Synthesis of Phosphorylated Graphene Oxide Based Multilayer

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Synthesis of Phosphorylated Graphene Oxides based Multilayer Coating: Self-Assembly Method and Application for Improving the Fire Safety of Cotton Fabrics Wei Wang, Xin Wang, Ying Pan, Kim Meow Liew, Ola A Mohamed, Lei Song, and Yuan Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 22, 2017

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Industrial & Engineering Chemistry Research

Synthesis of Phosphorylated Graphene Oxides based Multilayer Coating: Self-Assembly Method and Application for Improving the Fire Safety of Cotton Fabrics Wei Wang †, ‡, Xin Wang †, Ying Pan †, Kim Meow Liew ‡, Ola A. Mohamed ζ, Lei Song †, *, Yuan Hu †, * †

State Key Laboratory of Fire Science, University of Science and Technology of

China, 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China ‡

Department of Architecture and Civil Engineering, City University of Hong Kong,

Tat Chee Avenue, Kowloon, Hong Kong ζ

Tanning materials and leather technology department, National Research Centre

(NRC), Dokki, Giza 11622, Egypt. ⃰ Corresponding author. Tel/Fax: +86 551 63601664.

E-mail address: [email protected] (Y. Hu); [email protected] (L. Song)

KEYWORDS. Self-assembly, Phosphorylated Graphene Oxides, Cotton Fabrics, Fire Safety, Mechanism

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ABSTRACT

Phosphorylated graphene oxides (PGO) was synthesized by a one-pot method and fabricated on the surface of cotton fabrics for the improvement of fire safety. The chemical structure of PGO were well characterized by FTIR and XPS test. SEM images of the pure and coated cotton fabrics indicated that PGO had a better compatibility with water than GO and was more beneficial for self-assembly fabrication than GO. Vertical flame test and cone calorimeter test revealed that flame retardancy of cotton fabrics was obviously improved by PGO based multilayer coating. The plausible flame retardancy mechanism was proposed: PGO with large layered structures could effectively insulate the permeation of oxygen and volatile flammable gases, thereby decreased the heat release rate; on the other hand, the presence of phosphorus could play an important role on catalytic charring effect during combustion, which significantly promoted the formation of char residue and then further prevented the permeation of oxygen and pyrolysis products.


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INTRODUCTION Fire plays a great role throughout human progress, but it can bring serious property loss and human deaths as well. A wide range of commonly used materials are flammable especially cotton fabrics. Cotton fabrics are the most important natural fibre and widely used in the application of manufacture clothes and soft furnishings. However, this kind of cellulosic material has a low combustion temperature and can be highly ignited. The flame spreads fast once cotton fabrics are ignited, and thereby causing fatal burning within 15s. Therefore, cotton fabrics are potentially making human lives and properties in danger. Researchers have explored several methods trying to improve the fire safety of cotton fabric. Recently, self-assembly method as a surface modification technique has attracted considerable attention and been proved to be highly useful in fabricating coatings on substrates. Additionally, layer-by-layer technique is usually used to grow thin films through consecutive adsorption of oppositely charged materials onto polymer matrix and improve the fire safety of polymers. Grunlan et al firstly used chitosan and poly(vinyl sulfonic acid) sodium salt to improve the fire safety of polyurethane foam through layer-by-layer method1. Moreover, Ball et al prepared a flame retardant films by using montmorillonite and poly(allylamine)2. Earlier, Grunlan also alternately deposited polyelectrolyte and clay on cotton fabric for advanced flame retardancy3. These efforts confirm the feasibility by using layer-by-layer assembly method to improve the fire safety of cotton fabric. Among most of the reported works, layer-like materials are proved to play a prior 3 ACS Paragon Plus Environment


