High-resolution High-angle Annular Dark-field Scanning Transmission Electron Microscopy Research Articles

“Invisible” radioactive cesium atoms revealed: Pollucite inclusion in cesium-rich microparticles (CsMPs) from the Fukushima Daiichi Nuclear Power Plant

Understanding radioactive Cs contamination has been a central issue at Fukushima Daiichi and other nuclear legacy sites; however, atomic-scale characterization of radioactive Cs in environmental samples has never been achieved. Here we report, for the first time, the direct imaging of radioactive Cs atoms using high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). In Cs-rich microparticles collected from Japan, we document inclusions that contain 27 – 36 wt% of Cs (reported as Cs2O) in a zeolite: pollucite. The compositions of three pollucite inclusions are (Cs1.86K0.11Rb0.19Ba0.22)2.4(Fe0.85Zn0.84X0.31)2.0Si4.1O12, (Cs1.19K0.05Rb0.19Ba0.22)1.7(Fe0.66Zn0.32X0.41)1.4Si4.6O12, and (Cs1.27K0.21Rb0.29Ba0.15)1.9(Fe0.60Zn0.32X0.69)1.6Si4.4O12 (X includes other cations). HAADF-STEM imaging of pollucite, viewed along the [111] zone axis, revealed an array of Cs atoms, which is consistent with a simulated image using the multi-slice method. The occurrence of pollucite indicates that locally enriched Cs reacted with siliceous substances during the Fukushima meltdowns, presumably through volatilization and condensation. Beta radiation doses from the incorporated Cs are estimated to reach 106 – 107 Gy, which is more than three orders of magnitude less than typical amorphization dose of zeolite. The atomic-resolution imaging of radioactive Cs is an important advance for better understanding the fate of radioactive Cs inside and outside of nuclear reactors damaged by meltdown events.

Impact of varied zeolite materials on nickel catalysts in CO2 methanation

Ni catalysts supported on 4A, 5A, 13X, ZSM-5, and BEA zeolites were prepared using the vacuum-heating method for CO2 methanation. These support materials play a pivotal role in shaping the catalysts' properties and their catalytic performance. High-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) mappings suggest a significant concentration of Ni nanoparticles situated on the external surfaces of catalysts with low Si/Al ratios zeolites (Ni-4A, Ni-5A, and Ni-13X). These Ni nanoparticles exhibit characteristics of weaker metal-support interaction, lower metal reduction temperatures, and moderate H2 adsorption activity. Moreover, these low Si/Al ratio zeolites demonstrate robust CO2 adsorption activity. These properties endow Ni-5A and Ni-13X catalysts with heightened CO2 conversion (70.4–70.9%) and methane selectivity (92.4–96.4%). In contrast, high Si/Al ratio zeolite-based catalysts (Ni-ZSM-5 and Ni-BEA) exhibit smaller Ni particles, strong metal-support interaction and weaker CO2 adsorption activity, resulting in reduced CO2 methanation activity and decreased methane selectivity (71.2–73.4%). The normalized CO2 conversion rate presents a correlation with the average Ni particle size.

A precipitation pathway of T1 phase via heterogeneous nucleation on Li-rich particle in Al-Cu-Li alloy

The hexagonal T1 (Al2CuLi) phase is the main strengthening precipitate in Al-Cu-Li alloys and is usually considered to be formed heterogeneously at dislocations during artificial aging. The precipitation scenario in Al-Cu-Li-Mg alloy has been revisited using high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and in-situ heating inside TEM. Octahedral nano-complexes consisting of T1 precursors (T1p) on {111} facets of early-precipitated δ'(Al3Li) particles form at under-aging stage in the investigated alloy. This nano-complex with an average diameter of 5–10 nm is a transitional structure, which gradually dissolve to facilitate the transformation of a small portion of large precursors toward T1 precipitates. The T1p is determined to be isostructural to orthorhombic Ω phase based on atomic-resolution HAADF-STEM observations. The subsequent inward diffusion of Li is believed to promote the transition from Ω-like T1p to T1. An alternative precipitation path toward the key hardening T1 precipitate is thus proposed and could explain the formation of T1 precipitates apart from those at the dislocations. Our findings suggest complex multiple precipitation pathways for T1 phase should exist in Al-Cu-Li alloys.

Incorporating Sulfur Atoms into Palladium Catalysts by Reactive Metal–Support Interaction for Selective Hydrogenation

