Urchinlike CuO modified by reduced graphene oxide (rGO) was synthesized by a one-pot microwave-assisted hydrothermal method. The as-prepared composites were characterized using various characterization methods. A humidity sensor based on the CuO/rGO composites was fabricated and tested. The results revealed that the sensor based on the composites showed much higher impedance than pure CuO. Compared with the sensors based on pristine rGO and CuO, the sensor fabricated with the composites exhibited relatively good humidity-sensing performance in terms of response time and response value. The humidity-sensing mechanism was also briefly introduced. The enlargement of the impedance and improvement of the humidity-sensing properties are briefly explained by the Schottky junction theory.

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Organic-inorganic metal halide perovskites, particularly CH 3 NH 3 PbX 3 (X = Cl, Br, and I), have recently emerged as a promising optoelectronic material [1] because of their excellent properties such as large optical absorption, long carrier diffusion length, high carrier mobility, and low-cost solution production process. [2][3][4][5][6] Over the past decade, there have been conducted substantial research to utilize perovskites for diverse applications as solar cells, [2,7,8] photodetectors, [9,10] light emitting diodes, [11,12] and lasers. [13,14] Most of the research has focused on the control over crystallinity or chemical composition in a thin film form, in result, making great advances in material performance. [15][16][17][18][19] Continuous demands on optoelectronic devices with high integration density and new functions have raised the need for nanostructured perovskites. [20] Especially, nanowires, 1D nanostructures with controlled diameters and lengths, are the basic building blocks for creating miniaturized devices. Techniques to fabricate perovskite nanowires mainly rely on i) vapor-phase deposition [21,22] or ii) solution-mediated crystallization. [14,[23][24][25][26] The former offers an excellent crystal quality but lacks the ability to precisely position individual nanowires. In the latter that is based on supersaturation of solutes, there have been several remarkable attempts to fabricate and align individual nanowires by confinement of solution inside templates, [23,24] nanoimprint molds, [25] or nanofluidic channels [26] under evaporation of solvent.Recently, some clever methods based on inkjet printing have been devised for patterning perovskite micro/nanostructures. [27,28] These attempts have enhanced the freedom of nanostructures design beyond straight nanowires, potentially enabling a high-level integration of perovskite circuitries and devices. However, the developed patterning techniques for perovskites are still limited to in-plane fabrication and alignment.Since its invention in the 1980s, 3D printing, known as additive manufacturing, has attracted great attention as a facile method to produce tangible freeform structures. Beyond simple prototyping, there have recently been enormous efforts to improve or diversify the properties of 3D printed objects-for their practical use-by engineering materials' crystallinity [29,30] or molecular orientation. [31][32][33] In this context, owing to their As competing with the established silicon technology, organic-inorganic metal halide perovskites are continually gaining ground in optoelectronics due to their excellent material properties and low-cost production. The ability to have control over their shape, as well as composition and crystallinity, is indispensable for practical materialization. Many sophisticated nanofabrication methods have been devised to shape perovskites; however, they are still limited to in-plane, low-aspect-ratio, and simple forms. This is in stark contrast with the demands of modern optoelectronics with freeform circui...

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