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Irradiated Graphene Loaded with SnO2 Quantum Dots for Energy Storage Ruting Huang, Lijun Wang, Qian Zhang, Zhiwen Chen, Zhen Li, Dengyu Pan, Bing Zhao, Minghong Wu, C.M. Lawrence Wu, and Chan-Hung Shek ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b05146 • Publication Date (Web): 04 Oct 2015 Downloaded from http://pubs.acs.org on October 5, 2015

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Irradiated Graphene Loaded with SnO2 Quantum Dots for Energy Storage

Ruting Huang,☆,a,c Lijun Wang,☆,a,c Qian Zhang,☆,a,c Zhiwen Chen,*,a,c,d Zhen Li,a,c Dengyu Pan,b,c Bing Zhao,a,c Minghong Wu,*,a,b,c C. M. Lawrence Wu,d Chan-Hung Shek*,d a

Shanghai Applied Radiation Institute;

b

Institute of Nanochemistry and Nanobiology;

c

School of

Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, People's Republic of China; d Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong.

a

Shanghai Applied Radiation Institute;

b

Institute of Nanochemistry and Nanobiology;

c

School of

Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, People's Republic of China; d Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong. ☆

These authors contributed equally to this work.

E-mail: [email protected]; [email protected]; [email protected] ACS Paragon Plus Environment

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ABSTRACT: Tin dioxide (SnO2) and graphene are unique strategic functional materials with widespread technological applications, particularly in the fields such as solar battery, optoelectronic devices, and solid-state gas sensors owing to advances in optical and electronic properties. Versatile strategies for microstructural evolution and related performance of SnO2 and graphene composites are of fundamental importance in the development of electrode materials. Here we report that a novel composite, SnO2 quantum dots (QDs) supported by graphene nanosheets (GNSs), has been prepared successfully by a simple hydrothermal method and electron-beam irradiation (EBI) strategies. Microstructure analysis indicates that the EBI technique can induce the exfoliation of GNSs and increase their interlayer spacing, resulting in the increase of GNS amorphization, disorder and defects, and the removal of partial oxygen-containing functional groups on the surface of GNSs. The investigation of SnO2 nanoparticles supported by GNSs (SnO2/GNSs) reveals that the GNSs are loaded with SnO2 QDs, which are dispersed uniformly on both sides of GNSs. Interestingly, the electrochemical performance of SnO2/GNSs indicates that SnO2 QDs supported by 210 kGy irradiated GNS shows excellent cycle response, high specific capacity, and high reversible capacity. This novel SnO2/GNS composite has potential practical applications in SnO2 electrode materials during Li+ insertion/extraction.

KEYWORDS: SnO2 quantum dots; graphene nanosheets; electron-beam irradiation; microstructural evolution; electrochemical performance

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Developing new materials with excellent performance depend not only on the constituents but also on their micro/nanostructures. Multi-components and micro/nanostructures are the key issues to realize multifunctional materials. In recent years, composites have attracted far-reaching attention because of their potential to combine desirable properties of different components and micro/nanostructures to achieve advantageous optical, electronic, magnetic, and mechanic properties.1-3 The key scientific issues of tin dioxide (SnO2) and graphene composites and their applications and developments have driven scientists to persistently explore in depth their preparation, micro/nanostructure and performance. SnO2, an n-type wide-band-gap semiconductor (Eg = 3.64 eV at 300 K), has very wide applications in fields such as solid-state gas sensors,4 luminescent materials,5 solar battery,6 antistatic coating,7 and optoelectronic devices.8 Graphene nanosheet (GNS), a honeycomb network of sp2 carbon atoms, has become one of the most appealing carbon matrices due to its outstanding mechanical flexibility,9-11 excellent optical properties,12-14 large specific surface area,15 high thermal and chemical stability,16-18 and electrochemical properties.19-23 Functionalization of SnO2 nanoparticles supported by graphene nanosheets (GNSs) may be able to further enhance their properties. Furthermore, the development of metal oxide/graphene composites may also provide an important milestone to improve the application performance of metal oxide materials. Many types of metal oxide/graphene composites have aroused wide public concern, such as Cu2O/graphene,24 Ag2O/graphene,24 CoO/graphene,25 Co3O4/graphene,26 CuO/graphene,27

Mn3O4/graphene,28

Fe2O3/graphene,29

NiO/graphene,30

Fe3O4/graphene,31

TiO2/graphene,32 ZnO/graphene,33 and SnO2/graphene.34 Among these composites, SnO2 selected as an electrode material has attracted much attention because of its high theoretica1 capacity (782 mAh/g), low cost, and non-toxicity. A large amount of effort has been devoted to developing SnO2/graphene composites using various methods, including solution-based synthesis,35-37 solvothermal method,38 self-assembly,39 and ultrasonic spray pyrolysis. However, in all of these techniques, it is difficult to control the uniform distribution of ACS Paragon Plus Environment

