Modular reprogrammable 3D mechanical metamaterials with unusual hygroscopic deformation modes.

The majority of polymer-based materials demonstrate expansion upon absorbing water from the air. Mechanical metamaterials provide an interesting way to achieve unusual hygroscopic deformation. However, previous studies have only accommodated the limited tunability of negative hygroscopic expansion by theoretical analysis but have never involved other deformation modes. This work proposes modular reprogrammable 3D moisture-sensitive mechanical metamaterials with switchable hygroscopic deformation modes, which are built up of multi-material 3D-printed bi-material curved strips and cubic nodes. Depending on the geometrical parameters and spatial layouts of the curved strips, the metamaterials exhibit tunable coefficient of hygroscopic expansion from negative to positive. In addition to homogeneous deformation, complex 3D hygroscopic deformation modes can be achieved including shear and twist. More interestingly, the metamaterials are reprogrammable since all the deformation modes can be switched by modular disassembling and reassembling of the curved strips, just like LEGO building blocks. This work demonstrates a feasible approach to achieve customized 3D hygroscopic deformation through easy block building for specific engineering applications including eliminating hygroscopic stress, shape morphing structures, and smart actuators.

In situ fabrication of MIL-68(In)@ZnIn2S4 heterojunction for enhanced photocatalytic hydrogen production.

Metal-organic frameworks (MOFs), as a class of semiconductor-like materials, are widely used in photocatalysis. However, the limited visible light absorption and poor charge separation efficiency are the main challenges restricting their photocatalytic performance. Herein, the type II heterojunction MIL-68(In)@ZIS was successfully fabricated by in situ growth of ZnIn2S4 (ZIS) on the surface of a representative MOF, i.e. MIL-68(In). After composition optimization, MIL-68(In)-20@ZIS shows an extraordinary photocatalytic hydrogen production efficiency of 9.09 mmol g-1 h-1 and good photochemical stability, which far exceeds those of most photocatalysts. The hierarchical loose structure of MIL-68(In)-20@ZIS is conducive to the adsorption of reactants and mass transfer. Meanwhile, a large number of tight 2D contact interfaces significantly reduce the obstruction of charge transfer, paving the way for high-perform photocatalytic hydrogen evolution. The experimental results demonstrate that the MIL-68(In)@ZIS heterojunction achieves intensive photoresponse and effective charge separation and transfer benefiting from unique charge transport paths of a type II heterojunction. This study opens an avenue toward MOF-based heterojunctions for solar energy conversion.

Unexpected High-Performance Photocatalytic Hydrogen Evolution in Co@NCNT@ZnIn2 S4 Triggered by Directional Charge Separation and Transfer.

The structural design of photocatalysts is highly related to the separation and transfer of photogenerated carriers, which is essential for the improvement of photocatalytic hydrogen evolution performance. Here, the hybrid photocatalyst M@NCNT@ZIS (M: Fe, Co, Ni; NCNT: nitrogen-doped carbon nanotube; ZIS: ZnIn2 S4 ) with a hierarchical structure is rationally designed and precisely synthesized. The unique hollow structure with a large specific surface area offers abundant reactive sites, thus increasing the adsorption of reactants. Importantly, the properly positioned metal nanoparticles realize the directional charge migration from ZIS to M@NCNT, which significantly improves the efficiency of charge separation. Furthermore, the intimate interface between M@NCNT and ZIS effectively facilitates charge migration by shortening the transfer distance and providing numerous transport channels. As a result, the optimized Co@NCNT@ZIS exhibits a remarkable photocatalytic hydrogen evolution efficiency (43.73 mmol g-1 h-1 ) without Pt as cocatalyst. Experimental characterizations and density functional theory calculations demonstrate that the synergistic effect between hydrogen adsorption and interfacial charge transport is of great significance for improving photocatalytic hydrogen production performance.

Mass Diffusion Metamaterials with "Plug and Switch" Modules for Ion Cloaking, Concentrating, and Selection: Design and Experiments.

