Physico-Chemical nanomaterials science

Gas-phase bromination: a versatile approach for the surface modification of nanocarbons
Diyuk V. E.1, Horodetska D. S.1, Vakaliuk A. V.1, Tsapyuk G. G.1, Yatsymyrskyi A. V.1, Mariychuk R.2, Boldyrieva O. Yu.1, Lisnyak V. V.1,2
1 Faculty of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska Street, 64/13, Kyiv 01601, Ukraine.
E-mail: daria_horodetska@ukr.net; lisnyak@chem.univ.kiev.ua
2 Preљov Universirty in Preљov, Ul. 17 novembra, 1, Preљov 15 080 01, Slovakia.

The adsorption and catalytic properties of nanoporous materials depend significantly on surface chemistry. Some surface groups are formed during the synthesis and subsequent processing, but the formation of functional groups on the surface is possible only as a result of chemical modification. The most versatile method of modifying is halogenation and the subsequent halogen replacement by other functional groups. This approach allows obtaining the required functional groups at a given concentration on the carbon surface. However, modification using liquid bromine and its solutions caused a low yield of grafted bromine and, because of the high affinity of carbon to oxygen, is accompanied by significant oxidation of the carbon surface. The treatment with bromine vapor prevents unwanted parallel oxidation and more actively involves the active centers in bromine addition. Here gas-phase bromination of various nanoporous carbons, e.g., NACs, NACFs, soot, and MWCNTs, was performed. Bromination was carried out by Br2 vapors in an argon stream under isothermal conditions at 200, 300, 400, and 500 °C, under gravimetric control. Adsorption of N2, SEM, thermal analysis, and FTIR methods were used to characterize the samples. Depending on the carbon and the temperature on the surface, 0.4–2.5 mmol/g of chemisorbed bromine is formed. The rate of bromination depends on the texture of nanocarbons. The highest and the lowest contents of grafted bromine were obtained for NACs and MWCNTs, correspondingly. The kinetic analysis of the bromination reaction was shown that this process is realized in two kinetic modes in the range of 200–500 °C. The rapid process involves chemisorption of 65–80% bromine in the first 30–35 min. The slow process takes place over the next 100–250 min, depending on the bromination temperature. The surface brominated nanocarbons are convenient precursors with the high content of reactive bromine, which is the active center for substitution by N-, S- and O-containing functional groups.