Revealing the relationship of NO2 sensing with energy level in 2D van der Waals SnS1−xSex alloys

•2D van der Waals SnS1−xSex alloys with tunable Se content are synthesized via the ion-exchanged method.•The cause-and-effect of the energy level with response value for p-type SnS1−xSex van der Waals alloys is revealed.•Increasing the gap between Ef of alloys and molecular orbital of NO2 contributes to improving the response value.