It's not possible to create hourly intervals that are specific to half-hour increments as you may have assumed. If you truly need this functionality, you could list out all the exact times you want the task to run each day, similar to the approach discussed in this question. However, considering you currently have it set to run every minute, this may not be necessary for your situation.
The CRON command you provided is incorrect and won't function properly. It seems like you've omitted including the seconds in your statement. A corrected version of what you're aiming for might look like:
0 0/1 8-17 ? * MON-FRI
At seconds :00 of every minute,
starting at minute :00,
every hour between 08:00 AM and 05:00 PM,
every day from Monday to Friday, every month
You can verify CRON syntax with detailed explanations using tools such as Cron Expression Generator & Explainer.