There is a growing interest in the conversion of water and solar energy into clean and renewable H2 fuels using earth-abundant materials due to the depletion of fossil fuel and its serious environmental impact. This critical review highlights some key factors influencing the efficiency of heterogeneous semiconductors for solar water splitting (i.e. improved charge separation and transfer, promoted optical absorption, optimized band gap position, lowered cost and toxicity, and enhanced stability and water splitting kinetics). Moreover, different engineering strategies, such as band structure engineering, micro/nano engineering, bionic engineering, co-catalyst engineering, surface/interface engineering of heterogeneous semiconductors are summarized and discussed thoroughly. The synergistic effects of the different engineering strategies, especially for the combination of co-catalyst loading and other strategies seem to be more promising for the development of highly efficient photocatalysts. A thorough understanding of electron and hole transfer thermodynamics and kinetics at the fundamental level is also important for elucidating the key efficiency-limiting step and designing highly efficient solar-to-fuel conversion systems. In this review, we provide not only a summary of the recent progress in the different engineering strategies of heterogeneous semiconductors for solar water splitting, but also some potential opportunities for designing and optimizing solar cells, photocatalysts for the reduction of CO2 and pollutant degradation, and electrocatalysts for water splitting.

Engineering heterogeneous semiconductors for solar water splitting

Photocatalysis is the most promising method for achieving artificial photosynthesis, but a bottleneck is encountered in finding materials that could efficiently promote the water splitting reaction. The nontoxicity, low cost, and versatility of photocatalysts make them especially attractive for this application. This study demonstrates that small amounts of α-Fe2O3 nanosheets can actively promote exfoliation of g-C3N4, producing 2D hybrid that exhibits tight interfaces and an all-solid-state Z-scheme junction. These nanostructured hybrids present a high H2 evolution rate >3 × 104 µmol g-1 h-1 and external quantum efficiency of 44.35% at λ = 420 nm, the highest value so far reported among the family of g-C3N4 photocatalysts. Besides effectively suppressing the recombination of electron–hole pairs, this Z-scheme junction also exhibits activity toward overall water splitting without any sacrificial donor. The proposed synthetic route for controlled production of 2D g-C3N4-based structures provides a scalable alternative toward the development of highly efficient and active photocatalysts.

High Efficiency Photocatalytic Water Splitting Using 2D α-Fe2O3/g-C3N4 Z-Scheme Catalysts

Photoelectrochemical (PEC) water splitting is a promising technology for solar hydrogen production to build a sustainable, renewable and clean energy economy. Hematite (α-Fe2O3) based photoanodes offer promise for such applications, due to their high chemical stability, great abundance and low cost. Despite these promising properties, progress towards the manufacture of practical water splitting devices has been limited. This review is intended to highlight recent advancements and the limitations that still hamper the full utilization of hematite electrodes in PEC water splitting systems. We review recent progress in manipulating hematite for PEC water splitting through various approaches, focused on e.g. enhancing light absorption, water oxidation kinetics, and charge carrier collection efficiency. As the morphology affects various properties, progress in morphological characterization from thicker planar films to recent ultrathin nanophotonic morphologies is also examined. Special emphasis has been given to various ultrathin films and nanophotonic structures which have not been given much attention in previous review articles.

Using hematite for photoelectrochemical water splitting: a review of current progress and challenges.

Interpenetration control in metal–organic frameworks for functional applications

Bismuth vanadate (BiVO4 ) is a promising photoanode material for photoelectrochemical (PEC) water splitting. However, owing to the short carrier diffusion length, the trade-off between sufficient light absorption and efficient charge separation often leads to poor PEC performance. Herein, a new electrodeposition process is developed to prepare bismuth oxide precursor films, which can be converted to transparent BiVO4 films with well-controlled oxygen vacancies via a mild thermal treatment process. The optimized BiVO4 film exhibits an excellent back illumination charge separation efficiency mainly due to the presence of enriched oxygen vacancies which act as shallow donors. By loading FeOOH/NiOOH as the cocatalysts, the BiVO4 dual photoanodes exhibit a remarkable and highly stable photocurrent density of 5.87 mA cm-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination. An artificial leaf composed of the BiVO4 /FeOOH/NiOOH dual photoanodes and a single sealed perovskite solar cell delivers a solar-to-hydrogen conversion efficiency as high as 6.5% for unbiased water splitting.

