Accepted Manuscript Effect of humic acid on photocatalytic activity of ZnO nanoparticles Preethy Chandran, Suhas Netha, S. Sudheer Khan PII: DOI: Reference:
S1011-1344(14)00175-4 http://dx.doi.org/10.1016/j.jphotobiol.2014.05.013 JPB 9754
To appear in:
Journal of Photochemistry and Photobiology B: Biology
Received Date: Revised Date: Accepted Date:
26 January 2014 12 April 2014 19 May 2014
Please cite this article as: P. Chandran, S. Netha, S. Sudheer Khan, Effect of humic acid on photocatalytic activity of ZnO nanoparticles, Journal of Photochemistry and Photobiology B: Biology (2014), doi: http://dx.doi.org/ 10.1016/j.jphotobiol.2014.05.013
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.
1
Effect of humic acid on photocatalytic activity of ZnO nanoparticles
2 3
Preethy Chandran*, Suhas Netha, S. Sudheer Khan
4 5
CeNTAB, School of Chemical and Biotechnology, SASTRA University, Thanjavur – 638 401
6 7 8 9 10 11 12
*Corresponding author:
13
Dr. Preethy Chandran
14
Assistant Professor (Research)
15
CeNTAB, School of Chemical and Biotechnology
16
SASTRA University
17
Thanjavur – 638 401
18
Email: [email protected]
19
Phone: 91 9751275798
20
Fax: +91 4362 264120
21 22 23 1
24
Abstract
25
Zinc oxide nanoparticles (ZnO NPs) are widely used in consumer products including sunscreens,
26
textiles and paints. The indiscriminate use of such materials may leads to its release into the
27
environment. The present study evaluated the photocatalytic effect of ZnO NPs in presence of
28
humic acid (HA), which is an important factor present largely in the environment. ZnO NPs were
29
characterized by using UV-visible spectrophotometer, scanning electron microscopy, particle
30
size analyzer and X-Ray diffraction analysis. The mean diameter of the particles was found to be
31
55 ± 2.1 nm. The XRD patterns exhibited hexagonal structure for ZnO NPs. The photocatalytic
32
activity of ZnO NPs was evaluated based on the change in UV–visible absorption spectra of the
33
methylene blue solution as a function of reaction time under visible light source. The rate of
34
photocatalytic degradation of methylene blue was decreased with increase in HA concentration.
35 36
Keywords: ZnO NPs, Humic acid, Methylene blue, Visible light, Photocatalytic property.
37 38 39 40 41 42 43 44 45 46 2
47
1. Introduction
48
Nanoparticles (NPs) (1-100 nm) have been attracted by their unique surface properties
49
and their potential use in a wide range of applications including photocatalysis and biomedicine
50
[1-4]. Among that metal oxide nanoparticles (NPs) received considerable attention and they are
51
being manufactured and incorporated into variety of products based on catalytic capacity,
52
optoelectronic properties and antimicrobial activity [5] ZnO is a wide band-gap semiconductor
53
with a large excitation binding energy. The wurtzite crystal structure and piezoelectricity of ZnO
54
NPs makes it particularly attractive for electronic sensor, solar voltaics and transducer
55
applications. ZnO is used as an effective photocatalyst in variety of environmental control
56
technologies from remediation of environmental pollutants to medical disinfection [6]. Currently
57
ZnO NPs are used in products including plastics, ceramics, glass, cement, rubber, lubricants,
58
paints, pigments, micronutrients for plants, batteries and personal care products including
59
cosmetics and sunscreens [5]. According to Borm et al. [7] the estimated production of NPs in
60
sunscreen products alone is approximately 1000 tons during 2003/3004, consisting principally of
61
TiO2 and ZnO particles. The amount of ZnO NPs was estimated to be 430 µg/L in treated
62
wastewater in Europe [8]. The increased use of ZnO NPs has resulted in the release of such
63
particles to the environment there by increase the environmental availability of ZnO NPs [9]
64
The toxicological effect of ZnO NPs towards a broad range of organisms has been
65
studied extensively. Researchers were reported that ZnO NPs posses toxicity to environmentally
66
relevant bacterial species, algal species, invertebrates and vertebrates [2, 9-12]. ZnO is a good
67
photocatalyst and promotes generation of reactive oxygen species (ROS) under irradiation with
68
energy [5]. The toxic action of ZnO NPs can potentially involve due to the photocatalytic effect
69
[9]. 3
70
The toxicological effect of the NPs is limited by environmental factors such as the
71
presence of HA. HA has the ability to inhibit the aggregation of NPs due to its adsorption onto
72
NPs surface [13]. Hence the present study evaluated the impact of HA on the photocatalytic
73
effect of ZnO NPs.
74 75
2. Materials and methods
76
2.1. Materials
77
All the chemicals were obtained from Merck chemicals Ltd., India. Humic acid was
78
obtained from Sigma-Aldrich, USA. All the chemicals used for the study were of analytical
79
grade. UV- visible absorption spectra was recorded by using a double beam Lambda 25 UV-
80
visible spectrophotometer (Perkin Elmer, USA).
81 82
2.2. Preparation of ZnO nanoparticles
83
The ZnO NPs were prepared by sol-gel method where zinc acetate dihydrate (Zn
84
(CH3COO) 2. 2H2O) and tri-ethanolamine (TEA) were chosen as precursor and stabilizing agent
85
respectively. Ethanol and ammonium hydroxide takes care for the homogeneity and pH value of
86
the solution and helps to make a stoichiometric solution to get ZnO NPs. Briefly, 20 mL of water
87
was added with 30 mL of TEA, followed by the addition of ethanol (2 mL) drop wise under
88
continuous stirring to get a homogeneous solution. Thereafter, 0.5 M zinc acetate solution (50
89
mL) was added and stirred at 80oC for 30 min. Ammonia was added drop wise until the white
90
milky precipitate is formed. The precipitate was collected and dried in hot-air oven at 60o C. The
91
obtained sample was calcined at 500o C for 60 min in a muffle furnace.
