Cent. Eur. J. Chem. • 11(3) • 2013 • 364-370 DOI: 10.2478/s11532-012-0167-2
Central European Journal of Chemistry
Photocatalytic properties of ZnO/TiO2 powders obtained via combustion gel method Research Article
Albena D. Bachvarova-Nedelcheva1*, Reni S. Iordanova1, Angelina M. Stoyanova2, Radka D. Gegova1, Yanko B. Dimitriev3, Alexandre R. Loukanov4
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1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Medical University, 5800 Pleven, Bulgaria
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University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria
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University of Mining and Geology “St. Ivan Rilski”, Sofia 1700, Bulgaria
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Received 18 July 2012; Accepted 18 October 2012
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Abstract: The present study is a continuation of our previous investigations on the ZnO-TiO2 system. By applying different sol-gel methods we proved that the type of precursor and the order of adding the components influence the microstructure of the final product. This study focuses on the combustion sol-gel synthesis and photocatalytic properties of nanosized (∼7-20 nm) ZnO-TiO2 powders. The photocatalytic tests were performed toward two model organic dyes, Malachite Green and Reactive Black 5, in the UV and Vis region. For synthesis of ZnO/TiO2 powders, different precursors such as Zn(NO3)2•6H2O, Zn(CH3COO)2•2H2O, Ti(OC2H5)4 and Ti(OC4H9)4 were used. During the combustion process various phases (ZnO, TiO2 – anatase and rutile, ZnTiO3) were obtained. The structure and morphology of the resulting particles were characterized by XRD and SEM analysis. All samples exhibited a good photocatalytic activity in both UV and Vis regions. Keywords: Precursor • Gel combustion method • TiO2/ZnO composites © Versita Sp. z o.o.
1. Introduction
Recently, extensive research has been conducted on the synthesis and characterization of zinc oxide (ZnO) and titanium oxide (TiO2) due to their electric, optical, catalytic, magnetic, and biochemical properties as well as owing to their good chemical and thermal stability. Most of the obtained results are summarized in the review by Carp et al. [1] and in the Ozgur’s monograph [2]. In our previous investigations submicron ZnO/TiO2 powders were obtained by hydrolytic sol-gel method and their photocatalytic and antibacterial properties against E. Coli were studied [3-5]. Several authors established that the method of synthesis and morphology of
the nanoparticles have an essential impact on the photocatalytic processes [6,7]. Recently, the combustion synthesis (CS) became more popular and has emerged as an important technique for the synthesis and processing of oxide and non-oxide advanced materials [8]. This method has many potential advantages, such as low-processing cost, energy efficiency, and high production rate. Several books [9-12] and review articles [13-19] have been published on this subject in recent years. By combustion synthesis, TiO2, nanoparticles sized from 100 nm to 1000 nm were very quickly obtained through gasification, nucleation, and crystal growth [20,21]. Zinc oxide (ZnO) powders with different types of morphology were
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Table 1. Samples obtained by combustion gel method. samples N
Nominal composition, mol %
Precursors
order of adding the components
final product
particles size
1
10TiO2•90ZnO
Ti butoxide (sol A) + Zn acetate (sol B)
Solution A to solution B
ZnO
16 nm
2
5TiO2•95ZnO
Ti butoxide (sol A) + Zn nitrate (sol B)
Solution A to solution B
ZnO
7 nm
3
90TiO2•10ZnO
Ti etoxide (sol A)+ Zn nitrate (sol B)
Solution A to solution B
TiO2 (rutile, anatase) + ZnTiO3
-
2. Experimental procedure 2.1. Sample preparation
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Based on the results of our previous studies in this system [3-5], three samples with different nominal compositions 90ZnO•10TiO2 (sample 1), 95ZnO•5TiO2 (sample 2) and 10ZnO•90TiO2 (sample 3) were selected for investigation. The starting materials were: zinc nitrate – Zn(NO3)2•6H2O (Merck), Zn acetate Zn(CH3COO)2•2H2O (Merck), titanium (IV) ethoxide (Fluka AG) - Ti(OC2H5)4, titanium (IV) butoxide (SigmaAldrich) - Ti(OC4H9)4 and ethylene glycol (C2H6O2) (Table 1). The main scheme for the synthesis is presented in Fig. 1. Solutions (A and B) were prepared via dissolving the precursors in ethylene glycol and absolute alcohol by means of vigorous magnetic stirring. A white xerogel was obtained by drying at 110oC for 5 h. Subsequently, the as-obtained xerogel was subjected to evaporation in a water bath. During the heating on a hot plate up to 200-300oC, combustion process takes place and as a result gray-white powders are obtained. According to the classification for the CS made by [8], our method could be classified as a gel combustion method. The last step of synthesis consists of calcinations of the powders at 400-450oC for 2 hours in air, until obtaining white powders (samples 1, 2 and 3). The calcination temperature was selected on the basis of our previous investigations fro two reasons: because there is no presence of organic constituents above 400ºC and and in order to keep the small size of the obtained crystals. All samples were prepared by addition of Ti precursor solutions to the zinc nitrate. During the experiments the pH was measured and it varied depending on compositions. The pH for compositions with high ZnO (90, 95 mol%) content was 3-5, while for the sample with high TiO2 content (90 mol%) the pH was 7-8.
