Hierarchically Structured Fullerene C70 Cube for ... - ACS Publications

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Hierarchically Structured Fullerene C70 Cube for Sensing Volatile Aromatic Solvent Vapors Partha Bairi, Kosuke Minami, Waka Nakanishi, Jonathan P. Hill, Katsuhiko Ariga,* and Lok Kumar Shrestha* World Premier International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *

ABSTRACT: We report the preparation of hierarchically structured fullerene C70 cubes (HFC) composed of mesoporous C70 nanorods with crystalline pore walls. Highly crystalline cubic shape C70 crystals (FC) were grown at a liquid−liquid interface formed between tert-butyl alcohol and C70 solution in mesitylene. HFCs were then prepared by washing with isopropanol of the FC at 25 °C. The growth directions and diameters of C70 nanorods could be controlled by varying washing conditions. HFCs perform as an excellent sensing system for vapor-phase aromatic solvents due to their easy diffusion through the mesoporous architecture and strong π−π interactions with the sp2 carbon-rich pore walls. Moreover, HFCs offer an enhanced electrochemically active surface area resulting in an energy storage capacity 1 order of magnitude greater than pristine C70 and fullerene C70 cubes not containing mesoporous nanorods. KEYWORDS: fullerene crystals, interface, hierarchical structure, self-assembly, vapor sensing, specific capacitance

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carbon nanotube (CNT) on Sn exhibited excellent cyclical performance and rate capability for a prototype lithium-ion battery.31 Despite these promising properties, construction of complex hierarchical functional materials over different length scales using functional molecules, in particular fullerene C60 or C70, remains a challenging task.32−34 Fullerene C60 is a highly symmetric and isotropic functional molecule whose crystal structure is known from powder scattering studies.35,36 However, larger nanoscale or microscale X-ray diffraction cube-shaped crystals of C60 are not found unless it is cocrystallized with an inorganic salt such as silver(I) nitrate.37 The resulting cube-shaped crystals are composed of an exohedral Ag-n-alkene complex of fullerene with a network of nitrate anions bridging the fullerene-complexed silver cations, yielding a hybrid heteronanostructure of C60. In contrast, the less symmetric fullerene C70 molecule can be crystallized into cube-shaped crystals with well-defined edges (Scheme 1), allowing the preparation of larger nanoscale and microscale fullerene crystals, which can possess unique properties. Here, we demonstrate the preparation of hierarchically structured fullerene cubes (HFCs) composed of mesoporous fullerene C70 nanorods with crystalline pore walls, which function as an excellent sensing system selective for vaporphase aromatic solvents (Scheme 1). HFCs were prepared by a

roduction of shape-controlled micro- or nanosized objects through self-assembly of functional building blocks is a key aspect of bottom-up nanotechnology.1−10 Self-assembled nanostructures of functional molecules, such as fullerene (C60 or C70), have recently received considerable attention due to their unique physical and chemical properties, which are a result of their high symmetries and extended conjugated π-systems. Several potential applications have been demonstrated in a wide range of research fields including biomedicine, semiconductors, optics, electronics, and spintronics.11−15 Thus, materials’ design using fullerene crystals has led to innovative functions not available using conventional nanomaterials. However, since functions and performances of devices depend largely on their dimensions, morphologies, and the structural hierarchies of the nanomaterials, the strategic design of shape-controlled nanostructures and hierarchical superstructures is very important. Depending on the synthesis conditions, functional πconjugated fullerene molecules have been shown to form various bulk morphologies including self-assembled onedimensional (1D) nanowhiskers or two-dimensional (2D) nanosheets in the bulk phase or at liquid interface.16,17 However, none of those possess complex three-dimensional (3D) network structures or hierarchical superstructures.18,19 Hierarchical nanostructures are advantageous because of their high specific surface areas, synergistic interactions, and multiple functionalities, which are generally impossible to achieve using conventional materials.20−30 For example, a hierarchical nanostructure based on 3D hollow carbon cubes with 1D © 2016 American Chemical Society

