Highly photodegradation of organic dye pollutant RhB using TiO2-modied reduced graphene oxide

TiO 2 -modied reduced graphene oxide (TiO 2 -rGO) has been studied in one-process step synthesis via the hydrothermal method. In this study, the various concentrations of rGO were investigated and characterized by fourier transform infrared (FTIR), x-ray diffraction (XRD), scanning electron microscope (SEM), surface area analyzer (SAA), thermogravimetric analysis/differential thermal analysis (TGA/DTA), and spectroscopy UV-Vis. In addition, the photodegradation of rhodamine-B was studied by a batch system under visible light irradiation at wavelength 554 nm. The optimum experimental results showed that the presence of rGO in TiO 2 nanostructures could improve the specic surface area until 248.58 m 2 g − 1 and enhanced the photodegradation eciency until 70.66% under visible light irradiation. The photocatalyst stability was evaluated for ve cycles experiment and the performance have reduced 6.58% eciency.


Introduction
Water contamination becomes severe problem in the world because of its importance for human life. The lower water quality contains dangerous substance such as carcinogenic dyes, heavy metals, pharmaceutical waste [1]. Dyes contaminant comes from textile industry that contains soluble mineral salts, aromatic and halogen compound mixture [2]. The dye waste can be treated by ionic exchange precipitation, evaporation, reverse osmosis, ultra ltration, and solvent extraction [3]. Photodegradation technique is more interesting because of simple relatively, good stability, no expensive [4] and carbonaceous product [5]. TiO 2 material is attractive for photocatalysis because it has bandgap energy of 3.0-3.2 eV that can be active in wavelength of ~ 390 nm and free-electron movement with shorter line distance averages [6]. However, TiO 2 only work optimally in the ultraviolet range and electron-hole recombination that causes less optimal degradation e ciency [7,8].
The performance of TiO 2 depends on size, composition, crystal phase, crystallinity, speci c surface area and morphology [9,10]. The higher speci c surface area impacts on enhancement of substrate and photocatalyst interaction so that photodegradation effectiveness optimally increases. The sub-micron light scattered impacts the enhancement of the photon movement pathway's length in photoanode, then it can improve light absorption and e ciency [11]. Photocatalytic activity of TiO 2 can be improved again with composite or hybrid formation by metal or non-metal. Graphene-based composite with semiconductor has been reported as photocatalyst material such as TiO 2 /graphene, ZnO/graphene, Ta 2 O 5 /graphene [12], Cu 2 O/graphene [13] and CdS/graphene [14].
The TiO 2 modi cation with graphene-like (reduced graphene oxide) can improve the photocatalytic activity of semiconductor. The reduced graphene oxide (rGO) existence causes the redshift of light absorption and slower recombination. Besides that, enhancement of photocatalytic activity occurs because the rGO decorated TiO 2 (rGO-TiO 2 ) do strong interaction that initiates the electron injection and electron transport from TiO 2 to rGO so that inhibits electron-hole recombination [15]. In this work, TiO 2 will be synthesized with rGO simultaneous by a one-pot process using the hydrothermal method for improving photocatalytic properties by using rhodamine-B (RhB) as a dye for photocatalytic degradation experiment under visible light irradiation.

Procedure
The TTIP was reacted with 0.362 M oxalic acid in a ratio of 1 : 2.2 mol. After that, GO powder was added by 2%, 4%, and 10% (w/w). Then, 8 g of NaOH was mixed to the mixture slowly with a constant stirring until all of NaOH pellets were dissolved and formed gel. Gel was further treated by hydrothermal process at a temperature of 200 o C during 24 hours. The hydrogel was cooled and washed with 1 liters of 0.1 M HCl followed with 1 liters deionized water until reach pH of 8. The precipitate was dried by freeze drying technique until obtained powder, then heated at 100 o C for 12 hours. After that, the powder was heated at 300 o C for 3 hours to obtain rGO-TiO 2 powder.

Characterization
Spectrophotometer UV-Vis Perkin-Elmer Lambda 25 was used to observe absorption spectra in ultraviolet and visible wavelength. The functional groups were performed by Spectrophotometer Fourier Transform Infra-Red (FTIR) IR Prestige-21. The X-ray diffraction pattern was recorded with Powder XRD Rigaku Mini ex 600 Benchtop using CuKα radiation (λ = 0.15406 nm). Scanning Electron Microscopy (SEM) Vega 3 Tescan was used for morphology characterization. The surface area and pore distribution was measured by Surface Area & Pore Analyzer (SAA) Quantachrome Nova 1200e. Thermal analysis of decomposition material process was performed by Thermogravimetric Analysis/Differential Thermal Analysis (TG/DTA) Linseis STA PT-1600.

