Synthesis, characterization and daylight active photocatalyst with antiphotocorrosive property for detoxification of azo dyes

Separation and Purification Technology 164 (2016) 170–181

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Synthesis, characterization and daylight active photocatalyst with antiphotocorrosive property for detoxification of azo dyes B. Subash a, Balu Krishnakumar b,⇑, Abilio J.F.N. Sobral b, C. Surya d, N. Agnel Arul John d, A. Senthilraja a, M. Swaminathan c, M. Shanthi a,⇑ a

Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India Chemistry Department, University of Coimbra, 3004-535 Coimbra, Portugal Nanomaterials Laboratory, Kalasalingam University, Krishnankoil 626 126, Tamil Nadu, India d Department of Biochemistry, Srimad Andavan Arts and Science College, Tiruchirappalli 620 005, Tamil Nadu, India b c

a r t i c l e

i n f o

Article history: Received 2 November 2015 Received in revised form 14 March 2016 Accepted 16 March 2016 Available online 17 March 2016 Keywords: CdS loaded Ag–ZnO Antiphotocorrosive Sun light Degradation Azo dyes Dual mechanism

a b s t r a c t Utilization of solar energy is an energy efficient process for dye wastewater treatment. Photocatalytic degradation of toxic azo dyes was carried out using modified semiconductors under direct sun light. Concerning this, the different wt% of CdS loaded Ag–ZnO catalysts were prepared by the simple precipitation – thermal decomposition method and used for degradation studies. Cadmium sulfide (CdS) is a kind of semiconductor with fine band gap of 2.4 eV, and its valence electron can be effortlessly evoked to conduction band under solar or visible light illuminations. Among the different CdS prepared catalysts, highly efficient 2 wt% of CdS loaded Ag–ZnO was characterized by different characterization techniques. Metal sulfide loading increases the absorbance of ZnO into the entire visible region. XPS reveals that the presence of metallic silver in the catalyst. The photocatalytic activity of 2 wt% CdS loaded Ag–ZnO was compared with single metal doped, undoped, and other commercial catalysts, especially Degussa P25, a standard bench mark photocatalyst. The photodegradation of RR 120, RO 4, and RY 84 had been analyzed in detail. Mineralization of these dyes has been confirmed by chemical oxygen demand (COD) measurements. A dual mechanism has been proposed for the higher efficiency of CdS–Ag–ZnO at neutral pH under solar light. Antiphotocorrosive study reveals that bare ZnO suffers more dissolution by photocorrosion than our prepared photocatalyst CdS–Ag–ZnO. This catalyst is found to be more stable and reusable. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction The global energy crisis and related ecological concerns are among the prime technological challenges being confronted by chemists and technologists in the 21st century. The rate of total energy used by all of human civilization reached 15 TW in 2008 and is unsurprising to virtually twice by 2050 due to the emergent global creation and inhabitants [1]. On the contrary, our most important energy resources still instigate from restricted and non-renewable fossil fuels, such as firewood, oil and natural gas. Besides, the combustion of these fossil fuels has caused a succession of critical environmental tribulations, ranging from air and water contamination to worldwide humidity. For this reason, looking for renewable, clean as well as carbon–neutral unusual energy ⇑ Corresponding authors. E-mail addresses: [email protected] [email protected] (M. Shanthi). http://dx.doi.org/10.1016/j.seppur.2016.03.029 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.

(B.

Krishnakumar),

resources is very quickly needed to replace our dependence on fossil fuels. Solar energy is the primary source of energy for the life on our planet. It’s very safe, abundant and carbon neutral. Next to nuclear energy and a combine of other renewables, it is amongst the best options to substitute the fossil fuels, causing environmental problems [2]. On the other hand, solar energy is diffuse, sporadic and its collection, concentration and storage hinder the full utilization of its potential. Plants and organisms have learnt how to use it to exchange plentiful compounds such as water and CO2 into constructive chemicals for their intensification. Semiconductor photocatalysis like TiO2, ZnO and WO3 have attracted more extensive awareness in current years owing to their immense potential in environmental contaminant degradation and water splitting [3–11]. Among them, ZnO is almost considered to be an excellent sun light receptive photocatalyst material due to its relatively narrow band gap (3.2 eV). Conversely, concerning its application in environmental remediation, there are a number of critical negative aspects to be noted: (i) the separation efficiency

