Phosphorene, the monolayer form of black phosphorus, has emerged as a promising candidate for gas and chemical sensing due to its unique structural and electronic properties. Unlike graphene and other two-dimensional materials, phosphorene possesses a puckered honeycomb lattice that provides a high surface-to-volume ratio and anisotropic charge transport, making it highly sensitive to surface interactions with gas molecules. The material’s tunable bandgap, which ranges from approximately 0.3 eV in bulk to around 2.0 eV in monolayer form, further enhances its potential for detecting a wide range of analytes through electronic or optical transduction mechanisms.
One of the most significant advantages of phosphorene-based sensors is their exceptional sensitivity to nitrogen dioxide (NO2) and ammonia (NH3). Studies have demonstrated that phosphorene exhibits a strong response to NO2 at room temperature, with detection limits reaching parts-per-billion (ppb) levels. The adsorption of NO2 on phosphorene induces a substantial charge transfer due to the molecule’s strong electron-withdrawing nature, leading to measurable changes in electrical conductivity. Similarly, NH3, an electron-donating molecule, interacts with phosphorene through physisorption, causing a decrease in hole concentration in p-type phosphorene. The sensitivity to these gases is attributed to phosphorene’s high surface activity and the presence of lone-pair electrons on phosphorus atoms, which facilitate stronger interactions with polar molecules compared to non-polar ones.
Beyond NO2 and NH3, phosphorene has shown responsiveness to other gases such as CO, CO2, SO2, and volatile organic compounds (VOCs). The selectivity of phosphorene toward different analytes depends on the binding energy and charge transfer characteristics of each gas molecule. For instance, SO2 exhibits moderate adsorption on phosphorene due to its intermediate electronegativity, while CO and CO2 show weaker interactions. The anisotropic nature of phosphorene also plays a role in adsorption behavior, as gas molecules may bind differently along the armchair and zigzag directions.
Despite its high sensitivity, phosphorene-based sensors face challenges in selectivity and recovery. Pristine phosphorene often lacks the ability to distinguish between gases with similar electronic properties, such as NO2 and O2, both of which are strong oxidizers. To address this, researchers have explored several strategies, including surface functionalization, heterostructure engineering, and defect engineering. Functionalizing phosphorene with metal nanoparticles, such as gold or silver, can enhance selectivity by promoting specific gas adsorption sites. Similarly, creating heterostructures with other 2D materials like graphene or molybdenum disulfide (MoS2) can modulate charge transfer and improve discrimination between analytes. Defect engineering, such as introducing vacancies or dopants, can also alter the electronic structure of phosphorene to favor certain gas interactions.
Recovery remains another critical issue, as some gas molecules, particularly NO2, exhibit strong binding to phosphorene, leading to slow desorption rates. Thermal annealing and ultraviolet (UV) irradiation have been employed to accelerate recovery by providing energy to overcome adsorption barriers. However, excessive heating or UV exposure may degrade phosphorene due to its susceptibility to oxidation in ambient conditions. Encapsulation with inert layers like hexagonal boron nitride (hBN) has been proposed to improve environmental stability while maintaining sensing performance.
When compared to other 2D material-based sensors, phosphorene offers distinct advantages and limitations. Graphene, while highly conductive and stable, lacks an intrinsic bandgap, resulting in lower sensitivity and selectivity for gas sensing without additional modifications. Transition metal dichalcogenides (TMDCs) like MoS2 exhibit better selectivity than graphene but generally show lower sensitivity to certain gases compared to phosphorene. MXenes, another class of 2D materials, demonstrate excellent conductivity and surface chemistry but often require functionalization to achieve high sensitivity. Phosphorene’s inherent electronic properties and surface reactivity provide a balance between sensitivity and tunability, though its environmental instability remains a drawback relative to more robust materials like graphene or hBN.
Future developments in phosphorene-based sensors will likely focus on optimizing stability and scalability while maintaining high performance. Advances in passivation techniques, such as atomic layer deposition of protective oxides, could mitigate oxidation issues. Additionally, integrating phosphorene into hybrid or composite structures may enhance both selectivity and recovery without compromising sensitivity. The exploration of few-layer phosphorene, as opposed to strictly monolayer configurations, may also provide a trade-off between stability and sensing performance.
In summary, phosphorene’s high surface-to-volume ratio, tunable electronic properties, and selective adsorption characteristics make it a compelling material for gas and chemical sensing applications. Its sensitivity to NO2, NH3, and other analytes outperforms many conventional 2D materials, though challenges in selectivity and recovery persist. By leveraging surface modifications, heterostructures, and encapsulation strategies, phosphorene-based sensors can overcome these limitations and find practical use in environmental monitoring, industrial safety, and other applications requiring precise gas detection.