Mosquito's compound eyes as inspiration for fabrication of conductive

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Mosquito’s compound eyes as inspiration for fabrication of conductive superhydrophobic nanocarbon materials from waste wheat straw Yanbin Wang, Dong Zhang, June Deng, Feng Zhou, Zhiying Duan, Qiong Su, and Shaofeng Pang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Sustainable Chemistry & Engineering

Mosquito’s compound eyes as inspiration for fabrication of conductive superhydrophobic nanocarbon materials from waste wheat straw Yanbin Wang,a,b Dong Zhang,a,b June Deng,c Feng Zhou,d Zhiying Duan,a,b Qiong Su*,a,b and Shaofeng Pang*,a,b aChemical

Engineering Institute, Northwest Minzu University, No.1, Northwest Xincun, Lanzhou, 730030, P. R. China

bKey

Laboratory of Environmental Friendly Composite Materials and Biomass in Universities of Gansu

Province, Northwest Minzu University, No.1, Northwest Xincun, Lanzhou, 730030, P. R. China c

School of Foreign Languages, Northwest Minzu University, No.1, Northwest Xincun, Lanzhou, 730030, P. R. China dDalian

National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, No.457, Zhongshan Road, Dalian, 116023, P. R. China

*Corresponding author: [email protected] (S. Pang); [email protected] (Q. Su) KEYWORDS : Biomimetic, Wheat straw, Controllable construction, Superhydrophobicity, Conductivity, Nanocarbon materials

ABSTRACT : Converting waste wheat straw to conductive superhydrophobic nanocarbon materials is inspired by mosquito’s compound eyes, and one step pyrolysis process is used for controllable construction

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of micro/nano hierarchical structures. Their structures and hydrophobicity can be tuned efficiently by the annealing temperature. SEM, EA, XRD, Laser Raman, FT-IR and XPS analysis were used to reveal the structure of ECMs and a possible mechanism of the nanostructures formation. These results indicate that there are a big difference between the surface and the inner structure for the nanocarbon materials, and the releasing rates of multicomponent gas at different annealing temperature by further decomposition of waste wheat straw should be pivotal to the formation of nanostructures, there are also cause for superhydrophobicity and electroconductivity for the ECM-800. Considering the potential use of the novel ECM-800, a low-cost, green, sustainable, conductive and superhydrophobic coating strategy was developed by dip-coating of the different substrates with ECM-800/PDMS mixture, followed by thermal curing. The water contact angle of the prepared ECM-800 /PDMS coating is greater than 160o, while the sliding angle is less than 5o, showing excellent superhydrophobicity as well as good mechanical robustness and waterrepellency in the artificial rain tests simultaneously. In addition, a robust ECM-800 enables resistance to a variety of harsh environments, such as an ability to resist heat, UV aging as well as sustaining exposure to strong acid/alkaline corrosion. Moreover, This work should be conducive to the controllable construction of conductive superhydrophobic nanocarbon materials with well-defined structure, and open up a convenient sustainable way for disposal of waste biomass for production of value-added materials.

INTRODUTION Superhydrophobicity of material surface is an important property, the surface usually possesses superhydrophobicity, in which the water droplets roll down from the surface at a small inclination angle and sequentially removing contamination from material surface. Therefore, the superhydrophobic materials is an important backbone for many biological and industrial applications such as antifouling paint for ships, antennas and windows, self-cleaning windshields, antifouling fabrics, metal purifying, antifouling paint for buildings, blood-repellent surfaces, etc.1-4 To date, a number of construction protocols for fabrication of superhydrophobic surfaces have been explored, including the self-assembly technique,5-9 the sol-gel

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process,10,


11

the template method,12-16 electro-deposition,17-19 electro-spinning,20-22 hydrothermal

synthesis,22-24 spray coating,25-29 chemical vapor deposition,30-32 the displacement deposition,33-35 and so forth.36-39 However, most of these methods and techniques for fabrication of superhydrophobic coatings are expensive and time-consuming and often involve complex multi-stage processes. Besides, superhydrophobic surfaces often both have low surface energy and also electrically insulated, which impede mass production and diverse applications. Thus, it is necessary to develop a simple and economical protocol to fabricate conductive superhydrophobic surfaces.40-43 Generally, to achieve hydrophobicity or superhydrophobicity, two key aspects need to be considered. 4448

One is the surface structure. In nature, animals and plants exhibit the feature to protect their bodies from

water wetting in order to survive. A series of bionic research discovered that the combination of micro and nano hierarchical structures on many natural species surface, that is, nanostructures on top of microstructures, cause its superhydrophobicity. For example, the compound eyes of mosquitoes have excellent superhydrophobicity due to micro-ommatidia covered by nano-nipple. Importantly, substantial and regular nipples with average diameters of 101.1 ± 7.6 nm and interparticle spacings of 47.6 ± 8.5 nm were spread throughout the surface of each microhemisphere, it provided an air cushion to prevent the wetting of the mosquito eyes by the hierarchical micro and nanostructures.49 The other significant point is a suitable surface energy, which is often required for both molecular geometry and chemical composition. For example, a surface of conductive graphene with epitaxial structure usually shows better hydrophobicity.50-52



