Yajun Zhang

Ultrathin FeOOH Nanolayers with Abundant Oxygen Vacancies on BiVO4 Photoanodes for Efficient Water Oxidation

Photoelectrochemical (PEC) water splitting is a promising method for storing solar energy in the form of hydrogen fuel, but it is greatly hindered by the sluggish kinetics of the oxygen evolution reaction (OER). Herein, a facile solution impregnation method is developed for growing ultrathin (2 nm) highly crystalline β-FeOOH nanolayers with abundant oxygen vacancies on BiVO4 photoanodes. These exhibited a remarkable photocurrent density of 4.3 mA cm-2 at 1.23 V (vs. reversible hydrogen electrode (RHE), AM 1.5 G), which is approximately two times higher than that of amorphous FeOOH fabricated by electrodeposition. Systematic studies reveal that the excellent PEC activity should be attributed to their ultrathin crystalline structure and abundant oxygen vacancies, which could effectively facilitate the hole transport/trapping and provide more active sites for water oxidation.

Enhanced Solar Water Splitting by Swift Charge Separation in Au/FeOOH Sandwiched Single-Crystalline Fe2O3 Nanoflake Photoelectrodes

In this work, single crystalline α-Fe2 O3 nanoflakes (NFs) are formed in a highly dense array by Au seeding of a Fe substrate by a thermal oxidation technique. The NFs are conformally decorated with a thin FeOOH cocatalyst layer. Photoelectrochemical (PEC) measurements show that this photoanode, incorporating α-Fe2 O3 /FeOOH NFs rooted on the Au/Fe structure, exhibits significantly enhanced PEC water oxidation performance compared to the plain α-Fe2 O3 nanostructure on the Fe substrate. The α-Fe2 O3 /FeOOH NFs on Au/Fe photoanode yields a photocurrent density of 3.1 mA cm-2 at 1.5 VRHE , and a remarkably low onset potential of 0.5-0.6 VRHE in 1 m KOH under AM 1.5G (100 mW cm-2 ) simulated sunlight illumination. The enhancement in PEC performance can be attributed to a synergistic effect of the FeOOH top decoration and the Au underlayer, whereby FeOOH facilitates hole transfer at the interface of electrode/electrolyte and the Au layer provides a sink for the electron transport to the back contact. This results in a drastically improved charge-separation efficiency in the single crystalline α-Fe2 O3 NF photoanode.

One-dimensional hematite photoanodes with spatially separated Pt and FeOOH nanolayers for efficient solar water splitting

Charge separation plays a crucial role in determining the solar energy conversion efficiency of semiconductors for photoelectrochemical (PEC) water splitting. However, owing to the intrinsically high electron–hole recombination, the generally reported PEC performance is still far below that expected. Therefore, we have attempted to demonstrate that efficient charge separation can be achieved on the selective growth of spatial FeOOH cocatalyst and Pt nanoparticles on hematite photoanodes. Unlike traditional strategies – single cocatalyst decoration or element doping – this comprises an interfacial hole-transfer layer as the active sites (FeOOH) for water oxidation, and an electron collector and transfer layer (Pt) to facilitate water reduction. Here we demonstrate a simple but effective “bottom-up” method, combined metallic replacement with hydrolytic reactions, for the selective growth of the FeOOH cocatalyst (top) and Pt particles (bottom) on the hematite-nanoflake (NF) photoanodes. As expected, this novel FeOOH/α-Fe2O3 NF/Pt photoanode yields excellent photoresponse behavior for PEC water splitting reactions −2.0 mA cm−2 and 3.2 mA cm−2 obtained at 1.23 VRHE and at 1.5 VRHE under AM 1.5G illumination – which is much higher than the pristine hematite and/or exclusively FeOOH cocatalyst-decorated samples. These results demonstrate that selectively integrating electron and hole-transfer layers into photoanodes could effectively achieve spatial charge separation and greatly improve PEC performance for water splitting.

Facile synthesis of Fe3+/Fe2+ self-doped nanoporous FeVO4 photoanodes for efficient solar water splitting

We demonstrated a facile and effective method to fabricate nanoporous FeVO4 photoanodes for efficient solar water splitting. More importantly, the rationally self-doped Fe2+ or Fe3+ on the FeVO4 photoanode could further improve the photoelectrochemical (PEC) performance, which may provide an alternative approach for the design and construction of low-cost and highly efficient PEC systems.

The synergistic effect of Bi2WO6 nanoplates and Co3O4 cocatalysts for enhanced photoelectrochemical properties

Herein, we demonstrated the controllable fabrication of Bi2WO6 nanoplate arrays decorated with Co3O4 cocatalysts through the combination of hydrothermal reactions with annealing treatments. The synergistic effect of bulk charge transport and surface oxygen evolution could significantly improve the photoelectrochemical properties (0.45 mA cm−2, 1.23 VRHE) for water splitting under simulated solar light, which are much higher than those of traditional Bi2WO6 particle photoanodes (0.02 mA cm−2, 1.23 VRHE). These demonstrations clearly reveal that the rational construction of a photoanode structure and selective decoration of oxygen evolution cocatalysts could serve as an alternative strategy toward the development of highly efficient water splitting systems.

