Sustainable Synthesis of High-Surface-Area Graphite Oxide via Dry

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Sustainable synthesis of high-surface-area graphite oxide via dry ball milling Alaa El Din Mahmoud, Achim Stolle, and Michael Stelter ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00147 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Sustainable Chemistry & Engineering

Sustainable synthesis of high-surface-area graphite oxide via dry ball milling Alaa El Din Mahmoud, *,†,‡ Achim Stolle†, Michael Stelter†,II †Institue

for Technical Chemistry and Environmental Chemistry, Center for Energy and Environmental Chemistry, Friedrich-Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany. ‡Environmental IIFraunhofer

Sciences Department, Faculty of Science, Alexandria University, 21511, Alexandria, Egypt.

IKTS, Fraunhofer Institute for Ceramic Technologies and Systems, 07629 Hermsdorf, Germany.

*Corresponding author Email: [email protected] / [email protected] KEYWORDS: mechanochemistry, graphene, oxidation, methylene blue, treatment, adsorption. ABSTRACT: A sustainable route to produce graphite oxide (GO) is presented using dry ball milling. The production method was based on pristine graphite flakes in a planetary ball mill. The prepared GO was characterized using UV-vis spectroscopy, BET surface area analysis, thermal analysis, SEM-EDX, TEM, XPS, elemental analysis and Raman spectroscopy. The degree of graphite oxidation was controllable by the milling time and milling material, and the carbon-based yields ranged from 86 to 97%. The maximum oxygen/carbon ratios of the produced GOs were 0.16 and 0.15 after 24 h of ball milling with steel and zirconia balls, respectively. The BET surface area increased with an increasing milling time from 1 m2 g-1 for pristine graphite up to 730 m2 g-1 for the ball-milled samples. Furthermore, the intensity ratios of the D and G bands (ID/IG) from the Raman spectra were 0.84 and 0.77 for GO produced with the steel and zirconia balls, respectively. The in-plane sp2 crystallite sizes (La) of graphite (168 nm) decreased to 20 (steel balls) and 22 nm (zirconia balls). Additionally, the produced GO was tested as an adsorbent for methylene blue dye removal.

INTRODUCTION Graphene is a type of carbon nanomaterial that has gained rapidly expanded research attention since single-layer graphene was first prepared from the isolation of graphene from graphite in 2004.1,2 This isolation was accomplished by micromechanical cleavage, which earned Novoselov and Geim the Nobel Prize in 2010.1 This procedure is a suitable method for research applications but is not suited for large-scale graphene production or the production of related materials. Blake et al.3 and Marcano et al.4 concluded that no industrial technology could rely on the micromechanical cleavage technique because only low quantities of graphene can be obtained. Other routes to graphene/graphene oxide (cf. Electronic Supplementary Information, ESI) suffer from high investment or operative costs or are not sustainable due to the consumption of hazardous chemicals or the existence of impurities originating from wet chemical oxidation.5 Therefore, a scalable and low cost approach to obtain large quantities of graphene oxide/graphene is needed.6

ble strategy to synthesize different composites. The mechanochemical method was identified as a sustainable technique to synthesize graphene materials with less structural defects.10 Utilization of mechanical stress can be realized in ball mills. During operation, the milling balls roll down the surface of the jar in a series of parallel layers and transfer energy to the powder.11 Furthermore, the high speed of the planetary rotation and collisions of the jars and balls generate sufficient kinetic energy for bond cleavages in the aromatic graphite structure. This process introduces functional groups on the edges, surfaces and basal planes of graphene materials during the milling process.10 Consequently, comminution using ball mills not only reduces the particle size but also refines the grains into nanoscale sizes and increases the surface area of the solids. Ball milling is expected to increase the proportion of highly active regions on the material surface.12 The large-scale production of graphite oxide (GO)/graphene may be achieved using ball milling. As the moving balls apply their kinetic energy to the milled material, strong bonding interactions will break, producing fresh surfaces.13

The synthesis of such nanomaterials requires an interdisciplinary green chemistry principles that utilize nontoxic chemicals or environmentally benign solvents.7,8 Recently, AmbrosioMartin et al.9 reported ball milling as an efficient and sustaina-

In this process, the milling jar is not supposed to be filled completely by the balls and powder to allow the milling process to occur. Thus, air (or another gas) is present after closing the jar. The presence of air reduces the cold welding to a favorable

