Draft - University of Toronto T-Space · where hv is the photon energy; Eg, the band gap energy; and B, a characteristic constant for each semiconductor material. The optical band

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Synthesis, Characterization and Photocatalytic activity of silver and zinc codoped TiO2 nanoparticle for

photodegradation of methyl orange dye in aqueous solution

Journal: Canadian Journal of Chemistry

Manuscript ID cjc-2018-0308.R3

Manuscript Type: Article

Date Submitted by the Author: 21-Mar-2019

Complete List of Authors: Oladipo, Gabriel; D.S Adegbenro ICT Polytechnic, Science Laboratory Technology; Federal University of Agriculture Abeokuta, ChemistryAkinlabi, Akinola; Federal University of Agriculture Abeokuta, ChemistryAlayande, Samson; First Technical University, Industrial ChemistryMsagati, Titus; University of South Africa - Science CampusNyoni, H; University of South Africa - Science CampusOgunyinka, Opeyemi; D.S Adegbenro ICT Polytechnic, Science Laboratory Technology

Is the invited manuscript for consideration in a Special

Issue?:Not applicable (regular submission)

Keyword: photocatalyst, nanoparticles, photodegradation, Titanium (IV) oxide, Visible light

https://mc06.manuscriptcentral.com/cjc-pubs

Canadian Journal of Chemistry

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Synthesis, Characterization and Photocatalytic activity of silver and zinc codoped TiO2

nanoparticle for photodegradation of methyl orange dye in aqueous solution

Oladipo, Gabriel. O1,2, Akinlabi, Akinola. K1, Alayande, Samson. O3, Msagati, Titus. A.M4,

Nyoni, Hlengilizwe. H4, Ogunyinka, Opeyemi. O2

1. Department of Chemistry, Federal University of Agriculture, Abeokuta, Ogun State, Nigeria

2. Department of Science Laboratory Technology, D.S Adegbenro ICT Polytechnic, Itori-

Ewekoro, Ogun State, Nigeria

3. Department of Industrial Chemistry, First Technical University, Ibadan, Oyo State, Nigeria

4. Nanotechnology and Water Sustainability Research Unit, College of Science Engineering and

Technology, University of South Africa, Florida, South Africa.

E-mail addresses: [email protected] (Akinlabi A.K), [email protected] (Alayande S.O), [email protected] (Ogunyinka O.O), [email protected] (Nyoni, H.H), [email protected] (Msagati, T.A.M).Corresponding author: Oladipo G. O. [email protected], +2348039706811. D.S Adegbenro ICT Polytechnic, Itori-Ewekoro, Ogun State, Nigeria

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mailto:[email protected]






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ABSTRACT

In this study, TiO2 nanocrystals, 1 mol% Ag-doped and 1 mol% Ag and 0.6 mol% Zn codoped

TiO2 powders were synthesized by the sol-gel route. Their photocatalytic activities on methyl

orange dye under visible irradiation were investigated. The powders were characterized by X-

Ray Diffraction (XRD), UV–Visible spectroscopy (UV-Vis), Brunauer–Emmett–Teller (BET)

and Fourier Transform Infrared Spectroscopy (FT-IR). The XRD results revealed the presence of

a rutile phase with an average crystallite size of 9 and 11 nm. The UV-Vis spectra showed a red-

shift towards longer wavelength with the corresponding decrease in band gap from 2.9 to 2.5 eV.

The BET surface areas of the nanoparticles ranged from 4.7 to 11.8 m2g-1 with an average pore

size between 18.9 and 56.6 nm. The Ag-doped TiO2 has the largest surface area of 11.8 m2g-1,

while Ag-Zn co-doped TiO2 was found to have the highest pore size and volume. The absorption

bands at 750-500 cm-1 were attributed to -O-Ti-O- bond in the TiO2 lattice. The photocatalytic

efficiency was highest at optimum pH 4.1 for Ag-Zn codoped TiO2. The results confirmed that

Ag-doped and Ag-Zn co-doped TiO2 were more effective than pure TiO2. The kinetic data were

fitted into a pseudo-first-order equation using Langmuir-Hinshelwood kinetic model.

Keywords: Photocatalyst, Nanoparticles, Photodegradation, Titanium (IV) oxide, Visible light

1.0 INTRODUCTION

The recent development in nanotechnology has been able to address fundamental issues of the

environment and water sectors. In water treatment applications, it has been used for treatment,

remediation, sensing, detection, and pollution control [1, 2].

Nanotechnology holds great promise in remediation, desalination, filtration, purification and

water treatment. The development of nanomaterials for the treatment of water and wastewater

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https://en.wikipedia.org/wiki/Desalination

https://en.wikipedia.org/wiki/Water_purification

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cannot be overemphasized because it was proving to be more efficient in removing toxic

contaminants. Many different types of nanomaterials or nanoparticles are used in water treatment

processes such as TiO2, ZnO, and CdS [3-5] primarily due to their particle size, morphology,

surface area, electrical, optical, physical, chemical, or biological and antimicrobial properties.

