Surface Modification of C3N4 through Oxygen-Plasma Treatment: A

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Surface Modification of C3N4 through Oxygen Plasma Treatment: A Simple Way towards Excellent hydrophilicity Xiuming Bu, Jipeng Li, Siwei Yang, Jing Sun, Yuan Deng, Yucheng Yang, Gang Wang, Zheng Peng, Peng He, Xianying Wang, Guqiao Ding, Junhe Yang, and Xiaoming Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10516 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 21, 2016

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ACS Applied Materials & Interfaces

Surface Modification of C3N4 through Oxygen Plasma Treatment: A Simple Way toward Excellent Hydrophilicity Xiuming Bu,+,a,b Jipeng Li,+,c Siwei Yang,+,b,e Jing Sun,b,e Yuan Deng,c Yucheng Yang,b,e,f Gang Wang,d Zheng Peng,b,e Peng He,b,e Xianying Wang,a,* Guqiao Ding,b,e,* Junhe Yang,a and Xiaoming Xieb,e,f

a School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai, 200093(China) b State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai, 200500 (China) c Department of Opthalmology, Shanghai Ninth People’s Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, 20011(China) d Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo, Zhejiang, 315211(China) e CAS Center for Excellence in Superconducting Electronics (CENSE), Shanghai 200050, China f School of Physical Science and Technology, Shanghai Tech University, Shanghai, 200031(China) KEYWORDS: C3N4, Oxygen Plasma Treatment, Excellent Hydrophilicity, Tissue Repair, Nitrogen-containing Carbon Materials

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ABSTRACT: We developed a universal method to prepare hydrophilic carbon nitrogen (C3N4) nanosheets. By treating C3N4 nanosheets with oxygen plasma, hydroxylamine groups (N-OH) with intense protonation could be introduced on the surface, moreover, the content of N-OH groups increased linearly with the oxygen plasma treating time. Thanks to the excellent hydrophilicity, uniformly dispersed C3N4 solution were prepared, which was further translated into C3N4 paper by simple vacuum filtration. Pure C3N4 paper with good stability, excellent hydrophilicity and biocompatibility were proved to have excellent performance in tissue repair. Further research demonstrated that oxygen plasma treating method can also introduce N-OH groups into other nitrogen-containing carbon materials (NCMs) such as N doped graphene, N doped carbon nanotube and C2N, which offers a new sight on the surface modification and functionalization of these carbon nanomaterials.

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INTRODUCTION Carbon nitrogen (C3N4), as a kind of metal-free material, has attracted much attention in recent years because of its wide applications.1-4 Among all of these applications, the hydrophilic property of C3N4 should usually be taken into consideration. For example, photocatalytic reaction needs the effective interaction between target molecules/ions (i.e., water and organic pollutants) and C3N4 photocatalysts in the solution. Hydrophilic performance is also one of the key factors affecting the biological conformation of protein to avoid clotting because of the irreversible adsorption of the protein on the surface of biological material.5, 6 However, the strong van der Walls attractions (π-π stacking) between sp2 carbon atoms make C3N4 insoluble, hydrophobic and seriously agglomerated in most of solvents. 7-9 To solve this problem, many efforts have been done to introduce steric or/and electrostatic repulsion among adjacent layers to achieve stable dispersion of C3N4 with high concentration.10 Current approaches can be categorized into two strategies: (1) modification with hydrophilic groups11 and (2) auxiliary dispersing (introduction of stabilizers, such as surfactants or polymers).12Traditional molecular designing for modification aimed at oxygen-containing groups on carbon atoms (such as: C-OH, C-O-C and -COOH). Zhang et al. demonstrated that stable C3N4 colloidal suspension can be obtained via chemical protonation of C3N4 solids with strong oxidizing acids.13 However, the dispersive capacity of C3N4 is still unsatisfying. On the other hand, auxiliary dispersing approach can also increase the dispersive capacity of C3N4 to a certain extent, however, it is difficult to remove the stabilizers and hard to be redispersed.14, 15 Thus, by now, it remains a great challenge to increase the dispersity of C3N4. Herein, we developed a new simple yet effective modification technique to prepare excellent hydrophilic C3N4, where oxygen plasma treatment was used to introduce hydroxylamine structure (N-OH) on the surface. Thus the chemical formula can be expressed as C3Nx(N-OH)y 。 Characterization results demonstrate that the contents of N-OH groups on C3Nx(N-OH)y have 3



