Enhanced photocatalytic degradation of direct blue 71 dye using TiO2-PAA-GO composite in aqueous solution

In this work, we successfully synthesized a TiO2-PAA-GO hydrogel photocatalyst (GO: graphene oxide; PAA: poly acrylic acid) using a hydrothermal method. The XRD, FTIR, SEM, and XPS results demonstrated the formation of cross-link bond within the TiO2, GO, and PAA nanocomposite. TiO2 nanomaterials, with a particle size of approximately 5 nm, were uniformly distributed on GO layer, and have a high surface area (156 m2 g−1). We then applied the TiO2-PAA-GO composite to remove direct blue 71 (DB-71) from water, achieving up to 98.16% removal. This success was due to (i) the reduction in bandgap energy, (ii) reduced electron and hole recombination, (iii) increased output of generated electrons, and (iv) high specific surface area. We also investigated the efficiency of DB-71 degradation, considering the initial concentration of DB-71, pH, contact time, catalyst mass, and the role of reactive radicals. After six reaction cycles, over 95% of the DB-71 was removed from the aqueous medium using the TiO2-GO-PAA photocatalyst.

other materials to enhance the adsorption. When combined with TiO 2 , the electron-hole pairs of TiO 2 are separated by moving the excited electrons to GO. In addition, the stacked layered TiO 2 -graphene oxide nanostructure increases the contact area between the semiconductors and graphene sheets because of the increased surface area and creates a shorter diffusion distance of photoelectric charge, facilitating rapid electron transmission through substances separation [13,18,19].
Hydrogels that are eco-friendly and highly efficient for removing dye from water sources have recently been studies, using a combination of GO and TiO 2 (table 1). Rong Zhang et al [20] reported that PAN/β-CD/TiO 2 /GO (GO: graphene oxide; PAN: polyacrylonitrile; β-CD: β-cyclodextrin) composites, synthesized using ultrasonic-assisted electrospray method, can degraded 90.92 and 93.53% of MO and MB, respectively. Yuheng Zhang et al [21] reported that the Ag/β-cyclodextrin co-doped TiO 2 hydrogel removed up to 96.8% unsymmetrical dimethylhydrazine (UDMH) after 80 min of Sunlight irradiation. Abd-Elhamid et al [22] fabricated PAN/β-CD/graphene oxide films that could remove up to 98% of crystal violet (CV). Yian Chen et al [23] successfully fabricated cellulose/GO/TiO 2 hydrogels by hydrothermal method, which removed 93% MB in 120 min. A hydrogel based on graphene oxide-TiO 2 -β-cyclodextrin was able to removed up to 93% of new coccine (NC), after 60 min of reaction [24]. Thus, the combination of GO and TiO 2 with crosslinkers such as cellulose, β-cyclodextrin, had led to successful synthesis of 3D-structured hydrogels with increased efficiency in removing dyes. Furthermore, these 3D-structured hydrogels are synthesized using simple, environmentally friendly, biodegradable, and low-cost processes.
In this study, we synthesized a 3D structural hydrogel TiO 2 -PAA-GO, which proved to be a remarkably effective photocatalysts for degrading DB-71 dye in an aqueous medium. The TiO 2 -PAA-GO photocatalyst demonstrated a removed up to 98.16% for DB-71 by (i) reducing the bandgap energy, (ii) minimizing electron and hole recombination, (iii) enhancing the output of generated electrons, and (iv) providing a large specific surface area.

Synthesized of GO sample
GO was synthesized according to the method described below. First, graphite (3 g) was added to a flask containing H 2 SO 4 (98%, 360 ml) and H 3 PO 4 (90%, 40 ml) The mixture was stirred for 25 min. Subsequently, 18 g of KMnO 4 was carefully added using a spatula while vigorously stirring the solution. The solution was stirred at 80°C-90°C for 3 h, after which 120 ml of deionized (DI) water was slowly added to avoid partial reaction of the solution. The mixture was then allowed to cool to room temperature, and H 2 O 2 solution (30%, 10.5 ml) was added, followed by stirring for 20 min. The mixtures were washed with 35% HCl solution and the centrifuged at 5000 rpm for 15 min using a CN 650 centrifuge (Taiwan). Finally, the GO mixture was washed again in water, and the solutions were neutralized to pH 7 before being dried in an oven at 50°C for 10 h.