A notable nanomaterial is Titania (TiO2), it has been reported as an efficient and most

investigated nanomaterial as a photocatalyst [6-8]. This is attributed to its unique properties such

as low cost, non-toxicity, relatively high photocatalytic activity (PCA) in ultraviolet (UV) light,

high chemical stability, strong oxidizing power, and availability [9-12]. It is used in a wide range

of applications such as air purification, photoinduced hydrophilic coating, and self-cleaning

devices, self-sterilization, wastewater treatment, and production of hydrogen fuel [9].

TiO2 exists in three crystalline polymorphic phases namely: rutile, anatase, and brookite [13].

The anatase TiO2 has been extensively utilized as a photocatalyst due to its low electron-hole

pair recombination [14-17], while rutile TiO2 has received less attention as a photocatalyst

because of low photoelectrochemical performance. However, the rutile phase has some

advantages over the anatase phase such as higher chemical stability, higher refractive index and

cheaper production cost [16, 18]. The higher chemical stability of rutile TiO2 in different environments

(such as pH) and cheaper production cost make a decisive factor for utilization as catalysts in wastewater

treatment and in building a photoelectrochemical cell. Moreso, its photocatalytic activity can also be

enhanced when modified with metallic ions. The dopants (metallic ions) have been reported by

many researchers to cause a shift in the absorption edge of TiO2 nanoparticles to the visible

region of lower energy [9, 19, 20].

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Wet chemical methods have been used in synthesizing TiO2 nanoparticles. These include

precipitation [21], sonochemical [22], hydrothermal [23-25], microwave hydrothermal [14],

solvothermal [18] and sol-gel methods [26-28]. The sol-gel method is the most frequently used

among them because is simple, economical and gives accessibility for synthesizing TiO2

nanoparticles with different morphologies like sheets, tubes, particles, wires, rods, mesoporous

and aerogels

In recent times, researchers are using a photocatalytic approach capable of decolorizing and

degrading synthetic dyes in wastewater in an effective way without leaving a trace of

carcinogenic compounds. Most of the synthetic dyes used in many applications are stable to

light, non-biodegradable and hazardous to health [29]. In order to reduce the risk of

environmental pollution from such wastewater, it is paramount to treat them before discharging it

into the environment. The successive hydroxyl radical generations during photocatalysis are

responsible for the degradation of dyes. Moreso, doping inhibits the electron-hole recombination

and increase hydroxyl radical generation, and therefore enhance photocatalytic activity against

organic pollutants.

The present study focuses on the synthesizing of undoped, silver doped and silver and zinc co-

doped TiO2 nanoparticles by sol-gel route and their photocatalytic activities against methyl

orange dye under visible light irradiation were investigated.

2.0 MATERIALS AND METHODS

2.1 Materials

Titanium tetraisopropoxide (TTIP, 97%), 2-propanol (99%), concentrated trioxonitrate (VI)

acid, silver trioxonitrate (V) and zinc trioxonitrate (V) hexahydrate were purchased from Sigma

Aldrich, South Africa. Deionized water was supplied from the Q-Millipore purification system.

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The following instruments were used at the Nanotechnology and Water sustainability Resources

unit, University of South Africa, Florida: Scanning Electron Microscopy-Energy Dispersive X-

Ray, Spectroscopy, Electronic balance, Muffle furnace, Air drying oven, Heating magnetic

stirrer, Raman spectroscopy, X-Ray Diffractometer, Thermogravimetric Analyzer, Fourier

Transform Infrared Spectroscopy, Centrifuge machine, UV-Vis Spectrometer, and Brunauer-

Emmett-Teller.

2.2 Methods

2.2.1 Synthesis of TiO2 Nanoparticles

TiO2 nanopowders were prepared via the sol-gel method using the precursor titanium

tetraisopropoxide (TTIP, 97%, Sigma Aldrich), deionized water and 2-propanol (99%, Sigma

Aldrich) as the starting materials. 100 ml of 2-propanol was added to 15 ml of TTIP in 250ml

beaker. The mixture solution was stirred 10 minutes using magnetic stirrer at room temperature.

For hydrolysis reaction 10 ml of deionized water was added dropwise to the mixed solution.

Then the mixture solution was stirred continuously at 80oC for 1 hour. After 1 h, concentrated

HNO3 (8 ml) mixed with deionized water was added to the TTIP solution and kept under

constant stirring at 80 ºC for 5 h, a viscous sol-gel was obtained. The gel was centrifuged and

dried in an air oven at 60oC for 12 h. The dried TiO2 powder obtained was annealed at 550oC in a

Muffle furnace for 4 h to obtain TiO2 nanoparticles [3].

For the synthesis of Ag-doped, Ag and Zn co-doped TiO2 nanoparticles, 86.87 mg (1 mol%) of

AgNO3 and 91.21 mg (0.6 mol%) Zn(NO3)2.6H2O were dissolved in 10 ml of deionized water

and added dropwise to the mixed solution during hydrolysis.

