Atomic dynamics of electrified solid-liquid interfaces in liquid-cell TEM.

Electrified solid-liquid interfaces (ESLIs) play a key role in various electrochemical processes relevant to energy1-5, biology6 and geochemistry7. The electron and mass transport at the electrified interfaces may result in structural modifications that markedly influence the reaction pathways. For example, electrocatalyst surface restructuring during reactions can substantially affect the catalysis mechanisms and reaction products1-3. Despite its importance, direct probing the atomic dynamics of solid-liquid interfaces under electric biasing is challenging owing to the nature of being buried in liquid electrolytes and the limited spatial resolution of current techniques for in situ imaging through liquids. Here, with our development of advanced polymer electrochemical liquid cells for transmission electron microscopy (TEM), we are able to directly monitor the atomic dynamics of ESLIs during copper (Cu)-catalysed CO2 electroreduction reactions (CO2ERs). Our observation reveals a fluctuating liquid-like amorphous interphase. It undergoes reversible crystalline-amorphous structural transformations and flows along the electrified Cu surface, thus mediating the crystalline Cu surface restructuring and mass loss through the interphase layer. The combination of real-time observation and theoretical calculations unveils an amorphization-mediated restructuring mechanism resulting from charge-activated surface reactions with the electrolyte. Our results open many opportunities to explore the atomic dynamics and its impact in broad systems involving ESLIs by taking advantage of the in situ imaging capability.

Visualizing Facets Asymmetry Induced Directional Movement of Cadmium Chloride Nanomotor.

Nanomotors in solution have many potential applications. However, it has been a significant challenge to realize the directional motion of nanomotors. Here, we report cadmium chloride tetrahydrate (CdCl2·4H2O) nanomotors with remarkable directional movement under electron beam irradiation. Using in situ liquid phase transmission electron microscopy, we show that the CdCl2·4H2O nanoparticle with asymmetric surface facets moves through the liquid with the flat end in the direction of motion. As the nanomotor morphology changes, the speed of movement also changes. Finite element simulation of the electric field and fluid velocity distribution around the nanomotor assists the understanding of ionic self-diffusiophoresis as a driving force for the nanomotor movement; the nanomotor generates its own local ion concentration gradient due to different chemical reactivities on different facets.

Strong Structural and Electronic Coupling in Metavalent PbS Moiré Superlattices.

Moiré superlattices are twisted bilayer materials in which the tunable interlayer quantum confinement offers access to new physics and novel device functionalities. Previously, moiré superlattices were built exclusively using materials with weak van der Waals interactions, and synthesizing moiré superlattices with strong interlayer chemical bonding was considered to be impractical. Here, using lead sulfide (PbS) as an example, we report a strategy for synthesizing moiré superlattices coupled by strong chemical bonding. We use water-soluble ligands as a removable template to obtain free-standing ultrathin PbS nanosheets and assemble them into direct-contact bilayers with various twist angles. Atomic-resolution imaging shows the moiré periodic structural reconstruction at the superlattice interface due to the strong metavalent coupling. Electron energy loss spectroscopy and theoretical calculations collectively reveal the twist-angle-dependent electronic structure, especially the emergent separation of flat bands at small twist angles. The localized states of flat bands are similar to well-arranged quantum dots, promising an application in devices. This study opens a new door to the exploration of deep energy modulations within moiré superlattices alternative to van der Waals twistronics.

Hollow Multishell-Structured TiO2/MAPbI3 Composite Improves Charge Utilization for Visible-Light Photocatalytic Hydrogen Evolution.

