Atomic-engineered gradient tunable solid-state metamaterials.

Metamaterial has been captivated a popular notion, offering photonic functionalities beyond the capabilities of natural materials. Its desirable functionality primarily relies on well-controlled conditions such as structural resonance, dispersion, geometry, filling fraction, external actuation, etc. However, its fundamental building blocks-meta-atoms-still rely on naturally occurring substances. Here, we propose and validate the concept of gradient and reversible atomic-engineered metamaterials (GRAM), which represents a platform for continuously tunable solid metaphotonics by atomic manipulation. GRAM consists of an atomic heterogenous interface of amorphous host and noble metals at the bottom, and the top interface was designed to facilitate the reversible movement of foreign atoms. Continuous and reversible changes in GRAM's refractive index and atomic structures are observed in the presence of a thermal field. We achieve multiple optical states of GRAM at varying temperature and time and demonstrate GRAM-based tunable nanophotonic devices in the visible spectrum. Further, high-efficiency and programmable laser raster-scanning patterns can be locally controlled by adjusting power and speed, without any mask-assisted or complex nanofabrication. Our approach casts a distinct, multilevel, and reversible postfabrication recipe to modify a solid material's properties at the atomic scale, opening avenues for optical materials engineering, information storage, display, and encryption, as well as advanced thermal optics and photonics.

Closed-Loop Multi-Objective Optimization for Cu-Sb-S Photo-Electrocatalytic Materials' Discovery.

Copper antimony sulfides are regarded as promising catalysts for photo-electrochemical water splitting because of their earth abundance and broad light absorption. The unique photoactivity of copper antimony sulfides is dependent on their various crystalline structures and atomic compositions. Here, a closed-loop workflow is built, which explores Cu-Sb-S compositional space to optimize its photo-electrocatalytic hydrogen evolution from water, by integrating a high-throughput robotic platform, characterization techniques, and machine learning (ML) optimization workflow. The multi-objective optimization model discovers optimum experimental conditions after only nine cycles of integrated experiments-machine learning loop. Photocurrent testing at 0 V versus reversible hydrogen electrode (RHE) confirms the expected correlation between the materials' properties and photocurrent. An optimum photocurrent of -186 µA cm-2 is observed on Cu-Sb-S in the ratio of 9:45:46 in the form of single-layer coating on F-doped SnO2 (FTO) glass with a corresponding bandgap of 1.85 eV and 63.2% Cu1+ /Cu species content. The targeted intelligent search reveals a nonobvious CuSbS composition that exhibits 2.3 times greater activity than baseline results from random sampling.

Sub-10 nm Mixing and Alloying of Cu-Ag and Cu-Ni via Accelerated Solid Diffusion.

Development of nanoscale multicomponent solid inorganic materials is often hindered by slow solid diffusion kinetics and poor precursor mixing in conventional solid-state synthesis. These shortcomings can be alleviated by combining nanosized precursor mixtures and low temperature reaction, which could reduce crystal growth and accelerate the solid diffusion at the same time. However, high throughput production of nanoparticle mixtures with tunable composition via conventional synthesis is very challenging. In this work, we demonstrate that ∼10 nm homogeneous mixing of sub-10 nm nanoparticles can be achieved via spark nanomixing at room temperature and pressure. Kinetically driven Spark Plasma Discharge nanoparticle generation and ambient processing conditions limit particle coarsening and agglomeration, resulting in sub-10 nm primary particles of as-deposited films. The intimate mixing of these nanosized precursor particles enables intraparticle diffusion and formation of Cu/Ni nanoalloy during subsequent low temperature annealing at 100 °C. We also discovered that cross-particle diffusion is promoted during the low-temperature sulfurization of Cu/Ag which tends to phase-segregate, eventually leading to the growth of sulfide nanocrystals and improved homogeneity. High elemental homogeneity, small diffusion path lengths, and high diffusibility synergically contribute to faster diffusion kinetics of sub-10 nm nanoparticle mixtures. The combination of ∼10 nm homogeneous precursors via spark nanomixing, low-temperature annealing, and a wide range of potentially compatible materials makes our approach a good candidate as a general platform toward accelerated solid state synthesis of nanomaterials.

