We report a facile colloidal synthesis of gallium (Ga) nanoparticles with the mean size tunable in the range of 12–46 nm and with excellent size distribution as small as 7–8%. When stored under ambient conditions, Ga nanoparticles remain stable for months due to the formation of native and passivating Ga-oxide layer (2–3 nm). The mechanism of Ga nanoparticles formation is elucidated using nuclear magnetic resonance spectroscopy and with molecular dynamics simulations. Size-dependent crystallization and melting of Ga nanoparticles in the temperature range of 98–298 K are studied with X-ray powder diffraction, specific heat measurements, transmission electron microscopy, and X-ray absorption spectroscopy. The results point to delta (δ)-Ga polymorph as a single low-temperature phase, while phase transition is characterized by the large hysteresis and by the large undercooling of crystallization and melting points down to 140–145 and 240–250 K, respectively. We have observed size-tunable plasmon resonance in the ultraviolet and visible spectral regions. We also report stable operation of Ga nanoparticles as anode material for Li-ion batteries with storage capacities of 600 mAh g–1, 50% higher than those achieved for bulk Ga under identical testing conditions.

Transparent conductive oxides (TCO) are a unique class of materials exhibiting optical transparency combined with metallike electrical conductivity and thus are of utmost importance for the rapidly expanding fi elds of transparent electronics and sustainable energy generation. [ 1a-e ] For most applications the standard compound is ITO exhibiting best optoelectronic performance amongst all TCO materials but to ensure a sustainable supply of such materials earth abundant and inexpensive alternatives such as aluminum doped zinc oxide (AZO) are crucial. [ 2 ] In fact this is also refl ected in predicted markets of almost $1 Billion in 2016 for alternative TCOs. [ 3 ] Nowadays, plasma based (magnetron-sputtering), [ 4a ] pulsed laser deposition (PLD), [ 4b ] atomic layer deposition(ALD) [ 4c,d ] or chemical vapor deposition (CVD) [ 4e ] methods are employed on industrial scale to obtain high quality AZO thin fi lms with resistivity of 10 −3 -10 −4 Ω cm and visible transparency > 90%, although instrumental complexity poses high investment costs as well as limits scalability. In this respect low cost non-vacuum methods for AZO thin fi lms are of immense interest.A variety of solution based approaches have been described but to obtain good optoelectronic properties comparable to vacuum deposition techniques, high temperature annealing (300-600 ° C) preferably in vacuum (10 −1 -10 −4 mbar), and for long time (60-90 min) are necessary. Some approaches employ fl ammable or toxic organic solvents (e.g. 2-methoxyethanol), [ 5a-c ] and in the case of electrodeposition conductive substrates are inevitable. [ 5d ] The most straightforward yet challenging approach for deposition of ZnO thin fi lms is aqueous solution growth [ 6 ] (e.g., chemical bath deposition or hydrothermal synthesis) on seeded substrates as illustrated in Figure 1 a. The solution chemistry comprises a water soluble zinc salt and a complexant (usually ammonium salts, ethanolamine, or NH 4 OH) which is also used for adjusting the pH to a basic regime. The crystal growth is associated with the decreasing thermodynamic stability of the zinc complex leading to controlled supersaturation and the retrograde solubility of ZnO upon increased temperature. [ 7 ] Thus, phase pure ZnO thin fi lms can be obtained already at temperatures of 60 ° C. The use of water as solvent makes the method environmental friendly and due to the use of inexpensive chemicals -as mostly water soluble metal salts are employed -it can also be designated as low cost. A number of approaches have been reported to obtain intrinsic, undoped ZnO in the form of nanorod, nanoneedle, or nanopillar thin fi lms [ 8a ] using aqueous solution deposition, but only a few studies attempt to obtain conductive AZO thin fi lms. [ 8b,c ] However, achieving a compact, dense, and highly conductive (<100 Ω /sq) AZO thin fi lm at low process temperatures has not been successful yet, although for most of the applications high conductivity is essential.To overcome this bottleneck, we present a new concept to...