Interfacial photogenerated charge separation and transport have demonstrated great influence on photocatalytic performance. Herein, the composite photocatalysts of methylammonium lead iodide perovskite (MAPbI3) in TiO2 with a hollow multishell structure (HoMS) are designed and synthesized. The results indicate that the heterogeneous interface within the MAPbI3/Pt/TiO2-HoMS can help enhance the separation of photogenerated charges. HoMSs assembled with multiple shells can not only support large surfaces available for building a heterogeneous interface and photocatalytic reactions but also improve the light absorption capability of photocatalysts. Besides, the thin shell structure can also reduce the transmission distance of carriers so as to hinder charge recombination and improve charge utilization. As a result, samples of MAPbI3/Pt/triple-shelled TiO2 hollow structure displayed a H2 yield of 6856.2 µmol h-1 g-1 under visible light, which is greatly better than that of bare MAPbI3 (268.6 µmol h-1 g-1).

Heterogeneous Hollow Multi-Shelled Structures with Amorphous-Crystalline Outer-Shells for Sequential Photoreduction of CO2.

Constructing delicate nano-/microreactors with tandem active sites in hierarchical architectures is a promising strategy for designing photocatalysts to realize the challenging but attractive CO2 reduction. Herein, hollow multi-shelled structure (HoMS) based microreactors with spatial ordered hetero-shells are fabricated, which achieve two-step CO2 -to-CH4 photoreduction. The multiple inner CeO2 shells increase the number of active catalytic sites to ensure efficient first-step reaction for generating CO, along with enriching the local CO concentration. The second-step CO-to-CH4 reaction is consequently induced by amorphous TiO2 (A-TiO2 ) composites on the adjacent outer-most shell, thus realizing the CO2 -to-CH4 conversion capability using one CeO2 @CeO2 /A-TiO2 HoMS. In-depth explorations in the microreactors provide compositional, structural, and interfacial guidance for engineering HoMS-based microreactors with temporally-spatially ordered shells toward efficient tandem catalysis.

Hollow Multishelled Structured SrTiO3 with La/Rh Co-Doping for Enhanced Photocatalytic Water Splitting under Visible Light.

La- and Rh-co-doped SrTiO3 (STO:La/Rh) hollow multishelled structures (HoMSs) are fabricated by adding La3+ and Rh3+ ions during the hydrothermal process of converting TiO2 HoMSs to STO HoMSs. STO:La/Rh HoMSs have successfully expanded the light absorption edge to 520 nm. Accompanied with the benefits of the unique hierarchical structure and relatively thin shells, STO:La/Rh HoMSs exhibit elevated light-harvesting capacity and charge separation efficiency. Compared with STO:La/Rh nanoparticles (NPs), STO:La/Rh HoMSs demonstrate enhanced photocurrent response, photocatalytic hydrogen evolution activity, and the quantum efficiency. Moreover, overall water splitting is realized by a Z-scheme system combining STO:La/Rh HoMSs with BiVO4 (BVO) nanosheets with 1 wt% Pt as the co-catalyst. Steady evolution of hydrogen and oxygen is performed under both visible light and simulated sunlight irradiation. The solar-to-hydrogen efficiency of double-shelled STO:La/Rh HoMS-BVO photocatalysts reaches 0.08%, which is twofold higher than STO:La/Rh NP-BVO photocatalysts.

General Synthesis of Multiple-Cores@Multiple-Shells Hollow Composites and Their Application to Lithium-Ion Batteries.

Rational nanostructure design has proved fruitful in addressing the bottlenecks of diverse fields. Especially hollow multi-shelled structures (HoMS) have stood out due to their temporal-spatial ordering mass transfer and buffering effect. Localizing multiple cores in a HoMS is highly desired, which could endow it with more fascinating properties. However, such a structure has been barely reported due to the highly challenging fabrication. Here, we develop a controllable synthesis strategy to realize such a structure, which is applicable for diverse cores and shells. Additionally, cores and shells could be tuned to be homogeneous or heterogeneous, with the core and shell number well controlled. In situ TEM analysis verifies that the inner shell confines the expansion orientation of cores, while the outer shell maintains a stable interface. In addition to energy storage, such structure is also promising for multi-drug co-delivery and sequential responsive release as well as tandem catalysis applications.

Delicate Control on the Shell Structure of Hollow Spheres Enables Tunable Mass Transport in Water Splitting.

