Ternary hybrid formation and Photocatalytic activity of NiO/CuO/TiO2 microstructure over textile azo dye under solar light irradiation


 The photocatalytic degradation and mineralization of ReactiveOrange30 on NiO/CuO/TiO2 ternary composites has been studied using solar light irradiation. This NiO/CuO/TiO2 ternary composites were prepared by different mass ratios of NiO and CuO/TiO2(3wt% to 15wt.%) on the ethonalic dispersion and annealed at 300˚ C. SEM, UV- Vis DRS, PL, XRD and FTIR analysis has used to characterize the all photocatalysts. NiO/CuO/TiO2 ternary composites demonstrate enhanced photocatalytic activity than NiO/TiO2, CuO/TiO2and pure TiO2 due to separation of photogenerated electrons and holes charges. The NiO/CuO/TiO2ternary composites shows good photostability and the photocatalyst retain the 94% of its initial activity in the seventh cycle, respectively.


Introduction
Increased population and industrial activities have resulted in industrial e uent generation and pollution of water bodies, harmful to humans and aquatic life especially when they contain substances such as dyes, pesticides, heavy metals and pharmaceutical wastes [1]. The world production of dyes has increased dramatically over the past years and these azo dyes, reactive dyes, solvent dyes for instance are used by the textile, food, adhesive, cosmetics, arts, construction, paints, glass and ceramic industries extensively [2]. The release of these coloredorganic substances into our environment is a source of aesthetic pollution and is detrimental to the environmental ecosystems.
High chemical stability and low biodegradability of theseorganic pollutants have necessitated the search for suitabletreatment methods that can easily break down inherent chemical components. Techniques such as adsorption, biodegradation, ozonation and photo-Fenton processes have been adoptedin treating dyes and pesticides pollutants [3]. However, thesemethods have been reported to be ineffective in the treatment of polluted water bodies [4]. Recent studies have been focused on theuse of advance oxidation processes such as photocatalysis for the removal of dyes and pesticides from wastewaters [5].
Titanium Dioxide semiconductor has been widely investigated in waste water treatment using photocatalytic and sonocatalytic process [6,7]. As catalyst, TiO 2 has several advantages such as low cost, and nontoxic [8]. However single metal TiO 2 has several drawbacks that reduce its e ciencies. High rate recombination of electron and holes of TiO 2 is the main problem in waste water treatment [9]. The second problem is the di culty of TiO 2 to separate from the aqueous solution. Therefore we need to nd an effective method to enhance the e ciency of TiO 2 as a catalyst for water treatment. The couple metal oxide is one of the most common solutions for enhancing the degradation e ciency of TiO 2 .
Coupled metal oxide has an ability to reduce recombination of electron-hole pairs so it could enhance the degradation e ciency [10,11]. TiO 2 has coupled with many materials such as CuO, CeO 2 and many others [12]. Among other materials, CuO has been proved to be able to enhance the degradation e ciency compared to single TiO 2 metal oxide. Suitable band edge between n-type TiO 2 and p-type CuO semiconductors facilitates electron and holes to transfer easily between TiO 2 and CuO and vice versa [13].
NiO is chief and cheep semiconducting oxides, which has been mostly consumed in photocatalytic applications. The reason is e ciently suppress the electron-hole recombination and enhance the degradation kinetics by composing with ZnO because of the difference in band gap energy [14]. ZnO-NiO composite is one of the most important photocatalytic systems, which has attracted a lot of interest.
Some researchers indicated the development of properties and e ciency of these materials by compositing [15,16].
In recent years, more and more researchers have interested to the fabricating of ternary composites. In the whole ternary catalytic reaction process, photo-generated carriers can be transferred in multiple steps through the induction mechanism to achieve the purpose of electron and hole separation, thereby achieving photocatalytic activity beyond the binary catalytic system [17,18] In the typical synthesis of TiO 2 , 5mLof TiCl 4 was added into 50 mL of ice cold distilled water with stirring for 2 h and a clear solution was obtained after stirring, then to that solution, aqueous ammonia was added drop wise till the formation of a gel. The gel solution was washed repeatedly with distilled water to remove the entire chloride ions in the solutions and then it was kept for drying at 100° C to remove part of the absorbed water. The dry gel was milled and calcinaied at 450°C for 4 h to obtain crystalline TiO 2 .
Preparation of CuO mictrostructure CuO mictrostructure was synthesized by using micelle surfactant method.In typical synthesis 0.005 mol metal salts (Copper source)has been dissolved into 100 mL distilled water, which reacts with 0.01molof NaOH solution to produce nickel hydroxide precipitate. Then aqueous micelle solution of CTAB was added to the above solution, and it was further re uxed for 12 hours at 75°C. The supernatant was then decanted, and the solid residue was washed three times with water, dried in air at 100°C for 10 hours, and calcined in mu e furnace at different temperature, 600°C for 5 hours.

