TiO2, ZnO and nanobimetallic silica catalyzed photodegradation of methyl green

Materials Science in Semiconductor Processing 16 (2013) 185–192

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TiO2, ZnO and nanobimetallic silica catalyzed photodegradation of methyl green S. Senthilvelan n, V.L. Chandraboss, B. Karthikeyan, L. Natanapatham, M. Murugavelu Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India

a r t i c l e i n f o

abstract

Available online 25 May 2012

The present works describes a method for degradation and decolorization of methyl green (MG) on semiconducting oxides and nanobimetal (Ag/Pt) doped silica sol–gel powder by using UV and solar light. Bimetallic nanoparticles are the recent research interest because of the high catalytic activity and tunable surface plasmon resonances. In this study the nanobimetal (Ag/Pt) doped silica sol–gel powder were prepared and characterized. Silica sol–gel powder was characterized by FT-IR spectroscopy, SEM with EDX, TEM, DRS and XRD analysis. The results of photodegradation of methyl green on TiO2, ZnO and nanoAg/Pt bimetal doped silica sol–gel powder were presented and compared. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Silica sol–gel powder Semiconducting oxide Doping Photodegradation Methyl green

1. Introduction Every day a large amount of unconsumed dye produced by textile and printing industries are discharged in to the environment. The presence of dyes and pigments in water causes considerable damage to the aquatic environment [1–3]. These contaminants result in a high chemical oxygen demand (COD), high biochemical oxygen demand (BOD), toxicity, bad smell, and mainly, are responsible for the coloration of wastewaters [4]. Even at very low concentrations, the color of this kind of contaminants can be recognized, because the presence of dyes in water is highly visible. This effect is undesirable because the color blocks the sunlight access to aquatic flora and fauna, and it reduces the photosynthetic action within the ecosystem [5,6]. Among many dyes that are applied in manufacture products, malachite green must be highlighted. This dye has been used as a food-coloring additive, as a dye for silk, leather, wool, cotton and paper [7]. Moreover, this compound has also been used as a medical disinfectant, as well as, in aquaculture as a fungicide and antiseptic [8].

n

Corresponding author. Tel.: þ91 9486389270. E-mail address: [email protected] (S. Senthilvelan).

1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.04.018

Many processes have extensively applied the treatment of dye-containing wastewater such as: incineration, biological treatment, ozonation and adsorption on solid phases. However, these procedures have some limitations. The incineration can produce toxic volatiles; biological treatment demands long periods of treatment and bad smell; ozonation presents a short half-life, ozone stability is affected by the presence of salts, pH and temperature and the adsorption results in phase transference of contaminant, not degrading the contaminant and producing sludge [9–12]. In this way, the heterogeneous photocatalysis becomes an elegant alternative for dye degradation. This technique presents many advantages over conventional technologies such as the dye degradation into innocuous final products [13]. Advanced oxidation processes (AOPs) have been previously described as a promising option to remove persistent pollutants from contaminated water when conventional water treatment processes are not efficient enough. AOPs are based on physicochemical processes that are able to produce deep changes in the chemical structure of the pollutants and are defined as processes involving the in situ generation and use of highly oxidizing agents, mainly hydroxyl radicals. The hydroxyl radical possesses inherent properties that enable it to attack organic pollutants in water to obtain a complete

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mineralization into CO2, water and mineral acids such as sulfuric, hydrochloric and nitric acids [14]. The dye pollutants have become a major source of environmental pollution because the conventional technologies are not capable of reducing them at the lowest levels demanded by the environmental laws. In the last decades, the development of new technologies such as the advanced oxidation process (AOP) for treating both gas and water pollutants, has been promoted. Among the AOP, the heterogeneous photocatalyst has been reported as a powerful tool to solve environmental and energy problems [15]. The basic principles concerning this catalyst are well established [16] what fundamentally occurs, is that the photoradiation in a semiconductor material promotes the electron excitation from the valence band to the conduction band, leaving, thus, an electron deficiency or hole in the valence band, which generates electron/hole pairs. Both the reduction and oxidation processes can occur at/or near the surface of the photoexcited semiconductor particle. The sol–gel process is a wet-chemical technique used for the fabrication of both glassy and ceramic materials. In this process, the sol (or solution) evolves gradually towards the formation of a gel-like network containing both a liquid phase and a solid phase. The sol–gel technique is based on hydrolysis of liquid precursors and formation of colloidal sols. The precursors are usually organo silicates yielding silicate sol–gel materials [17]. The sol–gel technology finds applications in the development of new materials for catalysis [18], chemical sensors [19,20], membranes [21,22], optical gain media [23], photochronic applications [24] and solid state electrochemical devices and in a diverse range of scientific and engineering fields, such as ceramic industry, nuclear industry and electronic industry [25]. Sol–gel is one of the most exploited methods; it is used mainly to produce thin film and powder catalysts. Many studies revealed that different variants and modifications of the process have been used to produce pure thin films or powders in large homogeneous concentration and under stoichiometry-control [26–28]. In this work such nanoAg/Pt bimetallic doped silica sol–gel powder catalyzed photodegradation studies were

