Albertus D. Handoko

Stable and selective electrochemical reduction of carbon dioxide to ethylene on copper mesocrystals

Stable and selective electrochemical reduction of carbon dioxide to ethylene was achieved using copper mesocrystal catalysts in 0.1 M KHCO3. The Cu mesocrystal catalysts were facilely derived by the in situ reduction of a thin CuCl film during the first 200 seconds of the CO2 electroreduction process. At −0.99 V vs. RHE, the Faradaic efficiency of ethylene formation using these Cu mesocrystals was ~18× larger than that of methane and forms up to 81% of the total carbonaceous products. Control CO2 reduction experiments show that this selectivity towards C2H4 formation could not be replicated by using regular copper nanoparticles formed by pulse electrodeposition. High resolution transmission electron microscopy reveals the presence of both (100)Cu facets and atomic steps in the Cu mesocrystals which we assign as active sites in catalyzing the reduction of CO2 to C2H4. CO adsorption measurements suggest that the remarkable C2H4 selectivity could be attributed to the greater propensity of CO adsorption on Cu mesocrystals than on other types of Cu surfaces. The Cu mesocrystals remained active and selective towards C2H4 formation for longer than six hours. This is an important and industrially relevant feature missing from many reported Cu-based CO2 reduction catalysts.

Mechanistic Insights into the Enhanced Activity and Stability of Agglomerated Cu Nanocrystals for the Electrochemical Reduction of Carbon Dioxide to n-Propanol

The reduction of carbon dioxide (CO2) to n-propanol (CH3CH2CH2OH) using renewable electricity is a potentially sustainable route to the production of this valuable engine fuel. In this study, we report that agglomerates of ∼15 nm sized copper nanocrystals exhibited unprecedented catalytic activity for this electrochemical reaction in aqueous 0.1 M KHCO3. The onset potential for the formation of n-propanol was 200-300 mV more positive than for an electropolished Cu surface or Cu(0) nanoparticles. At -0.95 V (vs RHE), n-propanol was formed on the Cu nanocrystals with a high current density (jn-propanol) of -1.74 mA/cm(2), which is ∼25× larger than that found on Cu(0) nanoparticles at the same applied potential. The Cu nanocrystals were also catalytically stable for at least 6 h, and only 14% deactivation was observed after 12 h of CO2 reduction. Mechanistic studies suggest that n-propanol could be formed through the C-C coupling of carbon monoxide and ethylene precursors. The enhanced activity of the Cu nanocrystals toward n-propanol formation was correlated to their surface population of defect sites.

Hydrothermal synthesis of sodium potassium niobate solid solutions at 200 °C

For the first time, a series of sodium potassium niobate solid solutions with compositions around the morphotropic phase boundary (MPB) are hydrothermally synthesized at 200 °C using a simple KOH and NaOH mixture with Nb2O5 as a precursor powder. Rietveld refinement of X-ray diffraction data indicated the presence of a second sodium niobate perovskite phase when the concentration of NaOH (compared to the total hydroxyl ion concentration) is above 11.7%. The presence of the second phase is attributed to the different solubilities of the intermediate potassium and sodium hexaniobate species. It is also found that heat-treating the mixed-phase powders for two hours at a temperature of 800 °C is effective in obtaining the desired single-phase solid solution with compositions near the MPB, thereby opening the way to using hydrothermal synthesis in simplifying the laborious solid-state process.

Enhanced activity of H2O2-treated copper(II) oxide nanostructures for the electrochemical evolution of oxygen

The successful design and synthesis of earth-abundant and efficient catalysts for the oxygen evolution reaction (OER) will be a major step forward towards the use of electrochemical water splitting as an environmentally-friendly process for producing H2 fuel. Due to their poor activity, copper-based materials have not been considered apt for catalysing OER. In this work, we demonstrate that unique copper(II) oxide nanostructures obtained via hydrothermal synthesis and subsequent hydrogen peroxide treatment exhibit unusually high and sustainable OER activity. In 0.1 M KOH electrolyte, the CuO nanostructures catalyse OER with current densities of 2.6–3.4 mA cm−2 at 1.75 V (vs. RHE). The calculated turnover frequency (per Cu site) of ~2 × 10−3 s−1 for O2 production is markedly higher than that of high-surface area electrodeposited Cu metal nanoparticles by 40–68 times. The OER activity of the CuO nanostructures is also stable, approaching about half of 20% IrOx/Vulcan XC-72 after an hour-long OER. In situ Raman spectroscopy at OER-relevant potentials recorded compelling evidence that CuIII active species may be responsible for the unusual OER activity of the CuO nanostructures, as indicated by its signature vibration at 603 cm−1. This hitherto unobserved peak is assigned, with the aid of the model compound NaCuIIIO2, to the Cu–O stretching vibration of CuIII oxide. This feature was not found on electrodeposited Cu metal, which exhibited correspondingly weaker OER activity. The enhanced catalysis of O2 evolution by the CuO nanostructures is thus attributed to not just their higher surface area, but also the higher population of CuIII active sites on their surface.

Hydrothermal growth of piezoelectrically active lead-free (Na,K)NbO3–LiTaO3 thin films

Single phase, lead-free thin films based on the (Na,K)NbO3–LiTaO3 solid solution near the morphotropic phase boundary are grown hydrothermally at 130 °C for the first time. Formation of the unwanted Na-rich phase normally occurring during hydrothermal synthesis of solid solution powder was successfully averted in the film growth by the addition of small amounts of EDTA to the starting precursor solution, which also allowed better control of the film growth rate. The hydrothermally grown NKN-LT film was found to be ferroelectrically and piezoelectrically active, showing a piezoelectric constant (d33) of 25 pm V−1.

