Understanding hydrothermal transformation from Mn2O3 particles to Na0.55Mn2O4·1.5H2O nanosheets, nanobelts, and single crystalline ultra-long Na4Mn9O18 nanowires
Identifieur interne : 000098 ( Pmc/Corpus ); précédent : 000097; suivant : 000099Understanding hydrothermal transformation from Mn2O3 particles to Na0.55Mn2O4·1.5H2O nanosheets, nanobelts, and single crystalline ultra-long Na4Mn9O18 nanowires
Auteurs : Yohan Park ; Sung Woo Lee ; Ki Hyeon Kim ; Bong-Ki Min ; Arpan Kumar Nayak ; Debabrata Pradhan ; Youngku SohnSource :
- Scientific Reports [ 2045-2322 ] ; 2015.
Abstract
Manganese oxides are one of the most valuable materials for batteries, fuel cells and catalysis. Herein, we report the change in morphology and phase of as-synthesized Mn2O3 by inserting Na+ ions. In particular, Mn2O3 nanoparticles were first transformed to 2 nm thin Na0.55Mn2O4·1.5H2O nanosheets and nanobelts via hydrothermal exfoliation and Na cation intercalation, and finally to sub-mm ultra-long single crystalline Na4Mn9O18 nanowires. This paper reports the morphology and phase-dependent magnetic and catalytic (CO oxidation) properties of the as-synthesized nanostructured Na intercalated Mn-based materials.
Url:
DOI: 10.1038/srep18275
PubMed: 26667348
PubMed Central: 4678907
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PMC:4678907Le document en format XML
<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Understanding hydrothermal transformation from Mn<sub>2</sub>O<sub>3</sub> particles to Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets, nanobelts, and single crystalline ultra-long Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires</title><author><name sortKey="Park, Yohan" sort="Park, Yohan" uniqKey="Park Y" first="Yohan" last="Park">Yohan Park</name><affiliation><nlm:aff id="a1"><institution>Department of Chemistry, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></nlm:aff></affiliation></author><author><name sortKey="Woo Lee, Sung" sort="Woo Lee, Sung" uniqKey="Woo Lee S" first="Sung" last="Woo Lee">Sung Woo Lee</name><affiliation><nlm:aff id="a2"><institution>Center for Research Facilities & Department of Materials Science and Engineering, Chungnam National University</institution>, Daejeon 34134,<country>Republic of Korea</country></nlm:aff></affiliation></author><author><name sortKey="Kim, Ki Hyeon" sort="Kim, Ki Hyeon" uniqKey="Kim K" first="Ki Hyeon" last="Kim">Ki Hyeon Kim</name><affiliation><nlm:aff id="a3"><institution>Department of Physics, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></nlm:aff></affiliation></author><author><name sortKey="Min, Bong Ki" sort="Min, Bong Ki" uniqKey="Min B" first="Bong-Ki" last="Min">Bong-Ki Min</name><affiliation><nlm:aff id="a4"><institution>Center for Research Facilities, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></nlm:aff></affiliation></author><author><name sortKey="Kumar Nayak, Arpan" sort="Kumar Nayak, Arpan" uniqKey="Kumar Nayak A" first="Arpan" last="Kumar Nayak">Arpan Kumar Nayak</name><affiliation><nlm:aff id="a5"><institution>Materials Science Centre, Indian Institute of Technology</institution>, Kharagpur 721 302,<country>W.B., India</country></nlm:aff></affiliation></author><author><name sortKey="Pradhan, Debabrata" sort="Pradhan, Debabrata" uniqKey="Pradhan D" first="Debabrata" last="Pradhan">Debabrata Pradhan</name><affiliation><nlm:aff id="a5"><institution>Materials Science Centre, Indian Institute of Technology</institution>, Kharagpur 721 302,<country>W.B., India</country></nlm:aff></affiliation></author><author><name sortKey="Sohn, Youngku" sort="Sohn, Youngku" uniqKey="Sohn Y" first="Youngku" last="Sohn">Youngku Sohn</name><affiliation><nlm:aff id="a1"><institution>Department of Chemistry, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></nlm:aff></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmid">26667348</idno><idno type="pmc">4678907</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4678907</idno><idno type="RBID">PMC:4678907</idno><idno type="doi">10.1038/srep18275</idno><date when="2015">2015</date><idno type="wicri:Area/Pmc/Corpus">000098</idno><idno type="wicri:explorRef" wicri:stream="Pmc" wicri:step="Corpus" wicri:corpus="PMC">000098</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Understanding hydrothermal transformation from Mn<sub>2</sub>O<sub>3</sub> particles to Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets, nanobelts, and single crystalline ultra-long Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires</title><author><name sortKey="Park, Yohan" sort="Park, Yohan" uniqKey="Park Y" first="Yohan" last="Park">Yohan Park</name><affiliation><nlm:aff id="a1"><institution>Department of Chemistry, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></nlm:aff></affiliation></author><author><name sortKey="Woo Lee, Sung" sort="Woo Lee, Sung" uniqKey="Woo Lee S" first="Sung" last="Woo Lee">Sung Woo Lee</name><affiliation><nlm:aff id="a2"><institution>Center for Research Facilities & Department of Materials Science and Engineering, Chungnam National University</institution>, Daejeon 34134,<country>Republic of Korea</country></nlm:aff></affiliation></author><author><name sortKey="Kim, Ki Hyeon" sort="Kim, Ki Hyeon" uniqKey="Kim K" first="Ki Hyeon" last="Kim">Ki Hyeon Kim</name><affiliation><nlm:aff id="a3"><institution>Department of Physics, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></nlm:aff></affiliation></author><author><name sortKey="Min, Bong Ki" sort="Min, Bong Ki" uniqKey="Min B" first="Bong-Ki" last="Min">Bong-Ki Min</name><affiliation><nlm:aff id="a4"><institution>Center for Research Facilities, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></nlm:aff></affiliation></author><author><name sortKey="Kumar Nayak, Arpan" sort="Kumar Nayak, Arpan" uniqKey="Kumar Nayak A" first="Arpan" last="Kumar Nayak">Arpan Kumar Nayak</name><affiliation><nlm:aff id="a5"><institution>Materials Science Centre, Indian Institute of Technology</institution>, Kharagpur 721 302,<country>W.B., India</country></nlm:aff></affiliation></author><author><name sortKey="Pradhan, Debabrata" sort="Pradhan, Debabrata" uniqKey="Pradhan D" first="Debabrata" last="Pradhan">Debabrata Pradhan</name><affiliation><nlm:aff id="a5"><institution>Materials Science Centre, Indian Institute of Technology</institution>, Kharagpur 721 302,<country>W.B., India</country></nlm:aff></affiliation></author><author><name sortKey="Sohn, Youngku" sort="Sohn, Youngku" uniqKey="Sohn Y" first="Youngku" last="Sohn">Youngku Sohn</name><affiliation><nlm:aff id="a1"><institution>Department of Chemistry, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></nlm:aff></affiliation></author></analytic><series><title level="j">Scientific Reports</title><idno type="eISSN">2045-2322</idno><imprint><date when="2015">2015</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><p>Manganese oxides are one of the most valuable materials for batteries, fuel cells and catalysis. Herein, we report the change in morphology and phase of as-synthesized Mn<sub>2</sub>O<sub>3</sub> by inserting Na<sup>+</sup> ions. In particular, Mn<sub>2</sub>O<sub>3</sub> nanoparticles were first transformed to 2 nm thin Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets and nanobelts via hydrothermal exfoliation and Na cation intercalation, and finally to sub-mm ultra-long single crystalline Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires. This paper reports the morphology and phase-dependent magnetic and catalytic (CO oxidation) properties of the as-synthesized nanostructured Na intercalated Mn-based materials.