MXene-Based Composite Photocatalysts: Advances and Applications

MXenes, a family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, have garnered significant attention in materials science due to their unique physicochemical properties. Their high conductivity, tunable surface chemistry, hydrophilicity, and rich surface functionalities make MXenes attractive for various applications, including energy storage, sensing, and environmental remediation. Among these, MXene-based composite photocatalysts have emerged as a promising frontier for harnessing solar energy in photocatalytic processes.

Structure and Properties of MXenes
MXenes are typically derived from the selective etching of the A-layer elements (e.g., Al, Si) from MAX phases (Mn+1AXn, where M is a transition metal, A is a group III or IV element, and X is C and/or N). This etching process creates a 2D structure with abundant surface terminations, such as -OH, -O, and -F. These surface terminations influence their electronic structure, bandgap, and photocatalytic activity.

Key properties that make MXenes suitable for photocatalysis include:

• High electrical conductivity, which promotes efficient charge carrier separation.
• Tunable bandgap, allowing for absorption in the visible-light spectrum.
• Surface reactivity, which facilitates the anchoring of catalytic species or co-catalysts.

MXene-Based Composites for Photocatalysis
MXenes rarely function as standalone photocatalysts due to their narrow optical absorption range and susceptibility to oxidation. Instead, they are commonly integrated into composites to enhance their photocatalytic performance. These composites leverage the synergistic interactions between MXenes and other semiconductor or catalytic materials.

1. MXene-Semiconductor Heterojunctions

MXenes are often coupled with semiconductors like TiO₂, ZnO, g-C₃N₄, or CdS to form heterojunctions that improve charge separation and light absorption. The advantages include:

• Electron shuttle role: MXenes act as an electron sink, suppressing electron-hole recombination.
• Enhanced light harvesting: Their surface plasmon resonance can increase the absorption of visible light.
• Stability improvement: MXenes provide structural support, reducing photocorrosion of the semiconductor.

For instance, a Ti₃C₂/g-C₃N₄ composite has shown improved hydrogen evolution rates in photocatalytic water splitting due to enhanced charge transfer efficiency.

2. MXene-Based Z-Scheme Photocatalysts

Z-scheme photocatalysts mimic natural photosynthesis by creating a stepwise electron transfer mechanism. MXenes are integrated as conductive bridges or co-catalysts in Z-scheme systems. These systems enhance redox capabilities while minimizing recombination. For example, an MXene-TiO₂/Au nanocomposite demonstrated superior performance in CO₂ reduction due to its efficient charge transfer pathways.

3. MXene-Doped Materials

Doping MXenes with metals or non-metals further enhances their catalytic activity by modulating their electronic structure and extending light absorption to the visible or near-infrared regions. For example, Fe-doped MXenes have been reported to improve photocatalytic Fenton-like reactions for pollutant degradation.

4. MXene-Carbon-Based Hybrids

Integrating MXenes with carbonaceous materials like graphene or carbon nanotubes provides a platform for enhanced electrical conductivity and light absorption. These hybrids exhibit excellent stability and photocatalytic efficiency, particularly in organic pollutant degradation and hydrogen evolution reactions.

Applications

1. Photocatalytic Water Splitting: MXene composites can efficiently generate hydrogen by enhancing the HER (hydrogen evolution reaction) process. Their excellent conductivity and active sites ensure high efficiency under visible light irradiation.
2. Environmental Remediation: MXene-based photocatalysts have been employed in degrading organic pollutants like dyes, antibiotics, and pesticides. Their high surface area and reactive sites allow for efficient adsorption and degradation mechanisms.
3. CO₂ Reduction: MXene composites facilitate the photocatalytic conversion of CO₂ into value-added fuels. Their tunable surface chemistry optimizes CO₂ adsorption and activation.
4. Antimicrobial Applications: MXene composites have shown promise in antimicrobial photocatalysis by generating reactive oxygen species (ROS) under light irradiation, effectively killing bacteria and viruses.

Challenges and Future Perspectives
While MXene-based composite photocatalysts show immense potential, several challenges remain:

1. Stability: MXenes are prone to oxidation, which can degrade their performance.
2. Scalability: Synthesis methods need optimization for large-scale production.
3. Bandgap Engineering: Further research is required to tailor their electronic properties for specific photocatalytic processes.
4. Cost: The production of high-purity MXenes and their composites remains costly.

Future research directions include:

• Developing novel MXene derivatives with enhanced stability.
• Exploring multi-dimensional (2D/3D) MXene-based architectures.
• Integrating MXenes into tandem systems for multi-step photocatalytic processes.

MXene-based composite photocatalysts represent a transformative approach in the field of photocatalysis. Their unique structural and electronic properties, combined with their versatility in composite formation, open avenues for advancements in energy and environmental applications. Continued innovation in material design and synthesis will likely propel MXenes into mainstream applications, making them integral to sustainable energy and environmental technologies.