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2.2.2 UV-Vis Spectrophotometry

UV-Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination

of different analytes, such as transition metal ions, highly conjugated organic compounds, and

biological macromolecules. The optical properties of nanoTiO2, Ag-TiO2, and Ag-Zn-TiO2

powder were evaluated using PerkinElmer, Lambda 650 S UV-Vis spectrometer. A set of Teflon

sample holders was used to hold samples and reflectance standard. Samples were ground to fine

powders before measuring. Integrating sphere, with an internal PTFE-coating, measures the total

integrated reflectance of surfaces placed against the sphere’s sample port. The UV-Vis

spectrometer was operated at a wavelength range from 300 to 800nm. The absorbance spectra

were converted to reflectance spectra on the spectrometer. The reflectance values were converted

to the absorption coefficient F(R) according to the Kubelka–Munk Equation.

………………………………………………………………1𝐹(𝑅) = (1 ― 𝑅)2

2𝑅

Where R is reflectance of the sample and F(R) is equivalent to the absorption coefficient. The

band gap of nanoTiO2, Ag-TiO2, and Ag-Zn-TiO2 was obtained using Tauc’s equation

………………………………………………….2(𝐹(𝑅)ℎ𝑣)1/2 = 𝐵(ℎ𝑣 ― 𝐸g)

where hv is the photon energy; Eg, the band gap energy; and B, a characteristic constant for each

semiconductor material. The optical band gap energy Eg was determined by using the Tauc’s

equation by plotting (F(R)hν)1/2 versus photon energy (hν) and extrapolating the linear portion to

(F(R)hν)1/2 = 0. The linear nature of the plots above the absorption edge indicates that the

fundamental optical transition in these materials is indirect [30].

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https://en.wikipedia.org/wiki/Analytical_chemistry

https://en.wikipedia.org/wiki/Quantitative_analysis_(chemistry)

https://en.wikipedia.org/wiki/Transition_metal

https://en.wikipedia.org/wiki/Conjugated_system

https://en.wikipedia.org/wiki/Organic_compound

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2.2.3 X-Ray Diffraction (XRD)

X-ray Diffraction (XRD) is used for phase identification and also determines strain, preferred

orientation, crystallographic structure, and grain size of crystalline materials. A powder sample

was mounted on a sample holder plate and gradually rotated from one starting diffraction angle

to an ending diffraction angle (normally 2θ= 5° to 90°) with a step size of 0.02o while being

bombarded with X-rays, which generate a diffraction pattern. The characteristic peak

(2θ=27.42o) of rutile (110) was used for comparative and quantitative analysis. The average

crystallite size, ‘τ’ is estimated from the broadening of XRD peaks using Scherrer equation from

Eq. (3). The highest intensity peaks (110) were used to calculate the crystallite size, ‘τ’ of the

samples.

…………………………………………………………………3𝜏 = 𝐾𝜆

𝛽𝐶𝑂𝑆𝜃

where k is the dimensionless shape factor (k= 0.9), λ is the X-ray wavelength ( , θ 𝜆 = 1.54059)

is the Bragg angle (degree), and β is the line broadening at half the maximum intensity (FWHM)

in degree.

2.2.4 Brunauer-Emmett-Teller (BET)

Brunauer-Emmett-Teller is used to measure the specific surface area of a sample including the

pore size distribution. The BET (Brunauer−Emmett−Teller) surface area (SBET) of the samples

was determined from an N2 adsorption-desorption isotherm study at liquid nitrogen temperature

(77 K) using Micromeritics TRiStarII3020. Before the analysis, the samples were degassed under

vacuum at 110 °C for 2 h to evacuate the physisorbed moisture.

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2.3 Evaluation of Photocatalytic Performance

The photocatalytic degradation of methyl orange dye by undoped, Ag-doped and Ag-Zn co-

doped TiO2 nanoparticles was investigated under solar simulator (HAL-320 F.S, ASAHI

SPECTRA) using 100 LCL Compact Xenon Light Lamp as a visible light source. The

degradation experiments were carried out using an initial concentration of 4 ppm of methyl

orange. A mixture of 50 ml of methyl orange solution and 0.1g of TiO2, Ag-TiO2, Ag-Zn-TiO2

nanoparticles was stirred at 200 rpm for 30 min without visible light irradiation to check the

adsorption-desorption equilibrium in dark condition of all the nanoparticles. At every 30 min

intervals, 3 ml suspensions were pipetted out with a syringe fitted with Millipore Millex-LCR

hydrophilic PTFE 0.45 µm filter to remove the nanoparticles. This solution was transferred to a

quartz cuvette for absorption measurement. The UV–Vis spectroscopy measurement was

performed using the PerkinElmer Lamda 650 S spectrophotometer to determine the absorbance.