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Figure 1. The new concept for preparation of nanocarbon materials. In addition, the controllable method for the construction of modified graphene always can present fine hydrophobicity due to the surface with the special chemical composition. 53, 54 Therefore, it could be an important and intriguing idea if structural bionics45, 55-57 and controllable construction are to be used in exploitation and development of conductive superhydrophobic materials to integrate surface structure of mosquito’s compound eyes and conductive graphene or graphene-like materials (Figure 1). Biomass, in particular, has long been considered as a potential sustainable source of high added-value carbon. In particular, nanocarbon materials, other solid carbon forms (activated carbon, graphite, and porous carbon), and liquid fuels, etc.58-61 In these carbons, nanocarbon materials, including amorphous carbon, ordered mesoporous carbon, graphite/graphene (oxide), carbon nanotubes, etc., with well-defined molecular structures have been studied for many important application.62,

63

In addition, structures of

nanocarbon materials are easy to be modified by controllable construction. 64,

65

Thus, It is of great

significance to convert biomass into high value-added nanocarbon materials with different structures, especially for the purpose of "from waste to wealth".66 Wheat straw is a kind of lignocellulosic biomass, which usually contains cellulose, hemicellulose and lignin, and it is widely found in agricultural waste, can be a potential “zero-cost” carbon precursor.67,

68

Converting wheat straw into nanocarbon materials

(graphene or graphene-like materials) is of major interest to dispose this major rural waste. Motivated by this, here we present the results of converting wheat straw to nanocarbon materials with external hierarchical structure consisting of micro-lumps and nano-nipples by two facile steps, namely, the target materials were formed by controllable carbonization after chemical exfoliation of siliceous and wax impurities from natural wheat straw. Therefore, our strategy is easy, fast and safe operation, without any toxic organic solvents or by-products. Besides, the approach is environmentally friendly. A series of ECMs (electric conduction of carbon materials) was obtained and denoted as ECM-T (T = annealing temperature). Among them, the obtained ECM-600 and ECM-800, which have high surface roughness, and as a result,


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they exhibit the unique superhydrophobic properties. Moreover, the ECM-800 showed good mechanical robustness and superior property of heat / UV aging / acid / alkaline-resistant. As it is well known, both hierarchical rough structures and wax layers are vital to fabricate a surface with superhydrophobic behaviour.1,

69

As reported, polydimethylsiloxane (PDMS) can be used as curing

adhesive thanks to the most prominent elasticity and waxy property. 70 At this point, we developed a simple method to fabricate the coating with superhydrophobicity by using the special micro/nano-structures of the ECMs as building blocks and the PDMS with waxy property as a glue. Characteristics of the obtained ECMs were examined by scanning electron microscope (SEM), elemental analysis (EA), X-ray diffraction (XRD), laser Raman spectroscopy, fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). Water contact angle (WCA) and sliding angle (SA) measurements were chosen to characterized the hydrophobicity of ECMs/PDMS coating and these results show that the surface coating of ECM-800/PDMS for different substrates with superhydrophobicity, good adhesion and mechanical durability. Besides, the obtained superhydrophobic ECM-800 has exhibited heat, UV aging, strong acid and alkaline corrosion resistance. Based on the results, the mechanism of superhydrophobic nature of ECM-800 is proposed. The results also show that ECM-800/PDMS and ECM-1000/PDMS coatings exhibit excellent electrical conductivity. Among them, the ECM-800 may be used to make and design for chemical engineering materials,71 radar absorbing materials,72 microfluidic devices, which can transport of a small amount of corrosive fluids.73 Furthermore, this work is aimed not only to develop an very simple, cheap and effective way to convert waste wheat straw to potentially usable and valuable nanocarbon materials with superhydrophobic and conductive properties but also to explore a new possible strategy for controllable construction of special structural nanomaterials based on biomimetic structure design. To our knowledge, this is the first report of the simplest way to prepare such superhydrophobic material and could open the new way for many kinds of applications. MATERIALS AND METHODS