B. Subash et al. / Separation and Purification Technology 164 (2016) 170–181

of photogenerated electrons and holes is very low; (ii) it is prone to photocorrosion in aqueous media containing oxygen during photochemical reaction [12], (iii) it is usually present as fine or ultrafine particles, and it is a challenging and expensive task to separate the catalyst particles from the reaction systems. To overcome these problems, modifying ZnO photocatalysts to enhance light absorption and photocatalytic activity under sun light has been a main research direction in recent years. One of the efficient methods to modify semiconductor surface is by doping noble metals such as Cu, Ag, Ce, Au, and Mg [13–18]. Additionally, the formation of a coupled semiconductor structure can efficiently improve the optical absorption capacity and at the same time reduce the charge recombination under sun light irradiation because they can reimburse for the disadvantages of the individual component, and persuade a synergistic effect [6,7,19–25]. Chalcogenides such as Ag2S, CdS, CoS2, ZrS2 and ZnS have been studied extensively, since they have ideal edge positions of the valence and conduction bands for the redox reactions [26–31]. CdS is an important II–VI semiconductor with direct band-gap energy of 2.42 eV [32], which could be excited by visible light to produce photogenerated electrons and holes. As the conduction band of ZnO is about 0.5 eV more positive than that of CdS, the band position between CdS and ZnO favors the transfer of photogenerated electrons from the conduction band of CdS into that of ZnO efficiently. Furthermore, the improvement in the stability of CdS based photocatalysts utilizing visible range of the spectrum is a great challenge. In the present work, CdS loaded Ag–ZnO composite catalyst was synthesized by direct loading of CdS on Ag–zinc oxalate substrate with simple precipitation – thermal deposition method under mild condition. The photocatalytic activities of CdS–Ag–ZnO were evaluated with azo dyes degradation under solar light. 2. Experimental 2.1. Materials The commercial azo dyes Reactive Red 120 (RR 120) (Fig. S1, see Supplementary data), Reactive Orange 4 (RO 4), (Fig. S2, see Supplementary data) and Reactive Yellow 84 (RY 84) (Fig. S3, see Supplementary data), from Balaji Colour Company, Dyes and Auxiliaries (Chennai) were used as received. Oxalic acid dihydrate (99%) and zinc nitrate hexahydrate (99%) were obtained from Himedia chemicals. AgNO3
and CdS from sigma Aldrich, ZnO (Himedia), TiO2 (Merck) were used as received. A gift sample of Degussa TiO2-P25 was obtained from Evonik (Germany). It is a 80:20 mixture of anatase and rutile with the particle size of 30 nm and BET surface area of 50 m2 g
1. K2Cr2O7 (s.d.fine), Ag2SO4 (s.d.fine), HgSO4 and FeSO47H2O (Qualigens) were used as such. The double distilled water was used to prepare experimental solutions. The pH of the solution before irradiation was adjusted using H2SO4 or NaOH. 2.2. Preparation of CdS loaded Ag–ZnO CdS loaded Ag–ZnO was prepared by precipitation thermaldecomposition method. Aqueous solutions of 100 mL of 0.4 M zinc nitrate hexahydrate and 100 mL of 0.6 M oxalic acid in deionized water were brought to their boiling points separately. 5 mL of appropriate amount of silver nitrate solution was mixed with zinc nitrate solution. Solution of zinc nitrate and AgNO3 were mixed with oxalic acid solution. Precipitation of zinc oxalate with Ag occurred (2 wt% Ag related to ZnO, reported concentration refers to the initial concentration of the solution). To this solution 0.054 g of CdS was added and stirred for 1 h at 60–70 °C. The mixed suspension was stirred continuously to attain ambient tempera-

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ture. The mixed precipitate of CdS with Ag–zinc oxalate was somewhat fine and homogeneous in dimension. CdS–Ag–zinc oxalate crystals were washed several times with distilled water, air-dried overnight and dried at 100 °C for 5 h. It was calcined in the muffle furnace at the rate of 20 °C min
1 to attain the decomposition temperature of zinc oxalate (450 °C). After 12 h, the furnace was allowed to cool down to room temperature. The CdS loaded Ag–ZnO catalyst was collected and used for further investigation. This catalyst contained 2 wt% of CdS. Catalysts with 1, 3, 4 and 5 wt% of CdS were prepared with this procedure by the addition of appropriate amounts of CdS initially. The pure ZnO was prepared without addition of AgNO3 and CdS. Ag–ZnO, Ag–CdS and CdS–ZnO were prepared by the same procedure with relevant precursors. 2.3. Analytical methods Powder X-ray diffraction patterns were obtained using X’Per PRO diffractometer equipped with a Cu Ka radiation (wavelength 1.5406 Å) at 2.2 kW Max. Peak positions were compared with the standard files to identity the crystalline phase. For transmission electron microscope (TEM) images, the grids were dried under natural conditions and examined using a TEM Hitachi H-7500. FE-SEM images were taken using a JEOL JSM-6701F field emission scanning electron microscope (FE-SEM). Earlier than FE-SEM analysis, the samples were mounted on a gold platform located in the scanning electron microscope for succeeding analysis at various kinds of magnifications. DRS (diffused reflectance spectra) of the prepared materials were recorded using Shimadzu UV-2450. Photoluminescence (PL) spectra were recorded using a Perkin Elmer LS 55 fluorescence spectrophotometer. The semiconductor martial were dispersed in carbon tetrachloride and excited using light of wavelength 300 nm. X-ray photoelectron spectra of the catalysts were recorded in an ESCA-3 Mark II spectrometer (VG scientific Ltd., England) using Al Ka (1486.6 eV) radiation as the source. The spectra were referenced to the binding energy of C1s (285 eV). The specific surface areas of the samples were determined through nitrogen adsorption at 77 K on the basis of BET equation using a Micromeritics ASAP 2020 V3.00 H. A Shimadzu (Japan) AA6300 spectra Atomic Absorption spectrometer was used to measure the concentration of Zn2+ ions. UV spectral measurements were done using Hitachi-U-2001 spectrometer. 2.4. Photodegradation experiments Solar photocatalytic degradation was carried out under similar conditions on sunny days between 11 am and 2 pm. An open borosilicate glass tube of 50 mL capacity, 40 cm height and 20 mm diameter was used as the reaction tube vessel. Fifty milliliters of RR 120 (2  10–4 M), RO 4 (5  10–4 M) and RY 84 (5  10–4 M) with the suitable amount of catalyst was stirred for 30 min in dark prior to elucidation in order to achieve utmost adsorption of dye onto the catalyst surface. Irradiation was carried out in the open air with continuous aeration by a pump to provide oxygen and for the complete mixing of reaction solution as well as catalyst. During the illumination time no volatility of the solvent was noted. The temperature of the experimental solution is 32 °C. In all cases, 50 mL of reaction mixture was irradiated. At specific time intervals, 2–3 mL of the sample was withdrawn and centrifuged to remove the catalyst. One milliliter of the sample was suitably diluted and dye concentration was determined from the absorbance at the analytical wavelength (RR 120– 285 nm, RO 4–285 nm and RY 84–312 nm). 4-nitrophenol degradation was carried out under both UV and solar sources. For the degradation of 4-nitrophenol by UV-A light (365 nm), a Heber Multilamp-photoreactor HML MP 88 was used [33] (Fig. S4, see Supplementary data).

