Graphene Oxide Quantum Dots as the Support for the Synthesis of

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Article Cite This: ACS Omega 2018, 3, 7278−7287

Graphene Oxide Quantum Dots as the Support for the Synthesis of Gold Nanoparticles and Their Applications as New Catalysts for the Decomposition of Composite Solid Propellants Juan P. Melo,† Paulina L. Ríos,† Paula Povea,‡ Cesar Morales-Verdejo,† and María B. Camarada*,† †

Facultad de Ciencias, Centro de Nanotecnología Aplicada, Universidad Mayor, Camino la Pirámide 5750, 8580745 Santiago, Chile Laboratorio de Materiales Energéticos, Instituto de Investigaciones y Control del Ejército de Chile (IDIC), Av. Pedro Montt 2136, 8370899 Santiago, Chile

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‡

S Supporting Information *

ABSTRACT: Graphene oxide quantum dot (GOQD) and reduced GOOD (rGOQD) were synthetized using a simple and straight methodology based on an oxidative treatment and sonication. GOQD and rGOQD were used as supporting agents for the in situ generation of gold nanoparticles, avoiding the use of additional stabilizers. GOQD resulted as a better support than rGOQD because of the presence of higher functional groups that can interact with gold. Theoretical studies through density functional theory revealed the important role of the epoxy groups of GOQD on the stabilization of gold. GOQD and GOQD-Au were tested for the first time as catalysts for the decomposition of solid composite propellants. GOQD not only lowered the decomposition temperature of ammonium perchlorate (AP) but also enhanced the exothermic heat of AP, in comparison to graphene and GO. GOQD-Au increased the energy release; however, the effect on the decrease of the decomposition temperature of AP was not as significant as other previous reported catalysts.

1. INTRODUCTION During the last century, the development of commercial and military applications of rocketry has fomented advancements in rocket science. High-energy materials are most important ingredients to provide the driving force to escape Earth’s gravity. Among these materials, propellants are commonly used as the energy source for the propulsion of solid rocket motors.1 Propellants are defined as a combustible material that burns slowly, in a controlled manner, propelling a projectile, such as a missile, a rocket, or a space launch vehicle.2 For space exploration and military applications, both solid and liquid propellants are used. However, for safety reasons, reliability, simplicity, and long storage life, solid propellants are preferred over liquid propellants.3 Heterogeneous propellants are mainly represented by composite solid propellants, which are the major source of chemical energy in modern space vehicles and missiles. Composite solid propellants are essentially made up of three basic components: a binder, an oxygen-rich solid oxidizer, and a combustible metal additive.4 The oxidizer is the source of oxygen and the major component of the composite solid propellant. Ammonium perchlorate (AP) is the most common oxidizer because of its excellent properties such as high oxygen content, high density, high specific volume of combustion products, good stability in storage and use, and low price.5 One important factor in the general performance of composite solid propellants is the decomposition temperature, © 2018 American Chemical Society

which is directly related to the chemical properties of AP. Generally speaking, the lower the decomposition temperature of AP, the shorter the delay time of propellant ignition and thus the higher the combustion rate and the better the performance of the composite solid propellants. The lower the decomposition temperature of AP, the higher will be the burn rate of the propellant. Evidence has shown that the final decomposition temperature and the burn rate of composite solid propellants can be easily tuned by adding a burn rate catalyst.6 Currently, burn rate catalysts include mainly transition-metal oxides,7−11 metal nanoparticles (NPs),12−15 and ferrocene derivatives.16,17 However, most of them are not able to increase the decomposition rate and energy release significantly or require difficult and expensive synthesis paths. In the search of new catalysts, Sabourin and co-workers18 assayed graphene to improve the combustion performance of composite solid propellants. Their work highlighted the role of graphene in increasing the burn rate of the propellants, which not only catalyzes fuel combustion reactions but also participates energetically and is consumed without producing residual particulates. Graphene oxide (GO), which consists of defect-rich graphene and residual chemisorbed oxygenReceived: April 27, 2018 Accepted: June 19, 2018 Published: July 3, 2018 7278

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Figure 1. (a) FT-IR spectra of graphite and GO, (b) attenuated total reflection (ATR)−IR spectra of GOQD and rGOQD, and (c) UV−vis spectra of GO, GOQD, and rGOQD.

