Influence of stabilizing agents on the microstructure of Co-nanoparticles for removal of Congo red

Accepted Manuscript Influence of stabilizing agents on the microstructure of Co-nanoparticles for removal of Congo red Ommer Bashir, Mohammad Naved Khan, Tabrez Alam Khan, Zaheer Khan, Shaeel Ahmed AL-Thabaiti

PII: DOI: Reference:

S2352-1864(17)30233-X http://dx.doi.org/10.1016/j.eti.2017.07.005 ETI 142

To appear in:

Environmental Technology & Innovation

Received date : 16 August 2016 Revised date : 15 March 2017 Accepted date : 17 July 2017 Please cite this article as: Bashir, O., Khan, M.N., Khan, T.A., Khan, Z., AL-Thabaiti, S.A., Influence of stabilizing agents on the microstructure of Co-nanoparticles for removal of Congo red. Environmental Technology & Innovation (2017), http://dx.doi.org/10.1016/j.eti.2017.07.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

Highlights ● α- cobalt nano materials synthesized in presence and absence of stabilizers. ● Surfactants (CTAB ) and polymer (PVA) has significant impacts on the morphology. ● The efficiency of cobalt nano materials improved in presence of NaBH4 for the reduction of Congo red.

*Revised Manuscript with No Changes Marked

1 2

Influence of stabilizing agents on the microstructure of Co-nanoparticles for removal of Congo red

3 4

Ommer Bashir, Mohammad Naved Khan, Tabrez Alam Khan

5

Department of Chemistry, Jamia Millia Islamia, New Delhi-110025, India

6 7

Zaheer Khan*, Shaeel Ahmed AL-Thabaiti

8 9

Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah, 21589, Saudi Arabia

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

*Corresponding author. E-mail : [email protected] (Z. Khan).

2 31

Abstract

32

Removal of Congo red is a serious environmental problem. Metal nanoparticles are

33

emerging as a efficient catalysts and/or adsorbents to the degradation of dyes due to

34

the large surface area. UV–visible absorption spectroscopy was used to quantify the

35

decolorization of Congo red by Co-nanoparticles with and with out sun light at

36

different time intervals. The experimental results show that the 100 % Congo red (

37

2.1×10-4 mol dm-3 ) can be mineralized after ca. 1 min with Co-nanoparticles ( 3.2 ×

38

10-3 mol dm-3) in presence of sun light irradiation. The degradation pathway of

39

Congo red under the optimal experimental conditions is also proposed and discussed.

40

In this paper, a simple and nonexpensive method was used to the synthesis of Co-

41

nanoparticles in absence and presence of stabilizers. The morphology , stability and

42

color of cobalt sols strongly depends on the nature and/or presence of stabilizers.

43

Transmission electron microscopic data revealed that the Co-nanoparticles possesses

44

multi-layered aggregated sheet, and dumbly shaped

45

stabilizers.

46

Keywords: Co-nanoparticles; Congo red; Catalytic degradation; Stabilizers

47

Capsule: Cobalt nanoparticles synthesized with and without stabilizers, and

48

used as a adsorbent to the removal of Congo red in absence and presence of sun

49

light.

50 51 52 53

morphology with different

3 54

1. Introduction

55

Molecular structure of dyes contained mutagenic and carcinogenic aromatic coal tar-

56

based hydrocarbon(s). Industrial wastes contained dyestuffs and other coloring

57

materials, which can be mixed with surface water and then they may bring a chief

58

threat to human health. Therefore, it is necessary to prepare a suitable adsorbent for

59

the removal of the dye pollutions from our water resources. Synthesis, and structural

60

evaluation of cost effective, environmental friendly and higher efficient advanced

61

nano materials for the removal of toxic industrial wastes, especially organic dyestuffs

62

have been the subject of various investigations (Chiou et al., 2004; Gong et al., 2005;

63

Mittal et al., 2009; Cheng et al., 2011; Gupta et al., 2013). Size-dependent reactivity

64

and large surface area of nanomaterials lead to their use as efficient catalysts ( Pal et

65

al., 1998). The methods of preparations, and presence of stabilizers have significant

66

impact on the morphology (nanocones , nanoplates, nanodiscs , nanorods, nanosheets,

67

nanoneedles, nanowires , nanoflake, and butterfly-like structures ) of cobalt nano

68

materials (Jeevanandam et al., 2000; Hosono et al., 2005; Yang et al., 2010; Yan et

69

al., 2011; Wang et al. 2011), which also have potential applications in different

70

technologies, such as catalysts (Meyn et al., 1990), adsorbents (Nedez et al., 1996),

71

composite materials (Xue et al., 2003), ceramics (Philipse et al. 2004), rechargeable

72

batteries (Faure et al., 1991), gas sensing (Frost and Wain, 2008), ionic exchangers

73

(Liu et al., 2006), magnetic materials (Zhang et al., 2008). It has been established that

74

cobalt hydroxides exist in α-, and β- polymorphic forms (hydrotalcite-, and brucite-

75

like morphologies, respectively) and have a hexagonal layered structure (Gedanken et

76

al. , 2000 ; Zhu et al., 2002 ; Kobayashi et al., 2003; Sahiner et al., 2010).

