We have a long FAQ on outdoor motion sensors, so you can take a look at that for several different approaches that people have taken, but the short answer is that networked motion sensor devices that work well with smartThings don’t typically work well outdoors because they’re all heat-based and there are just too many things that can cause false alarms.
There are various workarounds. I think the most common are:
One) combine several sensors into a single “zone” and only react if they all show motion. But then that increases your cost because you are using multiple devices.
Two) use an outdoor camera or video doorbell that integrates with smartthings such as Arlo or Ring and then trigger off of its motion sensor. This can work very well, but unless you were already going to get the camera it’s an expensive option.
Three) use a small tube over the sensor lens to restrict the detection area and reduce the number of false alerts. This is inexpensive and easy, but will only work for some use cases because now you are going to a very narrow field of detection. It’s typically good for when you want the front porch lights to come on and you have a specific spot where you are looking for motion.
Anyway, all of these are discussed in much more detail in the FAQ, but those are the most popular approaches.