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Materials. Because it is abundant in China and also contains high cellulose, wheat straw was selected as the precursor material of biochar.74 Wheat straw was picked from a nearby farm of Yu Zhong campus in Northwest Minzu University, China. Let dry for 2 days, then bake at 80 oC overnight, wheat straw was ground and following sifted through 100-mesh (0.154 mm) sieve; PDMS (Sylgard 184) was purchased from Dow Corning and composed of two components including the vinyl-terminated prepolymer and curing agent. A commercial copper, iron and aluminum plate used as a model substrate with 0.5mm thick was purchased from a department store. Glass slide ( 75 mm x 25 mm x 2.5mm ) used as a glass substrate. All other solvents and chemicals were purchase and use directly from chemical reagent suppliers. Typical procedure for treated wheat straw powder preparation. 5g wheat straw powders was added to aqueous NaOH (100 mL, 5.0 M), and keep stirring the mixture at room temperature for two hours to remove siliceous and wax impurities. Then, aqueous HCl (1.0 M) was hereby dripped into the mixture until the pH value of the aqueous solution was ≈ 3. The mixture was centrifuged and then washed with deionized water until the pH value of filtrate was ≈ 7. Subsequently, the solid was dried in air at 80 oC for overnight. The final treated wheat straw powder yield was about 81%. The wheat varieties used in these experiments was Dingfeng 10 (Breeding unit: Dingxi dry farming agricultural research and extension center), which is widely planted in western China. In addition, element contents in the wheat straw and treated wheat straw were analyzed by Vario EL microanalyzer, the wheat straw contained of ∼42.67 wt % carbon, ∼5.51 wt % hydrogen,∼51.13 wt % oxygen and 0.75 wt % nitrogen, the treated wheat straw contained of ∼42.78 wt % carbon, ∼6.16 wt % hydrogen, ∼51.04 wt % oxygen and only small amounts of nitrogen was about 0.02 wt %. As the straw was treated with alkali solution, the corresponding element contents will also have changed. The wheat straw showed the presence of atomic % ∼69.39 % carbon, ∼25.32 % oxygen, ∼2.08 % nitrogen and ∼2.68 % silicon, the treated wheat straw showed the presence of atomic % ∼76.03 % carbon,∼23.62 % oxygen, ∼0.12 % nitrogen and a very small presence of ∼0.23 % silicon, which were examined by XPS characterization. XPS characterization showed that the treatment process can effectively remove silicon impurities from wheat straw.


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Typical procedure for ECM-400 preparation. 4g treated wheat straw powder was activated under a nitrogen gas flow (99.999%) at 400 °C for 1 h with heating rate of 5 °C min−1. Following, the reaction tube was cooled under the nitrogen gas flow until room temperature, and 0.9 g ECM-400 was obtained with a carbonization ratio of 22.5%. Different samples were prepared with the same procedure but different annealing temperatures, and denoted as ECM-600, ECM-800 and ECM-1000. The carbonization ratio of ECM-600, ECM-800 and ECM-1000 was 20.5%, 20.0% and 12.5%, respectively. A schematic of the preparation processes of ECM-800 is also set forth in Figure 2.

Figure 2. Preparation of ECM-800 by controllable carbonization. Preparation of superhydrophobic coating solution and coating. ECMs (0.1g) were added in 5 mL THF and ultrasonic treatment. Following, the component A of PDMS (The vinyl-terminated prepolymer of Sylgard 184, 0.05g) was added to the mixture. the mixture was submitted to 30 min of ultrasonic treatment and formed the uniformly solution A. The component B of PDMS (The curing agent of Sylgard 184, 0.005g) was added in the uniformly solution A at ambient temperature to prepare the superhydrophobic coating solution. Before varnishing, the copper plate, iron plate, aluminum plate and glass slide as substrates were cleaned with water and ethanol and dried at 80 °C in a vacuum drying oven for 0.5 h. after that, substrates were dipped into the obtained coating solution and formed the coating, and then dried at 80 °C for 0.5 h.



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Characterization Methods. The surface micro and nanostructures of as-prepared samples were observed by FE-SEM. SEM was performed on the JEOL JSM-6701F and installed a field emission gun. WCA (water contact angle) was measured by a POWEREACH JC2000D goniometer (Made in China). The wettability of the substrates coated with the ECMs/PDMS was evaluated by WCA measurement of 4μL water droplets on the substrates. The profiles of the contact angle were photographed using the digital camera of the goniometer. However, a 4 μL water droplets were difficult to adhere to the surface of substrates due to the superhydrophobicity of the coating (exhibited in the Schematic Video-1 from Supporting Information). Thus, the contact angles were calculated by injecting ~ 5μL water droplets on the surface using a micrometer syringe. At least five measurements were carried out to obtain each value. The error in contact angle measurements was ±2o. The Vario EL microanalyzer was used to analyze the element composition (N, C, O, H) of the samples. The sample was analyzed using XRD equipped with STADIP automatic transmission diffractometer (STOE), which installed an incident beam bending germanium monochromator with CuKα1 radiation. After the samples were dried in air, it was pressed onto glass slides for further analysis. X-ray diffraction were scanned from 5° to 80° in the 2θ range. Besides, WinXpow software (STOE) and the ICDD Powder Diffraction Database File (PDF) were used for data analysis. LabRAM HR Evolution Confocal Microscope (Horiba Jobin Longjumeau Cedex, France) was used to measure the raman spectra with a 532nm edge. FT-IR spectral feature of the samples at 500-4000 cm-1 by a Nicolet 5700 spectrometer with a resolution of 1 cm-1. For each FT-IR spectrum, the 0.5mg of carbon material was uniformly mixed with 100 mg of potassium bromide, and then the mixture was laminated with a tablet press for further analysis. The X-ray photoelectron spectroscopy (XPS) analysis were carried out by the instrument with the model as VG ES-CALAB 210 for investigation of structure and bonding environments of ECMs, X-rays were generated by a hemispherical capacitor analyzer (equipped with a 5 keV Ar+ ion gun) and a dual Mg/Al anode X-ray source. Among them, non-monochromatic MgKa (1253.6 eV) radiation were used for recording the photoelectron spectra. The samples were pasted on the stainless steel sample table with double-sided adhesive. The C1s peak (284.8 eV) was used as the reference peak to


