Mateusz CISZEWSKI, Andrzej MIANOWSKI ? Department of Chemistry, Inorganic Technology and Fuels, Faculty of Chemistry, Silesian University of Technology, Gliwice, Poland

Abstract:
Three most popular methods of chemical oxidation of graphite proposed by Brodie, Staudenmaier and Hummers has been compared here. Synthetic graphite has been oxidized using strong chemical oxidizing agents in highly-concentrated inorganic acids. Reaction time varied from several to several hundred hours depending on the applied method. Products were characterized with respect to the oxygen content, C/O ratio, full qualitative and quantitative analysis of the oxygencontaining groups. Based on these experiments it was determined that the most oxidized graphite was obtained using the Brodie method. In case of Staudenmaier method, that can be carried out in single step, well-oxidized graphite can be obtained but much longer oxidation time is required. Method proposed by Hummers is the shortest but graphite oxide possesses a plenty of unoxidized carbon atoms.

Please cite as: CHEMIK 2013, 67, 4, 267-274

 

Introduction
Introduction Carbon allotropes were until recently limited to graphite and diamond. Carbon balls called fullerenes [1] have been discovered only in eighties while carbon nanotubes and graphene completed the whole family lately. Graphene, discovered in 2004 [2], is a basic structural element for graphite -pile of graphene layers, carbon nanotubes -cylindrically rolled graphene layers and (0D) spherical fullerenes. Graphite is composed of graphene layers aligned in AB stacking sequence forming hexagonal lattice with sp2-hybridized carbon atoms. The interlayer distance between the layers is about 0.34 nm and is maintained by van der Waals forces. For many years graphite was mainly used in electrode fabrication and as a lining in high temperature furnaces. Graphene discovery has started development of graphite chemistry and has found new possible applications in electronic, automotive and pharmaceutical industries. The key point was an isolation of 2D material by Geim and Novoselov [2] who separated graphene sheets from graphite using scotch tape. A new material was thoroughly examined by Lee et al. [3] and Balandin et al. [4] resulting in high thermal conductivity 5000 W/(mK), high theoretical surface area 2600 m2/g and Young modulus as high as 1 TPa. This led to much interest in graphene and techniques of its preparation because mechanical exfoliation was inconvenient and insufficient. Nowadays many different graphene synthesis methods have been developed but the most important are chemical vapor deposition (CVD) [5÷7], epitaxial growth on SiC [8] and graphite oxide reduction [9]. Though some defects in the final structure graphite oxide reduction seems to be the best method in an industrial scale.

Graphite oxidation
Graphite oxide was first prepared by Brodie [10] in 1859 who treated graphitic powder with potassium chlorate in concentrated fuming nitric acid. This troublesome method was changed by Staudenmaier [11] forty years later by using higher excess of the oxidizing agent and additive of concentrated sulfuric acid. This modification enabled to carry out continuous process without the necessity of nitric acid addition during the reaction.

Fig. 1. Scheme of graphite oxidation using Staudenmaier method with subsequent thermal exfoliation [12]

But these two methods were still too long till Hummers [13] proposed in 1958 graphite oxidation using potassium permanganate and sodium nitrate in concentrated sulfuric acid that shortened oxidation time to several hours. In previous years graphite oxide was mainly used as an intermediate for expanded graphite but graphene discovery caused huge interest in graphite oxide itself. Although tremendous amount of papers concerning graphite oxide, graphene oxide and graphene have been recently published the precise structure of graphite oxide is not known. Probably the reason is some discrepancies in graphite substrates used in synthesis, reaction conditions and analytical methods. Graphite chemical oxidation involves intercalation of the oxygen-containing groups in interlayer space. Embedded oxygen breaks van der Waals forces that hold layers together and an increase in the interlayer distance is observed. Carbon-oxygen bonds cause partial change of carbon atoms hybridization from sp2 to sp3 and change in graphene layers arrangement into turbostratic (locally parallel) structure. Up to now several models of graphite oxide structures, presented below, have been proposed.

Fig. 2. Graphite oxide models commonly presented in literature [14]

Most-appreciated is Lerf-Klinowski model [15] that assumes coexistence of unoxidized benzene rings and corrugated alicyclic six-membered rings with C=C, C-OH, C-O-C and edge COOH groups. Some authors deny the presence of epoxy/ether groups and C-O signal interpret as hydroxyl group only. Graphite oxide is the interesting material possessing above 20% of oxygen, electrical conductivity much lower than graphite but several times higher surface area and can be easily converted to various composites by means of the oxygen-containing groups. Separation of graphene layers from graphite oxide involves desorption of the oxygen species as gases tearing the layers. The most common technique is a heat treatment at temperatures starting from several hundred degrees Celsius [16]. There exist numerous chemical reduction techniques mainly using hydrazine or metal hydrides but also lesser popular methods [17÷21]. Oxidized graphite as well as reduced graphene oxide may be widely applied in electronic industry or as a drug carrier in medicine, can form composites used in supercapacitors or lithium-ion batteries.

