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Tuning the Mechanical and Adsorption Properties of Silica with Graphene Oxide [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) Silica and graphene oxide (GO) were synthesized by modified Stöber and Hummer's methods, respectively. Silica/GO composite powders of different compositions were obtained by introduction of GO sheets in between silica particles through a hydrothermal synthesis. Composites were characterized by Fourier transform infrared spectroscopy, X-ray diffraction analysis, and scanning and transmission electron microscopy. Density measurements were performed with a pycnometer and chemical analyses of powders were performed by energy-dispersive X-ray and X-ray photoelectron spectroscopies. Composite powders showed an improvement in the Cu2+ ion adsorption property relative to pure silica. Composite powders were further consolidated at 1200°C under 50 MPa pressure in vacuum by pulsed electric current sintering. Transparency in the UV/Vis region, the electrical conductivity, and the toughness of consolidated samples were altered by varying the GO content in the starting materials. Studies suggest that the use of GO as a reinforcing agent not only improves the mechanical properties of transparent silica ceramic, but also improves the electrical conductivity and adsorption properties of silica. Keywords : ceramics * copper removal * graphene oxide * mechanical properties * optical properties (ProQuest: ... denotes formulae omitted.) Introduction The potential of the nanosize-enhanced properties of ceramic nanoparticles, such as silica, alumina, titanium dioxide, zirconia, silicon nitride, and silicon carbide, has been widely recognized in several areas of materials and surface science. Among them, nanosized silica (SiO2) has many interesting properties, such as low thermal expansion coefficient, low electrical conductivity, high chemical resistance, and high visible- and ultraviolet-light transparency.[1, 2] It is widely used for high-efficiency lamps, an- tenna windows, crucibles for melting high-purity silicon, inte- grated-circuit photomask substrates, lens material for excimer stepper equipment, and white-light phosphors.[3] Transparent SiO2 glass has been commonly fabricated by melting at high temperatures with optical loss, particularly in the middle to near-infrared (NIR) region.[1] Other techniques, such as sol-gel, vapor-phase axial deposition, pressureless sintering of floc- cast[4] or slip-cast[1] green body, and laser sintering have also been used to prepare SiO2 glass at low temperatures.[5] Howev- er, if a long sintering process and high sintering temperatures are employed for eliminating residual pores and enhancing transparency, crystallization of SiO2 glass and a loss of transpar- ency are induced.[6] Pulsed electric current sintering (PECS), also known as spark plasma sintering (SPS), employs a combi- nation of a pulsed direct current and uniaxial exerted pressure for compaction, and allows powders to be consolidated at comparably low temperatures in a very fast manner, thus effec- tively suppressing the grain growth and phase changes. In ad- dition, it is possible to reach densities close (> 99 %) to the the- oretical density.[7] SPS has been used to prepare a wide spec- trum of ceramic materials, including biomaterials,[8] compo- sites,[9] dielectrics,[10] and transparent ceramics.[11] Mayerhçfer et al.[12] reported on the feasibility of preparing glassy and transparent materials from amorphous SiO2 powders by the SPS method. With respect to both local structural features and the overall network hierarchy, the SPS-compacted SiO2 samples were found to be very similar to conventionally prepared silica glass. Graphene, a one atom thick 2 D lattice of sp2-bonded carbon, has attracted great attention from the science com- munity because of its multifunctional properties including high surface area, low density, high electron transport properties, quantum Hall effect, high thermal conductivity, and ease of functionalization.[13, 14] Graphene-based polymer nanocompo- sites, devices, and sensors have been developed and investi- gated over the past decade.[15-17] At present, there are five different reported methods to obtain graphene. These are chemical vapor deposition, micro- mechanical exfoliation of graphite, epitaxial growth on an elec- trically insulating surface, solvothermal synthesis, and the re- duction of graphene oxide (GO).