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calibrate the other electron binding energy peaks. After Shirley background subtraction, the gauss-lorentz curve was used to fit the peaks. The pressure inside the test chamber was less than 10-7 Pa. The CMTSR2000N (Advanced Instrument Technology) was used to measure sheet resistance and resistivity to the conductivity of ECMs/PDMS coatings by four probe system. Intelligent numerical control photothermal aging oven (Model LHX-205, China) was used for UV aging. Then the samples were put into the oven for UV aging, the light came from an ultraviolet high-pressure mercury lamp with 500 W output power, besides, the lamp with the wavelength was 365 nm. The UV intensity was adjusted at 950 and 1200 μW/cm2 to simulate the UV aging. The test temperature of the equipment was set to 60 oC, and all samples will be taken out after continuous 24 h aging test. In addition. All the digital photos were shot with Canon EOS550D camera. RESULTS AND DISCUSSION The morphology of ECMs formed after the temperature-controlled carbonation were observed by SEM. It is observed clearly that microstructure of ECMs consist of carbonaceous brick-like lumps by Figure 3ad. Importantly, the carbonaceous brick-like lumps with an irregular shape. Meanwhile, it is noteworthy that the brick-like lumps become larger and larger with increased annealing temperature. Notably, the annealing temperature is a critical factor to control the preparation process for construction of hierarchical surface structure. As annealing temperatures rise, the hierarchical surface structure containing many nanometerscale nano-nipples has gradually formed (Figure 3a1-c1). When annealing temperature is adjusted to 600 °C for 1.0 h, a spot of nano-nipples are observed. Obviously, when raising the annealing temperature to 800 °C, the morphology of the resulting ECM-800 (Figure 3c1) is dramatically different from other samples, the surface microstructure is covered by a number of nano-nipples in ECM-800.



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Figure 3. SEM images of obtained ECMs dispersed on carbon tape at different magnifications. The insets are the corresponding wetting experiments. The micro and nanostructure shown on the SEM images are marked with red and green circle lines, respectively. The number of nano-nipples on the image of ECM-800 (Figure S1) were measured from the particle size distribution (PSD). Average nano-nipple diameters of ECM-800 was 43.72 nm, and average interval of nano-nipples was 117.32nm. More detailed of PSD is shown that the proportion of nano-nipple sizes within 25-50 nm was 80% and interval size within 0-150 nm was 80%, these data basically obey normal distribution. However, the nano-nipples are all disappeared while the annealing temperature rise to 1000 °C (Figure 3d1). Moreover, the relationship between morphology and hydrophobicity of ECMs was further discussed, the ECMs/PDMS composites on the glass surface via dipping-drying-curing method was chosen as a simple technique because of the waxy properties and adhesive forces of PDMS. Insets of Figure 3a-d show the results of wetting experiments were carried out on a glass substrate with ECMs/PDMS coatings using a water droplet of 5μL. The water contact angle of baseline glass and PDMS are 43 o and 103o, respectively. In addition, in contrast, several commercially available carbon materials, such as graphite, multiwalled carbon nanotubes and Reduced Graphene Oxide(RGO) were studied by direct contact angle measurement, as expected, the water contact angle of graphite, multiwalled carbon nanotubes and Reduced


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Graphene Oxide(RGO) are 132o, 130o and 60o, respectively. Wettability tests were carried out on wheat straw and treated wheat straw to explore their hydrophobicity, The water contact angle of wheat straw and treated wheat straw are 58o and 35o, respectively. The treated wheat straw exhibited hydrophilicity. As expected, the water contact angle of ECM-400/PDMS is 141o, the contact angle of ECM-600/PDMS is 152o. Noticeably, the contact angle of ECM-800/PDMS is significantly increased to 162o, the water contact angle finally reaches to 149o for ECM-1000/PDMS, which indicate that ECM-600 and 800/PDMS coatings have superhydrophobic property. In the present study, the ECM-600 and 800/PDMS coating show superhydrophobic behavior heavily relies on the micro/nano hierarchical surface structure by the SEM observation.

Micro-nano

structures

provide

sufficient

roughness

to

facilitate

the

surface

superhydrophobicity as described in Wenzel’s theory. 75 Besides, these nano-nipples form a crowd of deep trenches that trap air inside to form a lot of air cushions. These air cushions can significantly increase the gas-liquid interface effectively when the ECM-800 is in contact with water, as a result, the ECM-800 exhibit superhydrophobic property. Table 1. Elemental Composition of ECMsa

a

ECM-400

ECM-600

ECM-800

ECM-1000

C

72.83

81.56

83.66

86.12

H

3.52

1.92

0.65

0.38

O

23.21

16.14

15.27

13.08

N

0.44

0.43

0.42

0.47

The Vario EL microanalyzer was used to analyze the element composition (N, C, O, H wt%) of the samples.