containing moieties,19 provides a less expensive alternative to high-purity graphene and may be equally effective for propellant applications. The positive effects of GO on energetic materials mainly rely on its high thermal20 and electrical conductivity21,22 and large specific surface area,23 which influence the high-temperature decomposition (HTD) and low-temperature decomposition (LTD) steps of AP, accelerating the electron transfer and thus speeding up its decomposition. GO can also act as the support and stabilizer of metal NPs, such as Fe2O3 and Mn3O4, enhancing their catalytic activities for the decomposition of AP.24,25 In the case of supported Mn3O4, a 5 wt % of the composite reduced the HTD of AP by 142 °C, which is one of the best decreases reported to date. GO sheets have also been successfully applied for the catalysis of the reduction of iron(III).26 Graphene quantum dots (GRQDs) have attracted increasing attention in nanoscience and nanotechnology because of their remarkable properties, such as high surface area, excellent solubility, low cytotoxicity, stable fluorescence, and an adjustable band gap.27 At present, a variety of methods have been developed to prepare GRQDs. The most common is cutting GO28 because of its low cost, easy processing, and mass production, resulting in GO quantum dots (GOQDs),29 which can be further functionalized or reduced to produce reduced GOQDs (rGOQDs). Most of the GRQD synthesis involves hydrothermal,30,31 solvothermal,28 or microwave-assisted32 paths that require several hours of reaction or expensive apparatus. More recently, Zhu and co-workers reported a highly efficient, simple, and fast ultrasonic strategy to synthesize GRQD from GO.33 GOQD and GRQD have been reported as the support for the synthesis of metal NPs. Silver,34,35 copper,36 iron,37 and palladium38 NPs have been successfully produced by the in situ reduction of cations previously stabilized by GOQD or GRQD. GRQDs conjugating with gold NPs (AuNPs) have also been described for the modification of electrode conductivity39 and as the catalyst for vapor deposition growth.40 However, both methodologies referred to the use of graphene derivatives only as the support of previously stabilized AuNPs by cysteamine or a silicon substrate as the pattern. To date, most of the studies do not refer to the use of GOQD as the direct stabilizer of AuNPs. Wu et al. reported in 2015 the effective in situ synthesis of AuNPs supported on GRQD to catalyze the oxidation of veratryl alcohol.41 However, the use of GOQD as the stabilizer has not been reported. Moreover, to our knowledge, neither GOQDs nor AuNPs have been tested as the catalyst for the decomposition of AP, the latter probably due to its elevated cost. Therefore, in this paper, we report the in situ synthesis and characterization of AuNPs supported on

GOQD and rGOQD and their further use as catalysts for the decomposition of AP. Graphene derivatives were obtained using a simple and direct oxidative methodology based on sonication, avoiding the use of expensive apparatus. GOQD increased the performance of solid composite propellants because of its improved heat transfer, higher thermal conductivity, and catalytic decomposition of AP. The interaction between GOQD and gold was also analyzed through ab initio calculations, revealing the important role of epoxy groups on the surface of the layer for the stabilization of the metal.

2. RESULTS AND DISCUSSION 2.1. Synthesis of GO, GOQD, and rGOQD. A Jasco FT/ IR 4100 spectrometer was used to measure Fourier transform infrared spectroscopy (FT-IR) spectra in the 500−4000 cm−1 frequency range. A Jasco V-630 UV−visible spectrophotometer was used to measure the UV−visible spectra. All measurements were performed at room temperature. The FT-IR spectra of graphite and GO are shown in Figure 1a. The GO spectrum shows oxygen functionalities at 1054 (C−O stretching vibrations), 1728 (CO stretching vibrations), and 3400 cm−1 (O−H stretching vibrations), supporting the effective oxidation of graphite to GO. Skeletal vibrations of CC from unoxidized graphitic diamonds appeared at 1622 cm−1. The pure graphite spectrum shows broad absorption bands at 1040 and 3440 cm−1 corresponding to the stretching vibrations of C−O and O− H, respectively.42 The 1640 cm−1 absorption peak was attributed to the skeletal vibrations of CC.43 The spectra of GO and graphite are quite different; the GO spectra show fewer oxygen-containing functional groups, confirming the successful oxidation of graphite. The as-obtained GO was submitted to an oxidative treatment using ultrasonic waves. It is known that the ultrasound can lead to the formation and collapse of small vacuum bubbles.44 The energy of ultrasonic waves cuts GO sheets into GOQDs. In this work, no microwave-assisted synthesis was applied, avoiding the necessity of costly apparatus or long hydrothermal treatments.30,32 The resulting GOQD can be dispersed in water without further sonication. As shown in Figure 1b, the ATR−IR spectrum of GOQD exhibits a C−O stretching peak at approx. 1000 cm−1, a C−H stretching peak at ca. 1335 cm−1, and a broad O−H stretching peak at 3400 cm−1. After reduction, the rGOQD presents a reduction in the C−O signal because of deoxygenation.45,46 All samples were prepared, and UV−visible spectra were recorded. As depicted in Figure 1c, the GO UV−vis spectrum (0.1 mg/mL in water) shows two bands: a shoulder at 7279

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Figure 2. PL of (a) GOQD and (b) rGOQD at different excitation wavelengths.