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level.14 Therefore, process control agents are generally not required. Once graphene/GO is prepared, these materials can be dispersed in water or organic solvents to form stable suspensions to use for various purposes.15 Previous reports have stated that graphene sheets can be prepared from wet ballmilling and active precursors. For instance, Zhao et al.16 conducted a wet ball milling experiment for the graphite with N,Ndimethylformamide (DMF) solvent for 30 h at 300 rpm to be exfoliated into graphene sheets. However, Coleman 17 reported that the liquid exfoliation of graphite in a majority of solvents requires their subsequent removal that hinders the preparation process. Other researchers used various active precursors with the starting graphite material. For example, melamine,18,19 ammonia borane,20 cellulose21 and dry ice13 have been applied to weaken the van der Waals interactions between graphite layers. In general, molecular adsorption on the graphite surface effectively weakens those interactions, facilitating the exfoliation of graphite particles.20 The previous methods required tedious multistep washing process or freeze drying to get the final desired product. In addition to applications in materials science, graphene/GO materials have potential environmental applications, such as attractive adsorbents for wastewater treatment, due to their high surface areas and sorption capacities as well as nontoxicity. 22,23 For instance, several authors demonstrated the adsorption capability of graphene oxide for heavy metals, such as As, Cd, Cr or Pb.24,25 Dyes, as examples of colored organic pollutants, exist in the effluents of textile, paper, polymer and food production. Moreover, dye wastewater affects the environment and human health because of its color depth and high content of organic pollutants with poor biodegradability.26,27 The annual production of dyes has been calculated to be more than 700,000 ton with a high variability in their chemical nature.28 In this study, methylene blue (MB) was applied as an example of a cationic dye with positively charged sulfur groups, which can have strong electrostatic attractions with the negative oxygen-containing functional groups on carbon materials.29 Here, we report a sustainable synthesis of GO using a dry ballmilling technique. We investigated the effects of two different milling materials and the ball-milling conditions on the produced GO. In addition, the adsorption efficiency of the produced GO for MB as a model contaminant was demonstrated. EXPERIMENTAL SECTION Chemicals and reagents. Graphite flakes were purchased from Sigma Aldrich Chemical Co., USA. Graphene oxide nanocolloids (2 mg ml-1) and silicon dioxide were obtained from Sigma Aldrich Chemical Co., Germany. MB was obtained from Merck, Germany. Deionized water was used throughout the experiments. Preparation of GO by ball milling. The feed sample; graphite is denoted as G. The ball-milling experiments were carried out in a planetary ball mill Pulverisette 7 (Fritsch GmbH, Germany) under ambient conditions. The milling parameters were set as described in Table 1. The ball size was 5 mm for both grinding materials. The rotation frequency was maintained at 600 rpm. After 8 h, the milling was interrupted for 10 min before restarting. The product samples were denoted as A-6h, B-16h, and C-24h (stainless steel) and D-6h, E-16h, and F-24h (zirconia). The mass yield of the milled product was determined by weighing the samples after ball

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milling. The temperatures and pressures inside the milling beakers were monitored using an Easy-GTM system (Fritsch GmbH, Germany). Table 1. Experimental conditions of the ball-milling experiments Ball-to-powder ratio [-] Rotation frequency

7:1

[min-1]

600

Ball size [mm]

5

Milling tool material

Stainless steel

Zirconia

Beaker volume Graphite weight [g]

80 1.49

80 1.40

Φtotal [-]

0.049

0.049

1-Φtotal [-]

0.951

0.951

Milling atmosphere [cm3]

76.1

75.9

[cm3]

Milling time [h] Sample notation

6, 16, 24

6, 16, 24

A-6h, B-16h,

D-6h, E-16h,

C-24h

F-24h

Characterization The suspensions were created by adding 50 mg of carbon material to 10 mL of deionized water followed by ultrasonication for 30 min. Each suspension (0.5 mg mL-1) was made transparent by diluting by a factor of 10 prior to UV-vis measurements (Perkin-Elmer Lambda 35; scan range: 800-200 nm; resolution: 1 nm; scan rate: 480 nm min-1). Brunauer-Emmett-Teller (BET) surface areas of the samples were obtained by N2-physisorption measurements (Quantachrome; degassing at 120 °C prior to measurement). Thermogravimetric analyses were performed with a DTA-TG Shimadzu (10 K min-1; air 30 ml min-1; up to 1000 °C). The morphologies of the samples before and after ball milling were investigated by scanning electron microscopy (SEM, Phenom) equipped with energy dispersive x-ray spectroscopy (EDX) and Transmission Electron Microscopy (TEM Fei Technai G2). TEM of the samples were dispersed by ultrasonicating in ethanol and loading one drop of the prepared suspensions onto a carbon coated copper grid. X-ray photoelectron spectroscopy (XPS) was analyzed using an SPECS EA200 analyzer with Mg Kα radiation (deconvolution was realized based on a Gaussian–Lorentzian model). The oxygen/carbon ratio (O/C) was calculated based on the elemental composition measured with an elemental analyzer (Elementar Vario Micro). Raman spectra were obtained using a Renishaw InVia spectrometer (UK) with the 514 nm line of an Ar+ ion laser in the range of 1200-3000 nm. Application of GO for MB removal The carbon material (12.5 mg) was added to the MB solution (50 ml; 30 mg L-1) in a 250 ml flask. The pH of the aqueous solutions was adjusted with 0.1 and 0.01 mol L-1 NaOH and HCl. Adsorption experiments were carried out in dynamic mode using a horizontal shaker (agitation speed: 200 rpm) at room temperature for 1-60 min. All experiments were conducted in duplicate, and the averages were obtained. After each adsorption experiment, the suspensions were centrifuged (6,000 rpm; 10 min). The final concentration of MB dye in the solution was analyzed by a UV-vis spectrophotometer (PerkinElmer Lambda 35; 664 nm, cuvette path length: 10 mm) using deionized water as a blank. The Lambert-Beer law was used to calculate the remaining MB concentration after adsorption. The removal efficiency of MB was calculated according to the following equation:30-32


2

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Removal% 

cMB ,initial  cMB ,t cMB ,initial

100

(1)

where cMB,initial and cMB,t are the initial MB and the MB concentration after a certain time t [mg L-1], respectively. RESULTS AND DISCUSSION Synthesis.

