INTRODUCTION
With advent of photocatalysis, wastewater treatment has witnessed tremendous developments that increases the hope to overcome environmental crises in the near future [1-3]. Oceans and seas all over the world are contaminated with toxic and persistent organic pollutants from industrial zones, farms and cities [4, 5]. As a result, emerging diseases are becoming prevalent, and health agencies have to spend large sum of money to treat these newly occurred diseases [6, 7]. Additionally, fighting with environmental crises stemming from water pollution requires billions of dollars, and developing countries need much more international supports in this regard [8, 9]. 
Photocatalysis is a new technology for removing organic pollutants from water media, which offers several advantages with respect to conventional treatment methods [10-12]. Photocatalysis is based on advanced oxidation processes, utilizing the photo-induced generation of oxidative species to mineralize pollutants [13]. Based on this, photocatalysis is considered as an environmentally friendly approach due to elimination of toxic chemicals used in conventional methods [14, 15]. Moreover, photocatalysis treatment is powerful enough to completely degrade persistent pollutants without generating dangerous intermediates [16]. 
Despite aforesaid advantages, the photocatalytic method suffers from some obstacles that largely limit its efficiency [17]. In this regard, the poor activation of common photocatalyst materials, including titanium dioxide (TiO2) and zinc oxide (ZnO), under visible light irradiation is significant limitation, hindering the widespread utilization of photocatalysis [18]. Consequently, numerous efforts have been devoted to exploiting the visible light spectrum for conducting photocatalytic reactions. In this context, nanocomposite materials benefit from synergistic electronic interactions between components, which effectively enhance the visible light activity of photocatalysts [19].
For examples, Saadati et al. reported synthesis of TiO2-x/FeTiO3 heterojunction photocatalyst for degradation of organic pollutants under the visible light irradiation [20]. Fatima et al. synthesized Yb2O3/ZnFe2O4 nanocomposite for photocatalytic degradation of dye and herbicide [21]. Also, a binary nanocomposite comprising WO3/CeTiO4 has been synthesized for solar activated photocatalytic degradation of reactive orange 30 [22].
In this work, TiO2/BiDyO3 nanocomposite (TiO2/BDO) was synthesized as a highly efficient visible light driven photocatalyst for degradation of organic pollutant. Aqueous solution of acid blue 113 (AB113) was used to examine the photocatalytic activity of the nanocomposite. Various parameters were investigated to study the efficiency of TiO2/BDO nanocomposite for removal of pollutants under different conditions. 

 
MATERIALS AND METHODS
Synthesis of BiDyO3 nanoparticles
BiDyO3 (BDO) nanoparticles were synthesized by a co-precipitation method in an aqueous medium. The required nitrate precursors, Bi(NO3)3.5H2O and Dy(NO3)3.5H2O, were dissolved in water and mixed to obtain a homogeneous precursor solution. Polyethylene glycol was used as a capping agent to influence particle growth of the precipitated solid. NH4OH solution (0.5 M) was then added gradually under continuous mixing until the solution reached neutral pH and precipitation reaction was completed. The solid was collected and washed with water and ethanol to remove soluble residual species. The washed solid was dried at 100 °C and subsequently calcined at 800 °C for 7 h to promote formation and crystallization of the BDO nanoparticles.

Synthesis of TiO2 nanoparticles 
The TiO2 nanoparticles were synthesized by a sol–gel route. Absolute ethanol was used as the reaction medium, followed by addition of a small amount of citric acid and ethylene glycol. Tetrabutyl orthotitanate (TBOT) was then introduced under mixing. The solution was maintained under mild heating in an oven at 60 °C until the sol gradually transformed into a gel. Further heating was applied to obtain a dry gel. The dried gel was finally calcined at 600 °C for 3 h to produce the pure TiO2 nanoparticles.

Synthesis of TiO2/BiDyO3 nanocomposite
The TiO2/BDO nanocomposites were synthesized by adding as-prepared BDO nanoparticles to the TiO2 sol at the amounts 2, 4, 6, and 8 wt.%. The BDO nanoparticles were dispersed into the TiO2 sol under identical preparation conditions for each composition. The resulting mixtures were dried and calcined under the same conditions, yielding TiO2/2-DBO, TiO2/4-DBO, TiO2/6-DBO, and TiO2/8-DBO. 

Characterization
X-ray diffraction (XRD) (Philips X’pert pro MPD) was used to identify crystalline phases and evaluate the structural characteristics and crystallinity of the synthesized nanocomposites. Field emission scanning electron microscopy (FESEM) (TESCAN mira 3) was used to examine particle morphology and surface features. Diffuse reflectance spectroscopy (DRS) (JASCO UV-Vis) was used to investigate optical absorption and the visible-light response of the samples. Photoluminescence (PL) spectroscopy (CARY ECLIPSE) was used to compare the recombination behavior of photogenerated charge carriers among the prepared nanocomposites. 

