Tuning the Electronic Properties of Two-Dimensional Lepidocrocite Titanium Dioxide-Based Heterojunctions.

Two-dimensional (2D) heterostructures reveal novel physicochemical phenomena at different length scales that are highly desirable for technological applications. We present a comprehensive density functional theory study of van der Waals (vdW) heterostructures constructed by stacking 2D TiO2 and 2D MoSSe monolayers to form the TiO2-MoSSe heterojunction. The heterostructure formation is found to be exothermic, indicating stability. We find that by varying the atomic species at the interfaces, the electronic structure can be considerably altered due to the differences in charge transfer arising from the inherent electronegativity of the atoms. We demonstrate that the heterostructures possess a type II or type III band alignment, depending on the atomic termination of MoSSe at the interface. The observed charge transfer occurs from MoSSe to TiO2. Our results suggest that the Janus interface enables the tuning of electronic properties, providing an understanding of the possible applications of the TiO2-MoSSe heterostructure.

Producing Bilayer Graphene Oxide via Wedge Ion-Assisted Anodic Exfoliation: Implications for Energy and Electronics.

Electrochemical synthesis has emerged as a promising approach for the large-scale production of graphene-based two-dimensional (2D) materials. Electrochemical intercalation of ions and molecules between graphite layers plays a key role in the synthesis of graphene with controllable thickness. However, there is still a limited understanding regarding the impact of intercalant molecules. Herein, we investigated a series of anionic species (i.e., ClO4-, PF6-, BF4-, HSO4-, CH3SO3-, and TsO-) and examined their wedging process between the weakly bonded layered materials driven by electrochemistry. By combining cyclic voltammetry, X-ray diffraction (XRD), and Raman spectroscopy, along with density functional theory (DFT) calculations, we found that stage-2 graphite intercalation compounds (GICs) can be obtained through intercalation of ClO4-, PF6-, or BF4- anions into the adjacent graphene bilayers. The anodic exfoliation step based on ClO4--GIC in (NH4)2SO4 (aq.) resulted in the formation of bilayer-rich (>57%) electrochemically exfoliated graphene oxide (EGO), with a high yield (∼85 wt %). Further, the physicochemical properties of these EGO can be readily customized through electrochemical reduction and modification with different surfactants. This versatility allows for precise tailoring of EGO, making it feasible for energy and electronic applications such as electrodes in electrochemical capacitors and functional composites in wearable electronics.

Polygonal gold nanocrystal induced efficient phase transition in 2D-MoS2for enhancing photo-electrocatalytic hydrogen generation.

Plasmonic nanocrystals (NCs) assisted phase transition of two-dimensional molybdenum disulfide (2D-MoS2) unlashes numerous opportunities in the fields of energy harvesting via electrocatalysis and photoelectrocatalysis by enhancing electronic conductivity, increasing catalytic active sites, lowering Gibbs free energy for hydrogen adsorption and desorption, etc. Here, we report the synthesis of faceted gold pentagonal bi-pyramidal (Au-PBP) nanocrystals (NC) for efficient plasmon-induced phase transition (from 2 H to 1 T phase) in chemical vapor deposited 2D-MoS2. The as-developed Au-PBP NC with the increased number of corners and edges showed an enhanced multi-modal plasmonic effect under light irradiations. The overpotential of hydrogen evolution reaction (HER) was reduced by 61 mV, whereas the Tafel slope decreased by 23.7 mV/dec on photoexcitation of the Au-PBP@MoS2hybrid catalyst. The enhanced performance can be attributed to the light-induced 2H to 1 T phase transition of 2D-MoS2, increased active sites, reduced Gibbs free energy, efficient charge separation, change in surface potential, and improved electrical conductivity of 2D-MoS2film. From density functional theory (DFT) calculations, we obtain a significant change in the electronic properties of 2D-MoS2(i.e. work function, surface chemical potential, and the density of states), which was primarily due to the plasmonic interactions and exchange-interactions between the Au-PBP nanocrystals and monolayer 2D-MoS2, thereby enhancing the phase transition and improving the surface properties. This work would lay out finding assorted routes to explore more complex nanocrystals-based multipolar plasmonic NC to escalate the HER activity of 2D-MoS2and other 2D transition metal dichalcogenides.

