Incorrect: It peaked his interest.
Correct: It piqued his interest.
This is now a commonly seen error in writing. These two words sound the same in verbal language, but they mean two different things.
"Peek" is rarely confused with "pique" or "peak," but is included for completeness.
Pique: Definition at dictionary.com
(v) to offend; to arouse
Peak:Definition at dictionary.com
Peek: Definition at dictionary.com
(n) quick or furtive glance
"Peak" can be a noun, verb, or adjective. It's often misused in the verb form: "His interest was peaked when he heard she'd been asking about him". As a verb, "to peak" means to reach the highest point of a value or time, viz: "The stock peaked at $46 a share,"' it doesn't mean to arouse interest, that's "pique."
"Pique" can be used as a noun or verb. As a noun, it means irritation or resentment: "When she heard the news, she threw the clock across the room in a fit of pique."
As a verb, to "pique" means to arouse interest or curiosity: "The blood splatters piqued his curiosity." It can also mean to feel irritated: "She was piqued at his carelessness."
The rationale I most often hear when people confuse "piqued interest" and "peaked interest" is that interest reaches a summit. No, no and no again. Learn the correct usage and then you won't have to explain yourself.