[3,18, 19] Mostly, the starting ma- terial is graphite oxide produced by acidic oxidation of graph- ite, that is, graphite intercalated by oxygen-containing func- tional groups, such as epoxy, carboxylic, carbonyl, and hydroxyl. Graphite oxide is then treated mechanically (i.e. , by ultrasonication) to obtain GO with a single to a few layers. Among graphene production methods the chemical reduction of GO with reducing agents, such as hydrazine hydrate, dime- thylhydrazine, hydroquinone, NaBH4,H2S, hexylamine, and sulfur-containing compounds (NaHSO3 and Na2S),[20, 21] is a promising approach for the large-scale production of re- duced graphene oxide (RGO). RGO can be dispersed easily compared with single-layer graphene owing to the presence of trace amounts of oxygen-containing functional groups, and it has high surface energy because of the high surface area in- trinsic to its geometry. These properties suggest RGO as a promising candidate for strong interfacial bonding with a ce- ramic matrix. Unfortunately, getting RGO is very slow and needs excessive usage of reducing agents, which are environ- mentally prejudicial. These are highly toxic and leave impurities in graphene sheets, which also alter their properties. Therefore there is increasing demand for nontoxic and effective ap- proaches for producing graphene chemically. Thermal reduc- tion of GO can be an eco-friendly method to produce high- quality RGO on a large scale without using any hazardous re- ducing agents.[22] Graphene with its outstanding mechanical properties, such as high tensile strength (130 GPa)[23] and high Young's modulus (0.5-1 TPa),[24] shows great potential as a nanofiller in compo- site materials. Recently, the use of graphene as nanofiller in ce- ramic matrix composites showed improvements in mechanical, electrical, and thermal properties.[25-27] Walker et al. have im- proved the fracture toughness by approximately 235 % and discussed the new toughening mechanisms for silicon nitride composite containing graphene nanoplatelets.[25] Moreover, Ramirez et al. have reported the effect of graphene on the electrical properties of a silicon nitride matrix by SPS compac- tion.[28] Copper in the form of Cu2 + is one of the hazardous metal ions present in wastewater discharge and effluent from many industries.[29] Its presence in the body can cause insomnia, skin rashes, and premenstrual syndrome. Copper can be accumulat- ed in the kidneys, brain, skin, and heart. A wide range of pro- cesses, such as chelation therapy, reverse osmosis, ion ex- change, adsorption, and biological treatment, are being ex- ploited for removal of heavy-metal ions from aqueous solu- tions.[30, 31] Several studies have shown that the efficiency of re- moving heavy metals from wastewater by adsorption methods is dependent on the physical and chemical composition of the adsorbents. For example, silica-based membranes have been employed for their simple, inexpensive, and effective removal of heavy-metal ions from water.[32, 33] The results suggested that the hydrophilic groups on the surface of silica nanoparticles contribute to the overall properties of the membrane, especial- ly water permeability, hydrophilicity, and antifouling ability. Furthermore, silica particles with smaller size possess a higher ratio of external surface area to volume, which is helpful for the water purification performance.[34] However, the inorganic materials, especially nanosized materials, always encounter a problem of their homogeneous dispersion or aggregation in the membrane matrix at higher concentration. This problem can be overcome by functionalizing the surface of silica with GO having hydrophilic functional groups at the surface. GO has a high specific surface area and many oxygen atoms on the graphitic backbone in the form of epoxy, hydroxyl, and car- boxyl groups. These oxygen groups can bind to metal ions, es- pecially multivalent metal ions,[35] through both electrostatic in- teractions and coordination bonds. An increase in surface area can influence the absorption capacity by providing more active sites for absorption with oxygen-containing functional groups.[36] In 2003, Bois et al.[37] utilized aminopropyl-functional- ized silica for efficient adsorption of heavy-metal ions such as Cu2+ ,Ni2 + , and Co2 + at pH 6. Recently, Liu et al.