Semiconductor Photocatalysis: Size Control of Surface-Capped CdS Nanocrystallites and the Quantum Size Effect in Their Photocatalysis

Abstract

Size-controlled CdS nanocrystallites were prepared by using thiophenol or hexanethiol as a capping reagent by controlling the ratio of Cd2+ to bis(trimethylsilyl) sulfide (S(TMS)2) as a source of the sulfide ion in reversed micelles. Their solubility and photocatalysis were examined. A series of size-controlled phenyl-capped CdS nanocrystallites catalyze the photoreduction of aromatic ketones in the presence of triethylamine as an electron donor under visible-light irradiation. The photocatalytic activity is affected by the particle size, i.e., the reducing power of the photogenerated electron on CdS nanocrystallites toward the ketones increases with decreasing the particle size. The enhancement of photocatalytic activity observed by reducing the particle size should be ascribed to not only a negative shift of the conduction band edge, but also suppression of the formation of surface defects.

Photocatalytic Hydrogenation on Semiconductor Particles

“…The apparent quantum yield for the formation of 2-butanol was 0.27 at 313 nm. [25,26,40]. Two types of the reduction products, secondary alcohols from two electron-transfer process and pinacols from one electron-transfer process, were observed as listed in Table 4, while the oxidation of TEA as a sacrificial electron donor by the VB holes afforded diethylamine and acetaldehyde.…”

“…Two types of the reduction products, secondary alcohols from two electron-transfer process and pinacols from one electron-transfer process, were observed as listed in Table 4, while the oxidation of TEA as a sacrificial electron donor by the VB holes afforded diethylamine and acetaldehyde. The yields of the hydrogenation involving two electron-transfer process were preferable for substrates possessing the electron-withdrawing group (-CN or -Cl) as shown in Table 4, and further became favorable with increasing light intensity [26] and with decreasing the particle size [40]. 1,2-Diketones such as camphorquinone, 1-phenyl-1,2-propanedione, and benzil were hydrogenated to the corresponding α-hydroxyketones in moderate to good yields on the UV-irradiated P25 TiO2 as shown in Scheme 4 [41,42].…”

Quantum Size Effects in Semiconductor Photocatalysis

“…Molybdenum(IV) sulfide, which does not in the form of microcrystals exhibit activity in the oxidative photodecomposition of phenol [26] and pentachlorophenol [27], can accelerate these photoreactions when added to the reaction mixture in the form of nanoparticles measuring 4-5 nm. The photocatalytic reduction of benzophenone only takes place with the participation of CdS nanoparticles with size not exceeding 4.0 nm [28]. Nanocrystals of ZnS exhibit photocatalytic activity in the reduction of CO 2 [17,29], whereas bulk crystals of zinc sulfide are inert in this reaction.…”

“…In [28,33] a relation was established between the size of the CdS nanoparticles and their photocatalytic activity in the reduction of methylviologen (MV 2+ ). With decrease in the size of the CdS particles from 5.0 to 3.0 nm the energy gap between the potential of the conduction band of the semiconductor and the redox potential of MV 2+ increases as a result of the quantum confinement effect by 0.2 eV, leading to a 4-5-fold increase in the quantum yield of the photoreaction.…”

Size-controlled electron transfer rates determine hydrogen generation efficiency in colloidal Pt-decorated CdS quantum dots

“…In order to design improved systems, it is crucial to understand dynamics and kinetic details of interfacial charge carriers transfer. [12][13][14][15][16][17][18][19][20] Whereas most of the photocatalytic studies focused on size dependent activity and then connected this with band edge shifts and surface area changes, [21][22][23][24][25] understanding of size dependent QD−metal interfacial electron-transfer dynamics, which is crucial to provide design principle for improved systems, is still lacking. A number of time-resolved PL measurements have been conducted on CdSe QD-Au NP systems, providing information on the size dependent overall radiative decay processes that extends into the nanoseconds time regime.…”