Journal of Nanomaterials
Volume 2015 (2015), Article ID 108297, 13 pages
http://dx.doi.org/10.1155/2015/108297

Research Article

Microwave-Assisted Synthesis of Porous ZnO/SnS₂ Heterojunction and Its Enhanced Photoactivity for Water Purification

A. B. Makama,¹ A. Salmiaton,¹ E. B. Saion,² T. S. Y. Choong,¹ and N. Abdullah¹

¹Department of Chemical and Environmental Engineering, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia
²Department of Physics, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia

Received 12 January 2015; Accepted 15 March 2015

Academic Editor: Xin Zhang

Copyright © 2015 A. B. Makama et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Porous ZnO/SnS₂ nanocomposites with adjustable SnS₂ contents were prepared via microwave-assisted heating of different aqueous solutions of SnS₂ precursors in the presence of fixed amount of ZnCO₃ nanoparticles at pH 7. The structures, compositions, BET specific surface areas, and optical properties of the as-prepared products were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, transmission electron microscopy, N₂ adsorption, and UV-Vis absorption spectra. Photocatalytic activities of the samples were tested by the removal of aqueous ciprofloxacin, , and methylene blue under visible-light ( nm) irradiation. The experimental results reveal that the as-prepared heterogeneous nanostructures exhibit much higher visible-light-driven photocatalytic activity for the degradation of the pollutants than pure SnS₂ nanocrystals. The photocatalytic degradation rates of the pollutants for the most active heterogeneous nanostructure are about 10, 49, and 9 times higher than that of pure SnS₂. The enhanced photocatalytic activities exhibited by the heterojunctions could be ascribed to the synergetic effect of enhanced absorption in the visible region and the reduced rate of charge carrier recombination because of efficient separation and electron transfer from the SnS₂ to ZnO nanoparticles.

1. Introduction

The occurrence of subnano- to microquantities of the synthetic broad-based antibiotic ciprofloxacin (CIP) in surface and ground waters is an emerging source of high concern [1, 2]. The presence of CIP in aquatic environments, albeit at low concentrations, may lead to serious environmental issues. Of immediate concern is the high likelihood of inducing the evolution and proliferation of multidrug resistant bacteria [3]. On the mid and long term, the concern stems from not knowing the risks posed by CIP to ecosystems and human health. Because existing conventional water treatment plants are not designed to treat pharmaceutical products, there is the urgent need to develop cost-effective treatment technology to cope with this problem. In this context, advanced oxidation processes (AOPs) are being intensively investigated as possible remediation techniques. Advanced oxidation processes hold much promise in this regard because of their ability to produce highly reactive oxidative species (ROS) that are capable of complete mineralization of recalcitrant organic, biological, and inorganic pollutants [4].

Many types of AOPs are being considered for water decontamination [5, 6]. These include electrochemical, photochemical, photocatalytic processes, or their combinations. The photochemical and electrochemical processes, such as ozonation [7, 8], photo/electro-Fenton [9, 10], and persulfate [11, 12], use expensive precursors (, , and coupled with UV photons or electricity) to generate ROS. On the other hand, the photocatalytic decomposition (PCD) produces ROS without the need of any chemical and can be operated at ambient conditions. Furthermore, PCD has the inherent potential to be simple, reusable, efficient, and clean and can be designed to utilize a significant portion of the infinitely free energy from the sun. For these reasons, PCD is widely regarded as a potentially viable AOP for water purification [13–15]. In heterogeneous photocatalytic decontamination (PCD) process, a semiconductor (SC) material harnesses energy from photons that pose energies equal to or greater than its band gap. The absorbed energy is utilized to excite electrons from the valence band to the conduction band of the semiconductor. The photogenerated electrons and holes then react with adsorbates such as , to initiate the production of ROS radicals that directly degrade pollutants [16–18].

The fate and economic viability of PCD in large-scale water purification process are contingent on the development of high-performance photocatalyst(s) [19]. From industrial perspective, the optimal catalyst must possess the ability to transform the pollutant(s) entirely to harmless molecule(s). Additionally, it must have a bandgap (with suitably positioned conduction and valence bands) that absorbs light in the visible range and remains stable in contact with the water matrix. Furthermore, it should be nontoxic, abundant, and cheap. Towards this goal, a considerable variety of semiconductor materials have been tested for this purpose in the past decades. Table 1 gives a summary of some of the most recent materials investigated as photocatalysts for CIP degradation.

