Developing a facile g-C3N4 coated stainless steel mesh with different superhydrophilic/underwater superoleophobic and superoleophilic behavior for oil-water separation

There is an increasing demand for the development of inexpensive and effective approaches for the oil– water separation due to the global concern in oil industries. The present study was conducted to fabricate graphitic carbon nitride/thermoplastic polyurethane (g-C 3 N 4 /TPU) coated stainless steel meshes via the dip-coating method to investigate the capability of g-C 3 N 4 nanosheets (CN-NS) in oil-water separation. CN-NS was synthesized using the polycondensation process followed by exfoliation with Hummer’s method. We studied the effect of TPU and CN-NS concentration on wettability behavior to obtain an optimized coating solution. CN-NS coated mesh showed superoleophilic/hydrophobic behavior at CN-NS:TPU ratio of 50:50 and it e�ciently passed oil from the emulsi�ed water-in-oil mixture (with 50%wt. oil) with the e�ciency of 99%. The wettability behavior of superhydrophilic/underwater superoleophobic was also obtained at CN-NS:TPU ratio of 80:20 and it was able to separate water from the emulsi�ed water-in-oil mixture with the e�ciency of 79% under gravity. Both �lters were able to separate free water and oil mixtures with �ux and e�ciency of 6114 L.m −2 .h −1 and ~99.99%, respectively. The mechanism of wettability behavior of the coating is mainly related to the functional groups on the edge of g-C 3 N 4 -NS, thus increasing the hydrophilic properties of the surface. In addition, the micro-nano hierarchical structure of the surface coating improves its roughness due to the presence of CN-NS, which is effectively embedded into the hydrophilic TPU. More importantly, commercially available TPU chemical and simple fabrication of g-C 3 N 4 from an inexpensive precursor, make the method reported herein as a signi�cant alternative for large-scale application.


Introduction
Oil and water separation has become a worldwide subject with growing challenges of the production of water in oil emulsions.The presence of water in oil causes problems, such as corrosion and catalyst deactivation, in downstream processes.Therefore, it is necessary to develop an effective method for oilwater separation.Recently, ltration media such as metal meshes, polymer membranes and cotton fabrics based on superoleophilic and superhydrophilic/underwater superoleophobic materials have been applied for highly e cient separation of water-in-oil (W/O) or oil-in-water (O/W) mixtures, respectively ( Various physical and chemical methods of surface modi cation have been used to render ltration media with speci c wetting properties.The prepared ltration media with appropriate pore size and engineering morphologies are capable to separate oil-water emulsions (Zhu, Dudchenko et al. 2017, Su, Liu et al. 2018).Inorganic materials as promising candidate coating materials with appropriate mechanical and thermal properties are utilized in coating materials.Stainless steel mesh (SSM) coated with many nanoparticles such as: silica nanoparticles ( (Song, Huang et al. 2014) and so on have been reported for oil/water and water/oil separation.However, these developed surface coatings are often operationally and chemically intensive and require expensive material or complex precursor fabrication processes.
Although many nanoparticles have been reported for water and oil separation in various studies, carbon nitride nanoparticles have rarely been used for oil-water separation.Simple fabrication process of carbon nitrides, low cost and high commercial availability of their raw materials, give them an advantage over other nanoparticles to be used in the preparation of lters for oil-water separation in industry.
Graphitic carbon nitride (g-C 3 N 4 ) has received tremendous attention as a novel carbon-nitride nanomaterial because of its special physicochemical characteristics, eco-friendly nature, suitable band gap, special electronic band structure and easy functionalization (Du, Zou et al. 2015, Ong, Tan et al. 2016).Other notable potentials of this material include its easy preparation from cheap and available precursors of urea (Liu, Li et al. 2015) or melamine (Fang, Fan et  Therefore, the unique characteristics of g-C 3 N 4 also make it a desirable membranes for advanced separation (Wang, Gao et al. 2020).
Another challenge in the preparation of nanostructure-based lters is the adhesion and stability of nanoparticles to the substrate in water-oil separation cycles, which is also a signi cant issue in this study.The adhesion of thin nanoparticle coatings to the substrate is performed using a combination of modi ed particles and modi ed substrates (Ong, Tan et al. 2016) or with the aid of crosslinking agents such as silane groups, polymeric binders and so on (Yoon 2007, Basnar, Litschauer et al. 2012, da Silva, Lucas et al. 2022).Thermoplastic polyurethane (TPU) is commonly used as adhesive to join different materials in automotive and general use adhesives industries (Oertel 1993, Tien andWei 2001).
Therefore, TPU as a binder to increase the adhesion of nanoparticles to the substrate will be investigate in this work.
Because of the previously mentioned advantages of g-C 3 N 4 -NS and TPU, they might be the favorable material for coating on SSM to render the surface with speci c wetting and durability properties.In this study, for the rst time, g-C 3 N 4 -NS is used to modify the surface in order to separate water and oil.In this work, the surface wettability, surface energy and oil-water separation ability of g-C 3 N 4 /TPU coated SSM was assessed.Ultimately, facile and scalable synthesis approach, commercially available chemicals of TPU and melamine as a precursor of g-C 3 N 4 , the g-C 3 N 4 -NS coated mesh has a great ability for largescale application for oil-water separation.
Material And Methods
Synthesis of Functionalized g-C 3 N 4 nanosheets (CN-NS) was performed using the improved Hummer's method (Hummers Jr and Offeman 1958).Brie y, 5.00 g of b-CN powder was suspended in 150 mL of sulfuric acid under continuous strirring at 333.2 K for 60 min.Then, 7 g of potassium permanganate was moderately added into the mixture while it was placed in an ice bath the solution.Mixture was heated from and stirred at 303.2 K for 60 min.Then, 1500mL deionized water was slowly poured into the mixture within 50 min.Finally, 5% hydrogen peroxide solution was dropped into the suspension to turn its color to milky.The precipitated CN-NS was washed by HCl solution (2%) several times, followed by washing with water and drying in vacuum oven at 303.15 K overnight.

