Gradient tantalum-doped hematite homojunction photoanode improves both photocurrents and turn-on voltage for solar water splitting

Host/Guest Nanostructured Photoanodes Integrated with Targeted Enhancement Strategies for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) hydrogen production from water splitting is a green technology that can solve the environmental and energy problems through converting solar energy into renewable hydrogen fuel. The construction of host/guest architecture in semiconductor photoanodes has proven to be an effective strategy to improve solar-to-fuel conversion efficiency dramatically. In host/guest photoanodes, the absorber layer is deposited onto a high-surface-area electron collector, resulting in a significant enhancements in light-harvesting as well as charge collection and separation efficiency. The present review aims to summarize and highlight recent state-of-the-art progresses in the architecture designing of host/guest photoanodes with integrated enhancement strategies, including i) light trapping effect; ii) optimization of conductive host scaffolds; iii) hierarchical structure engineering. The challenges and prospects for the future development of host/guest nanostructured photoanodes are also presented.

Surface-oxidized titanium diboride as cocatalyst on hematite photoanode for solar water splitting

The PEC performance of α-Fe2O3/SO-TiB2 is attributed to the enhancement of photogenerated charge separation and injection efficiency under the driving force of the interfacial electric field.

Advancing Photoelectrochemical Energy Conversion through Atomic Design of Catalysts

Powered by inexhaustible solar energy, photoelectrochemical (PEC) hydrogen/ammonia production and reduction of carbon dioxide to high added-value chemicals in eco-friendly and mild conditions provide a highly attractive solution to carbon neutrality. Recently, substantial advances have been achieved in PEC systems by improving light absorption and charge separation/transfer in PEC devices. However, less attention is given to the atomic design of photoelectrocatalysts to facilitate the final catalytic reactions occurring at photoelectrode surface, which largely limits the overall photo-to-energy conversion of PEC system. Fundamental catalytic mechanisms and recent progress in atomic design of PEC materials are comprehensively reviewed by engineering of defect, dopant, facet, strain, and single atom to enhance the activity and selectivity. Finally, the emerging challenges and research directions in design of PEC systems for future photo-to-energy conversions are proposed.

Enhancing the photocatalytic hydrogen production activity of BiVO4 [110] facets using oxygen vacancies

The activity of the hydrogen evolution reaction (HER) during photoelectrochemical (PEC) water-splitting is limited when using BiVO₄ with an exposed [110] facet because the conduction band minimum is below the H⁺/H₂O potential. Here, we enhance the photocatalytic hydrogen production activity through introducing an oxygen vacancy. Our first-principles calculations show that the oxygen vacancy can tune the band edge positions of the [110] facet, originating from an induced internal electric field related to geometry distortion and charge rearrangement. Furthermore, the induced electric field favors photogenerated electron-hole separation and the enhancement of atomic activity. More importantly, oxygen-vacancy-induced electronic states can increase the probability of photogenerated electron transitions, thus improving optical absorption. This study indicates that oxygen-defect engineering is an effective method for improving the photocatalytic activity when using PEC technology.

Sn-Controlled Co-Doped Hematite for Efficient Solar-Assisted Chargeable Zn-Air Batteries

The photoelectrochemical performance of a co-doped hematite photoanode might be hindered due to the unintentionally diffused Sn from a fluorine-doped tin oxide (FTO) substrate during the high-temperature annealing process by providing an increased number of recombination centers and structural disorder. We employed a two-step annealing process to manipulate the Sn concentration in co-doped hematite. The Sn content [Sn/(Sn + Fe)] of a two-step annealing sample decreased to 1.8 from 6.9% of a one-step annealing sample. Si and Sn co-doped hematite with the reduced Sn content exhibited less structural disorder and improved charge transport ability to achieve a 3.0 mA cm^-2 photocurrent density at 1.23 V_RHE, which was 1.3-fold higher than that of the reference Si and Sn co-doped Fe₂O₃ (2.3 mA cm^-2). By decorating with the efficient co-catalyst NiFe(OH)_x, a maximum photocurrent density of 3.57 mA cm^-2 was achieved. We further confirmed that the high charging potential and poor cyclability of the zinc-air battery could be dramatically improved by assembling the optimized, stable, and low-cost hematite photocatalyst with excellent OER performance as a substitute for expensive Ir/C in the solar-assisted chargeable battery. This study demonstrates the significance of manipulating the unintentionally diffused Sn content diffused from FTO to maximize the OER performance of the co-doped hematite.

Atomic layer deposition triggered Fe-In-S cluster and gradient energy band in ZnInS photoanode for improved oxygen evolution reaction

Vast bulk recombination of photo-generated carriers and sluggish surface oxygen evolution reaction (OER) kinetics severely hinder the development of photoelectrochemical water splitting. Herein, through constructing a vertically ordered ZnInS nanosheet array with an interior gradient energy band as photoanode, the bulk recombination of photogenerated carriers decreases greatly. We use the atomic layer deposition technology to introduce Fe-In-S clusters into the surface of photoanode. First-principles calculations and comprehensive characterizations indicate that these clusters effectively lower the electrochemical reaction barrier on the photoanode surface and promote the surface OER reaction kinetics through precisely affecting the second and third steps (forming processes of O* and OOH*) of the four-electron reaction. As a result, the optimal photoanode exhibits the high performance with a significantly enhanced photocurrent of 5.35 mA cm^-2 at 1.23 V_RHE and onset potential of 0.09 V_RHE. Present results demonstrate a robust platform for controllable surface modification, nanofabrication, and carrier transport.

