In situ sonochemical synthesis of luminescent Sn@C-dots and a hybrid Sn@C-dots@Sn anode for lithium-ion batteries

Abstract

A facile sonochemical approach is employed for the in situ formation of C-dots via ultrasonic irradiation of polyethylene glycol (PEG) solvent and its decomposition. Metallic bulk tin was added to the reaction vessel and heated to its melting point (234 °C) in the presence of polyethylene glycol 400. The two-phase mixture was sonicated to yield Sn@C-dots and subsequently to achieve Sn nanoparticles decorated with Sn@C-dots (Sn@C-dots@Sn). The fluorescence (luminescence) properties of Sn@C-dots are different from those of the C-dots alone and change as a function of excitation wavelength. The as-synthesized Sn@C-dots@Sn nanoparticles were directly deposited on the copper foil current collector as a promising anode for Li-ion batteries. Encouraging lithiation and delithiation properties are obtained with high coulombic efficiency and enhanced rate capabilities for the hybrid Sn@C-dots@Sn nanoparticles, owing to the conducting carbon dot network on the tin nanoparticles minimizing pulverization effects. Methodical studies on morphology (SEM, TEM), structure (XRD, HR-TEM) and compositions (XPS, EDS) are carried out on the Sn@C-dots and electroactive Sn@C-dots@Sn nanoparticles to understand the reaction mechanism and their luminescence and battery anode properties.

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1. Introduction

Nanosized carbons have recently emerged as a unique class of optical nanomaterials targeting both biomedical, energy conversion and storage applications.^1–5 Carbon-dots (C-dots) not only possess analogous properties to carbon nanotubes,⁶ fullerenes,^7,8 and graphene,^9,10 but also have additional applications due to their emission properties as fluorescent sensors,^9,10 in bioimaging,^11,12 drug delivery,¹³ photocatalysis,^14–16 solar cells^17,18 and supercapacitors.^3,19 These functional C-dots have been prepared using arc-discharge,²⁰ acidic oxidation,²¹ sonochemical,^11,22 laser-ablation,²³ hydrothermal,^24,25 microwave,^25,26 and electrochemical²⁷ synthetic techniques.

Recently, doping of C-dots is widely used to tune their fluorescence properties. Various doping methods with hetero-atoms (such as metal and non-metal N^14,28,29, S³⁰, B^31,32, P²¹ etc.) have been reported to provide more than 20% quantum yields. Widely accepted mechanism for the excitation-dependent fluorescence doped C-dots is known³³ with polymeric or oligomeric surface groups. Moreover, an excellent biocompatibility and optical properties can be achieved by doping C-dots with metals or non-metals.^34,35 Gong et al. synthesized polyol-mediated gadolinium-doped green fluorescent C-dots by microwave irradiation for bimodal magnetic resonance imaging.³⁶ Recently, we synthesized luminescent Ga@C-dots via a one-step sonochemical method.²²

Nowadays, energy consumption has increased drastically due to the exponential growth in human population and has become basic requirement of our lives. Traditional energy resources such as oil and gas will not be sufficient in the future to fulfill the growing demand, therefore alternative resources of energy generation and storage are needed.^37–39 In particular, batteries and capacitors are prime examples of traditional energy storage devices.^40–49 The rechargeable lithium ion batteries are used in portable electric devices due to its high efficiency, light weight, reasonable safety, and good energy density.⁴³ Traditional lithium ion anodes are graphite, but recently new forms of carbon such as carbon nanotubes, SnO₂@Carbon hollow particles,⁴⁴ nitrogen-doped graphene sheets,⁴⁵ carbon-coated SnO₂ nanoplates,⁴⁶ doped carbon nanotubes,^47,48 and graphene are playing an increasingly important role in the design and development of next generation energy conversion and storage devices.^18,49 Recently developed C-dots manifesting tunable properties,⁵⁰ including luminescence complementary to the traditional metal-containing quantum dots.⁵¹ The combination of multicolor and tunable emission, controlled surface chemistry, and solvent dispensability in one simple platform that has made C-dots attractive for a wide range of optical, sensing, and biomedical applications.^52–54 Moreover, these tunable C-dots may offer interesting electrochemical performance in rechargeable battery systems.

