How Is Photocatalytic Activity of TiO2/In2S3/Cu and TiO2/In2S3/Ag Nanocomposites under Visible Light?

INTRODUCTION
Recently, using photocatalysts for purifying water and wastewater has received incredible attention around the world because it works by a renewable source of energy. In addition, it operates at room temperature without additional power sources [1]. There are many different photocatalytic materials but TiO2 is recognized as the excellent photocatalytic material due to its high oxidation potential, brilliant photoactivity, nontoxicity, physical and chemical stability, and earth abundancy [2-4]. It was illustrated that contaminant removal started by redox reactions on the surface of the catalyst. First, photon absorbed by the surface of catalyst and generates electron-hole pairs. These generated charge carriers migrate to the surface of the catalyst and contribute to a degradation reaction [5-7]. The produced holes generate hydroxyl radical by reacting with the surface-trapped H2O.  Hydroxyl radical could oxidize most organic/inorganic pollutants [8]. On the other hand, the photogenerated electrons react with O2 and produced O2-.radicals [8].
Various synthesis methods have developed to increase the surface area of the catalyst, for example, producing TiO2-based nanoparticles, [9, 10] and nanorods, [11, 12]. An important issue that should be solved is that electron-hole pairs in TiO2 just generated by absorption the ultraviolet (UV) range (~5 % of the solar spectrum). This happens due to the large band gap of TiO2 (> 3.0 eV for rutile). In this case, much of the sunlight cannot be used for a photocatalytic reaction [13, 14]. 
Many types researches were done to solve this problem by decreasing the band gap by doping pure TiO2 with a dopant such as N, Fe, S, etc. [15, 16].  The additional energy states generated by adding dopant can extend the absorption spectrum of TiO2 to the visible range but intermediate energy states from the introduced atoms and defects can serve as traps and increases electron-hole recombination rate. This could drop off the degradation efficiency for doped TiO2 [17-19]. Therefore, decreasing the band gap of TiO2 by adding dopant is not a suitable solution. Physically or chemically attaching a heterogeneous material (called a co-sensitizer) on TiO2 could be a brilliant solution.  This can extend the absorption peak of TiO2 to visible range in the in an efficient and more flexible way. Typical photocatalytic materials such as CdS and CdSe have been used as a sensitizer to change the band gap and extend the absorption peak of TiO2 to the visible range [20, 21]. The Photogenerated electrons in sensitizer could transfer to the conduction band of TiO2 If the conduction band of sensitizer located above the conduction band of TiO2 [22-25]. However, choosing co- sensitizers are limited in terms of their water-solubility, toxicity, and performance. 
In present work, we prepared TiO2/In2S3/Ag and TiO2/In2S3/Cu nanocomposite to achieve high photocatalytic activity under visible light. TiO2/In2S3/Ag and TiO2/In2S3/Cu nanocomposite enjoy both co-sensitizer and plasmonic effects. TiO2/In2S3/Ag and TiO2/In2S3/Cu nanocomposites were prepared by photo reduction and hydrothermal method. We studied the effect of different parameters such as reaction time and temperature on the morphology of TiO2/In2S3/Ag and TiO2/In2S3/Cu nanocomposites. As-prepared TiO2/In2S3/Ag and TiO2/In2S3/Cu nanocomposites show promising photocatalytic activity under visible light. 

MATERIALS AND METHODS
Preparation of TiO2/In2S3 composite
Firstly, 0.55 g of InCl3.4H2O and 0.22 g of thioacetamide were dissolved into 40 mL of distilled water, and then TiO2 was added into the solution. The mixture was transferred into stainless steel autoclave and heated at 140 C for 8 h. The samples were collected after being filtered and washed with distilled water and finally dried at 60 ◦C in air. 

Preparation of TiO2/In2S3/Ag composite
Firstly, 0.25 g of AgNO3 was dissolved into 40 mL of distilled water, and then TiO2/In2S3 was added into the solution. The mixture was irradiated for 2 h under visible light. The samples were collected after being filtered and washed with distilled water and finally dried at 60 ◦C. 

Preparation of TiO2/In2S3 /Cu
Firstly, 0.15 g of Cu(NO3)2 was dissolved into 40 mL of distilled water, then TiO2/In2S3 composite was added into the solution. The mixture was irradiated for 2 h under visible light. The samples were collected after being filtered and washed with distilled water and finally dried at 60 ◦C.

