Photocatalytic degradation of herbicides under visible light using Ni-Pr2O3 nanocomposites

Journal of Alloys and Compounds xxx (2016) 1e6

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Photocatalytic degradation of herbicides under visible light using NiPr2O3 nanocomposites T. Sobahi Department of Chemistry, Faculty of Science, King Abdulaziz University, PO Box 80203 21589, Jeddah, Saudi Arabia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 September 2016 Received in revised form 12 October 2016 Accepted 26 October 2016 Available online xxx

Pr2O3 nanowires and Ni-Pr2O3 nanocomposites were successfully prepared by hydrothermal method and a photoassisted deposition methods, respectively. Pr2O3 was prepared as nanowires and nickel was deposited as metallic nickel. Photocatalytic degradation of 2,4-dichlorophenoxyacetic acid using visible light irradiation was used as a model reaction to measure photocatalytic performances of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites. Deposition of nickel on surface of Pr2O3 nanowires increase photocatalytic activity of Pr2O3 nanowires. Also, we noticed that wt % of deposited nickel play important role in control photocatalytic activity of Pr2O3 nanowires and the highest photocatalytic activity was achieved by using 0.3 wt % of nickel. 0.3 wt % Ni-Pr2O3 nanocomposites have photocatalytic stability for degradation of 2,4-dichlorophenoxyacetic acid for five times. © 2016 Elsevier B.V. All rights reserved.

Keywords: 2,4-Dichlorophenoxyacetic acid Pr2O3 Nickel metal Visible light

1. Introduction The growth inhibition of weeds and protection of crops from insect pests can be carried by using different types of pesticides and herbicides. Pesticides and herbicides caused many problems when them were transferred to rivers and groundwater. 2,4dichlorophenoxyacetic is the main component of greater than one thousands and fifty hundred of pesticides and herbicides [1]. 2,4-dichlorophenoxyacetic can be considered as highly toxic and carcinogenic pollutant, due to its high chemical and biological stability. Therefore, it is very hard to degrade [2,3]. Pesticides and herbicides can be removed by many methods such as biological, chemical and physical methods [4e7]. The disadvantages of these methods are high process costs, low removal efficiency, secondary pollution and high process time. Nowadays, The most effective method for removal of herbicides is the photocatalysis method [8e10]. Titanium dioxide is the most famous photocatalyst. The disadvantages for commercial using of titanium dioxide are high recombination rate and wide band gap. Many scientists used several ways to moving absorption edges of TiO2 from UV to visible region such as coupling [11,12], metal doping [13] and non metal doping [14,15]. The disadvantages of these methods are low doping concentration and low resistance for photocatalyst corrosion [16,17]. Recently, scientists prepared new materials have narrow

E-mail address: [email protected].

band gap [18e27]. According to our knowledge, this is first time to prepare Pr2O3 nanowires and Ni-Pr2O3 nanocomposites for degradation of 2,4-dichlorophenoxyacetic (2,4-D) under visible light irradiation. 2. Experimental 2.1. Photocatalyst preparation Pr2O3 nanowires were prepared by hydrothermal method as in the following steps: 4 mmol of Pr(NO3)3$6H2O was dissolved in 40 ml of deionized water and the pH of the aqueous solution was adjusted by HCL to be equal 1.3. Then, 8 mmol of glycine was added to above solution under stirring for 60 min. The resulting mixture was transferred to Teflon-lined stainless steel autoclave and was heated at 160
C for 24 h. The obtained materials were washed many times by deionized water and ethanol and finally, dried for 24 h at 100
C. Different of weight percentages of Nickel metal (0.1, 0.2, 0.3and 0.4 wt%) were deposited into surface of Pr2O3 nanowires by photo-assisted deposition method as in the following steps: dispersion one gram of Pr2O3 nanowires in an aqueous solution of nickel nitrate and the obtained dispersion mixtures were irradiated by strong UV light (Hg lamp, 150 W) for 24 h. The obtained materials were washed many times by deionized water and ethanol and finally, dried for 24 h at 100
C. Finally, The obtained materials were calcined for 2 h at 120
C in the presence of hydrogen gas (20 ml min1).

