Photodegradation of Methyl Orange in Aqueous Solution by the Visible Light Active Co:La:TiO2 Nanocomposite

In this study, the Co:La:TiO2 nanocomposite was prepared by the wet chemical method. Synthesized TiO2 and Co:La:TiO2 were characterized by X-Ray Diffractometer, SEM, TEM, UVvis, FT-IR, Band gap energy and BET. The TiO2 and Co:La:TiO2 were used as photocatalyst for the degradation of Methyl Blue. The XRD pattern confirmed the presence of anatase and rutile phase in the catalyst. The particle size was estimated by the Scherrer’s and found 68 and 32 nm for TiO2 and Co:La:TiO2 respectively. The particle morphology of the photocatalysts was found in nanodiamension. The surface area of the photocatalysts was found 37.52 and 106.68 m2/g for TiO2 and Co:La:TiO2 respectively. The band gap energy of TiO2 and Co:La:TiO2 were 3.2 and 3.0 eV. The photodegradation of Methyl Orange has been found 18% and 88% at 5 pH for TiO2 and Co:La:TiO2 respectively. The photodegradation of Methyl Orange has been found 19% and 98.9% at 25 ppm concentration of dye. The photodegradation of Methyl Orange has been found 25% and 96% at 800 mg/L amount of photocatalyst and 180 min illumination of visible light. The photodegradation was following the first order kinetics.


Introduction
Dyes are not safe aggravates that are found in modern waste water bringing about unfriendly natural issues. A large portion of the dyes utilized as a part of the pigmentation of materials, leather, paper, ceramic production, and nourishment handling are found from azo dyes. Dyes are lost with waste water in the midst of preparing [1][2][3][4][5]. This speaks to a tremendous danger to human and ecological wellbeing because of the poisonous quality of azo dyes [6]. The treatment of such defilements can be proficient by heterogeneous photocatalysis in light of its capability and negligible exertion and furthermore to the way that it grants complete poisonous quality of dyes to carbon dioxide and inorganic acids [7][8][9].
Titanium dioxide TiO 2 is a most vital nanomaterial's which has pulled in an awesome consideration because of its novel properties. Titanium dioxide TiO 2 have great merits in solar powered vitality exchanging and photocatalysis of toxic substance in environmental condition. The concoction inactivity and the non-poisonous quality of TiO 2 have likewise made it a predominant photocatalyst [10][11][12][13]. Titania has a substantial band hole (3.20 eV for anatase TiO 2 ) and along these lines, just a little part of sunlight based light can be absorbed [14]. Many endeavours have been made to sensitized titanium dioxide to the entire visible range, for example, doping with d block metals [15,16], d block metal ions [17], non-metal atoms [18] and organic materials [19]. An acquaintance of dopant permits Titania with assimilating in the visible region yet this does not really imply that the doped photocatalyst has a superior photocatalytic activity. In photocatalysis, light is consumed by an adsorbed substrate. Today, semiconductors are generally chosen as photocatalysts, on the grounds that semiconductors have a narrow band gap between the valence and conduction groups [20]. With the end goal for photocatalysis to continue, the semiconductors need to assimilated vitality equivalent to or more than its vitality crevice. When TiO 2 is irradiated by UV light (400 nm or less), electron is excited to generate electron (e -) hole (h + ) pairs [21]. This movement of electrons forms e -/h + or negatively charged electron/positively charged hole pairs. The hole can oxidize donor molecules. In photogenerated catalysis the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron-hole pairs, which generate free radicals able to undergo secondary reactions [22][23][24].
In this study, the nanocomposites of Cobalt, Lanthanum and Titania were prepared. The prepared nanocomposites used for the photodegradation of methyl orange at different parameters i.e., pH of solution, temperature of reaction, concentration of dye, amount of photocatalyst and irradiation time. The Kinetic and thermodynamic parameters were determined for the photodegradation of methyl orange.