To preliminarily interpret the relationship between the formation of nano-nipples and the annealing temperature, elemental analyses of ECMs are conducted after ending the carbonization process. The ECMs have been characterized by elemental analysis, the ECM-400 contained of ∼72.83 wt % carbon, ∼3.52 wt % hydrogen, ∼23.21 wt % oxygen, and only small amounts of nitrogen was about 0.44 wt % (Table 1, entry 1). Notably, the hydrogen and oxygen content obviously reduced while the annealing temperature increased



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during carbonization process, the elemental analysis proved that the ECM-800 contained certain carbon element (∼83.66 wt %) and very little hydrogen (∼0.65 wt %), oxygen (∼15.27 wt %), and nitrogen (∼0.42 wt %) (Table 1, entry 3). When the annealing temperature rises to 1000 oC, the ECM-1000 with ∼86.12 wt % carbon, ∼0.22 wt % hydrogen, ∼13.08 wt % oxygen, and 0.47 wt % nitrogen (Table 1, entry 4). Interestingly, the nitrogen content scarcely changed from ECM-400 to ECM-1000. One possibility is that this phenomenon is caused by the chemical reactions that occurred during annealing and produced the multicomponent gas containing carbon, hydrogen and oxygen. It can be imagined that the releasing rates of multicomponent gas with different annealing temperature should be the key factor in the formation of nano-nipples.

Figure 4. XRD diffraction patterns of ECM-400 (a), ECM-600 (b), ECM-800 (c) and ECM-1000 (d). XRD and Raman spectroscopy can deeply characterize the material structures, generally, they are widely used in the characterization of carbon materials. Thus, XRD and Raman spectroscopy have been applied to characterize a series of ECMs. The XRD spectra measured in a range of 2θ from 5o to 85o show (002) diffraction peak at 2θ = 22.7o and 26.7o (ECM-400) (Figure 4a), indicating a certain distance between graphene layers with a low graphitization degree,76-78 and the reﬂections of the overlapped (100) diffraction 12 ACS Paragon Plus Environment

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peak at 2θ = 42.26o. As expected, with the annealing temperature increasing, peaks obviously shift to the lower angle (from 22.7o (002) to 22.1o (002), 42.26o (100) to 42.1o (100)) (Figure 4a-d), which point out that the an enlarger interlayer distance of carbonaceous material produces, the reason for the large interlayer distance is the formation of a large number of oxygen-containing functional groups such as hydroxyl, epoxy and carboxyl groups.79 Meanwhile, it is noteworthy that the 22.1o and 26.7o (002) peaks from ECM-800 to ECM-1000 become intense and narrow (Figure 4c-d), indicating the improved degree of graphitization.

Figure 5. Raman spectra of ECMs at 100-2000 cm-1 (a) and 2000-3500 cm−1 (b). Figure 5 shows typical 532 nm Raman spectra observed for the four samples. Evidently, the two most intense features centered at 1340 and 1580 cm-1. Among them, the disorder and a significant number of defects induced the well-defined D-band at ∼1340 cm-1, the doubly degenerate zone center E2g mode cause the forming of the G-band (∼1598 cm-1), respectively (Figure 5a). It's worth noticing that the ID/IG ratios (peak area ratio) for the ECM-400, ECM-600, ECM-800 and ECM-1000 are 1.86, 1.19, 1.07 and 0.89, respectively, these results demonstrate a low degree of graphitization.80, 81 However, the ID/IG ratios (peak area ratio) are gradually decreased with increased annealing temperature; This is mainly due to the improvement in the degree of graphitization for the ECMs after high temperature annealing, which corresponding to the results of XRD diffraction patterns. Also, the Raman spectras of the 2D peak of ECMs with different annealing temperature are shown clearly in Figure 5b. Obviously, a broad 2D band was



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observed at ∼2760 cm-1, revealing that typical Raman characteristics for few-layer graphene sheets with abundant defects, which should be favorable to the improvement of its electrochemical properties. 82

Figure 6. FT-IR spectra of ECM-400 (a), ECM-600 (b), ECM-800 (c) and ECM-1000 (d). Although this mechanism is not fully understood, it is quite possible that wheat straw cells mainly produce the nano-nipples by controlled-releasing multicomponent gas due to the chemical reactions that occurred during annealing.83 Thus, the surface chemical compositions of the ECMs with different annealing temperatures were further studied and investigated by FT-IR spectra. Figure 6 shows FT-IR spectra of ECMs, which have similar absorption bands, and the ECMs exhibit typical absorption features of the peaks at 2920, 2850, 1400 and 1363 cm-1 (Figure 6a-d) are attributed to -CH2 groups84. The peak at 1600 cm-1 is assigned to the C=C and C=O stretching vibration,85,