Coating procedure
Stainless steel mesh was used as substrates in this study.for the preparation of coating materials, TPU (4 wt.%) as a binder was dissolved into DMF, and then CN-NS was added into the DMF solution and ultrasonicated for 3h under 100 W power to obtain CN-NS dispersions in DMF with different mass ratios of CN-NS and TPU (ratio of CN/TPU of 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:2, 2:1 and 1:0 which characterized by A1, A2, A3, A4, A5, A6, A7, A8, A9 and A10, respectively).The prepared solution was coated on the stainless steel mesh substrates using dip-coater with the speed of 3 cm/min.Then, the samples were dried at ambient temperature to let the solvent evaporate completely.The coating procedure was repeated twice again.In this way, The CN-TPU composite lm on stainless steel mesh was prepared and used for further testing.Fig. 1 shows a schematic diagram of the fabrication process of CN-TPU composite lm.

Emulsion preparation
Decalin (CAS Ref.-No.493-01-6, surface tension of 31.5 mN/m) as an alternative model for crude oil (20 mL), which mixed with 2 wt.% of SPAN80 (HLB=4.3)as an emulsi er, and deionized water (20 mL), which was colored with blue dye, were mixed and treated for 3h under stirring rate of 2500rpm to form a water/oil emulsion.
The as-prepared lter was xed between two glass tubes with diameter of 25 mm.The water/oil emulsion was poured onto the as-prepared mesh.The separation was achieved by the driving force of gravity.

Characterization
Fourier transform infrared (FTIR, Brucker Tensor27 model) spectroscopy and X-ray powder diffraction (XRD, Inel EQUINOX3000 model) analyses performed to identify functional groups and crystalline structure of b-CN and CN-NS.The surface morphology of CN-coated stainless steel mesh was observed using eld emission scanning electron microscopy (FESEM, model MIRA3TESCAN-XMU) equipped with energy dispersive spectroscopy (EDS) to obtain the elemental compositions of the surface of CN-coated stainless steel mesh.SEM images were also obtained with a eld emission scanning electron microscope to examine the surface morphology.
Atomic force microscope (AFM, BRUKER, model ICON) in direct contact mode was used to evaluate topographic studies such as roughness of the lter surface.Carbon, nitrogen and hydrogen contents of resultant material, the elemental analyses (CHN analysis, Costech ECS 4010) was carried out.Oil and water contact angle (OCA and WCA) and underwater oil contact angles as the wetting properties of the prepared lter was measured using a video optical contact angle system at ambient temperature.For this purpose, 5µL water droplet was dropped onto the coated substrate and microscopic images of these droplets were analyzed using ImageJ (National Institutes of Health, USA) by three times.Size distribution of dispersed phase in emulsion was measured using dynamic light scattering (DLS) (Cordouan Tech, model VASCO2).The separation e ciency of the prepared lter was calculated using equation ( 1 Where C 0 and C p are the water or oil content of permeate and retentate.