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2.5. Solar light intensity measurements Solar light intensity was measured for every 30 min and the average light intensity over the duration of each experiment was determined. The sensor was always set in the position of greatest intensity. The intensity of solar light was measured using LT Lutron LX-10/A Digital Lux meter and the intensity was (1250  100) ± 100 lux. The intensity was almost invariable during the experiment.

2.6. Chemical oxygen demand (COD) measurements To verify the complete mineralization process, COD was determined using the following procedure. Sample was refluxed with HgSO4, known volume of standard K2Cr2O7, Ag2SO4 and H2SO4 for two hours and titrated with standard ferrous ammonium sulfate (FAS) using ferroin as indicator. A blank titration was carried out with distilled water instead of dye sample. COD was determined using the following equation.

COD ¼

XRD. Lack of such a shift in XRD indicates Ag may be in the surface of catalyst, not in lattices. The crystallite sizes of bare ZnO and CdS–Ag–ZnO were determined using Debye–Scherrer equation.

D¼

Kk b cos h

where D is the crystal size of the catalyst, K is dimensionless constant (0.9), k is the wavelength of X-ray, b is the full width at half-maximum (FWHM) of the diffraction peak and h is the diffraction angle. The average crystalline size of CdS–Ag–ZnO is found to be 6.4 nm which is less than the size of bare ZnO (30.1 nm). The surface morphological properties of CdS–Ag–ZnO were examined by TEM and FE-SEM images. Fig. 2 shows the TEM images of CdS–Ag–ZnO at different magnifications (Fig. 2a–d). At higher magnifications, the hexagonal structure of ZnO particles is seen (Fig. 2c and d). It is found that the CdS–Ag–ZnO particle sizes are in the range from 5 to 80 nm. The FE-SEM images at different magnifications are given in Fig. 3a and b. At higher magnification (Fig. 3b) hexagonal structure of ZnO is seen (indicated by circle),

ðBlank titre value
dye sample titre valueÞ  normality of FAS  8  1000 Volume of sample

3. Results and discussion 3.1. Characterization of catalyst To find out the optimum amount of CdS loading, initially, we had carried out the degradation of three azo dyes RR 120, RO4 and RY 84 with different wt% of CdS in Ag–ZnO. The percentages of degradation with 1, 2, 3, 4, and 5 wt% were found to be 70, 93, 85, 81, and 75 for RR 120 (64%, 82%, 75%, 76% and 68% for RO 4, 69, 80, 78, 74 and 65 for RY 84), respectively at 20 min irradiation. When we raise the amount of CdS from 1 to 2 wt% the percentage of degradation also increases, further raising the amount of CdS, decrease the degradation percentage. The catalyst loaded with 2 wt% of CdS was found to be the most efficient in all three dyes (RR 120, RO4 and RY 84) under solar light. Hence, 2 wt% of CdS was taken as optimal concentration of CdS on Ag–ZnO and this catalyst was characterized by XRD, TEM, FE-SEM, EDS, DRS, PL, XPS, AFM and BET surface area measurements. XRD analysis of the bare ZnO and CdS–Ag–ZnO are shown in Fig. 1. The diffraction peaks of bare ZnO (Fig. 1a) at 31.68°, 34.36°, 36.18° and 56.56°, correspond to (1 0 0), (0 0 2), (1 0 1) and (1 1 0) planes of wurtzite ZnO (JCPDS 89-0511). No other peaks are identified in bare ZnO. Fig. 1b shows the XRD pattern of CdS–Ag–ZnO. XRD pattern of CdS–Ag–ZnO is dissimilar from that of bare ZnO. In the meantime, the additional peaks at 30.6°, 43.9° and 52.1° were observed in CdS–Ag–ZnO (Fig. 1b), which can be allocated for CdS cubic phase (2 0 0), (2 2 0) and (3 1 1) crystal planes [34] (JCPDS 89-0440), respectively. XRD clearly reveals that loading of CdS on Ag–ZnO and the wurtzite and cubic structures of ZnO and CdS, respectively. But, ‘Ag’ could not be detected by XRD (Fig. 1b), and this may be due to very low concentration of ‘Ag’ in catalyst [15]. Even though CdS has same wt%, and it could be detected by XRD. However, EDS and XPS analyses evidence the presence of Ag in the catalyst (discussion comes soon after). The doping possibility of silver is unlikely because of the difference in the ionic radii between Zn2+ (0.72 Å) and Ag+ (1.22 Å). If the silver is substituted in place of Zn, a corresponding peak shift is expected in

ð2Þ

ð1Þ

although the distinction of CdS in CdS–Ag–ZnO was unfeasible. This may be due to very low concentration of CdS (2 wt% respect to ZnO). Furthermore surface morphology of the CdS–Ag–ZnO catalyst particles is roughly hexagonal as well as nano chain like structures (Fig. 3a and b). The EDS analysis of CdS–Ag–ZnO shown in Fig. S5 reveals the presence of Cd, S, Ag, Zn and O. The percentage of Ag at particular region is also given in table (inset Fig. S5). The atomic percentages of Zn/O and Cd/S were nearly 1:1. The UV–Vis diffuse reflectance spectra of CdS–Ag–ZnO and bare ZnO photocatalysts are displayed in Fig. 4. All these spectra are recorded in the wavelength range of 200–800 nm. Fascinatingly, with increasing reflectance, both the catalyst clearly exhibits

Fig. 1. XRD patterns of (a) bare ZnO and (b) CdS–Ag–ZnO.