86

which indicates that the different annealing

temperature does not change the containing functional groups of ECMs obviously. However, the annealing process was carried out under anaerobic conditions, and the thermal cracking process leads to different changes in these peaks. The annealing produces volatile organic compounds, which are dehydrogenated & dihydroxylation of carbohydrates and then condensed to increase the aromatic structure content. 87 Such as the O-H groups, ECM-400 exhibits typical absorption features of the peak at 3331 cm-1 assigns to the OH groups stretching vibration, while increasing the annealing temperature result in obviously red shift (The band at 3420 cm-1), and the absorption band of -OH decreased slightly in intensity (peak area). This may 14 ACS Paragon Plus Environment

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indicate that a small amount of O-H groups on the cellulose are cracking under 400oC annealing process, but much more cellulose is cracking under 600-1000oC annealing process. Interestingly, as the annealing temperature increased (400 °C → 1000 °C), no obvious absorption bands shift for -CH2, C=C and C=O groups can be seen in ECMs, indicating that the biochars were formed under low carbonization temperature (400 °C), and the surface of biochars prepared by chemical redox processing unavoidably bonded with these groups. Whereas, as the annealing temperature is elevated, the intensity of absorption bands (peak area) for these groups (-CH2, C=C and C=O) decreased slightly, these results suggest that the annealing process with different temperature led to the continuous and different degrees of cracking of the biochars. Consequently, all discussed above indicate that the relatively low annealing temperature is for construction of microstructures (400 oC) while relatively high annealing temperature (600 °C and 800 °C) produces nanostructures since the latter requires dewatering and degasification process to break many more O-H, CH, C=C and C=O bonding to produce the water vapor, methane, carbon monoxide and carbon dioxide slowly for the nano-nipples formation. The chemical reactions of the pyrolysis process are expressed in Figure 7a and b. However, When heated to 1000 °C, the nano-nipples on the surface of ECM-1000 were completely disappeared (Figure 7c), which is possible because the formation of nano-nipples were terminated during the dewatering and degasification process with high annealing temperature (1000 °C). Furthermore, as the annealing temperature is elevated (400 → 1000 °C), The band at 1116 cm-1 has been observed gradually, which symbolizes formation of C-O-C groups. These results clearly indicate that the turbostratic crystallites formed from pyrogenic carbon at high temperature ranges.87, 88 Combining with all the characterization results above, which indicates some brick-like lumps of ECMs could be rough and porous.



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Figure 7. Schematic illustration of the formation of nano-nipples and chemical reactions by different annealing temperatures. Generally, compared with low temperature biochar, high temperature biochar has higher degree of carbonation, higher content of aromatic substances, lower content of aliphatic substances, higher aromaticity and lower polarity.89 Thus, the chemical composition of the obtained ECMs (ECM-600 to ECM-1000) were investigated by XPS. Typical XPS survey scans of the ECMs (ECM-600 to ECM-1000) are shown in Figure S2 revealing that the samples contains C and O as the main elements. Two fitting peaks are shown in C1s spectrum, one of the peak (284.7 eV) is assigned to sp2-bonded carbon and another peak (285.5 eV) for sp3-bonded carbon, these results indicates the constitution of abundant C=C π-conjugated systems90 (Figure 8).


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Figure 8. C1s spectra of ECM-600 (a), ECM-800 (b) and ECM-1000 (c). Whereas, during the different annealing temperature process (Figure 8a-c), also note that, the already existing peaks of C1s spectrums does not shift. In addition, C-O functional groups (such as C=O, C-O and O-C=O) in the higher-binding energy region of C1s spectrums can hardly be observed.91 However, two adsorbed oxygen species with O1s 531.1 and 532.5 eV can be formed on the ECMs surface (Figure S3). By reference to the XPS C1s spectra, which indicates that the former oxygen species is the dissociative atomic oxygen, and the latter species is constituted by the oxygen atoms association. Moreover, the O1s spectrums of atomic oxygen is in good agreement with literature, 92, 93 and also with DFT-calculations.94 Thus, we could infer that many nano-nipples of the surface of ECM-600 and ECM-800 formed the trenches and trapped air inside to form a lot of air cushions, which are composed of nanostructures. Because air itself is also a key part of the surface structure, the ECM-800/PDMS coating surface under the droplet can be seen as a composite surface bound to air. As a result, water droplets come into contact with air of rough surface, causing superhydrophobicity. Crucially, combined with the results of WCA, EA, XRD, Raman and FT-IR, implied that the oxygen functional groups may be entrapped inside the ECMs, and almost no oxygen-containing functional groups on the near-surfaces of ECMs. We assume that the difference between the surface structure and the inner structure was most likely due to a physical reaction rather than a chemical reaction. The superhydrophobic ECM-600 and ECM-800 were prepared by using treated wheat straw. Micro-nano hierarchical structure could only be obtained with precisely controlled architectures including the annealing temperature, the program tempering and retention time. In addition, the surface of the ECM800 was easily affected by the environment, which was caused by the special structure of the material surface.95 As is known, the hydrophility of graphene-like materials was mainly due to the formation of hydrogen bonding between the oxygen functional groups of the membrane surface and water droplets.96 Also, it has been reported that the oxygen functional groups prevent charge from passing through the electrical double layer by means of a thin graphene film covering the electrode surface.97 Therefore, this is



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also another reason that ECM-800/PDMS coating possess both superhydrophobicity and good electrical performance, so an schematic illustration of ECM-800 structures is given in Figure 9.