Synthesis of Titania by wet chemical method
In this method, both TiCl 4 solution (1000 mg/l) and NaOH solution (64.5 g/l) was added drop wise to water with stirring. After the resulting solution reaches pH to 7, the slurry was filtered, and the filter cake of TiO 2 was washed and redispersed in water to prepare 1 M of TiO 2 slurry. Resulting TiO 2 slurry and an aqueous solution of HNO 3 were refluxed at 95°C for 2 h, cooled to room temperature and neutralized with 28% of aqueous ammonia. Then, it was filtered, washed and calcined at 400°C [25,26].

Synthesis of Co:La:TiO 2 nanocomposite
In this study, Co:La:TiO 2 nanocomposites were prepared by solution impregnation method. In this method suitable quantity of prepared TiO 2 (2 g) was dispersed in alcoholic cobalt acetate 10% (w/v) and lanthanum nitrate 5% (w/v). The dispersion is agitated continuously for 4 hour at 100°C temperature. After the treatment the residue was removed through filtration and was sintered for 4 hour in presence of air at 600°C by kipping it in a silica crucible inside the muffle furnace. After sintering and slow anilling to room temperature, content was taken out from furnace and was stored in air tight bottles and was used as photocatalyst [27].

Characterization
The prepared samples were subjected to x-ray diffraction analysis on Powder X-Ray Diffractometer. The observed X-Ray diffractogram of samples were analyzed further to estimate average grain size in the sample by Scherrer's calculation. Since the absorption of light by photocatalyst is the most crucial step in any photocatalysed reaction, and is decided primarily by the band gap energy of catalyst. The morphology and size of the Titania particles were determined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

Phase identification by X-Ray diffraction analysis
The obtained X-Ray diffraction patterns of Titania and Co:La:TiO 2 are shown in Figures 1a and 1b. The observed pattern of peaks, when compared with the standard JCPDS database, suggested that, in prepared TiO 2 sample, major peaks at 2θ=25.5° 37.2, 48.3, and 54.5, which can be indexed to the (101), (004), (200), and (211) facet of anatase TiO 2 . While major peaks at 2θ=26.9° and 28.2° demonstrate the presence of rutile phase which can be indexed to the (110), (121), individually. In case of Co:La:TiO 2 sample, the observed XRD pattern indicates not only a change in the peak intensity, compared to TiO 2 , but even the absence of some originally observed TiO 2 peaks [28]. This is, probably, due to the change in the crystallinity and grain fragmentation, when the samples were wet impregnated by cobalt and Lanthanum.

Determination of average size of crystal in samples:
The Scherrer's calculations were attempted to know the average size of crystal in the samples [28]. Although, Scherrer's calculations are only approximate in nature, but definitely provide a first-hand idea of the average size of the crystal in the samples, which may be quite accurate, provided the size of crystal is below 100 nm. The results of Scherrer's calculations are presented in Table 1. The crystal size was found in the nm range.

Scanning Electron Microscopy (SEM)
The morphology of the samples was investigated by scanning electron microscopy and it resumes the most interesting outcomes. Figure 2 clearly show that both the prepared samples are obtained in nanometric dimension. The size of Co:La:TiO 2 was 160 nm whereas 200 nm for TiO 2 . This is due to the impregnation of metal ions in Titania [29].

Transmission Electron Microscope (TEM)
TEM images were clearly displayed the morphology and particle size of neat TiO 2 and Cobalt and lanthanum doped TiO 2 . From the Figure 3 we find that Cobalt and lanthanum doped modified TiO 2 change the size of neat TiO 2 significantly, as shown in Figure 3a and 3b. The sizes of both modified and neat TiO 2 are mono disperse about 100-200 nm. Moreover, the crystal lattice line can be clearly found in the TEM images. The aggregations of both kinds of particles are caused by high surface energy; however, the agglomeration of the modified one is alleviated obviously compared with that of the neat [30]. Figure 4 demonstrated the BET and adsorption and desorption plot for the TiO 2 and Co:La:TiO 2 . Figure 4 showed adsorption-desorption of nitrogen and with the help of nitrogen adsorption to determine the surface area, pore volume and average pore size of the TiO 2 and Co:La:TiO 2 photocatalyst. The surface parameters of TiO 2 and Co:La:TiO 2 was shown in Table 2. The TiO 2 modified by Cobalt and Lanthanum are fragmentation to some extent during heat treatment,

TiO 2 68
Co:La:TiO 2 32 leading to a marked increment of the BET surface areas and the average pore radius size and diminishing of the pore volume [31,32].