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role in enhancing flame retardancy of polymers such as flexible polyurethanes and cotton fabrics. As we know, graphene, a carbon monolayer with an indefinitely extended two dimensional lattice, was firstly isolated to a single-atom-thick layer by Novoselov and co-workers4. Since then, graphene had attracted considerable attention due to its unique characteristics, including excellent electrical and thermal conductivity, unparalleled flexibility and mechanical strength and high specific surface area5-8. Graphene has been developed to improve the flame retardancy of polymers9-14. The high specific surface area is like a physical barrier, which can retard the permeation of toxic gases and oxygen during the combustion of polymers. Therefore, it can be anticipated that as the surface of cotton is densely covered by graphene, the flame retardancy of cotton could be improved. There are a few functional groups on surface of graphene oxides, such as carboxyl, hydroxyl, carbonyl, etc. While, those weakly charged functional groups are unbeneficial for the layer-by-layer assembly technique, which needs excessive positive or negative charges. Hence, in order to make graphene oxides more suitable for self-assembly technique, corresponding modifications for graphene oxides are necessary. In this work, phosphorylated graphene oxides (PGO) was prepared and used as a negative charges in the progress of fabrication. Moreover, polyethyleneimine (PEI) was used as positively charged electrolyte. PGO with more phosphate groups are expected to achieve a better compatibility than GO in water, which can enlarge the content of negative charges in favor of fabrication process. On the other hand, phosphorus, an environment-friendly flame retardant element, has been proved to 4 ACS Paragon Plus Environment

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effectively improve the fire safety of polymers. Actually, the multilayer consisting of PEI and PGO and cotton fabrics form an intumescent-like flame retardancy system. Here, cotton fabrics are acted as “carbon source” due to its energetic oxygen hexa-heterocyclic groups15-17; PGO works as an “acidic source” producing in situ phosphoric acid and promoting the char formation of polymers13, 18; PEI is identified as “gas source” owing to its amino groups19. Therefore, this system is anticipated to improve the fire safety of cotton fabrics. PGO are synthesized and measured by FTIR and XPS tests. The fire safety of prepared cotton fabrics coated are characterized by cone colorimeter and vertical flame tests. Experimental Section Raw Materials. Expandable graphite is provided by Qingdao Tianhe Graphite Co., Ltd. (China). Flexible polyurethane foam (DW30) was obtained from Jiangsu Lvyuan New Material Co., Ltd. Branched polyethyleneimine (PEI) (Mw = 70000 g·mol-1), hydrochloric acid (HCl) (36.5-38%) and sodium hydroxide (NaOH) were supplied from Sinopharm Chemical Reagent Co. Ltd. Potassium permanganate (KMnO4), sodium nitrate (NaNO3), sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30%), tetrahydrofuran (THF) and triethylamine (TEA) were purchased from Sinopharm Chemical Reagent Co. Ltd. Phosphorus oxychloride (POC) was obtained by Aladdin Industrial Corporation. Synthesis of GO and PGO. The preparation of GO is according to the Hummers method20. The PGO was prepared by a one-pot method. Typically, 5 g of GO was dispersed in a three-necked flask with 200 mL of THF under constant stirring and 5 ACS Paragon Plus Environment


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nitrogen condition. Then, 30 g of TEA was added into the above suspension by stirring for 10 min. Subsequently, 24 g of POC dissolved in 100 mL of THF was slowly dropped into the GO suspension in 1 h. After the completion of the dropping, the reaction was allowed for 4 h. Finally, the products were obtained by centrifuging and washing by ethanol and deionized water for several times. The scheme diagram of the synthesis is listed in Supporting Information Fig. S1. Preparation of self-assembly components: PEI solution and PGO suspension. The preparation of PEI solution ( 0.5 wt %) is according to our previous work21. PEI (the value of pH was 9) was performed as positively charged electrolyte. Actually, the pH value of PEI solution will be 12 or more, it is due to the water dissociation promoted by amine groups. In order to obtain the positive charged state, HCl solution can provide protons, forming ammonium ions. PGO suspension as a negative component was prepared by a 0.5% wt. % and the pH value of the suspension are adjusted to 7 by using NaOH and HCl solution. As a control group, GO suspension was prepared by a same way as well. All the components were stirred throughout one night before fabrication. Self-assembly process. Prior to fabrication, cotton fabrics should be washed by deionized water and air-dried. In a typical process, cotton fabrics were alternately immersed in PEI solution and PGO suspension. The first dip needed a long time of 10 min and the following dip needed 2 min. The first layer on the surface of cotton fabric is important and difficult, longer time can guarantee the successful deposition of negative and positive charged materials. In addition, the samples after each dip should 6 ACS Paragon Plus Environment