Open AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022Incorporating Sulfur Atoms into Palladium Catalysts by Reactive Metal–Support Interaction for Selective Hydrogenation Zhen-Yu Wu†, Hang Nan†, Shan-Cheng Shen, Ming-Xi Chen, Hai-Wei Liang, Chuan-Qi Huang, Tao Yao, Sheng-Qi Chu, Wei-Xue Li and Shu-Hong Yu Zhen-Yu Wu† Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 †Z.-Y. Wu and H. Nan contributed equally to this work.Google Scholar More articles by this author , Hang Nan† Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 †Z.-Y. Wu and H. Nan contributed equally to this work.Google Scholar More articles by this author , Shan-Cheng Shen Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Ming-Xi Chen Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Hai-Wei Liang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Chuan-Qi Huang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Hangzhou Institute of Advanced Studies, Zhejiang Normal University, Hangzhou, Zhejiang 311231 Google Scholar More articles by this author , Tao Yao National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Sheng-Qi Chu Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Wei-Xue Li Department of Chemical Physics, School of Chemistry and Materials Science, iCHeM, CAS Center for Excellence in Nanoscience, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author and Shu-Hong Yu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101428 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Developing highly active and selective catalysts for the hydrogenation of nitroarenes, an environmentally benign process to produce industrially important aniline intermediates, is highly desirable but very challenging. Pd catalysts are generally recognized as active but nonselective catalysts for this important reaction. Here, we report an effective strategy to greatly improve the selectivity of Pd catalysts based on the reactive metal–support interaction, by which the sulfur atoms doped in the carbon supports are extracted out and react with Pd at 500 °C to form Pd4S nanoparticle catalysts. The Pd4S catalysts that are formed display high selectivity of >98% at complete conversion to diverse nitroarenes (with more than 20 examples) under mild conditions. Density functional theory calculations reveal that the incorporation of S atoms into Pd breaks up the Pd ensembles, which results in the preferential adsorption of nitro groups on the Pd4S surfaces and thus selective hydrogenation of substituted nitroarenes to corresponding anilines. Download figure Download PowerPoint Introduction Hydrogenation reactions have attracted continuing interest due to their great significance in the chemical industry and scientific research.1–7 A typical but important example is the hydrogenation of substituted nitroarenes, which produces substituted anilines as key intermediates for the manufacture of pharmaceuticals, agrochemicals, dyes, and functional polymers.8–14 A big challenge for this reaction is the selective reduction of the nitro group when more than one reducible group (e.g., –Cl, –C=C, –C=O, –OH) is present in the nitroarenes, as conventional supported noble metal catalysts (e.g., Pd or Pt) cannot discriminate different functional groups for selective hydrogenation.8,12,13,15–20 Developing new hydrogenation catalysts with high intrinsic chemoselectivity is a straightforward and effective strategy to tackle this tough problem.8,21–27 Some important progress has been achieved recently by employing Au-based, Ag-based, and pyrolyzed Fe(or Co)/nitrogen/carbon catalysts for selective reduction of the nitro group of substituted nitroarenes.8,21,25,28–30 Nevertheless, these catalysts always suffer from low activity and thus often require harsh reaction conditions, such as high temperature (≥120 °C), high H2 pressure (≥5.0 MPa), and long reaction time (≥12 h).8,21,25,28,31 Another effective strategy is improving chemoselectivity of conventional hydrogenation catalysts (mainly Pt-group catalysts), which possess poor catalytic selectivity but high intrinsic activity.13,15,20,31–35 A traditional approach is the modification of active metal species with inorganic or organic modifiers. However, those modifiers not only decrease the activity via coverage of the active sites, but also raise environmental problems.20,34,36 Other ways of improving selectivity of Pt-group catalysts mainly include size control of active components (even at the single-atomic level),11,12,18,31 formation of alloys or intermetallic compounds,17,32,37–40 and regulation by the pore structure of supports.33,35,41 Particularly, construction of intermetallic compounds, featuring atomically ordered and thermodynamically stable structures with defined stoichiometry, can change the adsorption/desorption properties of the relevant reaction species, which makes it possible to catalyze the nitroarene hydrogenation reaction in the desired direction.32,37,40 Furthermore, well-defined atomic arrangements provide an excellent platform to study the catalytic mechanism, enabling better performance optimization and catalyst design.32,37 Inspired by these promising works, we propose that incorporating nonmetallic elements (i.e., sulfur) into active Pd metal lattices to form compounds with well-defined structures is a feasible way to create selective nitroarene hydrogenation catalysts that will work well under mild conditions. Herein, we report a novel nitroarene hydrogenation catalyst, that is, crystalline Pd4S nanoparticles supported on S-doped carbon (Pd4S/SC), which is prepared through the reactive metal–support interaction (RMSI). RMSI, a chemical reaction that generally occurs on the metal–oxide support interface under high temperatures, was recently employed to prepare bimetallic structures.42–44 However, RMSI has never before been explored in the carbon-supported metal catalysts. Here, we demonstrate an example of carbon support-based RMSI, in which the sulfur atoms doped in the carbon supports are extracted out at 500 °C and incorporated into Pd to form crystalline Pd4S nanoparticle catalysts. In contrast to conventional Pd/C catalysts with poor selectivity, the prepared Pd4S catalyst gives high selectivity of >98% at complete conversion for structurally diverse nitroarenes (with more than 20 examples) under mild conditions. Density functional theory (DFT) calculations disclose that the geometric effect induced by the S incorporating into the Pd lattice results in the preferential adsorption of the nitro group on the Pd4S surfaces and thus selective hydrogenation of nitroarenes to corresponding anilines. Experimental Methods Chemicals and materials SiO2 fumed powder and commercial Pd/C catalyst were purchased from Sigma-Aldrich (St. Louis, MO). Commercial Pt/C catalyst was obtained from Alfa Aesar (Ward Hill, MA). Other chemicals were commercially procured from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used as received without further purification. Synthesis of SC supports and a-PdxS/SC-b catalysts In a typical experiment for synthesizing SC, 2.0 g of 2,2′-bithiophene, 1.0 g of Co(NO3)2·6H2O, and 2.0 g of SiO2 fumed powder (7 nm; Sigma-Aldrich S5130) were first added into 120 mL of tetrahydrofuran and stirred for ca. 2 h. Then, the solvent was removed by rotary evaporation. The obtained dried powder was subsequently carbonized under flowing N2 for 2 h at 800 °C. The carbonized product was then leached in 2.0 M NaOH for 3 days and 0.5 M H2SO4 at 90 °C for 8 h to remove SiO2 templates and metallic species, and finally afforded SC. An inductively coupled plasma-atomic emission spectrometer (ICP-AES) test showed that the residual Co content in SC was very low (0.24 wt %). To synthesize a-PdxS/SC-b catalysts, PdCl2 was first dissolved in HCl aqueous solution to generate a H2PdCl4 solution with a concentration of 2.815 mg mL−1. Then, a certain amount of H2PdCl4 aqueous solution and 60 mg of SC was added into 30 mL deionized water. After stirring for 6 h and sonication for another 0.5 h, the water was removed by rotary evaporation. The obtained powder was treated by thermal reduction at 300–700 °C under flowing 5% H2/Ar for 2 h to afford final a-PdxS/SC-b catalysts, where a and b are the weight ratios of Pd to SC and reduction temperatures, respectively. Catalyst characterizations Transmission electron microscopy (TEM) images were taken with a Hitachi H7700 transmission electron microscope with a charge-coupled device (CCD) imaging system and an accelerating voltage of 100 kV. High-angle annular dark-field scanning TEM (HAADF-STEM) and high-resolution TEM (HRTEM) images were acquired by using a JEM-ARM 200F atomic resolution analytical microscope (JEOL, Tokyo, Japan) operating at an accelerating voltage of 200 kV. X-ray diffraction (XRD) data were collected on a Philips X’Pert PRO SUPER X-ray diffractometer (Almelo, The Netherlands) equipped with graphite monochromatic Cu Kα radiation (λ = 1.54056 Å). The energy-dispersive spectroscopic (EDS) line-scan and EDS elemental mapping was carried out on a Talos F200X transmission electron microscope (FEI, Hillsboro, OR) at an accelerating voltage of 200 kV equipped with an energy dispersive detector. N2 sorption analysis was recorded on an ASAP 2020 accelerated surface area and porosimetry instrument (Micromeritics, Norcross, GA), equipped with automated surface area, at 77 K using Barrett–Emmett–Teller (BET) calculations for the surface area. X-ray photoelectron spectroscopy (XPS) was performed on an X-ray photoelectron spectrometer (ESCALab MKII, Thermo Scientific, Waltham, MA) with an excitation source of Mg Kα radiation (1253.6 eV). The X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of Pd K-edge were measured at BL14W1 beam line of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). The XANES tests of S K-edge were carried out at the 4B7A beamline of Beijing Synchrotron Radiation Facility (BSRF, Beijing, China). The XANES tests of S L-edge were performed at the BL11U beamline of National Synchrotron Radiation Laboratory (NSRL, Hefei, China). The S XANES spectra in the figures have been normalized to the background before and after the main features. The gas chromatograph (GC) instrument was equipped with a Restek-5 capillary column (5% diphenyl and 95% dimethylsiloxane, 0.32 mm diameter, 60 m length), and a flame-ionization detector (FID). GC (Shimadzu GC-2014, Nakagyo-ku, Kyoto, Japan) was used to calculate the conversion and selectivity of the catalytic products. Pd K-edge XAFS analysis The obtained EXAFS data were processed according to the standard procedures utilizing the ATHENA module in the IFEFFIT software packages. The EXAFS spectra were obtained by subtracting the postedge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, the χ(k) data were Fourier-transformed (FT) to real (R) space using a hanning windows (dK = 1.0 Å−1) to separate the EXAFS contributions from different coordination shells. Least-squares curve parameter fitting was carried out using the ARTEMIS module of the IFEFFIT software packages to obtain the quantitative structural parameters around central atoms. Catalytic reaction experiments The hydrogenation of 4-chloronitrobenzene was carried out in a stainless steel autoclave equipped with a pressure control system and a magnetic stirrer (1200 rpm). In a typical hydrogenation experiment, 1.0 mmol of 4-chloronitrobenzene and 0.094 mol % Pd of catalysts were mixed with 1 mL of ethyl acetate in a reaction glass vial. The reaction vial was then placed into a 100 mL steel Parr autoclave. The autoclave was purged three times with H2 and then charged with 0.6 MPa of H2 before the reaction mixture was stirred for 1 h at 80 °C. After the reaction was completed, the autoclave was placed into a cool water bath to stop the reaction. Then, the H2 was released slowly, and 60 μL of o-xylene was added to the system as the internal standard. Next, 10 mL of ethyl acetate was added to dilute the reaction solution. The products were analyzed by GC. In the recyclability test, all reactions were carried out for 0.75 h according to the procedures above. After each run, the used catalyst was separated by vacuum filtration and washed with deionized water and ethanol. Then the obtained sample was dried at 80 °C overnight before being tested in the next run. For the gram-scale hydrogenation of the 4-chloronitrobenzene test, the reaction was conducted by using 25 mmol of 4-chloronitrobenzene, 0.0188 mol % Pd of catalysts, and 15 mL of ethyl acetate as solvent under 1.0 MPa of H2 at 100 °C. The hydrogenation experiment procedure remained the same. After the reaction was performed for 1 h, the autoclave was placed into a cool water bath to stop the reaction. Then