Band gap energy determination
The band gap of samples was calculated by extrapolation of the (αhv) 2 versus hv plots, where α is the absorption coefficient and hv is the photon energy, hv=(1239/λ) eV. The value of hv extrapolated to α=0 gives an absorption energy, which corresponds to a band gap (Eg). Figure 5 yields an Eg value of 3.2 eV for TiO 2 and 3.0 for Co:La:TiO 2 [33]. The slight decrease in band gap energy in case of Co:La:TiO 2 , is due to formation of sub-band level between valence band and conduction band caused impregnation of Co +2 and La +3 in TiO 2 host.

Adsorption study
A control experiment was first carried out under two conditions, vis (i) dye+UV (no TiO 2 ) (ii) TiO 2 +dye in dark without any irradiation ( Figure 6). It can be seen that under dark conditions, after 20 min the amount of catalyst adsorbed becomes constant i.e., equilibrium adsorption is achieved. The reaction of methyl orange in presence of TiO 2 and Co:La:TiO 2 nanocomposites and UV irradiation is an example of heterogeneous catalysis. Rate laws in such reactions seldom follow proper law models and hence are inherently more difficult to formulate from the data. It has been widely accepted that heterogeneous catalytic reactions can be analyzed with the help of Langmuir Hinshelwood (LH) Model [34,35], with the following assumptions being satisfied, (i) there are limited number of adsorption sites on the catalyst and its surface is homogeneous, (ii) only one molecule can be adsorbed on one site and monolayer formation occurs (iii) the absorption reaction is reversible in nature, and (iv) The adsorbed molecules do not react amongst themselves [36][37][38]. According to LH Model, following steps take place in the kinetics mechanism [39][40][41][42][43] (Adsorption of dye onto the catalyst surface). There are three steps of adsorption, Surface reaction, Desorption of products from the surface.

Photo-degradation of dyes
The photodegradation of Methyl Orange has been studied in the presence of TiO 2 and Co:La:TiO 2 . The dye solution prepared in water and alcohol 10:1 (V/V) ratio. The definite amount (100-800 mg/L) of photocatalyst was dispersed in the dye solution. The dispersion was irradiated under visible light, although kept under agitation. After different time interval and different temperature the photocatalyst was separated from dispersion. The remaining concentration of dye in the dispersion mixture was determined spectrophotometrically. The photodegradation of Methyl Orange is showing in Figures 7-9.
Effect of temperature: The affected the temperature on photocatalysis reaction has not concerned enough interest. However, in this research, photodegradation of Methyl Orange has a vast effect of temperature. The photodegradation efficiency can be increased about 2-3 times if the temperature increased from 30°C to 40°C Because the solar energy include UV light, which can be used to activate the photocatalytic course, which is increase the temperature of photocatalytic system. The experiments showed that Methyl Orange were photodegraded in presence of photocatalyst and Visible light. The Methyl Orange was efficiently degraded shown in Figure 7. In presence of the Co:La:TiO 2 at 40°C, 88% photodegradation was observed and 30°C 76% photodegradation observed. The obvious decrease of concentration of dye shows that the Co:La:TiO 2 can serve as an effective photocatalyst as compared with pure TiO 2 but in the blank experiment only 0.2% photodegradation observed due to the light radiation which causes the thermal action on the dye [44]. Co and La ions have low band gap energy than the pure Titania. Therefore the band gap energy of Co:La:TiO 2 was reduce and capable to absorb the visible light radiation.