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be washed by deionized water and wringed out to expel the excess water. As the desired number of multilayers was obtained, the fabricated cotton fabrics were dried in oven at 60 ºC overnight. The fabrication process of GO suspension was the same with that of PGO suspension. Here, the PGO based multilayer with 5 layers was marked as Cotton/PGO-5L. Similarly, the others were thereby marked as Cotton/PGO-10L, Cotton/GO-5L and Cotton/GO-10L. The brief scheme diagram of the first layer was presented in Scheme 1. The zeta-potential data of GO, PEI and PGO are provided in supporting information Table S2. Experimental Methods. FTIR spectra were performed on a Nicolet MAGNA-IR 750 FTIR spectrometer. KBr ground in a mortar with a pestle and enough solid sample was ground with KBr to make a 1 wt. % mixture for making KBr pellets. Zeta-potential of inorganic particles were measured on a NanoBrook 90Plus PALS Zeta potential analyzer. The transition mode was used and the wavelength range was set from 4000 to 500 cm-1. X-ray photoelectron spectroscopy (XPS) was performed on a VG Escalab Mark II spectrometer (Thermo-VG Scientfic Ltd. UK), using Al K an excitation radiation (ht = 1486.6 eV). The thermogravimetric analysis (TGA) of samples was undertaken using TGA-Q5000 apparatus (TA Co., USA) from 50 ºC to 800 ºC at a heating rate of 20 ºC·min-1. Vertical flame tests were used according to ASTM D6413, using a vertical burning tester (CZF-3, Nanjing Jiangning Analytical Instrument Factory, China). The samples (300 mm × 76 mm), held 19 mm over the Bunsen burner, were first exposed to the flame for a period of 12 s and then removed rapidly. A combustion test was per-formed on the cone calorimeter (Fire Testing 7 ACS Paragon Plus Environment


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Technology, U.K.) according to ISO 5660 standard procedures, using 100*100 mm2* two layers specimens. Each specimen was exposed horizontally to 35 kW·m-2 external heat flux. Laser Raman spectroscopy measurements were carried out at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co., USA). RESULTS AND DISCUSSION Characterization of GO and PGO. The chemical structure of GO and PGO are characterized by FTIR, which is shown in Fig. 1. For GO, some typical characteristic peaks can be observed, such as the C-O-C stretching vibration in epoxy groups (1043 cm-1), C=C or H2O vibration (1623 cm-1); C=O stretching vibration (1727 cm-1) and O-H stretching vibration (3427 cm-1)22. After modification, the FTIR of PGO shows several new characteristic peaks locating at 840, 1086 and 1228 cm-1, which are attributed to P-O-C and P=O23-24, respectively. The analysis indicated that the synthesis process was well-organized and phosphate groups were successfully deposited on GO. XPS was used to further investigate the element composition and chemical state of GO and PGO, the XPS results was presented in Fig. 2. XPS survey spectra shown in Fig. 2a confirms the presence of phosphorus as well. The high resolutions of C1s, O1s and P2p are shown in Fig. 2b-d respectively. In Fig. 2b, the C1s XPS spectra can be deconvoluted into five peaks such as 284.6, 285.6, 287.1, 288.5 and 289.3 eV, ascribing to the graphitic carbon, C-O-P, C=O and C(O)OH, respectively25-26. O1s XPS spectra shown in Fig. 2c can be divided into two peaks with BE locations of 531.0 and 532.8 EV, mainly attributing C=O/P=O and C-O/C-O-P27-28, respectively. 8 ACS Paragon Plus Environment

