Photoluminescence and enhanced photocatalytic activity of ZnO nanoparticles through incorporation of metal dopants Al and Ca

Journal Pre-proof Photoluminescence and enhanced photocatalytic activity of ZnO nanoparticles through incorporation of metal dopants Al and Ca P. Visali, R. Bhuvaneswari

PII:

S0030-4026(19)31604-3

DOI:

https://doi.org/10.1016/j.ijleo.2019.163706

Reference:

IJLEO 163706

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Optik

Received Date:

22 July 2019

Revised Date:

28 October 2019

Accepted Date:

5 November 2019

Please cite this article as: Visali P, Bhuvaneswari R, Photoluminescence and enhanced photocatalytic activity of ZnO nanoparticles through incorporation of metal dopants Al and Ca, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163706

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Photoluminescence and enhanced photocatalytic activity of ZnO nanoparticles through incorporation of metal dopants Al and Ca P.Visalia, R.Bhuvaneswarib a.

Department of Physics, Kongu Arts and Science College, Erode-638107, India

b.

Department of Physics, Vellalar College for Women, Erode-638012, India

ABSTRACT

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In the present work influence of aluminium and calcium in the structural, optical properties and the photocatalytic performance of zinc oxide (ZnO) nanoparticles are studied. Pure, 2 wt% aluminium doped, 2 wt% calcium doped and 2 wt% aluminium 2 wt% calcium codoped ZnO nanoparticles are synthesized using sol-gel route. The structural, morphological

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and optical properties are investigated using XRD, SEM, EDAX, UV-Visible spectroscopy and photoluminescence spectroscopy. The synthesized nanoparticles are spherical in shape and are crystallized well with wurtize phase. Discrete narrow emission spectra are obtained in

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photoluminescence spectroscopy. The prepared samples were subjected to photocatalytic degradation with methylene blue dye and a substantial photocatalytic performance is

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observed with calcium doped ZnO nanoparticles.

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1. Introduction

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Keywords: ZnO, nanoparticles, codoped, photoluminescence, photocatalytic activity

Metal oxide semiconductors are widely studied as photocatalyst for the deduction of

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organic dyes due to its hazardous impact on environment and society. Recently semiconductors like TiO2 [1-2], SnO2 [3], Cu2O [4] and ZnO [5-7] etc., are reported to be the competent photocatalytic materials. Zinc oxide (ZnO) is a n-type semiconductor being costeffective, non-toxic and a promising material for removing organic pollutants with appreciable photocatalytic activity[5-7]. ZnO is a direct band gap material with a wide bandgap energy of 3.37eV, having a large exciton binding energy of 60 meV [5]. The problem associated with ZnO is the recombination of electron hole pair in a short interval of time. In order to overcome the problem of recombination of electron hole pair ZnO is doped

with suitable materials. Metals such as Al [8-12], Ca [13-14], Ag [15-17], Mn [18], Mg [19], Sr [20], Fe [21], V [22] and Ce [23] are in current use as dopants for improving the photocatalytic activity of ZnO. Literature survey reveals that Al doping enrich the photocatalytic performance of ZnO by introducing defects, facilitating charge carriers, reducing grain size and modifying the energy structure of ZnO [8-12]. Limited works have been reported on calcium doped ZnO nanoparticles possessing high adsorption capacity [13-14]. Duan et al [24]., attained a maximum degradation efficiency of 89% with Al/Mg codoped ZnO. Codoped ZnO nanoparticles are less studied for photocataytic application [24-28]. Elhalil et al[29].,

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observed significant degradation efficiency with Ca doped ZnO-Al2O3. Being economical and eco friendly, aluminium and calcium are chosen as dopants in our present work. It has been inferred from the literature survey that no studies have been reported so far with Al and Ca codoped ZnO nanoparticles. The present work intends to compare the impact of structural

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modification by Al doping, adsorption capacity by Ca doping and the combined effect by

2.Experimental Technique

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2.1 Materials used

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Al/Ca codoping on the photocatalytic performance of ZnO nanoparticles.

Zinc acetate dihydrate (ZnC4H6O4.2H2O), 0.1N sodium hydroxide (NaOH) solution, alumnium nitrate nonahydrate (Al(NO3)3.9H2O), calcium chloride (CaCl2) of analytical grade

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are used.

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2.2 Experimental Procedure

Simple sol – gel route is adopted in which 0.15M zinc acetate is dissolved in de

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ionised water under constant stirring and 0.1N NaOH solution is slowly added to the above solution to maintain a pH at 14. The above solution is stirred for 2 hours at 50ºC. A milky white solution is obtained which is then washed with de ionized water to remove the impurities and then heated on hot plate at 100 ºC for 1 hour to get dried powder and then annealed at 400 ºC for 3 hours to get pure ZnO nanoparticles (NPs). For preparing doped samples the same method is followed by the replacement of 2 wt% of zinc acetate with aluminium nitrate and calcium chloride to obtain Al and Ca doped ZnO NPs. Further to

obtain the Al/Ca codoped ZnO NPs 4 wt% of zinc acetate is replaced with 2wt% aluminium nitrate and 2wt% calcium chloride.

