Visible light efficient and photo stable nanostructure of GO/CuO/m-TiO2 ternary composite

Various combinations of mesoporous-titania (m-TiO2) in binary composites of GO/m-TiO2, CuO/m-TiO2 and ternary composite of GO/CuO/m-TiO2 with 4.0 wt% CuO and 10.0 wt% GO were prepared. XRD manifests mesoporous nature with anatase phase of m-TiO2. In binary and ternary composites, the peak of GO is shifted due to presence of Cu and Ti. In GO/CuO/m-TiO2 ternary composite, GO sheets appeared as irregular and flaky sheets showing successful distribution of clew like CuO and m-TiO2. UV-vis DRS indicated that all photo catalysts showed strong response in visible region. The N2 sorption desorption isotherms indicated meso-porous nature and high surface area of ternary composite. The synergistic effect of C–Cu–Ti and C–O–Ti linkages is studied by FTIR and Raman analyses. Ternary CuO/GO/m-TiO2 composite demonstrates the highest degradation of methyl orange (MO) reduction and phenol oxidation simultaneously under visible light. The free electrons are scavenged by MO and reduced it while the holes oxidized the phenol.


Introduction
Energy crises and environmental pollutions are becoming more rigorous issues due to the rapid growth of industries and population [1]. Organic dyes are extensively utilized in a number of industries. Most dyes are toxic and detrimental. Their direct ejection into water as a harmful organic pollutants can adversely deteriorate the environment [2,3]. Purification of waste water containing organic pollutants, dyes and aromatic compounds through photo catalysis are more addressable area of research [4]. TiO 2 based photo catalysts are promising material and gaining considerable attention owing to their good chemical stability, non-toxicity, biological inertness and cost efficiency for production of clean energy and degradation of organic pollutants into carbon dioxide and H 2 O [5]. But most of semi conductor based photo catalysts like ZnO, SnO 2 and TiO 2 [6][7][8] are only respondent to UV light (4%-5%) due to possessing the large band gap energy value which suppresses their photocatalytic effect under visible irradiation [9]. To overcome this drawback, coupling of TiO 2 with narrow band gap metal oxides and expand their applications in visible region which makes the foremost and imperative fraction (47%) of solar spectrum.
CuO is a transition metal oxide which has been broadly exploited for the degradation of waste materials in visible light. It has narrow band gap energy value, naturally profuse, low cost and high chemical stability [10][11][12][13]. The coupling of CuO with TiO 2 tuned the optical and electronic properties of TiO 2 on account of enhancement in photocatalytic oxidation rate by collecting the photo generated electrons through metal oxide and augment the production of free OH · / ·-O 2 radicals [14]. This combination makes a promising, environmental friendly and viable candidate which exhibits the sturdy photocatalytic activity under visible light [15]. Wu et al. fabricated the CuO nanotube decorated with TiO 2 nanoparticles which was utilized for better photocatalytic