Table 1: Photocatalysts and experimental conditions reported for the degradation of ciprofloxacin.

Photocatalyst
Catalyst
Concentration
Radiation
Illumination
CIP
References

load, mg
mg/L
time, min
removal, %

ZnO
20
5
UV-A
60
~50
[21]

Bio
50
10
NGb
720
100
[52]

KMn₈O₁₆
50a
—
NG
30
>90
[53]

Ag/Pt@TiO₂
80
10
Vis.
120
99
[54]

Ag@NC-TiO₂
80
10
Vis.
150
98
[55]

TiO₂
500
33
UV-Vis
—
—
[45]

rGOc-BiVO₄
20
10
UV
60
68.2
[56]

TiO₂-P25
500
14
UV
—
—
[57]

Pt-BiO₄
150
10
Vis.
60
91.97
[58]

Anatase-TiO₂
1000
50
UV
120
—
[59]

NaCl/TiO₂
100
10
Vis.
60
70.90
[13]

Molar ratio of catalyst to CIP; bNG = not given, crGO = reduced graphene oxide.

Among the materials under consideration are and ZnO semiconductors with bulk band gaps of about 2.2 eV and 3.37 eV, respectively [20]. They are among the most promising photocatalysts, owing to their low cost, nontoxic nature, and high photocatalytic activity [21–23]. Moreover, they have matched band potentials; that is, both the valence band and conduction band potentials of are more negative than those of ZnO [24]. Thermodynamically, this allows the photogenerated electron to easily transfer from the conduction band of to the conduction band of ZnO under visible-light ( > 420 nm) irradiation, hence, enhancing the separation of photogenerated electrons and holes in and bringing about the sensitization of ZnO. For this reason, the ZnO/ composites with appropriate compositions should have higher visible-light-driven photocatalytic activity than individual and ZnO.

On the basis of these considerations, we report a study on the synthesis of porous ZnO/ nanosphere heterojunctions via a three-step process, including (1) room temperature synthesis of in polyvinyl pyrrolidone-ethylene glycol solution, (2) microwave-assisted conformal deposition of layer on the nanospheres, and (3) calcination of the / nanospheres at . The objectives are to evaluate the effectiveness of porous ZnO/ as a photocatalyst for the removal of (a) CIP and (b) hexavalent chromium ion and methylene blue. In addition, the source of enhanced photoactivity (if any) will be investigated. To the best of our knowledge, no such work on porous / was ever reported.

2. Materials and Methods

2.1. Chemicals

All chemicals and reagents used in this work were of analytical grade and they were used as-received without any purification. The materials are ammonium hydrogen carbonate (, Friendemann Schimdt Chemical, Germany), tin (IV) chloride pentahydrate (, Acros, New Jersey, US), sodium sulfide (, QRec, Thailand), ethylene glycol (, Merck, Germany), zinc sulfate heptahydrate (, R & M Chemicals, UK), and polyvinyl pyrrolidone (PVP 58000, Alfa Aesar, UK), and denatured absolute ethanol (, Globe Chemicals, Germany). Model pollutants purchased include ciprofloxacin hydrochloride ( HCl, Cadila’s Ciprodac, India), potassium dichromate (, AR grade, Systerm, Malaysia), and methylene blue (MB) whose structures and concentrations in test liquids are shown in Table 2. Distilled deionized water 18.2 mΩ·cm was prepared using TKA instrument and used to prepare all aqueous solutions.

Table 2: Pollutants and some experimental conditions used in testing the photocatalytic activity of the porous ZnO/SnS₂ heterostructures. Unless otherwise indicated, reactions were performed at (30 ± 1)°C in a water-jacketed photoreactor, continuously air/argon-sparged and stirred at 400 rpm.

