Visible light driven degradation of brilliant green dye using titanium based ternary metal oxide photocatalyst

Results in Physics 12 (2019) 1850–1858

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Visible light driven degradation of brilliant green dye using titanium based ternary metal oxide photocatalyst

T

Debopriya Bhattacharyaa, Debopriyo Ghoshala, Dheeraj Mondalb, Biplab Kumar Paulc, ⁎ ⁎ Navonil Bosed, , Sukhen Dasb, Mousumi Basua, a

Department of Physics, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, India Jadavpur University, Kolkata 700032, India c CSIR-Central Glass and Ceramic Research Institute, Jadavpur, Kolkata 700032, India d Supreme Knowledge Foundation Group of Institutions, Mankundu, Hooghly 712139, India b

ARTICLE INFO

ABSTRACT

Keywords: CuCo0.5Ti0.5O2 nanocomposite Nanoporous Bandgap modification Visible light driven photocatalysis Brilliant green dye

We report a novel copper and cobalt impregnated titanium based ternary metal oxide nanocomposite (CuCO0.5Ti0.5O2) synthesized via simple chemical method. X-ray diffraction pattern and SAED pattern reveals the well crystallinity (R3̃m space group) and phase purity of the synthesized sample. TEM micrograph shows the nano size and heterostructure of the product. Nanoporous nature of the synthesized product is observed from BET analysis. The incorporation of the copper and cobalt in titanium oxide nanoparticle host modifies the band gap of the host and a broadband absorption spectrum (∼325 nm to 800 nm) of the nanocomposite is observed from the UV–Vis-NIR absorption spectroscopy analysis. Photoluminescence (PL) spectrum confirms generation of sufficient electron-hole pairs which could actively participate in photodegradation activity. Photocatalytic performance of the product has been investigated by degrading brilliant green (BG) dye, which shows excellent activity with increased catalytic material loading. The photocatalytic activity is enhanced at high pH level of the solution. Reusability experiments confirms that the catalyst material is reusable with almost same efficiency for degrading BG dye. Wavelength selective photocatalytic degradation of BG dye reveals that the CuCO0.5Ti0.5O2 nanocomposite shows the highest activity under blue-green illumination.

Introduction Everyday throughout the world a large amount of industrial waste contaminates the water resources near the human localities, which becomes a serious threat to human health as well as ecological system of the water body [1,2]. Among many of the industrial waste, dye is one of the abundant chemical having profound use in leather, textile, biological straining, medicine, food and beverages, paints and printing ink industries [3–6].Waste water containing dye is harmful to the living bodies of the surrounding environment and can cause malfunctions to the human body in direct or indirect way. Therefore, the elimination of this pollutant from water is of great concern. In this regard photodegradation of the dye under visible light is emerging as an economic, environment friendly method to eliminate the dye from water body [13–15]. Dyes are synthetic organic compounds of different types like acidic, basic, triphenylmethane, azo, sulphur, nitro [7]. Brilliant green (BG) is one of the triphenylmethane based dye which has an insightful use in leather, textile and biological industries [8]. But BG tainted water has adverse

⁎

effect on human and can cause hypertension, health issues related to heart, lungs, kidney, even it can be carcinogenetic to living bodies [9–12]. For the past few decades the nanostructured semiconductor oxide photocatalyst has drawn a considerable research interest due to its low cost, high activity, better stability and unique physicochemical as well as optical properties [16–19]. Extensive studies have been performed on the semiconductor oxide photocatalyst including ZnO [20–21], CuO [22,23], CoO [24], SnO2 [25–26] and TiO2 [27–33]. Among those TiO2 has found to be the most efficient waste water purifier and photocatalyst due to its high degradation efficiency, nontoxicity, and water insolvability, hydrophilicity, against photo-corrosion, constancy and inexpensive obtainability [15]. Furthermore TiO2 can be easily deposited on different kind of substrate like glass, steel, activated carbon, fibres, inorganic materials [15] which make it an easy to use and highly reusable photocatalyst. However, the mass use of the TiO2 as photocatalyst is restricted by its large band gap (∼3.2 eV for anatase phase and ∼3 eV for rutile phase) [34]. This allows TiO2 to work most efficiently under only UV-ray illumination. TiO2 can be used only for a very small

