Paper-Based Hydroelectric Generators for Water Evaporation-Induced Electricity Generation.

The research presented in this paper introduces a novel environmental energy-harvesting technology that harnesses electricity from the evaporation of water using porous structural materials. Specifically, a strategy employing paper-based hydroelectric generators (p-HEGs) is proposed to capture the energy produced during water evaporation and convert it into usable electricity. The p-HEGs offer several advantages, including simplicity in fabrication, low cost, and reusability. To evaluate their effectiveness, the water evaporation-induced electrical output performance of four different p-HEGs are compared. Among the variants tested, the p-HEG combining wood pulp and polyester fiber exhibits the best output performance. At room temperature, this particular p-HEG generates a short-circuit current and open-circuit voltage of ≈0.4 µA and 0.3 V, respectively, thereby demonstrating excellent electrical stability. Furthermore, the electrical current and voltage generated by the p-HEG through water evaporation are able to power an LED light, both individually and in series and parallel connections. This study delves into the potential of electricity harvesting from water evaporation and establishes it as a viable method for renewable energy applications.

Phase, Conductivity, and Surface Coordination Environment in Two-Dimensional Electrochemistry.

The booming frontier of electrochemistry is radically transforming the landscape of global chemical and energy industry. Most recent advancements in electrocatalysts have been built on trial and error, lacking model experiments to illuminate the fundamental factors hidden behind, such as phase, conductivity, and surface coordination environment. Here, we use phase-controllable, highly oriented two-dimensional MoTe2 as the model catalysts. The 2H phase MoTe2's conductivity can be engineered both extrinsically and intrinsically by single-layer graphene and lithiation, bringing down the sheet resistance from 0.95 MΩ/□ to 0.8 kΩ/□ and 0.6 kΩ/□. The corresponding electrocatalytic performance was unlocked from a silent state, catching up to its 1T' counterpart, with a parallel Tafel slope of 141 mV/dec. A focused ion beam further exposed the edge atoms, which exhibited a hydrogen evolution turnover frequency 104 times superior to that of basal plane atoms.

Largely Tunable Band Structures of Few-Layer InSe by Uniaxial Strain.

Because of the strong quantum confinement effect, few-layer γ-InSe exhibits a layer-dependent band gap, spanning the visible and near infrared regions, and thus recently has been drawing tremendous attention. As a two-dimensional material, the mechanical flexibility provides an additional tuning knob for the electronic structures. Here, for the first time, we engineer the band structures of few-layer and bulk-like InSe by uniaxial tensile strain and observe a salient shift of photoluminescence peaks. The shift rate of the optical gap is approximately 90-100 meV per 1% strain for four- to eight-layer samples, which is much larger than that for the widely studied MoS2 monolayer. Density functional theory calculations well reproduce the observed layer-dependent band gaps and the strain effect and reveal that the shift rate decreases with the increasing layer number for few-layer InSe. Our study demonstrates that InSe is a very versatile two-dimensional electronic and optoelectronic material, which is suitable for tunable light emitters, photodetectors, and other optoelectronic devices.

Optimizing Nonlinear Optical Visibility of Two-Dimensional Materials.

Two-dimensional (2D) materials have attracted broad research interests across various nonlinear optical (NLO) studies, including nonlinear photoluminescence (NPL), second harmonic generation (SHG), transient absorption (TA), and so forth. These studies have unveiled important features and information of 2D materials, such as in grain boundaries, defects, and crystal orientations. However, as most research studies focused on the intrinsic NLO processes, little attention has been paid to the substrates underneath. Here, we discovered that the NLO signal depends significantly on the thickness of SiO2 in SiO2/Si substrates. A 40-fold enhancement of the NPL signal of graphene was observed when the SiO2 thickness was varied from 270 to 125 nm under 800 nm excitation. We systematically studied the NPL intensity of graphene on three different SiO2 thicknesses within a pump wavelength range of 800-1100 nm. The results agreed with a numerical model based on back reflection and interference. Furthermore, we have extended our measurements to include TA and SHG of graphene and MoS2, confirming that SiO2 thickness has similar effects on all of the three major types of NLO signals. Our results will serve as an important guidance for choosing the optimum substrates to conduct NLO research studies on 2D materials.

Chemical and Bandgap Engineering in Monolayer Hexagonal Boron Nitride.

Monolayer hexagonal boron nitride (h-BN) possesses a wide bandgap of ~6 eV. Trimming down the bandgap is technically attractive, yet poses remarkable challenges in chemistry. One strategy is to topological reform the h-BN's hexagonal structure, which involves defects or grain boundaries (GBs) engineering in the basal plane. The other way is to invite foreign atoms, such as carbon, to forge bizarre hybrid structures like hetero-junctions or semiconducting h-BNC materials. Here we successfully developed a general chemical method to synthesize these different h-BN derivatives, showcasing how the chemical structure can be manipulated with or without a graphene precursor, and the bandgap be tuned to ~2 eV, only one third of the pristine one's.

Pristine Graphene Electrode in Hydrogen Evolution Reaction.

Graphene, the sp2 carbonaceous two-dimensional (2D) material, is gaining more attention in recent electrochemical studies. However, this atomic thick electrode usually suffers with surface contamination and poor electrochemical endurance. To overcome the drawbacks, we developed a PMMA-assisted, flipped transfer method to fabricate the graphene electrode with pristine surface and prolonged lifetime in hydrogen evolution reaction (HER). The HER performances of the single-layer graphene (SLG) were evaluated on various insulating and conductive substrates, including SiO2, polymers, SLG, highly oriented pyrolytic graphite (HOPG), and copper. The parallel Tafel slopes of SLG, bilayer graphene (BLG), and HOPG suggest they share the same electrochemical activities deriving from the sp2 carbon basal plane. Moreover, the atomic barriers, both for SLG and the single-layer h-BN (SLBN), are semitransparent in HER for the underneath copper, providing a new perspective for the 2D materials to protect and couple with the other electrochemical catalysts.