Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O), PA, 3H2NCA, 2H1NCA, 2-naphthol, 3-aminopropyltrimethoxysilane (APTMS), and cadmium nitrate tetrahydrate were purchased from Sigma-Aldrich. All other reagents were purchased from Wako Pure Chemicals, Ltd. Indium tin oxide-coated glasses (ITO) were purchased from Geomatec Co., Ltd. (Yokohama, Japan). All solutions were prepared with ultrapure water obtained from a water purification system (Millipore WR600A, Yamato Co., Japan).
The UV-visible spectra were recorded by an optical spectrophotometer, USB 2000, Ocean Optics, Inc. The transmission electron microscopic and X-ray photoelectron spectroscopic analyses were performed at the Korea Basic Science Institute, Busan Center, Korea. The scanning electron microscopic (SEM) images were observed using a field-emission scanning electron microscope (FE-SEM; JSM-7400 F, JEOL, Japan).
As a typical preparation method of AuNPs with PA, 7.9 mg PA was placed in a test tube and 9.0 ml of pure water was added followed by sonication for 15 min. Forty microliters of 1.0 M NaOH (aq.) was then added to the solution, and pure water subsequently added to make the volume 10.0 ml. The mixture was then sonicated for 15 min to make the 2.0-mM PA solution clear. Next, 100 μl of 1.0 M NaOH (aq.) was added and sonicated for 1 min; then, 10 ml of a 1.34-mM solution of HAuCl4 (aq.) was added under sonication and stored for 15 min. Finally, the solution was stored for 60 min undisturbed to allow the complete formation of the AuNPs. We checked the necessity of this process by observing the changes in the absorption spectra and found that further stirring after the mixing tended to be unfavorable for preparing the monodispersed AuNPs. Similar preparations were carried out with other reagents, i.e., 3H2NCA, 2H1NCA, or 2-naphtol, but, in these cases, the concentrations were increased to 4.0 mM, i.e., twice that of PA, to make the amount of the naphthol units the same.
In preparing some samples for the TEM and X-ray photoelectron spectroscopy (XPS) measurements, the prepared solution of the AuNPs with PA was centrifuged at 12,000 rpm and the obtained sediment was redispersed in 1 mM NaOH (aq.). The centrifugation and redispersion processes were repeated three times to remove any free or loosely bound molecules. For the TEM analysis, the AuNPs were transferred to a copper grid by dipping it into the purified alkaline solution of the AuNPs. For the XPS analysis, the purified alkaline solution of the AuNPs was dropped onto a cleaned ITO substrate and dried at 40 °C.
In preparing the AuNP-modified ITO, a piece of ITO was immersed overnight in ethanol containing 2 % APTMS (v/v) at room temperature, and the amine-terminated ITO was prepared. After washing with ethanol, the electrode was dried by flowing nitrogen. On the other hand, the solution of the AuNPs was centrifuged at 12,000 rpm and the supernatant was decanted. The obtained sediment was redispersed in water. The centrifugation and redispersion processes were repeated three times. The APTMS-modified ITO was dipped in the purified aqueous AuNPs solution for 2 h. After washing with water, the modified ITO electrode was dried at 40 °C.
We developed a thermal decomposition method for preparing NiO nanoparticles (NiONPs) using disodium salt of pamoic acid (Na2PA) as a complexing agent and Ni(NO3)2⋅6H2O as a nickel precursor. Prior to thermal decomposition, Ni(NO3)2⋅6H2O was mixed with Na2PA in ethanol, and the ethanol was evaporated succesively. The dried reaction mass was characterized using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermal gravimetric analysis. Thermal decomposition was then performed in air to obtain the NiONPs. The role of Na2PA in the synthesis of NiONP was evaluated by preparing NiONPs according to the protocol described above without the addition of Na2PA. The X-ray diffraction data indicated that crystalline NiO (bunsenite, cubic crystal system) formed with or without Na2PA; however, field emission scanning electron microscopy images showed that smaller monodisperse NiONPs formed only with the addition of Na2PA. Without Na2PA, the obtained NPs were quite large and polydisperse. The sizes of the NiONPs prepared in the presence of Na2PA were determined using transmission electron microscopy imaging to be 19.1 ± 3.2 nm. The electrocatalytic activity of the NiONPs toward water oxidation under alkaline conditions was evaluated by immobilizing the NPs onto an in-house prepared filter paper derived carbon electrode, and compared.