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. 2021 Nov 18;6(47):32166–32177. doi: 10.1021/acsomega.1c04966

Facile and Green Fabrication of Microwave-Assisted Reduced Graphene Oxide/Titanium Dioxide Nanocomposites as Photocatalysts for Rhodamine 6G Degradation

Andri Hardiansyah †, William J Budiman ‡, Nurfina Yudasari †, Isnaeni †, Tetsuya Kida §, Arie Wibowo ‡,∥,*
PMCID: PMC8638020  PMID: 34870037

Abstract

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Organic pollutants, such as synthetic dyes, are treated to prevent them from contaminating natural water sources. One of the treatment methods is advanced oxidation process using a photocatalyst material as the active agent. However, many photocatalysts are hindered by their production cost and efficiency. In this study, nanocomposites consisting of reduced graphene oxide and titanium dioxide (rGO/TiO2) were prepared by a simple and green approach using the microwave-assisted method, and we utilized a graphene oxide (GO) precursor that was fabricated through the Tour method. The ratios of rGO/TiO2 in nanocomposites were varied (2:1, 1:1, and 1:2) to know the influence of rGO on the photocatalytic performance of the nanocomposites for rhodamine 6G degradation. Transmission electron microscopy (TEM) observation revealed that a transparent particle with a sheetlike morphology was detected in the rGO sample, suggesting that a very thin film of a few layers of GO or rGO was successfully formed. Based on scanning electron microscopy (SEM) observation, the rGO/TiO2 nanocomposites had a wrinkled and layered rGO structure decorated by TiO2 nanoparticles with average diameters of 125.9 ± 40.6 nm, implying that rGO layers are able to prevent TiO2 from agglomeration. The synthesized product contained only rGO and TiO2 in the anatase form without impurities that were proven by Raman spectra and X-ray diffraction (XRD). The nanocomposite with rGO/TiO2 ratio 1:2 (composite C) was found to be the best composition in this study, and it was able to degrade 82.9 ± 2.4% of the rhodamine 6G after UV irradiation for 4 h. Based on a time-resolved photoluminescence study at wavelength emission 500 nm, the average decay lifetime of R6G-rGO/TiO2 composites (2.91 ns) was found to be longer than that of the R6G-TiO2 sample (2.05 ns), implying that the presence of rGO in rGO/TiO2 composites successfully suppressed the electron–hole recombination process in TiO2 and significantly improved their photocatalytic performance. This study showed that the rGO/TiO2 nanocomposites synthesized through relatively simple and eco-friendly processes display promising prospects for photocatalytic degradation of dyes and other recalcitrant pollutants in a water stream.

1. Introduction

In the past decades, industrial effluents that were released into the aquatic environment without proper treatment have contributed greatly toward water pollution. In textile industries alone, 1.5 million liters of effluents per day are released into natural water sources from a fabric mill producing 60 × 104 m of textile fabrics.1 Synthetic dyes, which are abundant in effluents, are used extensively in textile industries because of their higher color yield, lower water usage and steam consumption, shorter cycle time, and cheaper cost compared to natural dyes.1,2 On the other hand, most synthetic dyes are categorized into recalcitrant pollutants, which are difficult to degrade completely through conventional physical, chemical, and biological processes.1,3,4 Many serious problems such as color pollution, reduced water photosynthetic activity, physiological disorder, and cancer in the human body can occur if these synthetic dyes are released directly to the environment untreated.1 Thus, an alternative method is needed to overcome these problems.

Advanced oxidation process (AOP) technology such as Fenton-based catalysts3,5,6 and photocatalysts3,7 can eliminate persistent organic pollutants through total mineralization into CO2 and H2O, which are harmless to the environment.4 Photocatalytic degradation is an attractive solution for synthetic dye removal in a water stream because of the abundance of its energy source in sunlight and no other chemical substances being needed, making it cheaper compared to other methods.8−10 To date, there are various nanostructure materials that have been explored as photocatalysts such as graphitic carbon nitride-based composites,11−13 transition-metal selenides and diselenides,14 PrVO4,15 Tl4CdI6,16 and Nd2Sn2O7.17 Among them, titanium oxide (TiO2) is one of the semiconductors that is commonly used as an active material in the photocatalytic degradation process because TiO2 is relatively cheaper, commercially available, photochemically stable, and nontoxic.18,19 However, TiO2 has several weaknesses such as a fast recombination phenomenon, low specific surface area, particle agglomeration, and high band gap, which may reduce its performance and hinder it from visible-light photoexcitation.20,21 Thus, many researchers devoted their efforts to enhancing their performance of the TiO2-based photocatalytic degradation process.22

