Low-temperature synthesis of titanium oxide/gold nanoparticle composite powders using a combination of the sol–gel process and ultraviolet light irradiation
- 199 Downloads
Amorphous titanium oxide/plasmonic gold nanoparticle composite powders were synthesized by a combination of the sol–gel process and ultraviolet light irradiation using light-emitting diode at room temperature. The resultant composite powders were dried at ~50 °C. These amorphous titanium oxide/gold nanoparticle composite powders were heated to 450 °C to obtain crystalline titanium oxide/gold nanoparticles. The formation and microstructure of the titanium oxide/gold nanoparticle composite powders were confirmed by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, thermo gravimetry–differential thermal analysis, and optical absorption measurements. A clear plasmonic absorption band due to typical plasmonic gold nanoparticle was observed in both composite powders.
KeywordsGold nanoparticle Plasmon Titanium oxide Sol–gel process Photoinduced reduction Low-temperature synthesis
Titanium oxide has attracted attention because of its unique photoredox, photocatalytic, electron-accepting, and electron-transporting properties. Indeed, titanium oxide is used as an electron-accepting or electron-transporting material in several types of solar cells such as dye-sensitized solar cells [1, 2], perovskite solar cells [3, 4], and organic thin-film solar cells [5, 6, 7]. A thin film of amorphous titanium oxide can be used in practically the same way as crystalline titanium oxide as the electron-transporting material in organic thin-film solar cells . In general, amorphous titanium oxide can be prepared at considerably lower temperatures than crystalline titanium oxide. Specifically, the sol–gel process is well known to be a low-temperature process for the preparation of amorphous titanium oxide from the corresponding titanium alkoxide [8, 9].
When noble-metal (e.g., gold and silver) nanoparticles are irradiated by visible light, a localized enhanced electric field originating from localized surface plasmon resonance (LSPR) is generated around the nanoparticle within its radius. LSPR from such plasmonic metal nanoparticles can excite photoactive substances; this can be verified using surface-enhanced Raman scattering [10, 11], fluorescence emission [12, 13, 14], enhancement of photoelectric conversion [15, 16, 17, 18], etc.
Based on this background information, a composite material consisting of plasmonic nanoparticles and titanium oxide is attractive for the development of novel plasmonic functional materials. Such composite materials have been synthesized via thermal reduction of metal ions in titanium oxide media [19, 20, 21, 22, 23], photoinduced reduction of metal ions on the surface of titanium oxide [24, 25, 26], modification of gold nanoparticle colloids on the surface of titanium oxide [27, 28], etc.
From the viewpoint of the development of plasmonic applications, composite materials consisting of titanium oxide and plasmonic nanoparticles with diameters of several tens of nanometers to 100 nm are useful. In addition, the low-temperature preparation of such composite materials must contribute to the expanding the area of plasmonic science and applications of these materials in devices. Recently, Viana et al. [29, 30] reported the preparation of titanium oxide/silver nanoparticles and composite thin films, using a combination of the sol–gel process and photoinduced reduction of silver ions. In particular, they reported that silver nanoparticles were embedded in the titanium oxide thin film. This unique structure is useful and quite interesting for next-generation plasmonic materials. Inspired by the reports of Viana et al., we attempted to synthesize composite powders of titanium oxide with gold nanoparticles using a combination of the sol–gel process and ultraviolet (UV) light irradiation [31, 32, 33, 34]. Based on these previous trials and recent developments, herein, we report the low-temperature synthesis of a composite solution containing titanium oxide/gold nanoparticle; this was dried at ~50 °C to obtain the corresponding composite powder. We also report the effect of calcination of the composite powder.
The absorption spectra of the powder samples were measured by UV–visible–NIR spectroscopy (Jasco V-670) using the diffuse-reflection method. The microstructures of the samples were evaluated by transmission electron microscopy (TEM; Hitachi H-8100). Surface observations of the composite materials were performed by scanning electron microscopy (SEM; Jeol JSM-6500F). X-ray diffraction (XRD) analysis of the samples was performed using a Philips X’Pert MPD system, in which Cu Kα radiation was used. X-ray photoelectron spectroscopy (XPS) measurements were taken using a Shimadzu ESCA-3400 instrument with monochromatic Mg Kα radiation (1253.6 eV) used. Thermogravimetry and differential thermal analysis (TG–DTA) was carried out using a Rigaku TG8120 instrument with a heating rate of 10 °C/min under an air atmosphere.
3 Results and Discussion
The yellow color of the as-prepared reaction mixture 1 is due to the Au3+ ions from HAuCl4·4H2O. The color change of 1 from yellow to pale yellow can be explained by the reduction of Au3+ to Au+ by photoexcited titanium oxide, which is produced upon hydrolysis of titanium(IV) isopropoxide in the reaction mixture. Further, UV irradiation promotes the reduction of Au+ to Au atoms, which coalesce in the reaction mixture solution 1 to form gold nanoparticles. Then, the purple color of sample 2, which is due to the plasmon resonance absorption band of gold nanoparticles, was observed. These gold nanoparticles could be produced anywhere within sample 2; therefore, the gold nanoparticles of samples 2, 3, and 4 could be located not only “on” the surface of the bulk titanium oxide, but also “in” the titanium oxide medium.
Further systematic optimization of the reaction conditions may enable control over the diameters and densities of the embedded nanoparticles in titanium oxide media. In addition, the photocatalytic activity and photoinduced electron-transfer properties of these titanium oxide/gold nanoparticle composite powders should be evaluated; we are currently working on this investigation.
We demonstrated the low-temperature synthesis and characterization of titanium oxide/gold nanoparticle composite powders using a combination of the sol–gel process and photoinduced reduction.
This work was partially supported by the “Joint Usage/Research Program on Zero-Emission Energy Research” at the Institute of Advanced Energy, Kyoto University (ZE25B-19 and ZE26B-15). T.A. also wishes to thank the “Adaptable and Seamless Technology Transfer Program through Target-driven R&D (AS231Z00944C)” of the Japan Science and Technology Agency for its partial support of this study. The authors would like to acknowledge Professor B. Jeyadevan (The University of Shiga Prefecture) for the TG–DTA measurements.
- 7.Kuwabara T, Kuzuba M, Emoto N, Yamaguchi T, Taima T, Takahashi K (2014) Jpn J Appl Phys 53 (2 PART 2):02BE06 (6 pages)Google Scholar
- 16.Akiyama T, Nakada M, Terasaki N, Yamada S (2006) Chem Commun 395–397Google Scholar
- 23.Matsuoka J, Yoshida H, Nasu H, Kamiya K (1997) J Sol–Gel Sci Technol 9:145–155Google Scholar
- 31.Akiyama T, Matsumoto T, Oku T (2013) The 11th meeting of the Japanese sol–gel society. Hiroshima University, Japan 72 Google Scholar
- 32.Akiyama T, Sakaguchi H (2014) The 5th international symposium of advanced energy science. Kyoto University, Japan 185 Google Scholar
- 33.Matsumoto T, Akiyama T, Oku T (2014) The 17th SANKEN international symposium. Osaka University, Japan 135 Google Scholar
- 34.Akiyama T, Matsumoto T, Banya S, Oku T (2015) XVIII International Sol-Gel Conference, Kyoto, Japan P-Tu-4-18Google Scholar