Reduced graphene oxide (rGO) has emerged as a material of significant interest due to its tunable electrical and optical properties, which are closely tied to its reduction efficiency and residual functional groups. The reduction process of graphene oxide (GO) to rGO involves the removal of oxygen-containing groups, leading to partial restoration of the sp2 carbon network. This restoration directly influences the material's conductivity, carrier mobility, optical absorption, and transparency.

The electrical conductivity of rGO is highly dependent on the reduction method and the extent of oxygen group removal. Pristine graphene exhibits high conductivity due to its uninterrupted sp2 lattice, but GO is insulating because of the disruption caused by oxygen functional groups. Reduction processes, whether chemical, thermal, or photochemical, aim to restore conductivity by eliminating these groups. However, complete restoration is rarely achieved, leaving residual epoxide, hydroxyl, and carboxyl groups that act as scattering centers for charge carriers. The conductivity of rGO typically ranges from 100 to 10,000 S/m, depending on reduction efficiency. Hydrazine-reduced rGO, for example, can achieve conductivities around 2,000 S/m, while thermally reduced rGO may reach higher values but often with more structural defects.

Carrier mobility in rGO is another critical parameter affected by residual functional groups and structural disorder. While pristine graphene exhibits mobilities exceeding 200,000 cm²/V·s at room temperature, rGO mobilities are significantly lower, often between 1 and 100 cm²/V·s. The reduction process introduces defects such as vacancies and topological distortions, which hinder charge transport. Additionally, the presence of residual oxygen groups disrupts the π-conjugation, further reducing mobility. Strategies such as high-temperature annealing or chemical reduction with strong reducing agents can improve mobility by minimizing these defects, but trade-offs exist between reduction efficiency and structural integrity.

Optical properties of rGO, particularly absorption and transparency, are also influenced by the degree of reduction. GO is optically transparent with a pale yellow color due to its wide bandgap, whereas rGO becomes darker as reduction progresses, approaching the black appearance of graphene. The absorption spectrum of rGO shows a broad featureless profile, with increased absorption in the visible and near-infrared regions compared to GO. The optical transparency of rGO films is inversely related to thickness and reduction level. For instance, a monolayer rGO film may retain over 90% transparency in the visible range, while thicker films or highly reduced samples exhibit reduced transparency due to increased absorption and scattering.

The correlation between reduction efficiency and residual functional groups is crucial in tailoring rGO properties. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) are commonly used to quantify the C/O ratio and identify remaining functional groups. A higher C/O ratio generally indicates better reduction and improved electrical properties. However, excessive reduction can lead to excessive defect formation, degrading performance. Optimal reduction balances oxygen removal with minimal structural damage.

Doping strategies are employed to further enhance the electrical and optical performance of rGO. Heteroatom doping, such as nitrogen or sulfur incorporation, can modify the electronic structure and improve conductivity. Nitrogen doping, for example, introduces electron-donating groups that increase carrier density and mobility. The nitrogen content in doped rGO can range from 1% to 10%, with higher doping levels leading to conductivities comparable to chemically reduced graphene. Boron doping, on the other hand, creates p-type characteristics by introducing electron-deficient sites.

Another approach involves metal nanoparticle decoration, where nanoparticles such as gold or silver are deposited on rGO surfaces. These nanoparticles can enhance conductivity through improved charge transfer and introduce plasmonic effects that modify optical absorption. The interaction between metal nanoparticles and rGO depends on particle size, distribution, and the nature of the residual functional groups that facilitate anchoring.

Chemical doping with small molecules, such as hydrazine or hydroiodic acid, can also tune rGO properties. These dopants not only reduce GO but also introduce additional charge carriers. For instance, hydroiodic acid-treated rGO shows improved conductivity due to both reduction and iodine doping, which acts as a charge transfer enhancer.

In summary, the electrical and optical properties of rGO are intricately linked to its reduction efficiency and residual functional groups. Conductivity and carrier mobility improve with higher reduction but are limited by defect formation. Optical absorption and transparency are similarly influenced, with optimal performance requiring a balance between reduction and structural preservation. Doping strategies, including heteroatom incorporation, metal nanoparticle decoration, and chemical dopants, offer pathways to further enhance these properties. Understanding these relationships enables the tailored design of rGO for specific applications where electrical and optical performance are critical.

The following table summarizes key properties of rGO based on reduction methods:

Reduction Method | Conductivity (S/m) | Carrier Mobility (cm²/V·s) | C/O Ratio | Transparency (%)
Chemical (Hydrazine) | 200 - 2,000 | 1 - 50 | 5 - 10 | 60 - 90
Thermal (1000°C) | 1,000 - 10,000 | 10 - 100 | 10 - 20 | 40 - 80
Photochemical | 100 - 1,000 | 1 - 30 | 4 - 8 | 70 - 95

These values highlight the variability in rGO properties based on synthesis conditions, emphasizing the need for precise control over reduction and doping processes to achieve desired performance characteristics.