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Single-atom catalysts are playing a pivotal-role in understanding atomic-level photocatalytic processes. However, single-atoms are typically non-uniformly distributed on photocatalyst surfaces, hindering the systematic investigation of structure–property correlation at atomic precision. Herein, by combining material design, spectroscopic analyses, and theoretical studies, we investigate the atomic-level CO2 photoreduction process on TiO2 photocatalysts with uniformly stabilized transition metal single-atoms. First, the electronic interaction between single Cu atoms and the surrounding TiO2 affects the reducibility of the TiO2 surface, leading to spontaneous O vacancy formation near Cu atoms. The coexistence of Cu atoms and O vacancies cooperatively stabilizes CO2 intermediates on the TiO2 surface. Second, our approach allows us to control the spatial distribution of uniform single Cu atoms on TiO2, and demonstrate that neighboring Cu atoms simultaneously engage in the interaction with CO2 intermediates by controlling the charge localization. Optimized Cu1/TiO2 photocatalysts exhibit 66-fold enhancement in CO2 photoreduction performance compared to the pristine TiO2.

A large variety of nickel-based catalysts has been investigated for the oxygen evolution reaction (OER) in alkaline media. However, their reported activity, as well as Tafel slope values, vary greatly. To understand this variation, we studied electrodeposited Ni80Fe-20OOH catalysts with different loadings at varying rotation rates, hydroxide concentrations; with or without sonication. We show that, at low current density (5 mA/cm2), the Tafel slope value is ~30 mV/dec for Ni80Fe20OOH in 0.1M KOH. At higher polarization, the Tafel slope continuously increases and is dependent on rotation rate, loading, hydroxide concentration and sonication. These Tafel slope values are convoluted by non-kinetic effects, such as bubbles, potential-dependent changes in ohmic resistance and (internal) OH- gradients. As best practise, we suggest that Tafel slopes should be plotted vs. current or potential. In such a plot, it can be appreciated if there is a kinetic Tafel slope or if the observed Tafel slope is influenced by non-kinetic effects.

The development of efficient water-oxidizing electrocatalysts is a key issue for achieving high performance in the overall water electrolysis technique. However, the complexity of multiple electron transfer processes and large activation energies have been regarded as major bottlenecks for efficient water electrolysis. Thus, complete electrochemical processes, including electron transport, charge accumulation, and chemical bond formation/dissociation, need to be analyzed for establishing a design rule for film-type electrocatalysts. In light of this, complex capacitance analysis is an effective tool for investigating the charge accumulation and dissipation processes of film-type electrocatalysts. Here, we conduct complex capacitance analysis for the Mn3O4 nanocatalyst, which exhibits superb catalytic activity for water oxidation under neutral conditions. Charge was accumulated on the catalyst surface by the change in Mn valence between Mn(II) and Mn(IV) prior to the rate-determining O-O bond forming step. Furthermore, we newly propose the dissipation ratio (D) for understanding the energy balance between charge accumulation and charge consumption for chemical O-O bond formation. From this analysis, we reveal the potential- and thickness-dependent contribution of the charge accumulation process on the overall catalytic efficiency. We think that an understanding of complex capacitance analysis could be an effective methodology for investigating the charge accumulation process on the surface of general film-type electrocatalysts.

