Photocatalytic activity of Ag-N co-doped ZnO nanorods under visible and solar light irradiations for MB degradation

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Photocatalytic activity of Ag-N co-doped ZnO nanorods under visible and solar light irradiations for MB degradation Tesfay Welderfael ∗ , Manjunatha Pattabi, Rani M. Pattabi, Arun Kumar Thilipan G Department of Materials Science, Mangalore University, Mangalagangothri 574 199, India

a r t i c l e

i n f o

Article history: Received 28 April 2016 Accepted 3 November 2016 Available online xxx Keywords: Nanorods Dyes Degradation Irradiations

a b s t r a c t Zinc oxide nanorods (CZ), silver-doped zinc oxide nanorods (CAZ), nitrogen-doped zinc oxide nanorods (CNZ), and silver-nitrogen co-doped zinc oxide nanorods(CANZ) were synthesized using the chemical precipitation method. The as-synthesized nanorods were characterized using x-ray diffraction (XRD); scanning electron microscopy (SEM), and UV–vis spectroscopy (UV–vis). The photocatalytic degradation of methylene blue (MB) was studied using the as-synthesized nanorods under both solar and visible light irradiations. CANZ has the highest photocatalytic activity under both visible as well as solar light irradiations; with CNZ, CAZ and CZ show a decreasing order in their photocatalytic activities. In addition to this, the photocatalytic activity of the as-synthesized nanorods was compared under both light sources. The photocatalytic activity of all the nanorods is higher under solar light as compared to the visible light irradiation. Therefore, sun light is the best light source for the removal of dyes from the environment. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, environmental pollution is a critical problem of the world. Different factors may cause this environmental pollution. Organic dyes are commonly used in different industries, especially in developing countries, including the textile, paper printing, food processing, cosmetics, pharmaceutical and leather industries. The presence of strong aromatic rings in the dye molecules resists the destruction of the structure of these molecules by physico–chemical treatment methods, and therefore, it is essential to find out effective methods for the degradation of dyes [1]. Recently, ultrasonic irradiation has been attracting attention as an advanced oxidation process for the removal of the pollutants in water [2]. Sonication of water encourages the formation, growth and collapse of small gas bubbles. The collapsing of bubbles in aqueous solution produces extremely high temperature and pressure conditions [3]. The high temperature near the bubbles leads to thermal decomposition of H2 O molecules to H• and • OH radicals and these free radicals can directly be reacted with the organic dyes. The dye removal rate in this method is still very low in comparison to other methods and consumes more time and energy [2].

∗ Corresponding author. E-mail addresses: [email protected] (T. Welderfael), [email protected] (M. Pattabi).

Heterogeneous photocatalysis is an excellent technology for the removal of organic dyes from wastewater. It has great potential as an environmental friendly, low cost and sustainable technology. Several researches have made attempts to remove these dyes using various photocatalysts like TiO2 [4], Fe2 O3 [5], ZnO [6] and CuO [7] because of their capability for removal of dyes present in the wastewater. Zinc oxide nanorod is one of the various semiconductors, which stands out due to its properties and wide applications. Zinc oxide has a large band gap energy of 3.37 eV, larger binding energy (60 eV), efficient absorption of larger fraction of solar energy and of low cost. Therefore, it is taken as a competitive nanomaterial with potential uses in photocatalysis [8]. Much research has been focused on addressing the enhancement of the photocatalytic activities of the nanomaterials through different approaches such as alloying with low band gap semiconductors, doping with metals and non metals, co-doping with metal-metal, or metal-non metal, non metal-non metal etc. to achieve effective electron transport [9]. The photocatalytic activity can be improved by creation of polar surfaces and doping with metals [10]. Unsaturated coordination bonds are highly reactive and are favorable for the adsorption of oxygen molecules resulting in a large number of H2 O2 radicals which can improve the photocatalytic properties. Zinc oxide prism quantum dots with polar surfaces adsorbed OH− ions, resulting in improvement of its photocatalytic activity [10].

http://dx.doi.org/10.1016/j.jwpe.2016.11.001 2214-7144/© 2016 Elsevier Ltd. All rights reserved.

