zinc oxide hollow spheres: Visible light nanophotocatalysts

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CERAMICS INTERNATIONAL

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Cobalt/zinc oxide hollow spheres: Visible light nanophotocatalysts Reda M. Mohameda,b, David McKinneyc, Mohammad W. Kadia, Ibraheem A. Mkhalida, Wolfgang Sigmundd,n a

Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia Center of Excellence in Environmental Studies, King Abdulaziz University, P.O. Box 80216, Jeddah 21589, Saudi Arabia c MM Virtuoso, Gainesville, FL 32606 USA d University of Florida, College of Engineering, Department Materials Science and Engineering, 225 Rhines Hall, P.O. Box 116400, Gainesville, FL 32611-6400 USA b

Received 25 July 2015; received in revised form 5 September 2015; accepted 6 October 2015

Abstract Organic dyes used by food and textile industries, e.g. Malachite green dye (MG), contaminate surface and ground waters. Photocatalysis with ultraviolet light (UV) irradiation of zinc oxide (ZnO) nanoparticles can remove such industrial effluents. However, to more efficiently use solar energy, a photocatalyst active over a wider range of the visible light spectrum remains desirable for environmental remediation. Thus, we synthesized visible light photoactive cobalt/zinc oxide hollow sphere nanostructures with a hydrothermal process, and we used a number of characterization techniques to confirm that doped cobalt (Co) ions replace some zinc (Zn) ions in the lattice. The as-prepared Co doped hollow spheres performed 55.9 times better in Malachite green dye visible light photodegradation tests than similar ZnO nanoparticles. Furthermore, the Co/ZnO hollow spheres are recyclable. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Cobalt/zinc oxide; Dyes; Hollow spheres; Nanostructures; Visible light photocatalysis

1. Introduction Textile industries use highly toxic and non-biodegradable organic dyes, which can be carcinogenic and genotoxic to humans [1]. Malachite green dye (MG) is one ubiquitously found substance used to color silk, wool, leather, cotton, jute, paper, and even foods [2]. This dye, also known as C23H25N2Cl and 4-[(4-dimethylaminophenyl)-phenyl-methyl]N,N-dimethylaniline, and its reduced form, leucomalachite green, exhibit strong effects on the immune and reproductive systems [3]. Highly water or ethanol soluble [4], MG must first be removed from industrial effluents before the water is returned to the environment. Nanostructural zinc oxide's (ZnO) photocatalytic abilities to degrade organic contaminants in air and water and to convert them n

Corresponding author. E-mail address: sigmund@ufl.edu (W. Sigmund).

into benign materials safe for the environment and humans are well documented [5]. Nanoscale ZnO exhibits high photocatalytic activity because of its numerous active sites and significant surface area [6], has lower preparation costs than TiO2 [7], and is environmentally benign [8]. Unfortunately, its wide band gap (
3.37 eV) hinders photocatalytic applications on a wider scale because of required UV light. Similarly, its high degree of recombination of photogenerated species limits current photocatalysis performance [9]. Alterations in semiconductors can improve efficiencies and lengthen absorption wavelengths. Changes in design include the junction of a semiconductor with a conductor, the junction of two semiconductors [10], and semiconductor doped with metal ions, particularly of transition metals (TM) [11,12]. This type of change redshifts the wavelength of absorption onset in n-type semiconductors. Changes in the lattice and the junctions have the ability to reduce recombination. The combination of long wavelength absorption and exciton stabilization enhances ZnO's

http://dx.doi.org/10.1016/j.ceramint.2015.10.024 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: R.M. Mohamed, et al., Cobalt/zinc oxide hollow spheres: Visible light nanophotocatalysts, Ceramics International (2015), http://dx. doi.org/10.1016/j.ceramint.2015.10.024