undertaken along with the conventional metal oxide catalysis towards the above said dye methyl green. 2. Materials and methods 2.1. Chemicals Titanium oxide, zinc oxide, methyl green, chloroplanic acid, silver nitrate and trisodium citrate were the guaranteed reagents of Sigma Aldrich. Tetramethyl orthosilicate (TMOS), methanol and nitric acid are of analytical grade and used as received. The aqueous solutions were prepared by using double distilled water. The chemical structure of methyl green dye is shown in Fig. 1. 2.2. Preparation of Ag/Pt bimetallic colloid In a typical synthesis of Ag/Pt bimetallic nanoparticles, the Silver nitrate of 0.024 g was added to 100 mL of sterile double distilled water and the solution was heated for 1 h at 90 1C. Thereafter 20 mL of 1% trisodium citrate was added to the solution. After 30 min of boiling, 280 mL of 0.02 M H2PtCl6 was added dropwise directly to the round bottomed flask and the heating was continued with one and half hour more with the condenser. The color changes from colorless to yellowish green color and indicate the formation of Ag/Pt bimetallic nanoparticles [29]. 2.3. Preparation of nanobimetal (Ag/Pt) doped silica sol–gel powder The SiO2–Ag/Pt sol was prepared using tetramethyl orthosilicate (TMOS), methanol, double distilled water, Ag/Pt colloid and nitric acid as precursors. First, solution A was prepared by mixing 2 mL of TMOS and 2 mL of methanol in equal volume. Then, solution B was prepared by adding 4 mL of Ag/Pt colloid, 1 mL of water and 2 drops of 5% nitric acid (HNO3) together. Nitric acid was added to solution B simply to adjust the pH to approximately 2. Finally, solution B was added to solution A drop by drop stirring vigorously at room temperature. Following the

Fig. 1. Chemical structure of methyl green dye.

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mixing of two solutions (A and B), the sol was left to aging, drying and shrinking. It usually took a day for the sol to get ready, depending on the metal concentration. The influence of metal salts on the stability of the sol and the gelation process can be found in the literature. The sol–gel substrate was dried at 100 1C for 1 h in an isotemperature oven. The obtained sol–gel matrix is the bimetal doped one. Finally the sol–gel matrix is grind well to form silica sol–gel powder [30]. This silica sol–gel powder is used for further studies. The scheme for the nanobimetal doped silica sol–gel powder preparation is given in Scheme 1. 2.4. Photodegradation in reactors The solution of methyl green of desired concentration was prepared afresh and use. The photodegradation reactions were carried out in double distilled water and the volume of the reaction solution was maintained (25 mL in multi lamp photoreactor). Air was bubbled through the reaction solution using a micro-air pump that effectively stirs the solution and keep the suspended catalyst under constant motion. The concentration of methyl green at 630 nm was measured spectrophotometrically after centrifuging, the catalyst and diluting the solution five times to keep the absorptions within the Beer–Lambert law limit. 3. Results and discussion 3.1. Characterization of nanobimetal doped silica sol–gel powder 3.1.1. Fourier transformed infrared spectroscopic studies The IR spectrum of the bimetal doped silica sol–gel powder is shown in the (Fig. 2a). The bands are observed at 3434, 1635, 1383, 1086–1220, 958 and 798 cm  1. The bands at 1635 and 3434 cm  1 are assigned to OH bending and stretching vibrations respectively. The two vibrations in the range 1086–1220 and 958 cm  1 are assignable to Si–O– Si and Si–O vibration modes of isolated Si–OH groups, respectively. The peak at 798 cm  1 may be assigned to the O–Si–O vibration mode of SiO2. The IR spectrum demonstrates the presence of all the vibrational bands of Si–OH, O–Si–O and Si–O–Si typical of amorphous SiO2. 3.1.2. Surface morphology of nanobimetal doped silica sol–gel powder The morphology of the bimetal doped silica sol–gel powder was determined by scanning electron microscopy (Fig. 2b). The SEM micrograph shows that the particles are almost clustered and are in the size range of 500 nm. This observation confirms the advantages of the sol–gel process for obtaining amorphous nanoscale materials at lower temperature. It is also observed that the nanosized particles are clearly seen in the only bimetal doped sol– gel material. The EDX analysis confirms the Ag/Pt and Si in the sol–gel powder (Fig. 2c).

