Development of new, highly efficient and stable

Catalysis Communications 62 (2015) 39–43

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Short communication

Development of new, highly efficient and stable visible light active photocatalyst Ag2ZrO3 for methylene blue degradation Sanjay R. Thakare a,⁎, G.S. Gaikwad b, N.T. Khati b, A.V. Wankhade c a b c

Department of Chemistry, Science College, Congress Nagar, Nagpur 440012, India Department of Applied Chemistry, Priyadarshini College of Engineering, Nagpur, India Department of Chemistry, Visvesvaraya National Institute of Technology, Nagpur, India

a r t i c l e

i n f o

Article history: Received 2 October 2014 Received in revised form 29 November 2014 Accepted 30 December 2014 Available online 2 January 2015 Keywords: Silver zirconate Photocatalysis Solid state synthesis Methylene blue New photocatalyst

a b s t r a c t A novel visible light driven photocatalyst of Ag2ZrO3 was prepared by solid-state reaction technique for the first time and the photocatalytic properties were investigated. Reactive powder of the photocatalyst was obtained by heating a 1:2 molar mixture of zirconyl oxychloride and silver nitrate at 900 °C up to 24 h and characterized by XRD, SEM, TEM, EDAX, and UV–visible diffuse reflectance spectroscopy. The photocatalytic degradation of aqueous methylene blue dye with Ag2ZrO3 as catalyst was investigated under visible light irradiation. The photocatalyst is superior over the known photocatalyst and has high stability and reusability. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor photocatalysis fascinates many researchers because of its application for the destruction of chemical contaminants in waste water and industrial effluents. TiO2 is most commonly use for various photocatalytic oxidation techniques [1–3], as it has relatively high photocatalytic activity, biological and chemical stability, low cost, non-toxicity, long life span and is environment friendly. TiO2 appears to be more effective under UV light, which is the main hurdle in its commercialization. To overcome this problem several efforts have been taken to develop visible light active photocatalyst, hence the main part of the solar spectrum can be utilized which makes the process economically viable. The major limitation in development of visible light active (N400 nm) photocatalytic system is the quick recombination of charge carriers, as recombination has faster kinetics than surface redox reactions. Over the past 40 years, a lot of semiconductor photocatalyst materials such as TiO2-based, ZrO2, titanates, niobates and tantalates [4–10] have been developed for water splitting and degradation of organic pollutants. However, these catalysts only operate in the ultraviolet light region, accounting for only 4% of the incoming solar energy, which renders the photocatalytic water splitting and degradation of organic pollutants impractical. Thus, the development ⁎ Corresponding author at: Department of Chemistry, Institute of Science, Civil Line, Nagpur 440001, India. E-mail addresses: [email protected], [email protected] (S.R. Thakare).

http://dx.doi.org/10.1016/j.catcom.2014.12.027 1566-7367/© 2014 Elsevier B.V. All rights reserved.

of visible-light-driven photocatalyst has consequently become an imperative topic in current photocatalysis research. The search for such materials has focused mainly on metal-ion substitution and non-metal-ion substitution or solid-solution fabrication. Other than metal oxide and metal sulfide, visible light active photocatalysts are C3N4, metal tungstates, metal vanadates, metal chromates and metal phosphates [11–15]. It is reported that the insertion of silver ion in the catalyst improves the photocatalytic efficiency of the material. However, the number of photocatalyst working in the visible-light region for degradation of organic pollutants is limited, and the efficiency of these catalysts is still low and needs improvement. Herein, we developed a visible light active sliver zirconate (Ag2ZrO3) prepared by a simple solid state reaction method to function as efficient visible-light-driven photocatalyst for degradation of methylene blue. We also discussed the characteristics of photocatalyst, such as crystal structure, morphology, optical property and photocatalytic properties in detail. 2. Experimental section Ag2ZrO3 powder was prepared by a solid-state reaction. All reagents used in the synthesis were analytical grade and used without further purification. High-purity AgNO3 (99.9% Merck) and ZrOCl2·6H2O (99.9% Merck) were use as raw materials. Stoichiometric amounts of precursors (1:2) were accurately weighed so as to yield about 10 g of the Ag2ZrO3 after calcinations. The two components were mixed by milling for about 45 min. The slurry formed was dried at room temperature and

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S.R. Thakare et al. / Catalysis Communications 62 (2015) 39–43