Figure 3. UV−vis spectra of (a) GOQD-Au and (b) rGOQD-Au at different ratios and (c) comparison of both peaks obtained with a concentration of 0.3 mM HAuCl4.

synthesis of AuNPs by using an additional stabilizing agent such as cysteamine39 or a silicon substrate as the pattern.40 As shown in Figure 3, different concentrations of HAuCl4 were assessed. With both substrates, a broad and intense band was observed at approximately 540 nm, characteristic of zero-valent gold particles.52,53 Both GOQD and rGOQD resulted as adequate stabilizers for the synthesis of AuNPs. As the amount of HAuCl4 increased, the intensity of the Au0 band decreased, related to the maximum concentration of gold that can be correctly stabilized by the functional groups of the QDs. Figure 3c presents a comparison of the peaks obtained by using each QD as the support at the lower gold concentration, 0.3 mM. As can be seen, GOQD showed a higher intensity, related to the presence of a greater amount of functional groups such as epoxy, carboxylic acids, or alcohols, which can interact with gold atoms and generate more stable complexes. In this way, the GOQD system resulted as a more efficient support for the stabilization of AuNPs. Figure S2 shows the ATR−IR spectra of GOQD-Au. The oxygen-containing functional groups are present but with less intensity compared to GOQD because of the reductive effect of sodium citrate in the experimental procedure. These electron-rich sites effectively stabilized AuNPs, prevented the agglomeration, and resulted in higher stability than sole AuNPs in water.26 The coordination sites of GOQD and the affinity to gold were explored using computational tools and will be further discussed in the next section. The GOQD-Au sample at the lower concentration of gold was characterized by high-resolution scanning electron microscopy (HR-SEM). Figure 4 depicts the analysis of diameter distribution and the obtained microphotographs. Two main sizes were identified, in an average of 9 and 35 nm, with a higher prevalence of the smaller diameters. The wide distribution is probably connected to the low monodispersity of the GOQD sample. Low size NPs can be effectively

approximately 310 nm, corresponding to the n/π* transition of CO bonds, and a maximum at 230 nm, which can be assigned to the π/π* transition of aromatic CC bonds.47 GOQD in water shows an absorption band at approximately 296 nm48 because of the absorption of the graphitic structure, similar to GO. The weak shoulder at 344 nm is related to the n/π* transition.34 In the case of rGOQD, this band disappeared, which is related to the lower level of oxidation in the structure. The absorbance peak in rGOQD slightly redshifted to 293 nm, while the absorbance value increased in comparison to that of GOQD, suggesting that the electronic conjugation is restored after reduction.49 To further explore the optical characteristics of the asprepared GOQD and rGOQD, a photoluminescence (PL) study was carried out by using different excitation wavelengths. The spectra of most luminescent carbon materials are dependent on the excitation wavelength. In this work, as shown in Figure 2, the position of the PL spectral peaks remained almost invariable as the excitation wavelength increased, indicating an excitation-independent PL feature. GOQD presented a maximum PL peak at 430 nm with an excitation wavelength of 270 nm, whereas rGOQD exhibited a peak at 470 nm at the same excitation value. The fluorescence of these materials may originate from emissive free zigzag sites with a carbene triplet ground state.50,51 The size of the GOQD was assessed by transmission electron microscopy (TEM) (Figure S1). The average diameter of the obtained QDs was 17.7 nm, which is larger in comparison to the other reported synthesis with the 3−5 nm diameter.48 The higher diameter value is probably related to the application of lower temperature and no microwave equipment, resulting in lower energy during the GO cleavage process and thus larger GO fragments. 2.2. AuNPs Supported on GOQD and rGOQD. The resulting GOQD and rGOQD were used as the support for the in situ synthesis of AuNPs. Most of the studies report the 7280

