No abstract

Negative thermal expansion materials are important and desirable in science and engineering applications. However, natural materials with isotropic negative thermal expansion are rare and usually unsatisfied in performance. Here, we propose a novel method to achieve two- and three-dimensional negative thermal expansion metamaterials via antichiral structures. The two-dimensional metamaterial is constructed with unit cells that combine bimaterial strips and antichiral structures, while the three-dimensional metamaterial is fabricated by a multimaterial 3D printing process. Both experimental and simulation results display isotropic negative thermal expansion property of the samples. The effective coefficient of negative thermal expansion of the proposed models is demonstrated to be dependent on the difference between the thermal expansion coefficient of the component materials, as well as on the circular node radius and the ligament length in the antichiral structures. The measured value of the linear negative thermal expansion coefficient of the three-dimensional sample is among the largest achieved in experiments to date. Our findings provide an easy and practical approach to obtaining materials with tunable negative thermal expansion on any scale.

We report on the photoelectrochemical stability of electrodeposited films of cuprous oxide (Cu2O) in aqueous solutions. p-type Cu2O was electrodeposited from an alkaline lactate solution onto Au substrates with a (111) preferred orientation. The as-deposited films exhibit a strong (111) orientation and a columnar morphology, terminating at the surface with pyramidal features. Stability of the photoelectrochemical response was studied via repeated cycling polarization and by monitoring the photocurrent within a voltage window where only the CuO/Cu2O transformation would occur. The Cu2O film morphology changes from a dense structure to a more stable network of elongated leaf-like crystals. We attributed this behavior to the energy differences among the various crystallographic facets of Cu2O, with the {111} facets being the most stable. Photocurrent evolution over time was explained in terms of an initial increase in surface area at the start of the process, followed by a decrease due to increased recombination losses resulting from having only the {111} surface exposed to the electrolyte.

No abstract