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Moreover, the XPS spectra of P2p is shown in Fig. 2d. The peaks at 133.4 and 134.4 eV are respectively ascribed to P-OH and P-O-/P-O-C27-29. The high-resolution XPS spectra of C1s, O1s and P2p strongly proves the chemical state of PGO. The detail data of XPS test is provided in supporting information Table S1. The content of P is 1.4%, the low loading of P is possible due to the large surfaces of GO, which cause the low content of active groups and thereby reduce the content of P. Raman spectra of GO and PGO is shown in supporting information S2. As can be seen, PGO shows a higher value of ID/IG than GO, this is due to the modification process, which caused more structure defects30-31. The Surface Morphology of Pure and Coated Cotton Fabrics. SEM test was used to observe the morphology changes of coated cotton fabrics after fabrication. In Fig. 3a, pure cotton fabrics show a clean and smooth surface. After fabrication with GO, the surface of cotton/GO-5L is covered by multilayer coatings. With the layer number increases, the thickness of the coating increases as well. However, the surface of cotton/GO-5L and cotton/GO-10L are rough, this is mainly due to the incompatibility between GO and water. For cotton/PGO-5L and cotton/PGO-10L in Fig. 3d, e, the surfaces show relatively smooth, it can be explained that PGO possesses more negative groups after modification and have a good compatibility with water, which benefiting for self-assembly operation. Thermal Stabilities of Pure Cotton and Coated Cottons fabrics. Thermal stabilities of the control and coated cotton fabrics are tested by TGA under nitrogen condition, the results are shown in Fig. 4, and the corresponding data is 9 ACS Paragon Plus Environment


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presented in Table 1. It can be observed that all the coated cotton fabrics have a lower decomposition temperature than pure cotton, this is mostly caused by the removal of stable oxygen-containing groups of GO. Moreover, coated cotton fabrics exhibit higher char residues compared with the pure one, implying improved thermal stabilities. For the DTG results, the values of maximum decomposition rates of cotton/PGO-5L and cotton/PGO-10L are higher than that of cotton/GO-5L and cotton/GO-10L. This is mainly due to the pre-pyrolysis of phosphate group in PGO. Flammability of Pristine and Coated Cotton evaluated by Vertical Flame Tests and Cone Colorimeter tests. Vertical flame tests were employed to evaluate the flammability of cotton fabrics. As is shown in Fig. 5, the flame on the pure cotton shows more vigorous and brighter compared with the coated samples at 5 seconds after ignition. The flame on cotton/GO-5L and cotton/GO-10L are similar and milder than the control one, implying the improved flame retardancy. Significantly, after the coating of PGO, the flame on cotton/PGO-5L and cotton/PGO-10L perform shorter and softer than the control and GO based cotton fabrics. Additionally, the photos of the char residues and the corresponding Raman spectra of the control and coated cotton fabrics are shown in Fig. 6. After vertical flame tests, the pure cotton have little residues, showing a poor fire safety. After the coating of GO and PGO, the coated cotton fabrics leave more char residues. Nevertheless, the char residues of cotton/PGO-5L and cotton/PGO-10L perform more compact and intact than that of cotton/GO-5L and cotton/GO-10L, demonstrating that PGO exhibits better than GO in protecting cotton 10 ACS Paragon Plus Environment