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Modification of zinc oxide nanomaterial can delay electron–hole recombination, and broaden the absorption spectrum to facilitate some specific reactions on the surface of catalysts and to improve photocatalytic activity. Doping of zinc oxide with transition metals like Ag, Pd, Pt, Au and Fe have solved the problems of anodic photo-corrosion, and instability of zinc oxide nanorods in acidic and alkaline media [11]. Zinc oxide has high band gap (= 3.37 eV) which limits light photo-absorption to the UV region only. It is also possible to extend zinc oxide photo-absorption into visible region. Modification of zinc oxide with nonmetals such as N, F, S, P etc will reduce the band gap energy of zinc oxide nanorods. Nitrogen is a nonmetal element which is widely acceptable owing to its compatible size to oxygen and has the smallest ionization energy [12]. Doping nitrogen in zinc oxide will reduce its band gap and shift the photo-absorption edge to the visible region. There are some literature reports about nitrogen-doped zinc oxide [12] and silver-doped zinc oxide nanoparticles [11]. But no reports are found on silver-nitrogen co-doped zinc oxide nanorod. The only report found, to the best of our knowledge, is nitrogendoped zinc oxide/silver nanocomposite reported by Dou et al. [13]. Here, we describe a very simple and rapid method to synthesize zinc oxide nanorod and silver-nitrogen co-doped zinc oxide nanorods and their photocatalytic property under visible light and solar light. 2. EXPERIMENTAL 2.1. Materials All the chemicals are analytical grade and used without any further purification. Zinc nitrate hexahydrate Zn(NO3 )23 ·6H2 O, >99.5%, MERCK, and Ammonium hydroxide (NH3 ·OH2 , >98% MERCK) are the precursors. Ethanol (CH3 CH2 OH > 97.9% pure) is the solvent, Silver nitrate (AgNO3 > 99.9%, MERCK) acts as the source of silver, urea ((NH2 )2 CO MERCK) is the nitrogen source, Cetyltrimethyl ammonium bromide (CTABr), (C19 H42 BrN, Sigma Aldrich) acts as stabilizer, Methylene Blue (C16 H18 ClN3 S, MERCK) is the organic dye.

stirred well. A wet material was formed. The wet material was calcined in a furnace at 500 ◦ C. Then, silver-doped zinc oxide nanorods (CAZ) were obtained. iii Synthesis of nitrogen-doped zinc oxide nanorods Urea was added to the as-synthesized zinc oxide nanorod. The mixture was ground and mixed well and calcined in a furnace at 500 ◦ C. The nitrogen-doped zinc oxide nanorods (CNZ) were obtained [15]. iv Silver-nitrogen co-doped zinc oxide nanorods Nitrogen-doped zinc oxide nanorods were taken in a ceramic crucible. Silver nitrate solution (0.5 M) was added. It was stirred and mixed well. Then, the sample was heated in a furnace at 500 ◦ C. Silver-nitrogen co-doped zinc oxide nanorods (CANZ) are formed. 2.3. Degradation of dyes The photocatalytic activities of the as-synthesized nanorods were examined using the degradation of methylene blue (MB) under both visible and solar light irradiations. 200 mg of the assynthesized nanorods were dispersed in 125 ml of MB aqueous solution (20 mg/L). The mixed suspension was magnetically stirred for 30 min in dark to reach an adsorption–desorption equilibrium. Under ambient conditions and stirring, the mixed suspension was exposed to a 300 W power of xenon lamp (visible light) at 6 cm distance from the solution and solar light for different time duration. At 15 min time intervals, the mixed suspension was extracted and centrifuged at 5000 rpm (revolution per minute) to remove the photocatalyst. The degradation process was monitored by measuring the absorption of MB in the filtrate at 663 nm [16]. The photocatalytic degradation efficiency was calculated using the following equation: % of Degradation = (C0 − C)/C0 × 100

(1)

where C is the concentration at time t and C0 is the initial concentration.

2.2. Synthesis 2.4. Characterization i Pure zinc oxide nanorod Zinc nitrate hexahydrate was dissolved in distilled water (solution A), and Cetyltrimethyl ammonium bromide (CTABr) was dissolved in ethanol (solution B). Then, solution A was added to solution B under continuous stirring. Ammonium hydroxide solution was added to the above solution. Subsequently, the assynthesized solutions were heated at 100 ◦ C for eight hours. The white precipitates formed were filtered and dried in an oven to obtain zinc oxide nanorod. The sample was calcined in a furnace at 500 ◦ C and calcined zinc oxide nanorods (CZ) were obtained [14]. The reaction proceeds as follows: Zn(NO3 )2 · 6H2 O+2NH4 OH