Results And Discussion
The mechanism of the Hummer's method for the fabrication of exfoliated CN-NS is explained as follow: b-CN is protonated by H 2 SO 4 in the beginning of the oxidization process.Then, the edge of the carbon nitride layers is oxidized by KMnO 4 .
Powder X-ray Diffraction (XRD) measurements of b-CN and CN-NS (exfoliated g-C 3 N 4 using Hummer's method) was used to study changes of crystalline structures in the exfoliation process (Fig. 2a).The diffraction peak of b-CN at about 2 =13.1° can be seen in both b-CN and CN-NS.The corresponding angle is indexed as the (100) plane of b-CN caused by the intra-planar structural packing.The strong peak at 2 =27.8179° (d-spacing = 0.3207 nm) is indexed as (002) plane arising from the inter-layer diffraction in graphite-like carbon nitride structures (Niu, Zhang et al. 2012).The peak of Hummer's g-C 3 N 4 had moved to the higher angles in comparison with the peaks of b-CN.The diffraction peak (002) of the product is quite in accordance with the diffraction peak (002) and it had move to higher angle of 2 =28.2422° (d-spacing=0.3944nm) in Hummer's g-C 3 N 4 , which causes increasing in the interlayer distance based on Bragg's law.Distance between g-C 3 N 4 sheets increased by 0.7 ∘ A after Hammer's oxidation method.Thus, oxidization exfoliation process showed the desire performance in the protonation process and creates the positive charges on each single layer of graphene-like carbon nitride, which indicates that the spacing enlargement of (002) plane were aroused by the protonation.Moreover, the partly intercalation performance of H 2 SO 4 could induce the active site in the subsequent oxidization exfoliation process (Chen, Li et al. 2015).Thus Hummer's method enables nanoparticles to form a good structure and lm in the polymer structure or in combination with polymer binders such as TPU, which we will examine this.
The crystal size of g-C 3 N 4 , calculated from the Scherrer equation that relates the crystallite size to the broadening of a peak in a diffraction pattern, decreased from 16.9 to 8.6 nm with Hummer's oxidation method.
The surface wettability for water and/or oil in uences by the physical morphology and surface chemistry.Thus, to investigate the surface chemistry and morphology surface chemistry of the modi ed meshes, FESEM, FTIR, and AFM measurements were conducted.FT-IR was employed to get more insight of the functional groups in CN_NS (Fig. 2b).As shown in Fig. 2b, the characteristic absorption peaks of the tri-striazine ring and CN heterocycles are between 1200 and 1600 cm −1 .Peak at 801 cm −1 indicates the similar covalent bonds between carbon atoms and nitride atoms.The oxidation reaction in Hummer's method is con rmed with the more intense peak at 1071 cm-1 attributed to C=O stretching vibrations compared to that at bulk g-C 3 N 4 .Adsorption peak at 1678 cm-1 arising from carboxyl and carbonyl are stronger in the spectrum of CN_NS compared with b-CN.The broad peak arouses at around 3100 and 3600 cm −1 in the spectrum of CN_NS assigns the NH moieties, as well as to the adsorbed hydroxyl groups of H 2 O.
To investigate the nitrogen content of resultant b-CN and CN-NS, the elemental analyses of carbon, nitrogen and hydrogen are shown in Table 1.The C/N ratio increases from 0.634 for the bulk g-C 3 N 4 to 0.701 for the CN-NS one, which is due to nitrogen loss.In the exfoliation process, oxygen atoms substitutes by oxidized nitrogen atoms and consequently results in nitrogen loss in CN-NS sample.The C/N ratio for both b-CN and CN-NS was lower than 0.75 for the ideal g-C 3 N 4 crystal which justi ed with reduction of the hydrogen contents in polymerization time.The residual hydrogen atoms, in the incomplete polycondensation process, formed 2C-NH and C-NH2 bonds with bonding to the edges of g-C 3 N 4 sheet.
Additionally, Brunauer-Emmett-Teller (BET) method was carried out to measure the speci c surface area (SSA), pore volume and pore size of the as-prepared bulk g-C 3 N 4 and Hummer's g-C 3 N 4 (Table 1).Fig. 3 (a-c) represents the nitrogen adsorption-desorption isotherms and pore size distribution of bulk and Hummer's g-C 3 N 4 .Pore radius and SSA were determined by the BJH method (Fig. c,d).It can be clearly seen that both samples showed a typical type III isotherm with a H3 hysteresis loop according to the IUPAC classi cation.Using Hummer's exfoliation process, the SSA and pore volume of g-C 3 N 4 increased from 12.023 m 2 .g - and 0.06 cm 3 .g - to 114.4 m 2 .g - and 0.74 cm 3 .g - , respectively.The pore size of the g-C 3 N 4 decreased to 15.218 nm after Hummer's exfoliation process.These results strongly con rm that in Hummer's method, bulk-g-C3N4 has been successfully exfoliated into nanosheets and their porosity has increased.Large amount of oxygenous functional groups has given special importance to CN-NS for their application.For instant, CN-NS with oxygenous functional groups can apply as a reactant for graft copolymerization.This issue is especially important in this study because it has been tried to coat carbon nitrides on a stainless steel mesh surface with the aid of a polymer binder to make strong bonding of the lm to the substrate.
In order to investigate the uniform lm formation of CN-NS and TPU binder on stainless steel mesh, the images taken from FESEM are shown in the Fig. 4. It is obvious from FESEM images (Fig. 4a-c) that there is no area on the wires of mesh that is not covered with the coating material.This con rms lm formation and the successful assembly of CN-TPU on the mesh wires.In other words, there is no signi cant re-stacking of CN-NS in the composite and CN-NS is well dispersed in the polymer matrix.As can be observed, CN-NS creates the nano-micrometer scale roughness and formed many cavities because of its low surface energy, which is discussed later.The structure of the micro-nanometer roughness operates in such a way that could result in trapping air in oil and water droplets and consequently causes an increment in the contact angle of the coating layer with the solid surface (Facio and Mosquera 2013).Moreover, ower-like micrometer-scale aggregates from can be discovered on the surface of coated mesh (Fig. 4a-c).There are also smaller carbon nitride nanoparticles with an average diameter of 35 nm on the surface of coated mesh.
EDS spectra in Fig. 4d, which illustrates the chemical composition of CN-TPU lm coated on the surface of substrate, showed that the wires are enriched with carbon, nitrogen and oxygen on the surface of the mesh and con rms the successful synthesis of CN-TPU composite.The homogenous distribution of these elements on the mesh wires also con rms by the elemental mapping results, shown in Fig. 4d.

