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[Emily C. Hayward](https://orcid.org/0009-0004-9211-954X), [Masaki Takeguchi](https://orcid.org/0000-0002-0282-6020), [Harry J. Lloyd](https://orcid.org/0000-0002-1062-0794), Joshua M. Stratford, [Andrew J. Smith](https://orcid.org/0000-0003-3745-7082), [Tim Snow](https://orcid.org/0000-0001-7146-6885), [Joaquin Ramírez-Rico](https://orcid.org/0000-0002-1184-0756), [Zoe Schnepp](https://orcid.org/0000-0003-2171-067X)

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[<i>In situ</i> TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of cellulose](https://mdr.nims.go.jp/datasets/0989166d-02fd-4035-a1d3-035725378865)

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In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseJournal ofMaterials Chemistry APAPEROpen Access Article. Published on 16 July 2025. Downloaded on 9/11/2025 11:01:00 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View IssueIn situ TEM and saSchool of Chemistry, University of Birmingbham.ac.ukbCenter for Basic Research on Materials, NatSengen, Tsukuba, Ibaraki, 305-0047, JapancDiamond Light Source, Didcot, OxfordshiredDepartamento F́ısica de la Materia CondensSevilla, Universidad de Sevilla – CSIC, AveSpain† Electronic supplementary informahttps://doi.org/10.1039/d5ta03584hCite this: J. Mater. Chem. A, 2025, 13,26327Received 6th May 2025Accepted 15th July 2025DOI: 10.1039/d5ta03584hrsc.li/materials-aThis journal is © The Royal Society oynchrotron SAXS/WAXS study onthe impact of different iron salts on iron-catalysedgraphitization of cellulose†Emily C. Hayward, a Masaki Takeguchi, b Harry J. Lloyd, ac Joshua M. Stratford,aAndrew J. Smith, c Tim Snow, c Joaquin Ramı́rez-Rico d and Zoe Schnepp *aCarbon materials are essential for emerging energy applications and there is a pressing need to be able toproduce carbons with controlled properties from sustainable precursors. Iron-catalysed graphitization ofbiomass is an attractive approach, where simple iron salts are used to convert organic matter to graphiticcarbons at relatively low temperature. The choice of iron salt can have a significant impact on thechemical and structural properties of carbons derived from biomass. In this paper, we report a detailedmechanistic investigation of iron catalysed graphitization of cellulose by Fe(NO3)3 and FeCl3. In situ smalland wide angle X-ray scattering and electron microscopy show that the evolution of catalyst particlesfrom the two salts follows very different pathways. Remarkably, graphitization by FeCl3 is an order ofmagnitude faster than by Fe(NO3)3.IntroductionGraphitic carbon is an essential component of existing andemerging energy applications.1 Limitations on natural graphitesupply and process sustainability have meant that graphite islisted as a critical material by many governments.2,3 As analternative, graphite can be manufactured from petroleumproducts4 or biomass5 such as cellulosic materials. However,production of synthetic graphite requires very high tempera-tures (>2500 °C), making it a very energy intensive process. Inaddition, while biomass is renewable and abundant, its sourcesare non-graphitizable by traditional methods. A potential solu-tion is catalytic graphitization, where catalysts from abundantelements such as iron are used to convert amorphous carbon tographitic carbon at relatively low temperatures (∼800 °C).6,7 Theprocess of iron-catalysed graphitization of biomass is verysimple and involves mixing biomass with soluble iron salts thenpyrolyzing in an inert atmosphere. The biomass decomposes toamorphous carbon and the iron salts decompose to form ironnanoparticles dispersed throughout the carbon. On furtherheating, the iron nanoparticles move through the amorphousham, B15 2TT, UK. E-mail: z.schnepp@ional Institute for Materials Science, 1-2-1, OX11 0DE, England, UKada, Instituto de Ciencia de Materiales denida Reina Mercedes SN, 41012 Sevilla,tion (ESI) available. See DOI:f Chemistry 2025carbon matrix (Fig. 1a) to produce graphitic carbon nano-structures (Fig. 1b) via a dissolution–precipitation mechanism.8Previous studies have shown that the type of biomass canhave a dramatic effect on the structure and properties ofcarbons produced by iron-catalysed graphitization. Forexample, the branched gel network of starch produces amor-phous carbon that is resistant to graphitization even in thepresence of catalysts.9 This makes it easier to control the degreeof graphitization and, in turn, the porosity. The choice of ironcatalyst can also have a signicant effect on the mechanism ofcatalytic graphitization and thus on the microstructure of theresulting carbons.10,11 For example, our previous work showedthat