Effect of concentration of dye:
The effect of concentration (from 25 ppm to 100 ppm) of dye was studied on the photodegradation of dye. The photodegradation of methyl orange is given in Figure 8a. the photodegradation of methyl orange were found maximum 19% and 98.9% at 25 ppm concentration, for TiO 2 and Co:La:TiO 2 respectively. The rate of photodegradation of Methyl Orange was found to decrease with the gradual increase in concentration. This is due to the hindrance created in the path of light by the number of dye molecules, therefore the number of photons penetrating in the dye solution is reduced, and hence catalyst surface is not received complete photons. Thus the formation of the reactive • OH and O 2 radicals is reduced. Thus the optimum concentration of dye should be maintained in the photocatalytic reaction, where maximum efficiency of photodegradation can be found [45].

Effect of time on photocatalytic degradation of dye:
In presence of TiO 2 and Co:La:TiO 2 , the photodegradation of methyl Orange has been studied at different irradiation time. The photocatalytic degradation of methyl Orange was increased with increase of irradiation time. In case of Co:La:TiO 2 , The photodegradation was found 98% at 180 min irradiation of visible light but in case of pure titania only 18% photodegradation was observed. In case of blank experiment, there was no major change observed. The effect of irradiation time on photodegradation of methyl Orange is showing in Figure 8b. This is because of the dye molecule interaction with the surface of photocatalyst and the time of illumination increases, the interaction increased. Consequently the photodegradation efficiency of photocatalyst was increased [46].

Effect of pH of solution:
The effect of pH (from 2 to 9) on photodegradation of methyl orange was determined. The pH of the solution was adjusting with H 2 SO 4 and NaOH, while kept at constant amounts of photocatalyst 800 mg/ L and concentration of dye solutions (Figure 8c). The observed photodegradation was found low photodegradation rates at acidic ranges of pH. Although at pH 5 photodegradation was found maximum. This informed that less acidic situation are favourable towards the construction of the reactive intermediates that is hydroxyl radicals is extensively improved, which further help in enhancing the reaction rate. Alternatively in highly acidic conditions for the creation of reactive intermediates is comparatively less favourable and hence less spontaneous [47,48].

Effect of photocatalyst amount:
The effect of photocatalyst amount has been studied by applying the different amount (100 ppm  to 800 ppm) of the photocatalyst. The photodegradation rate was found to increase by increasing the amount of photocatalyst. It is clear from the results shown in Figure 8d, the photodegradation increased rapidly with increase of amount of Co:La:TiO 2 . This is due to the fact that introduction of Co 2+ and La 3+ into TiO 2 the band gap energy decreased up to 3.0 eV which enhance the photocatalytic activity [49]. Figure  8 that both TiO 2 and Co:La:TiO 2 are efficient photocatalyst for the photodegradation of Methyl Orange dye. Though Co:La:TiO 2 shows the good photocatalytic activity than pure TiO 2 for the degradation of Methyl Orange dye. The major photodegradation of methyl orange was observed in 3 hour irradiation time, in the presence of Co:La:TiO 2 [50]. This is due to the doping of La 3+ and Co 2+ ions in TiO 2 , the band gap energy decreased, due to the formation of sub-band by the La 3+ and Co 2+ ions. Therefore the photocatalytic activity of TiO 2 was enhanced by the doping of La 3+ and Co 2+ into TiO 2 .

Recyclability of photocatalyst
The photocatalyst and Methyl Orange mixture was agitated, illuminated with visible light and after desired time, the mixture was centrifuge to remove the photocatalyst. The obtained photocatalyst was washed three times with distilled water and kept in oven for 24 h at 60°C and reused for the photodegradation of methyl orange. The photodegradation of Methyl Orange by the recyclized Photocatalyst are showing in Figure 9. The result shows that the recyclized photocatalyst efficiency is slightly decreased probably due to the loss of some active sites and decrease of collection efficiency of photon [51].

