Towards Understanding the Solvent-Dynamic Control of the Transport and Heterogeneous Electron-Transfer Processes in Ionic Liquids

Effect of Surface Oxygen on the Wettability and Electrochemical Properties of Boron-Doped Nanocrystalline Diamond Electrodes in Room-Temperature Ionic Liquids

This paper reports on how the surface chemistry of boron-doped nanocrystalline diamond (BDD) thin-film electrodes (H vs O) affects the wettability and electrochemical properties in two room-temperature ionic liquids (RTILs): [BMIM][PF₆] and [HMIM][PF₆]. Comparative measurements were made in 0.5 mol L^-1 H₂SO₄. The BDD electrodes were modified by microwave or radio-frequency (RF) plasma treatment in H₂ (H-BDD), Ar (Ar-BDD), or O₂ (O-BDD). These modifications produced low-, medium-, and high-oxygen surface coverages. Atomic O/C ratios, as determined by X-ray photoelectron spectroscopy (XPS), were 0.01 for H-BDD, 0.08 for Ar-BDD, and 0.17 for O-BDD. The static contact angle of ultrapure water on the modified electrodes decreased from 110° (H-BDD) to 41° (O-BDD) with increasing surface oxygen coverage, as expected as the surface becomes more hydrophilic. Interestingly, the opposite trend was seen for both RTILs as the contact angle increased from 20° (H-BDD) to 50° (O-BDD) with increasing surface oxygen coverage. The cyclic voltammetric background current and potential-dependent capacitance in both RTILs were largest for BDD electrodes with the lowest O/C ratio (H-BDD) and smallest contact angle. Slightly larger voltammetric background currents and capacitance were observed in [HMIM][PF₆] than in [BMIM][PF₆]. Capacitance values ranged from 8 to 16 μF cm^-2 over the potential range for H-BDD and from 4 to 6 μF cm^-2 for O-BDD. The opposite trend was observed in H₂SO₄ as the voltammetric background current and capacitance were largest for BDD electrodes with the highest O/C ratio (O-BDD) and smallest contact angle. In summary, reducing the surface oxygen on BDD electrodes increases the wettability to two RTILs and this increases the voltammetric background current and capacitance.

A consolidated account of electrochemical determination of band structure parameters in II-VI semiconductor quantum dots: a tutorial review

Probing absolute electronic energy levels in semiconductor quantum dots (Q-dots) is crucial for engineering their electronic band structure and hence for precise design of composite nano-structure based devices. The use of electrochemistry has allowed us to investigate size, shape and composition dependent band structure parameters viz. the conduction band edge, valence band edge & quasi-particle gap and to establish novel charge induced phenomena in colloidal semiconductor Q-dots. The electrochemical behavior is also of special importance for the prediction of the stability of Q-dots in biological environments as well as for precise design of composite nanohetero-structures for opto-electronic (light emitting diodes) and photovoltaic (solar cells) applications. Several researchers have contributed to probing and predicting the positions of absolute energy levels of band edges and surface states as well as to the establishment of a potential window of stability for a wide variety of Q-dots both in aqueous media and in organic solution. The crucial point about these studies is that unlike spectroscopic methods, no unified approach has been followed and a variety of methods and protocols have been developed to carry out these measurements either on diffusing or thin films of Q-dots in different electrolyte media viz. aqueous, organic and ionic liquids, each having their own advantages over the others. However, a consolidated account of these methods and protocols is not available in the literature. The aim of this tutorial review is therefore to consolidate and compare the studies related to the determination of the band structure of II-VI semiconductor Q-dots through electrochemical measurements. A brief introduction to electrochemical techniques, especially cyclic voltammetry, is given, followed by a summary of experimental methods developed for these measurements. Finally, a concise protocol that can be easily applied universally and is attractive for other users dealing with semiconductor Q-dot based devices is discussed.