β-Ag2S–ZnO as a novel sunshine photocatalyst for the effective degradation of RR 120 dye

Powder Technology 241 (2013) 49–59

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β-Ag2S–ZnO as a novel sunshine photocatalyst for the effective degradation of RR 120 dye B. Subash, B. Krishnakumar 1, M. Swaminathan, M. Shanthi ⁎ Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 4 December 2012 Received in revised form 13 February 2013 Accepted 1 March 2013 Available online 13 March 2013 Keywords: β-Ag2S–ZnO Sunshine photocatalyst RR 120 Dual mechanism

a b s t r a c t A novel β-Ag2S loaded ZnO was successfully synthesized by precipitation of zinc oxalate and Ag2S and calcination of the mixed precipitate at 400 °C for 12 h. The catalyst was characterized by XRD, FE-SEM, EDS, TEM, DRS, PL, CV, XPS and BET surface area measurements. XPS reveals that Ag in the catalyst is present in the form of Ag+ before irradiation. The photocatalytic activity of β-Ag2S–ZnO was investigated for the degradation of RR 120 under solar light. It is found to be more efficient than commercial ZnO, prepared ZnO, TiO2-P25 and TiO2 (Merck) at neutral pH for the mineralization of RR 120. The effects of operational parameters have been analyzed, and the mineralization of RR 120 has been confirmed by COD measurements. The catalyst is found to be reusable. A dual mechanism involving dye sensitization has been proposed for the degradation of RR 120 by β-Ag2S–ZnO under day light illumination. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Environmental problems, such as organic pollutants and toxic water pollutants, provide the impetus for the fundamental and applied research in the environmental area. Photocatalysis using solar energy is favorably expected to be an ideal “Green” technology for sustainable development of human beings where an active photocatalytic material offers the potential for the elimination of toxic chemicals through its efficiency and broad applicability [1]. To date, TiO2 is well-known as a stable, low cost, environmental friendly and highly efficient photocatalytic material [1–4]. ZnO is found to be a suitable alternative to TiO2 [5]. Sunlight contains about 4% ultraviolet light [6,7]. Utilization of solar energy is a very interesting aspect of science. Solar photocatalysis has therefore become a most important area of research in which sunlight is the source of illumination to perform different photocatalytic reactions. Since visible light is the major component of solar radiation, the development of a stable photocatalytic system, which can be effected by visible light, is indispensable. Therefore, it is of great interest to develop new visible-light photocatalysts to extend the absorption into the visible light region. ZnO is an important wide band gap semiconductor, which is bio-safe and biocompatible material and can be directly used in heterogeneous photocatalysis [8]. Basically, ZnO exhibits more

⁎ Corresponding author. Tel./fax: +91 4144 237386. E-mail addresses: [email protected] (B. Krishnakumar), [email protected], [email protected] (M. Shanthi). 1 Present address: Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei, 106, Taiwan, ROC. 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.03.002

efficiency than TiO2 in photocatalytic degradation of some dyes under visible light illumination [9]. Many attempts have been made to enhance the utilization of solar energy and to inhibit the recombination of photogenerated e−–h+ pairs by doping the base photocatalyst with impurities. In the past, transition and noble metal ions have been used as dopants to broaden optical absorption in the visible light band for practical applications [10,11]. Other elements were doped with ZnO to enhance its photocatalytic efficiency [12]. For higher photo catalytic efficiency, modified ZnO should absorb both visible and UV light, while the bare ZnO can only absorb the latter. Aluminum doped zinc oxide shows improved solar cell characteristics [13]. In contrast to the single semiconductor photocatalyst, many coupled semiconductor systems, such as ZnO–Fe2O3 [14], ZnO–WO3 [14,15], ZnO–SnO2 [16], TiO2–WO3 [17], TiO2–SnO2 [18], TiO2–ZnO [19] have shown high photocatalytic efficiency due to increased charge separation and extended energy range of photoexcitation. Furthermore, the optimal photocatalytic activities of coupled semiconductor systems have been obtained through the controlled synthesis process. The metal sulfide photocatalysts such as ZnS–CdS [20], CdS–TiO2 [21], CdS–ZnO [22], were also extensively investigated, due to their suitable band structure. In particular, the low band gap energy (about 1.0 eV) of β-Ag2S renders the capability of absorbing a broad solar spectrum, which makes β-Ag2S–ZnO, an effective semiconductor material for photocatalytic applications. However, synthesis of Ag2S coupled with semiconductor oxide is rarely reported [23,24]. This stimulated our interest to prepare a composite material of β-Ag2S coupled with ZnO. Ag2S is known as a coinage mineral which undergoes a structural phase transition. At room temperature, Ag2S has a monoclinic structure called acanthite (β-Ag2S) space group P21/c and Z = 4. But above 865 K, Ag2S changes to argentite