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photocatalytic properties under visible light conditions. Finally, another alternative is to synthesize new shapes for photocatalysts. For example, nano-sized hollow sphere ZnO structures show improved photocatalytic activity [13]. In this paper, we further advance the field by combining above-mentioned alterations on ZnO, an n-type semiconductor. Improvements from TM-doped ZnO nanostructures (particles and rods) [14,15], positive results from tin oxide/zinc stannate hollow sphere experiments [16], and encouraging non-doped ZnO hollow sphere photocatalysis performances [13] motivate us to dope hollow sphere ZnO structures with cobalt, a transition metal. This combination redshifts ZnO's light absorption and yields improved visible light photocatalyst. Herein, we also elucidate differences between two ZnO nanostructures. We present a nanocomposite for ZnO as hollow shells doped with cobalt metal (Co), test its ability to photodegrade organic MG under visible light conditions, and compare results to other similarly prepared ZnO based nanocomposites.

2.2. Characterization Nanostructure morphology and sample dimensions were measured using JEOL-JEM-1230 transmission electron microscopy (TEM). Samples were suspended in ethanol and ultrasonicated for 30 m. A small amount was then dried on a carbon coated copper grid and loaded into the TEM. Also, N2-adsorption measurements were taken on treated samples (2 h under vacuum at 100 1C) with a Nova 2000 series Chromatech apparatus at 77 K to calculate surface area. Crystalline phase was determined by powder X-ray diffraction (XRD) using Bruker axis D8 with Cu Kα radiation (λ¼ 1.540 Å) at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific K-ALPHA spectrometer. Band gap performance was determined by Ultra violet–visible diffuse reflectance spectra (UV–vis-DRS), which was measured using a UV–vis-NIR spectrophotometer (V-570, Jasco, Japan) in air at room temperature to detect absorption over the range 200–800 nm. Lastly, photoluminescence emission spectra (PL) were obtained with a Shimadzu RF-5301 fluorescence spectrophotometer.

2. Experimental 2.1. Sample preparation

2.3. Photocatalytic tests

Two different nanostructures were prepared with a standard hydrothermal method, and they were doped with Co via UV irradiation. ZnO nanosized hollow spheres: 2.0 g dodecyl amine were dissolved under mild stirring in a mixture of 40 ml ethanol and 100 ml H2O; 8.0 ml zinc nitrate were added drop-wise to this solution; the resultant mixture was continually stirred at room temperature for 24 h. Afterward, it was transferred to an autoclave and maintained at 80 1C for 24 h. Finally, the obtained precipitate was centrifuged, washed several times with deionized water, dried at 100 1C for 24 h, and calcined at 550 1C for 5 h. Hereafter, ZHS denotes these as-prepared samples. ZnO nanoparticles: 180 mg zinc nitrate and 200 mg polyethylene glycol were dissolved in 200 ml H2O and stirred for 30 m; 2 ml of 6 mol sodium hydroxide were added drop-wise to this solution with constant stirring. Afterward, the gel was transferred to an autoclave and maintained at 80 1C for 10 h. Finally, the obtained precipitate was centrifuged, washed several times with distilled water, dried at 100 1C for 24 h, and calcined at 550 1C for 5 h. Hereafter, ZNP denotes these as-prepared samples. 3 wt% Co/ZnO nanoparticles and hollow spheres: ZHS or ZNP were dispersed in H2O; cobalt nitrate was dissolved into the suspension; N2 bubbling (100 ml/min) was conducted for 1 h. Afterward, a Hg–Xe lamp (UV intensity: ca. 10 m W/cm2 at 365 nm) was used to irradiate the mixture; the Co source was reduced by photogenerated electrons; Co was deposited on the light-reachable surface of the ZNP or ZHS. Finally, the obtained precipitate was washed several times with distilled water and dried at 60 1C for 2 h. Hereafter, Co/ZNP and Co/ ZHS denote these as-prepared Co/ZnO nanoparticle and hollow sphere samples.