77

The chemical literature contains abundant reports aimed towards understanding the

78

role of stabilizers in the synthesis and characterization of CONPs and/or its alloy with

4 79

and with out doped ( CoFe2O4, CoFe2-xGdxO4, and CoxCu1-X TiO3 ) having different

80

morphologies under various experimental conditions (Pouretedal et al., 2010; ; Liang

81

and Zhao, 2012; Wang et al. 2014; Hashemian and Foroghimoqhadam, 2014; Ding et

82

al., 2015). These investigators also used Co-nonmaterials for the degradation of congo

83

red with and without sunlight irradiations. The polyhedral Cu2O nanoparticles has

84

been used for the adsorption removal of Congo red from aqueous solution (Wang et

85

al. 2015).

86

Tri cobalt tetraoxide nanocubes have been prepared by a simple hydrothermal

87

reaction under external magnetic fields (Wang et al., 2011). Synthesis of uniform

88

cobalt nanoparticles by the reduction of CoCl2 with NaBH4 inside

89

micelles of didodecyldimethylammonium bromide has been reported (Chen et al.

90

1994). Synthesis of trioctylphosphine-coated 2D super lattices of magnetic cobalt

91

nanomaterials was discussed (Pileni et al., 1998, 1999). They used stiochiometric

92

ratio (1: 2) of cobalt bis(2-ethylhexyl)sulfosuccinate) and NaBH4 in two micellar

93

solution of same surfactant having the 0.25 mol dm-3 diameter, sodium bis(2-

94

ethylhexyl) sulfosuccinate and discussed their self-organization predisposition, which

95

converted nanoparticles into 2D superlattices. Generally, Co2+ - NaBH4 redox system

96

with and without stabilizers (organic solvents, mixture of surfactants, silica, ) was

97

used to the synthesis of stable cobalt nanoparticles (Kobayashi et al. 2003; Sahiner et

98

al. 2010) . However, the published articles on the effect of individual surfactant and

99

PVA on the nucleation and growth of cobalt nanoparticles are rather limited.

the reverse

100

It has been established that the morphology of the nanomaterials can be easily

101

controlled by using different kinds of stabilizers, such as surfactants, polymers,

102

proteins, phospholipids, etc. (Bakshi, 2016, 2011). In this work, Co2+-NaBH4 redox

103

system was used to the preparation of CoNPs in presence of two stabilizers, namely,

5 104

CTAB, and PVA. The degradation of congo red

105

which is capable of dying cotton directly. It is prepared by coupling tetrazotised

106

benzidine with two molecules of napthionic acid. Congo red containing effluents are

107

generated from textiles, printing and dyeing, paper, rubber and plastics industries.

108

Due to its structural stability, it is difficult to biodegrade) was preformed in presence

109

of CTAB-capped CoNPs under NaBH4 with and without sun light.

110

2. Experimental

111

2.1. Materials

112

Double-distilled deionized water was used as a solvent to the preparation and dilution

113

of all reagent solutions. Cobalt nitrate (Co(NO3)2 ; oxident), sodium borohydride

114

(NaBH4; reductant), stabilizers (cetyltrimethylammonium bromide, C19H42BrN and

115

poly(vinyl)alcohol, 99-100 % hydrolyzed), and Congo red (C32H22N6Na2O6S2) were

116

used as received from Merck India products (purity ≥ 99 % ). Stock PVA solutions

117

were prepared by slow stepwise addition of PVA to solvent, water, whilst rapidly

118

stirring to avoid their aggregation. Due to the instability and/or hydrolysis of NaBH4

119

in water, its aqueous solution contains certain amount of NaOH (Eq. 1). Therefore,

120

freshly prepared solutions were used to the synthesis of CoNPs (Cloutier et al.,

121

2007).

122

NaBH4 + 4H2O

(first synthetic anionic diazo dye,

NaOH + H3BO3 + 4H2

(1)

123

2.2. Preparation of CoNPs

124

In a typical experiment, the required NaBH4 solution was added into the reaction

125

mixture containing Co(NO3)2 and stabilizer (if necessary). The appearance of gray

126

turbidity, blue color, and light green color, indicating the formation of CoNPs having

6 127

different morphologies, and light pink color of Co2+ ions has been disappeared

128

completely (Guella et al. , 2006). The as prepared CoNPs were collected with a

129

magnet and washed them with deionized water and ethanol several times. The

130

transparent sols were centrifuged (10000 rpm for 30 min). Aqueous solutions were

131

decanted from the centrifuge tubes, and CoNPs were dispersed in water, filtered and

132

washed three times, and dried under vacuum for 3 to 4 h. The formation of CoNPs has

133

been summarized in Eq.2. Co2+ +BH4- + 4H2O

Co0 + H3BO3 + OH- + 4H2

(2)

stabilizer stabilized CoNPs 134 135

2.3. Characterization of cobalt nanomaterials by FT-IR, TEM, XRD, and SEM

136

To find out the preliminary information about the morphology, spectra of reaction