In this study, economically viable, facile sonochemical synthesis method is employed for in situ formation of C-dots by ultrasonic irradiation of polyethylene glycol (PEG). Furthermore, in situ formation of nano Sn yielding Sn@C dots and subsequently produced Sn nanoparticles decorated with Sn@C-dots (Sn@C-dots@Sn) is reported. The as-synthesized Sn@C-dots@Sn nanoparticles were directly mounted on the current collector, copper foil via sonication as a promising anode for Li-ion batteries. Sonochemical coating advantages (such as surface activation, oxidation, reduction, microjets etc.)^{11,22,38,55–57} are implemented in the fabrication of these hybrid materials. The synthesis does not require a strong acid, base, volatile organic solvent, or other post-synthetic surface passivation.

2. Experimental section

2.1 Required chemical

Tin (99.999%), polyethylene glycol-400 (PEG-400, 99.998%), and Cu-foils were purchased from Sigma-Aldrich. Isopropanol (99.7%), acetone, and ethanol were purchased from Daejung Chemicals & Metals Co, Korea.

2.2 Synthetic procedure

Recently, we have reported the formation of ultrafine C-dots¹¹ and Ga@C-dots²² by ultrasonic cavitation of PEG-400, or of molten Ga overlayered by PEG-400, respectively. The current synthesis (Scheme 1) was done as follows: (Step 1) 15 mL of polyethylene glycol (PEG-400) was transferred into a Quartz test tube, which was immersed in a water bath and maintained at 75 °C. The tip of an ultrasonic transducer (Sonics and Materials Inc., USA, model VCX 750, frequency 20 kHz, AC voltage 230 V) was dipped in the solution, about 2 cm above the bottom of the test tube. Ultrasonic irradiation was applied with sonication amplitude of 70% for 2.5 hours. (Step 2) Sn metal (99.999%, Sigma) was added to the sonication cell, which at this stage contained prepared C-dots and the residual PEG-400 (as a medium). The boiling point of the PEG (>250 °C) is higher than the melting point (234 °C) of the Sn metal. The vessel was heated (in an oil bath or by a direct Bunsen flame) until the Sn was molten, forming two immiscible liquid phases. (Step 3) The solution was then sonicated for 12–15 minutes. This caused the dispersion of the metal phase into sub-micro/nanoparticles of tin and simultaneously formation of hybrids of tin–carbon dots (Sn@C-dots) and Sn@C-dots@Sn nanoparticles (NPs). (Step 4) Upon centrifugation, heavier Sn@C-dots@Sn nanoparticles were collected at the bottom of the centrifuge tube, while the lighter Sn@C-dots stayed in the supernatant. The Sn@C-dots@Sn nanoparticles were washed with double distilled water and acetone (1 : 1) and then dried in vacuum at room temperature. This synthesis procedure guarantees the formation of both Sn@C-dots@Sn nanoparticles and Sn@C-dots. The structural and textural properties of the produced Sn@C-dots, as a primary carbon particle and secondary coating for metals, are closely linked to the reaction parameters of the sonolytic synthesis. Reaction parameters such as sonication time and polymer/metal ratio were varied to get best performance.

Scheme 1 Schematic overview of the sonochemical synthesis of C-dots, Sn@C-dots, and Sn@C-dots@Sn NPs. Sonication of polyethylene glycol promotes formation of Sn@C-dots that aggregate upon the surface of Sn nanoparticles.

We have found that carbon dots can be sonochemically deposited on a variety of substrates, such as polymers, glass, and metallic surfaces.¹¹ We attempted to in situ deposit a coating layer of Sn@C-dots@Sn on Cu-substrate by sonication. Few small pieces (2.5 cm × 2.5 cm) of Cu disks were immersed in a suspension of Sn@C-dots@Sn NPs in PEG. The suspension was sonicated at 30% amplitude at 10 °C for 30 min. The disks were then washed with water and ethanol to remove bigger particles or unattached Sn@C-dots@Sn nanoparticles and dried in a vacuum chamber. Prior to electorchemical testing, the coated electrodes were heated in Argon to 300 °C for 2 h to remove residual solvents from the product.