The Compact Xenon Light lamp was placed at 120 mm away from the solution surface during

the irradiation. The absorption spectra were recorded and the concentration of the degraded dye

solution was determined from the calibration curve to drown from six standard solutions as

shown in Figure 1. The degradation efficiency was calculated as:

………………………………….4𝐷𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (%) = 𝐶𝑜 ― 𝐶𝑡

𝐶𝑜 𝑥 100

The degradation of methyl orange was calculated using the ratio (C/Co) with respect to

irradiation time from absorbance change at the wavelength of 465 nm (Langmuir-Hinshelwood

adsorption kinetic model). Since the plots of ln(Co/C) against irradiation time were approximated

to be linear, the slope of the linear relation, i.e., rate constant k, will be determined on each

sample and used as a measure of the photocatalytic activity of the sample. Factor such as pH,

catalyst dose and concentration of methyl orange will be considered [30]

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…………………………………………………………5ln (𝐶𝑜𝐶) = 𝑘𝑎𝑝𝑝𝑡

where kappt is apparent rate constant, Co is the initial concentration of methyl orange while C is

the concentration after irradiation at a time interval, t

3.0 RESULTS AND DISCUSSION

3.1 Fourier Transform Infrared Spectroscopy

The spectra of undoped nanoTiO2, doped or co-doped nanoTiO2 are shown in Figure 2. The

strong absorption bands at 750–500 cm-1 located in the fingerprint region are attributed to

vibration stretching of Ti–O–Ti bond in the nanoTiO2 lattice. The broad absorption peaks

observed in the region 3500–3400 cm-1 for all the nanoparticles are due to a surface-bound

hydroxyl group (OH) stretching vibration which confirms the strong interaction of water on the

surface of a nanoTiO2 lattice [24]. The absorption peaks at 1630–1627 cm-1 are assigned to

bending vibration of surface adsorbed water molecules. No additional peaks are observed for Ag

and Zn doping and/or co-doping which supports the efficient dispersion of dopants and it

indicates the absence of clusters of silver and zinc.

3.2 UV–Vis Diffusion Reflectance Spectra (DRS) Analysis

The optical properties of nanoTiO2, Ag-TiO2, and Ag-Zn-TiO2 powder were evaluated using

Lambda 650 S UV-Vis spectrometer. Optical diffuse reflectance spectra are shown in Figure 3a-

c. From the reflectance spectra (Figure 3b), it can be observed that the reflectance onset is sharp

for nanoTiO2 and the variation of reflectance is gradual for 1 mol% Ag-doped and 1 mol% Ag

and 0.6 mol% Zn co-doped nanoTiO2. The band gap of nanoTiO2 is found to be 2.9eV with the

corresponding absorption wavelength of 427nm which is higher than 1 mol% Ag-doped (2.5eV)

and 1 mol% Ag and 0.6 mol% Zn co-doped nanoTiO2 (2.75eV) with the absorption wavelength

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of 496 and 451nm respectively. This clearly exhibits a redshift of absorption edge towards the

visible region upon doping of 1 mol% Ag and 1 mol% Ag and 0.6 mol% Zn into nanoTiO2 as

shown in Figure 3c. The dopants narrow the band gap between the valence band and the

conduction band of nanoTiO2 by introducing a new energy level.

3.3 X-Ray Diffractometry

Figure 4 shows the XRD pattern of pure TiO2. The XRD pattern shows the presence of the peaks

at 2θ = 27.44, 36.08, 39.19, 41.24, 44.04, 54.32, 56.63, 62.76, 64.05, 65.51, 69.00 and 69.80o,

regarded as an attributive indication of rutile TiO2, which is coincided with Joint Committee on

Powder Diffraction standard (JCPDS) (PDF Card No.: 9015662). The peaks are indexed as 110,

101, 200, 111, 210, 211, 220, 002, 310, 221, 301 and 112 in the order of increasing diffraction

angles respectively.

The XRD results indicate that the TiO2 powder has been predominantly crystallized into the

rutile phase. Figure 4 also shows the XRD pattern of Ag-TiO2 and Ag-Zn-TiO2 samples. The

presence of the rutile phase without any impurity phases such as Ag or AgNO3 and Zn or

Zn(NO3)2 confirms the complete doping of Ag or Zn in TiO2 lattice. The average crystallite size

of undoped TiO2, 1 mol% Ag-doped and 1 mol% Ag and 0.6 mol% Zn co-doped TiO2

nanoparticles is found to be 11, 10 and 9 nm respectively, demonstrating that doping TiO2 with

Ag and Zn slightly inhibit the growth of TiO2 nanoparticle. Ionic radius strongly influences the

ability of the dopant to enter into the TiO2 crystal lattice to form a stable solid solution. If the

dopants ionic radii are relatively much larger than Ti4+ ionic radius, the dopants ions are most

likely located in interstitial positions of the lattice rather than directly in Ti4+ sites [31] The

similarity of the ionic radius of Zn2+ ion (0.74 Å) with Ti4+ ion (0.745 Å) will result in an easy

substitution of Zn2+ ion with Ti4+ ion in TiO2 lattice [32]. However, the relative huge difference

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between Ag+ ion (1.26 Å) and Ti4+ ion (0.745 Å) radius may hinder substitution but a rather

interstitial substitution of Ag+ ion in TiO2 lattice. This suggests that the Ag+ and Zn2+ ions are

adsorbed on the surface of TiO2 lattice and suppressed the growth of TiO2 nanoparticles. It is

also observed that the peaks intensity of the doped or co-doped TiO2 is more intense than a

undopedTiO2 nanoparticle. This is attributed to the absorption of laser intensity by silver and zinc

present in the nanoTiO2 lattice

3.4 Brunauer-Emmett-Teller (BET)

The results of BET specific surface area values for undoped, doped and co-doped samples are

listed in Table 1. All the samples possess quite low specific surface area values, ranging from

about 4.7 to11.8 m2g-1. The average pore size ranges from 18.9-56.6 nm for all of the samples.

