Zirconium (Oxy)Nitrides for (Photo)Electrochemical Applications
Authors: Streibel Verena , Schönecker Johanna Leonie , Wagner Laura Idoya , Maier Thomas , Apetrei Teodor , Eichhorn Johanna , Santra Saswati , Sharp Ian D. 
Published: February 2024

Transition metal (TM) nitrides are an emerging class of (photo)electrocatalytic materials that have recently received growing research interest.1,2 In general, nitrogen-poor TM nitrides are usually refractory materials with metallic character while nitrogen-rich nitrides often possess semiconducting character.3 Hence, while the former are potential electrocatalyst candidates, the latter may qualify as photoelectrode absorber materials. For example, ZrN has recently been proposed as an electrocatalyst for both the electrochemical nitrogen4 and oxygen5 reduction reactions (ORR and NRR), while Zr3N4 and also Zr2N2O have been suggested as potential photoanode materials in photoelectrochemical water oxidation.2 In this contribution, we test these hypotheses regarding the (photo)electrochemical characteristics of Zr-based (oxy)nitrides by experiment. To this end, we investigate reactively sputtered thin films for the electrochemical NRR/ORR and the photoelectrochemical oxygen evolution reaction (OER). Previous experiments on Ta-based nitrides have shown that addition of oxygen during the reactive sputter process is necessary to access higher metal oxidation states.6 As we introduce controlled amounts of oxygen at otherwise fixed deposition conditions, we observe a transition from metallic ZrN to a disordered nitrogen-rich ZrxNy to a crystalline bixbyite-type Zr2N2O to nitrogen-doped cubic ZrO2. Crystalline Zr3N4 was not accessible under the used experimental conditions. In our experiments, we observe a lack of electrocatalytic activity for ZrN in NRR and ORR and instabilities of the disordered nitrogen-rich ZrxNy in the photoelectrochemical OER. Introducing more oxygen into the structure, however, leads to a more stable crystalline structure (Zr2N2O), the opening of a band gap in the visible range, and the emergence of photoelectrochemical activity for oxidation reactions. Based on chopped linear sweep voltammetry measurements, we show that Zr2N2O films are photoactive for the OER in alkaline electrolyte with low onset potentials, indicating an overall favorable band alignment of the material with respect to the water oxidation and reduction potentials. While the observed photocurrents are still about one order of magnitude lower than for the benchmark oxynitride photoanode TaON, further material optimization could potentially close this gap and provide a materials system functioning as sustainable photoanode.
References

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