A number of free Anti-Virus can be found on play store that can be installed easily on your device to protect your smartphones, but in reality, companies trick us by saying that they are providing free version, let's put a light here! Not all features and protection will be provided free but some, A reputed and the big company like "Kaspersky" "Norton" so on, would never allow you to use their services without paying anything, but of course, I have something for you that can really help you out.. and you don't need to pay for it. The first Anti-Virus I would offer you to install is "BitDefender Anti-Virus" it's free for android and works really well, not only viruses, but it would secure phone from spyware, malware, and ransomware, not only this but it supports real-time scan. just for it, others Anti-Virus like "Avira", "ESET", "Mcafee" are the better options but not all features will be free to use.