Hydrothermal Synthesis of Monoclinic VO2 Microparticles without Use of Hazardous Reagents: A Key Role for the W-Dopant.

Monoclinic vanadium dioxide (VO2 (M)) is a promising material for various applications ranging from sensing to signature management and smart windows. Most applications rely on its reversible structural phase transition to rutile VO2 (VO2 (R)), which is accompanied by a metal-to-insulator transition. Bottom-up hydrothermal synthesis has proven to yield high quality monoclinic VO2 but requires toxic and highly reactive reducing agents that cannot be used outside of a research lab. Here, we present a new hydrothermal synthesis method using nontoxic and safe-to-use oxalic acid as a reducing agent for V2O5 to produce VO2 (M). In early stages of the process, polymorphs VO2 (A) and VO2 (B) were formed, which subsequently recrystallized to VO2 (M). Without the presence of W6+, this recrystallization did not occur. After a reaction time of 96 h at 230 °C in the presence of (NH4)6H2W12O40 in Teflon-lined rotated autoclaves, we realized highly crystalline, phase pure W-doped VO2 (M) microparticles of uniform size and asterisk shape (ΔH = 28.30 J·g-1, arm length = 6.7 ± 0.4 µm, arm width = 0.46 ± 0.06 µm). We extensively investigated the role of W6+ in the kinetics of formation of VO2 (M) and the thermodynamics of its structural phase transition.

Polymeric Backbone Eutectogel Electrolytes for High-Energy Lithium-Ion Batteries.

This work introduces a polymeric backbone eutectogel (P-ETG) hybrid solid-state electrolyte with an N-isopropylacrylamide (NIPAM) backbone for high-energy lithium-ion batteries (LIBs). The NIPAM-based P-ETG is (electro)chemically compatible with commercially relevant positive electrode materials such as the nickel-rich layered oxide LiNi0.6Mn0.2Co0.2O2 (NMC622). The chemical compatibility was demonstrated through (physico)chemical characterization methods. The nonexistence (within detection limits) of interfacial reactions between the electrolyte and the positive electrode, the unchanged bulk crystallographic composition, and the absence of transition metal ions leaching from the positive electrode in contact with the electrolyte were demonstrated by Fourier transform infrared spectroscopy, powder X-ray diffraction, and elemental analysis, respectively. Moreover, the NIPAM-based P-ETG demonstrates a wide electrochemical stability window (1.5-5.0 V vs Li+/Li) and a reasonably high ionic conductivity at room temperature (0.82 mS cm-1). The electrochemical compatibility of a high-potential NMC622-containing positive electrode and the P-ETG is further demonstrated in Li|P-ETG|NMC622 cells, which deliver a discharge capacity of 134, 110, and 97 mAh g-1 at C/5, C/2, and 1C, respectively, after 90 cycles. The Coulombic efficiency is >95% at C/5, C/2, and 1C. Hence, gaining scientific insights into the compatibility of the electrolytes with positive electrode materials that are relevant to the commercial market, like NMC622, is important because this requires going beyond the electrolyte design itself, which is essential to their practical applications.

Sunlight-Powered Reverse Water Gas Shift Reaction Catalysed by Plasmonic Au/TiO2 Nanocatalysts: Effects of Au Particle Size on the Activity and Selectivity.

This study reports the low temperature and low pressure conversion (up to 160 °C, p = 3.5 bar) of CO2 and H2 to CO using plasmonic Au/TiO2 nanocatalysts and mildly concentrated artificial sunlight as the sole energy source (up to 13.9 kW·m-2 = 13.9 suns). To distinguish between photothermal and non-thermal contributors, we investigated the impact of the Au nanoparticle size and light intensity on the activity and selectivity of the catalyst. A comparative study between P25 TiO2-supported Au nanocatalysts of a size of 6 nm and 16 nm displayed a 15 times higher activity for the smaller particles, which can only partially be attributed to the higher Au surface area. Other factors that may play a role are e.g., the electronic contact between Au and TiO2 and the ratio between plasmonic absorption and scattering. Both catalysts displayed ≥84% selectivity for CO (side product is CH4). Furthermore, we demonstrated that the catalytic activity of Au/TiO2 increases exponentially with increasing light intensity, which indicated the presence of a photothermal contributor. In dark, however, both Au/TiO2 catalysts solely produced CH4 at the same catalyst bed temperature (160 °C). We propose that the difference in selectivity is caused by the promotion of CO desorption through charge transfer of plasmon generated charges (as a non-thermal contributor).

Photocatalytic Performance of Undoped and Al-Doped ZnO Nanoparticles in the Degradation of Rhodamine B under UV-Visible Light:The Role of Defects and Morphology.

Quasi-spherical undoped ZnO and Al-doped ZnO nanoparticles with different aluminum content, ranging from 0.5 to 5 at% of Al with respect to Zn, were synthesized. These nanoparticles were evaluated as photocatalysts in the photodegradation of the Rhodamine B (RhB) dye aqueous solution under UV-visible light irradiation. The undoped ZnO nanopowder annealed at 400 °C resulted in the highest degradation efficiency of ca. 81% after 4 h under green light irradiation (525 nm), in the presence of 5 mg of catalyst. The samples were characterized using ICP-OES, PXRD, TEM, FT-IR, 27Al-MAS NMR, UV-Vis and steady-state PL. The effect of Al-doping on the phase structure, shape and particle size was also investigated. Additional information arose from the annealed nanomaterials under dynamic N2 at different temperatures (400 and 550 °C). The position of aluminum in the ZnO lattice was identified by means of 27Al-MAS NMR. FT-IR gave further information about the type of tetrahedral sites occupied by aluminum. Photoluminescence showed that the insertion of dopant increases the oxygen vacancies reducing the peroxide-like species responsible for photocatalysis. The annealing temperature helps increase the number of red-emitting centers up to 400 °C, while at 550 °C, the photocatalytic performance drops due to the aggregation tendency.

