There are a number of hands-free door openers which either operate a knob or replace a knob together. These are typically used by people in wheelchairs. However, they tend to be pretty expensive, the least of them are around a $1500 range. And they move The door very slowly for safety reasons, which is often annoying to people who are ambulatory.
There are also electronic strike plates which are typically used in offices. These unlatch the door but do not open it. These are very rarely used in homes because it’s hard to figure out what behavior you want when the power goes out, particularly since the power may go out because of a fire in the residence.
What happens in a fire?
Most US building codes will require that if the building is a place where people sleep, and the power goes out, the electronic lock must be able to be opened from the inside without requiring a second device or a key or key card.
A deadbolt with a turnbolt on the inside meets this legal requirement and still keep the door locked from the outside. The person inside just turns the bolt. The person on the outside can’t get in even though the power is out.
The problem with a regular knob is that it’s extremely likely that to meet the code the knob will end up being unlocked from the outside if the power goes out. It’s just a design issue because the knob on the inside also turns the knob on the outside and vice a versa. Note that this is not true for a deadbolt. The deadbolt doesn’t have a turnbolt on the outside.
All of this put together is the reason why almost all residential smart locks Are deadbolts rather than just doorknobs. It allows them to meet the code requirement that the door can be opened from the inside when there is no power while still meeting the homeowner’s requirement that the door remains locked from the outside.
You can make one yourself
OK, having said all that, there are people who put together their own inexpensive door strikes that are controllable by smartthings. Those will work for a knob. However, they may either not be to code or might leave your door unlocked if the power fails.
Here is a typical project of that type.
So can you buy one for sale? Yes, for an office. Probably no for a home unless it is part of a system that physically opens the door as well, in which case it gets expensive.
Can you make one for yourself? Yes, but you need to think through the use case very carefully.