In the study of structure-property relationships for rational materials design, hollow multishell structures (HoMSs) have attracted tremendous attention owing to the optimal balance between mass transfer and surface exposure. Considering the shell structure can significantly affect the properties of HoMSs, in this paper, we provide a novel one-step strategy to continually regulate the shell structures of HoMSs. Through a simple phosphorization process, we can effectively modify the shell from solid to bubble-like and even duplicate the shells with a narrow spacing. Benefitting from the structure merits, the fabricated CoP HoMSs with close duplicated shells can promote gas release owing to the unbalanced Laplace pressure, while accelerating liquid transfer for enhanced capillary force. It can provide effective channels for water and gas and thus exhibits a superior electrocatalytic performance in the hydrogen and oxygen evolution reaction.

In Situ Phosphatizing of Triphenylphosphine Encapsulated within Metal-Organic Frameworks to Design Atomic Co1-P1N3 Interfacial Structure for Promoting Catalytic Performance.

The engineering coordination environment offers great opportunity in performance tunability of isolated metal single-atom catalysts. For the most popular metal-Nx (MNx) structure, the replacement of N atoms by some other atoms with relatively weak electronegativity has been regarded as a promising strategy for optimizing the coordination environment of an active metal center and promoting its catalytic performance, which is still a challenge. Herein, we proposed a new synthetic strategy of an in situ phosphatizing of triphenylphosphine encapsulated within metal-organic frameworks for designing atomic Co1-P1N3 interfacial structure, where a cobalt single atom is costabilized by one P atom and three N atoms (denoted as Co-SA/P-in situ). In the acidic media, the Co-SA/P-in situ catalyst with Co1-P1N3 interfacial structure exhibits excellent activity and durability for the hydrogen evolution reaction (HER) with a low overpotential of 98 mV at 10 mA cm-2 and a small Tafel slope of 47 mV dec-1, which are greatly superior to those of catalyst with Co1-N4 interfacial structure. We discover that the bond-length-extended high-valence Co1-P1N3 atomic interface structure plays a crucial role in boosting the HER performance, which is supported by in situ X-ray absorption fine structure (XAFS) measurements and density functional theory (DFT) calculation. We hope this work will promote the development of high performance metal single-atom catalysts.

Hollow Multishelled Structures for Promising Applications: Understanding the Structure-Performance Correlation.

The unique structural features of hollow multishelled structures (HoMSs) endow them with abundant beneficial physicochemical properties including high surface-to-volume ratio, low density, short mass transport length, and high loading capacity. As a result, HoMSs have been considered as promising candidates for various application areas including energy storage, electromagnetic wave (EW) absorption, catalysis, sensors, drug delivery, etc. However, for a long time, the general and controllable synthesis of HoMSs has remained a great challenge using conventional soft-templating or hierarchical self-assembly methods, which severely limits the development of HoMSs. Fortunately, the sequential templating approach (STA), which was first reported by our group and further developed by others, has been proven to be a versatile method for HoMS fabrication. By using the STA and through accurate physical and chemical manipulation of the synthesis conditions, the diversity of the HoMS family has been enriched in both compositional and geometrical aspects. Benefiting from the flourishing of synthetic methodology, various HoMSs have been fabricated and showed application prospect in diverse areas. However, the structure-performance correlation remained obscure, which hinders the design of optimal HoMSs to achieve the best application performance. This Account aims to explore the correlation between HoMS structural characteristics and their application performance. We first briefly summarize the achievements in the compositional and geometrical manipulation of HoMSs by physically and chemically tuning the synthesis process. Then, we systematically discuss the effect of structural engineering on optimizing performance in various application areas, especially for energy storage, EW absorption, catalysis, sensors, and drug delivery. Specifically, HoMSs with multiple thin shells can provide numerous active sites for energy storage, leading to a higher volumetric energy density than their single-shelled counterparts. The high shell porosity permits electrolyte access to the interior of HoMSs, along with shortened mass transport path through the thin shells, resulting in a high power density. The adequate inner cavity effectively buffers the ion-insertion strain, leading to prolonged cycling stability. For EW absorption, HoMSs with high surface-to-volume ratio can provide many sites for EW-sensitive material loading. The multiple separated shells with small intershell space enable multiple EW reflection and scattering, thus improving EW absorption efficiency. For catalysis and sensors, the increased reaction sites along with the facilitated transport of reactants and products can enhance the activity and sensitivity. The selectivity can be improved by optimizing the pore structure and hydrophobic or hydrophilic properties of the shells. Also the stability is improved with inner shells being protected by exterior ones. For drug delivery, the increased exposed sites and the inner cavity improve the drug loading capacity. The adjustable pore structure along with accurately designed shell composition leads to well-targeted drug release responding to different stimuli at different targeting sites. The multiple separated shells endow HoMSs with sustained drug release step-by-step from inside to outside. These in-depth understandings on the structure-performance correlation can guide the design of ideal HoMSs to satisfy the specific requirements for different application areas, thus further improving the application performance and expanding the HoMSs family.