The outstanding abilities of metamaterials to manipulate physical fields are extensively studied in both wave-based and diffusion-based fields. However, mass diffusion metamaterials, with the ability to manipulate diffusion with practical applications associated with chemical and biochemical engineering, have not yet been experimentally demonstrated. In this work, ion cloaking, concentrating, and selection in liquid solvents are verified by both simulations and experiments, and the concept of a "plug and switch" metamaterial is proposed based on scattering cancellation (SC) to achieve switchable functions by plugging modularized functional units into a functional motherboard. Plugging in any module barely affects the environmental diffusion field, but the module choice impacts different diffusion behaviors in the central region. Cloaking strictly hinds ion diffusion, and concentrating increase diffusion flux, while cytomembrane-like ion selection permits the entrance of some ions but blocks others. In addition, these functions are demonstrated in special applications like the catalytic enhancement by the concentrator and the protein protection by the ion selector. This work not only experimentally demonstrates the effective manipulation of mass diffusion by metamaterials, but also shows that the "plug and switch" design is extensible and reconfigurable. It facilitates novel applications including sustained drug release, catalytic enhancement, bioinspired cytomembranes, etc.

Moisture-sensitive mechanical metamaterials with unusual and re-programmable hygroscopic deformation modes.

Mechanical metamaterials are of great interest due to their counterintuitive deformation under various physical fields. However, the research on metamaterials responding to moisture is still rare and controllable hygroscopic deformation is vital for sensoring, actuating, and stress elimination in a moisture environment. Inspired by the hygroscopic deformation of pinecones, this work studies 2D moisture-sensitive mechanical metamaterials exploiting bi-material curved strips as building blocks by simulations and experiments, which especially demonstrates repeatable programming ability to realize customized unusual hygroscopic deformations. Depending on the structural design of geometrical parameters and material configurations, the metamaterials exhibit a tunable coefficient of hygroscopic expansion from negative to positive, and unusual hygroscopic deformation modes including anisotropic, shearing, gradient, bending, and 3D deformation of 2D structures. Programmable metamaterials of arbitrary hygroscopic deformation are achieved by pixelated design and coding the building blocks. More importantly, the hygroscopic deformation is re-programmable by adopting erasable moisture-proof coatings on specific areas of metamaterials, i.e., it can continuously provide different customized deformation modes in a sample.

Programmable Mechanical Metamaterials with Tailorable Negative Poisson's Ratio and Arbitrary Thermal Expansion in Multiple Thermal Deformation Modes.

Mechanical metamaterials pave a way for designing and optimizing microstructure topology to achieve counterintuitive deformation including negative Poisson's ratio (NPR) and negative thermal expansion (NTE). Previous studies were always limited to single anomalous mechanical or thermal deformation, but current applications for high-precision mechanical or optical equipment always require their combination and customized and anisotropic deformation parameters. This work develops programmable two-dimensional (2D) mechanical metamaterials based on chiral and antichiral structures constructed with curved bimaterial strips to produce tailorable NPR and arbitrary thermal deformation. The coefficient of thermal expansion of the mechanical metamaterials is tunable on a large scale across negative, near-zero, and positive values depending on the bimaterial configurations and geometrical parameters of curved strips, while the value of NPR is mainly determined by the radian. Furthermore, it is programmable by coding the unit cells to exhibit customized and anisotropic thermal deformation combining homogeneous, gradient, and shear modes. The proposed mechanical metamaterials are fabricated by multimaterial three-dimensional (3D) printing, and the unusual deformation modes are verified experimentally, which is well in agreement with the results of finite element analysis. This work demonstrates a feasible approach to achieving customized mechanical and thermal deformation through easy block building for specific engineering applications including eliminating thermal stress, shape morphing, and smart actuators.

Ultrasensitive Frequency Shifting of Dielectric Mie Resonance near Metallic Substrate.