Contact angle and interfacial tension measurements
Hydrophilicity and oleophilicity, indicating the tendency of the surface to water or oil, are decisive factors for a surface to apply for oil-water separation.These wettability factors were determined by water contact angle (WCA) in air and oil contact angle (OCA).Thus, at rst step we examined the effect of g-C 3 N 4 and TPU concentration on surface wettability (A1-A10, Table 2).Decalin as a sample oil was used to measure OCA.Unmodi ed bulk carbon nitride (bulk g-C 3 N 4 ) showed hydrophobic behavior with a WCA and underwater OCA of 98.6° and 135.55°, respectively, which by post treatment using Hammer's method, its wettability properties changed signi cantly towards hydrophilicity and superhydrophilivity from sample 1 to 10.
The results indicates that the WCA increases with increasing the concentration of g-C 3 N 4 -NS, but the OCA remained constant at 0° for all samples.As indicated in Fig. 5, WCA for lter coated with TPU (A10) was 88° and it will be gradually decrease with the addition of CN-NS.Hydrophilic TPU was used to make the coating more adhesive.However, all lters, including lter A1, show superoleophilic behavior in air with OCA of 0°.
Although hydrophilic behavior appeared for TPU coated mesh, WCA has been increased with addition of CN-NS.Increasing the concentration of CN nanoparticles in coating composition of A8 to A9 e ciently decreased the WCA from 32° to 0°.Indeed with increasing the CN-NS composite on coating materials, it can be seen that hydrophilicity gradually maximized for A1 sample with the CN/TPU ratio of 9:1, which con rms the superhydrophilic behavior.For A1 lter, underwater OCA was examined and results show that A1 has underwater oleophobic behavior with OCA of 155° (Fig. 6).Therefore, A1 is a superhydrophilic/underwater superoleophobic, while the other lters are superoleophilic/hydrophilic in air.Phase separation occurs in coating composition and carbon nitride CN-NS comes to the surface of the lter and plays a role in controlling the surface wettability.This improvement in properties caused by the formation of hydrogen bonds between the soft segments of the TPU and the hydroxyl groups on the CN-NS surface (Nunes, Fonseca et al. 2000, Nunes, Pereira et al. 2001).Carbon nitride nanoparticles are inherently hydrophilic with functional groups on their edge, thus increasing the hydrophilic properties of the surface.In addition, with the arrival of nanoparticles on the lter surface, in turn, the surface roughness increases according to the Cassie−Baxter state and e ciently causes more hydrophilic surface (Facio and Mosquera 2013).
AFM analysis was carried out to deals with the effect of CN nanoparticles on the surface roughness.Fig. 7 indicates the AFM micrographs and the surface topography of the CN-NS coated-mesh of A5 and A1.AFM results reveal that the Root Mean Square (RMS) roughness (Rq) is 0.14 µm and 0.154 µm for A5 and A1 (Fig. 7a,b), respectively.Therefore, the surface roughness of A1 is higher than of A5.Increasing the surface roughness has led to the conversion of the surface from hydrophilic (A5) to superhydrophilic (A1).Roughness increases the wettability of the hydrophilic CN-NS coated surface.Furthermore, the micrographs of AFM results indicate the micro/nanoscale roughness that is the result of packing the CN-NS together.