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B. Subash et al. / Powder Technology 241 (2013) 49–59

(α-Ag2S) with cubic structure [25–28]. Both α and β phases have body-centered cubic arrangement of sulfur atoms with good photocatalytic activity. β-Ag2S behaves like a semiconductor (dr/dT > 0, with activation energy of 1.3 eV), where α-Ag2S behaves like a metal (dr/dT b 0). Thus, the β–α transition of Ag2S has the character of semiconductor–metal transition [25–29]. The present study aims to prepare a β-Ag2S–ZnO composite system and to characterize this new composite material and its photocatalytic activity on the degradation of RR 120 dye. To the best of our knowledge this is the first report to use β-Ag2S–ZnO as an effective sunshine photocatalyst for efficient degradation of RR 120 dye.

washed several times with distilled water, air-dried overnight and dried at 100 °C for 3 h. Ag2S–zinc oxalate dihydrate was taken in a porcelain dish and heated in the muffle furnace at the rate of 20 °C min −1 to reach the decomposition temperature of zinc oxalate (400 °C). After 12 h, the furnace was allowed to cool down to room temperature. The β-Ag2S–ZnO catalyst was collected and used for further analysis. This catalyst contained 18.2 wt.% of Ag2S. Catalysts with 4.5, 9.1, 27.4 and 36.5 wt.% of β-Ag2S in the catalyst were prepared with this procedure. The bare ZnO was prepared using the same procedure without addition of Ag2S.

2.3. Analytical methods

2. Experimental 2.1. Materials The commercial azo dye Reactive Red 120 (Fig. S1, See supplementary data) from Balaji Colour Company, Dyes and Auxiliaries (Chennai) were used as such. Oxalic acid dihydrate (99%), ZnO and zinc nitrate hexahydrate (99%) were obtained from Himedia chemicals. AgNO3 and Na2S were obtained from Sigma Aldrich. TiO2 (Merck) was used as received. A gift sample of Degussa TiO2-P25 was obtained from Evonik (Germany). It is an 80:20 mixture of anatase and rutile. It has a particle size of 30 nm and BET surface area 50 m 2 g −1. The double distilled water was used to prepare experimental solutions. The pH of the solutions before irradiation was adjusted using H2SO4 or NaOH. 2.2. Preparation of β-Ag2S–ZnO A novel β-Ag2S loaded ZnO was prepared by precipitation–decomposition method (Scheme 1). Aqueous solution of equal volume of 0.4 M zinc nitrate hexahydrate and of 0.6 M oxalic acid dihydrate in deionized water were brought to their boiling points and zinc nitrate solution was added rapidly to the oxalic acid. The zinc oxalate is formed. 5 ml of solution of 0.004 M of AgNO3 (0.676 g) was mixed with 0.002 M of Na2S (0.184 g) in 5 mL. Formed Ag2S was added to the zinc oxalate. The zinc oxalate dihydrate crystals with Ag2S were

Zinc nitrate

Oxalic acid

Δ

Powder X-ray diffraction patterns were obtained using X′Per PRO diffractometer equipped with a CuKα radiation (wavelength 1.5406 Å) at 2.2 kW Max. Peak positions were compared with the standard files to identity the crystalline phase. The morphology of the catalyst was examined using a JEOL JSM-6701F cold field emission scanning electron microscope (FE-SEM). Before FE-SEM measurements, the samples were mounted on a gold platform placed in the scanning electron microscope for subsequent analysis at various magnifications. For transmission electron microscope (TEM), the grids were dried under natural conditions and examined using a TEM Hitachi H-7500. Diffuse reflectance spectra were recorded using Shimadzu UV-2450. X-ray photoelectron spectra of the catalysts were recorded in an ESCA-3 Mark II spectrometer (VG Scientific Ltd., England) using Al Kα (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 catalysts were determined using a Micromeritics ASAP 2020 sorption analyzer. The samples were degassed at 423 K for 12 h and analysis was performed at 77 K with N2 gas as the adsorbate. The Brunauer–Emmett–Teller (BET) multipoint method least-square fit provided the specific surface area. Photoluminescence (PL) spectra at room temperature were recorded using a Perkin Elmer LS 55 fluorescence spectrometer. The nanoparticles were dispersed in carbon tetrachloride and excited using light of wavelength 300 nm. Cyclic voltammetry (CV) measurements were carried out using CHI 60 AC Electrochemical Analyzer

Silver nitrate

Sodium sulfide

Δ

Zinc oxalate

Ag2S

Stirring

Ag2S with zinc oxalate Air oven at 100°C

Furnace at 400°C

β-Ag2S-ZnO Scheme 1. Schematic representation for preparation of β-Ag2S–ZnO.