http://dx.doi.org/10.1016/j.jallcom.2016.10.257 0925-8388/© 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: T. Sobahi, Photocatalytic degradation of herbicides under visible light using Ni-Pr2O3 nanocomposites, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.257

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T. Sobahi / Journal of Alloys and Compounds xxx (2016) 1e6

2.2. Photocatalyst characterization

Pr2O3 The samples' nanostructure morphologies and dimensions were measured using a JEOL-JEM-1230 transmission electron microscope (TEM). Samples were suspended in ethanol and ultrasonicated for 30 min, dried in small amounts on carbon-coated copper grids, and loaded into the TEM. Additionally, N2-adsorption measurements were taken on treated samples (2 h under vacuum at 100
C) with a Nova 2000 series Chromatech apparatus at 77 K to calculate surface area. The crystalline phases of samples were determined by powder X-ray diffraction (XRD) using a Bruker axis D8 with CuKa radiation (l ¼ 1.540 Å) at room temperature. Xray photoelectron spectroscopy (XPS) measurements were taken on a Thermo Scientific K-ALPHA spectrometer. The band gap of all samples was determined by collecting the diffuse reflectance UVevisible absorption spectra in air at room temperature over the 200e800 nm range, as measured using a UVeViseNIR spectrophotometer (V-570, Jasco, Japan). Finally, photoluminescence emission spectra were obtained with a Shimadzu RF-5301 fluorescence spectrophotometer.

Intensity( a.u.)

0.1 wt % Ni/Pr2O3 0.2 wt % Ni/Pr2O3

0.3 wt % Ni/Pr2O3

0.4 wt % Ni/Pr2O3

20

30

40

50

60

70

80

2-Theta(degree) Fig. 1. XRD patterns of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites.

B

A

Pr3d3/2

O 1s

Intensity (a.u.)

Intensity (a.u.)

Pr3d5/2

924

926

928

930

932

934

936

938

940

526

528

530

532

534

536

538

540

Binding energy (eV)

Binding energy (eV)

C

Intensity, a.u.

Ni 2P1/2 Ni 2P

850

3/2

855

860

865

870

875

880

Binding energy (eV) Fig. 2. XPS spectra for Pr3d (A), O1S(B) and Ni 2p (C) of 0.3 wt % Ni/Pr2O3 nanocomposites.

Please cite this article in press as: T. Sobahi, Photocatalytic degradation of herbicides under visible light using Ni-Pr2O3 nanocomposites, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.257

T. Sobahi / Journal of Alloys and Compounds xxx (2016) 1e6

2.3. Photocatalytic performance

3. Results and discussion

To measure the photocatalytic performance of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites, a dose of photocatalyst was dispersed in 200 mL of 2,4-D aqueous solution (40 ppm) under airbubbling at a 6 mL/min flow rate. A 300 W Xenon lamp with 0.96 W/cm2 intensity simulated sunlight, while a cut-off filter was used to remove UV light (l < 420 nm). Before the lamp was turned on, the heterogeneous sample mixture was stirred in the dark for 1 h. Samples (2 mL) were taken from the reaction mixture at certain intervals, after which their remaining concentration of 2,4-D and intermediates was measured using HPLC. HPLCeUV-DAD (Agilent Technologies, series 1100) using a C18 SUPELCO column (5 mm, 150  3 mm) and UV detection (l ¼ 229 nm) was used for the following the decay of 2,4-D aqueous solution.

