Accepted Manuscript Enhancement of visible light photocatalytic activity of CdO modified ZnO nanohybrid particles S. Sudheer Khan PII: DOI: Reference:

S1011-1344(14)00328-5 http://dx.doi.org/10.1016/j.jphotobiol.2014.11.001 JPB 9872

To appear in:

Journal of Photochemistry and Photobiology B: Biology

Received Date: Revised Date: Accepted Date:

12 September 2014 30 October 2014 3 November 2014

Please cite this article as: S. Sudheer Khan, Enhancement of visible light photocatalytic activity of CdO modified ZnO nanohybrid particles, Journal of Photochemistry and Photobiology B: Biology (2014), doi: http://dx.doi.org/ 10.1016/j.jphotobiol.2014.11.001

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1

Enhancement of visible light photocatalytic activity of CdO modified ZnO

2

nanohybrid particles

3 4

S. Sudheer Khana,b,*

5 6

a

7

Kumbakonam-612 001, Tamil Nadu, India.

8

b

9

Biotechnology, SASTRA University, Thanjavur- 613 401, Tamil Nadu, India.

Department of Chemistry and Biosciences, Srinivasa Ramanujan Centre, SASTRA University,

Centre for Nanotechnology and Advanced Biomaterials, School of Chemical and

10 11

*Corresponding author:

12

Dr. S. Sudheer Khan

13

Assistant Professor (Research)

14

Centre for Nanotechnology and Advanced Biomaterials, School of Chemical and Biotechnology,

15

SASTRA University, Thanjavur- 613401, Tamil Nadu, India.

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Email: [email protected]; [email protected]

17

Phone: 91 9751275798

18 19 20 21 22

1

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Abstract

24

Highly effective ZnO-CdO nanohybrid particles were synthesized via a hydrothermal co-

25

precipitation method and characterized by X-ray diffraction (XRD), field emission-scanning

26

electron microscopy (FESEM), particle size analyzer, zeta sizer and Brunauer–Emmett–Teller

27

(BET) surface area analysis. ZnO-CdO-3:1 nanohybrid photocatalyst exhibited significantly

28

enhanced photostability and photocatalytic activity for the degradation of methylene blue. The

29

formation of hydroxyl radicals was measured by photoluminescence (PL) spectroscopy.

30

Photocatalytic efficiency of ZnO was high at ZnO-CdO ratio of 3:1. Furthermore, this work

31

provides an insight into the development of a new photocatalyst for the degradation of organic

32

contaminants.

33 34

Keywords: ZnO-CdO nanohybrid particles; Methylene blue; Visible light; Photocatalysis;

35

Photodegradation.

36 37 38 39 40 41 42 43 44 45

2

46

1. Introduction

47

Photocatalysis has received considerable attention in recent years owing to its ability to

48

mineralize many organic pollutants in a short span of time and eliminate the environmental

49

pollution. Among the semiconductors, ZnO and TiO2 exhibit good photocatalytic performances

50

due to their high photosensitivity [1, 2]. However, the photocatalytic efficiency in

51

semiconductors is greatly affected due to the quick recombination of charge carriers. The

52

electron-hole pairs generated in semiconductor by optical irradiation can be easily recombined

53

due to the direct band gap [3]. Therefore, the photo induced charge separation in semiconductors

54

is important to improve the photocatalytic efficiency. Notable progress has been made in the

55

fabrication of various semiconductors to improve the photocatalytic efficiency, including ZnO

56

and their composites [4, 5]. Nanomaterials have ultra-fine sizes with high surface area which

57

make them an important candidate in catalysis [6]. A nanocomposite can be briefly described as

58

a matrix to which nanoparticles are added in order to improve a particular property of the

59

material [7, 8]. The preparation of complex nanostructures is a challenging field and the

60

development of nanohybrid particles by integrating different components is an advanced topic in

61

the field of materials science [9, 10].