the remaining hydrogen gas was discharged, and 0.1 mL liquid mixture was extracted to analyze the product by GC. Afterward, the autoclave was charged with pure H2 again to perform the next run. In the experiment of hydrogenation of substituted nitroarenes, the substrates and products were determined by GC or GC-mass spectrometry (GC-MS). The conversion and selectivity were calculated by GC analysis, which were determined by GC peak areas. DFT calculation method DFT calculations were performed using the Vienna ab initio Simulation Package (VASP) based on the projected augmented wave (PAW) method.45–48 The nonlocal correlation functional of van der Waals density functional (vdW-DF) scheme proposed by Dion et al.49 was used to account for vdW interaction. The exchange effect of the electron was described with the optB86b exchange functional.50 This combined optB86b-vdW functional was verified to have a good description of the vdW interaction between molecules and metal surfaces.50,51 Optimized lattice constants of 3.941 Å for Pd and a = 5.18 Å, c = 5.65 Å for Pd4S were used. For Pd(111), Pd(100), and Pd(110) surfaces, slabs of 4, 4, and 5 layers, with supercells of (5 × 5), (4 × 4), and (3 × 4) size were used, respectively. For Pd4S(110), Pd4S(100), Pd4S(001), Pd4S(102), and Pd4S(112) surfaces, (2 × 2) super cells of about 9 Å atomic layer thickness were used, respectively. Slabs were separated with 20 Å of vacuum. The upper two layers of each slab were relaxed. A plane wave basis set with energy cutoff of 400 eV and (2 × 2) k-points were used to ensure the convergence of total ground-state energy. All geometrical optimization was performed until the force acting on relaxing atoms was less than 0.02 eV Å−1. Results and Discussion Synthesis and characterization of the catalysts The first step of the synthesis is to prepare the SC support by carbonization of molecular precursors with SiO2 nanoparticles as hard templates (see Experimental Methods for details).52,53 The SC support possessed a high specific surface area of 1164 m2 g−1, a large pore volume of 3.24 cm3 g−1, and a particularly high S content of 13.2 wt % ( Supporting Information Figures S1 and S2a and Table S1). The Pd4S/SC catalyst was then prepared by the wet impregnation of H2PdCl4 (5 wt % Pd) and subsequent H2 reduction at 500 °C. The S-enriched carbon supports enabled the reaction of Pd with S atoms to form Pd4S nanoparticles based on the RMSI (Figure 1a). As far as we know, it is the first example of synthesizing metal sulfide catalysts by RMSI on the metal–carbon interface. Figure 1 | Synthesis and characterization of the catalyst. (a) Schematic illustration for the preparation processes of Pd4S/SC catalyst based on RMSI. (b) Low-magnification HAADF-STEM image of the Pd4S/SC catalyst. Inset shows the particle size distribution of Pd4S. (c) High-resolution HAADF-STEM of a Pd4S nanoparticle. (d) EDS elemental mapping images of the Pd4S/SC catalyst. (e) Line-scan EDS elemental distribution curves of an individual Pd4S nanoparticle. (f) XRD pattern of the Pd4S/SC catalyst. (g) Crystal structure model of Pd4S. Download figure Download PowerPoint We first made HAADF-STEM observations to characterize the microstructure of the prepared catalysts. HAADF-STEM images and the corresponding particle size distribution showed that the nanoparticles with an average diameter of 5.8 ± 1.8 nm were homogeneously distributed on the SC supports (Figure 1b, the inset in Figure 1b, and Supporting Information Figure S3). The high-resolution HAADF-STEM image of an individual nanoparticle revealed well-defined lattice fringes with spacings of 0.243 and 0.220 nm, which were consistent with (102) and (112) planes of Pd4S (Figure 1c). Additionally, HRTEM also showed two sets of lattice fringes from Pd4S ( Supporting Information Figure S4). EDS elemental mapping images indicated that the S element was distributed over the whole carbon matrix, while line-scan curves confirmed that both of Pd and S elements were concentrated in individual particles (Figures 1d and 1e). Subsequently, we performed XRD to identify crystalline phase compositions of the prepared catalyst. The XRD pattern displayed main diffraction peaks at 35.2°, 36.6°, 39.5°, 40.8°, 42.8°, 48.2°, and 51.8° (Figure 1f), which corresponded well to the (200), (102), (210), (112), (211), (202), and (212) planes, respectively, of tetragonal Pd4S (JCPDS 73-1387). The space group of Pd4S is P-421c with eight palladimn atoms in general positions and two sulfur atoms in positions (0, 0, 0) and (1/2, 1/2, 1/2) (Figure 1g).54 Overall, the above characterizations unambiguously proved the formation of highly crystalline Pd4S nanoparticles on the SC supports based on RSMI. We then explored the effect of reduction temperature and Pd contents on RSMI for the catalyst preparation. All of the catalysts are referred to as a-PdxS/SC-b, where a and b are the weight ratio of Pd to SC and reduction temperatures, respectively. XRD analyses clearly indicated the vital role of reduction temperature on RSMI for the catalyst synthesis ( Supporting Information Figure S5). Unlike the highly crystalline Pd4S phase of 5%-PdxS/SC-500 (i.e., aforementioned Pd4S/SC), no diffraction peaks were found for the sample prepared at 300 °C ( Supporting Information Figure S5a), indicating its amorphous structure or the ultrasmall size of the nanoparticles. For the 700 °C sample (5%-PdxS/SC-700), we observed the coexistence of diffraction peaks corresponding to Pd4S and metallic Pd ( Supporting Information Figure S5b), suggesting the partial decomposition of Pd4S to form metallic Pd phase at 700 °C. Along with the gradual evolution of Pd structures depending on temperatures, the particle size sharply increased from 0.90 ± 0.24 nm for 5%-PdxS/SC-300 to 14.1 ± 6.2 nm for 5%-PdxS/SC-700 when the H2-reduction temperature rose from 300 to 700 °C ( Supporting Information Figures S6 and S7). When we fixed the H2-reduction temperature at 500 °C and changed the Pd contents in the range of 2∼10 wt %, similar nanoparticle size distributions were found ( Supporting Information Figures S8 and S9). However, we also observed the emergence of the metallic Pd phase for the high-Pd-content (10 wt %) sample at 500 °C ( Supporting Information Figure S10), implying the deficiency of S atoms in the SC supports to form Pd4S for a high Pd content. Additionally, N2 adsorption–desorption tests revealed that all of the a-PdxS/SC-b catalysts possessed high specific surface areas of ca. 1000 m2 g−1 and large pore volumes of >3.5 cm3 g−1 ( Supporting Information Figures S2b–S2f and Table S1). Electronic properties of the catalysts We further conducted XAFS measurements to reveal the effects of reduction temperatures on the electronic and local structures of the catalysts based on RSMI. The Pd K-edge XANES profiles in Figure 2a showed that the near-edge feature of 5%-PdxS/SC-300 was extremely close to that of PdS, while those of 5%-PdxS/SC-500 and 5%-PdxS/SC-700 were quite similar to that of Pd foil, indicating that the valence states of Pd in 5%-PdxS/SC-300, 5%-PdxS/SC-500, and 5%-PdxS/SC-700 were close to Pd2+, Pd0, and Pd0, respectively. The white line, feature A, and feature B peaks increased or decreased gradually with the reduction temperature, demonstrating that the valence states of the Pd element in 5%-PdxS/SC-b decreased accordingly. Figure 2b shows the FT k2-weight EXAFS curves of 5%-PdxS/SC-b as well as those of reference PdS and Pd foil. 5%-PdxS/SC-300 exhibited a prominent peak at ca. 1.8 Å, agreeing well with Pd–S coordination of the PdS reference, and no other obvious peak for Pd–Pd was observed. When the reduction temperature was increased to 500 and 700 °C, a new peak located at ca. 2.6 Å pointing to Pd–Pd coordination was found. Of note, the intensity of Pd–Pd coordination gradually increased with the reduction temperature, accompanying the gradually weakened Pd–S scattering from 5%-PdxS/SC-300 to 5%-PdxS/SC-700. Figure 2 | Electronic properties of the catalysts. (a) XANES spectra at the Pd K-edge of the 5%-PdxS/SC-b catalysts, PdS, and Pd foil. The inset is the magnified image of pre-edge XANES spectra. (b) FT at the Pd K-edge of 5%-PdxS/SC-b catalysts, PdS, and Pd foil. (c) WT of 5%-PdxS/SC-b catalysts, PdS, and Pd foil. (d) Pd XPS spectra of 5%-PdxS/SC-b catalysts. Download figure Download PowerPoint Wavelet transform (WT) analysis was further conducted to probe the atomic dispersion of Pd in 5%-PdxS/SC-b (Figure 2c). The WT contour plots of PdS showed only one intensity maximum at 1.8 Å attributed to Pd–S coordination, while Pd foil displayed only one intensity maximum at 2.6 Å from Pd–Pd coordination. Clearly, the intensity of Pd–S coordination in WT contour plots of 5%-PdxS/SC-b catalysts decreased gradually when the reduction temperature increased from 300 to 700 °C, accompanying the breaking of Pd–S bonds and the increase of Pd–Pd coordination. Furthermore, we carried out EXAFS fitting to obtain the quantitative chemical configuration of the Pd atoms in the 5%-PdxS/SC-b catalysts ( Supporting Information Figure S11 and Table S2). For 5%-PdxS/SC-300, there was only one coordination configuration (i.e., Pd–S) at ca. 2.31 Å with a coordination number of ca. 3.9, very close to the PdS reference with an average Pd–S bond length of 2.32 Å and a coordination number of about 3.6. As expected, 5%-PdxS/SC-500 and 5%-PdxS/SC-700 possessed two coordination configurations, that is, Pd–S and Pd–Pd. The coordination numbers of Pd–S and Pd–Pd were 2.1 and 5.5 in 5%-PdxS/SC-500, and 1.7 and 6.5 in 5%-PdxS/SC-700, respectively. Pd EXAFS results in k-space for 5%-PdxS/SC-b catalysts, PdS and Pd foil are provided in Supporting Information Figure S12. We also performed XPS tests to further characterize the electronic properties of 5%-PdxS/SC-b catalysts (Figure 2d). The Pd 3d XPS spectrum of 5%-PdxS/SC-300 consisted of asymmetric Pd 3d5/2 and 3d3/2 peaks centered at 337.2 and 342.5 eV, respectively, which are the characteristics of Pd2+.55,56 The Pd 3d5/2 and Pd 3d3/2 peaks sharply dropped for 5%-PdxS/SC-500 and 5%-PdxS/SC-700, and two distinct peaks at 335.6 and 340.9 eV emerged, which were indicative of Pd0.55,56 The ratios of Pd0/Pd2+ were 0, 3.06, and 3.75 for 5%-PdxS/SC-300, 5%-PdxS/SC-500, and 5%-PdxS/SC-700, respectively. In the S K-edge, L-edge XANES and XPS spectra, we only identified the C–S–C bond from the SC support and did not find the Pd–S bond from PdxS ( Supporting Information Figures S13 and S14), probably due to the relatively low content of Pd–S bonds compared with that of C–S–C bonds in 5%-PdxS/SC-b catalysts. Given the above HADDF-STEM, XRD, XAFS, and XPS analyses, we proposed the evolution process of Pd species on the SC supports with the reduction temperature based on RMSI. At the low reduction temperature of 300 °C, the Pd precursors react with S atoms that are extracted out from SC supports to form loosely assembled PdxS nanoclusters with exclusive Pd–S coordination configuration. At 500 °C, along with the slight aggregation/ripening, the Pd and S atoms in the particles will rearrange into well-defined crystalline Pd4S structures. Further raising the temperature to 700 °C will cause some S atoms to leave the Pd4S lattice, that is, the partial decomposition of Pd4S into metallic Pd. Catalytic performance in hydrogenation of 4-chloronitrobenzene We chose hydrogenation of 4-chloronitrobenzene as a model reaction to investigate the catalytic performance of a-PdxS/SC-b catalysts, in which the selective hydrogenation of the nitro group to produce 4-chloroaniline is desired (Figure 3a).35,40 The catalytic reactions were performed under a mild condition (T = 80 °C, P (H2) = 0.6 MPa) for all the a-PdxS/SC-b catalysts. Commercial Pd/C and Pt/C were also investigated under the identical conditions for comparison. As expected, the commercial Pd/C and Pt/C catalysts showed the high conversion of 100% after 1 h reaction but poor 4-chloroaniline selectivity of 56.2% and 74.5%, respectively (Figure 3b), as dechlorination occurred on both catalysts to form the aniline as side product. The SC support alone did not show any catalytic activity toward the 4-chloronitrobenzene hydrogenation. Remarkably, the 5%-PdxS/SC-500 catalyst exhibited a superior 4-chloroaniline selectivity of >99.9%, along with the complete conversion with 1 h reaction (Figure 3b). Additionally, we conducted the hydrogenation reaction under harsh reaction conditions, including longer reaction time and higher reaction temperature, to further demonstrate the outstanding selectivity of the 5%-PdxS/SC-500 catalyst. As shown in Supporting Information Figure S15, 5%-PdxS/SC-500 still retained excellent selectivity of >99.9% when the reaction time

