Photocatalytic water splitting

Energy from the Sun can easily provide enough power for all of our energy needs if it can be efficiently harvested. While there already exist a number of devices that can capture and convert electromagnetic energy, the most common—a photovoltaic cell—produces electricity, which must be used immediately or stored in a secondary device such as a battery or a fly-wheel. A more elegant, practical, and potentially more efficient route to storing solar power is to convert the electromagnetic energy directly into chemical energy in the form of molecular bonds, analogous to the photosynthesis process exploited by nature.The photoelectrochemical (PEC) water splitting and photocatalytic (PC) solar energy conversion are the most appealing pathway for artificial photosynthesis. Indeed solar water splitting would form the basis for a sustainable hydrogen-based energy economy. [1]

Hierarchical hematite photoanodes by Plasma Enhanced Chemical Vapor Deposition (PE-CVD)
A family of new hierarchical hematite photoanodes for PEC water splitting are under development. The materials are synthesized by using a plasma enhanced chemical vapor deposition apparatus. The different structures obtained are characterized by a structural organization on multiple length scale and present buiding blocks that can be tuned in dimension and shape.

Plasma Enhanced Chemical Vapor Deposition (or Plasma Assisted OM-CVS organo-metallics chemical vapor synthesis) methodologies offer some peculiar and interesting advantages with respect other more “classical” oxide synthetic pathways: synthesis is relatively simple, mostly one step, it is a full dry process, the high energies, at low temperature, can lead to non equilibrium phases/morphologies, and dopants can be easily added.

The CVS (Chemical Vapor Synthesis) apparatus used for PE-CVD is composed by three stages:

• Sublimation Unit – An OrganoMetallic (OM) precursor is sublimated and the vapors are driven in the next stage by an orthogonal Ar gas flow and by an axial pulsed high pressure Ar gas flow
• Plasma Reactor – The OM precursor is injected inside a cold RF Plasma (13.56 MHz – 30W) fed by an Ar/O2 gas mixture (10%) or Ar/H2 gas mixture
• Deposition Unit – The target substrate is placed orthogonally in the plasma plume and a metallic or metal oxidefilm grow on crystalline and nanostrcure.

In the example below: an α-Fe2O3 photoanodes have two levels of hierarchical organization on multiple length scales: the smallest building blocks (tens of nm) assemble in the nanoplatelets (hundreds of nm) that constitute the nanofeatured film. The hematite grow as single hyper branched nanoplatelets (200 to 600) nm vertically aligned with a preferential crystal growth along the [110] direction, self-organizing into high crystalline nanosheets.

Ref: 

‘Hierarchical Hematite Nanoplatelets for Photoelectrochemical Water Splitting’ – M. Marelli, A. Naldoni, A. Minguzzi, M. Allieta, T. Virgili, G. Scavia, S. Recchia, R. Psaro, and V. Dal Santo - ACS Applied Materials & Interfaces, Vol. 6, 2014, 11997.

Engineered TiO^₂ nanoparticles

Role of metal nanoparticles in tuning charge trapping properties and photoefficiency [2]. Metal-loaded TiO2 is, by far, one of the most important class of photocatalysts in hydrogen production through photoreforming of organics and water photosplitting. In this study anatase loaded with Au and Pt nanoparticles (Au/TiO2 and Pt/TiO2) by an impregnation-reduction method was investigated in the photocatalytic  hydrogen production by methanol photoreforming. The electron and hole trapping centers, Ti3+ and O−, respectively, formed under UV–vis irradiation of the photocatalysts, were studied by in situ electron spin resonance (ESR) spectroscopy. The nature of the loaded metal affected both the H2 evolution rate and the distribution of the methanol oxidation products. The better performance of Pt/TiO2 is attributable to the greater ability of Pt with respect to Au to act as electron sink, slowering the recombination of photoproduced electron–hole couples. Direct evidence of this effect was obtained by ESR analysis, showing that the amount of Ti3+ active sites follows the order TiO2 > Au/TiO2 >> Pt/TiO2, thus confirming easier electron transfer from Ti3+ to Pt, where the H+ reduction to H2 occurs.

Reduced TiO₂ and Au/Pt bimetallic nanoparticles for H₂ generation from renewables [3,4]. Bimetallic Pt-Au nanoparticles supported on reduced anatase nanocrystals represent a new class of promising photocatalysts with high activity in hydrogen production by photoreforming of aqueous solution of renewable feedstock, such as ethanol and glycerol. he presence of bimetallic Pt-Au nanoparticles and of Ti³⁺ sites / O^2- vacancies in the bulk structure of titania are two key parameters to maximize light absorption and feedstock activation, finally resulting in good photocatalytic performances.

Black TiO² [5]. Compared to bare TiO₂, defective TiO₂ is more attractive for photovoltaics, photocatalysis, and fuel cells owing to its narrower bandgap (less than typical 3 eV value), enabling absorption of visible light, and relatively high electrical conductivity. Indeed, bandgap engineering is a crucial requirement for optimizing TiO₂ solar light harvesting capability. We have demonstrated that black TiO₂ nanoparticles obtained through a one-step reduction/crystallization process exhibit a reduced bandgap. Our black TiO₂ NPs exhibit unique crystalline core/disordered shell morphology. VO’s are present in the bulk anatase crystalline phase, while the disordered NP surface appears to be nearly stoichiometric. The bandgap narrowing is dictated by the synergistic presence of VO’s and surface disorder.

Two ongoing projects consist in the following topics: (a) photocatalytic H₂ production by using black TiO₂/plasmonic nanoparticles and (b) comparison of plasmonic oxidation activity of stoichiometric, N-doped, and black TiO₂.

New research topics
In addition to the development of the projects presented above, several new research topics are starting at the moment:
1) Synthesis and characterization of low cost multilayer photoelectrodes (both the photoanode and the photocathode) for artificial photosynthesis.
2) Synthesis of new electrocatalysts for OER with controller shape and dimension to be coupled to photoelectrodes for PEC water splitting.
3) Hierarchical plasmonic complex materials based on inverse opals.
4) Physicochemical/Electrochemical and synchrotron based characterization of the semiconductor/electrocatalyst/liquid interface.

References:
[1] Gratzel et. al. ChemSusChem 2011, 4, 432 – 449.
[2] "M@TiO₂ Pt and Au@TiO2 photocatalysts for methanol reforming: role of metal nanoparticles in tuning charge trapping properties and photoefficiency"  A. Naldoni, M. D’Arienzo, R.Scotti, M. Marelli, M. Altomare, E. Selli, F. Morazzoni, V. Dal Santo, Appl. Catal, B. 2013, 130– 131, 239–248.
[3] “Bimetallic Au-Pt/TiO₂ photocatalysts active under UV-A and Vis light for H₂ production from ethanol” A. Gallo, M. Marelli, R. Psaro, V. Gombac, T. Montini, P. Fornasiero, R. Pievo and V. Dal Santo, Green Chem., 2012, 14, 330-333.
[4] “H₂ production by renewables photoreforming on Au-Pt/TiO₂ catalysts”, A. Gallo, T. Montini, M. Marelli, A Minguzzi, V. Gombac, R. Psaro, P. Fornasiero, and V. Dal Santo, ChemSusChem 5 (2012) 1800–1811.
[5] “The effect of nature and location of defects on bandgap narrowing in black TiO₂ nanoparticles”, A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, S. Cappelli, Serena; C.M. Bianchi; R. Psaro, V. Dal Santo, J. Am. Chem. Soc. 2012, 134, 7600−7603