The utilization of p-p isotype heterojunctions is an effective strategy to enhance the gas sensing properties of metal-oxide semiconductors, but most previous studies focused on p-n heterojunctions owing to their simple mechanism of formation of depletion layers. However, a proper choice of isotype semiconductors with appropriate energy bands can also contribute to the enhancement of the gas sensing performance. Herein, we report nickel oxide (NiO)-decorated cobalt oxide (CoO) nanorods (NRs) fabricated using the multiple-step glancing angle deposition method. The effective decoration of NiO on the entire surface of CoO NRs enabled the formation of numerous p-p heterojunctions, and they exhibited a 16.78 times higher gas response to 50 ppm of CH at 350 °C compared to that of bare CoO NRs with the calculated detection limit of approximately 13.91 ppb. Apart from the p-p heterojunctions, increased active sites owing to the changes in the orientation of the exposed lattice surface and the catalytic effects of NiO also contributed to the enhanced gas sensing properties. The advantages of p-p heterojunctions for gas sensing applications demonstrated in this work will provide a new perspective of heterostructured metal-oxide nanostructures for sensitive and selective gas sensing.

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A facile, highly efficient approach to obtain molybdenum trioxide (MoO3)-doped tungsten trioxide (WO3) is reported. An annealing process was used to transform ammonium tetrathiotungstate [(NH4)2WS4] to WO3 in the presence of oxygen. Ammonium tetrathiomolybdate [(NH4)2MoS4] was used as a dopant to improve the film for use in an electrochromic (EC) cell. (NH4)2MoS4 at different concentrations (10, 20, 30, and 40 mM) was added to the (NH4)2WS4 precursor by sonication and the samples were annealed at 500 °C in air. Raman, X-ray diffraction, and X-ray photoelectron spectroscopy measurements confirmed that the (NH4)2WS4 precursor decomposed to WO3 and the (NH4)2MoS4–(NH4)2WS4 precursor was transformed to MoO3-doped WO3 after annealing at 500 °C. It is shown that the MoO3-doped WO3 film is more uniform and porous than pure WO3, confirming the doping quality and the privileges of the proposed method. The optimal MoO3-doped WO3 used as an EC layer exhibited a high coloration efficiency of 128.1 cm2/C, which is larger than that of pure WO3 (74.5 cm2/C). Therefore, MoO3-doped WO3 synthesized by the reported method is a promising candidate for high-efficiency and low-cost smart windows.

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