Carbon Nanodots for All-in-One Photocatalytic Hydrogen Generation

Abstract

Carbon nanodots (CNDs) were photochemically altered to produce dihydrogen under light irradiation. Within the complex structure of CNDs, photo-oxidation takes place at citrazinic acid molecular fluorophore sites. Important is the fact that the resulting CND materials have a dual function. On one hand, they absorb light, and on the other hand, they photo-and electrocatalytically produce dihydrogen from water and seawater, without any external photosensitizer or cocatalyst. Record HER activities of 15.15 and 19.70 mmol(H 2 ) g(catalyst) −1 h −1 were obtained after 1 h of 75 mW/cm 2 Xe lamp illumination, from water and seawater, respectively. This impressive performance outweighs the remaining structural uncertainties. A fullfledged physicochemical investigation based on an arsenal of steady-state and time-resolved spectroscopic characterizations together with microscopy enabled a comprehensive look into the reaction mechanism. For an efficient dihydrogen formation, a precatalytic activation by means of reduction with a sacrificial electron donor is imperative.

The role of molecular fluorophores in the photoluminescence of carbon dots derived from citric acid: current state-of-the-art and future perspectives

“…The possible clue about the chemical structure of the above red-emitting dyes synthesized from CA and urea has been provided recently by Jana et al 15 They reported extraordinarily high photocatalytic dihydrogen generation (15.15-19.70 mmol (H 2 ) g(catalyst) −1 h −1 ) catalyzed by photochemically altered CDs without any external photosensitizer or cocatalyst. It was inferred that the photooxidation of the CD materials is centered on the citrazinic acid fluorophores, which are present in the CDs and which strongly interact with the carbon dot nanoparticles.…”

“…8C). 15 It is important to note that indigoidine (nonfluorescent blue dye) exhibits absorption characteristics similar to red-emitting CDs synthesized from CA and urea. At this moment it is difficult to judge whether some unknown indigoidine derivatives are able to emit light in the red/near-infrared region of the electromagnetic spectrum or not but certainly this might be considered as an interesting starting point for future systematic studies.…”

“…40 This discovery led to many applications based on green emitting carbon dots containing HPPT on their surface. 15,20,[75][76][77][78][79][80][81] Recently, the above-mentioned 2-pyridone-based fluorophores have played an important role in the explanation of the fluorescence properties of numerous bottom-up synthesized CDs prepared from CA as a precursor, which will be further elaborated in the next sections. At this point, it is important to note the synergistic effect of the development of the CD field on the evolution of CA-based fluorescent dyes and their applications.…”

“…5 Furthermore, CDs have been proven to perform well in many applications ( e.g. , bioimaging, 6 selective sensing, 7 theranostics, 8,9 production of LEDs, 10 energy conversion, 11,12 and photocatalyzed hydrogen evolution 13–15 ) paving the path for more environmentally-benign alternatives to the traditional semiconductor nanocrystal quantum dots. 16…”

“…3,4 The latter caused almost an exponential increase in the number of reports published on the synthesis methods and optical properties of CDs, making them one of the hottest research topics in nanoscience. 5 Furthermore, CDs have been proven to perform well in many applications (e.g., bioimaging, 6 selective sensing, 7 theranostics, 8,9 production of LEDs, 10 energy conversion, 11,12 and photocatalyzed hydrogen evolution [13][14][15] ) paving the path for more environmentally-benign alternatives to the traditional semiconductor nanocrystal quantum dots. 16 In 2016, Cayuela et al classified CDs into three categories in terms of their structures and methods used for their synthesisgraphene quantum dots (GQDs), carbon quantum dots (CQDs) and carbon nanodots (CNDs); 17 in addition, polymer dots (PDs) and carbonized PDs were introduced.…”