activity [16]. Lei et al. synthesized the CuO-modified TiO 2 photo catalysts by impregnation method. Photocatalytic degradation of poly brominated diphenyl ethers were performed by TiO 2 and CuO/TiO 2 photo catalysts but the rapid degradation of 2, 2 / , 4,4 /_ tetra bromo diphenyl ether was conducted by CuO/TiO 2 photo catalysts [17]. Shamaila et al. prepared mesoporous TiO 2 by sol-gel process and incorporated Cu into mesoporous TiO 2 by wet chemical impregnation method. The interfacial Ti-O-Cu linkages improved photocatalytic activity in degradation of MO and 2, 4-DCP [18].
Carbon based nanomaterials including carbon nanotubes, graphene oxide (GO) and reduced graphene oxide (RGO) have been extensively used in many applications like photo catalysis, super capacitors, lithium-ion batteries, solar cells and chemical sensors [19][20][21][22][23] due to possessing the unique and fascinating properties. GO is a two-dimensional single sheet of oxidized carbonaceous material. Oxygen functionalities make it possible to dispense in water and other organic materials [24][25][26][27][28][29]. GO has the characteristics of high surface area, fast electron mobility, π-π conjugation, electrical, thermal conductivities and periphery bearing oxygen functionalities. It acts as an efficient photosensitizer when its surface is adapted with metal oxides in photo degradation of organic waste materials [30,31]. GO behaves as good transporter owing to unpaired π electrons which proficiently suppress rate of recombination of free charge carriers. The band gap reduction of GO based metal oxides hybrid structure leading to augmentation in photocatalytic activities [32,33] [35]. Shamaila et al. studied bismuth-modified ordered mesoporous TiO 2 for simultaneous phenol oxidation and chromium reduction under visible irradiation [36].
In the present work, GO was fabricated by modified Hummers method. Mesoporous TiO 2 and clew like CuO nanomaterials were prepared by hydrothermal treatment, respectively. 4.0 wt% CuO and 10.0 wt% GO nano-sheets on the surface of m-TiO 2 were introduced to produce GO/m-TiO 2 , CuO/m-TiO 2 , and GO/CuO/m-TiO 2 nanocomposite structure by simple and facile route of chemical impregnation method. The synergistic effect of nanocomposites for simultaneous photocatalytic degradation of MO reduction and phenol oxidation under visible light is observed. GO modifies the properties of binary and ternary composite nanomaterials. The formation of C-Cu-Ti, C-Cu-Ti and C-O-Ti linkages with high surface area of GO/CuO/m-TiO 2 nanocomposite is helpful in enhancement of photocatalytic degradation of organic pollutants under visible light. GO sheets play the crucial function in photo-electron excitation and then transfer to TiO 2 and CuO due to its photo-absorptive nature on exposure of light source and reduce the rate of recombination of free charge carriers. The ternary composite of GO/CuO/m-TiO 2 shows much superior photocatalytic activity as compared to single and hetero structure (binary composites) based on m-TiO 2 owing to high surface area, electron transfer, synergistic effect and low band gap value.

