Valorization of rambutan peel for the synthesis of silver-doped titanium dioxide (Ag/TiO₂) nanoparticles

2.1 Materials and synthesis of Ag/TiO₂ NPs

Fresh Rambutan fruit was purchased from the popular market near Universidad de las Fuerzas Armadas ESPE, Sangolqui, Ecuador. After being washed thoroughly, Rambutan fruit peel (5 g) was cut finely and heated (60–62°C) in 50 ml of deionized water for 60 min. After cooling, the dark red color RPE was filtered using Whatman No. 1 paper, England and stored at 4°C for further use. TiO₂ (99%, anatase) and DPPH· (>99.5%) were purchased from Sigma Aldrich, USA. Silver nitrate (AgNO₃, 99.0%) as the source of Ag⁺ ion and MB (99.5%) were purchased from the Spectrum, USA. Milli-Q water was used in all experiments. The Ag/TiO₂ NPs were synthesized based on the reduction of AgNO₃ in a TiO₂ suspension. RPE (2 ml) was sonicated with ultrasonic processors (DAIGGER GE 505, 500 W, 20 kHz) for (5 min, 72%, 30 s pulse on/off) immersed directly into a 12 ml solution of AgNO₃ (1 mm, 10 ml) and TiO₂ (5 mm, 2 ml), then stirred at room temperature (22–25°C) for 24 h until the color of the reaction mixture changed from turbid white to light brown.

2.2 Photocatalytic degradation activity

In order to evaluate the photocatalytic performances of Ag/TiO₂ NPs, photocatalytic degradation of MB under solar light irradiation (850–936 cd/m²) was performed. In the reaction set 1, 10 ml of MB (10 mg/l) was mixed with 2 ml H₂O and in another set 2, 10 ml MB (10 mg/l), Ag/TiO₂ NPs (250 μl) and H₂O (1750 μl) were mixed vigorously for 10 min in the dark to reach an adsorption-desorption equilibrium. Then, both sets were kept in the direct sunlight (solar light) and progress of the reaction was monitored at different time intervals. Finally, the rate of MB dye degradation was monitored by measuring the absorption of MB in the filtrate at the wavelength 664 nm, before and after degradation using a UV-vis absorption spectrometer. Photocatalytic degradation percentage of MB was calculated using Eq. (1):

where η is the rate of degradation of MB in terms of percentage, A₀ is the initial absorbance of the dye solution and A_t is the absorbance of the MB at time t.

2.3 Antioxidant activity

The antioxidant activity of the Ag/TiO₂ NPs was measured by the DPPH· assay using a method by Kumar et al. [16], [17] with slight modification. The DPPH· is a stable free radical with purple color and its activity is inhibited due to donation of electrons by an antioxidant molecule. An aliquot (1000–200 μl) of Ag/TiO₂ NPs or control and (1000–1800 μl) of H₂O was mixed with 2000 μl of DPPH· (0.2 mm in 96% ethanol). The mixture was vortexed vigorously and allowed to stand at room temperature for 30 min in the dark. Absorbance of the mixture was measured spectrophotometrically at 517 nm, and the free radical scavenging activity was calculated using Eq. (2):

The scavenging percentages of all samples were plotted. The final result was expressed as % of DPPH· free radical scavenging activity (ml).

2.4 Characterization of Ag/TiO₂ NPs

The UV-vis spectrum was measured using a Thermo Spectronic, GENESYS 8, England spectrophotometer. Size and selective area electron diffraction (SAED) patterns of nanoparticles were studied on TEM (FEI, Tecnai, G2 spirit twin, Holland). The particle size distributions of nanoparticles were determined using the Horiba, DLS Version LB-550 program, Japan. XRD studies on thin films of the nanoparticles were carried out using a PANalytical brand θ–2θ configuration (generator-detector) X-ray tube copper λ=1.54 A° and an Empyrean diffractometer. FTIR-ATR spectra were recorded on a Spectrum Two IR spectrometer (Perkin Elmer, USA).

3.1 Visual and UV-vis analysis

As shown in Figure 2, reduction of the Ag⁺ to AgNPs on the surface of TiO₂ using RPE could be followed by the appearance of a brown color, due to excitation of plasmon resonance in AgNPs [1], [2], [16]. Before doping, TiO₂ did not show any significant absorption peak in the range of 350–500 nm, but after doping with Ag⁺, the UV-vis spectra showed an increase of absorbance and appearance of new peaks in the range of 370–460 nm (showed a red shift). The red shift of the absorption curve results in a reduction of the band gap energy and also the recombination rate, and hence, enhanced photocatalytic activity. These results agreed with the observation reported by Gupta et al. [18].

Figure 2:

UV-vis spectrum of silver-doped titanium dioxide nanoparticles (Ag/TiO₂ NPs) synthesized by rambutan peel extract (RPE).

3.2 TEM and DLS analysis

The approximate size, shape, and size distribution of particles were characterized using TEM and DLS. The TEM image (Figure 3A) of TiO₂ particles was found to be almost spherical in shape and about 100–180 nm, respectively. Figure 3B and C show that the Ag/TiO₂ NPs have a strong interaction with the biomolecules of the RPE and the majority of Ag/TiO₂ NPs were agglomerated in addition to small Ag particles. From the morphological point of view, Ag/TiO₂ NPs showed a spherical shape with a size range of 30–200 nm, and Ag particles (black dots indicated) were located on the surface of the individual TiO₂ NPs (size of Ag<TiO₂). This result is very similar to the Ag loaded TiO₂ NPs reported by Anandan et al. [19]. The SAED patterns (Figure 3C′) show circular diffraction patterns, which are indicative of the crystalline nature of the Ag/TiO₂ NPs, and are in good agreement with the XRD data. The DLS size distribution of Ag/TiO₂ NPs (Figure 3D) showed that the average size of Ag/TiO₂ NPs is 163.3±249.2 nm, which is a slight deviations from the results obtained from the TEM analysis. This is due to the agglomeration of NPs, screening of small particles by bigger ones and attachment of RPE biomolecules on the surface of Ag/TiO₂ NPs [20].