</p></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Wu, Z S" uniqKey="Wu Z">Z.-S. Wu</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Lee, H W" uniqKey="Lee H">H.-W. 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<front><journal-meta><journal-id journal-id-type="nlm-ta">Sci Rep</journal-id><journal-id journal-id-type="iso-abbrev">Sci Rep</journal-id><journal-title-group><journal-title>Scientific Reports</journal-title></journal-title-group><issn pub-type="epub">2045-2322</issn><publisher><publisher-name>Nature Publishing Group</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="pmid">26667348</article-id><article-id pub-id-type="pmc">4678907</article-id><article-id pub-id-type="pii">srep18275</article-id><article-id pub-id-type="doi">10.1038/srep18275</article-id><article-categories><subj-group subj-group-type="heading"><subject>Article</subject></subj-group></article-categories><title-group><article-title>Understanding hydrothermal transformation from Mn<sub>2</sub>O<sub>3</sub> particles to Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets, nanobelts, and single crystalline ultra-long Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires</article-title></title-group><contrib-group><contrib contrib-type="author"><name><surname>Park</surname><given-names>Yohan</given-names></name><xref ref-type="aff" rid="a1">1</xref><xref ref-type="author-notes" rid="n1">*</xref></contrib><contrib contrib-type="author"><name><surname>Woo Lee</surname><given-names>Sung</given-names></name><xref ref-type="aff" rid="a2">2</xref><xref ref-type="author-notes" rid="n1">*</xref></contrib><contrib contrib-type="author"><name><surname>Kim</surname><given-names>Ki Hyeon</given-names></name><xref ref-type="aff" rid="a3">3</xref></contrib><contrib contrib-type="author"><name><surname>Min</surname><given-names>Bong-Ki</given-names></name><xref ref-type="aff" rid="a4">4</xref></contrib><contrib contrib-type="author"><name><surname>Kumar Nayak</surname><given-names>Arpan</given-names></name><xref ref-type="aff" rid="a5">5</xref></contrib><contrib contrib-type="author"><name><surname>Pradhan</surname><given-names>Debabrata</given-names></name><xref ref-type="corresp" rid="c1">a</xref><xref ref-type="aff" rid="a5">5</xref></contrib><contrib contrib-type="author"><name><surname>Sohn</surname><given-names>Youngku</given-names></name><xref ref-type="corresp" rid="c2">b</xref><xref ref-type="aff" rid="a1">1</xref></contrib><aff id="a1"><label>1</label><institution>Department of Chemistry, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></aff><aff id="a2"><label>2</label><institution>Center for Research Facilities & Department of Materials Science and Engineering, Chungnam National University</institution>, Daejeon 34134,<country>Republic of Korea</country></aff><aff id="a3"><label>3</label><institution>Department of Physics, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></aff><aff id="a4"><label>4</label><institution>Center for Research Facilities, Yeungnam University</institution>, Gyeongsan 38541,<country>Republic of Korea</country></aff><aff id="a5"><label>5</label><institution>Materials Science Centre, Indian Institute of Technology</institution>, Kharagpur 721 302,<country>W.B., India</country></aff></contrib-group><author-notes><corresp id="c1"><label>a</label><email>deb@matsc.iitkgp.ernet.in</email></corresp><corresp id="c2"><label>b</label><email>youngkusohn@ynu.ac.kr</email></corresp><fn id="n1"><label>*</label><p>These authors contributed equally to this work.</p></fn></author-notes><pub-date pub-type="epub"><day>15</day><month>12</month><year>2015</year></pub-date><pub-date pub-type="collection"><year>2015</year></pub-date><volume>5</volume><elocation-id>18275</elocation-id><history><date date-type="received"><day>25</day><month>09</month><year>2015</year></date><date date-type="accepted"><day>16</day><month>11</month><year>2015</year></date></history><permissions><copyright-statement>Copyright © 2015, Macmillan Publishers Limited</copyright-statement><copyright-year>2015</copyright-year><copyright-holder>Macmillan Publishers Limited</copyright-holder><license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by/4.0/"><pmc-comment>author-paid</pmc-comment>
<license-p>This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit <ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by/4.0/">http://creativecommons.org/licenses/by/4.0/</ext-link></license-p></license></permissions><abstract><p>Manganese oxides are one of the most valuable materials for batteries, fuel cells and catalysis. Herein, we report the change in morphology and phase of as-synthesized Mn<sub>2</sub>O<sub>3</sub> by inserting Na<sup>+</sup> ions. In particular, Mn<sub>2</sub>O<sub>3</sub> nanoparticles were first transformed to 2 nm thin Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets and nanobelts via hydrothermal exfoliation and Na cation intercalation, and finally to sub-mm ultra-long single crystalline Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires. This paper reports the morphology and phase-dependent magnetic and catalytic (CO oxidation) properties of the as-synthesized nanostructured Na intercalated Mn-based materials.</p></abstract></article-meta></front><body><p>Manganese (Mn) oxides are indispensable materials in many applications, particularly in batteries, fuel cells, supercapacitors, and catalysts<xref ref-type="bibr" rid="b1">1</xref><xref ref-type="bibr" rid="b2">2</xref><xref ref-type="bibr" rid="b3">3</xref><xref ref-type="bibr" rid="b4">4</xref><xref ref-type="bibr" rid="b5">5</xref><xref ref-type="bibr" rid="b6">6</xref><xref ref-type="bibr" rid="b7">7</xref><xref ref-type="bibr" rid="b8">8</xref><xref ref-type="bibr" rid="b9">9</xref><xref ref-type="bibr" rid="b10">10</xref>. Several attempts have been made to increase the efficiency of Mn materials (MnO<sub>2</sub>, Mn<sub>2</sub>O<sub>3</sub>, and Mn<sub>3</sub>O<sub>4</sub>) in the aforementioned applications. Tailoring the morphology has been a major approach and a range of morphologies have been reported, including wires/rods (1-D) and plates/sheets (2-D)<xref ref-type="bibr" rid="b4">4</xref><xref ref-type="bibr" rid="b11">11</xref><xref ref-type="bibr" rid="b12">12</xref><xref ref-type="bibr" rid="b13">13</xref><xref ref-type="bibr" rid="b14">14</xref><xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b16">16</xref><xref ref-type="bibr" rid="b17">17</xref><xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b19">19</xref><xref ref-type="bibr" rid="b20">20</xref><xref ref-type="bibr" rid="b21">21</xref><xref ref-type="bibr" rid="b22">22</xref><xref ref-type="bibr" rid="b23">23</xref><xref ref-type="bibr" rid="b24">24</xref>. Single-unit cell thick Mn<sub>3</sub>O<sub>4</sub> sheets were synthesized by a solution method using Mn(NO<sub>3</sub>)<sub>2</sub> and aminoethanol, which has shown a coercivity of 5.8 kOe at 5 K<xref ref-type="bibr" rid="b21">21</xref>. Tan <italic>et al.