Synthesis and characterization of Manganese doped ZnO ... - ijens

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The SEM of 1% Mn-doped ZnO illustrated that morphology is well ordered, has low aggregation, and homogeneous distribution of particle size.
International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 04

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Synthesis and Characterization of Manganese Doped ZnO Nanoparticles Y. Abdollahi1, A. H. Abdullah1,2, Z. Zainal1,2, N. A. Yusof2 1

Advanced Materials and Nanotechnology Laboratory, Institute of Advanced Technology Universiti Putra Malaysia, 43400 Serdang, Selangor D.E., Malaysia. 2 Department of Chemistry, Faculty of Science, Universiti Putra Malaysia 43400 UPM Serdang, Selangor, Malaysia [email protected]

Abstract-- Various levels of manganese (Mn)-doped ZnO were synthesized by precipitation method. Characterization was carried out by XRD, TEM, SEM, EDX, BET and the band gap measured by UV-visible reflectance. In the XRD pattern of samples, there is no signature of impurity peaks, which could indicate Mn-related secondary phases. The EDX show the amount of Mn doped on ZnO is slightly lower than the theoretical value. The SEM of 1% Mn-doped ZnO illustrated that morphology is well ordered, has low aggregation, and homogeneous distribution of particle size. High aggregation is observed, however, in other percentages of Mn-doped ZnO. Results of TEM show that more than 50% of the particles for undoped and Mn-doped ZnO use between 15 and 35 nm, with 1% Mn doped ZnO having the highest percentage (77%). The BET shows that the surface area of synthesized catalyst increases when the weight ratio of manganese increases up to 1% Mn, but decreases thereafter. The band gap of 1% Mn-doped ZnO is 2.2 eV which is smaller than the undoped ZnO band gap. The results of characterization show 1% Mn-doped ZnO has the highest surface area, the lowest particles size and the lowest agglomerate. Moreover the calculated band gap of 1% Mn-doped ZnO is lower than others except 0.5%Mn. Additionally, photodegradation of cresols under visible light showed that 1% Mn-doped ZnO had maximum adsorption and rate of photodegradation. In conclusion 1% Mn doped ZnO is suitable as the best photocatalyst to degrade cresols under visible light irradiation.

Index Term-- ZnO; Co-precipitation; Manganese-doping; Optical properties; Nanoparticles

1. INTRODUCTION ZnO is a II-VI semiconductor with wide direct-gap (3.37 eV) and exciton binding energy (60 meV) at room temperature [1]. It is an inexpensive and environmentally safe host material. Due to its properties, the interest in ZnO as a photocatalyst has increased, however it has been mainly used under ultra violet (UV) irradiation [2-6]. Photocatalytic reaction is initiated when a photoexcited electron is promoted from the filled valence band (VB) of a semiconductor photocatalyst (SC) to the empty conduction band (CB) as the absorbed photon energy, hν, equals or exceeds the band gap of the photocatalyst. The hole pair (e--h+) is generated at the surface of the photoexcited photocatalyst as shown below [7]. Photoexcitation : SC+hν→ e− +h+ (1) Adsorbed oxygen: (O2)ads +e→− O2−• (2) Ionization of water : H2O → OH− +H+ (3) Protonation of superoxides : O2−• +H+→ HOO• (4)

