It is better to use pull up resistor with GPIO0 in ESP8266 ESP-01

In ESP8266, GPIO has two two functionalities.

Low state: To keep the ESP8266 in firmware flash mode 

High state: To keep the ESP8266 in normal working mode

Open state: Works same as High state

Problem in keeping the GPIO0 in open state: Sometimes during boot, the device do not recognize GPIO0 as high/Low state and the device do not boot up properly.

Solution: while working in normal mode, use 1K resister with GPIO0 and make it high by connecting with power supply.