Photocatalytic Removal of Carcinogenic Reactive Red S3B Dye by using

ZnO and Cu doped ZnO Nanoparticles Synthesized by Polyol Method: A

Kinetic Study.Humaira Rashid Khan, a Ghulam Murtaza, b Muhammad Aziz Choudhary, a Zahoor Ahmed a

Mohammad Azad Malik c*

a Department of Chemistry, Mirpur University of science and Technology (MUST), Mirpur, AJ&K-

10250, Pakistanb School of Chemistry, The University of Manchester, Manchester, Oxford Road, M13 9PL, UKc School of Materials, The University of Manchester, Manchester, Oxford Road, M13 9PL, UK

Corresponding author Email: azad.malik@manchester.ac.uk

Abstract

Zinc oxide (ZnO) nanoparticles were synthesised from zinc acetate as a precursor and PVP as

a capping agent by polyol method. The as prepared ZnO nanoparticles were doped with

0.01M solution of copper acetate in PVP. Both ZnO and Cu-doped ZnO nanoparticles were

characterized by UV-Vis spectroscopy, X-rays diffraction (XRD), scanning electron

microscopy (SEM), photoluminescence (PL) and energy dispersion X-rays (EDX)

spectroscopy.

The sunlight irradiated photocatalytic degradation of Reactive Red S3B was carried out using

both ZnO and Cu doped ZnO nanoparticles. Both ZnO and Cu-doped ZnO decolourize the

respective dyes but Cu-doped ZnO nanoparticles were clearly more powerful catalyst than

ZnO. The kinetics of the photocatalytic degradation was also studied. It was found that the

degradation of Reactive Red S3B followed the pseudo first-order kinetics. These

photocatalysts are efficient and environment-friendly and can be highly useful for the

wastewater treatment contaminated with these carcinogenic synthetic dyes.

Key Words:

ZnO, Cu-Doped ZnO, Polyol method, Photo-catalysis, Reactive red S3B,

Introduction

Materials industry is one of the most essential industries that consumes large amount of water

and creates a lot of wastes containing numerous coloured substances.1 Dyeing and finishing

processes are the main source of pollution in wastewater.2 Synthetic dyes are extensively

utilized as a part of these procedures; in this way, their release into normal water causes

serious environmental issues.3

A large number of these dyes are carcinogenic, unsafe to human health and because of their

complex structures and synthetic nature, they are exceptionally stable and it is hard to

decolourize them by biological treatment approaches.4 At the point when the biological

treatment strategies can't treat polluted and harmful water, advanced oxidation processes

(AOPs) are one of the promising techniques to treat the polluted water. The principal

advantages of this process are its cost-effectiveness, the non-selective oxidation of substrates,

the possibility of using solar light and the fact that the catalyst is easy to obtain. Moreover,

the reactions can be carried out at room temperature and at atmospheric pressure under sun

light. Photodegradation of wastewater pollutants using solar light can make it an

economically feasible process in particular for large-scale aqueous-phase applications.5 In

AOP, the hydroxyl radicals (.OH) which are exceptionally stable and non-particular chemical

oxidants are produced in solution, and these are used for the oxidation of the natural effluents

to water and carbon dioxide.6

Various wide band gap semiconductors, for example, ZnS 7, SnO2 8, ZnO 9 and TiO2 10-11 have

been used as photocatalysts for this purpose. Usually, the mostly used catalyst has been TiO2

due to its activity below 390 nm, however in the last few years ZnO has attracted more

attention because of its high photocatalytic activity, cheap, and easily synthesised.11,12 ZnO

nanoparticles have been synthesized by hydrothermal13, co-precipitation14-15, sol-gel16,

evaporative decomposition of solution and gas-phase synthesis with various reagents.17 There

are only few reports in which poly (vinyl pyrrolidone) (PVP) was utilized as a capping agent

to prepare ZnO nanoparticles in various techniques.18-19 The polyol method is an

extraordinary type of solvothermal synthesis of ZnO which utilizes different diols as a

reaction medium. In addition to reaction medium it works as a stabilizing agent and confines

the particle growth. 20 The polyol method has the advantage of producing nanoparticles with

an organophilic surface layer that can be used as nanocomposites without any extra surface

modification. Polyvinylpyrrolidone (PVP), is an amorphous thermoplastic material with ideal

mechanical and handling properties, it replaces inorganic glass in numerous applications.21