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deposited SnO2 on GNSs. The synthesis and fabrication of high-quality SnO2/graphene composites are still very elusive. Thus, it is highly desirable to develop new methods for preparing SnO2/graphene composites with excellent performance. It is well-known that electron-beam irradiation (EBI) techniques have a number of highly advantageous properties. Moreover, EBI can be deliberately used to alter the chemical, mechanical, optical, and electronic properties of materials. The EBI technique achieved by an electron accelerator is of widespread technological applications in the fields of degraded organic compounds, medical treatment, industrial and agricultural applications.40 This method (i) is simple, rapid, and convenient, (ii) can be carried out at room temperature without any kind of catalyst, and (iii) is useful for mass production of materials. Teweldebrhan and Balandin reported that the changes in graphene lattice can be induced by EBI at low or medium energy.41 Compared with a previous heat-treatment technique commonly used in crystallization, the microstructure and electrical properties of graphene were also found to be tuned by the EBI.42-44 The microstructural evolution and performance induced by EBI strategies have also attracted much attention, for example, in the study of disorder and defects, oxygen-containing functional groups, and damages of materials, etc. In order to investigate the microstructural evolution and improve the performance of SnO2 quantum dots (QDs) supported by graphene nanosheets (GNSs), we introduced an electron beam intentionally to modify the surface structure of graphene. The incorporation of GNSs with well-dispersed metal oxides is an effective way to achieve excellent performance of SnO2 QDs. However, SnO2 assembled to irradiated GNSs is so far rarely reported. In this work, GNSs were irradiated by an electron beam with four absorbed doses of 70 kGy, 140 kGy, 210 kGy, and 280 kGy at room temperature with an irradiation rate of 5 mA/s. GNS loaded with SnO2 QDs (SGNS) without radiation exposure and irradiated GNSs loaded with SnO2 QDs (designated as 70 kGy SGNS, 140 kGy SGNS, 210 kGy SGNS, and 280 kGy SGNS) have been successfully prepared by a facile hydrothermal method.45 X-ray diffraction (XRD) analysis confirms that SnO2 has good crystallinity in the SGNS. From the field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) images, it can be observed ACS Paragon Plus Environment

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that the SnO2 nanoparticles are uniformly distributed on the surface of GNSs. High-resolution transmission electron microscopy (HRTEM) observation indicates that the size of SnO2 QDs is 3~5.5 nm. It is surprisingly found that the crystallinity of SnO2 in the irradiated GNSs is better than that of the one without radiation exposure. Brunauer-Emmett-Teller (BET) surface area analysis indicates that the specific surface area of the irradiated GNSs loaded with SnO2 QDs is much higher, and is almost at its largest value when the irradiation dose is 210 kGy. The electrochemical performance of the 210 kGy irradiated SGNS and un-irradiated SGNS as anode materials were measured by galvanostatic charge/discharge cycling. The 210 kGy irradiated SGNS and un-irradiated SGNS electrodes maintained a respective reversible capacity of 932.4 and 655.9 mAh/g after 30 cycles at 5 C. This shows excellent cycling performance, high specific capacity, and high reversible capacity for lithium ion battery with 210 kGy irradiated GNS loaded with SnO2 QDs.

RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of as-prepared un-irradiated GNS and irradiated GNS powders. The XRD pattern from the un-irradiated GNS clearly reveals the typical characteristics of graphene with a strong (002) diffraction peak. This peak indicates that the GNS is multilayer graphene stacked with an interlayer spacing of about 0.34 nm, implying that the graphene is similar to natural graphene sheets (d002 = 0.34 nm).46 Compared with the un-irradiated GNS, all irradiated GNSs have a diffraction peak at 2θ = 26.4°, which corresponds to the (002) peak of graphite, suggesting that the EBI has no obvious influence on the crystal structure of GNS. However, the interlayer spacing of the irradiated GNSs widens and a small increase from 3.4 Å to 3.52 Å (~3.5%) is apparent. This is attributed to the alteration of some functional groups and defects. In addition, the interlayer spacing of graphene is proportional to the degree of oxidation. Compared with the un-irradiated GNS, the increased interlayer spacing indicates that the irradiated GNSs are more oxidized and so less reduced eventually, which is in agreement with the Raman results as shown in Figure 2a. No diffraction peak assigned to graphene is ACS Paragon Plus Environment

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found in the XRD patterns of 70 kGy SGNS, 140 kGy SGNS, 210 kGy SGNS, and 280 kGy SGNS as shown in Figure 1b. This is due to the overlap of the diffraction peak for graphene (002) plane with SnO2 (110) plane, and the relatively weak intensity of the graphene peak compared with SnO2 due to the low amount of GNSs. In Figure 1b, the major diffraction peaks at 22.6°, 33.9°, 37.9°, 51.8°, and 65.9° can be indexed to (110), (101), (200), (211), and (301) planes of tetragonal SnO2 (JCPDS No.41-1445). These diffraction peaks are considerably broadened, suggesting that the SnO2 particles in these composites are crystallized with a small grain size. Therefore, we think the regular lamellar structures of the GNS have been broken in the SGNS and irradiated SGNS, forming exfoliated GNSs loaded with SnO2 grains. Comparing the five curves in Figure 1b, it confirmed that the growth of SnO2 nanocrystals is influenced significantly by the irradiated GNSs. For the irradiated SGNS, the diffraction peak for graphene (002) is relatively low, indicating that significant face-to-face stacking is absent because of the introduction of SnO2 particles on the irradiated GNSs. The above results prove that EBI is a potentially powerful technique to modulate microstructures.