Lowering of electron-hole recombination
Photoluminescence spectra have been used to examine the mobility of the charge carriers to the surface as well as the recombination process involved by the electron-hole pairs in semiconductor particles. PL emission results from the radiative recombination of excited electrons and holes. In other words, it is a critical necessity of a good photocatalyst to have minimum electron-hole recombination. To study the recombination of charge carriers, PL studies of synthesized materials have been undertaken. PL emission intensity is directly related to recombination of excited electrons and holes. Figure 10 shows the photoluminescence spectra of synthesized photocatalysts. In the PL spectra the intensity of TiO 2 is higher than Co:La:TiO 2 indicating rate of recombination of e --h + is higher in TiO 2 than that of Co:La:TiO 2 . The weak PL intensity of Co:La:TiO 2 may arise due to the impregnation of La 3+ and Co 2+ in Titania lattice, which for sub band level in band gap region of TiO 2 . This delays the electrons-holes recombination process and hence utilized in the redox, reaction leading to improved photocatalytic activity [52].

Hydroxyl radical formation
As hydroxyl radical performs the key role for the decomposition of the organic pollutants, it is necessary to investigate the amount of hydroxyl radicals produced by each photocatalyst. In this study terephthalic acid (TA) has been used as a probe reagent to evaluate • OH radical present in the photoreaction pathway. Figures 11 and 12 shows the PL spectra of TiO 2 and Co:La:TiO 2 recorded Methyl Orange solution in presence of 10 -3 M Terephthalic solution. OH radical attack Terephthalic, forming 2-hydroxyl terephthalic acid (TAOH) which gives a fluorescence signal at 426 nm. The fluorescent intensity is linearly related to the number of hydroxyl radicals formed by the photocatalysts. Higher the generation of hydroxyl radical, more will be yield of TAOH and hence more intense will be the fluorescence peak. The spectra show that the intensity of peak indicating in presence of Co:La:TiO 2 higher generation of more number of hydroxyl radicals compared to TiO 2 [53].   [3] It is recommended that the hydroxyl radical ( • OH) and superoxide radical anions (O 2 .-) are the primary oxidizing species in the photocatalytic oxidation processes [50]. These oxidative reactions would results in the degradation of the pollutants as shown in the following equations 4-5;

Mechanism of photooxidation process
Oxidation of the organic pollutants via successive attack by OH radicals or by direct reaction with holes R+h + → R + → CO 2 +H 2 O+NH 3 [5] Kinetic study The pseudo-first-order rate constant (k, min -1 ) for the photodegradation reaction of Methyl Orange was determined through the following relation where, k can be calculated from the plot of ln(C o / C t ) against time (t), C o and C t denote the initial concentration and reaction concentration, respectively. ln C o /C t =k 1 t [6] In addition, the linear feature of plots of ln(C o /C t ) versus time ( Figure 13) indicates that this photocatalytic degradation reactions follow the pseudo-first-order rate law [54,55]. The rate constant of the photocatalysis at 30°C is 0.04260 to 0.0234 min -1 . The effect of temperature and concentration are showing in Table 3.

Thermodynamic parameter study
In this section an attempt has been made to calculate different activation parameters. For this the reaction has been studied at two different temperatures and with the help of observed rate/rate constant, the energy of activation (∆E*), specific rate constant (k r ), entropy of activation (∆S*), enthalpy of activation (∆H*), free energy of activation (∆G*) and Arrhenius frequency factor (A) have been computed for different reactions (shown in Table 4) [56].     88% at 5 pH, 96% at 800 mg/L amount of photocatalyst and 98.5% at 180 min illumination of visible light in presence of Co:La:TiO 2 whereas in case of TiO 2 about 20% photodegradation was observed. Hence the prepared Co:La:TiO 2 nanocomposite is the efficient and superior photocatalyst than the neat TiO 2 . The photodegradation was following the first order kinetics.