Multi-byte character XML entity

I'm having a problem encoding a multi-byte character to an XML document

import java.io.ByteArrayOutputStream;
import java.io.UnsupportedEncodingException;
import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamWriter;

public class XmlWriter {
    static final XMLOutputFactory outputFactory = XMLOutputFactory.newFactory();
    static XMLStreamWriter streamWriter;

    public static String Write(String s) throws XMLStreamException, UnsupportedEncodingException {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        streamWriter = outputFactory.createXMLStreamWriter(out, "utf-16");
        streamWriter.writeCharacters(s);
        streamWriter.flush();
        return new String(out.toByteArray());
    }
}


public class XmlWriterTest extends TestCase {

    public void testWrite() throws Exception {
        System.out.println("Write");
        String s = "\uD803\uDC22";
        String expResult = "𐰢";
        String result = XmlWriter.Write(s);
        assertEquals(expResult, result);

    }

I've tried many contortions of charsets etc but to no avail; I keep getting an output of

�&#xdc22

This is part of an application which generates an Excel Workbook (*.xlsx) and is failing when the document is opened in Excel due to these characters.

What can I do to achieve the correct XML entity? I was hoping that this would be handled by the XML library (the original code used Apache's StringEscapeUtils.escapeXml()).