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Figure 1. XRD patterns of (a) un-irradiated GNS and irradiated GNS under different irradiation dose of 70 kGy, 140 kGy, 210 kGy, 280 kGy, and (b) SGNS and irradiated SGNS under different irradiation dose of 70 kGy, 140 kGy, 210 kGy, 280 kGy with an irradiation rate of 5 mA/s. ACS Paragon Plus Environment

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Raman spectroscopy is a powerful tool to characterize carbonaceous materials owing to its fine sensitivity to the crystal surface. The Raman spectra of un-irradiated and irradiated GNSs at 70 kGy, 140 kGy, 210 kGy, 280 kGy with an irradiation rate of 5 mA/s are presented in Figure 2a. As seen in Figure 2a, the Raman spectrum of un-irradiated GNS as shown in the inset of Figure 2a contains the D band (~1324 cm-1, k-point phonons of A1g symmetry) and G band (~1593 cm-1, E2g phonons of sp2 atoms).46 The peak at 1593 cm-1 is related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, while the peak at 1324 cm-1 could be related to the defects associated with vacancies, grain boundaries, amorphous carbon species, and disorder in the hexagonal graphitic layers. In Figure 2a, the D peak appears after irradiation and is due to the breathing modes of the sp2 rings that require a defect for activation.47 The intensity of G peak increases with increasing radiation dose except for 280 kGy. The G peak width becomes wider with higher radiation dose. This tendency was observed under all irradiation conditions. The full width at half maximum (FWHM) for D and G broadens under irradiation. The changes in all GNS spectra under irradiation are indicative of the disorder and defects introduced by the EBI. In order to rationalize the results, we plot the intensity ratio (ID/IG) of D and G peaks as a function of EBI. The ID/IG ratio increases along with the irradiated GNSs at 70 kGy, 140 kGy, and 210 kGy, reaching maximums, and then decreases in the irradiated GNSs at 280 kGy. The ID/IG ratio of the D to G bands is calculated as 1.00 for the GNS, 0.97, 0.98, 1.00, 0.99 for the irradiated GNSs at 70 kGy, 140 kGy, 210 kGy, and 280 kGy, respectively. This is similar to what was observed previously,41 indicating that defects are created by EBI. The EBI in the low and medium energy leads to similar results. The ID/IG ratio is a measure of the disorder degree and average size of the sp2 domains.48 EBI may result in lattice damage and disorder. The D peak intensity becomes stronger with increasing radiation dose. Graphene with damage induced by irradiation is further evaluated based on G peak shifts. The increase in the irradiation dose results in conversion into mainly sp2 amorphous carbon. The intensity of G peak also increases with increasing radiation dose. Comparing with the un-irradiation GNS, the ID/IG ratio of the D to G band decreases for the irradiated GNS. However, the ACS Paragon Plus Environment

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decrease is larger for the overall disorder of graphene, resulting in a decline in the ratio of GNS. The damage to the material, which consists of a single or few atomic layers, can be quite significant even at low radiation dose. Therefore, the D and G peak evolution with increasing radiation dose follows the amorphization trajectory, which suggests the graphene’s transformation to the nanocrystalline and then to amorphous form. The trends summarized by the amorphization trajectory indicate that the ID/IG ratio increases when crystalline graphite evolves into nanocrystalline graphite in 70 kGy irradiation dose and then decreases when nanocrystalline graphite becomes mainly sp2 amorphous carbon in 140 kGy irradiation dose. Our case appears to follow the amorphization trajectory irradiated at 70 and 140 kGy, which suggests that the crystalline graphene under irradiation transforms into nanocrystalline phase, possibly with localized defects. Then, as the radiation damage increases, it becomes more disordered, i.e. amorphous at 280 kGy irradiation dose. The original amorphization trajectory for bulk graphite is characterized by further decrease in ID/IG, which corresponds to the increase in sp3 content and the formation of tetrahedral amorphous carbon. The situation is different for irradiated graphene where the ID/IG ratio tends to saturate with increasing radiation dose. The latter can be related to the fact that we deal with just one or two atomic layers of materials, and sp3 phase does not form easily. We can exclude vacancies due to knock-on damage as possible mechanism for the observed GNS surface. The molecular residue signatures can also be distinguished from the regular G peak. As seen from Figure 2b, the Raman spectra of the SGNS composites also contain the G and D bands. The G band of SGNS composites is distinctly sharper than that of GNS. Moreover, the ID/IG ratio of the G to D band is also higher for SGNS composites than that for un-irradiated and irradiated GNS, indicating a decrease in the average size of the sp2 domains, and an increase in vacancies, grain boundaries, and amorphous carbon species. The ID/IG ratios of these five samples are 1.03 for SGNS, 1.00, 1.02, 1.03, and 1.00 for 70 kGy SGNS, 140 kGy SGNS, 210 kGy SGNS, and 280 kGy SGNS respectively. The ID/IG ratio for SGNS composites is larger than that of un-irradiated and irradiated GNS, and it increases with increasing radiation dose except for the sample irradiated at 280 kGy. This is ACS Paragon Plus Environment