Nickel oxide synthesis
For the synthesis of Nickel oxide, rstly 0.005 mol nickel nitrate was dissolved in 100 ml distilled water. 2.50 g citric acid was dissolved and stirred in 50 ml water in a separate beaker. The dissolved Citric acid was slowly added to the solution of nickel nitrate. After homogenization of the solution, aqueous ammonia was added drop wise awaiting the pH reached to 8. The solution was slowly heated up to 70 °C and it was allowed to be evaporated. Then it was placed in an oven at 100 °C for upto water drying. The obtained gel was calcined in the furnace at 500 °C for 5 h.

Preparation of CuO/TiO 2 binary composites
In the preparation of 4wt % CuO/TiO 2 binary composites, 0.04 g of CuO was rst dispersed in 40 ml of ethanol, to that suspension, 0.2750 g of oxalic acid was added, and the mixture was stirred in a magnetic stirrer to form a homogeneous suspension. To that suspension 0.96 g of TiO 2 was added, and the stirring was continued to overnight and then the suspension was dried and subsequently annealed at 300°C for 3 hours in a mu e furnace. The same procedure used for synthesis of further heterojunction photocatalysts (4wt% CuO/TiO 2, 8wt% CuO/TiO 2, 12wt% CuO/TiO 2, 16wt% CuO/TiO 2 and 20wt % CuO/TiO 2 binary composites) and were labelled as CuT4, CuT8, CuT12, CuT16 and CuT 20 respectively.

Synthesis of NiO/CuO/TiO 2 ternary composites
The typical preparation of NiO/CuO/TiO 2 ternary composites is as follows: rstly, 0.98g as-prepared CuO/TiO 2 binary composite are dispersed in 40 ml CH 3 CH 2 OH and stirred for 1 h. Then, 0.02 g NiO powder is added into the above solution and again stirred upto 12 h. After volatilization of the CH 3 CH 2 OH, dark greenish powder is obtained after drying at 80 °C in the air. The whitedark greenish powder is the NiO/CuO/TiO 2 ternary composites. Further, NiO/CuO/TiO 2 ternary composites prepared with various weight ratio of NiO and CuO/TiO 2 , these ternary composites labeled as 0NCT, 2NCT, 4NCT, 6NCT, 8NCT, 10NCT and 12NCT.
Photocatalytic degradation of NiO/CuO/TiO 2 ternary composites All the photocatalytic experiments were performed under natural sunlight on clear sky days during the period of October to december-2020. In a typical experiment, 50 ml of dye solution (concentration 50 mg l -1 ) was taken with 50 mg of photocatalyst in a 250 ml glass beaker. Then the dye solution was kept in direct sunlight with continuous aeration and the concentration of the dye remains was measured periodically by measuring its light absorbance at the visible λ max by using Elico SL-171 Visible spectrophotometer. In order to avoid the variation in results due to uctuation in the intensity of the sunlight, a set of experiments have been carried out simultaneously.The active species from photocatalytic mechanism were detected by using in the same procedure in presence of scavengers.
Characterization of Photocatalyst X-ray powder diffraction (XRD) patterns of the photocatalysts were recorded on a Philips X'pert-MPD diffractometer in the 2h range 20°-80° using Cu Ka radiation. The data were collected with a step of 0.028 (2h) at room temperature. The phase structure of the products was determined by comparing the experimental X-ray powder patterns to the standard compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS). The crystallite sizes were calculated from the peak width using the Scherer equation. The surface morphologies and particle size were observed by Scanning Electron Microscopy (JEOL.JSM-6360LV). The optical properties of the synthesized catalysts were measured by using UV-Vis DRS spectrophotometer (Shimadzu, UV-3600) with BaSO 4 as the background.The photoluminescence emission spectra of the samples were measured at room temperature via Perkin-Elmer LS 55 Luminescence spectrophotometer.Fourier transform infrared spectra of the samples were measured on an IR Prestige-21 spectrometer (Shimadzu). For this, the samples were formed into pellets with KBr.