92 4
93
2.3. Characterization of ZnO NPs
94
The preliminary characterization of NPs was done by using a double beam Lambda 25
95
UV- visible spectrophotometer. For XRD analysis, lyophilized nanoparticles were coated on
96
XRD grid and the spectra was recorded using Bruker AXS Diffractometer (D8 Focus, Germany)
97
operated at the voltage of 40 KV using Cu Kα radiation. The surface area was measured using a
98
Smart Sorb 93 Single point BET surface area analyzer (Smart Instruments Co. Pvt. Ltd.,
99
Mumbai, India). The surface state, morphology and structure of NPs were recorded using a field
100
emission scanning electron microscopy (JEOL JSM-6701F, Japan) at a magnification level of 6
101
K with an acceleration voltage of 3-35 kV. The zeta potential of the synthesized nanoparticles
102
was determined by zeta sizer (Nanoseries, Nano- ZS, UK). Size distribution of the particles was
103
determined using particle size analyzer (Microtrac Blue Wave, Nikkiso, Japan).
104 105
2.4. Preparation of HA
106
A stock solution of HA was prepared by hydrating 500 mg of HA in 1 L of MilliQ water
107
and the suspension was incubated in a rotary shaker at 150 rpm for 24 h. There after the solution
108
was filtered using 0.1 µm syringe filter. Then the HA stock solution was stored at 4 ºC for further
109
use.
110 111
2.5. Evaluation of photocatalytic property
112
The photocatalytic property of ZnO NPs was evaluated by interacting 10 mg/L of MB
113
solution 5 mg/L of NPs. The solution was exposed to a 500 W Xenon lamp (Oriel instruments),
114
placed 30 cm above the dishes. The photocatalytic efficiency was evaluated based on the
115
degradation of MB and it was monitored by UV–vis spectrophotometer at 10 min time intervals. 5
116
The characteristic absorption of MB was monitored at 665 nm. According to the Beer–Lambert
117
Law, the concentration of MB is directly proportional to the absorbance. Hence the degradation
118
efficiency can be calculated by following the equation [14].
119
C Ct R 0 C0
120
where C0 and Ct are the absorbance of MB at time 0 and t, respectively. The effect of HA on the
121
photocatalytic activity of ZnO NPs was evaluated in presence of 1, 10 and 100 mg/L HA.
100
122
The rate of ·OH formation during photocatalytic degradation under visible light was
123
evaluated by the photoluminescence technique as per the procedure described by Xiang et al.
124
[15]. The excitation wavelength and the scanning speed were adjusted to 332 nm and 1200
125
nm/min respectively. After visible light irradiation, the solution was filtrated to measure the
126
photoluminescence intensity at 456 nm.
127 128
3. Results and discussion
129
3.1. Characterization of ZnO NPs
130
Optical properties of the ZnO NPs were evaluated by using UV–visible
131
spectrophotometer. The UV-visible absorption spectra showed the absorption maximum at 385
132
nm (Fig. 1a) indicates the presence of ZnO NPs. Size and morphology of NPs were characterized
133
by SEM (Fig. 1b). The microscopic images showed that NPs were spherical in shape and
134
polydispersed. The specific surface area of the NPs was determined to be 0.23 m2/g. The particle
135
size distribution analysis of NPs showed a mean diameter of 55 ± 2.1 nm. The histogram of size
136
distribution of ZnO NPs is shown in Fig. 1c. The XRD pattern of ZnO NPs revealed that the
137
synthesized particles were pure and crystalline in nature. The facets (2θ value) observed at 31.7, 6
138
34.3, 36.3, 47.6 and 56.7 were assigned to (100), (002), (101), (102) and (110) reflection lines of
139
hexagonal ZnO NPs respectively (Fig. 1d). It indicates that the characteristic peaks represent the
140
ZnO NPs with hexagonal phase. The average crystalline size was obtained from the XRD peaks
141
by using Scherrer's equation [16] and the calculated particle diameter was 43 nm. The
142
broadening of the diffraction peaks indicates that the prepared particles are in nano form.
143 144
3.2. Adsorption process
145
The adsorption characteristics of MB on the surface of ZnO NPs are very important in
146
photodegradation process. The adsorption process may influenced by pH of the interaction
147
medium due to the modification of the electrical double layer of the solid electrolyte interface
148
[17]. Hence the present study used different pH in order to evaluate the effect of pH on
149
adsorption. The adsorption of MB on ZnO NPs surface was tested using suspensions of MB and
150
NPs in dark condition. The pH of samples was adjusted by using 0.1 N HCl or NaOH solutions.
151
The adsorption process completed in 40 min of interaction and further no more adsorption
152
occurred. The adsorption of MB onto ZnO NPs surface is shown in Fig. 2. Here the adsorption
153
increased quasi-linear with the increase of the pH.
154 155
3.3. Photodegradation process
156
The photocatalytic degradation process was performed at pH 5, since least adsorption
157
was observed at pH 5 and its slight adsorption onto the photocatalyst surface was neglected.
158
However, the photodegradation efficiency is expected to increase with pH, since higher the pH
159
provides higher concentration of hydroxyl ions to react with holes to form hydroxyl radicals
160
[18]. 7
161
Hydroxyl radical is considered to be one of the main reactive species during
162
photocatalytic reaction and to be responsible for oxidation decomposition of many organic
163
compounds. Hence it is clear that hydroxyl radical is the primary oxidant that degrading the MB,
164
it comes from the oxidation of adsorbed water or adsorbed OH-. The UV-visible spectral changes
165
of MB due to photocatalytic degradation as a function of irradiation time in the presence of ZnO
166
NPs are presented in Fig. 3. The photooxidation of MB was evaluated based on the changes in
167
UV-visible absorption spectra and its degradation efficiency. The intensity of the absorption
168
peaks corresponding to MB in presence of ZnO NPs diminished gradually as the exposure time
169
increased. After 60 min of NPs exposure under visible light, the absorption spectra of MB almost
170
disappeared indicates the degradation of MB by NPs. The intersection of C/C0 and 1-C/C0
171
indicates the half-life of the dye molecules, which is the time taken for the concentration of MB
172
to reduce or degrade by its half. The half period of MB by NPs was calculated to be 35 min.
173
Rahman et al. [19] studied the photocatalytic activity of ZnO NPs by the degradation of
174
rhodamine B dye, and they found that NPs could degrade 95 % dye in 70 min.