There is a good correlation between the wettability and surface energy.Therefore, the interface energy, which composed of non-dispersive or polar component (γ p ) and polar component (γ d ), was calculated for CN-NS/TPU (A1 and A5) by Geometric-mean relation (Wu 1982) as follow: Where; γ d SV and γ p SV are the dispersive and polar components of surface energy, and θ denotes the contact angle.Two probe liquids of water and decalin was used to calculate these two components.The values of the surface tension of these liquids and their polar and dispersive components are shown in Table 2.The obtained surface energy of A1 (74.07 mJ/m 2 ) is higher than of water's surface tension (72.8 mJ/m 2 ), While A5 has the surface energy (37.68 mJ/m 2 ) close to the surface tension of decalin (31.5 mJ/m 2 ).This implies that A1 and A5 have more a nity to become wet with water and decalin, respectively.Therefore, lters A1 (with 9:1 ratio of CN:TPU, underwater OCA=155° and WCA=0°) and A5 (with 5:5 ratio of CN:TPU, OCA=0° and WCA=78°), were mounted in the separation setup ,shown in Fig. 8, to investigate their ability to separate the water and oil emulsion.As can be seen in the gure, after pouring the emulsion, decalin passed through lter A5, which had superoleophilic behavior, and effectively separated from emulsion with the e ciency of 99%.Because A5 lter exhibits oil a nity rather than water, it demonstrates the water repellency.Decalin has turned yellow after passing through coated mesh due to presence of oil-soluble Span-80 emulsi er (Fig. 8a).
Pre-wetted lter of A1 with superhydrophilic and underwater superoleophobic behavior separated water from the prepared water-in-oil emulsion with the e ciency of 79% (Fig. 8b).Although this lter has passed oil along with water with some extent, its good performance in separating water is due to its prewetting, which allows it to show its wettability behavior of underwater superoleophobicity.Indeed, high degree of surface roughness leads to trap water and subsequently the transition of Wenzel to Cassie √ √ √ state of underwater oil droplet (Yong, Chen et al. 2017).It should be noted that the emulsion separation is partly due to the small pore size of the stainless steel mesh, which is about 5 µm.
Permeate ux of A5 for separation of decalin from water-in-decalin emulsion is about 50 L.m −2 .h−1 , while this value is lower for separation of water using pre-wetted A1 lter and it measured as 45 L.m −2 .h−1 .This is because the ux of the water-rich permeate in separation of emulsi ed water-oil mixture is limited by the sedimentation velocity of water droplets, which are the dispersed phase.Droplet size before and after separation was measured using DLS measurement and shown in Fig. 8(c,d).The droplet size of the water-in-decalin emulsion feed is distributed from 100 nm to about 1.2 µm, while the size distribution of water droplets has been increased to 500 nm to 10 µm in permeate, which showed the coalescence of water droplets once contacting with the coatings of the mesh.Therefore, coalescence separation is dominated in case the size of the emulsion droplets is smaller than the average diameter of pores of TPU/g-C 3 N 4 coated-mesh (Dang, Liu et al. 2016).This mechanism happens by the coalescence of droplets when contacting with the coatings of the mesh as shown in Fig. 8e.It is noted that the size distribution of dispersed phase in permeate justi ed that water-in-decalin emulsion could not be completely demulsi ed.
Generally, the separation mechanism of two lters with different wettability properties is justi ed as follow: 1) hierarchical nano-micrometer scale roughness structures formed by the CN-NS on the surface; 2) hydrophilic features and surface energy reduction using the high amount of hydrophilic CN-NS and 3) SSM with a smaller pore size of about 5 µm. it seems that the presence of more polar groups (OH and COOH), especially for A1 lter with higher concentration of CN-NS, combined with the special morphological structures observed by FESEM and AFM are responsible for the different surface wettability.
That being said, both A1 and A5 lters have a good ability to separate oil and water emulsions.It should be noted that since the lters are made of inexpensive carbon nitride materials and TPU polymeric binder, as well as do not require complex preparation methods, they can be a good alternative to other lters for commercialization.