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fabrics from the flame combustion. In addition, the Raman data of all samples are provided to study the graphitic structure of residues. As can be observed, all the spectra show two bands: the D-band (1350 cm-1) attributing to the vibration of carbon atoms with dangling bonds in the plane terminations of disordered graphite or glass carbons and the G-band (1580 cm-1) ascribing to the vibration of sp2-bonds carbon atoms in graphite layers. In general, the ratio of the intensity of D and G bands (ID/IG) is used to evaluate the graphitization degree of residue: the lower value of ID/IG, the lower structure defects of char residue. The higher ID/IG means lower structure defects of char residues, which can protect the underlying matrix well and prevent the permeation of organic volatiles and thereby improve the thermal stability of polymer matrix. After deposition of GO and PGO, the values of ID/IG decease compared with the control one. Furthermore, the value of ID/IG of cotton/PGO-5L and cotton/PGO-10L shows lower than that of cotton/GO-5L and cotton/GO-10L. This is mainly due to the catalytic charring effect from phosphorus and the physical barrier effect from graphene oxides. Additionally, as can be observed, cotton/GO-5L shows a lower ID/IG value compared to cotton/GO-10L and the difference of ID/IG values between cotton/PGO-5L and cotton/PGO-10L is negligible. This phenomenon indicates that as the layer number increases, the structure defects of char layer change little. Heat release rate (HRR) and total heat release (THR) curves were shown in Fig. 7. The coated cotton fabrics achieve lower PHRR values compared with bare cotton fabrics. Moreover, the PHRR value of cotton/PGO-10L is the lowest among all 11 ACS Paragon Plus Environment


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samples, indicating the improvement of flame retardancy, which is due to the physical barrier effect caused by graphene oxides and catalytic charring effect from phosphorus. Cotton/GO-5L and cotton/GO-10L show near THR values with the control one. Besides, compared with GO based cotton fabrics, the samples fabricated by PGO show the lower THR values. This results demonstrate that GO based multilayers have little influence on the THR values of cotton fabrics, and the reduction of THR values of PGO based cotton fabrics are ascribed to the catalytic charring effect from phosphorus element. The photos of the residue after cone calorimeter test were provided in Supporting Information Fig. S3. For the control one in Fig. S3a, there is nothing left. The residue of cotton/GO-5L and cotton/GO-10L in Fig. S3b, c can be observed better than the control cotton fabrics. The photos in Fig. S3d, e shows the char residues of cotton/PGO-5L and cotton/PGO-10L. The shapes of cotton fabrics are approximately preserved. SEM tests are used to study the microstructure of char residues after cone calorimeter test, which is presented in Fig. 8. All char residue show a similar latticed shape, indicating the well-preserved char structure. This is mainly attributed to the physical barrier effect from layered graphene oxides, which can effectively retard the permeation of oxygen and volatile flammable compounds. Under the same magnification, the char residues of cotton/PGO-5L and cotton/PGO-10L exhibit bulkier and more contact than that of cotton/GO-5L and cotton/GO-10L. This is due to the catalytic charring effect from phosphorus. Therefore, the probable flame retardancy mechanism of PGO based cotton fabrics 12 ACS Paragon Plus Environment

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was proposed: on one hand, the GO possesses large layered structure, which can effectively insulate the permeation of oxygen and volatile flammable gases, thereby decrease the heat release rate; on the other hand, the presence of phosphorus can play an important role on catalytic charring effect during combustion, which can significantly promote the formation of char residue and then further prevent the permeation of oxygen and pyrolysis products. CONCLUSION In this work, PGO was successfully synthesized and fabricated on the surface of cotton fabrics by self-assembly method for improving the fire safety of cotton fabrics. The synthesis process of PGO was well-organized and investigated by FTIR and XPS tests. SEM images of the pure and coated cotton fabrics indicated that PGO had a better compatibility with water than GO and was more beneficial for self-assembly fabrication than GO. TGA data revealed that PGO can improve the thermal stability of cotton fabrics. Moreover, the flammability of cotton fabrics was evaluated by vertical flame test and cone calorimeter test. The results showed that PGO based multilayer decrease HRR and THR values and significantly promoted the char formation of cotton fabrics during combustion. The corresponding flame retardancy mechanism was proposed as well: GO performed as a physical barrier retarding the permeation of oxygen and volatile flammable gases; on the other hand, phosphorus acted as a catalytic charring agent, which could obviously promote the char formation during combustion, thereby improving the fire safety of cotton fabrics.