Photocatalytic degradation efficiency of MG was evaluated by measuring the absorbance of an aqueous solution under illumination by visible light. In a typical sequence, an amount of a photocatalyst was suspended in MG aqueous solution in a 500 ml reactor. The mixture was stirred for 30 min in darkness to establish the adsorption–desorption equilibrium between photocatalyst and MG aqueous solution. A 300 W power Xenon lamp with 0.96 W/cm2 intensity simulated sunlight, while a cut-off filter was used to remove UV light (λ o 420 nm). Aliquots were taken from the suspension at different intervals of time and filtrated for analysis. Photodegradation efficiency was determined with a UV–vis spectrometer by measuring the solution's absorbance at the absorption wavelength (617 nm) at room temperature.

Intensity, a.u.

Intensity, a.u.

ZNP

35.0

ZHS Co/ZNP Co/ZHS

35.5

36.0

36.5

2θ( )

37.0

ZNP ZHS

Co/ZNP

Co/ZHS 20

30

40

50

60

70

80

2θ( )

Fig. 1. XRD patterns of ZNP, ZHS, Co/ZNP and Co/ZHS samples.

Please cite this article as: R.M. Mohamed, et al., Cobalt/zinc oxide hollow spheres: Visible light nanophotocatalysts, Ceramics International (2015), http://dx. doi.org/10.1016/j.ceramint.2015.10.024

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Fig. 2. TEM images of ZNP (a), Co/ZNP (b), ZHS (c), and Co/ZHS (d) samples.

3. Results and discussion

Co 2P3/2

3.1. Structural and chemical characteristics Co 2P1/2 Intensity (a.u.))

The as-prepared samples' crystal structure was investigated using XRD. As shown in Fig. 1, Co/ZnO samples exhibit XRD peaks matching the standard diffraction data for ZnO; as expected, so did the comparison ZnO samples. No obvious peaks corresponding to cobalt metal or a cobalt oxide phase were observed. Furthermore, the wurtzite structure of ZnO remained unchanged after cobalt modification. A subtle right shift in the [101] peak of ZnO in the region 2θ, 36.0–36.5 for the Co/ZnO samples suggests that Co2 þ has been doped into the ZnO lattice. Mean crystallite sizes, calculated by the Scherrer formula, for the samples were as follows: ZNP, 20 nm; Co/ZNP, 18 nm; ZHS, 15 nm; and Co/ ZHS, 12 nm. Thus, Co doped ZnO nanostructures had finer crystals than ZnO ones, and hollow sphere nanostructures had finer ones than their solid nanoparticle counterparts. Fig. 2 shows TEM images for all samples; all had spherical shapes. Moreover, ZHS and Co/ZHS samples display good uniformity. ZNP and Co/ZNP sizes were 140 and 180 nm, respectively. ZHS and Co/ZHS had shell thicknesses of 20–40 and 25–50 nm, respectively, with core diameters of 200 and 250 nm. Thus, adding cobalt to ZNP and ZHS increases their sizes. XPS spectra were used to investigate the doped cobalt in Co/ZHS samples. Fig. 3 shows peaks of the Co 2p region with binding energy at 794.9 and 779.7 eV for Co 2p1/2 and Co2p3/ 2. The difference in binding energies, 15.2 eV, matches that reported for Co2 þ . This information also suggests that Co2 þ ions incorporated into the ZnO lattice of ZHS.

810

805

800

795

790

785

780

775

770

Binding energy /eV

Fig. 3. XPS spectra of Co 2p for Co/ZHS sample.

N2-adsorption measurements from treated samples (2 h under vacuum at 100 1C) on a Nova 2000 series Chromatech apparatus at 77 K were used to calculate surface area of all samples. Fig. 4 shows their nitrogen adsorption–desorption isotherms. Nanoparticle samples exhibited isotherms type II, while hollow sphere samples exhibited isotherms type IV. Thus, ZHS and Co/ZHS were mesoporous. Fig. 5 shows a pore size distribution plot for Co/ZHS, calculated using the BJH equation. Results demonstrate a narrow distribution around 4 nm. This indicates that Co/ZHS samples should have large surface areas. Indeed, surface areas for the hollow sphere samples were significantly higher compared to particle samples. Measurements were as follows: ZNP, 30 m2/g;

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Adsorption Desorption Volume Adsorbed (cm /g)

3

3

Volume Adsorbed (cm /g)

Adsorption Desorption

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0

0

P/P

P/P

Adsorption Desorption

Volume Adsorbed (cm /g)

Volume Adsorbed (cm /g)

Adsorption Desorption

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

P/P

0.5

0.6

0.7

0.8

0.9

1.0

P/P

Fig. 4. Adsorption–desorption isotherms of ZNP (a), Co/ZNP (b), ZHS (c), and Co/ZHS (d) samples.

photocatalyst surface area. Thus, the hollow sphere samples should possess enhanced photocatalytic capability.

