How to initialize empty data frame (lot of columns at the same time) in R

I found how to initialize an empty data frame with 3 or 4 dimensions. It's like

df <- data.frame(Date=as.Date(character()),
             File=character(), 
             User=numeric(), 
             stringsAsFactors=FALSE)

However, What's the most effective way to initialize an empty data.frame with a lot of column names. like

mynames <- paste("hello", c(1:10000))

The wrong way I tried is:

df <- data.frame(mynames=numeric())

Thanks a lot beforehand