137

mixtures containing Co2+, Co2+ + NaBH4, Co2+ + CTAB + NaBH4 and Co2+ + PVA +

138

NaBH4 were recorded at different time intervals by using a UV/visible

139

spectrophotometer (UV-260 Shimadzu, with 1cm quartz cuvettes) at different time

140

intervals. All CoNPs samples, CTAB, NaBH4 and PVA were crushed and mixed to

141

KBr powder, pressed into a pellet and dried for 24 hours at room temperature. FTIR

142

spectra were recorded in the range of 4000-500 cm-1 collected after 40 scans at a

143

resolution of 4 cm-1 using FT-IR spectrophotometer (IR Prestige-21, IRAffinity-1,

144

FTIR-8400S , Shimadzu Corporation Analytical and Measuring Instrument Division)

145

. The size, shape, and the size distribution were investigated using a transmission

146

electron microscope (TECHNAI-320 KV JAPAN), operating at 80 kV (TEM;

147

together with selected area electron diffraction (SAED) experiments). X-ray

148

diffraction patterns were obtained using Ni-filtered Cu Kα radiation (λ = 1.54056 Å)

149

of a (Rigaku X-ray diffractometer, XRD) operating at 40 kV and 150 mA, the Bragg

150

angle (2θ) in the range from 10 to 800 ). Elemental composition analyses were

151

carried out using energy dispersive X-ray spectroscopy (EDX) by following on a

152

TECHNAI-320 KV JAPAN, operating at 80 kV system equipped with energy

153

dispersion X-ray spectroscopy. The surface

154

observed using a field emission scanning electron microscope (QUANTA FEG 450,

homogeneity and particle size were

7 155

FEI Company, Eindhoven, The Netherland). The samples for EDX analysis were

156

prepared by placing a drop of the as-synthesized colloids onto a carbon-coated Cu

157

grid (300 mesh), followed by slow evaporation of solvent at room temperature.

158

2.4. Congo red removal and/or degradation

159

A UV-260 Shimadzu, with 1cm quartz cuvettes UV/visible spectrophotometer was

160

used to monitor the Congo red degradation rates with as prepared CoNPs in an

161

aqueous solutions. The removal and/or catalytic activity was monitored by fading the

162

Congo red color in absence and under solar irradiation at different time intervals. In a

163

typical experiment, resulting CoNPs ( = 1.6 × 10-3 mol dm-3) was mixed with Congo

164

red aqueous solutions ( [Congo red ] = 1.4 × 10-4 mol dm-3 and 2.1×10-4 mol dm-3) at

165

room temperature under stirring for at least 20 min. We did not observed any

166

significant decay at 495 nm (λmax of Congo red). Then, NaBH4 (= 10.0 × 10-3 mol dm-

167

3

168

Congo red absorbance at 495 nm decreases, indicating the degradation of Congo red

169

by cobalt sols in presence of NaBH4. The reaction mixture containing the same

170

reactants concentrations (Congo red = 1.4 × 10-4 mol dm-3 + NaBH4 = 10.0 × 10-3 mol

171

dm-3) was irradiated in sun light for 5 min. The CoNPs = 1.6 × 10-3 mol dm-3 was

172

added in the reaction mixture and absorbance decay of Congo red recorded as a

173

function of time. The degradation efficiency (% D) was calculated by using Eq.(3).

) was added into the reaction solution under stirring. As the reaction-time increases,

%D = 100 [ (C0 - Ct ) / C0 ) ] %D = 100 [ (A0 - At ) / A0 ) ]

(3) (4)

174 175

where, C0 = initial concentration of Congo red solution, and Ct = concentration of the

176

Congo red solution at time t (min ). A0 , At and t are the initial absorbance of the

8 177

Congo red, absorbance of the sample at time t, and irradiation time of the sample,

178

respectively.

179

3. Results and discussion

180

3 .1. Removal ability for Congo red

181

Congo red removal from an aqueous solution using the as-prepared CoNPs was

182

investigated spectrophotometrically in absence and presence of sun light. Fig. 1 shows

183

the spectra of pure CoNPs, pure Congo red, and mixture of CoNPs + Congo red.

184

Interestingly, Congo red spectrum has two peaks at 340 nm and 495 nm due to the π–

185

π-* transition of –NH and azo groups, respectively. On the other hand, CoNPs

186

spectrum show a steep rise in absorption at very short wavelengths. Our spectra are in

187

good agreement to the observations regarding the formation of cobalt sols (optical

188

absorption increases smoothly in the UV region without a maximum up to 200 nm)

189

(Janata et al. 2000). Therefore, exact Co2+ / NaBH4 ratios can not be determined

190

under our experimental conditions. In the visible range the absorption was flat and

191

featureless (Creighton and Eadon, 1991). The absorption spectrum of mixed solution

192

of Congo red- CoNPs shows a blue shifted (from 340 nm to 335 nm) and red ( from

193

495 nm to 500 nm) shifted intense bands at 465 nm as well as high-energy blue

194

shifted band at 329 nm, indicating the adsorption of Congo red on to the surface of