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Fig. 2. TEM images of CdS–Ag–ZnO (a) 200 nm, (b) 100 nm, (c) 50 nm and (d) 20 nm.

almost the alike absorption profile with a steep absorption edge at 400 nm. A slight red shift was noted for CdS–Ag–ZnO (from 400 to 420 nm) due to the loading Ag and CdS on the ZnO material. Therefore slight red shifted absorption spectrum is construed as a good evidence for superior interaction between ZnO, Ag and CdS species. Consequently, the red shift of CdS–Ag–ZnO photocatalyst can be attributed to the charge transfer between CdS and ZnO. Due to this charge transfer, e––h+ recombination is reduced. Hence, we found a small red-shift for the samples, which indicates that there is a decrease in energy band gap of these samples when compared to the bare ZnO. The band gap energy (Eg) of the samples could be estimated with Tauc’s law from the intercept of a straight line fitted through the rise of the function [F(R)hm]2 plotted versus energy of absorbed light E, where F(R) is a Kubelka–Munk function and hm is the energy of the incident photon (E) (Eq. (3)) [35].

FðRÞE

1=2

" #1=2 ð1
RÞ2 ¼  hv 2R

ð3Þ

The band gap energy (Eg value) of CdS–Ag–ZnO and bare ZnO can be thus estimated from the plot of [F(R)hm]2 versus absorbed light energy E. The intercept of the tangent to the X-axis will give a good approximation of the band gap energy for the photocatalyst [36,37]. The estimated band gap energies of the resulting samples bare ZnO and CdS–Ag–ZnO were found to be 3.15 and 2.91 eV respectively (Fig. 5a and b). This reduction of band gap may enhance the visible light activity of the catalyst. Photoluminescence (PL) spectral analysis was used to investigate the migration, transfer and separation efficiencies of photo-generated electrons and holes in semiconductors, since PL

emission of the semiconductor mainly occurs from the charge carrier recombination. Fig. 6 shows the room temperature photoluminescence spectra of bare ZnO and CdS–Ag–ZnO photocatalysts. The CdS–Ag–ZnO gives a similar band-to-band fluorescence emission characteristic as bare ZnO. Clearly, the photocatalytic activity of CdS–Ag–ZnO can be substantially improved by introducing CdS and Ag. The much weakened emission intensity of CdS–Ag–ZnO compared to bare ZnO, suggests decreased recombination probability of photoexcited charge carriers in the CdS–Ag–ZnO photocatalyst, which increased the photocatalytic activity under solar light. In other words, the loading of CdS and Ag with ZnO do not shift the emission of ZnO but the intensity of PL emission is less when compared to bare one. This is because of suppression of recombination of electron–hole pairs by Ag and CdS, which enhances the photocatalytic activity of the catalyst. The binding energies of electrons were determined by X-ray photoemission spectroscopy (XPS). The XPS survey spectrum of the CdS–Ag–ZnO is shown in Fig. 7a. Besides, C signal coming from the instrument itself, only Cd, Ag, S, O and Zn were detected, confirming the oxidation state and high purity of the catalyst. Binding energy peak of Cd is given in inset of Fig. 7a. It is the Cd 2d core level spectrum, showing two obvious peaks at 405.3 and 412.5 eV assigned to the characteristic Cd 3d5/2 and Cd 3d1/2 peaks of Cd2+, respectively [38]. Binding energy peaks of Ag, 3d5/2 and 3d3/2 were observed at 374.4 and 368.4 eV, respectively (Fig. 7b). According to Zhang et al., [39] the peaks at 373.96 and 368.11 eV can be attributed to metallic silver (Ag0). In Fig. 7c, the O1 s profile is asymmetric and can be fitted to two symmetrical peaks (a and b locating at 530.6 and 532.3 eV, respectively), indicating two different kinds of O species in the sample. The peaks a and b should be associated with the lattice oxygen (OL) of ZnO and chemisorbed

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Fig. 5. Kubelka–Munk function [F(R)hm]2 versus energy of the light absorbed of (a) bare ZnO and (b) CdS–Ag–ZnO. Fig. 3. FE-SEM images of CdS–Ag–ZnO (a) 400 nm and (b) 300 nm.

Fig. 6. Photoluminescence spectra of (a) bare ZnO and (b) CdS–Ag–ZnO. Fig. 4. DRS of (a) bare ZnO and (b) CdS–Ag–ZnO.

oxygen (OH) caused by the surface hydroxyl [40], respectively. The Zn 2p core level spectrum is presented in Fig. 7d. Two strong peaks centered at 1022.6 and 1045.7 eV, corresponding to the Zn 2p3/2 and Zn 2p1/2, respectively. These values are good agreement with the binding energies of Zn2+ ion [41]. The S2p peaks are located

in the range of 162.5–172.4 eV (Fig. 7e), which indicates the presence of the S2
[42]. Fig. 8 shows that atomic force microscope (AFM) images of CdS–Ag–ZnO (2D and 3D images). Fig. 8a and b exhibits (2D images) of the formation of slightly fine agglomerations consisting of packed together photocatalyst with non-uniform size. Furthermore, the

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Fig. 7. XPS of CdS–Ag–ZnO (a) survey spectrum (b) Ag3d peak, (c) O1 s peak (d) Zn2p peak and (e) S2p peak.

roughness of the as-deposited ZnO 3D images on microscope glass was determined (Fig. 8c and d). It is obvious that the enhancement of surface roughness is in dependence on the plasma treatment conditions, since it can affect the photoinduced properties of the catalyst due to the enhancement of the adsorption of dye molecules on the catalyst surface, which can increase the photocatalytic response. The Brunauer–Emmet–Teller (BET) surface area and pore structure of the prepared samples were investigated using adsorption– desorption measurements. As shown in Table 1, it should be noted that the specific surface area of CdS–Ag–ZnO (19.9 m2 g
1) as well as total pore volume at single point (0.15 cm3 g
1) are higher when compared to the bare ZnO. Moreover, Figs. S6a and S6b show the nitrogen adsorption–desorption isotherms and the corresponding curves of the pore size distribution (inset) for prepared samples.