Lattice Distortion in Hollow Multi-Shelled Structures for Efficient Visible-Light CO2 Reduction with a SnS2 /SnO2 Junction.

Precise control of the micro-/nanostructures of nanomaterials, such as hollow multi-shelled structures (HoMSs), has shown its great advantages in various applications. Now, the crystal structure of building blocks of HoMSs are controlled by introducing the lattice distortion in HoMSs, for the first time. The lattice distortion located at the nanoscale interface of SnS2 /SnO2 can provide additional active sites, which not only provide the catalytic activity under visible light but also improve the separation of photoexcited electron-hole pairs. Combined with the efficient light utilization, the natural advantage of HoMSs, a record catalytic activity was achieved in solid-gas system for CO2 reduction, with an excellent stability and 100 % CO selectivity without using any sensitizers or noble metals.

Hollow Multi-Shelled Structures of Co3O4 Dodecahedron with Unique Crystal Orientation for Enhanced Photocatalytic CO2 Reduction.

Structure and facet control are considered to be effective routes to enhance catalytic performance. We successfully synthesized hollow multi-shelled structures (HoMSs) of a Co3O4 dodecahedron by adopting metal-organic frameworks (MOFs) as templates and using the sequential templating approach (STA). Importantly, owing to the topological arrangement of metal atoms in MOFs, the Co3O4 nanocrystals in HoMSs are assembled in the desired orientation, forming a unique shell with dominant exposure of (111) facets. This process is defined as "genetic inheritance" in this work. In addition, these exposed facets possess high activity for photocatalytic CO2 reduction. Adding this to the properties inherited from HoMSs, i.e., multiple interfaces and strong solar light harvesting, these Co3O4 HoMSs present high catalytic activity for CO2 photoreduction. The catalytic activity of quadruple-shelled (QS) Co3O4 HoMSs was about 5 and 3 times higher than that of Co3O4 nanoparticles and Co3O4 HoMSs without facet control, respectively.

A Rutile TiO2 Electron Transport Layer for the Enhancement of Charge Collection for Efficient Perovskite Solar Cells.

Interfacial charge collection efficiency has demonstrated significant effects on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, crystalline phase-dependent charge collection is investigated by using rutile and anatase TiO2 electron transport layer (ETL) to fabricate PSCs. The results show that rutile TiO2 ETL enhances the extraction and transportation of electrons to FTO and reduces the recombination, thanks to its better conductivity and improved interface with the CH3 NH3 PbI3 (MAPbI3 ) layer. Moreover, this may be also attributed to the fact that rutile TiO2 has better match with perovskite grains, and less trap density. As a result, comparing with anatase TiO2 ETL, MAPbI3 PSCs with rutile TiO2 ETL delivers significantly enhanced performance with a champion PCE of 20.9 % and a large open circuit voltage (VOC ) of 1.17 V.

Constructing SrTiO3 -TiO2 Heterogeneous Hollow Multi-shelled Structures for Enhanced Solar Water Splitting.