Synergistic effect of Ni-Ag-rutile TiO2 ternary nanocomposite for efficient visible-light-driven photocatalytic activity.

P25 comprising of mixed anatase and rutile phases is known to be highly photocatalytically active compared to the individual phases. Using a facile wet chemical method, we demonstrate a ternary nanocomposite consisting of Ni and Ag nanoparticles, decorated on the surface of XTiO2 (X: P25, rutile (R)) as an efficient visible-light-driven photocatalyst. Contrary to the current perspective, RTiO2-based Ni-Ag-RTiO2 shows the highest activity with the H2 evolution rate of ∼86 µmol g-1 W-1 h-1@535 nm. Together with quantitative assessment of active Ni, Ag and XTiO2 in these ternary systems using high energy synchrotron X-ray diffraction, transmission electron microscopy coupled energy dispersive spectroscopy mapping evidences the metal to semiconductor contact via Ag. The robust photocatalytic activity is attributed to the improved visible light absorption, as noted by the observed band edge of ∼2.67 eV corroborating well with the occurrence of Ti3+ in Ti 2p XPS. The effective charge separation due to intimate contact between Ni and RTiO2 via Ag is further evidenced by the plasmon loss peak in Ag 3d XPS. Moreover, density functional theory calculations revealed enhanced adsorption of H2 on Ti8O16 clusters when both Ag and Ni are simultaneously present, owing to the hybridization of the metal atoms with d orbitals of Ti and p orbitals of O leading to enhanced bonding characteristics, as substantiated by the density of states. Additionally, the variation in the electronegativity in Bader charge analysis indicates the possibility of hydrogen evolution at the Ni sites, in agreement with the experimental observations.

Correction: Analysis on the energetics, magnetism and electronic properties in a 45° ZnO grain boundary doped with Gd.

[This corrects the article DOI: 10.1039/C8RA00985F.].

Analysis on the energetics, magnetism and electronic properties in a 45° ZnO grain boundary doped with Gd.

The structural stability and magnetic properties of a grain boundary (GB) formed by aligning two ZnO single crystals oriented at an angle of 45° is investigated by density functional theory, using generalized gradient approximation (GGA) and taking the U parameter into consideration for the 4f impurity states. We found that the GB is stable with no dangling bonds and inter-granular structures. The stability of defects such as Gd substituted to the Zn site (GdZn), Zn vacancy (VZn) and O vacancy (VO) as well as defect complexes GdZn-GdZn, GdZn-VZn, and GdZn-VO are analyzed using formation energy calculations. It is found that GdZn-GdZn clusters prefers to form at the GB. The spin polarization at the GdZn sites is too localized and the exchange coupling energy is insufficient to overcome the thermal fluctuations. However, we show that the presence of VZn increases the hybridization between p orbitals of O as well as d orbitals of Zn, which can assist in increasing the magnetic polarization of the system. This work advances the understanding of the ferromagnetism in Gd-doped ZnO, indicating that Gd clustering at the GB is not likely to contribute to the ferromagnetism.

Enhanced Performance of MoS2 Photodetectors by Inserting an ALD-Processed TiO2 Interlayer.

2D molybdenum disulfide (MoS2 ) possesses excellent optoelectronic properties that make it a promising candidate for use in high-performance photodetectors. Yet, to meet the growing demand for practical and reliable MoS2 photodetectors, the critical issue of defect introduction to the interface between the exfoliated MoS2 and the electrode metal during fabrication must be addressed, because defects deteriorate the device performance. To achieve this objective, the use of an atomic layer-deposited TiO2 interlayer (between exfoliated MoS2 and electrode) is reported in this work, for the first time, to enhance the performance of MoS2 photodetectors. The TiO2 interlayer is inserted through 20 atomic layer deposition cycles before depositing the electrode metal on MoS2 /SiO2 substrate, leading to significantly enhanced photoresponsivity and response speed. These results pave the way for practical applications and provide a novel direction for optimizing the interlayer material.