Experimental and DFT studies of novel Z-scheme Bi-doped Bi2WO6/Bi2S3p-n/n homo/heterojunction and its application in cathodic photoelectrochemical immunosensing

Doping, as an important strategy, can change the conductivity type of semiconductor and has been used to construct p-n homojunction. However, the effect of doping style on the conductivity type has not been well ascertained. Herein, taking Bi2WO6 with layered crystal structure as the model, the effect of Bi doping type on the conductivity type of Bi2WO6 is studied through density functional theory (DFT) and experiments. Detailed investigations reveal that Bi substitutes W site rather than fills O site or interlayer, is critical for changing the conductivity type of Bi2WO6 (i.e. from n to p), thus producing a p-n homojunction structure in Bi2WO6 and enabling it cathodic photoelectrochemical (PEC) performance. To further study and improve the PEC properties of Bi-doped Bi2WO6 (marked as Bi2+xWO6), a novel p-n/n homo/heterojunction photocathode material of Z-scheme Bi2+xWO6/Bi2S3 has been synthesized by an in-situ ion-exchange reaction. Benefiting from the internal built-in electric field of homojunction and Z-scheme heterojunction, the bulk and interface charges of Bi2+xWO6/Bi2S3-3 (i.e. treated with 1 mM of Na2S) are sufficiently separated and transferred, and thus a quite high cathodic photocurrent response occurs, which is about 5 and 100 times of those induced by Bi2+xWO6 and Bi2S3, respectively. To explore the applicability of this homo/heterojunction structure, a novel "signal-on" cathodic PEC immunosensor is constructed with Z-scheme Bi2+xWO6/Bi2S3-3 as substrate and self-assembled 3D Ti3C2@Au as label for the ultrasensitive detection of neuron-specific enolase (NSE). The immunosensor exhibits high sensitivity and selectivity.