The BET specific surface area of 1 mol% Ag-doped and 1 mol% Ag and 0.6 mol% Zn co-doped

TiO2 is 11.8 and 6.1 m2g-1 respectively, which are higher than undoped TiO2 (4.7 m2g-1 ). The

increase in surface area and average pore size may be attributed to the incorporation of dopants

into the TiO2 lattice. But the effect is more pronounced in 1 mol% Ag and 0.6 mol% Zn co-

doped TiO2 nanoparticles.

3.5 Optimization of Variables in Photodegradation

The effect of various operational variables such as catalyst dose, initial concentration, and pH on

the photodegradation of methyl orange dye was investigated

3.5.1 Effect of Catalyst Dosage on Photodegradation

The effect of catalyst dosage is also studied in the range of 2–8 gL−1and the results are shown in

Figure 6 and illustrated by UV-Vis absorption spectra (Figure 7a-c). Degradation of methyl

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orange dye is observed to increase as the catalyst dosage increases from 2 to 8 gL−1, however,

degradation irradiation time is found to decrease as catalyst dosage increases. Ag-Zn-TiO2 and

Ag-TiO2 nanoparticle show complete mineralization of methyl orange dye at 8 gL−1 catalyst dose

within 60 and 120 min respectively, while TiO2 nanoparticle shows 63% degradation within 300

min at pH 4.1. This is because, the higher the catalyst dosage, the higher the number of active

sites that could absorb more photons and methyl orange [33, 34]. However, an excess dosage

may lead to the turbidity of suspension in the solution, and reduces the light penetration and

impede the photocatalytic process [35]. Gupta and Tripathi reported that high activity of TiO2

photocatalyst should satisfy two requirements: a large surface area for absorbing substrates and

high crystallinity to reduce the rate of photoexcited electron-hole recombination [36]. Therefore,

the higher percentages of degradation obtained for Ag-TiO2 and Ag-Zn-TiO2 nanoparticle are

due to narrow band gap and ease of separation of the electron-hole pair by their metal dopants.

Moreso, an increase in surface area, pore size and volume are also responsible. However, the

large average pore size and volume observed on Ag-Zn-TiO2 nanoparticles could be responsible

for its high photocatalytic activity.

3.5.2 Effect of pH on Photodegradation

The pH of the solution is an important controlling parameter in evaluating an aqueous phase for

photocatalytic reaction. Wastewater containing dyes is discharged at different pH; therefore, it is

important to study the effect of pH on the degradation of methyl orange dye by varying the pH

values (2.1, 4.1 6.7 and 11) at constant dye concentration (4 ppm) and catalyst dose (2g/L). The

catalyst efficiency for dye removal could be explained by the amphoteric performance of the

catalyst, which is based on its zero point charge (pHZPC). The pHZPC is found to be at pH 6.3;

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below this pH value the catalyst surface would be positively charged and attracted the negatively

charged methyl orange (anionic dye), and above the pH, its surface would be negatively charged

which will result to electrostatic repulsion of anionic dye [37, 38]. This may explain the highest

degradation at an acidic condition rather than alkaline. The natural pH value of methyl orange

dye is found to be 6.7 with an absorption peak at 465nm and is adjusted by adding drops of 1M

NaOH or HCl, while the absorption peaks at 506, 487 and 467 are assigned to pH 2.1, 4.1 and 11

respectively.

From Figure 5, it can be seen that Ag-Zn-TiO2 nanoparticle gives the highest degradation at pH

2.1 and pH 4.1with 84.4 and 93.1% respectively at 2 gL−1 catalyst dose, while Ag-TiO2 has the

highest degradation at pH 11 with 58.5%. At neutral pH 6.7, the entire nanoparticle shows low

degradation. TiO2 nanoparticle shows the lowest degradation at all pH. This may be due to the

poor photo-excitation by TiO2 catalyst under visible light irradiation as a result of its large band

gap energy and high electron-hole pair recombination which reduces hydroxyl radical generation

for degradation of dye.

3.5.3 Effect of Initial Concentration of Dye on Photodegradation

An optimum pH 4.1 and catalyst dose 2 gL−1 is used for photocatalytic degradation of methyl

orange dye with an initial concentration of 4, 5 and 6 ppm. From Figure 8; the highest percentage

degradation of 2.3, 79.9 and 93.1% are observed at the lowest concentration within 300 min. It

can be seen that as the initial concentration of the dye increases, percentage degradation

decreases. This is attributed to the reduced amount of photons absorbed by the catalyst in the dye

solution at higher concentrations, thereby reducing the formation of OH radical and vice versa.

