With the rapid development of industrialization and urbanization, as well as huge population increases, water pollution leading to the necessity of wastewater purification and recycling became a major issue in our modern societies. This urgent demand for clean water sources has attracted attention all over the world. The traditional processes of purification are based on pollutant removal by simple transfer on an adsorbent phase, but the development of methods allowing the in situ destruction of the contaminants seems to be much more efficient and advantageous for long terms applications.

To date, photocatalysis has been considered as one of the most appealing options for wastewater treatment, due to its great potential and rather high efficiency by using sunlight to remove organic pollutants and harmful bacteria with the aid of a solid photocatalyst.

Anatase TiO2 is one of the best-understood photocatalyst prototype that shows a high activity under UV illumination but needs to be improved in its quantum efficiency [1]. Development of heterostuctured photocatalysts [2,3] by depositing metals onto the surface of semiconductors or by coupling two semiconductors with suitable band edge position can reduce recombination phenomena by controlled vectorial transfer of charge carriers into different directions. Indeed, when a wide band-gap semiconductor is coupled with a narrow-band-gap semiconductor, enhanced electron-hole separation will be achieved since the minority carriers are separated from each other at the heterojunction. Thus the hybrid semiconductor systems can promote the separation of electron-hole pairs and leads to reduction and oxidation reactions at different reaction sites, which tremendously improves photocatalytic efficiency of semiconductor nanostructures.

In order to develop efficient and reusable photocatalytic films for application in water treatment, there is a great need to tailor the chemical, structural and electronic properties of material and enhance its photocatalytic activity. The photocatalytic performance of such a system is crucially related to the size, shape and surface area of the heterostructure [2]. Tuning the composition, chemical functionality and the morphology of the network provides a flexible design strategy to optimize the performance of such materials.

In order to ensure the success of the project, the PhD student will:

(1) Make an exhaustive review of the state-of-the-art related to heterostructured metal oxides (e.g. TiO2/NiO, ZnO/NiO, BiVO3/CuNbO3)  semiconductors by wet chemical routes for water treatment applications and select the most suitable candidates for the structuration process ;

(2) Develop adequate protocols and start the synthesis of the morphology-controlled heterostructures;

Two different approaches based on wet-templating processes could be envisaged:

The first one relies on arrays of oxide nanorods functionalized with co-catalysts.
1D nanostructures present a large surface-area-to-volume ratio as well as interesting intrinsic optical and electronic properties. Nanosphere lithography is a very promising structuration approach due to its compatibility with wafer-scale processes and low-cost [4]. It is based on the use of sacrificial nanobeads monolayers (such as polystyrene nanospheres) to tune the density of nanowires, which may influence the efficiency of the applications.

The second approach consists in the fabrication of (hierarchical) porous metal oxides containing different scales of pores by wet-chemical processes [5]. Moreover, an additional increase in the light harvesting efficiency can possibly be achieved due to photonic or backscattering effects resulting from periodic macroscopic structures. A strategy for the preparation of hierarchical metal oxides framework can be to spray a suspension containing both a metal oxide precursor and a structuring agent (e.g. polymer nanospheres or block copolymer). The porosity of the porous host scaffold is then freed by calcination. Finally, the colloidal crystal template can be impregnated with another mesoporous metal oxide phase and forms a three-dimensional hierarchical hetero-structured material after template removal.

3) Characterize the chemical, structural and morphological properties of the layers  (XRD, Raman, IR, SEM, TEM, 3D tomography, poro-ellipsometry, AFM,…)  as well as their opto-electronic properties using surface science techniques as mostly photoemission. The candidate will also study the adsorption and photodegradation of the pollutants.

The research will involve two Academic research groups with a wide interdisciplinary expertise (ULg-GREENMAT & TUDA surface science division), fully exploiting the complementary know-how of the partners in synthesis and characterization.
Moreover, the research will be performed in a close collaboration with the industrial partner Aquatic Science,  already active in the development of UV photoreactors for the treatment of water. Depending on the discussions with AquaticScience, the above-mentioned routes will be adapted to the practical issues and to the industrial’s expectations.

The control of the architecture of the heterostructured metal oxides materials through templating processes will bring a significant and innovative contribution to the field of chemistry of materials and will open new pathways in many other research fields (e.g. solar cells, batteries). This will broaden the scientific background of the candidate.

[1] M. N. Chong et al., Recent developments in photocatalytic water treatment technology: A review, Water research, (2010) 44, 2997
[2] S. Dong et al., Recent developments in heterogeneous photocatalytic water treatment using visible light-responsive photocatalysts: a review, RSC Adv., (2015) 5, 14610
[3] R. P. Daghrir, Modified TiO2 For Environmental Photocatalytic Applications: A Review, Ind. & Engin. Chem. Res., (2013) 52, 3581.
[4] P. Colson et al., ZnO Nanowire Arrays with Higher Surface Area and Reversible Wetting Properties Manufactured by Combined Nanosphere Lithography and Hydrothermal Growth J. Mater. Chem., (2012) 22, 17086
[5] C. Henrist et al., Hierarchical Porous TiO2 thin films by soft and dual templating: A quantitative approach of specific surface and porosity Thin solid films, (2013) 539, 188


Project Partners

GREENMAT Laboratory, University of Liege, Belgium: metal oxides coatings by wet-deposition processes, structural and morphological characterization

Surface Science Division, Department of Materials Science, Technical University (TU) Darmstadt, Germany: electronic characterization

Industry Partner: Aquatic Science, Belgium: design of water treatment system - conception of UV photoreactors

Host Country (employment): Belgium

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