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This article has an erratum: [https://doi.org/10.1051/epjap/210039s]


Issue
Eur. Phys. J. Appl. Phys.
Volume 94, Number 1, April 2021
Article Number 10402
Number of page(s) 4
Section Nanomaterials and Nanotechnologies
DOI https://doi.org/10.1051/epjap/2021200330
Published online 20 April 2021

© EDP Sciences, 2021

1 Introduction

Noble metals, such as Pt, Ag, and Au are widely investigated as catalysts on account of strong molecular absorption and electron transfer due to unfilled d electrons [119]. Nanosize noble metal such as nanoparticles can further improve the catalytic activity because of the enhanced specific surface area [15, 1219]. Surface plasmon resonance of such nanoparticles can offer applications such as bio-sensors in addition to the catalytic utilization [1, 5, 2026]. A reliable and low-cost method is required for available nanoparticles used as convenient and efficient catalysts and bio-sensors.

Normally, noble nanoparticles work together with porous carriers to effectively prevent the activity decline or deactivation and improve the dispersibility [14, 1417, 19]. Because of good chemical and thermal stability, aluminiferous materials such as alumina are the most commonly used carrier for noble nanoparticles [13, 15]. However, the carrier and nanoparticles are generally prepared separately and composited together, which complicates the fabricating process and pushes the cost up [14, 17, 19].

In this work, we demonstrated a one-step method for synthesizing gold nanoparticles (GNPs) with their carrier. Here, an unusual hydrothermal (HT) method was employed. As Al/Au bilayer films were directly immersed in pure hot water, the Au layer was reformed to be GNPs, while the Al layer was transformed to be boehmite with a special density-gradient porous structure [27]. The GNPs embedded in the boehmite carrier demonstrated uniform size and high dispersibility. The structure and optical characteristics of the GNPs and carrier were analyzed systematically.

2 Experimental

The as-deposited Al/Au bilayer films (Fig. 1a) were prepared on corning #1737 glass by an rf magnetron sputtering system. Both Al and Au layers were deposited in a pure Ar environment from high-purity metallic targets. The work pressure and rf power were fixed to be 1.33 Pa and 50 W respectively. The average thickness of the Al layer was fixed to be 100 nm for all samples, which ensure that the Al film can be reacted with water completely. The thickness of the Au layer varied from 1.5 to 3.7 nm by changing the deposition time from 2 to 5 s.

The HT-treatment in this work is simple and different from the traditional method. The as-deposited films were just HT-treated in hot water near the boiling point at 368 K. The water has high purity with a resistivity of more than 18.2 MΩ ⋅ cm. After HT-treated for 20 minutes, the Al film was reacted completely with water to be boehmite and Au film was reformed to be nanosize particles embedding in the boehmite carrier, as shown in Figure 1 [27]. X-ray diffraction, scanning electron microscope (SEM), transmission electron microscope (TEM), and ultraviolet-visible-near infrared spectrometer were used to evaluate the structure and optical characteristics of GNPs and their carrier.

thumbnail Fig. 1

(a) and (b) are the schematics of Al/Au bilayer films before and after the HT-treatment. After the HT-treatment, the Al/Au film becomes a plumose structure with embedding GNPs. (c) and (d) are the cross-section TEM images of Al/Au bilayer films before and after the HT-treatment.

3 Results and discussion

Figure 1c shows the cross-section image of the as-deposited Al/Au film. The Al film is layered with a uniform thickness of about 100 nm. Au film with a thickness of several nanometers is also continuously layered, which implies that surface adhesive force between metallic Al and Au are stronger than the adatom cohesive force of Au [18]. Such layer-by-layer growth Au is good for improvement of the size uniformity and dispersibility of GNPs synthesized by the HT-treatment. As Figure 1d shows, after the HT-treatment, the Al layer grew to be a density-gradient structure with a porous bottom layer of about 100 nm and a plumose surface layer of more than 300 nm. Such a self-organizing structure is consistent with the HT-treated Al monolayer film, which has been identified to be boehmite in our previous work [27]. On the other hand, the Au layer did not react with water and was reformed to be nanoparticles embedding in the boehmite at the same distance away from the glass substrate.

Figure 2a shows the XRD patterns of as-deposited Al film and HT-treated Al/Au films with different thicknesses of Au layers. For as-deposited Al film, the positions and intensities of strongest peaks well agree with powder diffraction patterns of Al [28], which suggests that Al film is polycrystalline without perfect orientation. For as-deposited Al/Au films, similar XRD patterns can be gained, in which the contribution of Au can not be distinguished because the Au layer is much thinner than the Al layer and has the same unit cell structure with very close lattice constant [29]. After the HT-treatment, Al was transformed to be porous boehmite and peaks attributed to Al disappeared [27]. Meanwhile, peaks belong to Au can be observed instead because it does not react with water. The intensities of those Au peaks increase with increasing thickness of Au film and the full width at half maximum is large indicating that the size of those GNPs is small.