Photocatalytic experiments
The Photocatalytic degradation processes were performed using 50 mL of an aqueous Acid Blue 113 (AB113) solution with an initial concentration of 50 ppm. The visible light irradiation was supplied by an LED lamp (100 W, white color) from the fixed distance of 30 cm, and the irradiation period was carried out at the interval time of 180 min. Before illumination, the AB113 solution containing the TiO2/BDO nanocomposite was stirred in the dark to establish adsorption equilibrium. The AB113 concentration was subsequently monitored during irradiation by UV–visible spectroscopy. The influence of nanocomposite loading amounts was examined using 0.01, 0.02, 0.03, 0.04, and 0.05 g of the nanocomposite. The initial pH was adjusted to 3, 5, 7, 9, or 11. The effect of H2O2 was investigated at concentrations of 0.01, 0.02, and 0.03 M. 

RESULTS AND DISCUSSION
XRD, FESEM, DRS, and PL results
Fig. 1 exhibits the XRD patterns of the TiO2/x-BDO nanocomposites containing different concentrations of DBO nanoparticles. The observed pattern for the BiDyO3 (BDO) nanoparticles clearly matches with the JCPDS reference code (027-1040) for the cubic structure of bismuth dysprosium oxide. Also, the synthesized TiO2 shows the pattern corresponds to the anatase phase of titanium dioxide (JCPDS code: 004-0477). As can be seen form the Fig. 1, the incorporation of BDO into the TiO2 matrix brings two diffraction peaks at 2ϴ around 28° and 32°, which are intensified as the BDO concentration increases. Also, a shoulder can be observed at around 47°, which is attributed to the BDO phase.   
The FESEM image in Fig. 2a illustrates the morphological features of the TiO2/6-BDO nanocomposite. The quasi-spherical particles in the size range around 100 nm are clearly observed. In addition, the EDX spectrum (Fig. 2b) revealed the composition of the nanocomposite, including Ti (71.31%), Bi (3.11%), Dy (2.14%), and O (23.44%).  
The optical properties of the synthesized nanocomposite were analyzed using DRS and PL spectroscopy. Fig. 3a shows the DRS spectra for the pure TiO2 and TiO2/x-BDO nanocomposites. As clearly observed, the light absorption in the visible region improved with increasing the concentration of BDO. The TiO2/6-BDO nanocomposite possesses the significant visible light absorption, while incorporation of 8 wt% of BDO into the TiO2 network led to slight decrease in the visible light characteristic. One the most important features observed in DRS spectra is the absorption edge, which shifted to the longer wavelengths when the BDO content increased. The optical band gap for the nanocomposites was calculated by plotting (αhν)2 with respect to hν and  extrapolating the linear portion of the curves to the hν axis [23]. The results showed that the band gap values were 3.29, 3.21, 2.84, 2.57, and 2.69 eV for the pure TiO2, TiO2/2-BDO, TiO2/4-BDO, TiO2/6-BDO, and TiO2/8-BDO, respectively.
Furthermore, the PL spectra in Fig. 3b provide further information on the optical properties of the TiO2/x-BDO nanocomposites. Given that the photoluminescence originates from returning the excited electrons to the ground states, the PL intensity is directly associated to the effectiveness of accomplished mechanisms to retard the recombination of the electron-hole pairs [24]. In this regard, the pure TiO2 represents the strong PL peaks, revealing the fast recombination of the photo-generated electrons and holes. The incorporation of the BDO into the TiO2 matrix forms the new energy levels that facilitate the electron transfer, thereby decreasing the recombination rate of the charge carriers [25, 26]. As seen, the TiO2/6-BDO nanocomposite shows the lowest PL intensity, confirming the improved separation of charge carriers and the photocatalytic activity. However, the nanocomposite containing 8 wt% of the BDO nanoparticles has the slight increase in the PL intensity compared with the TiO2/6-BDO nanocomposite.

Photocatalytic activity of TiO2/x-BDO nanocomposites
The visible light photocatalytic activity of the pure TiO2 and the TiO2/DBO nanocomposites was first compared at the fixed photocatalyst amount of 0.04 g. Fig. 4 shows after 180 min of the photocatalytic reaction, the pure TiO2 produced only 16.83% degradation of AB113. Incorporation of 2 wt% BDO nanoparticles increased the degradation to 49.11%. Increasing the BDO content to 4 wt% raised the removal efficiency of the AB113 to 89.92%, while TiO2/6BDO nanocomposite reached 91.77%. Further addition of BDO nanoparticles to 8 wt% caused the efficiency to decline to 80.35%.
The marked improvement after coupling BDO nanoparticles with TiO2 indicates that the BDO has the beneficial role in visible-light photocatalysis. Formation of the new energy levels can provide a pathway for photogenerated charge migration between the two components, resulting in the improved separation of electron–hole pairs [27]. These electronic interactions also enhance the utilization of the visible light irradiation, which was confirmed by PL and DRS results (Fig. 3). However, the decline observed at 8 wt% suggests that the amount of the second phase must be optimized. Excess BDO nanoparticles may cover active TiO2 sites, alter the accessibility of the interface, increase light scattering, or introduce defect sites that facilitate charge loss. 