CTABr

→

Zn(OH)2 + 2NH4 NO3 +6H2 O

H2 O,Ethanol

The absorption spectra of the nanorods were recorded in the range of 300–800 nm using a Shimadzu spectrophotometer model UV-1800 operated at a resolution of 1 nm. Powder XRD patterns of the samples were recorded using a Rigaku Miniflex 600 Powder X- ray diffractometer with Cu-K␣ radiation (␭ = 1.54056 Å). The scanning speed was 30 /min with step size of 0.05 from 20 to 800 . The acceleration voltage and emission current is 50 kV and 50 mA, respectively. Morphological characterization of CZ, CAZ, CNZ, and CANZ is carried out using a Carl Zeiss Field Emission Scanning Electron Microscope (FESEM) model igma operated at an accelerating voltage of 3 kV and at a working distance of ∼8 mm. 3. Result and discussion 3.1. XRD analysis

Heat

Zn(OH)2 → ZnO(S) + H2 O(L) at1000 C

ii Silver-doped zinc oxide nanorods The as-synthesized zinc oxide nanorod was taken in a ceramic crucible. Silver nitrate (AgNO3 ) solution (0.5 M) was added and

The XRD patterns of CZ, CAZ, CNZ and CANZ are shown in Fig. 1A–D. The diffraction peaks appearing at 2 = 31.890 , 34.40 , 36.60 , 47.60 , 56.70 , 62.90 , 66.50 , 68.40 , 69.60 , 72.50 , and 77.40 in all correspond to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (004) and (2 0 2) planes of the hexagonal wurtzite structure of CZ compared with (standard JCPDF card number of 79-2205). All the peaks are sharp and narrow. No additional

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0.04

b a

0.00 300

400

500

600

700

800

Wavelength (nm) Fig. 3. UV–vis absorption spectra of (a) CZ and (b) CAZ. Fig. 1. X-ray diffractogram of (A) CZ (B) CAZ (C) CNZ (D) CANZ.

Absorbance

0.08

b a

0.00

300

400

500

600

700

800

Wavelength (nm) Fig. 4. UV–vis absorption spectra of (a) CNZ and (b) CANZ. Fig. 2. The expanded X-ray diffractogram of (A) CAZ and (B) CANZ.

peaks are observed, indicating the high purity and crystallinity of the as-synthesized CZ [17]. Intense, narrow and sharp reflections corresponding to CZ appear in the pattern from all the samples. Two comparatively very small additional peaks are observed at 38.5 and 44.5 in the diffraction pattern from CAZ and CANZ. As they are not clearly seen in Fig. 1, the region where these peaks appear is expanded and reproduced in Fig. 2. The peaks can be indexed to the (111) and (200) planes of Ag. (standard JCPDF card number 89-3722). Silver is only deposited in the surface of CZ rather than going in to the lattice [18]. No additional peaks other than that of CZ and Ag are observed. The presence of these peaks does not affect the structure of CZ as no shifts in the diffraction peaks of CZ are observed. If silver is incorporated into the lattice of zinc oxide, the diffraction peaks would have shifted because the ionic radii of Ag+ (1.22 Å) are much larger than that of Zn2+ (0.74 Å). Therefore, we conclude that Ag does not get incorporated into the lattice of zinc oxide, but gets deposited on the surface of zinc oxide. No diffraction peaks are observed for CANZ except Ag and zinc oxide indicating that the presence of nitrogen is below the XRD detection limit. Iftekhar Uddin et al. [19], carried out similar work on Ag-C co-doped zinc oxide nanorods and obtained similar results.

The average crystallite size of the as-synthesized nanorods was estimated from the full width at half maximum of the most intense diffraction peaks using the Debye Scherer equation: D = 0.89

 ˇ Cos 

(2)

where, d is the average crystallite size, ␭ is the wavelength of radiation used, ␪ is the Bragg angle and ␤ is the angular width of the diffraction peak or full width at half maximum (FWHM) on the 2␪ scale. The average crystallite size of CZ, CAZ, CNZ and CANZ is 49.9, 48.3 46.6 and 45.3 nm, respectively. As can be seen, the average crystal sizes decrease from CZ through CAZ, CNZ and CANZ. Nipane calculated the average crystallite size of CZ obtained similar results for carbon-doped zinc oxide [20]. 3.2. The UV–vis analysis Figs. 3a, b, and 4a, b shows the UV–vis absorption spectra recorded in the wavelength range of 300–800 nm for CZ, CAZ, CNZ, and CANZ, respectively. CZ, CAZ, CNZ and CANZ showed absorption edges at 380, 387, 534 and 553 nm, respectively. For CZ and CAZ, the absorption edge is very sharp and it lies in the UV region, showing the larger band gap of the nanomaterial [21]. Compared to CZ the absorption spectra of CAZ are shifted towards visible region. Doping silver into CZ does not reduce much. But as shown later it assists to hinder the electron-hole recombination, enhancing the photocatalytic activity of CZ [11].