Conclusion
In conclusion, g-C 3 N 4 /TPU-coated stainless steel meshes were fabricated via dip-coating method to investigate the capability of graphitic carbon nitride (g-C 3 N 4 ) in oil-water separation.The uniform coating of g-C 3 N 4 /TPU endowed the mesh with superhydrophilicity/underwater superoleophobicity and superoleophilicity at the g-C 3 N 4 :TPU ratio of 9:1 and 1:1, respectively.The rst lter with superhydrophilic/underwater superoleophobic behavior permeated water from emulsi ed wate-in-oil mixture with 79% e ciency.The later one with superoleophilic behavior could selectively separate oil from emulsion driven by gravity with high water rejection (∼99.00%).Both lters were able to separate free water and oil mixtures with ux and e ciency of 6114 L.m −2 .h−1 and ~99.99%, respectively.Surface energy of both lters was calculated and implied that A1 with superhydrophilic/underwater superoleophobic behavior and A5 with superoleophilic behavior have more a nity to become wet with water and decalin, respectively.In addition, surface roughness, functional groups on the surface and stainless steel mesh with a smaller average pore size of about 5µm are responsible for e cient separation of water-in-oil emulsion.
Thus, we envision that this g-C 3 N 4 /TPU coated mesh has potentials for applications in the industrial scale for oil-water separation.Because this technology employs commercially available polymeric binder, a very simple and fast coating process, as well as facile preparation process of g-C 3 N 4 from available and inexpensive precursor, it has great potential for large-scale applications for oil-water separation.
Yang, Pi et al. 2010), TiO 2 (Yu, Chen et al. 2014, Agano, Villanueva et al. 2021), ZnO nanowires (Tian, Zhang et al. 2011, Tian, Zhang et al. 2012), Cu micro-akes al. 2015, Li, Lian et al. 2016).g-C 3 N 4 has a layered structure with weak intermolecular van der Waals forces or electrostatic forces (Niu, Zhang et al. 2012, Xie, Zhang et al. 2013).The structure of g-C 3 N 4 composes of tri-s-triazine and triazine units in 6membered rings with sp 2 -hybridized C and N atoms, which form an aromatic p-conjugated system.The unique stable structure of g-C 3 N 4 makes it durable under severely acidic and alkaline conditions (Wang, Maiyalagan et al. 2012, Martin, Reardon et al. 2014).In addition, The nonporous nature of g-C 3 N 4 accompanied by an extremely low speci c surface area and the stacking of thick g-C 3 N 4 layered structures hinders its application in various elds (Martin, Reardon et al. 2014, Ouyang, Xu et al. 2022).Graphene-like carbon nitride nanosheets (CN-NS) with 2D morphology and large speci c surface area has shown a potential performance in photo-catalysis, energy storage and bio-sensing (Wang, Blechert et al. 2012, Liu, Liu et al. 2015, Ong, Tan et al. 2016, Truong, Bae et al. 2021).

Figure 2 a
Figure 2

Figure 4 Field
Figure 4

Figure 5 An
Figure 5

Table 1 :
CHN analysis of g-C 3 N 4 bulk and modi ed g-C 3 N 4 with Hummer's method

Table 1
The composition of for different synthesized coatings on SSM

Table 2
Data of surface tension and its components of the probe liquids used in this work component γ p (mJ/m 2 ) γ d (mJ/m 2 ) γ (mJ/m 2 )