ZnO nanoparticles in pure and doped forms have been utilized for removal of dyes such as

methyl blue (MB) 22, methyl orange (MO) 23 and Rhodamine 6G by photo-catalytic method. 24-27

The semiconductor ZnO is vastly used in photocatalysis in the disinfection and

detoxification of the environment due to its special physicochemical properties. However, its

large exciton binding energy (60 meV) and the wide bandgap (3.37 eV) restricts the response

to visible portion of solar radiation highlighting an important challenge. Another challenge to

be dealt with is the fast recombination of photo-generated electron–hole pairs. Therefore,

many reports were presented to increase the sensitivity of ZnO towards visible portion of

solar spectrum and decrease the rate of electron/hole recombination. ZnO doped with

transition metals improve the absorption of visible radiation from solar light. Also the doping

improves the separation of hole-electron pair resulting in higher photodegradation activity.28

The use of ZnO has some evident disadvantages including high recombination of

photo-generated electron-hole, the constricted light responsive range, and photo corrosion.

These restrictions have been overcome by different methods, for example, doping of ZnO and

coupling of semiconductor oxides.29-31 ZnO nanoparticles doped with Cu were found to be

more photochemically active due to their higher substrate and reagent binding ability. These

doped nanoparticles found their applications in reducing a large number of cancer-causing

dyes32 especially in print industrial effluents which are too carcinogenic, and toxic for aquatic

life.

Herein we report the synthesis of ZnO and Cu-doped ZnO nanoparticles by polyol method,

their full characterization and their use as photo-catalyst for the removal of carcinogenic dye

Reactive Red S3B (Figure 1).

Experimental

Materials

Copper nitrate trihydrate Cu(NO3)2.3H2O,Mw = 241.6 (Merck), zinc acetate dihydrate

Zn(CH3COO)2.2H2O, Mw = 219.5 (Merck), Poly (vinylpyrrolidone) PVP (Daejung Reagent

Chemicals-Korea), Ethylene glycol (EG) 99% (Daejung), and reactive red S3B used for

photocatalytic degradation was taken from native fabric dye industries and was used without

additional purification.

Apparatus

SHIMADZU UV 1800 Spectrophotometer was used to measure the UV-Vis absorption

spectra. The structures of the nanoparticles were characterized by FESEM TESCAN

MIRA3XMU Scanning Electron Microscope (SEM) together with EDX (JEOL, USA). The

crystal structures of as synthesized nanoparticles were studied by D8 ADVANCE XRD

(Bruker, Germany). The photocatalytic activities of pure and Cu-doped ZnO nanoparticles

were revealed against the removal of typical dye i.e.; reactive red solution in the presence and

absence of 1mL H2O2 and 0.1g nanocatalyst at ambient temperature after specific intervals.

Synthesis of nanoparticles

A characteristic polyol reduction method was used to synthesize ZnO nanoparticles. In this

method ethylene glycol was used as a solvent and reducing agent and PVP assumed the part

of capping agent. 20 mL of EG was preheated at 170 oC for half an hour with a steady

stirring in a 250 mL round bottom flask. After attaining the constant temperature, 15 mL

solution of PVP was added to the flask followed by the dropwise addition of Zn (acac) 2 with

constant stirring. The concentration of both the solutions was 0.1 M. The whole reaction

mixture was heated for four hours at 170 oC. After the complete of the reaction, the white

suspension was obtained, which was allowed to cool at room temperature and washed with

distilled water and ethanol to remove the unreacted reagents. The similar procedure was

repeated for the doping of copper by adding 2 mL of 0.01 M copper nitrate solution to dop

the ZnO nanoparticles.

Photocatalytic Activity Estimation

Aqueous solution of Reactive Red S3B was selected to check the photocatalytic activity of

pure ZnO and Cu-ZnO nanoparticles at ambient temperature.