2.3 Photocatalytic measurement The prepared sample is mixed with methylene blue dye solution by stirring at 50 rpm followed by irradiation with UV light using 500 W halogen lamp for 120 minutes keeping the solution 20 cm away from the lamp. The absorption intensity of methylene blue dye is measured in each 15 minutes interval of time using Shimadzu UV – 1800 spectrophotometre.

3 Results and Discussion

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3.1 Structural and morphological properties Fig.1, illustrates the XRD pattern of synthesized pure, 2% Al doped, 2% Ca doped, 2% Al 2% Ca co-doped ZnO nanoparticles. All the samples are crystallized well and the observed peaks are in accordance with the JCPDS file no.89-0510 corresponding to wurtize phase of ZnO. The shifting of peaks to higher angle (Fig.2) with doping indicates the strain

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produced in the lattice by dopants. The broadening of peaks is attributed to the decrease in particle size. No changes in wurtize phase are observed with doping as there are no additional

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peaks. Table 1, lists the crystallite size, lattice parameters, volume of the unit cell and microstrain of the samples. The crystallite size calculated from Debye Scherrer formula [30]

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decreases with doping in the following order ZnO > Ca doped ZnO > Al doped ZnO > Al/Ca codoped ZnO NPs. The calculated lattice parameters are in accordance with the wurtize structure of standard ZnO. For doped materials the lattice parameters a and c are reduced,

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resulting in the reduction of the unit cell volume leaving a constant c/a ratio which indicates the substitution of the dopants [31] in the lattice without affecting crystal structure. Sundaram et al[32]., reported Mn substituted ZnO nanoparticles with constant c/a ratio. Micro strain is

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calculated using the relation ξ=βcos(θ/4)[33]. Microstrain increases with doping resulting in diminished particle size[33].

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The SEM images of the samples are shown in Fig.3 The particles are spherically

shaped and are agglomerated with uniform size distribution. No influence of dopants on morphology is observed. The EDAX images of these samples are shown in Fig.4 confirming the formation of ZnO and the presence of the dopants Al, Ca in the samples.

3.2 Optical Properties

The UV-visible absorption spectra of the as prepared samples are dipicted in Fig.6 with narrow absorption in the UV region compatible with excitonic absorption observed by Soumen Dhara et al[34]., on mechanochemically synthesized ZnO nanorods of particle size 15nm – 40nm range. The absorption wavelength and bandgap values of the materials are tabulated in Table 2. The bandgap is increased in the order of decreasing crystallite size. Blue shift with excitonic absorption is attributed to weak quantum confinement effect[35] due to smaller crystallite size of the samples. Duan et al[24]., reported blue shift and hence increasing bandgap energy with Al/Mg codoped ZnO NPs. The room temperature PL spectra of the samples are shown in Fig.6. High intensity

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emission in the UV region represents the high crystalline quality of the samples in consistent with the XRD results. In Al doped and Al/Ca codoped ZnO NPs the emission become more discrete and narrow as reported by Bekkari et al[36]., demonstrating the monodispersed and narrow size distribution of the system. Prominent peaks observed at 376.5nm and 389nm for

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pure ZnO, 373nm and 387nm for Ca doped ZnO, 370nm and 386nm for Al doped ZnO, 369.5nm and 384nm for Al/Ca codoped ZnO correspond to excitonic emission [34,37]. Khan

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et al[37]., reported excitonic emissions with Al doped ZnO NPs at room temperature as a result of large exciton binding energy of ZnO. A weak emission at 417nm in pure ZnO and weak

emission at 400nm in all the samples except Ca doped ZnO NPs are due to Zn defects[34,38].

high crystalline quality.

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Defect level emissions in the visible region are absent in calcium doped ZnO NPs exhibiting

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3.3 Photocatalytic Activity

The photocatalytic degradation of the samples are depicted in Fig.8. All the samples

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exhibited appreciable photocatalytic performance due to their high purity and crystallinity avoiding the electron hole recombination at defect levels [29]. Photocatalytic degradation

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kinetics follows first order equation

𝑙𝑛

𝐶0 𝐶

= 𝐾𝑡 [39], where, 𝐶0

the initial concentration

and 𝐶 the final concentration of the dye solution in a specific radiation time and K is rate constant (min-1). Graph between ln C0/C and irradiation time is given in Fig.7 showing linear variation of ln(C0/C) with irradiation time t. The degradation rate is maximum for Ca doped ZnO. The calculated Photocatalytic Degradation Percentage[39] against the samples is shown in Fig.9. Ca doped ZnO exhibits highest PDP of 89% comparable with the results of Duan et al[24]., Even though Al doped ZnO and Al/Ca codoped ZnO have similar optical properties

Al/Ca codoped ZnO NPs showed better degradation than pure and Al doped ZnO but not good as Ca doped ZnO. Resembling A. Elhalil et al.,[29] results with Ca doped ZnO-Al2O3 nanomaterial, high crystalline quality and good adsorptive power of calcium improve the photocatalytic efficiency implying calcium as an effective doping material to boost the photocatalytic performance of ZnO NPs.