Synthesized of the TiO 2 -PAA-GO sample
200 mg of GO sample was added to 40 ml of distilled water to obtain a 5 g l −1 GO solution. Separately, 67 mg of PAA was dissolved in 10 ml of distilled water. The GO mixture was added to the PAA solution and stirred for 1 h at room temperature, followed by ultrasonically irradiated for 30 min to obtain solution 1. In parallel, 1.25 ml of TIOT was dissolved in 10 ml of ethanol to obtain a colorless solution (solution 2). Solution 1 was then added to solution 2 and stirred for 2 h at room temperature. The resulting suspension was ultrasonicated for 1 h and transferred to a hydrothermal autoclave, where it was heated to 180°C for 6 h. Finally, the dark brown solid was dried in an oven at 60°C for 10 h to obtain TiO 2 -PAA-GO.

Photocatalytic activities of TiO 2 -PAA-GO sample
The photocatalytic degradation of TiO 2 -PAA-GO was monitored by observing the degradation of DB-71 under visible-light irradiation at room temperature. Specifically, 100 mg of the TiO 2 -PAA-GO photocatalyst was added to 100 ml of DB-71 (25 mg l −1 ) solution and stirred in the dark for 1 h to allow for adsorption equilibrium. The reaction system was then illuminated under visible light from a 36 W compact lamp while stitrring. At 15 min intervals, 2 ml of the DB-71 solution sample was collected, and the solids were separated psychologically. The liquid was then analyzed using the UV-vis method at a maximum wavelength of 587 nm to determine the concentration of BD-71 after the photocatalytic reaction [28]. The degradation performance of DB71 (H%) was determined using equation (1): Where, %H is the removal performance of DB-71, C o is the initial concentration of DB-71 (mg l −1 ), and C t is the concentration of DB-71 at various time intervals (mg l −1 ).
To control the pH of the mixture, 0.1 N HCl and 0.1 N NaOH solutions was using during the reaction.   The N 2 adsorption-desorption isotherms of the TiO 2 , GO, and TiO 2 -PAA-GO samples are shown in figure 5. The N 2 adsorption-desorption isotherms for the synthesized samples follow type IV according to IUPAC cloassification [41]. The TiO 2 -PAA-GO nanocomposite exhibites a smaller hysteresis loop compared to GO due to the filling of space between the GO layers with TiO 2 nanoparticles, leading to reduced capillary condensation [1].

