Gold nanoparticles supported on Al2O3 using Tabebuia aurea leaf extract and catalytic properties for methylene blue reduction

In this research, we reported a cost-effective and an environmentally friendly technique for synthesis of gold nanoparticles supported on alumina (Al2O3/AuNPs) using Tabebuia aurea leaf extract (TAE) as non-toxic reducing agent, efficient stabilizer, and weak base source without adding any surfactants. Al2O3/AuNPs was characterized using various techniques including UV-Visible Spectrophotometer, Fourier-transform Infrared (FT-IR) spectroscopy, X-Ray diffraction (XRD), and Particle Size Analyzer (PSA). AuNPs was successfully synthesized using HAuCl4 as precursor. The characterization of UV-Vis Spectrophotometer shows that AuNPs colloidal was formed at 0.15 % TAE. FTIR characterization of Al2O3 shows Al-OH band at 1505 cm−1 and Al-O band at 680 cm−1. Phase of Al2O3 was amorphous confirmed by XRD. The particle size distribution average of Al2O3/AuNPs was about 43 nm. The reduction percentage of methylene blue using Al2O3/AuNPs catalyst was 70.41 % for 120 min.


Introduction
Aluminum oxide (Al2O3) is a well support catalyst material due to its inert properties, good thermal stability, capable to bind with metals as catalyst, has high surface area and evenly dispersed pores on the surface [1][2][3][4]. On the other hand, gold nanoparticle (AuNPs) has important roles in several catalytic processes such as in reduction of nitro-aromatic compounds and azo dyes, low temperature CO oxidation, and organic synthesis [5][6][7][8]. However, the use of metal nanoparticles like AuNPs can cause some problems such as homogeneity, reusability, and catalyst separation from the system [9]. Thus, some previous researchers have conducted many studies of metal nanoparticle immobilization on the support material [10][11]. This result is in line with the property of Al2O3 as a support catalyst. AuNPs may enhance the catalytic activity of Al2O3 due to its active site. The immobilization of AuNPs on Al2O3 surface can convert AuNPs phase from colloidal into solid phase [12].
Tabebuia aurea is one of the medicinal plants in tropical countries like Indonesia [13]. This plant has secondary metabolite compounds that can be used to synthesize the nanoparticle. Tabebuia aurea leaf extract have been previously reported to contain alkaloid, flavonoid, polyphenols and saponins compounds which act as a weak base source and stabilizing agent in nanoparticles formation [14]. Green synthesis method has been widely used for both metal and metal oxide synthesis. In previous study, we prepared both metal and metal oxide by green synthesis approach using plant [15][16][17][18][19].
To the best of our knowledge, we reported about cost-effective and environmentally friendly synthesis and immobilization of AuNPs on Al2O3 surface (Al2O3/AuNPs) using TAE. The catalytic activity of Al2O3/AuNPs was observed in methylene blue reduction.

Preparation of leaf extract
Dried powder of Tabebuia aurea (TA) leave was added into methanol of 300 mL and macerated for 7 days at room temperature. The result was then partitioned by n-hexane. The methanol fraction was evaporated then added by distilled water to obtain TA extract (TAE) for nanoparticles fabrication.
2.3. Au NPs synthesis 0.05, 0.10, and 0.15% of TAE (w/v) were added dropwise into 1.0 x 10 -4 M HAuCl4 until forming a violet colloidal. The colloidal was further characterized using a UV-Vis spectrophotometer.
2.4. Al2O3 synthesis 0.15% of TAE was added dropwise into 0.03M Al(NO3)3 solution in volume ratio (1:3) and stirred continuously at 80 °C for 5 h to form a white colloid. The colloid was heated at 100 °C for 1 h and annealed at 500 °C for 4 h to obtain white powder of Al2O3.

Al2O3 /AuNPs synthesis
Al2O3 powder was dispersed into 30 mL of 1.0 x 10 -4 M HAuCl4 with sonication for 20 min. TAE 0.15% (w/v) was added dropwise into the previous mixture and stirred continuously for 100 min at room temperature. The dispersed system was annealed at 500 °C for 4 h to obtain the violet powder of Al2O3/AuNPs. 2.7. Catalytic activity analysis 5 mg of Au/Al2O3 NPs was added into the mixture of 5 mL of 2.0 × 10 −5 M methylene blue and 0.1 M NaBH4. The catalytic activity was observed by measuring the absorbance change using UV-Vis spectrophotometer for 120 min.

Optimization of AuNPs
Synthesis of AuNPs was varied in TAE concentrations of (0.05, 0.10 and 0.15) %. It aims to observe the effect of TAE concentration in AuNPs formation. The formation of AuNPs is signed by the presence of absorption peak at the maximum wavelength of 500-550 nm due to the SPR peak formation [20,21]. A color change of colloid occurs from yellow to violet in the AuNPs formation. AuNPs formation can be described by UV-Vis absorption spectra as shown in figure 1. 0.15 % TAE was the optimum concentration in AuNPs formation. Since this concentration resulted the absorption peak at the smallest maximum wavelength of 540 nm called by blue shift region. It has the conformity with the previous research that the maximum wavelength shift at blue region will produce the small size of nanoparticle [22]. Furthermore, 0.15 % TAE was used as optimum concentration for Al2O3/AuNPs fabrication. The particle size of AuNPs was further observed by PSA characterization.

FT-IR spectroscopy and XRD analysis
Functional groups of TAE, Al2O3, and Al2O3/AuNPs were identified by FT-IR as shown in figure 2a. TAE has broad vibrations band at 3318.7 cm -1 corresponds to -OH stretching of polyphenols, flavonoids and saponins that act as capping and reducing agents in AuNPs formation [19,22,23]. The vibrations band at 1384.0 cm -1 assigns to C-N stretching from alkaloids compound that plays a role as weak base source for the synthesis of metal oxide nanoparticle [15]. Figure 2 shows that both Al2O3 and Al2O3/AuNPs reported the typical Al-O stretching vibration at wavenumber of 715.6 and 713 cm -1 , respectively. Based on figure 2b, XRD pattern of Al2O3 shows no sharp peak, indicating the amorphous phase. The amorphous form of Al2O3 has an excellent performance on catalysis due to its better adsorption ability [24].

Particle size analyzer
PSA characterization was performed to determine the average of particle size distribution of Al2O3/AuNPs as presented in figure 3. Figure 3 shows that Al2O3 and Al2O3/AuNPs were distributed at particle size average of 37.76 and 43.29 nm, respectively. These results demonstrate that both of Al2O3 and Al2O3/AuNPs are nanoparticles.   Figure 4 shows the reduction percentage of MB calculated from the reduction of MB with NaBH4 through the observation of the fading blue color and the decrease of its absorbance. MB was reduced to be a leuco methylene blue (LMB) presented by a new absorption as shown in figure 4a. The formation of LMB occurs at the wavenumber of 300 nm [25]. The reduction percentage of MB using Al2O3 and Al2O3/AuNPs are 60.26 % and 70.41 %, respectively, as shown in figure 4b. The presence of AuNPs can accelerate the reduction of MB solution. AuNPs plays important role in electron transfer from BH4 -as a donor to MB as an electron acceptor. BH4 -are nucleophilic, while MB are electrophilic [26].