Corresponding authors. E-mail addresses: [email protected] (N. Bose), [email protected] (M. Basu).

https://doi.org/10.1016/j.rinp.2019.01.065 Received 22 January 2019; Accepted 23 January 2019 Available online 02 February 2019 2211-3797/ © 2019 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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D. Bhattacharya, et al.

part (5%) of solar flux incident on earth’s surface for photocatalytic activity [15]. This property can be enhanced by tuning the band gap of TiO2 towards visible light region, such that it can be able to use the abundant solar spectrum incidenting on earth’s surface. Till now numerous works has been reported on modification of band structure of TiO2 by adding another semiconductor with lower band gap [34], organic dye sensitization or sensitization of surface by metal composites [35], doping with metal ion/non-metal ions [36,37], two or more alien ions co-doping [38,39], surface fluorination [40], noble metal deposition [41], formation of metal-metal oxide composites [42,43]. TiO2 decorated with metal nanoparticles has drawn much interest among researchers due to its good efficiency and enhanced visible light absorbance capability. X. Zhang et al. [44] reported enhanced photo activity of ZnO decorated TiO2. D. Nagy et al. [45] reported WO3-TiO2 composite with enhanced efficiency. Titania (TiO2) doped with copper [46], cobalt [47], iron [48] have been reported earlier for enhances photocatalytic activity. Composites of titania (TiO2) have been reported by Y. Yin et al. [49], L. G. Devi et al. [50] with large visible light absorbance capability and efficient electron-hole pair generation ability. Following these methods one can design and tailor the electronic band structure and can construct modified as well as suitable surface structure which ascribed to better quantum efficiency and higher rate kinetics for the degradation of organic contaminants under solar light illumination [51]. Composites of TiO2 are basically the combination of one or more material with titania. These composites can have different structures like layered [52], heterostructures [53], core-shell [54], yolk-shell [55] etc. and can be synthesized by different routes like hydrothermal method [56], solvothermal method [57], chemical synthesis [58], solgel method [59], electro deposition [60], chemical vapour deposition [61], sono-chemical method [62], microwave synthesis [63] etc. In this work, we report a TiO2 based CuCo0.5Ti0.5O2 ternary metal oxide nanocomposite, for enhanced photocatalytic activity. The ternary nanocomposite was synthesized by facile chemical route. The crystallinity, phase purity and morphology of the composite were studied by X-Ray diffraction (XRD) and Transmission electron microscopy (TEM) method. UV–Vis and Photoluminescence spectra were recorded to study the optical property and band gap of the sample. BET study was conducted to evaluate the porosity and surface area. Taking brilliant green (BG) dye as a degradable product we also studied the photocatalytic activity of the synthesized sample.

was centrifuged and washed by water several times until the pH of the solution became 7. The solvent was evaporated at 85 °C to get the product. Finally the obtained product was calcined at 400 °C for 5 h. Characterization X-Ray powder diffraction method was used to study the structural property of the synthesized nanocomposite by a Bruker D8 X-Ray diffractometer with Cu-Kα radiation (1.541 Å), Bragg-Brentano goniometer geometry and θ–2θ mechanism. Transmission electron microscope (TEM) (FEI.TECHNAI.T-20 G2 SUPERTWIN) was used to get an idea of surface and structural morphology of the product. Photoluminescence spectra were recorded by a Perkin-Elmer fluorescence spectrometer. A JascoV750 spectrophotometer was used to record the UV–Vis absorbance data. N2 adsorption-desorption behaviour was evaluated by a Quanta chrome Autosorb 1C BET surface area & pore volume analyzer. The photodegradation activity of synthesized nanocomposite material was evaluated by degrading brilliant green (BG) dye solution under visible light irradiation. All the photocatalytic experiment were performed at room temperature with a projection lamp (200 W B22 250 V A80 CL 1SL/ 50 Phillips, 2950 lm) taken as visible light source. The source was kept at a distance of 10 cm from the catalytic reactor (100 ml beaker). Before illumination all the reaction mixtures were stirred in a magnetic stirrer for 20 min in dark condition to achieve adsorption–desorption equilibrium. To study the photodegradation activity of the nanocomposite, the filtered analytical samples were taken in adequate amount at every 10 min time interval and the UV–Vis absorbance spectrums of the samples were recorded by using Jasco-V750 spectrophotometer. The degradation efficiency (η) was calculated by [64],

