Final Report Summary - NANOHAP (Nanocomposite approaches for the deposition of highly active photocatalytic thin films with plasma technology)
Photocatalysis is a potential tool for environmental and effluent water cleaning. Large band gap semiconductors, first of all the TiO2, are commonly employed as photocatalysts. However, most of the photocatalytic materials possess band-gaps corresponding to the Ultra-Violet (UV) wavelengths (220-380 nm) and suffer from low efficiencies due to the high degree of recombination between the photogenerated charges carriers.
Several strategies have been employed to overcome these limitations and improve the photoefficiency of TiO2, including the impregnation or surface modification with noble and transition metals. For many applications it is essential to use photocatalytically active materials in the form of thin films, e.g. on architectural glass or for their application to the water treatment by photocatalysis, where the use of thin layers prevents the costly step of separating the photocatalyst from the contacting solution. The work performed within the framework of the Marie Curie Project NANOHAP aimed at developing new thin film photocatalytic materials (i.e. TiO2 and noble metals/TiO2 systems) for water and surface cleaning purposes by using innovative processes. The fabrication of the coatings was carried out by making use of plasma technologies, and especially of sputtering methods. In fact, these are very advantageous techniques for the deposition of crystalline TiO2 thin films at low temperatures. Moreover, sputtering is also one of the most feasible method for the deposition of metal ions and oxide nanoparticles over a thin film matrix, e.g. of TiO2, due to its versatility and the capability of controlling the physical properties of the particles by a proper choice of preparation parameters.
Two approaches were envisaged in order to obtain the new materials:
(1) Synthesis and deposition of metal nanoparticles (NPs: Cu, Ag, Pt) on magnetron sputtered TiO2 thin films by using a gas phase condensation technology, namely the gas flow sputtering (GFS);
(2) Deposition of NPs layers of TiO2 by gas flow sputtering.
The research activities performed during the two-year project can be schematically described as follows:
1. Characterization of the GFS process and study/optimization of the deposition conditions for the metal NPs (Cu, Ag and Pt). The deposition rate of the metals and its dependence on parameters like the power applied to the source and the pressure of the process were in this phase analyzed.
2. Selection of the best photocatalytic substrate for the subsequent metal deposition. The TiO2 photocatalytic thin films used as reference and as substrates for the subsequent NPs deposition were prepared by magnetron sputtering in the group of Precision Coatings led by Dr. Frach. The group has deep knowledge about sputtering processes, and the deposition conditions of TiO2 films with the use of a magnetron were already optimized. For that, the dependence of the photocatalytic activity on the nature of the glass used as substrate for the TiO2 coating, and on the TiO2 film thickness, was evaluated. Particularly, glass microscope slides, floated glass (FG), and fluorine doped tin oxide (FTO) glasses were used, and TiO2 films of 100, 300, and 500 nm thickness were deposited on them by magnetron sputtering. The combination of these variables led to a total of nine different test materials. The evaluation of the photocatalytic activity was made using the Methylene-Blue (MB) and the Stearic Acid (SA) decomposition tests, while the photoinduced superhydrophylicity properties of the samples were analyzed by measuring the water contact angles (WCA) of the films during UV-A illumination. Optical measurements (transmittance and reflectance) using an UV-Vis spectrometer were also realized for all samples.
3. Deposition of Cu, Ag and Pt NPs on the TiO2 substrates. Nature of the glass substrate, sputtering time and process parameters were changed in order to determine the best deposition conditions for an optimal photocatalytic activity in each case. The resulting materials were characterized as done before for the TiO2 films without metal NPs (optical characterization, MB and SA tests, WCA measurements). Moreover, some samples were analysed by scanning and transmission electron microscopy (SEM and TEM respectively) and by X-Ray Photoelectron Spectroscopy (XPS) in order to investigate the morphology of the deposited NPs and to quantify the amount of particles and the oxidation state of the metals.