High-valent metal-oxo moieties have been implicated as key intermediates preceding various oxidation processes. The critical O–O bond formation step in the Kok cycle that is presumed to generate molecular oxygen occurs through the high-valent Mn-oxo species of the water oxidation complex, i.e., the Mn4Ca cluster in photosystem II. Here, we report the spectroscopic characterization of new intermediates during the water oxidation reaction of manganese-based heterogeneous catalysts and assign them as low-spin Mn(IV)-oxo species. Recently, the effects of the spin state in transition metal catalysts on catalytic reactivity have been intensely studied; however, no detailed characterization of a low-spin Mn(IV)-oxo intermediate species currently exists. We demonstrate that a low-spin configuration of Mn(IV), S = 1/2, is stably present in a heterogeneous electrocatalyst of Ni-doped monodisperse 10-nm Mn3O4 nanoparticles via oxo-ligand field engineering. An unprecedented signal (g = 1.83) is found to evolve in the electron paramagnetic resonance spectrum during the stepwise transition from the Jahn–Teller-distorted Mn(III). In-situ Raman analysis directly provides the evidence for Mn(IV)-oxo species as the active intermediate species. Computational analysis confirmed that the substituted nickel species induces the formation of a z-axis-compressed octahedral C4v crystal field that stabilizes the low-spin Mn(IV)-oxo intermediates. Understanding reaction intermediates provides a foundation for active electrocatalysts’ design, but it remains elusive for heterogeneous electrocatalysts. Here, the authors report the spectroscopic characterization of low-spin Mn(IV)-oxo as the active intermediates during electrochemical water oxidation.

Identification of the surface structure of nanoparticles is important for understanding the catalytic mechanism and improving the properties of the particles. Here, we provide a detailed description of the coordination modes of ethylenediaminetetraacetate (EDTA) on Mn3O4 nanoparticles at the atomic level, as obtained by advanced electron paramagnetic resonance (EPR) spectroscopy. Binding of EDTA to Mn3O4 leads to dramatic changes in the EPR spectrum, with a 5-fold increase in the axial zero-field splitting parameter of Mn(II). This indicates significant changes in the coordination environment of the Mn(II) site; hence, the binding of EDTA causes a profound change in the electronic structure of the manganese site. Furthermore, the electron spin echo envelope modulation results reveal that two 14N atoms of EDTA are directly coordinated to the Mn site and a water molecule is coordinated to the surface of the nanoparticles. An Fourier transform infrared spectroscopy study shows that the Ca(II) ion is coordinated to the carboxylic ligands via the pseudobridging mode. The EPR spectroscopic results provide an atomic picture of surface-modified Mn3O4 nanoparticles for the first time. These results can enhance our understanding of the rational design of catalysts, for example, for the water oxidation reaction.

A highly efficient CO2 electrolysis system could be created by introducing biomass oxidation as an alternative anodic reaction to the sluggish oxygen evolution reaction in a CO2-saturated and near-neutral electrolyte. Here, we successfully demonstrate anodic biomass oxidation by synthesizing 5 nm nickel oxide nanoparticles (NiO NPs). NiO NPs show a unique electrocatalytic activity for 5-hydroxymethylfurfural (HMF) oxidation under near-neutral conditions, exhibiting an anodic current onset (1 mA cm-2) at 1.524 V versus the reversible hydrogen electrode and a total Faradaic efficiency of ≤70%. Electrokinetic and in situ ultraviolet-visible spectroscopic analyses suggest that a redox active nickel hydroxide species is formed on the surface of NiO electrocatalysts during HMF oxidation, and this oxidation of Ni(II) hydroxide to Ni(III) oxyhydroxide could be the rate-determining step. This mechanistic study of biomass oxidation in a CO2-saturated electrolyte provides insight into constructing a highly efficient system for the paired electrolysis of CO2 reduction and biomass oxidation.

Photocatalysis is an easily configurable and cost-effective technology for the conversion of solar energy into chemical energy. Recently, increasing attention has been given to metal halide perovskite (MHP) photocatalysts because of the development of stabilization strategies for MHPs under reaction conditions. From this perspective, we first describe several substantial breakthroughs in the photocatalytic application of MHPs. Performance trends in the solar fuel production applications of MHPs, including photocatalytic H2 generation and photocatalytic CO2 reduction reactions, are then described. Recent developments to extend the use of MHPs to various photocatalytic organic transformations are also highlighted. Finally, we propose several scientific challenges for the practical implications of MHPs for solar fuel production and various photoreactions.