</italic> controlled the Mn<sub>3</sub>O<sub>4</sub> morphology in the shape of nanowires, nanorods and nanoparticles by varying the relative amounts of cosolvents (CH<sub>3</sub>CN and water) using Mn(AC)<sub>3</sub> precursor, and reported a large coercivity, HC = 10.7 kOe at 5 K, for the nanowires<xref ref-type="bibr" rid="b22">22</xref>. Liu <italic>et al.</italic> prepared single-layer MnO<sub>2</sub> nanosheets via a simple one-step reaction of KMnO<sub>4</sub> and sodium dodecyl sulfate (SDS), where SDS acted as the precursor of dodecanol (a reducer) and a sheet-structure agent<xref ref-type="bibr" rid="b23">23</xref>. A graphene oxide–template method was used to synthesize the MnO<sub>2</sub> nanosheets with a high surface area of 157 m<sup>2</sup>/g and good capacitance (>1017 F/g) and rate capability (>244 F/g)<xref ref-type="bibr" rid="b24">24</xref>. For applications to batteries, the insertion/deinsertion behaviors of alkali ions (Li and Na) over Mn oxides<xref ref-type="bibr" rid="b25">25</xref><xref ref-type="bibr" rid="b26">26</xref><xref ref-type="bibr" rid="b27">27</xref><xref ref-type="bibr" rid="b28">28</xref>. and their synthesis/characterization have been studied<xref ref-type="bibr" rid="b29">29</xref><xref ref-type="bibr" rid="b30">30</xref><xref ref-type="bibr" rid="b31">31</xref><xref ref-type="bibr" rid="b32">32</xref>. Spinel LiMn<sub>2</sub>O<sub>4</sub> has attracted the most interest as a cathode martial because of its thermal stability and high performance<xref ref-type="bibr" rid="b2">2</xref><xref ref-type="bibr" rid="b7">7</xref><xref ref-type="bibr" rid="b14">14</xref><xref ref-type="bibr" rid="b25">25</xref><xref ref-type="bibr" rid="b33">33</xref><xref ref-type="bibr" rid="b34">34</xref>. Zhang <italic>et al.</italic> prepared LiMn<sub>2</sub>O<sub>4</sub> polyhedrons (with 200–1000 nm sizes) by a solid-state reaction using Mn<sub>3</sub>O<sub>4</sub> nanowires and LiOH·H<sub>2</sub>O at 750 °C for 6 hr, and achieved a discharge capacity of 115 mAh/g<xref ref-type="bibr" rid="b14">14</xref>. As potential alternative to Li-ion batteries, Na-inserted Mn materials have attracted considerable interest owing to their lower cost (and high abundance) and similar physicochemical properties (e.g., redox potential and intercalation behavior)<xref ref-type="bibr" rid="b29">29</xref><xref ref-type="bibr" rid="b30">30</xref><xref ref-type="bibr" rid="b31">31</xref><xref ref-type="bibr" rid="b35">35</xref><xref ref-type="bibr" rid="b36">36</xref><xref ref-type="bibr" rid="b37">37</xref>. Recently, orthorhombic Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> (referred to as Na<sub>0.44</sub>MnO<sub>2</sub>) has attracted a great deal of interest as a cathode material for Na-ion rechargeable batteries<xref ref-type="bibr" rid="b32">32</xref><xref ref-type="bibr" rid="b38">38</xref><xref ref-type="bibr" rid="b39">39</xref><xref ref-type="bibr" rid="b40">40</xref><xref ref-type="bibr" rid="b41">41</xref><xref ref-type="bibr" rid="b42">42</xref><xref ref-type="bibr" rid="b43">43</xref><xref ref-type="bibr" rid="b44">44</xref><xref ref-type="bibr" rid="b45">45</xref><xref ref-type="bibr" rid="b46">46</xref><xref ref-type="bibr" rid="b47">47</xref><xref ref-type="bibr" rid="b48">48</xref><xref ref-type="bibr" rid="b49">49</xref><xref ref-type="bibr" rid="b50">50</xref><xref ref-type="bibr" rid="b51">51</xref><xref ref-type="bibr" rid="b52">52</xref><xref ref-type="bibr" rid="b53">53</xref><xref ref-type="bibr" rid="b54">54</xref><xref ref-type="bibr" rid="b55">55</xref><xref ref-type="bibr" rid="b56">56</xref>. Several methods have been used to synthesize the material, including sol-gel/high- temperature calcinations<xref ref-type="bibr" rid="b32">32</xref><xref ref-type="bibr" rid="b42">42</xref><xref ref-type="bibr" rid="b43">43</xref><xref ref-type="bibr" rid="b52">52</xref>, solid-state reaction<xref ref-type="bibr" rid="b27">27</xref><xref ref-type="bibr" rid="b39">39</xref>, thermal-conversion of a precursor<xref ref-type="bibr" rid="b41">41</xref>,polymer-pyrolysis<xref ref-type="bibr" rid="b45">45</xref>, and hydrothermal method<xref ref-type="bibr" rid="b52">52</xref>. Hosono <italic>et al.</italic> used a hydrothermal method (Teflon-lined autoclave at 205 °C for 2 days) using Mn<sub>3</sub>O<sub>4</sub> powder in a 5.0 M NaOH solution and obtained single-crystalline Na<sub>0.44</sub>MnO<sub>2</sub> nanowires with superior capacity of 120 mAh/g and high charge-discharge cyclability<xref ref-type="bibr" rid="b52">52</xref>. In these cases, the efficiency of the material was shown to be dependent on the surface area and morphology; hence, an understanding of the change in morphology during Na (or Li and K) ion-insertion is very important. Liu <italic>et al.</italic> prepared Na<sub>0.44</sub>MnO<sub>2</sub> nanorods with recipes of MnSO<sub>4</sub>, KMnO<sub>4</sub> and NaOH solutions by a hydothermal method<xref ref-type="bibr" rid="b56">56</xref>. Le <italic>et al.</italic> reported a change in morphology (from nanosheets to nanowires) and crystal structure (from Mn<sub>2</sub>O<sub>3</sub> to birnessite and Na<sub>0.44</sub>MnO<sub>2</sub>) after the hydrothermal reaction of Mn<sub>2</sub>O<sub>3</sub> powder in a 5.0 M NaOH solution<xref ref-type="bibr" rid="b48">48</xref>. Although many studies have reported the electrochemical properties of Na-inserted MnO<sub>x</sub> materials<xref ref-type="bibr" rid="b32">32</xref><xref ref-type="bibr" rid="b38">38</xref><xref ref-type="bibr" rid="b39">39</xref><xref ref-type="bibr" rid="b40">40</xref><xref ref-type="bibr" rid="b41">41</xref><xref ref-type="bibr" rid="b42">42</xref><xref ref-type="bibr" rid="b43">43</xref><xref ref-type="bibr" rid="b44">44</xref><xref ref-type="bibr" rid="b45">45</xref><xref ref-type="bibr" rid="b46">46</xref><xref ref-type="bibr" rid="b47">47</xref><xref ref-type="bibr" rid="b48">48</xref><xref ref-type="bibr" rid="b49">49</xref><xref ref-type="bibr" rid="b50">50</xref><xref ref-type="bibr" rid="b51">51</xref><xref ref-type="bibr" rid="b52">52</xref><xref ref-type="bibr" rid="b53">53</xref><xref ref-type="bibr" rid="b54">54</xref><xref ref-type="bibr" rid="b55">55</xref><xref ref-type="bibr" rid="b56">56</xref>, this study examined the undiscovered Na-insertion and morphological behaviors of Mn<sub>2</sub>O<sub>3</sub> nanoparticles during a hydrothermal reaction process.