Toward a Metal Anode-Free Zinc-Air Battery for Next-Generation Energy Storage.

Rechargeable aqueous zinc-air batteries (ZABs) promise high energy density and safety. However, the use of conventional zinc anodes affects the energy output from the battery, so that the theoretical energy density is not achievable under operation conditions. A large portion of the zinc is shielded by anode passivation during the discharge process and remains electrochemically unused, making the operation of rechargeable ZABs inefficient up to date. In a metal anode-free ZAB, there is no unnecessary excess zinc if the zinc reservoir can be precisely adjusted by electrodeposition of zinc from the electrolyte. In this respect, an anode-free battery uses the electrolyte offering a dual-mode functionality not only providing ionic conductivity but also being the source of zinc. In addition, it is shown that a defined porous anode architecture is crucial for high rechargeability in this new type of ZAB. 3D-spatially arranged carbon nanotubes as geometrically defined host structures allow a homogeneous zinc deposition from the electrolyte. Together with carbon nanohorns as an active 2e- catalyst on the cathode side, the rechargeability of this new concept reaches up to 92%.

Atomic Layer Deposition of Ternary Indium/Tin/Aluminum Oxide Thin Films, Their Characterization and Transistor Performance under Illumination.

Multilayered heterostructures comprising of In2 O3 , SnO2 , and Al2 O3 were studied for their application in thin-film transistors (TFT). The compositional influence of tin oxide on the properties of the thin-film, as well as on the TFT characteristics is investigated. The heterostructures are fabricated by atomic layer deposition (ALD) at 200 °C, employing trimethylindium (TMI), tetrakis(dimethylamino)tin (TDMASn), trimethylaluminum (TMA), and water as precursors. After post-deposition annealing at 400 °C the thin-films are found to be amorphous, however, they show a discrete layer structure of the individual oxides of uniform film thickness and high optical transparency in the visible region. Incorporation of only two monolayers of Al2 O3 in the active semiconducting layer the formation of oxygen vacancies can be effectively suppressed, resulting in an improved semiconducting and switching behavior. The heterostacks comprising of In2 O3 /SnO2 /Al2 O3 are incorporated into TFT devices, exhibiting a saturation field-effect mobility (µsat ) of 2.0 cm2 ⋅ V-1 s-1 , a threshold-voltage (Vth ) of 8.6 V, a high current on/off ratio (IOn /IOff ) of 1.0×107 , and a subthreshold swing (SS) of 485 mV ⋅ dec-1 . The stability of the TFT under illumination is also altered to a significant extent. A change in the transfer characteristic towards conductive behavior is evident when illuminated with light of an energy of 3.1 eV (400 nm).

Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc. Thin Film Transistor and Sensoric Behaviour of ZnO Films and Rods.

Invited for the cover of this issue is Jörg J. Schneider and co-workers at Technical University Darmstadt, Helmholtz-Zentrum Dresden-Rossendorf and KIT Karlsruhe. The image depicts the application of high energy generated electron/positron couples which are able to detect defects sites in semiconducting zinc oxide thin films. Read the full text of the article at 10.1002/chem.202004270.

Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc: Thin-Film Transistor and Sensor Behavior of ZnO Films and Rods.

Zinc oxide thin films are fabricated by controlled oxidation of sputtered zinc metal films on a hotplate in air at temperatures between 250 and 450 °C. The nanocrystalline films possess high relative densities and show preferential growth in (100) orientation. Integration in thin-film transistors reveals moderate charge carrier mobilities as high as 0.2 cm2 V-1 s-1 . The semiconducting properties depend on the calcination temperature, whereby the best performance is achieved at 450 °C. The defect structure of the thin ZnO film can be tracked by Doppler-broadening positron annihilation spectroscopy as well as positron lifetime studies. Comparably long positron lifetimes suggest interaction of zinc vacancies (VZn ) with one or more oxygen vacancies (VO ) in larger structural entities. Such VO -VZn defect clusters act as shallow acceptors, and thus, reduce the overall electron conductivity of the film. The concentration of these defect clusters decreases at higher calcination temperatures as indicated by changes in the S and W parameters. Such zinc oxide films obtained by conversion of metallic zinc can also be used as seed layers for solution deposition of zinc oxide nanowires employing a mild microwave-assisted process. The functionality of the obtained nanowire arrays is tested in a UV sensor device. The best results with respect to sensor sensitivity are achieved with thinner seed layers for device construction.

Synthesis and Assembly of Zinc Oxide Microcrystals by a Low-Temperature Dissolution-Reprecipitation Process: Lessons Learned About Twin Formation in Heterogeneous Reactions.

