High-angle Annular Dark-field Scanning Research Articles

Interface tomography of GaInAs/AlInAs quantum cascade laser active regions

We present a high-resolution electron tomography study of buried ultra-thin layers and their interfaces from the active region of a (Ga,In)As/(Al,In)As quantum cascade laser (QCL) test structure. Using a high-angle annular dark-field scanning transmission electron microscopy image series, a three-dimensional (3D) reconstruction of the complex layer structure is obtained. From this 3D information, we determine quantitative values for the chemical width and, simultaneously and independently, the root mean square roughness (rms) and lateral correlation length of the individual interfaces of a cascade using topographic height maps. The interfacial widths are comparably small for all interfaces within a cascade and the layer thicknesses show only a small standard deviation of less than one monolayer. The rms roughness is systematically lower at the direct (Ga,In)As-on-(Al,In)As interface compared to its inverse. In addition, using the one-dimensional height–height correlation function, different lateral correlation lengths along the two perpendicular in-plane directions are detected indicating a distinct anisotropy of the interface morphology. Our accurate and comprehensive results on the QCL test structure will serve as feedback to evaluate the growth process and help to assess the performance of corresponding future laser devices in detail.

Controlled synthesis of Pd–Ag nanowire networks with high-density defects as highly efficient electrocatalysts for methanol oxidation reaction

Pd–Ag bimetallic catalysts containing different molar ratios of Pd/Ag (30/70, 50/50, and 70/30) were prepared using a facile visible-light-assisted liquid-phase method. The samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, selected-area electron diffraction, and inductively coupled plasma optical emission spectroscopy. Cyclic voltammetry and chronoamperometry measurements were used to evaluate their electrocatalytic properties toward the methanol oxidation reaction (MOR) in alkaline media. The alloyed Pd50Ag50 nanowire network with the optimized molar ratio, which had abundant defects (grain boundaries, lattice dislocations, lattice expansion, and distortion), provided more active sites for the MOR under alkaline conditions. The onset potential of the MOR on the Pd50Ag50-loaded glassy carbon electrode (GCE) was more negative than those on the Pd30Ag70, Pd70Ag30, and pure Pd electrodes. The Pd50Ag50 catalyst showed the best catalytic activity (2853 mA mgPd−1/6.85 mA cmPd−2) compared with those of the pure Pd catalyst (320 mA mgPd−1/2.68 mA cmPd−2) and other samples. This sample also exhibited a high tolerance (ib/if = 0.19) to the poisoning species and long stability (only lost 7.64 % after 500 cycles). The alloyed Pd50Ag50 network is a promising catalyst for the MOR. This work may be extended to construct advanced Pd–M (M = other metals) catalysts for fuel cell applications by surface defect engineering.

Bifunctional catalytic degradation of diclofenac over Cu–Pd co-modified sponge iron-based trimetal: Parameter optimization

Currently, the pharmaceutical and personal care products (PPCPs) have posed great challenge to advanced oxidation techniques (AOTs). In this study, we decorated sponge iron (s-Fe0) with Cu and Pd (s-Fe0-Cu-Pd) and further optimized the synthesis parameters with a response surface method (RSM) to rapidly degrade diclofenac sodium (DCF). Under the RSM-optimized conditions of Fe: Cu: Pd = 100: 4.23: 0.10, initial solution pH of 5.13, and input dosage of 38.8 g/L, 99% removal of DCF could be obtained after 60 min of reaction. Moreover, the morphological structure of trimetal was characterized with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS). Electron spin resonance (ESR) signals have also been applied to capture reactive hydrogen atoms (H*), superoxygen anions, hydroxyl radicals, and single state oxygen (1O2). Furthermore, the variations of DCF and its selective degradation products over a series of s-Fe0-based bi(tri)metals have been compared. Additionally, the degradation mechanism of DCF has also been explored. To our best knowledge, this is the first report revealing the selective dechlorination of DCF with low toxicity over Pd–Cu co-doped s-Fe0 trimetal.

Boosting the Catalytic Performance of AuAg Alloyed Nanoparticles Grafted on MoS2 Nanoflowers through NIR-Induced Light-to-Thermal Energy Conversion.

