输出Excel没有边框？？？

如何才能让输出的Excel有边框呢？？我的Excel如下：
    public void CreateExcel2(DataTable dt, string fileName, Page page)
    {
        HttpResponse resp;
        resp = page.Response;        //resp.ContentEncoding = System.Text.Encoding.GetEncoding("GB2312");
        //resp.AppendHeader("Content-Disposition", "attachment;filename=" + fileName);
        StringWriter sw = new StringWriter();
        string colHeaders = "", ls_item = "";        ////定义表对象与行对象，同时用DataSet对其值进行初始化
        //DataTable dt = ds.Tables[0];
        DataRow[] myRow = dt.Select();//可以类似dt.Select("id>10")之形式达到数据筛选目的
        int i = 0;
        int cl = dt.Columns.Count;
        colHeaders += "<table cellSpacing=\"0\" cellPadding=\"0\" width=\"100%\" >";
        colHeaders += "<tr>";
        //取得数据表各列标题，各标题之间以t分割，最后一个列标题后加回车符
        for (i = 0; i < cl; i++)
        {
            if (i == (cl - 1))//最后一列，加n
            {
                colHeaders += "<td height=\"25\" width=\"30%\" align=\"center\" style=\"width: 10%\">" + dt.Columns[i].Caption.ToString() + "</td>";
            }
            else
            {
                colHeaders += "<td height=\"25\" width=\"30%\" align=\"center\" style=\"width: 10%\">" + dt.Columns[i].Caption.ToString() + "</td>";
            }        }
        colHeaders += "</tr>";
        //sw.Write(colHeaders);
        //resp.Write(colHeaders);
        //向HTTP输出流中写入取得的数据信息        //逐行处理数据 
        foreach (DataRow row in myRow)
        {
            //当前行数据写入HTTP输出流，并且置空ls_item以便下行数据   
            ls_item += "<tr>";
            for (i = 0; i < cl; i++)
            {
                if (i == (cl - 1))//最后一列，加n
                {
                    ls_item += "<td height=\"25\" width=\"30%\" align=\"center\" style=\"width: 10%\">" + row[i].ToString() + "</td>";
                }
                else
                {
                    ls_item += "<td height=\"25\" width=\"30%\" align=\"center\" style=\"width: 10%\">" + row[i].ToString() + "</td> ";
                }            }
            //resp.Write(ls_item);
            ls_item += "</tr>";        }
        ls_item += "</table>";
        sw.Write(colHeaders + ls_item);
        sw.Close();
        resp.Clear();
        resp.Buffer = false;
        resp.Charset = "UTF8";
        resp.ContentEncoding = System.Text.Encoding.UTF8;
        resp.AppendHeader("Content-Disposition", "attachment;filename=" + fileName + "");
        //Response.ContentEncoding = System.Text.Encoding.GetEncoding("GB2312"); 
        resp.ContentType = "application/ms-excel";
        
        page.EnableViewState = false;
        resp.Write(sw.ToString());
        resp.End();     }