chapter 5 removal of dyes by...

Chapter 5

Removal of dyes by photocatalysis

Chapter 5

Dyes removal by photocatalysis

The work described in this chapter is directed towards the investigation of

removal of dyes: methyl orange, methylene blue and rhodamine B from water. TiO2 and

ZrO2 metal oxide was used as a photocatalyst and graphene was used to modify their

photocatalytic properties. The photocatalytic performances of above metal oxides have

been examined by varying pH, temperature and concentration. Further graphene has

been found to enhance the photocatalytic properties of TiO2 and ZrO2 by minimizing the

electron hole pair recombination.

5.1 Introduction

The synthetic dyes are brighter, fast, cheaper to produce and easy to apply on

fabrics. These bright colored dyes changed the world. However, the chemicals used to

produce these dyes are often toxic, carcinogenic or even explosive in nature and thus are

undesirable. Approximately 1–20% of overall dye production of the world is discharged

into water [1-3]. Due to good solubility, synthetic dyes are considered as a common water

pollutant and thus it is necessary to eliminate them from wastewater. There are various

methods reported in the literature to remove dyes from water as discussed in chapter 2.

However, out of these reported methods photocatalytic has become method of choice

because it does not cause secondary pollution. In photocatalytic method lots of materials

can be used as a photocatalyst such as ZnO, TiO2, CeO2, ZrO2, SnO2 [9-13]. Among these

materials, TiO2 has been most widely used as a photocatalyst [6-15] since it can be

supported on various substrates for example glass, stainless steel, inorganic materials and

completely degrades the intermediate products into non–toxic substances [4-9]. Hence, in

the present work, TiO2 was used as a photocatalyst and its photocatalytic properties was

further enhanced by using reduced graphene oxide (GR) to remove methyl orange (MO)

dye. But, one drawback of TiO2 is its solubility in water and thus cannot be reused

efficiently. So in present work, oxide of metal (ZrO2) belonging to the same group as that

of titanium has also been explored for its possible use as a photocatalyst to remove MO,

methylene blue (MB) and rhodamine B (RB) dyes. Additionally, to enhance the

photocatalytic properties of it, GR was used.

Chapter 5: Removal of dyes by photocatalysis Page 121

This chapter is divided into two sections.

The first section (A) describes:

Synthesis and characterization of TiO2 and TiO2/GR composite.

Photocatalytic properties of TiO2 and TiO2/GR composite

The second section (B) consists of

Synthesis and characterization of ZrO2 and ZrO2/GR composite.

Photocatalytic properties of ZrO2 and ZrO2/GR composite

5A.1 Synthesis and characterization of TiO2 and TiO2/GR composite

TiO2 is an environmental friendly semiconductor material suitable for water

treatment due to its properties such as non toxicity [4], low production cost, photo and

chemical stability [5]. Sol–gel method was used to synthesize TiO2 and its properties were

further modified by synthesizing its composites with GR.

Precursor chemicals

The following chemicals were used for the preparation of TiO2 particles by sol–gel

method:

Chemical Chemical formula Function

Titanium tetra isopropoxide

(TTIP)] Ti[OCH(CH3)2]4 Source of Titanium

Ethanol [C2H6O] Solvent

Methyl orange C14H14N3NaO3S Dye

Graphene oxide Source of graphene

Acetic Acid C2H4O2 Solvent

Hydrazine hydrate N2H4 H2O Reducing agent

5A.1.1 Preparation of TiO2

The preparation of TiO2 by sol–gel procedure was carried out by the following

steps:

Hydrolysis and condensation of the titanium precursor

Gelation and aging,

Thermal treatment, Physical grinding and annealing.


The activity of particles strongly depends on the preparation method so careful

attention was devoted to the preparation of catalysts including monitoring and controlling

variables such as mixing speed, timing, pH and temperature. The flow chart of synthesis of

TiO2 is shown in Figure 5.1.

Figure 5.1 Flow chart for synthesis of TiO2

(a) Hydrolysis and condensation (Sol formation)

TiO2 nanoparticles were synthesized from TTIP using sol–gel method. TTIP (16 ml)

was mixed with ethanol (27.6 ml) and acetic acid (318 µl) and continuously stirred for 1½

hours at 90°C in dark environment. The obtained sol was clear, homogenous with yellowish

appearance.

(b) Gelation and aging

In the obtained sol, mixture of ethanol and water in the ratio of 2:1 was added and

stirred for 1 hour at 75°C to convert it in to gel.

(c) Thermal treatment, Physical grinding and annealing

The gel was dried in oven at 100°C for a day. The solid was crushed into fine

powder. The obtained TiO2 powder was annealed at 500°C and 800°C for one hour in

furnace named as T5 and T8 respectively.

Dry TiO2 at 100°C in oven, crush and anneal the TiO2 at different temperature

Stir at 75°C for 1 hour and obtain the Gel of TiO2

Add 20 ml ethanol + 10 ml water (2:1) mixture dropwise with stirring

Stir for 1½ hours at 90°C in dark environment

Mix 16 ml TTIP + 27.6 ml ethanol + 0.318 ml acetic acid (1:9:0.5)


5A.1.2 Preparation of TiO2/GR catalyst

TiO2 and GO are used as precursor materials for the synthesis of TiO2/GR

composites. T5 (1.08 g, 0.135 mol) was dispersed in ethanol (100 ml) and GO (100 mg)

was dispersed in DI water (10 ml) by sonication for one hour. Subsequently, both solutions

were mixed and 0.1 ml of hydrazine hydrate and 5 ml HCl was added followed by refluxing

of 36 hours. HCl modifies the surface of TiO2 by inducing charge on it and TiO2 particles

get attached to GO sheet. Further, all of oxygen functionalities of GO get reduced by

hydrazine hydrate (act as reducing agent for GO) and GO was converted into GR. The

resulting composites (T5a) were collected by centrifuge and dried in oven at 60°C for 12

hours.

The same procedure was used to prepare series of composite catalysts by varying the

content of GO. TiO2/GR composite having 133 mg and 200 mg of GO are named as T5b

and T5c respectively. Similarly TiO2/GR composites were prepared from T8 catalyst, named

as T8a, T8b, T8c having 100 mg, 133 mg and 200 mg GO respectively.