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(Eint). It is well-known that GO has some functional groups such as carboxylic acids, ethoxy, and alcohols that can stabilize metal ions. The oxidative-ultrasonic methodology applied in this work induced the doping of nitrogen into GOQD through nitration.55,56 Figure 5a shows the optimized GOQD geometry, including oxygen- and nitrogen-derived functional groups according to the literature. 32 These potential coordination sites with high electron density, where a metal ion could be attached and generate complexes, were considered for the binding analysis. The Au8 cluster was placed near to the electron-rich sites of the GOQD structure to generate the starting geometries of GOQD-Au8 complexes for optimization. Six different coordination sites were selected (Figure 5a): the gold cluster interacting with (a) the carboxylic acid group (−COOH), (b) the hydroxyl group (−OH) at the edges of the GOQD layer, (c) the −N− group at the aromatic rings, (d) the double −N− groups belonging to the aromatic rings on the center of the structure, (e) the double coordination to −N− at the edge site, and (f) the epoxy groups (−O−) on the GOQD layer. In order to search for alternative local minima, three initial conformations were tested at each coordination site. Table 1 lists the most stable Eint at the selected coordination sites, distances between the Au8 cluster and the anchor atom, and the charge for the optimized complexes. The Eint analysis indicated that the most stable rGOQD-Au8 complexes corresponded to site (f) (Figure 5b) and then complexes (c) and (d). Figure S4 shows the optimized geometry of all complexes, in which bond lengths and natural bond order charges at selected atomic sites were depicted. The only complex that did not show stable geometries at the original coordination site was complex (d), where the coordination to −N− atoms at the center of the aromatic rings was not favorable, preferring the coordination to the epoxy groups on the surface of the GOQD layer. Moreover, the final optimized structure (d) was the only complex showing double coordination to an oxygen atom (Figure S4). In comparison to structure (f), the complex (d) was approx. 30 kcal·mol−1 less stable, related to the double coordination that strained the cluster structure. This fact is supported by the longer average bond distances of the gold cluster in structure (d), which presented a deviation from the original structure of Au8 in approx. 3%, and a much longer distance between both anchored gold atoms, which separated from 2.78 to 3.1 Å.

Figure 4. Size distribution analysis and SEM images of the AuNPs synthetized using GOQD as the supporting agent, with the lower concentration of gold (0.3 mM).

stabilized by the 17 nm GOQD; however, bigger sizes such as 35 nm require a sphere of GOQD to achieve the correct protection and stabilization. GOQD and GOQD-Au were also electrochemically characterized. Each glassy carbon electrode (GCE) was modified with 5 μL of the GOQD and GOQD-Au (HAuCl4 0.3 mM), respectively, and dried at room temperature. As shown in Figure S3, GOQD reduces the conductivity of the electrode surface, related to the lower current intensity recorded by cyclic voltammetry and the upper semicircle radius of the electrochemical impedance spectroscopy (EIS), agreeing with previous studies reporting much slower electrontransfer rate for GRQDs.54 In the case of GOQD-Au, the presence of gold decreases the resistance of GOQD, improving the modified GCE response to the Fe(II)/Fe(III) redox couple. GOQDs are not suitable for applying to a beneficial single electrode material, but the presence of AuNPs in GOQD-Au exhibits great potential. 2.3. Computational Description of GOQD-Au. Considering that GOQD resulted as the better support for the synthesis of AuNPs than rGOQD, the isolated GOQD structure was fully optimized at the B3LYP/6-311G level and used as the reference for the calculation of basis set superposition error (BSSE)-corrected interaction energy

Figure 5. Optimized structure at the B3LYP/6-311G level of (a) GOQD and the six possible coordination sites, (b) most stable GOQD-Au8 complex (f), natural population analysis (NPA) charges (a.u.) for the selected atoms are displayed in italics, and bond lengths are given in angstrom. (c) Energy of the Fermi level calculated for GOQD () and the most stable complexes of GOQD-Au (). 7281

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ACS Omega Table 1. Au−X (X = N or X = O) Anchor Bond Distances dX−Au in Åa complex

anchor bond

dX−Au

qX

qAu

Δqcluster

a −COOH b −OH c −N− d −N−

Au−O Au−O Au−N Au−O Au−O Au−N Au−O

2.345 2.468 2.224 2.208 2.372 2.350 2.128

−0.066 −0.728 −0.516 −0.862

0.157 0.186 0.166 0.238 0.111 0.114 0.289

−0.126 −0.114 −0.177 −0.343

8.6193 5.7618 19.197 15.037

(11.812) (9.1504) (22.311) (27.125)

