UV Light-Mediated Hydrolytic Reaction to Develop Magnetic Hydrogel Actuators with Spatially Distributed Ferriferous Oxide Microparticles.

Magnetic hydrogel actuators are developed by incorporating magnetic fillers into the hydrogel matrix. Regulating the distribution of these fillers is key to the exhibited functionalities but is still challenging. Here a facile way to spatially synthesize ferrosoferric oxide (Fe3 O4 ) microparticles in situ in a thermal-responsive hydrogel is reported. This method involves the photo-reduction of Fe3+ ions coordinated with carboxylate groups in polymer chains, and the hydrolytic reaction of the reduced Fe2+ ions with residual Fe3+ ions. By controlling the irradiation time and position, the concentration of Fe3 O4 microparticles can be spatially controlled, and the resulting Fe3 O4 pattern enables the hydrogel to exhibit complex locomotion driven by magnet, temperature, and NIR light. This method is convenient and extendable to other hydrogel systems to realize more complicated magneto-responsive functionalities.

Thermal and Near-Infrared Light-Responsive Hydrogel Actuators with Spatiotemporally Developed Polypyrrole Patterns.

Conjugated polymers are commonly adopted to develop electro- and photoresponsive materials due to their superior electronic conductivity and phototothermal convertibility. However, they are usually homogeneously polymerized within the network, which makes their functionalities challenging to spatiotemporally modulate. In this work, we report a convenient and extensible method to develop polypyrrole patterns in a thermally responsive sodium alginate/poly(N-isopropylacrylamide) hydrogel. The polypyrrole pattern is developed by spatial photoreduction of Fe3+ ions into Fe2+ ions and subsequently initiating oxidation polymerization of pyrrole by the residual Fe3+ ions. During this process, carboxylate groups coordinated with Fe3+ ions are also sacrificed in a gradient manner along the thickness direction, and the resulting concentration gradients of the carboxylate group endow the hydrogel with thermal-responsive actuation. The polymerized polypyrrole also renders the hydrogels' prominent temperature-rising behaviors upon NIR light irradiation. By designing the PPy pattern, hydrogels can exhibit versatile actuating behaviors and execute mechanical works such as lifting objects. This method is convenient and can be extended to develop other conjugated polymers in hydrogel systems for versatile applications.

Stretching-Induced 2D-to-3D Shape Transformation of an Elastic Composite for Sensitivity-Tailorable Soft Electronics.

The shapes of rubbers and elastomers are challenging to alter, and current methods relying on permanent plasticity and dynamic cross-linking strategies are usually laborious and can inevitably compromise the network elasticity. Here, we report a photoresponsive elastic composite that can be programmed into 3D shapes by first UV light irradiation and then stretching. The composite comprises ethylene propylene rubber as the elastic substrate and photoliquefiable azobenzene small molecules as the responsive filler. Upon UV light irradiation, the liquefication of the filler induces the destruction of the crystalline aggregates near the irradiated surface, and after stretching and subsequent stress release, the irradiated part bends to the irradiated side based on a gradient network orientation mechanism. The position and amplitude of bending deformation can be controlled to realize a 2D-to-3D shape transformation. We further show that the resulting 3D-shaped elastomer can integrate with silver conductive paste to develop soft conductive lines with tailorable strain-sensitive conductivities. This study may open a new door for the development of shape-tailorable elastomers and soft electronics with designable strain-sensitive conductivities.

Fabricating 3D Moisture- and NIR Light-Responsive Actuators by a One-Step Gradient Stress-Relaxation Process.

Polymeric materials that can actuate under the stimulation of environmental signals have attracted considerable attention in fields including artificial muscles, soft robotics, implantable devices, etc. To date, the improvement of shape-changing flexibility is mainly limited by their unchangeable shapes and structural and compositional distributions. In this work, we report a one-step treatment process to convert 2D poly(ethylene oxide)/sodium alginate/tannic acid thin films into 3D-shaped moisture- and NIR light-responsive actuators. Spatial surface wetting of the film leads to the release of residual stress generated in film formation in a gradient manner, which drives the wetted regions to bidirectionally bend. By controlling the position and bending amplitude of the wetted regions, designated 3D shapes can be obtained. Moreover, Fe3+ ions in the aqueous solution used for surface wetting can coordinate with carboxylate groups in sodium alginate chains to form a gradient cross-linking network. This gradient network can not only stabilize the resulting 3D shape but also render the film with moisture-responsive morphing behaviors. Fe3+ ions can also self-assemble with tannic acid molecules to form photothermal aggregates, making the film responsive to NIR light. We further show that films with versatile 3D shapes and different modes of deformation can be fabricated by a one-step treatment process. This strategy is convenient and extendable to develop 3D-shaped polymer actuators with flexible shape-changing behaviors.

