Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl3 solution using in situ high resolution x-ray reflectivity (also known as crystal truncation rod (CTR)) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y3+) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y3+ per 11.4 ± 1.6 Å2, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y3+ per ~240 Å2). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y3+ profile also shows a molecular-scale gap (≥5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y3+ ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Interaction Between Edge Dislocation and Single-Layered Graphene in Aluminum Matrix Investigated by Molecular Dynamics Simulation

The influence of graphene on dislocation movement and subsequent mechanical response of aluminum is investigated by the computational method of molecular dynamics simulation. A Lennard–Jones potential describing Al-C interaction was obtained through ab initio calculation. It was observed that the 2D graphene could reinforce Al matrix similar to the traditional Orowan mechanism. The Al/graphene interface first attract the gliding dislocation to reduce the system energy, which is unlike the grain boundary to repel gliding dislocations through pile-up mechanism. With the increase of stress, dislocation attracted and trapped at the front of graphene could glide along the interface and finally bypass it through climbing when graphene is orientated out of the shear plane. In addition, the strengthening ability of graphene is size dependent, showing a linear relationship between strength increment and graphene size.

Electron penetration triggering interface activity of Pt-graphene for CO oxidation at room temperature

Achieving CO oxidation at room temperature is significant for gas purification but still challenging nowadays. Pt promoted by 3d transition metals (TMs) is a promising candidate for this reaction, but TMs are prone to be deeply oxidized in an oxygen-rich atmosphere, leading to low activity. Herein we report a unique structure design of graphene-isolated Pt from CoNi nanoparticles (PtǀCoNi) for efficiently catalytic CO oxidation in an oxygen-rich atmosphere. CoNi alloy is protected by ultrathin graphene shell from oxidation and therefore modulates the electronic property of Pt-graphene interface via electron penetration effect. This catalyst can achieve near 100% CO conversion at room temperature, while there are limited conversions over Pt/C and Pt/CoNiOx catalysts. Experiments and theoretical calculations indicate that CO will saturate Pt sites, but O2 can adsorb at the Pt-graphene interface without competing with CO, which facilitate the O2 activation and the subsequent surface reaction. This graphene-isolated system is distinct from the classical metal-metal oxide interface for catalysis, and it provides a new thought for the design of heterogeneous catalysts.

Interface construction for charge transportation of ZnO/graphene multilayer films

In order to clarify the effect of interface construction on the charge transportation, the interfaces between zinc oxide (ZnO) and graphene layers were designed into the following types: the smooth interface by direct deposition ZnO layer onto the surface of fresh graphene/glass substrate; the nanoscale rough interface by Ar[Formula: see text] bombardment etching the surface of graphene/glass substrate before deposition of a ZnO layer, and rough ZnO/Ag/graphene interface by deposition Ag first and then ZnO layers on the rough graphene/glass substrate. The results showed that, compared to the morphology of the ZnO/graphene film with smooth surface, the particle sizes of the film with rough interface became fine and their shapes changed from sharp to round. The carriers’ mobility increased from 0.3 cm2 ⋅ V[Formula: see text] ⋅ s[Formula: see text] to 0.6 cm2 ⋅ V[Formula: see text] ⋅ s[Formula: see text] due to the enhancement of the nanocontact at the rough interface between ZnO and graphene layers. In order to improve the electrical properties of ZnO/graphene multilayer film, a 10 nm Ag layer was inserted into the rough graphene/glass and ZnO layer to construct the rough metal interface. The carrier concentration was enhanced from 10[Formula: see text] cm[Formula: see text] of ZnO/graphene to 10[Formula: see text] cm[Formula: see text] ZnO/Ag/graphene films, although the carrier mobility reduced slightly from ZnO/graphene 0.6 to ZnO/Ag/graphene 0.2 cm2 ⋅ V[Formula: see text] ⋅ s[Formula: see text]. The sheet resistance and resistivity of the ZnO/Ag/graphene multilayer film decreased dramatically by inserting the conductive Ag layer, which took the roles of both the provider of charge carriers from Ag layer and bridges of the carriers from graphene layer.

Deciphering asymmetric charge transfer at transition metal dichalcogenide–graphene interface by helicity-resolved ultrafast spectroscopy

Transition metal dichalcogenide (TMD)/graphene (Gr) heterostructures constitute a key component for two-dimensional devices. The operation of TMD/Gr devices relies on interfacial charge/energy transfer processes, which remains unclear and challenging to unravel. Fortunately, the coupled spin and valley index in TMDs adds a new degree of freedom to the charges and, thus, another dimension to spectroscopy. Here, by helicity-resolved ultrafast spectroscopy, we find that photoexcitation in TMDs transfers to graphene by asynchronous charge transfer, with one type of charge transferring in the order of femtoseconds and the other in picoseconds. The rate correlates well with energy offset between TMD and graphene, regardless of compositions and charge species. Spin-polarized hole injection or long-lived polarized hole can be achieved with deliberately designed heterostructures. This study shows helicity-resolved ultrafast spectroscopy as a powerful and facile approach to reveal the fundamental and complex charge/spin dynamics in TMD-based heterostructures, paving the way toward valleytronic and optoelectronic applications.