175
Fig. 4 shows the UV-visible absorption spectra of MB in presence of different
176
concentration of HA. The results show that the degradation of methylene blue content was
177
decreased with increase in concentration of HA. At 10 mg/L NPs, the MB degradation was
178
almost complete and it showed very low peak intensity. Whereas, when the concentration of HA
179
increased from 1 to 100 mg/L, a gradual increase in peak intensity can be clearly seen in the
180
figure. The presence of ZnO NPs with 100 mg/L HA showed the peak intensity of MB was
181
slightly less than the blank solution. The results say that the presence of humic acid reduced the
182
photocatalytic effect of NPs.
8
183
It is expected that the photocatalytic activity of ZnO NPs to be increased with increase in
184
ZnO NPs content. For this reason and to find the optimal condition, a series of experiments was
185
carried out with different concentrations of NPs. Fig. 5 illustrates the photodegradation of MB
186
treated with different concentrations of ZnO NPs, under exposure to visible light. The figure
187
shows the linear plots of ln(C/C0) for the photodegradation of MB with different NPs
188
concentrations. The slops of plots which express the photodegradation rate constants. Here the
189
rate of reaction in the degradation of MB increases with increase in NPs content. The
190
photocatalytic degradation reaction constant for 5, 10, 25 and 50 mg/L ZnO NPs was determined
191
to be 9.8 x 10-3 m-1, 14.1 x 10-3 m-1, 24.8 x 10-3 m-1 and 47.5 x 10-3 m-1 respectively. The
192
formation of ·OH was determined to be 0.15, 0.28, 0.49, and 0.81 µM/h for 5, 10, 25 and 50
193
mg/L ZnO NPs respectively. In order to evaluate the effect of HA on photocatalytic activity of
194
ZnO NPs, the photo degradation phenomenon observed under different HA conditions and
195
photolysis (in the absence of photo-catalyst) is presented in Fig. 6. The figure shows the linear
196
plots of ln(C/C0) for the photodegradation of MB with different HA concentrations. Here the rate
197
of reaction decreased when the concentration of HA increased. The rate constant for 10 mg/L
198
NPs was determined to be 16 x 10-3 m-1. In presence of HA the rate constant of the reaction was
199
decreased to 4.3 x 10-3 m-1, 3.3 x 10-3 m-1 and 1.9 x 10-3 m-1 for 1, 10 and 100 mg/L HA
200
respectively. The formation of ·OH was determined to be 0.15, 0.09, and 0.025 µM/h for 1, 10
201
and 100 mg/L HA respectively. Here, it can be seen that the formation of hydroxyl radicals was
202
reduced in presence of HA; hence it reduced the rate of MB degradation. Besides the essential
203
role played by oxygen in generating the oxidizing species (H2O2 and ·HO), the photogenerated
204
charge carriers at the semiconductor/liquid interface, is influenced to some extent by the surface
205
properties of the semiconductor, depending on pH, on surface hydroxyl groups and on adsorbed 9
206
charged molecules [20]. Yu et al. [21] was reported the influence of fluoride on photoactivity.
207
Bimodal mesoporous titania powders with high photocatalytic activity were observed when it
208
was prepared by hydrolysis of titanium tetraisopropoxide in the presence of HNO3 or NH4OH
209
under ultrasonic irradiation [22]. Here we can say that the photogenerated holes and electrons
210
could not react with OH-/H2O and O2 to form ·OH and O2-, respectively. Therefore, it was easy
211
to understand that the formation of lower level of ·OH in presence of HA.
212 213
Conclusion
214
The present study evaluated the effect of HA on the photocatalytic activity of ZnO NPs.
215
ZnO NPs exhibited excellent photocatalytic activity under visible light, but the presence of HA
216
decreased the photocatalytic effect of NPs. The study suggests that the presence of HA such
217
factors in the environment limited the photocatalytic activity of NPs; thereby reduce its toxic
218
effect on environmental organisms.
219 220 221 222
Acknowledgments Authors thank the management of SASTRA University for providing facility to carry out this work.
223 224 225 226 227 228 10
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References
230
[1] A. Nel, T. Xia, L. Mädler, N. Li, Toxic potential of materials at the nanolevel. Science 311
231
(2006) 622–627.
232
[2] M. Premanathan, K. Karthikeyan, K. Jeyasubramanian, G. Manivannan, Selective toxicity of
233
ZnO nanoparticles toward Gram-positive bacteria and cancer cells by apoptosis through lipid
234
peroxidation. Nanomed-Nanotechnol. 7 (2011) 184-192.
235
[3] R. Wahab, N.K. Kaushik, A.K. Verma, A. Mishra, I.H. Hwang, Y.B. Yang, H.S. Shin, Y.S.
236
Kim, Fabrication and growth mechanism of ZnO nanostructures and their cytotoxic effect on
237
human brain tumor U87, cervical cancer HeLa, and normal HEK cells, J. Biol. Inorg. Chem.
238
16 (2011) 431–442.
239
[4] G. S. Mital, T. Manoj, A review of TiO2 nanoparticles, Chin. Sci. Bull. 56 (2011) 1639–1657.
240
[5] Hongbo Ma, Phillip L. Williams, Stephen A. Diamond, Ecotoxicity of manufactured ZnO
241 242 243 244
nanoparticles-A review, Environ. Pollut. 172 (2013) 76-85. [6] M.R.Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis. Chem. Rev. 95 ( 1995) 69-96. [7] P. Borm, F.C. Klaessig, T.D. Landry, B. Moudgil, J. Pauluhn, K. Thomas, R. Trottier, S.
245
Wood, Research strategies for safety evaluation of nanomaterials, part V: role of dissolution
246
in biological fate and effects of nanoscale particles. Toxicol. Sci. 90 , (2006a) 23-32.
247
[8] F. Gottschalk, T. Sonderer, R.W. Scholz, B. Nowack, 2009. Modeled environmental
248
concentrations of engineered nanomaterials (TiO2, ZnO, Ag, CNT, Fullerenes) for different
249
regions. Environmental Science & Technology 43 (24), 9216-9222.
11
250
[9] Woo-Mi Lee, Youn-Joo An, Effects of zinc oxide and titanium dioxide nanoparticles on
251
green algae under visible, UVA, and UVB irradiations: No evidence of enhanced algal
252
toxicity under UV pre-irradiation, Chemosphere 91, (2013) 536–544..