MoS2 nanoflowers (NFs) obtained through a hydrothermal approach were used as the substrate for the deposition of tiny spherical bimetallic AuAg or monometallic Au nanoparticles (NPs), leading to novel photothermal-assisted catalysts with different hybrid nanostructures and showing improved catalytic performance under NIR laser irradiation. The catalytic reduction of pollutant 4-nitrophenol (4-NF) to the valuable product 4-aminophenol (4-AF) was evaluated. The hydrothermal synthesis of MoS2 NFs provides a material with a broad absorption in the Vis-NIR region of the electromagnetic spectrum. The in situ grafting of alloyed AuAg and Au NPs of very small size (2.0-2.5 nm) was possible through the decomposition of organometallic complexes [Au2Ag2(C6F5)4(OEt2)2]n and [Au(C6F5)(tht)] (tht = tetrahydrothiophene) using triisopropilsilane as reducing agent, leading to nanohybrids 1-4. The new nanohybrid materials display photothermal properties arising from NIR light absorption of the MoS2 NFs component. The AuAg-MoS2 nanohybrid 2 showed excellent photothermal-assisted catalytic activity for the reduction of 4-NF, which is better than that of the monometallic Au-MoS2 nanohybrid 4. The obtained nanohybrids were characterised by transmission electron microscopy (TEM), High Angle Annular Dark Field-Scanning Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy (HAADF-STEM-EDS), X-ray photoelectron spectroscopy and UV-Vis-NIR spectroscopy.

Cubic-Shaped and Rod-Shaped YPO4 Nanocrystal-Doped Optical Fibers: Implications for Next Generation of Fiber Lasers

Engineering silica optical fibers by nanoparticle doping is a promising technology that allows the introduction of new functionalities and extends their applicable fields. However, the knowledge gap about the impact of the extreme fabrication temperatures on the nanoparticle features prevents field progress. Herein, we demonstrate that the particularities of fiber fabrication, such as fast-heating rates and quenching heat treatments, can be leveraged to explore unlikely phenomena at the nanoscale under standard laboratory conditions. Tetragonal cubic-shaped and monoclinic rod-shaped YPO4 nanocrystals are in situ nucleated in a silica-based fiber core glass, slightly modified with Ge and P, which shows for the first time, the possibility of doping optical fibers with this type of nanostructures, in terms of shape, composition, and structure of the nanocrystals. Structural and anisotropic differences allow engineering differently their shape and composition in the fiber core by tailoring the drawing temperature, as revealed by a thorough study consisting of scanning electron microscopy (SEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), electron energy loss spectroscopy (EELS), and high-resolution transmission electron microscopy (HRTEM). This work demonstrates, for the first time, the possibility of doping optical fibers fabricated by modified chemical vapor deposition (MCVD) with anisotropic nanostructures, as well as the stability of the monoclinic YPO4 phase. These findings open up new avenues to study shape-dependent properties of rare-earth orthophosphate (REPO4) nanostructures in optical fibers which will allow incorporating unprecedented functionalities and will have an impact in several fields of application, such as fiber lasers and optical fiber amplifiers, among others.

Complexions at carbide / binder grain boundaries of Re-containing submicron cemented carbide

Samples of submicron WC-Co-Re cemented carbide were produced and examined by different techniques. The microstructure of the samples is fine and extremely uniform indicating the strong inhibiting effect of Re with respect to the WC grain growth during liquid-phase sintering. Results of examination of hot hardness at a temperature of 500°C provide evidence for the significantly higher hardness value of the WC-Co-Re cemented carbide in comparison with that of a conventional submicron WC-Co grade. Studies of creep rates of WC-Co-Re and WC-Co cemented carbides at 800°C indicated dramatically improved high-temperature creep-resistance of the Re-containing cemented carbide. Carbide / binder grain boundaries in a FIB lamella prepared from the WC-Co-Re cemented carbide were examined by high-angle annular dark field scanning transmission electron microscopy and energy dispersive X-ray spectroscopy at a high resolution. Rhenium was found to segregate at the grain boundaries forming complexions of 2 to 3 atomic monolayers, which presumably consist of mixed W-Re carbide. Such complexions are believed to suppress the phenomena of grain boundary sliding at elevated temperatures and WC grain growth at sintering temperatures thus dramatically improving the high-temperature creep-resistance of the WC-Co-Re material in comparison with conventional WC-Co cemented carbide.