In photocatalytic experiment, 0.10 g of ZnO or Cu doped ZnO was dispersed in 50 ppm

Reactive red S3B solution. The suspension was carefully mixed by magnetic stirring for 15

minutes to establish the adsorption-desorption equilibrium among the nanocatalyst, the dye

and water. This mixture was then subjected to visible light radiation. After every 10 minutes,

3 mL aliquot was taken out from the reaction mixture and was centrifuged for 10 min at 1000

rpm to separate the catalyst. The progress of the reaction was checked by calculating the

absorbance (A) of the purified filtrate solution at 542 nm (λmax) using UV-Vis

spectrophotometer (Shimadzu UV 2550). The photocatalytic degradation efficiency (%) was

estimated by using the following formula:

Decolorization Efficiency (%) = Co−Ct

Co x 100 (1)

Where C0 is the initial concentration of dye and Ct is the concentration of dye after photo

irradiation at different time intervals (expressed in mol/L).

Result and Discussion

In order to determine the transient energy levels of pure and Cu-doped ZnO nanoparticles

UV-Vis spectroscopy was carried out. Figure 2 shows an absorbance peak for ZnO

nanoparticle at 360 nm and for Cu-doped ZnO nanoparticles at 390 nm respectively. The 390

nm peak clearly shows the shift due to Cu-doping of ZnO. There was no evidence of the

presence of any Cu or CuO due the absence of any peak at 350 nm.33 The absence of any

metallic copper shows that copper is doped in the ZnO rather than an impurity.

Tauc relationship was applied to calculate the direct band gap of pure and Cu-doped ZnO

nanoparticles. With respect to the absorption spectra measured, (ᾳhυ)2 against hυ curves of

ZnO nanoparticles are revealed in Figure 3 (a, b). The values of band gap at about 3.03 and

2.7 eV were calculated for both ZnO and Cu-doped ZnO respectively. The doping of Cu in

ZnO showed a significant decrease in the band gap i.e. from 3.03 to 2.7 eV. The decrease in

band gap was also the strong mixing of p-d orbitals of O and Cu.

The photoluminescence (PL) spectra of ZnO and Cu-ZnO nanoparticles are presented in

figure 4. The intensity of the peaks is directly proportional to the rate of electron-hole

recombination. The ZnO exhibited UV emission peak at 349 and a broad emission band at

700 nm while the Cu doped ZnO showed emission peak at 379 nm and broad emission band

at 760 nm respectively which is a significant change. The UV emission is usually attributed

to the near-band-edge emissions of ZnO. The broad emission bands at about 700 nm and 760

nm are mostly attributed to the radiative recombination of a photo-generated hole with an

electron occupying the oxygen vacancy.34

Figure (5) shows an XRD pattern of ZnO and Cu-ZnO nanoparticles. The XRD

pattern of ZnO shows the reflection peaks at 2θ values of 31.82°, 34.33° ,36.49° and 47.56°

which correspond to (100), (002), (101) and (102) planes. These value match to the standard

value reported in the literature (ICPDS card No.36-1451). There was no noticeable shift in

the peaks when copper was doped in zinc oxide nanoparticles. A very intensity peak was

observed for copper in the copper doped zinc oxide which indicates that not all copper was

gone inside the crystal lattice of zinc oxide. The size of crystallites was determined by using

Debye’s Scherer’s formula.

Average crystallite size (D) = 0.9 λβcosθ (2)

Where λ is wavelength of X-ray used (0.15418nm), β is FWHM (101) plane and θ is the

diffraction angle. It is clear from the figure 4 that the average particle size was reduced from

11.51 to 6.36 nm. The broadening of the peaks in the XRD pattern of Cu-ZnO show the

smaller size of the nanoparticles in Cu-ZnO.35

Scanning electron microscopy shows the size, shape and general morphology of the

nanoparticles. The SEM images of ZnO (Figure 6a, b) nanoparticles show the spherical

shape of the crystallites with varying size. There are few larger particles with a diameter of

approx. 1µm dispersed in smaller spherical particles with diameter of around 200 nm. Figure

5 (a) and (b) show images with different magnifications. The SEM images of Cu-doped ZnO

(Figure 5c, d) show the non-uniform size distribution. Most of the smaller spherical particles

form clusters of different sizes and shapes which range from 1µm to 300 nm. Clear can be

observed by the ZnO and Cu.ZnO images e.g. there are no large size spherical particles

observed in Cu.ZnO as shown by ZnO images.

