Phase-segregated NiP x @FeP y O z core@shell nanoparticles: ready-to-use nanocatalysts for electro- and photo-catalytic water oxidation through in situ activation by structural transformation and spontaneous ligand removal

Revealing Structural Evolution of Nickel Phosphide-Iron Oxide Core-Shell Nanocatalysts in Alkaline Medium for the Oxygen Evolution Reaction

Metal phosphide-containing materials have emerged as a potential candidate of nonprecious metal-based catalysts for alkaline oxygen evolution reaction (OER). While it is known that metal phosphide undergoes structural evolution, considerable debate persists regarding the effects of dynamics on the surface activation and morphological stability of the catalysts. In this study, we synthesize NiP x -FeO x core-shell nanocatalysts with an amorphous NiP x core designed for enhanced OER activity. Using ex situ X-ray absorption spectroscopy, we elucidate the local structural changes as a function of the cyclic voltammetry cycles. Our studies suggest that the presence of corner-sharing octahedra in the FeO x shell improves structural rigidity through interlayer cross-linking, thereby inhibiting the diffusion of OH-/H2O. Thus, the FeO x shell preserves the amorphous NiP x core from rapid oxidation to Ni3(PO4)2 and Ni(OH)2. On the other hand, the incorporation of Ni from the core into the FeO x shell facilitates absorption of hydroxide ions for OER. As a result, Ni/Fe(OH) x at the surface oxidizes to the active γ-(oxy)hydroxide phase under the applied potentials, promoting OER. This intriguing synergistic behavior holds significance as such a synthetic route involving the FeO x shell can be extended to other systems, enabling manipulation of surface adsorption and diffusion of hydroxide ions. These findings also demonstrate that nanomaterials with core-shell morphologies can be tuned to leverage the strength of each metallic component for improved electrochemical activities.

Flexible nanosheets for plasmonic photocatalysis: microwave-assisted organic synthesis of Ni-NiO@Ni2CO3(OH)2 core-shell@sheet hybrid nanostructures

Visible light-active nickel-based plasmonic photocatalysts provide a cost-effective alternative to noble metals. However, their rarity, fragility, and limited understanding pose challenges. This work presents a microwave-assisted organic synthesis of a Ni-NiO@Ni2CO3(OH)2 core-shell@sheet plasmonic photocatalyst. By employing time and power dependent synthesis, this catalyst exhibits flexible Ni2CO3(OH)2 nanosheets enveloping the Ni-NiO structure, surpassing the pristine Ni@NiO/NiCO3 core-shell counterpart. Chemical reaction mechanisms suggest that irradiation of pristine Ni-NiO/NiCO3 nano structures leads to breakage of amorphous NiCO3 to Ni2+ and CO32-, which further, in the presence of water solvent, interacts with OH- ions leading to the formation of Ni(CO3)·Ni(OH)2. With enhanced light absorption and photocatalytic properties, the resulting core-shell@sheet photocatalyst demonstrates double the hydrogen evolution reaction yield (40 μmol g-1 h-1) compared to the pristine catalyst (20 μmol g-1 h-1). The enhanced H2 yield is attributed to the flexible sheets, cross-dimensional photocatalyst structure, increased surface area for surface reactions, and higher H2 activity of Ni2CO3(OH)2. This research showcases the potential of microwave-assisted synthesis in developing flexible nanosheets with superior solar water splitting performance.

Function of Internal and External Fe in a Ni-Based Precatalyst System Toward Oxygen Evolution Reaction

It is well known that the "iron" impurity will influence the oxygen evolution reaction (OER) in an alkaline electrolyte, especially for the Ni-based electrocatalyst. Many research studies have investigated the function of Fe in the OER active phase, such as M(OH)₂/MOOH (M = Ni and/or Fe), while, surprisingly, very few studies have examined the function of Fe in the "precatalyst" system. Accordingly, in this work, the Ni_3-xFe_xP (x = 0, 0.5, 1) series as an Ni-based precatalyst was employed to inspect the function of internal and external Fe in the Ni-based precatalyst system. It was realized that the sample with internal Fe (i.e., Ni_2.5Fe_0.5P and Ni₂FeP) exhibits efficient OER activity compared to that of the Fe-free one (i.e., Ni₃P) owing to the large amount of active M(OH)₂/MOOH formed on the surface. This indicates that the internal Fe in the present system may have the ability to facilitate the phase transformation; it was later rationalized from electronic structural calculations that the d band center of the internal Fe (middle transition metal) and Ni (late transition metal) holds the key for this observation. Adding excessive ferrous chloride tetrahydrate (FeCl₂·4H₂O) as the external Fe in the electrolyte will greatly improve the OER performances for Ni₃P; nevertheless, that the OER activity of Ni₂FeP is still much superior than that of Ni₃P corroborates the fact that the Fe impurity is not the only reason for the elevated OER activity of Ni₂FeP and that internal Fe is also critical to the phase transformation as well as OER performance.

