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Electrochemical supercapacitor performance of SnO2 quantum dots

Metal oxide nanostructures are widely used in energy applications like super capacitors and Li-on battery. Smaller size nanocrystals show better stability, low ion diffusion time, higher-ion flux and low pulverization than bigger size nanocrystals during electrochemical operation. Studying the distinct properties of smaller size nanocrystals such as quantum dots (QDs) can improve the understanding on reasons behind the better performance and it will also help in using QDs or smaller size nanoparticles (NPs) more efficiently in different applications. Aqua stable pure SnO2 QDs with compositional stability and high surface to volume ratio are studied as an electrochemical super capacitor material and compared with bigger size NPs of size 25 nm. Electron energy-loss spectroscopic study of the QDs revealed dominant role of surface over the bulk. Temperature dependent study of low frequency Raman mode and defect Raman mode of QDs indicated no apparent volume change in the SnO2 QDs within the temperature range of 80-300 K. The specific capacitance of these high surface area and stable SnO2 QDs has showed only 9% loss while increasing the scan rate from 20 mV/S to 500 mV/S. Capacitance loss for the QDs is less than 2% after 1000 cycles of charging discharging, whereas for the 25 nm SnO2 NPs, the capacitance loss is 8% after 1000 cycles. Availability of excess open volume in QDs leading to no change in volume during the electro-chemical operation and good aqua stability is attributed to the better performance of QDs over bigger sized NPs.


Introduction
Semiconductor quantum dots (QDs) plays vital role in technological and biological applications. However for an effective use of QDs, it is very important to understand their distinct physical and chemical properties with respect to their bulk counterparts. Wide band gap n-type semiconducting SnO 2 nanostructures are well established for the use in technologically important applications like gas sensing, transparent electrodes, and as a catalyst [1][2][3][4]. It is also found that the SnO 2 is a potential material for much needed applications like solar cells [1,5,6], as anode materials in Li-ion battery [1,[7][8][9], and electrochemical supercapacitors [11][12][13]. When compared with other metal oxides, SnO 2 holds high electric conductivity (21.1 Ω.cm), high theoretical capacity, low potential of Li ion intercalation, as well as superior electron mobility (100-200 cm 2 /V.s). These properties made this material appealing for the energy storage applications [14,15]. However, SnO 2 or Sn-based anodes undergo a large volume changes of 200, 300% during the charging/discharging process for super capacitor and Li-ion battery applications, which leads to pulverization and loss of electrical contact between particles, subsequently, resulting in low capacity value and poor rate capability [16,17], To overcome this problem, structures are inter-connected to compensate the volume change [18]. Alternatively, nano-sized crystals with dense stacking is used to reduce the volume variation [19], along with mixture structures are formed with other inactive substances, e.g., SnO 2 with carbon [20,21], in which the inactive carbon behave as a confining buffer for the volume variations. However these techniques could not solve the problem completely. Because of the in-active carbon, the total capacitance value reduces. Smaller size nanocrystals can provide good stability, an increased interface for the interaction between SnO 2 and other species, leading to an enhanced performance in specific capacitance, power density [11,16]. Cui et al. [11] reported pseudo capacitance of 3 SnO 2 QDs by adding 20% acetylene black, which they attributed to Faradic reactions between SnO 2 QDs and the electrolyte. Shin et al. [22] reported maximum specific capacitance of 40.5 µF/cm 2 and 8.9% loss of specific capacitance after 1000 cycles for the pure hierarchical SnO 2 nanobranches without any carbon materials. SnO 2 QDs exhibit a typical behavior in comparison with their bulk counterparts due to high surface to volume ratio and excitonic confinement effects [23,24]. Thus, it is important to understand this distinct nature of QDs such that it can be used more effectively in applications. In the past, even though electrochemical studies were performed with the SnO 2 QDs, however, the reason behind the better performance of the smaller size NPs is not well understood.
In the present work we have studied electrochemical super capacitor performance of the pure 2.4 and 25 nm nanoparticles (NPs) without adding any additives like carbon materials to observe the pure material behavior. These nanostructures are studied for their stoichiometry by using electron energy-loss spectroscopy (EELS), specific surface area by using Brunauer-Emmett-Teller (BET) method and aqua stability by studying the time dependent absorption spectra and Zeta potential measurements. Possible change in the volume of the NPs is probed by utilizing the temperature dependent Raman spectroscopy. Density functional theory (DFT) calculations are also carried out to calculate the charge distribution for each atom and the relaxed surface of the atoms. The stoichiometry, aqua stability, volume changes of these 2.4 and 25 nm NPs are compared for their electrochemical super capacitor performance.

