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'Black' TiO2 -in the widest sense, TiO2 reduced by various treatments-has attracted tremendous scientific interest in recent years because of some outstanding properties; most remarkably in photocatalysis. While the material effects visible light absorption (the blacker, the better), black titania produced by high pressure hydrogenation was recently reported to show another highly interesting feature; noble-metal-free photocatalytic H2 generation. In a systematic investigation of high-temperature hydrogen treatments of anatase nanoparticles, TEM, XRD, EPR, XPS, and photoelectrochemistry are used to characterize different degrees of surface hydrogenation, surface termination, electrical conductivity, and structural defects in the differently treated materials. The materials' intrinsic activity for photocatalytic hydrogen evolution is coupled neither with their visible light absorption behavior nor the formation of amorphous material, but rather must be ascribed to optimized and specific defect formation (gray is better than black). This finding is further confirmed by using a mesoporous anatase matrix as a hydrogenation precursor, which, after conversion to the gray state, even further enhances the overall photocatalytic hydrogen evolution activity.

Over the past five years a number of different synthesis approaches has been reported to obtain so-called ‘black’ titania. One of the outstanding features of the material is that certain synthesis processes lead to the formation of an intrinsic co-catalytic center and thus enable noble-metal free photocatalytic H2-generation. In the present work, using TiO2 nanotube layers, we compare three common ‘blackening’ approaches, namely i) the original high-pressure hydrogenation (HPT-H2), ii) a classic high temperature reduction in Ar, and iii) an electrochemical (cathodic) reduction. We demonstrate that except for high pressure hydrogenation also cathodic reduction leads to an activation of TiO2 - that is, it exhibits noble-metal-free photocatalytic H2 generation. Moreover, we show that a combination of cathodic reduction/high pressure hydrogenation leads to a synergistic effect, that is, a significant enhancement of the combined co-catalytic activity.

The high-pressure hydrogenation of commercially available anatase or anatase/rutile TiO2 powder can create a photocatalyst for H2 evolution that is highly effective and stable without the need for any additional co-catalyst. This activation effect cannot be observed for rutile; however, for anatase/rutile mixtures, a strong synergistic effect can be found (similar to results commonly observed for noble-metal-decorated TiO2). EPR and PL measurements indicated the intrinsic co-catalytic activation of anatase TiO2 to be due to specific defect centers formed during hydrogenation. These active centers can be observed specifically for high-pressure hydrogenation; other common reduction treatments do not result in this effect.

Summary We analyze a class of linear-quadratic optimal control problems with an additional L1-control cost depending on a parameter β. To deal with this nonsmooth problem, we use an augmentation approach known from linear programming in which the number of control variables is doubled. It is shown that if the optimal control for a given is bang-zero-bang and the switching function has a stable structure, the solutions are Lipschitz continuous functions of the parameter β. We also show that in this case the optimal controls for β * and a with | β − β * | sufficiently small coincide except on a set of measure . Finally, we use the augmentation approach to derive error estimates for Euler discretizations. Copyright © 2014 John Wiley & Sons, Ltd.

The Front Cover shows the classic hydrothermal equipment needed to synthesise TiO(2) nanostructures versus the simple room temperature, “test tube” approach described in the article on the synthesis of titanium dioxide ‘hedgehog’ nanowires (SEM picture). These different morphologies were then grown on conductive substrates and tested for their photoelectric properties. More information can be found in the Communication by C. Schneider et al. on page 817 in Issue 7, 2019 (DOI: 10.1002/open.201900013).[Image: see text]