A Versatile Self-Assembly Approach toward High Performance

May 14, 2014 - Exploiting the functionalization chemistry of graphene, long-range electrostatic and short-range covalent interactions were harnessed t...
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A Versatile Self-Assembly Approach toward High Performance Nanoenergetic Composite Using Functionalized Graphene Rajagopalan Thiruvengadathan,†,§ Stephen W. Chung,† Sagnik Basuray,† Balamurugan Balasubramanian,‡ Clay S. Staley,†,§ Keshab Gangopadhyay,†,§ and Shubhra Gangopadhyay*,† †

Department of Electrical and Computer Engineering, University of Missouri, Columbia, Missouri 65211, United States Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, United States § NEMS/MEMS Works, LLC., Columbia, Missouri 65203, United States ‡

S Supporting Information *

ABSTRACT: Exploiting the functionalization chemistry of graphene, longrange electrostatic and short-range covalent interactions were harnessed to produce multifunctional energetic materials through hierarchical self-assembly of nanoscale oxidizer and fuel into highly reactive macrostructures. Specifically, we report a methodology for directing the self-assembly of Al and Bi2O3 nanoparticles on functionalized graphene sheets (FGS) leading to the formation of nanocomposite structures in a colloidal suspension phase that ultimately condense into ultradense macrostructures. The mechanisms driving self-assembly were studied using a host of characterization techniques including zeta potential measurements, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), particle size analysis, micro-Raman spectroscopy, and electron microscopy. A remarkable enhancement in energy release from 739 ± 18 to 1421 ± 12 J/g was experimentally measured for the FGS self-assembled nanocomposites. capabilities at low costs.11−18 Nanocomposite energetic materials are generally described as heterogeneous mixtures of metallic fuels (aluminum (Al), boron, magnesium, etc.) and inorganic oxidizers (cupric oxide, bismuth trioxide (Bi2O3), ferric oxide, etc.) with nanoscale dimensions. The organization, intimacy, and dimensions of the discrete fuels and oxidizers in the nanocomposites largely influence their combustion kinetics. It is well understood that increasing fuel and oxidizer interfacial contact area enhances the reaction rate of a nanocomposite, and consequentially, mechanisms to self-assemble the fuels and oxidizers into dense and arranged structures are recently a subject of great interest.15,16,19,20 Nanocomposite energetic materials have been self-assembled using complementary DNA strands,15 electrostatically charged aerosols,19 and molecular polymer linkers.16 Regardless of the self-assembly method employed, significant combustion performance improvements have been shown using self-assembly over randomly mixing the fuels and oxidizers together. Here we demonstrate a unique methodology for preparing highly reactive Al/Bi2O3 energetic nanocomposites using FGS as a self-assembly directing agent. Al and Bi2O3 nanoparticles were selected to form the nanocomposite systems in this work due to excellent combustion properties including a fast burn

1. INTRODUCTION Functionalized graphene sheets (FGS) have recently attracted considerable attention for use in nanocomposites as researchers attempt to exploit the unique electrical, physical, mechanical, and thermal properties of FGS for a spectrum of applications.1−5 FGS are two-dimensional carbon sheets with high surface area, where the carbon to oxygen (C/O) ratio and surface functionalities are molecularly engineered based on synthesis parameters.5 To successfully utilize the interesting properties of FGS in nanocomposites, mechanisms to homogeneously distribute, structurally organize, and selfassemble the FGS incorporated nanocomposites at multiple length scales (from nano to macro) are required. FGS can serve as a versatile building block to self-assemble a broad range of nanomaterials because the properties of FGS can be tailored to accommodate various self-assembly approaches.6,7 Wang et al. used surfactant chemistry to direct the self-assembly of metal oxide and FGS nanostructures with interest toward energy storage applications.8 Shen et al. demonstrated that complementary charged FGS chemically modified with polyelectrolytes can electrostatically assemble into layer-by-layer FGS structures.9 Patil et al. have reported the synthesis of lamellar bionanocomposites prepared using FGS with DNA functionalization.10 Engineering nanocomposites for energetic materials is a new and emerging field of research attributed to the potential of developing multifunctional combustion systems with enhanced © XXXX American Chemical Society

Received: February 11, 2014 Revised: May 12, 2014

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The GO precipitate was dispersed in deionized water with mechanical stirring and centrifuged at 4000 rpm for 10−15 min. This process was repeated at least 4−5 times until all impurities were removed. At this point, the measured pH value was about 5. The GO was then dispersed in DI water to form a hydrosol with a typical concentration of about 2 mg/mL. To form GO paper, the GO dispersion in DI water was further centrifuged for 30 min at 4000 rpm to remove any thick unoxidized graphite. The clear dispersion was then processed by heating at 80 °C for about 30 min, resulting in a thin GO paper membrane formed on the top of the dispersion. The DI water was then pipetted out, and the GO paper was heated again at 80 °C to remove any remaining water. Lastly, the GO paper was removed, cut, and weighed for use as GO in the nanocomposites. Using GO for self-assembly was of interest in this work because the high density of oxygen-containing functional groups of GO was expected to offer numerous covalent binding sites for Bi2O3 and Al nanoparticles. Spectroscopic measurements of the GO revealed a large number of sp2 carbon domains (associated with defects), a C/O ratio of 1.95, and the presence of hydroxyl, carbonyl, epoxide, and carboxylic acid functional groups. The measured FTIR and Raman spectra are shown and discussed in the Supporting Information. Preparation of Dispersions and Nanocomposites. Similarly, Al and Bi2O3 dispersions were prepared in 1:1 v/v of DMF:IPA with appropriate amounts given in Table 1 for different nanocomposites.