HOO• + e− → HO2− (5) HOO− +H+→ H2O2 (6) H2O2+e-→ OH− +OH• (7) H2O+h+→H++OH• (8) Since 46% of solar energy consists of visible light, it is more economical to use visible light than UV light in large scale operations. To use visible light, ZnO however has several drawbacks including the low surface area or big particles size and an unsuitable band gap energy (Eg=3.7 eV). It was observed that doping with 3d metals reduced the E g of semiconductors by forming interband-gap localized levels [8]. The charge-transfer transition between the d-electron of dopant (t2g levels) and the CB or VB was reported [9]. On the other hand, the metal d-orbital’s are mixed with the CB and VB of semiconductors [10]. This overlap is because wide VB or CB directly decreases Eg. Recently, there has been much attention focused on modifying ZnO by doping with transition metals such as Ag [11], Ni [12], Cu [13], Co [14], Cr [15], Ti [16]. These studies demonstrated that the metals can change Eg of ZnO, and that the dopants can control ZnO grain size. It demonstrated the presence of the d-electron because the t2g of manganese (Mn) is very close to the VB [10, 17]. More recently, synthesis of a Mn-doped ZnO nano-crystal (not powder) was reported by wet-chemical techniques [18]. Moreover, ZnO nanocrystalline powders doped with 1, 3 and 5% Mn were prepared using a sol-gel process [19]. Unfortunately, up to now the value of the Eg and optimum amount of Mn as dopant is not clear. Additionally, reported results of Mn-doped ZnO being applied as a photocatalyst are quite different. In this work, Mn-doped ZnO nano-powder was prepared with up to 2% Mn by the co-precipitate method. The samples were characterized to obtain particle size, surface area and the Eg. The synthesised Mn-doped ZnO was applied as photocatalyst to photodegradation of ortho, meta and paracresol (o, m, p-cresol) under visible light irradiation. 2. EXPERIMENTAL 2.1. Materials Zinc acetate (99%, Merck), Manganese acetate (98%, Sigma, Alorich), NaOH (99% Merck), H2SO4 (95%-97%), Ethanol (absolute), o-cresol (99%, Merck), m-cresol (99%, Merck), pcresol (99.5%, Fluka), commercial ZnO (C-ZnO, 99%, Merck), and other required chemicals are of reagent grade, obtained from Merck and were used without further purification.

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International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 04

2.2. Preparation For synthesizing the catalyst, 75mM of ethanolic Zinc solution was mixed with different concentrations of an ethanolic manganese solution. The mixed solutions were heated at 75 ◦C for 45 min to reduce the amount of the solution and increase the concentration of the metal ions, after which it was cooled at room temperature. NaOH solution was then added to the mixed solution with constant stirring (150 rpm) until pH 8.3 was reached. The new colorless solution was kept in a water bath at 67 ◦C for two hours. It was observed that the solution started precipitating after one hour in water bath. After cooling the solution to room temperature for four hours, the colloidal solution was centrifuged for 20 minutes at 4000 rpm. The resulting precipitate was washed with ethanol, sonicated (30 min), and centrifuged (4000 rpm). The above procedure was repeated five times to remove unreacted ions. The separated precipitate was dried overnight at 110◦C. The catalysis was ground and then calcined under compressed air at 650ºC for 3.5h in the tubular furnace. Undoped ZnO (UZnO) was synthesized with a similar procedure except for the addition of manganese acetate 2.3. Characterization X-ray Diffraction (XRD) analysis of the photocatalysts for structural characterization was carried out using a Shimadzu XRD-6000 Diffractometer. Chemical composition and morphology of the samples was carried out using a scanning electron microscope (SEM), model Hitachi S-3400N and Energy Dispersive X-ray (EDX) thermo electron corporation. The nanostructure and particle size of the nanocrystals in these samples was determined from Transmission Electron Microscopy (TEM) [20] images obtained using a Hitachi H7100 electron microscope[21]. The Brunauer-Emmett-Teller (BET) method, using nitrogen adsorption at liquid nitrogen temperature 77 K, was employed to measure total surface area of the photocatalysts. This was done using a Thermo Finnigan Sorptomatic Instrument model 1900. The optical absorption spectra were obtained using a Perkin Elmer Lambda 35 UVVis-NIR diffuse reflectance spectrometer. Various concentrations of cresols solutions were prepared using deionized water. Photodegradation of cresols was performed in a batch photoreactor, designed in a column 14.5cm height with an 11.5-cm diameter (Fig. not showed). To volatilize produced gas (may be CO2), increase solution fluidization and access oxygen for eq. (2), air was blown into the reaction solution by an air pump at a flow rate of 10m3/h. the cooled air into the solution eliminated the lamp’s heat effect and kept the temperature at around 25 ºC. Magnetic stirring at a speed of 200 rpm was applied to make the suspension solution, during the reaction. The photoreactor is located in an aluminum-sealed tube to enhance the radiation by reflection. A Philips lamp (23watt) was used as light source. Throughout the experiment, the appropriate concentration of cresols solution was contacted with the correct amount of photocatalyst in the photoreactor. At specific time intervals, samples were withdrawn from the bulk