Different loading amounts of TiO2/6-BDO nanocomposite
Fig. 5 shows the photodegradation of the AB113 solution using different amounts of the TiO2/6-BDO nanocomposite. As can be seen, 0.01 g of the TiO2/6-BDO nanocomposite produced 32.61% degradation efficiency after 180 min. In addition, the gradual increase in the photocatalytic efficiency was provided using increasing the amount of the nanocomposite. The highest removal of AB113 was approximately 96.12%, which was obtained with 0.05 g of the TiO2/6-BDO nanocomposite. The observed enhancement in the photocatalytic activity is attributed to the greater number of available active surface sites upon addition of more amount of the nanocomposite [28, 29]. The increased number of active sites imply that the more AB113 molecules can adsorb on the surface of the nanocomposite and then degrade using generated radicals.  

Different pH of AB113
Electrostatic interaction between the AB113 molecules and the TiO2/6-BDO surface is particularly important. Hence, the effect of AB113 solution pH on the photocatalytic efficiency was studied. As shown in Fig. 6, the degradation efficiency at pH 3 reached approximately 98%, which was slightly higher than the degradation results obtained pH 7 (96%). However, the significant reductions in the photocatalytic activity were observed at pH 9 and 11. 
Due to negative charge of AB113 molecules [30], their adsorption on the active surface sites of the nanocomposite is higher at acidic conditions compared to that of under alkaline pHs [31, 32]. At lower pH, the surface of the nanocomposite is protonated and carries positive charges, which can electrostatically interact with the AB113 molecules.  However, under alkaline conditions, de-protonation of surface groups can increase the negative character of the nanocomposite, increasing the electrostatic repulsion between the negatively charged surface and the anionic dye.

Different concentrations of H2O2
H2O2 was introduced as an auxiliary oxidant to promote the visible-light photocatalysis for removal of AB113. The H2O2 concentration was varied from 0.01 to 0.03 M and its effect was investigated on the removal efficiency of AB113 using the TiO2/6-BDO nanocomposite, as shown in Fig. 7. The results showed that increasing the H2O2 concentration initially improved AB113 degradation, with the best performance obtained at 0.01 M and degradation approaching complete removal. The improvement can be associated with the ability of H2O2 to participate in the generation of highly reactive radical species and thereby provide additional oxidation levels [33]. However, the benefit did not continue indefinitely. At 0.02 and 0.03 M, the degradation decreased, indicating that excessive H2O2 interferes with the radical production [34].

Reusability of nanocomposite
The reusability of the TiO2/6-BDO nanocomposite was evaluated over six consecutive photocatalytic cycles. After each cycle, the nanocomposite was recovered using centrifugation at 5500 rpm for 5 min, washed with water and ethanol, dried, and reused under the same reaction conditions. Fig. 8 shows the reusability results for the degradation of AB113 using 0.5 g of the TiO2/6-BDO nanocomposite. As can be seen, the nanocomposite maintained the high level of activity throughout the repeated tests. However, after the sixth cycle, the degradation efficiency decreased by approximately 2.27%, which is attributed to the gradual loss of the nanocomposite during recovery, washing, and transfer between cycles. Overall, the degradation efficiency remained unchanged through the third cycle, indicating good stability of the synthesized TiO2/6-BDO nanocomposite under repeated operation. 

CONCLUSION
In conclusion, the TiO2/BiDyO3 (TiO2/BDO) nanocomposite showed good potential for the photocatalytic degradation of AB113 under visible light. The results indicates that the addition of BiDyO3 to TiO2 improved the potential properties of the nanocomposite by decreasing the band gap to 2.57 eV and enhancing the separation of charge carriers. Among the synthesized samples, TiO2/6-BDO showed the highest photocatalytic activity. It was also found that 0.05 g of this nanocomposite was the optimum photocatalyst amount for AB113 degradation. The degradation process was more effective under acidic conditions, which can be related to the positively charged surface of the nanocomposite and its favorable interaction with the anionic AB113 molecules. The presence of H2O2 also increased the degradation efficiency. Furthermore, the photocatalyst showed good stability after six consecutive cycles, confirming its potential for repeated use. These results suggest that the synthesized TiO2/6-BDO nanocomposite can be effectively used for the degradation of AB113 in the visible-light-driven photocatalytic processes.  

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.