golang make the first character in a string lowercase/uppercase

import (
     "unicode"
)
func UcFirst(str string) string {
    for i, v := range str {
        return string(unicode.ToUpper(v)) + str[i+1:]
    }   
    return ""
}

func LcFirst(str string) string {
    for i, v := range str {                                                                                                                                            
        return string(unicode.ToLower(v)) + str[i+1:]
    }   
    return ""
}