Journal of Sol-Gel Science and Technology

, Volume 78, Issue 3, pp 692–697 | Cite as

Low-temperature synthesis of titanium oxide/gold nanoparticle composite powders using a combination of the sol–gel process and ultraviolet light irradiation

  • Taisuke Matsumoto
  • Tsuyoshi Akiyama
  • Shoto Banya
  • Daisuke Izumoto
  • Hiroshi Sakaguchi
  • Takeo Oku
Original Paper: Sol-gel, hybrids and solution chemistries


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.

Graphical Abstract


Gold nanoparticle Plasmon Titanium oxide Sol–gel process Photoinduced reduction Low-temperature synthesis 

1 Introduction

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 [6]. 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.

2 Experimental

The synthesis of the titanium oxide/gold nanoparticle composite is summarized in Fig. 1. All chemicals were used as received. First, HAuCl4·4H2O (Wako Pure Chemical Industries, 1 g) was dissolved in 2-propanol (Wako Pure Chemical Industries, 25 mL). Then, the HAuCl4 solution (3.3 mL), titanium(IV) isopropoxide (Tokyo Chemical Industry, 2.6 mL), 2-propanol (26.7 mL), and water (0.3 mL) were mixed to prepare the precursor solution (1) for the titanium oxide/gold nanoparticle composite. The precursor solution was stirred and irradiated with UV light from a light-emitting diode (OptoCode, LED365-SPT, 365 nm, ~17 mW/cm2) at room temperature for 3 h. The color of the reaction mixture gradually changed from yellow to pale yellow to purple during UV irradiation. Subsequently, 0.3 mL of water was added to the reaction mixture, and the mixture was stirred for 3 h under irradiation with UV light at room temperature. The process was repeated to obtain the purple solution (2). Finally, water (0.65 mL) and 2-propanol (10 mL) were added to the reaction mixture and followed by stirring for an additional 1 h without UV irradiation at room temperature. After centrifugation (6000 rpm for 30 min) of the reaction mixture and removal of the supernatant liquid, a dark purple gel was obtained. The purple gel was dried under vacuum at about 50 °C to obtain a purple powder (3). 3 was crushed and heated to 450 °C for 1 h, resulting in a dark blue powder (4).
Fig. 1

Schematic illustration of synthetic processes for the preparation of titanium oxide/gold nanoparticle composite powders

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.

Figure 2 shows the X-ray photoelectron spectra of samples 3 and 4. Clear photoelectron intensity peaks corresponding to titanium, gold, and oxygen atom regions were observed, which indicates the presence of those atoms in samples 3 and 4.
Fig. 2

XPS spectra of sample 3 and sample 4

TEM images and selected electron-diffraction patterns of samples 3 and 4 are shown in Figs. 3a–d. In the TEM image of sample 3 (Fig. 3a), high-contrast particles with diameters of several tens of nanometers were observed in a matrix of very small particles. In sample 3, the grain sizes of the gold nanoparticles were 6–32 nm. The electron-diffraction pattern of sample 3 (Fig. 3c) suggests that the dark spheres and matrix shown in Fig. 3a are gold nanoparticles and amorphous titanium oxide, respectively; this was verified by the Debye–Sherrer rings and diffuse halo ring. In the TEM image of sample 4 (Fig. 3b), similar image consisting of the dark spheres and the matrix was observed. In sample 4, the grain sizes of the gold nanoparticles were 8–56 nm. The corresponding diffraction pattern of sample 4 (Fig. 3d) also suggests that the dark spheres and matrix in Fig. 3b are gold nanoparticles and titanium oxide microcrystals, respectively; this was verified by the Debye–Sherrer rings.
Fig. 3

TEM images and diffraction patterns of (a, c) sample 3 and (b, d) sample 4

Figure 4a, b shows SEM images of samples 3 and 4, respectively. In Fig. 4a, the white spots with diameters of 10–100 nm in the magnified SEM images appear to be gold nanoparticles. Considerably fewer nanoparticles are evident on the surface of the samples when compared with the density of the gold nanoparticles in the corresponding TEM images (Fig. 3a). One of the most plausible reasons is that most of the gold nanoparticles are embedded “in” the titanium oxide medium. A similar relationship was observed upon comparing Fig. 4b and Fig. 3b.
Fig. 4

SEM images of (a, c) sample 3 and (b, d) sample 4

XRD patterns of samples 3 and 4 are shown in Fig. 5. Diffraction peaks corresponding to gold and no obvious peaks due to crystalline titanium oxide were observed in the XRD pattern of sample 3. In contrast, in the case of sample 4, diffraction peaks corresponding to crystalline gold and crystalline anatase titanium oxide were observed. These results indicate that sample 3 is an amorphous titanium oxide/gold nanoparticle composite and sample 4 consists of anatase crystalline titanium oxide/gold nanoparticles. The crystallites of the gold nanoparticles in samples 3 and 4 were 15 nm, as estimated using Sherrer’s equation and the full-width-at-half-maximum values of the corresponding XRD peaks for gold. The TEM images of samples 3 and 4 show considerably larger gold nanoparticles than the crystalline sizes of gold (i.e., 15 nm). Therefore, the larger gold nanoparticles in samples 3 and 4 must be polycrystalline gold particles.
Fig. 5

XRD patterns of sample 3 and sample 4. Closed circle, TiO2 (anatase): open square, gold

The TG–DTA results for crushed sample 3 are shown in Fig. 6. The DTA curve profile corresponds well with the TG curve. The endothermic peak and weight loss between 50 and 150 °C correspond to the evolution of water. The exothermic peaks and weight loss between 200 and 500 °C were attributed to the degradation and vaporization of organic compounds. No obvious weight loss was observed above 450 °C, which suggests that almost all the amorphous titanium oxide of sample 3 converted to anatase titanium oxide to form sample 4.
Fig. 6

TG-DTA curves obtained for sample 3. Solid line, TG: dotted line, DTA

The diffuse-reflectance spectra of samples 3 and 4 after Kubelka–Munk conversion are shown in Fig. 7. Characteristic absorption bands appear around 580 and 610 nm for samples 3 and 4, respectively. These absorption-peak wavelengths correspond to the typical plasmon absorption band of gold nanoparticles. The difference in the peak wavelengths of sample 3 and sample 4 can be explained by the differences in the sum of the refractive indices of amorphous and anatase titanium oxide around the gold nanoparticles, and by the differences in the diameters of the gold nanoparticles in samples 3 and sample 4.
Fig. 7

Reflectance absorption spectra of sample 3 and sample 4

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.

4 Conclusion

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.


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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  • Taisuke Matsumoto
    • 1
  • Tsuyoshi Akiyama
    • 1
  • Shoto Banya
    • 1
  • Daisuke Izumoto
    • 1
  • Hiroshi Sakaguchi
    • 2
  • Takeo Oku
    • 1
  1. 1.Department of Materials Science, School of EngineeringThe University of Shiga PrefectureHikoneJapan
  2. 2.Institute of Advanced EnergyKyoto UniversityGokasho, UjiJapan

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