Is it professional to use emojis in work emails?

not at all.

emojis are for friendly talk, and can't be considered in professional communications at all. 

As of today emojis are still not considered 'professional'. They are more for a friendly communication. So by work emails if you mean emails to communicate with someone senior or someone you don't know then no it is not professional.

It depends. Simple emojis, like smile or Sad, could be used in work emails. Myself, do use it but quite rarely and only for the people whom I know well and feel comfortable with. But in general, it is not recommended.