</p><p>This paper reports a facile process to control the morphology and phase of alkali metal intercalated Mn oxides using a simple hydrothermal technique. Three different alkali metals (Li, Na, and K) were intercalated into the Mn<sub>2</sub>O<sub>3</sub> powder (particles) to nanosheets, nanobelts and nanowires. In particular, quantum-thick Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets, nanobelts and single crystalline ultra-long Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires were produced by inserting Na with different concentrations and reaction durations. The magnetic and catalytic (CO oxidation) properties of the as-synthesized Mn oxides are reported in detail. In addition to the new findings of the morphological behaviors (by Na-insertion)/detailed characterization and magnetic properties, the laser-induced Na-deinsertion behavior was also examined by Raman spectroscopy. The present study provides several new insights into the development of alkali metal ion intercalated Mn materials.</p><sec disp-level="1"><title>Results and Discussion</title><p><xref ref-type="fig" rid="f1">Figure 1</xref> presents powder XRD patterns and scanning electron microscopy (SEM) images of the starting materials (Mn<sub>3</sub>O<sub>4</sub> and Mn<sub>2</sub>O<sub>3</sub>) and the synthesized Na-intercalated Mn oxides by varying the reaction conditions. The insets in the SEM images in <xref ref-type="fig" rid="f1">Fig. 1</xref> also show photographs of the powder, indicating the change in color of the sample from black (for Mn<sub>2</sub>O<sub>3</sub>) to brown (for Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub>), as the hydrothermal reaction time was increased. The XRD patterns (□) of the initial starting material synthesized by a hydrothermal method at 120 °C for 12 hrs revealed tetragonal Mn<sub>3</sub>O<sub>4</sub>. Upon annealing at 750 °C for 4 hrs, the crystal structure of tetragonal Mn<sub>3</sub>O<sub>4</sub> (■) changed to cubic phase (la-3) Mn<sub>2</sub>O<sub>3</sub> (JCPDS 1-071-0636). A hydrothermal reaction was then performed with the Mn<sub>2</sub>O<sub>3</sub> nanoparticles (NPs) dispersed in 1.0 and 10 M NaOH solutions at 200 °C for different durations. With increasing hydrothermal reaction time in a 10 M NaOH solution, new XRD peaks (Δ) appeared at 12.5° and 25.1° 2θ and their intensity increased. The 2θ position of these two new peaks were in good agreement with the (001) and (002) planes of monoclinic (C2/m) Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O (JCPDS 43-1456). At the same time, the intensity of the XRD peaks (■) of cubic phase Mn<sub>2</sub>O<sub>3</sub> decreased gradually. On the other hand, for the sample prepared by treating Mn<sub>2</sub>O<sub>3</sub> NPs hydrothermally in a 1.0 M NaOH solution at 200 °C, the intensity of these new peaks (Δ) did not increase significantly, even though the reaction was performed for 3 weeks, which was attributed to the lack of Na<sup>+</sup> ions. On the other hand, in the 10 M NaOH solution, these two diffraction peaks (Δ) for Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O showed significant intensities upon a reaction for less than 3 days. Upon the reaction for 1 week, the XRD peaks corresponding to the cubic phase Mn<sub>2</sub>O<sub>3</sub> were disappeared completely. Interestingly, several new diffraction peaks (ο) appeared. With further increases in the reaction time to 1~3 weeks, the two peaks (Δ) for Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O at 12.5° and 25.1° 2θ decreased in intensity. After a reaction for 3 weeks, the newly appeared peaks (ο) were mainly present, which matched orthorhombic (Pbam) Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> (JCPDS 27-0750) (<xref ref-type="supplementary-material" rid="S1">Supporting Information Fig. S1</xref> and <xref ref-type="supplementary-material" rid="S1">S2</xref>)<xref ref-type="bibr" rid="b32">32</xref><xref ref-type="bibr" rid="b42">42</xref><xref ref-type="bibr" rid="b46">46</xref>. This suggests a complete change in the crystal phase of Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O to Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> with increasing hydrothermal reaction duration to 3 weeks in 10 M NaOH at 200 °C. The high purity Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires were finally obtained after the intermediate mixture; a mixture of Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>•1.5H<sub>2</sub>O and Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> followed by a mixture of Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>•1.5H<sub>2</sub>O and Mn<sub>2</sub>O<sub>3</sub>. High purity Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>•1.5H<sub>2</sub>O nanosheets were not observed in the hydrothermal method.</p><p>Rietveld analysis was performed for a sample with mixed crystal phases (Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O:Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> = 22.7%:77.3%). The inset in <xref ref-type="fig" rid="f1">Fig. 1</xref> shows the observed and Rietveld refinement XRD patterns (see <xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S3</xref>). The crystal structures were fully refined, and the detailed structural parameters are provided in the <xref ref-type="supplementary-material" rid="S1">Supporting Information Fig. S3</xref>, <xref ref-type="supplementary-material" rid="S1">Tables S1 and S2</xref>.</p><p>The SEM and TEM/HRTEM images of the corresponding samples were examined to further understand the recrystallization mechanism of Mn<sub>2</sub>O<sub>3</sub> NPs in a NaOH solution under hydrothermal conditions at 200 °C for the specified duration. <xref ref-type="fig" rid="f2">Figure 2</xref> shows SEM images of the starting materials (Mn<sub>3</sub>O<sub>4</sub> and Mn<sub>2</sub>O<sub>3</sub>) and the synthesized materials prepared by a hydrothermal method in 1.0 M NaOH, LiOH and KOH solutions for 24 hrs. The starting Mn<sub>3</sub>O<sub>4</sub> and Mn<sub>2</sub>O<sub>3</sub> showed particle morphologies with different sizes. On the other hand, after a hydrothermal reaction (1.0 M NaOH) at 200 °C, the surface morphology had changed entirely to ultrathin nanosheets. Under LiOH and KOH solution conditions, the surface morphologies were also changed to nanosheets, but were thicker than those prepared in the NaOH solution. <xref ref-type="supplementary-material" rid="S1">Supporting Information</xref>, <xref ref-type="supplementary-material" rid="S1">Fig. S4</xref> provides additional SEM images of the nanosheets obtained by Na, Li and K intercalation. The SEM images and the XRD patterns (<xref ref-type="fig" rid="f1">Fig. 1</xref>) indicate that the sheet morphology originated from the monoclinic Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O phase, which was formed by the exfoliation of Mn<sub>2</sub>O<sub>3</sub> upon Na and H<sub>2</sub>O concomitant intercalation. On the other hand, the presence of a Mn<sub>2</sub>O<sub>3</sub> phase for the samples prepared in a