that the real active sites are farther from (rather than near) the oxygen vacancies.

Integration of an Electron Transport Layer and a p‐n Heterojunction in a ZnO photoanode for Photoelectrochemical Water Oxidation

“…The selective equivalent circuits (EC) for the charge transfer process of ZnO‐based photoanode under illumination are shown in Figure 4a, where R S represent the series resistance (including the solution resistance and the contact resistance), R CT and C SC represents the charge transport resistance at the ZnO‐electrolyte interface and the space charge capacitance, respectively. The fitting parameters are summarized in Table S2, and the FTO/TiO 2 /ZnO/NiO photoanode shows the smallest transport resistance, which is consistent with the optimal PEC‐WS performance [55] . The Mott‐Schottky spectra are employed to analyse the effect of TiO 2 and NiO on the charge density ( N D ) of ZnO photoanode (Figure 4b), and the N D can be calculated according to the slope [56] .…”

“…The fitting parameters are summarized in Table S2, and the FTO/TiO 2 /ZnO/NiO photoanode shows the smallest transport resistance, which is consistent with the optimal PEC-WS performance. [55] The Mott-Schottky spectra are employed to analyse the effect of TiO 2 and NiO on the charge density (N D ) of ZnO photoanode (Figure 4b), and the N D can be calculated according to the slope. [56] The positive slope implies that the ZnO photoanode belongs to n-type semiconductor.…”

Metal–organic Framework-Derived CoSx/NiS Co-Decorated Heterostructures: Toward Simultaneous Acceleration of Charge Carrier Separation and Catalytic Kinetics

“…The fitting peaks for Co 2+ and Co 3+ species implied the co-existence of both Co 2+ and Co 3+ in TiO 2 /CoS x /NiS and TiO 2 /CdS/CoS x /NiS. Moreover, in the Ni 2p spectra (Figure b), the binding energy of Ni 2p 3/2 and Ni 2p 1/2 could be divided into two peaks, suggesting the successful decoration of CoNi-MOF or NiS . As demonstrated in Figure c,d, after the ion exchange with Cd 2+ , the peaks of Cd 3d 3/2 and Cd 3d 5/2 appeared while the peaks of S 2p 1/2 and S 2p 3/2 negatively shifted, indicating the successful construction of TiO 2 /CdS/CoS x /NiS and strong interaction between CoS x /NiS and CdS.…”

Advanced oxygen evolution reaction catalysts for solar-driven photoelectrochemical water splitting

“…Compared with the original Sn@α-Fe 2 O 3 photoanode, enhanced photocurrent and decreased onset potential could be achieved after reconstruction of oxygen vacancies, owing to the improved charge separation. 78…”

“…Compared with the original Sn@a-Fe 2 O 3 photoanode, enhanced photocurrent and decreased onset potential could be achieved aer reconstruction of oxygen vacancies, owing to the improved charge separation. 78 Recently, other strategies such as heteroatom incorporation, in situ activation and precursor regulation have also been demonstrated to construct oxygen defects in OER catalysts for further promoting the PEC water splitting activity. For instance, Wang et al reported a two-step photo-assisted electrodeposition and subsequent calcination process for incorporating Fe into CoO x to form FeCoO x with abundant oxygen defects, which drastically increased the photocurrent density of the BiVO 4 photoanode to 4.82 mA cm −2 at 1.23 V RHE (Fig.…”