[38] have report- ed the efficient removal ([asymptotically =]95 %) of pesticides from water by utilizing graphene-coated silica particles. Therefore, silica/GO composite can be expected to be a good candidate as an ad- sorbent for the removal of Cu2 + ions. Herein, by taking GO as a starting material, we have exploit- ed the multifunctional properties of graphene. The thermal re- duction method was used to reduce GO while consolidating the composites by PECS. In particular, we have studied the me- chanical and electrical properties of SiO2 ceramic matrix in rela- tion to the transparency of the ceramic in the UV/Vis region. At the same time, we have also investigated the ability of GO to improve Cu2 + ion adsorption by SiO2 particles from water. Results and Discussion After hydrothermal treatment in water the SiO2/GO composite suspensions showed increasingly darker color with an increas- ing amount of GO, as illustrated in Figure 1. Suspensions were dried in an oven at 60°C for 12 hours for further characteriza- tions. As it is expected that the density and the specific surface area of the composite powders change with the addition of GO, they were measured. Figure 2 shows the plot between density and specific surface area versus the amount of GO. It is evident that an increase in the amount of GO reduces the den- sity of the composites whereas the specific surface area in- creases. The transmission spectra of the compacts of silica and silica/ GO composites obtained through PECS are depicted in Figure 3. PECS-compacted silica shows a transmission of 89 %, whereas the composites show variation in transmission from 4 to 84 % depending on the amount of RGO in the ceramic matrix. It is clear from the plot that increasing the amount of RGO in the silica matrix, that is, from SG-0.001 to SG-1, reduces the transmission of ceramics. The SG-10 compact was opaque and electrically conducting with a sheet resistance of 30 O *-1. The Raman spectra of amorphous silica powder, GO, and silica/GO are compared in Figure 4. The Raman spectrum of amorphous silica powder has a weak broad peak at 396 cm-1 and a sharper peak at 481 cm-1. The former may be attributed to an asymmetrical Si-O-Si stretching mode and the latter to the Si-O-Si symmetric vibration, denoted as the D1 band.[39] The Raman spectra of GO and silica/GO display a prominent G peak at 1596 cm-1 along with a D peak at 1358 cm-1 corre- sponding to the first-order scattering of the E2g mode and A1g mode, respectively,[4] with an ID/IG ratio of 0.98. The Raman spectra of PECS-consolidated silica and silica/GO composite are compared in Figure 5. In the case of PECS-con- solidated silica, the D1 peak becomes much less intense, which could result from the rearrangement of the basic structural units. A new peak appears at 600 cm-1 attributed to the Si-O- Si symmetric vibration of the three-membered ring of the SiO4 tetrahedra and is represented as the D2 peak. Furthermore, new peaks at 796 and 1072 cm-1 correspond to symmetric Si- O-Si stretching.[12] Consolidated silica/GO composite, on the other hand, shows formation of three new sharp peaks at 110, 230, and 416 cm-1 corresponding to Si-O stretching.[40] The appearance of these new peaks indicates the formation of a-crystobalite. Further- more, the ID/IG ratio changes from 0.98 to 1.05 as compared to the powder sample. An increase in the ID/IG ratio may be ex- plained in terms of an increase in the quantity of amorphous carbon, a higher density of defects in the structure, or a reduc- tion in the crystallite size or domains. To understand the distribution of RGO over silica after PECS consolidation, SEM mapping over cross sections of PECS-con- solidated silica/GO composite was performed. The scanning area is 250-150 mm2, indicated by the box in Figure 6 a. SEM mapping clearly indicates a uniform distribution of carbon- based sheets over silica particles. The FTIR spectra of silica, GO, and silica/GO nanocomposites are depicted in Figure 7 a. In silica, a broad peak at 3424 cm-1 and moderate peak at 810 cm-1 correspond to the stretching vibration of -OH and the Si-O-Si bending vibration, respec- tively. The weak peak at 960 cm-1 can be attributed to silanol groups (Si-OH). The presence of two intense peaks at 1120 and 472 cm-1 can be ascribed to the Si-O antisymmetric vibra- tion and Si-O-Si bending vibration, respectively.[41] In GO, the band at 3383 cm-1 can be attributed to -OH stretching, whereas absorption peaks appearing at 1721 and 1407 cm-1 are because of C=O and O-H deformation vibration, respec- tively. Furthermore, bands at 1241 and 1112 cm-1 are because of epoxy symmetrical ring deformation and C-O stretching mixed with C-OH bending, respectively. The peak correspond- ing to liberation and bending vibrations of H-O-Hat 2127 cm-1 is not present with a 1601 cm-1 peak ;[42] therefore, the peak at 1601 cm-1 can be assigned to the skeletal vibra- tions of the graphene sheets.