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solution, and filtered through 0.2μm PTFE filters. The concentration of cresols was measured using a UV-visible spectrophotometer (shimadzu, uv-1650pc), 3. RESULTS AND DISCUSSION 3.1. Phase and Element analysis The XRD patterns of U-ZnO and Mn-doped ZnO with different Mn concentration are presented in Fig. 1. The XRD pattern shows only the peaks correspond to wurtzite crystal structure of ZnO (reference code 01-089-1397). In the XRD pattern of samples, there is no signature of impurity peaks which might belong to Mn-related secondary phases. However, EDX analysis confirmed the presence of Mn in the samples (Table 1). One reason is that the majority of Mn atoms are located at substitutional sites for lower doping concentrations [22]. As observed in Fig. 1, the (101) peak (crystal plate) of 1% Mn is broader than other peaks, because 1% Mn photocatalyst has a different crystalline size. Using the Debye-Scherrer equation [23] and the half-width of the XRD lines, the average values of crystalline size of the samples were calculated based on the 101 crystal plate and are shown in Table 1. The crystalline size of photocatalyst decreased with increasing percentage of Mn over 1%. Beyond this, the crystal size increased again. It may be the doping of Mn in ZnO can control crystal size, as observed for other transition metals such as Co [14], Cr [15].

Fig. 1. X-ray diffraction patterns of the undoped powders (a) undoped ZnO, and (b) 0.5%, (c) 1 %, (d) 1.5%, (e) 2 % Mn-doped ZnO after heat treatment.

Table I XRD data, Mn determined by EDX and particles size of U-ZnO and Mn doped ZnO

%Mn (Theoretical)

%Mn (EDX)

Crystallite size (nm)

0

0

51

0.5

0.4

42

1

0.8

13

1.5

1.4

15

2

1.9

30

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International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 04 3.2. Morphology studies The influence of different percentages of Mn on the surface morphologies of photocatalysts (0-2% Mn-doped ZnO) was studied by SEM (Fig. not show). The SEM image of 1% Mndoped ZnO illustrates the morphology is well ordered; a lower aggregation and better particle size distribution (Fig. 2) than other percentages of Mn-doped ZnO. TEM were conducted to investigate the morphology and particles size of the samples. The TEM morphology of 1% Mn-doped ZnO confirmed the low aggregation and better distribution in SEM images (Fig. 3). In addition, the TEM images showed spherical nanocrystallites for 1%Mn-doped ZnO (Expanded part in Fig. 3). The measured particles size of samples showed (Table 2) more than 50% of frequency was between 15 and 35 nm while the highest percentage (77%) belonged to 1% Mn doped ZnO. This indicates that the addition of Mn may be control the growth of ZnO grains.

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control of growth of ZnO grains by Mn. However, the surface area was shown to decrease above 1% Mn, which confirms the TEM and SEM observations. Agglomeration is a limiting factor for surface area. SEM and TEM images showed that with above 1% Mn, the agglomeration increases, and this may be due to a decrease in surface area.

Fig. 3. TEM images of the 1% Mn doped ZnO nanoparticles

Fig. 2. SEM micrograph for 1% doped ZnO

Table II the particles size distribution of the U-ZnO and Mn-doped ZnO

Particle size

Frequency (Mn-doped ZnO)

(nm)

ZnO

0.5%

1.0%

1.5%

2.0%

15-20

5

8

6

5

3

20-25

9

11

17

10

9

25-30

14

16

29

22

16

30-35

23

24

25

19

27

35-40

26

29

13

25

18

40-45

15

7

7

11

16

45--50

8

3

3

8

11

The surface area (BET) of U-ZnO and Mn doped ZnO are presented in Fig. 4. As TEM images show, the photocatalyst particle size decreases with increasing % of Mn up to 1% Mn. Results of BET showed the surface area increased when the weight ratio of Mn increases up to 1% Mn, confirming the