The mass yield becomes a vital parameter that must be considered and optimized for the large-scale production of graphite/graphene oxide. Thus, the weight of the starting material (graphite) before and after ball milling were measured (Table 2). Although carbon yields of > 85% were obtained for the recovered material in all experiments, increasing the milling time provided little advantage with respect to the yield, as some powder stuck on the balls and the milling jar walls. A similar effect was observed for the mass yield, showing a less pronounced effect of the milling time. Values of 96% to complete recovery were achieved

Table 2. Yields obtained from the ball-milling synthesis of GOs and BET data of the samples and pristine graphite. Samples

Total yield [g (%)]

Carbon yield

ABET

[g (%)]

[m2

Graphite (G)

g-1]

Amicropores

Aexternal surface

[m2

[m2 g-1]

g-1]

1

1

0

A-6h1)

1.49 ± 0.00 (100 ± 0.0)

1.40 ± 0.00 (97.1 ± 0.1)

250

93

195

B-16h1)

1.46 ± 0.01 (98.3 ± 0.5)

1.25 ± 0.01 (88.3 ± 0.3)

621

231

386

C-24h1)

1.45 ± 0.01 (97.6 ± 0.5)

1.21 ± 0.01 (86.1 ±0.4)

666

265

396

D-6h2)

1.40 ± 0.00 (100 ± 0.0)

1.31 ± 0.03 (96.9 ± 0.2)

128

34

94

E-16h2)

1.36 ± 0.01 (96.8 ± 0.5)

1.21 ± 0.00 (92.3 ± 0.4)

469

166

302

F-24h2) 1.35 ± 0.01 (96.2 ± 0.3) 1.15 ± 0.01 (87.9 ± 0.1) 730 293 437 Reaction conditions: see Table 1. 1) Milling tools: stainless steel, jar volume: 80 ml, initial mass of graphite = 1.49 g. 2) Milling tools: zirconia, jar volume: 80 ml, initial mass of graphite = 1.40 g. Note: the total yield of one milling cycle is 2.89 g (steel) and 2.80 g (zirconia). causes a temperature increase in the grinding stock, the milling The total filling degree of the milling beaker Φtotal, the volumettools (balls and beaker) and the atmosphere inside the beaker, ric filling degree of the milling balls ΦMB,material and the volumetwhich are responsible for the observed temperature increase. ric filling degree of the grinding stock ΦGS,packing were found to be 0.049, 0.017 and 0.032, respectively, for the ball milling with stainless-steel balls (see Table 1; for calculation see SI S1-S4). The filling degree is an essential parameter that influences the trajectories of the milling balls and the contact between the balls and powder. Additionally, optimizing the filling degree is crucial for large-scale synthesis with ball mills, without requiring an increase in the jar volume and/or a replacement ball mill. Stolle et al.33 reported that Φtotal ranges from 0 to 1, which corresponds to the empty and completely filled milling beaker, respectively. Furthermore, the residual free volume for the milling ball movement (1-Φtotal) in the present experiments was 0.951. The calculated volumetric filling degree of the milling balls in regard to the packing volume ΦMB,packing (eq. S4) was 0.2, which is < 0.4. This indicated that the stress energy distribution of the grinding media was reduced, leading to the observed effect regarding the yield (Table 2).34 The pressure and temperature inside the milling beakers during graphite milling were recorded (Figure 1). Within 90 min of milling at 600 rpm, the temperature inside the stainless-steel jar increased from room temperature to 34 °C. Afterwards, only a minor increase up to 37 °C was observed. The temperature and pressure dropped after 8 h because of a 10 min milling pause. Similar behavior was found for the experiment with zirconia milling tools. Wieczorek-Ciurowa35 assumed that after a period of comminution and activation, agglomerates begin to form and increase in size. A powder layer can be attached to the surface of milling balls or the inner wall of the beaker. If a ball hits this layer, part of its kinetic energy is transferred to the powder as heat, followed by a local temperature increase. As multiple collisions occur during milling, the dissipated energy

Figure 1. Temperature and pressure measured on-line during the ball milling of graphite with stainless-steel and zirconia milling tools. Beaker volume was 80 ml for both materials. The pressure increase is directly proportional to the temperature during the first 90 min, with a maximum after 45 min independent from the milling material. A pressure drop was observed at t > 45 min, which is presumably due to oxygen uptake by the powders. Using the ideal gas law, the amount of oxygen uptake by graphite during milling was calculated from the pressure drop (starting value: 21 mg; for milling atmosphere volumes, see Table 1). Based on the pressure drop, the oxygen contents were 12 and 13 mg after milling for 16 h with milling