Structural change of electrocatalysts is typically considered a negative effect but can be utilized for controlling interfacial kinetics. In the December issue of Matter, Wang et al. demonstrate a new strategy for enhancing the performance of NiMoFe electrocatalyst by MoO42− dissolution and subsequent reconstruction into the active species.

Scalable and facile synthesis of highly active alkaline hydrogen evolution reaction catalysts was demonstrated by controlling the microstructures of Al–Ni alloys and making porous foam and powder. The microstructures were carefully controlled in the binary alloys and the subsequent selective etching resulted in highly active catalysts, which are comparable to conventional Pt-based catalysts. Large-scale coating of a dealloyed Al–Ni alloy powder was also demonstrated, showing the practical applicability of this method.

Water oxidation is considered an important reaction to attain sustainable energy systems. Inspired by the biological Mn4CaO5 cluster, considerable effort has been devoted to developing efficient manganese-based heterogeneous catalysts for the water oxidation reaction. This review first describes the structural characteristics of the Mn4CaO5 cluster and synthetic manganese oxides. Important features related to the electrocatalytic activity of manganese-based heterogeneous catalysts as well as recent in situ spectroscopic results for intermediate characterization are then summarized. Current understanding of the reaction mechanism at the atomic level and the rate-determining step is highlighted. Finally, future perspectives for the development of highly active and stable manganese-based water-oxidizing catalysts are proposed.

The charge transport in the film-type electrocatalysts for the oxygen evolution reaction is a significant factor affecting the overall catalytic performance. For instance, transition metal oxide-ba...

Although NiFeOxHy has attracted enormous attention as an alkaline water electrolyzer owing to its high performance in the oxygen evolution reaction (OER), it still requires further improvement in t...

Electrochemical water oxidation is considered as one of the most important reactions for a sustainable future. However, in heterogeneous water oxidation, atomistic understanding of this four-electr...

Electrochemical water oxidation is a key counter reaction in obtaining value-added chemicals by reduction in aqueous solution. However, slow kinetics is a problem in this process, so the quantitati...

The reversible and cooperative activation process, which includes electron transfer from surrounding redox mediators, the reversible valence change of cofactors and macroscopic functional/structural change, is one of the most important characteristics of biological enzymes, and has frequently been used in the design of homogeneous catalysts. However, there are virtually no reports on industrially important heterogeneous catalysts with these enzyme-like characteristics. Here, we report on the design and synthesis of highly active TiO2 photocatalysts incorporating site-specific single copper atoms (Cu/TiO2) that exhibit a reversible and cooperative photoactivation process. Our atomic-level design and synthetic strategy provide a platform that facilitates valence control of co-catalyst copper atoms, reversible modulation of the macroscopic optoelectronic properties of TiO2 and enhancement of photocatalytic hydrogen generation activity, extending the boundaries of conventional heterogeneous catalysts. Reversible and cooperative activation processes are important characteristics of biological enzymes and can be used in designing catalysts. Highly active TiO2 photocatalysts incorporated with site-specific single copper atoms are now shown to exhibit such a photoactivation process.

The reaction mechanism of electrochemical chloride oxidation at neutral pH is different from that at acidic pH, in which a commercial chlor-alkali process has been developed. Different proton concentrations and accelerated hydrolysis of the generated chlorine into hypochlorous acid at high pH can change the electrokinetics and stability of reaction intermediates. We have investigated a unique reaction mechanism of Co3O4 nanoparticles for chloride oxidation at neutral pH. In contrast with water oxidation, the valency of cobalt was not changed during chloride oxidation. Interestingly, a new intermediate of Co-Cl was captured spectroscopically, distinct from the reaction intermediate at acidic pH. In addition, Co3O4 nanoparticles exhibited high selectivity for active chlorine generation at neutral pH, comparable to commercially available RuO2-based catalysts. We believe that this study provides insight into designing efficient electrocatalysts for active chlorine generation at neutral pH, which can be practically applied to electrochemical water treatment coupled to hydrogen production.

