Promoting Charge Separation and Injection by Optimizing the Interfaces of GaN:ZnO Photoanode for Efficient Solar Water Oxidation

Photoelectrochemical water oxidation by a MOF/semiconductor composite

Herein, we report the development of a MOF-semiconductor composite film active for water oxidation at a thermodynamic underpotential.

Machine Learning Guided Dopant Selection for Metal Oxide-Based Photoelectrochemical Water Splitting: The Case Study of Fe2 O3 and CuO

Doping is an effective strategy for tuning metal oxide-based semiconductors for solar-driven photoelectrochemical (PEC) water splitting. Despite decades of extensive research effort, the dopant selection is still largely dependent on a trial-and-error approach. Machine learning (ML) is promising in providing predictable insights on the dopant selection for high-performing PEC systems because it can uncover correlations from the seemingly ambiguous linkages between vast features of dopants and the PEC performance of doped photoelectrodes. Herein, the authors successfully build ML model to predict the doping effect of 17 metal dopants into hematite (Fe₂ O₃ ), a prototype photoelectrode material. Their findings disclose the critical parameters from the 10 intrinsic features of each dopant. The model is further experimentally validated by the coherent prediction on Y and La dopants' behaviors. Further interpretation of the ML model suggests that the chemical state is the most significant selection criteria, meanwhile, dopants with higher metal-oxygen bond formation enthalpy and larger ionic radius are favored in improving the charge separation and transfer (CST) in the Fe₂ O₃ photoanodes. The generic feature of this ML guided selection criteria has been further extended to CuO-based photoelectrodes showing improved CST by alkaline metal ions doping.

A ZnO@CuO core–shell heterojunction photoanode modified with ZnFe-LDH for efficient and stable photoelectrochemical performance

A highly efficient ZnO@CuO core–shell heterojunction photoanode modified with cocatalyst ZnFe-layered double hydroxides was designed and synthesized in this work.

α-Fe2O3/Ag/CdS ternary heterojunction photoanode for efficient solar water oxidation

By taking full advantage of the α-Fe2O3/Ag/CdS ternary heterojunction in charge separation and transfer, light harvesting and electrocatalytic water oxidation, obviously improved water oxidation performance was achieved on the photoanode.

Designing efficient Bi2Fe4O9 photoanodes via bulk and surface defect engineering

An efficient Bi₂Fe₄O₉ photoanode can be fabricated through surface and bulk defect engineering to achieve a state-of-the-art photoresponse.

Enhanced Photoelectrochemical Water Splitting with Er- and W-Codoped Bismuth Vanadate with WO3 Heterojunction-Based Two-Dimensional Photoelectrode

A novel two-dimensional (2D) heterojunction photoelectrode composed of WO₃ and (Er,W):BiVO₄ is proposed for water oxidation with efficient photoinduced charge carrier separation and transfer. Er stoichiometric along with W nonstoichiometric codoping was introduced to simultaneously manage vacancy creation during substitutional doping, enhance light absorption, and reduce overall impedance. It was found that Er³⁺ is substituted at the Bi³⁺ sites in the BiVO₄ lattice to provide expanded light absorption from 400 to 680 nm. The fabricated WO₃/(Er,W):BiVO₄ electrode shows photocurrent densities of 4.1 and 7.2 mA cm^-2 at 1.23 and 2.3 V (vs reversible hydrogen electrode, RHE), respectively, under a 1 sun illumination in K₂HPO₄ electrolyte. This electrode has shown remarkably high charge separation efficiency of 93% at 1.23 V (vs RHE). With the addition of a standard surface catalyst (i.e., Co-Pi), the WO₃/(Er,W):BiVO₄/Co-Pi electrode exhibits the highest photocurrent of 5.6 ± 0.3 mA cm^-2 at 1.23 V (vs RHE), nearing the theoretical limit (i.e., 7.5 mA cm^-2) while retaining 98% of the photoelectrochemical cell performance after 3 h. By concomitantly doping the Bi³⁺ and V⁵⁺ sites to enhance absorption, this study demonstrates for the first time a planar WO₃/BiVO₄ heterojunction that reaches 88% of the record-high performance of its nanostructured counterpart. Through a detailed characterization of the electrodes, it is concluded that the stoichiometric Er and nonstoichiometric W codoping extend light absorption region and improve charge separation efficiency by reducing bulk resistance. The photoactive materials with 2D morphology were synthesized using a facile ultrasonic spray-coating technique without any complex process steps and thus it can be scaled for commercial development.