cellulose pyrolyzed with Fe(NO3)3 produces a mesoporousgraphitic carbon whereas cellulose pyrolyzed with FeCl3 gener-ates a carbon with a much wider range of pore sizes.12 This isbelieved to be partially due to the different iron salts drivingdifferent decomposition mechanisms in the cellulose. In addi-tion, cellulose treated with FeCl3 produced very large interme-diate iron oxide particles whereas no crystalline iron oxideintermediates were observed in the cellulose-Fe(NO3)3 system.Our in situ transmission electron microscopy of the cellulose-Fe(NO3)3 system showed that the graphitization catalyst movesin a liquid-like way through the amorphous carbon.8 However,no such experiments have been conducted with cellulose-FeCl3.Therefore, the role of the large iron oxide crystallites inproducing the active catalyst and multimodal porosity isunknown. Furthermore, since all the experiments with cellu-lose-FeCl3 have been ex situ it is not known whether the largeiron oxide crystallites actually play a role in the mechanism orwhether they only form on quenching.J. Mater. Chem. A, 2025, 13, 26327–26336 | 26327http://crossmark.crossref.org/dialog/?doi=10.1039/d5ta03584h&domain=pdf&date_stamp=2025-08-08http://orcid.org/0009-0004-9211-954Xhttp://orcid.org/0000-0002-0282-6020http://orcid.org/0000-0002-1062-0794http://orcid.org/0000-0003-3745-7082http://orcid.org/0000-0001-7146-6885http://orcid.org/0000-0002-1184-0756http://orcid.org/0000-0003-2171-067Xhttps://doi.org/10.1039/d5ta03584hhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5ta03584hhttps://pubs.rsc.org/en/journals/journal/TAhttps://pubs.rsc.org/en/journals/journal/TA?issueid=TA013032Fig. 1 (a) Schematic showing proposed mechanism of iron-catalysedgraphitization and (b) scanning electron microscope image ofgraphitic nanotubes produced by iron-catalysed graphitization ofsoftwood (image reproduced with permission from ref. 13).Journal of Materials Chemistry A PaperOpen Access Article. Published on 16 July 2025. Downloaded on 9/11/2025 11:01:00 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineCellulose is the most abundant biopolymer on Earth andcomprises many different types of renewable biomass. If cellu-losic biomass is to become a viable precursor for syntheticgraphite, it is therefore essential that we understand howdifferent graphitization catalysts inuence the properties ofcarbons produced from cellulose. In this paper, we use in situsynchrotron small and wide angle X-ray scattering (SAXS andWAXS) and in situ transmission electron microscopy (TEM) tocompare the graphitization mechanism of microcrystallinecellulose pyrolyzed with Fe(NO3)3 and FeCl3. These techniquesenable us to determine the evolution and progression of crys-talline phases and also the distribution of particle sizes atdifferent stages of the graphitization process. This offersunprecedented insight into the mechanism of iron-catalysedgraphitization of cellulose.Results and discussionFig. 2a shows a heatmap of in situ WAXS data for a sample ofmicrocrystalline cellulose treated with Fe(NO3)3 and heated to800 °C under nitrogen, followed by a 30 minute dwell and then26328 | J. Mater. Chem. A, 2025, 13, 26327–26336cooling to 100 °C. The bright orange/pink bands representcrystalline phases and, as expected based on previous studies,there is little evidence of crystalline content below 700 °C.12Between 700 °C and 800 °C, a broad band emerges at q z 1.8Å−1, corresponding to the 002 peak for graphitic carbon.Alongside this, peaks in the q = 2.5–3.5 Å−1 region indicate theformation of crystalline iron phases. In contrast, a heatmap fora sample of microcrystalline cellulose with FeCl3 (Fig. 2b)displays well-dened orange bands from ∼500 °C. There isa peak shi to lower q at ∼60 minutes (700 °C) followed bya sudden transition to a different crystalline phase. At the samepoint, there is a sudden appearance of a strong graphitic carbonband. In both heatmaps, there is very little change during thedwell stage of the experiment. During the cooling stage of theexperiment, the shi in peak positions from ∼110 minutesrepresents crystalline contraction and there also appears to bea phase transformation.The change in crystalline phase composition can be morereadily seen in plots of the Bragg peaks (Fig. 2c and d). Around600 °C in the cellulose-Fe(NO3)3 system, the system is largelyamorphous, with peaks for iron-containing crystalline phasesonly appearing above 700 °C. In contrast, the cellulose-FeCl3system undergoes a large number of phase transformations.The gradual emergence of the graphite peak in the cellulose-Fe(NO3)3 system also contrasts very clearly with the very fastgrowth of the same peak in the cellulose-FeCl3 system. A plot ofnormalised