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[Display omitted] • Operates without external bias voltage due to the photovoltaic effect in the NbSe 2 -MoS 2 junction. • Detects NH 3 gas at concentrations ranging from 10 ppm to 500 ppm with varying responses (8.8 % to 28.8 %). • The built-in electric field and charge-transfer mechanism enhance gas specificity. • The gas sensing mechanism relies on physisorption and charge transfer at the surface of the nanostructure. Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention for their optical and gas-sensing applications due to their exceptional sensitivity. Reliable selectivity and low power consumption are two major requirements for photodetector and gas sensor applications in next-generation electronic devices and the Internet of Things. Self-powered sensors (especially photovoltaic gas sensors) can solve these problems. In this study, for the first time, we report 2D TMDs (NbSe 2 -MoS 2 hybrid) on a SiO 2 /Si substrate to fabricate photovoltaic self-powered gas sensors. The gas sensors are operated by the photovoltaic effect of the NbSe 2 -MoS 2 nanostructure, which is prepared using the liquid phase exfoliation process. Initially, it was revealed that the present hybrid material exhibits photovoltaic properties under light illumination, with a circuit current of 0.25 µA and a circuit voltage of 34 mV. The NbSe 2 -MoS 2 nanostructure characteristics were then used for NH 3 gas sensing at different concentrations, and the gas sensing response was detected from low (8.8 % at 10 ppm) to high (28.8 % at 500 ppm) concentrations. The built-in electric field occurred between the NbSe 2 -MoS 2 junction and eventually operated as a driving force for NbSe 2 -MoS 2 gas sensing without an external bias voltage. The physisorption of gas molecules on their surface prompts a charge-transfer mechanism that improves the gas sensor response. The combined outcome of NbSe 2 -MoS 2 heterostructures could pave way to next-generation gas sensing device fabrications. [ABSTRACT FROM AUTHOR]