(Gold core)/(titania shell) nanostructures for plasmon-enhanced photon harvesting and generation of reactive oxygen species

C. Fang, H. Jia, S. Chang, Q. Ruan, P. Wang, T. Chen, J. Wang
Energy & Environmental Science, 7, 3431-3438, (2014)

(Gold core)/(titania shell) nanostructures for plasmon-enhanced photon harvesting and generation of reactive oxygen species

Keywords

nanostructures, plasmon-enhanced photon, oxygen

Abstract

Titania is an important oxide semiconductor. It is widely studied for applications on catalysis,1 solar cells,2 photochromism,3 photocatalytic water splitting4 and degradation of organic pollutants,5 owing to its low toxicity, chemical and thermal stability, resistance to photocorrosion, and relative abundance.6 However, for photo-driven applications, its photoresponse is limited to the ultraviolet region because of its wide band gap around 3.2 eV. To extend its photoresponse to the visible and even near-infrared regions, methods, such as doping with metal or non-metal elements,7 introducing disorder,8 sensitizing with organic dyes,9 and coupling with other semiconductors,10 have been developed. For most of these methods, the photoresponse of TiO2 can only be extended slightly to the visible region. In addition, the obtained absorption coefficient in the extended spectral region is generally small.      

Plasmonic metal nanocrystals have extremely large absorption/scattering cross-sections and can strongly focus light close to the metal surface.11–13 They can therefore broaden and enhance the light absorption of TiO2 through scattering, absorption enhancement, sensitization, and hot-electron injection.14–17 For example, Au nanoparticles have been applied to TiO2 for enhancing the photo-degradation of organic dyes,18,19 photocatalytic activity in organic synthesis,20 photocatalytic and photoelectrochemical activity in water splitting,21,22 and performance of dye-sensitized solar cells (DSSCs).23,24 In these studies, Au nanoparticles are adsorbed on or embedded in TiO2 through molecular or electrostatic interactions. In addition, mainly spherical Au nanoparticles with plasmon wavelengths limited in a narrow range are employed. Until now, only in a few studies have Au nanorods, including colloidal22,25,26 and templated ones,27 been integrated with TiO2.

Coating TiO2 onto Au nanocrystals to form a nanoscale core/shell architecture can increase the active interfacial area between the two materials. Such core/shell nanostructures are beneficial for studying plasmon-enhanced/enabled processes and of great potential in light-harvesting applications. On the other hand, they can also protect the Au nanocrystal core from reshaping, aggregation, and chemical corrosion. Many studies have been performed on coating TiO2 onto Au nanoparticles.18,28–44 The involved Au nanoparticles are mostly spherical or nearly spherical, with only two exceptions,40,42 where Au nanorods and (Au core)/(Ag shell) nanocrystals have recently been coated with TiO2. The TiO2 shell coated on Au nanorods has a thickness of 4.5 nm.40 Neither coating the TiO2 shell with controllable thicknesses on Au nanorods nor good dispersibility of the obtained (Au nanorod core)/(TiO2 shell) nanostructures has been demonstrated. Mainly three types of TiO2 precursors have been used in these coating experiments. They are TiF4, titanium(IV) alkoxides, and titanium(IV) complexes. The hydrolysis and condensation rates of these TiO2 precursors are generally fast. The coating entails delicate control of synthetic conditions and often multiple steps of reaction, separation, and surface functionalization.

Gold nanorods, compared with Au nanospheres, exhibit larger local electric field enhancements. Their longitudinal plasmon wavelength can be synthetically varied from the visible to near-infrared region.45 Herein we report on a facile and versatile route to the preparation of (Au nanorod core)/(TiO2 shell) nanostructures. TiCl3, which has recently been used to coat TiO2 on relatively large metal nanorods that are templated and embedded in the alumina pores,46 is employed as the TiO2 precursor. Our coating method has been readily extended to other monometallic and bimetallic Pd, Pt, and Au nanocrystals. The plasmon band of the (Au core)/(TiO2 shell) nanostructures can be synthetically varied from ∼700 nm to over 1000 nm. The controllable plasmonic properties and synergistic interactions between the metal core and the TiO2 shell make the (Au core)/(TiO2 shell) nanostructure a multifunctional nanomaterial. The (Au nanorod core)/(TiO2 shell) nanostructures have been shown to function as a scattering layer in DSSCs to enhance the absorption of visible light, with the resultant cells exhibiting a 13.3% increase in the power conversion efficiency and a 75% decrease in the scattering-layer thickness. Moreover, under near-infrared resonant excitation, they can efficiently utilize low-energy photons to generate reactive oxygen species (ROS), including singlet oxygen (1O2) and hydroxyl radicals (˙OH). Our (Au core)/(TiO2 shell) nanostructures are therefore of great potential in improving the performance of inorganic and organic thin-film solar cells, functioning as photocatalysts in organic synthesis as well as photosensitizers in photodynamic therapy.

Code

DOI: 10.1039/C4EE01787K

Sources

Website PDF

See all publications 2014