195

nanoparticles. From the successive UV-visible spectrum of Congo red adsorption

196

(Fig. 1), we can directly observed that the CoNPs is not in position to remove and/or

197

degrade the Congo red concentration under our experimental conditions. Therefore, in

198

the next experiment, a required [NaBH4] ( = 10.0 × 10-3 mol dm-3 ) was used to

199

monitor the degradation. The degradation of the Congo red by CoNPs in presence of

200

NaBH4 was confirmed by the gradual decrease of its absorption peak intensity at 340

201

and 495 nm. Congo red color was completely discharged within ca. 7 min of the

202

reaction time as indicated by an arrow (typical example ; Fig. 1) i.e., the completed

203

reaction was, the peak at 495 nm due to the π–π-* transition of azo groups were no

204

longer observed, whereas control experiment showed no appreciable change in peak

205

intensity for at least 40 min to the observation period. Interestingly, the appearance of

206

a new peak ( at 295 nm ; indicated by an arrow) and disappearance of azo groups peak

207

at 495 nm, respectively, were noticed simultaneously (Fig. 1). Finally, reaction

9 208

mixture shows a peak at 285 nm after of the completion of the reaction, which might

209

be due to the degradation and/or reduction of -N=N- group into -NH2 group. These

210

results (appearance of peak at 285 to 290 nm) are in good agreement to the

211

observations of Pal et al. regarding the use of AgNPs as a catalyst for the reduction of

212

aromatic nitro compounds in to corresponding amino compounds in presence of

213

NaBH4( Kundo et al., 2004).

214

3.0

495 nm

Time (min) Pure CR CR&NaBH4

2.5

1 2 3 4 5 6 7 8 9 10 CoNPs

340 nm

Absorbance

2.0

1.5

1.0

0.5

0.0 300

350

400

450

500

550

600

Wavelength(nm)

215 216

Fig.1. Degradation of Congo red ( = 1.4 ×10-4 mol dm-3 ) using CoNPs (= 1.6 × 10-3

217

mol dm-3 ) as a catalyst in presence of NaBH4 (= 10.0 × 10-3 mol dm-3 ) at different

218

time intervals.

219

The Langmuir- Hishelwood, all adsorption and desorption pressure are in

220

equilibrium, (Langmuir, 1916; Pouretedal and Keshavarz, 2010) kinetic model is used

221

to explain the degradation of congo red in presence of heterogeneous catalyst (Eq. 5).

 = - d[C] / dt =

kKad ( 1 + KC )

where C = congo red. 222

(5)

10 223

Eq.(5) can be modified to the pseudo-first order rate law at low [congo red].

ln(C0 / Ct) = k Kad t = k t

(6)

224 225

where d[C] / dt = rate of congo red degradation ( mol dm-3 s-1), k = apparent first-

226

order rate constant (s-1), and Kad = absorption coefficient of the congo red onto the

227

catalyst particle. The half-life time , t

228

Eq(7).

t 1/2 = 0.693 / k

(1/2),

can be calculated using the following

(7)

229 230

Congo red concentrations were measured by the absorbance value at 495 nm in UV-

231

visible spectra as the absorbance is directly proportional to the [Congo red] (Khan et

232

al. 2016). Fig. 2 shows plots of ln (C / C0) versus reaction time, indicating the

233

degradation follows the excellent pseudo-first order kinetic rate-law. It has been

234

observed that the CoNPs is highly active for catalyzing for Congo red reduction ca. 98

235

% in only 5 min (Fig. 3).

236

Fig. 2A

11

4

C / C0

3

Congo red + CoNPs Congo red + NaBH4 Congo red + CoNPs +NaBH4

2

Congo red +CoNPs +NaBH4 + SL 1

0

0

1

2

3

4

5

Time (min)

237 238

Fig. 2B

0.5 0.0 -0.5

ln(C / C 0)

-1.0 -1.5 -2.0

-

CR+ BH4 -

-2.5

CR+BH4 +CoNPs -

CR+BH4 +CoNPs+SL

-3.0 0

1

2

3

4

5

Time (min)

239 240

Fig.2. (A) Catalytic efficiency of CoNPs under different systems for CR reduction as

241

a function of time. (B) representative plot of ln (C/C0) versus time. Reaction

242

conditions: [CR] = 2.1×10-4 mol dm-3, [CoNPs] = 3.2 × 10-3 mol dm-3 and [NaBH4]

243

= 10.0 × 10-3 mol dm-3 .

12

Congo red degradation (%)

100

3

-3

10 [CoNPs] (mol dm ) 3.2 1.6

80

60

40

20

0

0

1

2

3

4

5

6

7

8

9

10

11

12

Time (min)

244 245

Fig.3. Variation of CR % degradation ( = 2.1×10-4 mol dm-3 ) with time.

246

In order to determine the reduction rates of Congo red with CoNPs + NaBH4, kinetics

247

of these reactions was studied spectrophotometrically by monitoring the absorption

248

peak decay at 495 nm with varying [CoNPs], [NaBH4], and [Congo red] under

249

pseudo-first-order conditions with respect to [NaBH4] over [Congo red] ([NaBH4] ≥

250

10 times). The representative plots of log (absorbance) versus time are shown in Fig.