62

Photocatalysis is a green technology and this process can be used to clean the

63

environmental contaminants with the help of solar energy [11, 12]. In this view, the development

64

of an efficient photocatalyst is inevitable for the efficient utilization of solar spectrum and

65

artificial visible light sources. In this work, we reported a facile method to synthesize ZnO-CdO

66

nanohybrid particles without adding any linking molecules. The photocatalytic efficiency of the

67

ZnO-CdO hybrid nanoparticles was evaluated based on the ability to degrade methylene blue

68

(MB). MB is a commonly used as a model pollutant to evaluate the photocatalytic performance

3

69

of particles [13, 14]. The degradation mechanism of MB is well known, hence we chosen MB as

70

a model dye molecule. The photocatalytic performance of the hybrid particles was evaluated, and

71

the experimental results indicate that the photocatalytic activity of ZnO is influenced by the

72

density of CdO. It is demonstrated that the catalytic activity firstly increases and then decreases

73

as CdO NPs amount increases. In addition, it is found that ZnO-CdO with optimal density (3:1)

74

exhibited excellent photocatalytic activity. The ZnO-CdO nanohybrid particles provide a new

75

insight to enhance the photocatalytic performance of particles by developing desirable

76

nanohybrid particles.

77 78

2. Materials and methods

79

2.1. Materials

80

ZnSO4, Cd(NO3)2 and NaOH were purchased from Merck, India. All the chemicals used

81

were of analytical grade. All the experiments were performed in triplicate. The UV-visible

82

spectrophotometer used in the study was purchased from Elico, India.

83 84

2.2. Synthesis of NPs

85

ZnSO4 and Cd(NO3)2 were used as precursors for ZnO and CdO respectively. 0.1 M

86

concentration of ZnSO4 and Cd(NO3)2 were prepared separately and mixed in a ratio of 3:1, 2:2

87

and 1:3 and were named as ZnO-CdO-3:1, ZnO-CdO-2:2 and ZnO-CdO-1:3 respectively. The

88

solution without Cd(NO3)2 was used to prepare ZnO NPs and the solution without ZnSO4 was

89

used to prepare CdO NPs. The flask containing solution was placed in a magnetic stirrer and the

90 91

8

C. The equal volume of 0.4 M concentration of

NaOH was added drop wise to the above solution followed by vigorous stirring for 3 h. The

4

92

T

precipitate was centrifuged 6

93

C.

94 95 96

2.3. Characterization of NPs. Field emission scanning electron microscopy (JEOL JSM-6701F, Japan) and

97

transmission electron microscopy (TEM, Tecnai G-20) were used to observe the surface state,

98

morphology and structure of ZnO-CdO at a magnification level of 6 k with an acceleration

99

voltage of 3-35 kV. The ZnO-CdO was coated in XRD grid and the spectra were recorded using C Kα

100

Bruker diffractometer (D8 Focus) operated at voltage of 40 kV

101

surface area was measured using a Smart Sorb 93 Single point BET surface area analyzer (Smart

102

Instruments Co. Pvt. Ltd., Mumbai, India). XPS equipped with an Al Kα X-ray source at energy

103

of 1486.6 eV was employed at pressure less that 10−7 Pa to investigate the chemical composition

104

of the synthesized materials. The hydrodynamic size distribution of NPs was determined using

105

Malvern particle size analyzer (Malvern, UK). The zeta potential of NPs was measured using

106

Malvern zeta sizer (Malvern, UK).

The

107 108

2.4. Photocatalytic measurement

109

Methylene Blue (MB) was used as the model organic dye to evaluate the photocatalytic

110

activity of the ZnO-CdO nanohybrid particles. A 300 W halogen lamp was used as the light

111

source providing visible light irradiation for the photodegradation process. For comparison,

112

commercial P25 TiO2 powder was adopted as the reference and tested under the same

113

experimental condition. 2 mg of particles were suspended into 20 mL of 10 mg/L MB solution

114

and stirred for 30 min in the dark to reach an adsorption/desorption equilibrium for MB at the 5

115

surface of catalysts. Then the solution was exposed to visible light irradiation. At specific time

116

intervals, small aliquots were collected, centrifuged, and then filtered to remove the catalyst

117

particles. The MB left in the solution was quantified by measuring the absorbance at 665 nm

118

using UV–visible spectrophotometer.