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linear relationship with the plasma treating time. Due to the intense protonation of N-OH, C3Nx(N-OH)y shows good hydrophilic nature with the zeta potential ca. -96 mV, which is much lower than that of C3N4 with oxygen-containing groups. The C3Nx (N-OH)y aqueous dispersion with high mass concentration shows excellent stability up to 30 days. Due to the excellent dispersing performance, C3Nx(N-OH)y paper was successfully fabricated via the routine filtration technique, which was directly used as the biologically active substrate. The C3Nx(N-OH)y paper with good mechanical properties has great potentials in biological fields such as the regeneration of bone, vessel and skins. This modification approach is also adaptable to some nitrogencontaining carbon materials (NCMs) such as N doped graphene (N-G), N doped carbon nanotube (N-CNT) and C2N.

EXPERIMENTAL SECTION Chemicals All the chemicals were purchased from Aladdin (Shanghai, China) and used without further purification. The water used throughout all experiments was purified using a Millipore system. Synthesis The C3Nx (N-OH)y was obtained by controllable plasma treatment on C3N4 nanosheet, and the detailed synthesizing progress can be found in the supporting materials. Oxygen plasma treating experiments were carried out via a Seren PM613 microwave radical generator. The operating power was kept at 100 kW and the chamber pressure was 200 Torr with the O2 flow rate of 35 cc min-1.The typical treating time was 90 min. 5 g of C3N4 nanosheets were positioned at 3 cm downstream from the plasma source to minimize possible surface damage. At this position the plasma generated ions were energetically relaxed upon arrival at the sample. Characterization methods

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The morphologies of the plasma treated C3N4 nanosheets were characterized by scanning electron microscope (SEM, FEI Quanta FEG) and transmission electron microscopy (TEM, Hitachi H-8100). Atomic Force Microscope (AFM) images were obtained in a Bruker Dimension Icon with a tapping mode. X-ray Diffractometer (Bruker D8 ADVANCE) with a monochromatic source of Cu Kα1 radiation (λ= 0.15405 nm) at 1.6 kW (40 kV, 40 mA) was used to analyze the structures. The compositions were carried out with X-ray photoelectron spectra (XPS, PHI Quantera II system). Fourier transform infrared spectroscopy (FTIR) was recorded using FT-IR NICOLET-4700 in the range 500-4000 cm−1.The ultraviolet-visible (UV-vis, UV5800) spectra were used to monitor the dispersion performance of the samples. Biotic experiments The culture and encapsulation of Rat adiposetissue-derivedstromalcells (rADSCs) were obtained from subcutaneous adipose tissue in the inguinal groove of 6-week-old male Sprague Dawley rats (Shanghai Animal Experimental Center, China) and F12/DMEM (Dulbecco's Modified Eagle Media: Nutrient Mixture F-12) supplemented with 10% FBS (Invitrogen) and 100 unitsmL-1 penicillin-streptomycin (Invitrogen) according to our protocol. Once C3N4 paper was put at the bottom of a 24-well plate, rADSCs were seeded to player with the density of 2×104 cells well-1. Also, 1.0 mL F12/DMEM medium was added into the 24-well plate and the each well was cultured at 37 oC in the atmosphere of 5% CO2. Live/dead assay and CCK-8 A live/dead assay was performed on days 1, 2, 3, 4, 5, 6 and 7. The cell culture medium was removed and the C3Nx(N-OH)y paper was washed with phosphate-buffered saline. 1.0 mL medium (pH=7.2, without serum) containing 0.001 M Mcalcein-AM and 0.001 mM ethidium homodimer1

(Invitrogen) was added to each specimen. The cells were then observed via Leica TCS SP8 mi-

croscope (Leica Microsystems, Germany). Live cells were stained green while dead cells were 5



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red. First of all, the percentage of live cells was measured, which was defined as PLive = NLive/(NLive +NDead), where NLive is the number of live cells and NDead is the number of dead cells in the same image. Three specimens of each material were tested (n = 3). Two randomly selected fields were photographed for each specimen. Then the morphologies and distributions of live cell were also measured. The proliferations of rADSCs were assessed using a cell counting kit-8 (CCK-8) as previously reported. One hundred microliters of CCK-8 solution (Dojindo Molecular Technologies, Inc., Japan) was added to each well and incubated with the cells for 4 h at 37 oC. The absorbance spectra were measured at the wavelength of 450 nm.