251

5 for varying [NaBH4] and [Congo red]. The pseudo-first-order rates constant ( kobs /

252

s-1) were calculated from the slopes of these plots (kobs = 0.0, 2.5 and 9.2 × 10-4 s-1 for

253

[NaBH4] = 0.0, 6.7 and 8.0 × 10-3 mol dm-3 at constant [Co2+] = 1.6 × 10-3 mol dm-3

254

and [CTAB] =

255

degradation of Congo red has an induction period. Whereas at higher [CoNPs] ( ≥ 3.2

256

× 10-3 mol dm-3 ), the concentration of the Congo red changed linearly with time.

257

Induction period has also been abolished with higher [NaBH4] = 20.0 × 10-3 mol dm-3.

258

This becomes obvious from Fig. 4, which shows the smooth plots of log (absorbance)

259

versus time obtained for the degradation of CR. The peak position of CR aqueous

260

solutions depends of the pH of the working reaction mixture (Wenqi et al. , 2011).

0.8 × 10-3 mol dm-3 ). At lower [CoNPs] (≤ 1.6 × 10-3 mol dm-3 ),

13 261

Amino groups protonation of CR takes place at pH 4.5-5.5 , which should be

262

associated with spectral transition ( Stopa et al., 2007). At low pH, CR become

263

cationic and shows two tautomeric form of protonated CR , i.e. ammonium rich and

264

azonium rich species. The fresh solution shows the predomination of ammonium rich

265

variety whereas after 1 h, azonium rich variety predominated and the isoelectric point

266

was found to be at pH = 3.0. Therefore, a series of experiments were performed to

267

determine the pH under different experimental conditions (pH = 4.9, 4.7, 4.8, 5.0, 4.8

268

and 4.8 for [CoNPs] = 3.2 × 10-3 mol dm-3 at different CR = 1.2, 1.7, 2.1, 2.5, 3.0 ×10-

269

4

270

presence of [CTAB] = 0.8×10-3 mol dm-3. On the other hand, addition of CoNPs and

271

NaBH4 solutions has no significant effect on the pH of CR solutions under our

272

experimental conditions.

mol dm-3 ) . The pH values was found to be nearly constant with increasing [CR] in

0.6

3

-3

10 [CoNPs] (mol dm ) 1.6 3.2 3.2

log (Absorbance)

0.3

0.0

-0.3

-0.6

-0.9

0

1

2

3

4

5

Time (min) 273

6

7

8

9

14 274

Fig.4. Plot of log (Absorbance) versus time for the degradation of CR using CoNPs as

275

a catalyst in the presence of NaBH4 = 10.0 × 10-3 (■ , ●) and 20.0 × 10-3 mol dm-3

276

(▲).

5.0

Absorbance

4.5 4.0

Pure congo red congo red &NaBH4

3.5

NPs,congo red &NaBH4 NPs,congo red &NaBH4

3.0

NPs,congo red &NaBH4

2.5 2.0 1.5 1.0 0.5 0.0

300

400

500

600

700

800

900

Wavelength(nm) 277 278

Fig.5. Degradation of Congo red ( = 2.1×10-4 mol dm-3 ) using CoNPs (= 3.2 × 10-3

279

mol dm-3 ) as a catalyst in presence of NaBH4 (= 10.0 × 10-3 mol dm-3 ) at different

280

time intervals in presence of sun light.

281

On the basis of observed results, Scheme 1 is proposed for the protonation of CR.

15 NH2

NH2 N

N

O

S

O

-

O

O

+

N

N

N

H S

O

H O

O

2

N

N

H

O

S

+NH

2

282

-

O

O

S

O

+

+NH

O

S

N

(Azonium species)

-

-

O NH2

- 2H+ + 2H+

NH2

O

N

N

O

-

N

N

H O

(Ammonium species)

-

S

O

O

283

Scheme 1. Protonation of CR in aqueous solution

284

In aqueous solution CR participates in the acid-base equilibrium and ammonium-, and

285

and azonium-rich species exist in solution (Scheme 1), concentrations of these species

286

depends on the pH. The azonium species is converted into ammonium species due to

287

the resonance delocalization processes. Between about pH 5.0 to - 6.0 (Table 1),

288

azonium rich species is the principal species.

289

presence of CoNPs, proceeds through the formation of an adsorption complex

290

between the CR, CoNPs and NaBH4 . The redx reaction have been considered to be

291

an electron transfer reaction in which NaBH4 , CR , and metal nanoparticles acts a

292

donor, acceptor, and electron transfer mediator, respectively. The reduction of 4-

293

nitriphenol by NaBH4 with AgNPs also formed a complex between the reactants and

294

the nanocatalyst (Pal et al., 1998). Interestingly, CR degradation occurs only in

295

presence of both CoNPs and NaBH4 in the present studies (Figs. 1 and 5). The

296

adsorption of CoNPs and BH4- ions on the -N=N- of CR might be responsible for the

The CR reduction by NaBH4 in

16 297

degradation mechanism of the CR, as summarized in the schematic diagram ((Scheme

298

2). NH2

NH2

.. N

O

N

H

H O

+ 2BH4-

nCo0

-

S

O

O NH2

---

N

H

H

N

N

---

NH2

(Co0)n BH4 S

N

-

O

O

..

