Doping strategies play a critical role in tailoring the electronic and chemical properties of graphene for applications in transparent conductors and catalysis. Two primary approaches—substitutional doping and surface doping—offer distinct mechanisms for modifying graphene’s behavior without altering its intrinsic structure. These methods enable precise control over carrier concentration, work function, and surface reactivity, making graphene suitable for advanced technological applications.

Substitutional doping involves replacing carbon atoms in the graphene lattice with heteroatoms such as nitrogen, boron, or sulfur. Nitrogen doping, for instance, introduces additional electrons into the graphene lattice due to nitrogen’s higher valence electron count compared to carbon. This n-type doping increases the carrier concentration and reduces sheet resistance, enhancing graphene’s performance as a transparent conductor. Studies have shown that nitrogen-doped graphene can achieve sheet resistances as low as 150 Ω/sq with optical transmittance exceeding 85%, making it competitive with indium tin oxide (ITO) in optoelectronic devices. The electronic structure is further modified through the formation of new energy states near the Fermi level, which can be leveraged in catalytic applications. For example, nitrogen-doped graphene exhibits improved activity for the oxygen reduction reaction (ORR) in fuel cells, with onset potentials comparable to platinum-based catalysts.

Boron doping, in contrast, creates p-type behavior by introducing electron-deficient sites. The incorporation of boron atoms lowers the Fermi level and increases the work function, which is beneficial for hole transport layers in organic electronics. The work function of boron-doped graphene can reach up to 5.2 eV, compared to 4.5 eV for pristine graphene, facilitating better charge injection in devices. Additionally, boron-doped graphene demonstrates enhanced catalytic performance for hydrogen evolution reactions due to the optimized adsorption of intermediates on the doped sites.

Surface doping, on the other hand, involves the adsorption of molecules or atoms onto graphene without disrupting the lattice. A common example is the use of gold chloride (AuCl₃) as a p-type dopant. When AuCl₃ molecules adsorb onto graphene, they withdraw electrons through charge transfer, increasing hole concentration and reducing sheet resistance. This method is particularly advantageous for preserving graphene’s high carrier mobility while tuning its electronic properties. Surface-doped graphene with AuCl₃ has achieved sheet resistances below 100 Ω/sq while maintaining optical transmittance above 90%. The work function can also be increased to over 5.1 eV, making it suitable for use as transparent electrodes in organic light-emitting diodes (OLEDs).

Another surface doping strategy employs metal nanoparticles such as palladium or platinum to enhance catalytic activity. These nanoparticles introduce localized electronic states at the graphene-metal interface, facilitating charge transfer during chemical reactions. For instance, platinum-decorated graphene shows superior performance in methanol oxidation, a critical reaction for direct methanol fuel cells. The doping effect here is not limited to electronic modulation but also includes spillover effects, where reactive species migrate from the metal nanoparticles to the graphene surface, further enhancing catalytic efficiency.

The choice between substitutional and surface doping depends on the desired application. Substitutional doping is more stable and permanent, making it ideal for applications requiring long-term performance, such as transparent conductors in displays or solar cells. However, the introduction of heteroatoms can sometimes degrade carrier mobility due to increased scattering. Surface doping offers reversibility and fine-tunability, which is advantageous for catalytic systems where reaction conditions may necessitate adaptive properties. Yet, surface dopants may be susceptible to environmental degradation or desorption over time.

In transparent conductors, doped graphene bridges the gap between conductivity and transparency. While pristine graphene exhibits high transparency, its conductivity is insufficient for many applications. Doping addresses this limitation by increasing carrier density without significantly compromising optical properties. For catalysis, doping creates active sites that lower activation energies and improve reaction kinetics. Nitrogen-doped graphene, for example, facilitates four-electron pathways in ORR, reducing the formation of detrimental peroxide byproducts.

The interplay between doping and graphene’s electronic structure is further evident in work function engineering. By adjusting the type and concentration of dopants, the work function can be tailored to match adjacent materials in device architectures, minimizing energy barriers for charge injection. This principle is exploited in organic photovoltaics, where doped graphene electrodes enhance open-circuit voltage and power conversion efficiency.

In summary, substitutional and surface doping strategies provide versatile tools for optimizing graphene’s electronic and catalytic properties. Substitutional doping modifies the lattice directly, offering stability and significant shifts in carrier concentration, while surface doping allows dynamic control through adsorbed species. Both approaches have demonstrated success in transparent conductors and catalysis, underscoring graphene’s adaptability as a functional material. Future advancements may focus on hybrid doping schemes that combine the benefits of both methods, further expanding graphene’s utility in next-generation technologies.