II - VI 족 반도체 나노 결정들의 용액상 합성과 광촉매적 특성

Abstract

In this thesis, I have done solution phase synthesis of II-VI semiconductor nanocrystals such as ZnO, ZnS, ZnSe, CdS, and specially interested in their photocatalytic properties. In chapter 1, N-doped ZnS nanoparticles with wurtzite phase was synthesized at 150oC, derived from an inorganic-organic complex, ZnS∙(piperazine)0.5. The meta-stable ZnS∙(piperazine)0.5 nanohybrid materials could be described as the layered structure where wurtzite ZnS layers are connected to each other through the bondings of nitrogen atoms in piperazine. It was found that with the progress of the synthetic reaction, the interlayer piperazine molecules were moved out of the layers and the phase was transformed into wurtzite ZnS. Interestingly, nitrogen atoms in piperazine could be doped into ZnS in the extraction of the interlayer molecules. Phase transition was studied by using various techniques, including powder XRD, FE-SEM, and HR-STEM. The N-doping was characterized with UV-vis spectroscopy, and experimental and theoretical analyses of X-ray Absorption Structure (XANES). The N-doped ZnS was applied to the photocatalytic degradation of dye under visible light irradiation. In chapter 2, Visible light irradiation removes organic impurities inherited from the synthesis of CdS and ZnS nanoparticles, while preserving crystalline phase and nanoscale structure of as-synthesized semiconductors as well as creating mesopores, thus providing a better post-treatment procedure for photo-active semiconductor nanoparticles than conventional thermal annealing. In chapter 3, we report a method for synthesizing exposed crystal face-controlled 3D ZnO superstructures under mild conditions (at room temperature or 90°C under 1 atm) without organic additives. The exposed crystal faces of the building blocks of the 3D structures were controlled by varying the reactant concentrations and the reaction temperatures. On the basis of the experimental results, we speculated a possible mechanism for the formation of the four distinct 3D ZnO superstructures (Structures I, II, III, and IV) under the different growth conditions. The optical properties of the 3D ZnO superstructures were probed by UV-Vis diffuse reflectance spectroscopy. The spectra were shifted depending on the dimensions and sizes of the building blocks of the 3D superstructures. The photocatalytic activities of the 3D superstructures varied according to the exposed crystal faces, which could be controlled by this method (Structure I > Structure IV > Structure III > Structure II). In chapter 4, we report a method for synthesizing three distinct type II 3D ZnO/ZnSe heterostructures through simple solution-based surface modification reactions in which polycrystalline ZnSe nanoparticles formed on the surfaces of singlecrystalline ZnO building blocks of 3D superstructures. The experimental results suggested a possible formation mechanism for these heterostructures. The formation of the ZnO/ZnSe heterostructures was assumed to result from a dissolution_ recrystallization mechanism. The optical properties of the 3D ZnO/ZnSe heterostructures were probed by UV_vis diffuse reflectance spectroscopy. The 3D ZnO/ZnSe heterostructures exhibited absorption in the visible spectral region. The visible photocatalytic activities of 3D ZnO/ZnSe heterostructures were much higher than those of the 3D pure ZnO structures. The activities of the 3D ZnO/ZnSe heterostructures varied according to the structures under visible light. The morphologies and exposed crystal faces of pure ZnO building blocks prior to surface modification had a significant effect on the visible light photocatalytic processes of ZnO/ZnSe heterostructures after surface modification. In chapter 5, we report a method for synthesizing ZnO/ZnSe heterostructure nanowire arrays for use in photoelectrochemical (PEC) water splitting. The surfaces of ZnO nanowires immobilized on a conducting glass substrate were modified to form ZnO/ZnSe heterostructure nanowire arrays through a reaction with an aqueous sodium selenite and hydrazine solution. ZnO/ZnSe heterostructure nanowires with different morphologies were synthesized by varying solution concentrations and reaction times. The ZnO nanowire/ZnSe nanoparticle heterostructures (ZS1) were synthesized by a dissolution-recrystallization mechanism. At longer reaction times and higher solution concentrations, the nanostructure arrays transformed into ZnO nanowire/ZnSe nanosphere heterostructure arrays (ZS2) via Ostwald ripening. ZnO/ZnSe heterostructure arrays (ZS1 and ZS2) yielded higher photocurrents than the pristine ZnO nanowire arrays in a PEC water splitting test under AM 1.5G simulated solar light. The ZnO/ZnSe heterostructure array photoanodes exhibited absorption in the visible spectrum (<550 nm in wavelength) with a high incident-photon-to-current-conversion efficiency (IPCE) of up to 47% (ZS1) or 57% (ZS2) at 0.0 V vs. Ag/AgCl. The photoanode yielded a relatively high photocurrent density of 1.67 mA/cm2 (ZS1) or 2.35 mA/cm2 (ZS2) at 0.3 V compared to the ZnO nanowire arrays (0.125 mA/cm2). Structural differences between ZS1 and ZS2 yielded different PEC performances. A comparison to ZS2 revealed that ZS1 exhibited a higher photocurrent density under a low applied potential (from –0.78 V to –0.07 V) and a lower photocurrent density under a high applied potential (above –0.07 V). In chapter 6, we report the synthesis of carbon-doped zinc oxide nanostructures using vitamin C, and their visible light photocatalytic activity. Amorphous/crystalline vitamin C–ZnO (VitC–ZnO) structures were obtained from a solution of zinc nitrate hexahydrate, HMT, and vitamin C through heating at 95°C for 1 h. VitC–ZnO structures were calcined in air at 500°C for 2 h to create C-doped ZnO nanostructures. Calcined structures were polycrystalline, with an average crystal domain size of 7 nm. EDS, XPS, and XRD analysis revealed the substitution of oxygen with carbon and the formation of Zn-C bonds in the C-doped ZnO nanostructures. The carbon concentrations, in the form of carbide, were controlled by varying the concentrations of vitamin C (more than 1 mM) added to reaction solutions. On the basis of these experimental results, we propose a possible formation mechanism for C-doped ZnO nanostructures. The C-doped ZnO nanostructures exhibited visible light absorption bands that were red-shifted relative to the UV exciton absorption of pure ZnO nanostructures. The visible light (λ ≥ 420 nm) photocatalytic activities of C-doped ZnO nanostructures were much better than the activities of pure ZnO nanostructures.