Received: March 3, 2016 Accepted: June 24, 2016 Published: June 24, 2016 6631

DOI: 10.1021/acsnano.6b01544 ACS Nano 2016, 10, 6631−6637

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crystalline material, which is further confirmed by selected area electron diffraction (SAED) patterns (Figure S5). The crystal form of FC was identified by powder X-ray diffraction (pXRD). The pXRD pattern of FC (Figure S6) was indexed as a simple cubic (SC) structure with the cell dimension ca. a = 1.05 nm.38,39 In contrast, pristine C70 has a hexagonal close-packed (HCP) structure40 with lattice parameters ca. a = 1.08 and c = 1.74 nm. The solid solvate structure of FC with mesitylene molecules trapped at the interstitial site of the SC lattice was confirmed by FT-IR study (Figure S7) and is supported by thermogravimetric analysis (Figure S8).39 For the fabrication of HFC (Figure 1a), solid FC (0.5 mg) was added to IPA (1 mL) and gentle hand shaking was applied for 30 s, followed by storing in an incubator at 25 °C for 1 h. SEM observations revealed that IPA causes structural rearrangements of the FC resulting in the formation of fullerene C70 nanorods, which protrude from the cube surface (Figures 1d,e and S9). The nanorod formation process is very rapid (∼3 min), and the diameters of the nanorods increase with time finally attaining a size of ca. 118 ± 29 nm after 24 h (Figure S10). Microscopic observation reveals that most of the nanorods are grown vertically from the cube surface (assigned as the z direction) with only a few rods growing in plane (assigned the x,y directions). When hand shaking was not applied, most of the nanorods grow in the x,y plane (Figures 1f,g and S11). These results suggest that growth direction of the nanorods depends on the diffusion kinetics of solvent (IPA). Growth in the z direction could be prevented by restricting the motion of FC in the IPA medium. For example, when FC was dropcast on a silicon substrate and immersed slowly in IPA then maintained undisturbed for an hour, nanorods are mostly oriented with long axes in the x,y plane giving a net-like structure (Figures 1h,i and S12) with only very few nanorods grown in the z direction. This process is rather slow and involves the formation of nanopores on the surfaces of FC after 5 min (Figure S13). From these observations, it can be concluded that when solvent diffusion is very fast (hand shaking of 30 s), supersaturation of fullerene C70 in IPA occurs instantaneously causing an almost immediate recrystallization (dissolution of C70 in IPA is energetically unfavorable) of C70 into randomly oriented and z direction nanorods. However, in the case of a slow diffusion rate (1 h) of IPA, any dissolved C70 has sufficient time to recrystallize directionally from the active surface of the FC. The degree of structural transformation from cube to rod was studied by sectioning the HFC using a focused ion beam. The results (Figure S14) show that the transformation of the cube structure occurs to a depth of ∼2 μm, which corresponds to about 14% of the crystal edge length in this case. The remaining crystalline core does not undergo further transformation under successive treatment with fresh IPA. Note that cube-to-rod structural transformation does not occur when FC is treated with other alcohols including methanol, ethanol, TBA, and butanol (Figure S15), a feature attributed to the differences in the solubilities of solid solvates of C70 in different alcohols and their types. Only IPA causes the unique cube-to-rod transformation (solubility of C70 in IPA 0.0021 mg mL−1).41 Moreover, when FC fabricated using a different solvent system (mesitylene/IPA) was treated with fresh IPA, HFC formation did not take place.38,39 Rather, the cube-to-rod transformation led to randomly oriented masses of nanorods (Figure S16). Similar randomly arranged nanorods were also observed upon

Scheme 1. Schematic Representation of the Preferential Assembly of Fullerene C70 into Cube-Shaped Crystals and Its Structural Rearrangement into HFC with Mesoporous Nanorods

structural rearrangement of solvated fullerene C70 cube (FC) upon simple treatment with isopropanol (IPA) at 25 °C. The growth directions and diameters of the nanorods could be controlled by altering the synthetic conditions. In addition to excellent sensing performance, the HFC showed enhanced electrochemical capacitive performance over pristine C70 and FC lacking mesoporous nanorods, demonstrating their possible future use in energy storage applications.