graphite peak intensity against time and tempera-ture (Fig. 2e) shows this even more strikingly, with themaximum rate of peak growth nearly an order of magnitudehigher for the cellulose-FeCl3 system. It should be noted thatthe data shown here for the cellulose-FeCl3 system is fora sample repeated at a higher scan rate to conrm the rapidphase changes and provide more data points during the tran-sition. The data for the original sample, run at the same scanrate as the cellulose-Fe(NO3)3 system, can be seen in (Fig. S1†).Rietveld renement of the diffraction data was used toextract phase fractions of the iron-containing crystalline phasesthroughout the experiments. In the cellulose-Fe(NO3)3 system(Fig. 2f), the peaks above q = 2.5 Å−1 correspond to a mixture ofFe3C, a-Fe and g-Fe. The decrease in a-Fe and concurrentincrease in g-Fe corresponds to the expected transition in theiron–carbon phase diagram.14 The increase of the g-Fe phasefraction occurs alongside the emergence of the graphite phase(from 60 minutes), suggesting that g-Fe may be an active cata-lyst for graphitization. During the same time frame, there isa decrease in Fe3C fraction, which may indicate a second oralternative route of graphitic carbon formation via Fe3Cdecomposition. Both catalysts have been identied duringchemical vapour deposition (CVD) synthesis of carbon nano-tubes, with the conclusion in that paper that two routes ofgraphitization occur simultaneously.15 There is very littlechange during the dwell stage of the experiment, other thana slight decrease in Fe3C and increase in g-Fe fraction. Thiscould correspond to decomposition of the Fe3C to graphiticcarbon. However, the bulk of graphitization occurs before thedwell phase of the experiment. On cooling, a small fraction ofthe g-Fe transforms to the lower temperature a-Fe, butThis journal is © The Royal Society of Chemistry 2025http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5ta03584hFig. 2 Heatmaps of WAXS data for samples of (a) cellulose + Fe(NO3)3 and (b) cellulose + FeCl3 heated to 800 °C under nitrogen. Also diffractiondata for (c) cellulose + Fe(NO3)3 and (d) cellulose + FeCl3 with an additional panel showing the temperature for each pattern during heating to800 °C. Phase fractions extracted from Rietveld refinement of data for (e) cellulose + Fe(NO3)3 and (f) cellulose + FeCl3. A plot of (g) normalisedgraphite (002) peak intensity vs. time and temperature. (h) Plot showing expansion of FeO lattice parameter a with increasing temperature.This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. A, 2025, 13, 26327–26336 | 26329Paper Journal of Materials Chemistry AOpen Access Article. Published on 16 July 2025. Downloaded on 9/11/2025 11:01:00 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5ta03584hJournal of Materials Chemistry A PaperOpen Access Article. Published on 16 July 2025. Downloaded on 9/11/2025 11:01:00 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinesurprisingly a large amount remains trapped as g-Fe. Trappingof g-Fe has been demonstrated during chemical vapour depo-sition synthesis of carbon nanotubes due to stabilization bycarbon in the interstitial sites.16A plot of phase fractions for the cellulose-FeCl3 system(Fig. 2g) shows the presence of various iron oxide phases duringthe heating stage. Interestingly, FeO (wüstite, more accuratelywritten as Fe1−xO, where 0.04 < x < 0.1) is the rst iron oxidephase to emerge, followed by Fe3O4. Rather than some of theFeO transforming to Fe3O4 (unlikely in the reducing conditions)we believe that the two phases form via different mechanisms.The sample preparation involves combining cellulose powderwith aqueous FeCl3. FeCl3 hydrolyses on contact with water and,as a result, there will be some iron(oxyhydr)oxide speciespresent alongside the FeCl3.17 These iron (oxyhydr)oxide speciesdecompose to FeO. Separately, we suggest that the FeCl3undergoes halide vapour hydrolysis,18 where volatile iron chlo-ride reacts with water vapour released from decomposingcellulose to produce crystalline magnetite.19 Themagnetite thenundergoes carbothermal reduction to form additional FeO. At∼700 °C, there is a shi in the FeO peaks to a smaller q value,indicating a lattice expansion. The trend in lattice parametera is shown in Fig. 2h and could indicate strain in the lattice inthe initial stages of carbothermal reduction. At 755 °C, the FeOquickly transforms to a mixture of g-Fe and Fe3C. This occurs atthe same time as the rapid emergence of the peak for graphiticcarbon. The presence of both phases again makes it impossibleFig. 