−3.882 −3.801 −4.014 −4.066

−0.194 −0.344

3.5431 (5.5885) 43.670 (53.395)

−4.091 −4.108

e −N− f −O−

−0.256 −0.841

Eint

EFL

NPA-derived atomic charges of the anchor atom qX, the bonded gold atom qAu, and the total charge of the metal cluster Δqcluster in a.u. BSSEcorrected and -uncorrected (in parenthesis) interaction energy (Eint, kcal·mol−1) for the studied complexes. EFL corresponds to the Fermi level in electronvolt. a

Figure 6. (a) DSC curves for the thermal decomposition of pure AP and AP with 4 wt % of GOQD, GOQD-Au, and AuNP and (b) summary of the effect of GR or GO derivatives on the HTD of AP.

GOQD. As shown in Figure 5c, the coordination of the Au cluster to GOQD lowered the EFL in both complexes (f) and (c), indicating a higher conductivity. The decrease of EFL in GOQD-Au can be related with the cyclic voltammetry results (Figure S3) that showed higher current intensities for GOQDAu, indicating an easier oxidation of the Fe(II)/Fe(III) redox couple because of the better conductivity of GOQD-Au. 2.4. Application as Propellant Catalysts. The synthetized systems were evaluated as catalysts for the decomposition of AP, the main component of composite solid propellants. With the aim of comparing the effect of AuNPs and GOQDAu on the decomposition of AP, 13 nm AuNPs were synthetized following the traditional sodium citrate procedure.57 Because the thermal decomposition of AP involves electronic transfers, the introduction of an electron transport agent could facilitate and accelerate the decomposition reaction of AP. In this sense, it has been reported that GR and GO derivatives are capable of constructing a conductive network that facilitates the heat and electron transfers during fuel decomposition, resulting in improved energy release and decomposition of AP.58,59 Moreover, nanocomposites of GO and Al@Fe2O3 also provide a pathway for electrostatic discharges, improving the overall safety.59 GR and GO have also been mixed with NPs such as cobalt and iron,25,60 resulting in a decrease in the higher decomposition temperature of AP. To our knowledge, both GOQD and gold have never been tested as the catalyst for the decomposition of AP, the latter probably due to its elevated cost. GOQD and GOQD-Au synthetized at the lower HAuCl4 concentration (0.3 mM) were tested as catalysts for the decomposition of AP at 4 wt %. The same concentration and weight percentage were used for AuNPs. Figure 6 shows the differential scanning calorimetry (DSC) curves for pure AP and the tested materials.

On the other hand, system (f) was approximately 25 kcal· mol−1 more stable than (c), indicating that the gold nanocluster improved the delocalization of the electronic charge around GOQD rings. Complex (f) also presented the shorter anchor bond distance (dX−Au) of the set, associated with stronger interaction and stability. The total NPA charge of the gold cluster (Δqcluster) was also analyzed. In all cases, the ligand (GOQD) transferred part of its charge to the cluster. Complex (f) presented the greatest amount of transferred electron density, with approximately 70% more transferred charge than complex (b), the less stable of the series. Figure S5 shows the electrostatic potential surface and frontier molecular orbitals of the optimized structure of GOQD. As can be seen, most of the electronic density is situated at the epoxy functional groups, specifically at the oxygen atoms. This evidence indicates that the lone pairs belonging to these atoms are highly available for the interaction with metal atoms. Therefore, AuNPs will tend to grow close to the epoxy groups on the surface of the GOQD layer rather than the functional groups located at the edge sites, as demonstrated by the Ebind analysis, where complex (f) was the most stable. Taking into account the average size of the synthetized GOQD (17.7 nm), it is possible to state that the smaller NPs of 9 nm can be effectively stabilized by the epoxy groups on the surface of a single GOQD, whereas the bigger NPs of approx. 35 nm require a sphere of GOQD interacting with the metal atoms through the different functional sites. As revealed by computational calculations, oxygen-containing groups result as better stabilizers than the nitrogen-rich groups. With the aim of exploring the effect of gold on the conductivity of GOQD, the energy of the Fermi level (EFL) was calculated for all complexes, GOQD, and the gold cluster (Table 1). GOQD presents a value of −3.69 eV and Au8 −5.06 eV. This indicates that the cluster is a better conductor than 7282