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The hydrothermal growth of cobalt oxide spinel (Co O ) nanocrystals from cobalt acetate precursors was monitored with in situ powder X-ray diffraction (PXRD) in combination with ex situ electron microscopy and vibrational spectroscopy. Kinetic data from in situ PXRD monitoring were analyzed using Sharp-Hancock and Gualtieri approaches, which both clearly indicate a change of the growth mechanism for reaction temperatures above 185 °C. This mechanistic transition goes hand in hand with morphology changes that notably influence the photocatalytic oxygen evolution activity. Complementary quenching investigations of conventional hydrothermal Co O growth demonstrate that these insights derived from in situ PXRD data provide valuable synthetic guidelines for water oxidation catalyst production. Furthermore, the ex situ analyses of hydrothermal quenching experiments were essential to assess the influence of amorphous cobalt-containing phases arising from the acetate precursor on the catalytic activity. Thereby, the efficient combination of a single in situ technique with ex situ analyses paves the way to optimize parameter-sensitive hydrothermal production processes of key energy materials.

Transition metal carbodiimides MNCN (M = Co, Ni, CoNi, Mn and Cu), were studied by simultaneous operando Raman and X-ray absorption spectroscopy (XAS) with focus on surface oxide detection during electrocatalytic water oxidation. As a proof of concept, easily modifiable screen-printed electrodes were used in this unified operando synchrotron setup for a trade-off between convenience of electrochemical anodization and spectroscopic data acquisition. Monitoring of chemical and structural transformations at the electrode surface during initial anodic electrode polarization shows stability for MNCN with M = Co, Ni, CoNi and Mn. While MnNCN is inactive, CoNCN emerges as the most active representative of the series. CuNCN displays pronounced side reactions and the formation of a surface copper oxide layer leading to lower current density attributed to water oxidation, as evident from an irreversible variation of the CuNCN redox behaviour in rotating ring-disc voltammetry. Furthermore, the accompanying structural and vibrational spectroscopy properties of the different MNCN compounds were explored with complementary ex situ analytical methods.

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The development of efficient, stable, and economic water oxidation catalysts (WOCs) is a forefront topic of sustainable energy research. We newly present a comprehensive three-step approach to systematically investigate challenging relationships among preparative history, properties, and performance in heterogeneous WOCs. To this end, we studied (1) the influence of the preparative method on the material properties and (2) their correlation with the performance as (3) a function of the catalytic test method. Spinel-type Co3O4 was selected as a clear-cut model WOC and synthesized via nine different preparative routes. In search of the key material properties for high catalytic performance, these cobalt oxide samples were characterized with a wide range of analytical methods, including X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, powder X-ray diffraction, Raman spectroscopy, BET surface area analysis, and transmission electron microscopy. Next, the corresponding catalytic water oxidation activities were assessed with the three most widely applied protocols to date, namely, photocatalytic, electrocatalytic, and chemical oxidation. The activity of the Co3O4 samples was found to clearly depend on the applied test method. Increasing surface area and disorder as well as a decrease in oxidation states arising from low synthesis temperatures were identified as key parameters for high chemical oxidation activity. Surprisingly, no obvious property–performance correlations were found for photocatalytic water oxidation. In sharp contrast, all samples showed similar activity in electrochemical water oxidation. The substantial performance differences between the applied protocols demonstrate that control and comprehensive understanding of the preparative history are crucial for establishing reliable structure–performance relationships in WOC design.

Water oxidation is the bottleneck reaction for overall water splitting as a direct and promising strategy toward clean fuels. However, the development of robust and affordable heterogeneous water oxidation catalysts remains challenging, especially with respect to the wide parameter space of synthesis and resulting material properties. Oxide catalysts performance in particular has been shown to depend on both synthetic routes and applied catalytic test methods. We here focus on spinel-type Co 3 O 4 as a representative case for an in-depth study of the influence of rather subtle synthetic parameter variations on the catalytic performance. To this end, a series of Co 3 O 4 samples was prepared via time-saving and tunable microwave-hydrothermal synthesis, while systematically varying a single parameter at a time. The resulting spinel-type catalysts were characterized with respect to key materials properties, including crystallinity, oxidation state and surface area using a wide range of analytical methods, such as PXRD, Raman/IR, XAS and XPS spectroscopy. Their water oxidation activity in electrocatalytic and chemical oxidation setups was then compared and correlated with the obtained catalyst properties. Both water oxidation methods displayed related trends concerning favorable synthetic parameters, namely higher activity for lower synthesis temperatures, lower precursor concentrations, addition of hydrogen peroxide and shorter ramping and reaction times, respectively. In addition to the surface area, structural features such as disorder were found to be influential for the water oxidation activity. The results prove that synthetic parameter screening is essential for optimal catalytic performance, given the complexity of the underlying performance-properties relationships.