Controlled Synthesis of Hierarchically Assembled Porous ZnO Microspheres with Enhanced Gas-Sensing Properties

Figure 6

SEM images of the as-prepared ZnO microspheres synthesized under the different amount of trisodium citrate and urea: (a) 0 mmol trisodium citrate; (b) 0.5 mmol trisodium citrate; (c) 3 mmol urea; (d) 4 mmol urea.

(a)

(b)

(c)

(d)