2.3 Characterization of synthesized materials

The fluorescence of the Sn@C-dots was measured by a spectrofluorometer (Varian Cary Eclipse). The scanning electron microscope (SEM, FEI Megallon 400L microscope) and transmission electron microscope (TEM, Tecnai G2, FEI which is a high contrast/Cryo TEM, Oregon USA, equipped with bottom CCD camera 1k × 1k) was used to evaluate the shape, size, and surface morphology of Sn@C-dots@Sn NPs. Elemental analysis was performed by the energy dispersive X-ray spectroscopy (EDS). Samples for TEM were prepared by making a suspension of the particles in isopropanol using water-bath sonication. Two small droplets of Sn@C-dots and Sn@C-dots@Sn suspension were applied on a carbon coated copper TEM grid separately, and dried in vacuum. X-ray diffraction (XRD) was performed with a Bruker D8 Advance or with Philips PW1050 X-ray diffractometer using Cu Kα radiation operated at 40 kV/40 mA with a 0.0019 step size per 0.5 s. DSC measurements were performed using a NETZSCH instrument model 200 F3 MAIA. The thermogravimetric analysis was performed using the TGA-GC-MS (EI/CI) Clarus 680/Clarus SQ 8C, by Perkin Elmer at scan rate of 10 K min⁻¹, from 30 to 800 °C. X-ray photoelectron spectroscopy (XPS) analyses of samples were recorded using an ESCALAB 250 spectrometer with a monochromatic X-ray source with Al Kα excitation (1486.6 eV). Binding energy calibration was based on C1s at 285 eV. The doping concentration was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES) using the Horiba instrument model Ultima 2 for the determination of Sn concentration.

2.4 Electrochemical evaluation

The electrochemical performance of as-prepared Sn@C-dots@Sn deposited on copper current collector (electrodes) was studied in half cell configuration, where pure lithium metal foils were used as reference electrodes and Celgard 2500 as the separator. The electrolyte used is 1 M LiPF6 in EC : DEC : DMC (1 : 1 : 1 volume ratio). Following the construction of 2032 coin cells, they were cycled galvanostatically using a Neware battery cycler at room temperature.

3. Results and discussions

After sonication and centrifugation two phases were obtained: (1) the supernatant, which is a pale yellow transparent solution containing Sn@C-dots in PEG-400 and (2) the precipitate, which contains grey colour non-transparent solid of Sn@C-dots@Sn NPs. The sonochemically prepared Sn@C-dots (supernatant) have shown broad absorption (centered at 435 nm) as well as a broad emission in the blue-green spectral range (centered at 510 nm, Fig. 1A) when excited at 430 nm. The fluorescence of Sn@C-dots is almost similar in shape to C-dots fluorescence pattern but is shifted to the red.⁵⁸ Furthermore, the fluorescence spectra was recorded at different excitation wavelengths (330, 350, 370, 390, 410, 430, 450, 470, and 490 nm) as shown in Fig. 1C. As mentioned the Sn@C-dots emission spectra are different from the pristine C-dots Fig. 1B. The shift in emission peak is due to the insertion of the metal and enlargement of particles size. The Sn@C-dots suspension exhibited a pale yellow transparent color in day light and greenish-blue in UV-light (365 nm) as shown in Fig. 1D, and the fluorescence remained stable for several months. Fig. 1E shows a typical transmission electron microscopy (TEM) image of the as-prepared Sn@C-dots. The size distribution of the Sn@C-dots were plotted on the basis of HRTEM images (counted ∼200 nanoparticles) and are presented in Fig. 1F. It is apparent that the Sn@C-dots are in the size range of 3 to 8 nm with an average of ∼7 nm, while the average size of C-dots without Sn is ∼5 nm.

Fig. 1 The as-prepared supernant product obtained by ultrasonic irradiation of PEG-400 and tin: (A) excitation and emission spectra of Sn@C-dots, (B) fluorescence spectra of C-dots, (C) fluorescence spectra of Sn@C-dots, (D) photographs of a suspension of the Sn@C-dots illuminated in day light and UV-light, (E) TEM images of the Sn@C-dots, inset: HR-TEM of crystal lattice of nanosized Sn particles, and (F) size distribution plot of Sn@C-dots.

