icon_emoji is not working with slack client

I made a java application to send message to slack.

1. I use slack-api-client libary.

// build.gradle
implementation "com.slack.api:slack-api-client:1.9.0"

2. I use bot token.
And bot has chat:write, chat:write.public, chat:write.customize permissions.

Slack slack = Slack.getInstance();
AsyncMethodsClient client = slack.methodsAsync("xoxb-xxxxx");

3. I set the message as follows.

{
  "channel": "channel_name",
  "username": "slack-bot",
  "icon_emoji": ":bell:",
  "text": "hi"
}

4. I use chat.postMessage method.

ChatPostMessageResponse response = client.chatPostMessage(message);

But I got a message with default app's image as user icon instead of :bell: emoji. How can I solve it?