rate and large gas production by weight, which have inspired many fascinating studies.12,14,21−23 FGS is particularly attractive for self-assembly as it supports combustion enhancement through beneficial properties, such as a high enthalpy of combustion (32.8 kJ/g for carbon−oxygen reaction and 1.6 kJ/ g for carbon−Al reaction), large surface area, exceptional optical and thermal characteristics that promote radiative heat transfer, and greater thermal conductivity within the nanocomposite.24,25 Additionally, the potential to tailor the chemical functionality of graphene at the molecular level with energetic groups such as nitro (−NO2) and amine (−NH2) offers tremendous opportunity to further enhance combustion performance.25 Recently, a few works have reported improvements in combustion characteristics for various energetic materials by incorporating FGS additives.24,25 However, we are the first to utilize FGS to direct the self-assembly of fuels and oxidizers and to serve as a combustion promoting additive. The self-assembly of Bi2O3 and Al nanoparticles on FGS leads to the formation of nanocomposite structures in a colloidal suspension phase that ultimately condense into ultradense macrostructures. The process reported here is spontaneous and does not utilize surfactant chemistry or other chemical and biological moieties,8−10 which can unfavorably hinder reaction kinetics by extending heat and mass transfer lengths. It is postulated that self-assembly is initiated through long-range electrostatic forces that leads to covalent and noncovalent interactions during nanocomposite formation. The mechanisms driving self-assembly were investigated using a host of characterization techniques including zeta potential measurements, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), particle size analysis, Raman spectroscopy, and electron microscopy. Lastly, differential scanning calorimetry (DSC) was used to measure the energy release of selfassembled nanocomposites against identical nanocomposite formulations prepared by random mixing.

Table 1. Experimental Parameters Used in the Synthesis of GO(X)/Al/Bi2O3 Nanocomposites GO (wt %) per total solid content

concn of GO in DMF

GO (mg)

Al/Bi2O3 nanothermite (mg)

Bi2O3 (mg)

Al (mg)

0 1 2 3.5 5 10

0 0.5 0.5 0.5 0.5 0.5

0 4 8 14 20 40

400 396 392 386 380 360

349.5 346.0 342.5 337.1 332.0 314.5

50.5 50.0 49.5 48.9 48.0 45.5

DMF and IPA were selected as suspension agents here because our prior works on Al/Bi2O3 nanopowders have shown that very good dispersions are obtained using IPA, and GO is known to exfoliate and disperse well in polar solvents such as DMF.28−31 Al/Bi2O3 control samples (without GO) were made following standard procedures reported in prior works.12,22 GO/Al/Bi2O3 nanocomposites were prepared by dispersing calculated amounts of GO paper in dimethylformamide (DMF) at 0.5% weight/volume (w/v) concentrations using ultrasonic bath agitation (output power of 250 W at 44 kHz frequency) for 8 h. Simultaneously, Bi2O3 and Al were separately dispersed in 1:1 volume ratios of DMF and isopropanol (IPA) for 4 h under ultrasonic agitation. Al suspensions were then added to the GO suspensions and ultrasonically mixed for 1 h further. Bi2O3 suspensions were then mixed with the GO/Al suspensions and ultrasonically agitated for an additional 1 h. Table 1 shows the measured amounts of all ingredients used to synthesize nanocomposites, with the appropriate amounts of Al and Bi2O3 defined by stoichiometric chemical reaction. Following the last step of ultrasonic agitation during sample preparation, all suspensions were removed from the ultrasonic bath, left undisturbed, and visually monitored as a function of time. In some experiments, dispersed Bi2O3 was not added to the Al/GO suspensions to exclusively study the self-assembly of Al nanoparticles on GO. It is important to note that the effects of constituent mixing order on the self-assembly process were also investigated by adding dispersed Al to Bi2O3/GO suspensions or by adding Al/Bi2O3 suspensions to dispersed GO, while keeping the remaining parameters (concentration, solvent system, mixing duration) constant. Similar selfassembled nanocomposites were produced regardless of mixing order, and the results disseminated herein were acquired from nano-

