Advances in new generation photoactive materials
Advances in New Generation Photoactive Materials.
Saint-Petersburg State University, Saint-Petersburg, Russia. email@example.com
To overcome three major challenges existing for photoactive materials: higher activity, higher sensitivity to the visible light, and higher selectivity in heterogeneous photoreactions, a new concept of the third generation of photoactive materials has been considered recently . The concept is based on the nanoconstruction of heterostructured materials as a combination of materials with different properties that results in improvement of the functional characteristics of the nanocomposite materials and enhancement of required reaction pathways comparing to the side processes. In this presentation a review of the concept and recent advances in the studies related to the third generation of photoactive materials performed in the Laboratory “Photoactive Nanocomposite Materials” of SPbSU and elsewhere in the world is given. The major attention focuses on the heterostructured materials realizing Z-scheme of two-photon excitation and charge separation, enhancement of the surface activity due to the effect of the surface localized plasmon resonance, and application of up-conversion materials to achieve higher spectral sensitivity of hybrid photoactive materials.
Second generation photoactive materials.
The major advantages achieved with the second generation photoactive materials produced by either metal or non-metal doping and co-doping, is an extension of the spectral range of the surface photochemical activity toward visible light. As demonstrated by spectral dependences of the surface selectivity (see Fig. 1), this extension is due to formation of the localized electronic states, which photoexcitation results in generation of the different types of the surface active centers (see Fig. 2).
Fig. 1 Spectral dependence of the Pt-TiO2 selectivity toward formation of hydroquinone during phenol photodegradation.
Thus, purposeful alteration of photoexcitation mechanism of such materials should result in alteration of the activity and selectivity of photoactive materials. This alteration is a distinguishable feature of the new generation materials. It can be achieved by formation of multi-component heterostructures.
Fig. 2. Mechanism of photoexcitation of doped photoactive materials resulting in the spectral selectivity.
Surface localized plasmon resonance (SLPR).
The distinguished features of the SLPR effect is the are the dependence of the position of the resonance absorption band on the size and shape of the metal nanoparticles and redistribution of the electromagnetic field around the metal particles. In other word, the metal nanoparticle serves as an wavelength selective antenna and energy re-transmitter. This phenomenon allows selectively to enhance the excitation of electronic states in photoactive semiconductor materials (see Fig. 2) as a component of the heterostructured new generation materials, due to near-resonance energy transfer (see Fig. 3).
Fig. 3 Mechanism of energy transfer from plasmonic metal states to electronic states in semiconductor.
Thus, the SLPR effect can tune and enhance visible light activity and selectivity of photoactive materials.
The summary of the recent results related to the application of SLRP effect in new generation photoactive materials obtained in the Laboratory “Photoactive Nanocomposite Materials” of SPbSU and elsewhere will be given during presentation.
The effect of up-conversion provides the ability to utilize IR-light to reach the excited states that otherwise correspond to photoexcitation with either visible or UV-light due to energy transfer processes in up-converters (see Fig 4). The possibility of energy transfer from the up-converter component to semiconductor in hybrid heterostructured photoactive materials provides the condition for excitation of the semiconductor component to initiate surface chemical sequences.
Manipulation with the concentration of active elements in up-converters allows to tune the transferred energy and therefore, provides the conditions for selective tuning and enhancement of the activity and selectivity of semiconductor surface.
Fig. 4 Mechanism of energy transfer from up-converter states to electronic states in semiconductor.
The summary of the recent results related to the application of up-conversion effect for photoexcitation of new generation photoactive materials obtained in the Laboratory “Photoactive Nanocomposite Materials” of SPbSU and elsewhere will be given during presentation.
Z-scheme of photoexcitation.
The problem of the spectral sensitization of wide band gap semiconductors and enhancment of their activity through the effective charge separation can be solved by application of the concept of heterostructured materials realizing so called Z-scheme of multi-photon excitation and charge separation at heterojunctions (see Fig. 5).
Fig. 5. Z-scheme oft wo-photon excitation and charge separation in heterostructured materials.
One of the promising systems with proper position of the energy states of the components can be the heterostructure TiO2/WO3/CdS/TiO2. The summary of the recent results related to the application of Z-scheme mechanism in new generation photoactive materials for photoelectrochemistry and photocatalysis will be given during presentation.
 N. Serpone, A.V. Emeline, J. Phys. Chem. Lett. 3 (5), (2012) 673.