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Fig. 5. The plot of (␣h)2 versus h for CZ, CAZ, CNZ and CANZ.

The absorption edge of CZ is extended to 534 nm by doping with nitrogen and then, further to 553 nm by co-doping with silver and nitrogen. The optical band gaps (Eg,) of the as-synthesized nanorods are calculated from the Tauc plot of the variation of the absorption coefficient (␣) with photon energy using the equation [22]:



˛hv = A

˛hv − Eg

(3)

where A is constant, h is Planck’s constant, v is frequency, ␣ is absorption coefficient, and Eg band gap. The band gap can be obtained by plotting (␣h)2 versus h by extrapolating the linear portions of the plots onto the energy axis, as shown in Fig. 5. The Eg values obtained are tabulated in Table 1. An apparent decrease in Eg from 3.10 to 2.17 eV is observed by co-doping CZ with silver and nitrogen.

Table 1 Band gap energies of CZ, CAZ, CNZ and CANZ. Name of nanorods Band gap (eV)

CZ 3.10

CAZ 3.07

CNZ 2.25

CANZ 2.17

3.3. The SEM analysis The SEM images of CZ, CAZ, CNZ, and CANZ are shown in Fig. 6A–D, respectively. The as-synthesized CZ shows a rod shape structure. The diameters of as-synthesized CZ, CAZ, CNZ and CANZ are around 200 nm and have a length of 1 ␮m. CZ showed agglomerated nanorods; however, the boundaries between crystallites are clearly observable. Fu and Fu, [23] studied the morphology of CZ, and similar results are obtained (with diameter of 180 nm). The surface morphology of CAZ is homogeneous and relatively smooth suggesting the high dispersion of silver metal on the surface of CZ. The structure of CZ is maintained and do not change during deposition of silver metal on zinc oxide. A great amount of rod-like nanocrystals were connected with each other forming multi-linked rods [24]. The size and shape of CNZ seem to be rod shaped, and remains unchanged after coverage of nitrogen to the nanorod. Moreover, it is very difficult to visualize the changes in size of CZ after nitrogen doping in the SEM images. Again, larger amounts of rod-like nanomaterials are connected with each other forming multi-linked rod [25]. As seen from Fig. 6 D the structure of CZ has changed into flat forms. This may be because of the heating for longer hours. As seen in Fig. 6B and C the structure of CZ are not changed much in comparison to D. In the case of CANZ the sample was heated for about

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Fig. 6. The SEM image of (A) CZ, (B) CAZ, (C) CNZ and (D) CANZ.

3.4. Photocatalytic degradation of MB Fig. 7 shows the degradation of MB (methylene blue) dye using CZ, CAZ, CNZ and CANZ under visible light irradiations. MB dye was removed by about 30, 35, 49 and 53% using CZ, CAZ, CNZ and CANZ, respectively within 135 min under visible light irradiations. From the above data, CANZ has the highest photocatalytic activity while CZ has the lowest photocatalytic activity for the removal of MB dye within the same time interval. CANZ is the only nanorod degrading the dye more than 50% within 135 min whereas for the other three nanorods the dye removal efficiency is less than 50%. The photocatalytic activity of CNZ is higher than CAZ and CZ, because CZ has two main problems with the degradation of MB dye [27]. These are (1). The recombination of electron and holes from the conduction and valence band respectively, after a few microseconds of separation (2) The large band gap of CZ. Doping of CZ with metals such as silver, gold, copper etc. hinders the recombination of electrons and holes, which enhances the photocatalytic activity of CZ [28]. Doping with non-metals such as nitrogen, sulfur, fluorine, etc which reduces the band gap of CZ by extending photo-absorption edge to the visible light region [29]. The combination of both silver and nitrogen improves the photocatalytic activity because silver hinders the recombination of electrons and holes, and nitrogen reduces the band gap energy by extending the

80

Percentage of Degradation

8 h due to the addition of silver and nitrogen [26]. Therefore, most of the structure of CANZ is flat.

CZ CAZ CNZ CANZ

60

40

20

0

0

30

60

90

120

150

Time (Min) Fig. 7. Degradation of MB using CZ, CAZ, CNZ and CANZ under visible light irradiations.

photo-absorption edge to the visible light region. Senthilraja et al. [30] carried out similar work on the degradation of acid red (AR 18) using Au-Sr co-doped zinc oxide, AR 18 degradation was completed within short period of time due to the enhancements of the nanomaterial.