According to the BET classification, the majority of physisorption isotherms can be grouped into many types [43]. Characteristically, CdS–Ag–ZnO and bare ZnO catalysts had the isotherm of type II hysteresis loops and the presence of large macropores. Greater specific surface area of photocatalyst can furnish more surface active sites and make charge carriers transport easier, leading to an enhancement of the photocatalytic performance [7,15]. Thus, CdS–Ag–ZnO catalyst may play a role in enhancing the photocatalytic activity. 3.2. Photodegradability of RR 120, reusability and antiphotocorrosive property of the catalyst All the prepared samples were tested for RR 120 dye degradation under solar light irradiation with suitable concentration of

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Fig. 8. AFM images of CdS–Ag–ZnO: 2D images (a and b) and 3D images (c and d).

Table 1 Surface properties of the catalysts. Properties

Bare ZnO

CdS–Ag–ZnO

BET surface area Total pore volume (single point)

11.5 (m2 g
1) 0.07 (cm3 g
1)

19.9 (m2 g
1) 0.15 (cm3 g
1)

dye solution, and the conditions were optimized only for CdS–Ag– ZnO (Fig. 9). A reference experiment exhibited that no dye degradation was observed when the experiment was carried out by taking pure dye solution in the absence of either photocatalyst (result not shown) or irradiation (curve b). The initial decrease in the dye concentration for CdS–Ag–ZnO (59.5%, curve b) is due to adsorption of the dye in dark on the surface of the catalyst. Among all the modified, unmodified and commercial catalysts, CdS– Ag–ZnO was able to produce almost complete degradation in 30 min (curve a) under sun light irradiation, whereas with Ag–ZnO, CdS–ZnO, Ag–CdS, CdS, bare ZnO, commercial ZnO, TiO2-P25 and TiO2 (Merck) under same conditions only 70.6 (curve c), 47.9 (curve d), 43.9 (curve e), 34.9 (curve f), 71.2 (curve g), 69.9

(curve h), 59.9 (curve i) and 71.1 (curve j) percentages of degradation occurred, respectively. This process clearly exhibit that CdS–Ag–ZnO is more efficient in RR 120 degradation than all other prepared and commercial catalysts. The initial adsorption of dye molecules in dark by CdS–Ag–ZnO (59.5%) is higher than the adsorption by other commercial and prepared catalysts. After irradiation for 30 min dye free water was obtained with CdS–Ag–ZnO photocatalyst. The dark adsorption and the degradation efficiency of the used CdS–Ag–ZnO catalyst were found to be similar to those of fresh catalyst. This confirms that the molecules adsorbed in dark underwent complete degradation during irradiation. In order to confirm this, FT-IR spectra of the CdS–Ag–ZnO photocatalyst with the adsorbed dye molecules before and after irradiation were taken and are shown in Figs. S7b and S7c along with the FT-IR spectrum of the fresh catalyst (Fig. S7a). Fig. 7b shows the characteristics peaks of the dye demonstrating adsorption of dye on the surface of catalyst. FT-IR spectrum of the catalyst after irradiation (i.e.,) after complete degradation (Fig. S7c) make known that the characteristics peaks of dye present on the adsorption spectrum disappeared and Fig. S7c is similar to Fig. S7a. This confirms that the dye molecules

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98.8% and 97.0%, respectively, after 30 min of irradiation time. The photocatalyst was stable and the dye degradation occurred without decreasing its activity. The present study demonstrated the good anti-photocorrosion capability of our developed ZnO based photocatalysts. It is well known that leaching of Zn ions happened under light irradiation of ZnO in aqueous medium [15,44,45]. So, in order to know the stability of ZnO, atomic absorption spectroscopy (AAS) studies of bare ZnO and CdS–Ag–ZnO have been carried out after the photocatalytic experiment and the results are shown in Table 2 for 1000 mg of ZnO and 1041 mg of CdS–Ag–ZnO (equivalent to 1000 mg of ZnO). The AAS result exhibited that the loss of Zn in ZnO is considerably higher than that of CdS–Ag–ZnO. The loss of Zn content in bare ZnO is around 2.49% after the photocatalytic reaction, whereas in the case of CdS–Ag–ZnO, the loss of Zn is much less and is around 0.652%, which is 3.6 times less than bare ZnO. The loss of Zn is strongly inhibited by the modification with CdS and Ag particles. These results revealed that the CdS and Ag modification drastically enhanced the stability of ZnO in the aqueous medium. Fig. 9. Photodegradability of RR 120; [RR 120] = 2  10
4 M, catalyst suspended = 3 g L
1, pH = 7, airflow rate = 8.1 mL s
1, Isolar = (1250  100) ± 100 lux.