3.1. Characterization of photocatalysts

3

3.1.1. XRD examinations XRD patterns of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites are shown in Fig. 1. The results reveal that all samples are contain only Pr2O3, which means there is no peaks for nickel or nickel oxides, due to weight percent of nickel is low detection limit of XRD or nickel is highly dispersed above surface of Pr2O3. Also, we noticed that increase weight percent of nickel decrease intensity of characteristic peaks of Pr2O3, which means weight percent of nickel play important role in determining size of Pr2O3. Mean crystallite sizes calculated by the Scherrer formula for the samples were as follows: Pr2O3 16 nm;, 0.1 wt % Ni/Pr2O3 14 nm; 0.2 wt % Ni/Pr2O3

Fig. 3. TEM images of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites, where the weight percent of Ni is (A) 0.0; (B) 0.1; (C) 0.2; (D) 0.3 and (E) 0.4.

Please cite this article in press as: T. Sobahi, Photocatalytic degradation of herbicides under visible light using Ni-Pr2O3 nanocomposites, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.257

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T. Sobahi / Journal of Alloys and Compounds xxx (2016) 1e6 Table 1 BETsurface area nanocomposites.

of

Pr2O3

nanowires

and

SBET (m2/g)

Sample Pr2O3 0.1 wt 0.2 wt 0.3 wt 0.4 wt

% % % %

Ni-Pr2O3 nanocomposites. The results reveal that the values of BET specific surface areas are 60, 57, 55, 52 and 50 m2/g for Pr2O3, 0.1 wt % Ni/Pr2O3, 0.2 wt % Ni/Pr2O3, 0.3 wt % Ni/Pr2O3 and 0.4 wt % Ni/ Pr2O3, respectively. Therefore, the BET surface areas of Pr2O3 nanowires are higher than those of Ni-Pr2O3 nanocomposites. Due to block of some pore of Pr2O3 by deposition of nickel.

Ni-Pr2O3

60.00 57.00 55.00 52.00 50.00

Ni/Pr2O3 Ni/Pr2O3 Ni/Pr2O3 Ni/Pr2O3

12 nm; 0.3 wt % Ni/Pr2O3 10 nm and 0.4 wt % Ni/Pr2O3 9 nm. Thus, Ni: Pr2O3 nanowires had finer crystals than both pure Pr2O3 nanostructures. 3.1.2. XPS examinations XPS spectra for Pr3d (A), O1S(B) and Ni 2p (C) of 0.3 wt % NiPr2O3 nanocomposites are shown in Fig. 2. The Pr was present as Pr3þ ion by presence of two peaks of Pr3d5/2 and Pr3d 3/2 at 933.4 and 929.0 eV, respectively as shown in Fig. 2 A. The oxygen was present as O2 ion by presence of one peak of O1s at 531.3 eV, as shown in Fig. 2 B. Therefore, Pr and O form Pr2O3, which is agree with XRD results. The Ni was present as metallic nickel by presence of two peaks of Ni2p3/2 and Ni2p1/2 at 852.5 and 870.0 eV, respectively as shown in Fig. 2C. 3.1.3. TEM examinations TEM images of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites are shown in Fig. 3. The results reveal that shape of Pr2O3 and NiPr2O3 samples are nanowires. Therefore, deposition of nickel metal on surface of Pr2O3 has no significant effect on its shape. Also, we noticed that nickel was deposited on surface of Pr2O3 as dotes as shown in Fig. 3 B to E. Increase weight percent of deposited nickel from 0.1 to 0.3 wt % increase dispersion of nickel on surface of Pr2O3. But, the aggregation of deposited nickel was observed by increase weight percent of deposited nickel above 0.3 wt % as shown in Fig. 3 E. Thus, dispersion of deposited nickel on surface of Pr2O3 can be controlled by control weight percent of deposited nickel. 3.1.4. BET surface area examination Table 1 shows BET specific surface areas of Pr2O3 nanowires and