Electrochemical water splitting is a promising means to produce eco-friendly hydrogen fuels. Inspired by the Mn4 CaO5 cluster in nature, substantial works have been performed to develop efficient manganese (Mn)-based heterogeneous catalysts. Despite improvements in catalytic activity, the underlying mechanism of the oxygen evolution reaction (OER) is not completely elucidated owing to the lack of direct spectroscopic evidence for the active Mn-oxo moieties. We identify water oxidation intermediates on the surface of Mn3 O4 nanoparticles (NPs) in the OER at neutral pH by in situ Raman spectroscopy. A potential-dependent Raman peak was detected at 760 cm-1 and assigned to the active MnIV =O species generated during water oxidation. Isotope-labeling experiments combined with scavenger experiments confirmed the generation of surface terminal MnIV =O intermediates in the Mn-oxide NPs. This study provides an insight into the design of systems for the observation of reaction intermediates.

The high refractive index of hydrogenated amorphous silicon (a-Si:H) at optical frequencies is an essential property for the efficient modulation of the phase and amplitude of light. However, substantial optical loss represented by its high extinction coefficient prevents it from being utilized widely. Here, the bonding configurations of a-Si:H are investigated, in order to manipulate the extinction coefficient and produce a material that is competitive with conventional transparent materials, such as titanium dioxide and gallium nitride. This is achieved by controlling the hydrogenation and silicon disorder by adjusting the chemical deposition conditions. The extinction coefficient of the low-loss a-Si:H reaches a minimum of 0.082 at the wavelength of 450 nm, which is lower than that of crystalline silicon (0.13). Beam-steering metasurfaces are demonstrated to validate the low-loss optical properties, reaching measured efficiencies of 42%, 62%, and 75% at the wavelengths of 450, 532, and 635 nm, respectively. Considering its compatibility with mature complementary metal-oxide-semiconductor processes, the low-loss a-Si:H will provide a platform for efficient photonic operating in the full visible regime.

Identification of the surface structure of nanoparticles is important for understanding the catalytic mechanism and improving the properties of the particles. Here, we provide a detailed description of the coordination modes of ethylenediaminetetraacetate (EDTA) on Mn

A highly efficient CO

Nitric oxide, a gaseous neurotransmitter, can be electrochemically generated inside the brain to activate calcium ion channels, paving the way for implantable neurotransmitter probes.

We found the correlation between atomic bonding structures and the refractive index in the visible spectrum. We found that optical hydrogenated amorphous silicon exhibits the extinction coefficient of 0.082 at the wavelength of 450 nm.

The reaction mechanism of electrochemical chloride oxidation at neutral pH is different from that at acidic pH, in which a commercial chlor-alkali process has been developed. Different proton concentrations and accelerated hydrolysis of the generated chlorine into hypochlorous acid at high pH can change the electrokinetics and stability of reaction intermediates. We have investigated a unique reaction mechanism of Co

Electrochemical water splitting to produce hydrogen energy is regarded as a promising energy conversion process for its environmentally friendly nature. To improve cell efficiency, the development of efficient water oxidation catalysts is essentially demanded. For several decades, 3d transition metal oxides have been intensively investigated for their high activity, good durability and low-cost. This review covers i) recent progress on 3d transition metal oxide electrocatalysts and ii) the reaction mechanism of oxygen evolving catalysis, specifically focused on the proposed pathways for the O-O bond formation step.

Organolead halide perovskites are used for low-operating-voltage multilevel resistive switching. Ag/CH3 NH3 PbI3 /Pt cells exhibit electroforming-free resistive switching at an electric field of 3.25 × 10(3) V cm(-1) for four distinguishable ON-state resistance levels. The migration of iodine interstitials and vacancies with low activation energies is responsible for the low-electric-field resistive switching via filament formation and annihilation.