Figure 9. Pictorial description of ECM-800. To investigate the electrical performance, the four-point probe technique was used to measure the electrical conductivity of ECMs/PDMS coatings on the glass substrate. The ECM-400/PDMS and ECM600/PDMS show lower conductivity of ∼7.2×10-5 and ∼8.0×10-2 S m-1, respectively. This phenomenon is likely due to their relatively low degree of graphitization. Whereas, more remarkable, the average conductivity of the obtained ECM-800/PDMS is 240 S m-1. When annealing temperature at 1000 oC for 1 h in N2, the average conductivity of ECM-1000/PDMS rise to to 450 S m-1, which is almost equivalent to the electrical conductivity of annealed graphene graphite film prepared by reduced graphite oxide at high temperature treatment and reduction under H2.98-100 These results are consistent with XRD and Raman characterizations. Furthermore, that ECM-800/PDMS coating is used as part of the conductor and the voltage of 1.5 V battery produce enough current (flowing electrons) to light a bulb readily (Figure S4).


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Figure 10. Digital photos of ECM-800/PDMS composites coated (a) glass substrate, (b) aluminum substrate, (c) copper substrate and (d) iron substrate. The insets are digital profile of the WCAs. In order to reveal the interactions between ECMs/PDMS coatings and many different kinds of substrates, we selected the ECM-800 as a model matrix with nano-nipple diameter ranging 25-50 nm and PDMS prepolymer as adhesive to examine the superhydrophobic and self-cleaning property of the ECM800/PDMS coating. We performed water contact-angle measurements on ECM-800/PDMS composites coated on a variety of substrates (Figure 10a-d), in which glass slide, aluminium sheet, copper sheet, and iron sheet were selected as substrates. To our delight, using one-step dip-coating process that dipped ECM800/PDMS particles tetrahydrofuran suspensions onto four kinds of substrates, All prepared superhydrophobic coatings still have a WCA higher than 160o and SA lower than 5o. As the results show, the dip-coating process can enhance the superhydrophobic surface of different substrates effectively. The concept of "super hydrophobic + adhesive" can be applied to large-scale industrial applications simply, flexibly and steadily. Furthermore, the coated surfaces exhibit water-proofing and mechanical properties from the artificial rain test (exhibited in the Schematic Video-2 from Supporting Information), in this test, the mixture of deionized water and methylene blue was dripped down to the surface of ECM-800/PDMSglass with a 15° tilt and a 1.5 cm distance. The volume of the liquid per drop was about 0.05 mL, and the 19 ACS Paragon Plus Environment


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rate of dripping was about 1.5 mL/min, the WCA data showed that the surface of coating was not affected obviously after 3 h of the dripping process. These results proved that the coating had good self-cleaning and adhesion properties when used on many different substrates, especially for hard materials, such as those which may be used in making radar-absorbing material72 and stealth aircraft material.101

Figure 11. SEM images of the prepared ECM-800 heated at 300 °C for 3 h (a), kept under UV-radiation for 24 h (b), immersed in strong acid (c) and base (d) solution for 24 h. The insets are the corresponding wetting experiments. Harsh environments usually involve high temperatures and ultraviolet aging.102 Besides, harsh environments also involve conditions that may cause corrosion and further leading to serious degradation of materials by strong acid and alkaline.103, 104 Generally, the development and exploitation of new materials have attracted considerable attention, one reason for this due to its potential practical applications in harsh environments. Thus, in this work, our expectation is that ECM-800 not only possess superhydrophobic and conductive property but also exhibit superior property of resistant to harsh environment. For that reason, several important robustness tests were performed to assess harsh environment resistance of ECM-800. Thermal robustness is the main challenge for superhydrophobic materials and was thus considered first. Firstly, ECM-800 was treated by heating under air at 300 oC for 3 h in the tube furnace (Figure 11a). 20 ACS Paragon Plus Environment