119

The rate of ·OH formation during photocatalytic degradation under visible light was

120

evaluated by the photoluminescence technique described by Xiang et al. [15]. The excitation

121

wavelength and the scanning speed were adjusted to 332 nm and 1200 nm/min respectively.

122

After visible light irradiation, the solution was filtrated to measure the photoluminescence

123

intensity at 456 nm.

124 125

3. Results and discussion

126

3.1. Characterization of NPs

127

The different ratio of ZnSO4 and Cd(NO3)2 was used to prepare ZnO-CdO nanohybrid

128

particles. The hydrodynamic size distribution ZnO-CdO was determined using Malvern particle

129

size analyzer, UK by dynamic light scattering method. Fig. 1a shows the size distribution of

130

ZnO-CdO-3:1 and the mean diameter of ZnO-CdO-3:1 was determined to be 35 ± 3 nm. The

131

mean diameter of ZnO-CdO-2:2, ZnO-CdO-1:3, ZnO and CdO were determined to be 40 ± 2, 45

132

± 3, 30 ± 2 and 45 ± 4 respectively. The stability of the particles was determined based on zeta

133

potential. The zeta potential of ZnO-CdO-3:1, ZnO-CdO-2:2, ZnO-CdO-1:3, ZnO and CdO were

134

determined to be -29.51 ± 2.13, -32.34 ± 1.8, -31.56 ± 2.1 and -31.94 ± 1.9 respectively.

135

Scanning electron microscope (SEM) image of ZnO-CdO-3:1 is shown in Fig. 1b. Hexagonal

136

shaped particles were observed in TEM image and the image is displayed in Fig. 1c. The surface

137

area of the prepared NPs was determined to be 26.5 m2/g. Surface characterization of the ZnO-

6

138

CdO has been carried out by XPS and the survey spectra indicated the presence of zinc, oxygen

139

and cadmium. The spectra of the Cd (3d) that consists of the Cd (3d5/2) and Cd (3d3/2) spin orbit

140

components, located at binding energy of 405.1 and 411.9 eV, respectively. The spectra of Zn

141

(2p) that consists of the Zn (2p3/2) and Zn (2p1/2) spin orbit components located at binding energy

142

of 1022.3 and 1045.4 eV, respectively. The binding energy of the Cd (3d) and Zn (2p) are

143

attributed to the Cd2+ bonding state of the CdO and Zn2+ of the ZnO respectively. The phase

144

purity and crystallographic structure of the particles were determined by using X-ray diffraction

145

(XRD) analysis (Fig. 1d). XRD

146

assigned to (100), (002), (101), (102) and (110) reflection lines of hexagonal ZnO. 2θ

147

33.4 and 66.9 are assigned to (111) and (311) crystalline plane of CdO. Debye-Sc

148

formula was used to derive the average crystallite sizes of ZnO-CdO and the crystallite sizes was

149

calculated to be around 34 ± 3 nm. The Williamson–Hall equation was used to find the effect of

150

strain as well as size of the NPs.

2θ

of 31.8, 34.4, 36.3, 47.7 and 56.6 were

151

’

(1)

152

where β

153

c

154

I

x c

z βc θ = λ

c

c c

k

λ=1 54 6 Å

θ = λ,

θ

,D

η X-

des a total strain in the NPs

155

whereas the inverse of interception on the Y axis offers effective particle size. The tensile strains

156

of ZnO-CdO amounting to 2 % contributed strongly to the broadening of the XRD pattern. The

157

deduced effective particles size from the same plot is of 31 ± 3 nm. Since the broadening of the

158

XRD pattern was influenced by the strain, the average size of ZnO-CdO was found be less than

7

159

the particle size calculated using the Scherrer's formula. Similar crystallographic structure was

160

observed in the XRD pattern of ZnO-CdO-2:2 and ZnO-CdO-1:3.