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RESULTS AND DISCUSSION Characterization

Figure 1.(a) SEM image of C3Nx(N-OH)y. (b, c) AFM image and height profiles of C3Nx(NOH)y. (d) TEM image of C3Nx(N-OH)y. (e) XRD pattern of C3Nx(N-OH)y (black curve) and C3N4 (red curve). Figures 1 show the morphologies of as-treated C3Nx(N-OH)y nanosheets, where wrinkled C3Nx(N-OH)y sheets with good flexibility can be observed. No distinct defects such as holes, tears etc. can be found, indicating that the physical damage caused by the plasma treatment is negligible. AFM image (Figures 1b and 1c) demonstrates that the thickness of C3Nx (N-OH)y is 7



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2.5-3.0 nm, which is corresponding to 6 and 8 layers.16 Both the thickness and lateral size of C3Nx(N-OH)y have no obvious changes after the plasma treatment (the morphology of pristine C3N4 in Figure S1). Figure 1e shows the XRD pattern of C3N4 and C3Nx(N-OH)y. The (002) peak of C3Nx(N-OH)y is located at 26.5o, shifted to the left direction in comparison with that of C3N4 (27.3o). According to the Bragg’s law, the interlayer distance was calculated to be 0.173 nm and 0.168 nm respectively, indicating that the layers were expanded.17 This may be attributed to the thermal etching of bulk g-C3N4 at high-temperature during the plasma treatment. Water dispersibility of C3Nx(N-OH)y We further examined the excellent hydrophily and water dispersibility of C3Nx (N-OH)y. As shown in Figure 2, the C3Nx (N-OH)y can be easily dispersed in water and diluted to dispersion with specific concentrations. The stability of this dispersion was monitored by UV-vis absorption spectroscopy. Figure 2a presents the typical UV-vis absorption spectra of the pristine C3N4 and C3Nx(N-OH)y dispersion. Compared with untreated C3N4, the absorption peak of C3Nx(N-OH)y of shifted to the left side, which can be due to the formation of the π-π* transition mode. The absorbance at 660 nm was selected as an indicator to reflect the change of the C3N4 content against storing time in the dispersions.18 To study the stability, we prepared five different solutions with the concentration of 30, 35, 40, 45 and 50 mg mL-1 were monitored. From Figure 2b, it is clear that the absorbance decrease gradually with the elongation of the time when the concentration is 50 mg mL-1. The stability tends to be better with the decrease of the concentration. For the 30 mg mL-1 solution, no significant loss in absorbance can be found for the C3Nx(N-OH)y aqueous dispersion at a long storing period of 30 days. The inset of Figure 2b shows the digital photograph of 5.0 mL homogeneous C3Nx(N-OH)y aqueous dispersion with the concentration of 30 mg mL-1 stockpiled for 1 days and 30 days. Tyndall effect exhibits in both solutions when a laser beam is passing through, demonstrating the uniform dispersion of C3Nx(N-OH)y in water and the good stability. 8


