Synthesis of Chromium ( III ) Oxide Nanoparticles by Electrochemical Method and Mukia Maderaspatana Plant Extract , Characterization , KMnO 4 Decomposition and Antibacterial Study

Chromium oxide nanoparticles were synthesized by the reduction of potassium dichromate solution with Mukia Maderaspatana plant extract. In electrochemical methods, Cr2O3 nanoparticles were synthesized by two ways, using platinum (Pt) electrodes and K2Cr2O7 solution with H2SO4 as medium in the first case. And chromium doped platinum electrode (Pt/Cr) in presence of NaHCO3 solution in second case. The resulting Cr2O3 nanoparticles were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV-VIS absorption and Fourier-transform infrared (FTIR) spectroscopy. The enhancing influence of Cr2O3 nanoparticles as a catalyst for the decomposition of KMnO4 has been studied. The antibacterial effect of Cr2O3 nanoparticles against E. coli was investigated. These particles were shown to have an effective bactericide.


Introduction
The study of fine and ultrafine particles has received increasing interest due to new properties that material may show when the grain size is reduced [1].During the past decades, considerable progress in the synthesis of nanoparticles has been achieved.Nanomaterials, particularly transition-metal oxides play an important role in many areas of chemistry, physics and material science [2].In technological applications, metal oxides have traditionally been used in the fabrication of microelectronic circuits, sensors, piezoelectric devices, fuel cells, coatings for the passivation of surface against corrosion, and as catalyst [2].In the emerging field of nanotechnology, a goal is to make nanostructures or nanoarrays with special properties with respect to those of bulk or single particles species.Metal oxides as nanoparticles can exhibit unique chemical properties due to their limited size and high density of corner or edge surface sites [2,3].Among metal oxides, special attention has been focused on the formation and properties of chromia (Cr 2 O 3 ) which is important as heterogeneous catalyst [4], coating material, wear resistance [5,6], advanced colorant [7], pigment [8] and solar energy collector [9].

Experimental
Chromium oxide nanoparticles are synthesized by three different methods and the comparative study for the above synthesized nanoparticles is undertaken.

Method 1 (Biological Method)
Potassium dichromate from Rankem was used without further purification.Mukia Maderaspatana plants were collected from Hassan district and the edible part of the whole plant was shade dried and pulverized using a mechanical grinder.The powdered plant material (50 g) was extracted with methanol (200 ml) by soxhlet apparatus for 24 hrs.The extract was evaporated using a rotaryvaccum evaporator at 40˚C to provide dry extract.The extract was kept at −20˚C until use.The preliminary phytochemical analysis of the extract revealed the presence of various bioactive components, such as alkaloids, flavonoids, phenolics, aminoacids and glycosides [19].An amount of 10.0 g of potassium dichromate was dissolved in 50 ml distilled water and stirred for 10 min.An orange colored solution was obtained.
Preparation of Cr 2 O 3 nanoparticles: In an experiment, 20 ml of potassium dichromate solution was mixed with 20 ml of plant extract in a beaker and stirred for 10 -15 min.The colour of the solution changed from orange to green indicating the formation of chromium(III) oxide nanoparticles.The solution was kept at room temp for evaporation of aqueous phase.The green solid product was dried in hot air oven at 65˚C -70˚C for an hour.The resulting solid was calcined at 650˚C -700˚C for 3 hrs.The addition of potassium dichromate solution to the plant extract containing mild reducing agents causes the reduction of orange dichromate(VI) ions to green chromium(III) ions.As an example, the reduction of Cr 6+ to Cr 3+ by reducing sugars resulting in the formation of Cr 2 O 3 nanoparticles is shown below.
The chemical reaction takes place according to the following mechanism (Scheme 1) Combining that with the half equation for the oxidation of an aldehyde in aqueous condition The reduction of Cr 6+ to Cr 3+ plays a main role in this process and then Cr 2 O 3 is generated.