Exploring the Synthesis, Band Edge Insights, and Photoelectrochemical Water Splitting Properties of Lead Vanadates

Exploring the ideal and stable semiconductor material for the efficient photoelectrochemical (PEC) overall water splitting reaction has remained a major challenge. Herein, we develop a facile hydrothermal method for the fabrication of monoclinic Pb₃[VO₄]₂ and orthorhombic PbV₂O₆ thin films for the efficient and stable PEC overall water splitting applications. Detailed characterization was performed to study the crystal structure and optical, electrical, and electrochemical properties. The band edge positions of Pb₃[VO₄]₂ and PbV₂O₆ are determined using spectroscopic data, revealing the conduction band edge positioned near the water reduction potential [∼0 V vs reversible hydrogen electrode (RHE)] and the valence band edge positioned well above the water oxidation potential, indicating the possible utilization of photogenerated electrons and holes for efficient water reduction and oxidation, respectively. With the optimized PbV₂O₆ thin films, a maximum photocurrent of 0.35 mA cm^-2 was obtained at 1.23 V versus RHE and the stable production of both O₂ and H₂ is observed with >90% Faradaic efficiency. Importantly, this work demonstrates the possibility of utilizing lead vanadate materials for PEC water splitting applications.

Built-in electric field for photocatalytic overall water splitting through a TiO2/BiOBr P-N heterojunction

Photocatalytic overall water splitting to simultaneously obtain abundant hydrogen and oxygen is still the mountain that stands in the way for the practical applications of hydrogen energy, in which composite semiconductor photocatalysts are critical for providing both electrons and holes to promote the following redox reaction. However, the interface between different components forms a deplete layer to hinder the charge transfer to a large extent. In order to enhance the charger transfer from an interface to the surface and promote the spatial separation of electron-hole pairs, a built-in electric field induced by a p-n heterojunction emerges as the best choice. As a touchstone, a p-n heterojunction of TiO2/BiOBr with a strong built-in electric field has been constructed, which presents a wide spectrum response owing to its interleaved band gaps after composition. The built-in electric field greatly enhances the separation and transportation of photogenerated carriers, resulting in fluorescence quenching due to the carrier recombination. The sample also displayed exceptional photoelectron responses: its photocurrent density (43.3 μA cm-2) was over 10 times that of TiO2 (3.5 μA cm-2) or BiOBr (4.2 μA cm-2). In addition, the sample with a molar ratio of 3 : 1 between TiO2 and BiOBr showed the best photocatalytic overall water splitting performance under visible light (λ > 420 nm): the hydrogen and oxygen production rate were 472.7 μmol gcat.-1 h-1 and 95.7 μmol gcat.-1 h-1, respectively, which are the highest values under visible light without other cocatalysts to have been reported in literature for the photocatalyst.

Multistep Functional Embellishment for p-ZnTe as a Cathode to Boost the Faraday Efficiency of Nitrogen Conversion

An electrochemical N₂ reduction reaction (N₂RR) is a selective sustainable approach to obtain NH₃ at mild conditions and has been proposed as an alternative to the full-blown Haber-Bosch process. However, achieving high yields of NH₃ and high faraday efficiency (FE) at a low overpotential remains a big challenge but has high expectations for the electrocatalytic N₂RR. Herein, a novel p-ZnTe cathode multistep embellished with NiO_x and ZnO thin films was prepared for boosting faraday efficiency to 9.89% for N₂RR at -0.2 V vs reversible hydrogen electrode (RHE), about 12 times of p-ZnTe@ZnO. All components within the NiO_x@p-ZnTe/ZnO electrode work cooperatively. A N source was determined through a ¹⁵N isotopic-labeling experiment. Using steady-state photoluminescence, electrochemical impedance spectroscopy, and control experiments, a possible model of charge transformation is built. In particular, a NiO_x layer has an important impact on increasing interfacial contact between a bare fluorine-doped tin oxide (FTO) glass and p-ZnTe and further reinforcing interfacial electron transfer. This work provides a practical application and a feasible strategy to develop highly efficient catalysts for N₂ reduction and also affords a guideline for the fabrication of a flat electrode.

In situ construction of 3D TiO2 photoelectrode with multilevel facet heterojunctions towards the efficient removal of bisphenol A

An innovative three-dimensional (3D) TiO2 photoelectrode with multilevel facet heterojunctions (FHs) is rationally designed based on in situ 1D rutile TiO2 nanorods with top {111} facets on a Ti mesh substrate. The 3D configuration composed of nanosheets and nanorods is provided with large specific area. The stepped band structure of the multilevel FH gives further impetus to spatial charge separation. The obtained FH-{111}TiO2/Ti photoelectrode achieves a 100% removal of bisphenol A (BPA) in only 20 min and presents an outstanding stability even after 10 cycles. Briefly, this work provides a reference pathway for the highly efficient removal of BPA.