Experimental section
SnO 2 QDs were synthesized by soft chemical method. NH 4 OH (2M) drops were added to aqueous SnCl 4 (1M) solution under continuous magnetic stirring at 80 °C. Precipitated white gel 4 was washed with mille-pore water (18.2 MΩ.cm) several times to remove the Cl ions. The washed gel was dried at 100 o C and then crushed with a rotor. The as-prepared material was further annealed in air atmosphere using horizontal quartz tube furnace for 1h at a temperature of 800 o C. Detailed synthesis process of NPs was reported in our earlier report [3].
Morphological and crystallographic information of the NPs were probed with the aid of field emission scanning electron microscopy (FESEM; Zeiss SUPRA 55) and transmission electron microscopy (TEM; Zeiss Libra 200), respectively. EELS studies were carried out using in-column second order corrected omega energy filter type spectrometer with an energy resolution of 0.7 eV in a high resolution transmission electron microscope (HRTEM, LIBRA 200FE Zeiss). Raman active vibrational modes were studied by using micro-Raman spectroscopy (InVia, Reinshaw) with 514.5 nm excitation of an Ar + laser with 1800 grooves/mm grating, and thermo electric cooled CCD detector in the back scattering configuration.The temperature dependent Raman spectra measurements were performed using an adiabatic stage (Linkam LNP95, UK). Optical absorption properties of the NPs were studied with the aid of UV-Vis spectroscopy (Avantes, Netherlands) in the range from 200 to 750 nm. For absorption study SnO 2 NPs of concentration 0.2 mg/ml were well dispersed in mille-pore water using ultrasonicator. Similar solution was used for the zeta potential (Malvern's Zetasizer-Nano) measurements. The specific surface area of the NPs powders were measured using the BET method (MONOSORB, MS16, Quantachrome, USA) employing nitrogen gas adsorptiondesorption measurement at 77 K. In order to remove the moisture content on the surface, the NPs powders were degassed at 150 °C for 3 h in flowing argon prior to the measurement.
The charge distribution for each atom including surface atoms of the relaxed slab is calculated. The ab-initio DFT calculations are carried out using the VASP plane wave code [25] and PAW pseudo potentials with the PBE GGA flavor for the exchange and correlation interactions [26]  . This is due to the crystal size of the as-prepared sample is lesser than the resolution limit of the used FESEM.

Results and discussion
Low magnification TEM image (Fig. 1a) shows spherical as-prepared SnO 2 NPs. Average size of the as-prepared NPs ( Fig. 1b) is around 2.4 nm, which is less than the Bohr radius of Since the NPs radius is below the Bohr radius, the electron hole motion is not correlated. In that case, the effective band gap energy is given by eqn. 1 [28].
Here, E g is the bulk band gap energy (3.6 eV), r is the particle radius, μ is the effective reduced mass (0.27m e for SnO 2 ) [29], and the dielectric constant,  (Fig. S2 a and b). The high specific surface area of the 2.4 nm NPs is due to their 10 time high surface to volume ratio than the 25 nm NPs.
These results are again supporting the EELS data.
Absorption spectra of the QDs (Fig. 4) and the 25 nm NPs (Fig. 5) were recorded with respect to time. Both the NPs were well dispersed on the first day of experiment by ultrasonication. By using Tauc's plot, the band gap of the QDs (inset Fig. 4) and the 25 nm NPs were reported in our earlier report [35]. The absorption spectrum of the 25 nm NPs is seen to decrease in intensity within 12 hours (Fig. 5). This decrease in the absorption intensity reflects the precipitation of 25 nm NPs due to reagglomeration. Fu et al. [36] reported that the ZnO QDs remained stable in water due to surface hydroxyl (OH) groups. It is noteworthy that the presence of high density of OH¯ on the surface of the SnO 2 QDs were disseminated by using FTIR and Raman spectroscopic studies [23]. The 25 nm NPs obtained after annealing at high temperature did not show such widely covered presence of OH¯ groups on the surfaces [23]. The dominance of the surface in SnO 2 QDs is clear from the EELS spectrum. Bonded -OH groups allow help 9 building van-der Walls bonding with the spread solvent. In addition, high surface to volume ratio in QDs provides buoyancy required for floating in the medium as oppose to 25 nm NPs which tend to settle due to gravitational force. Zeta potential measurements are carried out to find the stability of the NPs in aqua media (supplementary Fig. S4 a and b). Average zeta potential values are measured to be ~ +21 and +13 mV for the QDs and the 25 nm NPs, respectively. These to -2.2. So the theoretical study demonstrates that the -OH groups helps in forming static charges which further cause for the stability of the NPs.
Temperature dependent Raman spectroscopic measurements were carried out on the QDs as well as 25 nm NPs (Figs. 6 and 7). Fig. 6 shows symmetry allowed Raman modes at 473, 632, 773 cm -1 corresponding to E g , A 1g , and B 2g of rutile tetragonal SnO 2 [37]. Raman spectrum of 25 nm NPs at 80 K shows highest intensity for symmetry allowed Raman modes along with a blue shift. Whereas defect related forbidden modes (A 2u (TO), and A 2u (LO)) [37], which are prominent at high temperatures are not observed at 80 K (Fig. 6). Increase in the Raman intensity at 80 K may be a result of the decrease in bond length, where a reduction in the full width half maxima (FWHM) of the peak as well as increase in intensity can be observed [38,39]. The blue shift in the peak position with decreasing temperature is due to reduction of anharmonicity in the lattice [39]. Inset in Fig 6 shows the linearly fitted plot for A 1g peak position vs. temperature. The slope of the linear fit that is temperature coefficient of the A 1g peak found to be 0.0184 cm -1 /K.