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The good dispersing performance of C3Nx(N-OH)y allows it to be applied to prepare macroscopic materials. To demonstrate this, C3Nx(N-OH)y paper was directly fabricated through the routine filtration technique (Figure 2c). Typically, 10.0 mL of 1.0 mg mL-1 C3Nx(N-OH)y aqueous dispersion was filtrated through a membrane with the pore size of 100 nm (AAO, Shangmu Technology, China), followed by drying overnight at 60 oC. The filtration of C3Nx(N-OH)y aqueous dispersion could be completed within 30-45 min, much faster than the filtration of graphene oxide (GO) dispersion with equal volumes. A free-standing and bendable C3Nx(N-OH)y paper with a diameter of ca. 4.0 cm (Figure 2d) was obtained after peeling off the membrane. This C3Nx(N-OH)y paper shows excellent flexibility (Figure 2e) and high mechanical strength (Young’s modulus of 4.1 GPa). Figure 2f and 2g present the surface and cross sectional SEM images of C3Nx(N-OH)y paper. Evidently, the C3Nx(N-OH)y paper displays laminated structure, indicating the layer-by-layer deposition on the filter membrane. The layered structure and uniform thickness (3.8 µm) of the paper further give strong evidences that the C3Nx(N-OH)y sheets are fully outstretched and homogeneously dispersed in the aqueous solutions. The as-prepared C3Nx(N-OH)y paper also shows outstanding hydrophilic nature, as can be proved by the low water contact angle of 4.43o (Figure 2h). In contrast, the contact angle of original C3N4 with water is 68o.19 The superhydrophilic nature of C3Nx(N-OH)y can be attributed to the strong protonation of N-OH groups.

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Figure 2. Stable C3Nx(N-OH)y aqueous dispersion. (a) UV-vis spectrum of C3Nx(N-OH)y aqueous dispersion. (b) Sedimentation behavior of C3Nx(N-OH)y aqueous dispersion with different concentrations (30, 35, 40, 45 and 50 mg mL-1). A0 and A is the absorbance (at 660 nm) of C3Nx(N-OH)y aqueous dispersion stockpiled for 0 to different days. (c) Digital photograph of the preparation process of C3Nx(N-OH)y paper by routine filtration technique. (d, e) Photographs of the flexible C3Nx(N-OH)y paper with metallic luster. (f) SEM image of the surface of C3Nx(NOH)y paper. (g) Cross section SEM image of C3Nx(N-OH)y paper showing the uniform thickness (3.8 µm) and layered structure. (h) Contact angle of C3Nx(N-OH)y paper with water.

To further understand the origin of excellent hydrophilic performance of the oxygen plasma treated C3N4, XPS and FTIR spectra were further carried out to investigate molecular structure 10


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and the functional groups. From the XPS survey spectra of C3Nx(N-OH)y, C 1s peak at ca. 288 eV, along with O 1s peak at ca. 532 eV and N 1s peak at ca. 399.8eV can be found (Figure S2).20, 21

The C/N atomic ratio is 4/2.98. High resolution C1s and N1s spectra (Figure 3) of C3Nx(N-

OH)y were analyzed and deconvoluted to account for the nature of the oxygen functional groups. For the oxygen plasma treated sample, C 1s peak at 284.6 eV and 288.1 eV (Figure 3a) can be observed, which is assigned to sp2-bond carbon in C-C and N-C=N.22-25 The well fitted N 1s spectrum shows two peaks at 399.8 and 402.3 eV (Figure 3b), which is commonly attributed to sp2 N atoms involved in triazine rings and N-OH.26, 27 At the same time, the O 1s spectrum shows the singlet at 531.8 eV, indicating the existence of N-OH (Figure S3a).28, 29 The atomic ratio of N-OH and total N atoms is 0.82:4. Thus the chemical formula of these obtained C3Nx(N-OH)y is C3N3.18(N-OH)0.82. Moreover, the FTIR spectrum (Figure S4) of C3N3.18(N-OH)0.82 doubly confirmed the XPS results. The broad peaks located at 1000 cm-1, 1158 cm-1, 2148 cm-1 and 3000 cm-1correspondto the signals of N-OH groups.30, 31 These results collectively prove that the oxygen plasma treatment leads to the incorporation of N-OH on the surface of the C3N4 (Figure 3c).