Figure 2b shows the SEM images of the as-deposited Al film and HT-treated Al/Au bilayer films with various thicknesses of Au layers. There is almost no contrast in the SEM image for as-deposited Al film, which suggests that the surface is smooth. After the HT-treatment, all samples exhibit similar surface structures. The thicknesses of the structures are slightly different for samples with different Au layer thickness.

The optical characteristics of GNPs together with their carrier are shown in Figure 3. Due to the density-gradient structure of HT-treated films, all samples show high transparency, the transmittance of which are even higher than that of the glass substrate itself [27]. Such a feature is beneficial to optical applications, which improves the utilization of light. In addition, the HT-treated samples with the Au layer exhibit characteristic dips in the transmittance spectra (Fig. 3a) and corresponding peaks in the absorption spectra (Fig. 3b) at visible wavelength, which can be attributed to the surface plasmon resonance of GNPs [20]. The intensities of absorption peaks increase with increasing thickness of the Au layer. Comparing with samples without the boehmite carrier (Fig. 3c), the absorption peaks of GNPs with boehmite carrier are sharper. This implies the better size uniformity of GNPs with boehmite carrier, which can also be observed in Figure 1d.

Figure 4 shows the positions and intensities of absorption peaks of Au with and without the boehmite carrier. The position of the absorption peak represents the average size of GNPs [30]. For GNPs with boehmite carrier, the absorption peaks tend to appear at shorter wavelengths than those of GNPs without boehmite carrier, which implies that GNPs are smaller in size with boehmite carrier (Fig. 4a). The wavelengths of absorption peak positions increase with increasing of the Au layerthickness for all samples with and without the boehmite carrier. Compared with those samples without boehmite carrier, the peak position of samples with boehmite carrier change gently with Au layer thickness increasing, especially when the Au layer is thinner than 3 nm. This suggests that GNPs synthesized by the HT-treatment are better in size stability while the Au layer thickness is changed. The absorption peak intensities exhibit a similar trend for samples with and without the boehmite carrier, which linearly increases with the Au thickness increasing (Fig. 4b). It is possible to increase the density and keep the size of GNPs in the HT-treated sample by increasing the layer number of the as-deposited film in order to match the requirement of practical applications as catalysts and bio-sensors.

thumbnail Fig. 2

(a) X-ray diffraction patterns of as-deposited Al film and HT-treated Al/Au films with different thicknesses of Au layers. (b) The surface SEM images of the samples corresponding to XRD patterns.

thumbnail Fig. 3

(a) The wavelength-dependent transmittance of HT-treated films on the glass substrate. (b) The wavelength-dependent absorption of HT-treated films on the glass substrate. (c) The wavelength-dependent absorption of as-deposited Au films on the glass substrates with various thicknesses.

thumbnail Fig. 4

Peak positions (a) and intensities (b) of surface plasmon resonance for HT-treated Al/Au bilayer films and Au monolayer films with various Au layer thickness.

4 Conclusion

In this work, we demonstrate a method to synthesize the GNPs carried by boehmite by HT-treating the Al/Au bilayer films in pure water. The boehmite carrier exhibits a porous and density-gradient structure, which is suitable for the reaction of solution and can improve the transmittance of light. The GNPs synthesized by this one-step method demonstrate good size uniformity and strong surface plasmon resonance, which have great potential in the applications of catalysts and bio-sensors.

Author contribution statement

All the authors were involved in the preparation of the manuscript. All the authors have read and approved the final manuscript.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No.11874098), the LiaoNing Revitalization Talents Program (Grant No.XLYC1807156), and the Fundamental Research Funds for the Central Universities (DUT20LAB111).

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Cite this article as: Mengyi Wang, Rongxin Sha, Ziyang Zhang, Ailiang Zou, Yuekui Xu, Min Liu, Yibo Peng, Zhiyong Qiu, One-step synthesis of gold nanoparticles carried by boehmite, Eur. Phys. J. Appl. Phys. 94, 10402 (2021)

All Figures

thumbnail Fig. 1

(a) and (b) are the schematics of Al/Au bilayer films before and after the HT-treatment. After the HT-treatment, the Al/Au film becomes a plumose structure with embedding GNPs. (c) and (d) are the cross-section TEM images of Al/Au bilayer films before and after the HT-treatment.

In the text
thumbnail Fig. 2

(a) X-ray diffraction patterns of as-deposited Al film and HT-treated Al/Au films with different thicknesses of Au layers. (b) The surface SEM images of the samples corresponding to XRD patterns.

In the text
thumbnail Fig. 3

(a) The wavelength-dependent transmittance of HT-treated films on the glass substrate. (b) The wavelength-dependent absorption of HT-treated films on the glass substrate. (c) The wavelength-dependent absorption of as-deposited Au films on the glass substrates with various thicknesses.

In the text
thumbnail Fig. 4

Peak positions (a) and intensities (b) of surface plasmon resonance for HT-treated Al/Au bilayer films and Au monolayer films with various Au layer thickness.

In the text

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