WooCommerce - Show free products shortcode

<?php
  // show free products in the shop
  // Use: [free_products columns="3" orderby="title" order="asc"]
  // code goes in functions.php for your child theme
  add_shortcode('free_products', 'free_products');
  function free_products( $atts ) {
    $atts = shortcode_atts( array(
      'columns' => '4',
      'orderby' => 'title',
      'order'   => 'asc',
    ), $atts );

    $query_args = array(
      'post_type'           => 'product',
      'post_status'         => 'publish',
      'ignore_sticky_posts' => 1,
      'orderby'             => $atts['orderby'],
      'order'               => $atts['order'],
      'posts_per_page'      => 12,
      'meta_query' => array(
        array(
        'key' => '_price',
        'value' => 0,
        'compare' => '=',
        'type' => 'NUMERIC'
        )
      )
    );

    $loop = new WP_Query($query_args);
    print '<div class="clear"></div>';
    print '<ul class="products-list">';
    if ( $loop->have_posts() ) {
      while ( $loop->have_posts() ) : $loop->the_post();
        wc_get_template_part( 'content', 'product' );
      endwhile;
    } else {
      echo __( 'No products found' );
    }
    print '</ul>';
    print '<div class="clear"></div>';
    wp_reset_postdata();
    return;
  }