short duration (<3 weeks in 1 M NaOH or <3 days in 10 M NaOH) was attributed to the incomplete conversion of Mn<sub>2</sub>O<sub>3</sub> present primarily in the core part of the powder, whereas the surface consisted mainly of ultra-thin nanosheets (<xref ref-type="supplementary-material" rid="S1">Fig. S5,SI</xref>). TEM, HRTEM images and electron diffraction patterns were also obtained for the ultrathin nanosheets, as shown in <xref ref-type="fig" rid="f2">Fig. 2</xref>. The TEM image (top right, <xref ref-type="fig" rid="f2">Fig. 2</xref>) supports the nanosheet morphology shown in the SEM images. High resolution TEM (HRTEM) (bottom right, <xref ref-type="fig" rid="f2">Fig. 2</xref>) revealed the continuous lattice, indicating the crystalline nature of the nanosheets with a lattice spacing of 0.25 nm, corresponding to the (200) plane of monoclinic Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O<xref ref-type="bibr" rid="b48">48</xref>. The selected area electron diffraction (SAED) patterns of the distinct spots on the rings shown as an inset of the HRTEM image further confirmed the crystalline nature of these nanosheets. More TEM and HRTEM images were provided in the <xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S5</xref>. For comparison, Ma <italic>et al.</italic> employed a similar hydrothermal (170 °C for 12 hrs to 1 week) method using Mn<sub>2</sub>O<sub>3</sub> powder in a 10 M NaOH solution<xref ref-type="bibr" rid="b57">57</xref>. On the other hand, they reported Na<sup>+</sup>-ion free birnessite-related layered MnO<sub>2</sub> nanobelts (5–15 nm width), which is inconsistent with the present study.</p><p>To measure the accurate thickness of the ultrathin nanosheets discussed above, a more skillful technique was employed, as described in <xref ref-type="fig" rid="f3">Fig. 3</xref>. The nanosheets were first sandwiched between epoxy supported by disks, as illustrated in the Figure. Various treatment steps such as bonding, slicing, disk cutting, and ion milling, were then performed to make a suitable TEM specimen. The thickness of the TEM specimen was finally less than 5 μm. TEM, HRTEM and high-angle annular dark field (HAADF) images were taken, which clearly showed the edge of the nanosheets. Mn in the nanosheets edge was also confirmed by an EDX profile (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S6</xref>). The HRTEM image showed lattice fringes with neighboring distances of 0.25 nm, corresponding to the (200) plane of monoclinic Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O as mentioned above. The thickness of the nanosheet edge was measured to be 2 nm, which is close to the unit cell thickness (also see <xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S7</xref>).</p><p>Because the crystal phase of Mn<sub>2</sub>O<sub>3</sub> was not completely changed using 1.0 M NaOH, the NaOH concentration was increased to 10.0 M and a hydrothermal reaction was performed for various reaction durations. The morphologies and microstructures of the samples obtained by the hydrothermal treatment of Mn<sub>2</sub>O<sub>3</sub> in 10 M NaOH for 20 h at 200 °C were examined further by SEM and TEM/HRTEM, as shown in <xref ref-type="fig" rid="f4">Figs 4</xref> and <xref ref-type="fig" rid="f5">5</xref>. The Mn<sub>2</sub>O<sub>3</sub> particles initially changed to nanosheets and nanobelts with a few nanowires (or nanothreads) for a reaction duration of less than 1 week (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S8</xref>), whereas the Mn<sub>2</sub>O<sub>3</sub> nanoparticles were still present in the synthesized samples. This was supported by the corresponding XRD patterns (<xref ref-type="fig" rid="f1">Fig. 1</xref>). As the reaction time increased, the nanobelts evolved slowly to ultra-long nanowires. Mixed morphologies were observed in the SEM images (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S9</xref>). For the corresponding XRD results (<xref ref-type="fig" rid="f1">Fig. 1</xref>), the XRD patterns (∆) of Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O were diminished slowly and those (ο) of Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> were remarkable. Upon the reaction for 3 weeks, the SEM image in <xref ref-type="fig" rid="f5">Fig. 5</xref> showed mostly ultra-long (sub-mm) nanowires (also see <xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S10</xref>). The corresponding optical microscopy images showed that the black color of the Mn<sub>2</sub>O<sub>3</sub> (with particle morphology) changed to a brown color as the crystal phase changed to Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O and Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S11</xref>). The morphology appeared like nanofibers for the final Na-intercalated Mn product. HRTEM images of the nanobelts showed a clear lattice spacing of 0.23 nm, corresponding to the (200) plane of monoclinic Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O (<xref ref-type="fig" rid="f4">Fig. 4</xref>). This was also observed for the ultrathin nanosheets (<xref ref-type="fig" rid="f2">Figs 2</xref> and <xref ref-type="fig" rid="f3">3</xref>), suggesting a similar growth direction of nanosheets and nanobelts. The SAED pattern confirmed the single crystal nature of the Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanobelts. <xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S12</xref> shows the corresponding simulated diffraction patterns. A structure projection model in <xref ref-type="fig" rid="f4">Fig. 4</xref> displays the corresponding [001] planes of Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O. <xref ref-type="fig" rid="f5">Figure 5</xref> shows representative SEM, TEM, and HRTEM images of the Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires obtained using 10 M NaOH at 200 °C for 3 weeks. The HRTEM image shows a lattice spacing of 0.442 nm for the nanowires, which is in accordance with the (200) plane of orthorhombic Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub><xref ref-type="bibr" rid="b32">32</xref>. The spot SAED pattern confirms the single crystal structure of these nanowires. The wire grew along the [100] direction. <xref ref-type="fig" rid="f6">Figure 6</xref> shows the corresponding structure projections and crystal models of the Na-intercalated samples. In the case of the Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanobelts, H<sub>2</sub>O and Na cations were concomitantly intercalated between the skeletons of Mn-O sheets. For the <italic>ab</italic> plane structure of the Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires, Na was embedded in the Mn-O tunnel frame, which is consistent with the MnO<sub>5</sub> square pyramids and MnO<sub>6</sub> octahedra<xref ref-type="bibr" rid="b58">58</xref>. The Na cations are situated in two different sites (with a unique tunnel structure) and the <italic>c</italic>-axis is the charge-discharge paths of Na cation diffusion<xref ref-type="bibr" rid="b27">27</xref><xref ref-type="bibr" rid="b32">32</xref><xref ref-type="bibr" rid="b44">44</xref>. The SAED and simulated patterns of the starting material, i.e. Mn<sub>2</sub>O<sub>3</sub>, are provided in the <xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S13</xref>.