then in an air oven overnight at 80 °C. The dried cake was crushed into fine powder in an agate mortar and pestle. The dry powder was then calcined at 900 °C in a muffle furnace for 24 h using silica crucibles and was pulverized again to fine powder which was then used for characterization. Phase purity and crystal structure of the sample were determined by using an X-ray diffractometer (XRD) (XPERT-PRO Diffractometer) with monochromatic Cu Kα radiation (45 kV, 40 mA). Microstructural characterization was performed by scanning electron microscopy (SEM) by Zeiss DSM 940A. The diffuse reflectance spectra (DRS) of the photocatalyst were measured by a UV–visible spectrophotometer (UV-1800, Shimadzu). The photocatalytic activity determination in detail is given in the Supplementary information. The photocatalytic activity of the prepared Ag2ZrO3 Powder calcined at 900 °C for 24 h was evaluated using methylene blue as a model organic compound. Photolysis of aqueous suspensions of methylene blue of different concentrations (1 × 10−4 mol L−1, 1 × 10−5 mol L−1, 1 × 10−6 mol L−1) and Ag2ZrO3 powder of various quantities (0.5 g L−1, 1 g L−1, 2 g L−1) was carried out in a circular glass reactor (designed and fabricated in our laboratory). Halogen lamp (40 W/230 V/36D, Phillips) was used as a visible light source. During photolysis, suspensions were kept under constant air-equilibrated conditions. Before irradiation, the suspensions were magnetically stirred in the dark for about 30 min to ensure the establishment of an adsorption–desorption equilibrium among the photocatalyst, methylene blue and water before the lamp was turned on. The photocatalyst that contained methylene blue solution was irradiated from the bottom of the reactor. At given time intervals, 3 mL of aliquots was sampled after every 20 min, and centrifugated to remove the particles. In filtrates decomposition of methylene blue was analyzed by recording the variations of the absorption band in the UV–visible spectra of the suspension using a spectrophotometer.

Counts

3. Result and discussion Fig. 1 shows XRD pattern of Ag2ZrO3 calcined at 900 °C for 24 h and the sharp peaks of XRD pattern indicate that the synthesized Ag2ZrO3 has a well crystalline nature. XRD analysis resulted in similar Na2ZrO3 crystalline structures as shown in Fig. 1 for Ag2ZrO3 with no other phases being observed. However, the structure of Ag2ZrO3 samples, presented a slight shift to 2-theta values of the characteristic peaks in the diffraction patterns as the Ag substituted Ag2ZrO3 with respect to Na2ZrO3 [16]. This can be attributed to an increase in the inter-planar distance of the crystalline structure of Na2ZrO3 assuming a substitution of the Na by Ag atoms in the overall structure. We assume that the structure of Ag2ZrO3 is similar to Ag2SnO3 [17] having a layered structure. XRD analysis of the material shows that further investigation is needed to explore the structural chemistry of Ag2ZrO3. Structural morphology of the sample was revealed by SEM and Energy Dispersive X-ray Analysis (EDAX) analysis as illustrated in Fig. 2. The agglomerate nature is observed for Ag2ZrO3 powder heat treated at 900 °C. The EDAX analysis of the synthesized product reveals the presence of silver, zirconium and oxygen in the product. Strong signals were observed for Ag, Zr and O. Atomic percentage shown in Table 1 demonstrates that the synthesized product is Ag2ZrO3. A transmission electron microscope (TEM) micrograph of the sample is shown in Fig. 2. The Ag2ZrO3 powder calcined at 900 °C was uniform, with a well-distributed spherical particle with a size of about 50 nm. The average particle size may be increased with increasing calcination temperature, suggesting a gradual growth of the nanoparticles during the heating process. The average particle size was about 50 and 53 nm. The FTIR spectrum of the Ag2ZrO3 nanoparticles is shown in the Supplementary information of this article. The broad band at 3000–3200 cm − 1 shows the presence of a water molecule on the

AGZ-SSR

200

100

0 20

30

40

50

60

70

80

Position [°2Theta]

Fig. 1. X-ray diffraction patterns of prepared Ag2ZrO3 powder after calcinations at 900 °C.

90


A

41

B

C

D

Fig. 2. SEM, TEM and EDAX for Ag2ZrO3 powder calcined at 900 °C. A & B. SEM images, C. EDAX, D. TEM image of Ag2ZrO3.

surface which increases the wettability of the material and has advantage to separate easily after the photocatalytic reactions. The strong peak at lower wavelength shows that the presence of metal oxygen bond was also confirmed by Raman spectroscopy study (Supplementary information). The strong peak at 625 and 478 in Raman spectra shows the presence of Ag–O and Zr–O bond corresponding to stretching vibration respectively [18]. Information about the absorptive properties of silver zirconate can be obtained from diffuse reflectance UV–visible spectroscopy (detailed in the Supplementary information). Fig. 3 illustrates the light absorption properties of silver zirconate powder calcined at 900 °C from ultraviolet to visible light (200–800 nm), showing that the visible absorption spectra of Ag2ZrO3 are characteristic of photocatalyst able to respond to visible light. In this spectrum, a broad absorption edge situated at 425 nm indicates the optical band gap attributed to the O2 − → Ag+ charge-transfer interaction. The light absorption spectra were also dominated by the broad intense absorption in the visible region from 400 to 800 nm according to the heavily colored Ag2ZrO3. The UV–visible spectrum of Ag2ZrO3 shows that the material is ideal for the utilization of solar radiation. The band gap of the samples was determined by the equation Eg = 1239.8 / λ, where Eg is the band gap (eV) and λ (nm) is the wavelength of the absorption edges in the spectrum. The band gap of Ag2ZrO3 was found to be 2.9 eV. These values show that the sample possesses visible light activity and has low band gap than TiO2 and ZrO2.