Results
TiO 2 decorated rGO (rGO-TiO 2 ) have been synthesized by a base hydrothermal method in a one-pot process. TiO 2 was obtained from titanium oxalate precursors that ti-oxalate was obtained from the reaction of TTIP with oxalic acid refer to reference [9]. Figure 1 shows that synthesized TiO 2 material has a broadening peak indicating an amorphous crystal structure where the results were not the same as shown by Huang  and [303] only appear in small peaks, which indicate that the TiO 2 crystals have not yet grown perfectly so that the diffractogram pattern shows a low of crystallinity (peak broadening). The anatase phase is the metastable phase of TiO 2 before it becomes rutile, where the anatase phase is thermodynamically stable at smaller particle sizes (< 11 nm) while rutile is stable at larger sizes (> 35 nm) [ decreases with increasing GO addition that covers the existence of other peaks and it can also be known that TiO 2 in uences the formation of rGO, but also TiO 2 growth is affected by GO, so the growth of TiO 2 crystals is inhibited. Because the presence of GO is increasing and the amount of TiO 2 is relatively xed so GO cannot interact to TiO 2 uniformly. At the same reduction temperature, more GO are converted to rGO for the addition of GO 2%.
The X-ray diffraction results are supported by FTIR characterization data that is shown in Fig. 2 C atoms from rGO [19,21].
The broadening absorption at the wavenumber of 3400 cm − 1 indicates the existence of the remaining O-H functional groups of irreducible rGO, which is useful as a site of interaction between rGO and TiO 2 [19].
Absorption around the wavenumber of 1600 cm − 1 in the modi ed material spectra indicates the existence of vibrations from Ti-O-C bonds [22]. The results of X-ray diffraction characterization and FTIR spectrophotometry show that there is an interaction between TiO 2 and rGO. SEM imaging obtained the micrograph that is shown in Fig. 3. Figure 3 (a) shows that it has an agglomerated shape so that it appears micro-sized agglomeration of TiO 2 . Whereas the modi cation with 10% GO, Fig. 3 (b) shows that the graphene sheets (rGO) have micro-scale wrinkled layers which TiO 2 grows on its surface. The growth TiO 2 has a different morphology when growing alone (without rGO). TiO 2 is not agglomerated but instead shaped a nanometer size rod-like. It shows that rGO is used as an anchor agent for TiO 2 preventing agglomeration [19].
According to Lv et al. (2017), if the hysteresis area of the loops is increasing, become less mesoporous. In this study, the higher GO addition making the hysteresis area loops increase, then the mesoporous site increases. The BET active surface area from TiO 2 material is 231.58 m 2 / g ( Table 1). The higher the active surface area, can improve the increasing ability to interact with reactants. The higher GO addition obtains a more considerable surface area value as well. Increasing surface area can also be caused by reduced aggregation of TiO 2 [25]. The decrease in aggregation due to rGO as an anchor agent is following SEM data in Fig. 3.
The maximum pore size distribution is shifting as more GO are added which is caused by a decrease in the aggregation of TiO 2 [24]. The pattern of broadening the pore size distribution (Fig. 5) also indicates the presence of mesoporous sites in the material that is by the adsorption-desorption data isothermal.
The bandgap energy indicates the minimum energy to excite electrons from the valence band to the conduction band for initiating catalysis reaction which the decrease in the bandgap shows the photocatalyst ability to work in visible light increases [18].

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The bandgap energy measurement shows that TiO 2 , TiO 2 -rGO 2, TiO 2 -rGO 4 and TiO 2 -rGO 10 materials have Eg values of respectively 3.01, 2.85, 2.83 and 2.82 eV. The Eg indicates that there has been a redshift (in the direction of visible light), as shown in Fig. 6. Testing of photodegradation performance of material modi ed against RhB was performed using the UV-Vis spectrophotometry method at a maximum λ of 554 nm. Measurements were performed in the range of 240 minutes by taking every 30 minutes. The initial 30 minutes measurement was performed with the lamp not turned on to achieve adsorption-desorption equilibrium [10,20].  The electron transport in TiO 2 -rGO causes the activation energy of electron excitation to decreases with Ti-O-C interaction, so the number of electron-holes photogenerated is increasing. Then, the excited electron does not directly recombine with the hole because of the presence of rGO that the electron will be transferred to the surface of rGO which has electronic conductivity properties.
The photodegradation performance of the modi ed material was also carried out under UV irradiation, shown in Fig. 7 (b). The degradation percentage of the modi ed material also increases to 86.81% for the addition of 10% GO (Table 2). This enhancement shows that rGO can impact an increase of TiO 2 photodegradation activity.
The stability of photodegradation activity is shown in the graph in Fig. 8. Figure 8 shows that the photocatalyst is still working well after being used for ve cycles, as evidenced by an insigni cant decrease. The TiO 2 -rGO 10 material even shows only a decrease of 6.58% after using ve cycles. The ability of photocatalyst activity and good stability shows that modi cation with rGO can improve the photocatalytic properties of TiO 2 where both materials work together to produce a good photodegradation performance [26] [27].

Conclusions
The characterization of FTIR and XRD showed Ti-O-C interaction; then it supported with morphology appearance. SEM showed that TiO 2 grows on rGO surface so the modi cation of TiO 2 with rGO in oneprocess synthesis has been done by the hydrothermal method. The TiO 2 -rGO interaction reduces bandgap energy from 3.0 eV to 2.8 eV that indicates redshift (visible light). The photocatalytic performance had been evaluated toward RhB, which was known to improve about 70.66% from 20.11-34.32% under visible light irradiation and 71.98-86.81% (above 20%) under ultraviolet light irradiation. TiO 2 -rGO has good stability in ve cycles tests; the photocatalytic performance only decreased 6.58%, so photocatalytic activity of TiO 2 toward RhB increases with rGO modi cation. X-ray Diffractogram of modi ed material rGO-TiO compare with TiO2 standard, TiO2 (without rGO) and rGO.

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