Controlling the Surface Energetics and Kinetics of Hematite Photoanodes Through Few Atomic Layers of NiO_<i>x</i>

Photoanodes for solar water splitting require the use of a large applied potential to sustain high photocurrent. This has been conventionally addressed by depositing electrocatalysts on semiconductors to boost the kinetics of water oxidation. Herein, it is shown that the advancement in onset potential (<i>V</i>_ON) of hematite (α-Fe₂O₃) is not regulated by the overlayer activity but rather by how it modifies the surface properties of the electrode. NiO_<i>x</i> catalysts deposited with diverse methodologies induce different effects on the <i>J–V</i> curve of α-Fe₂O₃. Electrodeposited NiO_<i>x</i> produces only an increase in photocurrent at high bias, while photodeposited NiO_<i>x</i> induces also a 200 mV cathodic shift of <i>V</i>_ON, which reaches 0.58 V_RHE. Cathodoluminescence spectroscopy reveals that only through photodeposition is an effective passivation of surface defects achieved. This produces a decrease in electron–hole recombination and a substantial shift of the quasi-Fermi level in light. Concurrently, the Fermi level in the dark reaches a new energetic level. Electrochemical impedance spectroscopy on NiO_<i>x</i> photodeposited shows a 4-fold reduction of charge transfer resistance accounting for the low <i>V</i>_ON. This study shows how surface energetics and kinetics of photoanodes are tightly connected. Only the complete control of energetics allows achieving new performing interfaces for low-bias water splitting

A Flexible Electrode Based on Al-Doped Nickel Hydroxide Wrapped around a Carbon Nanotube Forest for Efficient Oxygen Evolution

The development of highly active, cheap, and stable electrocatalysts for overall water splitting is strategic for industrial electrolysis processes aiming to achieve sustainable hydrogen production. Here, we report the impressive electrocatalytic activity of the oxygen evolution reaction of Al-doped Ni­(OH)₂ deposited on a chemically etched carbon nanotube forest (CNT-F) supported on a flexible polymer/CNT nanocomposite. Our monolithic electrode generates a stable current density of 10 mA/cm² at an overpotential (η) of 0.28 V toward the oxygen evolution reaction in 1 M NaOH and reaches approximately 200 mA/cm² at 1.7 V versus the reversible hydrogen electrode in 6 M KOH. The CNT-F/NiAl electrode also shows an outstanding activity for the hydrogen evolution reaction under alkaline conditions. When CNT-F/NiAl is used both at the anode and at the cathode, our device can sustain the overall water splitting, reaching 10 mA/cm² at η = 1.96 V. The high electrocatalytic activity of the CNT-F/NiAl hydroxide is due to the huge surface area of the CNT forest, the high electrical conductivity of the nanocomposite substrate, and the interactions between the NiAl catalyst and the CNTs

α‑Fe₂O₃/NiOOH: An Effective Heterostructure for Photoelectrochemical Water Oxidation

The study of the semiconductor/electrocatalyst interface in electrodes for photoelectrochemical water splitting is of paramount importance to obtain enhanced solar-to-fuel efficiency. Here, we take into consideration the multiple effects that a thin layer of photodeposited amorphous Ni-oxyhydroxide (NiOOH) induces on hematite (α-Fe₂O₃) photoanodes. The reduction of overpotential produced a photocurrent onset potential advance of 150 mV and an increase of photocurrent of about 50% at 1.23 V vs RHE. To give an interpretation to these phenomena, we carried out deep electrochemical investigations by cyclic voltammetry and electrochemical impedance spectroscopy. The effective charge injection into the electrolyte due to the reduction of the charge transfer resistance at the electrode/electrolyte interface was observed and increased along with the amount of deposited NiOOH. The benefits of NiOOH deposition are ascribable to its ability to scavenge holes from hematite surface traps. This effect is mitigated at a potential higher than 1.25 V, since a fraction of photogenerated holes is consumed into the Ni redox cycle

Decentralized Solar-Driven Photothermal Desalination: An Interdisciplinary Challenge to Transition Lab-Scale Research to Off-Grid Applications