Construction of Bi2WO6 with double active sites of tunable metallic Bi and oxygen vacancies for photocatalytic oxidation of cyclohexane to cyclohexanone.

Oxygen-containing organics, which are generated from the selective oxidation of their corresponding hydrocarbons, have high value in the chemical and pharmaceutical industries. However, their oxidation reactions are very challenging as the products are more active than the substrates, especially for the oxidation of cyclohexane (CHA). Herein, we focused on the one-step preparation of Bi2WO6 with double active sites of tunable metallic Bi and oxygen vacancies (OV-Bi/Bi2WO6) by a facile solvothermal treatment. Then, OV-Bi/Bi2WO6 was used as an efficient photocatalyst for the partial oxidation of CHA to cyclohexanone (CHA-one) for the first time in air as an oxidant under solvent-free and room temperature conditions. The Bi : Bi2WO6 ratio in the as-prepared OV-Bi/Bi2WO6 heterojunction could be tailored from 0.08 to 8.43 by controlling the solvothermal temperature, and the synergistic effect between DMF and EG could increase the reduction of MDF/EG and promote the production of Bi. Moreover, OV-Bi/Bi2WO6-160 yielded 4.4 and 8.8 times more CHA-one (128.8 µmol) than pure Bi2WO6 and metallic Bi, respectively, and achieved 93.6% selectivity to CHA-one in air as an oxidant under solvent-free conditions. The results revealed that the highly enhanced photocatalytic activity was mainly attributed to the superior specific surface area, outstanding photo-absorption, abundant oxygen vacancies, and efficient electron-hole separation. Moreover, for the unique double active sites in OV-Bi/Bi2WO6, oxygen vacancies can enhance the adsorption and activation capacity of Bi2WO6 for O2, while metallic Bi can improve the adsorption and activation capacity of Bi2WO6 for CHA. Meanwhile, OV-Bi/Bi2WO6 also exhibited excellent durability due to the strong interaction between metallic Bi and Bi2WO6. The present work provides a flexible approach for tailoring the Bi : Bi2WO6 ratio and outlines an effective method for producing CHA-one from CHA under mild conditions.

Programmable Thermo-Responsive Actuation of Hydrogels via Light-Guided Surface Growth of Active Layers on Shape Memory Substrates.

Hydrogel shape memory and actuating functionalities are heavily pursued and have found great potential in various application fields. However, their combination for more flexible and complicated morphing behaviors is still challenging. Herein, it is reported that by controlling the light-initiated polymerization of active hydrogel layers on shape memory hydrogel substrates, advanced morphing behaviors based on programmable hydrogel shapes and actuating trajectories are realized. The formation and photo-reduction-induced dissociation of Fe3+ -carboxylate coordination endow the hydrogel substrates with the shape memory functionality. The photo-reduced Fe2+ ions can diffuse from the substrates into the monomer solutions to initiate the polymerization of the thermally responsive active layers, whose actuating temperatures and amplitudes can be facially tuned by controlling their thicknesses and compositions. One potential application, a shape-programmable 3D hook that can lift an object with a specific shape, is also unveiled. The demonstrated strategy is extendable to other hydrogel systems to realize more versatile and complicated actuating behaviors.

Thermal-Responsive Hydrogel Actuators with Photo-Programmable Shapes and Actuating Trajectories.