RESULTS AND DISCUSSION For the preparation of fullerene C70 cubes (FC) solid solvate, we used an ultrasound-assisted liquid−liquid interfacial precipitation (ULLIP) method.32 In a typical crystallization of FC, a solution of C70 in mesitylene (1.0 mL, conc. 1.0 mg mL−1) was placed in a clean glass bottle (13.5 mL), and tertbutyl alcohol (TBA) (5 mL) was slowly added so that an interface was formed between the C70 solution and TBA. The resulting mixture was stored in an incubator at 25 °C for 3 h without mechanical agitation. Mild sonication was applied for 30 s to complete the mixing of the two solvents, and the mixture was stored in the same incubator for 48 h. The resulting FC was isolated by centrifugation then dried at 60 °C under reduced pressure for 48 h. Figure S1 shows scanning electron microscopy (SEM) images of FC. We have also investigated the effect of the volume ratio of C70 solution against TBA and also considered the preincubation time on the morphology (Figures S2 and S3). SEM images of FC (Figure S1) fabricated under optimized conditions display well-defined 3D cube-shaped crystals with sharp edges. A histogram of the size distribution of 100 randomly selected FC (Figure S4) shows a narrow size distribution (average size ∼9 μm) apparently demonstrating the homogeneous nucleation of C70 molecules during interfacial precipitation. High-resolution transmission electron microscopy (HR-TEM) images clearly demonstrate that the FC is 6632

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leads to a much larger BET surface area (ca. 97.5 m2 g−1) compared to those of either FC (ca. 19 m2 g−1) or pristine C70 (ca. 6.5 m2 g −1). Nitrogen adsorption isotherms and corresponding pore size distributions are supplied in Figure S22. The pXRD pattern of HFC (Figure S23) was indexed as mixed phases of hexagonal (a = 2.527 nm and c = 1.669 nm) and orthorhombic (a = 2.370 nm, b = 3.228 nm, c = 1.070 nm) structures.45−47 Similar mixed phases have previously been found in porous fullerene C60 crystals.42,48 The mechanism of structural rearrangement relies initially on the solubility of solvated C70 cube surface in IPA, and solvating mesitylene also plays a key role for this transformation. To establish the mechanism of nanorods formation, morphological evolution of HFC structure was observed by SEM imaging. SEM images were recorded immediately after addition of FC crystals into IPA (10 s) without hand shaking and at different time intervals after applying hand shaking (for 30 s). Dissolution of the FC surface commences within 10 s (Figure S24a,b). The FC surface roughness increases after applying hand shaking; the porous structure on the surface has developed and nanorods started to grow (Figure S24c,d). HFC structure can be seen within ∼3 min (Figure S24g,h). When FC is treated with IPA, solvated C70 at the surface is dissolved in IPA thus releasing solvating mesitylene, which further contributes to the spontaneous dissolution of FC surface leaving an active surface for the growth of nanorods. Dissolved C70 is then rapidly recrystallized as highly crystalline porous nanorods at the cube surfaces. The key role of solvating mesitylene was further confirmed by a control experiment. Solvating mesitylene was removed from the FC by heat treatment at 220 °C under reduced pressure. Structural transformation into HFC was not observed when heat-treated FC was treated with IPA even after 24 h (Figure S25). Regarding the directional growth of the nanorods, we have found that, depending on the washing condition (fast and slow diffusion of IPA), surface morphology of the FC prior to the development of nanorods is different (Figure S26). For instance, the surface of FC became very rough upon shaking by hand (fast IPA diffusion condition) compared to the FC surface without any agitation (slow IPA diffusion condition). A rough surface prevents the growth of the nanorods in the x,y directions (along the surface) allowing nanorod growth in the z direction. On the other hand, smooth surfaces promote nanorod growth in the x,y directions.49 Molecular sensing of toxic organic substances has received considerable attention and has been investigated extensively to address growing concerns regarding environmental or workplace contamination. Previous investigations have revealed that sensitivity and selectivity of sensors strongly depends on effective design of host sensing structures. Thus, the high surface area and crystallized pore walls of the mesoporous nanorods contained in the HFC material were considered for selective sensing of aromatic solvent vapors and employed as a sensing system in conjunction with a quartz crystal microbalance (QCM). Figure 2a shows a typical example of the time dependencies of frequency shifts (Δf) for the HFC (Z directed nanorods) modified QCM electrode upon exposure to different solvent vapors (hexane, benzene, pyridine, and toluene). Note that the frequency shift is very rapid upon exposure of HFC-modified (Z directed nanorods) QCM sensor to the solvent vapor, and the frequency shift is strongly dependent on the nature of the vapor-phase guest molecules. Also, note that the adsorption of aromatic solvent vapors such as toluene (721