2. EXPERIMENTAL SECTION Reagents and Materials. Graphite nanoplatelets obtained from XG Sciences, Lansing, MI, were used as the main precursor for the synthesis of graphene oxide (GO). The as-purchased aluminum (Al) nanoparticles from Novacentrix TX have an average diameter of 80 nm, aluminum oxide (Al2O3) shell thickness of 2.2 nm, and 80% active Al content as specified by the manufacturer. Bismuth oxide (Bi2O3) nanoparticles with an average diameter of 90−210 nm were purchased from Accumet Materials Co., Ossining, NY. ACS reagent grade chemicals such as sulfuric acid (H2SO4), sodium nitrate (NaNO3), potassium permanganate (KMnO4), and hydrogen peroxide (H2O2) used in the synthesis of GO were purchased from Sigma-Aldrich. Solvents such as N,N-dimethylformamide (DMF; anhydrous, 99.8%) and 2-propanol (HPLC grade, 99.9%) were purchased from SigmaAldrich and used in the preparation of dispersions of Bi 2O3 nanoparticles, Al nanoparticles, and GO sheets as well energetic nanocomposites. Synthesis of Graphene Oxide. FGS in the form of GO was synthesized from GO paper made by following the modified Hummers method.26,27 Graphite nanoplatelets (1 g) and NaNO3 (1 g) were mixed with 46 mL of concentrated H2SO4 in a beaker cooled in an ice bath, followed by the addition of 6 g of KMnO4. The mixture was stirred at 35 °C for 1 h. Then, 80 mL of deionized water was added dropwise, and the reaction mixture was maintained in the water bath at 90 °C for 30 min. Subsequently, 200 mL of deionized water was slowly added, followed by addition of 6 mL of H2O2 to the resultant dispersions, and the solution was left to cool at room temperature. The GO dispersion was centrifuged to remove most of the ionic impurities. B

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composites prepared using the first process stated above. Mixing order having no impact on the final nanocomposite structures could be due to the strong covalent bonding occurring between the GO and Al nanoparticles exclusively, while only electrostatic and van der Waals interactions occur between Bi2O3 and GO as described in further detail later. Six Al/Bi2O3 nanocomposites were prepared with varying GO contents ranging from 0.0% (randomly mixed, control sample) to 10.0% by weight. The nanocomposites will henceforth be referred to as GO(X%)/Al/Bi2O3, where X denotes the weight percentage of GO. GO(X%)/Al/Bi2O3 nanocomposites were also prepared using differing proportions of Al to Bi2O3 in accordance with equivalence ratio (Φ) calculations defined by the stoichiometric Bi2O3 and Al reaction.16,32−34 Composites with a Φ value of 1.0 are described as stoichiometric, less than 1.0 as fuel-lean, and greater than 1.0 as fuelrich. Nanocomposites with Φ values of 1.0, 1.2, 1.4, 1.6, and 1.8 were used for this work. Since only up to 10% GO by weight was incorporated in any given composition, the role of GO was neglected from Φ calculations. Regardless of GO weight percentage or Φ value, the total solids mass was kept constant at 400 mg for preparing all compositions in this work. Material Characterization. Zeta potential and particle size measurements were performed using Delsa Nano C instrument (Beckman Coulter). X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis 165 spectrometer and vacuum level within the chamber remained near 1.0 × 10−8 Torr during acquisition. Fourier-transform infrared absorption measurements were made using a Thermo Nicolet spectrometer. Micro-Raman measurement was performed at room temperature with a Renishaw spectrometer system equipped with an argon ion laser (20 mW) excitation at 514 nm. Atomic force microscope (AFM) was used in ac mode to determine the thickness of GO sheets. For this purpose, GO dispersions were drop-casted onto a mica substrate to form an ultrathin film and dried at 65 °C under vacuum. Imaging was performed to determine the nature and the extent of self-assembly of Al and Bi2O3 nanoparticles on GO sheets using a transmission electron microscope (TEM; model: JEOL, 120 kV) and scanning electron microscope (SEM; model: FEI Quanta 600). Nitrogen adsorption−desorption isotherms were measured using a Quantachrome Autosorb-1 automated gas sorption system, and the surface area of GO was computed using the BET method. Simultaneous differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed to measure the heat flow and weight loss characteristics as a function of temperature using SDT Q600 thermal analyzer (TA Instruments). The measurements on all of the samples prepared in this work were carried out in argon ambient under identical experimental conditions (heating rate of 20 °C/min and argon flow rate of 200 mL/min).