</p><p>The change in crystal phase was further confirmed by FT-IR spectroscopy (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S14</xref>). The characteristics of the Mn-O vibrational peaks were observed between 500 and 800 cm<sup>−1</sup> for all samples<xref ref-type="bibr" rid="b13">13</xref>. No OH stretching bands at approximately 3400 cm<sup>−1</sup> was observed for the starting material, i.e. Mn<sub>2</sub>O<sub>3</sub> powder. Upon the formation of Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O, strong OH stretching bands were observed at 3430 and 3350 cm<sup>−1</sup>. On the other hand, the FTIR peaks became weaker and broader for the Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires (<xref ref-type="fig" rid="f5">Fig. 5</xref> and <xref ref-type="supplementary-material" rid="S1">Fig. S10</xref>). The much weaker broad band at 3400 cm<sup>−1</sup> for Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> was attributed to the adsorbed H<sub>2</sub>O (and OH) species.</p><p><xref ref-type="fig" rid="f7">Figure 7</xref> shows the Raman spectra of the Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires measured with different laser powers (0.004 mW to 2.7 mW). At a low laser power (<0.012 mW), no obvious signal was observed. With increasing laser power to 0.19 mW, the Raman signals became clear at 637.9 cm<sup>−1</sup> and a shoulder was observed at 561.8 cm<sup>−1</sup> (see <xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S15</xref>). Upon further increases in the laser power to 2.7 mW, a strong fluorescence signal was observed (also see <xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S16</xref>) and the peak at 637.9 cm<sup>−1</sup> was decreased significantly. Upon reducing the laser power to 0.19 mW, critically different Raman signals were obtained (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S15</xref>). This suggests that the crystal phase of Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> had changed irreversibly to Mn<sub>2</sub>O<sub>3</sub> by the high power laser irradiation. The laser light induces the de-insertion of Na cations in the structure, which requires further study. The newly obtained Raman spectrum shows peaks at 312.7, 374.3 and 656.8 cm<sup>−1</sup>, which match the bulk Mn<sub>2</sub>O<sub>3</sub><xref ref-type="bibr" rid="b17">17</xref>. Similar Raman spectral profiles and behaviors were also observed for the Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O sample (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S15</xref>, <xref ref-type="supplementary-material" rid="S1">S16</xref> and <xref ref-type="supplementary-material" rid="S1">S17</xref>).</p><p>X-ray photoelectron spectroscopy (XPS) was used to examine the chemical states of Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires and compared with those of the starting material, <italic>i.e.</italic>, hydrothermally synthesized Mn<sub>2</sub>O<sub>3</sub> powders, as displayed in <xref ref-type="fig" rid="f8">Fig. 8</xref>. A typical survey XPS scan of Mn<sub>2</sub>O<sub>3</sub> showed Mn, O and impurity carbon signals, whereas that of Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> showed additional Na as well as Mn, O and C (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S18</xref>). The distinct peaks at ~653.8 and ~642.1 eV (<xref ref-type="fig" rid="f8">Fig. 8</xref>, top left) were assigned to the Mn 2p<sub>1/2</sub> and Mn 2p<sub>3/2</sub> XPS peaks, respectively, with a spin-orbit energy splitting of 11.7 eV<xref ref-type="bibr" rid="b49">49</xref>. The Mn 2p XPS peaks for Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> were shifted slightly to a lower binding energy, confirming the Na insertion and reduction of the oxidation state of Mn<xref ref-type="bibr" rid="b59">59</xref><xref ref-type="bibr" rid="b60">60</xref>. The O 1s XPS spectra showed two broad peaks at 532.0 and 529.7 eV (<xref ref-type="fig" rid="f8">Fig. 8</xref>, top right) due to the absorbed surface oxygen (e.g., OH, H<sub>2</sub>O, and O<sub>2</sub>) species and lattice oxygen atoms of the Mn oxides, respectively<xref ref-type="bibr" rid="b13">13</xref>. The Na 1s XPS and Na KLL Auger peaks for Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> (<xref ref-type="fig" rid="f8">Fig. 8</xref>, bottom panel) were observed at 1070.7 and 494.2 eV, respectively<xref ref-type="bibr" rid="b49">49</xref>.</p><p>The magnetic properties of the Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires were examined by SQUID. <xref ref-type="fig" rid="f9">Figure 9</xref> presents zero-field-cooling (ZFC) and field-cooling (FC) magnetization curves measured at an applied field of H = 100 Oe (0.1 kOe) over the temperature range of 5−300 K. The top inset in <xref ref-type="fig" rid="f9">Fig. 9</xref> shows the magnetization (M−H) curves measured at various temperatures from 5 K to 300 K and magnetic fields from −50 to 50 kOe. An ideal linear plot (with no hysteresis loop) of magnetization was obtained with an applied magnetic field at temperatures between 300 K and 50 K, indicating the paramagnetic and antiferromagnetic properties of the Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires. The M−H curves showed no saturation magnetism in the external fields up to 50 kOe. A magnetization of 2.19 emu g<sup>−1</sup> was measured at 50 kOe and 300 K. The mass magnetic susceptibility of the nanowires at 300 K was 4.39 × 10<sup>−5</sup> emu·g<sup>−1</sup>·Oe<sup>−1</sup>. This increased with decreasing temperature and was determined to be 5.58 × 10<sup>−5</sup> emu·g<sup>−1</sup>·Oe<sup>−1</sup> at 50 K. Interestingly, a magnetic hysteresis loop was clearly observed at 5 K (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S19</xref>), suggesting typical ferromagnetic behavior. On the other hand, the M−H curve showed no saturation, indicating antiferromagnetic property. The residual magnetism (or remanence) and coercive force were measured to be 0.136 emu·g<sup>−1</sup> and 475 Oe, respectively. A coercivity of 10.7 kOe at 5 K was reported for the Mn<sub>3</sub>O<sub>4</sub> nanowires<xref ref-type="bibr" rid="b22">22</xref>. For single unit cell thickness Mn<sub>3</sub>O<sub>4</sub> nanosheets, Huang <italic>et al.</italic> observed paramagnetic and ferromagnetic (with a coercivity of 5.8 kOe) behaviors at room temperature and 5K, respectively<xref ref-type="bibr" rid="b21">21</xref>. The FC magnetization curve increased with decreasing temperature. On the other hand, the ZFC curve was increased slowly with decreasing temperature to 25 K, and decreased below that temperature. The ZFC curve showed a maximum at 25 K. This suggests a clear transition from paramagnetic to ferromagnetic at a temperature below 25 K. The FC and ZFC curves showed no overlap at all temperatures up to 300 K.