Fig. 4. Effect of doping level on the specific surface area (BET) of the undoped ZnO and various percent of Mn doped ZnO

3.3. Band gap measurement The diffuse reflectance (R%) spectra of the samples (0-2% Mn-doped ZnO catalysts) after Kubelka-Munk treatment [(Fhν)1/2] vs. һν (eV) are shown by Tauc’s plots [24] in Fig. 5. The intersection between the linear fit and the photon energy axis gives the value to Eg. The Eg of undoped ZnO of 2.75 eV was a little less than C-ZnO’s Eg. It may be due to a defect in the synthesised photocatalyst [25]. The Eg of 0.5% Mn-doped ZnO is 1.5 eV. It may be due to s-d and p-d exchange interaction [26-27]. This electron-hole may improve photodegradation but most of them are trapped by sub–surface recombination [28]. The Eg of 1% Mn-doped ZnO was 2.2

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eV. The d-electron of Mn (t2g level) can easily overlap with the ZnO’s valence band (VB) because t2g of Mn is very close to VB of ZnO [17]. This overlap causes a wide VB and consequently decreases the Eg of ZnO photocatalyst so that the VB electrons can be excited by visible light irradiation. The E g of 1.5% and 2% Mn-doped ZnO was 2.6 eV, which is higher than the Eg of 1% Mn-doped ZnO. Similar behavior was observed for vanadium doped ZnO, with E g higher with an increased vanadium concentration [29]. This might be due to the influence of numerous factors such as structural parameters, carrier concentrations and the presence of defects such as oxygen vacancies, which may lead to the Burstein Moss shift [30].

Fig. 5. Kubelka-Munk transformed reflectance spectra of undoped and Mndoped ZnO

3.4. Photodegradation studies In order to determine the optimum amount of Mn doped for a photocatalyst, photodegradation of cresols was studied under visible-light irradiation by C-ZnO and synthesised photocatalyst (0-2% Mn doped ZnO). Whereas the activity of photocatalyst seriously depends on the adsorption of an amount of removable pollutants over photocatalysis [31-32], the amount of adsorbed cresols on the C-ZnO, U-ZnO and Mn-doped ZnO is tabulated in Table 3. Results show the adsorption on the C-ZnO ≤ U-ZnO ≤ 0.5% Mn ≤ 1% Mn doped ZnO. However the adsorption is decreased above 1% Mn. As observed, the adsorption of cresols on 1% Mn-doped ZnO is the highest. It may be due to the 1% Mn doped ZnO having the highest BET surface area as previously discussed.

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Table III Amount of adsorbed cresols on catalyst during reaction time; initial condition: cresols concentration = 35 ppm, photocatalyst = 1.5g/L and natural pH

Photocatalyst (1.5g/L)

o-cresol (mg/L)

m-cresol (mg/L)

p-cresol (mg/L)

C-ZnO

1.8

1.2

1.8

U-ZnO

2.8

2.3

2.9

0.5% Mn-

3.6

2.8

3.7

1.0% Mn-

4.3

3.3

4.6

1.5% Mn-

3.4

2.6

3.4

2.0% Mn-

2.9

2.4

3.0

Photodegradation of cresols by using C-ZnO, U-ZnO and 1%Mn-doped ZnO were plotted against irradiation time (Fig. 6-8). As Table IV shows, the photoreaction rates and the photodegradation efficiency were 1%Mn-doped > U-ZnO > C-ZnO. This can be attributed to the characteristics of the photocatalyst, with a decrease in particle size, agglomeration size and increase in surface area of 1% Mn. Another reason could be related to overlap of the t2g of Mn with the VB of ZnO [17]. On the other hand %photodegradation by the synthesised ZnO is a little better than C-ZnO. Average particles size of synthesised ZnO is 30–40 nm, while C-ZnO particles size is 400-500 nm and surface area of undoped ZnO (18.7 m2/g) is greater than the surface area of C-ZnO (3.3 m2/g). Improvement of photodegradation efficiency and rate of reaction may be due to this surface area difference.