While the constituent suspensions of Al or GO remained well dispersed for over 16 h, the Bi2O3 dispersions were stable for 4 h. The Al/Bi2O3 (control sample) suspension remained dispersed for 4 h until the phase separation of Bi2O3 was visually observed by distinct yellow (Bi2O3) lower regions and gray (Al) top regions in the supernatant. Phase separation is highly undesirable for composite energetic materials as it reduces fuel and oxidizer interfacial contact and leads to poor and unreliable combustion performance. Al and GO produced more stable suspensions than Bi2O3 because they are less dense and contain more hydrophilic surface functionalities (such as hydroxyl groups) that result in favorable interactions with the IPA and DMF, thus promoting greater stability. As opposed to Al/Bi2O3, nanocomposite suspensions containing GO exhibited homogeneous solid phase precipitation over times ranging from minutes to hours. The precipitates from all GO/Al/Bi2O3 suspensions were a uniform dark green color, which implied that the precipitation of all solid phases occurred simultaneously without any phase separation (Figure 1). The concentrations shown in Figure 1 for (a) Bi2O3, (b) Al, (c) Al/Bi2O3, (d) GO, and (e) GO(5%)/ Al/Bi2O3 are nearly 70, 10, 40, 2, and 40 mg/mL, respectively, which are relatively high concentrations. Nanocomposites with higher GO content by weight (3.5% and 5%) exhibited complete solid phase precipitation within 2 min after being left undisturbed, while lower GO content nanocomposites (≤2%) fully precipitated from the solvent phase within 24−36 h. For nanocomposites with 3.5 and 5 wt % GO content, the assemblies of dispersed nanoparticles forming macroscopic structures could be easily observed without the assistance of a microscope. Most importantly, the Al and Bi2O3 nanoparticle dispersions were stable until the addition of the GO, which promoted very rapid solids precipitation as a result of selfassembly relative to the stability times observed for the Al and Bi2O3 suspensions. Interestingly, nanocomposites prepared with 10% GO content did not precipitate even after being left undisturbed for 3 days. The lack of precipitation along with the absence of any visible solids phase separation implies that higher GO content (10 wt %) leads to a stable dispersion. It is well established that high surface charge density will result in longrange repulsive double-layer interactions dominating over short-range attractive van der Waals forces to prevent coagulation and ensure a stable colloid.35,36 Hence for 10% GO nanocomposites, the precipitation of a dense macroscopic structure did not occur. This observation also alludes to the possible mechanistic pathway for the self-assembly process as is described next. As our primary interest here was in obtaining highly reactive energetic material macrostructures, we have focused on the characterization and subsequent analysis of nanocomposites with up to 5 wt % GO content. The dynamics of GO/Al/Bi2O3 self-assembly in colloidal suspension are governed by the relative magnitudes of various long-range and short-range forces that change as a function of particle-to-particle separation.35,36 In order to understand the role of these interactions on self-assembly process, the zeta potential (ζ) was measured for the constituent and nanocomposite suspensions as shown in Table 2. Suspensions used for ζ measurements were very dilute, ultrasonically agitated, and centrifuged to ensure uniform dispersions. The contrasting surface charge polarities between Al, Bi2O3, and GO suggest the potential for long-range electrostatic attraction to initiate the self-assembly process. The magnitude of zeta potential values

3. RESULTS AND DISCUSSION Figure 1 shows a photograph of the constituent and nanocomposite suspensions captured after leaving them undisturbed for 16 h following the last ultrasonic mixing step.

Figure 1. Single constituent and nanocomposite suspensions after sitting for nearly 16 h period undisturbed: (a) Bi2O3, (b) Al, (c) Al/ Bi2O3 control showing separation of solid phases, (d) GO, (e) GO(5%)/Al/Bi2O3 showing uniform solids precipitation. The concentrations shown here for (a) Bi2O3, (b) Al, (c) Al/Bi2O3, (d) GO, and (e) GO(5%)/Al/Bi2O3 are nearly 70, 10, 40, 2, and 40 mg/ mL, respectively, which are relatively high concentrations. C

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and Bi2O3 drives the iterative self-assembly process of the macrostructures into either layer by layer (Figure 2e) or random (Figure 2f) orientations. Electron microscopy images captured at different stages of the self-assembly process confirm our hypothesis and will be discussed later. It is important to note that the colloidal self-assembly process in polar solvents may not be straightforward due to the interplay of many plausible long-range and short-range interactions, and our hypothesis is derived from the published literature and materials characterization technique analysis.38,39 FTIR was used to identify the compatible surface functionalities of the nanocomposite constituents for selfassembly. FTIR absorbance of GO (see Supporting Information) confirms the presence of hydroxyl (C−OH groups: 3050−3800 cm−1 stretch and 1070 cm−1 bending), carboxylic acid (COOH groups: 1650−1750 cm−1), epoxide (C−O−C groups: 1230−1320 cm−1 asymmetric stretch and 850 cm−1 bending mode), and ketonic species (CO groups: 1600− 1650 cm−1 , 1750−1850 cm−1).40 Similarly, the FTIR absorbance spectrum of Al passivated with Al2O3 indicates the presence of hydroxyl groups by a broad peak at 3700−3200 cm−1, as shown in the Supporting Information.41−43 The presence of oxygen-containing functional groups on GO and Al augurs well for covalent interactions at close particle-to-particle distances following electrostatic attraction. A schematic representing two potential covalent interactions that could occur between Al and GO is shown in Figure 3. In

Table 2. Measured Zeta Potential for Constituent and Nanocomposite Suspensions nanomaterial

solvent (1:1 v/v)

starting concn (mg/mL)