Table 1 Elemental composition of the silver zirconate powder. Element

wt.%

at.%

OK Zr L Ag L Totals

30.85 25.90 43.26 100.00

33.79 10.86 20.35

Photocatalytic activities of the resulting samples were investigated by the degradation of methylene blue in aqueous solution. Blue color of the solution gradually diminished upon the visible light irradiation in the presence of the photocatalyst, illustrating the degradation of methylene blue. Total concentrations of all methylene blue species were simply determined by the maximum absorption measurement. Fig. 4 shows the decrease of the absorbance of methylene blue versus irradiation time in the presence of the as-prepared silver zirconate under visible light irradiation. It was found that almost all methylene blue degradation take place within 160 min of time. In view of the fact that organic dyes can also harvest visible light, experiments with methylene blue photodegradation over Ag2ZrO3 powders under different monochromatic visible-light irradiation conditions were designed to test the possibly evolved effects of photosensitization. From ultraviolet visible absorption spectra, it is clear that the methylene blue dyes are most excited by a wavelength of approximately 600–700 nm and least excited by a wavelength of 350–500 nm, whereas the Ag2ZrO3 semiconductors can be excited only by a wavelength shorter than 600 nm, as described above. Therefore, we first used monochromatic visible light centered at a wavelength of 420 (± 5 nm) to excite the Ag2ZrO3 semiconductor but minimize the excitation of the methylene blue. The methylene blue was still exhausted very quickly. In contrast, when monochromatic visible light centered at a wavelength of 640 (± 5 nm) was used to excite the methylene blue molecules but not the Ag2ZrO3 semiconductor, it is difficult to observe any methylene blue degradation at all. The effect of photosensitization during methylene blue decomposition over Ag2ZrO3 is thus negligible. In other words, methylene blue decomposition can definitely be attributed to the intrinsic strong photooxidative activity of the Ag2ZrO3. For comparison purposes, the results of methylene blue decomposition over the well known photocatalysts Ag3PO4, TiO2 (Degussa P-25) and Ag2ZrO3 under the same conditions (Supplementary information) were carried out. The comparative photocatalytic degradation efficiency is shown in Fig. 5. It was found that the process of methylene blue decomposition over Ag2ZrO3 was 1.5 times quicker than that over the

42


A

B

Fig. 3. A) Band gap determination using diffuse reflectance spectra. B) Diffuse reflectance spectra (DRS) of the prepared Ag2ZrO3 powder.

0 Min

160 Min

reference materials. The reuse of the photocatalyst is an important parameter for the technical point of view. Hence we use the Ag2ZrO3 photocatalyst material over the cycle by cycle up to five under same experimental condition. There was no significant change in degradation efficiency Ag2ZrO3 photocatalyst material observed. Further the XRD and diffuse reflectance spectra before the photo degradation study and after 5 cycles do not show any change in their characteristics which confirmed that the material has very high stability and reusability for the degradation of dye molecule. On the basis of earlier study, we assume that Ag2ZrO3 exhibited the ability to absorb visible light, which is attributed to the transition of the electrons from the VB (hybrid orbitals of O 2p and Ag 4d) to the CB (Zr 4d orbital) in the catalyst. Furthermore, the hybridization of the Ag 4d and O 2p levels makes the VB largely dispersed which favors the mobility of photoholes in the VB and is beneficial to the oxidation reaction. Thus, it has been found that silver is one of the elements that is able to make a valence-band position higher than O 2p orbitals.

100 90 80 70 60 50 40 30 20 10 0

% Degradation of MB using AgZ-SSR, Ag3PO4 and TiO2P25

TiO2 P25 MB Fig. 4. The changes in absorbance of methylene blue solution by Ag2ZrO3. The conc. of the catalyst was 1 g L−1, concentration of methylene blue (1 × 10−5 mol L−1).

AgZ SSR MB

Ag3PO4 MB

Fig. 5. Comparative degradation efficiency of different photocatalysts. The conc. of the catalyst was 1 g L −1, concentration of methylene blue (1 × 10−5 mol L−1) (AgZ SSR stand for Ag2ZrO3).


4. Conclusion We here synthesized Ag2ZrO3 by solid state reaction. This method is convenient and easy to handle. As prepared, Ag2ZrO3 was found to be active under visible light irradiation for degradation of dye methylene blue. Further the photocatalyst material is highly stable and reusable several times without losing its activity. The photocatalyst material Ag2ZrO3 has superior photocatalytic activity over the known photocatalysts Ag3PO4 and TiO2. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.12.027. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] M.A. Fox, M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1993) 341–357. [3] A. Mills, R.H. Davies, D. Worsley, Water purification by semiconductor photocatalysis, Chem. Soc. Rev. 22 (1993) 417–425. [4] X. Wang, J. Yu, Y. Chen, L. Wu, X. Fu, ZrO2-modified mesoporous nanocrystalline TiO2–xNx as efficient visible light photocatalysts, Environ. Sci. Technol. 40 (2006) 2369–2374. [5] S.R. Thakare, N.S. Bhave, Photocatalytic degradation of phenol as a model pollutant by immobilized TiO2, Indian J. Chem. A 44 (2005) 2262–2265.

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