Synthesis of GO
GO sheets were synthesized all the way through modified Hummers' method [37]. 3.0 g of graphite powder and 1.5 g of NaNO 3 dissolved in 80 mL of H 2 SO 4 (98%) to expand the graphite powder under magnetic stirring for 1 h at room temperature. Then solution was placed in ice bath keeping the temperature below 4°C. 9.0 g KMnO 4 was added slowly in solution by keeping the temperature constant under continuous stirring. The greenish purple colored hybrid solution was obtained. The mixture was transferred to water bath keeping temperature at 35-40°C and stirred continuously for 2 h. The thick paste of dark brown colored was diluted by slow addition of 200 mL distilled water. Then temperature was increased up to 98°C under magnetic stirring for 15 min. 300 mL more water was added. 30 mL H 2 O 2 (30%) slowly introduced into solution to dissolve manganese species. Ther resultant color of product changed into orange brown colloidal solution of GO, centrifuged (6000 rpm min −1 ) and rinsed with 10% HCl solution. In order to remove residual acids and un-exfoliated particles, GO slurry was washed with distilled water throughout to adjust the pH between 6 and 7. Final product was dried in oven at 60°C for 48 h to obtain GO sheets. The synthesis of GO is given in Scheme 1.

Synthesis of catalysts
For the preparation of mesoporous TiO 2, 10.0 g Pluronic P123 as soft template was dissolved into 50 mL of 99% ethanol under mild magnetic stirring until fully dissolved. 20 mL TBT as TiO 2 precursor dissolved into 100 mL mixture of ethanol and distilled water drop wise under vigorous stirring to control the hydrolysis process. A milky solution was formed. A few drops of concentrated HCl were added for maintaining pH level between 3 and 4. The solution containing P123 was added drop wise into above solution. The homogenization of mixture was occurred under sonication of 30 min. Then transferred into a Teflon-lined stainless-steel autoclave (100 mL capacity) and heated at 150°C for 8 h. After cooling it naturally at room temperature, white colored precipitate was collected by centrifugation (4000 rpm, 10 min) and washed with distilled water for several times. Then dried it overnight at 60°C and calcined at 500°C for 3 h.
Cu (NO 3 ) 2 is used as a precursor for CuO nanomaterials. 0.25 M solution of Cu (NO 3 ) 2 was dissolved in 85 mL of distilled water under magnetic stirring for 1 h. In another beaker, 1M solution of NaOH was dissolved in 85 mL of distilled water under mild magnetic stirring for 1 h. NaOH solution was mixed into above solution at room temperature (pH=10). The homogenization of solution was made under sonication for 30 min. The mixture was transferred into stainless-steel autoclave (100 mL capacity) and heated at 60°C for 8 h. For washing, above steps are repeated to obtain CuO nanomaterials. The black colored precipitate was annealed at 350°C for 3 h.
In order to form the binary composites of GO/m-TiO 2 and CuO/m-TiO 2 and ternary composite of GO/CuO/m-TiO 2 , 10.0 wt% GO and 4.0 wt% CuO was incorporated in m-TiO 2 by chemical wet impregnation method.

Characterization
The crystalline structure of the prepared samples was determined by a Rigaku D/MAX 2550 x-ray diffractometer. X-ray diffraction patterns of all samples were collected in the range of 5°-80°(2θ) using (Cu Kα1 radiation, λ=1.5406 Å), operated at 40 kV and 100 mA. The crystalline size was determined by applying the Scherrer equation. D=kλ/βcosθ, where β is the half height width of the diffraction peak K=0.89 is a coefficient, θ is the diffraction angle and λ is the X-ray wavelength corresponding to the CuKα irradiation. Scanning electron microscopic images and energy dispersive x-ray spectra were observed by MIRA 3 TESCAN instrument. The morphology of m-TiO 2 and ternary composite was determined by JEOL JEM1010 transmission electron microscope operating at 80 kV. FT-IR spectrometer of manufacturing model Nicolet 740 equipped with beam splitter of KBr along with TGS detector was used for obtaining spectra of the samples. UV visible spectroscopy was performed using Scan UV-vis-NIR spectrophotometer equipped with an integrated sphere assembly, using BaSO 4 as a reflectance sample. Raman spectrum of GO/CuO/m-TiO 2 nanocomposite was recorded by uRAMBOSS Dangoo Optron Co. Ltd Raman spectrometer at room temperature with excitation wavelength of 532.0 nm. Nitrogen adsorption and desorption isotherms were obtained at 77 K with a Micromeritics 2010 ASAP system.

Photocatalytic activity
The textile industry effluents contain azo dyes and organics. MO is an azo dye which is very stable due to -N=Nlinkage. Similarly the phenol is very toxic and carcinogenic chemical. It is demanding to degrade it under visible light. The photo catalytic activity was performed in a 100 mL quartz photochemical reactor with 1000 W Xenon lamp. The photocatalytic activity of the prepared samples is examined by using the initial concentration of 20 mg L −1 of each MO and phenol in same solution simultaneously under visible light source for 5 h. All photo catalysts (0.05 g) were dispersed in 50 mL of MO and phenol mixed solution and kept at constant magnetic stirring under darkness to obtain adsorption-desorption equilibrium for 30 min. After regular intervals of time, the suspended solutions were withdrawn and centrifuged to examine the degradation of MO and phenol. The absorption of 4.0 mL of each irradiated MO and phenol photo catalyst solution was determined at 464 and 270 nm, respectively using UV-visible spectrophotometer. The measurements were repeated for all catalysts and the experimental error was found to be ±3%.

