I2C protocol in mbed online IDE using two STM32F429ZI board
Your sending address and slave address are not matched.

You set accessing address as 0xA0 in master, and you set the slave address as 0x01. They have to match each other. 

Also, in I2C addressing, the least significant bit is used for Read/Write bit. so the LSB bit in the slave address is ignored, meaning 0x01 for slave address is invalid and the slave address might be general call address, 0x00. So try changing it to slave.address(0xA0); in your slave code.

Added: Don't forget to add pull up resistors on the two wires.