Water evaporation is a ubiquitous natural process exploiting thermal energy from ambient environment. Hydrovoltaic technologies emerged in recent years offer one prospective route to generate electricity from water evaporation, which has long been overlooked. Herein, we developed a hybrid hydrovoltaic generator driven by natural water evaporation, integrating an “evaporation motor” with an evaporation-electricity device and a droplet-electricity device. A rotary motion of the “evaporation motor” relies on phase change of ethanol driven by water-evaporation induced temperature gradient. This motion enables the evaporation-electricity device to work under a beneficial water-film operation mode to produce output of ~4 V and ~0.2 µA, as well as propels the droplet-electricity device to convert mechanical energy into pulsed output of ~100 V and ~0.2 mA. As different types of hydrovoltaic devices require distinctive stimuli, it was challenging to make them work simultaneously, especially under one single driving force. We here for the first time empower two types of hydrovoltaic devices solely by omnipresent water evaporation. Therefore, this work presents a new pathway to exploiting water evaporation-associated ambient thermal energy and provides insights on developing hybrid hydrovoltaic generators.

Artificial biomaterials with dynamic mechano-responsive behaviors similar to those of biological tissues have been drawing great attention. In this study, we report a TiO₂-based nanowire (TiO₂NWs) scaffolds, which exhibit dynamic mechano-responsive behaviors varying with the number and amplitude of nano-deformation cycles. It is found that the elastic and adhesive forces in the TiO₂NWs scaffolds can increase significantly after multiple cycles of nano-deformation. Further nanofriction experiments show the triboelectric effect of increasing elastic and adhesive forces during the nano-deformation cycles of TiO₂NWs scaffolds. These properties allow the TiO₂NW scaffolds to be designed and applied as intelligent artificial biomaterials to simulate biological tissues in the future.

Since its first discovery in 2017, evaporation-induced electricity has attracted extensive attention and shown significant advantages in green energy conversion. While the streaming potential-related electrokinetic effect has been intensively explored and widely recognized as the underlying mechanism, the role of coupling between water molecules and charge carriers in the material remains elusive. Here we show through carefully designed experiments that the streaming potential effect indeed plays a role but can only contribute about half to the total water-evaporation-induced voltage occurring within the partially-wetted region of the carbon black film where the solid-liquid-gas three-phase interface exists. It is also shown that water evaporation from carboxyl and amino-functionalized carbon black films produces opposite voltage signals. Detailed first-principles calculations unveil that the adsorption of water molecules can lead to reversed charge transfer in the carboxyl and amino-functionalized carbon substrates. Finally, an evaporation-driven charge transport mechanism is proposed for the induced electricity mediated by the coupling between water molecules and charge carriers in the material. The results reveal the important role of direct interaction between water molecules and materials, deepening our understanding of the mechanism for evaporation-induced hydrovoltaic effect beyond streaming potential.

The last decade has witnessed the emergence of hydrovoltaic technology, which can harvest electricity from different forms of water movement, such as raindrops, waves, flows, moisture, and natural evaporation. In particular, the evaporation-induced hydrovoltaic effect received great attention since its discovery in 2017 due to its negative heat emission property. Nevertheless, the influence of electrode reactions in evaporation-induced power generation is not negligible due to the chemical reaction between active metal electrodes and water, which leads to “exceptional” power generation. Herein, we designed a series of experiments based on air-laid paper devices with electrodes of different activities as the top and bottom electrodes. To verify the contribution of electrodes, we compared the output performance of different electrode combinations when the device was partially-wetted and fully-wetted. The device hydrophilicity, salt concentration, and acidity or basicity of solutions were also comprehensively investigated. It is demonstrated that the chemical reaction of active metals (Zn, Cu, Ag, etc.) with different aqueous solutions can generate considerable electrical energy and significantly distort the device performance, especially for Zn electrodes with an output voltage from ~ 1.26 to ~ 1.52 V and current from ~ 1.24 to ~ 75.69 μA. To promote the long-term development of hydrovoltaic technology, we recommend use of inert electrodes in hydrovoltaic studies, such as Au and Pt, especially in water and moisture environment.

Pseudo-ferroelectric transistors have attracted particular interest owing to their applications in the non-volatile memories and neuromorphic circuits; however, it remains to be explored in the limit of few-layer devices. Here we reveal a pseudo-ferroelectric phenomenon in the ultrathin graphene/black phosphorene (G/BP) heterostructure by first-principles calculations. Putting forward an excitation-assisted mechanism, the ferroelectric-like hysteresis loop can be explained by a combined effect of the external electric fields dependent bipolarity and anisotropy in the G/BP heterostructure. Considering the build-in electric field, the bipolar behavior results in the multistate effect of the G/BP heterostructure when modulating the applied electric field. The anisotropic hybridization caused by the susceptible Dirac electrons in graphene and the large in-plane anisotropy in BP provides the interfacial states, which trap excitations and stabilize the multistate. The pseudo-ferroelectric behavior should be useful for interpreting transport experiments in gated G/BP devices and exploring its applications in memories or synaptic devices.