3.1.5. UVeVis spectral examinations Fig. 4 shows UVeVis spectra of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites. The results reveal that Pr2O3 nanowire absorb in UV region and has absorption edge at about 363 nm. Ni-Pr2O3 nanocomposites samples absorb in visible region. Also, we noticed that weight percent of deposited nickel plays important role in determining amount of absorption edge of Pr2O3. Increase weight percent of nickel from 0.1 to 0.2 to 0.3 wt % increase absorption edges of Pr2O3 from 427 to 450e470 nm, respectively. But, increase weight percent of nickel from 0.3 to 0.4 wt % has no significant effect on absorption edge of Pr2O3. We used the following formula for calculating band gaps from obtained diffuse reflectance spectra:

band gap energy ðEÞ ¼ h 

C

l

where h is Planck's constant, C is speed of light, and l is cut off wavelength of recorded spectral data. The values of band gap energy are 3.41, 2.90, 2.75, 2.64 and 2.62 eV for Pr2O3, 0.1 wt % Ni/ Pr2O3, 0.2 wt % Ni/Pr2O3, 0.3 wt % Ni/Pr2O3 and 0.4 wt % Ni/Pr2O3. Thus, weight percent of nickel deposited on Pr2O3 is very important factor for control band gap energy of Pr2O3. 3.1.6. PL spectral examinations Fig. 5 shows PL spectra of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites. The results reveal that Pr2O3 nanowire has PL emission spectra greater than that Ni-Pr2O3 nanocomposites. Also, we noticed that weight percent of deposited nickel plays important role in determining PL emission spectra of Pr2O3. Increase weight percent of nickel from 0.1 to 0.2 to 0.3 wt % decrease PL emission spectra of Pr2O3. But, increase weight percent of nickel from 0.3 to 0.4 wt % has no significant effect on PL emission spectra of Pr2O3. Thus, weight percent of nickel deposited on Pr2O3 is very important factor for control e-h recombination rate by control PL emission

Pr2O3

Pr2O3

0.1 wt % Ni/Pr2O3

0.1 wt % Ni/Pr2O3

0.2 wt % Ni/Pr2O3

0.2 wt % Ni/Pr2O3 0.4 wt % Ni/Pr2O3

200

300

400

500

600

700

0.3 wt % Ni/Pr2O3

Intensity, a.u.

Absorbance( a.u.)

0.3 wt % Ni/Pr2O3

800

Wavelength (nm) Fig. 4. UVeVis spectra of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites.

0.4 wt % Ni/Pr2O3

330

360

390

420

450

480

510

540

Wavelength,nm Fig. 5. PL spectra of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites.

Please cite this article in press as: T. Sobahi, Photocatalytic degradation of herbicides under visible light using Ni-Pr2O3 nanocomposites, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.257

100

100

90

90

80 70 60 50 40

Pr2O3

30

0.1 wt % Ni/Pr2O3 0.2 wt % Ni/Pr2O3

20 10 0

Photocatalytic removal of 2,4 D, %

Photocatalytic removal of 2,4 D, %

T. Sobahi / Journal of Alloys and Compounds xxx (2016) 1e6

80 70 60 50 40 30 20

0.3 wt % Ni/Pr2O3

10

0.4 wt % Ni/Pr2O3

0

1 0

10

20

30

40

50

60

70

80

5

st

90 100 110 120

2

nd

rd 3

th 4

th 5

Number of cycles

Reaction time, min Fig. 6. Effect of weight percent of nickel on removal of 2,4-D.

Fig. 8. Photocatalytic stability of 0.3 wt % Ni/Pr2O3 nanocomposites for removal of 2,4D for five times.

spectra of Pr2O3. The values of band gap energy which calculated from PL emission spectra are 3.42, 2.92, 2.76, 2.65 and 2.63 eV for Pr2O3, 0.1 wt % Ni/Pr2O3, 0.2 wt % Ni/Pr2O3, 0.3 wt % Ni/Pr2O3 and 0.4 wt % Ni/Pr2O3, respectively, which are very close to values calculated from UVeVis spectra.

further increase above 0.3 wt % has no significant effect on photocatalytic activity. Therefore, wt % of Ni plays important role in controlling photocatalytic activity of Pr2O3 photocatalyst, which are in agreement with UVeVis, PL and BET results.