B. Subash et al. / Powder Technology 241 (2013) 49–59

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2.4. Irradiation experiments All photocatalytic experiments were 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 vessel. The suspensions were magnetically stirred in the dark for 30 min to attain adsorption–desorption equilibrium between dye and β-Ag2S–ZnO. Irradiation was carried out in open-air condition. Fifty milliliters of dye solution with β-Ag2S–ZnO was continuously aerated by a pump to provide oxygen and for the complete mixing of reaction solution. During the illumination time no volatility of the solvent was observed. After dark adsorption the first sample was taken. At specific time intervals 2–3 mL of the sample was withdrawn and centrifuged to separate the catalyst. One milliliter of the centrifugate was suitably diluted and its absorbance at 285 nm was measured. The absorbance at 285 nm represents the aromatic content of RR 120 and its decrease indicates the degradation of dye. 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 calculated. The sensor was always set in the position of maximum intensity. The intensity of solar light was measured using LT Lutron LX-10/A Digital Lux meter and the intensity was 1250 × 100 ± 100 lx. The intensity was nearly constant during the experiments.

Fig. 1. XRD pattern of a) bare ZnO and b) β-Ag2S–ZnO.

2.6. Chemical oxygen demand (COD) measurements (CHI Instruments Inc. USA). UV spectral measurements were done using Hitachi-U-2001 spectrometer. The pH of the solution was measured by using ELICO (LI-10T model) digital pH meter.

COD was determined using the following procedure. Sample was refluxed with HgSO4, known volume of standard K2Cr2O7, AgSO4

(a)

(b)

(c)

(d)

Fig. 2. FE-SEM images of β-Ag2S–ZnO a) 20 nm, b) 20 nm, c) 100 nm and d) 100 nm.

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3. Results and discussion 3.1. Characterization of catalyst

Fig. 3. EDS of β-Ag2S–ZnO.

and H2SO4 for 2 h 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

ðBlank titre valuedye sample titre valueÞ  normality of FAS  8  1000 Volume of sample

ð1Þ

Primary analysis of photocatalytic degradation of RR 120 with different β-Ag2S loaded ZnO catalysts was carried out. Pseudo-first order rate constants determined for 4.5, 9.1, 18.2, 27.4 and 36.5 wt.% β-Ag2S loading were 0.0846, 0.0915, 0.1010, 0.0929 and 0.0778 min −1, respectively. The catalyst loaded with 18.2 wt.% of β-Ag2S was found to be most efficient. Hence, 18.2 wt.% of β-Ag2S was taken as optimum concentration of Ag2S on ZnO and this catalyst was characterized by X-ray diffraction (XRD), filed emission scanning electron microscope (FE-SEM) images, energy dispersive spectra (EDS), transmission electron microscope (TEM) images, diffuse reflectance spectra (DRS), photoluminescence (PL), cyclic voltammetry (CV) measurements, X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) surface area measurements. The XRD pattern of bare ZnO is given in Fig. 1a. All the marked diffraction peaks of ZnO in Fig. 1a can coincidently be indexed by the known hexagonal standard wurtzite ZnO. The crystallographic phase of bare ZnO belongs to the wurtzite type ZnO (SG: P63mc). Diffraction peaks at 31.68°, 34.36°, 36.18° and 56.56°, correspond to (100), (002), (101) and (110) planes of wurtzite ZnO. The relatively high intensity of the (101) peak is indicative of anisotropic growth and implies a preferred orientation of the crystallites. Fig. 1b shows the XRD pattern of β-Ag2S–ZnO. The pattern of β-Ag2S–ZnO

Fig. 4. TEM images of β-Ag2S–ZnO a) 200 nm, b) 100 nm, c) 50 nm and d) 20 nm.