= 1

c × 100 c0

(1)

where C and C0 are the concentration of BG solution after certain time interval and initial BG concentration respectively. The first order rate constant (k) was calculated using the relation [64],

K= ln(C/C0 )/t

(2)

where, t is the time interval in minute. Result and discussion

Experimental

XRD analysis

Materials

Fig. 1 shows the XRD pattern of the synthesized product, which reveals the high crystallinity and phases of the prepared sample. The synthesized 4+ CuCo0.5Ti0.5O2 compound is a AB2+ 0.5 B0.5 O2 type delafossite oxide structure as reported by M.A. Marquard et al. [65]. In the synthesized sample two polytypes of the delafossite structure are formed, depending on the ~ stacking of the combined layers. One polytype is 3R type with R 3 m space group symmetry and rhombohedral structure. The other one is 2H type with hexagonal structure and P63/mmc space group symmetry [66]. The observed peaks at 2θ values of 31° (3R), 35.4° (3R), 37.6° (2H) , 40° (3R), 43.3° (3R), 47.5° (2H), 50.09° (3R), 53.4° (3R), 55° (3R), 61° (2H), 63.5° (3R),64.8° (3R), 65.7° (3R), 70.2° (3R), 75.2° (2H), 75.7° (2H), 75.5° (3R) are well indexed to the (0 0 6), (0 1 2), (0 1 2), (1 0 4), (0 1 5), (0 0 6), (1 0 7), (1 0 5), (0 1 8), (1 1 0), (1 1 3), (10 10), (10 12), (1 1 6), (0 2 1), (2 0 2), (2 0 5) planes of CuCo0.5Ti0.5O2 compound (JCPDF No.-41-0904) [66]. Along with composite phase it can be observed from the XRD pattern that phases of TiO2 (both rutile [R] and anatase [A]), Copper Oxide (CuO, Cu2O), and cobalt oxide (CoO, Co3O4) are present in the synthesized sample due to low temperature synthesis condition.

Titanium(IV) oxide [TiO2] (Mw = 79.87 g/mol), cobalt acetate[Co (CH3COO)2] (Mw = 249.08 g/mol),copper(II) acetate monohydrate [Cu (C2H3O2)2,H2O] (Mw = 199.65 g/mol)were obtained from Merck, Germany. Poly(Vinyl Alcohol) (Mw = 1,15,000 g/mol) was procured from Loba Chemicals, India. Materials were used without any further purification. Synthesis of CuCo0.5Ti0.5O2 nanocomposite The CuCo0.5Ti0.5O2 nanocomposite was synthesized by simple chemical method. At first, three different solutions of copper(II) acetate monohydrate [Cu(C2H3O2)2,H2O] and cobalt acetate (Co(CH3COO)2) and titanium(IV) oxide(TiO2) precursor were prepared by dissolving the 400 mg [Cu(C2H3O2)2,H2O], 270 mg of Co(CH3COO)2 and by dispersing 110 mg of TiO2 in 100 ml water. On the other hand 500 mg of poly (vinyl alcohol) (PVA) was completely dissolved in 250 ml of de-ionised water at 80 °C by vigorous stirring, then the PVA solution was set to cool at room temperature. The dissolved PVA works as a capping agent to restrict the growth of bulk CuCo0.5Ti0.5O2 material. After the fully dissolved PVA solution was cooled down to room temperature, the three solutions of [Cu(C2H3O2)2,H2O], Co(CH3COO)2 and TiO2 precursor (100 ml each) were mixed with PVA solution keeping the pH of the mixture at 10. Then the solution was vigorously stirred for 24 h. After that the mixed solution

Electron microscopy analysis In Fig. 2a, TEM micrograph shows the structure of the synthesized CuCo0.5Ti0.5O2 nanocomposite. The dark regions over the hexagonal 1851

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Fig. 3. Nitrogen adsorption-desorption isotherm and corresponding pore size distribution curve (inset) of CuCo0.5Ti0.5O2 nanocomposite.

