Cu₂O/ZnO p–n Junction Decorated with NiO_x as a Protective Layer and Cocatalyst for Enhanced Photoelectrochemical Water Splitting

Abstract

Cuprous oxide (Cu 2 O) has attracted much interest as a photocathode for photoelectrochemical (PEC) water splitting because of its elemental abundance and the favorable band gap, but its poor stability in aqueous solutions hinders the practical PEC application. Compared to the mostly used TiO 2 and noble metal cocatalysts for coating the Cu 2 O photocathode, this work demonstrates a strategy to fabricate a noble metal-free photocathode. We construct a Cu 2 O/ZnO p−n junction photocathode decorated with the NiO x layer as both the protective layer and the hydrogen evolution reaction (HER) cocatalyst. The NiO x cocatalyst exhibits a small Tafel slope of 35.9 mV/ dec and a very low overpotential of 115 mV to drive a current of 10 mA/cm 2 , which are very close to the HER activity of the noble metal platinum. With decorated NiO x , the Cu 2 O/ZnO/NiO x photocathode exhibits significantly improved stability and photocurrent density with a Faradaic efficiency of H 2 gas evolution of 95 ± 4%, distinctly outperforming the Cu 2 O, Cu 2 O/ZnO, and Cu 2 O/ZnO/TiO 2 photocathodes. Moreover, electrochemical impedance analysis evidenced that NiO x as a cocatalyst also facilitates the transfer of photogenerated electrons across the electrode/electrolyte interface for water reduction. This work demonstrates that NiO x is not only a stable protective layer against corrosion but also a highly active H 2 evolution cocatalyst. These findings provide new insights for the design of noble metalfree photocathodes toward solar fuel development.

Copper Oxide-Based Photocatalysts and Photocathodes: Fundamentals and Recent Advances

“…Thence, the p-n heterojunction can enhance the performance and block the self-oxidation of Cu 2 O as a protective layer. Many n-type semiconductors, Ga 2 O 3 [ 220 , 221 , 222 , 223 ], ZnO [ 224 , 225 ], TiO 2 [ 226 , 227 ] and NiO x [ 50 ], deposited over Cu 2 O can enhance the performance, since the n-type semiconductor and p-type semiconductor can provide holes and electrons for the water splitting, respectively, as well as relative band positions. Several Cu 2 O-based n-p heterojunctions behave as schematized in Figure 18 .…”

Toward Simultaneous Achievement of Outstanding Durability and Photoelectrochemical Reaction in Cu2O Photocathodes via Electrochemically Designed Resistive Switching

“…From a comparison with reported photocathodes consisting of similar structures containing AZO, the Cu 2 O photocathode subjected to the ECF-CC3 process showed a considerably superior PEC performance, with a cathodic photocurrent density of 11.9 mA cm −2 (Table 1). [20,28,[47][48][49][50][51][52][53][54][55][56][61][62][63][64] Additionally, our photocathode exhibited an impressive PEC performance compared with various other types of protection layers. Comprehensively, it is verified that the trade-off relationship between photocurrent generation and protection layer stability can be creatively redesigned using the proposed ECF process for producing homogenous and dense filaments, with selective Pt photodeposition at filament sites.…”

Nanoscale ZnO/α‐Fe2O3 Heterostructures: Toward Efficient and Low‐Cost Photoanodes for Water Splitting

“…In this regard, ZnO‐TiO 2 core–shell NWs, [ 138 ] FeVO 4 ‐passivated ZnO NRs, [ 139 ] ZnO/MnO 2 /TiO 2 NRs, [ 140 ] Cu 2 O/ZnO p–n junctions, [ 141 ] ZnO/V 2 O 5 , [ 142 ] GaON/ZnO, [ 143 ] and ZnO‐WO 3 [ 144 ] heterostructures have been reported in the literature to have remarkable PEC performances. Combining properly two different semiconductors (heterostructure formation) may promote the charge carrier separation.…”