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ACS Omega AP shows the classical first endothermic peak corresponding to the crystal phase transition from orthorhombic to cubic phase (240−250 °C).61 After that, the exothermic LTD and sublimation take place at approx. 300 °C with a proton transfer, which is the most accepted mechanism.62 Finally, the HTD (415 °C) step occurs. When GOQD is mixed with AP at 4 wt %, the first endothermic transition of AP occurs at lower temperature. A second endothermic peak is registered because of the decomposition of GOQD. The exothermic LTD peak was shifted to lower temperature (293 °C), whereas the HTD peak occurred at 392 °C, 23 °C before pure AP. Dey and coworkers63 evaluated GR as the catalyst for the decomposition of AP with a 5 wt %, registering a first exothermic peak at 368 °C and a HTD process at 395 °C. Zhao et al. tested GO at 2 wt %, observing two peaks for the LTD and HTD at 345 and 430 °C, respectively.60 A study of Memon and co-workers58 reported that fast crash AP with hand-mixed GO decreased the second decomposition temperature of AP from 435 to 409 °C, whereas fast crash AP and GO reduced the final decomposition temperature to 403 °C. These previous results confirm that GOQD exhibits better performance as the catalyst for the decomposition of AP than GO and GR. Both exothermic decomposition processes of AP + GOQD resulted in lower temperatures, indicating that GOQD accelerated the electrons to speed up the decomposition of AP. Figure 6b summarizes the effect of graphene or GO derivatives on the HTD of AP. The 13 nm AuNPs synthetized by the traditional methodology were also tested. The LTD appeared at higher temperature in comparison to AP (approx. 329 °C). The DSC profile was similar to the one recorded for AP, with very close HDT. GOQD-Au, as AuNP, shifted the first endothermic process to higher temperature. The LTD started at an upper temperature than AP and AP + GOQD (approx. 325 °C) and was almost in the middle temperature between AuNP and GOQD. The LTD and HTD merged into one step, the proton- and electron- transfer routes occurred in a single and continuous process. This result suggests a synergic effect of GOQD and AuNP, which participate actively in the mechanism of decomposition of AP, facilitating the electron transfer. It is well-known that the size of the catalysts affects the final decomposition of AP.64 Unlike the AuNP sample, GOQD-Au presented high dispersity of NP sizes, mainly 9 and 35 nm. This heterogeneous mixture can be related to the observed continuity of the HDT process. The AP + GOQDAu high-temperature peak (371 °C) appeared at lower temperature than AP + GOQD (392 °C) and AP + AuNP (406 °C), close to the performance exhibited by a graphene− iron oxide nanocomposite 1 wt %, which presented a second decomposition temperature of approx. 372 °C.63 However, this change was lower than the decrease registered by other metal NP catalysts supported on graphene derivatives, such as ultrafine Mn3O4 NPs dispersed on graphene, which decreased the second peak temperature to 291 °C24 or 2 wt % GO/ Co3O4, which lowered the HTD of AP to 297 °C.60 When the energy release is analyzed, it can be noted that GOQD (1305 W·g−1) released a higher amount than pure AP (1016 W·g−1) and also in comparison to the value reported previously for GO at 2 wt % (1215 W·g−1).60 GOQD not only lowers the decomposition temperature but also enhances the exothermic heat of AP. The AuNP released approx. 40% more energy (1471 W·g−1) than pure AP. However, the HDT was

not effectively reduced, resulting as an unsuitable catalyst to accelerate the decomposition of AP. GOQD-Au released 80% more energy than AP (1806 W·g−1), a value approx. 200 W·g−1 bigger than GO/Co3O4 at 2 wt %.60 Although the energy release was bigger than that of other systems, the effect on the decrease of the HDT was not as successful as other reported metal catalysts supported on graphene derivatives (Figure 6b). Therefore, the results obtained in this study suggest that GOQD is a promising material for increasing the performance of solid composite propellants because of its improved heat transfer, higher thermal conductivity, and catalytic decomposition of AP. The presence of a larger quantity of functional groups on the edges of the QDs than GO can be related to the superior performance of this material in the decomposition of AP, which increases the electron-transfer rate and at the same time serve as the fuel. Probably, the synthesis of new NPs supported on QDs will produce even better catalytic properties and will be considered in the future work.