The chemical composition of Sn@C-dots was analyzed by XPS to prove the doping of Sn in C-dots. The characteristic peaks corresponding to C 1s (284.951 eV), O 1s (532.625 eV), Sn 3d (487.500 eV) were observed in the XPS scan spectrum (Fig. 2a), confirming that Sn@C-dots are composed of C, O, and Sn. The high-resolution XPS C 1s spectrum (Fig. 2d) could be deconvoluted to three Gaussian peaks. Specifically, the peak at 284.951 eV is attributed to C atoms in the carbon dots, implying that the as-prepared Sn@C-dots possess predominantly sp² carbon. The other two peaks were assigned to the carbon atoms in C–O (286.560 eV), and COO (288.924 eV), verifying the presence of hydroxyl, carbonyl, and carboxylic acid groups on the surface of Sn@C-dots.^22,30 The O 1s peak at 532.625 eV shown in Fig. 2b is assigned with oxygen in the form of C–O and C–OH/C–O–C. The Sn 3d_5/2 and 3d_3/2 peaks located at 487.500. and 495.480 eV, respectively shown in Fig. 2c. These binding energy indicate that tin exists in the form of Sn⁴⁺, due to unavoidable surface oxidation of 7 nm Sn@C-dots.

Fig. 2 (a) Full XPS spectrum for Sn@C-dots, (b) XPS spectrum of O 1s, (c) XPS spectrum of Sn 3d (d) XPS spectrum of C 1s of Sn@C-dots.

To evaluate the effect of sonication time on the particles size of prepared Sn@C-dots@Sn NPs, DLS measurement was performed to determine the size distribution of samples prepared with different sonication time. As shown in Fig. S1a and b (see the ESI†), the average particles size is 160 nm for 12 min sonication and 190 nm for 15 min sonication. Since smaller particles would favor shorter lithium diffusion path hence better rate capabilities, we adopted 12 minutes as the optimum sonication time.

After optimization of sonication time, the effects of Sn to PEG weight ratio on particle size, coating thickness, and Sn loading on C-dots were evaluated systematically. Sn to PEG ratio of 1 : 80, 1 : 40, 1 : 20, 1 : 10 (w/v) were utilized as precursors for sonochemical synthesis of Sn@C-dots@Sn NPs. It was found that the particles size (∼160 nm) and coating thickness of C-dots/Sn@C-dots (10–12 nm) stayed the same regardless of the Sn to PEG ratios. However, the amount of Sn doped in C-dots exhibits a linear relationship with Sn to carbon ratio. The ICP-OES analysis was employed to determine the amount of Sn-doping/loaded in Sn@C-dots. Based on ICP analysis, it was observed that when Sn concentration was increased from 40 mg to 800 mg the Sn in C-dots also raised from 20 ppm to 400 ppm. However, the Sn content approached a limit, when its content in solution increased above 490 mg. Therefore, 500 mg of Sn, 20 mL PEG-400, and 12 minutes' time of sonication was determined to be the optimum synthesis conditions to obtain the smallest Sn@C-dots@Sn NPs with highly Sn-doped C-dots coating.

The precipitated sample (Sn@C-dots@Sn NPs) was analyzed using the XRD, TEM, DLS, SEM, EDS, and DSC. The TEM data of the precipitate is shown in Fig. 3. Here, Sn@C-dots surrounded or decorated onto metallic tin nanoparticles. Fig. 3a indicates that small tin doped C-dots are coated on larger Sn nanoparticles (50–300 nm) after 12 minutes sonication. Fig. 3b shows that Sn@C-dots are uniformly coated on the Sn NPs after 15 min sonication. The thickness of the Sn@C-dots on Sn nanoparticles was found to be ∼10 nm and the product looks like a core–shell structure. The selected area electron diffraction (SAED) pattern shown in Fig. 3c confirms a mixture of tin nanoparticles and Sn@C-dots. Fig. 3a and b show that Sn@C-dots@Sn particles are spherical and in the range of 50–300 nm. X-ray analysis of Sn@C-dots@Sn particles (Fig. 3d) shows multiple peaks, all matching with the database of Sn and a small broad diffraction around 22.2° belongs to C-dots (marked by an arrow). Fig. 3d, showing metallic Sn diffractions at 2θ = 30.63, 32.02, 43.87, 44.90, 55.34, 62.53, 64.59, 72.41, 73.17, 79.51, and 89.40 that are assigned to the diffraction planes (200), (101), (220), (211), (301), (112), (400), (321), (420), (411), (312), and (431), respectively. No other peaks (tin oxide or tin carbide) were observed to correspond to any other contaminant. It is worth mentioning that XRD of the same sample that was recorded after 3 months exhibited identical diffraction peaks, indicating no changes resulting from oxidation. However, it is still possible that a very thin oxidation layer is undetected under the current measurements or that the oxide is amorphous. The existence of Sn metal in its typical crystalline form suggested that Sn@C-dots coating could slow down the oxidation of tin nanoparticles. Both the XRD pattern and selected area electron diffraction (SAED) indicate that the particles are polycrystalline, tetragonal structure with I4₁/amd (141) space group. The lattice parameters are found to be a = b = 5.831, c = 3.181 Ǻ. These particles do not recombine to form a bulk metal at room temperature. It remains suspended in the medium or precipitate at the bottom of the vessel due to the high density of tin and covering of Sn@C-dots on the surface of Sn nanoparticles.

