Synthesis, Characterization, and Visible Light Photocatalytic Activity of Nanosized Carbon Doped Zinc Oxide

ZnO precursor was prepared using microemulsion method. Precursor was calcined in a furnace in a temperature range of 300– 500Cwith an interval of 100C. Precursor and calcined nanopowders were characterized using TG/DTA, XRD, FT-IR, EDX, TEM, SEM, and particle size analyzer. Precursor calcined at 300C for 2 h contains 3.87% carbon (C). Increase in calcination temperature of precursor shows decrease in C content. Precursor calcined at 500C for 2 h yielded pure ZnO. UV-visible spectrophotometer was used to analyze the concentration of MG during the degradation process. In presence of visible light C doped ZnO obtained by calcining precursor at 300C shows better photocatalytic activity for MG degradation. Parameters affecting the photocatalytic process such as calcination temperature of catalyst, catalyst loading, MG concentration, and pH of solution have been investigated.


Introduction
Environmental pollution has become a major threat to the human lives.Pollutants from polluted air and industry effluents create severe ecological problems.Photocatalytic oxidation of organic pollutants from industrial waste water by using semiconducting oxides can be an alternative to conventional methods for environmental remediation.Due to its mild operating conditions and the fact that it can be powered by sunlight, it allows green mineralization of organic pollutants [1][2][3][4].Among the semiconductors TiO 2 photocatalyst has been extensively investigated.Anatase TiO 2 shows better photocatalytic activity and is used as photocatalyst.It is difficult to prepare phase pure anatase TiO 2 , and also preparation of TiO 2 is costly.Zinc oxide (ZnO) can be considered as a suitable alternative to TiO 2 photocatalyst due to its nontoxic nature, good environmental stability, strong oxidizing power, and same band gap energy and is relatively cheaper.The major advantage of ZnO is that it absorbs a large fraction of solar spectrum than TiO 2 [5][6][7].ZnO has been used for the photodegradation of various dye pollutants [8][9][10].ZnO has a wide band gap of about 3.2 eV and can absorb UV light with wavelength below 387 nm, due to the fact that its photocatalytic activity is limited to irradiation wavelength in UV region only.Unfortunately, solar light consists of less than 5% UV radiations and indoor lighting consists of less than 0.1% UV radiations.Therefore it is necessary to modify ZnO in order to utilize major portion of sunlight/indoor lights.
For the development of visible light sensitive ZnO several methods are used.It includes dye sensitization [11,12], semiconductor coupling [13,14], metal or nonmetal doping [15][16][17][18][19], surface organic coating [20], and surface hybridization of ZnO with carbon [21].Doping of ZnO with nonmetals such as carbon (C), nitrogen (N), and sulfur (S) leads to the formation of intermediate energy level between the band gap, enabling it to absorb visible light [22,23].Among the nonmetal dopants, C has been found to be very efficient for visible light induced photocatalysis.Its additional advantage is that it can promote the separation of photoelectrons and holes by channelizing the photoexcited electrons to nanosized C on the surface of catalyst, thereby reducing the rate of recombination [24][25][26].
In this paper we report the synthesis of C doped ZnO nanorods by reverse microemulsion method.Also its photocatalytic activity is examined for the degradation of MG in presence of visible light as a model photocatalytic reaction.

Synthesis of
Precursor. 1 M solution of zinc nitrate was prepared by dissolving 29.74 g of Zn(NO 3 ) 2 ⋅6H 2 O in 100 mL distilled water. 2 M sodium hydroxide solution was prepared by dissolving 8 g of NaOH in 100 mL distilled water.To 28.8 mL 1 M zinc nitrate, 35.5 mL cyclohexane, 8 mL butanol, and 5.90 g cetyltrimethylammonium bromide (CTAB) were added.In another solution 28.8 mL 2 M NaOH, 35.5 mL cyclohexane, 8 mL butanol, and 5.90 g CTAB were mixed.Both these solutions were stirred continuously with the help of magnetic stirrer to form clear solutions.These clear solutions were mixed with each other.The mixture was transferred to 250 mL Teflon lined autoclave and heated in an oven at 150 ∘ C for 1 h.After 1 h autoclave was cooled to room temperature.The solid product formed was separated by filtration, washed with distilled water followed by ethanol and finally with acetone, and dried in an oven at 60 ∘ C. The hydrothermal product (precursor) thus obtained was characterized using various characterization techniques.