Figure 5.2 Flow chart of synthesis for TiO2/GR (T5a) composite

5A.2 Phase analysis

X–ray diffraction is a powerful approach to the study phase of the materials. The most

common phases of TiO2 are anatase, rutile and brookite was analyzed by XRD. The

Dry in oven at 60°C overnight, crush the sample to obtain TiO2/ GR composite

Centrifuge the solution and wash with ethanol several time

Reflux for 36 hours

Mix both the solution (Also add 0.1 ml of hydrazine hydrate and HCl)

Sonicate for 1 hour

Disperse TiO2 (1.08 g, 0.135 mol) in ethanol (100 ml) and GO (100 mg) in DI water (10 ml)


annealing temperature influences the crystalline nature, particle size and the phase of TiO2

nanoparticles.

5A.2.1 Effect of annealing temperature

As prepared TiO2 particles are amorphous phase in nature. As a result of annealing

TiO2 become crystalline. The enhanced crystallinity is due to the fact that at higher

temperature the condensation of free OH group on the TiO2 surface takes place. Two

different annealing temperatures (500°C and 800°C) were used to investigate the effect of

annealing on the phase of TiO2. Figure 5.3 (a, b) depicts the XRD pattern of T5, T8 and their

TiO2/GR composites (T5a, T5b, T5c, T8a, T8b and T8c) obtained by sol–gel route respectively.

Figure 5.3 Phase analysis of (a) TiO2 annealed at 500°C (T5) and TiO2/GR composite (T5a, T5b and T5c), (b)

TiO2 annealed at 800°C (T8) and TiO2/GR composite (T8a, T8b and T8c)

The synthesized T5 nanoparticles shows crystalline nature with 2θ peaks lying at

25.2° (101), 37.7° (004), 48° (200), 55° (105) corresponds to anatase phase and peak at 54°

(211) for rutile phase. T8 nanoparticles exhibits peaks at 27.47° (110), 36.09° (101), 41.31°

(111), 54.41° (211), 56.64° (220) and 69.41° (301) corresponds to rutile phase. The relative

composition of the anatase and rutile phase can be determined by comparing the intensity

of the anatase (101), Ianatase, and rutile (110), Irutile, reflection planes by applying the

following equation [15]:


Xanatase= (1+

-1

(equation 5.1)

where Xanatase is the share of anatase phase in the mixture.

In the XRD of T5 no peak was observed corresponding to (110) rutile phase

showing that T5 was purely anatase. Similarly there was no peak corresponding to anatase

(101) phase in T8 suggests T8 has rutile structure. The XRD data confirms that TiO2 has

anatase phase at 500°C and rutile at 800°C [16].

5A.2.2 Effect of GR

Characteristic peak of GO at 10.56° disappeared in the XRD pattern of TiO2/GR

composites (Figure 5.3 (a, b)), reveals that GO is reduced by hydrazine hydrate during the

reaction. It should be noted that there is no separate peak for GR in the TiO2/GR composite,

possibly due to the low intensity GR peak (near 24.11°) screened by the main peak of

anatase TiO2 at 25.2°. Also, the peak corresponding to graphite was absent in the

composites, indicating the decoration of TiO2 onto GR sheets. Moreover, it was observed

that GR in TiO2 does not affect the phase of the particles.

The powder size was calculated by using Debye scherrer formula

D=Kλ/ (βcosθ) (equation 5.2)

where D is the crystal size; λ is the wavelength of the X–ray radiation (λ=0.154 nm)

for CuKα; K is taken as 0.89; β is the line width at

half–maximum height and θ is the Bragg’s angle

[17]. The average value of crystallite size

calculated using equation 5.2 was 7.01 nm and

37.3 nm for T5 and T8, respectively. The larger

crystalline size of T8 particles shows that particles

grow at higher temperature.

5A.3 Raman analysis

The Raman spectrum (Figure 5.4) of T8

indicates the presence of crystalline particles and

showed Raman bands at 435 (Eg) and 600 (A1g)

Figure 5.4 Raman spectra of T8 and T8b

showing peaks corresponding to

rutile phase

http://en.wikipedia.org/wiki/Bragg_diffraction


for the rutile structure [16] which was in good

agreement with the reported XRD results.

In addition to different TiO2 modes, the

broad D–band (defect–induced mode) at 1379 cm-1

and G band (E2g graphite mode) at 1593 cm-1

were

observed in T8b composites as shown in Figure 5.5.

As position of Eg and A1g Raman bands does not

change by addition of GR further confirms that GR

does not affect the phase of TiO2.

5A.4 Effect of GR on the band gap

The band gap is a crucial parameter to understand the photocatalytic properties of

material. The band gap can be extracted from UV–Visible spectra. In order to investigate

change in the band gap of TiO2 with the addition of GR the UV–Visible absorption spectra

of T8 and T8b composite was recorded. The band gap is calculated using Tauc’s expression

[17]:

αhν = A (hν – Eg) n

(equation 5.3)

where α is absorption coefficient, A is a constant which is almost independent of the

chemical composition of the semiconductor, hν is the photon energy and Eg is the optical

band gap, n is 2 for an indirect transition and ½ for a direct transition. The band gap of TiO2

can be estimated from the plot of (αhν)2 versus photon energy (hν) as shown in Figure 5.6.

Figure 5.6 Tauc Plot (αhν)2 vs. Photon Energy (hν) of the synthesized (a) T8 and (b) T8b

The extrapolated intercept correspond to the band gap energy at 2.98 eV for T8 and

3.23 eV for T8b. The band gap estimated for T8 (2.98 eV) was in close agreement with the

Figure 5.5 Raman spectra of T8b from

1200 to 2000 cm-1


reported value for rutile nanostructures (3.0–3.1 eV). With the addition of GR in T8, the

band gap increased to 3.23 eV indicates the improvement in the photocatalytic properties of

TiO2.

5A.5 Photocatalytic analysis

Photocatalytic process involves acceleration of the reaction in the presence of catalyst

such as TiO2. When photon of energy greater than band gap of TiO2 falls on it electron/hole

pair is generated. As shown in Figure 5.7 the photo excited electrons react with oxygen

molecule to form super oxide anion (O2-) and positive hole breaks apart the water molecule

to form hydroxyl radical (OH*). This cycle continues until light is incident.

Figure 5.7 Mechanism of photocatalysis in TiO2

5A.5.1 Experimental procedure for degradation

500 ml solution of MO (4 mg) was prepared in water and 1 ppm of H2O2 was added

to remove excess of electrons generated during reaction. The pH of the solution was

maintained to 7 by using sodium hydroxide and HCl. The reaction temperature was

maintained at room temperature for all the experiments.T5 (30 mg) was added into 100 ml

of the solution of MO. The solution was kept under UV light and small amount of solution

was withdrawn from the reaction mixture at regular intervals and analyzed using UV

spectrophotometer. The experiment was repeated by varying the amount (0.3 g/L, 0.5 g/L,

1.0 g/L and 1.5 g/L) of catalyst (TiO2 and TiO2/GR) in the solution.