To solve this problem, TiO2 can be paired up with reduced graphene oxide (rGO) to become rGO/TiO2 nanocomposites. The presence of rGO in rGO/TiO2 nanocomposites could significantly improve the photocatalytic performance of TiO2 because rGO has a high surface area that prevents agglomeration of TiO2 particles, allowing electron transfer from TiO2 to rGO to suppress the electron–hole recombination process.23,24 This binary nanocomposite could be prepared by covalent attachment,25 alkaline hydrothermal synthesis,26 microwave-assisted synthesis,27 and sol–gel synthesis preceded by GO spray-drying.26−28

It is to be noted that most of the mentioned studies used graphene oxide (GO) synthesized through the Hummers method with little modification. However, this method might be harmful to the environment because it may produce toxic NO and NO2 gases from the decomposition of HNO3.29 In this context, utilization of the Tour method is more eco-friendly than the Hummers method to produce the GO precursor because the Tour method employs H3PO4 instead of HNO3, avoiding toxic gas production, to obtain a high yield of oxidized GO as its product.30 Thus, in this study, rGO/TiO2 nanocomposites were prepared by a green and simple method, where the GO precursor was synthesized by the Tour method and GO reduction was performed by microwave irradiation. Microwave irradiation was used to reduce GO because (i) this method is relatively safe and harmless compared to chemical reduction that used toxic chemicals, such as hydrazine, as the reducing agent;31 (ii) this method is able to produce uniform and rapid heat throughout the samples while consuming less energy and time;32 (iii) Iskandar et al. found that rGO that was produced by the microwave-assisted method achieve higher electrical conductivity (1180 S m–1) than the rGO synthesized by the Hummers (5 S m–1) and Tour (10 S m–1) methods.31 To the best of our knowledge, this combination method (Tour method for GO production and microwave irradiation for GO reduction) to prepare rGO/TiO2 nanocomposites has not been explored yet. The structure and morphology of the synthesized rGO/TiO2 nanocomposites were evaluated through microscopy and spectroscopy. The effect of rGO on the nanocomposite’s photocatalytic performance through the photocatalyst was investigated by varying the rGO/TiO2 ratio in nanocomposites.

2. Results and Discussion

2.1. GO Formation through the Tour Method

To confirm that GO was successfully prepared from the graphite precursor through the Tour method, both the graphite precursor and the synthesized product were characterized using XRD and the Raman spectroscopy method. As can be seen in Figure 1a, XRD patterns of the graphite precursor and the synthesized product showed main peaks at 2θ value of 26.5 and 9.4°, respectively, which are correlated with the interlayer spacing distances of graphite (3.4 Å) and GO (9.4 Å).33 The increased interlayer spacing distance value in the synthesized product can be attributed to the presence of hydroxyl, carboxyl, or epoxy functional groups that commonly occurred in GO. Since the peak that was associated with GO (9.4°) was observed and the peak that was associated with graphite (26.5°) disappeared in the synthesized product, it might be considered that GO was successfully prepared in the synthesized product. Further characterization using Raman spectroscopy was implemented to confirm this hypothesis.

Figure 1.

Figure 1

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Characterization results of the graphite precursor (blue) and the synthesized product (red) using (a) XRD and (b) Raman spectroscopy.

Raman spectroscopy results of the graphite precursor and the synthesized product are presented in Figure 1b. Raman spectroscopy of the graphite precursor showed two characteristic peaks at 1578 and 2715 cm–1, which correspond to the G band and the 2D band, respectively. The G band represents in-plane carbon sp2 stretching,34 while the 2D or G′ band represents the number of graphene layers in graphitic materials and is caused by second-order phonon processes.35,36 Raman spectroscopy of the synthesized product displayed the absence of the 2D band’s peak and the presence of the G band’s peak (1601 cm–1) with intensity higher than the G band of the graphite precursor. Significant increment of the G band in the synthesized product suggested that the exfoliation process of graphite occurred because the G band’s peak is narrow and intense in single-layered graphene and becomes broad and weak with the addition of new layers in graphite. Interestingly, a new peak was observed in the synthesized product at 1325 cm–1, which is correlated with the D band and represents the structural disorder of GO. The appearance of two characteristic peaks of GO (G and D bands) and the absence of a characteristic peak of graphitic materials (2D band) suggested that GO might be formed in the synthesized product. It is to be noted that the higher ratio between the D band and G band (ID/IG value) suggests the higher quality of the GO because it may indicate a higher degree of oxidation, which may cause the disordered structure in GO.30 Considering that the ID/IG value of the synthesized GO in this study (2.33) is higher than the ID/IG value of GO that was synthesized by the Hummers method (0.95–1.60),27,35 it can be concluded that the GO that was obtained from the Tour method has a better quality than that obtained by the Hummers method.