3


tools made from stainless steel and zirconia, respectively, and thus, the amounts of oxygen uptake by the carbon material were 9 and 8 mg, respectively. We assume that only oxygen is converted in this process, as no increase in the nitrogen content in the solid products was detected from elemental analysis (cf. following section). Product morphology The medium particle sizes (d50, Figure S2) of pristine graphite, A-6h and D-6h were 612, 14 and 29 µm, respectively. An increase in the milling time resulted in a decreased d50: 11 (B16h), 13 (E-16h) 8 (C-24h) and 10 µm (F-24h). The morphologies of graphite and the ball-milled samples are shown in Figure 2. The starting material (Figure 2a, SEM and TEM) consisted of flakes. Micrographs revealed a change in the sample morphology after ball milling.36-38 Figure 2c-f of SEM showed a significant reduction of the particle size after ball milling. Figure 2c-g of TEM depicted the irregular and stacked layered of the milled samples which a characteristic of graphite oxide as stated in Ma et al.39 and Mhamane et al.40. The graphite structure changed as a result of the graphite layers sliding due to the impact forces from ball collisions.38 For instance, SEM of graphite milled under 0.3 MPa nitrogen for 24 h at 150 min-1 (ball-topowder-ratio: 132:1, stainless steel) confirmed the absence of flake-like structures and, instead, showed irregular sphere-like particles with a broad particle size distribution.36 Furthermore, Energy dispersive X-ray spectroscope (EDX) measurements were conducted for all the studied materials (Figure 3). EDX showed the relative elements concentration on the surface of our products to check the possible contamination during the ball milling. It is proven that there was no iron contamination from the milling materials and the inset table showed the presence of oxygen in the samples in addition to the carbon as indicated in elemental analysis (EA; Table S3). The surface areas of the samples were determined using the BET method (Figure 4). The products generated by ball milling with stainless steel and zirconia possessed average surface areas of 666 and 730 m2 g-1, respectively. Notably, the present method acquired higher surface areas for the prepared GO samples than those previously reported in the literature. Using a combination of 5 and 7 mm milling balls, Dash et al. obtained GO with a 188 m2 g-1 surface area after 24 h of milling.41 Antisari et al.42 reported that 20 h of ball milling of graphite in the presence of water increased its surface area from 6 to 43 m2 g-1. Using DMF as a solvent resulted in a surface area increase from 23 to 35 m2 g-1 after 15 h of milling, also indicating a higher degree of graphite exfoliation.43 During graphite milling, as the surface area increased, the particle size reduced and a new surface was generated,44 indicating that no large clusters formed. Štefanić et al.45 reported that the level of contamination gradually dropped with a prolonged milling time, which can be attributed to a reduction in the grain size (Figure 2). Adsorption-desorption experiments confirmed the existence of micropores in the ball-milled samples (Table 2). Noticeably, the difference between the surface areas of samples B-16h and C-24h was rather small. On the other hand, for the milling experiment with zirconia, larger differences in the surface areas after 16 (E-16h) and 24 h milling (F-24h) were observed due to the higher energy input, as the stainless-steel milling balls provided a higher kinetic energy. Therefore, the surface area increased faster. According to IUPAC classification, all the samples showed type IV isotherms, characterized by the existence

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of mesopores (1.5-100 nm)46 and high adsorption energies. The average pore size of graphite was 8.9 nm. However, the average pore sizes of the graphite samples milled with stainless steel for 6, 16 and 24 h were 4.4, 4.6 and 4.3 nm, respectively. For ball milling with the zirconia balls, the average pore sizes were 4.7, 4.5 and 4.2 nm. Similar average pore sizes of GO (4.8 nm; Hummers method) have been reported in the literature. 47 Product composition For milling with stainless-steel balls, elongation of the milling time resulted in an oxygen increase from 2.54 to 13.24% for pristine graphite and C-24h, respectively (Figure 5). For the use of zirconia balls, the oxygen concentration increased to 13.10% after 24 h of ball milling. To understand the reason for this oxygen content increase further experiments were conducted under argon (Ar) and oxygen (O2) gas conditions, and the O/C ratios are provided in the ESI (Figure S1). The oxygen content increased under air, Ar and O2 conditions, but this increase was slower during milling under Ar. This increase may be due to the introduction of oxygen into the milling jar from vessel leakage during the milling process.48 Therefore, the experiments were conducted under air conditions to facilitate the scale-up process. These results indicated that the milled samples possessed a large amount of oxygen functional groups on their surface.41 Moreover, the ball-milled samples possessed different oxidation degrees after different milling times.41 Using stainless-steel balls, the O/C ratios of the produced GOs were 0.06 (A-6h), 0.15 (B-16h) and 0.16 (C-24h). Figure 5 illustrates that changing the milling material to zirconia results in a similar O/C ratio after 24 h (0.15). Posudievsky et al.49 reported an increase in the O/C ratio from 0.017 to 0.125 after 3 h of ball milling of graphite in the presence of Na2SO4. Comparison of the GO produced with different chemical oxidation methods resulted in materials with O/C ratios ranging from 0.062 to 0.5.50 Thermal analyses of graphite and the milled samples revealed a general decreasing thermal stability as the milling time increased (Figure 6). Mass loss was attributed to three events: i) the release of sorbed water and the desorption of gases (T < 200 °C), ii) the removal of oxygen-containing groups and amorphous carbon (200 ≤ T < 400 °C)51 and iii) the pyrolysis/combustion of more-stable oxygen-containing groups and the remaining carbon skeleton (T ≥ 400 °C).52 Under high-temperature pyrolysis and combustion (700 ≤ T ≤ 1,000 °C), the weight losses of graphite (G), A-6h, B-16h and C-24h were 10, 50, 88 and 95%, respectively. A similar trend was observed for the samples prepared with zirconia milling tools, revealing 18, 55 and 80% weight losses (for details see ESI; Table S4). The polydispersities of the graphene materials were estimated from TGA using the temperature ranges (ΔT) and the average combustion temperatures (T1/2).53 Figure 6c shows that the produced GOs are located in the temperature region of 580710 °C, whereas the characteristic temperature range for graphitic materials is 800-950 °C. The determined T1/2 values are in accordance with previously reported results.41,54 Furthermore, the ΔT value is related to the polydispersity of the product. The produced GOs (B-16h, C-24h, E-16h and F-24h) have lower polydispersities than those of graphite, A-6h and D-6h.