Functional graffiti of nanoparticles onto target surface is an important issue in the development of nanodevices. A general strategy has been introduced here to decorate chemically diverse substrates with gold nanoparticles (AuNPs) in the form of a close-packed single layer by using an omni-adhesive protein of α-synuclein (αS) as conjugated with the particles. Since the adsorption was highly sensitive to pH, the amino acid sequence of αS exposed from the conjugates and its conformationally disordered state capable of exhibiting structural plasticity are considered to be responsible for the single-layer coating over diverse surfaces. Merited by the simple solution-based adsorption procedure, the particles have been imprinted to various geometric shapes in 2-D and physically inaccessible surfaces of 3-D objects. The αS-encapsulated AuNPs to form a high-density single-layer coat has been employed in the development of nonvolatile memory, fule-cell, solar-cell, and cell-culture platform, where the outlying αS has played versatile roles such as a dielectric layer for charge retention, a sacrificial layer to expose AuNPs for chemical catalysis, a reaction center for silicification, and biointerface for cell attachment, respectively. Multiple utilizations of the αS-based hybrid NPs, therefore, could offer great versatility to fabricate a variety of NP-integrated advanced materials which would serve as an indispensable component for widespread applications of high-performance nanodevices.

Water electrolysis has been regarded an attractive approach to produce sustainable energy without undesirable byproducts. The anodic half reaction, oxygen evolving reaction (OER), is a major bottleneck for overall water electrolysis process because of its complex four electron transfer steps and high activation energy for O-O bond formation. Thus, transition-metal-based film-type electrocatalysts have been extensively reported with moderate efficiency due to their low price and abundance. Interestingly, transition metal based OER catalyst film generally exhibited optimal catalytic performance at considerable thickness (~ hundreds nanometers). The overall catalytic performance for the film-type catalysts are determined by various electrochemical processes, such as surface catalysis and electron transport. Therefore, to establish rational design rule for efficient OER electrocatalyst, it is necessary to extract well-defined kinetic parameters describing each electrochemical processes. In this regard, electrochemical impedance spectroscopy (EIS) is effective analysis to decouple and parametrize complicated electrochemical behavior in the catalyst film during OER. Here, we conducted EIS analysis for sub 10 nm-sized partially oxidized MnO nanoparticles (Mn-oxide NPs) operating with superb activity under neutral condition. Furthermore, we proposed a transmission line model with Havriliak-Negami capacitors to obtain reliable kinetic parameters for OER on the Mn-oxide NPs film. From this analysis, we explained the rationale for the optimum thickness. Also, we revealed that protons are involved in transport on the surface of p-MnO NPs, which directly proved the ongoing hypothesis of the oxo-hopping mechanism during catalysis. Furthermore, from correlation between the reaction rate constant (22.1 s-1 at 1.35 V vs. NHE) and TOF values, only small ratio of Mn sites served as factual active sites for OER. We believe that this mechanistic investigation using EIS analysis with our circuit model could be an effective platform to investigate comprehensive electrochemical processes in general film-type electrocatalysts. Figure 1

Developing alternatives to replace fossil fuels is an important issue for maintaining sustainable energy system in the future. Hydrogen fuel is an attractive alternative owing to its high energy density and environmentally friendly by-products. Electrical water splitting powered by a renewable energy source is a promising way to produce hydrogen fuel. However, anodic water oxidation is considered to be a bottleneck due to its high overpotential. Therefore, designing efficient catalyst for water splitting is regarded as an important issue. Previously, Ir and Ru-based catalysts have been shown to exhibit remarkable catalytic activity toward oxygen evolving reaction (OER) under acidic condition. Despite their high activity, it is difficult to use noble-metal-based catalysts for commercial applications because of the scarcity of precious metals and their costs. Therefore, non-precious transition metal oxide electrocatalysts were investigated for water oxidation reaction such as cobalt oxides, nickel oxides, iron oxides and manganese oxides.1,2 Their activities were only retained under basic conditions and advancement in electrocatalysts that show moderate activity at neutral pH has been relatively insufficient. Nature has highly efficient OER catalysts in photosystem II which is Mn4CaO5 cluster also called as oxygen evolving complex. Inspired by unique structure and catalytic mechanism of this cluster, Mn-based catalysts have been actively studied. Among various manganese oxide, nanoparticle system showed superior catalytic activity due to its totally different reaction mechanism compared to the bulk counterparts.3,4 However, after synthesis of manganese oxide nanoparticles, long alkyl organic ligands are attached on the surface of nanoparticles interrupting the approach of water molecules, which inhibits water oxidation. Previously, hydrophobic organic ligands were removed by thermal decomposition with sufficient annealing process. Alternative way to remove alkyl ligands is replacing it with different ligands through surface ligand treatment such as BF4 -, PF6 -, OH-, thiol, SCN- and PbCl3 -. Surface ligand exchange is beneficial in that it does not need high temperature annealing and possibility of fine surface control. We have successfully synthesized 6 nm Mn3O4 nanoparticles (NPs) and used for further surface treatments. Mn3O4 NPs were OH ligand exchanged as a first step and followed by methylamine treatment in aqueous solution. Identical TEM images and XRD patterns showed that the size of NPs was unchanged and the phase of NPs was maintained as Mn3O4. Electrokinetic study of the OER was analyzed by cyclic voltammetry (CV) at pH 7 and compared between OH ligand exchanged Mn3O4 NPs and further methylamine treated Mn3O4 NPs. Methylamine treatment reduced the overpotential for OER by nearly 100 mV at the current density 5 mA cm-2. Tafel slopes was also decreased after methylamine treatment to 83.1 mV dec-1. To understand the phenomenon of increased catalytic activity after methylamine treatment, further analysis of the surface of NPs were conducted using spectroscopic measurement. In the XPS O 1s spectra, surface hydroxyl group (Mn-OH) was increased up to 57.3 % after OH ligand exchange. Also, FTIR spectra showed increased ratio of Mn-OH to lattice oxygen (Mn-O-Mn) for OH ligand exchanged Mn3O4 NPs compared to Mn3O4 NPs which indicates successful OH ligand exchange on the surface of Mn3O4 NPs. After methylamine treatment of OH ligand exchanged Mn3O4 NPs, methylamine was not attached to the surface of NPs. XPS N 1s spectra did not show characteristic methylamine peak, FTIR did not show C-N vibration mode and elemental analysis indicates that N does not exist in the NPs. To figure out the role of methylamine on the surface of OH ligand exchanged Mn3O4 NPs, titration and zeta potential was measured. After methylamine treatment surface was more deprotonated. Deprotonation resulted in loss of positively charged proton which induced negative charge on the surface of Mn3O4 NPs and showed negative zeta potential. On the basis of nonexistence of methylamine on the surface and surface deprotonation after methylamine treatment, we hypothesized that methylamine work as Brønsted-Lowry base. Methylamine was replaced by ethylamine, ethylenediamine, dimethylamine, trimethylamine and ammonia water, then after reaction with different amines, overpotential was decreased for all cases. Interestingly, amines with larger pK a showed larger decrease in overpotential. Based on these results, we first proposed correlation between the surface charge and oxygen evolving activity. Deprotonating the surface of the catalyst through simple acid-base reaction induced a change in the surface state followed by catalytic activity improvement. This understanding can be applied for developing efficient nano-catalysts by deprotonating the nanoparticle surface to induce surface state change. [1] J. Am. Chem. Soc., 2014, 136, 4201-4211 [2] J. Am. Chem. Soc., 2014, 136, 7435-7443 [3] Sci. Rep., 2015, 5, 10279 [4] J. Am. Chem. Soc., 2017, 139, 2277-2285 Figure 1

Organometal halide perovskite materials have been the subject of intensive research efforts in the context of photovoltaic applications. Here the authors exploit dynamic equilibrium to photocatalytically generate hydrogen from aqueous HI solutions using methylammonium lead iodide.