Comprehensive Study of Li+/Ni2+ Disorder in Ni-Rich NMCs Cathodes for Li-Ion Batteries

The layered oxides LiNixMnyCozO2 (NMCs, x + y + z = 1) with high nickel content (x ≥ 0.6, Ni-rich NMCs) are promising high-energy density-positive electrode materials for Li-ion batteries. Their electrochemical properties depend on Li+/Ni2+ cation disordering originating from the proximity of the Li+ and Ni2+ ionic radii. We synthesized a series of the LiNi0.8Mn0.1Co0.1O2 NMC811 adopting two different disordering schemes: Ni for Li substitution at the Li site in the samples finally annealed in air, and close to Ni↔Li antisite disorder in the oxygen-annealed samples. The defect formation scenario was revealed with Rietveld refinement from powder X-ray diffraction data, and then the reliability of semi-quantitative parameters, such as I003/I104 integral intensity ratio and c/(2√6a) ratio of pseudocubic subcell parameters, was verified against the refined defect concentrations. The I003/I104 ratio can serve as a quantitative measure of g(NiLi) only after explicit correction of intensities for preferred orientation. Being normalized by the total scattering power of the unit cell, the I003/I104 ratio depends linearly on g(NiLi) for each disordering scheme. The c/(2√6a) ratio appears to be not reliable and cannot be used for a quantitative estimate of g(NiLi). In turn, the volume of the R3¯m unit cell correlates linearly with g(NiLi), at least for defect concentrations not exceeding 5%. The microscopy techniques such as high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron diffraction tomography (EDT) allow us to study the materials locally, still, there is no proper quantitative approach for comprehensive analysis of defects. In the present work, the TEM-assisted quantitative Li+/Ni2+ disordering analysis with EDT and HAADF-STEM in six Ni-rich NMC samples with various defects content is demonstrated. Noteworthy, while PXRD and EDT methods demonstrate overall defect amounts, HAADF-STEM allows us to quantitatively distinguish regions with various disordering extents. Therefore, the combination of mentioned PXRD and TEM methods gives the full picture of Li+/Ni2+ mixing defects in Ni-rich NMCs.

Nanostructured Polymer Composite Electrolytes with Self-Assembled Polyoxometalate Networks for Proton Conduction

