Improvement of eosin visible light degradation using PbS-sensititized TiO2

doi:10.1016/j.jphotochem.2007.08.008

Journal of Photochemistry and Photobiology A: Chemistry

Volume 194, Issues 2–3, 20 February 2008, Pages 173-180

https://doi.org/10.1016/j.jphotochem.2007.08.008 Get rights and content

Abstract

PbS was prepared by a simple chemical method for use as a photosensitizer with TiO₂. Its physico-chemical characterisation reveals a semiconducting behaviour with a band gap energy of 0.41 eV showing efficient visible light absorption. The valence and conduction band energy levels are −0.78 and −1.19 V, respectively. Interparticle electron injection (IPEI) from photoactivated PbS to inactivated TiO₂ was demonstrated by the increase of eosin degradation under visible light. Different parameters affecting the photoactivity such as the PbS amount, eosin concentration and initial pH were investigated. Optimal degradation of eosin occurs at a PbS/TiO₂ ratio of 10:90, an eosin concentration of 30 mg l⁻¹, a pH ∼7, and when using a 2× 200 W tungsten lamp. These conditions result in a constant decomposition rate of 0.116 min⁻¹ corresponding to an eosin half-life of 6.4 min. In this case, the rate of degradation of eosin is respectively 20 and 40 times greater than when in contact with pure TiO₂ or through direct photolysis.

The photoeffeciency of PbS(10%)/TiO₂ was compared to that of CdS(10%)/TiO₂, Bi₂S₃(10%)/TiO₂, Cu₂O(10%)/TiO₂ and Bi₂O₃(10%)/TiO₂ using visible and UV–vis light. PbS(10%)/TiO₂ has the best efficiency regardless of the wavelength of light used. The contribution of the electromotive force and the band gap of the narrow band gap semiconductor to the photoactivity improvement were discussed.

Introduction

Photocatalytic degradation of organic pollutants and organic dyes using semiconductors (SC) such as TiO₂ continues to attract interest as a method to mitigate their impact on the environment [1], [2], [3]. Dyes in particular contribute to eutrophication of water supplies and their incomplete photodegradation can also lead to production of carcinogenic intermediate molecules depending on the wavelength of the incident light [4], [5], [6].

TiO₂ is a well-known wide band gap semiconductor (E_g ∼3.2 eV) and as a consequence can aid in dye degradation using UV light with the dye acting as both a sensitising agent and pollutant. However, the rate of degradation remains very low and the process is restricted to organic dyes whose lowest unoccupied molecular orbital (LUMO) is lower in energy than the conduction band (CB) energy of TiO₂. The low degradation efficiency of TiO₂ can be ascribed to charge recombination between the electrons injected into CB-TiO₂ and the oxidized sensitizer, a situation also often observed in solar cells (n-DSC) [7], [8]. Another problem with such degradation processes is buildup of colourless by-products whose degradation competes with the dye itself by adsorbing preferentially to TiO₂ surface and preventing electron injection phenomena [1].

All of these drawbacks to TiO₂ degradation of organic dyes can be resolved by mixing a narrow band gap SC into TiO₂ as a sensitizer. The narrow band gap SC is able to generate and inject electrons through the CB of wide band gap SC (TiO₂) without actually binding to the surface of the TiO₂. This remote charge injection is possible by virtue of an electromotive force generated by the difference between the CBs of both the wide and narrow band gap semiconductors.

Recently, different heterosystems such as CdS/TiO₂, Bi₂S₃/TiO₂, Bi₂O₃/TiO₂, Cu₂O/TiO₂ [9], [10], [11] have been studied for their potential application to photocatalytic degradation of organic pollutants. The heterojunctions were investigated under different conditions of irradiation, using visible and UV–vis light as well as several different pollutants (dye, aromatic ring containing acidic or basic functional groups). The results suggest that the efficiency of the heterojunction can be maximized by a judicious choice of the narrow band gap SC with high visible light absorption and adequate CB position being the most important properties for consideration.

Recently, the photoelectrochemical properties of the PbS/TiO₂ heterojunction were reported to exhibit relatively high photoconversion efficiency [12], [13]. High efficiencies suggest that this heterojunction may also exhibit high efficiency in degradation of organic dye and pollutants using visible light. Here we evaluate the suitability and efficiency of the PbS/TiO₂ heterojunction for its use in decomposing organic pollutants by examining the physico-chemical properties of PbS and correlating these properties with the photoefficiency of the heterojunction with TiO₂.

Eosin (EOS) was used as a model molecule for investigating mechanisms of degradation under a variety of conditions. The PbS/TiO₂ system was compared to other heterojunctions previously studied [9], [10] using both visible and UV–vis irradiation providing an overview of the contribution of the electromotive force and the band gap energy to the performance enhancement of the heterojunctions.

Section snippets

Experimental

PbS was prepared by a direct precipitation process in which Pb(NO₃)₂ was added drop wise to an alkaline solution containing Na₂S until a 1:1 molar ratio of Pb²⁺ and S²⁻ was reached. The resulting precipitate and solution was heated at 80 °C for 1 h, filtered, washed several times with distilled water and then acetone and was then dried at 110 °C overnight. The resulting black powder was heated at 250 °C for 1 h. TiO₂ was purchased from Degussa (TiO₂-P25) and used as received. CdS, Bi₂S₃, Bi₂O₃ were

PbS characterization

Fig. 1 shows the powder XRD pattern of PbS after heat-treatment at 250 °C. All the peaks were indexed according to JCPDS card (5-0592) indicating single phase formation. PbS crystallized in a face centered cubic system (rock salt NaCl structure). The cell parameter (a = 0.5945 nm) was determined from the most intense peak (2 2 0), which is in good agreement with that reported in the literature [15]. The crystallite size was determined from the Scherrer formula using the full-width at half-maximum

Conclusion

PbS was prepared by a direct precipitation process. XRD analysis showed a single phase formation and a crystallization in a face centered cubic system. A crystallite size averaging 4 nm was obtained. An E_g value of 0.41 eV was obtained which results from a direct optical transition. PbS exhibits a semiconducting behavior with an activation energy ΔE_σ equal to 0.25 eV. The photoelectrochemical study allowed us to determine a flat band V_fb of −1.03 V and a fill factor FF of 0.34. This study proved

Acknowledgement

The authors would like to thank Dr. Tom Custer from Max-Planck Institute for Chemistry in Mainz, Germany for the English improvement of the manuscript.

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Improvement of eosin visible light degradation using PbS-sensititized TiO2

Abstract

Introduction

Section snippets

Experimental

PbS characterization

Conclusion

Acknowledgement

J. Photochem. Photobiol. A: Chem.

Appl. Catal. B: Environ.

Vacuum

Mutat. Res.

Chemosphere

Appl. Catal. B: Environ.

J. Photochem. Photobiol. A: Chem.

Catal. Today

J. Photochem. Photobiol. A: Chem.

Improvement of eosin visible light degradation using PbS-sensititized TiO₂