Photocatalytic discoloration of the azo dye methylene blue in the presence of irradiated TiO 2 /Pt nano-composite

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1 Photocatalytic discoloration of the azo dye methylene blue in the presence of irradiated TiO 2 /Pt nano-composite Vojka Žunič 1,2 1 Advanced Materials Department, Jožef Stefan Institute, Ljubljana, Slovenia 2 Jožef Stefan International Postgraduate School, Ljubljana, Slovenia vojka.zunic@ijs.si Abstract. An efficient photocatalytic material TiO2/Pt was prepared via the sonochemical synthesis followed by the thermal treatment. The TiO2/Pt nanocomposite was able to photocatalytically degrade the azo dye methylene blue (MB) under UV (ultraviolet) and Vis (visible) irradiation. The enhanced photocatalytic activity of Pt/TiO2 for methylene blue degradation is attributed to the following factors; to the presence of Pt particles which store photogenerated electron thus contribute to an efficient charge carrier s separation and to the adsorption of the dye on the surface of the composite, which acts as a photosensitizer. Keywords: TiO2 nano-powders, TiO2/Pt nano-composites, photocatalytic discoloration, methylene blue 1 Introduction Waste waters originating from industrial discharges represent a global problem which demands the development of an effective, economic, and environmental friendly water treatment technology 1. A high environmental impact has the textile industry since its discharge waters contain large amounts of non-fixed dyes among which are also the azo dyes 2. It is well known that some of azo dyes and their degradation products such as aromatic amines are highly carcinogenic 3. Chemical methods which are able to mineralize organic pollutants to carbon dioxide, water and inorganics or, at least, transform them into harmless products are the advanced oxidation processes (AOP) 1, 4. One of the AOP is the heterogeneous photocatalysis, which is based on the generation of highly reactive and oxidizing hydroxyl radicals in the presence of an irradiated semiconductor

2 metal oxide 1. The most interesting semiconductor for the photocatalytic applications is titanium dioxide (TiO 2 ). However, the most active TiO 2 crystal form anatase is active only when it is irradiated with UV light 1. Since the sunlight contains only a small part of the UV light, many efforts have been made to improve the photocatalytic activity of TiO 2 in the near UV and Vis portion, as well as to shift the TiO 2 anatase absorption edge to the Vis part. Among the different methods for the improvement of the TiO 2 photocatalytic efficiency is the attachment of TiO 2 with noble metals, such as platinum (Pt), gold (Au) and silver (Ag) 5. If the work function ( SB ) of the metal is higher than that of TiO 2, the photogenerated conduction band electrons are removed from the TiO 2 in the vicinity of the metal particle (Fig. 1). As a consequence, a Schottky barrier occurs at the contact metal-semiconductor, which leads to a decrease in the electron-hole recombination as well as to an efficient charge separation 1, 6. Therefore the TiO 2 photocatalytic efficiency should be significantly improved. Figure 1. A schematic representation of the photoinduced electron transfer between TiO 2 and Pt particles. The highest Schottky barrier is produced with Pt 1. Therefore, to improve the photocatalytic activity under UV irradiation, we choose to attach the TiO 2 particles with the Pt particles. For the photocatalytic activity test the organic azo dye methylene blue was used. Since the dye absorbs Vis light, we expected that the photosensitization effect that would be caused with the TiO 2 surface adsorbed dye would induce a Vis light performance.

3 2 Experimental 2.1. Synthesis of TiO 2 /Pt The TiO 2 nano-powders and TiO 2 /Pt nano-composites were prepared by using an alkoxide Ti precursor. Titanium(IV) n-butoxide (TNB; TiO 4 H 36 C 16, 98%) was dissolved in 1-Bultanol (C 4 H 9 OH, 99%) to form a Solution 1. Nitric acid (HNO 3, 65%) was diluted in ultrapure water to a form Solution 2. Afterwards a Solution 2 was added dropwise to the Solution 1. A transparent Solution 3 (ph=1) was formed. The Pt precursor chloroplatinic acid hexahydrate (H 2 PtCl 6 6H 2 O) was dissolved in ultrapure water and added to the Solution 3, which was transferred into a Suslick reactor and heated to the temperature 80 C. Afterwards the sonication was initiated. The following parameters were used: time of sonication t=3h, pulse on:off = 02:01 s, amplitude 80%, power P=600W and frequency f=20 khz. The formed precipitates were separated with centrifugation, dried and thermally treated in a reducing atmosphere (Ar/H 2 =96/4) at 400 C for 3h Characterization techniques The phase composition and the average crystallite size were evaluated utilizing the X-ray powder diffraction analysis. The specific surface area (s BET ) was measured by the Brunauer-Emmett-Teller method and the morphological characteristics were analyzed with the transmission electron microscopy (TEM, HRTEM, SAED). UV- Vis spectra were recorded using a UV-Vis-NIR in which the BaSO 4 standard was used as the reference spectrum. The photocatalytic activity was evaluated in an aqueous methylene blue solution. A 7.5 ml of the dye solution ( M; 10mg/l) and 15 mg of the TiO 2 powder (2g/l) were tested under UV and Vis irradiation. The change in the absorbance of the dye solution was measured utilizing the UV-Vis-NIR spectrometer (Shimadzu UV-Vis-NIR 3600). 3 Results and discussion The phase composition analysis of the TiO 2 nano-powder and TiO 2 /Pt nanocomposite before the thermal treatment revealed that the materials were a semicrystalline TiO 2 anatase. The following thermal step led to an improvement of TiO 2 crystallinity and to the oxidation state reduction of Pt particles deposited on the TiO 2 surface. The crystallinity and phase composition were also confirmed with the selected area electron diffraction (SAED) analysis (Fig. 2).