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Nanostructured Polymer Composite Electrolytes with Self-Assembled Polyoxometalate Networks for Proton Conduction Gang Wang, Jialin Li, Lichao Shang, Haibo He, Tingting Cui, Shengchao Chai, Chengji Zhao, Lixin Wu and Haolong Li Gang Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Jialin Li Key Laboratory of High Performance Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Lichao Shang Key Laboratory of High Performance Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Haibo He State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Tingting Cui State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Shengchao Chai State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Chengji Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of High Performance Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Lixin Wu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Haolong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Key Laboratory of High Performance Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000608 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The key challenge for the use of polymer electrolytes is to realize a high ionic conductivity without scarifying their mechanical performance. Herein, we report a facile strategy to prepare a nanostructured polymer electrolyte with both high proton conductivity and high modulus, based on the electrostatic self-assembly of polyoxometalate cluster H3PW12O40 (PW) and comb copolymer poly(ether-ether-ketone)-grafted-poly(vinyl pyrrolidone) (PEEK-g-PVP). The incorporation of protonic acid PW can enable the PEEK-g-PVP to be highly proton conductive and create flexible composite electrolyte membranes. Moreover, nanoscale phase separation between PEEK domains and PVP/PW domains spontaneously occurs in these membranes, forming a bicontinuous structure with three-dimensional (3D)-connected PW networks. Due to the dual role of PW networks as both proton transport pathways and mechanical enhancers, these membranes exhibit proton conductivities higher than 30 mS cm−1 and modulus over 4 GPa. Notably, the direct methanol fuel cells equipped with these membranes show good cell performance. Given the wide tunability of comb copolymers and polyoxometalates, this system can be extended to develop a variety of functional electrolyte materials, for example, the lithium-ion conductive electrolytes by using polyoxometalate-based lithium salts, which provides a promising platform to explore versatile electrolyte materials for energy and electronic applications. Download figure Download PowerPoint Introduction Ion-conducting polymer electrolytes have tremendous potential in a wide range of energy applications such as fuel cells, batteries, supercapacitors, and other energy storage and conversion devices.1–12 Despite these promising applications, a key issue in polymer electrolytes is maximizing the ionic conductivity while simultaneously optimizing the mechanical performance, so as to prepare high conductive and high stable electrolyte membranes for practical use.13 However, the facilitation of ion transport generally involves increasing the content of ionic groups or accelerating the segmental dynamics of polymer electrolytes, which inevitably causes the electrolyte membranes to become swollen or softened and thus sacrifice their mechanical strength. Nanostructured polymer electrolytes can provide a platform to address the trade-off between ionic conductivity and mechanical performance. In these materials, the nanoscale phase-separated domains can independently implement different tasks such as ion transport and mechanical support, which can decouple the interferent between these properties and realize their enhancement simultaneously.14–20 Among various polymer nanostructures, the bicontinuous structure shows a unique structural advantage in electrolyte materials due to its dual three-dimensional (3D) spatial connectivity. In a bicontinuous electrolyte, the 3D interconnected ion-conducting domains can reduce the energy barriers for ion diffusion and promote a highly efficient transport; meanwhile, the continuity of the mechanically robust domains can render superior mechanical properties.21–26 Thus, extensive attention has been paid to constructing bicontinuous polymer electrolytes by covalent or noncovalent modification. For example, by precisely placing the ionic groups at the periodic interval sites or the terminal sites of linear polymer backbones, Park group27 and Winey group28 constructed ordered bicontinuous structures, the gyroidal ionic phase, in homopolymers and block copolymers, respectively, achieving remarkably enhanced ionic conductivities. Lodge and co-workers29,30 presented the use of polymerization-induced microphase separation to obtain irregular bicontinuous electrolytes based on cross-linked block copolymers. Besides covalent modification, small nanoparticles like polyhedral oligomeric silsesquioxane can also be employed as nanoadditives to adjust the phase behavior of block copolymers, which can form bicontinuous composite polymer electrolytes, as reported by the Balsara group.31 These advances demonstrate that topological control and nanocomposite formation are both useful strategies toward bicontinuous polymer electrolytes. Polyoxometalates (POMs), the molecularly well-defined clusters of metal oxides,32 are widely used as functional inorganic nanobuilding blocks for the preparation of hybrid polymer nanocomposites.33–47 Moreover, the multicharged anionic characteristic and the high ion-dissociation ability of POMs make them promising molecular electrolytes to transport cations such as protons and lithium ions for energy applications.48–50 Recently, the excellent proton conductivity, electrochemical stability, and water-retention ability of POMs have attracted special interest to develop POM-polymer composite electrolytes for proton conduction.51–61 However, it is difficult to manipulate the distribution of POMs in polymer electrolytes to form well-connected proton conductive pathways, as the strong electrostatic interaction easily induces the disordered aggregation of POMs. Recently, Guiver and co-workers reported a breakthrough they obtained: one-dimensional (1D) POM-based proton-conducting channels in a homopolymer electrolyte matrix through an external magnetic field alignment.57 Nonetheless, a facile, large-area, and cost-efficient strategy to construct highly continuous POM-based channels, for example, networks, in polymer electrolyte matrices, is still needed. Herein, we report the fabrication of high modulus and high proton conductive polymer composite electrolyte membranes that integrate both a bicontinuous structure and a 3D-connected POM-based network by a facile electrostatic self-assembly method, as shown in Figure 1. This whole idea takes full account of the topological structure and mechanical strength of matrix polymers, as well as the complexation feature between matrix polymers and POMs. To facilitate the formation of a bicontinuous structure, an amphiphilic comb-shaped copolymer, poly(ether-ether-ketone)-grafted-poly(vinyl pyrrolidone) (PEEK-g-PVP, abbreviated as PGP) is designed as the matrix polymer, as the hydrophobic main chains and the hydrophilic side chains in such comb copolymers tend to form a continuously structured and microphase-separated morphology.62,63 PEEK, a well-known engineering plastic with high strength and durability, is used to impart mechanical stability; as for the PVP component, it is easily wettened and softened by water, and thus can efficiently promote the dissociation and conduction of protons. Besides, PVP performs a relatively weak electrostatic interaction with POMs, which can suppress the aggregation of POMs and favor the self-assembly of POMs into uniform networks in PVP domains. As expected, the 3D-connected POM networks remarkably enhance the proton conductivity and the mechanical strength of the resultant composite electrolytes. Figure 1 | (a) Synthetic route of PEEK-g-PVP. (b) Preparation strategy of PGP/PW nanocomposites electrolytes. Download figure Download PowerPoint Experimental Methods Synthesis of PGP Here, a general synthetic procedure of PGP is present, and the details are provided in the Supporting Information. Methyl-PEEK with a degree of polymerization of ca. 80, an Mw of ca. 25,000, and a polydispersity index (PDI) of ca. 3.3 was synthesized as the main chain, according to our previous work.64 The benzylmethyl groups on PEEK backbones were used as the grafting sites. By brominating the benzylmethyl groups and a subsequent nucleophilic substitution reaction, ethylxanthate groups were modified onto PEEK, leading to a PEEK-based macromolecular chain transfer agent (PEEK-CTA). Then, different lengths of PVP side chains were grafted from the PEEK-CTA by reversible addition-fragmentation chain transfer (RAFT) polymerization. Moreover, a shuttle CTA was added to regulate the RAFT polymerization to achieve a narrow distribution of PVP side chains.65 In this procedure, N-vinyl pyrrolidone monomer, PEEK-CTA, the shuttle CTA, and 2,2'-Azobis(2-methylpropionitrile) (AIBN) were mixed with N-methyl-2-pyrrolidinone solvent in an ampule bottle, subjected to three freeze–pump–thaw cycles, backfilled with nitrogen gas, and then reacted at 70 °C in an oil bath for 10 h. The solution was dropped into methanol to remove PVP homopolymers and the unreacted monomers. This procedure was repeated twice. The resulting copolymer was washed with water and methanol and then dried under vacuum at 60 °C for 24 h. Preparation of nanocomposite electrolytes H3PW12O40 (PW) was codissolved with PGP in N-methyl pyrrolidone (NMP) to get a homogeneous solution. Then after casting and being dried in air at 60 °C for 48 h, composite electrolyte membranes were readily obtained. Characterizations Proton nuclear magnetic resonance (1H NMR) spectra were measured on a Bruker UltraShield 500 MHz spectrometer (Bruker, Switzerland) in CDCl3 at room temperature. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) spectra were obtained on a Bruker Autoflex MALDI-TOF mass spectrometer (Bruker, Germany). Fourier transform infrared (FTIR) spectra were measured on a Bruker VERTEX 80 V FTIR spectrometer (Bruker, Germany) using KBr pellets in the wavelength range of 4000–400 cm−1. X-ray diffraction (XRD) patterns were obtained on a Rigaku X-ray diffractometer (D/max 2550; Rigaku, Japan) using an X-ray wavelength of 1.542 Å. XRD samples were prepared by PGP/PW nanocomposite films and PW powder. Small-angle X-ray scattering (SAXS) data were obtained on an Anton Paar instrument SAXSess mc2 (Anton Paar instrument, Austria) using an X-ray wavelength of 1.542 Å. Transmission electron microscopy (TEM) was recorded on a JEM-2100F electron microscope (JEOL, Japan). The TEM samples of films were embedded in resin (SPI-Pon 812) and cured at 60 °C overnight. A Leica EM UC7 microtome (Leica Microsystems, Germany) was used to prepare the ultrathin sections of the PGP/PW composite membranes with a thickness of about 80 nm. Mechanical measurements The tensile moduli of the samples were obtained on a tensile tester (SHIMADZU AG-I 1KN; SHIMADZU, Japan) at room temperature at a speed of 2 mm min−1. The compression moduli of the samples were obtained using the continuous stiffness measurement (CSM) method and XP-style actuator on an Agilent Nano Indenter G200 (Agilent Technologies Inc., United States) at room temperature. Ionic conductivity measurement The resistance value (R) was measured by alternating current (AC) impedance spectroscopy on a Solartron 1260A (AMETEK Scientific Ins., United States) impedance analyzer (1 Hz to 10 MHz, two-probe method). The proton conductivity (σ) of the composite membranes was calculated according to σ = L/(R × A), where L is the membrane thickness between the two electrodes, and A is the membrane area in contact with the electrodes. Direct methanol fuel cells The catalyst and the Nafion solution were mixed at room temperature and then sprayed evenly on both sides of the membranes. After drying, the catalyst-coated membranes (CCMs) were obtained. Then the gas diffusion layers (GDLs) were pressed on both sides of the CCMs. The cathode side was provided humidified oxygen at a flow rate of 30 mL min−1, and the anode side was provided methanol solution (2 M) at a flow rate of 2 mL min−1 when testing at 80 °C. Results and Discussion The synthetic route of PEEK-g-PVP is shown in Figure 1a, through grafting the PVP side chains onto a main chain of Methyl-PEEK (Mw of ca. 25,000) by RAFT polymerization. In the resultant PEEK-g-PVP, the polymerization degree of PVP side chains was controlled at 8, 17, 23, and 43, with the same grafting density of 0.1, which means that approximately 10% benzylmethyl groups on PEEK were grafted by PVP. Detailed synthetic procedures and characterizations like NMR are shown in Supporting Information Figures S1–S3. The four comb copolymers were named PGP8, PGP17, PGP23, and PGP43, respectively. Note that the molecular weights of PVP side chains have a narrow distribution, as exemplified by the MALDI-TOF results of PGP23, which displays a PDI of ca. 1.1 ( Supporting Information Figure S4). For the preparation of proton-conducting composite electrolytes, we selected a Keggin-type POM, PW, as the inorganic component, which is a well-known high proton conductor.50 The composite electrolyte membranes were prepared by simply casting the NMP solution of PW and PGP (Figure 1b). The content of PW and PVP in the membranes was optimized to realize both a high proton conduction and a good mechanical property. For polymer electrolytes, their ionic conductivity mainly depends on two factors: the ion-exchange capacity (IEC) and the connectivity of ion-conductive pathways. In our system, the protons from PW clusters contributed to the IEC, and thus a high content of PW can lead to a high IEC. On the other hand, the connectivity of proton transport pathways depends on the percolating degree of PVP/PW hybrid domains. A high volume fraction of PVP improves this connectivity. Based on these discussions, we optimized the composition of PGP/PW nanocomposites by gradually loading more PW or using the PGP with longer PVP side chains. However, we found that the composite membranes become fragile when the content of PW is higher than 30 wt % or the polymerization degree of PVP is more than 43 (Figure 2a and Supporting Information Figure S5a). Thus, PGP23-30 (where 30 represents the PW content of 30 wt %) is the sample with the optimal composition. For a systematical investigation of the influence of PVP length on the resultant composites, we also prepared PGP8-30 and PGP17-30. The three samples are all flexible transparent membranes, implying that PW is compatible with the PGP matrices (Figure 2a). Moreover, XRD patterns of these samples showed no characteristic peaks corresponding to crystalline PW aggregates, which further supports that PW clusters are homogeneously dispersed in the PVP domains (Figure 2b). To evaluate the contribution of the comb polymeric structure of PGP on the composite formation, we also prepared a ternary composite membrane of PW, PEEK, and PVP. Unfortunately, a serious macrophase separation occurred in this sample, which reflects that covalently anchoring PVP onto PEEK is necessary to improve the macroscopic compatibility of different components in this system ( Supporting Information Figure S5b). Figure 2 | (a) Photographs of PGP/PW nanocomposites. (b) XRD patterns of PGP8-30, PGP17-30, PGP23-30 and PW. (c) FTIR spectra of PW, PGP8-30, PGP17-30, and PGP23-30. (d) SAXS profiles of PGP8-30, PGP17-30, and PGP23-30. The inset of (d) corresponds to the high q region. Download figure Download PowerPoint In the composite electrolytes, the hydrophilic PVP chains that contain N-heterocycles can electrostatically interact with the acidic PW clusters through protonation, forming proton conductive domains; meanwhile, the hydrophobic PEEK chains provide mechanical support. The complexation between PVP and PW was studied by using FTIR spectroscopy (Figure 2c). In the IR spectrum of pristine PW, four kinds of oxygen atoms in PW exhibit the characteristic vibration bands at 1080 cm−1 νas(P–Oa), 984 cm−1 νas(W=Od), 890 cm−1 νas(W–Ob–W), and 808 cm−1 νas(W–Oc–W), (Oa, oxygen in PO4 tetrahedron; Ob, corner-sharing oxygen; Oc, edge-sharing oxygen; Od, terminal oxygen). These vibration bands were all displayed in the IR spectra of PGP8-30, PGP17-30, and PGP23-30, indicating that the PW clusters retained their structural integrity in the nanocomposites. In contrast to pristine PW, slight shifts were observed in these vibration bands for all three nanocomposites. The νas(P–Oa) and νas(W=Od) peaks shifted to the low-frequency region, appearing at 1079 and 977 cm−1, whereas the νas(W–Ob–W) and νas(W–Oc–W) peaks shifted to the high-frequency region, appearing at 897 and 818 cm−1, respectively. Similar phenomena were also found in the IR spectra of other PVP/POM composites in the literature, which were supposed to be due to the varied electrostatic environment of POM anions.52,66 The nanoscale morphologies of the composite membranes of PGP8-30, PGP17-30, and PGP23-30 were studied by using the SAXS method and the TEM. Under the same SAXS testing conditions, no obvious scattering signal was observed for PGP8-30. However, PGP17-30 and PGP23-30 displayed scattering peaks at the wave vector q of 0.18 and 0.20 nm−1, corresponding to the d-spacing of ca. 34.9 and 31.4 nm, respectively, given by 2π/q, which indicated that PGP17-30 and PGP23-30 both possess typical microphase-separated morphologies (Figure 2d). Notably, the SAXS profiles of PGP8-30, PGP17-30, and PGP23-30 at the high q region show the scattering peaks at 5.01, 4.44, and 3.04 nm−1, corresponding to the d-spacing of 1.3, 1.4, and 2.1 nm, respectively (the inset of Figure 2d). These peaks reflect the distribution distance between PW clusters in polymer matrices.58 In these samples, the contents of POMs are the same, but the volume fractions of PVP are increased. The increasing distribution distance of PW reflects that PW clusters are dispersed more and more sparsely when the PVP domains become larger. Such an increasable spacing between PW further supports that PW clusters are homogeneously distributed in the PVP domains, without forming crystalline aggregation in which the positions of PW should be fixed. To further clarify the microphase structures of the composite membranes, we microtomed the membranes into ultrathin sections with a thickness of ca. 80 nm for TEM characterizations. As the existence of heavy element tungsten increases the electron density, the PVP/PW domains are darker than the PEEK domains, and thus they are easily distinguished under the TEM observation. The TEM image of PGP8-30 shows a structure similar to the disordered spherical phase, in which most of the dark-colored PVP/PW domains are separated by the light-colored PEEK domains, like isolated islands with irregular shapes (Figures 3a and 3d). Thus, the PW-based proton-conducting domains are not continuous in PGP8-30. For the cases of PGP17-30 and PGP23-30, they both exhibit a bicontinuous structure in which the PVP/PW domains and the PEEK domains are highly intertwined (Figures 3b and 3c). Note that the PVP/PW and the PEEK domains in PGP23-30 are narrower than those in PGP17-30. This tendency is consistent with the decreased d-spacing from PGP17-30 to PGP23-30 in the SAXS results. Under higher magnification, the PVP/PW domains in PGP17-30 and PGP23-30 are observed as highly interconnected networks, with the average mesh diameters concentrating at around 20 and 24 nm, respectively (Figures 3e and 3f). Figure 3 | Bright-field TEM images of the ultrathin microtomed sections of the nanocomposite membranes. (a and d) PGP8-30, (b and e) PGP17-30, and (c and f) PGP23-30. Download figure Download PowerPoint Upon focusing on a mesh in PGP23-30 by TEM, we can reveal that the networks are formed by the nanosized PW clusters (Figure 4a). Because the PGP23-30 sections have a certain thickness, the PW clusters at different vertical heights in the thin sections are all projected into the TEM image, and their projections are overlapped. As a consequence, the observed dark dots are 2–5 nm, larger than the actual diameter of an individual PW, which is 1 nm. Even so, several dots still display the crystalline lattices of PW clusters (inset of Figure 4a). To further confirm that the networks are formed by PW, we employed high-angle annular dark-field scanning TEM (HAADF-STEM) to observe a mesh of the networks. In the HAADF-STEM mode, the heavier components appeared brighter against the polymer matrices. We found that the mesh backbone is brighter than the surrounding region, which confirmed that it is formed by PW (Figure 4b). Also, the elemental mapping by energy-dispersive X-ray spectroscopy (EDXS) confirmed the distribution of tungsten in the mesh backbone (Figure 4c). These results demonstrated the formation of PW-based 3D networks in PGP17-30 and PGP23-30. To get an insight into the forming conditions of bicontinuous structure in PGP17-30 and PGP23-30, we analyzed the volume fractions of PVP/PW domains (fPVP/PW) which is the ionic phase (detailed calculations are shown in the Supporting Information). The fPVP/PW of PGP17-30 and PGP23-30 was 43% and 49%, respectively. In contrast, the fPVP/PW of PGP8-30 was only 28% (Table 1). Therefore, a similar volume fraction of the ionic phase and the neutral phase in the nanocomposites was beneficial to the formation of bicontinuous microphase structures. Figure 4 | A magnified mesh in the ultrathin microtomed sections of PGP23-30, imaged by (a) high-resolution bright-field TEM, (b) HAADF-STEM, and (c) EDXS mapping of W. Note that (b) and (c) correspond to the same region. The inset of (a) shows the crystalline lattice phase of PW clusters in the mesh. Download figure Download PowerPoint Table 1 | Properties of the PGP/PW Nanocomposites Sample fPVP/PW (%)a σ (mS cm−1)b Ea (eV)c Tensile Modulus (Mpa) Compression Modulus (Gpa) PGP8-30 28 7.6 0.16 723 3.2 PGP17-30 43 18.2 0.13 861 4.0 PGP23-30 49 32.7 0.12 942 4.5 aVolume fraction of PVP/PW domains. bProton conductivity at 353 K. cActivation energy for proton conduction. The PW networks endow the PGP/PW nanocomposites with enhanced mechanical properties. The tensile modulus of PGP8-30, PGP17-23, and PGP23-30 is 723, 861, and 942 MPa, respectively, exhibiting an increasing tendency (Table 1). When assembling fuel cells, the proton-conducting membranes are tightly pressed together with the bipolar plates, which are used to ensure a sealed system and prevent the leakage of fuels. Considering this point, we also tested the compression modulus of the membranes on silicon wafers. Due to the influence of substrates, the obtained compression moduli were higher than the tensile modulus. We also observed an increasing tendency of the compression moduli from 3.2, 4.0 to 4.5 GPa, for the cases of PGP8-30, PGP17-23, and PGP23-30. This tendency is in agreement with the tensile testing results (Table 1 and Supporting Information Figure S6), which reflects that the presence of connective and compact PW networks can remarkably enhance the mechanical strength of PGP/PW nanocomposites. The role of POMs as nanoenhancers in polymer nanocomposites has also been reported elsewhere.37,41 Particularly in our previous works, we found that POMs can offer a remarkable mechanical enhancement when they self-assemble into certain percolated structures in polymer matrices, for example, networks.41 The present work belongs to such a case. The proton conductivities of the wet membranes were measured in the temperature range of 303–353 K by AC impedance measurements (Figures 5a and 5b). The detailed conductivity values are shown in Supporting Information Table S1. We found that the conductivities of PGP8-30, PGP17-23, and PGP23-30 at 353 K are increased from 7.6, 18.2 to 32.7 mS cm−1, accompanied by a decreased activation energy from 0.16, 0.13 to 0.12 eV by Arrhenius fitting (Figure 5c and Table 1), respectively, which corresponds to a Grotthuss-type proton conduction mechanism.60 The contents of PW in PGP8-30, PGP17-23, and PGP23-30 are all 30 wt % and thus lead to the same IEC of 0.31 mmol g−1. Given this situation, the enhanced conductivities and the decreased Ea from PGP8-30, PGP17-23 to PGP23-30 should be ascribed to the improved connectivity of PW networks but not the IEC. It should be mentioned that the proton conductivity of PGP23-30 is 16.9 mS cm−1 at 303 K. This value is 100-fold higher than the bicontinously structured poly(styrene-block-2-vinylpyridine)/H4SiW12O40 nanocomposite (0.13 mS cm−1 at 303 K) reported in our previous work,54 although the content of H4SiW12O40 in the latter is higher than 45 wt %. We believe that the enhanced proton conductivity is due to the better water absorption of PVP than that of poly(2-vinylpyridine), which introduces more water into the system to facilitate proton transport. Therefore, when designing polymer/POM composite electrolytes for proton conduction, it is necessary to select the polymer component with strong hydrophilicity to construct the ionic phase region. Figure 5 | The Nyquist plots of PGP/PW nanocomposites at (a) 303 and (b) 353 K. (c) Temperature-dependent conductivity of PGP8-30, PGP17-30, and PGP23-30 from 303 to 353 K. (d) Polarization curves and power density curves of the DMFCs using PGP23-30 membranes. Download figure Download PowerPoint Since PGP23-30 shows the best performance in both mechanical strength and proton conductivity among all the samples, we chose it to assemble the membrane electrodes for direct methanol fuel cells (DMFCs). The cell performance was evaluated at 80 °C under the operating conditions by using a 2 M aqueous solution of methanol on the anode side and oxygen gas on the cathode side. The open-circuit voltage and the maximum power density reached 0.82 V and 10.2 mW cm−2, respectively (Figure 5d). It is worthy of note that such a cell performance is obtained at a relatively low IEC value. The IEC of PGP23-30 is only 0.31 mmol g−1. For the Nafion-based DMFCs, the maximum power densities at 80 °C are generally 25–50 mW cm−2,67,68 but Nafion has a high IEC of ca. 0.9 mmol g−1 which is threefold the IEC of our samples.63 Thus, our PGP/PW composite electrolytes still have the potential to be improved. Conclusion We demonstrate the preparation of bicontinuously structured polymer composite electrolytes containing 3D-connected POM-based nanonetworks, through the electrostatic self-assembly of PW and a comb copolymer PEEK-g-PVP. The resultant nanocomposites can effectively overcome the trade-off between ionic conductivity and mechanical strength, exhibiting a high proton conductivity over 30 mS cm−1 and a modulus higher than 4 GPa, wherein the 3D-connected PW networks contribute a dual role of both proton-conducting pathways and mechanical reinforcers. The D