At higher concentration of dye solution, photon penetration into the dye solution is reduced due

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to an increase in light scattering and consequently, the degradation rate is decreased. This

observation is similar to the report of Neppolian et al. [39] and Saggioro et al. [40].

3.5.4 Effect of Irradiation Time on Photodegradation

The effect of irradiation time on the photodegradation of methyl orange dye is investigated using

4 ppm initial concentrations with 2-8 gL-1 of catalyst dose. It is observed that the photocatalytic

degradation percentage is found to be minimal at 60 min. The concentration of methyl orange

dye decreases, as the irradiation time increases as shown in Figure 9a-f. However, as the catalyst

dose increases, the rate of concentration decrease becomes faster and irradiation time reduces. At

8 gL-1 catalyst dose, the highest percentage photodegradation of methyl orange dye is observed

at irradiation time of 60 and 120 min for 1 mol% Ag and 0.6 mol% Zn co-doped, and 1 mol%

Ag-doped TiO2 nanoparticle respectively.

3.5.6 Kinetics of Photocatalytic Degradation of Methyl Orange dye (MO)

The rate of decolorization is recorded with respect to the change in the intensity of absorption

peaks at 465 nm for MO. The degradation rates of methyl orange dye are also studied using the

Langmuir–Hinshelwood (L–H) model [41] as shown in Figure 10a-f. It relates the degradation

rate and reactant concentration C, in water with respect to time t. The pseudo-first-order kinetics

with an apparent first-order rate constant is observed when the adsorption is relatively weak

and/or the reactant concentration is low. From the plots of ln(Co/C) against irradiation time, it is

seen that the data are roughly fitted to a regression or straight line, indicating the photocatalytic

degradation followed pseudo-first-order kinetics models [42].

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Table 2 shows the results of the pseudo apparent first-order rate constant (Kapp) and linear

correlation coefficient (R2) for photodegradation of methyl orange dye by undoped and doped or

co-doped TiO2 under visible light excitation. The results revealed the dependency of the

apparent-first-order rate constant on the varying catalyst dose and pH. It is observed that the

apparent-first-order rate constant is lowest at pH 6.7 and 11, albeit highest at pH 2.1 and 4.1 for

all the nanoparticles. However, the highest apparent-first-order rate constant (0.03788 min-1) is

observed at pH 4.1 for 8 gL-1 Ag-Zn-TiO2 nanoparticle doses with a correlation coefficient (R2)

value of 0.8251. The correlation coefficient value obtained suggests that the apparent-first-order

rate constant is dependent on the catalyst dose as it tends towards unity. This is followed by an 8

gL-1 Ag-TiO2 nanoparticle dose with the apparent-first-order rate constant of 0.01154 min-1 with

a correlation coefficient (R2) of 0.8817. This indicates that the apparent-first-order rate constant

is dependent on varying catalyst dose and pH. This may be due to the effect of 1 mol% Ag and

combine effect of 1 mol% Ag and 0.06 mol% Zn dopants on the TiO2 lattice to separate electron-

hole pairs and prevent their recombination which enhances photodegradation. Moreso, it is

observed that the apparent-first-order rate constant increases as the catalyst dose increases, thus,

an increase in the photodegradation of methyl orange dye. The apparent-first-order rate constant

for homogeneous photolysis and TiO2 nanoparticle at pH 4.1, 6.7 and 11 is approximately

negligible due to poor photodegradation which results from insufficient photogeneration under

visible irradiation. But as TiO2 nanoparticle dose increases from 2-8 gL-1, a significant apparent-

first-order rate constant is observed at 0.00303 and 0.00317 min-1 with a correlation coefficient

(R2) of 0.9750 and 0.9079 respectively.

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CONCLUSION

The study revealed the photocatalytic activity of pure TiO2, Ag-doped and Ag-Zn codoped TiO2

nanoparticles on the degradation of methyl orange dye as a model organic pollutant under visible

light irradiation. The Ag and Zn dopants are found to narrow the optical absorption band gap and

increase the surface area and pore size of the TiO2 nanoparticle and consequently, enhance the

photocatalytic activity of TiO2 nanoparticle due to reducing electron-hole pair recombination.

The XRD pattern confirmed that the nanoparticles were typical rutile structures with an average

crystallite size of 9, 10 and 11 nm. The experimental results showed that the rate of degradation

is sensitive to the operational parameters such as pH, catalyst dose, and initial concentration of

dye and irradiation time. The photodegradation efficiency of the photocatalysts was higher in an

acid medium than the basic medium. The catalyst dosage was found to influence the

photodegradation of methyl orange dye, which increases as the catalyst dose increases. Complete

mineralization was observed at 8 gL-1 catalyst dose within 60 and 120 min for Ag-Zn-TiO2 and

Ag-TiO2 nanoparticles respectively.

The rate of photodegradation was also found to decrease as the initial concentration of dye

increases. The kinetic study showed that the reaction followed pseudo-first-order kinetics. The

pseudo-apparent-first-order rate constant was found to be highest for Ag and Zn co-doped TiO2

nanoparticle at 8 gL-1 catalyst dose.

