PhD Defence Kai Han

Photocatalytic overall water splitting using modified SrTiO₃

Kai Han is a PhD student in the PhotoCatalytic Synthesis (PCS) group. His supervisor is prof.dr. G. Mul from the Faculty of Science and Technology. 

Solar driven water splitting is considered a promising way to produce renewable energy-based resources. Among other materials, SrTiO₃ has previously been shown to be capable of driving photocatalytic overall water splitting. Usually the advantages of SrTiO₃ are summarized by the following: i) SrTiO₃ is a earth-abundant and non-toxic material with good stability under illumination; ii) it has a suitable band gap to allow for solar light absorption and the positions of the valence and conduction band straddle the thermodynamic redox potentials of water reduction and oxidation; iii) the typical ABO₃ structure allows for modification of its electronic structure by substituting the metal cations in A or B position. In this thesis, SrTiO₃ is also used as the photocatalyst for water splitting. Due to these benefits, SrTiO₃ is also used as the light absorbing material/photocatalyst in this thesis. Several strategies have been applied to understand particular functions of the SrTiO₃-based photocatalyst and improve the performance for POWS. Surface modification by Ni/NiO co-catalysts is shown to improve reaction kinetics, the incorporation of Mg into the SrTiO₃ bulk is shown to increase charge separation and the deposition of CrO_x allows for improvements in photocatalyst stability.  Generally, the effect of the various modifications on the photocatalytic gas evolution rates has been revealed by reliable on-line GC measurements. Due to the fast detection mode of the GC, the applied setup allows to determine transients in gas-evolution to reveal activity and stability of the tested photocatalysts. In the following the individual chapters are shortly summarized, before several recommendations are given. In contrast to chapter 6, the outlook and the recommendations are more specific. Here recommendations are given providing guidelines to further progress the development and understanding of, in particular SrTiO₃-based, photocatalysts for overall water splitting.

In chapter 2, surface modification of SrTiO₃ is achieved by deposition of core/shell Ni/NiO structures. The performed photocatalytic water splitting measurements show that the composite 1) produces non-stoichiometric product mixtures and 2) continuously deactivates. With the help of TEM and XPS measurements, the transients in hydrogen evolution rates, and corresponding morphological changes of Ni/NiO core-shell particles are explained by the in situ formation of NiOOH upon illumination. Moreover, the metallic Ni core is proposed to serve as sacrificial agent in the water splitting process, and during regeneration. Theoretically, the photocatalytic reaction should stop when all the Ni is sacrificed. Certainly long term experiments and in-situ studies are required to further explore the dynamic behavior of Ni/NiO core/shell co-catalysts.

In chapter 3, a simple two-step solid state method has been described to prepare Mg-modified SrTiO₃ photocatalysts. As shown by XRD and Raman characterization, variation of Mg-content results in materials consisting of various phases. Only for a stoichiometric ratio of (Mg+Ti):Sr, phase pure Mg:SrTiO₃ materials are obtained. After deposition of Ni/NiO or Pt co-catalyst, the synthesized materials show dramatic improvement in the photocatalytic overall water splitting (POWS) rates compared to unmodified SrTiO₃. Among all the Mg-modified SrTiO₃ samples, pure ABO₃-phase type of Mg:SrTiO₃ provides the highest steady-state apparent quantum yield of 9.1% (based on a range of 300-400 nm solar light illumination). Still significant deactivation during the initial hours of illumination is observed likely due to a similar rearrangement of the co-catalyst as discussed in chapter 2. Mott-Schottky measurements and photocatalytic reactions in the presence of sacrificial agents indicated that the beneficial effect of Mg is due to an extension of the depletion layer (from 25 to 50 nm), allowing for better oxygen evolution. The results also indicate that the electron transfer to the surface is limited and the presence of an appropriate interface with a hydrogen evolution catalysts is of importance. It needs to be emphasized that Mg is most probably leaching out of the photocatalyst during washing. Still, as observed later (chapter 4), leaching also occurs during pre-conditioning (reactor purging overnight), so the Mg content in the material must be verified at different steps of the photocatalytic water splitting procedure. Also, further measurement needs to be conducted to fully reveal how  Mg is incorporated in the SrTiO₃ structure.

In chapter 4, Cr₂O₃ is photodeposited on NiO_x modified SrTiO₃ and Mg:SrTiO₃ materials. TEM and TEM-EDX measurements indicate that Ni and Cr are well distributed on the surface. Furthermore, photocatalytic measurements reveal that an apparent quantum efficiency of 30% is obtained for the Mg:SrTiO₃-NiO_x-Cr₂O₃ catalyst under 365 nm LED light illumination. The increase is assigned to an improved stability of the material. The stability is preserved for at least 70 hours. CrO_x is usually reported to be a protective layer used to prevent the back reaction of formed H₂ and O₂. However, for the Mg:SrTiO₃-NiO_x-Cr₂O₃ catalyst, this is only of minor importance. The analysis of the reaction solution by ICP at different stages of the photocatalytic process reveals that Cr₂O₃ prevents both Mg and Ni from leaching to the solution, in turn improving the stability of the material and maintaining its outstanding photocatalytic properties. A detailed investigation to understand the mechanism preventing leaching of Mg and Ni still has to be performed, particularly as the most recent result (in-situ ICP analysis) also indicate leaching of Sr is significant.

In chapter 5, a comparative performance analysis of two emerging semiconductor materials, namely Al:SrTiO₃ and Mg:SrTiO₃, is performed. The direct comparison of materials prepared in different laboratories (Al:SrTiO₃ prepared in the group of F. Osterloh, UC Davis) highlights the importance of reaction conditions when comparing photocatalyst efficiencies. As shown, the photocatalytic performance of both materials, Al:SrTiO₃ and Mg:SrTiO₃, is very similar, but their individual optimal activity is obtained at slightly different reaction conditions, e.g. photocatalyst concentrations. The measurable difference in their individual optimal efficiency might due to the difference in materials’ light absorption and particularly in the properties of the dispersion of the particles. Further work has to be done to fully reveal the light distribution within the reactor and in-situ measurements of the absorption and scattering behavior of the two different semiconductors must be performed.

Finally in chapter 6, recent literatures based on frequently applied co-catalysts (Pt, Rh/Cr₂O₃ or Ni/NiO) systems are summarized, discussed and similarities with other fields of research and necessary advances in the field are outlined, especially in the development of a better understanding of co-catalyst/semiconductor interfaces.