Model pollutant
Chemical structure
Initial concentration,
pH

mgL⁻¹ (molL⁻¹)

Dichromate ion

35.5 (120.7)
5.21

Ciprofloxacin

40 (120.7)
6.10

Methylene blue

38.6 (120.7)
7.60

2.2. Synthesis of ZnCO₃

Zinc carbonate () was prepared according to an earlier reported procedure with slight modification [25]. 2.9 g (0.01 g mol) of (Solution A) and 1.6 g (0.02 g mol) of (Solution B) were each dissolved in 50 mL of 5 wt% PVP-ethylene glycol solution at . Solution B was added in a dropwise manner to Solution A under magnetic stirring at 800 rpm. The mixture was stirred for 1 h, and the white precipitate formed was collected by centrifuge at 6 000 rpm for 3 min. The precipitate was washed with distilled deionized water (3×) and then with denatured absolute ethanol (3×). Finally, the moist precipitate obtained was dried in an oven at for 6 h.

2.3. Microwave-Assisted Synthesis of ZnO/SnS₂

A predetermined mass of powder was dispersed by sonication for 30 min in 100 mL (NaOH neutralized) solution of (pH 7) in argon sparged distilled deionized (DDI) water. After that, 20 mL solution of also in argon sparged DDI was dropped in the dispersion under vigorous stirring (800 rpm). The colloidal solution under vigorous stirring was irradiated using microwave in a pulsed regime of 5 s on, 20 s off at 20% power (180 W) in an oven for 10 min. The whitish brown / precipitate formed was collected by centrifuge and washed as explained in Section 2.2. The cleaned solid was calcined in a stagnant air furnace at for 3 h. Three different samples were prepared at various mass ratios of source and source that were controlled as 25, 50, and 75 wt%. Details of the experimental condition are given in Table 3. The as-prepared ZnO/ powders are designated ZOSS-1, ZOSS-2, and ZOSS-3, respectively, and referred to as such in the text.

Table 3: Designation and summary of synthesis conditions, BET specific area, BJH pore volume, band gaps, and valence band potential of the as-synthesized specimens.

Name
aZn²⁺
bSn⁴⁺
Na₂S
cMass

BJH

mmol
mmol
mmol
%
m²/g
cm³/g
eV
eV

ZnO
40
0
0
0
62.7
0.823
3.3
2.9

ZOSS-1
40
3.6
7.2
25
39.4
0.571
2.9
2.2

ZOSS-2
40
7.1
14.3
50
109.1
0.767
2.8
2.1

ZOSS-3
40
10.7
21.4
75
69.7
0.229
2.6
2.0

SnS2
0
14.3
28.5
100
19.7
0.665
2.4
2.2

ZnCO₃; bSnCl₄·5H₂O; cSnCl₄·5H₂O:ZnCO₃.

Table 4: Pseudo-first-order rate constants and the respective chi-square test statistics of the model fit for the photocatalytic transformations of the model pollutants over the as-prepared catalysts.

Pollutant
ZnO
SnS₂
ZOSS-1
ZOSS-2
ZOSS-3
mm-ZOSS

CIP
1.2b
0.0002
6.2
0.0010
27.2
0.0785
57.1
0.0102
33.3
0.0867
6.8
0.0201

CrVI
~0c
—
6.8
0.0003
NAd
NA
311.9
0.0020
NA
NA
NA
NA

MB
~0
—
10.9
0.0017
NA
NA
99.4
0.0016
NA
NA
NA
NA

All have units of min⁻¹; bsee text for explanation; cZnO is a large band gap semiconductor that is not active in visible-light. dExperiment not performed.

For the purpose of comparison, ZnO and were also produced in 5% PVP-EG solution using microwave heating as above.