Fig. 1. X-Ray diffraction pattern of the synthesized sample CuCo0.5Ti0.5O2.

TiO2nanoparticles infer that the Cu and Co are embedded in TiO2 matrix, confirming successful formation of the nanocomposite. The EDS spectra shown in Fig. 2b confirms the presence of Cu, Co, Ti and O which infers the phase purity of the synthesized sample. The SAED pattern with lattice planes shown in Fig. 2c. Prominent lattice planes, indexed as (0 0 6), (1 0 1), (0 0 3), (0 1 2), (1 0 4), (0 1 5), (0 0 9), (0 1 8), (1 1 0), were obtained from SAED pattern of the sample which are in accordance with the XRD result and JCPDF No.-41-0904. The SAED pattern confirms the polycrystalline nature of the sample.

H3 like isotherm can be seen from the figure which is suitable for nano porous material according to IUPAC classification [64]. The surface area of the nanocomposite was found to be 31.84 m2/g according to density function theory (DFT) [67] and 30.72 m2/g according to Barrett-Joyner-Halenda (BJH) method [25].The pore size and pore volume are found to be 2.27 nm and 0.33 cm3/g respectively according to DFT [68] and 2.16 nm and 0.31 cm3/g according to BJH method. The result infers that the nanoporous composite has high surface area, leading towards enhanced photocatalytic activity of the composite.

Nitrogen adsorption-desorption (BET) isotherms analysis

UV–Vis-NIR spectroscopy analysis

The effective surface area and pore size of the as synthesized CuCo0.5Ti0.5O2 nanocrystals were evaluated from the N2 adsorptiondesorption isotherm as shown in Fig. 3 at room temperature. A type IV

Fig. 4a shows the UV–Vis-NIR absorption spectrum of the as synthesized CuCo0.5Ti0.5O2 sample. A broad absorption spectrum (300 nm-

Fig. 2. (a): TEM image, (b) EDS spectra and (c) SAED pattern of CuCo0.5Ti0.5O2 nanocomposite. 1852

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Fig. 4. (a): UV–Vis absorption spectrum of CuCo0.5Ti0.5O2 nanocomposite; (b) Plot of (αhν)2 versus photon energy (hν) for the sample.

1000 nm) with peaks positioned at 327 nm, 490 nm, and 747 nm is evident from the figure. The absorption peak positioned at 327 nm is attributed to TiO2 phase [64], whereas the incorporation of Co and Cu generates defects (oxygen vacancies) which leads to narrower bang gap and broad range of absorption. This broad absorption range of the sample allows the sample to absorb radiation from UV to visible region, which makes it an efficient photocatalyst under visible and NIR light irradiation. The band gap of the sample is obtained from Tau’c plot as shown in Fig. 4b where (αhν)2 is plotted as a function of photon energy (hν) [69,71]. The estimated band gap (Eg ) of the composite is Eg ∼ 2.56 eV.