3 Snapshots from in situ TEM videos showing heating of a cellulose600 °C (scale bar = 100 nm) and (c) held at 600 °C (scale bar = 100 nm26330 | J. Mater. Chem. A, 2025, 13, 26327–26336to identify a single graphitization catalyst, suggesting that bothmay be active. During the dwell stage, there is a similar decreasein Fe3C content and concurrent increase in g-Fe, suggestinggradual decomposition of the Fe3C to graphitic carbon. Oncooling, a signicant fraction of the g-Fe transforms to thelower temperature a-Fe phase. A nal point to note is thatenergy dispersive X-ray analysis (ex situ) of a sample heated to800 °C under N2 in a furnace showed no evidence of chlorine(Fig. S2†), supporting the theory that all the FeCl3 is convertedinto iron oxide.Further insight into the different mechanisms operating inthe Fe(NO3)3 and FeCl3 systems comes from in situ transmissionelectron microscopy (TEM) footage. We have previously re-ported that catalyst particles in the cellulose-Fe(NO3)3 systemmove in a liquid-like manner during the graphitization step.8Before this point, the catalyst particles emerge from materialthat appears amorphous, consistent with the in situWAXS data.In the cellulose-FeCl3 system, large particles can be observed insnapshots (Fig. 3a) of in situ TEM data (Video S1†) even below400 °C. As the system is heated, the large particles can be seen tovaporise (Video S2†), consistent with the halide vapour hydro-lysis mechanism proposed above. The disappearance of thelarge particles occurs alongside the appearance of smallerparticles, presumed to be caused by deposition of iron oxide.20During continued heating from 500 °C to 600 °C, the particlesgrow (Fig. 3b) and then start to move in a similar erratic mannerto those observed in the cellulose-Fe(NO3)3 system (Fig. 3c and+ FeCl3 sample (a) up to 500 °C (scale bar = 50 nm), (b) from 500 °C to). Circles show the positions of a single particle as it moves.This journal is © The Royal Society of Chemistry 2025http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5ta03584hFig. 4 Snapshots from TEM video showing a region of the cellulose-FeCl3 sample during the graphitization step, showing (a) amorphous carbonwith some Fe nanoparticles and (b) a trail of graphitic carbon nanotubes left by a moving catalyst particle. Scale bar = 20 nm.Paper Journal of Materials Chemistry AOpen Access Article. Published on 16 July 2025. Downloaded on 9/11/2025 11:01:00 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineVideo S3†). The particles move through the even-texturedamorphous carbon of the decomposed cellulose, leavinga trail of graphitic carbon nanotubes (Fig. 4 and Video S4†),again as observed in the cellulose-Fe(NO3)3 system. Remark-ably, however, the particles from the cellulose-FeCl3 systemmove a lot faster than those in the cellulose-Fe(NO3)3 system(Video S5,† data for cellulose-Fe(NO3)3 reproduced withpermission from ref. 8). For example, the fastest particleobserved is highlighted in Fig. 3c and has an estimated speed of300 nm s−1, which is an order of magnitude faster than even thefastest particles observed in the cellulose-Fe(NO3)3 system.Other particles did not move quite as quickly (Fig. S3 and S4†)but were still considerably faster than the average speed ofparticles in the cellulose-Fe(NO3)3 system. This is surprising butis consistent with the in situWAXS data above, which shows therate of graphitization to be considerably faster for the FeCl3-cellulose system. It should be noted that the temperature ofgraphitization in the TEM experiments is lower than recordedin the in situ WAXS experiment. This could be due to electronbeam heating, but this is unlikely as it would also have affectedthe cellulose-Fe(NO3)3 sample. Given that the cellulose-FeCl3decomposition mechanism is believed to involve a vapour-phase step, it is more likely that the vacuum conditions of theTEM accelerated the progress of the pyrolysis or changed thecatalyst formation mechanism. In situ SAXS/WAXS experimentsat 700 °C show that graphitization is possible at lower temper-atures (Fig. S5†). However, TEM experiments under atmo-spheric pressure or vacuum phase synchrotron experimentswould be necessary to verify the exact reason for variation in thissystem.In situ small angle X-ray scattering (SAXS) offers furtherinsight into the different graphitization mechanisms. Fig. 5ashows the SAXS data for the microcrystalline cellulose-Fe(NO3)3system during heating from 400 °C to 800 °C at 25 °C intervals.During the early stages of heating (400–700 °C), there isa gradual increase in scattering intensity in the q range 0.2–1.2nm−1, indicating an increase in scattering features around 5–30 nm in size. Above 700 °C, the data shows an increase inscattering intensity at lower q, indicating the formation of largerscattering features. Selected scans were tted and analysedThis journal is © The Royal Society of Chemistry 2025using McSAS, a Monte Carlo method for extracting form-freesize distributions, for q range 0.027 # q (nm−1) $ 2.88 (datawith t lines in Fig. S6†).21 The histograms support observationsfrom the raw data and correlate with WAXS and TEM data. At700 °C, the sample consists of only very small scatteringfeatures (Fig. 5b), presumably the emerging nuclei of Fe3C andFe nanoparticles. The distribution moves to larger scatteringfeatures at 750 °C (Fig. 5c) and 800 °C (Fig. 5d) as the catalystparticles grow. It should be noted that the graphitic nanotubescreated by the catalyst particles will also contribute to scatteringdue to the presence of a carbon–air interface within the pore ofeach nanotube. The scattering length density contrast for theiron–carbon interface is roughly twice that of the carbon–airinterface. This corresponds to ∼4× difference in scatteringintensity. However, as there is a high proportion of porescompared to iron particles in the sample, the contribution inthe SAXS pattern from the pores is likely to be signicant. Togain more insight into the evolution of different features, nor-malised scattering intensity was plotted alongside graphite(002) peak intensity for two different q values. The graph for q =0.439 nm−1 (corresponding to scattering features ∼7 nm inradius) shows both scattering intensity and graphite peakevolution track each other exactly (Fig. 5e). In contrast, thescattering intensity at q = 0.133 nm−1 (corresponding to scat-tering features ∼23 nm in radius) increases in advance of thegraphite peak evolution (Fig. 5f). The graphitic nanotubesformed in these systems are multi-walled, meaning that theinternal pore will be smaller than the catalyst nanoparticle.Therefore, the features ∼23 nm in radius are likely to be thecatalyst particles and the smaller scattering features are likely tobe the graphitic nanotubes. This data therefore showsconvincing evidence of the emergence of catalyst particles fol-lowed by the growth of graphitic nanostructures, with bothcontributing to the broad distribution of sizes of scatteringfeatures. The SAXS pattern did not change during the dwell andcooling phase of the experiment (Fig. S7†).For the microcrystalline cellulose-FeCl3 system, the SAXSdata is very different (Fig. 6a), with no signicant change in thescattering prole until 750 °C, when there is a very suddenincrease in scattering intensity. This corresponds to the fastJ. Mater. Chem. A, 2025, 13, 26327–26336 | 26331http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5ta03584hFig. 5 (a) SAXS data for the heating range 400–800 °C and corresponding particle size histograms for data scans at (b) 700 °C, (c) 750 °Cand (d) 800 °C for cellulose-Fe(NO3)3. Normalised graphite (002) peak intensity alongside scattering intensity at (e) q = 0.439 nm−1 and(f) q = 0.133 nm−1.Fig. 6 (a) SAXS data for the heating range 400–800 °C and corresponding particle size histograms for data at (b) 750 °C, (c) 760 °C and (d) 770 °Cfor cellulose-FeCl3. Normalised graphite (002) peak intensity alongside scattering intensity at (e) q = 0.439 nm−1 and (f) q = 0.133 nm−1.26332 | J. Mater. Chem. A, 2025, 13, 26327–26336 This journal is © The Royal Society of Chemistry 2025Journal of Materials Chemistry A PaperOpen Access Article. Published on 16 July 2025. Downloaded on 9/11/2025 11:01:00 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5ta03584hPaper Journal of Materials Chemistry AOpen Access Article. Published on 16 July 2025. Downloaded on 9/11/2025 11:01:00 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinegraphitization step observed in the WAXS data. Selected scanswere tted and analysed using McSAS, a Monte Carlo methodfor extracting form-free size distributions, for q range 0.027 # q(nm−1)$ 2.88 (data with t lines in Fig. S8†).21 The particle sizehistogram extracted from the SAXS data at 750 °C (Fig. 6b)shows particles in the size range 10–100 nm (radius). The shapeof the data suggests that there may also be a distribution ofparticles above 100 nm in radius. This is consistent with thewide range of particle sizes observed in both in situ and ex situTEM experiments.12 At 760 °C (Fig. 6c), the approximatetemperature for onset of graphitization, the histogram showsthe presence of features below 10 nm in radius, consistent withthe observation of Fe/Fe3C particles less than 10 nm in radius byin situ TEM (Fig. 3c and 4b). A simple calculation shows thata FeO particle of radius 50 nm would contract to a Fe particlewith a radius of ∼40 nm. Therefore, the distribution of scat-tering features <10 nm cannot be due to particle contractionduring carbothermal reduction. Another possibility is that theFeO–Fe transition could involve