Scheme 1. Preparation route of silica sol–gel powder .

3.1.3. X-ray diffraction analysis The obtained XRD of the nanobimetal doped silica sol– gel powder is shown in Fig. 2d. The peaks at 38.11, 39.71and 64.51, are the diffractions of the Ag (111),

Pt (111), and Ag (220) crystal planes, respectively. The Ag/Pt-SiO2 sample exhibited a diffraction pattern of FCC crystal structure of Ag and Pt.

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Fig. 2. (a) IR spectrum of the bimetal doped silica sol–gel powder, (b) SEM image of bimetal doped silica sol–gel powder, (c) EDX spectrum of bimetal doped silica sol–gel powder and (d) XRD spectrum of bimetal doped silica sol–gel powder.

3.1.4. Transmission electron microscopy and High resolution-transmission electron microscopy analysis The nanobimetal doped silica sol–gel powder has been analyzed by TEM and HR-TEM studies. The obtained TEM of the Ag/Pt doped sol–gel material is given in Fig. 3a. TEM observation indicated the nanoparticles are nearly spherical shaped. The core and shell components can be easily differentiated by brightness difference using TEM. In the present study we have obtained bimetallic nanoparticles, the darker nucleus corresponds to the initial Ag and the lighter shell corresponds to the Pt nanoparticle. The HR-TEM image of nanobimetal doped silica sol–gel material is given in Fig. 3b. It further confirmed the above observations and the bimetallic nanoparticles are in the size range of 20–60 nm.

photoreactor with mercury UV lamps of wavelength 365 nm. The measured wavelength of methyl green reaction solution is 624 nm.

3.1.5. Ultra Violet–visible diffuse reflectance spectral analysis The UV–visible DRS spectra of TiO2, ZnO and nanobimetal doped silica sol–gel powder are shown in Fig. 4. The DRS observation indicated that Ag/Pt doped silica material is showing enhanced and shifted band when compared to the TiO2 and ZnO. The enhanced photocatalytic activity is facilitated by the presence of nanobimetal in the silicate material.

3.2.3. Effect of initial dye concentrations of methyl green The progress of the photodegradation and decolorization of methyl green represent the increase of illumination with all catalysts. The degradation and decolorization rate measurements at different methyl green dye concentrations showed the degradation and decolorization rates increases with and the variation is according to Langmuir–Hinshelwood kinetic model (Table 1, Fig. 5a).

3.2. Photodegradation and decolorization of methyl green

3.2.4. Effect of catalyst loadings on methyl green The effect of catalyst loading on the photocatalytic degradation and decolorization of the methyl green has been carried out in the range 0.01–0.05 g of the catalyst for 25 mL of solution. Variation of the amount of semiconducting oxides and sol–gel powder suspended in the reaction medium leads to increase of the degradation and decolorization rate of

3.2.1. Photodegradation of methyl green with artificial UV light The photodegradation of methyl green in aqueous medium in the presence of atmosphere air on TiO2, ZnO and sol–gel powder were studied using multi lamp

3.2.2. Factors influencing photodegradation and decolorization of methyl green The semiconducting oxides and sol–gel powder photodegradation of methyl green in aqueous medium was followed by UV–visible spectrophotometrically. Initially dye solution is green in color, after the photodegradation and decolorization, the color of the solution changes to colorless. The reaction time affords the photodegratation and decolorization of methyl green. The calculated rate and the related results are reproducible.