Bridging electrocatalyst and cocatalyst studies for solar hydrogen production via water splitting

Solar-driven water-splitting has been considered as a promising technology for large-scale generation of sustainable energy for succeeding generations. Recent intensive efforts have led to the discovery of advanced multi-element-compound water-splitting electrocatalysts with very small overpotentials in anticipation of their application to solar cell-assisted water electrolysis. Although photocatalytic and photoelectrochemical water-splitting systems are more attractive approaches for scaling up without much technical complexity and high investment costs, improving their efficiencies remains a huge challenge. Hybridizing photocatalysts or photoelectrodes with cocatalysts has been an effective scheme to enhance their overall solar energy conversion efficiencies. However, direct integration of highly-active electrocatalysts as cocatalysts introduces critical factors that require careful consideration. These additional requirements limit the design principle for cocatalysts compared with electrocatalysts, decelerating development of cocatalyst materials. This perspective first summarizes the recent advances in electrocatalyst materials and the effective strategies to assemble cocatalyst/photoactive semiconductor composites, and further discusses the core principles and tools that hold the key in designing advanced cocatalysts and generating a deeper understanding on how to further push the limits of water-splitting efficiency.

Transformations of Ionic Nanocrystals via Full and Partial Ion Exchange Reactions

ConspectusElaborate chemical synthesis methods allow the production of various types of inorganic nanocrystals (NCs) with uniform shape and size distributions. Many single-step synthesis approaches, such as the reduction of metal ions, the decomposition of metal complexes, double replacement reactions, and hydrolysis, have been adapted to promote the generation of monodisperse metal and ionic NCs. However, the question has become, how can we synthesize NCs with thermodynamically metastable phases or very complex structures? The transformation of already-synthesized NCs via elemental substitutions, such as ion exchange reactions for ionic NCs and galvanic replacement reactions for metal NCs, can overcome the difficulties facing conventional one-step syntheses. In particular, NC ion exchange reactions have been studied with numerous combinations of foreign ions and ionic NCs with various shapes. They have been investigated extensively because the reactions proceed under relatively mild conditions thanks to the large surface-to-volume ratio of the NCs relative to their bulk form. The functionality of the resulting ionic NCs, including semiconducting and plasmonic properties, can be easily tuned in a wide range, from the visible to near-infrared. Because anions generally have much larger ionic radii than cations within the frameworks of NCs, the cation exchange reactions proceed much faster than the anion exchange reactions. For ionic NCs above a critical size, the anion framework remains intact, and the original shape of the parent NCs is retained throughout the cation exchange reaction. In contrast, the anion exchange reaction often provides the new NCs with unique structures, such as hollow or anisotropically phase-segregated assemblies.This Account focuses on the full and partial ion exchange reactions involving ionic NCs, which have been thoroughly investigated by our group and others while highlighting important aspects such as the preservation of appearance and dimensions. First, we discuss how each type of ion exchange reaction progresses to understand the morphologies and crystal structures of their final products. This discussion is supported by emphasizing important examples, which help to explore the formation of NCs with thermodynamically metastable phases and complex structures, and other significant features of the ion exchange reactions leading to structure-specific functions. As a special case, we examine how the shape-dependent anionic framework (surface anion sublattice and stacking pattern) of polyhedral Cu₂O NCs determines the crystalline structure of the anion-exchanged products of hollow Cu_xS NCs. In addition, we review the characteristic anion exchange behavior of metal halide perovskite NCs observed in our laboratory and other laboratories. Finally, a general outline of the transformation of NCs via ion exchange reactions and future prospects in this field are provided.

Increasing Electrocatalytic Oxygen Evolution Efficiency through Cobalt-Induced Intrastructural Enhancement and Electronic Structure Modulation

Electrolytic water splitting using surplus electricity represents one of the most cost-effective and promising strategies for hydrogen production. The high overpotential of the oxygen-evolution reaction (OER) caused by the multi-electron transfer process with a high chemical energy barrier, however, limits its competitiveness. Here, a highly active and stable OER electrocatalyst was designed through a cobalt-induced intrastructural enhancement strategy combined with suitable electronic structure modulation. A carved carbon nanobox was embedded with tri-metal phosphide from a uniform Ni-Co-Fe Prussian blue analogue (PBA) nanocube by sequential NH3  ⋅ H2 O etching and thermal phosphorization. The sample exhibited an OER activity in an alkaline medium, reaching a current density of 10 mA cm-2 at an overpotential of 182 mV and displayed a small Tafel slope of 47 mV dec-1 , superior to the most recently reported OER electrocatalysts. Moreover, it showed impressive electrocatalytic durability, increasing by approximately 2.7 % of operating voltage after 24 h of continuous testing. The excellent OER activity and stability are ascribed to a favorable transfer of mass and charge provided by the porous carbon shell, synergistic catalysis between the three-component metal phosphides originating from appropriate electronic structure modulation, more exposed catalytic sites on the hollow structure, and chainmail catalysis resulting from the carbon protective layer. It is foreseen that this successfully demonstrated design concept can be easily extended to other heterogeneous catalyst designs.