Additional characterizations using UV–vis and FTIR spectroscopies were carried out on the synthesized product sample for further confirmation (Figure 2). UV–vis spectroscopy results of the synthesized product (Figure 2a) showed a main peak at 230 nm with a shoulder at 300 nm, which belongs to electronic transitions in GO.37 The 230 nm peak was correlated with the π → π* electronic transition from C=C bonding, while the 300 nm peak was correlated with the n → π* electronic transition from the free electron pair of the carbonyl group.

Figure 2.

Figure 2

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Characterization results of the synthesized product using (a) UV–vis and (b) FTIR spectroscopies.

The FTIR result of the synthesized product (Figure 2b) showed various peaks, which were assigned to various bonding vibrations.38−40 The broad peak at 3433 cm–1 was assigned to the O–H vibration in the hydroxyl group. Peaks at 1710 and 1632 cm–1 were attributed to the C=O vibration in the carbonyl or carboxyl group. The peak at 1369 cm–1 was attributed to the C–OH vibration in the carboxyl group. Peaks at 1171, 1064, and 856 cm–1 were attributed to the C–O–C vibration in the epoxide group. It can be said that GO was detected in the synthesized product due to the existence of peaks that correlated with oxygen-containing functional groups such as hydroxyl, carbonyl, carboxyl, or epoxide. Based on sample evaluation using XRD, Raman, UV–vis, and FTIR methods, it can be concluded that GO was successfully prepared from the graphite precursor through the Tour method.

2.2. Fabrication of rGO/TiO2 Composites

After achieving the GO precursor through the Tour method, GO was mixed with TiO2 with various compositions followed by microwave-assisted reduction to obtain rGO/TiO2 nanocomposites, and their morphologies were evaluated by TEM and SEM methods, while their elemental distributions were determined by the EDS method. The TEM observation revealed that a transparent particle with a sheetlike morphology was detected in the rGO sample (Figure 3a), which is consistent with the TEM image of rGO observed by Stobinski et al.41 The high transparency of sheet particles in the rGO sample suggested that a very thin film of a few layers of GO or rGO was successfully formed as a result of the exfoliation process that occurred during GO formation through the Tour method and followed by a reduction step.

Figure 3.

Figure 3

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TEM observation of (a) rGO and (b) composite C. (c) Low-magnification SEM observation of composite C as a representative of a nanocomposite sample.

Meanwhile, TEM observation of composite C as a representative of a nanocomposite sample unveiled the presence of a transparent sheet particle and irregular particles with a particle size of 127.4 nm (Figure 3b). Transparency of sheet particles in the composite sample is lower than that in the rGO sample, implying that rGO particles might be covered by other particles. Further characterization using the selected area electron diffraction (SAED; inset of Figure 3b) method confirmed the presence of TiO2 in the composite sample with Miller indices of (101), (004), (200), (211), and (204) (JCPDS No. 21-1272). Based on the TEM and SAED results, the irregular particles and sheet particles may be identified as TiO2 and rGO, respectively.42 Low-magnification SEM observations on composite C as a representative of nanocomposite samples revealed that micron-sized particles (125.9 ± 40.6 μm) with a sheet structure were observed (Figure 3c). These sheet particles might be associated with rGO in the nanocomposites sample.43

SEM observation at a higher magnification unveiled that nanoparticles were observed on the surface of sheet particles in all rGO/TiO2 nanocomposites with average sizes of 125.9 ± 40.6, 127.3 ± 27.2, and 128.6 ± 40.5 nm for composites A (Figure 4a), B (Figure 4e), and C (Figure 4i), respectively. EDS mapping of O (Figure 4b,f,j) and Ti (Figure 4c,g,k) elements of nanocomposites is correlated with nanoparticles, and their intensity increased as the TiO2 content in nanocomposites increased, while EDS mapping of the C element (Figure 4d,h,l) showed a reduced intensity as the TiO2 content in the composites increases. Summary of the elemental composition of rGO/TiO2 samples from EDS results is presented in Table 1. According to SEM and EDS results, it can be concluded that TiO2 nanoparticles were formed on the surface of rGO with the sheet structure.

Figure 4.

Figure 4

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FE-SEM images of (a) composite A and its EDS mapping results for (b) O, (c) Ti, and (d) C elements. FE-SEM images of (e) composite B and its EDS mapping results for (f) O, (g) Ti, and (h) C elements. FE-SEM images of (i) composite C and its EDS mapping results for (j) O, (k) Ti, and (l) C elements.