Thermal-responsive hydrogel actuators have aroused a wide scope of research interest and have been extensively studied. However, their actuating behaviors are usually monotonous due to their unchangeable shapes and structures. Here, we report thermal-responsive poly(isopropylacrylamide-co-2-(dimethylamino)ethyl methacrylate)/alginate hydrogels with programmable external shapes and internal actuating trajectories. The volume phase transition temperatures of the resulting hydrogels can be tuned in a wide temperature range from 32 to above 50 °C by adjusting the monomer composition. While the formation and photo-dissociation of Fe3+-carboxylate tri-coordinates within the entire hydrogel network enable photo-responsive shape memory property, the insufficient dissociation of the tri-coordinates along the irradiation path gives rise to gradient crosslinking for realizing thermal-responsive actuation. Controlling the evolution of the gradient structure facilitates the regulation of the actuating amplitude. Furthermore, we show that the combination of these two types of shape-changing functionalities leads to more flexible and intricate shape-changing behaviors. One interesting application, a programmable hook with changeable actuating behaviors for lifting different objects with specific shapes, is also demonstrated. The proposed strategy can be extended to other types of actuating hydrogels with more advanced actuating behaviors.

In Situ Anodic Oxidation Tuning of NiFeV Diselenide to the Core-Shell Heterojunction for Boosting Oxygen Evolution.

Developing non-noble metal-based core-shell heterojunction electrocatalysts with high catalytic activity and long-lasting stability is crucial for the oxygen evolution reaction (OER). Here, we prepared novel core-shell Fe,V-NiSe2@NiFe(OH)x heterostructured nanoparticles on hydrophilic-treated carbon paper with high electronic transport and large surface area for accelerating the oxygen evolution rate via high-temperature selenization and electrochemical anodic oxidation procedures. Performance testing shows that Fe,V-NiSe2@NiFe(OH)x possesses the highest performance for OER compared to as-prepared diselenide core-derived heterojunctions, which only require an overpotential of 243 mV at 10 mA cm-2 and a low Tafel slope of 91.6 mV decade-1 under basic conditions. Furthermore, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) confirm the morphology and elementary stabilities of Fe,V-NiSe2@NiFe(OH)x after long-term chronopotentiometric testing. These advantages are largely because of the strong synergistic effect between the Fe,V-NiSe2 core with high conductivity and the amorphous NiFe(OH)x shell with enriched defects and vacancies. This study also presents a general approach to designing and synthesizing more active core-shell heterojunction electrocatalysts for OER.

Light-Guided Growth of Gradient Hydrogels with Programmable Geometries and Thermally Responsive Actuations.

Hydrogel actuators have gained considerable interest and experienced significant advancements in recent years. However, the programming of their actuating behaviors is still challenging. Herein, we report the development and regulation of gradient structures of hydrogels for programmable thermally responsive actuating behaviors. The hydrogel actuators are developed by controlling the photoreduction of Fe3+ ions coordinated with carboxylate groups from the substrates and their limited diffusion into the precursor solutions to act as both initiators and crosslinkers. The developed hydrogels show well-defined external geometries and controllable thicknesses under spatiotemporal control of ultraviolet irradiation. The shapes and the actuation amplitudes of the hydrogel actuators can be independently regulated by controlling the formation and photodissociation of Fe3+-carboxylate coordination in the formed gradient networks. Some interesting applications such as the lifting of an object with a specific shape and directional walking are realized. The proposed method can be extended to other hydrogel actuators with different compositions and stimuli-responsive behaviors.

Research progress of CO2 oxidative dehydrogenation of propane to propylene over Cr-free metal catalysts.

CO2-assisted oxidative dehydrogenation of propane (CO2-ODHP) is an attractive strategy to offset the demand gap of propylene due to its potentiality of reducing CO2 emissions, especially under the demands of peaking CO2 emissions and carbon neutrality. The introduction of CO2 as a soft oxidant into the reaction not only averts the over-oxidation of products, but also maintains the high oxidation state of the redox-active sites. Furthermore, the presence of CO2 increases the conversion of propane by coupling the dehydrogenation of propane (DHP) with the reverse water gas reaction (RWGS) and inhibits the coking formation to prolong the lifetime of catalysts via the reverse Boudouard reaction. An effective catalyst should selectively activate the C-H bond but suppress the C-C cleavage. However, to prepare such a catalyst remains challenging. Chromium-based catalysts are always applied in industrial application of DHP; however, their toxic properties are harmful to the environment. In this aspect, exploring environment-friendly and sustainable catalytic systems with Cr-free is an important issue. In this review, we outline the development of the CO2-ODHP especially in the last ten years, including the structural information, catalytic performances, and mechanisms of chromium-free metal-based catalyst systems, and the role of CO2 in the reaction. We also present perspectives for future progress in the CO2-ODHP.