Cobalt-doped zinc oxide single crystals with the shape of hexagonal platelets were synthesized by thermohydrolysis of zinc acetate, cobalt acetate, and hexamethylenetetramine (HMTA) in mixtures of ethanol and water. The mineralization proceeds by a low-temperature dissolution-reprecipitation process from the liquid phase by the formation of basic cobalt zinc salts as intermediates. The crystal shape as well as twin formation of the resulting oxide phase can be influenced by careful choice of the solvent mixture and the amount of doping. An understanding of the course of the reaction was achieved by comprehensive employment of analytical techniques (i.e., SEM, XRD, IR) including an in-depth HRTEM study of precipitates from various reaction stages. In addition, EPR as well as UV/Vis spectroscopic measurements provide information about the insertion of the cobalt dopant into the zincite lattice. The Langmuir-Blodgett (LB) technique is shown to be suitable for depositing coatings of the platelets on glass substrates functionalized with polyelectrolyte multilayers and hence is applied for the formation of monolayers containing domains with ordered tessellation. No major differences are found between deposits on substrates with anionic or cationic surface modification. The adherence to the substrates is sufficient to determine the absolute orientation of the deposited polar single crystals by piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM) studies.

Molecular Precursors for ZnO Nanoparticles: Field-Assisted Synthesis, Electrophoretic Deposition, and Field-Effect Transistor Device Performance.

Zinc complexes with multidentate Schiff base ligands are suitable precursors for ZnO in microwave-assisted transformation reactions. [Bis(acetylacetonato)ethylenediimine]zinc(II) and [bis(methylacetoacetato)ethylenediimine]zinc(II) have been synthesized with high purity and good yield from the direct reaction of the respective diimine ligand with diethylzinc in tetrahydrofuran. The thermal decay is studied by thermogravimetry coupled with online infrared spectroscopy. The ceramization reaction in ethoxyethanol yields stable dispersions of spherical ZnO nanoparticles with very small particle sizes (around 5-6 nm), which can be employed for coating and thin-film deposition processes. Field-effect transistors (FETs) composed of thin films fabricated from these semiconducting ZnO particles possess charge-carrier mobilities of 6.0 × 10-3 and 5.4 × 10-2 cm2/(V s) after processing at 350 and 450 °C, respectively. Electrophoretic deposition affords dense film coatings composed of these ZnO nanoparticles with thicknesses of 30-90 nm on ITO (indium tin oxide) glass-electrodes. The positive ζ-potentials of the ZnO nanoparticles in these dispersions are in agreement with the electrocoating process at the cathode.

Gas adsorption capacity in an all carbon nanomaterial composed of carbon nanohorns and vertically aligned carbon nanotubes.

Whereas vertically aligned carbon nanotubes (VACNTs) typically show a promising adsorption behavior at high pressures, carbon nanohorns (CNHs) exhibit superior gas adsorption properties in the low pressure regime due to their inherent microporosity. These adsorption characteristics are further enhanced when both materials are opened at their tips. The so prepared composite material allows one to investigate the effect of physical entrapment of CO2 molecules within the specific adsorption sites of VACNTs composed of opened double walled carbon nanotubes (CNTs) and in specific adsorption sites created by spherically aggregated opened single walled carbon nanohorns. Combining 50 wt% of tip opened CNTs with tip opened CNHs increases the CO2 adsorption capacity of this material by ∼24% at 30 bar and 298 K compared to opened CNHs alone.

Evidence for electrohydrodynamic convection as a source of spontaneous self-ordering in porous anodic alumina films.

A comparative study of self-ordering behaviour of anodic alumina films fabricated in a series of diluted (down to 0.05 M) oxalic acid electrolytes allowed developing a relationship between the supporting electrolyte concentration and self-ordering voltages for the formation of porous oxide materials. Besides its practical importance, this work elucidates some fundamental principles of porous alumina formation, e.g. it suggests that the cell patterning arises from the electrohydrodynamic (EHD) convection process rather than the interfacial tension gradients near the anode surface (Marangoni-type instability).

Structural Polymorphism and Thin Film Transistor Behavior in the Fullerene Framework Molecule 5,6;11,12-di-o-Phenylenetetracene.