Results And Discussion
Crystalline properties analysis The XRD patterns of pure TiO 2 ,CuO, NiO and binary 16wt % CuO/TiO 2 , and 3,6,12wt% ratio of In the case of NiO/CuO/TiO 2 ternary composites, new much weaker diffraction peaks are found at representative areawhich can be ascribed to the planes of CuO and NiO. The results can be attributed to the relatively low diffraction intensity caused by the minimal content of CuO and NiO.
The anatase TiO 2 appears to have highly agglomerated particles as shown in Fig. 2a. The CuO/TiO2 binary powder after being irregular shaped grains and were agglomerated in the morphology (Fig. 2d). The FTIR spectrum of as prepared samples such as TiO 2 and 12 wt% of NiO/CuO/TiO 2 compositeswas shown in Figure 3. The prepared TiO 2 photocatalyst reveals the presence of some absorption bands in the ranges from 400 cm -1 to 4000 cm -1 .
In the FTIR spectrum a wide band from 400 to 650 cm -1 was observed that corresponds to the metal-

Photoluminescence spectra
The luminescence (PL) measurements were performed using an excitation wavelength of 360 nm to investigate the lifetime and separation of photo-induced electron-hole pairs in TiO 2 . The photoluminescence (PL) spectra of pure TiO 2 , 16 wt% CuO/TiO 2 , 9 wt% NiO/CuO/TiO 2 and12% NiO/CuO/TiO 2 composites are displayed in Figure 5.
From the PL data, there was close link between the PL intensity and the recombination process of photogenerated electron-hole pairs of the photocatalyst. Generally, a higher value of PL peak intensity indicates the rapid recombination of the photo-generated electron-hole pairs of the catalyst, while a lower value indicates the higher separation of photo-generated electron-hole pairs [25].
The PL spectra of TiO 2 representeda peak at 594 nm, indicating the surface and irradiative electron-hole recombination of the below conduction band and valance band. After incorporating NiO on the CuO/TiO 2 surface, the PL emission intensity is signi cantly reduced, indicating lower chargecarrier recombination. Furthermore, a signi cant PL quenching was observed when NiO was added to CuO/TiO 2 composites.
Indeed, the effect of NiO on the charge carrier separation was phenomenal and also revealed that TiO 2 hindered or suppressed electron−hole pair recombination, which improved photocatalytic activity.

