Effect of Copper Substitution, Calcination Temperature, and Photo-sensitizers on Photocatalytic Activity of Cu0.05Zn0.95O

A successful series of CuxZn1-xO (variable x = 0.05, 0.1, 0.15 and 0.2) were characterized by thermogravimetric (TG-DTA), Fourier Transform Infra-Red (FTIR) spectroscopy, and X-ray Diffraction (XRD) techniques. The photocatalytic activity of prepared samples was accurately assessed by the photocatalytic decomposition of LASER dye in an aqueous solution under irradiation of solar light and was compared favourably to non-dope commercially available ZnO photo-catalyst. The effect of various parameters like the amount of a catalyst, the calcination temperature on photocatalytic activity is also studied. The direct effect of various photosensitizing salts like NaCl, Na2CO3, and Na2S2O3 on photocatalytic activity of ZnO and Cu0.05Zn0.95O was carefully studied.


Introduction
Photocatalytic treatment of chemical pollutants using semiconductors as the photocatalyst has been important method among advanced reaction techniques. Many published studies precisely on the photocatalytic activity of a semiconductors like TiO 2 . TiO 2 and most of the other photo-catalysts can barely respond to UV irradiation [2] that takes up 4% of solar energy, which limits the practical application of photo-catalysts to broad extent. Since direct sunlight typically contains favorably nothing but 4% of Ultra-Violet (UV) light as compared to visible light which is 43% of unlimited solar energy hence use of TiO 2 is largely impaired [2]. In recent years ZnO shows its unique applications in the optics, opto electronics, catalysis, pyro-electricity, and piezo-electricity. ZnO is one of the important photo-catalyst because of its unique and novel advantages, like non-toxicity, minimum price, and high photocatalytic activity [4]. However, the disadvantages of this catalyst is that its catalytic activity is still not enough for the commercial applications. An effective and practical approach to improve the photocatalytic activity is doping by adding some hetero elements, through the presence of doping metal ions in the ZnO crystalline matrix significantly affects the photocatalytic activity charge carrier recombination rate and interfacial electron-transfer rate [5]. Extensively speaking the metal ions used as a dopant are often the transition metal ions e.g., Co 2+ , Mn 2+ , Mn 4+ etc.
[6]. In the past several years, semiconductors of ZnO doped with narrow-band-gap metals [1], including Fe, W, Cd and Ga have been reported. Rhodamine as shown in figure no. 1. Fluorescein, coumarin, stilbene, umbelliferone, tetracene, malachite green and many other dyes are commonly used as LASER dye.

Literature Survey
A lot of work has been done on the removal and photocatalytic degradation of harmful dyes and organic compounds. The degradation is carried out by using sunlight as well as artificial UV sources. TiO 2 and ZnO have good photocatalytic properties for photo-degradation of water pollutants including the good activity range of the solar radiation. TiO 2 photo-catalysed degradation of phenol and o-substituted phenol compounds also investigated. Photo assisted dehalogenation and mineralization of chloro/ fluoro-benzoic acid derivatives in aqueous media using TiO 2 is done. Removal of organic chlorine compounds by chemical action dehydrochlorination for the refinement of municipal waste plastic derived oil. Photo degradation of 3, 5, 6-trichloro-2-pyridinol in aqueous solution. Photo degradation of chlorinated pesticides dispersed on sand is also done.

Nature of Problem
The main purpose of the project is to degrade harmful laser dye compounds released from industries like textile industry and laser industries. LASER dyes are highly colored dye solution containing Kiton red, Rhodamine-6G, Coumarin, are shown in figure 1, Malachite green and others. These dyes are frequently utilized in LASER industries and these dye solution have to be replaced by fresh dye solution and it can't be directly discharged into water as effluent without treatment. These dyes contain large amount of organic matter which is highly toxic and carcinogenic, therefore very dangerously. The dye solution is stable and remains in the environment for longer periods. LASER dye solutions result in water pollution and this polluted water can penetrates through the soil and mixed with underground water, which is undesirable.

Procedure for Synthesis of Copper Doped Zinc Oxalate Precursors
The co-precipitation method was used for synthesis of Cu doped ZnO.

Characterizations of Precursor
The characterization of precursor was done by using TG-DTA and FTIR techniques.

Characterization by TG-DTA Analysis
Mass loss of substance is measured by thermogravimetric analysis as function of temperature. DTA used to study the thermal decomposition of precursor obtained in presence of air provides information about its thermal stability. The precursor CuxZn1-XC2O4 material was characterized by TG-DTA to find out the calcination temperature. Thermal decomposition of precursors was recorded on Matter TA 4000 instrument of Perkin-Elmer instrument.

Characterization by FTIR
IR spectra provides us full information about the molecular structure quickly. In this technique the majority cluster absorb characteristically inside definite vary. The shift within the position of absorption for specific teams with the modification in a molecule structure such as substitution or addition of a groups or an atoms in a molecule affects the relative mode of vibration of group resulting in to Change in band position, relative intensity & appearance of latest bands, splitting of single peaks into no. of peaks and the IR spectra of precursor were recorded within the region 4000-500 cm -1 .