161 162

Fig. 1. (a) Particle size distribution, (b) scanning electron microscopic image, (c) transmission

163

electron microscopic image and (d) XRD pattern of ZnO-CdO-3:1 hybrid nanoparticles.

164 165

3.2. Photodegradation process

166

To understand the relationship between the CdO contents and the photocatalytic property,

167

the photocatalytic activity of the hybrid particles has been investigated under visible-light

168

irradiation. MB was adopted as a representative organic dye to evaluate the photocatalytic

169

performance. The photocatalytic capability of ZnO-CdO was evaluated based on the degradation

170

of organic dye methyl blue (MB). The concentration (C) of MB solution was characterized by

171

UV–

172

monitoring the normalized change in the absorption spectra as a function of irradiation time in

173

the presence of nanoparticles under visible light. The concentration of MB was remained

174

unchanged under dark condition, indicating that less than UV-detectable amount of MB was

175

decomposed (data not shown). Furthermore, illumination in the absence of ZnO-CdO did not

176

result in the photocatalytic decolorization of MB. Fig. 2 shows the reduction in the absorption

177

spectra of MB ( c

178

exposure under visible light. The progression of the catalytic reduction of MB can be easily

179

c

c

c

λmax = 665 nm. The photodegradation of MB was demonstrated by

c

λmax value) with respect to time due to ZnO-CdO

c

(c

λmax value). Evidently, the absorption

180

spectra decreased gradually with reaction time, together with the color change from blue to

181

colorless. The decrease in absorption spectra is probably due to the degradation of MB 8

182

chromophore [16]. The degradation of MB without photocatalysts is negligible under visible

183

light illumination. The intersection of these two curves C/C0 and 1-C/C0 indicates the time taken

184

for the concentration of MB to decrease by half (half-life period of MB). The half life period of

185

MB by ZnO-CdO-3:1 was comparatively faster than ZnO-CdO-2:2, ZnO-CdO-1:3, ZnO and

186

CdO (Fig. 2). The half life period of MB by nanohybrid particles was determined to be 50, 104,

187

83, 70 and 108 min for ZnO-CdO-3:1, ZnO-CdO-2:2, ZnO-CdO-1:3, ZnO and CdO respectively.

188

Here, ZnO-CdO in the ratio 3:1 showed efficient photocatalytic performance than other particles

189

owing to the difference in band gap. The results show that the presence of CdO reduced the band

190

gap energy and thereby increase the photocatalytic performance of ZnO under visible light

191

irradiation. Samadi et al. [17] found that addition of CdO to the ZnO nanofibers resulted in band

192

gap narrowing and exhibited a better photocatalytic activity under visible light irradiation. Few

193

other studies were also reported the reduction in band gap energy of mixed ZnO and CdO

194

composition as compared to pure ZnO [18, 19]. The comparison of photocatalytic effect of

195

particles at different condition is shown in Fig. 3. Pant et al. [20] reported that ZnO nanoflower

196

containing TiO2 NPs was found to be an effective photocatalyst. Yin et al. [21] studied the

197

photocatalytic effect of Ag/ZnO nanocomposites and they found a good photocatalytic

198

performance with composite particles. Similarly our results also showed excellent photocatalytic

199

performance for hybrid particles. The shape-dependent photocatalytic activity by controlling the

200

photocatalyst crystal facets has received attention in recent years [22]. Few artificial

201

heterogeneous Z-scheme photocatalytic systems are reported that can mimick the natural

202

photosynthesis process which overcome the drawbacks of single-component photocatalysts and

203

satisfy those aforementioned requirements [23].

204

9

205

Fig. 2. Absorption spectral changes and photodegradation of MB by a) ZnO-CdO-3:1, b) ZnO-

206

CdO-2:2, c) ZnO-CdO-1:3, d) ZnO and e) CdO NPs under visible light.

207 208

Fig. 3. UV–visible absorption spectra of MB in presence and absence of nanoparticles after 120

209

min of visible light irradiation.

