Graphene oxide is manufactured typically by modified Hummers' method. There are a number of conditions but basically it uses oxidizing compounds to chemically oxidize graphite which is near perfect fused ring 2-dimensional sheet stacked up with many layers. Upon chemical oxidation, many defects (chemical functionalities) are inserted in these perfect lattice and weakens the interaction between individual sheets, which allows to separate each sheet. Some of those functionalities are internal epoxy group, phenolic group, and carboxylic acid groups. When you say "graphene" oxide, you imply a single sheet as graphene is a single sheet of the graphitic layers. One unique aspect of graphene oxide is the presence of chemically reactive functionalities on the basal plane in addition to the edge plane. This makes difference from typical silicate nanofillers which has available chemical functionalities (typically SiOH groups) only at the edge plane. Those that exist on the basal plane is embedded deep from the surface and cannot form covalent bonds with external chemicals, though it might weakly form hydrogen bonds. As a result of the insertion of chemical defects on graphene, graphene oxide's reduces electrical conductivity drastically from graphene, though the majority of the mechanical strength is maintained.

Expanded graphite is not a single layer. It is manufactured by inserting concentrated sulfuric acid into graphite and the sulfuric acid intercalated graphite is suddenly heated to a high temperature. This action will expand the d-spacing of the graphite and will be easy to further expand upon certain applications, such as a flame retardant additive. Since individual layers are still aggregated, the expanded graphite is not regarded as nano fillers nor it has the ability to reinforce polymer matrix like graphene nano filler since the aspect ratio of the filler is so small. Since concentrated sulfuric acid is used (and sometimes with nitric acid mixed), it is natural to expect the chemical reaction with the graphite layer by inserting sulfonate groups and carboxylic acid groups in a similar manner as the chemical surface treatment of traditional carbon fibers by those chemicals (though commercially, carbon fibers are oxidized by electrochemical means). Again, by inserting these chemical groups, the electrical conductivity reduces significantly from the pure graphite.

Thus, the similarity of graphene oxide and expanded graphite is that both have chemical functional groups inserted into the graphitic lattice with the corresponding chemical groups used to treat. The main difference is that graphene oxide is typically a single or few layers of oxidized graphene (nano fillers) whereas expanded graphite is a stack of so many layers and is still a usual filler size (many micrometers or even millimeters in both thickness and length. It is a usual filler size). Discussion of d-spacing is meaningless for graphene oxide (ideal graphene oxide has only single layer and thus usual layer-to-layer distance does not exist. If you are interested in the thickness of the layer, it is close to 1 nm). On the other hand, the expanded graphite has so much breadth in d-spacing and again it is almost meaningless to discuss about it.

The term exfoliated graphite is seldom used, but the meaning of this corresponds to graphene. When you exfoliate graphite, each layer is so distributed without the position alignment (random) and each single layer is distributed without any preferred position. This is the definition of graphene.

Graphene oxide is manufactured typically by modified Hummers' method. There are a number of conditions but basically it uses oxidizing compounds to chemically oxidize graphite which is near perfect fused ring 2-dimensional sheet stacked up with many layers. Upon chemical oxidation, many defects (chemical functionalities) are inserted in these perfect lattice and weakens the interaction between individual sheets, which allows to separate each sheet. Some of those functionalities are internal epoxy group, phenolic group, and carboxylic acid groups. When you say "graphene" oxide, you imply a single sheet as graphene is a single sheet of the graphitic layers. One unique aspect of graphene oxide is the presence of chemically reactive functionalities on the basal plane in addition to the edge plane. This makes difference from typical silicate nanofillers which has available chemical functionalities (typically SiOH groups) only at the edge plane. Those that exist on the basal plane is embedded deep from the surface and cannot form covalent bonds with external chemicals, though it might weakly form hydrogen bonds. As a result of the insertion of chemical defects on graphene, graphene oxide's reduces electrical conductivity drastically from graphene, though the majority of the mechanical strength is maintained.

Expanded graphite is not a single layer. It is manufactured by inserting concentrated sulfuric acid into graphite and the sulfuric acid intercalated graphite is suddenly heated to a high temperature. This action will expand the d-spacing of the graphite and will be easy to further expand upon certain applications, such as a flame retardant additive. Since individual layers are still aggregated, the expanded graphite is not regarded as nano fillers nor it has the ability to reinforce polymer matrix like graphene nano filler since the aspect ratio of the filler is so small. Since concentrated sulfuric acid is used (and sometimes with nitric acid mixed), it is natural to expect the chemical reaction with the graphite layer by inserting sulfonate groups and carboxylic acid groups in a similar manner as the chemical surface treatment of traditional carbon fibers by those chemicals (though commercially, carbon fibers are oxidized by electrochemical means). Again, by inserting these chemical groups, the electrical conductivity reduces significantly from the pure graphite.

Thus, the similarity of graphene oxide and expanded graphite is that both have chemical functional groups inserted into the graphitic lattice with the corresponding chemical groups used to treat. The main difference is that graphene oxide is typically a single or few layers of oxidized graphene (nano fillers) whereas expanded graphite is a stack of so many layers and is still a usual filler size (many micrometers or even millimeters in both thickness and length. It is a usual filler size). Discussion of d-spacing is meaningless for graphene oxide (ideal graphene oxide has only single layer and thus usual layer-to-layer distance does not exist. If you are interested in the thickness of the layer, it is close to 1 nm). On the other hand, the expanded graphite has so much breadth in d-spacing and again it is almost meaningless to discuss about it.

The term exfoliated graphite is seldom used, but the meaning of this corresponds to graphene. When you exfoliate graphite, each layer is so distributed without the position alignment (random) and each single layer is distributed without any preferred position. This is the definition of graphene.

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Hatsuo Ishida Thank you for such an elaborate and informative answer!