Controllable growth of two-dimensional materials on noble metal substrates

Heteroepitaxial Growth of Narrow Band Gap Carbon-Rich Carbon Nitride Using In Situ Polymerization to Empower Sunlight-Driven Photoelectrochemical Water Splitting

We describe an in situ-polymerized conformal thin layer coating of narrow band gap carbon-rich carbon nitride (NBG-CRCN) on titania nanorod arrays to design a binary semiconductor heterojunction photocatalyst. The in situ polymerization creates a strong interaction between the TiO2 nanorod substrate and the carbon nitride film, which prevents leaching of CRCN in liquid electrolytes. A unique aspect of our work is developing an easy and inexpensive technique for the heteroepitaxial growth of mechanically and photochemically stable carbon nitride thin films with intimate contact at the CN:TNR heterojunction interface. This method aids in overcoming one of the main problems with carbon nitride (CN), namely, the inability to produce an evenly distributed CN coating on a substrate. The synthesized NBG-CRCN@TNR extends the visible light absorption to 700 nm (Eg = 1.7 eV) and red-shifts the photoluminescence (PL) emission peak to 580 nm. The peak shifts and broadening in the Raman spectra of the NBG-CRCN@TNR hybrid compared to those in TNR confirm an unusually strong interaction between TiO2 and NBG-CRCN. An easy and inexpensive technique to heteroepitaxially grow CRCN (002) on rutile TiO2 (110) is confirmed by advanced characterization. High-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and grazing-incidence wide-angle X-ray scattering (GIWAXS) suggest the heteroepitaxial growth of (002) CRCN on rutile TiO2 (110). Under AM1.5G solar illumination, the NBG-CRCN@TNR hybrid shows superior performance in photoelectrochemical water splitting, generating a photocurrent density as high as 4.3 mA cm-2 in 1 M KOH under 0.6 V external bias, rising to 8.4 mA cm-2 in the presence of a hole scavenger (methanol). An impressive hydrogen evolution rate of 26.51 μmol h-1 with 88.12% Faradaic efficiency is recorded. Establishing a high-quality interface between g-C3N4 and titania permits effective charge carrier separation, leading to enhanced photocatalytic activity.

Time-Resolved Growth of 2D WSe2 Monolayer Crystals

Understanding and controlling the growth evolution of atomically thin monolayer two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) are vital for next-generation 2D electronics and optoelectronic devices. However, their growth kinetics are not fully observed or well understood due to the bottlenecks associated with the existing synthesis methods. This study demonstrates the time-resolved and ultrafast growth of 2D materials by a laser-based synthesis approach that enables the rapid initiation and termination of the vaporization process during crystal growth. The use of stoichiometric powder (e.g., WSe₂) minimizes the complex chemistry during the vaporization and growth process, allowing rapid initiation/termination control over the generated flux. An extensive set of experiments is performed to understand the growth evolution, achieving subsecond growth as low as 10 ms along with a 100 μm/s growth rate on a noncatalytic substrate such as Si/SiO₂. Overall, this study allows us to observe and understand the 2D crystal evolution and growth kinetics with time-resolved and subsecond time scales.

Tunable Electronic Properties of Two-Dimensional GaSe1-xTex Alloys

In this work, we performed a theoretical study on the electronic properties of monolayer GaSe1-xTex alloys using the first-principles calculations. The substitution of Se by Te results in the modification of a geometric structure, charge redistribution, and bandgap variation. These remarkable effects originate from the complex orbital hybridizations. We demonstrate that the energy bands, the spatial charge density, and the projected density of states (PDOS) of this alloy are strongly dependent on the substituted Te concentration.

Scalable-doped Nanoporous 1T″ ReSe2 via a General Surface Co-Alloy Strategy

Reliable and controllable doping of 2D transition metal dichalcogenides is an efficient approach to tailor their physicochemical properties and expand their functional applications. However, precise control over dopant distribution and scalability of the process remains a challenge. Here, we report a general method to achieve scalable in situ doping of centimeter-sized bicontinuous nanoporous ReSe₂ films with transition metal atoms via surface coalloy growth. The distinct strains induced by the bending curvature of nanoporous structures and uniform dopants result in a local 1T' to 1T″ structure phase transition over nanoporous ReSe₂ films. The as-prepared nanoporous Ru-ReSe₂ with high 1T″ phase exhibits preferable electrochemical activity in hydrogen evolution reaction. The work demonstrates a unique and general approach to synthesize uniformly-doped transition metal dichalcogenides with 3D bicontinuous nanoporous structure, which can be scaled up to batch production for various applications.