4

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Figure 2. SEM micrographs (left) and TEM images (right) of (a) pristine graphite, (b) A-6h, (c) B-16h, (d) C-24h, (e) D-6h, (f) E-16h and (g) F-24h. For sample notation see Table 1.

Figure 3. Graphical representation of EDX for: (a) graphite, (b) A-6h, (c) B-16h, (d) C-24h, (e) D-6h, (f) E-16h and (g) F-24h. For sample notation see Table 1.


5


Figure 4. N2-adsorption-desorption isotherms of pristine graphite and the ball-milled samples (for sample notation see Table 1).

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Figure 5. Elemental compositions of pristine graphite and the ball-milled samples (for sample notation, see Table 1).

Spectroscopic characterization The overview XPS spectra (Figure 7) showed C 1s and O 1s peaks in all the samples. An increase in the O 1s intensity upon prolonged ball milling is in accordance with the elemental analysis results (Figure 5) and Raman spectroscopy (cf. following text). The C 1s spectra of the feed and ball-milled samples are illustrated in Figure 7. For pristine graphite (Figure 7a), a peak at 284.1 eV indicates the presence of sp2 carbon. The prepared GO samples (Figure 7b-g) displayed the characteristic peaks of the C–C skeleton and hydroxyl (C–OH), ether and epoxy (C–O– C) and carbonyl/carboxyl (C=O/COOH) groups at 284.0-284.5, 286.0-286.2, 286.5-286.6 and 288.4-288.6 eV, respectively. The decreased peak area % corresponding to the C–C skeleton (284 eV) combined with the increasing peak area corresponding to oxidized carbon indicates the oxygenation of graphite to yield GO (Table 3). In addition, prolonged milling seemed to enhance the formation of ether, epoxy, carbonyl and carboxyl groups. Deconvoluted data for the O 1s peaks is shown in the ESI (Figure S3 and Table S5). Furthermore, XRD is shown in Figure S5.


6

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(a)


(b)

(c)

Figure 6. Thermogravimetric curves for (a,b) pristine graphite and the ball-milled samples (for sample notation, see Table 1) as well as (c) a thermo-phase diagram.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 7. XPS overview spectra for the graphite samples milled with stainless-steel (top left) and zirconia (top middle) milling tools. Deconvolution of the C 1s peak for (a) pristine graphite and (b) samples A-6h, (c) B-16h, (d) C-24h, (e) D-6h, (f) E-16h and (g) F-24h.


7


Table 3. Carbon functionalities of pristine graphite and the ball-milled samples from the XPS spectra (Figure 7). O/C ratio based on the ratio of the integrated O 1s and C 1s peaks in the overview spectra and elemental analysis (EA).

Graphite (G)

C-C 82.6

Area percentage [%] C-O C-O-C O-C=O 17.4 -

A-6h1)

67.3

20.7

12.0

-

O/C ratio [-] XPS EA 0.03 0.05 0.06 0.07

B-16h1)

60.4

17.3

14.0

8.3

0.17

0.15

C-24h1)

55.1

29.5

5.3

10.1

0.21

0.16

D-6h2)

62.0

38.1

-

-

0.06

0.05

0.11 E-16h2) 62.1 15.8 12.8 9.3 0.11 2 F-24h ) 53.6 14.2 15.4 16.8 0.20 0.15 Reaction conditions, see Table 1. 1) Milling tools: stainless steel, jar volume: 80 ml, initial mass of graphite = 1.49 g. 2) Milling tools: zirconia, jar volume: 80 ml, initial mass of graphite = 1.40 g. Note: the total yield of one milling cycle is 2.89 g (steel) and 2.80 g (zirconia). steel (c) 14 Raman spectroscopy is a powerful analytical technique to characterize carbon-based nanomaterials10. The main features in the Raman analysis of graphene and its derivatives are the positions of the G and D bands and their overtones (2D). The G band is attributed to sp2 hybridized carbon bonds (in-plane vibrations of the aromatic carbon structure), while the D band is an indication of structural defects (loss of hexagonal symmetry of disordered graphite).55,56 The Raman spectra of graphite and the ball-milled samples are shown in Figure 8a,b, and the Raman shifts for the D, G, D` and 2D bands are summarized in Table 4. The Raman spectrum of graphite revealed D, G and 2D bands at 1350, 1576 and 2711 cm-1, respectively. The intense G band in graphite is due to first-order scattering of the E2g mode, and the small D band is evidence for the presence of defects, such as bond-angle disorder and vacancies,57 in the graphite material. After ball milling, the samples revealed more intense D bands and broadened G bands. The observed line broadening is a result of the presence of more defects.58 Furthermore, the shoulder of the G band (D`) is characteristic of the presence of few-layered graphene.19,20 Figure 8c shows the relationship between the IG/ID´ ratio and the oxygen content after ball milling. When the IG/ID´ ratios are compared between the steel and zirconia ball-milled samples, the intensity ratio decreased from 6.6 (A-6h) and 7.1 (D-6h) to 1.9 (C-24h and F24h) for the steel and zirconia milled samples, respectively (Figure S4). Consequently, the degree of oxidation was estimated from the IG/ID´ ratio.59 The position of 2D band not only shifted to lower values but also became more symmetrical with an increasing milling time. This position reached 2696 (C-24h) and 2695 cm-1 (F-24h). In the literature, ball milling of graphite in the presence of KMnO4 or (NH4)2S2O8 resulted in a carbonaceous material with a 2D band at 2690 cm-1, characteristic of monolayer GO sheets.60