Photoprogrammable Moisture-Responsive Actuation of a Shape Memory Polymer Film.

Humidity-responsive polymeric actuators have gained considerable interest due to their great potential in the fields including soft robotics, artificial muscles, smart sensors, and actuators. However, most of them can only exhibit invariable shape changes, which severely restricts their further exploration and practical use. Herein, we report that programmable humidity-responsive actuating behaviors can be realized by introducing photoprogrammable hygroscopic patterns into shape memory polymers. Poly(ethylene-co-acrylic acid) is selected as a model polymer and the solvent-processed thin films are soft and elastic, whose external shapes can be programmed by a modified shape memory process. On another aspect, an Fe3+-carboxylate coordinating network formed by surface treatments can be spatially dissociated under UV, resulting in transient hygroscopic gradients as active joints for moisture-driven actuation. Moreover, we show that the shape memory effect can be an effective means to adjust the direction as well as the amplitude of the moisture-driven actuating behavior. The proposed strategy is convenient and can be generally extended to other shape memory polymers to realize programmable moisture-responsive actuating behaviors.

Photo-Dissociable Fe3+-Carboxylate Coordination: A General Approach toward Hydrogels with Shape Programming and Active Morphing Functionalities.

An extendable double network design for hydrogels with programmable external geometries and actuating trajectories is presented. Chemically cross-linked polyacrylamide as the first network penetrated with linear alginate chains is prepared for demonstration. The coordination of Fe3+ ions with carboxylate groups in alginate chains acts as the second network, and its dissociation through photoreduction is utilized to realize the photoresponsive shape memory property; the shape fixity ratio and shape recovery ratio both exceed 90%. The gradient dissociation of Fe3+-carboxylate coordination under UV facilitates 3D programming of hydrogel geometry. On another aspect, the resulted cross-linking gradient differentiates the extent and rate of solvent-induced volume change of the PAAm network, endowing the hydrogel with photo-programmable solvent-driven actuating behavior. Furthermore, by inducing the formation of Fe3+-carboxylate coordination within the entire network for shape programming and cross-linking gradients in specific regions as active joints, hydrogels with designed actuating behaviors based on specific 3D shapes are realized. The shape memory and active morphing functionalities enabled by photo-dissociable Fe3+-carboxylate coordination in PAAm hydrogel can be generally extended to other hydrogels.

Controlled direct synthesis of single- to multiple-layer MWW zeolite.

The minimized diffusion limitation and completely exposed strong acid sites of the ultrathin zeolites make it an industrially important catalyst especially for converting bulky molecules. However, the structure-controlled and large-scale synthesis of the material is still a challenge. In this work, the direct synthesis of the single-layer MWW zeolite was demonstrated by using hexamethyleneimine and amphiphilic organosilane as structure-directing agents. Characterization results confirmed the formation of the single-layer MWW zeolite with high crystallinity and excellent thermal/hydrothermal stability. The formation mechanism was rigorously revealed as the balanced rates between the nucleation/growth of the MWW nanocrystals and the incorporation of the organosilane into the MWW unit cell, which is further supported by the formation of MWW nanosheets with tunable thickness via simply changing synthesis conditions. The commercially available reagents, well-controlled structure and the high catalytic stability for the alkylation of benzene with 1-dodecene make it an industrially important catalyst.

Programmable Humidity-Responsive Actuation of Polymer Films Enabled by Combining Shape Memory Property and Surface-Tunable Hygroscopicity.

Most humidity-responsive polymeric actuators can only exhibit shape transformations between a planar shape in the dry state and a bended three-dimensional (3D) shape when exposed to moisture, and it is challenging to design and prepare hygroscopic actuators with programmable actuating behaviors displayed from sophisticated 3D structures. Herein, we demonstrate that the integration of shape memory property and surface treatment enabled hygromorphic responsivity endows a single-component polymer film with programmable moisture-driven actuating behaviors. The solvent-processed polyethylene-co-acrylic acid (EAA) copolymer film is soft and stretchable at room temperature, and has a good thermal-responsive shape memory property. By surface treatment using base/acid solutions, the reversible gradient conversion between carboxyl groups and carboxylate salts along the thickness direction enables the film to exhibit designed hygroscopic actuations. The shape memory property and moisture-driven actuating behaviors can be combined to realize 3D-3D morphing by first programming the films into 3D shapes and then conducting the surface treatments. Both shape programming and surface treatment processes can be reprogrammed to make the actuation behavior readily tunable. We also show that the created surface patterns can act as moisture-sensitive conducting paths to detect human breathes, and the combination of shape memory, moisture-responsive morphing and conductivity change leads to some interesting applications such as smart switch in conducting circuit. This work provides a new and general strategy for the design of advanced humidity-responsive actuators.