Method 2 (Electrochemical Method in Presence of K 2 Cr 2 O 7 and H 2 SO 4 )
Chromium oxide nanoparticles are synthesized electrochemically using platinum electrodes.A solution of potassium dichromate (0.3 M) was prepared.The electrochemical cell consists of reaction chamber a voltage power supply and platinum electrodes.The experiment was performed with 20 ml volume of potassium dichromate solution along with 5.0 ml of conc.H 2 SO 4 as a supporting medium.A positive voltage of 12 V was applied using battery eliminator (Neulite India) and current output of 70 mA -90 mA.The experiment was run for 3 hrs with continuous stirring.A change in colour from orange to dark green was observed.The above solution was allowed for the slow evaporation in a hot air oven at 100˚C for 2 hrs.The dried solid which was obtained showed positive results for sulphate test.Further, the solid was calcined at 650˚C -700˚C for removal of moisture and sulfate as sulfur dioxide.The reduction equation is as shown in equation below:

Method 3 (Electrochemical Method in the
Presence of (Pt/Cr) and NaHCO 3 ) In this method a thin film of chromium was deposited electrochemically on a platinum electrode (Pt/Cr) from chromium nitrate solution (0.1 M).The preparation of Cr 2 O 3 nanoparticles was carried in a reaction chamber containing 20 ml of NaHCO 3 solution.Voltage power supply of 12 V, current of 30 mA and Pt/Cr electrode as anode, Pt electrode as cathode were used.The experiment was run for 3 hrs with continous stirring at constant temperature.The anodic dissolution of chromium to give Cr 3+ ions due to electrolytic reaction, which are electrochemically reacted with aqueous NaHCO 3 to form Cr 3+ oxides/hydroxides, which is shown in Scheme 2. The synthesis takes place at the electrode-electrode interface or close to the electric double layer [20].The product formed floats in the electrolyte solution the resulting gel was filtered [21], washed several times with distilled water till complete removal of unreacted NaHCO 3 and dried at 100˚C for dehydration and removal of hydroxides.The dried compound was calcined for 3 hrs at 650˚C -700˚C in muffle furnance in order to decompose the hydroxides of chromium and to get chromium(III) oxide.The Electrochemical reaction takes place according to the following mechanism:

Results and Discussion
Characterization of the chromium oxide nanoparticles was carried out by different techniques.UV-Visable spectra were measured using (ELICO SL171) model double beam spectrophotometer.FTIR spectra were recorded with Fourier-transform infrared instrument between 4000 cm −1 to 400 cm −1 .The morphological properties of the Cr 2 O 3 nanoparticles were examined by scanning electron microscopy (SEM), X-Ray diffraction (XRD) pattern was recorded with pananalytical X-ray diffractometer using CuKα radiation (λ = 1.5406Ǻ).The IR spectra of chromium oxide synthesized from three different methods are shown in Figures 1(a)-(c).It shows that the characteristic bands are at 967 cm −1 -1037 cm −1 , 585 cm −1 -641 cm −1 and 1046 cm −1 -1085 cm −1 .Bands at 967 cm −1 -1037 cm −1 are assigned to Cr=O vibrations, 585 cm −1 -641 cm −1 are assigned to Cr-O vibrations and 1046 cm −1 -1085 cm −1 are relatively assigned to Cr-O-Cr vibrations.The X-ray diffraction pattern obtained for the chromium oxide nanoparticles from three different methods are as shown in Figures 2(a)-(c).The XRD spectrum contains peaks that are clearly distinguishable.All of them can be perfectly indexed to crystalline Cr 2 O 3 not only in peak position, but also in their relative intensity.The peaks with 2θ values of 24.6˚, 36.3˚,50.2˚ and 63.62˚ correspond to the crystal planes of (012), ( 110), (024) and (214) of crystalline Cr 2 O 3 , respectively.An average crystalline size, D hkl was estimated using the Debye-Scherrer equation given below for all X-ray diffraction peaks.
Where K is a shape factor which normally ranges between 0.9 and 1.0 (in our case K = 0.9), λ is the X-ray wavelength, and β and θ are the half width of the peak and half of the Bragg angle, respectively.Using the equation, the crystalline sizes of Cr 2 O 3 nanoparticles which were synthesized from method (1), electrochemical method (2) and method (3) were found to be 65 nm, 79 nm and 41 nm respectively.
The UV-Visible spectrum of Cr 2 O 3 is shown in Figure 3 which shows maximum absorption at 430 nm which is a characteristic value for Cr 2 O 3 as reported in the literature [22].
The morphological studies of synthesized Cr 2 O 3 nanoparticles analyzed by scanning electron microscopy are shown in Figures 4(a)-(c).Compared to three different methods the nanoparticles were well separated and no agglomeration was observed in method (3).