253 254 255
[10] I.Blinova, A. Ivask, M. Heinlaan, M. Mortimer, A. Kahru, Ecotoxicity of nanoparticles of CuO and ZnO in natural water. Environ. Pollut. 158 , (2010) 41-47. [11] L.Z.Li, D.M. Zhou, W.J.G.M. Peijnenburg, C.A.M. van Gestel, S.Y. Jin, Y.J. Wang, P.
256
Wang, Toxicity of zinc oxide nanoparticles in the earthworm, Eisenia fetida and subcellular
257
fractionation of Zn. Environ. Int. 37 (2011a) 1098-1104.
258
[12] P.Khare, M. Sonane, R. Pandey, S. Ali, K.C.Gupta, A. Satish, Adverse effects of TiO2 and
259
ZnO nanoparticles in soil nematode, Caenorhabditis elegans, J. Biomed. Nanotechnol. 7
260
(2011) 116-117.
261
[13] B.J.R.Thio, D. Zhou , A.A.Keller, Influence of natural organic matter on the aggregation
262
and deposition of titanium dioxide nanoparticles. J Hazard Mater 189 (2011) 556–563.
263 264 265 266
[14] D. Sun, et al., Effects of nitrogen content in monocrystalline nano-CeO2 on the degradation of dye in indoor lighting, Appl. Surf. Sci. 280 (2013) 693–697. [15] Q. Xiang, J. Yu, P.K. Wong, Quantitative characterization of hydroxyl radicals produced by various photocatalysts, J. Colloid Interf. Sci. 357 (2011) 163–167.
267
[16] A. Becheri ,M. Dürr , P.L. Nostro , P. Baglioni, Synthesis and characterization of zinc oxide
268
nanoparticles: application to textiles as UV-absorbers, J. Nanopart. Res. 10 (2008) 679–689.
269
[17] A. Franco, M.C. Neves, M.M.L.R. Carrott, M.H. Mendonc¸ a, M.I. Pereira, O.C. Monteiro,
270
Photocatalytic decolorization of methylene blue in the presence of TiO2/ZnS
271
nanocomposites, J. Hazard. Mater. 161 (2009) 545–550.
12
272
[18] H.R. Rajabi, O. Khani, M. Shamsipur, V. Vatanpour, High-performance pure and Fe3+-ion
273
doped ZnS quantum dots as green nanophotocatalysts for the removal of malachite green
274
under UV-light irradiation, J. Hazard. Mater. 250–251 (250) (2013) 370–380.
275 276 277
[19] Q.I. Rahman, M. Ahmad, S.K. Misra, M. Lohani, Effective photocatalytic degradation of rhodamine B dye by ZnO nanoparticles, Mater. Lett. 91 (2013) 170–174. [20] M.L. Curri, R. Comparelli, P.D. Cozzoli, G. Mascolo, A. Agostiano, Colloidal oxide
278
nanoparticles for the photocatalytic degradation of organic dye, Mater. Sci. Eng. C 23 (2003)
279
285–289.
280
[21] Jiaguo Yu, Shengwei Liu, Huogen Yu, Microstructures and photoactivity of mesoporous
281
anatase hollow microspheres fabricated by fluoride-mediated self-transformation, J. Catalysis
282
249, 2007, 59-66.
283
[22] Jiaguo Yu, Jimmy C. Yu, Mitch K.-P. Leung, Wingkei Ho, Bei Cheng, Xiujian Zhao, Jincai
284
Zhao, Effects of acidic and basic hydrolysis catalysts on the photocatalytic activity and
285
microstructures of bimodal mesoporous titania, J. Catalysis 217, 2003, 69-78.
286 287 288 289 290 291 292 293 294 13
295
Figure captions
296
Figure 1. Characterization of ZnO NPs. (a) UV-visible absorption spectra (b) scanning electron
297
microscopic image, (c) particle size distribution and (d) X-ray diffraction pattern for ZnO NPs.
298
Figure 2. Effect of pH on the adsorption of MB on the surface of ZnO NPs in dark condition.
299
Figure 3. UV-visible absorption spectral changes and photodegradation of MB by ZnO NPs
300
under visible light.
301
Figure 4. UV-visible absorption spectra of MB solutions and the degraded dye solutions under
302
different conditions at the end of 60 min under visible light. Blank indicates the MB solution
303
simulated under visible light without NPs.
304
Figure 5. Linear plots of ln(C/C0) for the photodegradation of MB under visible light in the
305
absence and presence of different concentration of ZnO NPs.
306
Figure 6. Linear plots of ln(C/C0) for the photodegradation of MB by ZnO NPs under visible
307
light in the absence and presence of different concentration of HA.
308
14
Figure Figure 1
Figure 2
Adsorption (1-C/C0)
0.8 0.7
0.6 0.5 0.4 0.3 0.2 0.1 0 5
6
7
8
Initial pH value
9
10
Figure 3 0 min 10 min 20 min 30 min 40 min 50 min 60 min
0.7 Absorbance
0.6 0.5 0.4 0.3 0.2 0.1
1.2
Ct/C0 and 1-C/C0
0.8
C/C0 1-C/C0
1 0.8
0.6 0.4 0.2
0 450
550 650 Wave length (nm)
750
0 0
20
40 Time (min)
60
Absorbance
Figure 4 0.7
Original
0.6
Blank
0.5
10 mg/L NPs 1 mg/L HA
0.4
10 mg/L HA 0.3
100 mg/L HA
0.2 0.1 0
450
550
650 Wavelength (nm)
750
Figure 5 0.2 0 0
20
40
60
80
ln(C/C0)
-0.2
-0.4
Blank 5 mg/L NPs
-0.6
10 mg/L NPs -0.8
25 mg/L NPs 50 mg/L NPs
-1
Time (min)
Figure 6 0.2
0 0
20
40
60
80
ln(C/C0)
-0.2 -0.4
Blank
-0.6
10 mg/L NPs
-0.8
1 mg/L EPS 10 mg/L HA
-1
100 mg/L HA
-1.2 Time (min)
Research Highlights
ZnO NPs were synthesized by chemical co-precipitate method.
The mean diameter was determined to be 55 nm.
ZnO NPs exhibited excellent photocatalytic activity.
