Visible light-responsive photocatalyst of SnO₂/rGO prepared using Pometia pinnata leaf extract

2.1 Materials

The chemicals utilized for this research consisted of methylene blue (MB), graphite flake, SnCl₂·2H₂O, citric acid, KMnO₄, H₂O₂, and H₂SO₄ (Merck-Millipore). The Pometia pinnata leaf was obtained from a tree cultivated in Sleman District, Yogyakarta Province. The PPE extract was obtained by boiling 10 g of PP dry in 100 mL aquadest followed by filtering using Whatman 41 paper.

2.2 Preparation of GO and rGO

Graphene oxide (GO) was prepared using Hummers’ method using graphite flake as a raw material. Graphite flake (2 g) was added to 46 mL of concentrated H₂SO₄ (in an ice bath) followed by the addition of 6 g of KMnO₄. The resulting mixture was stirred for 2 h at room temperature followed by increasing the temperature to 35°C. 92 mL of deionized water was added slowly, and then 10 mL of H₂O₂ (30 wt%) was added slowly with the appearance of vigorous bubbles and a change in the color of the suspension from dark brown to yellow. The suspension was aged to settle, and the clear supernatant was decanted. The remaining suspension was filtered and washed with acetone and was then dried in an oven overnight at 40^oC.

Reduction onto GO to produce rGO was performed by mixing 90 mL PE extract with 25 mg GO. The mixture was refluxed for 48 h at 100^oC. The mixture was then aged for 24 h for settling and then decanted and filtered.

2.3 Preparation of SnO₂/rGO

SnO₂/rGO was prepared by mixing rGO with a SnCl₂ solution at the theoretical Sn content of 20 wt%. The mixture was aged in an autoclave overnight before being filtered, dried, and calcined at 400°C for 2 h.

2.4 Physicochemical characterization

The physicochemical characterization to investigate the chemical change during synthesis was performed by X-ray diffraction (XRD), Fourier-transform infrared (FTIR), scanning electron microscope-energy dispersive X-ray (SEM-EDX), and a transmission electron microscope (TEM). A Rigaku Miniflex-600 (Tokyo, Japan) XRD instrument was employed for the phase change analysis using a Ni-filtered Cu-Kα source with the voltage of 40 kV and a 20 mA current. FTIR Spectroscopy was carried out using Perkin Elmer–ATR (Singapore). X-ray Photoelectron spectra were recorded using a PHOIBOS 225 (Specs GmbH) spectrometer. The surface morphology was studied using FESEM taken on a JX-JEOL electron microscope, while the TEM was recorded using the JSM-JEOL (Tokyo, Japan).

2.5 Photocatalytic activity test

The evaluation of the photocatalytic activity of the materials was carried out for MB photodegradation. Each experiment was conducted with the photocatalyst dosage of 0.5 g/L and the MB concentration of 20 mg/L. The photocatalytic reaction was conducted for a duration of 90 min, while the samples were collected at the predetermined 0, 5, 15, 30, 45, 60, and 90 min. The collected samples were centrifuged before colorimetric analyzation using the UV-Visible spectrophotometry method. The varied photocatalytic conditions consist of the effect of the light source (UV and Visible light), and the effect of the H₂O₂ addition was investigated. For the reaction using H₂O₂, particularly 1 mL H₂O₂, 10⁻³ M was added to the solution. The degradation efficiency (DE) of each treatment was calculated using the following formula (equation 1):

C ₀ and C ₉₀ are the initial concentrations of the MB concentration after treatment for 90 min.

1. Ethical approval: The conducted research is not related to either human or animal use.

3.1 Material characterization

The GO reduction was initially monitored by recording the UV-vis absorption spectroscopy analysis. The comparison of the GO and the rGO spectra is shown in Figure 1. GO exhibited an absorption peak at 230 nm, which attributed to the π–π* transitions of the aromatic C═C bonds, and there were no other peaks identified for another transition. After reduction, the spectrum exhibited other peaks, which confirmed the reduction located at around 220–380 nm [15,16]. The identified absorption peak at 266.5 nm in rGO confirmed the restoration of electronic conjugation within the graphene nano sheets that redshifted from 230 nm to 266.5 nm due to the reduction of the GO [17,18]. These chemical changes can be confirmed by the FTIR analysis of both materials (Figure 2).

Figure 1

UV-vis absorption spectra of GO and rGO prepared by PPE.

Figure 2

FTIR spectra of materials.

The FTIR spectra (Figure 2) illustrate the difference between the functional groups of GO and rGO obtained by varied reduction times. GO showed a broadband at 3,338 cm⁻¹, which corresponded to the stretching mode of the OH group, along with absorption at 1,154 cm⁻¹, which attributed to the presence of C–OH. An absorption peak at 1,724 cm⁻¹ due to C═O also indicated the more oxygenated state of GO. After reduction, the absorptions correlated with oxygenate disappeared on the reduction for either 12 or 48 h, which reveals the absence of the OH group after reduction. These results indicated that the partial functional groups in GO had been effectively eliminated by the reduction.