Sunlight-Fueled, Low-Temperature Ru-Catalyzed Conversion of CO2 and H2 to CH4 with a High Photon-to-Methane Efficiency.

Methane, which has a high energy storage density and is safely stored and transported in our existing infrastructure, can be produced through conversion of the undesired energy carrier H2 with CO2. Methane production with standard transition-metal catalysts requires high-temperature activation (300-500 °C). Alternatively, semiconductor metal oxide photocatalysts can be used, but they require high-intensity UV light. Here, we report a Ru metal catalyst that facilitates methanation below 250 °C using sunlight as an energy source. Although at low solar intensity (1 sun) the activity of the Ru catalyst is mainly attributed to thermal effects, we identified a large nonthermal contribution at slightly elevated intensities (5.7 and 8.5 sun) resulting in a high photon-to-methane efficiency of up to 55% over the whole solar spectrum. We attribute the excellent sunlight-harvesting ability of the catalyst and the high photon-to-methane efficiency to its UV-vis-NIR plasmonic absorption. Our highly efficient conversion of H2 to methane is a promising technology to simultaneously accelerate the energy transition and reduce CO2 emissions.

Understanding the Importance of Cu(I) Intermediates in Self-Reducing Molecular Inks for Flexible Electronics.

Fast and scalable low-temperature deposition of microscale metallic features is of utmost importance for the development of future flexible smart applications including sensors, wireless communication, and wearables. Recently, a new class of metal-organic decomposition (MOD) copper inks was developed, consisting of self-reducing copper formate containing amine complexes. From these novel inks, copper metal features with outstanding electrical conductivity (±105 S cm-1) are deposited at a temperature of 150 °C or less, which is well below the reduction temperature of orthorhombic α-copper formate (around 225 °C). However, the underlying principle of this reaction mechanism and the relationship between the corresponding temperature shift and the amine coordination are still under debate. The current study provides a full explanation for the shift in reduction temperatures via in situ characterization. The results clearly indicate that the structural resemblance and stability of the Cu(II) starting compound and the occurring Cu(I) intermediate during the in situ reduction are the two main variables that rationalize the temperature shift. As such, the thermal compatibility of copper MOD inks with conventional plastic substrates such as polyethylene terephthalate can be explained, based on metal-organic complex properties.

Wet-Chemical Synthesis of 3D Stacked Thin Film Metal-Oxides for All-Solid-State Li-Ion Batteries.

By ultrasonic spray deposition of precursors, conformal deposition on 3D surfaces of tungsten oxide (WO3) negative electrode and amorphous lithium lanthanum titanium oxide (LLT) solid-electrolyte has been achieved as well as an all-solid-state half-cell. Electrochemical activity was achieved of the WO3 layers, annealed at temperatures of 500 °C. Galvanostatic measurements show a volumetric capacity (415 mAh·cm-3) of the deposited electrode material. In addition, electrochemical activity was shown for half-cells, created by coating WO3 with LLT as the solid-state electrolyte. The electron blocking properties of the LLT solid-electrolyte was shown by ferrocene reduction. 3D depositions were done on various micro-sized Si template structures, showing fully covering coatings of both WO3 and LLT. Finally, the thermal budget required for WO3 layer deposition was minimized, which enabled attaining active WO3 on 3D TiN/Si micro-cylinders. A 2.6-fold capacity increase for the 3D-structured WO3 was shown, with the same current density per coated area.

Steering the Properties of MoOx Hole Transporting Layers in OPVs and OLEDs: Interface Morphology vs. Electronic Structure.

The identification, fine-tuning, and process optimization of appropriate hole transporting layers (HTLs) for organic solar cells is indispensable for the production of efficient and sustainable functional devices. In this study, the optimization of a solution-processed molybdenum oxide (MoOx) layer fabricated from a combustion precursor is carried out via the introduction of zirconium and tin additives. The evaluation of the output characteristics of both organic photovoltaic (OPV) and organic light emitting diode (OLED) devices demonstrates the beneficial influence upon the addition of the Zr and Sn ions compared to the generic MoOx precursor. A dopant effect in which the heteroatoms and the molybdenum oxide form a chemical identity with fundamentally different structural properties could not be observed, as the additives do not affect the molybdenum oxide composition or electronic band structure. An improved surface roughness due to a reduced crystallinity was found to be a key parameter leading to the superior performance of the devices employing modified HTLs.

Hydrothermal synthesis of ZnO nanorods: a statistical determination of the significant parameters in view of reducing the diameter.

In this paper a 2(8-4) fractional factorial design of experiments is applied to identify the important parameters that affect the average diameter of ZnO rods, synthesized by means of a hydrothermal procedure. A water-based Zn(2+) precursor is used for the formation of one-dimensional ZnO particles, without the presence of an organic additive. Results indicate that, at the investigated levels, four of the parameters have a significant effect on the mean diameter. These are the temperature, the heating rate, stirring and an ultrasonic pre-treatment of the precursor solution. Experiments carried out with zinc acetate and zinc chloride do not show a significant difference in rod diameter. Other parameters that do not show a significant effect are the concentration of Zn(2+), the molar ratio between the hydroxyl and the zinc ions, and the reaction time. Interactions are observed between stirring and an ultrasonic pre-treatment and between the zinc concentration and the OH:Zn ratio. By fixing the significant factors at their optimal value it is possible to decrease the mean diameter. The particles are characterized by means of x-ray diffraction (XRD) and transmission electron microscopy (TEM).