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Table 1: Specific surface area, pore volume and pore size of undoped TiO2, Ag-doped, Ag and

Zn co-doped TiO2 nanoparticles.

Sample Specific surface area (m2/g) Pore size (nm) Pore volume (cm3/g)

TiO2 4.7 18.9 0.02

Ag-TiO2 11.8 24.4 0.07

Ag-Zn-TiO2 6.1 56.6 0.09

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Table 2: The Pseudo Apparent First-Order Rate Constant (Kapp) and Linear Correlation

Coefficient (R2) for Photocatalytic Degradation of Methyl Orange Dye at Different pH Values

and Catalyst Dosage

Catalyst pH Catalyst dose (g/L) kappt R2

TiO2 2.1 2 0.00112 0.8348

4.1 4 0.00303 0.9750

4.1 8 0.00317 0.9079

Ag-TiO2 2.1 2 0.00291 0.9592

4.1

6.7

2

2

0.00369

0.00157

0.8859

0.9696

4.1 4 0.00675 0.8906

4.1 8 0.01154 0.8817

11 2 0.00173 0.9640

Ag-Zn-TiO2 2.1 2 0.00621 0.9573

4.1

6.7

2

2

0.00660

0.00174

0.9479

0.9764

4.1 4 0.00821 0.9686

4.1 8 0.03788 0.8251

11 2 0.00230 0.9246

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List of Tables

Table 1: Specific surface area, pore volume and pore size of undoped TiO2, Ag-doped, Ag and

Zn co-doped TiO2 nanoparticles

Table 2: The Pseudo Apparent First-Order Rate Constant (Kapp) and Linear Correlation

Coefficient (R2) for Photocatalytic Degradation of Methyl Orange Dye at Different pH

Values and Catalyst Dosage

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List of Figures

Figure 1: Calibration curve of methyl orange dye solution using UV–Vis spectroscopy.

Figure 2: FT-IR spectra of TiO2, Ag-TiO2 and Ag-Zn-TiO2 nanoparticles

Figure 3: UV-Vis absorption spectra (a), UV-diffuse reflectance spectra (b) and (F(R)hv)1/2

against band gap energy (hv) (c) for TiO2, Ag-TiO2 and Ag-Zn- TiO2 nanoparticle

Figure 4: XRD pattern of TiO2, Ag-TiO2 and Ag-Zn-TiO2 nanoparticle annealed at 550oC for 4

hours

Figure 5: Effect of pH on photodegradation of methyl orange dye at 2 gL-1 catalyst dose

Figure 6: Effect of catalyst dosage on photodegradation of methyl orange at pH 4.1

Figure 7: UV-Vis absorption spectra of methyl orange by (a) 2 gL-1 (b) 4 gL-1 (c) 8 gL-1 of Ag-

Zn-TiO2 nanoparticle at pH 4.1

Figure 8: Effect of initial concentration on photodegradation of methyl orange dye by 2 gL-1

catalyst dose at pH 4.1

Figure 9: Photocatalytic degradation ability of TiO2, Ag-TiO2 and Ag-Zn-TiO2 nanoparticles

against methyl orange dye at (a) pH 2.1 (2 gL-1) (b) pH 4.1 (2 gL-1) (c) pH 6.7 (2 gL-1) (d)

pH 11 (2 gL-1) (e) pH 4.1 (4 gL-1) (f) pH 4.1 (8 gL-1)

Figure 10: Effect of catalysts on Kinetics of photodegradation of methyl orange dye at (a) pH 2.1

(2 gL-1) (b) pH 4.1 (2 gL-1) (c) pH 6.7 (2 gL-1) (d) pH 11 (2 gL-1) (e) pH 4.1 (4 gL-1) (f)

pH 11 (8.1 gL-1)

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Draft0 1 2 3 4 5 6 70

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Concentration (ppm)

Abso

rban

ce

Figure 1: Calibration curve of methyl orange dye solution using UV–Vis spectroscopy.

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4000 3500 3000 2500 2000 1500 1000 500

% T

ransm

ittan

ce

Wavelength (cm-1)

Ag-Zn-TiO2

Ag-TiO2

TiO2

Figure 2: FT-IR spectra of TiO2, Ag-TiO2 and Ag-Zn-TiO2 nanoparticles

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Figure 3: UV-Vis absorption spectra (a), UV-diffuse reflectance spectra (b) and (F(R)hv)1/2

against band gap energy (hv) (c) for TiO2, Ag-TiO2 and Ag-Zn- TiO2 nanoparticle

a

b

c

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Inte

nsity

(cps

)

2-Theta (Degree)

Ag-Zn-TiO2 Ag-TiO2 TiO2

110

101

200

111

210

211

220

002 310

301112

Figure 4: XRD pattern of TiO2, Ag-TiO2 and Ag-Zn-TiO2 nanoparticle annealed at 550oC for 4

hours

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pH=2.1 pH=4.1 pH=6.7 pH=110

10

20

30

40

50

60

70

80

90

100

TiO2

Ag-TiO2

Ag-Zn-TiO2

Degr

adat

ion

effic

ienc

y (%

)