210 211

The proposed mechanism of photocatalytic activity of ZnO-CdO is as follows. The

212

photocatalytic activity is based on the band gap of photocatalyst, the oxidation potential of

213

photogenerated holes, and the efficiency in separation of photogenerated holes and electrons

214

[24]. The ZnO-CdO acts as a source for electron and hole. When ZnO-CdO is irradiated under

215

visible Light source, the photogenerated electrons will transfer from the valence band to the

216

conduction band, leaving the holes in the valence band. The electrons on the conduction band

217

(CB) relax to the defect level and then react with electron acceptors O2 to form superoxide anion

218

radicals O2.-, and the holes in valence band (VB) react with water to form hydroxyl radicals OH.-

219

[25, 26]. The photocatalytic activity of ZnO-CdO is mainly based on the amount of O2.- formed

220

[27]. But the electrons for the formation of O2.- can be lost by the recombination of electron-hole

221

pairs. The hydroxyl radicals formed due to the oxidation of adsorbed water molecules or

222

adsorbed OH-, is the primary oxidant that can degrade Methylene Blue. The presence of oxygen

223

prevents the recombination of electron-hole pairs. CdO act as sinks of photogenerated electrons

224

and induce a shift of the Fermi level toward more negative potentials [28]. The ZnO-CdO

225

interface can transfer electrons from ZnO to CdO by a charge equilibration process and lower the

226

electron-hole pair recombination to enhance the photocatalytic activity.

227

10

•h+ + e-

228

ZnO-CdO + ν

229

H2O + h+

230

O2 + e-

O2 -

(4)

231

O2·- + H+

HOO·

(5)

232

e- + HOO· + H+

233

H2O2 + e-

234

OH- + h+

235

MB + •OH

236

These oxidizing substances, as compare to common oxygen molecules, have higher reaction

237

activity which can completely destroy organic pollutants in wastewater [29].

(2)

•OH + H+

(3)

H2O2

(6)

·OH + OH-

(7)

•OH

(8)

CO2 + H2O

(9)

238

The photocatalytic degradation of MB by different particles observed under different

239

conditions were compared and the results are displayed in Fig. 4. The figure shows the reduction

240

in MB concentration with respect to time under visible light irradiation. Blank indicates the MB

241

absorbance after the irradiation at the same condition. ZnO-CdO-3:1 exhibited greater activity

242

than other particles.

243

The rate of MB degradation by nanoparticles under visible light irradiation could be

244

compared in terms of first-order rate constants. It can be determined by measuring the intensity

245

of λmax = 665 nm. The photocatalytic reaction rate depends on concentration of the MB and can

246

be described by the following kinetic model.

247

rate  

dC dt



kKC 1  KC

(10)

11

248

where C is concentration of MB (mol/L) at any time, t is the irradiation time, k is first-order rate

249

constant of the reaction and K is adsorption constant. This equation can be simplified to a

250

pseudo-first-order equation:

251

ln

C  kKt  k obst C0

(11)

252

in which kobs is the observed first-order rate constant of the photodegradation reaction which can

253

be calculated using the plot of ln C/C0 versus illumination time [30]. CdO-ZnO showed a

254

reaction rate constant of 62 x 10-4 min-1, which was comparatively higher than the rate constants

255

of ZnO-CdO-2:2 (32 x 10-4 min-1), ZnO-CdO-1:3 (39 x 10-4 min-1), ZnO (48 x 10-4 min-1) and

256

CdO (28 x 10-4 min-1). The reduction rate of MB was faster in the order of ZnO-CdO-3:1> ZnO>

257

ZnO-CdO-1:3> ZnO-CdO-2:2> CdO. The higher efficiency of ZnO-CdO-3:1 can be explained

258

by two reasons: one is that CdO act as electron traps to impede electron-hole pair recombination

259

[31] and the other one is that Fermi level equilibration between CdO and ZnO may decrease the

260

band gap of ZnO and hence diminishes the rapid electron-hole pair recombination [32, 33]. The