Constructing hollow multi-shelled structures (HoMSs) has a significant effect on promoting light absorption property of catalysts and enhancing their performance in solar energy conversion applications. A facile hydrothermal method is used to design the SrTiO3 -TiO2 heterogeneous HoMSs by hydrothermal crystallization of SrTiO3 on the surface of the TiO2 HoMSs, which will realize a full coverage of SrTiO3 on the TiO2 surface and construct the SrTiO3 /TiO2 junctions. The broccoli-like SrTiO3 -TiO2 heterogeneous HoMSs exhibited a fourfold higher overall water splitting performance of 10.6 µmol h-1 for H2 production and 5.1 µmol h-1 for O2 evolution than that of SrTiO3 nanoparticles and the apparent quantum efficiency (AQE) of 8.6 % at 365 nm, which can be mainly attributed to 1) HoMS increased the light absorption ability of the constructed photocatalysts and 2) the SrTiO3 -TiO2 junctions boosted the separation efficiency of the photogenerated charge carriers.

Hollow Multi-Shelled Structure with Metal-Organic-Framework-Derived Coatings for Enhanced Lithium Storage.

Herein, we present heterogeneous hollow multi-shelled structures (HoMSs) prepared by exploiting the properties of the metal-organic framework (MOFs) casing. Through accurately controlling the transformation of MOF layer into different heterogeneous casings, we can precisely design HoMSs of SnO2 @Fe2 O3 (MOF) and SnO2 @FeOx -C(MOF), which not only retain properties of the original SnO2 -HoMSs, but also structural information from the MOFs. Tested as anode materials in LIBs, SnO2 @Fe2 O3 (MOF)-HoMSs demonstrate superior lithium-storage capacity and cycling stability to the original SnO2 -HoMSs, which can be attributed to the topological features from the MOF casing. Making a sharp contrast to the electrodes of SnO2 @Fe2 O3 (particle)-HoMSs fabricated by hydrothermal method, the capacity retention after 100 cycles for the SnO2 @Fe2 O3 (MOF)-HoMSs is about eight times higher than that of the SnO2 @Fe2 O3 (particle)-HoMS.

Quantitative Study of Charge Carrier Dynamics in Well-Defined WO3 Nanowires and Nanosheets: Insight into the Crystal Facet Effect in Photocatalysis.

Photocatalysts with different morphologies and specific exposed facets usually exhibit distinguished activities. Previous researches have focused on revealing the essence of the facet effect in photocatalysis; however, quantitative analyses on the differences of carrier dynamic between different facets are scarce. Herein, we successfully synthesized WO3 nanosheets and nanowires with dominant exposed facets of {001} and {110}, respectively. The lower hole effective mass on {110} (0.94 m0) than on {001} (1.28 m0) calculated by density functional theory leads to the higher hole mobility on {110} (4.92 cm2 V-1 s-1) than on {001} (3.14 cm2 V-1 s-1). Combined with the Einstein equation and the lifetime of the hole, the calculated hole diffusion length on {110} (74.8 nm) is larger than on {001} (53.4 nm). Overall, the lower hole effective mass, higher hole mobility, and greater hole diffusion length on {110} collectively result in a photocatalytic activity on benzyl alcohol oxidation 2.46 times as high as that on {001}.

Metal (Hydr)oxides@Polymer Core-Shell Strategy to Metal Single-Atom Materials.

Preparing metal single-atom materials is currently attracting tremendous attention and remains a significant challenge. Herein, we report a novel core-shell strategy to synthesize single-atom materials. In this strategy, metal hydroxides or oxides are coated with polymers, followed by high-temperature pyrolysis and acid leaching, metal single atoms are anchored on the inner wall of hollow nitrogen-doped carbon (CN) materials. By changing metal precursors or polymers, we demonstrate the successful synthesis of different metal single atoms dispersed on CN materials (SA-M/CN, M = Fe, Co, Ni, Mn, FeCo, FeNi, etc.). Interestingly, the obtained SA-Fe/CN exhibits much higher catalytic activity for hydroxylation of benzene to phenol than Fe nanoparticles/CN (45% vs 5% benzene conversion). First-principle calculations further reveal that the high reactivity originates from the easier formation of activated oxygen species at the single Fe site. Our methodology provides a convenient route to prepare a variety of metal single-atom materials representing a new class of catalysts.

