Two-dimensional heterostructures have emerged as highly efficient catalysts for critical reactions such as the hydrogen evolution reaction (HER) and CO2 reduction. Their unique interfacial properties and synergistic effects between layers enable enhanced catalytic performance compared to their individual monolayer counterparts. The design and optimization of these heterostructures rely on understanding the atomic-level interactions at the interfaces, charge transfer mechanisms, and the creation of active sites that lower reaction barriers.

The hydrogen evolution reaction is a key process in sustainable energy technologies, particularly in water splitting for hydrogen production. Transition metal dichalcogenide (TMDC) heterostructures, such as MoS2/WS2, exhibit improved HER activity due to favorable electronic structure modifications at the interface. The lattice mismatch between MoS2 and WS2 induces strain, leading to a higher density of exposed sulfur edge sites, which are catalytically active for proton adsorption and reduction. Studies have shown that the overpotential for HER in MoS2/WS2 heterostructures can be as low as 140 mV at 10 mA/cm², significantly lower than that of standalone MoS2 or WS2 monolayers. The charge redistribution at the interface facilitates electron transfer to adsorbed hydrogen intermediates, accelerating the reaction kinetics.

Similarly, graphene-based heterostructures with TMDCs or metal nanoparticles demonstrate enhanced HER performance. For instance, graphene/MoS2 heterostructures benefit from the high electrical conductivity of graphene, which compensates for the poor charge transport in MoS2. The hybrid structure reduces charge recombination and improves electron mobility, leading to higher turnover frequencies. The introduction of defects or dopants in graphene further tunes the work function and optimizes hydrogen adsorption free energy, approaching the ideal value of zero for optimal HER activity.

CO2 reduction is another critical reaction where 2D heterostructures show promise. The conversion of CO2 into value-added chemicals like methane, methanol, or ethylene requires precise control over intermediate binding energies and selectivity. Heterostructures such as g-C3N4/TiO2 or Cu/graphene exhibit tailored electronic properties that favor CO2 activation and multi-electron transfer processes. In g-C3N4/TiO2 systems, the staggered band alignment promotes efficient charge separation, with electrons accumulating on g-C3N4 and holes on TiO2. This spatial separation suppresses recombination and enhances the availability of electrons for CO2 reduction. Experimental results indicate a Faradaic efficiency of up to 60% for methane production in such systems under visible light irradiation.

Copper-based heterostructures, particularly those involving graphene or reduced graphene oxide (rGO), are effective for CO2 reduction to multi-carbon products. The Cu/rGO interface stabilizes key intermediates like *CO, enabling C-C coupling reactions that lead to ethylene or ethanol. The presence of oxygen functional groups on rGO modulates the local electronic environment of Cu, lowering the energy barrier for CO dimerization. Faradaic efficiencies exceeding 40% for ethylene have been reported in these systems at moderate overpotentials.

The interfacial active sites in 2D heterostructures play a crucial role in determining catalytic performance. In TMDC-based systems, the edges and defects at the interface often serve as the primary active sites. For example, in vertically stacked MoSe2/WSe2 heterostructures, the Se vacancies at the interface create unsaturated metal centers that bind reaction intermediates more strongly than pristine surfaces. These sites exhibit higher turnover rates for HER due to their optimized adsorption energetics. Similarly, in lateral heterostructures where two materials share an in-plane boundary, the strain and charge polarization at the junction can generate highly reactive sites for CO2 activation.

Synergistic effects between the constituent layers further enhance catalytic activity. In van der Waals heterostructures, the weak interlayer coupling allows for independent tuning of each layer’s electronic properties while maintaining interfacial interaction. For instance, combining a p-type semiconductor with an n-type semiconductor in a heterostructure creates a built-in electric field that drives charge separation. This effect is particularly beneficial for photocatalytic CO2 reduction, where efficient electron-hole separation is essential. The heterojunction between black phosphorus and Bi2WO6 has demonstrated improved charge carrier mobility and extended lifetimes, leading to higher photocatalytic activity compared to individual components.

The role of defects and doping in 2D heterostructures cannot be overlooked. Intentional introduction of sulfur vacancies in MoS2/ReS2 heterostructures has been shown to increase the density of active sites for HER. Nitrogen doping in graphene/MoS2 systems alters the charge distribution, enhancing the binding strength of hydrogen atoms and reducing the overpotential. Similarly, transition metal doping in TMDC-based heterostructures can modify the d-band center, optimizing the adsorption energies of reaction intermediates for CO2 reduction.

Stability and scalability are important considerations for practical applications. While 2D heterostructures exhibit excellent catalytic performance, their long-term stability under operational conditions varies. Encapsulation with inert layers like hexagonal boron nitride (hBN) has been explored to protect reactive interfaces from degradation. For large-scale deployment, solution-based assembly techniques and chemical vapor deposition (CVD) growth of heterostructures offer promising pathways for scalable production.

In summary, 2D heterostructures provide a versatile platform for designing high-performance catalysts for HER and CO2 reduction. The interfacial engineering of active sites, combined with synergistic effects between layers, enables precise control over reaction pathways and efficiencies. Continued advancements in synthesis and characterization will further unlock the potential of these materials for sustainable energy applications.