261

above results reveal that the photocatalytic efficiency of ZnO-CdO-2:2 and ZnO-CdO-1:3 were

262

comparatively less than the photocatalytic performance of ZnO NPs. It may be the reason that

263

higher content of CdO remarkably inhibit the photocatalytic efficiency of ZnO-CdO-2:2 and

264

ZnO-CdO-1:3. The main reason for the reduction in photocatalytic performance is that higher

265

amount of CdO reduced the surface contact area between MB and ZnO-CdO. From the above

266

results, we suggest that the density of CdO on ZnO is an important factor for the enhancement of

267

photocatalytic activity. The ZnO-CdO ratio of 3:1 showed excellent efficiency than other ratios

268

of ZnO-CdO. Higher CdO content could be an obstacle to photocatalytic performance. The

269

reason is that when the amount of CdO is below the optimum density, CdO act as electron–hole

270

separation centers, which improves the photocatalytic performance of ZnO. When the amount of 12

271

CdO is higher than the optimum density CdO act as electron-hole recombination center, thus

272

decreasing the photocatalytic performance of ZnO. Similar result were observed by Sun et al.

273

[34] which suggests that the electron-hole pair recombination was increased when a large

274

number of Au NPs were attached onto ZnO surface resulting in reduced charge separation

275

efficiency and photocatalytic performance. Similar results were observed when ZnO was doped

276

with Ta and Eu [35, 36]. T

277

0.26 µM/h for ZnO-CdO-3:1, ZnO, ZnO-CdO-1:3, ZnO-CdO-2:2 and CdO respectively. Here, it

278

can be seen that the formation of hydroxyl radicals were reduced at higher density of CdO

279

thereby reducing the rate of MB degradation. The stability and recyclability of ZnO-CdO was

280

evaluated for 6 consecutive cycles and the particles were found active for every cycle with

281

complete transformation of NPs. The reduction in photocatalytic activity was negligible even in

282

the 6th cycle compared to the first cycle. The photostability of ZnO is attributed due the presence

283

of ZnO. Dai et al. [37] studied the photostability of Ag2CO3 under visible light, and they found

284

that the photocorrosion of Ag2CO3 was efficiently inhibited by the addition AgNO3 in the

285

photocatalytic reaction system. Hu et al. [38] reported that coating of NPs enhanced the

286

photostability and photocatalytic activity.

•OH

43, 0.37, 0.32, 0.28 and

287 288

Figure. 4. Photodegradation of MB under visible light at different conditions and linear plots of

289

lnC/C0 for the photodegradation.

290 291

In order to find the optimal photocatalytic performance, a series of experiments was

292

carried out on different concentration of ZnO-CdO-3:1 and the result are displayed in Fig. 5. The

293

degradation of MB by ZnO-CdO-3:1 decreased with increase in concentration. It was further

13

294

confirmed by evaluating the rate constant for the degradation reaction by plotting ln C/C 0 versus

295

time. It could be seen that the rate constant for the reaction increases with increasing

296

concentrations of CdO-ZnO-3:1. The rate constant for 2, 4, 6, 8 and 10 mg of ZnO-CdO-3:1 was

297

determined to be 18 x 10-4, 26 x 10-4, 36 x 10-4, 44 x 10-4 and 62 x 10-4 min-1 respectively. It is

298

revealed that the reaction rate constants increased with increasing amount of catalyst. The

299

formation

300

mg of ZnO-CdO-3:1 respectively.

•OH

to be 0.43, 0.59, 0.78, 0.95 and 1.12 µM/h for 2, 4, 6, 8 and 10

301 302

Figure 5. Photodegradation of MB at different ZnO-CdO-3:1 concentration and linear plots of ln

303

C/C0 for the photodegradation.

304 305

4. Conclusion

306

In summary, we have presented a simple method to prepare ZnO-CdO nanohybrid

307

particles, which showed better photocatalytic efficiency under visible light. The improved

308

photocatalytic efficiency is attributed due to CdO loading to ZnO, which decreased the

309

recombination of electrons and holes. The CdO density influenced the photocatalytic efficiency,

310

and ZnO-CdO ratio of 3:1 showed the highest photocatalytic activity. It is believed that as-

311

synthesized ZnO-CdO-3:1 is stable and efficient candidate for the environmental purification of

312

organic pollutants in aqueous solution.