Fig. 3 (a) TEM image (12 min sonication), (b) TEM image (15 min sonication) (c) SAED pattern (12 min sonication) of sonochemical decoration of Sn@C-dots on Sn nanoparticles, and (d) X-ray diffractogram of the Sn@C-dots@Sn NPs, showing the matching of the peaks with the database of pure Sn (red).

Determination of the composition of the product was done by energy dispersive X-ray spectroscopy (EDS). EDS analysis (Fig. S2a†) of the 12 min sonication sample shows the presence of only three elements (Sn, O and C). The oxygen presence is due to C-dots containing both the C and O. Using the selected area electron diffraction, dark field imaging was performed. Fig. S2b† shows the carbon are on the surface (white thin layer) and Sn are in the core of the sphere (dark black region). This Sn@C-dots are preventing coagulation of the Sn nanoparticles during the sonochemical reaction even though the melting temperature is lower than the reaction temperature.

The DSC curve for Sn@C-dots@Sn NPs is presented in Fig. 4a. Starting at −50 °C, the temperature was raised to 400 °C followed by full cycles of cooling and heating in that temperature range. The DSC curve for the particles shows a single endothermic signal with an onset temperature of 225 °C and a peak at 238 °C, whereas the melting onset temperature of bulk Sn is 234 °C, and 230 °C for the Sn nanoparticles.⁵⁹ The distinct differences in melting temperature indicate that the Sn@C-dots are playing the role to slightly lowering the melting temperature of the Sn@C-dots decorated on Sn nanoparticles. The small deviation is due to the slight melting-temperature depression that occurs with such particles in the sub-micrometric and nano dimension.⁶⁰

Fig. 4 (a) DSC curve, (b) TGA plot of a sample of Sn@C-dots@Sn NPs synthesized by ultrasonic cavitation.

The weight fraction of Sn@C-dots@Sn NPs is determined by themogravimetric analysis (TGA) as shown in Fig. 4b. The TGA curve illustrates a 4% weight loss at 285 to 370 °C, which is attributed to the removal of solvent residues.

3.1 Optimization of C-dots, Sn@C-dots and Sn@C-dots@Sn deposition onto Cu foil

Fig. 5a and b presents a HR-SEM image of the Sn@C-dots deposited on the copper foil substrate. The typical size of these decorated tin particles are around 35 nm, which is larger than the size of the individual C-dots that were observed before. It is possible that these features contain clusters of several C-dots around tin particles. Fig. 5c and d represents Sn@C-dots@Sn nanoparticles on Cu-substrate and the typical size of these features is between 50 and 200 nm. The loading of the active material is around 0.90 mg cm⁻², a relatively low value for ideal electrode loading. As one can imagine, both sides of the copper foil were coated with Sn@C-dots@Sn nanoparticles. Future work will optimize the dispersity, population, and size distribution of the formed Sn@C-dots@Sn nanoparticles on solid substrates.

Fig. 5 Sonochemical deposition of (a, b) Sn@C-dots, (c, d) Sn@C-dots@Sn nanoparticles on Cu-substrate.

In the current manuscript, we synthesized carbon dots due to decomposition of low molecular weight organic solvent (PEG-400) via polymeric cross-linking. The sonochemical degradation and carbonization of PEG under ultrasonic conditions could be a complex chemical process. This polymerization is induced by intermolecular dehydration of the organic solvent due the extreme conditions of pressure (ca. 500 atm.) and temperature (ca. 5500 K) which develop for extremely short times during the collapse of the acoustic bubble. Even if the vapor pressure of PEG-400 is low, and only a small amount of PEG vapors is found inside the collapsing bubble, the temperature in the 200 nm ring around the collapsing bubble is still very high (1900 K) for the carbonization of the PEG molecules.

