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We use metallic MoS2 as a catalyst for N2 fixation and NH3 generation. Based on the current literature with regard to MoS2 catalytic performance for NH3 generation, it is tracking with catalytic performance for hydrogen evolution. For example, if you increase edge sites with 2D MoS2 structuring or incorporate more vacancies/defect sites, the hydrogen evolution reaction improves. The same is true of NH3 formation. If we take this one step further, hydrogen generation further increases if MoS2 is converted to its thermodynamically unstable metallic state and is stabilized in this state with functional groups. Therefore, we extend this correlation to NH3 generation. We expect that the N2 fixation and NH3 generation will improve and be stable if we use metallic, functionalized MoS2 nanosheets. This hypothesis is tested and reported. We will also comment on the selectivity for the H2 versus NH3 generation from this catalyst with various functional groups.

Transition metal dichalcogenides (TMDCs) show promising electrochemical properties for the ultrathin and flexible light-harvesting devices due to their absorption in the visible range and high conversion efficiencies. In this work, we demonstrate a strong enhancement in photoelectrochemical conversion using porphyrin-functionalization on TMDCs and elucidate related light-harvesting mechanisms. We prepared large-area samples from Zn-centered protoporphyrins (ZnPPs) functionalized on films of liquid-exfoliated MoS2, WS2, MoSe2, and WSe2 flakes. WS2 and MoS2 demonstrate an order of magnitude greater anodic photocurrents with ZnPP functionalization, where ZnPP is working as photoactive layers. In contrast, MoSe2 and WSe2 serve as charge transfer media, demonstrating strong cathodic photocurrents with the porphyrins. Two distinct mechanisms were identified for observed photocurrent behaviors, which largely depend on the differences in the energy levels of TMDCs against the HOMO/LUMO of ZnPP. Our findings should be useful for designing high-performance TMDC-based photoelectrochemical devices.

Increasing the lifetimes of photoexcited charge carriers in two-dimensional transition metal dichalcogenides (2D-TMDCs) is an important goal for realizing efficient TMDC-based optoelectronic and photoelectrochemical devices. However, the path toward sustaining sufficiently long-lived carriers in high yields is not yet well defined. A promising strategy for overcoming ultrafast decay of the tightly bound excitons inherent to these materials is charge separation using energy level offsets at heterojunction interfaces. For example, in this study we separate charge using a model Type-II heterojunction with single-walled carbon nanotubes as the electron donor and MoS2 as the acceptor. This system can sustain charge past 10 µs (orders of magnitude longer than other reported TMDC-based heterojunctions), but the carrier kinetics vary depending on the TMDC microstructure. In particular, we observe longer-lived charge carriers in monolayer TMDCs containing a small fraction of multilayer islands. This highlights the challenge of optimizing TMDC-based heterostructures while TMDC synthetic methods are still developing, where chemical or structural (i.e. thickness) variations are often evident across neat TMDC substrates. Although obtaining TMDC thickness uniformity across wafer scales is a highly sought-after goal, here we propose that a small fraction of multilayer sites can be beneficial for extending carrier lifetimes. We discuss the photophysical processes that occur at monolayer/multilayer TMDC homojunctions, and we present transient absorption data where we use well-defined carbon nanotube spectral signatures to quantify carrier generation in various monolayer-only or monolayer-rich TMDC heterostructures. Our results provide valuable insight for understanding the role of thickness variation in TMDC dynamic photophysical processes.

We use metallic MoS2 as a catalyst for N2 fixation and NH3 generation. Based on the current literature with regard to MoS2 catalytic performance for NH3 generation, it is tracking with catalytic performance for hydrogen evolution. For example, if you increase edge sites with 2D MoS2 structuring or incorporate more vacancies/defect sites, the hydrogen evolution reaction improves. The same is true of NH3 formation. If we take this one step further, hydrogen generation further increases if MoS2 is converted to its thermodynamically unstable metallic state. Therefore, we extend this correlation to NH3 generation. We expect that the N2 fixation and NH3 generation will improve if we use metallic MoS2 nanosheets. This hypothesis is tested and reported. We will also comment on the selectivity for the H2 versus NH3 generation from this catalyst.

Electrochemical supercapacitors are hybrids of a capacitor and battery that rely on materials capable of storing charges via faradaic redox reactions or pseudocapacitive reactions in addition to conventional electrostatic double-layer charge storage. MXenes, a relatively new class of two-dimensional (2D) transition metal carbides and nitrides, are ideal candidates for supercapacitors due to their high electronic conductivity, high surface area, and ability to store charges viapseudocapacitive mechanisms. Nitride MXenes such as Ti2NTxare predicted to have higher pseudocapacitance than carbide MXenes but have not been explored experimentally. Here, we report on the synthesis, characterization, and pseudocapacitive charge storage mechanism in the Ti2NTxnitride MXene. Successful formation of nanolayered Ti2NTxMXene is characterized by XRD, SEM, and N2physisorption analyses. The identity of the surface terminating groups Txare assigned to primarily O and/or OH based on Raman, FTIR, and STEM-EELS. When tested in various electrolytes, the nanolayered Ti2NTxMXene exhibits pronounced reversible redox peaks and high areal capacitances (~1350 uF cm–2 in 1 M MgSO4aqueous electrolyte) well exceeding that expected from a double-layer charge storage (~50 uF cm–2) indicating that charge is stored in the material viaa pseudocapacitive mechanism. We report a trend in the capacitance as a function of cation as follows: Mg2+> Al3+> H+> Li+> Na+> K+, that matches theoretical predictions. Remarkably, nanolayered Ti2NTxMXene exhibits >200 F g–1 capacitance over a 1.0 V range in the Mg-ion electrolyte, and the capacitance increases to 160% of its initial value after 1000 cycles owing to the two-electron reaction and the unique multilayer adsorption behavior of the Mg2+cation on the Ti2NTxMXene. These findings identify Ti2NTxMXene as a new pseudocapacitive material that possesses high capacitance and wide working voltage in a safe and environmentally friendly Mg-ion electrolyte.