Figure 1. (a−c) Schemes illustrating how growth direction of the nanorods was controlled by varying the relative diffusion rate of solvent. (d,e) SEM images showing that nanorods grow roughly in the z direction (if z direction is perpendicular to the cube face) after 1 h with fast solvent diffusion (shaking 30 s). (f,g) SEM images of nanorods growing randomly in the z direction but also in the x and y directions after 1 h with moderate solvent diffusion (without shaking). (h,i) SEM images of nanorods growing systematically in the x and y directions after 1 h with very slow solvent diffusion. (j) TEM image of nanorods and (k) HR-TEM image of a single nanorod. Inset of panel (k) represents SAED pattern.

exposure of FC to IPA vapor for longer times ≥72 h (Figure S17) or when FC was treated with IPA at 60 °C (Figure S18). TEM observations of HFC indicate that the nanorods are highly porous (Figures 1j and S19−21). The pore size distribution is narrow with average pore size ca. 8 nm (mesoporous) (inset; Figure S21a). Coherently extended lattice fringes of fullerene C70 over several mesopores with SAED patterns (inset of Figure 1k) confirmed the crystallinity of the pore walls.42−44 Bright-field STEM images (Figure S21e,f) also support the presence of a mesoporous structure of the nanorods. The well-developed mesoporous structure of HFC 6633

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adsorption of toluene vapors is normalized with the mass of HFC on the QCM resonator. We found that adsorption of toluene vapors caused higher QCM frequency shifts for the HFC with z directed nanorods (404 Hz/μg) compared to the HFC with x,y directed nanorods (289 Hz/μg) (Figure S28). The better sensing performance of HFC with z directed nanorods is attributed to the easy availability and accessibility of a greater number of sites (mesopores) for the adsorption of toluene vapors, which is limited in HFC with nanorods oriented along the surface because vapors cannot be adsorbed on all faces of the rods since they do not protrude from the cube surface. It should be noted that toluene and pyridine are good solvents for C70, and it is expected that morphology of nanorods may change after repeated exposure of toluene vapor to the HFC. However, we did not observe any changes in the nanorod morphology in SEM images recorded after exposure to toluene vapors for 1 h (Figure S29). This demonstrates that exposure of HFC to the toluene vapors (for at least for 1 h) does not result in any permanent changes of morphology to the nanorods. In order to estimate the electrochemically accessible surface area, cyclic voltammetry (CV) measurements were performed. A glassy carbon electrode (GCE) was modified using HFC, then CV measurements were performed using 1 M H2SO4 as an electrolyte within the potential range 0 to 0.8 V (vs Ag/ AgCl) at different scan rates 5−100 mV s−1. The CV curves indicate very rapid current response and exhibit quasirectangular shape reminiscent of electrical double layer capacitors (Figure 3), which is further confirmed by symmetric

Figure 2. (a) QCM frequency shifts (Δf) upon exposure to hexane, benzene, toluene, and pyridine. (b) Summary of sensing performance of HFC (Z directed nanorods). (c) Typical repeatability test up to a few cycles of an HFC electrode involving exposure and evacuation of pyridine and toluene vapors. (d) Schematic of HFC as a sensing antenna system.

Hz) and pyridine (538 Hz) cause large frequency shifts (higher selectivity) compared to the vapors of aliphatic hydrocarbons such as cyclohexane (70 Hz) and hexane (40 Hz) despite their similar molecular dimensions. A summary of the sensing performance is presented in Figure 2b. The selectivity of sensing decreases in the following order: toluene > pyridine > benzene > ethanol > methanol > ethyl acetate > acetone > cyclohexane> hexane > water. Note that vapor pressure [log(pressure/mmHg)] of aromatic solvents benzene (1.978), toluene (1.462), and pyridine (1.316) is lower compared to the aliphatic hydrocarbon, n-hexane (2.179).50,51 Therefore, the observed higher sensing selectivity toward toluene or pyridine vapors cannot be due to the difference in saturated vapor pressure. It is caused a feature of the unique structure (HFC) of our material. Thus, QCM results demonstrate that HFC functions as an excellent sensing antenna selective for aromatic guest molecules because of easy diffusion of aromatic solvent vapors into the mesoporous architecture of the nanorod likely favored by strong π−π interactions between host and guest. Sensing performance of FC without mesoporous nanorods (sensing antenna) is very poor compared to HFC (Figure S27a). It is interesting to note that although benzene and pyridine have similar structures, response to pyridine is greater (benzene caused frequency shifts of about 300 Hz) due to a very strong donor−acceptor charge-transfer interaction between pyridine and C70.52,53 We have also investigated the repeatability of the electrode sensing operation by recording frequency shifts during alternate exposure and removal of both pyridine and toluene (Figures 2c and S27b,c). Careful observations revealed that release rate of toluene vapors is slower than pyridine vapors demonstrating that the host−guest type interactions between toluene and fullerene C70 mesoporous nanorods are more stable than with pyridine (Figure 2c). Sensing performances of HFC with nanorods vertically directed (z axis) and HFC with nanorods on the surface (major population of rods in the x,y plane) are compared for toluene vapors (Figure S28). The frequency shift caused by the