Al Bi2O3 Al−Bi2O3 GO GO +Al GO + Bi2O3 GO + Al + Bi2O3

DMF:IPA DMF:IPA DMF:IPA DMF:IPA DMF:IPA DMF:IPA DMF:IPA

0.001 0.001 0.003 0.001 0.001 0.001 0.003

zeta potential (mV) 70 40 63 −59 −27 −23 1

± ± ± ± ± ± ±

2.5 8.6 3 5.8 7.0 2 0.3

reduced from the constituent suspensions to −27 mV for GO/ Al, −23 mV for GO/Bi2O3, and finally +1 mV for GO/Al/ Bi2O3 to suggest colloidal instability as a result of the selfassembly process. A schematic of the likely self-assembly process is presented in Figure 2. We propose that upon adding a positively charged Al

Figure 2. Schematic of the self-assembly process: (a) electrostatic attraction of Al to GO, (b) covalent bonding of GO/Al existing as a stable GO/Al dispersion, (c) electrostatic attraction of Bi2O3 to GO/ Al nanostructures, and (d) noncovalent assembly of Bi2O3 on GO/Al; instability of GO/Al/Bi2O3 dispersion continues the self-assembly process to form ultradense macrostructures.

nanoparticle dispersion to a negatively charged GO dispersion, Al and GO migrate toward each other through long-range electrostatic interactions (Figure 2a) and, once close enough together, covalently bond (Figure 2b). This leads to a reduction in the measured ζ-potential from −59 ± 5.8 mV for neat GO to −27 ± 7.0 mV for GO/Al and implies that the surface charge of GO is partially neutralized by the Al.37 However, the reduced ζpotential for the GO/Al dispersion is still sufficient for stabilization as predicted by classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory.35,36 Positively charged Bi2O3 nanoparticles (ζ = 40 ± 8.6 mV) are then added to the GO/Al dispersion. The positively charged Bi2O3 electrostatically attract toward the negatively charged GO/Al nanocomposites (Figure 2c) and eventually assemble through shortrange, noncovalent interactions to form GO/Al/Bi2O3 nanocomposites (Figure 2d). Covalent bonding between Al and GO and the absence of covalent bonding between Bi2O3 and GO were verified using various spectroscopic methods and will be discussed later. The significant reduction in ζ-potential for GO/ Al/Bi2O3 suspensions to 1 ± 0.3 mV then leads to spontaneous coagulation to form ultradense macrostructures as predicted by DLVO theory.35,36 It is likely that the relative ratio of GO to Al

Figure 3. Chemical interactions between (a) hydroxyl groups of GO and surface hydroxyl groups of Al nanoparticles leading to C−O−Al covalent bond and (b) carboxylic groups of GO and hydroxyl groups of Al nanoparticles leading to OC−O−Al covalent bond.

Figure 3a, an oxygen atom from the OH of the Al reacts with the hydrogen atom of the OH group on GO to protonate the surface hydroxyl group of the Al and produce an alkoxide anion (GO−CO−) on the GO. The alkoxide reacts with the Al and creates a covalent linkage between the GO and Al while releasing a water molecule. Figure 3b shows the formation of a covalent bond, resulting from an interaction between a carboxylic acid (COOH) group of the GO and a surface hydroxyl group of the Al. In this case, the oxygen atom from the OH group on Al is protonated by the hydrogen of the COOH group on GO, which creates a carboxylate anion (GO−COO−) group on the GO. The oxygen atom of the carboxylate anion D

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then reacts with Al, releasing a water group and creating a covalent bond between GO and Al. To validate the occurrence of covalent interactions taking place during the self-assembly process, XPS measurements on GO, Al, and GO/Al were taken, and the carbon (C 1s spectra), aluminum (Al 2p), and oxygen (O 1s) spectra were analyzed. Since Bi2O3 nanoparticles do not contain any hydroxyl groups as evidenced by FTIR and TGA measurements (see Supporting Information), we do not expect covalent bond formation to result from short-range interactions between GO and Bi2O3. Therefore, XPS analysis was limited to the study of the GO/Al system. However, the long-range electrostatic and the shortrange noncovalent interactions (e.g., van der Waals interactions) between GO or GO/Al and Bi2O3 exist as seen from ζ measurements, leading to the self-assembled GO/Al/Bi2O3 nanocomposites. The XPS measurements were carried out using a Kratos Axis 165 spectrometer, and the vacuum level within the chamber remained near 1.0 × 10−8 Torr during acquisition. The C 1s and O 1s spectra for GO and GO/Al are shown in Figures 4a and 4b, respectively. It is evident from Figure 4a that