B. Subash et al. / Powder Technology 241 (2013) 49–59

D¼

Kλ β cos θ

ð2Þ

where D is the crystal size of the catalyst, K is dimensionless constant, λ is the wavelength of X-ray, β is the full width at half-maximum (FWHM) of the diffraction peak and θ is the diffraction angle. The average crystalline size of β-Ag2S–ZnO (4.5 nm) is lower than bare ZnO (30.1 nm). The structure and morphology of the catalyst are very important parameters as they influence the photocatalytic activity. The surface morphology of β-Ag2S–ZnO has been analyzed by FE-SEM images. The FE-SEM images of two different magnifications at two different locations are given in (Fig. 2a–d). At higher magnification of 20 nm, big particles with hexagonal structure of ZnO are clearly seen (indicated by arrow marks) (Fig. 2a and b). Lower magnification of (100 nm) at two different locations (Fig. 2c and d) clearly indicates the dispersion of β-Ag2S on the surface of ZnO. Presence of elements in β-Ag2S–ZnO composite has been confirmed by EDS shown as Fig. 3. Fig. 4 shows the TEM images of prepared β-Ag2S–ZnO at different magnifications (Fig. 4(a)–(d)). At lower magnifications, the Ag2S and ZnO particles are not distinguishable from each other (Fig. 4a and b). The hexagonal structure of ZnO is clearly seen from its higher magnifications (Fig. 4c and d). The sizes of the particle are in the range of 4 to 50 nm. The diffuse reflectance spectra of bare ZnO and β-Ag2S–ZnO are displayed in Fig. 5a and b, respectively. The threshold wavelength required for excitation of a semiconductor is given by the equation [31]. λðnmÞ ¼ 1240=Ebg ðeVÞ

ð3Þ

Hence wavelengths determined for excitation of ZnO and β-Ag2S– ZnO are 386 and 428 nm respectively. This reveals that the absorption of edge of ZnO is extended to visible region by loading of Ag2S. Furthermore Ag2S loading increased the absorption of visible light as shown in Fig. 5b. This makes β-Ag2S–ZnO more visible light sensitive. The band gap energies of bare ZnO and β-Ag2S–ZnO are found to be 3.26 eV and 2.90 eV, respectively. Fig. 6 presents the

300

250

(a) PL intensity

is different from that of ZnO catalyst. In β-Ag2S–ZnO system, there are five peaks with 2θ values of 22.19°, 28.07°, 31.06°, 38.09° and 44.28° corresponding to β-Ag2S [30]. This confirms the loading of β-Ag2S on ZnO. Broadening of peaks of β-Ag2S-ZnO indicates the reduction of size of the particle when compared to bare ZnO. The crystalline size of 18.2 wt.% of β-Ag2S–ZnO was determined using Debye– Scherrer equation.

53

200

150

(b)

100

50

0 300

350

400

450

500

Wavelength (nm) Fig. 6. Photoluminescence spectra of a) bare ZnO and b) β-Ag2S–ZnO.

photoluminescence (PL) spectra of the bare ZnO and β-Ag2S–ZnO. Photoluminescence occurs due to the recombination of electron–hole pair in the semiconductor. The PL spectra reveal that the PL intensity of β-Ag2S–ZnO is less than bare ZnO. This is because of suppression of recombination of the photogenerated electron–hole pairs by the β-Ag2S loaded on ZnO. Inhibition of electron–hole recombination makes this catalyst more photoactive. In cyclic voltammetry measurements, bare ZnO and β-Ag2S–ZnO, were used in the redox reaction of potassium ferrocyanide (3 mM concentration). Bare ZnO does not give any anodic potential and current (Fig. 7a) whereas for β-Ag2S–ZnO, the anodic potential and current are E = 0.7142 V and I = 5.875 × 10−6A respectively (Fig. 7b). This increase in current indicates presence of β-Ag2S in the catalyst [12]. In order to confirm the presence of Ag and its oxidation state in this catalyst, the XPS spectra of this sample has been taken. The XPS survey spectrum (Fig. 8a) of the β-Ag2S–ZnO indicates the peaks of elements Zn, O, Ag and S. Fig. 8b, c, d and e show the binding energies of Zn, O, Ag and S, respectively. Binding energy peaks of Zn2p occur at 1022.2 and 1045.2 eV (Fig. 8b), which confirm the presence of Zn in the catalyst. In Fig. 8c, the O1s profile is asymmetric and can be fitted to two symmetrical peaks (α and β located at 530.9 and 532.9 eV, respectively), indicating two different kinds of O species in the sample. The peaks α and β should be associated with the lattice oxygen (OL) -1.4

ZnO

80

-1.2

-0.8

40

Current (1e–5A)

60

% of Reflectance

β-Ag2S-ZnO

-1.0

(a)

(b)

-0.6

b

-0.4

a

-0.2 0

20 0.2 0.4

3.26 2.90 0

0.6 200

300

400

500

600

700

Wavelength (nm) Fig. 5. DRS of a) bare ZnO and b) β-Ag2S–ZnO.