2.4. Characterization

Investigation of the crystallinity and the phase structures of the samples was carried out on a Phillips Analytical X-ray diffractometer. All measurements were done with radiation between values of 20∘ and 80∘ at a scan rate of using accelerating voltage and current of 40 mV and 40 mA, respectively. Ultraviolet-Visible (UV-Vis) absorbance spectra of samples were obtained on a Shimadzu model UV-1800 spectrophotometer. Absorbance measurements are converted to absorption coefficient () from , where is the path length of the quartz cuvette and is the absorbance [26, 27]. The calculated are used in Tauc equation [] to estimate the energy gap of the nanocomposite [28, 29]. Morphological structures of the samples were viewed and photographed on a Hitachi scanning electron microscope (FESEM) model S-3400N. Energy-dispersive X-ray spectrometry (EDS) was also taken on the same machine. Fine structural details were investigated with a Hitachi transmission electron microscope, model H-7100 STEM. Surface area of the samples was determined from BET measurements on a 3Flex Surface Characterization Analyzer (Micrometrics, USA). Isoelectric point (IEP) of ZOSS-2 was determined using ZetaSizer Nano ZS (Malvern, UK) instrument.

2.5. Batch Photocatalytic Activity Measurement

Photocatalytic activities of the nanocomposite were investigated by photocatalytic degradation of ciprofloxacin (CIP) in 400 mL immersion well photoreactor (model RQ400, Photochemical Reactors, UK). In a run, 200 mg of a catalyst was added to 300 mL aqueous solution of the pollutant in the reaction flask at natural pH (see Table 2). The suspension was stirred at 400 rpm for 30 min (based on preliminary contact time test result) in the dark to allow for adsorption-desorption equilibrium. Visible-light irradiation ( > 420 nm) was generated by a 200 W quartz tungsten halogen lamp (Osram Sylvania Inc., US) which was wrapped in a UV filtering plastic sheet (F20 UV Yellow, Fluorolite Plastics, US) to remove stray UV radiation. The lamp was switched on for 10 min–15 min to reach maximum output prior to coupling to the reaction flask. After that, the light source was coupled to the reaction flask to start the photoreaction under continuous stirring at 400 rpm and supply. The lamp and reactor temperature was maintained at by cooling water. About 3 mL of the reaction mixture was drawn from the reactor vessel at scheduled time intervals to the maximum specified time. The aliquots were immediately centrifuged at 6 000 rpm for 3 min to separate the photocatalyst from the supernatant.

2.6. Analytical Methods

The temporal concentrations of the pollutants in the different supernatants were determined using established methods in the technical literature based on optical absorbance measurements. Absorbance of was taken at 540 nm according to the 1,5-diphenylcarbazide (DPC) procedure [30]. Those of CIP and MB were taken at 275 nm [31] and 664 nm, respectively. Where necessary, the absorbances were converted to concentration using calibration curves for the different pollutants.

3. Results and Discussions

3.1. Characterization of As-Synthesized Photocatalysts

3.1.1. Crystallinity and Structural

The crystalline phases of the as-prepared samples were investigated with XRD and the results are shown in Figure 1. The peaks on the diffractogram of pure ZnO could be indexed to reflections from (100), (002), (101), (102), (110), (103), and (112) planes of pure wurtzite structure of ZnO (PDF number: 36-1451). Those on the pure diffractogram were indexed to the (100), (101), (110), and (201) planes of pure hexagonal phase of (PDF number: 23-0677). On the other hand, the diffractograms of the heterojunctions (ZOSS-1, ZOSS-2, and ZOSS-3) show peaks coming from the planes of ZnO and those of , hence confirming that the composite powders consist of ZnO and phases. All the XRD patterns exhibit strong reflections and no trace of impurity peaks could be detected indicating the high crystallinity and phase purity of the products. The peaks on the diffractograms of ZOSS-1, ZOSS-2, and ZOSS-3 exhibit an evolutionary development with increased content from ZOSS-1 to ZOSS-3. The evolutionary trend is a commonly observed XRD feature of composite materials [32].

Figure 1: XRD patterns of the as-prepared ZnO and and the heterojunctions.

3.1.2. Surface Morphological and Elemental Analysis

The morphology and structure of ZnO, heterojunctions, and are photographed with SEM as shown in Figures 2(a)–2(e), respectively. The SEM images of zincite and in Figures 2(a) and 2(e) show that they consist of uniformly shaped and sized spherical aggregates with sizes between 0.2 μm and 0.8 μm. After compounding (Figures 2(b)–2(d)), the resulting composites show rough faced agglomerates that consist of spheres of ZnO covered and cemented together by mass of . The degree of agglomeration increases with content from 25% to 75%. At 75%, petal like features of began to appear as shown in Figure 2(d).

