The place for the intention is the heart, and it is not permissible to utter it out loud for prayer, fasting or zakaah. See the answer to question no. 13337. 

Shaykh al-Fawzaan (may Allah preserve him) said: … Except in two cases. 

The first case is when entering ihram for Hajj or ‘Umrah, in which case one says “Labbayka ‘umratan (here I am for ‘Umrah)” or “Labbayka Hajjan (here I am for Hajj)”. 

The second case is when slaughtering the hadiy or udhiyah or ‘aqeeqah, when one should say Bismillah and state which type it is, whether it is ‘aqeeqah or ‘udhiyah or hadiy, and on whose behalf it is being offered. So he should say: In the name of Allah and on behalf of So and so” or “In the name of Allah on my behalf and on behalf of my household,” then he should slaughter it. 

In these two cases it is prescribed to utter the intention out loud; in all other cases it is not permissible to utter the intention out loud for any act of worship, prayer or anything else. 

End quote from al-Muntaqa min Fataawa al-Fawzaan, 5/30 

Based on that, whoever wants to give the zakaah of his wealth should form the intention in his heart that this wealth is the zakaah of his wealth, and it is not prescribed for him to utter his intention verbally. 

And Allah knows best.