Presence of HA reduced the photocatalytic efficiency of ZnO NPs.
Fig. 1
S1011-1344(14)00175-4 http://dx.doi.org/10.1016/j.jphotobiol.2014.05.013 JPB 9754
To appear in:
Journal of Photochemistry and Photobiology B: Biology
Received Date: Revised Date: Accepted Date:
26 January 2014 12 April 2014 19 May 2014
Please cite this article as: P. Chandran, S. Netha, S. Sudheer Khan, Effect of humic acid on photocatalytic activity of ZnO nanoparticles, Journal of Photochemistry and Photobiology B: Biology (2014), doi: http://dx.doi.org/ 10.1016/j.jphotobiol.2014.05.013
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.
1
Effect of humic acid on photocatalytic activity of ZnO nanoparticles
2 3
Preethy Chandran*, Suhas Netha, S. Sudheer Khan
4 5
CeNTAB, School of Chemical and Biotechnology, SASTRA University, Thanjavur – 638 401
6 7 8 9 10 11 12
*Corresponding author:
13
Dr. Preethy Chandran
14
Assistant Professor (Research)
15
CeNTAB, School of Chemical and Biotechnology
16
SASTRA University
17
Thanjavur – 638 401
18
Email: [email protected]
19
Phone: 91 9751275798
20
Fax: +91 4362 264120
21 22 23 1
24
Abstract
25
Zinc oxide nanoparticles (ZnO NPs) are widely used in consumer products including sunscreens,
26
textiles and paints. The indiscriminate use of such materials may leads to its release into the
27
environment. The present study evaluated the photocatalytic effect of ZnO NPs in presence of
28
humic acid (HA), which is an important factor present largely in the environment. ZnO NPs were
29
characterized by using UV-visible spectrophotometer, scanning electron microscopy, particle
30
size analyzer and X-Ray diffraction analysis. The mean diameter of the particles was found to be
31
55 ± 2.1 nm. The XRD patterns exhibited hexagonal structure for ZnO NPs. The photocatalytic
32
activity of ZnO NPs was evaluated based on the change in UV–visible absorption spectra of the
33
methylene blue solution as a function of reaction time under visible light source. The rate of
34
photocatalytic degradation of methylene blue was decreased with increase in HA concentration.
35 36
Keywords: ZnO NPs, Humic acid, Methylene blue, Visible light, Photocatalytic property.
37 38 39 40 41 42 43 44 45 46 2
47
1. Introduction
48
Nanoparticles (NPs) (1-100 nm) have been attracted by their unique surface properties
49
and their potential use in a wide range of applications including photocatalysis and biomedicine
50
[1-4]. Among that metal oxide nanoparticles (NPs) received considerable attention and they are
51
being manufactured and incorporated into variety of products based on catalytic capacity,
52
optoelectronic properties and antimicrobial activity [5] ZnO is a wide band-gap semiconductor
53
with a large excitation binding energy. The wurtzite crystal structure and piezoelectricity of ZnO
54
NPs makes it particularly attractive for electronic sensor, solar voltaics and transducer
55
applications. ZnO is used as an effective photocatalyst in variety of environmental control
56
technologies from remediation of environmental pollutants to medical disinfection [6]. Currently
57
ZnO NPs are used in products including plastics, ceramics, glass, cement, rubber, lubricants,
58
paints, pigments, micronutrients for plants, batteries and personal care products including
59
cosmetics and sunscreens [5]. According to Borm et al. [7] the estimated production of NPs in
60
sunscreen products alone is approximately 1000 tons during 2003/3004, consisting principally of
61
TiO2 and ZnO particles. The amount of ZnO NPs was estimated to be 430 µg/L in treated
62
wastewater in Europe [8]. The increased use of ZnO NPs has resulted in the release of such
63
particles to the environment there by increase the environmental availability of ZnO NPs [9]
64
The toxicological effect of ZnO NPs towards a broad range of organisms has been
65
studied extensively. Researchers were reported that ZnO NPs posses toxicity to environmentally
66
relevant bacterial species, algal species, invertebrates and vertebrates [2, 9-12]. ZnO is a good
67
photocatalyst and promotes generation of reactive oxygen species (ROS) under irradiation with
68
energy [5]. The toxic action of ZnO NPs can potentially involve due to the photocatalytic effect
69
[9]. 3
70
The toxicological effect of the NPs is limited by environmental factors such as the
71
presence of HA. HA has the ability to inhibit the aggregation of NPs due to its adsorption onto
72
NPs surface [13]. Hence the present study evaluated the impact of HA on the photocatalytic
73
effect of ZnO NPs.
74 75
2. Materials and methods
76
2.1. Materials
77
All the chemicals were obtained from Merck chemicals Ltd., India. Humic acid was
78
obtained from Sigma-Aldrich, USA. All the chemicals used for the study were of analytical
79
grade. UV- visible absorption spectra was recorded by using a double beam Lambda 25 UV-
80
visible spectrophotometer (Perkin Elmer, USA).
81 82
2.2. Preparation of ZnO nanoparticles
83
The ZnO NPs were prepared by sol-gel method where zinc acetate dihydrate (Zn
84
(CH3COO) 2. 2H2O) and tri-ethanolamine (TEA) were chosen as precursor and stabilizing agent
85
respectively. Ethanol and ammonium hydroxide takes care for the homogeneity and pH value of
86
the solution and helps to make a stoichiometric solution to get ZnO NPs. Briefly, 20 mL of water
87
was added with 30 mL of TEA, followed by the addition of ethanol (2 mL) drop wise under
88
continuous stirring to get a homogeneous solution. Thereafter, 0.5 M zinc acetate solution (50
89
mL) was added and stirred at 80oC for 30 min. Ammonia was added drop wise until the white
90
milky precipitate is formed. The precipitate was collected and dried in hot-air oven at 60o C. The
91
obtained sample was calcined at 500o C for 60 min in a muffle furnace.