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because the ethanol and aqueous solutions from the process of hydrothermal synthesis provide a large amount of oxygen-containing groups and amorphous carbon species for SGNS. The increase of ID/IG in the SGNS (inset of Figure 2b) compared to that in the GNS (inset of Figure 2a) is due to the G (~1595.67 cm-1) and D (~1340.36 cm-1) peaks in the SGNS happen to move towards the direction of high wavelength (red shift). This is because the C=C double bond of graphene surface was damaged under stirring or in the hydrothermal synthesis process and the SnO2 was adsorbed onto the graphene surface. This reduced the number of oxygen-containing functional groups. This change suggests an increase in oxygen vacancies, grain boundaries, and amorphous carbon species. It is noticeable that a decreased ID/IG in the irradiated SGNS composites is observed in comparison with that of un-irradiation SGNS composite (1.03). Because of the damage, disordering and defects induced by EBI may lead to more attachment points for Sn4+ on the graphene surface, leading to further decrease in overall disorder. In addition, the ID/IG ratio of the SGNS composites irradiated at 70 kGy, 140 kGy, and 210 kGy first increases with increasing radiation dose, reaching maximums, and then decreases in the irradiated SGNS composites irradiated at 280 kGy. This is the same as the results of irradiated GNS. These results suggest that the EBI is the main reason for the change in intensity ratio. It is noteworthy that the ID/IG ratio for the SGNS is almost the same as that of the GNS, indicating no modification in the average size of the sp2 graphitic domains in the composites. Thus, the SnO2 nanocrystals and EBI have not changed the pristine laminated structure of GNS.


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Figure 2. Room-temperature Raman spectra of (a) un-irradiated GNS and irradiated GNS under different irradiation dose: 70 kGy, 140 kGy, 210 kGy, and 280 kGy, (b) SGNS and irradiated SGNS under different irradiation dose: 70 kGy, 140 kGy, 210 kGy, and 280 kGy. The insets show the Raman spectra of un-irradiated GNS in (a) and un-irradiated SGNS in (b). ACS Paragon Plus Environment

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Figure 3 shows the FE-SEM images of (a) un-irradiated GNS and irradiated GNS under different irradiation dose: (c) 70 kGy, (e) 140 kGy, (g) 210 kGy, and (i) 280 kGy, and (b) un-irradiated SGNS and irradiated SGNS under different irradiation dose: (d) 70 kGy, (f) 140 kGy, (h) 210 kGy, (j) 280 kGy with an irradiation rate of 5mA/s on Si(100) substrates. The layered platelets composed of curled nanosheets are displayed in Figure 3a. These are representative structures of GNS. Compared with Figure 3a, the multilayer structure of GNS does not change after irradiation. Interestingly, the irradiated GNSs have more layers, especially under high irradiation dose (e.g. 280 kGy). As shown in Figure 3b, d, f, h, and j, the SnO2 particles are uniformly distributed on the GNS, implying a strong interaction between the SnO2 particles and GNSs. It is also found that the un-irradiated and irradiated GNSs and SGNS composites have similar morphologies from the FE-SEM images, suggesting that a fine structural manipulation of GNS is successfully achieved even after the reassembling process with SnO2 particles. Interestingly, it can be seen clearly that the SnO2 particles are uniformly distributed on the GNS surface. GNS restacking and SnO2 particle aggregation have been prevented. With the increase in irradiation dose, it can be seen clearly that the SnO2 particles become more and more uniformly distributed on the GNS surface, especially in Figure 3h.


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Figure 3. FE-SEM images of (a) un-irradiated GNS and irradiated GNS under different irradiation dose: (c) 70 kGy, (e) 140 kGy, (g) 210 kGy, and (i) 280 kGy, (b) SGNS and irradiated SGNS under different irradiation dose: (d) 70 kGy, (f) 140 kGy, (h) 210 kGy, and (j) 280 kGy on Si(100) substrates.