5A.5.2 Degradation of MO using TiO2

The photocatalytic activities of TiO2 (annealed at 500°C and 800°C) were measured

by the degradation of MO under UV light irradiation. Figure 5.8 shows the degradation of

MO with time under irradiation of UV light using pure TiO2 (annealed at 500°C) as


photocatalyst. The absorption peak diminishes with increased irradiation time of UV light

indicates the degradation of MO as TiO2 absorb more energy from light source for longer

exposure time.

(a) Effect of catalyst loading

In order to determine the effect of catalysts (TiO2 and TiO2/GR) on the degradation

the experiments were carried out with and without catalyst loadings.

In the absence of catalyst, degradation of MO takes place in 215 minutes. To study

the effect of TiO2, the amount of photocatalyst was varied from 0.3 to 1.5 g/L in the dye

solution. It is clear from Figure 5.8 (a) that 0.3 g/L of catalyst (T5) degrade MO in 73

minutes and it reduced to 54 minutes when amount of catalyst increased to 0.5 g/L (Figure

5.8 (b)). It was found that at a loading of 1.0 g/L of T5 catalyst MO degrade in 46 minutes

(Figure 5.8 (c)).

Figure 5.8 UV–Visible absorption spectra of degradation of MO with T5 w.r.t. time for different amount of

catalyst loading (0.3 g/L, 0.5 g/L, 1.0 g/L and 1.5 g/L)


The rate of degradation increased from 0.3 g/L to 1.0 g/L catalyst loading. As

amount of catalyst in solution was increased to 1.5 g/L the degradation take place in 50

minutes (Figure 5.8 (d)). This revealed that degradation time first decreases (up to 1.0 g/L)

and then increases with increase in catalyst amount. This phenomenon can be explained by

the fact that the penetration of light through the solution becomes difficult with increased

dose of catalyst. Hence, decrease in photo absorption reduces the degradation of the dye by

reducing the reaction rates at the surface of the catalyst. Similar trend of degradation was

recorded for T8 catalyst. But in this case degradation time decreased up to 0.5 g/L and then

increased for higher concentration of catalyst. The degradation time of T5 and T8 catalysts

is summarized in Table 5.1.

(b) Effect of annealing temperature

It is clear from Table 5.1 that T5 degrade MO more quickly than T8. The difference in

photocatalytic activity was due to particle size and phase. Small crystalline size gives high

surface area and hence provides more sites for the adsorption. As T5 has smaller crystalline

size (7.01 nm) and exists in anatase phase which is more photoactive [18] than T8 having

larger crystalline size (37.3 nm) and exist in rutile phase. Thus, TiO2 (annealed at 500°C)

give better performance than TiO2 (annealed at 800°C).

Table 5.1 Degradation time of TiO2 (annealed at 500˚C and 800˚C) and TiO2/GR composites

Concentration

(g/L) Degradation time (minutes)

TiO2 annealed at 500°C TiO2 annealed at 800°C

T5 T5a T5b T5c T8 T8c

0.3 73 56 50 48 90 75

0.5 54 53 48 44 72 68

1.0 46 32 38 30 85 80

1.5 50 37 42 40 100 86

5A.5.3 Effect of GR

The photodegradation activity of TiO2 was significantly enhanced by TiO2/GR

composites. The activity of TiO2/GR was enhanced because GR provide large surface area

to absorb UV light. Moreover, the generated electrons were transferred to GR and the

recombination of photo generated electron–hole pairs was reduced (Figure 5.9).

The possible reaction kinetics on TiO2/GR composite is as follows:


TiO2/GR + hν TiO2 (h+) + GR (e

-)

GR (e-) + O2 GR+ O2

-

TiO2 (h+) + H2O TiO2 + OH*

OH* + MO Degradation products

The amount of GR in composite influences

the photocatalytic activity of TiO2/GR catalysts.

Figure 5.10, 5.11 and 5.12 show the degradation

curves of MO with time, under irradiation of UV

light using T5a, T5b and T5c photocatalyst. It has been

observed that the addition of GR enhance the

photocatalytic activities of TiO2 as it promote

electron–hole separation during reaction. The MO

degraded fast with increase in concentration of GR

in TiO2 (Table 5.1).

It is observed from Figure 5.10 (a) that MO degrade in 56 minutes with 0.3 g/L of

T5a as compared to 1.0 g/L which degrade MO in just 32 minutes (Figure 5.10 (c)).

Figure 5.10 UV–Visible absorption spectra of degradation of MO with T5a w.r.t. time for different amount of


Figure 5.9 Transfer of electron from TiO2 to GR

in photocatalysis process

Chapter 5: Removal of dyes by photocatalysis 1

When concentration was increased beyond 1.0 g/L the degradation time increases

which may be due to the layer of catalyst particles formed on the dye solution. Thus the

light does not efficiently reach to the solution. It was observed from Figure 5.10 (d) that for

1.5 g/L catalyst in solution the degradation take longer time. However, the degradation was

very fast in initial 10 minutes as compared to 1.0 g/L catalyst loading and after that

degradation rate was slowed down.

T5b composite has more concentration of GR as compared to T5a so it was chosen to

study out the effect of GR. As shown in Figure 5.11 the degradation time was decreased to

50 minutes for 0.3 g/L and to 48 minutes for 0.5 g/L concentration of T5b as compared to

T5a catalyst.

Figure 5.11 UV–Visible absorption spectra of degradation of MO with T5b w.r.t. time for different amount of


In case of T5c which have largest amount of GR in all T5 composites, the degradation

time was found to be minimum (30 min) for 1.0 g/L (Figure 5.12(c)). Similar studies were

also obtained in T8 composites (degradation time for T8c is given in Table 5.1). Although

there was small effect of GR in T8 due to rutile phase which does not have good


photocatalytic properties like anatase phase. Degradation of MO take longer duration (68

minutes) for T8c as compared to T5c (44 minutes) using 0.5 g/L of catalyst loading.

Figure 5.12 UV–Visible absorption spectra of degradation of MO with T5c w.r.t. time for different amount of


To find out the degradation of MO, the % degradation constant (DC) for reaction at

any time was calculated by equation:

(equation 5.4)

where Cinitial is the initial concentration and Cfinal is the concentration of MO at any

time t.

