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Design and fabrication of photo-catalytic materials for efficient solar water splitting
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posted on 21.02.2017by Gujral, Satnam Singh
The efficient conversion of solar energy into clean energy remains a significant challenge to the scientific communities. Developing photo-catalytic materials that use sunlight to generate hydrogen from water splitting offers a truly renewable and sustainable route to harvest solar energy and break the dependence on fossil fuels. Photo-electrochemical water splitting over semiconducting catalytic materials and using sunlight is a promising approach, which has potential to generate H₂ and O₂ (by-product). However, this involves a complex multi-electron process driven by photo-generated electrons and holes in a semiconductor catalyst upon exposure to sunlight.
Semiconducting metal oxides, such as TiO₂ , are amongst the most widely studied electrode materials in water splitting devices. Unfortunately, they typically have large band gaps (>3 eV) and do not absorb efficiently in the visible region of the solar spectrum. Metal oxynitrides, such as TaON, have emerged as promising materials to achieve solar-driven water splitting due to a lower band gap (EG = 2.5 eV for TaON) and appropriate positioning of conduction and valence band edge levels for the hydrogen evolution and oxygen evolution reactions, respectively. However, as with most metal oxynitrides, TaON undergoes oxidative photo-degradation, a side process in which photo-generated holes (due to short diffusion lengths) in the bulk of TaON react with nitride ions to form nitrogen gas and Ta₂O₅ instead of becoming involved in interfacial water oxidation. Insulating Ta₂O₅ layers formed in this competitive side-reaction are (photo)catalytically inactive due to a large band gap of 3.9 eV. This deteriorates the performance of TaON electrodes under irradiation during water splitting and leads to poor efficiencies. Therefore, a key research goal remains to improve the performance of TaON in terms of activity, stability and efficiency, in particular by suppressing the degradation process, and to achieve enhanced visible light driven water splitting efficiencies.
The objective of this thesis was to investigate the effect of TaON surface modification by conductive coatings and integration of oxygen evolving catalysts on activity and stability. These multicomponent photo-anodes achieved enhanced water splitting efficiencies under visible light irradiation. Further, these multicomponent photo-anode materials enabled photo-induced processes to be explored, and their composition and structure were tailored for optimal catalytic performance. In this work, titania as conductive coatings and metal oxide oxygen evolving co-catalysts, viz., MnOₓ, CoOₓ and NiOₓ, have been deposited onto the surface of screen-printed TaON photo-anodes by different methods in order to optimize the device performance in terms of activity and stability.
A post-necking treatment of TaON photo-anodes with TiO₂ was optimized by applying either TiCl₄ or Ti(ⁱOPr)₂(acac)₂ solutions to the TaON films followed by calcination at 450°C in air. A 25-fold enhancement in photo-current density was obtained for TaON post-necked by ethanolic TiCl₄, while aqueous TiCl₄ and Ti(ⁱOPr)₂(acac)₂ precursors showed improvements of 8- and 6-fold, respectively (in 0.1 M Na₂SO₄, pH = 6). Optimized TiO₂/TaON films had titania loadings of 10-20 wt.% with respect to TaON. Photo-electrochemical and microscopic studies revealed that titania coatings improve the interconnectivity of the TaON particles, and thus, the conductivity of the films.
The synergistic effect of integrating inexpensive metal oxides, MOₓ (where M = Mn, Ni, Co) as co-catalysts to the TiO₂/TaON photo-anodes was investigated. Fabrication procedures were optimized to selectively functionalize the TiO₂-TaON surface with the MOₓ nanostructures. The resulting multicomponent photo-anodes, MOₓ/TiO₂-TaON, produced by using photo-assisted electrodeposition exhibit improved activity and significantly higher long-term stability during water splitting under visible light irradiation when compared to those produced in dark and parent TiO₂-TaON photo-anodes. A 15-fold enhancement in the photo-current density was achieved when a TiO₂-TaON photo-anode was modified with CoOₓ deposits by the photo-assisted electrodeposition, while TiO₂-TaON photo-anode modified with MnOₓ and NiOₓ deposits showed 7- and 6-fold improvements, respectively. During continuous water photo-oxidation, both CoOₓ/TiO₂-TaON and NiOₓ/TiO₂-TaON films showed improved stability ca 20 and 45% deterioration in the activity (in 1 h measurement at 1.5 V vs. RHE) as compared to the more than 90% deterioration in the activity for MnOₓ/TiO₂-TaON and parent TiO2-TaON films. Moreover, during 24 h long-term performance at 1.2 V, the CoOₓ/TiO₂-TaON films remains stable for up to 22 h period after an initial loss (ca 12% deterioration) in activity over the first two hours. The fine structure of MnOₓ and NiOₓ deposits was significantly degraded during the 1 h stability test that was reflected in a slow deterioration of activity, whereas only minor structural coarsening was observed for CoOₓ/TiO₂-TaON films even after 15 h.