The surface morphology, microstructural evolution, and SnO2 particle size of these SGNS composites were further investigated by TEM and HRTEM. Figure 4 shows the typical HRTEM images of (a) un-irradiated GNS and irradiated GNS under different irradiation dose: (d) 70 kGy, (g) 140 kGy, (j) 210 kGy, and (m) 280 kGy, TEM images of (b) un-irradiated SGNS and irradiated SGNS under different irradiation dose: (e) 70 kGy, (h) 140 kGy, (k) 210 kGy, and (n) 280 kGy, and HRTEM images of (c) un-irradiated SGNS and irradiated SGNS under different irradiation dose: (f) 70 kGy, (i) 140 kGy, (l) 210 kGy, and (o) 280 kGy. In Figure 4a, d, g, j, and m, it can be seen that the GNSs are not single layer. After EBI on GNS, the electron-beam induced exfoliation of GNS, increasing their interlayer spacing, and forming the multilayer GNS. These changes in surface morphology can be observed clearly (Figure 4d, g, j, and m). With the increase in irradiation dose, the size of layered GNS increases and reaches maximum at 280 kGy. The surface of irradiated GNS at 280 kGy shows obvious damage and fold. It can be concluded that the high radiation energy can cause permanent damage and decrease the number of oxygen-containing groups. In Figure 4d, g, and j, we can observe that the number of irradiated GNS layers has almost no increase. That is to say, when the cumulative radiation dose adds up to 210 kGy, even though there is no influence on the GNS layers, the surface defects, grain boundaries, and the number of oxygen-containing groups are affected. As shown in Figure 4b, e, h, k and n, it can be seen that the GNSs are loaded with SnO2 nanoparticles, which can be called QDs. A large number of SnO2 QDs are uniformly distributed on the surface of GNSs. The GNSs are nearly transparent and very thin. The SnO2 nanoparticles deposited on the GNS surface prevent the GNS from stacking into multilayers. The GNS distributed between SnO2 nanoparticles also prevents the agglomeration of SnO2 nanoparticles, which are beneficial for the ACS Paragon Plus Environment

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formation of SnO2 QDs. Moreover, the EBI created lattice defects or damage in GNS. These defects and damage may induce the formation of more and smaller SnO2 QDs since defects and grain boundaries are favorable nucleation sites, culminating in the formation of new and smaller SnO2 QDs. Comparing the five samples, the SGNS irradiated at 210 kGy exhibits much better homogeneous distribution and much smaller size distribution of SnO2 QDs. This implies that it is better to use EBI to modify GNS to produce changes in distribution and size of SnO2 QDs. When the GNS was irradiated at 280 kGy, it is found that SnO2 QDs have large agglomeration as shown in Figure 4n. It is because the oxygen-containing functional groups and free radicals gather on and damage the edges of GNS layer due to high radiation dose. This causes the collapse or deterioration of the irradiated GNS, leading to SnO2 QDs agglomeration. This is consistent with the results observed from the Raman spectra. The above results indicate that the density of SnO2 QDs increases after EBI. In Figure 4c, f, i, l and o, it is found that the SnO2 QDs can be formed in the un-irradiated and irradiated GNSs. The clear lattice fringes demonstrate that the SnO2 QDs are composed of tetragonal SnO2. From HRTEM images of Figure 4c, f, i, l and o, the size distribution of SnO2 QDs is estimated to be ranging from 3.5 to 5.5 nm and average size is calculated to be about 3.8 ± 0.2 nm, 4.8 ± 0.2 nm, 4.2 ± 0.2 nm, 3.6 ± 0.2 nm, and 4.9 ± 0.2 nm, respectively. HRTEM analysis indicates that the crystal planes of SnO2 QDs become more perfect with the increase of irradiation dose. This is due to the EBI induced growth of SnO2 QDs. The EBI induced nucleation of new SnO2 QDs leads to the increase of SnO2 QD density, whereas the EBI induced growth of old SnO2 QDs leads to the perfect lattices of SnO2 QDs. In addition, the HRTEM images of typical SnO2 QDs shown in the insets of Figure 4c, f, i, l and o prove that the clear crystal lattice with a spacing of 0.33 nm corresponds to the (110) face of SnO2 rutile phase. Therefore, it is certain that the SnO2 QDs distributed on the surface of GNS are well-crystallized.


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Figure 4. HRTEM images of (a) un-irradiated GNS and irradiated GNS under different irradiation dose: (d) 70 kGy, (g) 140 kGy, (j) 210 kGy, and (m) 280 kGy, TEM images of (b) un-irradiated SGNS and irradiated SGNS under different irradiation dose: (e) 70 kGy, (h) 140 kGy, (k) 210 kGy, and (n) 280 kGy, HRTEM image of (c) un-irradiated SGNS and irradiated SGNS under different irradiation dose: (f) 70 kGy, (i) 140 kGy, (l) 210 kGy, and (o) 280 kGy.

N2 adsorption/desorption isotherms were employed to investigate the pore structure of the un-irradiated and irradiated SGNS composites under different irradiation dose: 70 kGy, 140 kGy, 210 kGy, and 280 kGy. The results are shown in Figure 5a, c, e, g, and i, respectively. The hysteresis loops in the nitrogen adsorption/desorption isotherms indicate that the SGNS composites are porous. The hysteresis loop resembles type-H3 IUPAC (International Union of Pure and Applied Chemistry) classification resulting from slit-shaped pores between parallel layers.49 It can be seen clearly that the adsorption branch increases sharply, after reaching a certain value to start desorption, whereas the desorption branch moves slowly to the middle P/P0 signal point and decreases sharply, stripping absorption isotherm form the hysteresis loop. It is found that the N2 adsorption/desorption isotherms have the same trend of adsorption and desorption in the un-irradiated and irradiated SGNS composites under different irradiation dose. It indicates that the irradiation conditions have not caused irreparable damage to GNS, and just produced a small change in the internal structure of GNS. This change is more easily combined with SnO2 QDs. The BET surface areas of the un-irradiated SGNS composite (Figure 5b) and irradiated SGNS under different irradiation dose: 70 kGy (Figure 5d), 140 kGy (Figure 5f), 210 kGy (Figure 5h), and 280 kGy (Figure 5j) are about 102.8 m2/g, 149.5 m2/g, 155.8 m2/g, 157.0 m2/g, 125.6 m2/g, respectively. Obviously, the BET specific surface areas of the irradiated SGNS composites are bigger than that of the un-irradiated one. With the increase of irradiation dose, the BET values gradually increase. However, at 280 kGy the BET value decreases slightly to 125.6 m2/g. From the overall trend of BET values, it is reasonable to speculate that more and new SnO2 crystal nucleus could ACS Paragon Plus Environment