92 4
93
2.3. Characterization of ZnO NPs
94
The preliminary characterization of NPs was done by using a double beam Lambda 25
95
UV- visible spectrophotometer. For XRD analysis, lyophilized nanoparticles were coated on
96
XRD grid and the spectra was recorded using Bruker AXS Diffractometer (D8 Focus, Germany)
97
operated at the voltage of 40 KV using Cu Kα radiation. The surface area was measured using a
98
Smart Sorb 93 Single point BET surface area analyzer (Smart Instruments Co. Pvt. Ltd.,
99
Mumbai, India). The surface state, morphology and structure of NPs were recorded using a field
100
emission scanning electron microscopy (JEOL JSM-6701F, Japan) at a magnification level of 6
101
K with an acceleration voltage of 3-35 kV. The zeta potential of the synthesized nanoparticles
102
was determined by zeta sizer (Nanoseries, Nano- ZS, UK). Size distribution of the particles was
103
determined using particle size analyzer (Microtrac Blue Wave, Nikkiso, Japan).
104 105
2.4. Preparation of HA
106
A stock solution of HA was prepared by hydrating 500 mg of HA in 1 L of MilliQ water
107
and the suspension was incubated in a rotary shaker at 150 rpm for 24 h. There after the solution
108
was filtered using 0.1 µm syringe filter. Then the HA stock solution was stored at 4 ºC for further
109
use.
110 111
2.5. Evaluation of photocatalytic property
112
The photocatalytic property of ZnO NPs was evaluated by interacting 10 mg/L of MB
113
solution 5 mg/L of NPs. The solution was exposed to a 500 W Xenon lamp (Oriel instruments),
114
placed 30 cm above the dishes. The photocatalytic efficiency was evaluated based on the
115
degradation of MB and it was monitored by UV–vis spectrophotometer at 10 min time intervals. 5
116
The characteristic absorption of MB was monitored at 665 nm. According to the Beer–Lambert
117
Law, the concentration of MB is directly proportional to the absorbance. Hence the degradation
118
efficiency can be calculated by following the equation [14].
119
C Ct R 0 C0
120
where C0 and Ct are the absorbance of MB at time 0 and t, respectively. The effect of HA on the
121
photocatalytic activity of ZnO NPs was evaluated in presence of 1, 10 and 100 mg/L HA.
100
122
The rate of ·OH formation during photocatalytic degradation under visible light was
123
evaluated by the photoluminescence technique as per the procedure described by Xiang et al.
124
[15]. The excitation wavelength and the scanning speed were adjusted to 332 nm and 1200
125
nm/min respectively. After visible light irradiation, the solution was filtrated to measure the
126
photoluminescence intensity at 456 nm.
127 128
3. Results and discussion
129
3.1. Characterization of ZnO NPs
130
Optical properties of the ZnO NPs were evaluated by using UV–visible
131
spectrophotometer. The UV-visible absorption spectra showed the absorption maximum at 385
132
nm (Fig. 1a) indicates the presence of ZnO NPs. Size and morphology of NPs were characterized
133
by SEM (Fig. 1b). The microscopic images showed that NPs were spherical in shape and
134
polydispersed. The specific surface area of the NPs was determined to be 0.23 m2/g. The particle
135
size distribution analysis of NPs showed a mean diameter of 55 ± 2.1 nm. The histogram of size
136
distribution of ZnO NPs is shown in Fig. 1c. The XRD pattern of ZnO NPs revealed that the
137
synthesized particles were pure and crystalline in nature. The facets (2θ value) observed at 31.7, 6
138
34.3, 36.3, 47.6 and 56.7 were assigned to (100), (002), (101), (102) and (110) reflection lines of
139
hexagonal ZnO NPs respectively (Fig. 1d). It indicates that the characteristic peaks represent the
140
ZnO NPs with hexagonal phase. The average crystalline size was obtained from the XRD peaks
141
by using Scherrer's equation [16] and the calculated particle diameter was 43 nm. The
142
broadening of the diffraction peaks indicates that the prepared particles are in nano form.
143 144
3.2. Adsorption process
145
The adsorption characteristics of MB on the surface of ZnO NPs are very important in
146
photodegradation process. The adsorption process may influenced by pH of the interaction
147
medium due to the modification of the electrical double layer of the solid electrolyte interface
148
[17]. Hence the present study used different pH in order to evaluate the effect of pH on
149
adsorption. The adsorption of MB on ZnO NPs surface was tested using suspensions of MB and
150
NPs in dark condition. The pH of samples was adjusted by using 0.1 N HCl or NaOH solutions.
151
The adsorption process completed in 40 min of interaction and further no more adsorption
152
occurred. The adsorption of MB onto ZnO NPs surface is shown in Fig. 2. Here the adsorption
153
increased quasi-linear with the increase of the pH.
154 155
3.3. Photodegradation process
156
The photocatalytic degradation process was performed at pH 5, since least adsorption
157
was observed at pH 5 and its slight adsorption onto the photocatalyst surface was neglected.
158
However, the photodegradation efficiency is expected to increase with pH, since higher the pH
159
provides higher concentration of hydroxyl ions to react with holes to form hydroxyl radicals
160
[18]. 7
161
Hydroxyl radical is considered to be one of the main reactive species during
162
photocatalytic reaction and to be responsible for oxidation decomposition of many organic
163
compounds. Hence it is clear that hydroxyl radical is the primary oxidant that degrading the MB,
164
it comes from the oxidation of adsorbed water or adsorbed OH-. The UV-visible spectral changes
165
of MB due to photocatalytic degradation as a function of irradiation time in the presence of ZnO
166
NPs are presented in Fig. 3. The photooxidation of MB was evaluated based on the changes in
167
UV-visible absorption spectra and its degradation efficiency. The intensity of the absorption
168
peaks corresponding to MB in presence of ZnO NPs diminished gradually as the exposure time
169
increased. After 60 min of NPs exposure under visible light, the absorption spectra of MB almost
170
disappeared indicates the degradation of MB by NPs. The intersection of C/C0 and 1-C/C0
171
indicates the half-life of the dye molecules, which is the time taken for the concentration of MB
172
to reduce or degrade by its half. The half period of MB by NPs was calculated to be 35 min.
173
Rahman et al. [19] studied the photocatalytic activity of ZnO NPs by the degradation of
174
rhodamine B dye, and they found that NPs could degrade 95 % dye in 70 min.