Experimental
Synthetic graphite (Fisher Scientific) was oxidized using three methods: Brodie, Staudenmaier and Hummers and products were labeled GO-B, GO-S and GO-H respectively. In every experiment round-bottomed flask equipped with reflux, thermometer and icecooled was used. Using Brodie method 10 g of graphite (Fisher Scientific) was magnetically mixed with 100 ml 100% fuming HNO3 (Sigma Aldrich, p.a.) in an ice bath. Then 85 g of KClO3 (POCh) was added. Then graphite oxide slurry was mixed for 24 h and heated at ~55 °C for 6 h. Thick paste was diluted with deionized water (Millipore, 0.0067 S/m) and vacuum-filtrated. Dry product was a substrate for the next oxidation step. In this method oxidation was repeated three times. In Staudenmaier method 10 g of graphite were mixed with 175 ml 95-97 % sulfuric acid (Acros) and 90 ml 100% fuming nitric acid and was oxidized by 110 g of potassium chlorate for 300 h. Hummers method based on 10 g of graphite, 230 ml 95-97% sulfuric acid and 5 g of sodium nitrate (POCh). Next 30 g of potassium permanganate (POCh) were added and brown mixture was stirred for 30 minutes at 35°C. After addition of 460 ml deionized water temperature raised to 100 °C. Slurry was diluted with water and 20 ml 30 % H2O2 was added to reduce manganese, consequently color of the mixture turned to bright yellow. Product was vacuum-filtrated and washed with plenty of water. All oxides were dried in desiccator with P2O5. Graphite oxides were characterized with powder X-ray diffraction (XRD, Seifert 3003) with a step size 0.02° and Cu Ka radiation, l = 0.154 nm in the region of 2q 5-50°. The IR-spectra were measured on a Nicolet 6700 FT-IR spectrophotometer with Attenuated Total Reflectance (ATR method). Presence and atomic percent of the oxygen-containing groups were confirmed by X-ray photoelectron spectroscopy (XPS, ESCALAB-210, VG Scientific).

Results
Turbostratic structure of graphite oxide stems from breaking of p-p bonds that can be observed by color change during oxidation. Matuyama [22] observed that graphite treated with oxidizing mixture of 2:1 sulfuric to nitric acid had a blue color while addition of chlorate changed color to brown or even green. In our experiments very dark blue mixture of GO-S was obtained that turned into dark green after dilution with 10 l of deionized water, however, finally filter cake was brown after drying. Graphite oxide slurry prepared by Brodie method was intensively yellow and darkened to yellowishbrownish after drying. In case of Hummers graphite oxide mixture was at first brown and turned yellow after manganese reduction. Solid product was the darkest from the all graphite oxides. XRD analysis is probably the most convenient method to study oxidation progress. Oxygen embedded between graphene layers causes an increase in the interlayer distance from 0.34 nm in graphite to about 0.7 nm in graphite oxide. Consequently characteristic graphitic signal shift from 26 ° to 10-15 ° is observed. Fig 3 presents XRD patterns for GO-B, GO-S and GO-H.

Fig. 3. XRD patterns of graphite oxides prepared by Brodie, Staudenmaier and Hummers method

Signals broadening and decrease in intensities are caused by crystallites size diminishing. Using Scherrer formula it was calculated that the crystallites height (Lc) decreased after oxidation from 47 nm in graphite to 8.7, 9.3 and 4.6 nm for GO-B, GO-S and GO-H respectively. In all graphite oxides signal at 26 ° disappeared and newer one located close to 10 ° appeared. The interlayer distances for GO-B, GO-S and GO-H were 0.74, 0.75 and 0.73 nm respectively. The qualitative analysis of the oxygen-containing groups was performed by FT-IR ATR. Figure 4 presents infrared spectra that confirmed a lot of oxygen within graphite.

