Synthesis of Chromium Doped Cobalt Oxide (Cr:Co3O4) Nanoparticles by Co-Precipitation Method and Enhanced Photocatalytic Properties in the Visible Region

In the present work, Co3O4, 1% chromium doped Co3O4 (1% Cr:Co3O4), and 5% chromium doped Co3O4 (5% Cr:Co3O4) was successfully prepared by simple chemical co-precipitation method followed by calcination at 400°C for 3 h. The synthesized nanoparticles materials were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), Brunauer Emmett Teller analysis (BET), and UV-visible spectroscopy. XRD confirmed the formation of cubic nature of nanoparticles while SEM images shown spherical structure. BET analysis confirmed the mesoporous behaviour of nanoparticles. UV-Visible spectra have been used to determine band gap and photo-oxidation behaviour of organic dye methyl orange. Experimental data suggested that 5% Cr:Co3O4 nanoparticles catalyst possessed the highest catalytic activity towards MB degradation in aqueous solution at the tested concentration level of 50 mg/L, higher than that of pure Co3O4. *Corresponding author: Deepak Kumar, Research Scholar, Department of Applied Chemistry, Babasaheb Bhimrao Ambedkar University, Lucknow, India, Tel: 9412818506; E-mail: dpk.4470@yahoo.in Received December 13, 2017; Accepted January 23, 2018; Published February 05, 2018 Citation: Hitkari G, Sandhya S, Gajanan P, Shrivash MK, Deepak K (2018) Synthesis of Chromium Doped Cobalt Oxide (Cr:Co3O4) Nanoparticles by CoPrecipitation Method and Enhanced Photocatalytic Properties in the Visible Region. J Material Sci Eng 7: 419. doi: 10.4172/2169-0022.1000419 Copyright: © 2018 Hitkari G, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
From the several last decades, more attentions have been paid for the preparation of nanocrystalline materials with appropriate size, geometry and stoichiometry to enhanced the magnetic activity as compare to their bulk counterpart. Essentially, chemical composition, distribution of cation, shape and size of the particle are responsible for change in morphology and various properties [1][2][3][4]. There are new opening corridor of advanced properties of the particles like detection of hazardous gasses, spintronic equipment, magnetic seals, biomedical application [5] and photo-disappearance of organic toxic pollutants by photocatalysis as well as easily magnetic recoverable photocatalyst [6][7][8][9]. As reduction in the size of particle into nanometre scale, then its surface consequence develops into more and more important due to the enhancing fraction of surface atoms, and the symmetry of the crystal is diminished for those metallic cations which accommodated on the surface because of the occurrence of incomplete atomic coordination.
Among the magnetic materials, oxides of transition metal with spinel structure are one of the most important magnetic materials [10], the generalised formulation for the magnetic spinel can be denoted as AB 2 O 4 (A=Zn, Fe, Co, Ni, Mg, etc.; B=Fe, Cr, Mn, Al, etc.), where A and B symbolizes the divalent and trivalent cat ions, respectively. The coordination of the presented ions at both A and B sites can be tetrahedral and octahedral with oxygen atoms. The magnetic performance of these compounds is very conscious of the nature of cations and their distribution among the two interstitial sites of spinel lattice, that it can be adjusted through modulation of cat ion composition and cat ion distribution [11]. Cobalt oxide (Co 3 O 4 ) is a normal spinel among the transition metal oxide with crystal structure established on a cubic close packing array of oxide ions, in which Co (II) ions occupy 1/8 tetrahedral A sites and Co (III) ions occupy 1/2 octahedral B sites. Moreover, Co 3 O 4 is an intrinsic p-type semiconductor with extraordinary awareness because of its prospective applications as heterogeneous catalysts, sensors, electrochemical devices, lithium ion batteries and magnetic materials. Lou et al. described the photo catalytic behaviour of Co 3 O 4 nano rods synthesized by the complex pyrogenation technique for the elimination of organic contaminants in water. Co 3 O 4 is also a capable applicant for the utilization in high-power lithium ion batteries, because of its outstanding rate ability and good cycle life [12].
There is numerous type of methods have been described for the preparation of Co 3 O 4 nanoparticles, He et al. used solubility controlled method for the production of Co 3 O 4 nano crystals by applying surfactant [13], Gu et al. have also been reported facile combustion method for the preparation of Co 3 O 4 nanocrystals [14]. A lot of different methods have also been applying for the fabrication of spinel Co 3 O 4 nano crystals like as polyol process, a sol-gel method, polymer assisted synthesis, solvothermal synthesis, thermal decompositions and hydrothermal synthesis for Co 3 O 4 nanorods [15][16][17][18][19][20].
In the present paper, we have employed a simple co-precipitation method to achieve nano-sized chromium doped cobalt oxide (Cr:Co 3 O 4 ) nanoparticles and their photocatalytic activity was checked out by applying methylene blue (MB) dyes as toxic materials under the radiation of visible light. This method does not require any type of special conditions such as a special surfactant, or temperature and pressure controlling. The character of the as-synthesized material was characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Ultraviolet-visible spectroscopy and BET techniques.