These results are similar to those reported by previous research [19,20]. After being composited with SnO₂, there was no specific change in the absorption compared to rGO, which implies the absence of an oxidative change by the composite formation.

Based on a previous investigation of the chemical constituent of PPE, the chemicals, which consisted of flavonoids, polyphenols, polysaccharides, etc., contained in the PPE play an important role in reducing the oxygen-containing groups decorated on GO sheets [13]. The reducibility of the phenolic and hydroxyl groups was reflected by the disappearance of the C═O and C–OH functional groups in the FTIR measurement.

Furthermore, the presence of SnO₂ in SnO₂/rGO was identified by the XRD analysis, and the reflections are presented in Figure 3.

Figure 3

XRD pattern of GO, rGO, and SnO₂/rGO.

The pattern depicted a typical GO pattern at 10.2° corresponding to the (001) plane with the d-spacing of 0.81 nm. The rGO exhibited a broad peak at 23.2° (002) along with the disappearance of the (001) reflection, which confirmed the successful reduction by PPE [21,22]. Moreover, the SnO₂/rGO composite displayed diffraction peaks at 26.5°, 37.9°, 39.2°, and 53.6°, which correspond to (110), (101), (200), and (211) reflections, respectively, indicating the formation of the tetragonal rutile SnO₂ (JCPDS 41-1445).

By using the Debye-Scherer formula for the crystallite size measurement (equation 2) [23,24]:

where B(2θ) is the full width at half maximum (FWHM), χ is the wavelength of the incident X-rays, L is the particle size, and 2θ is the Bragg angle of the each peak. The calculated size is 72.2 nm. Refer to the data listed in Table 1.

The presence of SnO₂ in the SnO₂/rGO composite was confirmed by the EDX analysis, which is related to the change in the surface profile identified by the SEM analysis, as presented in Figure 4. The rGO presents the layered surface structure, which was similar as previously reported [22,25]. After modification with SnO₂, the composite showed a distributed particle in irregular forms, reflecting the SnO₂ nanoparticles on the surface. The EDX analysis confirmed the Sn peak, which is associated with the SnO₂ content at a 51 wt%. This interpretation is in line with the TEM image (Figure 5). The layer structure of the rGO obtained from the SEM analysis was consistent with the sheet structures (shown by arrows). Moreover, the dispersed SnO₂ NPs were represented by irregular forms with a size ranging from 50 to 80 nm. This size range was consistent with the crystal size determined by the XRD measurement.

Figure 4

SEM profile of rGO and SnO₂/rGO.

Figure 5

TEM image of rGO and SnO₂/rGO.

The supported SnO₂ nanoparticles on the surface were confirmed by the EDX analysis results, and they also influence the surface profile as identified by the reduced adsorption–desorption isotherm presented in Figure 6. The calculated parameters are listed in Table 2. The Sn content in the SnO₂/rGO was about 26 at.%, which is slightly higher compared to the theoretical Sn content set up in the synthesis (25%). The nanoparticles tended to block the pores and to reduce the specific surface area, which is characterized by reducing the specific surface area from 63.4 to 32.2 m²/g along with reducing the pore volume.

Figure 6

Adsorption–desorption profile of rGO and SnO₂/rGO.

3.2 Photocatalytic activity

The photocatalytic activity of SnO₂/rGO was evaluated by MB degradation with and without the H₂O₂ addition as an oxidant. The comparison of the utilization of UV and visible light was performed to evaluate the effectiveness of the degradation of varied light sources. The degradation rate was estimated from the C/C ₀, plotted against time, after the initial evaluation of the existence of the degradation mechanism by the UV-Vis spectral analysis. Figure 6 shows the different patterns of the UV-Vis spectral changes on photodegradation with and without H₂O₂ under UV light illumination. A rapid degradation was indicated by the H₂O₂ addition to the system as the MB peak at 635 nm disappeared beginning at 15 min of the treatment with the DE of −99%. The degradation without H₂O₂ had a DE of about 65% at 15 min and increased at the extended time of treatment (Figure 7).

Figure 7

UV-Vis spectroscopy analysis of the treated MB solution over SnO₂/rGO (a) without H₂O₂ (b) with the H₂O₂ addition.

The comparison of the C/C ₀ plot from the MB degradation by SnO₂/rGO and rGO photocatalysts with and without H₂O₂ addition is depicted in Figure 8.

Figure 8

Kinetics plot of MB photodegradation over SnO₂/rGO (a) without H₂O₂ (b) with H₂O₂ addition.

The kinetics plots represent the photocatalytic activity exhibited by rGO under visible and UV light illumination. Moreover, an increasing photocatalytic activity was achieved by the dispersion of SnO₂ in the composite form of SnO₂/rGO.