Characterization.
TG and DTA curves of precursor were recorded using simultaneous TG/DTA recording system (Rigaku, Model-Thermo Plus TG8120).For recording TG/DTA curves 14.20 mg precursor was heated in a nitrogen atmosphere with a heating rate 10 ∘ C min −1 .For recording DTA curve, alumina was used as a reference material.FT-IR spectra of precursor and the precursor calcined at 300, 400, and 500 ∘ C were recorded using FT-IR spectrophotometer (Bruker).Precursor was calcined at 300, 400, and 500 ∘ C for 2 h and the XRD patterns of the product obtained at different temperatures were recorded using X-ray diffractometer (Rigaku, Model-Miniflex II) using monochromatized CuK radiation ( = 0.15405 nm) with scanning rate of 2 ∘ 2 min −1 .Qualitative elemental analysis was carried out using energy dispersive X-ray spectroscopy (EDX) (JEOL-JSM6360A) technique.TEM images of C doped/pure ZnO powder prepared in the present work were recorded using transmission electron microscope (Philips, CM200).For TEM sample was prepared by dispersing C doped/pure ZnO powder in isopropyl alcohol and solution was sonicated for 15 min.The drop of solution was placed over C coated copper grid and solvent was evaporated off under IR lamp.Particle size distribution study of product obtained at 300 ∘ C was carried out using particle size analyzer (Nanosight, Model-NTA-LM-20).Band gap energies of synthesized nanopowders were evaluated from UV-visible spectra recorded using UV-visible spectrophotometer (Shimadzu, Model-1800).

Photocatalytic Activity Study.
Photocatalytic activity of nanosized pure/C doped ZnO photocatalyst was tested for degradation of malachite green (MG) solution.Reaction suspension was prepared by adding 0.05 g ZnO (obtained at 300 ∘ C) photocatalyst in 100 mL, 10 ppm MG solution.This aqueous suspension was stirred in the dark for 30 min to attain adsorption-desorption equilibrium.Later, the solution was irradiated with visible light.The visible light irradiation was carried out in a photoreactor using a compact fluorescent lamp (65 W,  > 420 nm, Philips).Temperature of test solution was maintained constant throughout the experiment by circulating water around the solution.The amount of MG was monitored by sampling out 5 mL of aliquot solution at regular time intervals.The catalyst was first separated by centrifugation and the concentration of MG in the supernatant solution was estimated using UV-visible spectrum recorded in the wavelength range of 200-800 nm.
To evaluate the effect of various parameters on photocatalytic activity of ZnO, the same experiment was repeated by using the catalyst calcined at different temperatures, varying the amount of catalyst used, and changing the pH and concentration of MG.

Results and Discussion
3.1.TG and DTA Study.Simultaneous TG and DTA curves were recorded for thermal decomposition studies of precursor and are presented in Figure 1.TG curve shows that there is gradual weight loss from 25 to 200 ∘ C, and this can be attributed to vaporization of absorbed water molecules and cyclohexane.Also, this change is observed in DTA curve as an endothermic peak in the same temperature range with minima located near 104 ∘ C. In temperature range 200-330 ∘ C, TG curve exhibits a sudden weight loss, which corresponds to burning of residual surfactant and gives weak endothermic peak in DTA curve near 300 ∘ C.  Above 400 ∘ C there is no weight loss and TG curve gives a stable line.It indicates that precursor yields stable product above 400 ∘ C.This information was used to select the calcination temperature to get ZnO powder from its precursor.Precursor was calcined at 300, 400, and 500 ∘ C for 2 h and the product obtained was analyzed using X-ray diffractometer.

X-Ray Diffraction (XRD) Study.
Figure 2 shows XRD patterns of precursor and product obtained after heating the precursor at 300, 400, and 500 ∘ C. XRD pattern of precursor shows that it is the mixture of Zn(OH) 2 and as-grown ZnO particles.These patterns suggest that product obtained by heating the precursor at different temperatures is crystalline in nature and is in good agreement with the wurtzite ZnO phase (JCPDS 36-1451).The average crystallite sizes of the product obtained were calculated from their XRD patterns using Debye-Scherrer equation and were found to be between 20 and 35 nm.

FT-IR Study.
FT-IR spectra of the precursor and product obtained after calcinations of the precursor at different temperatures for 2 h are presented in Figure 3.It shows that precursor gives absorption peaks at 3452 and 1625 cm −1 which corresponds to the O-H stretching and bending This spectrum also shows absorption peaks below 560 cm −1 which corresponds to the stretching frequency of Zn-O bond in zinc oxide.FT-IR spectra of product obtained after heating the precursor at 300, 400, and 500 ∘ C for 2 h show no absorption peak in higher frequency region but give the sharp peak below 600 cm −1 which may be due to stretching frequency of Zn-O bond in zinc oxide.It indicates that precursor heated above 300 ∘ C for 2 h gives ZnO as a product.

Energy Dispersive X-Ray Spectroscopy (EDX) Study.
EDX spectra of the precursor heated at various temperatures were recorded.From EDX spectra amount of C present in the sample was estimated and data obtained is presented in Table 1.It shows that C is present in the product obtained after calcining the precursor at 300 and 400 ∘ C and the amount of C is 3.87 and 0.72%, respectively (by weight).However the precursor heated at 500 ∘ C for 2 h yielded pure ZnO.