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be formed in irradiated SGNS. This can also be proved by the TEM and HRTEM results. Due to the confinement effect of GNS, the SnO2 QDs can be separated one by one. In addition, due to the presence of EBI damage, the SnO2 nanoparticles on both sides of GNS are expected to become smaller. The smaller crystal nuclei, which can be called SnO2 QDs, lead to an increase of the BET surface area of the irradiated samples. Thus, the specific surface area of irradiated SGNS becomes larger. However, when the irradiated dose is increased to 280 kGy, the specific surface area of SnO2 QDs decreases on the GNS surface due to agglomeration. Barretl-Joyner-Halenda (BJH) measured the pore volumes of the un-irradiated SGNS (Figure 5b) and irradiated SGNS under different radiation dose: 70 kGy (Figure 5d), 140 kGy (Figure 5f), 210 kGy (Figure 5h), and 280 kGy (Figure 5j). The pore volumes were 0.459 cm3/g and 0.632 cm3/g, 0.648 cm3/g, 0.664 cm3/g and 0.389 cm3/g and the pore diameters were 3.711 nm, 3.892 nm, 4.014 nm, 4.210 nm, and 3.919 nm, respectively. The pore volume and pore size gradually increase with increasing irradiation dose. It is possible that the EBI can induce the formation of more pore structure on the irradiated GNS surface, and this would increase the diameter of the pores. Thus, it will be apt to combine SnO2 QDs on the irradiated GNS surface, which is in accord with the TEM and HRTEM results. When the irradiation dose is increased to 280 kGy, the pore volume and pore diameter become significantly lower. It suggests that the 210 kGy dose is an appropriate irradiation dosage to modify GNSs. The larger pore size is more convenient for the insertion and extraction of lithium ions, providing good lithium ion channels. The large space structure can reduce volume expansion in the electrochemical reaction process. The number of large pore decreases and that of micro-pores increases in the composites. This shows that the SnO2 QDs are densely and uniformly distributed on the GNS surface, partially blocking the pores of GNSs. It implies that EBI can significantly enhance the BET surface area.


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Figure 5. Nitrogen adsorption/desorption isotherms of (a) un-irradiated SGNS and irradiated SGNS under different irradiation dose: (c) 70 kGy, (e) 140 kGy, (g) 210 kGy, and (i) 280 kGy, and pore diameter distribution of (b) un-irradiated SGNS and irradiated SGNS under different irradiation dose: (d) 70 kGy, (f) 140 kGy, (h) 210 kGy, and (j) 280 kGy.

From the above evidence, the irradiated SGNS composites are optimized at the 140 and 210 kGy doses. Thus, the reaction process is further investigated by FTIR spectroscopy. Figure 6 shows the FTIR spectra of the un-irradiated SGNS (Figure 6a) and irradiated SGNS composites at 140 kGy and 210 kGy (Figure 6b). In Figure 6a, the peak at 3620 cm-1 is attributed to the stretching vibration of O-H bond. The peaks at 2363 cm-1 and 1765 cm-1 are associated with the stretching vibration of the C=O bond of carboxyl groups or triple bond. The peak at 1571 cm-1 can be attributed to the bending vibration of C=C. The peak at 798 cm-1 corresponds to the O-Sn-O bond. A relatively weak absorption peak in the range of 3000~3700 cm-1 can be ascribed to a lower degree of O-H bond reduction and adsorption of water molecules. Moreover, the comparison of the FTIR spectra of irradiated SGNS in Figure 6b reveals the large decrease in the intensity of the characteristic absorption peaks of various oxygen-containing groups such as O-H, C=O and C-O after the irradiation treatments. The peaks at 1584 cm-1 and 1560 cm-1 attributed to the C=C bending vibrations are attenuated. The peak near 1180 cm-1 belongs to C-O-C stretching vibration. Two peaks at 557 cm-1 (210 kGy) and 615 cm-1 (140 kGy) correspond to the Sn-O bond. The above evidence suggests that the irradiation treatments remove the partial oxygen-bearing groups so that they begin to evolve into different kinds of amorphous carbon. It is a kind of effective way to change the functional groups of GNSs by EBI.


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Figure 6. FTIR spectra of un-irradiated SGNS (a) and irradiated SGNS at 140 kGy and 210 kGy (b).