The elemental analysis of ZnO and Cu.ZnO nanoparticles were studied with EDX (Table 1).

EDX of ZnO showed the atomic ration of Zn and O as 55.64:44.36 which is close to 1:1 ratio

as expected despite the difficulty of measuring the oxygen with high accuracy. Cu.ZnO

sample showed the presence of 0.94 atomic percentage of Cu in ZnO which correspond to the

amount added i.e., 10% Cu in ZnO

Photodegradation studies

The UV/Vis spectra of Reactive red S 3B dye is represented in figure 7a. The degradation of

this dye was studied under four different experimental conditions viz. solar energy /ZnO,

solar energy /ZnO/H2O2, solar energy/Cu.ZnO and Dark/ZnO. The removal rate was

monitored by the change in intensity of UV/Vis peak at 542 nm for Reactive red S 3B dye.

Figure 6a shows the UV/Vis spectra of Reactive red S 3B dye solution. Initially the reaction

was started by illuminating the dye solution to sun light without the presence of any catalyst.

The UV/Vis measurements showed that the peak intensity decreased by 6.47% (Figure 7 b).

The adsorption of the dye over the photocatalyst was monitored under the four different

conditions mentioned earlier (solar energy /ZnO, solar energy /ZnO/H2O2, solar

energy/Cu.ZnO and Dark/ZnO) at fixed amount of dye solution (50 mg/L) and photocatalyst

loaded (0.1 g/L). The dye was exposed to daylight at a 10 minutes interval for 120 minutes to

check the degradation effect. It was observed that the absorption peak decreases slowly with

the decrease in the dye concentration and is shown in figure 7 (c, d, e, f). The gradual change

in absorption peak showed that the organic compound has been decomposed in the solution.

The percentage removal rate of the dye at different conditions are presented in the Figure 8.

The degradation efficiency is increased as the catalyst load was increased upto 0.1 g/L and

afterward it decreases gradually. The decrease in the rate of degradation with increase of

catalysts above 0.10 g/L may be because of the scattering of light and low penetration of light

through the dye solution. With a higher catalyst concentration, the activated molecules start

colliding with the ground state molecules and become deactivated, resulting in lowering the

rate of reaction. The percentage degradation of the dye using ZnO, Cu.ZnO, H2O2/ZnO in the

presence of light and ZnO in the dark was 79.68%, 88.98%, 79.04% and 26.24% respectively

and is shown in the figure 8.

Figure 9 shows the concentrations of the dye left after reacting with the photocatalysts.

Initially the concentration of pure dye was exposed to the sunlight without adding any

catalyst. It shows that there was no change in the absorption of the dye solution. So it was

labelled as the pure dye solution without any environmental effects. Similarly, when ZnO

was added to the dye but placed in the dark there was only 26.24% degradation of the dye.

However, when the dye solution was treated with H2O2, ZnO and Cu-doped nanoparticles

under sun light then a significant degradation was observed. It is cleared from the graph that

the rate of degradation was almost same (79.68%, 79.04%) with ZnO/H2O2 and ZnO exposed

to the light for 120 min. But the doped nanoparticles (Cu-ZnO) showed the most degradation

(88.98%) of the dye.

The kinetics of degradation of dye was roughly calculated. The kinetic model of the pseudo-

first-order model were applied to analyse the kinetics rate in the photodegradation process of

reactive red S3B onto ZnO and Cu.ZnO nanoparticles and are generally stated by the

following equation.

ln (CtCo

¿ = kt

Where k is the rate constant (min−1) and Ct and Co are the concentrations (mg/L) of dye at

various time intervals and initially used, respectively. The graph between ln (Ct/C0) and time

of reaction t gives a straight line which shows that the reaction follows a pseudo-first-order

kinetics behaviour.36-37 The results are shown in Figure 10.

