Continue taking pictures of Nitro Splicers until you have maxed out the research. These Splicers are somewhat rare up until Olympus Heights, so you will most likely finish your research on them then.
To fill the progress bar as fast as possible, take pictures of the Nitro Splicers when they are fully in the frame, during an action shot, and when they are with other enemies.
You must fill the research bar FIVE times, with a bonus obtained each time you fill it. The bonuses for the Nitro Splicer are as follows:
1. Increased damage +
2. 15% chance a Nitro Splicer's grenade is a dud
3. Increased damage ++
4. 35% chance a Nitro Splicer's grenade is a dud
5. Increased damage +++