Deconvolution of the C 1s spectra generated four Gaussian peaks at 284.5, 285.7, 286.6, and 288.8 eV, assigned to C−C (sp2), C−OH (carbon atom directly bonded to oxygen in hydroxyl configuration), C−O−C (epoxide), and HO−CO (carboxyl) groups, respectively, in the GO sample.44−46 The peak positions of C−OH and OC−OH in GO shifted to lower binding energy by 0.4−0.5 eV upon self-assembly of Al on GO or vice versa in the GO/Al. However, the peak positions of C−C (sp2) and C−O−C (epoxide) bonds remained unchanged in the C 1s spectrum of GO/Al as expected. These observations suggest the occurrence of short-range covalent interactions as presented in Figure 3. The peaks centered at 285.3 and 288.3 eV in the C 1s XPS spectrum of the self-assembled GO/Al are assigned to Al−O−C and O C−O−Al. In agreement with this observation, Al−O−C bonding was confirmed by a positive shift (higher binding energy) in the Al 2p XPS spectrum by about 0.4 eV (see Supporting Information). A similar shift in the magnitude is shown upon Al−O−C bond formation in a recently reported study on the wetting behavior of cotton cellulose fibers coated with Al2O3.47 To verify the peak fitting procedure and the assignments, the carbon to oxygen atomic ratios of COOH, C−OH, and C−O− C functional groups were calculated for GO using the areas and the relative sensitivity factors for oxygen and carbon. These values for COOH, C−OH, and C−O−C are 0.54, 1.12, and 2.12, respectively, which are close to the expected theoretical values of 0.5, 1, and 2, respectively. The relative amounts of C− OH, C−O−C, and OC−OH groups in the GO were 52%, 16%, and 32%, respectively. Furthermore, the overall carbon to oxygen atomic ratio in GO is 1.95. These values from quantitative analysis of XPS data clearly confirm that the GO produced in this work contains a high amount of oxygencontaining functional groups. An overlay of the measured O 1s spectra of Al, GO, and GO/ Al (see Supporting Information) showed a positive peak shift of 0.8 eV for GO/Al in comparison to Al. The positive shift (higher binding energy) is attributed to the removal of oxygen upon covalent binding of Al to GO. Interestingly, the FWHM of the convoluted O 1s peak for GO/Al is 2.7 eV, while that for GO is 3.5 eV. These observations confirm charge transfer between Al and GO sheets as a result of their interaction.47,48 Deconvolution of the broad O 1s spectrum generated four peaks at 530.9, 532.2, 533.5, and 534.7 eV, assigned to CO, C−O, C−O−C, and oxygen from adsorbed water groups, respectively, in GO.44,46 Alternatively, deconvolution of the broad O 1s spectrum for GO/Al yielded only three peaks at 531.4, 532.6, and 533.5 eV as a result of narrowing of the spectrum. Figure 4b shows the peak positions of OC−OH and C− OH for GO shifted to higher binding energy by 0.4−0.5 eV after Al self-assembly on GO or vice versa. The two peaks at 531.4 and 532.6 eV are assigned to OC−O−Al and C−O− Al for GO/Al, respectively. The peak position and FWHM of C−O−C (epoxide) bonding configuration for GO/Al remained unaltered with reference to GO within acceptable fitting error since these bonds do not participate in the chemical interactions during assembly. The carbon to oxygen atomic ratio of C−O−C peak for GO/Al remained constant at nearly 2.05 (close to the expected value of 2) in comparison to GO (2.12). For GO/Al, the carbon to oxygen atomic ratio in C− O−Al and OC−O−Al peaks were 0.17 and 0.33, respectively. This significant decrease in the atomic ratios is

Figure 4. Deconvoluted (a) C 1s spectra and (b) O 1s spectra of neat graphene oxide (GO) and GO/Al self-assembled composite. The change in the spectral shape and width shown in these spectra is a signature of covalent interactions leading to self-assembly of Al nanoparticles on GO sheets.

the C 1s spectrum of GO/Al exhibits significant changes in the spectral shape and width in comparison to that of GO. Similarly, the convoluted O 1s spectrum of GO/Al composite (Figure 4b) is considerably narrow in comparison to that of GO. Such changes observed in the spectra are signature of modifications in the electronic configuration of GO surface due to interactions with Al nanoparticles.44−46 E