2.0

3.2. Optical properties

1.5

Fig. 6 compares UV–vis spectra of the hollow spheres and Co/ ZNP with ZNP. Absorption edges red-shift from the characteristic ZnO spectrum that ZNP displays. Of interest in this case, ZHS displayed the second narrowest band gap of the four structures. So while Co is responsible for the red shift from ZNP to Co/ZNP and from ZHS to Co/ZHS, the hollow morphology itself also seems to contribute to a reduced band width. Calculated band gaps from the UV–vis spectra follow: ZNP, 3.11 eV; Co/ZNP, 2.95 eV; ZHS, 2.79 eV; Co/ZHS, 2.65 eV. Because XRD and XPS results point to doped Co2 þ in the ZnO structures, dramatically different diffuse reflectance spectra of Co doped samples compared to pure ZnO samples should also appear. Other researchers found an absorption band from 520 to 700 nm attributed to Co2 þ in tetrahedral coordination with just 0.5% Co [15]. However, we observed no such absorption band with our 3% Co doped nanostructures. Instead, the spectra mimicked typical ZnO spectrum, merely red shifted

3

Volume Adsorbed (cm /g)

2.5

1.0

0.5

0.0 0

2

4

6

8

10

12

14

Pore Size (nm)

Fig. 5. Pore size distribution of Co/ZHS sample.

ZHS, 70 m2/g; Co/ZNP, 27 m2/g; Co/ZHS, 67 m2/g. Increased surface area is advantageous because it improves the e–h separation process by adding a number of active sites. Co doping did not significantly interfere with the goal of increasing the

Please cite this article as: R.M. Mohamed, et al., Cobalt/zinc oxide hollow spheres: Visible light nanophotocatalysts, Ceramics International (2015), http://dx. doi.org/10.1016/j.ceramint.2015.10.024

R.M. Mohamed et al. / Ceramics International ] (]]]]) ]]]–]]] 0.2

100

Co/ZHS ZHS Co/ZNP ZNP

90

0.0 -0.2

70

-0.4

60

-0.6

Ln C/Co

Reflectance, %

80

50

-0.8

40

-1.0

30

-1.2

20

-1.4

10

-1.6

0 200

5

ZNP Co/ZNP ZHS Co/ZHS

-1.8

250

300

350

400

450

500

550

600

650

700

750

5

800

10

Wavelength, nm

Fig. 6. UV–vis spectra of ZNP, ZHS, Co/ZNP and Co/ZHS samples.

ZNP 3.1 Co/ZNP ZHS Co/ZHS

20

25

30

Fig. 8. Effect of type of photocatalyst on photocatalytic degradation of MG dye.

3.01

0.3 g/l 0.6 g/l 0.9 g/l 1.2 g/l 1.5 g/l

0.0

-0.5

2.80

Intensity, a.u.

15

Reaction time, min

Ln C/Co

-1.0

2.68

-1.5

-2.0

-2.5

3.4

3.2

3.0

2.8

2.6

Photon energy, eV Fig. 7. Pl spectra of ZNP, ZHS, Co/ZNP and Co/ZHS samples.