Multilayer In-Plane Heterostructures Based on Transition Metal Dichalcogenides for Advanced Electronics.

In-plane heterostructures of transition metal dichalcogenides (TMDCs) have attracted much attention for high-performance electronic and optoelectronic devices. To date, mainly monolayer-based in-plane heterostructures have been prepared by chemical vapor deposition (CVD), and their optical and electrical properties have been investigated. However, the low dielectric properties of monolayers prevent the generation of high concentrations of thermally excited carriers from doped impurities. To solve this issue, multilayer TMDCs are a promising component for various electronic devices due to the availability of degenerate semiconductors. Here, we report the fabrication and transport properties of multilayer TMDC-based in-plane heterostructures. The multilayer in-plane heterostructures are formed through CVD growth of multilayer MoS2 from the edges of mechanically exfoliated multilayer flakes of WSe2 or NbxMo1-xS2. In addition to the in-plane heterostructures, we also confirmed the vertical growth of MoS2 on the exfoliated flakes. For the WSe2/MoS2 sample, an abrupt composition change is confirmed by cross-sectional high-angle annular dark-field scanning transmission electron microscopy. Electrical transport measurements reveal that a tunneling current flows at the NbxMo1-xS2/MoS2 in-plane heterointerface, and the band alignment is changed from a staggered gap to a broken gap by electrostatic electron doping of MoS2. The formation of a staggered gap band alignment of NbxMo1-xS2/MoS2 is also supported by first-principles calculations.

FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta-Coordinated Sites.

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O2 ) reduction at the cathode of proton exchange membrane fuel cells are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O2 reduction, their controlled synthesis and stability for practical applications remain challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilization remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, this issue is addressed by coordinating Fe in a highly porous nitrogen-doped carbon support (≈3295m2 g-1 ), prepared by pyrolysis of inexpensive 2,4,6-triaminopyrimidine and a Mg2+ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54×1019 sitesgFeNC -1 and a record 52% FeNx electrochemical utilization based on in situ nitrite stripping are achieved. The Fe single atoms are characterized pre- and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which are further studied by density functional theory calculations.

Ag Aerogel-Supported Single-Atom Hg Nanozyme Enables Efficient SERS Monitoring of Enhanced Oxidase-Like Catalysis.

In this work, three-dimensional (3D) Ag aerogel-supported Hg single-atom catalysts (SACs) were explored as an efficient surface-enhanced Raman scattering (SERS) substrate to monitor the enhanced oxidase-like reaction. The influence of the concentrations of Hg2+ to prepare 3D Hg/Ag aerogel networks on their SERS properties to monitor the oxidase-like reaction has been investigated, and a specific enhancement with an optimized addition of Hg2+ has been achieved. The formation of Ag-supported Hg SACs with the optimized Hg2+ addition was identified from a high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image and X-ray photoelectron spectroscopy (XPS) measurement at an atomic level. This is the first discovery of Hg SACs for enzyme-like reaction applications inferred by SERS techniques. And density functional theory (DFT) was used to further reveal the oxidase-like catalytic mechanism of Hg/Ag SACs. This study provides a mild synthetic strategy to fabricate Ag aerogel-supported Hg single atoms to display promising prospects in various catalytic fields.

Partially Thiolated Au25 Cluster Anchored on Carbon Support via Noncovalent Ligand–Support Interactions: Active and Robust Catalyst for Aerobic Oxidation of Alcohols

