TiO2/Ni2CuO3 Nanocomposite as the Efficient Visible-Light Photocatalyst for Degradation of Methyl Orange

INTRODUCTION
In recent decades, a wide range of pollutants, such as pesticides, dyes, oil and chemicals, have been released into rivers and seas [1, 2]. As a result, incident of fatal diseases, including cancer, has increased, promoting public health authorities to search for effective solutions [3]. At the same time, these pollutants have severely damaged ecosystems, leading to the extinction of many species [4, 5].
Recently, environmentally-friendly processes have been emphasized for removing pollutants, aiming to minimize the harmful impacts associated with the use of harsh chemical treatment methods [6, 7]. In this context, photocatalysis have received considerable attention due to its advantages, such as no needs for toxic reagents, producing low hazardous by-products, and utilizing renewable energy resource—light [8, 9]. However, common photocatalysts, including TiO2 and ZnO, have limitations to harness the visible light, as they are activated by absorption of UV light—consisting only 5% of light spectrum [10]. Additionally, their efficiency is affected by fast recombination rate of the photo-generated electrons and holes, thereby reducing their effectiveness for practical uses [11].
To overcome these obstacles, many scientific communities are endeavoring to synthesize the visible-light sensitized photocatalysts [12]. There are several proposed approaches for enhancing the light absorption abilities of the common photocatalyst materials. In this regard, doping metal and non-metal ions have been reported that significantly increase the photoactivity of TiO2 [13, 14]. Moreover, coupling two or more metal oxides can effectively reduce the recombination rate of the electrons and holes [15, 16]. It materializes with the electronic transitions between the different energy levels of the different coupled materials [17, 18]. For instance, it has been reported that electronic transitions between TiO2 and WO3 can enhance the charge separation and the photocatalytic activity [19]. Also, Zhang and coworkers, reported the robust visible light photoactivity for TiO2@Bi2O3@TiO2 [20]. Kumar et al. studied photocatalytic degradation of pollutant using CeO2-TiO2@CNT [21].
Considering the photocatalyst materials previously reported in literature, a mixed metal oxide nanocomposite of TiO2 and Ni2CuO3 was synthesized. To this end, two different synthetic methods were used. The NiCuO3 nanoparticles were synthesized through precipitation reaction. Then, varied amounts of the NiCuO3 nanoparticles were embedded to TiO2 using sol-gel method. The photocatalytic activity of the synthesized nanocomposite was investigated by degradation of methyl orange (MO) under visible light irradiation. Also, the photoactivity of the synthesized nanocomposite was carefully assessed under different experimental conditions.

MATERIALS AND METHODS
Coprecipitation synthesis of Ni2CuO3 (NCO) nanoparticles
The NCO nanoparticles were synthesized by simple coprecipitation method, as follows: specific amount (0.1 g) of polyethylene glycol (PEG 400) was dissolved into 100 mL of distilled water as a capping agent. Then, Ni(NO3)2.6H2O (2.0 mmol) and Cu(NO3)2.3H2O (1.0 mmol) were added and the solution was stirred for 30 min. After that, the pH of solution was adjusted to about 9 using adding NaOH (0.1 M) dropwise. The solution was further stirred for 1 h. Next, the solid was separated using centrifugation at 6000 rpm for 10 min and then washed several times using ethanol/water solution. Finally, the solid was dried in an oven at 100 °C overnight.

Sol-Gel synthesis of TiO2 nanoparticles
First, 0.05 g of polypropylene glycol (PPG) was dissolved into absolute ethanol and then specific amount of tetrabutyl orthotitanate (TBOT) was added. The solution was stirred for more than 1 h. Next, the solution was heated in an oven at 60 °C to form a viscous gel, which then further heated in a furnace at 600 °C for 4 h to form pure and crystalline TiO2 powder. 

TiO2/Ni2CuO3 (TiO2/NCO) nanocomposite
Different amounts of the as-synthesized nanoparticles were mixed to prepare TiO2/NCO nanocomposite. For this purpose, NCO nanoparticles (2 wt%, 4 wt%, and 6 wt%) were dispersed into the TiO2 sol solution, and the solution was vigorously stirred for 2 h. The solution was converted to a viscous gel by heating and subsequently calcined to synthesized TiO2/NCO nanocomposite. The drying and calcination temperatures were the same as those described in Section 2.2. The TiO2/CNO-2, TiO2/CNO-4, and TiO2/CNO-6 stand for the TiO2/CNO nanocomposite containing different amounts of CNO nanoparticles 2 wt%, 4 wt%, and 6 wt%, respectively.  