</p><p>The surface resistance of Na<sub>4</sub>Mn<sub>9</sub>O<sub>8</sub> nanowires was measured as a function of temperature (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S20</xref>). The resistance of 12.5 MΩ at room temperature decreased linearly to 1.0 MΩ with increasing temperature to 200 °C. For the Mn<sub>3</sub>O<sub>4</sub> (in <xref ref-type="fig" rid="f2">Fig. 2</xref>) and Mn<sub>2</sub>O<sub>3</sub> powder samples, the surface resistance could not be measured because of the high resistance.</p><p>The CO oxidation activities (<xref ref-type="supplementary-material" rid="S1">Supporting Information, Fig. S21</xref>) of Mn<sub>3</sub>O<sub>4</sub> (in <xref ref-type="fig" rid="f2">Fig. 2</xref>), Mn<sub>2</sub>O<sub>3</sub> (in <xref ref-type="fig" rid="f2">Fig. 2</xref>), and Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets (or Mn<sub>2</sub>O<sub>3</sub>@Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O core-shell structures; sample prepared with NaOH solution in <xref ref-type="fig" rid="f2">Fig. 2</xref>) was tested for catalytic applications, such as CO oxidation using low cost materials<xref ref-type="bibr" rid="b13">13</xref>. In the first CO oxidation runs, the CO oxidation onsets were observed in the order of Mn<sub>2</sub>O<sub>3</sub> (200 °C) < Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O (250 °C) < Mn<sub>3</sub>O<sub>4</sub> (280 °C). The T<sub>10%</sub> (the temperature at 10% CO conversion) for Mn<sub>2</sub>O<sub>3</sub>, Mn<sub>3</sub>O<sub>4</sub> and Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O was observed at 240 °C, 280 °C and 320 °C, respectively. In the second runs, the order was the same as the onset temperatures of 180 °C (Mn<sub>2</sub>O<sub>3</sub>), 260 °C (Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O) and 300 °C (Mn<sub>3</sub>O<sub>4</sub>). The T<sub>10%</sub> for Mn<sub>2</sub>O<sub>3</sub>, Mn<sub>3</sub>O<sub>4</sub> and Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O was observed at 230 °C, 320 °C and 365 °C, respectively. Only the Mn<sub>2</sub>O<sub>3</sub> nanoparticles showed an increase in CO oxidation activity in the second run. The Na-insertion into Mn<sub>2</sub>O<sub>3</sub> (forming Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets on the surface) showed no synergistic effect for CO oxidation. Ji <italic>et al.</italic> prepared α- Mn<sub>2</sub>O<sub>3</sub> nanowires (by a molten salt method), Mn<sub>2</sub>O<sub>3</sub> nanoparticles and mixed Mn<sub>2</sub>O<sub>3</sub>/Na<sub>2</sub>Mn<sub>8</sub>O<sub>16</sub> (a ratio of 9/1) samples, and tested their CO oxidation activities<xref ref-type="bibr" rid="b13">13</xref>. They reported that α- Mn<sub>2</sub>O<sub>3</sub> nanowires (T<sub>10%</sub> ≈ 180 °C) showed much catalytic activity than the others (T<sub>10%</sub> ≈ 220 °C) and Na<sub>2</sub>Mn<sub>8</sub>O<sub>16</sub> did not relate to their high catalytic activity. Their conclusions are in good agreement with the present study.</p></sec><sec disp-level="1"><title>Conclusion</title><p>Na-ion intercalation into Mn<sub>2</sub>O<sub>3</sub> was initially transformed into ultra-thin monoclinic Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets and nanobelts. The nanobelts were then evolved to single crystalline ultra-long orthorhombic Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires with a (Na-ion mobile) tunnel structure. This synthesis process was extended further to other alkali metals (Li and K) using a simple hydrothermal method in a Mn<sub>2</sub>O<sub>3</sub>–dispersed alkali hydroxide (LiOH, NaOH and KOH) solution. SEM and TEM confirm the transformation of the morphology. XRD and HRTEM were used to examine the crystal phase change and microstructure. Detailed crystal structural parameters were obtained by Rietveld refinement analysis. XPS confirmed the presence of inserted Na cation. Moreover, high power laser irradiation readily induces the irreversible Na-deinsertion behavior from Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> to Mn<sub>2</sub>O<sub>3</sub>, as confirmed by Raman spectroscopy. The Na<sub>4</sub>Mn<sub>9</sub>O<sub>8</sub> nanowires exhibited ferromagnetic behavior at temperatures below 25 K and paramagnetic behavior at above that temperature. The surface resistance of Na<sub>4</sub>Mn<sub>9</sub>O<sub>8</sub> nanowires was 12.5 MΩ at room temperature and decreased linearly to 1.0 MΩ with increasing temperature to 200 °C. The CO oxidation activity (T<sub>10%</sub> = 230 °C) of the Mn<sub>2</sub>O<sub>3</sub> nanoparticles was substantially decreased after Na-intercalation. The very detailed transformation mechanism and the new fundamental characterization provide new insights into the development of alkali metal cation intercalated Mn oxides.</p></sec><sec disp-level="1"><title>Methods</title><sec disp-level="2"><title>Material synthesis</title><p>Mn<sub>3</sub>O<sub>4</sub> was synthesized by a hydrothermal method, as described below. Briefly, 10 mL of 0.1 M Mn(II) nitrate tetrahydrate (Sigma-Aldrich. >97.0%) was mixed with 10 mL of deionized water (18.2 MΩ cm resistivity) in a Teflon jar (120 mL capacity), and 1.0 mL of an ammonia solution was then added to obtain the precipitates. The reaction jar was capped tightly and placed in an oven (120 °C) for 12 hours, after which the oven was cooled naturally to room temperature. The brown precipitate was collected after washing with deionized water followed by ethanol, and then dried in an air convection oven (80 °C). Bulk Mn<sub>2</sub>O<sub>3</sub> was obtained by the post-annealing of Mn<sub>3</sub>O<sub>4</sub> at 750 °C for 4 hrs. To synthesize the Na(or Li and K)-intercalated Mn materials, the Mn<sub>2</sub>O<sub>3</sub> (~25 mg) was dispersed in a 20.0 mL 1.0 M (or 10 M) NaOH (or LiOH and KOH) solution. The solution in a Teflon-lined stainless autoclave was placed at 200 °C for a reaction time, which was varied from 12 hrs to 3 weeks. After a specified time (12 hrs, 1 day, 3 days, 1, 2 and 3 weeks were selected to show in the present article), the oven was stopped and cooled naturally to room temperature and the powder product was collected by centrifuging. The powder was finally washed and dried for further characterization. Although the slow reaction process took time and patience (and somewhat industrially impractical) we employed the slow process to disclose new findings and to carefully examine change in morphology which has never been reported for Mn oxide material.