Green reduction of the nanoparticles on the surface of the zinc hydroxide layered by a novel one-pot method

Fernando Júnior Quites1, Natasha Maira de Souza Pereira1, Everton Willian Oliveira1, José Carlos Germino2, Ailton José Terezo1

1Universidade Federal de Mato Grosso, Department of Chemistry, Mato Grosso, Brazil 2Universidade Estadual de Campinas, Instituto de Química, SP, Brazil e-mail: [email protected] or [email protected]

Abstract: In this present study a reproducible and facile approach to synthesis of nanocomposites based on gold metal nanoparticles (AuNPs) and layered compound is reported. Thus, zinc hydroxide-layered (here named as ZHL-layered), a layered inorganic material, was used as host for the deposition of the gold nanoparticles. We demonstrated that the presence of the ZHL-layered in chloroauric solution, at room temperature, is able in situ reduce Au3+ ions to the AuNPs, where ethanol was used as solvent and as reducing agent. Au3+ ions were reduced by solvent leading to production of AuNPs and formation to the acetaldehyde. Regarding the spectroscopic properties of the Au-ZHL materials produced, the UV-vis diffuse reflectance spectroscopy presented plasmon ressonance band at 532 nm region attributed to the spherical morphology of AuNPs. Transmission electronic microscopy further supported these observations. This article shows that Au3+ ions can be easily reduced, without the use of external reducing agents, in the presence of the ZHL-layered being important contribution to the synthesis of novel nanocomposites.

Keywords: gold nanoparticles, layered materials, in situ reduction, nanocomposites

Introduction Synthesis of AuNPs with homogeneous sizes and shapes has enormous importance in nanotechnology, because of their size-dependent optical, magnetic, electronic and catalytic properties [1,2]. Although various physical and chemical methods have been developed for nanoparticle synthesis, the major challenges remain use of new routines for the reduction from gold ions precursors to AuNPs without using of the external reducing agents and new strategies to avoid agglomeration of these nanoparticles in the state due removal of the stabilizing ligand resulting the loss stability and aggregation of the particles [2]. Thus, in this present study a facile one-step synthetic was used to in situ reduction from Au3+ ions to gold nanoparticles and its dispersion on the external surfaces of the zinc hydroxide-layered in nitrate form (here named as ZHL-layered). Zinc hydroxide-layered is a double layered salt presenting excellent adsorption ability [3], which is especially interesting for the impregnation of nano-size metals in the interlamelar space or on the external surfaces. Therefore, it was demonstrated that gold nanoparticles can be rapidly formed when ZHL-layered is added to the ethanol chloroauric acid solution. In this system the ethyl alcohol acted as a solvent of the gold precursor and as a reducing agent, where the Au3+ ions were mainly reduced by redox reaction between the metallic precursor and the solvent [2]. The AuNPs produced were adsorbed on the ZHL-layered support. The best our knowledge this is the first study that deals with the in situ and one-pot reduction of AuNPs in the presence of a layered compound and ethyl alcohol, without the use of an external reducing compound.

Experimental Procedure Preparation of zinc hydroxide-layered in nitrate form (ZHL-layered) was adopted from our recent work [3]. The in situ reduction procedure for nanocomposite formation was realized at room temperature. A portion of ZHL-layered (about 0.5 g) was added into 20 mL of ethanol chloroauric acid (HAuCl4) solution at different concentration. This suspension was stirring vigorously for 24 h, instantly, the color suspension changed from yellow to pink. The suspension was then washed with ethanol, filtered and dried gently at room temperature. The composites obtained were named as Au(1)-ZHL, Au(2)-ZHL and Au(3)-ZHL, where the number in parentheses indicated the amount of gold used in the synthesis: 0.05 , 0.10 and 0.20 mmol, respectively.

Results and Discussion The first evidence to the in situ reduction of AuNPs was changed color of the HAuCl4 yellow ethanolic solution with addition of the ZHL-layered compound. Immediately, this suspension turned to pink indicating the formation of Au0. To further study the structural features of Au-ZHL nanocomposites, a series of Au-ZHL materials synthesized under different original weight ratios of HAuCl4 to ZHL-layered at 1:10, 1.6:10 and 3.2:10 were carried out, and the XRD patterns were obtained (not shown). The profile of the pristine ZHL- layered where the characteristic diffraction peaks with 2θ at 8.96 and 18.1, were indexed to (200) and (400) lattices of inorganic layered compound (JCPDS card 24-1460). These same diffraction peaks are clearly observed to the Au-ZHL nanocomposites indicating the preservation of the layered structural features of pristine ZHL-layered after the in situ reduction of AuNPs. The basal spacing of ZHL-layered (d200 = 0.98 nm) did not change upon the reduction reaction with AuNPs suggesting that they are localized on the external surface of the support. It can be also seen that for the Au(3)-ZHL nanocomposite the new peaks appear at 2θ= 38.2° and 2θ = 44.4° corresponding to (111) and (200) planes, respectively, of the cubic phase of Au [1,2]. From elemental analysis (C, H and N) was further observed to decrease the amount of nitrate ions in the Au-ZHL composites, indicating exchange from - - NO3 to [AuCl4] , which is reduced to AuNPs releasing chloride ions. Thermal analysis (not shown) also supported these observations. UV-vis diffuse reflectance spectra of Au-ZHL powders at different Au loadings are shown in Figure 1 where a broad absorption band with λmax at 532 nm is observed related to the absorption of spherical-like morphology of the AuNPs [1,2]. This clearly demonstrates the formation and deposition of nanoparticles in the surface of inorganic support. The UV-band at 292 nm, related to the molecular absorption of nitrate groups [3] in ZHL-layered, was - - decreased in the nanocomposites suggesting exchange from NO3 to Cl with AuNPs formation. By the SEM images (not shown) the surface morphology of Au(1)-ZHL was identical to that of ZHL-layered. To study the existence of AuNPs in the ZHL-layered matrix, the HRTEM characterization for the Au-ZHL nanocomposites were also carried out and the HRTEM image is shown in Figure 1e. It is clearly observed that AuNPs are uniformly distributed on the external surface of inorganic host. The gold nanoparticles presented spherical morphology, as suggest by the plasmon resonance band, and particle size of the Au(1)-ZHL nanocomposite was estimated to be around 5-10 nm from the HRTEM image shown in Figure 1e.

Figure 1: UV-vis diffuse reflectance spectra of the (a) ZHL-layered and of the nanocomposites at different gold concentration: (b) Au(1)-ZHL, (c) Au(2)-ZHL and (d) Au(3)-ZHL and (e) TEM image of Au(1)-ZHL (Scale bar: 100 nm). Inset shows photographs of the corresponding powders before and after to the deposition of AuNPs.

Conclusions In summary, we demonstrated that ZHL-layered can be decorated with gold nanoparticles by a spontaneous redox reaction between the ethanol solvent and tetrachloroauric acid in presence of inorganic layered compound. This way, the resulting gold nanoparticles tend to grow at surface of the sheet of ZHL-layered. The basal spacing of ZHL- layered after the reduction of AuNPs did not change indicating that the nanoparticles are deposited on the external surfaces of inorganic support (ZHL-layered). Au-decorated ZHL- layered sheets could become a base material for exploring surface-plasmon-enhanced photo- and electrocatalysts for the oxidation of organic molecules. Our research group has investigated the reduction of other metal precursors, such as silver and palladium, and further investigations are in progress.

Acknowledgments We gratefully acknowledge financial support from FAPEMAT, CNPq and INEO for a fellowship. We also thank the Dr. Douglas Siqueira by acquisition of the HRTEM images.

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