800

900

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

Potential (V) Fig. 7. Cyclic voltammogram of a) bare ZnO and b) β-Ag2S–ZnO.

-0.8

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B. Subash et al. / Powder Technology 241 (2013) 49–59

of ZnO and chemisorbed oxygen (OH) caused by the surface hydroxyl [32], respectively. In Fig. 8d, the peaks at 368.5 and 374.5 eV are assigned to Ag 3d5/2 and Ag 3d3/2, respectively. It was reported earlier during the preparation of AgBr/H2WO4 composite, the Ag3d5/2 peak was further divided into two different peaks at 367.48 and 368.11 eV and the Ag3d3/2 was also divided into two different peaks at 373.47 and 373.96 eV [33]. According to Zhang et al. [34], the peaks at 368.11 and 373.96 eV can be attributed to metal Ag 0, whereas the peaks at 367.48 and 373.47 eV can be attributed to the Ag + of AgBr. In our XPS spectra of β-Ag2S–ZnO composite, there is no split of Ag 3d5/2 and Ag 3d3/2 peaks. (Fig. 8d). This reveals that Ag in β-Ag2S–ZnO is present only as Ag+ ion [12]. The binding energy peaks of the S2p are observed in the range of 162.5–172.4 eV (Fig. 8e).

In general the surface area of the catalyst is the most important factor influencing the catalytic activity. The surface area of catalysts (bare ZnO and β-Ag2S–ZnO) was determined using the nitrogen gas adsorption method. N2 adsorption–desorption isotherms of bare ZnO and β-Ag2S–ZnO are shown in Fig. 9a and b, respectively. The isotherms of bare ZnO and β-Ag2S–ZnO reveal type II hysteresis loop. The pore size distribution of the bare ZnO and β-Ag2S–ZnO is given in inset of Fig. 9a and b, respectively. The BET surface area and pore volume of bare ZnO and β-Ag2S–ZnO are given in Table 1. BET surface area of β-Ag2S–ZnO (18.9 m 2 g −1) is higher than the bare ZnO (11.5 m 2 g −1). The higher BET surface area makes the catalyst more photoactive. Even though surface area of the β-Ag2S–ZnO is less than that of TiO2-P25, the photocatalytic activity is higher than that

Fig. 8. XPS of β-Ag2S–ZnO a) survey spectrum, b) Zn2p peak, c) O1s peak, d) Ag3d peak and e) S2p peak.

B. Subash et al. / Powder Technology 241 (2013) 49–59

Desorption Adsorption 0.12

30 25 20

100

0.10

3

35

a Concentration of RR 120 (%)

40

Pore volume (cm /g)

Quantity adsorbed (cm2/g STP)

45

0.08 0.06 0.04 0.02

15

0.00 10

100

0

200

400

300

80

(e)

60

20

0

10

20

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative pressure (P/Po) 90

Desorption Adsorption

3

50 40

b

40

Ag2S-ZnO/dark (b)

Bare ZnO (d)

TiO2-P25 (e)

Commercial ZnO (c)

TiO2 (Merck) (f)

0.20

71.5, 51.3 and 57.2% degradations occurred, respectively. This shows that solar/β-Ag2S–ZnO process is more efficient in RR 120 degradation than solar/bare ZnO, solar/commercial ZnO, solar/TiO2P25 and solar/TiO2 (Merck) processes. The influence of operational parameters had been carried out with β-Ag2S–ZnO to find out the optimum conditions. The photocatalytic degradation of RR 120 dye containing β-Ag2S– ZnO obeys pseudo-first order kinetics. At low initial dye concentration the rate expression is given by

0.15 0.10 0.05

30

0.00 20

100

0

200

400

300

Pore radius (A°)

d½C=dt ¼ k′½C

10 0 0.0

Ag2S-ZnO (a)

Fig. 10. 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.