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Table 1 Photodegradation rates of methyl green. Methyl green (mM)

0.060 0.090 0.100

Rate (mol s  1) TiO2

ZnO

Sol–gel powder

0.0150 0.0222 0.0255

0.0138 0.0208 0.0247

0.0147 0.0211 0.025

Catalyst loading ¼ 0.01 g/25 mL, air flow rate ¼7.8 mL s–1, l ¼ 365 nm, photon flux ¼2.25  10  5 einstein L  1 s  1, total volume of reaction solution ¼25 mL.

methyl green. The rate is linearly related to the amount of catalyst. Catalyst loading increases, only sol–gel powder enhances the rate compared to the other catalyst (Fig. 5b). 3.2.5. Effect of atmosphere air flow rates on methyl green The investigation of the semiconducting oxide and sol–gel powder photocatalyzed photodegradation and decolorization of methyl green as a function of atmosphere O2 flow rate reveals enhancement of photocatalysis by oxygen. The variation of atmosphere O2 flow rate increases the photodegratation and decolorization rate is of Langmuir–Hinshelwood model (Fig. 5c). The reaction was also carried out without bubbling O2 but the solutions were not de-aerated. The dissolved oxygen itself brings out the reaction but the photocatalysis is weak.

Fig. 3. (a) TEM image of bimetal doped silica sol–gel powder and (b) HRTEM image of bimetal doped silica sol–gel powder.

3.2.6. Effect of different photon fluxes on methyl green The most important parameter that influences the photocatalytic degradation and decolorization is the photon flux. The light intensity highly influences the reaction. The reactions were studied with eight, four and two 8 W UV mercury lamps, the angles sustained by the adjacent bulbs at the sample are 451, 901 and 1801, respectively, which represents the variation of rate as a function of photon flux given in terms of einstein (Fig. 5d). The reactions do not occur in dark. 3.3. Kinetic analysis The photodegratation and decolorization of methyl green for the photocatalysis on reactive as well as nonreactive surfaces is the adsorption of dye and oxygen molecule on the catalyst surface. The rate of degradation and decolorization of methyl green depends on the fraction of the surface on which dye molecule is adsorbed, the surface area of the catalyst and the intensity of light. Rate ¼

Fig. 4. The UV–visible DRS spectra of (a) TiO2, (b) ZnO and (c) nanobimetal doped silica sol–gel powder.

kK 1 K 2 SIC½MGg 1 þ K 1 ½MG þ K 2 g þ K 1 K 2 ½MGg

ð1Þ

where K1 and K2 are the adsorption coefficients of methyl green and O2 molecules on the catalyst surface, k is the specific rate of degradation of methyl green. g is the air flow rate, S is the specific surface area of the catalyst, C is the amount of catalyst suspended per liter and I is the intensity of illumination. The fitment of the experimental data to the Langmuir–Hinshelwood model, drawn using a software program conforms the rate equation (Eq.(1)).

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Fig. 5. (a) Photodegradation rate of methyl green: catalyst loading¼ 0.01 g/25 mL, air flow rate¼ 7.8 mL s  1, l ¼365 nm, photon flux¼ 2.52  10  5 einstein L  1 s  1, total volume of reaction solution¼ 25 mL; (b) methyl green—photodegradation at different catalyst loading: [methyl green] ¼ 0.100 mM, air flow rate¼ 7.8 mL s  1, l ¼365 nm, photon flux¼ 2.52  10  5 einstein L  1 s  1, total volume of reaction solution ¼25 mL; (c) methyl green—photodegradation at different air flow rate: [methyl green] ¼ 0.100 mM, catalyst loading ¼0.01 g/25 mL, l ¼ 365 nm, photon flux ¼2.52  10  5 einstein L  1 s  1, total volume of reaction solution ¼ 25 mL and (d) methyl green - photodegradation at different photon fluxes: [methyl green] ¼0.100 mM, air flow rate¼ 7.8 mL s  1, l ¼365 nm, total volume of reaction solution ¼ 25 mL.

3.4. Mechanisms 3.4.1. The mechanism of dye degradation and decolorization on semiconducting oxides The photocatalytic degradation of organic dyes in wastewater utilizing titanium dioxide (TiO2, an n-type semiconductor) Then, electrons are excited from the valence band to the conduction band, generating positive holes and free electrons. The produced electron–hole pairs can recombine or interact with other organic substrates on the surface of TiO2 particles via the oxidation and reduction reactions. In aqueous solution, the positive holes are scavenged by surface hydroxyl groups to produce the very reactive oxidizing hydroxyl radicals (dOH), which can promote the degradation process and subsequently lead to the total mineralization of the organic substrate [31–34]. Recently, there are a few groups of researcher examining the degradation mechanism of several dyes under visible light irradiation. They had suggested a new method for the treatment or pre-treatment of dye-containing wastewater. The process is inspired by the principle of photosensitization of wide band gap semiconductors [35]. When a colored organic compound is present, the adsorbed dye molecule is excited by visible light, thus acts as a photosensitizer capable of injecting an electron into the conduction band of

semiconductor particles to form an oxidized radical. The oxidized form of the dye molecules will then undergo further degradation. Detailed mechanism of dye degradation under visible light irradiation is described by Eqs. (2–7) [36]. The photodegradation mechanism of ZnO has been proven to be similar to that of TiO2 and other photocatalyts. Dye þhn-Dye**