Table 1. EDS Elemental Composition of rGO/TiO2 Samples.

  wt %
sample O Ti C
rGO/TiO2 (2:1) 29.6 ± 1.9 26.5 ± 1.2 43.9 ± 1.8
rGO/TiO2 (1:1) 32.4 ± 1.3 30.8 ± 1.0 36.8 ± 0.4
rGO/TiO2 (1:2) 45.6 ± 2.2 41.9 ± 1.6 12.5 ± 2.3
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Raman spectra of pristine TiO2, rGO, and their nanocomposites are presented in Figure 5. Raman spectra of the pristine TiO2 sample showed peaks at 136, 196, 395, 514, and 638 cm–1, which belong to TiO2 in the anatase form.44 While Raman spectra of the pristine rGO sample showed a pair of weak peaks at 1344 and 1598 cm–1 (inset of Figure 5), which are associated with the D band and G band of rGO, respectively.26 Raman spectra of rGO/TiO2 nanocomposite samples demonstrated the presence of anatase and rGO peaks in all nanocomposite samples, suggesting that TiO2 has successfully been integrated into rGO layers and no other substances were formed. Anatase peaks of rGO/TiO2 nanocomposite samples had a lower intensity than the pristine TiO2 sample because TiO2 fractions in nanocomposite samples are lower than those of the pristine TiO2 sample. On the other hand, rGO peaks in the nanocomposite samples had a higher intensity than the pristine rGO sample because the integrated TiO2 nanoparticles may act as disordered and defect structures while also stretching the sp2 bond of rGO.26 These phenomena lead to the higher D band and G band intensities. The ID/IG values of D and G bands of GO, rGO, rGO/TiO2 (2:1), rGO/TiO2 (1:1), and rGO/TiO2 (1:2) were calculated to be 2.33, 2.21, 1.94, 2.39, and 2.42, respectively. The higher ID/IG value of GO compared to rGO was correlated with the degree of structural disorder caused by an abundance of oxide groups in GO. An increase in the ID/IG value in rGO/TiO2 samples with increasing TiO2 content could be assigned to the increased structural disorder in the graphitic structure because of TiO2 particle’s presence in the rGO layers.26

Figure 5.

Figure 5

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Raman spectra of pristine TiO2, rGO, and rGO/TiO2 nanocomposite samples with an inserted magnification at the rGO peak region. Certain peaks of TiO2 and rGO are marked with markers.

The XRD spectra of TiO2 and the composites (Figure 6) showed diffraction peaks that are correlated with the anatase peaks from JCPDS No. 21-1272. These XRD results are consistent with the Raman results, indicating the presence of anatase in TiO2 and the composite sample. rGO peaks could not be observed in XRD results of composite samples due to their broad peak and amorphous nature.41 The crystallite size of TiO2 and the nanocomposite samples can be calculated from their XRD results using the Scherrer formula as follows.45

2.2. 1

where D is the crystallite size, K is the shape factor, λ is the X-ray wavelength, β is the line broadening at full width at half-maximum (FWHM), and θ is the Bragg angle.

Figure 6.

Figure 6

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XRD spectra of pristine TiO2 and rGO/TiO2 nanocomposite samples. Certain peaks of anatase TiO2 according to the JCPDS No 21-1272 database are marked with markers.

According to the Scherrer formula, the crystallite sizes of TiO2, composite A, composite B, and composite C are 131.7, 75.5, 77.1, and 83.9 nm, respectively. The rGO/TiO2 composites with a higher rGO content have smaller anatase crystallite sizes that were also reported by other studies.46,47 The presence of rGO inhibited the TiO2 crystallite growth due to many possible nucleation sites on rGO.48,49

The dissolution–crystallization process of TiO2 can be controlled by tuning the pH of the solution. TiO2 powder can be dissolved in an acidic condition of a Tour solution to form TiO2+ ions. Dissolved TiO2+ species may attach to C=O, COOH, and C–OH bonds of GO oxide groups as the anchor of further growth. Adding NaOH to the solution will provide an alkaline condition that will lead to crystallization of Na+, OH–, and TiO2+ ions into the titanate complex such as Na2Ti3O7.50,51 Titanate crystals nucleated at these GO sites, especially the C=O site.49 The heating process of microwave irradiation reduced GO layers into rGO through the removal of oxide groups.52 Then, H+ ions can substitute the Na site on Na2Ti3O7 during the washing process using HCl and finally the titanate complex would transform into TiO2 during the drying process through the reaction described in eq 2.22 Based on these results, the formation mechanism of rGO/TiO2 nanocomposites in this study is illustrated in Figure 7.

2.2. 2

Figure 7.