175
Fig. 4 shows the UV-visible absorption spectra of MB in presence of different
176
concentration of HA. The results show that the degradation of methylene blue content was
177
decreased with increase in concentration of HA. At 10 mg/L NPs, the MB degradation was
178
almost complete and it showed very low peak intensity. Whereas, when the concentration of HA
179
increased from 1 to 100 mg/L, a gradual increase in peak intensity can be clearly seen in the
180
figure. The presence of ZnO NPs with 100 mg/L HA showed the peak intensity of MB was
181
slightly less than the blank solution. The results say that the presence of humic acid reduced the
182
photocatalytic effect of NPs.
8
183
It is expected that the photocatalytic activity of ZnO NPs to be increased with increase in
184
ZnO NPs content. For this reason and to find the optimal condition, a series of experiments was
185
carried out with different concentrations of NPs. Fig. 5 illustrates the photodegradation of MB
186
treated with different concentrations of ZnO NPs, under exposure to visible light. The figure
187
shows the linear plots of ln(C/C0) for the photodegradation of MB with different NPs
188
concentrations. The slops of plots which express the photodegradation rate constants. Here the
189
rate of reaction in the degradation of MB increases with increase in NPs content. The
190
photocatalytic degradation reaction constant for 5, 10, 25 and 50 mg/L ZnO NPs was determined
191
to be 9.8 x 10-3 m-1, 14.1 x 10-3 m-1, 24.8 x 10-3 m-1 and 47.5 x 10-3 m-1 respectively. The
192
formation of ·OH was determined to be 0.15, 0.28, 0.49, and 0.81 µM/h for 5, 10, 25 and 50
193
mg/L ZnO NPs respectively. In order to evaluate the effect of HA on photocatalytic activity of
194
ZnO NPs, the photo degradation phenomenon observed under different HA conditions and
195
photolysis (in the absence of photo-catalyst) is presented in Fig. 6. The figure shows the linear
196
plots of ln(C/C0) for the photodegradation of MB with different HA concentrations. Here the rate
197
of reaction decreased when the concentration of HA increased. The rate constant for 10 mg/L
198
NPs was determined to be 16 x 10-3 m-1. In presence of HA the rate constant of the reaction was
199
decreased to 4.3 x 10-3 m-1, 3.3 x 10-3 m-1 and 1.9 x 10-3 m-1 for 1, 10 and 100 mg/L HA
200
respectively. The formation of ·OH was determined to be 0.15, 0.09, and 0.025 µM/h for 1, 10
201
and 100 mg/L HA respectively. Here, it can be seen that the formation of hydroxyl radicals was
202
reduced in presence of HA; hence it reduced the rate of MB degradation. Besides the essential
203
role played by oxygen in generating the oxidizing species (H2O2 and ·HO), the photogenerated
204
charge carriers at the semiconductor/liquid interface, is influenced to some extent by the surface
205
properties of the semiconductor, depending on pH, on surface hydroxyl groups and on adsorbed 9
206
charged molecules [20]. Yu et al. [21] was reported the influence of fluoride on photoactivity.
207
Bimodal mesoporous titania powders with high photocatalytic activity were observed when it
208
was prepared by hydrolysis of titanium tetraisopropoxide in the presence of HNO3 or NH4OH
209
under ultrasonic irradiation [22]. Here we can say that the photogenerated holes and electrons
210
could not react with OH-/H2O and O2 to form ·OH and O2-, respectively. Therefore, it was easy
211
to understand that the formation of lower level of ·OH in presence of HA.
212 213
Conclusion
214
The present study evaluated the effect of HA on the photocatalytic activity of ZnO NPs.
215
ZnO NPs exhibited excellent photocatalytic activity under visible light, but the presence of HA
216
decreased the photocatalytic effect of NPs. The study suggests that the presence of HA such
217
factors in the environment limited the photocatalytic activity of NPs; thereby reduce its toxic
218
effect on environmental organisms.
219 220 221 222
Acknowledgments Authors thank the management of SASTRA University for providing facility to carry out this work.
223 224 225 226 227 228 10
229
References
230
[1] A. Nel, T. Xia, L. Mädler, N. Li, Toxic potential of materials at the nanolevel. Science 311
231
(2006) 622–627.
232
[2] M. Premanathan, K. Karthikeyan, K. Jeyasubramanian, G. Manivannan, Selective toxicity of
233
ZnO nanoparticles toward Gram-positive bacteria and cancer cells by apoptosis through lipid
234
peroxidation. Nanomed-Nanotechnol. 7 (2011) 184-192.
235
[3] R. Wahab, N.K. Kaushik, A.K. Verma, A. Mishra, I.H. Hwang, Y.B. Yang, H.S. Shin, Y.S.
236
Kim, Fabrication and growth mechanism of ZnO nanostructures and their cytotoxic effect on
237
human brain tumor U87, cervical cancer HeLa, and normal HEK cells, J. Biol. Inorg. Chem.
238
16 (2011) 431–442.
239
[4] G. S. Mital, T. Manoj, A review of TiO2 nanoparticles, Chin. Sci. Bull. 56 (2011) 1639–1657.
240
[5] Hongbo Ma, Phillip L. Williams, Stephen A. Diamond, Ecotoxicity of manufactured ZnO
241 242 243 244
nanoparticles-A review, Environ. Pollut. 172 (2013) 76-85. [6] M.R.Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis. Chem. Rev. 95 ( 1995) 69-96. [7] P. Borm, F.C. Klaessig, T.D. Landry, B. Moudgil, J. Pauluhn, K. Thomas, R. Trottier, S.
245
Wood, Research strategies for safety evaluation of nanomaterials, part V: role of dissolution
246
in biological fate and effects of nanoscale particles. Toxicol. Sci. 90 , (2006a) 23-32.
247
[8] F. Gottschalk, T. Sonderer, R.W. Scholz, B. Nowack, 2009. Modeled environmental
248
concentrations of engineered nanomaterials (TiO2, ZnO, Ag, CNT, Fullerenes) for different
249
regions. Environmental Science & Technology 43 (24), 9216-9222.
11
250
[9] Woo-Mi Lee, Youn-Joo An, Effects of zinc oxide and titanium dioxide nanoparticles on
251
green algae under visible, UVA, and UVB irradiations: No evidence of enhanced algal
252
toxicity under UV pre-irradiation, Chemosphere 91, (2013) 536–544..