3.3. Photodegradability of RO 4 and reusability of the catalyst The photocatalytic activity of CdS loaded Ag–ZnO catalyst was evaluated by photocatalytic decomposition of RO 4 in its aqueous solution under sun light illumination. Fig. 11 shows the percentage of RO 4 on irradiation of an aqueous solution of RO 4 (5  10
4 M) in the absence and the presence of photocatalyst under daylight (curve b). Almost complete degradation of RO 4 takes place at the time of 50 min with CdS–Ag–ZnO under solar light (curve a). In the presence of CdS–Ag–ZnO, without irradiation, only 20.1% decrease in dye concentration was observed (curve b) due to the adsorption of the dye on the surface of the catalyst. These observations reveal that both sun light and CdS–Ag–ZnO are needed for effective destruction of RO 4. When Ag–ZnO, CdS–ZnO, Ag–CdS, CdS, bare ZnO, commercial ZnO, TiO2-P25 and TiO2 (Merck) were used under same conditions only 71.6 (curve c), 67.9 (curve d), 55.6 (curve e), 50.3 (curve f), 57.2 (curve g), 62.9 (curve h), 68.1 (curve i) and 63.0 (curve j) percentages of degradation occurred, respectively. This reveals that CdS–Ag–ZnO is more efficient in

Fig. 10. Catalyst reusability, [RR 120] = 2  10
4 M, 2 wt% CdS loaded Ag–ZnO suspended = 3 g L
1, pH = 7, airflow rate = 8.1 mL s
1, Isolar = (1250  100) ± 100 lux, irradiation time = 30 min.

Table 2 Concentration of Zn2+ ions mg L
1in the solution mixture after photoreaction. No. of cycles

CdS–Ag–ZnO

Bare ZnO

First run Second run Third run Fourth run Average Initial ZnO Dissolution (%)

3.54 4.62 5.72 13.4 6.82 1000.0 0.682

2.1 16.2 31.4 49.7 24.9 1000.0 2.49

[RR 120] = 2  10
4 M, catalyst suspended = 3 g L
1, pH = 7, rate = 8.1 mL s
1, irradiation time = 30 min, Isolar = (1250  100) ± 100 lux.

airflow

adsorbed on the CdS–Ag–ZnO photocatalyst in dark have been completely degraded at the time of 30 min irradiation. In order to know the stability of the developed photocatalysts, the time course photocatalytic degradation over CdS–Ag–ZnO was carried out with the used catalyst and is presented in Fig. 10. The degradations for 1st, 2nd, 3rd and 4th cycle were 100%, 100%,

Fig. 11. Photodegradability of RO 4; [RO 4] = 5  10
4 M, catalyst suspended = 2 g L
1, pH = 7, airflow rate = 8.1 mL s
1, Isolar = (1250  100) ± 100 lux.

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RO 4 degradation than the other prepared and commercial catalysts. The catalysts lifetime is an important parameter of the photocatalytic process, due to the fact that a longer period of time leads to a significant cost reduction of the treatment. The results from four consecutive runs with the recovered catalyst are shown in Fig. S8. Although the degradation efficiency of CdS–Ag–ZnO is slightly decreased at 4th run, the catalyst exhibited 97.0% activity after three successive cycles under the solar irradiation. This indicates that CdS– Ag–ZnO catalyst remains effective and reusable under solar light. 3.4. Photodegradability of RY 84 and reusability of the catalyst The degradation rate of RY 84 vs. time for different kinds of photocatalysts is shown in Fig. 12. CdS loaded Ag–ZnO catalyst exhibits almost complete degradation (100%) (curve a) at the time of 50 min under sun light irradiation. This was contrasted with negligible degradation (0.2%) when the reaction was allowed to occur in the presence of solar light without any catalyst (result not shown). When the experiment was carried out with the catalyst in dark, dye concentration was decreased by 18.6% (curve b). This is due to the adsorption of the dye on the catalyst. When the photocatalyst of Ag–ZnO, CdS–ZnO, Ag–CdS, CdS, bare ZnO, commercial ZnO, TiO2-P25 and TiO2 (Merck) were used under same conditions only 81.9 (curve c), 71.0 (curve d), 44.0 (curve e), 43.1 (curve f), 60.0 (curve g), 80.0 (curve h), 55.9 (curve i) and 72.0 (curve j) percentages of degradation occurred, respectively. This reveals that CdS– Ag–ZnO is more efficient in RY 84 degradation than other catalysts. Reusability of the catalyst was tested by carrying out the degradation with the used catalyst. The results for four cycles of the catalyst are shown in Fig. S9. The CdS–Ag–ZnO exhibited remarkable photostability without any appreciable loss of photocatalytic activity after four cycles. The catalyst exhibited 100%, 98.0%, 97.4% and 97.1% activity at first, second, third and fourth cycles, respectively. These results indicate that CdS–Ag–ZnO catalyst is more stable and reusable under solar light. 3.5. UV–visible spectra of the dyes at different time of illumination Fig. S10 shows a typical time dependent UV–vis spectrum of dyes (RR 120, RO 4 and RY 84) solution during photoirradiation.

