CdS Nanostructures with Diverse Morphology as Heterogeneous Lewis Acid Catalysts

Abstract

CdS is one of the semiconductor nanomaterials (band gap: ∼2.42 eV) that has been exploited for its unique physical, chemical, and optical properties, notably for its good charge carrier capacity and photocatalytic activity. Additionally, CdS possesses good Lewis acidity to contend as an emerging efficient heterogeneous catalyst for organic transformation reactions. In this work, a very simple and easy-to-handle method has been utilized to fabricate diverse CdS nanostructures (CdS_1a–CdS_6a) from a mixture of {[Cd2(bpta)2(adc)2]·2H2O} n or {Cd2(bpea)2(adc)2} n (where bpta = N,N′-bis(pyridyl-t-butyl) amine, bpea = N,N′-bis(pyridylethyl) amine, and adc = acetylene dicarboxylate) and thiourea as the Cd2+ and S2– sources, respectively, in three different solvents (methanol, ethanol, and tert-butanol) under solvothermal conditions at 120 °C for 6 h. Moreover, a time-dependent evolution study was utilized for the understanding of their formation at optimized conditions. Their varied morphologies (hollow and porous) were confirmed by field-emission scanning electron microscopy (FESEM). The crystallinity and bulk phase purity of all CdS nanostructures was confirmed by powder X-ray diffraction (PXRD), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). On the basis of their solid-state ultraviolet–visible (UV–vis) diffuse reflectance spectra, the band gap values in the range of 2.441–2.5 eV were calculated from the Tauc plot. Their porosity and surface area were measured by a N2 adsorption experiment. Furthermore, the emission spectra showed maximum intensity in the case of nanostructures fabricated in ethanol, followed by those fabricated in methanol and tert-butanol, respectively. Among all of the nanostructures, as-synthesized CdS_3a was found to be the best heterogeneous catalyst to prepare several bis(indolyl)methanes (BIMs) at room temperature under solvent-free conditions. The recyclability up to three cycles and the stability of the catalyst were also confirmed.

Green and sustainable approaches for the Friedel–Crafts reaction between aldehydes and indoles

“…More recently, in 2022, Kaur et al utilized CdS nanostructures which possess good Lewis acidity for the heterogeneous catalysis of various organic transformations including the synthesis of BIMs as shown by Chabukswar and his research group [ 126 ]. Kaur et al synthesized various CdS nanostructures with differences in their morphology and screened them for their catalytic potential, while avoiding the use of surfactants, which were too difficult to remove from the reaction mixture ( Scheme 24 ).…”

“…While the mild reaction conditions and the lack of solvent or conventional heating aided the environmental impact of this approach, the limited substrate scope of just aromatic aldehydes bearing electron-withdrawing groups hindered its application and generality considerably leaving room for further improvements for the employment of CdS nanoparticles. However, the employment of CdS nanoparticles is not considered sustainable, despite their recyclability, due to their toxicity and carcinogenic properties ( Scheme 24 ) [ 126 ].…”