Sunlight can power thermal desalination as a carrier of electromagnetic energy if efficiently turned into heat. In the search for technologies to relieve global water scarcity, thermal desalination has key advantages in terms of sustainability, robustness, and limited salinity dependence. Solar-driven photothermal desalination (SDPD) can enable decentralized water purification, improved accessibility, and reduced environmental impact overcoming limitations of conventional, infrastructure-heavy desalination practices. However, there remains a lack of consensus on how to best evaluate the efficiency of diverse light-driven systems. While developing advanced absorbers, evaporators, and materials for desalination is essential, we have concluded that more efforts should focus on system-wide optimization, with particular attention paid to thermal energy recovery and loss mitigation. This Perspective offers a blueprint for achieving efficient and scalable SDPD under varying solar irradiation, emphasizing the need for interdisciplinary approaches to accomplish this goal

Significant Enhancement of Photoactivity in Hybrid TiO₂/g‑C₃N₄ Nanorod Catalysts Modified with Cu–Ni-Based Nanostructures

Light-driven processes such as photocatalytic environmental remediation and photoelectrochemical (PEC) water splitting to produce hydrogen under sunlight are key technologies toward energy sustainability. Despite enormous efforts, a suitable photocatalyst fulfilling all the main requirements such as high photoactivity under visible light, chemical stability, environmental friendliness, and low cost has not been found yet. A promising approach to overcome these limitations is to use hybrid nanostructures showing improved activity and physicochemical properties when compared with single components. Herein, we present a novel photocatalytic nanocomposite system based on titania (TiO₂): titania nanorod wrapped with Ni­(OH)₂ and Cu­(OH)₂ composite carbon nitride (CuNi@g-C₃N₄/TiO₂). This carefully tuned photoanode nanostructure shows almost one order of magnitude higher photocurrent density compared to unsensitized TiO₂ nanorods for PEC water splitting upon solar-light illumination. The heterostructured g-C₃N₄ strongly improves visible absorption of light, separation of electrons and holes, and surface catalysis due to the effect of Cu­(OH)₂ nanoparticles and Ni­(OH)₂ nanosheets, respectively. The improved photoperformance ascribed to the integrative cooperation effect of all the counterparts resulting in a one-dimensional hydrid nanostructured photoanode with improved light absorption, facile charge separation, and efficient surface catalysis toward PEC oxygen evolution

Defect-Mediated Energy States in Brookite Nanorods: Implications for Photochemical Applications under Ultraviolet and Visible Light

The photochemical properties of brookite nanorods are systematically explored using light-induced electron-paramagnetic resonance (EPR) techniques at different wavelengths spanning the UV–vis region of the electromagnetic spectrum (355–650 nm). Under UV irradiation, electron–hole pairs are generated, leading to the stabilization of paramagnetic centers, primarily Ti3+ and O– species at the surface. Visible light irradiation at low temperature results in a unique pair of EPR signals, including electrons trapped at titanium cations and a distinct signal resonating at g = 2.004. The pair of signals disappears after annealing at room temperature, indicating that recombination pathways with trapped electrons are available. The chemical reactivity of the different photogenerated species is tested using electron and holes scavengers. While peculiar light-harvesting capabilities are observed for the brookite nanorods, experiments carried out in the presence of a hole scavenger indicate a limited potential for oxidative processes under visible light

Hierarchical Hematite Nanoplatelets for Photoelectrochemical Water Splitting

A new nanostructured α-Fe₂O₃ photoelectrode synthesized through plasma-enhanced chemical vapor deposition (PE-CVD) is presented. The α-Fe₂O₃ films consist of nanoplatelets with (001) crystallographic planes strongly oriented perpendicular to the conductive glass surface. This hematite morphology was never obtained before and is strictly linked to the method being used for its production. Structural, electronic, and photocurrent measurements are employed to disclose the nanoscale features of the photoanodes and their relationships with the generated photocurrent. α-Fe₂O₃ films have a hierarchical morphology consisting of nanobranches (width ∼10 nm, length ∼50 nm) that self-organize in plume-like nanoplatelets (350–700 nm in length). The amount of precursor used in the PE-CVD process mainly affects the nanoplatelets dimension, the platelets density, the roughness, and the photoelectrochemical (PEC) activity. The highest photocurrent (<i>j</i> = 1.39 mA/cm² at 1.55 V_RHE) is shown by the photoanodes with the best balance between the platelets density and roughness. The so obtained hematite hierarchical morphology assures good photocurrent performance and appears to be an ideal platform for the construction of customized multilayer architecture for PEC water splitting

Effect of Nature and Location of Defects on Bandgap Narrowing in Black TiO₂ Nanoparticles

The increasing need for new materials capable of solar fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO₂ nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which matches well with visible light absorption. The electronic structure of black TiO₂ nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications