General Strategy to Optimize Gas Evolution Reaction via Assembled Striped-Pattern Superlattices

Patterned Gold Nanoparticle Superlattice Film for Wearable Sweat Sensors

Nanoparticle superlattices are beneficial in terms of providing strong and uniform signals in analysis owing to their closely packed uniform structures. However, nanoparticle superlattices are prone to cracking during physical activities because of stress concentrations, which hinders their detection performance and limits their analytical applications. In this work, template printing methods were used in this study to prepare a patterned gold nanoparticle (AuNP) superlattice film. By adjustment of the size of the AuNP superlattice domain below the critical size of fracture, the mechanical stability of the AuNP superlattice domain is improved. Thus, long-term sustainable high-performance signal output is achieved. The patterned AuNP superlattice film was used to construct a wearable sweat sensor based on surface-enhanced Raman scattering (SERS). The designed sensor showed promise for long-term reliable use in actual scenarios in terms of recommending water replenishment, monitoring hydration states, and tracking the intensity of activity.

Capillary-Driven Separate Gas-Liquid Transport: Alleviating Mass Transport Losses for Efficient Hydrogen Evolution

Developing earth-abundant transition metal electrodes with high activity and durability is crucial for efficient and cost-effective hydrogen production. However, numerous studies in the hydrogen evolution reaction (HER) primarily focus on improving the inherent activity of catalysts, and the critical influence of gas-liquid countercurrent transport behavior is often overlooked. In this study, we introduce the concept of separate-path gas-liquid transport to alleviate mass transport losses for the HER by developing a novel hierarchical porous Ni-doped cobalt phosphide electrode (CoNix-P@Ni). The CoNix-P@Ni electrodes with abundant microvalleys and crack structures facilitate the gas-liquid cotransport by separating the bubble release and water supply paths. Visualization and numerical simulation results demonstrate that cracks primarily serve as water supply paths, with capillary pressure facilitating the transport of water from the cracks to the microvalleys. This process ensures the continuous wetting of electrolytes in the electrode, reduces hydrogen supersaturation near the active site, and increases hydrogen transport flux to the microvalleys for accelerating bubble growth. Additionally, the microvalleys act as preferential sites for bubble evolution, preventing bubble coverage on other active sites. By regulating the amount of nickel, the CoNi1-P@Ni electrode exhibited the smallest and densest microvalleys and cracks, achieving superior HER performance with an overpotential of 51 mV at 10 mA cm-2. The results offer a promising direction for constructing high-performance HER electrodes.

Self-Encapsulation of High-Entropy Alloy Nanoparticles inside Carbonized Wood for Highly Durable Electrocatalysis

High-entropy alloy nanoparticles (HEAs) show great potential in emerging electrocatalysis due to their combination and optimization of multiple elements. However, synthesized HEAs often exhibit a weak interface with the conductive substrate, hindering their applications in long-term catalysis and energy conversion. Herein, a highly active and durable electrocatalyst composed of quinary HEAs (PtNiCoFeCu) encapsulated inside the activated carbonized wood (ACW) is reported. The self-encapsulation of HEAs is achieved during Joule heating synthesis (2060 K, 2 s) where HEAs naturally nucleate at the defect sites. In the meantime, HEAs catalyze the deposition of mobile carbon atoms to form a protective few-layer carbon shell during the rapid quenching process, thus remarkably strengthening the interface stability between HEAs and ACW. As a result, the HEAs@ACW shows not only favorable activity with an overpotential of 7 mV at 10 mA cm-2 for hydrogen evolution but also negligible attenuation during a 500 h stability test, which is superior to most reported electrocatalysts. The design of self-encapsulated HEAs inside ACW provides a critical strategy to enhance both activity and stability, which is also applicable to many other energy conversion technologies.

Unlocking Efficiency: Minimizing Energy Loss in Electrocatalysts for Water Splitting

Catalysts play a crucial role in water electrolysis by reducing the energy barriers for hydrogen and oxygen evolution reactions (HER and OER). Research aims to enhance the intrinsic activities of potential catalysts through material selection, microstructure design, and various engineering techniques. However, the energy consumption of catalysts has often been overlooked due to the intricate interplay among catalyst microstructure, dimensionality, catalyst-electrolyte-gas dynamics, surface chemistry, electron transport within electrodes, and electron transfer among electrode components. Efficient catalyst development for high-current-density applications is essential to meet the increasing demand for green hydrogen. This involves transforming catalysts with high intrinsic activities into electrodes capable of sustaining high current densities. This review focuses on current improvement strategies of mass exchange, charge transfer, and reducing electrode resistance to decrease energy consumption. It aims to bridge the gap between laboratory-developed, highly efficient catalysts and industrial applications regarding catalyst structural design, surface chemistry, and catalyst-electrode interplay, outlining the development roadmap of hierarchically structured electrode-based water electrolysis for minimizing energy loss in electrocatalysts for water splitting.

Lattice oxygen activation and local electric field enhancement by co-doping Fe and F in CoO nanoneedle arrays for industrial electrocatalytic water oxidation

Oxygen evolution reaction (OER) is critical to renewable energy conversion technologies, but the structure-activity relationships and underlying catalytic mechanisms in catalysts are not fully understood. We herein demonstrate a strategy to promote OER with simultaneously achieved lattice oxygen activation and enhanced local electric field by dual doping of cations and anions. Rough arrays of Fe and F co-doped CoO nanoneedles are constructed, and a low overpotential of 277 mV at 500 mA cm-2 is achieved. The dually doped Fe and F could cooperatively tailor the electronic properties of CoO, leading to improved metal-oxygen covalency and stimulated lattice oxygen activation. Particularly, Fe doping induces a synergetic effect of tip enhancement and proximity effect, which effectively concentrates OH- ions, optimizes reaction energy barrier and promotes O2 desorption. This work demonstrates a conceptual strategy to couple lattice oxygen and local electric field for effective electrocatalytic water oxidation.

Nanoparticle Assembly: From Self-Organization to Controlled Micropatterning for Enhanced Functionalities

Nanoparticles form long-range micropatterns via self-assembly or directed self-assembly with superior mechanical, electrical, optical, magnetic, chemical, and other functional properties for broad applications, such as structural supports, thermal exchangers, optoelectronics, microelectronics, and robotics. The precisely defined particle assembly at the nanoscale with simultaneously scalable patterning at the microscale is indispensable for enabling functionality and improving the performance of devices. This article provides a comprehensive review of nanoparticle assembly formed primarily via the balance of forces at the nanoscale (e.g., van der Waals, colloidal, capillary, convection, and chemical forces) and nanoparticle-template interactions (e.g., physical confinement, chemical functionalization, additive layer-upon-layer). The review commences with a general overview of nanoparticle self-assembly, with the state-of-the-art literature review and motivation. It subsequently reviews the recent progress in nanoparticle assembly without the presence of surface templates. Manufacturing techniques for surface template fabrication and their influence on nanoparticle assembly efficiency and effectiveness are then explored. The primary focus is the spatial organization and orientational preference of nanoparticles on non-templated and pre-templated surfaces in a controlled manner. Moreover, the article discusses broad applications of micropatterned surfaces, encompassing various fields. Finally, the review concludes with a summary of manufacturing methods, their limitations, and future trends in nanoparticle assembly.

Dynamically Adaptive Bubbling for Upgrading Oxygen Evolution Reaction Using Lamellar Fern-Like Alloy Aerogel Self-Standing Electrodes

Adopting renewable electricity to produce "green" hydrogen has been a critical challenge because at a high current density the mass transfer capability of most catalytic electrodes deteriorates significantly. Herein, a unique lamellar fern-like alloy aerogel (LFA) electrode, showing a unique dynamically adaptive bubbling capability and can effectively avoid stress concentration caused by bubble aggregation is reported. The LFA electrode is intrinsically highly catalytic-active and shows a highly porous, resilient, hierarchically ordered, and well-percolated conductive network. It not only shows superior gas evacuation capability but also exhibits significantly improved stability at high current densities, showing the record lowest oxygen evolution reaction (OER) overpotential of 244 mV at 1000 mA cm-2 and stably over 6000 h. With the merits of mechanical robustness, excellent electron transport, and efficient bubble evacuation, LFA can be self-standing catalytic electrode and gas diffusion layers in anion-exchange-membrane water electrolysis (AEMWE), which can achieve 3000 mA cm-2 at a low voltage of 1.88 V and can sustain stable electrolysis at 2000 mA cm-2 for over 1300 h. This strategy can be extended to various gas evolution reactions as a general design rule for multiphase catalysis applications.

Bubble Management for Electrolytic Water Splitting by Surface Engineering: A Review

During electrocatalytic water splitting, the management of bubbles possesses great importance to reduce the overpotential and improve the stability of the electrode. Bubble evolution is accomplished by nucleation, growth, and detachment. The expanding nucleation sites, decreasing bubble size, and timely detachment of bubbles from the electrode surface are key factors in bubble management. Recently, the surface engineering of electrodes has emerged as a promising strategy for bubble management in practical water splitting due to its reliability and efficiency. In this review, we start with a discussion of the bubble behavior on the electrodes during water splitting. Then we summarize recent progress in the management of bubbles from the perspective of surface physical (electrocatalytic surface morphology) and surface chemical (surface composition) considerations, focusing on the surface texture design, three-dimensional construction, wettability coating technology, and functional group modification. Finally, we present the principles of bubble management, followed by an insightful perspective and critical challenges for further development.

Efficient Alkaline Hydrogen Evolution Reaction Using Superaerophobic Ni Nanoarrays with Accelerated H2 Bubble Release

Despite the adverse effects of H2 bubbles adhering to catalyst's surface on the performance of water electrolysis, the mechanisms by which H2 bubbles are effectively released during the alkaline hydrogen evolution reaction (HER) remain elusive. In this study, a systematic investigation on the effect of nanoscale surface morphologies on H2 bubble release behaviors and HER performance by employing earth-abundant Ni catalysts consisting of an array of Ni nanorods (NRs) with controlled surface porosities is performed. Both aerophobicity and hydrophilicity of the catalyst's surface vary according to the surface porosity of catalysts. The Ni catalyst with the highest porosity of ≈52% exhibits superaerophobic nature as well as the best HER performance among the Ni catalysts. It is found that the Ni catalyst's superaerophobicity combined with the effective open pore channels enables the accelerated release of H2 bubbles from the surface, leading to a significant improvement in geometric activities, particularly at high current densities, as well as intrinsic activities including both specific and mass activities. It is also demonstrated that the superaerophobicity enabled by highly porous Ni NRs can be combined with Pt and Cr having optimal binding abilities to further optimize electrocatalytic performance.

Engineering composition-varied Au/PtTe hetero-junction-abundant nanotrough arrays as robust electrocatalysts for ethanol electrooxidation

Pt-based multi-metallic electrocatalysts containing hetero-junctions are found to have superior catalytic performance to composition-equivalent counterparts. However, in bulk solution, controllable preparation of Pt-based hetero-junction electrocatalyst is an extremely random work owing to the complexity of solution reactions. Herein, we develop an interface-confined transformation strategy, subtly achieving Au/PtTe hetero-junction-abundant nanostructures by employing interfacial Te nanowires as sacrificing templates. By controlling the reaction conditions, composition-varied Au/PtTe can be easily obtained, such as Au₇₅/Pt₂₀Te₅, Au₅₅/Pt₃₄Te₁₁, and Au₅/Pt₆₉Te₂₆. Moreover, each Au/PtTe hetero-junction nanostructure appears to be an array consisting of side-by-side Au/PtTe nanotrough units and can be directly used as a catalyst layer without further post-treatment. All Au/PtTe hetero-junction nanostructures show better catalytic activity towards ethanol electrooxidation than commercial Pt/C because of the combining contributions of Au/Pt hetero-junctions and the collective effects of multi-metallic elements, where Au₇₅/Pt₂₀Te₅ exhibits the best electrocatalytic performance among three Au/PtTe nanostructures owing to its optimal composition. This study may provide technically feasible guidance for further maximizing the catalytic activity of Pt-based hybrid catalysts.

A Programmable Plasmonic Gas Microsystem for Detecting Arbitrarily Combinated Volatile Organic Compounds (VOCs) with Ultrahigh Resolution

For gas sensors, the ultrasensitive and highly selective detection of multiple components is of great significance in a wide range of applications extending from environment to healthcare, which is still a long-term challenge due to the single sensing mechanism of most sensors. Here, we combine the advantages of microfluidic chips and surface-enhanced Raman spectroscopy (SERS) spectra to fabricate a smart single-chip for simultaneously detecting an arbitrary combination of VOCs that incorporates different detection units, working on either a physisorption or chemisorption mechanism. Full integration of microfluidic and multiplex nanostructure components on one chip permits programmable design for sensing multifarious volatile compounds, and enables on-chip signal amplifications with increased reproducibility. As a proof-of-principle experiment, we demonstrate the simultaneous identification of 9 different gases that belong to aromatic compounds, aldehydes, ketones, or sulfides in one mixture, with high sensitivity (ppb level), high selectivity, and high robustness (error ∼8%). We further evaluated the application of our universal gas sensor in two scenarios including indoor air pollution monitoring and exhaled breath-based disease diagnosis. We expect that our design will improve the various practical applications of gas sensors.

Tuning the Interface of Co1-xS/Co(OH)F by Atomic Replacement Strategy toward High-Performance Electrocatalytic Oxygen Evolution

The construction of heterostructures is one of the most promising strategies for engineering interfaces of catalysts to perform high-efficiency oxygen evolution reaction (OER). However, accurately tuning heterostructures' interface during operation remains a challenge. Herein, we fabricated the needled-like heterostructure Co_1-xS/Co(OH)F supported on flexible carbon fiber cloth via an atomic substitution strategy, in which sulfur atoms are simultaneously grafted into F vacancies after the partial removal of F atoms from Co(OH)F during the electrodeposition, thus achieving the growth of cobalt sulfide on the interface of Co(OH)F. This electrocatalyst with such design exhibits the following advantages: (1) The lattice distortion caused by atomic substitution leads to the increase of active sites; (2) Co_1-xS constructed on the surface of Co(OH)F by the atomic replacement strategy optimizes the adsorption (OH^-) and desorption (O₂) energy in the OER process; (3) the needle-like structure possesses the tip-enhanced local electric field effect. As a result, the Co_1-xS/Co(OH)F/CC catalyst exhibits very high OER catalytic performance with an overpotential of 269 mV at a current density of 10 mA cm^-2 and a Tafel slope of 71 mV dec^-1. The asymmetric electrode shows superior catalytic activity and stability in overall water splitting. The catalytic mechanism of these highly efficient Co_1-xS/Co(OH)F/CC catalysts was investigated via DFT theoretical calculations and ex situ characterizations. This atomic substitution strategy displays universality for other transition metal sulfides (metal = Ni, Mn, Cu).

Self-Assembly Bifunctional Tetranuclear Ln2Ni2 Clusters: Magnetic Behaviors and Highly Efficient Conversion of CO2 under Mild Conditions

A series of heterometallic tetranuclear clusters, Ln₂Ni₂(NO₃)₄L₄(μ₃-OCH₃)₂·2(CH₃CN) (Ln = Gd(1), Tb(2), Dy(3), Ho(4), Er(5); HL = methyl 3-methoxysalicylate), were synthesized solvothermally. The intramolecular synergistic effect of two metal centers of Ln(III) and Ni(II) and the exposed multimetallic sites serving as Lewis acid activators greatly increase the efficiency of the CO₂ conversion, and the yield for cluster 3 can be achieved at 96% at atmospheric pressure and low temperature. In particular, the self-assembly multimetal center with polydentate ligand shows good generality and enhanced recyclability. The design of such 3d-4f heterometallic clusters provides an effective strategy for the conversion of CO₂ under greener conditions. Meanwhile, magnetic investigations indicate that cluster 1 is a good candidate for magnetic refrigerant materials with a relatively large magnetocaloric effect (MCE) (-ΔS_m = 28.5 J kg^-1 K^-1 at 3.0 K and 7.0 T), and cluster 3 shows single-molecular magnet behavior under zero dc field. Heterometallic clusters with special magnetic properties and good catalytic behavior for the conversion of CO₂ are rare. Thus, they are potential bifunctional materials applied in practice.

Confinement of Pt NPs by hollow-porous-carbon-spheres via pore regulation with promoted activity and durability in the hydrogen evolution reaction

Electrochemical water splitting is a promising method to generate pollution-free and sustainable hydrogen energy. However, the specific activity and durability of noble metal catalysts is the main hindrance to the hydrogen evolution reaction. Based on the continuous pore regulation of hollow porous carbon spheres (N-HPCSs) by hexadecyl trimethyl ammonium bromide, the 6.21 wt% Pt/N-HPCSs exhibited good dispersibility, according to a low overpotential of 45 mV (10 mA cm^-2/1 M KOH). Its mass activity was 4 times that of the commercial 20 wt% Pt/C at -0.07 V (vs. RHE) potential. We analyse that the excellent activity is due to the interaction between Pt nanoparticles and N-HPCSs so that the electron density around the Pt atoms increases, which is beneficial for H₂O to obtain electrons and transform into H_ad. Meanwhile the sea urchin-like structure of N-HPCSs facilitates the desorption of H₂. Furthermore, the overpotential showed no obvious decrease in the long-term durability test, which should be attributed to the confinement of Pt nanoparticles by the well-defined pores in N-HPCSs to avoid the aggregation of Pt nanoparticles during long-term testing.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Characterizing the Onset Potential Distribution of Pt/C Catalyst Deposition by a Total Internal Reflection Imaging Method

A catalytic electrode with extraordinary performances for hydrogen evolution reaction (HER) should achieve a low onset potential of the bulk electrode, as well as its uniform distribution. Herein, a total internal reflection imaging (TIRi) method to characterize the onset potential distribution of the catalytic electrode surface is presented. When the potential scans toward negative in a linear sweep voltammetry, the equivalent refractive index of the electrolyte on the electrode surface will decrease due to H₂ microbubbles generation, leading to the increase in optical intensity. Analysis of the relationship between the optical intensity and potential in each region results in the onset potential distribution. The TIRi method reveals poor uniformity and repeatability in the catalytic electrodes which are fabricated by depositing Pt/C catalysts on a porous carbon support with polymer binders (e.g., Nafion). Further electrochemical stability test also shows poor durability, whose HER onset potential deteriorates from the edge to the middle of these catalytic electrodes. The present TIRi method realizes direct visualization of the activity distribution on the bulk electrode surface, which provides a powerful tool for better fabrication and evaluation of large-area HER electrodes in industrial energy devices.

Oxygen Generation Using Catalytic Nano/Micromotors

Gaseous oxygen plays a vital role in driving the metabolism of living organisms and has multiple agricultural, medical, and technological applications. Different methods have been discovered to produce oxygen, including plants, oxygen concentrators and catalytic reactions. However, many such approaches are relatively expensive, involve challenges, complexities in post-production processes or generate undesired reaction products. Catalytic oxygen generation using hydrogen peroxide is one of the simplest and cleanest methods to produce oxygen in the required quantities. Chemically powered micro/nanomotors, capable of self-propulsion in liquid media, offer convenient and economic platforms for on-the-fly generation of gaseous oxygen on demand. Micromotors have opened up opportunities for controlled oxygen generation and transport under complex conditions, critical medical diagnostics and therapy. Mobile oxygen micro-carriers help better understand the energy transduction efficiencies of micro/nanoscopic active matter by careful selection of catalytic materials, fuel compositions and concentrations, catalyst surface curvatures and catalytic particle size, which opens avenues for controllable oxygen release on the level of a single catalytic microreactor. This review discusses various micro/nanomotor systems capable of functioning as mobile oxygen generators while highlighting their features, efficiencies and application potentials in different fields.

Design of Aligned Porous Carbon Films with Single-Atom Co-N-C Sites for High-Current-Density Hydrogen Generation

Metal- and nitrogen-doped carbon (M-N-C) materials as a unique class of single-atom catalysts (SACs) have increasingly attracted attention as the replacement of platinum for the hydrogen evolution reaction (HER); however, their employment as HER electrodes at high current densities of industrial level remains a grand challenge. Herein, an aligned porous carbon film embedded with single-atom Co-N-C sites of exceptional activity and stability at high current densities is designed. Within the film, the atomic CoN_x moieties exhibit high intrinsic activity, while the multiscale porosity of the carbon frameworks with vertically aligned microchannels afford facilitated mass transfer under the conditions of high production rate and ultrathick electrodes. Moreover, the superwetting properties of the film promote electrolyte wetting and ensure the timely removal of the evolving H₂ gas bubbles. The as-designed film can work as an efficient HER electrode to deliver 500 and 1000 mA cm^-2 in acid at overpotentials of 272 and 343 mV, respectively, and can operate uninterruptedly and stably at 1000 mA cm^-2 for at least 32 h under static conditions. These findings pave the road toward the rational design of SACs with improved activity and stability at high current densities in gas-evolving electrocatalytic processes.

Energy-saving hydrogen production by chlorine-free hybrid seawater splitting coupling hydrazine degradation

Seawater electrolysis represents a potential solution to grid-scale production of carbon-neutral hydrogen energy without reliance on freshwater. However, it is challenged by high energy costs and detrimental chlorine chemistry in complex chemical environments. Here we demonstrate chlorine-free hydrogen production by hybrid seawater splitting coupling hydrazine degradation. It yields hydrogen at a rate of 9.2 mol h-1 gcat-1 on NiCo/MXene-based electrodes with a low electricity expense of 2.75 kWh per m3 H2 at 500 mA cm-2 and 48% lower energy equivalent input relative to commercial alkaline water electrolysis. Chlorine electrochemistry is avoided by low cell voltages without anode protection regardless Cl- crossover. This electrolyzer meanwhile enables fast hydrazine degradation to ~3 ppb residual. Self-powered hybrid seawater electrolysis is realized by integrating low-voltage direct hydrazine fuel cells or solar cells. These findings enable further opportunities for efficient conversion of ocean resources to hydrogen fuel while removing harmful pollutants.

Periodic nanostructures: preparation, properties and applications

Periodic nanostructures, a group of nanomaterials consisting of single or multiple nano units/components periodically arranged into ordered patterns (e.g., vertical and lateral superlattices), have attracted tremendous attention in recent years due to their extraordinary physical and chemical properties that offer a huge potential for a multitude of applications in energy conversion, electronic and optoelectronic applications. Recent advances in the preparation strategies of periodic nanostructures, including self-assembly, epitaxy, and exfoliation, have paved the way to rationally modulate their ferroelectricity, superconductivity, band gap and many other physical and chemical properties. For example, the recent discovery of superconductivity observed in "magic-angle" graphene superlattices has sparked intensive studies in new ways, creating superlattices in twisted 2D materials. Recent development in the various state-of-the-art preparations of periodic nanostructures has created many new ideas and findings, warranting a timely review. In this review, we discuss the current advances of periodic nanostructures, including their preparation strategies, property modulations and various applications.

A Bubble-STORM Approach for Super-Resolved Imaging of Nucleation Sites in Hydrogen Evolution Reactions

Stochastic optical reconstruction microscopy (STORM) is a powerful strategy to achieve super-resolved imaging of biological structures by virtue of the stochastic photoactivation of fluorophores and superlocalization algorithm. Herein, we report a fluorophore-free bubble-STORM approach for super-resolved imaging of nucleation sites in hydrogen evolution reactions (HER). When applying an appropriate pulse potential to the electrode, rapid electro-reduction of protons created a local oversaturation of hydrogen molecules and thus the nucleation of sparsely distributed hydrogen nanobubbles. A surface plasmon resonance microscopy was employed to monitor the process and report the localization of each nanobubble via superlocalization fitting. The withdrawal of electrode potential, or the microconvection, led to the immediate disappearance of nanobubbles and recovered the electrode surface before the next pulse. By repeating the procedures for thousands of cycles, one was able to reconstruct a map of nucleation sites with a spatial resolution beyond the optical diffraction limit. This approach does not require a model fluorogenic reaction or fluorescent labeling to the nanobubbles, thus revealing the intrinsic nucleation sites in the natural states. Our results further indicated the fast growth, coalescence, and detachment behaviors of nanobubbles on a time scale of sub-milliseconds, underscoring the significance of high temporal resolution for studying nanobubble nucleation.

Nanoassembled Interface for Dynamics Tailoring

The properties and performance of solid nanomaterials in heterogeneous chemical reactions are significantly influenced by the interface between the nanomaterial and environment. Oriented tailoring of interfacial dynamics, that is, modifying the shared boundary for mass and energy exchange has become a common goal for scientists. Although researchers have designed and constructed an abundance of nanomaterials with excellent performances for the tailoring of reaction dynamics, a complete understanding of the mechanism of nanomaterial-environment interfacial interaction still remains elusive. To predictively understand the nanomaterial-environment relationship over a wide range of time scale, a deep and dynamic insight is required urgently. In this Account, our recent works including advances in the design and construction of nanoassembled interfaces and understanding the dynamic interaction mechanisms between different combinations of nanoparticle (NP) assembly environment interfaces for tailoring the reaction dynamics.NP assemblies with well-defined structures and compositions are inherently suitable for replacing bulk-type nanomaterials for the research on interfaces. We primarily introduced two most relevant nanoassembled surfaces that were fabricated in our laboratory, namely, ordered self-assembly interface and animate nanoassembled interface. The disordered nanoparticles can be arranged into an ordered superlattice based on the self-assembly method and patterned-assembly method. In addition, we used NPs with flexible properties to construct three-dimensional (3D) animate assemblies. On the basis of a thorough understanding of the structure-property correlation, a series of nanoassembled interfaces with various structures have been developed for practice. In comparison with traditional nanomaterial-environment interfaces, the nanoassembled interfaces can change the mode of contact between the nanomaterial and environment, thereby maximizing the number of active sites and driving interferent/product off the nanoassembled interface. The geometry, porosity, and deformable/motional properties in the nanoassembled interface can be applied to enhance the mass transfer dynamics in the chemical reaction. Moreover, the nanoassembled interface can be used to strengthen the affinity between the NP assemblies and targets, thereby enhancing the adsorption efficiency. As shown in these examples, the nanoassembled interface can effectively change the speed, intensity, and mode of interactions between the NP assemblies and environment in spatiotemporal scales.The overall performance of the interfacial dynamics can be improved by the nanoassembled interface, thereby facilitating practical application in flowing systems. We have extended the applications of nanoassembled interfaces from simple adsorption to complex reactions in flowing systems, including in vivo magnetic resonance imaging, electrocatalytic gas evolution reaction, bacterial capture, sensing of exhaled volatile organic compounds, and heterogeneous catalysis. Our current endeavors to explore the applicability of animate nanoassembled interfaces for dynamic tailoring have widened the scope of research, and attempts to construct intelligent interfaces for applications are underway.

Highly efficient oxygen evolution reaction via facile bubble transport realized by three-dimensionally stack-printed catalysts

Despite highly promising characteristics of three-dimensionally (3D) nanostructured catalysts for the oxygen evolution reaction (OER) in polymer electrolyte membrane water electrolyzers (PEMWEs), universal design rules for maximizing their performance have not been explored. Here we show that woodpile (WP)-structured Ir, consisting of 3D-printed, highly-ordered Ir nanowire building blocks, improve OER mass activity markedly. The WP structure secures the electrochemically active surface area (ECSA) through enhanced utilization efficiency of the extended surface area of 3D WP catalysts. Moreover, systematic control of the 3D geometry combined with theoretical calculations and various electrochemical analyses reveals that facile transport of evolved O₂ gas bubbles is an important contributor to the improved ECSA-specific activity. The 3D nanostructuring-based improvement of ECSA and ECSA-specific activity enables our well-controlled geometry to afford a 30-fold higher mass activity of the OER catalyst when used in a single-cell PEMWE than conventional nanoparticle-based catalysts.

Improved design of three-dimensionally nanostructured catalysts for oxygen evolution reaction (OER) can play a key role in maximizing the catalytic performance. Here, the authors show that woodpile-structured iridium consisting of 3D-printed, highly-ordered nanowire building blocks significantly improve OER mass activity.

Efficient Oxygen Evolution and Gas Bubble Release Achieved by a Low Gas Bubble Adhesive Iron-Nickel Vanadate Electrocatalyst

Surface chemistry is a pivotal prerequisite besides catalyst composition toward advanced water electrolysis. Here, an evident enhancement of the oxygen evolution reaction (OER) is demonstrated on a vanadate-modified iron-nickel catalyst synthesized by a successive ionic layer adsorption and reaction method, which demonstrates ultralow overpotentials of 274 and 310 mV for delivering large current densities of 100 and 400 mA cm^-2 , respectively, in 1 m KOH, where vigorous gas bubble evolution occurs. Vanadate modification augments the OER activity by i) increasing the electrochemical surface area and intrinsic activity of the active sites, ii) having an electronic interplay with Fe and Ni catalytic centers, and iii) inducing a high surface wettability and a low-gas bubble-adhesion for accelerated mass transport and gas bubble dissipation at large current densities. Ex situ and operando Raman study reveals the structural evolution of β-NiOOH and γ-FeOOH phases during the OER through vanadate-active site synergistic interactions. Operando dynamic specific resistance measurement evidences an accelerated gas bubble dissipation by a significant decrease in the variation of the interfacial resistance during the OER for the vanadate-modified surface. Achievement of a high catalytic turnover of 0.12 s^-1 suggests metallic oxo-anion modification as a versatile catalyst design strategy for advanced water oxidation.

Molybdenum-titanium oxo-cluster, an efficient electrochemical catalyst for the facile preparation of black titanium dioxide film

Black-TiO₂ has become increasingly interesting as a promising photoactive material. Most of the preparations for black-TiO₂ involve either high temperature calcination, plasma, lengthy chemical reactions or dealing with dangerous or toxic chemicals. We found, by accident, that Mo-Ti oxo-clusters are efficient catalysts for the hydrogenation of a TiO₂ electrode to black-TiO₂ at room temperature. A series of Mo-Ti oxo-clusters, [Ti₄Mo₄O₁₀(OR)₁₄(X-BA)₂] (BA = benzoate, X = H (1), F (2), Cl (3), and Br (4)), were prepared and were characterized by crystallography. They have a Mo₄Ti₄ structure with Mo(v)-Mo(v) metal-metal interactions. The activated hydrogen (H*) generated by electrochemically catalytic water splitting turns the TiO₂ electrode to black-TiO₂ at room temperature, due to the reduction of Ti(iv) to H⁺Ti(iii). The potentials applied for water reduction must generally be higher than the overpotential at the TiO₂ electrode (-1.0 V vs. RHE). In this work, the onset potential of hydrogen evolution significantly decreased to -0.1 V vs. RHE. Using this blackened 1-TiO₂ electrode, the effective electrochemical catalytic degradation of a dye was examined in comparison with the degradation using the white TiO₂ electrode. This work provides a method for the facile preparation of a black-TiO₂ film, and is a step forward in black-TiO₂ research.

Self-assembly of anisotropic nanoparticles into functional superstructures

Self-assembly of colloidal nanoparticles (NPs) into superstructures offers a flexible and promising pathway to manipulate the nanometer-sized particles and thus make full use of their unique properties. This bottom-up strategy builds a bridge between the NP regime and a new class of transformative materials across multiple length scales for technological applications. In this field, anisotropic NPs with size- and shape-dependent physical properties as self-assembly building blocks have long fascinated scientists. Self-assembly of anisotropic NPs not only opens up exciting opportunities to engineer a variety of intriguing and complex superlattice architectures, but also provides access to discover emergent collective properties that stem from their ordered arrangement. Thus, this has stimulated enormous research interests in both fundamental science and technological applications. This present review comprehensively summarizes the latest advances in this area, and highlights their rich packing behaviors from the viewpoint of NP shape. We provide the basics of the experimental techniques to produce NP superstructures and structural characterization tools, and detail the delicate assembled structures. Then the current understanding of the assembly dynamics is discussed with the assistance of in situ studies, followed by emergent collective properties from these NP assemblies. Finally, we end this article with the remaining challenges and outlook, hoping to encourage further research in this field.

Effect of structure: A new insight into nanoparticle assemblies from inanimate to animate

Nanoparticle (NP) assemblies are among the foremost achievements of nanoscience and nanotechnology because their interparticle interactions overcome the weaknesses displayed by individual NPs. However, previous studies have considered NP assemblies as inanimate, which had led to their dynamic properties being overlooked. Animate properties, i.e., those mimicking biological properties, endow NP ensembles with unique and unexpected functionalities for practical applications. In this critical review, we highlight recent advances in our understanding of the properties of NP assemblies, particularly their animate properties. Key examples are used to illustrate critical concepts, and special emphasis is placed on animate property-dependent applications. Last, we discuss the barriers to further advances in this field.

Synergistically enhanced hydrogen evolution reaction by ruthenium nanoparticles dispersed on N-doped carbon hollow nanospheres

Ultrafine Ru nanoparticles dispersed on 3D N-doped carbon hollow nanospheres were firstly prepared by a feasible templating strategy.