Transmission Electron Microscopy (TEM)
Study.TEM images of precursor calcined at different temperatures were recorded and are presented in Figure 4.It shows that C doped ZnO prepared at 300 ∘ C is having a rod-like structure with diameter and length varying between 20-40 and 100-400 nm, respectively.Product obtained at 400 ∘ C is heterogeneous and consists of a mixture of rods and spheres.However ZnO particles obtained at 500 ∘ C are spherical in shape and size of particles ranges from 15 to 35 nm.This is may be due to the addition of NaOH into aqueous solution of Zn(NO 3 ) 2 leads to the formation of white precipitate of Zn(OH) 2 , which during hydrothermal treatment decomposes to ZnO.Thus the hydrothermal product obtained is a mixture of Zn(OH) 2 and as-grown ZnO particles.This mixture was heated in air and gives ZnO nanoparticles with different morphologies [26].The probable reaction that occurred during thermal treatment could be given as Selected area electron diffraction patterns (SAED) of all the samples (inset) show distinct rings that correspond to the diffraction pattern of ZnO indicating crystalline nature.

Scanning Electron Microscopy (SEM) Study. SEM images
of precursor calcined at different temperatures were recorded and are presented in Figure 5.It shows that C doped ZnO prepared at 300 ∘ C is having irregular shape.At 400 ∘ C this irregular shape initiated to transform into spherical shape and therefore it shows mixture of rods and spheres.ZnO particles obtained at 500 ∘ C are spherical in shape and at this temperature it shows aggregation.

Particle Size Analysis.
Particle size distribution of ZnO synthesized in present work was studied using particle size analyzer.The particle size distribution curve for the precursor calcined at 300 ∘ C for 2 h is presented in Figure 6.This figure shows that particle size of C doped ZnO prepared in the present work varies over range from 20 to 70 nm with average particle size 43 nm, which is in agreement with the size obtained from TEM studies.

UV-Visible Spectrophotometery Study. UV-visible spectra
of ZnO nanopowders synthesized in present work are presented in Figure 7. Figure 7(c) shows that pure ZnO (obtained at 500 ∘ C) gave band gap absorption edge at 403 nm whereas C doped ZnO, that is, ZnO obtained by calcining precursor at 400 and 300 ∘ C for 2 h, gave absorption peaks at higher wavelength whose absorption edges are at 430 and 461 nm, respectively.This distinct difference in absorption characteristics indicates that C is successfully doped on ZnO.The band gap energy of ZnO obtained at various temperatures was estimated using Tauc plot (Figure 8) and is presented in Table 1.The band gap energy of ZnO obtained at 300, 400, and 500 ∘ C is 2.69, 2.88, and 3.08 eV, respectively.Band gap of ZnO obtained at 300 ∘ C is less as compared to ZnO obtained at 400 and 500 ∘ C; due to this reason it absorbs higher wavelength (visible) light.MG.Mechanism of this degradation can be explained as follows: h h Figure 9 shows that in presence of photocatalyst ∼98% MG is degraded in 60 min.Several parameters affect the rate of photocatalytic degradation.To optimize the photocatalysis process it is necessary to study these parameters.Various parameters such as calcination temperature, amount of catalyst, pH, and concentration of MG which affect the rate of photocatalytic degradation were studied and are explained in the following section.within 60 min.This can be due to difference in size and shape as well as C content doped on ZnO at different temperatures.

Effect of Calcination
The photodegradation efficiency of MG using photocatalyst calcined at different temperatures is estimated and presented in Table 1.dye concentration increases, adsorption of dye on the catalyst surface sites increases by decreasing OH − concentration on the same sites.Also according to Beer-Lambert's law, as initial dye concentration increases, the intensity of photons entering the solution decreases which results in lowering the absorption of photons on catalyst surface; hence the rate of photodegradation process decreases.

Recycling of Photocatalyst.
For practical applications stability of photocatalyst during photodegradation is a crucial factor.C doped ZnO prepared at 300 ∘ C exhibits better photocatalytic activity; therefore its stability was studied.Stability tests were performed by repeating the reaction three times using recovered photocatalyst.The data obtained is presented in Figure 14.The data reveals that there is no

Figure 1 :
Figure 1: TG and DTA curves of ZnO precursor recorded in flowing nitrogen.

Figure 2 :
Figure 2: XRD patterns of precursor and precursor calcined at different temperatures.

Figure 3 :
Figure 3: FT-IR spectra of precursor and precursor calcined at different temperatures.
Temperature of Catalyst.MG solution was exposed to visible light in presence of pure and C doped ZnO.The UV-visible spectra of the MG

Figure 9 :
Figure 9: UV-visible spectra of MG solution irradiated with visible light at different time intervals in presence of C doped ZnO photocatalyst prepared at 300 ∘ C.

Table 1 :
Properties of C doped and pure ZnO photocatalyst.
* Zn(OH) 2 plotted and presented in Figure12.The figure indicates that photodegradation efficiency of MG is less at lower pH values and increases with increase in pH.At pH = 9 almost 100% dye degrades within 45 min.In acidic conditions, it is difficult for cationic dye to get adsorbed on catalyst surface.Also at low pH concentration of active • OH radicals is usually low; therefore photodegradation rate of MG remains slow.At high pH formation of active • OH radicals favors due to improved transfer of holes and increases the photodegradation rate.
3.9.3.Effect of pH.Photodegradation process is also strongly affected by pH of the solution.Effect of pH of MG solution on photocatalytic degradation rate was studied and rate constant of photodegradation process as a function of pH of solution ) Figure 12: Photodegradation rate of MG as function of pH.is