The degradation curves of TiO2 and TiO2/GR catalysts are shown in Figure 5.13,

5.14. It can be observed that degradation was more in T5 (74.2% for 0.5 g/L) shown in

Figure 5.13 (a) as compared to T8 (66.6% for 0.5 g/L) shown in Figure 5.14 (a). This shows

that the anatase phase degrade MO efficiently and hence more photoactive as compared to

rutile phase. With increasing GR concentration in TiO2, degradation increases up to 88.25%

with 1.0 g/L of T5c (Figure 5.14 (d)). Figure 5.15 shows image of decolorization of MO

dyes with T5a catalyst (1.0 g/L) in 0, 10, 15, 20, 25, 30, 32 minutes.


Figure 5.13 Degradation constant curve (%) of TiO2 annealed at 500°C (T5) and TiO2/GR composite (T5a, T5b

and T5c)

Figure 5.14 Degradation constant curve (%) of TiO2 annealed at 800°C (T8) and TiO2/GR (T8c) composite

Figure 5.15 (Left) Photograph of decolourization of MO solution containing T5a (1.0 g/L) in 0, 10, 15, 20, 25,

30, 32 minutes and (right) the characteristic UV−Vis absorption spectra of the MO degradation


5B.1 Synthesis and characterization of ZrO2 and ZrO2/GR composite

TiO2 is water soluble and thus its high dose may have side effects on the animals.

According to International Agency for Research on Cancer TiO2 dust when inhaled has

possibility of respiratory tract cancer in humans [19, 20]. The mechanism by

which TiO2 may cause cancer is not clear. TiO2 nanoparticles cause inflammatory response

and genetic damage in mice [21-23]. Thus, due to various limitations and solubility in water

TiO2 must be replaced by some other suitable photocatalyst for purification of water. ZrO2

can used in place of TiO2, since it has poor solubility in water and the bottom of conduction

band is more negative than redox potential of H+/H2. This section includes:

Synthesis and characterization of ZrO2 and ZrO2/GR composite.

Photocatalytic properties of ZrO2 and ZrO2/GR composite for the removal of

following dyes:

Methyl orange (MO)

Methylene blue (MB)

Rhodaminne B (RB)

Selection of Precursor

ZrO2 was synthesized by using the chemicals listed below:

Chemical Chemical formula Action

Zirconium oxychloride ZrOCl2.8H2O Source of ZrO2

Graphene oxide Source of GR

Hydrazine hydrate N2H4 Reducing agent

Ammonium hydroxide, Sodium

hydroxide NH4OH, NaOH To maintain pH

Hydrochloric acid HCl To maintain pH

MO C14H14N3NaO3S Dye

Methylene Blue C16H18N3SCl Dye

Rhodamine B C28H31ClN2O3 Dye

5B.1.1 Preparation of ZrO2

ZrO2 powder was prepared by combustion method at different annealing

temperatures. The flow chart of process used for the synthesis of ZrO2 is given below:

http://en.wikipedia.org/wiki/International_Agency_for_Research_on_Cancer


Figure 5.16 Flow chart for synthesis of ZrO2

To prepare ZrO2 powder, ZrOCl2.8H2O was dissolved in DI water and zirconium

hydroxide was precipitated by addition of ammonium hydroxide (maintaining constant pH

at 10.5) while stirring [23]. The resulting mixture was filtered and washed several times

with hot water. Finally, the filtered paste was kept in oven at 100oC for 12 hours and

annealed at different temperatures (400oC, 600

oC and 1000

oC) for 2 hours in furnace.

5B.1.2 Preparation of ZrO2/GR composites

The prepared ZrO2 and GO were used as precursors to synthesize the ZrO2/GR

composites. ZrO2 solution was prepared by addition of ZrO2 (1.64 g, 0.1 mol) in 100 ml DI

water. Subsequently, GO (100 mg) and hydrazine hydrate (10 µl, 0.2x10-3

mol) was added

in to it. After that, mixture was refluxed for 36 hours at 130oC. Then, the solution was

filtered and washed with deionized water and kept in oven at 80oC for drying resulting

5.7% weight fraction of GR in ZrO2.

Figure 5.17 shows the flow chart of synthesis of ZrO2/GR composite having 5.7%

weight fraction of GR. The same procedure was repeated at different concentrations of GO

(132 mg and 150 mg) keeping the constant amount (1.64 g) of ZrO2 to obtain 7.3% and

8.3% weight fraction of GR in ZrO2 respectively.

Anneal at different temperatures (400oC, 600oC and 1000oC) for 2 hours

Dry in oven for a 12 hours

Filter and wash several times

Obtain precipitate of zirconium hydroxide by addition of ammonium hydroxide while stirring

(maintaining constant pH at 10.5)

Dissolve ZrOCl2.8H2O in DI water


Figure 5.17 Flow chart of synthesis of ZrO2/GR composite

5B.2 Results and discussion

5B.2.1 Thermogravimetry Analysis

Thermal stability plots of ZrO2 and ZrO2/GR composites are shown in Figure 5.18.

Figure 5.18 TGA plots of ZrO2 and ZrO2/GR (8.3%) composite

annealed at 400oC, 600

oC and 1000

oC

TGA curves revealed that ZrO2 exhibit almost no weight loss during the whole

heating process (room temperature to 700oC). But after addition of GR (8.3%) in ZrO2, the

composite shows decrease in thermal stability with increasing temperature as compared to

Dry at 80oC

Filter and wash the solution

reflux at 130 oC for 36 hours

Add hydrazine hydrate (10 µl, 0.2x10-3 mol)

Dissolve ZrO2 (1.64 g, 0.1 mol) in 100 ml DI water and 100 mg GO in 100 ml water


pure ZrO2. The thermal stability did not reduce by more than 6% for ZrO2/GR (5.7%, 7.3%

and 8.3%)

5B.2.2 Phase analysis

XRD patterns of ZrO2 as a function of annealing temperatures are shown in Figure

5.19. As prepared ZrO2 shows the amorphous nature while sample annealed at 400oC shows

prominent peaks at 2θ = 30.18°, 35.17°, 50.07°, 60.07°, 62.61° and 74.41° corresponding to

(101), (110), (200), (211), (202) and (220) tetragonal phase (t) [24, 25] respectively.