O

-

N

---

S

+

N

(8)

BH4 (Co0)n ---

O

+

O

-

S

O

O

O NH2 NH2

2 O

299

S

O

+ H2N-

-NH2 + Boric acid + nCo0

(9)

-

O

300

Scheme. 2. Reductive degradation mechanism of CR with BH4- and CoNPs

301

By analogy with previous results (Florence , 1965; Weber, 1991) we assume that CR-

302

borohydride-CoNPs complex under goes multi-electrons transfer oxidation-reduction

303

mechanism. The proposed mechanism is in accordance to the involvement of colloidal

304

particles in the electron-transfer reactions via electron relay effect (Miller et al.,

305

1981).

306

3.2. Characterization of CoNPs

307

It is well known that the aqueous NaBH4 solutions were unstable and alkaline in

308

nature due to the fast hydrolysis with water and formation of NaOH (Kojima et al.

309

2004; Guella et al. 2006). Therefore, freshly prepared NaBH4 solutions were used to

310

the synthesis of CoNPs with and with out CTAB and PVA. The optical images to the

17 311

appearance of different cobalt sols as a function of time are given in Fig. 6. Addition

312

of NaBH4 , in a Co2+ ions (= 1.6 × 10-3 mol dm-3) solutions with and without required

313

[CTAB] and [PVA] , the appearance of perfect transparent stable different color were

314

observed. The color of Co 2+ ions changes from light pink to dark blue and light blue (

315

SET A; Fig. 6B and SET C; Fig. 6B) and SET B; Fig. 6B) during the mixing of the

316

reactants. The intensity of the colored increase and/or decrease with reaction time,

317

which might be due to the presence of CTAB and PVA. Finally, sky blue color

318

appeared in all cases (Fig.6). Interestingly, the reduction of Co2+ into Co0 has

319

occurred by NaBH4, but the resulting blue sols are not stable, and finally, blue-green

320

precipitate deposited at the walls of the reaction vessel (SET A; Fig. 6 F) after ca. 150

321

min. On the other hand, we did not observed the deposition any type of precipitate

322

with CTAB and PVA for up to 5h. Thus we may stated confidently that the presence

323

of stabilizer(s) is essential to the formation of stable and prefect transparent CoNPs.

324

Stability of the resulting blue colored cobalt sols depends on the capping action of the

325

CTAB and PVA. It has been established that the blue (α-cobalt hydroxide) and pink

326

(β - cobalt hydroxide) forms are exist in an aqueous solutions, but the former is

327

unstable, rapidly converted to the latter and only the pink

328

particularly characterized and reported in the literature (Jeevanandam et al. , 2000).

329 330 331

SET :1

β -phase has been

18

332 333 334

SET:2

335 336 337

SET: 3

19

338 339 340

Fig.6. Optical images of Co2+ aqueous solution (= 0.01 mol dm-3 A), formation and

341

stability of nano-size α-cobalt hydroxide without stabilizer (SET-1), with CTAB

342

(SET-2), and PVA (SET-3). Reaction conditions: [Co2+] = 1.6 × 10-3 mol dm-3,

343

[NaBH4] = 3.3×10-3 mol dm-3, [CTAB] = 0-8 × 10-3 mol dm-3, [PVA] = 2 ml ( 4% w /

344

v),

345

min),Temperature = 30 0C.

346

Fig. 7 shows the FTIR spectra of CONPs and compare it with that of pure NaBH4 ,

347

CTAB, and PVA. In all spectra, broad band centered at ca. 3441, 3441 and 3429 cm-1

348

for BH4--, CTAB-, and PVA-capped materials , respectively,

349

hydroxyls are involved in hydrogen bonding with the intercalated water molecules.

350

The band at ca. 1614, 1627, 1627 cm-1 might be attributed to the bending vibration

351

mode of the interlayer water molecules in all samples. We did not observed any peak

352

at 3630 cm-1 in the all FT-IR spectra for the β-cobalt hydroxide. All spectra shows the

353

peak at 3500-3000 cm-1 O-H vibration region, which might be due to the presence of

354

bending water molecules. These spectral features are in good agreement with the

Time: 1 min (B), 25 min (C), 35 min (D), 90 min (E), and 150 F (

revealing that the

20 355

results of others investigators (Portemer et al., 1992;

356

Gedanken et al. 2000) regarding the synthesis of α-cobalt hydroxide nano materials

357

under different experimental conditions. Fig. 7A has no vibration frequencies in the

358

boron-hydrogen stretching region (from 2300 to 2100 cm-1) , conforms that all BH4-

359

has been oxidized by Co2+ ions and/or hydrolyzed by water simultaneously. The

360

nitrate group is FI-IR active and shows various strong vibrations at 1400 to 900 cm-1