Photocatalytic activity test 
Certain amount of catalyst was dispersed in 50 mL water containing different organic pollution with 5 ppm in ‎concentration. This suspension kept in dark place for 2 h to equilibrium dye absorption on the surface of the catalyst. Afterward, photocatalysis test was started by irradiation visible light with 400 W in power.   

RESULTS AND DISCUSSION 
In this research, we prepared TiO2/In2S3/Ag and TiO2/In2S3/Cu composites as efficient visible driven photocatalysts. TiO2/In2S3/Ag and TiO2/In2S3/Cu composites enjoy both co-sensitizer and plasmonic effects. The XRD patterns for TiO2/In2S3, TiO2/In2S3/Ag, and TiO2/In2S3/Cu composites are presented in Fig. 1 a-c, respectively. As can be seen in Fig. 1 a, TiO2 has anatase phase and shows good agreement with JCPDS No. 21-1272. The XRD pattern of In2S3 could be assigned to β-In2S3 structure (JCPDS No. 65-0459). There are no impurities such as In2O3, InS or In(OH)3, are detected. Fig. 1 b shows that TiO2/In2S3/Ag nanocomposite successfully prepared. We indicated peaks related to the Ag in Fig. 1 b. XRD pattern related to the TiO2/In2S3/Cu composites is illustrated in Fig. 1 c. Bedside the peaks related to the TiO2/In2S3, peaks related to the Cu appeared.  Fig. 2 a-c shows the EDX results for TiO2/In2S3 nanocomposite, TiO2/In2S3/Ag nanocomposite, and TiO2/In2S3/Cu composites, respectively. Fig. 2 a shows the presence of Ti, O, In, and S elements related to the TiO2/In2S3 nanocomposite. EDX presented in Fig. 2 b indicted that sample containing Ti, O, In, S, and Ag which could be assigned to TiO2/In2S3/Ag nanocomposite. According to Fig. 2 c, the sample is containing Ti, O, In, S, and Cu which are in good agreement with TiO2/In2S3/Cu nanocomposite. Fig. 3 a- e shows the effect of hydrothermal time and temperature on the morphology of TiO2/In2S3/Ag nanocomposite. As seen in Fig. 3 a, TiO2/In2S3/Ag nanocomposite with an average size of 40- 150 nm were prepared when the reaction time and temperature were 6h and 140 °C. By changing the reaction time to 8h, very uniform TiO2/In2S3/Ag nanocomposite with 20- 40 nm in diameters were formed (Fig. 3 b). According to Fig. 3 c, particles with an average size of 40- 60 nm were formed when the reaction time was 10 h. Fig. 3 d shows that particles size are about 10-20 nm when the temperature was 120 °C. Particles size increased up to 1µm when reaction temperature was 160 C (Fig. 3 e). We studied the effect of reaction time on the morphology of TiO2/In2S3/Cu nanocomposite as well as. For these three different times including 6, 8, and 10 h was studied. Fig. 4 a shows TiO2/In2S3/Cu nanocomposites with 50-100 nm in diameters were formed when the reaction time was 6 h. When reaction time was 8 h, Particles with size about 20- 50 nm were fabricated (Fig. 4 b). By changing the reaction time to 10 h, particles size increased to 50-150 nm (Fig. 4 c). The effect of reaction temperature on the morphology of TiO2/In2S3/Cu nanocomposites was evaluated by preparing TiO2/In2S3/Cu nanocomposites at 120, 140, and 160 C. As can be seen in Fig. 4 d, large size distribution was observed when the reaction temperature was 120 °C (Fig. 4 d). By increasing the reaction temperature to the 140 C, more uniform particles are observed (Fig. 4 b). Particles stack together and form larger particles (about 200 nm) when reaction temperature was 160 C (Fig. 4 e).  
We used Diffuse Reflectance Spectroscopy (DRS) to study the effect of In2S3, Ag, and Cu on the absorption of TiO2. According to Fig. 5 a, TiO2/In2S3 has the broad absorption peak from 200- 600 nm. As can be seen in Fig. 5 b and c, the absorption intensity of TiO2/In2S3 was increased by adding Ag and Cu. It seems Ag had more significant effect due to the higher plasmonic effect [26, 27]. Based on these results, we can expect that TiO2/In2S3/Ag nanocomposites and TiO2/In2S3/Cu nanocomposites show highly photocatalytic activity under visible light. 
