Arduino串口通讯十六进制少前导零问题

#include <SoftwareSerial.h>
char item[7] = {0x55, 0x55, 0x01, 0x01, 0x00, 0x00, 0xAC};  //16进制命令
unsigned long buf[20];  //接收到的16进制字符串
String data = "";
String data_to_hex = "";
SoftwareSerial RS485(3, 2);   //RX, TX
void setup() {
  // put your setup code here, to run once:
  RS485.begin(9600);
  Serial.begin(115200);
}

void loop() {
  // put your main code here, to run repeatedly:
  delay(1000);
  data = "";
  RS485.write(item, 7);
  while(RS485.available()>0){
    int i = 0;
    buf[i] = (unsigned char)RS485.read();
    Serial.print(buf[i]);
    Serial.print(",");
    data += String(buf[i], HEX);
    data += ",";
    i++;
  }
  Serial.println("");
  Serial.println(data);
}
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