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Bi2O3 tended to condense into layered macrostructures. Layered macrostructure assembly orientation likely arises from the preference of larger, more two-dimensional GO(5%)/Al/Bi2O3 to geometrically align with one another, probably due to weak van der Waals interaction. Regardless of macrostructure organization, the GO/Al/Bi2O3 nanocomposites appeared to exhibit excellent fuel to oxidizer contact in comparison to randomly mixed Al/Bi2O3, which suggests the potential for enhanced reaction kinetics. DSC and thermogravimetric analysis (TGA) measurements were performed to characterize the thermophysical properties of the nanocomposites prepared in this work and quantify energy release. The effects of GO directed self-assembly on energy release and ignition characteristics were evaluated against GO weight content and equivalence mixing ratio (Φ) taking into account the active Al content of the Al nanoparticles.34 GO(X%)/Al/Bi2O3 nanocomposites were obtained by separating the precipitates from the supernatant (solvent) phase through decantation. Subsequently, the precipitates were dried at 65 °C under rough vacuum for 16 h. Randomly mixed Al/Bi2O3 samples were collected by drying well-dispersed suspensions of Bi2O3 mixed with Al at 65 °C for 16 h. Simultaneous DSC/TGA measurements were performed using a TA Instruments SDT Q600 thermal analyzer under 200 mL/min argon flow and 20 °C/min heating rate for all samples. A minimum of three measurements were taken for each nanocomposite under identical experimental conditions to quantify the error. DSC plots of GO(5%)/Al/Bi2O3 prepared with various Φ values against Al/Bi2O3 prepared at Φ = 1.0 are shown in Figure 6a. The Al/Bi2O3 trace showed an exothermic peak at 599−618 °C (with onset temperature at 556 °C) and an endothermic peak at 900−1100 °C, which are attributed to the Al−Bi2O3 reaction and the decomposition of the reaction products, respectively.14 Alternatively, all GO(5%)/Al/Bi2O3 traces had endothermic peaks at ∼100 °C and 900−1100 °C and an exothermic peak in the 475−750 °C range (with onset temperature of 500 °C) that appeared to be a convolution of two discrete exothermic reactions. Similar to Al−Bi2O3, the endothermic peak observed at 900−1100 °C is due to reaction product decomposition. However, the new endothermic peak at ∼100 °C is attributed to the decomposition of functional groups from GO that suggests the formation of C from the GO.49 DSC/TGA traces of pure GO are provided in the Supporting Information. Because of the formation of C at the early stages of heating, C−O, C−Bi2O3, and C−Al reactions are all potential reaction pathways in addition to Al−Bi2O3 and Al− O for the GO(5%)/Al/Bi2O3 self-assembled nanocomposites. Most importantly, the total area under the major exothermic DSC trace (signifying the energy release from the exothermic reaction) was greater for all GO(5%)/Al/Bi2O3 regardless of Φ in comparison to Al−Bi2O3. The maximum energy release was observed for GO(5%)/Al/Bi2O3 prepared at Φ = 1.4 due to balanced reaction stoichiometry. Figure 6b shows DSC traces for GO/Al/Bi2O3 nanocomposites prepared with varying GO content at a Φ = 1.4. For the GO/Al/Bi2O3 nanocomposites, the first exothermic peak observed at ∼560 °C is likely due to C−Bi2O3 reactions. C−Bi2O3 reactions are expected to occur preferentially over Al−Bi2O3 because the C is not passivated with an inert Al2O3 barrier, and therefore less thermal energy is required to incite reactivity.50 The second exothermic peak at 599−618 °C is associated with Al−Bi2O3 reactions as similarly observed for the

attributed to the presence of additional oxygen atoms that originate from the Al2O3 shell as a result of covalent bonding. To summarize, XPS analysis confirmed the role of covalent interactions in facilitating the self-assembly of Al on GO or vice versa at short particle to particle distances. Microscopic imaging techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were employed to characterize the physical structures of the nanocomposites and visualize the effects of self-assembly. A few representative images of GO(5%)/Al/Bi2O3 nanocomposites and the individual constituents used to prepare them are presented in Figure 5. The TEM image shown in Figure 5d

Figure 5. TEM (a−d) and SEM (e, f) images of GO(5%)/Al/Bi2O3 composites self-assembled from nano to macro length scales. (a−d) TEM images: (a) a few layered GO sheets, (b) Al nanoparticles with 80 nm average diameter, (c) Bi2O3 nanoparticles with 90−210 nm size range, and (d) layered nanoscale building block of a GO densely decorated with Al first and then Bi2O3.. The inset in (d) shows a ultradense assembly of Al and Bi2O3 on GO. (e, f) SEMs of ultradense macro structures self-assembled from GO(5%)/Al/Bi2O3 nanocomposites in layered (e) and random (f) orientations.

demonstrated the formation of self-assembled GO(5%)/Al/ Bi2O3 where both sides of the GO sheets were decorated with densely packed Al first layer (light contrast) and Bi2O3 (dark contrast) second layer. In agreement with electron microscopy images, dynamic light scattering (DLS) measurements showed a clear increase in average hydrodynamic diameter for GO(5%)/Al/Bi2O3 in comparison to the individual constituents (see Supporting Information). Low magnification TEM (subset of Figure 5d) revealed the formation of ultradense assemblies of Al/Bi2O3 on GO. Lower magnification SEM showed three-dimensional macrostructures formed from the GO(5%)/Al/Bi2O3 nanocomposite assemblies with dimensions larger than a few tens of micrometers in both random (Figure 5f) and layered orientations (Figure 5e). The orientation of the GO(5%)/Al/Bi2O3 nanostructures within the larger macrostructures appeared to be driven by the two particle size modes (see DLS data in Supporting Information), where smaller sized, less planar GO(5%)/Al/Bi2O3 formed randomly oriented macrostructures and larger sized, more planar GO(5%)/Al/ F

