Electrochemical supercapacitor performance of SnO₂ quantum dots

Abstract

Metal oxide nanostructures are widely used in energy applications like super capacitors and Li-ion 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 SnO₂ 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 SnO₂ 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], [10], and electrochemical supercapacitors [11], [12], [13]. When compared with other metal oxides, SnO₂ 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 cm²/V.s). These properties made this material appealing for the energy storage applications [14], [15]. However, SnO₂ 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., SnO₂ 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 SnO₂ and other species, leading to an enhanced performance in specific capacitance, power density [11], [16]. Cui et al. [11] reported pseudo capacitance of SnO₂ QDs by adding 20% acetylene black, which they attributed to Faradic reactions between SnO₂ QDs and the electrolyte. Shin et al. [22] reported maximum specific capacitance of 40.5 μF/cm² and 8.9% loss of specific capacitance after 1000 cycles for the pure hierarchical SnO₂ nanobranches without any carbon materials. SnO₂ 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 SnO₂ 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.