Polaron Delocalization Dependence of the Conductivity and the Seebeck Coefficient in Doped Conjugated Polymers.

Conjugated polymers are promising materials for thermoelectrics as they offer good performances at near ambient temperatures. The current focus on polymer thermoelectric research mainly targets a higher power factor (PF; a product of the conductivity and square of the Seebeck coefficient) through improving the charge mobility. This is usually accomplished via structural modification in conjugated polymers using different processing techniques and doping. As a result, the structure-charge transport relationship in conjugated polymers is generally well-established. In contrast, the relationship between the structure and the Seebeck coefficient is poorly understood due to its complex nature. A theoretical framework by David Emin (Phys. Rev. B, 1999, 59, 6205-6210) suggests that the Seebeck coefficient can be enhanced via carrier-induced vibrational softening, whose magnitude is governed by the size of the polaron. In this work, we seek to unravel this relationship in conjugated polymers using a series of highly identical pro-quinoid polymers. These polymers are ideal to test Emin's framework experimentally as the quinoid character and polaron delocalization in these polymers can be well controlled even by small atomic differences (<10 at. % per repeating unit). By increasing the polaron delocalization, that is, the polaron size, we demonstrate that both the conductivity and the Seebeck coefficient (and hence PF) can be increased simultaneously, and the latter is due to the increase in the polaron's vibrational entropy. By using literature data, we also show that this phenomenon can be observed in two closely related diketopyrrolopyrrole-conjugated polymers as well as in p-doped P3HT and PANI systems with an increasing molecular order.

One-Step Facile Synthesis of Cobalt Phosphides for Hydrogen Evolution Reaction Catalysts in Acidic and Alkaline Medium.

Catalysts for hydrogen evolution reaction are in demand to realize the efficient conversion of hydrogen via water electrolysis. In this work, cobalt phosphides were prepared using a one-step, scalable, and direct gas-solid phosphidation of commercially available cobalt salts. It was found that the effectiveness of the phosphidation reaction was closely related to the state of cobalt precursors at the reaction temperature. For instance, a high yield of cobalt phosphides obtained from the phosphidation of cobalt(II) acetate was related to the good stability of cobalt salt at the phosphidation temperature. On the other hand, easily oxidizable salts (e.g., cobalt(II) acetylacetonate) tended to produce a low amount of cobalt phosphides and a large content of metallic cobalt. The as-synthesized cobalt phosphides were in nanostructures with large catalytic surface areas. The catalyst prepared from phosphidation of cobalt(II) acetate exhibited an improved catalytic activity as compared to its counterpart derived from phosphidation of cobalt(II) acetylacetonate, showing an overpotential of 160 and 175 mV in acidic and alkaline electrolytes, respectively. Both catalysts also displayed an enhanced long-term stability, especially in the alkaline electrolyte. This study illustrates the direct phosphidation behavior of cobalt salts, which serve as a good vantage point in realizing the large-scale synthesis of transition-metal phosphides for high-performance electrocatalysts.

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.

Highly efficient photocatalytic H2 evolution from water using visible light and structure-controlled graphitic carbon nitride.

The major challenge of photocatalytic water splitting, the prototypical reaction for the direct production of hydrogen by using solar energy, is to develop low-cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported. The g-C3N4 exhibits an extraordinary hydrogen-evolution rate (ca. 20,000 µmol h(-1) g(-1) under full arc), which leads to a high turnover number (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5% under visible light, which is nearly an order of magnitude higher than that observed for any other existing g-C3N4 photocatalysts. Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen-evolution rate is significantly enhanced.

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.