A fluorescent nanoprobe based on HgS/ZnS core/shell quantum dots for in-situ rapid visual detection of Cr3+

Heavy metal Cr3+ has been regarded as a threat to the environment and human health. So it is crucial to find a rapid method for detecting Cr3+, especially for the trace amounts. Here, HgS/ZnS core/shell quantum dots (HgS/ZnS QDs) with a quantum yield of 22.6% were synthesized as a fluorescent nanoprobe for detection of Cr3+ with a detection limit of 0.19 nM. With captopril as stabilizer, the formation of the core/shell structure of HgS/ZnS QDs was demonstrated by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HR-HAADF-STEM). The substantial fluorescence (580 nm) quenching of HgS/ZnS QDs which could be seen immediately by the naked eyes was induced selectively and sensitively by Cr3+, even in the case of Cr3+ mixed with other interfering ion. The HgS/ZnS QDs exhibited a stable visual response to Cr3+ from 1 μM, which just conformed to World Health Organization (WHO) standards for drinking water. Moreover, further experiments indicated that the mechanism of fluorescence quenching lied on the aggregation of the HgS/ZnS QDs. These results provide a novel insight for in situ rapid visual detection of heavy metal ions.

The maximum a posteriori probability rule for atom column detection from HAADF STEM images

Recently, the maximum a posteriori (MAP) probability rule has been proposed as an objective and quantitative method to detect atom columns and even single atoms from high-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images. The method combines statistical parameter estimation and model-order selection using a Bayesian framework and has been shown to be especially useful for the analysis of the structure of beam-sensitive nanomaterials. In order to avoid beam damage, images of such materials are usually acquired using a limited incoming electron dose resulting in a low contrast-to-noise ratio (CNR) which makes visual inspection unreliable. This creates a need for an objective and quantitative approach. The present paper describes the methodology of the MAP probability rule, gives its step-by-step derivation and discusses its algorithmic implementation for atom column detection. In addition, simulation results are presented showing that the performance of the MAP probability rule to detect the correct number of atomic columns from HAADF STEM images is superior to that of other model-order selection criteria, including the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Moreover, the MAP probability rule is used as a tool to evaluate the relation between STEM image quality measures and atom detectability resulting in the introduction of the so-called integrated CNR (ICNR) as a new image quality measure that better correlates with atom detectability than conventional measures such as signal-to-noise ratio (SNR) and CNR.

Atomic-scale structural characterization of grain boundaries in epitaxial Ge/Si microcrystals by HAADF-STEM

The atomic structure of grain boundaries in Ge micro-crystals grown on Si pillars for the fabrication of a monolithically integrated X-ray detector was studied by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Three different boundaries are found in Ge: Σ3{111} coherent twin boundaries, Σ3{112} incoherent twin boundaries, and Σ9{122} and Σ27{552} grain boundaries. They are described using the structural unit models containing single columns. Remarkably, we find for the first time a Σ3{112} incoherent twin boundary exhibiting two different atomic structures; one symmetric and one asymmetric. Their co-occurrence is explained by the presence of a small step in the boundary plane and the introduction of dislocations. Likewise, the atomic structure of junctions formed by the interaction of twin boundaries which result in Σ9{122} and Σ27{552} grain boundaries is also revealed. Geometrical phase analysis is applied to map the strain fields at two triple junctions and to uncover the position of the dislocations.

A tool for automatic recognition of [110] tilt grain boundaries in zincblende-type crystals

The local atomic structure of [110] tilt grain boundaries (GBs) formed in ∼100 nm-sized GaAs nanocrystals, which crystallize in the non-centrosymmetric zincblende-type structure with face-centred cubic lattice symmetry, was imaged and analysed by means of high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The nanocrystals were grown by metal–organic vapour phase epitaxy on top of (001) Si nanotips embedded in an oxide matrix. This paper introduces an automatic analysis method and corresponding processing tool for the identification of the GBs. The method comprises (i) extraction of crystallographic parameters,i.e.misorientation angles and transformation matrices for the different crystal parts (grains/twins) observed by HAADF-STEM, and (ii) determination of their common plane(s) by modelling all possible intersections of the corresponding three-dimensional reciprocal lattices. The structural unit model is also used to characterize the GB structures and to validate the data obtained by the developed algorithm.

Synthesis of ε-Keggin-Type Cobaltomolybdate-Based 3D Framework Material and Characterization Using Atomic-Scale HAADF-STEM and XANES.

We describe the preparation of ε-Keggin-type cobaltomolybdate-based 3D frameworks with sodium cations, NaH9[ε-CoIIMoV8MoVI4O40CoII2], and their characterization by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy. Atomic-scale HAADF-STEM images of ε-Keggin compounds were obtained for the first time, and positions of Mo and Co were confirmed. Furthermore, clear evidence of the presence of a CoO4 tetrahedron was obtained by X-ray absorption near-edge structure (XANES) analysis. Their characterization clearly revealed that ε-Keggin-type cobaltomolybdate units, [ε-CoMo12O40]n-, constructed by a central CoIIO4 tetrahedron and 12 surrounding MoO6 octahedra, are linked with CoII to form 3D frameworks.

Encapsulation of Two Potassium Cations in Preyssler-Type Phosphotungstates: Preparation, Structural Characterization, Thermal Stability, Activity as an Acid Catalyst, and HAADF-STEM Images.

Dipotassium cation (K+)-encapsulated Preyssler-type phosphotungstate, [P5W30O110K2]13-, was prepared by heating monobismuth (Bi3+)-encapsulated Preyssler-type phosphotungstate, [P5W30O110Bi(H2O)]12-, in acetate buffer in the presence of an excess amount of potassium cations. Characterization of the isolated potassium salt, K13[P5W30O110K2] (1a), and its acid form, H13[P5W30O110K2] (1b), by single crystal X-ray structure analysis, 31P and 183W nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, cyclic voltammetry (CV), high-resolution electrospray ionization mass spectroscopy (HR-ESI-MS), and elemental analysis revealed that two potassium cations are encapsulated in the Preyssler-type phosphotungstate molecule with formal D5h symmetry, which is the first example of a Preyssler-type compound with two encapsulated cations. Incorporation of two potassium cations enhances the thermal stability of the potassium salt, and the acid form shows catalytic activity for hydration of ethyl acetate. Packing of the Preyssler-type molecules was observed by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).

Atomic Structure of Wurtzite CdSe (Core)/CdS (Giant Shell) Nanobullets Related to Epitaxy and Growth.

Heteronanocrystals consisting of a CdSe core and a giant CdS shell have shown remarkable optical properties which are promising for applications in opto-electrical devices. Since these properties sensitively depend on the size and shape, a morphological characterization is of high interest. Here, we present a high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) study of CdSe (core)/CdS (giant shell) heteronanocrystals. Electron tomography reveals that the nanocrystals have a bullet shape, either ending in a tip or a small dip, and that the CdSe core is positioned closer to the tip (or dip) than to the hexagonal base. Based on a high resolution HAADF-STEM study, we were able to determine all the surface facets. We present a heuristic model for the different growth stages of the CdS crystal around the CdSe core.

Control of Tetragonal Distortion and Ferroelectricity in Bi(Zn1/2Ti1/2)O3-Based Lead-Free Ferroelectric Thin Films