The molecular structure of the hydrocarbon 5,6;11,12-di-o-phenylenetetracene (DOPT), its material characterization and evaluation of electronic properties is reported for the first time. A single-crystal X-ray study reveals two different motifs of intramolecular overlap with herringbone-type arrangement displaying either face-to-edge or co-facial face-to-face packing depicting intensive π-π interactions. Density functional theory (DFT) calculations underpin that a favorable electronic transport mechanism occurs by a charge hopping process due to a π-bond overlap in the DOPT polymorph with co-facial arene orientation. The performance of polycrystalline DOPT films as active organic semiconducting layer in a state-of-the-art organic field effect transistor (OFET) device was evaluated and proves to be film thickness dependent. For 40 nm layer thickness it displays a saturation hole mobility (µhole ) of up to 0.01 cm(2) V(-1) s(-1) and an on/off-ratio (Ion /Ioff ) of 1.5×10(3) .

Genetically improved monolayer-forming tobacco mosaic viruses to generate nanostructured semiconducting bio/inorganic hybrids.

The genetically determined design of structured functional bio/inorganic materials was investigated by applying a convective assembly approach. Wildtype tobacco mosaic virus (wt TMV) as well as several TMV mutants were organized on substrates over macroscopic-length scales. Depending on the virus type, the self-organization behavior showed pronounced differences in the surface arrangement under the same convective assembly conditions. Additionally, under varying assembly parameters, the virus particles generated structures encompassing morphologies emerging from single micrometer long fibers aligned parallel to the triple-contact line through disordered but dense films to smooth and uniform monolayers. Monolayers with diverse packing densities were used as templates to form TMV/ZnO hybrid materials. The semiconducting properties can be directly designed and tuned by the variation of the template architecture which are reflected in the transistor performance.

Double-walled carbon nanotube array for CO2 and SO2 adsorption.

Grand-canonical Monte Carlo simulations and adsorption experiments are combined to find the optimized carbon nanotube (CNT) arrays for gas adsorption at low pressures and 303 K. Bundles of 3D aligned double-walled carbon nanotube (DWCNT) with inner diameter of 8 nm and different intertube distances were made experimentally. The experimental results show that decreasing intertube distance leads to a significant enhancement in carbon-dioxide (CO2) adsorption capacity at 1 bar. The molecular simulation study on CO2 adsorption onto bundles of 3D aligned DWCNT with inner diameters of 1, 3, and 8 nm and intertube distance of 0-15 nm shows that the intertube distance plays a more important role than the CNT diameter. The simulation results show that decreasing the intertube distance up to 1 nm increases the excess adsorption generally in all the studied systems at pressures 0 < p < 14 bars (the increase can be up to ∼40% depending on the system and pressure). This is in agreement with the experimental result. Further reduction in intertube distance leads to a decrease in the excess adsorption in the pressure range 9 < p < 14 bars. However, at lower pressure, 0 < p < 9 bars, intertube distance of 0.5 nm is found to have the highest excess adsorption. This result is indifferent to tube diameter. Furthermore, molecular simulations are conducted to obtain the optimal parameters, for the DWCNT bundle, for SO2 adsorption, which are similar to those observed for CO2 in the pressure range 0 < p < 3 bars.

Experimental validation of the novel theory explaining self-organization in porous anodic alumina films.

The lack of a reliable method for theoretical prediction of nanoporous anodic alumina films obtained from non-familiar electrolytes prompted the search of a viable solution to this problem. The theory explaining the self-assembly mechanism was described in our preceding work. Here, the results of an extensive validation test are presented.

Assembly of one dimensional inorganic nanostructures into functional 2D and 3D architectures. Synthesis, arrangement and functionality.

This review will focus on the synthesis, arrangement, structural assembly, for current and future applications, of 1D nanomaterials (tubes, wires, rods) in 2D and 3D ordered arrangements. The ability to synthesize and arrange one dimensional nanomaterials into ordered 2D or 3D micro or macro sized structures is of utmost importance in developing new devices and applications of these materials. Micro and macro sized architectures based on such 1D nanomaterials (e.g. tubes, wires, rods) provide a platform to integrate nanostructures at a larger and thus manageable scale into high performance electronic devices like field effect transistors, as chemo- and biosensors, catalysts, or in energy material applications. Carbon based, metal oxide and metal based 1D arranged materials as well as hybrid or composite 1D materials of the latter provide a broad materials platform, offering a perspective for new entries into fascinating structures and future applications of such assembled architectures. These architectures allow bridging the gap between 1D nanostructures and the micro and macro world and are the basis for an assembly of 1D materials into higher hierarchy domains. This critical review is intended to provide an interesting starting point to view the current state of the art and show perspectives for future developments in this field. The emphasis is on selected nanomaterials and the possibilities for building three dimensional arrays starting from one dimensional building blocks. Carbon nanotubes, metal oxide nanotubes and nanowires (e.g. ZnO, TiO(2), V(2)O(5), Cu(2)O, NiO, Fe(2)O(3)), silicon and germanium nanowires, and group III-V or II-VI based 1D semiconductor nanostructures like GaS and GaN, pure metals as well as 1D hybrid materials and their higher organized architectures (foremost in 3D) will be focussed. These materials have been the most intensively studied within the last 5-10 years with respect to nano-micro integration aspects and their functional and application oriented properties. The critical review should be interesting for a broader scientific community (chemists, physicists, material scientists) interested in synthetic and functional material aspects of 1D materials as well as their integration into next higher organized architectures.