loss is essential for achieving efficient photocatalytic activity [71–73]. CuCo0.5Ti0.5O2 nanocomposite contains large amount of oxygen related vacancies through incorporation of Cu and Co in TiO2 matrix. These defects act as recombination centres at much lower energy levels resulting low intensity peaks of PL emission spectrum in UV–Vis region. The lower energy recombination process promotes improved separation of charge carrier and enhances photocatalytic activity of the nanocomposite. Photocatalytic activity The photocatalytic efficiency of CuCo0.5Ti0.5O2 nanocomposite was investigated by degrading brilliant green dye. TiO2 efficiently works as photocatalyst under UV light irradiation as bandgap of TiO2 is ∼3.2 eV [69]. The incorporation of Cu and Co in TiO2develops a heterostructure of reduced bandgap which enables absorbance of visible light by this ternary composite. In our study the photocatalytic activity of the sample was investigated under different reaction conditions. The effect of loading concentration on photocatalytic efficiency of the nanocomposite was studied by adding different amounts of catalyst (0.25 g/L, 0.5 g/L, 0.75 g/L) in the BG solution (5 ppm). Degradation of BG dye solution of different concentration (5 ppm, 10 ppm, 20 ppm) under visible light irradiation was studied with 0.75 g/L catalyst loading. Variation of photocatalytic activity with pH of the dye solution was also studied by varying the pH level (in the range of 6 to 8) of a 5 ppm dye solution for 0.75 g/L photocatalyst loading. Reusability capacity of the product was studied by using the same sample repeatedly three times for degrading 5 ppm BG dye solution. Fig. 6a shows the photocatalytic degradation of 5 ppm BG dye solution by CuCo0.5Ti0.5O2 catalyst of different loading concentration (0.25 g/L, 0.50 g/L and 0.75 g/L) under visible light irradiation. The figure depicts the change in C/C0 ratio with time where C0 and C are the initial concentration of BG solution and concentration of the BG solution after certain time interval. Degradation of BG is increased with increasing loading of nanocomposite photocatalyst. The dye is degraded to 10% of its initial concentration within 120 min of visible light irradiation for 0.75 g/L photocatalyst loading. This phenomenon can be attributed to the fact that greater amount of catalyst produces greater amount of electron-hole pair by absorbing incident photons, which eventually results larger amount of oxidizing agent. Hence the photodegradation activity is accelerated. Fig. 6b shows the variation of degradation rate (k) with irradiation time for different amount of catalyst loading. The figure depicts that the degradation rate increases with increase in catalyst loading. Degradation efficiency (η) and first order rate constant (k) for different amount of catalyst loading are mentioned in Table 1. Maximum Degradation efficiency (η) and degradation rate (k) are obtained for 0.75 g/L catalyst loading. Fig. 7a and b shows photon assisted decomposition of BG dye solutions of different concentration (5 ppm, 10 ppm, 20 ppm) in presence

Photoluminescence spectrum analysis The room temperature PL emission spectrum of the synthesized CuCo0.5Ti0.5O2 nanocomposite is recorded under 425 nm excitation. From Fig. 5 it can be seen that three distinct but weak peaks are occurred around 486 nm, 530 nm and 587 nm. Inset of Fig. 5 shows the excitation spectra of CuCo0.5Ti0.5O2 nanocomposite, justifying the use of 425 nm excitation wavelength. The photocatalytic activity depends largely on PL property of the catalyst as the separation of charge carrier centre of the catalyst plays an important role here. During the photoexcitation process, a significant amount of energy can be wasted through recombination of generated electron-hole pairs [70,71] and those electron-hole pairs cannot contribute in photocatalytic process. Hence reduction of recombination related energy

Fig. 5. Photoluminescence emission spectrum of CuCo0.5Ti0.5O2 under 425 nm excitation (inset shows the excitation spectrum for CuCo0.5Ti0.5O2). 1853

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Fig. 6. (a): Time dependent degradation of BG dye under different amount of CuCo0.5Ti00.5O2 catalyst loading; (b) Rate kinetics of photodegradation under different amount of catalyst loading; (c) Digital images of photodegraded 5 ppm dye solution (at 0.75 g/L catalyst loading) at different intervals of time.

solution and consequently the overall photocatalytic activity of the nanocomposite is changed with change in pH of the solution[77]. Fig. 9 shows the fairly good stability and reusability of the CuCo0.5Ti0.5O2 nanocomposite as a photocatalyst for degrading BG dye. A fixed amount (0.75 g/L) of nanocomposite is reused in five cycles to degrade five different solution of BG dye (5 ppm) keeping all the initial conditions (pH-7.2) unchanged. Fig. 9(a) depicts that the activity of the nanocomposite as photocatalyst is not much diminished even after 5th cycle. Fig. 9(b) shows the variation of degradation efficiency of the catalyst confirming the reusability of the synthesized catalyst. Initially, the degradation efficiency is reduced from 92% to 75% after the third cycle of reuse and then, the degradation efficiency remains stable up to fifth cycle.