4. Deposition of TiO2 NPs layer by gas flow sputtering and analysis of their photocatalytic activity. The deposition of TiO2 was realized by using Ti targets in presence of variable contents of O2. The photocatalytic properties of the coatings and their structural/morphological characteristic were evaluated using the methods described above.
It was observed that the type of glass used as substrate for the TiO2 deposition plays an important role in the final photocatalytic activity of the samples. Particularly, the float glass resulted to be the best material and was chosen for further experiments. Actually, TiO2 films deposited on it showed the highest photocatalyic activity and reproducibility of the results.
Regarding the thickness of the TiO2 layer, 500 nm assured the highest performance of the samples, so that the glass substrates covered with 500 nm TiO2 were chosen as substrates for the next deposition experiments.
The comparison of the systems TiO2/Cu, TiO2/Ag and TiO2/Pt showed that in general, the highest improvement in the photocatalytic activity was achieved by the deposition of Pt NPs on the TiO2 films. The sputtering time plays also an important role in determining the best performance of the materials, as it determines the presence of different metal amounts on the semiconductor surface. In fact, if too many particles can block the active sites on the TiO2 surface, preventing the light to be absorbed. On the contrary, if the sputtering time is too low, most probably the number of metal particles on the surface is not enough to exert a beneficial effect on the photocatalytic activity of the TiO2.The optimal amount (i.e. the optimal deposition time) depends anyway on the sputtered metal.
TiO2 nanoparticle coatings prepared by GFS were active in the tested reactions, which confirmed the feasibility of this process for the production of new photocatalytic materials.
From what been said so far, it can be inferred that the application of the GFS technology to the deposition of modified and non-modified TiO2 thin films is a quite novel approach that would allow obtaining highly photoactive and large area coating for the purification of aqueous streams and self-cleaning surfaces. The work started during the NANOHAP project will be carried on thanks to the collaboration between the researcher and the host institution Fraunhofer FEP, and the improvement of the methodology and up-scaling of the technology appear realistic in the near future. The likelihood of using the GFS technology for preparing materials with potential photocatalytic water splitting properties is also being studied, thanks to a collaboration started during the project. Furthermore, resulting materials could also be photocatalytically active against gaseous contaminants and may have additional self-sterilizing or anti-fogging properties, which have to be still checked.
Several strategies have been employed to overcome these limitations and improve the photoefficiency of TiO2, including the impregnation or surface modification with noble and transition metals. For many applications it is essential to use photocatalytically active materials in the form of thin films, e.g. on architectural glass or for their application to the water treatment by photocatalysis, where the use of thin layers prevents the costly step of separating the photocatalyst from the contacting solution. The work performed within the framework of the Marie Curie Project NANOHAP aimed at developing new thin film photocatalytic materials (i.e. TiO2 and noble metals/TiO2 systems) for water and surface cleaning purposes by using innovative processes. The fabrication of the coatings was carried out by making use of plasma technologies, and especially of sputtering methods. In fact, these are very advantageous techniques for the deposition of crystalline TiO2 thin films at low temperatures. Moreover, sputtering is also one of the most feasible method for the deposition of metal ions and oxide nanoparticles over a thin film matrix, e.g. of TiO2, due to its versatility and the capability of controlling the physical properties of the particles by a proper choice of preparation parameters.
Two approaches were envisaged in order to obtain the new materials:
(1) Synthesis and deposition of metal nanoparticles (NPs: Cu, Ag, Pt) on magnetron sputtered TiO2 thin films by using a gas phase condensation technology, namely the gas flow sputtering (GFS);
(2) Deposition of NPs layers of TiO2 by gas flow sputtering.
The research activities performed during the two-year project can be schematically described as follows:
1. Characterization of the GFS process and study/optimization of the deposition conditions for the metal NPs (Cu, Ag and Pt). The deposition rate of the metals and its dependence on parameters like the power applied to the source and the pressure of the process were in this phase analyzed.