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Scheme 1. The Scheme diagram of the first Layer of fabrication process.

1623 1727

GO PGO

1440

840 1228 1086 3427

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Figure 1. FTIR spectra of GO and PGO.


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Figure 2. (a) XPS survey spectra of GO and PGO, high-resolution XPS of (b) C1s, (c) O1s and (d) P2p peaks of PGO.

Figure 3. SEM images of (a) Pure cotton, (b) cotton/GO-5L, (c) cotton/GO-10L, (d) cotton/PGO-5L and (e) cotton/PGO-10L.



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Figure 4. (a) TG and (b) DTG curves of control and fabricated cotton fabrics under nitrogen condition.

Figure 5. Photos of vertical flame testing of the pure and coated cotton fabrics 5 s after ignition.


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Figure 6. Photos of uncoated and coated cotton fabrics following the vertical flame test, Raman spectra of residual char of (a) pure cotton, (b) cotton/GO-5L, (c) cotton/GO-10L, (d) cotton/PGO-5L and (e) cotton/PGO-10L after vertical flame test.

Figure 7. (a) HRR and (b) THR curves of the uncoated and coated cotton fabrics.



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Figure 8. SEM images of (a, b) cotton/GO-5L, (c, d) cotton/GO-10L, (e, f) cotton/PGO-5L and (g, h) cotton/PGO-10L.

Samples

T-5%/ºC

Tmax/ºC

Char Residue at 800 ºC/%

HRR/kW/m2

THR/kJ/m2

Cotton

399.3

420.9

Cotton/GO-5L

299.6

359.4

9.1

294.7

3.20

10.9

266.3

3.12

Cotton/GO-10L

302.5

357.6

12.3

249.7

3.10

Cotton/PGO-5L

310.8

361.1

12.6

245.6

2.61

Cotton/PGO-10L

310.0

379.9

14.9

213.7

2.54

Table 1. The corresponding data of TGA and cone colorimeter tests for pure and coated cotton fabrics.


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ACKNOWLEDGEMENTS This work was supported by National Key Research and Development Program of China (2016YFC0802802); National Basic Research Program of China (973 Program) (2014CB931804); the National Natural Science Foundation of China (21374111); the Research Grants Council of the Hong Kong Special Administrative Region, China (9042354, CityU 11261216); and Fundamental Research Funds for the Central Universities (WK2320000032). Supporting Information. The Scheme diagram of PGO suspension, Raman spectra of GO and PGO, Photos of char residues of the control and coated cotton fabrics after cone calorimeter test, XPS data of GO and PGO and zeta-potential data of GO, PEI and PGO. REFERENCES (1) Laufer, G.; Kirkland, C.; Morgan, A. B.; Grunlan, J. C. Exceptionally flame retardant sulfur-based multilayer nanocoating for polyurethane prepared from aqueous polyelectrolyte solutions. ACS Macro Lett. 2013, 2, 361-365. (2) Laachachi, A.; Ball, V.; Apaydin, K.; Toniazzo, V.; Ruch, D. Diffusion of polyphosphates into (poly (allylamine)-montmorillonite) multilayer films: flame retardant-intumescent films with improved oxygen barrier. Langmuir 2011, 27, 13879-13887. (3) Li, Y.-C.; Schulz, J.; Mannen, S.; Delhom, C.; Condon, B.; Chang, S.; Zammarano, M.; Grunlan, J. C. Flame retardant behavior of polyelectrolyte-clay thin film assemblies on cotton fabric. ACS Nano 2010, 4, 3325-3337. 19 ACS Paragon Plus Environment


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Table of Contents Graphic and Synopsis


Synthesis of Phosphorylated Graphene Oxide Based Multilayer

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