Figure 3:

Transmission electron microscopy (TEM) micrograph of (A) titanium dioxide nanoparticles (TiO₂ NPs), (B, C) silver-doped titanium dioxide nanoparticles (Ag/TiO₂ NPs), (C′) selective area electron diffraction (SAED) pattern of Ag/TiO₂ NPs and (D) dynamic light scattering (DLS) size distribution pattern of Ag/TiO₂ NPs.

3.3 XRD and FTIR analysis

The XRD patterns of the as-synthesized Ag/TiO₂ NPs are shown in Figure 4A. The peaks located at 2θ values of 25.295°, 37.943° and 48.085°, can be indexed to (101), (112) and (200) diffractions of the anatase phase of TiO₂ (ICSD No. 98-002-4276). The diffraction peaks originating from Ag are hardly detected in the Ag/TiO₂ NPs, although the XRD peak at 37.9° is a common diffraction peak for both counterparts and Ag peaks overlapped with those of TiO₂. It also suggests a successful reduction of Ag⁺ into Ag nanoparticles on the surface of the TiO₂ crystallites [21], [22]. Another peak observed at 29.847° is due to the low crystallinity of the organic biomolecules and the broadened diffraction peaks indicate that the sizes of the Ag/TiO₂ NPs are small. The FTIR spectra provided information on the nature of the produced Ag/TiO₂ NPs. As shown in Figure 4B, the absorption peaks at 3270 cm^-1 and 2924 cm^-1 are attributed to the stretching mode of hydroxyl O-H and aliphatic C-H groups, respectively [15]. The absorption peaks at 1633 cm^-1 corresponds to bending vibration of H-O-H bond on TiO₂ [23] and C=O stretching vibration, whereas the peaks at 1417 cm^-1, 1318 cm^-1 and 1245 cm^-1 indicate the C=C and C-O stretching vibrations of flavonoids/phenolic groups of RPE [16]. The high intensity at 1030 cm^-1 corresponds to secondary -OH vibrations and the broad spectrum below 700 cm^-1 reveals the existence of metal oxide vibrations, including Ag-O, Ti-O and Ag-O-Ti [23]. The XRD and FTIR results confirm the formation of the Ag/TiO₂ NPs using RPE.

Figure 4:

(A) X-ray diffraction patterns and (B) Fourier transform infrared (FTIR) spectrum of silver-doped titanium dioxide nanoparticles (Ag/TiO₂ NPs).

3.4 Photocatalytic activity

The photocatalytic activity of Ag/TiO₂ NPs is evaluated by the degradation of MB in aqueous solution. The prominent peak intensity of MB (λ_max=664 nm) decreases gradually with the addition of a very low quantity of Ag/TiO₂ NPs under sunlight and shows more than 81% degradation within 600 min (Figure 5A). Pure TiO₂ has low photocatalytic activity under solar light, due to its wide band gap. So, Ag/TiO₂ NPs improved the photodegradation efficiency compared to pure TiO₂. The rate of MB degradation catalyzed by Ag/TiO₂ NPs assumed to be fitted by a first order rate law [15], shows a linear relationship between ln (A₀/A_t) and reaction time (t) and the slope gives the rate constant (K) (Figure 5B). The observed “K” is 0.00249545 min^-1. The basic mechanism behind the degradation of MB is that photogenerated limited charges can generate ·OH radicals and O₂· intermediate species by reacting with adsorbed H₂O molecules for the photodecomposition of MB [24]. Ag can act as an electron trap and promote the interfacial charge transfer processes in the composite systems, which reduces the recombination of the photo-induced electron-hole pairs, thus improving the photocatalytic activity of TiO₂ [25].

Figure 5:

(A) Solar light-induced photocatalytic degradation and (B) first order kinetic plot of methylene blue (MB) using the silver-doped titanium dioxide nanoparticles (Ag/TiO₂ NPs).

3.5 Antioxidant activity

Antioxidant activity refers to the inhibition of the reactive oxygen species including hydroxyl radicals, peroxyl radicals, super oxide radicals, hydrogen peroxide, singlet oxygen, and various lipid peroxides, that play an important role in the initiation and progression of various diseases [26]. In Figure 6, it was found that the DPPH· scavenging activity of RPE decreased less significantly in comparison to Ag/TiO₂ NPs with increasing doses and also demonstrated a higher antioxidant activity than Ag/TiO₂ NPs. The maximum scavenging efficacies for the RPE and Ag/TiO₂ NPs were found to be 91.80% and 70.65%, respectively, in 0.2 ml. This is due to the plant extract itself being responsible for the majority of the antioxidant activity, whereas, Ag/TiO₂ NPs are less soluble and undergo agglomeration and sedimentation, which causes an increase in the particle size and decrease in specific surface area. In general, chemically synthesized nanoparticles are not contributing much to the antioxidant activity, but Ag/TiO₂ NPs showed antioxidant efficacy, which may be attributed due to the adsorption of biomolecules of RPE over spherically shaped Ag/TiO₂ NPs [16], [27].

Figure 6:

Antioxidant activity of (A) rambutan peel extract (RPE) and (B) silver-doped titanium dioxide nanoparticles (Ag/TiO₂ NPs).