“…ZnO has also attracted attention for its low cost, ease of preparation, very good chemical stability, suitability to doping, non-toxicity, and ease of processing, for example in the form of thin films [ 84 , 128 ]. This is a material that can be easily synthesized in various types of nanostructures, such as nanorods ( Figure 2 a) [ 122 ], nanosheets ( Figure 2 b) [ 132 ], nanotubes ( Figure 2 c) [ 133 ], nanoflowers ( Figure 2 d) [ 1 ], microspheres ( Figure 2 e) [ 134 ], nanoplates ( Figure 2 f) [ 113 ], nanoflakes ( Figure 2 g) [ 135 ], nanowires ( Figure 2 h) [ 136 ], and nanofibers ( Figure 2 i) [ 137 ]. These types of microstructures ensure a large specific surface area and allow the penetration of gas molecules into the volume of the material, which is a great advantage for RT detection with SAW sensors [ 128 , 138 , 139 , 140 , 141 ].…”