253 254 255
[10] I.Blinova, A. Ivask, M. Heinlaan, M. Mortimer, A. Kahru, Ecotoxicity of nanoparticles of CuO and ZnO in natural water. Environ. Pollut. 158 , (2010) 41-47. [11] L.Z.Li, D.M. Zhou, W.J.G.M. Peijnenburg, C.A.M. van Gestel, S.Y. Jin, Y.J. Wang, P.
256
Wang, Toxicity of zinc oxide nanoparticles in the earthworm, Eisenia fetida and subcellular
257
fractionation of Zn. Environ. Int. 37 (2011a) 1098-1104.
258
[12] P.Khare, M. Sonane, R. Pandey, S. Ali, K.C.Gupta, A. Satish, Adverse effects of TiO2 and
259
ZnO nanoparticles in soil nematode, Caenorhabditis elegans, J. Biomed. Nanotechnol. 7
260
(2011) 116-117.
261
[13] B.J.R.Thio, D. Zhou , A.A.Keller, Influence of natural organic matter on the aggregation
262
and deposition of titanium dioxide nanoparticles. J Hazard Mater 189 (2011) 556–563.
263 264 265 266
[14] D. Sun, et al., Effects of nitrogen content in monocrystalline nano-CeO2 on the degradation of dye in indoor lighting, Appl. Surf. Sci. 280 (2013) 693–697. [15] Q. Xiang, J. Yu, P.K. Wong, Quantitative characterization of hydroxyl radicals produced by various photocatalysts, J. Colloid Interf. Sci. 357 (2011) 163–167.
267
[16] A. Becheri ,M. Dürr , P.L. Nostro , P. Baglioni, Synthesis and characterization of zinc oxide
268
nanoparticles: application to textiles as UV-absorbers, J. Nanopart. Res. 10 (2008) 679–689.
269
[17] A. Franco, M.C. Neves, M.M.L.R. Carrott, M.H. Mendonc¸ a, M.I. Pereira, O.C. Monteiro,
270
Photocatalytic decolorization of methylene blue in the presence of TiO2/ZnS
271
nanocomposites, J. Hazard. Mater. 161 (2009) 545–550.
12
272
[18] H.R. Rajabi, O. Khani, M. Shamsipur, V. Vatanpour, High-performance pure and Fe3+-ion
273
doped ZnS quantum dots as green nanophotocatalysts for the removal of malachite green
274
under UV-light irradiation, J. Hazard. Mater. 250–251 (250) (2013) 370–380.
275 276 277
[19] Q.I. Rahman, M. Ahmad, S.K. Misra, M. Lohani, Effective photocatalytic degradation of rhodamine B dye by ZnO nanoparticles, Mater. Lett. 91 (2013) 170–174. [20] M.L. Curri, R. Comparelli, P.D. Cozzoli, G. Mascolo, A. Agostiano, Colloidal oxide
278
nanoparticles for the photocatalytic degradation of organic dye, Mater. Sci. Eng. C 23 (2003)
279
285–289.
280
[21] Jiaguo Yu, Shengwei Liu, Huogen Yu, Microstructures and photoactivity of mesoporous
281
anatase hollow microspheres fabricated by fluoride-mediated self-transformation, J. Catalysis
282
249, 2007, 59-66.
283
[22] Jiaguo Yu, Jimmy C. Yu, Mitch K.-P. Leung, Wingkei Ho, Bei Cheng, Xiujian Zhao, Jincai
284
Zhao, Effects of acidic and basic hydrolysis catalysts on the photocatalytic activity and
285
microstructures of bimodal mesoporous titania, J. Catalysis 217, 2003, 69-78.
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Figure captions
296
Figure 1. Characterization of ZnO NPs. (a) UV-visible absorption spectra (b) scanning electron
297
microscopic image, (c) particle size distribution and (d) X-ray diffraction pattern for ZnO NPs.
298
Figure 2. Effect of pH on the adsorption of MB on the surface of ZnO NPs in dark condition.
299
Figure 3. UV-visible absorption spectral changes and photodegradation of MB by ZnO NPs
300
under visible light.
301
Figure 4. UV-visible absorption spectra of MB solutions and the degraded dye solutions under
302
different conditions at the end of 60 min under visible light. Blank indicates the MB solution
303
simulated under visible light without NPs.
304
Figure 5. Linear plots of ln(C/C0) for the photodegradation of MB under visible light in the
305
absence and presence of different concentration of ZnO NPs.
306
Figure 6. Linear plots of ln(C/C0) for the photodegradation of MB by ZnO NPs under visible
307
light in the absence and presence of different concentration of HA.
308
14
Figure Figure 1
Figure 2
Adsorption (1-C/C0)
0.8 0.7
0.6 0.5 0.4 0.3 0.2 0.1 0 5
6
7
8
Initial pH value
9
10
Figure 3 0 min 10 min 20 min 30 min 40 min 50 min 60 min
0.7 Absorbance
0.6 0.5 0.4 0.3 0.2 0.1
1.2
Ct/C0 and 1-C/C0
0.8
C/C0 1-C/C0
1 0.8
0.6 0.4 0.2
0 450
550 650 Wave length (nm)
750
0 0
20
40 Time (min)
60
Absorbance
Figure 4 0.7
Original
0.6
Blank
0.5
10 mg/L NPs 1 mg/L HA
0.4
10 mg/L HA 0.3
100 mg/L HA
0.2 0.1 0
450
550
650 Wavelength (nm)
750
Figure 5 0.2 0 0
20
40
60
80
ln(C/C0)
-0.2
-0.4
Blank 5 mg/L NPs
-0.6
10 mg/L NPs -0.8
25 mg/L NPs 50 mg/L NPs
-1
Time (min)
Figure 6 0.2
0 0
20
40
60
80
ln(C/C0)
-0.2 -0.4
Blank
-0.6
10 mg/L NPs
-0.8
1 mg/L EPS 10 mg/L HA
-1
100 mg/L HA
-1.2 Time (min)
Research Highlights
ZnO NPs were synthesized by chemical co-precipitate method.
The mean diameter was determined to be 55 nm.
ZnO NPs exhibited excellent photocatalytic activity.
Presence of HA reduced the photocatalytic efficiency of ZnO NPs.
Fig. 1