Highly Efficient Oxidative Cyanation of Aldehydes to Nitriles over Se,S,N-tri-Doped Hierarchically Porous Carbon Nanosheets.

Oxidative cyanation of aldehydes provides a promising strategy for the cyanide-free synthesis of organic nitriles. Design of robust and cost-effective catalysts is the key for this route. Herein, we designed a series of Se,S,N-tri-doped carbon nanosheets with a hierarchical porous structure (denoted as Se,S,N-CNs-x, x represents the pyrolysis temperature). It was found that the obtained Se,S,N-CNs-1000 was very selective and efficient for oxidative cyanation of various aldehydes including those containing other oxidizable groups into the corresponding nitriles using ammonia as the nitrogen resource below 100 °C. Detailed investigations revealed that the excellent performance of Se,S,N-CNs-1000 originated mainly from the graphitic-N species with lower electron density and synergistic effect between the Se, S, N, and C in the catalyst. Besides, the hierarchically porous structure could also promote the reaction. Notably, the unique feature of this metal-free catalyst is that it tolerated other oxidizable groups, and showed no activity on further reaction of the products, thereby resulting in high selectivity. As far as we know, this is the first work for the synthesis of nitriles via oxidative cyanation of aldehydes over heterogeneous metal-free catalysts.

Gallium nitride catalyzed the direct hydrogenation of carbon dioxide to dimethyl ether as primary product.

The selective hydrogenation of CO2 to value-added chemicals is attractive but still challenged by the high-performance catalyst. In this work, we report that gallium nitride (GaN) catalyzes the direct hydrogenation of CO2 to dimethyl ether (DME) with a CO-free selectivity of about 80%. The activity of GaN for the hydrogenation of CO2 is much higher than that for the hydrogenation of CO although the product distribution is very similar. The steady-state and transient experimental results, spectroscopic studies, and density functional theory calculations rigorously reveal that DME is produced as the primary product via the methyl and formate intermediates, which are formed over different planes of GaN with similar activation energies. This essentially differs from the traditional DME synthesis via the methanol intermediate over a hybrid catalyst. The present work offers a different catalyst capable of the direct hydrogenation of CO2 to DME and thus enriches the chemistry for CO2 transformations.

Insight into the Intermolecular Interaction and Free Radical Polymerizability of Methacrylates in Supercritical Carbon Dioxide.

High pressure in situ Fourier transfer infrared/near infrared technology (HP FTIR/NIR) along with theoretical calculation of density functional theory (DFT) method was employed. The solvation behaviors and the free radical homopolymerization of methyl methacrylate (MMA), methacrylate acid (MAA), trifluoromethyl methacrylate (MTFMA) and trifluoromethyl methacrylate acid (TFMAA) in scCO2 were systematically investigated. Interestingly, the previously proposed mechanism of intermolecular-interaction dynamically-induced solvation effect (IDISE) of monomer in scCO2 is expected to be well verified/corroborated in view that the predicted solubility order of the monomers in scCO2 via DFT calculation is ideally consistent with that observed via HP FTIR/NIR. It is shown that MMA and MAA can be easily polymerized, while the free radical polymerizability of MTFMA is considerably poor and TFMAA cannot be polymerized via the free radical initiators. The α trifluoromethyl group (-CF3) may effectively enhance the intermolecular hydrogen bonding and restrain the diffusion of the monomer in scCO2. More importantly, the strong electron-withdrawing inductive effect of -CF3 to C=C may distinctly decrease the atomic charge of the carbon atom in the methylene (=CH2). These two factors are believed to be predominantly responsible for the significant decline of the free radical polymerizability of MTFMA and the other alkyl 2-trifluoromethacrylates in scCO2.