Nature designs circulatory systems with hierarchically organized networks of gradually tapered channels ranging from micrometer to nanometer in diameter. In most hard tissues in biological systems, fluid, gasses, nutrients and wastes are constantly exchanged through such networks. Here, we developed a biologically-inspired, hierarchically-organized structure in ceramic to achieve effective permeation with minimum void region, using fabrication methods that create a long-range, highly-interconnected nanochannel system in a ceramic biomaterial. This design of a synthetic model-material was implemented through a novel pressurized sintering process formulated to induce a gradual tapering in channel diameter based on pressure-dependent polymer agglomeration. The resulting system allows long range, efficient transport of fluid and nutrients into sites and interfaces that conventional fluid conduction cannot reach without external force. We demonstrate the ability of mammalian bone-forming cells placed at the distal transport termination of the nanochannel system to proliferate in a manner dependent solely upon the supply of media by the self-powering nanochannels. This approach mimics the significant contribution that nanochannel transport plays in maintaining living hard tissues by providing nutrient supply that facilitates cell growth and differentiation, and thereby makes the ceramic composite ‘alive’.

In two-dimensional interfacial assemblies, there is an interplay between molecular ordering and interface geometry, which determines the final morphology and order of entire systems. Here we present the interfacial phenomenon of spontaneous facet formation in a water droplet driven by designed peptide assembly. The identified peptides can flatten the rounded top of a hemispherical droplet into a plane by forming a macroscopic two-dimensional crystal structure. Such ordering is driven by the folding geometry of the peptide, interactions of tyrosine and crosslinked stabilization by cysteine. We discover the key sequence motifs and folding structures and study their sequence-specific assembly. The well-ordered, densely packed, redox-active tyrosine units in the YYACAYY (H-Tyr-Tyr-Ala-Cys-Ala-Tyr-Tyr-OH) film can trigger or enhance chemical/electrochemical reactions, and can potentially serve as a platform to fabricate a molecularly tunable, self-repairable, flat peptide or hybrid film.

Background: Adult onset Still’s disease (AOSD) is a rare systemic inflammatory disorder of unknown aetiology, frequently accompanying multiple lymphadenopathy. It often mimics malignant lymphoma, and immunohistochemical and molecular studies are needed for definite diagnosis. Aims: To aid in diagnosis and understand the pathogenesis of the disease by clarifying lymph node (LN) pathology in AOSD. Methods: Thirteen biopsies (one follow up biopsy) and medical records of 12 patients were reviewed. Immunohistochemistry, polymerase chain reaction for T cell receptor γ chain (TCRγ) and immunoglobulin heavy chain gene rearrangement, and Epstein-Barr virus in situ hybridisation were performed. Results: Histologically, LN lesions were classified into four patterns. The most common (six biopsies) showed paracortical hyperplasia, with prominent vascular proliferation, scattered large B/T immunoblasts, and infiltration by reactive lymphocytes and inflammatory cells. In the second pattern (two biopsies), paracortical hyperplasia was accompanied by massive sinus histiocytosis and S-100 positive histiocyte aggregates. The third pattern (three patients) showed an exuberant immunoblastic reaction, in the form of patchy/diffuse infiltration of large T immunoblasts with high mitotic activity, although clonal rearrangement of the TCRγ gene was not detected. The fourth pattern showed distinct follicular hyperplasia (two cases). One patient with a follow up biopsy showed a pattern change from pronounced follicular hyperplasia to atypical paracortical hyperplasia. Conclusions: AOSD LN lesions show a dynamic histological spectrum, including atypical paracortical hyperplasia, burnt out histiocytic reaction, exuberant immunoblastic reaction, and follicular hyperplasia. During the course of disease, LN reactivity changes and mixed B and T cells are involved in the pathogenesis.