To determine the kinetics constant of the reaction rate (k), a kinetic study of MB photodegradation was carried out according to the classic heterogeneous kinetic model (pseudo first-order model) [26] (equations 3–5):

where C _o and C _t are the initial concentration and concentration at time of t, k _app is the apparent kinetics constant, K is the adsorption–desorption equilibrium constant, and k = k app K is the measured kinetics constant.

The calculated kinetics constants and the correlation coefficient of the pseudo first-order kinetics are listed in Table 3. The data lead to the conclusion that the SnO₂/rGO photocatalyst is superior under visible light, as shown by the higher DE at a visible light exposure reaction compared to under UV illumination. The photodegradation reaction was also accelerated by the addition of H₂O₂ as an oxidant in the degradation system. The comparison on the kinetics plot at varied light sources indicates that both SnO₂/rGO and rGO materials showed a higher photoactivity under visible light rather than under UV light. The effect of the light source and the presence of H₂O₂ did not significantly affect the kinetics of photodegradation using rGO.

This result is interesting because many experiments on photocatalysis have been attempted to create a visible light sensitive photocatalyst, which means that the photocatalyst can intensively function under lower energy consumption and even under solar irradiation.

The role of SnO₂ in a faster degradation is related to its n type semiconductor performance, whereas if the light source impinges the material, an excitation of electrons from the valence band to the conduction band occurs. The excitation produces holes (h ⁺), and as it interacts with ^–OH, the radical hydroxyl radical (˙OH) is generated as a strong oxidizing agent. The excited electrons at the conductance band will also be activated with its interaction with O₂ for the released superoxide radicals (˙O₂ ⁻). The possible mechanism due to these possibilities is as follows [27]:

The fitness of a photocatalyst for a visible light region is indicated by the photoluminescence spectra analysis presented in Figure 9. The rGO has a high PL intensity along the visible light region (300–800 nm), which supports the capability of rGO to catch light for electron excitation.

Figure 9

PL spectra of rGO.

The comparison of the photocatalytic activity of SnO₂/rGO prepared for this work with other previous related works presented in Table 4 implies that the material has a relatively high capability. Compared to the DE using rGO reported by previous works (32.68%), the DE obtained in this study is higher (69.23%), even though the specific surface area of the rGO synthesized from this work (63 m²/g) is less than that of rGO (94 m²/g) [29]. Moreover, the SnO₂/rGO prepared for this work is also superior in DE (98.28%) compared to that reported by previous researchers (89%) [32]. Similarly, a higher activity was exhibited due to the higher kinetics constant of MB degradation by visible light compared to SnO₂/rGO prepared by the dominant SnO₂ [9]. The specific surface area and the Sn content in SnO₂/rGO for this work compared with that of previous studies suggest the effect of the rGO surface profile in enhancing the photodegradation mechanism. The composed functional groups from PPE-reduction significantly influenced an increase in the adsorption mechanism, which strongly influences the photodegradation mechanism. Moreover, the synthesized SnO₂/rGO showed superior properties related to visible light-responsive capability.

3.3 Material stability

Photocatalyst stability is an important parameter for application purposes. Because photocatalyst activity is influenced by many factors, such as pH, oxidation condition, etc., the properties of photocatalyst materials can be changed, leading to reduced activity. For this study, XPS and XRD analyses of the SnO₂/rGO material before and after use were performed.

The XPS survey scan spectra presented in Figure 10 indicate that the surface chemical composition of the SnO₂/rGO was not significantly different after use. The survey spectra before and after use exhibited the peaks of C 1s, O 1s, and Sn. The deconvolution of the C 1s spectrum represented three peaks centered at 284.4, 286.5, and 288.6 eV, which correspond to the C–C, C–O, and –COO– groups, respectively. This indicates the existence of oxygen-containing groups in the rGO as support. Furthermore, the presence of Sn was identified by Sn 3p, 3d, and 4d. In particular, Sn 3d was characterized by two peaks with a binding energy of 486.3 and 494.7 eV, respectively, which corresponded to Sn 3d_5/2 and Sn 3d_3/2, respectively. These peaks are attributed to SnO₂ in nanoparticle form [34]. The same pattern of the C and Sn peaks implies that there was an unchanged electronic state of Sn after usage [35,36]. The maintained structure after use was also evidenced by the unchanged XRD reflections before and after use, as presented in Figure 10. All peaks corresponding to the interpretation based on Figure 3 still had relatively similar intensities, representing the stability of SnO₂ nanoparticles (Figure 11).

Figure 10

(a) XPS survey spectra of SnO₂/rGO before and after use (b) Deconvoluted C 1s spectrum and (c) Sn 3d spectra in SnO₂/rGO.

Figure 11

(a) XRD pattern of SnO2/rGO before and after use.