Toward Optimized Radial Modulation of the Space-Charge Region in One-Dimensional SnO₂-NiO Core-Shell Nanowires for Hydrogen Sensing

The gas-sensing properties and mechanism and the role of the shell thickness of structurally well-defined SnO₂/NiO heterostructures are studied. One-dimensional (1D) SnO₂/NiO core-shell nanowires (CSNWs) were produced by a two-step process; single-crystalline SnO₂-core nanowires (NWs) were synthesized by vapor-liquid-solid (VLS) deposition and then decorated with a polycrystalline NiO-shell layer by atomic layer deposition (ALD). The thickness of the NiO-shell layer was precisely controlled between 2 and 8.2 nm. The electrical conductance of the sensors was decreased many orders of magnitude with the NiO coating, suggesting that the conductivity of the sensors is dominated by Schottky barrier junctions across the n(core)-p(shell) interfaces. The gas-sensing response of pristine SnO₂ NWs and SnO₂/NiO CSNWs sensors with various thicknesses of the NiO-shell layers was investigated toward hydrogen at various temperatures. The response of the SnO₂/NiO-X (X is the number of ALD cycles) CSNWs significantly depends on the thickness of the NiO-shell layer. The SnO₂/NiO-100 sensor showed the best performance (NiO-shell thickness ca. 4.1 nm), where the radial modulation of the space-charge region is maximized. The sensing response of the SnO₂/NiO-100 sensor was 114 for 500 ppm of hydrogen at 500 °C, which was about four times higher than the response of pristine SnO₂ NWs. The sensing mechanism is mainly based on the formation of a p-n junction at the p-NiO-shell and the n-SnO₂-core interface and the modulation of the hole-accumulation region in the NiO-shell layer. The remarkable performance of the SnO₂/NiO CSNWs sensors toward hydrogen is attributed to the high surface to volume ratio of the 1D SnO₂ core-NWs, the conformal NiO shell layer, and the optimized shell layer thickness radially modulating the space-charge regions.