Chemicals and materials
Analytical grade cobalt chloride hexahydrate (CoCl 2 ·6H 2 O, Merck India), chromium chloride hexahydrate (CrCl 2 ·6H 2 O, Himedia India), methylene blue (MB) and ammonia solution (Merck India) were used without any further purification. Doubled de-ionized water was used as a solvent. All the glassware's were cleaned with concentrated acid. The dried glassware's were used in all the experiments.

Preparation of chromium doped Co 3 O 4 nanoparticles
A homogeneous solution of Cr 2+ and Co 2+ was obtained by mixing appropriate amounts of CrCl 2 ·6H 2 O and CoCl 2 ·6H 2 O in double distilled deionized water. The solution of the diluted NH 3 ·H 2 O (25%) was added to the above solution with continuous stirring till the pH 11 was reached. After stirring at room temperature for 30 min, the reaction mixture was reflux at 180°C for two hours. The product was aged for 24 h and the resulting precipitates were centrifuged, followed by washing with doubled distilled water. The sample was then dried at 80°C for 2 h and the obtained sample was further annealed at 400°C for 3 h. In this way, Cr:Co 3 O 4 nanoparticles containing different concentrations of chromium (0, 1, and 5 mole%) were prepared and annealed at 400°C for 3 h.

Catalyst characterization
The crystal structure of the prepared materials was implemented by the X-ray diffraction (XRD, a Pananalytical's X'Pert Pro X-ray diffractometer, =1.5406 A) in the range 2 of 10-80° with scan rate of 10°/min and Cu K radiation, 40 KV. Surface morphology was achieved by applying Scanning electron microscopy (SEM) images were obtained on a JEOL: JSM-6490 LV Scanning Electron Microscope. Chemical composition was investigated by using energy dispersive X-ray (EDX) spectroscopy. The sample was heated for pretreatment for 2 h under vacuum at 120°C before measurements. Then specific surface areas and pore size distributions were investigated from the consequence of N 2 adsorption-desorption measurement at 77 K (BELSORP MINI II) by using the BET (Brunauer-Emmett-Teller) and BJH (Barrett-Joyner Halenda). The spectrum of UV-Visible spectra was verified in absorption mode with a carry 100 spectrophotometer in the 200-800 nm regions.

Photocatalytic activity of chromium doped Co 3 O 4 nanoparticles
The application of the synthesized product was predicted by photo degradation of methylene blue (MB) in presence of visible light radiation in a photocatalytic chamber. 200 mg quantity of prepared Co 2 O 3 materials initially dissolved in 100 ml of 1 × 10 -5 M, MB standard solution and mixture solution was stirred for 30 min in the dark condition in order for attaining the adsorption-desorption equilibrium. Finally, the solution was irradiated with visible light from the fluorescent lamp (9 W) in a photocatalytic chamber. The solution was agitated, during irradiation by using a magnetic stirrer and air was supply into the reaction mixture to implement a constant supply of oxygen. After the preferred time interaction, an aliquot amount of the solution was withdrawn, centrifuged and take its absorbance on a UV-visible spectrophotometer to measure the percentage degradation. The same procedure was reciprocated for 1% chromium doped cobalt oxide and 5% chromium doped cobalt oxide NPs also. The degradation efficiency of photocatalytic was measured by applied the following equation: Where A 0 represents the initial absorbance of the dye solution and A t ; the absorbance after irradiation at a particular time t.

