Highly dispersed phase of SnO2 on TiO2 nanoparticles synthesized by polyol-mediated route: Photocatalytic activity for hydrogen generation

https://doi.org/10.1016/j.ijhydene.2009.02.085Get rights and content

Abstract

TiO2–SnO2 mixed oxides (Ti:Sn = 98:2 (TS2), 95:5 (TS5) and 90:10 (TS10) by atomic weight) of large surface area and small particle size, in which SnO2 is in a dispersed phase on TiO2, have been synthesized by a polyol-mediated route. Characterization by various techniques has shown that a highly dispersed phase of SnO2 on anatase TiO2 is formed in TS2 sample. No separate discernible phases corresponding to cassiterite SnO2 or rutile TiO2 is seen in TS2 sample, whereas rutile TiO2 and SnO2 are observed besides the anatase phase of TiO2 in TS5 and TS10 samples. The average particle size of the mixed oxide samples is ∼20 nm. All samples absorb visible light and the onset of absorption was ∼425 nm. These mixed oxides show emission from defect levels arising due to the anion vacancies present in TiO2. The visible light absorption of these samples is attributed to the presence of defect levels in the bandgap of TiO2. Photocatalytic activity of these samples for hydrogen generation from water using methanol as sacrificial reagent was studied under sunlight type radiation. The results indicate that mixed oxides have better activity compared to pure TiO2 synthesized by the same method and the activity decreases with increasing SnO2 concentration in TiO2. The enhanced activity of TS2 sample is ascribed to the efficient charge separation from TiO2 to SnO2 owing to the high dispersion of SnO2 in TiO2. The decreased photocatalytic activity with increased SnO2 concentration is due to the aggregation of SnO2 on TiO2, which results in relatively poor dispersion of SnO2 and decreased charge transfer efficiency, but still maintains better photocatalytic activity compared to TiO2. In addition loading Pd co-catalyst produces a pronounced increase in the hydrogen yield due to the accumulation of electrons in the metal from the TiO2 and SnO2 semiconductors and the increased reductive power of the Pd loaded mixed oxide nanoparticles.

Introduction

Titanium oxide is a well-known photocatalyst working in the ultraviolet region of light. It is widely used for photocatalytic reactions like oxidation of organic pollutants in air and water as well as hydrogen generation from water [1], [2], [3], [4]. However, the wide bandgap of this material limits its application for the effective utilization of solar radiation. Different strategies are being employed to improve the photocatalytic efficiency of TiO2. Doping TiO2 with different anions like N, S, Cl and Br [5], [6] or cations like Gd, V, Cu, Sn, Fe, Cr etc. [7], [8], [9], [10], [11], [12], [13], [14] has been tried to reduce the bandgap and to increase the visible light absorption. However, these dopants can form isolated localized levels within the bandgap and trap electrons and can lead to adverse effect in phtocatalytic hydrogen generation. If the defect levels can form a continuous band with the conduction band of TiO2, the electrons can reach the conduction band of TiO2 and photocatalytic hydrogen generation can be enhanced. It has been reported that self doping of TiO2 (doping TiO2 with Ti3+) results in the formation of defect levels due to the creation of anion vacancies in TiO2 lattice, which overlap with the conduction band of TiO2 [15]. This leads to visible light absorption and photcatalytic activity. When N is doped in TiO2, the N 2p states can form a continuous band with the O 2p valence band of TiO2. This results in the narrowing of the bandgap of TiO2 and visible light photocatalytic activity [16]. However, some studies point out that N doping generates isolated narrow band above the valence band of TiO2 rather than forming a continuous band with the valence band of TiO2. Nevertheless, in this case also, visible light photocatalytic activity was observed due to the excitation of electrons from the narrow band formed by the N doping [17].

