Understanding the charge transfer mechanism in CsPbBr3 nanocrystals and nitrogen-doped carbon quantum dot heterostructures: effect of nanocrystal encapsulation.

Recently, lead halide perovskite nanocrystal (NC)-based heterostructures have demonstrated significant promise in various research areas, including solar cells, CO2 reduction, and photocatalysis. These hybrid structures have also played a crucial role in advancing our fundamental conception of charge transfer mechanisms occurring at the interface. A thin shell around the NCs is not suitable for the formation of stable and luminescent materials. However, such NCs are suitable for solar cells, LEDs, CO2 reduction, and photocatalytic applications due to higher carrier mobility. Thick-shelled NCs are highly stable but hinder charge transport among the NCs which is beneficial for bio-imaging and color-converted LED fabrication. So, understanding the mechanism of charge transport among the NCs dependent on the shell materials is important. Here, we synthesized CsPbBr3 NCs with various coating materials to vary the effective distance between the perovskite and nitrogen-doped carbon quantum dots (NCQDs) to understand the charge transfer process among them. We encapsulated the NCs with different coating materials (i.e., oleic acid, oleylamine, polyvinylpyrrolidone, and silica) such that the thickness of the NCs' shell can differ. We observed that the charge transfer rate between thick-shelled NCs and NCQDs is slow. The faster charge transfer among the thinner-shelled NCs and NCQDs is feasible due to the bonding of the N-state of NCQDs with Pb-atoms of the CsPbBr3 structure. The density functional theory (DFT) calculations of the heterostructure indicate that the electron acceptor state of the N-atom in NCQDs lies below the conduction band of perovskite NCs, which is accountable for such charge transfer. This study has immense significance as it provides crucial insights into the design and application of heterostructures, which can be extended to various novel opportunities for progress and innovation.

Theoretical Insights into the Hydrogen Evolution Reaction on VGe2N4 and NbGe2N4 Monolayers.

Catalytically active sites at the basal plane of two-dimensional monolayers for hydrogen evolution reaction (HER) are important for the mass production of hydrogen. The structural, electronic, and catalytic properties of two-dimensional VGe2N4 and NbGe2N4 monolayers are demonstrated using the first-principles calculations. The dynamical stability is confirmed through phonon calculations, followed by computation of the electronic structure employing the hybrid functional HSE06 and PBE+U. Here, we introduced two strategies, strain and doping, to tune their catalytic properties toward HER. Our results show that the HER activity of VGe2N4 and NbGe2N4 monolayers are sensitive to the applied strain. A 3% tensile strain results in the adsorption Gibbs free energy (ΔG H*) of hydrogen for the NbGe2N4 monolayer of 0.015 eV, indicating better activity than Pt (-0.09 eV). At the compressive strain of 3%, the ΔG H* value is -0.09 eV for the VGe2N4 monolayer, which is comparable to that of Pt. The exchange current density for the P doping at the N site of the NbGe2N4 monolayer makes it a promising electrocatalyst for HER (ΔG H* = 0.11 eV). Our findings imply the great potential of the VGe2N4 and NbGe2N4 monolayers as electrocatalysts for HER activity.

Functional Pyromellitic Diimide as a Corrosion Inhibitor for Galvanized Steel: An Atomic-Scale Engineering.