Photocatalytic experiments
The photocatalytic efficiency of the TiO2/NCO nanocomposites was studied using degradation of methyl orange (MO) under visible light irradiation. For that, a constant concentration of MO solution (40 ppm) was subjected to photocatalysis using varying amounts (0.03, 0.05, 0.07 g) of the prepared nanocomposites containing different NCO nanoparticles loadings (2 wt%, 4 wt%, and 6 wt%).
In order to illumination, an Osram lamp (400 W) was used as the visible light source, and illuminating was performed from constant distance of 50 cm. Before starting the illumination, the solution was stirred for 30 min in the darkness to reach the adsorption/desorption equilibrium. After that, the photocatalysis was initiated and the degradation level of MO was monitored by measuring the adsorption of MO at maximum wavelength (465 nm) using UV/Vis spectrophotometer. The procedure was carried out every 30 min by collecting 1 mL of the MO solution. The photocatalyst particles were separated using centrifugation at 700 rpm for 15 min. 
The photocatalysis of MO solution was conducted under different experimental conditions to find the optimum performance of the synthesized nanocomposites, including: different pH of MO solution and different amount of nanocomposites (TiO2/CNO-2, TiO2/CNO-4, and TiO2/CNO-6).    

Characterization
Crystallinity and phase structure of the synthesized nanocomposites were studied using X-ray diffraction (XRD) patterns by Philips X’pert Pro MPD. Optical properties and optical band gap values were investigated for the different nanocomposites using diffuse reflectance spectroscopy (DRS) by Shimadzu UV-670 spectrophotometer. Composition and morphology of the synthesized nanocomposites were studied using field emission electron microscopy (FESEM) equipped with energy dispersive X-ray analyzer by TESCAN mira 3. Photoluminescence (PL) analysis was carried out by Varian Cary Eclipse fluorescence spectrophotometer at excitation wavelength of 300 nm. 

RESULTS AND DISCUSSION
XRD results
Fig. 1 shows the XRD patterns for the different synthesized samples, including pure TiO2, Ni2CuO3, TiO2/NCO-2, TiO2/NCO-4, and TiO2/NCO-6. As can be seen, the XRD pattern of the pure TiO2 is consistent with diffraction planes corresponded to the anatase phase of TiO2 (JCPDS file no.04-0477). The synthesized Ni2CuO3 nanoparticles represents the reflections which are matched with the orthorhombic phase of nickel copper oxide (JCPDS file. No 040-0959). As for the synthesized nanocomposites, the obtained patterns are consisted of both TiO2 (marked with) and Ni2CuO3 (marked with ⨂) phases. The diffraction peaks at 2ϴ = 37.1°, 43.7°, and 61.9° are related to the Ni2CuO3 phase which are intensified with increasing the concentrations of the Ni2CuO3 within the nanocomposites. 

DRS results
The DRS spectra for the pure TiO2, TiO2/NCO-2, TiO2/NCO-4, and TiO2/NCO-6 samples are provided in Fig. 2a. As seen, by increasing the concentration of Ni2CuO3 phase in the nanocomposite to 4wt%, the light absorption in the visible light region increases. This observation can be attributed to the formation of new energy states, which effectively narrows the band gap of TiO2 [22]. The decreased band gap of the TiO2/NCO-4 nanocomposite favors for the visible-light harvesting, reflecting to the enhanced absorption in the range of 400 nm to 700 nm. However, more amount of the Ni2CuO3 (6 wt%) have no positive effect on the light absorption ability of the nanocomposite. For comparison, pure TiO2 showed the weak light absorption in the visible light region. 
Fig. 2b shows the (αhν)2 versus hν plots for the prepared samples. As expected, the TiO2/NCO-4 nanocomposite possess lower band gap value of 2.65 eV, while pure TiO2, TiO2/NCO-2, and TiO2/NCO-6 have the band gaps of 3.46, 2.84, 2.76 eV, respectively. 

PL results
Fig. 3 represents the PL spectra of the samples, showing the lower PL intensity for the TiO2/NCO-4 which reveals the decreased recombination rate of the charge carriers caused by addition of the Ni2CuO3 phase into TiO2 structure. In agreement with the DRS results, the TiO2/NCO-4 have the enhanced charge separation due the induced electron transitions between the energy states of the TiO2 and Ni2CuO3 phases. Incorporation of 6wt% of the Ni2CuO3 into the nanocomposite caused to the increased recombination rate of the charges, highlighting in the slightly elevated PL intensity for the TiO2/NCO-6 nanocomposite. Additionally, pure TiO2 have the highest PL intensity, which implies that the photogenerated electrons and holes participate mainly to the radiative relaxation. These observations confirmed the efficient charge transitions between different energy levels of the incorporated TiO2 and Ni2CuO3 nanoparticles [23, 24].