[15, 22, 43] In comparison with silica/ GO composites, a peak of C=O gradually starts appearing with the reduction of peak intensity at 810 cm-1 as the amount of GO increases. This indicates in- teractions between oxygen-con- taining functional groups of GO and silica particles as proposed in Scheme 1. Figure 7 b compares the FTIR spectra of silica and silica/GO nanocomposites after PECS treatment. After PECS treatment all samples show a new peak at 780 cm-1 owing to Si-O-Si stretching. The peak at 1601 cm-1 becomes more promi- nent at a higher amount of GO with the disappearance of C=O and C-O groups, which indicates the reduction of GO. In compari- son to silica, composites with a higher concentration of GO show broadening of the peak at 960 cm-1 and the appearance of a new peak at 615 cm-1. The peak at 615 cm-1 may be attributed to a-cristobalite.[44] Peaks at 1105 cm-1 and at approximately 1260-1300 cm-1 corre- sponding to Si-O-C[40] and Si-O-C(=O),[45] respectively, are absent. This clearly indicates there is no direct Si-O-C bond from silica to graphene. The XRD spectra of synthetic graphite, starting GO, silica powder, and silica/GO composite are compared in Figure 8. Synthetic graphite shows diffraction peaks at 2q = 26.36, 44.44, and 54.488 corresponding to (002), (101), and (004) reflections, respectively. The (002) diffraction peak for GO is found to be shifted to approximately 10.078, which indicates an interlayer spacing of approximately 8.93 Å. The highly oxidized GO sheets are expected to be thicker as a result of intercalated water molecules trapped in between adjacent GO sheets.[41] Silica powder shows only a diffuse scattering maximum cen- tered at 2q = 22.48 without any other crystalline peaks, where- as silica/GO composites show characteristic GO (002) indicating the presence of GO in the silica matrix. XRD spectra of PECS-consolidated samples are compared in Figure 9. PECS-consolidated silica does not show any crystalline peak, whereas PECS-consolidated silica/GO composites show disappearance of the (002) diffraction peak of GO indicating the reduction of GO. Furthermore, composites show sharp dif- fraction peaks of a-cristobalite at 2q= 21.93, 28.38, 31.44, and 36.348 corresponding to (101), (111), (102), and (112) reflec- tions, respectively. The peak intensities of cristobalite increase with increasing amount of GO in composites, which indicates the change in bond angle of Si-O-Si owing to removal of functional groups from the surface of amorphous silica, as shown in Scheme 1 b. The XPS measurements have been exploited over PECS con- solidated composite to confirm the extent of thermal reduc- tion of GO over SiO2 particles. Figure 10 shows the XPS survey scan spectra of PECS-consolidated SG-10 composite, deconvo- luted XPS C 1s spectra of GO and SG-10 composite, and high- resolution XPS Si2p spectra of SG-10. Figure 10a clearly con- firms the presence of three elements corresponding to C (C 1s at 284.6eV), Si (Si2p at 103eVand Si2s at 151eV), and O (O1s at 531 eV). In the case of GO, four bands observed at 285, 286.9, 288.1, and 290 eV correspond to graphitic C=C bonds, C-O bonds, carbonyl carbon (C=O), and carboxylate carbon (O-C=O; Figure 10 b).[4] However, in the PECS-consolidated composite, the peak at 286.9 eV decreased significantly and peaks at 288.1 and 290 eV almost disappeared (Figure 10 c). The ratio of C/O changes from 0.63 to 3.22, which indicates a considerable degree of re- duction. Figure 10d shows the high-resolution XPS Si 2p spec- tra of SG-10 composite. The symmetrical peak is attributed to the chemical state of Si (Si4 + from SiO2).[41] SEM and TEM images of GO, silica, and silica/GO composite are shown in Figure 11. The SEM image of GO shows a wrinkled and layered morphology, whereas the silica image shows the presence of spherical particles of size approximately 540 nm. In the case of SiO2/GO composite, the image clearly shows that silica particles are completely covered by GO sheets. Energy- dispersive X-ray spectroscopy (EDS) of composite powder shows the distribution of GO sheet over silica particles (Figure 12). The cross-sectional SEM mapping of PECS-consolidated com- posite confirms the presence of compacted GO stacks in be- tween the SiO2 matrix (Figure 13). The TEM image of GO shows a wrinkled, few-layered GO sheet (Figure 14 a). Furthermore, TEM images