When ZrO2 annealed at 600oC, new

peaks at 2θ = 24.25°, 28.20°, 31.47°, 34.33°,

38.40°, 44.66°, 50.18°, 60.21°, 62.85° and

78.20° corresponding to monoclinic phase (m)

and two other peaks at 30.25° and 35.28°

corresponding to tetragonal phase of ZrO2 was

observed. If annealing temperature increased

to 1000oC some new peaks at 49.26°, 57.15°,

58.13° and 71.09° corresponding to

monoclinic phase and at 31.49° corresponding

to cubic phase (Figure. 5.19) also emerged.

The XRD pattern indicates that ZrO2

transform into monoclinic structure with

annealing at higher temperature. The amount

of monoclinic, tetragonal or cubic phase

present in ZrO2 was calculated using intensity

of the monoclinic peaks (111) and (111) vs. tetragonal peak (111) using equation [26-29]:

% M=I(111)t + I(111)m + )m 1 I(11

I(111)m + )m 1 I(11

(equation 5.5)

where I(111)m, I(111)m and I(111)t are intensity of the monoclinic peaks (111),

(111) and tetragonal peak (111) respectively.

Figure 5.19 Phase analysis of ZrO2 before

and after annealing at 400oC, 600

oC

and 1000oC for 2 hours


It was found that ZrO2 annealed at 400°C was purely tetragonal in nature but when

annealed at 600°C, it has 45.5% monoclinic phase and 54.5% tetragonal phase. Monoclinic

phase increased to 92.3% in sample annealed at 1000°C. So, monoclinic phase dominate at

higher annealing temperature and at intermediate temperatures (between 400°C to 1000°C)

it transform to monoclinic phase (Table 5.2). The crystalline size of ZrO2 powder was

calculated by Debye–Scherrer formula using equation 5.2. The average values of crystalline

size of ZrO2 annealed at 400°C, 600°C and 1000°C are 14.32 nm, 26.5 nm and 45.3 nm

respectively (Table 5.2). The crystalline size of ZrO2 increase with annealing temperature

as at higher temperature growth of particles takes place.

5B.2.3 Role of GR in phase transformation

The XRD patterns of ZrO2 annealed at 400°C and with three different compositions

of GR (5.7%, 7.3% and 8.3%) are shown in Figure 5.20. The study was carried out to

investigate the effect of annealing temperature for each composition. After the

incorporation of GR in ZrO2 (400oC), it was observed that in ZrO2/GR (7.3% GR) the

monoclinic phase has slightly increased as a new peak corresponding to monoclinic phase

(m(111)) at 28.2° was appeared (Figure 5.20).

Monoclinic phase dominates in ZrO2 annealed at 600oC as compared to ZrO2

annealed at 400oC as shown in Figure 5.21 (a). The two peaks corresponding to m(111) and

m(111) appeared in ZrO2 (600oC) which were absent in ZrO2 (400

oC). This clearly

indicates that the transformation of ZrO2 from tetragonal phase to monoclinic phase took

place.

Table 5.2 Average crystalline size and phase of ZrO2 and ZrO2/GR (annealed at different temperatures)

Temp

(°C )

ZrO2 ZrO2/GR (5.7%) ZrO2/GR (7.3%) ZrO2/GR (8.3%)

Avg

D

(nm)

Phase

Avg

D

(nm)

Phase

Avg

D

(nm)

Phase

Avg

D

(nm)

Phase

400 14.32 t 24.4 t 34.3 t 25.7 t

600 26.5

45.5%(m)

54.5%(t)

30.53 69.5%(m)

30.5%(t) 27

73.3%(m)

26.7%(t) 27.2

73%(m)

27%(t)

1000 45.3

92.3%(m)

07.7%(t)

34.36 m 33.45 m 33.9 m


In case of ZrO2/GR composite (ZrO2

annealed at 600oC) for all composition of GR, the

intensity of t(101) peak was diminished as

compared to intensity of m(111) and m(111)

(Figure 5.21(a)). In ZrO2 (600oC) the intensity of

t(302) peak was diminished by coupling of GR with

ZrO2 and a new peak corresponding to m(200) was

observed at 49.2°.

A peak corresponding to cubic phase c(111)

was observed in ZrO2 annealed at 1000oC as shown

in Figure 5.21 (b). But in its composites with GR,

the intensity of c(111) peak reduced and

disappeared in sample (8.3%). Slight shift in peak

positions of ZrO2/GR composites is due to the

interactions of GR and ZrO2.

Figure 5.21 XRD patterns of ZrO2/GR showing increase in monoclinic phase with increase in the

concentration of GR when ZrO2 annealed at (a) 600oC, (b) 1000

oC

This is clear from the XRD analysis that the monoclinic phase dominate as

compared to tetragonal or cubic phase with increase in concentration of GR. The

corresponding percentage of phases and crystalline size of ZrO2 and ZrO2/GR composite

(annealed at different temperatures) is summarized in Table 5.2.

Figure 5.20 XRD patterns of ZrO2/ GR

(ZrO2 annealed at 400oC)


5B.2.4 Raman analysis

ZrO2 has eighteen (9Ag + 9Bg) Raman active modes for the monoclinic ZrO2 and six

(A1g + 2B1g + 3Eg) for the tetragonal ZrO2 [30, 31]. Raman spectrum of ZrO2 annealed at

400oC shows Raman peaks corresponding to tetragonal phase as shown in Figure 5.22 (a).

The tetragonal peaks are assigned as follows: B1g at 144 cm-1

, Eg at 267 cm-1

, 462 cm-1

and

645 cm-1

[31, 32]. Raman spectrum of ZrO2 annealed at 600oC shows Raman peaks

corresponding to monoclinic phase assigned as follows: Ag at 173 cm-1

, Ag at 306 cm-1

, Bg

at 329 cm-1

, Bg at 374 cm-1

, Ag at 469 cm-1

, Ag at 552 cm-1

and Ag at 622 cm-1

and a

tetragonal peak at 253 cm-1

(Bg).

Figure 5.22 Raman spectra of (a) ZrO2 annealed at 400oC, 600

oC and 1000

oC, (b) ZrO2/GR (8.3%) (ZrO2

annealed at 400oC, 600

oC and 1000

oC)

Raman spectrum of ZrO2 annealed at 1000oC shows monoclinic peaks

corresponding to Ag at 177 cm-1

, Bg at 333 cm-1

, Bg at 381 cm-1

, Ag at 475 cm-1

, Ag at 558

cm-1

and Ag at 637 cm-1

. Thus Raman spectra in accordance with XRD results indicate that

at 400oC the ZrO2 was tetragonal and at 1000

oC it was monoclinic but at 600

oC

combination of tetragonal and monoclinic phase is observed.