Fig. 4. FTIR-ATR spectra of graphite oxides

In all spectra broad signal located at 3700-2800 cm-1 attributed to the stretching vibration in hydroxyl groups and water molecules can be observed as well as signal at 1710-1705 cm-1 belonging to the carboxylic groups, signal at 1350 cm-1 due to deformation vibration in C-OH, 1040-1023 cm-1 represented by C-O groups and a peak around 950 cm-1 from epoxy groups. GO-S and GO-B have a signal located at 1620 cm-1 belonging to OH groups while GO-H possesses signal at 1566 cm-1 from C=C bonds and small signal at 1214 cm-1 that can be either attributed to C-O groups or sulfates [23]. Pumera [24] suggests that large peak at 1600 cm-1 corresponds to the unoxidized sp2 regions but in fact signal slightly above 1600 cm-1 is more often attributed to OH groups while signal around 1580 cm-1, very common in less-oxidized graphite, may result from sp2 domains [25]. The low small signals located at 2160 and1960 cm-1 may belong to nitrogen compounds and aromatic ring respectively.

These results were confirmed by XPS analysis. Fig 5 presents C 1s spectra deconvoluted into five separate peaks representing C=C sp2, C-C sp3, C-O/ C-OH, C=O and COOH.

Fig. 5. Deconvoluted C 1s spectra with respective carbon bonds formed

The observed signals are attributed to hydroxyl/epoxy (~287 eV), carbonyl (~288 eV) and carboxyl groups (~289 eV). Additionally signals representing oxidized and unoxidized carbon atoms with hybridization sp3 (~286 eV) and sp2 (~285 eV) respectively were found.

In comparison to the latter GO-H has much higher amount of sp2 carbons that indicates its lower oxidation. In GO-B very intense signals of hydroxyl and carboxyl groups were observed. Detailed information about content of the particular bonding was collected in Table 1.

Relatively small amount of the edge carboxyl groups were identified in all spectra. Hydroxyl/epoxy group content is comparable in all graphite oxides. XPS technique confirmed higher amount of the sp2 carbon atoms in GO-H. Though Hummers method is the fastest to obtain graphite oxide the final material possesses unoxidized domains. Atomic composition of graphite oxides was presented in table 2. It should be pointed out that in GO-B very small amount of potassium and chlorine was additionally detected but generally this method results in highly-oxidized graphite with the highest oxygen content and the lowest C/O ratio. Higher amount of nitrogen in form of NO2 and NO3 is compensated by lack of the sulfur contaminations. The lower C/O ratio in GO-H with respect to the GO-S is caused by the 2 % higher content of oxygen in GO-H that was assigned on O 1s spectrum to some oxides and probably is a consequence of humidity.

Conclusions
Review of the most important methods of graphite chemical oxidation was reported. In all graphite oxides comparatively small amount of carboxyl groups and high amount of hydroxyl groups were observed. Prolonged drying over phosphorus pentoxide at ambient temperature allowed to preserve epoxy groups that easily desorbed at slightly higher temperature. It can be concluded that:

• GO-B is most-oxidized, has lack of sulfur contaminations but oxidation time is about 150 h with an additional drying of the intermediates
• GO-S is well-oxidized, process can be carried out continuously but reaction time is 100-300 h
• GO-H is the worst-oxidized, possesses some amount of unoxidized sp2 carbons but is the shortest method that usually takes about several hours.

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Translation into English by the Author

Mateusz CISZEWSKI, M.Sc., is a graduate of the Faculty of Chemistry of the Silesian University of Technology (2009). At the moment, Mr Ciszewski is completing his doctoral studies in the Department of Chemistry, Inorganic Technology and Fuels of the Silesian University of Technology. Scientific interests: synthesis of carbon composite materials, graphene, graphene oxide, EDLCs. He is co-author of 1 patent application.
e-mail: ,
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Professor Andrzej MIANOWSKI ? (Sc.D., Eng.), Professor, is a graduate of the Faculty of Chemistry of the Silesian University of Technology (1970). Mr Mianowski obtained his doctoral degree in 1976 and the degree of habilitated doctor in 1988. He was awarded the title of Full Professor in the field of technical sciences in 2001. At the moment Mr Mianowski is employed as full professor in the Department of Chemistry, Inorganic Technology and Fuels of the Silesian University of Technology, and the Institute for Chemical Processing of Coal in Zabrze. Scientific interests: coal technology, solid waste disposal, technological and industrial aspects of thermal analysis. He is the author or co-author of over 150 publications, 30 patents granted and pending, including several implemented patents, the co-author of several books, course books and over 120 lectures. He promoted 11 doctors of technical sciences, including 3 with distinction, and is currently supervising another two doctoral candidates.