(2)

Dye* þTiO2-Dye þ þTiO2(e)

(3)

–

TiO2(e)þ O2-TiO2 þO2

–

(4)

O2 þTiO2(e)þ2H þ -H2O2

(5)

H2O2 þTiO2 (e)-OHþOH–

(6)

–

Dye þ þO2 (or O2 or OH)-Peroxylated or hydroxylated intermediates--Degraded or mineralized products (7)

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3.4.2. The mechanism of photocatalytic effect of Ag/Pt nanoparticles (NPs) doped with SiO2 matrix The photoactive sites formed on the surface of SiO2 nanoparticles prepared by the sol–gel method [37]. SiO2 nanoparticles absorb UV light and an electron from its valence band (vb) get excited to the conduction band (cb) generating a positively charged hole in the valence band þ  (hvb) and negative charge in the conduction band (ecb) according to Eq. (8). The chemisorbed H2O molecules interact with the valence band holes forming OH radicals (Eq. (9)), which attack dye molecules successively to make degradation. Furthermore, the conduction band electrons ecb interact with dissolved O2; producing superoxide radical anion þ  (O2 ) as shown in Eq. (10). On the other hand, hvb could  interact with donor –OH and HO2 forming OH radical which attack the dye as in Eq. (11). The main factor affecting the efficiency of SiO2 nanoparticles is the amount of OH radicals as described above. Consequently any factor enhancing the generation of OH radicals will increase the rate of the photocatalytic degradation of dye. The mechanism of photocatalytic degradation of dye by Ag/Pt nanoparticles doped with SiO2 matrix layer could be controlled by the nanoparticles, which affect the electron–hole recombination process [38–40] as shown in Fig. 6a. –

þ

SiO2 þhv-ecb þhvb

(8)

191

þ

(H2O-H þ þOH–)þhvb-H þ þ dOH –

(9)

O2 þecb-(O2 )þ(H þ þOH–)-HO2 þ –OH d–

d

(10)

þ

HO2 þ –OH þhvb-dOH d

(11)

–

Ag/Pt þecb-Ag/Pt(ecb–)

(12)

Ag/Pt þH þ -Ag/Pt þH2

(13)

Ag/Pt(ecb–)þO2-Ag/Ptþ O2 þ(H þ þ –OH)-–OHþHO2 (14) d–

d

The major role of nano-Ag/Pt doped silica matrix is attributed to the consumption of the electrons and reducþ  tion of the recombination of charges (hvb and ecb) and  favors the formation of OH radical [41,42] as illustrated (Eqs. (12–14)). According to thermodynamical point of view, the electrons transfer from the SiO2 nanoparticles conduction band to Ag/Pt nanoparticles at the interface is allowed since the Fermi level of SiO2 nanoparticles is higher than that of Ag/Pt nanoparticles. Therefore the Schottky barrier at Ag/Pt nanoparticles contact region is formed, which improves the charge separation and thus enhances the photocatalytic activity of SiO2 nanoparticles. Photoconduction energy level diagram in small metal clusters is shown in Fig. 6b. Similar work function can be assumed in this catalyst. 4. Conclusions The photodegradation of methyl green on TiO2, ZnO and silica sol–gel powder in an aqueous medium has been studied as a function of initial dye concentration, different air flow rates, amount of catalyst suspended and different photon fluxes. Degradation rate of methyl green dye increases with increase of concentration, catalyst loading, air flow rate and different photon fluxes. The photodegradation of methyl green in dark and absence of catalyst loading does not influence the reaction. Catalyst loading increases only sol–gel powder enhances the rate compared to the other catalyst. The mechanism of photocatalytic effect of semiconducting oxides and nanobimetal doped SiO2 matrix has been discussed.

Acknowledgments The author (S.S.V) is highly thankful to UGC, New Delhi for granting a major research project. References

Fig. 6. (a) Ag/Pt nanoparticles (NPs) doped silica matrix and (b) photoconduction energy level diagram of small metal clusters.

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