How to style ajaxSubmitButton, place icon instead of label

Question

I'm new to Yii and I need to use ajaxSubmitButton to get the $_POST variable from the controller and pass it to the next view/page. Now the problem is how do I make it look like the normal button which is:

<a href="<?php echo Yii::app()->request->baseUrl?>/sample/Things/Upload">
    <span class="btn"><i class="icon-upload-2"></i></span>
</a>

So far I've been experimenting but to no avail:

<?php echo CHtml::ajaxSubmitButton('',Yii::app()->createUrl('/sample/Things/Upload'),
    array(
        'type'=>'POST',
        'data'=> array('cat_id'=> $cat_id),
        'icon'=>'icon-upload-2'
    ),array('class'=>'btn'));
?>

How do I specify the class for tags <span> and <i> in the array?