of SiO2/GO are shown in Figure 14 b and c. From the images it is clear that the surface of the silica nanoparticle is completely coated with GO sheets, and the magnified image exhibits single-layered GO sheet over the surface. Indents were created on samples using a 49 N applied load. Figure 15 a and b show representative hardness indentation marks over silica and SG-10 nanocomposite. Figure 15 c and d show high-resolution SEM images of the hardness-induced cracks revealing the branched crack structure in SG-10 nano- composite. From Figure 15 c there is clear evidence of "sheet pull-out" and GO sheets that are bridging the cracks. The pres- ence of such bridges of GO sheet anchor the matrix, thereby giving improved values of fracture toughness for ceramics. Based on the indentation-induced crack lengths, the fracture toughness was calculated for each composite. Figure 16 shows a plot of the calculated toughness values for the SiO2/GO nanocomposites as a function of GO concentration from 0 to 10 vol % GO. The plot confirms the increase in toughness from approximately 2.4 to 3.6 MPa m1/2 with increasing GO concen- tration. Table 1 summarizes the density, theoretical density, hardness, modulus, and toughness values for each nanocom- posite. The case of another amount of graphene (0.02 to 15 vol %) in the ceramic matrix shows a significant improve- ment in the fracture toughness (50-235 %) at high holding temperature (> 1300°C).[25,46] Silica/GO composites are not only highly functionalized but also show high specific surface area, which may affect their po- tential use in the removal of contaminating Cu2 + ions. We measured the UV/Vis absorbance at 540 nm (Figure 17) to cal- culate the adsorption isotherm of Cu2 + ions for each sample, to estimate the performance of the composite for Cu2 + ion re- moval by silica and SGO-1 (Figure 18). At higher Ce values (> 20 mgg-1), the adsorption capacity of all the samples approxi- mately reaches a constant. The Langmuir and Freundlich equations were fitted to the isotherm as shown in Figures 19 and 20, respectively. Results are summarized in Table 2. The qm value [see Eq. (6)] increases with the increasing amount of GO in the composite, thus indi- cating an improvement in ad- sorption capacity of composites as compared to pure silica. Com- parison of the removal capacity of SGO-10 with that of SGO- 1 and silica suggests that com- posites with a highly functional- ized surface and high specific surface area attain higher adsorption capacity of Cu2 + ions owing to an increase in the number of active sites for adsorp- tion on the surface of the composites. Moreover, n values cal- culated through the Freundlich model are greater than 1, which indicates favorable adsorption of Cu2 + ions.[47] Com- pared with other reports, 2-acrylamido-2-methylpropanesulfon- ic acid and o-dihydroxybenzene-functionalized silica[48, 49] show lower qm values of 19.90 and 22.11 mg g-1 than SGO-10. Conclusion Silica and silica/graphene oxide (GO) composite powders have been synthesized and further consolidated by pulsed electric current sintering (PECS). Spherical silica particles of size ap- proximately 540 nm and GO were synthesized by a sol-gel method, whereas hydrothermal synthesis has been exploited for the synthesis of silica/GO composites. Powders and PECS- consolidated samples were compared for their optical, me- chanical, and adsorption properties at various GO loadings. PECS-consolidated samples were also studied for their tough- ness. GO was converted to its reduced form without using any hazardous reducing agent in a ceramic matrix by utilizing the environmental conditions of sintering. Powdered samples were used to check their adsorption properties. Chemical and struc- tural characterizations based on FTIR, Raman, and X-ray photo- electron spectroscopy and XRD analysis reveal the synthesis of silica/GO composites and reduction of GO. The transparency of ceramic in the 200-1000 nm range has been altered by GO and at the same time fracture toughness has been improved with a very low sheet resistance value of approximately 30 O *-1. The presence of a highly functionalized surface and high specific surface area of composites improve the Cu2 + ion adsorption capacity compared to silica particles. The above re- sults suggest GO is a potential candidate as a reinforcing agent to improve the toughness of transparent silica ceramic for developing tough, electrically conductive, transparent ce- ramics. At the same time GO shows improvement in making composites an efficient sorbent for Cu2 + ions. Experimental Section Starting materials Graphite powder (< 20 mm synthetic) and potassium permanga- nate (KMnO4, ACS reagent, =99 %, low in mercury), sulfuric acid (H2SO4, ACS reagent, 98 %), phosphorus pentoxide (P2O5, = 99.99 % with trace metals), potassium persulfate (K2S2O8, ACS reagent, =99.0%), H2O2 (30 wt% in H2O, ACS reagent), hydrochloric acid (ACS reagent, 37 %), tetraethyl orthosilicate (TEOS, =99.0%), and ethanol (ACS reagent, = 99.5%) were purchased from Sigma-Al- drich. Synthesis of GO GO was prepared by oxidizing graphite powder based on a modi- fied Hummer's method.