4 Figure 2. The phase composition of thermally treated TiO 2 and TiO 2 /Pt obtained with a) X-ray analysis and with the SAED analysis for b) TiO 2 and c) TiO 2 /Pt. The average size of TiO 2 particles calculated from the X-ray patterns was 7 nm for the TiO 2 and 10 nm for the TiO 2 /Pt. Since the TiO 2 nano-powders consisted of smaller particles than the TiO 2 /Pt nano-composites, they exhibited a higher specific surface. The measured specific surface area was 87 m 2 /g for the TiO 2 nano-powders and 54 m 2 /g for the TiO 2 /Pt nano-composites. Morphologically, the sonication method followed by the thermal treatment resulted in the formation of uniformly sized sphere-like TiO 2 nano-particles which tended to agglomerate (Fig. 3). The ultrasound induced agglomeration of TiO 2 could be due to the collision of two particles which caused melting at the point of impact resulting in the agglomeration 7. The observed TiO 2 particle size was in agreement with the calculated one. The formed Pt nanostructures in the TiO 2 /Pt nano-composites were present in the form of sphere-like (up to 5 nm) and polyhedral (up to 25 nm) particles (Fig. 4). The formed TiO 2 /Pt nano-composites exhibited a blue shift of the fundamental absorption edge as analyzed with the diffuse reflectance spectroscopy (Fig. 5). Such a blue shift of the fundamental absorption edge is usually observed with TiO 2 nano-materials which consist of particles from 5 to 10 nm due to the quantum size

5 effect 1. Since the formed TiO 2 /Pt material consisted of larger particles than the TiO 2, we believe that the addition of chloroplatinic ions led to changes in the electronic band structures of the TiO 2 /Pt nano-composites. Figure 3. The TEM image (a) and the HRTEM image (b) of the TiO 2 nanopowder. Figure 4. The TEM image a) and the HRTEM (b) of the TiO 2 /Pt nanocomposite. The kinetics of the photocatalytic discoloration of the model organic pollutant, the azo dye methylene blue, follows an apparent first order reaction mechanism (Eq. 1) which is in agreement with the generally observed Langmuir-Hinshelwood model 8 : lnc = ln(c 0 ) k app t, (1) where C 0 and C are the initial concentrations of the dye at time zero and at time t, respectively, and k app is the apparent first-order reaction constant. The degradation

6 reaction constants were determined based on this apparent first-order kinetic mechanism (Table 1). Figure 5. The Diffuse reflectance spectra of the prepared materials TiO 2 and TiO 2 /Pt. The photocatalytic activity measurements showed that the prepared TiO 2 /Pt nanocomposites were characterized with an improved photocatalytic efficiency when compared to bare TiO 2. The efficiency of TiO 2 /Pt under UV irradiation was two times higher than that of TiO 2. Under Vis irradiation bare TiO 2 was not able to degrade the methylene blue. However, after the TiO 2 particles were attached with Pt particles there was a noticeable degradation of the dye under Vis irradiation. Table 1 The UV and Vis first-order reaction constants k app (min -1 ) for TiO 2 and TiO 2 /Pt Sample k app (min -1 ) UV x 10 3 k app (min -1 ) Vis x 10 3 TiO TiO 2 /Pt 23 7 The enhancement of the UV photocatalytic activity of the prepared TiO 2 /Pt composite, when compared to bare TiO 2 could, be ascribed to the presence of the TiO 2 surface attached Pt particles which acted as the an electron storage 5 thus contributing to better separation of charge carriers. On the contrary, we believe