Efficient hydrogen production from MIL-53(Fe) catalyst-modified Mo:BiVO4 photoelectrodes

Photoelectrochemical (PEC) water splitting to produce hydrogen energy has attracted considerable attention for solving current energy and environmental crises. However, the intrinsically high electron–hole recombination and low charge mobility greatly diminish the PEC efficiency. Herein, we demonstrated that metal organic framework MIL-53(Fe) could serve as an efficient hole-transfer co-catalyst to significantly improve the PEC performance of Mo-doped BiVO4 photoanodes toward water oxidation under solar irradiation, which is much higher than that of the traditional FeOOH co-catalyst under the same conditions. More specifically, owing to the unique 3D interlinked nanochannel and confinement effect of Fe atom sites, the charge separation, hole transport at the interface, and interactions with H2O molecules have been effectively facilitated, which thus allows for enhancing PEC water oxidation properties.

Synergistic effects of P-doping and a MnO2 cocatalyst on Fe2O3 nanorod photoanodes for efficient solar water splitting

Herein, we design and fabricate hematite (Fe2O3) nanorod photoanodes modified with P-doping and a MnO2 oxygen evolution cocatalyst for photoelectrochemical (PEC) water splitting. This novel MnO2/P:Fe2O3 photoanode exhibited a remarkably enhanced PEC water oxidation activity, and its photocurrent density is approximately 5-fold higher than that of pristine Fe2O3. The significant improvement of PEC performance is mainly attributed to the synergistic effects of P-doping and MnO2 cocatalysts. More specifically, the Mott–Schottky and Nyquist plots clearly reveal that P-doping could not only effectively increase the density of charge carriers but also enhance the electron and hole transfer mobilities of Fe2O3 photoanodes. Furthermore, the MnO2 cocatalyst modification can significantly facilitate charge separation and hole transport to the photoanode/electrolyte interface for water oxidation. Therefore, these demonstrations may provide an alternate strategy for designing and fabricating highly efficient Fe2O3-based PEC systems for solar energy conversion.

Ultrathin FeFx Nanolayers Accelerating Hole Transfer for Enhanced Photoelectrochemical Water Oxidation

In photoelectrochemical (PEC) water splitting systems, the sluggish oxygen evolution reaction (OER) kinetics dominated by hole transfer greatly limits the photo-conversion efficiency. Herein, we demonstrate the in situ growth of ultrathin FeFx nanolayers on Fe2O3 photoanodes, acting as a highly efficient OER cocatalyst to accelerate hole transfer for improved PEC activity. By virtue of its ultrathin structure and high electronegativity, ultrathin FeFx not only promotes the rapid hole transfer among the anode/cocatalyst interfaces but also maximizes the contact areas between the electrode and the reactants. As expected, this photoanode exhibits a remarkably improved photocurrent density of 2.4 mA cm−2 at 1.23 V (reversible hydrogen electrode, AM 1.5 G), up to 12 times enhancement with respect to the pristine Fe2O3 sample. Moreover, a notably negative shift (0.1 VRHE) on the onset potential has been simultaneously achieved. This work represents a new, simple and highly efficient approach to suppress charge recombination and accelerate the hole transfer for water oxidation, which may serve as an alternate candidate for creating high-efficiency PEC water splitting systems.

Rapid Activation of Co3O4 Cocatalyst with Oxygen Vacancies on TiO2 Photoanodes for Highly Efficient Water Splitting

Cobalt oxide (Co3O4) has been extensively utilized as a promising oxygen evolution reaction (OER) cocatalyst for solar photoelectrochemical (PEC) water splitting. However, the relationship between its oxygen vacancies and PEC conversion efficiency has yet to be determined. Herein, we demonstrate a rapid and highly efficient strategy for rationally constructing oxygen vacancies on Co3O4 cocatalysts by Ar-plasma treatment, which could significantly enhance the PEC water oxidation reactivity of TiO2-based photoanodes. The photocurrent density could be achieved up to 2.5 mA cm−2 (1.23 VRHE) under AM 1.5G irradiation (100 mW cm−2), up to 2 times higher than that of the untreated Co3O4/TiO2 samples. More specifically, the oxygen vacancy formation on Co3O4 cocatalysts not only facilitates the hole trapping ability for promoting interfacial charge separation and migration, but also provides more active sites for water oxidation. The rapid and highly efficient activation of OER cocatalysts by producing oxygen vacancies may be potentially a universal strategy for improving the photo-conversion efficiency of PEC photoanodes now available.

ZnO nanowire arrays decorated with PtO nanowires for efficient solar water splitting

Photoelectrochemical (PEC) water splitting is a promising method for storing solar energy in the form of hydrogen fuel. However, the high recombination ratios of electron–hole pairs and the sluggish kinetics of the oxygen evolution reaction (OER) severely restrict the PEC efficiency. Herein, we demonstrate that vertically-oriented PtO nanowires could be highly selectively grown on ZnO nanowire arrays to construct a novel cross-linked heterostructure by a light-controlled growth method. The key to PtO nanowire growth and interactions is based on the use of a photogenerated electric field from ZnO nanowire arrays. More importantly, these PtO nanowires are utilized, for the first time, as highly efficient OER cocatalysts in PEC water splitting systems to produce hydrogen and exhibit much higher activities than other Pt species decorated samples. Systematic studies reveal that the excellent PEC performance should be attributed to the pure PtO crystalline phase and vertically-oriented nanowire structure, which could effectively facilitate the hole trapping/transport and provide more active sites for water oxidation. These results provide a new insight and strategy to construct special structured OER cocatalysts on PEC photoanodes for enhancing water splitting performance.