Quantum Confinement Controls Photocatalysis: A Free Energy Analysis for Photocatalytic Proton Reduction at CdSe Nanocrystals

Abstract

The ability to adjust the mechanical, optical, magnetic, electric, and chemical properties of materials via the quantum confinement effect is well-understood. Here, we provide the first quantitative analysis of quantum-size-controlled photocatalytic H2 evolution at the semiconductor-solution interface. Specifically, it is found that the hydrogen evolution rate from illuminated suspended CdSe quantum dots in aqueous sodium sulfite solution depends on nanocrystal size. Photoelectrochemical measurements on CdSe nanocrystal films reveal that the observed reactivity is controlled by the free energy change of the system, as determined by the proton reduction potential and the quasi-Fermi energy of the dots. The corresponding free energy change can be fitted to the photocatalytic activity using a modified Butler-Volmer equation for reaction kinetics. These findings establish a quantitative experimental basis for quantum-confinement-controlled proton reduction with semiconductor nanocrystals. Electrochemical data further indicate that proton reduction occurs at cadmium sites on the dots, and that charge separation in these nanocrystals is controlled by surface effects, not by space charge layers.

Nanohybrids of Two‐Dimensional Transition‐Metal Dichalcogenides and Titanium Dioxide for Photocatalytic Applications

“…Up to now,s everal semiconducting materials including TiO 2 , [15] ZnO, [16] Nb 2 O 5 , [17] CdS, [18] CuInS 2 , [19] and CdSe [20] have been utilized for photocatalytic hydrogen production. Among these semiconductors, TiO 2 is the most famousm ember that has received widespread attention for photocatalytic applications mainlyd ue to its strong redox ability,a bundant availability, and extraordinary stability under harsh photocatalytic reactions.…”

Nanoscale Effects in Water Splitting Photocatalysis

“…In 2013, the author's group established a similar rate dependence for proton reduction with CdSe quantum dots (Fig. 8) [98,99]. For the study, monodisperse CdSe QDs with diameter 1.8-6.0 nm were synthesized in the presence of 2-mercaptoethanol as a ligand [100], followed by size-selective precipitation with 2-propanol.…”

“…Diffuse reflectance optical spectra reveal an absorption edge of 430 nm, consistent with a band gap of 2.88 eV, compared to 2.68 eV for bulk . Reproduced with permission from [99]. Copyright 2013, American Chemical Society WO 3 .…”

“…This shifts the Fermi energy in (5) and with it the driving force for photochemical charge transfer. In general, the variation of the electron transfer rate constant with E F can be understood using free energy relationships [122], including Butler-Volmer [99,101] and Marcus theory [94].…”

Hole‐Accepting‐Ligand‐Modified CdSe QDs for Dramatic Enhancement of Photocatalytic and Photoelectrochemical Hydrogen Evolution by Solar Energy

“…A suitable hole‐accepting ligand should satisfy the energy requirement for successive hole transfer and interact with CdSe QDs intimately to make ultrafast hole transfer feasible. Phenothiazine (PTZ) is likely to serve as an ideal ligand for such application because the highest occupied molecular orbital (HOMO) level of PTZ (≈0.9 V vs NHE) is more negative than the valence band of CdSe QDs (≈1.3 V vs NHE;11 ≈1.9 nm in diameter), implying that hole transfer from CdSe QDs to PTZ is exothermic. Moreover, the electron‐rich heterocyclic structure of PTZ not only allows ready electron‐donating ability but also ensures intimate interaction with QDs through sulphur‐cadmium coordination and van der Waals interaction 12.…”