Interfacial nitrogen modulated Z-scheme photoanode for solar water oxidation

Abstract

The fabrication of heterojunctions with an appropriate energy band alignment is an attractive strategy to achieve highly efficient solar water splitting. Despite unique advantages, conventional type II heterojunctions face the problem of a relatively low redox power of the charge carriers. Although Z-scheme systems can overcome this problem, their introduction to photoelectrochemical (PEC) devices is challenging due to the specific band alignment. Herein, as an example, an ultrathin MoO x N y layer is deposited on the surface of TiO 2 nanoarrays via an in situ conversion process, and the optimized MoO x N y /TiO 2 sample generates a photocurrent density that is approximately 2.4 times that for pure TiO 2 . In addition, a similar surface modification leads to 2.5- and 2.6-fold higher photocurrent values for BiVO 4 and WO 3 nanostructures, respectively. Interestingly, the impedance of the MoO x N y /TiO 2 photoanode is larger than that of the pure TiO 2 in the dark, while illumination reverses the trend. This phenomenon, combined with the measured flat band positions, suggests a Z-scheme mechanism, which is further supported by surface potential measurements. The nitrogen (N) atoms play a major role in the boosted photoconversion efficiency for the MoO x N y /TiO 2 Z-scheme heterojunction, not only enhancing the light sensitivity but, importantly, substantially promoting charge separation. • Z-scheme MoO x N y /TiO 2 in situ constructed with 2.4 times photocurrent of pure TiO 2 . • Z-scheme structure could be supported by the CPD shift value from KPFM. • Control experiment reveals the vital role of N atoms for Z-scheme construction. • 2.5- and 2.6-fold photocurrent values realized for BiVO 4 and WO 3 , respectively.