Chromium Oxide as a Catalyst in the Course of KMnO 4 Decomposition
It was elucidated that the more p-type character the solid catalyst have, the more pronounced is the effect on the reaction rate.Trying to compare the influence of chromium oxide catalyst as a p-type semiconductor in the course of other decomposition reaction will have great value in view of the practical importance for oxygen evolution.It has been suggested that its decomposition process includes an electron transfer from one permanganate ion to another with formation of stable 2 4 MnO  ions and unstable MnO 4 radicals [23].Dowden [24] stated that the most active oxide catalyst for oxygen abstraction is solid oxides with vacancies in the d-orbital's.It is found that MnO 2 enhances the rate of decomposition.Markowitz and Boryta [25] suggested that the effect of metal oxide on the decomposition can be attributed to the abstraction of atomic oxygen.On the other hand, Freeman and Coworkers [26] considered it to be due to a charge transfer mechanism.Therefore, the reaction can be represented for p-type catalyst as Where  is a positive hole in the oxide, O oxide is an oxygen atom abstracted by the oxide and MnO 4 is a radical.In this work an investigation has been carried out to shed light on the reaction kinetics by which chromium oxide can act as a catalyst for the decomposition of KMnO 4 .

Effect of KMnO 4 on the rate
The reaction was performed in the presence of 10 ml of KMnO 4 (0.0001 M, 0.00005 M, and 0.0002 M) taken in three separate beakers consisting of 20.0 mg of Cr 2 O 3 nanoparticles which were synthesized from three different methods, namely method 1, method 2 and method 3. The experiment was carried out at room temperature and the rate of reaction was followed with respect to the change in concentration of KMnO 4 by using a spectrophotometer.A plot of log%T verses time was recorded where the rate of decomposition of KMnO 4 increases in the order of method 3 > method 2 > method 1, which is shown in Table 1 and Figures 5(a)-(c).The results show   2 and Figures 6(a) and (b).The catalytic activity was observed greater for Cr 2 O 3 synthesized by method 3 as its size is lower than the other two methods.

Antibacterial Assay
The Cr O nanoparticles synthesize ent methods were tested for a diffusion method against Es monas aregunosa.The pure bacterial culture was subcultured on nutrient agar media.The activity was compared against standard Gentamycin.The concentration of nanoparticles was 50 mg/ml and 50 µl of each solution was  The values from the table indicate that the nano particles of Cr 2 O 3 synthesized from all the three methods show inhibiting effect towards different bacteria.However, the nano Cr 2 O 3 (41 nm) synthesized from method 3 shows a very good inhibiting effect close to the standard.The figure shows the zone inhibition of bacterial growth on agar plates.Hence, the results clearly demonstrate that the newly synthesized Cr 2 O 3 nanoparticles were promising antimicrobial agents against bacteria.Copyright © 2013 SciRes.MRC

Figure 6 . 3 s
Figure 6.(a) Effect of Cr 2 O 3 (10.0mg) on the rate of its decomposition; (b) Effect of Cr O (30.0 mg) on the rate of it decomposition.
logical and electro by UV-Visible, IR, SEM and XRD.The nanoparticles synthesized by Pt/Cr and NaHCO 3 and by electrochemical method act as very good catalysts for KMnO 4 decomposition and as promising antibacterial agents.