Table 1 Degradation efficiency, rate constant and linear regression coefficient for the photo degradation of BG with different amounts of material loading. Nanocomposite loading (g/L)

Degradation efficiency (η) (%)

First Order Rate Constant(k) (min−1)

Adj.R2

25 50 75

76.0 82.0 91.3

0.02018 0.02302 0.02899

0.99385 0.99607 0.99795

of CuCo0.5Ti0.5O2 nanocomposite catalyst (0.75 g/L). Table 2 showing the corresponding values of η and k for different dye concentrations confirms that the photodegradation activity diminishes with increasing dye concentration. The photodegradation efficiency (η) decreases with increasing dye concentration and value of η is reduced to 75% for 20 ppm. Light penetration power in the dye solution decreases with increasing dye concentration, which leads to a decrease in generation of oxidizing agent from the photocatalyst and consequently photocatalytic activity is retarded. Photodegradation efficiency of a catalyst is altered by the varying pH of the dye solution [74,75]. According to WHO the average pH of the water body around the world lies in the range of 6.5–8.5 [76], hence the real life application of a photocatalyst depends on its efficiency at different pH levels of the dye solution. The effect of pH on photodegradation activity of the nanocomposite is studied for three different pH levels of the dye solution such as 6, 7.2 and 8. Fig. 8a and b shows the photodegradation of BG dye solution at different pH condition of the solution where 0.75 g/L catalyst was used to degrade 5 ppm dye solution. The figures infer that the photodegradation efficiency and first order rate constant are increased with increasing pH value of the solution as mentioned in Table 3. The surface potential of the nanocomposite catalyst plays a key role in photodegradation activity, however the surface potential is modified by the varying pH of the

Proposed mechanism of photocatalysis by CuCo0.5Ti0.5O2 Nanocomposite The BG dye degradation mechanism by CuCo0.5Ti0.5O2 catalyst under visible light irradiation can be explained via combination of two simultaneous mechanism, photooxidation and photosensitization [51] (see Fig. 10). In photooxidation mechanism, with the help of visible light irradiation the catalyst generates photoelectron- hole pair in the conduction and valance band. These photo generated electron-hole pairs interact with the free oxygen (O2) and water (H2O) molecule of the solution to generate hydroxyl radicals (%OH−) and super oxide anions (%O2 ). Hydroxyl radical is supposed to be the main oxidizing agent for degrading the dye [78,79].

CuCo0.5 Ti 0.5O2 + h = CuCo0.5 Ti0.5 O2 (eC . B + hV+. B )

(3)

e−C.B + O2 = %O2−

(4)

– h+ V.B + OH

−

(5)

+ OH

(6)

hV+. B 1854

= OH %

+ H2 O =

H+

Results in Physics 12 (2019) 1850–1858

D. Bhattacharya, et al.

Fig. 7. (a): Time dependent degradation of BG dye and (b) Rate kinetics of photodegradation with different initial dye concentrations. Table 2 Degradation efficiency, rate constant and linear regression coefficient for the photo degradation of BG with different solution concentration.

%

Table 3 Degradation efficiency, rate constant and linear regression coefficient for the photo degradation of BG with different solution pH.

Solution Concentration (ppm)

Degradation efficiency (η) (%)

First Order Rate Constant (k) (min−1)

Adj.R2

pH level of the dye solution

Degradation efficiency (η) (%)

First Order Rate Constant(k) (min−1)

Adj.R2

5 10 20

91.6 85.0 76.0

0.0299 0.0225 0.0174

0.99342 0.99660 0.99454

6 7.2 8

78.0 91.6 95.0

0.01570 0.02875 0.03378

0.98619 0.98947 0.98854

O2− +

+

H = HO2 %

CuCo0.5Ti0.5O2 (e−C.B) + O2 = %O2− + CuCo0.5Ti0.5O2

(7)

Dye + %OH− = Degradation product

(8)

Dye + h(+V . B )=Oxidation Product

(9)

Dye + e(C . B)=Reduction Product

(10)

%

Dye∗ + CuCo0.5Ti0.5O2 = %Dye+ + CuCo0.5Ti0.5O2 (e−C.B)

Dye → degradation products

(13) (14)

Also the excited cationic dye radicals interact with the hydroxyl radicals and oxide radicals to generate degraded products [51,81] %

Dye+ + OH− = Dye + HO%

Dye + 2 HO → H2O + Oxidation products %

The photosensitization mechanism generally takes place under visible light irradiation. In this mechanism adsorbed dye molecules are excited by absorbing the energy from incident visible light. These excited dye molecules (Dye∗) form dye radicals (%Dye+) by injecting electrons to the conduction band of CuCo0.5Ti0.5O2 catalyst, which again produces oxygen radicals (%O2−) [79–82].

