The benefits of increasing the number of surface hydroxyls on TiO2 nanoparticles (NPs) are known for environmental and energy applications; however, the roles of the hydroxyl groups have not been characterized and distinguished. Herein, TiO2 NPs with abundant surface hydroxyl groups were prepared using commercial titanium dioxide (ST-01) powder pretreated with alkaline hydrogen peroxide. Through this simple treatment, the pure anatase phase was retained with an average crystallite size of 5 nm and the surface hydroxyl group density was enhanced to 12.0 OH/nm2, estimated by thermogravimetric analysis, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Especially, this treatment increased the amounts of terminal hydroxyls five- to six-fold, which could raise the isoelectric point and the positive charges on the TiO2 surface in water. The photocatalytic efficiency of the obtained TiO2 NPs was investigated by the photodegradation of sulforhodamine B under visible light irradiation as a function of TiO2 content, pH of solution, and initial dye concentration. The high surface hydroxyl group density of TiO2 NPs can not only enhance water-dispersibility but also promote dye sensitization by generating more hydroxyl radicals.

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All surface-excitation studies of Au in the past focused on the well-known 2.4 eV surface plasmon polariton in the visible spectral regime. The existence of surface exciton polaritons is believed to be pristine to the spectral regimes, showing strong excitonic absorptions ͓F. Yang et al., Phys. Rev. Lett. 64, 559 ͑1990͒; Phys. Rev. B 44, 5855 ͑1991͔͒. The presence of surface exciton polaritons in far-UV in Au ͑Ն10 eV͒, where the optical and electronic properties of Au are dominated by broad interband transitions that display characters of rather weak and diffused excitonic oscillator strengths, is not expected and has never been discussed. Reexamining the reports of Yang et al. and using electron energy-loss spectroscopy with a 2 Å electron probe in aloof ͑optical near-field͒ setup and real-space energy-filtered imaging, we firmly establish the existence of surface exciton polaritons in individual Au nanoparticles in the far-UV spectral regime. These results indicate that surface exciton polaritons indeed can be excited in weak excitonic onsets in addition to their general believing for the sharp excitonic oscillations. Our experimental observations are further confirmed by the theoretical calculations of electron energy-loss spectra. The unmatched spatial resolution ͑2 Å͒ of the electron spectroscopy technique enables an investigation of individual nanomaterials and their surface excitations in aloof setup. The surface exciton polaritons in individual Au nanoparticles thus represent an example of surface excitations of this type beyond the visible spectral regime and could stimulate further interests in surface exciton polaritons in various materials and applications in novel plasmonics and nanophotonics at high energies via manipulations of the associated surface near fields.

Anthracene hydrogenation in aqueous micellar solutions is catalyzed at room temperature by ionic-surfactantprotected gold and silver nanoparticles (NPs) with well-controlled particle sizes. When sodium borohydride was used as the hydrogen source in the presence of these NPs, the only product obtained in the reaction is 9,10-dihydroanthracene. During the course of the reaction, a blue shift in the surface plasmon resonance band of silver NPs was more prominent and lasted longer than that of gold NPs. The difference in the optical property of metal NPs was related to the charging of their surfaces, indicating that the metal NPs play a role as the nanoelectrode storing electrons from hydrides. At the same time, a remarkable size-dependence of catalytic activity was found for both gold and silver NPs. Mechanism about the electron transfer-relaying effects of metal NPs is proposed for the hydrogenation reaction.