Another method to improve the photocatalytic activity is to increase the separation efficiency of electrons and holes so that their recombination can be minimized. The electrons and holes formed during photoexcitation of TiO2 recombine very fast (∼10 ns) [18] and hence only a fraction of these charges are available for the photoreaction. If TiO2 is mixed with another semiconductor whose conduction band is at a lower potential than TiO2, the electrons present in the conduction band of TiO2 can be transferred to the second semiconductor and the recombination rate comes down. A coupled TiO2–SnO2 photocatalyst synthesized by ball milling is found to work efficiently in this method [19]. Though the bandgap energy of SnO2 (3.8 eV) is higher than that of TiO2 (3.2 eV), the conduction band of SnO2 is at a lower level (ECB = 0 V versus NHE at pH 7) than that of TiO2 (ECB = −0.5 V versus NHE at pH 7). Because of this potential difference, the photoexcited electrons can easily migrate to the conduction band of SnO2 from TiO2 and can give enhanced photocatalytic activity. This catalyst was found to have better photocatalytic activity for the degradation of monocrotophos compared to pure TiO2. Similarly, a bilayer structure of titanium–tin composite oxide films prepared by magnetron sputtering showed visible light absorption and photocatalytic activity for the decomposition of stearic acid [20]. The photocatalytic activity for methylene blue degradation on SnO2-TiO2 nanotube composites synthesized by solvothermal route showed a dependency on the concentration of SnO2 in the composite oxide [21]. The highest activity was obtained for the composition having 5 wt% SnO2 in TiO2.The decreased activity with increased SnO2 concentration was attributed to the decreased adsorption of the reactant on TiO2 surface and decreased efficiency of charge separation. A bicomponent TiO2–SnO2 prepared by electrospinning process showed enhanced photocatalytic activity for the oxidation of RhB dye compared to pure TiO2 under UV light [22].

In the present work, TiO2 containing different concentrations of SnO2 with homogeneous dispersions, was synthesized by a polyol-mediated route. Our idea was to bring about an intimate contact of SnO2 with TiO2 in these systems so that efficient charge transfer occurs from TiO2 to SnO2 when irradiated. Many previous reports have shown that polyol method of synthesis can generate crystalline nanoparticles with large surface area [12], [23]. The small particle size and large surface area can help in the formation of a highly dispersed phase of SnO2 on TiO2. Photocatalytic activity for hydrogen generation from water using methanol as sacrificial reagent was studied using different compositions of SnO2 dispersed on TiO2. It is noteworthy that the photocatalytic hydrogen generation using this system has not been studied so far.

Section snippets

Experimental

TiO2–SnO2 mixed oxides (Ti:Sn = 98:2 (TS2), 95:5 (TS5) and 90:10 (TS10) by atomic weight) in which SnO2 is in a dispersed state on TiO2 have been synthesized by a polyol-mediated route. Ti metal (2 g) was dissolved in dil. HCl. The volume of the solution was reduced by evaporation to 20 cm3 and mixed with 25 cm3 of ethylene glycol (EG). The solution was refluxed at 100 °C for 10 min followed by the addition of urea (13 g, Ti:urea = 1:4 molar ratio) dissolved in 25 cm3 of EG. The temperature was raised to

Results and discussion

Fig. 1A shows the XRD patterns of TS2, TS5, TS10 and TiO2-Pol samples. The pattern of TiO2-Pol is indexed as anatase phase of TiO2 and the reflections corresponding to the (hkl) planes are marked in Fig. 1. The pattern of TS2 sample shows peaks corresponding to anatase phase of TiO2 and peaks corresponding to SnO2 are not seen. But the patterns of TS5 and TS10 indicate the presence of a second phase in addition to anatase TiO2. Fig. 1B shows the XRD pattern of all samples in the region 2θ = 

Conclusions

Highly dispersed SnO2 on TiO2 nanoparticles could be synthesized by a polyol-mediated route followed by calcination at 500 °C. The dispersion of SnO2 was better in TiO2–SnO2 mixed oxide having 2% Sn than on samples with 5% and 10% Sn. All samples absorbed visible light due to the presence of defect levels arising from the oxygen anion vacancies present in TiO2. TiO2 containing SnO2 dispersed phase showed enhanced photocatalytic activity for hydrogen generation compared to pure TiO2 due to the

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