361

region that can be seen in FT-IR spectra of the α-cobalt hydroxide nanomaterials

362

intercalated with nitrate anions (Nakamoto , 1963; He et al., 2005). The presence of

363

various vibration peaks at bands at 978-819 cm-1 are ascribed to metal-hydroxide (M-

364

OH) bending modes. The infrared spectrum is characterized by a broad band centered

365

at 3450 cm-1, characteristic of the OH stretching vibration.The peaks observed at 3021

366

cm-1, 2931 cm-1 , 2848 cm-1, and 1474 cm-1 are assigned to the CH2 stretching

367

vibrations, anti symmetric deformation of alkyl chains, and C-N vibrations in pure

368

CTAB, respectively (Fig. 7B). The C-N vibrations peak of pure CTAB shifted (little

369

blue shift) from 1474 cm-1 to 1376 cm-1 and peak intensity also decreased for the

370

CTAB-capped cobalt nanomaterials. FT-IR spectrum of pure PVA and PVA-capped

371

cobalt nanomaterials are showed in Fig. 7C. It clearly reveals the major peaks

372

associated with PVA at 3600-3650 cm-1, 3200-3570 cm-1 , and 2850-3000 cm-1 for

373

free alcohol non bonded –OH stretching, hydrogen bonded band, and C–H broad

374

alkyl stretching band, respectively. The absorption peak was observed at 1142 cm-1

375

for the intra molecular and intermolecular hydrogen bonding among PVA chains due

376

to high hydrophilic forces. In general the FT-IR absorption bands of PVA are all quite

377

broad and severally overlapped in the 600-1500 cm-1 region, which might be due to

378

the several reasons such as wagging vibration of CH2 and CH, degree of crystallinity,

379

C-O stretching, syndiotactic structure, C-C stretching and out of plane OH bending.

Fernandez et al., 1994;

21 380

All these vibration stretching frequencies were also observed for PVA-capped cobalt

381

nano materials (Fig. 7C ; red line PVA-CoNPs).

382

A

130

NaBH4

120 110

CoNPS

100 90

30 20 10

2296 2225

3549 3429 3409

2391

40

0

824 615

1003 946

50

1614

3237

60

1487

70

-

NO3

1124

Transmittance (a.u.)

80

-40

4000

3500

3000

2500

2000

1455 1379

-30

1614

3441

-20

1168

-10

1500

-1

1000

500

Wavenumber (cm )

383

B

384

110

CTAB CTAB + CoNPs

105 100

HOH bendC-N

2913

70 65

2848

75

723

1627 1474

80

3021

85

3435

Transmittance (a.u.)

90

1162 1041 959 908

95

40

4000

3500

3000

2500

2000 -1

Wavenumber (cm )

385 386

C

1500

1000

672

1029

1379

2919 2856

45

3441

50

1627

55

1156

60

500

22 387

90 88 86 84 82 80 78 76 74 72 70 68 66 64 62 60 58 56

PVA PVA + CoNPS

4000

3500

3000

2500

615

946 843 1105

564

856 780 685

1029

1373 1251

1500

1162 1111 1054

1735 1627

2000

1455 1379

1436

1728 1633

2358 2372 2339

2334

2926 2856 2926 2869

3429

3416

Transmittance (a.u.)

C-O

1029

1000

500

-1

Wavenumber (cm )

388 389

Fig. 7. FT-IR of CoNPs with NaBH4 (A), CTAB (B), and PVA (C). Reaction

390

conditions: [Co2+] = 1.6 × 10-3 mol dm-3, [NaBH4] = 3.3 × 10-3 mol dm-3, [CTAB]

391

=0.8×10-3 mol dm-3, and [PVA] = 2ml (4%) .

392

The SEM of sky blue α-cobalt hydroxide are shown in Fig. 8, which clearly suggests

393

that CoNPs were composed of smaller clusters and reveals the aggregated nature of

394

the nano particles. CoNPs formed in water solutions without stabilizer are poorly

395

aggregated in an unsymmetrical manner and tend to coalesce forming larger

396

aggregates (Fig. 9A). Inspection of SEM images shows that the sample has an

397

irregular needle-spinal-, multilayer-, aggregated interconnected needles- , and stone-

398

shape morphology without (Fig. 8A) stabilizer and with CTAB (Fig. 8B), and PVA

399

(Fig. 8C), respectively, which might be attributed to the different capping properties

400

of used CTAB and PVA.

401

A

23

402 403

B

404 405 406

407

C

24 408 409

Fig.8. SEM images of CoNPs without stabilizer (A), with CTAB (B), and PVA (C).

410

Reaction conditions: same as in Fig. 6.

411

Fig. 9 shows the TEM images of images of CoNPs , CTAB-capped and PVA-capped

412

CoNPs, which have different morphologies (fibrillar turbostratic, needles-like sheet,

413

and spherical in absence and presence of CTAB , and PVA, respectively). These

414

results are in good agreement to the observations of CoNPs having turbostratic

415

morphology (Oliva et al. 1982). We did not observed hexagonal platelet like size

416

distribution. Thus ruled out the possibility to the formation of β-cobalt hydroxides

417

under our experimental conditions (Guella et al. 2006). Our TEM data are in

418

accordance to the optical images of the CoNPs (Fig. 6).