0.25 Pore volume (cm /g)

Quantity adsorbed (cm2/g STP)

80

60

30

Time (min)

0

70

(b) (f) (c) (d) (a)

40

0

Pore radius (A°)

5

55

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

ð4Þ

where k′ is the pseudo-first order rate constant. The dye is adsorbed onto the β-Ag2S–ZnO surface and the adsorption–desorption equilibrium is

Relative pressure (P/Po) 1.0 Fig. 9. N2 adsorption–desorption isotherms of a) bare ZnO b) β-Ag2S–ZnO.

of TiO2-P25. β-Ag2S–ZnO shows greater photocatalytic activity per unit surface area when compared with TiO2-P25.

Fig. 10 shows the percentage of RR 120 on irradiation of an aqueous solution of RR 120 (2 × 10−4 M) using solar light. It has been observed that almost complete degradation of the pollutant takes place at the time of 30 min with β-Ag2S–ZnO under solar light. This is in contrast to a 49.5% decrease in dye concentration which occurred for the same experiment performed with β-Ag2S–ZnO in the absence of solar light, which is due to adsorption of dye on the surface of the catalyst. Negligible degradation (0.2%) was observed when the reaction was allowed to occur in the presence of solar light without any catalyst. These observations reveal that solar light and photocatalyst are needed for effective destruction of RR 120. When the photocatalysts bare ZnO, commercial ZnO, TiO2-P25 and TiO2 (Merck) were used under same conditions only 76.9,

Absorbance

3.2. Photodegradability of RR 120

0.5

a d

0 Table 1 Surface properties of the catalysts.

200

400

600

800

Wavelength (nm)

Properties

Bare ZnO

β-Ag2S–ZnO

BET surface area Total pore volume (single point)

11.5 (m2 g−1) 0.07 (cm3 g−1)

18.9 (m2 g−1) 0.14 (cm3 g−1)

Fig. 11. The changes in UV–vis spectra of RR 120 on irradiation with solar light in the presence β-Ag2S–ZnO: [RR 120] = 2 × 10−4 M; pH = 7; β-Ag2S–ZnO suspended = 3 g L−1; airflow rate = 8.1 mL s−1; (a) 0 min, (b) 10 min, (c) 20 min and (d) 30 min.

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B. Subash et al. / Powder Technology 241 (2013) 49–59

0.11

70

0.10 60

0.09

% of RR 120 remaining

k min -1

0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 1

3

5

7

9

50 40 30 20 10

11

Initial pH

0

Fig. 12. Effect of solution pH; [RR 120] = 2 × 10−4 M, 18.2 wt.% β-Ag2S–ZnO suspended = 3 g L−1, airflow rate = 8.1 mL s−1, irradiation time = 20 min.

0

30

lnðC0 =CÞ ¼ k′t

ð5Þ

where C0 is the equilibrium concentration of dye and C is the concentration at time t. The UV–vis spectra of RR 120 (2 × 10 −4 M) solution at different irradiation times are shown in Fig. 11. There is no significant change in UV maxima during irradiation but the intensities at 285 and 512 nm decrease gradually during the degradation. This reveals that the intermediates do not absorb at the analytical wavelengths of 285 and 512 nm. 3.3. Effect of operational parameters

90

120

150

Time (min) I Run

reached in 30 min. After adsorption, the equilibrium concentration of the dye solution is determined and it is taken as the initial dye concentration for kinetic analysis. Integration of Eq. (4) (with the limit of C = C0 at t = 0 with C0 being the equilibrium concentration of the bulk solution) gives Eq. (5),

60

II Run

III Run

IV Run

Fig. 14. Effect of reusability, [RR 120] = 2 × 10−4 M, 18.2 wt.% β-Ag2S–ZnO suspended = 3 g L−1, airflow rate = 8.1 mL s−1, pH = 7.

controlled during the course of the reaction. The effect of initial pH on RR 120 degradation is shown in Fig. 12. After 20 min of irradiation the pseudo-first order rate constants for β-Ag2S–ZnO at pH 3, 5, 7, 9 and 11 are 0.0261, 0.0829, 0.101, 0.0895 and 0.0722 min −1, respectively. It is observed that increase in pH from 3 to 7 increases the removal efficiency of RR 120 and then decreases. The optimum pH for efficient RR 120 removal on β-Ag2S–ZnO is 7. At acidic pH range the removal efficiency is less and it is due to the dissolution of ZnO present in β-Ag2S–ZnO. ZnO can react with acids to produce the corresponding salt at low acidic pH values. At high pH value β-Ag2S–ZnO surface is negatively charged by means of adsorbed OH − ions. The presence of large quantities of OH − ions on the particle surface as well as in the reaction medium favors the formation of •OH. Since the photocatalytic efficiency depends on adsorption of dye molecules,