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2: Low magnification SEM micrographs of (a) ZnO, (b) ZOSS-1, (c) ZOSS-2, (d) ZOSS-3, (e) , and (f) EDX spectrum of ZOSS-2. Scale bar 2 μm.

The morphology of the smaller particles is further elucidated by the TEM micrograph. The TEM image of ZOSS-2 is shown in Figure 3(a). From this picture, it is clear that the ZOSS-2 heterostructure is made up of uniformly sized spheroidal particles. The dotted rectangular part in Figure 3(a) was further magnified, as shown in the inset. The enlarged portion displays that the surface of the ZnO (black spheroids) particles is entirely encased by SnS (gray mass) material. Particle size estimate obtained by counting shows the average diameter of ZOSS-2 to be  nm (Figure 3(b)). EDX analysis was performed to determine the elemental compositions of the ZOSS-surfaces. The analysis result for ZOSS-2 is shown on the EDX plot (Figure 2(f)), and it further confirms the coexistence of the elements Zn, O, S, and Sn.

(a)

(b)

Figure 3: (a) TEM image and (b) particle size distribution of ZOSS-2.

3.1.3. Optical Absorbance Analysis

Optical absorbance spectral studies in the UV-Vis region were done to estimate the band gaps of the synthesized photocatalysts. Figure 4 displays the plots for pristine ZnO and and for the ZnO/ (ZOSS-1 to -3) heterojunctions. It is seen that zincite exhibits no optical absorption in visible-light region due to its large energy band gap (3.3 eV). The spectra of the heterojunctions showed steep absorption over the visible-light region, indicating that the visible-light response of ZnO/ materials is a manifestation of the transition between the VB and CB not from hybrid energy levels [33, 34]. The band gaps estimated from the Tauc plot (inset of Figure 4) are tabulated in Table 3. They range from 2.6 eV to 2.9 eV. The visible-light absorption abilities of the heterojunctions gradually increased with increasing . The present findings seem to be consistent with other research that reported similar visible-light absorption trend of semiconductor nanocomposite materials [35].

Figure 4: Room temperature optical absorbance spectra ZnO, ZOSS1–3, and . Inset shows the Tauc (()² versus ) plots and band gap estimates for the and the different heterojunctions. The band gap of ZnO was estimated from the onset of absorption on the main plot.

3.1.4. Surface Area and Pore Size Distribution

The specific surface areas and pore sizes of the different photocatalysts were determined by nitrogen adsorption-desorption isotherms. The isotherms obtained revealed that all the pure phases and the heterojunctions have typical type IV isotherms according to the IUPAC classification of sorption isotherms. Figure 5(a) shows the adsorption-desorption isotherms for ZOSS-2. The isotherm with its characteristic type H1 hysteresis loop shows that the synthesized ZOSS-2 is purely a mesoporous material that contains similar sized nonintersecting cylindrical mesopores [36]. The specific surface area for ZOSS-2 calculated according to the BET theory was found to be 109.1 m²g⁻¹. This is much bigger than the areas of pure ZnO (62.7 m²g⁻¹) and (19.7 m²g⁻¹). The monolayer capacity and the BET constant of ZOSS-2 were found to be 25.1 cm³g⁻¹ STP and 84.5, respectively. The material also shows a moderate pore volume of 0.72 cm³g⁻¹. The pore size distribution (inset of Figure 5(b)) calculated by BJH method shows that the pore diameters are in the mesoporous range with an average pore diameter equal to 3.8 nm. Similar properties for the other samples were calculated and summarized in Table 3. It is observed that the surface area increases < ZOSS-1 < ZnO < ZOSS-3 < ZOSS-2.

(a)

(b)

(c)

Figure 5: (a) Representative nitrogen adsorption-desorption isotherms and the BET surface area plot (inset) for ZOSS-2. (b) BJH pore size distribution. (c) potential of porous ZnO/ suspension as a function of pH showing the isoelectric point.

3.2. Proposed Mechanism for the Synthesis of Porous ZnO/SnS₂

Scheme 1 is a pictorial illustration of the proposed steps for the formation of porous ZOSS-2 nanosphere heterojunctions.

