One of the main reasons for the limited efficiency of photocatalytic water splitting devices is the recombination of excited electrons and holes in the photocatalyst. One way to suppress this recombination is the vectorial separation of free charges. This can be achieved through the combination of materials with different band energy positions leading to an internal electric field. This field enhances electron transfer to one material, while holes are transferred to the other material. Thus, the vectorial separation decreases the probability of charge recombination.

For development of functional material combinations, the knowledge if charge transfer between the materials occurs and in which direction this transfer takes place is crucial. The efficiency of charge separation not only depends on the differences of the band energy positions of the two materials but also on the interface between them. Defects in the crystal structure can serve as recombination centers decreasing the overall efficiency.

In the present study, we analyzed the charge transfer between TiO₂ and WO₃ as a reference system, because both directions of charge transfer between these two materials are reported in literature. For this purpose, we applied the novel two-step layer transfer process for fabrication of highly porous nanoparticle layers on various substrates [1]. Here, nanoparticles are synthesized in the gas phase via Flame-Spray-Pyrolysis and accumulated on a filter paper. The aerosol deposition enables the combination of various materials within different sections of the nanoparticle layer. In a second step, the filter deposit is transferred to diverse substrates through mechanical compression, leading to mechanically stabilized highly porous percolating nanoparticle layers withstanding liquid environments. With this process, we synthesized layers of pure TiO2 and WO3 as well as layers with the combination of these two materials and transferred them to fluorine doped tin oxide coated glass slides. The material combinations were either simply one material on top of the other or material gradients with increasing amount of one material and decreasing amount of the other in the direction of the layer’s height. These layers were applied as electrodes in a photoelectrochemical cell to investigate the influence of the material combinations on the occurring photocurrents. From these measurements, the effect and the direction of charge transfer between these two materials can be determined. 

[1] S. O. Schopf, S. Salameh, and L. Mädler, "Transfer of Highly Porous Nanoparticle Layers to Various Substrates through Mechanical Compression," Nanoscale, 2013, 5(9), 3764-3772.