No abstract

Designing metal hydroxide electrocatalysts with high efficiency to overcome the slow reaction kinetics of the oxygen evolution reaction (OER) is considered as a significant approach for renewable energy resources. We report here a simple methodology to synthesize 2D thin nickel hydroxide/nickel oxyhydroxide sheets that show efficient activity towards OER. Further, by doping with a heteroatom, Fe, thinner sheets of nickel hydroxide/nickel oxyhydroxide are developed, which exhibit enhanced electrocatalytic activity towards OER with high durability. Fe‐doped Ni(OH)2/NiOOH requires only 200 mV overpotential to produce 10 mA/cm2, whereas bare Ni(OH)2/NiOOH needs 290 mV overpotential. Moreover, Fe‐doped nickel hydroxide/nickel oxyhydroxide shows a minimal Tafel value of 48 mV/decade, which is even lower than RuO2/CC (82 mV/decade). X‐ray photoelectron spectroscopy indicates that in the case of Fe‐doped Ni(OH)2/NiOOH, the Ni3+ signal enhances, which indicates the favourable stabilization of Ni3+ in the presence of Fe3+ dopant. Under electrochemical OER conditions, in Fe‐doped Ni(OH)2/NiOOH, Fe3+ species help to generate more Ni3+, which function as the active species. Fe0.06Ni0.94(OH)2/NiOOH shows long‐term stability for at least 24 hours in alkaline medium. This work unveils a green strategy for Fe‐doping in 2D thin sheets of Ni(OH)2/NiOOH, which show improved electrocatalytic activity compared to bare Ni(OH)2/NiOOH. The mechanism of OER activity enhancement after Fe‐doping is proposed here.

The efficient utilization of solar power is becoming an important strategy for its conversion into a storable, clean, and renewable energy source like H 2 . To generate H 2 as a chemical fuel from solar power, attempts are being made to establish photoelectrochemical (PEC) water splitting as an efficient, greener pathway. Here, the surfaces of ZnO 2D nanosheets are adorned by graphite-like carbon nitride (g-C 3 N 4 ) quantum dots (QDs) with the intention of developing efficient photoanodes. Sensitization of ZnO nanosheets with C 3 N 4 QDs leads to a more enhanced PEC performance than that of bare ZnO. The observed enhancement in PEC is due to the high light absorbance and photon-generated charge-carrier separation. The best-obtained ZnO/C 3 N 4 photoanode exhibits a nearly 2.29 times as high photocurrent density compared to bare ZnO. ZnO 2D sheets can generate a photocurrent density of 0.414 mA cm −2 at 0.5994 V versus reversible hydrogen electrode (RHE), whereas ZnO/C 3 N 4 can produce 0.952 mA cm −2 at 0.5994 V versus RHE under uninterrupted conditions of light illumination. Further, there is improvement in the observed PEC activity of the heterostructure because of enhancement in the carrier density. The carrier density enhances nearly 2.2 times in the heterostructure compared to the bare ZnO sheet. ZnO/C 3 N 4 shows a maximum photoconversion efficiency (η) of 0.70%. Both ZnO 2D sheets and the ZnO/C 3 N 4 heterostructure show efficient stability under chopped irradiation of light for 1000 s. The stability of ZnO/C 3 N 4 is also determined for 1 h under continuous illumination.

Hydrogen evolution through ecofriendly photoelectrochemical (PEC) water splitting is considered to be one of the most cost-effective and desirable methods for meeting ever-growing energy demands. However, the low photoconversion efficiency limits the practical applicability of PEC water splitting. To develop an efficient photoelectrode, here the morphology of ZnO is tuned from 0D to 3D. It is observed that vertically grown 2D nanosheets outperform other morphologies in PEC water splitting by generating nearly 0.414 mA cm–2 at 0 V vs Ag/AgCl. Furthermore, these perpendicularly developed 2D nanosheets of ZnO are sensitized by metal-free carbon (C) dots to improve the photoconversion efficiency of ZnO. The prepared ZnO/C dots work as an effective photoanode, which can produce a 0.831 mA cm–2 photocurrent density upon application of 0 V vs Ag/AgCl under constant illumination, which is 2 times higher than that of bare ZnO. The enhanced PEC performance of ZnO/C dots is confirmed by the photoconversion efficiency (η). The ZnO/C dots exhibit a 2-fold-higher photoconversion efficiency (η) compared to that of ZnO. Additionally, the enhancement in PEC activity of ZnO/C dots is attributed to the higher carrier concentrations in the heterostructure. Bare ZnO has a 1.77 × 1020 cm–3 carrier density, which becomes 3.70 × 1020 cm–3 after sensitization with C dots. Enhanced carrier density successively leads to higher PEC water splitting efficiency. Band alignments of ZnO and C dots indicate the creation of the type-II heterostructure, which facilitates successful charge transportation among C dots and ZnO, producing a charge-carrier separation. Two-dimensional sheets of ZnO and ZnO/C dots exhibit appreciable stability under continuous illumination for 1 and 2 h, respectively.

CdS nanoparticles attached to ZnO 2D sheets help to improve light absorbance, leading to enhanced photoelectrochemical water splitting performance.