FESEM and EDX results
The FESEM image for the pure TiO2 (Fig. 4a) shows the spherical nanoparticles in the size range below 100 nm. Due to the high surface energy of the nanoparticles, the agglomerated particles also are observed in the FESEM image [25]. Additionally, Fig. 4b exhibits the morphological properties of the TiO2/NCO-4 nanocomposite. The very fine nanoparticles attached to the surface of the larger particles are observed, which can be attributed to the presence of Ni2CuO3 nanoparticles.  
The EDX spectra for the pure TiO2 and TiO2/NCO-4 are provided in Fig. 4c-d, respectively, verifying the presence of the constituents elements with their relative amounts given below: of Ti (51.22%), O (48.78%) for the pure TiO2 sample; Ti (46.91%), Ni (2.37%), Cu (1.15%), and O (49.57%) for the TiO2/NCO-4 nanocomposite.

Photocatalysis results
The photocatalytic activity of the synthesized samples, including pure TiO2, TiO2/NCO-2, TiO2/NCO-4, and TiO2/NCO-6, was assessed for degradation of MO solution under visible light irradiation. Fig. 5 shows the degradation of MO solution under 150 min visible light illumination. As seen, the TiO2/NCO-4 nanocomposite exhibits the highest photoactivity about 94.17% for the degradation of MO solution. This observation is reasoned with the results of DRS and PL analyses, confirming the excellent photoactivity of the TiO2/NCO-4 nanocomposite. The pure TiO2, TiO2/NCO-2, and TiO2/NCO-6 showed the photocatalytic activities of the 27.48%, 51.14%, and 82.37%, respectively.
The effect of the different amounts of the loaded photocatalyst was studied for the TiO2/NCO-4 nanocomposite. Fig. 6 shows that using more than 0.05 g of the TiO2/NCO-4 nanocomposite lead to the decrease in the photocatalytic degradation of MO solution. This result can be attributed to the limitation of light beam to activate the photocatalyst in the turbid solution. The suspended photocatalyst particles, in exceeded amount, scatter the light beams, thereby limiting their penetration through the turbid solution [26, 27].
In addition, the effect of the different pH values of MO solution on the photocatalytic activity of the TiO2/NCO-4 nanocomposite was studied, provided in Fig. 7. Due to anionic nature of the MO dye [28, 29], the positively charged surface of the nanocomposite under acidic conditions facilitates the higher degradation level of the MO solution [30]. As seen from the Fig. 7, the photocatalytic activity of the MO solution increases to 99% by addition of HCl solution (0.5 M) until reaching the pH of 3. In contrast, under alkaline condition (pH = 9), the photocatalytic degradation of MO solution was dramatically decreased to 41.39%.     
The reusability experiments were accomplished for the TiO2/NCO-4 nanocomposite over 6 successive reaction cycles. After each reaction cycle, the nanocomposite particles were separated using centrifugation at 700 rpm for 15 min, washed several times with distilled water, and then dried at 100 °C for 5 h. Fig. 8 exhibits the stability of the TiO2/NCO-4 nanocomposite over 6 reaction cycles. The TiO2/NCO-4 nanocomposite showed the excellent stability until 4th reaction cycles, without any significant decrement in the photocatalytic efficiency. But, after 5th and 6th reaction cycles, the decrease in the photocatalytic efficiency was 1.86% and 2.53%, respectively.

CONCLUSION
In summary, the synthesis of TiO2/NCO nanocomposites containing different concentrations of Ni2CuO3 was reported as the efficient visible-light photocatalyst for degradation of organic dye. Using DRS and PL analyses, it was confirmed that the TiO2/NCO-4 containing 4 wt% of the Ni2CuO3 has the lower optical band gap (2.65 eV), along with enhanced the visible-light absorption and improved the charge separation. 
To assess the photocatalytic activity of the TiO2/NCO nanocomposites, the visible light photocatalysis of the MO solution was performed under 150 min illumination. During this time, the TiO2/NCO-4 showed the highest photocatalytic efficiency (94.17%) for removing of the MO solution. Conducting the photocatalytic reactions under different experimental conditions revealed that the optimal photocatalytic degradation was achieved with the 0.05 g of the TiO2/NCO-4 nanocomposite in the acidic MO solution. Furthermore, the TiO2/NCO-4 nanocomposite exhibited the appreciable stability over 6 consecutive reaction cycles.

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