Partially thiolated atomically precise Au cluster catalysts were developed by controlled calcination of presynthesized thiolate-protected Au clusters on a porous carbon support. X-ray absorption fine structure analysis, powder X-ray diffraction, and aberration-corrected high-angle annular dark field scanning transmission electron microscopy revealed that calcination of 1 wt % of Au25(SR)18 (RS = 2-phenylethanethiolate, 1-dodecanethiolate) on carbon at 450 °C for 8–12 h produced partially thiolated Au25 clusters Au25(SR)/C without sintering. The Au25(SR)/C catalyst exhibited comparable catalytic activity to that of the thiolate-free Au25 cluster Au25/C for benzyl alcohol oxidation, whereas Au25(SR)/C showed higher durability than Au25/C. These results suggested that the residual RS ligands were distributed at the interface between the clusters and the support and suppressed the diffusion of the clusters by multiple van der Waals interactions. Large kinetic isotope effects indicated that the hydride abstraction from the α-carbon of the alkoxide was the rate-determining step. The O2 pressure dependence on rate constants was comparable to that reported for negatively charged Au clusters stabilized by poly(N-vinyl-2-pyrrolidone). This comparison suggests that electron-rich Au atoms left behind by desorbed RS served as active sites, which was supported by density functional theory calculations on model clusters. The partially thiolated Au25 cluster catalysts oxidized various p-substituted benzyl alcohols and benzylic secondary alcohols, with turn-over numbers exceeding 1 × 104. This work demonstrates that partially ligated, atomically precise metal clusters anchored by noncovalent ligand–support interaction are promising model catalysts with both high activity and durability.

Engineered Cobalt Single-Atoms@BiFeO3 Heteronanostructures for Highly Efficient Solar Water Oxidation.

Efficient charge-carrier separation and their utilization are the key factors in overcoming sluggish four-electron reaction kinetics involved in photocatalytic oxygen evolution. Here, a novel study demonstrates the significance of Na2 S2 O8 as a sacrificial agent in comparison to AgNO3 . Resultantly, BiFeO3 (BFO) and titanium doped-oxygen deficient BiFeO3 (Ti-BFO-R) nanostructures achieve ≈64 and 44.5 times higher O2 evolution in the presence of Na2 S2 O8 compared to AgNO3 as a sacrificial agent, respectively. Furthermore, the presence of Co single atoms (Co-SAs) deposited via immersion method on BFO and Ti-BFO-R nanostructures led to achieving outstanding O2 evolution at a rate of 16.11 and 23.89mmol g-1 h-1 , respectively, which is 153 and 227.5 times higher compared to BFO (in the presence of AgNO3 ), the highest O2 evolution observed for BFO-based materials to date. The successful deposition of Co-SAs is confirmed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) and X-ray absorption near-edge structure (XANES). The charge transfer investigations confirm the significance of Co-SAs on BFO-based photocatalysts for improved charge-carrier separation, transport, and utilization. This novel study validates the excellent role of Na2 S2 O8 as a sacrificial agent and Co-SAs as a cocatalyst for BFO-based nanostructures for efficient O2 evolution.

Characterization and theoretical calculations of the T([formula omitted])/Al interface in 2024 alloys: TEM and DFT studies

In this study, we systematically investigated several typical interfaces of the T-phase (a precipitated phase in 2024 alloy), namely the T(200)/Al(403), T(101)/Al(4‾03) and T(200)/Al(301) interface and analyzed interface properties through first-principles calculations and HAADF-STEM (High-angle annular dark-field scanning transmission electron microscope) observations. The calculation results show that AlMn- terminated interface is the most thermodynamically stable of the different terminal interfaces in T(200)/Al(403) and T(101)/Al(4‾03) interface. Therefore, we also speculate that the T precipitated phase forms interface with aluminum matrix through the AlMn- terminated. By investigating the DOS(density of states) and the electron density difference of the interface, we prove that Al–Mn covalent bond is formed on the outer surface of the T phase. In addition, this work provides some basis for the analysis of the T phase and T/Al interface from an energy point of view through various data, which helps us to further investigate the properties of the interface.

Effect of aging temperature on the hardening behavior and precipitation evolution of Mg-10Gd alloy

Mg alloys containing high Gd are featured by the outstanding aging hardening response. Here we conduct a systematic study on the hardening behavior and precipitation evolution in a Mg10Gd alloy during isothermal aging at 200–250 °C by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The optimum aging temperature of Mg10Gd is 200 °C with peak hardness of 102.4 HV. Hardness improvement of the aged samples at 200 °C mainly originates from precipitation strengthening, and β’ nano-precipitates with fine size of 10 nm are the main strengthening phases. The samples exhibit a certain aging hardening effect at 225 °C with peak hardness of 89.4 HV, due to the gourd-shaped β’ precipitates with coarse size of 100 nm. Furthermore, various metastable nanoscale structures (including short-range ordered GP zones, β”, βT, βM and βF′ phases) are observed in the aged samples, and their formation mechanisms are rationalized in detail.