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Figure 7. (a) The initial charge-discharge curves of un-irradiated SGNS and irradiated SGNS at 210 kGy at 0.1 C rate in the voltage range of 0.05-3 V. (b) Comparison of the discharge specific capacity of un-irradiated SGNS and irradiated SGNS at 210 kGy at different rate capability. ACS Paragon Plus Environment

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The charge/discharge performance of the un-irradiated and irradiated SGNS composites was further investigated using coin-type cells. The charge/discharge curves and cyclic curves tested at different current densities are shown in Figure 7. The 1st charge-discharge curves of the un-irradiated SGNS and irradiated SGNS at 210 kGy electrodes at a constant current density of 0.1 C are shown in Figure 7a, in which the voltage range is 5 mV to 3 V. In the first cycle, the irradiated SGNS at 210 kGy and un-irradiated SGNS show the discharge and charge capacity of about 2200.5 and 1242.6 mAh/g, 1958.4 and 945.4 mAh/g, respectively. The corresponding Coulombic efficiencies at the first cycle are about 56.47% and 48.3% respectively. The irreversible capacity reaches 957.9 and 1013 mAh/g, which accounts for 43.5% and 51.7% of the respective discharge capacity. It is found that the irreversible capacity of this irradiated SGNS is lower than the theoretical irreversible capacity of pure SnO2 (47.6%) for the first cycle. This shows that the irradiated SGNS prepared by the hydrothermal synthesis method and the GNS irradiated at 210 kGy possess high specific capacity and high reversible capacity. These results indicate that the irradiated SGNS at 210 kGy is suitable for use as electrode materials. There are two reasons why the first irreversible capacity of based SnO2 composites is larger.1,35 First, due to the irreversible conversion of SnO2 to Sn and Li2O upon lithiation, Li+ is reversibly inserted into Sn as LixSn alloys in the process of the first discharge. Second, SnO2 formed at the solid electrolyte interface (SEI) during the first discharge process is combined with a certain amount of lithium ions. The nanostructure of the samples leads to a large specific surface area with the presence of large SEI. Compound crystal is not quite perfect according to XRD and TEM analysis results. There are defects, vacancies, micro-pores, and other unstructured areas on the particle surface. Thus, Li+ cannot be completely released form these locations, leading to a large irreversible capacity loss after the first cycle. The cycling responses of the irradiated SGNS at 210 kGy and un-irradiated SGNS electrodes at different C-rates were evaluated and the results are shown in Figure 7b. The discharge capacities of the electrodes at current densities of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 5 C respectively are shown. The ACS Paragon Plus Environment

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irradiated SGNS at 210 kGy and un-irradiated SGNS electrodes maintain a reversible capacity of 932.4 and 655.9 mAh/g after 30 cycles at a current density of 5 C, corresponding to 42.8% and 33.5% of the initial capacity respectively. These are higher than the value of 570 mAh/g reported by Honma et al.49 These results indicate that there is a beneficial effect of irradiated GNS on the enhanced stability of SnO2 nanoparticles during the Li alloying/dealloying process. In addition, the GNS could also act as a good conductive medium, which could promote electron transfer during the lithiation and delithiation processes. When SnO2 reacts with the Li+, there is a huge volume expansion, leading to electrode cracking and pulverization. The great improvement of cyclic and rate performance of the SGNS irradiated at 210 kGy achieved in this anode is attributed to the use of irradiation. The irradiation effectively enlarges the layer spacing of GNS and increase the buffer area. This can limit the huge volume expansion of SnO2. These results indicate that there is still plenty of scope for improving electrode performance as long as we design the electrode from the view of whole system. By embedding SnO2 nanoparticles in GNSs matrix, the volume expansion and contraction of SnO2 nanoparticles can be buffered by the flexible GNSs. Improved energy storage can be similarly attributed to the irradiation between SnO2 and GNS in the layered composites during the phase transformation of lithiation/delithiation that usually leads to capacity strengthening.

CONCLUSIONS In summary, a combined route to synthesize SnO2/graphene nanosheets (SGNS) composites was developed. The EBI strategies for the GNS assembled SnO2 QDs were investigated in detail by various experimental techniques. It was surprisingly found that irradiated GNS is beneficial for SnO2 QDs assembly. Under different irradiation dose of 70 kGy, 140 kGy, 210 kGy, and 280 kGy, high quality SnO2 QDs with diameters ranging from 3 to 5.5 nm can be obtained. Clear lattice fringes of SnO2 QDs indicate that the crystal planes of the QDs are structurally perfect and uniform. BET surface area analysis indicates that the specific surface area of irradiated GNS is much higher, and reaches a peak at ACS Paragon Plus Environment

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about 210 kGy. The irradiated SGNS at 210 kGy shows large reversible capacity and excellent cycling performance. This excellent cycling ability is, on one hand, beneficial for the SnO2 QDs, which provides a minimal transport pathway for the alloying/dealloying of lithium ions and minimizes alloying deformation in SGNS composites. On the other hand, the major contribution comes from the irradiation and intimate contact of the conductive GNSs with SnO2 QDs. The GNSs prevent the aggregation of SnO2 QDs and protect the electrode materials from large volume change during Li+ insertion/extraction, resulting in enhanced cycling performance. The results reveal that EBI is a potentially powerful technique to achieve SnO2 QDs formation and modify SGNS composites for the improvement of lithium storage properties.