Dielectric resonators on metallic surface can enhance far-field scattering and boost near-field response having promising applications in nonlinear optics and reflection-type devices. However, the dependence of gap size between dielectric resonator and metallic surface on Mie resonant frequency is complex and desires a comprehensive physical interpretation. Here, we systematically study the effect of metallic substrate on the magnetic dipole (MD) resonant frequency at X-band by placing a high permittivity CaTiO3 ceramic block on metallic substrate and regulating their gap size. The simulated and experimental results show that there are two physical mechanisms to codetermine the metallic substrate-induced MD frequency. The greatly enhanced electric field pair in the gap and the coupling of MD resonance with its mirror image are decisive for small and large gaps, respectively, making the MD resonant frequency present an exponential blue shift first and then a slight red shift with increasing gap size. Further, we use the two mechanisms to explain different frequency shifting properties of ceramic sphere near metallic substrate. Finally, taking advantage of the sharp frequency shifting to small gaps, the ceramic block is demonstrated to accurately estimate the thickness or permittivity of thin film on metallic substrate through a governing equation derived from the method of symbolic regression. We believe that our study will help to understand the resonant frequency shifting for dielectric particle near metallic substrate and give some prototypes of ultrasensitive detectors.

Engineering of g-C3N4-based photocatalysts to enhance hydrogen evolution.

The technology of photocatalytic hydrogen production that converts abundant yet intermittent solar energy into an environmentally friendly alternative energy source is an attractive strategy to mitigate the energy crisis and environmental pollution. Graphitic carbon nitride (g-C3N4), as a promising photocatalyst, has gradually received focus in the field of artificial photosynthesis due to its appealing optical property, high chemical stability and easy synthesis. However, the limited light absorption and massive recombination of photoinduced carriers have hindered the photocatalytic activity of bare g-C3N4. Therefore, from the perspective of theoretical calculations and experiments, many valid approaches have been applied to rationally design the photocatalyst and ameliorate the hydrogen production performance, such as element doping, defect engineering, morphology tuning, and semiconductor coupling. This review summarized the latest progress of g-C3N4-based photocatalysts from two perspectives, modification of pristine g-C3N4 and interfacial engineering design. It is expected to offer feasible suggestions for the fabrication of low-cost and high-efficiency photocatalysts and the photocatalytic mechanism analyses assisted by calculation in the near future. Finally, the prospects and challenges of this exciting research field are discussed.

Ultrahigh piezocatalytic capability in eco-friendly BaTiO3 nanosheets promoted by 2D morphology engineering.

Piezocatalysis, converting mechanical vibration into chemical energy, is an emerging technology to address environmental issues. In this work, we propose an efficient method to significantly improve the piezocatalytic activity by morphology engineering rather than composition design. The catalytic property in BaTiO3 nanocrystallites with diverse morphologies is investigated by dye degradation and hydrogen production under ultrasonic vibration. The BaTiO3 nanosheets exhibit an excellent piezocatalytic activity with a degradation rate of 0.1279 min-1 for Rhodamine B, far beyond those in previous piezocatalytic literature and even comparable to excellent photocatalysts, and also a high hydrogen production rate of 92 µmol g-1 h-1. Compared with nanowires and nanoparticles, the 2D morphology greatly enhances the piezocatalytic activity in nanosheets owing to much larger piezoelectric potential. This proves that the piezocatalytic property is dominated by the morphology-dependent piezoelectricity, rather than specific surface area as other catalysis. Dominated by bending vibrating mode, the piezocatalytic activity reaches a maximum at the piezoelectric resonating frequency, and it increases with the ultrasonic power. Moreover, it has good reusability and wide versatility for catalytic degradation. This work gives an in-depth understanding of piezocatalytic mechanism and provides a way to develop high performance and eco-friendly piezocatalysts.

Fully Controllable Pancharatnam-Berry Metasurface Array with High Conversion Efficiency and Broad Bandwidth.

Metasurfaces have powerful abilities to manipulate the properties of electromagnetic waves flexibly, especially the modulation of polarization state for both linearly polarized (LP) and circularly polarized (CP) waves. However, the transmission efficiency of cross-polarization conversion by a single-layer metasurface has a low theoretical upper limit of 25% and the bandwidth is usually narrow, which cannot be resolved by their simple additions. Here, we efficiently manipulate polarization coupling in multilayer metasurface to promote the transmission of cross-polarization by Fabry-Perot resonance, so that a high conversion coefficient of 80-90% of CP wave is achieved within a broad bandwidth in the metasurface with C-shaped scatters by theoretical calculation, numerical simulation and experiments. Further, fully controlling Pancharatnam-Berry phase enables to realize polarized beam splitter, which is demonstrated to produce abnormal transmission with high conversion efficiency and broad bandwidth.