XRD profile
In order to verify the crystal phases, purity and crystalline size of synthesized nanomaterials, XRD analysis is performed. Figure 1 shows XRD patterns of pure samples of m-TiO 2 , GO and CuO and binary composites of GO/m-TiO 2 , CuO/m-TiO 2 and ternary composite of GO/CuO/m-TiO 2 nanomaterials. The other diffraction peaks correspond to (101), (004), (112), (200) and (105) index planes indicate the anatase phase of TiO 2 [38,39]. In addition to the above peaks, a small intensity peak at 2θ=5.0º manifests the porous nature of TiO 2 [18]. In XRD pattern of GO ( figure 1(b)), a small characteristic peak at 26.1°(002) appears due to graphite as a starting material. After the oxidation process and interaction of acidic groups with graphitic layer, this peak is shifted to the lower angle region. A sharp peak is appeared at ∼11.3°(001) of GO correspond to larger interlayer distance of 7.8 Å due to possessing the oxygen functionalities as compared to graphite which is noted as 3.4 Å [40][41][42]. The diffraction peaks of CuO (figure 1(c)) at 2θ=32.8°, 35.6°, 38.8°, 48.7°and 58.4°correspond to (110) (002), (111), (202) and (202) planes of monoclinic phase of CuO [43]. The XRD spectra of binary composites of GO/m-TiO 2 and CuO/m-TiO 2 and ternary composite of GO/CuO/m-TiO 2 are indicated in figures 1(d)-(f), respectively. In composites, the peak of GO is slightly shifted into higher angle region due to Cu-C-Ti linkage. No diffraction peak of CuO is observed in CuO/m-TiO 2 and GO/CuO/m-TiO 2 composite due to its small concentration or elevated dispersion. In binary and ternary composite samples, all characteristics peaks of TiO 2 reflect the anatase phase. It is also noted that small angle peak of anatase TiO 2 in composite samples is slightly shifted to higher angle indicating distorted d-spacing of host TiO 2 material. The small pores are compacting into larger pores and pore channels are totally covered by foreign materials also confirmed by BET analysis [36]. The average crystalline size of all synthesized samples is calculated by Scherer's equation as shown in table 1. The decline trend in crystalline size in composite structures is assigned that average distance covered by photo generated charge carriers to reach on the surface of catalyst at reaction site becomes smaller. At reactive sites, they suffered in oxidation and reduction process which alternatively responsible in enhancement of photocatalytic activity by lowering the rate of recombination process in binary and ternary structures as compared to single one.  figure 2(a). It is clearly seen that during hydrothermal treatment, re-precipitation of m-TiO 2 nanomaterial leads to formation of aggregated and irregular shaped primary particles. It is observed that granular texture of prepared sample is similar to porous structure of TiO 2 [39]. The morphology of GO is shown in figure 2(b). It is clearly observed that GO has an ultrathin, flexible and sheet like texture. The edges of GO are folded, wavy and wrinkled which reflect the occurrence of H 2 O molecules, hydroxyl or carboxyl groups. GO possesses many aggregated closely associated conducting sheets which are soluble in water. CuO particles with vertical elongated terminal edges at pH=10 are observed by SEM image (figure 2(c)). It reveals that formation of agglomerated clew like CuO nanoparticles

Nitrogen sorption studies
The textural properties of the synthesized photo catalysts were examined through N 2 adsorption-desorption behaviors. N 2 adsorption desorption isotherms of m-TiO 2 , GO/m-TiO 2 and GO/CuO/m-TiO 2 are presented in figure 5(A). All isotherms are of characteristic type IV which indicate capillary condensation of adsorbent in the mesopores. It is worth seen that the isotherms are obtained at relative pressure range of 0.4-0.8 with a sloppy adsorption and sharp steep desorption branch. All the hysteresis loops represents H 2 type according to the IUPAC classification systems associated with narrow pore sizes. Figure 5(B) shows Barrett-Joyner-Halenda (BJH) pore-size distribution curves of photo catalysts. The BET surface area and BJH pore sizes results of photo catalysts are summarized in table 3. Higher BET surface area of TiO 2 nanoparticles presents sufficient space for infiltration of GO and CuO nanomaterials. Due to the high surface area, light is more proficiently harvested in Where h is Planck's constant, n is the photon's frequency, α is the absorption coefficient, E g is the band gap and A is a proportionality constant. The value of the exponent denotes the indirect allowed electronic transition [45].  The calculated optical band gap value of CuO is 2.      OH radicals which can reduce in electron-hole pair recombination and ultimately augment the photocatalytic process.