Rational Design and Construction of Cocatalysts for Semiconductor-Based Photo-Electrochemical Oxygen Evolution: A Comprehensive Review

Photo-electrochemical (PEC) water splitting, as an essential and indispensable research branch of solar energy applications, has achieved increasing attention in the past decades. Between the two photoelectrodes, the photoanodes for PEC water oxidation are mostly studied for the facile selection of n-type semiconductors. Initially, the efficiency of the PEC process is rather limited, which mainly results from the existing drawbacks of photoanodes such as instability and serious charge-carrier recombination. To improve PEC performances, researchers gradually focus on exploring many strategies, among which engineering photoelectrodes with suitable cocatalysts is one of the most feasible and promising methods to lower reaction obstacles and boost PEC water splitting ability. Here, the basic principles, modules of the PEC system, evaluation parameters in PEC water oxidation reactions occurring on the surface of photoanodes, and the basic functions of cocatalysts on the promotion of PEC performance are demonstrated. Then, the key progress of cocatalyst design and construction applied to photoanodes for PEC oxygen evolution is emphatically introduced and the influences of different kinds of water oxidation cocatalysts are elucidated in detail. Finally, the outlook of highly active cocatalysts for the photosynthesis process is also included.

Understanding the visible-light photocatalytic activity of GaN:ZnO solid solution: the role of Rh2-y Cr y O3 cocatalyst and charge carrier lifetimes over tens of seconds

A persistent challenge for the widespread deployment of solar fuels is the development of high efficiency photocatalysts combined with a low-cost preparation and implementation route. Since its discovery in 2005, GaN:ZnO solid solution has been a benchmark overall water splitting photocatalyst. Notably, GaN:ZnO functionalised with an appropriate proton reduction cocatalyst is one of the few particulate photocatalyst systems that can generate hydrogen and oxygen directly from water using visible light. However, the reasons underlying the remarkable visible light activity of GaN:ZnO are not well understood and photophysical studies of GaN:ZnO have been limited to date. Using time-resolved optical spectroscopies, we investigated the charge carrier dynamics of GaN:ZnO and the effect of Rh2-y Cr y O3 proton reduction cocatalyst. Here we show that charge trapping and trap state filling play an important role in controlling the photophysics of GaN:ZnO. We also find that electrons transfer to Rh2-y Cr y O3 on sub-microsecond timescales, important to reduce the electron concentration within GaN:ZnO and promote hole accumulation. Operando measurements showed that the water oxidation process is the rate determining process, and that the dependence of the rate of water oxidation on the accumulated hole density is similar to common metal oxides photoanodes such as TiO2, α-Fe2O3, and BiVO4. Remarkably, we show that the recombination timescale of holes accumulated on the surface of GaN:ZnO is on the order of 30 s, distinctly longer than for metal oxides photoanodes. We conclude that the unusual visible light activity of GaN:ZnO is a result of large electron-hole spatial separation due to the preferential flow of holes to the GaN-rich surface and efficient electron extraction by the cocatalyst. Our studies demonstrate that in depth spectroscopic investigations of the charge carrier dynamics of photocatalysts yield important information to understand their behaviour, and identify key properties to deliver outstanding performance.

Energy loss analysis in photoelectrochemical water splitting: a case study of hematite photoanodes

The energy loss of photoelectrochemical processes can be quantitatively evaluated by analyzing the decoupled photovoltaic and electrocatalytic process.