First, the process is called "Sonochemical process" not "SOLOchemical process".

 Sonochemical synthesis is different from sol-gel synthesis. 

Sonochemistry is the application of ultrasound to chemical reactions and processes. Here, the precursors are mixed together and then subjected to various direct ultrasonic irradiation. Take note: Sonication intensity (W/cm2) and sonication time affect the product morphology. 

Sol–gel process is based on the hydrolysis and condensation of molecular precursors. Here, precursors are being turned into "Sol" and then continuously stirred under heat to obtain "gel". 

To answer specific questions:

1. Mechanism of the sonochemical process involves;

Liquid phase reactions – chemical reactions driven by cavitation effects; radical generation, excited species formation, mechanochemical bond scission, …

Solid state reactions – introduction of defects.

2. Sonochemical method mechanism for ZnO:

a. The first reaction will be the formation of primary ions before ultrasound irradiation by dissociation of water, sodium hydroxide (OH-) and zinc nitrate (Zn2+).

b. In the presence of ultrasound following reaction take place: The [Zn(OH)4]2- is formed. Then, In the initial stage of the sonochemical reaction, precipitation of Zn(OH)2 is obtained. Then, the surface of the Zn(OH)2 crystals will slowly be dissolved while some  ZnO nucleus is initiated in Zn(OH)2. The process proceeds until ZnO is produced. 

The solochemical synthesis of ZnO nanostructures consists of chemical reactions between a heated alkaline solution, such as NH4OH or NaOH, and a zinc precursor solution under controlled temperature and slow reagent mixture. In this process, the reactions take place at relatively low temperatures (below 100 °C) using a short reaction time. Furthermore, the solochemical processing has other advantages such as easy procedure, simple equipment, low-cost, and the products do not require subsequent thermal treatment after the drying step. In this method, by controlling the growth parameters like pH value and reaction temperatures, rounded nanoparticles and rod-like structures can be achieved, and the size and optical properties of the ZnO nanostructures can also be tuned. No templates, surface capping agents, catalysts, or other additives are necessary to produce the ZnO nanostructures through this technique. Thus, this processing may be one of the simplest and appropriate chemical routes to meet the industrial needs of ZnO nanocrystals.

The formation mechanism of the ZnO nanostructures is a complex process and mostly considered to include two main steps: the generation of a ZnO nuclei, and subsequent ZnO crystal growth. In a typical aqueous solution approach, the Zn(OH)42- complexes serve as basic growth units for the preparation of ZnO nanostructures. Therefore, in the solochemical reaction, the Zn2+ ions are produced from the zinc precursor and react with the OH ions to produce Zn(OH)42-

complexes. When their concentration reaches the degree of supersaturation, a part of these growth units decompose to form ZnO nuclei, and the nucleation process is initiated. At last, the growth of the nuclei took place along the polar c-axis of the ZnO structure by means of the incorporation of growth units on the growth interfaces (0001), resulting in ZnO nanostructures. I hope this will help You.

@Akeem Oladipo

Thank you for your response. At first I thought that solochemical is a typo, but I found at least three journal talking about solochemical process in nanoparticles synthesis which is using metal precursor and alkaline solution to produce e.g. metal oxide without ultrasonic irradiation. It's simpler yet has similar reaction where formation of Zn(OH)42- and Zn(OH)2 occurred. But thank you for your answer. I really appreciate it.

@Dr. Anuradha Yadav

Thank you for your response. So If I want to synthesize e.g. ZnO from Zn(NO3)2.6H2O and NaOH, how many NaOH (molar ratio)  do I need to convert all Zn(NO3)2.6H2O (stoichiometry)?

Does molar ratio Zn(NO3)2.6H2O:NaOH 1:2 is enough to convert all Zn(NO3)2.6H2O?

Does the nanoparticle produced by solochemical process is good for photocatalysis application? Or does annealing process is needed (just like so-gel method)?

Hi Muhammad,

 according to my calculation, 1:2 is sufficient to convert hydrated zinc nitrate.  I suggest you for Annealing for better crystallisation as well as for the morphology. 

@Dr. Anuradha

How do you calculate the alkaline needed for this reaction? I calculate it based on Zn(NO3)2.6H2O + 2 NaOH -> Zn(OH)2 + 2 NaNO3 + 6 H2O

SInce 2 NaOH is needed, so I guess 1:2 is sufficient.

So, even though solochemical process can produce wurtzite crystalline structure (no amorphous phase), annealing process is still needed to improve the crystallization process which could improve its photocatalytic activity?

Thank you for your response.