Stepwise photoassisted decomposition of carbohydrates to H2

Stepwise photoassisted decomposition of carbohydrates to H2

Hardening effect and precipitation evolution of an isothermal aged Mg-Sm based alloy

The age-hardening behavior and precipitation evolution of an isothermal aged Mg−5Sm−0.6Zn−0.5Zr (wt.%) alloy have been systematically investigated by means of transmission electron microscopy (TEM) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The Vickers hardness of the present alloy increases first and then decreases with ageing time. The sample aged at 200 °C for 10 h exhibits a peak-hardness of 90.5 HV. In addition to the dominant β0′ precipitate (orthorhombic, a = 0.642 nm, b = 3.336 nm and c = 0.521 nm) formed on {11-20}α planes, a certain number of γ” precipitate (hexagonal, a = 0.556 nm and c = 0.431 nm) formed on basal planes are also observed in the peak-aged alloy. Significantly, the basal γ” precipitate is more thermostable than prismatic β0′ precipitate in the present alloy. β0′ precipitates gradually coarsened and were even likely to transform into β1 phase (face centered cubic, a = 0.73 nm) with the increase of ageing time, which accordingly led to a gradual decrease in number density of precipitates and finally resulted in the decreased hardness and mechanical property in the over-aged alloys.

Laser-enabled localized synthesis of Mo1-xWxS2 alloys with tunable composition

Alloying can tailor the optoelectronic properties of two-dimensional transition metal dichalcogenides (2D-TMDC) by bandgap tuning and shallowing of the deep-level defects, leading to enhancement in their photodetection performance. Hence, seeking for a scalable composition-controlled alloy preparation technology is essential. Here, direct laser-synthesis of micro-patterned Mo1-xWxS2 alloys is demonstrated. By irradiating composite single source precursor films of molybdenum and tungsten sulfides with a laser beam at visible wavelengths, photo-thermal decomposition takes place, thus forming high-quality Mo1-xWxS2 alloys in ambient conditions without any need for high-vacuum based environment. The Mo1-xWxS2 alloys composition is simply regulated by adjusting the volume ratio of the partial single source precursor solutions, in the final mixture solution.The stoichiometry, the structural, morphological and optical characteristics of laser-written alloys with different compositions have been investigated. Analysis of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) revealed the formation of the long-range (∼30 nm), few-layered crystals with randomly distributed molybdenum and tungsten metal atoms with no evidence of clustering. A photosensing device has been fabricated from laser-written Mo0·5W0·5S2 alloys to demonstrate the functionality of the material.

Second harmonic generation and simplified bond hyperpolarizability model analyses on the intermixing of Si/SiGe stacked multilayers for gate-all-around structure

Si/SiGe stacked multilayers are key elements in fabrication of gate-all-around (GAA) structures and improvement of electrical properties, with the evolution of the Si/SiGe interfaces playing a crucial role. In this work, a model is developed based on the simplified bond hyperpolarizability model (SBHM) to analysis the anisotropic reflective second harmonic generation (Ani-RSHG) on a three-period stacked Si/Si1−x Ge x multilayer, which builds on Si(100) diamond structures. The C 4v symmetry of the Si(100) structure enables the second harmonic generation (SHG) contribution from the bonds to be simplified and the effective hyperpolarizabilities of the interfacial and bulk sources to be obtained. The effective interface dipolar and bulk quadrupolar SHG hyperpolarizabilities in the Si1−x Ge x sample with various Ge concentration profiles are modeled by interpreting the concentration of a component element as the probability of the element occupying an atomic site. On the basis of the developed model, the Ani-RSHG spectra of the as-grown samples with various Ge ratios for each layer and the samples annealed at 850 °C and 950 °C are analyzed to inspect the change in Ge distribution and its gradient in depth. The ani-RSHG analysis on as-grown samples showed difference in Ge distribution in samples with the multi Si/SiGe structure, which is not well observed in synchrotron x-ray diffraction (XRD) spectra. For the annealed samples, the response to changes in Ge concentration and its gradient in depth reveal the Si/Si1−x Ge x interface intermixing. Results of high-angle annular dark-field scanning transmission electron microscopy and energy dispersive x-ray spectroscopy agree well with the Ani-RSHG with SBHM findings. Compared with the Raman and synchrotron XRD spectra, the Ani-RSHG with SBHM simulation result demonstrates much better response to changes in compositions of the Si/Si1−x Ge x stacked multilayered structures, verifying the potential for characterizing the concentration distribution in stacked multilayered thin films for GAA structures.