3. CONCLUSIONS In this work, the synthesis of GOQD and rGOQD was performed by the oxidation and sonication of GO. The experimental characterization confirmed the effective cut of GO into small layers, which exhibited excitation-independent PL feature. GOQD and rGOQD were applied as the support and stabilizing agents for the in situ synthesis of AuNPs. GOQD resulted as a better substrate because of the presence of a higher amount of functional groups that can interact and bind to gold. Computational calculations highlighted the key role of the epoxy groups on the surface of the GOQD layer in the stabilization of AuNPs. Electrochemical measurements revealed the lower conductivity of GOQD in comparison to that of GOQD-Au, a property that was correlated with the higher energy of the Fermi level of GOQD. DSC assays confirmed better performance of GOQD as the catalyst for the decomposition of AP than that of GO and GR. GOQD not only lowered the decomposition temperature but also enhanced the exothermic heat of AP. GOQD-Au merged the LTD and HTD into one step, the proton- and electron-transfer routes occurred in a single process, suggesting an active participation of gold and GOQD on the decomposition mechanism of AP. Although the energy release of GOQD-Au + AP was bigger than that of GOQD + AP, the effect on the decrease of the HDT was not as important as other previous reported metal catalysts supported on graphene derivatives. The results obtained in this work suggest that GOQD is a promising material for increasing the performance of solid composite propellants. Future work will consider the synthesis of new NPs supported on QDs to produce enhanced catalytic properties. 4. EXPERIMENTAL DETAILS 4.1. Reagents and Characterization Methods. All reagents used were of analytical grade or the highest commercially available purity and were used as received. KBr, graphite, H2SO4, K2S2O8, KMnO4, H2O2, HNO3, NaOH, NaBH4, sodium citrate, KCl, K3[Fe(CN)6], and K4[Fe(CN)6] were acquired from Merck. Phosphate-buffered saline (PBS) was acquired from Medicago. P2O5 and HAuCl4·3H2O were acquired from Sigma-Aldrich. All solutions, including the ones used in electrochemical measurements, were prepared with in7283

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(CH Instruments, CHI 760E) at room temperature (20 °C). All measurements were performed in a common threecompartment cell. A platinum (Pt) disk electrode (2 mm diameter) was used as the working electrode. A coiled Pt wire of large area was used as the counter electrode, and it was separated from the electrolytic solution in all time by a sintered glass frit. All potentials are referred to as Ag/AgCl (KCl, 1 M) electrode. Prior to each experiment, the Pt disk electrode was polished to a mirror finish with an alumina slurry (particle sizes 0.3 and 0.05 μm) on microcloth pads, rinsed exhaustively with Milli-Q water, and dried. Also, prior to each experiment, the working solution was purged for 15 min with high-purity argon, and a reduced flow was maintained during the measurements. Cyclic voltammetry characterization profiles were performed in 0.2 M PBS, 0.5 mM K3[Fe(CN)6], and 0.5 mM K4[Fe(CN)6] in an electrochemical window of −0.2 to +0.6 V. EIS was performed in the same solution at a frequency range from 0.1 Hz to 1000 kHz and at an ac voltage amplitude of 0.01 V at open-circuit potential. 4.5. Computational Details. The nanoarchitecture of GOQD and GOQD-Au was explored by theoretical methodologies. Because of the size of the system, the GOQD layer was represented by a 2 × 2 nm2 structure. The functional groups were distributed in accordance with previous experimental results.32 It is well-known that GO has some functional groups such as carboxylic acids, ethoxy, and alcohols that can stabilize metal ions. The oxidative-ultrasonic methodology applied in this work induced the doping of nitrogen into GOQD through nitration.55,56 The geometry of GOQD was fully optimized at the density functional theory level using the Gaussian 16 software.66 AuNPs were represented by an eight-atom cluster (Au8). In order to understand the interaction between GOQDs and AuNPs, all complexes were fully optimized using the Becke’s three-parameter nonlocal hybrid exchange potential with the nonlocal correlation functional of Lee, Yang, and Parr (B3LYP).67−69 No symmetry restrictions were applied. Light atoms, such as C, H, O, and N, were described with the triple-ζ 6-311G(d,p) basis set, whereas gold atoms were described with a relativistic effective core potential basis set with pseudopotentials (LANL2DZ70). A tight self-consistent field convergence criterion (10−8 a.u.) was used in all calculations. The charge distribution of intermolecular interactions was calculated by the NPA method,71 as implemented in Gaussian 16. The interaction energy (Eint) was defined according to the following expression: Eint = EGOQD‑Au8 − EGOQD(GOQD‑Au8) − EAu8(GOQD‑Au), which represents the energy difference between the complex and the energies of constituent monomers. It is well-known that the estimation of Eint with finite basis sets introduces an error known as BSSE. The BSSE is related to the use of different numbers of basic functions to describe the complex and monomers for the same basis set. BSSE-corrected interaction energies were computed using the Boys−Bernardi counterpoise correction scheme. 72 The effect on the conductivity of the coordination of gold to GOQDs was estimated by the energy of the Fermi level (EFL), which corresponds to the average of the highest occupied molecular orbital and the lowest unoccupied molecular orbital energies.