Results and discussion
As shown in table 2, the GO material displays the largest surface area (119 m 2 g −1 ), medium diameter (6.68 nm) and the high pore volume (0.17 cm 3 g −1 ), while the TiO 2 sample displays the high pore volume   material has a higher surface area than the bare GO and TiO 2 samples because GO effectively controls the formation of TiO 2 nanoparticle size (see SEM images in figure 8).
The UV-vis DRS spectra of the GO, TiO 2 , and TiO 2 -PAA-GO samples recorded in the 200-800 nm range are shown in figure 6. The TiO 2 sample exhibited strong UV absorption with an absorption edge wavelength of 390 nm ( figure 6(a)). Meanwhile, the GO sample showed a broad light absorption capacity within the 200-800 nm range. In contrast, the TiO 2 -PAA-GO sample showed an absorption peak shift towards the visible light region, accompanied by an significantly increase in the visible light absorption intensity compared to the TiO 2 sample. The enhanced visible light absorption capacity of the TiO 2 -PAA-GO nanocomposite could be result from the chemical interaction between the TiO 2 and GO or the formation of a common electronic system between them, leading to an reduction in the band-gap energy [42,43]. In the TiO 2 -PAA-GO nanocomposite, TiO 2 acted as an electron donor, while GO acted as an electron acceptor. In the presence of GO and PAA electron acceptors, photoexcited electrons are transferred to the GO and PAA layers via π-π * electrons, emptying the conduction band [36]. As a result, the Fermi energy level is reduced and bends towards the valence band, causing a decrease in the bandgap energy value [44]. The narrowing of the band gap energy of TiO 2 -PAA-GO nanocomposites is due to the transfer of photoconductive electrons in the region, leading to the surface of GO   across the TiO 2 -PAA-GO interface via carboxyl groups, thereby shortening the electron transmission pathway [44]. The ability to absorb light is crucial for efficient excitation of holes and electrons [3]. Therefore, the photocatalytic activity depends on both the number of photo-excited carriers and the efficiency of the electron separation process [1]. The band gap energies of TiO 2 and TiO 2 -PAA-GO were also estimated using a Tauc graph [45].  figure 7(a). The TiO 2 sample showed a dominant luminescence peak at 435 nm, indicating the recombination of holes in the valence band and electrons in the conduction band, known as excitonic emission [46]. The PL spectrum of the TiO 2 -PAA-GO composite was similar to that of the TiO 2 sample, indicating no new luminescence was generated upon adding GO and PAA [47]. However, the intensity of the PL spectrum decreased significantly upon combining TiO 2 and GO due to the prevention of electron-hole recombination. GO trapped electrons, which created surface defects in the TiO 2 nanostructures, affecting their luminescence efficiency. Alternatively, electrons can be trapped by defective oxygen vacancies, reducing the recombination of electrons and holes [48]. Thus, the hybridization of TiO 2 nanostructures with GO significantly changed the luminescence properties of TiO 2 materials [49].
To further elucidate the separation and transfer features of the photoelectron-generating electron-hole pairs, photocurrent response tests of the GO, TiO 2 , and TiO 2 -PAA-GO samples were performed. The transient optical currents measured between 0 and 270 s in the off and on cycles was shown in figure 7(b). The TiO 2 -PAA-GO composite exhibited a higher current density than the TiO 2 and GO samples, indicating more efficient separation of electron-hole pairs (e − /h + ) due to the synergistic effect between the TiO 2 nanoparticles and GO [1]. Furthermore, the Ti-O-C bond formation caused electrons in the conduction band (CB) of TiO 2 to be excited to the GO surface. The low-resistance path in the TiO 2 structure promoted direct electron transport and increased the optical current [44].
The charge transfers rates of TiO 2 and TiO 2 -PAA-GO samples were determined by the EIS method. For EIS measurements, a 1 M Na 2 SO 4 solution was used as the electrolyte with a frequency range of 0.01 to 100 kHz. In figure 6(d), the Nyquist plot of the TiO 2 -PAA-GO sample displayed a smaller semicircle compared to that of TiO 2 sample, indicating a faster charge transfer rate. This outcome can be attributed to the strong interaction between the TiO 2 and GO phases [23].
The SEM image of the GO carrier shows a thin plate structure with multiple folds and stacked layers [50]. The TiO 2 material exhibits a spherical shape, with nanoparticle size ranging from 5 to10 nm, which tend to cluster together forming particles of 50-200 nm. For the TiO 2 -PAA-GO nanocomposite, the TiO 2 nanoparticles with a particle size of approximately 5 nm were uniformly coated and dispersed on the GO thin layers. This result indicated that the TiO 2 -PAA-GO sample was successfully synthesized using the hydrothermal method.
The TEM images ( figure 9) show that the GO sheets have a light color, while the TiO 2 nanoparticles have a darker color. TiO 2 nanoparticles exhibit high crystallinity, a uniform particle size of approximately 5 nm, and a face distance (101) of approximately 0.35 nm. This result indicated that the TiO 2 -PAA-GO sample was successfully synthesized using the hydrothermal method.
The swelling properties of TiO 2 -PAA-GO materials were determined by immersing the material with 25 ml of water at 25°C for 24 h. TiO 2 -PAA-GO sample exhibited a lower a swelling at 25°C reaching 40% compared to PAA sample. This decrease in swelling properties can be attributed to two factors: (i) the formation of H bonds between the oxygen-containing functional groups of GO/TiO 2 and the hydroxyl groups of PAA, and (ii) the water-insoluble TiO 2 nanoparticles reduce the diffusion of water molecules into the material [20].

