Carbon quantum dots (CQDs) are a class of carbon nanomaterials that have recently gained recognition as current entrants to traditional semiconductor quantum dots (QDs). CQDs have the desirable advantages of low toxicity, environmental friendliness, low cost, photostability, favorable charge transfer with enhanced electronic conductivity, and their comparable easy synthesis protocols. This article examines advancements in CQD research and development, with a focus on their synthesis, functionalization, and energy applications. Initially, various synthesis methods are discussed briefly with pros and cons. Herein, first top-down methods including arc discharge technique, laser ablation technique, plasma treatment, ultrasound synthesis technique, electrochemical technique, chemical exfoliation, and combustion were discussed briefly. The later section presents bottom-up (microwave synthesis, hydrothermal synthesis, thermal pyrolysis, and MOF template-assisted approach) and waste-derived CQDs synthesis methods. The next section is focused on the energy applications of CQDs including supercapacitors, lithium-ion batteries, photovoltaics, hydrogen (HER), and oxygen evolution reaction (OER). Finally, challenges and perspectives in this exciting and promising area are presented.

Considering the potential applications of transition metal doped nanostructured materials and the advantages of novel, cost-effective and environmental friendly biosynthesis methods, Ni-doped SnO2 nanomaterials have been synthesized using remnant water (ideally kitchen waste) collected from soaked Bengal gram beans (Cicer arietinum L.) extract. The structural and optical properties of the Ni-doped SnO2 nanostructures were studied using various techniques such as UV/visible spectroscopy, FT-IR spectroscopy, X-ray powder diffraction (XRD), Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The SEM, TEM images and XRD of biosynthesized Ni-SnO2 nanoparticles reveal uniform size distribution with the average size of 6 nm and confirmed the formation of rutile structure with space group (P42/mnm) and nanocrystalline nature of the products with spherical morphology.Subsequently, Ni-doped biosynthesized SnO2 nanoparticles were coated onto the glass substrate using doctor blade method to form thin films. The NO2 sensing properties of the materials have been studied in comparison with other gases.The reported gas sensing results are promising, which suggest that the Ni-dopant is a promising noble metal additives to fabricate low cost SnO2 based sensor.

Constructing two-dimensional (2D), free-standing, nonprecious, and robust electrocatalysts for oxygen evolution reactions (OERs) is of primary importance in the commercial water-splitting technology. Herein, we have constructed a 2D heterostructured NiFe 2 O 4 /NiFe layered double hydroxides (LDH) mixed composite on a low-cost stainless-steel mesh substrate using a lowtemperature one-step wet chemical synthesis method and have also investigated the effect of starting material concentration on the formation of the NiFe 2 O 4 /NiFe LDH mixed composite. The as-prepared NiFe 2 O 4 /NiFe LDH-25 electrocatalyst drives a 100 mA/cm 2 OER with the lowest reported overpotential of 190 mV and a Tafel slope 21.5 mV/dec and drives a stable 100 mA/cm 2 OER process in 1 M KOH. These OER activities are superior to that of the state-of-the-art RuO 2 OER electrocatalyst. The excellent OER activity appears to be due to the synergetic effect of NiFe LDHs and NiFe 2 O 4 . In addition, the vertically aligned heterostructure of the NiFe 2 O 4 /NiFe LDH composite thin sheets provides a large number of active edge sites, directly attached to the highly conducting substrate, which contributes to improving the electronic conductivity of the electrocatalyst. This work provides valuable insight into the design and one-step synthesis of NiFe 2 O 4 /NiFe LDH bimetallic mixed oxide and hydroxide composite thin films with enhanced OER activity and stability as well as deciphering the origin of the OER enhancement by metal oxides and metal hydroxides.

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