as described above, on all accounts. This indicates that only a small amount of Co is present as dopant. We surmise that other forms of cobalt exist in forms below the detection threshold for the applied techniques. PL spectra for all samples were used to corroborate the absorption shift found in the UV–vis data and to determine whether PL intensity is observable at all with 3% cobalt. Fig. 7 shows a high resolution scan of the samples' PL spectra band edge peaks. The PL spectra confirmed the UV–vis results. PL intensities decreased in the same order as the samples' UV–vis absorption edges red shifted. Ultimately, as do others, we attribute the red-shift to the sp–d exchange between ZnO band electrons and localized D-electrons associated with doped Co2þ species. Furthermore, other reports showed PL intensity disappears with 5% and above Co doping, while 2% was still observable [17]. We found 3% still measurable. 3.3. Photocatalytic performance enhancements To determine how well the as-prepared samples performed under visible light only conditions, photodegradation of MG

5

10

15

20

25

30

Reaction time, min

Fig. 9. Effect of amount of Co/ZHS photocatalyst on photocatalytic degradation of MG dye.

was measured in several scenarios. For the first tests, 0.3 g weight of the photocatalyst was suspended in 500 ml of 100 ppm concentration MG aqueous solution. Fig. 8 shows data for each sample. The ZNP samples had virtually no photocatalytic activity under visible light, while Co/ZNP nanocatalysts performed better. We attribute the enhancement to the increased absorption in the visible light range due to added Co. Interestingly, moreover, our findings showed that simply by altering ZnO morphology, photocatalytic activity improved significantly. Ultimately, Co/ZHS exhibited the best photocatalytic activity. This result aligns with predictions made from the characterization analyses. The initial data points for samples in Figs. 8 and 9 are calculated from the linear regressions of the other plotted points. These lines suggest that MG degradation follows the Langmuir–Hinshelwood mechanism. To explain why MG amounts in hollow sphere sample solutions are lower compared to particle sample solutions at the beginning of the experiment, we postulate that the dye was pre-adsorbed into

Please cite this article as: R.M. Mohamed, et al., Cobalt/zinc oxide hollow spheres: Visible light nanophotocatalysts, Ceramics International (2015), http://dx. doi.org/10.1016/j.ceramint.2015.10.024

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Table 1 Rate constants of reaction kinetics for photocatalytic oxidation of malachite green dye. Sample

k  10
5, min
1

ZNP Co/ZNP ZHS Co/ZHS

111 1111 3544 6210

Table 2 Rate constants of reaction kinetics for effect of catalyst amount on oxidation of malachite green dye. Sample (g/l)

k  10
5, min
1

0.3 0.6 0.9 1.2 1.5

3544 6210 7453 6827 4768

the hollow spheres while being stirred for 30 min in the dark prior to the visible light test. Table 1 summarizes the rate constant of reaction kinetics for each sample type. The second condition tested was how varying concentrations of Co/ZHS photocatalysts altered the performance. Fig. 9 shows that by increasing the photocatalyst weight from 0.3 g to 0.9 g, degradation time shortened. However, increasing photocatalyst concentration above 0.9 g slowed the degradation time. The initial increase of photocatalyst amounts helps speed MG degradation due to more available sites for photocatalytic reaction. However, higher amounts hinder light penetration during the reaction, thus slowing degradation. Table 2 summarizes the rate constants of reaction kinetics for the different catalyst amounts. The third scenario was to recycle one Co/ZHS sample solution and test it multiple times. The exact same results shown in Fig. 8 were achieved every time, even after five runs. Thus, Fig. 10 shows that Co/ZHS is a stable photocatalyst which can easily be recycled and separated.

4. Conclusions Herein, we report on an efficient, hydrothermal synthesis of Co/ZHS. The resultant characterized photocatalysts performed MG photodegradation excellently under visible light irradiation; moreover, they are recyclable. Thus, Co/ZHS possess potential applications in solar energy environmental remediation of pollutant dyes. The hollow sphere morphology increases the surface area of the photocatalyst. Finally, Co doping performs a critical role in the electron–hole separation process and red shifts the absorption edge of ZnO to the visible region.

100

Photocatalytic degradation of malachite green dye, %

6

80

60

40

20

0

1st

2nd

3rd

4th

5th

Number of runs

Fig. 10. Recycling and reuse of Co/ZHS photocatalysts for photocatalytic degradation of MG dye.

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