Nanostructure of chromium doped Co 3 O 4 nanoparticles
The crystal structural and phase of the as prepared sample Co 3 O 4 was recorded by XRD analysis as shown in Figure 1. All peaks have a good agreement with the standard spinel cubic Co 3 O 4 suggesting that the sample produced is well crystallized Co 3 O 4 . However some small intense peaks correspond to CoO also found in the XRD patterns as an impurity. As presented curve in Figure 1a, Where d is the average particle size of crystallites; λ is the X-ray wavelength; is the full width at half maximum (FWHM) of the peak and θ the Braggs angle thus confirming that the crystal size of Co 3 O 4 .
Structural, morphological and compositional analysis of asprepared materials was accomplished by SEM and EDX investigated.  Figure 2b. From the observation of the SEM image, the morphology of 1% Cr:Co 3 O 4 NPs is similar to that of cobalt oxide; however there is no change in the shape and size. Corresponding EDX spectrum of 1% Cr: Co 3 O 4 Figure  2e indicates that the nanoparticle is composed of chromium, cobalt and oxygen element. In the SEM image Figure 2c the morphology of the 5% chromium doped cobalt oxide (5% Cr:Co 3 O 4 ) NPs is consist of spherical nanocrystals and micropores [22] but size is smaller to the measurements. The results are systematically shown in Table 1. It is observed from the Barrett-Joyner-Halenda (BJH) plot that the addition of dopants to the base Co 3 O 4 photocatalyst leads to an increase in surface area. The observed order of considerable surface areas in terms of booster like chromium added is 5% Cr:Co 3 O 4 >1% Cr:Co 3 O 4 >Co 3 O 4 , respectively. The large surface area of the doped materials might be associated to that metal ions contribution as additional nucleation sites during the precipitation and subsequent growth steps and this leads to solid materials with higher surface areas. The average pore diameter of the prepared samples is in the following sequence Co 3 O 4 >1% Cr:Co 3 O 4 >5% Cr:Co 3 O 4 . The addition of increase percentage of chromium over Co 3 O 4 base catalyst caused a drastic decrease in average pore diameter Table 1. The lowest pore size is obtained over 5% Cr:Co 3 O 4 doped cobaltite sample. Furthermore; it is remarkable to note that among doped catalysts, surface area increases and pore diameter decreases as percentage of chromium metal increases. Small pore diameters are obtained with sample having higher surface area.
In Figure 4, measurements of pore-size distributions by the BJH method are presented for all the materials. The graph indicates that the photocatalyst materials consumed broad pore size distributions in the mesoporous region, illustrating the volatile-sized mesopores in the photocatalyst structure. The 1% Chromium doped Co 3 O 4 (1% Cr:Co 3 O 4 ) materials shows a peak around 60 Å, but also a shoulder at a lower pore diameter of 90 Å. The 5% Chromium doped (5% Cr:Co 3 O 4 ) materials reveals a peak around 120 Å. The un-doped Co 3 O 4 nanoparticles reveal a peak at 90 Å as about the similar distribution profiles.

Optical and photocatalytic properties of chromium doped Co 3 O 4 nanoparticles
The UV-Visible spectra of all the materials were recorded in the aqueous solution and direct band gap energy was calculated from the Tauc relation: Where ε is the molar extinction coefficient, h is plank constant, ν is the frequency of light, Eg is the band gap energy and P is the arbitrary constant. The plot of (εhν) 2 against energy of photon 'hν' shown sharp absorption edge of the powder catalyst and Eg was calculated by extrapolation of the linear region of the plot as shown in Figure 5. For other NPs. All together Figure 2f (the EDX spectrum) indicates that the particle is also composed of chromium, cobalt, and oxygen.
Nitrogen sorption measurements are carried out to further study of the surface and porosity of as-synthesized chromium doped Co 3 O 4 materials. Typical nitrogen adsorption-desorption isotherms are demonstrated in Figure 3. These isotherms display type IV isotherms with H3 hysteresis loops, characteristics of mesoporous materials. The textural behaviour of prepared catalysts was also examined by the BET surface area, pore size distribution and total pore volume