3.2. Photocatalytic activity 3.2.1. Effect of weight percent of nickel on removal of 2,4-D The results of adsorption of Pr2O3 nanowires and Ni-Pr2O3 nanocomposites under dark conditions reveal that almost all samples have no adsorption ability and the reaction is photocatalytic reaction only. Fig. 6 shows effect of weight percent of nickel on removal of 2,4-D. The results reveal that Pr2O3 nanowires have no photocatalytic activity, because Pr2O3 nanowires absorb in UV region and reaction was carried out under visible region. Also, we noticed that deposition of metallic nickel on surface of Pr2O3 nanowires increase photocatalytic removal of 2,4-D from 3 to 100% by deposited 0.3 wt % of nickel. Increase wt % of Ni from 0.1 to 0.3 wt % increase photocatalytic activity from 69 to 100%, respectively and

110

0.3 g/l 0.6 g/l 0.9 g/l 1.2 g/l 1.6 g/l

Photocatalytic removal of 2,4 D, %

100 90 80

3.2.2. Effect of the dose of 0.3 wt % Ni/Pr2O3 nanocomposites on removal of 2,4-D Fig. 7 shows effect of the dose of 0.3 wt % Ni/Pr2O3 nanocomposites on removal of 2,4-D. The results reveal that increase photocatalyst dose of 0.3 wt % Ni/Pr2O3 nanocomposites from 0.3 to 0.6 g/l increase photocatalytic activity for removal of 2,4-D from 85 to 100%, respectively after 120 min. Increase photocatalyst dose of 0.3 wt % Ni/Pr2O3 nanocomposites from 0.6 to 0.9e1.2 g/l decrease reaction time which required for complete removal of 2,4-D from 120 to 90 to 60 min, respectively. Due to, increase photocatalyst dose increase available active sites for photocatalytic reaction and so increase photocatalytic performance of 0.3 wt % Ni/Pr2O3 nanocomposites which decrease reaction time. Also, we noticed that increase photocatalyst dose of 0.3 wt % Ni/Pr2O3 nanocomposites above 1.2 g/l decrease photocatalytic activity and increase photocatalytic reaction time, due to large dose of photocatalyst dose of 0.3 wt % Ni/Pr2O3 nanocomposites can hinder the penetration of light to surface of photocatalyst and hence decrease photocatalytic activity. 3.2.3. Photocatalytic stability of 0.3 wt % Ni/Pr2O3 nanocomposites for removal of 2,4-D for five times Fig. 8 shows photocatalytic stability of 0.3 wt % Ni/Pr2O3 nanocomposites for removal of 2,4-D for five times. The results reveal that 0.3 wt % Ni/Pr2O3 nanocomposites has photocatalytic stability for degradation of 2,4-D for five times.

70 60 50 40

4. Conclusions

30 20 10 0

0

10

20

30

40

50

60

70

80

90 100 110 120

Reaction time, min Fig. 7. Effect of the dose of 0.3 wt % Ni/Pr2O3 nanocomposites on removal of 2,4-D.

Herein we report on an efficient, hydrothermal synthesis of Ni/ Pr2O3 nanowires. wt % of deposited nickel was critical to controlling Pr2O3's band gap. Adding 0.3 at % Ni red shifted the absorption edge to the visible light region. The resultant, characterized Ni/Pr2O3 photocatalysts performed excellently under visible light irradiation for 2,4-dichlorophenoxyacetic acid; moreover, they are easily recyclable. Thus, Ni/Pr2O3 nanowires possess potential applications in solar energy environmental remediation of pollutant herbicides.

Please cite this article in press as: T. Sobahi, Photocatalytic degradation of herbicides under visible light using Ni-Pr2O3 nanocomposites, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.257

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Acknowledgements This work was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number (130-4-D1437). The author, therefore, thankfully acknowledges DSR's technical and financial support. References
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Please cite this article in press as: T. Sobahi, Photocatalytic degradation of herbicides under visible light using Ni-Pr2O3 nanocomposites, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.257