Figure 3. High-resolution C 1s (a) and N 1s (b) spectra of oxygen plasma treated C3N4, respectively. Schematic diagram of the structure of (c) C3Nx(N-OH)y. 11



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For comparison, alkali (NaOH) treatment method was also used to modify the surface of C3N4 and the molecule structures were characterized with XPS. As shown in Figure S3, the C 1s spectrum shows two peaks located at 286.9 eV and 288.8 eV, which can be assigned to sp2 C atoms in the aromatic ring attached to the –NH2 group and C-OH, respectively.22, 32 Meanwhile, the well-fitted N 1s spectrum of mixed acid treated C3N4 shows singlet at 399.8 eV, which indicates the existence of sp2 N atoms involved in triazine rings.22 The atomic ratio of C-OH and total C atoms is 0.3:3. O 1s XPS spectrum further confirmed the existence of C-OH. As shown in Figure S3b, the well-fitted O 1s spectrum of mixed acid treated C3N4 shows the singlet at 531.8 eV, demonstrating that the exclusive existence form of oxygen-containing groups is C-OH. 28, 30 Thus, the chemical formula of these alkali treated C3N4 is C2.7(C-OH)0.3N4 (Figure S3c). The particular existing form of oxygen-containing groups (N-OH) in C3Nx (N-OH)y can be attributed to the higher chemical activity of pyridinic nitrogen on the C3N4. 25 C3N4 is a graphitelike material. There are mounts of tris-triazine units connected with planar amino groups in each layer and weak van der waals force between layers. 33, 34 The existence of N in each tris-triazine unit can be classified into two forms: pyridinic-N and graphitic-N. The ratio of graphiticN/pyridinic-N is 1:3. Previous reports have demonstrated that carbon materials doped with pyridinic-N configuration are excellent platform to trap a series of atoms, 35 so we believe that large amounts of pyridinc-N in C3N4 are inclined to attach other atoms under oxygen plasma treatments. 12


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Figure 4.The relationship between N-OH content (yN-OH, black curve) / C-OH content (yC-OH, red curve) and reaction time (t). It deserved to be noted that the N-OH content (yN-OH) can be simply controlled by changing the plasma treating time (t) (See Table S1). As shown in Figure 4, the yN-OH is 0.07, 0.21, 0.41, 0.62 and 0.82 when the t is 10, 30, 60, 90 and 120 min, respectively. The linear relationship between yN-OH and t is as follows: yN-OH=0.0069 t

(1)

By contrast, as shown in Figure 4, there is no evident regularity between the C-OH content (yCOH)

of alkali treated C3N4 and the treating time (t).

Biologically active properties The superhydrophilicity of oxygen plasma treated C3N4 makes it good candidate for bioapplications. In this study, rADSCs were used to evaluate the biocompatibility of C3N3.18(NOH)0.82 paper. Adipose tissue was harvested from the subcutaneous tissue of 4-week-old Spra13



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gue-Dawleyrt and then the isolated ADSCs were adherent to the plastic culture plates. The pure cells lineage displayed spindle-shaped, fibroblast-like morphology. The clonal cell clusters were found in passages 2 reflecting rADSCs maintained favorable stem cell pluripotency and multiple differentiation potentials. Cells proliferation was assessed using CCK-8 assay. As shown in Figure 5a, the absorbance (OD) for rADSCs on the C3N3.18(N-OH)0.82 paper is 0.31, 0.62, 0.93, 1.2, 1.4, 1.7 and 2.2 at day 1, 2, 3, 4, 5, 6 and 7, respectively. Obviously, remarkable proliferation of rADSCs can be observed after 2 days of incubation. The OD value for rADSCs on the C3N3.18(N-OH)0.82 paper at day 7 is about 8 times as much as that on day 1. By contrast, the negative control shows low multiplication rate of rADSCs. The OD value for negative control is 0.17, 0.38, 0.48, 0.62, 0.70, 0.77 and 0.83 at day1, 2, 3, 4, 5, 6 and 7 respectively.

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Figure 5. Biologically active properties of C3N3.11(N-OH)0.82 paper. (a) OD for rADSCs on the C3N3.18(N-OH)0.82 paper (red curve) and in negative control (black curve) at day 1, 2, 3, 4, 5, 6 and 7.(b) CCK-8 assay for the rADSCs proliferation on the C3N3.18(N-OH)0.82 paper. (c) Cell density for rADSCs on the C3N3.18(N-OH)0.82 paper (red curve) and in negative control (black curve) at day 1, 2, 3, 4, 5, 6 and 7.