Figure 5: Effect of pH on photodegradation of methyl orange dye at 2 gL-1 catalyst dose

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Draft2 4 8

0

20

40

60

80

100

120

TiO2

Ag-TiO2

Ag-Zn-TiO2

Catalyst loading (g/L)

Degr

adat

ion

effic

ienc

y (%

)

Figure 6: Effect of catalyst dosage on photodegradation of methyl orange at pH 4.1

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DraftFigure 7: UV-Vis absorption spectra of methyl orange by (a) 2 gL-1 (b) 4 gL-1 (c) 8 gL-1 of Ag-

Zn-TiO2 nanoparticle at pH 4.1

200 400 600 800

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Abs

orba

nce

Wavelength (nm)

0 min 30 min 60 min 90 min 120 min 150 min 180 min 210 min 240 min 270 min 300 min

Ag-Zn-TiO2

pH=4.1

200 400 600 800-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Abs

orba

nce

Wavelength (nm)

0 min 30 min 60 min 90 min 120 min 150 min 180 min 210 min

Ag-Zn-TiO2

pH=4.1

200 400 600 800

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Abs

orba

nce

Wavelength (nm)

0 min 30min 60 min

Ag-Zn-TiO2

pH=4.1

a b c

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4 5 60

10

20

30

40

50

60

70

80

90

100

TiO2

Ag-TiO2

Ag-Zn-TiO2

Initial concentration (ppm)

Degr

adat

ion

effic

ienc

y (%

)

Figure 8: Effect of initial concentration on photodegradation of methyl orange dye by 2 gL-1

catalyst dose at pH 4.1

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Figure 9: Photocatalytic degradation ability of TiO2, Ag-TiO2 and Ag-Zn-TiO2 nanoparticles

against methyl orange dye at (a) pH 2.1 (2 gL-1) (b) pH 4.1 (2 gL-1) (c) pH 6.7 (2 gL-1) (d) pH 11

(2 gL-1) (e) pH 4.1 (4 gL-1) (f) pH 4.1 (8 gL-1)

0 30 60 90 120 150 180 210 240 270 300 3300.0

0.2

0.4

0.6

0.8

1.0

C/C

o

Irradiation time (min)

Photolysis TiO2

Ag-TiO2

Ag-Zn-TiO2

pH=2.1

0 30 60 90 120 150 180 210 240 270 300 3300.0

0.2

0.4

0.6

0.8

1.0

1.2

C/C

o


Photolysis TiO2

Ag-TiO2

Ag-Zn-TiO2

pH=4.1

0 36 72 108 144 180 216 252 288 324 3600.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

C/C

o


Photolysis TiO2

Ag-TiO2

Ag-Zn-TiO2

pH=6.7

0 30 60 90 120 150 180 210 240 270 300 3300.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

C/C

o


Photolysis TiO2

Ag-TiO2

Ag-Zn-TiO2

pH=11

0 30 60 90 120 150 180 210 240 270 300

0.0

0.2

0.4

0.6

0.8

1.0

C/C

o


Photoysis TiO2

Ag-TiO2

Ag-Zn-TiO2

pH=4.1

0 30 60 90 120 150 180 210 240 270 300 330

0.0

0.2

0.4

0.6

0.8

1.0

1.2

C/C

o


Photolysis TiO2

Ag-TiO2

Ag-Zn-TiO2

pH=4.1

a b c

d e f

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Figure 10: Effect of catalysts on Kinetics of photodegradation of methyl orange dye at (a) pH 2.1

(2 gL-1) (b) pH 4.1 (2 gL-1) (c) pH 6.7 (2 gL-1) (d) pH 11 (2 gL-1) (e) pH 4.1 (4 gL-1) (f) pH 11

(8.1 gL-1)

0 30 60 90 120 150 180 210 240 270 300 330-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

ln(C

o/C)


Ag-Zn-TiO2

Ag-TiO2

TiO2

pH=2.1

0 30 60 90 120 150 180 210 240 270 300 3300.0

0.5

1.0

1.5

2.0

ln(C

o/C)


Ag-TiO2

Ag-Zn-TiO2

pH= 4.1

0 30 60 90 120 150 180 210 240 270 300 3300.0

0.1

0.2

0.3

0.4

0.5

0.6

ln(C

o/C)


Ag-TiO2

Ag-Zn-TiO2

pH=6.7

0 30 60 90 120 150 180 210 240 270 300 3300.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

ln(C

o/C)


Ag-Zn-TiO2

Ag-TiO2

pH=11

0 30 60 90 120 150 180 210 240 270 3000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

ln(C

o/C)


TiO2

Ag-TiO2

Ag-Zn-TiO2

pH=4.1

0 30 60 90 120 150 180 210 240 270 3000.0

0.5

1.0

1.5

2.0

2.5

ln(C

o/C)


TiO2

Ag-TiO2

Ag-Zn-TiO2

pH=4.1

a b c

d e f

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Draft - University of Toronto T-Space · where hv is the photon energy; Eg, the band gap energy; and B, a characteristic constant for each semiconductor material. The optical band