Figure 7

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Schematic illustration of the formation mechanism of rGO/TiO2 nanocomposites.

2.3. Photocatalytic Degradation of Rhodamine 6G (R6G)

The R6G degradation profile of each sample under UV irradiation is shown in Figure 8. Prior to studying the photocatalytic performance of composites, the R6G degradation profiles of the pristine R6G solution, rGO, and TiO2 were examined to know the effect of each component on the photocatalytic performance of composites under UV irradiation. As can be seen in Figure 8, the R6G concentration of the pristine R6G solution remains the same as their original concentration after UV irradiation for 4 h, suggesting that photodegradation did not occur on the pristine R6G solution.

Figure 8.

Figure 8

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Degradation of R6G measured through UV–vis spectroscopy in the photocatalysis experiment.

The effect of each component of rGO/TiO2 composites on the R6G degradation profile was explored by mixing the R6G solution with rGO and TiO2 separately and exposing to UV irradiation for 4 h. In the presence of rGO in the R6G solution, the R6G concentration in a solution decreases to about 10.9 ± 1.9% after UV irradiation for 4 h. It is to be noted that rGO has a residual oxide function group and vacancies as leftovers from the GO reduction process may facilitate R6G adsorption through π–π and/or electrostatic interaction, leading to the slight decrease of the R6G concentration in solution.53 Considering that the curve was stagnant at around 10.9%, it could be implied that rGO’s adsorption capacity in this study was reached, which prevented further R6G adsorption. The reduction of the R6G concentration is lower compared to that of a previous study that introduced continuous stirring during the adsorption process.54 Meanwhile, the R6G concentration in solution was reduced by about 30.6 ± 3.2% if the R6G solution containing TiO2 was irradiated with UV for 4 h. It is noteworthy that Pu et al. reported that the R6G adsorption behavior of pure TiO2 was ∼5% with a TiO2:R6G ratio (w/w) of 50:1.54 Since the TiO2:R6G ratio (w/w) that was used in this study (1:1) is significantly lower than the work of Pu et al. (50:1), it could be expected that the role of adsorption on reduction of the R6G concentration is insignificant in this study. Thus, the R6G degradation profile of the TiO2 sample could be associated with photocatalytic degradation of R6G in the presence of TiO2 as a photocatalyst. It should be noted that most studies used a higher concentration of the catalyst compared to its pollutant model, which explained its relatively low degradation by 4 h of photocatalysis.26,55,56

A greater decrease of the R6G concentration in solution could be obtained by mixing the R6G solution with the rGO/TiO2 composites. As can be seen in Figure 8, the R6G concentration in the rGO/TiO2 composite samples decreases by around 42.7–82.9% after UV irradiation for 4 h. These results indicate that combining rGO and TiO2 as composites led to a substantial improvement on R6G degradation, which is significantly higher than the sole adsorption effect of pristine rGO (10.9 ± 1.9%) and the sole photocatalytic degradation effect of pristine TiO2 (30.6 ± 3.2%). This phenomenon might have occurred due to the presence of rGO in the rGO/TiO2 composites, which could have acted as dye adsorption sites and electron acceptors that were generated during photoexcitation. By moving the electron from TiO2 to rGO, which has excellent electron-transfer performance,26 the recombination process could be prevented,28 thus improving the photocatalytic activity of the photocatalyst.57,58

As can be seen in Figure 8, composite C has a higher R6G degradation (82.9 ± 2.4%) than composite A (42.7 ± 9.3%) and composite B (67.7 ± 0.7%). It is well known that tuning the ratio of rGO and TiO2 in the rGO/TiO2 composites may affect the composite’s photoactivity. In this case, a higher rGO content in the system may disturb the absorption of the photon by TiO2 due to scattering and absorption by carbon in rGO.57 On the other hand, the low rGO content can be insufficient to facilitate electron transfer from photogeneration in TiO2 to prevent electron–hole recombination.28 In this case, composite C, which had the lowest rGO composition, was the optimal composition as indicated by its high R6G degradation.

To understand this phenomenon, further characterizations are needed to reveal the role of rGO on the catalytic performance of rGO/TiO2 composites. In this study, the diffuse reflectance UV–vis (DR UV–vis) spectroscopy method was performed on TiO2 and rGO/TiO2 nanocomposite samples to understand the influence of rGO on the TiO2 band-gap value in rGO/TiO2 composites. The resulting reflectance was processed into band-gap energy using the Tauc plot curve through the Kubelka–Munk equation as follows.59

2.3. 3

where R is the reflectance (R = R sample/R reference), α is the absorption coefficient, and s is the scattering coefficient. This is the equation of a straight line whose interception with the abscissa axis corresponds to a band-gap energy.