Dimensionally and compositionally controlled growth of calcium phosphate nanowires for bone tissue regeneration

Nanostructured biomaterials with controlled morphology and composition are of high interest for bone tissue regeneration. As resorbable and biocompatible materials for bone tissue engineering, calcium phosphate nanowires and nanoneedles with different aspect ratios and compositions have been first synthesized without the use of any toxic surfactants via an energy efficient microwave assisted process. Correlation between solvent composition, mixing methodology and reagent stoichiometric ratios was investigated with the aim of producing orientated growth and varied biphasic composition, resulting in dimensionally controlled growth of materials containing varying hydroxyapatite (HA)/monetite quantities. It was observed that the HA/monetite content and dimensionality could be manipulated by changing the initial ethanol (EtOH) volume in the H2O/EtOH solvent mixture. Three dimensional particles with minute amounts of HA were produced when a H2O/EtOH volumetric ratio of 20/80 was used. Conversely, high aspect ratio (ca. 54) nanowires containing ca. 38 wt% HA were obtained with a 60/40 H2O/EtOH volumetric ratio. Importantly, the quantity of HA in the high aspect ratio nanowires/needles was controlled by varying the stoichiometric ratio of the reactants, demonstrating that one-dimensional materials with close to 100% HA can be achieved when the Ca/P ratio is increased to 1.67. Additionally, significant correlation between the extent of orientated growth of the materials and the point of EtOH addition during the mixing method was observed. The findings highlight that solvent composition, reactant stoichiometric ratio and mixing procedure can be used in tandem to tailor the morphology and composition of calcium phosphate materials, which are of very high importance in developing excellent materials suitable for bone tissue regeneration.

Piezoelectrically active hydrothermal KNbO3 thin films

In this study, a post-growth treatment has been developed for epitaxial KNbO3 films grown hydrothermally at 200 °C so that they are ferroelectrically active. The treatment comprised of an O2 plasma followed by thermal annealing that removed lattice hydroxyls and also reduced the concentration of oxygen vacancies. This lowered the leakage current sufficiently such that films could be poled to saturation. This has led to the first demonstration of piezoelectrically active KNbO3 films grown hydrothermally.

Hydrothermal synthesis of (00l) epitaxial BiFeO3 films on SrTiO3 substrate

Epitaxial thin films of multiferroic BiFeO3 (BFO) are successfully grown on (100) SrTiO3 substrates using a hydrothermal synthesis method at 120–200 °C in a relatively short time of 2–8 h. It is found that the morphology evolution of the (001)PC-oriented BFO films over time and temperature involve unique dissolution–reprecipitation reactions, whereby initial large sparse islands dissolve, and re-precipitate into dense mosaic-like structures that further grow vertically, resembling those of terrace-like structures. High-resolution transmission electron microscopy (HRTEM) and ψ-ω-2θ high resolution thin film X-ray diffraction scans suggest that the BFO films have relaxed to a monoclinic structure—different from the rhombohedral bulk BFO powders. Peak fittings of the powders and films reveal that the lattice parameters contract when either reaction time or temperature is increased. Similar lattice contraction is also observed when the films/powder are annealed at 550 °C in N2 atmosphere for 10 min. Dielectric constant and loss, leakage current and ferroelectric properties of the BFO film improve with annealing, attributed to a reduced amount of defects.

Rational Design of Sulfur‐Doped Copper Catalysts for the Selective Electroreduction of Carbon Dioxide to Formate

The selective electroreduction of CO2 to formate (or formic acid) is of great interest in the field of renewable-energy utilization. In this work, we designed a sulfur-doped Cu2 O-derived Cu catalyst and showed that the presence of sulfur can tune the selectivity of Cu significantly from the production of various CO2 reduction products to almost exclusively formate. Sulfur is doped into the Cu catalysts by dipping the Cu substrates into ammonium polysulfide solutions. Catalyst films with the highest sulfur content of 2.7 at % showed the largest formate current density (jHCOO- ) of -13.9 mA cm-2 at -0.9 V versus the reversible hydrogen electrode (RHE), which is approximately 46 times larger than that previously reported for Cu(110) surfaces. At -0.8 V versus RHE, the faradaic efficiency of formate was maintained at approximately 75 % for 12 h of continuous electrolysis. Through the analysis of the evolution of the jHCOO- and jH2 values with the sulfur content, we show that sulfur doping increases formate production and suppresses the hydrogen evolution reaction. Ag-S and Cu-Se catalysts did not exhibit any significant enhancement towards the reduction of CO2 to formate. This demonstrates clearly that sulfur and copper acted synergistically to promote the selective formation of formate. A hypothesis about the role of sulfur is proposed and discussed.

Low temperature formation of (NaxK1-x) NbO3 from hydrothermally synthesised NaNbO3

Abstract Na0·5K0·5NbO3 was synthesised by heating a mixture of KNbO3 and NaNbO3 powders to 800°C and cooling immediately. The KNbO3–NaNbO3 powder mixture was obtained after 10 h of reacting NaNbO3 in a 6M KOH solution at 200°C. This reaction caused the dissolution of NaNbO3 and precipitation of KNbO3. The dissolution–reprecipitation process led to an intimate mixture of KNbO3 and NaNbO3 powders with much smaller particle sizes and a surface area that was 3·5 times greater than the separately hydrothermally synthesised KNbO3 and NaNbO3 powders. The increased surface area, smaller grain sizes and intimate mixing of the powders were responsible for the much faster conversion to a Na0·5K0·5NbO3 solid solution powder.