Silver-modified titanium dioxide thin films for efficient photodegradation of methyl orange

doi:10.1016/S0926-3373(02)00233-3

Applied Catalysis B: Environmental

Volume 42, Issue 2, 8 May 2003, Pages 187-201

https://doi.org/10.1016/S0926-3373(02)00233-3 Get rights and content

Abstract

Silver-modified, rough, high surface area titanium dioxide thin films resulting via a two-step dipping and UV-irradiation process were examined for their catalytic activity towards photodegradation of methyl orange (MO). Optimization of the photocatalyst’s performance as a function of the dipping time, irradiation time and the dipping solution concentration was performed. The optimum silver nitrate concentration of the dipping solution was found to be 10⁻³ M. The modified materials present enhanced photocatalytic efficiency and can decompose the organic pollutant three-times faster than the undoped original films (Degussa P25). A further Ag⁺-ion concentration increase in dipping solution results a decrease of the films photocatalyst efficiency due to a shading of the available semiconductor surface by the silver layer. This performance is consistent with the unique structural, morphological, and surface characteristics of the composite silver/titania materials. The lower the average particle size, roughness and fractal dimension, the higher the photodegradation percentage and rate constants. The surface doping effect is synergetic to the charge separation process and the photocatalytic results are explained on the basis of a mechanism involving efficient separation of electron–hole pairs induced by silver-ions (Ag⁺). Reproducibility tests proved that the photocatalytic activity of the silver-modified films remains intact even after six consecutive experiments of new added pollutant quantities.

Introduction

TiO₂ heterogeneous photocatalysis has been the subject of numerous investigations in recent years as it is an attractive technique for the complete destruction of undesirable contaminants (pollutants) in both the liquid [1], [2], [3] and gaseous phase [4], [5] by using solar or artificial light illumination. Titania photocatalysis advantages, such as low operation temperature, low cost, significantly low energy consumption, have led the relevant applications to the stage of commercialization [1], [6].

Recent investigations on TiO₂ photocatalysis are oriented towards the photocatalyst immobilization in the form of a thin film. This technique enables industrial application benefits [5], [7] eliminating the majority of the problems encountered with slurries: (a) the need for separation or filtration steps, (b) the problematic use in continuous flow systems and (c) the particles aggregation, especially at high concentrations. However, it is well established that the efficiency of the immobilized systems is significantly lower than that of the corresponding slurries. An efficiency increase places the photocatalyst immobilization technique even more appealing for wide range applications. A proficient way to enhance the photocatalytic reaction rate is the addition of transition metals to the semiconductor [8]. So far noble metal cations doping has been performed on TiO₂ matrixes [9], [10], [11], [12] in order to hinder the photogenerated electron/hole pair recombination and accelerate the photoexcitation and formation of oxidizing species. In particular, deposition of silver on titania has been of considerable interest for both mechanistic [13], [14], [15] and applicability [16], [17] reasons. It is experimentally evident [18], [19] that silver particles can act as electron traps aiding electron–hole separation. Silver particles also facilitate electron excitation by creating a local electrical field. Dopant incorporation in titania during catalyst preparation processes is described to increase the total surface of titania exposed [20].

This work aims to develop new composite immobilized photocatalysts of increased efficiency, consisting of silver particles photochemically deposited on rough high surface area nanocrystalline TiO₂ thin films. The surface modified catalyst is characterized by means of spectroscopy and microscopy in order to elucidate the silver valence state and the morphological structure. To evaluate the photocatalytic efficiency of the silver-modified catalyst, the photodegradation of the well known organic azo-dye methyl orange (MO), a typical pollutant in the textile industry [21], was investigated as model compound under near UV irradiation (350 nm).

Section snippets

Materials

Methyl orange (4-[[(4-dimethylamino)phenyl]-azo]benzenesulfonic acid sodium salt) and all reagents, utilized for the synthesis of TiO₂ thin film catalysts were of analytical grade from Fluka (Switzerland). Silver nitrate (AgNO₃) was purchased from BDH Laboratory supplies (England). Ultrapure water was obtained by a USF Purelab plus (Germany) apparatus. Drying N₂ stream was of 99.999% purity.

Titania films

Opaque Degussa P25 TiO₂ thin films were prepared on optically transparent microscopy glass substrates

Catalyst characterization

The doctor-blade technique [25] can be easily employed as a fast and non-energy consuming procedure to mass production of titania thin films (of ∼3.5 μm in thickness) with good uniformity and reproducible properties. After dipping of the TiO₂ films in the silver nitrate solution and subsequent UV irradiation at 254 nm for 2 h, the photocatalyst appears dark brown. Increasing the Ag⁺ concentration of dipping solution produces darker films at a gradient rate. Macroscopic observation of the film

Conclusions

Doctor-blade titanium dioxide Degussa P25 films developed on microscope glass slides were modified by silver deposition, characterized and successfully tested for the photocatalytic degradation of the pollutant methyl orange. The improvement in the photocatalytic efficiency of titania films by silver-ions deposition is about 100% and can be compared with the slurry’s performance. This is twice the improvement found by Lee et al. [45] and eight times greater than that reported by Herrmann et al.

Acknowledgements

We appreciate valuable assistance from Fotini Papadimitriou for obtaining the AFM images. Thanks must be addressed to Delis AE Athens, Greece and Degussa AG Frankfurt, Germany for generously providing the TiO₂ Degussa P25 powder. Financial support from Greece–France bilateral cooperation (PLATON) and NATO (EST.CLG.976641 grant) is also greatly acknowledged.

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Citation Excerpt :
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Silver-modified titanium dioxide thin films for efficient photodegradation of methyl orange

Abstract

Introduction

Section snippets

Materials

Titania films

Catalyst characterization

Conclusions

Acknowledgements

J. Photochem. Photobiol. C Photochem. Rev.

J. Photochem. Photobiol. A Chem.

J. Photochem. Photobiol. A

J. Photochem. Photobiol. A Chem.

Mater. Sci. Eng. B

Appl. Catal. A Gen.

J. Catal.

J. Photochem. Photobiol. A Chem.

Appl. Catal. B Environ.

Mutat. Res.

Thin Solid Films

Chem. Phys. Lett.

J. Catal.

Appl. Surf. Sci.

J. Catal.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

J. Photochem. Photobiol. A Chem.

Catal. Today

Appl. Catal. B Environ.

Chemosphere

J. Photochem. Photobiol. A Chem.