3.3.1. Effect of solution pH The solution pH plays an important role in the photocatalytic degradation process of various pollutants [35,36]. The effect of pH on the photodegradation of RR 120 was studied in the pH range 3–11. The pH of the solution was adjusted before irradiation and it was not

k min-1

0.105

0.095

0.085

0.075 0

1

2

3

4

5

6

7

Catalyst loading (g L–1) Fig. 13. Effect of catalyst loading, [RR 120] = 2 × 10−4 M, pH = 7, catalyst suspended = β-Ag2S–ZnO, airflow rate = 8.1 mL s−1, irradiation time = 20 min.

Fig. 15. XRD pattern of β-Ag2S–ZnO (a) before irradiation and (b) after irradiation.

B. Subash et al. / Powder Technology 241 (2013) 49–59

cycles for the degradation of RR 120 under solar light and the results are shown in Fig. 14. The complete degradation occurred in 1st, 2nd and 3rd runs at 30 min whereas 4th run gave 96.0% degradation. The results show our prepared catalyst is found to be stable and reusable. The stability of β-Ag2S in the catalyst was tested by XRD. Fig. 15a and b show the XRD of β-Ag2S–ZnO before and after its use in photocatalytic process respectively. The XRD patterns of β-Ag2S–ZnO before and after irradiation is same but the intensity of peak at 2θ = 38.09° is slightly increased (Fig. 15b). The diffraction of (111) plane of metallic silver (Ag0) also appears at 2θ = 38.09° [39–41]. This indicates the photoreduction of Ag2S in the catalyst during the degradation on irradiation. This type of photoreduction was reported earlier [42,43]. The solution obtained after 1 h irradiation with β-Ag2S–ZnO was tested for the ions present (Ag or S) in that solution. After irradiation the solution was neutralized using nitric acid and addition of Pb(NO3)2 or sodium bromide to the solution gave no precipitate indicating that there is no leaching of Ag or S ion during the degradation.

Table 2 COD measurements. Time (min)

% COD reduction

10 20 30

29.8 63.6 98.9

[RR 120] = 2 × 10−4 M; 18.2 wt.% β-Ag2S–ZnO suspended = 3 g L−1; pH = 7; airflow rate = 8.1 mL s−1.

an experiment to verify the dark adsorption of RR 120 under different pH was carried out. The percentages of adsorption at pH 3, 5, 7, 9 and 11 were found to be 34.8, 41.8, 49.5, 44.3 and 40.7 after the attainment of adsorption equilibrium (20 min). As the adsorption is high at pH 7 the degradation is more efficient at this pH. 3.3.2. Effect of catalyst loading Experiments were carried out to assess the optimum catalyst loading by varying the amount of catalyst from 2–6 g L −1 (Fig. 13). After 20 min of irradiation the pseudo-first order rate constants are 0.0820, 0.1010, 0.1003, 0.1002 and 0.0976 min −1 at catalyst loading of 2, 3, 4, 5 and 6 g L −1, respectively. It is interesting to note that the rate constant of RR 120 degradation increases with increase in catalyst amount from 2 to 3 g L −1 and then decreases. Enhancement of removal rate is due to (i) the increase in the amount of catalyst which increases the number of dye molecules adsorbed, (ii) the increase in the density of catalyst particles in the area of illumination. The decrease in the removal efficiency of RR 120 at higher amount (above 3 g L −1) is due to the light reflectance by catalyst particles. Similar results have been reported for the photodegradation of dyes by TiO2 and ZnO [37,38].

3.3.4. Chemical oxygen demand (COD) analysis To confirm the mineralization of RR 120, the degradation was also analyzed by COD values. The % of COD reduction is given in Table 2. After one hour irradiation with β-Ag2S–ZnO, 98.9. % of COD reduction is obtained. This indicates the almost complete mineralization of the dye. 3.3.5. Mechanism of degradation Since β-Ag2S–ZnO is found to be stable and reusable and more active when compared with bare ZnO and commercial catalysts a mechanism based on the energy levels of β-Ag2S and ZnO is proposed for the degradation of the dye. It was earlier reported that, even though the CB level of electron donor (β-Ag2S) is lower than that of electron acceptor (ZnO), electron transfer may also happen from β-Ag2S to ZnO [30]. Band energy levels for β-Ag2S and ZnO with respect to NHE are shown in Scheme 2. Electrons generated by solar light are transferred from CB of β-Ag2S to CB of ZnO while holes are