</p></sec><sec disp-level="2"><title>Material characterization</title><p>The surface morphology of the synthesized powder samples was examined by field emission scanning electron microscopy (FE-SEM, Hitachi SE-4800). High resolution transmission electron microscopy (HRTEM) and the electron diffraction patterns were obtained using a FEI Tecnai G2 F20 at an operating voltage of 200 kV. The powder X-ray diffraction (XRD) patterns were obtained using a PANalytical X’Pert Pro MPD diffractometer operated at 40 kV and 30 mA using Cu Kα radiation. The Rietveld refinement was performed using the TOPAS software program (ver. 4.2, Bruker 2005). Further details are described elsewhere<xref ref-type="bibr" rid="b61">61</xref>. The Fourier-transform infrared (FT-IR) spectroscopy was performed using a Thermo Scientific Nicolet iS10 spectrometer in ATR (attenuated total reflectance) mode. The X-ray photoelectron spectra were obtained using a Thermoscientific K-alpha X-ray photoelectron spectrometer with a monochromated Al Kα X-ray source, a pass energy of 20.0 eV, and an analyzed spot size of 400 μm. Confocal Raman microscopy (PRISM, NOST Co., South Korea) was conducted to take the Raman spectra for the powder samples at a laser wavelength of 532 nm and a 100 ×, 0.9NA microscope objective. The laser intensity was varied from 0.004 mW to 2.7 mW. All the Raman spectra were referenced to the Raman spectrum of cyclohexane. The magnetic properties of the Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires were examined using a MPM5-XL-7 superconducting quantum interference device (SQUID) magnetometer (Quantum Design, Inc.) at various temperatures.</p></sec><sec disp-level="2"><title>CO oxidation and surface resistance tests</title><p>The CO oxidation experiments were performed on a continuous flow quartz U-tube reactor with a 10 mg sample. A mixed gas (1% CO and 2.5% O<sub>2</sub> in N<sub>2</sub> balance) was introduced into the reactor at a flow rate of 40 mL/min. The temperature heating rate was fixed to 20 °C/min. The reaction gas products were analyzed using a SRS RGA200 quadrupole mass spectrometer. The surface resistance of the pelletized sample was measured using a home-built four-probe resistance measurement instrument.</p></sec></sec><sec disp-level="1"><title>Additional Information</title><p><bold>How to cite this article</bold>: Park, Y. <italic>et al.</italic> Understanding hydrothermal transformation from Mn<sub>2</sub>O<sub>3</sub> particles to Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanosheets, nanobelts, and single crystalline ultra-long Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires. <italic>Sci. Rep.</italic><bold>5</bold>, 18275; doi: 10.1038/srep18275 (2015).</p></sec><sec sec-type="supplementary-material" id="S1"><title>Supplementary Material</title><supplementary-material id="d33e28" content-type="local-data"><caption><title>Supplementary Information</title></caption><media xlink:href="srep18275-s1.pdf"></media></supplementary-material></sec></body><back><ack><p>This work was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2014R1A1A2055923), and the Science and Education Research Board (SERB), Department of Science and Technology, New Delhi through the grant SB/S1/IC-15/2013. 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Sci.</source><volume>307</volume>, <fpage>736</fpage>–<lpage>743</lpage> (<year>2014</year>).</mixed-citation></ref></ref-list><fn-group><fn><p><bold>Author Contributions</bold> Y.S. designed the main experimental concepts and prepared the manuscript. D.P. analyzed HRTEM data and prepared the manuscript. Y.P. mainly performed the material synthesis. S.W.L. contributed to structural analysis. K.H. Kim performed magnetic measurements and analysis. B.K.M. performed the thickness measurement. A.K.N. performed HRTEM measurements.</p></fn></fn-group></back><floats-group><fig id="f1"><label>Figure 1</label><caption><title>XRD patterns of the starting materials (Mn<sub>3</sub>O<sub>4</sub> and Mn<sub>2</sub>O<sub>3</sub>) and the synthesized materials according to the reaction time in the 1.0 and 10 M NaOH solution.</title><p>The insets show the corresponding SEM images (left) and Rietveld refinement powder XRD patterns of a mixed phase sample (top right). The additional Figures are provided in the <xref ref-type="supplementary-material" rid="S1">Supporting Information</xref> (<xref ref-type="supplementary-material" rid="S1">Figs S1, S2</xref>, and <xref ref-type="supplementary-material" rid="S1">S3a, S3b</xref>) to understand the change in the crystal phase with varying reaction conditions. The reaction time was written on the right of the corresponding XRD.</p></caption><graphic xlink:href="srep18275-f1"></graphic></fig><fig id="f2"><label>Figure 2</label><caption><title>SEM images of Mn<sub>3</sub>O<sub>4</sub>, Mn<sub>2</sub>O<sub>3</sub> and the synthesized materials in 1.0 M NaOH, LiOH, and KOH solutions.</title><p>TEM and HRTEM images of the nanosheets synthesized in 1.0 M NaOH solution. The inset shows the SAED pattern of the nanosheets.</p></caption><graphic xlink:href="srep18275-f2"></graphic></fig><fig id="f3"><label>Figure 3</label><caption><title>TEM sample preparation procedures (top), HRTEM image of the edge of nanosheets (bottom left).</title><p>The inset shows the illustrated crystal planes. HAADF image (bottom right).</p></caption><graphic xlink:href="srep18275-f3"></graphic></fig><fig id="f4"><label>Figure 4</label><caption><title>SEM (left column), low-magnification TEM, and HRTEM images of Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O nanobelts.</title><p>SAED and a model of the corresponding crystal planes are shown on the lower right.</p></caption><graphic xlink:href="srep18275-f4"></graphic></fig><fig id="f5"><label>Figure 5</label><caption><title>SEM (left column), low-magnification TEM and HRTEM images of Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires.</title><p>SAED and the model of the corresponding crystal planes are shown on the lower right.</p></caption><graphic xlink:href="srep18275-f5"></graphic></fig><fig id="f6"><label>Figure 6</label><caption><title>Structure projections and crystal models of Na<sub>0.55</sub>Mn<sub>2</sub>O<sub>4</sub>·1.5H<sub>2</sub>O (top) and Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> (bottom).</title></caption><graphic xlink:href="srep18275-f6"></graphic></fig><fig id="f7"><label>Figure 7</label><caption><title>Raman spectra of the Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires with increasing laser power.</title><p>The inset shows an image of the analyzed area.</p></caption><graphic xlink:href="srep18275-f7"></graphic></fig><fig id="f8"><label>Figure 8</label><caption><title>High resolution Mn 2p, O1s, N 1s, and Na KLL photoelectron spectra of the Mn<sub>2</sub>O<sub>3</sub> particles and Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires.</title></caption><graphic xlink:href="srep18275-f8"></graphic></fig><fig id="f9"><label>Figure 9</label><caption><title>Mass-normalized FC and ZFC curves of Na<sub>4</sub>Mn<sub>9</sub>O<sub>18</sub> nanowires from 5 to 300 K in H = 100 Oe.</title><p>The inset show the magnetization (M−H) curves measured at various temperatures.</p></caption><graphic xlink:href="srep18275-f9"></graphic></fig></floats-group></pmc></record>Pour manipuler ce document sous Unix (Dilib)
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