[50,51] First of all, concentrated H2SO4 (50mL) was heated to 908C in a 300mL beaker with K2S2O8 (10g) and with P2O5 (10 g) added under stirring until all reactants had completely dissolved. The mixture was cooled to 80°C. Graphite powder (10 g) was then added to the H2SO4 solution, resulting in bubbling, which subsided within 30 min. The mixture was kept at 80°C for 4.5 h using a hotplate, after which the heating was stopped and the mixture diluted with deionized (DI) water (2 L) and left overnight for 8 h. After one day, the mixture was filtered and the residue was washed to remove all traces of acid. The solid was transferred to a petri dish and allowed to dry in air for 12 h. For the oxidation, H2SO4 (460 mL) was placed into a round-bot- tomed flask and cooled to 0°C using an ice bath. The pretreated graphite was then added to the acid and the mixture was stirred. KMnO4 (60 g) was added slowly and allowed to dissolve with stir- ring, while the temperature was closely monitored so as not to allow the mixture to go above 5°C. This mixture was then allowed to react at 35°C for 2 h after which distilled water (920 mL) was added. The mixture was stirred for 2 h with additional DI water (2.8 L). Then a dilution of 30 % H2O2 (50 mL) was added to the mix- ture, which resulted in a brilliant yellow color along with bubbling. The mixture was allowed to settle for at least a day after which the clear supernatant was decanted. The remaining mixture was centri- fuged and the residue was washed with 10 % HCl solution followed by DI water to remove the acid. The product was then vacuum dried at 40°C. Finally, the graphite oxide was dispersed into water by ultrasonication to obtain the GO suspension. Synthesis of SiO2 particles SiO2 particles were synthesized by exploiting a modified Stçber method.[52,53] This process involves hydrolysis and condensation of metal alkoxide in the presence of base as catalyst. Synthesis was performed by mixing ethanol (200 mL) with TEOS (50 mL). DI water (100 mL) was poured into the mixture followed by addition of am- monium hydroxide (NH4OH, 100 mL). The solution was left to react for 2 h at 30°C. Finally, the obtained white suspension was centri- fuged for 30 min at 3500 rpm to get a silica gel. The gel was dried under vacuum at 60°C and calcined at 500°C for 3 h to remove impurities. The average size of the obtained silica particles was ap- proximately 540 nm. The general reactions can be written as [Eqs. (1)-(3)]:[54] ... (1) ... (2) ... (3) Synthesis of SiO2/GO composites Silica particles were dispersed in 3 m hydrochloric acid for 24 h and then washed with DI water and dried under vacuum at 60°C. Silica and GO mixtures with different volumetric percentage were pre- pared by ultrasonication of homogeneous GO aqueous suspension (50 mL) of different concentrations with acid-treated silica (2 g) for 30 min using a Misonix ultrasonic liquid processor (XL2010). The samples were labeled as SGO-0.001, SGO-0.01, SGO-0.1, SGO-1, and SGO-10 corresponding to compositions of 99.999 vol % SiO2 + 0.001 vol% GO, 99.990 vol% SiO2+0.010 vol% GO, 99.900 vol% SiO2+0.100 vol% GO, 99.000vol% SiO2+1.000 vol% GO, and 90.000 vol% SiO2 +10.000 vol% GO, respectively. Subsequently, the mixture was sealed in a 50 mL autoclave, retained at 170°C for 12 h, and then filtered. Synthesis of RGO and pulsed electric current sintering proc- essing Synthesis of RGO inside a ceramic matrix was performed by ther- mal reduction of GO in situ during the consolidation process. The consolidation was performed in vacuum using an FCT HP D 25-2 (FCT Systeme GmbH, Germany) PECS system. Powders were com- pressed in a 20 mm diameter graphite die, heated at a rate of 100°C min-1 up to the holding temperature of 1200°C, and held under a pressure of 50 MPa for 1 min. The samples were labeled as SG-0.001, SG-0.01, SG-0.1, SG-1, and SG-10 corresponding to the