Unlike 25 nm NPs the QDs exhibit a completely different Raman spectrum and an unusual
behavior with respect to temperature. Fig. 7 shows the temperature dependent Raman spectra of SnO 2 QDs. There is hardly any signature from the Raman allowed modes of rutile tetragonal SnO 2 . Other than the low frequency Raman mode, there is only one broad peak at 570 cm -1 (D), which is characteristically observed for the SnO 2 QDs [23,37]. The D peak shows complete temperature independence in the temperature range from 80 to 300 K. Unlike the defect modes in 25 nm NPs (A 2u (TO), and A 2u (LO)) the D peak does not disappear or reduce in its intensity at low temperature of 80 K. Analogous to the EELS measurements, the Raman modes also depend on the coordination or local environment (leading to a specific local symmetry) of the atoms [39]. In our earlier report, we have showed that the D peak gradually disappears with increasing particle size [23]. The peak was assigned to the in-plane 'O' vacancies from the results of DFT calculations [40]. The reasons behind temperature independence of the D peak intensity may lie in the fact that the local co-ordination and hence the local symmetry of the atoms is not altered for the non-stoichiometric QDs (discussed in the EELS analysis Fig. 3) and also there is no significant anharmonicity arising in the temperature range of 80-300K. This is further supported by the size dependent low frequency Raman mode. The low frequency Raman scattering (LFRS) from the QDs exhibits an unusual behavior with respect to the change in temperature. Unlike symmetry allowed Raman modes in 25 nm NPs (Fig. 6), the LFRS mode in QDs increases in intensity with increasing temperature from 80 to 300 K. All the LFRS modes in Fig. 7 are fitted with Lorentzian function and the fitted parameters are depicted (supplementary information Fig.   S5 and Table S1) to show that there are no appreciable change in the FWHM and peak positions.
Wu et al. [41] observed a similar effect with respect to temperature for the Ge 0.54 Si 0.46 nanocrystals embedded in SiO 2 matrix and have explained it with the help of the collective modes produced by the interference of Raman scattering from the individual QDs. They also found that the intensity of the LFRS mode increased with increasing temperature and the FWHM was independent of the temperature, similar to our observations. If the lattice parameter of QDs increases with increasing temperature then the total volume or size of the QDs also increases.
Consequently the LFRS mode is supposed to show a red shift [23,42]. In contrast, the LFRS Smaller particle size reduces the diffusion length and increases the reaction site for the ion due to its high surface area. This is certainly very important in regard to both capacitance and cyclic performance. Specific capacitance versus scan rate study has drawn immense attention in this direction (Fig. 8 c and d). Specific capacitance value for the QDs is 10 F/g at a scan rate of 20 mV/S and there is only 9% loss in the capacitance value after increasing the scan rate to 500 mV/S. Retention capacity of the 2.4 nm and the 25 nm particle is about 91 and 88% respectively ( Fig. 8d). This result further concludes that the QDs are stable against the strain originated due to volume change at the time of electrochemical process. Similar effect was seen in the temperature dependent Raman spectroscopic studies. Such volume change creates a discontinuity between the particles subsequently responsible for reduced specific capacitance with scan rate as seen in the case of 25 nm NPs.
Galvanostatic charge discharge of the NPs was carried out to study the electrochemical behavior of materials with respect to time in a specific potential window in the same 0. 14 respectively (Fig. 9). Thus, SnO 2 QDs posses extremely high electrochemical stability with retaining more than 98% of capacitance even after 1000 charge discharge cycles. Whereas, in case of the 25 nm NPs retainment of the capacitance is around 92% (Fig. 9). This further supports the superior stability of the SnO 2 QDs.

Supporting Information.
FESEM images of the deposited thin films of as-prepared and annealed NPs. Tauc's plot of the 25 nm NPs, Zeta potential measurements, Lorentzian fitting of the LFRS peaks in the Fig.   7 and Table containing Lorentzian fitted parameters of the LFRS mode in Fig. 7.      Table S1. Lorentzian fitted parameters of the LFRS mode in Fig. S3.