3.2 Electrochemical activity of Sn@C-dots@Sn nanoparticles

In situ synthesized and decorated electrochemically-active Sn@C-dots@Sn hybrid electrode is tested for the lithiation-delithiation process against lithium. Generally, when tin based materials lithiate to form SnLi_4.4 alloy, their volume expands for more than 300%. Consequently, the electrode material could experience severe structural degradation (particles may become detached, electrically inactive, and aggregated) and excessive SEI growth upon repeated cycling.⁶¹ In this work, the conductive carbon dot shell could serve as a breathable coating to accommodate the extreme volumetric changes associated with the tin lithiation process.

Galvanometric cycling study of Sn@C-dots@Sn NPs on copper substrate in the voltage range of 0 to 3 V is shown in Fig. 6. The dQ/dV plot of the 10th cycle is plotted to illustrate the lithiation and delithiation mechanism (Fig. 6a). The dominating lithiation peak at 0.35 V and the delithiation peaks are 0.48 V, 0.57 V, and 0.78 V are in good agreement with those characteristic of tin lithiation and delithiation.⁶² The intense lithiation peak close to 0 V is attributed to lithium intercalation into the carbon dots. Voltage profile as a function of areal capacities at various cycles (corresponding to different cycling rates) is given in Fig. 6b. At slow cycling rates (10^th, 20^th, and 30^th), the two plateaus at 0.35 V and 0 V (as observed in Fig. 6a) are clearly visible in the discharge curves. However, as the rate increases further (for 40^th and 50^th cycles), the voltage profile becomes smooth with no plateaus, suggesting the decline in alloying reactions due to the lack of sufficient reaction time. Consequently, the charge curves also become smoother at faster rates.

Fig. 6 Electrochemical performance of in situ prepared Sn@C-dots@Sn NPs electrode as LIB anode. (a) cyclic voltammetry, (b) charge and discharge profiles at various cycles (last cycles at each current density in (c)), (c) rate capability study followed by a constant rate cycling at 0.52 mA cm⁻².

Fig. 6c summarizes the cycling performance of the Sn@C-dots@Sn NPs electrode at various rates (1^st to 60^th cycles) and then at constant cycling rate of 0.52 mA cm⁻² (61^th to 90^th). The 1^st cycle discharge capacity is about 2.25 mA h cm⁻² (corresponding to 430 mA h g⁻¹ of coating) with initial Coulombic efficiency of 61.3%. Within three cycles, the Coulombic efficiency quickly rises to >99% for the subsequent cycles, suggesting good reversibility of the electrochemistry. The capacity stabilizes to 0.49 mA h cm⁻² at 0.13 mA cm⁻² and 0.2 mA h cm⁻² at 2.62 mA cm⁻². Upon returning to cycling rate of 0.13 mA cm⁻², the electrode is able to regain its capacity of 0.50 mA h cm⁻². A constant rate cycling is immediately followed; and the result indicates that the electrode can cycle efficiently with minimal capacity fading (from 0.34 mA h cm⁻² at 61^st cycle to 0.30 mA h cm⁻² at 90^th). While good cycling stability has been demonstrated in this work, cycling capacity of the in situ formed electrodes is less ideal due to the imperfection in coating uniformity and C-dot/Sn NP ratio. For future work, parameters such as NP deposition time (sonication time), NP concentration in the reactor, and pretreatment of coating substrate will be explored to yield better electrodes. Moreover, deposition of other premade electroactive, high capacity metals (e.g., Ge, Co, Si) nanoparticles with carbon dots onto flexible current collectors (e.g. carbon cloth) will be conducted for flexible energy storage devices.

4. Conclusions

In conclusion, a straightforward sonochemical approach is employed for the in situ formation of C-dots via ultrasonic irradiation of polyethylene glycol (PEG) solvent. Bulk metallic tin acted as a source of tin nanoparticles in presence of synergistically added heat and sonication process, yielding Sn@C-dots and subsequently Sn nanoparticles decorated with Sn@C-dots (Sn@C-dots@Sn). The measured luminescence properties of Sn@C-dots depend on the excitation wavelengths and are similar but not identical to those of the pristine C-dots. The as-synthesized and directly deposited Sn@C-dots@Sn nanoparticles electrodes showed promising anode activities in Li-ion batteries. Reaction mechanism is studied by systematic morphological, structural and compositional analysis of functional luminescent Sn@C-dots and electroactive Sn@C-dots@Sn nanoparticles.

Acknowledgements

Purdue University (PU) authors thank PU for the generous start-up funding.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09926b

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