Comment on Pashchanka, M. Conceptual Progress for Explaining and Predicting Self-Organization on Anodized Aluminum Surfaces. Nanomaterials 2021, 11, 2271.

In the review article "Conceptual Progress for Explaining and Predicting Self-Organization on Anodized Aluminum Surfaces" [...].

Hierarchical structures of carbon nanotubes and arrays of chromium-capped silicon nanopillars: formation and electrical properties.

The formation of stochastically oriented carbon-nanotube networks on top of an array of free-standing chromium-capped silicon nanopillars is reported. The combination of nanosphere lithography and chemical vapor deposition enables the construction of nanostructures that exhibit a hierarchical sequence of structural sizes. Metallic chromium serves as an etching mask for Si-pillar formation and as a nucleation site for the formation of carbon nanotubes through the chemical vapor deposition of ethene, ethanol, and methane, respectively, thereby bridging individual pillars from top to top. Iron and cobalt were applied onto the chromium caps as catalysts for CNT growth and the influence of different carbon sources and different gas-flow rates were investigated. The carbon nanotubes were structurally characterized and their DC electrical properties were studied by in situ local- and ex situ macroscopic measurements, both of which reveal their semiconductor properties. This process demonstrates how carbon nanotubes can be integrated into Si-based semiconductors and, thus, this process may be used to form high-surface-area sensors or new porous catalyst supports with enhanced gas-permeation properties.

Synthesis, characterization, electronic and gas-sensing properties towards H2 and CO of transparent, large-area, low-layer graphene.

Low-layered, transparent graphene is accessible by a chemical vapor deposition (CVD) technique on a Ni-catalyst layer, which is deposited on a <100> silicon substrate. The number of graphene layers on the substrate is controlled by the grain boundaries in the Ni-catalyst layer and can be studied by micro Raman analysis. Electrical studies showed a sheet resistance (R(sheet)) of approximately 1435 Ω per □, a contact resistance (R(c)) of about 127 Ω, and a specific contact resistance (R(sc)) of approximately 2.8×10(-4) â€…Ω cm(2) for the CVD graphene samples. Transistor output characteristics for the graphene sample demonstrated linear current/voltage behavior. A current versus voltage (I(ds)-V(ds)) plot clearly indicates a p-conducting characteristic of the synthesized graphene. Gas-sensor measurements revealed a high sensor activity of the low-layer graphene material towards H(2) and CO. At 300 °C, a sensor response of approximately 29 towards low H(2) concentrations (1 vol %) was observed, which is by a factor of four higher than recently reported.

Gas phase synthesis and adsorption properties of a 3D ZIF-8 CNT composite.

The metal organic framework structure ZIF-8 has been grown directly on vertically aligned carbon nano tubes (VACNT) by a solid vapour transformation of a ZnO@VACNT composite with gaseous 2-methylimidazole. The ZnO@VACNT composite was synthesised by atomic layer deposition (ALD) using diethylzinc and water as precursors resulting in a homogeneous distribution of crystalline ZnO particles with an average size of 13 nm within the 3D VACNT host structure. The ZnO@VACNT composite was transformed to ZIF-8 by reaction with 2-methyl-imidazole (Hmim) while maintaining the 3D VACNT structure employing a solid vapour transformation reaction. Reaction time and temperature were identified as key parameters to control the generated surface area and the degree of conversion of the nanoscaled ZnO particles. 80 °C and 72 h were found to be sufficient for a complete conversion while longer reaction times result in even higher surface areas of the formed ZIF-8@VACNT composite. Surface areas of up to 1569 m2 g-1 could be achieved. Temperatures below 80 °C led to an incomplete conversion even under longer reaction times of up to 6 weeks. Finally, the CO2 adsorption properties of the ZIF-8@VACNT composite were evaluated. A composite with a 27 w% content of CNTs and a surface area of 1277 m2 g-1 shows an adsorption of 6.05 mmol g-1 CO2 at 30 bar. From the comparison with the pristine materials ZIF-8 and VACNT alone the observed overall CO2 adsorption behaviour of the composite is a combination of the behaviour of the individual components, ZIF-8 and VACNTs. Namely the typical steep rise of the ZIF-8 in the low-pressure regime with a nearly linear steady progression in the medium pressure size regime, the latter typical for VACNTs, proves that the combination of both components leads to enhanced adsorption properties of the ZIF-8@VACNT composite compared to the sole components ZIF-8 and VACNTs.