Scheme 1: Schematic illustration of the conversion processes from nanospheres to porous ZnO/SnS₂ heterojunction.

First, uniformly shaped nanospheres of are produced by precipitation in 5% PVP-EG solution:

Secondly, nanocrystal layer is deposited on the nanospheres in an aqueous solution (pH 7) to produce the core-shell nanospheres according to the net ionic equation:

Finally, the / core-shell nanospheres are converted to porous ZnO/ nanosphere heterojuctions by thermal decomposition at for 3 h:

3.3. Measurement of Photocatalytic Activity

3.3.1. Degradation of Ciprofloxacin

Figure 6 shows the decay in concentration of ciprofloxacin with time as a function of photocatalysts type under visible-light. It also shows that, in the absence of the catalyst, the degradation of CIP does not occur. In the presence of the as-synthesized photocatalysts (except ZnO) however, appreciable fall in the concentrations of the pollutant was observed, indicating that the catalysts are photoactive under visible-light. The negligible activity exhibited by ZnO could be attributed to its large band gap [21] which makes it most photoactive under UV light but not under visible-light. The percentage removal of CIP over the different catalysts in 60 min is shown as inset of Figure 6. From the figure, it is clear that the photoactivity catalysts have various activities in removing CIP. The heterojunctions (ZOSS-1–3) exhibit much higher photocatalytic activities than single phase ZnO and . After 60 min of visible-light illumination, the CIP removal over ZnO and was <10% and about 30%, respectively. In contrast, the percentage decomposition in 60 min over each heterojunction was >80%. Moreover in 120 min, CIP was completely decomposed over the heterojunctions. It is worth noting that the coating of a small amount (25%) of on ZnO results in a sharp increase of CIP decomposition from 5.9% to 82.3% (ZOSS-1). As the content increases to 50% (ZOSS-2), the highest photocatalytic activity was recorded. A further increase in content to 75% (ZOSS-3) however results in a drop in photocatalytic activity.

Figure 6: Visible-light ( > 420 nm) induced photodegradation of CIP by photolysis and by photocatalysis over different catalysts. Inset shows the comparison of the quantity of CIP removed in 60 min by photocatalysis over the different catalysis. Rate coefficient for each reaction is tabulated in Table 4.

The slight differences in activities of the heterojunctions under visible-light could be tied to the differences in their content. When the content of is low as in ZOSS-1, it is very likely that a significant portion of ZnO is not covered. Thus, the visible-light response of this sample might be weak due to less content. On the other hand, more content of will result in greater coating and also lead to the development of high density of interfacial defects. Depending on the density of defects, they could either promote or impede charge carrier separation [37–39]. Low density of defects is believed to be beneficial to separation and mobility of charge carriers [40]. However, when the defects become more, they could act as electron-hole recombination centers. Consequently, electrons and holes are quenched at these centers instead of separating and thus lead to decrease of photocatalytic activity. Therefore, it is probable that the fall in activity noticed for ZOSS-3 could be ascribed to high density of charge defects as a result of more ions being incorporated into the structure of ZnO. Thus, there must exist an optimum composition for ZnO/ nanocomposite to achieve the highest photocatalytic activity. For these reasons, ZOSS-2 with content of 50% possessed the highest photocatalytic activity among heterojunctions.

To evaluate the role of chemically induced interfacial binding on the photocatalytic activity of the heterojunction systems, 2 parts pure phase ZnO and 1 part pure phase SnS were mechanically mixed to produce a system with same composition as ZOSS-2 and used to photooxidized CIP under the same conditions. The activity recorded is also shown in Figure 6. The mechanically mixed specimen (mm-ZOSS) shows a rather low photocatalytic performance (32.2%) for decomposition of CIP compared to chemically produced heterojunctions. The performance of mm-ZOSS is marginally higher than the performance of the pure phase (28.2%). The low activity shown by mm-ZOSS could be explained by the relatively loose binding between the two semiconductors which hinders efficient charge transfer between them. As reported [39, 41], the main advantage of the wet synthesis of the heterojunctions is the formation of tight chemically bonded interfaces between the two materials (see Figure 3(a)). The tight binding between the two semiconductors makes charge transfer between them spatially smooth [42–44].