A relatively new class of two-dimensional (2D) materials called MXenes have garnered tremendous interest in the field of energy storage and conversion. Thus far nearly all MXenes reported experimentally have been described as metals, with a lone report of a mixed-metal carbide phase exhibiting semiconducting character. Here, we report the optical, electrocatalytic and electrical properties of the 2D Ti4N3TxMXene (Tx= basal plane surface terminating groups) and show this material exhibits both metallic and semiconducting behavior. We provide complete structural characterization of exfoliated Ti4N3TxMXene and assign Tx= O and/or OH and find that this material is susceptible to surface oxidation. Optical experiments indicate that the exfoliated Ti4N3TxMXene forms a hybrid with a thin surface oxide layer resulting in visible light absorption at energies greater than ~2.0 eV and an excitation wavelength-dependent defect-state emission over a broad range centered at ~2.9 eV. As an electrocatalyst for the hydrogen evolution reaction, the exfoliated Ti4N3Txshows an overpotential of ~300 mV at –10 mA cm–2and a Tafel slope of ~190 mV/decade. Finally, we observe a clear semiconductor-to-metal transition at ~90 K from temperature-dependent transport measurements under 5 T magnetic field likely resulting from the thin oxide layer. These results unveil the intriguing optical, electrocatalytic, and electrical properties of this 2D Ti4N3TxMXene that expands the potential of these new 2D materials into electrocatalysis and (opto)electronic applications.

Layered transition metal dichalcogenides (TMDs) represent a diverse, emerging source of two-dimensional (2D) nanostructures with intriguing optoelectronic and catalytic properties. Gold nanoparticles (AuNPs), supporting localized surface plasmon resonances, can complement TMDs, allowing their optoelectronic properties to be tuned for use in a broad range of applications. Plasmonic AuNPs offer an extraordinary ability to concentrate electromagnetic fields and confine light to sub-wavelength dimensions. While non-plasmonic enhancement of electrocatalytic HER performance in AuNP-decorated TMDs has been reported, light-induced plasmonic excitation and dissipation mechanisms and their effect on light absorption, charge transfer, and band structure in adjacent TMDs are of keen interest. Mechanisms of plasmon-active enhancement includes light scattering, local field confinement, resonant energy transfer, and interfacial charge transfer pathways. This work utilizes transient absorption spectroscopy, with pump tunability and broadband visible probing, to monitor the carrier excitation and dynamics in various AuNP-TMD heterostructures. In particular, a combination of both time-resolved and steady-state optical spectroscopies unveils the presence of a sub-gap absorption feature in AuNP-decorated molybdenum diselenide (MoSe2) nanosheets. Optical excitation of this low-energy feature generates long-lived, energetic carriers in MoSe2. This development could represent an important advance in the design and implementation of hybrid metal NP-TMD heterostructures for use in photoelectrochemistry.

Functionalizing nanosheets of metallic molybdenum disulfide (MoS2) provides a synthetic chemical route for controlling the electronic properties and stability within the traditionally thermally unstable metallic state. Metallic MoS2 nanosheets have been studied for HER due to their higher reactivity for HER, earth abundance, low-cost, and non-toxicity, which makes metallic MoS2 a candidate to replace platinum for HER. The functionalized nanosheets are more stable to the thermally initiated phase transformation from the metallic 1T phase to the semiconducting 2H phase. Furthermore, we show for an exemplary functionalized sample (Et2NPh-MoS2) that functionalization leads to better stability and long-term performance under HER conditions. These results provide a framework for understanding and controlling the balance between catalytic activity and stability for these unique 2D materials. Chemically-exfoliated, metallic (1T) MoS2 nanosheets are functionalized with organic phenyl rings containing electron donating or withdrawing groups. We find that MoS2 functionalized with the most electron donating functional group (p-(CH3CH2)2NPh-MoS2) is the most efficient catalyst for HER in this series, with initial activity similar to the pristine metallic phase of MoS­­­­­­ 2. The p-(CH3CH2)2NPh-MoS2 is more stable than unfunctionalized metallic MoS2 and outperforms unfunctionalized metallic MoS2 for continuous H2 evolution within 10 min under the same conditions. With regards to the entire studied series, the overpotential and Tafel slope for catalytic HER are both directly correlated with the electron donating strength of the pendant group on the phenyl ring. The results are consistent with a mechanism involving ground-state electron donation or withdrawal to/from the MoS2 nanosheets, which modifies the electron transfer kinetics and catalytic activity of the MoS2 sheet. We show that the functional groups preserve the metallic feature of the MoS2 films, inhibiting conversion to the thermodynamically stable semiconducting state (2H) when annealed at 150 °C for 24 h in a nitrogen atmosphere. We propose that this protection is critical to maintaining the catalytically active state of 1T MoS2 nanosheets. Our results demonstrate a strong correlation between the electron donating strength of substituents and the surface energetics, electron transfer resistance, and the HER catalytic activity of functionalized MoS2 nanosheets.