Procedure for Synthesis of Copper Doped Zinc Oxide
The previously synthesized precursor was calcined at 600 o C temperature in muffle furnace for 2 hr. to give C ux

Characterization by Chemical Analysis
All synthesized CuxZn1-x C2O4 (x = 0.05, 0.1, 0.15 and 0.2) was characterized by wet chemical methods, first sample was disintegrated by acid treatment. Then from the disintegrated samples Zn and Cu are separated by using group reagents. These separated metal compound solutions are diluted to known volume. Finally from these dilute solutions Zn was quantitatively determine by EDTA titration method using Erichrome Black T as indicator and Cu was determined by Iodometric method using Starch as indicator.

Characterization by Powder XRD Studies
The characterization of synthesized CuxZn1-xO samples was also done by using XRD (Model PW-1729) with auto divergent slit using Cu Kα radiation. The XRD pattern was used to determine the Inter planar distances (d Where, D = diameter (particle size) Β = β + π/180 β + = Full width at half maximum λ = Wave length θ = Glancing angle

Characterization by FTIR
The IR spectra are shown in fig. 7 to 10 From the IR spectra the synthesis of copper doped zinc oxides was confirmed.

General Mechanism of Photocatalytic Degradation
The semiconductors (TiO2 & ZnO) are good photo-catalyst, which shows photosensitivity, stability, and band gap used for the degradation of various environmental chemical contaminants. The photo-catalyst generates electron-hole pairs produced the empty conduction band leaving positive holes in valance band, which are capable of initiating a series of chemical reactions that eventually mineralized the pollutants. Many toxic chemicals can be degraded by this process. Moreover the formation of harmless eco-friendly end products represents another attractive features of this process as shown in figure no. 2.

Characterization by TG-DTA analysis
The TGA and DTA curves in the range of 25 to 800 0 C are shown below is shown in figure no. 3. The total weight loss was 56.50% and could be two distinct processes. Weight loss of approximately 18% occurred at 140 0 C and could be attributed to the evaporation of surface water molecules on the particle surface. At 400 0 C, an additional observed weight reduction of approximately 38.50% was consistent with the decomposition oxalate molecules. No additional weight loss occurred above the decomposition temperature, indicating that the final decomposition products were copper doped zinc oxide. In this similar way the calcination temperature and weight loss were determined for the remaining compounds.

Characterization of FTIR
The IR spectra's of synthesized precursors shows prominent bands at 3600-3300 cm -1 may be due to water molecules, band at 1300-1200 cm -1 and 1700-1600 cm -1 may be due to characteristic band due to carbonyl C-O and C=O vibrational stretching frequency respectively. Also other bands at 560-450 cm -1 are due to M-O vibrational stretching frequencies.
(IR spectra are shown in Figure. No. 4 to 9 and 14 to 15)

Characterization by powder XRD studies
The XRD patterns of CuxZn1-Xc2o4 (x= 0.05, 0.      The average particle size (D) for all samples was calculated by using Scherrer equation. The results are summarized in Table: 6.

Characterization by FTIR
From the IR spectra are shown in fig.14 to 17. From the IR spectra's of synthesized copper doped zinc oxides it is seen that the band at 3600-3300cm -1 , 1750-1650cm -1 and 1300-1200cm -1 which was seen in IR spectra's of synthesized precursors are not seen. This indicates that the organic moiety is loosed completely to form M-O bond. But in all oxides spectra's band at 550-450 cm -1 was seen, which are characteristic of M-O vibrational stretching frequency, indicating that final products was copper doped zinc oxides.

Figure 14.
The M-O vibrational stretching frequency for all compounds are summarized in following table.

550-450 M-O
In comparison with pure ZnO IR spectra, the bands of copper doped zinc oxide spectra are slightly shifted this indicates that the copper was doped in ZnO.

Photocatalytic Degradation of LASER dye Using Pure and Copper Doped Zinc Oxide
The photocatalytic degradation of LASER dye using pure and copper doped zinc oxide is done in triplet. The average results are summarized in Table No. 8 and bar graph is shown in Figure No. 18.   The results are shown in Table No. 10. It is seen that the different photo-sensitizers shows different effect on photocatalytic degradation efficiency. NaCl and Na2CO3 show positive effect, while Na2S2O3 shows negative effect. As amount of photo-sensitizer increases photocatalytic property of NaCl and Na2CO3 increases and photocatalytic property of Na2S2O3 decreases.

Conclusion
The copper doped zinc oxides (Cu x Zn 1-x O; where x = 0.05, 0.1, 0.15, 0.2) was synthesized by co-precipitation method. The doping was confirmed by XRD and FTIR studies. The photocatalytic degradation of LASER dye solution using Cu x Zn 1-x O (where x = 0.05, 0.1, 0.15, 0.2) photo-catalyst showed that Cu 0.05 Zn 0.95 O shows better activity than other synthesized catalyst. The photocatalytic degradation of LASER dye solution using Cu 0.05 Zn 0.95 O photo-catalyst calcined at various temperature (400, 500 and 600 o C) showed sample calcined at 600 o C shows maximum efficiency. It is also found that the different photosensitizers shows different effect on photocatalytic degradation efficiency. NaCl and Na 2 CO 3 shows positive effect, while Na 2 S 2 O 3 shows negative effect. As amount of photosensitizer increases photocatalytic property of NaCl and Na 2 CO 3 increases and photocatalytic property of Na 2 S 2 O 3 decreases. Among the all photo-sensitizers Na 2 CO 3 is act best photo-sensitizer for both ZnO and Cu 0.05 Zn 0.95 O.