Figure 3. (a) CVs of HFC at different scan rates (5, 10, 20, 50, 80, and 100 mV s−1); (b) corresponding calculated specific capacitance (Cs) vs scan rates; (c) comparison of CV curves of HFC, FC, and pristine C70 (scan rate 5 mV s−1); and (d) corresponding calculated specific capacitance. In panel (c) CD stands for current density.

triangular shape charge−discharge curve (Figure S30).54,55 HFC displayed improved electrochemical performance over pristine C70 and FC because of the presence of mesoporous nanorods (Figure 3c). The specific capacitance of HFC is ca. 39, which is ∼32 and 47 times greater than FC (lacking mesopores) and pristine C70, respectively (Figure 3d). 6634

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Focused Ion Beam (FIB) Cutting. FIB sectioning of HFC were carried out using a gallium ion beam of a Hitachi Model FB-2100 scanning ion microscope (SIM) operating at a voltage of 40 kV and current of 1.5 μA. CV. Electrochemical capacitive performances of HFC, FC, and pristine C70 were studied by CV with a three-electrode system in 1 M aqueous H2SO4 solution at 25 °C. Measurements were carried out using ALS/CH Model 850D electrochemical analyzer in the potential range from 0 to 0.8 V (vs Ag/AgCl). Platinum wire was used as a counter electrode. A bare GCE used as working electrode was mirror polished with Al2O3 slurry and cleaned with double-distilled water and sonicated in acetone for 5 min. Pristine C70 (1 mg) was dispersed in water−ethanol mixture (4:1) mixture (1 mL) and sonicated for 1 h. Then 3 μL of this dispersion was placed on the GCE surface and dried at 60 °C for 1 h. After the solvent was evaporated, 5 μL of Nafion solution (0.5%) was added as binder on the surface of the GCE and dried at 60 °C for 12 h. Working electrode for FC was also prepared following the similar method. For the preparation of HFC electrode, dry FC (0.5 mg) was added in IPA (1 mL), and a gentle hand shaking was applied for 30 s. The mixture was then stored in an incubator at 25 °C for 1 h. The HFC dispersion (5 μL) was then placed on the GCE and dried at 60 °C for 1 h. After drying, 5 μL of Nafion solution (0.5%) was added as binder on the surface of the GCE and dried at 60 °C for 12 h. Vapor Sensing by QCM. The QCM can be used to measure the mass per unit area by measuring the change in frequency of a quartz crystal resonator. It has become a potent technique for detection of small mass changes during thin-film deposition, gas sensing, etc. In this study, we used a resonance frequency of 9 MHz (AT-cut). The frequency of the QCM electrode was measured during adsorption and was recorded when it became stable, i.e., QCM frequency in air was stable within ±2 Hz over 10 min. FC (0.5 mg) was suspended in IPA (1 mL) and shaken by hand for 30 s followed by standing for about 1 h. Three μL of the resulting HFC preparation was drop casted onto the QCM electrode. The electrode was dried at 60 °C under reduced pressure for 12 h prior to measurement. The modified QCM electrode was then installed in the QCM instrument and exposed to different solvents (10 mL) within a sealed chamber to prevent the escape of vapors during the adsorption measurements. The electrode was exposed to air to desorb the solvent vapor between measurements. The time dependence of frequency shift (Δf) was plotted during alternate exposure and removal of the solvent vapors. All experiments were performed at room temperature. The change in mass m (g cm−2) of the HFC sample deposited onto a QCM electrode can be measured by the oscillating frequency of the quartz electrode. The frequency change (Δf) corresponds to amount of sample loaded on the QCM electrode, which can be calculated from the Sauerbrey equation:

CONCLUSION In summary, we have prepared a unique class of carbon nanostructure: hierarchically structured fullerene C70 cubes composed of mesoporous nanorods with a crystalline framework. Mesoporous materials with crystallized pore walls are in strong demand for various applications including as optoelectronic devices and for photocatalysis. In this work, we have explored innovative functions of the mesoporous fullerene C70 nanorods as a highly selective sensing antenna system for toxic aromatic solvent vapors. Moreover, these hierarchically structured fullerene C70 cube crystals display enhanced electrochemical capacitive performance compared to pristine C70 or fullerene crystals without pores leading to improved energy storage capacity an order of magnitude greater than for the unstructured materials. We believe that successful development of hierarchically structured fullerene C70 cubes would be an asset to the carbon nanomaterials field enabling the discovery and exploration of other complex hierarchical nanomaterials useful in a variety of technological and biomedical applications. METHODS Synthesis of FC and HFC. Fullerene C70 cube (FC) was synthesized by using an ULLIP method. A solution of fullerene C70 in mesitylene (1 mg mL−1) was prepared by dissolving pristine C70 powder (20 mg, 99%, MTR Ltd., USA) in mesitylene (20 mL) with ultrasonication for 1 h followed by filtration. In a typical crystallization, C70 solution (1 mL) was placed in a cleaned 13.5 mL glass bottle and stored in an incubator at 25 °C. After 1 h, TBA (5 mL), which had also been stored at 25 °C, was slowly added to the C70 solution leading to a layered solution with a clear interface between the C70 solution and TBA. The resulting mixture was stored at 25 °C without mechanical disturbance for 3 h. Light agitation (shaking) by hand was then applied followed by sonication for 30 s. The resulting mixture was again stored at 25 °C in an incubator for 48 h to obtain FC. For the synthesis of HFC, dry FC crystals (0.5 mg) were added to IPA (1 mL) at 25 °C, then the mixture was gently shaken by hand for 30 s followed by storing in an incubator at 25 °C for 1 h. For directional growth of nanorods, FC crystals were drop casted onto a clean and dry silicon wafer and dried at 25 °C under reduced pressure. The resulting silicon wafer with FC crystals was immersed very slowly into a beaker containing IPA (2 mL) at 25 °C and allowed to stand for 1 h without disturbance. Characterizations. SEM images of FC and HFC were obtained using a Hitachi Model S-4800 field-effect scanning electron microscope (FE-SEM) operating at an accelerating voltage of 10 kV. SEM samples were prepared by dropping suspensions of FC or HFC on cleaned silicon wafers followed by drying at 80 °C. All the samples were coated with platinum (∼2 nm) by sputtering using a Hitachi S2030 ion coater. TEM images (TEM and HR-TEM) and SAED were obtained using a transmission electron microscope (JEOL Model JEM-2100F operated at 200 kV). TEM samples were prepared by dropping suspensions of FC and HFC onto standard carbon-coated copper grids. TEM samples were dried at 80 °C for 24 h under reduced pressure prior to TEM measurements. Solvation of FC was investigated by using Fourier transform infrared (FT-IR) spectroscopy. FT-IR spectra of pristine fullerene C70 and FC were recorded using a Nicolet 4700 FT-IR instrument, ThermoElectron Corporation. Thermogravimetric analysis was performed using a SII Instrument (Model Exstar 600) under an argon gas atmosphere at a heating rate of 10 °C min−1. pXRD patterns of the pristine C70, FC, and HFC were recorded at 25 °C on a Rigaku RINT2000 diffractometer with Cu−Kα radiation (λ = 0.1541 nm). Nitrogen adsorption−desorption isotherms were recorded on an automatic adsorption instrument (Quantachrome Instruments, Autosorb-iQ2, USA) at liquid nitrogen temperature 77.35 K. For each measurement, about 20 mg of sample was taken and degassed for 24 h at 100 °C prior to the measurements.

Δf = (2f0 2 / ρQ μQ )m′

(1)

where f 0 (Hz) is the natural frequency of the quartz crystal, ρQ is quartz density (2.649 g cm−3), and μQ is the shear modulus (2.947 × 1011 g cm−1 s2).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b01544. Additional SEM, TEM, and HR-TEM images, histograms cube size distributions, nitrogen adsorption isotherms, thermogravimetric plot, additional sensing results, and CV data (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 6635

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The authors declare no competing financial interest.

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