INVESTIGATION OF THE FUNCTIONAL GROUPS OF GRAPHENE OXIDE BASED ON ITS C-13 NMR SPECTRUM

ABSTRACT

This article presents an analysis of the C-13 nuclear magnetic resonance (NMR) spectrum obtained by the direct excitation method for a graphene oxide (GO) sample. Peaks corresponding to epoxy, hydroxyl, lactol, carboxyl, and carbonyl groups are identified in the spectrum, confirming the presence of oxygen-containing functional groups attached to the GO surface. Based on the experimental results, the mass percentage composition of the functional groups on the GO surface is calculated. The obtained chemical shift values are compared with literature data and found to be in complete agreement. This study provides a deeper understanding of the chemical structural composition of GO, its degree of oxidation, and its potential application areas.

АННОТАЦИЯ

В данной статье представлен анализ спектра ЯМР C-13, полученного методом прямого возбуждения для образца оксида графена (GO). В спектре выявлены пики, соответствующие эпоксидным, гидроксильным, лактольным, карбоксильным и карбонильным группам, что подтверждает наличие кислородсодержащих функциональных групп, прикрепленных к поверхности GO. На основе экспериментальных данных рассчитан массовый процентный состав функциональных групп на поверхности GO. Полученные значения химических сдвигов сопоставлены с данными литературы и выявлено их полное соответствие. Данное исследование способствует более глубокому пониманию химического состава GO, степени его окисления и потенциальных областей применения.

Keywords: graphene oxide (GO), C-13 NMR, functional groups, epoxy groups, hydroxyl groups, lactol structures, carboxyl groups, carbonyl groups, chemical shifts, oxygen-containing groups, surface chemistry, oxidation degree.

Ключевые слова: оксид графена (GO), ЯМР C-13, функциональные группы, эпоксидные группы, гидроксильные группы, лактольные структуры, карбоксильные группы, карбонильные группы, химические сдвиги, кислородсодержащие группы, химия поверхности, степень окисления.

1. Introduction

Graphene oxide (GO) is a two-dimensional material enriched with a high content of functional groups, formed as a result of the chemical oxidation of graphite. The chemical structure of GO, the distribution and proportion of its functional groups determine the physicochemical properties of the material. Therefore, accurate analysis of the composition of GO is of significant importance in the fields of nanomaterials chemistry, composite materials, electronics, catalysis, biomedicine, and sensor systems.

The structure of GO mainly consists of the following functional groups:

1. C=C bonds in the sp²-hybridized graphene plane;
2. Hydroxyl (–OH) and epoxy (–O–) groups attached to sp³-hybridized carbon;
3. Ring-shaped lactol groups;
4. Terminal carboxyl (–COOH) groups;
5. Carbonyl (C=O) groups formed as a result of oxidation.

One of the most effective spectroscopic methods for determining the quantitative composition of these groups is C-13 NMR spectroscopy. In this article, the C-13 NMR spectrum of GO is analyzed, and the presence and proportion of functional groups are evaluated.

2. Experimental Section

The GO sample was synthesized using a method based on the oxidation of graphite in the presence of strong oxidizing agents. The obtained material was dried and prepared for C-13 NMR analysis. The spectrum was recorded using the direct excitation method, which allows all types of carbon atoms to be observed under the same conditions.

The C-13 NMR spectrum exhibited the following main peaks.

Table 1.

Main peaks

The chemical shift values were found to be in complete agreement with the data reported in references 266–270.

3. Results and Discussion

3.1. General Analysis of the Spectrum

The C-13 NMR spectrum of GO clearly reflects its chemical heterogeneity. Graphene oxide does not consist solely of sp² carbons like fully crystalline graphene; rather, due to the incorporation of oxidizing groups, it contains carbon atoms in a variety of chemical environments.

3.1.1. 60 ppm – Epoxy Groups

Epoxy groups are attached to the graphene layer in a bridge-like manner, and their signal in the spectrum is observed at approximately 60 ppm. The presence of this peak confirms the oxidation of the material. The low intensity indicates that the epoxy groups are present in relatively small amounts.

3.1.2. 69 ppm – Hydroxyl groups

The most intense peak in the spectrum is observed at 69 ppm. This signal corresponds to sp³-hybridized carbon atoms bearing –OH groups. The high intensity of this signal is a key factor determining the hydrophilic nature of graphene oxide (GO).

3.1.3. 100 ppm – Lactol groups

The low-intensity peak recorded at 100 ppm indicates the presence of cyclic lactol structures. These groups reflect the complex chemical nature of the GO structure, showing that the graphene basal plane has formed not only open-chain oxidation products but also partially closed-ring oxidation products.

3.1.4. 129 ppm – C=C bonds in the graphene plane

This peak represents sp²-hybridized carbon atoms that remain preserved in the GO structure. If graphene were fully oxidized, this peak would be very weak. The moderate intensity of the peak indicates that GO is partially oxidized.

3.1.5. 165 ppm – Carboxyl groups

Carboxyl groups are mainly located at the edge sites of graphene oxide and produce a small peak at around 165 ppm in the spectrum. These functional groups significantly affect the dispersion behavior of GO.

3.1.6. 191 ppm – Carbonyl groups

The broad peak observed at 191 ppm indicates the presence of carbonyl groups in different chemical environments. These groups originate from a wide variety of structural sites formed on the graphene plane under the action of oxidizing reagents.

3.2. Percentage composition of functional groups

Based on the C-13 NMR spectrum, the mass-based composition of functional groups on the surface of GO was calculated as follows:

Table 2.

Calculations

The above results indicate that GO is a highly oxidized material. In particular, the proportion of hydroxyl groups is very high—55 %—which makes the sample strongly hydrophilic and allows it to function effectively as a well-dispersed colloid in water. The content of epoxy groups is also relatively high (22 %), weakening the interlayer interactions of graphene sheets and enhancing the mechanical flexibility of GO. Carboxyl and carbonyl groups together account for 4 %, increasing the material’s acidity and its reactivity in ion-exchange processes. Interestingly, the total fraction of functional groups reaches 85 % by mass, indicating that GO is highly oxidized, with graphite almost entirely saturated with oxidizing functional groups.

3.3. Comparison with literature

According to the literature, the chemical shift values for GO are typically reported as follows:

1. Epoxy: 55–62 ppm
2. Hydroxyl: 68–72 ppm
3. Lactol: 95–105 ppm
4. Aromatic C=C: 125–133 ppm
5. Carboxyl: 160–170 ppm
6. Carbonyl: 185–200 ppm

The values obtained in this study fully correspond to those reported in references 266–270, indicating that the synthesized GO sample is pure, free from contaminants, and possesses a classical GO structure.

4. Conclusion

The C-13 NMR analysis conducted above provides a comprehensive understanding of the complex, multifunctional structure of GO. The spectrum clearly identified epoxy, hydroxyl, lactol, carbonyl, carboxyl groups, as well as C=C bonds characteristic of the graphene plane. Their mass fractions were determined, showing that hydroxyl and epoxy groups constitute the main components of GO. The results confirm that GO is a highly oxidized and hydrophilic material. These properties make GO a promising material for applications in composite materials, catalyst preparation, ion-exchange membranes, drug delivery nanoplatforms, and other advanced technologies.

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