Fig. 7. Photodegradation of m-cresol by C-ZnO, U-ZnO and 1%Mn- doped ZnO under visible-light irradiation. Initial conditions: m-cresol= 35ppm; photocatalyst = 1.5g/L and pH =7.63

Fig. 8. Photodegradation of p-cresol by C-ZnO, U-ZnO and 1%Mn- doped ZnO under visible-light irradiation. Initial conditions: p-cresol= 35ppm; photocatalyst = 1.5g/L and pH =7.49 Table IV the rate of photodegradation, photodegradation% and R2 of cresols photodegradation by C-ZnO, U-ZnO and 1%Mn-doped ZnO under visible irradiation

Cresol

Photocatalyst

-r (10-3)

R2

Remove%

commercial

2

0.99

67

undoped

2.3

0.99

79

1%Mn-doped

2.5

0.98

88

commercial

1.7

0.95

58

undoped

2.1

0.99

72

1%Mn-doped

2.2

0.98

78

commercial

2.1

0.99

72

undoped

2.4

0.97

81

1%Mn-doped

2.5

0.98

89

ortho

Fig. 6. Photodegradation of o-cresol by C-ZnO, U-ZnO and 1%Mn- doped ZnO under visible-light irradiation. Initial conditions: o-cresol= 35ppm; photocatalyst = 1.5g/L and pH =7.37

meta

para

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International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 04 4. CONCLUSION Various levels of Mn-doped ZnO were synthesized by precipitation method. Characterization was carried out by XRD, TEM, SEM, EDX, BET, and the band gap measured by UV-visible reflectance. In order to evaluate the synthesised photocatalyst, cresol photodegradation was studied under visible-light irradiation with C-ZnO and synthesised photocatalyst. In the XRD pattern of samples, there is no signature of impurity peaks, which might belong to Mnrelated secondary phases, while the EDX confirmed the amount of Mn doped on ZnO which was very close to the theoretical value. The SEM of 1% Mn-doped ZnO illustrated the morphology is well ordered, has low aggregation, and has a homogeneous distribution of particle size. Results of TEM showed that more than 77% of the 1% Mn doped ZnO particles were between 15-35 nm. The BET show that the surface area of the 1% Mn doped ZnO was the highest. The band gap of 1% Mn-doped ZnO is 2.2 eV which is smaller than U-ZnO band gap. The characterization of photocatalyst indicated that 1% Mn-doped ZnO in comparison with U-ZnO, 0.5%, 1.5% and 2% Mn-doped ZnO has a higher surface area, lower particle size and lower agglomerate. Moreover, the calculated band gap of 1% Mn-doped ZnO is lower than the others except 0.5% Mn. Additionally, photodegradation of cresols under visible light showed that 1% Mn doped ZnO had maximum adsorption and rate of photodegradation. In conclusion, 1% Mn-doped ZnO is suitable as the best photocatalyst for removing cresols.

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[30] Chakraborty, P.; Datta, G.; Ghatak, K., A Simple Theoretical Analysis of the Effective Electron Mass in Heavily Doped III–V Semiconductors in the Presence of Band-Tails. Physica Scripta 2003, 68, 368-77. [31] Abou-Helal, M. O.; Seeber, W. T., Preparation of TiO2 thin films by spray pyrolysis to be used as a photocatalyst. Applied Surface Science 2002, 195 (1-4), 53-62. [32] Anandan; Vinu, A.; Sheeja Lovely, K.; Gokulakrishnan, N.; Srinivasu, P.; Mori, T.; Murugesan, V.; Sivamurugan, V.; Ariga, K., Photocatalytic activity of La-doped ZnO for the degradation of monocrotophos in aqueous suspension. Journal of Molecular Catalysis A: Chemical 2007, 266 (1-2), 149-57.

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