Solution synthesis and dielectric properties of alumina thin films: understanding the role of the organic additive in film formation.

Alumina thin films are synthesized by combustion synthesis of mixtures of aluminium nitrate (ALN) and methylcarbazate (MCZ). The interdependence of the ratio of oxidizer and reducing agent on composition, microstructure and electronic properties of the resulting oxide layers is investigated. The dielectric and insulating behaviour is improved by addition of different amounts of MCZ (MCZ : ALN = 0.67 or 2.5). In this way films (thickness ∼140 nm) with a dielectric constant κ of 9.7 and a dielectric loss tan δ below 0.015 can be achieved. Medium concentrations of MCZ (MCZ : ALN = 1.0 or 1.5) lead to films with lower performance, though. Our studies indicate two opposing effects of the organic additive. Removal of organic residues during film formation as combustion gases is potentially detrimental. Larger amounts of MCZ, however, cause condensation reactions in the precusor mixture, which improve the microstructure. The porosity of the films can be sucessfully analyzed by positron annihilation liftetime studies. In this way the impact of the organic ligand sphere on the resulting microstructure can be quantified. Samples prepared from ALN alone exhibit mesopores and also larger micropores. In contrast, the formation of mesopores can be inhibited by addition of MCZ.

Conductive Geopolymers as Low-Cost Electrode Materials for Microbial Fuel Cells.

Geopolymer (GP) inorganic binders have a superior acid resistance compared to conventional cement (e.g., Portland cement, PC) binders, have better microbial compatibility, and are suitable for introducing electrically conductive additives to improve electron and ion transfer properties. In this study, GP-graphite (GPG) composites and PC-graphite (PCG) composites with a graphite content of 1-10 vol % were prepared and characterized. The electrical conductivity percolation threshold of the GPG and PCG composites was around 7 and 8 vol %, respectively. GPG and PCG composites with a graphite content of 8 to 10 vol % were selected as anode electrodes for the electrochemical analysis in two-chamber polarized microbial fuel cells (MFCs). Graphite electrodes were used as the positive control reference material. Geobacter sulfurreducens was used as a biofilm-forming and electroactive model organism for MFC experiments. Compared to the conventional graphite anodes, the anode-respiring biofilms resulted in equal current production on GPG composite anodes, whereas the PCG composites showed a very poor performance. The largest mean value of the measured current densities of a GPG composite used as anodes in MFCs was 380.4 µA cm-2 with a standard deviation of 129.5 µA cm-2. Overall, the best results were obtained with electrodes having a relatively low Ohmic resistance, that is, GPG composites and graphite. The very first approach employing sustainable GPs as a low-cost electrode binder material in an MFC showed promising results with the potential to greatly reduce the production costs of MFCs, which would also increase the feasibility of MFC large-scale applications.

Binary Au/MWCNT and ternary Au/ZnO/MWCNT nanocomposites: synthesis, characterisation and catalytic performance.

Gold nanoparticles of 10-24 and 5-8 nm in size were obtained by chemical citrate reduction and UV photoreduction, respectively, on acid-treated multiwalled carbon nanotubes (MWCNTs) and on ZnO/MWCNT composites. The shape and size of the deposited Au nanoparticles were found to be dependent upon the synthetic method used. Single-crystalline, hexagonal gold particles were produced in the case of UV photoreduction on ZnO/MWCNT, whereas spherical Au particles were deposited on MWCNT when the chemical citrate reduction method was used. In the UV photoreduction route, n-doped ZnO serves as the e(-) donor, whereas the solvent is the hole trap. All materials were fully characterised by UV/Vis spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and BET surface analysis. The catalytic activity of the composites was studied for the selective hydrogenation of alpha,beta-unsaturated carbonyl compound 3,7-dimethyl-2,6-octadienal (citral). The Au/ZnO/MWCNT composite favours the formation of unsaturated alcohols (selectivity=50% at a citral conversion of 20%) due to the presence of single-crystalline, hexagonal gold particles, whereas saturated aldehyde formation is favoured in the case of the Au/MWCNT nanocomposite that contains spherical gold particles.