7 that the Vis induced photocatalytic activity was caused with the surface adsorbed dye methylene blue. Since methylene blue absorbs Vis light, it can be excited by the Vis light irradiation thus acting as a photosensitizer 9. The excited dye injects an electron to the TiO 2 conduction band, where it is scavenged by preadsorbed oxygen (O 2 ), forming and forms oxygen radicals which are able to drive the photodegradation or mineralization 9. We believe that this phenomenon is responsible for a Vis light photocatalytic activity of the TiO 2 /Pt nano-composites. 4 Conclusions The TiO 2 /Pt nano-composites which consisted of Pt particles (up to 25 nm) and TiO 2 particles in the anatase crystal form (up to 10 nm) were synthesized via the sonochemical method. Such materials are shown to be an efficient photocatalytic material for the discoloration of the azo dye methylene blue under UV and Vis irradiation. The TiO 2 surface attachment with the Pt particles led to the significant improvement of the UV photocatalytic activity and the Vis light photocatalytic activity was induced with TiO 2 surface adsorbed dye. References: [1] O. Carp, C.L. Huisman, A. Reller. Photoinduced reactivity of titanium dioxide. Progress in Solid State Chemistry, 32: , [2] N. Tüfekci, N. Sivti, İ. Toroz. Pollutants of textile industry wastewater and assessment of its discharge limits by water quality standards. Turkish Journal of Fisheries and Aquatic Science, 7: , [3] H. Lachheb, E. Puzenat, A. Houas, M.Ksibi, E. Elaloui, C. Guillard, J.-M. Herrmann. Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania. Applied Catalysis B: Environmental, 39: 75-90, [4] R. Andreozzi, V. Caprio, A. Insola, R. Marotta. Advanced oxidation processes (AOP) for water purification and recovery. Catalysis Today, 53: 51-59, [5] H. Tada, T. Kiyonaga, S. Naya. Rational design and applications of highly efficient reaction systems Photocatalyzed by noble metal nanoparticle-loaded titanium(iv) dioxide. Chemical Society Reviews, 38: 1849 (2009).

8 [6] B. Kreaeutler, A.J. Bard. Heterogeneous photocatalytic preparation of supported catalysts. Photodeposition of platinum on TiO 2 powder and other substrates. Journal of the American chemical society, 100(13): 4317, [7] T. Prozorov, R. Prozorov, K.S. Suslick. High velocity interparticle collisions driven by ultrasound. Journal of American chemical society, 126: (2004). [8] R.W. Matthews. Photocatalytic oxidation and adsorption of methylene blue on thin films of near-ultraviolet-illuminated TiO 2. Journal of the Chemical Society, Faraday Transactions, 85(6): 1291 (1989). [9] T. Wu, T. Lin, J. Zhao, H. Hidaka, N. Serpone. TiO2-assisted photodegradation of dyes. 9. Photooxidation of a squarylium cyanine dye in aqueous dispersion under visible light irradiation. Environmental Sciences and Technology, 33: 1379 (1999).

9 For wider interest It is well know that TiO 2 is characterized with photocatalytic properties by utilizing UV (ultraviolet) light. This phenomena is already been used for commercial applications such as self-cleaning concrete (Italcement Group) in building facades (Jubilee Church (also known as the Dives in Misericordia) in Rome) and pavements (Municipal District of Bergamo, Italy Borgo Palazzo Street), self-cleaning windows (Pilkington), ect. Another field, in which the photocatalytic properties of TiO 2 can be of advantage, is the water purification. Water contamination due to the industrial wastewaters which contain organic dyes has become a global problem. About 1-20% of organic dyes are lost during the industrial dyeing processes and released into the environment. The dyes itself and their degradation products represent toxic substances which cause diverse effects on animal and human health. Therefore, the purification and remediation of discharged waters generated from industrial processes is a necessity. Having in mind such problems, the idea of this work was to prepare TiO 2 which could be used for azo dyes degradation in water. Since the UV light represents only a small part of the sunlight (only 2-3%) the goal of our work was to synthesize a TiO 2 which exhibits improved photocatalytic properties under UV irradiation and also is active under Vis (visible) light irradiation. Since such TiO 2 is able to degraded organic dyes utilizing solar energy (UV and Vis) it represents an economic and efficient method for water purification. We prepared such photocatalyst by forming a TiO 2 /Pt nano-composite which is able to effectively photocatalytically degraded the azo dye methylene blue under UV and Vis irradiation.