Effects of metal doping (Cu, Ag, Eu) on the electronic and optical behavior of nanostructured TiO2
Graphical abstract
Metal doping of TiO2 by Cu, Ag and Eu leads to the formation of different configurations for the metal environments as function of the metallic element and contribute to modulate the photoinduced electronic and optical responses of the doped structures.
Introduction
Titanium dioxide (TiO2) is known as one of the most popular photoactive materials which has emerged as an excellent photocatalyst for environmental preservation and generation of a new source of sustainable energy such as hydrogen production [1]. For environmental applications it has been used for the degradation of air pollutants, for water purification, removal of residual pesticides, self-cleaning glass, etc. Moreover, TiO2 has been widely applied for water splitting and in photovoltaic process in dye sensitized solar cells since 1991 [2], [3], [4].
The photocatalytic process leading to the photodegradation of organic pollutants in solutions occurs when TiO2 is emerged in the solution and submitted to radiations with energies equal or higher than the band gap; i.e. UV radiation is required due to the optical band gap of 3.2 eV for anatase polymorph [5], [6]. In such conditions, photogenerated electron-hole pairs are created with the electrons being promoted in the conduction band (CB) and holes in the valance band (VB). Depending on the lifetime of the electron-holes pairs, the excited CB electrons and VB holes can either recombine and then dissipate the absorbed energy as heat or photons or catalyze redox reactions which occur at the interfaces between the semiconductor (SC) and the surrounding media.
The incorporation of noble metal nanoparticles (NPs) into the host matrix TiO2 is a recent strategy to overcome the problem of low lifetimes for the photogenerated electron-hole pairs. This criterion improves the photocatalytic activity of metal doped titanium dioxide [7]. Doping with metal ions has an effect on the photocatalytic activity of TiO2. Some previous reports underlines that metal doping contributes to lower the threshold absorption energy of doped TiO2 able to enhance the photocatalysis efficiency. Moreover, other references on doping effects point out the tendency of doping sites to act as recombination centres for electrons and holes contributing then to increase the lifetime of photogenerated electron-hole pairs with the benefit of the increasing photoactivity [8]. As a photocatalyst, TiO2 absorbs only ultraviolet region (wavelength < 390 nm) due to the wide band gap. Although, solar light contains only about 2–3% UV light, therefore, TiO2 based materials which can harvest solar light are always in need. Modifications by various strategies have been applied to make the TiO2 as visible light active material by doping with metal and non metals or co-dopoing [9].
Whatever the situation in which TiO2 based photocatalysts are used, the concentration of photogenerated charge carriers and the lifetimes of electron/hole recombination are the main parameters acting on the photocatalytic efficiency.
Also, the present work focused on comparative studies of synthesis and characterization of physical features as well as an evaluation of the photocatalytic efficiency of Cu-, Ag- and Eu- doped TiO2 nanoparticles prepared by the sol-gel method. As a doping agent, Cu has been employed in semiconducting materials where it acts as traps of charge carriers interceding then in the interfacial transfer by reducing the electron-hole recombination rates. The ionic radius of Cu2+ (0.68 Å) is quite closer to the Ti4+ (0.74 Å) therefore the incorporation of this element into TiO2 is expected to be of a substitutional nature in the crystalline sites. This situation contributes to modify the electronic density of states inside the band gap of a pure structure [10]. For Ag doping, the tendency is to form Ag rich clusters superimposed to TiO2 particles with the involvement of plasmon resonance at the interfaces. The co-existence of Ag-clusters with TiO2 contribute in one hand to enhance the electromagnetic field at the interfaces and reinforcing the photoinduced charge carriers and on the other hand they promote interfacial charge-transfer process limiting also the recombination rates. Indeed, electronic band alignment at the interfaces Ag-clusters-TiO2 favors the migrations of the photogenerated electrons to metallic particles. This process increases the lifetime of holes leading then to the redox reactions required for the PC oxidation of organic pollutants [11], [12] as it was also demonstrated by our group on other family of photoactive oxides as BiVO4. For lanthanide ions, they are well known for their ability to form complexes with various Lewis bases (e.g., acids, amines, aldehydes. Alcohols, thiols, etc) in the interaction of these functional groups with the f-orbitals of lanthanides [13]. Eu ions doping may contribute to locate some Eu rich composition on the surface of TiO2. In such situation, organic pollutants may have tendency to concentrate on the semiconductor surfaces where the probability is enough high to fix easily superoxide or hydroxyl radicals [14]. Moreover, as earlier reported [15], lanthanide ions allows an efficient separation of electron-hole pairs in accordance with photocurrent response measurements which might also apply to photocatalytic process.
So, as outlined above for the different considered metal doping elements, the expected behavior for the mode of doping may be different. Substitutional doping can be realized for Cu but for Ag, it will have tendency to form clusters while Eu ions may adopt different configurations (substitution, phase segregation). The originality of the work is to address the main features of the doping process as function of the metal nature and to evaluate its consequence on the physical response related to TiO2 driven heterogeneous photocatalysis.
Section snippets
Material synthesis
Pure and M-doped TiO2 powders, were synthesized by soft chemical sol-gel method using titanium (IV) isopropoxide as a precursor. The anatase phase was obtained using different annealing temperatures, depending on the dopant. Titanium (IV) isopropoxide (TTIP), was purchased from Strem Chemicals and the acetic acid from Fisher Scientific. The solvent 2-Propanol (99.5%) and anhydrous copper (II) acetate (98%) were purchased from Alfa Aesar GmbH while hydrated europium (III) acetate (99.9%) and
Conclusion
Comparative investigations were devoted to metal doped TiO2 where doping process has shown versatile behavior and dependence on the nature of the used elements. Even with different doping elements which apparently can adopt the same valence state (2+) such as (Cu2+, Ag2+, Eu2+), different behaviors were demonstrated for the effective incorporation of these ions in the host structure of TiO2. The discrepancy between ionic radii of the different used elements modulates the ratio of the
Acknowledgment
Jesus Vargas would like to thank CONACYT - Mexico for scholarship and the doctoral school 3MPL University du Maine – France for providing facilities in IMMM Institute UMR CNRS 6283 and financial support during the PhD thesis cotutelle between CIITEC-IPN and University du Maine.
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