Photocatalytic Overall Water Splitting Activity of Templateless Structured Graphitic Nanoparticles Obtained from Cyclodextrins

Abstract

Pyrolysis of α-, β-, and γ-cyclodextrins at 900 °C under Ar forms porous 3D graphitic carbon nanoparticles with remarkable crystallinity, in which the pore dimensions range from 0.74 to 1.1 nm in the ultra/micropore range as determined by N2, Ar, and CO2 adsorption. These materials behave as semiconductors with the potential energy of the conduction and valence bands being remarkably dependent in as much as 0.57 eV on the dimensions of the micropores. The photogeneration of electrons and holes was confirmed by photodeposition of Pt (electrons) and PbO2 (holes) nanoparticles by reduction and oxidation, respectively. Importantly, these 3D porous graphitic carbons generate H2 and O2 from H2O in the absence of any metal cocatalyst. Theoretical calculations at the DFT level confirm in models the influence of the dimensions of the pores, the presence of defects, and residual oxygen on the band energy and that the occurrence of H2O dissociation is favored inside the pores by a confinement effect. Considering the interest in developing metal-free photocatalyst, the present results show the possibility to tune the band alignment of porous carbon semiconductors by appropriate selection of precursors and the convenient generation of defects.

A Novel Porous Ti‐Squarate as Efficient Photocatalyst in the Overall Water Splitting Reaction under Simulated Sunlight Irradiation

“…The valence band energy maximum was estimated from XPS by determining the minimum energy to observe the appearance of electrons, resulting in a potential of 1.364 V versus normal hydrogen electrode (NHE) after correction by the work function of the instrument (see the Supporting Information for a detailed description of the calculations). [ 42 ] From this value, the conduction band energy minimum was deduced by adding the optical bandgap. Figure illustrates the alignment of the valence and conduction band energy values with respect to the potential required for H 2 and O 2 generation from H 2 O at pH 0.…”

Band Engineering of Semiconducting Microporous Graphitic Carbons by Phosphorous Doping: Enhancing of Photocatalytic Overall Water Splitting

“…It was observed that the relative photocatalytic activity follows the order mp-(P)­C α > mp-(P)­C β > mp-(P)­C γ . This is in line with the previous observation of the positive influence of a small pore size on the (photo)­catalytic activity. , This relative photocatalytic activity order was also measured at various P contents, showing the positive influence of H 2 O confinement and small pore size on the photocatalytic reaction.…”

“…Under optimal conditions, the mp-(P)C exhibits a much enhanced photocatalytic activity for overall water splitting under simulated sunlight illumination as compared to the parent mp-C material, reaching values that are among the highest ever reported for a C-based photocatalyst in the absence of any metal for overall water splitting. As for the parent mp-C materials, an influence of pore size on the photocatalytic activity has been observed for mp-(P)­C, with the most active material being the one with the smallest pore diameter.…”

“…14 In this context, we have recently reported the preparation of a novel type of structured microporous graphitic carbon [mp-C, mp-indicates microporous, C corresponds to graphitic carbon] with semiconducting properties by pyrolysis of cyclodextrins. 15 The special molecular geometry of cyclodextrins having a truncated conical structure with a hollowed interior determines their self-assembly, probably in the melt state above 300 °C, during the thermal treatment leading to graphitization. The resulting graphitic carbon is constituted by nanoparticles about 20−30 nm in diameter having channels with a strictly regular microporosity.…”

Band gap alignment of structured microporous graphitic carbons by N doping and its influence on photocatalytic overall water splitting

“…[1][2][3] In this regard, we recently reported the uniqueness of cyclodextrin pyrolysis, since these organic capsules form particulate graphitic carbon residues with (ultra) micropores of uniform subnanometric dimensions. 4,5 Cyclodextrins are cyclic molecular oligomers of 1,4-glucopyranose with a rigid geometry of truncated cones of 0.79 nm height. [6][7][8][9] The diameter of the cone is a function of the number of glucose units ranging for external larger diameters from 1.37 to 1.69 nm for aand g-cyclodextrins, respectively.…”