2. Selection of the best photocatalytic substrate for the subsequent metal deposition. The TiO2 photocatalytic thin films used as reference and as substrates for the subsequent NPs deposition were prepared by magnetron sputtering in the group of Precision Coatings led by Dr. Frach. The group has deep knowledge about sputtering processes, and the deposition conditions of TiO2 films with the use of a magnetron were already optimized. For that, the dependence of the photocatalytic activity on the nature of the glass used as substrate for the TiO2 coating, and on the TiO2 film thickness, was evaluated. Particularly, glass microscope slides, floated glass (FG), and fluorine doped tin oxide (FTO) glasses were used, and TiO2 films of 100, 300, and 500 nm thickness were deposited on them by magnetron sputtering. The combination of these variables led to a total of nine different test materials. The evaluation of the photocatalytic activity was made using the Methylene-Blue (MB) and the Stearic Acid (SA) decomposition tests, while the photoinduced superhydrophylicity properties of the samples were analyzed by measuring the water contact angles (WCA) of the films during UV-A illumination. Optical measurements (transmittance and reflectance) using an UV-Vis spectrometer were also realized for all samples.
3. Deposition of Cu, Ag and Pt NPs on the TiO2 substrates. Nature of the glass substrate, sputtering time and process parameters were changed in order to determine the best deposition conditions for an optimal photocatalytic activity in each case. The resulting materials were characterized as done before for the TiO2 films without metal NPs (optical characterization, MB and SA tests, WCA measurements). Moreover, some samples were analysed by scanning and transmission electron microscopy (SEM and TEM respectively) and by X-Ray Photoelectron Spectroscopy (XPS) in order to investigate the morphology of the deposited NPs and to quantify the amount of particles and the oxidation state of the metals.
4. Deposition of TiO2 NPs layer by gas flow sputtering and analysis of their photocatalytic activity. The deposition of TiO2 was realized by using Ti targets in presence of variable contents of O2. The photocatalytic properties of the coatings and their structural/morphological characteristic were evaluated using the methods described above.
It was observed that the type of glass used as substrate for the TiO2 deposition plays an important role in the final photocatalytic activity of the samples. Particularly, the float glass resulted to be the best material and was chosen for further experiments. Actually, TiO2 films deposited on it showed the highest photocatalyic activity and reproducibility of the results.
Regarding the thickness of the TiO2 layer, 500 nm assured the highest performance of the samples, so that the glass substrates covered with 500 nm TiO2 were chosen as substrates for the next deposition experiments.
The comparison of the systems TiO2/Cu, TiO2/Ag and TiO2/Pt showed that in general, the highest improvement in the photocatalytic activity was achieved by the deposition of Pt NPs on the TiO2 films. The sputtering time plays also an important role in determining the best performance of the materials, as it determines the presence of different metal amounts on the semiconductor surface. In fact, if too many particles can block the active sites on the TiO2 surface, preventing the light to be absorbed. On the contrary, if the sputtering time is too low, most probably the number of metal particles on the surface is not enough to exert a beneficial effect on the photocatalytic activity of the TiO2.The optimal amount (i.e. the optimal deposition time) depends anyway on the sputtered metal.
TiO2 nanoparticle coatings prepared by GFS were active in the tested reactions, which confirmed the feasibility of this process for the production of new photocatalytic materials.
From what been said so far, it can be inferred that the application of the GFS technology to the deposition of modified and non-modified TiO2 thin films is a quite novel approach that would allow obtaining highly photoactive and large area coating for the purification of aqueous streams and self-cleaning surfaces. The work started during the NANOHAP project will be carried on thanks to the collaboration between the researcher and the host institution Fraunhofer FEP, and the improvement of the methodology and up-scaling of the technology appear realistic in the near future. The likelihood of using the GFS technology for preparing materials with potential photocatalytic water splitting properties is also being studied, thanks to a collaboration started during the project. Furthermore, resulting materials could also be photocatalytically active against gaseous contaminants and may have additional self-sterilizing or anti-fogging properties, which have to be still checked.