Electrochemical Method To Prepare Graphene Quantum Dots and Graphene Oxide Quantum Dots

In this study, we present the preparation of graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs). GQDs/GOQDs are prepared by an easy electrochemical exfoliation method, in which two graphite rods are used as electrodes. The electrolyte used is a combination of citric acid and alkali hydroxide in water. Four types of quantum dots, GQD1–GQD4, are prepared by varying alkali hydroxide concentration in the electrolyte, while keeping the citric acid concentration fixed. Variation of alkali hydroxide concentration in the electrolyte results in the production of GOQDs. Balanced reaction of citric acid and alkali hydroxide results in the production of GQDs (GQD3). However, three variations in alkali hydroxide concentration result in GOQDs (GQD1, GQD2, and GQD4). GOQDs show tunable oxygen functional groups, which are confirmed by X-ray photoelectron spectroscopy. GQDs/GOQDs show absorption in the UV region and show excitation-dependent photoluminescence behavior. The obtained average size is 2–3 nm, as revealed by transmission electron microscopy. X-ray diffraction peak at around 10° and broad D band peak at 1350 cm^–1 in Raman spectra confirm the presence of oxygen-rich functional groups on the surface of GOQDs. These GQDs and GOQDs show blue to green luminescence under 365 nm UV irradiation

Mechanism of Formation of Faceted Titania Nanoparticles from Anodized Titania Nanotubes

Though researchers worldwide have attempted to fabricate faceted titania nanoparticles with a higher fraction of {001} facets, which have high surface energy, the approaches have focused on use of either a very aggressive heating schedule or highly corrosive chemicals like HF. The current article reports a simple method for the transformation of the titania nanotubes to faceted nanoparticles (size varying from 15–120 nm) at relatively low temperatures and heating rates, without the use of any other corrosive chemicals, utilizing only the electrolyte inside the titania nanotubes remnant from the anodization of the titanium substrate. The formation of faceted nanoparticles was found to be strongly dependent on fluorine concentration and on initial state of titania nanotubes (amorphous/crystalline) and annealing temperature. The formation of the unique “nanorod in nanoporous” structures has been reported for the first time. The current article deals with a detailed study of the formation of these unique nanostructures and proposes a mechanism for the same

Combinatorial Chemical Bath Deposition of CdS Contacts for Chalcogenide Photovoltaics

Contact layers play an important role in thin film solar cells, but new material development and optimization of its thickness is usually a long and tedious process. A high-throughput experimental approach has been used to accelerate the rate of research in photovoltaic (PV) light absorbers and transparent conductive electrodes, however the combinatorial research on contact layers is less common. Here, we report on the chemical bath deposition (CBD) of CdS thin films by combinatorial dip coating technique and apply these contact layers to Cu­(In,Ga)­Se₂ (CIGSe) and Cu₂ZnSnSe₄ (CZTSe) light absorbers in PV devices. Combinatorial thickness steps of CdS thin films were achieved by removal of the substrate from the chemical bath, at regular intervals of time, and in equal distance increments. The trends in the photoconversion efficiency and in the spectral response of the PV devices as a function of thickness of CdS contacts were explained with the help of optical and morphological characterization of the CdS thin films. The maximum PV efficiency achieved for the combinatorial dip-coating CBD was similar to that for the PV devices processed using conventional CBD. The results of this study lead to the conclusion that combinatorial dip-coating can be used to accelerate the optimization of PV device performance of CdS and other candidate contact layers for a wide range of emerging absorbers