Dye + h = Dye*

+

O2− +

%

(17)

%

HO2 + H+ + (e−C.B) = H2O2

(18)

H2O2 + (e−C.B) = %HO + HO−

(19)

% %

(12)

H = HO2

+

%

+

%

+

%

−

Dye + O2 = degradation products Dye + HO2 = degradation products Dye + HO = degradation products

Fig. 8. (a): Time dependent degradation of BG dye and (b) Rate kinetics of photodegradation with different pH levels of the solution. 1855

(16)

%

%

(11)

+

(15)

(20) (21) (22)

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Fig. 9. (a): The reusability of the catalyst CuCo0.5Ti0.5O2for degradation of BG dye at comparable rates for five cycles; (b) variation of degradation efficiency with cycle number.

Fig. 10. Schematic diagram describing the process of photocatalysis. 1856

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Fig. 11. (a): Time dependant degradation of BG dye under different wavelength illumination; (b) Rate kinetics of photodegradation under variable wavelength illumination.

Wavelength selective photo-catalytic reaction

References

Wavelength selective performance of CuCo0.5Ti0.5O2 photocatalyst was investigated by degrading 5 ppm BG dye solution under irradiation of blue (460 nm), green (522 nm), yellow (574 nm) and red (662 nm) light. Light of different colours but of same intensity were produced from a visible light source (200 W B22 250 V A80 CL 1SL/50 Phillips, 2950 lm) using different wavelength selective filters. The wavelength and luminous flux of the respective irradiation were measured by an optical multimeter (ILX Lightwave OMM-6810B) and found to be ∼1875 lm. The degradation curves are shown in the Fig. 11a and b. It can be seen from the Fig. 11a that after 2 h. of illumination the degradation efficiency is found to be 53%, 58%, 47%, 43% for blue, green, yellow and red filter respectively. This difference in efficiency can be explained on the basis of UV–Vis absorption spectra. The synthesized CuCo0.5Ti0.5O2 nanocomposite has the strongest absorption peak around 490 nm, in blue-green light region. Therefore a greater amount of photonic energy is absorbed by the sample in this wavelength region, which leads to generation of a larger number of electron-hole pair under blue-green light irradiation enhancing the photodegradation efficiency.

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Conclusion In summary, TiO2 based CuCo0.5Ti0.5O2nanocomposite was successfully synthesized via a simple cost effective chemical method. XRD histogram confirms the phase purity and crystallinity of the synthesized nanocomposite. TEM micrograph is also in agreement with the XRD results. BET analysis confirms the formation of reasonably high porosity of the nanocomposite which makes it an eligible photocatalyst. UV–Vis spectrograph shows a broad range of absorption band of the nanocomposite over UV–Vis-NIR region, which allows it to absorb abundant solar irradiation. Generation of sufficient number of electron-hole pair actively contributing in photocatalytic activity is explained through the obtained results of photoluminescence spectroscopy of the synthesized sample. Photocatalytic experiment proves the high efficiency of the nanocomposites for degrading BG dye with faster rate constant (0.02899 min−1). Photocatalytic behaviour of CuCo0.5Ti0.5O2nanocomposite was also studied by varying pH and concentration of the BG dye solution.0.75 g/L amount of photocatalyst was able to degrade 5 ppm dye solution of pH ∼8 with a rate constant of 0 0.03378 min−1. Irradiation wavelength dependence of the photocatalytic behaviour of the sample was also investigated. The synthesized porous metal oxide nanocomposite with modified optical band gap and high reusability is found to be a suitable candidate for photocatalysis application. 1857

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