Thermal reduction of graphene oxide is a critical process for obtaining reduced graphene oxide (rGO) with tailored properties for various applications. This method involves the removal of oxygen-containing functional groups from graphene oxide (GO) through heat treatment, restoring the sp2 carbon network and improving electrical conductivity. The thermal reduction process can be achieved through several techniques, including furnace annealing, rapid thermal processing, and laser-assisted reduction, each with distinct temperature ranges, atmospheric conditions, and resulting material properties.

Furnace annealing is one of the most common thermal reduction methods. It typically involves heating GO in a controlled environment, with temperatures ranging from 200°C to 1000°C. The choice of temperature depends on the desired level of reduction and the intended application. Lower temperatures (200°C–400°C) result in partial reduction, retaining some oxygen groups, while higher temperatures (800°C–1000°C) lead to near-complete reduction. The process is often conducted under inert atmospheres such as argon or nitrogen to prevent oxidation. However, reactive gases like hydrogen can be introduced to enhance reduction efficiency by facilitating the removal of oxygen groups. The resulting rGO exhibits improved electrical conductivity, with sheet resistance values dropping from several MΩ/sq for GO to below 1 kΩ/sq for highly reduced rGO. A challenge with furnace annealing is the potential for defect formation due to the violent release of CO and CO2 gases during decomposition of oxygen functionalities, which can disrupt the carbon lattice.

Rapid thermal processing (RTP) offers a faster alternative to conventional furnace annealing, with heating durations ranging from seconds to minutes. Temperatures in RTP typically span 500°C–1200°C, allowing for efficient reduction while minimizing prolonged exposure to high heat, which can exacerbate defect formation. The process is often performed under vacuum or inert gas flow to prevent oxidation. RTP yields rGO with fewer defects compared to furnace annealing due to the shorter processing time, but achieving uniform heating across large samples remains a challenge. The electrical conductivity of RTP-reduced rGO can reach values comparable to furnace-annealed samples, but scalability is limited by the size constraints of RTP equipment.

Laser-assisted reduction is a localized and precise method for reducing GO, often used for patterning conductive rGO tracks on flexible substrates. The process involves irradiating GO with a laser beam, typically in the ultraviolet or infrared range, which induces rapid heating and decomposition of oxygen groups. Laser power and scanning speed determine the extent of reduction, with higher energy densities leading to greater restoration of the sp2 network. The temperature during laser reduction can exceed 1000°C in the irradiated zones, but the short interaction time (microseconds to milliseconds) limits thermal damage to surrounding areas. Laser-reduced rGO exhibits high conductivity, with sheet resistance values as low as 100 Ω/sq, but the method is not suitable for large-scale production due to its serial nature.

Atmospheric conditions play a crucial role in thermal reduction. Inert atmospheres prevent oxidation but may leave residual oxygen groups if the temperature is insufficient. Reactive gases like hydrogen can further reduce oxygen content but may introduce hydrogenation defects. Vacuum environments facilitate the removal of gaseous byproducts, reducing defect formation. The choice of atmosphere also affects the final properties of rGO, including its thermal stability and mechanical strength.

Defect formation is a persistent challenge in thermal reduction. The decomposition of epoxy, hydroxyl, and carboxyl groups releases gases that create vacancies and topological defects in the carbon lattice. These defects can degrade electrical and mechanical properties. Strategies to mitigate defects include controlled heating rates, two-step reduction processes (low-temperature pre-treatment followed by high-temperature annealing), and post-reduction treatments such as thermal healing in hydrocarbon atmospheres.

Scalability is another concern for thermal reduction methods. Furnace annealing is batch-processed and energy-intensive, while RTP and laser reduction are limited by equipment throughput. Continuous thermal reduction systems, such as roll-to-roll annealing, are being explored to address these limitations.

Chemical reduction methods, such as those using hydrazine or sodium borohydride, offer an alternative to thermal reduction. These methods operate at lower temperatures (below 100°C) and can achieve high reduction degrees. However, they often leave residual reducing agents or byproducts that can degrade material performance. Thermal reduction avoids chemical contamination but requires higher energy input and careful control of process parameters to minimize defects.

In summary, thermal reduction of graphene oxide is a versatile approach for producing rGO with tunable properties. Furnace annealing, rapid thermal processing, and laser-assisted reduction each offer distinct advantages and challenges in terms of temperature control, atmospheric conditions, defect formation, and scalability. The choice of method depends on the specific requirements of the application, balancing factors such as conductivity, defect density, and production scale. Continued advancements in thermal processing techniques aim to improve the quality and scalability of rGO for broader industrial adoption.