Understanding the Visible Absorption of Electron Accepting and Donating CNDs

Abstract

Carbon nanodots (CNDs) synthesized from citric acid and formyl derivatives, that is, formamide, urea, or N‐methylformamide, stand out through their broad‐range visible‐light absorbance and extraordinary photostability. Despite their potential, their use has thus far been limited to imaging research. This work has now investigated the link between CNDs’ photochemical properties and their chemical structure. Electron‐rich, yellow carbon nanodots (yCNDs) are obtained with in situ addition of NaOH during the synthesis, whereas otherwise electron‐poor, red carbon nanodots (rCNDs) are obtained. These properties originate from the reduced and oxidized dimer of citrazinic acid within the matrix of yCNDs and rCNDs, respectively. Remarkably, yCNDs deposited on TiO2 give a 30% higher photocurrent density of 0.7 mA cm−2 at +0.3 V versus Ag/AgCl under Xe‐lamp irradiation (450 nm long‐pass filter, 100 mW cm−2) than rCNDs. The difference in overall photoelectric performance is due to fundamentally different charge‐transfer mechanisms. These depend on either the electron‐accepting or the electron‐donating nature of the CNDs, as is evident from photoelectrochemical tests with TiO2 and NiO and time‐resolved spectroscopic measurements.

Alteration of the Optoelectronic Properties of CuInS2 Quantum Dots via Colloidal Annealing

“…The fit lifetimes from the temporal cuts deviate only slightly while remaining in the same order of magnitude for each annealing time. These lifetimes likely emerge from the same recombination processes or states. − …”

Red-Emissive Center Formation within Carbon Dots Based on Citric Acid and Formamide

“…It is important to note that during the CDs synthesis from CA and FA multiple reaction byproducts are formed, and in addition to purification by dialysis and filtration, supplementary purification steps are necessary, such as chromatography 24 or precipitation in an antisolvent (Table S1). 25,26 Meanwhile, the chemical yield of nanoparticles is not indicated in the published works, but from our experience it does not exceed 3%, relative to the mass of CA precursor used. In this study, to reduce the consumption of precursors and amount of byproduct, we optimized CDs synthesis from CA and FA in DMF and water.…”

“…23,24 For this type of CDs, when dissolved in polar aprotic solvents (DMF, dimethyl sulfoxide (DMSO)), a vibrational structure of optical transitions is observed in the longer-wavelength spectral range, which may be attributed to sp 2 -domains or organic dye derivatives at CA/ FA-based CDs' surface. 25 The high overall PL QY up to 42% was achieved for these CA/FA-based CDs dissolved in DMSO.…”

“…) synthesized according to a published procedure. 25 The PL−PLE maps show the redistribution of intensity between those 3 optical centers� with a decrease in CA/FA molar ratio, the red emission becomes more intense compared to the blue one. The ratio of the "red" at 600 nm to the "blue" at 410 nm absorption increases sharply starting from the CD d -0.…”

“…The synthesized CDs are well soluble in aprotic polar solvents, such as DMSO, and have a worse solubility in water or alcohols and are not soluble in nonpolar solvents, which agrees reported data. 25 In addition, in water and alcohols, a strong blue PL appears (Figures S3 and S4). To establish whether the polarity or the type of a solvent affects the red optical bands, CD w -0.25p was dissolved in solvents with a close dielectric constant (ε) and polarity index (p): acetone (polar aprotic, ε = 21, p = 5.1), ethanol (polar protic, ε = 24.55, p = 5.2), and methanol (polar protic, ε = 33, p = 5.1).…”