ZnO, with direct wide band gap of 3.37 eV and high excitonic binding energy of 60 meV has been attracting much attention due to its wide range of applications, for transparent electronics, solar cells, and other optoelectronics device. We present a simple, single step process to produce ZnO nanotrees using co-precipitation method. As a precursor, zinc nitrate dehydrate was stabilized by hexamethylene tetraamine (HMTA) and 3-9 mM polyethylene glycol (PEG) was added at 180°C for 3-6 hours followed by residual polymer removal. Scanning Electron Microscopy revealed the typical rod-like branched nanostructures were achieved. For longer annealing time the PEG-assisted growth process indeed exhibited a distinctive c-direction inhibition responsible for the lateral growth and subsequent branching of ZnO, in which the branch growth in sample with PEG amount of 0.05 g is the slowest. Some amounts of PEG up to 0.03 g are sensitive to affect several parameters, such as, lattice stress, unit cell volume, density of film and dislocation density.

ZnO nanoflowers were synthesized by co-precipitation method at low temperature (80°C) for dye-sensitized solar cell applications. ZnO nanostructures were grown at various pH values from Zinc nitrate tetrahydrate [Zn (NO3)2.4H2O], HMTA (Hexamethylenetetramine) and Potassium Hydroxide (KOH). The results indicate that by increasing pH from 6 to 12, the ZnO morphology evolves from aggregrated particles, prism-like and eventually flower-like structures. According the Scherrer equation the particle size of (100), (002), and (101) peaks tends to drop as the pH increased. HMTA is known as an effective reductant to produce hexagonal ZnO rods of various lengths and cross-sections. Low HMTA led to smaller petal and further addition caused the petal became wider. Annealing temperature should not be higher than 200°C.