419

A

420 421

422

B

25

423

C

424

425 426 427

Fig.9. TEM images of CoNPs without stabilizer (A), with CTAB (B), and PVA (C).

428

Reaction conditions: same as in Fig.6.

429

In order to determine the crystalline phase, a series of XRD spectra were also

430

recorded for the CTAB and PVA capped CoNPs. The XRD pattern of resulting

431

nanomaterials were (Fig. 10) were compared and interpreted with standard data of

26 432

International Centre of Diffraction Data (ICDD). We did not observed the well

433

defined characteristic diffraction peaks for Co, indicating the amorphous nature of the

434

resulting nanomaterials. Fig. 10 shows that the α-hydroxides are poorly ordered

435

phases and exhibit broad bands in their X-ray diffraction patterns (XRD patterns of

436

CoNPs are featureless, which might be due to the to the presence of interlamellar

437

water molecules bound to the -OH groups through hydrogen bonding. It has been

438

established the the α-cobalt hydroxides shows broad peaks and scaly particles in the

439

XRD patterns and TEM images in presence of organic additives (Kobayashi et al.,

440

2003). However, the weak peak positions at 2θ = 44.8, 47.1 and 49.50 can be

441

indexed as α-cobalt nanoparticles (Sun and Murray, 1999). In the present study, we

442

did not used an aqueous NaOH to maintain the high pH condition to avoid the

443

formation of β- hydroxides (Guella et al. 2006). The absence of Co3+ ions in the α-

444

hydroxides is also checked by reacting with excess ferrous ammonium sulfate and

445

back titrating the excess ferrous ammonium sulfate with standard potassium

446

dichromate. Comparison with a blank titration confirms the absence of any Co3+ .

447

A

1000

900

900

800

800

700

700

600

600

500

500

400

400

Intensity

1000

10

20

30

40

2 448

50



60

70

80

27

B

449

900

Intensity

800

700

600

500

10

20

30

40

2

50

60

70

80



450 451

Fig.10. XRD of CoNPs under different experimental conditions. (A) with CTAB and

452

(B) PVA. Reaction conditions were same as those in Fig. 6.

453

On the basis of above results and discussion, the following flow diagram is proposed

454

for the different morphologies of CTAB and PVA capped CoNPs (Scheme 3).

BH4 Co2+ + CTAB

Co2+ + Micelles

CoNPs Capped With CTAB

BH4 -

Co2+ + PVA

Co2+ + Capped With PVA

CoNPs Capped With PVA

455 456

Scheme 3. Role of different stabilizers on the morphologies of CoNPs.

457

Scheme 3 clearly shows that the stabilizers have significant impact on the

458

morphology of CoNPs. CTAB (cationic surfactant) formed aggregates, which

28 459

solubilized and /or incorporated the reactants (Co2+ ions and NaBH4) into the Stern

460

layer of cationic micelles through electrostatic interactions. Such type of situation

461

does on persist in presence of PVA.

462

3.3. Morphology of used CoNPs

463

In order to see insight into the morphological changes of used catalyst, a magnet bar is

464

added into the reaction mixture for the separation of CoNPs. The aqueous solution

465

was decanted and the magnetic bar-CoNPs was dispersed in water. The solution was

466

centrifuged when all nanoparticles settled at the bottom of the centrifuge tube and the

467

aqueous solution was carefully decanted out. The SEM and XRD were also recorded

468

of the separated black CoNPs (Fig. 11). We did not observed any significant change

469

in the XRD patterns of used CoNPs and those prepared initially. Thus we may stated

470

confidently that the as prepared CoNPs are very stable under normal conditions.

471 472

29

Intensity

1000

1000

900

900

800

800

700

700

600

600

500

500

400

400

10

20

30

40

2

50

60

70

80



473 474

Fig.11. SEM image and XRD of recovered CoNPs. Reaction conditions were same as

475

those in Fig. 7.

476

4. Conclusions

477

We demonstrated a simple method to the synthesis of CoNPs in absence and presence

478

CTAB and PVA by using Co2+-NaBH4 redox reaction. The UV–visible, FT-IR, SEM,

479

TEM and XRD techniques were used for the resulting CoNPs. The CTAB capped

480

CoNPs are stable in aqueous medium for a long time without any aggregation. These

481

results confirmed that CTAB and PVA have significant impacts on the morphology of

482

CoNPs, which might be due to the different capping action of stabilizers. The addition

483

of stabilizers affected the presence of interlamellar water molecules bound to the -OH

484

groups (). We used resulting sols as a adsorbent materials for the removal of industrial

485

wastes azo dye, CR in an aqueous solution with in ca. 6 min at room temperature,

486

which may be attributed to the adsorption of BH4-, CR, and CoNPs

487

electrostatic as well as van der Waals interaction. The degradation process has an

488

induction period followed by an auto catalyst path. Induction period strongly depends

489

on the [CoNPs] and [NaBH4] under pseudo-first-order conditions.

through

30 490

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