splitting of the particles intofragments. Crack formation and propagation is a well-knownphenomenon on a bulk scale in the carbothermal reductionof iron ore.22 This occurs due to volume/density changes and gasevolution. Therefore, particle splitting during carbothermalreduction is a plausible explanation for the observed change inscattering intensity. Once graphitization is complete, at 770 °C(Fig. 6d), the most signicant contribution in the histogram isfrom features below 10 nm in radius. As above, this is likely tobe due to a combination of catalyst nanoparticles and pores.This is corroborated by TEM footage, which shows that thecatalyst particles do not grow signicantly in size duringgraphitization. As above, more insight can be gained bycomparing normalised scattering intensity at certain q values tographite (002) peak intensity. For q = 0.439 nm−1 (corre-sponding to scattering features ∼7 nm in radius), the normal-ised scattering and graphite peak intensities track each otherclosely (Fig. 6e). However, the data for q = 0.133 nm−1 (corre-sponding to scattering features ∼23 nm in radius) shows rstlyFig. 7 Schematic showing the two proposed mechanisms of graphitizaThis journal is © The Royal Society of Chemistry 2025a gradual increase in scattering intensity up to 70 minutes(Fig. 6f). We know there is a population of FeO that comes fromiron hydroxide (from hydrolysis of FeCl3 in sample preparation)and the growth and sintering of these particles could cause thissmall intensity increase. The most dramatic observation is thatthe fast increase in scattering intensity occurs more quicklythan the corresponding evolution of the graphite peak. As withthe cellulose-Fe(NO3)3 system, this indicates that the catalystparticles are larger than the internal pores of the multiwalledgraphitic nanotubes they produce. The catalyst particles alsoform rst, with a time lag during which the graphitic nanotubesare produced. As with the cellulose-Fe(NO3)3 system, the SAXSpatterns for cellulose-FeCl3 do not change signicantly duringthe dwell and cooling phase of the experiment (Fig. S9†).The data above enable us to propose a mechanism for thevery different graphitization behaviour of catalyst particlesformed from Fe(NO3)3 and FeCl3. The rst point is the verysudden onset of graphitization for the cellulose-FeCl3 systemcompared to the gradual increase in graphitic content in thecellulose-Fe(NO3)3 system. Pyrolysis of cellulose with Fe(NO3)3results in the formation of a largely amorphous mixture ofcarbon with iron presumably present as very small amorphousiron oxide clusters or dispersed throughout the carbon (Fig. 7a).Carbothermal reduction at ∼700 °C is followed by gradualgrowth of the Fe/Fe3C crystals and then the onset of graphiti-zation. These data point to the need for catalyst particles toreach a critical size before graphitization can commence. Aninteresting exercise we can do at this point is estimate theminimum carbon concentration required for precipitation ofsingle-atom thick graphene sheets around catalyst particles ofdifferent diameters. Because of surface/volume ratio effects, theconcentration of carbon needed to form a graphene ‘cap’ orlayer around an iron catalyst particle rises steeply withdecreasing catalyst diameter (Fig. 8). The model is extremelysimple, but it clearly shows that catalyst particles would have toachieve a minimum diameter before graphitization can occur,given that the solubility of carbon in g-Fe at the 723 °C eutectiction catalyst formation from cellulose with (a) Fe(NO3)3 and (b) FeCl3.J. Mater. Chem. A, 2025, 13, 26327–26336 | 26333http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5ta03584hFig. 8 Estimation of carbon concentration required to cover a catalystsphere with a single-atom thick layer of graphene.Fig. 9 Schematic of the evolution of iron phases and graphitic carbonduring pyrolysis of microcrystalline cellulose with (a) Fe(NO3)3 and (b)FeCl3 from 700 °C.Journal of Materials Chemistry A PaperOpen Access Article. Published on 16 July 2025. Downloaded on 9/11/2025 11:01:00 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineis only 0.8%. In contrast, pyrolysis of cellulose with FeCl3 resultsin the formation of large iron oxide crystallites (Fig. 7b). Theseappear to break apart during carbothermal reduction, followedby a very sudden onset of graphitization. If breaking up of thelarge oxide particles results in a lot of g-Fe particles above theminimum size for graphitization, this could explain the very fastonset of graphitization.The very fast rate of catalyst movement in the cellulose-FeCl3is harder to rationalize. We concluded in our previous study ofthe cellulose-Fe(NO3)3 system that the process of graphitizationoccurs