Raman study
The Raman spectra of ternary composite of GO/CuO/m-TiO 2 photo catalyst are shown in figure 9. Raman brands of GO/CuO/m-TiO 2 nanocomposite indicate the signals of the m-TiO 2 at 156 cm −1 and 403 cm −1 . Raman bands of anatase TiO 2 are typically observed at 146.0, 196.0, 397.0, 516.0 and 638.0 cm −1 attributed to the E g , B 1g , A 1g , B 2g and E g vibrational modes, respectively [18]. The signals at range shift of 279 cm −1 and 600 cm −1 indicates the presence of CuO nanomaterial. The existence of carbon based material in prepared sample is confirmed by appearance of small intensity D (1350 cm −1 ) and G (1565 cm −1 ) bands corresponding to symmetrical feature of sp 3 defects and in-plane stretching vibrations of sp 2 C-C bonds, respectively [47]. The existence of characteristic signals of small intensity D (1361 cm −1 ) and G (1568 cm −1 ) bands showed little higher shift in comparison to pure GO indicating the synergistic effect of GO and CuO in mesoporous TiO 2 .

Photocatalytic activity
The photocatalytic activities of synthesized nanomaterial photo catalysts i.e m-TiO 2 , GO, CuO, CuO/mTiO 2 , GO/m-TiO 2 and GO/CuO/m-TiO 2 over simultaneous reduction of MO and phenol oxidation under visible light are evaluated in figures 10(A) and (B), respectively. The highest decomposition rate of MO reduction and phenol oxidation is observed in ternary composite of GO/CuO/m-TiO 2 . Pure TiO 2 shows very less degradation activity due to the large band gap values. In the absence of photo catalyst, no noticeable degradation of MO (photo-fading) and phenol is observed. The photocatalytic degradation process of MO and phenol follow pseudo first order kinetics; Where C o is the initial concentration at t=0, C is the concentration of solute remaining in the solution, t is the time of irradiation and k app is degradation rate constant. The degradation rate constant for the decomposition of MO and phenol for synthesized different photo catalysts is calculated. It is noticed that GO/CuO/m-TiO 2 ternary composite has the highest value of k app as compared to other photo catalysts as shown in figures 11(A) and (B) respectively. The calculated rate constant values of different samples are given in table 4. Being a photo catalyst, CuO could absorb light in UV-vis region due to possessing the small band gap value. But its less photo catalytic activity is noted which could be attributed to its low charge transportation rate which enhance the chances of charge recombination rate. A rapid photo catalysis is observed when surface of m-TiO 2 is modified with CuO and GO to form the binary and tri-junctions. In case of CuO/m-TiO 2 heterojunction, photo catalytic reduction of MO and oxidation of phenol is noted as ∼64% and ∼55%, respectively. The enhancement in catalytic degradation of mixed organic species is due to the generation of reduced form of CuO which leads to formation of oxygen vacancies. These vacancies acts as the electron donor by binding the photo generated electrons and fruitful in the formation of holes and hydroxyl radicals ( · OH.). These free hydroxyl radicals provoke mineralization of the mixed organic species. m-TiO 2 has large surface area and provides sufficient     [50]. GO acts as an electron acceptor. The impregnation of GO to m-TiO 2 creates the interfacial C-O-Ti linkages. Most of organic pollutants have aromatic compounds that create π-π stacking with the delocalized electrons of GO and enhance adsorption affinity on the surface of GO. The adsorption of organic species with the photo catalysts facilitate to overcome rate of recombination of photo generated charge carriers and improve photo degradation of organic species in visible region. In GO/CuO/m-TiO 2 tri-junction nanocomposite, the highest photocatalytic activity is observed for mixed organic species used in photocatalytic test. GO/CuO/m-TiO 2 composite demonstrates the reduction of MO to hydrazine derivative and phenol solution was observed ∼93% and ∼90%, respectively. GO acts as photo-electron acceptor when light source is irradiated due to presence of oxygen-containing functional groups like hydroxyl, epoxy and carboxyl groups at the edge of sheet. These functional groups may facilitate the electron transfer process among m-TiO 2 , CuO and GO sheets and reduce probability of charge recombination process. In GO/CuO/m-TiO 2 ternary composite, mesoporous texture is helpful in improving the photocatalytic degradation rate of organic pollutants by providing a large interacting surface area. In order to determine the stability of photo catalyst, GO/CuO/m-TiO 2 nanocomposite was exploited four times in degradation of MO and phenol with diminutive dropped efficiency as shown in figure 12. This result indicates that GO/CuO/m-TiO 2 nanocomposite has high stability under visible light source and cost effective. Further stability of GO/CuO/m-TiO 2 nanocomposite is also confirmed by XRD pattern, SEM image and EDX analysis ( figure 13). XRD profile still exhibits reflected diffraction peaks corresponding to GO and TiO 2 . SEM photographs and EDX analysis indicate the original morphology and composition of GO/CuO/m-TiO 2 nanocomposite after four repeated cycles of photocatalytic tests. These results confirmed the stability of photocatalyst in photocatalytic reaction.