We used Rhodamine B (RhB), Methyl orange (MO), Acid Black 1 (AB1), and Acid Brown 214 (AB214) as organic contaminations to study the photocatalytic activity of TiO2/In2S3/Cu nanocomposites and TiO2/In2S3/Ag nanocomposites. In all photocatalytic tests, the concentration of pollution was 5 ppm and 1 g/ L was catalyst was used.  The results for the degradation of these four dyes by TiO2/In2S3/Cu nanocomposites under visible light for 120 min are presented in Fig. 6. As can be seen in Fig. 6 a, degradation efficiency for RhB was 82, 87, 80, 82, and 79 % for sample 1-5, respectively. The degradation efficiency was 78, 86, 74, 83, and 79 % for wastewater containing Mo (Fig. 6 b).  When the organic pollutant was AB1, the degradation yield changes to 74, 87, 73, 86, and 63 % for sample 1-5, respectively (Fig. 6 c). Finally, the degradation rate of 81, 89, 73, 76, and 86 % were achieved when AB214 was used as an organic contaminant (Fig. 6 d). 
When TiO2/In2S3/Ag nanocomposites were used as photocatalysts, the degradation efficiency was 85, 87, 80, 82, and 79 % for RhB (Fig. 7 a).  As can be seen in Fig. 7 b, the degradation yield changes to 78, 84, 74, 82, and 78 % for sample 1-5 when the organic pollutant was MO. The degradation efficiency was 74, 84, 72, 78, and 76 % for wastewater containing AB1 (Fig. 7 c).  By applying TiO2/In2S3/Ag nanocomposites to purify water containing AB214, 80, 87, 74, 78, and 85 % of AB214 was degraded in 120 min (Fig. 7 d). 
Fig. 8 a and b illustrates the degradation results for RhB by TiO2, TiO2/In2S3, TiO2/In2S3/Cu nanocomposites, and TiO2/In2S3/Ag nanocomposites under visible light. Bare TiO2 only degrades 17 % of RhB under 120 min of irradiation, while TiO2/In2S3 degrades 72 of RhB during same irradiation time. This shows that In2S3 successfully boosted the photocatalytic activity of TiO2 under visible light. By adding Cu and Ag to TiO2/In2S3, degradation efficiency increased to 87 and 90 % from 72 %. Fig. 9 schematically describes how In2S3, Cu, and Ag boost degradation efficiency. As seen from Fig. 9, the electrons in the conduction band of In2S3 transferred to the conduction band of TiO2. These electrons reacted with O2 and generated active radicals that could degrade organic pollutions. On the other hand, holes in valance band of TiO2 could jump to the valence band of In2S3 and reacted with H2O and generate radical that could degrade organic pollutions. Cu and Ag help to increased absorption and generated more electron and holes by their plasmonic effect. 

CONCLUSION
In this research TiO2/In2S3/Cu nanocomposites and TiO2/In2S3/Ag nanocomposites were prepared by simple hydrothermal method. The aim was to extend the absorption peak of TiO2 to the visible range and boost its photocatalytic activity under the visible light. Rhodamine B (RhB), Methyl orange (MO), Acid Black 1 (AB1), and Acid Brown 214 (AB214) as organic contaminations to study the photocatalytic activity of TiO2/In2S3/Cu nanocomposites and TiO2/In2S3/Ag nanocomposites were used to study the catalytic activity of TiO2/In2S3/Cu nanocomposites and TiO2/In2S3/Ag nanocomposites under the visible light. Results show TiO2/In2S3/Cu nanocomposites and TiO2/In2S3/Ag nanocomposites are highly efficient photocatalysts under visible light. Adding In2S3/Cu, and In2S3/Ag boosted the degradation efficiency to 87 and 90 % for RhB. TiO2/In2S3/Cu nanocomposites and TiO2/In2S3/Ag nanocomposites are characterized by using XRD, SEM, TEM, DRS, and EDX.  

ACKNOWLEDGMENT
We would like to show our gratitude to the University of Raparin for supporting this work.

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