The spectrum of dye in the UV region exhibits a main band with a maximum at 200–400 nm. As it is obvious from this figure, the absorption peaks diminished and finally vanished during the reaction, which indicated that the dye had been degraded successfully. Practically complete (100%) removal of all the three (RR 120, RO 4 & RY 84) dyes was observed after with respective time of irradiation under desired conditions. It is understood that the catalytic degradation reaction of organic pollutants take places on the surface of catalyst; O2 and H2O are compulsory for the photocatalytic degradation. Oxygen adsorbed on the catalyst surface avoids the recombination of electron–hole pairs by trapping electrons; super  oxide radical ions (O– 2 ) are thus formed. OH radicals are formed from holes reacting with either H2O or OH– adsorbed on the CdS–Ag–ZnO surface. Moreover to the point the anti-photocorrosion ability, the lowest particle size, highest surface area, lowest PL intensity and higher absorption in DRS analysis are important factors for the CdS–Ag– ZnO for concerting the best catalytic activity for dye degradation under solar light irradiation. In most cases, if the particle size is very small, the electron can simply migrate from the immensity to the surface and initiate the catalytic reaction. So the small particle size of CdS–Ag–ZnO favors the electrons reaching the surface easily to initiate the photocatalytic reaction of dye degradation and hence greatly enhances the photocatalytic activity. 3.6. Chemical oxygen demand (COD) analysis In order to confirm the mineralization of dyes, COD measurements were made for the degradation of RR 120, RO 4 and RY 84 with CdS–Ag–ZnO catalyst. The mineralization of these three dyes (RR 120, RO 4 and RY 84) is reported by measuring the COD values by various times of irradiation under optimized conditions in solar light. The percentages of COD reduction at different times of irradiation are presented in Table 3. About 99.1% (RR 120), 97.4% (RO 4) and 94.2% (RY 84) of COD reductions were observed for these dyes under optimum conditions. It indicates the complete mineralization of dye molecules. 3.7. Comparison of degradation efficiencies of catalyst with different dyes Degradation percentages of RR 120, RO 4 and RY 84 with CdS–Ag–ZnO for 30 min UV irradiation under optimal conditions are given in Table 4. If the efficiency of catalyst is compared for the three dyes, it is found that RR 120 underwent higher degradation when compared to other two dyes RO 4 and RY 84. Both dyes (RO 4 and RY 84) are more or less equally degraded by CdS–Ag–ZnO catalyst. Degradation efficiency of dyes depends on so many factors such as adsorption of dye on catalyst, absorption of light by the catalyst,

Table 3 COD measurements.

Fig. 12. Photodegradability of RY 84; [RY 84] = 5  10
4 M, catalyst suspended = 3 g L
1, pH = 7, airflow rate = 8.1 mL s
1, Isolar = (1250  100) ± 100 lux.

Time (min)

COD removal (RR 120)a

COD removal (RO 4)b

COD removal (RY 84)c

10 20 30 40 50

23.9 64.7 99.1 – –

14.3 34.5 60.7 81.9 97.4

17.7 40.5 67.4 80.2 94.2

a [RR 120] = 2  10–4 M; 2 wt% CdS loaded Ag–ZnO suspended = 3 g L–1; pH = 7; airflow rate = 8.1 mL s
1; Isolar = (1250  100) ± 100 lux. b [RO 4] = 5  10–4 M; 2 wt% CdS loaded Ag–ZnO suspended = 2 g L–1; pH = 7; airflow rate = 8.1 mL s
1; Isolar = (1250  100) ± 100 lux. c [RY 84] = 5  10–4 M; 2 wt% CdS loaded Ag–ZnO suspended = 3 g L–1; pH = 7; airflow rate = 8.1 mL s
1; Isolar = (1250  100) ± 100 lux.

B. Subash et al. / Separation and Purification Technology 164 (2016) 170–181 Table 4 Comparison of photocatalytic degradability of dyes with CdS–Ag–ZnO. Dyes

RR 120a

RO 4b

RY 84c

CdS–Ag–ZnO (Degradation % at 30 min)

100.00

72.8

80.2

59.5

20.1

CdS–Ag–ZnO (Dark adsorption %) a

–4

18.6 –1

[RR 120] = 2  10 M; 2 wt% CdS loaded Ag–ZnO suspended = 3 g L ; pH = 7; airflow rate = 8.1 mL s
1; Isolar = (1250  100) ± 100 lux. b [RO 4] = 5  10–4 M; 2 wt% CdS loaded Ag–ZnO suspended = 2 g L–1; pH = 7; airflow rate = 8.1 mL s
1; Isolar = (1250  100) ± 100 lux. c [RY 84] = 5  10–4 M; 2 wt% CdS loaded Ag–ZnO suspended = 3 g L–1; pH = 7; airflow rate = 8.1 mL s
1; Isolar = (1250  100) ± 100 lux.

the dye structure, and concentration of dye. An experiment on adsorption of dyes on the catalysts was carried out at optimum conditions. Adsorption percentages of three dyes after the attainment of equilibrium with CdS–Ag–ZnO are given in Table 4. The adsorption percentage of RR 120 was higher than RO 4 and RY 84 and hence RR 120 undergoes higher degradation then the other two dyes.

3.8. Mechanism of dye degradation A mechanistic scheme of the charge separation and photocatalytic reaction for CdS–Ag–ZnO photocatalyst is shown in Scheme 1. This shows the conduction band (CB) position of ZnO is lower than that of CdS, so that the former can act as a sink for the photogenerated electrons in the mixed semiconductors [46]. Ever since, the holes move in the conflicting direction from the electrons, the photogenerated electrons in CdS might be trapped within the ZnO particle, making charge separation highly efficient; in addition to this, presence of ‘Ag’ trap the electron from both ZnO as well as CdS then the recombination of electrons and holes in CdS–Ag–ZnO is significantly suppressed. This may increase the photocatalytic activity of CdS–Ag–ZnO.