Phase segregated Cu2-x Se/Ni3Se4 bimetallic selenide nanocrystals formed through the cation exchange reaction for active water oxidation precatalysts

Control over the composition and nanostructure of solid electrocatalysts is quite important for drastic improvement of their performance. The cation exchange reaction of nanocrystals (NCs) has been reported as the way to provide metastable crystal structures and complicated functional nanostructures that are not accessible by conventional synthetic methods. Herein we demonstrate the cation exchange-derived formation of metastable spinel Ni₃Se₄ NCs (sp-Ni₃Se₄) and phase segregated berzelianite Cu_2−xSe (ber-Cu_2−xSe)/sp-Ni₃Se₄ heterostructured NCs as active oxygen evolution reaction (OER) catalysts. A rare sp-Ni₃Se₄ phase was formed by cation exchange of ber-Cu_2−xSe NCs with Ni²⁺ ions, because both phases have the face-centered cubic (fcc) Se anion sublattice. Tuning the Ni : Cu molar ratio leads to the formation of Janus-type ber-Cu_2−xSe/sp-Ni₃Se₄ heterostructured NCs. The NCs of sp-Ni₃Se₄ and ber-Cu_2−xSe/sp-Ni₃Se₄ heterostructures exhibited high catalytic activities in the OER with small overpotentials of 250 and 230 mV at 10 mA cm⁻² in 0.1 M KOH, respectively. They were electrochemically oxidized during the OER to give hydroxides as the real active species. We anticipate that the cation exchange reaction could have enormous potential for the creation of novel heterostructured NCs showing superior catalytic performance.

Bimetallic selenide nanocrystals formed by cation exchange reaction work as a precursor of efficient water oxidation electrocatalyst.

The role of nanoparticle size and ligand coverage in size focusing of colloidal metal nanoparticles

Controlling the size distribution of nanoparticles is important for many applications and typically involves the use of ligands during synthesis. In this study, we show that the mechanism of size focusing involves a dependence of the growth rate on the size of the nanoparticles and the ligand coverage on the surface of the nanoparticles. To demonstrate these effects, we used in situ small angle X-ray scattering (SAXS) and population balance kinetic modeling (PBM) to investigate the evolution of size distribution during the synthesis of colloidal Pd metal nanoparticles. Despite temporal overlap of nucleation and growth, our in situ SAXS show size focusing of the distribution under different synthetic conditions (different concentrations of metal and ligand as well as solvent type). To understand the mechanism of size focusing using PBM, we systematically studied how the evolution of the nanoparticle size distribution is affected by nucleation rate, and dependence of the growth rate constant on ligand surface coverage, and size of the nanoparticles. We show that continuous nucleation contributes to size defocusing. However, continuous nucleation results in different reaction times for the nanoparticle population leading to time and size-dependent ligand surface coverage. Using density functional theory (DFT) calculations and Brønsted-Evans-Polanyi relations, we show that as the population grows, larger nanoparticles grow more slowly than smaller ones due to lower intrinsic activity and higher ligand coverage on the surface. Therefore, despite continuous nucleation, the faster growth of smaller nanoparticles in the population leads to size focusing. The size focusing behaviour (due to faster growth of smaller nanoparticles) was found to be model independent and similar results were demonstrated under different nucleation and growth pathways (e.g. growth via ion reduction on the surface and/or monomer addition). Our results provide a microscopic connection between kinetics and thermodynamics of nanoparticle growth and metal-ligand binding, and their effect on the size distribution of colloidal nanoparticles.

Elaborately Modified BiVO4 Photoanodes for Solar Water Splitting

Photoelectrochemical (PEC) cells for solar-energy conversion have received immense interest as a promising technology for renewable hydrogen production. Their similarity to natural photosynthesis, utilizing sunlight and water, has provoked intense research for over half a century. Among many potential photocatalysts, BiVO₄ , with a bandgap of 2.4-2.5 eV, has emerged as a highly promising photoanode material with a good chemical stability, environmental inertness, and low cost. Unfortunately, its charge transport properties are modest, at most a hole diffusion length (L_p ) of ≈70 nm. However, recent rapid developments in multiple modification strategies have elevated it to a position as the most promising metal oxide photoanode material. This review summarizes developments in BiVO₄ photoanodes in the past 10 years, in which time it has continuously broken its own performance records for PEC water oxidation. Effective modification techniques are discussed, including synthesis of nanostructures/nanopores, external/internal doping, heterojunction fabrication, surface passivation, and cocatalysts. Tandem systems for unassisted solar water splitting and PEC production of value-added chemicals are also discussed.