Photocatalyst
Total surface area (m 2 g -1 ) Total pore volume (cm 3 g -1 ) Average pore diameter (Å) Crystallite size (nm)  [24]. The change in the E g2 (valence to conduction band excitation) of Co 3 O 4 with 1.0, and 5.0% doped Cr was 2.44, and 2.37 eV respectively, and the experimental data its represented that 5.0% Cr:Co 3 O 4 has the more advanced photocatalytic application in some oxidation process.
The photo degradation capability of organic pollutants by the abovesynthesized Co 3 O 4 , 1% Cr:Co 3 O 4 and 5% Cr:Co 3 O 4 photo catalysts was calculated by measuring photo degradation performance of organic dye methyl blue (MB). In the current investigation, uninterrupted air bubbling was supplied to confirm the presence of O 2 , which was used as an oxidizing agent. Figure 6a-6c represent the deviation in the UV-visible absorbance spectra of MB solution (50 mg/L) with different irradiation time in the presence of prepared photo catalysts Co 3 O 4 , 1% Cr:Co 3 O 4, and 5% Cr:Co 3 O 4 . Figure 6 illustrate the amount of degradation of the MB dye by Co 3 O 4 , 1% Cr:Co 3 O 4, and 1% Cr:Co 3 O 4 under the irradiation of visible light for 35 min time. It is noticeable from the UV-Visible spectra that photo degradation capacity of 5% Cr:Co 3 O 4 is the highest and Co 3 O 4 is the lowest while in the case of 1% Cr:Co 3 O 4 , it lies in between Co 3 O 4 and 5% Cr:Co 3 O 4 for the degradation of MB. Almost complete degradation of adsorbed dye molecules (~99%) was observed within 35 min using 5% Cr:Co 3 O 4 nanoparticles. It is shown that the photocatalytic efficiency of semiconductor photo catalysts depends on various properties like band gap, phase/crystal structure, particle size, shape, surface area, surface defects and presence of bound molecules on the surface. The photocatalytic activity of Co 3 O 4 , 1% Cr:Co 3 O 4 and 5% Cr:Co 3 O 4 has a direct correlation with surface area [25]. The photocatalytic activity for 5% Cr:Co 3 O 4 is the highest because its surface area is the highest (99.12 m 2 /g -1 ), for Co 3 O 4 , it is the lowest because the surface area of the same is the lowest (54.79 m 2 /g -1 ) while in the case of 1% Cr:Co 3 O 4 , it lies in between two samples, because the surface area of 1% Cr:Co 3 O 4 (79.00 m 2 /g -1 ) lies in between 5% Cr:Co 3 O 4 and Co 3 O 4 .
Here Co is the initial concentration of dye solution, C the concentration after time t and k is the rate constant [27]. In order to further investigate the effect of the chromium doping level on the catalytic activities of the nanoparticle catalysts, Figure 7 shows the curves of C/C o at various time points after being treated with different catalysts. A near linear correlation between C/C o and time was observed. A graph has been plotted between ln C/C o versus t (Figure 8) (Figure 8).
The above data confirm that the photocatalytic activity of the materials has direct correlation with the surface area of the photocatalysts, along with the crystallinity of the materials. Apart from the high surface area, the presence of high density of oxygen vacancies and/or defects in the 5% Cr:Co 3 O 4 , calcinated at 400°C, played an important role on high photo-oxidation capacity. The band gap energy of Co 3 O 4 is too high to be excited by visible light. However, its band gap can be manipulated to such an extent where visible light induced photocatalysis could take place effectively. Such modification can be achieved by doping or by the addition of other transition metal to Co 3 O 4 to make diluted like 1% Cr:Co 3 O 4 and 5% Cr:Co 3 O 4 . Figure 5 shows that the band gap of pure Co 3 O 4 (2.7eV value) is lowered to 2.5 eV for the 1% Cr:Co 3 O 4 and further to 2.4 eV in the case of 5% Cr:Co 3 O 4 . The improvement of photocatalytic activity of 1% Cr:Co 3 O 4 and 5% Cr:Co 3 O 4 NPs could be achieved due to a decrease of band gap values, which in turn cause an increase of electron-hole formation under visible light irradiation.
The fact that the synthesized photocatalysts adsorb significantly more dye during the reaction, suggests a possible adsorptiondesorption mechanism. Visible light irradiation generates an electronhole pair on the surface of above-prepared Co 3 O 4 , 1% Cr:Co 3 O 4, and 5% Cr:Co 3 O 4 catalysts. The electron-hole pair enables the formation of some intermediate radicals such as hydroxyl radicals, hydroperoxyl radicals and superoxide radical anions in the catalytic solution by their interaction with water and oxygen from the supplied air. Continuous air bubbling facilitates the formation of these radicals; mainly the hydroxyl radicals, which participates directly in the oxidative photodegradation of dye molecules. As the degradation involves adsorption followed reaction, the adsorbed dye molecules can easily interact with the photogenerated oxidizing radical species, resulting in the degradation of dye. The high photocatalytic activity of 1% Cr:Co 3 O 4 and 5% Cr:Co 3 O 4 may be due to the diminishing of the recombination of electron-hole pairs in these materials and hence encouraging their catalytic activity. In fact, a large amount of oxygen vacancies present in 1% Cr:Co 3 O 4 and 5% Cr:Co 3 O 4 function as electron acceptors and trap electrons of the conduction band, thus decreasing the rate of recombination of electrons and holes and increasing rate of photo catalysis [26].
Since the band gap of Co 3 O 4 becomes narrower and narrower while oxygen vacancies go richer and richer by the addition of 1% and 5% chromium transition metal, the photocatalytic activity of 1% Cr:Co 3 O 4 found better than Co 3 O 4 while in the case of 5% Cr:Co 3 O 4 , it is the best among three.
The photocatalytic degradation of Methylene blue under visible light irradiation using Co 3 O 4 , 1% Cr:Co 3 O 4, and 5% Cr:Co 3 O 4 materials is an example of diluted semiconductor catalysis and these reactions seldom follow the proper rate law model and hence it is inherently difficult to formulate a rate equation from the observed data. The

Conclusion
In summary, Co 3