Raman spectra of ZrO2/GR composite with 8.3% of GR is shown in Figure 5.22 (b),

verify the coupling of GR with ZrO2 composites. In ZrO2/GR (ZrO2 annealed at 400oC) D

band was observed at 1330 cm-1

. G peak at 1565 cm-1

and second–order two–phonon mode

corresponding to GR at 2273 cm-1

suggest interaction between GR and ZrO2. Small shift in


the ZrO2 peaks of ZrO2/GR was due to the interaction between GR and ZrO2 [33, 34].

Similarly peaks corresponding to GR were observed at 1600 cm-1

, 1640 cm-1

, 2184 cm-1

for

ZrO2 (600oC)/GR (8.3%) and at 1345 cm

-1, 1579 cm

-1, 2167 cm

-1 for ZrO2

(1000oC)/GR(8.3%) as shown in Figure 5.22 (b).

5B.2.5 FTIR spectroscopy

FTIR spectra of ZrO2 (400oC)/GR (8.3%) is shown in Figure 5.23, in this peak

observed at 3302 cm-1

is assigned to the bending and stretching vibrations of the O–H bond

due to absorbed water molecules. The band at 1304 cm-1

was attributed to the absorption of

non–bridging OH groups. The peaks at 1034 cm-1

, 980 cm-1

, 903 cm-1

and 849 cm-1

correspond to Zr–O bonds and the peak near 680 cm-1

was typical of tetragonal phase of

ZrO2 [35]. Based on the above information, it is clear that ZrO2/GR composite has been

successfully synthesized.

Figure 5.23 FTIR spectra of ZrO2 (400oC)/GR (8.3%) composite

5B.3 Photocatalytic analysis

The photocatalytic activity of ZrO2 and ZrO2/GR was examined by analyzing the

degradation of MO, MB and RB dyes in the presence UV light. In the present work, the

effect of annealing temperature, pH, and catalyst/GR concentration on the degradation of

dyes is investigated.

Solutions of MO, MB and RB were prepared by dissolving 4 mg of dye in 500 mL

of DI water. In the MO solution 1ppm H2O2 was added to remove excess electron from the

surface but it was not required for degradation of MB and RB. The pH of the solution was

maintained to 7 for degradation study of MO. In prepared solution of dye, the different

amount (0.1 g/L, 0.3 g/L, 0.5 g/L, 1 g/L, 1.5 g/L and 1.8 g/L) of catalyst (ZrO2 or ZrO2/GR)


was added and kept under UV irradiation. Photodegradation of these dyes was studied at

different intervals of time using UV–Visible spectroscopy.

5B.3.1 Degradation of MO

Experiment performed in the absence of catalyst, indicates that slow

photodegradation of dyes take place. This may be due to the natural self–fading of the dye

or because of presence of H2O2 but this takes long time (3 hours 35 minutes for MO).

(a) Effect of catalyst concentration

Photocatalysis was carried out at different concentration of catalyst in the dye

solution as shown in Figure 5.24. The initial slope of the curves is found to decrease by

increasing catalyst loading from 0.1 g/L to 1.5 g/L thereafter the degradation time was

found to increase. This may be explained on the basis that the total active surface area of

catalyst for adsorption of dyes increases with increase in catalyst amount [36]. However, as

the amount of catalyst in the dye solution increases beyond a limit, an increase in the

turbidity of the solution as a result UV light cannot efficiently reach into the solution and

hence photo activated volume of suspension decreases [37]. Thus, high dose of catalyst is

not useful for degradation of dyes, in view of aggregation of particles and reduced light

field due to light scattering. The trend of degradation was similar for each catalyst but the

time taken for degradation depends on the amount and type of catalyst in the dye solution.

It was also observed that degradation time is large (>2 hours) for 0.5 g/L concentration of

catalysts (ZrO2) annealed at 400°C and 600°C in the dye solution. Low concentration of

ZrO2 gives poor efficiency for MO degradation. It was found that degradation takes

minimum time at 1.5g/L concentration therefore this concentration was selected for MO

degradation.

(b) Effect of annealing temperature and UV irradiation time

Figure 5.25 (a) shows the absorbance spectra of MO using pure ZrO2 (annealed at

400°C) as photocatalyst. It was observed that the photocatalytic activity shows strong

dependence on the UV irradiation time. ZrO2 surface gain more energy from UV light

source at longer exposures. Therefore, more electron hole pairs are generated on the surface

of ZrO2 resulting in fast degradation of MO. It was observed that the photodegradation

further increase with annealing. This may be due to:

Increase in monoclinic phase of ZrO2 with annealing temperature [38],


Reduction in recombination centers and

Improved crystallinity at higher temperatures.

Moreover, grain size of ZrO2 increases with annealing temperature. As a result of it

defects and impurities tend to disappear causing a reorganization of the structure [31].

Improved crystallinity is important factor for OH group formation which acts as active sites

for photodegradation [39]. The degradation of MO occurs in 110 minutes, 90 minutes, 80

minutes for ZrO2 annealed at 400°C, 600°C and 1000°C respectively (Figure 5.25 (a), 5.26

(a), 5.27 (a)).

Figure 5.24 Effect of catalysts concentration on the degradation of MO


Figure 5.25 Photocatalytic degradation of MO using ZrO2/GR catalyst (1.5g/L in dye solution) annealed at

400°C (a) without GR (b) 5.7% GR (c) 7.3% GR and (d) 8.3% GR, respectively

Figure 5.26 Photocatalytic degradation curve of MO using ZrO2/GR composite (1.5g/L in dye solution)

annealed at 600°C (a) without GR (b) 5.7% GR (c) 7.3% GR and (d) 8.3% GR, respectively


Figure 5.27 Photocatalytic degradation curve of MO using ZrO2/GR composite (1.5g/L in dye solution)

annealed at 1000°C (a) without GR (b) 5.7% GR (c) 7.3% GR and (d) 8.3% GR, respectively

(c) Effect of GR

The degradation of MO was further enhanced by ZrO2/GR composites. It was found

that the degradation of MO occurs in 100 minutes, 70 minutes, 60 minutes for ZrO2

annealed at 400°C having 5.7%, 7.3%, 8.3% GR (Figure 5.25 (b, c, d)) respectively. The

degradation time decreased from 110 minutes (for ZrO2) to 60 minutes with ZrO2/GR

(8.3%) catalyst. Lowering of the degradation time shows the effectiveness of GR for the

removal of MO. For ZrO2 annealed at 600°C having 5.7%, 7.3% and 8.3% of GR the

degradation time further reduced to 80 minutes, 60 minutes and 51 minutes respectively

(Figure 5.26 (b, c, d)). The degradation time reduced to 51 minutes for ZrO2/GR (8.3%) as

compared to ZrO2 annealed at 600°C (90 minutes). Moreover, ZrO2/GR (5.7%, 7.3% and

8.3%) in which ZrO2 annealed at 1000°C, the MO degraded in 67 minutes, 55 minutes and

45 minutes (Figure 5.27 (b, c, d)) respectively. From above data it may be concluded that

presence of GR and annealing temperature of ZrO2 enhanced the degradation rate of MO.