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Compared with the untreated one by high magnification SEM images (Figure 3c and c1), it could not be observed the changes in the surface structure of ECM-800, and lots of nano-nipples were still densely arranged on the surface of the graphene sheets, indicating that ECM-800 have the higher thermal stability under air. In addition, UV aging-resistant is also an important property of superhydrophobic material as well as the driving force for practical application. ECM-800 was kept under UV-radiation at 950 μW/cm2 for 12 h and following 1200 μW/cm2 for 12 h. Interestingly, with the enhance of UV radiation strength (from 950 to 1200 μW/cm2), a spot of small size nano-nipples have became larger after aged under UVradiation for 24 h but not a significant change in the surface structure were observed (Figure 11b). These results show that using UV-radiation only slightly affects nanostructure, and ECM-800 has good resistance to the UV aging. Furthermore, resistance of ECM-800 to strong acid and alkaline was tested, we used aqueous H2SO4 (pH ≈ 1) ̶ a strongly acidic and aqueous KOH solution (pH ≈ 14). Although this extreme severity of chemical corrosion is not common in practice, establishing the chemical robustness of materials is a meaningful approach. ECM-800 was soaked in sulfuric acid solution (pH ≈ 1) or sodium hydroxide solution (pH ≈ 14) for 24h, washed with water, following, dried at 80 oC in a vacuum. To our delight, after treatment with sulfuric acid solution or sodium hydroxide solution, it still not could be found that the changes in ECM-800 surface structure (Figure 11c and d), the number of nano-nipples per unit area on the surface of ECM-800-a (treated with aqueous H2SO4) was almost equal to that on the surface of ECM-800b (treated with KOH solution), the diameter distribution of which is indicated in (Figure S5). In consideration of the superhydrophobicity of four types of treated (heat/UV-aging/acid/alkaline-resistant) ECM-800, PDMS was then added and formed ECM-800/PDMS coatings on glass substrate. In the following, the superhydrophobicity of four kinds of obtained ECM-800/PDMS coatings was measured by typical water resistance test. Water droplets can still form a small ball on glass slides which coated with ECM-800/PDMS composites. After treatment with heating, UV aging, strong acid and alkaline, the corresponding changes in the WCA was 160°, 158°, 160° and 159°, respectively. Average nano-nipple diameters of four treated samples discussed above were 44.67 nm, 50.21 nm, 42.52 nm and 53 nm, average



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interval of nano-nipples of samples were 107.44 nm, 114.25 nm, 105.6 nm and 116.37nm, respectively, which indicated that the surface structure and morphology were completely unaffected. CONCLUSIONS In summary, we have developed a mosquito’s compound eyes bioinspired superhydrophobic material via an effective process to convert rural waste wheat straw into high value-added carbon nanomaterials with unique superhydrophobic property as well as the electrical conductivity property. Besides, no complex and expensive external conditions are required in the whole preparation process. Characterization of a series of ECMs reveals that the ECM-800 combination of micro/nano hierarchical structures, the possible mechanism of nano-nipples formation and the relationship between the surface and inner structure of ECMs. To further explore the and potential applications of the ECM-800, a sustainable, low-cost, conductive and eco-friendly coating with superhydrophobicity was prepared with the ECM-800/PDMS solution by dip-coating of the different substrates, then heat curing was carried out. Also, a robust ECM800 as a kind of nanocarbon material that sustained structural stability under a different of harsh environments, which included many resistance properties, such as heat resistance, UV aging, and sustaining exposure to strong acid/alkaline corrosion. In addition, good mechanical robustness and water-repellency of the ECM-800/PDMS coating were established by the artificial rain tests. To our knowledge this is the first case of the simplest and economic route for preparation of superhydrophobic and conductive carbon nanomaterials with micro/nano hierarchical structures. Perhaps more significantly, we are now developed a approach that could both addresses a serious environmental problem by converting agricultural residue in to nanocarbon materials with high additional value and could extend a variety of practical applications of conductive superhydrophobic nanocarbon materials in real life. ASSOCIATED CONTENT Supporting Information Figures S1-S5 include the following sections : SEM image of ECM-800 and the corresponding PSD histograms, XPS spectra of prepared ECMs, O1s spectra of prepared ECMs, ECM-800/PDMS coating


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as part of the conductor to light a bulb, SEM image of the ECM-800 treated with aqueous H2SO4 or KOH solution. Schematic Video-1 and Schematic Video-2 show the hydrophobic property of the ECMs/PDMS and the artificial rain test, respectively. AUTHOR INFORMATION Corresponding Author * Email: [email protected] or [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (no.51563022) and the Fundamental Research Funds for the Central Universities (1001050227), the Scientific Research Foundation of Northwest University for Nationalities (xbmuyjrc 201705). REFERENCES (1) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Bioinspired surfaces with special wettability. Acc. Chem. Res. 2005, 38 (8), 644-652. (2) Li, X. M.; Reinhoudt, D.; Crego-Calama, M. What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces. Chem. Soc. Rev. 2007, 36 (8), 1350-1368. (3) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Progess in superhydrophobic surface development. Soft matter 2008, 4 (2), 224-240. (4) Jokinen, V.; Kankuri, E.; Hoshian, S.; Franssila, S.; Ras, R. H. Superhydrophobic Blood-Repellent Surfaces. Adv. Mater. 2018, 1705104.



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We demonstrate a facile route: converting waste wheat straw to conductive superhydrophobic nanocarbon materials.


Mosquito's compound eyes as inspiration for fabrication of conductive

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