The Tauc plot of TiO2 and rGO/TiO2 nanocomposites and their band-gap energy determination are presented in Figure 9. As can be seen in Figure 9a, the TiO2 band-gap value was 3.22 eV, which is similar to the TiO2 band gap in the anatase form (3.2 eV).60 This result is consistent with Raman and XRD results, indicating the presence of anatase in the TiO2 sample. Interestingly, the band-gap value of all composite samples (Figure 9b–d) was similar to the TiO2 band-gap value (3.22 eV), suggesting that the presence of rGO in the composite did not affect the TiO2 band-gap value.61 These results imply that band-gap modification might not be the reason for the increment phenomena of the catalytic performance of rGO/TiO2 composites.

Figure 9.

Figure 9

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Tauc plot and band-gap value of (a) TiO2, (b) composite A, (c) composite B, and (d) composite C derived from their DR UV–vis spectra.

Further study using the photoluminescence spectroscopy method was conducted on R6G, R6G with rGO, R6G with TiO2, and R6G with composite C samples as the representative of the rGO/TiO2 composite sample. The photoluminescence spectra results of all samples are shown in Figure 10a. After UV irradiation, R6G-rGO, R6G-TiO2, and R6G-composite samples showed a lower luminescent intensity than the pristine R6G sample, with the greatest reduction of luminescent intensity being observed on the R6G-composite sample. Even though the laser that was used in this RT-PL study (420 nm = 2.95 eV) has a lower energy than the anatase band gap (3.2 eV60), a small and broad peak at around 500 nm was only observed on the TiO2-containing samples (R6G-TiO2 and R6G-rGO/TiO2), suggesting that this peak might correlate with TiO2. Mathew et al. found that TiO2 nanoparticles that were prepared by the hydrothermal method showed emission peaks at 387, 421, 485, 530, and 574 nm.60 These peaks are correlated with the presence of defect levels on TiO2 that were caused by the oxygen vacancies. Thus, the presence of a broad peak at around 500 nm on TiO2-containing samples might indicate the presence of the defect level of TiO2 nanoparticles in our sample.

Figure 10.

Figure 10

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(a) Photoluminescence of R6G samples with and without a catalyst after UV irradiation; time-resolved photoluminescence of R6G samples with and without a catalyst after UV irradiation at (b) λems = 500 nm and (c) λems = 550 nm.

Charge separation and energy transfer on our samples were studied by conducting time-resolved photoluminescence (TRPL) at two wavelength emissions, i.e., 500 nm (luminescence of TiO2 nanoparticles) and 550 nm (luminescence of dye R6G). The TRPL signals for 500 and 550 nm are shown in Figure 10b and c, respectively. The average decay lifetime (τave), energy-transfer rate (kET), and energy-transfer efficiency (nET) were calculated from the TRPL signal using eqs 4–6 as follows62,63

2.3. 4
2.3. 5
2.3. 6

The calculation results are presented in Table 2. For TRPL at 500 nm (2.48 eV), the decay lifetime of the R6G sample was found to be shorter than the others, implying the negative values of kET and nET for the R6G sample at this wavelength (data is not shown). Thus, it can be concluded that there is no energy transfer from R6G to TiO2 or rGO at 500 nm. Furthermore, the average decay lifetime of R6G-rGO/TiO2 composites (2.91 ns) is longer than the R6G-TiO2 sample (2.05 ns), implying that the presence of rGO in rGO/TiO2 composites successfully suppressed the electron–hole recombination process in TiO2. This phenomenon occurred because the photogenerated electron can be transferred from TiO2 to rGO, and this phenomenon might be crucial for the significant increment of the photocatalytic performance.23,24

Table 2. Fitting Results of Time-Resolved Photoluminescence and Energy-Transfer Efficiency.

sample τave (ns) at λ = 500 nm τave (ns) at λ = 550 nm kET (108 s–1) at λ = 550 nm nET (%) at λ = 550 nm
R6G 1.35 3.99    
R6G + TiO2 2.05 3.15 6.68 21.05
R6G + rGO 2.25 3.89 0.64 2.51
R6G + composite 2.91 2.46 15.58 38.35
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On the other hand, TRPL at 550 nm (2.25 eV) showed that energy transfer can occur at 550 nm, with R6G composites showing the highest energy-transfer efficiency (around 38.35%). This means that more electrons will be transferred to TiO2 defect levels for generating radicals. Therefore, luminescent quenching will be strongly observed. The generation of radicals is one of the main reasons for luminescent quenching and degradation of the dye R6G.64

Based on these TRPL results, a simple diagram of energy transfer in our study is illustrated in Figure 11. There is an energy-transfer process that occurs between dye R6G and TiO2 or rGO. Excited electrons from R6G were then transferred to defect levels of TiO2, as noted by ET in the diagram. These electrons can be used for the oxidation process to make O2 radicals. Some electrons may return back to the valance band of R6G by giving out luminescence. Meanwhile, another radical was generated due to charges (from holes) in the R6G. We assume that holes and electrons will generate radicals. Each photocatalysis process that uses TiO2 is strongly predicted to have O2– and –OH radicals, since charge carriers (electrons and holes) near the surface of TiO2 will easily react with surrounding molecules or dyes and generate radicals.65

Figure 11.