https://espace.library.uq.edu.au/view/UQ:725506/UQ725506_OA.pdf

New BiVO4 Dual Photoanodes with Enriched Oxygen Vacancies for Efficient Solar-Driven Water Splitting

We present a surface corrosion method to shift the photocurrent onset potential cathodically for water oxidation on a Ti4+ doped Fe2O3 by about 100 mV. After the surface treatment, the doped hematite photoanodes showed a similar photocurrent onset potential to the lowest values obtained by loading with electrocatalysts or depositing functional over-layers. Moreover, the cathodic shift of the onset potential was preserved well even after a long operating time. The results indicated the effectiveness of this simple surface treatment. In order to make clear the reason for the onset potential shift, the doped hematite samples before and after surface corrosion were investigated by SEM, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICP-MS), photoluminescence spectroscopy (PL), electrochemical impedance spectroscopy (EIS), Mott–Schottky and so on. Based on the experimental evidence, we proposed a new mechanism for the onset potential shift. The cathodic shift of the onset potential was due to decreasing the back reaction, but not accelerating water oxidation kinetics, passivating surface states or ions adsorption as reported in the previous studies. This strategy of suppressing the back reaction can offer a reference to reduce the overpotential for other photoelectrodes.

Cathodic shift of onset potential for water oxidation on a Ti4+ doped Fe2O3 photoanode by suppressing the back reaction

Surface exfoliation: A Ta3 N5 photoanode prepared by a thermal oxidation and nitridation method shows a high solar photocurrent. This photocurrent is currently the highest achieved by a Ta3 N5 photoanode. The photocurrent is obtained mainly because of facile thermal and mechanical exfoliation of the surface passivation layer of the Ta3 N5 photoanode.

A co-catalyst-loaded Ta(3)N(5) photoanode with a high solar photocurrent for water splitting upon facile removal of the surface layer.

Four cadmium and cobalt coordination polymers with unique structures and topologies have been successfully synthesized under solvothermal conditions by employing an elongated triangular rigid N-containing ligand tris(4-(1H-imidazol-1-yl)phenyl)amine (TIPA) and 5-hydroxyisophthalic acid (5-OH-H2bdc) as anion coligand. Compounds 1−4 were characterized by single crystal X-ray structure analyses, thermogravimetric analyses, SHG, and photoluminescent measurements. Compound 1 crystallizes in the chiral space group C2 (No. 5) and features a 2D → 3D parallel/parallel inclined polycatenated framework. Compound 2 crystallizes in trigonal symmetry with a high-symmetry space group R3 and features a 2D porous noninterpenetrating coordination network. Compound 3 crystallizes in the monoclinic space group P21/n and shows a 2D → 3D parallel/parallel polycatenation framework, and compound 4 crystallizes in the orthorhombic chiral space group P212121 (No. 19) and shows a very rarely 3D + 3D heterogeneous 2-fold interpenet...

Chiral and Porous Coordination Polymers Based on an N-Centered Triangular Rigid Ligand

Recent progress in photoelectrochemical water splitting for solar hydrogen production

Ion doping is a very effective method to improve the photoelectrochemical performance of photoelectrodes. Generally, it is difficult to prepare doped samples by hydrothermal methods. In this study, we prepare Ti4+ doped α-Fe2O3 films by a facile hydrothermal reaction and find that Ti4+ also plays a shielding effect on the Fe2O3 growth process for the first time. The thickness of the pure Fe2O3 film increases initially and then decreases, while the thickness of the Ti4+ doped samples always increases and reaches plateau values when the hydrothermal time is prolonged, which suggests that Ti4+ plays a role in preventing the α-Fe2O3 films from dissolving into the acid aqueous solution during the hydrothermal process. Moreover, the ion doping effect on the doped Fe2O3 is also compared with that of doped samples prepared by other methods in the literatures. The results indicate that the increase of carrier concentrations, rather than morphology change or surface passivation, plays a main role in the photocurrent improvement after doping.

Dapeng Cao

Papers

Cathodic shift of onset potential for water oxidation on a Ti4+ doped Fe2O3 photoanode by suppressing the back reaction

A co-catalyst-loaded Ta(3)N(5) photoanode with a high solar photocurrent for water splitting upon facile removal of the surface layer.

Chiral and Porous Coordination Polymers Based on an N-Centered Triangular Rigid Ligand

Recent progress in photoelectrochemical water splitting for solar hydrogen production

A transparent Ti4+ doped hematite photoanode protectively grown by a facile hydrothermal method