Thermally Reduced Graphene Oxide Heat Capacity Varies with Compaction, Reveals Anomalies Between 2, 300 K

The thermal properties of thermally reduced graphene oxide are critical for its application in advanced materials, and a new study by A. I. Krivchikov, A. Jezowski, M. S. Barabashko, and colleagues investigates how processing affects its heat capacity. Researchers systematically varied both the compaction pressure applied to the material and the temperature used during its thermal reduction, revealing a complex interplay between structural order and thermal response. The team demonstrates that heat capacity is governed by a combination of different vibrational modes, including those related to defects and the material’s layered structure, and that increasing compaction and annealing temperature alters these modes in predictable ways. Significantly, the absence of a specific vibrational feature observed in similar materials suggests that the two-dimensional nature of graphene oxide dominates its thermal behaviour, offering valuable insight for designing carbon-based materials with tailored thermal properties.

Low-Temperature Thermal Properties of Graphene Oxide

Investigations into graphene oxide (GO), reduced graphene oxide (rGO), and related carbon nanostructures reveal complex low-temperature thermal behaviour not easily explained by standard models. Researchers consistently observe anomalies, suggesting localized vibrational modes, glassy behaviour, or unique characteristics influenced by the degree of reduction. These anomalies are frequently attributed to two-level systems arising from defects or structural disorder. Studies highlight the importance of flexural phonons, out-of-plane vibrations crucial for thermal transport in 2D materials, particularly at low temperatures.

Both acoustic and optical phonons contribute to heat capacity, with their relative contributions dependent on material structure, dimensionality, and temperature. The temperature at which GO is reduced significantly impacts the final structure and properties of rGO, influencing its heat capacity profile. Controlling the reduction process, such as with pulsed high-frequency discharge, allows tailoring of material properties. Researchers also investigate the link between reduction temperature and the hydrogen sorption capacity of rGO, demonstrating a connection between structural changes and gas adsorption.

Computational methods, such as molecular dynamics simulations, are employed to study vibrational properties and thermal transport. A recurring theme is the presence of glassy behaviour in rGO, suggesting structural disorder plays a significant role in its thermal properties. Understanding these properties is crucial for developing advanced materials for thermal management, energy storage, and sensing.

Thermal Properties of Reduced Graphene Oxide Layers

Scientists meticulously investigated the low-temperature heat capacity of thermally reduced graphene oxide, systematically varying compaction pressure and annealing temperature to control material properties. Precise calorimetry measurements, conducted between 2 and 300 Kelvin, characterized the specific heat capacity of the resulting materials. The study revealed that thermal response is governed by phonons, encompassing a Schottky-type anomaly, a defect-related linear term, a Debye term representing acoustic phonons, and a dispersive term with a negative coefficient associated with out-of-plane flexural phonons. Researchers discovered that increasing compaction pressure altered interlayer coupling, leading to non-monotonic changes in heat capacity, while higher annealing temperatures enhanced graphitization, reduced disorder, and modified phonon dispersion. Notably, the experiments failed to detect a boson peak, commonly observed in carbon nanotubes, supporting the dominance of two-dimensional vibrational modes within the samples. This finding, combined with detailed analysis of phonon contributions, elucidates the relationship between dimensionality, structural disorder, and processing parameters in shaping phonon dynamics.

Heat Capacity Varies with Reduction and Pressure

Scientists meticulously investigated the low-temperature heat capacity of thermally reduced graphene oxide, systematically varying compaction pressure and annealing temperature to control oxygen content and structural order. Heat capacity measurements, conducted between 2 and 300 Kelvin, reveal that thermal response is governed by phonons, encompassing a Schottky-type anomaly, a defect-related linear term, a Debye term, and a unique negative contribution from out-of-plane flexural phonons. Experiments demonstrate that increasing compaction pressure alters interlayer coupling, resulting in non-monotonic changes in heat capacity, while higher annealing temperatures enhance graphitization, reduce disorder, and modify phonon dispersion. Researchers observed that at 2 Kelvin, the heat capacity of their samples was approximately twice that of previously published data for a similar material, highlighting the sensitivity of low-temperature heat capacity to synthesis routes and structural morphology.

The absence of a boson peak supports the dominance of two-dimensional vibrational modes within the material. Further analysis reveals that the temperature dependence of heat capacity can be described by an equation incorporating a Schottky-like contribution, a linear term, and phonon contributions. Researchers identified a negative term arising from the contribution of out-of-plane flexural phonon mode, and observed that experimental data align well with the model below 10 Kelvin. These findings elucidate the relationship between dimensionality, structural disorder, and processing parameters in shaping phonon dynamics, providing guidance for tailoring its thermal behaviour in advanced carbon-based functional materials.

Reduced Graphene Heat Capacity and Processing Parameters

This research presents a detailed investigation into the low-temperature heat capacity of thermally reduced graphene oxide, revealing how processing parameters influence its thermal behaviour. Scientists systematically varied both compaction pressure and annealing temperature to control the material’s oxygen content and structural order, then meticulously measured the resulting changes in heat capacity. The data demonstrate that the thermal response is governed by a combination of phonon contributions, including a Schottky-type anomaly, a linear term related to defects, a Debye term, and a unique negative contribution from out-of-plane flexural phonons. The team found that increasing compaction pressure causes non-monotonic changes in heat capacity, reflecting the anisotropic nature of the layered structure and variations in interlayer coupling.