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exothermic C−Al reaction peak become more pronounced with greater Φ values due to the excess Al present in the more fuelrich nanocomposites. A plot of energy release as functions of GO content and Φ for all the nanocomposites is presented in Figure 6c. DSC traces of Al/Bi2O3 versus Φ are provided in the Supporting Information. Randomly mixed Al/Bi2O3 showed an increasing trend in energy release with greater Φ value (more fuel-rich) that peaked at 739 ± 18 J/g for Φ = 1.6. Published literature confirms that the optimum Φ with respect to highest energy release and reactivity is generally fuel-rich for various nanocomposite energetic systems.16,32−34 For GO/Al/Bi2O3 prepared with 3.5 and 5 wt % GO, the energy release increased from Φ = 1.0 to 1.4 and then decreased at Φ = 1.6. The optimum energy release for both 3.5 and 5 wt % GO nanocomposites was observed at Φ = 1.4 with average values of 1120 ± 15 and 1421 ± 12 J/g, respectively. The reduction in optimum Φ from 1.6 for Al/Bi2O3 to 1.4 for GO/Al/Bi2O3 indicates that GO participates as a fuel during combustion and further supports the likelihood of C−Bi2O3 reactions occurring before the primary Al−Bi2O3 reaction as observed in the DSC traces. Most importantly, a 92% increase in total energy release was realized when comparing the optimum energy release for randomly mixed Al/Bi2O3 at Φ = 1.6 to the optimum energy release for GO(5%)/Al/Bi2O3 at Φ = 1.4. The substantial increase in energy release achieved through the incorporation of GO is a direct result of the intimate self-assembly of Al and Bi2O3 and the introduction of C into the nanocomposites to serve as a tertiary reactant.15,50 Although the maximum energy release for GO(5%)/Al/Bi2O3 was substantially higher than Al/ Bi2O3, it was still lower than the theoretical value of 2118 J/g calculated for Al/Bi2O3 likely due to the inert Al2O3 passivation of the Al nanoparticles.15,55

4. CONCLUSION In summary, we have demonstrated a surfactant-free process for directing the self-assembly of Al/Bi2O3 nanocomposites on functionalized graphene sheets in the form of GO. Long-range electrostatic attraction followed by short-range covalent (between GO and Al) and noncovalent interactions (between GO/Al and Bi2O3) produces nanostructured GO/Al/Bi2O3 that further condense into ultradense, highly reactive macrostructures. The mechanisms driving self-assembly were hypothesized and confirmed through zeta potential, DLS, electron microscopy, and chemical spectroscopy techniques. Contrasting surface charge polarities between the Al, Bi2O3, and GO as determined from zeta potential measurements of +70.14, +39.71, and −58.57 mV, respectively, suggests longrange electrostatic attraction initiates the self-assembly process. Spectroscopic analysis confirms the presence of short-range covalent interactions between GO sheets and Al nanoparticles. More specifically, XPS data showed covalent bonding between Al and GO. Significant enhancements in energy release up to 92% were observed when comparing randomly mixed (739 ± 18 J/g) to self-assembled nanocomposites (1421 ± 12 J/g) due to the benefits of self-assembly and role of GO as an energetic reactant. Although not investigated here, GO has the potential to be molecularly rendered with energetic functional groups such as nitro (−NO2) and amine (−NH2) to foster further combustion enhancements beyond self-assembly. Macroscale energetic materials prepared by the self-assembly of nanoscale constituents are expected to have major implications in the field of energetics due to the ease in handling and further processing,

Figure 6. DSC plots of (a) GO(5%)/Al/Bi2O3 nanocomposites at different equivalence ratios (Φ) and (b) GO(X%)/Al/Bi2O3 (X = 0, 3.5, and 5) nanocomposites prepared at Φ = 1.4 obtained from thermal analysis performed under argon ambient at a heating rate of 20 °C/min. The insets in (a) and (b) show the heat flow measurements of different compositions in the full temperature range of 25−1200 °C. (c) Energy release as a function of equivalence ratio and GO content.

pure Al/Bi2O3 samples. The Al−Bi2O3 reaction peak has a larger area underneath the curve (greater energy release) and peaks at a slightly lower temperature for the GO/Al/Bi2O3 nanocomposites in comparison to Al/Bi2O3 due to improved Al and Bi2O3 intermixing from self-assembly. This supports more complete combustion and consequentially greater energy release.15,16,19 Greater energy release is observed for nanocomposites with larger GO content likely due to improved selfassembly and fuel−oxidizer intermixing. Following the Al− Bi2O3 reaction, there is a small endothermic peak at ∼660 °C attributed to the melting of excess Al, and shortly thereafter the exothermic peak at ∼665 °C is due to C−Al reactions.51−54 As shown in Figure 6a, the endothermic Al melting peak and G

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for example, as additives in propellants and explosives compared to nanopowders. Moreover, FGS-directed selfassembly is applicable to alternative fuels and oxidizers outside of Bi2O3 and Al. The development of GO/Al/Bi2O3 selfassembled nanocomposites represents a major milestone toward establishing the fundamental science necessary to engineer, synthesize, and implement multifunctional and high performance energetic material systems in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed analysis of FTIR absorbance, micro-Raman vibrational spectrum, surface area analysis, hydrodynamic size distribution, and AFM image. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.G.). Notes

The authors declare no competing financial interest.

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REFERENCES

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