The value of correlation coefficient (R2) was 0.9945, which specifies that the

photodegradation of dye satisfies the kinetic model. The rate constants of the dye in the

presence of ZnO, Cu.ZnO, in the dark and with H2O2/ZnO were 0.78, 0.68, 0.27 and 1.45

min−1, respectively. The ZnO (in the pure and doped forms) used in the photocatalytic

treatment was washed with distilled water and ethanol many times, dried at 70 °C in an oven

and it was reused further in the next photocatalytic experiments. It was observed that renewed

nanocatalysts have nearly the same efficiency for the dye degradation and can be used for

further photocatalytic decolourization of given dye solution.

Conclusion

Wurtzite ZnO and Cu-doped ZnO nanoparticles have been synthesized by polyol method

which is simple and easily scalable. These polyol nanoparticles are stable for periods of

several months at room temperature. Both ZnO and Cu-doped ZnO showed a good

photocatalytic activity against Reactive Red S3B, a carcinogenic dye. These non-toxic

nanoparticles are useful photocatalyst for the industrial effluents containing these types of

dyes and for the degradation of many other carcinogenic dyes which were used in print and

textile industry. We believe that the polyol method used to synthesise these materials is

highly effective to produce high quality products. The work on the degradation of other dyes

is under investigation and will be published elsewhere.

Acknowledgement

The author appreciatively acknowledges the financial support from the Higher Education

Commission (HEC) Pakistan and Office of Research Innovation and Commercialization

(ORIC), MUST, for the arrangement of funds required for the chemicals and samples

characterizations.

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Figures

Fig.1 Structure of Reactive red S3B

Fig. 2 UV-Visible spectra of ZnO and Cu-doped ZnO nanoparticles

IRVisibleUV

Fig. 3 Tauc plots showing the band gaps of (a) ZnO (b) Cu-Doped ZnO nanoparticles

(b)(a)

Fig.4 PL spectra of ZnO and Cu-ZnO nanoparticles

Fig.5 p-XRD pattern for (a) ZnO (b) Cu-ZnO nanoparticles

Fig. 6 SEM images of (a, b) ZnO and (c, d) copper doped ZnO nanoparticles at different magnifications.

(d)

1 µm

(c)

5 µm

(a)

5 µm 1 µm

(b)

(c)

(b)

(d)

(a)

(a)

Fig.7 UV-Vis absorbance spectra of (a) RR S3B solution (b) RRS3B solution exposure to

sun light without adding catalyst (c) In Dark, (d) RRS3B solution in the presence of ZnO (e)

RR S3B solution in the presence of Cu-ZnO, (f) RR S3B solution in presence of H2O2/ZnO.

(f)

(c)

(e)

(d)

(b)(a)

Fig. 8 Percentage degradation of (a) RR S3B solution in presence of H2O2/ZnO, (b) RR S3B

solution in the presence of Cu-ZnO, (c) RRS3B solution in the presence of ZnO, (d) RR S3B

dye in the presence of dark.

Fig. 9 Effect of catalysts on photocatalytic degradation of (a) RRS3B dye solution, (b) RR S3B solution in the dark, (c) RRS3B solution in the presence of ZnO, (d) RR S3B solution in presence of H2O2/ZnO, (e) ) RR S3B solution in the presence of Cu-ZnO (Conditions: Initial Dye concentration = 50 ppm, Catalyst Loaded = 0.1 g/L).

Fig. 10 Photocatalytic degradation kinetic curves for photocatalytic degradation of (a) RR S3B dye in the presence of dark, (b) RRS3B solution in the presence of ZnO, (c) RR S3B solution in the presence of Cu-ZnO, (d) RR S3B solution in presence of H 2O2/ZnO. (50 ppm initial concentration of solution, 0.1 g/L catalysts)

Table 1. EDX data for ZnO and Cu.ZnO nanoparticles

Materials Elements Weight% Atomic%

ZnO

O 23.49 55.64

Zn 76.51 44.36

Cu.ZnO

O 19.53 49.77

Zn 79.00 49.29

Cu 1.47 0.94