SGO-0.001, SGO-0.01, SGO-0.1, SGO-1, and SGO-10 powders, re- spectively. Materials characterization Materials were characterized by recording Fourier transform infra- red (FTIR) spectra of the samples on an FTIR spectrometer Model Nicolet 380. The test specimens were prepared by the KBr disk method. Raman spectra were measured on a Witec Alpha confocal Raman microscope with 532 nm laser excitation. Wide-angle X-ray diffraction (XRD) analyses were performed on a Philips X'Pert Pro system by using CuKa (l= 0.154 nm) radiation at 45 kV and 40 mA in the range of 2q=5-708 with scan rate of 28min and a resolution of 0.0018. Powder density measurements were performed by Accu- Pyc 1330 pycnometer. The surface area was evaluated by a Micro- meritics TriStar II 3020 surface area analyzer. Scanning electron mi- croscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) analysis were performed by using a Hitachi S-4700 micro- scope, and transmission electron microscopy (TEM) images were obtained utilizing a Tecnai G2 F30 S-Twin (FEI) microscope. X-ray photoelectron spectroscopy (XPS) was recorded on an AXIS Ultra photoelectron spectrometer. The UV/Vis-NIR spectroscopic meas- urements were performed with a Lambda 950 UV/Vis spectrometer (PerkinElmer) in transmission mode at plan-parallel polished sam- ples. The spectra were recorded from 200 to 1000 nm with a resolu- tion of 1 nm. Air was used as reference. The transmission spectra were normalized to a sample thickness of 1 mm. Measurement of sheet resistance was performed by using a four-probe sheet resis- tivity meter SD-600. Nanoindentation experiments were performed using a Hysitron Triboindenter T1-900 with a 130 nm Berkovich tip to estimate the modulus of samples. The tip area calibration was done using a standard fused-quartz substrate of known modulus (69.6 GPa). To study the effect of RGO concentration on the tough- ness of the ceramic, a Nexus 4000 Vickers hardness tester was used to induce radial cracking from the corners of the indentation. The length of the cracks was measured to calculate toughness values by using the Anstis equation [Eq. (4)]:[55] ... (4) This equation uses the measured hardness (H), applied load (P), modulus (E as measured by the nanoindenter), crack length (co), and a constant for Vickers-produced radial cracks in brittle ceram- ics (0.16) to calculate a toughness value. The Vickers hardness number (H) used to calculate the toughness value was measured using an applied load of 9.8 N to avoid forming radial cracks. An applied load of 49 N was used to create reproducible radial cracks that were used to measure the crack length values (co) used in Equation (1). Cu2 + ion adsorption capacity of SiO2/GO composites CuCl2 (at final concentrations of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 mm) was mixed with samples (10 mg) of silica, SGO-1, and SGO-10 in different vials. Each suspension was fully mixed by a vortex mixer. After mixing at room temperature for 12 h, the sus- pensions were filtered through 0.45 mm membrane filters and the concentration of Cu2 + ions was determined by an acetaldehyde- bis(cyclohexanone) oxaldihydrazone (BCO) method.[35] Briefly, triam- monium citrate (0.7 mL ; 20 g triammonium citrate dissolved in 100 mL DI water), NH3/NH4Cl buffer solution (0.5 mL ; 4 mL NH3*H2O and 4 g NH4Cl dissolved in 100 mL DI water), acetaldehyde (0.1 mL ; 40 %), BCO (1.1 mL; 0.2 g dissolved in a mixture of 50 mL DI water and 50 mL alcohol), and DI water (2.1 mL) were added to the su- pernatant solution (0.5 mL) and mixed by a vortex mixer. After 10 min, the absorbance at 540 nm was recorded. The Cu2 + ion concentration was calculated by referring to the standard plot (Figure 17). The equilibrium adsorption capacity was calculated using Equation (5): ... (5) in which qe is the quantity of Cu2 + ions adsorbed on the compo- sites at the time of equilibrium (mg g-1), C0 is the initial concentra- tion of Cu2+ ions in the aqueous solution of CuCl2 (mg L-1), Ce is the final concentration of Cu2 + ions in the aqueous solution of CuCl2 at the time of equilibrium (mg L-1), V is the total volume of the solution (L), and m is the mass of adsorbents (g). Equilibrium Cu2 + concentration (Ce) and the equilibrium adsorption capacity (qe) values were fitted by the Langmuir model and Freundlich model [Eqs. (6) and (7)]:[35] ... (6) ... 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Johansson Department of Forest Products Technology Aalto University POB 16400, 00076 Aalto (Finland) (c) 2014 Blackwell Publishing Ltd. |