Atomic-scale characterization of contact interfaces between thermally self-assembled Au islands and few-layer MoS2 surfaces on SiO2

The interaction between metallic nanoparticles and transition metal chalcogenides (TMDs) can realize new functionalities in thriving technologies such as optoelectronics and nanoengineering. Here we have investigated the self-assembly of triangular-shaped crystalline Au nanoislands on MoS2 flakes mechanically exfoliated or grown by chemical vapor deposition (CVD). The density and size of the islands are determined by substrate temperature, deposition flux, and subsurface morphology. The thickness of the MoS2 layers is measured by Raman spectroscopy, which also enables the evaluation of the strain and doping distributions induced by the Au islands. Top and cross-sectional images of the Au-MoS2 interface are obtained by scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Sub-nanometer resolution of the Au, Mo and S layers reveals that the MoS2 flakes follow the corrugation of the SiO2 substrate, with flattening and wrinkling effects induced by the growth of the Au islands on top.

Polar–Nonpolar Transition-Type Negative Thermal Expansion with 11.1% Volume Shrinkage by Design

Chemical substitution for the tuning of the working temperature of phase-transition-type negative thermal expansion (NTE) materials generally reduces the volume shrinkage during the transition. We have investigated the effects of electron doping and reduction of 6s2 lone-pair activity in PbVO3 with a large polar distortion (c/a = 1.23) and found that the combination of Bi and Sr substitutions for Pb enables a temperature-induced polar to non-polar transition with 11% volume shrinkage, even larger than the pressure-induced volume collapse of PbVO3 (∼10.6%), and is the largest value among the NTE materials reported so far. The domain structure of the coexisting cubic and tetragonal phases with such a huge volume difference was successfully observed by high-angle annular dark-field scanning transmission electron microscopy and the spatial distribution of domains by Bragg coherent X-ray diffraction imaging. The temperature hysteresis is reduced by repeated heating/cooling cycles, suggesting that the changes in the domain structure dominate the NTE properties.

Scalable Synthesis of Holey Deficient 2D Co/NiO Single-Crystal Nanomeshes via Topological Transformation for Efficient Photocatalytic CO2 Reduction.

Preparation of holey, single-crystal, 2D nanomaterials containing in-plane nanosized pores is very appealing for the environment and energy-related applications. Herein, an in situ topological transformation is showcased of 2D layered double hydroxides (LDHs) allows scalable synthesis of holey, single-crystal 2D transition metal oxides (TMOs) nanomesh of ultrathin thickness. As-synthesized 2D Co/NiO-2 nanomesh delivers superior photocatalytic CO2 -syngas conversion efficiency (i.e., VCO of 32460µmol h-1 g-1 CO and of 17840µmol h-1 g-1 H2 ), with VCO about 7.08 and 2.53 times that of NiO and 2D Co/NiO-1 nanomesh containing larger pore size, respectively. As revealed in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the high performance of Co/NiO-2 nanomesh primarily originates from the edge sites of nanopores, which carry more defect structures (e.g., atomic steps or vacancies) than basal plane for CO2 adsorption, and from its single-crystal structure adept at charge transport. Theoretical calculation shows the topological transformation from 2D hydroxide to holey 2D oxide can be achieved, probably since the trace Co dopant induces a lattice distortion and thus a sharp decrease of the dehydration energy of hydroxide precursor. The findings can advance the design of intriguing holey 2D materials with well-defined geometric and electronic properties.