Perovskite solar cells (PSCs) have attracted much attention due to their rapidly increased power conversion efficiencies, however, their inherent poor long-term stability hinders their commercialization. The degradation of PSCs first comes from the degradation of hole transport materials (HTMs). Here, we report the construction of periodic π-columnar arrays and ionic interfaces over the skeletons by introducing cationic covalent organic frameworks (C-COFs) to the HTM. Periodic π-columnar arrays can optimize the charge transport ability and energy levels of the hole transport layer and suppress the degradation of HTM, and ionic interfaces over the skeletons can produce stronger electric dipole and electrostatic interactions, as well as higher charge densities. The C-COFs were designed and synthesized via Schiff base reaction by using 1,3,5-triformylphloroglucinol as a neutral knot and dimidium bromide as cationic linker. The neutral COFs (N-COFs) were also synthesized as a reference by using 3,8-diamino-6-phenylphenanthridine as neutral linker. PSCs with cationic COF exhibit the highest efficiency of 23.4% with excellent humidity and thermal stability. To the best of our knowledge, this is the highest efficiency among the meso-structured PSCs fabricated by a sequential process.

The Au (100) surface has been a subject of intense studies due to excellent catalytic activities and its model character for surface science. However, the spontaneous surface reconstruction buries active Au (100) plane and limits practical applications, and how to controllably eliminate the surface reconstruction over large scale remains challenging. Here, we experimentally and theoretically demonstrate that simple decoration of the Au (100) surface by tellurium (Te) atoms can uniquely lift its reconstruction over large scale. Scanning tunneling microscopy imaging reveals that the lifting of surface reconstruction preferentially starts from the boundaries of distinct domains and then extends progressively into the domains with the reconstruction rows perpendicular to the boundaries, leaving a Au (100)-(1 × 1) surface behind. The Au (100)-(1 × 1) is saturated at ~ 84% ± 2% with respect to the whole surface at a Te coverage of 0.16 monolayer. With further increasing the Te coverage to 0.25 monolayer, the Au (100)-(1 × 1) surface becomes reduced and overlapped by a well-ordered (2 × 2)-Te superstructure. No similar behavior is found for Te-decorated Au (111), Cu (111), and Cu (100) surfaces, nor for the decorated Au (100) with other elements. This result may pave the way to design Au-based catalysts and, as an intermediate step, even potentially open a new route to constructing complex transition metal dichalcogenides.

We reveal the ultralow friction or superlubricity of water nanodroplets containing cations and anions on graphene substrates at high ion concentration by molecular dynamics simulations. When the ion concentration is higher than 7 wt.% and the nanodroplet diameter is larger than 10 nm, the friction coefficients of water nanodroplets are lower than 10⁻², and can decrease to the order of 10⁻³ with increasing the ion concentration further. At a certain ion concentration, the optimal nanodroplet diameter of 17–20 nm exists at which the friction coefficient is the lowest. The ultralow friction behaviors of water nanodroplets containing cations and anions are mainly attributed to the opposite variation trends between the interfacial adhesion energy and surface energy of water nanodroplet with ion concentration, and the interfacial hydrophobicity sustained by high ion concentration. These results unveil the essential role of ions in achieving the superlubricity of water nanodroplets.

Lubrication induced by a vertical electric field or bias voltage is typically not applicable to two-dimensional (2D) van der Waals (vdW) crystals. By performing extensive first-principles calculations, we reveal that the interlayer friction and shear resistance of Janus transition metal dichalcogenide (TMD) MoXY (X/Y = S, Se, or Te, and X ≠ Y) bilayers under a constant normal force mode can be reduced by applying vertical electric fields. The maximum interlayer sliding energy barriers between AA and AB stacking of bilayers MoSTe, MoSeTe, and MoSSe decrease as the positive electric field increases because of the more significant counteracting effect from the electric field energy and the more significant enhancement in interlayer charge transfer in AA stacking. Meanwhile, the presence of negative electric fields decreases the interlayer friction of bilayer MoSTe, because the electronegativity difference between Te and S atoms reduces the interfacial atom charge differences between AA and AB stacking. These results reveal an electro-lubrication mechanism for the heterogeneous interfaces of 2D Janus TMDs.