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synthesized by the combustion synthesis method using zinc nitrate, metallic zinc, and glycine as precursors [22]. Nanosized ZnO powder was synthesized by solution combustion method. The obtained powder showed three times higher photocatalytic efficiency than any other commercial photocatalysts [23]. Quite recently, Zn-doped TiO2 was obtained by the combustion method with better photocatalytic activity compared to pure TiO2 [24]. Photocatalysis of N-doped TiO2/ZnO composite powder was studied towards methyl orange dye [24]. The authors found that the enhanced photocatalytic activity of the composite powder is related to the good crystallization, the presence of anatase phase, and the particle size reduction. Several authors [25,26] reported that the coupling of TiO2 with ZnO is useful to achieving higher photocatalytic reaction rate. The results showed that the photocatalytic activity of ZnO/TiO2 coupled photocatalysts was higher than that of the single phase. Due to the above stated advantages, we applied sol-gel combustion method for synthesis of powders in the ZnO-TiO2 system. Despite of the large number of investigations, the effect of the synthesis method on the morphology and photocatalytic properties of ZnO/TiO2 composite materials has still not been clarified. The present investigation is a continuation of our previous work on obtaining powders in ZnO-TiO2 system by applying different sol-gel methods [3-5]. As was mentioned above, the order of adding the components influences the microstructure of the final product, however the other important factor is the selection of an appropriate scheme for synthesis. In this study we continue the investigations on the synthesis and photocatalytic activity of nanosized ZnO/TiO2 powders applying combustion gel method. The purpose was to study the effect of the synthesis method on the microstructure and properties of the final product. The photocatalytic tests in the visible and ultraviolet range of the spectra were performed toward two organic dyes Malachite Green (MG) and Reactive Black 5 (RB5).
2.2. Samples characterization
The phase formation and morphology of the obtained powders were established by X-ray diffraction (Bruker D8 Advance X-ray apparatus) and SEM (JEOL Superprobe
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Figure 1.
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Photocatalytic properties of ZnO/TiO2 powders obtained via combustion gel method
Scheme for combustion gel synthesis of ZnO/TiO2 powders.
3. Results and discussion
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733). The optical absorption of the obtained powders was measured by UV-Vis analysis (Spectrophotometer Evolution 300). The actual composition of selected samples was determined by laser ablation inductively coupled plasma (LA-ICP-MS) measurements. The specific surface area of the samples was measured using a modified BET method. The photocatalytic activities of the obtained powders were evaluated by degradation of model aqueous solutions of Malachite Green (MG) and Reactive Black 5 (RB 5) upon UV- and VIS-light irradiation. The illumination was carried out by UV-lamp (Sylvania BLB, 18W, λ ~ 315–400 nm) and by VIS lamp (TUNGSRAM lamp, 500 W, maximum emission at about λ ~ 700 nm). MG and RB 5 solutions (150 mL, 10 ppm) containing 0.1 g of as-prepared powders were placed in a glass beaker. Before the light was turned on, the solution was first ultrasonicated for 10 min and then stirred for 10 min in the darkness to ensure adsorption/desorption equilibrium. All photocatalytic tests were performed at constant stirring (400 rpm) and room temperature of 25oC. Volumes of 3 ml solution were taken at given time intervals and the catalyst was separated through centrifugation (5000 rpm, 5 min). Then the change in concentration of MG and RB 5 in the supernatant solutions was measured using a Jenway 6400 spectrophotometer at wavelengths of 618 nm and 595 nm, respectively. 366
The X-ray diffraction patterns of ZnO/TiO2 powders are shown in Fig. 2. Zinc oxide (ZnO - JCPDS 36-1451), was only identified in samples 1 and 2. The characteristic peaks for TiO2 (anatase) were not detected and probably this phase remains in the amorphous state. A Rietvield refinement was performed for sample 2 and the result showed only one phase ZnO (Fig. 3). The average crystallite size of all powders calculated from the broadening of the diffraction line using Sherrer’s equation is below 20 nm. Although, only ZnO was found by XRD in samples 1 and 2, by using LA-ICP-MS method for sample 2, the presence of TiO2 was also proved (Table 2). A mixture of anatase (JCPDS 78-2486), rutile (JCPDS 89-0555) and ZnTiO3 (JCPDS 26-1500) were found in sample 3 (Fig. 2). Our XRD results are compatible with the data reported for ZnO obtained by the solution combustion method, where the size of the powders was about 30 nm [23]. According to SEM observations (Figs. 4a, 4b), during the calcinations, sample 1 is characterized by a strong tendency to agglomeration, while sample 2 (Figs. 4c, 4d) showed a porous agglomerate structure, irrespective that both samples were obtained by the same method. Probably, the type of the precursor influenced on the particles porosity.