In the present work, prepared ZrO2/GR (8.3%) composite (ZrO2 annealed at

1000°C) reduced the degradation time by 2.45 times than the ZrO2 annealed at 400°C. The

degradation time of various samples are shown in Table 5.3.


Table 5.3 MO degradation time using ZrO2 and ZrO2/GR composites (annealed at different temperature)

Catalyst

ZrO2 Annealed

at (400°C)

Degrada

tion time

Catalyst

ZrO2 Annealed at

(600°C)

Degrada

tion time

Catalyst

ZrO2 Annealed at

(1000°C)

Degrada

tion time

ZrO2 110 ZrO2 90 ZrO2 80

ZrO2/GR(5.7%)

100 ZrO2/GR(5.7%)

80 ZrO2/GR(5.7%)

67

ZrO2/GR(7.3%)

70 ZrO2/GR(7.3%)

60 ZrO2/GR(7.3%)

55

ZrO2/GR(8.3%)

60 ZrO2/GR(8.3%)

51 ZrO2/GR(8.3%)

45

To determine the degradation of MO the % degradation constant at any time is

calculated using equation 5.4. Degradation constant curves shown in Figure 5.28 (a, b, c)

illustrate that for ZrO2 annealed at a fixed temperature, degradation of MO was larger for

composite having higher concentration of GR.

Figure 5.28 Degradation constant (%) vs. time variation of ZrO2 and ZrO2/GR having different concentration

of GR (5.7%, 7.3%, 8.3%) when ZrO2 annealed at (a) 400°C, (b) 600°C and (c) 1000°C


This may be due to the fact that GR heighten the reaction rate by separating

electron–hole pair. The e- shift to GR and react with oxygen to create superoxide oxygen

anion (O2-*). ZrO2 having excess hole (ZrO2 (h

+)) react with hydroxyl anion and change it

to hydroxyl radical (OH*). Both superoxide oxygen anion and hydroxyl anion react with

dye and degrade it [39-43]. The reaction occurs at the time of photocatalysis on the surface

of ZrO2/GR composite are shown below:

ZrO2+ hv e- + h

+ (at the ZrO2 surface)

ZrO2 (e-) + GR ZrO2 + GR (e

-)

GR (e-) + O2 GR + O2

-*

ZrO2 (h+) + OH- ZrO2+ OH*

OH* + MO dye Degradation products

Additionally, the sp2 conjugated bond in the carbon lattice of GR enhances the light

absorption range, promote charge separation [44] and increase adsorption of pollutants.

Highest degradation (98%) of MO in 45 minutes was obtained for ZrO2/GR

composite (ZrO2 annealed at 1000°C) having 8.3% of GR.

5B.4 Degradation of MB and RB dyes

In the previous section it is clearly evident that ZrO2/GR composites degrade MO

successfully. So, work was further extended to study the degradation of MB and RB dyes

from water. The photodegradation of MB and RB dyes was examined as a function of

catalyst/GR concentration and pH. Experiments were performed in the absence of catalyst

and it was observed that self fading of dyes occur in 108 minutes for MB and 90 minutes

for RB.

(a) Catalyst selection

For the photodegradation of MB and RB dyes, the prepared ZrO2/GR (ZrO2

annealed at 1000°C) catalysts having 8.3% GR concentration was chosen as evident from

the previous section. To further optimizing the catalyst, test experiments were performed

using 0.5 g/L concentration of catalyst. Figure 5.29 (a) depicts that if the concentration of

GR increased in ZrO2 annealed at 1000°C the degradation time reduces and degradation

efficiency increases. The ZrO2 having 8.3% of GR shows maximum degradation. Also, the

effect of annealing temperature was studied as shown in Figure 5.29 (b) and it was found

that composite of ZrO2 and GR (8.3%) annealed at 1000°C shows maximum degradation.


In view of above observations, ZrO2/GR (ZrO2 annealed at 1000°C) having 8.3% GR was

fixed to degrade MB and RB since both are basic dyes.

Figure 5.29 Optimization of catalyst to remove MB from water by varying (a) the amount of GR in ZrO2 (b)

annealing temperature of ZrO2

5B.4.1 Effect of catalyst concentration

To study the effect of catalyst concentration the experiments were carried out

between 0.1 g/L to 1 g/L concentration and degradation constant (%) was calculated by

equation 5.4. As shown in Figure 5.30 it was observed that degradation efficiency increases

with increase in the concentration of catalyst up to 0.5g/L and above it the effect was

reversed. As explained earlier it is due to the agglomeration of particles on the surface due

to which light does not reach to dye solution efficiently. Also excess of catalyst prevents

the formation of –OH radical essential for photodegradation [45].

Figure 5.30 Effect of catalyst concentration on degradation of MB dye (a) ZrO2 and (b) ZrO2/GR

5B.4.2 Effect of pH

Photocatalytic degradation of MB and RB was investigated as a function of pH in

the range of 3.0–12.0 keeping constant amount of catalyst (0.5g/L). The strong effect of pH

on the photo degradation efficiency of MB and RB solution was observed. The highest dye


removal rate was obtained at pH of 10 and 11 for MB and RB respectively as shown in

Figure 5.31. Above it as pH increases the degradation efficiency decreases. Thus for

efficient degradation, RB needs more basic solution as compared to MB.

Figure 5.31 Photocatalytic degradation curves (%) of MB and RB as a function of pH for (a) MB dye using

ZrO2, (b) RB using ZrO2, (c) MB dye using ZrO2/GR and (d) RB dye using ZrO2/GR

It was observed that degradation was faster in alkaline medium as compared to acidic

medium for both the dyes. This is because ZrO2 particles are either positively or negatively

charged depending upon the value of pH [46]. The point of zero charge (pzc) of ZrO2

sample was reported as pH 5.5. This means if pH < pHpzc, the surfaces of ZrO2 particles are

positively charge and if pH > pHpzc, ZrO2 particles are negatively charged. As MB and RB

are basic dyes and in water these produces cations, so catalysts (ZrO2 or ZrO2/GR) repel

MB and RB particles in acidic medium [47, 48]. Therefore degradation was less in acidic

medium. On the other hand in basic medium catalyst is negatively charged and adsorption

become possible so that degradation rate is enhanced. Moreover, in the alkaline medium the

generation of –OH radicals is easier hence degradation rate was further enhanced.