via dissolution of carbon atoms at the front face of the Fecatalyst particle, diffusion of carbon atoms through the solid g-Fe particle and precipitation of graphitic carbon structures atthe rear face of the catalyst. Movement of the catalyst occursthrough self-diffusion of iron atoms away from the site ofprecipitation. The speed of catalyst movement in the cellulose-Fe(NO3)3 system was consistent with diffusion constants listedin the literature. If catalyst movement is considerably faster inthe cellulose-FeCl3 system, then there are several possibleconsiderations. Firstly, it is possible that different diffusionmechanisms operate in the two systems (i.e. bulk diffusion of Fein the Fe(NO3)3 system and surface diffusion in the FeCl3system). Surface diffusion is a lot faster than bulk diffusion butgiven the two systems have very similar crystalline compositionsduring graphitization, it seems unlikely that diffusion mecha-nisms would be signicantly different. Another possibility isthat the catalyst particles in the cellulose-FeCl3 system areliquid, compared to the solid catalyst particles in the cellulose-Fe(NO3)3 system. Again, this seems unlikely. The iron–carbonphase diagram and molecular dynamics simulations bothprovide strong evidence that Fe and Fe3C cannot be liquidunder the conditions of these experiments.8 Another possibilityis the properties of the amorphous carbon. We know that theLewis acidic nature of FeCl3 changes the decompositionpathway of cellulose.12 We also know that changes in themolecular structure and composition of biochar can lead todifferent rates of graphitization, e.g. with high-nitrogen biocharundergoing very slow graphitization.23 The different26334 | J. Mater. Chem. A, 2025, 13, 26327–26336decomposition pathway for cellulose with FeCl3 could result inan amorphous carbon with a different structure, which may bemore soluble, accelerating graphitization.A nal possibility is that the primary graphitization catalyst isdifferent between the two systems. Fig. 9 is a schematic of thedifferent crystalline phases (from Rietveld renements in Fig. 2).The solid black vertical line in each panel marks the onset ofgraphitization and it can be clearly seen that graphitization in thecellulose-Fe(NO3)3 system begins when Fe3C is the dominantcrystalline iron phase, with some a-Fe also present. As graphiti-zation progresses (and the temperature increases), the a-Feconverts to g-Fe. In contrast, the sudden onset of graphitization inthe cellulose-FeCl3 is concurrent with the carbothermal reductionof FeO to g-Fe, with Fe3C as a signicant secondary phase. BothFe3C and g-Fe are known to act as catalysts for graphitization butthe diffusion coefficients of carbon in Fe3C (Dz 10−11 cm2 s−1)24and in g-Fe (D z 10−8 cm2 s−1)25 at 725 °C indicate that carbonmoves much faster in g-Fe. Given that g-Fe is the dominantcrystalline iron phase at the onset of graphitization in the cellu-lose-FeCl3 system, it is reasonable to presume that this is thedominant catalyst. The much larger carbon diffusion rate in g-Fewould be consistent with the much faster catalyst particle speed.In contrast, Fe3C is the dominant phase in the cellulose-Fe(NO3)3system. Given themuch smaller C diffusion coefficient for Fe3C, itis reasonable that catalysis will be much slower in this system.Additionally, it has been shown that carbon diffusion in super-saturated g-Fe is much slower.26,27 It is possible that the suddenformation of g-Fe in the cellulose-FeCl3 system results in a lowerconcentration of carbon compared to the slow evolution of g-Fe inthe cellulose-Fe(NO3)3 system. Based on this, it is reasonable thatg-Fe-catalysed graphitization would be faster in the cellulose-FeCl3 system, even though g-Fe is present in both systems.ConclusionThe in situ data in this paper show that the choice of iron saltcan have a big impact on the graphitization pathway in catalyticgraphitization of cellulose. This is due to the decomposition ofthe salt, the impact of the salt on cellulose decomposition andThis journal is © The Royal Society of Chemistry 2025http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5ta03584hPaper Journal of Materials Chemistry AOpen Access Article. Published on 16 July 2025. Downloaded on 9/11/2025 11:01:00 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinethe inuence of the changed decomposition pathways onparticle size of solid-state catalyst precursors. Cellulose-Fe(NO3)3 decomposes to very small iron–oxygen clusters whichare carbothermally reduced to small Fe3C catalyst particles. Thisresults in a gradual onset of graphitization. In contrast, thelarger iron oxide particles formed during pyrolysis of cellulose-FeCl3 undergo sudden carbothermal reduction