Rational Design of Single Molybdenum Atoms Anchored on N-Doped Carbon for Effective Hydrogen Evolution Reaction.

The highly efficient electrochemical hydrogen evolution reaction (HER) provides a promising pathway to resolve energy and environment problems. An electrocatalyst was designed with single Mo atoms (Mo-SAs) supported on N-doped carbon having outstanding HER performance. The structure of the catalyst was probed by aberration-corrected scanning transmission electron microscopy (AC-STEM) and X-ray absorption fine structure (XAFS) spectroscopy, indicating the formation of Mo-SAs anchored with one nitrogen atom and two carbon atoms (Mo1 N1 C2 ). Importantly, the Mo1 N1 C2 catalyst displayed much more excellent activity compared with Mo2 C and MoN, and better stability than commercial Pt/C. Density functional theory (DFT) calculation revealed that the unique structure of Mo1 N1 C2 moiety played a crucial effect to improve the HER performance. This work opens up new opportunities for the preparation and application of highly active and stable Mo-based HER catalysts.

Low-temperature solution-processed tin oxide as an alternative electron transporting layer for efficient perovskite solar cells.

Lead halide perovskite solar cells with the high efficiencies typically use high-temperature processed TiO2 as the electron transporting layers (ETLs). Here, we demonstrate that low-temperature solution-processed nanocrystalline SnO2 can be an excellent alternative ETL material for efficient perovskite solar cells. Our best-performing planar cell using such a SnO2 ETL has achieved an average efficiency of 16.02%, obtained from efficiencies measured from both reverse and forward voltage scans. The outstanding performance of SnO2 ETLs is attributed to the excellent properties of nanocrystalline SnO2 films, such as good antireflection, suitable band edge positions, and high electron mobility. The simple low-temperature process is compatible with the roll-to-roll manufacturing of low-cost perovskite solar cells on flexible substrates.

The Non-Denatured Processing of Brasenia schreberi Mucilage-Characteristics of Hydrodynamic Properties and the Effect on In Vivo Functions.

Food non-denatured processes, such as freeze-drying and grinding, are commonly applied to raw materials with good bioactive functions. Although the functional components are maintained, whether structural and physical changes impact the in vivo function is often ignored in practical situations. Brasenia schreberi mucilage (BSM) has a significant alleviation effect on DSS-induced colitis. This work focused on the influence of non-denatured manufacture on the colonic benefits of BSM-based products. First, three forms of products including fresh mucilage (FM), freeze-dried products (FS), and freeze-dried powder (FP) were prepared. Then, their in vitro physiochemical properties were compared, analyzing their influence on the gut inflammation degree, microbial composition, and SCFA production in mice. The results suggested that the water retention rate of FS and FP was decreased to 34.59 ± 3.85%, and 9.93 ± 1.76%. The viscosity of FM, FS, and FP was 20.14 Pa∙s, 4.92 Pa∙s, and 0.41 Pa∙s, respectively. The freeze-drying and grinding process also damaged the lamellar microstructure of BSM. Then, animal tests showed that colitis mice intervened with FM, FS, and FP had disease activity scores of 2.03, 3.95, and 4.62. Meanwhile, FM notably changed the gut microbial composition and significantly increased propionate and butyrate levels. It seemed that the distinct colitis alleviation efficacy of BSM-based products is attributed to different hydrodynamic properties in the gut. FM had relatively higher viscosity and correspondingly high nutritional density in the gut lumen, which stimulates Firmicutes growth and promotes butyrate production, and thereby exhibited the best efficiency on protecting from colitis.