An alternative route is hydrothermal crystallization. Normally crystallization goes through nucleation, primary and secondary crystal growth. If we stop the crystallization at primary crystal growth stage I hope we can get nano crystals.

if available please see mybook with prof A.Subramania

Hello Venkat, I have experience producing nanoparticles in aqueous phase. I may be able to give some suggestions if this route is acceptable with you.

fine

Hi Venkat, it depends on what type of nanocrystals you are trying to get. If it is gold or silver or other noble metals, you can easily get them as colloidal crystals or nanocrystals jst by simple reflux method with suitable reagents. If you try to get for others, it will depend on the nature of the materials. For oxide or suplphides materials, you would get nanocrystals by hydrothermal method. Often, after preparation of the samples, crystalline nature can be improved by simple air heat treatment. In this case, you have to be careful on the temperature range. Generally, for oxides or sulphides, if you stop the growth at primary nucleation step, the amorphous materials is expected to obtain. 

  I walked through my memory lane

In 1973 i worked on the electro crystallization on steel - phosphate and on Zinc zinc oxide.They are nano single crystals  see my papers in  B Corr Journal 1978 and J Appl electro chem, corrosion science 1978's

A single crystal is nano  if you grow a single crystal it is nano'

Anna university India has the facility since 1990's

However it is difficult to characterize amorphous (nano?)

Hello,if you just want to obtain metal or metal oxide nanocrystals. You can try to use thermal evaportion method. Controlling the chamber argon pressure, you can get various sizes of nanocrystals.

Thanks for the answers.  Actually I am trying to prepare porous elemental (or metal) oxide nanomaterials.  In this case, How can we prepare.

Most oxides and mixed-metal oxides are easily made through flame spray pyrolysis. The concept is that you produce a solution containing the metals of interest in an organic solvent. This mixture is then combusted. The resulting soot is your product. I've made this work for most of the periodic table minus my nemesis niobium.

Please try laser ablation in liquid. 

Dear Venkat

If you are actually trying to prepare porous metal oxide crystalline nanomaterials. You should first look at crystallization ability of the metal oxide itself, Thus: 

If the targeted oxide crystallize easily at RT like bohemite AlOOH, you can get nanocrystals at RT with now calcinations see my article in journal of catalysis 178/1998 " Synthesis of Short Fibrous Boehmite Suitable for Thermally Stabilized Transition Aluminas Formation"

But, some other system can be prepared in nanosize range at low temp and crystallization can be induced to the nano sized particles by controlled calcination provided that you have a separator (second phase) between these nanosized crystals. I have tried this withr many Metal oxide composite with alumina, silica or MCM41, pls see my following articles for example 

Formation and characterization of different ceria/silica composite materials via dispersion of ceria gel or soluble ceria precursors in silica sols. Formation and characterization of different ceria/silica composite materials via dispersion of ceria gel or soluble ceria precursors in silica sols.

Humidity sensing properties of NiO/Al2O3 nanocomposite materials Solid State Ionics. 10/2003;