(b) D

G

2D

D

G D`

2D

Oxygen content [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46(a) 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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zirconia

12 10 8 6 4 2 0 0

1.5

3

4.5

6

7.5

IG/ID´ [-]

Figure 8. (a,b) Raman spectra of pristine graphite and the ballmilled samples. (c) Relationship between IG/ID´ and the oxygen content % after ball milling (experimental conditions, see Table 1). Table 4. Positions of the D, G, D` and 2D bands in the Raman spectra and the ID/IG, IG/ID´ and La values of pristine graphite (G), commercial graphene oxide (GO) and the ball-milled samples (Figure 8).

G

D G D` 2D ID/IG [cm-1] [cm-1] [cm-1] [cm-1] [-] 1350 1576 2711 0.10

IG/ID´ [-] -

La [nm] 168

GO

1349 1599 -

-

17

A-6h1) 1354 1574 1614 2708 0.26

6.6

64

B-16h1) 1350 1576 1613 2700 0.70

2.4

24

C-24h1) 1350 1574 1608 2696 0.84

1.9

20

D-6h2) 1352 1573 1612 2679 0.25

7.1

67

E-16h2) 1346 1570 1605 2694 0.31

4.6

54

F-24h2) 1350 1577 1605 2695 0.77

1.9

22

2710 0.97

Reaction conditions, see Table 1. 1) Milling tools: stainless steel, jar volume: 80 ml, initial mass of graphite = 1.49 g. 2) Milling tools: zirconia, jar volume: 80 ml, initial mass of graphite = 1.40 g.

D`

D` D`

D` D`

The intensity ratios of the D and G bands (ID/IG), representing the structural defects, are summarized in Table 5 for graphite, commercial GO and the ball-milled samples. The ID/IG ratio of graphite is rather low (0.1) due to its large grain size (cf. Figure 2 and Figure S2). After 24 h of ball milling, the ID/IG ratio gradually increased to 0.84 (C-24h) and 0.95 (F-24h) with an


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increasing milling time (ID/IG for GO: 0.97). This indicates increased structural distortion (structural imperfections created by the hydroxyl, carboxyl and epoxide groups on the carbon basal plane) and size reduction of the in-plane sp2 domain caused by the induced mechanical stress.41 According to McDonald et al.,61 the ID/IG ratio of GO is always ~ 0.95, indicating a large amount of defects within the crystal lattice. Moreover, the ID/IG ratio is useful to calculate the in-plane sp2 crystallite size (La):19,41

La  2.4 1010 

4 laser

I D / IG

(2)

where λ is the wavelength of the laser source (nm). In graphite, La is 168 nm, and Table 5 reveals that the La values of the samples decrease with an increasing milling time. A La of 20 nm was found for sample C-24h, whereas the sample prepared with zirconia (F-24h) possessed a value of 22 nm. The decreased La with an increasing milling time is likely due to the breaking of crystallites due to the increasing degree of oxidation.41 Similar values have been reported for samples prepared with stainless-steel balls of different sizes for 24 h (18.5 nm) 41. Krishnamoorthy et al.57 calculated the in-plane sp2 crystallite size (La) of GO synthesized using a modified Hummers method (KMnO4 and conc. H2SO4, followed by ultrasonication). Increasing the amount of oxidant from 1 to 6 g resulted in decreased La values of 18.2 and 13.8 nm. Notably, the ID/IG ratio is inversely proportional to the in-plane sp2 crystallite size (La), as shown in Table 5. Herdman et al.62 confirmed the same inverse relationship between the ID/IG ratio and La with La values greater than 2 nm with relatively low disorder. GO solutions. The ball-milled carbon materials were dispersed in deionized water. Suspensions prepared from samples milled for 6 h showed no significant absorbance (A-6h, D-6h). Aqueous dispersions of samples prepared by milling with stainless-steel balls revealed absorbances at 265 and 263 nm for B-16h and C-24h, respectively (Figure 9).