Photoresponsive Shape Memory Hydrogels for Complex Deformation and Solvent-Driven Actuation.

A new design for photoresponsive shape memory hydrogels and their possible applications are demonstrated in the present study. We show that the photodissociable Fe3+-carboxylate coordination can be utilized as a molecular switch to realize photocontrol of shape memory on both macroscopic and microscopic scales and enable a number of functions. Indeed, Fe3+-carboxylate coordination can fix a large tensile strain (up to 680%) of the sodium alginate/polyacrylamide hydrogel through cross-linking of sodium alginate chains, and subsequent UV irradiation allows strain energy release in spatially selected regions through reduction of Fe3+ to Fe2+. By manipulating light irradiation, complex 3D structures are obtained from 2D hydrogel sheets, and they exhibit complex solvent-driven actuation behaviors due to a light-changeable modulus and cross-linking density in the hydrogel. Based on the same approach, micropatterns can be inscribed on the hydrogel surface using mask-assisted irradiation, and they exhibit chain orientation-mediated anisotropic topography change upon solvent exchange. Moreover, light-controlled strain energy release also enables changing hydrogel surface wettability by solvent replacement. The demonstrated mechanism for photoresponsive hydrogels is highly efficient and applicable to many systems, which offers new perspectives in developing hydrogels with multiple photoresponsive functions.

Controlled 3D Shape Transformation Activated by Room Temperature Stretching and Release of a Flat Polymer Sheet.

Shape transformation of polymeric materials, including hydrogels, liquid crystalline, and semicrystalline polymers, can be realized by exposing the shape-changing materials to the effect of a variety of stimuli such as temperature, light, pH, and magnetic and electric fields. Herein, we demonstrate a novel and different approach that allows a flat sheet or strip of a polymer to transform into a predesigned 3D shape or structure by simply stretching the polymer at room temperature and then releasing it from the external stress, that is, a 2D-to-3D shape change is activated by mechanical deformation under ambient conditions. This particular type of stimuli-controlled shape-changing polymers is based on suppressing plastic deformation in selected regions of the flat polymer sheet prior to stretching and release. We validated the design principle by using a polymer blend composed of poly(ethylene oxide) (PEO), poly(acrylic acid) (PAA), and tannic acid (TA) whose plastic deformation can be locally inhibited by surface treatment using an aqueous solution of copper sulfate pentahydrate (Cu2+ ink) that cross-links PAA chains through a Cu2+-carboxylate coordination and, consequently, increases the material's Young's modulus and yield strength. After room temperature stretching and release, elastic deformation in the Cu2+ ink-treated regions leads to 3D shape transformation that is controlled by the patterned surface treatment. This facile and effective "stretch-and-release" approach widens the scope of preparation and application for shape-changing polymers.

2D-to-3D Shape Transformation of Room-Temperature-Programmable Shape-Memory Polymers through Selective Suppression of Strain Relaxation.

Although shape-memory polymers (SMPs) can alter their shapes upon stimulation of environmental signals, complex shape transformations are usually realized by using advanced processing technologies (four-dimensional printing) and complicated polymer structure design or localized activation. Herein, we demonstrate that stepwise controlled complex shape transformations can be obtained from a single flat piece of SMP upon uniform heating. The shape-memory blends prepared by solution casting of poly(ethylene oxide) and poly(acrylic acid) (PAA) exhibit excellent mechanical and room-temperature shape-memory behaviors, with fracture strain beyond 800% and both shape memory and shape recovery ratio higher than 90%. After plastic deformation by stretching under ambient conditions, the material is surface-patterned to induce the formation of an Fe3+-coordinated PAA network with gradually altered cross-linking density along the thickness direction at desired areas. Upon subsequent heating for shape recovery, strain release is restricted by the PAA network to different extents depending on the cross-linking density, which results in bending deformation toward the nonpatterned side and leads to three-dimensional shape transformation of the SMP. More interestingly, by sequentially dissociating the PAA network via UV or visible light-induced photoreduction of Fe3+ to Fe2+, residual strains can be removed in a spatially controlled manner. Using this approach, a series of origami shapes are obtained from a single SMP with a tailored two-dimensional initial shape. We also demonstrate that by incorporating polydopamine nanoparticles as photothermal fillers into the material, the whole shape transformation process can be carried out at room temperature by using near-infrared light.