Figure 11

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Proposed energy-transfer and charge separation diagram.

3. Conclusions

Based on characterization results, it can be concluded that the rGO/TiO2 nanocomposites were successfully prepared by the microwave-assisted method with the GO precursor fabricated through the Tour method. The presence of rGO in the rGO/TiO2 nanocomposites was able to enhance the photocatalytic performance of TiO2 due to effective electron transfer from TiO2 to rGO, which prevents fast recombination, good ability to promote formation of TiO2 nanoparticles, and efficient dye adsorption into nanocomposites. Composite C with rGO:TiO2 ratio 1:2 was found to be the best composition in this study with R6G degradation 82.9 ± 2.4% after UV irradiation for 4 h. Based on time-resolved photoluminescence study at wavelength emission 500 nm, the average decay lifetime of R6G-rGO/TiO2 composites (2.91 ns) is longer than that of the R6G-TiO2 sample (2.05 ns), implying that the presence of rGO in rGO/TiO2 composites successfully suppressed the electron–hole recombination process in TiO2 and significantly improved their photocatalytic performance. It is hoped that the simple and green synthesis of rGO/TiO2 nanocomposites proposed in this study may encourage a new perspective of eco-friendly fabrication of rGO/TiO2 nanocomposites and their utilization as a promising photocatalyst for photocatalytic degradation of dyes and other recalcitrant pollutants in a water stream.

4. Experimental Section

4.1. Materials

Graphite, potassium permanganate (KMnO4), phosphoric acid (H3PO4), sulfuric acid (H2SO4), hydrogen peroxide (H2O2), hydrochloric acid (HCl), titanium (IV) oxide (TiO2, in the anatase form), sodium hydroxide (NaOH), and rhodamine 6G (R6G) were purchased from Sigma-Aldrich. All chemicals were of proanalytical (p.a.) grade and used as received without further purification.

4.2. Synthesis of Reduced Graphene Oxide/Titanium Dioxide (rGO/TiO2)

Graphene oxide (GO) was synthesized by the Tour method.30,66 Briefly, expanded graphite was oxidized with KMnO4 in an acidic solution of H2SO4/H3PO4 (9:1), stopped with the addition of ice and H2O2, and washed with 5% HCl and deionized (DI) water three times each to yield graphene oxide as the product. Then, 6.6 mL of water-dispersed GO (3.8 mg/mL) was diluted with DI water into 50 mL of GO (0.5 mg/mL). Three different quantities of TiO2 (anatase) powder were added into the GO dispersion to create three different compositions of GO:TiO2. These were 2:1 as composite A, 1:1 as composite B, and 1:2 as composite C. An ultrasound bath (75 kHz, 40% amplitude, pulse 4 s, stop 2 s) with 30 min of pulse total duration was used to disperse the mixture homogeneously. The pH of the ultrasonicated mixture was adjusted with 10 M NaOH until its value reached pH 9. The GO/TiO2 was treated with microwave irradiation heating in a commercial microwave oven Sharp 900 W with 40% power (360 W) for 10 min. Then, the water solvent was evaporated in an ambient condition at 25 °C. Eventually, the dried product was washed with 5% HCl and water three times each until it reached neutral pH. After the washing process, the rGO/TiO2 nanocomposites were dried in an ambient condition at 25 °C. The dried product was recovered as the synthesis result. As a comparison, rGO was synthesized using the aforementioned method without the addition of TiO2.

4.3. Characterization of Structure and Morphology

The GO and rGO/TiO2 were characterized using various methods. Raman spectroscopy was carried out for GO, its precursor graphite, and rGO/TiO2 by a Raman microscope (Horiba Jobin Yvon, iHR320, Japan) with 1800 I/mm grating, 40 s acquisition, and two accumulations per sample using a 100X objective lens magnification. The excitation light source was a He–Ne laser with a wavelength of 532 nm. The ratios of GO and rGO/TiO2 peaks (D band to G band ratio, ID/IG) were calculated from the Raman spectra using the Peak Analyzer from Origin, a scientific data and graphic analysis tool.