179

In addition to this, dye sensitized mechanism is also possible in RR 120 degradation. We had carried out the degradation of RR 120 with 365 nm UV light (IUV = 1.381  10
6 Einstein L
1 s–1) under the identical condition used for solar light. It was found that RR 120 endures 87.2% degradation with UV light (365 nm). But under the same conditions 93.4% degradation occurred with solar light for 20 min irradiation. Higher efficiency of CdS–Ag–ZnO in solar light point out the presence of dye sensitized mechanism along with the usual ZnO sensitization. This is favored when more dye molecules are adsorbed on the semiconductor face. This improves the photoexcited electron transfer from solar light sensitized dye molecule to the CB of ZnO and afterwards increases the electron transfer to the adsorbed oxygen molecule. Higher adsorption of dye molecules by CdS–Ag–ZnO also confirms this (59.5% adsorption at pH = 7). Addition to the degradation of dye by the usual ZnO sensitization mechanism, the dye molecules are also degraded by the super oxide radicals produced by dye sensitization mechanism (Eqs. (4)–(6)). Further to verify this mechanism we had also carried out an experimentation for the degradation of 4-nitrophenol by CdS–Ag–ZnO with UV and solar light. We found that the degradation was more efficient in UV light than in solar light (UV: 87.5%, solar: 40.8% in 60 min), demonstrating presence of dye sensitized mechanism for the degradation of RR 120 dye by CdS–Ag–ZnO. Similar result has been reported for solar photodegradation of dyes [15].

Dye þ CdS—Ag—ZnO ! Dyeþ CdS—Ag—ZnO þ ecb

ð4Þ

e
cb þ O2 ! O
2

ð5Þ

Dyeþ þ O2 =O
2 ! degradation products

ð6Þ

4. Conclusion In summary, we have demonstrated cost-effective precipitation-thermal decomposition method for the production

Scheme 1. Mechanism of degradation of dyes by CdS–Ag–ZnO.

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of CdS loaded Ag–ZnO photocatalyst. This synthesis is simple and cost efficient. TEM and XRD reveal the presence of hexagonal and wurtzite structure of ZnO, respectively. EDS shows the presence of Ag, S and Cd in the catalyst. Presence of Ag and CdS increase the absorption of ZnO to entire visible region. DRS spectra indicate the reduction of band gap of the CdS–Ag–ZnO, when compared to ZnO. The PL spectra show the suppression of recombination of the photogenerated electron–hole pairs by the CdS and Ag loading on ZnO. XPS reveal the presence of metallic silver in the catalyst. Furthermore, it can be concluded that the CdS and Ag strongly interacts with the ZnO and help to prevent the loss of Zn during the photocatalytic reaction. So, the anti-photocorrosion ability of CdS–Ag–ZnO is one of the most responsible factors for showing the best result towards azo dyes degradation under solar light irradiation. COD measurements confirm the complete mineralization of three azo dye (RR 120, RO 4 & RY 84) molecules. A dual mechanism involving dye sensitization has been proposed for the efficient dye degradation with RR 120 dye by CdS–Ag–ZnO under solar light. This catalyst was found to be ‘‘stable” and ‘‘reusable”. Moreover, prepared CdS–Ag–ZnO had greater photocatalytic activity and photostability towards RR 120, RO 4 and RY 84 dye degradations under these optimum conditions. This process, using CdS–Ag–ZnO photocatalytic material would be more useful for industrial effluent treatment, due to its advantage of ‘‘effortlessness”, ‘‘reusability” and ‘‘excellent concert.” Acknowledgements M. Shanthi is highly thankful to UGC, New Delhi, India for financial support through research project F.No 41-288/2012 (SR). This work was supported by FCT/QREN-COMPETE through the project PTDC/AAC-CLI/118092/2010 and grant SFRH/BPD/86971/2012 (Balu Krishnakumar). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2016.03. 029. References [1] P.W. Du, R. Eisenberg, Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting: recent progress and future challenges, Energy Environ. Sci. 5 (2012) 6012–6021. [2] V. Balzani, A. Credi, M. Venturi, Photochemical conversion of solar energy, ChemSusChem. 1 (2008) 26–58. [3] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemannt, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [4] A. Mills, R.H. Davies, D. Worsley, Water purification by semiconductor photocatalysis, Chem. Soc. Rev. 22 (1993) 417–425. [5] B. Subash, B. Krishnakumar, R. Velmurugan, M. Swaminathan, Photodegradation of an azo dye with reusable SrF2–TiO2 under UV light and influence of operational parameters, Sep. Purif. Technol. 101 (2012) 98–106. [6] B. Subash, B. Krishnakumar, V. Pandiyan, M. Swaminathan, M. Shanthi, An efficient nanostructured Ag2S–ZnO for degradation of Acid Black 1 dye under day light illumination, Sep. Purif. Technol. 96 (2012) 204–213. [7] B. Subash, B. Krishnakumar, M. Swaminathan, M. Shanthi, Enhanced photocatalytic performance of WO3 loaded Ag–ZnO for Acid Black 1 degradation by UV-A light, J. Mol. Catal. A: Chem. 366 (2013) 54–63. [8] N. Lakshminarasimhan, E. Bae, W. Choi, Enhanced photocatalytic production of H2 on mesoporous TiO2 prepared by template-free method: role of interparticle charge transfer, J. Phys. Chem. C 111 (2007) 15244–15250. [9] Yunqi Li, B.P. Bastakoti, M. Imura, S.M. Hwang, Z. Sun, J.H. Kim, S.X. Dou, Y. Yamauchi, Synthesis of mesoporous TiO2/SiO2 hybrid films as an efficient photocatalyst by polymeric micelle assembly, Chem. Eur. J. 20 (2014) 6027– 6032. [10] N. Suzuki, X. Jiang, L. Radhakrishnan, K. Takai, K. Shimasaki, Y.T. Huang, N. Miyamoto, Y. Yamauchi, Hybridization of photoactive Titania nanoparticles with mesoporous silica nanoparticles and investigation of their photocatalytic activity, Bull. Chem. Soc. Jpn. 84 (2011) 812–817.

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