Figure 5.32 (a) shows that degradation time of MB was reduced from 108 minutes

(absence of catalyst) to 60 minutes for 0.5 g/L concentration of ZrO2 in dye solution. As

shown in Figure 5.32 (a) MB degraded to 55.89% for 0.1 g/L concentration and

degradation increased to 84.6% for 0.5 g/L concentration of ZrO2 at pH 10. The RB

degraded to 88.5% for 0.5 g/L concentration of ZrO2 at pH 11 as compared to 0.1 g/L for

which it degraded 63.2% (Figure 5.32(b)).

Figure 5.32 Photocatalytic degradation curves (%) (a) MB degradation at pH = 10 using ZrO2 as a catalyst,

(b) RB degradation at pH = 11 using ZrO2 as a catalyst, (c) for MB degradation at pH = 10 using ZrO2/GR

as a catalyst and (d) for RB degradation at pH = 11 using ZrO2/GR as a catalyst

5B.4.3 Effect of GR

To enhance the photocatalytic degradation of ZrO2, GR was used because of its high

surface area. The high surface area of GR provides more efficient absorption of light so that

the generation of –OH radical become faster.

It was found that degradation time was further reduced by using ZrO2/GR

composite as a catalyst. It was observed that catalyst dependency of ZrO2/GR was same as

that of ZrO2 but the degradation occurs fast in case of ZrO2/GR (Figure 5.30 (b)). The

degradation of MB occurs in 32 minutes with ZrO2/GR (0.5 g/L) at pH 10 as shown in

Figure 5.32 (c) as compared to ZrO2 which degrade MB in to 60 minutes with less


efficiency (Figure 5.32(a)). For RB dye, degradation takes place in 80 minutes with ZrO2

and 40 minutes with ZrO2/GR (Figure 5.32 (d)) for 0.5 g/L concentration.

It was found that the degradation efficiency for the removal of MB at pH 10

increased from 84.6% to 98.1% for 0.5g/L concentration when ZrO2 was replaced with

ZrO2/GR and for RB it was increased from 88.5% to 99%. The absorption spectra of MB

and RB degradation are shown in Figure 5.33. The variation in the color of dyes with time

is also shown in Figure 5.34. The highest degradation was found for RB (99%) than the

MB (98.1%) and MO (98%). Hence, ZrO2/GR remove dyes more efficiently compared to

ZrO2.

Figure 5.33 Absorption spectra of MB and RB using 0.5 g/ L ZrO2/GR as a catalyst

Figure 5.34 Decolorization with time in (a) MB for 0, 10, 20, 30 and 32 minutes and (b) RB for 0, 10, 20, 30

and 40 minutes using ZrO2/ GR (0.5 g/L) as a catalyst

5B.5 Comparison of degradation of dyes using different catalysts

The different types of catalysts have been reported in literature to remove MO, MB

and RB dyes. In this work, TiO2, TiO2/GR, ZrO2 and ZrO2/GR catalysts were prepared and

used to degrade the MO, MB and RB dyes. Table 5.4 shows the comparison of performance

of prepared and literature reported catalysts for the removal of MO, MB and RB dyes.


Table 5.4 Comparison of degradation of MO, MB and RB dyes using metal oxide

Catalyst Dye %

Degradation

Time

(minutes)

Concentration

(g/L) of

catalyst

References

ZnO MO 99 60 1 [49]

Copper

porphyrins-TiO2

MO 98 90 [50]

TiO2/GR MO 100 120 [51]

TiO2/Zinc MO 100 120 0.6 [52]

α-Fe2O3 MO 80 210 0.2 [53]

TiO2/GR MO 88.25 30 1.0 Performed

work

ZrO2/GR MO 98 45 1.5 Performed

work

TiO2 MB 120 0.1wt% [54]

TiO2 MB 98 56 0.9 [55]

CeO2 MB 90.6 125 1 [56]

Bismuthoxy

bromide/GO

MB 98 30 0.5 [57]

ZrO2/GR MB 98.1 32 0.5 Performed

work

α-Fe2O3 RB 90.13 40 1 [58]

Bismuthoxy

bromide/GO

RB 95 45 0.5 [57]

ZrO2/GR RB 99 40 0.5 Performed

work

5.2 Conclusion

TiO2/GR composites were used to degrade MO from the water using photocatalysis

process. The rate of photo degradation increases with increase in catalyst dose up to an

optimum loading, after which degradation of MO decreases as higher concentration act as a

barrier to transfer UV light in to the solution. The degradation time decreases with increase

in the concentration of GR in TiO2 since, GR efficiently separate out the electron hole pair

by capturing the electrons during reaction. Degradation of MO was achieved up to 88.25%

in 30 minutes with T5c as compared to TiO2 which degrade 81.25% MO in 46 minutes.

ZrO2 catalyst of crystalline size in the range of about 14–45 nm was synthesized by

combustion method. It was found that the phase of ZrO2 changes from tetragonal to

monoclinic with increasing annealing temperature. Further, its properties were modified by

preparing its composites with GR. It was observed that GR does not cause any significant

change in the thermal stability but affects the structural properties of ZrO2. ZrO2/GR


composite shows presence of monoclinic phase at lower annealing temperature as

compared to ZrO2. Raman analysis indicates that inside the composite, GO was

successfully reduced to GR and there was strong interaction between ZrO2 and GR.

Photocatalytic degradation of the three textile dyes using ZrO2 photocatalyst is

found to depend on the amount of catalyst, concentration of dye, pH and concentration of

GR. The degradation of MO was up to 98% using ZrO2/GR (8.3%) in which ZrO2 annealed

at 1000oC. The degradation of RB and MB dyes is enhanced in alkaline medium. ZrO2 was

found to be more effective than TiO2 due to its large band gap and non-corrosive nature. In

comparison of MO, the degradation of MB and RB was easier as there was no requirement

of continuous supply of oxygen or addition of hydrogen peroxide for removal of excess

electron from the surface. Hence, ZrO2/GR may be a viable solution for the treatment of

large volume of textile wastewater.


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