METHODS Synthesis of SnO2/Graphene Composites. Graphene nanosheets (GNSs) were synthesized using a modified Hummers method followed by rapid heating of graphene oxide at 500 °C for 3 h in a N2 atmosphere.45 50 mg of the as-prepared GNS was added to 30 mL distilled water and pure ethanol solution (1:1 in volume) followed by ultrasonic treatment for 30 min to form a colloidal suspension. Four identical suspensions were irradiated by using the GJ-2-II electron accelerator (Shanghai Xianfeng Electrical Plant, China) at ambient temperature with 1.8 MeV, and an irradiation rate of 5 mA/s. The four identical suspensions were respectively placed in the radiation-field about 30 cm away from the radiation source, and the absorbed doses for these four suspensions were 70, 140, 210, and 280 kGy. Then, cetyltrimethyl ammonium bromide (CTAB) was added to the above irradiated colloidal suspensions. Subsequently, 0.175 g of SnCl4·5H2O and 0.04 g of NaOH were added to the above irradiated suspensions with vigorous stirring. The resulting mixtures with Sn/graphene ratio = 1:1 in weight were continuously stirred for 30 min. The resulting black suspensions were transferred and sealed in a high pressure teflon vessel and kept at 160 oC for 20 h. The four systems were then cooled naturally to room temperature. The as-synthesized products were washed with ethanol and distilled ACS Paragon Plus Environment

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water several times to remove the Cl- by centrifugation, followed by vacuum drying in an oven at 60 oC for 8 h. The un-irradiated product is labeled as SGNS, the irradiated products are labeled as 70 kGy SGNS, 140 kGy SGNS, 210 kGy SGNS, and 280 kGy SGNS, respectively. Materials Characterization. Powder XRD patterns were obtained from a Japanese Regaku D/max-2500 machine using Cu Kα radiation in reflection geometry. An operating voltage of 40 kV and a current of 40 mA were used. XRD patterns were recorded at a scanning rate of 0.08os-1 in the 2θ range from 10 to 80°. Raman scattering measurements were obtained by backscattering geometry with a SPEX-1403 laser Raman spectrometer. The excitation source was an argon-ion laser operating at a wavelength of 514.5 nm in the backscattering configuration and a low incident power to avoid thermal effects. FE-SEM imaging was carried out with a field emission scanning electron microanalyzer (JEOL-6700F, 15 kV). HRTEM observations were performed on a JEOL JEM-2010F electron microscope operating at 200 kV. Surface area determinations were performed by BET method using an ASAP-2000 Surface Area Analyzer (Micromeritics Instrument Corporation). Fourier transform infrared (FTIR) spectra were recorded using a Bruker FTIR instrument. Electrochemical Performance. Electrochemical tests were carried out using two-electrode cells with lithium metal as the counter and reference electrode. The working electrode was composed of an active material (e.g., SGNS), a conductive agent (acetylene black) and binder polytetrafluoroethylene (PTFE) in a weight ratio of 80:10:10, and pasted onto a copper foil. A Celgard 2300 porous membrane was used as a separator. The electrolyte solution was 1 mol/L LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 by volume). The cell assembly was carried out in a glovebox filled with pure argon in the presence of an oxygen scavenger and a sodium-drying agent. Charge-discharge tests were performed using a battery testing system (Shanghai Chenhua Instrument Company) at different current densities with a voltage window of 0.05-3.0 V.


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AUTHOR INFORMATION Corresponding Author *

Phone: +86 21 66137503. Fax: +86 21 66137787. E-mail: [email protected] (Z.C.);

[email protected] (M.W.); [email protected] (C.-H.S.). ☆ These authors contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The work described in this article was financially supported by the National Natural Science Foundation of China (11375111, 11428410, 11074161, 11575105, 41373098 and 41430644), the Research Fund for the Doctoral Program of Higher Education of China (20133108110021), the Key Innovation Fund of Shanghai Municipal Education Commission (14ZZ098 and 10ZZ64), the Science and Technology Commission of Shanghai Municipality (14JC1402000 and 10JC1405400), the Shanghai Pujiang Program (10PJ1404100), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13078). This work was also supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China [Project No. (RGC Ref. No.), CityU 119212].

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Tin dioxide (SnO2) and graphene are unique strategic functional materials with widespread technological applications, particularly in the fields such as solar battery, optoelectronic devices, and solid-state gas sensors owing to advances in optical and electronic properties. In this work, a novel composite, graphene nanosheets (GNSs) loaded with SnO2 quantum dots (QDs), has been prepared successfully by a simple hydrothermal method and electron-beam irradiation (EBI) strategies. The electrochemical performance of SnO2/GNSs indicates that SnO2 QDs supported by 210 kGy irradiated GNS shows excellent cycle response, high specific capacity, and high reversible capacity. This novel SnO2/GNS composite has potential practical applications in SnO2 electrode materials during Li+ insertion/extraction.


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Irradiated Graphene Loaded with SnO2 Quantum ... - ACS Publications

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