Lead-free ferroelectric materials with superior property have been demanded, yet a few lead-free ferroelectrics such as alkali-based (K,Na)NbO3 family and bismath-based (Bi,Na)TiO3 family have been reported. Pb(Zr,Ti)O3[PZT] is one of very famous ferroelectric because their ferroelectric and piezoelectric properties can be controlled by their composition and domain structure. And almost of all for application use, PZT having tetragonal and morphotropic phase boundary[MPB] composition have been used. From a view point of material design in PZT case, mother material of tetragonal PbTiO3 is very important for designing MPB. However, it is know that there are only BaTiO3 and (Bi,K)TiO3 in the field of tetragonal perovskite materials. Novel lead-free tetragonal ferroelectric materials are really wanted. Recently, novel metastable phase materials prepared by high pressure synthesis have been reported. Among them, BiCoO3 1 and Bi(Zn0.5Ti0.5)O3 2 are promising ferroelectric materials due to their high tetragonality, c/a, of over 1.2. Large spontaneous polarization is expected by first principal calculation. BiCoO3 is a one of multiferroic, but unfortunately it is very leaky because of containing Co3+(d 6). On the other hand, Bi(Zn0.5Ti0.5)O3 is constructed by only d 0 and d 10 ions. Unfortunately, nobody reported their ferroelectricity. We can expect that leakage property will be better than that of BiCoO3. Therefore, we have focused on Bi(Zn0.5Ti0.5)O3 and prepare novel ferroelectric materials. Epitaxial Bi(Zn0.5Ti0.5)O3-BiFeO3 thin films with about 300 nm thickness were grown at 700oC on (100)cSrRuO3//(100)SrTiO3 by pulsed metalorganic chemical vapor deposition using Bi[(CH3)2(2-(CH3)2NCH2C6H4)] (Tosoh Co. Ltd), Zn(C14H25O2)2, Ti(O∙i-C3H7)4, Fe(C2H5C5H4)2 and oxygen gas as the source materials. SrRuO3 films were grown by RF magnetron sputtering method. Film thickness and composition were confirmed by surface profilometry and X-ray fluorescence (XRF) calibrated using standard samples. Crystal structure of the deposited films was characterized by X-ray diffraction (XRD) analysis using a four-axis diffractometer with CuKa radiation. Atomic microstructures of the films were observed by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Pt/[Bi(Zn1/2Ti1/2)O3- BiFeO3]/SrRuO3 capacitor structure was used for the electrical measurements after making circular Pt top electrodes of 100 mm in diameter, which were deposited by e-beam evaporation. Piezoresponse mappings and longitudinal piezoelectric coefficient (d 33) was measured using piezoresponse force microscopy. High resolution Advanced X-ray reciprocal space mapping leads us to know all diffractions from the films, suggesting that crystal structure and domain structure could be investigated easily. This mapping reveals d-spacing information along both the substrate surface normal and off-normal directions. It is observed that all diffraction peaks from the films can be identified to tetragonal P4mm symmetry as same as Bi(Zn1/2Ti1/2)O3, and a single perovskite phase is achieved to fabricate. The out-of-plane (c-axis) and in-plane (a-axis) lattice parameters calculated from diffractions are 0.465 nm and 0.381 nm, respectively, resulting in a giant tetragonal distortion of (c/a)-1 = 22%. This value is 3.5 times larger than the 6.3% of PbTiO3. In addition, this [(c/a)-1] is larger than the reported one for pure Bi(Zn1/2Ti1/2)O3, [(c/a)-1] = 21%. Similar enlargement of the [(c/a)-1] by the substitution with BiFeO3 has been reported in PbTiO3-BiFeO3, even if the substituted BiFeO3 has rhombohedral symmetry. We have also conducted cross sectional high resolution HAADF-STEM for visualization of the crystal structure directly. The “90o domain boundary” has a 51o/39o angle due to the large tetragonality, which is in good agreement with 50.7o/40.3 o value based on the crystal structure information obtained by XRD measurement. Z-contrast image to perovskite unit cell shows that the B-site ions are also displaced along [001] significantly away from the center. There are in good agreement with structure analysis of the Bi(Zn1/2Ti1/2)O3 prepared by high pressure synthesis. In addition, as XRD results showed no super lattice peaks, B-site cations in perovskite are not ordered. These results clearly indicate that this material is characterized as displacive-type ferroelectric, and unusual angle of the 90o domain is originated from large tetragonal distortion. We will also perform to measure ferroelectricity for discussing relationship between ferroelectricity/piezoelectricity and tetragonality in these materials. 1A. A. Belik, S. Iikubo, K. Kodama, N. Igawa, S. Shamoto, S. Niitaka, M. Azuma, Y. Shimakawa, M. Takano, F. Izumi, and E. Takayama-Muromachi, Chem. Mater. 18 (2006) 798. 2M. R. Suchomel, A. M. Fogg, M. Allix, H. Niu, J. B. Claridge, M. J. Ro sseinsky, Chem. Mater. 18 (2006) 4987.

Quantification of Bi distribution in MOVPE-grown Ga(AsBi) via HAADF STEM

The importance of dilute bismide III/V semiconductors increases and their physical properties open up a wide range for applications. Therefore, high quality layers are required, what is difficult to achieve, as these alloys are highly metastable and phase separation can occur. We use HAADF (high angle annular dark field) imaging in aberration-corrected STEM (scanning transmission electron microscopy) to quantify the Bi distribution in MOVPE (metal organic vapor phase epitaxy) grown material at large length scales as well as down to the nanoscale. This is done for different Bi fractions in the solid, which are achieved by changing the MOVPE growth conditions. The composition of the Ga(AsBi) was determined by comparing frozen lattice annular dark field simulations of different Ga(AsBi) supercells with the experimental HAADF STEM images. The derived compositions are in quantitative agreement with results of HR-XRD (high resolution X-ray diffraction) and SIMS (secondary ion mass spectroscopy) of the same samples. We furthermore show a homogeneous Bi distribution for the investigated samples, which contain up to 5% Bi. By separating the group III intensities from the group V intensities in high resolution HAADF STEM images, we can investigate the group V intensity distribution only. Moreover from the statistical evaluation, we conclude that the Bi is distributed homogeneously across the group V lattice positions, confirming the excellent structural quality of the layers. This result is also important for device applications of dilute bismide alloys, as homogeneous layers are a prerequisite for optimizing optoelectronic applications.

Analysis of edge threading dislocations b→=12〈110〉 in three dimensional Ge crystals grown on (001)-Si substrates

Threading dislocations (TDs) in germanium (Ge) crystals epitaxially grown on a patterned (001)-silicon (Si) substrate are investigated using transmission electron microscopy (TEM) techniques. Analysis of dislocations performed on the Ge crystals reveals 60° and edge TDs with Burgers vector b→=12〈110〉. High-angle annular dark-field scanning TEM (HAADF-STEM) is used to observe the core of the edge TDs at atomic scale. Pairs of TDs with b→=12〈110〉 are present in the material running parallel at small distances between them (0.5–1.5 nm). The observation of such parallel dislocation pairs in Ge has not been documented before. The interaction between the edge dislocation pairs is obtained experimentally from the high-resolution HAADF-STEM images by applying geometrical phase analysis. The experimental strain maps are compared to analytical calculations based on the anisotropic elastic theory demonstrating a good match between them.

Comprehensive study of phase transformation in age-hardening of Mg–3Nd–0.2Zn by means of scanning transmission electron microscopy

Effects of thermal history on precipitation behavior and precipitation kinetics in Mg–3Nd–0.2Zn (wt.%) alloy were studied. Microstructure analysis of the aged alloys revealed the presence of new phases in the microstructure in addition to the old description: Super saturated solid solution (SSSS)→G. P. zones→β″→β′/β1→β, reported for Mg–Nd system. In this regard, the sequence of precipitation for the studied alloy was identified as: Super saturated solid solution (SSSS)→Clusters of atoms→G. P. zones (I, II, III)→β′→β2→β1/γ′→β. The formation of clusters of solute atoms in this system was confirmed by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and atom probe tomography analysis. In addition, new metastable phases designated as G. P. zones (I, II, III) and β2 were identified in the microstructure of the aged alloys by means of high resolution HAADF-STEM. The β2 phase was found to be a transition phase in transformation of β′ to β1. Furthermore, the microstructure analysis revealed that the structure of β1 phase is indeed a body centered tetragonal structure in spite of face centered cubic structure that has been reported for this phase in the literature.

Conformal and atomic characterization of ultrathin CdSe platelets with a helical shape.

Currently, ultrathin colloidal CdSe semiconductor nanoplatelets (NPLs) with a uniform thickness that is controllable up to the atomic scale can be prepared. The optical properties of these 2D semiconductor systems are the subject of extensive research. Here, we reveal their natural morphology and atomic arrangement. Using cryo-TEM (cryo-transmission electron microscopy), we show that the shape of rectangular NPLs in solution resembles a helix. Fast incorporation of these NPLs in silica preserves and immobilizes their helical shape, which allowed us to perform an in-depth study by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Electron tomography measurements confirm and detail the helical shape of these systems. Additionally, high-resolution HAADF-STEM shows the thickness of the NPLs on the atomic scale and furthermore that these are consistently folded along a ⟨110⟩ direction. The presence of a silica shell on both the top and bottom surfaces shows that Cd atoms must be accessible for silica precursor (and ligand) molecules on both sides.

Growth and local structure of BiFeO3 thin films with giant tetragonality on SrRuO3-buffered SrTiO3(001) substrate by ion beam sputtering

Rhombohedral (R-) and tetragonal-like (T-) BiFeO3 (BFO) mixed-phase thin films and single R-phase BFO thin films have been grown on SrRuO3-buffered SrTiO3 (STO) (001) substrate by ion beam sputtering. From X-ray diffraction study, it was found that T-BFO and R-BFO mixed-phase thin films can be grown at a low oxygen partial pressure of 1 mPa, and the lattice parameters of the T-BFO and R-BFO were a = 0.381 nm, c = 0.461 nm (c/a = 1.22), and a = 0.394 nm and c = 0.400 nm (c/a = 1.02), respectively. High-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation and energy dispersive X-ray analysis (EDXA) mappings have revealed a clear T-BFO lattice and a–c domain in the middle of the thin films. However, Bi-rich and Fe-rich secondary phases were also found with the T-BFO phase. Therefore, it can be concluded that secondary phases such as Bi2O3 or Bi-poor phase are needed for T-BFO formation on STO (001) substrate, and the a–c domain formation does not occur by stress relaxation but matching of the growth directions of different domains.

Statistical distribution of single atoms and clusters of supported Au catalyst analyzed by global high-resolution HAADF-STEM observation with morphological image-processing operation

We have developed a quantitative particle size analytical method at the single atomic level employing electron microscopy and image processing for the investigation of supported metal catalysts. In the present study, a supported gold (Au) catalyst containing sub-nano clusters and individual atoms was globally observed by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) using spherical aberration (Cs)-corrected TEM. To fully extract structural information of the Au clusters and individual atoms from the HAADF-STEM images, a morphological image-processing operation was applied. The resulting mean particle size was in good agreement with particle sizes estimated from average information provided by X-ray absorption fine structure analysis. It is demonstrated that the present HAADF-STEM image analysis gives a quantitative particle size distribution measurement of supported Au clusters and individual atoms.