pristine graphite (Figure S2), the surface morphology changes, which enhances the dispersive ability and hydrophilicity due to oxygen functionalization36. Moreover, solutions of the prepared GOs showed a linear relationship between the absorbance and the GO solution concentration, thus obeying the Beers-Lambert law (cf. Figure S6 in the SI). Application for MB adsorption The ball-milled graphite samples (cf. Table 1) were tested for their suitability to remove MB from aqueous solution. The removal efficiencies (eq. 1) of MB (30 mg L-1) for B-16h and C24h increased to 99.3 and 99.6%, respectively, after 15 min. The pH of the solution, which can change the surface charges of the adsorbent and adsorbate, is an important factor.63 According to the literature,64,65 the pH values of the solutions were adjusted to 8-9 to obtain high MB removal efficiencies. Here, the removal efficiencies at pH 8.5 reached 99.3% and 99.5% for B16h and C-24h, respectively (Figure 10). The removal rate reached equilibrium values at an adsorption time of 30 min. Notably, the remaining solution was almost colorless. The small difference between B-16h and C-24h in MB adsorption is likely due to their similar surface areas (cf. Table 2). The fast adsorption rate is a merit of the prepared GO. In comparison, graphite (G) removed only 48.1% of MB after 15 min at pH 4.5, while at pH 8.5, a removal efficiency of only 54.2% was found. The increasing removal efficiency is in agreement with the increased oxygen% in the ball-milled samples (see Figure 5 and Table 3), indicating that oxygen-containing functional groups, especially hydroxyl, epoxy, carbonyl and carboxyl groups, play key roles in the adsorption process. Furthermore, Pérez-Ramírez et al.66 stated that small adsorbent sizes endow advantages, such as rapid equilibrium rates, high adsorption capacity, and effectiveness over a broad pH range. Yang et al.67 found that the adsorption of MB onto activated carbon or carbon nanotubes requires more than 1 h to reach equilibrium. In recent years, different routes have been established to synthesize GO materials and test these materials for their MB removal efficiencies. Table 5 summarizes these results, indicating that samples produced by ball milling are comparable or even better regarding their removal efficiencies.

(a)

Figure 9. UV-vis absorption of graphene oxide and the ballmilled samples dispersed in water (for sample notation, see Table 1).

(b)

However, the absorbance peaks of GO produced with zirconia (E-16h and F-24h) occurred at 267 and 264 nm. In comparison, the absorbance peak of commercial graphene oxide nanocolloids ( 0.4 mg mL-1) was observed at 240 nm with a shoulder 335 nm. The small shifts in the red spectral region is related to the lower oxidation degree of the ball-milled GO. Although Posudievsky et al.60 applied (NH4)2S2O8 as an additional oxidant during ball milling, these authors reported similar absorption spectra (255 nm). As the particle size is reduced compared to


9


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Figure 10. Effects of the absorption time and pH on the MB removal efficiency from aqueous solutions of (a) pristine graphite, (b) B-16h andC-24h. Table 5. Comparison of the experimental conditions for MB removal with different adsorbents. cMB,initial madsorbent Vsolution pH t MB removal Ladsorbent Adsorbent Ref [mg L-1] [mg] [mL] [-] [min] [%] [mg g-1] 67 125 0.6 250 Graphite oxide 1) 1.2 6 5 99 68 Graphite oxide 1) 12 25 50 9 300 98.8 243 29 Porous graphite oxide 2) NA NA NA 6.8 NA 99 200 69 Graphite oxide 3) 45 1 20 9 360 24 216 69 Graphite oxide 4) 45 1 20 9 360 35 306 1 59 PVA/graphene oxide composite ) 80 25 50 9.2 NA 90 122 70 Agar/graphene oxide composite 5) 200 20 50 6 1,560 95.4 476 GO 6) 30 12.5 50 5 15 99.3 278 This study GO 6) 30 12.5 50 8.5 1 99.3 278 This study 1) Modified Hummers method. 2) Hydrothermal method. 3) Commercial. 4) Magnetic graphene oxide by co-precipitation method. 5) Modified Hummers method and vacuum freeze drying. 6) Dry ball milling (sample B-16h).

Corresponding Author * Alaa El Din Mahmoud. Email: [email protected]/ [email protected]

CONCLUSIONS Mechanochemical ball milling is an environmentally friendly synthetic technology for the production of GO. A one-step, dry production route was developed to synthesize GO without the need for additional oxidants other than air. The absorbance peaks of the produced GO reached 263 and 264 nm with a small peak at 338 nm. Small shifts in the red spectral region were related to lower oxidation degrees compared to GO produced from chemical methods. However, other characterization techniques showed that the produced ball-milled GO was comparable to GO synthesized via chemical pathways. For instance, TGA showed that the weight loss was caused by the pyrolysis of oxygen-containing functional groups. The oxidation degree of the produced GO increased with an increasing milling time, as indicated by with XPS, elemental analysis and EDX. EDX analysis proved no iron contamination arised from the milling process. This oxidation degree was independent of the material used for the milling balls and jars (stainless steel or zirconia). However, the rate of oxygen increase in the material as the milling time increased was more pronounced for stainless-steel ball-milling materials (due to their higher density) than that for zirconia materials. The intensity ratios of the D and G bands (ID/IG) for GO produced with stainless-steel (0.84) and zirconia balls were lower than that for GO produced by chemical methods, indicating a smaller amount of defects within the crystal lattice of the ball-milled samples. Moreover, the BET surface area of GO synthesized via ball milling was > 650 m2 g-1 containing micro- and mesopores, enabling its application as an interesting material for energy and environmental applications. The produced GO proved efficient for MB removal (greater than 99% removal after 1 min).