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Percentage of Degradation

150

O2

CZ CAZ CNZ CANZ

O2

Ag

e-

CB

hv

100

3.10 eV = ZnO

H2 O

50

2.25 eV = N-ZnO

h

OH

+

VB

H2O2 OH + Dye

+ Dye 0

Degradation product 0

30

60

90

120

150

Degradation product

Fig. 10. Diagram of the degradation mechanism of MB.

Time (Min) Fig. 8. Degradation of MB using CZ, CAZ, CNZ and CANZ nanorods under solar light irradiations.

Percentage of Degradation

250

CZ - visible CAZ - visible CNZ - visible CANZ - visible

200

CZ - solar CAZ - solar CNZ - solar CANZ - solar

150

100

50

0

0

30

60

90

120

150

Time (Min) Fig. 9. Comparison of degradation of MB using CZ, CAZ, CNZ and CANZ under visible and solar light irradiations.

The photocatalytic degradation of MB using CZ, CAZ, CNZ and CANZ under solar light irradiations is shown in Fig. 8. The removal of MB dye was completed by about 68.6, 82.1, 100% using CZ, CAZ; CNZ within 135 min respectively, and the degradation of MB was completed 100% in the presence of CANZ within 75 min. CANZ has the highest photocatalytic activity for solar irradiation showing that the combinations of the dopants and the light source are the best solution for the removal of the organic dye while CZ has the lowest photocatalytic activity. The comparison of the degradation of MB using the assynthesized nanorods under both visible and solar light is shown in Fig. 9. The degradation of MB dye is greater under solar than visible light irradiations in the presence of the as-synthesized nanorods. The power source of the sun light being stronger than the visible light help in the formation of more holes in the valence band (VB) and electrons in the conduction band (CB) as well as formation of hydroxide radicals for attacking the dyes. Therefore, solar light radiations are better sources of light for the degradation of dyes in the presence of modified and unmodified nanorods [31]. According to Tesfaye and Aynalem [31], the degradation of malachite green was

completed by more than 85% under solar light irradiations whereas only 80% of the dye was degraded under visible light irradiations. In general, solar light is a good source for the photocatalytic degradation of organic dyes.

3.4.1. Mechanism Fig. 10 shows the photocatalytic degradation mechanism of MB using CZ, CAZ, CNZ and CANZ under light irradiations. When a semiconductor is irradiated by light, electrons are excited from the VB to CB, and holes are formed in the VB. In general, after a few micro seconds the electron and holes recombine. The metal silver traps electron from CB of zinc oxide hindering the electron-hole recombination [32]. As Ag traps more number of electrons, more number of superoxide radical anions is formed and simultaneously the VB holes of zinc oxide react with water to produce highly reactive hydroxyl (• OH) radicals. The production of more superoxide radical anions and hydroxyl radicals are essential for attacking the dyes [33]. In addition to this modification of the semiconductor with non metals is essential for the degradation mechanism of the dye. When nitrogen is doped into zinc oxide, the wavelength of the absorption edge of zinc oxide is shifted to the visible light region. Therefore, there is a formation of new valence band. The production of new VB by doping with nitrogen helps to narrow the band gap, and the electrons on VB of nitrogen-doped zinc oxide nanorods could absorb the energy from sunlight more easily than pure zinc oxide nanorods [31].

4. Conclusion CZ, CAZ, CNZ and CANZ were synthesized by chemical precipitation method under the same calcination temperatures. The structure of the as-synthesized nanorods, the morphology, and the absorption spectra of the as-synthesized nanorods were studied using XRD, SEM, and UV–vis spectroscopy. The as-synthesized nanorods acts as photocatalysts for degrading MB dye. CANZ has the highest photocatalytic activity for MB degradation under both solar and visible light irradiations; while CZ has the lowest photocatalytic activity for the degradation of MB. In addition to this CNZ has lower photocatalytic activity than CANZ nanorods. The degradation of MB was also conducted using the nanorods under both solar and visible light irradiations. The degradation of the dye was highest using the nanorods under solar light irradiation but smaller in case of visible light irradiations.

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Please cite this article in press as: T. Welderfael, et al., Photocatalytic activity of Ag-N co-doped ZnO nanorods under visible and solar light irradiations for MB degradation, J. Water Process Eng. (2016), http://dx.doi.org/10.1016/j.jwpe.2016.11.001