Live and dead staining was further presented to exhibit the excellent biocompatibility of C3N3.18(N-OH)0.82 paper. As shown in Figure 5b, live cells were stained green while dead cells were red. The rADSCs seeded on C3N3.18(N-OH)0.82 paper exhibited excellent viability which kept above 97 % at day 1, 5, 7 respectively and dead cells were barely seen. The cells density even increased during the prolonged culture time. The cell density for rADSCs on the C3N3.18(NOH)0.82 paper is 80, 160, 242, 314, 366, 430 and 561 cells mm-2 at day 1, 2, 3, 4, 5, 6 and 7, respectively, which is much higher than that of negative control (Figure 5c). All these results suggest that the C3N3.18(N-OH)0.82 paper has the excellent biocompatibility and cytocompatibility which can be used as a bio-active scaffold. The excellent biological properties can be attributed to both good hydrophilicity and low electrical conductivity. These features make the C3N3.18(NOH)0.82 paper potentially attractive for tissue engineering, especially as a new biomaterial for bone, vessel and skin regeneration.

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Figure 6. (a) Sedimentation behaviour of N-G, N-CNT and C2N aqueous dispersion. (b) Variation of the N-OH contents in N-G, N-CNT and C2N with treating time.

To verify the universal adaptability of the oxygen plasma treating method, similar experiments were carried out to modify N doped graphene (N-G), N doped carbon nanotube (N-CNT) and C2N samples. The detailed synthesizing process of these NCMs can be found in the supplemental materials. The effect of oxygen plasma treatment was evaluated by the dispersing performance of the N-doped carbon materials in aqueous solutions. From the digital photos in the inset of Figure 6a, it is remarkable that after 6 days of standing, the plasma treated samples have much better dispersity compared with the untreated ones. The sedimentation process in the 6 days was monitored by measuring the absorbance of the solutions. For the oxygen plasma treated N-G, N-CNT and C2N samples, the absorbance of their aqueous dispersion had no significant loss within 6 days. However, obvious reduction of absorbance was observed for the original untreated samples. Moreover, the N-OH content (yN-OH) of N-G, N-CNT and C2N can also be simply controlled by changing the plasma treating time (t), as shown in Figure 6b. The linear relationships between yN-OH and t can be expressed by the following equations: as follows: yN-OH=0.0012t

(2)

yN-OH=0.0019t

(3)

yN-OH=0.0039t

(4)

CONCLUSION In summary, we proposed a simple and universal method to modify the surface properties of C3N4. OH groups can be grafted to the N atoms during oxygen plasma treatment because N atoms are more active in the C3N4 structure, and the content of N-OH groups increases linearly with the treating time. Due to the intense protonation of N-OH groups, C3N3.18(N-OH)0.82 shows excellent hydrophilic nature and has good dispersing performance in aqueous solutions when the 16


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concentration can reach at 30 mg/mL, which makes it possible for various applications. Macroscopic C3N3.18(N-OH)0.82 paper with good mechanical property was successfully fabricated by routine infiltration method for the first time. Studies further proved that the hydrophilic C3N4 paper is biocompatible and can be used for the carrier for tissue repair. Experiments on other NCMs exhibit similar results. We believe that this study provides a new insight for the rational modification of NCMs towards various applications. various NCMs including C3N4, N doped graphene, N doped CNTs etc.

ASSOCIATED CONTENT Supporting Information Available Experimental details, XPS survey, O1s spectra, FTIR spectrum, the morphology of primary rADSCs proliferation ability and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Xianying Wang: [email protected] *Guqiao Ding: [email protected] Author Contributions +

These authors contributed equally.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

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This work was supported by projects from the National Science and Technology Major Project (2011ZX02707), the Chinese Academy of Sciences (KGZD-EW-303 and XDA02040000), Science and Technology Commission of Shanghai Municipality The Sailing Program (15YF1406800), NSFC (51072119, 81501605) and STCSM (1552072030).

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Surface Modification of C3N4 through Oxygen-Plasma Treatment: A

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