Photodegradability of Reactive Orange 30
The photocatalytic activity of the pure TiO 2 , CuO/TiO 2 and NiO/CuO/TiO 2 ternary composites was evaluated in the photodegradation of Reactive Orange 30 (RO 30) at room temperature, under solar light were shown in Figure 6 (a).
The changes in the RO 30 dye concentration with and without the catalysts (Fig. 10) showed a slight decrement under visible-light irradiation. After 90 min, about 25%, 36%, 59%, 79.7% and 86.6%,of the RO 30 was degraded by the pure CuO, NiO, TiO 2 , CuO/TiO 2 , and NiO/CuO/TiO 2 composites, respectively. The CuO/TiO 2 binary composite exhibited better degradation e ciency due to the synergistic effect resulting from the simultaneous presence of both CuO and TiO 2 phases [1]. The lowest degradation e ciency of pure TiO 2 is mainly because it could not be excited for photocatalytic reaction under visible light. The extended photoresponse and effective separation of the electron-hole pairs are responsible for the enhanced photocatalytic behavior of the CuO/TiO 2 composite. In the NiO/CuO/TiO 2 composites, NiO acts as the prime absorber of visible light and can be easily excited to produce electron-hole pairs. However, its degradation e ciency is low due to its lowest bandgap value, which allows easy recombination of electrons and holes. The low degradation e ciency observed for pure TiO 2 may be due to its poor visiblelight harvesting ability. These demerits are tackled when they are synthesized as a composite. The enrichment in the RO 30 degradation using ternary NiO/CuO/TiO 2 heterojunction is most likely due to the existence of heterojunctions between NiO, TiO 2 and CuO, which e ciently prevent charge-carrier recombination than those in binary CuO/TiO 2 heterojunction. In addition, this development may be due to the larger surface area of the ternary NiO/CuTiO 3 /ZnO heterojunction compared to those of the binary CuTiO 3 /ZnO heterojunction.
The data obtained from the degradation studies were analysedwith the Langmuir-Hinshelwood kinetic model: where r S is the speci c degradation reaction rate the dye (mgl -1 min −1 ), C the concentration of the dye (mgl -1 ), k the reaction rate constant (min −1 ) and K is the dye adsorption constant. When the concentration (C) is small enough, the above equation can be simpli ed in an apparent rst-order equation: After integration, we will get Where C 0 is the initial concentration (mg l −1 ), C is the concentration of the dye after (t) minutes of illumination. The data obtained from the degradation of RB 5 ts well the apparent rst order kinetics ( Figure 7) and their rate constant values are given in the Table 1. The electrons in the conduction band can be transferred to surface adsorbed oxygen molecules and form superoxide anions, which can further transform to OH • and initiate the degradation of RO 30.
The enhanced sunlight photocatalytic e ciency of CeO 2 /FeTiO 3 /TiO 2 composites can be attributed to two mean factors such as the superior visible light absorbance and e cient charge separation and charge transfer by various pathways as proposed in Figure 8.
To understand the electrons-holes separation mechanism, the valence band (VB) and conduction band (CB) potentials of synthesized NiO, CuO, and TiO 2 should be con rmed. The VB and CB edge potentials of a semiconductor photocatalysts can be calculated from generalized equations. To investigate the improved photocatalytic activity, we proposed a mechanism in Fig. 8, which we suggest the formation of possible pathway in the coupled NiO/CuO/TiO 2 composites.In photocatalytic pathway, we assume the presence of TiO 2 between CuO and NiO. From Figure 8, we nd that the CB of TiO 2 is more negative than NiO whereas VB of TiO 2 is more negative than NiO. Furthermore, the CB of CuO is too positive than that of TiO 2. Inaddition with VB of TiO 2 is positive than that of CuO. Hence photoinduced electrons (e -) in CB of TiO 2 can transfer to the CB of NiO and the photoinduced electrons (e -) in CB of TiO 2 cannot transfer to the CB of CuO, because the conduction band potential is too low than that of TiO 2. Simultaneously, the generated holes (h + ) can transfer from the VB of TiO 2 to the VB of CuO, and then not holes move from NiOof VB to TiO 2 VB. From the dye degradation pathway, the photogenerated charges (eand h + ) are highly separated. Hence the photocatalytic degradation process highly enhanced.
Role of Reactive Species Figure 9 shows the photocatalytic degradation results in the presence of different scavengers under solar light irradiation. When BQ was addedto the photocatalyst mixed composition,a dramatic change in the photocatalytic activity was observed compared with the absence of scavenger, con rming that the dissolved oxygen (O 2-), has a main source on the photodegradation process under sunlight irradiation.
However, a some of different change in the photocatalytic activity was observed upon the addition of 2P as a . OH scavenger, which indicates that the O 2and . OH are the main reactive species in the NiO/CuO/TiO 2 composites [2]. The degradation e ciency of RO 30 decreased slightly upon in presence of AO compared with the without scavenger, which indicated the photogenerated holes are not the main active species for degradation of RO 30.

Stability and Reusability
In order to evaluate the stability of ternary CuO/CuO/TiO 2 composites during photocatalytic reaction, cycling experiments were also carried out by repeated degradation of RO 30 for seventimes under solar light irradiation.
As shown in Fig. 10, the photocatalytic activity of ternary NiO/CuO/TiO 2 composites is high even with third cycles.Further, the degradation rate was slight decrease, which could be due to the loss and catalytic poisoning of the photocatalyst during the recycling experiments.

Effect of Oxidants on initial dye concentrations
From mechanistic and application point of view, one must realizethe importance of photocatalytic reaction rate on the initialconcentration of organic dyes.
When the dye concentration washigh, then it was absorb the bulk amount of photons, thuspreventing the same to be absorbed by the photocatalyst surface.Hence, the production of oxidizing species like • OH and O 2 •obviously gets forbidden. Another problem is that more dyemolecules are adsorbed on the photocatalyst surface when thedye concentration in the solution was high. This high surfacecoverage of photocatalyst by the dye molecules also reduces itsphotocatalytic activity. This screening of photons by dye molecules can be reduced by carrying out the degradation in the presence of green oxidant such as H 2 O 2 .
This oxidant produces highly reactive free radicals which can react with thedye molecules and reduces their concentration. The effect of concentration of RO 30 solution on photocatalytic e ciency ofternary NiO/CuO/TiO 2 compositeswas studied by varyingdye concentration from 25 to 150 mg/L and results obtainedwere shown in Fig.11.The results show that the only less than 50% of the dye was photocatalytically degraded by ternary NiO/CuO/TiO 2 compositeswhen the dye concentration above 50 mg /L. However, the ternary system shows high e ciencyfor the degradation of dye in the presence of   Tables   Table 1 kinetic   Stability test of ternaryNiO/CuO/TiO2 composites for seven cycles, Figure 11 Effect of oxidants in initial concentrations