4. CONCLUSION The observed results show that the sol-gel synthesized undoped, Al doped, Ca doped and Al/Ca codoped ZnO NPs exhibit spherical shape and wurtize phase with crystallite size of 27.23nm, 26.09nm, 25.23nm and 24.5nm respectively. Blue shift is observed with doping

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and the bandgap increases with decreasing crystallite size. Exciton emissions corresponding to weak confinement effect are observed in PL studies of all the prepared samples. In Al doped and Al/Ca codoped ZnO NPs narrow discrete PL emission is observed making the materials suitable for optical devices. All the samples showed appreciable photocatalytic

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degradation. Amongst all, calcium doped ZnO nanoparticle showed highest degradation efficiency of 89% confirming calcium as a good dopant for ZnO for photocatalytic

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Declaration of interests

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application.

The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

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1802. DOI:10.1007/s11426-012-4721-8

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6000 4000

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2000 0 6000 4000 2000

Zn98Ca2O

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Intensity a.u

Zn96Al2Ca2O

0 6000

Zn98Al2O

4000

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2000

0

(111)

(111)

(111)

(103)

2000

(110)

(102)

(100) (002)

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4000

(101)

ZnO

6000

0 20

30

40

50

60

70

80

2in degrees

Fig.1 The XRD pattern of undoped, 2% Al doped, 2% Ca doped, 2% Al and2% Ca codoped ZnO nanoparticles

6000

Al/Ca codoped

4000 2000 0 6000

Al doped

4000

Intensity (a.u)

2000 0 6000

Ca doped

4000 2000 0

ZnO

4000 2000 0 30

32

34

36

2in degree

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6000

38

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Fig.2 The XRD pattern of ZnO nanoparticles showing shift in diffraction peaks with doping

Fig.3 SEM images of a) Ca doped ZnO NPs b) Al doped ZnO NPs c) Al/Ca codoped ZnO NPs

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Fig.4 EDAX spectrum of a) Al doped ZnO NPs, b) Ca doped ZnO NPs and c) Al/Ca

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codoped ZnO NPs

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ZnO Zn98Al2O

1.6

Zn98Ca2O Zn96Al2Ca2O

1.4

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absorbance (a.u)

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1.2 1.0 0.8 0.6 0.4

300

400

500

600

700

Wavelength (nm)

Fig.5 UV-Vis spectra of the synthesized samples

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Fig.6 PL spectra of the prepared samples

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2.5

2.0

ZnO Al doped ZnO Ca doped ZnO Al, Ca codoped ZnO

ln(C0/C)

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1.5

1.0

0.5

0.0 0

20

40

60

80

100

120

Irradiation time in minute

Fig.7 Graph showing the photocatalytic degradation rate

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Fig.8 Photocatalytic degradation of methylene blue by a) pure ZnO NPs, b) Al doped ZnO

60

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PDP% At 120 min

80

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NPs, c) Ca doped ZnO NPs and d) Al/Ca codoped ZnO NPs

40

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20

0

ZnO

ZnAlO

ZnCaO

ZnAlCaO

SAMPLE

Fig.9 Photocatalytic Degradation Percentage of various samples in 120 minutes

Table 1: Crystallite size, lattice parameters and volume of the unit cell of the synthesized pure and doped ZnO nanoparticles. Lattice Crystallite

Strain

size D (nm)

ξ10-3m

Volume of

parameter

Index

c/a ratio

a(Ẳ)

c(Å)

the unit cell (Å)3

27.23

1.93

3.247

5.222

1.608

47.68

Zn98Ca2O

26.09

2.01

3.230

5.176

1.603

46.77

Zn98Al2O

25.23

2.08

3.240

5.192

1.603

47.20

Zn96Al2Ca2O

24.50

2.11

3.234

5.187

1.604

46.98

Standard ZnO

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-

3.248

5.205

1.603

47.58

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ZnO

S.NO Index

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Table 2: Bandgap energy of the synthesized samples Wavelength

Bandgap (eV)

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(nm) 384

3

Zn98Al2O

379

3.28

4

Zn96Al2Ca2O

378

3.29

1

3.23

Zn98Ca2O

380

3.27

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2

ZnO