Carbon soot with arbitrary wettability deposited on solid surface by ethanol flame method
Graphical abstract
Introduction
Superhydrophobic surfaces [1], which have the static water contact angle (WCA) greater than 150° and rolling angle less than 10°, are widely observed in nature and are mimicked as promising artificial materials for various household and industrial applications such as oil-water separation [2,3], self-cleaning, anti-bioadhesion [4], anti-frost [5] and drag-reducing [6] because of their special surface properties. Over several decades, versatile man-made superhydrophobic materials were developed, including films, fibers, glass, metals, and particles [[7], [8], [9]].
Superhydrophobic soot is a material that attracted wide attention because of its convenient preparation process, low cost, excellent temperature resistance, and anti-corrosion performance. Yuan et al. [10,11] have prepared a flexible carbon fabric via a simple and low-cost flame synthesis process using ethanol as soot precursor. Iqbal and Majhy [12] have prepared a stable and biocompatible superhydrophobic and superhemophobic surface from polydimethylsiloxane (PDMS) and candle soot. Zhao et al. [13] have obtained superhydrophobic and super-oleophilic nickel foam by loading a soot layer on commercial nickel foam. With the painstaking research of scientists, carbon soot coatings with controllable optical transmittance can be made by in-situ single step control of their physicochemical profile [14]. Bandosz et al. [15] firstly reported a kind of magnetic soot which possess a high adsorption capacity and an easiness of a mechanical separation in removing oil contamination from water. Nowadays, superhydrophobic soot can be even applied to in-vitro fertilization analyses [16]. Carbon soot, once regarded as a pollutant, becomes as a functional material now [17]. In some studies, superhydrophobic soot is obtained by burning fossil fuels, biomass, or candles [[18], [19], [20], [21]], and super-hydrophobicity is originated from the remaining unburnt long chain carbon compounds. Kozbial and Zhou et al. considered that the common, freshly peeled graphite is intrinsically mildly hydrophilic, but the adsorbed hydrocarbons in the air results in its hydrophobic property [22]. These low surface energy organic residues can generate super-hydrophobicity on rough but fine soot carbon particles; however, they still have the same disadvantages of being easily damaged and leached as in the case of common superhydrophobic materials, resulting in the loss of super-hydrophobicity.
However, we observed a behavior of a carbon-deposited bottom of a pan as a hydrophobic surface, in which an alcohol lamp or a natural gas stove serves as a heating source. Water dropped onto such surfaces can be repelled and rolled away quickly. Considering that the main content of natural gas is methane and that of alcohol is ethanol, which are both small molecule organic fuels without long chains, the possibility of introduction of low surface energy substances with long chains as modifiers on the generated soot in naturally burning flame of methane and ethanol can be excluded. In addition, some examples, Ag2O and AgCl, whose particle size decided wettability without introducing any low-surface-energy substance has been reported recently [23,24]. These results inspired us to further investigate this occurrence. Therefore, it is necessary to determine the mechanism of generation of such superhydrophobic soot in the absence of modifiers.
Thus, a simple method is designed to investigate the mechanism of generation of superhydrophobic soot by vapor deposition of flaming ethanol. An alcohol lamp with pure ethanol was used as the heating source to investigate the possible effects of long chain chemicals and wax. Different substrates were used to deposit soot. The wettability and stability of deposited soot under different conditions were determined, and the generation mechanism of wettability was analyzed by characterization.
Section snippets
Materials
Ethanol (≥99.7%) was purchased from Chengdu Cologne Chemical Co., Ltd. (China) and was used as a source for carbon soot particles. Ceramic crucible was supplied by Chengdu Changzheng Glass Co., Ltd. 316 stainless Steel foil (06Cr17Ni12Mo2) was produced by Shanghai Shengzhuo Materials Co., Ltd. Cu foil (purity ≥99.8%) was obtained from Dongguan Mingjue Metal Materials Co., Ltd. (China). Quartz glass sheet was purchased from Donghai County Weida Quartz Products Co., Ltd. All reagents were used
Preparation of soot samples with different wettability
A clean porcelain crucible (Fig. S1-A) was smoked by a common alcohol lamp, and some water was dropped onto the black bottom containing deposited soot. As shown in Fig. S1-B, the droplet at the center of crucible bottom maintained its spherical shape, but water at the edge of the black soot layer spread out immediately to form an even water layer on soot. This occurrence showed that the soot obtained in same ethanol flame contained two types: hydrophobic soot in center, and hydrophilic soot at
Conclusions
In summary, two soot products exhibiting super-hydrophobicity and superhydrophilicity were simultaneously prepared by a simple carbon deposition method using flaming ethanol. Super-hydrophilic soot was always generated at the edge of the soot layer and superhydrophobic soot at the center. In this process, no low surface energy modifiers were introduced to cause wettability transformation, and no residual long-chain organic molecules were detected. The super-hydrophobicity and
Acknowledgement
We are thankful for the financial support provided by the National Natural Science Foundation of China Project (Nos. 21676168 & 21476146).
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