to g-Fe andFe3C, alongside strain-induced shattering of the particles. Theonset of graphitization is equally sudden. This data indicatesthat there is a critical size required for the onset of graphitiza-tion to occur. In situ TEM and SAXS/WAXS data show thatgraphitization is considerably faster in the cellulose-FeCl3system and we ascribe that to g-Fe being a much faster catalyst,consistent with reported diffusion coefficients. Given thatcellulose is the most abundant component of biomass, thisunderstanding of cellulose graphitization mechanisms isessential for moving towards carbon materials with controlledstructure and properties.ExperimentalPreparation of iron-doped celluloseFor cellulose samples, 0.68 mmol of iron salt (Fe(NO3)3$9H2O orFeCl3$6H2O) was dissolved in 10 mL of water and added to 5 g ofmicrocrystalline cellulose. The mixtures were manually stirreduntil all the solution was absorbed. Samples were then dried ina 70 °C oven overnight. The samples were then preheated forexperiments by heating in an alumina crucible in a tube furnaceand heated to 400 °C at a rate of 5 °C per minute under the owof nitrogen and held for 1 hour. This is to remove water and avoidexpansion in the capillary during the SAXS/WAXS experiment.In situ synchrotron small and wide angle X-ray scatteringmeasurementsIn situ SAXS/WAXS experiments were performed at DiamondLight Source using the I22 beamline. Pre-heated samples wereplaced in a quartz capillary (1 mm OD) and packed at either endwith quartz wool to prevent movement during the experiment.Samples were heated at 20 °C per minute to 400 °C and held for5 minutes to equilibrate before beginning the experiment. Itshould be noted that although every effort was made to ensureconsistency between experiments, it was difficult to ensure thatthe capillary was the exact same distance from the hot airblower in every experiment. Therefore there may be some errorin the temperatures stated. The capillary was heated using a hotair blower. Measurements were performed using a 14 keV beam(wavelength = 0.8856 Å), a sample to detector distance of 2.730m, and a beam size of 200 mm × 180 mm. The scattered X-rayswere detected using a Pilatus P3-2M unit from Dectris, whichhas a pixel size of 172 mm× 172 mm. A schematic of the set-up ofthe experiment is shown in Fig. S10,† alongside details of therenement method.TEM measurementsApprox. 50 mg of amorphous carbon (prepared from FeCl3-doped cellulose at 400 °C) was dispersed in 1 mL of ethanol byThis journal is © The Royal Society of Chemistry 2025sonication for 10 minutes. One drop of the dispersion waspipetted on to a Protochips Thermal E-chip (E-FHDC). In situTEM footage was collected on a JEOL JEM-ARM200F equippedwith a Schottky eld emission gun. The sample was heated at 1 °C per second up to 500 °C. Samples were then heated at 0.5 °Cper second up to 600 °C and held for approx. 10 minutes insidethe microscope.Estimation of critical catalyst diameterFor catalyst spheres of diameter 1–50 nm, the surface area andvolume were calculated and the mass of iron calculated usingan austenite density of 7.65 g cm−3 (800 °C).28 The areal densityof a single graphene sheet can be calculated as 7.6× 10−4 g m−2and from this, we can estimate the mass of carbon required toform a single-atom thick graphene layer around a catalystsphere. From this, we can calculate the mass% of carbonrequired if all the carbon was to precipitate from a catalystsphere to form a single-atom thick graphene layer.Data availabilityAll the data for this project can be found at the following https://doi.org/10.25500/edata.bham.00001281.Conflicts of interestThere are no conicts to declare.AcknowledgementsThe authors acknowledge the Leverhulme Trust (RPG-2020-076)for funding and Diamond Light Source for synchrotron beam-time access.References1 Y. Liu, H. Shi and Z. S. Wu, Energy Environ. Sci., 2023, 16,4834–4871.2 European Commission, Critical Raw Materials Factsheets(2020), 2020.3 BEIS, Resilience for the Future: the United Kingdom's CriticalMinerals Strategy, 2022.4 J. L. Rowlandson, K. J. Edler, M. Tian and V. P. Ting, ACSSustainable Chem. Eng., 2020, 8, 2186–2195.5 Z. Shi, S. Wang, Y. Jin, L. Zhao, S. Chen, H. Yang, Y. Cui,R. Svanberg, C. Tang, J. Jiang, W. Yang, P. G. Jönsson andT. Han, SusMat, 2023, 3, 402–415.6 M. Mennani, A. 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Shahriari, E. Ben Fredj, J.-B. Morin andM. Jahazi, J. Manuf. Mater. Process., 2018, 2(2), 34.This journal is © The Royal Society of Chemistry 2025http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h In situ TEM and synchrotron SAXS/WAXS study on the impact of different iron salts on iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03584h