313 314 315 316

Acknowledgement Financial support from Science and Engineering Research Board, Department of Science and Technology, Government of India (SB/FT/LS-281/2012) is gratefully acknowledged.

14

317 318

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17

Figures Figure 1

1

Figure 2 0 min 20 min 40 min 60 min 80 min 100 min 120 min

a

1.2 0.8 0.6 0.4

0.8 0.6 0.4 0.2

0

0 550 650 Wavelength (nm)

0

750 0 min 20 min 40 min 60 min 80 min 100 min 120 min 140 min 160 min

Absorbance

20

40

60 80 100 120 Time (min)

1.2 C/C0 & 1-C/C0

450

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

C/C0 1-C/C0

1

0.8

0.6 0.4 0.2 0

450

Absorbance

1

0.2

b

C/C0 1-C/C0

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

0

550 650 750 Wavelength (nm) 0 min 20 min 40 min 60 min 80 min 100 min 120 min

c

20 40 60 80 100 120 140 160 Time (min)

1.2 C/C0

1

1-C/C0

C/C0 & 1-C/C0

Absorbance

1

1.2 C/C0 & 1-C/C0

1.4

0.8 0.6 0.4 0.2 0

450

550 650 Wavelength (nm)

750

0

2

20

40 60 80 Time (min)

100

120

C/C0 1-C/C0

C/C0 & 1-C/C0

1 0.8 0.6 0.4

0 550 650 Wavelength (nm)

750

0

0 min 20 min 40 min 60 min 80 min 100 min 120 min 140 min 160 min

e

1.2 Absorbance

1.2

0.2 450

1.4

0 min 20 min 40 min 60 min 80 min 100 min 120 min

d

1 0.8 0.6

0.4 0.2

1.2

20

40 60 80 Time (min)

100

120

C/C0 1-C/C0

1 C/C0 & 1-C/C0

Absorbance

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

0.8 0.6 0.4 0.2 0

0

450

550 650 Wavelength (nm)

0

750

3

20 40 60 80 100 120 140 160 Time (min)

Figure 3 1.4 Blank 1.2

CdO ZnO-CdO-2:2

Absorbance

1

ZnO-CdO-1:3 0.8

ZnO ZnO-CdO-3:1

0.6 0.4 0.2 0

400

500

600 Time (min)

700

4

800

Figure 4 1.2

Blank CdO ZnO-CdO-2:2 ZnO-CdO-1:3 ZnO ZnO-CdO-3:1

1

C/C0

0.8 0.6 0.4 0.2 0 0

20

40

60 80 Time (min)

100

120

0.1 0 0

50

100

150

lnC/C0

-0.1 -0.2 Blank CdO ZnO-CdO-2:2 ZnO-CdO-1:3 ZnO ZnO-CdO-3:1

-0.3 -0.4 -0.5 -0.6 -0.7

Time (min)

5

Figure 5 Blank

1.2

ZnO-CdO-100

1 C/C0

ZnO-CdO-80

0.8

ZnO-CdO-60

0.6

ZnO-CdO-40 ZnO-CdO-20

0.4 0.2 0

0

20

40

60

80 100 120

Time (min) 0.1 0

lnC/C0

-0.1 0

50

100

150

-0.2 -0.3 -0.4

Blank

-0.5

ZnO-CdO-100

-0.6

ZnO-CdO-80

-0.7

ZnO-CdO-60

-0.8

Time (min)

ZnO-CdO-40 ZnO-CdO-20

6

Research Highlights



ZnO-CdO nanohybrid particles have been synthesized by hydrothermal co-precipitation method.



ZnO-CdO-3:1 nanohybrid particles exhibited excellent photocatalytic activity under visible light.



Higher density of CdO decreased the photocatalytic efficiency.