Notes The authors declare no competing financial interest. ACKMOWLEDGMENTS Alaa El Din thanks the ‘‘Deutscher Akademischer Austausch Dienst’’ (DAAD) and the Egyptian Ministry of Higher Education for his PhD scholarship. Authors express their thanks to Prof. Dr. Lothar Wondraczek (Otto-Schott-Institute for Material Research, Jena) for Raman measurements and Dr Bernd Schroeter (Institute for Solid State Physics, Jena) for XPS measurements. REFERENCES

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ASSOCIATED CONTENT Supporting Information. “This material is available free of charge via the Internet at http://pubs.acs.org.” Details of the calculations of the ball-milling parameters and characterization methods (EA, TGA, XPS, particle size distribution, Raman spectroscopy, XRD and UV-vis absorption spectroscopy).

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AUTHOR INFORMATION

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Synopsis Alternative route was developed to access graphite oxide powders via a solvent-free mechanochemical process. It didn’t require harsh chemicals.


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Figures List - Figure 1. Temperature and pressure measured on-line during the ball milling of graphite with stainlesssteel and zirconia milling tools. Beaker volume was 80 ml for both materials. - Figure 2. SEM micrographs (left) and TEM images (right) of (a) pristine graphite, (b) A-6h, (c) B-16h, (d) C-24h, (e) D-6h, (f) E-16h and (g) F-24h. For sample notation see Table 1. - Figure 3. Graphical representation of EDX for: (a) graphite, (b) A-6h, (c) B-16h, (d) C-24h, (e) D-6h, (f) E16h and (g) F-24h. For sample notation see Table 1. - Figure 4. N2-adsorption-desorption isotherms of pristine graphite and the ball-milled samples (for sample notation see Table 1). - Figure 5. Elemental compositions of pristine graphite and the ball-milled samples (for sample notation, see Table 1). - Figure 6. Thermogravimetric curves for (a,b) pristine graphite and the ball-milled samples (for sample notation, see Table 1) as well as (c) a thermo-phase diagram. - Figure 7. XPS overview spectra for the graphite samples milled with stainless-steel (top left) and zirconia (top middle) milling tools. Deconvolution of the C 1s peak for (a) pristine graphite and (b) samples A-6h, (c) B-16h, (d) C-24h, (e) D-6h, (f) E-16h and (g) F-24h. - Figure 8. (a,b) Raman spectra of pristine graphite and the ball-milled samples. (c) Relationship between IG/ID´ and the oxygen content % after ball milling (experimental conditions, see Table 1). -Figure 9. UV-vis absorption of graphene oxide and the ball-milled samples dispersed in water (for sample notation, see Table 1). - Figure 10. Effects of the absorption time and pH on the MB removal efficiency from aqueous solutions of (a) pristine graphite, (b) B-16h andC-24h.

1 ACS Paragon Plus Environment

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Figure 1. Temperature and pressure measured on-line during the ball milling of graphite with stainlesssteel and zirconia milling tools. Beaker volume was 80 ml for both materials.

Figure 2. SEM micrographs (left) and TEM images (right) of (a) pristine graphite, (b) A-6h, (c) B-16h, (d) C24h, (e) D-6h, (f) E-16h and (g) F-24h. For sample notation see Table 1



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Figure 3. Graphical representation of EDX for: (a) graphite, (b) A-6h, (c) B-16h, (d) C-24h, (e) D-6h, (f) E16h and (g) F-24h. For sample notation see Table 1.

Figure 4. N2-adsorption-desorption isotherms of pristine graphite and the ball-milled samples (for sample notation see Table 1).


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Figure 5. Elemental compositions of pristine graphite and the ball-milled samples (for sample notation, see Table 1).



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Figure 6. Thermogravimetric curves for (a,b) pristine graphite and the ball-milled samples (for sample notation, see Table 1) as well as (c) a thermo-phase diagram.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 7. XPS overview spectra for the graphite samples milled with stainless-steel (top left) and zirconia (top middle) milling tools. Deconvolution of the C 1s peak for (a) pristine graphite and (b) samples A-6h, (c) B-16h, (d) C-24h, (e) D-6h, (f) E-16h and (g) F-24h.


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(a)

(b) D

G

2D

D

G

2D

D`

D`

D`

D` D`

D`

(c) 14 Oxygen content [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


steel zirconia

12 10 8 6 4 2 0 0

1.5

3

4.5

6

7.5

IG/ID´ [-]

Figure 8. (a,b) Raman spectra of pristine graphite and the ball-milled samples. (c) Relationship between IG/ID´ and the oxygen content % after ball milling (experimental conditions, see Table 1).

Figure 9. UV-vis absorption of graphene oxide and the ball-milled samples dispersed in water (for sample notation, see Table 1).



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Figure 10. Effects of the absorption time and pH on the MB removal efficiency from aqueous solutions of pristine graphite, B-16h and C-24h.


Sustainable Synthesis of High-Surface-Area Graphite Oxide via Dry

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