Photocatalytic activities of TiO 2 -PAA-GO
The decomposition of DB-71 under visible-light irradiation over TiO 2 , TiO 2 -GO, and TiO 2 -PAA-GO samples is shown in figure 10(a). The TiO 2 -GO samples, synthesized with PAA (TiO 2 -PAA-GO) and without PAA  (TiO 2 -GO), exhibite significantly higher photocatalytic performance than TiO 2 under visible light irradiation. After 60 min of dark adsorption, TiO 2 , TiO 2 /GO and TiO 2 -PAA-GO samples removed 6.4, 20.34, and 32.42% of DB-71 dye, respectively. Thus, the addition of PAA increased the dye adsorption capacity because PAA contains numerous −OH and −COOH groups that facilitate the π-π * interaction between the dye and the TiO 2 -PAA-GO material [36]. Specifically, after 90 min of irradiation, the remaining concentrations of DB-71 after the photocatalytic reaction were 59.13, 12.64, and 1.98% for the TiO 2 , TiO 2 -GO, and TiO 2 -PAA-GO samples, respectively. Thus, the DB-71 dye was almost completely decomposed over the TiO 2 -PAA-GO photocatalyst after 90 min of reaction, while TiO 2 only degraded 40% of the DB-71 dye. This improvement can be due to the ehanced band-gap energy (figure 6), reduced electron-hole recombination rate ( figure 7(a)), and generation of more electrons ( figure 7(b)). In addition, the addition of PAA facilitates faster electrons movement, and the formation of Ti-O-C bonds generates more electrons (figures 7(b) and (c)).
The TiO 2 -PAA-GO photocatalyst was used to investigate the effects of different DB-71 dye concentrations, pH, catalyst dose, and radicals trap experiments. As shown in figure 10(b), the DB-71 removal efficiency was over 96% when the DB-71 dye concentration ranged from 5 to 25 mg l −1 after 90 min of visible light irradiation. However, the efficiency decreased to 82% when the DB-71 dye concentration increased to 35 to 40 mg l −1 . As shown in figure 10(c), increasing the catalyst dose from 20 to 100 mg l −1 enhanced the the DB-71 dye removal efficiency from 70.11 to 98.16%. The increase in DB-71 removal efficiency increased can be due to the more favorable contact between the active sites of the catalyst and dye [2]. Nonetheless, an excessive amount of catalyst (100-140 mg l −1 ) interfered with the interaction between the inner layers of the catalyst, reducing in the number of e − /h + pairs [51]. The pH values influence the surface area of the TiO 2 -PAA-GO sample (figure 11), whose isoelectric point (IEP) is 6.9. Thus, the TiO 2 -PAA-GO material carries a positive charge at pH values < 6.8 and a negative charge at pH values > 6.9. The effects of pH on the dye removal efficiency are presented in figure 10(d). The DB-71 removal rate increased from 72.35 to 98.26% when the pH was raised from 2 to 6. In contrast, as the pH was increased from 6 to 10, the DB-71 removal efficiency decreased from 98.26% to 80.13%. This decline resulted from the negative charge on the TiO 2 -PAA-GO catalyst surface, which hindered the reaction process and interactions with the negatively charged DB-71 when the pH reached 10. In addition, the accumulated hydroxide groups interfered with the radical scavenging reactions at pH 10, decreasing photocatalytic activity [34]. Thus, under optimal conditions (dye concentration DB-71, photocatalyst mass, pH of 25 mg l −1 , 100 mg l −1 , pH = 6), TiO 2 -PAA-GO can remove up to 98.26% of the DB-71 dye. Experiments to conducted to understand the mechanism of action of the    hydrogel exhibited higher DB-17 degradation ability compared to other materials, indicating it is a highly efficient photocatalyst in dye degradation. The hydrogel is easily separable from the aqueous medium, has no secondary pollution, is conveniently fabricated, and exhibits good reusability.

Conclusions
A TiO 2 -PAA-GO photocatalyst was successfully synthesized using a hydrothermal method. The XRD, FTIR, SEM and XPS results demonstrated the formation of double bonds in TiO 2 , GO and PAA composite. TiO 2 nanomaterials had a particle size of approximately 5 nm, were uniformly distributed on GO layers, and had a high surface area (156 m 2 g −1 ). The presence of −OH and −COOH groups in PAA increased the adsorption capacity of the material, resulting in the enhancement in removal efficiency of DB-71. In addition, the increased visible light adsorption capacity, reduced electron-hole recombination, high surface area, and more generated electrons contributed to the increased the DB-71 removal efficiency. Under optimal conditions, the dye concentration DB-71, photocatalyst dose, pH of 25 mg l −1 , 100 mg l −1 , pH = 6, the photocatalyst TiO 2 -PAA-GO could remove up to 98.26% of the DB-71 dye. The • O 2 − radicals are the main reactants in the DB-71 decomposition reaction. The photocatalyst TiO 2 -PAA-GO material was stable and reusable, maintaining over 90% removal efficiency for DB-71 after six reaction cycles.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).