Hiroji Hosokawa

Preparation of size-controlled hexagonal CdS nanocrystallites and the characteristics of their surface structures

Hexagonal CdS nanocrystallites have been easily prepared by reacting Cd2+ and S2- (H2S or Na2S) in N,N-dimethylformamide (DMF) without adding any surface stabilizers. The mean diameter of the nanocrystallites can be rigorously controlled over the range 1.8–4.2 nm with keeping the hexagonal crystalline structure by varying the sulfur source (H2S and Na2S) and by regulating the preparation temperature. Use of Na2S as a sulfur source produces smaller CdS nanocrystallites than use of H2S. The size control and stability of CdS nanocrystallites are discussed in terms of the characteristics of the surface structures based on insitu Cd K-edge EXAFS analysis.

Semiconductor photocatalysis. Part 20.—Role of surface in the photoreduction of carbon dioxide catalysed by colloidal ZnS nanocrystallites in organic solvent

Colloidal solutions of monodispersed ZnS nanocrystallites, ZnS–DMF, ZnS–AN and ZnS–MeOH, have been prepared by reacting zinc perchlorate with H2S in N,N-dimethylformamide, acetonitrile or methanol, respectively. In a ZnS–DMF (hexagonal nanocrystallites, average size ca. 2 nm) solution, carbon dioxide undergoes effective photoreduction in the presence of triethylamine as a sacrificial electron donor, under UV light (λ > 290 nm) irradiation, giving formate and CO. Formate is formed exclusively when ZnS–DMF is prepared stoichiometrically. The system shows a blue emission at ca. 325 nm which is attributed to emission from the conduction band or the shallow electron-trap sites of ZnS–DMF. The strength and the lifetime of the blue emission were enhanced by the addition of CO2 to the system, which can be explained by assuming the stabilization of photoformed electrons in the condition band or in shallow electron-trap sites via the adsorptive interaction of a CO2 molecules on the ZnS–DMF surface. The addition of a zinc ion to the system changes the product distribution without losing efficiency or changing emission behaviour: i.e. the competitive formation of CO with formate and the appearance of a red emission at ca. 460 nm under continuous light excitation can be observed, which may be ascribed to the formation of surface sulfur vacancies. Theoretical molecular orbital calculations using a density-functional method support the preferential adsorptive interaction of a CO2 molecule with a Zn atom in the vicinity of a sulfur vacancy on hexagonal ZnS.

Photoluminescence from surface-capped CdS nanocrystals by selective excitation

We have studied photoluminescence (PL) properties of hexagonal CdS nanocrystals whose surface is capped by pentafluorothiophenol and N,N-dimethylformamide, by means of florescence-line-narrowing spectroscopy. Both the sharp band-edge and the broad trap-state emission are strongly modified by exciton-phonon interactions enhanced by quantum confinement. The LO-phonon modified structures in the band-edge emission spectra show the existence of two excitonic states at the band edge. The broad trap-state luminescence is modified by the strong coupling between localized excitons and polar vibrations at the surface. The selectively excited luminescence properties and the coupling between electronic and vibrational excitations in CdS nanocrystals are discussed.

Controlling Microscopic Surface Structure, Crystalline Size and Crystallinity of CdS and ZnS Nanocrystallites

Size controlled CdS and ZnS nanocrystallites with well-defined crystallinity were simply prepared by reacting M2+ (M=Cd2+ or Zn2+) and S2- (H2S or Na2S) in N,N-dimethylformamide (DMF) without adding any surface stabilizers. The mean diameter of CdS nanocrystallites can be rigorously controlled over a range from 1.8 to 4.2 nm maintaining the hexagonal crystalline structure by varying the sulfur source and by regulating the preparation temperature. The band-gap dependence estimated from the absorption peak wavelength on the size of the hexagonal CdS nanocrystallites was in good agreement with the pseudopotential calculation using lattice parameters of hexagonal CdS. In the system of ZnS nanocrystallites, crystallinity was controlled by changing the surface modifying reagent. Reactive organic molecules, which can bind onto the surface of ZnS nanocrystallites, induce phase transition of ZnS nanocrystallites from the hexagonal to the cubic phase, maintaining a crystalline size of 3 nm at ambient temperature and pressure.

Phase transition of ZnS nanocrystallites induced by surface modification at ambient temperature and pressure confirmed by electron diffraction

Pentafluorothiophenol, thiophenol, 1-decanethiol, 1-hexanethiol, benzoic acid and cyanoacetic acid, which are confirmed to bind to the surface of ZnS nanocrystallites, induce phase transition of ZnS nanocrystallites from the hexagonal to the cubic phase, with retention of the crystalline size of 3 nm at ambient temperature and pressure.

Chiroselective electron transfer at enantiomer-capped ZnO nanocrystalline surfaces

Abstract Chiral recognition of amino acid, d - and l -tryptophan, is achieved using (R)-(−)- or (S)-(+)-1,1′-binaphthyl-2,2′-diylhydrogen phosphate (BHP) -capped quantized ZnO nanocrystallites as a fluorescent sensor. The chiroselective electron transfer is caused by the formation of the BHP capping layer on the surface of ZnO nanocrystallites. The ability for chiral recognition depends on the surface coverage and binding strength of the capping molecules on the nanocrystalline surface. Structural analysis of the capping molecules on the surface using circular dichroic spectra proved that screwness of the chiral molecules depends on the surface coverage. The ability of the BHP-capped ZnO system was improved when a well-packed surface capping layer of one enantiomeric form of chiral molecules was formed on the surface of ZnO nanocrystallites.

SURFACE MODIFICATION OF CDS QUANTUM DOTS WITH FLUORINATED THIOPHENOL

CdS nanocrystallites prepared by capping with pentafluorothiophenol, 2,3,5,6-tetrafluorothiophenol or 4-fluorothiophenol are characterized as quantum dots by TEM, FTIR, NMR, UV–VIS and fluorescence spectroscopy. The crystalline size tends to increase with an increase in the number of fluorine atoms in the capping molecules while maintaining high solubility in organic solvents, with solubility in alcohols depending on the number of fluorine atoms in the capping molecules. Pentafluorophenyl-capped CdS nanorystallites have the highest solubilities in alcohols, and exhibit quantum dot photocatalysis in methanol, leading to the efficient two-electron transfer photoreduction under visible-light irradiation.