Graphite oxide (GO) is a compound obtained by treating graphite with strong oxidizers. In GO carbon — Physical Chemistry — Kinetics Chemistry Question
Graphite oxide
Graphite oxide (GO) is a compound obtained by treating graphite with strong oxidizers. In GO carbon honeycomb layers (Fig. 1a) are decorated with several types of oxygen containing functional groups. A net molecular formula of GO is СОXНY, where X and Y depend on the method of oxidation. In recent years GO has attracted much attention as a promising precursor of graphene, the most famous two-dimensional carbon nanomaterial with unique electrical properties. The exfoliation of graphite oxide produces atomically thin graphene oxide sheets (Fig. 1b). The reduction of the latter produces graphene.
Figure 1. а) Crystal lattice of graphite. GO retains the layer structure of graphite, but the interlayer spacing is almost two times larger (~12 Å instead of 6.69 Å in the figure) and part of the carbon atoms are oxidized. b) Single sheet in the GO crystal lattice. Several oxygen containing functional groups are shown. Absolute and relative number of functional groups depends on the particular synthesis method.
Give two reasons why GO is more favorable precursor of graphene, compared to graphite itself? What in your opinion is the most serious disadvantage of GO as a graphene precursor?
Model Answer
In GO the interplane spacing is larger. This facilitates exfoliation of GO. Graphite is hydrophobic, whereas GO is hydrophilic due to the formation of the functional groups. This makes GO soluble in water, which is very important for chemical exfoliation. The grave disadvantage of GO as a precursor of graphene is the necessity of reduction of single sheets after exfoliation. Graphene produced from GO is always defective.
The simplest model of the GO sheet (the Hoffman model) is presented in Fig. 2а.
It was assumed that only one functional group, namely (–O–) is formed in the carbon plane as a result of the graphite oxidation. Calculate Х in the net formula СОХ of GO, if 25% of carbon atoms in GO keep the sp2 hybridization. What is the maximum Х in the Hoffman model?
Model Answer
25 % of carbon atoms retain the sp2 hybridization, which means that they are not bonded to oxygen atoms. 75% of carbon atoms form chemical bonds with oxygen. Each oxygen atom is bonded to the pair of carbon atoms. The net formula is СО0.375. Maximum Х in the Hoffman model is 0.5. The net formula is СО0.5.
The up-to date model of a single GO sheet (Lerf-Klinowski model) is shown in Fig. 2b.
Name functional groups shown in the Figure.
Model Answer
The four groups are the phenol (OH sp2), hydroxyl (OH sp3), and epoxide groups in the basal plane, and the carboxylic acid groups at the edges.
Let all the sheets in a GO lattice look like it was predicted in the Lerf-Klinowski model (Fig. 2b). The net formula of the material is СН0.22О0.46. Estimate the amount of carbon atoms (in %) which were not oxidized. Give the upper and lower limits.
Model Answer
Each hydrogen atom corresponds to one oxidized carbon atom. 22 % of carbon atoms are bonded to the hydroxyl or phenol group, or are in the carboxylic acid group. Let all the hydrogen atoms be in the carboxylic acid groups. Then 44 % of oxygen atoms are in the carboxylic acid groups and 2% are in the epoxy groups. In this case 22 % + (2×2) % = 26 % of all the carbon atoms are oxidized. 74 % of the total amount of carbon atoms do not form chemical bonds with oxygen. This is the upper limit. Let all the hydrogen atoms be in the hydroxyl or phenol groups. This means that there are no carboxylic acid groups in the particular GO sample! Then 24 % of oxygen atoms are in the epoxy groups. In this case 22 % + 2×24 % = 70 % of all the carbon atoms are bonded to oxygen. 30 % of carbon atoms are not oxidized. This is the lower limit.
GO absorbs water in between the GO sheets. This is one of the most important properties of the material. Absorption occurs due to the formation of hydrogen bonds between molecules of water and functional groups (Fig. 3).
Let GO have the net formula СН0.22О0.46. What maximum amount of water molecules can be absorbed per atom of carbon in this case? What is the net formula of the corresponding GO hydrate? Use the Lerf-Klinowski model. Consider only contacts depicted in Fig.3 (one molecule of water between two epoxy and/or between two OH groups).
Model Answer
Acid groups do not participate in the hydrogen bonding network (Fig. 3). It means that maximum degree of water absorption will be reached in case of the absence of such groups in GO. Then each pair of hydrogen atoms holds one molecule of Н2О (0.11), and each pair of epoxy groups also holds one molecule of Н2О (0.46 – 0.22) / 2 = 0.12. Altogether there are 0.23 molecules of water per one carbon atom. The chemical formula of GO hydrate is СН0.22О0.46 ⋅ 0.23 Н2О.