Photocatalytic mechanism
Photocatalytic mechanism demonstrates the redox properties of GO/CuO/m-TiO 2 nanocomposite in case of simultaneous photo reduction of MO and phenol oxidation as illustrated in scheme 2. It has been found that GO sheets possess the different oxidation levels. The indirect band value of GO is calculated in range of 2.5-3.5 eV.
The conduction band edge of GO is located at −0.75 V and valence band has minimum value at 1.5 V (versus NHE) [48]. GO plays the crucial function in photo-electron excitation and carrying to m-TiO 2 and CuO due to its photo-absorptive nature on exposure of light source. Upon irradiation of visible light, electrons and holes are produced on the surface of GO/CuO/m-TiO 2 catalyst. GO acts as both photo-sensitizer and electrons acceptor. GO transfers the electrons to the conduction band of m-TiO 2 and CuO. Photo-induced electrons of m-TiO 2 could switch to the valence band of CuO. MO is reduced to hydrazine derivatives by scavenging photo-generated electrons on the surface of conduction band of CuO. The photo-generated holes on valence band of CuO could be transferred to the TiO 2 and GO surface to contribute in oxidation reactions of phenol [51]. GO/CuO/m-TiO 2 ternary composite can produce high amount of free electrons and holes which in turn degrade the mixed organic pollutants. In the photo-degradation reaction, GO produces electrons and transfer them to m-TiO 2 and CuO under visible-light. This is helpful in the reduction of recombination rate of electronhole pairs. Essential steps involved in oxidation reduction reaction of GO/CuO/m-TiO 2 nanomaterial and organic species are represented by chemical equations given below;   elongated clew like CuO nanomaterials and agglomerated irregular shaped porous d TiO 2 on the entire surface of the GO sheet are confirmed by SEM and TEM photographs. UV-vis spectra indicate that all nanocomposites show strong response in the visible region. FTIR studies indicate C-Cu-Ti and C-O-Ti linkages which confirm the formation of GO/CuO/m-TiO 2 tri-junction. Large surface area of nanocomposite with small pore size is helpful in enhancement of photocatalytic degradation of mixed organic species under visible light. In Raman spectrum, the existence of characteristics signals related to TiO 2 , CuO and GO indicates that GO/CuO/m-TiO 2 nanomaterial is successfully synthesized. The highest rate of simultaneous MO reduction and phenol oxidation is noted by GO/CuO/m-TiO 2 . On visible light irradiation , the C-Cu-Ti linkage of GO/CuO/m-TiO 2 photo catalyst enhance adsorption of organic species and pollutants. On the absorption of photon, the electrons are excited from GO sheets and shifted towards the m-TiO 2 and CuO. π electrons of GO and reduced form of CuO leads to formation of oxygen vacancies which facilitate to overcome rate of recombination of photo-generated charge carriers by producing the high amount of free electrons and holes pairs. The free electrons are helpful in the reduction of MO while holes are responsible to oxidize the phenol into simple compounds. The high catalytic activity is due to both the unique combination and high surface area, reduced electron-hole recombination and low band gap. The ternary nanocomposite of GO/CuO/m-TiO 2 is cost effective and highly photostable.