3.3.2. Multiactivity Test and Kinetics

In order to better assess the activity of ZOSS-2, its performance to transform anionic (hexavalent chromium ) and cationic (methylene blue (MB)) pollutant was investigated. The results of the investigation are shown in Figure 7 and they show that ZOSS-2 interacts differently with the model pollutants under dark and illuminated conditions. Under the experimental conditions tabulated in Table 2, ZOSS-2 shows excellent interaction with the pollutants. It reversely adsorbed 15.2% of from its solution and when the solution was exposed to visible-light, 81.3% of the residual was reduced to in 10 min. By 20th min of illumination, the amount of was practically reduced to zero. About 6.1% of CIP was adsorbed onto ZOSS-2 and upon irradiation with light, 90.2% of the residual CIP was removed. In the case of MB, 0.2% of the dye was adsorbed after 30 min and after 60 min of irradiation, 98.2% of the residual dye content was removed by photocatalysis. These findings show that adsorption plays only a nominal role in the transformation of the three model pollutants over ZOSS-2, suggesting that the transformation of the pollutants (especially for CIP and MB) mainly occurs in the bulk fluid, not on the catalyst surface.

Figure 7: Visible-light ( > 420 nm) photodegradation of , CIP, and MB over ZOSS-2 photocatalyst. Inset shows a comparison of CIP removed by adsorption and photocatalysis.

To quantitatively understand the pollutants’ transformation kinetics, experimental data were analyzed with the heterogeneous pseudo-first-order model as expressed by (2). The model has been applied successively [14, 16, 21, 42, 45] to represent the photocatalytic degradation process if the initial concentration of pollutant is low:

where and are the concentrations of each pollutant in solution at time 0 (optimum contact time to achieve adsorption-desorption equilibrium) and , respectively, and is the pseudo-first-order rate constant. The rate constants calculated from the nonlinear fit (NLF) of data to (2) and their corresponding goodness of fit values for all the catalysts tested are tabulated in Table 4. A careful inspection of the -values reveals that a reasonably good fit of experimental data to the kinetic model was achieved (). It is clearly evident from the table that the rate constants of pollutant transformation by ZOSS-2 are much higher than the other catalysts. For the same initial molar concentration of pollutants, CIP, , and MB, the rate of their removal over ZOSS-2 is about 9.2, 49, and 9 faster than by single phase . The higher activity exhibited by ZOSS-2 could be attributed to its large surface area that offers more surface for visible-light absorption compared to the others. Similar findings have been reported in many reports [19, 42, 46–48]. Moreover, the dramatic increase in photoactivity of the nanocomposite is usually explained as a consequence of the formation of tight binding interface between the constituting semiconductors which favors charge carrier separation.

3.4. Apparent Quantum Yield

A measure of the energy utilization efficiency of the reactor-photocatalyst system was assessed using the apparent quantum yield () concept. On the basis of time required to achieve 90% conversion level, is defined [49] as the ratio of the rate of 90% converted molecule over the number of photons with the required energy to cause excitation of the photocatalyst that enter the reactor. Accordingly, for the degradation of CIP can be expressed mathematically as

where is the number of pollutant molecules degraded at the 90% conversion level, is the time required to achieve 90% conversion in seconds,    is the spectral irradiance intensity, (m²) is the irradiated catalyst surface area (), () is Plank’s constant, () is the speed of light in vacuum, and , are the lower and upper wavelengths of the spectral band of interest. In this evaluation, the numerical integration of (3) is restricted within the 420 nm ()–443 nm () spectral bandwidth for ZOSS-2. A visible high resolution spectra-photo radiometer (model BLK-HR-VIS, StellarNet Inc., US) was used to obtain the spectral distribution of the 200 W tungsten halogen lamp between 390 nm and 500 nm, as shown in Figure 8. The spectral irradiance of the lamp at the surface of outer wall of the immersion well was used to estimate the number of photons entering the reacting mixture. The apparent quantum yield was computed using (3) to be 0.225%.