Structures and morphologies of rGO/TiO2 were observed using a field-emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) with an electron microscope and EDS elemental analysis (Jeol JIB-4610F, Japan) with a Schottky electron gun with a maximum ion current of 200 nA. The samples were gold-coated with a GSL-1100X-SPC-12 Compact Plasma Sputtering Coater. Both low and high magnifications on FE-SEM were used to observe the composites, and EDS mapping for O, Ti, and C elements was conducted. A transmission electron microscope (TEM) instrument (FEI Tecnai G2 20S-Twin, the United States) was also used to observe morphologies at a higher magnification with 200 kV voltage and the distance between sample and electron source of 680 mm.

GO, graphite, TiO2, and rGO/TiO2 nanocomposites were characterized using an X-ray diffraction (XRD) diffractometer (Rigaku SmartLab, Japan) with a Cu Kα light source (λ = 1.54056 Å). For GO and graphite, about 1 g of sample was used with diffraction angle (2θ) between 5 and 60°. On the other hand, about 0.1 g of TiO2 and its nanocomposites was used with diffraction angle (2θ) between 5 and 100°. The interlayer spacing distance (d) of GO and graphite was identified from their highest peak and calculated using Bragg’s law with the previously mentioned variables.

GO was characterized even further using ultraviolet–visible (UV–vis) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The UV–vis spectrum was obtained using a UV–vis spectrometer (Hitachi UH5300, Japan) with wavelength between 190 and 1100 nm. The FTIR spectrum was obtained using an FTIR spectrometer (Thermoscientific Nicolet iS-10) with the wavelength range between 400 and 4000 cm–1. About 5 mg of the GO sample was mixed with potassium bromide (KBr) powder and was manually pressed with the aid of a pressing device.

Further characterizations were conducted on TiO2 and rGO/TiO2 nanocomposites. Diffuse reflectance (DR) UV–vis was conducted with a simple spectrometer with an Ocean Optics DH-mini UV–vis–NIR light source and a high-sensitivity Maya Pro Series (Ocean Optics Inc., FL) as the detector to measure the sample’s reflectance.

4.4. Photocatalytic Experiment of Rhodamine 6G

Photocatalytic performances of TiO2, rGO, and the obtained rGO/TiO2 nanocomposites were determined by studying the dye degradation profile under UV irradiation (λ = 352 nm64) for 4 h with rhodamine 6G (R6G) as the dye model. Briefly, six different samples, namely, R6G, rGO, TiO2, composite A, composite B, and composite C, were prepared by mixing 500 mL of R6G solution with 25 mL of catalyst solutions containing DI water, rGO, TiO2, composite A, composite B, and composite C, respectively. The final concentration of R6G and the catalyst in the testing samples was 10 ppm to obtain R6G/catalyst ratio 1:1. All of the samples were mixed until homogeneous right before the experiment began. During photocatalytic experiments, samples were taken at every 15 min time intervals for the first 3 h and at 60 min time intervals at the fourth hour and characterized using UV–vis spectroscopy to determine the R6G concentration in the solution. These concentration values would be divided by the initial R6G concentration to yield the R6G degradation profile (in percentage/%) as a function of UV irradiation time.

Time-resolved photoluminescence (TRPL) was conducted on samples before and after the photocatalysis experiment using TimeHarp 260 from PicoQuant and a picosecond laser at 420 nm wavelength, 20 ps pulse width, 50 mW power, and 10 MHz repetition rate. The laser beam was focused onto the sample, and the emission was then collected into a photon microdevice (PMD) detector. A band-pass filter was placed in front of the detector to get only emission signals at 500 and 550 nm wavelengths. For photoluminescence measurements, the 420 nm laser was focused onto the samples and the emission spectra were recorded using an Aurora4000 spectrometer.

The analysis of time-resolved photoluminescence was done using a two-component exponential decay fitting to get ai (fractional contribution of decay lifetime, i = 1, 2) and τi (decay lifetime, i = 1, 2).

Acknowledgments

A.W. would like to acknowledge the ITB Research Fund 2021 scheme from the Institut Teknologi Bandung (ITB). A.H. would like to acknowledge all support in characterization on the ELSA LIPI system, materials, and facilities from the Research Center of Physics, Indonesian Institute of Sciences.

Author Contributions

A.H.: resources, supervision, data analysis, writing—review. W.J.B.: writing the original draft, editing, data acquisition, data analysis. N.Y.: resources, data acquisition. Isnaeni: resources, data acquisition. T.K.: data analysis, writing—review. A.W.: supervision, resources, writing the original draft, writing—review. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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