In this work we lay out design guidelines for catalytically more efficient organic photocathodes achieving stable hydrogen production in neutral pH. We propose an organic photocathode architecture employing a NiO hole selective layer, a PCDTBT:PCBM bulk heterojunction, a compact TiO2 electron selective contact and a RuO2 nanoparticle catalyst. The role of each layer is discussed in terms of durability and function. With this strategically designed organic photocathode we obtain stable photocurrent densities for over 5 h and discuss routes for further performance improvement.

Photocharge extraction in the visible-light energy range is essential to sustain efficient photoelectrochemical processes driven by solar irradiation of photoanode materials. Recent literature has shown that crystallographic orientation can have...

a b s t r a c tA kinetic study was conducted using five metallo-tetraphenylporphyrins (MTPPs) as catalysts for the aerobic oxidation reaction of ␤-isophorone (␤-IP) to ketoisophorone (KIP). The oxidation reaction catalyzed by MTPPs (M = Cr, Mn, Fe, Co and Cu) was observed over a seven hour period under a range of experimental conditions. Changes in the specificity of conversion of reactant to the product, KIP, and the main side-product, ␣-isophorone (␣-IP), were observed at temperatures ranging from 60 • C to 75 • C, in solvents with varying compositions of pyridine and ␤-IP, with different MTPP catalysts, oxygen gas flow rates, and solution agitation frequencies. Control experiments show that MTPP catalysts modestly increase the rate of KIP formation, but significantly improve specificity through the apparent suppression of ␣-IP formation. Analysis revealed that MnTPP (CH 3 COO − ) produced the highest product specificity ratio [KIP]/ [␣-IP]. Given that all of the metals were in the 3+ oxidation state in this study and there was no observed binding by O 2 to any of the MTPP catalysts, the catalytic mechanism is suggested to involve the binding of ␤-IP to the MTPP metal center. Binding of ␤-IP is most favorable when pyridine is the trans ligand, but ␤-IP is also observed to weakly bind when trans to acetate in the MnTPP adduct. This conclusion is supported by electronic absorption spectroscopy, resonance Raman spectroscopy and density functional theory (DFT) calculations. The role played by the catalyst appears to be the activation of hydrogen abstraction following the substrate's ligation to the metal, rather than the more traditional role of MTPPs as O 2 activation catalysts by ligation of diatomic O 2 . ␤-IP oxidation is an example of a case where the catalyst appears to have a more important role for improving the specificity of the reaction (through decreasing side-product formation) rather than increasing the rate of product formation.

This work reveals that photoanodes based on TiNb 2 O 7 (TNO)p owder show remarkablew ater-oxidation properties including nearly ideal charge-transfer and chargeinjectione fficiencies. Furthermore, using as implified photoanode construction and carefully surveying the structural and morphological characteristics of oriented and polycrystalline thin films and powder-based samples revealedt hat the water-splitting kinetics of TNO is negligibly effected by surface morphology; instead, internal grain boundaries likely play ad riving role. The current powder-based TNO photoanodes exhibit ideal water-oxidation kinetics and oxidize water at minimal applied biases under illumination; consequently, TNO exhibits an early onset photocurrent voltage (0.4 Vv s. RHE) that rivals that of other state-of-the-art photoanode materials. Figure 10. Linear voltage sweep comparison of a) TNO-powder-based photoanodes and 400 nm polycrystalline TNO film and b) thick (400 nm) and thin (100 nm) TNO film photoanodes (the 100 nm TNO photoanode is shownwith chopped-lightillumination for easy dark-current comparison).