house produced ultrapure water with resistivity less than 18 MΩ cm (Milli-Q, USA). A Jasco FT/IR 4100 spectrometer was used for measuring FT-IR spectra, using the 500−4000 cm−1 frequency range, on the KBr pellet. A Jasco V-630 UV−visible spectrophotometer was used for measuring the UV−visible spectra. All measurements were completed at room temperature. HR-SEM and HR-TEM were carried out using a FEI INSPECT-F50 and a Tecnai ST F20 FEI equipment, respectively. PL measurements were taken with a Jasco spectrophotometer FP-8200. DSC analysis was performed on an 822e Mettler Toledo instrument at a heating rate of 5 °C·min−1 under a nitrogen blanket in the range of 140−500 °C. To investigate the catalytic performance of the compounds for the thermal decomposition of AP, specific amounts of the complexes and AP were mixed and ground in a certain weight ratio for the DSC analysis. 4.2. Synthesis of GO. GO was synthetized using the modified Hummers method.65 Shortly, graphite powder (4 g) was dissolved in H2SO4 (10 mL), heated (90 °C) with vigorous stirring, and then preoxidized using K2S2O8 and P2O5 (2 g each). The solution reacted under stirring (4 h) at high temperature (80 °C). Then, Milli-Q water (500 mL) was added, and the solution was stirred overnight. The preoxidized graphite was filtered and washed (deionized water) until reaching neutral pH and then dried at room temperature overnight. This powder was oxidized even more with H2SO4 (92 mL) and KMnO4 (12 g) in an ice bath and then stirred (2 h) at 35 °C. Then, Milli-Q water (184 mL) was added. The reaction was terminated, reducing the manganese in the solution, by adding H2O2 (10 mL) and Milli-Q water (560 mL). The solution turned bright yellow and was stirred overnight. GO was centrifuged (3000 rpm) and washed (deionized water) until reaching neutral pH. Then, it was vacuum-dried at 30 °C. 4.3. Synthesis of GOQDs, rGOQDs, and AuNPs Supported on GOQD (GOQD-Au) and rGOQD (rGOQDAu). GOQDs were prepared following the procedure reported by Li and co-workers with some modifications.32 Briefly, 0.05 g of GO was suspended in 40 mL of concentrated sulfuric acid and nitric acid (1:3 ratio). The mixture was stirred for 24 h and then sonicated for 24 h. The mixture was cooled and diluted with deionized water (20 mL). The pH of the resulting dark brown solution was adjusted by adding the concentrated solution of NaOH until reaching pH 8, avoiding temperatures higher than 80 °C. The resulting mixture was filtered through a microporous membrane (0.22 μm), obtaining a yellowish solution. The liquid excess was evaporated by a rotary evaporator. The concentrated solution was dialyzed five times using a 3500 Da dialysis bag. The resulting solution corresponded to GOQDs. rGOQDs were prepared by mixing 5 mL of GOQD with 1 g of NaBH4 to reduce the functional groups. This solution was filtrated through a microporous membrane (0.22 μm) and further dialyzed using a 3500 Da dialysis bag. The GOQD and rGOQD solutions (0.1 mL) were mixed with 0.1 mL of HAuCl4 of different concentrations: 0.3, 0.6, 0.9, 1.2, and 1.5 mM. After 15 min agitation, a 0.1 mL sodium citrate solution (0.1 mM) was added as the reducing agent and sonicated for 5 min. A red-pink solution was obtained after sonication, related to the formation of the NPs. 4.4. Electrochemical Methods and Characterization. The electrochemical properties of the samples were measured using a potentiostat−galvanostat electrochemical workstation

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TEM images of GOQD, electrochemical measurements, and ATR−IR and computational description of the GOQD-Au explored structures (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.B.C.). ORCID

María B. Camarada: 0000-0001-5408-3073 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.B.C. is grateful to Fondecyt for funding this research (Project Regular 1180023). C.M.-V. thanks Fondecyt Chile for supporting this work (Project Regular 1161297). Powered@ NLHPC: This research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02).

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