Publication date: 15th December 2014
The semi-conductor TaON is an efficient photocatalyst for visible-light-driven water splitting. However, further activity and stability enhancement is desired. Especially the oxidation of water can be improved by the deposition of co-catalysts that act as reaction centers and hole scavengers. We have studied the effect of different deposition methods for MnOx on phase-pure TaON produced via ammonolysis of Ta2O5 at elevated temperatures. Different synthesis protocols lead to differences in the properties of MnOx, which were carefully analyzed using various methods including high-resolution TEM and synchrotron XPS before and after catalysis. Finally, the composite catalysts were tested in the oxygen evolution reaction to establish correlations of synthesis parameters, material properties and catalytic activity.
One series was produced by impregnation with Mn(II)-nitrate and subsequent calcination at 300°C. Two other series were obtained using the pH-controlled symproportionation of KMnO4 and Mn(II)-nitrate yielding Mn(IV), followed by calcination at the same temperature. Both synthesis methods yield electrocatalytically active Mn(III)/Mn(IV) species. The dispersion of MnOx species was found to be very sensitive to the base used during symproportionation to control the pH during synthesis (high dispersion for NH3 and low dispersion for NaOH). Furthermore, the Mn-loading was varied as an additional parameter.
The activity of the resulting Mn/TaON-photocatalysts was studied under illumination with visible light using an MS-coupled batch reactor containing silver nitrate as a sacrificial electron acceptor. The initial oxygen production rates showed that the activity of the MnOx/TaON system was doubled compared with the pristine TaON for a Mn-loading as low as 0,1 wgt.% using both impregnation and symproportionation synthesis with NH3. These samples showed a high dispersion of Mn, uniform distribution at the surface and high contact area with TaON. No such highly active system was obtained with NaOH, where larger and segregated MnOx particles were found. In all cases, larger aggregates were also observed when the loading was increased. These can form a light-absorbing layer on the surface of TaON and inhibit the photocatalytic reaction.
These results highlight the need for a thin and homogenous MnOx-layer in this system. By successful deposition of such a co-catalyst layer, the photocatalytic activity of MnOx/TaON was substantially enhanced. In addition, selected MnOx/TaON powders were deposited on FTO coated substrates by electrophoretic deposition and their PEC performance for light-induced water oxidation evaluated.
This work is supported by the German Science Foundation (DFG) in the frame of the priority program 1613.
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