Figure 8: Spectral irradiance of the 200 W tungsten halogen lamp measured at the outer wall of the immersion wall ( cm away from the lamp). Shaded area represents the spectral band of interest.

3.5. Stability of the ZnO/SnS₂ Photocatalyst

Although the as-synthesized ZOSS-2 heterojunction has shown excellent photocatalytic activity, this positive characteristic will amount to naught if the catalyst is not stable for repeat operation. For this reason, its stability was investigated by recycling it over and over in the decomposition of CIP under visible-light. Before any new cycle of the experimental run, the used catalyst was first cleaned, dried, and weighed. The result of the stability test is shown in Figure 9 and from it, it is apparent that there is very little lost in the activity of the ZOSS-2 photocatalyst after the first five reuse cycles. From this finding, it can be supposed that, under the current set of experimental conditions, ZOSS-2 is a stable photocatalyst.

Figure 9: Photocatalytic performances of ZnO/ in the first five reuse cycles.

3.6. Charge Transfer between the SnS₂ and ZnO Crystals and Enhancement of Photocatalytic Activity of ZnO/SnS₂

It is widely believed [19, 35, 39, 44] that the much better performance exhibited by composite semiconductor photocatalysts over single phase materials is mainly due to the phenomenon of electric-field-assisted photon induced charge separation and transfer across the systems’ boundary. Thus, in order to understand the charge separation process between and ZnO interface, the valence band (VB) and conduction band (CB) potentials at the point of zero charge were calculated according to the electronegativity concept [50] using the empirical equations

where is the absolute electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms. The absolute electronegativity of the constituent atoms is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy. The values for and ZnO were calculated with data from Pearson [51] to be 5.17 eV and 5.79 eV, respectively. is the energy of free electrons on the hydrogen scale (ca. 4.5 eV) and is the band gap of the semiconductor. The values for ZnO and obtained in the experiments are 3.3 eV and 2.4 eV, respectively. Accordingly, the VB and CB energy values of crystals were calculated to be 1.87 eV and −0.53 eV while those for ZnO crystals were determined to be 2.89 eV and −0.36 eV, respectively.

Using the computed band potentials, the electronic band diagram of the heterojunction is drawn and shown in Scheme 2. As seen from the scheme, the ZnO/ heterojunction is a “staggered” type II system. The CB edge potential of (−0.53 eV) is at a more negative potential than that of ZnO (−0.36 eV). Hence, visible-light excited electrons in the CB of crystal can readily move across the system boundary to the CB of ZnO. At the other end, the holes () created as a result of photon excitation of electrons remain in the VB of because it is at a lower potential than the VB of ZnO. In addition, the separation of electron-hole pairs is also aided by the internal electric fields generated within the depletion region of the two semiconductors. In other words, the small built-in voltage also induces the separation of electron-hole pairs both at the interface and in the individual semiconductors [43]. This also helps to reduce the probability of electron-hole recombination within the crystals. As a result, electrons are localized on the ZnO surface and holes on the surface. The separation and localization of electrons and holes in different semiconductors could effectively inhibit their recombination. Consequently, the lifetimes of these charge carriers are prolonged. This gives them the higher probability to partake in redox reactions which manifests as increased activity for the heterojunctions [42, 48].

Scheme 2: Electronic band diagram of the ZnO/ system.

4. Conclusions

Porous ZnO/ heterostructures were synthesized through simple microwave-assisted heating chemical method at pH 7. The presence of extended the spectral response of ZnO from the UV to the visible region, enabling the heterostructures to efficiently remove ciprofloxacin, hexavalent chromium, and methylene blue from aqueous solutions under visible-light irradiation. The characterization results confirmed the formation of heterojunction and revealed that the heterojunction could promote the transfer of photoelectrons and thus inhibit their recombination with holes, leading to enhanced photocatalytic activity. The study suggests that the porous ZnO/ heterostructures are new types of visible-light-driven photocatalysts for environmental applications.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors gratefully acknowledge the financial support provided by the Ministry of Education, Government of Malaysia, through Grant no. FGRS/1/11/TK/UPM/02/34, dated 1 August 2011, and RU Project Grant no. 05-02-12-2193RU.

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