The Role of Hole Localization in Sacrificial Hydrogen Production by Semiconductor–Metal Heterostructured Nanocrystals

Abstract

The effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals is investigated. By tuning the energy balance at the semiconductor-ligand interface, we demonstrate that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play an important role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation. In particular, we show that the energy of oxidizing holes can be efficiently transferred to a ligand moiety, leaving the semiconductor domain intact. This allows reusing the inorganic portion of the "degraded" nanocrystal-ligand system simply by recharging these nanoparticles with fresh ligands.

Enhanced hydrogen evolution rates at high pH with a colloidal cadmium sulphide–platinum hybrid system

“…While the former is slightly lower compared to the H 2 evolution in the presence of SO 3 2− hole scavenger (5 mmol h −1 g −1 ), 15,19 a remarkable six-fold increase in H 2 generation rates for pH 13 and 14 in comparison with pH 11 and 12 follows the recently reported trend for Ni-decorated CdS nanorods. 22 This strong increase in efficiencies also favourably compares to the previously reported CdS/ZnSe/Pt hybrid system by Acharya et al (0.2 mmol h −1 g −1 ) 18 and the CdS/Pt system published by Wang et al (13.8 mmol h −1 g −1 ), 17 and is ascribed to a fast removal process of the photohole. Instead of the slower direct photohole transfer to TEA at low pH, 23 the OH − ions at high pH effectively promote the scavenging of photohole transfer by forming • OH radicals.…”

“…10,[15][16][17] Other factors such as the chemical nature and the redox potential of surfactants and sacrificial hole scavengers, and solution pH are relevant factors determining rates of H 2 evolution. [18][19][20] In this letter, we report enhanced hydrogen generation rates over a colloidal CdS-Pt system at high pH. We compare the hydrogen production at four different pH values ranging from 11 to 14, and show a large increase in the hydrogen generation rates at pH higher than 13.…”

Engineered inorganic core/shell nanoparticles

“…Among more complex anisotropic semiconductor/semiconductor core/shell structures CdSe/CdS tetrapods [121], CdSe/CdE (E:S, Se, Te) octapods [122], CdSe/CdS "dot-in-plate" (spherical core, disk-shaped shell) [123] and ZnE/CdS-Pt (E=Se, Te) dots in rods heterostructures comprising a Pt seed at one end [124] can be found. The latter exhibit interesting photocatalytic properties for hydrogen production.…”

Aqueous synthesis of CdS and CdSe/CdS tetrapods for photocatalytic hydrogen generation

“…[11][12][13] Very recently, they also started to gain an increasing attention as components of photocatalytic systems for hydrogen generation from water. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] These studies commonly combine colloidal semiconductor nanocrystals possessing superior light harvesting properties--often of rod-like or tetrapod-like shapes--with a metal-based co-catalyst possessing catalytic properties, such as platinum, palladium, or nickel. 15,29,30 For the chosen semiconductor nanocrystals, it is important to consider a suitable band gap as well as the absolute position of the conduction and valence band minima/maxima, 31,32 in order to absorb as much light as possible from the solar spectrum, and at the same time to match the reduction and oxidation potentials of water.…”

“…Such structures were shown to possess not only the superior light-harvesting ability but also advantageous charge separation at the interface between the two components, thus improving their possible use for photocatalytic hydrogen generation. 15,[20][21][22] MPA-capped CdSe nanocrystals prepared in aqueous solution by our previously published procedure 39 have been added as seeds (1 ml; 5 × 10 −5 mmol) to the water/ethylendiamine mixture, followed by the injection of sulfur precursor as described above. The absorption spectrum of the sample (inset in Figure 2) shows both the contribution of CdSe in the spectral region between 500 and 600 nm and strong absorption from CdS with the well-resolved maximum at 440 nm.…”