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Figure 2. XRD patterns of the investigated samples 1, 2 and 3.
Figure 3. Rietvield refinement of sample 2. Table 2. Quantitative analysis made by LA-ICP-MS method for the sample with nominal composition 5TiO2.95ZnO (sample 2). Sample Oxides N
2
Initial Composition
Quantitative analysis
mol %
wt %
LA-ICP-MS, wt %
TiO2
5
4.91
3.55 ±0.1
ZnO
95
95.09
95.09
The UV-Vis absorption spectra of samples 1 and 2 are shown in Fig. 5. As can be seen, both powder samples are transmittant in the visible range of the spectra above 400 nm. Figs. 6 and 7 present the results for photocatalytic experiments of investigated samples toward Reactive Black 5 and Malachite Green. The decolouration of RB 5 and MG under UV irradiation completed in approximately 45 min in both cases (Figs. 6a, 7a). Generally, the decolouration of the dyes under Vis irradiation takes more time and it completes in 80 min for RB5 and in 60 min for MG (Figs. 6b, 7b). As is seen from the figures, the photocatalytic activity of samples 1 and 2 containing higher ZnO content (90, 95 mol%) is comparable under UV and Vis irradiation but it is higher than that of sample 3 (Figs. 6, 7). Additionally, the values of their specific surface areas are in the range of 40-60 m2 g-1. Obviously, the type of precursor does not influence the specific surface area and photocatalytic activity. Our results did not confirm those obtained earlier by other authors [25], who claimed that the type of precursor (zinc nitrate or acetate) influenced the sample porosity. The photocatalytic tests of pure ZnO and TiO2 synthesized by us were also the subject of previous investigations 367
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Photocatalytic properties of ZnO/TiO2 powders obtained via combustion gel method
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Figure 4. SEM micrographs of samples 1 (a, b) and 2 (c, d) obtained at different magnifications.
Figure 5. UV-Vis spectra of samples 1 and 2. [3-5]. The comparison between the present and previous results showed that as obtained ZnO/ТiО2 composite powders exhibited a comparable to the pure ZnO and TiO2 photocatalytic activity (∼30 min toward MG and RB 5) under UV irradiation. Also, quite recently Xu et al. [27] reported that the photodegradation of methyl orange by TiO2/ZnO composite powders completed in 3 h after UV irradiation. According to Pawar et al., the Zn doped TiO2 powders obtained by combustion method possess improved photocatalytic activity in comparison 368
to pure TiO2 [28]. Their photocatalytic activity towards phenol under UV irradiation was completed in 60 min. Bearing in mind these facts, we may conclude that the ZnO/TiO2 powders prepared by combustion gel method possess better photocatalytic activity and they are suitable photocatalysts especially in the VIS range of the spectrum. The reason for that may be attributed to the changes in the band gap and increasing the number of surface defects as a result of combustion synthesis [29,30].
Figure 6.
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Photocatalytic degradation of Reactive Black 5 (RB5) by the investigated samples under UV (a) and Vis (b) irradiation
4. Conclusions
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It was established that the combustion gel method is a fast and appropriate one for the synthesis of photocatalytic active powders. By this method nanosized (about 7-20 nm) ZnO/TiO2 powders in the investigated system were prepared. The powders obtained by the method presented here showed good photocatalytic activity against Malachite Green and Reactive Black 5
Figure 7.
Photocatalytic degradation of Malachite Green (MG) by the investigated samples under UV (a) and Vis (b) irradiation.
dyes in the ultraviolet and visible range of the spectrum. Samples containing higher ZnO contents exhibited better photocatalytic activities toward both dyes.
Acknowledgements Authors are grateful to the financial support of National Science Fund of Bulgaria, Contract No DDVU 02/36.
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