3.3.3. Catalyst reusability Main advantage of heterogeneous catalyst used for any reaction whether it is organic transformation or photodegradation, is its reusability. The reusability of β-Ag2S–ZnO was tested by four successive

Energy (eV) Vs NHE

O2• -

57

O2

O2• -

e–

-3.0

Dye•

e– CB 0.0

hv

– CB e

hv

hv

β-Ag2S

1.0

ZnO

Dye

+ VB h

H2O

2.0

Dye

+

H2O

3.0

h+ VB

•

OH

•

OH Dye / Dye* + HO• Dye / Dye* +

O2•

Mineral acids + CO2 Mineral acids + CO2

Scheme 2. Mechanism of degradation of RR 120 by β-Ag2S–ZnO.

+ H2O + H2O

58

B. Subash et al. / Powder Technology 241 (2013) 49–59

transferred from VB of ZnO to VB of Ag2S. This electron transfer process is faster than electron–hole recombination in β-Ag2S. The electrons in CB of ZnO produce O2• – which degrades the dyes. The holes in the β-Ag2S and the holes produced in ZnO by photoexcitation react with H2O and − OH to produce •OH radicals for the degradation of the dye. 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 same condition used for solar light. It was found that RR 120 underwent 75.1% degradation with UV light (365 nm). But under the same conditions 93.3% degradation occurred with solar light for 20 min. Higher efficiency of β-Ag2S–ZnO in solar light indicates the presence of dye sensitized mechanism along with the usual ZnO sensitization. This occurs when more dye molecules are adsorbed on the semiconductor surface. This enhances the photoexcited electron transfer from solar light sensitized dye molecule to the conduction band of ZnO and subsequently increases the electron transfer to the adsorbed oxygen. Higher adsorption of dye molecules by β-Ag2S–ZnO also confirms this (49.5% adsorption of pH = 7). In addition to the degradation of the dye by the usual ZnO sensitization mechanism, the dye molecules are also degraded by the super oxide radicals produced by dye sensitization mechanism (Eqs. (6)–(8)). Further to prove this mechanism we had also carried out an experiment for the degradation of 4-nitrophenol by β-Ag2S–ZnO with UV and solar light. We found that the degradation was more efficient in UV light than in solar light (UV: 79.5%, solar: 29.0% in 60 min), indicating that the presence of dye sensitized mechanism is operative for the degradation of RR 120 dye by β-Ag2S–ZnO. Similar results have been reported for photodegradation of dye [44]. þ•

Dye⁎ þ β  Ag2 S  ZnO→Dye ecb

−

þ O2 →O2

þ•

Dye

þ O2 =O2

þ β  Ag2 S  ZnO þ ecb

•−

•−



ð6Þ ð7Þ

→degradation products:

ð8Þ

4. Conclusions A novel β-Ag2S loaded ZnO was synthesized by precipitation– decomposition method. The loading of β-Ag2S in ZnO has been revealed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) images, transmission electron microscope (TEM) images, energy dispersive spectra (EDS), diffuse reflectance spectra (DRS), photoluminescence (PL), cyclic vloltammetry (CV) measurements, X-ray photoelectron spectroscopy (XPS) and BET surface area measurements. Loading of β-Ag2S increases the visible light absorption of ZnO, which makes this catalyst, more photoactive in day light illumination. The PL spectra reveal the suppression of recombination of the photogenerated electron–hole pairs by the β-Ag2S loaded on ZnO. XPS reveals that all the Ag in the catalyst are present in the form of Ag + before irradiation. As a result, the synthesized β-Ag2S loaded ZnO exhibited excellent photocatalytic activity than commercial, bare ZnO, TiO2-P25 and TiO2 (Merck) for degradation of RR 120 under solar light. Because of the efficient solar light absorption capability, the β-Ag2S loaded ZnO might be used for other photo-applications such as photo electrochemical cells, solar cells and photocatalysts for hydrogen production from water splitting, in addition to the photodegradation of pollutants. The optimum pH and catalyst loading for efficient removal of dye are found to be 7 and 3 g L −1, respectively. The catalyst is found to be reusable. COD analysis confirms the almost complete mineralization of RR 120 molecule. Mechanism of dye degradation by β-Ag2S–ZnO is proposed on the basis of band energy levels of Ag2S and ZnO. The presence of dye sensitized mechanism for the degradation has also been proved.

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