Corrosion of metal/steel is a major concern in terms of safety, durability, cost, and environment. We have studied a cost-effective, nontoxic, and environmentally friendly pyromellitic diimide (PMDI) compound as a corrosion inhibitor for galvanized steel through density functional theory. An atomic-scale engineering through the functionalization of PMDI is performed to showcase the enhancement in corrosion inhibition and strengthen the interaction between functionalized PMDI (F-PMDI) and zinc oxide (naturally existing on galvanized steel). PMDI is functionalized with methyl/diamine groups (inh1 (R = -CH3, R' = -CH3), inh2 (R = -CH3, R' = -CH2CH2NH2), and inh3 (R = -C6H3(NH2)2, R' = -CH2CH2NH2). The corrosion inhibition parameters (e.g., orbital energies, electronegativity, dipole moment, global hardness, and electron transfer) indicate the superior corrosion inhibition performance of inh3 (inh3 > inh2 > inh1). Inh3 (∼182.38 kJ/mol) strongly interacts with ZnO(101̅0) compared to inh2 (∼122.56 kJ/mol) and inh1 (∼119.66 kJ/mol). The superior performance of inh3 has been probed through charge density and density of states. Larger available states of N and H (of inh3) interact strongly with Zn and Osurf (of the surface), respectively, creating N-Zn and H-Osurf bonds. Interestingly, these bonds only appear in inh3. The charge accumulation on Osurf, and depletion on H(s), further strengthens the bonding between inh3 and ZnO(101̅0). The microscopic understanding obtained in this study will be useful to develop low-cost and efficient corrosion inhibitors for galvanized steel.

Sculpting Artificial Edges in Monolayer MoS2 for Controlled Formation of Surface-Enhanced Raman Hotspots.

Hotspot engineering has the potential to transform the field of surface-enhanced Raman spectroscopy (SERS) by enabling ultrasensitive and reproducible detection of analytes. However, the ability to controllably generate SERS hotspots, with desired location and geometry, over large-area substrates, has remained elusive. In this study, we sculpt artificial edges in monolayer molybdenum disulfide (MoS2) by low-power focused laser-cutting. We find that when gold nanoparticles (AuNPs) are deposited on MoS2 by drop-casting, the AuNPs tend to accumulate predominantly along the artificial edges. First-principles density functional theory (DFT) calculations indicate strong binding of AuNPs with the artificial edges due to dangling bonds that are ubiquitous on the unpassivated (laser-cut) edges. The dense accumulation of AuNPs along the artificial edges intensifies plasmonic effects in these regions, creating hotspots exclusively along the artificial edges. DFT further indicates that adsorption of AuNPs along the artificial edges prompts a transition from semiconducting to metallic behavior, which can further intensify the plasmonic effect along the artificial edges. These effects are observed exclusively for the sculpted (i.e., cut) edges and not observed for the MoS2 surface (away from the cut edges) or along the natural (passivated) edges of the MoS2 sheet. To demonstrate the practical utility of this concept, we use our substrate to detect Rhodamine B (RhB) with a large SERS enhancement (∼104) at the hotspots for RhB concentrations as low as ∼10-10 M. The single-step laser-etching process reported here can be used to controllably generate arrays of SERS hotspots. As such, this concept offers several advantages over previously reported SERS substrates that rely on electrochemical deposition, e-beam lithography, nanoimprinting, or photolithography. Whereas we have focused our study on MoS2, this concept could, in principle, be extended to a variety of 2D material platforms.

Tuning the Electrocatalytic Activity of Co3O4 through Discrete Elemental Doping.

To gain constructive insight into the possible effect of doping on the electrocatalytic activity of materials, a catalytic framework with a discrete distribution of dopants is an appropriate model system. Such a system assures well-defined active centers, maximum atom utilization efficiency, and hence enhanced selectivity, catalytic activity, and stability. Herein, a comprehensive investigation of the electrocatalytic activity of iron-doped cobalt oxide (Fe-Co3O4) nanosheets is presented. In order to understand the contribution of dopants, a series of materials with controlled doping levels are investigated. By controlled iron inclusion into the structure of Co3O4, an apparent improvement in the oxygen evolution reaction activity which is reflected in the decrease of 160 mV in the overpotential to reach the current density of 10 mA/cm2 is manifested. Additionally, it is shown that there exists an optimum doping content above which the catalytic activity fades. Further investigation of the system with density functional calculations reveals that, along with the optimization of adsorption energy toward the reaction intermediates, substantial downshift of the Fermi level and delocalization of electron density occurs on introducing iron ions into the structure.